Study On Super-Cooled Austenite Low Carbon Steel By Twin-Bollstrip Casting

Strip casting processing is praised as the greatest progress that wasachieved in 20 century in metallurgy, in which continuous casting androlling is connected, the strip also can be cast forming without onlinerolling. Twin-Roll Strip Casting processing is the rapid solidificationprocess, with short coagulation time, high cooling speed, small secondarydendrite arm spacing, and large austenite grain size, which make thistechnology a large development in high strength micro-alloy product.The microstructure and precipitates of low-carbon as-cast strip(referred to LC strip) and Nb micro-alloyed low-carbon as-cast strip(referred to Nb-LC strip) are studied in this paper. The process ofsuper-cooled austenite transformation during the continuous coolingprocess of LC strip and Nb-LC strip is conducted with a laser scanningconfocal microscope (LSCM), in order to research on the effect ofcooling rate, austenite grain size and niobium element on super-cooled austenite. During the process of bainitic ferrite nucleation and growth,gradual change in growth direction is confirmed both by observation andelectron backscattering diffraction (EBSD) analysis.The solidification structure of strip is composed of columnar zonesextending from the surface of strip and a equiaxed zone in the center ofthe strip. The primary dendrite spacing is about 16-20μm and secondarydendrite arm spacing is 12-14μm. It is shown the average size of originalaustenite grains of the as-cast strip are about 100-200μm in widths and200-350μm in lengths. The final micro-structure of LC strip is composedof polygonal ferrite, pearlite and bainite. while, the final micro-structureof Nb-LC strip is composed of bainite and polygonal ferrite. There arelarge of 0°~15 low-angle grain boundaries in Nb-LC strip, above 84%,while low-angle grain boundaries in LC strip is about 70%. The strip isisotropic, without obvious texture.Bainite phase transformation temperature is from about 700℃to590℃in LC strip, and it's from about 650℃in Nb-LC strip. So, Nbreduces the phase transition starting temperature, restrains the diffusiontransformation, such as the pearlite, and promotes formation of bainite.It is found that the higher cooling rate, or bigger austenite grain size,or more Nb in sample steel, the more disequilibrium final microstruture inthe as-cast strip, this three parameters affect super-cooled austenite phasetransformation comprehensively.