The present invention relates to a hot-rolled steel sheet for a high strength galvanized steel sheet, having excellent surface quality, and a method for producing the same, the hot-rolled steel sheet comprising, by weight %: C: 0.05 to 0.15%, Si: 0.03 to 0.10%, Mn: 0.7 to 1.39%, P: 0.001 to 0.05%, S: 0.001 to 0.03%, Al: 0.002 to 0.035%, and the remainder being Fe and unavoidable impurities. The weight ratio of Mn/Si is 15 to 25, the weight ratio of C/Si is 1 to 5, and the weight ratio of Si/P is 3 to 10. The hot-rolled steel sheet has a microstructure consisting of, in area fraction, 10 to 40% of bainite, 20 to 30% of pearlite and 40 to 60% of ferrite, and includes a ternary eutectic compound of FeO, Fe.sub.2SiO.sub.4 and Fe.sub.3(PO).sub.4 formed within 50 μm from the surface.

Provided are structural ultra-thick steel having excellent resistance to brittle crack propagation and a production method therefor. The structural ultra-thick steel comprises 0.02-0.1 wt % of C, 0.8-2.5 wt % of Mn, 0.05-1.5 wt % of Ni, 0.005-0.1 wt % of Nb, and 0.005-0.1 wt % of Ti with the remainder being Fe and other inevitable impurities, and has microstructures including one structure selected from the group consisting of a single-phase structure of ferrite, a single-phase structure of bainite, a complex-phase structure of ferrite and bainite, a complex-phase structure of ferrite and pearlite, and a complex-phase structure of ferrite, bainite, and pearlite. The ultra-thick structural steel has excellent resistance to brittle crack propagation, excellent yield strength and an excellent impact transition temperature in the center.

A canted coil spring includes a core wire 10 formed of steel having a pearlite structure; and a copper plating layer 20 formed of copper or a copper alloy and covering an outer circumferential surface 11 of the core wire 10. The steel contains 0.5 mass % or more and 1.0 mass % or less carbon, 0.1 mass % or more and 2.5 mass % or less silicon, and 0.3 mass % or more and 0.9 mass % or less manganese, with the balance being iron and inevitable impurities. The copper plating layer 20 has a crystallite size of 220±50 Å.

A steel reinforcing bar contains 0.06 wt % to 0.11 wt % carbon, more than 0 and not more than 0.25 wt % silicon, 0.8 wt % or more and less than 2.0 wt % manganese, more than 0 and not more than 0.01 wt % phosphorus, more than 0 and not more than 0.01 wt % sulfur, 0.01 to 0.03 wt % aluminum, 0.50 to 1.00 wt % nickel, 0.027 to 0.125 wt % molybdenum, more than 0 and not more than 0.25 wt % chromium, more than 0 and not more than 0.28 wt % copper, more than 0 and not more than 0.01 wt % nitrogen, and the remainder being iron and unavoidable impurities. The reinforcing bar has a surface layer and a core. The surface layer has a hardened layer of tempered martensite, and the core has a mixed structure of bainite, ferrite and pearlite.

A high-strength steel product is described that includes a composition consisting of, in terms of weight percentages, 0.02% to 0.05% C, 0.1% to 0.6% Si, 1.1% to 2.0% Mn, 0.01% to 0.15% Al, 0.01% to 0.08% Nb, 0.5% or less Cu, 0.5% or less Cr, 0.7% or less Ni, 0.03% or less Ti, 0.1% or less Mo, 0.1% or less V, 0.0005% or less B, 0.015% or less P, 0.005% or less S, and the remainder being Fe and inevitable impurities, wherein the steel product has a microstructure comprising a matrix consisting of, in terms of volume percentages, 40% to 80% quasi-polygonal ferrite, 20% to 40% polygonal ferrite, 20% or less bainite, and the remainder being pearlite and martensite of 20% or less.

Provided are high-strength steel having superior brittle crack arrestability and a production method therefor. The structural ultra-thick steel comprises 0.05-0.1 wt % of C, 0.9-1.5 wt % of Mn, 0.8-1.5 wt % of Ni, 0.005-0.1 wt % of Nb, 0.005-0.1 wt % of Ti, 0.1-0.6 wt % of Cu, 0.1-0.4 wt % of Si, at most 100 ppm of P, and at most 40 ppm of S with the remainder being Fe and other inevitable impurities, has microstructures including one structure selected from the group consisting of a single-phase structure of ferrite, a single-phase structure of bainite, a complex-phase structure of ferrite and bainite, a complex-phase structure of ferrite and pearlite, and a complex-phase structure of ferrite, bainite, and pearlite, and has a thickness of at least 50 mm. The high-strength steel has high yield strength and superior brittle crack arrestability.

A method for manufacturing a hot-rolled and coated steel sheet having a thickness between 1.8 mm and 5 mm. The method contains the steps of: providing a semi-product having a composition containing: 0.04%≤C≤0.38%, 0.40%≤Mn≤3%, 0.005%≤Si≤0.70%, 0.005%≤Al≤0.1%, 0.001%≤Cr≤2%, 0.001%≤Ni≤2%, 0.001%≤Ti≤0.2%, Nb≤0.1%, B≤0.010%, 0.0005%≤N≤0.010%, 0.0001%≤S≤0.05%, 0.0001%≤P≤0.1%, Mo≤0.65%, W≤0.30%, Ca≤0.006%, hot-rolling with a final rolling temperature FRT, to obtain a hot-rolled steel product having a thickness between 1.8 mm and 5 mm, then cooling down to a coiling temperature T.sub.coil satisfying: 450° C.≤T.sub.coil≤T.sub.coilmax with T.sub.coilmax=650−140×fγ, T.sub.coilmax being expressed in degrees Celsius and fγ designating the austenite fraction just before the coiling, and coiling to obtain a hot-rolled steel substrate, pickling and coating the hot-rolled steel substrate with Al or an Al alloy by continuous hot-dipping in a bath, to obtain a hot-rolled and coated steel sheet containing a hot-rolled steel sheet and an Al or an Al alloy coating, having a thickness between 10 and 33 μm, on each side of the hot-rolled steel sheet.

Provided are high-strength steel having superior brittle crack arrestability and a production method therefor. The high-strength steel comprises 0.05-0.1 wt % of C, 0.9-1.5 wt % of Mn, 0.8-1.5 wt % of Ni, 0.005-0.1 wt % of Nb, 0.005-0.1 wt % of Ti, 0.1-0.6 wt % of Cu, 0.1-0.4 wt % of Si, at most 100 ppm of P, and at most 40 ppm of S with the remainder being Fe and other inevitable impurities, and has microstructures including one structure selected from the group consisting of a single-phase structure of ferrite, a single-phase structure of bainite, a complex-phase structure of ferrite and bainite, a complex-phase structure of ferrite and pearlite, and a complex-phase structure of ferrite, bainite, and pearlite. The high-strength steel has high yield strength and superior brittle crack arrestability.

Manufacturing a steel sheet for a steel belt includes hot rolling a steel slab containing, in mass %, 0.60 to 0.80% of C, 1.0% or less of Si, 0.10 to 1.0% of Mn, 0.020% or less P, 0.010% or less S, 0.1 to 1.0% of Cr, 0 to 0.5% of V, 0 to 0.1% of Ti, 0 to 0.1% of Nb, and 0 to 0.01% of B, the balance Fe and unavoidable impurities, under a finish hot rolling temperature of 800 to 900° C. An average cooling rate from finish rolling to coiling is 20° C. per second or more. A coiling temperature is 450 to 650° C. The hot-rolled slab is cold rolled with a total rolling reduction ratio of 40% or more and a reduction ratio per one pass of less than 12%, without performing a heat treatment. The cold-rolled slab is aged at 200 to 500° C. for 0.5 to 30 hours.

A hot-dip galvanized steel sheet wherein the hot-dip galvanized steel sheet comprises a base steel sheet and a hot-dip galvanized layer, a ferrite phase is, by volume fraction, 40% or more and 97% or less in a range of ⅛ thickness to ⅜ thickness centered at a position of ¼ thickness from the surface of the base steel sheet, a hard structure is 3% or more in total, wherein the hot-dip galvanized steel sheet has the hot-dip galvanized layer in which Fe is 5.0% or less and Al is 1.0% or less, and columnar grains formed of a ζ phase is 20% or more in an entire interface between the plated layer and the base steel sheet on the surface of the base steel sheet in which a ratio of a volume fraction of the hard structure in a surface layer range of 20 μm depth in a steel sheet direction from an interface between the hot-dip galvanized layer and the base steel sheet is 0.10 times or more to 0.90 times or less of a volume fraction of the hard structure in the range of ⅛ thickness to ⅜ thickness, and wherein the hot-dip galvanized steel sheet has a refined layer at the side of the interface in the base steel sheet, and wherein an average thickness of the refined layer, an average grain size of ferrite in the refined layer and a maximum size of the oxide included in the refined layer are defined respectively.