Method for the manufacture of a galvannealed steel sheet includes the following steps: A) the provision of a pre-coated steel sheet coated with a first coating comprising iron and nickel, such steel sheet having the following chemical composition in weight percent 0.10<C<0.40%, 1.5<Mn<3.0%, 0.7<Si<2.0%, 0.05<Al<1.0%, 0.75<(Si+Al)<3.0% and on a purely optional basis, one or more elements such as Nb≤0.5%, B≤0.005%, Cr≤1.0%, Mo≤0.50%, Ni≤1.0%, Ti≤0.5%, the remainder of the composition making up of iron and inevitable impurities resulting from the elaboration, B) the thermal treatment of such pre-coated steel sheet at a temperature between 600 to 1000° C., C) the hot-dip coating of the steel sheet obtained in step B) with a second coating based on zinc and D) an alloying treatment to form a galvannealed steel sheet.

A production method for a high-strength steel sheet having a tensile strength TS of 780 MPa or more is provided. The production method comprises: heating a steel slab having a predetermined chemical composition; hotrolling the steel slab; coiling the hot-rolled sheet; subjecting the hot-rolled sheet to pickling treatment; holding the hot-rolled sheet in a pre-determined temperature range for predetermined time; cold rolling the hot-rolled sheet to obtain a cold-rolled sheet; subjecting the cold-rolled sheet to first annealing treatment; cooling the cold-rolled sheet at a pre-determined average cooling rate; cooling the cold-rolled sheet to room temperature; reheating the clod-rolled sheet to perform second annealing treatment; cooling the cold-rolled sheet at a first average cooling rate; cooling the cold-rolled sheet at a second average cooling rate; reheating the cold-rolled sheet to a predetermined reheating temperature range; and holding the cold-rolled sheet in the reheating temperature range.

A production method for a high-strength steel sheet having a tensile strength TS of 780 MPa or more is provided. The production method comprises: heating a steel slab having a predetermined chemical composition; hotrolling the steel slab; coiling the hot-rolled sheet; subjecting the hot-rolled sheet to pickling treatment; holding the hot-rolled sheet in a pre-determined temperature range for predetermined time; cold rolling the hot-rolled sheet to obtain a cold-rolled sheet; subjecting the cold-rolled sheet to first annealing treatment; cooling the cold-rolled sheet at a pre-determined average cooling rate; cooling the cold-rolled sheet to room temperature; reheating the clod-rolled sheet to perform second annealing treatment; cooling the cold-rolled sheet at a first average cooling rate; cooling the cold-rolled sheet at a second average cooling rate; reheating the cold-rolled sheet to a predetermined reheating temperature range; and holding the cold-rolled sheet in the reheating temperature range.

Disclosed is a method for manufacturing high-carbon bearing steel, which include: heating a billet at a temperature of about 950 to 1,050° C. for about 70 to 120 minutes, rolling the billet to manufacture a wire rod, winding the wire rod to manufacture a wire rod coil, cooling the wire rod coil, and subsequently heat treating the wire rod coil for spheroidizing and carbonitriding, respectively. The bearing steel may include an amount of about 0.9 to 1.3 wt % of carbon (C), an amount of about 1.1 to 1.6 wt % of silicon (Si), an amount of about 1.0 to 1.5 wt % of manganese (Mn), an amount of about 1.5 to 1.9 wt % of chromium (Cr), an amount of about 0.2 to 0.6 wt % of nickel (Ni), an amount of about 0.1 to 0.3 wt % of molybdenum (Mo), and the balance iron (Fe) based on the total weight thereof.

Disclosed is a method for manufacturing high-carbon bearing steel, which include: heating a billet at a temperature of about 950 to 1,050° C. for about 70 to 120 minutes, rolling the billet to manufacture a wire rod, winding the wire rod to manufacture a wire rod coil, cooling the wire rod coil, and subsequently heat treating the wire rod coil for spheroidizing and carbonitriding, respectively. The bearing steel may include an amount of about 0.9 to 1.3 wt % of carbon (C), an amount of about 1.1 to 1.6 wt % of silicon (Si), an amount of about 1.0 to 1.5 wt % of manganese (Mn), an amount of about 1.5 to 1.9 wt % of chromium (Cr), an amount of about 0.2 to 0.6 wt % of nickel (Ni), an amount of about 0.1 to 0.3 wt % of molybdenum (Mo), and the balance iron (Fe) based on the total weight thereof.

A preferable aspect of the present invention provides a high-strength high-toughness hot-rolled steel sheet and a manufacturing method therefor, wherein the hot-rolled steel sheet contains, by weight, 0.07-0.13% C, 0.20-0.50% Si, 0.5-0.9% Mn, 0.03% or less P, 0.02% or less S, 0.005-0.03% Nb, 0.3-0.6% Cr, 0.005-0.03% Ti, 0.1-0.35% Cu, 0.05-0.3% Ni, 0.01-0.15% Mo, 0.007% or less N, 0.001-0.006% Ca, 0.01-0.05% Al, and the balance Fe and other unavoidable impurities, the alloy elements satisfying the following relational formulas [Relational formula 1] 1.6≤(Mo/96)/(P/31)≤6, [Relational formula 2] 1.6≤(Ca/S)≤3, and [Relational formula 3] 3.5≤(3*C/12+Mn/55)*100≤5; wherein a microstructure comprises, by area fraction, 85% or more of polygonal ferrite and 15% or less of pearlite, the crystal grain size of the polygonal ferrite being 10 μm or less; and wherein a variation in yield strength in a width direction is 35 MPa or lower.

A preferable aspect of the present invention provides a high-strength high-toughness hot-rolled steel sheet and a manufacturing method therefor, wherein the hot-rolled steel sheet contains, by weight, 0.07-0.13% C, 0.20-0.50% Si, 0.5-0.9% Mn, 0.03% or less P, 0.02% or less S, 0.005-0.03% Nb, 0.3-0.6% Cr, 0.005-0.03% Ti, 0.1-0.35% Cu, 0.05-0.3% Ni, 0.01-0.15% Mo, 0.007% or less N, 0.001-0.006% Ca, 0.01-0.05% Al, and the balance Fe and other unavoidable impurities, the alloy elements satisfying the following relational formulas [Relational formula 1] 1.6≤(Mo/96)/(P/31)≤6, [Relational formula 2] 1.6≤(Ca/S)≤3, and [Relational formula 3] 3.5≤(3*C/12+Mn/55)*100≤5; wherein a microstructure comprises, by area fraction, 85% or more of polygonal ferrite and 15% or less of pearlite, the crystal grain size of the polygonal ferrite being 10 μm or less; and wherein a variation in yield strength in a width direction is 35 MPa or lower.

A steel sheet has a composition comprising 0.060%≤C≤0.085%, 1.8%≤Mn≤2.0%, 0.4%≤Cr≤0.6%, 0.1%≤Si≤0.5%, 0.010%≤Nb≤0.025%, 3.42N≤Ti≤0.035%, 0≤Mo≤0.030%, 0.020%≤Al≤0.060%, 0.0012%≤B≤0.0030%, S≤0.005%, P≤0.050%, 0.002%≤N≤0.007% and optionally 0.0005%≤Ca≤0.005%, the remainder of the composition being iron and unavoidable impurities. The microstructure consists of 34% to 80% bainite, 10% to 16% martensite, and 10% to 50% of ferrite. The surface fraction of unrecrystallized ferrite, with respect to the whole structure, is of less than 30%. The martensite consists of self-tempered martensite and fresh martensite, the surface fraction of self-tempered martensite being comprised between 4% and 10%.

A steel sheet has a composition comprising 0.060%≤C≤0.085%, 1.8%≤Mn≤2.0%, 0.4%≤Cr≤0.6%, 0.1%≤Si≤0.5%, 0.010%≤Nb≤0.025%, 3.42N≤Ti≤0.035%, 0≤Mo≤0.030%, 0.020%≤Al≤0.060%, 0.0012%≤B≤0.0030%, S≤0.005%, P≤0.050%, 0.002%≤N≤0.007% and optionally 0.0005%≤Ca≤0.005%, the remainder of the composition being iron and unavoidable impurities. The microstructure consists of 34% to 80% bainite, 10% to 16% martensite, and 10% to 50% of ferrite. The surface fraction of unrecrystallized ferrite, with respect to the whole structure, is of less than 30%. The martensite consists of self-tempered martensite and fresh martensite, the surface fraction of self-tempered martensite being comprised between 4% and 10%.

An embodiment of the present invention provides a steel wire rod and a method for producing same, the steel wire rod comprising 0.3-0.5 wt % of C, 0.02-0.4 wt % of Si, 1.0-1.5 wt % of Mn, 0.3-0.7 wt % of Cr, 0.003 wt % or less of B, 0.03 wt % or less of Ti, 0.03 wt % or less of P, 0.01 wt % or less of S, 0.02-0.05 wt % of Al, and 0.001-0.01 wt % of N, with the balance being Fe and inevitable impurities, and having a microstructure in which the main phase thereof is a complex structure of ferrite+pearlite, and contains at most 5 area % (including 0 area %) of at least one of bainite or martensite, wherein the average pearlite colony size in a region extending from the ⅖ point to the ⅗ point of the diameter is at most 7 μm.