Nippon Steel Corporation

Nippon Steel & Sumitomo Metal Corporation

Jun Haga

Takuya Nishio

Toshiro Tomida

Norio Imai

Kengo Hata

Hiroyuki Kawata

Akihiro Uenishi

Kohichi Sano

Yasuaki Tanaka

Koutarou Hayashi

Masayuki Wakita

Kunio Hayashi

Masaharu Kameda

Takayuki Nishi

Mitsuru Yoshida

Seiji Furuhashi

Yoshiaki Nakazawa

Riki Okamoto

Kazuo Hikida

Hiroshi Takebayashi

Kaori Kawano

Nobusato Kojima

Masahito Tasaka

Suguhiro Fukushima

In a metal coated steel sheet, a chemical composition contains, in mass %, at least C: 0.03% to 0.70%, Si: 0.25% to 2.50%, Mn: 1.00% to 5.00%, P: 0.100% or less, S: 0.010% or less, sol. Al: 0.001% to 2.500, N: 0.020% or less, and a balance composed of iron and impurities, a metal structure contains greater than 5.0 vol % of retained austenite and greater than 5.0 vol % of tempered martensite, and satisfies a C content in the retained austenite being 0.85 mass % or more.

Metal coated steel sheet, manufacturing method of hot-dip galvanized steel sheet, and manufacturing method of alloyed galvannealed steel sheet

The entirety or a part of this hot stamped product includes, as a chemical composition, by mass %, C: 0.001% or more and less than 0.080%, Si: 2.50% or less, Mn: 0.01% or more and less than 0.50%, P: 0.200% or less, S: 0.0200% or less, sol.Al: 0.001% to 2.500%, N: 0.0200% or less, Cr: 0.30% or more and less than 2.00%, and a remainder: Fe and impurities, in which a metallographic structure contains, by vol %, ferrite: more than 60.0%, martensite: 0% or more and less than 10.0%, and bainite: 0% or more and less than 20.0%, a tensile strength is less than 700 MPa, and &#x394;TS, which is a decrease in the tensile strength after a heat treatment at 170&#xb0; C. for 20 minutes, is 100 MPa or less.

Hot stamped product, steel sheet for hot stamp, and manufacturing method thereof

A hot dip galvanized steel sheet and hot dip galvannealed steel sheet improved in uniform ductility and local ductility, yield strength and tensile strength, and low temperature impact property, characterized by having a predetermined chemical composition, having a metal structure containing, by volume %, retained austenite: over 5.0% and tempered martensite: over 5.0%, having retained austenite containing C: 0.85 mass % or more, and having a ratio [C]/[P]of an amount of segregation of C (number of atoms/nm): [C]to an amount of segregation of P (number of atoms/nm): [P]at prior austenite grain boundaries of 4.0 or more.

Hot dip galvanized steel sheet and hot dip galvannealed steel sheet

A hot-dip galvanized cold-rolled steel sheet has a tensile strength of 750 MPa or higher, a composition consisting, in mass percent, C: more than 0.10% and less than 0.25%, Si: more than 0.50% and less than 2.0%, Mn: more than 1.50% and 3.0% or less, and optionally containing one or more types of Ti, Nb, V, Cr, Mo, B, Ca, Mg, REM, and Bi, P: less than 0.050%, S: 0.010% or less, sol. Al: 0.50% or less, and N: 0.010% or less, and a main phase as a low-temperature transformation product and a second phase as retained austenite. The retained austenite volume fraction is more than 4.0% and less than 25.0% of the whole structure, and has an average grain size of less than 0.80 &#x3bc;m. A number density of retained austenite grains having a grain size of 1.2 &#x3bc;m or more is 3.0&#xd7;10/&#x3bc;mor less.

Hot-dip galvanized cold-rolled steel sheet and process for producing same

A steel sheet for galvannealed steel contains, by mass %, C: 0.25 to 0.70%, Si: 0.25 to 2.50%, Mn: 1.00 to 5.00%, Al: 0.005 to 3.50%, P: 0.15% or less, S: 0.03% or less, N &#xa3; 0.02%, O &#xa3; 0.01%, and optionally one or more selected from Ti, Nb, V, Cr, Mo, Cu, Ni, B, Ca, REM, Mg, W, Zr, Sb, Sn, As, Te, Y, Hf and Co, a balance being Fe and impurities. The microstructure consists of, by vol. %, retained g: 10.0 to 60.0%, high-temperature tempered martensite5.0%, low-temperature tempered martensite5.0%, fresh martensite &#xa3; 10.0%, a: 0 to 15.0%, P: 0 to 10.0%, a balance being bainite. Total volume ratio of tempered martensite and bainite is 30.0% or more, tensile strength is 1470 MPa or more, tensile strength&#xd7;uniform elongation is 13000 MPa % or more, and tensile strength&#xd7;local elongation is 5000 MPa % or more.

Galvannealed steel sheet and method for producing the same

A high-strength cold-rolled steel sheet having excellent ductility and stretch flangeability includes: a chemical composition consisting, in mass %, C: 0.06 to 0.3, Si: 0.6 to 2.5%, Mn: 0.6 to 3.5%, P: at most 0.1%, S: at most 0.05%, Ti: 0 to 0.08%, Nb: 0 to 0.04%, total of Ti and Nb: 0 to 0.10%, sol.Al: 0 to 2.0%, Cr: 0 to 1%, Mo: 0 to 0.3%, V: 0 to 0.3%, B: 0 to 0.005%, Ca: 0 to 0.003%, REM: 0 to 0.003% and the remainder of Fe and impurities; a microstructure having a main phase including at least 40 area % in total of martensite and/or bainite; and a texture in which proportion of an average X-ray intensity in an {100}&#x3c;011&#x3e; to {211}&#x3c;011&#x3e; orientations relative to an average X-ray intensity of a random structure not having a texture is less than 6.

Cold-rolled steel sheet and process for manufacturing same

A base material () included in a plated steel sheet () includes a structure, at a &#xbc; sheet thickness position, represented by, in volume fraction: tempered martensite: 3.0% or more; ferrite: 4.0% or more; and retained austenite: 5.0% or more. An average hardness of the tempered martensite in the base material () is 5 GPa to 10 GPa, and a part or all of the tempered martensite and the retained austenite in the base material form an M-A. A volume fraction of ferrite in a decarburized ferrite layer () included in the plated steel sheet () is 120% or more of the volume fraction of the ferrite in the base material () at the &#xbc; sheet thickness position, an average grain diameter of the ferrite in the decarburized ferrite layer () is 20 &#x3bc;m or less, a thickness of the decarburized ferrite layer () is 5 &#x3bc;m to 200 &#x3bc;m, a volume fraction of tempered martensite in the decarburized ferrite layer () is 1.0 volume% or more, a number density of the tempered martensite in the decarburized ferrite layer () is 0.01/&#x3bc;mor more, and an average hardness of the tempered martensite in the decarburized ferrite layer () is 8 GPa or less.

Plated steel sheet

A hot-rolled steel sheet having a composition comprising, in mass %, C: 0.06 to 0.3%, Si: 0.4 to 2.5%, Mn: 0.6 to 3.5%, P: at most 0.1%, S: at most 0.05%, Ti: 0 to 0.08%, Nb: 0 to 0.04%, a total content of Ti and Nb: 0 to 0.10%, sol.Al: 0 to 2.0%, Cr: 0 to 1%, Mo: 0 to 0.3%, V: 0 to 0.3%, B: 0 to 0.005%, Ca: 0 to 0.003%, REM: 0 to 0.003%, the remainder Fe and impurities, is cold rolled, then heated at an average heating rate of at least 15&#xb0; C./sec so that a proportion of an unrecrystallization of a region not transformed to austenite at a time of reaching (Acpoint+10&#xb0; C.) is at least 30 area %, and then held for 30 seconds at temperature of at least (0.9&#xd7;Acpoint+0.1&#xd7;Acpoint) and at most (Acpoint+100&#xb0; C.).

Process for manufacturing cold-rolled steel sheet

A method for producing a high-tensile cold-rolled steel sheet includes subjecting a slab having a composition containing C: more than 0.020% and less than 0.30%, Si: more than 0.10% and 3.00% or less, and Mn: more than 1.00% and 3.50% or less to hot rolling wherein the roll draft of the final one pass is higher than 15%, and rolling is finished in the temperature region of Arpoint or higher, optionally annealing wherein the hot-rolled steel sheet is heated to 300&#xb0; C. or higher after being cooled to 780&#xb0; C. or lower, coiling higher than 400&#xb0; C. or lower than 400&#xb0; C., cold rolling the hot-rolled steel sheet or the annealed steel sheet, and annealing wherein the cold-rolled steel sheet is soaked in the temperature region of (Acpoint&#x2212;40&#xb0; C.) or higher, cooling to 500&#xb0; C. or lower and 300&#xb0; C. or higher, and holding in that temperature region for 30 seconds or longer.

Method for producing cold-rolled steel sheet

A steel sheet suitable as a starting material for a vehicle impact absorbing member with high absorption of impact energy and resistance to cracking contains, by mass %, C: 0.08-0.30%, Mn: 1.5-3.5%; Si+Al: 0.50-3.0%, P: 0.10% or less, S: at most 0.010%, and N: at most 0.010%, and optionally, one or more types selected from Cr: at most 0.5%, Mo: at most 0.5% , B: at most 0.010%, Ti: less than 0.04%, Nb: less than 0.030%, V: less than 0.5%, Ca: at most 0.010%, Mg: at most 0.010%, REM: at most 0.050%, and Bi: at most 0.050%. The microstructure contains, by area %, bainite: more than 50%, martensite: 3-30%, and retained austenite: 3-15%, the remainder comprising ferrite having an average grain diameter of less than 5 mm. The product of uniform elongation and hole expansion ratio is at least 300%and 5% effective flow stress is at least 900 MPa.

Steel sheet suitable for impact absorbing member and method for its manufacture

A high strength cold-rolled steel sheet has excellent surface appearance and a composition consisting essentially of, in mass %, C: 0.0005-0.025%, Si: at most 0.2%, Mn: 0.3-2.5%, P: at most 0.15%, S: at most 0.02%, N: at most 0.006%, sol. Al: less than 0.005%, Ti: 0.005-0.05%, and Nb: 0.020-0.200% with the mass ratio (Nb/Ti) of the contents of Nb and Ti being at least 2, either boron at most 0.0020% or one or more of Cr, Mo, V, W, Cu, and Ni, each at most 1%, and a remainder of Fe and impurities. The sheet has excellent press formability and tensile strength by having an r value in a direction at 45&#xb0; with respect to the rolling direction (r) of at least 1.80 and/or a mean r value (r) of at least 1.60, and a tensile strength of at least 340 MPa.

Cold-rolled steel sheet

A high-strength cold-rolled steel sheet includes a composition having controlled amounts of carbon, silicon, manganese, phosphorous, sulfur, titanium, niobium, sol. Aluminum, chromium, molybdenum, vanadium, boron, calcium, REM, and iron. A microstructure thereof has a main phase of ferrite of at least 40 area %, and a second phase of a low-temperature transformation phase consisting either or both of martensite and bainite, which comprises at least 10 area % in total and retained austenite (&#x3b3;) at least comprising 3 area % . An average grain diameter of ferrite has a tilt angle of at least 15&#xb0; is at most 5.0 mm, an average grain diameter of the low-temperature transformation-produced phase is at most 2.0 mm, an average grain diameter of lump-like retained &#x3b3; having an aspect ratio of less than 5 is at most 1.5 mm, and an area fraction of the lump-like retained &#x3b3; relative to the retained &#x3b3; is at least 50%.

Cold-rolled steel sheet with controlled microstructure, grain diameters, and texture

A high-strength cold-rolled steel sheet excellent in ductility, work hardenability, and stretch flangeability, and having tensile strength of 780 MPa or more includes: a chemical composition containing, in mass percent, C: more than 0.020% to less than 0.30%, Si: more than 0.10% to 3.00% or less, Mn: more than 1.00% to 3.50% or less; and metallurgical structure whose main phase is a low-temperature transformation product, and whose secondary phase contains retained austenite. The retained austenite has a volume fraction relative to overall structure of more than 4.0% to less than 25.0% and an average grain size of less than 0.80 &#x3bc;m, and of the retained austenite, the number density of retained austenite grains whose grain size is 1.2 &#x3bc;m or more is 3.0&#xd7;10grains/&#x3bc;mor less.

A high strength cold-rolled steel sheet and a high strength plated steel sheet which have an excellent surface appearance required for outer panels of automobiles and which have an extremely high r value in a direction at 45&#xb0; with respect to the rolling direction and which have excellent press formability and a tensile strength of at least 340 MPa and a process for their manufacture are provided. The steel sheets have a chemical composition consisting essentially of, in mass %, C: 0.0005-0.025%, Si: at most 0.2%, Mn: 0.3-2.5%, P: at most 0.15%, S: at most 0.02%, N: at most 0.006%, sol. Al: less than 0.005%, Ti: 0.005-0.05%, and Nb: 0.020-0.200% with the mass ratio (Nb/Ti) of the contents of Nb and Ti being at least 2, and a remainder of Fe and impurities, and they have an r value in a direction at 45&#xb0; with respect to the rolling direction (r) of at least 1.80 and/or a mean r value (r) of at least 1.60, and a tensile strength of at least 340 MPa.

Cold-rolled steel sheet and galvannealed steel sheet

A high-strength cold-rolled steel sheet providing a product with a good surface condition after press forming, having excellent bake hardenability and anti room temperature aging property, and having a dual phase structure with a tensile strength of at least 340 MPa is provided. A high-strength cold-rolled steel sheet has a structure comprising a main phase which is a ferrite and a secondary phase which is a low temperature transformation product including a martensite and has a hardness distribution of the ferrite phase in an arbitrary cross section having a length of 10 mm in the widthwise 10 direction of the sheet which satisfies the relationship prescribed by (HV&#x2212;HV)&#x3c;0.5&#xd7;(Hv. HVis the maximum Vickers hardness of ferrite grains in a region at a distance of from (&#x215b;)t to (&#xbc;)t in the thickness direction from the surface when the thickness of the high-strength cold-rolled steel sheet is t, and Hvis the average Vickers hardness of ferrite grains in this region.

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Tokyo, Japan

Jun Haga