A cold-rolled steel plate (1) and a manufacturing method therefor. The chemical composition of the steel plate (1) in percentage by weight is: C 0.15-0.25%, Si 1.50-2.50%, Mn 2.00-3.00%, P≤0.02%, S≤0.01%, Al 0.03-0.06%, N≤0.01%, with the balance being Fe and impurities. The surface layer has an inner oxide layer (2) with a thickness of 1-5 μm, and there is no enrichment of Si or Mn on the surface. The steel plate (1) has good phosphating performance and formability, with a tensile strength of ≥1180 MPa and an elongation of ≥14%, and has a complex-phase structure of ferrite, martensite, and retained austenite, the content of the retained austenite being not lower than 5%. A dew point is at −25° C. to 10° C. in continuous annealing, such that external oxidation transitions to internal oxidation.

Provided is a hot rolled steel sheet having excellent formability and fatigue properties comprising, in percentage by weight: 0.3-0.8% of C; 13-25% of Mn; 0.1-1.0% of V; 0.005-2.0% of Si; 0.01-2.5% of Al; 0.03% or less of P; 0.03% or less of S; 0.04% or less (excluding 0%) of N; and the balance being Fe and inevitable impurities, wherein, when viewed in a cross section in the thickness direction, the hot rolled steel sheet comprises, by area fraction, 20-70% of a non-recrystallized structure and 30-80% of a recrystallized structure.

The invention relates to a stainless steel. The stainless steel consists of in weight % (wt. %): TABLE-US-00001 C 0.32-0.50 Si 0.1-1.0 Mn 0.1-0.8 Cr11-14 Mo 1.8-2.6 V 0.35-0.70 N 0.05-0.19 optional elements, balance Fe and impurities.

A blank material is formed from a steel sheet, a first quenching of the blank material is performed, and a second quenching of the blank material is performed after the first quenching. When the first quenching is performed, the blank material is heated to a first temperature of not lower than (Ac3 point—50)° C. nor higher than 1200° C. at an average heating rate of 2° C./sec or more, and the blank material is cooled from the first temperature to a second temperature of 250° C. or lower. When the second quenching is performed, the blank material is heated from the second temperature to a third temperature of not lower than (Ac3 point—50)° C. nor higher than 1200° C. at an average heating rate of 2° C./sec or more, and the blank material is cooled from the third temperature to a fourth temperature of 250° C. or lower. Forming of the blank material is performed in the first quenching or the second quenching or both of the above.

A hot-rolled coated steel sheet including: in wt %, C: 0.05-0.14%, Si: 0.1-1.0%, Mn: 1.0-2.0%, P: 0.001-0.05%, S: 0.001-0.01%, AI: 0.01-0.1%, Cr: 0.005-1.0%, Ti: 0.005-0.13%, Nb: 0.005-0.03%, N: 0.001-0.01%, Fe residues, and other inevitable impurities; a mixed structure of ferrite and bainite as a main phase; and as a remaining structure, one or more selected from the group consisting of martensite, austenite, and phase martensite (MA), wherein a fraction of the ferrite and bainite is 95-99 area % and Equation 1 is satisfied. [Equation 1] FCO.sub.{110}<112>+FCO.sub.{112}<111>≥10 where, FCO.sub.{110}<112> and FCO.sub.{112}<111>, each representing an area fraction occupied by a structure having ac crystal orientation of {110}<112> and {112}<111>.

Steel material whose constituent grains comprise a matrix in which precipitates are incorporated, the precipitates comprising at least one metallic element selected from a metallic element M, a metallic element M′, a metallic element M″ or mixtures thereof; the microstructure of the steel being such that the grains are equiaxed and the average grain size being such that the average of their largest dimension “Dmax” and/or the average of their smallest dimension “Dmin” is comprised between 10 μm and 50 μm. The steel material has optimized, stable and isotropic mechanical properties, in particular so that the steel material can best withstand mechanical and/or thermal stresses.

What is provided is a high-strength steel sheet including, by mass %: C: 0.05% to 0.15%; Si: 1.5% or less; Mn: 2.00% to 5.00%; P: 0.100% or less; S: 0.010% or less; Al: 0.001% to 2.000%; N: 0.010% or less; and a remainder of Fe and impurities, in which Ceq defined by Ceq=C+Si/90+Mn/100+1.5P+3S is less than 0.21, the high-strength steel sheet contains martensite in an area ratio of 98% or more, and a residual structure is in an area ratio of 2% or less, a two-dimensional homogeneous dispersion ratio S defined by S=Sy.sup.2/Sx.sup.2 (Sx.sup.2 is a dispersion value of Mn concentration profile data in a sheet width direction, and Sy.sup.2 is a dispersion value of the Mn concentration profile data in a sheet thickness direction) is 0.85 or more and 1.20 or less, and a tensile strength is 1200 MPa or more.

A hot rolled steel sheet having a composition including the following elements, 0.18%≤Carbon≤0.3%, 1.8%≤Manganese≤4.5%. 0.8%≤Silicon≤2%,0 001%≤Aluminum ≤0.2%, 0.1%≤Molybdenum≤1%, 0.001%≤Titanium≤0.2%, 0.0001%≤Boron≤0.01%, 0%≤Phosphorus≤0.09%, 0%≤Sulfur≤0.09%.,0%≤Nitrogen≤0.09%, 0%≤Chromium≤0.5%, 0%≤Niobium≤0.1%, 0%≤Vanadium≤0.5%, 0%≤Nickel≤1%, 0%≤Copper≤1%, 0%≤Calcium≤0.005%, 0%≤Magnesium≤0.0010% the remainder composition being composed of iron and unavoidable impurities caused by processing, the microstructure of said steel sheet including in area fraction, of at least 70% martensite, 8% to 25% residual austenite, wherein the shape factor of the residual austenite is between 4 and 12.

In the present invention, provided is a steel sheet having a predetermined chemical composition and a metallographic structure, in which A/B, which is a ratio of a length A of an interface between epitaxial ferrite and ferrite to a length B of an interface between the epitaxial ferrite and martensite in a cross section that is along a rolling direction and perpendicular to a surface of the steel sheet at a position of ¼ of a sheet thickness from the surface of the steel sheet is more than 1.5, a ratio of a major axis to a minor axis of the martensite is 5.0 or more, and a tensile strength is 980 MPa or more.

This hot-rolled steel sheet has a predetermined chemical composition, in which a metallographic structure contains, by area %, less than 3.0% of residual austenite, 15.0% or more and less than 60.0% of ferrite, and less than 5.0% of pearlite, has a ratio L.sub.60/L.sub.7 of a length L.sub.60 of a grain boundary having a crystal misorientation of 60° to a length L.sub.7 of a grain boundary having a crystal misorientation of 7° about a <110> direction of 0.60 or more, has a standard deviation of a Mn concentration of 0.60 mass % or less, and has a tensile strength of 980 MPa or more.