Thick, high tensile-strength hot-rolled steel sheets with excellent low temperature toughness and manufacturing method therefor

Abstract

There is provided a thick-walled high-strength hot rolled steel sheet having a high tensile strength TS of 521 MPa or more and excellent low-temperature toughness. Specifically, a steel material containing 0.02%-0.08% C, 0.01%-0.10% Nb, and 0.001%-0.05% Ti is heated, C, Ti, and Nb satisfying (Ti + (Nb / 2)) / C < 4. After hot rolling including rough rolling and finish rolling is performed, cooling is performed at an average cooling rate of 10 °C / s or more at a middle position of the steel sheet in the thickness direction to a specific cooling stop temperature or lower, the cooling stop temperature being dependent on the amounts of alloy elements and the cooling rate. Then coiling is performed at a specific coiling temperature or lower, the coiling temperature being dependent on the amounts of alloy elements, thereby producing the thick-walled hot rolled steel sheet having excellent uniformity of a microstructure in the thickness direction and having the microstructure in which the difference ”D between the average grain size of a ferrite phase serving as a main phase at a position 1 mm from a surface of the steel sheet in the thickness direction and the average grain size of the ferrite phase at a middle position of the steel sheet in the thickness direction is 2 µm or less and in which the difference AV between the fraction (percent by volume) of a second phase at the position 1 mm from the surface of the steel sheet in the thickness direction and the fraction (percent by volume) of the second phase at the middle position of the steel sheet in the thickness direction is 2% or less. This results in significant improvement in low-temperature toughness. In particular, DWTT characteristics and CTOD characteristics are significantly improved, in which DWTT and CTOD are tests using full-thickness test specimens.

Description

technical field

[0001] The present invention relates to a line pipe suitable for transporting crude oil, natural gas, etc., and suitable as a raw material for a high strength electric resistance welded steel pipe or a high strength spiral steel pipe requiring high toughness Thick-walled high-strength hot-rolled steel sheet (thick-walled high-strength hot-rolled steel sheet) and its manufacturing method, particularly relating to improvement of low-temperature toughness. In addition, a "steel sheet" includes a steel plate and a steel strip. In addition, the "high-strength hot-rolled steel sheet" referred to here refers to a high-strength hot-rolled steel sheet (hot rolled steel sheet) having a tensile strength (tensile strength) TS of 510 MPa or more. In addition, a "thick-walled" steel sheet is Refers to steel plates with a thickness of 11 mm or more. Background technique

[0002] In recent years, due to the high price of crude oil since the oil crisis and the diversificati...

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