Effects of Coiling Temperature on Microstructure and Mechanical Properties of High-strength Hot-rolled Steel Plates Containing Cu, Cr and Ni.

In this study, four kinds of high-strength hot-rolled steel plates were fabricated with additions of tramp elements of Cr and Ni to a basic C–Mn–Si–Cu steel, and their microstructures and mechanical properties were investigated to analyze the effects of coiling temperature and tramp elements. When the steels werecoiled at 400°C, their volume fraction of retained austenite was lower than the case of coiling at 450°C. The steels containing Cr individually or together with Ni had the volume fraction of retained austenite above 10%, but showed the characteristics of dual phase steels having high tensile strength and low elongation as a large amount of austenite was transformed to martensite during cooling. In the case of coiling at 450°C, both elongation and strength–elongation balance were increased owing to the increase in the fraction of retained austenite and the transformation induced plasticity (TRIP) effect. The addition of Ni resulted in a great increase in the fraction of retained austenite at the higher coiling temperature, thereby improving tensile strength and elongation.