Cooling process and mechanical properties design of hot-rolled low carbon high strength microalloyed steel for automotive wheel usage

For the purpose of developing Nb-V-Ti microalloyed, hot rolled, high strength automotive steel for usage in heavy-duty truck wheel-discs and wheel-rims, appropriate cooling processes were designed, and microstructures and comprehensive mechanical properties (tension, bending, hole-expansion, and Charpy impact) of the tested steels at two cooling schedules were studied. The results indicate that the steel consists of 90% 5 mu m polygonal ferrite and 10% pearlite when subjected to a cooling rate of 13 degrees C/s and a coiling temperature of 650 degrees C. The yield strength, tensile strength, and hole-expansion ratio are 570 MPa, 615 MPa, and 95%, respectively, which meet the requirements of the wheel-disc application. The steel consists of 20% 3 mu m polygonal ferrite and 80% bainite (granular bainite and a small amount of acicular ferrite) when subjected to a cooling rate of 30 degrees C/s and a coiling temperature of 430 degrees C. The yield strength, tensile strength, and hole-expansion ratio are 600 MPa, 655 MPa, and 66%, respectively, which meet the requirements of the wheel-rim application. Both the ferrite-pearlite steel and ferrite-bainite steel possess excellent bendability and Charpy impact property. The precipitation behavior and dislocation pattern are characterized and discussed. (C) 2013 Elsevier Ltd. All rights reserved.