A cold rolled steel flat product for packaging made of a low carbon steel having a thickness of less than 0.49 mm and a method of making. The steel flat product has a martensite-free microstructure and represents a standard grade for packaging with tensile strengths from 300 to 550 MPa, which can be produced from a cold-rolled steel sheet with a carbon content from 0.01% to 0.1% by weight by inductive annealing of the steel sheet and subsequent water cooling for quenching the recrystallization-annealed steel sheet. To achieve flatness of 5 I-units or less, the induction annealed steel sheet is first primarily cooled in the manufacturing process to a take-off temperature at a rate of less than 1000 K/s, with the take-off temperature being below the transformation temperature of 723° C., and thereafter a secondary cooling by water cooling with a water temperature of less than 80° C. at a rate of more than 1000 K/s.

Provided is a steel reinforcement including an amount of 0.07 to 0.43 wt % of carbon (C), an amount of 0.5 to 2.0 wt % of manganese (Mn), an amount of 0.05 to 0.5 wt % of silicon (Si), an amount greater than 0 and less than or equal to 0.5 wt % of chromium (Cr), an amount greater than 0 and less than or equal to 4.5 wt % of copper (Cu), an amount greater than 0 and less than or equal to 0.003 wt % of boron (B), an amount greater than 0 and less than or equal to 0.25 wt % of vanadium (V), an amount greater than 0 and less than or equal to 0.012 wt % of nitrogen (N), an amount greater than 0 and less than or equal to 0.03 wt % of phosphorus (P), an amount greater than 0 and less than or equal to 0.03 wt % of sulfur (S), an amount of 0.01 to 0.5 wt % of the sum of one or more of nickel (Ni), niobium (Nb) and titanium (Ti), the balance of iron (Fe), and other inevitable impurities. A final microstructure includes ferrite, bainite, pearlite, retained austenite, and precipitates comprising copper.

The purpose of one aspect of the present invention is to provide: a thick steel plate capable of removing a conventional normalizing treatment required for ensuring toughness low temperature and cryogenic environments, and having properties equal to or better than those of a conventional steel subjected to the normalizing treatment; and a method for manufacturing the method.

A method for manufacturing a high-strength steel bar can include the steps of: reheating a steel slab at a temperature ranging from 1000° C. to 1100° C., the steel slab including a certain amount of carbon (C), silicon (Si), manganese (Mn), phosphorus (P), sulfur (S), chromium (Cr), copper (Cu), nickel (Ni), molybdenum (Mo), aluminum (Al), vanadium (V), nitrogen (N), antimony (Sb), tin (Sn), and iron (Fe) and other inevitable impurities, The method can further include finish hot-rolling the reheated steel slab at a temperature of 850° C. to 1000° C., and cooling the hot-rolled steel to a martensite transformation start temperature (Ms (° C.)) through a tempcore process.

This hot-rolled steel sheet has a predetermined chemical composition and predetermined metallographic structure, a ratio between a maximum depth of a region where, on one surface, a rotation angle between a normal line of the one surface and a (011) pole near the normal line of the one surface is 5° or less and a maximum depth of a region where, on the other surface, a rotation angle between a normal line of the other surface and a (011) pole near the normal line of the other surface is 5° or less is 1.00 to 1.20, and a tensile strength is 1150 MPa or more.

The present invention relates to a rail which has a predetermined chemical composition and satisfies expressions of 1.00<Mn/Cr≦4.00 and 0.30≦0.25×Mn+Cr≦1.00 and in which a structure to a depth of 25 mm from an outer surface of a head portion as the origin includes 95% or greater of a pearlite structure, the hardness of the structure is in a range of Hv 350 to 480, 50 to 500 V carbonitride having an average grain size of 5 to 20 nm are present per 1.0 μm.sup.2 of an area to be inspected in a transverse cross section at a position having a depth of 25 mm from the outer surface of the head portion, and the value obtained by subtracting the hardness of the position having the depth of 25 mm from the outer surface of the head portion from the hardness of the position having a depth of 2 mm from the outer surface of the head portion is in a range of Hv 0 to Hv 40.

The present invention relates to a rail which has a predetermined chemical composition and in which at least 90% of a metallographic structure from an outer surface of the rail bottom portion, as the origin, to a depth of 5 mm is a pearlite structure, a surface hardness HC of a foot-bottom central portion is in a range of Hv 360 to 500, a surface hardness HE of a foot-edge portion is in a range of Hv 260 to 315, and HC, HE, and a surface hardness HM of a middle portion positioned between the foot-bottom central portion and the foot-edge portion satisfy HC≧HM≧HE.

A steel for wheel contains, in mass %, C: 0.65 to 0.84%, Si: 0.4 to 1.0%, Mn: 0.50 to 1.40%, Cr: 0.02 to 0.13%, S: 0.04% or less and V: 0.02 to 0.12%, wherein Fn1 expressed by formula (1) is 32 to 43, and Fn2 expressed by formula (2) is 25 or less, the balance being Fe and impurities. P, Cu and Ni as impurities are P: 0.05% or less, Cu: 0.20% or less and Ni: 0.20% or less:
Fn1=2.7+29.5'C+2.9'Si+6.9'Mn+10.8'Cr+30.3'Mo+44.3'V   (1)
Fn2=exp(0.76)'exp(0.05'C)'exp(1.35'Si)'exp(0.38'Mn)'exp(0.77'Cr)'exp(3.0'Mo)'exp(4.6'V)  (2).
The steel has excellent properties for use as a wheel.

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 are: a boron-added high strength steel for bolt excellent in delayed fracture resistance even having a tensile strength of 1100 MPa or more without addition of large amounts of expensive alloy elements such as Cr and Mo: and a high strength bolt made from the boron-added high strength steel for bolt. The high strength steel for bolt contains C of 0.23% to less than 0.40%, Si of 0.23% to 1.50%, Mn of 0.30% to 1.45%, P of 0.03% or less (excluding 0%), S of 0.03% or less (excluding 0%), Cr of 0.05% to 1.5%, V of 0.02% to 0.30%, Ti of 0.02% to 0.1%, B of 0.0003% to 0.0050%, Al of 0.01% to 0.10%, and N of 0.002% to 0.010%, with the remainder being iron and inevitable impurities. The steel has a ratio ([Si]/[C]) of the Si content [Si] to the C content [C] of 1.0 or more and has a ferrite-pearlite mixed microstructure.