Variations in the Microstructure and Properties of Mn-Ti Multiple-phase Steel with High Strength under Different Tempering Temperatures  

作　　者：LI Dazhao LI Xiaonan CUI Tianxie LI Jianmin WANG Yutian FU Peimao 

机构地区：[1]School of Materials Scinece and Engineering, North University of China [2]Technology Center,Taiyuan Iron & Steel (Group) Co. [3]School of Materials Scinece and Engineering, Tianjin University

出　　处：《Chinese Journal of Mechanical Engineering》2015年第2期430-436,共7页中国机械工程学报（英文版）

基　　金：Supported by National Natural Science Foundation of China(Grant No.51201154)

摘　　要：There are few relevant researches on coils by tempering,and the variations of microstructure and properties of steel coil during the tempering process also remain unclear.By using thermo-mechanical control process（TMCP）technology,Mn-Ti typical HSLA steel coils with yield strength of 920 MPa are produced on the 2250 hot rolling production line.Then,the samples are taken from the coils and tempered at the temperatures of 220℃,350℃,and 620℃respectively.After tempering the strength,ductility and toughness of samples are tested,and meanwhile microstructures are investigated.Precipitates initially emerge inside the ferrite laths and the density of the dislocation drops.Then,the lath-shaped ferrites begin to gather,and the retained austenite films start to decompose.Finally,the retained austenite films are completely decomposed into coarse and short rod-shape precipitates composed of C and Ti compounds.The yield strength increases with increasing tempering temperature due to the pinning effect of the precipitates,and the dislocation density decreases.The yield strength is highest when the steel is tempered at 220℃because of pinning of the precipitates to dislocations.The total elongation increases in all samples because of the development of ferrites during tempering.The tensile strength and impact absorbed energy decline because the effect of impeding crack propagation weakens as the retained austenite films completely decompose and the precipitates coarsen.This paper clarifies the influence of different tempering temperatures on phase transformation characteristics and process of Mn-Ti typical multiphase steels,as well as its resulting performance variation rules.There are few relevant researches on coils by tempering,and the variations of microstructure and properties of steel coil during the tempering process also remain unclear.By using thermo-mechanical control process（TMCP）technology,Mn-Ti typical HSLA steel coils with yield strength of 920 MPa are produced on the 2250 hot rolling production line.Then,the samples are taken from the coils and tempered at the temperatures of 220℃,350℃,and 620℃respectively.After tempering the strength,ductility and toughness of samples are tested,and meanwhile microstructures are investigated.Precipitates initially emerge inside the ferrite laths and the density of the dislocation drops.Then,the lath-shaped ferrites begin to gather,and the retained austenite films start to decompose.Finally,the retained austenite films are completely decomposed into coarse and short rod-shape precipitates composed of C and Ti compounds.The yield strength increases with increasing tempering temperature due to the pinning effect of the precipitates,and the dislocation density decreases.The yield strength is highest when the steel is tempered at 220℃because of pinning of the precipitates to dislocations.The total elongation increases in all samples because of the development of ferrites during tempering.The tensile strength and impact absorbed energy decline because the effect of impeding crack propagation weakens as the retained austenite films completely decompose and the precipitates coarsen.This paper clarifies the influence of different tempering temperatures on phase transformation characteristics and process of Mn-Ti typical multiphase steels,as well as its resulting performance variation rules.

关 键 词：multiple-phase microstructure lath-shaped ferrite polygonal ferrite precipitate retained austenite film 

分 类 号：TG142.1[一般工业技术—材料科学与工程] TG156.5[金属学及工艺—金属材料]