Nkk Corporation

Jfe Steel Corporation

Nippon Kokan Kabushiki Kaisha

Toshiaki Urabe

Akihiko Nishimoto

Yoshihiro Hosoya

Kunikazu Tomita

Masaharu Jitsukawa

Takeshi Fujita

Fusato Kitano

Yuji Yamasaki

Nobuyuki Nakamura

Kohei Hasegawa

Katsumi Nakajima

Hideyuki Tsurumaru

Kozo Harada

Yasunobu Nagataki

Akio Kobayashi

Kaneharu Okuda

Akira Sakamoto

Masayuki Kinoshita

Toru Inazumi

Shogo Sato

Hiromi Yoshida

Kenichi Mitsuzuka

Katsutoshi Ito

Shunsaku Node

Akihide Yoshitake

Yoshihara Hosoya

A high-strength steel sheet is useful for applications to automobile steel sheets and the like and has excellent deep drawability, a tensile strength (TS) of as high as 440 MPa or more, and a high r value (average r value &#x2267;1.2), and a process for producing the steel sheet. The steel sheet has a composition containing, by % by mass, 0.010 to 0.050% of C, 1.0% or less of Si, 1.0 to 3.0% of Mn, 0.005 to 0.1% of P, 0.01% or less of S, 0.005 to 0.5% of Al, 0.01% or less of N, and 0.01 to 0.3% of Nb, the Nb and C contents in steel satisfying the relation, (Nb/93)/(C/12)=0.2 to 0.7, and the balance substantially including Fe and inevitable impurities. The steel microstructure contains a ferrite phase and a martensite phase at area ratios of 50% or more and 1% or more, respectively, and the average r value is 1.2 or more.

High-strength steel sheet excellent in deep drawing characteristics and method for production thereof

The present invention provides a method for manufacturing an ultra high strength cold-rolled steel sheet, comprising the step of continuously annealing a cold-rolled steel sheet consisting essentially of, in terms of weight percentages, 0.07 to 0.15% C, 0.7 to 2% Si, 1.8 to 3% Mn, 0.02% or less P, 0.01% or less S, 0.01 to 0.1% Sol. Al, 0.005% or less N, 0.0003 to 0.003% B, and the balance being Fe, in which such continuous annealing comprises the steps of: heating the cold-rolled steel sheet at from 800&#xb0; C. to 870&#xb0; C. for 10 seconds or more; slowly cooling the heated steel sheet down to from 650&#xb0; C. to 750&#xb0; C.; rapidly cooling the slowly cooled steel sheet down to 100&#xb0; C. or less at a cooling speed of over 500&#xb0; C./sec; reheating the rapidly cooled steel sheet at from 325&#xb0; C. to 425&#xb0; C. for from 5 minutes to 20 minutes; cooling the reheated steel sheet down to room temperature; and coiling the cooled steel sheet. According to the invention, the ultra high strength cold-rolled steel sheet, for use in a structural member of automobile, which has a tensile strength of 980 MPa or more and is excellent in stretch-flangeability, ductility, and spot-weldability can be obtained.

Method for producing cold rolled steel plate of super high strength

A steel sheet containing 0.004 to 0.02% C, 1.0% or less Si, 0.7 to 3.0% Mn, 0.02 to 0.15% P, 0.02% or less S, 0.01 to 0.1% Al, 0.004% or less N, 0.2% or less Nb, by mass %, optionally Ti, Bi or at least one element selected from the group consisting of Cr Mo, Ni and Cu, and the balance being Fe; the Nb content satisfying a formula of(12/93)&#xd7;Nb*/C&#x2267;1.0,wherein Nb*=Nb&#x2212;(93/14)&#xd7;N, and wherein C, N and Nb designate the content in mass % of carbon, nitrogen and niobium, respectively; and a yield strength and an average grain size of the ferritic grains which satisfy a formula of&#x2266;&#x2212;120+1280,wherein YP designates yield strength in MPa, and d designates an average size of ferritic grains in &#x3bc;m.

Steel sheet

The present invention relates to a steel sheet that consists essentially of 0.10 to 0.37% C, 1% or less Si, 2.5% or less Mn, 0.1% or less P, 0.03% or less S, 0.01 to 0.1% sol.Al, 0.0005 to 0.0050% N, 0.0003 to 0.0050% B, by mass, and balance of Fe, &lsqb;14B/(10.8N)&rsqb; being 0.5 or more, average particle size of precipitate BN being 0.1 &mgr;m or more, and grain size of prior austenite after the hardening treatment being 2 to 25 &mgr;m. The steel sheet according to the present invention allows an inexpensive hardening treatment method to be applied to improve the strength to a level required for the structural components and the functional components of automobiles, while providing excellent toughness after the hardening treatment.

Steel sheet and method for manufacturing the same

The steel sheet comprises: a ferritic phase having ferritic grains of 10 or more grain size number and ferritic grain boundaries; and at least one kind of Nb precipitates and Ti precipitates. The ferritic grain has a low density region with a low precipitate density in the vicinity of grain boundary. The low density region has a precipitate density of 60% or less to the precipitate density at center part of the ferritic grain. The steel sheet consists essentially of 0.002 to 0.02% C, 1% or less Si, 3% or less Mn, 0.1% or less P, 0.02% or less S, 0.01 to 0.1% sol.Al, 0.007% or less N, at least one element of 0.01 to 0.4% Nb and 0.005 to 0.3% Ti, by mass %, and the balance being Fe.

An ultra-high strength cold rolled steel sheet having a tensile strength of 880 to 1170 MPa, that consists essentially of 0.01 to 0.07% C, 0.3% or less Si, 0.1% or less P, 0.01% or less S, 0.01 to 0.1% sol.Al, 0.0050% or less N, 1.6 to 2.5% of the sum of at least one element selected from the group consisting of Mn, Cr, and Mo, by mass, and a balance of Fe, and that has an inner zone deeper than 10 &mgr;m from the surface of the steel sheet being substantially a martensitic single phase structure. Since the steel sheet has a hole expansion ratio of 75% or more, specified by the Standard of Japan Iron and Steel Federation, JFST1001-1996, the steel sheet has a tensile strength in a range of from 880 to 1170 MPa, and an excellent mechanically joining property, and is suitable for fabricating automobile seat frames.

Ultra-high strength cold rolled steel sheet and method for manufacturing the same

A hot-dip galvanized steel sheet is produced by rough rolling a steel, finish rolling the rough rolled steel at a temperature of Ar3 point or more, coiling the finish rolled steel at a temperature of 700&deg; C. or less, and hot-dip galvanizing the coiled steel at a pre-plating heating temperature of Ac1 to Ac3. A continuous hot-dip galvanizing operation is performed by soaking a pickled strip at a temperature of 750 to 850&deg; C., cooling the soaked strip to a temperature range of 600&deg; C. or less at a cooling rate of 1 to 50&deg; C. per second, hot-dip galvanizing the cooled strip, and cooling the galvanized strip so that the residence time at 400 to 600&deg; C. is within 200 seconds. The steel sheet has a structure consisting essentially of ferrite and martensite.

Hot-dip galvanized steel sheet and method for producing the same

Method of making non-oriented electrical steel sheets

Method of making non-oriented electrical steel sheets having excellent

A method for producing a non-oriented electrical steel sheet with precise thickness and homogeneous magnetic property comprising the steps of: making a steel ingot which has: 0.01 wt. % or less C, 0.003 wt. % or less N, 0.01 to 1.0 wt. % Mn, Al and Si satisfying, in wt. %, the formulas of: ##EQU1## provided that (Si %) represents the Si (wt. %) and (Al %) represents Al content (wt. %), and the balance being Fe and inevitable impurities to produce a steel slab; hot-rolling the slab at a finishing temperature of 700.degree. to 900.degree. C. into a steel strip and coiling the hot rolled strip; and cold-rolling the hot-rolled strip into a cold-rolled strip, followed by annealing the cold-rolled strip.

Method for producing non-oriented steel sheets

For enabling the manufacture of non-oriented steel strips by HDR, the amounts of AlN and MnS which precipitate on the way of HDR are decreased to such a level that they do not affect the magnetic properties by regulating the Al and S contents, and further unavoidable precipitating nitrides are precipitated as coarse BN. With regard to the steel composition, the amounts of C, Si and P are not only regulated, but also the amounts of Mn, Al, S and N are regulated from the above standpoint, and a proper amount of B is added if required. In addition, in regard to treatment conditions, in order to secure necessary finishing and coiling temperatures, the lower limit of the slab temperature at the starting time of HDR is specified. Moreover, to secure precipitation of BN and recrystallization of a ferrite structure, the lower limits of the finishing and coiling temperatures are specified. Furthermore, to secure the magnetic properties after cold rolling, the strips are finally continuously annealed at the determined temperature.

Method of making non-oriented magnetic steel strips

For enabling the making of non-oriented magnetic steel strips by direct rolling, the amounts of AlN and MnS which precipitate on the way of the direct rolling are decreased to such a level that they do not affect the magnetic properties by regulting the Al and S amounts, and further unavoidable precipitating nitrides are precipitated coarsely as BN. With regard to the steel composition, the amount of C, Si and P are not only regulated, but also the amounts of Mn, Al, S and N are regulated from the above standpoint, and a proper amount of B is added if required. In addition, in regard to treatment conditions, in order to secure necessary finishing and coiling temperatures, the lower limit of the slab temperature at the starting time of the direct rolling is specified. Moreover, to promote refining of ferrite, the upper limit of the finishing temperature is specified, and also to prevent non-uniform recrystallization after coiling, the upper limit of the coiling temperature is specified. Furthermore, to secure satisfied magnetic properties, the annealing of the hot rolled strip is performed under specific conditions determined by a soaking temperature and a soaking time. In addition, to secure the magnetic properties of the cold rolled strip, the strip is finally continuously annealed at determined temperatures.

A method for producing cold-rolled steel sheets which comprises the steps of: producing slabs containing 0.001-0.003 wt. % C, 0.004 wt. % or less N and 0.03-0.20 wt. % P, providing that P(wt. %).times.N(wt. %).ltoreq.3.times.10.sup.-4 wt. % is satisfied; hot-rolling the slabs into hot-rolled steel strips; cold-rolling the hot-rolled steel strips at 60-95% reduction ratio into cold-rolled steel strips; and annealing continuously the cold-rolled strips at temperature of recrystallization temperature to Ac.sub.3 transformation point temperature. The slabs further include 0.0005-0.0030 wt. % B. The hot-rolling is performed at finishing temperature ranging Ar.sub.3 transfromation point to Ar.sub.3 transformation +100.degree. C., at cooling temperature of 750.degree. C. and less.

Growing community of inventors

Tokyo, Japan

Toshiaki Urabe