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Precipitation Strengthening by Nanometer-sized Carbides in Hot-rolled Ferritic Steels

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Journal of Iron and Steel Research International 2014年第12期21卷 1140-1146页

作者： Xiao-pei WANG Ai-min zhao zheng-zhi zhao Yao HUANG zhi-da GENG Yang YU Sheet Metal Research Institute Shougang Research Institute of Technology Metallurgical Engineering Research Institute University of Science and Technology Beijing

The mechanical properties of the hot-rolled plates of Ti steel and Ti-Mo steel after isothermal transformation in a temperature range of 600 700 ℃ for 60 min have been tested, and the microstructures of the matrix and the characteristics of precipitated nanometer-sized carbides have also been examined by scanning electron microscopy and transmission electron microscopy. The precipitation regularity of nanometer-sized carbides has been studied by thermodynamic method and the contributions of corresponding strengthening mechanisms to the total yield strength have been calculated. The tensile strength of hot-rolled Ti-Mo ferritie steel can achieve 780 MPa with an elongation of 20.0% after being isothermally treated at 600 ℃ for 60 rain, and the tensile strength of Ti steel is 605 MPa with an elongation of 22.7%, according to the results of tensile tests. The critical nucleation size of （Ti,Mo）C is smaller than that of TiC at a given isothermal temperature, but the nucleation rate of （Ti, Mo）C is larger than that of TiC. The grainrefinement strengthening and precipitation strengthening contribute the main amount of the total yield strength. The major increase in yield strength with the decrease of isothermal temperature results from the contribution of precipi tation strengthening. The contribution of precipitation strengthening to the yield strength of the steels has been esti mated. The ferrite phase can be strengthened by about 400 MPa through precipitation strengthening in Ti-Mo steel isothermally treated at 600 ℃ for 60 rain, which is about 200 MPa higher than that of Ti steel under the same conditions.

关键词： ferritie steel nanometer sized carbide thermodynamic method precipitation strengthening

Mechanical properties and characteristics of nanometer-sized precipitates in hot-rolled low-carbon ferritic steel

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International Journal of Minerals,Metallurgy and Materials 2014年第3期21卷 266-272页

作者： Xiao-pei Wang Ai-min zhao zheng-zhi zhao Yao Huang Liang Li Qing He Engineering Research Institute University of Science and Technology Beijing

The microstructures and properties of hot-rolled low-carbon ferritic steel have been investigated by optical microscopy, field-emission scanning electron microscopy, transmission electron microscopy, and tensile tests after isothermal transformation from 600°C to 700°C for 60 min. It is found that the strength of the steel decreases with the increment of isothermal temperature, whereas the hole expansion ratio and the fraction of high-angle grain boundaries increase. A large amount of nanometer-sized carbides were homogeneously distributed throughout the material, and fine（Ti, Mo）C precipitates have a significant precipitation strengthening effect on the ferrite phase because of their high density. The nanometer-sized carbides have a lattice parameter of 0.411-0.431 nm. After isothermal transformation at 650°C for 60 min, the ferrite phase can be strengthened above 300 MPa by precipitation strengthening according to the Ashby-Orowan mechanism.

关键词： ferritic steel nanoparticles mechanical properties carbides precipitation strengthening

Effect of microstructure evolution on Luders strain and tensile properties in an intercritical annealing medium-Mn steel

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Journal of Iron and Steel Research International 2021年第6期28卷 762-772页

作者： Shuai Wang Wei-jian Chen zheng-zhi zhao Xiao-long zhao Xiao-yang Luo Qiang Wang Collaborative Innovation Center of Steel Technology University of Science and Technology BeijingBeijing 100083China Beijing Laboratory for Modern Transportation Advanced Metal Materials and Processing Technology University of Science and Technology BeijingBeijing 100083China Carbon Steel and Thin Slab Factory of Hongxing Iron and Steel Co. Ltd.Jiuquan Iron and Steel GroupJiayuguan 735100GansuChina

The influence of microstructural characteristics on Lu¨ders strain and mechanical properties was explored by means of altering thermo-mechanical circumstances in an intercritical annealing(IA)medium-Mn Fe-11Mn-0.09C-0.25Si(wt.%)*** IA of cold-rolled samples with severe plastic deformation,exclusively equiaxed dual phases were obtained because of active recovery and *** equiaxed austenite(gamma E)with a larger size and inadequate chemical concentration was more readily transformed into martensite,and subsequent transformation-induced plasticity(TRIP)effect was triggered actively at relatively higher IA temperature,lessening localized *** addition,grown-in dislocations were prone to multiply and migrate around a broad mean free path for coarser equiaxed ferrite(alpha E)due to weakening dynamic recovery;therefore,it was the ensuing increased mobility of dislocations instead of reserving plentiful initial dislocation density that facilitated the propagation velocity of Luders bands and the accumulation of work *** contrast,the bimodal-grained microstructure with lath-like and equiaxed austenite(gamma L+gamma E)satisfactorily contributed to a smaller yield point elongation(YPE)without compromise of comprehensive mechanical properties on the grounds that austenitic gradient stability gave rise to discontinuous but sustainable TRIP effect and incremental work ***,Luders strain is closely related to the absence of work hardening in the region which yields *** follows that the decreased stability of retained austenite,favorable mobility of dislocations and the bimodal-grained structure all prominently make up for the insufficiency of work hardening,thereof resulting in a limited YPE.

关键词： Luders strain Medium-Mn steel Work hardening Dislocation Mechanical property

Effect of vanadium on hydrogen embrittlement susceptibility of high-strength hot-stamped steel

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Journal of Iron and Steel Research International 2021年第2期28卷 211-222页

作者： Wei-jian Chen Peng-fei Gao Shuai Wang Hong-zhou Lu zheng-zhi zhao Collaborative Innovation Center of Steel Technology University of Science and Technology BeijingBeijing 100083China Beijing Laboratory for Modern Transportation Advanced Metal Materials and Processing Technology University of Science and Technology BeijingBeijing 100083China CITIC Metal Co. Ltd.Beijing 100004China

Based on the chemical composition of traditional hot-stamped steel(e.g.,22MnB5 and 30MnB5),Nb and V microalloying elements are added into 30MnB5 steel to meet the requirements of ultra-high strength,excellent ductility and potent resistance to hydrogen embrittlement(HE)at the same *** influence of hot-stamped steel on HE was studied by conducting a hydrogen permeation method and pre-charged hydrogen slow strain rate ***,the experimental steel microstructures and corresponding fracture surfaces are observed and analyzed to characterize HE *** results show that a finer microstructure,a lower apparent diffusion coefficient of hydrogen and a smaller percentage of strength and plasticity reduction are obtained due to the addition of the vanadium element into hot-stamped *** to the V free experimental steel,the steel with 0.14 wt.% V has a large number of dispersive precipitates and more grain boundary areas,which makes hydrogen atoms dispersedly distribute.

关键词： Hot-stamped steel Vanadium Hydrogen embrittlement Hydrogen permeation test Slow strain rate test Microstructure

Prediction and evaluation of optimum quenching temperature and microstructure in a 1300 MPa ultra-high-strength Q&P steel

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Journal of Iron and Steel Research International 2022年第2期29卷 307-315页

作者： Peng-fei Gao Ju-hua Liang Wei-jian Chen Feng Li zheng-zhi zhao Collaborative Innovation Center of Steel Technology University of Science and Technology BeijingBeijing100083China Beijing Laboratory of Metallic Materials and Processing for Modern Transportation Beijing100083China Key Laboratory of Materials Physics Institute of Solid State PhysicsChinese Academy of SciencesHefei230031AnhuiChina

The quenching and partitioning steel is the representative of the third generation of advanced high-strength *** effect of quenching temperature on the microstructure and mechanical property of ferrite-containing quenching and partitioning steel was studied by intercritical annealing quenching and partitioning *** preparing a test steel with a tensile strength of 1300 MPa and total elongation of 19%,it is found that the actual optimum quenching temperature was lower than that calculated according to the constrained carbon *** results indicate that the martensite start temperature of the austenite was overestimated when considering the diffusion of carbon *** grain size which is affected by low temperature and the existence of ferrite during intercritical annealing influenced the optimum quenching temperature.A scheme considering the diffusion of various alloying elements and austenite grain size was proposed and *** this scheme,the optimum quenching temperature of intercritically annealed quenching and partitioning steel with complex microstructures was well predicted.

关键词： Quenching and partitioning Constrained carbon equilibrium Retained austenite Martensite start temperature Plasticity Quenching temperature

Effect of Cr and isothermal holding temperature on microstructure and properties of complex phase steel with high formability(CH steel)

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Journal of Iron and Steel Research International 2023年第2期30卷 328-337页

作者： Xiao-hong Chu Peng-fei Gao Wei-jian Chen Feng Li Tao Kang Yan zhao Xian-dong Yin zheng-zhi zhao Collaborative Innovation Center of Steel Technology University of Science and Technology BeijingBeijing100083China Beijing Laboratory for Modern Transportation Advanced Metal Materials and Processing Technology University of Science and Technology BeijingBeijing100083China Technical Center Shougang Jingtang United Iron and Steel Co.Ltd.Tangshan063200HebeiChina

The effects of Cr contents(0.3 and 1.0 wt.%)and isothermal holding temperatures(400,440,and 480℃)on the microstructure evolution and properties of complex phase steel with high formability(CH steel)were investigated using dilatometry,scanning electron microscopy,transmission electron microscopy(TEM),and X-ray *** results show that the microstructures of CH steel with 0.3 wt.%Cr are ferrite,granular bainite,martensite,and retained austenite,while no ferrite is observed in the microstructure of CH steel with 1.0 wt.%Cr in the same *** promotes the precipitation of(Nb,Ti)C in the high-temperature austenite region through theoretical calculations and TEM *** retards the bainite transformation and refines the grain size of CH ***,as isothermal holding temperature increases from 400 to 480℃,the bainite and retained austenite fractions of two CH steels decrease,while the martensite fraction increases in the steels after final ***,the strength has an increasing tendency and the total elongation has a decreasing tendency with increasing isothermal temperature.

关键词： Complex phase steel High formability steel CH steel Cr Isothermal holding temperature Microstructure Mechanical property

Effect of Annealing Process on Microstructure and Mechanical Property of 1100 MPa Grade TRIP Steel

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Journal of Iron and Steel Research International 2015年第7期22卷 622-629页

作者： Hong-xiang YIN zheng-zhi zhao Ai-min zhao Xiao LI Han-jiang HU Jiang-tao LIANG Metallurgical Engineering Research Institute University of Science and Technology Beijing Beijing Laboratory of Metallic Materials and Processing for Modern Transportation

To develop super-high strength TRIP steel for automobile application with good combination of strength and plasticity, an ultra high-strength TRIP steel 0.2 %C-2.0% Si-1.8% Mn was investigated at different intercritical annealing temperatures and bainitic isothermal temperatures. Microstructures were observed by scanning electron mi- croscopy, transmission electron microscopy, electron backscatter diffraction, and X-ray diffraction, and mechanical properties were tested under a uniaxial tensile testing machine. It shows that the best comprehensive mechanical properties were obtained for the experimental steel particularly annealed at 820 ℃ and austempered at 410 ℃. Yield strength of the tested steel mainly depends on ferrite, while the high strength of the tested steel is related to mar- tensite which forms through blocky austenite transformation during cooling and subsequent tensile test attributed to the TRIP effect. Hence, it could be concluded that the grain size of the retained austenite and carbon content in re- tained austenite are the main reasons for high elongation of the tested steel.

关键词： TRIP steel retained austenite intercritical annealing bainitic isothermal temperature automobile steel

Influence of original microstructure on the transformation behavior and mechanical properties of ultra-high-strength TRIP-aided steel

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International Journal of Minerals,Metallurgy and Materials 2015年第3期22卷 262-271页

作者： Hong-xiang Yin Ai-min zhao zheng-zhi zhao Xiao Li Shuang-jiao Li Han-jiang Hu Wei-guang Xia Metallurgical Engineering Research Institute University of Science and Technology Beijing Beijing Laboratory of Metallic Materials and Processing for Modern Transportation Key Laboratory for Mechanics in Fluid Solid Coupling Systems Institute of Mechanics Chinese Academy of Sciences

The transformation behavior and tensile properties of an ultra-high-strength transformation-induced plasticity （TRIP） steel （***） were investigated by different heat treatments for automobile applications. The results show that F-TRIP steel, a tradi- tional TRIP steel containing as-cold-rolled ferfite and pearlite as the original microstructure, consists of equiaxed grains of intercritical ferrite surrounded by discrete particles of M/RA and B. In contrast, M-TRIP steel, a modified TRiP-aided steel with martensite as the original mi- crostlucture, containing full martensite as the original microstructure is comprised of lath-shaped grains of ferrite separated by lath-shaped martensite/retained austenite and bainite. Most of the austenite in F-TRIP steel is granular, while the austenite in M-TRIP steel is lath-shaped. The volume fraction of the retained austenite as well as its carbon content is lower in F-TRIP steel than in M-TRIP steel, and austenite grains in M-TRIP steel are much finer than those in F-TRIP steel. Therefore, M-TRIP steel was concluded to have a higher austenite stability, re- sulting in a lower transformation rate and consequently contributing to a higher elongation compared to F-TRIP steel. Work hardening be- havior is also discussed for both types of steel.

关键词： high strength steels transformation-induced plasticity phase transformations mechanical properties original microstructure work hardening

Microstructure evolution and mechanical properties influenced by austenitizing temperature in aluminum-alloyed TRIP-aided steel

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Journal of Iron and Steel Research International 2017年第11期24卷 1115-1124页

作者： Ju-hua Liang zheng-zhi zhao Cai-hua Zhang Di Tang Shu-feng Yang Wei-ning Liu Collaborative Innovation Center of Steel Technology University of Science and Technology BeijingBeijing 100083China Technology Center HBIS Handan Iron and Steel Group Co.Ltd.Handan 056015HebeiChina School of Metallurgical and Ecological Engineering University of Science and Technology BeijingBeijing 100083China AC Construction Branch State Grid Corporation of ChinaBeijing 100031China

The Fe-0.21C 2.2Mn 0.49Si-1.77A1 transformation induced plasticity （TRIP） aided steel was heat trea- ted at various austenitizing temperatures under both TRiP-aided polygonal ferrite type （TPF） and an- nealed martensite matrix （TAM） processes. The microstructure evolution and their effects on mechanical properties were systematically investigated through the microstructure observation and dilatometric analysis. The microstructure homogeneity is improved in TPF steel heated at a high temperature due to the reduced banded martensite and the increased bainite. Compared with the mechanical properties of the TPF steels, the yield strength and elongation of the TAM steels are much higher, while the tensile strength is lower than that of TPF steels. The stability of intercritical austenite is affected by the heating tempera- ture, and thus the following phase transformation influences the mechanical properties, such as the bain- ite transformation and the precipitation of polygonal ferrite. Obvious dynamic bainite transformation occurs at TAM850, TAM900 and TAM950, More proportion of polygonal ferrite is found in the sample heated at 950 ℃. The bainite transformation beginning at a higher temperature results in the wider bainitic ferrite laths. The more proportion of polygonal ferrite and wide bainitic ferrite laths commonly contribute to the lower strength and better elongation. The uniform microstructure with lath-like morphology and retained austenite with high average carbon content ensures a good mechanical property in TAM850 with the product of strength and elongation of about 28 GPa ·%,

关键词： Retained austenite Aluminum Phase transformation High strength steel Mechanical property