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 invention relates to a high strength steel with a careful selection of the normal alloying elements C, Mn, Si and Al, together with micro-element addition. This a steel with a high strength and high hole expansion ratio can be produced. The invention also relates to the method to manufacture this high strength steel.

Provided is a steel sheet and a method for manufacturing same, the steel sheet, which is optimized a composition and microstructure, and which can be used for automobile parts and the like, having superb bendability, and superior balance of strength and ductility and of strength and hole expansion ratio.

Disclosed are a low-silicon and low-carbon equivalent GPa grade multi-phase steel plate/steel strip and a manufacturing method therefor. The steel plate/steel strip comprises the following components in percentages by weight: 0.03-0.07% of C, 0.1-0.5% of Si, 1.7-2.0% of Mn, P<0.02%, S<0.01%, N<0.01%, 0.01-0.05% of Al, 0.4-0.7% of Cr, 0.001-0.005% of B, and 0.07-0.15% of Ti, and also comprises one or both of 0.15-0.4% of Mo or 0.02-0.08% of Nb, with the balance being Fe and other inevitable impurities; and at the same time, the steel plate/steel strip satisfies: the content of available B*>0.001, the content of available B*=B-[Ti-3.4N-1.2(C—Nb/7.8)]/22, CE<0.58, and CE=C+Mn/6+(Cr+Mo+V)/5+(Si+Ni+Cu)/15. The steel plate has a tensile strength of >980 MPa and a yield strength of >780 MPa, and the hole expansion rate satisfies: if an original hole is a punched hole, the hole expansion rate is >50%; and if the original hole is a reamed hole, the hole expansion rate is >60%. The steel plate is mainly used in the preparation of vehicle chassis and suspension system parts.

Provided is a steel sheet which can be used for automobile parts and the like, and relates to a steel sheet having a superior balance of strength and ductility and strength and hole expansion ratio and superior bending formability, and a method for manufacturing same.

Disclosed are a low-cost ultra-high-strength multiphase steel plate/steel strip and its manufacturing method. Said steel plate/steel strip comprises the following components in percentage by weight: 0.03 to 0.07% of C, 0.1 to 0.5% of Si, 1.3 to 1.9% of Mn, less than or equal to 0.02% of P, less than or equal to 0.01% of S, 0.01 to 0.05% of Al, 0.2 to 0.5% of Cr, 0.07 to 0.14% of Ti, less than 0.03% of (Ni+Nb+Mo+V), and the balance being Fe and other inevitable impurities; and Mn+1.5Cr+5(Ti+Al+Cu)+10(Mo+Ni)+20(Nb+V)<3.0; Mn+2Cr+4Ti+4Nb+4V+4Mo—Si/3+2C≤3.0. The steel plate is mainly used for the manufacturing of automotive chassis and suspension system parts.

The present disclosure relates to a wear resistant steel material that is not cracked even after being cut using gas, etc., and a method of manufacturing the wear resistant steel material.

Provided is a steel sheet which can be used for automobile parts and the like, and relates to a steel sheet having a superior balance of strength and ductility and strength and a hole expansion ratio and superior bending formability, and a method for manufacturing same.

A nickel-containing steel for low temperature according to an aspect of the present invention has a chemical composition within a predetermined range, in which a metallographic structure of a thickness middle portion contains 2.0 vol % to 20.0 vol % of an austenite phase, an average grain size of prior austenite grains is 3.0 μm to 15.0 μm, an average aspect ratio of the prior austenite grains is 1.0 to 2.4, a plate thickness is 4.5 mm to 30 mm, the chemical composition and the average grain size of the prior austenite grains are further limited depending on the plate thickness, a yield stress at room temperature is 460 MPa to 710 MPa, and a tensile strength at the room temperature is 560 MPa to 810 MPa.

A Ni steel has a chemical composition within a predetermined range, in which a metallographic structure of a thickness middle portion contains 2.0 vol % to 20.0 vol % of an austenite phase, an average grain size of prior austenite grains is 3.0 μm to 12.0 μm, an average aspect ratio of the prior austenite grains is 2.6 to 10.0, a plate thickness is 4.5 mm to 20 mm, a yield stress at room temperature is 590 MPa to 710 MPa, and a tensile strength at the room temperature is 690 MPa to 810 MPa, when the plate thickness is more than 16 mm, the Ni steel contains Ni: 11.5% or more, and when the plate thickness is 16 mm or less and the Ni steel contains Ni: less than 11.5%, the average grain size of the prior austenite grains is 6.0 μm or less.