The microstructural evolution for varied cooling strategies is investigated in a cold rolled bainitic steel. Special attention is given to the effect of slow cooling on bainitic transformation kinetics as well as the microstructure and properties. The results indicate that the introduction of a slow cooling stage resulted in the formation of a small amount of ferrite, which slowed the subsequent bainitic transformation. With the decreasing of austempering temperature, bainitic transformation kinetics slows down. As the austempering temperature decreased to below Ms, the initial rate of bainite transformation is accelerated. Whereas, the introduction of pre-formed martensite could only accelerate the bainitic transformation kinetics in the early stage rather than the overall stage. The incorporation of the slow cooling stage significantly improved ductility due to the strain accommodation between soft and hard phase as well as the increased fraction of RA, yielding a higher combination of strength and ductility.
ToloueiRTitheridgeH. Vehicle mass as a determinant of fuel consumption and secondary safety performance. Trans Res Part D: Trans Env2009; 14: 385–399.
2.
BroughtonJ. The British index for comparing the accident record of car models. Accid Anal Prev 1996; 28: 101.
3.
TimokhinaIBBeladiHXiongXY, et al. Nanoscale microstructural characterization of a nanobainitic steel. Acta Mater 2011; 59: 5511–5522.
4.
CaballeroFGMillerMKBabuSS,et al. Atomic scale observations of bainite transformation in a high carbon high silicon steel. Acta Mater 2007; 55: 381–390.
5.
HuHJXuGWangL, et al. The effects of Nb and Mo addition on transformation and properties in low carbon bainitic steels. Mater Des2015; 84: 95–99.
6.
QianLHLiZWangTL, et al.Roles of pre-formed martensite in below-ms bainite formation, microstructure, strain partitioning and impact absorption energies of low-carbon bainitic steel. J Mater Sci Technol2022; 96: 69–84.
7.
LiuXLuCLuoH, et al. The effect of solution and precipitation of Nb and V on the continuous cooling transformation of undercooling austenite in high-strength 20MnSi steel. Mater Today Commun2024; 40: 110046.
8.
VettersHDongJBornasH, et al. Microstructure and fatigue strength of the roller-bearing steel 100Cr6 (SAE 52100) after two-step bainitisation and combined bainitic-martensitic heat treatment. Int J Mater Res2006; 97: 1432–1440.
9.
GongWTomotaYHarjoS, et al.Effect of prior martensite on bainite transformation in nanobainite steel. Acta Mater2015; 85: 243–249.
10.
SugimotoKItohMHojoT, et al.Microstructure and mechanical properties of ausformed ultra high-strength TRIP-aided steels. Mater Sci Forum2007; 539–543: 4309–4314.
11.
KobayashiJInaDYoshikawaN,et al.Effects of the addition of cr, mo and Ni on the microstructure and retained austenite characteristics of 0.2% C^|^ndash;si^|^ndash;mn^|^ndash;nb ultrahigh-strength TRIP-aided bainitic ferrite steels. ISIJ Int2012; 52: 1894–1901.
12.
CongQLiJZhouL,et al.Simultaneous improvement of strength and uniform elongation of ferrite-bainite steel with heterogeneous networked microstructure. Mater Lett2024; 370: 136815.
13.
LiuPGuanLLiuJ, et al. Effect of annealing process on structure and properties of a low cost 980 MPa cold-rolled dual-phase steel (in Chinese). Heat Treat Metals2024; 49: 122–127.
14.
YangYChuXLuH, et al. Effect of continuous annealing process parameters on microstructure and properties of 1180 MPa grade Nb microalloyed dual phase steel with high formability (in Chinease). China Metall 2024; 34: 72–80.
15.
HouXLiuWWangJ, et al. Microstructure control and enhancement mechanism of strength-plasticity for ultra-high strength complex phase steel (in Chinese). China Metall 2024; 34: 61–71.
16.
LiYZhangYLongX, et al.Effect of undercooled austenite cooling rate on the low cycle fatigue properties of an austempering bainitic steel. Int J Fatigue2025; 193: 108809.
17.
LongXLiuWZhuR, et al.Effect of the cooling rate in the medium temperature zone on the phase transformation and microstructure of carbide-free bainitic steel. J Mater Res Technol2024; 29: 50–66.
18.
KickingerCSuppanCHebesbergerT, et al.Microstructure and mechanical properties of partially ferritic Q&P steels. Mater Sci Eng A2021; 815: 141296.
19.
RochaROMeloTMFPerelomaEV,et al.Microstructural evolution at the initial stages of continuous annealing of cold rolled dual-phase steel. Mater Sci Eng A2005; 391: 296–304.
QuidortDBrechetY. Isothermal growth kinetics of bainite in 0.5% C steels. Acta Mater2001; 49: 4161–4170.
22.
ZhuKChenHMasseJ, et al.The effect of prior ferrite formation on bainite and martensite transformation kinetics in advanced high-strength steels. Acta Mater2013; 61: 6025–6036.
23.
LuzginovaNZhaoLSietsmaJ. Evolution and thermal stability of retained austenite in SAE 52100 bainitic steel. Mater Sci Eng A2007; 448: 104–110.
24.
Jimenez-MeleroEvan DijkNHZhaoL, et al.Characterization of individual retained austenite grains and their stability in low-alloyed TRIP steels. Acta Mater2007; 55: 6713–6723.
25.
KungCYRaymentJJ. An examination of the validity of existing empirical formulae for the calculation of ms temperature. Metall Trans A1982; 13: 328–331.
26.
WaterschootTVerbekenKCoomanBCD. Tempering kinetics of the martensitic phase in DP steel. ISIJ Int2006; 46: 138–146.
27.
LeivaJAVMoralesEVVillar-CociñaE. Kinetic parameters during the tempering of low-alloy steel through the non-isothermal dilatometry. J Mater Sci2010; 45: 418–428.
28.
PashangehSSomaniMCGhasemi BanadkoukiSS, et al.On the decomposition of austenite in a high-silicon medium-carbon steel during quenching and isothermal holding above and below the ms temperature. Mater Charact2020; 162: 110224.
29.
RaviAMNavarro-LópezASietsmaJ,et al.Influence of martensite/austenite interfaces on bainite formation in low-alloy steels below M. Acta Mater2020; 188: 394–405.
30.
Navarro-LopezASietsmaJSantofimiaMJ. Metall Mater Trans A2016; 47: 1028.
31.
TianJXuGZhouM,et al.Refined bainite microstructure and mechanical properties of a high-strength low-carbon bainitic steel treated by austempering below and above MS. Steel Res Int2018; 89: 1700469.
32.
KawataHHayashiKSugiuraN, et al.Effect of martensite in initial structure on bainite transformation. Mater Sci Forum2010; 638–642: 3307–3312.
33.
BhadeshiaHKDH. Bainite in Steels: Transformation, microstructure and properties. 3rd ed. London: The Institute of Materials2001, p.136.
34.
PashangehSGhasemiSSBSomaniM,et al.Characteristics and kinetics of bainite transformation behaviour in a high-silicon Medium-carbon steel above and below the ms temperature. Materials (Basel)2022; 15: 539.
35.
ChenSWangCShanL, et al.Effect of isothermal time and alloy elements on bainitic transformation below ms in medium mn steels. IOP Conf Ser: Mater Sci Eng2019; 592: 12018.
36.
ChenHZhangCZhuJ, et al. Austenite/ferrite interface migration and alloying elements partitioning: an overview. Acta Metall2018; 54: 217–227.
RaviAMKumarAHerbigM, et al.Impact of austenite grain boundaries and ferrite nucleation on bainite formation in steels. Acta Mater2020; 188: 424–434.
39.
ChenXRolfeBAbdollahpoorA, et al.Effect of pro-eutectoid ferrite on subsequent bainite transformation kinetics simulation. Mater Sci Technol2019; 35: 429–436.
40.
TimokhinaIBMillerMKBeladiH,et al.The influence of fine ferrite formation on the γ/α interface, fine bainite and retained austenite in a thermomechanically-processed transformation induced plasticity steel. J Mater Res Technol2016; 31: 806–818.
41.
WangTQianLYuW, et al.Effect of ferrite-austenite morphology and orientation relationship on bainite transformation in low-alloy TRIP steels. Mater Charact2022; 184: 111656.
42.
GuoHGaoXBaiY, et al.Variant selection of bainite on the surface of allotriomorphic ferrite in a low carbon steel. Mater Charact2012; 67: 34–40.
43.
LiuMXuGTianJ, et al.The effect of primary ferrite on bainitic transformation, microstructure, and properties of low carbon bainitic steel. Met Sci Heat Treat2020; 62: 306.
44.
LongXZhangYSunD, et al.Study on the influence of Pre-formed phase on accelerating bainitic transformation. Coatings2023; 13: 1700.
45.
GonzagaRA. Influence of ferrite and pearlite content on mechanical properties of ductile cast irons. Mater Sci Eng A2013; 567: 1–8.
46.
TanXPongeDLuW, et al.Carbon and strain partitioning in a quenched and partitioned steel containing ferrite. Acta Mater2019; 165: 561–576.
47.
KangTZhaoZLiangJ, et al.Effect of the austenitizing temperature on the microstructure evolution and mechanical properties of Q&P steel. Mater Sci Eng A2020; 771: 138584.
48.
LeeCKimSLeeT,et al.Effects of volume fraction and stability of retained austenite on formability in a 0.1C–1.5Si–1.5Mn–0.5Cu TRIP-aided cold-rolled steel sheet. Mater Sci Eng A2004; 371: 16–23.
