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Effect of Retained austenite on the fracture behavior of a novel press-hardened steel

Journal of Materials Science & Technology 2023年第4期135卷 34-45页

作者： Zhisong Chai Qi Lu Jun Hu Lingyu Wang Zhou Wang Jianfeng Wang Wei Xu State Key Laboratory of Rolling and Automation Northeastern UniversityShenyang 110819China China Science Laboratory General Motors Global Research&DevelopmentShanghai 201206China

Tensile and bending properties are two critical attributes of press hardened steels(PHSs)for automotive body structural components.However,the research on these properties of PHSs with Retained austenite(RA),which is introduced to improve the mechanical properties,has not been well reported.In this study,the effect of RA on the quasi-static uniaxial tensile and three-point bending behaviors has been systematically investigated for a newly developed Cr and Si alloyed PHS.RA with various degrees of mechanical stabilities was obtained by tuning the auto-tempering of martensite through the control of the die contact pressure during the press hardening process.Mechanically stable RA provides the optimal combination of tensile and bending performance,while the PHS with RA of poor mechanical stability has a deteriorated bending toughness as manifested by a reduction of bending angle from approximately 61.6°to56.8°.However,itstensileproperties,incontrast,arethebestintermsof theproductofulti-mate tensile strength and total elongation(15.9 GPa%).This is mainly attributed to the unstable RA on the outermost surface of the bending sample.The unstable RA can easily transform into brittle martensite under local plane strain and strain gradient conditions in the bending test compared with uniaxial stress in the tensile test,promoting crack initiation and propagation.Furthermore,the effect of martensitic auto-tempering or contact pressure on the quantity and stability of RA are also discussed.This study would provide a useful reference to guide hot stamping process optimization for the newly developed Cr and Si-containing press-hardened steel.

关键词： Press-hardened steel Retained austenite Tensile property Bending toughness Crack initiation

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Improving hydrogen embrittlement resistance of a modified press hardening steel by introducing Retained austenite as hydrogen trap

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Journal of Iron and Steel Research(International) 2022年第11期29卷 1864-1872页

作者： Wei Ding Kai Yang Yu Gong Li Wang Ji-yao Hong Wei Li Xue-jun Jin Shanghai Key Laboratory of Materials Laser Processing and Modification Shanghai Jiao Tong UniversityShanghai 200240China Institute of Advanced Steels and Materials School of Materials Science and EngineeringShanghai Jiao Tong UniversityShanghai 200240China State Key Laboratory of Development and Application Technology of Automotive Steels BaoSteel Research InstituteShanghai 201900China

The effect of Retained austenite(RA)with higher mechanical stability on hydrogen embrittlement resistance of a modified 22MnB5(C 0.22,Si 0.8,Mn 1.5,B 0.002,Fe balance,in wt.%)press hardening steel(PHS)has been studied.One-step quenching and partitioning(Q&P)treatment was applied to PHS,and around 6 vol.%ultra-fine RA was obtained.The ultra-fine RA was found to act as stronger hydrogen trap than dislocations and grain boundaries in martensitic matrix and can decrease the apparent diffusion coefficient of hydrogen from 5.97×10^(-7) to 3.83×10^(-7) cm^(2) s^(-1),which was verified by the combination of thermal desorption spectroscopy analysis and hydrogen permeation test.The higher mechanical stability of the ultrafine RA assures enough stability of the hydrogen trap,which results in better hydrogen embrittlement resistance in Q&P-treated PHS than the conventional directly quenched PHS.

关键词： Hydrogen embrittlement Press hardening steel Quenching and partitioning Retained austenite Thermal desorption spectroscopy Hydrogen permeation

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Effect of Retained austenite on Corrosion Behavior of Ultrafine Bainitic Steel in Marine Environment

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Acta Metallurgica Sinica(English Letters) 2023年第5期36卷 717-731页

作者： Xian Zhang Li Gong Yanpeng Feng Zhihui Wang Miao Yang Lin Cheng Jing Liu Kaiming Wu Hubei Province Key Laboratory of Systems Science in Metallurgical Process The State Key Laboratory of Refractories and MetallurgyCollaborative Innovation Center for Advanced SteelsWuhan University of Science and TechnologyWuhan430081China School of Chemistry Chemical Engineering and Life SciencesWuhan University of TechnologyWuhan430070China

In this study,the impact of Retained austenite on corrosion initiation,propagation,and resistance of ultrafine bainitic steels in marine environments based on the one-step and two-step bainitic isothermal transformation was investigated using a combination of theoretical calculations and experimental methods.According to the results,the microstructure of ultrafine bainitic steels was composed of parallel arranged bainite ferrite(BF)and Retained austenite(RA).The fraction of the block RA was significantly reduced,whereas the amount of the film RA increased through the two-step bainitic transformation.In the initial stage of corrosion,micro-galvanic effects occurred between the multiphases due to the difference in nobility,leading to the selective dissolution of BF sheaves,which is adjacent to the block RA in the one-step bainitic steel.However,uniformly distributed film RA in the two-step bainitic steel acted as a barrier against corrosion propagation.The electrochemical measurements and neutral salt spray tests revealed a relatively lower corrosion rate for the two-step bainitic steel,indicating a higher corrosion resistance than the one-step bainitic steel.The corrosion products layer mainly consisted ofα-FeOOH,γ-FeOOH,and Fe_(3)O_(4),which were stable and favorable for the formation of a protective rust layer.

关键词： Ultrafine bainitic steel Retained austenite Marine environment Corrosion behavior First principle modeling

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Effect of Indirect Transformation of Retained austenite During Tempering on the Charpy Impact Toughness of a Low-Alloy Cr-Mo-V Steel

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Acta Metallurgica Sinica(English Letters) 2020年第10期33卷 1346-1358页

作者： Yong-Han Li Zhong-Hua Jiang Zhen-Dan Yang Jue-Shun Zhu Shenyang National Laboratory for Materials Science Institute of Metal ResearchChinese Academy of Sciences72 Wenhua RoadShenyang 110016China

A modified tempering treatment has been designed in order to avoid the direct transformation of Retained austenite(Ar)during tempering of a low-alloy Cr-Mo-V steel.Instead of the direct transformation of Ar into ferrite and M23C6 carbides during conventional tempering at 700℃,transformation into aggregate of ferrite and cementite has been forced by a pre-tempering at 455℃ before conventional tempering.Experiments have been performed on specimens quenched with cooling rates 1.5,3 and 12℃/s,providing different types of Ar within the as-quenched microstructures.The results show that the tempering modification does not improve the Charpy impact toughness at the highest quenching rate of 12℃/s,where the specimens cannot incur cleavage cracking induced from fine and discontinuous M23C6 carbides along lath interfaces.For the lowest quenching rate 1.5℃/s,the Charpy impact toughness can be improved,and the failure is dominated by carbide aggregates,which originate from the decomposed products of blocky Ar.This is because the tempering modification effectively suppresses the formation of coarse M23C6 carbides at interfaces between the carbide aggregate and bainitic matrix,thereby resulting in a relatively homogeneous distribution of M23C6 carbides inside carbide aggregates.Therefore,the tempering modification is recommended for large-scale forgings,in which relatively high quenching rates are difficult to achieve.

关键词： Bainite steel Retained austenite Impact toughness

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Effect of lower bainite/martensite/Retained austenite triplex microstructure on the mechanical properties of a low-carbon steel with quenching and partitioning process

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International Journal of Minerals,Metallurgy and Materials 2016年第3期23卷 303-313页

作者： Wan-song Li Hong-ye Gao Zhong-yi Li Hideharu Nakashima Satoshi Hata Wen-huai Tian School of Materials Science and Engineering University of Science and Technology Beijing Interdisciplinary Graduate School of Engineering Sciences Kyushu University

We present a study concerning Fe-0. 176C-1.31Si-1.58Mn-0.26Al-0.3Cr （wt%） steel subjected to a quenching and partitioning （Q＆P） process. The results of scanning electron microscopy, transmission electron microscopy, X-ray diffraction, and tensile tests demon- strate that the microstructures primarily consist of lath martensite, Retained austenite, lower bainite （LB）, and a small amount of tempered martensite; moreover, few twin austenite grains were observed. In the microstrucmre, three types of Retained austenite with different sizes and morphologies were observed： blocky Retained austenite （-300 nm in width）, film-like Retained austenite （80-120 nm in width）, and ul- tra-fine film-like Retained austenite （30-40 nm in width）. Because of the effect of the Retained austenite/martensite/LB triplex microstructure, the specimens prepared using different quenching temperatures exhibit high ultimate tensile strength and yield strength. Furthermore, the strength effect of LB can partially counteract the decreasing strength effect of martensite. The formation of LB substantially reduces the amount of Retained austenite. Analyses of the Retained austenite and the amount of blocky Retained austenite indicated that the carbon content is critical to the total elongation of Q＆P steel.

关键词： low-carbon steel quenching partitioning Retained austenite rnicrostructure mechanical properties

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Work Hardening Behavior and Stability of Retained austenite for Quenched and Partitioned Steels

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Journal of Iron and Steel Research(International) 2016年第2期23卷 130-137页

作者： Cun-yu WANG Ying CHANG Jie YANG Wen quan CAO Han DONG Yi-de WANG Central Iron and Steel Research Institute Beijing 100081China Taiyuan Iron and Steel Group Co. Ltd.Taiyuan 030003ShanxiChina National Key Laboratory of Industrial Equipment Structural Analysis School of Automotive EngineeringDalian University of TechnologyDalian 116024LiaoningChina Society of Automotive Engineers of China Beijing 100055China

Both microstrueture and mechanical properties of low alloy steels treated by quenching and partitioning （Q＆P） process were examined. The mixed microstructure of martensite and large-fractioned Retained austenite （about 27.3%） was characterized and analyzed, excellent combinations of total elongation of 19% and tensile strength of 1 835 MPa were obtained, and three-stage work hardening behavior was demonstrated during tensile test. The en hanced mechanical properties and work hardening behavior were explained based on the transformation induced plas ticity effect of large fractioned austenite.

关键词： Q＆P process work hardening Retained austenite martensite mechanical property TRIP effect

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Effects of Sub-zero Celsius Treatment and Tempering on the Stability of Retained austenite in Bearing Steel

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Acta Metallurgica Sinica(English Letters) 2015年第6期28卷 787-792页

作者： Xiao-Hui Lu Wei Li Cheng-Lin Wang Hong-Shan Zhao Xue-Jun Jin State Key Lab of Metal Matrix Composite School of Materials Science and Engineering Shanghai Jiao Tong University Institute of Advanced Steels and Materials School of Materials Science and Engineering Shanghai Jiao Tong University

In this work, the influence of sub-zero Celsius treatment and tempering on the mechanical and thermal stability of Retained austenite in beating steel were assessed by tensile test and DSC. Compared with traditional quenched and tempered treatment, sub-zero Celsius treatment obviously decreases the volume fraction of Retained austenite. Moreover, the mechanical stability of Retained austenite was enhanced due to the accumulation of compressive stresses in Retained austenite after sub-zero Celsius treatment and tempering. Meanwhile, the morphology of Retained austenite changed from film-like to blocky with austenitization temperature increasing, and the mechanical stability of film-like Retained austenite is higher than that of blocky one. The DSC results showed that the activation energy of Retained austenite decomposition slightly increased through sub-zero Celsius treatment and tempering. This result may probably be ascribed to partitioning of carbon during tempering. However, the temperature at which Retained austenite starts to decompose is unchanged.

关键词： Bearing steel Sub-zero Celsius treatment Retained austenite Stability

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TRIP Steel Retained austenite Transformation under Cyclic V-bending Deformation

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Journal of Wuhan University of Technology(Materials Science) 2014年第3期29卷 594-600页

作者：李洪洋 ZHANG Yanjing ZHAO Wenjun GU Zhifei LI Xianchao MA Jinlong School of Material Science and Engineering Beijing Institute of Technology National Key Laboratory of Science and Technology on Materials under Shock and Impact Inner Mongolia First Machinery Group Co. Ltd

To investigate the transformation behavior of TRIP steel Retained austenite under cyclic load, cyclic V-bending deformation of low carbon Si-Mn TRIP600 was studied by experiment and finite element in this paper. The results showed that, under cyclic V-bending deformation, Retained austenite in TRIP steel transformed into martensite gradually with the increasing of bending times, and for the symmetrical characteristic, upper surface and lower surface presented the same transformation tendency. From the first to the fourth V-bending deformatiort, Retained austenite volume fraction decreased nearly linearly and then attained saturation step by step. Compressive stress state was helpful for martensite transformation than tension stress state with V-bending deformation, and strain magnitude was the determining factor for retaining anstenite martensitic transformation. With the increasing of bending times effective stress increased and the relationship between maximum effective stress and bending times was nearly linear. Effective stress and effective strain distribution were non-uniform, the maximum effective stress and effective strain were present in the center of the samples. The relationships between Retained austenite and V-bending times, and Retained austenite with effective strain were set up as Eqs.（1）-（5）. The relationship was typical quadric function, decreased linearly for the initial deformation and attained saturation finally.

关键词： TRIP steel martensitic transformation Retained austenite cyclic load

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Effects of Si on the stability of Retained austenite and temper embrittlement of ultrahigh strength steels

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International Journal of Minerals,Metallurgy and Materials 2011年第5期18卷 543-550页

作者： Li-jun Wang Qing-wu Cai Hui-bin Wu Wei Yu National Engineering Research Center for Advanced Rolling Technology University of Science and Technology Beijing Beijing 100083 China

Effects of silicon （Si） content on the stability of Retained austenite and temper embrittlement of ultrahigh strength steels were investigated using X-ray diffraction （XRD）,transmission electron microscopy （TEM）,and other experimental methods.The results show that Si can suppress temper embrittlement,improve temper resistance,and hinder the decomposition of Retained austenite.Reversed austenite appears gradually with the increase of Si content during tempering.Si has a significant effect on enhancing carbon （C） partitioning and improving the stability of Retained austenite.Si and C atoms are mutually exclusive in lath bainite,while they attract each other in austenite.ε-carbides are found in 1.8wt% Si steel tempered at 250℃,and they get coarsened obviously when tempered at 400℃,leading to temper embrittlement.Not ε-carbides but acicular or lath carbides lead to temper embrittlement in 0.4wt% Si steel,which can be inferred as cementites and composite compounds.Temper embrittlement is closely related to the decomposition of Retained austenite and the formation of reversed austenite.

关键词： silicon ultrahigh strength steel Retained austenite temper embrittlement reversed austenite

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Mechanical Properties and Retained austenite Transformation Mechanism of TRIP-Aided Polygonal Ferrite Matrix Seamless Steel Tube

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Journal of Iron and Steel Research(International) 2011年第8期18卷 73-78页

作者： ZHANG Ming-ya ZHU Fu-xian ZHENG Dong-sheng State Key Laboratory of Rolling and Automation Northeastern University Shenyang 110819 Liaoning China

Through the comparison of microstructure for polygonal ferrite （PF） matrix transformation induced plasticity （TRIP） seamless steel tube at different positions before and after tensile rupture, the transformation behavior of Retained austenite （RA） was studied. The results showed that there were no yield points in tensile process and the splendid elongation and tensile strength were contributed by the uniform ferrite/bainite grains and the transformation of RA. The stability of RA was to some extent in inverse proportion with the ability of transformation induced plas ticity. The coarse Retained austenite located in ferrite and ferrite/bainite laths were all transformed into martensite during the tensile process.

关键词： TRIP steel Retained austenite austempering heat treatment

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