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Effect of Austempering Route on Microstructural Characterization of Nanobainitic Steel

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Acta Metallurgica Sinica(English Letters) 2014年第1期27卷 19-26页

作者： huifang lan linxiu du Na Zhou xianghua liu State Key Laboratory of Rolling and Automation Northeastern University Hebei Shougang Qian'an Iron & Steel Co. Ltd

A low-temperature nanobainitic steel was obtained through one-step and two-step austempering. The effect of austempering temperature, time, and route on bainitic lath width, volume fraction of retained austenite, carbon concen- tration in retained austenite, and nanohardness of bainitic lath and retained austenite was studied. Results showed that the transformation kinetics was slowed down and the bainitic lath was refined as the austempering temperature decreased from 300 to 250 C. Both coarser and finer bainitic laths were obtained with the two-step austempering, which was consistent with the lath size at 300 and 250 C austempering, respectively. X-ray diffraction analysis showed that both volume fraction of retained austenite and its carbon concentration decreased with the decrease of austempering temperature for the one-step austempering, and especially the carbon concentration is obviously increased when the two-step austempering is adopted. The nanohardness of the bainitic lath in the sample after two-step austempering treated lies between that of the samples after 300 and 250 C austempering treated. The product of tensile strength and total elongation of the two-step austempered sample is the highest, which increases monotonously with the product of retained austenite fraction and its carbon concentration. Higher strengthluctility balance may be resulted by the fine bainitic lath, high volume fraction, and high stability of retained austenite in the sample after two-step austempered.

关键词： Nanobainitic steel Stability Retained austenite Nanohardness Strength-ductility balance

High performance low cost steels with ultrafine grained and multi-phased microstructure

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Science China(Technological Sciences) 2009年第8期52卷 2245-2254页

作者： liu xianghua lan huifang du linxiu liu WeiJie State Key Laboratory of Rolling and Automation Northeastern UniversityShenyang 110004China Graduate Institute of Ferrous Technology Pohang University of Science and TechnologyPohang790-784Korea

Ultrafine grained ferrite was obtained through tempering cold rolled martensite with an average grain size of 200―400 nm in a low carbon and a microalloyed steel. Thermal and mechanical stability of the two steels was studied. due to the pinning effect of microalloyed precipitates on the movement of dislocations and grain boundaries, the recrystallization and grain growth rate were retarded, and the thermal stability of ultrafine grained microstructure was improved. The ultrafine grained ferritic steel was strengthened, but its strain hardening rate was reduced. It seems that the tiny carbide precipitates have no significant effect on work hardening rate. The ultrafine grained ferrite+martensite dual phase microstructure was obtained in the microalloyed steel through intercritically annealing cold rolled martensite. The resulting multiphase microstructure has a tensile strength higher than 1.0 GPa with a yield ratio lower than 0.7. Another type of multiphase microstructure with nanoscaled lath bainite+ retained austenite was obtained through an isothermal heat treatment in low temperature bainite transformation region in high carbon steel. The tensile strength was as high as 1.64 GPa with a yield ratio of 0.84.

关键词： ultrafine grained steel multiphase high strength yield ratio

Dynamic Recrystallization and Precipitation Behavior of Mn-Cu-V Weathering Steel

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Journal of Materials Science & Technology 2011年第12期27卷 1131-1138页

作者： Hongyan Wu linxiu du xianghua liu State Key Laboratory of Rolling and Automation Northeastern University Shenyang 110819 China

The hot deformation behavior of a Mn-Cu-V weathering steel was investigated at temperatures ranging from 850 to 1050℃ and strain rates ranging from 0.01 to 5 s-1 using MMS-300 thermal-mechanical simulator. The activation energy for dynamic recrystallization and stress exponent were calculated to be 551 kJ/mol and 7.73, respectively. The accurate values of critical strain were determined by the relationship between work hardening rate and flow stress （θ-σ） curves. The hyperbolic sine constitutive equation was employed to describe the relationship between the peak stress and Zener-Hollomon parameter during hot deformation. The interaction between dynamic recrystallization and dynamic precipitation of V（C,N） at a low strain rate was analyzed. The results showed that precipitation particles size of weathering steel increased with increasing strain at deformation temperature g50~C and strain rate 0.1 s-I. The calculation results of the recrystallization driving force and pinning force showed that dynamic precipitation could retard the progress of dynamic recrystallization but not prevent it while the pinning forces is less than driving force. On the contrary, dynamic precipitation can effectively prevent the progress of dynamic recrystallization.

关键词： Weathering steel Critical strain Dynamic recrystallization Dynamic precipitation

Microstructure and mechanical properties of X80 pipeline steels in different cooling schedules

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Acta Metallurgica Sinica(English Letters) 2010年第3期23卷 171-175页

作者： Min ZHOU linxiu du xianghua liu State Key Laboratory of Rolling Technology and Automation Northeastern University Shenyang 110819 China

The microstructures and mechanical properties of three X80 pipeline steel plates were investigated in this *** plates were hot rolled by two stages, and cooled by three different cooling schedules to obtain different microstructures and mechanical *** electron microscope（TEM） was used to observe the *** results indicated that the amount of polygonal ferrite（PF） and quasi-polygonal ferrite（QPF） decreased with decreasing of the finish cooling temperature（FCT）, while the amount and size of M-A islands were *** promoted the Charpy V-notch（CVN） impact toughness at-20℃, but not the drop weight tear testing（DWTT） property at-15℃, and decreased the strength of the steel.

关键词： Microstructure Mechanical properties Cooling schedule Relaxation

Growth behavior of ultrafine austenite grains in microalloyed steel

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Acta Metallurgica Sinica(English Letters) 2009年第1期22卷 7-12页

作者： linxiu du Shengjie YAO xianghua liu Guodong WANG State Key Laboratory of Rolling and Automation Northeastern University Shenyang 110004 China

Ultrafine austenite gains （UFAGs） with size of 1-5 μm were prepared through repetitive treatment, four times, of rapid heating and quenching, and the growth behaviors of these UFACs during both the reheating and cooling stages were investigated. The results indicated that UFAGs without pinning particles appeared with significant coarsening when the reheating temperature reached 1000 ℃. Although coarsening still occurred in the cooling stage, the growth was obscured during the isothermal holding process at temperatures between 900 ℃ and At3.

关键词： Ultrafine austenite grain Growth Microalloyed steel Second-phase particle

Static Recrystallization and Precipitation Behavior of a Weathering Steel Microalloyed with Vanadium

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Journal of Materials Science & Technology 2013年第12期29卷 1197-1203页

作者： Hongyan Wu linxiu du Zhengrong Ai xianghua liu State Key Laboratory of Rolling and Automation Northeastern University

The static recrystallization （SRX） and precipitation behavior of a weathering steel microalloyed with vanadium were investigated through double-pass compression tests under controfled conditions using the MMS-300 thermal-mechanical simulator. The deformation temperatures ranged from 800 °C to 1000 °C, and the inter- pass time from 1 s to 500 s. The simulation results showed that SRX occurred after 5-10 s at the first compression deformation. The softening fraction of SRX was found to increase with increasing the deformation temperature and the pre-strain. However, the softening fraction scarcely changed during the process of strain-induced precipitation. In addition, the kinetics of SRX was described by the Avrami equation, and the Avrami exponent appeared to be closely associated with the deformation temperature. The microstructure evolution was investigated at the initiation and completion of recrystallization. The amount and distribution of the precipitates were analyzed. The relationship between the driving force of SRX and the pinning force of precipitation was discussed. Besides, the recrystallization inhibition was detected at the early stage of precipitation, and the pinning forces were found to be of a magnitude comparable to the driving force. Moreover, the pinning forces were found to increase with the degree of precipitation and reach a peak at the intermediate stage of precipitation, and finally reduce as the particles coarsened.

关键词： Vanadium-microalloyed weathering steel Static recrystallization Kinetics of SRX Microstructural evolution Precipitation

Isothermal Growth Kinetics of Ultra-fine Austenite Grains in a Nb-V-Ti Microalloyed Steel

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Journal of Materials Science & Technology 2009年第5期25卷 615-618页

作者： Shengjie Yao linxiu du xianghua liu Guodong Wang State Key Lab. of Rolling and Automation Northeastern University Shenyang 110004 China

Ultra-fine austenite grains with size of i-3 μm were prepared in a Nb-V-Ti steel through repetitive treatment of rapid heating and quenching. A model for the growth kinetics of these ultra-fine austenite grains was successfully created through successive 2 processes, and the activation energy Q for growth was estimated to be about 693.2 kJ/mol, which directly shows the inhibition effect of microalloy elements on the growth of ultra-fine austenite grains.

关键词： Ultra-fine austenite grain Grain growth kinetics Microalloyed steel

Effect of Coiling Temperature on Ti(C,N) Precipitation and Properties of Batch Annealed DC06EK Enamel Steel

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Journal of Wuhan University of Technology(Materials Science) 2018年第3期33卷 697-702页

作者：薛飞董福涛 TIAN Yaqiang 陈连生 du linxiu liu xianghua College of Electrical Engineering North China University of Science and Technology Key Laboratory of the Ministry of Education for Modern Metallurgy Technology North China University of Science and Technology The State Key Laboratory of Rolling and Automation Northeastern University

The effects of hot-strip coiling temperature on Ti（C,N） precipitation, texture and hydrogen permeation behavior in DC06 EK enamel steel were investigated by TEM, EBSD test and electrochemical hydrogen permeation experiment. It was found that the Ti（C,N） particles in hot-strip coarsened with increasing coiling temperature, whereas after cold-rolling and annealing, the size difference of Ti（C,N） particles was lessened. The hot-strip coiling temperature has a significant impact on the recrystallized texture in the subsequent cold-rolled and annealed sheet. Hot-strip using high temperature（700 ℃） coiling leads to strong {111} recrystallized texture in annealed sheet, with peak intensity 9.2. On the contrary, in annealed sheets using hot-strip coiling at 650 ℃, their {111} recrystallized textures were weaker, which was also reflected in their rmvalues. Even though the hydrogen diffusion coefficient is slightly lower（7.76×10^（-5） mm^2/s） in annealed sheet using low temperature coiling（600 ℃）, appropriately higher coiling temperature is more suitable for DC06 EK enamel steel combining both good drawability and fish-scale resistance.

关键词： enamel steel coiling temperature Ti(C N) γ-fiber texture hydrogen diffusion

A New Attempt to Obtain Bulk Nanocrystalline Steel

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Journal of Materials Science & Technology 2009年第1期25卷 81-84页

作者： Shengjie Yao linxiu dut xianghua liu Guodong Wang The State Key Lab. of Rolling Technology and Automation Northeastern University Shenyang 110004 China

Bulk nanocrystalline steel sample was obtained in laboratory through refining of austenite grains and controlled rolling. Transmission electron microscopy micrographs show that some textures are evolved in the process of the treatment and two typical carbides are classified according to their size and location. The tensile strength of the nanocrystalline sample is obviously lower than conventional consideration, and scanning electron microscopy observation shows that the existence of the first type of carbide is considered as the main reason for the failure.

关键词： Bulk Nanocrystalline Deformation Carbide Fracture