A cold rolled steel sheet having a composition including of 0.05 % ≤ Carbon ≤ 0.15 %, 1.8% ≤ Manganese ≤ 2.7%, 0.1% ≤ Silicon ≤ 1%,0.01% ≤ Aluminum ≤ 0.8%,0.1% ≤ Chromium ≤ 0.9%,0% ≤ Phosphorus ≤ 0.09%,0.0001% ≤ Titanium ≤ 0.1%,0.0005% ≤ Boron ≤ 0.003%, 0.01% ≤ Niobium ≤ 0.1%, 0 % ≤ Sulfur ≤ 0.09 %, 0 % ≤ Nitrogen ≤ 0.09%, 0% ≤ Vanadium ≤ 0.2%, 0%≤Molybdenum≤0.2%, 0%≤Nickel≤2%, 0% ≤ Copper ≤ 2%, 0% ≤ Calcium ≤ 0.005%, 0% ≤ Cerium ≤ 0.1%, 0% ≤ Magnesium ≦ 0.05%, 0% ≤ Zirconium ≦ 0.05%, the remainder being composed of iron and unavoidable impurities caused by processing, the microstructure of the steel sheet including in area fraction, 40% to 60% martensite, 15 to 40% of inter-critical ferrite, a cumulated amount of 10 to 35% of transformed ferrite and bainite and 0% to 5% of residual austenite.

A high-strength hot-dip galvanized steel sheet contains predetermined amounts of C, Si, Mn, P, S, N, O, sol. Al, Ti, and B, 0.1 to 1.5 mass % of Cr+2×Mo, and a balance in a form of Fe and inevitable impurities. A steel structure includes, in area %, ferrite: 1 to 50%, martensite: 20 to 70%, residual austenite: 0 to 5%, pearlite: 0 to 5%, MA and cementite having 0.2 μm or more grain size: 0 to 5% in total, and a balance in a form of bainite. A number density of MA or cementite having 0.2 μm or more grain size and isolated in ferrite or bainite grains is 100 pcs/1000 μm.sup.2 or less, and an average hardness of martensite is in a range from 330 to 500 Hv.

In a metal coated steel sheet, a chemical composition contains, in mass %, at least C: 0.03% to 0.70%, Si: 0.25% to 2.50%, Mn: 1.00% to 5.00%, P: 0.100% or less, S: 0.010% or less, sol. Al: 0.001% to 2.500, N: 0.020% or less, and a balance composed of iron and impurities, a metal structure contains greater than 5.0 vol % of retained austenite and greater than 5.0 vol % of tempered martensite, and satisfies a C content in the retained austenite being 0.85 mass % or more.

One aspect of the present invention relates to a plated steel sheet for hot press forming, having an excellent impact property.

A high strength steel strip having medium amounts of C, Mn, Si, Cr and Al, wherein the steel strip has a microstructure consisting of, in vol. %: ferrite and bainite together 50-90%, martensite<15%, retained austenite 5-15%, the remainder being pearlite, cementite, precipitates and inclusions together up to 5%.

The present invention relates to a cold rolled steel sheet containing C: 0.15% or more and 0.40% or less, Si: 0.50% or more and 4.00% or less, Mn: 1.00% or more and 4.00% or less, and sol. Al: 0.001% or more and 2.000% or less, having a metallic structure consisting of 35 to 65 area % of ferrite phases, 35 to 65 area % of hard second phases, and 0 to 5 area % of remaining phases, wherein 60% or more of the ferrite phases are recrystallized ferrite phases, an average crystal grain size defined by 15° grain boundaries is 5.0 μm or less, a maximum connecting rate of the hard second phases is 10% or more, and a two-dimensional isoperimetric constant of the hard second phases is 0.20 or less.

A plated steel sheet includes: a steel sheet; and a plating layer that is formed on at least a part of a surface of the steel sheet, in which a chemical composition of the plating layer includes, by mass %, Al: more than 5.00% and 35.00% or less, Mg: 3.00% to 15.00%, Si: 0% to 2.00%, Ca: 0% to 2.00%, and a remainder of Zn and impurities, in which in a cross section of the plating layer in a thickness direction, the area ratio of a lamellar structure in which an (Al—Zn) phase and a MgZn.sub.2 phase are arranged in layers is 10% to 90%, a lamellar spacing of the lamellar structure is 2.5 μm or less, and the area ratio of an (Al—Zn) dendrite is 35% or less.

A method for producing a zinc or zinc-alloy coated steel sheet with a tensile strength higher than 900 MPa, for the fabrication of resistance spot welds containing in average not more than two Liquid Metal Embrittlement cracks per weld having a depth of 100 μm or more, with steps of providing a cold-rolled steel sheet, heating cold-rolled steel sheet up to a temperature T1 between 550° C. and Ac1+50° C. in a furnace zone with an atmosphere (A1) containing from 2 to 15% hydrogen by volume, so that the iron is not oxidized, then adding in the furnace atmosphere, water steam or oxygen with an injection flow rate Q higher than (0.07%/h×α), α being equal to 1 if said element is water steam or equal to 0.52 if said element is oxygen, at a temperature T≥T1, so to obtain an atmosphere (A2) with a dew point DP2 between −15° C. and the temperature Te of the iron/iron oxide equilibrium dew point, then heating the sheet from temperature T.sub.1 up to a temperature T.sub.2 between 720° C. and 1000° C. in a furnace zone under an atmosphere (A2) of nitrogen containing from 2 to 15% hydrogen and more than 0.1% CO by volume, with an oxygen partial pressure higher than 10.sup.−21 atm., wherein the duration to of heating of the sheet from temperature T.sub.1 up to the end of soaking at temperature T.sub.2 is between 100 and 500 s, soaking the sheet at T.sub.2, then cooling the sheet at a rate between 10 and 400° C./s, then coating the sheet with zinc or zinc-alloy coating.

The present invention relates to a manufacturing method of a steel strip, a steel strip with controlled decarburized depth, a spot welded joint and the use of said steel strip or said spot welded joint. This invention is particularly well suited for the automotive industry due to the improvement of the Liquid Metal Embrittlement (LME) resistance along with target mechanical properties.

Low-density hot dip galvanized steel, comprising a steel substrate (1) located at a core portion and a coating layer (3) located on the surface. An interface layer is disposed between the steel substrate (1) and the coating layer (3), the interface layer comprises an iron particle layer (4), iron particles dispersed on the steel substrate (1) and covering the steel substrate (1) are disposed in the iron particle layer (4), and the iron particles are covered by a first inhibition layer (5). The low-density hot dip galvanized steel contains element Al in a mass percentage of 3.0% to 7.0%. Correspondingly, the present invention also comprises a manufacturing method for the low-density hot dip galvanized steel. The low-density hot dip galvanized steel has a low density, a high strength and high galvanizability and coating layer adhesion.