A high-strength steel having excellent low-yield-ratio characteristics, according to one embodiment of the present invention, comprises, by wt %, 0.06-0.12% of C, 0.2-0.5% of Si, 1.5-2.0% of Mn, 0.003-0.05% of Al, 0.01% or less of N, 0.02% or less of P, 0.003% or less of S, 0.05-0.5% of Cr, 0.05-0.5% of Mo, 0.01-0.05% of Nb, 0.0005-0.005% of Ca and the balance of Fe and other inevitable impurities, and comprises polygonal ferrite as a microstructure, wherein the area fraction of the polygonal ferrite is 10-30% and the average hardness of the polygonal ferrite can be 180 Hv or less.

Provided is a high-strength steel sheet having a tensile strength of 780 MPa or higher. The high-strength steel sheet has a low yield ratio and excellent ductility (El) and strain hardening exponent (n) and thus has enhanced processability.

A press hardening method includes the following steps: A. the provision of a steel sheet for heat treatment, precoated with a zinc- or aluminum-based pre-coating for anti-corrosion purpose, B. the deposition of a hydrogen barrier pre-coating over a thickness from 10 to 550 nm, C. the batch annealing of the precoated steel sheet in an inert atmosphere to obtain a pre-alloyed steel sheet, D. the cutting of the pre-alloyed steel sheet to obtain blank, E. the thermal treatment of the blank to obtain a fully austenitic microstructure in the steel, F. the transfer of the blank into a press tool, G. the hot-forming of the blank to obtain a part, H. the cooling of the part obtained at step G).

A press hardening method including the following steps: A. the provision of a steel sheet for heat treatment being optionally coated with a zinc- or aluminum-based pre-coating, B. the flexible rolling of the steel sheet in the rolling direction so as to obtain a steel sheet having a variable thickness, C. the cutting of the rolled steel sheet to obtain a tailored rolled blank, D. the deposition of a hydrogen barrier pre-coating over a thickness from 10 to 550 nm, E. the heat treatment of the tailored rolled blank to obtain a fully austenitic microstructure in the steel, F. the transfer of the tailored rolled blank into a press tool, G. the hot-forming of the tailored rolled blank to obtain a part having a variable thickness,H. the cooling of the part having a variable thickness obtained at step G).

This hot-rolled steel sheet has a predetermined chemical composition, in a microstructure at a 1/4 position of a sheet thickness in a sheet thickness direction from a surface, by area ratios, a primary phase is 95.00% to 98.00% of bainite, a secondary phase is 2.00% to 5.00% of tempered martensite, an average grain size of the secondary phase is 1.5 μm or less, a pole density in a (110)<112> orientation is 3.0 or less, an average grain size of an iron-based carbide is 0.100 μm or less, in a microstructure from the surface to a 1/16 position of the sheet thickness in the sheet thickness direction from the surface, a pole density in a (110)<1-11> orientation is 3.0 or less, and a tensile, strength TS is 980 MPa or more.

A method for producing a coated steel sheet having a tensile strength TS of at least 1450 MPa and a total elongation TE of at least 17% includes the successive steps of providing a cold rolled steel sheet made of a steel having a chemical composition comprising, in weight %: 0.34%≤C≤0.45%, 1.50%≤Mn≤2.30%, 1.50≤Si≤2.40%, 0%<Cr≤0.7%, 0%≤Mo≤0.3%, 0.10%≤Al≤0.7%, and optionally 0%≤Nb≤0.05%, the remainder being Fe and unavoidable impurities, annealing the cold-rolled steel sheet at an annealing temperature AT higher than the Ac3 transformation point of the steel, quenching the annealed steel sheet by cooling it down to a quenching temperature QT lower than the Ms transformation point of the steel and comprised between 150° C. and 250° C., and reheating the quenched steel sheet to a partitioning temperature PT between 350° C. and 450° C. and maintaining the steel sheet at the partitioning temperature PT for a partitioning time Pt of at least 80 s, and coating the steel sheet by galvannealing, with an alloying temperature GAT comprised between 470° C. and 520° C.

Provided is a hot rolled steel sheet comprising a predetermined composition wherein the hot rolled steel sheet comprises ferrite with an average orientation difference in the same grain of 0.5 to 5.0° in 30 to 70 vol %, the ferrite and martensite in a total of 90 vol % or more, and a balance microstructure of 10 vol % or less, has an average grain size of the ferrite of 0.5 to 5.0 μm, and has an average grain size of the martensite and the balance microstructure of 1.0 to 10 μm. Provided is a method for producing a hot rolled steel sheet comprising rolling where two or more consecutive passes of rolling including a final pass are performed under conditions of a rolling temperature: A point or more and less than Ae.sub.3 point, a strain rate: 1.0 to 50/sec, and a time between passes: within 10 seconds and where a total strain amount of all passes satisfying the conditions is 1.4 to 4.0, cooling by a 20° C./sec or more average cooling rate, and coiling the steel sheet at room temperature or more and less than 300° C.

The steel sheet of the present invention has a steel microstructure containing, in area fraction, martensite: from 20% to 100%, ferrite: from 0% to 80%, and another metal phase: 5% or less, and in which a ratio of a dislocation density in metal phases on a surface of the steel sheet to a dislocation density in the metal phases in a thicknesswise central portion of the steel sheet is from 30% to 80%. The maximum amount of warpage of the steel sheet when the steel sheet is sheared to a length of 1 m in a rolling direction is 15 mm or less.

A pillar for a vehicle including at least two different localized areas of different tensile strengths. The pillar includes a body defining a width that merges into sidewalls at a transition. The body having a first tensile strength and the transition has a second tensile strength, wherein the first tensile strength is smaller than the second tensile strength. The variety in tensile strength resulting from at least one of varying the material treatment and varying the gauge. The pillar is press-hardened until it reaches a tensile strength of 1500 MPa to 2000 Mpa.

This hot-rolled steel sheet has a predetermined chemical composition, in a microstructure at a ¼ position of a sheet thickness in a sheet thickness direction from a surface, a primary phase is bainite, a secondary phase is martensite or a martensite-austenite mixed phase, an average grain size of the secondary phase is 1.5 μm or less, an average grain size of particles having grain diameters that are largest 10% or less out of all particles in the secondary phase is 2.5 μm or less, a pole density in a (110)<112> orientation is 3.0 or less, and, in a microstructure from the surface to a 1/16 position of the sheet thickness in the sheet thickness direction from the surface, a pole density in a (110)<1-11> orientation is 3.0 or less.