Automobile cold heading steel wire rod ML40Cr annealing reduction production method

Abstract

The invention relates to the technical field of production of metal wires, in particular to a production method for reducing annealing of an automobile cold heading steel wire rod ML40Cr. The automobile cold heading steel wire rod is prepared from the following chemical components in percentage by weight: 0.35 percent to 0.45 percent of C, 0.10 percent to 0.30 percent of Si, 0.60 percent to 1.00 percent of Mn, 0.90 percent to 1.20 percent of Cr and the balance of Fe and inevitable impurities. And through controlled rolling and water bath cooling, grains are refined, and the structure uniformity is improved. The pearlite content in the wire rod structure can be increased to 80% or above, and the austenite grain size is controlled to be 10-11 grade. The steel wire rod can achieve the spheroidizing grade 4 by adopting one-time spheroidizing annealing in the automobile fastener machining process, the cold heading machining requirement is met, one-time spheroidizing annealing is reduced, and therefore the production cost is reduced, and the production efficiency is improved.

Description

Technical Field

[0001] This invention relates to the field of metal wire production technology, specifically to a method for producing ML40Cr automotive cold heading steel wire rod with reduced annealing. Background Technology

[0002] ML40Cr cold heading steel wire rod is a key raw material for manufacturing 10.9 grade high-strength automotive fasteners (such as chassis, wheel hubs, engine bolts, etc.). Given its use in automotive safety components, the quality requirements for the wire rod are extremely stringent.

[0003] Currently, ML40Cr hot-rolled wire rods produced using traditional processes typically undergo Stellmore cooling after rolling. The specific process involves cooling the rolled piece to 850-950°C in a water tank after the final rolling pass, followed by slow cooling to complete the phase transformation. However, this traditional cooling method has inherent drawbacks: firstly, the resulting microstructure contains a high amount of ferrite, resulting in relatively coarse grains; secondly, during fan cooling, uneven cooling occurs at the overlap points and in the middle of the wire rod, leading to significant differences in microstructure between the overlap points on both sides and the middle section. Traditional ML40Cr wire rods have high strength and a microstructure dominated by ferrite and pearlite. Therefore, automotive fastener manufacturers must perform two spheroidizing annealing treatments (costing approximately 600-800 RMB / ton) to meet the requirements of cold heading processes. This not only significantly increases production costs but also results in long production cycles and low efficiency, becoming a technical bottleneck restricting the industry's development.

[0004] To overcome this challenge, the industry has made numerous attempts. For example, Chinese patent application CN116334469A discloses "ML40Cr non-spheroidizing annealed hot-rolled wire rod and its preparation method," and Chinese patent CN116334469B discloses "ML40Cr non-spheroidizing annealed hot-rolled wire rod and its preparation method." Both methods use a traditional air-cooled line with an insulation cover to control the cooling rate of the wire rod after coiling to above 0.6℃ / s, cooling it to 660~700℃ to obtain a granular annealed structure. Although this approach has made some progress, the resulting structure is essentially a semi-annealed structure. Its spheroidization rate, carbide morphology, and uniformity of distribution have not yet reached ideal levels, with a spheroidization grade of only about level 1, making it unsuitable for direct cold heading. No practical application has been seen in fastener companies.

[0005] Currently, the microstructure of ML40Cr wire rod obtained using conventional air-cooling processes is ferrite + pearlite, with a pearlite content of 50%~70% and an austenite grain size generally around grade 8. When producing automotive fasteners, this type of wire rod requires two spheroidizing annealing processes to achieve a spheroidization grade of 4 or higher to meet the requirements of cold heading. Summary of the Invention

[0006] To overcome the shortcomings of existing technologies, this invention provides a method for producing ML40Cr automotive cold-heading steel wire rod with reduced annealing: by controlling rolling and water bath cooling, the grain size is refined and the uniformity of the microstructure is improved. This allows the pearlite content in the wire rod microstructure to be increased to over 80%, and the austenite grain size to be controlled at grade 10-11. This wire rod can achieve a spheroidization grade of 4 in automotive fastener processing with a single spheroidizing annealing process, meeting the requirements of cold heading, reducing the need for one spheroidizing annealing step, thereby lowering production costs and improving production efficiency.

[0007] To achieve the above objectives, the present invention employs the following technical solution: An automotive cold heading steel wire rod ML40Cr is composed of the following chemical composition in weight percentages: C: 0.35%~0.45%, Si: 0.10%~0.30%, Mn: 0.60%~1.00%, Cr: 0.90%~1.20%, with the remainder being Fe and unavoidable impurities.

[0008] The microstructure of the automotive cold heading steel wire rod ML40Cr consists of pearlite and ferrite, with the volume content of pearlite being not less than 80%, and the original austenite grain size level of the steel being greater than or equal to grade 10.

[0009] The ML40Cr automotive cold heading steel wire rod has a spheroidization grade greater than or equal to level 4 after one spheroidization process.

[0010] The effect of selecting the above alloying elements and their contents: Carbon (C) is the element with the most significant solid solution strengthening effect in steel. As the C content increases, the strength and hardness of the steel increase, while its plasticity and toughness decrease. Therefore, this invention precisely controls the C content to be 0.35%~0.45%.

[0011] Si is a ferrite solid solution strengthening element that can effectively inhibit grain growth, thereby improving the hardness and strength of steel. Si can improve the tempering stability of fasteners, but it can also reduce the cold deformation performance of cold heading steel. Therefore, this invention precisely controls the Si content to be 0.10%~0.30%.

[0012] Mn plays a role in solid solution strengthening and grain refinement strengthening, and delays the transformation of pearlite and ferrite, thereby improving the hardenability of steel; therefore, the present invention precisely controls the Mn content to be 0.60%~1.00%.

[0013] Cr can improve the strength and toughness of steel and enhance its resistance to atmospheric corrosion. It also effectively improves hardenability, ensuring the strength and hardness of the material after quenching. Therefore, this invention precisely controls the Mn content of Cr to be 0.90%~1.20%.

[0014] The aforementioned method for producing ML40Cr automotive cold-heading steel wire rod with reduced annealing includes the following sequential processes: molten iron pretreatment, converter, LF furnace, continuous casting, continuous rolling, billet cleaning, heating, rolling, controlled cooling, finishing, wire rod pickling, drawing, and spheroidizing annealing. The method at least controls the LF furnace refining, continuous casting, rolling and controlled cooling processes, and the spheroidizing annealing process as follows: 1) LF Refining: Two composition adjustments are made: during the coarse adjustment, 0.5~0.8 kg / t of aluminum powder is added for deoxidation, and during the fine adjustment, the C, Si, Mn and Cr contents in the steel are controlled within the target range.

[0015] 2) Continuous casting: The cross-sectional dimensions of the continuously cast billet are (250~350)mm×(350~450)mm. The superheat of the tundish is controlled at 20~30℃, and the billet casting speed is controlled at 0.53~0.57m / min. The voltage of the electromagnetic stirring in the crystallizer is controlled at 100~200V, the current intensity is 100~300A, and the frequency is 3~8Hz.

[0016] 3) Rolling: The initial rolling temperature is controlled at 930~980℃, the finishing rolling inlet temperature is 880~950℃, the sizing inlet temperature is 850~920℃, and the wire drawing temperature is 850~950℃.

[0017] 4) Controlled cooling: After spinning, the wire rod is cooled by a water bath. The wire rod first enters a water tank with a cooling water temperature of 80~100℃, and is cooled to 600~700℃ at a cooling rate of 10~15℃ / s before exiting the water tank.

[0018] It then enters the heat insulation cover for slow cooling, with the cooling rate inside the cover controlled at 0.5~1.0℃ / s, until the temperature outside the cover reaches 450~600℃.

[0019] Finally, air-cool it to room temperature on a cooling bed.

[0020] 5) Spheroidizing annealing: The wire rod is heated from room temperature to 770~790℃ and held for 2.5 hours; then cooled to 700~720℃ at a cooling rate of 20℃ / h and held for 8 hours; finally cooled to 500℃ in the furnace at a cooling rate of 2℃ / min and then cooled to room temperature after being removed from the furnace.

[0021] Furthermore, in step 1), quicklime and fluorite are used for slag formation, and the electrode heating time is controlled to be 20-30 minutes.

[0022] Furthermore, in step 2), a low-oxidizing alkaline covering agent is added to the surface of the molten steel in the ladle and tundish.

[0023] Furthermore, in step 2), protective sleeves are used between the main package and the intermediate package, and between the intermediate package and the crystallizer, and argon blowing is performed for sealing.

[0024] Furthermore, in step 2), a protective slag is added inside the crystallizer.

[0025] Furthermore, in step 4), the insulation cover is completely closed.

[0026] Compared with the prior art, the beneficial effects of the present invention are: 1. Significantly Optimized Rolled Microstructure, Laying a Solid Foundation for "Single-Stage Spheroidizing Annealing": The core innovation of this invention lies in the unique "forced water bath cooling" method used as the first-stage cooling method after wire drawing. By rapidly placing the high-temperature wire rod (850~950℃) into a constant-temperature water bath at 80~100℃ and quickly cooling it to the phase transformation temperature range of 600~700℃ at a relatively high cooling rate of 10~15℃ / s, the austenite grains are greatly refined, and the precipitation of proeutectoid ferrite is effectively suppressed. This results in a uniform, fine initial microstructure dominated by fine lamellar pearlite. This microstructure has high internal energy, numerous interfaces, and dispersed carbides. As the initial microstructure in the subsequent spheroidizing annealing process, it has a large spheroidizing driving force and good kinetic conditions, making it extremely easy to spheroidize. This is the fundamental reason for reducing the annealing process from two stages to one stage.

[0027] 2. Significantly reduces production costs and improves efficiency for downstream users: Because the wire rod provided by this invention has a uniform and fine rolled microstructure that is easy to spheroidize, downstream fastener manufacturers no longer need to perform the traditional two-stage spheroidizing annealing process. Only one spheroidizing annealing is required to obtain a level 4 spheroidized microstructure, fully meeting the stringent requirements for material plasticity in the cold heading of 10.9 grade automotive fasteners. This directly saves downstream users approximately 300-400 RMB / ton in annealing costs, while significantly shortening the production cycle and improving production efficiency, resulting in extremely significant economic benefits.

[0028] 3. Significantly Improves Product Structure Uniformity and Performance Consistency: The inherent problems of uneven cooling at "overlap points" and "center of the coil" in traditional air-cooling processes are fundamentally solved in the "water bath cooling" stage of this invention. Water cooling has strong penetration, providing uniform and intense cooling, ensuring highly consistent cooling conditions throughout the entire coil before the critical phase transformation stage. This results in extremely uniform microstructure and hardness distribution along the entire length and cross-section of the coil. This effectively avoids fluctuations in cold heading cracking rate caused by uneven structure, greatly improving the quality stability and consistency of the final fastener products.

[0029] 4. Ensuring Material Cleanliness and Uniformity from the Source, Eliminating Internal Defects: This invention effectively controls nitrogen through the LF refining process and significantly promotes inclusion flotation, refines impurity particles, and greatly reduces center segregation and banded structures through protective casting (alkaline covering agent, protective sleeve, and argon blowing seal) throughout the continuous casting process and optimized crystallizer electromagnetic stirring (parameters: voltage 100~200V, current 100~300A, frequency 3~8Hz) throughout the continuous casting process. This ensures high cleanliness and compositional uniformity of the billet from the source, completely eliminating the risk of cold heading cracks caused by inclusions and segregation, and providing a fundamental guarantee for subsequent controlled cooling and excellent cold heading performance.

[0030] In summary, this invention, through precise and coordinated control of the entire process, particularly the cooling process in the water tank after spinning, yields a uniform and fine microstructure dominated by pearlite. This facilitates spheroidizing annealing in downstream processes, enabling a single spheroidizing annealing step to meet cold heading requirements. This represents a significant technological leap for downstream users, moving from "two spheroidizing annealing processes" to "one spheroidizing annealing," achieving a perfect balance of cost reduction, efficiency improvement, and quality assurance. Attached Figure Description

[0031] Figure 1 The spheroidized microstructure of ML40Cr obtained using the present invention (first spheroidization, grade 4).

[0032] Figure 2 The spheroidized microstructure of ML40Cr obtained by conventional methods (first spheroidization, grade 3). Detailed Implementation

[0033] This invention discloses a method for producing ML40Cr automotive cold-heading steel wire rod with reduced annealing. Those skilled in the art can refer to this document and appropriately modify the process parameters to achieve the desired result. It should be particularly noted that all similar substitutions and modifications are obvious to those skilled in the art and are considered to be included in this invention. The methods and applications of this invention have been described through preferred embodiments, and those skilled in the art can clearly modify or appropriately change and combine the methods and applications described herein without departing from the content, spirit, and scope of this invention to realize and apply the technology of this invention.

[0034] An automotive cold heading steel wire rod ML40Cr is composed of the following chemical composition in weight percentages: C: 0.35%~0.45%, Si: 0.10%~0.30%, Mn: 0.60%~1.00%, Cr: 0.90%~1.20%, with the remainder being Fe and unavoidable impurities.

[0035] The aforementioned controlled cooling method for the reduced annealing of ML40Cr automotive cold heading steel wire rod includes the following sequential processes: molten iron pretreatment, converter, LF furnace, continuous casting, continuous rolling, billet cleaning, heating, rolling, controlled cooling, finishing, inspection, and warehousing. The method at least controls the LF furnace refining, continuous casting, rolling, and controlled cooling processes as follows: 1. LF Refining: The main tasks of LF refining are composition fine-tuning, temperature control, and slag-forming operations. Nitrogen enrichment in molten steel is controlled using a combination of slightly positive pressure operation, large slag volume, and limited heating time. Quicklime and fluorite are used for slag-forming, with electrode heating time controlled at 20-30 minutes. Two composition adjustments are performed: during coarse adjustment, 0.5-0.8 kg / t of aluminum powder is added for deoxidation; during fine adjustment, the C, Si, Mn, and Cr contents in the steel are controlled according to the target.

[0036] 2. Continuous casting: The cross-sectional dimensions of the continuously cast billet are (250~350)mm × (350~450)mm. The superheat in the tundish is controlled at 20~30℃, and the casting speed is constant (the casting speed of the billet is controlled at 0.55±0.02m / min). Protective casting is performed (low-oxidizing and alkaline covering agents are added to the molten steel in the ladle and tundish; protective sleeves are used from the ladle to the tundish and from the tundish to the crystallizer, and argon blowing is used for protection; high-quality protective slag is added to the crystallizer). The crystallizer is electromagnetically stirred. The voltage is 100~200V, the current intensity is 100~300A, and the frequency is 3~8Hz. The electromagnetic stirring process can promote the floating of non-metallic inclusions in molten steel, reduce the size of inclusion particles in steel, and make inclusions evenly distributed; reduce element segregation in steel, thereby homogenizing the structure and reducing banded defects in steel; reduce the number and depth of surface defects such as transverse cracks, longitudinal cracks, and corner cracks in steel billets, and provide high-quality raw materials for rolling scratch-free wire rods.

[0037] 3. Rolling process: The initial rolling temperature is controlled at 930~980℃, the finishing rolling inlet temperature is 880~950℃, the sizing inlet temperature is 850~920℃, and the wire drawing temperature is 850~950℃.

[0038] 4. Control the cooling process: Water bath cooling method: After the wire rod is spun, it enters the water tank. The cooling water temperature in the water tank is 80~100℃. The wire rod is cooled to 600~700℃ at a cooling rate of 10~15℃ / s and then exits the water tank into the insulation cover. The insulation cover is completely closed. The cooling rate inside the cover is 0.5~1.0℃ / s. The temperature at the exit of the cover is 450~600℃. The cooling bed is air-cooled.

[0039] By controlling the heating, rolling, and water tank cooling, a uniform wire rod structure dominated by fine pearlite is obtained, which is beneficial for subsequent spheroidizing annealing.

[0040] 5. Spheroidizing annealing process: After pickling and phosphating, the wire rod undergoes a single drawing process, followed by spheroidizing annealing in a spheroidizing furnace. The process is as follows: The wire rod is heated from room temperature to 770~790℃ → held for 2.5h → cooled to 700~720℃ at 20℃ / h → held for 8h → cooled in the furnace to 500℃ (cooling rate ≤2℃ / min) → removed from the furnace and cooled to room temperature.

[0041] The chemical composition of the embodiments and comparative examples of the present invention is shown in Table 1. The LF refining and continuous casting processes of the embodiments of the present invention are shown in Table 2. The rolling process of the embodiments of the present invention is shown in Table 3. The cooling process of the embodiments of the present invention is shown in Table 4. The spheroidizing annealing process of the embodiments of the present invention is shown in Table 5. The microstructure inspection results of the embodiments and comparative examples of the present invention are shown in Table 6.

[0042] Table 1. Chemical composition (wt%) of the embodiments and comparative examples of the present invention. Table 2 LF refining and continuous casting processes in embodiments of the present invention Table 3 Rolling process of embodiments of the present invention Table 4 Cooling process of embodiments of the present invention Table 5 Spheroidizing Annealing Process in Embodiments of the Invention Table 6. Tissue testing results of embodiments and comparative examples of the present invention. As can be seen from the data comparison in the table above, the wire rod produced using the method of this invention has a higher pearlite content and finer grains in its rolled microstructure, effectively eliminating coarse network ferrite. This makes the microstructure more favorable for subsequent spheroidizing annealing. Therefore, the wire rod of this invention only requires one spheroidizing annealing to meet the requirements of cold heading. Compared with the two annealings required in Comparative Example 1, it can save approximately 300-400 yuan per ton in annealing costs, and significantly improves production efficiency and product yield.

[0043] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A method for producing automotive cold heading steel wire rod ML40Cr with reduced annealing, the preparation process comprising the sequential steps of molten iron pretreatment, converter, LF furnace, continuous casting, continuous rolling, billet cleaning, heating, rolling, controlled cooling, finishing, wire rod pickling, drawing, and spheroidizing annealing, characterized in that, The method at least controls the following processes: LF furnace refining, continuous casting, rolling and controlled cooling, and spheroidizing annealing: 1) LF Refining: Two composition adjustments are made: during the coarse adjustment, 0.5-0.8 kg / t of aluminum powder is added for deoxidation; during the fine adjustment, the C, Si, Mn, and Cr contents in the steel are controlled within the target range. 2) Continuous casting: The cross-sectional dimensions of the continuously cast billet are (250~350)mm×(350~450)mm. The superheat of the tundish is controlled at 20~30℃, and the billet casting speed is controlled at 0.53~0.57m / min. The voltage of the electromagnetic stirring in the crystallizer is controlled at 100~200V, the current intensity is 100~300A, and the frequency is 3~8Hz. 3) Rolling: The initial rolling temperature is controlled at 930~980℃, the finishing rolling inlet temperature is 880~950℃, the sizing inlet temperature is 850~920℃, and the wire drawing temperature is 850~950℃. 4) Controlled cooling: After spinning, the wire rod is cooled by a water bath. The wire rod first enters a water tank with a cooling water temperature of 80~100℃, and is cooled to 600~700℃ at a cooling rate of 10~15℃ / s before exiting the water tank. It then enters the heat insulation cover for slow cooling, with the cooling rate inside the cover controlled at 0.5~1.0℃ / s, until the temperature outside the cover reaches 450~600℃; Finally, air-cool the bed to room temperature. 5) Spheroidizing annealing: The wire rod is heated from room temperature to 770~790℃ and held for 2.5 hours; then cooled to 700~720℃ at a cooling rate of 20℃ / h and held for 8 hours; finally cooled to 500℃ in the furnace at a cooling rate of 2℃ / min and then cooled to room temperature after being removed from the furnace.

2. The production method of ML40Cr automotive cold heading steel wire rod under reduced annealing according to claim 1, characterized in that, In step 1), quicklime and fluorite are used for slag formation, and the electrode heating time is controlled to be 20-30 minutes.

3. The production method of ML40Cr automotive cold heading steel wire rod under reduced annealing according to claim 1, characterized in that, 2) In this process, a low-oxidizing alkaline covering agent is added to the surface of the molten steel in the ladle and tundish.

4. The production method of ML40Cr automotive cold heading steel wire rod with reduced annealing according to claim 1, characterized in that, 2) In this process, protective sleeves are used between the main package and the intermediate package, and between the intermediate package and the crystallizer, and argon blowing is performed for sealing.

5. The production method of ML40Cr automotive cold heading steel wire rod under reduced annealing according to claim 1, characterized in that, 2) In this step, a protective slag is added to the crystallizer.

6. The method for producing ML40Cr automotive cold-heading steel wire rod under reduced annealing according to claim 1, characterized in that, 4) The insulation cover is completely closed.

7. The production method of ML40Cr automotive cold heading steel wire rod under reduced annealing according to claim 1, characterized in that, The ML40Cr automotive cold heading steel wire rod has the following chemical composition by weight percentage. composition: C: 0.35%~0.45%, Si: 0.10%~0.30%, Mn: 0.60%~1.00%, Cr: 0.90%~1.20%, with the remainder being Fe and unavoidable impurities.

8. The method for producing ML40Cr automotive cold-heading steel wire rod with reduced annealing according to claim 7, characterized in that, The microstructure of the automotive cold heading steel wire rod ML40Cr consists of pearlite and ferrite, with the volume content of pearlite being not less than 80%, and the original austenite grain size level of the steel being greater than or equal to grade 10.

9. The method for producing ML40Cr automotive cold-heading steel wire rod under reduced annealing according to claim 7, characterized in that, The ML40Cr automotive cold heading steel wire rod has a spheroidization grade greater than or equal to level 4 after one spheroidization process.

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