Provided is a steel sheet suitably used for an automobile structural member and the like, and more particularly, to a steel sheet having high strength and excellent formability that may prevent processing defects such as cracks or wrinkles occurring during the press-forming, and a method for producing the same.

Provided is a low-density clad steel sheet having excellent formability and fatigue properties, including a base material; and cladding materials provided on both side surfaces of the base material, wherein the base material is a lightweight steel sheet including, by weight, C: 0.3 to 1.0%, Mn: 4.0 to 16.0%, Al: 4.5 to 9.0%, and a remainder of Fe and inevitable impurities, and each of the cladding materials is martensitic carbon steel including, by weight, C: 0.1 to 0.45%, Mn: 1.0 to 3.0%, and a remainder of Fe and inevitable impurities.

Provided is a high-strength cold rolled steel sheet, a high-strength hot-dip galvanized steel sheet manufactured using the cold rolled steel sheet, and manufacturing methods therefor, the high-strength cold rolled steel sheet comprising, by wt %, 0.17-0.21% of carbon (C), 0.3-0.8% of silicon (Si), 2.7-3.3% of manganese (Mn), 0.3-0.7% of chromium (Cr), 0.01-0.3% of aluminum (Al), 0.01-0.03% of titanium (Ti), 0.001-0.003% of boron (B), 0.04% or less of phosphorus (P), 0.02% or less of sulfur (S), 0.01% or less of nitrogen (N) and the balance of iron (Fe) and other inevitable impurities, wherein the amounts of carbon (C), silicon (Si) and aluminum (Al) satisfy the following mathematical relation (1). [Mathematical relation (1)] [C]+([Si]+[Al])/5≤0.35% (wherein [C], [Si] and [Al] respectively mean the wt % of C, Si and Al.)

A hot-rolled flat steel product having a thickness of <1.5 and consisting of, in % by mass, C: 0.04-0.23%, Si: 0.04 0.54%, Mn: 1.4-2.9%, Ti+V, wherein 0.005%<%Ti+%V<0.15%, and optionally one or more elements of Al, Cr, Mo, and B, where AI: 0.01-1.5%, 0.02<%Mo+%Cr<1.4%, and B: 0.0005-0.005%, and the remainder consisting of iron and inevitable impurities. The structure of the flat steel product consists of, in percent by area, in sum, 50-90% ferrite and bainite ferrite, 5-50% martensite, 2-15% residual austenite and <10% other structure elements. The flat steel product has a yield point Rp0.2>290 MPa, a tensile strength Rm>490 MPa and an elongation at break A80 where A80[%]=B−Rm/37 with 31<B<51. To at least one surface of the flat steel product, a Zn coating is applied by hot-dip coating. Also a method for producing a flat steel product of this kind.

In accordance with one aspect of the present disclosure, there is provided a steel material for a tailor-welded blank, including 0.04 to 0.06 wt % of carbon (C), 1.2 to 1.5 wt % of manganese (Mn), 0.01 to 0.10 wt % of titanium (Ti), 0.01 to 0.10 wt % of niobium (Nb), and the balance of iron (Fe) and inevitable impurities; having a tensile strength (TS) of 550 MPa or greater, a yield strength (YS) of 300 MPa or greater, and an elongation (EL) of 20% or greater; and having a dual-phase structure of ferrite and martensite.

An ultra low carbon interstitial free steel having an improved composition, wherein Ti+Nb+V is max 0.10 weight %. This steel has a very good ductility.

A method for producing a high strength uncoated steel sheet having an improved strength and an improved formability, including the steps of: providing an uncoated steel sheet; annealing the sheet at an annealing temperature TA higher than 865° C. but less than 1000° C. for a time of more than 30 s; cooling the sheet down to a quenching temperature QT between 310° C. and 375° C., at a cooling speed of at least 30° C./s; heating the sheet up to a partitioning temperature PT between 370° C. and 470° C. and maintaining the sheet at the partitioning temperature for a partitioning time Pt between 50 s and 150 s; and cooling the sheet down to the room temperature.

A rolled steel sheet, for press hardening is provided, having a chemical composition where Ti/N>3.42, and the carbon, manganese, chromium and silicon contents satisfy:

[00001]$2.6C+\frac{\mathrm{Mn}}{5.3}+\frac{\mathrm{Cr}}{13}+\frac{\mathrm{Si}}{15}\ge 1.1\%.$

The sheet has a nickel content Ni.sub.surf at any point of the steel in the vicinity of the surface over a depth Δ, such that: Ni.sub.surf>Ni.sub.nom, Ni.sub.nom denoting the nominal nickel content of the steel, and such that, Ni.sub.max denoting the maximum nickel content within Δ:

[00002]$\frac{\left({\mathrm{Ni}}_{\max }+{\mathrm{Ni}}_{\mathrm{nom}}\right)}{2}\times \left(\Delta \right)\ge 0.6,$

and such that:

[00003]$\frac{\left({\mathrm{Ni}}_{\max }-{\mathrm{Ni}}_{\mathrm{nom}}\right)}{\Delta }\ge 0.01$

and the surface density of all of the particles D.sub.i , and the surface density of the particles D.sub.(>2 μm) larger than 2 micrometers satisfy, at least to a depth of 100 micrometers in the vicinity of the surface of said sheet:

D.sub.i+6.75 D.sub.(>2 μm)<270

D.sub.i and D.sub.(>2 μm) being expressed as number of particles per square millimeter, and said particles denoting all the oxides, sulfides, and nitrides, either pure or combined such as oxysulfides and carbonitrides, present in the steel matrix.

A steel sheet for the manufacture of a press hardened part is provided, having a composition of: 0.15%≤C≤0.22%, 3.5%≤Mn<4.2%, 0.001%≤Si≤1.5%, 0.020%≤Al≤0.9%, 0.001%≤Cr≤1%, 0.001%≤Mo≤0.3%, 0.001%≤Ti≤0.040%, 0.0003%≤B≤0.004%, 0.001%≤Nb≤0.060%, 0.001%≤N≤0.009%, 0.0005%≤S≤0.003%, 0.001%≤P≤0.020%. A microstructure has less than 50% ferrite, 1% to 20% retained austenite, cementite, such that the surface density of cementite particles larger than 60 nm is lower than 10{circumflex over ( )}7/mm.sup.2, and a complement of bainite and/or martensite, the retained austenite having an average Mn content of at least 1.1*Mn %. Press-hardened steel part obtained by hot forming the steel sheet, and manufacturing methods thereof.

A heat treated and cold rolled steel sheet having a composition including of the following elements 0.09%≤Carbon≤0.15%, 1.8%≤Manganese≤2.5%, 0.2%≤Silicon≤0.7%, 0.01%≤Aluminum≤0.1%, 0%≤Phosphorus≤0.09%, 0%≤Sulfur≤0.09%, 0%≤Nitrogen≤0.09%, 0%≤Niobium≤0.1%, 0%≤Titanium≤0.1%, 0%≤Chromium≤1%, 0%≤Molybdenum≤1%, 0%≤Vanadium≤0.1%, 0%≤Calcium≤0.005%, 0%≤Boron≤0.01%, 0%≤Cerium≤0.1%, 0%≤Magnesium≤0.05%, 0%≤Zirconium≤0.05% the remainder composition being composed of iron and unavoidable impurities caused by processing, the microstructure of said steel sheet comprising in area fraction, 65 to 85% Tempered Martensite, 0% to 5% Residual Austenite and a cumulative presence of Ferrite and Bainite between 15 and 35%.