The austenitic stainless steel containing niobium according to an exemplary embodiment of the present invention includes: 16 to 26 wt. % of chromium (Cr), 8 to 22 wt. % of nickel (Ni), 0.02 to 0.1 wt. % of carbon (C), 0.2 to 1 wt. % of niobium (Nb), 0.015 to 0.025 wt. % of titanium (Ti), 0.004 to 0.01 wt. % of nitrogen (N), and 0.5 to 2 wt. % of manganese (Mn), wherein the austenitic stainless steel containing niobium has an austenitic matrix structure, a fine niobium carbide and a fine titanium nitride are precipitated in the austenitic matrix structure, and the fine niobium carbide is uniformly dispersed in the austenitic matrix structure.

The austenitic stainless steel containing niobium according to an exemplary embodiment of the present invention includes: 16 to 26 wt. % of chromium (Cr), 8 to 22 wt. % of nickel (Ni), 0.02 to 0.1 wt. % of carbon (C), 0.2 to 1 wt. % of niobium (Nb), 0.015 to 0.025 wt. % of titanium (Ti), 0.004 to 0.01 wt. % of nitrogen (N), and 0.5 to 2 wt. % of manganese (Mn), wherein the austenitic stainless steel containing niobium has an austenitic matrix structure, a fine niobium carbide and a fine titanium nitride are precipitated in the austenitic matrix structure, and the fine niobium carbide is uniformly dispersed in the austenitic matrix structure.

A high-strength steel sheet includes a predetermined chemical composition, a microstructure contains, by volume percentage, 80% or more of tempered martensite, and a remainder consists of ferrite and bainite, the microstructure contains 5.0×10.sup.11 pieces/mm.sup.3 or more of, per unit volume, precipitate containing Ti and having an equivalent circle diameter of 5.0 nm or less, Hvs/Hvc which is a ratio of an average hardness Hvs at a position of a depth of 20 μm from a surface to an average hardness Hvc at a position of 0.20 to 0.50 mm from the surface is 0.85 or more, and the tensile strength is 1,180 MPa or more.

A steel material according to the present disclosure has a chemical composition consisting of, in mass %: C: 0.20 to 0.45%, Si: 0.05 to 1.00%, Mn: 0.01 to 1.00%, P: 0.030% or less, S: 0.0050% or less, Al: 0.005 to 0.100%, Cr: 0.60 to 1.50%, Mo: more than 1.00 to 2.00%, Ti: 0.002 to 0.020%, V: 0.05 to 0.30%, Nb: 0.005 to 0.100%, B: 0.0005 to 0.0040%, N: 0.0100% or less, O: less than 0.0020%, and the balance being Fe and impurities, and satisfying Formula (1) described in the specification. A grain diameter of a prior-austenite grain is 11.0 μm or less, and an average area of precipitate which is precipitated in a prior-austenite grain boundary is 10.0×10.sup.−3 μm.sup.2 or less. A yield strength is 758 to 862 MPa.

A steel alloy including, by weight percent: Ni: 18 to 19%; Co: 11.5 to 12.5%; Mo: 4.6 to 5.2%; Ti: 1.3 to 1.6%; Al: 0.05 to 0.15%; Nb: 0.15 to 0.30%; B: 0.003 to 0.020%; Cr: max 0.25%; Mn: max 0.1%; Si: max 0.1%; C: max 0.03%; P: max 0.005%; and S: max 0.002%, the balance being iron plus incidental impurities.

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.

Provided is a high-strength steel sheet including, in % weight, carbon (C): 0.04 to 0.15%, silicon (Si): 0.01 to 1.0%, manganese (Mn): 1.8 to 2.5%, molybdenum (Mo): 0.15% or less (excluding 0%), chromium (Cr): 1.0% or less (excluding 0%), phosphorus (P): 0.1% or less, sulfur (S): 0.01% or less, aluminum (Al): 0.01 to 0.5%, nitrogen (N): 0.01% or less, boron (B): 0.01% or less (excluding 0%), antimony (Sb): 0.05% or less (excluding 0%), one or more of titanium (Ti): 0.003 to 0.06% and niobium (Nb): 0.003 to 0.06%, a balance of Fe and other unavoidable impurities, and contents of the C, the Si, the Al, the Mo and the Cr satisfy the following Expression 1: Expression 1: {(2×(Si+Al))+Mo+Cr}/C≥20. The high-strength steel sheet comprises: a ferrite phase, a bainite phase, a martensite phase, and a residual austenite phase, the ferrite phase being less than 40% of area fraction in the microstructure.

A high-ductility, high-strength electrolytic zinc-based coated steel sheet includes an electrolytic zinc-based coating on a base steel sheet, in which the base steel sheet has a predetermined composition and a steel microstructure in which the total area percentage of one or two of martensite containing a carbide having an average particle size of 50 nm or less and bainite containing a carbide having an average particle size of 50 nm or less is 90% or more in the entire steel microstructure, the total area percentage of one or two of the martensite containing a carbide having an average particle size of 50 nm or less and the bainite containing a carbide having an average particle size of 50 nm or less is 80% or more in a region extending from the surface of the base steel sheet to a depth of ⅛ of the thickness of the base steel sheet.

The invention concerns a method for heat treatment of an austenitic steel of the High Nitrogen Steel or austenitic HNS type, or of an austenitic steel of the High Interstitial Steel or austenitic HIS type, said austenitic HNS or austenitic HIS containing precipitates of nitrides, carbides or carbonitrides of chromium and/or of molybdenum, this method comprising the step which consists, after machining the austenitic HNS or austenitic HIS containing the precipitates, in redissolving the precipitates by bringing the austenitic HNS or austenitic HIS to its austenitizing temperature, then cooling the austenitic HNS or austenitic HIS sufficiently rapidly to avoid the re-formation of precipitates. The invention also concerns different heat treatment methods allowing chromium and/or molybdenum nitride, carbide or carbonitride type precipitates to appear in an austenitic HNS or austenitic HIS. Indeed, the presence of these precipitates in the matrix of the austenitic HNS or austenitic HIS makes machining operations easier by promoting the formation and removal of chips during machining of the components.