METHOD FOR MANUFACTURING HIGH STRENGTH STEEL SHEET AND SHEET OBTAINED

To provide a method for manufacturing a steel sheet having a yield strength of more than 1,000 MPa, a tensile strength of more than 1,150 MPa and a total elongation of more than 8%.SOLUTION: A method comprises a step of preparing a steel sheet through rolling from a steel containing by wt.% of 0.19-0.22% C, 2-2.6% Mn, 1.45-1.55% Si, 0.15-0.4% Cr, less than 0.020% P, less than 0.05% S, less than 0.008% N, 0.015-0.070% Al, and the balance Fe with impurities, then uniformly heating the steel sheet at 860-890°C for 100-210 seconds, cooling the steel sheet to 220-330°C from 500°C or more at a cooling rate of 15°C/s or more, heating the steel sheet to a first temperature more than 380°C for 115-240 seconds, heating the steel sheet to a second temperature of 420-450°C for 300-610 seconds, and cooling the steel sheet to a temperature less than 100°C at a cooling rate less than 5°C/s. The structure of the steel contains more than 80% of tempered martensite, more than 5% of retained austenite, less than 5% of ferrite, less than 5% of bainite and less than 6% of fresh martensite.SELECTED DRAWING: None 【課題】降伏強度１０００ＭＰａ超、引張強度１１５０ＭＰａ超、全伸び８％超の鋼板の製造方法の提供。【解決手段】重量％で０．１９〜０．２２％のＣ、２〜２．６％のＭｎ、１．４５〜１．５５％のＳｉ、０．１５〜０．４％のＣｒ、０．０２０％未満のＰ、０．０５％未満のＳ、０．００８％未満のＮ、０．０１５〜０．０７０％のＡｌ、残部がＦｅと不純物の鋼を鋼板圧延後、８６０〜８９０℃で１００〜２１０秒均熱化、５００℃以上から１５℃／秒以上で２２０〜３３０℃まで冷却、１１５〜２４０秒かけて３８０℃超の第１の温度まで加熱、次いで３００〜６１０秒かけて４２０〜４５０℃の間の第２の温度まで加熱、５℃／秒未満で１００℃未満まで冷却する工程を含み、鋼の組織は８０％超の焼き戻しマルテンサイト、５％超の残留オーステナイト、５％未満のフェライト、５％未満のベイナイトおよび６％未満のフレッシュマルテンサイトを含有する方法である。【選択図】なし