40Cr钢方坯表面淬火工艺及组织转变规律

热送裂纹限制了热送技术的推广应用,而表面淬火技术可以有效改善热送裂纹的质量问题.以某钢厂180 mm×180mm断面小方坯连铸机为对象,40Cr钢为淬火试验钢种,通过开展现场离线淬火试验研究,并利用光学显微镜(OM)及数值仿真技术,进行显微组织分析与淬火-自回火过程传热分析,研究了表面淬火对铸坯表层组织转变的影响规律....

Surface quenching technology and microstructure evolution law of 40Cr steel billet

Surface crack,which generates in the process of hot charging,restricts the application of hot charging,and surface quenching technology can effectively improve this problem. This paper takes a 180 mm×180 mm cross-section billet caster as the object,and selects 40Cr steel as quenching experiment steel to carry out off-line quenching experiment research,combined with analysis of microstructure and heat transfer of quenching and self-tempering process by means of optical microscope and numerical simulation technique,the influence of quenching on the surface microstructure evolution of the billet was studied. The results showed that surface quenching can effectively eliminate the film-like ferrite,and form a certain hardening depth. In the quenching time range of 40-100 s,a tempered sorbite layer with the depth of 2-10 mm can form at the surface layer,and film-like ferrite begins to form at the depth of 19-38 mm from the surface. At the same time,with the extension of quenching time,the quenching depth increases significantly. Based on the purpose of quenching,it is proposed to use the depth at which film-like ferrite begins to form as the basis for judging the quenching depth of casting billet after quenching. Combined with heat transfer analysis and types and characteristics of the microstructure after quenching,two online quenching process schemes were proposed. The expected surface microstructure after quenching is refined ferrite with pearlite(weak cooling scheme) and tempered sorbite(strong cooling scheme) respectively. For the weak cooling scheme,the expected hardening depth is 7-13 mm,for the strong cooling scheme,the expected hardening depth can reach 2-10 mm(taking tempered sorbite as the criterion for determining the hardening depth) or 19-38 mm (taking film-like ferrite as the criterion for determining the hardening depth). Based on the off-line quenching experiment combined with heat transfer analysis in this paper,the determined on-line quenching process schemes can make a certain theoretical foundation for the implementation of surface quenching technology at the plant.