Development of IF High Strength Steel with Fine Grain Structure for Exposure Panel

New type of IF high strength steel (HSS) has been developed by hybridizing the grain refinement and the supplemental solid-solution hardening. Grain refinement was achieved by the fine distribution of carbide under the appropriate combination of the higher carbon content near 60ppm and niobium. 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 formability. While this steel has the fine grain structure, yield ratio is decreased due to the unique carbide distribution, and r-value is improved by the favorable texture formation. As the result, it has been clarified that a new type of 440MPa grade grain-refined IF-HSS improved the press-formability and exhibited a superior secondary work-embrittlement to the conventional IF-HSS.