2001 Volume 41 Issue 11 Pages 1402-1410
High strength cold-rolled steel sheets (HSS) with sufficient formability have been developed for the IF steel-bases in the last decade, in which the major strengthening method was solid-solution hardening with silicon, manganese and phosphorous. When the IF steel is strengthened with the high amount of solid-solution elements, it becomes susceptible to the secondary work embrittlement because of the lack of grain boundary strength, which is the essential drawback of interstitial free steel. Although the grain refinement is an effective method to improve the toughness of steel, this method has not been taken into consideration in view of press-formability because it leads the steel to higher yield ratio, lower n-value and lower r-value.
A new type of IF-HSS was strengthened by hybridizing the grain refinement and the supplemental solidsolution hardening. The grain refinement was achieved by means of the fine distribution of carbide under the appropriate combination of the relatively higher carbon content near 60 ppm with a suitable carbideforming element. While this steel has the fine grain structure, yield strength hardly increases due to the formation of unique microstructure containing PFZ, and the γ fiber texture sufficiently develops. As the result, a new type of grain-refined IF-HSS has been successfully developed to reach a higher r-value and a superior secondary work embrittlement as compared with the conventional IF-HSS.
