A cemented carbide comprising 55-90 parts by mass of WC particles and 10-45 parts by mass of a Fe-based binder phase; the binder phase having a composition comprising 0.5-10% by mass of Ni, 0.2-2% by mass of C, 0.5-5% by mass of Cr, 0.2-2.0% by mass of Si, and 0.1-5% by mass of W, the balance being Fe and inevitable impurities, and containing 0.05-2.0% by area of Fe—Si—O-based particles.

Provided is a high-strength steel sheet including: 0.12% to less than 0.17% of carbon (C), 0.3% to 0.8% of silicon (Si), 2.5% to 3.0% of manganese (Mn), 0.4% to 1.1% of chromium (Cr), 0.01% to 0.3% of aluminum (Al), 0.01% to 0.03% of niobium (Nb), 0.01% to 0.03% of titanium (Ti), 0.001% to 0.003% of boron (B), 0.04% or less of phosphorus (P), 0.01% or less of sulfur (S): 0.01% or less of nitrogen (N), and a balance of iron (Fe) and inevitable impurities. The contents of C, Si, and Al satisfy: [C]+[Si]+[Al])/5≤0.35 wt. A microstructure includes more than 1% to 4% or less of retained austenite, more than 10% to 20% or less of fresh martensite, 5% or less (excluding 0%) of ferrite, more than 50% to 70% or less of tempered martensite, and a balance of bainite.

The present invention provides a high-strength steel sheet, which can be used in various applications including automobile parts and exhibits excellent collision safety and excellent moldability, and a method for manufacturing the high-strength steel sheet. The high-strength steel sheet according to an aspect of the present invention satisfies a predetermined chemical composition and has a metallographic microstructure having ferrite fraction: 0% to 10%, MA fraction: 0% to 30%, hard phase other than ferrite and MA: 70% to 100% in terms of area proportion and retained austenite fraction: 5% to 30% in terms of volume proportion, and in the high-strength steel sheet, the skewness of IQ as analyzed by the EBRD method is −1.2 to −0.3 when the skewness is expressed by a predetermined relational expression in a case where crystal grains having a bcc structure and a bct structure are regarded as an aggregation of regions having an area of 0.05 μm.sup.2.

The present invention provides steel sheet having both bendability and hydrogen embrittlement resistance. The steel sheet of the present invention includes a central part of sheet thickness and a surface sort part formed at one side or both sides of the central part of sheet thickness. The microstructure of the central part of sheet thickness comprises, by volume ratio, 60% or more of tempered martensite, respectively less than 30% of ferrite, bainite, pearlite, and retained austenite, and less than 5% of as-quenched martensite. A thickness of the surface soft part is more than 10 μm per side and 15% or less of a thickness of the central part of sheet thickness, an average hardness of the surface soft part is 0.90 time or less of an average hardness of the central part of sheet thickness, the surface soft part includes carbides in a number density of 1×10.sup.4/mm.sup.2 or more, an average particle size of the carbides is 0.250 μm or less, and a standard deviation of a log of a particle size is 0.05 or less.

This hot-stamping formed body has a predetermined chemical composition and has a metallographic structure consisting of, by area ratio, a total of 10% to 30% of ferrite and granular bainite and a remainder in microstructure consisting of one or more of martensite, bainite, and tempered martensite, and, in textures of a surface layer region and an inside region, ratios between a pole density of an orientation group consisting of {001}<1-10> to {001}<−1-10> and a pole density of an orientation group consisting of {111}<1-10> to {111}<−1-12> are controlled.

A heat treated cold rolled steel sheet having a composition comprising of the following elements, 0.1%≤Carbon≤0.25, 2.15%≤Manganese≤3.0%, 1%≤Silicon≤0.8%, 0.1%≤Aluminum≤0.9%, 0.05%≤Chromium≤0.5%, 0%≤, Phosphorus≤0.09%, 0%≤Sulfur≤0.09%, 0%≤Nitrogen≤0.09%, 2.4%≤C+Mn≤3%, 0%≤Niobium≤0.1%, 0% ≤Titanium≤0.1%, 0%≤Vanadium≤0.1%, 0%≤Molybdenum≤1%, 0%≤Nickel≤1%, 0%≤Calcium≤0.005%, 0%≤Boron≤0.01%, 0%≤Cerium≤0.1%, 0%≤Magnesium≤0.05%, 0%≤Zirconium≤0.05% the remainder being composed of iron and unavoidable impurities, the microstructure of said steel sheet including, 20% to 70% Martensite, 5 to 60% of Inter-critical Ferrite, 5 to 30% of Transformed Ferrite, 8% to 20% of Residual Austenite and 1 to 20% Bainite, wherein the cumulated amount of Inter-critical and Transformed Ferrite is between 15% and 65%.

A high-strength cold-rolled steel sheet having a high yield ratio and excellent stretch flangeability and a method for manufacturing the steel sheet. The high-strength cold-rolled steel sheet has a chemical composition including, by mass %, C: 0.10 to 0.30%, Si: 0.50 to 2.00%, Mn: 2.5 to 4.0%, P: 0.050% or less, S: 0.020% or less, Al: 0.10% or less, N: 0.01% or less, Ti: 0.100% or less, and B: 0.0003 to 0.0030%, with the balance being Fe and incidental impurities. N and Ti satisfy a specified formula, and the total area fraction of martensite and bainite is 95% or more. The number density of bainite grains having an area of 3 μm.sup.2 or more and a carbon concentration of less than 0.7C is 1200 grains/mm.sup.2 or less.

The present invention is to provide a steel plate with physical properties superior to existing steel plates used in fields such as industrial machinery, especially excellent low-temperature impact toughness along with high strength and high hardness, and a method for manufacturing same.

Disclosed in the present invention is complex-phase steel having high hole expansibility. The complex-phase steel has a microstructure of ferrite and bainite. The complex-phase steel having high hole expansibility comprises the following chemical elements in percentage by mass: C: 0.06-0.09%, Si: 0.05-0.5%, Al: 0.02-0.1%, Mn: 1.5-1.8%, Cr: 0.3-0.6%, Nb≤0.03%, Ti: 0.05-0.12%, and the balance of Fe and inevitable impurities. In addition, also disclosed in the present invention is a manufacturing method for the foregoing complex-phase steel having high hole expansibility. The method comprises the following steps: (1) smelting and casting; (2) heating; (3) hot-rolling; (4) phosphorous removal; (5) laminar cooling: a relaxation time period is controlled to be 0-8 s, and a laminar cooling rate is 40-70° C./s; (6) coiling; (7) leveling; and (8) pickling. The complex-phase steel having high hole expansibility can simultaneously satisfy the requirements for hole expansibility and good plasticity.

Provided are a cryogenic steel plate and a method for manufacturing the same, the cryogenic steel plate comprising, in wt %, 0.04 to 0.08% carbon (C), 8.9 to 9.3% nickel (Ni), 0.6 to 0.7% manganese (Mn), and 0.2 to 0.3% silicon (Si), and 50 ppm or less of P, 10 ppm or less of S, and the remainder in iron (Fe) and various unavoidable impurities, and the microstructure at a ¼t location of the steel plate, where t is a thickness of the steel plate, comprising, in % surface area, 10% or more of tempered bainite, 10% or less of residual austenite, and the remainder of tempered martensite.