Nippon Steel Corporation

Nippon Steel Sumitomo Metal Corporation

Kengo Hata

Toshiro Tomida

Norio Imai

Takuya Nishio

Jun Haga

Yasuaki Tanaka

Mitsuru Yoshida

Masayuki Wakita

Yuji Arai

Hiroshi Takebayashi

Suguhiro Fukushima

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

The steel material according to the present invention contains, in mass %, C: 0.15 to 0.45%, Si: 0.10 to 1.0%, Mn: 0.10 to 0.8%, P: 0.050% or less, S: 0.010% or less, Al: 0.01 to 0.1%, N: 0.010% or less, Cr: 0.1 to 2.5%, Mo: 0.35 to 3.0%, Co: 0.05 to 2.0%, Ti: 0.003 to 0.040%, Nb: 0.003 to 0.050%, Cu: 0.01 to 0.50%, and Ni: 0.01 to 0.50%, and satisfies the following Formulae. A prior-austenite grain diameter of its microstructure is less than 5 &#x3bc;m, and a block diameter of its microstructure is less than 2 &#x3bc;m. The microstructure contains a total of 90% by volume or more of tempered martensite and tempered bainite.C+Mn/6+(Cr+Mo+V)/5+(Cu+Ni)/15&#x2212;Co/6+&#x3b1;&#x2265;0.70&#x2003;&#x2003;(1)(3C+Mo+3Co)/(3Mn+Cr)&#x2265;1.0&#x2003;&#x2003;(2).

Steel material and oil-well steel pipe

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 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 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 cold-rolled steel sheet having a refined structure in which grain growth during annealing is suppressed has a chemical composition containing, in mass percent, controlled amounts of carbon, manganese, niobium, titanium, vanadium, sol. Aluminum, chromium, molybdenum, boron, calcium, and REM and a microstructure which contains at least 50% by area of ferrite as a main phase, a second phase containing at least 10% by area of a low temperature transformation phase and 0-3% by area of retained austenite and which satisfies the following Equations (1)-(3), in addition to a particular texture,&#x3c;2.7+10000/(5+300&#xd7;C+50&#xd7;Mn+4000&#xd7;Nb+2000&#xd7;Ti+400&#xd7;V)&#x2003;&#x2003;(1),&#x3c;4.0&#x2003;&#x2003;(2),and&#x2266;1.5&#x2003;&#x2003;(3),wherein dis the average grain diameter (&#x3bc;m) of ferrite defined by a high angle grain boundary having a tilt angle of at least 15&#xb0;, and dis the average grain diameter (&#x3bc;m) of the second phase.

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Tokyo, Japan

Kengo Hata