664 results about "Cementite" patented technology

The invention discloses a high-strength plastic accumulation TRIP (Transformation-Induced Plasticity) steel plate and a preparation method thereof, wherein the high-strength plastic accumulation TRIP steel plate consists of the following elements: 0.08%-0.5% of C, 0.4%-2.0% of Si, 3%-8% of Mn, smaller than or equal to 0.10% of P, smaller than or equal to 0.02% of S, 0.02%-4% of Al, smaller than or equal to 0.01% of N, 0-0.5% of Nb, 0-0.5% of V, 0-0.5% of Ti, 0-2% of Cr, 0-1% of Mo, and the balance of Fe and unavoidable impurities. In a microstructure, martensites account for 30%-90% by area occupation ratio, austenites account for 5%-30% by volume fraction, and the remaining is few ferrites and cementites. The hot rolling heating-up temperature is 1100-1250 DEG C, the holding time is more than or equal to 2 hours, the initial rolling temperature is higher than or equal to 1100 DEG C, the final rolling temperature is 850-950 DEG C, and the coiling temperature is lower than 720 DEG C; a hot rolled plate is 2-4mm in thickness; and the cover annealing is furnace heating for heat preservation at 550-750 DEG C for 1-20 hours and then furnace cooling.

The invention discloses a high-strength weatherabile steel and manufacturing method in the low-alloy steel making domain, which is characterized by the following: adding low-carbon and Cu-Cr-Ni-Mo-Nb into two or more composite additives; controlling carbon content with maximum solubility in the alpha-Fe at 0.0218% under normal temperature; reducing carbon-permeating evolution; affirming main control tissue evenly.

The invention relates to 700MPa high strength hot rolling Q&P steel and a manufacturing method thereof. The steel has the following chemical components in percentage by weight: 0.15-0.40% of C, 1.0-2.0% of Si, 1.5-3.0% of Mn, less than or equal to 0.015% of P, less than or equal to 0.005% of S, 0.3-1.0% of Al, less than or equal to 0.0065% of N, 0.005-0.015% of Ti and the balance of Fe. The yield strength is greater than or equal to 70MPa, the strength of extension is greater than or equal to 1300MPa, and the ductility is greater than 10%. According to the invention, through a reasonable compound design, based on common C-Mn steel components, austenite crystal is refined by micro Ti treatment by improving the content of Si to inhibit separation of cementite, so that the austenite transformation kinetics in the air cooling process is accelerated by improving the content of Al. Meanwhile, a hot continuous rolling process is matched with a sectional cooling process, so that proeutectoid ferrite+martensite+retained austenite tissues are obtained. The cost of the alloy is greatly lowered.

A rolling member excellent in pitting strength, spalling strength and bending strength of dedendum has a first quench hardened layer 1 which is formed on a surface layer of the rolling member and has a parent phase taking the form of martensite phase which forms a solid solution with carbon of 0.35 to 0.8 wt %, and a second quench hardened layer 2 which is formed at a deeper layer under the first quench hardened layer and has a parent phase containing at least either one of martensite phase or bainite phase which forms a solid solution with carbon of 0.07 to 0.3 wt % and contains cementite dispersed therein in a content of 2 to 20% by volume.

The invention relates to low-yield-ratio high-strength hot-rolled Q&P steel and a manufacturing method thereof. The steel comprises the chemical components of 0.20-0.40% of C, 1.0-2.0% of Si, 1.5-3.0% of Mn, no more than 0.015% of P, no more than 0.005% of S, 0.02-0.08% of Al, no more than 0.006% of N, 0.005-0.015% of Ti, and balance of Fe and unavoidable impurities. According to the invention, through low-cost component design, on a basis of common C-Mn steel, Si content is increased such that cementite precipitation is inhibited, and austenite grains are micronized through micro-Ti treatment. Also, with a combination of hot continuous rolling process and sectioned cooling process, a structure comprising proeutectoid ferrite, martensite, and residual austenite is obtained. The yield strength is lower than 700MPa, a tensile strength is higher than 1000MPa, a yield ratio is 0.50-0.60, and alloy cost is greatly reduced.

The invention relates to a method for producing color-coated steel sheet for a structure. The method comprises the following steps of: smelting, namely desulfurizing by molten iron, smelting by a converter, casting continuously to form a blank and performing hot rolling; performing acid washing; performing cold milling; annealing continuously; performing hot dipping on aluminum and zinc; and polishing, straightening, performing color coating and packaging for later use. In the method, the yield strength RP 0.2 (or ReH) is between 550 and 600 MPa; the tensile strength Rm is between 560 and 610MPa; and the percentage elongation after break A80 mm is more than or equal to 6 percent. A neutral salt mist test is performed for 1,200 hours, a coating does not have bubbling and corrosion phenomena. At present, the annealed color-coated plate of which the metallographic structure is ferrite and pearlite or the ferrite and free cementite or the ferrite and the pearlite and less free cementite cannot achieve the effect, and the production cost can be reduced by 10 to 15 percent. By the method, special occasions which have high-yield strength and are used for cooling chambers of automobiles,arch coverings and the like can be met, and a process is simple and easy to implement.

A high-strength cold-rolled steel sheet includes, by mass %, C: 0.10% to 0.40%, Mn: 0.5% to 4.0%, Si: 0.005% to 2.5%, Al: 0.005% to 2.5%, Cr: 0% to 1.0%, and a balance of iron and inevitable impurities, in which an amount of P is limited to 0.05% or less, an amount of S is limited to 0.02% or less, an amount of N is limited to 0.006% or less, the microstructure includes 2% to 30% of retained austenite by area percentage, martensite is limited to 20% or less by area percentage in the microstructure, an average particle size of cementite is 0.01 μm to 1 μm, and 30% to 100% of the cementite has an aspect ratio of 1 to 3.

A wire material comprises at least 0.90 to 1.30 mass% of C, 0.1 to 1.2 mass% of Si, 0.1 to 1.0 mass% of Mn, 0.1 to 0.6 mass% exclusive of Al, 0 to 0.02 mass% of P, 0 to 0.02 mass% of S, 10 to 60 ppm of N, and 10 to 40 ppm of O, with the remainder being Fe and unavoidable impurities, wherein a pearlite structure comprises 97% or more of the surface area of a cross-section surface perpendicular to the lengthwise direction of the wire material, and a pro-eutectoid cementite structure comprises 0.5% or less of the center region of the cross-section surface and also comprises 0.5% or less of a first surface layer region of the cross-section surface.

A method for manufacturing wire rods and steel wire for use in making bead wires, wire ropes and springs is disclosed. That is, high drawability wire rods and a manufacturing method therefor are disclosed, in which the wire drawing is possible without carrying out patenting (an intermediate heat treatment). The high drawability wire rods for making high strength steel wire includes a steel containing, in wt %, 0.4-0.65% of C, 0.1-1.0% of Si, 0.1-1.0% of Mn, 0.3% or less of Cr, 100 ppm or less of B, Fe and other unavoidable impurities. The steel further contains 0.02% or less of one or more elements selected from a group consisting of Ti, Nb and V. The steel has a degenerated pearlite structure with pro-eutectoid ferrite of 10% or less, the remaining part being a discontinuously formed cementite. A billet having the above composition is hot-rolled, and is cooled at a cooling rate of 10-30° C. / sec.

The present invention relates to a wire material comprising 0.95 to 1.30 mass% of C, 0.1 to 1.5 mass% of Si, 0.1 to 1.0 mass% of Mn, 0 to 0.1 mass% of Al, 0 to 0.1 mass% of Ti, 0 to 0.02 mass% of P, 0 to 0.02 mass% of S, 10 to 50 ppm of N, 10 to 40 ppm of O, and the remainder containing at least Fe and unavoidable impurities, wherein 97% or more of the area of a cross-sectional surface perpendicular to the length-wise direction of the wire material is occupied by a pearlite structure, and wherein both a 0.5% or less of the area of the center region of the cross-sectional surface and 0.5% or less of the area of a first surface layer region of the cross-sectional surface are occupied by a pro-eitectoid cementite structure.

A hot rolled steel sheet comprised of, by mass, 0.01 to 0.1% C, 0.01 to 0.1% Si, 0.1 to 3% Mn, 0.1% or less P, 0.03% or less S, 0.001 to 1% Al, 0.01% or less N, 0.005 to 0.08% Nb, 0.001 to 0.2% Ti andthe balance Fe and unavoidable impurities, satisfying the formula ¢Nb! x ¢C!=4.34 x 10<-3>, wherein the density of solid solution C grain boundary quantity is in the range of 1 to 4.5 grain boundaries / nm<2>, and wherein the diameter of cementite grains precipitated on the grain boundaries within the steel sheet is 1 ¢mu!m or less. This hot rolled steel sheet is manufactured according to the following process. A steel piece with the above composition of the hot rolled steel sheet is heated at temperature SRTmin ( DEG C) to 1170 DEG C and subjected to rough rolling in the condition of 1080 DEGto 1150 DEG C finalization temperature. Thereafter, finishing rolling is started at 1000 DEG to below 1080 DEG C within 30 to 150 sec and terminated at temperature ranging from the Ar3 transformation point temperature to 950 DEG C so that the draft upon final pass is in the range of 3 to 15%. The resultant steel sheet is subjected to cooling from the start to the temperature range of 450 DEG to550 DEG C at a cooling rate of over 15 DEG C / sec and wound up.

The present invention discloses on-line medium carbon steel nodularizing and rolling process. The process includes low temperature finish rolling at 680-850 deg.c and accumulated cross section deformation of 50 %-80 %, controlled cooling after rolling at 3-15 deg.c / s to 660-720 deg.c, isothermal course, and naturally cooling to room temperature. The said technological scheme results in metallographic structure of the rolled piece with high ferrite component, less pearlite component, small and dispersed pearlite and granular cementite, lowered strength and hardness, raised plasticity and excellent cold machining performance.

The invention discloses a high-strength hot-rolled Q and P steel and a manufacturing method thereof. The high-strength hot-rolled Q and P steel contains the chemical components in percentage by weight: 0.20-0.50% of C, 0.8-2.0% of Si, 1.5-3.0% of Mn, less than or equal to 0.015% of P, less than or equal to 0.005% of S, 0.02-0.08% of Al, less than or equal to 0.006% of N, 0.005-0.015% of Ti, 0.5-1.5% of Cr and the balance of Fe and unavoidable impurities. On the basis of the traditional C-Mn steel components, the content of Si is increased to inhibit the separation of a cementite, a trace of Ti is added to control the size of an austenite grain during heating, so that fine lath martensite is obtained during quenching; and a certain quantity of Cr element is added to improve the hardenability of a steel plate and delay the conversion from a residual austenite to a bainite at a distribution stage, so that the high-strength hot-rolled wear-resistant steel is manufactured at low cost, and the steel has the tensile strength of greater than 1500MPa and favorable elongation of greater than 10% and is excellent in strength and plasticity matching.

The invention discloses a gradient cemented carbide with binding phases being in gradient change and a preparation method thereof. The method is characterized by comprising the following steps that WC-Co powder is prepared; the carbon content of the WC-Co powder is adjusted, and the upper limit and the lower limit of the carbon content are calculated sub-stoichiometrically; a forming agent is added; pressing is conducted for blank forming; the forming agent is removed; vacuum sintering is conducted; carburizing heat treatment is conducted, the temperature is increased to 900-1200 DEG C, a vacuum sintered body is subjected to carburizing heat treatment, and gas is pumped in a pulse mode; the temperature is increased to 1275-1325 DEG C, and Co phase transport treatment is conducted; the temperature is increased to 1380-1450 DEG C, Ar gas is pumped, and the pressure is maintained to be 10-20 millibars; pressure sintering is conducted; a blank is rapidly cooled to 1270 DEG C; and then the blank is cooled from 1270 DEG C to the room temperature, and the gradient cemented carbide is obtained after being discharged from a furnace. The cobalt content in the gradient cemented carbide disclosed by the invention is in gradient distribution, the cobalt content of the surface is low, cementite phases are avoided, the hardness is high, the cobalt content of a core is high, eta phases are avoided, the toughness is high, and the whole performance is good.

A thick-walled steel pipe for linepipes is provided in which the decrease in yield stress caused by the Bauschinger effect is inhibited by optimizing the metallographic structure of a steel plate and in which the base metal and the weld heat-affected zone have excellent toughness. The welded steel pipe for linepipes, which has superior compressive strength and superior toughness, contains, in terms of mass%, 0.03-0.08% C, up to 0.10% Si, 1.00-2.00% Mn, up to 0.010% P, up to 0.0030% S, up to 0.06% Al, 0.005-0.020% Nb, 0.005-0.025% Ti, and 0.0010-0.0060% N, Ti (%) / N (%) being 2-4, the value of Ceq being 0.30 or more, and the remainder comprising Fe and incidental impurities. The steel pipe is characterized in that the metallographic structure of the base metal has specific values of the contents of bainite, island martensite (MA), and cementite, etc. and that the weld heat-affected zone has a specific metallographic structure. Also provided is a process for producing the welded steel pipe.

A cold-rolled steel sheet of the present invention which has a composition containing, in terms of % by mass, C: 0.05-0.30%, Si: 3.0% or less (including 0%), Mn: 0.1-5.0%, P: 0.1% or less (including 0%), S: 0.010% or less (including 0%), and Al: 0.001-0.10%, and remainder being mainly iron, and which has a structure comprising, in terms of area ratio, 10-80% ferrite, less than 5% (including 0%) of the sum of retained austenite and martensite, and a hard phase as the remainder. The steel sheet gives a KAM value frequency distribution curve in which the relationship between the proportion of frequency having a KAM value ≦0.4, XKAM≦0.4°, and the area ratio of ferrite, Vα satisfies XKAM≦0.4° / Vα≧0.8 and the proportion of frequency having a KAM value in the range of 0.6-0.8, XKAM=0.6-0.8° is 10-20%. In the hard phase adjoining the ferrite, cementite, grains having an equivalent circle diameter of 0.1 μm or larger exist so that three or less such cementite grains are dispersed per μm2 of the hard phase. The steel sheet has improved balance between elongation and stretch flangeability and has better formability.

The invention relates to a high-chromium cast iron composite inoculant, a preparation method and an application thereof, in particular to cast iron alloy containing chromium. The high-chromium cast iron composite inoculant is a nanocrystalline high-chromium cast iron composite inoculant containing rare-earth ferrosilicon and ferroboron. The high-chromium cast iron composite inoculant comprises Fe-Ce-Si-Ca intermediate alloy and Fe-B intermediate alloy, wherein the weight ratio between the Fe-Ce-Si-Ca intermediate alloy and the Fe-B intermediate alloy is 1:0.07-0.13. The nanocrystalline crystal grains of the inoculant are less than 100nm, the inoculant is a flaky inoculant obtained by carrying out melt fast-quenching processing on raw materials which comprise commercially purchased rare-earth ferrosilicon and commercially purchased ferroboron, the inoculant is used for carrying out crystal grain and tissue refining processing on the tissue of the high-chromium cast iron alloy of high-chromium cast iron wear parts used for engineering machinery, the processing method is a metal melt casting method, the casting defect of the existing casting technology of the high-chromium cast iron alloy is overcame, and matrix crystal grains and cementite phases are obviously refined, so that the overall mechanical properties of the high-chromium cast iron alloy are obviously improved.

A steel wire rod or bar steel, the chemical composition of which comprises, in terms of mass%, 0.1-0.6% of C, 0.01-1.5% of Si, 0.05-2.5% of Mn, 0.015-0.3% of Al, and 0.004-0.015% of N, optionally together with definite arbitrary element(s), with the balance consisting of iron and impurities, wherein the impurities contain not more than 0.035% of P and not more than 0.025% of S. In the steel wire rod or bar steel: a surface layer area to a depth equivalent to 15% of the section radius from the surface is a steel tissue that is configured from ferrite having an average particle diameter of 1-15 mum and spherical cementite having an average aspect ratio of 2 or less and an average particle diameter of 0.1-2 mum; an inner area ranging from the point of a depth equivalent to 25% of the section radius from the surface to the core is a steel tissue that is configured from ferrite having an average particle diameter of 15-40 mum and pearlite and / or spherical cementite; after removing surface scale, the surface roughness (Ra) of the surface in the circumferential direction is not more than 4 mum; and the depth of a grain boundary oxidation layer on the surface is not more than 30 mum.

The present invention provides spring steel used for spring steel wire achieving both high strength and cold coilability and spring steel wire, that is, spring steel containing, by mass %, C: 0.45 to 0.70%, Si: 1.0 to 3.0%, Mn: 0.05 to 2.0%, P: 0.015% or less, S: 0.015% or less, N: 0.0015 to 0.0200%, and t-O: 0.0002 to 0.01 and further limiting Al≦0.01% and Ti≦0.003%. Further, it is characterized by satisfying the following regarding the cementite-based spherical carbides present at an observed plane, an occupied area ratio of grains with a circle equivalent diameter of 0.2 μm or more of 7% or less, a density of presence of grains with a circle equivalent diameter of 0.2 to 3 μm of 1 / μm2 or less, and a density of presence of grains with a circle equivalent diameter 3 μm or more of 0.001 / μm2 or less, having an prior austenite grain size number of #10 or higher and a residual austenite of 15 mass % or less, and having an area ratio of poor regions with a small density of presence of cementite-based carbides of a circle equivalent diameter of 2 μm or more of 3% or less.

The invention belongs to the technical field of cold-rolled plate and strip production, and specifically relates to a method for producing cold-rolled enamelled steel by a continuous annealing production process. Cold-rolled enamelled steel is composed of the following ingredients in percentage by weight: 0.01-0.07% of C, not greater than 0.05% of Si, 0.10-0.60% of Mn, not greater than 0.030% of P, not greater than 0.025% of S, 0.015-0.070% of Als, not greater than 0.01% of N, 0.001-0.01% of B and the balance Fe and inevitable impurities. The cold-rolled enamelled steel prepared by the method disclosed by the invention has the characteristics of being excellent in surface quality, good in forming property and fish scaling resistance, good in economic benefits and the like; the continuous annealing method is used for producing the cold-rolled enamelled steel, and is simple in process and low in cost, the cooling speed of a rapid-cooling section of a steel plate is increased, and lots of cementites are separated out from the steel plate as hydrogen storage traps, thus improving the fish scaling resistance and the like of the steel plate.

It is intended to provide a high strength high toughness steel wire suitable for use in PC steel wires, zinc plated steel twisted wires, steel wires for springs, suspension bridge cables, etc. A piano wire rod or hard steel wire rod of 170 kgf / mm<2> or greater tensile strength and 30% or more rupture drawing, having a structure composed mainly of pearlite, wherein the average value of pro-eutectoid cementite area ratio in a central region extending from the center of the wire rod to less than 20% of the radius of the wire rod is 5% or below or wherein the micromartensite size in C-section is 100 mum or less is obtained by subjecting a hard steel wire rod whose steel chemical components, chemical composition of contained inclusions, size and numerical density have been specified to hot rolling and subsequently to either directly patenting treatment or, via reaustenitization, patenting treatment.

A high-strength and high-fatigue-strength steel having a base metal strength of 1000 MPa or more and a rotating bending fatigue strength of 550 MPa or more is provided. The steel contains 0.3-0.8 percent by mass of C, 0.01-0.9 percent by mass of Si, 0.01-2.0 percent by mass of Mn, and Fe and unavoidable impurities as the remainder. The steel has a ferrite-cementite structure having a grain size of 7 mum or less or a ferrite-cementite-pearlite structure having a grain size of 7 mum or less. A surface metal of the steel after high-frequency induction quenching has a martensite structure having a prior austenite grain size of 12 mum or less. Alternatively, a surface metal of the steel after nitriding has a fine structure having a ferrite grain size of 10 mum or less.

A computer determines a subsequent state of a steel volume, based on an instantaneous initial state of said steel volume and at least one volumetric surface, the temporary influence quantities acting on said steel volume, by resolution of an equation of thermal condition and phase change. The states include for at least one volumetric element of the steel volume, a local distribution in concentration of a alloy element mobile in the steel, the local proportions of the modeled phases of the steel and a quantity describing a local energy content of the steel. The phases include austenite and another phase, generally, ferrite or cementite. In the context of the change equation, the concentration levels of the mobile alloy element, which are located on either side of the phase boundary, between the austenite and the other phase are determined by resolution of a Stephan problem.

The present invention concerns a computer which determines a subsequent state of a steel volume, based on an instantaneous initial state of said steel volume and at least one volumetric surface, the temporary influence quantities acting on said steel volume, by resolution of an equation of thermal condition and phase change. The states include for at least one volumetric element of the steel volume, a local distribution in concentration of a alloy element mobile in the steel, the local proportions of the modeled phases of the steel and a quantity describing a local energy content of the steel. The phases comprise austenite and another phase, generally, ferrite or cementite. In the context of the change equation, the concentration levels of the mobile alloy element, which are located on either side of the phase boundary, between the austenite and the other phase are determined by resolution of the Stephan problem.

This hot rolled steel contains, in terms of mass %, C: 0.01 to 0.1%, Si: 0.01 to 0.1%, Mn: 0.1 to 3%, P: not more than 0.1%, S: not more than 0.03%, Al: 0.001 to 1%, N: not more than 0.01%, Nb: 0.005 to 0.08%, and Ti: 0.001 to 0.2%, with a remainder being iron and unavoidable impurities, wherein a formula: [Nb]×[C]≦4.34×10−3 is satisfied, a grain boundary density of solid solution C is not less than 1 atom / nm2 and not more than 4.5 atoms / nm2, and a grain size of cementite grains precipitated at grain boundaries within the steel sheet is not more than 1 um. This method for manufacturing a hot rolled steel sheet includes: heating a steel slab having the same composition as the above hot rolled steel sheet at a temperature that is not less than a temperature of SRTmin (° C.) and not more than 1,170° C.; performing rough rolling at a finishing temperature of not less than 1,080° C. and not more than 1,150° C.; subsequently starting finish rolling within not less than 30 seconds and not more than 150 seconds at a temperature of not less than 1,000° C. but less than 1,080° C.; completing the finish rolling within a temperature range from not less than an Ar3 transformation point temperature to not more than 950° C. so as to achieve a final pass reduction ratio of not less than 3% and not more than 15%; and conducting cooling at a cooling rate exceeding 15° C. / sec from a cooling start temperature to a temperature within a range from not less than 450° C. to not more than 550° C., and then coiling the steel sheet.

The invention discloses chemical compositions of ferrite ductile iron used at low temperature and a manufacturing method of the ferrite ductile iron, determines the range of the chemical compositions of the ferrite ductile iron used at low temperature being minus 40 DEG C and provides a quality standard for the production of qualified and stable ductile iron workpieces. The manufacturing method comprises the steps of adopting pure iron, a carburant and silicon iron as raw materials, synthesizing and melting the raw materials in an electric furnace, conducting spheroidization on the raw materials by utilizing a low-rare-earth spheroidizing agent, and producing the ferrite ductile iron by adopting a combined inoculation method. According to the method, the purity of molten iron is improved stably, and the difficulties of high phosphorus, sulphur, titanium, other imputies and interference elements contents in the molten iron, large fluctuation and instability are solved. According to the method, a novel technical scheme is provided for the production of ferrite ductile iron workpiece which contains small, circular and smooth ductile iron and does not contain cementite and iron phosphide eutectic basically, the ferrite ductile iron can meet the demands of high-performance ferrite ductile iron workpiece in electrical industry, high-speed rail industry and the like, the dependence on imported products can be broken; and the running safety of nuclear power and the high-speed rail can be improved substantially.

A hot rolled wire material which comprises 0.005 to 0.6 mass % of C, and has a structure wherein pseudo-pearlite, bainite and ferrite account for 10 vol % or more, 75 vol % or less and 60 vol % or less, respectively, and the relationship that the sum of (pseudo-pearlite + bainite + ferrite) = 90 vol % is satisfied. A method for producing the above hot rolled wire material which comprises rolling a specific steel, followed by winding up at 750 to 1000 DEG C, cooling the steel from an upper limit temperature of 750 to 1000 DEG C to a lower limit temperature of 400 to 550 DEG C at a cooling rate of 20 DEG C / sec or higher, holding the steel for 20 sec or longer at 400 to 550 DEG C, and then cooling the steel. A steel wire comprises 0.005 to 0.6 mass % of C, and has a structure wherein the distances between cementite particles have a value of (standard deviation / average) of 0.50 or less. A method for producing the steel wire includes subjecting the above hot rolled wire material to spheroidizing annealing.

The present invention provides a high strength heat treated steel wire for spring having a tensile strength of 2000 MPa or more which is coiled in the cold state and can achieve both sufficient atmospheric strength and coilability and spring steel used for that steel wire, that is, a high strength heat treated steel wire for a spring characterized by comprising, by mass %, C: 0.5 to 0.9%, Si: 1.0 to 3.0%, Mn: 0.1 to 1.5%, Cr: 1.0 to 2.5%, V: over 0.15 to 1.0%, and Al: 0.005% or less, controlling N to 0.007% or less, further containing one or two of Nb: 0.001 to less than 0.01% and Ti: 0.001 to less 0.005%, and having a tensile strength of 2000 MPa or more, having cementite-based spheroidal carbides and alloy-based spheroidal carbides in a microscopic visual field satisfying an area percentage of carbides with a circle equivalent diameter of 0.2 μm or more of 7% or less and a density of carbides with a circle equivalent diameter of 1 grain / μm2 or less, having a prior austenite grain size number of #10 or more, and having retained austenite of 15 mass % or less.

Disclosed is a high-strength bolt having a tensile strength of 1,200 N / mm2 or more and superior in delayed fracture resistance and relaxation resistance, prepared by wire-drawing a bolt steel containing the following elements: C: 0.5 to 1.0% (mass %, the same shall apply hereinafter), Si: 0.55 to 3%, Mn: 0.2 to 2%, P: 0.03% or less (but not 0%), S: 0.03% or less (but not 0%), and Al: 0.3% or less (but not 0%), and containing proeutectoid ferrite, proeutectoid cementite, bainite and martensite at a total areal rate of less than 20% and pearlite in balance; cold-heading the wire into a bolt shape; and then bluing the bolt in a temperature range of 100 to 500° C.