Annealing-free carbon cold heading steel wire rod and production method thereof

Abstract

The present invention relates to the technical field of cold-heading steel wire rod production, specifically to an annealing-free carbon cold-heading steel wire rod and its production method. The chemical composition of the wire rod, by weight, is as follows: C: 0.34% to 0.36%, Si ≤ 0.05%, Mn: 0.70% to 0.90%, P ≤ 0.010%, S ≤ 0.010%, Al: 0.02% to 0.04%, Ti: 0.015% to 0.040%, N ≤ 0.0050%, with the remainder being Fe and unavoidable impurities. The production method specifically comprises the following steps: 1) heating the steel billet; 2) rolling and spinning; 3) controlling the wire rod dimensions; and 4) cooling the wire rod using the EDC process, followed by slow cooling with a hood. The wire rod structure is composed of ferrite and pearlite. The ferrite content is not less than 50% by area, the ferrite grain size is 10-25μm, the cross-sectional shrinkage rate is not less than 55%, the hardness is not higher than 85HRB, and the extreme difference in hardness in the circumferential direction is not greater than 20HV, which reduces the production cost of the wire rod.

Description

Technical Field

[0001] The invention relates to the technical field of cold heading steel wire rod production, in particular to an annealing-free carbon cold heading steel wire rod and a production method thereof. Background Art

[0002] Currently, the typical production process for high-strength cold-heading steel wire is: wire rod → (spheroidizing annealing) → pickling (mechanical descaling) → phosphating → drawing → spheroidizing annealing → pickling (mechanical descaling) → phosphating → drawing. In this process, annealing effectively reduces the wire's hardness, facilitating subsequent drawing and cold-heading deformation. However, this process incurs costs and investment in energy, efficiency, equipment, space, and labor. As upstream and downstream industries squeeze the fastener industry, annealing-free products are increasingly popular in the market due to their cost and efficiency advantages. Literature related to the development of annealing-free cold-heading steel products can be found both domestically and internationally.

[0003] The Chinese patent document with application number CN202310627713.X discloses "A method for producing annealing-free cold heading steel", the Chinese patent document with application number CN202211672915.8 discloses "A method for controlling hot-rolled wire rods of easy-pickling annealing-free medium-carbon cold heading steel", and the Chinese patent document with application number CN202110031191.8 discloses "A method for producing annealing-free high-strength cold heading steel and its manufacturing method".

[0004] However, most of the above-mentioned documents require that the final rolling and spinning temperature of the wire rod reach below 820°C, and the influence of the chemical composition and processing process of the wire rod on the hardness of the finished wire is less involved. Compared with conventional production processes, the load of the wire rod high-speed rolling mill is significantly increased, the expected life of rolling-related components is reduced, and the cost of wire rod production enterprises is increased. Summary of the Invention

[0005] In order to overcome the shortcomings of the existing technology, the present invention provides an annealing-free carbon cold heading steel wire rod and a production method thereof. The wire rod structure is composed of ferrite and pearlite, the ferrite content calculated by area is not less than 50%, the ferrite grain size is 10-25μm, the cross-sectional shrinkage rate is not less than 55%, the hardness is not higher than 85HRB, the extreme difference in hardness in the circumferential direction is not greater than 20HV, thereby reducing the production cost of the wire rod.

[0006] In order to achieve the above object, the present invention adopts the following technical solutions:

[0007] An annealing-free carbon cold-heading steel wire rod, the chemical composition of the wire rod is as follows by weight percentage:

[0008] C: 0.34%~0.36%, Si≤0.05%, Mn: 0.70%~0.90%, P≤0.010%, S≤0.010%, Al: 0.02%~0.04%, Ti: 0.015%~0.040%, N≤0.0050%, and the balance is Fe and unavoidable impurities.

[0009] The role of selecting the above chemical components and their contents, as well as the design reasons are as follows:

[0010] Carbon is the primary strengthening element in steel and fundamental to ensuring the strength of finished fasteners. Therefore, a certain amount of carbon must be retained in the steel. Furthermore, carbon significantly increases steel's strength while simultaneously reducing its plasticity. Increasing the carbon content will affect the performance of the wire rod, so excessive carbon content is not recommended. Furthermore, annealing-free products cannot adjust the wire hardness through the annealing process, making them more sensitive to compositional fluctuations. The carbon content in this invention is controlled within a range of 0.34% to 0.36%.

[0011] Silicon is the primary deoxidizing and strengthening element in steel. Silicon does not form carbides in steel, but rather exists as a solid solution in ferrite. This significantly increases the strength of low-carbon steel and improves the cold work hardening rate. Annealing-free products cannot adjust the wire hardness through annealing, and reducing the silicon content does not significantly reduce the strength of the steel after quenching and tempering. Unless sufficient silicon is added to ensure deoxidation, the silicon content in the steel should be minimized. Therefore, the silicon content in the present invention is controlled to ≤ 0.05%.

[0012] Manganese is also an element that improves wire rod strength, increasing the hardness and strength of ferrite. Furthermore, because manganese lowers the critical transition temperature in steel, it facilitates pearlite refinement and increases its strength. To ensure the strength of the finished fastener, steel should contain a certain amount of manganese. However, to control the hot-rolled strength of the wire rod and the hardness of the steel wire, the manganese content should not be too high. Therefore, the manganese content in the present invention is controlled to 0.70% to 0.90%.

[0013] Phosphorus and sulfur are harmful impurity elements in steel. The lower the content, the better, provided they do not cause other effects. The present invention requires that the content of [P] in the wire rod is ≤ 0.010% and the content of [S] is ≤ 0.010%.

[0014] Aluminum is a strong deoxidizing element and also acts as a nitrogen-fixing agent. The AlN formed by aluminum and nitrogen acts as a heterogeneous core, refining the grains and improving the uniform deformation capability of the wire rod. However, excessive aluminum content can worsen the deformability of inclusions in the steel and create difficulties in the steelmaking and continuous casting processes. Therefore, the aluminum content in the present invention is controlled to 0.02% to 0.04%.

[0015] Titanium acts as a deoxidizer in steel, and titanium carbide forms before solidification, becoming a heterogeneous core during solidification and refining the grains. The present invention utilizes titanium and aluminum composite microalloying to further enhance grain refinement, thereby eliminating uneven grain size, improving the wire rod's ability to coordinate deformation, and preventing cold heading cracking. This fully utilizes the titanium element while avoiding the inclusion of hard and brittle TiN phases and significant cost increases caused by excessive addition. The content is controlled between 0.015% and 0.040%.

[0016] Solid-dissolved nitrogen in steel plays a role in precipitation strengthening and improves the work hardening rate of steel. Therefore, the lower the nitrogen content, the better, without causing other effects. The present invention requires that N≤0.0050%.

[0017] The wire rod structure is composed of ferrite and pearlite, with the ferrite content calculated by area not less than 50%, and the ferrite grain size ranging from 10 to 25 μm. The wire rod has a cross-sectional reduction rate of not less than 55%, a hardness not higher than 85 HRB, and a circumferential hardness range of not more than 20 HV.

[0018] The manufacturing method of the above-mentioned annealing-free boron-containing cold heading steel wire rod is as follows:

[0019] 1) Billet heating: The purpose of billet heating is to fully austenitize the billet, reduce the deformation resistance of the billet, and make the wire rolling process go smoothly.

[0020] The total heating time of the steel billet in the furnace is 2.5 to 3 hours, and the temperature in the soaking section is controlled at 950 to 1050°C.

[0021] 2) Rolling and spinning:

[0022] During wire rod rolling, the finishing inlet temperature and the spinning temperature affect the austenite grain size before phase transformation and, to a certain extent, the phase transformation onset temperature. Relatively low finishing and spinning temperatures help reduce austenite grain size, thereby increasing the nucleation sites for ferrite at grain boundaries and ultimately promoting the formation of a uniform, fine structure. On the other hand, as finishing and spinning temperatures decrease, the deformation resistance of high-speed rolling mills increases significantly. Using excessively low finishing and spinning temperatures can shorten mill life and accelerate aging of the groove.

[0023] The finishing rolling inlet temperature is 800-850℃, and the spinning temperature is 800-850℃.

[0024] 3) Wire rod size control:

[0025] The actual diameter of the control wire rod is larger than the nominal diameter, and the deviation range is +0.1mm to +0.3mm.

[0026] By adopting positive tolerance rolling, the deformation required to draw the wire rod to the target fine wire specification is reduced, and the work hardening effect during the drawing process is reduced.

[0027] 4) Wire rod cooling:

[0028] The wire rod is cooled using the EDC process, followed by slow cooling with a cover.

[0029] Under the condition of a constant spinning temperature, the cooling method and cooling rate of the wire rod directly affect the residence time and microstructure of the wire rod in each phase region. Through EDC cooling, the cooling uniformity of the wire rod in the range from spinning temperature to water outlet temperature is improved, and the microstructure differences between different positions of the wire rod are reduced, which is conducive to the uniform deformation of the wire rod in the subsequent drawing process and prevents the phenomenon of flange out-of-round after cold heading. Through slow cooling with a buckle cover and a lower cooling rate, the time the wire rod passes through the ferrite phase region can be extended, the amount of ferrite precipitation can be increased, and the ferrite can be fully grown, forming blocky ferrite on the grain boundary, reducing the strength of the wire rod and improving the plasticity of the wire rod.

[0030] Furthermore, in step 1), the cross-sectional dimensions of the steel billet used for the wire rod are: (150-180) mm×(150-180) mm.

[0031] Furthermore, in step 2), the final rolling process of the rolled piece adopts a double-module rolling mill, the double-module inlet temperature is 800-850°C, and the wire laying temperature is 800-850°C.

[0032] Furthermore, in step 4), the wire rod entering water temperature is 780-820°C, the initial roller speed is 0.6-0.90 m / s, and the water outlet temperature is 650-720°C.

[0033] Furthermore, in step 4), the entire line is covered and slowly cooled on an air-cooled roller conveyor.

[0034] Furthermore, in step 4), the average cooling rate of the wire rod overlap point after exiting the water is 0.5-1.5°C / s.

[0035] Compared with the prior art, the present invention has the following beneficial effects:

[0036] 1. The carbon content of the present invention is controlled within a range of 0.34% to 0.36%. Carbon is the primary strengthening element in steel and is essential for ensuring the strength of finished fasteners. Therefore, a certain amount of carbon must be retained in the steel. Furthermore, carbon significantly increases steel's strength while simultaneously decreasing its plasticity. Increasing the carbon content will affect the performance of the wire rod, so excessive carbon content is not recommended. Furthermore, annealing-free products cannot adjust the wire hardness through the annealing process, making them more sensitive to compositional fluctuations.

[0037] 2. The heating of the steel billet of the present invention makes the steel billet fully austenitized, reduces the deformation resistance of the steel billet, and enables the wire rolling process to proceed smoothly.

[0038] 3. The present invention controls the finishing rolling inlet temperature to 800-850°C and the spinning temperature to 800-850°C. This increases the nucleation sites for ferrite at grain boundaries, ultimately facilitating the formation of a uniform, fine structure. Furthermore, as the finishing and spinning temperatures decrease, the deformation resistance of the high-speed rolling mill increases significantly. By adopting appropriate finishing and spinning temperatures, the present invention extends the life of the rolling mill and prevents rapid aging of the groove.

[0039] 4. The present invention adopts positive tolerance rolling to reduce the deformation required for drawing the wire rod to the target fine wire specification, thereby reducing the work hardening effect during the drawing process.

[0040] 5. The present invention improves the cooling uniformity of the wire rod from the spinning temperature to the water outlet temperature through EDC cooling, reduces the structural differences between different positions of the wire rod, facilitates uniform deformation of the wire rod during subsequent drawing, and prevents the occurrence of out-of-round flanges after cold heading. By using a buckle cover for slow cooling and a lower cooling rate, the wire rod's transit time in the ferrite phase region can be extended, increasing the amount of ferrite precipitation and allowing for full ferrite growth, forming massive ferrite at the grain boundaries, reducing the strength of the wire rod and improving its plasticity.

[0041] In summary, the annealing-free boron-containing cold heading steel wire rod produced by the chemical composition and method of the present invention is composed of ferrite and pearlite, the ferrite content calculated by area is not less than 50%, the ferrite grain size is 10-25 μm, the cross-sectional shrinkage rate is not less than 55%, the hardness is not higher than 85 HRB, and the extreme difference in hardness in the circumferential direction is not greater than 20 HV. This reduces the load of the wire rod high-speed rolling mill, increases the expected life of rolling-related components, increases the cost of wire rod production enterprises, and creates conditions for efficient and green production of wire rod production enterprises and cold heading steel fine wire production enterprises. DETAILED DESCRIPTION

[0042] The present invention discloses an annealing-free carbon cold-heading steel wire rod and a production method thereof. Those skilled in the art can refer to the contents of this article and appropriately improve the process parameters to achieve the desired result. It should be noted that all similar substitutions and modifications are obvious to those skilled in the art and are considered to be included in the present invention. The methods and applications of the present invention have been described through preferred embodiments, and relevant personnel can obviously modify or appropriately change and combine the methods and applications described herein without departing from the content, spirit and scope of the present invention to implement and apply the technology of the present invention.

[0043] The chemical composition of the embodiment of the present invention is shown in Table 1, the manufacturing method of the embodiment of the present invention is shown in Table 2, and the microstructural characteristics and mechanical properties of the embodiment of the present invention are shown in Table 3.

[0044] Table 1 Chemical composition of the embodiments of the present invention, wt%

[0045]

[0046] Table 2 Manufacturing method of the embodiment of the present invention

[0047]

[0048] Table 3 Microstructure characteristics and mechanical properties of the embodiments of the present invention

[0049]

[0050] As shown in Table 3, the microstructure of the annealing-free boron-containing cold heading steel wire rod produced by the chemical composition and method of the present invention is composed of ferrite and pearlite, the ferrite content calculated by area is not less than 50%, the ferrite grain size is 10-25 μm, the cross-sectional reduction rate is not less than 55%, the hardness is not higher than 85 HRB, and the extreme difference in hardness in the circumferential direction is not greater than 20 HV. This reduces the load of the wire rod high-speed rolling mill, increases the expected life of rolling-related components, reduces the cost of wire rod production enterprises, and creates conditions for efficient and green production of wire rod production enterprises and cold heading steel fine wire production enterprises.

[0051] 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 annealing-free carbon cold heading steel wire rod, characterized in that: The chemical composition of the wire rod is as follows in percentage by weight: C: 0.34% to 0.36%, Si ≤ 0.05%, Mn: 0.70% to 0.90%, P ≤ 0.010%, S ≤ 0.010%, Al: 0.02% to 0.04%, Ti: 0.015% to 0.040%, N ≤ 0.0050%, the balance is Fe and unavoidable impurities; The wire rod structure is composed of ferrite and pearlite. The ferrite content is not less than 50% by area, and the ferrite grain size is 10~25μm. The cross-sectional shrinkage of the wire rod shall not be less than 55%, the hardness shall not be higher than 85HRB, and the extreme difference in hardness in the circumferential direction shall not be greater than 20HV; The details are as follows: 1) Billet heating: The total heating time of the steel billet in the furnace is 2.5 to 3 hours, and the temperature in the soaking section is controlled at 950 to 1050°C; 2) Rolling and spinning: The final rolling process adopts a double-module rolling mill with a double-module inlet temperature of 800-850℃ and a spinning temperature of 815-850℃. 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 is cooled using the EDC process, followed by slow cooling with a cover; the wire rod inlet water temperature is 780-820°C, the initial roller speed is 0.6-0.90 m / s, and the outlet water temperature is 650-720°C; the average cooling rate of the wire rod overlap point after exiting the water is 0.5-1.5°C / s.

2. The method for manufacturing annealing-free carbon 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 the wire rod are: (150-180) mm × (150-180) mm.

3. The method for manufacturing annealing-free carbon cold heading steel wire rod according to claim 1, characterized in that: In step 4), the entire line is covered and slowly cooled on an air-cooled roller conveyor.

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