Effect of Final Cooling Temperature on Microstructure and Mechanical Properties of a Cr-Ni-Mo-V Bainite Steel
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JMEPEG https://doi.org/10.1007/s11665-018-3288-8
Effect of Final Cooling Temperature on Microstructure and Mechanical Properties of a Cr-Ni-Mo-V Bainite Steel Jian Zhang, Chang-Sheng Li, Bin-Zhou Li, Zhen-Xing Li, and Qi-Wen Wang (Submitted September 26, 2016; in revised form February 3, 2018) The microstructural evolution and mechanical properties of a low-carbon high-strength bainitic steel under different final cooling temperatures were studied. The microstructures of the experimental steel at different final cooling temperatures were composed of acicular ferrite and granular bainite. A decrease in final cooling temperature resulted in an increase in volume fraction of granular bainite and a decrease in volume fraction of acicular ferrite. The specimen with the lowest final cooling temperature (459 °C) exhibited the highest yield strength, tensile strength, and yield ratio, and a lower total elongation than the specimens with the highest final cooling temperatures (501 and 535 °C) because of the higher volume fraction of granular bainite and finer bainitic ferrite lath. The specimen with the lowest final cooling temperature had a lower absorbed energy than that of the highest final cooling temperature, because the strip-like martensite– austenite (M-A) constituents that existed between the bainitic ferrite or in prior austenite grain boundaries reduced its toughness. At an intermediate final cooling temperature of 501 °C, the experimental steel exhibited excellent mechanical properties with a yield strength, tensile strength, and absorbed energy of 825 MPa, 1232 MPa, and 102 J, respectively. Keywords
bainitic microstructure, Cr-Ni-Mo-V steel, final cooling temperature, mechanical property, thermomechanical control process
1. Introduction Advanced structural steels with a high strength and good toughness often require a bainitic microstructure (Ref 1). To ensure the perfect combination of strength and toughness, the general method is to add alloying elements such as chromium, niobium, and molybdenum into the steel to increase the hardenability to obtain a bainitic microstructure. However, this addition can lead to a decrease in ductility and toughness because of the formation of a hard brittle phase (Ref 2-5). A thermomechanical control process (TMCP) has been proposed for the perfect combination of a high strength and toughness by promoting the formation of a fine bainitic microstructure (Ref 6). In recent years, the TMCP has been combined with accelerated cooling as an effective method for microstructure controlling to produce high-strength and tough bainitic steel plates (Ref 7, 8). The main TMCP parameters, which include a final rolling temperature, cooling rate, and final cooling temperature, provide the final mechanical properties of the products (Ref 9-12). Recent studies have shown that the final cooling temperature affects the morphology of the bainitic microstructure significantly (Ref 13). As the final cooling temperature decreases, the bainitic ferrite substructure changes,
Jian Zhang, State Key Laboratory of Rolli
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