Disclosed are a pressure vessel steel sheet and a method for manufacturing the same, the steel sheet comprising: by wt %, 0.10-0.20% of C, 0.15-0.40% of Si, 1.15-1.50% of Mn, 0.45-0.60% of Mo, 0.03-0.30% of Cu, 0.025% or less of P, 0.025% or less of S and 0.005-0.06% of sol. Al; two or more selected from the group consisting of 0.03-0.30% of Cr, 0.002-0.025% of Nb and 0.002-0.025% of Zr, and the balance of Fe and inevitable impurities, wherein the structure comprises a mixture structure of ferrite, perlite and tempered bainite after post weld heat treatment (PWHT) for 60 hours at 600-660° C., and the area fraction of the tempered bainite is at least 10% (excluding 100%).

An electric resistance welded steel pipe, and a method for manufacturing the same are provided. An electric resistance welded steel pipe has a welded portion that includes a heat-affected zone having a steel microstructure principally including a bainitic ferrite phase and/or a bainite phase. The steel microstructure at half the wall thickness includes a bainitic ferrite phase and/or a bainite phase in a total area ratio of 90% or more. In the steel microstructure located 1 mm in the wall thickness direction, the bainitic ferrite phase and/or the bainite phase has an average grain size of 20 μm or less. The average grain size of the bainitic ferrite phase and/or the bainite phase located 1 mm in the wall thickness direction is 0.5 times or more and 2 times or less the average grain size of the bainitic ferrite phase and/or the bainite phase at half the wall thickness.

A method for manufacturing of steel tube from a long steel strip, including providing a length of steel strip material to the process, forming a tube of the steel strip material, welding the formed tube in longitudinal direction, giving the tube a heat treatment wherein the mentioned steps are performed in one continuous in-line manufacturing line and the heat treatment includes a heating regime such that in successive cross-sections of the tube a microstructure is achieved which holds at least 50 vol % austenite and a cooling trajectory to re-introduce ferrite, and/or bainite in desired volume fractions.

A method for manufacturing of steel tube from a long steel strip, including providing a length of steel strip material to the process, forming a tube of the steel strip material, welding the formed tube in longitudinal direction, giving the tube a heat treatment wherein the mentioned steps are performed in one continuous in-line manufacturing line and the heat treatment includes a heating regime such that in successive cross-sections of the tube a microstructure is achieved which holds at least 50 vol % austenite and a cooling trajectory to re-introduce ferrite, and/or bainite in desired volume fractions.

Provided is an electric resistance welded steel pipe or tube having excellent fatigue durability after rapid and short-time heating quenching treatment. An electric resistance welded steel pipe or tube comprises: a base metal being a steel sheet having a specific chemical composition and an electric resistance weld portion having a bond width of 40×10.sup.−6 m or more and 120×10.sup.−6 m or less, wherein C.sub.0-C.sub.1 is 0.05 mass % or less, where C.sub.0-C.sub.1 is a difference between C.sub.1 in mass % which is a minimum C content of the electric resistance weld portion and C.sub.0 in mass % which is a C content of the steel sheet, and a depth of a total decarburized layer in each of an inner surface layer and an outer surface layer of the electric resistance welded steel pipe or tube is 50×10.sup.−6 m or less.

A spot welded joint of at least two steel sheets is provided. At least one of the steel sheets presents yield strength above or equal to 600 MPa, an ultimate tensile strength above or equal to 1000 MPa, uniform elongation above or equal to 15%. The base metal chemical composition includes 0.05≤C≤0.21%, 4.0≤Mn≤7.0%, 0.5≤Al≤3.5%, Si≤2.0%, Ti≤0.2%, V≤0.2%, Nb≤0.2%, P≤0.025%, B≤0.0035%, and the spot welded joint contains a molten zone microstructure containing more than 0.5% of Al and containing a surface fraction of segregated areas lower than 1%, said segregated areas being zones larger than 20 μm.sup.2 and containing more than the steel nominal phosphorus content.

A spot welded joint of at least two steel sheets is provided. At least one of the steel sheets presents yield strength above or equal to 600 MPa, an ultimate tensile strength above or equal to 1000 MPa, uniform elongation above or equal to 15%. The base metal chemical composition includes 0.05≤C≤0.21%, 4.0≤Mn≤7.0%, 0.5≤Al≤3.5%, Si≤2.0%, Ti≤0.2%, V≤0.2%, Nb≤0.2%, P≤0.025%, B≤0.0035%, and the spot welded joint contains a molten zone microstructure containing more than 0.5% of Al and containing a surface fraction of segregated areas lower than 1%, said segregated areas being zones larger than 20 μm.sup.2 and containing more than the steel nominal phosphorus content.

The steel sheet of the present invention has a steel microstructure containing, in area fraction, martensite: 20% to 100%, ferrite: 0% to 80%, and another metal phase: 5% or less, in which, on a surface of the steel sheet, a ratio of dislocation density in metal phases at a widthwise edge of the steel sheet to dislocation density in the metal phases at a widthwise center of the steel sheet is 100% to 140%, and, at a thicknesswise center of the steel sheet, a ratio of dislocation density in the metal phases at the widthwise edge of the steel sheet to dislocation density in the metal phases at the widthwise center of the steel sheet is 100% to 140%. 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 method of manufacturing a hot-stamped part includes: inserting a blank into a heating furnace including a plurality of sections with different temperature ranges; step heating the blank in multiple stages; and soaking the blank at a temperature of about Ac3 to about 1,000° C., wherein in the step of heating the blank, a temperature condition in the heating furnace satisfies the following equation: 0<(Tg−Ti)/Lt<0.025° C./mm, where Tg denotes a soaking temperature (° C.), Ti denotes an initial temperature (° C.) of the heating furnace, and Lt denotes a length (mm) of step heating sections.

To provide a method and apparatus for manufacturing a joined member that inhibit occurrence of cracks in a joined member even when the joined portion is quenched when members are welded together. The method includes placing the first member D and the second member E with a joint target portion Df and a joint target portion Ef being in contact with each other, welding the joint target portions by heating, subjecting the first member D after the welding to a process for inhibiting occurrence of cracks, and tempering a portion where the first and second members have been welded to each other by electromagnetic heating. The apparatus includes a first electrode 11 to contact with the first member D; a second electrode 12 to contact with the second member E; and an induction heating coil 23 for performing induction heating of a portion where a joint target portions Df and Ef have been contacted and joined to each other, and the induction heating coil 23 is placed between the two electrodes 11 and 12 when the induction heating is performed.