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 1 100 and 1 150°C, while austenite grains can be further refined for lower deformation temperature of 1 000°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.

High-Precision Header Flux Control of Ultra-Fast Cooling in Hot Strip Mill By PI Controlle

“…As shown in Fig.4, the A-E curve was obtained by using the A-E equation, which was provided by equation (2). The A-E of the adjusting valve has a nonlinear relationship with the opening rate of the adjusting valve.…”

“…The thermo-metallurgical phase transformations of the steel take place during the cooling process on the run-out table (ROT). The microstructure and mechanical properties of steel are directly related to the cooling process [1][2][3][4]. Ultra-fast cooling(UFC) can provide flexible cooling paths in order to achieve the desirable phase transformations, which makes it an appealing selection for the cooling line after rolling [5,6].…”

Microstructural Characteristics with Various Finish Rolling Temperature and Low Temperature Toughness in Hot Rolled Nb–Ti Ferritic Steel

“…5,18) Besides finish rolling temperature, cooling rate after hot rolling also significantly influences grain size and precipitation behavior, and ultra fast cooling (UFC) technology is used to obtained higher strength steel. 19,20) Compared with the normal cooling after hot rolling, UFC can effectively suppress the growth of austenite grain and refine the size of ferrite grain and precipitation. Tang et al 20) reported that the yield strength of hot rolled microalloyed steels can be increased by more than 100 MPa by using UFC over conventional accelerated cooling, which is associated with grain refinement and precipitation.…”

Tailoring Strength and Ductility of an Fe–Mn–Al–C Low‐Density Duplex Steel by Controlling the Cooling Path after Hot Rolling

“…Extensive efforts have been made on optimizing the microstructure and mechanical properties of alloy steels by controlling cooling. [17][18][19] However, the studies on controlling cooling after hot rolling of duplex low-density steels are still rarely reported. In the present work, different cooling rates were adopted in a low-density duplex steel after hot rolling for investigating the relevant influencing mechanism.…”