热处理工艺对稀土微合金化1200 MPa级高强钢性能的影响研究

Effect of Heat Treatment on Properties of Rare Earth Microalloyed 1200 MPa High Strength Steel

结合目前国内以2250mm热连轧生产机组为代表的先进控轧控冷工艺,开发了轧制结束利用加密层流冷却进行分段淬火-等温配分-低温卷取自回火的在线热处理工艺(DQ&P&T),与传统的离线调质热处理工艺(RQ&T)生产稀土微合金化高强钢(R_(m)≥1200MPa)进行对比研究。利用光学显微镜、扫描电子显微镜及配备的电子背散射衍射仪以及拉伸试验机、冲击试验机、维氏硬度计,以及湿砂/橡胶轮磨损试验机分析检测手段,系统地分析了实验钢在2种热处理制度下组织转变形态和机械性能。研究表明:实验钢在2种热处理工艺的室温组织均由板条马氏体、贝氏体铁素体以及少量残余奥氏体组成。相比离线热处理(RQ&T),采用在线热处理(DQ&P&T)实验钢的板条马氏体含量降低,贝氏体铁素体含量增加。等温转变形成的贝氏体铁素体穿插分割形变过冷奥氏体,促进组织晶粒细化和大角度晶界比例增加,提高了材料的强韧性和耐磨性。但在产品厚度方向硬度的均匀性劣于经RQ&T工艺处理的高强钢。在线热处理相比离线热处理生产R_(m)≥1200 MPa高强钢效率提高,制造成本降低,且产品各项性能满足标准要求,证明在线热处理工艺是可行的。

Combined with the advanced controlled rolling and cooling process,which is represented by the 2250 mm hot continuous rolling production line,in China,an on-line heat treatment process(DQ&P&T)is developed,which uses dense laminar flow cooling to conduct sectional quenching,isothermal distribution and low-temperature coil after tempering followed by rolling,and the rare-earth-microalloyed ultra-high strength steel(R_(m)≥1200 MPa)was prepared through the traditional off-line quenching and tempering heat treatment process(RQ&T)for comparison.The microstructure transformation morphology and mechanical properties of the test steels under two heat treatments were systematically analyzed by OM,SEM equipped with electron backscatter diffractometer,tensile testing machine,impact testing machine,Vickers hardness tester,and wet sand/rubber wheel wear testing machine.The results show that the microstructure of the test steels at room temperature is composed of lath martensite,bainite ferrite and a small amount of residual austenite under the two heat treatment processes.Compared with that prepared by RQ&T process,the lath martensite content of the steel prepared by DQ&P&T process decreases and the bainite ferrite content increases.The bainitic ferrite formed by isothermal transformation penetrates and divides the deformed undercooled austenite,which promotes the grain refinement and the increase in the proportion of large angle grain boundaries,thereby improving the strength,toughness and wear resistance of the material.However,the uniformity of hardness in the thickness direction of the product prepared by DQ&P&T process is more inferior than treated by RQ&T process.Compared with the off-line heat treatment,the on-line heat treatment has higher efficiency and lower manufacturing cost in the production of high strength steel,and the product performance meets the standard requirements,which proves that the on-line heat treatment process is feasible.