Ti微合金化高强钢性能波动原因分析与工艺优化

目前,700MPa级高强钢主要采用Ti微合金化成分体系,利用纳米级Ti析出相来提高强度,但存在塑性不足、性能波动大等问题。利用OM、SEM和物理化学相分析法,对比研究了带钢头、中、尾部微观组织、力学性能以及析出相的类型与大小。结果表明,带钢头、中、尾部均为准多边形铁素体组织,头部晶粒尺寸细小,中部和尾部晶粒尺寸相对较大,然而带钢头部强度显著低于中部和尾部。通过分析强化机制,发现带钢头部M(CN)析出不充分,沉淀强化作用相对较小是其强度偏低的主要原因。为此,对控轧控冷工艺进行了优化,采取提高头部卷取温度、轧后送入缓冷坑等措施后,带钢头、中、尾部析出相数量基本相当,性能均匀性得到了明显改善。

Cause analysis of performance fluctuation of Ti microalloyed high strength steel and process optimization

At present,700 MPa grade high strength steel mainly uses a Ti microalloyed composition system,and uses nano-scale Ti precipitated phases to improve strength,but there are problems such as insufficient plasticity and large performance fluctuations.The microstructures,mechanical properties,types and sizes of precipitated phase in the head,middle and tail part of 700 MPa grade(YS) high strength steel coils were investigated by means of OM,SEM and physicochemical phase analysis.The results show that microstructure of the strip is quasi polygonal ferrite in different coil parts and the grain size in the coil head is relatively finer than that in the coil middle and tail.However,the strength of the strip head is significantly lower than that of the middle and tail.Through the analysis of strengthening mechanism,the cause is found that M(CN) phase precipitate insufficient and the precipitation strengthening has relatively little effect in the coil head.Therefore,the process of controlled rolling and controlled cooling was optimized.After taking measures such as increasing the coiling temperature of the strip head and delivering the strip to slow cooling pit after rolling,the number of precipitated phases in the strip head,middle and tail was basically equal,and the uniformity of performance was significantly improved.