980 MPa级冷轧超高强度双相钢边裂原因分析与工艺优化

汽车用超高强度双相钢CR550/980DP冷轧边裂问题,严重影响热轧/冷轧工序界面生产顺行,易造成冷轧机架间及连退炉内断带事故,成为超高强度双相钢生产的难题。基于高温热塑性曲线和热轧动态CCT曲线,采用对显微组织、力学性能、裂纹扩展分析等手段明确冷轧边裂产生原因。试验结果分别指出,精轧阶段带钢横向温度分布不均匀、边部温降大,导致在第Ⅲ脆性区轧制;同时,受Nb作用再结晶温度提高,边部低温区为未再结晶区轧制;当应变量超过塑性极限、轧制力超过边部热强度时,形成热轧卷边裂。边部形成细小弥散的铁素体(F)和马氏体(M)两相组织,不协调应变将导致F/M相界面产生应力集中而形成裂纹;裂纹以微孔聚集方式进行扩展,形成热轧卷无边裂-冷轧边裂现象。通过投用边部加热器和优化初轧定宽量、精轧入口温度、精轧机架间冷却水、终轧温度、卷取温度等措施,实现热轧卷边部质量改善、解决边裂问题。

Cause analysis and process optimization of edge crack for 980 MPa cold rolled ultra high strength dual-phase steel

The edge cracking problem of coldrolled ultra high strength steel (UHSS) CR550/980DP severely affects hotrolling/coldrolling process interface with strip broke at cold rolling stands and continuous annealing furnace, which has been a difficulty for UHSS production. Based on high temperature thermoplastic and hot rolling dynamic CCT curves, microstructure, mechanical properties and crack propagation were analyzed to study the cause of edge cracking. It is pointed out that inhomogeneous temperature distribution in transverse direction at finish mill and large heat loss at the edge of strip steel will lead to strip deformed at the third brittle region. Furthermore, with Nb improves recrystallization temperature, the edge of strip stays in non-recrystallization region which deteriorates the plasticity of strip, and it is easy to form edge cracks when the strain exceeds the plastic limit and rolling force exceeds edge thermal strength at hot rolling process. The microstructure of hot rolled strip edge shows fine and dispersed ferrite and martensite, which will form stress concentration and crack defects at the interface of martensite and ferrite due to strain discordance when deformed at cold rolling process. The crack propagates in the form of micropore aggregation, and occurs phenomenon that hot rolling has no edge crack and cold rolling has edge crack. Using edge heaters and optimizing of slab size press, finish entrance temperature, cooling water between finishing rolling stands, finish temperature, coiling temperature etc, the edge quality has been efficiently improved, and the edge cracking problem is solved.