Disclosed is a hot-pressed member that can exhibit very high tensile strength after hot pressing of 1780 MPa or more, excellent delayed fracture resistance, and high cross tensile strength after resistance spot welding by properly adjusting its chemical composition and its microstructure such that a prior austenite average grain size is 8 μm or less, a volume fraction of martensite is 90% or more, and at least 10 cementite grains having a grain size of 0.05 μm or more are present on average per 200 μm.sup.2 of a cross section parallel to a thickness direction of the member, and such that at least 10 Ti-based precipitates having a grain size of less than 0.10 μm are present on average per 100 μm.sup.2 of the cross section parallel to the thickness direction of the member in a range of 100 μm in the thickness direction from a surface of the member.

Disclosed are a high-strength and non-plated steel sheet which is for hot forming and may be suitable for use in automotive structural members that require collision resistance characteristics, a hot-formed member, and a method for manufacturing same.

A steel sheet for hot forming and a hot-formed member according to an embodiment of the present disclosure contain, in percent by weight (wt %), 0.05 to 0.3% of carbon (C), 0.5 to 3.0% of silicon (Si), 0.1 to 2.0% of manganese (Mn), 3.0 to 9.0% of chromium (Cr), more than 0% and less than 0.2% of nitrogen (N), and 0.03 to 1.0% of niobium (Nb), with the remainder comprising iron (Fe) and inevitable impurities.

The invention relates to a maraging steel containing C: 0.02% (mass %, hereinafter the same) or less, Si: 0.3% or less, Mn: 0.3% or less, Ni: 7.0 to 15.0%, Cr: 5.0% or less, Co: 8.0 to 12.0%, Mo: 0.1 to 2.0%, Ti: 1.0 to 3.0%, and Sol.Al: 0.01 to 0.2%, where the balance includes Fe and unavoidable impurities of P: 0.01% or less, S: 0.01% or less, N: 0.01% or less, and O: 0.01% or less. The parent phase of the maraging steel includes a martensitic phase. The parent phase contains a martensitic phase obtained by reverse transformation from a martensitic phase to an austenitic phase and then transformation from the austenitic phase, in an area fraction of 25% to 75%.

A steel for a sawing device (100) containing in wt. %: C: 0.7-1.2 Mn: 0.3-0.7 Cr: 0-1.05 Ni: 0-1.5 Al: 0-0.5 Si: 0-0.5 wherein the total amount of C, Mn, Cr, Ni, Al, and Si is 1.5-4.5 wt. % and the balance being Fe and incidental elements and wherein the microstructure of the steel alloy is bainitic or a mixture of bainite and martensite with dispersed Fe.sub.3C-particles.

Disclosed is a hot-pressed member that can exhibit very high tensile strength after hot pressing, excellent delayed fracture resistance, and high tensile shear stress after resistance spot welding by properly adjusting its chemical composition and its microstructure such that at least 20 Nb-based precipitates having a grain size of less than 0.10 μm are present on average per 100 μm.sup.2 of a cross section parallel to a thickness direction of the member, a prior austenite average grain size is 8 μm or less, an average aspect ratio of prior austenite grains is 2.5 or less, and a volume fraction of martensite is 90% or more, and such that a standard deviation of Vickers hardness measured every 200 μm on a surface of the member is 40 or less.

A cylindrical part of a rotor for an eddy current deceleration device of the present embodiment has a chemical composition consisting of, by mass %, C: 0.05 to 0.15%, Si: 0.10 to 0.40%, Mn: 0.50 to 1.00%, P: 0.030% or less, S: 0.030% or less, Mo: 0.20 to 1.00%, Nb: 0.020 to 0.060%, V: 0.040 to 0.080%, sol. Al: 0.030 to 0.100%, B: 0.0005 to 0.0050%, N: 0.003 to 0.010%, Cu: 0 to 0.20%, Ni: 0 to 0.20%, Cr: 0 to 0.10%, and the balance: Fe and impurities, and in which the total area fraction of martensite and bainite in a microstructure is more than 95.0%, and the number density of carbides having an equivalent circular diameter of 100 to 500 nm is 0.35 to 0.75 particles/μm.sup.2.

Provided in the present disclosure is a method of heat treating a high-strength steel, wherein the high-strength steel comprises, by weight: 0.30-0.45% C, 1.0% or less Si, 0.20-2.5% Mn, 0.20-2.0% Cr, 0.15-0.50% Mo, 0.10-0.40% V, 0.2% or less Ti, 0.2% or less Nb, and a balance of Fe and other alloy elements and impurities, wherein the above alloy elements make Eq(Mn) according to the following formula (1) no less than 1.82, which method comprises the steps of 1) austenitizing; 2) carbide precipitation; and 3) tempering. The heat-treated steel in accordance with the present invention has high strength, high ductility and high toughness at the same time, especially improved reduction in area of tensile sample, so that it is particularly suitable for preparing spring members for vehicle suspension.

Eq(Mn)=Mn+0.26Si+3.50P+1.30Cr+2.67Mo  (1)

A series of welding filler wires with innovative composition design for fusion welding precipitation-hardened lightweight austenitic Fe—Mn—Al—C alloys. The first class of the welding filler wires is composed of 23-34 wt. % Mn, 7.5-11.5 wt. % Al, 1.35-1.95 wt. % C, with the balance being essentially Fe. After fusion welding, there are high-density of nano-sized (˜3-5 nm) (Fe,Mn).sub.3AlC carbides (κ-carbides) uniformly distributed within the austenite dendrite cells in the fusion zone (FZ). The amount of nano-sized (˜6-10 nm) κ-carbides existing within the eutectic regions is significantly increased and the size of the austenite dendrite cells is substantially reduced. The second class of welding filler wires has the composition of 23-34 wt. % Mn, 7.5-11.5 wt. % Al, 1.40-1.95 wt. % C, 0.1-2.5 wt. % Ti, 0.1-3.0 wt. % Nb, 0.1-2.5 wt. % V, with the balance being essentially Fe. The microstructure of the FZ in the as-welded condition results in formation of substantial amount of nano-sized (˜6-10 nm) face-centered-cubic structured ductile Ti-rich Ti-carbides, Nb-rich Nb-carbides and V-rich V-carbides within the eutectic regions. These carbides are extremely hard (2000˜3500 Hv), enhancing hardness of the obtained FZ.

A series of welding filler wires with innovative composition design for fusion welding precipitation-hardened lightweight austenitic Fe—Mn—Al—C alloys. The first class of the welding filler wires is composed of 23-34 wt. % Mn, 7.5-11.5 wt. % Al, 1.35-1.95 wt. % C, with the balance being essentially Fe. After fusion welding, there are high-density of nano-sized (˜3-5 nm) (Fe,Mn).sub.3AlC carbides (κ-carbides) uniformly distributed within the austenite dendrite cells in the fusion zone (FZ). The amount of nano-sized (˜6-10 nm) κ-carbides existing within the eutectic regions is significantly increased and the size of the austenite dendrite cells is substantially reduced. The second class of welding filler wires has the composition of 23-34 wt. % Mn, 7.5-11.5 wt. % Al, 1.40-1.95 wt. % C, 0.1-2.5 wt. % Ti, 0.1-3.0 wt. % Nb, 0.1-2.5 wt. % V, with the balance being essentially Fe. The microstructure of the FZ in the as-welded condition results in formation of substantial amount of nano-sized (˜6-10 nm) face-centered-cubic structured ductile Ti-rich Ti-carbides, Nb-rich Nb-carbides and V-rich V-carbides within the eutectic regions. These carbides are extremely hard (2000˜3500 Hv), enhancing hardness of the obtained FZ.

A steel alloy intended for cutting applications and hot working tools, comprising, in weight percent (wt. %), C: 0.40-1.2 wt. %, Si: 0.30-2.0 wt. %, Mn: max 1.0 wt. %, Cr: 3.0-6.0 wt. %, Mo: 0-4.0 wt. %, W: 0-8.0 wt. %, wherein (Mo+W/2)≥3.5 wt. %, Nb: 0-4.0 wt. %, V: 0-4.0 wt. %, wherein 1.0 wt. %≤(Nb+V)≤4.0 wt. %, Co: 25-40 wt. %, S: max 0.30 wt. %, N: max 0.30 wt. %, the balance being Fe and unavoidable impurities.