A delayed fracture resistant 14.9 grade quenched and tempered cold heading steel hot rolled wire rod for bolts and its preparation method

Abstract

The invention belongs to the technical field of cold heading steel, and specifically relates to a cold heading steel hot rolled wire rod for delayed fracture resistant 14.9 grade quenched and tempered bolts and a preparation method thereof. The composition of the cold heading steel hot rolled wire rod by weight percentage is: C: 0.58-0.64%, Si: 0.35-0.45%, Mn: 1.70-1.90%, P: ≤0.010%, S: 0.025-0.035%, Al: 0.020-0.040%, B: 0.0025-0.0035%, Cr, Ni, Cu, Mo≤0.10%. The steel is prepared by converter smelting, LF refining, RH vacuum refining, billet continuous casting, billet rolling, billet flaw detection, billet grinding, and wire rod rolling. The material's plasticity, strength and hardenability are effectively improved, and the problem of delayed fracture of bolts is solved, meeting the production requirements of downstream industries for producing delayed fracture resistant 14.9 grade quenched and tempered bolts.

Description

Technical Field

[0001] The invention belongs to the technical field of cold heading steel, and relates to a hot-rolled wire rod of cold heading steel for delayed fracture resistant 14.9-grade quenched and tempered bolts, and also relates to a preparation method of the hot-rolled wire rod of cold heading steel for delayed fracture resistant 14.9-grade quenched and tempered bolts. Background Art

[0002] With the rapid development of industries such as aviation, automobiles, high-speed rail, ships, wind power, military industry, steel structures, and bridges, the demand for high-strength fasteners has increased, and the strength level of fasteners has been further increased. Take the automobile industry as an example. As the largest consumer of fastener products, the automotive industry requires a large number and variety of fasteners. With the increasing pressure on energy conservation and emission reduction in the industry, lightweighting of automobiles has become an important direction for the development of the industry. The use of high-strength fasteners can reduce weight and increase installation space by reducing their own size under the same clamping force. Therefore, the function and volume of the connected parts can be optimized, so that the equipment can achieve the purpose of overall weight reduction and performance optimization, which requires fasteners to have higher strength.

[0003] At present, the high-strength fasteners produced in industrialized mass production in the domestic industry include four grades: 8.8, 9.8, 10.9 and 12.9. The higher-strength fasteners that have been developed include 13.9 and 14.9, but most have not yet been put into industrialized mass production. With the continuous improvement of the strength level of fasteners, the concept of ultra-high-strength fasteners is being proposed. Now, fasteners of grade 14.9 and above can be preliminarily called ultra-high-strength fasteners. From the perspective of practical application, grade 14.9 is the most widely and urgently needed grade among the current ultra-high-strength fasteners, and its prospects for industrialized mass production are relatively large.

[0004] At present, some domestic steel mills have successively developed hot-rolled wire rod raw materials for manufacturing 14.9 grade fasteners. For hot-rolled wire rod raw materials, medium carbon steel with a carbon content of about 0.40% is usually used, and higher Cr and Mo alloying elements are added, and a certain amount of V and Nb microalloying elements are added to improve the strength and hardenability of the wire rod, and ensure the mechanical properties of the fasteners after quenching and tempering heat treatment. Since C is the main element affecting the cold plastic deformation of steel, the higher the carbon content, the lower the plasticity of the steel; the two alloying elements Cr and Mo can significantly improve the hardenability of steel, but also reduce the plasticity of steel, and significantly affect the metallographic structure and cold heading forming performance of hot-rolled steel; the two elements V and Ni can improve the strength and plasticity of steel, but due to their relatively expensive prices, they will cause a substantial increase in the manufacturing cost of fasteners, so they are not suitable as raw materials for industrial mass production of fasteners. On the other hand, the existing 14.9 grade fasteners have a more prominent delayed fracture problem, most of which is hydrogen-induced delayed fracture, which is also a bottleneck problem in the manufacturing and application of 14.9 grade fasteners.

[0005] Using the existing hot-rolled wire rod as raw material, the representative manufacturing process flow of 14.9 grade bolts is: hot-rolled wire rod - spheroidizing annealing - pickling - phosphating - saponification - drawing - spheroidizing annealing - pickling - phosphating - saponification - drawing - cold heading - tempering heat treatment - surface treatment - dehydrogenation treatment. Its preparation requires two spheroidizing annealings, two picklings, two phosphating and saponifications, and two drawing, followed by cold heading and tempering heat treatment of the material. Since the hot-rolled wire rod itself has poor plasticity and cannot be directly drawn and cold headed, the production process flow of the bolts is long and the processing cost is high. In addition, the cost of the hot-rolled wire rod itself is high, which ultimately leads to the high cost of 14.9 grade bolts and limited production.

[0006] In summary, in order to meet the manufacturing and application needs of ultra-high strength fasteners, it is urgent to develop a cold-heading steel hot-rolled wire rod with high strength and hardenability, good plasticity, and resistance to delayed fracture, shorten the manufacturing process of 14.9 grade bolts, reduce manufacturing costs, and solve the problem of delayed fracture of bolts. Summary of the invention

[0007] In order to solve the above problems, the present invention optimizes the chemical composition design, eliminates the use of alloy elements such as Cr, Mo, V, and Ni, and improves the strength and hardenability of steel through the combined use of C-Si-Mn-B elements. In particular, the large coil production line with hot curling function is combined to control rolling, and after rolling, it enters the insulation tunnel to control cooling, which significantly improves the plasticity of the hot-rolled wire rod; the Mn-S combination in the chemical composition is combined with the manufacturing process flow to effectively control the hydrogen content, build a hydrogen trap, and improve the delayed fracture resistance of the material. The cold-headed steel hot-rolled wire rod of the present invention is used as the raw material, without annealing, and quenched and tempered to produce 14.9-grade quenched and tempered bolts, and the bolts have no delayed fracture problem.

[0008] The chemical composition of the cold heading steel hot-rolled wire rod for delayed fracture resistant 14.9 grade quenched and tempered bolts of the present invention is designed to be C: 0.58-0.64%, Si: 0.35-0.45%, Mn: 1.70-1.90%, P: ≤0.010%, S: 0.025-0.035%, Al: 0.020-0.040%, B: 0.0025-0.0035%, Cr, Ni, Cu, Mo≤0.10% by weight.

[0009] Compared with the existing hot-rolled wire rod for manufacturing 14.9 grade bolts, the wire rod composition of the present invention eliminates the use of alloy elements such as Cr, Mo, V, and Ni; the combined use of C-Si-Mn-B elements improves the strength and hardenability of the steel, and provides a basis for the subsequent large coil production line with hot coiling function to control rolling and enter the insulation tunnel for controlled cooling after rolling to improve the plasticity of the wire rod; through the Mn-S combination, more irreversible hydrogen traps are constructed to prevent hydrogen embrittlement and improve the delayed fracture resistance of the wire rod.

[0010] A method for preparing a cold heading steel hot rolled wire rod for delayed fracture resistant 14.9 grade quenched and tempered bolts comprises a converter smelting process, an LF refining process, an RH vacuum refining process, a billet continuous casting process, a billet rolling process, a billet flaw detection process, a billet grinding process, and a wire rod rolling process.

[0011] Converter smelting process:

[0012] The converter loading capacity of the converter smelting process is 130 tons / furnace, of which 90% is molten iron and 10% is scrap steel; argon is blown bottom and stirred throughout the converter smelting process, the steel tapping temperature is ≥1600℃, the steel tapping carbon is controlled in the range of 0.08% to 0.12%, and the steel tapping P is ≤0.008%; deoxidizer (silicon carbide), alloy (silicon manganese alloy, high carbon ferromanganese), recarburizer, and slag are added in sequence starting from 1 / 4 of the steel tapping.

[0013] LF refining process:

[0014] In the early stage of LF refining process, a small amount of aluminum particles is added for deoxidation, and then appropriate aluminum wire is fed; in the later stage of LF smelting, boron iron wire, sulfide iron wire, high carbon ferromanganese and carburizer are added to adjust the chemical composition to the required range.

[0015] RH vacuum refining process:

[0016] The vacuum degree of the RH vacuum smelting process is <67Pa, the cycle time is ≥30 minutes, and the total RH smelting time is 45 to 60 minutes; 5 minutes before the end of RH, take samples to test the chemical composition and do gas analysis; after RH breaks the air, feed an appropriate amount of pure calcium wire, and soft blowing is ≥15 minutes.

[0017] Billet continuous casting process:

[0018] In the billet continuous casting process, the cross-section of the continuous casting billet is 300mm*325mm. The continuous casting superheat is controlled at 20-25℃, and the constant pulling speed control of 1.8±0.2m / min is implemented. The secondary cooling adopts strong cooling intensity. Alkaline covering agent is used in the continuous casting process to strengthen the argon protection between the large ladle shroud and the long shroud to maintain a slight positive pressure of 1.5-2.0Pa. Special protective slag for high carbon steel is used. After the continuous casting billet is off the line, it enters the slow cooling pit for slow cooling. The pit entry temperature is 800-900℃ and the slow cooling time is 48 hours.

[0019] Billet rolling process:

[0020] The billet rolling process uses a walking-beam heating furnace to heat the continuous casting billet at a temperature of 900-1200°C and a heating time of 3-4 hours. The heated billet is rolled into a 160mm*160mm billet with a cross-section of 300mm*325mm using 7 horizontal and vertical alternating rolling mills. The billet rolling temperature is 1050-1100°C. After the billet is off the line, it enters a slow cooling pit for slow cooling. The pit temperature is 700-800°C and the slow cooling time is 36 hours.

[0021] Rolling billet flaw detection process:

[0022] The billet flaw detection process uses magnetic particle flaw detectors and ultrasonic flaw detectors to check the surface quality and internal quality of the billet to ensure that the internal and external quality of the billet is normal.

[0023] Billet grinding process:

[0024] The billet grinding process uses a grinding wheel type peeling machine to grind the billet, wherein the grinding depth of the billet surface is 1.8 to 2.0 mm, and the grinding width of the billet corner is 15.0 to 20.0 mm.

[0025] Wire rod rolling process:

[0026] The wire rod rolling process adopts a large coil production line with hot coiling function. The rolled billet is heated at 750-1050℃ for 1.5-2.5 hours in a three-stage step heating furnace. The roughing, intermediate rolling and pre-finishing rolling units are temperature-controlled at 900-950℃, and the sizing unit is temperature-controlled at 750-800℃. After rolling, it is cooled to 700-730℃ in a water tank and hot-coiled and collected. The rolled coil immediately enters the insulation tunnel for controlled cooling, and is cooled to below 120℃ at a cooling rate of 2.5-3.0℃ / min, and then exits the insulation tunnel and naturally cools to room temperature in the air.

[0027] The present invention produces bolts by quenching and tempering heat treatment (quenching temperature is 860±10°C, quenching medium is quenching oil, tempering temperature is 450±10°C) of rolled cold heading steel wire rod without annealing treatment. The produced bolts can meet the requirements of grade 14.9 and the bolts have no delayed fracture problem.

[0028] Beneficial Effects

[0029] The present invention improves the strength and hardenability of steel by using a combination of C-Si-Mn-B elements, especially by combining a large coil production line with a hot curling function to control rolling, and entering a heat preservation tunnel to control cooling after rolling, which significantly improves the plasticity of the hot-rolled wire rod; the combination of Mn-S in the chemical composition, combined with the manufacturing process, effectively controls the hydrogen content, constructs a hydrogen trap, and improves the delayed fracture resistance of the material. The wire rod of the present invention does not need to add alloy elements such as Cr, Mo, V, and Ni, and its production cost is significantly reduced;

[0030] The production of 14.9-grade bolts using the wire rod of the present invention as raw material only requires one pickling, one phosphating saponification and one drawing, and no spheroidizing annealing and dehydrogenation treatment are required; compared with the prior art, the comprehensive production cost is significantly reduced, the production efficiency is significantly improved, the energy consumption is greatly reduced, and the pollutant emission is greatly reduced, thus solving the industry bottleneck of high manufacturing cost and limited output of 14.9-grade bolts. DETAILED DESCRIPTION

[0031] The present invention: The specific composition of the cold heading steel hot-rolled wire rod for delayed fracture resistant 14.9 grade quenched and tempered bolts is as follows:

[0032] Wt, %

[0033] C Si Mn P S Al B Cr, Ni, Cu, Mo 0.58-0.64 0.35-0.45 1.70-1.90 ≤0.010 0.025-0.035 0.020-0.040 0.0025-0.0035 0.032-0.065

[0034] The wire rod preparation process is: converter smelting - LF refining - RH vacuum refining - billet continuous casting - billet rolling - billet flaw detection - billet grinding - wire rod rolling.

[0035] The bolt preparation process is: hot-rolled wire rod - pickling - phosphating - saponification - drawing - cold heading - tempering heat treatment - surface treatment.

[0036] Example 1

[0037] 1. Chemical composition

[0038] The chemical composition is designed as C: 0.59%, Si: 0.44%, Mn: 1.75%, P: 0.006%, S: 0.027%, Al: 0.029%, B: 0.0028%, Cr: 0.04%, Ni: 0.01%, Cu: 0.02%, and Mo: 0.02% in weight percentage.

[0039] 2. Converter smelting

[0040] The converter loading capacity of the converter smelting process is 130 tons / furnace, of which 90% is molten iron and 10% is scrap steel; argon is blown bottom and stirred throughout the converter smelting process, the steel tapping temperature is 1605°C, the steel tapping carbon content is 0.09%, and the steel tapping P content is 0.003%; from 1 / 4 of the steel tapping, deoxidizer (120kg of silicon carbide / furnace), alloy (1500kg of silicon manganese alloy / furnace, 1100kg of high carbon ferromanganese / furnace), recarburizer (650kg / furnace), and slag (500kg of lime, 300kg of slag-reducing agent) are added in sequence.

[0041] 3. LF refining

[0042] In the early stage of LF refining process, a small amount (50kg / furnace) of aluminum particles is added for deoxidation, and then an appropriate amount (100kg / furnace) of aluminum wire is fed; in the later stage of LF smelting, boron iron wire, sulfide iron wire, high carbon ferromanganese and carburizer are added to adjust the chemical composition to the required range.

[0043] 4. RH vacuum refining

[0044] The vacuum degree of the RH vacuum smelting process is 65Pa, the cycle time is 38 minutes, and the total RH smelting time is 52 minutes; 5 minutes before the end of RH, samples are taken to test the chemical composition and perform gas analysis; after RH breaks through the air, an appropriate amount of pure calcium wire is fed and soft blowing is performed for 23 minutes.

[0045] 5. Continuous casting of steel billets

[0046] In the billet continuous casting process, the cross-section of the continuous casting billet is 300mm*325mm. The continuous casting superheat is controlled at 25℃, and the constant pulling speed control of 1.8m / min is implemented. The secondary cooling adopts strong cooling intensity. Alkaline covering agent is used in the continuous casting process to strengthen the argon protection between the large ladle shroud and the long shroud to maintain a slight positive pressure of 1.8Pa. Special protective slag for high carbon steel is used. After the continuous casting billet is off the line, it enters the slow cooling pit for slow cooling. The pit entry temperature is 858℃ and the slow cooling time is 48 hours.

[0047] 6. Billet rolling

[0048] The billet rolling process uses a walking-beam heating furnace to heat the continuous casting billet, with a heating temperature of 915~1170℃ and a heating time of 3.5 hours; the heated billet is rolled into a 160mm*160mm billet with a cross-section of 300mm*325mm using 7 sets of horizontal and vertical alternating rolling mills, and the billet rolling temperature is 1058~1077℃; after the billet is off the line, it enters a slow cooling pit for slow cooling, with an entry pit temperature of 726℃ and a slow cooling time of 36 hours.

[0049] 7. Rolling billet flaw detection

[0050] The billet flaw detection process uses magnetic particle flaw detectors and ultrasonic flaw detectors to check the surface quality and internal quality of the billet to ensure that the internal and external quality of the billet is normal.

[0051] 8. Rolling mill grinding

[0052] The billet grinding process uses a grinding wheel type peeling machine to grind the billet, where the grinding depth of the billet face is 1.8mm and the grinding width of the billet corner is 20.0mm.

[0053] 9. Wire rod rolling

[0054] The wire rod rolling process adopts a large coil production line with hot coiling function. The rolled billet is heated at 758-1040℃ for 1.8 hours in a three-stage step heating furnace. The roughing, intermediate rolling and pre-finishing rolling units are temperature-controlled at 935℃, and the sizing unit is temperature-controlled at 762℃. After rolling, it is cooled to 710℃ in a water tank and hot-coiled and collected. The rolled coil immediately enters the insulation tunnel for controlled cooling, and is cooled to 102℃ at a cooling rate of 2.6℃ / min, and then naturally cooled to room temperature in the air out of the insulation tunnel.

[0055] Example 2

[0056] 1. Chemical composition

[0057] The chemical composition is designed as C: 0.62%, Si: 0.36%, Mn: 1.73%, P: 0.005%, S: 0.033%, Al: 0.037%, B: 0.0032%, Cr: 0.02%, Ni: 0.01%, Cu: 0.03%, and Mo: 0.01% in weight percentage.

[0058] 2. Converter smelting

[0059] The converter loading capacity of the converter smelting process is 130 tons / furnace, of which 90% is molten iron and 10% is scrap steel; argon is blown bottom and stirred throughout the converter smelting process, the steel tapping temperature is 1608°C, the steel tapping carbon content is 0.10%, and the steel tapping P content is 0.006%; from 1 / 4 of the steel tapping, deoxidizer (120kg / furnace of silicon carbide), alloy (1500kg / furnace of silicon manganese alloy, 1050kg / furnace of high carbon ferromanganese), recarburizer (700kg / furnace), and slag (500kg of lime, 300kg of slag-reducing agent) are added in sequence.

[0060] 3. LF refining

[0061] In the early stage of LF refining process, a small amount (60kg / furnace) of aluminum particles is added for deoxidation, and then an appropriate amount (110kg / furnace) of aluminum wire is fed; in the later stage of LF smelting, boron iron wire, sulfide iron wire, high carbon ferromanganese and recarburizer are added to adjust the chemical composition to the required range.

[0062] 4. RH vacuum refining

[0063] The vacuum degree of the RH vacuum smelting process is 61Pa, the cycle time is 40 minutes, and the total RH smelting time is 55 minutes; 5 minutes before the end of RH, samples are taken to test the chemical composition and perform gas analysis; after RH breaks through the air, an appropriate amount of pure calcium wire is fed and soft blowing is performed for 18 minutes.

[0064] 5. Continuous casting of steel billets

[0065] In the billet continuous casting process, the cross-section of the continuous casting billet is 300mm*325mm. The continuous casting superheat is controlled at 21℃, and the constant pulling speed control of 1.9m / min is implemented. The secondary cooling adopts strong cooling intensity. Alkaline covering agent is used in the continuous casting process to strengthen the argon protection between the large ladle shroud and the long shroud to maintain a slight positive pressure of 2.0Pa. Special protective slag for high carbon steel is used. After the continuous casting billet is off the line, it enters the slow cooling pit for slow cooling. The pit entry temperature is 886℃ and the slow cooling time is 48 hours.

[0066] 6. Billet rolling

[0067] The billet rolling process uses a walking-beam heating furnace to heat the continuous casting billet, with a heating temperature of 931~1195℃ and a heating time of 3.6 hours; the heated billet is rolled into a 160mm*160mm billet with a cross-section of 300mm*325mm using 7 horizontal and vertical alternating rolling mills, and the billet rolling temperature is 1066~1082℃; after the billet is off the line, it enters a slow cooling pit for slow cooling, with an entry pit temperature of 751℃ and a slow cooling time of 36 hours.

[0068] 7. Rolling billet flaw detection

[0069] The billet flaw detection process uses magnetic particle flaw detectors and ultrasonic flaw detectors to check the surface quality and internal quality of the billet to ensure that the internal and external quality of the billet is normal.

[0070] 8. Rolling mill grinding

[0071] The billet grinding process uses a grinding wheel type peeling machine to grind the billet, where the grinding depth of the billet surface is 2.0mm and the grinding width of the billet corner is 15.0mm.

[0072] 9. Wire rod rolling

[0073] The wire rod rolling process adopts a large coil production line with hot coiling function. The rolled billet is heated at 766-1049℃ for 2.3 hours in a three-stage step heating furnace. The roughing, intermediate rolling and pre-finishing rolling units are temperature-controlled at 945℃, and the sizing unit is temperature-controlled at 763℃. After rolling, it is cooled to 716℃ in a water tank and hot-coiled and collected. The rolled coil immediately enters the insulation tunnel for controlled cooling, and is cooled to 99℃ at a cooling rate of 2.8℃ / min, and then exits the insulation tunnel and naturally cools to room temperature in the air.

[0074] Example 3

[0075] 1. Chemical composition

[0076] The chemical composition is designed as C: 0.63%, Si: 0.35%, Mn: 1.89%, P: 0.006%, S: 0.032%, Al: 0.036%, B: 0.0033%, Cr: 0.02%, Ni: 0.01%, Cu: 0.03%, and Mo: 0.02% in weight percentage.

[0077] 2. Converter smelting

[0078] The converter loading capacity of the converter smelting process is 130 tons / furnace, of which 90% is molten iron and 10% is scrap steel; argon is blown bottom and stirred throughout the converter smelting process, the steel tapping temperature is 1608°C, the steel tapping carbon content is 0.10%, and the steel tapping P content is 0.006%; starting from 1 / 4 of the steel tapping, deoxidizer (120kg of silicon carbide / furnace), alloy (1500kg of silicon manganese alloy / furnace, 1100kg of high carbon ferromanganese / furnace), recarburizer (700kg / furnace), and slag (500kg of lime, 300kg of slag-reducing agent) are added in sequence.

[0079] 3. LF refining

[0080] In the early stage of LF refining process, a small amount (60kg / furnace) of aluminum particles is added for deoxidation, and then an appropriate amount (110kg / furnace) of aluminum wire is fed; in the later stage of LF smelting, boron iron wire, sulfide iron wire, high carbon ferromanganese and recarburizer are added to adjust the chemical composition to the required range.

[0081] 4. RH vacuum refining

[0082] The vacuum degree of the RH vacuum smelting process is 61Pa, the cycle time is 40 minutes, and the total RH smelting time is 55 minutes; 5 minutes before the end of RH, samples are taken to test the chemical composition and perform gas analysis; after RH breaks through the air, an appropriate amount of pure calcium wire is fed and soft blowing is performed for 18 minutes.

[0083] 5. Continuous casting of steel billets

[0084] In the billet continuous casting process, the cross-section of the continuous casting billet is 300mm*325mm. The continuous casting superheat is controlled at 21℃, and the constant pulling speed control of 1.9m / min is implemented. The secondary cooling adopts strong cooling intensity. Alkaline covering agent is used in the continuous casting process to strengthen the argon protection between the large ladle shroud and the long shroud to maintain a slight positive pressure of 2.0Pa. Special protective slag for high carbon steel is used. After the continuous casting billet is off the line, it enters the slow cooling pit for slow cooling. The pit entry temperature is 885℃ and the slow cooling time is 48 hours.

[0085] 6. Billet rolling

[0086] The billet rolling process uses a walking-beam heating furnace to heat the continuous casting billet, with a heating temperature of 933~1196℃ and a heating time of 3.6 hours; the heated billet is rolled into a 160mm*160mm billet with a cross-section of 300mm*325mm using 7 sets of horizontal and vertical alternating rolling mills, and the billet rolling temperature is 1068~1087℃; after the billet is off the line, it enters a slow cooling pit for slow cooling, with an entry pit temperature of 753℃ and a slow cooling time of 36 hours.

[0087] 7. Rolling billet flaw detection

[0088] The billet flaw detection process uses magnetic particle flaw detectors and ultrasonic flaw detectors to check the surface quality and internal quality of the billet to ensure that the internal and external quality of the billet is normal.

[0089] 8. Rolling mill grinding

[0090] The billet grinding process uses a grinding wheel type peeling machine to grind the billet, where the grinding depth of the billet surface is 2.0mm and the grinding width of the billet corner is 15.0mm.

[0091] 9. Wire rod rolling

[0092] The wire rod rolling process adopts a large coil production line with hot coiling function. The rolled billet is heated at 767-1044℃ for 2.3 hours in a three-stage step heating furnace. The roughing, intermediate rolling and pre-finishing rolling units are temperature-controlled at 946℃, and the sizing unit is temperature-controlled at 765℃. After rolling, it is cooled to 717℃ in a water tank and hot-coiled and collected. The rolled coil immediately enters the insulation tunnel for controlled cooling, and is cooled to 98℃ at a cooling rate of 2.8℃ / min, and then naturally cooled to room temperature in the air out of the insulation tunnel.

[0093] Comparative Example 1

[0094] The B content in the chemical composition in step 1 of Example 1 is replaced with 0.0010% by weight, and other conditions are the same as in Example 1.

[0095] Comparative Example 2

[0096] The S content in the chemical composition in step 1 of Example 1 is replaced with 0.005% by weight, and other conditions are the same as in Example 1.

[0097] Comparative Example 3

[0098] Replace the "after the continuous casting billet comes off the line, it enters the slow cooling pit for slow cooling, the pit entry temperature is 858°C, and the slow cooling time is 48 hours" in step 5 of Example 1 with "after the continuous casting billet comes off the line, it is pile cooled and naturally cooled to room temperature in the air", and replace the "after the rolled billet comes off the line, it enters the slow cooling pit for slow cooling, the pit entry temperature is 726°C, and the slow cooling time is 36 hours" in step 6 of Example 1 with "after the rolled billet comes off the line, it is pile cooled and naturally cooled to room temperature in the air", and other conditions are the same as in Example 1.

[0099] Comparative Example 4

[0100] The following steps in step 5 of Example 1 are replaced with "after rolling, the coil is cooled to 710°C in a water tank and hot-rolled and collected; the rolled coil immediately enters a heat preservation tunnel for controlled cooling and is cooled to 102°C at a cooling rate of 2.6°C / min, and then exits the heat preservation tunnel and is naturally cooled to room temperature in the air" with "after rolling, the coil is spun by a spinning machine at a spinning temperature of 720°C, and after rolling, it is controlled to be cooled by a Stelmor controlled cooling line at a cooling rate of 6°C / min to 450°C, and then is naturally cooled to room temperature in the air when it comes off the line", and other conditions are the same as those in Example 1.

[0101] The hot-rolled wire rods produced in the embodiments of the present invention and the comparative examples are used as raw materials. The bolt manufacturing process is as follows: hot-rolled wire rod (Φ26.0 mm) - pickling - phosphating - saponification - drawing (Φ25.2 mm) - cold heading - quenching and tempering heat treatment (quenching and tempering heat treatment, quenching temperature 860±10°C, quenching medium is quenching oil, tempering temperature 450±10°C) - surface treatment. The mechanical properties, cold heading cracking rate, and delayed fracture of the produced bolts are compared as shown in Table 1 below:

[0102] Table 1

[0103] category Yield strength, MPa Tensile strength, MPa Yield-to-strength ratio Cold heading cracking rate, % Delayed brittle fracture ratio, % Require ≥1260 ≥1400 ≥0.9 ≤5 ≤0.01 Example 1 1360 1462 0.93 1.35 0 Example 2 1392 1481 0.94 1.60 0 Example 3 1409 1499 0.94 1.55 0 Comparative Example 1 1045 1215 0.86 2.50 0 Comparative Example 2 1275 1466 0.87 4.50 25 Comparative Example 3 1235 1403 0.88 3.20 18 Comparative Example 4 1305 1450 0.90 68.66 10

Claims

1. A hot-rolled wire rod of cold heading steel for 14.9 grade quenched and tempered bolts with delayed fracture resistance, characterized in that: The chemical composition of the cold heading steel hot rolled wire rod is calculated by weight percentage as follows: C: 0.58-0.64%, Si: 0.35-0.45%, Mn: 1.70-1.90%, P: ≤0.010%, S: 0.025-0.035%, Al: 0.020-0.040%, B: 0.0025-0.0035%, and the total weight of Cr, Ni, Cu, and Mo is ≤0.10%; The preparation method of the cold heading steel hot rolled wire rod for delayed fracture resistant 14.9 grade quenched and tempered bolts is: converter smelting, LF refining, RH vacuum refining, billet continuous casting, billet rolling, billet flaw detection, billet grinding, and wire rod rolling; During the continuous casting process, the cross-section of the continuous casting billet is 300mm*325mm, the continuous casting superheat is controlled at 20-25℃, the constant casting speed control is 1.8±0.2m / min, and the secondary cooling adopts strong cooling intensity; the continuous casting process uses alkaline covering agent, strengthens the argon protection between the large ladle nozzle and the long nozzle, and maintains a slight positive pressure of 1.5~2.0Pa; uses special protective slag for high carbon steel; the continuous casting billet enters the slow cooling pit for slow cooling after it comes off the line, the pit temperature is 800-900℃, and the slow cooling time is 48 hours; The billet rolling adopts a walking beam heating furnace to heat the continuous casting billet, the heating temperature is 900-1200℃, and the heating time is 3-4 hours; the heated billet is rolled into a 160mm*160mm billet with a cross-section of 300mm*325mm by 7 horizontal and vertical alternating rolling mills, and the billet rolling temperature is 1050-1100℃; after the billet is rolled off the line, it enters the slow cooling pit for slow cooling, the pit temperature is 700-800℃, and the slow cooling time is 36 hours; The wire rod rolling adopts a large coil production line with hot coiling function. The rolled billet is heated at 750-1050℃ for 1.5-2.5 hours in a three-stage step heating furnace. The roughing, intermediate rolling and pre-finishing rolling units are temperature-controlled at 900-950℃, and the sizing unit is temperature-controlled at 750-800℃. After rolling, it is cooled to 700-730℃ in a water tank and hot-coiled and collected. The rolled coil immediately enters the insulation tunnel for controlled cooling, and is cooled to below 120℃ at a cooling rate of 2.5-3.0℃ / min, and then naturally cooled to room temperature in the air out of the insulation tunnel.

2. The delayed fracture resistant 14.9 grade quenched and tempered cold heading steel hot rolled wire rod for bolts according to claim 1, characterized in that: During the converter smelting process, the converter load is 130 tons / furnace, of which 90% is molten iron and 10% is scrap steel; argon is blown bottom and stirred throughout the converter smelting process, the steel tapping temperature is ≥1600℃, the steel tapping carbon is controlled in the range of 0.08%~0.12%, and the steel tapping P is ≤0.008%; deoxidizer, alloy, recarburizer and slag are added in sequence starting from 1 / 4 of the steel tapping.

3. The hot-rolled cold heading steel wire rod for delayed fracture resistant 14.9 grade quenched and tempered bolts according to claim 1, characterized in that: In the early stage of LF refining, aluminum particles are added for deoxidation, and then aluminum wire is fed; in the later stage of LF smelting, boron iron wire and sulfide iron wire are added, and high carbon ferromanganese and carburizer are added to adjust the chemical composition to the required range; the vacuum degree of RH vacuum smelting process is <67Pa, the cycle time is ≥30 minutes, and the total RH smelting time is 45 to 60 minutes; 5 minutes before the end of RH, samples are taken to test the chemical composition and perform gas analysis; pure calcium wire is fed after RH breaks the air, and soft blowing is ≥15 minutes.

4. The hot-rolled wire rod of cold heading steel for delayed fracture resistant 14.9 grade quenched and tempered bolts according to claim 1, characterized in that: The billet flaw detection uses magnetic particle flaw detector and ultrasonic flaw detector to check the surface quality and internal quality of the billet to ensure the normal internal and external quality of the billet; the billet grinding process uses a grinding wheel peeling machine to grind the billet, where the grinding depth of the billet surface is 1.8~2.0mm, and the grinding width of the billet corner is 15.0~20.0mm.