Abstract: An extra high tensile steel having an excellent stress corrosion cracking resistance in sea water and a yield strength of 1080 MPa or more is provided. A slab comprising, in terms of % by weight, 0.04 to 0.09% of C, 0.01 to 0.10% of Si, 0.05 to 0.65% of Mn, 8.0 to 11.0% of Ni, 0.5 to 1.5% of Mo, 0.2 to 1.5% of Cr, 0.02 to 0.20% of V and 0.01 to 0.08% of Al with the balance consisting of iron and unavoidable impurities is heated to a temperature between 1000.degree. C. and 1250.degree. C., hot rolled at a temperature of Ar' point or above, air-cooled, reheated at a rate of 120.degree. C./min or less to a temperature region of from (A.sub.C3 point--40.degree. C.) to (A.sub.C3 point+40.degree. C.), quenched and subsequently tempered at a temperature of the A.sub.C1 point or below.

Abstract: An extra high tensile steel having an excellent stress corrosion cracking resistance and a yield strength of 1080 MPa or more is provided. A slab comprising, in terms of % by weight, 0.03 to 0.08% of C, 0.01 to 0.10% of Si, 0.05 to 0.65% of Mn, 8.0 to 11.0% of Ni, 0.5 to 1.5% of Mo, 0.2 to 1.5% of Cr, 0.02 to 0.20% of V and 0.01 to 0.08% of Al with the balance consisting of iron and unavoidable impurities is heated to a temperature between 1000.degree. C. and 1250.degree. C., hot-rolled in an austenite recrystallization temperature region with a reduction ratio of 30 to 70%, subsequently rolled in an austenite nonrecrystallization temperature region with a reduction ratio of 20 to 60%, subjected to roll finishing, water-cooled from a temperature of 600.degree. C.

Abstract: A method of producing non-oriented magnetic steel plate having high magnetic flux density in a low magnetic field and uniform magnetic properties through the thickness direction, comprising selection of a heating temperature and finish rolling temperature to coarsen the size of the austenite grains and prevent refinement of the grain size in the rolling process, and annealing the steel after it has been rolled.

Abstract: A method of high tension steel having superior weldability and low temperature toughness, comprises the steps of preparing a steel billet having a composition consisting, by weight, of 0.02 to 0.05% C, 0.02 to 0.5% Si, 0.4 to 1.5% Mn, 0.5 to 4.0% Ni, 0.20 to 1.5% Mo, 0.005 to 0.03% Ti, 0.01 to 0.08% Al, not more than 0.0002% B, 0.5 to 2.0% Cu, not more than 0.01% N, and the balance Fe and incidental impurities; heating said steel billet to a temperature of 900.degree. C. to 1000.degree. C.; hot-rolling the heated steel billet first at a rolling reduction of 30 to 70% in a temperature range in which austenite recrystallizes and then at a rolling reduction of 20 to 60% in another temperature range in which austenite does not recrystallize; hardening the hot-rolled billet by water-cooling from a temperature not lower than the Ar.sub.3 transformation point and terminating the cooling at a temperature not higher than 250.degree. C.

Abstract: A method of producing non-oriented magnetic steel plate that comprises hot-rolling high-purity steel and adjusting the grain size together with the dehydrogenation treatment to produce a uniform ferrite grain diameter and impart uniform magnetic properties in a low magnetic field through the thickness direction.

Abstract: High-strength steel having a tensile strength of 80 kgf/mm.sup.2 or more is produced by one-line quenching or on-line quenching and tempering. The steel has also excellent low-temperature toughness and excellent weldability, which makes high temperature preheating unnecessary prior to welding. The composition is: C=0.04% to 0.11%; Si.ltoreq.1.0%; Mn=0.5% to 2.00%, Mo=0.10% to 1.0%, Nb=0.005% to 0.05% B=0.0009% to 0.0012% Al.ltoreq.0.1%; and N.ltoreq.0.0060%.

Abstract: A method of producing non-oriented magnetic steel heavy plate that has good magnetic properties in a low magnetic field that comprises hot-rolling high-purity steel, adjusting the crystal grain size and dehydrogenation treatment, whereby the a uniform ferrite grain diameter is imparted to the steel.