Abstract: A micro-alloy plate having not only high tensile strength and high toughness both at room temperature and low temperature but which also displays superior weldability and excellent toughness at a heat affected zone (HAZ) of welding.The steel contains 0.005-0.08 C, not more than 0.6% Si, 1.4-2.4% Mn, 0.01-0.03% Nb, 0.005-0.025% Ti, 0.005-0.08% Al, not more than 0.003% S, 0.0005-0.005% Ca, not more than 0.005% O, not more than 0.005% N, all being represented by weight and the balance being incidental impurities, further the steel must satisfy the following requirements; ##EQU1## The steel thus prepared is heated at a temperature range of 900.degree.-1000.degree. C., hot rolled with a rolling reduction of more than 60% below 900.degree. C. with a rolling finishing temperature within a range from 20.degree. C. above the Ar.sub.3 point down to 10.degree. C. below the Ar.sub.3 point and, immediately after the rolling, the steel stock is cooled down to 300.degree. C. or lower at a cooling rate of 15.degree.-60.

Abstract: Due to increasing demands for steel to be used for construction such as buildings, pressure vessels, pipe lines or the like, various kinds of high tension steels, particularly steels suitable for welding have increasingly been developed.Heretofore, proposed methods of making such high tension steel have relied on so-called cold rolling and/or rolling followed by quenching and tempering, however, these conventional steels have suffered from drawbacks such as a tempering step indispensable after quenching, softening of welded zone and lack of uniformity in the metal structure in the direction of plate thickness.A compositional feature of the new method resides in addition of minor amounts of Ti and B along with Nb as contributing to grain refining or precipitation hardening elements in addition to limited amounts of other ingredients such as C, Si, Mn, S, Al and N.Further addition of at least one of V, Ni, Cu, Cr, Mo, Ca and REM also acts to improve the properties of the steel.

Abstract: A method for producing a steel sheet having excellent low-temperature toughness, comprising a step of heating to a temperature not higher than 1150.degree. C., a steel slab containing 0.01 - 0.13% C, 0.05 - 0.8% Si, 0.8 - 1.8% Mn, 0.01 - 0.08% total Al, 0.08 - 0.40% Mo and not more than 0.015% S with the balance being iron and unavoidable impurities, and a step of hot rolling the steel slab thus obtained by at least three passes with a minimum reduction percentage not less than 2% by each rolling pass in a temperature range of 900 - 1050.degree. C, a total reduction percentage not less than 50%, and with a finishing temperature not higher than 820.degree. C.