A duplex stainless steel having excellent carbon dioxide corrosion resistance, excellent sulfide stress corrosion cracking resistance, and excellent sulfide stress cracking resistance. The duplex stainless steel comprises, by mass %, C: 0.03% or less, Si: 1.0% or less, Mn: 0.10 to 1.5%, P: 0.030% or less, S: 0.005% or less, Cr: 20.0 to 30.0%, Ni: 5.0 to 10.0%, Mo: 2.0 to 5.0%, Cu: 2.0 to 6.0%, N: less than 0.07%, at least one selected from Al: 0.05 to 1.0%, Ti: 0.02 to 1.0%, and Nb: 0.02 to 1.0%, and the balance being Fe and unavoidable impurities, and has a structure that is 20 to 70% austenite phase, and 30 to 80% ferrite phase in terms of a volume fraction.

The present invention provides a heat-resistant cast steel, and a preparation method and use thereof. Based on the total mass of the heat-resistant cast steel, the heat-resistant cast steel includes the following elements in mass percentage: 0.08 wt %-0.18 wt % of C, 0.10 wt %-0.40 wt % of Si, 0.30 wt %-0.70 wt % of Mn, 9.80 wt %-10.70 wt % of Cr, 3.00 wt %-3.50 wt % of Co, 1.60 wt %-2.00 wt % of W, 0.45 wt %-0.85 wt % of Mo, 0.10 wt %-0.30 wt % of V, 0.02 wt %-0.08 wt % of Nb, 0.010 wt %-0.035 wt % of N, 0.001 wt %-0.010 wt % of B, <0.20 wt % of Ni and 79 wt %-85.5 wt % of Fe. The heat-resistant cast steel can satisfy the use requirements of turbine parts with a working temperature of 635° C. and below 635° C.

The present invention provides a heat-resistant cast steel, and a preparation method and use thereof. Based on the total mass of the heat-resistant cast steel, the heat-resistant cast steel includes the following elements in mass percentage: 0.08 wt %-0.18 wt % of C, 0.10 wt %-0.40 wt % of Si, 0.30 wt %-0.70 wt % of Mn, 9.80 wt %-10.70 wt % of Cr, 3.00 wt %-3.50 wt % of Co, 1.60 wt %-2.00 wt % of W, 0.45 wt %-0.85 wt % of Mo, 0.10 wt %-0.30 wt % of V, 0.02 wt %-0.08 wt % of Nb, 0.010 wt %-0.035 wt % of N, 0.001 wt %-0.010 wt % of B, <0.20 wt % of Ni and 79 wt %-85.5 wt % of Fe. The heat-resistant cast steel can satisfy the use requirements of turbine parts with a working temperature of 635° C. and below 635° C.

Please amend the Abstract as originally filed as shown below wherein additions are indicated using underlining and deletions are indicated using strikethrough or double brackets in accordance with 37 C.F.R. § 1.121(b)(2):

Realized is ferritic stainless steel which has excellent high-temperature strength and excellent red scale resistance. The ferritic stainless steel contains not more than 0.025% by mass of C, 0.05% by mass to 3.0% by mass of Si, 0.05% by mass to 2.0% by mass of Mn, not more than 0.04% by mass of P, not more than 0.003% 0.03% by mass of S, not more than 0.5% by mass of Ni, 10.5% by mass to 25.0% by mass of Cr, not more than 0.025% by mass of N, 0.05% by mass to 1.0% by mass of Nb, not more than 3.0% by mass of Mo, not more than 1.8% by mass of Cu, not more than 0.2% by mass of Al, and not more than 0.5% by mass of Ti. The sum of the concentrations of Cr and Si, each of which is present as oxide or hydroxide, at a surface of the ferritic stainless steel and at depths to 6 nm from the surface is a given value or more.

Provided herein is a stainless steel seamless pipe for oil country tubular goods. A method for manufacturing such a stainless steel seamless pipe is also provided. The stainless steel seamless pipe has: a composition that contains, in mass %, C: 0.10% or less, Si: 0.5% or less, Mn: 0.05 to 0.50%, P: 0.030% or less, S: 0.005% or less, O: 0.0040% or less, Ni: 3.0 to 8.0%, Cr: 10.0 to 14.0%, Mo: 0.5 to 2.8%, Al: 0.1% or less, V: 0.005 to 0.2%, N: 0.10% or less, Cu: 0.01 to 1.0%, Co: 0.01 to 1.0%, and Ca: 0.0005 to 0.0030%, and in which the balance is Fe and incidental impurities; a microstructure containing at most 20 non-metallic inclusions having a predetermined composition ratio of CaO and Al.sub.2O.sub.3 and a major axis of 5 μm or more per 100 mm.sup.2; and a yield stress of 655 MPa or more.

Provided herein is a stainless steel seamless pipe for oil country tubular goods. A method for manufacturing such a stainless steel seamless pipe is also provided. The stainless steel seamless pipe has: a composition that contains, in mass %, C: 0.10% or less, Si: 0.5% or less, Mn: 0.05 to 0.50%, P: 0.030% or less, S: 0.005% or less, O: 0.0040% or less, Ni: 3.0 to 8.0%, Cr: 10.0 to 14.0%, Mo: 0.5 to 2.8%, Al: 0.1% or less, V: 0.005 to 0.2%, N: 0.10% or less, Cu: 0.01 to 1.0%, Co: 0.01 to 1.0%, and Ca: 0.0005 to 0.0030%, and in which the balance is Fe and incidental impurities; a microstructure containing at most 20 non-metallic inclusions having a predetermined composition ratio of CaO and Al.sub.2O.sub.3 and a major axis of 5 μm or more per 100 mm.sup.2; and a yield stress of 655 MPa or more.

An alloy material is provided which contains elements including, in mass %, C: 0.010 to 0.10%, Si: more than 0.10% to 0.50% or less, Mn:0.05 to 0.50%, Ni:34.5 to 37.0%, and Nb:0.001 to 1.0%, and which satisfies [T.sub.0≤T.sub.1-2], [C—Nb/7.7-Ta/15≤0.045], [Nb-7.7C≤0.30], and [Ta-15C≤0.30]. Where, each symbol of an element in the above formulas represents a content (mass %) of the corresponding element, T.sub.0 represents a Curie temperature (° C.) of the alloy material, and T.sub.1 represents a Curie temperature (° C.) of the alloy material after the alloy material is held at 900° C. for one minute and thereafter is cooled under conditions such that an average cooling rate in a temperature range from 600 to 300° C. is 0.2° C./s.

Method of producing cast-steel molded parts especially suited to low-temperature applications, particularly, handling liquid hydrogen. Conventional high-nickel alloy austenitic stainless steels must be used as forged, not cast, products with high wall thicknesses to lend them the mechanical properties sufficient for handling liquid hydrogen and preventing hydrogen embrittlement. According to the method, an alloy consisting essentially of 2.5-4.5% Si, 10.5-19.0% Cr, 13.5-20.0% Ni, 0.5-1.5% Mn, 1.0-2.0% Co, and 0.5-1.5% Mo is melted; the melt is poured into a mold; and the molded part is solution heat-treated at a temperature of from 950° C. to 1150° C. The cast steel parts have a high content of hydrogen-embrittlement curtailing silicon, and nickel, chromium and other components lending them properties not essentially due to the conventional-steel presence of carbon. The molded parts thus produced have sufficient fracture toughness even at liquid-hydrogen temperatures.

Method of producing cast-steel molded parts especially suited to low-temperature applications, particularly, handling liquid hydrogen. Conventional high-nickel alloy austenitic stainless steels must be used as forged, not cast, products with high wall thicknesses to lend them the mechanical properties sufficient for handling liquid hydrogen and preventing hydrogen embrittlement. According to the method, an alloy consisting essentially of 2.5-4.5% Si, 10.5-19.0% Cr, 13.5-20.0% Ni, 0.5-1.5% Mn, 1.0-2.0% Co, and 0.5-1.5% Mo is melted; the melt is poured into a mold; and the molded part is solution heat-treated at a temperature of from 950° C. to 1150° C. The cast steel parts have a high content of hydrogen-embrittlement curtailing silicon, and nickel, chromium and other components lending them properties not essentially due to the conventional-steel presence of carbon. The molded parts thus produced have sufficient fracture toughness even at liquid-hydrogen temperatures.

[Object] To provide a steel sheet for carburizing that demonstrates improved ductility, and a method for manufacturing the same. [Solution] A steel sheet consisting of, in mass %, C: more than or equal to 0.02%, and less than 0.30%, Si: more than or equal to 0.005%, and less than 0.5%, Mn: more than or equal to 0.01%, and less than 3.0%, P: less than or equal to 0.1%, S: less than or equal to 0.1%, sol. Al: more than or equal to 0.0002%, and less than or equal to 3.0%, N: less than or equal to 0.2%, Ti: more than or equal to 0.010%, and less than or equal to 0.150%, and the balance: Fe and impurities, in which the number of carbides per 1000 μm.sup.2 is 100 or less, percentage of number of carbides with an aspect ratio of 2.0 or smaller is 10% or larger relative to the total carbides, average equivalent circle diameter of carbide is 5.0 μm or smaller, and average crystal grain size of ferrite is 10 μm or smaller.