One aspect of the present invention relates to a plated steel sheet for hot press forming, having an excellent impact property.

Provided is a steel sheet having: ferrite and pearlite composing 80% or more, in area fraction, of the microstructure; yield strength of 60 ksi or more; elongation of at least 23%; an n-value of at least 0.14; incidental impurities; and, in weight percent: C: 0.03˜0.10 Si: 0˜0.6 Mn: 0.5˜1.5 Cu: 0˜1.0 Ni: 0˜1.0 Nb: 0˜0.06 Ti: 0˜0.1 Mo: 0˜0.5% Cr: 0˜1.0 Al: 0˜0.06 N: 0.0001˜0.006 Ca: 0˜0.006 P: 0˜0.02 S: 0˜0.005.

A process for manufacturing compact coils of ultra-fine grained, martensite-free steel bars, the process comprising the following stages:

a) rolling a steel billet by means of a roughing rolling mill producing a steel bar;

b) performing at least one first cooling stage so that the steel bar has a surface temperature higher than the martensite start temperature, and performing at least one first equalization stage in air;

c) rolling the steel bar by means of at least one intermediate rolling mill;

d) performing at least one second cooling stage always maintaining the surface temperature higher than the martensite start temperature, and performing at least one second equalization stage in air;

e) rolling the steel bar by means of a finishing rolling mill in a non-recrystallization temperature range, maintaining the whole cross-section of the steel bar within said non-recrystallization temperature range, and with a total reduction between 25 and 50% with respect to the cross-section of the steel bar at the entry of the finishing rolling mill, in order to obtain an ultra-fine-grained austenitic matrix;

f) winding the steel bar in a compact coil, by means at least one spooling device, so that the ultra-fine-grained austenitic matrix transforms in a mixture of ferrite and pearlite.

After the winding operation is completed, the compact coil can be transferred to a storage area through a transferring device, for example a walking beam, where a natural or forced or retarded cooling is applied to the coil.

A cold-rolled steel sheet is provided that has a tensile strength of 980 MPa or more, and has a prescribed chemical composition. The microstructure is composed of, in area %, ferrite: 1 to 29%, retained austenite: 5 to 20%, martensite: less than 10%, pearlite: less than 5%, and the balance: bainite and/or tempered martensite. The total sum of the lengths of phase boundaries where ferrite comes in contact with martensite or retained austenite having a circle-equivalent radius of 1 μm or more is 100 μm or less per 1000 μm.sup.2. The cold-rolled steel sheet is excellent in workability and low-temperature toughness, and in particular is excellent in low-temperature toughness after introduction of plastic strain.

This hot-rolled steel sheet has a predetermined chemical composition, in a microstructure, in terms of area %, residual austenite is less than 3.0%, ferrite is 15.0% or more and less than 60.0%, and pearlite is less than 5.0%, an E value that indicates periodicity of the microstructure is 10.7 or more, and an I value that indicates uniformity of the microstructure is less than 1.020, a standard deviation of a Mn concentration is 0.60 mass % or less, and a tensile strength is 980 MPa or more.

A method for producing a machine component excellent in pitting resistance characteristics and toughness includes a carburizing step, performed on a steel material containing 0.13-0.30% C and 0.90-2.00% Cr in mass % and at least one of Si, Mn, Ni, Mo, Nb, V, Ti, B, Al, and N, balance Fe and unavoidable impurities; heating the material to 850-1030° C. to attain carbon concentration in a surface of 0.8-1.5%; cooling the material at an average rate of 5° C./sec or lower from a temperature higher than the A.sub.cm point of a surface layer to a cooling end temperature that is at least 50° C. lower than the A.sub.1 point to cause the surface layer to have a pearlite or bainite structure with dispersion; spheroidizing annealing at a temperature not higher than the A.sub.cm point at the surface layer; heating the material to not higher than the A.sub.cm point at the surface layer; and performing tempering.

A high-strength steel sheet satisfies a predetermined chemical composition in which a content of martensite is 93% by volume or more, contents of ferrite, pearlite, and bainite are 2% by volume or less in total, and a content of retained austenite is 7% by volume or less, based on an entire metal structure, a number of laths in martensite measured by a cutting method in a total length of 300 μm in an image of the metal structure observed under a scanning electron microscope is 240 or more, and a tensile strength of the high-strength steel sheet is 1470 MPa or more.

Steel for a crankshaft includes 0.37 to 0.42 wt % of carbon (C), 0.55 to 0.70 wt % of silicon (Si), 1.45 to 1.65 wt % of manganese (Mn), 0.025 wt % or less (excluding 0 wt %) of phosphorus (P), 0.020 to 0.035 wt % of sulfur (S), 0.15 to 0.30 wt % of chromium (Cr), 0.035 to 0.055% of vanadium (V), and the remainder of Fe and other inevitable impurities. The steel for a crankshaft has strength that is maintained high even when reducing the amount of vanadium.

A rail includes a foot, a web, and a head. The web has a chemical composition containing C: 0.70 to 1.20 mass%, Si: 0.20 to 1.20 mass%, Mn: 0.20 to 1.50 mass%, P: 0.035 mass% or less, and Cr: 0.20 to 2.50 mass%, with the balance being Fe and incidental impurities. The area fraction of pearlite in the web is 95% or more, and the average size of pearlite blocks is 60 .Math.m or less.

A high-strength steel sheet includes a chemical composition including: by mass %, C: 0.080 to 0.500%, Si: 2.50% or less, Mn: 0.50 to 5.00%, P: 0.100% or less, S: 0.0100% or less, Al: 0.001 to 2.500%, N: 0.0150% or less, O: 0.0050% or less, and the balance: Fe and inevitable impurities. The high-strength steel sheet satisfying a predetermined formula has a microstructure in a region from ⅛t to ⅜t from a steel sheet surface. The microstructure includes: by volume %, 20% or more of acicular ferrite, 20% or more of an island-shaped hard structure including residual austenite, 2% to 25% of residual austenite, and 20% or less of aggregated ferrite.