Production method of cold forging steel for cold rolling

Abstract

The invention belongs to the technical field of smelting, and discloses a production method of cold forging steel for cold rolling, which comprises the following steps: 1) raw material component design: (1) basic components (in percentage by mass): less than or equal to 0.07% of C, less than or equal to 0.06% of Si, 0.10-0.25% of Mn, less than or equal to 0.020% of P, less than or equal to 0.010% of S and the balance of Fe and inevitable impurities; (2) microalloying elements: Alt is greater than or equal to 0.030%, grains are refined, and plasticity is enhanced; (3) gas component content requirements are as follows: N content is less than or equal to 45ppm, and [O] is less than or equal to 25ppm; 2) smelting and continuous casting process; and 3) rolling process. Components are specially designed on the basis of traditional low-carbon cold forging steel, the carbon content is strictly controlled by adopting a post-furnace decarburization technology on the aspect of smelting process control, and the plasticity of a wire rod is greatly improved; and finally, the use requirements of customers on bending forming process products after cold rolling are stably met.

Description

Technical Field

[0001] The present invention belongs to the technical field of smelting, and relates to a production method of cold heading steel for cold rolling applications. Background Art

[0002] As a key material for manufacturing high-strength fasteners (such as bolts, rivets, special-shaped parts) and industrial products used in daily life, the plastic deformation ability, mechanical property stability and surface quality of cold heading steel directly affect the efficiency of cold forming processes and the quality of products. The traditional production methods of cold heading steel mainly have the following deficiencies:

[0003] 1. The carbon and manganese contents of traditional cold heading steel for low carbon are relatively high (such as C≥0.10%, Mn≥0.25%), resulting in insufficient plasticity and easy cracking or surface cracks during bending after cold drawing.

[0004] 2. The control of gas content and inclusions during the smelting process is not strict, affecting the material uniformity.

[0005] 3. The rolling process parameters are broad, resulting in large fluctuations in mechanical properties, insufficiently fine grain size, and difficulty in meeting the stable performance requirements for forming after cold rolling. Therefore, there is an urgent need to develop a production method of cold heading steel with high plasticity, low inclusion content and stable mechanical properties. Summary of the Invention

[0006] To solve the above technical problems, the present invention provides a production method of cold heading steel for cold rolling. By special chemical composition design, strict control of gas content in steel, and precise regulation of rolling and cooling processes, the above technical bottlenecks are effectively solved, realizing the high-performance and precision production of cold heading steel products for cold rolling applications, and meeting the good use of customer products while reducing production costs.

[0007] The technical solution adopted by the present invention is: a production method of cold heading steel for cold rolling, the chemical composition of the steel by weight percentage is C≤0.10%, Si≤0.10%, Mn≤0.30%, P≤0.025%, S≤0.020%, Al≥0.020%;

[0008] The key process steps include:

[0009] S1: Converter smelting, adopting a large slag volume and double slag smelting process, the carbon content at the end of the converter is ≤0.05%, the oxygen content at the end is 450 - 650 ppm, the end temperature is 1590 - 1620 °C, lime is added for slag washing during tapping, no deoxidizer is added for deoxidation, a slide gate is used for slag blocking during tapping, the slag layer thickness is ≤25 mm, and after tapping, large argon stirring is carried out for 2 - 3 minutes for decarburization;

[0010] S2: LF refining. After the molten steel enters the furnace, the oxygen content is determined first. According to the oxygen content, Al wire is fed, and the target Al content is 0.030 - 0.050%. During the smelting process, lime and synthetic slag are added for slag making and temperature raising. During the power supply process, Al pellets and calcium carbide are used for rapid slag making and deoxidation. A slightly positive pressure atmosphere is maintained in the furnace, and it is strictly prohibited to expose the arc during smelting. Ca treatment is carried out before tapping, and the soft blowing time is ≥15 min;

[0011] S3: Continuous casting process. The long nozzle of the tundish is protected by Ar sealing, the tundish is covered with a high - basicity covering agent, the casting speed is 2.4 - 2.6 m / min, the specific water consumption is 0.86 L / kg, and mold electromagnetic stirring is used;

[0012] S4: Rolling process. The steel billet is heated to 1050 - 1120 °C, and then undergoes rough rolling, medium rolling and finish rolling. The temperature for entering the sizing mill is 840 - 870 °C, and the coiling temperature is 810 - 840 °C. It is rapidly cooled to 600 °C and then slowly cooled.

[0013] Preferably, the basic composition of the steel by weight percentage is C ≤ 0.07%, Si ≤ 0.06%, Mn: 0.10 - 0.25%, P ≤ 0.020%, S ≤ 0.010%, and the balance is Fe and unavoidable impurities; the micro - alloying element Alt ≥ 0.030%, the gas component N content ≤ 45 ppm, and [O] ≤ 25 ppm.

[0014] Furthermore, 200 - 250 kg / furnace of lime is added during the tapping process of the S1 converter for slag washing.

[0015] Furthermore, 700 - 900 kg / furnace of lime and 200 - 300 kg / furnace of synthetic slag are added in the S2 LF refining.

[0016] Furthermore, the N content of the steel billet is stabilized within 40 ppm, the total oxygen content is stabilized within 25 ppm, the tensile strength is stably controlled within 350 Mpa, and the grain size of the wire rod is stably within 8.5 - 10.0 grades.

[0017] The innovation points of the present invention are as follows: Combining the processing technology and application characteristics of downstream customers, special design of the composition (mainly referring to Mn and Al) is carried out on the basis of traditional low-carbon cold-heading steel. In terms of smelting process control, post-furnace decarburization technology is adopted to strictly control the carbon content. From the converter to the continuous casting process, 200 - 250 kg of lime is added during converter tapping for slag washing, a slightly positive pressure atmosphere is maintained in the furnace, arc exposure during smelting is strictly prohibited, and the long nozzle of the ladle is protected by Ar sealing to strictly control nitrogen increase. Refining is carried out to produce a high-alkalinity reducing slag to strictly control the total oxygen content and improve the work hardening during cold rolling of the wire rod. By increasing the Al content in the steel and adopting a suitable controlled cooling process, the grain size of the wire rod is refined, which is 2 - 2.5 grades finer than that of ordinary low-carbon cold-heading steel, greatly improving the plasticity of the wire rod. Finally, it can stably meet the usage requirements of the products processed by the customer's cold rolling and bending forming process.

[0018] The beneficial effects of the present invention are as follows:

[0019] 1. Achieve a special composition design that meets the customer's usage requirements, and the cost is lower than that of cold-heading steel processed by ordinary processes;

[0020] 2. The N content in the billet is stably within 40 ppm, and the total oxygen content is stably within 25 ppm;

[0021] 3. The tensile strength is stably controlled within 350 Mpa;

[0022] 4. The grain size of the wire rod is stably within 8.5 - 10.0 grades;

[0023] 5. The process is stable, suitable for mass production, and there is no cracking in the customer's cold rolling and bending forming. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, the drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

[0025] Figure 1 It is the grain size diagram of the wire rod of ordinary cold-heading steel with a Φ10 mm specification;

[0026] Figure 2 It is the grain size diagram of the wire rod of cold-heading steel produced in Example 1;

[0027] Figure 3 It is the bending diagram of ordinary cold-heading steel after cold rolling;

[0028] Figure 4 It is the bending diagram of cold-heading steel produced in Example 1 after cold rolling. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0029] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0030] A production method of cold heading steel for cold rolling, the chemical composition of the steel in weight percentage is C≤0.10%, Si≤0.10%, Mn≤0.30%, P≤0.025%, S≤0.020%, Al≥0.020%; the key process steps include:

[0031] 1. Raw material composition design: ① Basic composition (by mass percentage): C: ≤0.07%, Si≤0.06%, Mn: 0.10 - 0.25%, P≤0.020%, S≤0.010%, the balance is Fe and inevitable impurities; ② Microalloying elements: Alt≥0.030%, refining grains and enhancing plasticity; ③ Requirements for gas component content: N content≤45ppm, [O]≤25ppm. Composition optimization principle: By reducing the contents of C and Mn, increasing the content of Al, reducing the tensile strength of the original wire rod, refining the grain size of the wire rod, improving the plasticity of the material, reducing the gas content in the steel, improving the work hardening during cold rolling, and enhancing the processing performance of the product.

[0032] 2. Smelting and continuous casting process

[0033] ① Converter smelting: Adopt the smelting process of large slag amount and double slag method. Lime, dolomite and iron ore are added to the converter for slag making. The pre-stage slag is poured during the smelting process, and slag materials are re-added after pouring the slag for smelting. The carbon content at the end of the converter is ≤0.05%, the oxygen content at the end is 450 - 650ppm, the temperature at the end is 1590 - 1620°C. During the tapping process, 200 - 250kg of lime is added for slag washing, no deoxidizer is added for deoxidation, the tapping is carried out by using a slide gate to block the slag, the thickness of the slag layer is ≤25mm, and after the tapping is completed, the molten steel is stirred with large argon gas for 2 - 3 minutes for decarburization.

[0034] ② LF refining: After the molten steel enters the station, the oxygen is first determined, and Al wire is fed according to the oxygen content. The target Al content is 0.030 - 0.050%. During the smelting process, 700 - 900kg / ladle of lime and 200 - 300kg / ladle of synthetic slag are added for slag making and temperature raising. During the power supply process, Al pellets and calcium carbide are used for rapid slag making and deoxidation to form a high-alkalinity reducing white slag as much as possible. A slightly positive pressure atmosphere is adopted in the furnace, and it is strictly prohibited that the arc is exposed during smelting. Ca treatment is carried out before tapping, and the soft blowing time is ≥15min.

[0035] ③Continuous casting process: The long tundish nozzle is protected by Ar seal, the tundish is covered with a high-alkaline covering agent, the casting speed is 2.4 - 2.6 m / min, the specific water ratio is 0.86 L / kg, and mold electromagnetic stirring is used.

[0036] 3. Rolling process: The steel billet is heated to 1050 - 1120 °C, and then undergoes rough rolling, intermediate rolling, and finish rolling. The temperature for sizing and reducing is 840 - 870 °C, and the wire laying temperature is 810 - 840 °C. It is rapidly cooled (specifically, the temperature is reduced within a few seconds) to 600 °C and then slowly cooled.

[0037] The content of the present invention will be further described below in conjunction with embodiments.

[0038] The embodiments are produced according to the raw material composition design, converter smelting, LF refining, continuous casting, and rolling production process steps described in the present invention. The key process parameters of each embodiment are as follows:

[0039] Example 1

[0040] (1) Melting composition: C: 0.0556%, Mn: 0.1563%, [O]: 17 ppm, N: 39 ppm, Al: 0.037%; the balance is iron;

[0041] (2) Converter smelting: The large slag volume + double slag smelting process is adopted. The amount of lime added to the converter is 5.3 tons, dolomite is 1.5 tons, and 1.5 t of iron ore. The slag is poured once in the early stage of smelting. The C content of the molten steel at tapping is 0.04%, the end point temperature is 1598 °C, and the end point oxygen content is 502 ppm. The slide gate slag stopper is used for tapping, the slag layer thickness is measured to be 20 mm, 223 kg of lime is added during tapping, and argon stirring is carried out for 2 minutes;

[0042] (3) LF refining: After the molten steel enters the station, the oxygen content is determined to be 240 ppm. The Al content is sampled by feeding Al wire and is 0.035%. High gear is used for power transmission and temperature increase. During the process, Al pellets and calcium carbide are used for slag making and deoxidation. 854 kg of lime and 265 kg of synthetic slag are added during the process to quickly form a high-alkalinity reducing slag. After Ca treatment, the soft blowing time is 17 minutes.

[0043] (4) Continuous casting: The long tundish nozzle is protected by Ar for the whole process of casting. The tundish uses a high-alkalinity covering agent, the casting speed is 2.4 m / min, the specific water ratio is 0.86 L / kg, and mold electromagnetic stirring is used.

[0044] (5) Rolling process: The steel billet is heated to 1100 °C, and then undergoes rough rolling, intermediate rolling, and finish rolling. The temperature for sizing and reducing is 850 °C, and the wire laying temperature is 815 °C. It is rolled to Φ10 mm. During the rolling process, it is rapidly cooled to 600 °C and then slowly cooled.

[0045] The grain size diagram of the cold heading steel wire rod produced according to the above process is as Figure 2 shown, and the bending situation of the cold heading steel after cold rolling is asFigure 4 as shown

[0046] Example 2

[0047] (1) Melting composition: C: 0.0632%, Mn: 0.1473%, [O]: 19 ppm, N: 42 ppm, Al: 0.033%; the balance is iron;

[0048] (2) Converter smelting: Adopt the smelting process of large slag amount + double slag method. The amount of lime added to the converter is 5.5 tons, dolomite is 1.6 tons, and 1.2 t of iron ore. Slag is poured once in the early stage of smelting. The C content of the molten steel tapped is 0.035%, the end point temperature is 1610 °C, and the end point oxygen content is 522 ppm. Slide plate slag blocking is used for tapping. The thickness of the slag layer is measured to be 27 mm. 233 kg of lime is added during tapping, and argon stirring is carried out for 3 minutes;

[0049] (3) LF refining: After the molten steel enters the station, the oxygen content is determined to be 270 ppm. The Al content is 0.032% after feeding the Al wire for sampling. High gear is used for power transmission and temperature rise. During the process, Al pellets and calcium carbide are used for slag making and deoxidation. 814 kg of lime and 295 kg of synthetic slag are added during the process to quickly form a high alkalinity reducing slag. After Ca treatment, the soft blowing time is 19 minutes.

[0050] (4) Continuous casting: The tundish long nozzle is used for full protection casting with Ar. The tundish uses a high alkalinity covering agent. The drawing speed is 2.4 m / min, the specific water volume is 0.86 L / kg, and mold electromagnetic stirring is used.

[0051] (5) Rolling process: The steel billet is heated to 1090 °C, and then undergoes rough rolling, medium rolling and finish rolling. The temperature entering the sizing mill is 860 °C, and the coiling temperature is 820 °C, and it is rolled to Φ6.5 mm. During the rolling process, it is quickly cooled to 600 °C and then slowly cooled.

[0052] Comparative Example 1:

[0053] Chemical composition of ordinary cold heading steel: C ≤ 0.08%, Si ≤ 0.08%, Mn: 0.25% - 0.50%, P ≤ 0.025%, S ≤ 0.015%, Al ≥ 0.02%.

[0054] Production process of ordinary cold heading steel: Converter smelting - LF smelting - continuous casting - rolling, without the post-furnace decarburization process of the present invention. The grain size diagram of the cold heading steel wire rod produced is as Figure 1 shown, and the bending situation of the cold heading steel after cold rolling is as Figure 3 shown

[0055] Experimental data comparison: Compare the traditional process with the process of the present invention:

[0056] Project Traditional process Process of the present invention Mn content ≥0.27％ ≤0.20％ Grain size of wire rod Grade 7.5 - 8.5 Grade 8.5 - 10 Bending after customer cold rolling Cracking No cracking

[0057] It can be seen that the present invention uses a special composition design to reduce the content of Mn. In the control of the smelting process, post-furnace decarburization technology is adopted to strictly control the carbon content. However, the crystal grain size of the wire rod is stable at grades 8.5 - 10.0 and there is no cracking after cold rolling and bending.

[0058] The above are only the preferred embodiments of the present invention and are not intended to limit the protection scope of the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention are all included in the protection scope of the present invention.

Claims

1. A method for producing cold-rolled cold heading steel, characterized in that: The chemical composition of the steel is C≤0.10%, Si≤0.10%, Mn≤0.30%, P≤0.025%, S≤0.020%, Al≥0.020% by weight; The key process steps include: S1: Converter smelting, using large slag and double slag smelting process, converter end carbon ≤ 0.05%, end oxygen: 450-650ppm, end temperature 1590-1620℃, lime is added for slag washing during steel tapping, no deoxidizer is added for deoxidation, slide plate is used to block slag for steel tapping, slag layer thickness ≤ 25mm, and large argon gas is used for stirring for 2-3 minutes for decarburization after steel tapping; S2: LF refining, oxygen is determined after the molten steel enters the station, and Al wire is fed according to the oxygen content, with the target Al content of 0.030-0.050%. Lime and synthetic slag are added during the smelting process to make slag and raise the temperature. Al balls and calcium carbide are used for rapid slag making and deoxidation during the power transmission process. A slightly positive pressure atmosphere is used in the furnace. It is strictly forbidden to expose the arc during smelting. Ca treatment is carried out before leaving the station, and the soft blowing time is ≥15min; S3: Continuous casting process, the long water inlet of the ladle is protected by Ar seal, the tundish uses a high alkalinity covering agent, the casting speed is 2.4-2.6m / min, the specific water volume is 0.86L / kg, and the crystallizer is electromagnetically stirred; S4: rolling process, the steel billet is heated to 1050-1120℃, rough rolled, intermediate rolled and finish rolled, the sizing temperature is 840-870℃, the spinning temperature is 810-840℃, and it is quickly cooled to 600℃ and then slowly cooled.

2. The method for producing cold-rolled cold heading steel according to claim 1, characterized in that: The basic components of the steel are C≤0.07%, Si≤0.06%, Mn: 0.10-0.25%, P≤0.020%, S≤0.010%, and the balance is Fe and inevitable impurities; micro-alloying element Alt≥0.030%, gas component N content≤45ppm, [O]≤25ppm.

3. The method for producing cold-rolled cold heading steel according to claim 1, characterized in that: During the steelmaking process of the S1 converter, 200-250 kg of lime is added per furnace for slag washing.

4. The method for producing cold-rolled cold heading steel according to claim 1, characterized in that: The S2 LF refining adds 700-900 kg / furnace of lime and 200-300 kg / furnace of synthetic slag.

5. A method for producing cold-rolled cold heading steel according to any one of claims 1 to 4, characterized in that: The N content of the steel billet is stable within 40pp, the total oxygen content is stable within 25ppm, the tensile strength is stably controlled within 350Mpa, and the grain size of the wire rod is stable at 8.5-10.0.

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