Effects of Ultra-Fast Cooling After Hot Rolling and Lamellarizing on Microstructure and Cryogenic Toughness of 5% Ni Steel

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 degrees C. is 185 J). The ductile brittle transition temperature of 5% Ni steel treated by QT and UFC-LT heat treatments were -152 degrees C. and lower than -196 degrees C.,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.