metals

Review Tempforming as an Advanced Processing Method for Carbon

Anastasiya Dolzhenko, Rustam Kaibyshev and Andrey Belyakov * Laboratory of Mechanical Properties of Nanostructured Materials and Superalloys, Belgorod State University, 308015 Belgorod, Russia; [email protected] (A.D.); [email protected] (R.K.) * Correspondence: [email protected]; Tel.: +7-4722-585457

 Received: 28 October 2020; Accepted: 20 November 2020; Published: 24 November 2020 

Abstract: The microstructural mechanisms providing delamination toughness in high-strength low-alloyed steels are briefly reviewed. Thermo-mechanical processing methods improving both the strength and impact toughness are described, with a close relation to the microstructures and textures developed. The effect of processing conditions on the microstructure evolution in steels with different carbon content is discussed. Particular attention is paid to tempforming treatment, which has been recently introduced as a promising processing method for high-strength low-alloyed semi-products with beneficial combination of strength and impact toughness. Tempforming consists of large strain warm rolling following . In contrast to ausforming, the steels subjected to tempforming may exhibit an unusual increase in the impact toughness with a decrease in test temperature below room temperature. This phenomenon is attributed to the notch blunting owing to easy splitting (delamination) crosswise to the principle crack propagation. The relationships between the crack propagation mode, the delamination fracture, and the load-displacement curve are presented and discussed. Further perspectives of tempforming applications and promising research directions are outlined.

Keywords: high-strength carbon steels; thermo-mechanical treatment; ausforming; tempforming; strength; impact toughness; ductile-brittle transition temperature

1. Introduction Carbon steels are widely used structural materials. One of drawbacks of such steels is their relatively high temperature of ductile-brittle transition in the hardened state that makes the steels brittle at temperatures just below room temperature and, correspondingly, restricts their applications at lowered temperatures. Comprehensive studies of the fracture mechanisms and the conditions of the ductile-brittle transition have been carried out from the middle of the last century. According to the Yoffee diagram, an increase in the resistance to brittle fracture and/or a decrease in the effective yield stress should decrease the ductile-brittle transition temperature [1]. A common approach to suppress the brittle intercrystalline fracture involves the grain refinement and grain boundary precipitations. On the other hand, a decrease in the effective flow stress at the crack tip can be achieved in the microstructure, which spontaneously delaminates ahead of the crack crosswise to the crack propagation direction and, thus, blunts the crack tip. Both approaches, i.e., the grain refinement and the delamination, have been utilized through thermo-mechanical treatment that is commonly known as ausforming, under the conditions of stable or meta-stable austenite. The hot working of austenite is intended to refine the deformation microstructure along with a certain work [2]. The latter changes the intergranular fracture mode along prior austenite grain boundaries to a transgranular fracture across prior austenite grains. Moreover, the hot rolling of austenite results in heterogeneous banding of the original cast structure, which leads to crack

Metals 2020, 10, 1566; doi:10.3390/met10121566 www.mdpi.com/journal/metals Metals 2020, 10, 1566 2 of 20 blunting and increases the impact strength, owing to delamination along the segregation/inclusion bands. The fracture mechanisms and ausforming treatments improving the impact toughness are briefly reviewed and discussed in Sections2 and3. An alternative approach to increase the toughness and decrease the ductile-brittle transition temperature of plain carbon steels has been suggested by Kimura et al. [3]. The proposed method consists in the formation of a submicrocrystalline lamella type microstructure with a uniform dispersion of secondary phase particles by means of warm rolling under conditions of tempering. Such thermo-mechanical treatment, which has been referred to as tempforming, provides an outstanding combination of mechanical properties in low-alloyed carbon steels. Processed steel has high strength due to the reduction of grain size and as well as high impact toughness owing to delamination toughness. A superior combination of the ultimate tensile strength of 1800 MPa and impact toughness (KCV) of almost 240 J/cm2 at room temperature has been obtained in a medium carbon low-alloyed steel [3]. The present review is aimed to highlight the various benefits of tempforming in a brief comparison with ordinary thermo-mechanical treatments used to improve impact toughness of high-strength steels and outline the most promising directions for further research. The developed microstructures and fracture behavior of low-alloyed steel subjected to tempforming are considered in Section4. The e ffects of alloying extent and tempforming conditions on the strength and impact toughness of low-alloyed steels are critically reviewed and discussed with a reference to underlying structural mechanisms responsible to deformation behavior and fracture. Specific emphasis is placed on the fracture mechanisms that may unexpectedly lead to increasing the impact toughness in carbon steels while decreasing the test temperature. Finally, prospective investigations and applications of tempforming and related phenomena are summarized in Section5.

2. Impact Toughness

2.1. Fracture Mechanisms Generally, structural steels and alloys may exhibit two types of fracture behavior during impact tests, either ductile or brittle [3–5]. The dominant fracture mechanism is governed by microstructure and testing conditions. The ductile fracture is characterized by general yield when remarkable plastic deformation precedes the failure. The specimen fracture in such a case is associated with a gradual change in the cross section. The ductile fracture of impact specimen is accompanied by stable crack propagation when the crack motion follows local plastic deformation and is controlled by applied load. The ductile crack growth occurs with relatively large energy consumption, which results in high impact toughness. The most frequently referred to mechanism of ductile fracture consists in steady void nucleation followed by their growth and coalescence, finally leading to specimen failure. The corresponding fracture surface is composed with many dimples with tearing edges (Figure1a) [ 6]. The void nucleation is commonly attributed to dislocation storage caused by some obstacles for dislocation motion and microstrain incompatibilities [5,7–9]. Various non-metallic inclusions and second phase particles were frequently considered as void nucleation centers [10]. In contrast to ductile fracture, brittle fracture is associated with unstable crack propagation, which starts under applied stress below the general yield stress [3,4]. Generally, the brittle fracture takes place when the effective yield stress exceeds the critical stress for material detachment (or cleavage) at the crack tip. Therefore, the brittle fracture occurs without notable plastic deformation at the front of the growing crack. In this case, the crack grows spontaneously and does not consume remarkable energy, resulting in pretty low impact toughness. The microstructural mechanisms of brittle fracture of structural steels and alloys consist in intercrystalline splitting and/or plane cleavage of crystal lattice. The latter is characterized by wavy steps on the fracture surface (Figure1b) [ 6]. A plane of {100} is a typical cleavage plane in ferrite/martensite [4]. Metals 2020, 10, x FOR PEER REVIEW 2 of 20

segregation/inclusion bands. The fracture mechanisms and ausforming treatments improving the impact toughness are briefly reviewed and discussed in Sections 2 and 3. An alternative approach to increase the toughness and decrease the ductile‐brittle transition temperature of plain carbon steels has been suggested by Kimura et al. [3]. The proposed method consists in the formation of a submicrocrystalline lamella type microstructure with a uniform dispersion of secondary phase particles by means of warm rolling under conditions of tempering. Such thermo‐mechanical treatment, which has been referred to as tempforming, provides an outstanding combination of mechanical properties in low‐alloyed carbon steels. Processed steel has high strength due to the reduction of grain size and precipitation hardening as well as high impact toughness owing to delamination toughness. A superior combination of the ultimate tensile strength of 1800 MPa and impact toughness (KCV) of almost 240 J/cm2 at room temperature has been obtained in a medium carbon low‐alloyed steel [3]. The present review is aimed to highlight the various benefits of tempforming in a brief comparison with ordinary thermo‐mechanical treatments used to improve impact toughness of high‐strength steels and outline the most promising directions for further research. The developed microstructures and fracture behavior of low‐alloyed steel subjected to tempforming are considered in Section 4. The effects of alloying extent and tempforming conditions on the strength and impact toughness of low‐alloyed steels are critically reviewed and discussed with a reference to underlying structural mechanisms responsible to deformation behavior and fracture. Specific emphasis is placed on the fracture mechanisms that may unexpectedly lead to increasing the impact toughness in carbon steels while decreasing the test temperature. Finally, prospective investigations and applications of tempforming and related phenomena are summarized in Section 5.

2. Impact Toughness

2.1. Fracture Mechanisms Generally, structural steels and alloys may exhibit two types of fracture behavior during impact tests, either ductile or brittle [3–5]. The dominant fracture mechanism is governed by microstructure and testing conditions. The ductile fracture is characterized by general yield when remarkable plastic deformation precedes the failure. The specimen fracture in such a case is associated with a gradual change in the cross section. The ductile fracture of impact specimen is accompanied by stable crack propagation when the crack motion follows local plastic deformation and is controlled by applied load. The ductile crack growth occurs with relatively large energy consumption, which results in high impact toughness. The most frequently referred to mechanism of ductile fracture consists in steady void nucleation followed by their growth and coalescence, finally leading to specimen failure. The corresponding fracture surface is composed with many dimples with tearing edges (Figure 1a) [6]. The void nucleation is commonly attributed to dislocation storage caused by Metalssome2020, 10obstacles, 1566 for dislocation motion and microstrain incompatibilities [5,7–9]. Various non‐metallic3 of 20 inclusions and second phase particles were frequently considered as void nucleation centers [10].

Metals 2020, 10, x FOR PEER REVIEW 3 of 20

Figure 1. Examples of ductile (a) and brittle (b) fracture surfaces of 10Cr9WMoVNbB steel samples subjected to impact tests at 20 °C and −80 °C, respectively. Reproduced with permission from [6], Springer Nature, 2016.

In contrast to ductile fracture, brittle fracture is associated with unstable crack propagation, which starts under applied stress below the general yield stress [3,4]. Generally, the brittle fracture takes place when the effective yield stress exceeds the critical stress for material detachment (or

cleavage) at the crack tip. Therefore, the brittle fracture occurs without notable plastic deformation at (a) (b) the front of the growing crack. In this case, the crack grows spontaneously and does not consume Figureremarkable 1. Examples energy, of resulting ductile ( ain) andpretty brittle low (impactb) fracture toughness. surfaces The of 10Cr9WMoVNbB microstructural mechanisms steel samples of subjectedbrittle fracture to impact of testsstructural at 20 steelsC and and80 alloysC, respectively. consist in Reproducedintercrystalline with splitting permission and/or from plane [6], ◦ − ◦ Springercleavage Nature, of crystal 2016. lattice. The latter is characterized by wavy steps on the fracture surface (Figure 1b) [6]. A plane of {100} is a typical cleavage plane in ferrite/martensite [4]. 2.2. Ductile-Brittle Transition 2.2. Ductile‐Brittle Transition The impact toughness of plain carbon and high-strength low-alloyed steels depends substantially on temperatureThe impact of impact toughness tests (Figure of plain2a) [ 1carbon]. Typically, and high the steel‐strength impact low toughness‐alloyed steels slightly depends decreases with asubstantially decrease in on the temperature test temperature of impact to or tests below (Figure room 2a) temperature [1]. Typically, followed the steel by impact drastic toughness drop as the temperatureslightly furtherdecreases approaches with a decrease cryogenic in the test range. temperature Therefore, to or the below temperature room temperature dependence followed of impactby drastic drop as the temperature further approaches cryogenic range. Therefore, the temperature toughness is characterized by two shelves, i.e., the lower shelf in the region of low temperature and dependence of impact toughness is characterized by two shelves, i.e., the lower shelf in the region of upper shelf in the domain of around room temperature and upward. The inflection corresponds to low temperature and upper shelf in the domain of around room temperature and upward. The ductile-brittleinflection transition corresponds temperature. to ductile‐brittle The changetransition in temperature. the impact toughness The change with in the the impact test temperature toughness is associatedwith the with test the temperature change in is fracture associated mechanism. with the change Namely, in fracture commonly mechanism. ductile Namely, fracture commonly at relatively high temperaturesductile fracture becomes at relatively brittle high as temperaturetemperatures decreases.becomes brittle The as transition temperature from decreases. ductile to The brittle fracturetransition occurs infrom rather ductile wide to temperaturebrittle fracture interval, occurs wherein rather the wide percentage temperature of brittle interval, fracture where gradually the increasespercentage to 100% of atbrittle expense fracture of ductilegradually fracture increases [4]. to 100% at expense of ductile fracture [4].

(a) (b)

FigureFigure 2. Schematic 2. Schematic relationship relationship between between impact impact toughness toughness and temperature and temperature of ductile-brittle of ductile‐ transition,brittle TDBT (transition,a) and eff ectsTDBT of(a) brittle and effects fracture of brittle stress, fractureσB, and stress, effective σB, and yield effective stress, yieldσY, on stress, TDBT σY(,b on). ReproducedTDBT (b). with permissionReproduced from with [permission1], AAAS, from 2008. [1], AAAS, 2008.

The changeThe change in the in fracture the fracture mechanisms mechanisms and and the impactthe impact toughness toughness can can be illustratedbe illustrated by by the the Yo ffee diagram,Yoffee which diagram, compares which compares the temperature the temperature effects oneffects brittle on brittle fracture fracture stress stress and and effective effective yield yield stress (Figurestress2b) [ (Figure1]. The 2b) former [1]. isThe almost former temperature is almost temperature independent, independent, while the latter while rapidly the latter increases rapidly with increases with a decrease in temperature. Upon mechanical loading, the growing stress at the crack a decrease in temperature. Upon mechanical loading, the growing stress at the crack tip can reach tip can reach either general yield stress (σY) leading to plastic deformation and subsequent ductile either general yield stress (σY) leading to plastic deformation and subsequent ductile crack propagation crack propagation or brittle fracture stress (σB), when fracture occurs in brittle manner. Since the or brittlefracture fracture mode stressis controlled (σB), whenby the fracturelower stress occurs among in the brittle brittle manner. fracture Sinceand yield the stresses, fracture brittle mode is controlledfracture by occurs the lower at low stress temperatures among the while brittle ductile fracture fracture and is more yield probable stresses, at brittle higher fracture temperatures; occurs at the intersection point of the effective yield stress and the brittle fracture stress can be considered as Metals 2020, 10, 1566 4 of 20 low temperatures while ductile fracture is more probable at higher temperatures; the intersection point of the effective yield stress and the brittle fracture stress can be considered as ductile-brittle transition temperature. According to the Yoffee diagram, the temperature range of ductile fracture can Metals 2020, 10, x FOR PEER REVIEW 4 of 20 be expanded toward low temperatures due to the increase in brittle fracture stress. This can be realized owingductile to grain‐brittle refinement. transition In temperature. fact, the grain According refinement to the increases Yoffee diagram, both the the brittle temperature fracture stressrange andof the effectiveductile yield fracture stress. can However, be expanded an increment toward low in thetemperatures brittle fracture due to stress the increase with a in decrease brittle fracture in the grain size exceedsstress. This the can corresponding be realized owing increase to grain in the refinement. effective In yield fact, the stress grain [11 refinement]. This beneficial increases e ffbothect the of grain size onbrittle mechanical fracture propertiesstress and the has effective been encouraging yield stress. the However, development an increment of processing in the methodsbrittle fracture involving stress with a decrease in the grain size exceeds the corresponding increase in the effective yield stress the grain refinement. [11]. This beneficial effect of grain size on mechanical properties has been encouraging the 3. Thermo-Mechanicaldevelopment of processing Treatment methods involving the grain refinement.

3.1. Ausforming3. Thermo‐Mechanical Treatment Starting3.1. Ausforming from the middle of the last century, the great attention has been paying to thermal and thermo-mechanicalStarting from treatments the middle in of combination the last century, with the the great modification attention has of been the paying chemical to thermal composition and of high-strengththermo‐mechanical low-alloyed treatments steels in combination order to obtain with athe specific modification microstructure, of the chemical which composition should provide of a highhigh level‐strength of strength low‐alloyed and toughness.steels in order It to has obtain been a specific observed microstructure, that surprisingly which should high strength provide a could be obtainedhigh level in of low- strength and (mainly toughness. nickel–chromium) It has been observed steels that by surprisingly warm working high strength of austenite could prior be to martensiteobtained (or in bainite) low‐alloy transformation (mainly nickel–chromium) [12–14]. Such steels thermo-mechanical by warm working treatmentof austenite involving prior to the deformationmartensite of meta-stable(or bainite) transformation austenite has been[12–14]. called Such ausforming thermo‐mechanical [15]. Ausforming treatment involving consists of the plastic deformation of meta‐stable austenite has been called ausforming [15]. Ausforming consists of plastic deformation in the temperature range between Ar1 and MS followed by to martensite before deformation in the temperature range between Ar1 and MS followed by quenching to martensite bainite/pearlite transformation (Figure3a). Then, the martensite in ausformed steels is subjected to before bainite/pearlite transformation (Figure 3a). Then, the martensite in ausformed steels is conventionalsubjected tempering to conventional treatment. tempering Some earlytreatment. examples Some of early the improvementexamples of the of mechanical improvement properties of owingmechanical to ausforming properties are listedowing in to Tableausforming1[12–14 are,16 listed]. in Table 1 [12–14,16].

(a) (b)

FigureFigure 3. Schematics 3. Schematics of of ausforming ausforming ((aa) and modified modified ausforming ausforming (b) ( treatments.b) treatments.

Table 1. Offset yield strength ( ), ultimate tensile strength (UTS), elongation ( ), and impact toughness Table 1. Offset yield strengthσ0.2 (σ0.2), ultimate tensile strength (UTS), elongationδ (δ), and impact (KCV)toughness in some steels(KCV) subjectedin some steels to ausforming subjected orto conventionalausforming or quenching-tempering conventional quenching treatment.‐tempering treatment. 2 Steel T, ◦C σ0.2 *, MPa UTS *, MPa δ *, % KCV *, J cm− Ref. −·2 0.35%CSteel 20T, °C 2050 σ/0.21690*, MPa 2260UTS*,/2030 MPa δ 7.5*, /%8.5 KCV*, J∙cm - Ref. [12] 0.40%C0.35%C 2020 21902050/1690/1790 23702260/2030/2160 7.5/8.5 6.5/8.1 ‐ -[12] [12] 0.49%C0.40%C 2020 21202190/1790/1830 23002370/2160/2080 6.5/8.1 5.5/7.6 ‐ -[12] [12] 43400.49%C 2020 20502120/1830/1820 22802300/2080/2100 5.5/7.6 8/10 ‐ 14.7/7.8[12] [13] 4340 80 - - - 11.8/4.9 [13] 4340 − 20 2050/1820 2280/2100 8/10 14.7/7.8 [13] 30CrNiMo4340 ‐ 2080 ‐ ‐ ‐ 1800/1500 2100/1800 6//1011.8/4.9 -[13] [16] 40Cr3Mo3SiV30CrNiMo20 20 21001800/1500/1800 23702100/1800/2160 6//10 4/5 ‐ -[16] [14] 40CrNi2Mo 20 1970/1650 2180/1930 10/10 - [14] 40Cr3Mo3SiV 20 2100/1800 2370/2160 4/5 ‐ [14] * Numerator40CrNi2Mo corresponds to ausforming,20 1970/1650 denominator 2180/1930 - conventional 10/10 ‐ quenching-tempering[14] treatment. * Numerator corresponds to ausforming, denominator ‐ conventional quenching‐tempering treatment. It has been suggested that the structural mechanisms responsible for superior strength owing It has been suggested that the structural mechanisms responsible for superior strength owing to to ausformingausforming are are associated associated with with direct direct inheritance inheritance by the by martensite the martensite of defects of defects generated generated during the during Metals 2020, 10, 1566 5 of 20 the warm working of the austenite [14,17]. Recently, the dislocation density that inherited from the deformed austenite has been confirmed as the main strengthener, irrespective of grain refinement caused by plastic deformation of austenite [18]. The most important inherited structural/substructural elements are dislocation tangles, sub-boundaries, and finely dispersed carbide particles. The ausforming is, therefore, controlled by two principal processes; namely, plastic deformation of meta-stable austenite and subsequent martensitic transformation. The plastic deformation for ausforming can be carried out by various methods like rolling and forging for large scale production. The most important deformation parameters for a certain processing method are the deformation temperature and total strain [14]. The different effect of the warm working strain on the subsequent martensitic transformation has been reported depending on the transformation mechanisms [19]. The plate martensite formation is scarcely affected by previous warm working, whereas austenite tends to be mechanically stabilized at a pre-strain level above 20% against the lath martensite formation. This different strain effect during ausforming on martensitic transformation has been attributed to different effect of austenite dislocation substructure on the growth of martensite plate and laths. On the other hand, the strength of lath martensite depends significantly on the deformation of austenite in the range of small strains, where the dislocation density is greatly affected by the strain. MS temperature increases with an increase in pre-strain to a critical value, and then decreases approaching a constant level with further pre-straining [20]. An increase in ausforming temperature increases the probability of detrimental phase transformations like formation of coarse granular and/or plate-like bainite during warm working of austenite that increases the size of final structural elements and reduced the strengthening [21]. In contrast, decreasing the ausforming temperature enhances the effectiveness of warm working to create the highly dislocated substructure in austenite. Moreover, a decrease in the ausforming temperature promotes the refinement of microstructure that is desirable for impact toughness [22]. The temperature and strain of warm deformation of austenite should be optimized in order to minimize the prior austenite grain size with sufficiently high dislocation density on the one hand, and reduce the working load as much as possible on the other. Similar to conventional heat treated structural steels, carbon is the essential alloying element governing the mechanical properties of ausformed steels [14]. Generally, an increase in the carbon content increases the strengthening of ausformed steels, although effectiveness of strengthening by carbon decreases as its content increases. Other alloying elements may also enhance the ausforming effect [14,23]. Such elements as chromium and molybdenum are very beneficial for steels intended for ausforming. These elements separate the ranges of pearlite and bainite transformations and, therefore, expand the range of meta-stable austenite, making easier the selection of appropriate processing conditions. On the other hand, the carbide forming elements result in finely dispersed particles that precipitate in austenite during warm working, increasing the strength of ausformed steels.

3.2. Modified Ausforming In spite of a great success in using the ausforming for improvement of mechanical properties of structural steels, its applicability has some limitations. The most serious drawback of ausforming is a requirement of certain stability of undercooled austenite for hardenable steels. An increased demand for high-strength low-alloyed steels with greatly improved impact toughness at lowered temperatures has encouraged extensive research works dealing with relevant thermo-mechanical treatments. An original solution to the problem has been found. This consists in hot rolling of the steels in the temperature range of stable austenite followed by quenching to martensite and tempering (Figure3b). Such processing technique has been referred to as modified ausforming, to distinguish it from ordinary ausforming, which involves warm working of metastable austenite [24,25]. The modified ausforming is also known as high temperature thermo-mechanical treatment [14,23]. Of particular interest is an application of modified ausforming to increase the low temperature impact properties Metals 2020, 10, 1566 6 of 20 of low- and medium-alloyed steels, whose S-curve locates in the range of short times [25]. Arbitrary selected examples of steel properties after modified ausforming are shown in Table2[23,26–30].

Table 2. Offset yield strength (σ0.2), ultimate tensile strength (UTS), elongation (δ), and impact toughness (KCV) of some steels subjected to modified ausforming or conventional quenching-tempering treatment.

2 Steel T, ◦C σ0.2 *, MPa UTS *, MPa δ *, % KCV *, J cm Ref. · − 50CrNi4Mo 20 1850/1650 2110/1870 25/29 - [26] 40CrNiMo 20 - 2460/2280 9.8/9.7 - [27] 0.5%C, 2.6%Si 20 2130/1930 2460/2250 5/5 - [28] 0.6%C, 1.6%Si 20 2080/1820 2550/2160 7/5.5 - [28] 0.6%C, 2.2%Si 20 2320/1900 2700/2200 7/6 - [28] 0.6%C, 2.6%Si 20 2230/1930 2700/2250 7/5 - [28] 20CrMnB 20 - 1630/1570 39.5/30 - [29] 0.4C-Ni-Cr-Mo 20 1680/1500 1940/1800 15/10 15/10 [30] 0.4C-Ni-Cr-Mo 80 - - - 13/6 [30] − 4340 20 1720/1460 2200/1850 9.8/5.8 13.5/10.8 [23] 4340 80 - - - 12.6/5 [23] − * Numerator corresponds to modified ausforming, denominator - conventional quenching-tempering treatment.

Early studies on modified ausforming have revealed two factors affecting the impact strength [24]. Both of them are associated with the change in the fracture mode. First is the change from intergranular fracture along prior austenite grain boundaries to transgranular fracture. The former frequently occurs if the austenite experienced recrystallization during hot deformation treatment, whereas the latter happens in the steels with non-recrystallized (work hardened) austenite. The second is associated with the mechanism of transgranular fracture, which should proceed as delamination. This fracture mechanism is closely connected with segregations leading to sulfides with brittle interfaces. The steels subjected to modified ausforming are characterized by planar arrangement of sulfide inclusions that delamination along the rolling plane, resulting in crack blunting and high impact toughness in longitudinal specimens [24,31]. The beneficial effect of modified ausforming on the mechanical behavior of high-strength steels consists in simultaneous enhancement of both impact toughness and strength. An increase in rolling reduction during modified ausforming leads to direct increase in the strength while ductility does not reduce. Moreover, the steels subjected to modified ausforming exhibit lower ductile-brittle transition temperature as compared to those processed by conventional quenching and tempering [32]. The desirable microstructural state that may provide this useful effect is associated with cell substructure with high dislocation density that evolves in austenite and then is inherited by martensite [25]. The cell substructure consisting of movable dislocations that is introduced to martensite through the transformation has been suggested as being responsible for relieving the high stress concentration at a crack tip, improving ductility. In addition, the dislocation cell substructure enhances the dispersion of second-phase particles in martensite that is favorable for strength and ductility [33]. It should be noted here retained austenite itself does not contribute to the excellent mechanical properties of the steels subjected to modified ausforming, because these steels are commonly characterized by a rather small amount of retained austenite [25]. The cooling regime following the modified ausforming is another important factor affecting the final microstructure and properties of the steels. In order to maintain the beneficial ausforming effect, cooling of the processed steels should be carried out as fast as possible to prevent any static recrystallization and grain growth processes that can remove the developed dislocation substructures [25]. On the other hand dynamic recrystallization during hot working under conditions of modified ausforming may lead to useful microstructure consisting of fine grains with irregular boundaries and dislocation cell substructure [34–37]. Dynamic recrystallization rapidly develops in austenite with a rather low stacking fault energy during hot deformation. The new grains are formed due to local migration of grain boundaries (bulging) towards the high dislocation density [35]. Such a mechanism of dynamic recrystallization is classified as discontinuous [38]. The dynamic grain size decreases with Metals 2020, 10, x FOR PEER REVIEW 7 of 20

density [35]. Such a mechanism of dynamic recrystallization is classified as discontinuous [38]. The dynamicMetals 2020grain, 10 , sizex FOR decreases PEER REVIEW with decreasing temperature and/or increasing strain rate 7(Figure of 20 4) Metals 2020, 10, 1566 7 of 20 [39–45]. Note in Figure 4 that the strong temperature/strain rate dependence of dynamic grain size underdensity conditions [35]. Such of ahot mechanism working of becomesdynamic recrystallizationweaker with ais decreaseclassified asin discontinuous deformation [38]. temperature The dynamic grain size decreases with decreasing temperature and/or increasing strain rate (Figure 4) decreasingbecause of temperature the change and in dynamic/or increasing recrystallization strain rate (Figure mechanism.4)[ 39–45 Thus,]. Note the in required Figure4 thatstructural the strong state [39–45]. Note in Figure 4 that the strong temperature/strain rate dependence of dynamic grain size with a grain size of less than a micrometer can be obtained by processing at relatively low temperatureunder conditions/strain rate of dependence hot working of becomes dynamic weaker grain with size undera decrease conditions in deformation of hot working temperature becomes weakertemperatures.because with aof decrease the change in deformationin dynamic recrystallization temperature because mechanism. of the Thus, change the inrequired dynamic structural recrystallization state mechanism.with a grain Thus, size the of required less than structural a micrometer state withcan be a grainobtained size by of processing less than aat micrometer relatively low can be obtainedtemperatures. by processing at relatively low temperatures.

Figure 4. Effect of temperature‐compensated strain rate on the grain size evolved by dynamic FigureFigure 4. E 4.ffect Effect of temperature-compensatedof temperature‐compensated strain rate rate on on the the grain grain size size evolved evolved by bydynamic dynamic recrystallization. Reproduced with permission from from [45], Elsevier, 2016. recrystallization.recrystallization. Reproduced Reproduced with with permission permission from from fromfrom [45], [45], Elsevier, Elsevier, 2016. 2016.

AbilityAbilityAbility of of grain grainof grain boundaries boundaries boundaries to to to migrate migrate migrate decreases decreases with with decreasing decreasing decreasing deformation deformation deformation temperature temperature temperature that that that slowsslowsslows down down down the the kinetics the kinetics kinetics of of discontinuous of discontinuous discontinuous dynamic dynamic recrystallization recrystallization recrystallization (Figure (Figure (Figure5 5))[ [46]. 465) ].[46]. Under Under Under conditions conditions conditions of warmof warmof working warm working working at relatively at atrelatively relatively low temperatures, lowlow temperatures,temperatures, the austenite thethe microstructureaustenite austenite microstructure microstructure evolution evolution is accompanied evolution is is byaccompanied theaccompanied development by bythe ofthe development so-called development continuous of of soso‐‐called dynamic continuouscontinuous recrystallization dynamic dynamic recrystallization recrystallization resulting from resulting anresulting increase from from in an increase in the misorientation of deformation sub‐boundaries to typical values of ordinary grain thean misorientationincrease in the misorientation of deformation of sub-boundaries deformation sub to‐boundaries typical values to typical of ordinary values grain of ordinary boundaries grain boundaries and the formation of a continuous net of strain‐induced grain boundaries [38]. andboundaries the formation and ofthe a continuousformation netof ofa strain-inducedcontinuous net grain of strain boundaries‐induced [38 ].grain Continuous boundaries dynamic [38]. Continuous dynamic recrystallization is characterized by low kinetics, so the formation of a Continuous dynamic recrystallization is characterized by low kinetics, so the formation of a recrystallizationhomogeneous is characterizedultrafine‐grained by microstructure low kinetics, soduring the formation warm working of a homogeneousrequires large strains ultrafine-grained [38,46]. homogeneous ultrafine‐grained microstructure during warm working requires large strains [38,46]. microstructureA promising during way warm to obtain working ultrafine requires grained large strainsaustenite [38 is,46 the]. A treatment promising under way to intermediate obtain ultrafine grainedA promisingtemperature austenite way conditions is the to treatment obtain between ultrafine under discontinuous intermediate grained and austenite continuous temperature is dynamic the conditions treatment recrystallization, between under discontinuous when intermediate the andtemperature continuoustwo mechanisms conditions dynamic accelerate recrystallization,between each discontinuous other (Figure when 5). the and Onset two continuous mechanismsof discontinuous dynamic accelerate dynamic recrystallization, each recrystallization other (Figure when 5 the). Onsettwo inmechanisms of the discontinuous transition accelerate region dynamic follows each recrystallization the other high (Figure density 5).of in strainOnset the transition‐induced of discontinuous high region‐angle follows grain dynamic boundaries. the highrecrystallization density of strain-inducedin the transition high-angle region follows grain boundaries.the high density of strain‐induced high‐angle grain boundaries.

Figure 5. Effect of deformation conditions on the initiation of discontinuous dynamic recrystallization (DDRX) corresponding roughly to peak stress strain or on the development of continuous dynamic recrystallization (CDRX) indicated by various percentage of high-angle grain boundaries (HAB). Reproduced with permission from from [46], Elsevier, 2013. Metals 2020, 10, x FOR PEER REVIEW 8 of 20

Figure 5. Effect of deformation conditions on the initiation of discontinuous dynamic recrystallization (DDRX) corresponding roughly to peak stress strain or on the development of continuous dynamic recrystallization (CDRX) indicated by various percentage of high‐angle grain Metals 2020, 10, 1566 8 of 20 boundaries (HAB). Reproduced with permission from from [46], Elsevier, 2013.

The thermo-mechanicalthermo‐mechanical treatment involving hot working under conditions of discontinuousdiscontinuous dynamic recrystallization followed by gradual decrease in temperature down to two-phasetwo‐phase (austenite-ferrite)(austenite‐ferrite) domain domain of of low low-alloyed‐alloyed steels steels has has been been referred referred to as to controlled as controlled rolling rolling [47]. [ 47An]. Anadvantage advantage of this of technique this technique is the isbeneficial the beneficial microstructure microstructure evolved evolved in two stages, in two i.e., stages, the austenite i.e., the austenitegrain refinement grain refinement owing to discontinuous owing to discontinuous dynamic recrystallization dynamic recrystallization followed by followed work hardening by work hardeningassisted by assisted partial by γ‐ partialα phaseγ-α transformation.phase transformation. An increase An increase in strain in strain in inthe the two two phase phase domain improves both the strength and impact toughness along with a decrease inin ductile-brittleductile‐brittle transition temperature. TheThe highly highly elongated elongated grains grains/sub/sub-grains‐grains with with dislocation dislocation cell substructure cell substructure evolved evolved during controlledduring controlled rolling have rolling been have considered been considered as the main as factorsthe main responsible factors responsible for the improved for the mechanical improved propertiesmechanical [ 23properties,30,47]. An [23,30,47]. embrittlement An embrittlement across the rolled across plate the thickness rolled causedplate thickness by crystallographic caused by texturecrystallographic and plan sulphidetexture and distribution plan sulphide is responsible distribution to delamination is responsible and notchto delamination blunting upon and impact notch tests,blunting while upon the impact high dislocation tests, while density the high strengthens dislocation the density steels. strengthens the steels. In orderorder to to obviate obviate the the harmful harmful effect effect of static of recrystallizationstatic recrystallization and grain and growth grain on growth the properties on the ofproperties steels subjected of steels to modifiedsubjected ausforming,to modified the ausforming, steels are alloyed the steels with are some alloyed carbide with forming some elements, carbide typicallyforming elements, niobium. typically The effect niobium. of solute The drag effect and of dispersed solute drag precipitations and dispersed on precipitations the recrystallization on the kineticsrecrystallization in plain-carbon kinetics (C)in plain and‐carbon Nb-modified (C) and (Nb) Nb‐ steelsmodified is schematically (Nb) steels is shownschematically in Figure shown6[ 48 in]. BothFigure the 6 recrystallization [48]. Both the startrecrystallization (Rs) and finish start (Rf) (Rs) are significantlyand finish (Rf) delayed are insignificantly Nb-modified delayed steel. It in is importantNb‐modified to note steel. that It Nbis important solute plays to anote crucial that role Nb in solute the retardation plays a crucial of recovery role in and the recrystallization. retardation of Correspondingrecovery and timesrecrystallization. for recrystallization Corresponding start and finishtimes arefor indicated recrystallization by Rs(S) andstart Rf(S), and respectively, finish are inindicated Figure6 by. The Rs(S) dispersed and Rf(S), particles respectively, e ffectively in Figure retard 6. The static dispersed recrystallization particles effectively when recrystallization retard static nucleationrecrystallization is already when delayed recrystallization by solute additions.nucleation is already delayed by solute additions.

Figure 6. EffectEffect of solid solution and precipitation on dynamic recrystallizationrecrystallization behavior through comparison of plain plain-carbon‐carbon (C) and Nb Nb-modified‐modified (Nb) steels. Reproduced Reproduced with with permission permission from from [48], [48], Taylor && Francis,Francis, 1979.1979. 4. Tempforming 4. Tempforming 4.1. Microstructure 4.1. Microstructure Recently, an original approach to solute the problems of low impact toughness and high Recently, an original approach to solute the problems of low impact toughness and high temperature of ductile-brittle transition of high-strength carbon steels has been revealed by Japanese temperature of ductile‐brittle transition of high‐strength carbon steels has been revealed by Japanese group [3,49]. The suggested method consists of warm rolling with large reductions following group [3,49]. The suggested method consists of warm rolling with large reductions following the the tempering of quenched martensite. The warm rolling has been suggested to carry out at a tempering of quenched martensite. The warm rolling has been suggested to carry out at a tempering tempering temperature; thus, this processing technique has been called tempforming. In contrast temperature; thus, this processing technique has been called tempforming. In contrast to ausforming to ausforming and modified ausforming, the tempforming treatment is carried out after quenching and modified ausforming, the tempforming treatment is carried out after quenching and tempering. and tempering. Tempforming, therefore, is the final processing stage for technology production of Tempforming, therefore, is the final processing stage for technology production of rolled steel rolled steel semi-products. Originally proposed for caliber bar rolling [49], tempforming has been also semi‐products. Originally proposed for caliber bar rolling [49], tempforming has been also revealed revealed as a promising alternative for modified ausforming applied for plate rolling [50]. as a promising alternative for modified ausforming applied for plate rolling [50]. The enhancement of mechanical properties is closely related to the deformation microstructure evolved in tempered martensite during large strain warm rolling. The developed microstructure in steels subjected to tempforming is commonly characterized by highly elongated grains/sub-grains along the rolling direction [50] (Figure7a). For instance, the transverse grain and sub-grain sizes in S700MC Metals 2020, 10, x FOR PEER REVIEW 9 of 20

MetalsThe 2020 enhancement, 10, x FOR PEER of REVIEW mechanical properties is closely related to the deformation microstructure9 of 20 evolved in tempered martensite during large strain warm rolling. The developed microstructure in steels Thesubjected enhancement to tempforming of mechanical is commonly properties characterized is closely related by highly to the elongated deformation grains/sub microstructure‐grains alongevolved the inrolling tempered direction martensite [50] (Figure during 7a). large For straininstance, warm the rolling. transverse The graindeveloped and sub microstructure‐grain sizes in in

S700MCsteelsMetals 2020subjected steel, 10, 1566are toabout tempforming 500 nm and is 100commonly nm, respectively, characterized after by warm highly rolling elongated to ε = 1.5grains/sub at 650 °C‐grains [50].9 of 20 Largealong thestrain rolling warm direction rolling [50] following(Figure 7a). martensitic For instance, transformation the transverse grainresults and in sub lamellar‐grain sizes‐type in microstructureS700MC steel arewith about low 500‐ to nm high and‐angle 100 nm,boundaries/sub respectively,‐boundaries after warm rollingin the totempformed ε = 1.5 at 650 steels °C [50]. as revealedLargesteel are strainby about transmission 500warm nm rolling and electron 100 nm,following microscopy respectively, martensitic using after converged warmtransformation rolling beam to techniqueε =results1.5 at 650(Figurein ◦lamellarC[ 507b).]. Large ‐Thetype warmmicrostructurestrain working warm rolling iswith usually following low‐ carriedto martensitichigh ‐outangle at transformationboundaries/subelevated temperatures results‐boundaries in (the lamellar-type inrange the tempformedof microstructurehigh temperature steels with as tempering).revealedlow- to high-angle by Hence, transmission boundariesthe developed electron/sub-boundaries microstructures microscopy in using the do tempformed notconverged contain beam steelshigh dislocation astechnique revealed (Figuredensity, by transmission 7b). which The remainwarmelectron atworking microscopythe level is ofusually usingtempered carried converged martensite, out beam at elevatede.g., technique of about temperatures (Figure 8×1014 7mb). −2(the in The a rangehigh warm‐ strengthof working high lowtemperature is‐alloyed usually steeltempering).carried [50]. out at Hence, elevated the temperatures developed microstructures (the range of high do temperature not contain tempering). high dislocation Hence, density, the developed which remainmicrostructures at the level do of not tempered contain martensite, high dislocation e.g., of density, about 8×10 which14 m remain−2 in a high at the‐strength level oflow tempered‐alloyed 14 2 steelmartensite, [50]. e.g., of about 8 10 m− in a high-strength low-alloyed steel [50]. ×

(a) (b)

Figure 7. Elongated grains/sub‐grains along the rolling direction (RD) in an S700MC steel subjected to tempforming(a) at 650 °C to total rolling strain of 1.5 Colors in (a)correspond(b) to the direction along theFigure normal 7. Elongated direction grains/sub grains(ND). /Thesub-grains‐ grainsnumbers along in (the b) indicaterolling direction the boundary (RD) in misorientations an S700MC steel in subjecteddegrees. Reproducedto tempforming with at permission 650 °C◦C to fromtotal [50],rolling Elsevier, strain of 2018. 1.5 Colors in (a)correspond to the direction along the normal direction (ND). The numbers in in ( b) indicate the boundary misorientations in in degrees. degrees. TheReproduced large strain with permissionwarm rolling from of [[50],50 tempered], Elsevier, 2018.2018.martensite results in strong texture in the steels subjected to tempforming. Caliber bar rolling is accompanied by the development of α‐fiber of <110> along Thethe largerollinglarge strain strain direction warm warm rolling(Figure rolling of 8) tempered of[51]. tempered Plate martensite rolling martensite under results resultsconditions in strong in texturestrong of warm intexture the working steels in the subjected should steels leadsubjectedto tempforming. to more to tempforming. complicated Caliber bar textureCaliber rolling barcomposed is rolling accompanied isby accompanied a part by theof α development ‐byfiber the from development {001}<110> of α-fiber of ofαto‐fiber <{111}<110>,110 of> along<110> γalongthe‐fiber rolling andthe θ‐rolling directionfiber, direction i.e., (Figure {111} (Figureand8)[ 51{001}]. 8) Plate parallel[51]. rolling Plate with rolling under the rolling conditionsunder plane, conditions of respectively, warm of working warm [52], working should although should lead the to texturesleadmore to complicated inmore tempformed complicated texture steel composed texture plates havecomposed by anot part been ofbyα studieda-fiber part from ofin αdetail. {001}‐fiber< Generally,110from> to{001}<110> {111} an , {111}<110>,γ -fiberin warm and rollingγθ-fiber,‐fiber strainand i.e., θ‐ {111} duringfiber, and i.e., tempforming {001} {111} parallel and {001} strengthens with parallel the rolling with the theα plane,‐ fiberrolling respectively,in plane,rolled respectively, rods [52 ],and although the [52], partial although the textures α‐and the γtexturesin‐fibers tempformed in in rolled tempformed steelplates plates [52]. steel haveOn plates the not otherhave been nothand, studied been the studied in effect detail. of in tempering Generally, detail. Generally, temperature an increase an increase in on warm the intexture rolling warm developedrollingstrain during strain seems tempforming during to depend tempforming strengthens on the temperature strengthens the α-fiber range. inthe rolled αReported‐fiber rods in and resultsrolled the partial suggestrods andα-and that the γthere-fibers partial might in α rolled‐ andbe aplatesγ temperature‐fibers [52 in]. rolled On interval, the plates other e.g., [52]. hand, around On the the e ff600 ectother of°C tempering hand,for 0.6C–2Si–1Cr the effect temperature of steel,tempering on that the is temperature texture favorable developed for on the the seemstexture texture to developmentdevelopeddepend on theseems [51]. temperature to depend range. on the Reported temperature results range. suggest Reported that there results might suggest be a temperature that there might interval, be ae.g., temperature around 600 interval,◦C for 0.6C–2Si–1Cr e.g., around steel,600 °C that for is 0.6C–2Si–1Cr favorable for steel, the texture that is development favorable for [51 the]. texture development [51].

Figure 8. Inverse pole figures for the rolling direction of a 0.6C-2Si-1Cr steels ubjected to tempforming at indicated temperatures.

Another feature of tempformed steel microstructure inherits from tempered martensite and is associated with dispersed particles, mainly and alloy carbides. Plastic deformation accelerates the second-phases precipitation, leading to more complete and dispersed distribution of the particles [53]. Therefore, the tempformed steels are characterized by a homogeneous distribution of second-phase Metals 2020, 10, x FOR PEER REVIEW 10 of 20

Figure 8. Inverse pole figures for the rolling direction of a 0.6C‐2Si‐1Cr steels ubjected to tempforming at indicated temperatures.

Another feature of tempformed steel microstructure inherits from tempered martensite and is

Metalsassociated2020, 10 ,with 1566 dispersed particles, mainly iron and alloy carbides. Plastic deformation accelerates10 of 20 the second‐phases precipitation, leading to more complete and dispersed distribution of the particles [53]. Therefore, the tempformed steels are characterized by a homogeneous distribution of particlessecond‐phase at various particles low-to-high-angle at various low (sub)boundaries.‐to‐high‐angle (sub)boundaries. The size and volume The size fraction and volume of dispersed fraction particlesof dispersed depends particles on alloying depends extent on and alloying processing extent conditions. and processing In low-alloyed conditions. high-strength In low‐alloyed steel subjectedhigh‐strength to tempforming steel subjected at 650 ◦toC, tempforming the dispersed particlesat 650 °C, are mainlythe dispersed Cr23C6-type particles carbides are with mainly an averageCr23C6‐type size ofcarbides 50 nm with (Figure an9 average)[50]. size of 50 nm (Figure 9) [50].

Figure 9. Cr C -type carbide particles and their size distribution in an S700MC steel subjected to Figure 9. Cr2323C66‐type carbide particles and their size distribution in an S700MC steel subjected to tempformingtempforming at at 650 650◦ C.°C. Reproduced Reproduced with with permission permission from from [50 [50],], Elsevier, Elsevier, 2018. 2018. 4.2. Delamination Toughness 4.2. Delamination Toughness High-strength low-alloyed steels subjected to tempforming possess outstanding impact toughness at loweredHigh‐ temperaturesstrength low for‐alloyed the impact steels loading subjected crosswise to tempforming to the rolled rodpossess axis oroutstanding along the normal impact directiontoughness for at the lowered rolled plates temperatures (Figure 10 fora) the [3,50 impact]. In the loading latter case, crosswise the specimens to the rolled for impact rod axis toughness or along evaluationthe normal correspond direction for to so-calledthe rolled crack plates arrester (Figure orientation 10a) [3,50]. [ 54In ,55the]. latter The tempformedcase, the specimens steels are for characterizedimpact toughness by enhanced evaluation impact correspond toughness asto comparedso‐called tocrack conventional arrester quenched-temperedorientation [54,55]. andThe ausformedtempformed steels steels of the are same characterized chemical composition.by enhanced Itimpact is worth toughness nothing as that compared the impact to toughness conventional of tempformedquenched‐tempered steels tends and to ausformed even increase steels with of the decreasing same chemical temperature composition. in the It range is worth down nothing to about that the impact toughness of tempformed steels tends to even increase with decreasing temperature in 100 ◦C. The load-displacement curves are normally characterized by general yield (PGY) followed by −the range down to about −100 °C. The load‐displacement curves are normally characterized by an increase in the load to maximum (PM) and subsequent gradual decrease in the load until the start general yield (PGY) followed by an increase in the load to maximum (PM) and subsequent gradual of unstable crack propagation (PF) and then the load drop to characteristic value of crack arrest (PA) decrease in the load until the start of unstable crack propagation (PF) and then the load drop to irrespectiveMetals 2020, 10 of, x FOR test temperaturePEER REVIEW in a wide range down to 90 ◦C (Figure 10b) [50]. 11 of 20 characteristic value of crack arrest (PA) irrespective of test− temperature in a wide range down to −90 °C (Figure 10b) [50].

(a) (b)

FigureFigure 10.10. Variations ofof impactimpact toughnesstoughness ofof 0.4C-2Si-1Cr-1Mo0.4C‐2Si‐1Cr‐1Mo andand S700MCS700MC steelssteels subjectedsubjected toto tempformingtempforming (TF), (TF), or or quenching-tempering quenching‐tempering (QT), (QT), or or ausforming ausforming (AF) (AF) with with impact impact test test temperature temperature (a) and(a) load-displacementand load‐displacement curves curves obtained obtained during impactduring tests impact of tempformed tests of tempformed S700MC (b). S700MC Reproduced (b). withReproduced permissions with from permissions [50], Elsevier, from [50], 2018. Elsevier, 2018.

The enhancement of impact toughness in the tempformed steels is attributed to delamination of the steel samples crosswise to the impact direction (Figure 11) [3]. The excellent impact properties have been reported for the laminated steel composites and discussed as a result of notch blunting because of the composite delamination [54]. The delamination nullifies the stress concentration at the crack tip, leading to gradual specimen bending accompanied by a decrease in the impact load (from PM to PF in Figure 10b) and absorbing a large energy. The break of separate bent layer results in a drop of the impact load to the crack arrest (PA), which is directly related to the next delamination event; the PA level corresponds to the bend resistance of the specimen remains.

Figure 11. Impact specimens of tempformed 0.4C‐2Si‐1Cr‐1Mo steel exhibiting delamination toughness. Reproduced with permission from [3], AAAS, 2008.

The impact toughness of notched and unnotched laminated steel composites has been almost the same [54]. The reason for delamination can be associated with internal tensile stresses, which are created ahead of the crack tip [56]. These stresses comprise about 0.2 of the peak stress concentration and act along the crack propagation direction. Delamination toughness has been also observed in a microalloyed steel with banded microstructure consisting of alternate ferrite and pearlite layers as a consequence of hot rolling [57]. The ferrite/pearlite interfaces have experienced splitting upon impact tests, changing one thick sample to a series of thin samples; an increase in the number density of ferrite/pearlite layers has thinned the delaminated segment and improved the impact toughness. Similar splitting frequently takes place in steels subjected to controlled rolling with finishing Metals 2020, 10, x FOR PEER REVIEW 11 of 20

(a) (b)

Figure 10. Variations of impact toughness of 0.4C‐2Si‐1Cr‐1Mo and S700MC steels subjected to tempforming (TF), or quenching‐tempering (QT), or ausforming (AF) with impact test temperature Metals(a2020) and, 10, 1566load‐displacement curves obtained during impact tests of tempformed S700MC (b11). of 20 Reproduced with permissions from [50], Elsevier, 2018.

The enhancement of impact toughness in the tempformed steels is attributed to delamination of the steelsteel samplessamples crosswise crosswise to to the the impact impact direction direction (Figure (Figure 11)[ 11)3]. [3]. The The excellent excellent impact impact properties properties have havebeen reportedbeen reported for the for laminated the laminated steel composites steel composites and discussed and discussed as a result as of a notch result blunting of notch because blunting of becausethe composite of the delaminationcomposite delamination [54]. The delamination [54]. The delamination nullifies the nullifies stress concentration the stress concentration at the crack tip, at theleading crack to tip, gradual leading specimen to gradual bending specimen accompanied bending byaccompanied a decrease inby the a decrease impact load in the (from impact PM toload PF (fromin Figure PM to10 Pb)F in and Figure absorbing 10b) and a large absorbing energy. a Thelarge break energy. of separate The break bent of separate layer results bent in layer a drop results of the in aimpact drop of load the to impact the crack load arrest to the (P crackA), which arrest is (P directlyA), which related is directly to the related next delamination to the next delamination event; the PA event;level corresponds the PA level tocorresponds the bend resistance to the bend of the resistance specimen of the remains. specimen remains.

Figure 11.11.Impact Impact specimens specimens of tempformed of tempformed 0.4C-2Si-1Cr-1Mo 0.4C‐2Si‐1Cr steel‐1Mo exhibiting steel exhibiting delamination delamination toughness. toughness.Reproduced Reproduced with permission with permission from [3], AAAS, from 2008.[3], AAAS, 2008.

The impact toughness of notched and unnotched laminated steel composites has been almost the same [54]. [54]. The The reason reason for for delamination can be associated with internal internal tensile stresses, which are created ahead of the crack tip [[56].56]. These stresses comprise about 0.2 of the peak stress concentration and act along the crack propagation direction. Delamination toughness has been also observed in a microalloyed steel with banded microstructure consisting of alternate ferrite and pearlite layers as a consequence ofof hot hot rolling rolling [57 [57].]. The The ferrite ferrite/pearlite/pearlite interfaces interfaces have have experienced experienced splitting splitting upon impact upon impacttests, changing tests, changing one thick one sample thick sample to a series to a series of thin of samples;thin samples; an increase an increase in the in the number number density density of offerrite ferrite/pearlite/pearlite layers layers has has thinned thinned the delaminated the delaminated segment segment and improved and improved the impact the impact toughness. toughness. Similar Similarsplitting splitting frequently frequently takes place takes in steels place subjected in steels to subjected controlled to rolling controlled with finishing rolling temperaturewith finishing in ferrite-austenite region [31,58]. Elongated sulfide inclusions along prior austenite grain boundaries have been considered as preferred sites for splitting crack nucleation [47]. The effect of delamination on the impact toughness can be discussed, considering the difference in coherence lengths on cleavage planes after tempforming (Figure 12)[59,60]. The coherence length of {001} cleavage planes in the tempformed steels is highly anisotropic. The coherence length is maximal in the rolling direction and minimal in the transverse direction. Correspondingly, the cleavage fracture stress along the transverse direction (σC//SD in Figure 12) is minimal, that along the rolling direction (σC//RD) is maximal, and that at 45◦ to rolling direction (σC//45) is intermediate. The resolved tensile stress (σt) can be characterized by similar anisotropy, but their variation is much smaller. Therefore, the delamination fracture occurs along the cleavage plane (i) at temperatures below T1 in Figure 12, when σt//SD exceeds σC//SD. This may lead to increasing the impact toughness as temperature decreases.

Upon further decrease in temperature below T2, when σt//45◦ exceeds σC//45◦ , the impact toughness decreases, because delamination happens on the cleavage planes (ii) inclined at 45◦ to the rolling direction. Then, the brittle fracture with low absorbed energy takes place at temperatures below T3, when σt//RD exceeds σC//RD. The fracture surface of specimens made of tempformed steels is commonly characterized by terracing, with brittle fracture on the terraces and ductile fracture on some steps (Figure 13)[50]. It is interesting to note that mainly brittle fracture during delamination of tempformed steels improves their impact toughness at lowered temperatures. Metals 2020, 10, x FOR PEER REVIEW 12 of 20

temperature in ferrite‐austenite region [31,58]. Elongated sulfide inclusions along prior austenite grain boundaries have been considered as preferred sites for splitting crack nucleation [47]. The effect of delamination on the impact toughness can be discussed, considering the difference in coherence lengths on cleavage planes after tempforming (Figure 12) [59,60]. The coherence length of {001} cleavage planes in the tempformed steels is highly anisotropic. The coherence length is maximal in the rolling direction and minimal in the transverse direction. Correspondingly, the cleavage fracture stress along the transverse direction (σC//SD in Figure 12) is minimal, that along the rolling direction (σC//RD) is maximal, and that at 45° to rolling direction (σC//45) is intermediate. The resolved tensile stress (σt) can be characterized by similar anisotropy, but their variation is much smaller. Therefore, the delamination fracture occurs along the cleavage plane (i) at temperatures below T1 in Figure 12, when σt//SD exceeds σC//SD. This may lead to increasing the impact toughness as temperature decreases. Upon further decrease in temperature below T2, when σt//45° exceeds σC//45°, the impact toughness decreases, because delamination happens on the cleavage planes (ii) inclined at 45° to the rolling direction. Then, the brittle fracture with low absorbed energy takes place at temperatures below T3, when σt//RD exceeds σC//RD. The fracture surface of specimens made of tempformed steels is commonly characterized by terracing, with brittle fracture on the terraces and ductile fracture on some steps (Figure 13) [50]. It is interesting to note that mainly brittle fracture Metalsduring2020, 10 , 1566delamination of tempformed steels improves their impact toughness at lowered12 of 20 temperatures.

FigureFigure 12. Modified 12. Modified Yoff Yoffeeee diagram diagram for for tempformed tempformed steels steels with with highly highly elongated elongated grainsgrains/subgrains/subgrains and strongMetalsand 2020 {110} strong, 10, fiber x FOR {110} texture. PEER fiber REVIEW texture. Reproduced Reproduced with permissionwith permission from from [60 ],[60], Elsevier, Elsevier, 2016. 2016. 13 of 20

FigureFigure 13. Terraced 13. Terraced fracture fracture surface surface of of tempformed tempformed S700MC S700MC steel steel specimen specimen after after impact impact test at test 20 at°C. 20 ◦C. ReproducedReproduced with with permission permission from from [50 [50],], Elsevier, Elsevier, 2018.2018.

AnotherAnother advantage advantage of tempforming of tempforming is its is suitabilityits suitability for steelsfor steels with with high high concentration concentration of harmful of elementsharmful like elements phosphorus like [60phosphorus,61]. In contrast [60,61]. to In structural contrast plainto structural carbon steelsplain aftercarbon conventional steels after heat conventional heat treatment, the phosphorus segregation has promoted delamination toughness and treatment, the phosphorus segregation has promoted delamination toughness and increased impact increased impact toughness of a 0.09% P steel subjected to tempforming. On the one hand, toughnesssegregations of a 0.09% on cleavage P steel subjectedplanes arranged to tempforming. along the rolling On the direction one hand, in the segregations tempformed on steel cleavage planesrods/plates arranged weakens along the the rollingbonds and direction advances in fracture, the tempformed and on the steelother, rods such/ plateseasy splitting weakens results the in bonds and advancescrack blunting fracture, and increases and on the the other, impact such toughness. easy splitting results in crack blunting and increases the impact toughness.It is worth noting that impact toughness of tempformed steels depends significantly on the Itloading is worth direction. noting The that microstructural impact toughness anisotropy of tempformed of highly elongated steels depends (after significantlybar rolling) or on flattened the loading direction.(after Theplate microstructural rolling) microstructure anisotropy results of highly in corresponding elongated (aftermechanical bar rolling) anisotropy. or flattened In contrast (after to plate rolling)rolled microstructure bars/rods, when results all inimpact corresponding direction crosswise mechanical to the anisotropy. rolling direction In contrast are kinds to rolled of crack bars/ rods, arrester orientations, the impact load along only normal direction corresponds to crack arrester when all impact direction crosswise to the rolling direction are kinds of crack arrester orientations, the orientation for rolled plates. Thus, tempformed plates exhibit outstanding impact toughness in crack impact load along only normal direction corresponds to crack arrester orientation for rolled plates. arrester orientation, whereas their impact toughness in the case of loading direction along transverse Thus,direction tempformed (crack plates divider exhibit orientation outstanding [55]) is impact much lower toughness and may in crack rank arrester below that orientation, of ordinary whereas thermo‐mechanically treated steel [50]. It is obvious that impact toughness in crack delamination orientation [55] of the tempformed steels is quite low, which is, probably, the major drawback of tempforming treatment.

4.3. Strengthening The development of lamellar microstructure with the transverse grain size well below 1 m along with rather high dislocation density in grain/sub‐grain interior and uniform dispersion of second‐phase particles (mainly carbides) in tempformed steels result in significant strengthening [3,50]. For instance, the yield strength of high‐strength low‐alloyed steel subjected to tempforming at 650 °C approaches 1100 MPa, which is greater by almost 200 MPa than that in the ausformed state (Figure 14) [50]. The effect of grain size (D) on the yield strength (σ0.2) is generally expressed by the Hall–Petch relationship [62,63], Metals 2020, 10, 1566 13 of 20 their impact toughness in the case of loading direction along transverse direction (crack divider orientation [55]) is much lower and may rank below that of ordinary thermo-mechanically treated steel [50]. It is obvious that impact toughness in crack delamination orientation [55] of the tempformed steels is quite low, which is, probably, the major drawback of tempforming treatment.

4.3. Strengthening The development of lamellar microstructure with the transverse grain size well below 1 µm along with rather high dislocation density in grain/sub-grain interior and uniform dispersion of second-phase particles (mainly carbides) in tempformed steels result in significant strengthening [3,50]. For instance, the yield strength of high-strength low-alloyed steel subjected to tempforming at 650 ◦C approaches 1100 MPa, which is greater by almost 200 MPa than that in the ausformed state (Figure 14)[50]. The effect of grain size (D) on the yield strength (σ0.2) is generally expressed by the Hall–Petch relationship [62,63], 0.5 σ0.2 = σ0 + ky D− , (1) where σ0 is the strength of the same material with infinite grain size and ky is the grain boundary 0.5 strengthening factor. Taking σ0 = 70 MPa and ky = 240 MPa µm for low-alloyed ferrite [64–66] and effective grain size of 1 µm, the grain size strengthening of tempformed steel comprises about 310 MPa. The dislocation strengthening (∆σρ) is commonly expressed by Taylor-type equation [67],

0.5 ∆σρ = αGb ρ , (2) · where α is the numerical factor of about 0.9 [68], G is the shear modulus (81,000 MPa for low-alloyed steels [69]), b is the Burgers vector (2.5 10 10 m 1 in ferrite [69]) and ρ is the dislocation density. Thus, × − − the dislocation strengthening of ausformed steels with ρ = 8 1014 m 2 can be as high as 515 MPa. × − The dispersion strengthening (∆σOR) is usually expressed by Orowan relationship [70],

1 0.5 ∆σ = 0.4 MGb (πλ)− (1 ν)− ln(0.8d/b), (3) OR · · − · where M is the Taylor factor, d is the mean particle size, λ—is the interparticle distance, which can be evaluated as λ = d (0.36 F 0.5 1) for randomly distributed particles with volume fraction of F [71]. · V− − V Taking the latter of about 0.8% for particles located in grain/sub-grain interiors and at low-angle dislocation sub-boundaries of high-strength low-alloyed steels and d = 50 nm, the dispersed particles provide about 250 MPa strength increment. Then, assuming that the strengthening mechanisms above are independent and linear additive [72,73], the calculated yield strength comprises 1075 MPa that is almost the same with experimental one (Figure 14). The speculation above suggests that the dislocation strengthening is the major contributor to the strengthening of high-strength low-alloyed steels subjected to tempforming. It should be noted that an increase in the yield strength after tempforming is not accompanied by a decrease in ductility and the upper-shelf energy of impact toughness as frequently observed in the steels processed by ausforming/modified ausforming [51]. This beneficial effect of tempforming on the mechanical properties seems to be attributed to the development of a kind of ultrafine grained microstructure. The ultrafine grained metals and alloys have been frequently reported to possess a unique combination of high strength and plasticity [65,74–76]. The grain refinement down to sub-micrometer scale may strengthen metallic materials without a degradation of plasticity. The effect of hot working of austenite on the microstructure and properties of tempformed steel has not been studied in sufficient detail, although a decrease in the prior austenite grain size has been shown to be favorable for ductility and toughness of steels subjected to tempforming [77]. After tempforming, the transverse size of largely elongated grains/sub-grains and the second-phase particle dispersion depend mainly on the tempforming conditions, i.e., tempering time/temperature and subsequent strain, irrespective of the size of prior austenite grains. On the other hand, the size of Metals 2020, 10, 1566 14 of 20 prior austenite grains should affect the size of martensite packets [77]. Generally, the refinement of microstructural elements promotes plasticity. Moreover, the dislocation substructure in the hot worked austenite is expected to have a beneficial effect on the mechanical properties of tempformed steels Metals 2020, 10, x FOR PEER REVIEW 14 of 20 similar to that in the ausforming and modified ausforming.

Metals 2020, 10, x FOR PEER REVIEW 15 of 20

particle dispersion depend mainly on the tempforming conditions, i.e., tempering time/temperature and subsequent strain, irrespective of the size of prior austenite grains. On the other hand, the size of prior austenite grains should affect the size of martensite packets [77]. Generally, the refinement of microstructural elements promotes plasticity. Moreover, the dislocation substructure in the hot worked austenite is expected to have a beneficial effect on the mechanical properties of tempformed steels similar to that in the ausforming and modified ausforming. Figure 14. Tensile stress-strain curves for a high-strength low-carbon steel processed by tempforming Figure 14. Tensile stress‐strain curves for a high‐strength low‐carbon steel processed by or ausforming. Reproduced with permission from [50], Elsevier, 2018. 4.4. Effecttempforming of Tempforming or ausforming. Temperature Reproduced with permission from [50], Elsevier, 2018. 4.4. ETemperingffect of Tempforming temperature Temperature controls the softening of tempered martensite and the developed σ0.2 = σ0 + ky D−0.5, (1) microstructure,Tempering temperaturei.e., dislocation controls density, the lath softening size (for of tempered the case martensiteof lath martensite), and the developed size and microstructure,distributionwhere  0 ofis i.e.,thesecond strength dislocation‐phase of theparticles density, same material lath [78]. size Subsequent with (for infinite the case rollinggrain of lath size at martensite), andthe kytempering is the size grain and temperature boundary distribution is 0.5 ofaccompanied second-phasestrengthening by particlesdynamic factor. Taking [recovery78]. σ Subsequent0 = 70 of MPa tempered and rolling ky dislocation= 240 at theMPa tempering μ msubstructure for low temperature‐alloyed and leads ferrite is to accompanied [64–66] the elongation and by effective grain size of 1 m, the grain size strengthening of tempformed steel comprises about 310 dynamicor pancaking recovery of microstructural of tempered dislocation elements, substructuregrains/sub‐grains, and leads along to the the rolling elongation direction or pancaking [59]. The MPa. The dislocation strengthening (σ) is commonly expressed by Taylor‐type equation [67], oftransverse microstructural grain/sub elements,‐grain size grains commonly/sub-grains, reduces along while the rollingthe dislocation direction and [59]. particle The transverse densities grainincrease/sub-grain with a decrease size commonly in rolling reduces temperature. whileσρ the = αThus,Gb dislocation ρ0.5 ,warm rolling and particle following densities tempering increase increases(2) with a decreasethe whereyield in αstrength rolling is the numerical temperature. of tempered factor Thus, of aboutsteel warm 0.9that [68], rolling is G isassociated followingthe shear modulus temperingwith characteristic (81,000 increases MPa for the changeslow yield‐alloyed strength in the ofmicrostructural temperedsteels [69]), steel bparameters. is that the isBurgers associated On vector the with (2.5other × characteristic 10 hand,−10 m−1 thein ferrite effect changes [69]) of and intempforming the ρ is microstructural the dislocation temperature density. parameters. on the Onultimate theThus, other tensile the hand, dislocation strength the eff strengtheningectsubstantially of tempforming of depends ausformed temperature on steelsthe alloying with on the ρ ultimate=extent. 8×1014 Them tensile−2 can ultimate be strength as hightensile substantially as 515strength dependsincreasesMPa. on Theby the tempforming dispersion alloying strengthening extent. in steels The ultimatewith (σOR relatively) is usually tensile small strengthexpressed carbon increases by Orowan content, by relationship tempforming whereas that[70], in can steels be witheven relativelydecreased small after carbontempforming content, of whereas steelsσOR = 0.4MGbwith that canrather∙(πλ be)−1∙(1 evenhigh− ν)−0.5 decreased∙carbonln(0.8d/b), content after tempforming of about 0.6% of (Figure steels(3) with 15) rather[51,79]. high carbon content of about 0.6% (Figure 15)[51,79]. where M is the Taylor factor, d is the mean particle size, λ—is the interparticle distance, which can be evaluated as λ = d∙(0.36FV−0.5 − 1) for randomly distributed particles with volume fraction of FV [71]. Taking the latter of about 0.8% for particles located in grain/sub‐grain interiors and at low‐angle dislocation sub‐boundaries of high‐strength low‐alloyed steels and d = 50 nm, the dispersed particles provide about 250 MPa strength increment. Then, assuming that the strengthening mechanisms above are independent and linear additive [72,73], the calculated yield strength comprises 1075 MPa that is almost the same with experimental one (Figure 14). The speculation above suggests that the dislocation strengthening is the major contributor to the strengthening of high‐strength low‐alloyed steels subjected to tempforming. It should be noted that an increase in the yield strength after tempforming is not accompanied by a decrease in ductility and the upper‐shelf energy of impact toughness as frequently observed in the steels processed by ausforming/modified ausforming [51]. This beneficial effect of tempforming on the mechanical properties seems to be attributed to the development of a kind of ultrafine grained microstructure. The ultrafine grained metals and alloys have been frequently reported to possess a unique combination of high strength and plasticity [65,74–76]. The grain refinement down to sub‐micrometer scale may strengthen metallic materials without a degradation of plasticity. (a) (b) The effect of hot working of austenite on the microstructure and properties of tempformed steel FigurehasFigure not 15.15. been Tensile studied stress-strainstress in‐ strainsufficient curvescurves detail, ofof 0.6C–2Si–1Cr0.6C–2Si–1Cr although a decrease steelsteel [[51]51]( in(aa ))the and prior (0.2–0.6)C-2Si-1Cr-1Mo(0.2–0.6)C austenite‐2Si‐ 1Crgrain‐1Mo size steelssteels has (been(bb)) subjectedsubjected shown to toto be tempformingtempforming favorable for (TF)(TF) ductility oror conventionalconventional and toughness quenching-temperingquenching of steels‐tempering subjected (QT). (QT).to tempforming [77]. After tempforming, the transverse size of largely elongated grains/sub‐grains and the second‐phase A decrease in tempforming temperature enhances the delamination along the rolling plane, increasing the impact toughness at lowered temperatures (Figure 16) [51,80]. Correspondingly, an increase tempering temperature weakens the effect of tempforming on the impact toughness. The latter increases with an increase in tempering temperature and does not significantly change by subsequent rolling. On the other hand, tempforming at elevated temperatures results in increased upper‐shelf energy, although advantage of tempforming at elevated temperatures over conventional quenching and tempering is not so pronounced. Therefore, in order to make the best use of tempforming, it should be carried out at relatively low tempering temperatures, when the effect of delamination toughness can be achieved in combination with a high strength. Metals 2020, 10, 1566 15 of 20

A decrease in tempforming temperature enhances the delamination along the rolling plane, increasing the impact toughness at lowered temperatures (Figure 16)[51,80]. Correspondingly, an increase tempering temperature weakens the effect of tempforming on the impact toughness. The latter increases with an increase in tempering temperature and does not significantly change by subsequent rolling. On the other hand, tempforming at elevated temperatures results in increased upper-shelf energy, although advantage of tempforming at elevated temperatures over conventional quenching and tempering is not so pronounced. Therefore, in order to make the best use of tempforming, Metals 2020, 10, x FOR PEER REVIEW 16 of 20 itMetals should 2020, be 10, carriedx FOR PEER out REVIEW at relatively low tempering temperatures, when the effect of delamination16 of 20 toughness can be achieved in combination with a high strength.

(a) (b) (c) (a) (b) (c) Figure 16. Effect of tempforming (TF) at different temperatures on the impact toughness of S700MC FigureFigure 16.16. EEffectffect ofof tempforming tempforming (TF) (TF) at at di differentfferent temperatures temperatures on on the the impact impact toughness toughness of S700MCof S700MC (a) (a) and 0.6C‐2Si‐1Cr (b,c) steels in comparison with ordinary quenching‐tempering (QT). and(a) and 0.6C-2Si-1Cr 0.6C‐2Si‐1Cr (b,c ()b steels,c) steels in comparison in comparison with with ordinary ordinary quenching-tempering quenching‐tempering (QT). (QT). 4.5. Carbon Effect 4.5.4.5. Carbon EEffectffect The first impressive results on the combination of high strength and impact toughness achieved TheThe firstfirst impressiveimpressive resultsresults onon thethe combinationcombination ofof highhigh strength andand impact toughness achieved by tempforming have been obtained in medium carbon (0.4%) high‐strength steel [3,49]. Then, byby tempformingtempforming havehave beenbeen obtainedobtained inin mediummedium carboncarbon (0.4%)(0.4%) high-strengthhigh‐strength steelsteel [3[3,49].,49]. Then, Kimura et al. have carried out a series of studies, using steels with 0.2%C to 0.6%C [51,59–61,79,81]. KimuraKimura etet al.al. havehave carriedcarried outout aa seriesseries ofof studies,studies, using steels with 0.2%C to 0.6%C [51,59–61,79,81]. [51,59–61,79,81]. The obtained results can be summarized as follows. The upper‐shelf energy in the steels subjected to TheThe obtained results can be be summarized summarized as as follows. follows. The The upper upper-shelf‐shelf energy energy in in the the steels steels subjected subjected to tempforming increases with a decrease in carbon content (Figure 17) [79]. The temperature of totempforming tempforming increases increases with with a adecrease decrease in in carbon carbon content content (Figure (Figure 17)17)[ [79].79]. The The temperature ofof ductile‐brittle transition in the tempformed steels also decreases with a decrease in carbon content ductile-brittleductile‐brittle transition in the tempformed steels also decreases with a decreasedecrease inin carboncarbon contentcontent (Figure 17). It is worth noting that the same tendency for the upper‐shelf energy dependence on (Figure(Figure 17 17).). It It is is worth worth noting noting that that the the same same tendency tendency for the for upper-shelf the upper‐ energyshelf energy dependence dependence on carbon on carbon is observed in these steels after conventional quenching and tempering. iscarbon observed is observed in these in steels these after steels conventional after conventional quenching quenching and tempering. and tempering.

FigureFigure 17. 17.E ffEffectect of carbonof carbon content content on impact on toughnessimpact toughness of high-strength of high steels‐strength subjected steels to tempformingsubjected to Figure 17. Effect of carbon content on impact toughness of high‐strength steels subjected to (TF)tempforming or conventional (TF) or quenching-tempering conventional quenching (QT).‐tempering (QT). tempforming (TF) or conventional quenching‐tempering (QT). TheThe carbon carbon e ffeffectect on on the the strengthening strengthening of of steels steels subjected subjected to tempformingto tempforming is illustrated is illustrated in Figure in Figure 17. The carbon effect on the strengthening of steels subjected to tempforming is illustrated in Figure It17. is clearlyIt is clearly seen thatseen the that strengthening the strengthening effect ofeffect tempforming of tempforming diminishes diminishes with increasing with increasing the carbon the 17. It is clearly seen that the strengthening effect of tempforming diminishes with increasing the content.carbon content. Therefore, Therefore, proficiency proficiency of tempforming of tempforming in improving in improving the mechanical the mechanical properties properties of carbon of carbon content. Therefore, proficiency of tempforming in improving the mechanical properties of steelscarbon decreases steels decreases as the carbon as the content carbon increases. content increases. The effect ofThe tempforming effect of tempforming temperature temperature on the impact on carbon steels decreases as the carbon content increases. The effect of tempforming temperature on the impact toughness at low temperature is also more pronounced in the steels with low carbon the impact toughness at low temperature is also more pronounced in the steels with low carbon content (Figure 15b), although the tempforming effect on the properties of steels with various content (Figure 15b), although the tempforming effect on the properties of steels with various alloying extent should be further clarified. It can be concluded that tempforming is very effective alloying extent should be further clarified. It can be concluded that tempforming is very effective thermo‐mechanical treatment for obtaining high‐strength medium carbon steels with high impact thermo‐mechanical treatment for obtaining high‐strength medium carbon steels with high impact toughness at lowered temperatures that expand significantly the service conditions of these steels toughness at lowered temperatures that expand significantly the service conditions of these steels towards low temperature applications. On the other hand, concurrent increase in impact toughness towards low temperature applications. On the other hand, concurrent increase in impact toughness and strength of low‐alloyed steels opens new perspectives for high‐strength low‐alloyed steels as and strength of low‐alloyed steels opens new perspectives for high‐strength low‐alloyed steels as structural material for crucial applications. structural material for crucial applications.

Metals 2020, 10, 1566 16 of 20 toughness at low temperature is also more pronounced in the steels with low carbon content (Figure 15b), although the tempforming effect on the properties of steels with various alloying extent should be further clarified. It can be concluded that tempforming is very effective thermo-mechanical treatment for obtaining high-strength medium carbon steels with high impact toughness at lowered temperatures that expand significantly the service conditions of these steels towards low temperature applications. On the other hand, concurrent increase in impact toughness and strength of low-alloyed steels opens new perspectives for high-strength low-alloyed steels as structural material for crucial applications.

5. Summary and Perspectives Tempforming is a very promising thermo-mechanical treatment, particularly for high-strength carbon steels. Conventional heat treatment, i.e., quenching and tempering, is very effective for producing the steels for various structural applications with appropriate strength level, which is controlled by tempering conditions. However, exploitation of such steels processed in high strength state under conditions of lowered temperature requires solving a problem of low impact toughness, which is associated with relatively high ductile-brittle transition temperature, leading frequently to brittle fracture at ambient temperatures. Ordinary ausforming and/or modified ausforming impose certain conditions on the alloying design of steels, which can be processed. The austenite in these steels should possess rather slow transformation kinetics in the temperature range between Ar1 and Ms, or should be stable against discontinuous recrystallization at temperatures above Ar3. Moreover, corresponding temperature intervals should be sufficiently wide to use conventional rolling technology. Hence, various carbon steels are far beyond the required conditions and can be hardly improved by ausforming/modified ausforming. In contrast to ordinary approach for improving the impact toughness owing to enhanced ductile fracture, tempforming utilizes brittle fracture in order to increase impact toughness. The brittle delamination in tempformed steels blocks the crack propagation across the rolled steel semi-product, resulting in high impact toughness even at low temperatures. It is important to note that tempforming increases the impact toughness without any degradation of strength. High-strength low-alloyed steels are other perspective candidates for processing by tempforming. Tempforming of these steels provides simultaneous increase in both impact toughness and strength. It is worth noting that impact toughness after tempforming increases, especially, at lowered temperatures that is commonly problem domain for high-strength carbon steels processed by ordinary quenching and tempering. Variety of high-strength steels applicable to tempforming makes this treatment very attractive for steel production, although the benefits and disadvantage of tempforming for steels with different alloying design are not clear yet and require further investigations. Current studies on tempforming of hot worked steels suggest an approach for further improvement of mechanical properties of high-strength steels. More beneficial combination of high strength and impact toughness can be achieved by thermo-mechanical treatment combining the grain refinement in austenite followed by tempforming of tempered martensite. The refinement of prior austenite grains as a result of dynamic recrystallization during hot to warm working is expected to accelerate the formation of desired ultrafine lamellar microstructure during subsequent tempforming. In turn, reduction of rolling strain during tempforming increases productivity of the process and allows us widening the diversity of processed steels owing to variety of applicable regimes.

Author Contributions: Conceptualization, R.K. and A.B.; software, A.D.; formal analysis, A.D.; resources, A.D. and A.B.; writing—original draft preparation, A.B.; writing—review and editing, R.K.; supervision, R.K.; project administration, A.B.; funding acquisition, R.K. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the Russian Science Foundation, Agreement No. 20-19-00497. Acknowledgments: Authors are grateful to the personnel of the Joint Research Center, Technology & Materials, Belgorod State University, for their assistance with instrumental analysis. Conflicts of Interest: The authors declare no conflict of interest. Metals 2020, 10, 1566 17 of 20

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