Patent ID: 8758528
Filing Date: 2014-06-24
Classification: C21D,C22C

Abstract:
1. A high-strength steel plate comprising a steel having the following components: by % by mass, 0.03 to 0.12% of C, 0.01 to 0.5% of Si, 1.5 to 3% of Mn, 0.01 to 0.08% of Al, 0.01 to 0.08% of Nb, 0.005 to 0.025% of Ti, 0.001 to 0.01% of N, 0.003% or less of O, 0.001% or less of S, and 0.0005 to 0.01% of Ca; and at least one component of 0.01 to 2% of Cu, 0.01 to 3% of Ni, 0.01 to 1% of Cr, 0.01 to 1% of Mo, and 0.01 to 0.1% of V; wherein the contents of Ca, O, and S satisfy the equation (1) below, and the balance is composed of Fe and inevitable impurities; 1≦(1−130×[O])×[Ca]/(1.25×[S])≦3 . . . (1) wherein [O], [Ca], [S] are the contents (% by mass) of the respective elements in steel; and having a tensile strength of at least 900 MPa, an impact energy by a Charpy V notch test at −30° C. of 200 J or more, and a shear area by a DWTT test at −30° C. of 75% or more, and the steel plate further contains a microstructure in which: the area fraction of any one of ferrite+bainite, ferrite+martensite, and ferrite+bainite+martensite is 90% or more; the area fraction of ferrite is 10 to 30%; cementite in bainite and/or martensite has an average grin size of 0.5 μm or less; and the total amount of Nb, Ti, Mo, and V contained in a single carbide containing at least one of Nb, Ti, Mo, and V present in steel or a composite carbide containing two or more of these elements is 10% or less of the total of Nb, Ti, Mo, and V contained in steel.