A high strength interstitial free low density steel and method for producing the steel.

An austenitic Fe—Ni—Cr alloy comprises C: 0.005˜0.03 mass %, Si: 0.17˜1.0 mass %, Mn: not more than 2.0 mass %, P: not more than 0.030 mass %, S: not more than 0.0015 mass %, Cr: 18˜28 mass %, Ni: 21.5˜32 mass %, Mo: 0.10˜2.8 mass %, Co: 0.05˜2.0 mass %, Cu: less than 0.25 mass %, N: not more than 0.018 mass %, Al: 0.10˜1.0 mass %, Ti: 0.10˜1.0 mass %, Zr: 0.01˜0.5 mass %, and the balance being Fe and inevitable impurities; wherein Cr, Mo, N and Cu satisfy PRE=Cr+3.3×Mo+16×N≧20.0 and PREH=411-13.2×Cr-5.8×Mo+0.1×Mo.sup.2+1.2×Cu≦145.0 and wherein Al, Ti, and Zr satisfy 0.5≦Al+Ti+1.5×Zr≦1.5, and has an excellent corrosion resistance in air or under a wet environment even at a surface state having an oxide film formed by an intermediate heat treatment.

A hot-rolled steel sheet for nitriding or a cold-rolled steel sheet for nitriding, in which a dislocation density within 50 μm in the sheet thickness direction from the surface is not less than 2.0 times nor more than 10.0 times as compared to a dislocation density at the position of ¼ in the sheet thickness direction; and a method of manufacturing the same. The manufacturing method comprises, on a hot-rolled steel sheet or a cold-rolled steel sheet, performing pickling, and then performing skin pass rolling under the condition that a reduction ratio is 0.5 to 5.0% and FIT, defined as a ratio of a line load F (kg/mm) of a rolling mill load divided by a sheet width of the steel sheet and a load T (kg/mm.sup.2) per unit area to be applied in the longitudinal direction of the steel sheet, is 8000 or more.

Provided is a ferrite-based stainless steel having superior moldability when molding a fuel cell divider sheet from a material by controlling yield point elongation in accordance with alloy components. The ferrite-based stainless steel comprises, in weight percentages: no more than 0.02% of C; no more than 0.02% of N; no more than 0.4% of Si; no more than 0.2% of Mn; no more than 0.04% of P; no more than 0.02% of S; 25.0-32.0% of Cr; 0-1.0% of Cu; no more than 0.8% of Ni; no more than 0.01-0.5% of Ti; no more than 0.01-0.5% of Nb; no more than 0.01-1.5% of V; and residual Fe and inevitable elements, wherein the content of Ti, Nb, V, C, and N in terms of weight % of steel uses Formula (1) to render a yield point elongation of the material of no more than 1.1%, and wherein a steel material has superior moldability.
9.1C−1.76V+5.37(C+N)/Ti−1.22Nb≦0.7.  Formula (1)

According to aspects of the invention, provided are an Fe—Cr—Al-based stainless steel sheet which has improved manufacturability by improving the toughness of a hot-rolled steel sheet and a cold-rolled steel sheet without deteriorating oxidation resistance at a high temperature and shape stability when used at a high temperature, and a stainless steel foil which is manufactured by rolling the stainless steel sheet. V and B are added in combination to Fe—Cr—Al-based stainless steel in amounts within specified ranges. Specifically, V content is controlled to be 0.010% or more and 0.050% or less and B content is controlled to be 0.0001% or more and 0.0050% or less, in which the relationship {V content (V %)}/{B content (B %)}>10 is satisfied.

Provided are a ferritic stainless steel foil excellent in terms of corrugating workability, shape change resistance at a high temperature, and manufacturability and a method for manufacturing the steel foil. A ferritic stainless steel foil having a chemical composition containing, by mass % , C: 0.020% or less, Si: 2.0% or less, Mn: 1.0% or less, S: 0.010% or less, P: 0.050% or less, Cr: 10.0% or more and 25.0% or less, Ni: 0.05% or more and 0.50% or less, Ti: 0.14% or more and 0.25% or less, Al: 0.001% or more and 0.10% or less, V: 0.02% or more and 0.10% or less, N: 0.020% or less, and the balance being Fe and inevitable impurities, and a Vickers hardness of higher than 200 and lower than 350.

Provided is a material for a cold-rolled stainless steel sheet having a chemical composition containing, by mass %, C: 0.01% to 0.05%, Si: 0.02% to 0.75%, Mn: 0.1% to 1.0%, P: 0.04% or less, S: 0.01% or less, Cr: 16.0% to 18.0%, Al: 0.001% to 0.10%, N: 0.01% to 0.06% and the balance being Fe and inevitable impurities. The material has a metallographic structure including a martensite phase having an area ratio of 5% to 50% and the balance being a ferrite phase. A ferrite phase in portions extending from surface layers of front and back surfaces of a steel sheet has an average grain diameter of 20 μm or more and 50 μm or less, and a ferrite phase in a central portion of the sheet includes an unrecrystallized ferrite phase.

A high-toughness hot-rolled high-strength steel with a yield strength of Grade 800 MPa with its chemical components, in weight percentages, being C 0.02-0.05%, Si≦0.5%, Mn 1.5-2.5%, P≦0.015%, S≦0.005%, Al 0.02-0.10%, N≦0.006%, Nb 0.01-0.05%, Ti 0.01-0.03%, 0.03%≦Nb+Ti≦0.06%, Cr 0.1%-0.5%, Mo 0.1-0.5%, B 0.0005-0.0025%, and the balance of Fe and unavoidable impurities, and a preparation method thereof. The present invention acquires, via direct quenching, an ultra-low carbon martensite structure with a yield strength of 800 Mpa and an impact energy of more than 100J under a temperature of −80° C.

This ferritic stainless steel contains, by mass %, C: 0.001% to 0.030%; Si: 0.01% to 1.00%, Mn: 0.01% to 2.00%, P: 0.050% or less, S: 0.0100% or less, Cr: 11.0% to 30.0%, Mo: 0.01% to 3.00%, Ti: 0.001% to 0.050%, Al: 0.001% to 0.030%, Nb: 0.010% to 1.000%, and N: 0.050% or less, with a remainder being Fe and inevitable impurities, wherein an amount of Al, an amount of Ti, and an amount of Si (mass %) satisfy Al/Ti≧8.4Si−0.78.

Provided is a method for stably obtaining a non-oriented electrical steel sheet with high magnetic flux density and excellent productivity, at a low cost by casting in a continuous casting machine a slab having a chemical composition including by mass %, C≦0.0050%, 3.0%<Si≦5.0%, Mn≦0.10%, Al≦0.0010%, 0.040%<P≦0.2%, N≦0.0040%, 0.0003%≦S≦0.0050%, Ca≦0.0015%, and total of at least one element selected from Sn and Sb: 0.01% or more and 0.1% or less, balance including Fe and incidental impurities, subjecting the slab to heating, then subjecting the slab to hot rolling to obtain a hot rolled steel sheet, then subjecting the steel sheet to hot band annealing, pickling, subsequent single cold rolling to obtain a final sheet thickness, then subjecting the steel sheet to final annealing, wherein in the hot band annealing, soaking temperature is 900° C. or higher and 1050° C. or lower, and cooling rate after soaking is 5° C/s or more.