无碳化物析出的无硅和无铝贝氏体钢热处理工艺开发

Design of carbide-free bainite in Si/Al-free steel by chemical heterogeneity

采用锰配分珠光体为初始组织的非均质贝氏体转变(PB)工艺,研究了非均质锰分布对Fe-0.39C-3.69Mn(质量分数,%)钢贝氏体转变的影响。结果表明,经盐浴炉快速加热到730℃短时保温90 s后,高温奥氏体继承了珠光体的非均质锰分布,从而调控了贝氏体转变过程,因此获得了由富锰片层残余奥氏体和贫锰贝氏体铁素体构成的鬼珠光体。特别地,虽然试验钢中未添加Si、Al元素,但是在鬼珠光体区域却并没有发现碳化物的析出,表明得到了无碳化物贝氏体组织。这是由于,非均质锰分布片层式分割了高温奥氏体,其中原铁素体处的贫锰奥氏体稳定性不足,在等温阶段迅速发生贝氏体转变,而原渗碳体处的富锰奥氏体稳定性强,能够保持稳定。同时,C、Mn元素的强烈相互作用,驱动了C元素自贫锰区域向富锰区域的扩散,而且富锰区域和贫锰区域的纳米片层结构缩短了C元素的扩散距离,导致富锰奥氏体的C含量增加,从而进一步抑制了贝氏体转变和碳化物的析出。得益于鬼珠光体区域的相互堆叠的片层结构,在相同贝氏体转变温度下,PB工艺能够获得与传统贝氏体转变(CB)工艺相似抗拉强度(1043 MPa vs.1027 MPa),同时大幅提升了试样的塑性,其中断后伸长率从6.1%增加到11.5%,均匀伸长率从4.3%增加到7.6%。

Instead of conventional bainite austempering(CB)process,a bainite austempering(PB)process purposely based on Mn-partitioned pearlite to investigate the effect of Mn distribution on bainite transformation was proposed.In present study,carbide-free bainite by Mn-heterogeneous austenite which was originated from the Mn-partitioned pearlite in the Fe-0.39C-3.69Mn(wt.%)steel was obtained.The results show that the Mn-heterogeneous austenite can tailor the austenite-to-bainite transformation during austempering,leading to the formation of ghost pearlite which was assembled by bainite ferrite lath and film retained austenite.During the austempering stage,Mn-depleted austenite was transformed into bainite ferrite quickly,while Mn-enriched austenite can be stabilized to the austempering temperature.Specifically,carbide precipitation was not detected in ghost pearlite without Si and Al addition.This is mainly due to the strong interaction between carbon and manganese elements,which drives the diffusion of C from the carbon-depleted region to the manganese-enriched region.Moreover,the nano-scale lamellar structure of ghost pearlite reduced the diffusion distance,resulting in an increased C content in the manganese-enriched austenite,and further inhibited the bainite transformation and carbide precipitation.As a result,the PB samples significantly improve ductility than CB samples under the same austempering condition,with a total elongation of 11.5%,uniform elongation of 7.6%.Meanwhile,PB samples keep a constant strength with CB samples,with an ultimate tensile strength of 1043MPa.