Hot rolled steel sheets having tensile strength of about 300MPa with excellent press formability have been developed by using RH-degassing process and hot rolling technology. The basic chemical composition of new steel is chosen as extra low carbon, low manganese and low nitrogen to improve ductility. In addition, controlled cooling on runout table or micro-alloying of niobium is applied to prevent the grain coarsening, which is caused by leaner compositions.
These steel sheets have high elongation and excellent press formability. Furthermore, these steel sheets exhibit a high resistance to cold-work embrittlement. These properties are compared with those of DQ grade mild steel sheets and interstitial free steel sheets.
In this paper, the metallurgical basis of these new steel sheets and their characteristics are described.