THICK, HIGH TENSILE-STRENGTH HOT-ROLLED STEEL SHEETS WITH EXCELLENT LOW TEMPERATURE TOUGHNESS AND MANUFACTURING METHOD THEREFOR

摘要

Provided are thick, high tensile-strength hot-rolled steel sheets that have high strengths of TS: 521 MPa or higher and excellent low temperature toughness. Specifically, steel raw material of a composition that comprises C: 0.02-0.08%, Nb: 0.01-0.10% and Ti: 0.001-0.05%, and such that C, Ti and Nb satisfy (Ti + (Nb/2))/C < 4, is heated, and after hot-rolling that comprises roughing-down and finish rolling, cooling of 10°C/s by mean cooling rate at the center of the sheet thickness is performed to below a specified end cooling temperature that is dependent on the amounts of alloy elements and the cooling rate. Then the steel is taken up at below a specified take-up temperature that is dependent on the amounts of alloy elements. It thereby becomes a thick hot-rolled steel sheet of excellent structural uniformity in the thickness direction that has a structure wherein the difference DD in mean crystal particle diameters of the main ferrite phases between a position that is 1 mm in the thickness direction from the surface of the steel sheet and at the center of the steel sheet thickness is less than 2 µm, and the difference DV in proportion (volume%) of secondary phase structures is less than 2%. Thereby, low temperature toughness, particularly DWTT characteristics and CTOD characteristics, that are toughness tests over the entire thickness, are markedly improved.