Development of high-strength hot-rolled sheet products for automobile application

The requirement to the steel industry for the supply of high-strength sheet products seems to be accelerated to reduce the car weight from the point of saving energy. In this case stretch-flangebility is the most important parameter in order to meet the above requirement. At present the focuses of development are wheel discs, members and arms. The authors have been developing much more formable high strength sheet products with a high stretch-flangebility. Metallurgical view points for these products were to reduce the amount of carbide particles and the hard second phase and to restrict the formation of the band structure. Those were achieved both by adjusting the chemical composition and by applying thermomechanical treatment through hot rolling. Basically that is the combination technology with higher finishing rolling temperature control and lower coiling temperature control. Formability could be further improved by increasing silicon content if the surface defect was solved, namely due to scale. Now high strength hot-rolled sheet products over 540 N/mm2 grade have been commercially accepted by automotive industry.