A method of manufacturing steel strip including the steps of: casting molten steel into slabs; reheating the slabs at 1150° C. or more for 1 hour or more; hot rolling the steel into a strip, preferably with an average F1 slab entry temperature above 1000° C.; coiling the hot rolled steel strip; batch annealing the steel strip: at an intercritical temperature (i.e. between Ac1 and Ac3), preferably below 700° C.; in non-oxidising and non-nitrogenated atmosphere; total annealing time at least 5 hours, preferably at least 10 hours to get Mn enrichment in austenite such that Mn content is at least 1.25 times bulk Mn content of the steel and C enrichment such that C content is at least 1.2 times bulk C content of the steel; cooling the steel after batch annealing in air, forced air or water quench.

A method and an apparatus for environmentally-friendly batch hot-dip coating of high-performance alloy are provided. The method is that workpiece is heated to the process temperature in the heating box with inner gas before galvanizing. The heating box body consist of two or three zones, which are waiting zone, heating zone and post-plating turnover zone (the post-plating turnover zone can be omitted). A zinc pot is arranged in the heating zone, and the zinc pot is configured for hot-dip coating. Workpieces can be processed with zinc or zinc-based alloys. A transporting device is configured to successively transport in a sealed state the workpiece to be processed to the waiting zone, the heating zone, the zinc pot, and the post-plating turnover zone (the post-plating turnover zone can be omitted). The new method realizes hot-dip coating with zinc and other zinc-based alloys without the use of the flux.

A steel sheet has a specific chemical composition and has a structure represented by, by area ratio, ferrite: 5 to 60%, and bainite: 40 to 95%. When a region that is surrounded by a grain boundary having a misorientation of 15° or more and has a circle-equivalent diameter of 0.3 μm or more is defined as a crystal grain, the proportion of crystal grains each having an intragranular misorientation of 5 to 14° to all crystal grains is 20 to 100% by area ratio. A precipitate density of Ti(C,N) and Nb(C,N) each having a circle-equivalent diameter of 10 nm or less is 10.sup.10 precipitates/mm.sup.3 or more. A ratio (Hvs/Hvc) of a hardness at 20 μm in depth from a surface (Hvs) to a hardness of the center of a sheet thickness (Hvc) is 0.85 or more.

Disclosed is a hot-dip Al—Zn alloy coated steel sheet having excellent anti-corrosion property after coating, and a method for producing the same. In the disclosure, the hot-dip Al—Zn alloy coated steel sheet has a hot-dip coating layer containing by mass %, Al: 25% to 90%, and at least one of Sn: 0.01% to 2.0%, In: 0.01% to 10%, and Bi: 0.01% to 2.0%.

A cold-rolled steel according to the present invention has a predetermined chemical composition, satisfies (5×[Si]+[Mn])/[C]>10 when [C] is the amount of C by mass %, [Si] is the amount of Si by mass %, and [Mn] is the amount of Mn by mass %, includes 40% to 95% ferrite and 5% to 60% martensite in area fraction, and optionally further includes 10% or less pearlite in area fraction, 5% or less retained austenite in volume fraction, and less than 40% bainite in area fraction. The total of the area fraction of ferrite and the area fraction of martensite is 60% or more, the hardness of martensite measured with a nanoindenter satisfies H2/H1<1.10 and σHM<20.

A tailored welded blank produced from at least two blank parts, where at least one is a press-hardenable manganese-boron steel and at least one has a coating of aluminum or an aluminum-based alloy. The parts are welded by laser-beam welding or hybrid laser/gas-metal-arc welding, while retaining the coating, using shielding gas and a filler wire having in % by weight: C: 0.41 to 0.9; Si: 0.4 to 4; Mn: 0.4 to 3; optionally Cr: 0 to 10; and with optional alloying of one or more of: Mo: 0.01 to 1.0; B: 0.0008 to 0.0040; Ti: 2.5×B<=Ti<=5×B; V: 0.01 to 0.4; Nb: 0.01 to 0.2; W: 0.01 to 0.2; the remainder Fe and unavoidable impurities. The high proportion of C and Cr or additionally or alternatively of Mo, V, Nb and/or W enables hardening by carbide formation in a weld-seam region after welding.

A method and an apparatus for environmentally-friendly batch hot-dip coating of high-performance alloy are provided. The method is that workpiece is heated to the process temperature in the heating box with inner gas before galvanizing. The heating box body consist of two or three zones, which are waiting zone, heating zone and post-plating turnover zone (the post-plating turnover zone can be omitted). A zinc pot is arranged in the heating zone, and the zinc pot is configured for hot-dip coating. Workpieces can be processed with zinc or zinc-based alloys. A transporting device is configured to successively transport in a sealed state the workpiece to be processed to the waiting zone, the heating zone, the zinc pot, and the post-plating turnover zone (the post-plating turnover zone can be omitted). The new method realizes hot-dip coating with zinc and other zinc-based alloys without the use of the flux.

A cold-rolled steel according to the present invention has a predetermined chemical composition, satisfies (5[Si]+[Mn])/[C]>10 when [C] is the amount of C by mass %, [Si] is the amount of Si by mass %, and [Mn] is the amount of Mn by mass %, includes 40% to 95% ferrite and 5% to 60% martensite in area fraction, and optionally further includes 10% or less pearlite in area fraction, 5% or less retained austenite in volume fraction, and less than 40% bainite in area fraction. The total of the area fraction of ferrite and the area fraction of martensite is 60% or more, the hardness of martensite measured with a nanoindenter satisfies H2/H1<1.10 and HM<20.

Provided is a high-strength cold-rolled steel sheet having excellent ductility and stretch-flangeability as well as weldability in a range in which a tensile strength is 980 MPa or higher and a 0.2% yield strength is less than 700 MPa (preferably 500 MPa or higher). In the high-strength cold-rolled steel sheet of the present invention, the chemical composition is adjusted as appropriate, and the area ratio of below-mentioned metal structures at a position of sheet thickness in the steel sheet satisfies following requirements: tempered martensite: 10 area % to less than 30 area %, bainite: more than 70 area %, total of tempered martensite and bainite: 90 area % or more, ferrite: 0 area % to 5 area %, and retained austenite: 0 area % to 4 area %. The high-strength cold-rolled steel sheet has excellent ductility, stretch-flangeability, and weldability, and has a tensile strength of 980 MPa or higher and a 0.2% yield strength of less than 700 MPa.

A hot-stamped steel according to the present invention has a predetermined chemical composition, satisfies (5[Si]+[Mn])/[C]>10 when [C] is the amount of C by mass %, [Si] is the amount of Si by mass %, and [Mn] is the amount of Mn by mass %, includes 40% to 95% ferrite and 5% to 60% martensite in area fraction, and optionally further includes 10% or less pearlite in area fraction, 5% or less retained austenite in volume fraction, and less than 40% bainite in area fraction. The total of the area fraction of ferrite and the area fraction of martensite is 60% or more, the hardness of martensite measured with a nanoindenter satisfies H2/H1<1.10 and HM<20, and TS which is product of tensile strength TS and hole expansion ratio is 50000 MPa.Math.% or more.