A peening method which can sufficiently improve fatigue properties of a lap fillet welded joint having a thin steel sheet as a base sheet, in which a knocking pin having a predetermined shape is continuously knocked as a series of knocking toward a direction inclined relative to the welding direction, the series of knocking is repeatedly performed in the welding direction, at that time, a knocking mark group made of a plurality of knocking marks formed by the series of knocking is superimposed on at least a part of an adjacent knocking mark group while an end part in the direction orthogonal to the welding direction of the knocking mark group is separated from an end part in the direction orthogonal to the welding direction of the adjacent knocking mark group.

A peening method which can sufficiently improve fatigue properties of a lap fillet welded joint having a thin steel sheet as a base sheet, in which a knocking pin having a predetermined shape is continuously knocked as a series of knocking toward a direction inclined relative to the welding direction, the series of knocking is repeatedly performed in the welding direction, at that time, a knocking mark group made of a plurality of knocking marks formed by the series of knocking is superimposed on at least a part of an adjacent knocking mark group while an end part in the direction orthogonal to the welding direction of the knocking mark group is separated from an end part in the direction orthogonal to the welding direction of the adjacent knocking mark group.

The present invention relates to a motor vehicle crash box with a working direction in vehicle's longitudinal axis manufactured out of a tube which is expanded into different zones, wherein the zones are different in strength and diameter by using steel after forming a homogenous austenitic microstructure with a strain hardening effect. The present invention further relates to the manufacturing method of such a component.

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 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.

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.

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.

A fabrication method for a press hardened part is provided. A sheet or a steel substrate blank for heat treatment is provided. A pre-coating is applied. The pre-coating has at least one layer of aluminum or aluminum alloy in contact with the steel substrate on at least one of the principal faces of the sheet or blank. Then a polymerized layer is deposited on the pre-coating. The polymerized layer has a thickness between 2 and 30 μm. The polymerized layer does not contain silicon, has a nitrogen content of less than 1% by weight and carbon pigments in a quantity between 3 and 30% by weight. The blank or the sheet is heated to obtain an interdiffusion between the steel substrate and the pre-coating and to give the steel a partly or totally austenitic structure. Then the blank or the sheet is hot stamped to obtain a part. The part is cooled by holding the part in a stamping tool so that the microstructure of the steel substrate includes, at least in a portion of the part, martensite or bainite.

A fabrication method for a press hardened part is provided. A sheet or a steel substrate blank for heat treatment is provided. A pre-coating is applied. The pre-coating has at least one layer of aluminum or aluminum alloy in contact with the steel substrate on at least one of the principal faces of the sheet or blank. Then a polymerized layer is deposited on the pre-coating. The polymerized layer has a thickness between 2 and 30 μm. The polymerized layer does not contain silicon, has a nitrogen content of less than 1% by weight and carbon pigments in a quantity between 3 and 30% by weight. The blank or the sheet is heated to obtain an interdiffusion between the steel substrate and the pre-coating and to give the steel a partly or totally austenitic structure. Then the blank or the sheet is hot stamped to obtain a part. The part is cooled by holding the part in a stamping tool so that the microstructure of the steel substrate includes, at least in a portion of the part, martensite or bainite.

Heat treatment of an Alloy 282 which has been subjected to an initial solution annealing followed by cooling can be heat treated by heating the Alloy 282 to a temperature between 954° C. and 1010° C. until the gamma prime (γ′) phase is sufficiently dissolved, and cooling the Alloy 282 to a temperature a sufficiently low temperature, and at a sufficiently high cooling rate, to suppress gamma prime precipitation. A component such as a turbine exhaust case and a gas turbine engine made of said alloy can be heat treated in the above manner.