CN113322410A

CN113322410A - High-strength bolt steel with excellent delayed fracture resistance and preparation method thereof

Info

Publication number: CN113322410A
Application number: CN202010130887.1A
Authority: CN
Inventors: 金峰; 赵浩洋; 翟瑞银; 姚赞
Original assignee: Baoshan Iron and Steel Co Ltd
Current assignee: Baoshan Iron and Steel Co Ltd
Priority date: 2020-02-28
Filing date: 2020-02-28
Publication date: 2021-08-31
Anticipated expiration: 2040-02-28
Also published as: CN113322410B

Abstract

A high-strength bolt steel with excellent delayed fracture resistance and a preparation method thereof are disclosed, and the steel comprises the following components in percentage by weight: carbon: 0.35-0.50%, manganese: 0.20 to 0.70%, chromium: 0.75-1.35%, molybdenum: 0.55-1.45%, vanadium: 0.10 to 0.50%, aluminum: 0.005-0.10%, sulfur: 0-0.005%, silicon: 0.02 to 0.05%, titanium: 0.005-0.15%, oxygen: 0.0005 to 0.003%, nitrogen: 0.001-0.007% of iron and other inevitable impurities as the rest; and the content of the titanium is more than or equal to 3.5 times of the content of the nitrogen; the sum of the contents of the three elements of chromium, molybdenum and vanadium is not less than 1.75 percent; the sum of the contents of the sulfur, the oxygen and the nitrogen is not higher than 0.01 percent. The steel for the high-strength bolt has high steel purity and refined crystal grains, can be used for manufacturing the high-strength bolt with the tensile strength of not less than 1400MPa, has the reduction of area of not less than 50 percent, has excellent delayed fracture resistance, and can be used for automobile and mechanical parts.

Description

High-strength bolt steel with excellent delayed fracture resistance and preparation method thereof

Technical Field

The invention relates to steel for bolts, in particular to high-strength steel for bolts with excellent delayed fracture resistance and a preparation method thereof.

Background

Screw and bolt connections are among the most common means of connection between automotive and mechanical components. The threaded connection has the advantages of convenient disassembly and assembly, simple structure, reliable connection and the like. According to the regulation of the national standard GB/T3098.1 bolt, screw and stud with mechanical performance of fastener, the currently used fastener is 8.8-12.9 grade (equivalent to the tensile strength grade of 800-1200 MPa). The highest strength class of fasteners available in the newly revised standards is class 12.9, and fasteners of higher strength classes have not yet formed standards. At present, cold heading steel grades for manufacturing 12.9-grade bolts in various steel mills and fastener production enterprises generally adopt 35CrMo (SCM435) and 42CrMo (SCM440), and bolts meeting strength grade requirements are obtained through corresponding heat treatment processes.

The 35CrMo and 42CrMo materials adopted by the bolts of 12.9 grade and below do not require addition of Ti, V, Nb and other microalloy elements, and the principle that the addition of Cr and Mo elements can increase the hardenability of steel is mainly utilized to ensure that the core microstructure of a formed part can be quenched into martensite in the modulation heat treatment process, and the martensite is tempered to obtain the comprehensive mechanical property. Meanwhile, the addition of 0.35 to 0.42 percent of carbon element and the addition of alloy elements such as Cr, Mo and the like can ensure that the steel can obtain a martensite structure meeting the hardness requirement after quenching, and fine carbide precipitation is formed after tempering to play a role in dispersion strengthening.

With the development of various production departments such as automobiles, machinery, buildings, light industry and the like, higher requirements are put forward on materials for manufacturing various fasteners (bolts, screws, nuts and the like). In the field of high performance and weight reduction of automobiles, high-rise building structures, and super-lengthening bridges, high design stress and light weight are required for bolts as coupling members. The most effective measure for this is to increase the strength of the bolt steel. At present, the requirement that the tensile strength of some bolts for automobiles and construction machinery is more than 1400MPa is already provided.

The screw threads are used for connection and fastening, but the structure of the screw threads is equivalent to notches in uniform material and shape. The bolt can be subjected to continuous tensile stress or alternating tensile stress to form stress concentration in the assembly service process, and the stress concentration and the alternating fatigue load function can be formed at the thread. When the tensile strength of the bolt exceeds 1000MPa, sudden fracture occurs after a certain period of time after loading (assembling, tightening), and the time interval is several hours and several days and months, which is called a delayed fracture phenomenon of the high-strength bolt. At present, the delayed fracture phenomenon is generally considered to be related to the segregation and precipitation of hydrogen elements in the material. Therefore, it is necessary to consider avoiding or reducing the delayed fracture phenomenon of the material when developing a high-strength bolt.

The high-strength bolt steel with excellent delayed fracture resistance disclosed in Chinese patent CN00105872.X comprises the following components: 0.25 to 0.50 percent of C, 0.10 to 0.40 percent of Si, less than or equal to 0.50 percent of Mn, 0.50 to 1.50 percent of Cr0.30 to 1.50 percent of Mo0.30, 0.10 to 0.40 percent of V, 0.001 to 0.05 percent of RE, 0.005 to 0.10 percent of Al0.020 percent of N, less than or equal to 0.015 percent of P, less than or equal to 0.010 percent of S, and 0.005 to 0.10 percent of one or the sum of two of Ti and Nb, and the balance of Fe and inevitable impurities. The patent does not control the content of Si element to an extremely low level, which can reduce the service life of a die in the cold heading process and the low-temperature impact property of the material, and is unfavorable for the performance of the bolt under high strength.

A method for producing a high-strength bolt excellent in delayed fracture resistance and relaxation resistance, disclosed in Chinese patent CN01800818.6, comprises alloying components of 0.50 to 1.0% by mass of C (hereinafter abbreviated as "%"), 0.5% or less (excluding 0%), 0.2 to 1% of Mn, 0.03% or less (including 0%) of P, and 0.03% or less (including 0%) of S. The steel has a total content of proeutectoid ferrite, proeutectoid cementite, bainite and martensite structures of less than 20%, and a pearlite structure as the remainder. The production method of the bolt comprises the steps of drawing steel (severe) to obtain a steel wire, manufacturing the steel wire into a bolt shape through cold heading, and carrying out bluing treatment on the formed steel wire at the temperature of 100-400 ℃. This invention is a high carbon series, and is made into a high strength part by deformation strengthening, and the reason for the delayed fracture resistance is not described.

Chinese patent CN200410074410.7 discloses a high strength bolt steel with excellent delayed fracture resistance and cold workability, which comprises the following components: 0.25 to 0.4 percent of C, less than or equal to 0.1 percent of Si, 0.4 to 1.4 percent of Mn, less than or equal to 0.01 percent of P, less than or equal to 0.008 percent of S, 0.1 to 0.5 percent of Cr0.15 to 0.35 percent of Mo, 0.0005 to 0.003 percent of B, 0.01 to 0.1 percent of Ti, 0.005 to 0.04 percent of RE, 0.005 to 0.05 percent of Al, 0.004 to 0.01 percent of N, and the balance of Fe. The steel can also be added with any one or the sum of two of Nb and Zr of 0.01-0.1 percent in specific chemical components (weight percent). The content of alloy elements such as Cr, Mo and the like in the invention is low, the strength of more than 1400MPa is difficult to achieve in practical industrial use, and the delayed fracture resistance is not high.

Chinese patent CN200610105980.7 discloses a high strength bolt with excellent delayed fracture resistance and a manufacturing method thereof, wherein the chemical components are C: 0.2 to 0.6%, Si: 0.05-0.5%, Mn: 0.1% -2%, Mo: 0.5-6%, Al: 0.005-0.5%, tensile strength of 1400MPa or above, and compressive residual stress of the surface layer of the thread bottom of 10-90% of the tensile strength, and the processing process of the bolt is specified. The invention does not utilize the grain refining effect of microalloy elements, and the grain size of the material is not specified, and because the delayed fracture of the high-strength material is often generated on the grain boundary of coarse grains, the delayed fracture resistance is obviously deteriorated by the overlarge grain size.

Disclosure of Invention

The invention aims to provide high-strength bolt steel with excellent delayed fracture resistance and a preparation method thereof, wherein the high-strength bolt steel has high steel purity and refined crystal grains, can be used for manufacturing high-strength bolts with tensile strength of not less than 1400MPa, has the reduction of area of not less than 50 percent, has excellent delayed fracture resistance, and can be used for automobile and mechanical parts.

In order to achieve the purpose, the technical scheme of the invention is as follows:

a high-strength bolt steel with excellent delayed fracture resistance comprises the following components in percentage by weight: carbon: 0.35-0.50%, manganese: 0.20 to 0.70%, chromium: 0.75-1.35%, molybdenum: 0.55-1.45%, vanadium: 0.10 to 0.50%, aluminum: 0.005-0.10%, sulfur: 0-0.005%, silicon: 0.02 to 0.05%, titanium: 0.005-0.15%, oxygen: 0.0005 to 0.003%, nitrogen: 0.001-0.007% of iron and other inevitable impurities as the rest; and the titanium content/nitrogen content is more than or equal to 3.5; the sum of the contents of the three elements of chromium, molybdenum and vanadium is not less than 1.75 percent; the sum of the contents of the sulfur, the oxygen and the nitrogen is not higher than 0.01 percent.

Further, it contains, boron: 0.0005 to 0.0050%, niobium: 0.003-0.10% of one or more.

The microstructure of the high-strength bolt steel provided by the invention is ferrite, lamellar pearlite and bainite.

The bolt microstructure made of the high-strength bolt steel is tempered sorbite and precipitates, the precipitates comprise iron carbide, chromium carbide, molybdenum carbide and vanadium carbonitride, and the sizes of the precipitates are 5-120 nm; the grain size is not more than 50 μm.

The tensile strength of the bolt made of the high-strength bolt steel is 1400-1650 MPa, and the reduction of area is higher than 50%.

In the composition design of the steel of the invention:

carbon: carbon is added into steel as a cheap element, so that the hardenability and the hardenability of steel can be improved, and simultaneously, the carbon and iron and a plurality of alloy elements form a compound to play a role in second-phase dispersion strengthening in the tempering process, so that the service strength of the material can be effectively improved by adding a certain amount of carbon. When the carbon content is less than 0.35%, the strength of the material after quenching and tempering is less than 1400MPa, and when the carbon content exceeds 0.5%, the plasticity of the material is reduced, and the tendency of delayed fracture after loading is increased. Therefore, the carbon content is set to 0.35 to 0.50%.

Manganese: manganese is an austenite stabilizing element, hardenability and strength can be effectively improved, and meanwhile, a certain amount of manganese element has little influence on the plasticity of steel, but the manganese element is easy to segregate in the solidification process of the ferroalloy, so more than 0.20% of manganese is required to be added for ensuring the alloy strength and hardenability, but the manganese content is controlled below 0.70% for avoiding the material uniformity deterioration caused by manganese segregation. Therefore, the manganese content is set to 0.20 to 0.70%

Chromium: the method has the effect of improving the hardenability of the iron alloy, fine and dispersed carbide particles are precipitated in the tempering process, the effect of dispersion strengthening is achieved, the strength of the material is improved, and the chromium element has the effect of refining the structure, so that the solid solution strengthening and precipitation strengthening effects of the chromium are exerted in the design of the method, and the content of the material structure is controlled to be 0.70-1.60%.

Molybdenum: is a ferrite forming element, is beneficial to improving the hardenability of the steel, and leads the steel to form bainite and martensite in the quenching process. Because the molybdenum element has larger atomic weight and large atomic radius and is not easy to diffuse in the iron alloy, the molybdenum mainly exists in the steel in a solid solution form to play a solid solution strengthening effect during tempering in a lower temperature range, and fine carbide can be formed to improve the strength of the steel during tempering at a higher temperature, so that the tempering resistance of the material can be improved on the whole. The high tempering temperature can effectively reduce the amount of dislocation and subgrain boundary in the material, thereby avoiding delayed cracking caused by gathering hydrogen element at the dislocation and the subgrain boundary. Molybdenum is a precious alloying element, and the addition of higher molybdenum leads to increased costs. Therefore, 0.55 to 1.45 percent of molybdenum is added in the design of the invention.

Vanadium: vanadium is a strong carbide forming element, and can be dissolved into austenite and uniformly distributed in the quenching and heating process due to moderate dissolving temperature of vanadium carbide in the austenite, fine vanadium carbide particles can be formed in the tempering process, the fine vanadium carbide can play a certain role in hindering dislocation, and simultaneously the vanadium carbide can effectively organize grain boundary movement, refine grains and improve the strength of steel. Under the condition of high-temperature tempering, more vanadium content can form coarse vanadium carbide particles to reduce the impact property of steel, and the design of the invention combines other alloy elements and adds 0.10-0.50% of vanadium to ensure the comprehensive mechanical property of steel.

Chromium, molybdenum and vanadium are carbide forming elements, and in order to ensure good delayed fracture resistance of the material, the total amount of the three elements is above a certain content, so that the carbide in the material after quenching and tempering forms a high-energy hydrogen trap with enough energy, the residual hydrogen elements in the material and hydrogen atoms permeated in the service process of the part are fixed, and the delayed fracture caused by hydrogen generated by hydrogen overflow is avoided. The sum of the contents of the three elements of chromium, molybdenum and vanadium in the design of the invention is not less than 1.75 percent.

Sulfur: the solubility of sulfur in iron alloy is very low, and the sulfur is easy to form large-size sulfide inclusions with iron, manganese and the like, and hydrogen-induced cracking is finally caused by the fact that hydrogen is easily enriched on the interface of sulfide and an alloy matrix, so that the delayed fracture resistance of the material is obviously influenced. The steel grade in the design of the invention controls the content of residual sulfur not to exceed 0.005.

Similar to the action of sulfur in steel, nitrogen and oxygen mainly form nitride inclusions and oxide inclusions in steel, and the interfaces of the inclusions and a matrix are easy to enrich hydrogen to finally cause hydrogen-induced cracking, so that the sum of the contents of the sulfur, the nitrogen and the oxygen is not higher than 0.01 percent.

Silicon: silicon replaces iron atoms in steel in a replacement mode, dislocation movement is hindered, the strength of a ferrite phase can be obviously improved, the hardness of the annealed material is improved due to the high silicon content, cold heading forming is not facilitated, and the service life of a forming die can be shortened. At the same time, a higher silicon content reduces the low-temperature impact toughness of the steel. Therefore, in the design of the present invention, the silicon content is set as follows: 0.02-0.05%.

Titanium: titanium is likely to form compounds with carbon and nitrogen in steel, and titanium nitride is formed at a temperature of 1400 ℃ or higher, and is generally precipitated in a liquid phase or delta ferrite, thereby achieving the purpose of refining austenite grains. Too high a titanium content results in the formation of coarse titanium nitride precipitates, which lead to a reduction in the impact properties and fatigue properties of the steel. During the tempering, if the titanium content is too high, the fluctuation range of the low-temperature impact energy is increased. Therefore, the titanium content in the design of the invention is controlled to be 0.005-0.15%, and the titanium content/nitrogen content is more than or equal to 3.5.

Boron: in the steelmaking process, boron can change the solidification process and the cast structure of steel, and reacts with molten steel to form fine particles which become non-spontaneous nucleation cores in the solidification process, so that the nucleation work is reduced, and the nucleation rate is increased; meanwhile, boron belongs to surface active elements in steel, is easily adsorbed on the surface of a solid crystal nucleus, prevents atom supply required by crystal growth, prevents crystal growth, and can greatly improve the hardenability of the steel. Therefore, boron can refine the cast structure, reduce dendrite segregation and zone segregation, improve the uniformity of the steel and improve the hardenability. Therefore, the boron content is 0.0005-0.0050% in the design of the invention.

Niobium: niobium is added into steel to play a role in inhibiting steel recrystallization, and exists in the steel as replacement solute atoms, and the niobium atoms have larger size than niobium atoms, are easy to be deviated at dislocation lines and generate strong dragging effect on dislocation movement. Meanwhile, niobium can form interstitial mesophases such as niobium carbide and niobium nitride in steel, and the interstitial mesophases have the effects of blocking the pinning of dislocation and the migration of subgrain boundary in the recrystallization process, can effectively refine crystal grains, have high niobium content, can form coarse niobium carbide particles under the condition of high-temperature tempering, and deteriorate the low-temperature impact energy of steel. In combination with other alloy elements, 0.003-0.15% of niobium is added in the material design to ensure the mechanical property of the steel.

The preparation method of the high-strength bolt steel with excellent delayed fracture resistance, provided by the invention, comprises the following steps of:

1) smelting and casting

Smelting according to the components, adopting a continuous casting machine to cast round billets or square billets, wherein the size of the round billets or the square billets is 320-500mm, and adopting larger billet shape size can ensure sufficient rolling compression ratio, reduce and eliminate inevitable component segregation and loose cores;

2) blooming in blooming

The micro segregation can be further reduced by adopting a secondary firing process, the initial rolling and cogging of the round billet or the square billet at the temperature of 1050-;

3) heating the blank, wherein the heating of the heating furnace is controlled at 920-1150 ℃, and the heat preservation time is 1.0-3.0 h; because the components have higher alloy content, the rolling resistance is obviously improved by excessively low heating temperature, and obvious surface decarburization is easily generated by excessively high heating temperature, which is not beneficial to the fatigue performance of the final product;

4) wire rod rolling

Controlling the rolling speed to be 15-115m/s in the rolling process of the high-speed wire rod; the finish rolling temperature is 880-1050 ℃, and the spinning temperature is 800-950 ℃; the dimension specification of the rolled wire rod is phi 5-28 mm;

5) controlled cooling

After the wire rod is rolled, the component of a Stelmor line fan is adjusted to control the structure transformation of the wire rod, and the air quantity adjusting range of 14 Stelmor line fans is as follows: the air volume of the F1-F3 fan is 500-1200 cubic meters/minute, and the air volume of the F4-F6 fan is 0-800 cubic meters/minute; and (2) covering a heat-insulating cover after the fan blows, wherein the cooling speed in the heat-insulating process is not higher than 0.15 ℃/s, the cooling process ensures that the wire rod quickly enters a phase-change temperature interval, and the phase-change process is in an isothermal state, and the microstructure of the obtained high-strength bolt steel is ferrite and lamellar pearlite and a small amount of bainite and does not contain martensite.

Further, spheroidizing annealing → acid washing, phosphorization → drawing → cold heading → tempering → thread processing → dehydrogenation → surface treatment; wherein,

the annealing heat treatment is carried out by heating in an atmosphere protection furnace or a vacuum furnace at 775-795 ℃ for 3.5-6 hours; reducing the temperature to 690-710 ℃ after 4-6 hours, and preserving the heat for 3-5 hours; finally, the material is discharged from the furnace when the temperature is reduced to below 550 ℃, and the annealing process can ensure that the hardness of the material is reduced to below 180HB, so that the drawing and cold heading processes are easier to perform;

the quenching and tempering heat treatment adopts a furnace production or continuous production mode, the quenching and heating temperature is 915-945 ℃, the heating time is not lower than 90 minutes, and the higher quenching and heating temperature and the longer heating time ensure that carbides of various alloys are fully dissolved to obtain uniform and stable high-temperature tissues; then, cooling the mixture to below 80 ℃ by adopting mineral oil at the temperature of 40-60 ℃; finally, the alloy is heated for more than 90 minutes at the temperature of not less than 550 ℃, and the high tempering heating temperature and the long tempering time can ensure that carbides of various alloys are fully dispersed and precipitated, play a role in strengthening the parts, and serve as stable hydrogen traps to relieve the delay cracking phenomenon caused by inevitable solid solution hydrogen and external hydrogen;

and (3) dehydrogenation treatment, wherein the heating temperature is 180-250 ℃, and the heating time is not less than 4 hours, so that solid-solution hydrogen and foreign hydrogen in the part are fully separated out, and the delayed cracking risk of the part is reduced.

Preferably, the surface treatment is oiling or passivation treatment.

Preferably, an electric furnace or a converter is adopted in the smelting in the step 1), and then external refining is carried out, wherein the external refining adopts an LF furnace and VD or RH degassing treatment process, the composition and alkalinity of synthetic slag are adjusted in the LF refining process, and the content of S element in steel is controlled to be lower than 0.005%; the VD or RH vacuum degassing time needs to be more than 30 minutes, the O content is controlled to be 0.0005 to 0.0040 percent at the end point, the N content is controlled to be 0.0010 to 0.0090 percent, and the H content is controlled to be less than 2 ppm; the steel ladle sedation time is more than 15min after refining, which is beneficial to floating of large-particle inclusion and controlling the inclusion in the molten steel to be less than 30 um.

The invention has the beneficial effects that:

the material related by the invention has reasonable components and lower cost, can be used for producing bolts with the strength of more than 1400MPa, and simultaneously ensures good plasticity and toughness of the bolts; because the strong carbide forming element is added, nano-scale dispersed carbide particles can be formed in the final bolt part, the strengthening and toughening effects on the part are achieved, and the dispersed carbide can also be used as a hydrogen element in a stable hydrogen trap fixing material, so that the delayed cracking phenomenon caused by hydrogen is relieved and eliminated; the moderate content of carbon element and alloy element can ensure the good hardenability and hardenability of the material, so that the part is easier to be thermally treated and formed; the added trace alloy elements further ensure the quenching performance and the tempering stability of the material, so that the material has a wider heat treatment process range and is easy to produce and control.

The material smelting, processing and deep processing processes can further ensure and improve the mechanical property and the delayed cracking resistance of finished parts. Harmful gases such as hydrogen and the like in the material can be removed through vacuum treatment in the smelting process, the content of oxide inclusions in steel can be fully reduced through deoxidation treatment, the condition that unstable hydrogen is gathered near large-size inclusions is reduced, and the occurrence of delayed cracking is relieved; the continuous casting of the steel billet with larger size can ensure the compression ratio in the processing process and reduce the unstable performance caused by segregation and looseness; the rolling and heat treatment processes both adopt higher heating temperature and heating time, so that the microsegregation in the material can be further eliminated, and carbides and the like are fully dissolved to form uniform and stable high-temperature tissues; the higher heating temperature in the processing process is limited, so that the decarburization of the material can be avoided, and the influence on zero performance caused by abnormal growth of crystal grains can be avoided;

the invention ensures that the finished parts have high mechanical property, long fatigue life and good delayed cracking resistance through the optimized design of components and reasonable processing technology, and can meet the use requirements of light weight of automobiles and high strength and long service life in the mechanical industry.

Detailed Description

The present invention will be further described with reference to the following examples.

The chemical components of the high-strength bolt steel of the invention of the examples A1-A10 and the three comparative steel grades B1-B3 are shown in Table 1, and the preparation processes of the high-strength bolt steel of the invention of the examples A1-A10 and the comparative examples B1-B3 are shown in Table 2.

The preparation method comprises the following steps:

according to the invention, the alloys A1-A5, B2 and B3 are smelted by an electric furnace, the alloys A6-A10 and B1 are smelted by a converter, and then are smelted by an external furnace, wherein the alloys A1-A4, A8-A10 and B1 are refined by an LF furnace and VD, the alloys A5-A7, B2 and B3 are treated by LF and RH, the structure and alkalinity of the synthetic slag are optimized, the vacuum degassing time of the alloys A1-A6 and B1 is 35 minutes, the vacuum degassing time of the alloys A7-10#, B2 and B3 is 40 minutes, the end point O content is controlled to be 0.0005-0.0040%, and the N content is controlled as follows: 0.001-0.009%, and H content is less than 2 ppm.

After smelting, the round billets of 300mm are cast in examples A1-A3 and B1, the round billets of 450mm are cast in examples A4-A6, the square billets of 320 multiplied by 420mm are cast in examples A7, A8 and B2, the square billets of 500mm are cast in examples A9, A10 and B3, and the tundish covering agent and the crystallizer covering slag with good sealing property are adopted in the casting process. In the examples A1-A3 and B1, the blooming temperature of the continuous casting billets was 1050 ℃, and the end face size of the rolled billet was 115 mm. Examples A4-A8 and B2 square billets were heated at 1120 ℃ and rolled to a billet size of 125 mm. The heating temperature of the square billets of examples A9, A10 and B3 was 1200 ℃ and the billet size was 170 mm.

The furnace temperatures of the furnaces of examples A1 to A7 and B1 were controlled at 920 ℃ and the holding time was 1.2 hours, and the furnace temperatures of the furnaces of examples A8 to A10, B2 and B3 were controlled at 1150 ℃ and the holding time was 2.8 hours. The rolling speed is controlled to be 15-115m/s in the rolling process of the high-speed wire rod. The online temperature control scheme is as follows: in the examples A1-A7 and B1, the inlet temperature of the alloy finishing mill group is 890-960 ℃, the inlet temperature of the reducing and sizing mill group is 840-930 ℃, and the spinning temperature is 820-890 ℃. In the examples A8-A10, B2 and B3, the inlet temperature of the alloy finishing mill group is 950-1050 ℃, the inlet temperature of the reducing and sizing mill group is 940-970 ℃, and the spinning temperature is 880-950 ℃. The alloy rolled wire rods of the embodiments A1-A5, B1 and B2 have the dimension specification of phi 5-15 mm respectively, and the alloy rolled wire rods of the embodiments A6-A10 and B3 have the dimension specification of phi 16-28 mm.

The whole wire rod is subjected to annealing heat treatment, in the examples A1-A4 and B1, a nitrogen atmosphere protection heat treatment furnace is adopted, nitrogen is introduced after charging, the temperature is increased to 780 ℃ along with the furnace, the temperature is maintained for 4 hours, then the temperature is reduced to 690 ℃ along with the furnace, the temperature is maintained for 4 hours, then the temperature is reduced to 300 ℃ along with the furnace, and the wire rod is taken out of the furnace. In the examples A5-A10, B2 and B3 adopt vacuum heat treatment furnaces, the vacuum heat treatment furnaces are charged and heated to 790 ℃ along with the furnaces, the temperature is maintained for 4 hours, the temperature is reduced to 710 ℃ along with the furnaces for 4 hours, the temperature is maintained for 4 hours, the vacuum heat treatment furnaces are cooled to 400 ℃ along with the furnaces, and the materials are discharged. All the examples are subjected to acid washing and phosphating processes, and then are subjected to drawing and cold heading.

Examples A1, A2, A5 to A7 and B1 were prepared by heating at 920 ℃ for 100 minutes in a mesh belt type heat treatment furnace, cooling with 40 ℃ mineral oil to room temperature, heating at 560 ℃ for 100 minutes, and then cooling naturally. In the examples A3, A4, A8 to A10, B2 and B3, the materials are fed into a bell-type furnace by a charging basket, heated at 940 ℃ for 100 minutes, cooled to room temperature by mineral oil at 60 ℃, heated at 610 ℃ for 100 minutes and then naturally cooled. And then the finished bolt is manufactured through thread processing and surface heat treatment.

The properties of inventive examples A1-A10 and comparative examples B1-B3 are shown in Table 3. The tensile strength of the steel is 1400-1650 MPa, the reduction of area is higher than 50%, the size of austenite crystal particles is not more than 50 μm, and the performance loss in a hydrogen charging environment is small.

Claims

1. A high-strength bolt steel with excellent delayed fracture resistance comprises the following components in percentage by weight: carbon: 0.35-0.50%, manganese: 0.20 to 0.70%, chromium: 0.75-1.35%, molybdenum: 0.55-1.45%, vanadium: 0.10 to 0.50%, aluminum: 0.005-0.10%, sulfur: 0-0.005%, silicon: 0.02 to 0.05%, titanium: 0.005-0.15%, oxygen: 0.0005 to 0.003%, nitrogen: 0.001-0.007% of iron and other inevitable impurities as the rest; and the titanium content/nitrogen content is more than or equal to 3.5; the sum of the contents of the three elements of chromium, molybdenum and vanadium is more than or equal to 1.75 percent; the sum of the contents of the sulfur, the oxygen and the nitrogen is less than or equal to 0.01 percent.

2. The steel for high-strength bolts excellent in delayed fracture resistance according to claim 1, characterized in that: further contains, boron: 0.0005 to 0.0050%, niobium: 0.003-0.10% of one or more.

3. The steel for high-strength bolts excellent in delayed fracture resistance according to claim 1 or 2, characterized in that: the microstructure of the high-strength bolt steel is ferrite, lamellar pearlite and bainite.

4. The steel for high-strength bolts excellent in delayed fracture resistance according to claim 1, 2 or 3, characterized in that: the microstructure of the bolt made of the high-strength bolt steel is tempered sorbite and precipitates, the precipitates comprise iron carbides, chromium carbides, molybdenum carbides and vanadium carbonitrides, and the sizes of the precipitates are 5-120 nm; the grain size is not more than 50 μm.

5. The steel for high-strength bolts excellent in delayed fracture resistance according to claim 1, 2, 3 or 4, characterized in that: the tensile strength of the bolt made of the high-strength bolt steel is 1400-1650 MPa, and the reduction of area is higher than 50%.

6. The method for producing a steel for high strength bolts excellent in delayed fracture resistance as claimed in any one of claims 1 to 5, characterized by comprising the steps of:

1) smelting and casting

Smelting according to the components of claim 1 or 2, casting into a round billet or a square billet by a continuous casting machine, wherein the size of the round billet or the square billet is 320-500 mm;

2) blooming in blooming

Adopting a two-fire forming process, performing primary rolling and cogging on a round billet or a square billet into a 115-plus 170-mm square billet at the temperature of 1050-plus 1270 ℃, wherein the total rolling reduction is higher than 40%;

3) heating the blank, wherein the heating of the heating furnace is controlled at 920-1150 ℃, and the heat preservation time is 1.0-3.0 h;

4) wire rod rolling

Controlling the rolling speed to be 15-115m/s in the rolling process of the high-speed wire rod; the inlet temperature of the finishing mill unit is 880-1050 ℃, the inlet temperature of the reducing sizing mill unit is 840-970 ℃, and the spinning temperature is 800-950 ℃; the dimension specification of the rolled wire rod is phi 5-28 mm;

5) controlled cooling

After the wire rod is rolled, the component of a Stelmor line fan is adjusted to control the structure transformation of the wire rod, and the air quantity adjusting range of 14 Stelmor line fans is as follows: the air volume of the F1-F3 fan is 500-1200 cubic meters/minute, and the air volume of the F4-F6 fan is 0-800 cubic meters/minute; and (3) covering a heat preservation cover after the fan blows air, wherein the cooling speed in the heat preservation process is not higher than 0.15 ℃/s, and the microstructure of the obtained high-strength bolt steel is ferrite, lamellar pearlite and bainite.

7. The method for producing a high-strength bolt steel excellent in delayed fracture resistance according to claim 6, characterized by further comprising spheroidizing annealing → pickling, phosphating → drawing → cold heading → tempering → screw processing → dehydrogenation → surface treatment; wherein,

the annealing heat treatment is carried out by heating in an atmosphere protection furnace or a vacuum furnace at 775-795 ℃ for 3.5-6 hours; reducing the temperature to 690-710 ℃ after 4-6 hours, and preserving the heat for 3-5 hours; finally, the temperature is reduced to below 550 ℃ along with the furnace, and the mixture is discharged;

the quenching and tempering heat treatment adopts a furnace production or continuous production mode, the quenching and heating temperature is 915-945 ℃, and the heating time is not less than 90 minutes; then, cooling the mixture to below 80 ℃ by adopting mineral oil at the temperature of 40-60 ℃; finally, heating for more than 90 minutes at the temperature of not less than 550 ℃;

and (4) carrying out dehydrogenation treatment, wherein the heating temperature is 180-250 ℃, and the heating time is not less than 4 hours.

8. The method for producing a steel for high strength bolts excellent in delayed fracture resistance as claimed in claim 7, wherein said surface treatment is oiling or passivating.

9. The method of producing a high-strength bolt steel excellent in delayed fracture resistance as claimed in claim 6, wherein the step 1) of smelting uses an electric furnace or a converter, and then external refining is performed, the external refining uses an LF furnace plus VD or RH degassing treatment process, the composition and basicity of the synthetic slag are adjusted during the LF refining, and the content of the S element in the steel is controlled to be less than 0.005%; the VD or RH vacuum degassing time needs to be more than 30 minutes, the end point O content is controlled to be 0.0005 to 0.0030 percent, the N content is controlled to be 0.0010 to 0.0070 percent, and the H content is controlled to be lower than 2 ppm; the steel ladle sedation time is more than 15min after refining, which is beneficial to floating of large-particle inclusion and controlling the inclusion in the molten steel to be less than 30 um.

10. The method for producing a high-strength bolt steel excellent in delayed fracture resistance according to any one of claims 6 to 9, wherein a bolt made of the high-strength bolt steel has a tensile strength of 1400 to 1650MPa and a reduction of area of more than 50%.