控轧后的冷却速度对Q420qENH园林钢组织和力学性能的影响

Effect of Cooling Rates after Controlled Rolling on Microstructure and Mechanical Properties of Q420qENH Steel for Garden

对15 mm×100 mm的Q420qENH园林钢试样,在Gleeble-3800型热模拟试验机上进行控制轧制和控制冷却。采用金相显微镜、扫描电镜和透射电镜等设备研究了控轧后的冷却速度对Q420qENH钢显微组织和力学性能的影响。结果表明:控制轧制后以6~18℃/s速率冷却的钢的力学性能均达到了要求,以12和18℃/s速率冷却的钢的强度高于要求值49 MPa以上,屈强比小于0.85;随着冷却速度从2℃/s提高至18℃/s,钢的组织从铁素体+少量珠光体转变为以粒状贝氏体为主的组织,M/A岛面积分数和平均尺寸减小,不同取向晶粒的尺寸减小;随着控轧后冷速的增大,钢中小角度晶界的比例减小,大角度晶界的比例增大;控轧后以不同速度冷却的Q420qENH钢的晶粒尺寸与屈服强度之间存在线性关系,控轧控冷的园林钢的屈服强度可采用拟合的霍尔-佩奇公式预测。

Controlled rolling and controlled cooling test for Q420qENH garden steel specimen 15 mm in diameter by 100 mm long was done in a Gleeble-3800 thermal simulation tester.The effect of cooling rate after controlled rolling on microstructure and mechanical properties of the Q420qENH steel were investigated by means of metallographic microscope,scanning electron microscope and transmission electron microscope.The results showed that(a)the mechanical properties of the steel cooled at 6 to 18℃/s after controlled rolling conformed with the requirements,the steel cooled at 12 and 18℃/s exhibited strength value at least 49 MPa higher than required value and yield ratio less than 0.85;(b)with the increase of cooling rate from 2 to 18℃/s,the microstructure of the steel changed from ferrite plus a small amount of pearlite to predominant granular bainite,the area fraction and average size of M/A islands decreased,and the size of grains with different orientations decreased;(c)as the cooling rate subsequent to controlled rolling was increased,the proportion of small angle grain boundary decreased but that of large angle grain boundary increased in the steel;(d)there was a linear relationship between grain size and yield strength for the steel cooled at different rates after controlled rolling,and the fitted Hall-Petch formula was likely to be used to predict yield strength of garden steel after controlled rolling and controlled cooling.