CN110923545B

CN110923545B - High-weather-resistance cold forging steel for 12.9-grade fastener and production method thereof

Info

Publication number: CN110923545B
Application number: CN201911247222.2A
Authority: CN
Inventors: 汪开忠; 姜婷; 于同仁; 郭湛; 张晓瑞; 龚梦强; 尹德福; 余良其; 丁雷; 孙凯; 吴建曦
Original assignee: Maanshan Iron and Steel Co Ltd
Current assignee: Maanshan Iron and Steel Co Ltd
Priority date: 2019-12-09
Filing date: 2019-12-09
Publication date: 2021-06-22
Anticipated expiration: 2039-12-09
Also published as: CN110923545A

Abstract

The invention discloses high weather-resistant cold forging steel for a 12.9-grade fastener and a production method thereof, belonging to the technical field of cold forging steel production. The main chemical components and the mass percentage content are as follows: c: 0.35-0.45%, Si: 0.30-0.50%, Mn: 0.60% -1.00%, Cr: 0.60% -0.83%, Ni: 0.50-0.80%, Cu: 0.20-0.40%, Mo: 0.05-0.15%, Ti: 0.01% -0.06%, Alt: 0.015% -0.040%, RE: 0.01% -0.10%, P: 0.010-0.030 percent of the total weight of the alloy, less than or equal to 0.0015 percent of O, less than or equal to 0.006 percent of N, and the balance of Fe and inevitable impurities. Tensile strength Rm is more than or equal to 1220MPa, yield strength R_p0.2The elongation A after fracture is more than or equal to 1100MPa, the face shrinkage Z is more than or equal to 44%, the yield ratio is more than or equal to 0.9, and the performance requirements of 12.9 grade cold forging steel grades are met.

Description

High-weather-resistance cold forging steel for 12.9-grade fastener and production method thereof

Technical Field

The invention belongs to the technical field of cold heading steel production, and particularly relates to high-weather-resistance cold heading steel for a 12.9-grade fastener and a production method.

Background

The cold heading steel is mainly used for producing fasteners such as bolts, screws, nuts and the like, railway construction in China keeps developing rapidly, new high-specification railways and existing lines are transformed into high-speed motor train units, high-power locomotives, heavy goods vehicles and the like, new market demands are provided for the fasteners, and market space is provided for the fasteners due to the updating of in-service equipment on railway lines. The Chinese territory is wide, the natural climate environment is complex, along with the development of the rail transit career, higher requirements are put forward on the cold forging steel for the fastener, the cold forging steel material with excellent weather resistance is urgently needed to be developed, and the fields of wind power, bridges and the like in China also have urgent requirements on the steel with high weather resistance for the fastener.

At present, the weather-resistant building steel such as plates and section steel in China is researched more, and the product is relatively mature. Although some research is accumulated on weather-resistant cold heading steel, most of the weather-resistant cold heading steel has lower strength level or higher cost and is not widely applied, and the research on steel for fasteners with high weather resistance is almost blank. With the complication of the application environment of the fastener for rail transit, the development of a high-strength cold forging steel material with high weather resistance at low cost is urgently needed. In the process of manufacturing the weathering steel, years of experiments show that when the weather resistance index I is larger than or equal to 7.0, the steel shows high weather resistance, the strength of the cold heading steel is improved under the condition of ensuring high weather resistance, and the problem to be solved urgently in manufacturing high-performance fasteners is solved.

The Chinese patent application numbers are: 200710192043.4, publication date is: 2008.07.09 which discloses a steel for high-strength corrosion-resistant fastener, the chemical composition (weight percentage) of the material is: c: 0.14% -0.17%, Si: 0.17% -0.24%, Mn: 0.50-0.70%, Cr: 0.85% -0.95%, Ni: 0.25-0.35%, Cu: 0.20-0.25%, Al: 0.25 to 0.35 percent of the total weight of the alloy, and the balance of Fe. The fastener made of the steel produced by the method has better oxidation resistance and corrosion resistance, the yield strength reaches 320-370 MPa, the tensile strength is 490-520 MPa, the fastener has good processing technological properties such as cold drawing, cold heading, hot upsetting and the like, and simultaneously has better oxidation resistance and corrosion resistance, and can meet the working requirements under high stress and high relaxation conditions in a corrosion environment; however, the tensile strength is 520MPa or less, and the method cannot be used for manufacturing a high-strength fastener.

The Chinese patent application numbers are: 200810120807.3, publication date is: 2009.04.01A 08CrNiCu low-alloy high-strength annealing-free cold forging steel with atmospheric corrosion resistance and a production process thereof comprises the following chemical components in percentage by weight: 0.5-0.90%, Si: 0.15 to 0.25%, Mn: 0.55-0.65%, P: 0.005-0.025%, S: 0.005-0.02%, Cr: 0.65-0.75%, Ni: 0.25-0.30%, Cu: 0.35-0.40%, and the balance of Fe and inevitable impurities. The produced steel can resist oxidation corrosion on the surface of the material in the atmosphere and water environment, and has longer service life; the quenching-free direct cold heading forming process does not need a softening annealing procedure before cold machining, so that the energy consumption is greatly reduced, the metal consumption is low, the production period is shortened, and the working efficiency is high; the cost is reduced due to the reduction of the addition amount of the alloy, but the strength is high, so that the alloy can be used for processing 8.8-grade bolts and matched nuts; but the strength grade is not high, and the method cannot meet the increasing strength requirement of cold forging steel.

The Chinese patent application numbers are: 201711215120.3, publication date is: 2018.05.25 'an anti-delayed fracture 1040 MPa-grade weather-proof bolt', which comprises the following chemical components in percentage by mass: c: 0.21 to 0.32, Si: 0.10 to 0.50, Mn: 0.60 to 1.00, P: 0.008 to 0.020, S: less than or equal to 0.005, Cr: 0.82 to 1.20, Ni: 0.25 to 0.50, Cu: 0.25 to 0.50, Mo: 0.05 to 0.20, Nb: 0.015 to 0.060, V: 0.015 to 0.090, Ti: 0.008-0.035, B: 0.0008 to 0.0035, Al: 0.015 to 0.040, Ca: 0.003 to 0.007, Zr: 0.015 to 0.045, RE: 0.010-0.045, and the balance of Fe and inevitable impurities; the metal raw material is prepared into the steel material for the delayed fracture resistant and weather resistant bolt, which can resist delayed fracture and atmospheric corrosion; however, the weathering index I value of the steel in the method is basically below 7.0, and the steel only has general weathering resistance.

Disclosure of Invention

1. Problems to be solved

Aiming at the problem that the existing high-weather-resistance cold heading steel is not high in strength, the invention provides high-weather-resistance cold heading steel for a 12.9-grade fastener; tensile strength Rm is more than or equal to 1220MPa, yield strength R_p0.2The elongation A after fracture is more than or equal to 1100MPa, the face shrinkage Z is more than or equal to 44%, the yield ratio is more than or equal to 0.9, and the performance requirements of 12.9 grade cold forging steel grades are met.

2. Technical scheme

In order to solve the problems, the technical scheme adopted by the invention is as follows:

the invention relates to high weather-resistant cold heading steel for a 12.9-grade fastener, which comprises the following main chemical components in percentage by mass: c: 0.35-0.45%, Si: 0.30-0.50%, Mn: 0.60% -1.00%, Cr: 0.60% -0.83%, Ni: 0.50-0.80%, Cu: 0.20-0.40%, Mo: 0.05-0.15%, Ti: 0.01% -0.06%, Alt: 0.015% -0.040%, RE: 0.01% -0.10%, P: 0.010-0.030 percent of the total weight of the alloy, less than or equal to 0.0015 percent of O, less than or equal to 0.006 percent of N, and the balance of Fe and inevitable impurities.

As a further illustration of the invention, the chemical composition satisfies the following weather resistance calculation relationship:

26.01(％Cu)+3.88(％Ni)+1.20(％Cr)+1.49(％Si)+17.28(％P)-7.29(％Cu)(％Ni)-9.10(％Ni)(％P)-33.39(％Cu)2)≥7.0。

the invention discloses a production method of high weather-resistant cold heading steel for a 12.9-grade fastener, which comprises the following steps of:

s1, smelting in a converter or an electric furnace;

s2, refining in an LF furnace;

s3, RH or VD vacuum degassing;

s4, square billet continuous casting;

s5, hot charging of a casting blank, and heating the casting blank in a heating furnace;

s6, rolling the high-speed wire rod at low temperature to prepare a wire rod;

and S7, slowly cooling the wire rod according to a stelmor cooling line to obtain a finished wire rod product.

As a further explanation of the present invention, in the step S1, oxygen is determined before tapping and steel is left during tapping in the electric furnace or converter smelting.

In step S2, in the LF furnace refining process, elements such as C, Si, Mn, Cr, Ni, Cu, V, RE, and P are adjusted to target values.

As a further explanation of the present invention, in the step S3, the degassing time is not less than 15 minutes, and the [ H ] content after the vacuum treatment is not more than 1.5 ppm.

As a further explanation of the invention, in the step S4, the temperature of the tundish molten steel is controlled to be 10-40 ℃ above the liquidus line, protective casting is adopted, the content of Alt is controlled, and N is prevented from being increased.

In the step S5, the soaking temperature of the heating furnace is 1030 to 1100 ℃.

As a further description of the present invention, in step S6, the high-speed wire rod low-temperature rolling includes:

a. starting a rolling stage, wherein the rolling temperature is 890-960 ℃;

b. in the diameter reducing and sizing stage, the diameter reducing and sizing temperature is 770-810 ℃;

c. in the spinning stage, the spinning temperature is 780-820 ℃.

As a further description of the present invention, in step S7, when the wire rod is slowly cooled according to stelmor cooling line, the wire rod is placed in a heat-preserving cover for heat preservation, the heat-preserving cover is completely closed, and the phase change cooling speed of the wire rod is controlled within 1 ℃/S.

3. Advantageous effects

Compared with the prior art, the invention has the beneficial effects that:

(1) according to the 12.9-grade high-weather-resistance cold heading steel for the fasteners, in order to ensure that the steel for the fasteners achieves high weather resistance, the steel needs to be matched according to the contribution of the corrosion resistance of each element to strengthen and generate a compact rust layer with strong adhesion, so that the corrosion resistance is better increased, according to a field test for many years, the value I is controlled to be more than or equal to 7.0, the steel has high weather resistance, and is further preferably more than or equal to 7.2, the carbon equivalent is calculated at the same time, and the strength requirement and other mechanical property requirements of the cold forged steel are met under the conditions of reducing the cost and reducing the addition amount of alloy elements;

(2) the high weather-resistant cold forging steel for the 12.9-grade fastener has the tensile strength Rm of more than or equal to 1220MPa and the yield strength R_p0.2The elongation A after fracture is more than or equal to 1100MPa, the face shrinkage Z is more than or equal to 44%, the yield ratio is more than or equal to 0.9, and the corresponding requirements of 12.9 grade cold forging steel grades are met;

(3) according to the production method of the high weather-resistant cold heading steel for the 12.9-grade fastener, the soaking temperature of the heating furnace is controlled to be 1030-1100 ℃, and when the soaking temperature is lower than 1030 ℃, insoluble elements such as Ni and Cu cannot be dissolved in austenite, so that the steel is segregated at a crystal boundary during rolling, the steel is high in brittleness, and steel heaping is likely to occur; the soaking temperature is higher than 1100 ℃, austenite grains begin to become coarse, and the decarburization tendency is greatly increased;

(4) according to the production method of the high weather-resistant cold heading steel for the 12.9-grade fastener, the initial rolling temperature is controlled to be 890-960 ℃, and when the initial rolling temperature is lower than 890 ℃, the load on steel rolling equipment is large, and the rolling is difficult; the initial rolling temperature is higher than 960 ℃, the temperature drop in the rolling process is large, abnormal structures are easy to form, and the performance of the cold forging steel is influenced;

(5) the invention relates to a production method of high weather-resistant cold heading steel for a 12.9-grade fastener, which is characterized in that the reduced diameter temperature is controlled to be 770-810 ℃, and when the reduced diameter temperature is lower than 770 ℃, the steel enters a phase change stage before entering a heat preservation section, and a martensite structure is possibly formed in the air; when the sizing temperature is reduced to be higher than 810 ℃, the subsequent spinning temperature is possibly increased, the phase transition temperature is not reached before entering the heat-insulating cover, and the complete phase transition on a stelmor line is difficult to complete, so that the generation of abnormal structures is caused, and the performance of cold forging steel is influenced;

(6) according to the production method of the high weather-resistant cold heading steel for the 12.9-grade fastener, the spinning temperature is controlled to be 780-820 ℃, and when the spinning temperature is lower than 780 ℃, the steel enters a phase change stage before entering a heat-insulating cover; when the spinning temperature is higher than 820 ℃, the phase transition temperature is not reached before entering the heat-insulating cover, and the whole phase transition on a stelmor line is difficult to complete, so that an abnormal structure is generated, and the performance of cold forging steel is influenced;

(7) according to the production method of the high weather-resistant cold heading steel for the 12.9-grade fastener, the cover of the heat preservation section is controlled to be completely closed so as to control the phase change cooling speed of the wire rod to be within 1 ℃/s, an ideal structure of pearlite, ferrite and a small amount of bainite is obtained, and if the heat preservation cover is not completely closed, the cooling speed is difficult to guarantee, a large amount of martensite and bainite structures are easily generated, and the mechanical property of the cold heading steel is influenced.

Drawings

The technical solutions of the present invention will be described in further detail below with reference to the accompanying drawings and examples, but it should be understood that these drawings are designed for illustrative purposes only and thus do not limit the scope of the present invention.

FIG. 1 is a schematic representation of a high weather resistant cold heading steel delayed fracture specimen for a grade 12.9 fastener of the present invention.

Detailed Description

Exemplary embodiments of the present invention are described in detail below. Although these exemplary embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, it should be understood that other embodiments may be realized and that various changes to the invention may be made without departing from the spirit and scope of the present invention. The following more detailed description of the embodiments of the invention is not intended to limit the scope of the invention, as claimed, but is presented for purposes of illustration only and not limitation to describe the features and characteristics of the invention, to set forth the best mode of carrying out the invention, and to sufficiently enable one skilled in the art to practice the invention. Accordingly, the scope of the invention is to be limited only by the following claims.

The following detailed description and example embodiments of the invention are set forth.

The main chemical component compositions and the mass percentage contents of the high weather resistant cold forging steel for the 12.9-grade fastener are shown in the table 1:

TABLE 1 chemical compositions and weight percentages (mass%, balance Fe) of the examples and comparative examples

The contents of the elements in the above examples all satisfy the following ranges:

c: 0.35-0.45%, Si: 0.30-0.50%, Mn: 0.60% -1.00%, Cr: 0.60% -0.83%, Ni: 0.50-0.80%, Cu: 0.20-0.40%, Mo: 0.05-0.15%, Ti: 0.01% -0.06%, Alt: 0.015% -0.040%, RE: 0.01% -0.10%, P: 0.010-0.030 percent of the total weight of the alloy, less than or equal to 0.0015 percent of O, less than or equal to 0.006 percent of N, and the balance of Fe and inevitable impurities.

The specific effects of the element composition in the scheme are as follows:

c: c is the most effective strengthening and hardenability element in steel, but if the content of C is too high, the ductility of the steel can be reduced, the delayed fracture risk of bolts is increased, the mechanical property of cold heading steel is influenced, the use of the cold heading steel is not facilitated, if the content of C is too low, the strength of the cold heading steel can be influenced, and for the high weather-resistant cold heading steel for the 12.9-grade fastener, the content of C is controlled to be 0.35-0.45%.

Si: si is an important element for strengthening in steel, the strength and the hardness of the steel are improved through solid solution, the sag resistance of the cold forging steel is improved, silicon is mainly enriched on the surface of the steel, the stability of a rust layer is improved, and the corrosion resistance is improved, but the increase of the Si element can increase the diffusion of carbon in the steel and aggravate the decarburization of steel, so that excessive Si element is not added according to the carbon content, and the Si content of the high weather-resistant cold forging steel for the 12.9-grade fastener is controlled to be 0.30-0.50%.

Mn: mn and Fe form a solid solution, so that the hardness and strength of ferrite and austenite in the steel are improved, and meanwhile, Mn is used for improving the stability of an austenite structure and remarkably improving the hardenability of the steel. However, excessive Mn lowers the plasticity of the steel. The addition of Mn is simultaneously beneficial to forming a rust layer on the surface of steel, the corrosion resistance of steel is improved, excessive Mn can cause the growth of corrosion product particles, and the corrosion rate is improved. For the 12.9-grade high-weather-resistance cold forging steel for the fastener, the Mn content is controlled to be 0.60-1.00 percent.

Cr: cr element remarkably improves the toughness and the heat strength in steel, is precipitated in a carbide form, increases hydrogen capture points and improves delayed fracture resistance. Cr can increase the hardenability of steel, but excessive Cr increases the temper brittleness tendency of steel, and Cr can form a compact oxide film on the surface of steel, thereby improving the passivation capability of steel. For the 12.9-grade high weather resistant cold heading steel for the fastener, the Cr content is controlled to be 0.60-0.83 percent.

Ni: ni can generate an infinitely miscible solid solution with Fe, has the function of enlarging a phase region, and does not form carbide. Nickel stabilizes austenite and enhances hardenability of steel. Ni is an effective element for reducing the ductile-brittle transition temperature and obviously improving the low-temperature toughness. The Ni element is a noble metal element, and excessive addition thereof results in excessive cost. For the 12.9-grade high weather resistant cold heading steel for the fastener, the Ni content is controlled to be 0.50-0.80%.

Cu: the prominent role of Cu in steel is to improve the corrosion resistance of steel. The cathode contact between the steel and the Cu secondarily precipitated on the surface can promote the anodization of the steel and form a rust layer with better protection. Copper also changes the moisture absorption of the rust layer, thereby increasing the critical humidity. However, Cu causes high crack sensitivity in steel, increases hydrogen embrittlement sensitivity of steel, and deteriorates delayed fracture resistance. For the 12.9-grade high weather resistant cold heading steel for the fastener, the Cu content is controlled to be 0.20-0.40%.

Mo: mo is generally dispersed in a matrix material as second phase particles or inclusions after steel is subjected to heat treatment, and atoms have a large adsorption effect on hydrogen, namely, the Mo has good hydrogen corrosion resistance and is an element which effectively delays delayed fracture of a fastener. Meanwhile, Mo is also an element for increasing hardenability and precipitation hardening, and effectively improves the strength of the steel after heat treatment and tempering. However, excessive Mo content deteriorates cold workability of the steel. For the 12.9-grade high-weather-resistance cold forging steel for the fastener, the Mo content is controlled to be 0.05-0.15%.

Ti: the Ti element has active performance, forms TiC and TiN in the steel to form hydrogen trapping points, has the strongest trap energy, and effectively improves the delayed fracture resistance of the high-strength steel. However, when the amount of titanium added is too large, coarse Ti (C, N) inclusions are easily formed, and the workability of the steel is deteriorated. For the 12.9-grade high weather resistant cold forging steel for the fastener, the Ti content is controlled to be 0.01-0.06%.

Al: al is a strong deoxidizing element, and simultaneously, the oxidation resistance of the steel is improved. However, as the Al content increases, the amount of coarse carbonitride-based inclusions increases, and the delayed fracture resistance decreases. The content of Alt is controlled to be 0.015-0.040%.

RE: the addition of right amount of rare earth RE in the steel can change MnS, A12O3 and other impurities into rare earth impurities, and has good deoxidation and desulfurization effects. The tiny solid particles of the rare earth RE compound provide heterogeneous crystal nuclei or are eccentrically polymerized on a crystallization interface, so that the growth of unit cells is hindered, the normal-temperature mechanical property of steel is improved, and the excessive RE effect is not obvious any more. For the 12.9-grade high-weather-resistance cold forging steel for the fastener, the RE content is controlled to be 0.01-0.10%.

P: p promotes the steel to generate an amorphous rust layer, improves the stability of the rust layer and increases the weather resistance of the steel, but the P is an element with strong segregation tendency, generally causes the co-segregation of sulfur and manganese and is harmful to the uniformity of the product structure and performance. For the 12.9-grade high weather resistant cold forging steel for the fastener, the P content is controlled to be 0.010-0.030%.

O and N: T.O forms oxide inclusions in the steel, and the T.O is controlled to be less than or equal to 0.0015 percent; fe4N is precipitated from the steel by N, the diffusion speed is slow, the steel has timeliness, and meanwhile, the cold processing performance of the steel is reduced by N, and the N is controlled to be less than or equal to 0.006%.

The above elements also need to satisfy the relation (1):

26.01(％Cu)+3.88(％Ni)+1.20(％Cr)+1.49(％Si)+17.28(％P)-7.29(％Cu)(％Ni)-9.10(％Ni)(％P)-33.39(％Cu)2)≥7.0；

relation (2): 0.70 ≦ C + (% Mn)/6+ (% Cr)/6+ (% Mo)/5+ (% Ni)/15+ (% Cu)/20+ (% Si)/10 ≦ 0.85.

Wherein the relational expression (1) is a calculation formula of weather resistance index, and the calculated value is called I value. In the invention, in order to ensure that the steel for the bolt achieves high weather resistance, the steel needs to be matched according to the contribution of the corrosion resistance of each element so as to strengthen and generate a compact rust layer with strong adhesion and better increase the corrosion resistance. According to years of field tests, the value I is controlled to be not less than 7.0, and the steel has high weather resistance, and more preferably the value I is not less than 7.2.

The calculated value of the relational expression (2) is called a beta value, is equivalent to a calculation formula of the carbon equivalent of the steel, and is obtained by summarizing steel making data of the cold forging steel according to the corresponding formula. In the present invention, the C content is first controlled to 0.45% or less, reducing the risk of hydrogen-induced delayed fracture. However, in order to make the fastener meet the requirements of strength and toughness of 12.9 grade, the carbon equivalent, that is, the β value, should be controlled to 0.7 or more, while the cost is increased when the alloy content is increased, so the upper limit is controlled to 0.85 or less.

s1, smelting in a converter or an electric furnace; during smelting in an electric furnace or a converter, oxygen is determined before tapping, and steel is left in the tapping process to avoid slag falling.

S2, refining in an LF furnace; in the refining process of the LF furnace, elements such as C, Si, Mn, Cr, Ni, Cu, V, RE, P and the like are adjusted to target values.

S3, RH or VD vacuum degassing; the degassing time is more than or equal to 15 minutes, the content of [ H ] after vacuum treatment is less than or equal to 1.5ppm, and the hydrogen embrittlement phenomenon of the bolt is avoided.

S4, square billet continuous casting; the temperature of the tundish molten steel is controlled to be 10-40 ℃ above a liquidus line, protective casting is adopted, the Alt content is controlled, N is prevented from being increased, and 150mm multiplied by 150 mm-250 mm multiplied by 250mm square billets can be continuously cast in the actual continuous casting process.

S5, hot charging of a casting blank, and heating the casting blank in a heating furnace; the soaking temperature of the heating furnace is 1030-1100 ℃. The soaking temperature of the heating furnace is controlled to be 1030-1100 ℃, and when the soaking temperature is lower than 1030 ℃, insoluble elements such as Ni, Cu and the like cannot be dissolved in austenite, so that the steel is segregated at a grain boundary during rolling, the steel is relatively brittle, and steel piling is likely to occur; the soaking temperature is higher than 1100 ℃, austenite grains begin to become coarse, and the decarburization tendency is greatly increased.

S6, rolling the high-speed wire rod at low temperature to prepare a wire rod; the high-speed wire low-temperature rolling comprises the following steps:

a. starting a rolling stage, wherein the rolling temperature is 890-960 ℃; controlling the initial rolling temperature to be 890-960 ℃, and when the initial rolling temperature is lower than 890 ℃, the load on steel rolling equipment is large, and the rolling is difficult; the initial rolling temperature is higher than 960 ℃, the temperature drop in the rolling process is large, abnormal structures are easy to form, and the performance of the cold forging steel is influenced.

b. In the diameter reducing and sizing stage, the diameter reducing and sizing temperature is 770-810 ℃; controlling the temperature of the reducing diameter to be 770-810 ℃, and when the temperature of the reducing diameter is lower than 770 ℃, entering a phase change stage before entering a heat preservation section, wherein a martensite structure can be formed in the air; when the reducing diameter temperature is higher than 810 ℃, the subsequent spinning temperature is possibly increased, the phase transition temperature is not reached before entering the heat-insulating cover, and the complete phase transition on a stelmor line is difficult to complete, so that the generation of abnormal structures is caused, and the performance of the cold heading steel is influenced.

c. In the spinning stage, the spinning temperature is 780-820 ℃; controlling the spinning temperature to be 780-820 ℃, and entering a phase change stage before entering a heat-insulating cover when the spinning temperature is lower than 780 ℃; when the spinning temperature is higher than 820 ℃, the phase transition temperature is not reached before entering the heat-insulating cover, and the whole phase transition on a stelmor line is difficult to complete, so that an abnormal structure is generated, and the performance of cold forging steel is influenced;

s7, slowly cooling the wire rod according to a stelmor cooling line to obtain a finished wire rod product; and when the wire rod is slowly cooled according to the stelmor cooling line, placing the wire rod in a heat-insulating cover for heat insulation, completely closing the heat-insulating cover, and controlling the phase change cooling speed of the wire rod within 1 ℃/s. The ideal structure of pearlite, ferrite and a small amount of bainite is obtained, and if the heat-insulating cover is not completely closed, the cooling speed is difficult to ensure, a large amount of martensite and bainite structures are easy to generate, and the mechanical property of the cold forging steel is influenced.

The method for detecting the performance of the high weather-resistant cold heading steel for the fastener comprises the following steps:

organizing: taking a test sample with the length of 15mm from a wire rod, polishing the cross section, corroding by using 4% nitric acid alcohol, and performing organization evaluation according to GB/T13298 metal microscopic organization inspection method; whether the wire rod has excellent use performances such as drawing and cold heading during the processing of the fastener can be judged through the structure.

Cold heading: the wire rod is taken out for cold upsetting according to the following requirements: x h1/h 1/3; (wherein h is the height of the sample before cold upsetting (twice the diameter of the wire rod) and h1 is the height of the sample after cold upsetting.) after the cold upsetting test, no cracks, fissures, cracks and hairline defects can be seen by naked eyes on the surface of the sample, as shown in figure 1, 30 groups of cold upsetting tests are carried out on each number, and the cracking rate is counted. The cold heading performance of the fastener during processing can be judged through the cold heading cracking rate.

Tensile properties after heat treatment: the wire rod adopts the following quenching and tempering heat treatment process: quenching at 870 ℃, oil cooling, tempering at 540 ℃ and air cooling. Straightening after heat treatment, performing a tensile test, and testing the tensile strength Rm and the yield strength R_p0.2Elongation after fracture A, face shrinkage Z, yield ratio, etc. Judging whether the steel meets the requirement of 12.9 grade or not through the stretching performance after heat treatment.

Weight loss rate in 100h week immersion test: sampling on the square billet, carrying out quenching and tempering heat treatment (the heat treatment process is the same as the above), processing the sample according to TB/T2375 & lt 1993 & gt periodic infiltration corrosion test method for weathering steel for railway, completing 100-hour immersion test, calculating corrosion weight loss rate, and calculating the average value of 10 groups of each number. The weather resistance of the steel can be judged through the weekly immersion test, and the weight loss rate of the 100h weekly immersion test is less than or equal to 0.5m²High weather resistance at h, 0.5m²H < weight loss rate not more than 1.2m²H is judged as general weather resistance, and the weight loss rate is more than 1.2m²H is judged to have no weather resistance. Wherein the corrosion weight loss ratio (W) is calculated according to the following formula:

in the formula: w-weight loss ratio, g/(m)²·h)；G₀-sample original weight, g; g₁-the post-test weight of the sample, g; a-specimen length, mm; b-sample width, mm; c-specimen thickness, mm; t-test time, h.

Example 1

The main chemical composition and the mass percentage content of the high weather resistant cold heading steel for the 12.9-grade fastener in the embodiment are shown in the embodiment 1 in the table 1.

The production method of the high weather resistant cold heading steel for the 12.9-grade fastener comprises the following steps based on the components of the embodiment:

s1, smelting in an electric furnace; in the smelting process, oxygen is determined before tapping, and steel is left in the tapping process to avoid slag falling.

S3, RH vacuum degassing; the degassing time is more than or equal to 15 minutes, the content of [ H ] after vacuum treatment is less than or equal to 1.5ppm, and the hydrogen embrittlement phenomenon of the bolt is avoided.

S4, square billet continuous casting; the temperature of the tundish molten steel is controlled to be 10-40 ℃ above a liquidus line, protective casting is adopted, the content of Alt is controlled, N is prevented from being increased, and the size of a continuous casting square billet is 220mm multiplied by 220mm

S5, hot charging of a casting blank, and heating the casting blank in a heating furnace; the soaking temperature of the heating furnace is 1044 ℃.

a. starting a rolling stage, wherein the rolling temperature is 892 ℃;

b. reducing and sizing at 773 deg.C;

c. in the spinning stage, the spinning temperature is 781 ℃;

s7, slowly cooling the wire rod according to a stelmor cooling line to obtain a finished wire rod product; and when the wire rod is slowly cooled according to the stelmor cooling line, placing the wire rod in a heat-insulating cover for heat insulation, completely closing the heat-insulating cover, and controlling the phase change cooling speed of the wire rod within 1 ℃/s.

The wire rod obtained in this example had a size of 12mm, and the metallographic structure obtained was pearlite + ferrite + a small amount of bainite.

Example 2

The main chemical composition and the mass percentage content of the high weather resistant cold heading steel for the 12.9-grade fastener in the embodiment are shown in the embodiment 2 in the table 1.

S3, VD vacuum degassing; the degassing time is more than or equal to 15 minutes, the content of [ H ] after vacuum treatment is less than or equal to 1.5ppm, and the hydrogen embrittlement phenomenon of the bolt is avoided.

S5, hot charging of a casting blank, and heating the casting blank in a heating furnace; the soaking temperature of the heating furnace is 1031 ℃.

a. starting a rolling stage, wherein the rolling temperature is 910 ℃;

b. in the diameter reducing and sizing stage, the diameter reducing and sizing temperature is 791 ℃;

c. in the spinning stage, the spinning temperature is 799 ℃;

The wire rod obtained in this example had a size of 6.5mm, and the metallographic structure obtained was pearlite + ferrite + a small amount of bainite.

Example 3

The main chemical composition and the mass percentage content of the high weather resistant cold heading steel for the 12.9-grade fastener in the embodiment are shown in the embodiment 3 in the table 1.

s1, smelting in a converter; in the smelting process, oxygen is determined before tapping, and steel is left in the tapping process to avoid slag falling.

S5, hot charging of a casting blank, and heating the casting blank in a heating furnace; the soaking temperature of the heating furnace is 1092 ℃.

a. starting a rolling stage, wherein the rolling temperature is 957 ℃;

b. reducing and sizing at 801 deg.c;

c. in the spinning stage, the spinning temperature is 819 ℃;

The wire rod obtained in this example has a size of 25mm, and the obtained metallurgical structure is pearlite + ferrite + a small amount of bainite.

Example 4

The main chemical composition and the mass percentage content of the high weather resistant cold heading steel for the 12.9-grade fastener in the embodiment are shown in the embodiment 4 in the table 1.

S5, hot charging of a casting blank, and heating the casting blank in a heating furnace; the soaking temperature of the heating furnace is 1085 ℃.

a. beginning a rolling stage, wherein the rolling temperature is 943 ℃;

b. reducing and sizing at 808 ℃;

c. in the spinning stage, the spinning temperature is 815 ℃;

The wire rod obtained in this example has a size of 24mm, and the obtained metallurgical structure is pearlite + ferrite + a small amount of bainite.

Example 5

The main chemical composition and the mass percentage content of the high weather resistant cold heading steel for the 12.9-grade fastener in the embodiment are shown in the embodiment 5 in the table 1.

S4, square billet continuous casting; the temperature of the tundish molten steel is controlled to be 10-40 ℃ above a liquidus line, protective casting is adopted, the content of Alt is controlled, N is prevented from being increased, and the size of a continuous casting square billet is 250mm multiplied by 250mm

S5, hot charging of a casting blank, and heating the casting blank in a heating furnace; the soaking temperature of the heating furnace is 1052 ℃.

a. starting a rolling stage, wherein the rolling temperature is 937 ℃;

b. reducing and sizing at 801 deg.c;

c. in the spinning stage, the spinning temperature is 815 ℃;

Example 6

The main chemical composition and the mass percentage content of the high weather resistant cold heading steel for the 12.9-grade fastener in the embodiment are shown in the embodiment 6 in the table 1.

S5, hot charging of a casting blank, and heating the casting blank in a heating furnace; the soaking temperature of the heating furnace is 1048 ℃.

a. starting a rolling stage, wherein the rolling temperature is 925 ℃;

b. reducing and sizing at 808 ℃;

c. in the spinning stage, the spinning temperature is 799 ℃;

The wire rod obtained in this example has a size of 18mm, and the obtained metallurgical structure is pearlite + ferrite + a small amount of bainite.

Example 7

The main chemical composition and the mass percentage content of the high weather resistant cold heading steel for the 12.9 grade fastener of the embodiment are shown in the example 7 in the table 1.

S4, square billet continuous casting; the temperature of the tundish molten steel is controlled to be 10-40 ℃ above a liquidus line, protective casting is adopted, the content of Alt is controlled, N is prevented from being increased, and the size of a continuous casting square billet is 150mm multiplied by 150mm

S5, hot charging of a casting blank, and heating the casting blank in a heating furnace; the soaking temperature of the heating furnace is 1039 ℃.

a. starting a rolling stage, wherein the rolling temperature is 899 ℃;

b. reducing and sizing at 785 ℃;

c. in the spinning stage, the spinning temperature is 806 ℃;

Example 8

The main chemical composition and the mass percentage content of the high weather resistant cold heading steel for the 12.9 grade fastener of the embodiment are shown in the example 8 in the table 1.

S5, hot charging of a casting blank, and heating the casting blank in a heating furnace; the soaking temperature of the heating furnace is 1061 ℃.

a. starting a rolling stage, wherein the rolling temperature is 910 ℃;

b. in the diameter reducing and sizing stage, the diameter reducing and sizing temperature is 798 ℃;

c. in the spinning stage, the spinning temperature is 813 ℃;

The wire rod obtained in this example has a size of 16mm, and the obtained metallurgical structure is pearlite + ferrite + a small amount of bainite.

Example 9

The main chemical composition and the mass percentage content of the high weather resistant cold heading steel for the 12.9 grade fastener of the embodiment are shown in the example 9 in the table 1.

S5, hot charging of a casting blank, and heating the casting blank in a heating furnace; the soaking temperature of the heating furnace is 1030 ℃.

a. starting a rolling stage, wherein the rolling temperature is 890 ℃;

b. reducing the sizing temperature to 770 ℃ in a sizing reducing stage;

c. in the spinning stage, the spinning temperature is 780 ℃;

Example 10

S5, hot charging of a casting blank, and heating the casting blank in a heating furnace; the soaking temperature of the heating furnace is 1100 ℃.

a. starting a rolling stage, wherein the rolling temperature is 960 ℃;

b. reducing and sizing at 810 ℃;

c. in the spinning stage, the spinning temperature is 820 ℃;

Example 11

S5, hot charging of a casting blank, and heating the casting blank in a heating furnace; the soaking temperature of the heating furnace is 1050 ℃.

a. starting a rolling stage, wherein the rolling temperature is 931 ℃;

b. reducing and sizing at 800 ℃;

c. in the spinning stage, the spinning temperature is 808 ℃;

Comparative example 1

The main chemical composition and the mass percentage content of the high weather resistant cold heading steel for the 12.9-grade fastener of the embodiment are shown in comparative example 1 in table 1.

S5, hot charging of a casting blank, and heating the casting blank in a heating furnace; the soaking temperature of the heating furnace is 1156 ℃.

a. starting a rolling stage, wherein the rolling temperature is 992 ℃;

b. reducing and sizing at 889 ℃;

c. in the spinning stage, the spinning temperature is 887 ℃;

The wire rod obtained in this example had a dimension of 25mm, and the metallographic structure obtained was martensite + bainite + ferrite.

Comparative example 2

The main chemical composition and the mass percentage content of the high weather resistant cold heading steel for the 12.9-grade fastener of the embodiment are shown in comparative example 2 in table 1.

S5, hot charging of a casting blank, and heating the casting blank in a heating furnace; the soaking temperature of the heating furnace is 1045 ℃.

a. starting a rolling stage, wherein the rolling temperature is 935 ℃;

b. reducing and sizing at 912 ℃;

c. in the spinning stage, the spinning temperature is 906 ℃;

The wire rod obtained in this example had a size of 25mm, and the metallographic structure obtained was bainite + pearlite + ferrite.

Comparative example 3

The main chemical composition and the mass percentage content of the high weather resistant cold heading steel for the 12.9-grade fastener of the embodiment are shown in comparative example 3 in table 1.

S5, hot charging of a casting blank, and heating the casting blank in a heating furnace; the soaking temperature of the heating furnace is 1053 ℃.

a. starting a rolling stage, wherein the rolling temperature is 935 ℃;

b. reducing the sizing temperature to 803 ℃;

c. in the spinning stage, the spinning temperature is 814 ℃;

s7, slowly cooling the wire rod according to a stelmor cooling line to obtain a finished wire rod product; and when the wire rod is slowly cooled according to a stelmor cooling line, placing the wire rod in a heat-preserving cover for heat preservation, and fully opening the heat-preserving cover.

The wire rod obtained in this example had a size of 20mm, and the obtained metallographic structure was martensite + bainite + ferrite.

Comparative example 4

The main chemical composition and the mass percentage content of the high weather resistant cold heading steel for the 12.9-grade fastener of the embodiment are shown in comparative example 4 in table 1.

S5, hot charging of a casting blank, and heating the casting blank in a heating furnace; the soaking temperature of the heating furnace is 1038 ℃.

a. starting a rolling stage, wherein the rolling temperature is 922 ℃;

b. reducing the sizing temperature to 803 ℃;

c. in the spinning stage, the spinning temperature is 801 ℃;

s7, slowly cooling the wire rod according to a stelmor cooling line to obtain a finished wire rod product; and when the wire rod is slowly cooled according to a stelmor cooling line, placing the wire rod in a heat-preserving cover for heat preservation, wherein the heat-preserving cover is a whole half.

The wire rod obtained in this example had a size of 13.5mm, and the obtained metallurgical structure was bainite + pearlite + ferrite.

Comparative example 5

The main chemical composition and the mass percentage content of the high weather resistant cold heading steel for the 12.9-grade fastener of the embodiment are shown in comparative example 5 in table 1.

S5, hot charging of a casting blank, and heating the casting blank in a heating furnace; the soaking temperature of the heating furnace is 1072 ℃.

a. starting a rolling stage, wherein the rolling temperature is 917 ℃;

b. reducing and sizing at 804 ℃;

c. in the spinning stage, the spinning temperature is 806 ℃;

The wire rod obtained in this example has a size of 8mm, and the obtained metallurgical structure is pearlite + ferrite + a small amount of bainite.

Comparative example 6

The main chemical composition and the mass percentage content of the high weather resistant cold heading steel for the grade 12.9 fastener of the embodiment are shown in comparative example 6 in table 1.

S5, hot charging of a casting blank, and heating the casting blank in a heating furnace; the soaking temperature of the heating furnace is 1036 ℃.

a. starting a rolling stage, wherein the rolling temperature is 910 ℃;

b. in the diameter reducing and sizing stage, the diameter reducing and sizing temperature is 794 ℃;

c. in the spinning stage, the spinning temperature is 798 ℃;

The wire rod obtained in this example had a size of 8mm, and the metallographic structure obtained was ferrite + pearlite.

Comparative example 7

The main chemical composition and the mass percentage content of the high weather resistant cold heading steel for the grade 12.9 fastener of the embodiment are shown in comparative example 7 in table 1.

a. starting a rolling stage, wherein the rolling temperature is 904 ℃;

b. reducing and sizing at 801 deg.c;

c. in the spinning stage, the spinning temperature is 795 ℃;

The wire rod obtained in this example had a size of 10mm, and the metallographic structure obtained was pearlite + ferrite + a small amount of bainite.

Comparative example 8

The main chemical composition and the mass percentage content of the high weather resistant cold heading steel for the grade 12.9 fastener of the embodiment are shown in comparative example 8 in table 1.

a. starting a rolling stage, wherein the rolling temperature is 908 ℃;

b. in the diameter reducing and sizing stage, the diameter reducing and sizing temperature is 796 ℃;

c. in the spinning stage, the spinning temperature is 798 ℃;

The wire rod obtained in this example has a size of 20mm, and the obtained metallurgical structure is pearlite + ferrite + a small amount of bainite.

Comparative example 9

The main chemical composition and the mass percentage content of the high weather resistant cold heading steel for the 12.9-grade fastener of the embodiment are shown in comparative example 9 in table 1.

S5, hot charging of a casting blank, and heating the casting blank in a heating furnace; the soaking temperature of the heating furnace is 1041 ℃.

a. starting a rolling stage, wherein the rolling temperature is 900 ℃;

c. in the spinning stage, the spinning temperature is 797 ℃;

The results of the weather resistance index I and the β values calculated for the above examples and comparative examples are shown in table 2:

TABLE 2 weather resistance index I and beta values calculated for the ingredients of the examples and comparative examples

	Value of I	Beta value
			Example 1	7.39	0.76
Example 2	7.39	0.76
			Example 3	7.38	0.79
Example 4	7.38	0.79
			Example 5	7.57	0.71
Example 6	7.57	0.71
			Example 7	7.35	0.78
Example 8	7.35	0.78
			Example 9	7.39	0.76
Example 10	7.32	0.79
			Example 11	7.45	0.78
Comparative example 1	7.39	0.76
			Comparative example 2	7.38	0.79
Comparative example 3	7.57	0.71
			Comparative example 4	7.35	0.78
Comparative example 5	1.62	0.78
			Comparative example 6	7.18	0.57
Comparative example 7	1.80	0.74
			Comparative example 8	6.51	0.73
Comparative example 9	7.04	0.66

The results of the performance tests performed in the above examples and comparative examples are shown in table 3:

TABLE 3 results of performance test of each example and comparative example

In the above examples and comparative examples:

the cold heading steel of the embodiments 1 to 11 has appropriate chemical composition, production method and corresponding process parameters, the chemical composition of the cold heading steel ensures that the weather resistance index I is more than or equal to 7.0, the steel has the strength grade of 12.9 grade and above, and simultaneously has high weather resistance and excellent delayed fracture resistance, and the production method also ensures that the wire rod structure is pearlite, ferrite and a small amount of bainite, thereby realizing excellent service performance in the downstream fastener processing, simplifying annealing and saving cost.

The cold heading steel of comparative examples 1 to 4 has proper chemical components, and the components meet the requirement that the weather resistance index I is more than or equal to 7.0, but in the production process, the defects of cold heading steel performance caused by improper use of heating temperature, rolling temperature or heat-insulating cover are overcome, in the comparative example 1, because the heating temperature is too high, the surface of a wire rod is subjected to full decarburization, the surface strength of a wire rod is lower, the cross section strength is not uniformly distributed, the toughness of the material is reduced, and simultaneously, martensite and bainite tissues exist in the tissues, so that cold heading is easy to crack; comparative examples 2-4 are that the cooling speed is too high due to higher rolling temperature or incomplete closing of a cover, a large amount of bainite and martensite exist in a wire rod microstructure, the cracking rate is high when a standard part is upset, the annealing and heat preservation time of a user is obviously increased, and the processing cost of the user is obviously increased.

Comparative examples 5 and 7 are examples in which the chemical composition is not properly controlled and the weather resistance index I is too low, and although the strength grade of the steel reaches 12.9 grade by adding a proper amount of strengthening elements such as C, Mn, Cr, V, or Mo, the corrosion weight loss rate is high in the similar ratio and the weather resistance is poor in the immersion test. Meanwhile, the comparative example 5 mainly depends on C, Si and Mn elements to improve the strength, so that the toughness and the delayed fracture resistance are poorer; comparative example 7 is that no RE element was added, and brittle inclusions were large, resulting in deviation of cold heading property.

Comparative example 6 is an example in which the weather resistance index I is satisfactory but the chemical composition is not proper, and the strength grade is low and cannot satisfy the requirement of grade 12.9.

Comparative example 8 is an example with chemical components in the required range, but the weather resistance index I is lower due to improper proportioning, the corrosion weight loss rate is higher, and the weather resistance is general.

Comparative example 9 is an example in which the chemical composition is within the required range, but the carbon equivalent β value is lower due to improper proportioning, and the strength grade is reduced and the requirement of grade 12.9 is not met.

Claims

1. The high-weather-resistance cold forging steel for the 12.9-grade fastener is characterized by comprising the following main chemical components in percentage by mass: c: 0.35-0.45%, Si: 0.30-0.50%, Mn: 0.60% -1.00%, Cr: 0.60% -0.83%, Ni: 0.51-0.80%, Cu: 0.20-0.40%, Mo: 0.05-0.15%, Ti: 0.01% -0.06%, Alt: 0.015% -0.040%, RE: 0.01% -0.10%, P: 0.010-0.030 percent of the total weight of the alloy, less than or equal to 0.0015 percent of O, less than or equal to 0.006 percent of N, and the balance of Fe and inevitable impurities; the chemical composition satisfies the following weather resistance calculation formula:

26.01(％Cu)+3.88(％Ni)+1.20(％Cr)+1.49(％Si)+17.28(％P)-7.29(％Cu)(％Ni)-9.10(％Ni)(％P)-33.39(％Cu)²)≥7.0。

2. a production method of high weather resistant cold heading steel for 12.9-grade fasteners is characterized by comprising the following steps:

s1, smelting in a converter or an electric furnace;

s2, refining in an LF furnace;

s3, RH or VD vacuum degassing;

s4, square billet continuous casting;

s6, rolling the high-speed wire rod at low temperature to prepare a wire rod;

3. The method for producing high weather-resistant cold heading steel for fasteners of grade 12.9 as claimed in claim 2, wherein in step S1, oxygen is determined before tapping during electric furnace or converter smelting, and steel is left in the tapping process.

4. The method of claim 2, wherein in step S2, elements such as C, Si, Mn, Cr, Ni, Cu, V, RE and P are adjusted to target values during LF furnace refining.

5. The method for producing a high weather-resistant cold heading steel for a grade 12.9 fastener according to claim 2, wherein in the step S3, the degassing time is not less than 15 minutes, and the [ H ] content after the vacuum treatment is not more than 1.5 ppm.

6. The method for producing high weather resistant cold heading steel for grade 12.9 fastener according to claim 2, wherein the tundish molten steel temperature is controlled to be 10-40 ℃ above the liquidus line in step S4, and the content of Alt is controlled by protective casting while preventing the increase of N.

7. The method for producing a high weather resistance cold heading steel for a grade 12.9 fastener according to claim 2, wherein in the step S5, the heating furnace soaking temperature is 1030 to 1100 ℃.

8. The method for producing high weather resistant cold heading steel for grade 12.9 fastener according to claim 2, wherein the step S6 of high speed wire cold rolling comprises:

a. starting a rolling stage, wherein the rolling temperature is 890-960 ℃;

c. in the spinning stage, the spinning temperature is 780-820 ℃.

9. The method for producing a high weather-resistant cold heading steel for a grade 12.9 fastener according to any one of claims 3 to 8, wherein in the step S7, when slow cooling is performed according to a stelmor cooling line, the wire rod is placed in a heat-preserving cover for heat preservation, the heat-preserving cover is completely closed, and the phase transition cooling speed of the wire rod is controlled within 1 ℃/S.