Low-Carbon Bainite Steel with High Strength and Toughness Processed by Recrystallization Controlled Rolling and Ultra Fast Cooling (RCR+UFC)

Abstract

Effects of temperature, strain, strain rate and cooling rate on austenite grain size were investigated at first. The results show that by increasing the strain from 0.0 to 0.5 can significantly refine austenite grains for different deformation temperatures studied. However, by increasing the strain from 0.5 to 0.8 can not continue to refine austenite grains for higher deformation temperatures of 1100 and 1150°C, while austenite grains can be further refined for lower deformation temperature of 1000°C. The austenite grain size is proportional to ( is strain rate and p is the strain rate exponent) and v^–q (v is cooling rate and q is cooling rate exponent). Moreover, using ultra fast cooling after hot rolling can significantly refine austenite grains. Then a low-carbon bainite steel with yield strength of 811 MPa and ductile-brittle transition temperature (DBTT) of –49°C produced by optimum recrystallization controlled rolling designed based on thermosimulation results and ultra fast cooling. The microstructural characteristics, mechanical properties and the mechanism of toughening were investigated in details. On the one hand, fine austenite grains with equivalent diameter of 16.5 μm is obtained by recrystallization repeated and suppressing recrystallized austenite grain growth using ultra fast cooling. On the other hand, recrystallized austenite grains are divided by fine bainite blocks with higher misorientation between each other. So the fine bainite microstructure is obtained. In addition, Prior austenite grain boundaries and bainite packet and block boundaries with higher misorientation can effectively arrest cracks propagation, resulting in better low-temperature toughness.