Thick steel plate and method for manufacturing the same

Abstract

To provide a thick steel plate excellent in tensile strength in a plate thickness direction and having a plate thickness of 130 mm or less and a producing method thereof.SOLUTION: A plate thickness is 130 mm or less, and the chemical components are C, Si, Mn, P, S, Al, Cr, Mo, Nb, and N in a predetermined range, and the metallographic structure is any single phase structure of bainite, tempered bainite, tempered martensite, or a mixed structure made of any two or more among bainite, tempered bainite and tempered martensite, a major axis of each of the MnS inclusions and the NbCN inclusions is 10 μm or less, an aperture of the plate thickness direction tensile test is 35% or more on average of three tested, and a minimum value of the aperture of the three plate thickness directional tensile test is 25% or more.SELECTED DRAWING: None

Claims

1. The plate thickness is 130 mm or less. The chemical composition is expressed in mass percent. C: 0.02-0.13%, Si: 0.02-0.50%, Mn: 0.30-0.70%, P: 0.030% or less, S: 0.006% or less, Al: 0.060% or less, Cr: 8.0-14.0%, Mo: 0.03-2.20%, Nb: 0.01 to 0.10%, N: 0.003 to 0.080%, It contains, moreover, Cu: 0.50% or less, Ni: 0.40% or less, V: 0.40% or less, Ti: 0.15% or less, B: 0.006% or less, Ca: 0.0050% or less, Mg: 0.0050% or less, W: 2.0% or less, It contains one or more selected from the following: The remainder consists of Fe and unavoidable impurities. The metallic structure is either a single-phase structure of bainite, tempered bainite, or tempered martensite, or a mixed structure consisting of two or more of bainite, tempered bainite, and tempered martensite. The major axis of each of the MnS-based inclusions and NbCN-based inclusions is 10 μm or less. A thick steel plate in which the reduction of area in a thickness-direction tensile test is 35% or more on average over three tests, and the lowest reduction of area among the three thickness-direction tensile tests is 25% or more.

2. A continuous cast slab with a thickness of 195 mm or more having the chemical composition described in claim 1 is heated to 1050 to 1300°C. Hot forging is performed with a hot forging temperature of 900°C or higher, a reduction ratio of 5% or higher per pass, and a total reduction ratio of 10% or higher in the thickness direction. Cooling is performed, and after cooling, it is reheated and hot-rolled to a plate thickness of 130 mm or less. Cooling is performed, and after cooling, the product is baked at a temperature of 900 to 1100°C. The metallic structure is either a single-phase structure of bainite, tempered bainite, or tempered martensite, or a mixed structure consisting of two or more of bainite, tempered bainite, and tempered martensite. The major axis of each of the MnS-based inclusions and NbCN-based inclusions is 10 μm or less. The reduction of area in the thickness direction tensile test is 35% or more on average over three tests, and the lowest reduction of area among the three thickness direction tensile tests is 25% or more. A method for manufacturing thick steel plates with a thickness of 130 mm or less.

3. A method for manufacturing a thick steel plate according to claim 2, wherein after the normalizing process, cooling is performed, and after the cooling, tempering is performed at a temperature of 700 to 850°C.

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