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Morphologies of secondary austenite in 2507 duplex stainless steel after heat treatment

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Journal of Iron and Steel Research(International) 2022年第6期29卷 994-1003页

作者： Xiao-han Wu Zhi-gang Song Bao-shun Wang Ming-hua Wu Qi-liang Nai Liang Yao Han Feng Wen-jie Zheng School of Material Science and Engineering University of Science and Technology BeijingBeijing 100083China Special Steel Institute Central Iron&Steel Research InstituteBeijing 100081China Engineering Research Center of High-Performance Nuclear Power Pipe Forming of Zhejiang Province Huzhou 313028ZhejiangChina Yongxing Special Stainless-Steel Co. Ltd.Huzhou 313005ZhejiangChina Zhejiang Jiuli Hi-Tech Metals Co. Ltd.Huzhou 313028ZhejiangChina

The microstructure evolution of secondary austenite in 2507 duplex stainless steel was investigated by means of optical metallography,scanning electron microscopy and confocal scanning laser microscopy.Four types of secondary austenite(γ2)morphologies including partially transformed austenite,grain boundary austenite,Widmannstätten austenite(WA)and intragranular austenite could be formed during cooling after solution treatment.It was concluded that secondary austenite morphology was mainly dependent on cooling rates.Two mechanisms of WA formation were proposed.WA nucleated at grain boundary of ferrite or at the formed phase boundary.Transmission electron microscopy(TEM)analysis displayed that WA grew in parallel to the ferrite.The effects of Cr_(2)N on secondary austenite precipitation were discussed in detail.Once Cr_(2)N was involved,a mechanism was proposed that secondary austenite formed by element diffusion,leading to the migration of the austenite–ferrite interface.Cr_(2)N acted as the nucleation sites forγ2 and provided the nitrogen for the transformation;meanwhile,the precipitation of Cr_(2)N during the rapid cooling was captured by TEM.A process of nucleation followed by diffusion was concluded to be the formation of secondary austenite with no Cr_(2)N precipitated.The decomposition of secondary austenite was also studied,which was found to be a diffusion mechanism followed by displacement in connection with the element distribution.

关键词： Secondary austenite Duplex stainless steel Heat treatment Cooling rate Element diffusion

austenite transformation and work hardening of medium manganese steel

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Journal of Iron and Steel Research(International) 2018年第12期25卷 1265-1269页

作者： Lei Zhang Cun-yu Wang Heng-chang Lu Wen-quan Cao Chang Wang Han Dong Leng Chen School of Materials Science and Engineering University of Science and Technology BeijingBeijing 100083China Special Steel Institute of Central Iron and Steel Research Institute Beijing 100081China Mechanical Engineering Department of Tangshan Polytechnic College Tangshan 063299HebeiChina

The austenite transformation behavior and mechanical properties of medium manganese steel subjected to tensile strain were studied by electron backscatter diffraction,X-ray diffraction and tensile tests.The results show that the austenite phases are mainly distributed on the grain boundary in the duplex microstructure of austenite and ferrite,and it is easy for the big-size austenite to transform at small beginning tension strain following the mechanisms of both austenite (fcc)→ε- martensite (hcp)→α-martensite (bcc)and austenite (fcc)→α-martensite (bcc).Both yield strength and tensile strength increase with the increase in pre-strain,and the total elongation decreases,while the value of pre-strain plus total elongation almost keeps constant.During tensile deformation,transformation from austenite into martensite improves work-hardening rate remarkably.

关键词： Medium manganese steel austenite Martensite Microstructure Mechanical property

Effect of grain size on the intrinsic mechanical stability of austenite in transformation-induced plasticity steels:The competition between martensite transformation and dislocation slip

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Journal of Materials Science & Technology 2023年第31期162卷 38-43页

作者： D.P.Yang T.Wang Z.T.Miao P.J.Du G.D.Wang H.L.Yi State Key Laboratory of Rolling and Automation Northeastern UniversityShenyang 110819China

The deformation accompanied by austenite to martensite trans-formation was first studied in Fe-Ni alloys,high-Mn steels,and austenitic stainless steels with a full austenite microstructure.The plasticity of these steels can be improved by the trans-formation during deformation and hence they were named as transformation-induced plasticity(TRIP)steels[1].Over the past few decades,a series of TRIP steels with multiphase microstructure for automotive applications were developed.These steels contain-ing a certain amount of austenite were called TRIP-assisted steels,such as low-alloy TRIP steels[2],carbide-free bainitic steels[3],δ-TRIP steels[4],quenching and partitioning steels[5],and medium-Mn steels[6].The mechanical property of TRIP steels is largely determined by the mechanical stability of austenite[7]which de-pends on the chemical composition[8],grain size[9],orientation[10],and morphology[11]of austenite grains,as well as the nature of neighboring phases[12].In most cases of TRIP-assisted steels,larger austenite grains transform to martensite earlier or faster than smaller austenite grains with the increase of macrostrain[13-18],indicating lower mechanical stability.However,larger austen-ite grains usually contain less C and Mn[13,17-19]and suffer more strain than smaller grains during deformation[20],it doesn’t mean that grain refinement increases the mechanical stability of austen-ite.

关键词： austenite martensite transformation

A review of the thermal stability of metastable austenite in steels:Martensite formation

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Journal of Materials Science & Technology 2021年第32期91卷 200-214页

作者： Yong Li David San Martín Jinliang Wang Chenchong Wang Wei Xu State key laboratory of rolling and automation Northeastern UniversityShenyang 110819China Materalia Research Group Centro Nacional de Investigaciones Metalúrgicas(CENIM-CSIC)Avda Gregorio del Amo 8Madrid E-28040Spain School of Mechanical and Power Engineering Guangdong Ocean UniversityZhanjiang 524088China

Metastable austenite plays a critical role in achieving improved combinations of high strength and high ductility/toughness in the design of advanced high-strength steels(AHSS). The thermal stability of metastable austenite determines the transformation characteristics of AHSS and thus primarily determines the microstructure evolution during complex processes, e.g., the quenching and partitioning process, to achieve the desirable microstructure. This study provides a review of the thermal stability of austenite and its influence on martensitic transformation from both experimental and theoretical modeling perspectives. From the experimental perspective, factors affecting the thermal stability are analyzed,the relative sensitivities are compared, and their corresponding mechanisms are discussed. From the theoretical modeling perspective, the most representative kinetic models that describe athermal and isothermal martensitic transformation are reviewed. The advantages, shortcomings, and applicability of each model are discussed. The systematic review of both experimental and theoretical aspects reveals critical factors in tailoring the stability of metastable austenite and, therefore, provides guidance for the design of advanced steels.

关键词： austenite Thermal stability Martensitic transformation Kinetic models

austenite Grain Refinement and Isothermal Growth Behavior in a Low Carbon Vanadium Microalloyed Steel

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Journal of Iron and Steel Research(International) 2014年第8期21卷 757-764页

作者： Geng-wei YANG Xin-jun SUN Qi-long YONG Zhao-dong LI Xiao-xian LI Key Laboratory for Ferrous Metallurgy and Resources Utilization of Ministry of Education Wuhan University of Science and Technology Department of Structural Steel Central Iron and Steel Research Institute Department of Materials Science and Engineering Kunming University of Science and Technology

The austenite grain refinement through control of the grain growth during reheating process after thermomechanical controlled process （TMCP） in a vanadium microalloyed steel was achieved. The formation of ultra-fine grained austenite was attributed to the high density of austenite nucleation at the ferrite/martensite structure and to the inhibition of austenite growth by （Ti~ V）C particles at the relatively low reheating temperature. Corresponding with the precipitation behavior of （Ti,V）C with temperature, the growth behavior of austenite in the vanadium mi- croalloyed steel could be divided into two regions. At lower reheating temperature, austenite grains grew slowly, and ultra-fine grained austenite smaller than 5 ~m was successfully obtained. By contrast, the austenite grains grew rap- idly at high temperature due to the dissolution of （Ti, V）C particles. According to the measured and predicted results of austenite growth kinetics, two models were developed to describe the growth behavior of austenite grains in two different temperature regions, and the apparent activation energy Qapp for grain growth was estimated to be about 115 and 195 kJ/mol, respectively.

关键词： grain refinement austenization （Ti, V） C precipitationl grain growth austenite

austenite grain growth behavior of Q1030 high strength welded steel

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International Journal of Minerals,Metallurgy and Materials 2012年第8期19卷 711-716页

作者： Rong-ting Xiao Hao Yu Ping Zhou School of Materials Science and Engineering University of Science and Technology Beijing Beijing 100083 China Laiwu Iron & Steel Group Co. Ltd. Laiwu 271104 China

The austenite grain growth behavior of Q1030 steel was studied under different heating conditions. The austenite grain size increases with the heating temperature and holding time increasing. austenite grains grow in an exponential manner with rising heating tem- perature and in a parabolic manner with prolonging holding time. A mathematical model for describing the austenite grain growth behavior of Q 1030 steel was obtained on the basis of experimental results using regression analysis. When the heating temperatures lie between 1000 and 1100℃ at a certain holding time, abnormal grain growth appears, which causes mixed grains in Q1030 steel.

关键词： high strength steel heat treatment austenite grain size grain growth mathematical models

Kinetic transitions and Mn partitioning during austenite growth from a mixture of partitioned cementite and ferrite:Role of heating rate

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Journal of Materials Science & Technology 2020年第14期47卷 70-80页

作者： Geng Liu Zongbiao Dai Zhigang Yang Chi Zhang Jun Li Hao Chen Key Laboratory for Advanced Materials of Ministry of Education School of Materials Science and EngineeringTsinghua UniversityBeijing100084China Research Institute of Baoshan Iron and Steel Co. LtdShanghai201900China

austenite formation from a ferrite-cementite mixture is a crucial step during the processing of advanced high strength steels(AHSS).The ferrite-cementite mixture is usually inhomogeneous in both structure and composition,which makes the mechanism of austenite formation very complex.In this contribution,austenite formation upon continuous heating from a designed spheroidized cementite structure in a model Fe-C-Mn alloy was investigated with an emphasis on the role of heating rate in kinetic transitions and element partitioning during austenite formation.Based on partition/non-partition local equilibrium(PLE/NPLE)assumption,austenite growth was found alternately contribute by PLE,NPLE and PLE controlled interfaces migration during slow-heating,while NPLE mode predominately controlled the austenitization by a synchronous dissolution of ferrite and cementite upon fast-heating.It was both experimentally and theoretically found that there is a long-distance diffusion of Mn within austenite of the slow-heated sample,while a sharp Mn gradient was retained within austenite of the fast-heated sample.Such a strong heterogeneous distribution of Mn within austenite cause a large difference in driving force for ferrite or martensite formation during subsequent cooling process,which could lead to various final microstructures.The current study indicates that fast-heating could lead to unique microstructures which could hardly be obtained via the conventional annealing process.

关键词： Cementite austenite Kinetics Elements partitioning Fast heating

Analysis of retained austenite in TRIP590 steel by electron backscattered diffraction method

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Journal of Iron and Steel Research(International) 2020年第3期27卷 355-365页

作者： Yu-cheng Zhang Yang Meng Gui-bin Cui Hui-ping Jia Xin-hua Ju Zhong-hang Jiang Ze-jun Ma Bruker(Beijing)Scientific Technology Co. Ltd.Beijing 100192.China Research Institute of Technology Shougang Group Co.Ltd.Beijing 100043China

The retained austenite content in TRIP590 steel was first measured by X-ray diffraction(XRD)method.Then,the effect of different test parameters(indexing rate,step size,field number,etc.)on the measured results of retained austenite content in TRIP590 steel by electron backscattered diffraction(EBSD)method was studied.By comparing the measured results between XRD and EBSD method,the optimal test parameters for the analysis of retained austenite content using EBSD were determined.Finally,the total content of retained austenite and the retained austenite contents with different aspect ratios in TRIP590 steel were analyzed,respectively.The results showed that the test parameters have a great influence on the measured results of retained austenite content in TRIP590 steel.In order to improve the test accuracy,high indexing rates,small step sizes and more field number are necessary for the analysis of retained austenite content in TRIP590 steel by EBSD method.

关键词： TRIP590 steel Retained austenite Electron backscattered diffraction Indexing rate Step size

Prediction model for austenite grains growth during reheating process in Ti micro-alloyed cast steel by coupling precipitates dissolution and coarsening behavior

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Journal of Iron and Steel Research(International) 2019年第2期26卷 162-172页

作者： Tao Liu Mu-jun Long Wen-jie He Deng-fu Chen Zhi-hua Dong Xian-guang Zhang Hua-mei Duan State Key Laboratory of Coal Mine Disaster Dynamics and Control College of Materials Science and EngineeringChongqing UniversityChongqing 400030China Chongqing Key Laboratory of Vanadium-Titanium Metallurgy and New Materials Chongqing UniversityChongqing 400030China Applied Materials Physics.Department of Materials Science and Engineering KTH-Royal Institute of TechnologyStockholm 10044Sweden Institute for Materials Research Tohoku UniversitySendai 980-8577Japan

A combined model to predict austenite grains growth of titanium micro-alloyed as-cast steel during reheating process was established.The model invoIves the behaviors of austenite grains growth in continuous heating process and isothermal soaking process,and the variation of boundary pinning efficiency caused by the dissolution and coarsening kinetics of sec on d-phase particles was also con sidered into the model.Furthermore,the experimental verificatio ns were performed to examine the prediction power of the model.The results revealed that the mean austenite grains size increased with the increase in reheating temperature and soaking time,and the coarsening temperature of austenite grains growth was 1423 K under the current titanium content.In addition,the reliability of the predicted results in continuous heating process was validated by continuous heating experimenls.Moreover,an optimal regression expression of austenite grains growth in isothermal soaking process was obtained based on the experimental results.The compared results indicated that the combined model in conjunction with precipitates dissolution and coarsening kinetics had good reliability and accuracy to predict the austenite grains growth of titanium micro-alloyed casting steel during reheating process.

关键词： austenite grains growth Reheating process Precipitate dissolution Precipitate coarsening Prediction model