Steel plate and method for manufacturing the same

Abstract

To provide a steel plate excellent in intrinsic and mechanical properties and a producing method thereof.SOLUTION: A steel plate characterized by having a component composition composed of at mass%, C: 0.03-0.18%, Si: 0.03-0.70%, Mn: 0.30-2.50%, P: 0.030% or less, S: 0.0200% or less, Al: 0.001-0.100%, O: 0.0100% or less and N: 0.0100% or less, and the remainder of a component composition consisting of Fe and unavoidable impurities, and in a region of 30 mm thickness including a plate thickness center, an average area of porosity is not more than or equal to 4.0 mm2, the standard deviation of the area of porosity is not more than 4.0 mm2, the porosity area ratio is not more than 2.0%, the tensile strength is not less than 430 MPa, and the Charpy absorption energy is not less than or equal to 27 J at -50°C.SELECTED DRAWING: None

Claims

1. In mass percent, C: 0.03-0.18%, Si: 0.03-0.70%, Mn: 0.30-2.50%, P: 0.030% or less, S: 0.0200% or less, Al: 0.001-0.100%, O: 0.0100% or less and N: 0.0100% or less It contains, with the remainder being Fe and unavoidable impurities, and has a component composition. In a 30 mm thick region including the center of the plate thickness, Using an ultrasonic flaw detection device, the region where the echo height measured with a frequency of 10 MHz, focal length of 15 mm thickness, transducer diameter of 0.375 inches, detection speed of 100.0 mm / s, and detection pitch of 0.1 mm is 25% or more is defined as porosity. The average area of ​​the aforementioned porosity is 4.0 mm ２ The following: The standard deviation of the area of ​​the aforementioned porosity is 4.0 mm ２ The following and The area ratio of the aforementioned porosity is 2.0% or less. The tensile strength is 430 MPa or more. A steel plate having a Charpy absorption energy of 27 J or more at -50°C.

2. The aforementioned component composition is further expressed in mass%, Cu: 2.00% or less, Ni: 2.50% or less, Cr: 1.50% or less, Mo: 1.00% or less Nb: 0.100% or less, Ti: 0.100% or less, V: 0.30% or less, B: 0.0100% or less, W: 0.50% or less, Ca: 0.0200% or less, Mg: 0.0200% or less, Sn: 0.0500% or less and REM: 0.0500% or less The steel plate according to claim 1, comprising one or more selected from the group consisting of the following.

3. A method for manufacturing a steel sheet according to claim 1 or 2, comprising heating a slab to 1050°C or higher and 1200°C or lower, and then subjecting it to one or more passes of hot rolling, In the aforementioned hot rolling, In passes where the plate thickness center temperature is 950°C or higher and 1200°C or lower, the cumulative value of X expressed by the following formula (1) in each pass satisfies 1.0 or higher and 10.0 or lower. A method for manufacturing steel plates, wherein in passes where the core temperature of the plate thickness is 800°C or higher and less than 950°C, the cumulative value of X, expressed by the following formula (1), in each pass satisfies 3.0 or more. [Math 1] However, in equation (1), t ０ : This is the plate thickness (mm) on the roll entry side of each hot rolling pass. t １ : This is the plate thickness (mm) at the roll exit side of each hot rolling pass. R: Roll radius (mm) T ｍｉｄ : This is the absolute temperature (K) at the center of the plate thickness on the roll entry side of each hot rolling pass. T ｓｕｒ : This is the absolute temperature (K) of the plate surface on the roll entry side of each hot rolling pass. A: This is a constant determined by the following equation (2). A=2851+2968C-1120C ２ ・・・Form (2) In equation (2), C represents the carbon content (mass%).

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