高强度低碳贝氏体钢的工艺与组织细化

MICROSTRUCTURE REFINEMENT OF HIGH STRENGTH LOW CARBON BAINITIC STEEL

在超低碳贝氏体钢中,采用弛豫-析出-控制相变(RPC)技术可得到细化的中温转变组织。组织类型为细化的板条贝氏体及少量不规则粒状贝氏体或针状铁素体.与一般控轧空冷和调质处理组织比较,除细化外,所得贝氏体类型及形貌均有所不同,通过这种工艺细化的低碳贝氏体钢板其强度比控轧后空冷或轧后再加热-淬火(调质处理)钢有明显提高.在采用RPC工艺时,轧后弛豫时间长短对最终组织细化程度和形貌也有明显影响,从而造成性能有所差别.终轧后弛豫阶段形成并被应变诱导析出物钉扎的位错胞状组织或亚晶结构是细化相变组织、阻碍贝氏体生长的主要原因.冷却过程中,在贝氏体相变前形成的不规则粒状贝氏体或针状铁素体,分割了压扁的原奥氏体晶粒,同样限制了贝氏体板条束的长度和宽度.

The ultra-fine bainitic microstructure in a high strength low carbon bainitic Mn-Nb-B steel has been obtained by a newly developing relaxation-precipitation-controll phase transformation (RPC) processing. In a pan-cake like prior-austenite grain, the microstructure consists of lath bainite, and some amount of abnormal granular bainites or/and acicular ferrites. The laths are 3-5 mum in length and less than 1 mum in width, the packet size of lath bainite is refined to 3-5 mum. In spite of refining the effective grain, the morphologies of bainite and retained austenite are different in RPC processed steel comparing with the commonly CR or RQ-T processing. The relaxation time after finishing rolling also influences the refinement of the microstructure. The yield strength of RPC processed steel is increased by about 30% comparing with the RQ-T processed steel with the same composition. The refining mechanism by RPC process is the formation of polygonal dislocation substructure pined by strain induced precipitates during relaxation, and the substructure would be benefit of nucleating bainite and acicular ferrite or limiting the growth of bainitic lath.