返温工艺对微合金化S355钢高温塑性的影响

Influence of Reheating Processing on High Temperature Plasticity of Micro-alloyed S355 Steel

针对S355连铸坯的横裂纹问题,利用Gleeble热模拟设备,研究了常规工艺下S355钢的高温力学性能,测定了其第三脆性区的温度范围。结合热拉伸试样显微组织观察、断口形貌观察,分析了S355钢第三脆性区的脆性机理,探讨了返温工艺对S355钢高温塑性的影响。常规工艺实验结果表明,微合金化S335钢的第三脆性区温度范围为667~850℃,750℃时断面收缩率最小,断口呈现为冰糖状,裂纹沿奥氏体晶界扩展,为典型的沿晶脆性断裂。返温工艺实验结果表明,S335钢在750℃变形时的断面收缩率最小,但较常规工艺提高了21%,第三脆性区的塑性得到明显改善,且断口上出现少量韧窝,是以脆性断裂为主的复合断裂类型。结合金相组织观察及高温力学性能实验结果可知第三脆性区脆性出现的主要原因是沿奥氏体晶界形成的网状铁素体导致。返温工艺明显改善S355钢高温延塑性的原因可归结为奥氏体晶粒的细化、铁素体百分比增加及其网状特征的弱化。

To solve the transverse cracks of the S355 cast slab,the high temperature plasticity under conventional processing was studied by the hot simulation Gleeble equipment.The temperature range of third brittleness zone was measured.Combined with the optical micrograph observation and fracture morphology,the brittleness mechanism at the third brittleness zone was analyzed.Meanwhile,the effect of reheating processing on the high temperature plasticity was studied.The conventional processing experiment result showed that the temperature range of the third brittleness zone for the tested steel is 667-850℃.The reduction of area is smallest and the fracture morphology is candy shape at 750℃.The reheating processing experiment result showed that the reduction of area is smallest at 750℃,but enhanced 21%compared with the conventional processing.The brittleness at the third brittleness zone was improved largely.Meanwhile,the fracture mode is composite fracture mode with mainly brittleness fractures.Combined with OM and high temperature plasticity result,it can be concluded that the occurrence of the third brittleness zone is caused by the net-shape film ferrite formed at the prior austenite grain boundary.The reason that reheating processing can improve the high temperature plasticity of the S355 steel obviously is attributed to the austenite grain size refinement,the ferrite percent improving and the weakness of the net-shape.