This application discloses thin metal strips and methods of making thin metal strip. Particular embodiments of such methods include cooling the thin metal strip to a temperature equal to or less than a bainite or a martensite start transformation temperature B.sub.S or M.sub.S to thereby form bainite and/or martensite, respectively, within the thin metal strip, reheating the thin metal strip to a reheat temperature equal to or greater than transformation temperature Ac.sub.3 and holding the thin metal strip at the reheat temperature for at least 2 seconds and thereby forming austenite within the thin metal strip with at least 75% of austenite grains having a grain size equal to or less than 15 m, and rapidly recooling the thin metal strip to a temperature equal to or less than the martensite start transformation temperature M.sub.S and thereby providing finer martensite within the thin metal strip from a finer prior austenite.

High strength steel sheet having a tensile strength of 800 MPa or more comprising a middle part in sheet thickness and a soft surface layer arranged at one side or both sides of the middle part in sheet thickness, wherein each soft surface layer has a thickness of more than 10 M and 30% or less of the sheet thickness, the soft surface layer has an average Vickers hardness of more than 0.60 time and 0.90 time or less the average Vickers hardness of the sheet thickness 1/2 position, and the soft surface layer has a nano-hardness standard deviation of 0.8 or less is provided.

A preferable aspect of the present invention provides: an ultra-high-strength hot-rolled steel sheet containing, by weight, one or two of 0.40-0.60% of C, 0.7-1.5% of Mn, 0.3% or less (excluding 0%) of Si, 0.03% or less (including 0%) of P, 0.004% or less (including 0%) of S, 0.04% or less (excluding 0%) of Al, 0.3% or less (excluding 0%) of Cr, 0.3% or less (excluding 0%) of Mo, 0.9-1.5% of Ni, and 0.9-1.5% of Cu, 1.1% or more of Cu+Ni, 0.04% or less (excluding 0%) of Ti, 0.005% or less (excluding 0%) of B, 0.006% or less (excluding 0%) of N, and the balance Fe and other impurities, the alloy elements satisfying relational formulas 1 and 2 below, wherein a microstructure of the hot-rolled steel sheet comprises, by volume, 7% or more of ferrite and 93% or less of perlite; a steel pipe and a member each using the same; and manufacturing methods therefor. [Relational formula 1] (Mn/Si) #3 (weight ratio) [Relational formula 2] (Ni/Si) #1 (weight ratio)

A method for producing a part includes providing a first and a second precoated sheet (1,2), butt welding the first and second precoated sheets (1) to obtain a blank (15), and heating the blank (15) to a heat treatment temperature at least 10 C. lower than the full austenitization temperature of the weld joint (22) and at least 15 C. higher than a minimum temperature T.sub.min: $T_{\min }\left(C.\right)=\mathrm{AC}3\left(\mathrm{WJ}\right)-\frac{{}_{\mathrm{IC}}^{\max }}{100}\left(\mathrm{Ac}3\left(\mathrm{WJ}\right)-673-40\mathrm{Al}\right).$ where Ac3(WJ) is the full austenitization temperature of the weld joint (22) ${}_{\mathrm{IC}}^{\max }=(1-\frac{\left(1+\right)(}{}$

According to one aspect of the present invention, provided are a high-strength hot-dipped galvanized steel sheet having excellent surface quality and spot weldability, and a manufacturing method therefor.

A steel sheet including a steel micro-structure containing, in volume fraction, tempered martensite: 85% or more, retained austenite: 5% or more to less than 15%, and ferrite, pearlite, bainite, and as-quenched martensite being less than 10% in total, when contents of Mn and C in the retained austenite are denoted by Mn.sub.A and C.sub.A, and when contents of Mn and C in a matrix are denoted by Mn.sub.M and C.sub.M, respectively, following Formulas (1) to (3) are satisfied, and the number of carbides having an equivalent circle radius of 0.1 m or more is 100 or less in a region measuring 20000 m.sup.2, and the steel sheet has a tensile strength of 1100 MPa or more. The steel sheet is excellent in crash resistance and formability.
Mn.sub.A/Mn.sub.M1.2(1)
C.sub.A/C.sub.M5.0(2)
C.sub.A1.0(3)

There is provided a ferritic lightweight steel which contains 2.0 to 3.0 wt % manganese (Mn), 5.0 to 6.0 wt % aluminum (Al) and 0.1 to 0.3 wt % carbon (C) and has a tensile strength of 900 MPa to 1,108 MPa. The lightweight steel includes ferrite-austenite dual grains as a result of performing low-temperature tempering-induced partitioning (LTP) at 300? C. for 10 minutes after isothermal annealing.

A method for heat treating a manganese steel product whose alloy comprises: a carbon fraction (C) between 0.09 and 0.15 wt. %, and a manganese fraction (Mn) in the range of 3.5 wt. %Mn4.9 wt. %, the method comprising: performing a first annealing process (S4.1) with the substeps heating (E1) the steel product to a first holding temperature (T1), which lies above 780 C., holding (H1) the steel product during a first time period (1) at the first holding temperature (T1), cooling (A1) the steel product, performing a second annealing process (S4.2) with the substeps heating (E2) the steel product to a holding temperature (T2), which lies above 630 C. and below 660 C., holding (H2) the steel product during a second time period (2) at the holding temperature (T2), cooling (A2) the steel product.

A multipurpose continuous processing line able for heat treating and hot dip coating a steel strip containing: an annealing section (1) for heating the steel strip to a predetermined annealing temperature and for maintaining the steel strip at said annealing temperature, a first transfer section (2), an overaging section (3) able to maintain the temperature of the steel strip between 300? C. and 700? C., a second transfer section (4) able to adjust the temperature of the steel strip to allow the hot dip coating of the strip and, a hot dip coating section (5), wherein the first transfer section (2) includes, in sequence, a cooling section (21) and a heating section (22).

Provided are a 1500 MPa-grade steel with a high product of strength and elongation for vehicles and a manufacturing method thereof. The mass percentages of the chemical elements thereof are: 0.1-0.3% of C, 0.1-2.0% of Si, 7.5-12% of Mn, 0.01-2.0% of Al, and the balance of iron and other inevitable impurities. The microstructure of the steel with a high product of strength and elongation for vehicles is austenite+martensite+ferrite or austenite+martensite. The steel for vehicles can reach a grade of 1500 MPa, and has a product of strength and elongation of no less than 30 GPa %.