Nippon Steel Corporation

Mamoru Ohashi

Hiroshi Tamehiro

Yasumitsu Onoe

Hiroo Mazuda

Shinogu Tamukai

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.degree. 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.

Method of producing steel having high strength and toughness

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. The steel prepared to have the aforesaid composition is subjected to controlled heating, subsequent rolling under prescribed rolling reduction ratio, temperature for terminating rolling and to a specified cooling rate. The steel plate thus processed has a structure having fine bainite grains alone or a duplex grain structure consisting of fine bainite and fine ferrite particularly finer toward the surface so as to satisfy both ductility and toughness without tempering operation and displays good weldability and toughness even at a welded zone as well as stable hardness distribution throughout the plate thickness.

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Kimitsu, Japan

Mamoru Ohashi