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 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).

A press hardening method including: A. provision of a steel sheet for heat treatment, being optionally precoated with a zinc- or aluminum-based pre-coating, B. deposition of a hydrogen barrier pre-coating comprising chromium and not comprising nickel over a thickness from 10 to 550 nm, C. cutting of the precoated steel sheet to obtain a blank, D. heat treatment of the blank at a furnace temperature from 800 to 970° C., during a dwell time from 1 to 12 minutes, in an atmosphere having an oxidizing power equal or higher than that of an atmosphere consisting of 1% by volume of oxygen and equal or smaller than that of an atmosphere consisting of 50% by volume of oxygen, such atmosphere having a dew point between −30 and +30° C., E. transfer of the blank into a press tool, F. hot-forming at a temperature from 600 to 830° C. to obtain a part, G. cooling of the part obtained at step E).

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 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 method is for producing a high strength coated steel sheet having a yield stress YS>800 MPa, a tensile strength TS>1180 MPa, and improved formability and ductility. The steel contains: 15%≤C≤0.25%, 1.2%≤Si≤1.8%, 2%≤Mn≤2.4%, 0.1%≤Cr≤0.25%, Al≤0.5%, the remainder being Fe and unavoidable impurities. The sheet is annealed at a temperature higher than Ac3 and lower than 1000° C. for a time of more than 30 s, then quenched by cooling it to a quenching temperature QT between 250° C. and 350° C., to obtain a structure consisting of at least 60% of martensite and a sufficient austenite content such that the final structure contains 3% to 15% of residual austenite and 85% to 97% of martensite and bainite without ferrite, then heated to a partitioning temperature PT between 430° C. and 480° C. and maintained at this temperature for a partitioning time Pt between 10 s and 90 s, then hot dip coated and cooled to the room temperature.

A method of manufacturing a steel sheet plated with an Al—Fe alloy for hot forming. The method includes: aluminum-plating and coiling a base steel sheet to obtain an aluminum-plated steel sheet, where an amount of the aluminum-plating is 30 to 200 g/m.sup.2 based on one surface of the base steel sheet, and tension in the coiling is 0.5 to 5 kg/mm.sup.2; after the aluminum-plating, performing cooling to 250° C. at a rate of 20° C./sec or less; annealing the aluminum-plated steel sheet to obtain the steel sheet plated with the Al—Fe alloy; and cooling the steel sheet plated with the Al—Fe alloy. The annealing is carried out for 30 minutes to 50 hours within a heating temperature range of 550 to 750° C. in a batch annealing furnace.

A method of manufacturing a steel sheet plated with an Al—Fe alloy for hot forming. The method includes: aluminum-plating and coiling a base steel sheet to obtain an aluminum-plated steel sheet, where an amount of the aluminum-plating is 30 to 200 g/m.sup.2 based on one surface of the base steel sheet, and tension in the coiling is 0.5 to 5 kg/mm.sup.2; after the aluminum-plating, performing cooling to 250° C. at a rate of 20° C./sec or less; annealing the aluminum-plated steel sheet to obtain the steel sheet plated with the Al—Fe alloy; and cooling the steel sheet plated with the Al—Fe alloy. The annealing is carried out for 30 minutes to 50 hours within a heating temperature range of 550 to 750° C. in a batch annealing furnace.

A steel sheet has a predetermined chemical composition and a metal structure represented by, in area fraction, polygonal ferrite: 40% or less, martensite: 20% or less, bainitic ferrite: 50% to 95%, and retained austenite: 5% to 50%. In area fraction, 80% or more of the bainitic ferrite is composed of bainitic ferrite grains that have an aspect ratio of 0.1 to 1.0 and have a dislocation density of 8×10.sup.2 (cm/cm.sup.3) or less in a region surrounded by a grain boundary with a misorientation angle of 15° or more. In area fraction, 80% or more of the retained austenite is composed of retained austenite grains that have an aspect ratio of 0.1 to 1.0, have a major axis length of 1.0 μm to 28.0 μm, and have a minor axis length of 0.1 μm to 2.8 μm.