Development of a New 450 N/mm2 Grade Ultra-Low Carbon Sheet Steel for Automotive Panels

For the automotive exposed panels, several types of 350N/mm² grade bake-hardenable sheet steel have been developed and actually applied. However for further weight reduction of automotive body panels, especially inner panels, a 450N/mm² grade sheet steel with excellent formability has been required. For this demand a new 450N/mm² grade sheet steel has been developed. As the result it was found, that by the co-addition of Mn and P to ultra-low carbon interstitial free steel the recrystallization texture favorable for deep drawability can be formed, accompanied with the increase in tensile strength, when hot band coiled temperature is lower than 773K.

In order to improve the property of the 450N/mm² grade steel, the effect of Si content has been studied. It was found that the deep drawability is not deteriorated by the addition of Si into the Mn and P co-added ultra-low carbon IF-steel. This steel is produced through both continuous annealing and continuous galvannealing processes and exhibits a lower yield strength and better deep drawability comparing with conventional low carbon high strength steels. The new product is going to be applied for automotive unexposed panels and members.