An anneal-free boron-containing cold heading steel wire rod and its production method

Abstract

This invention relates to the field of cold heading steel wire rod production technology, specifically to a non-annealed boron-containing cold heading steel wire rod and its production method. The chemical composition of the wire rod, by weight percentage, is as follows: C: 0.31%–0.33%, Si ≤ 0.05%, Mn: 0.70%–0.90%, P ≤ 0.010%, S ≤ 0.010%, Al: 0.02%–0.06%, B: 0.0010%–0.0025%, Cr: 0.20%–0.30%, Ti: 0.015%–0.040%, N ≤ 0.0050%, with the balance being Fe and unavoidable impurities. The wire rod structure consists of ferrite and pearlite. The ferrite content is not less than 55% by area, the ferrite grain size is 10-25μm, the reduction of area is not less than 60%, the hardness is not higher than 85HRB, and the hardness difference in the circumferential direction is not greater than 20HV, which reduces the production cost of wire rod.

Description

Technical Field

[0001] This invention relates to the field of cold heading steel wire rod production technology, specifically to a non-annealing boron-containing cold heading steel wire rod and its production method. Background Technology

[0002] Currently, the general production process for high-strength cold-heading steel wire is as follows: wire rod → (spheroidizing annealing) → pickling (mechanical descaling) → phosphating → drawing → spheroidizing annealing → pickling (mechanical descaling) → phosphating → drawing. In this process, the annealing step effectively reduces the hardness of the wire, facilitating subsequent drawing and cold-heading deformation. However, it incurs costs and investments in energy, efficiency, equipment, space, and labor. With increasing pressure from upstream and downstream industries on the fastener industry, non-annealed products are becoming increasingly popular due to their cost and efficiency advantages. Relevant literature on the development of non-annealed cold-heading steel products can be found both domestically and internationally.

[0003] Chinese patent document with application number CN202310627713.X discloses "A method for producing non-annealed cold heading steel", Chinese patent document with application number CN202310039557.5 discloses "A non-annealed high-strength low-carbon boron-containing cold heading steel and its preparation method", Chinese patent document with application number CN202211672915.8 discloses "A method for controlling the hot-rolled wire rod of easily pickled non-annealed medium-carbon cold heading steel", and Chinese patent document with application number CN202110031191.8 discloses "A non-annealed high-strength cold heading steel and its manufacturing method".

[0004] However, most of the aforementioned literature requires the final rolling and wire drawing temperature of the wire rod to be below 820°C. The chemical composition of the wire rod and the processing of the wire rod have little impact on the hardness of the fine wire. Compared with conventional production processes, this significantly increases the load on high-speed wire rod mills, reduces the expected life of rolling-related components, and increases the cost for wire rod manufacturers. Summary of the Invention

[0005] To overcome the shortcomings of existing technologies, this invention provides a non-annealing boron-containing cold heading steel wire rod and its production method. The wire rod's microstructure consists of ferrite and pearlite, with a ferrite content of not less than 55% by area, a ferrite grain size of 10~25μm, a reduction of area of ​​not less than 60%, a hardness of not more than 85HRB, and a circumferential hardness difference of not more than 20HV, thereby reducing the production cost of the wire rod.

[0006] To achieve the above objectives, the present invention employs the following technical solution:

[0007] An anneal-free boron-containing cold heading steel wire rod, the chemical composition of which is as follows by weight percentage:

[0008] C: 0.31%–0.33%, Si ≤ 0.05%, Mn: 0.70%–0.90%, P ≤ 0.010%, S ≤ 0.010%, Al: 0.02%–0.06%, B: 0.0010%–0.0025%, Cr: 0.20%–0.30%, Ti: 0.015%–0.040%, N ≤ 0.0050%, with the balance being Fe and unavoidable impurities.

[0009] The functions of selecting the above chemical components and their concentrations, as well as the rationale for the design, are as follows:

[0010] Carbon is the main strengthening element in steel and is fundamental to ensuring the strength of finished fasteners; therefore, a certain amount of carbon must be retained in steel. However, carbon is an element that significantly increases the strength of steel while reducing its plasticity. Increasing the carbon content will affect the performance of the wire rod; therefore, the carbon content should not be too high. Furthermore, non-annealed products cannot adjust the hardness of the steel wire through the annealing process, making them more sensitive to fluctuations in composition. In this invention, the carbon content is controlled within the range of 0.31% to 0.33%.

[0011] Silicon is a major deoxidizing and strengthening element in steel. Silicon does not form carbides in steel but exists as a solid solution in ferrite, thus significantly improving the strength of low-carbon steel. It also increases the cold work hardening rate. For products that do not require annealing, the hardness of the steel wire cannot be adjusted through the annealing process. Furthermore, reducing the silicon content does not significantly reduce the strength of the steel after quenching and tempering. When it is not necessary to add sufficient silicon to ensure deoxidation, the silicon content in the steel should be reduced as much as possible. Therefore, the silicon content in the steel of this invention is controlled at ≤0.05%.

[0012] Manganese is also an element that improves the strength of wire rod. It can increase the hardness and strength of ferrite. Simultaneously, because manganese can lower the critical transformation temperature in steel, it is beneficial for pearlite refinement, thus improving the strength of pearlite. From the perspective of ensuring the strength of finished fasteners, steel should contain a certain amount of manganese. However, from the perspective of controlling the hot-rolled strength of wire rod and the hardness of steel wire, the manganese content in steel should not be too high. Therefore, the manganese content in steel should be controlled between 0.70% and 0.90%.

[0013] Phosphorus and sulfur are harmful impurity elements in steel, and their levels should be as low as possible without causing other adverse effects. This invention requires that the phosphorus content of wire rod be ≤0.010% and the sulfur content be ≤0.010%.

[0014] Boron can significantly improve the hardenability of steel, and an addition of 0.0020% is sufficient to achieve a significant improvement in hardenability. This allows for a reduction in the amount of other alloying elements that improve hardenability. To fully utilize the hardenability of boron while avoiding the weakening of material grain boundaries caused by boron segregation, the boron content in this invention is controlled at 0.0010% to 0.0025%.

[0015] Chromium can improve the hardenability and strength of steel without significantly deteriorating its plasticity or causing work hardening. It also inhibits decarburization of wire rod. The chromium content in this invention is controlled at 0.20% to 0.30%.

[0016] Titanium can be used as a deoxidizer in steel. Furthermore, titanium carbide can form before steel solidification, becoming a heterogeneous nucleus during solidification and refining the grain size. In boron-containing steel, it can be used as a nitrogen-fixing agent, preventing boron from forming BN and thus losing its ability to improve hardenability. In this invention, the titanium content is controlled at 0.015%~0.040%.

[0017] Aluminum is a strong deoxidizing element and is also used as a nitrogen-fixing agent. The AlN formed by aluminum and nitrogen can act as a heterogeneous nucleus, refining the grain size and improving the uniform deformation capacity of wire rod. However, excessive aluminum content will worsen the deformability of inclusions in the steel and cause difficulties in the steel smelting and continuous casting processes. Therefore, the aluminum content of wire rod is controlled between 0.02% and 0.06%.

[0018] Nitrogen dissolved in steel plays a precipitation strengthening role and also increases the work hardening rate of the steel. Therefore, the lower the nitrogen content, the better, without causing other adverse effects. This invention requires that the [N] content of the wire rod be ≤0.0050%.

[0019] The above-mentioned wire rod microstructure consists of ferrite and pearlite, with a ferrite content of not less than 55% by area and a ferrite grain size of 10~25μm. The wire rod has a reduction of area of ​​not less than 60%, a hardness of not more than 85HRB, and a circumferential hardness range of not more than 20HV.

[0020] The manufacturing method of the above-mentioned non-annealed boron-containing cold heading steel wire rod is as follows:

[0021] 1) Heating of steel billets:

[0022] The purpose of heating the billet is to fully austenitize it, reduce its deformation resistance, and ensure the smooth progress of the wire rod rolling process.

[0023] The total heating time of the steel billet in the furnace is 2.5 to 3 hours, and the temperature of the soaking zone is controlled at 950 to 1050℃.

[0024] 2) Rolling and spinning:

[0025] During wire rod rolling, the finishing mill inlet temperature and the wire drawing temperature affect the grain size of austenite before phase transformation and also influence the phase transformation initiation temperature to some extent. Relatively lower finishing and wire drawing temperatures are beneficial for reducing the austenite grain size, thereby increasing the nucleation sites of ferrite at the grain boundaries and promoting the final formation of a uniform and fine microstructure. On the other hand, as the finishing and wire drawing temperatures decrease, the deformation resistance of the high-speed rolling mill increases significantly. Using excessively low finishing and wire drawing temperatures will lead to rapid aging of the rolling mill and the rolling groove.

[0026] The inlet temperature of the finishing mill is 800-850℃, and the wire drawing temperature is 800-850℃.

[0027] 3) Wire rod size control:

[0028] The actual diameter of the control bar is larger than the nominal diameter, and the deviation range is +0.1mm to +0.3mm.

[0029] By adopting positive tolerance rolling, the amount of deformation required to draw wire rod to the target precision wire specification is reduced, thereby reducing the work hardening effect during the drawing process.

[0030] 4) Wire rod cooling:

[0031] The wire rod is cooled using the EDC process, followed by slow cooling with a cover.

[0032] At a constant coiling temperature, the cooling method and rate of the wire rod directly affect the residence time and microstructure of the wire rod in each phase region. EDC cooling improves the cooling uniformity of the wire rod between the coiling temperature and the outlet water temperature range, reduces the microstructure differences between different locations on the wire rod, and facilitates uniform deformation during subsequent drawing, preventing flange non-roundness after cold heading. Slow cooling with a cover, using a lower cooling rate, can prolong the time the wire rod spends in the ferrite phase region, increasing the amount of ferrite precipitation and allowing for sufficient ferrite growth, forming blocky ferrite at the grain boundaries, reducing the wire rod's strength, and increasing its plasticity.

[0033] Furthermore, in step 1), the cross-sectional dimensions of the steel billet used for the wire rod are (150~180) mm × (150~180) mm.

[0034] Furthermore, in step 2), the final rolling process of the rolled piece adopts a dual-module rolling mill, with a dual-module inlet temperature of 800-850℃ and a wire drawing temperature of 800-850℃.

[0035] Furthermore, in step 4), the wire rod inlet water temperature is 780~820℃, the initial roller speed is 0.6~0.90m / s, and the outlet water temperature is 650~720℃.

[0036] Furthermore, in step 4), the entire line of the air-cooled roller conveyor is then covered and slowly cooled.

[0037] Furthermore, in step 4), the average cooling rate at the overlap point of the wire rod after water discharge is 0.5~1.5℃ / s.

[0038] Compared with the prior art, the present invention has the following beneficial effects:

[0039] 1. This invention employs a boron-added composition design. Boron can significantly improve the hardenability of steel, and an addition of 0.0020% is sufficient to achieve a significant improvement in hardenability. This allows for a reduction in the amount of other alloying elements that improve hardenability. To fully utilize the hardenability of boron while avoiding the weakening of material grain boundaries caused by boron segregation, the boron content in this invention is controlled at 0.0010% to 0.0025%.

[0040] 2. The present invention heats the billet to fully austenitize it, reducing the deformation resistance of the billet and allowing the wire rod rolling process to proceed smoothly.

[0041] 3. This invention controls the finishing mill inlet temperature to 800–850℃ and the wire drawing temperature to 800–850℃. On the one hand, this increases the nucleation sites of ferrite at the grain boundaries, which is beneficial for the final formation of a uniform and fine microstructure. On the other hand, as the finishing rolling temperature and wire drawing temperature decrease, the deformation resistance of the high-speed rolling mill is significantly improved. This invention uses appropriate finishing rolling temperature and wire drawing temperature to improve the mill life and avoid rapid aging of the rolling groove.

[0042] 4. This invention reduces the amount of deformation required to draw wire rod to the target precision wire specification by using positive tolerance rolling, thereby reducing the work hardening effect during the drawing process.

[0043] 5. This invention improves the cooling uniformity of the wire rod from the wire drawing temperature to the outlet water temperature range through EDC cooling, reduces the microstructure differences between different locations on the wire rod, which is beneficial for uniform deformation during subsequent drawing and prevents the flange from becoming out of round after cold heading. By using a cover for slow cooling at a lower rate, the time the wire rod spends in the ferrite phase region can be extended, increasing the amount of ferrite precipitation and allowing for sufficient ferrite growth, forming blocky ferrite at the grain boundaries, reducing the wire rod's strength and increasing its plasticity.

[0044] In summary, the non-annealed boron-containing cold heading steel wire rod produced using the chemical composition and method described in this invention has a microstructure composed of ferrite and pearlite, with a ferrite content of not less than 55% by area, a ferrite grain size of 10~25μm, a reduction of area of ​​not less than 60%, a hardness of not more than 85HRB, and a circumferential hardness range of not more than 20HV. This reduces the load on high-speed wire rod mills, improves the expected life of rolling-related components, increases the cost for wire rod manufacturers, and creates conditions for efficient and green production for both wire rod manufacturers and cold heading steel precision wire rod manufacturers. Detailed Implementation

[0045] This invention discloses a non-annealed boron-containing cold heading steel wire rod and its production method. Those skilled in the art can refer to the content of 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.

[0046] The chemical composition of the embodiments of the present invention is shown in Table 1, the manufacturing method of the embodiments of the present invention is shown in Table 2, and the microstructure characteristics and mechanical properties of the embodiments of the present invention are shown in Table 3.

[0047] Table 1 Chemical composition of the embodiments of the present invention, wt%

[0048]

[0049] Table 2 Manufacturing method of embodiments of the present invention

[0050]

[0051] Table 3 Microstructure characteristics and mechanical properties of embodiments of the present invention

[0052]

[0053] As shown in Table 3, the microstructure of the non-annealed boron-containing cold heading steel wire rod produced using the chemical composition and method described in this invention consists of ferrite and pearlite. The ferrite content, calculated by area, is not less than 55%, the ferrite grain size is 10~25μm, the reduction of area is not less than 60%, the hardness is not higher than 85HRB, and the hardness difference in the circumferential direction is not greater than 20HV. This reduces the load on high-speed wire rod mills, improves the expected life of rolling-related components, and reduces the cost for wire rod manufacturers, creating conditions for efficient and green production for both wire rod manufacturers and cold heading steel precision wire rod manufacturers.

[0054] The above description is only a preferred specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any technician familiar with the technical field, within the technical scope disclosed by the present invention, who makes equivalent replacements or changes based on the technical solution and inventive concept of the present invention, should be covered by the scope of protection of the present invention.

Claims

1. A method for manufacturing boron-containing cold heading steel wire rod without annealing, characterized in that, The chemical composition of the wire rod, by weight percentage, is as follows: C: 0.31%~0.33%, Si≤0.05%, Mn: 0.70%~0.90%, P≤0.0092%, S≤0.010%, Al: 0.02%~0.06%, B: 0.0010%~0.0025%, Cr: 0.20%~0.30%, Ti: 0.015%~0.040%, N≤0.0050%, with the balance being Fe and unavoidable impurities; The wire rod microstructure consists of ferrite and pearlite, with a ferrite content of not less than 55% by area and a ferrite grain size of 10~25μm. The section reduction rate of wire rod is not less than 60%, the hardness is not higher than 85HRB, and the hardness range in the circumferential direction is not greater than 20HV. Specifically as follows: 1) Heating of steel billets: The total heating time of the steel billet in the furnace is 2.5 to 3 hours, and the temperature of the soaking zone is controlled at 985 to 1050℃. 2) Rolling and spinning: The final rolling process uses a dual-module mill with an inlet temperature of 822–850℃ and a wire exit temperature of 813–850℃. 3) Wire rod size control: The actual diameter of the control wire rod is larger than the nominal diameter, and the deviation range is +0.1mm to +0.3mm; 4) Wire rod cooling: The wire rod cooling adopts the EDC process. The wire rod inlet water temperature is 780~820℃, the initial roller speed is 0.6~0.9m / s, and the outlet water temperature is 650~720℃. After that, the entire line is covered and slowly cooled on the air-cooled roller conveyor. After exiting the water, the average cooling rate of the wire rod overlap point is 0.5~1.5℃ / s.

2. The method for manufacturing a non-annealed boron-containing cold heading steel wire rod according to claim 1, characterized in that, In step 1), the cross-sectional dimensions of the steel billet used for wire rod are (150~180) mm × (150~180) mm.

Images