Effect of nb on hot rolled high strength steel sheets produced by thin slab casting and hot direct rolling process

In order to meet to the demands from automobile industry such as weight reduction and safety improvement, the hot rolled high strength steel sheets have been used mainly for suspension parts. While, the steel production by thin slab casting (TSC) and hot direct rolling (HDR) process has been noticed. In this study, the characteristics of the high strength hot rolled steels produced by TSC-HDR were discussed comparing with the conventional process and concluded as follows; (1) The effect of difference of production route on the relationship between tensile strength and elongation or tensile strength and stretch flangeability was small. (2) Strength obtained by TSC-HDR route was higher than those of conventional production route in No bearing steel. This means Nb effectively works as a strengthening element in TSC-HDR route. (3) The addition of Nb and Mn was effective on the improvement of tensile strength and stretch flangeabilty relationship. Especially when the stretch flangeabilty was compared in the same TS level, Nb bearing steels exhibited more than two times of the value of plain C-Mn steel.