Low-carbon cold heading steel wire rod and preparation method and application thereof

Abstract

The invention relates to a low-carbon cold heading steel wire rod. The low-carbon cold heading steel wire rod comprises the following chemical components in percentage by weight: 0.06-0.09% of C; less than or equal to 0.30% of Si; 0.25 to 0.32 percent of Mn; p is less than or equal to 0.03%; s is less than or equal to 0.03%; 0.012% to 0.025% of Al; and the balance of iron and inevitable impurities. The invention further provides a preparation method of the low-carbon cold heading steel wire rod, and the low-carbon cold heading steel wire rod is prepared through a molten steel smelting procedure, an LF refining procedure, a continuous casting procedure, a heating procedure, a temperature control rolling procedure and a slow cooling procedure which are sequentially carried out. According to the mechanical properties of the low-carbon cold heading steel wire rod, the percentage reduction of area is larger than or equal to 75% and smaller than or equal to 81%, the percentage elongation after fracture is larger than or equal to 33% and smaller than or equal to 37%, the tensile strength ranges from 335 MPa to 380 MPa, the yield strength ranges from 225 MPa to 275 MPa, the hardness ranges from 52 HRB to 56 HRB, the 1 / 4 cold heading qualification rate can reach 100%, and the low-carbon cold heading steel wire rod can be applied to cold heading parts.

Description

Technical Field

[0001] The invention relates to the field of steel material production, in particular to a low-carbon cold heading steel wire rod and a preparation method and application thereof. Background Art

[0002] Cold heading steel wire and wire rod are mainly used to manufacture fasteners such as bolts, nuts, screws, rivets, self-tapping screws and cold heading parts. According to the characteristics of cold heading steel products, the cold heading method is used to produce super-standard parts, which has a fast deformation speed, large deformation degree, and uneven deformation. After the standard parts are formed, heat treatment and surface treatment are required, and they need to withstand various stresses during use. Therefore, cold heading steel must have excellent surface quality and good cold formability. And the cold heading steel deformation must have as little resistance as possible and the highest possible deformation capacity.

[0003] According to statistics from standard parts factories, 80% of cold cracks are caused by defects such as horizontal lines, scratches, and microcracks on the surface of the steel wire. Therefore, the surface quality requirements for cold heading steel wire are very strict, with the dimensional tolerance range controlled at ±0.20mm and the depth of cracks and scratches less than 0.07mm.

[0004] As the proportion of ingredients in cold heading products becomes increasingly perfect, the surface quality of wire rod becomes the decisive factor for the excellent quality of cold heading products. Summary of the invention

[0005] The purpose of the present invention is to provide a low-carbon cold-heading steel wire rod and a preparation method and application thereof, so that the quality and performance of the prepared low-carbon cold-heading steel wire rod are optimized.

[0006] In order to achieve the above object, the technical solution adopted by the present invention is: The first aspect of the present invention provides a low carbon cold heading steel wire rod, wherein the chemical composition of the low carbon cold heading steel wire rod is as follows in terms of weight percentage: C: 0.06~0.09%; Si≤0.30%; Mn: 0.25~0.32%; P≤0.03%; S≤0.03%; Al: 0.012~0.025%; the balance is Fe and unavoidable impurities.

[0007] Furthermore, the chemical composition of the low carbon cold heading steel wire rod is calculated by weight percentage as follows: C: 0.07%; Si≤0.10%; Mn: 0.27%; P≤0.025%; S≤0.025%; Al: 0.015%; the remainder is Fe and unavoidable impurities.

[0008] The second aspect of the present invention provides a method for preparing a low-carbon cold heading steel wire rod, by sequentially performing a molten steel smelting process, a LF refining process, a continuous casting process, a heating process, a temperature-controlled rolling process and a slow cooling process to obtain the low-carbon cold heading steel wire rod.

[0009] Furthermore, the molten steel smelting process includes: feeding the smelting raw materials consisting of molten iron and scrap steel into a converter for smelting, and adding aluminum blocks, alloys, and lime into the molten steel in the order of tapping; wherein, the target sample of the ladle is: C≤0.06%; Si≤0.08%; Mn: 0.20%~0.27%, Al: 0.007%~0.030%; the final molten steel tapping C: 0.03~0.06%, P≤0.030%; and the tapping temperature is 1620~1670℃.

[0010] Furthermore, the LF refining process includes: adding lime to the molten steel obtained in the molten steel smelting process, blowing argon and stirring during the process, feeding the aluminum wire into the LF at the station, adjusting the content of other alloys to meet the target range, and then feeding the new granular silicon calcium wire for soft blowing, the time is ≥7 minutes.

[0011] The continuous casting process includes: performing continuous casting protection casting on the molten steel obtained in the LF refining process, the start pouring temperature of the molten steel is 1554-1579°C, and the start pouring superheat is 25-50°C; the continuous pouring temperature is 1544-1559°C, and the continuous pouring superheat is 15-30°C.

[0012] Furthermore, the heating process includes: heating the steel billet obtained in the casting process in sections, wherein the temperature of the preheating section is 700-800°C, the temperature of the first heating section is 770-890°C, the temperature of the second heating section is 880-990°C, and the temperature of the soaking section is 980-1050°C.

[0013] Furthermore, the temperature-controlled rolling process includes: descaling the steel billet obtained in the heating process with high-pressure water, rolling it into wire rod, and then water-cooling it for spinning, wherein the descaling water pressure is 10-15 MPa; the starting rolling temperature is 920-960°C, the finishing mill inlet temperature is ≤900°C; and the spinning temperature is 880-920°C.

[0014] Furthermore, the slow cooling process includes: placing the wire rod obtained in the temperature-controlled rolling process on the Stelmor slow cooling line for insulation and slow cooling, and then performing a second section of insulation corridor slow cooling; wherein, the slow cooling inlet temperature of the second section of insulation corridor is 700~770℃, the insulation and slow cooling time is 75~150min, and the wire rod exiting the insulation corridor temperature: wire rod with specification of φ5.5mm~φ8mm is less than 400℃, and wire rod with specification of φ9mm~φ24mm is less than 500℃.

[0015] A third aspect of the present invention provides a use of a low-carbon cold heading steel wire rod in cold heading parts.

[0016] Due to the application of the above technical solution, the present invention has the following advantages compared with the prior art: The invention reasonably designs and limits the weight percentages of chemical components in a cold heading steel wire rod, specifically, C: 0.06-0.09%; Si≤0.30%; Mn: 0.25-0.32%; P≤0.03%; S≤0.03%; Al: 0.012-0.025%; the balance is iron and inevitable impurities, thereby providing a low-carbon cold heading steel wire rod, wherein the prepared wire rod has a cross-sectional shrinkage of 75-81%, an elongation after fracture of 33-37%, a tensile strength of 335-380 MPa, a yield strength of 225-275 MPa, a hardness of 52-56 HRB, and a 1 / 4 cold heading qualified rate of 100%, and the low-carbon cold heading steel wire rod has good plasticity and tensile strength, and is excellent in surface quality and cold heading performance. DETAILED DESCRIPTION

[0017] The technical solution of the present invention will be described clearly and completely below. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0018] The present invention provides a low carbon cold heading steel wire rod, the chemical composition of the low carbon cold heading steel wire rod is calculated by weight percentage: C: 0.06~0.09%; Si≤0.30%; Mn: 0.25~0.32%; P≤0.03%; S≤0.03%; Al: 0.012~0.025%; the balance is iron and unavoidable impurities.

[0019] In a specific embodiment of the present invention, the chemical composition of the low carbon cold heading steel wire rod is calculated by weight percentage as follows: C: 0.07%; Si≤0.10%; Mn: 0.27%; P≤0.025%; S≤0.025%; Al: 0.015%; the remainder is Fe and unavoidable impurities.

[0020] Specifically, since the addition of C element to steel can improve the hardenability and hardenability of the steel, but the increase in C content will lead to increased hardness, reduced plasticity and poor cold heading performance of the steel, so the C content is controlled within 0.06-0.09% in the present invention.

[0021] The Si element can replace iron atoms in steel by substitution, which can hinder dislocation movement and significantly improve the strength of the ferrite phase. However, the Si content in the steel should not be too high. A higher Si content will increase the hardness of the material after annealing, which is not conducive to cold deformation and will reduce the life of the forming mold. At the same time, a higher Si content will also reduce the low-temperature impact toughness of the steel. Based on this, the weight percentage content of the Si element is controlled to ≤0.30% in the present invention.

[0022] Mn is an austenite stabilizing element. Adding an appropriate amount of Mn to steel can effectively improve the hardenability and strength of the steel. Adding a certain amount of Mn has little effect on the plasticity of the steel, but Mn is prone to segregation during the solidification process of the ferroalloy. Therefore, in order to ensure the strength of the alloy and a certain hardenability, the Mn content in the steel is controlled at 0.25~0.32%.

[0023] The phosphorus element is a harmful element in steel. The plasticity and toughness of steel with a high phosphorus content are close to zero at low temperatures, resulting in cold brittleness. At the same time, phosphorus can dissolve in ferrite, reducing the plasticity of ferrite and increasing its strength and hardness. Therefore, the phosphorus content in the present invention is controlled to be ≤0.03%.

[0024] The addition of S element will deteriorate the hot working performance, reduce the plasticity and toughness of steel, make the steel easy to crack, tensile deformation and fracture, and also reduce the pressure bearing and impact strength of steel and reduce the fatigue resistance of steel. Therefore, in the present invention, the S content is controlled to ≤0.03%, and the content should not be too high.

[0025] The Al element can refine the steel grains and improve the plasticity. At the same time, as a deoxidizer, it can improve the purity. However, if the Al content is too high, it is easy to generate aluminum oxide inclusions. Therefore, in the present invention, the Al content is controlled to 0.012-0.025%.

[0026] The low carbon cold heading steel wire rod is prepared by sequentially performing a molten steel smelting process, a LF refining process, a continuous casting process, a heating process, a temperature-controlled rolling process and a slow cooling process. Specifically, each of the above processes is described in detail below.

[0027] (1) Molten steel smelting process: Molten steel is obtained by converter steelmaking with molten iron and scrap steel as raw materials. During the steel tapping process, aluminum block-alloy-lime are added in order, and silicon manganese and ferrosilicon are used for alloying. On average, 1kg of aluminum block, 4kg of lime, 0.5kg of ferrosilicon and 1-3kg of silicon manganese are added to every ton of steel. The amount of alloy added is added according to the amount of residual manganese content at the end point, and the target sample of the ladle is controlled: C≤0.06; Si≤0.08; Mn: 0.20%~0.27%, Al: 0.007%~0.030% and the end point molten steel tapping C: 0.03~0.06%, P≤0.030%; the tapping temperature of the molten steel is controlled at 1620~1670℃.

[0028] (2) LF refining process Add 3~4kg / t of lime to the molten steel in the steelmaking process, AF-5-F to adjust the slag fluidity, weakly stir and control the argon stirring intensity before slag formation in the early stage; LF refining is frequently bleached, and calcium carbide is bleached in small amounts and multiple times to maintain a reducing atmosphere, and a slightly positive pressure operation is maintained throughout the process. LF arrives at the station to feed 100~200m of aluminum wire (reference amount), and take samples after the slag surface is clean and the molten steel temperature is ≥1560℃ after 10 minutes of power-on, and adjust the composition in time according to the first sample. Feed 60 meters of new silicon calcium wire (100 meters in the pouring furnace), and the soft blowing time is not less than 7 minutes.

[0029] (3) Continuous casting process The molten steel in the above LF refining process is subjected to continuous casting protection casting, and the pouring temperature is controlled at 1554-1579℃ (overheat temperature 25-50℃) for the open pouring furnace and 1544-1559℃ (overheat temperature 15-30℃) for the continuous pouring furnace; the crystallizer vibration adopts small amplitude and high frequency to reduce the depth of vibration marks; the secondary cooling adopts mist cooling: the distribution ratio of the secondary cooling zone is: 28% for zone one, 26% for zone two, 22% for zone three, and 24% for zone four, the compressed air pressure is 0.20MPa, and long water inlet argon seal + immersed water inlet protection casting are adopted throughout the process to prevent secondary oxidation.

[0030] (4) Heating process The steel billet obtained by the above continuous casting process is heated in sections by a regenerative blast furnace gas heating furnace so that the temperature difference between the head, tail and middle of the steel billet does not exceed 30°C, wherein the preheating section temperature is 700~800°C, the first section heating temperature is 770~890°C, the second section heating temperature is 880~990°C, and the equalizing section temperature is 980~1050°C.

[0031] (5) Temperature controlled rolling process The steel billet obtained by the heating process is descaled under high-pressure water, and the descaling water pressure is 10~15Mpa to effectively remove the iron oxide scale on the surface of the steel billet. The steel billet is rolled after descaling, and the starting rolling temperature is 920~960℃, the inlet temperature of the finishing mill is ≤900℃, and the outlet temperature is ≤1020℃. According to different specifications of products, the final rolling speed is 15~110m / s, and the plasticity of the rolled wire rod is improved by low-temperature rolling.

[0032] Specifically, the steel billet needs to undergo rough rolling (rolling piece size φ73mm±1mm, rolling piece temperature 850-950℃), intermediate rolling (rolling piece size φ27~32mm±0.6mm, rolling piece temperature 920~1000℃), pre-finishing rolling (rolling piece size is φ16.8~24.2mm±0.2mm round rolled piece, rolling piece temperature 950~1020℃) and finishing rolling (10-stand top-cross 45° twist-free rolling mill, the future material will be rolled into φ5.5mm~24mm wire finished product according to different finished product specifications) to obtain wire.

[0033] After the final rolling, the wire rod is water-cooled before spinning, and the spinning temperature is 880-920℃, that is, water cooling is carried out between the rolling mill and the spinning machine - three or more water cooling tanks are arranged in sequence between the rolling mill and the spinning machine. When the wire rod passes through the three water cooling tanks after leaving the rolling mill, only one or two water cooling tanks close to the spinning machine are opened, and the rest of the water cooling tanks are closed. Of course, it is not limited to this.

[0034] (6) Slow cooling process The wire rod obtained by the above-mentioned temperature-controlled rolling process is kept warm and slowly cooled on the Stelmor slow cooling line, the insulation cover and the fan tuyere are all closed, and the cooling rate of the wire rod in the phase change temperature range is ≤0.5℃ / s, thereby extending the residence time of the wire rod in the phase change temperature range.

[0035] Specifically, the wire rods in the STEL slow cooling line are hot coiled after passing through four rollers. According to different product specifications, the entrance roller speed is 0.160~0.400m / s, and the exit roller speed is 0.184~0.462m / s (actually there may be a ±5% deviation). This can further ensure that a lower cooling rate is maintained, and the residence time in the phase change temperature range is extended to improve the uniformity of the internal structure of the wire rod.

[0036] The wire rods after slow cooling on the Stelmore line are collected at a temperature of 730~850℃, and then enter the insulation corridor at a temperature of 700~770℃ for the second insulation slow cooling. According to the different specifications of the wire rods, the slow cooling time in the insulation corridor is controlled at 75~150min to make the metallographic transformation of the product more complete and obtain a good balanced structure. At the same time, the stress concentration of the wire rod is reduced. Finally, the temperature of the cold heading steel wire rod is controlled to be less than 500℃ when it leaves the insulation corridor.

[0037] The specific implementation methods of the present invention are further introduced below by setting Examples 1-3 and Comparative Example 1.

[0038] Example 1 This embodiment provides a low-carbon cold heading steel wire rod and a preparation method thereof, which is prepared by a molten steel smelting process, a LF refining process, a continuous casting process, a heating process, a temperature-controlled rolling process and a slow cooling process. The composition control of the wire rod is shown in Table 1, and the specific process parameters are controlled as follows: The tapping temperature of the converter is 1620℃, the final C content is 0.06%, and the P content is 0.03%. When 1 / 3 of the converter is tapped, aluminum block, alloy, and lime are added in sequence, and silicon manganese and ferrosilicon are used for alloying. The amount of aluminum block added is 1kg / t, the amount of lime added is 4kg / t, the amount of ferrosilicon added is 0.5kg / t, and the amount of silicon manganese added is 1kg / t. Argon is blown from the bottom of the steelmaking process to obtain molten steel.

[0039] After the molten steel LF is refined to the station, 3kg / t lime is added, and argon is blown during stirring. 100m of aluminum wire is fed into the LF station. After 10 minutes of power-on, the slag surface is clean and the molten steel temperature is ≥1560℃, and then sampling is performed. According to the target steel grade component content, alloy is added to adjust the composition. Raw and auxiliary materials are prohibited from being added 5 minutes before steelmaking. Then 60 meters of new granular calcium silicon wire is fed (100 meters is fed in the pouring furnace), and argon is blown softly for 7 minutes. The intensity of argon blowing ensures that the liquid surface is slightly turned over and the molten steel is not exposed. Inclusions in the molten steel are removed. The molten steel temperature is 1590℃ after argon blowing.

[0040] Continuous casting, the molten steel obtained from the above LF refining process is cast under full protection, the pouring temperature is 1554℃, the superheat is controlled at 25℃ (the continuous pouring temperature is 1544℃, the superheat is 15℃), the pulling speed is 1.8m / min, the water volume of the second cooling zone is 0.53L / kg, it is divided into 4 zones, and the water distribution ratio of each zone is 28:26:22:24, and the square billet with a specification of 180*180mm is obtained.

[0041] After the quality inspection of the ingot, it is heated for 90 minutes, the preheating section temperature is 700°C, the first heating section temperature is 770°C, the second heating section temperature is 880°C, the soaking section temperature is 980°C, and then it is rolled after dephosphorization with 10MPa high-pressure water.

[0042] High-speed wire rolling: the starting rolling temperature is 920℃, the finishing rolling entrance temperature is 900℃, and the wire laying temperature is 880℃.

[0043] Cooling after rolling: After spinning, the wire rod first enters the Stelmor slow cooling line for insulation and slow cooling, where the entrance roller speed is 0.16m / s, the exit roller speed is 0.184m / s, the wire rod cooling rate is ≤0.5℃ / s, and then hot coiling is carried out, the coiling temperature is 770℃, and then enters the insulation corridor at 740℃ for the second insulation and slow cooling, the slow cooling time is controlled at 150min, and the temperature out of the insulation corridor is less than 400℃. Finally, the steel wire rod with a specification of φ5.5mm is inspected, weighed and stored.

[0044] Example 2 This embodiment provides a low-carbon cold heading steel wire rod and a preparation method thereof, which is prepared by a molten steel smelting process, a LF refining process, a continuous casting process, a heating process, a temperature-controlled rolling process and a slow cooling process. The composition control of the wire rod is shown in Table 1, and the specific process parameters are controlled as follows: The tapping temperature of the converter is 1670℃, the final C content is 0.03%, and the P content is 0.025%. When 1 / 3 of the converter is tapped, aluminum block, alloy, and lime are added in sequence, and silicon manganese and ferrosilicon are used for alloying. The amount of aluminum block added is 1kg / t, the amount of lime added is 4kg / t, the amount of ferrosilicon added is 0.5 kg / t, and the amount of silicon manganese added is 3kg / t. Argon is blown from the bottom of the ladle during the whole tapping process to obtain molten steel.

[0045] After the molten steel LF is refined to the station, 3kg / t lime is added, and argon is blown during stirring. 200m of aluminum wire is fed into the LF station. After 10 minutes of power-on, the slag surface is clean and the molten steel temperature is ≥1560℃, and then sampling is performed. According to the target steel grade component content, alloy is added to adjust the composition. Raw and auxiliary materials are prohibited from being added 5 minutes before steelmaking. Then 60 meters of new granular calcium silicon wire is fed (100 meters for pouring furnace), and argon is blown softly for 10 minutes. The intensity of argon blowing ensures that the liquid surface is slightly turned over and the molten steel is not exposed, and the inclusions in the molten steel are removed. The molten steel temperature is 1585℃ after argon blowing.

[0046] Continuous casting, the molten steel obtained from the above LF refining process is cast under full protection, the pouring temperature is 1579℃, the superheat is controlled at 50℃ (the continuous pouring temperature is 1559℃, the superheat is 30℃), the pulling speed is 1.8m / min, the water volume of the second cooling zone is 0.85L / kg, it is divided into 4 zones, and the water distribution ratio of each zone is 33:39:18:10, and the square billet with a specification of 180*180mm is obtained.

[0047] After the quality inspection of the ingot, it is heated for 100 minutes, the preheating section temperature is 800°C, the first heating section temperature is 890°C, the second heating section temperature is 990°C, the soaking section temperature is 1050°C, and then it is rolled after dephosphorization with 15MPa high-pressure water.

[0048] High-speed wire rolling: the starting rolling temperature is 960℃, the finishing rolling entrance temperature is 900℃, and the wire laying temperature is 920℃.

[0049] Cooling after rolling: The wire rod after spinning first enters the Stelmor slow cooling line for insulation and slow cooling, where the entrance roller speed is 0.400m / s, the exit roller speed is 0.462m / s, the wire rod cooling rate is ≤0.5℃ / s, and then hot coiling is carried out, the coiling temperature is 850℃, and then enters the insulation corridor at 817℃ for the second insulation and slow cooling, the slow cooling time is controlled at 75min, and the temperature out of the insulation corridor is less than 500℃. Finally, the steel wire rod with a specification of φ24mm is inspected, weighed and stored.

[0050] Example 3 The tapping temperature of the converter is 1650℃, the final C content is 0.05%, and the P content is 0.021%. When 1 / 3 of the converter is tapped, aluminum block, alloy, and lime are added in sequence, and silicon manganese and ferrosilicon are used for alloying. The amount of aluminum block added is 2.2kg / t, the amount of lime added is 4kg / t, the amount of ferrosilicon added is 0.5 kg / t, and the amount of silicon manganese added is 1kg / t. Argon is blown from the bottom of the ladle during the whole tapping process to obtain molten steel.

[0051] After the molten steel LF is refined to the station, 3kg / t lime is added, and argon is blown during stirring. 150m of aluminum wire is fed to the LF station. After 10 minutes of power-on, the slag surface is clean and the molten steel temperature is ≥1560℃, and then sampling is performed. According to the target steel grade component content, alloy is added to adjust the composition. Raw and auxiliary materials are prohibited from being added 5 minutes before steelmaking. Then 60 meters of new granular calcium silicon wire is fed (100 meters for pouring furnace), and argon is blown softly for 12 minutes. The intensity of argon blowing ensures that the liquid surface is slightly turned over and the molten steel is not exposed, and the inclusions in the molten steel are removed. The molten steel temperature is 1579℃ after argon blowing.

[0052] Continuous casting, the molten steel obtained from the above LF refining process is cast under full protection, the pouring temperature is 1568℃, the superheat is controlled at 30℃ (the continuous casting temperature is 1550℃, the superheat is 23℃), the pulling speed is 1.8m / min, the water volume of the second cooling zone is 0.55L / kg, it is divided into 5 zones, and the water distribution ratio of each zone is 25:31:21:14:9, and the square billet with a specification of 180*180mm is obtained.

[0053] After the quality inspection of the ingot, it is heated for 95 minutes, the preheating section temperature is 750°C, the first heating section temperature is 820°C, the second heating section temperature is 950°C, the soaking section temperature is 1010°C, and then it is rolled after dephosphorization with 13MPa high-pressure water.

[0054] High-speed wire rolling: starting rolling temperature is 940℃, finishing rolling entrance temperature is 886℃, and wire laying temperature is 888℃.

[0055] Cooling after rolling: After spinning, the wire rod first enters the Stelmor slow cooling line for insulation and slow cooling, where the entrance roller speed is 0.34m / s, the exit roller speed is 0.357m / s, the wire rod cooling rate is ≤0.5℃ / s, and then hot coiling is carried out, the coiling temperature is 830℃, and then enters the insulation corridor at 801℃ for the second insulation and slow cooling. The slow cooling time is controlled at 100min, and the temperature out of the insulation corridor is less than 500℃. Finally, the steel wire rod with a specification of φ18mm is inspected, weighed, and stored.

[0056] Comparative Example 1 The tapping temperature of the converter is 1580℃, the final C content is 0.08%, and the P content is 0.045%. When 1 / 3 of the converter is tapped, aluminum block, alloy, and lime are added in sequence, and silicon manganese and ferrosilicon are used for alloying. The amount of aluminum block added is 1kg / t, the amount of lime added is 4kg / t, the amount of ferrosilicon added is 0.5 kg / t, and the amount of silicon manganese added is 0.8kg / t. Argon is blown from the bottom of the ladle during the whole tapping process to obtain molten steel.

[0057] After the molten steel LF is refined to the station, 3kg / t lime is added, and argon is blown during stirring. 250m of aluminum wire is fed to the LF station. After 10 minutes of power-on, the slag surface is clean and the molten steel temperature is ≥1560℃, and then sampling is performed. According to the target steel grade component content, alloy is added to adjust the composition. Raw and auxiliary materials are prohibited from being added 5 minutes before steelmaking. Then 60 meters of new granular calcium silicon wire is fed (100 meters for pouring furnace), and argon is blown softly for 5 minutes. The intensity of argon blowing ensures that the liquid surface is slightly turned over and the molten steel is not exposed, and the inclusions in the molten steel are removed. The molten steel temperature is 1602℃ after argon blowing.

[0058] Continuous casting, the molten steel obtained from the above LF refining process is cast under full protection, the pouring temperature is 1554℃, the superheat is controlled at 25℃ (the continuous casting temperature is 1544℃, the superheat is 15℃), the pulling speed is 2.6m / min, the water volume of the second cooling zone is 1L / kg, it is divided into 5 zones, and the water distribution ratio of each zone is 39:39:10:7:5, and the square steel billet with a specification of 150*150mm is obtained.

[0059] After the quality inspection of the ingot, it is heated for 110 minutes, the preheating section temperature is 830°C, the first heating section temperature is 920°C, the second heating section temperature is 1013°C, the soaking section temperature is 1086°C, and then it is rolled after dephosphorization with 8MPa high-pressure water.

[0060] High-speed wire rolling: the starting rolling temperature is 988°C, the finishing rolling entrance temperature is 920°C, and the wire laying temperature is 918°C.

[0061] Cooling after rolling: After spinning, the wire rod first enters the Stelmor slow cooling line for insulation and slow cooling, where the entrance roller speed is 0.4m / s, the exit roller speed is 0.462m / s, the wire rod cooling rate is ≤10℃ / s, and then hot coiling is carried out, the coiling temperature is 681℃, and then enters the insulation corridor at 655℃ for the second insulation and slow cooling. The slow cooling time is controlled at 60min, and the temperature out of the insulation corridor is less than 500℃. Finally, the φ24mm steel wire rod is inspected, weighed, and stored.

[0062] Table 1 Chemical composition / % C Si Mn P S Al Example 1 0.06 0.20 0.32 0.030 0.030 0.012 Example 2 0.07 0.10 0.27 0.020 0.020 0.015 Example 3 0.09 0.30 0.25 0.025 0.025 0.025 Comparative Example 1 0.98 0.42 0.47 0.038 0.035 0.038 At the same time, the performance of the wire rods prepared in the above embodiments and comparative examples was tested, and the test results are shown in Table 2.

[0063] Table 2 Performance test results Specification / mm Tensile strength / Mpa Yield strength / Mpa Sectional shrinkage% Elongation after break / % Hardness / HRB 1 / 4 Cold upsetting qualified rate / % Abnormal tissue conditions Example 1 5.5 335 258 80 36 52 100% none Example 2 24 368 275 81 37 52 100% none Example 3 18 380 225 75 33 56 100% none Comparative Example 1 24 460 201 72 31 70 78% Martensite, Bainite It can be seen from the above embodiments and comparative examples that the cold heading steel wire rod and the preparation method thereof in the embodiments of the present invention, through the control of composition and the research on the process temperature, slow cooling speed, etc., the low carbon cold heading steel wire rod in the embodiment is controlled to have excellent mechanical properties, uniform internal structure distribution, no abnormal structure, a small number of internal inclusions and very few large-sized inclusions, excellent surface performance, and the 1 / 4 cold heading qualified rate reaches 100%. However, the steel wire rod in the comparative example has a high C content in its composition, resulting in a high tensile strength and low plasticity of the cold heading steel wire rod, a high internal inclusion content and a large number of large-sized inclusions. The abnormal structure present therein will cause the cold heading parts produced in the subsequent production to have quality problems such as plastic cracking, and the 1 / 4 cold heading qualified rate is too low, only 78%.

[0064] In summary, based on the reasonable design of the components and mass percentage of cold heading steel wire rod, the prepared wire rod has a cross-sectional shrinkage of 75~81%, an elongation after fracture of 33~37%, a tensile strength of 335~380Mpa, a yield strength of 225~275Mpa, a hardness of 52~56HRB, and a 1 / 4 cold heading pass rate of 100%, which can be used in the production of low-strength fasteners with high requirements for anti-cracking performance.

[0065] The above embodiments are only for illustrating the technical concept and features of the present invention, and their purpose is to enable people familiar with this technology to understand the contents of the present invention and implement them accordingly. They cannot be used to limit the protection scope of the present invention. Any equivalent changes or modifications made according to the spirit of the present invention should be included in the protection scope of the present invention.

Claims

1. A low carbon cold heading steel wire rod, characterized in that: The chemical composition of the low carbon cold heading steel wire rod is calculated by weight percentage: C: 0.06~0.09%; Si≤0.30%; Mn: 0.25~0.32%; P≤0.03%; S≤0.03%; Al: 0.012~0.025%; the balance is Fe and unavoidable impurities.

2. The method for preparing a low carbon cold heading steel wire rod according to claim 1, characterized in that: The chemical composition of the low carbon cold heading steel wire rod is calculated by weight percentage as follows: C: 0.07%; Si≤0.10%; Mn: 0.27%; P≤0.025%; S≤0.025%; Al: 0.015%; the remainder is Fe and unavoidable impurities.

3. A method for preparing a low-carbon cold heading steel wire rod according to claim 1 or 2, characterized in that: The low-carbon cold heading steel wire rod is obtained by sequentially performing a molten steel smelting process, a LF refining process, a continuous casting process, a heating process, a temperature-controlled rolling process and a slow cooling process.

4. The method for preparing a low carbon cold heading steel wire rod according to claim 3, characterized in that: The molten steel smelting process includes: sending smelting raw materials consisting of molten iron and scrap steel into a converter for smelting, and adding aluminum blocks, alloys, and lime into the molten steel in the order of tapping; wherein, the target sample of the ladle is: C≤0.06%; Si≤0.08%; Mn: 0.20%~0.27%, Al: 0.007%~0.030%; the final molten steel tapping C: 0.03~0.06%, P≤0.030%; the tapping temperature is 1620~1670°C.

5. The method for preparing a low carbon cold heading steel wire rod according to claim 3, characterized in that: The LF refining process includes: adding lime to the molten steel obtained in the molten steel smelting process, blowing argon and stirring during the process, feeding the aluminum wire after the LF arrives at the station, adjusting the content of other alloys to meet the target range, and then feeding the new granular silicon calcium wire for soft blowing, the time is ≥7 minutes.

6. The method for preparing a low carbon cold heading steel wire rod according to claim 3, characterized in that: The continuous casting process includes: performing continuous casting protection casting on the molten steel obtained in the LF refining process, the start pouring temperature of the molten steel is 1554-1579°C, and the start pouring superheat is 25-50°C; the continuous pouring temperature is 1544-1559°C, and the continuous pouring superheat is 15-30°C.

7. The method for preparing a low carbon cold heading steel wire rod according to claim 3, characterized in that: The heating process includes: heating the steel billet obtained in the casting process in sections, wherein the preheating section temperature is 700-800°C, the first heating section temperature is 770-890°C, the second heating section temperature is 880-990°C, and the soaking section temperature is 980-1050°C.

8. The method for preparing a low carbon cold heading steel wire rod according to claim 3, characterized in that: The temperature-controlled rolling process comprises: descaling the steel billet obtained in the heating process with high-pressure water, rolling it into wire rod, and then cooling it with water to spin wire, wherein the descaling water pressure is 10-15 MPa; the starting rolling temperature is 920-960°C, the finishing mill inlet temperature is ≤900°C; and the spinning temperature is 880-920°C.

9. The method for preparing a low carbon cold heading steel wire rod according to claim 3, characterized in that: The slow cooling process comprises: placing the wire rod obtained in the temperature-controlled rolling process on a Stelmor slow cooling line for heat preservation and slow cooling, and then performing a second section of heat preservation corridor slow cooling; wherein the second section of heat preservation corridor slow cooling inlet temperature is 700-770°C, and the heat preservation slow cooling time is 75-150 minutes. The temperature of the wire rod leaving the insulation corridor: when the wire rod specification is φ5.5mm~φ8mm, the temperature leaving the insulation corridor is <400℃; when the wire rod specification is φ9mm~φ24mm, the temperature leaving the insulation corridor is <500℃.

10. Use of the low carbon cold heading steel wire rod according to any one of claims 1 to 2 in cold heading parts.