采用“控制轧制、轧后超快冷、亚温淬火+回火(UFC-LT)”工艺制备了5%Ni钢,系统研究了这一工艺条件下5%Ni钢的微观组织与力学性能,并与常规调质工艺(QT)和离线淬火、亚温淬火加回火工艺(QLT)进行了对比。结果表明,经UFC-LT工艺处理后,5%Ni钢的显微组织由回火马氏体、临界铁素体和5.83%的逆转奥氏体组成,并可以获得优于QT和QLT工艺的强韧性匹配(抗拉强度为608 MPa,屈服强度为491 MPa,-196 ℃时Charpy冲击功为185 J),韧脆转变温度由QT工艺的-152 ℃下降到-196 ℃以下。与QT工艺相比,UFC-LT工艺改善韧性的因素主要有渗碳体的溶解、高密度的大角度晶界及5.83%的逆转奥氏体。
In recent years, the demands for liquefied ethylene gas (LEG) are rapidly increased in China. 5%Ni steel is being widely used to build LEG tanks, due to the excellent toughness, high strength and ductility of the material. Along with the continuous increase in the size of LEG tanks, higher cryogenic toughness has been required for new generation 5%Ni steel. In this work, controlled rolling (CR) has been developed in the aim of microstructure refinement for Ni-containing steels, and ultra-fast cooling (UFC) after hot rolling has been successfully applied to replace on-line direct quenching, which formed the integrated CR-UFC for 5%Ni steel. A new processing technologies, named UFC-LT treatment which consisted of CR-UFC, lamellarizing and tempering has been developed for 5%Ni steel in this work. The microstructure and mechanical properties of 5%Ni steel treated by UFC-LT were investigated, as well as quenching and tempering (QT), quenching, lamellarizing and tempering (QLT) treatments. The results show that the microstructure of 5%Ni steel treated by UFC-LT treatment consisted of tempered martensite, intercritical ferrite and about 5.83% reversed austenite. The reversed austenite has two types of morphologies: one type is acicular reversed austenite which forms along the lath boundaries; another type is block reversed austenite which mainly forms at prior austenite grain boundaries. An optimum combination of strength and toughness were obtained by UFC-LT treatment (ultimate tensile strength is 608 MPa, yield strength is 491 MPa, elongation is 34%, Charpy impact energy at -196 ℃ is 185 J). The ductile-brittle transition temperature of 5%Ni steel treated by QT and UFC-LT heat treatments were -152 ℃ and lower than -196 ℃, respectively. The superior cryogenic toughness compared to QT treatment contributed to the dissolution of cementite, high percentage of large angle grain boundaries and the formation of 5.83% reversed austenite.