Cold heading steel wire rod and production method

Abstract

The present application relates to a kind of cold heading steel wire rod and production method, eliminate the addition B element and Ti element, reasonable wire rod rolling and off-line salt bath process, accurately control wire rod microstructure to be ferrite and pearlite, and the pearlite content is 40~50%, so that wire rod has suitable strength and excellent shear performance;Then wire rod is produced by drawing, cold rolling and obtains through core nut wire, and the tensile strength is 850~950MPa, and the hardness is 270~300HV;Through core nut wire is produced by shearing and cold heading and is self-punching nut of automobile, and shear break is flush, without collapse, and the mechanical property, shear performance and import material are equivalent, and import substitution can be realized.

Description

Technical Field

[0001] This invention relates to a cold heading steel wire rod and its production method, belonging to the field of iron and steel smelting technology. Background Technology

[0002] Wire rod (also known as wire) is produced by drawing, cold rolling or drawing + cold rolling to produce steel wires with cross-sections of round, rectangular, Z-shaped and other shapes. It is then further processed into fasteners, wire ropes, springs, pipes and other products, which are widely used in the automotive, machinery and energy industries.

[0003] Through-hole nut wire is a special-shaped steel wire used for automotive self-piercing nuts. The wire has a grooved cross-section (see attached image). Figure 1 (As shown). Through-hole nut wire is produced from wire rod through drawing and cold rolling, and then processed into automotive self-piercing nuts through shearing and cold heading. Currently, the main problems with domestically produced through-hole nut wire are wire shearing fracture and deformation, resulting in automotive self-piercing nuts whose dimensional accuracy cannot meet assembly requirements. Therefore, domestic automotive self-piercing nut manufacturers mainly use imported through-hole nut wire.

[0004] However, with the increasing domestic demand, the reliance on imported wire rods has led to a significant increase in costs, becoming an issue that cannot be ignored in automobile production. Referring to wire rods currently produced in related fields, such as the invention with application number 2026100215491, a wire rod for automotive caliper guide rods, a production method, and the resulting automotive caliper guide rod are disclosed. The chemical composition of the wire rod is: C 0.18~0.22%, Si 0.15~0.25%, Mn 0.80~1.00%, P≤0.015%, S≤0.010%, B 0.0005~0.0030%, Ti 0.01~0.04%, Al 0.015~0.035%, N≤0.004%, Cr≤0.05%, Ni≤0.05%, and Cr+Ni≤0.08%, with the remainder being Fe and unavoidable impurities. The production methods include converter smelting, LF refining, small billet continuous casting, high-speed wire rod rolling, and online salt bath processing, or converter smelting, LF refining, small billet continuous casting, high-speed wire rod rolling, Stellmore cooling, and offline salt bath processing. The wire rod produced by this method has a tensile strength of 700-800 MPa and a reduction of area of ​​70-78%. After drawing and cold heading necking, the resulting automotive caliper guide rod has a tensile strength of 1000-1150 MPa, a reduction of area of ​​58-65%, and a hardness of 29-36 HRC.

[0005] However, the main technical defects of the above-mentioned technology for the through-hole nut wire required in this application are as follows: 1. The addition of B and Ti elements to the chemical composition improves the hardenability of the steel. By controlling the microstructure of the wire rod to be bainite through online or offline salt bath treatment, the strength of the wire rod is too high, i.e., the tensile strength is 700~800MPa; 2. The microstructure of the wire rod is a single bainite structure, which deteriorates the shear performance of the wire rod and easily leads to shear fracture collapse and deformation.

[0006] Therefore, in order to solve the above problems, it is urgent to propose a production method for the through-hole nut line used in the production of self-piercing nuts for automobiles, so as to achieve import substitution. Summary of the Invention

[0007] This invention provides a cold heading steel wire rod and its production method, which eliminates the addition of B and Ti elements, and uses a reasonable wire rolling and offline salt bath process. The resulting wire rod, through drawing and cold rolling, produces through-hole nut wire with mechanical and shear properties comparable to imported materials, thus achieving import substitution.

[0008] The technical solution adopted by this invention to solve its technical problem is: A cold heading steel wire rod, said wire rod being used to manufacture through-hole nut wire, the chemical composition of which, by mass percentage, comprises: C: 0.18~0.23%, Si: 0.20~0.30%, Mn: 0.70~1.00%, P≤0.012%, S≤0.010%, Cr≤0.30%, Al: 0.020~0.035%, N≤0.0040%, Mn+Cr≤1.00%, with the remainder being Fe and unavoidable impurities; The microstructure of the wire rod is ferrite and pearlite, wherein the pearlite content is 40-50%; The method for producing the cold heading steel wire rod includes the following steps: Step S1, converter smelting, the temperature is set to 1600~1640℃ when the steel is tapped from the converter; Step S2, LF refining: The ladle is transported to the LF station, bottom-blown argon gas is used for stirring, and the molten steel is heated. During the heating process, lime and fluorite are added to create high-alkalinity refining slag, and the white slag refining time lasts for 10-15 minutes. Temperature is measured and samples are taken multiple times during the refining process until the temperature and composition are within the preset range. Step S3, RH vacuum treatment: transport the ladle to the RH station, start the vacuum pump, reduce the pressure in the vacuum chamber to a vacuum degree of 1.0~1.8mbar, and carry out deoxidation and denitrification treatment; blow argon through the riser pipe for soft stirring, and continuously circulate the molten steel between the vacuum chamber and the ladle. The argon soft stirring time lasts for 15~20 minutes. Step S4, large billet continuous casting: molten steel is cast using the large billet continuous casting process. The cross-section of the continuously cast billet is 300mm×390mm. The casting speed is set to 0.65±0.2m / min, the water content in the secondary cooling zone is 0.50±0.2L / kg, and the total reduction is controlled between 11.6 and 14.4mm. Step S5, billet opening: The continuously cast billet is heated and then rolled into a 140mm×140mm square billet through nine alternating vertical and horizontal rolling mills. Step S6, grinding: The billet is fully ground using a mechanical grinding machine. After grinding, the billet surface is free of cracks, pits, and grinding burrs. The surface roughness Ra of the billet is ≤50μm. Step S7, wire rod rolling: the billet is heated, rolled, and cooled in Stellmore to become wire rod with a diameter of 10-14 mm; the wire rod is collected into coils by a winding drum. Step S8, offline salt bath: the rolled wire rod is fed clockwise and enters the heating furnace through the straightening rollers. The heating temperature is 950±10℃, the heating time is 9~10min, and the carbon potential in the furnace is 0.22±0.01%. The heated wire rod is then immersed in the salt bath liquid in the salt tank. The salt bath temperature is set to 450~530℃ and the salt bath time is 130~150s. Furthermore, in step S1, the basicity of the final converter slag is 4.0~4.5; In step S2, the basicity of the refining slag is 3.5~4.5, and the bottom-blown argon flow rate is controlled at 200~400 NL / min; Furthermore, in step S3, the vacuum treatment time is maintained at 15-20 min, and the argon gas soft stirring intensity is 50-90 NL / min; Furthermore, in step S4, the segmented pressing amounts of the pressing rollers 1 to 11 are as follows: 0 mm, 0 mm, 0 mm, 0 mm, 0.5±0.2 mm, 0.5±0.2 mm, 2.5±0.2 mm, 3.0±0.2 mm, 2.5±0.2 mm, 2.5±0.2 mm, 1.5±0.2 mm; Furthermore, in step S5, the heating temperature of the continuously cast billet is 1260~1300℃, and the heating time lasts for 240~280min; Furthermore, in step S6, the single-sided grinding depth is set to 1.0±0.3mm, and the grinding wheel roughness is 24 mesh; Furthermore, in step S7, the billet heating temperature is 1100~1140℃, the initial rolling temperature is 990~1020℃, the finishing rolling inlet temperature is 880~920℃; the wire rod extrusion temperature is 860~880℃, the inlet section roller speed is 0.8~1.2m / s, all the insulation covers of the Stellmore cooling line are opened, and all the fans are turned off; Furthermore, in step S8, the clockwise wire feeding speed is set to 4.0~4.5m / min; Furthermore, the produced wire rod has a tensile strength of 530~560MPa, a reduction of area of ​​72~78%, and an elongation of 30~35%; the tensile strength fluctuation range within the same coil is ≤10MPa; after drawing and cold rolling, the wire rod obtained has a tensile strength of 850~950MPa and a hardness of 270~300HV; the drawing area reduction rate is 25~35%, and the cold rolling deformation rate is 30~40%.

[0009] By employing the above technical solutions, the present invention has the following beneficial effects compared to the prior art: 1. The cold heading steel wire rod provided by this invention has a reasonable content of elements such as carbon, silicon, manganese and chromium in its composition design, and strictly prohibits the addition of elements B and Ti, so as to avoid the improvement of steel hardenability caused by elements B and Ti from the source; 2. The cold heading steel wire rod production method provided by the present invention, based on the design of the chemical composition of the wire rod, matches reasonable wire rolling and offline salt bath processes to control the microstructure of the wire rod to ferrite + pearlite, and the pearlite content reaches 40~50%, which significantly improves the shear performance of the wire rod. The mechanical properties and shear performance of the through-hole nut wire obtained by drawing and cold rolling of this wire rod are comparable to those of imported materials. Attached Figure Description

[0010] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0011] Figure 1 This is a schematic diagram of the cross-section of the through-nut wire; Figure 2 This is a schematic diagram of the microstructure of wire rod ferrite + pearlite obtained in Example 1 of the present invention; Figure 3 This is a schematic diagram of the microstructure of wire rod ferrite + pearlite obtained in Comparative Example 2 provided by the present invention; Figure 4 This is a schematic diagram of the microstructure of wire rod bainite obtained in Comparative Example 3 provided by the present invention. Detailed Implementation

[0012] The invention will now be described in further detail with reference to the accompanying drawings.

[0013] Figure 1The image shows an irregular cross-section of the through-hole nut wire with grooves. When using domestically produced through-hole nut wire to produce automotive self-piercing nuts, problems such as fracture collapse and deformation easily occur during the shearing process, resulting in the dimensional accuracy of the automotive self-piercing nuts failing to meet assembly requirements. Therefore, this application improves upon this by addressing two aspects: First, the chemical composition is designed, with appropriate additions of elements such as carbon, silicon, manganese, and chromium, while excluding the addition of boron and titanium; second, a suitable wire rolling and offline salt bath process is matched. Ultimately, the microstructure of the wire rod is controlled to be ferrite and pearlite, with a pearlite content reaching 40-50%. Only after drawing and cold rolling can the through-hole nut wire obtained from this type of wire rod achieve mechanical and shearing properties comparable to imported materials.

[0014] This application relates to cold heading steel wire rod used for making through-hole nut wire, the chemical composition of which, by mass percentage, includes: C: 0.18~0.23%, Si: 0.20~0.30%, Mn: 0.70~1.00%, P≤0.012%, S≤0.010%, Cr≤0.30%, Al: 0.020~0.035%, N≤0.0040%, Mn+Cr≤1.00%, with the remainder being Fe and unavoidable impurities.

[0015] In the above composition, boron and titanium are excluded. As for other elements, carbon (C) is the most basic and cheapest strengthening element in steel, but as the carbon content increases, the plasticity of the wire rod deteriorates, reducing the cold heading performance and weldability of the steel. Therefore, the carbon content in this invention is controlled at 0.18~0.23%.

[0016] Silicon (Si) is a strengthening element in steel, which can improve the shear properties of steel, but excessive silicon will reduce the plasticity of wire rod. In this invention, the silicon content is controlled at 0.20~0.30%.

[0017] Manganese (Mn) is a strengthening element in steel, which can improve the strength and hardenability of steel. However, excessive manganese will reduce the plasticity and weldability of steel. In this invention, the manganese content is controlled at 0.70~1.00%.

[0018] Sulfur (S) is an impurity element in steel. Its segregation at grain boundaries causes grain embrittlement, thereby reducing the strength and plasticity of the steel. This invention controls the phosphorus content to below 0.012% and the sulfur content to below 0.010%.

[0019] Cr is a strengthening element in steel, which can improve the strength of steel, but excessive Cr will reduce the plasticity of steel. In this invention, the chromium content is controlled below 0.30%.

[0020] Al is a deoxidizing element in steel, effectively removing oxygen and improving its cleanliness. Additionally, Al reacts with nitrogen to form AlN, which helps refine the grain size. In this invention, the aluminum content is controlled between 0.020% and 0.035%.

[0021] Nitrogen (N) can reduce the plasticity of steel, affecting its cold heading properties. High nitrogen content can also coarsen AlN, hindering fine-grain strengthening. In this invention, the nitrogen content is controlled below 0.0040%.

[0022] When combined with subsequent production methods, the wire rod with this composition design exhibits a microstructure combining pearlite and ferrite, resulting in suitable strength and plasticity, as well as excellent uniformity throughout the rod. A pearlite content of 40-50% significantly improves shear performance.

[0023] The following provides a method for producing the cold heading steel wire rod, characterized by the following steps: Step S1, converter smelting, the temperature is set at 1600~1640℃ when the steel is tapped from the converter; the basicity of the final slag in the converter is 4.0~4.5.

[0024] Step S2, LF refining: The ladle is transported to the LF station, and bottom-blown argon is used for stirring, with the argon flow rate controlled at 200~400 NL / min. The molten steel is heated, and lime and fluorite are added during the heating process to create a high-basicity refining slag. The basicity of the refining slag is 3.5~4.5, and the white slag refining time lasts for 10~15 minutes. Temperature is measured and samples are taken multiple times during the refining process until the temperature and composition are within the preset range.

[0025] Step S3, RH vacuum treatment: transport the ladle to the RH station, start the vacuum pump, reduce the pressure in the vacuum chamber to a vacuum level of 1.0~1.8 mbar, and perform deoxidation and denitrification treatment. The vacuum treatment time is maintained for 15~20 min. Argon is blown through the riser pipe for soft stirring. The argon soft stirring intensity is 50~90 NL / min. The molten steel is continuously circulated between the vacuum chamber and the ladle. The argon soft stirring time lasts for 15~20 min.

[0026] Step S4, large billet continuous casting: molten steel is cast using the large billet continuous casting process. The cross-section of the continuously cast billet is 300mm×390mm. The casting speed is set to 0.65±0.2m / min, and the water content in the secondary cooling zone is 0.50±0.2L / kg. The overall total reduction is controlled between 11.6 and 14.4mm. The segmented reductions of the 1st to 11th reduction rollers are as follows: 0 mm, 0 mm, 0 mm, 0 mm, 0.5±0.2 mm, 0.5±0.2 mm, 2.5±0.2 mm, 3.0±0.2 mm, 2.5±0.2 mm, 2.5±0.2 mm, 1.5±0.2 mm.

[0027] Step S5, billet preparation: heat the continuous casting billet, set the heating temperature to 1260~1300℃, and the heating time to last 240~280min; then roll it into a 140mm×140mm square billet through nine alternating vertical and horizontal rolling mills.

[0028] Step S6, grinding: The billet is fully ground using a mechanical grinding machine. The grinding depth on one side is set to 1.0±0.3mm, the grinding wheel roughness is 24 mesh, and the surface of the billet after grinding is free of cracks, pits, and grinding burrs. The surface roughness Ra of the billet is ≤50μm.

[0029] Step S7, wire rod rolling: The billet is heated, rolled, and cooled in Stellmore to become a wire rod with a diameter of 10-14 mm. The billet heating temperature is 1100-1140℃, the initial rolling temperature is 990-1020℃, and the finishing mill inlet temperature is 880-920℃. The wire rod extrusion temperature is 860-880℃, and the inlet section roller speed is 0.8-1.2 m / s. All insulation covers of the Stellmore cooling line are opened, and all fans are turned off. The wire rod is collected into coils by a coiling drum and awaits offline salt bath treatment.

[0030] Step S8, offline salt bath: the rolled wire rod is fed clockwise and enters the heating furnace through the straightening rollers. The clockwise feeding speed of the wire rod is set to 4.0~4.5m / min, the heating temperature is 950±10℃, the heating time is 9~10min, and the carbon potential in the furnace is 0.22±0.01%. The heated wire rod is then immersed in the salt bath liquid in the salt tank. The salt liquid temperature is set to 450~530℃, and the salt bath time is 130~150s.

[0031] The final produced wire rod has a tensile strength of 530~560MPa, a reduction of area of ​​72~78%, and an elongation of 30~35%; the tensile strength fluctuation range within the same coil is ≤10MPa; after drawing and cold rolling, the wire rod obtained has a tensile strength of 850~950MPa and a hardness of 270~300HV; the drawing area reduction rate is 25~35%, and the cold rolling deformation rate is 30~40%.

[0032] To make the objectives, technical solutions, and advantages of this invention clearer, the design scheme of the steel smelting components and the corresponding preparation process of this invention will be described in detail below with reference to specific embodiments. It should be understood that the specific embodiments described herein are for illustrative purposes only and are not intended to limit the invention. Raw materials, equipment, and operating steps not specifically described herein are all conventional technologies in the field of steel smelting.

[0033] This application provides Embodiment 1, Embodiment 2, and Embodiment 3. Correspondingly, Comparative Example 1 is provided for Embodiment 1, Comparative Example 2 is provided for Embodiment 2, and Comparative Example 3 is provided for Embodiment 3, in order to better verify the advantages of the embodiments of this application.

[0034] The chemical composition designs of Examples 1, 2, and 3, and Comparative Examples 1, 2, and 3 are shown in Table 1.

[0035] Table 1 Chemical composition / %

[0036] Example 1:

[0037] The provided method for producing cold heading steel wire rod includes the following steps: Step S1, converter smelting, the temperature is set to 1600℃ when the steel is tapped from the converter; the basicity of the final slag in the converter is 4.5.

[0038] Step S2, LF refining: The ladle is transported to the LF station, and bottom-blown argon is used for stirring, with the argon flow rate controlled at 400 NL / min. The molten steel is heated, and lime and fluorite are added during the heating process to create a high-basicity refining slag. The basicity of the refining slag is 3.5, and the white slag refining time lasts for 15 minutes.

[0039] Step S3, RH vacuum treatment: The ladle is transported to the RH station, the vacuum pump is started, the vacuum chamber is depressurized to a vacuum degree of 1.0 mbar, and deoxidation and denitrification are performed. The vacuum treatment time is maintained for 15 minutes. Argon is blown through the riser pipe for soft stirring. The argon soft stirring intensity is 50 NL / min. The molten steel is continuously circulated between the vacuum chamber and the ladle. The argon soft stirring time lasts for 20 minutes.

[0040] Step S4, large billet continuous casting: molten steel is cast using the large billet continuous casting process. The cross-section of the continuously cast billet is 300mm×390mm. The casting speed is set to 0.63m / min, and the water content in the secondary cooling zone is 0.52L / kg. The total reduction is controlled at 11.6mm. The segmented reductions of the 1st to 11th reduction rollers are as follows: 0 mm, 0 mm, 0 mm, 0 mm, 0.3mm, 0.3mm, 2.3mm, 2.8mm, 2.3mm, 2.3mm, 1.3mm.

[0041] Step S5, billet preparation: The continuous casting billet is heated, and the heating temperature is set to 1260℃ for 280 minutes. Then, it is rolled into a 140mm×140mm square billet by a rolling mill with nine alternating vertical and horizontal rolling mills.

[0042] Step S6, grinding: The billet is fully ground using a mechanical grinding machine. The grinding depth on one side is set to 0.7 mm, and the grinding wheel roughness is 24 mesh. After grinding, the billet surface is free of cracks, pits, and grinding burrs. The surface roughness of the billet Ra=45μm.

[0043] Step S7, wire rod rolling: The billet is heated, rolled, and cooled in Stellmore to become a 10mm diameter wire rod. The billet heating temperature is 1100℃, the initial rolling temperature is 990℃, and the finishing mill inlet temperature is 920℃. The wire rod extrusion temperature is 880℃, the inlet section roller speed is 1.2m / s, all insulation covers of the Stellmore cooling line are opened, and all fans are turned off. The wire rod is collected into coils by the coiling drum and awaits offline salt bath treatment.

[0044] Step S8, offline salt bath: the rolled wire rod is fed clockwise and enters the heating furnace through the straightening rollers. The clockwise feeding speed of the wire rod is set to 4.0 m / min, the heating temperature is 960℃, the heating time is 10 min, and the carbon potential in the furnace is 0.21%. The heated wire rod is then immersed in the salt bath liquid in the salt tank. The salt liquid temperature is set to 530℃ and the salt bath time is 150 s.

[0045] Comparative Example 1: The provided method for producing cold heading steel wire rod includes the following steps: Step S1, converter smelting, the temperature is set to 1600℃ when the steel is tapped from the converter; the basicity of the final slag in the converter is 4.5.

[0046] Step S2, LF refining: The ladle is transported to the LF station, and bottom-blown argon is used for stirring, with the argon flow rate controlled at 400 NL / min. The molten steel is heated, and lime and fluorite are added during the heating process to create a high-basicity refining slag. The basicity of the refining slag is 3.5, and the white slag refining time lasts for 15 minutes.

[0047] Step S3, RH vacuum treatment: The ladle is transported to the RH station, the vacuum pump is started, the vacuum chamber is depressurized to a vacuum degree of 1.0 mbar, and deoxidation and denitrification are performed. The vacuum treatment time is maintained for 15 minutes. Argon is blown through the riser pipe for soft stirring. The argon soft stirring intensity is 50 NL / min. The molten steel is continuously circulated between the vacuum chamber and the ladle. The argon soft stirring time lasts for 20 minutes.

[0048] Step S4, large billet continuous casting: molten steel is cast using the large billet continuous casting process. The cross-section of the continuously cast billet is 300mm×390mm. The casting speed is set to 0.63m / min, and the water content in the secondary cooling zone is 0.52L / kg. The total reduction is controlled at 11.6mm. The segmented reductions of the 1st to 11th reduction rollers are as follows: 0 mm, 0 mm, 0 mm, 0 mm, 0.3mm, 0.3mm, 2.3mm, 2.8mm, 2.3mm, 2.3mm, 1.3mm.

[0049] Step S5, billet preparation: The continuous casting billet is heated, and the heating temperature is set to 1200℃ for 230 minutes. Then, it is rolled into a 140mm×140mm square billet by a rolling mill with nine alternating vertical and horizontal rolling mills.

[0050] Step S6, grinding: The billet is fully ground using a mechanical grinding machine. The grinding depth on one side is set to 0.7 mm, and the grinding wheel roughness is 24 mesh. After grinding, the billet surface is free of cracks, pits, and grinding burrs. The surface roughness of the billet Ra=45μm.

[0051] Step S7, wire rod rolling: The billet is heated, rolled, and cooled in Stellmore to become a 10mm diameter wire rod. The billet heating temperature is 1060℃, the initial rolling temperature is 940℃, and the finishing mill inlet temperature is 870℃. The wire rod exiting temperature is 840℃, the inlet section roller speed is 0.5m / s, all insulation covers of the Stellmore cooling line are opened, and all fans are turned off. The wire rod is collected into coils by the coiling drum and awaits offline salt bath treatment.

[0052] Step S8, offline salt bath: the rolled wire rod is fed clockwise and enters the heating furnace through the straightening rollers. The clockwise feeding speed of the wire rod is set to 3.5 m / min, the heating temperature is 910℃, the heating time is 11.5 min, and the carbon potential in the furnace is 0.21%. The heated wire rod is then immersed in the salt bath liquid in the salt tank. The salt liquid temperature is set to 540℃ and the salt bath time is 172 s.

[0053] Example 2:

[0054] The provided method for producing cold heading steel wire rod includes the following steps: Step S1, converter smelting, the temperature is set to 1620℃ when the converter taps the steel; the basicity of the final slag in the converter is 4.2.

[0055] Step S2, LF refining: The ladle is transported to the LF station, and bottom-blown argon is used for stirring, with the argon flow rate controlled at 200 NL / min. The molten steel is heated, and lime and fluorite are added during the heating process to create a high-basicity refining slag. The basicity of the refining slag is 4.5, and the white slag refining time lasts for 10 minutes.

[0056] Step S3, RH vacuum treatment: The ladle is transported to the RH station, the vacuum pump is started, the vacuum chamber is depressurized to a vacuum degree of 1.8 mbar, and deoxidation and denitrification are performed. The vacuum treatment time is maintained for 20 minutes. Argon is blown through the riser pipe for soft stirring. The argon soft stirring intensity is 90 NL / min. The molten steel is continuously circulated between the vacuum chamber and the ladle. The argon soft stirring time lasts for 15 minutes.

[0057] Step S4, large billet continuous casting: molten steel is cast using the large billet continuous casting process. The cross-section of the continuously cast billet is 300mm×390mm. The casting speed is set to 0.65m / min, and the water content in the secondary cooling zone is 0.50L / kg. The overall total reduction is controlled at 14.4mm. The segmented reductions of the 1st to 11th reduction rollers are as follows: 0 mm, 0 mm, 0 mm, 0 mm, 0.7mm, 0.7mm, 2.7mm, 3.2mm, 2.7mm, 2.7mm, 1.7mm.

[0058] Step S5, billet preparation: The continuous casting billet is heated, and the heating temperature is set to 1300℃ for 240 minutes. Then, it is rolled into a 140mm×140mm square billet by a rolling mill with nine alternating vertical and horizontal rolling mills.

[0059] Step S6, grinding: The billet is fully ground using a mechanical grinding machine. The grinding depth on one side is set to 1.3 mm, and the grinding wheel roughness is 24 mesh. After grinding, the billet surface is free of cracks, pits, and grinding burrs. The surface roughness of the billet Ra=50μm.

[0060] Step S7, wire rod rolling: The billet is heated, rolled, and cooled in Stellmore to become a 14mm diameter wire rod. The billet heating temperature is 1140℃, the initial rolling temperature is 1020℃, and the finishing mill inlet temperature is 880℃. The wire rod exiting temperature is 860℃, the inlet section roller speed is 0.8m / s, all insulation covers of the Stellmore cooling line are opened, and all fans are turned off. The wire rod is collected into coils by the coiling drum and awaits offline salt bath treatment.

[0061] Step S8, offline salt bath: the rolled wire rod is fed clockwise and enters the heating furnace through the straightening rollers. The clockwise feeding speed of the wire rod is set to 4.5 m / min, the heating temperature is 940℃, the heating time is 9 min, and the carbon potential in the furnace is 0.22%. The heated wire rod is then immersed in the salt bath liquid in the salt tank. The salt liquid temperature is set to 450℃ and the salt bath time is 130 s.

[0062] Comparative Example 2: The provided method for producing cold heading steel wire rod includes the following steps: Step S1, converter smelting, the temperature is set to 1620℃ when the converter taps the steel; the basicity of the final slag in the converter is 4.2.

[0063] Step S2, LF refining: The ladle is transported to the LF station, and bottom-blown argon is used for stirring, with the argon flow rate controlled at 200 NL / min. The molten steel is heated, and lime and fluorite are added during the heating process to create a high-basicity refining slag. The basicity of the refining slag is 4.5, and the white slag refining time lasts for 10 minutes.

[0064] Step S3, RH vacuum treatment: The ladle is transported to the RH station, the vacuum pump is started, the vacuum chamber is depressurized to a vacuum degree of 1.8 mbar, and deoxidation and denitrification are performed. The vacuum treatment time is maintained for 20 minutes. Argon is blown through the riser pipe for soft stirring. The argon soft stirring intensity is 90 NL / min. The molten steel is continuously circulated between the vacuum chamber and the ladle. The argon soft stirring time lasts for 15 minutes.

[0065] Step S4, large billet continuous casting: molten steel is cast using the large billet continuous casting process. The cross-section of the continuously cast billet is 300mm×390mm. The casting speed is set to 0.65m / min, and the water content in the secondary cooling zone is 0.50L / kg. The overall total reduction is controlled at 14.4mm. The segmented reductions of the 1st to 11th reduction rollers are as follows: 0 mm, 0 mm, 0 mm, 0 mm, 0.7mm, 0.7mm, 2.7mm, 3.2mm, 2.7mm, 2.7mm, 1.7mm.

[0066] Step S5, billet preparation: The continuous casting billet is heated, and the heating temperature is set to 1300℃ for 240 minutes. Then, it is rolled into a 140mm×140mm square billet by a rolling mill with nine alternating vertical and horizontal rolling mills.

[0067] Step S6, grinding: The billet is fully ground using a mechanical grinding machine. The grinding depth on one side is set to 1.3 mm, and the grinding wheel roughness is 24 mesh. After grinding, the billet surface is free of cracks, pits, and grinding burrs. The surface roughness of the billet Ra=50μm.

[0068] Step S7, wire rod rolling: The billet is heated, rolled, and cooled in Stellmore to become a 14mm diameter wire rod. The billet heating temperature is 1140℃, the initial rolling temperature is 1020℃, and the finishing mill inlet temperature is 880℃. The wire rod extrusion temperature is 860℃, the inlet section roller speed is 0.8m / s, all the insulation covers of the Stellmore cooling line are opened, and 1-8 fans are turned on at 100% airflow. The wire rod is collected into coils by a coiling drum.

[0069] Example 3:

[0070] The provided method for producing cold heading steel wire rod includes the following steps: Step S1, converter smelting, the temperature is set to 1640℃ when the steel is tapped from the converter; the basicity of the final slag in the converter is 4.0.

[0071] Step S2, LF refining: The ladle is transported to the LF station, and bottom-blown argon is used for stirring, with the argon flow rate controlled at 300 NL / min. The molten steel is heated, and lime and fluorite are added during the heating process to create a high-basicity refining slag. The basicity of the refining slag is 4.0, and the white slag refining time lasts for 12 minutes.

[0072] Step S3, RH vacuum treatment: The ladle is transported to the RH station, the vacuum pump is started, the vacuum chamber is depressurized to a vacuum degree of 1.3 mbar, and deoxidation and denitrification are performed. The vacuum treatment time is maintained for 18 minutes. Argon is blown through the riser pipe for soft stirring. The argon soft stirring intensity is 70 NL / min. The molten steel is continuously circulated between the vacuum chamber and the ladle. The argon soft stirring time lasts for 18 minutes.

[0073] Step S4, large billet continuous casting: molten steel is cast using the large billet continuous casting process. The cross-section of the continuously cast billet is 300mm×390mm. The casting speed is set to 0.67m / min, and the water content in the secondary cooling zone is 0.48L / kg. The overall total reduction is controlled at 13.0mm. The segmented reductions of the 1st to 11th reduction rollers are as follows: 0 mm, 0 mm, 0 mm, 0 mm, 0.5mm, 0.5mm, 2.5mm, 3.0mm, 2.5mm, 2.5mm, 1.5mm.

[0074] Step S5, billet preparation: The continuous casting billet is heated, and the heating temperature is set to 1280℃ for 260 minutes. Then, it is rolled into a 140mm×140mm square billet by a rolling mill with nine alternating vertical and horizontal rolling mills.

[0075] Step S6, grinding: The billet is fully ground using a mechanical grinding machine. The grinding depth on one side is set to 1.0 mm, and the grinding wheel roughness is 24 mesh. After grinding, the billet surface is free of cracks, pits, and grinding burrs. The surface roughness of the billet Ra=48μm.

[0076] Step S7, wire rod rolling: The billet is heated, rolled, and cooled in Stellmore to become a 12mm diameter wire rod. The billet heating temperature is 1120℃, the initial rolling temperature is 1000℃, and the finishing mill inlet temperature is 900℃. The wire rod extrusion temperature is 870℃, the inlet section roller speed is 1.0m / s, all insulation covers of the Stellmore cooling line are opened, and all fans are turned off. The wire rod is collected into coils by the coiling drum and awaits offline salt bath treatment.

[0077] Step S8, offline salt bath: the rolled wire rod is fed clockwise and enters the heating furnace through the straightening rollers. The clockwise feeding speed of the wire rod is set to 4.3 m / min, the heating temperature is 950℃, the heating time is 9.6 min, and the carbon potential in the furnace is 0.23%. The heated wire rod is then immersed in the salt bath liquid in the salt tank. The salt liquid temperature is set to 480℃ and the salt bath time is 140 s.

[0078] Comparative Example 3: The provided method for producing cold heading steel wire rod includes the following steps: Step S1, converter smelting, the temperature is set to 1640℃ when the steel is tapped from the converter; the basicity of the final slag in the converter is 4.0.

[0079] Step S2, LF refining: The ladle is transported to the LF station, and bottom-blown argon is used for stirring, with the argon flow rate controlled at 300 NL / min. The molten steel is heated, and lime and fluorite are added during the heating process to create a high-basicity refining slag. The basicity of the refining slag is 4.0, and the white slag refining time lasts for 12 minutes.

[0080] Step S3, RH vacuum treatment: The ladle is transported to the RH station, the vacuum pump is started, the vacuum chamber is depressurized to a vacuum degree of 1.3 mbar, and deoxidation and decarburization are performed. The vacuum treatment time is maintained for 18 minutes. Argon is blown through the riser pipe for soft stirring. The argon soft stirring intensity is 70 NL / min. The molten steel is continuously circulated between the vacuum chamber and the ladle. The argon soft stirring time lasts for 18 minutes.

[0081] Step S4, large billet continuous casting: molten steel is cast using the large billet continuous casting process. The cross-section of the continuously cast billet is 300mm×390mm. The casting speed is set to 0.67m / min, and the water content in the secondary cooling zone is 0.48L / kg. The overall total reduction is controlled at 13.0mm. The segmented reductions of the 1st to 11th reduction rollers are as follows: 0 mm, 0 mm, 0 mm, 0 mm, 0.5mm, 0.5mm, 2.5mm, 3.0mm, 2.5mm, 2.5mm, 1.5mm.

[0082] Step S5, billet preparation: The continuous casting billet is heated, and the heating temperature is set to 1280℃ for 260 minutes. Then, it is rolled into a 140mm×140mm square billet by a rolling mill with nine alternating vertical and horizontal rolling mills.

[0083] Step S6, grinding: The billet is fully ground using a mechanical grinding machine. The grinding depth on one side is set to 1.0 mm, and the grinding wheel roughness is 24 mesh. After grinding, the billet surface is free of cracks, pits, and grinding burrs. The surface roughness of the billet Ra=48μm.

[0084] Step S7, wire rod rolling: The billet is heated, rolled, and cooled in Stellmore to become a 12mm diameter wire rod. The billet heating temperature is 1120℃, the initial rolling temperature is 1000℃, and the finishing mill inlet temperature is 900℃. The wire rod extrusion temperature is 870℃, the inlet section roller speed is 1.0m / s, all insulation covers of the Stellmore cooling line are opened, and all fans are turned off. The wire rod is collected into coils by the coiling drum and awaits offline salt bath treatment.

[0085] Step S8, offline salt bath: the rolled wire rod is fed clockwise and enters the heating furnace through the straightening rollers. The clockwise feeding speed of the wire rod is set to 4.3 m / min, the heating temperature is 950℃, the heating time is 9.6 min, and the carbon potential in the furnace is 0.23%. The heated wire rod is then immersed in the salt bath liquid in the salt tank. The salt liquid temperature is set to 480℃ and the salt bath time is 140 s.

[0086] The mechanical properties and microstructure detection results of the wire rods obtained in Examples 1, 2, and 3, and Comparative Examples 1, 2, and 3 are shown in Table 2.

[0087] Table 2

[0088] As shown in Table 2, the tensile strength of the wire rods obtained in Examples 1, 2, and 3 is 530~560MPa, the reduction of area is 72~75%, the elongation is 30~35%, and the tensile strength of the same coil fluctuates within a range of 6~8MPa. Figure 2 A schematic diagram of the microstructure of the wire rod obtained in Example 1 is provided. Comparative Example 1 and Comparative Example 2 ( Figure 3 (A schematic diagram of the microstructure of the wire rod obtained in Comparative Example 2 is provided.) The tensile strength and pearlite content of the obtained wire rod are both lower than the requirements of this invention. In particular, the tensile strength of the wire rod obtained in Comparative Example 2 fluctuates significantly within the same coil, which does not meet the requirements of this invention. Comparative Example 3, due to the addition of B and Ti elements, has a bainitic microstructure in its wire rod. Figure 4 As shown), it obviously does not meet the requirements of the present invention; due to the presence of bainite, the tensile strength of the wire rod obtained in Comparative Example 3 is too high; the tensile strength, reduction of area and elongation of the wire rod obtained in Comparative Example 3 do not meet the requirements of the present invention.

[0089] Next, the wire rods obtained in Example 2 and Comparative Example 2 were drawn and cold-rolled to produce through-hole nut wire, and the tensile strength and hardness of the through-hole nut wire were tested. The obtained through-hole nut wire was sheared and cold-forged to form automotive self-piercing nuts, and the shear fracture surface was observed to determine whether there was collapse or deformation, which was used to evaluate the shear performance of the through-hole nut wire. Imported through-hole nut wire was used as a test comparison, and the test results are shown in Table 3.

[0090] Table 3

[0091] As shown in Table 3, the tensile strength, hardness, and shear properties of the through-hole nut wire produced from the wire rod obtained in Example 2 after drawing and cold rolling are comparable to those of the imported through-hole nut wire. The wire rod obtained in Comparative Example 2, due to its low pearlite content, produces through-hole nut wire with poor shear properties and a collapsed shear fracture surface. Furthermore, because the wire rod obtained in Comparative Example 2 has a lower tensile strength, the tensile strength and hardness of the produced through-hole nut wire do not meet the requirements of this invention.

[0092] In summary, the mechanical properties and shear properties of the cold heading steel wire rod and production method provided in this application are comparable to those of imported materials.

[0093] Those skilled in the art will understand that, unless otherwise defined, all terms used herein (including technical and scientific terms) have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains. It should also be understood that terms such as those defined in general dictionaries should be understood to have the same meaning as in the context of the prior art, and should not be interpreted in an idealized or overly formal sense unless defined as herein.

[0094] The meaning of "and / or" as used in this application includes situations where each exists alone or both exist simultaneously.

[0095] The term "connection" as used in this application can mean a direct connection between components or an indirect connection between components through other components.

[0096] Based on the above-described preferred embodiments of the present invention, and through the foregoing description, those skilled in the art can make various changes and modifications without departing from the inventive concept. The technical scope of this invention is not limited to the contents of the specification, but must be determined according to the scope of the claims.

Claims

1. A cold heading steel wire rod, characterized in that: The wire rod is used to manufacture through-hole nut wire, and its chemical composition, by mass percentage, includes: C: 0.18~0.23%, Si: 0.20~0.30%, Mn: 0.70~1.00%, P≤0.012%, S≤0.010%, Cr≤0.30%, Al: 0.020~0.035%, N≤0.0040%, Mn+Cr≤1.00%, with the remainder being Fe and unavoidable impurities; The microstructure of the wire rod is ferrite and pearlite, wherein the pearlite content is 40-50%.

2. The method for producing cold heading steel wire rod according to claim 1, characterized in that: Includes the following steps: Step S1, converter smelting, the temperature is set to 1600~1640℃ when the steel is tapped from the converter; Step S2, LF refining: The ladle is transported to the LF station, bottom-blown argon gas is used for stirring, and the molten steel is heated. During the heating process, lime and fluorite are added to create high-alkalinity refining slag, and the white slag refining time lasts for 10-15 minutes. Temperature is measured and samples are taken multiple times during the refining process until the temperature and composition are within the preset range. Step S3, RH vacuum treatment: transport the ladle to the RH station, start the vacuum pump, reduce the pressure in the vacuum chamber to a vacuum degree of 1.0~1.8mbar, and carry out deoxidation and denitrification treatment; blow argon through the riser pipe for soft stirring, and continuously circulate the molten steel between the vacuum chamber and the ladle. The argon soft stirring time lasts for 15~20 minutes. Step S4, large billet continuous casting: molten steel is cast using the large billet continuous casting process. The cross-section of the continuously cast billet is 300mm×390mm. The casting speed is set to 0.65±0.2m / min, the water content in the secondary cooling zone is 0.50±0.2L / kg, and the total reduction is controlled between 11.6 and 14.4mm. Step S5, billet opening: The continuously cast billet is heated and then rolled into a 140mm×140mm square billet through nine alternating vertical and horizontal rolling mills. Step S6, grinding: The billet is fully ground using a mechanical grinding machine. After grinding, the billet surface is free of cracks, pits, and grinding burrs. The surface roughness Ra of the billet is ≤50μm. Step S7, wire rod rolling: the billet is heated, rolled, and cooled in Stellmore to become wire rod with a diameter of 10-14 mm; the wire rod is collected into coils by a winding drum. Step S8, offline salt bath: the rolled wire rod is laid out clockwise and enters the heating furnace through the straightening rollers. The heating temperature is 950±10℃, the heating time is 9~10min, and the carbon potential in the furnace is 0.22±0.01%. The heated wire rod is then immersed in the salt bath liquid in the salt tank. The salt liquid temperature is set to 450~530℃ and the salt bath time is 130~150s.

3. The method for producing cold heading steel wire rod according to claim 2, characterized in that: In step S1, the basicity of the final converter slag is 4.0~4.5; In step S2, the basicity of the refining slag is 3.5~4.5, and the bottom-blown argon flow rate is controlled at 200~400 NL / min.

4. The method for producing cold heading steel wire rod according to claim 2, characterized in that: In step S3, the vacuum treatment time is maintained at 15~20 min, and the argon gas soft stirring intensity is 50~90 NL / min.

5. The method for producing cold heading steel wire rod according to claim 2, characterized in that: In step S4, the segmented pressing amounts of the pressing rollers 1 to 11 are as follows: 0 mm, 0 mm, 0 mm, 0 mm, 0.5±0.2 mm, 0.5±0.2 mm, 2.5±0.2 mm, 3.0±0.2 mm, 2.5±0.2 mm, 2.5±0.2 mm, 1.5±0.2 mm.

6. The method for producing cold heading steel wire rod according to claim 2, characterized in that: In step S5, the heating temperature of the continuously cast billet is 1260~1300℃, and the heating time lasts for 240~280min.

7. The method for producing cold heading steel wire rod according to claim 2, characterized in that: In step S6, the single-sided grinding depth is set to 1.0±0.3mm, and the grinding wheel roughness is 24 mesh.

8. The method for producing cold heading steel wire rod according to claim 2, characterized in that: In step S7, the billet heating temperature is 1100~1140℃, the initial rolling temperature is 990~1020℃, the finishing rolling inlet temperature is 880~920℃; the wire rod extrusion temperature is 860~880℃, the inlet section roller speed is 0.8~1.2m / s, all the insulation covers of the Stellmore cooling line are opened, and all the fans are turned off.

9. The method for producing cold heading steel wire rod according to claim 2, characterized in that: In step S8, the clockwise wire feeding speed is set to 4.0~4.5m / min.

10. The method for producing cold heading steel wire rod according to claim 2, characterized in that: The produced wire rod has a tensile strength of 530~560MPa, a reduction of area of ​​72~78%, and an elongation of 30~35%; the tensile strength fluctuation range within the same coil is ≤10MPa; after drawing and cold rolling, the wire rod obtained has a tensile strength of 850~950MPa and a hardness of 270~300HV; the drawing area reduction rate is 25~35%, and the cold rolling deformation rate is 30~40%.

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