HIGH-STRENGTH STEEL SHEETS AND PROCESSES FOR PRODUCTION OF THE SAME

摘要

The invention provides high-strength steel sheets having excellent elongation and excellent stretch flangeability. The invention relates to a high-strength steel sheet which contains by mass C: 0.05 to 0.3%, Si: 0.01 to 3.0%, Mn: 0.5 to 3.0% and Al: 0.01 to 0.1% with the balance consisting of Fe and unavoidable impurities and has a structure composed mainly of tempered martensite and annealed bainite. The space factors of tempered martensite and annealed bainite are 50 to 95% and 5 to 30% respectively, while the mean grain size of the tempered martensite is 10µm or below in term of circle-equivalent diameter and the tensile strength is 590MPa or above. The invention also relates to a high-strength steel sheet having a metal structure which comprises martensite phases as the main component at a space factor of 80% or above and in which the mean grain size of martensite phases is 10µm or below in terms of circle-equivalent diameter and the space factor of martensite phases having grain sizes of 10µm or above in terms of circle-equivalent diameter is 15% or below, with the space factor of retained austenite phases in the metal structure being 3% or below. The invention also relates to a high-strength steel sheet which has a dual-phase structure comprising ferrite phases and martensite phases as the main components at space factors of 5 to 30% and 50 to 95% respectively and in which the ferrite phases are made of annealed martensite.