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Effect of volume fraction and mechanical stability of austenite on ductility of medium Mn steel

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Journal of Iron and Steel Research International 2019年第11期26卷 1209-1218页

作者： Si-lian Che Zhao-xi cao Chang Wang Chong-xiang Huang Dirk Ponge wen-quan cao Central Iron and Steel Research Institute(CISRI) Beijing 100081China Science Faculty University of SydneySydney NSW 2006Australia School of Aeronautics and Astronautics Sichuan UniversityChengdu 610065SichuanChina Max-Planck-Institute for Iron Research Duesseldorf 40237Germany

A hot-rolled medium Mn(0.2C5Mn)steel is annealed at 650℃ to produce an ultrafine-grained duplex microstructure with different austenite volume fractions by austenite reverted transformation(ART)annealing,and the orientation relationship strictly obeys K-S orientation relationship before *** tests are carried out in a temperature range from-196 to 400℃ to examine the effects of the austenite volume fraction and the deformation temperature on the tensile properties and the austenite *** observations reveal that the metastable austenite gradually transformed into a-martensite,which is controlled by the deformation strain,the temperature and the austenite volume *** strain hardening behavior and ductility of the studied steel are dependent on austenite volume fraction and deformation temperature *** stress-strain curves of ART-annealed 0.2C5Mn steel assume an S shape and a very large work hardening rate of about 10 GPa is obtained at liquid nitrogen deformation *** on the experimental data,a quantitative relation is proposed to describe the ductility dependence on both the austenite volume fraction and its mechanical stability.

关键词： TRIP effect Ductility Phase transformation Austenite volume fraction Mechanical stability

Intercritical Rolling Induced Ultrafine Lamellar Structure and Enhanced Mechanical Properties of Medium-Mn Steel

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Journal of Iron and Steel Research International 2014年第5期21卷 551-558页

作者： Wei ZHAN Li-qiang cao Jun HU wen-quan cao Jian LI Han DONG College of Materials Science and Engineering Huazhong University of Science and Technology Institute for Structural Materials Central Iron and Steel Research Institute(CISRI)

The medium-Mn steel with ferrite and austenite structure was rolled in the intercritical region down to dif- ferent rolling reduction. The microstructure and mechanical properties of the rolled steels were investigated by scan- ning electron microscopy, transmission electron microscopy, X-ray diffraction and tensile tests. It was found that the ferrite and austenite structure gradually evolved into an ultrafine structure from the random directional lath structure to lamellar structure with lath longitudinal direction parallel to the rolling direction with increasing rolling strain. It was found that the thickness of the laths was gradually refined with increasing rolling strain. The lath thickness is about 0. 15 9m stored with high density dislocations and the austenite volume fraction of the steel is about 24% after 80% rolling reduction. Furthermore, it was interesting to find that yield strength, tensile strength and total elongation of the 80% rolled medium-Mn steel are about 1000 MPa, 1250 MPa and 24%, respectively, demonstrating an excellent combination of the strength and ductility. Based on the microstructure examination, it was proposed that the grain refinement of the medium-Mn steels could be attributed to the duplex structure and the low rolling temperature. Analysis of the relationship between the microstructure and the mechanical properties indicated that the high yield strength mainly resulted from the ultrafine grain size and the high density dislocation, but the improved ductili- ty may be attributed to the large fractions of austenite retained after intercritical rolling.

关键词： intercriticaI roiling medium manganese steel rolling reduction ultrafine lameUar structure

Kinetics of austenite grain growth in medium-carbon niobium-bearing steel

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Journal of Zhejiang University-Science A(Applied Physics & Engineering) 2011年第3期12卷 171-176页

作者： Ying-li ZHAO Jie SHI wen-quan cao Mao-qiu WANG Gang XIE School of Material and Metallurgy Engineering Kunming University of Science and Technology Kunming 650093 China Institute for Structural Materials Central Iron and Steel Research Institute Beijing 100081 China

In order to locate a reasonable heating system,the austenite grain growth behavior of Nb microalloyed medium carbon steel has been experimentally studied at various austenitizing temperatures and for different holding *** is indicated that austenite grain growth increases with increasing austenitizing temperatures and holding *** when the austenitizing temperature was above 1100 ℃,austenite grains grew rapidly,and an abnormal austenite grain growth was *** the austenitizing temperature was lower than 1100 ℃,austenite grain size and growth rate were *** activation energy of grain growth in the tested steel is 397 679.5 J/*** ensure an absence of coarse grains in microstructures,the heating technology of the tested steel should be controlled for 1 h at 1100 ℃.The relationships of austenite average grain size with soaking temperature and time of tested steel were obtained by mathematical calculation,and austenite average grain size was found to be in agreement with the measured size for different holding times.

关键词： Microalloyed steel Grain growth Modeling

Effect of direct quenching on microstructure and mechanical properties of medium-carbon Nb-bearing steel

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Journal of Zhejiang University-Science A(Applied Physics & Engineering) 2010年第10期11卷 776-781页

作者： Ying-li ZHAO Jie SHI wen-quan cao Mao-qiu WANG Gang XIE School of Material and Metallurgy Engineering Kunming U~iversity of Scienee and Technology Kunming 650093 China Institute for Structural Materials Central Iron and Steel Research Institute Beijing 100081 China

The influence of direct quenching (DQ) on microstructure and mechanical properties of 0.19C-1.7Si-1.0 Mn-0.05Nb steel was studied. The microstructure and mechanical properties of reheat quenched and tempered (RQ&T) steel plate were compared with those of direct quenched and tempered (DQ&T) steel plates which were hot rolled at different finish rolling tem-peratures (1173 K and 1123 K), i.e., recrystallization-controlled-rolled direct-quenched (RCR&DQ) and controlled-rolled direct-quenched (CR&DQ), respectively. The strengths generally increased in the following order: RQ&T

关键词： Direct quenching (DQ) Block Mechanical properties Electron backscattering diffraction (EBSD) Nb steel

Microstructure, hardness and contact fatigue properties of X30N high nitrogen stainless bearing steel

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Journal of Iron and Steel Research International 2018年第9期25卷 954-967页

作者： Hai-feng Xu Gui-lin Wu Chang Wang Jian Li wen-quan cao Special Steel Institute Central Iron and Steel Research Institute Beijing 100081 China College of Materials Science and Engineering Chongqing University Chongqing 400045 China Central Laboratory Central Iron and Steel Research InstituteBeijing 100081 China Center for Fuel Cell Innovation State Key Laboratory of Material Processing and Die & Mould Technology School of Materials Science and Engineering Huazhong University of Science and Technology Wuhan 430074 Hubei China

Microstructure, hardness and fatigue properties of X30N high nitrogen stainless bearing steel were investigated. It was found that nitrogen addition could effectively reduce the amount and size of coarse carbides. The original austenite grain size was obviously refined. Additionally, more retained austenite was found in X30N steel after quenching at 1050 ℃, which could be reduced from about 30% to about 6% by cold treatment at - 73 ℃ and subsequent tempering, and thus, the ultimate hardness was increased up to about 61 HRC with reduction of austenite and precipitation of carbonitrides. Furthermore, the rolling contact fatigue lives of X30N steel ate superior to those of 440C steel, which was attributed to the enhanced hardness and a certain retained austenite in the high nitrogen steel.

关键词： Nitrogen addition Grain refining High hardness Ultrafine precipitate Contact fatigue life High nitrogen stainless bearing steel

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

Comparison of microstructure and property of high chromium bearing steel with and without nitrogen addition

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Journal of Iron and Steel Research International 2017年第2期24卷 206-213页

作者： Hai-feng Xu Feng Yu Chang Wang Wan-li Zhang Jian Li wen-quan cao Center for Fuel Cell Innovation State Key Laboratory of Material Processing and Die & Mould TechnologySchool of Materials Science and EngineeringHuazhong University of Science and Technology Special Steel Institute Central Iron and Steel Research Institute

Microstructure and property of bearing steel with and without nitrogen addition were investigated by microstructural observation and hardness measurement after different heat treatment processing. Based on the microstructural observation of both 9Cr18 steel and X90N steel, it was found that nitrogen addition could effectively reduce the amount and size of coarse carbides and also refine the original austenite grain size. Due to addition of nitrogen, more austenite phase was found in X90N steel than in 9Cr18 steel. The retained austenite of X90N steel after quenching at 1050℃ could be reduced from about 60% to about 7 9% by cold treatment at -73℃ and subsequent tempering, and thus finally increased the hardness up to 60 HRC after low temperature tempering and to 63 HRC after high temperature tempering. Furthermore, both the wear and corrosion resistance of X90N steel were found much more superior than those of 9Cr18 steel, which was attributed to the addition of nitrogen. It was proposed at last that nitrogen alloying into the high chromium bearing steel was a promising way not only to refine the size of both carbides and austenite, but also to achieve high hardness, high wear property and improved corrosion resistance of the stainless bearing steel.

关键词： Nitrogen addition High temperature tempering High hardness Austenite grain size Carbide Wear resistance High chromium bearing steel

High Ductility and Toughness of a Micro-duplex Medium-Mn Steel in a Large Temperature Range from—196 ℃ to 200 ℃

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Journal of Iron and Steel Research International 2015年第12期22卷 1126-1130页

作者： Si-lian CHEN Jun HU Xiao-dan ZHANG Han DONG wen-quan cao Central Iron and Steel Research Institute School of Materials Science and Engineering Huazhong University of Science and Technology Danish-Chinese Centre for Nanometals Section for Materials Science and Advanced CharacterizationDepartment of Wind EnergyTechnical University of Denmark

A medium-Mn steel （0.2C5Mn） was processed by intercritical annealing at different temperatures （625 ℃ and 650 ℃ ）. An ultrafine-grained micro-duplex structure consisting of alternating austenite and ferrite laths was de- veloped by austenite reverse transformation （ART） during intercritical annealing after forging and hot rolling. Ultra- high ductility with a total elongation higher than 30% was achieved in the temperature range from -196 ℃ to 200 ℃, and high impact toughness no less than 200 J at -40 ℃ was obtained. Based on the analysis of microstructure and mechanical properties, it was found that the enhanced ductility was determined by the phase transformation effect of austenite （TRIP effect）, while the delayed ductile to brittle transition was controlled by austenite stability.

关键词： high strength high ductility intercritical annealing medium-Mn steel ultrafine grain size

Ultrafine Grained Duplex Structure Developed by ART-annealing in Cold Rolled Medium-Mn Steels

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Journal of Iron and Steel Research International 2014年第2期21卷 208-214页

作者： Jie SHI Jun HU Chang WANG Cun-yu WANG Han DONG wen-quan cao National Engineering Research Center of Advanced Steel Technology(NERCAST) Central Iron and Steel Research Institute(CISRI) College of Materials Science and Engineering Huazhong University of Science and Technology

The microstructural evolutions of the cold rolled Fe-0.1C-5Mn steel during intercritical annealing were ex- amined using combined advanced techniques. It was demonstrated that intercritical annealing results in an ultrafine granular ferrite and austenite duplex structure in cold rolled 0.1C-5Mn steel. The strong partitioning of manganese and carbon elements from ferrite to austenite was found during intercritical annealing by scanning transmission elec- tron microscopy （STEM） and X-ray diffraction （XRD）. Strong effects of boundary characters on the austenite for- mation were indicated by austenite fast nucleation and growth in the high angle boundaries but sluggish nucleation and growth in the low angle boundaries. The ultrafine grained duplex structure in 0.1C-5Mn was resulted from the the sluggish Mn-diffusion and the extra high Gibbs free energy of ferrite phase. Based on the analysis of the micro- structure evolution, it was pointed out that the intercritical annealing of the medium Mn steels could be applied to fabricate an ultrafine duplex grained microstructure, which would be a promising approach to develop the 3rd genera- tion austomobile steels with excellent combination of strength and ductility.

关键词： intercritical annealing partitioning segregation ultrafine grained duplex structure boundary character Gibbs free energy