53results about How to "Excellent resistance to delayed fracture" patented technology

The disclosed steel contains, by mass, 0.10-0.55% carbon, 0.01-3% silicon, and 0.1-2% manganese. Said steel also contains one or more of the following: 0.05-1.5% chromium; 0.05-0.2% vanadium; 0.05-0.40% molybdenum; 0.001-0.05% niobium; 0.01-4% copper; 0.01-4% nickel; and 0.0001-0.005% boron. The remainder comprises iron and unavoidable impurities. The structure of the disclosed steel is primarily tempered martensite. Said steel, which is a high-strength steel that exhibits excellent resistance to delayed fracture, is characterized by the following being formed in a surface layer thereof: (a) a nitrided layer with a thickness of at least 200 [mu]m from the surface of the steel and a nitrogen concentration by mass that is at least 0.02% higher than that of the steel and no greater than 12.0%; and (b) a low-carbon region, at a depth between 100 [mu]m and 1000 [mu]m from the surface of the steel, with a carbon concentration by mass that is at most 0.9 times that of the steel and no less than 0.05%.

Provided is an ultrahigh-strength cold-rolled steel sheet that has a steel composition in which neither transition metal elements which considerably raise the alloy cost, such as V and Mo, nor Al, which may induce casting defects, is contained in excess and that, despite the composition, has excellent delayed-fracture resistance and a tensile strength of 1,320 MPa or higher. Also provided is a process for producing the steel sheet. The steel sheet having excellent ductility and delayed-fracture resistance is characterized by containing, in terms of mass%, 0.15-0.25% C, 1.0-3.0% Si, 1.5-2.5% Mn, up to 0.05% P, up to 0.02% S, 0.01-0.05% Al, and below 0.005% N, with the remainder comprising Fe and incidental impurities. The steel sheet is further characterized by having a metallographic structure which comprises 40-85 vol.% tempered martensite phase and 15-60 vol.% ferrite phase and by having a tensile strength of 1,320 MPa or higher.

The invention discloses a high-strength alloyed hot-dipped galvanized steel sheet having excellent workability and delayed fracture resistance, and a method for producing the same. Provided is an alloyed hot-dipped galvanized steel sheet which has excellent workability in terms of platability, bendability and hole expandability, while having excellent delayed fracture resistance, and which has a tensile strength of 1,180 MPa or more and an yield ratio (YR) of 73.0% or more. The high-strength plated steel sheet according to the present invention comprises a plating layer on the surface of a base steel sheet, and contains a specific steel component. This high-strength plated steel sheet sequentially comprises, from the interface between the base steel sheet and the plating layer toward the base steel sheet: an internal oxide layer which is present at the interface between the base steel sheet and the plating layer and contains at least one element selected from the group consisting of Si and Mn; a soft layer which includes the internal oxide layer and has a Vickers hardness equal to 90% or less of the Vickers hardness of the base steel sheet at a position of t / 4, where t is the sheet thickness of the base steel sheet; and a hard layer that is configured of a structure mainly composed of martensite. The average depth (D) of the soft layer is 20 [mu]m or more, and the average depth (d) of the internal oxide layer is 4 [mu]m or more and less than (D). The variation coefficient of KAM of the base steel sheet at a position of t / 4 is 0.66 or less.

Provided are a hot-dip galvanized steel sheet and an alloyed hot-dip galvanized steel sheet having a tensile strength of at least 980 MPa, having excellent plating properties, and having excellent workability in terms of extensibility, bendability and hole expandability, and delayed fracture resistance. Disclosed is a high-strength plated steel sheet that is a plated steel sheet having a hot-dip galvanized layer or an alloyed hot-dip galvanized layer on the surface of a base material steel sheet, and that comprises, in order from the interface between the base material steel sheet and the galvanized layer toward the base material steel sheet side: an inner oxide layer including at least one type of oxide selected from the group consisting of Si and Mn; a soft layer that is a layer including said inner oxide layer, and that has a Vickers hardness which is 90% or less of the Vickers hardness at a t / 4 position of the base material steel sheet, where t is the thickness of the base material steel sheet; and a hard layer constituted by a structure including, with respect to the entire metallographic structure, from 20 to 85% by area of a low-temperature transformation-forming phase, greater than 10% and less than or equal to 70% by area of polygonal ferrite, and greater than or equal to 5% by volume of retained austenite. The average depth D of the soft layer is 20 mu m or greater, the average depth d of the inner oxide layer is 4 mu m or greater and less than D, and the tensile strength is 980 MPa or greater.

Provided are: a high-strength cold-rolled steel plate having a tensile strength (TS) of 1150 MPa or more and excellent elongation and delayed fracture resistance; and a method for producing same. The high-strength cold-rolled steel plate has a component composition comprising, in mass%, 0.16-0.30% of C, 1.20-2.20% of Si, 1.50-2.50% of Mn, 0.05% or less of P, 0.005% or less of S, 0.10% or less of Al, 0.007% or less of N, at least one of Ti and Nb (with the total of Ti + Nb being 0.04-0.15%), and a remainder of Fe and unavoidable impurities. The high-strength cold-rolled steel plate is configured so as to have a composite structure comprising 45-65% by volume fraction of ferrite having an average crystal grain size of 3 [mu]m or less, 35-55% by volume fraction of tempered martensite having an average crystal grain size of 2 [mu]m or less, and 5% or less (including 0%) of non-recrystallized ferrite having an average crystal grain size of 5 [mu]m or less.

The invention relates to spring steel and a production process thereof, in particular to 55Si2MnVNbN spring steel and a production process thereof. The 55Si2MnVNbN spring steel comprises the following components in percentage by weight: 0.52 to 0.60 percent of C, 1.60 to 1.90 percent of Si, 0.60 to 0.80 percent of Mn, 0.15 to 0.35 percent of Cr, 0.06 to 0.12 percent of V, 0.015 to 0.025 percent of Nb, 0.80 to 1.50 percent of N, less than or equal to 0.0035 of P, less than or equal to 0.0035 percent of S and the balance of Fe and inevitable impurities. The process comprises electric furnace electrosmelting, LF refining, VD furnace vacuum degassing, continuous casting and rolling. The spring steel can meet the requirements of springs on various severe working environments, high strength andlong service life and has high tensile strength, high delayed fracture resistance, high corrosion resistance, high fatigue resistance and high elasticity attenuation resistance.

The chemical composition of this high-strength PC steel wire comprises, in mass%, C: 0.90-1.10%, Si: 0.80-1.50%, Mn: 0.30-0.70%, P: 0.030% or less, S: 0.030% or less, Al: 0.010-0.070%, N: 0.0010-0.010%, Cr: 0-0.50%, V: 0-0.10%, B: 0-0.005%, Ni: 0-1.0%, Cu: 0-0.50%, and the remainder is Fe and impurities. The ratio of the Vickers hardness (HvS) of the area 0.1D [D: diameter of steel wire] from thesurface of the steel wire (the surface layer) and the Vickers hardness (HvI) of the area interior to the surface layer satisfies [1.10 < HvS / HvI <= 1.15]. The metal composition of the region from thesurface of the steel wire up to 0.01D (the outermost layer) is, by area%, a pearlite structure: less than 80%, and the remainder is a ferrite structure and / or a bainite structure; the metal structurein the region interior to the aforementioned outermost layer region is, by area%, a pearlite structure: greater than or equal to 95%, and has a tensile strength of 2000-2400 MPa. This high-strength PCsteel wire has a simple manufacturing method and has excellent delayed fracture resistance.

Provided are a hot-dip galvanized steel sheet and an alloyed hot-dip galvanized steel sheet having at least 980 MPa, and having excellent plating properties, workability in terms of bendability, hole expandability, and balance between strength and ductility, and delayed fracture resistance. This high-strength plated steel sheet is a plated steel sheet having a plating layer on the surface of a base material steel sheet, includes predetermined steel components, and comprises, in order from the interface between the base material steel sheet and the plating layer toward the base material steel sheet side: a soft layer having a Vickers hardness that is 90% or less of the Vickers hardness at a t / 4 position of the base material steel sheet, where t is the thickness of the base material steel sheet; and a hard layer including martensite, bainite, and ferrite in predetermined ranges. The average depth D of the soft layer is 20 [mu]m or greater, and the average depth d of an inner oxide layer is 4 [mu]m or greater and less than D.

Provided are a hot-dip galvanized steel sheet and an alloyed hot-dip galvanized steel sheet having a tensile strength of at least 980 MPa, having excellent plating properties, and having excellent workability in terms of extensibility, bendability and hole expandability, and delayed fracture resistance. Disclosed is a high-strength plated steel sheet that is a plated steel sheet having a hot-dip galvanized layer or an alloyed hot-dip galvanized layer on the surface of a base material steel sheet, and that comprises, in order from the interface between the base material steel sheet and the galvanized layer toward the base material steel sheet side: an inner oxide layer including at least one type of oxide selected from the group consisting of Si and Mn; a soft layer that is a layer including said inner oxide layer, and that has a Vickers hardness which is 90% or less of the Vickers hardness at a t / 4 position of the base material steel sheet, where t is the thickness of the base materialsteel sheet; and a hard layer constituted by a structure including, with respect to the entire metallographic structure, from 20 to 85% by area of a low-temperature transformation-forming phase, greater than 10% and less than or equal to 70% by area of polygonal ferrite, and greater than or equal to 5% by volume of retained austenite. The average depth D of the soft layer is 20 mu m or greater, the average depth d of the inner oxide layer is 4 mu m or greater and less than D, and the tensile strength is 980 MPa or greater.

In the hot-pressed member according to the present invention, after the component composition thereof is properly adjusted, the microstructure thereof is configured so that there are an average of 5 or more Ti-based deposits having a grain size of 0.10 [mu]m or less per 100 [mu]m2 of a cross section parallel to the thickness direction of the member in a range from the member surface to a depth of100 [mu]m in a sheet thickness direction, the microstructure has a volume fraction of martensite of 95-100% in a depth range of 20-100 [mu]m in the sheet thickness direction from the member surface, and there are an average of 10 or more cementites having a grain size of less than 0.20 [mu]m among prior austenite grains, whereby the hot-pressed member not only has an extremely high tensile strength TS of 1780 MPa or greater after hot pressing, but excellent delayed fracture resistance after projection welding can also be obtained.

The present invention addresses the problem of providing: a high-strength steel plate having excellent delayed fracture resistance and a tensile strength of at least 980 MPa; and a method for producing same. This high-strength steel plate has a specific composition, wherein the degree of Mn segregation in a specific region is 1.5 or less, the maximum concentration of P in a specific region is 0.08 mass% or less, specific MnS particles are included in a cross section parallel to the rolling direction of the steel plate in a region within 100 [mu]m from the surface of the plate in the thickness direction, the number of the specific MnS particles is 2.0 or less per 1 mm<2>, the number of specific oxide inclusions is 8 or less per 1 mm<2>, at least 80% by number of the oxide inclusions have a specific composition, the microstructure of the steel comprises 30-95% of martensite, 5-70% of ferrite phase, less than 30% (including 0%) of bainite, and less than 2.0% (including 0%) of austenite phase by volume percentage, and the tensile strength is at least 980 MPa.

Provided are: a bolt exhibiting excellent delayed fracture resistant properties at a high strength level not less than 1200 MPa but less than 1600 MPa in terms of tensile strength where the possibility of delayed fracture occurring is extremely high; and a bolt steel material serving as a material for said bolt. The bolt has a compositional makeup satisfying formulae (1) and (2), and has a tensile strength not less than 1200 MPa but less than 1600 MPa. Herein, (1): 0.48<=Mo / 1.4+V<1.10. (2): 0.80<Mo / V<3.00, wherein the contents (mass%) of Mo and V contained in the bolt are respectively substituted into Mo and V.

The invention discloses industrial stable high-performance bolt steel and a manufacturing method thereof and belongs to the technical field of high-strength bolt steel. The chemical components of the steel comprise, by mass, 0.28-0.37% of C, equal to or less than 0.10% of Si, 0.15-0.30% of Mn, equal to or less than 0.010% of P, equal to or less than 0.005% of S, 1.10-1.20% of Cr, 0.15-0.40% of Mo, 0.0005-0.003% of B, 0.03-0.08% of Ti, 0.015-0.030% of Al and equal to or less than 0.005% of N, and the balance is Fe, on the basis, 0.15-0.30% of V or 0.02-0.04% of Nb can be added. Through the adoption of a reasonable controlled rolling and controlled cooling technology, the original austenite grain size of the steel after phase change is further refined by 1-2 grades, 3-8 um superfine austenite grains and precipitated phase with the size of 1-20 nm are obtained, and therefore the high-temperature tempering strength of the steel is improved. The industrial stable high-performance bolt steel and the manufacturing method thereof have the advantages that compared with the prior art, the carbon content of the steel in the same strength grade is reduced, the hardness fluctuation after quenching+tempering is small, the strength after high-temperature tempering is high, the stability of batch quality is high, the cost is low, and the delayed fracture resistance is excellent.

Provided are: a high-strength hot-dip zinc-coated steel sheet having high strength and excellent delayed fracture resistance properties; and a method for manufacturing the high-strength hot-dip zinc-coated steel sheet. A high-strength hot-dip zinc-coated steel sheet having a specified component composition and also having such a steel structure that the total content of martensite and bainite containing a carbide is 60 to 100% by area in a region extending to the depth of 300 to 400 [mu]m from the surface layer of the steel sheet as observed in the sheet thickness direction, the average grain diameter of prior austenite is 15 [mu]m or less, and the ratio of the height of a peak of an Auger electron spectrum of P at a position 5 nm or more away from a prior austenite grain boundary to that at the prior austenite grain boundary in the region extending to the depth of 300 to 400 [mu]m from the surface layer of the steel sheet as observed in the sheet thickness direction is 0.20 or more, wherein a hot-dip zinc coating layer is arranged on the surface of the steel sheet.

Provided is a high-strength cold-rolled steel sheet to which Cu is added to improve delayed fracture resistance characteristics, which has good chemical conversion treatability during low-temperaturechemical conversion treatment and for which excellent post-painting corrosion resistance and delayed fracture resistance characteristics can be attained. The high-strength cold-rolled steel sheet contains at least one of Si, Mn, and Al and 0.05-0.25 mass% of Cu, wherein: the steel structure is a recrystallized structure; no oxide layer containing at least one of Al, Si, and Mn is present on the surface of the steel sheet; and cross-sectional coverage by a deposited metal Cu on the steel sheet surface is 30% or less. A film of a pickling liquid on the surface of the steel sheet reacts with thesteel sheet between a period after pickling until washing with water causing Cu to be eluted from the steel plate and deposited on the surface of the steel plate. This deposited metal Cu lowers the chemical conversion treatability, and so, by keeping the cross-sectional coverage by this deposited metal Cu to a low level, chemical conversion treatability can be improved.

The present invention provides a steel plate suitable mainly for strength parts for automobiles and building materials, the steel plate has a tensile strength of 1180 MPa or more, is excellent in delayed fracture resistance, and is also excellent in temporary rust resistance. It has a film on the surface of a cold-rolled steel sheet with a tensile strength of 1180 MPa or more, and the film contains one or more metal salts and phosphorus compounds selected from molybdate and tungstate, and the metal salt is a metal (Mo, W) conversion calculated the total adhesion amount is 10 ~ 1000mg / m 2 , preferably 50-1000mg / m 2 , the attachment amount of phosphorus compound in P conversion is 10~1000mg / m 2 .

The present invention provides a high strength bolt superior in delayed fracture resistant property having a bolt tensile strength of 1200 N / mm<2> or more and a steel material for the same. <??>A high strength bolt comprised of a head portion and shaft portion formed by a steel material, wherein the relation between a bolt tensile strength TS (N / mm<2>) and tempering temperature T ( DEG C) satisfies the following expression (1), the relation between the bolt tensile strength TS (N / mm<2>) and the carbon equivalent Ceq (%) calculated from the chemical composition of the steel material used for the high strength bolt satisfies the following expression (2), and the tensile strength of the bolt is refined to a range of 1200 N / mm<2> to 1600 N / mm<2> by quenching and tempering: <DF NUM="(1)">TS≤.1T+850 < / DF> <DF NUM="(2)">TS≤550Ceq+1000 < / DF> where, <DF>Ceq=C+(Mn / 6)+(Si / 24)+(Ni / 40) +(Cr / 5)+(Mo / 4)+(V / 14) < / DF>

The purpose of the present invention is to provide: a hot-pressed member having tensile strength of 1780 MPa or more and improved in delayed fracture resistance properties; a method for producing the hot-pressed member; a cold-rolled steel sheet for a hot-pressed member; and a method for producing the cold-rolled steel sheet. A hot-pressed member having a specified component composition, wherein the average crystal grain diameter of prior austenite is 8 [mu]m or less and the volume fraction of martensite in a region extending from the surface to the depth of 30 [mu]m or less is 95% or more in the micro structure of a steel sheet, there is an Ni-diffused region having a thickness of 0.5 [mu]m or more in a surface layer, the standard deviation of Vickers hardness is 35 or less, the Mndif (mass%) value in the thickness direction is 0.20 or less wherein Mndif (mass%) represents an Mn segregation degree, and the tensile strength is 1780 MPa or more.

A steel sheet according to one embodiment of the present invention has a predetermined chemical composition, and the microstructure of a 1 / 4 portion of the sheet thickness of the steel sheet contains, by volume percentage, tempered martensite: 30 to 70%, and ferrite and bainite. One or two types of: the total amount is 20% or more and 70% or less, and in the structure of the above-mentioned 1 / 4 part of the plate thickness, the retained austenite is less than 10% by volume percentage, and the fresh martensite is 10% or less, Pearlite is 10% or less, their total volume percentage is 15% or less, and the number density of iron-based carbides with a major diameter of 5 nm or more in the tempered martensite at the 1 / 4 part of the plate thickness is 5×10 7 Pieces / mm 2 In the above, the ratio of the number of ε-based carbides in the 1 / 4 part of the sheet thickness to the number of the iron-based carbides with a major diameter of 5 nm or more is 20% or more, and the tensile strength of the steel sheet is 780 MPa or more. .

The present invention addresses the problem of providing: a high-strength steel plate having excellent delayed fracture resistance and a tensile strength of at least 980 MPa; and a method for producing same. This high-strength steel plate has a specific composition, wherein the degree of Mn segregation in a specific region is 1.5 or less, the maximum concentration of P in a specific region is 0.08 mass% or less, the number of the specific MnS particles in a specific region is 2.0 or less per 1 mm2, the number of specific oxide inclusions is 8 or less per 1 mm2, at least 80% of the oxide inclusions by number have a composition comprising 50 mass% of alumina, 20 mass% or less of silica, and 40 mass% or less of calcia, and the microstructure of the steel comprises 30-95% in total of martensite and bainite, 5-70% of a ferrite phase, and less than 3% (including 0%) of an austenite phase by volume percentage, and the tensile strength is at least 980 MPa.