采用全自动热模拟试验机测定了新开发的800 MPa级相变诱发塑性钢的CCT曲线,据此制定了12种工艺对试验钢进行退火处理;通过拉伸试验测定了经不同工艺退火处理试验钢的力学性能,确定出了最优热处理工艺;对经最优工艺退火处理钢的显微组织和残余奥氏体的稳定性进行了研究.结果表明:各种工艺处理钢均获得了800 MPa以上的抗拉强度,获得最佳综合力学性能(强塑积最大)的热处理工艺为830℃退火120 s后,先以20℃·s-1的速率缓冷至700℃,再以40℃·s-1的速率冷至400℃,并在400℃等温处理400 s,最后以20℃·s-1的速率冷至室温;经最优工艺退火处理后钢的显微组织为50%铁素体+38%贝氏体+12%残余奥氏体,残余奥氏体主要分布在铁素体晶界处,或铁素体与贝氏体的晶界处,还有小部分存在于大的铁素体晶粒内;在拉伸过程中试验钢中残余奥氏体的相变大部分发生变形量为10%～20%阶段.
12 kind of annealing processes were worked out based on the CCT curves of the new developed 800 MPa (TRIP) steel which were determined by full automatic thermal simulation machine. The optimal heat treatment process was found out after the mechanical properties of testing steels annealed with different processes were obtained by tension tests. The microstructure of the steel treated with the optimal annealing process and the mechanical stability of the retained austenite were investigated. The results show that 800 MPa tensile strength could be obtained by all kinds of processes, the optimal comphrehensive mechanical properties (the biggest Rm ×A value) could be obtained using following process: annealing at 830℃ for 120 s, then slow cooling to 700 ℃ at 20 ℃·s-1, and fast cooling to 400 ℃ at 40 ℃ ·s-1, after isothermal treating at 400 ℃ for 400 s, cooling to room temperature at 20 ℃·s-1. The microstructure of the steel with optimal annealing process was 50% ferrite,38% bainite and 12% retained austenite. The retained austenite grains mainly dispersed at ferrite grains boundaries,or the boundaries between ferrite and bainite grains, and a fraction of retained austenite located inside big ferrite grains. Most retained austenite phases of the testing steel transformed during 10%-20% tensile deformation.
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