A high-strength and high-fatigue-life steel for a cable, which comprises, in addition to Fe, the following chemical elements in percentages by mass: 0.90-1.00% of C; 0.90-1.50% of Si; 0.25-0.58% of Mn; 0.20-1.00% of Cr; 0.03-0.12% of V; and 0.0008-0.0025% of Ca. In addition, further provided are a wire rod made of the high-strength and high-fatigue-life steel for a cable and a preparation method for the wire rod.

A hot-pressed member having a specified chemical composition in which Ti/Sb 2.0≤Ti/Sb≤20.0 is satisfied and a method for producing the hot-pressed member. The hot-pressed member has a microstructure in which a volume fraction of martensite is 95% or more, an average grain size of prior austenite is 7 μm or less, the number of cementite grains with a grain size of 0.10 μm or more is 2 grains/μm.sup.2 or more, and the number of Ti-based carbide grains with a grain size of 0.10 μm or less is 0.20 grains/μm.sup.2 or more, and a tensile strength of 2100 MPa or higher.

This steel sheet has a predetermined chemical composition, at a depth position of ¼ of a sheet thickness from a surface, an area fraction of GAM.sub.0.5-1.7 is 50% or more and 100% or less, an area fraction of GAM.sub.>1.7 is 0% or more and 20% or less, an area fraction of GAM.sub.≤0.5 is 0% or more and less than 50%, an area fraction of residual austenite is 0% or more and less than 4%, a total area fraction of the residual austenite, fresh martensite, cementite and pearlite is 0% or more and 10% or less, an average grain size is 15.0 μm or less, an average dislocation density is 1.0×10.sup.14/m.sup.2 or more and 4.0×10.sup.15/m.sup.2 less, a total of pole densities of {211}<011> and {332}<113> in a thickness middle portion is 12.0 or less, and a tensile strength is 980 MPa or more.

A high-strength member having excellent delayed fracture resistance, a method for manufacturing the high-strength member, and a method for manufacturing a steel sheet for the high-strength member. The high-strength member has a bent ridge portion obtained by using a steel sheet having a tensile strength of 1470 MPa or more, an edge surface of the bent ridge portion has a residual stress of 800 MPa or less, and a longest crack among cracks that extend from the edge surface of the bent ridge portion in a bent ridge direction D1 has a length of 10 μm or less.

A flat steel product and a method of making a flat steel product having a hole expansion of at least 60%, a yield strength of at least 660 MPa, a tensile strength of at least 760 MPa, and an elongation at break of at least 10%. The flat steel product is made from a complex-phase steel, which includes (in wt %) C: 0.01-0.1%, Si: 0.1-0.45%, Mn: 1-2.5%, Al: 0.005-0.05%, Cr: 0.5-1%, Mo: 0.05-0.15%, Nb: 0.01-0.1%, Ti: 0.05-0.2%, N: 0.001-0.009%, P: <0.02%, S: <0.005%, Cu: ≤0.1 %, Mg: ≤0.0005 %, O: <0.01 %, optionally one or more of Ni, B, V, Ca, Zr, Ta, W, REM, and Co, where Ni: ≤1%, B: ≤0.005%, V: ≤0.3%, Ca: 0.0005-0.005%, Zr, Ta, W: in total ≤2%, REM: 0.0005-0.05%, and Co: <1%, and iron and unavoidable impurities as the remainder, where % Ti>(48/14)% N+(48/32)% S and % Nb<(93/12)% C+(45/14)% N+(45/32)% S. The structure of the flat steel product includes (in area %) ≥80% bainite, <15% ferrite, <15% martensite, <5% cementite, and <5 vol % retained austenite.

A medium manganese cold-rolled steel intermediate product having an improved fts value is disclosed, the alloy having a carbon fraction within the range 0.003 wt %<C<0.12 wt %, a manganese fraction (Mn) within the range 3.5 wt %<Mn<12 wt %, a silicon fraction (Si) and/or an aluminium fraction (Al) as alloy fractions, where Si wt %+Al wt %<1, optionally further alloy fractions, optional microalloy fractions, in particular a titanium fraction (Ti) and/or a niobium fraction (Nb) and/or vanadium fraction (V), and the remainder of the alloy has iron (Fe) and unavoidable impurities of a melt. A method is also disclosed having the following step that is carried out after the cold-rolling step performing an intercritical box annealing process at a maximum annealing temperature of 684° C.−(517° C.*the carbon fraction in wt %).

A high-strength steel sheet having high delayed fracture resistance and a method for manufacturing the high-strength steel sheet. The high-strength steel sheet has a specified chemical composition. Relative to the whole microstructure of the steel sheet, the total area fraction of at least one of (i) bainite containing carbide grains having an average grain size of 50 nm or less and (ii) martensite containing carbide grains having an average grain size of 50 nm or less is 90% or more. The average number of inclusions having an average grain size of 5 μm or more that are present in a section of the steel sheet perpendicular to a rolling direction is 5.0/mm.sup.2 or less.

An aspect of the present invention relates to a cold-rolled steel plate for hot forming, which is excellent in corrosion-resistance and spot-weldability, contains, by weight %, C: 0.1-0.4%, Si: 0.5-2.0%, Mn: 0.01-4.0%, Al: 0.001-0.4%, P: 0.001-0.05%, S: 0.0001-0.02%, Cr: 0.5% to less than 3.0%, N: 0.001-0.02%, and a balance of Fe and inevitable impurities, satisfying formula (1) below, and includes an Si amorphous oxidation layer continuously or discontinuously formed at a thickness of 1 nm-100 nm on the surface thereof. Formula (1): 1.4≤0.4*Cr+Si≤3.2 (wherein element symbols denote measurements of respective element contents by weight %).

Provided is a high-strength cold rolled steel sheet, a high-strength hot-dip galvanized steel sheet manufactured using the cold rolled steel sheet, and manufacturing methods therefor, the high-strength cold rolled steel sheet comprising, by wt %, 0.17-0.21% of carbon (C), 0.3-0.8% of silicon (Si), 2.7-3.3% of manganese (Mn), 0.3-0.7% of chromium (Cr), 0.01-0.3% of aluminum (Al), 0.01-0.03% of titanium (Ti), 0.001-0.003% of boron (B), 0.04% or less of phosphorus (P), 0.02% or less of sulfur (S), 0.01% or less of nitrogen (N) and the balance of iron (Fe) and other inevitable impurities, wherein the amounts of carbon (C), silicon (Si) and aluminum (Al) satisfy the following mathematical relation (1). [Mathematical relation (1)] [C]+([Si]+[Al])/5≤0.35% (wherein [C], [Si] and [Al] respectively mean the wt % of C, Si and Al.)

A cold-rolled and heat treated steel sheet, has a composition comprising 0.1% ≤ C≤0.4%, 3.5%≤Mn≤8.0%, 0.1%≤Si≤1.5%, Al≤ 3%, Mo≤0.5%, Cr≤1%, Nb≤0.1%, Ti≤0.1 %, V≤0.2%, B≤0.004%, 0.002%≤N≤0.013%, S≤0.003%, P≤0.015%. The structure consists of, in surface fraction: between 8 and 50% of retained austenite, at most 80% of intercritical ferrite, the ferrite grains, if any, having an average size of at most 1.5 .Math.m, and at most 1% of cementite, the cementite particles having an average size lower than 50 nm, martensite and/or bainite.