热处理工艺对高碳贝氏体钢组织与力学性能的影响

Effect of heat treatment process on the microstructure and mechanical properties of high-carbon bainitic steel

借助OM、SEM、XRD等手段,对比研究了一步、两步等温贝氏体转变工艺及QPB(淬火+配分+贝氏体转变)工艺对高碳贝氏体钢(w(C)=0.79%)显微组织与力学性能的影响。结果表明,采用一步等温贝氏体转变工艺处理试验钢时,当等温温度同为250℃,随着保温时间的延长,钢中贝氏体转变越充分,块状残余奥氏体尺寸降低,组织更为均匀细小;而在较低温度下(200℃)等温处理时,钢中残余奥氏体含量显著降低,贝氏体铁素体板条更细小,材料的强度和硬度提高,而塑性和韧性下降。两步等温贝氏体转变工艺处理(250℃×24 h+200℃×72 h)的试验钢中贝氏体铁素体板条平均尺寸约为82 nm,残余奥氏体体积分数为21.4%,获得了最佳的综合力学性能,抗拉强度达到2040 MPa,伸长率为12.5%,冲击韧性为21 J。QPB工艺提高了贝氏体转变速率,大大缩短了热处理时间,最终得到马氏体+贝氏体铁素体+残余奥氏体的组织,试验钢同时也获得了良好的强度和塑韧性。

By means of OM,SEM,XRD,the effects of heat treatment processes including one-step and two-step bainitic isothermal transformation as well as QPB(quenching+partitioning+bainite transformation)approach on the microstructure and mechanical properties of a high-carbon bainitic steel(w(C)=0.79%)were investigated comparatively.The results indicate that when the tested steel is treated by one-step bainitic isothermal transformation approach at the same isothermal temperature of 250℃,as the holding time is extended,the bainite transformation is more sufficient,the size of the block retained austenite is reduced,and the microstructure is more uniform and fine.When the isothermal temperature is lowered to 200℃,the volume fraction of retained austenite in the steel is significantly reduced and the bainitic ferrite lath is finer,resulting in the increase of strength and hardness and the decrease of plasticity and toughness.After two-step isothermal bainite transformation treatment,the average size of bainitic ferrite lath in steel is around 82 nm and the volume fraction of retained austenite is 21.4%.The best comprehensive mechnical properties of the steel are obtained with the tensile strength of 2040 MPa,elongation rate of 12.5%and impact toughness of 21 J.Besides,the QPB approach can effectively improve the bainite transformation rate and therefore shorten the heat treatment time of the steel,and finally the microstructure consisting of martensite,bainitic ferrite and retained austenite is obtained.The relatively good strength,toughness and plasiticity can also be achieved for the as-treated steel.