Nippon Steel Corporation

Yasumitsu Onoe

Hiroshi Katoh

Hiroshi Tamehiro

Osamu Akisue

Hiroo Mazuda

Mamoru Ohashi

Takashi Fujita

Koe Nakajima

Atsuhiko Yoshie

Yoshikazu Matsumura

Masaaki Fujioka

Kishio Mochinaga

Shuji Aihara

Keiji Fukuda

Shozo Sekino

Shinogu Tamukai

Koichi Kawamura

Masaki Umeno

A steel plate having a high toughness, low yield ratio and high fatigue strength is provided by preserving the fine metallographical microstructure of martensite or bainite while austenitizing extremely fine portions of the microstructure, and during cooling, dispersing the portions as martensite, retained austenite, cementite or mixture thereof in a tempered martensite or tempered bainite phase.

High toughness low yield ratio, high fatigue strength steel plate and

A cooling apparatus for uniformly cooling a thick steel plate in order to obtain a very flat thick steel plate constituted by a cooling water supply apparatus provided with more than two header systems by which a desired water volume crown is given in the width direction of the thick steel plate.

Cooling apparatus for thick steel plate

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.

Method of making wrought high tension steel having superior low

A method for producing a low carbon deep drawing quality steel or high strength steel in the form of a strip or sheet. At least one of carbide or nitride forming elements is added to the low carbon steel to provide therein 0.015 to 0.10% Sol Al, 0.01 to 0.10% Nb, 0.01 to 0.10% Ti and 0.01 to 0.15% V. The steel is cast or bloomed to form a slab which is held above the Ar.sub.3 point for keeping the carbide and nitride forming elements in a dissolved state. Then, without allowing the temperature to fall below the Ar.sub.3 point, hot rolling and hot finishing rolling are directly carried out at a temperature above the Ar.sub.3 point. Thereafter, if necessary and desirable, the thus-rolled slab or sheet is subjected to cold rolling and annealing.

Method for producing steel strip or steel sheet containing carbide and

A method for treating a continuously cast steel slab suitable for production of a hot rolled or cold rolled steel sheet, the steel slab being obtained by continuously casting a molten steel containing 0.01 to 2.5% by weight of Al, which method comprises holding the slab at a temperature within the range of from the Ar.sub.3 point to 650.degree. C for at least 20 minutes to precipitate nitrides, and hot rolling the slab.

Method for treating continuously cast steel slabs

A process for producing a steel having excellent strength and toughness, which comprises continuously casting a molten steel containing 0.01 - 0.10% Al and 0.002 - 0.009% N with an average cooling rate of not lower than 3.degree. C/min. down to 500.degree. C to obtain a steel slab heating the steel slab to a temperature ranging from 900.degree. to 1150.degree. C and rolling the steel slab.

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Kitakyushu, Japan

Yasumitsu Onoe