CN114107822A

CN114107822A - 15.9-grade high-strength bolt steel and production method and heat treatment method thereof

Info

Publication number: CN114107822A
Application number: CN202111441588.0A
Authority: CN
Inventors: 张晓瑞; 汪开忠; 于同仁; 姜婷; 尹德福; 龚梦强; 郭湛; 丁雷; 孙凯; 余良其; 葛晴晴
Original assignee: Maanshan Iron and Steel Co Ltd
Current assignee: Maanshan Iron and Steel Co Ltd
Priority date: 2021-11-30
Filing date: 2021-11-30
Publication date: 2022-03-01
Anticipated expiration: 2041-11-30
Also published as: CN114107822B

Abstract

The invention provides a 15.9-grade high-strength bolt steel, a production method and a heat treatment method thereof, and the steel comprises the following components: 0.51 to 0.54 percent of C, 0.60 to 1.00 percent of Si, 0.40 to 0.60 percent of Mn, 1.15 to 1.30 percent of Cr, 1.20 to 1.40 percent of Mo, 0.50 percent of V0.80 percent of percentage-0.80 percent, 0.08 to 0.10 percent of Nb, 0.06 to 0.10 percent of Ti, 0.045 to 0.080 percent of Alt, less than or equal to 0.008 percent of P, less than or equal to 0.008 percent of S, less than or equal to 0.0020 percent of O and less than or equal to 0.0040 percent of N; the balance of Fe and other inevitable impurities. The mechanical property of the alloy after the thermomechanical treatment and the high-frequency tempering heat treatment meets the tensile strength R_mNot less than 1500MPa, good strength and plasticity and toughness, and excellent delayed fracture resistance.

Description

15.9-grade high-strength bolt steel and production method and heat treatment method thereof

Technical Field

The invention belongs to the technical field of steel for fasteners, and particularly relates to 15.9-grade high-strength bolt steel and a production method and a heat treatment method thereof.

Background

The fastener is a basic industry of equipment manufacturing industry and is widely applied to various fields of national economy. In recent years, with the rapid development of various industries such as automobiles, wind power, machinery, buildings and the like, the requirements of design stress and light weight are provided for materials used for manufacturing parts such as various fasteners (such as bolts, nuts and the like), the most effective measure is to improve the strength of the fasteners, and bolts for automobiles and construction machinery even require the strength to be more than 1400 MPa. A series of high-strength bolted steels with excellent delayed fracture resistance, such as ADS series of Japanese Sumitomo metal company, KNDS series of Shenhu steel-making, ADF series of Beijing Steel research institute, etc., have been developed at home and abroad.

The high-strength fastener connection has the advantages of high bearing capacity, good stress, fatigue resistance, no looseness, safety, simple and convenient construction, detachability and the like, and is widely used in the steel structure connection of the infrastructure. From the viewpoint of weight reduction and performance improvement of building structures and machine parts, the higher the strength level of bolts, the more desirable, but when the tensile strength of bolts exceeds 1200MPa, the problem of delayed fracture, particularly when the strength reaches 1400MPa, is very prominent, and therefore, bolts with ultra-high strength levels of 1400MPa or more have not been put into practical use, and how to improve the delayed fracture resistance of high-strength bolts while ensuring high strength has been a major research focus in various countries.

Chinese patent CN1329179A published in 2002, 1, 2 discloses a high-strength bolt steel which is suitable for producing high-strength bolt steel with strength of 1400-1600MPa and delayed fracture resistance. The concrete components are as follows: 0.35 to 0.5 percent of C, 0.01 to 0.09 percent of Si, less than or equal to 0.30 percent of Mn, less than or equal to 0.010 percent of P, less than or equal to 0.008 percent of S, 0.5 to 1.5 percent of Cr0.7 to 1.5 percent of Mo0.20 to 0.50 percent of V, 0.01 to 0.08 percent of Nb0.002 to 0.04 percent of Re0.005 to 0.05 percent of Al, 0.006 to 0.015 percent of N, 0.01 to 0.155 percent of any one or the sum of two of Ti and Zr, and the balance of Fe and inevitable impurities. The method has the advantages that the chemical components are added with the rare earth element Re, and the production cost is obviously increased due to the lower rare earth yield.

Chinese patent CN105579603A published on 5, 11/2016 discloses a high-strength bolt steel and bolt excellent in delayed fracture resistance and bolt formability, which have the following chemical components: 0.10 to 0.30 percent of C, 0.4 to 0.7 percent of Ni, more than 0 percent and less than 0.2 percent of Si, 0.3 to 0.8 percent of Mn, more than 0 percent and less than 0.03 percent of P, more than 0 percent and less than 0.03 percent of S, 0.8 to 1.2 percent of Cr, 0.8 to 1.5 percent of Mo, 0.05 to 0.13 percent of V, 0.02 to 0.08 percent of Ti, 0.01 to 0.1 percent of Al, N: 0.001-0.01%, the balance being iron and unavoidable impurities. The method is a high-strength bolt with the tensile strength of 1100-1400MPa and excellent delayed fracture resistance, and the strength level is not up to 1500 MPa.

Disclosure of Invention

The invention aims to provide a 15.9-grade high-strength bolt steel and a production method thereof, wherein the tensile strength of a product is more than 1500MPa, and the product has excellent delayed fracture resistance.

Still another object of the present invention is to provide a heat treatment method for a steel for a high strength bolt of grade 15.9.

The specific technical scheme of the invention is as follows:

the steel for the 15.9-grade high-strength bolt comprises the following components in percentage by mass: 0.51 to 0.54 percent of C, 0.60 to 1.00 percent of Si, 0.40 to 0.60 percent of Mn, 1.15 to 1.30 percent of Cr, 1.20 to 1.40 percent of Mo, 0.50 to 0.80 percent of V, 0.08 to 0.10 percent of Nb, 0.06 to 0.10 percent of Ti, 0.045 to 0.080 percent of Alt, less than or equal to 0.008 percent of P, less than or equal to 0.008 percent of S, less than or equal to 0.0020 percent of O and less than or equal to 0.0040 percent of N; the balance of Fe and other inevitable impurities.

Furthermore, the chemical composition should satisfy 2.0. gtoreq.10 XTi/V. gtoreq.0.75, preferably 1.6. gtoreq.10 XTi/V. gtoreq.0.9.

The invention provides a production method of a 15.9-grade high-strength bolt steel, which comprises the following process flows of:

batching → electric furnace smelting → LF furnace refining, RH vacuum degassing → large round billet continuous casting → rolling square billet → flaw detection, coping → high-speed wire heating furnace heating → controlled rolling and controlled cooling → reduced sizing mill treatment → laying head treatment → obtaining finished wire rod → packaging and warehousing.

The electric furnace smelting specifically comprises the following steps: the end point C of the electric furnace is controlled to be 0.06-0.20 percent, and P is less than or equal to 0.010 percent; slag-stopping and tapping, adding refining slag and lime when tapping 1/5-1/4 molten steel, and adding deoxidizer and alloy when tapping 1/3-1/2, wherein the sequence is as follows: aluminum iron → slag charge → silicomanganese → high carbon ferrochrome → ferromolybdenum → carburant, after tapping, according to the amount of slag, uniformly spraying aluminum particles on the surface of the steel slag.

And refining in the LF furnace: argon is blown from the bottom of the ladle in the whole process, so that the homogenization of aluminum in steel is ensured. The argon flow is based on the steel tapping ladle without splashing molten steel; adding premelted refining slag and lime for slagging, wherein the alkalinity is R3-6, the white slag time is more than or equal to 20 minutes, and adding alloy to adjust the contents of Si, Mn, Cr, Mo, V, Nb and Ti before and during refining according to the analysis result of components before entering the LF furnace.

The RH vacuum degassing: in the early stage of vacuum, if the vacuum degree is less than or equal to 100 Pa, the vacuum maintaining time is more than or equal to 10 minutes, and if the vacuum degree is more than 100 Pa and less than or equal to 200 Pa, the vacuum maintaining time is more than or equal to 15 minutes; the later-period holding time of the vacuum is more than or equal toFor 10 minutes. According to the analysis result of the composition in the early stage of vacuum, if the composition adjustment is required in the middle stage, the vacuum holding time of more than 5 minutes must be ensured after the adjustment. Calcium wire feeding treatment is carried out after vacuum breaking, calcium aluminate inclusion is promoted to float upwards, and Al is avoided₂O₃The inclusion-like adhering nozzle causes the problem of casting nodulation. And carrying out soft argon blowing treatment before the station is out, wherein the soft argon blowing time is more than or equal to 15 min.

And (3) continuously casting the large round billet: the large round billet continuous casting is adopted to prepare for obtaining a wire rod finished product with good surface quality, and the cold heading cracking phenomenon in the subsequent drawing process is avoided. The whole-process protection casting is adopted, a protective sleeve and argon sealing are adopted between a steel ladle and a tundish, the tundish is protected by using a molten steel covering agent and argon blowing, an immersion water gap is adopted between the tundish and a crystallizer, the pressure of primary cooling water is 5.0-9.0bar, the pressure of secondary cooling water is 9.0-15.0bar, the liquid level, the pulling speed and the superheat degree in the casting process are stable, and a defect-free casting blank is obtained.

The square billet is rolled into 150 square billet by utilizing a large round billet, the temperature of the soaking section of the heating furnace is controlled to be 1300 ℃ in 1200-year mode, the total heating time is controlled to be 350min in 250-year mode, all V, Ti elements can be ensured to be dissolved in solid solution only at the temperature, and the existence of large-size V, Ti nitride is avoided. The initial rolling temperature is controlled to be 1100 +/-50 ℃, and the temperature of the pile cooling is more than or equal to 400 ℃ after rolling.

After rolling, the surface and the end part of the 150 square billet are polished, so that good surface quality is provided for subsequent high-speed wire rolling, and the surface decarburization sensitivity of the wire rod is reduced.

Heating by using the high-speed wire heating furnace: in order to meet the requirement of a rolling process and enable carbonitride to be dissolved in austenite in a solid mode, the heating temperature is controlled at 1100-1250 ℃, the soaking time is more than 30min, the Nb, V and Ti elements are ensured to be dissolved in a solid mode fully, and the heated casting blank enters a continuous rolling unit for rolling after being descaled by high-pressure water;

the controlled rolling and controlled cooling adopts a low-temperature controlled rolling and controlled cooling process, the rolling temperature is 950-;

the reducing sizing machine comprises the following specific steps: reducing the sizing temperature 770-810 ℃;

the laying head treatment specifically comprises the following steps: the spinning temperature is 760-780 ℃, and then the spinning tube is slowly cooled in a heat-preserving cover at the cooling rate of 0.5-0.9 ℃/s, so that the martensite structure is prevented from being generated and the subsequent drawing and other deep processing are influenced.

Under the production process conditions, the steel of the invention is subjected to large deformation rolling and slow cooling at low temperature to obtain a microstructure which is as follows: pearlite, ferrite and bainite, no martensite hard phase structure harmful to drawing, fine and uniform dispersion precipitation of carbides of V, Nb and Ti, and obtaining a fine crystal structure of more than or equal to 10.0 grade, thereby providing tissue preparation for mechanical properties of 1500MPa after heat treatment and good plasticity and toughness.

The invention provides a heat treatment method of a 15.9-grade high-strength bolt steel, which comprises a deformation heat treatment and a high-frequency tempering heat treatment.

The thermomechanical treatment process comprises the following specific steps: preserving heat at the temperature of 1000-1200 ℃ for 320 seconds at the temperature of 280-850 ℃, cooling to the temperature of 750-850 ℃ at the cooling rate of 4-7 ℃/s, performing deformation treatment for 20-50 percent, and then performing water cooling;

the specific process of the high-frequency tempering heat treatment comprises the following steps: an electron tube high-frequency heating power supply is adopted, the temperature rise time is 15-25s, the heat preservation temperature is 500-.

The structure obtained after heat treatment was: tempering sorbite and dispersing and separating out carbide.

The mechanical property of the product after heat treatment meets the tensile strength R_mNot less than 1500MPa and yield ratio R_P0.2/R_mMore than or equal to 0.9 percent, the elongation A after fracture is more than or equal to 15 percent, the reduction of area Z is more than or equal to 48 percent, and the normal temperature impact absorption power KV₂The fatigue strength is more than or equal to 45J, the fatigue strength is more than or equal to 680MPa, and the delayed fracture resistance R is more than or equal to 2.

The design principle of the steel is as follows:

c: element C is necessary for obtaining high strength of the steel for high-strength fasteners. The high C content is advantageous for the strength of steel, etc., but is extremely disadvantageous for the cold heading property, plasticity and toughness of steel, and decreases the yield ratio, increases the decarburization sensitivity, and deteriorates the fatigue resistance and workability of steel. Therefore, the content of C is controlled to be 0.51-0.54%.

Si: si is a main deoxidizing element in steel, and contributes to an increase in strength as a solid solution hardening element, and when the Si content is less than 1.3%, the strength of the main body tends to be insufficient. However, silicon can significantly improve the deformation resistance of steel, is unfavorable for cold heading and cold extrusion, reduces the plasticity and toughness of steel, deteriorates the fatigue resistance of steel, and increases the surface decarburization sensitivity, so the silicon content is not too high, and the Si content is controlled to be 0.60-1.00%.

Mn: mn is an effective element for deoxidation and desulfurization, but Mn and P have a strong tendency of intergranular co-segregation during high-temperature tempering of quenched steel, promoting temper brittleness, and excessively high Mn content deteriorates the weather resistance of the steel. Therefore, the Mn content in the steel is properly reduced and controlled to be 0.40-0.60 percent.

Cr: cr element is the most commonly used alloying element in high strength bolting steel. Cr is effective in improving hardenability and temper resistance of steel to obtain a desired high strength. Too low content is difficult to perform the above-mentioned effects, but too high content deteriorates toughness and cold workability of the steel, and thus the Cr content is controlled to be 1.15% to 1.30%.

Mo: the Mo element is an alloy element which is commonly adopted in high-strength bolt steel. Not only can obviously improve the tempering resistance of the steel, but also forms carbide Mo₂C has a trapping effect on hydrogen, and can improve the delayed fracture resistance of the steel; it is also possible to improve the grain boundary bonding strength of the steel by giving segregation in the prior austenite grain boundary. In addition, Mo element can reduce the amount of hydrogen invading the steel surface, inhibit the generation of corrosion pit and improve the corrosion resistance, so that the Mo content is controlled to be 1.20-1.40%.

V: v is an excellent deoxidizer of steel, and vanadium is added into the steel to refine structure grains and improve the strength and the toughness. In addition, the V element is also an element with strong tempering and softening resistance, the tempering temperature is increased under the condition of keeping the strength unchanged, fine and uniform vanadium carbonitride precipitated during tempering at a higher temperature can generate secondary hardening to further improve the strength of the steel, and the vanadium carbonitride has strong trap energy and can trap hydrogen to be uniformly dispersed in crystal, so that the diffusion of the hydrogen and the segregation of crystal boundary are inhibited, and the delayed fracture resistance of the steel is improved. The content of V is controlled between 0.50 percent and 0.80 percent.

Nb: the Nb element can finely refine grains, and the grain refinement not only can improve the toughness of the steel, but also can improve the low-temperature performance of the steel. Meanwhile, the tempering resistance is improved, and the delayed fracture resistance of the high-strength steel is improved. The hydrogen trap binding energy of the carbide is lower than that of vanadium carbide and titanium carbide, and the trapping effect on hydrogen is slightly poor. The range of Nb can be controlled between 0.08 percent and 0.10 percent.

Ti: titanium carbide formed by Ti element can pin austenite grain boundary refined grains, and the titanium carbide has stronger trapping effect on hydrogen, thereby improving the delayed fracture resistance of steel, simultaneously improving the corrosion resistance, and inhibiting the absorption and generation of hydrogen in the corrosion environment. The Ti content is controlled to be 0.06-0.10%.

S and P: s, P, etc., will greatly reduce delayed fracture resistance due to segregation of impurity elements at grain boundaries. The P element can form micro segregation when molten steel is solidified, and then is deviated and gathered at a crystal boundary when being heated at an austenitizing temperature, so that the brittleness of steel is obviously increased, and the delayed fracture sensitivity of the steel is increased; the S element forms Mn-S inclusion and is segregated in the grain boundary, thereby increasing the delayed fracture sensitivity of the steel, and therefore, the content of P, S is controlled to be less than or equal to 0.008 percent of P and less than or equal to 0.008 percent of S.

O and N: oxygen forms various oxide inclusions in the steel. Under the action of stress, stress concentration is easily generated at the oxide inclusions, and microcrack is initiated, so that the mechanical properties, particularly toughness and fatigue resistance, of the steel are deteriorated. Therefore, measures are needed to be taken to reduce the content of O in the metallurgical production as much as possible and control the content of O to be less than or equal to 0.0020 percent; n precipitation of Fe in steel₄N, the diffusion speed is slow, so that the steel has timeliness, and simultaneously, the cold processing performance of the steel is reduced by N, and the N is controlled to be less than or equal to 0.0040%.

And (3) Alt: alt is a strong deoxidizing element, improves the oxidation resistance of steel, and can also refine austenite grains and improve delayed fracture resistance. In addition, the high Alt element is added to be combined with nitrogen to form AlN, so that the pinning effect of dislocation is reduced, the blue brittleness tendency is obviously reduced, the impact toughness is improved, and good comprehensive mechanical properties are obtained after heat treatment. The Alt content is controlled to be 0.045% -0.080%.

In order to obviously improve the delayed fracture resistance of the material, the invention adopts V-Ti composite microalloying, fully exerts the hydrogen trapping effect of TiC and VC, on one hand, the hydrogen trapping energy of TiC is far higher than that of VC and is about 10 times of that of VC at the same tempering temperature, the delayed fracture performance of Ti is far better than that of V, on the other hand, the precipitation temperature of VC at the same strength is higher than that of TiC, the secondary hardening effect of VC dispersed precipitation at tempering is higher than that of TiC, the size of VC is 1/5 of TiC, namely, the delayed fracture performance of V is better than that of Ti, therefore, the comprehensive consideration needs to satisfy 2.0 more than or equal to 10 multiplied by Ti/V more than or equal to 0.75, preferably 1.6 more than or equal to 10 multiplied by Ti/V more than or equal to 0.9, and the N is required to be less than or equal to 0.0040 for ensuring that the nitride such as TiN, VN and the like is not precipitated. Meanwhile, in order to ensure good toughness under high-temperature tempering, a certain amount of Nb element and V element are added to refine grains together, and the delayed fracture resistance is improved to a certain extent. In addition, the invention improves the content of Alt element refined crystal grains, improves the delayed fracture resistance and simultaneously improves the toughness, thereby being beneficial to the subsequent processing of fasteners.

The hot rolled wire rod produced by the method is processed by the working procedures of drawing → annealing → cold heading → heat treatment → coating. In order to improve the delayed fracture resistance of the steel, the improvement of the strength is ensured by adopting a mode of thermomechanical treatment and high-frequency tempering heat treatment, and meanwhile, the ductility and toughness are not obviously reduced. The method reduces and refines grain boundary carbides by quenching after deformation of an austenite non-recrystallization region, and comprises the following specific processes: the temperature is kept at 1000-1200 ℃ for 280-320 seconds, then the temperature is cooled to 750-850 ℃ at the cooling speed of 4-7 ℃/s, the deformation treatment is carried out for 20-50 percent, and then the water cooling is carried out. And through high-frequency tempering heat treatment, carbides in prior austenite grains are finely dispersed, the delayed fracture resistance is further improved, the stress concentration coefficient is reduced, the fatigue strength and the service life of the steel are improved, the notch sensitivity is reduced, and the specific process comprises the following steps: an electron tube high-frequency heating power supply is adopted, the temperature rise time is 15-25s, the heat preservation temperature is 500-. The mechanical property after heat treatment reaches 15.9 grades, and the product has good delayed fracture resistance.

Compared with the prior art, the 15.9-grade high-strength bolt steel produced by adopting the chemical components and the process flow has the advantages that the austenite grain size of the finished hot-rolled wire rod is more than or equal to 10.0 grade, and the mechanical property after heat treatment meets the tensile strength R_mNot less than 1500MPa and yield ratio R_P0.2/R_mMore than or equal to 0.9 percent, the elongation A after fracture is more than or equal to 15 percent, the reduction of area Z is more than or equal to 48 percent, and the normal temperature impact absorption power KV₂Greater than or equal to 45J, fatigue strength greater than or equal to 680MPa, good strength and ductility and toughness, and excellent delayed fracture resistance, and R ═ Hc is adopted]/[He]Evaluation of delayed fracture resistance R.gtoreq.2 (where [ Hc ]]Critical diffusion hydrogen content, [ He ]]The larger the R value, the better the delayed fracture resistance, for the diffusible hydrogen content invading from the environment).

Drawings

FIG. 1 shows the hot rolled structure (500X) in example 1;

FIG. 2 is the microstructure (500X) of example 2 after heat treatment;

FIG. 3 shows a hot rolled structure (500X) of comparative example 5.

Detailed Description

Examples 1 to 5

The steel for the 15.9-grade high-strength bolt comprises the following components in percentage by mass: as shown in table 1, the balance not shown in table 1 is Fe and inevitable impurities.

Comparative examples 1 to 5

The steel for the bolt 1 comprises the following components in percentage by mass: as shown in table 1, the balance not shown in table 1 is Fe and inevitable impurities.

TABLE 1 chemical composition (wt%) of inventive and comparative examples

The production method of the steel for the bolt, which is described in the above examples 1 to 5 and comparative examples 1 to 7, comprises the following process flows:

proportioning according to given chemical composition proportion → electric furnace smelting → LF furnace refining, RH vacuum degassing → large round billet continuous casting → rolling 150 square billet → fault detection, coping → high-speed wire heating furnace heating → controlled rolling and controlled cooling → reducing and sizing mill processing → wire laying machine processing → obtaining finished wire rod → packaging and warehousing. The specific operating points are as follows:

smelting in an electric furnace: the end point C of the electric furnace is controlled to be 0.06-0.20 percent, and P is less than or equal to 0.010 percent; and (3) slag-stopping and tapping, wherein refining slag and lime are added when molten steel about 1/5 is tapped, and a deoxidizer and an alloy are added when molten steel about 1/3 is tapped, and the sequence is as follows: aluminum iron → slag charge → silicomanganese → high carbon ferrochrome → ferromolybdenum → carburant, after tapping, according to the amount of slag, a proper amount of aluminum particles are uniformly thrown to the surface of the steel slag.

The RH vacuum degassing: in the early stage of vacuum, if the vacuum degree is less than or equal to 100 Pa, the vacuum maintaining time is more than or equal to 10 minutes, and if the vacuum degree is more than 100 Pa and less than or equal to 200 Pa, the vacuum maintaining time is more than or equal to 15 minutes; the vacuum later-period holding time is more than or equal to 10 minutes. According to the analysis result of the composition in the early stage of vacuum, if the composition adjustment is required in the middle stage, the vacuum holding time of more than 5 minutes must be ensured after the adjustment. And (5) performing calcium wire feeding treatment after vacuum breaking. And carrying out soft argon blowing treatment before the station is out, wherein the soft argon blowing time is more than or equal to 15 min.

The 150 square billet is rolled into the 150 square billet by utilizing the large round billet, the temperature of the soaking section of the heating furnace is controlled to be 1300 ℃ plus 1200 ℃, the total heating time is controlled to be 350min plus 250, and the V, Ti elements can be completely dissolved only at the temperature, so that the existence of large-size V, Ti nitride is avoided. The initial rolling temperature is controlled to be 1100 +/-50 ℃, and the pile cooling temperature is more than or equal to 400 ℃ after rolling;

Heating by using a high-speed wire heating furnace: in order to meet the requirement of a rolling process and enable carbonitride to be dissolved in austenite in a solid mode, the heating temperature is controlled at 1100-1250 ℃, the soaking time is more than 30min to ensure that Nb, V and Ti elements are fully dissolved in a solid mode, and the heated casting blank enters a continuous rolling unit for rolling after being descaled by high pressure water;

the low-temperature controlled rolling and cooling process is adopted, the rolling temperature is 950-;

reducing the sizing temperature 770-810 ℃;

the spinning temperature is 760-780 ℃, and then the spinning tube is slowly cooled in a heat-preserving cover at the cooling rate of 0.5-0.9 ℃/s, so that the martensite structure is prevented from being generated and the subsequent drawing and other deep processing are influenced.

The steel with the formula realizes fine and uniform dispersion and precipitation of V, Nb and Ti carbides through large deformation rolling and slow cooling at low temperature under the production process conditions, obtains a fine grain structure more than or equal to 10.0 grade, and realizes the mechanical property more than or equal to 1500MPa after heat treatment.

Examples 1 to 5 and comparative examples 1 to 7 the specific process parameters in the production process according to the above method are shown in tables 2 and 3.

TABLE 2 Steel making, continuous casting and billet rolling parameters for the examples of the present invention

The wire rod rolling process of each example of the present invention and comparative example is shown in table 3. The comparative examples 3 and 4 both adopt rolling processes meeting the requirements of the invention, the grain size does not reach 10.0 grade, and the comparative example 5 adopts the same chemical components of the example 1 and adopts a conventional wire rolling process, namely, the diameter reduction temperature is 830-; the structure pearlite + ferrite + bainite + martensite of comparative example 5 has a grain size of grade 9.0, and the coefficients of 10 × Ti/V of comparative example 6 and comparative example 7 do not satisfy the requirements of the present invention, and have a grain size of grade 9.5.

Table 3 example of the invention and comparative example wire rod rolling process

The samples with tensile property, impact property, fatigue property and delayed fracture resistance are processed into blanks of standard samples in each example and comparative example, then heat treatment is carried out according to the heat treatment process of the invention, the heat treatment process is shown in table 4, the comparative example 8 adopts the same chemical components and rolling process as in example 1, only the heat treatment process does not meet the heat treatment process method of the invention, the strength grade is less than 1500MPa, the mechanical property and delayed fracture property after heat treatment are shown in table 5, the strength of each example reaches 1500MPa, the elongation reaches more than 15%, the surface shrinkage reaches more than 48%, and the samples are punched at normal temperatureImpact absorption energy KV₂The fatigue strength is more than or equal to 45J, the fatigue strength is more than or equal to 680MPa, and the austenite grain size of the steel is more than or equal to 10.0 grade, which shows that the embodiment has better toughness and fatigue property. Simultaneously has good delayed fracture resistance and critical diffusible hydrogen content [ Hc]And diffusible hydrogen content [ He ] invading from the environment]The ratios are all greater than 2.

Table 4 heat treatment process of each example of the present invention and comparative example

TABLE 5 mechanical Properties after Heat treatment of inventive and comparative examples

The comparative examples 1 and 2 are respectively low in Si and Mo element contents, and the strength level is less than 1500 MPa; comparative example 3 is a microalloy element without V, Nb and Ti, and compared with the examples, the grain is obviously coarse and the ductility and toughness are insufficient; comparative example 4 is a lower content of Alt and lower impact toughness; and comparative examples 1, 2, 3, 4, 5 and 6 are insufficient in delayed fracture resistance.

Claims

1. The steel for the 15.9-grade high-strength bolt is characterized by comprising the following components in percentage by mass: 0.51 to 0.54 percent of C, 0.60 to 1.00 percent of Si, 0.40 to 0.60 percent of Mn, 1.15 to 1.30 percent of Cr, 1.20 to 1.40 percent of Mo, 0.50 to 0.80 percent of V, 0.08 to 0.10 percent of Nb, 0.06 to 0.10 percent of Ti, 0.045 to 0.080 percent of Alt, less than or equal to 0.008 percent of P, less than or equal to 0.008 percent of S, less than or equal to 0.0020 percent of O and less than or equal to 0.0040 percent of N; the balance of Fe and other inevitable impurities.

2. The steel for high-strength bolt of grade 15.9 according to claim 1, wherein the steel for high-strength bolt of grade 15.9 satisfies a composition of 2.0. gtoreq.10 xTi/V. gtoreq.0.75.

3. A production method of the steel for grade 15.9 high strength bolts according to claim 1 or 2, characterized by comprising the following process flows: batching → electric furnace smelting → LF furnace refining, RH vacuum degassing → large round billet continuous casting → rolling square billet → flaw detection, coping → high-speed wire heating furnace heating → controlled rolling and controlled cooling → reduced sizing mill treatment → laying head treatment → obtaining finished wire rod → packaging and warehousing.

4. The production method according to claim 3, wherein the electric furnace smelting is specifically: the end point C of the electric furnace is controlled to be 0.06-0.20 percent, and P is less than or equal to 0.010 percent.

5. The production method according to claim 3, characterized in that the LF furnace refining: adding premelted refining slag and lime for slagging, wherein the alkalinity is R3-6, and the white slag time is more than or equal to 20 minutes.

6. The production method according to claim 3, wherein the RH vacuum degassing: if the vacuum degree is less than or equal to 100 Pa, the vacuum holding time is more than or equal to 10 minutes; if the vacuum degree is more than 100 Pa and less than or equal to 200 Pa, the vacuum retention time is more than or equal to 15 minutes; the vacuum later-period holding time is more than or equal to 10 minutes.

7. The production method according to claim 3, wherein the round billet continuous casting: the pressure of the primary cooling water is 5.0-9.0bar, and the pressure of the secondary cooling water is 9.0-15.0 bar.

8. The production method according to claim 3, wherein the temperature of the soaking section of the heating furnace is controlled to be 1200-1300 ℃, the total heating time is controlled to be 250-350min, the initial rolling temperature is controlled to be 1100 +/-50 ℃, and the temperature of the pile cooling after rolling is more than or equal to 400 ℃.

9. The production method according to claim 3, wherein the high-line heating furnace heats: the heating temperature is controlled at 1100-1250 ℃, and the soaking time is more than 30 min.

10. The production method as claimed in claim 3 or 8, wherein the controlled rolling and controlled cooling are carried out at a start rolling temperature of 950 ℃ and 1030 ℃.

11. The production method according to claim 3, characterized in that the reducing sizing machine processes are in particular: reducing the diameter temperature 770-810 ℃.

12. A production method according to claim 3, characterized in that said laying head treatment is in particular: the spinning temperature is 760-780 ℃, and then the spinning solution is slowly cooled in a heat-preserving cover, wherein the cooling rate is 0.5-0.9 ℃/s.

13. A heat treatment method for the steel for high-strength bolts of grade 15.9 according to claim 1 or 2, characterized by comprising a thermomechanical treatment and a high-frequency tempering heat treatment.

14. The heat treatment method according to claim 13, wherein the thermomechanical heat treatment comprises the following specific processes: the temperature is kept at 1000-1200 ℃ for 280-320 seconds, then the temperature is cooled to 750-850 ℃ at the cooling speed of 4-7 ℃/s, the deformation treatment is carried out for 20-50 percent, and then the water cooling is carried out.

15. The heat treatment method according to claim 13 or 14, wherein the high-frequency tempering heat treatment comprises the following specific processes: an electron tube high-frequency heating power supply is adopted, the temperature rise time is 15-25s, the heat preservation temperature is 500-.