Abstract: A Ni-added steel plate includes, by mass %, C: 0.04% to 0.10%, Si: 0.02% to 0.12%, Mn: 0.3% to 1.0%, Ni: more than 7.5% to 10.0%, Al: 0.01% to 0.08%, T.O: 0.0001% to 0.0030%, P: limited to 0.0100% or less, S: limited to 0.0035% or less, N: limited to 0.0070% or less, and the balance consisting of Fe and unavoidable impurities, in which a Ni segregation ratio at an area of ¼ of a plate thickness away from a plate surface in a thickness direction is 1.3 or less, a fraction of austenite after a deep cooling is 0.5% or more, an austenite unevenness index after the deep cooling is 3.0 or less, and an average equivalent circle diameter of the austenite after the deep cooling is 1 ?m or less.

Abstract: A Ni-added steel plate contains, by mass %, C: 0.03% to 0.10%, Si: 0.02% to 0.40%, Mn: 0.3% to 1.2%, Ni: 5.0% to 7.5%, Cr: 0.4% to 1.5%, Mo: 0.02% to 0.4%, Al: 0.01% to 0.08%, T.O: 0.0001% to 0.0050%, P: limited to 0.0100% or less, S: limited to 0.0035% or less, and N: limited to 0.0070% or less with a remainder composed of Fe and inevitable impurities, in which a Ni segregation ratio at a position of ¼ of a plate thickness away from a plate surface in a thickness direction is 1.3 or less, a fraction of austenite after deep cooling is 2% or more, an austenite unevenness index after deep cooling is 5.0 or less, and an average equivalent circle diameter of austenite after deep cooling is 1 ?m or less.

Abstract: A Ni-added steel plate includes, by mass %, C: 0.04% to 0.10%, Si: 0.02% to 0.12%, Mn: 0.3% to 1.0%, Ni: more than 7.5% to 10.0%, Al: 0.01% to 0.08%, T.O: 0.0001% to 0.0030%, P: limited to 0.0100% or less, S: limited to 0.0035% or less, N: limited to 0.0070% or less, and the balance consisting of Fe and unavoidable impurities, in which a Ni segregation ratio at an area of ¼ of a plate thickness away from a plate surface in a thickness direction is 1.3 or less, a fraction of austenite after a deep cooling is 0.5% or more, an austenite unevenness index after the deep cooling is 3.0 or less, and an average equivalent circle diameter of the austenite after the deep cooling is 1 ?m or less.

Abstract: A Ni-added steel plate contains, by mass %, C: 0.03% to 0.10%, Si: 0.02% to 0.40%, Mn: 0.3% to 1.2%, Ni: 5.0% to 7.5%, Cr: 0.4% to 1.5%, Mo: 0.02% to 0.4%, Al: 0.01% to 0.08%, T•O: 0.0001% to 0.0050%, P: limited to 0.0100% or less, S: limited to 0.0035% or less, and N: limited to 0.0070% or less with a remainder composed of Fe and inevitable impurities, in which a Ni segregation ratio at a position of ¼ of a plate thickness away from a plate surface in a thickness direction is 1.3 or less, a fraction of austenite after deep cooling is 2% or more, an austenite unevenness index after deep cooling is 5.0 or less, and an average equivalent circle diameter of austenite after deep cooling is 1 ?m or less.

Abstract: The present invention provides a steel plate that exhibits excellent low-temperature toughness in a base material and a weld heat-affected zone and has small strength anisotropy, wherein the steel includes, by mass, C: 0.04%-0.10%; Si: 0.02%-0.40%; Mn: 0.5%-1.0%; P: 0.0010%-0.0100%; S: 0.0001%-0.0050%; Ni: 2.0%-4.5%; Cr: 0.1%-1.0%; Mo: 0.1%-0.6%; V: 0.005%-0.1%; Al: 0.01%-0.08%; and N: 0.0001%-0.0070%, with the balance including Fe and inevitable impurities, a Ni segregation ratio at a portion located at one-fourth of a thickness of the steel plate in a steel-plate thickness direction from a surface of the steel plate is 1.3 or lower, a degree of flatness of a prior austenite grain is in a range from 1.05 to 3.0, an effective diameter of crystal grain is 10 ?m or lower, and a Vickers hardness number is in a range of 265 HV to 310 HV.

Abstract: The present invention provides a steel plate that exhibits excellent low-temperature toughness in a base material and a weld heat-affected zone and has small strength anisotropy, wherein the steel includes, by mass, C: 0.04%-0.10%; Si: 0.02%-0.40%; Mn: 0.5%-1.0%; P: 0.0010%-0.0100%; S: 0.0001%-0.0050%; Ni: 2.0%-4.5%; Cr: 0.1%-1.0%; Mo: 0.1%-0.6%; V: 0.005%-0.1%; Al: 0.01%-0.08%; and N: 0.0001%-0.0070%, with the balance including Fe and inevitable impurities, a Ni segregation ratio at a portion located at one-fourth of a thickness of the steel plate in a steel-plate thickness direction from a surface of the steel plate is 1.3 or lower, a degree of flatness of a prior austenite grain is in a range from 1.05 to 3.0, an effective diameter of crystal grain is 10 ?m or lower, and a Vickers hardness number is in a range of 265 HV to 310 HV.

Abstract: The present invention provides a low alloy steel and a weld joint thereof excellent in hydrochloric acid corrosion resistance and sulfuric acid corrosion resistance, said low alloy steel containing, in mass, C: 0.001 to 0.2%, Si: 0.01 to 2.5%, Mn: 0.1 to 2%, Cu: 0.1 to 1%, Mo: 0.001 to 1%, Sb: 0.01 to 0.2%, P: 0.05% or less, and S: 0.05% or less, with the balance consisting of Fe and unavoidable impurities; and the acid corrosion resistance index AI of said low alloy steel being zero or positive. Here, said AI is given by the following expression, AI/10,000=0.0005+0.045×Sb %?C %×Mo %, where % means mass %.

Abstract: The present invention provides a low alloy steel and a weld joint thereof excellent in hydrochloric acid corrosion resistance and sulfuric acid corrosion resistance, said low alloy steel containing, in mass, C: 0.001 to 0.2%, Si: 0.01 to 2.5%, Mn: 0.1 to 2%, Cu: 0.1 to 1%, Mo: 0.001 to 1%, Sb: 0.01 to 0.2%, P: 0.05% or less, and S: 0.05% or less, with the balance consisting of Fe and unavoidable impurities; and the acid corrosion resistance index AI of said low alloy steel being zero or positive. Here, said AI is given by the following expression, AI/10,000=0.0005+0.045×Sb %?C %×Mo %, where % means mass %.

Abstract: The present invention provides a low alloy steel and a weld joint thereof excellent in hydrochloric acid corrosion resistance and sulfuric acid corrosion resistance, said low alloy steel containing, in mass, C: 0.001 to 0.2%, Si: 0.01 to 2.5%, Mn: 0.1 to 2%, Cu: 0.1 to 1%, Mo: 0.001 to 1%, Sb: 0.01 to 0.2%, P: 0.05% or less, and S: 0.05% or less, with the balance consisting of Fe and unavoidable impurities; and the acid corrosion resistance index AI of said low alloy steel being zero or positive. Here, said AI is given by the following expression, AI/10,000=0.0005+0.045×Sb %?C %×Mo %, where % means mass %.

Abstract: The present invention provides a low alloy steel and a weld joint thereof excellent in hydrochloric acid corrosion resistance and sulfuric acid corrosion resistance, said low alloy steel containing, in mass, C: 0.001 to 0.2%, Si: 0.01 to 2.5%, Mn: 0.1 to 2%, Cu: 0.1 to 1%, Mo: 0.001 to 1%, Sb: 0.01 to 0.2%, P: 0.05% or less, and S: 0.05% or less, with the balance consisting of Fe and unavoidable impurities; and the acid corrosion resistance index AI of said low alloy steel being zero or positive. Here, said AI is given by the following expression, AI/10,000=0.0005+0.045×Sb %?C %×Mo %, where % means mass %.