CN114807738B - High-strength steel for bolts and manufacturing method thereof - Google Patents

High-strength steel for bolts and manufacturing method thereof Download PDF

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CN114807738B
CN114807738B CN202110113145.2A CN202110113145A CN114807738B CN 114807738 B CN114807738 B CN 114807738B CN 202110113145 A CN202110113145 A CN 202110113145A CN 114807738 B CN114807738 B CN 114807738B
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steel
strength
quenching
temperature
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CN114807738A (en
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赵浩洋
黄宗泽
赵四新
姚赞
翟瑞银
金峰
余子权
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Baoshan Iron and Steel Co Ltd
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    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21C—MANUFACTURE OF METAL SHEETS, WIRE, RODS, TUBES OR PROFILES, OTHERWISE THAN BY ROLLING; AUXILIARY OPERATIONS USED IN CONNECTION WITH METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL
    • B21C37/00—Manufacture of metal sheets, bars, wire, tubes or like semi-manufactured products, not otherwise provided for; Manufacture of tubes of special shape
    • B21C37/04—Manufacture of metal sheets, bars, wire, tubes or like semi-manufactured products, not otherwise provided for; Manufacture of tubes of special shape of bars or wire
    • B21C37/047—Manufacture of metal sheets, bars, wire, tubes or like semi-manufactured products, not otherwise provided for; Manufacture of tubes of special shape of bars or wire of fine wires
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/18—Hardening; Quenching with or without subsequent tempering
    • C21D1/25—Hardening, combined with annealing between 300 degrees Celsius and 600 degrees Celsius, i.e. heat refining ("Vergüten")
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/06—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of rods or wires
    • C21D8/065—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of rods or wires of ferrous alloys
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C33/00—Making ferrous alloys
    • C22C33/04—Making ferrous alloys by melting
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium with molybdenum or tungsten
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/24—Ferrous alloys, e.g. steel alloys containing chromium with vanadium
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/001—Austenite
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/009—Pearlite
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
    • Y02P10/00—Technologies related to metal processing
    • Y02P10/20—Recycling

Abstract

The invention discloses steel for high-strength bolts, which comprises the following components in percentage by mass: c:0.35 to 0.45 percent; si:0.15 to 0.24 percent; mn:0.20 to 0.40 percent; cr:0.75 to 1.35 percent; mo:0.55% -1.00%; v:0.08 to 0.30 percent; al:0.005% -0.10%; s:0 to 0.005 percent. Also disclosed is a method for producing a high-strength steel for bolts which has excellent delayed fracture resistance.

Description

High-strength steel for bolts and manufacturing method thereof
Technical Field
The invention relates to the field of alloy steel, in particular to steel for a high-strength bolt and a manufacturing method thereof.
Background
The high-strength bolt belongs to a notch part, has high notch sensitivity, is easy to generate corrosion to different degrees after long-term exposure in humid air, rainwater and other environments, diffuses and enriches trace hydrogen generated by corrosion reaction at a corrosion pit and trace hydrogen existing in the bolt under the action of stress, and is easy to cause hydrogen-induced delayed fracture of the high-strength bolt.
Hydrogen-induced delayed fracture is associated with plastic deformation, i.e., hydrogen promotes dislocation emission and movement. When hydrogen promotes the development of localized plastic deformation to critical conditions, the stress concentration in the localized region (e.g., the front end of the dislocation population) is equal to the atomic bonding force reduced by hydrogen, thereby causing hydrogen induced cracking to nucleate there. Grain boundaries can block dislocation movement and easily cause dislocation accumulation in front of the grain boundaries, so that grain-by-grain fracture is the most common delayed fracture mode of high-strength bolt steel in a hydrogen-containing environment. While as the grains are refined, the dislocation number of the product before the grain boundary is reduced, and under the same variable, the deformation is dispersed in more grains, so that the stress concentration degree is reduced, and the delayed fracture resistance of the material is further improved.
Disclosure of Invention
The invention aims to solve the problem of low delayed fracture resistance of high-strength bolt steel. The invention provides a high-strength steel for bolts, which has excellent delayed fracture resistance, and a method for manufacturing the same.
In order to solve the technical problems, the embodiment of the invention discloses steel for high-strength bolts, which comprises the following components in percentage by mass: c:0.35 to 0.45 percent; si:0.15 to 0.24 percent; mn:0.20 to 0.40 percent; cr:0.75 to 1.35 percent; mo:0.55% -1.00%; v:0.08 to 0.30 percent; al:0.005% -0.10%; s:0 to 0.005 percent.
By adopting the technical scheme, the high-strength bolt steel has excellent delayed fracture resistance.
According to another embodiment of the present invention, an embodiment of the present invention discloses a steel for high strength bolts, the balance being Fe and unavoidable impurities.
According to another specific embodiment of the invention, the embodiment of the invention discloses steel for high-strength bolts, wherein the mass percentage of V, the mass percentage of C and the quenching temperature satisfy the following relation: log { [ V ] [ C ] } > 6.72-9500/T; wherein [ V ] is the mass percentage of V in the steel for the high-strength bolt, [ C ] is the mass percentage of C in the steel for the high-strength bolt, T is the quenching temperature, and the unit of the quenching temperature T is K.
By adopting the technical scheme, the high-strength bolt steel meets the relation, so that the microstructure of the high-strength bolt steel still maintains a certain volume of undissolved VC precipitated phases, and the VC precipitated phases can pin the prior austenite grain boundaries so as to prevent the prior austenite grain boundaries from growing up, thereby preventing the microstructure from coarsening and further achieving the aim of improving the delayed fracture resistance.
According to another embodiment of the invention, the embodiment of the invention discloses a high-strength bolt steel, wherein the microstructure of the high-strength bolt steel is tempered sorbite, and the prior austenite grain size of the tempered sorbite is less than or equal to 20 mu m.
By adopting the technical scheme, the microstructure coarsening of the steel for the high-strength bolt can be prevented, and the aim of further improving the delayed fracture resistance is fulfilled.
Here, the prior austenite grain size has an important influence on the mechanical properties of the steel material.
The prior austenite grain size and uniformity are one of the important indexes affecting the mechanical properties of steel,
the size of the original austenite grains directly affects the structure change in the rolling and cooling control process, so that the mechanical property of the material is greatly affected, and the original austenite grains of the steel are accurately displayed to have important significance.
According to another embodiment of the invention, the embodiment of the invention discloses a high-strength bolt steel, wherein the tensile strength of the high-strength bolt steel is more than or equal to 1400MPa, the yield strength is more than or equal to 1260MPa, and the area reduction is more than or equal to 50%.
By adopting the technical scheme, the steel for the bolt has high strength and good plasticity and toughness.
According to another embodiment of the present invention, a high strength bolt steel having a strength loss rate of < 20% in a hydrogen charging environment and a plastic loss rate of < 50% in a hydrogen charging environment is disclosed.
By adopting the technical scheme, the steel for the bolt has excellent delayed fracture resistance.
The embodiment of the invention also discloses a manufacturing method of the steel for the high-strength bolt, which comprises the following steps:
smelting and casting: smelting and casting into a continuous casting blank or a steel ingot according to the components, wherein the ratio of the cross sectional area of the continuous casting blank or the steel ingot to the cross sectional area of the wire rolled blank is more than 6, and the cross sectional area of the continuous casting blank or the steel ingot is more than or equal to 320 multiplied by 425mm;
soaking treatment: soaking the continuous casting blank or the steel ingot, wherein the soaking temperature is 1200-1250 ℃, and the heat preservation time is more than or equal to 24 hours;
blooming and cogging: cogging the continuous casting blank or the steel ingot to form a steel billet;
rolling wire rods: heating the steel billet to 1000-1080 ℃, and rolling wires, wherein the final rolling temperature is 830-900 ℃, and the wire laying temperature is 780-810 ℃ to obtain wire rods;
quenching and tempering: and (3) carrying out quenching and tempering on the wire rod, carrying out oil quenching after preserving heat for 60-80 min at the quenching temperature T, tempering at the temperature of more than or equal to 500 ℃ for the heating time of more than or equal to 90min, and carrying out air cooling to obtain the steel for the high-strength bolt.
By adopting the technical scheme, the manufactured high-strength bolt steel has excellent delayed fracture resistance.
Detailed Description
Further advantages and effects of the present invention will become apparent to those skilled in the art from the disclosure of the present specification, by describing the embodiments of the present invention with specific examples. While the description of the invention will be described in connection with the preferred embodiments, it is not intended to limit the inventive features to the implementation. Rather, the purpose of the invention described in connection with the embodiments is to cover other alternatives or modifications, which may be extended by the claims based on the invention. The following description contains many specific details for the purpose of providing a thorough understanding of the present invention. The invention may be practiced without these specific details. Furthermore, some specific details are omitted from the description in order to avoid obscuring the invention. It should be noted that, without conflict, the embodiments of the present invention and features of the embodiments may be combined with each other.
Hereinafter, the component system of the present invention will be described.
C：0.35％～0.45％
C is added into steel as an inexpensive element, can improve the hardenability and hardenability of the steel, and forms fine dispersed carbide after quenching and tempering, is critical to the strength of the steel, and when the C content is higher than 0.45%, fe in the steel is caused 3 Since the amount of C increases, the plasticity of the steel decreases, and thus the C content is set to 0.35% to 0.45%.
Si：0.15％～0.24％
Si replaces iron atoms in steel in a replacement mode to block dislocation movement, strength of ferrite phase can be remarkably improved, higher silicon content can improve hardness of annealed materials, cold heading forming is not facilitated, service life of forming dies is shortened, low-temperature impact toughness of the steel is reduced, and therefore the Si content is set to be 0.15% -0.24%.
Mn：0.20％～0.40％
Mn has stronger solid solution strengthening effect, is an important strengthening element, can effectively improve hardenability and strength, and meanwhile, a certain amount of Mn is added to have little influence on plasticity of steel, but Mn is easy to segregate in the solidification process of the ferroalloy, when the Mn content is more than 0.20%, the strength and hardenability of the alloy are ensured, and when the Mn content is less than 0.40%, the phenomenon of material uniformity deterioration caused by Mn segregation is avoided, so that the Mn content is set to be 0.20% -0.40%.
Cr：0.75％～1.35％
Cr has the function of improving the hardenability of the ferroalloy, and simultaneously precipitates fine dispersed carbide particles in the tempering process to play a role of dispersion strengthening, thereby improving the strength of the steel, and also has the function of refining the structure, so that the Cr content is set to be 0.75-1.35% in order to exert the solid solution strengthening and precipitation strengthening functions of Cr and improve the structure of the steel.
Mo：0.55％～1.00％
Mo is a ferrite forming element, which is beneficial to improving the hardenability of steel, so that bainite and martensite are formed in the steel during quenching. Since Mo has a large atomic weight and a large atomic radius, and is not easily diffused in the iron alloy, mo exists mainly in a solid solution form in steel at a low temperature to achieve a solid solution strengthening effect, and forms fine carbides by tempering at a high temperature to improve the strength of steel and the tempering resistance of steel as a whole. The higher tempering temperature can effectively reduce the dislocation and subgrain boundary amount in the steel, thereby avoiding the delayed cracking caused by the aggregation of hydrogen elements at the dislocation and the grain boundary, but Mo is a noble alloy element, and the higher Mo content can cause the cost to rise, so the Mo content is set to be 0.55% -1.00%.
V：0.08％～0.30％
V is a strong carbide forming element, and the formed carbide is divided into two types, one is undissolved vanadium carbide in the quenching process and the other is precipitated vanadium carbide in the tempering process. Undissolved vanadium carbide belongs to a high-temperature precipitated phase in the quenching process, the complete dissolution temperature of the vanadium carbide is higher than the quenching temperature, and the undissolved vanadium carbide can pin the prior austenite grain boundary to prevent the prior austenite grain from growing up, so that the effects of refining the prior austenite grain and refining the martensite structure are achieved; fine vanadium carbide particles can be formed in the tempering process, the fine vanadium carbide belongs to an irreversible hydrogen trap, hydrogen in steel can be effectively trapped, and the hydrogen is prevented from gathering at a stress concentration position, so that hydrogen induced cracking cracks are generated; in addition, the vanadium carbide precipitated phase can prevent interaction between hydrogen and dislocation, reduce increment of dislocation and reduce local plastic deformation. At higher tempering temperatures, the V content is too high, which tends to form coarse vanadium carbide particles, reducing the impact properties of the steel, and therefore, in combination with the other alloying elements of the invention, the V content is set at 0.08% to 0.30%, preferably 0.25% to 0.30%.
Al：0.005％～0.10％
Al is a deoxidizer added in steel, and the content of Al is controlled within the range of 0.005-0.10%, so that grains are refined, and the toughness of the steel is improved.
S：0～0.005％
The S content is maintained at a low level because of the hot shortness problem caused by the formation of FeS from S, and thus is limited to 0.005% or less. The lower limit is not defined because the lower the S content is for the same reason, the better.
The method for manufacturing the steel for the high-strength bolt comprises the following steps:
smelting and casting
Because more alloying elements such as Cr, mo, V and the like are added in the steel, the alloying elements are easy to segregate in the solidification process, and the segregation is inherited, and the segregation of the continuous casting blank or steel ingot is transmitted to the wire rod, so that the uniformity of the material is reduced, and compared with the conventional casting process, the following requirements are met for the cross section area of the continuous casting blank or steel ingot: the ratio of the cross sectional area of the continuous casting blank or steel ingot to the cross sectional area of the wire rolled blank is larger than 6, and the cross sectional area of the continuous casting blank or steel ingot is larger than or equal to 320 multiplied by 425mm, wherein the measuring method of the cross sectional area of the continuous casting blank or steel ingot is that the measuring method of the measuring tape is adopted to measure the continuous casting blank or the steel ingot at room temperature, and the cross sectional area is calculated.
Soaking treatment and blooming
Compared with the conventional wire rod production process, the soaking treatment and blooming steps are added to reduce the segregation of Cr, mo, V and other alloy elements in steel, improve the uniformity of the material, and the continuous casting billet or steel ingot needs to be subjected to the soaking treatment step before the blooming step to reduce the segregation of the material, wherein the soaking treatment step of the continuous casting billet or steel ingot is particularly carried out at 1200-1250 ℃ for more than 24 hours, and the blooming step is particularly carried out by rolling the continuous casting billet or steel ingot with larger cross section area into the steel billet with smaller cross section area.
Wire rod rolling
Heating a steel billet to 1000-1080 ℃ for wire rod rolling, wherein the final rolling temperature is 830-900 ℃, the wire rod spinning temperature is 780-810 ℃, and the stelmor wire (the stelmor wire is a controlled cooling process designed based on the transformation rule of the steel structure during cooling) is treated according to the conventional process, so as to obtain the wire rod of the fine-grain high-strength bolt steel with delayed fracture resistance.
Quenching and tempering
The wire rod is subjected to quenching and tempering, the selected quenching temperature is required to meet the requirement that log { [ V ] [ C ] } > 6.72-9500/T (wherein [ V ] is the mass percent of V in the steel for the high-strength bolt, [ C ] is the mass percent of C in the steel for the high-strength bolt, T is the quenching temperature, the unit of the quenching temperature T is K), the wire rod is subjected to oil quenching after being kept at the quenching temperature for 60-80 min, mineral oil at 40-60 ℃ is adopted to cool to below 80 ℃, then the temperature of not lower than 500 ℃ is adopted to heat for more than 90min for tempering, and the steel for the high-strength bolt is obtained through air cooling.
Through the tempering treatment step, the obtained steel for the high-strength bolt has tensile strength of more than 1400MPa, yield strength of more than 1260MPa and reduction of area of more than 50 percent, and the microstructure of the steel for the high-strength bolt is tempered sorbite, and the prior austenite grain size of the tempered sorbite is less than or equal to 20 mu m.
The beneficial effects of the invention are as follows:
the high-strength bolt steel provided by the invention has the tensile strength reaching above 1400MPa, the yield strength reaching above 1260MPa and the area reduction rate reaching above 50%, and the microstructure of the high-strength bolt steel is tempered sorbite, and the original austenite grain size of the tempered sorbite is less than or equal to 20 mu m. The alloy cost of the steel for the high-strength bolt is lower, the strength of the steel for the high-strength bolt is improved through nano-scale precipitates, meanwhile, the steel for the high-strength bolt has good toughness and delayed fracture resistance, and the finished bolt has higher fatigue life by refining tissues and controlling the composition and the size of inclusions, so that the requirements of high strength and long service life in the automobile light weight and mechanical industry can be met, the technical level of the industry is improved, and good economic benefits are realized.
Hereinafter, the present invention will be described in more detail by way of examples.
Examples
According to the embodiment of the invention, smelting and casting are carried out according to the chemical component requirements, and a continuous casting blank with the thickness of 320 multiplied by 425mm is cast. Examples 1 to 10 and comparative examples 11 to 13 were shown in Table 1.
Soaking the continuous casting blank in a soaking pit for more than 24 hours at the temperature of 1200-1250 ℃ to perform blooming and rolling to form a steel billet with the thickness of 142 multiplied by 142mm, wherein the compression ratio is 6.7, and meets the requirement that the compression ratio is more than 6, and the compression ratio is the ratio of the cross section area of the continuous casting blank or steel ingot to the cross section area of the steel billet.
Rolling a steel billet of 142X 142mmIs produced into
Figure BDA0002919738300000062
After the wire rod is subjected to thermal refining, the wire rod is processed to form the steel for the high-strength bolt with delayed fracture resistance. />
Wherein, the quenching temperature T in the quenching and tempering process meets the following relation: log { [ V ] [ C ] } > 6.72-9500/T; wherein [ V ] is the mass percent of V in steel, [ C ] is the mass percent of C in steel, and T is the quenching temperature (unit is K); preserving heat for 60-80 min at the quenching temperature T, and cooling to below 80 ℃ by adopting mineral oil at 40-60 ℃; heating above 500 deg.C for 90min, and air cooling.
The quenching and tempering process parameters are shown in Table 2, the prior austenite grain size and the mechanical properties are shown in Table 3, wherein the strength loss rate and the plasticity loss rate of comparative examples 11-13 under the hydrogen charging environment are obviously higher than those of examples 1-10, the V content in example 2 is in the preferred range of 0.25% -0.30%, and the steel for the high-strength bolt shows better delayed fracture resistance.
The measurement method of the prior austenite grains refers to a straight line intercept point method in the GB/T6394-2017 standard.
The method for measuring the strength loss rate and the plasticity loss rate in the hydrogen charging environment comprises the following steps: by adopting a slow stretching experiment, placing a sample in a hydrogen-containing environment in the slow stretching process, soaking the sample in 0.1mol/L hydrochloric acid solution, wherein the strain rate is 10 -6 s -1 The mechanical property of the steel for the high-strength bolt in the hydrogen filling environment is obtained, the mechanical property of the steel for the high-strength bolt in the hydrogen filling environment is compared with the mechanical property (including tensile strength and reduction of area) of the steel which is slowly stretched in the air environment, and the strength loss rate and the plasticity loss rate of the steel in the hydrogen filling environment are obtained, wherein the strength loss rate and the plasticity loss rate are represented by the following formula: i σ ＝(σ 0 -σ H )/σ 0 ×100％；I Z ＝(Z 0 -Z H )/Z 0 X 100%; in which I σ 、I Z The strength loss rate and the plasticity loss rate, sigma, respectively 0 、σ H Tensile strength of the sample in air and in hydrogen-containing environment, Z 0 、Z H The samples were air-conditioned and hydrogen-containing respectivelyArea reduction in the environment.
TABLE 1 chemical Components (in%) of examples and comparative examples
Figure BDA0002919738300000061
Figure BDA0002919738300000071
TABLE 2 thermal refining process parameters
Examples Quenching and tempering
1 Heat preservation at 900 ℃ for 60min and oil quenching; tempering after heat preservation at 530 ℃ for 90min, and air cooling
2 Keeping the temperature at 880 ℃ for 80min, and then carrying out oil quenching; tempering after heat preservation at 600 ℃ for 90min, and air cooling
3 Heat preservation at 870 ℃ for 70min and oil quenching; tempering after heat preservation at 530 ℃ for 90min, and air cooling
4 Preserving heat at 890 ℃ for 75min, and then carrying out oil quenching; tempering after heat preservation at 520 ℃ for 90min, and air cooling
5 Heat preservation at 920 ℃ for 70min and oil quenching; tempering after keeping the temperature at 580 ℃ for 90min, and air cooling
6 Heat preservation at 920 ℃ for 60min and oil quenching; tempering after heat preservation at 570 ℃ for 90min, and air cooling
7 Preserving heat at 910 ℃ for 80min, and then quenching with oil; tempering after heat preservation at 560 ℃ for 90min, and air cooling
8 Heat preservation at 920 ℃ for 65min and oil quenching; tempering after 90min of heat preservation at 590 ℃ and air cooling
9 Preserving heat at 890 ℃ for 75min, and then carrying out oil quenching; tempering after heat preservation at 560 ℃ for 90min, and air cooling
10 Heat preservation at 900 ℃ for 70min and oil quenching; tempering after heat preservation at 500 ℃ for 90min, and air cooling
11 Comparative example Heat preservation at 850 ℃ for 70min and oil quenching; tempering after preserving heat at 400 ℃ for 90min, and air cooling
12 Comparative example Preserving heat at 910 ℃ for 70min, and then quenching with oil; tempering after keeping the temperature at 420 ℃ for 90min, and air cooling
13 Comparative example Heat preservation at 960 ℃ for 70min and oil quenching; heat preservation at 550 ℃ for 90min, tempering and air cooling
TABLE 3 prior austenite grain size and mechanical Properties
Figure BDA0002919738300000072
As is clear from tables 1 to 3, the prior austenite grain size in tempered sorbite of the steel for bolts of examples 1 to 10 is 20 μm or less, the tensile strength is 1400MPa or more, the yield strength is 1260MPa or more, the reduction of area is 50% or more, the strength loss rate in hydrogen-filled environment is < 20%, the plastic loss rate in hydrogen-filled environment is < 50%, while the prior austenite grain size in tempered sorbite of the steel for bolts of comparative example 11 is > 20 μm, because the content of V element in comparative example 11 is 0.01%, the content of V element is too low, the formed carbide such as vanadium carbide is less, the effect of refining the prior austenite grain cannot be achieved, and the combination of lower tempering temperature results in the reduction of area of the steel for bolts of comparative example 11 being < 50%, the strength loss rate in hydrogen-filled environment is > 20%, the plastic loss rate in hydrogen-filled environment is > 50%, namely the plastic toughness and the delayed fracture resistance are poor.
The lower content of Mo and V elements in comparative example 12 resulted in the steel for the bolts of comparative example 12 having poor hardenability and the original austenite grain size of tempered sorbite > 20 μm, and the lower tempering temperature resulted in the steel for the bolts of comparative example 12 having a reduction of area of < 50%, a strength loss rate of > 20% in a hydrogen-charged environment and a plastic loss rate of > 50% in a hydrogen-charged environment, i.e., poor toughness and delayed fracture resistance.
In comparative example 13, the mass percentages of the V element and the C element were 0.22 and 0.41, respectively, and according to the formula log { [ V ] [ C ] } > 6.72-9500/T, the quenching temperature T was calculated to be less than 1223.44K, namely, the quenching temperature T was less than 950 ℃, whereas according to Table 3, the quenching temperature adopted in comparative example 13 was 960 ℃ and exceeds 950 ℃, therefore, the higher quenching temperature was selected for comparative example 13, resulting in the prior austenite grain size of the tempered sorbite of the steel for bolts being > 20 μm, and the poor delayed fracture resistance (the strength loss rate in the hydrogen charging environment was > 20%, and the plastic loss rate in the hydrogen charging environment was > 50%).
While the invention has been described with reference to certain preferred embodiments thereof, it will be understood by those skilled in the art that the foregoing is a further detailed description of the invention in connection with specific embodiments, and it is not intended to limit the invention to the specific embodiments described. Various changes in form and detail may be made therein by those skilled in the art, including a few simple inferences or alternatives, without departing from the spirit and scope of the present invention.

Claims (2)

1. A high-strength steel for bolts is characterized by comprising the following components in percentage by mass:
c:0.35% -0.45%; si:0.15% -0.24%; mn:0.20% -0.40%; cr:0.75% -1.35%; mo:0.55% -1.00%; v:0.08% -0.30%; al:0.005% -0.10%; s:0 to 0.005%, the balance being Fe and unavoidable impurities,
wherein, the mass percent of V, the mass percent of C and the quenching temperature satisfy the following relation: lg { [ V ] [ C ] } > 6.72-9500/T; wherein [ V ] is the mass percentage of V in the high-strength bolt steel, [ C ] is the mass percentage of C in the high-strength bolt steel, T is the quenching temperature, the unit of the quenching temperature T is K, and
the microstructure of the steel for the high-strength bolt is tempered sorbite, the prior austenite grain size of the tempered sorbite is less than or equal to 20 mu m, and
the tensile strength of the steel for the high-strength bolt is more than or equal to 1400MPa, the yield strength is more than or equal to 1260MPa, the area reduction is more than or equal to 50 percent, and
the strength loss rate of the steel for the high-strength bolt in the hydrogen charging environment is less than 20%, and the plasticity loss rate in the hydrogen charging environment is less than 50%.
2. A method of manufacturing the steel for high-strength bolts according to claim 1, characterized by comprising the steps of:
smelting and casting: smelting and casting into a continuous casting blank or a steel ingot according to the components, wherein the ratio of the cross sectional area of the continuous casting blank or the steel ingot to the cross sectional area of the wire rolled blank is more than 6, and the cross sectional area of the continuous casting blank or the steel ingot is more than or equal to 320 multiplied by 425mm;
soaking treatment: soaking the continuous casting blank or the steel ingot, wherein the soaking temperature is 1200-1250 ℃, and the heat preservation time is more than or equal to 24 hours;
blooming and cogging: rolling the continuous casting blank or the steel ingot to form a steel billet;
rolling wire rods: heating the steel billet to 1000-1080 ℃, and rolling wires, wherein the final rolling temperature is 830-900 ℃, and the wire laying temperature is 780-810 ℃ to obtain wire rods;
quenching and tempering: and carrying out quenching and tempering on the wire rod, carrying out oil quenching after preserving heat for 60-80 min at the quenching temperature T, tempering at the temperature of more than or equal to 500 ℃ for the heating time of more than or equal to 90min, and carrying out air cooling to obtain the steel for the high-strength bolt.
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