A method for producing a steel plate member (SPM), including: a quenching step for heating the SPM to a temperature higher than an austenite transformation finish temperature A3 and subsequently cooling the SPM at a cooling rate (CR) faster than an upper critical CR; and a tempering step for reheating a second region of the SPM to a temperature higher than an austenite transformation start temperature A1 without reheating a first region of the SPM after quenching and subsequently cooling the SPM at a CR slower than a lower critical CR. In the cooling process of the tempering step, the shape of the second region is corrected in a temperature range from a temperature equal to or lower than A1 to a temperature equal to or higher than a temperature at which transformation into ferrite and pearlite is finished while maintaining the CR slower than the lower critical CR.

A method for manufacturing a forged component includes: performing hot forging on a material; heating the hot forged material to a first set temperature; and performing warm coining to correctly shape the heated material. The material may be heated to a second set temperature before hot forging. The material heated to the second set temperature may be hot forged. The second set temperature may be higher than the first set temperature. The hot forged material may be subjected to controlled cooling to a third set temperature at a predetermined cooling rate. The controlled cooled material may be heated to the first set temperature. The third set temperature may be lower than or equal to the first set temperature.

This plated steel includes: a steel; and a plating layer formed on the steel, in which the plating layer contains, as a chemical composition, by mass %, Zn: 1.0% to 30.0%, Mg: 0% to 10.0%, Si: 0.05% to 10.0%, Fe: 0 to 10.0%, 0% to 5.00% in total of one or two or more selected from Ca: 0% to 3.00%, Sb: 0% to 0.50%, Pb: 0% to 0.50%, Sr: 0% to 0.50%, Sn: 0% to 1.00%, Cu: 0% to 1.00%, Ti: 0% to 1.00%, Ni: 0% to 1.00%, Mn: 0% to 1.00%, Cr: 0% to 1.00%, La: 0% to 1.00%, Ce: 0% to 1.00%, Zr: 0% to 1.00%, and Hf: 0% to 1.00%, and a remainder of Al and impurities, a microstructure of the plating layer contains an phase which is a solid solution of Al and Zn, and the phase contains a Zn phase having a grain size of 10 to 200 nm in a number density of 10/100 m.sup.2 or more.

This plated steel includes: a steel; and a plating layer formed on the steel, in which the plating layer contains, as a chemical composition, by mass %, Zn: 1.0% to 30.0%, Mg: 0% to 10.0%, Si: 0.05% to 10.0%, Fe: 0 to 10.0%, 0% to 5.00% in total of one or two or more selected from Ca: 0% to 3.00%, Sb: 0% to 0.50%, Pb: 0% to 0.50%, Sr: 0% to 0.50%, Sn: 0% to 1.00%, Cu: 0% to 1.00%, Ti: 0% to 1.00%, Ni: 0% to 1.00%, Mn: 0% to 1.00%, Cr: 0% to 1.00%, La: 0% to 1.00%, Ce: 0% to 1.00%, Zr: 0% to 1.00%, and Hf: 0% to 1.00%, and a remainder of Al and impurities, a microstructure of the plating layer contains an phase which is a solid solution of Al and Zn, and the phase contains a Zn phase having a grain size of 10 to 200 nm in a number density of 10/100 m.sup.2 or more.

One aspect, this invention relates to a carbide-free bainite and retained austenite steel including a composition designed and processed such that the carbide-free bainite and retained austenite steel meets property objectives comprising a yield strength in a range of about 1000-2000 MPa, a uniform ductility, a desired total elongation and hole-expansion ratio, a desired level of weldability and an austenite stability designed to have an austenite start temperature M.sub.s.sup. to be equal to an application temperature in range from about 50 C. to 50 C. The property objectives are design specifications of the carbide-free bainite and retained austenite steel.

One aspect, this invention relates to a carbide-free bainite and retained austenite steel including a composition designed and processed such that the carbide-free bainite and retained austenite steel meets property objectives comprising a yield strength in a range of about 1000-2000 MPa, a uniform ductility, a desired total elongation and hole-expansion ratio, a desired level of weldability and an austenite stability designed to have an austenite start temperature M.sub.s.sup. to be equal to an application temperature in range from about 50 C. to 50 C. The property objectives are design specifications of the carbide-free bainite and retained austenite steel.

This invention generally relates to a method for producing parts of AHS steel via a controlled local cooling by a cooling medium and an interrupted cooling at a required temperature, without immersion in a cooling bath, to thereby create a multiphase microstructure. Typically, the steel part may be cooled by a jet of cooling medium so that, depending on the amount of heat which needs to be removed from the surface of the part, the locations from which a larger amount of heat needs to be removed are cooled at a higher intensity.

This hot-rolled steel sheet has a desired chemical composition, a microstructure contains, in area %, ferrite: 10 to 30%, bainite: 40 to 85%, retained austenite: 5 to 30%, fresh martensite: 5% or less, and pearlite: 5% or less, the ferrite has an average particle size of 5.00 m or less, a difference between an average nanoindentation hardness of the ferrite and an average nanoindentation hardness of the bainite is 1,000 MPa or less, and the tensile strength is 980 MPa or more.

This hot-rolled steel sheet has a desired chemical composition, a microstructure contains, in area %, ferrite: 10 to 30%, bainite: 40 to 85%, retained austenite: 5 to 30%, fresh martensite: 5% or less, and pearlite: 5% or less, the ferrite has an average particle size of 5.00 m or less, a difference between an average nanoindentation hardness of the ferrite and an average nanoindentation hardness of the bainite is 1,000 MPa or less, and the tensile strength is 980 MPa or more.

Provided are a hot rolled steel having small compressive strength loss after being processed into a steel pipe, and a manufacturing method therefor. The hot rolled steel of the present invention comprises, by wt %, 0.15% or less of carbon (C), 2.5% or less of silicon (Si), 2.0% or less of manganese (Mn), 0.05% or less of aluminum (Al), nitrogen (N) and boron (B) in a sum of 0.002 to 0.008%, and a balance of Fe and inevitable impurities, wherein 250<450C+95Si+70Mn is satisfied, a microstructure is a mixed structure of ferrite and pearlite, the average grain size of the ferrite is 8 to 25 m, and a value defined by relation 1 is less than 20%.