2009 Volume 49 Issue 1 Pages 109-114
High strength cold-rolled bake hardenable steel sheets for automotive use have been developed. In the last decade, the major strengthening methods of extra low carbon (ELC) steel and interstitial free (IF) steel were the solid-solution strengthening with silicon, manganese and phosphorous and the precipitation strengthening with microalloying elements, such as niobium and titanium. When the steels are strengthened with the high amount of solid-solution elements such as silicon and manganese, it shows the low surface quality due to surface oxidation. In addition, phosphorous added ELC and IF steels become susceptible to the secondary work embrittlement because of the lack of grain boundary strength, which is the essential drawback of the steels.
In this paper, we propose the extra low carbon bake hardenable steel which is precipitation strengthened by means of the fine distribution of Copper sulfide (Cu2S) precipitates instead of solid-solution strengthening and precipitation strengthening using silicon, manganese, niobium and titanium. These Cu sulfides are oriented to the body-centered cubic α-Fe matrix in (001)Cu2S//(001)α-Fe and [001]Cu2S//[001]α-Fe relationships.
