Disclosed is a clad steel plate with further improved low temperature toughness along with excellent HIC resistance while ensuring a tensile strength of 535 MPa or more. A clad steel plate includes: a base steel; and a clad metal made of a corrosion resistant alloy bonded to one surface of the base steel, in which the base steel has: a chemical composition with appropriately controlled values of ACR and P.sub.HIC; and a steel microstructure in which bainite is present in an area fraction of 94% or more at a ½ thickness position in a thickness direction of the base steel, and with an average crystal grain size of 25 μm or less, and shear strength at a bonded interface between the base steel and the cladding metal is 300 MPa or more.

The steel plate has a specific chemical composition, and a microstructure where a volume fraction of martensite at a depth of 1 mm from a surface of the steel plate is 95% or more, and at a depth of 1 mm from a surface of the steel plate, a Vickers hardness at 400° C. is 288 or more, and a Brinell hardness at 25° C. is 360 HBW10/3000 to 490 HBW10/3000.

A steel sheet has a tensile strength of 1310 MPa or higher, a specified chemical composition, and a steel microstructure containing martensite at an area ratio of 70% or more, bainite at an area ratio of 30% or less, and ferrite and retained austenite at a total area ratio of 5% or less, in which, at a ¼ thickness position of the steel sheet, a number density of carbides having long axes of 0.5 μm or more is 60000 carbides/mm.sup.2 or less, in a ¼-to-¾ thickness region of the steel sheet, a number density of inclusion grains having equivalent circle diameters of 4.0 μm or more is 10 grains/mm.sup.2 or more and 30 grains/mm.sup.2 or less, and, in a surface-to-¼ thickness region of the steel sheet, a number density of inclusion grains having equivalent circle diameters of 4.0 μm or more is 27 grains/mm.sup.2 or less.

This application discloses thin metal strips and methods of making thin metal strip. Particular embodiments of such methods include cooling the thin metal strip to a temperature equal to or less than a bainite or a martensite start transformation temperature B.sub.S or M.sub.S to thereby form bainite and/or martensite, respectively, within the thin metal strip, reheating the thin metal strip to a reheat temperature equal to or greater than transformation temperature Ac.sub.3 and holding the thin metal strip at the reheat temperature for at least 2 seconds and thereby forming austenite within the thin metal strip with at least 75% of austenite grains having a grain size equal to or less than 15 μm, and rapidly recooling the thin metal strip to a temperature equal to or less than the martensite start transformation temperature M.sub.S and thereby providing finer martensite within the thin metal strip from a finer prior austenite.

This hot-rolled steel sheet has a predetermined chemical composition and predetermined metallographic structure, a ratio between a maximum depth of a region where, on one surface, a rotation angle between a normal line of the one surface and a (011) pole near the normal line of the one surface is 5° or less and a maximum depth of a region where, on the other surface, a rotation angle between a normal line of the other surface and a (011) pole near the normal line of the other surface is 5° or less is 1.00 to 1.20, and a tensile strength is 1150 MPa or more.

The invention relates to a wear-resistant steel with excellent surface quality, which is composed of C: 0.12-0.20%, Si: ≤0.1%, Mn: 0.6-1.20%, Nb: 0.010-0.040%, V: ≤0.01%, Ti: 0.010%-0.030, Al: ≤0.04%, Ni: ≤0.1%, Cu: ≤0.1%, Cr: 0.10-0.40%, Mo: ≤0.1%, B: 0.001-0.005%, Ca: 0.0010-0.0050%, P: ≤0.010%, S: ≤0.0015%, O: ≤0.0012%, N: ≤0.0035%, H: ≤0.0002%, the balance is Fe, and the carbon equivalent CEV≤0.4; PCM≤0.25. The production process flow is: converter smelting->LF refining->VD or Rh high vacuum degassing->continuous casting->heating->rolling->shot blasting->quenching->tempering. The wear-resistant steel of the invention has better surface quality, and there are no surface defects such as air pit, inclusion, hemp pit and pressed iron oxide scale. The depth of surface spots caused by the peeling off iron oxide scale is ≤0.1 mm, and the surface grinding of steel plate cannot be carried out. Based on element design, non-preheating welding and excellent toughness can be further realized.

Provided are structural ultra-thick steel having excellent resistance to brittle crack propagation and a production method therefor. The structural ultra-thick steel comprises 0.02-0.1 wt % of C, 0.8-2.5 wt % of Mn, 0.05-1.5 wt % of Ni, 0.005-0.1 wt % of Nb, and 0.005-0.1 wt % of Ti 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 ultra-thick structural steel has excellent resistance to brittle crack propagation, excellent yield strength and an excellent impact transition temperature in the center.

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 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.