Low-carbon free-cutting steel wire rod capable of being subjected to large-deformation cold heading and production method of low-carbon free-cutting steel wire rod

Abstract

According to the production method, an extremely-cold fine grain layer with the thickness being 10 mm or above and a large-size rectangular blank are obtained by means of the large specific water flow of primary cooling water, and the two-phase region controlled rolling, rapid cooling and isothermal slow cooling processes are matched, so that the low-carbon free-cutting steel wire rod capable of being subjected to large-deformation cold heading is obtained. And a structure of fine grain ferrite with large deformability and a very small amount of sorbite is obtained, and the 1 / 3 cold upsetting non-cracking requirement is met. After the produced free-cutting steel wire rod is subjected to oxide scale removal, drawing sizing and blanking, direct large-deformation cold heading and punching machining can be achieved, and the requirement for high-speed cutting machining is met.

Description

Technical Field

[0001] The invention belongs to the technical field of iron and steel metallurgy, and in particular relates to a low-carbon free-cutting steel wire rod capable of large deformation and cold heading and a production method thereof. Background Art

[0002] With the development of mechanical processing technology, especially the development of automobile industry, precision instrument industry and household appliances, the output and variety of free-cutting steel are increasing, and the quality is also improving. Low-carbon, low-silicon, high-sulfur free-cutting steel is widely used in the field of mechanical manufacturing due to its excellent cutting and processing properties. In addition, this steel is also favored for its good processing properties when mass-producing on automated machine tools.

[0003] The production and parts processing technology of low-carbon, low-silicon, high-sulfur free-cutting steel mainly includes steelmaking, rolling, cutting and other processes. In the steelmaking process, it is necessary to accurately control the ratio of raw materials and the smelting temperature to ensure that the chemical composition and mechanical properties of the steel meet the requirements. In the rolling process, high-sulfur free-cutting steel has poor hot working performance and is prone to slipping and steel piling. In the processing of parts, a large amount of surface cutting and drilling are generally required, so the overall material utilization rate is less than 40%, and the cutting cost is relatively high.

[0004] The mainstream free-cutting steel wires on the market are SAE1215 and others, with an annual demand of more than 300,000 tons. Traditional SAE1215 production methods can only improve the cutting performance of the material, but cannot achieve large deformation cold heading processing of the material. The overall utilization rate of the material is low, resulting in high processing and manufacturing costs of parts. In response to this problem, the invention patent with application number 201711315893.9 proposes a method for improving the cold heading performance of graphitized free-cutting steel. It uses warm deformation technology to perform warm deformation of ferrite dynamic recrystallization on the graphitized free-cutting steel wire rod after graphitization treatment, which can achieve cold heading with a small upsetting ratio. The composition design of the graphite free-cutting steel in this method has a high Si content, and elements such as Al and B need to be added, and it must be combined with post-rolling annealing treatment. The overall production cost is high, and the addition of 0.01-0.25% aluminum will produce more hard inclusions such as Al2O3 in the steel, resulting in an increase in the surface roughness of the parts during the cutting process, and a significant reduction in the service life of the tool.

[0005] In summary, the low-carbon, low-silicon, high-sulfur free-cutting steel produced by traditional processes cannot achieve large deformation cold heading processing, and the overall material utilization rate is reduced, resulting in high processing and manufacturing costs of parts. Graphite free-cutting steel can only achieve small upsetting ratio processing, but its production process has high manufacturing costs, and its material contains more hard inclusions such as Al2O3, which significantly reduces the service life of the tool and is not suitable for market promotion and application. Summary of the invention

[0006] Aiming at the technical problem that low-carbon, low-silicon, high-sulfur free-cutting steel produced by traditional processes cannot be subjected to large deformation cold heading processing, the present invention proposes a wire rod of low-carbon free-cutting steel that is compatible with large deformation cold heading and high-speed cutting processing and a production method thereof.

[0007] In order to achieve the above object, the technical solution adopted by the present invention is:

[0008] The invention discloses a low-carbon free-cutting steel wire rod capable of being cold headed with a large deformation amount. The chemical composition thereof is C≤0.09%, Si≤0.01%, Mn 1.00-1.30%, P 0.035-0.060%, S0.30-0.60%, Cr≤0.10%, Ni≤0.10%, Cu 0.05-0.20%, Al≤0.005%, Nb 0.003-0.010%, and the balance is Fe and unavoidable impurities.

[0009] Preferably, its chemical composition is C 0.07%, Si 0.008%, Mn 1.15%, P0.04%, S 0.32%, Cr 0.01%, Ni0.005%, Cu 0.11%, Al 0.002%, Nb 0.009% in weight percentage, and the balance is Fe and inevitable impurities.

[0010] The present invention also provides a method for producing a low-carbon free-cutting steel wire rod, comprising the following steps:

[0011] Converter smelting: low basicity slag making is used for P removal operation, and the oxygen level at the LF furnace outlet is 40-60ppm;

[0012] Refining: LF furnace is used to refine molten steel;

[0013] Continuous casting: The mold uses special protection slag for free-cutting steel, with a cooling water of 190m 3 / h or more high-volume cooling, to obtain 180mm × 240mm rectangular billets, to ensure that the large-size rectangular billets have a surface layer of more than 10mm of extremely cold fine grains. 3 / h or more high-volume cooling ensures that the 180mm×240mm large-size rectangular billet obtains an extremely cold fine-grained layer of more than 10mm on the surface, so that the FeS inclusions on the surface of the billet cannot gather and grow, avoiding the large particles of FeS on the surface as a crack source during the subsequent wire rod cold heading process to cause cracking; in addition, the 180mm×240mm large-size rectangular billet is used to roll into a finished φ5.5-26.0mm wire rod, which can greatly improve the compression ratio compared to the traditional small square billet of 160mm×160mm and below, ensuring the refinement of the wire rod grains and improving the plasticity.

[0014] Heating furnace heating: control the heating temperature at 1160-1200℃, the soaking zone temperature at 1180-1240℃;

[0015] Low temperature rolling: Control the rolling start temperature to 1080-1140℃, control the delay of no water penetration, and do not penetrate water for more than 5m of the front of the rolled material during the rolling process. At the same time, control the temperature of the rolled material entering the finishing mill and the sizing reduction to 800-850℃, which can ensure that the front of the billet does not split, and carry out low temperature dual-phase rolling. Do not penetrate water for more than 5m of the front of the rolled material during the rolling process to avoid splitting due to the low temperature of the front of the rolled material during the rolling process, which leads to steel promotion during the rolling process.

[0016] Rapid cooling: Use 780-820℃ low temperature spinning, after spinning, turn on the fan for strong cooling, make the cooling rate>10℃ / s, cool to 640-690℃, and then cover with insulation cover for isothermal slow cooling.

[0017] The two-phase zone controlled rolling + rapid cooling + isothermal slow cooling processes cooperate with each other, that is, the rolled material is rolled in the low-temperature two-phase zone (austenite and ferrite phase zone) of 800-850℃ in the finishing mill and the sizing is reduced, combined with low-temperature wire drawing at 780-820℃, strong cooling after rolling and isothermal slow cooling. The ferrite grain size of the final finished wire rod is level 10 or above, and a fine-grained ferrite + very small amount of troostite structure with large deformation ability is obtained, which meets the requirement that 1 / 3 of the hot-rolled wire rod will not produce cracks during cold forging.

[0018] Preferably, in the converter smelting step, the basicity is controlled at 1.6-2.5, the amount of slag lime added is 3-4.5 kg / t, the amount of ferroaluminum added is 2.5-3.5 kg / t, the amount of low-carbon ferromanganese added is 4-5.5 kg / t, the amount of ferrophosphorus added is 1.5-1.0 kg / t, and the amount of ferrosulfur added is 9-25 kg / t.

[0019] Preferably, in the refining step, the basicity is controlled at 2.7-3.3, 2.5-5.5 kg / t of pre-melted wollastonite and 1-2 kg / t of lime are added; the temperature is measured and the oxygen is determined after oxygen blowing into the station, and the active oxygen [O] out of the LF furnace is 45-90 ppm; at the same time, through alloying matching, the manganese-sulfur ratio is controlled at 4.0-6.5, and the ladle temperature is controlled at 1590-1620°C.

[0020] Preferably, when the superheat degree in the continuous casting step is 25-55°C, the superheat degree is controlled to decrease from low pulling speed to high pulling speed, and the pulling speed is controlled at 1.5-3.0 m / min to maintain a constant pulling speed.

[0021] Preferably, in the low-temperature rolling step, the first 5.5 m of the rolled material is not subjected to water penetration, and the temperature of the rolled material entering the finishing mill and reducing the diameter is 810-830°C.

[0022] Compared with the prior art, the advantages and positive effects of the present invention are:

[0023] (1) The present invention utilizes the existing equipment and process conditions of the steel plant to reduce the content of Mn, P, and S elements in SAE1215, thereby reducing production costs; adding a small amount of Cu element and cooperating with P element to improve the corrosion resistance of the wire rod as a whole. Adding a trace amount of Nb element to form NbC and other particles can greatly refine the grains without affecting the cutting performance.

[0024] (2) Using a large specific water volume of cold water, an extremely cold fine-grained layer with a thickness of more than 10 mm and a large-sized rectangular billet is obtained. Combined with the two-phase zone controlled rolling + rapid cooling + isothermal slow cooling process, a fine-grained ferrite with large deformation capacity + a very small amount of troostite structure is obtained, which meets the 1 / 3 cold forging without cracking requirements.

[0025] (3) The free-cutting steel wire rod produced by the present invention can be directly cold-forged and punched with large deformation after the iron oxide scale is removed, drawn and sized, and blanked, and can meet the requirements of high-speed cutting. The overall cutting material utilization rate of parts produced by cold forging has been increased from less than 40% to more than 80% at present, and the complex cutting process has been reduced, the production efficiency has been increased by 30%, and the overall production cost has been reduced by more than 60%, which has greatly improved the market competitiveness of the products. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 200X metallographic structure diagram of the wire rod produced by the embodiment of the present invention;

[0027] Figure 2 200X metallographic structure diagram of the wire rod produced by the comparative example of the present invention;

[0028] Figure 3 Photos of 1 / 3 cold forging test blocks of hot-rolled wire rods from different batches produced in the embodiments of the present invention;

[0029] Figure 4 The photos of 1 / 3 cold forging test blocks of hot-rolled wire rods produced in different batches of the comparative examples of the present invention;

[0030] Figure 5 1 is a process diagram of cold heading and punching of test blocks of an embodiment of the present invention; (a) is a top view, (b) is a main view, and the processing sequence is from left to right. DETAILED DESCRIPTION

[0031] In order to better understand the present invention, a detailed description is given below in conjunction with the accompanying drawings and embodiments.

[0032] Example:

[0033] The invention discloses a low-carbon free-cutting steel wire rod capable of being cold headed with a large deformation amount. The chemical composition thereof is C0.07%, Si 0.008%, Mn 1.15%, P 0.04%, S 0.32%, Cr0.01%, Ni0.005%, Cu 0.11%, Al0.002%, Nb 0.009% by weight, and the balance is Fe and unavoidable impurities.

[0034] The above-mentioned low-carbon free-cutting steel wire rod capable of large deformation and cold heading is produced by the following steps:

[0035] (1) Converter smelting: basicity is controlled at 1.6-2.5, slag lime is added in an amount of 3-4.5 kg / t, ferroaluminum is added in an amount of 2.5-3.5 kg / t, low carbon ferromanganese is added in an amount of 4-5.5 kg / t, ferrophosphorus is added in an amount of 1.5-1.0 kg / t, and ferrosulfur is added in an amount of 9-25 kg / t;

[0036] (2) Refining: basicity is controlled at 2.7-3.3, 2.5-5.5kg / t pre-melted wollastonite and 1-2kg / t lime are added. Temperature measurement and oxygen determination are performed after oxygen blowing into the furnace. The active oxygen [O] out of the LF furnace is 60-80ppm. At the same time, the manganese-sulfur ratio is controlled at 4.0-6.5 through alloy matching, and the ladle temperature is controlled at 1590-1620℃.

[0037] (3) Continuous casting: When the superheat is controlled at 25-55℃, the superheat is reduced from low pulling speed to high pulling speed, and the pulling speed is controlled at 1.5-3.0m / min to maintain a constant pulling speed. The continuous casting crystallizer uses special protective slag for free-cutting steel, with a cooling water of 193m 3 / h large water volume cooling; billet size: 180mm×2400mm.

[0038] (4) Heating in the heating furnace: the heating temperature is controlled at 1160-1200°C, the temperature in the soaking section is controlled at 1180-1240°C, and the rolling temperature is controlled at 1100-1120°C. During this process, no splitting occurs at the front of the ingot.

[0039] (5) Low temperature rolling: Control the delay of no water penetration. During the rolling process, the first 5.5m of the rolled material will not be penetrated by water. The temperature of the rolled material entering the finishing mill and reducing the diameter is 810-830℃.

[0040] (6) Rapid cooling: Use low temperature spinning at 790-815℃. After spinning, turn on the fan for strong cooling at a cooling rate of 10.5℃ / s. After cooling to 655-675℃, cool slowly at isothermal temperature.

[0041] (7) Packaging, inspection and storage: Use an offline fan to cool the wire rod to below 100°C before packaging. After sampling and testing, it can be stored.

[0042] like Figure 1As shown, the finished wire rod structure obtained in this embodiment is fine-grained ferrite + a very small amount of troostite, and the ferrite grain size is 10.0. Figure 3 As shown in Figure 1, no cracking occurred in the cold forging of 1 / 3 of the finished wire rods from different batches. Figure 5 As shown, the sample did not crack during the cold heading and punching process, and its high plastic cutting performance is equivalent to that of conventional SAE1215. The finished parts processed meet the use requirements.

[0043] Comparative Example:

[0044] A low-carbon, low-silicon, high-sulfur free-cutting steel wire rod, whose chemical composition by weight percentage is C 0.07%, Si 0.007%, Mn 1.25%, P 0.05%, S 0.39%, Cr 0.01%, Ni 0.006%, Cu 0.04%, Al 0.003%, Nb 0.008%, and the balance is Fe and inevitable impurities.

[0045] The above-mentioned low-carbon, low-silicon, high-sulfur free-cutting steel wire rod is produced by the following steps: continuous casting of square billets with a cross section of 150mm×150mm and a rolling specification of Ф20.0mm. The key process steps include:

[0046] The converter adopts low basicity slagging to carry out weak S and P removal operation;

[0047] LF furnace outgoing oxygen level is 35ppm;

[0048] The continuous casting mold uses special protection slag for free-cutting steel, and the first cooling water volume is set to 135m 3 / h; high-speed wire rolling temperature 1140-1170℃, high-speed wire rolling process temperature 1030-1060℃;

[0049] After spinning at 920-940°C, the fan is turned on for forced cooling, the cooling rate is 5.5°C / S, and then air cooling. Other process parameters are the same as those in the embodiment.

[0050] like Figure 2 As shown, the finished wire rod structure obtained in this comparative example is ferrite + a small amount of pearlite, and the ferrite grain size is 7.5. Figure 4 As shown, 1 / 3 of the specimen cracked during cold forging and could not meet the requirements of large deformation cold heading and cutting processing.

[0051] The above description is only a preferred embodiment of the present invention and does not limit the present invention in other forms. Any technician familiar with the profession may use the technical content disclosed above to change or modify it into an equivalent embodiment with equivalent changes and apply it to other fields. However, any simple modification, equivalent change and modification made to the above embodiment based on the technical essence of the present invention without departing from the content of the technical solution of the present invention still falls within the protection scope of the technical solution of the present invention.

Claims

1. A low-carbon free-cutting steel wire rod capable of large deformation and cold heading, characterized in that: Its chemical composition is C≤0.09%, Si≤0.01%, Mn 1.00-1.30%, P 0.035-0.060%, S 0.30-0.60%, Cr≤0.10%, Ni≤0.10%, Cu 0.05-0.20%, Al≤0.005%, Nb 0.003-0.010% in terms of weight percentage, and the balance is Fe and inevitable impurities.

2. The low-carbon free-cutting steel wire rod capable of large deformation and cold heading according to claim 1, characterized in that: Its chemical composition by weight percentage is C 0.07%, Si 0.008%, Mn 1.15%, P 0.04%, S 0.32%, Cr 0.01%, Ni 0.005%, Cu 0.11%, Al 0.002%, Nb 0.009%, and the balance is Fe and inevitable impurities.

3. A method for producing a low-carbon free-cutting steel wire rod, characterized in that: The following steps are involved: Converter smelting: low basicity slag making is used for P removal operation, and the oxygen level at the LF furnace outlet is 40-60ppm; Refining: LF furnace is used to refine molten steel; Continuous casting: The mold uses special protection slag for free-cutting steel, with a cooling water of 190m 3 / h or more high-volume water cooling to obtain a rectangular billet with a size of 180mm×240mm, ensuring that the large-size rectangular billet has an extremely cold fine-grained layer of more than 10mm on the surface; Heating furnace heating: control the heating temperature at 1160-1200℃, the soaking section temperature at 1180-1240℃; low temperature rolling: control the start rolling temperature at 1080-1140℃, control the delay time for not penetrating water, and during the rolling process, the first 5m of the rolled material will not penetrate water, and at the same time, control the temperature of the rolled material entering the finishing mill and reducing the sizing to 800-850℃; Rapid cooling: Use 780-820℃ low temperature spinning, after spinning, turn on the fan for strong cooling, make the cooling rate>10℃ / s, cool to 640-690℃, and then cover with insulation cover for isothermal slow cooling.

4. The method for producing low-carbon free-cutting steel wire rod according to claim 1, characterized in that: In the converter smelting step, the basicity is controlled at 1.6-2.5, the amount of slag lime added is 3-4.5kg / t, the amount of aluminum iron added is 2.5-3.5kg / t, the amount of low carbon manganese iron added is 4-5.5kg / t, the amount of ferrophosphorus added is 1.5-1.0kg / t, and the amount of ferrosulfur added is 9-25kg / t.

5. The method for producing low-carbon free-cutting steel wire rod according to claim 1, characterized in that: In the refining step, the basicity is controlled at 2.7-3.3, and 2.5-5.5kg / t of pre-melted wollastonite and 1-2kg / t of lime are added; the temperature is measured and the oxygen is determined after oxygen blowing into the station, and the active oxygen [O] out of the LF furnace is 45-90ppm; at the same time, through alloying matching, the manganese-sulfur ratio is controlled at 4.0-6.5, and the ladle temperature is controlled at 1590-1620℃.

6. The method for producing low-carbon free-cutting steel wire rod according to claim 1, characterized in that: When the superheat degree is 25-55°C in the continuous casting step, the superheat degree is controlled to decrease from low pulling speed to high pulling speed, and the pulling speed is controlled at 1.5-3.0m / min to maintain a constant pulling speed.

7. The method for producing low-carbon free-cutting steel wire rod according to claim 1, characterized in that: In the low-temperature rolling step, the first 5.5 m of the rolled material is not subjected to water penetration, and the temperature of the rolled material entering the finishing mill and reducing the diameter is 810-830°C.

Images