Cause analysis and process optimization of edge crack for 980 MPa cold rolled ultra high strength dual-phase steel
CAO Xiao-en1,2, ZHAO Hong-qi1, LI Shou-hua1, LIU Zi-quan1, XUE Ren-jie1, ZHANG Zhi-yang1
1. Automotive and Appliance Sheet R & D Center, HBIS Group Hansteel Company Technology Center, Handan 056015, Hebei, China; 2.Hebei Key Laboratory of Advanced Steel Materials for Auto, Handan 056015, Hebei, China
Abstract：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.
曹晓恩, 赵红旗, 李守华, 刘自权, 薛仁杰, 张志扬. 980 MPa级冷轧超高强度双相钢边裂原因分析与工艺优化[J]. 中国冶金, 2022, 32(9): 90-96.
CAO Xiao-en, ZHAO Hong-qi, LI Shou-hua, LIU Zi-quan, XUE Ren-jie, ZHANG Zhi-yang. Cause analysis and process optimization of edge crack for 980 MPa cold rolled ultra high strength dual-phase steel[J]. China Metallurgy, 2022, 32(9): 90-96.
2022, Vol. 32 
