Hot-rolled steel sheet and method for manufacturing the same

Abstract

The present invention aims to provide a hot-rolled steel sheet having a tensile strength of 1180 MPa or more and excellent bendability, as well as a method for manufacturing the same. Having a specific component composition, at the 1 / 4 thickness position, martensite accounts for 90% or more by area ratio, and in the surface layer of the steel sheet located in the range of 50 to 150 μm from the surface to the thickness depth direction, upper bainite accounts for 80% or more by area ratio, and martensite accounts for 0 to 20% by area ratio. A hot-rolled steel sheet in which the R / t, obtained by dividing the limit bending radius R (where no cracking occurs in a bending test) by the sheet thickness t, is 2.5 or less.

Claims

1. In mass percent, C: 0.03-0.20%, Si: 0.1-2.0%, Mn: 0.5-3.5%, Ti: 0.005% or more and less than 0.205% P: 0.100% or less, S: 0.02% or less, The composition has less than 1.55% Al, with the remainder being Fe and unavoidable impurities. The metallic structure shows that at the 1 / 4 thickness position, martensite accounts for 90% or more of the area. In the surface layer of the steel plate located in the range of 50 to 150 μm from the surface of the steel plate in the thickness direction, upper bainite accounts for 80% or more of the area, and martensite accounts for 0 to 20% of the area. A hot-rolled steel sheet in which the R / t, obtained by dividing the limit bending radius R (where no cracking occurs in a bending test) by the sheet thickness t, is 2.5 or less.

2. The aforementioned component composition is further expressed in mass%, Cr: 0.05% or more and less than 2.05% Mo: 0.05% or more and less than 2.05%, V: 0.05% or more and less than 1.05%, Cu: 0.05% or more and less than 4.05% Ni: 0.005% or more and less than 2.050%, Nb: 0.005% or more and less than 0.205% B: 0.0003 to 0.0050%, Ca: 0.0001-0.0050%, REM: 0.0001-0.0050%, Sb: 0.0010% or more and less than 0.1050% The hot-rolled steel sheet according to claim 1, comprising one or more types of Sn selected from 0.0010% to less than 0.5050%.

3. A method for manufacturing a hot-rolled steel sheet according to Claim 1 or 2, wherein a slab having the above-mentioned component composition is heated, rough-rolled, and then, in a finish-rolling process, the amount of reduction for each rolling pass is set such that the sum of the reduction strains Sr ΣSr of the rolling passes performed at an entry temperature Tn satisfying formula (1) is between 1.0 and 3.0, and the rolling is performed accordingly. A method for manufacturing a hot-rolled steel sheet, wherein the time from the end of finish rolling to the start of cooling is 10.0 s or less, and the temperature range from the cooling start temperature after the end of finish rolling to 300°C is cooled under conditions where the average cooling rate is 50°C / s or more, and the coiling temperature is 300°C or less. Tn≦74.35×t －０．６８５ -5500;0.1-(Ti+Nb)}+950・・・(1) Here, t is the time it takes for the slab to pass from the current finishing rolling stand to the next stand (or the time until cooling begins in the case of the final pass). Also, each element symbol in equation (1) represents the mass %) of each element in the slab, and 0 is used if the element is not present. The reduction strain Sr is defined by the following equation (2). Sr=ln(WTe / WTd)...(2) Here, WTe is the thickness of the entry side plate of the path (mm), and WTd is the thickness of the exit side plate of the path (mm).

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