奥氏体化温度对900MPa级HSLA钢显微组织和晶体学演变的影响

Effect of Austenitizing Temperature on Microstructure and Crystallographic Evolution of 900 MPa Grade HSLA Steel

采用OM和SEM研究了奥氏体化温度对HSLA钢组织演变和低温韧性的影响。结果表明:奥氏体化温度由850℃升高至950℃(实验钢的A_(C3)温度为819℃)并保温30 min后,奥氏体的平均晶粒尺寸由7.22μm增大到17.39μm,在850~950℃淬火后的显微组织均为板条马氏体,屈服强度和抗拉强度均呈下降趋势,延伸率没有明显的变化,但是-20℃下的冲击韧性由97 J显著降低到31 J。使用EBSD和ARPGE软件的晶体学分析结果表明,随着奥氏体化温度的升高晶粒尺寸增大且变体选择增强,表现为奥氏体晶粒大部分区域内由单个变体对占据。同时,850℃试样变体之间呈现较明显的CP(Close packed)组合方式,奥氏体化温度升高到950℃后变体之间的组合方式更倾向以Bain group组合,同一组的变体在极图上的位置较近,变体之间的取向差也较小,表明大角度错取向的操作因子比例减少,大角度晶界密度降低,阻碍裂纹扩展的能力降低和恶化冲击韧性。

The effect of austenitizing temperature on the microstructure evolution and low temperature toughness of high strength low alloy(HSLA) steel was investigated by OM and SEM. The results show that with the increase of austenitizing temperature from 850℃ to 950℃ while heat treated for 30 min, the average austenite grain size increases from 7.22 μm to 17.39 μm(the temperature of A_(C3)is 819℃). After quenching at 850~950℃, the microstructure is lath martensite. The yield strength and tensile strength decreased respectively, and there was no obvious variation in elongation. However, the toughness decreased significantly from 97 J to 31 J. The crystallographic analysis results by EBSD and ARPGE software show that the grain size increased and the variants selection enhanced with the increase of quenching temperature, which show that austenite grain is mainly occupied by a single pair of variants. In addition, the combination mode of the variants for the 850 A sample tends to show a CP(Close packed) combination mode. When the austenitizing temperature increased to 950℃, the combination mode of the variants is more likely to be Bain group combination, and the proportion of operation factors representing high angle misorientation decreases, which leads to the decrease of high angle grain boundary density,and the ability to hinder crack propagation is reduced, further deteriorating the impact toughness.