An object is to provide a steel plate having excellent deformability in the central portion in the thickness direction and a method for manufacturing the steel plate. A steel plate having a chemical composition containing, by mass %, C: 0.01% to 0.15%, Si: 0.01% to 1.00%, Mn: 0.10% to 2.00%, P: 0.010% or less, S: 0.0050% or less, Al: 0.002% to 0.100%, Ni: 5.0% to 10.0%, N: 0.0010% to 0.0080%, and with a balance being Fe and incidental impurities and a percentage reduction of area of 30% or more in a tensile test in the thickness direction performed on the central portion in the thickness direction.

An object is to provide a steel plate having excellent deformability in the central portion in the thickness direction and a method for manufacturing the steel plate. A steel plate having a chemical composition containing, by mass %, C: 0.01% to 0.15%, Si: 0.01% to 1.00%, Mn: 0.10% to 2.00%, P: 0.010% or less, S: 0.0050% or less, Al: 0.002% to 0.100%, Ni: 5.0% to 10.0%, N: 0.0010% to 0.0080%, and with a balance being Fe and incidental impurities and a percentage reduction of area of 30% or more in a tensile test in the thickness direction performed on the central portion in the thickness direction.

A cold rolled steel sheet having a composition including of 0.05 % ≤ Carbon ≤ 0.15 %, 1.8% ≤ Manganese ≤ 2.7%, 0.1% ≤ Silicon ≤ 1%,0.01% ≤ Aluminum ≤ 0.8%,0.1% ≤ Chromium ≤ 0.9%,0% ≤ Phosphorus ≤ 0.09%,0.0001% ≤ Titanium ≤ 0.1%,0.0005% ≤ Boron ≤ 0.003%, 0.01% ≤ Niobium ≤ 0.1%, 0 % ≤ Sulfur ≤ 0.09 %, 0 % ≤ Nitrogen ≤ 0.09%, 0% ≤ Vanadium ≤ 0.2%, 0%≤Molybdenum≤0.2%, 0%≤Nickel≤2%, 0% ≤ Copper ≤ 2%, 0% ≤ Calcium ≤ 0.005%, 0% ≤ Cerium ≤ 0.1%, 0% ≤ Magnesium ≦ 0.05%, 0% ≤ Zirconium ≦ 0.05%, the remainder being composed of iron and unavoidable impurities caused by processing, the microstructure of the steel sheet including in area fraction, 40% to 60% martensite, 15 to 40% of inter-critical ferrite, a cumulated amount of 10 to 35% of transformed ferrite and bainite and 0% to 5% of residual austenite.

A cold rolled steel sheet having a composition including of 0.05 % ≤ Carbon ≤ 0.15 %, 1.8% ≤ Manganese ≤ 2.7%, 0.1% ≤ Silicon ≤ 1%,0.01% ≤ Aluminum ≤ 0.8%,0.1% ≤ Chromium ≤ 0.9%,0% ≤ Phosphorus ≤ 0.09%,0.0001% ≤ Titanium ≤ 0.1%,0.0005% ≤ Boron ≤ 0.003%, 0.01% ≤ Niobium ≤ 0.1%, 0 % ≤ Sulfur ≤ 0.09 %, 0 % ≤ Nitrogen ≤ 0.09%, 0% ≤ Vanadium ≤ 0.2%, 0%≤Molybdenum≤0.2%, 0%≤Nickel≤2%, 0% ≤ Copper ≤ 2%, 0% ≤ Calcium ≤ 0.005%, 0% ≤ Cerium ≤ 0.1%, 0% ≤ Magnesium ≦ 0.05%, 0% ≤ Zirconium ≦ 0.05%, the remainder being composed of iron and unavoidable impurities caused by processing, the microstructure of the steel sheet including in area fraction, 40% to 60% martensite, 15 to 40% of inter-critical ferrite, a cumulated amount of 10 to 35% of transformed ferrite and bainite and 0% to 5% of residual austenite.

A color coating layer is formed on the surface of a stainless steel plate by a chemical coloring method or an electrolytic coloring method. Thereafter, a colored stainless steel plate having the color coating layer is cold-rolled, the thickness of the color coating layer is adjusted to between 0.05 μm and 1.0 μm, and an entire plate thickness is adjusted to 0.5 mm or less. By the cold rolling a Vickers hardness Hv is between 250 and 550 to form a deformed band. As surface roughness, an arithmetic average roughness Ra is adjusted to between 0.05 μm and 5.0 μm. In this manner, the strength and rigidity of a thin colored stainless steel plate can be secured, and a color stainless steel plate and a colored stainless steel coil which do not easily cause galling and are excellent in press moldability can be obtained.

A color coating layer is formed on the surface of a stainless steel plate by a chemical coloring method or an electrolytic coloring method. Thereafter, a colored stainless steel plate having the color coating layer is cold-rolled, the thickness of the color coating layer is adjusted to between 0.05 μm and 1.0 μm, and an entire plate thickness is adjusted to 0.5 mm or less. By the cold rolling a Vickers hardness Hv is between 250 and 550 to form a deformed band. As surface roughness, an arithmetic average roughness Ra is adjusted to between 0.05 μm and 5.0 μm. In this manner, the strength and rigidity of a thin colored stainless steel plate can be secured, and a color stainless steel plate and a colored stainless steel coil which do not easily cause galling and are excellent in press moldability can be obtained.

A hot-pressed member has a predetermined chemical composition. In the hot-pressed member, a steel sheet has a microstructure in which a prior austenite average grain diameter is 8 μm or less, and martensite is present in a volume fraction of 95% or greater in a region within 30 μm of a surface; a Ni diffusion region having a thickness of 0.5 μm or greater exists in a surface layer; a standard deviation of Vickers hardness values is 35 or less; Mndif (mass %) in a sheet thickness direction ≤0.20, where Mndif (mass %) is a degree of Mn segregation; and a tensile strength is 1780 MPa or greater.

A hot-pressed member has a predetermined chemical composition. In the hot-pressed member, a steel sheet has a microstructure in which a prior austenite average grain diameter is 8 μm or less, and martensite is present in a volume fraction of 95% or greater in a region within 30 μm of a surface; a Ni diffusion region having a thickness of 0.5 μm or greater exists in a surface layer; a standard deviation of Vickers hardness values is 35 or less; Mndif (mass %) in a sheet thickness direction ≤0.20, where Mndif (mass %) is a degree of Mn segregation; and a tensile strength is 1780 MPa or greater.

A method is used to fabricate a hot-rolled steel having a yield strength greater than 550 MPa and an impact toughness of at least 27 J at a temperature of −40° F. In one embodiment, the yield strength is greater than 690 MPa. The method includes melting steel to create melted steel. The melted steel is poured into a mold. The metal steel is continuously cast into a steel slab. The steel slab is heated to maintain a predetermined temperature. The steel slab is rolled to reduce the thickness to a predetermined thickness to create a hot-rolled steel sheet.

A method is used to fabricate a hot-rolled steel having a yield strength greater than 550 MPa and an impact toughness of at least 27 J at a temperature of −40° F. In one embodiment, the yield strength is greater than 690 MPa. The method includes melting steel to create melted steel. The melted steel is poured into a mold. The metal steel is continuously cast into a steel slab. The steel slab is heated to maintain a predetermined temperature. The steel slab is rolled to reduce the thickness to a predetermined thickness to create a hot-rolled steel sheet.