Microstructure Evolution and Mechanical Properties of 0.4C-Si-Mn-Cr Steel During High Temperature Deformation

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Article Microstructure Evolution and Mechanical Properties of 0.4C-Si-Mn-Cr Steel during High Temperature Deformation

Fei Zhang 1,2, Yang Yang 1,2, Quan Shan 1,2,* , Zulai Li 1,2,*, Jinfeng Bi 1,2 and Rong Zhou 1,2 1 School of Material Science and Engineering, Kunming University of Science and Technology, Kunming 650093, China; [email protected] (F.Z.); [email protected] (Y.Y.); [email protected] (J.B.); [email protected] (R.Z.) 2 National & Local Joint Engineering Laboratory of Advanced Metal Solidiﬁcation Forming and Equipment Technology, Kunming University of Science and Technology, Kunming 650093, China * Correspondence: [email protected] (Q.S.); [email protected] (Z.L.)

Received: 8 November 2019; Accepted: 27 December 2019; Published: 1 January 2020

Abstract: Herein, the effects of height-diameter ratios (H/D) on the microstructure evolution and mechanical properties of 0.4C-Si-Mn-Cr steel during high temperature deformation are reported. The compression experiments were performed on steel samples using Gleeble to obtain a reasonable deformation temperature, and the degree of deformation was assessed in the range of 1.5 to 2.0 H/D via forging. The forged specimens were quenched using the same heat treatment process. The hardness and impact toughness of the steel samples were tested before and after heat treatment. Grain sizes gradually increased with an increase in the compression temperature from 950 ◦C to 1150 ◦C, and the grain sizes decreased with an increase in H/D. The microstructure of the steel samples contained pearlite, bainite, martensite, and retained austenite phase. The microstructure after forging was more uniform and finer as compared to that of as-cast steel samples. The hardness and impact toughness of the steel samples were evaluated after forging; hardness first increased and then decreased with an increase in H/D, while the impact toughness continuously increased with an increase in H/D. Hence, the microstructure and properties of steel could be improved via high temperature deformation, and this was primarily related to grain refinement.

Keywords: deformation; height-diameter ratio; pearlite; impact toughness

1. Introduction Owing to their excellent comprehensive performance, medium carbon steels have been widely used in essential fields such as energy development, machine manufacturing, and rail traffic [1–4]. In recent years, there has been an intense focus on the improvement of material properties through deformation technologies such as hot rolling, cold rolling, and forging [5–8]. Forging is a method for refining the grain size of steel via severe plastic deformation to enhance the strength and toughness. Internal cracks and defects of raw materials can be reduced during forging, and the microstructure and grain distribution are more uniform and compact after forging [9,10]. Grain refinement during deformation has a positive effect on the microstructure and properties of steel. G.P. Chaudhari et al. [11–14] found that the grain was clearly refined after multidirectional forging of steel, which induced fine austenite grains because of continuous dynamic recrystallization and dynamic recovery. The formation of fine grains is beneficial for enhancement of tensile strength and hardness. Crystal slips are the primary mode of deformation, and the mechanism of ferrite refinement is comprised grain segmentation and recovery. Sumit et al. [15] found that when the steel was forged in pure ferrite regions, the mechanical properties of steel had the best combination of yield

Materials 2020, 13, 172; doi:10.3390/ma13010172 www.mdpi.com/journal/materials Materials 2020, 13, 172 2 of 11

strength and ductility. Hot compression deformation at different temperatures (950–1150 ◦C) and 1 rates (0.01–10 s− ) can significantly promote grain refinement of as-cast samples [16]. Yang et al. [17] conducted hot deformation experiments of 35CrMoV steel with a degree of deformation that was about 50% at temperatures of 850 ◦C, 950 ◦C, and 1050 ◦C and concluded that grain size increased gradually with an increase in the deformation temperature under the same strain rate. Furthermore, the performance of the forging can be further enhanced via heat treatment. Smaropoulos [18] achieved excellent tensile strength, hardness, and toughness of a low alloy steel after forging followed by heat treatment. The as-obtained medium carbon steel had extraordinary mechanical properties, and this was achieved by alternating timed quenching in water and air [19,20]. However, the current research on microstructure evolution and mechanical properties of 0.4C-Si-Mn-Cr steel under different degrees of deformation is still limited, especially in the high temperature forging process. Therefore, understanding of microstructure evolution in steels and the associated mechanism is beneficial for further formulating and improving the deformation process. Herein, to determine an appropriate forging temperature, Gleeble simulated compression deformation of medium carbon (0.4C-Si-Mn-Cr) steel under different temperatures. Samples with different height-diameter ratios (H/D) were forged and quenched. The microstructures of specimens were characterized using optical microstructure (OM) analysis, scanning electron microscopy (SEM), X-ray diffraction (XRD), and transmission electron microscopy (TEM). The hardness and impact toughness were investigated, and the microstructure evolution and mechanical properties of 0.4C-Si-Mn-Cr steel contributed towards providing a theoretical basis for the change rule of the deformation process.

2. Experimental Methods The chemical composition of the steel investigated in this work is listed in Table1. The steel was smelted and poured into a cylindrical billet in an intermediate frequency furnace. A direct reading spectrometer was used to determine the chemical composition of the tested alloy. A schematic diagram of the experimental process is provided in Figure1. The billet was processed into a round rod of size Φ8 12 mm. Specimens were heated to 50 C below the predetermined temperature at a rate of × ◦ 10 ◦C/s on a Gleeble-1500D dynamic thermal simulator (Data Sciences International INC, St. Paul, MI, 1 USA), and then, the heating rate was reduced to 2 ◦C/s. The strain rate was 5 s− , and deformation was 50%. Compression tests were conducted at temperatures of 950 ◦C, 1050 ◦C, and 1150 ◦C and maintained at the speciﬁed temperatures for 5 min followed by water quenching to cool the sample to room temperature (Figure1a,b). The round bar was subjected to multidirectional force and then formed into a ball in a high temperature forging process (Figure1c). Cylindrical billets with di ﬀerent H/D values (1.5, 1.7, and 2.0) were taken to 1050 ◦C for hot forging, and the billets were forged into a sphere of 150 mm (Figure1d). The heat treatment was carried out by alternating the cycle cooling process in water and air (Figure1e). After forging, samples were subjected to a quenching treatment under the same conditions. Moreover, all the samples were heated to 900 ◦C for 3 h, followed by alternating cycle cooling in water and air (after water cooling, the sample was cooled in air). Because the cooling rate of steel in water was higher than that in air, the temperature of samples could be reduced relatively uniformly to the isothermal region. As the quenching temperature decreased, the slope of the curve during water cooling was reduced gradually. The time of water cooling was 10 s, and that of air cooling was 15 s. The samples were treated at 330 ◦C for 3 h in a salt bath and further cooled to room temperature in air.

Table 1. Chemical composition of the 0.4C-Si-Mn-Cr steel.

Element C Si Mn Cr Fe Content (wt.%) 0.37–0.60 1.60–2.0 2.0–2.5 0.55–0.85 Balance Materials 2019, 13, x FOR PEER REVIEW 3 of 11

Japan) was performed on a diffractometer (Copper Kα radiation) at 10°/min for the counting time from 30° to 100°. Furthermore, for TEM, the specimens were prepared via electrolytic double spraying of 5% perchloric acid and 95% alcohol solution. The microstructure of the steel was observed using an optical microscope (Nikon ECLIPSE MA200, Japan), scanning electron microscope (ZEISS EVO18, Baden-Wurttemberg, Germany), and transmission electron microscope (FEI G220, Hillsboro, Oregon State, U.S.).

Table 1. Chemical composition of the 0.4C-Si-Mn-Cr steel.

Element C Si Mn Cr Fe Materials 2020, 13, 172 3 of 11 Content (wt.%) 0.37–0.60 1.60–2.0 2.0–2.5 0.55–0.85 Balance

Figure 1. SchematicSchematic diagram diagram of of the experimental process. ( (aa)) High High temperature temperature compression deformation processprocess at at T =T950 = ◦950C, 1050°C, ◦1050C, and °C, 1150 and◦C; 1150 (b) specimens °C; (b) specimens under unidirectional under unidirectional compression compressionin Gleeble; (c )in stress Gleeble; and ( deformationc) stress and deformation of a cylinder of during a cylinder high during temperature high temperature forging; (d) forgingforging; and(d) forgingpressing and process pressing of casting process rod of withcasting height-diameter rod with height-diameter ratios (H/D) ratios values (H/D) of 1.5, values 1.7, and of 1.5, 2.0; 1.7, (e) heatand 2.0;treatment (e) heat process treatment of casting process and of casting forging. and forging.

3. ResultsThe specimens were prepared at half the distance from the center of the instrument before and after heat treatment. The samples were then processed into unnotched impact specimens with dimensions of 10Stress–strain10 55 mm. curves Hardness and the and grain impact size tests of the were compressed performed steels using at a Rockwellvarious temperatures tester (HR-150A, are × × shownDongshan, in Figure China) 2. The and shape semi-automatic of the flow curve tester wa (JB-300B,s mainly Dongshan, unimodal. China).At different The temperatures, specimens used with a anmixture increase of saturated in strain, picric the true acid, stress purified first water, increased detergent, to a maximum and hydrochloric value an acidd then to corrode decreased the grains.to the steady-stateThe diameter region. of the grainWhen in the diff temperatureerent fields was increase measuredd from using 950 to the 1150 cut line°C, the method maximum in the experiment,flow stress decreasedand then, thefrom average 219.93 value to 117.23 was countedMPa. This as themay grain be because size. The of samples the increase were corrodedin temperature using 4%and nitric the activationacid alcohol energy solution of thermal and picric deformation. acid solution. Furtherm XRD (MiniFlexore, the 600,kinetic Rigaku, energy Japan) of atoms was performedincreased, onand a thediff ractometercritical shear (Copper stress slip Kα lineradiation) decreased. at 10 ◦The/min be forhavior the countingof strain timesoftening from was 30◦ toconsidered 100◦. Furthermore, to be due for TEM, the specimens were prepared via electrolytic double spraying of 5% perchloric acid and 95% alcohol solution. The microstructure of the steel was observed using an optical microscope (Nikon ECLIPSE MA200, Japan), scanning electron microscope (ZEISS EVO18, Baden-Wurttemberg, Germany), and transmission electron microscope (FEI G220, Hillsboro, Oregon State, U.S.).

3. Results Stress–strain curves and the grain size of the compressed steels at various temperatures are shown in Figure2. The shape of the flow curve was mainly unimodal. At di fferent temperatures, with an increase in strain, the true stress first increased to a maximum value and then decreased to the steady-state region. When the temperature increased from 950 to 1150 ◦C, the maximum flow Materials 2020, 13, 172 4 of 11

stressMaterials decreased 2019, 13, x FOR from PEER 219.93 REVIEW to 117.23 MPa. This may be because of the increase in temperature4 andof 11 the activation energy of thermal deformation. Furthermore, the kinetic energy of atoms increased, andto new the dynamic critical shear recrystallization stress slip line (DRX), decreased. and the The asymptotic behavior behavior of strain was softening characteristic was considered of dynamic to berecovery due to (DRV) new dynamic [21,22]. recrystallizationThe shape of the (DRX), grains and was the a asymptoticpolygon or behaviora block. wasWith characteristic an increase ofin dynamiccompression recovery temperature, (DRV) [21 the,22 grain]. The size shape increased of the grainssignificantly. was a polygon The maximu or a block.m value With grain an size increase was inobtained compression under a temperature, compression the temperature grain size of increased 1150 °C. signiﬁcantly.When the compression The maximum temperature value grainwas more size wasthanobtained 1050 °C, under the microstructure a compression appeared temperature to be of 1150clearly◦C. the When feature the compressionof an equiaxed temperature crystal. DRX was moreincompletely than 1050 produced◦C, the microstructure granular crystal appeared grains at to bea low clearly temperature. the feature Moreover, of an equiaxed the average crystal. grain DRX incompletelysize gradually produced increased granularfrom 6.2 crystalto 17.8 grainsμm with at aan low increase temperature. in the compression Moreover, thetemperature. average grain The sizevariation gradually rule of increased the actual from grain 6.2 size to 17.8 wasµ mobtained with an via increase numerical in the fitting. compression The relationship temperature. between The variationgrain size ruleand ofcompression the actual graintemperature size was is obtainedas follows: via numerical ﬁtting. The relationship between grain size and compression temperature is as follows:

−42 GeTT=−+2.954 2 0.561 273.2125 (1) G = 2.95e− T 0.561T + 273.2125 (1) − where GG isis thethe graingrain sizesize ((µμm)m) andand TT isis thethe compressioncompression temperaturetemperature ((°C).◦C).

Figure 2.2. Stress–strain curves ( a) and grain size data (b) ofof compressedcompressed 0.4C-Si-Mn-Cr0.4C-Si-Mn-Cr steel underunder

temperatures fromfrom 950950 toto 11501150 ◦°CC..

The graingrain sizes sizes were were fine fine for thefor compressionthe compression temperatures temperatures in the rangein the of range 950–1050 of ◦950–1050C. Therefore, °C. toTherefore, obtain fine to obtain original fine austenite original grains austenite before grains heat before treatment, heat thetreatment, forging the temperature forging temperature was selected was to beselected 1050 ◦toC, be which 1050 is°C, in which the range is in of the 950–1050 range of◦ C.950–1050 The sizes °C. of The grains sizes with of grains different with H different/D values H/D are providedvalues are in provided Figure3. in The Figure results 3. The showed results that showed the shape that of the the shape samples of the was samples elliptical was and elliptical band-like, and andband-like, the grain and size the decreased grain size with decreased an increase with in an the in Hcrease/D value. in the For H/D cast value. steel, theFor graincast steel, size was the aboutgrain 26.4size wasµm, about and the 26.4 grains μm, wereand the shaped grains as were irregular shaped blocks. as irregu Furthermore,lar blocks.the Furthermor distributione, the was distribution relatively dispersed,was relatively and dispersed, the grain roundnessand the grain was roundnes not goodsenough. was not Whengood enough. the H/D When value wasthe H/D from value 1.5 to was 2.0, thefrom grains 1.5 to were 2.0, mainlythe grains elliptical. were mainly After forging, elliptical. the grainAfter sizeforging, decreased the grain to 3.6 sizeµm, decreased and the distribution to 3.6 μm, ofand grains the distribution was relatively of grains uniform was and relatively dense. unifor With forging,m and dense. the variations With forging, in grain the sizevariations with di inff grainerent Hsize/D with values different were obtained H/D values via numericalwere obtained fitting: via numerical fitting:

G = 0.755x3 + 7.545x2 29.78x + 51.02(i R, i , 0) (2) i =−i32 +i −i +() ∈ ≠ GxxxiRiiiii−0.755 7.545− 29.78 51.02∈ , 0 (2) where Gi is the grain size (µm) and xi is the height-diameter ratio (H/D).

where Gi is the grain size (μm) and xi is the height-diameter ratio (H/D).

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Figure 3. GrainGrain size size of of 0.4C-Si-Mn-Cr 0.4C-Si-Mn-Cr steel steel with with differe diﬀerentnt H/D H/ Dvalues values for for the the deformation deformation process. process. (a) (As-cast,a) As-cast, (b) (H/Db)H /=D 1.5,= 1.5, (c) H/D (c)H =/D 1.7,= 1.7,and and (d) H/D (d)H =/D 2.0.= 2.0.

The optical microstructure (OM) of 0.4C-Si-Mn-Cr steel before and after heatheat treatmenttreatment withwith didifferentfferent HH/D/D valuesvalues isis shownshown inin FigureFigure4 4.. TheThe microstructure microstructure waswas primarily primarily composed composed ofof pearlite pearlite and ferrite before the heat treatment, and the pearli pearlitete and ferrite were distributed among each each grain. grain. AsAs shownshown in in Figure Figure4a–d, 4a–d, the the size size of the of pearlitethe pearlite in the in forged the forged samples samples was finer was than finer that than of thethat pearlite of the inpearlite the as-cast in the steel. as-cast The steel. pearlite The pearlite in the forged in the steel forged was steel mainly was distributedmainly distributed on the shear on the bands, shear and bands, the pearliteand the waspearlite deformed was deformed in the growth in the direction. growth Compared direction. withCompared forged steels,with forged the as-cast steels, specimen the as-cast had aspecimen distinct grainhad a orientation,distinct grain which orientation, could be which because could a temperature be because a gradient temperature field wasgradient formed field in was the crystalformed body in the due crystal to the body quantitative due to the loss quantitative of heat in thelossuncompressed of heat in the uncompressed state. This led state. to the This formation led to ofthe fibrous formation tissue of parallelfibrous tissue to the parallel direction to of the heat dire dissipation.ction of heat The dissipation. microstructures The microstructures of samples after of heatsamples treatment after heat for treatment different H for/D different values are H/D shown values in are Figure shown4e–h. in TheFigure microstructure 4e–h. The microstructure was mainly composedwas mainly of composed acicular bainite, of acicular martensite, bainite, and martensi retainedte, austenite.and retained Compared austenite. with Compared Figure4e, with the bainite Figure and4e, the martensite bainite and of specimensmartensite wereof specimens finer and were had fi aner dispersed and had distribution, a dispersed distribution, as shown in as Figure shown4f–h. in AccordingFigure 4f–h. to According the color metallographic to the color metallographic techniques [19 techniques], with an increase [19], with in Han/D, increase the bainite in H/D, content the increasedbainite content gradually increased while gradually the martensite while content the martensite decreased. content decreased.

Figure 4.4. Optical microstructure (OM) of 0.4C-Si-Mn-Cr steel before and after heat treatmenttreatment with didifferentﬀerent HH/D/D valuesvalues forfor thethe deformationdeformation process.process. ( a–d) Before heat treatment and ((e–h) afterafter heatheat treatment.treatment. P: pearlite, F: ferrite, B: bainite, M:M: martensite,martensite, and RA:RA: retainedretained austenite.austenite.

Figure5 5 shows shows SEM SEM images images of of steel steel before before and and after after heat heat treatment. treatment. Figure Figure5a is5a an is SEMan SEM image image of theof the sample sample with with an H /anD valueH/D ofvalue 2.0 beforeof 2.0 heatbefore treatment. heat treatment. Different pearliteDifferent clusters pearlite were clusters distributed were anddistributed grew along and thegrew grain along boundary. the grain The boundary. sizes of ferriteThe sizes and of cementite ferrite and in all cementite the grains in wereall the di ffgrainserent andwere regularly different arranged and regularly in a specific arranged direction. in a specific The SEM direction. microstructure The SEM of micros steel aftertructure heat of treatment steel after is heat treatment is shown in Figure 5b, and the microstructure was similar to that shown in Figure 4e– h. Figure 6 shows the XRD spectra for the steel samples before and after heat treatment, and the peaks

Materials 2020, 13, 172 6 of 11 shown in Figure5b, and the microstructure was similar to that shown in Figure4e–h. Figure6 shows Materials 2019, 13, x FOR PEER REVIEW 6 of 11 the XRD spectra for the steel samples before and after heat treatment, and the peaks observed before andobserved after heatbefore treatment and after corresponded heat treatment to the correspon body-centeredded to cubicthe body-centered structure. These cubic results structure. showed These that theresults sample showed had thethat face the centeredsample had cubic the structure face center aftered heatcubic treatment. structure Withafter theheat combined treatment. analysis With the of Figurescombined4 and analysis5, it could of Figures be conﬁrmed 4 and 5, that it could before be treatment,confirmed thethat body before centered treatment, cubic the structure body centered of the steelcubic was structure ferrite, of while the steel after was treatment, ferrite, there while was after a mixed treatment, structure there of was bainite a mixed and martensite.structure of The bainite face centeredand martensite. cubic structure The face wascentered the retainedcubic structure austenite was phase the retained after quenching, austenite andphase these after results quenching, were consistentand these results with the were experimental consistent results.with the experimental results.

Figure 5. SEMSEM micrographs micrographs of of the the microstr microstructureucture in in the the tested tested steel. steel. (a)( Beforea) Before heat heat treatment treatment and and ((b)) afterafter heatheat treatment.treatment.

Figure 6. XRDXRD spectra spectra of the tested steel treated under different diﬀerent conditions.

Figure7 7 shows shows TEM TEM images images and and the the selected selected area area electron electron di ff ractiondiffraction (SAED) (SAED) pattern pattern of steel of withsteel anwith H /anD valueH/D ofvalue 2.0. of As 2.0. shown As inshown Figure in7 a,Figure after forging,7a, after the forging, steel had the asteel large had number a large of dislocationnumber of linesdislocation and dislocation lines and dislocation tangles between tangles the between ferrite and the cementite.ferrite and Furthermore,cementite. Furthermore, cementite (cementite<100 nm) is(<100 observed nm) is inobserved Figure7 inb. Figure The deformation 7b. The deformatio ability ofn pearliteability of with pearlite ferrite with and ferrite cementite and cementite was variable was becausevariable ofbecause different of different stresses duringstresses high during temperature high temperature forging. forging. A dislocation A dislocation in the ferrite in the formed ferrite becauseformed because of the plastic of the deformation plastic deformation of the pearlite of the during pearlite forging. during When forging. there When was athere large was amount a large of deformation,amount of deformation, the layer size the and layer grain size sizeand ofgrain pearlite size of were pearlite fine. Thewere deformation fine. The deformation resistance decreasedresistance viadecreased adjustments via adjustments in the orientation in the orientation of lamella duringof lamella continuous during continuous impact compression. impact compression. The cementite The incementite the pearlite in the grain pearlite group grain gradually group fractured, gradually and fractured, the ferrite and formed the ferrite a high formed density a high of dislocations density of anddislocations a substructure and a thatsubstructure could reduce that the could strain reduce energy the [23 strain]. TEM energy images [23]. of the TEM morphology images of of the steelmorphology after heat of the treatment steel after with heat an treatment H/D value with of 2.0 an areH/D shown value inof Figure2.0 are 7shownc–d. After in Figure heat 7c–d. treatment, After theheat steel treatment, obtained the the steel multiphase obtained microstructure the multiphase that mi includedcrostructure dislocation-type that included lath dislocation-type martensite, bainite, lath andmartensite, retained bainite, austenite and and retained a number austenite of dislocations and a number gathered of around dislocations the martensite. gathered According around the to themartensite. inserted According SAEDP in Figureto the7 d,inse therted orientation SAEDP relationshipsin Figure. 7d, of the the multiphaseorientation microstructurerelationships of were the (110)α // (111)γ. In addition, the retained_ austenite_ _ was mainly found to be film-like and blocks. multiphase microstructure were (110)α // (111)γ. In addition, the retained austenite was mainly found to be film-like and blocks.

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FigureFigure 7.7.7. TEM TEMTEM images imagesimages of theofof thethe morphology morphologymorphology and SAEDandand SAEDSAED of steel ofof with steelsteel an with Hwith/D valueanan H/DH/D of 2.0 valuevalue in the ofof deformation 2.02.0 inin thethe process.deformationdeformation (a,b ) process.process. Before heat ((aa––bb treatment)) BeforeBefore heatheat and treatmenttreatment (c,d) after andand heat ((cc treatment.––dd)) afterafter heatheat treatment.treatment.

TheThe mechanicalmechanical propertiespropertiesproperties of ofof steel steelsteel with withwith di differendifferenfferentt Ht H/DH/D/D values valuesvalues are areare shown shownshown in inin Figure FigureFigure8. As8.8. AsAs shown shownshown in inFigurein FigureFigure8a, 8a, the8a, the hardnessthe hardnesshardness values valuesvalues (40–45 (40–45(40–45 HRC) HRC)HRC) of the ofof steel thethe steelsteel samples samplessamples after afterafter heat treatmentheatheat treatmenttreatment were were largerwere largerlarger than thanthethan hardness thethe hardnesshardness values valuesvalues (21–29 (21–29(21–29 HRC) HRC)HRC) of the ofof steel thethe steelsteel samples sasamplesmples before beforebefore heat heatheat treatment. treatment.treatment. As shownAsAs shownshown in Figure inin FigureFigure8b, 2 8b,the8b, the impactthe impactimpact toughness toughnesstoughness after afterafter heat heatheat treatment treatmenttreatment was waswas greater greatergreater than thanthan 12 J1212/cm J/cmJ/cm, which22,, whichwhich was waswas higher higherhigher than thanthan that thatthat of 2 oftheof thethe steel steelsteel samples samplessamples before beforebefore heat heatheat treatment treatmenttreatment (3–9 (3–9(3–9 J/ cmJ/cmJ/cm).22).). The TheThe hardness hardnesshardness and andand the thethe impactimpactimpact toughnesstoughnesstoughness werewere significantlysignificantlysignificantly enhanced enhanced after after heat heat tr treatment.treatment.eatment. Before Before heat heat treatment, treatment, with with anan increaseincrease inin thethe H/DH/D H/D value,value, hardnesshardness firstfirstfirst increased increasedincreased and andand then thenthen decreased. decreased.decreased. However, However,However, the thethe impact impactimpact toughness toughnesstoughness of the ofof the samplesthe samplessamples before beforebefore and andafterand afterafter heat heatheat treatment treatmenttreatment increased increasedincreased linearly. linearly.linearly.

FigureFigure 8.8. MechanicalMechanical properties properties of of the the 0.4C-Si-Mn 0.4C-Si-Mn-Cr0.4C-Si-Mn-Cr-Cr steel steel showing showing the the development development of of ( (aa)) hardness hardness andand ( (bb)) impactimpact toughnesstoughness beforebefore andand afterafterafter heatheatheat treatment.treatment.treatment.

4.4. Discussion Discussion FigureFigure 9 99 presents presentspresents a aa series seriesseries of ofof schematic schematicschematic diagrams diagramsdiagrams of ofof the thethe microstructure microstructuremicrostructure evolution evolutionevolution of ofof steel steelsteel during duringduring the process of compression-forging-heat treatment. When the temperature was greater than point thethe processprocess ofof compression-forging-heatcompression-forging-heat treatment.treatment. WhenWhen thethe temperaturetemperature waswas greatergreater thanthan pointpoint AA11,, Apearlitepearlite1, pearlite andand andferriteferrite ferrite dissolveddissolved dissolved andand formedformed and formed austeniteaustenite austenite grains.grains. grains. WhenWhen When thethe specimensspecimens the specimens beganbegan to beganto yield,yield, to plasticplastic yield, plasticstrainstrain andand strain recrystallizationrecrystallization and recrystallization drivingdriving driving forceforce increasedincreased force increased withwith anan with increaseincrease an increase inin compressioncompression in compression temperature.temperature. temperature. TheThe Thegraingrain grain boundaryboundary boundary migrationmigration migration raterate rateaccelerataccelerat accelerated,ed,ed, andand andthisthis thispromotedpromoted promoted rapidrapid rapid graingrain grain growth.growth. growth.

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FigureFigure 9.9. MicrostructureMicrostructure evolutionevolution mechanism of medi mediumum carbon carbon 0.4C-Si-Mn-Cr 0.4C-Si-Mn-Cr steel steel during during the the processprocess of of compression-forging-heat compression-forging-heat treatment.treatment.

AccordingAccording toto thethe kineticskinetics ofof graingrain growthgrowth [[24]:24]:

Gm Gm = k(t t ) (3) t mmt0 0 GGktt−−=−()− (3) tt0 0 where Gt0 and Gt are the grain diameter (µm) at the times t0 and t, respectively. m is the average mobility of the grain boundary, and m depends on the index of Gt/G . k = k exp( Q/RT), where Q is the where Gt0 and Gt are the grain diameter (μm) at the times t0 andt0 t, respectively.0 − m is the average activationmobility of energy the grain of the boundary, grain boundary and m depends and T is theon the heating index temperature. of Gt/Gt0. k = k0exp(−Q/RT), where Q is the activationAccording energy to Equation of the (3),grain when boundary time is and constant, T is them heatingis proportional temperature. to exp( Q/RT); thus, the rate − of grainAccording growth isto relatedEquation to (3),m. Whenwhen time the temperatureis constant, m is is higher, proportional the grain to exp( boundary−Q/RT); migration thus, the rate and theof grain growth growth rate were is related faster to due m. When to the increasedthe temperature atomic is di higher,ffusion the rate; grain also, boundary the grain migration size increased and exponentiallythe growth rate with were an faster increase due into the temperature. increased at Temperatureomic diffusion aff rate;ects also, the plasticthe grain deformation size increased of a material,exponentially which with as a consequence,an increase in directly temperature. influences Temperature the microstructure affects the evolution. plastic Thedeformation growth rate of a of grainsmaterial, at a which low temperature as a consequence, was lower directly than influenc that ates a highthe microstructure temperature, which evolution. was becauseThe growth recovery rate andof grains recrystallization at a low temperature were more was pronounced lower than at that a high at a high temperature. temperature, which was because recovery and Therecrystallization diameter of thewere cross-section more pronounced of the roundat a high bar temperature. decreased during forging. Under the action of externalThe diameter force, the of roundthe cross-section bar was deformed of the round and bar expanded decreased outward during in forging. the radial Under direction. the action The twoof external end faces force, were the elongated round bar along was deformed the vertical and axis expanded (Figure outward1c). The in dislocation the radial densitydirection. of The pearlite two duringend faces the were forging elongated deformation along the process vertical increased, axis (Figure and 1c). its The value dislocation changed de fromnsity 2.02of pearlite1014 duringm 2 to × − 6.07the forging1014 m deformation2; this provided process nucleation increased, sites and for its cementite value changed and promoted from 2.02 diff usion× 1014 ofm− iron2 to 6.07 and carbon× 1014 × − atoms.m−2; this An provided increase innucleation the H/D valuesites for accelerated cementite the an migrationd promoted of diffusion atoms along of iron the dislocationand carbon line, atoms. and thisAn ledincrease to a decrease in the H/D in the value deformation accelerated resistance. the migrat Whenion of the atoms strain along was the higher dislocation than the line, critical and strain, this theled lattice to a decrease was distorted in the becausedeformation the number resistance. of slippages When the and strain twinning was higher increased than viathe atomscritical through strain, dislocation.the lattice was The distorted initial grains because began the to number break, andof slippages the slip resistanceand twinning increased increased in this via case. atoms Dislocation through densitydislocation. and deformationThe initial grains energy began were to formedbreak, and in thethe ferrite,slip resistance and this increased was beneficial in this forcase. obtaining Dislocation fine grainsdensity during and deformation DRX [25,26 ].energy Furthermore, were formed the temperature in the ferrite, of and the this core was slowly beneficial decreased for obtaining because of fine the largegrains volume during of DRX steel, [25,26]. providing Furthermore, sufficient the time temperature for grain of recovery the core in slowly the forging decreased process. because However, of the withlarge a volume continuous of steel, decrease providing in temperature sufficient duringtime for the grain forging recovery process, in the it wasforging diffi process.cult to recrystallize However, deformedwith a continuous austenite decrease because delayedin temperature nucleation during inhibited the forging DRX process, [27]. In addition,it was difficult the high to recrystallize content of Si increaseddeformed the austenite hot deformation because delayed energy andnucleation reduced inhibi the rateted DRX of DRX, [27]. which In addition, made the the steel high deformation content of diSiffi increasedcult, and thethe grain hot growthdeformation gradually energy stopped and [redu28–30ced]. Thethe microstructurerate of DRX, ofwhich the undeformed made the steel billet wasdeformation relatively stabledifficult, under and same the grain heating growth conditions gradua andlly still stopped retained [28–30]. a large The grain microstructure size. When the of billetthe undeformed billet was relatively stable under same heating conditions and still retained a large grain was air cooled to room temperature after forging, transformation of the ferrite and pearlite occurred size. When the billet was air cooled to room temperature after forging, transformation of the ferrite during the cooling process [31]. Because of nucleation and grain growth during the transformation and pearlite occurred during the cooling process [31]. Because of nucleation and grain growth during process, it was difficult to obtain quantitative information for DRX during deformation. the transformation process, it was difficult to obtain quantitative information for DRX during The hardness and impact toughness of the steel sample obviously increased under the same deformation. quenching conditions. The grain size decreased after forging, and microstructure refinement improved The hardness and impact toughness of the steel sample obviously increased under the same the hardness and toughness of the metal [32]. After heat treatment, the pearlite of the forged state quenching conditions. The grain size decreased after forging, and microstructure refinement transformed into bainite, martensite, and retained austenite. The increase in the martensite content was improved the hardness and toughness of the metal [32]. After heat treatment, the pearlite of the beneficial for enhancing the hardness, and bainite and film-like retained austenite clearly improved forged state transformed into bainite, martensite, and retained austenite. The increase in the the impact toughness [33]. The transformation induced plasticity (TRIP) effects resulted from the martensite content was beneficial for enhancing the hardness, and bainite and film-like retained deformation during forging, and this contributed to the enhanced toughness because the austenite austenite clearly improved the impact toughness [33]. The transformation induced plasticity (TRIP) was rich in manganese and carbon [34]. Smaller prior austenite grains formed a finer martensite effects resulted from the deformation during forging, and this contributed to the enhanced toughness because the austenite was rich in manganese and carbon [34]. Smaller prior austenite grains formed

Materials 2020, 13, 172 9 of 11 lath after heat treatment, whereas the longer martensite in the specimens also meant that there were larger grains before austenite transformed to bainite and martensite [35,36]. With an increase in deformation, the hardness of the forged billet slightly decreased before and after heat treatment. The work hardening effect was inherited by the subsequent microstructural transformation process due to thermal deformation. The material appeared to undergo dynamic recovery and flow softening because of the influence of the annihilation of dislocation when the strain was further increased to some extent [37,38]. With increased deformation at different values of H/D, the specimens were completely recrystallized, and this reduced the deformation energy storage and led to decreased hardness. However, the hardness of steel was improved compared to that of the as-cast steel because of deformation.

5. Conclusions

(1) With an increase in deformation temperature in the range of 950–1150 ◦C, the grain size of samples increased from 6.2 µm to 17.8 µm. The relationship between the grain size and temperature conformed to the fitting formula (Equation (1)). The steel had an abundance of blocky grains during the deformation process. Moreover, the grain size decreased from 26.4 µm to 3.6 µm with an increase in the H/D value. The relationship between the grain size and H/D was close to Equation (2). (2) The microstructure was pearlite, ferrite, bainite, martensite, and retained austenite with different H/D values. The microstructure was clearly more uniform and finer than that of the as-cast steel. The optimum deformation process was a heating temperature that was less than 1050 ◦C, and the H/D value was 2.0. Changes in grain size depended on the grain nucleation and dislocation movement during forging. (3) Compared with steel that was not subjected to heat treatment, the hardness and impact toughness of the steel samples after heat treatment showed obvious improvements. When the H/D value was 2.0, the maximum impact toughness was 87 J/cm2. Heat treatment had a significant contribution to improving the mechanical properties of the forged samples. Hardness decreased slightly with an increase in the H/D value. Furthermore, the impact toughness gradually increased, and this was related to the microstructure, dynamic recovery, and softening of the grains.

Author Contributions: The study was designed by Q.S., Z.L. and R.Z. The data was collected by F.Z., Y.Y. and J.B., and the literature was searched by Y.Y. and J.B. The figures and data analysis wereperformedby F.Z. and Z.L. The data interpretation was carried out by Q.S. The manuscript was written by F.Z. and Q.S. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by [National key R&D Program of China] grant number [2017YFB0305100, 2017YFB0305101] and [National Natural Science Foundation of China] grant number [51561018, 51871116]. Conflicts of Interest: The authors declare no conflict of interest.

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