Nkk Corporation

Fusato Kitano

Takeshi Fujita

Yasunobu Nagataki

Yuji Yamasaki

Yoshihiro Hosoya

Toshiaki Urabe

Masaya Morita

Toru Inazumi

Hiroshi Matsuda

Tadashi Inoue

Kohei Hasegawa

Moriaki Ono

Masahiro Iwabuchi

Katsumi Nakajima

Takeo Ishii

Akio Kobayashi

Kentaro Sato

Kunikazu Tomita

Akira Gamou

Shogo Sato

Shunsaku Node

Michihiro Hiasa

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 invention relates to a high strength hot-dip galvanized steel sheet consisting essentially of 0.03 to 0.25% C, 0.7% or less Si, 1.4 to 3.5% Mn, 0.05% or less P, 0.01% or less S, 0.05 to 1% Cr, 0.005 to 0.1% Nb, by mass, and balance of Fe, and being made of a composite structure of ferrite and secondary phase, and having an average grain size of the composite structure of 10 &#x3bc;m or smaller. Since the high strength hot-dip galvanized steel sheet of the present invention hardly induces softening at HAZ during welding, it is applicable to structural members of automobiles for 'Tailor Welded Blanks' (TWB).

High strength hot-dip galvanized 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.

Steel sheet and method for manufacturing the same

A high strength cold rolled steel sheet consists essentially of 0.0040 to 0.01% C, not more than 0.05% Si, 0.1 to 1.0% Mn, 0.01 to 0.05% P, not more than 0.02% S, 0.01 to 0.1% sol.Al, not more than 0.004% N, 0.01 to 0.14% Nb, by weight, and a balance of substantially Fe and inevitable impurities, and having an n value of not less than 0.21 calculated from two points (1% and 10%) of nominal strain determined by the uniaxial tensile test, and a method for manufacturing the cold rolled steel sheet. The high strength cold rolled steel sheet has excellent combined formability, resistance to embrittlement during secondary operation, formability at welded portions, and anti-burring performance, and has a desirable surface appearance and uniformity of material in a coil, and thus can be desirably used for automobile exterior panels.

High strength cold rolled steel sheet and method for manufacturing the same

Cold-rolled steel sheet and galvanized steel sheet, which are excellent in formability, panel shapeability, and dent-resistance, and method of manufacturing the same

The present invention relates to a very low C&mdash;Nb cold rolled steel sheet giving 340 to 440 MPa of tensile strength. For example, the cold rolled steel sheet consists essentially of 0.0040 to 0.01% C, not more than 0.05% Si, 0.1 to 1.0% Mn, 0.01 to 0.05% P, not more than 0.02% S, 0.01 to 0.1% sol.Al, not more than 0.004% N, 0.01 to 0.14% Nb, by weight, and balance of substantially Fe and inevitable impurities, and has not less than 0.21 of n value calculated from two points (1% and 10%) of nominal strain determined by the uniaxial tensile test, and relates to a method for manufacturing the cold rolled steel sheet. The present invention provides a high strength cold rolled steel sheet for automobile exterior panels having excellent combined formability, resistance to embrittlement during secondary operation, formability at welded portions, anti-burring performance, good surface appearance, and uniformity of material in a coil.

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

The present invention provides a method of manufacturing a cold rolled steel sheet and a hot-dip zinc-coated steel sheet having excellent surface properties, an excellent resistance to natural aging and an excellent dent resistance of panel and, thus, adapted to the manufacture of steel sheet for outer panels of vehicles. The method comprises the steps of preparing steel consisting of 0.005 to 0.012% by weight of C, 0.01 to 0.4% by weight of Si, 0.15 to 1.0% by weight of Mn, 0.01 to 0.08% by weight of P, at most 0.02% by weight of S, 0.01 to 0.1% by weight of sol. Al, at most 0.004% by weight of N, and at least one element selected from the group consisting of 0.01 to 0.2% by weight of Nb and 0.04 to 0.1% by weight of Ti, and satisfying the condition 1.2&lE;(12/93) (Nb%/C%)&plus;(12/48) (Ti*%/C%)&lE;2.5, applying a hot rolling and a cold rolling to the steel, soaking the cold rolled steel sheet at T(&deg; C.) meeting a formula Ac3&gE;T(&deg; C.)&gE;157log(X)&plus;737, and cooling the steel sheet after the soaking step at a cooling rate meeting a formula R(&deg; C./sec)&lE;&minus;35&plus;162/X. Ti*% (noted above)&equals;Ti%&minus;(48/14)N%&minus;(48/32)S%. When Ti* in the above equation is not larger than 0, Ti* is regarded as 0.

Method of manufacturing cold rolled steel sheet excellent in resistance of natural aging and panel properties

A cold-rolled steel sheet or a zinc or zinc alloy layer coated steel sheet containing 0.0010 to 0.01 wt % of C and having a steel composition containing one or two kinds of 0.005 to 0.08 wt % of Nb and 0.01 to 0.07 wt % of Ti in the ranges given by {(12/93)Nb+(12/48)Ti*} .gtoreq.0.0005, 0.ltoreq.C-{(12/93)Nb+(12/48)Ti*}.ltoreq.0.0015, and Ti*=Ti-{(48/32)S+(48/14)N } , in which a bake hardenability BH f the steel sheet at 170.degree. C..times.20 min after 2% tensile prestrain is 10 to 35 MPa and the BH (MPa) and a yield strength YP (MPa) of the steel sheet satisfy the ranges given by BH.gtoreq.exp(-0.115.multidot.YP+23.0) and 0.67.multidot.BH+160.ltoreq.YP.ltoreq.-0.8.multidot.BH+280.

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Fukuyama, Japan

Fusato Kitano