NIPPON STEEL STAINLESS STEEL CORPORATION

Provided is an austenitic stainless steel material, wherein a chemical composition includes, in mass %, C: 0.20% or less, Si: 2.0% or less, Mn: 6.0 to 20.0%, P: 0.060% or less, S: 0.0080% or less, Cr: 10.0 to 18.0%, Ni: 4.0 to 12.0%, N: 0.01 to 0.30%, Cu: 4.0% or less, Mo: 3.0% or less, an optional element, and the balance: Fe and impurity, an A value is 30.0 to 60.0, and in a metallurgical structure of a surface layer, a proportion Gs of high angle grain boundaries satisfies [0.1<Gs<0.6].

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 0.03% 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.

A ferritic-austenitic duplex stainless steel material has a composition including, on a mass basis, C: 0.001 to 0.050%, Si: 0.01 to 0.50%, Mn: 1.0 to 4.5%, P: 0.050% or less, S: 0.030% or less, Ni: 1.5 to 3.5%, Cr: 19.6 to 24.0%, Mo: 0.01 to 1.00%, Cu: 0.01 to 1.20%, and N: 0.010 to 0.090%, C+N being less than 0.130%, the balance being Fe and impurities. The ferritic-austenitic duplex stainless steel material has a metallographic structure having: an Md value as represented by the following equation (1) of 50.0 to 150.0° C.; and 25 to 49% by volume of austenite phase. The value of Md of the austenite phase represented by the following equation (1) is 35.0 to 100.0° C. Md = 551 - 462 ⁢ ( C + N ) - 9.2 Si - 8.1 Mn - 29 ⁢ ( Ni + Cu ) - 13.7 Cr - 18.5 Mo ( 1 ) in which each of the element symbols represents a content (% by mass) of each element.

The present invention has an object to provide a ferritic stainless steel having a broad range of suitable quenching temperature and a high hardness and excellent corrosion resistance after quenching and beautiful and useful as a material for martensitic stainless steel products and provide an industrially stable method for production. The ferritic stainless steel of the present invention is characterized by having a steel composition comprising, by mass %, C: 0.45% or more and 0.55% or less, Si: 0.10% or more and 1.00% or less, Mn: 0.1% or more and 1.0% or less, Cr: 12.0% or more and 15.0% or less, Ni: 0% or more and 1.0% or less, Mo: 0.50% or more and 0.80% or less, V: 0.10% or more and 0.20% or less, N: 0.015% or more and 0.100% or less, P: 0% or more and 0.040% or less, S: 0% or more and 0.030% or less, balance: Fe and impurities, having an average crystal grain size of a ferrite phase of 10 μm or less, and having carbides having a diameter of 1.5 μm or less of 0.8/μm 2 or more.

An austenitic stainless steel, wherein when an amount of hydrogen emitted at a heating rate of 100° C./h and in a temperature range from 25 to 800° C. is measured by using thermal desorption analysis, an amount of emitted hydrogen [H] is less than 10.0 ppm, a peak temperature [Tp] of hydrogen emission is 350° C. or more, a peak rate [Rmax] of hydrogen emission is 0.050 ppm/min or less, and a number density of compounds having a diameter of 0.1 to 2 μm is 1.0 to 10.0/mm 2 .

The stainless steel contains 0.0001 mass % or more and 0.15 mass % or less of C, 0.30 mass % or more and 2.0 mass % or less of Si, 0.1 mass % or more and 15 mass % or less of Mn, 5 mass % or more and 30 mass % or less of Ni, 0.0001 mass % or more and 0.01 mass % or less of S, 16 mass % or more and 25 mass % or less of Cr, 0 mass % or more and 5 mass % or less of Mo, 0 mass % or more and 0.005 mass % or less of Al, 0 mass % or more and 0.0010 mass % or less of Mg, 0.0010 mass % or more and 0.0060 mass % or less of O, and 0.0001 mass % or more and 0.5 mass % or less of N, and at least includes an inclusion with an equivalent circle diameter of 5 μm or more, having the average composition of 5 mass % or more of MnO, 20 mass % or more of Cr 2 O 3 +Al 2 O 3 , 1 mass % or more of Al 2 O 3 , and 5 mass % or less of CaO. The number density of the inclusion having the composition is 0.5 inclusions/mm 2 or less.

An austenitic stainless steel material that has a passivation film on a surface is provided. The austenitic stainless steel material has a chemical composition consisting of, in mass %, C: 0.10% or less, Si: 1.0% or less, Mn: 8.0-10.0%, P: 0.030% or less, S: 0.003% or less, Cr: 15.0-18.0%, Ni: 7.0-9.0%, N: 0.15-0.25%, Al: 0.005-0.20%, Ca: 0.0005-0.01%, Cu: less than 1.0%, Mo: less than 1.0%, B: 0-0.0050%, Nb: 0-0.50%, Ti: 0-0.50%, V: 0-0.50%, W: 0-0.50%, Zr: 0-0.50%, Co: 0-0.50%, Mg: 0-0.005%, Ga: 0-0.005%, Hf: 0-0.10%, REM: 0-0.10%, and the balance: Fe and impurities. An f value, namely, [Ni+0.72Cr+0.88Mo+1.11Mn−0.27Si+0.53Cu+12.93C+7.55N], is more than 29.5 and less than 32.5.

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 0 ≤T 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 0 represents a Curie temperature (° C.) of the alloy material, and T 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.

A bar-shaped stainless steel product contains, by mass %, 0.001 to 0.030% of C, 0.01 to 4.00% of Si, 0.01 to 2.00% of Mn, 0.01 to 4.00% of Ni, 6.0 to 35.0% of Cr, 0.01 to 5.00% of Mo, 0.01 to 2.00% of Cu, and 0.001 to 0.050% of N. In the product, an F value is 20 or less, a rolling-direction-crystal-orientation RD//<100> fraction is 0.05 or more, and preferably a rolling-direction-crystal-orientation RD//<334> fraction is 0.2 or less. The above RD//<100> fraction means an area ratio of crystal having 25 degrees or less of an orientation difference between a <100> orientation and a rolling direction, and the above RD//<334> fraction means an area ratio of crystal having 10 degrees or less of an orientation difference between a <334> orientation and a rolling direction. F value=700C+800N+20Ni+10Cu+10Mn−6.2Cr−9.2Si−9.3Mo−74.4Ti−37.2Al−3.1Nb+63.2.

There is provided a ferritic stainless steel sheet including a chemical composition that consists of, in mass %, C: 0.02% or less, Si: 1.0% or less, Mn: 1.0% or less, P: 0.01 to 0.10%, S: 0.0001 to 0.005%, N: 0.02% or less, Cr: 17.0 to 20.0%, Cu: 1.0 to 1.5%, Ti: 0.05 to 0.3%, Nb: 0.005 to 0.2%, Mo: 0.02 to 0.5%, B: 0.0001 to 0.0030%, Al: 0.005 to 0.5%, Ni: 0.01 to 0.2%, V: 0.01 to 0.2%, optional elements, with the balance: Fe and impurities and satisfies [86 P+33Nb+Mo+4Al≥5.0], in which an area fraction of Cu particles having a diameter of 20 nm or less is 2.0% or less.