HIGH-STRENGTH STEEL PLATE AND PROCESS FOR PRODUCTION THEREOF, AND HIGH-STRENGTH STEEL PIPE

摘要

The invention provides a high-strength steel plate which exhibits excellent cutting-crack resistance, Charpy absorption energy and DWTT characteristics, a low yield ratio and a tensile strength of 900 MPa or above; a process for the production thereof; and high-strength steel pipes made by using the same. The high-strength steel plate is one which contains by mass C: 0.03 to 0.12%, Si: 0.01 to 0.5%, Mn: 1.5 to 3%, Al: 0.01 to 0.08%, Nb: 0.01 to 0.08%, Ti: 0.005 to 0.025%, N: 0.001 to 0.01%, and at least one of Cu: 0.01 to 2%, Ni: 0.01 to 3%, Cr: 0.01 to 1%, Mo: 0.01 to 1%, and V: 0.01 to 0.1% and satisfies the following relationship (1) as to the contents of Ca, O and S, whose microstructure is composed of ferrite and a hard second phase at an area fraction of ferrite of 10 to 50% with the cementite contained in the hard second phase having a mean particle diameter of 0.5µm or below, and in which the content of Nb contained as carbide in the steel is at most 10% based on the whole Nb content of the steel. 1 < (1 - 130 × [O]) × [Ca]/(1.25 × [S]) < 3 ... (1)