CN110938732A

CN110938732A - Titanium-containing atmospheric corrosion-resistant 14.9-grade high-strength bolt steel and production method thereof

Info

Publication number: CN110938732A
Application number: CN201911230955.5A
Authority: CN
Inventors: 张晓瑞; 汪开忠; 于同仁; 尹德福; 姜婷; 龚梦强; 郭湛; 丁雷; 孙凯; 余良其
Original assignee: Maanshan Iron and Steel Co Ltd
Current assignee: Maanshan Iron and Steel Co Ltd
Priority date: 2019-12-05
Filing date: 2019-12-05
Publication date: 2020-03-31

Abstract

The invention discloses titanium-containing atmospheric corrosion resistant 14.9-grade high-strength bolt steel, which comprises the following chemical components in percentage by weight: 0.45 to 0.50 percent of C, less than or equal to 0.10 percent of Si, 0.40 to 0.70 percent of Mn, 1.20 to 1.50 percent of Cr, 0.80 to 1.00 percent of Mo, 0.20 to 0.40 percent of V, 0.030 to 0.050 percent of Ti, 0.20 to 0.30 percent of Ni, 0.15 to 0.35 percent of Cu, 0.015 to 0.040 percent of Alt, less than or equal to 0.010 percent of P, less than or equal to 0.010 percent of S, less than or equal to 0.0015 percent of O and less than or equal to 0.006 percent of N. The balance of Fe and other inevitable impurities. The invention also provides a production method of the steel, and the mechanical property of the steel after heat treatment can reach: r_mNot less than 1400MPa, yield ratio R_P0.2Rm is more than or equal to 0.9, elongation A after fracture is more than or equal to 10 percent, reduction of area Z is more than or equal to 45 percent, austenite grain size of steel is more than or equal to 10.0 grade, and normal temperature impact absorption power KV₂Not less than 45J, good strength and plasticity and toughness, excellent delayed fracture resistance and good corrosion resistance.

Description

Titanium-containing atmospheric corrosion-resistant 14.9-grade high-strength bolt steel and production method thereof

Technical Field

The invention belongs to the technical field of steel for fasteners, and particularly relates to titanium-containing atmospheric corrosion resistant 14.9-grade high-strength bolt steel and a production method thereof.

Background

The fastener is a basic industry of equipment manufacturing industry in China 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 in China, the requirements of design stress and light weight are provided for materials used for manufacturing various fastener parts, the most effective measure is to improve the strength of the fastener, and bolts for some 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. However, the steel structure is corroded when used in various atmospheric environments, and high-strength bolts used for connecting steel structure buildings, bridges, electric power and the like for prolonging the service life are all subjected to corrosion prevention in a coating mode, such as electroplating or hot galvanizing. However, the corrosion prevention time limit of methods such as surface spraying is generally 5-10 years, maintenance needs to be carried out for 3-5 years regularly and corrosion prevention coating needs to be carried out again for 10-15 years due to aging and differentiation of a coating, the problems of environmental pollution and high cost exist in the coating process, and meanwhile, the problem of delayed fracture caused by hydrogen in the bolt using process is increased by an H element introduced in the processes of high-strength bolt electroplating and hot galvanizing, so that the demand of the weather-resistant high-strength bolt steel is more and more urgent, the demand is higher and higher, and along with the development of steel structures such as large-span bridges and the like and the application of weather-resistant steel with higher strength level, the weather-resistant high-strength bolt steel is required to have excellent atmospheric corrosion resistance and excellent delayed fracture resistance so as to meet the requirements of light weight and.

In recent years, the development of high strength bolts resistant to atmospheric corrosion has been a hot issue. For example, JP2014-1442 proposes a high-strength bolt steel having weather resistance and delayed fracture resistance, but the strength level thereof is 1000 to 1200MPa class, and it is only used for manufacturing a 10.9 class high-strength bolt. Chinese application patents 2017101375650.8 and 201710375670.5 respectively propose industrial atmospheric corrosion resistant high-strength bolted steel for a coating-free steel structure and a manufacturing method thereof, industrial atmospheric corrosion resistant high-strength bolted steel for a coating-free bridge structure and a manufacturing method thereof, wherein the tensile strength level after quenching and tempering is 1000-1200 MPa, and the high-strength bolted steel can be used for manufacturing 10.9-level high-strength bolts, and chinese patent CN 108754303 a, a high-strength bolted steel with excellent atmospheric corrosion resistance and delayed fracture resistance, proposes a high-strength bolted steel with tensile strength of more than 1200MPa and excellent atmospheric corrosion resistance and delayed fracture resistance, but can be used for manufacturing 12.9-level high-strength bolts.

As described above, with the development of weight reduction of automobiles, there is an increasing demand for high-strength bolt materials that achieve both atmospheric corrosion resistance and delayed fracture resistance, but at present, there are few reports of high-strength bolt steels that are excellent in bolt strength highest grade only at the 12.9 grade and 14.9 grade and have excellent atmospheric corrosion resistance and delayed fracture resistance, and there are gaps in the product and result.

Disclosure of Invention

The invention aims to provide high-strength bolt steel with tensile strength of more than 1400MPa and excellent atmospheric corrosion resistance and delayed fracture resistance, which can be used for manufacturing high-strength bolts for 14.9-grade steel structures with tensile strength of more than 1400MP, and also provides a production method of the steel.

The technical scheme adopted by the invention is as follows: (1) on the basis of 42CrMo steel, the Mn element content in the steel for the fastener is properly reduced, and meanwhile, the contents of an impurity element P, S and the like in the steel are strictly controlled, so that the grain boundary segregation is reduced, and the grain boundary embrittlement is prevented; (2) refined grain elements such as V, Ti and the like are properly added to generate dispersed and precipitated carbonitride to refine austenite grains, so that the strength is improved, the toughness is improved, meanwhile, the carbon nitride can be used as a hydrogen trap to inhibit the diffusion of hydrogen and enable the hydrogen to be uniformly distributed, and the hydrogen-induced delayed fracture resistance is improved; (3) the content of Mo element is properly increased, meanwhile, element V with strong tempering softening resistance is added, the tempering temperature is increased under the condition of keeping the strength unchanged, carbides are fine and uniformly spheroidized, and in addition, the Mo element can also reduce the amount of hydrogen invading the surface of steel and inhibit the generation of corrosion pits; (4) and elements such as Ni, Cu and the like are added, so that the atmospheric corrosion resistance is improved, and high notch toughness is obtained. The key point of the invention is that the optimization adjustment of the components and the metallurgical quality control are organically combined, so that the high strength is obtained, and simultaneously, the excellent atmospheric corrosion resistance and delayed fracture resistance are obtained.

The titanium-containing atmospheric corrosion resistant 14.9-grade high-strength bolt steel comprises the following chemical components in percentage by weight: 0.45 to 0.50 percent of C, less than or equal to 0.10 percent of Si, 0.40 to 0.70 percent of Mn, 1.20 to 1.50 percent of Cr1, 0.80 to 1.00 percent of Mo, 0.20 to 0.40 percent of V, 0.030 to 0.050 percent of Ti, 0.20 to 0.30 percent of Ni0.15 to 0.35 percent of Cu, 0.015 to 0.040 percent of Alt, less than or equal to 0.010 percent of P, less than or equal to 0.010 percent of S, less than or equal to 0.0015 percent of O and less than or equal to 0.006 percent of N. The balance of Fe and other inevitable impurities.

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 the steel, but is extremely disadvantageous for the cold heading property, plasticity and toughness of the steel, and the C content is controlled to 0.45 to 0.50% because the yield ratio is lowered, the decarburization sensitivity is increased, and the fatigue resistance and workability of the steel are deteriorated.

Si: si is a main deoxidizing element in steel and has a strong solid solution strengthening effect, but Si can obviously improve the deformation resistance of the steel and is extremely unfavorable for cold heading and cold extrusion, and Si can promote the grain boundary segregation of impurity elements, increase the delayed fracture sensitivity of the steel, reduce the plasticity and toughness of the steel and deteriorate the fatigue resistance of the steel, so the Si content is not too high, and the Si content is controlled to be less than or equal to 0.10 percent.

Mn: mn is an effective element for deoxidation and desulfurization, but Mn and P have strong tendency of grain boundary co-segregation during high temperature tempering of quenched steel, thereby promoting temper brittleness, and excessively high Mn content deteriorates the weather resistance of the steel, so that the Mn content in the steel is appropriately reduced to 0.40-0.70%.

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. Meanwhile, the alloy is compounded with Cu, so that the weather resistance of the steel can be obviously improved. The above effect is hardly obtained when the content is less than 0.5%, but the toughness and cold workability of the steel deteriorate when the content exceeds 1.50%, so that the Cr content is controlled to be 1.20% to 1.50%.

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 also reduce the amount of hydrogen invading the steel surface, inhibit the generation of corrosion pits and improve the corrosion resistance, thereby controlling the Mo content to be 0.80-1.00%.

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 can improve the tempering resistance of the steel, vanadium carbonitride precipitated during tempering at higher temperature can generate secondary hardening to further improve the strength of the steel, and the vanadium carbonitride has stronger trap energy and can trap hydrogen to uniformly disperse the hydrogen in crystal and inhibit the diffusion of the hydrogen and the grain boundary segregation, thereby improving the delayed fracture resistance of the steel, and the content of V is controlled to be 0.20-0.40%.

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, and controlling the Ti content to be 0.030-0.050%.

Ni: ni can stabilize austenite, enhance hardenability of steel and improve low-temperature toughness. The addition of the Ni element can improve the structure of a rust layer, improve the compactness and the cohesiveness to the surface of the steel, improve the corrosion resistance of the steel, inhibit the absorption of hydrogen, and further is beneficial to improving the delayed fracture resistance, and the content of Ni is controlled to be 0.20-0.30%.

Cu: the Cu element can obviously improve the corrosion resistance of the steel, and the cathodic 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, too high Cu content can reduce the high-temperature plasticity of the steel and easily generate cracks in the hot working process, so that the Cu content is controlled to be 0.15-0.35%.

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 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 0.010 percent of P and less than or equal to 0.010 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 the iron oxide in the metallurgical production as much as possible and control the T.O 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 invention provides a method for producing titanium-containing atmospheric corrosion-resistant 14.9-grade high-strength bolt steel, which comprises the steps of proportioning according to given chemical components → electric furnace smelting → LF + RH vacuum refining → large round billet continuous casting → heating → blooming in first rolling → continuous rolling → rolling square billet → flaw detection, coping → high-speed wire heating furnace heating → controlled rolling and controlled cooling → sizing mill → laying head → wire rod finished product → packaging and warehousing.

The heating temperature of the billet during rolling is controlled in the range of 1050-1150 ℃ for the requirements of the rolling process and the solid dissolution of carbon and nitride in austenite. The rough rolling and the medium rolling are finished at the temperature of more than 950 ℃, namely the rolling is basically finished in an austenite recrystallization region to realize recrystallization and refinement, and the existing rough rolling and medium rolling steel rolling equipment and process are easy to realize. The finishing mill adopts low-temperature controlled rolling, which is beneficial to further thinning the final product structure. The controlled cooling adopts a slow cooling process to obtain softened tissues to facilitate drawing.

Because the secondary hardening peak value is narrower when the steel is tempered, in order to fully utilize the secondary hardening effect, the mechanical property of the steel meets the use requirement of the 14.9-grade high-strength bolt steel, and the heat treatment process system comprises the following steps: quenching temperature: quenching with oil at 900 +/-30 ℃; tempering temperature: 610 ℃ C. + -. 10 ℃.

The atmospheric corrosion resistant 14.9-grade high-strength bolt steel produced by adopting the chemical components and the process flow has the following mechanical properties after heat treatment: r_mNot less than 1400MPa, 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 10 percent, the reduction of area Z is more than or equal to 45 percent, the austenite grain size of steel is more than or equal to 10.0 grade, and the impact absorption power KV at normal temperature₂The fatigue life is more than or equal to 45J, the fatigue life is more than or equal to 1000 ten thousand times under the condition of 650MPa cyclic stress, and the structure of the steel after quenching and tempering heat treatment is fine tempered sorbite with orientation and a large amount of dispersed short rod-shaped or spherical fine carbides. The delayed fracture resistance was evaluated and analyzed by a slow strain rate tensile test (SSRT) at room temperature, and the delayed fracture strength ratio R is R_BN/R_BN0Greater than 0.5 (notched tensile strength R of the hydrogen-charged specimen)_BN(ii) a Notched tensile strength R of the non-hydrogen-charged specimen_BN0) The atmospheric corrosion resistance index I is more than or equal to 6.8.

Drawings

FIG. 1 is a microstructure diagram of austenite grain size (11 grades);

FIG. 2 is a drawing of a tempered sorbite microstructure;

FIG. 3 is a notched tensile specimen for SSRT;

FIG. 4 shows the fracture morphology of a notched tensile specimen for SSRT.

Detailed Description

The invention is further described in the following with reference to the drawings and examples of the specification.

The invention provides a high-strength bolt steel with tensile strength of more than 1400MPa and excellent atmospheric corrosion resistance and delayed fracture resistance, which comprises the following components in percentage by weight: the invention adopts wire rod with specific components, which comprises the following components in percentage by weight: 0.45 to 0.50 percent of C, less than or equal to 0.10 percent of Si, 0.40 to 0.70 percent of Mn, 1.20 to 1.50 percent of Cr, 0.80 to 1.00 percent of Mo, 0.20 to 0.40 percent of V, 0.030 to 0.050 percent of Ti, 0.20 to 0.30 percent of Ni, 0.15 to 0.35 percent of Cu, 0.015 to 0.040 percent of Alt0.010 percent of P, less than or equal to 0.010 percent of S, less than or equal to 0.0015 percent of O and less than or equal to 0.006 percent of N. The balance of Fe and other inevitable impurities.

The invention adopts wire rod with specific components, the components of each group of examples and comparative examples are shown in table 1, and the production process is as follows:

electric furnace smelting: oxygen is determined before tapping, and slag is strictly controlled during tapping;

and (4) LF furnace: C. adjusting elements such as Si, Mn, Cr, Mo, V, Ti, Ni, Cu and the like to target values;

vacuum degassing: the pure degassing time is more than or equal to 15 minutes, and the content of H after vacuum treatment is ensured;

continuous casting: the target temperature of the tundish molten steel is controlled to be 10-40 ℃ above the liquidus temperature, and a round billet with phi of 380mm is continuously cast.

Wire rod rolling route: round blank with phi 380mm → heating → square blank with 150mm multiplied by 150mm → scalping → heating → high-speed wire controlled rolling → stelmor cooling by wire → finished product of wire rod with phi 18 mm. Wherein, when the wire rod is rolled, the heating temperature is controlled to 1050-1150 ℃, the initial rolling temperature is controlled to 950-1000 ℃, the final rolling temperature is controlled to 790-815 ℃, and the spinning temperature is controlled to 780-815 ℃.

The wire rod chemistry and austenite grain size are shown in table 1. Wherein the austenitizing heat treatment process comprises the following steps: quenching at 880 ℃, oil cooling, wherein the temperature of a quenching medium is 18-35 ℃, and metallographic sample preparation and austenite grain size grading are carried out after cooling.

TABLE 1 chemical composition and organization (wt%) of examples of the invention

Note: atmospheric corrosion resistance index I ═ 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

Table 2 shows the specific process parameters of the manufacturing methods of the example wire rods of the present invention and the comparative example wire rods.

TABLE 2

The test steel sample blanks for tensile test, impact test, notch sensitivity test, fatigue test and notch tensile delayed fracture test are processed into standard round bar tensile test samples, notch sensitivity test samples and notch tensile delayed fracture test sample blanks after the annealing (furnace cooling to room temperature) treatment of a finished wire rod product at 850 ℃ for 60 min. The sample blank is quenched by oil at 900 +/-30 ℃, then tempered in the temperature range of 610 +/-10 ℃ for 120min and naturally cooled by air. The mechanical properties after heat treatment are shown in Table 3, the strength of each group of examples reaches 1400MPa, the elongation reaches more than 10%, and the surface shrinkage reaches more than 45%, which indicates that the examples have better toughness.

The material is quenched and tempered, and subjected to a 72-hour salt spray corrosion test, wherein a comparative example is 42CrMo commonly used in the market at present, and the corrosion rate of each group of examples is lower than 1.5.

TABLE 3 mechanical properties and corrosion resistance after quenching and tempering heat treatment in the examples of the present invention

Therefore, the mechanical property of the high-strength bolt steel produced by the method of the invention after heat treatment can reach: r_mNot less than 1400MPa, yield ratio R_P0.2Rm is more than or equal to 0.9, elongation A after fracture is more than or equal to 10 percent, reduction of area Z is more than or equal to 45 percent, austenite grain size of steel is more than or equal to 10.0 grade, and normal temperature impact absorption power KV₂Not less than 45J, good strength and plasticity and toughness, excellent delayed fracture resistance and good corrosion resistance.

Claims

1. The titanium-containing atmospheric corrosion resistant 14.9-grade high-strength bolt steel is characterized by comprising the following chemical components in percentage by weight: 0.45 to 0.50 percent of C, less than or equal to 0.10 percent of Si, 0.40 to 0.70 percent of Mn, 1.20 to 1.50 percent of Cr, 0.80 to 1.00 percent of Mo, 0.20 to 0.40 percent of V, 0.030 to 0.050 percent of Ti, 0.20 to 0.30 percent of Ni, 0.15 to 0.35 percent of Cu, 0.015 to 0.040 percent of Alt, less than or equal to 0.010 percent of P, less than or equal to 0.010 percent of S, less than or equal to 0.0015 percent of O and less than or equal to 0.006 percent of N. The balance of Fe and other inevitable impurities.

2. The method for producing the steel for the titanium-containing atmospheric corrosion-resistant 14.9-grade high-strength bolt according to claim 1, characterized by comprising the steps of: batching → electric furnace smelting → LF + RH vacuum refining → large round billet continuous casting → heating → blooming and cogging → continuous rolling → rolling square billet → flaw detection, coping → high-speed wire heating furnace heating → controlled rolling and controlled cooling → reducing and sizing mill → laying head → finished wire rod → packaging and warehousing;

in order to meet the requirements of the rolling process and make the carbon and the nitride be dissolved in the austenite, the heating temperature of the billet during rolling is controlled according to the range of 1050-1150 ℃; finishing the rolling of a rough rolling mill and a medium rolling mill at the temperature of more than 950 ℃; the finishing mill adopts low-temperature controlled rolling;

the heat treatment process system comprises the following steps: quenching at 900 +/-30 ℃ in oil; the tempering temperature is 610 ℃ plus or minus 10 ℃.

3. The method for producing the steel for titanium-containing atmospheric corrosion resistant 14.9-grade high-strength bolts as claimed in claim 2, wherein the mechanical properties of the produced steel for titanium-containing atmospheric corrosion resistant 14.9-grade high-strength bolts after heat treatment are as follows: r_mNot less than 1400MPa, 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 10 percent, the reduction of area Z is more than or equal to 45 percent, the austenite grain size of steel is more than or equal to 10.0 grade, and the impact absorption power KV at normal temperature₂The fatigue life is more than or equal to 45J, the fatigue life is more than or equal to 1000 ten thousand times under the condition of 650MPa cyclic stress, and the structure of the steel after quenching and tempering heat treatment is fine tempered sorbite with orientation and a large amount of dispersed short rod-shaped or spherical fine carbides.

4. The method for producing the steel for titanium-containing atmospheric corrosion resistant 14.9-grade high-strength bolts as claimed in claim 2, wherein the produced steel for titanium-containing atmospheric corrosion resistant 14.9-grade high-strength bolts has delayed fracture resistance as follows: delayed fracture strength ratio R ═ R_BN/R_BN0Is more than 0.5, and the atmospheric corrosion resistance index I is more than or equal to 6.8.