JPS601929B

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; (A) (B) The steel thus prepared is heated at a temperature range of 900 DEG -1000 DEG C., hot rolled with a rolling reduction of more than 60% below 900 DEG C. with a rolling finishing temperature within a range from 20 DEG C. above the Ar3 point down to 10 DEG C. below the Ar3 point and, immediately after the rolling, the steel stock is cooled down to 300 DEG C. or lower at a cooling rate of 15 DEG -60 DEG C./sec. The steel may further contain small amounts of at least one alloying element selected from the group of Ni, Cu, Cr, Mo, V and B. Due to these composition controls together with controlled heating, rolling and cooling, the product steel stock obtains a very fine grained and uniform microstructure and thereby satisfies the required mechanical properties suitable for use in welded constructions in many fields such as buildings, pressure vessels, the ship building industry and pipe lines.