Low-cost delayed-fracture-resistant high-strength quenched and tempered steel wire rod for fastener and preparation method thereof

Abstract

The invention relates to a wire rod for a low-cost delayed fracture resistant high-strength quenched and tempered fastener and a preparation method of the wire rod. The wire rod comprises the following chemical components in percentage by weight: 0.38 to 0.43 percent of C, less than or equal to 0.10 percent of Si, 0.80 to 1.10 percent of Mn, less than or equal to 0.015 percent of P, less than or equal to 0.005 percent of S, 1.30 to 1.50 percent of Cr, 0.25 to 0.35 percent of Mo, 0.10 to 0.15 percent of Ni, 0.02 to 0.04 percent of Nb, 0.10 to 0.15 percent of V, 0.03 to 0.06 percent of Ti, 0.0010 to 0.0050 percent of B, 0.002 to 0.050 percent of Al, less than or equal to 0.00005 percent of H and the balance of iron and trace impurities. The manufacturing process of the hot-rolled wire rod comprises the following steps: preparing raw materials according to chemical components, and sequentially carrying out converter / electric furnace-LF refining-vacuum degassing-casting-cogging-intermediate billet surface treatment-wire rod controlled rolling-wire rod controlled cooling. The steel wire rod produced by the invention is subsequently subjected to steel wire rod modification, cold heading and thermal refining to produce a fastener, and the fastener has the characteristics of low production cost and excellent delayed fracture resistance, and meets the requirements of 13.9-14.9 grade fasteners.

Description

Technical Field

[0001] The invention belongs to the technical field of special steel smelting, and specifically relates to a low-cost delayed fracture-resistant high-strength quenched and tempered fastener wire rod and a preparation method thereof. Background Art

[0002] High-strength fasteners are widely used in automobiles, engineering machinery, etc. At present, the strength levels of high-strength fasteners generally used are divided into four strength levels: 8.8, 9.8, 10.9 and 12.9. With the requirements of lightweight, high-power miniaturization and continuous improvement of vehicle performance, the performance requirements for high-strength fasteners are also increasing. When the stress on the fastener increases, and the increase in cross-sectional size is limited by the design structure, the strength level of the fastener becomes particularly important. From the perspective of practical application, 13.9-14.9 grades are the most widely and urgently needed grades among high-strength fasteners, and their prospects for industrial mass production are relatively large. A series of high-strength fastener steels with excellent delayed fracture resistance have been developed at home and abroad, such as the ADS series of Sumitomo Metal of Japan, the MB series of Nippon Steel, the KNDS series of Kobe Steel, and the ADF series of Beijing Iron and Steel Research Institute.

[0003] For high-strength quenched and tempered steel for fasteners, high Mo design is generally adopted to ensure delayed fracture resistance. For example, in the technical solutions disclosed in publication numbers CN 111041356 A and CN 110468340 A, Mo is controlled above 0.6% to ensure the tempering temperature of quenched and tempered heat treatment, but the increase in Mo content greatly increases the material cost. The content of impurity elements such as P and S in steel is strictly controlled to reduce grain boundary segregation and prevent grain boundary embrittlement. At the same time, the Mn content that promotes P and S segregation is reduced, but the role of Mn in steel, especially the impact on strength, is ignored. For high-strength fasteners, delayed fracture resistance is an important indicator. Improper control will lead to early failure of fasteners. Summary of the invention

[0004] The technical problem to be solved by the present invention is to provide a low-cost delayed fracture resistant high-strength quenched and tempered fastener wire rod and a preparation method thereof in response to the above-mentioned prior art, which has the characteristics of meeting the strength requirements of high-strength fasteners at a relatively low alloy cost and having excellent delayed fracture resistance.

[0005] The technical solution adopted by the present invention to solve the above problems is: a low-cost delayed fracture resistant high-strength quenched and tempered fastener wire rod, whose chemical composition mass percentage is C: 0.38-0.43%, Si: ≤0.10%, Mn: 0.80-1.10%, P: ≤0.015%, S: ≤0.005%, Cr: 1.30-1.50%, Mo: 0.25-0.35%, Ni: 0.10-0.15%, Nb: 0.02-0.04%, V: 0.10-0.15%, Ti: 0.03-0.06%, B: 0.0010-0.0050%, Al: 0.002-0.050%, H: ≤0.00005%, and the rest is Fe and unavoidable impurities.

[0006] To achieve the above purpose, the reasons for limiting the chemical elements in the present invention are as follows: 1) Determination of C content C is the most basic strengthening element in steel, but the increase in carbon content in tempered martensite will increase the sensitivity of delayed fracture and have an adverse effect on the toughness and plasticity of steel. The C content of the present invention is determined to be in the range of 0.38-0.43%.

[0007] 2) Determination of Si content Si is the main deoxidizing element in steel and has a strong solid solution strengthening effect. However, silicon can significantly improve the deformation resistance of steel, which is unfavorable to the cold heading and cold extrusion of steel. In addition, Si can promote the grain boundary segregation of impurity elements, increase the delayed fracture sensitivity of steel, and have an adverse effect on the toughness and plasticity of steel. The range of C content in the present invention is determined to be ≤0.10%.

[0008] 3) Determination of Mn content As a deoxidizing element in the steelmaking process, Mn can improve the hardenability of steel. Mn can also fix the form of sulfur in steel and form MnS and (Fe, Mn)S, which are less harmful to the performance of steel, reduce or inhibit the production of FeS, and prevent the effect of thermal embrittlement. In addition, manganese improves the strength of steel by solid solution strengthening, and is one of the most effective low-cost alloying elements in steel to improve strength. However, Mn has the effect of promoting grain growth, which increases the overheating sensitivity and crack tendency of steel, which is not conducive to the delayed fracture resistance of steel. In order to obtain excellent microstructure, mechanical properties and delayed fracture resistance, the range of Mn content in the present invention is determined to be 0.80-1.10%.

[0009] 4) Determination of P and S content P causes serious segregation during solidification in steel. P dissolves in ferrite to cause grain distortion and coarsening, and increases cold brittleness, which is not conducive to the delayed fracture resistance of steel. The range of P content in the present invention is determined to be ≤0.015%. S causes hot brittleness in steel, reduces the ductility and toughness of steel, and is not conducive to the delayed fracture resistance of steel. The range of S content in the present invention is determined to be ≤0.005%.

[0010] 5) Determination of Cr content Cr is the most commonly used alloying element in high-strength fasteners. It can effectively improve the hardenability and temper softening resistance of steel to obtain the required high strength. However, if the content exceeds 1.50%, the toughness and cold workability of the steel will deteriorate. The range of Cr content in the present invention is determined to be 1.30-1.50%.

[0011] 6) Determination of Mo content Mo is an alloying element commonly used in high-strength fasteners. It can significantly improve the hardenability and temper softening resistance of steel. The formed carbides have a hydrogen trapping effect and can improve the delayed fracture resistance of steel. The Mo content of the present invention is determined to be within the range of 0.25-0.35%.

[0012] 7) Determination of Ni content Ni can stabilize austenite, enhance the hardenability of steel, improve low-temperature toughness, inhibit hydrogen adsorption, and thus be beneficial to improving delayed fracture resistance. The Ni content of the present invention is determined to be in the range of 0.10-0.15%.

[0013] 8) Determination of Nb content Nb is a typical microalloying element, has a strong affinity with C and N, and can form stable and fine carbonitrides. The precipitation strengthening and grain refinement of its carbonitrides can significantly improve the strength of steel. Fine carbonitrides can also serve as hydrogen traps, inhibiting hydrogen diffusion and making hydrogen uniformly distributed, thereby improving delayed fracture resistance. The content of Nb in the present invention is determined to be in the range of 0.02-0.04%.

[0014] 9) Determination of V content It can significantly improve the hardenability and temper softening resistance of steel. The formed carbonitride has a trapping effect on hydrogen. Vanadium carbonitride has a strong trapping energy, which can trap hydrogen and make it evenly dispersed in the crystal, inhibiting the diffusion of hydrogen and grain boundary segregation, thereby improving the delayed fracture resistance of steel. At the same time, adding vanadium to steel can refine the grain structure and improve the strength and toughness. The range of V content in the present invention is determined to be 0.10-0.15%.

[0015] 10) Determination of B content B is the most significant element for improving the hardenability of steel. However, if the B content exceeds 0.0050%, a brittle phase of BN will easily appear, which will deteriorate the delayed fracture resistance of the steel. Therefore, the B content of the present invention is controlled to be 0.0010-0.0050%.

[0016] 11) Determination of Ti content Ti mainly plays the role of nitrogen fixation. It can combine with free nitrogen in steel in the early stage of continuous casting solidification to form TiN particles in the steel, thereby reducing the combination of free nitrogen and B, giving full play to the role of B in improving hardenability. At the same time, adding vanadium to the steel can refine the microstructure grains and improve the strength and toughness. Therefore, the present invention controls the Ti content to 0.03-0.06%.

[0017] 12) Determination of Al content Al is added as a deoxidizing element in steel. In addition to reducing the dissolved oxygen in molten steel, Al and N form dispersed and fine aluminum nitride, which can refine the grains and increase the temperature of steel grain coarsening. However, when the Al content is high, brittle inclusions such as Al2O3 are easily formed during the molten steel smelting process, which reduces the purity of the molten steel. When the Al content is high, the grain size of the steel is more likely to grow and coarsen. The Al content of the present invention is determined to be in the range of 0.002-0.050%.

[0018] 13) Determination of H content H is a direct factor in delayed fracture. The present invention strictly controls the hydrogen content in the steel to below 0.5 ppm to reduce the risk of hydrogen-induced delayed fracture.

[0019] In order to obtain the above-mentioned wire rod, the present invention provides a method for preparing a low-cost delayed fracture resistant high-strength quenched and tempered fastener wire rod, which specifically comprises the following steps: Step 1: Smelting and casting: The raw materials are smelted in converter / electric furnace, refined, vacuum degassed and mold cast or continuous cast in turn to produce steel billets with precise chemical composition, good uniformity and high purity.

[0020] Step 2: Opening: The continuous casting billet is opened by a reversible rolling mill. The intermediate billet after opening is inspected by online fully automatic infrared flaw detection (detection accuracy 0.3mm). The defects found are spot ground and then re-inspected by magnetic powder to ensure that the depth of surface defects of the intermediate billet is ≤0.3mm. After the flaw detection, the surface is automatically ground online with a grinding depth ≥0.5mm to ensure the surface quality of the intermediate billet.

[0021] Step 3: Controlled rolling and controlled cooling of wire rod: The billet is heated in sections in the heating furnace, and the temperature of the preheating section is controlled to be ≤720℃, the temperature of the high temperature section is controlled at 1150~1250℃, the holding time of the high temperature section is not less than 60min, and the residual oxygen content in the furnace is ≤2.0%. After the insulation is completed, the intermediate billet is descaled to remove the surface oxide layer after being taken out of the furnace, and then rolled. Rough rolling stage: the starting rolling temperature is 850~950℃, and the total compression ratio is ≥60%; secondary descaling is performed before medium rolling; medium finishing rolling stage: the medium finishing rolling temperature is 820~900℃, and the total compression ratio is ≥90%; the temperature of the sizing mill is 750~820℃, and the compression ratio of the last two passes is ≥30%; the wire laying temperature is 780~830℃, and the slow cooling process is adopted. The roller speed is 0.10m / s~0.35m / s, 2~4 insulation covers are opened, and the rest are closed to ensure that the cooling rate is ≤0.6℃ / S.

[0022] Preferably, in step one, high-quality molten iron from a blast furnace, scrap steel and raw and auxiliary materials are used for smelting to reduce the content of harmful elements in molten steel. Molten steel smelting involves primary smelting in a converter / electric furnace, LF refining, vacuum degassing, KR molten iron pretreatment and converter / electric furnace slag making to achieve C and P removal, and slag should be avoided when tapping steel. One of the purposes of vacuum treatment is to control the hydrogen content to be lower than 0.5ppm, effectively reducing the hazard of hydrogen embrittlement of high-strength fasteners. The molten steel is cast into a billet by a continuous casting or mold casting process. If a continuous casting process is adopted, the pouring superheat is 20 to 35°C during the continuous casting process. The continuous casting process adopts a light pressure reduction and electromagnetic stirring process to improve the segregation of the billet. The entire continuous casting process adopts protective casting to isolate the billet from air to prevent secondary oxidation.

[0023] Preferably, in step 2, the billet is subjected to high temperature diffusion to further remove the hydrogen content in the steel, and the billet is heated at high temperature (1150-1270°C) in a heating furnace for a holding time of ≥5 hours to fully dissolve the alloy components in the steel and make the components uniform. The large square billet is cut into □150mm 2 ~□200mm 2 Intermediate billet, where □ represents the cross-sectional shape of the intermediate billet. After the billet is opened, it is slowly cooled off the production line. After completion, the surface is processed to ensure the surface quality of the intermediate billet.

[0024] The specification of the wire rod for producing low-cost delayed fracture resistant high-strength quenched and tempered fasteners according to the present invention is Φ5.5-Φ18 mm. The wire rod with a mixed metallographic structure of bainite+martensite+ferrite+pearlite is subsequently annealed, drawn and converted into small-size steel wire (Φ3-Φ17 mm), cold headed and quenched and tempered. The specific quenching and tempering process is: heating the wire rod to 860-930° C. and keeping it warm for 60-120 min, water cooling or oil cooling, tempering at 480-550° C. and keeping it warm for 60-90 min. The fasteners produced have the characteristics of low production cost and excellent delayed fracture resistance, the structure is tempered structure, the tensile strength is 1320-1530 MPa, the yield strength is 1188-1377 MPa, the elongation is 10-18%, and the room temperature KV2 impact energy is above 42 J, which meets the requirements of 13.9-14.9 grade fasteners. Compared with the prior art, the advantages of the present invention are: 1) Composition design: (1) On the basis of 42CrMo steel, strictly control the content of impurity elements such as P and S in the steel to reduce grain boundary segregation, prevent grain boundary embrittlement, and improve the resistance to hydrogen-induced delayed fracture; (2) Appropriately add V, Nb, and Ti to refine the grains and generate dispersed carbonitrides to refine the austenite grains. While improving the strength, it can also improve the toughness. The grain boundaries can also serve as hydrogen traps to inhibit the diffusion of hydrogen and make hydrogen uniformly distributed, thereby improving the resistance to hydrogen-induced delayed fracture; (3) Appropriately increase the content of Cr and Mo elements. Compared with 14.9 grade fastener steel, the amount of Mo is reduced, and V is added. The tempering temperature can be increased while ensuring the strength. The tempering temperature is 480-550℃, making the carbonitrides fine and uniformly spheroidized, which can capture diffusible hydrogen, less harmful hydrogen, and improve the resistance to hydrogen-induced delayed fracture; (4) Appropriately add Ni and B elements to increase the hardenability of the steel and improve the strength; the addition of Ni can also improve the low-temperature toughness and inhibit the adsorption of hydrogen, which is beneficial to improving the delayed fracture resistance.

[0025] 2) During molten steel smelting, converter / electric furnace is used to produce molten steel with low S and P content. LF refining + vacuum degassing is conducive to the production of molten steel with uniform chemical composition and purity. The vacuum treatment should control the hydrogen content below 0.5ppm, and diffuse the blank at the same time, effectively reducing the harm of hydrogen embrittlement of high-strength fasteners and improving the delayed fracture resistance.

[0026] 3) Controlled rolling: The rough rolling and medium finishing rolling of wire rods use a large compression ratio, forming a large number of dislocations and deformation bands inside the steel, and accumulating a large amount of storage energy, preparing for the subsequent sizing rolling. When the temperature of the sizing mill is controlled at 750-820°C, combined with the last two rollings with a compression ratio of ≥30%, the austenite is rolled in the non-recrystallized area. Usually, the recrystallization area corresponds to a large compression ratio. After rolling in the non-recrystallized area, the grains do not grow, and the grains can be refined. Affected by the compression ratio and rolling temperature, the grains are refined and the delayed fracture resistance is improved. Due to the high alloy content, slow cooling is used to prevent the steel from brittle fracture after too much martensite.

[0027] 4) The fasteners produced by the wire rod after subsequent wire rod reforming, cold heading and tempering treatment have the characteristics of low production cost and excellent delayed fracture resistance, meeting the requirements of 13.9-14.9 grade fasteners. DETAILED DESCRIPTION

[0028] The technical solution of the present invention is described in more detail in conjunction with the preferred embodiments of the present invention. However, these embodiments are only descriptions of the preferred implementation methods of the present invention and cannot impose any limitation on the scope of the present invention. Example 1

[0029] The specification of the low-cost delayed fracture resistant high-strength quenched and tempered fastener wire rod involved in this embodiment is Φ8mm, and the components and their mass percentages contained are: C: 0.39%, Si: 0.05%, Mn: 0.88%, P: 0.010%, S 0.002%, Cr: 1.35%, Mo: 0.28%, Ni: 0.12%, Nb: 0.025%, V: 0.12%, Ti: 0.0353%, B: 0.0021%, Al: 0.030%, H: 0.00002%, and the rest are Fe and unavoidable impurities.

[0030] According to the chemical composition, the raw materials are configured and the converter / electric furnace is carried out successively - LF refining - vacuum degassing - casting (390mm*510mm) - blanking (150-200mm 2 Square) - intermediate billet surface treatment (defect detection + grinding) - wire controlled rolling - wire controlled cooling. Billet thickness, billet opening process, intermediate billet specifications, surface treatment, etc. During the continuous casting process, the pouring superheat is 26°C. Soft pressure reduction and electromagnetic stirring technology are used to improve the segregation of the ingot. The entire continuous casting process adopts protective pouring to isolate the ingot from air to prevent secondary oxidation.

[0031] Controlled rolling and controlled cooling of wire rod: the billet is heated in sections in the heating furnace, the temperature of the preheating section is controlled at 705℃, the temperature of the high temperature section is controlled at 1190℃, the holding time of the high temperature section is 65min, and the residual oxygen content in the furnace is 0.5%. After the holding is completed, the intermediate billet is descaled to remove the surface oxide layer after being taken out of the furnace, and then rolled. Rough rolling stage: the opening rolling temperature is 935℃, and the total compression ratio is 68%; secondary descaling is performed before medium rolling; medium finishing rolling stage: the medium finishing rolling temperature is 868℃, and the total compression ratio is 98%; the temperature of the sizing mill is 811℃, and the compression ratio of the last two passes is 37%; the wire laying temperature is 813℃, and the slow cooling process is adopted. The roller speed is 0.30m / s, 4 insulation covers are opened, and the rest are closed, and the slow cooling rate is 0.5℃ / S.

[0032] The wire rod produced by the above manufacturing method is subsequently subjected to wire rod reforming, cold heading, and tempering treatment (heating to 880°C for 80 minutes, oil cooling, and tempering at 530°C for 60 minutes) to produce fasteners with a tensile strength of 1380 MPa, a yield strength of 1270 MPa, an elongation of 13%, and a room temperature impact KV2: 51J, meeting the requirements of grade 13.9 fasteners. Example 2

[0033] The specification of the low-cost delayed fracture resistant high-strength quenched and tempered fastener wire rod involved in this embodiment is Φ16mm, and the components and their mass percentages contained are, C: 0.42%, Si: 0.07%, Mn: 0.92%, P: 0.008%, S 0.002%, Cr: 1.42%, Mo: 0.32%, Ni: 0.13%, Nb: 0.028%, V: 0.14%, Ti: 0.0453%, B: 0.0025%, Al: 0.025%, H: 0.00002%, and the rest are Fe and unavoidable impurities.

[0034] The raw materials are configured according to the chemical composition, and the processes are carried out in sequence: converter / electric furnace—LF refining—vacuum degassing—casting—blanketing—intermediate billet surface treatment—wire controlled rolling—wire controlled cooling.

[0035] The pouring superheat during the continuous casting process is 22°C. Soft pressure reduction and electromagnetic stirring technology are used in the continuous casting process to improve the segregation of the ingot. Protective pouring is used in the entire continuous casting process to isolate the ingot from air and prevent secondary oxidation.

[0036] Controlled rolling and controlled cooling of wire rod: the billet is heated in sections in the heating furnace, the temperature of the preheating section is controlled at 688℃, the temperature of the high temperature section is controlled at 1215℃, the holding time of the high temperature section is 63min, and the residual oxygen content in the furnace is 1.0%. After the insulation is completed, the intermediate billet is descaled to remove the surface oxide layer after being taken out of the furnace, and then rolled. Rough rolling stage: the starting rolling temperature is 945℃, and the total compression ratio is 68%; secondary descaling is carried out before medium rolling; medium finishing rolling stage: the medium finishing rolling temperature is 850℃, and the total compression ratio is 96.5%; the temperature entering the sizing mill is 795℃, and the compression ratio of the last two passes is 34%; the wire laying temperature is 803℃, and the slow cooling process is adopted. The roller speed is 0.25m / s, 3 insulation covers are opened, and the rest are closed. Slow cooling rate: 0.3℃ / S The wire rod produced by the above manufacturing method is subsequently subjected to wire rod reforming, cold heading, and tempering treatment to produce fasteners that meet the requirements of 14.9 grade fasteners.

[0037] The wire rod produced by the above manufacturing method is subsequently subjected to wire rod reforming, cold heading, and tempering treatment (heating to 900°C for 70 minutes, oil cooling, and tempering at 490°C for 70 minutes) to produce fasteners with a tensile strength of 1440Mpa, a yield strength of 1320Mpa, an elongation of 11%, and a room temperature impact KV2: 46J, meeting the requirements of grade 14.9 fasteners.

[0038] Although the preferred embodiments of the present invention are described in detail above, it should be clearly understood that various modifications and variations are possible for those skilled in the art. Any modification, equivalent substitution, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A low-cost delayed fracture resistant high-strength quenched and tempered fastener wire rod, characterized in that: The chemical composition of the wire rod is C: 0.38-0.43%, Si: ≤0.10%, Mn: 0.80-1.10%, P: ≤0.015%, S: ≤0.005%, Cr: 1.30-1.50%, Mo: 0.25-0.35%, Ni: 0.10-0.15%, Nb: 0.02-0.04%, V: 0.10-0.15%, Ti: 0.03-0.06%, B: 0.0010-0.0050%, Al: 0.002-0.050%, H: ≤0.00005%, and the rest is Fe and unavoidable impurities.

2. The low-cost delayed fracture resistant high-strength quenched and tempered fastener wire rod according to claim 1, characterized in that: The specification of the wire rod is Φ5.5~Φ18mm.

3. The low-cost delayed fracture resistant high-strength quenched and tempered fastener wire rod according to claim 1, characterized in that: The mechanical properties of the wire rod meet the requirements of tensile strength of 1320-1530 MPa, yield strength of 1188-1377 MPa, elongation of 10-18%, and KV2 impact energy of more than 42J at room temperature, meeting the requirements of 13.9-14.9 grade fasteners.

4. The low-cost delayed fracture resistant high-strength quenched and tempered fastener wire rod according to claim 1, characterized in that: The metallographic structure of the wire rod is a mixed metallographic structure of bainite+martensite+ferrite+pearlite.

5. A method for preparing a low-cost delayed fracture resistant high-strength quenched and tempered fastener wire rod as claimed in claim 1, characterized in that: The method comprises the following steps: Step 1, smelting and casting: the smelting raw materials are successively smelted in a converter / electric furnace, refined, vacuum degassed and mold cast or continuous cast to smelt a steel billet with precise chemical composition, good uniformity and high purity; Step 2, billet opening: The continuous casting billet is billeted by a reversible rolling mill. The intermediate billet after billet opening is subjected to online fully automatic infrared flaw detection. The defects found are spot-grinded and then re-detected with magnetic powder to ensure that the depth of surface defects of the intermediate billet is ≤0.3mm. After the flaw detection, the surface is fully automatically ground online, and the grinding depth is ≥0.5mm to ensure the surface quality of the intermediate billet; Step 3: Controlled rolling and controlled cooling of wire rod: The billet is heated in sections in the heating furnace. After the insulation is completed, the intermediate billet is taken out of the furnace to descale and remove the surface oxide layer, and then rolled. Rough rolling stage: the starting rolling temperature is 850-950℃, and the total compression ratio is ≥60%; secondary descaling is carried out before intermediate rolling; intermediate finishing rolling stage: the intermediate finishing rolling temperature is 820-900℃, and the total compression ratio is ≥90%; the temperature entering the reducing and sizing mill is 750-820℃, and the compression ratio of the last two passes is ≥30%; the wire laying temperature is 780-830℃, and a slow cooling process is used to finally obtain a wire rod that meets the requirements.

6. The method for preparing a low-cost delayed fracture resistant high-strength quenched and tempered fastener wire rod according to claim 5, characterized in that: In step 1, high-quality molten iron from a blast furnace, scrap steel and raw and auxiliary materials are used for smelting to reduce the content of harmful elements in the molten steel. The smelting of molten steel involves primary smelting in a converter / electric furnace, LF refining, vacuum degassing, KR molten iron pretreatment and slag making in a converter / electric furnace to achieve C and P removal, and slag is avoided during steel tapping; the hydrogen content is controlled to be lower than 0.5ppm; the molten steel is cast into a billet by a continuous casting or mold casting process, the pouring superheat is 20 to 35°C during the continuous casting process, the continuous casting process adopts light pressure reduction and electromagnetic stirring technology to improve the segregation of the billet, and the entire continuous casting process adopts protective pouring to isolate the billet from air to prevent secondary oxidation.

7. The method for preparing a low-cost delayed fracture resistant high-strength quenched and tempered fastener wire rod according to claim 5, characterized in that: In step 2, the billet is diffused at high temperature to further remove the hydrogen content in the steel. The billet is heated in a heating furnace at a temperature of 1150-1270°C for ≥5 hours to fully dissolve the alloy components in the steel and make the components uniform. The large square billet is cut into 150mm square billets. 2 ~□200mm 2 Intermediate billet, where □ represents the cross-sectional shape of the intermediate billet. After the billet is opened, it is slowly cooled off the production line. After completion, the surface is processed to ensure the surface quality of the intermediate billet.

8. The method for preparing a low-cost delayed fracture resistant high-strength quenched and tempered fastener wire rod according to claim 5, characterized in that: In step three, the temperature of the preheating section is controlled to be ≤720°C, the temperature of the high-temperature section is controlled to be between 1150 and 1250°C, the insulation time of the high-temperature section is not less than 60 minutes, and the residual oxygen content in the furnace is ≤2.0%.

9. The method for preparing a low-cost delayed fracture resistant high-strength quenched and tempered fastener wire rod according to claim 5, characterized in that: In step 3, the roller speed during the slow cooling process is 0.10m / s~0.35m / s, 2~4 insulation covers are opened, and the rest are closed to ensure that the cooling rate is ≤0.6℃ / S.

10. The method for preparing a low-cost delayed fracture resistant high-strength quenched and tempered fastener wire rod according to claim 5, characterized in that: The obtained wire rod is subsequently annealed, drawn into small-sized steel wire, cold headed, and tempered to obtain fasteners that meet the requirements of grade 13.9-14.

9. The tempering treatment is to heat the wire rod to 860-930℃ and keep it for 60-120min, water-cool or oil-cool it, and temper it at 480-550℃ and keep it for 60-90min.