The steel material according to the present disclosure has a chemical composition consisting of, in mass %, C: more than 0.20 to 0.35%, Si: 0.05 to 1.00%, Mn: 0.02 to 1.00%, P: 0.025% or less, S: 0.0100% or less, Al: 0.005 to 0.100%, Cr: 0.40 to 1.50%, Mo: 0.30 to 1.50%, Ti: 0.002 to 0.050%, B: 0.0001 to 0.0050%, N: 0.0100% or less and O: 0.0100% or less, with the balance being Fe and impurities, and satisfies Formula (1) and Formula (2) described in the description. The yield strength is 862 MPa or more. A numerical proportion of precipitates having an equivalent circular diameter within a range of 20 to 300 nm among precipitates having an equivalent circular diameter of 20 nm or more in the steel material is 0.85 or more.

The steel plate has a specific chemical composition, and a microstructure where a volume fraction of martensite at a depth of 1 mm from a surface of the steel plate is 95% or more, and at a depth of 1 mm from a surface of the steel plate, a Vickers hardness at 400° C. is 288 or more, and a Brinell hardness at 25° C. is 360 HBW10/3000 to 490 HBW10/3000.

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.

A soft magnetic alloy comprising an internal area having a soft magnetic type alloy composition including Fe and Co, a Co concentrated area existing closer to a surface side than the internal area and having a higher Co concentration than in the internal area, a SB concentrated area existing closer to the surface side than the Co concentrated area and having a higher concentration of at least one element selected from Si and B than in the internal area, and a Fe concentrated area including Fe existing closer to the surface side than the SB concentrated area; wherein a crystalized area ratio of the SB concentrated area represented by S.sub.SB.sup.cry/S.sub.SB and a crystalized area ratio of the Fe concentrated area represented by S.sub.Fe.sup.cry/S.sub.Fe, satisfy a relation of (S.sub.SB.sup.cry/S.sub.SB)<(S.sub.Fe.sup.cry/S.sub.Fe).

A soft magnetic alloy comprising an internal area having a soft magnetic type alloy composition including Fe and Co, a Co concentrated area existing closer to a surface side than the internal area and having a higher Co concentration than in the internal area, a SB concentrated area existing closer to the surface side than the Co concentrated area and having a higher concentration of at least one element selected from Si and B than in the internal area, and a Fe concentrated area including Fe existing closer to the surface side than the SB concentrated area; wherein a crystalized area ratio of the SB concentrated area represented by S.sub.SB.sup.cry/S.sub.SB and a crystalized area ratio of the Fe concentrated area represented by S.sub.Fe.sup.cry/S.sub.Fe, satisfy a relation of (S.sub.SB.sup.cry/S.sub.SB)<(S.sub.Fe.sup.cry/S.sub.Fe).

A zinc-plated steel sheet of an aspect of the present invention includes a steel sheet having a predetermined chemical composition and a zinc-plated layer. In the steel sheet, steel microstructures in a range of ⅛ thickness to ⅜ thickness, having the center at ¼ thickness from a steel sheet surface, include, by vol %, ferrite: 0% to 10%, bainite: 0% to 20%, tempered martensite: 70% or more, fresh martensite: 0% to 10%, retained austenite: 0% to 10%, and pearlite: 0% to 5%. In the zinc-plated steel sheet, the amount of hydrogen emitted when the steel sheet is heated to 200° C. from a room temperature after removal of the zinc-plated layer is 0.40 ppm or less per mass of the steel sheet, the tensile strength is 1470 MPa or more, and no cracking occurs in a U-shape bending test where a stress equivalent to 1000 MPa is applied for 24 hours.

A zinc-plated steel sheet of an aspect of the present invention includes a steel sheet having a predetermined chemical composition and a zinc-plated layer. In the steel sheet, steel microstructures in a range of ⅛ thickness to ⅜ thickness, having the center at ¼ thickness from a steel sheet surface, include, by vol %, ferrite: 0% to 10%, bainite: 0% to 20%, tempered martensite: 70% or more, fresh martensite: 0% to 10%, retained austenite: 0% to 10%, and pearlite: 0% to 5%. In the zinc-plated steel sheet, the amount of hydrogen emitted when the steel sheet is heated to 200° C. from a room temperature after removal of the zinc-plated layer is 0.40 ppm or less per mass of the steel sheet, the tensile strength is 1470 MPa or more, and no cracking occurs in a U-shape bending test where a stress equivalent to 1000 MPa is applied for 24 hours.

Provided is steel for nitrocarburizing with excellent surface fatigue strength. The steel has a nitride compound layer with a thickness of 5.0 μm to 30.0 μm and a hardened layer in an order from a steel surface to steel inside, where a thickness of a porous layer on an outermost surface of the compound layer is 3.0 μm or less and 40.0% or less of a compound layer's thickness, the hardened layer has hardness of HV600 or more, HV400 or more and HV250 or more at 50 μm inward from the steel surface, from the steel surface to the steel inside of 400 μm, and from the steel surface to the steel inside of 600 μm, respectively, an unhardened portion excluding the compound and hardened layers has a predetermined chemical composition, and the hardened layer has a chemical composition with a higher N content than the unhardened portion.

Provided is steel for nitrocarburizing with excellent surface fatigue strength. The steel has a nitride compound layer with a thickness of 5.0 μm to 30.0 μm and a hardened layer in an order from a steel surface to steel inside, where a thickness of a porous layer on an outermost surface of the compound layer is 3.0 μm or less and 40.0% or less of a compound layer's thickness, the hardened layer has hardness of HV600 or more, HV400 or more and HV250 or more at 50 μm inward from the steel surface, from the steel surface to the steel inside of 400 μm, and from the steel surface to the steel inside of 600 μm, respectively, an unhardened portion excluding the compound and hardened layers has a predetermined chemical composition, and the hardened layer has a chemical composition with a higher N content than the unhardened portion.

In a steel sheet according to the present embodiment, a Ti content and a N content satisfy Ti−3.5×N≥0.003, at a sheet thickness ¼ position, a metallographic structure includes 90% or more of martensite in terms of volume fraction, at the sheet thickness ¼ position, a number density of TiC having a circle equivalent diameter of 1 to 500 nm is 3.5×10.sup.4 particles/mm.sup.2 or more, at the sheet, thickness ¼ position, a value of a median value of a Mn concentration+3σ is 5.00% or less, and a hardness measured at the sheet thickness ¼ position is 1.30 times or more a hardness measured at a position 50 μm deep from a surface of the steel sheet.