A high-strength steel sheet with excellent formability and high yield ratio that has TS of 980 MPa or more and YR of 68% or more is obtained by providing a predetermined chemical composition and a steel microstructure that contains, in area ratio, 15 to 55% of polygonal ferrite, 8% or more of non-recrystallized ferrite, and 15 to 30% of martensite, and that contains, in volume fraction, 12% or more of retained austenite, in which the polygonal ferrite has a mean grain size of 4 μm or less, the martensite has a mean grain size of 2 μm or less, the retained austenite has a mean grain size of 2 μm or less, and a value obtained by dividing an Mn content in the retained austenite (in mass %) by an Mn content in the polygonal ferrite (in mass %) equals 2.0 or more.

Provided are high-strength steel having superior brittle crack arrestability and a production method therefor. The high-strength steel comprises 0.05-0.1 wt % of C, 0.9-1.5 wt % of Mn, 0.8-1.5 wt % of Ni, 0.005-0.1 wt % of Nb, 0.005-0.1 wt % of Ti, 0.1-0.6 wt % of Cu, 0.1-0.4 wt % of Si, at most 100 ppm of P, and at most 40 ppm of S with the remainder being Fe and other inevitable impurities, and has microstructures including one structure selected from the group consisting of a single-phase structure of ferrite, a single-phase structure of bainite, a complex-phase structure of ferrite and bainite, a complex-phase structure of ferrite and pearlite, and a complex-phase structure of ferrite, bainite, and pearlite. The high-strength steel has high yield strength and superior brittle crack arrestability.

High strength steel sheet having a tensile strength of 800 MPa or more comprising a middle part in sheet thickness and a soft surface layer arranged at one side or both sides of the middle part in sheet thickness, wherein each soft surface layer has a thickness of more than 10 μm and 30% or less of the sheet thickness, the soft surface layer has an average Vickers hardness of 0.60 time or less the average Vickers hardness of the sheet thickness ½ position, and the soft surface layer has a nano-hardness standard deviation of 0.8 or less is provided.

Disclosed is a steel sheet having a predetermined chemical composition and a steel microstructure that contains, in area ratio, 35% or more and 80% or less of polygonal ferrite and 5% or more and 25% or less of martensite, and that contains, in volume fraction, 8% or more of retained austenite, in which the polygonal ferrite, the martensite, and the retained austenite have a mean grain size of 6 μm or less, 3 μm or less, and 3 μm or less, respectively, and each have a mean grain aspect ratio of 2.0 or less, and in which a value obtained by dividing an Mn content in the retained austenite in mass % by an Mn content in the polygonal ferrite in mass % equals 2.0 or more.

Disclosed is a steel composition including specified ranges of Ni; Mo; Co; Mo+Co+Si+Mn+Cu+W+V+Nb+Zr+Ta+Cr+C; Co+Mo; Ni+Co+Mo; and traces of Al; Ti; N; Si; Mn; C; S; P; B; H; O; Cr; Cu; W; Zr; Ca; Mg; Nb; V; and Ta in specified ranges; the remainder being iron and impurities. The inclusion population, as observed by image analysis over a polished surface measuring 650 mm.sup.2 if hot-formed or hot-rolled; and measuring 800 mm.sup.2 if cold-rolled, does not contain non-metallic inclusions of diameter >10 μm, and, in the case of a hot-rolled sheet, does not contain more than four non-metallic inclusions of diameter 5-10 μm over 100 mm.sup.2, the observation being performed by image analysis over a polished surface measuring 650 mm.sup.2.

One embodiment of the present invention provides an ultra-thick structural steel having excellent brittle crack initiation resistance, and a manufacturing method therefor, the ultra-thick structural steel comprising, by wt %, 0.03-0.08% of C, 1.6-2.2% of Mn, 0.6-1.3% of Ni, 0.005-0.03% of Nb, 0.005-0.02% of Ti, 0.1-0.4% of Cu, 100 ppm or less of P, 40 ppm or less of S, 1.5 ppm or less of H, and the balance of Fe and other inevitable impurities, wherein the sum of acicular ferrite and granular bainite in the microstructure is 80% or more by area fraction, the sum of the total length of cracks having a size of 30 μm or more per unit area of 1 mm2 in a ±1 mm region on the basis of the thickness center of the steel is 130 μm or less, and the yield strength is 500 MPa or more.

A steel strip for an electric-resistance-welded steel pipe or tube having a strength of X70 grade or more and excellent HIC resistance and SSC resistance is provided. A steel strip for an electric-resistance-welded steel pipe or tube has a chemical composition containing, in mass %: C: 0.02% to 0.06%; Si: 0.1% to 0.3%; Mn: 0.8% to 1.3%; P: 0.01% or less; S: 0.001% or less; V: 0.04% to 0.07%; Nb: 0.04% to 0.07%; Ti: 0.01% to 0.04%; Cu: 0.1% to 0.3%; Ni: 0.1% to 0.3%; Ca: 0.001% to 0.005%; Al: 0.01% to 0.07%; and N: 0.007% or less, with a balance being Fe and incidental impurities, contents of C, Nb, V, and Ti satisfying the following Expression (1)

[C]−12([Nb]/92.9+[V]/50.9+[Ti]/47.9)≦0.03%  (1),

wherein a ferrite area ratio is 90% or more.

There is provided a high-strength steel material for oil well having a chemical composition consisting, by mass percent, of C: 0.70-1.8%, Si: 0.05-1.00%, Mn: 12.0-25.0%, Al: 0.003-0.06%, P: ≦0.03%, S: ≦0.03%, N: ≦0.10%, V: >0.5% and ≦2.0%, Cr: 0-2.0%, Mo: 0-3.0%, Cu: 0-1.5%, Ni: 0-1.5%, Nb: 0-0.5%, Ta: 0-0.5%, Ti: 0-0.5%, Zr: 0-0.5%, Ca: 0-0.005%, Mg: 0-0.005%, B: 0-0.015%, the balance: Fe and impurities, satisfying [0.6≦C-0.18V-0.06Cr<1.44], wherein a metal micro-structure is consisting essentially of an austenite single phase, V carbides having circle equivalent diameters of 5 to 100 nm exist at a number density of 20 pieces/μm.sup.2 or higher, and a yield strength is 654 MPa or higher.

A high-strength hot-pressed part having a specified chemical composition, a microstructure including, in terms of volume fraction, 80% or more of a martensite phase, in a range of 3.0% to 20.0% of a retained austenite phase, a tensile strength TS of 1500 MPa or more, and a uniform elongation uEl of 6.0% or more. A method for manufacturing the high-strength hot-pressed part, the method comprising performing a heating process and a hot press forming process on a raw material steel sheet in order to obtain a hot-pressed part having a specified shape.

A high strength and high toughness cast steel material of the invention has a composition comprising 0.10 to 0.20% by mass of C, 0.10 to 0.50% by mass of Si, 0.40 to 1.20% by mass of Mn, 2.00 to 3.00% by mass of Ni, 0.20 to 0.70% by mass of Cr, and 0.10 to 0.50% by mass of Mo, and further comprising Fe and unavoidable impurities. The high strength and high toughness cast steel material of the invention is produced by subjecting an ingot having the above composition to annealing at 1,000 to 1,100° C., quenching at 850 to 950° C., tempering at 610 to 670° C., and then, if desired, stress-relief annealing at less than 610° C.