The Role of Retained Austenite and Its Carbon Concentration on Elongation of Low Temperature Bainitic Steels at Different Austenitising Temperature

Article The Role of Retained Austenite and Its Carbon Concentration on Elongation of Low Temperature Bainitic Steels at Different Austenitising Temperature

Baoqi Dong, Tingping Hou *, Wen Zhou, Guohong Zhang and Kaiming Wu * The State Key Laboratory for Refractories and Metallurgy, Hubei Collaborative Innovation Center on Advanced Steels, International Research Institute for Steel Technology, Wuhan University of Science and Technology, Wuhan 430081, China; [email protected] (B.D.); [email protected] (W.Z.); [email protected] (G.Z.) * Correspondence: [email protected] (T.H.); [email protected] (K.W.); Tel.: +86-27-6886-2266 (T.H.); +86-27-6886-2772 (K.W.) Received: 25 October 2018; Accepted: 7 November 2018; Published: 11 November 2018

Abstract: The inﬂuence of austenitising temperature on the tensile properties of low temperature bainitic steel was investigated. With the increasing austenitising temperature, a signiﬁcant change of elongation was found between 850 and 950 ◦C, which was changed from 1.0 ± 0.5 to 10.7 ± 2.0%; while there was a slight increase between 950 to 1050 ◦C (11.2 ± 1.5%). By characterising the retained austenite at necking and matrix, we found that the elongation is obviously correlated with the retained austenite content, and also determined by the volume change of retained austenite during the tensile test. The transformation induced plasticity (TRIP) effect, which contributes to the improve elongation, almost did not occur at 850 ◦C due to the relatively low volume percentage of retained austenite and its high carbon concentration, which resulted in a very low martensite transformation temperature. With the austenitising, the temperature was increased up to 950 and 1050 ◦C, and a large volume percentage of retained austenite was observed in the matrix. Meanwhile, a considerable amount of retained austenite has occurred by the TRIP effect because of a moderate carbon content.

Keywords: low temperature bainitic steel; elongation; retained austenite; carbon content; austenite grain size

1. Introduction Bainitic steels are a kind of steel used in armor, bearing, and wear-resistant plates owing to their ultra-high strength, good ductility, and toughness properties [1,2]. The microstructure of bainitic steel contains a mixture of fine bainitic ferrite plates, retained austenite, and some martensite [3,4]. The strength is mainly determined by bainitic ferrite (harder phase) and its dislocation density; and the ductility is controlled by the retained austenite, which is a softer phase compared with bainitic ferrite and martensite [5,6]. It is believed that the retained austenite stability is closely related to the ductility of the bainitic steel. Retained austenite with a different stability contributes to the transformation induced plasticity (TRIP) effect at different strain stages until necking or damage occurs. In a previous work, the retained austenite stability was observed to be influenced by its chemical composition [4,7]. It is found that the carbon concentration in retained austenite has the strongest influence on the ductility of bainitic steel. The film-like retained austenite, which promotes the elongation in the TRIP steel, are more stable, and less likely to transform into martensite, because of their relatively high carbon content [8]. However, it was also demonstrated that too stable austenite does not guarantee better ductility, since it is difficult to transform into martensite under stress [4]. On the other hand, the TRIP effect will easily

Metals 2018, 8, 931; doi:10.3390/met8110931 www.mdpi.com/journal/metals Metals 2018, 8, 931 2 of 11 occur under small stress due to lower stability of retained austenite, which is also not conducive to the improvement of ductility. More recently, it confirmed that the retained austenite stability also strongly depends on the size of the austenite grain [9,10]. In general, the size of the austenite grain will increase with the increasing of the austenitising temperature. It has been suggested that the size of the austenite grain has an optimal range for the improvement of the TRIP effect, and too large or too fine grains are not conducive to the mechanical property enhancement [9]. Researchers also found that the prior austenite grain size has a substantial contribution to the austenite stability when the austenite grain size is submicron [11]. However, it is discovered that there is no correlation between the mechanical stability of austenite and the grain size, the thermal stability of austenite increases significantly through grain refinement in stainless steel [12]. The most advantageous variants are selected in the TRIP effect to release tensile strain and leads to the grain size independence of the mechanical stability of austenite. Therefore, the relationship between the austenite grain size and retained austenite stability is not the same in different steels. The aim of this study is to explore the retained austenite content and its carbon concentration influence on the elongation of low temperature bainitic steels treated at different austenitising temperatures. In addition, the variation of retained austenite volume fraction and its carbon concentration at the necking area is also investigated.

2. Experiment The chemical analysis of the investigated steel is presented in Table1. In the present work, the use of medium carbon steel as an experimental steel is necessary. The reason is that the high carbon content in the matrix will form a large number of Fe-C clusters, which is extremely detrimental to the plasticity of the low-temperature bainite steel [13,14]. In contrast, low carbon content reduces the stability of retained austenite, which is also not conducive to the improvement of plasticity. The as-received steel was homogenized at 1200 ◦C for two days in a vacuum furnace followed by slow cooling to an ambient temperature. The slow cooling rate to an ambient temperature was applied to avoid spontaneous crack formation due to the martensite transformation. The homogenized specimens were austenitized at 850 ◦C, 950 ◦C, and 1050 ◦C for 30 min, and then were isothermally transformed to bainite at 220 ◦C for 10 days. Finally, the specimens were air cooled to room temperature.

Table 1. Chemical composition of the investigated steel (wt.%).

C Mn Si Mo Ni Co Cr S Fe 0.58 2.21 1.76 0.33 0.76 0.45 0.54 0.02 balance

Quantitative XRD (X-ray diffraction) analysis (Rigaku D/Max-1200X diffractometer, Tokyo, Japan) was used to determine the volume percentage of retained austenite and its carbon concentration. Grinding and final polishing were carried out and the samples were etched in 4 vol.% nital solution for the phase characterization. The samples were set in the Bragg-Brentano configuration using a Rigaku D/Max-1200X diffractometer (Tokyo, Japan) with unfiltered Co Kα radiation (wavelength 1.78901 Å). The 2θ scanning angles were kept between 20◦ and 120◦ and the scanning speed was less than 0.1◦/min. The machine was operated at 40 kV and 30 mA. The retained austenite volume percentage was calculated from the integrated intensities of (200), (220), and (311) austenite peaks and (200) and (211) of ferrite peaks [15,16]. The carbon content of retained austenite was measured using the lattice parameter and empirical equation [17], expressed as:

aγ/Å = 3.578 + 0.044Xc (1) where aγ represents the lattice parameters of austenite, and Xc is the weight percent of the carbon atom. Metals 2018, 8, 931 3 of 11 Metals 2018, 8, x FOR PEER REVIEW 3 of 11

Cylindrical tensiletensile samplessamples withwith 55 mm mm in in diameter diameter and and 25 25 mm mm in in gauge gauge length length were were machined machined in thein the longitudinal longitudinal direction direction of bars. of bars. The The tensile tensile experiment experiment were were conducted conducted with with a 100 a kN 100 load kN cell load using cell anusing Zwick an Zwick HTM 5020HTM (ZWICK/ROELL, 5020 (ZWICK/ROELL Ulm, Germany), Ulm, Germany ﬁtted with) fitted a crosshead with a crosshead speed of speed 0.03 mm/s of 0.03 at ambientmm/s at ambient temperature. temperature. The specimens The specimens were extracted were extracted from the from tensile the fracture tensile tofracture the matrix to the at matrix every 2at mmevery thickness. 2 mm thickness. A total of A ﬁve total pieces, of five numbered pieces, numbered a–e, were obtaineda–e, were as obtained shown inas Figureshown1. in These Figure thin 1. slicesThese werethin slices used towere characterize used to characterize the microstructure the microstructure and to measure and theto measure retained the austenite retained content austenite and itscontent carbon and concentration its carbon concentration at the necking at zonethe necking after the zone tensile after test. the tensile test.

Figure 1. 1. (a)( Tensilea) Tensile sample sample of experiment of experiment steels; steels; (b) five ( bslices) ﬁve at slicesevery at2 mm every thickness, 2 mm numbered thickness, numberedthrough a to through e, were a obtained. to e, were obtained.

To measuremeasure the carboncarbon concentrationconcentration of retained austenite (RA),(RA), electron probe microanalysis (EPMA, JXA-8530F,JXA-8530F, JEOL, Tokyo, Japan)Japan) was also used,used, and its X-rayX-ray detectordetector has a lower content limit ofof 0.001%. The The operation operation and and precautions precautions have have been been reported reported in in detail detail in in an an earlier earlier work work [ [1818].]. The carbon content of the measured point was determined at 15.0 kV and 20 mA. Every sample was assigned 30 points to detect the carbon concentration of RA, and th thenen the average value of the carbon content was calculated. The microstructural features of the matrix and austenite grain size were determined by using optical microscopy. The surfaces of tensile fractured specimens were observed by a scanning electron microscope. A t transmissionransmission electron microscope (TEM, JEM JEM-2100,-2100, JEOL, Tokyo, Japan) was utilized to measure the thickness of bainite ferrite with at least 10 micrographs.micrographs. The austenite grain size was measured byby thethelinear linear intercept intercept method method with with the the data data obtained obtained from from different different ﬁelds fields of the of samplesthe samples [19]. The[19]. VickersThe Vickers hardness hardness tests (THV-1MD)tests (THV-1MD) were notedwere asnoted an averageas an average of at least of at 15 least indentations 15 indentations using a loadusing of a 1load Kg. of 1 Kg.

3. Results 3. Results 3.1. Microstructural Analysis 3.1. Microstructural Analysis The typical microstructure as resolved by optical microscope (OM) and scanning electron The typical microstructure as resolved by optical microscope (OM) and scanning electron microscope (SEM) micrographs of the steel after austenitising at 850, 950, and 1050 ◦C for 30 min microscope (SEM) micrographs of the steel after austenitising at 850, 950, and 1050 °C for 30 min and and then isothermally at 220 ◦C for 10 days is given in Figure2. In Figure2a–c, bainitic ferrite then isothermally at 220 °C for 10 days is given in Figure 2. In Figure 2a–c, bainitic ferrite (dark phase), (dark phase), retained austenite (light phase), and a small part of martensite can be seen in the three retained austenite (light phase), and a small part of martensite can be seen in the three types of bainitic types of bainitic steels. The microstructure became much coarser with an increasing austenitising steels. The microstructure became much coarser with an increasing austenitising temperature. This temperature. This feature reduces the nucleation density of bainitic ferrite and increases the needle feature reduces the nucleation density of bainitic ferrite and increases the needle length of the bainite length of the bainite lath, since the austenite grain size is greater at a higher austenitising temperature. lath, since the austenite grain size is greater at a higher austenitising temperature. Retained austenite Retained austenite is divided into two forms in low temperature bainitic steel, namely film-like is divided into two forms in low temperature bainitic steel, namely film-like retained austenite retained austenite distributed between the bainitic ferrite lath, and blocky retained austenite distributed distributed between the bainitic ferrite lath, and blocky retained austenite distributed between bainite between bainite sheaves. At a much higher SEM magnification, the retained austenite, in a blocky sheaves. At a much higher SEM magnification, the retained austenite, in a blocky shape, can clearly shape, can clearly be seen, while there was no sign of a film-like structure (Figure2d–f). It has been be seen, while there was no sign of a film-like structure (Figure 2d–f). It has been observed that fine observed that fine film-like retained austenite can be obtained through bainite transformation at a film-like retained austenite can be obtained through bainite transformation at a low temperature (220 low temperature (220 ◦C). Figure3 shows the TEM micrographs treated at different austenitising °C). Figure 3 shows the TEM micrographs treated at different austenitising temperatures. The temperatures. The measured thickness of bainitic ferrite was 45 ± 5 nm, 50 ± 5 nm, and 70 ± 15 nm measured thickness of bainitic ferrite was 45 ± 5 nm, 50 ± 5 nm, and 70 ± 15 nm for the specimens for the specimens treated at 850 ◦C, 950 ◦C, and 1050 ◦C, respectively, which showed an upward trend treated at 850 °C, 950 °C, and 1050 °C, respectively, which showed an upward trend with the with the austenitising temperature. austenitising temperature.

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Figure 2.2 .. Optical micrograpmicrographshs ((OM)OM)) and and scanning scanning electron electron microscope microscope ((SEM) (SEM)) images images at at different different austenitisingaustenitising temperatures.temperatures. ((aa)) andand ((dd)) 850850 ◦°C;°C;C; ( (b)) and and ( (ee)) 950 950 °C;◦°C;C; ( (cc)) and and ( (ff)) 1050 °C.◦°C.C.

Figure 3. 3.. TTransmissionransmissionransmission electron electron electron microscope microscope microscope (( (TEM)TEM)) imagesimages treated treated at at different different austenitising austenitising ◦ ◦ ◦ temperatures.temperatures. ( (aa)) 850 850 °C;°C;C; ( (b)) 950 950 °C;°C;C; (( cc)) 10501050 °C.°C.C. 3.2. Tensile Properties 3.2.. Tensile Properties The tensile strength, elongation, and hardness of the tested steels by three different austenitising The tensile strength, elongation,, and hardness of the tested steels by three different austenitising treatments are shown in Table2. The hardness decreased gradually with the increasing austenitising treatmentstreatments areare shownshown inin TableTable 2.2. The hardness decreased gradually with the increasing austenitising temperature. There was a slight difference in the tensile strength at 850 ◦C and 950 ◦C, but it decreased temperature.temperature. ThereThere waswas aa slightslight differencedifference inin thethe tensiletensile strengthstrength atat 850850 °C°C andand 950950 °C,°C, but it decreased sharply from 1900 ± 60 MPa at 950 ◦C to 1794 ± 65 MPa at 1050 ◦C. For elongation, there is a marked sharplysharply fromfrom 19001900 ±± 6060 MPaMPa atat 950950 °C°C toto 17941794 ±± 6565 MPaMPa atat 10501050 °C.°C. ForFor elongation,elongation, therethere isis aa markedmarked increase between 850 ◦C and 950 ◦C, which was increased from 1.0 ± 0.5% to 10.7 ± 2.0%; while the increaseincrease betweenbetween 850 °C°C andand 950950 °C,°C, whichwhich waswas increasedincreased fromfrom 1.01.0 ±± 0.5%0.5% toto 10.710.7 ±± 2.0%;2.0%; whilewhile thethe elongation at 1050 ◦C (10.7 ± 2.0) has a slight increase compared to 950 ◦C (11.2 ± 1.5%). elongation at 1050 °C (10.7 ± 2.0)) hashas aa slightslight increaseincrease comparedcompared toto 950950 °C°C (11.2(11.2 ±± 1.5%).1.5%). Table 2. Tensile properties of the investigated steel. Table 2.. Tensile properties of the investigatedinvestigated steelsteel.. Austenitising Temperature (◦C) Tensile Strength (MPa) Elongation (%) Hardness (HV) Austenitising Temperature (°C)(°C) Tensile Strength (MPa)(MPa) Elongation (%) Hardness (HV) 850 1910 ± 65 1.0 ± 0.5 619 ± 10 850850 19101910 ±± 6565 1.01.0 ±± 0.50.5 619619 ±± 1010 950 1900 ± 60 10.7 ± 2.0 599 ± 10 950 1900 ± 60 10.7 ± 2.0 599 ± 10 9501050 17941900± ±65 60 11.210.7± 1.5 ± 2.0 586 ±5995 ± 10 10501050 17941794 ±± 6565 11.211.2 ±± 1.51.5 586586 ±± 55

Figure4 displays the tensile fractography of the tensile specimens at different austenitising Figure 4 displays thethe tensile tensile fractography fractography of of the the tensile tensile specimensspecimens at different austenitising temperatures. The fracture surface at 850 ◦C (Figure4a) consists of large quasi-cleavage facets and temperatures.temperatures. TheThe fracturefracture surfacesurface atat 850850 °C°C (Figure(Figure 4a)4a) consistsconsists ofof largelarge quasiquasi--cleavagecleavage facetsfacets andand an an intergranular fracture, indicating a brittle fracture mode. The fracture surface with different size intergranularintergranular fracture, fracture, indicating indicating a a brittle fracture mode. The fracture surface with different size dimples at 950 °C is shown in Figure 4b, which indicates a better toughness in comparison to that illustratedillustrated inin Figure 4a. Austenitising at 1050 °C (Figure 4c) resulted in an extremely high density of

Metals 2018, 8, 931 5 of 11 dimples at 950 ◦C is shown in Figure4b, which indicates a better toughness in comparison to that Metals 2018,, 8,, xx FORFOR PEER REVIEW 5 of 11 illustrated in Figure4a. Austenitising at 1050 ◦C (Figure4c) resulted in an extremely high density of dimplesdimples,,, indicating indicating a a typical typical ductile mode mode of fracture. It It can be deduced that a higher austenitising temperaturetemperature produces a positive effect on improvingimproving elongation.elongation..

FigureFigure 4 4... TheThe fracture fracture morphology morphology of specimens of specimens after after the tensile the tensile test. ( test.a) 850 ( a°C;) 850 (b) ◦950C; (°C;b) 950and ◦(cC;) 1050and (°C.c) 1050 ◦C.

3.33.3... The The Austenite Austenite Grain Grain Size Size at at Different Different Austenitising Temperatures OpticalOptical micrographs micrographs (OM) (OM) of of the the austenite austenite grain grain boundaries boundaries are are presented presented in in Figure Figure 5 5aa–c.–c. ◦ Obviously,Obviously, the the fine ﬁne austenite austenite grain grain can can be be obtained obtained when when austenitising austenitising temperature temperature at 850 at 850°C andC and950 ◦ °C.950 TheC. Theaustenite austenite grain grain size sizewas was measured measured using using the thelinear linear intercept intercept method method... TheThe The austeniteaustenite austenite grain grain is coarsenedis coarsened due due to to an an increase increase in in the the austenitising austenitising temperature. temperature. The The average average austenite austenite grain grain size, size, ◦ ◦ ◦ comparedcompared with that atat 850850 °CC (33(33 ± 88 μm)µm) and and 950 950 °CC (35 (35 ± 66 μmµm),), was was significantly signiﬁcantly larger larger at at 1050 °CC µ ◦ ◦ (82(82 ± 1111 μm),m), while while only only a a little little difference difference was was seen seen between between 850 850 °CC and and 950 950 °CC (Figure (Figure 5d)5d)...

FigureFigure 5 5... TheThe OM OM images images of of austenite austenite grain grain of of the the steel steel at atdifferent different austenitising austenitising temperatures temperatures for for30 ◦ ◦ ◦ ◦ min30 min followed followed by byiso isothermalthermal transformation transformation atat 220 220 °C Cfor for 10 10 days. days. (a ()a )850 850 °C;C; (b (b) )950 950 °C;C; ( (cc)) 1050 1050 °C;C; andand ( d)) dependence dependence of of the the austenite grain size on austenitising temperatures. 3.4. The Retained Austenite Contents of Necking Sites at Different Austenitising Temperatures 3.4.. The Retained Austenite Contents of Necking Sites at Different Austenitising Temperatures Representative XRD patterns (Figure6a–c) conﬁrmed the volume percentage of retained austenite Representative XRD patterns (Figure 6a–c) confirmed the volume percentage of retained dependence of necking sites. All test specimens were consisted of bainitic ferrite, martensite, austenite dependence of necking sites. All test specimens were consisted of bainitic ferrite,, and retained austenite. With the necking sites from a–e, the volume fractions of the retained austenite martensite,, and and retained retained austenite. austenite. With With the the necking necking sites sites from from a–e,, the the volume volume fractions fractions of of the the were increased with approaching to site e (Figure7a). It is evident from site e that the volume retained austenite were increased with approaching to site e (Figure 7a). It is evident from site e that the volume percentage of the retained austenite of 850 °C (11.68%) is slightly higher than that of 950 (11.15%) and 1050 °C (11.22%). However, from the investigation of site a, the retained austenite content at 850 °C (8.70%) was significantly higher compared with that at 950 (4.52%) and 1050 °C

Metals 2018, 8, 931 6 of 11 Metals 2018, 8, x FOR PEER REVIEW 6 of 11 Metals 2018, 8, x FOR PEER REVIEW 6 of 11 percentage of the retained austenite of 850 ◦C (11.68%) is slightly higher than that of 950 (11.15%) and (4.06%), 1050indicating◦C (11.22%). that However,only a small from thepart investigation of the retained of site a,austenite the retained was austenite transformed content atin 850to martensite◦C (4.06%), indicating that only a small part of the retained austenite was transformed into martensite during the(8.70%) tensile was signiﬁcantlyprocess for higher 850 compared°C. The three with that straight at 950 (4.52%) lines andrepresent 1050 ◦C (4.06%),the trend indicating of the that content of retainedduring the onlyaustenite tensile a small with partprocess of necking the for retained 850 sites °C. austenite at The different wasthree transformed straightaustenitising intolines martensite representtemperature during thes theaftertrend tensile linear of processthe fitting. content The of ◦ carbonretained concentration foraustenite 850 C. The with three of necking retained straight linessites austenite represent at different at the different trend austenitising of the necking content temperature of sites retained are austenite showns after with inlinear Figure necking fitting. 7b. The sites at different austenitising temperatures after linear ﬁtting. The carbon concentration of retained experimentalcarbon concentration results show of retained that the austenitecarbon concentration at different in necking the retained sites areaustenite shown of in different Figure necking 7b. The experimentalaustenite results at different show neckingthat the sites carbon are shown concentration in Figure7 b.in Thethe experimentalretained austenite results show of different that the necking sites is similarcarbon concentrationto that of the in thematrix. retained This austenite means of differentthat the necking tensile sites process is similar has to thatessentially of the matrix. no obvious sites is similar to that of the matrix. This means that the tensile process has essentially no obvious impact onThis the means carbon that concentration the tensile process in hasthe essentially retained noaustenite. obvious impact on the carbon concentration in impact onthe the retained carbon austenite. concentration in the retained austenite.

Figure 6. X-ray diffraction (XRD) patterns of retained austenite content at five necking sites at (a) 850 Figure 6Figure. X-ray 6. diffractionX-ray diffraction (XRD (XRD)) patterns patterns of retainedretained austenite austenite content content at ﬁve neckingat five sitesnecking at (a) 850sites◦C at; (a) 850 °C; (b) 950 °C; ◦(c) 1050 °C,◦ respectively. (d) The retained austenite content of matrix. °C; (b) 950(b) °C; 950 (C;c) (1050c) 1050 °C,C, respectively. respectively. (d ()d The) The retained retained austenite austenite content content of matrix. of matrix.

Figure 7.Figure (a) The 7. (retaineda) The retained austenite austenite contents contents of of five ﬁve necking necking sites sites and and (b) ( itsb) carbon its carbon content content at different at different austenitisingFigure 7.austenitising (a) The temperatures retained temperatures austenite followed followed contents by by numbering numbering of five throughneckingthrough a tositesa e.to Notee. and Note that (b )that site its acarbon site is close a is content to close the tensile to at the different tensile fracture, and number e is close to the matrix. fracture,austenitising and numbertemperatures e is close followed to the by matrix. numbering through a to e. Note that site a is close to the tensile fracture,4. Discussion and number e is close to the matrix. 4. DiscussionFigure 5d presents the variation of austenite grain size dependence of austenitising temperatures. 4. DiscussionAustenite grain size slightly increased as the temperature increased from 850 ◦C to 950 ◦C, while it Figure 5d presents the variation◦ of◦ austenite grain size dependence of austenitising◦ Figurerapidly 5d increased presents from the 950 variationC to 1050 ofC. austeniteThere exists grain a critical siz temperaturee dependence (about of 1000 austenitisingC), temperatures.below whichAustenite it is not grain signiﬁcant size slightly and above increased which is aas distinct the temperature temperature effect.increased The growth from of850 the °C to 950 °C,temperatures. while it rapidly Austenite increased grain from size slightly950 °C toincreased 1050 °C. as There the temperature exists a critical increased temperature from 850 (about °C to 1000 950 °C)°C, , whilebelow it which rapidly it iincreaseds not significant from 950 and °C above to 1050 which °C. Thereis a distinct exists temperaturea critical temperature effect. The (about growth 1000 of the°C) ,austenite below which grain it dependsis not significant on the ratesand aboveof the which austenite is a distinctgrain boundary temperature migration effect. The[20]. growth With the of increasethe austenite of the grain austenitising depends temperature, on the rates the of migrationthe austenite rate grain of the boundary grain boundary migration is also [20 accelerated.]. With the Belowincrease this of the critical austenitising austenitising temperature, temperature the migration the migration rate of ratethe grain is slightly boundary increased is also accelerated. (essentially unchanged),Below this critical while above austenitising this temperature temperature it is thesignificantly migration increased. rate is slightly The sudd increaseden change (essentially in grain growthunchanged), behavior while at above950 °C this and temperature 1050 °C may it be is attributedsignificantly to theincreased. dissolution The suddof theen pinning change particles in grain ofgrowth MnS. behavior The pinning at 950 force °C and produced 1050 °C may by MnS be attributed was gradually to the dissolution decreased atof the higher pinning austenitising particles temperatureof MnS. Thes due pinning to spherical force produced sulfides ( Figure by MnS 8) . wasThe statistical gradually results decreased showed at higherthat the austenitising aspect ratio oftemperature MnS are 2.65s due ± 1.11,to spherical 2.21 ± 0.67, sulfides and 1.69(Figure ± 0.41 8). atThe 850 statistical °C, 950 °C,results and showed1050 °C, that respectively. the aspect It ratio has beenof MnS suggested are 2.65 that ± 1.11, large 2.21 and ± 0.67,long andstring 1.69 MnS ± 0.41 inclusions at 850 °C,are more950 °C, significantly and 1050 °C, effective respectively. in hindering It has been suggested that large and long string MnS inclusions are more significantly effective in hindering

Metals 2018, 8, 931 7 of 11 austenite grain depends on the rates of the austenite grain boundary migration [20]. With the increase of the austenitising temperature, the migration rate of the grain boundary is also accelerated. Below this critical austenitising temperature the migration rate is slightly increased (essentially unchanged), while above this temperature it is significantly increased. The sudden change in grain growth behavior at 950 ◦C and 1050 ◦C may be attributed to the dissolution of the pinning particles of MnS. The pinning force produced by MnS was gradually decreased at higher austenitising temperatures due to spherical sulfides (Figure8). The statistical results showed that the aspect ratio of MnS are 2.65 ± 1.11, 2.21 ± 0.67, and 1.69 ± 0.41 at 850 ◦C, 950 ◦C, and 1050 ◦C, respectively. It has been Metalssuggested 2018, 8, that x FOR large PEER and REVIEW long string MnS inclusions are more significantly effective in hindering7 of the 11 migration of austenite grain boundaries than spherical ones [21]. In addition, the relationship between theelongation migration and of MnS austenite has been grain investigated boundaries according than spherical to the McClintock ones [21]. In mode addition, [22]: the relationship between elongation and MnS has been investigated according to the McClintock mode [22]: dσ/(σ·dE) < KF2 (f /1 − f ) (2λ + 1)1/2 (2) dσ/(σ·dE) < KF2 (f/1 − f) (2λ + 1)1/2 (2) where σ is the true stress,stress, ff is the volume fractionfraction ofof inclusions,inclusions,λ λis is the the aspect aspect ratio ratio of of inclusions, inclusionsK, Kis is a aconstant, constantF, Fis is a a hole-growth hole-growth factor, factor and, and d dσσ//ddEEis is thethe work-hardeningwork-hardening rate.rate. InIn the present work, f cancan ◦ ◦ ◦ bebe regarded regarded as a constant at 850 °C,C, 950 °C,C, an andd 1050 °CC since the volume fraction of MnS having nearlynearly the samesame valuevalue (5.42 (5.42 wt.%), wt.%), which which was was calculated calculated by by JMatpro JMatpro (V7.0, (V7.0 Sente, Sen Software,te Software Boston,, Boston UK)., UKAccording). According to the model, to the themodel, decrease the decreasein the aspect in the ratio aspect of MnS ratio causes of the MnS shear caus bands’es the formation shear bands to be’ formationmore difficult to be at more the same difficult volume at the fraction same ofvolume MnS ( ffraction), thereby of resultingMnS (f), thereby in a higher resulting elongation in a higher during elongationthe tensile during process. the Table tensile2 showed process. that Table the 2 tensileshowed strength that the presentstensile strength a downward presents trend a downward with the trendincrease with of austenitethe increase grain of size.austenite Similar grain experimental size. Simil resultsar experimental have been reportedresults have in other been high-strength reported in othersteels high [23–-25strength]. It suggested steels [23 that–25]. the It suggested tensile strength that the gradually tensile strength decreases gradually due to adecrease coarsers austenite due to a coarsergrain with austenite the austenitising grain with temperaturethe austenitising increasing. temperature increasing.

FigureFigure 8 8.. MnSMnS inclusions inclusions at different at different austenitizing austenitizing temperature temperaturess of (a) 850 of °C, (a ()b 850) 950◦ °C,C, and (b) ( 950c) 1050◦C, °C.and The (c) 1050inset ◦inC. the The upper inset right in the represents upper right the represents composition the compositionof MnS. of MnS.

InIn addition, addition, the the tensile tensile strength strength is is also also affected by the size of bainitic ferrite. The The size size of of the the austeniteaustenite grain grain has has a asigniﬁcant significant impact impact on theon length, the length, width, width and, thickness and thickness of bainite of ferritebainite by ferrite affecting by affectingthe density the of density nucleation of nucleation sites and growth sites and rate. growth It has beenrate. provedIt has been that theproved thickness that ofthe bainitic thickness ferrite of bainiticin the coarse ferrite austenite in the coarse grain austenite is larger grain than thatis larger of ﬁner than ones that [of26 ].finer In lowones temperature [26]. In low bainitictemperature steel, bainiticthe tensile steel, strength the tensile is mainly strength determined is mainly by the determined thickness ofby the the bainitic thickness ferrite. of the The bainitic measured ferrite. thickness The ◦ ◦ measuredof bainitic thickness ferrite (45 of± bainitic5 nm, 50 ferrite± 5 nm, (45 ± and 5 nm, 70 ±5015 ± 5 nm nm for, and the 70 specimens ± 15 nm for treated the specimens at 850 C, treated 950 C, ◦ ◦ atand 850 1050 °C, 950C, respectively)°C, and 1050 showed°C, respectively) that the specimenshowed that treated the specimen at 1050 C treated had the at largest1050 °C thickness. had the ◦ largestTherefore, thickness it had.lower Therefore, tensile it strengthhad lower at highertensile austenitisingstrength at higher temperatures austenitising(1050 temperatureC) due to itss coarse(1050 °C)bainitic due to ferrite its coarse plates. bainitic ferrite plates. TheThe tensile tensile elongation elongation depends depends on on the the content content of of retained retained austenite, austenite, which which is is a a softer phase comparedcompared with with the the bainitic bainitic ferrite ferrite and martensite [2,5] [2,5].. The The XRD XRD test test was was performed performed to to estimate estimate the the volume percentage of retained austenite and its carbon concentration concentration.. It is is suggested suggested that that the the retained retained austeniteaustenite percentage percentage of of the the matrix matrix can can be be expected expected to to increase increase with with an increase in the austenitising ◦ ◦ tempertemperatureature (Table (Table 33).). AsAs thethe austenitisingaustenitising temperaturetemperature increasedincreased fromfrom 850850 °CC to 950 °C,C, the the volume volume percentage of retained austenite increased from 12.07% to 16.23%, while there was only a little increase between 950 °C and 1050 °C. In low temperature bainitic steel, the content of retained austenite is a significant factor for controlling elongation. Thus, the sample has poor elongation at 850 °C compared with that at 950 °C and 1050 °C, which was caused by the lesser content of retained austenite (12.07%).

Table 3. The volume percentage of retained austenite and its carbon concentration at different austenising temperatures.

The Carbon Content The Carbon Content in Austenitising The Volume Fraction of in Retained Austenite Retained Austenite by Temperature (°C) Retained Austenite (%) by XRD (wt.%) EPMA (wt.%)

Metals 2018, 8, 931 8 of 11 percentage of retained austenite increased from 12.07% to 16.23%, while there was only a little increase between 950 ◦C and 1050 ◦C. In low temperature bainitic steel, the content of retained austenite is a signiﬁcant factor for controlling elongation. Thus, the sample has poor elongation at 850 ◦C compared with that at 950 ◦C and 1050 ◦C, which was caused by the lesser content of retained austenite (12.07%).

Table 3. The volume percentage of retained austenite and its carbon concentration at different austenising temperatures.

The Carbon Content in The Carbon Content in Austenitising The Volume Fraction of Retained Austenite by Retained Austenite by Temperature (◦C) Retained Austenite (%) XRD (wt.%) EPMA (wt.%) 850 12.07 1.510 1.553 ± 0.085 950 16.23 1.258 1.275 ± 0.070 1050 16.68 1.247 1.250 ± 0.090

Further enhancement in the elongation can be achieved when the retained austenite transforms to martensite because of the TRIP effect. To make the TRIP effect fully effective, a suitable stability of retained austenite is necessary. The chemical composition is one of the most critical aspects influencing the retained austenite stability, especially the carbon concentration [27]. There are two conditions that are disadvantageous for improving the elongation of bainitic steel. The first is that the retained austenite is too stable to cause a TRIP effect because of the high carbon concentration. Secondly, a low carbon content causes low stability of the retained austenite so that the TRIP effect occurs under lower stress. Thus, it is meaningful to measure the carbon concentration in retained austenite to identify stability (Table3). The carbon content at 850 ◦C is 1.553 ± 0.085% and the corresponding elongation is only 1.0 ± 0.5%, while the carbon content is 1.275 ± 0.070% at 950 ◦C and 1.250 ± 0.090% at 1050 ◦C, which correspond to the elongation rates of 10.7 ± 2.0% and 11.2 ± 1.5%, respectively. Owing to the higher carbon concentration in retained austenite at 850 ◦C, the transformation to martensite also cannot occur at the same stress compared with that at 950 ◦C and 1050 ◦C. This result was also proved by changing the retained austenite content at the fracture necking. As shown in Figure7a, the change of retained austenite content at 850 ◦C is significantly less than that at 950 ◦C and 1050 ◦C. The retained austenite content of 850 ◦C is 8.70% and 12.07% at the site a and the matrix, respectively. It means that only 3.37% of the retained austenite transforms into martensite at site a. The difference of the retained austenite content between the fracture and the matrix at 950 ◦C and 1050 ◦C indicated that a large volume percentage of retained austenite has almost entirely transformed into martensite during the tensile test. The content of the retained austenite, which has transformed into martensite (site a), reached 11.71% and 12.62% at 950 ◦C and 1050 ◦C, respectively. Compared with 850 ◦C, the TRIP effect of 950 ◦C and 1050 ◦C is stronger. Therefore, the quantity of retained austenite that caused the TRIP effect is also determined by its stability. To further explore the plastic deformation at different necking sites, the local tensile elongation was also investigated with the rule of constant volume in plastic deformation (Figure9). It can be 2 2 expressed as: L% = Rs /RN − 1 × 100%. Rs and RN represent the radius of the sample before the tensile test and the radius of the necking site after the test, respectively. The results showed that the local tensile elongation decreases with the distance away from the fracture. It also demonstrated that it has a negative correlation with the content of retained austenite at the same necking sites. Metals 2018, 8, x FOR PEER REVIEW 8 of 11

850 12.07 1.510 1.553 ± 0.085 950 16.23 1.258 1.275 ± 0.070 1050 16.68 1.247 1.250 ± 0.090

Further enhancement in the elongation can be achieved when the retained austenite transforms to martensite because of the TRIP effect. To make the TRIP effect fully effective, a suitable stability of retained austenite is necessary. The chemical composition is one of the most critical aspects influencing the retained austenite stability, especially the carbon concentration [27]. There are two conditions that are disadvantageous for improving the elongation of bainitic steel. The first is that the retained austenite is too stable to cause a TRIP effect because of the high carbon concentration. Secondly, a low carbon content causes low stability of the retained austenite so that the TRIP effect occurs under lower stress. Thus, it is meaningful to measure the carbon concentration in retained austenite to identify stability (Table 3). The carbon content at 850 °C is 1.553 ± 0.085% and the corresponding elongation is only 1.0 ± 0.5%, while the carbon content is 1.275 ± 0.070% at 950 °C and 1.250 ± 0.090% at 1050 °C, which correspond to the elongation rates of 10.7 ± 2.0% and 11.2 ± 1.5%, respectively. Owing to the higher carbon concentration in retained austenite at 850 °C, the transformation to martensite also cannot occur at the same stress compared with that at 950 °C and 1050 °C. This result was also proved by changing the retained austenite content at the fracture necking. As shown in Figure 7a, the change of retained austenite content at 850 °C is significantly less than that at 950 °C and 1050 °C. The retained austenite content of 850 °C is 8.70% and 12.07% at the site a and the matrix, respectively. It means that only 3.37% of the retained austenite transforms into martensite at site a. The difference of the retained austenite content between the fracture and the matrix at 950 °C and 1050 °C indicated that a large volume percentage of retained austenite has almost entirely transformed into martensite during the tensile test. The content of the retained austenite, which has transformed into martensite (site a), reached 11.71% and 12.62% at 950 °C and 1050 °C, respectively. Compared with 850 °C, the TRIP effect of 950 °C and 1050 °C is stronger. Therefore, the quantity of retained austenite that caused the TRIP effect is also determined by its stability. To further explore the plastic deformation at different necking sites, the local tensile elongation was also investigated with the rule of constant volume in plastic deformation (Figure 9). It can be 2 2 expressed as: LRR% s / N  1  100%. Rs and RN represent the radius of the sample before the tensile test and the radius of the necking site after the test, respectively. The results showed that the local tensile elongation decreases with the distance away from the fracture. It also demonstrated that it has Metalsa negative2018, 8, 931correlation with the content of retained austenite at the same necking sites. 9 of 11

FigureFigure 9. 9The. The local local tensile tensile elongation elongation of of ﬁve five necking necking sites sites was was calculated calculated with with the the rule rule of of constant constant volumevolume in in plastic plastic deformation. deformation.

ExceptExcept for for carbon carbon concentration concentration in in retainedretained austenite,austenite, the retained austenite austenite stability stability can can also also be bedemonstrated demonstrated by by its its martensite martensite transformation temperature, Ms.s. It It is is more more difﬁcult difficult to to transform transform retainedretained austenite austenite into into martensite martensite if if the theM Ms iss is too too low. low. As As the the alloying alloying elements elements except except carbon carbon were were homogeneouslyhomogeneously distributed distributed during during isothermal isothermal treatment, treatment, the theM Mss of of the the retained retained austenite austenite can can be be calculatedcalculated by by the the following following equation equation [28 [2,298,2]:9]:

Ms (◦C) = 539 − 423C − 30.4Mn − 7.5Si + 30Al (wt.%) (3)

According to Equation (3) and the experimental result of EPMA (Table4), the Ms temperatures of the retained austenite were −197 ◦C, −79 ◦C, and −69 ◦C for the samples treated at 850, 950, and 1050 ◦C, respectively. Therefore, the retained austenite at the 850 ◦C specimen is more stable than that of the 950 ◦C and 1050 ◦C specimens. Excessively stable retained austenite does not induce martensite transformation during the tensile process, and thus does not contribute to elongation.

Table 4. The content of C, Mn, Si, and Al in retained austenite (wt.%) and its Ms at different austenising temperatures.

Austenitising Temperature (◦C) C Mn Si Al Ms (◦C) 850 1.553 ± 0.085 2.182 ± 0.055 1.657 ± 0.050 - −197 950 1.275 ± 0.070 2.186 ± 0.080 1.662 ± 0.090 - −79 1050 1.250 ± 0.090 2.190 ± 0.075 1.660 ± 0.065 - −69

In this work, it has been proved that the elongation is a combined effect of retained austenite content and its carbon concentration. In the meantime, the Ms of the retained austenite also inﬂuences its stability and thus plays a role on the elongation.

5. Conclusions Tensile tests were performed on low temperature bainitic steel treated at different austenitising temperatures. The characteristics of the microstructures and the factors causing the difference in tensile properties were also discussed. Conclusions are drawn as follows: (1) In the necking area, the retained austenite content increased with the distance away from the fracture at three different austenitising temperatures. At 850 ◦C, the content of retained austenite at the fracture is signiﬁcantly higher than that at 950 ◦C and 1050 ◦C, while the content of it transformed into martensite is lower during the tensile test. (2) The elongation is a combination function of the volume percentage of retained austenite and its carbon concentration. The TRIP effect makes a great contribution in improving the elongation at 950 ◦C and 1050 ◦C, owing to a higher volume percentage of retained austenite and its appropriate carbon concentration. Metals 2018, 8, 931 10 of 11

(3) When the austenitising temperature is 850 ◦C, the retained austenite has an extremely low Ms temperature. This proved that the too stable retained austenite cannot play a positive effect on the elongation.

Author Contributions: Conceptualization, B.D., T.H. and K.W.; Methodology, B.D.; Software, B.D. and G.Z.; Validation, K.W., T.H. and B.D.; Formal Analysis, B.D. and W.Z.; Investigation, B.D. and W.Z.; Resources, K.W.; Data Curation, B.D.; Writing—Original Draft Preparation, B.D.; Writing—Review & Editing, T.H., K.W. and G.Z.; Visualization, B.D.; Supervision, K.W.; Project Administration, K.W.; Funding Acquisition, K.W. Funding: The authors sincerely thank the support from the Major Technology Innovation of Hubei Province (No. 2016AAA022), the National Natural Science Foundation of China (No. U1532268), and the Hubei Provincial Natural Science Foundation of Innovation Team (No. 2016CFA004). Conﬂicts of Interest: The authors declare no conﬂict of interest.

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