High carbon free-cutting steel wire rod and method for producing the same

Abstract

The application discloses a high-carbon free-cutting steel wire rod and a production method thereof, and belongs to the field of free-cuting steel heat treatment processes. The high-carbon free-cutting steel wire rod is characterized in that the content of C element in the wire rod is 0.60% to 1.0%, and the metallographic structure of the wire rod is composed of ferrite and carbon-free bainite. The production method of the high-carbon free-cutting steel wire rod mainly comprises the following steps: hot rolling, wire drawing, Stelmor air cooling line slow cooling, salt quenching, high-pressure water desalting and the like. The application adopts a unique Stelmor air cooling line + on-line salt bath process route, which not only fully guarantees that no harmful carbide is precipitated after the wire rod is quenched, but also maximally weakens thermal stress and organizational stress formed in the quenching process of the wire rod, effectively reduces the hardness and cracking tendency of the wire rod after quenching, and thus promotes the industrialized stable production of the high-carbon free-cutting steel.

Description

Technical Field

[0001] This invention belongs to the field of free-cutting steel and its heat treatment technology, and more specifically, relates to a high-carbon free-cutting steel wire rod and its production method. Background Technology

[0002] Free-cutting steels are mainly used to manufacture automotive parts such as crankshafts, connecting rods, gears, brakes, and spark plugs. Currently, the most widely produced and used free-cutting steels domestically and internationally are sulfur-based and sulfur + lead-based free-cutting steels. Due to the toxicity of sulfur and lead and the serious pollution they cause during smelting and recycling, their application in free-cutting steels is increasingly restricted. Steel companies are actively developing lead-free, low-sulfur, and environmentally friendly free-cutting steels. Graphite free-cutting steels, with their excellent machinability, cold formability, high fatigue strength, and environmental friendliness, have attracted widespread attention from domestic and international parts manufacturers and metallurgists in recent years. Their research and development strategy involves transforming carbon, which exists as cementite in the steel, into the free-cutting phase graphite.

[0003] To achieve industrial-scale production, research on graphite free-cutting steel has focused primarily on shortening the graphitization time, with in-depth exploration into the application of elements promoting graphitization (Si, Al, B, etc.) and improvements in heat treatment processes. Currently, the commonly used heat treatment process for graphite free-cutting steel is quenching followed by graphitization tempering. The quenching process (as disclosed in Chinese invention patents CN101906597A, CN102660666A, CN100535171C, CN101899555A, and CN106947907A) involves heating the steel to full or partial austenitization and holding it at that temperature for a period of time, followed by rapid water cooling to below the martensitic transformation temperature. This controls the carbon atoms in the steel to exist in a supersaturated solid solution state, while ensuring the matrix has a high lattice distortion energy, thereby accelerating the graphitization process of carbon during subsequent tempering and effectively shortening the tempering time. However, hot-rolled graphite free-cutting steel wire rods are prone to cracking to varying degrees during the existing water quenching process, which leads to a decline in product quality and an increase in scrap rate, seriously affecting their stable industrial production.

[0004] It should be noted that in recent years, the inventors of this application have been committed to the research of graphite free-cutting steel materials and their preparation processes, and have achieved many research results. For example, Chinese patent applications No. 202110800295.0 and 202210890492.0 respectively disclose a high-carbon high-aluminum steel for drilling locks and a medium-carbon hoist lock body material and its preparation method. However, the above methods are only applicable to the production of graphite free-cutting steel lock body materials with large cross sections. When used to produce graphite free-cutting steel wire rods with small cross sections, it is difficult to guarantee the uniformity of internal and external stresses during hot-state wire rod water quenching and to prevent cracking.

[0005] Therefore, with the increasing demand for environmentally friendly graphite free-cutting steel wire rods for mechanical parts, there is an urgent need to effectively solve the problem of quenching cracking of hot graphite free-cutting steel wire rods, so as to improve the product quality and meet the requirements of stable industrial production. Summary of the Invention

[0006] 1. Technical problems to be solved

[0007] The purpose of this invention is to provide a high-carbon free-cutting steel wire rod and its production method, thereby solving the technical problem that existing graphite free-cutting steel hot-rolled wire rods are prone to cracking during quenching, leading to a decline in product quality, and greatly improving the qualification rate of the obtained products.

[0008] 2. Technical Solution

[0009] This invention provides a high-carbon free-cutting steel wire rod with a carbon content of 0.60%~1.0%, and its microstructure consists of ferrite and carbon-free bainite. Currently available high-carbon free-cutting steel wire rods, due to their high carbon content, are prone to cracking during quenching, leading to decreased product quality and increased scrap rates, severely impacting the stability of industrial production. The high-carbon free-cutting steel wire rod of this invention, with its microstructure composed of ferrite and carbon-free bainite, effectively improves the plasticity and toughness of the high-carbon free-cutting steel wire rod and prevents cracking during quenching.

[0010] Furthermore, the wire rod also contains the following components by mass percentage: Si: 1.60%~2.20%, Mn: 0.15%~0.35%, Al: 0.03%~0.50%, Si+Al: ≥2.10%.

[0011] Furthermore, the wire rod contains P: ≤0.035%, S: ≤0.015%, and N: ≤0.008%.

[0012] The present invention also provides a method for producing the above-mentioned high-carbon free-cutting steel wire rod, including hot rolling, wire drawing, slow cooling on a Steyrmo air-cooling line, and salt quenching production processes.

[0013] Currently, high-carbon free-cutting steel wire rods are typically produced using a hot-rolling-water-quenching process. This application, however, involves hot rolling and wire drawing followed by slow cooling on a Stellmore air-cooling line, followed by online salt bath treatment. This approach ensures no harmful carbide precipitation after quenching and minimizes thermal and structural stresses generated during quenching, thereby reducing the tendency for cracking and achieving the goals of lowering the scrap rate, reducing production costs, and improving production efficiency and product quality. The salt-quenched wire rods are then tempered and can undergo other special processing as needed.

[0014] Furthermore, the spinning temperature is controlled at A c3 +30~A c3 +60℃ or A ccm +30~A ccm The temperature is +60℃, and the wire rod is cooled at a rate of ≤5℃ / s after spinning. This invention further improves the uniformity of the internal and external structure of the hot wire rod during cooling by employing a high spinning temperature and a low Stellmo air-cooling line roller conveyor, allowing for rapid release of internal stress and reducing localized stress concentration in the cross-sectional direction of the wire rod.

[0015] Furthermore, the hot wire rods are cooled to A temperature on the Steyrmo air-cooled line. c3 -30℃~A c3 Or A ccm -30~A ccm At ℃, it is subjected to isothermal salt bath treatment, with a salt bath temperature of 390~450℃ and a salt bath time of 3~5min.

[0016] This invention further optimizes and controls the quenching temperature and salt bath temperature and shortens the temperature difference between them. This not only effectively avoids the intensification of thermal stress caused by rapid cooling, but also makes full use of the soft phase characteristics of ferrite to weaken the structural stress generated during the salt quenching process, thereby obtaining a low-brittle ferrite + carbon-free bainite fine-grained structure. This avoids the quality problem of cracks caused by uneven internal and external structure and intensified internal stress after salt quenching of hot graphite free-cutting steel wire rod.

[0017] Furthermore, the salt bath solution is a mixed solution of KNO3 and NaNO3, and the mass ratio of KNO3 to NaNO3 is 1:1.

[0018] Furthermore, after the wire rod leaves the salt bath line and cools naturally for 1 minute, high-pressure water is used to remove the residual nitrates on the surface of the wire rod.

[0019] Furthermore, the diameter of the wire rod is controlled to be 5.5~13mm, which can effectively ensure the consistency of its cross-sectional structure and machinability after quenching and tempering. Attached Figure Description

[0020] Figure 1 The low-magnification morphology of the salt-quenched specimen in Example 1 of this invention;

[0021] Figure 2 The metallographic structure after salt quenching in Example 1 of this invention;

[0022] Figure 3 The low-magnification morphology of Comparative Example 1 after water quenching;

[0023] Figure 4 The metallographic structure of Comparative Example 1 after water quenching;

[0024] Figure 5 The metallographic structure after salt quenching is shown in Comparative Example 2;

[0025] Figure 6 The values ​​represent the hardness from the surface to the core of the quenched wire rods in Examples 1-4 and Comparative Examples 1-2 (dot positions from 1 to 6 represent the wire rod from the surface to the core). Detailed Implementation

[0026] To further understand the content of this invention, the invention will be described in detail below with reference to the embodiments.

[0027] Example 1

[0028] The high-carbon free-cutting steel wire rod of this embodiment has the following composition by mass percentage: 0.63% C, 2.15% Si, 0.16% Mn, 0.032% Al, 0.031% P, 0.012% S, 0.0077% N, with the remainder being Fe and unavoidable impurities. The production process of the high-carbon free-cutting steel wire rod of this embodiment is as follows: the free-cutting steel is hot-rolled into wire rods with a diameter of 13mm. The wire rod extrusion temperature is controlled at 910±5℃. Before the wire rod enters the Stellmore air-cooling line roller conveyor, the fan is turned off, and an insulation cover is placed on top. The cooling rate is controlled to ≤5℃ / s. When the surface temperature of the wire rod drops to 860±5℃, it is placed in a 450℃ salt bath (KNO3:NaNO3 (mass ratio) = 1:1) for isothermal treatment for 3 minutes. The wire rod is then removed and allowed to cool naturally for 1 minute. Finally, high-pressure water is used to remove residual nitrates from the surface of the wire rod. The low-magnification morphology and microstructure of the sample after salt quenching in this embodiment are as follows: Figure 1 , Figure 2 As shown.

[0029] Example 2

[0030] The high-carbon free-cutting steel wire rod of this embodiment has the following composition by mass percentage: 0.75% C, 2.03% Si, 0.32% Mn, 0.11% Al, 0.028% P, 0.008% S, 0.0044% N, with the remainder being Fe and unavoidable impurities. This free-cutting steel is hot-rolled into wire rods with a diameter of 10 mm, and the wire drawing temperature is controlled at 895±5℃ (its A... c3 (Temperature is 873℃). Before the wire rod enters the Stellmore air-cooled line roller conveyor, the fan is turned off and the insulation cover is put on. After the surface temperature of the wire rod drops to 850±5℃, it is placed in a 420℃ salt bath (KNO3:NaNO3 (mass ratio) = 1:1) for isothermal treatment for 4 minutes. Then the wire rod is taken out and naturally cooled for 1 minute. Finally, high-pressure water is used to remove the residual nitrates on the surface of the wire rod.

[0031] Example 3

[0032] The high-carbon free-cutting steel wire rod of this embodiment has the following composition by mass percentage: 0.82% C, 1.83% Si, 0.28% Mn, 0.287% Al, 0.026% P, 0.010% S, 0.0053% N, with the remainder being Fe and unavoidable impurities. This free-cutting steel is hot-rolled into wire rods with a diameter of 8 mm. The wire rod's extrusion temperature is controlled at 890±5℃. Before the wire rod enters the Stellmore air-cooling line's roller conveyor, the fan is turned off, and the wire rod is covered with an insulation cover. Once the surface temperature of the wire rod drops to 855±5℃, it is placed in a 390℃ salt bath (KNO3:NaNO3 (mass ratio) = 1:1) for isothermal treatment for 3.5 minutes. Afterward, the wire rod is removed and allowed to cool naturally for 1 minute. Finally, high-pressure water is used to remove residual nitrates from the wire rod's surface.

[0033] Example 4

[0034] The high-carbon free-cutting steel wire rod of this embodiment has the following composition by mass percentage: 0.97% C, 1.67% Si, 0.24% Mn, 0.482% Al, 0.025% P, 0.006% S, 0.0061% N, with the remainder being Fe and unavoidable impurities. This free-cutting steel is hot-rolled into wire rods with a diameter of 5.5 mm. The wire rod's extrusion temperature is controlled at 900±5℃. Before the wire rod enters the Stellmore air-cooling line's roller conveyor, the fan is turned off, and the wire rod is covered with an insulation cover. Once the surface temperature of the wire rod drops to 845±5℃, it is placed in a 420℃ salt bath (KNO3:NaNO3 (mass ratio) = 1:1) for isothermal treatment for 4.5 minutes. Afterward, the wire rod is removed and allowed to cool naturally for 1 minute. Finally, high-pressure water is used to remove residual nitrates from the wire rod's surface.

[0035] Comparative Example 1

[0036] This comparative example describes a method for producing high-carbon free-cutting steel with the same chemical composition as in Example 1. The free-cutting steel is hot-rolled into wire rods with a diameter of 13 mm. The wire rod extrusion temperature is controlled at 910±5℃. Before the wire rod enters the Stellmore air-cooling line roller conveyor, the fan is turned off and the insulation cover is placed on it. After the surface temperature of the wire rod drops to 860±5℃, it is placed in a 25℃ circulating water tank for isothermal treatment for 3 minutes. Then, the wire rod is taken out and allowed to cool naturally.

[0037] Comparative Example 2

[0038] This comparative example describes a method for producing high-carbon free-cutting steel, with the same chemical composition as in Example 3. The free-cutting steel is hot-rolled into 8mm diameter wire rods. The wire rod drawing temperature is controlled at 890±5℃. After drawing, the wire rod is quickly immersed in a 350℃ salt bath (KNO3:NaNO3 (mass ratio) = 1:1) for isothermal treatment for 3.5 min. The wire rod is then removed and allowed to cool naturally for 1 min. Finally, residual nitrates on the surface of the wire rod are removed using high-pressure water. The low-magnification morphology and microstructure of the sample after salt quenching in this comparative example are shown below. Figure 5 As shown.

[0039] The cracking of the surface and core of each quenched wire rod was observed visually. The metallographic structure of the edges and core of each quenched wire rod was observed using a Leica metallographic microscope. The Vickers hardness from the surface to the core of the quenched wire rod was measured using a Vickers hardness tester. The specific test results are shown in Table 1 below. Figure 6 As shown.

[0040] Table 1. Cracking of quenched wire rods in Examples 1-4 and Comparative Examples 1-2, and their surface and core microstructures.

[0041]

[0042] Through Table 1 and Figure 6 It can be seen that the graphite free-cutting steel wire rods in Examples 1-4 showed no cracks on the surface or in the core after salt quenching, and the microstructure (combination) was good. Figure 2 All are ferrite + carbon-free bainite, with Vickers hardness values ​​between 320 and 350 HV. Compared to Examples 1-4, Comparative Example 1 (combined with...) Figure 4 The quenched microstructure of Comparative Example 1 is ferrite + acicular martensite, with a significantly higher hardness (680~700HV). However, its wire rod exhibits obvious cracking after quenching, with cracks extending from the surface to the core. In contrast, the quenched microstructure of Comparative Example 2 (combined with...) Figure 5 It is composed of acicular martensite and has a high hardness value (450~480HV). After quenching, its wire rod has obvious small cracks on the surface.

[0043] Example 5

[0044] The high-carbon free-cutting steel wire rod of this embodiment has the same composition and production process as in Embodiment 2, except that the salt quenching temperature is 850±5℃ and the salt bath temperature is 450℃.

[0045] Example 6

[0046] The high-carbon free-cutting steel wire rod of this embodiment has the same composition and production process as in Embodiment 2, except that the salt quenching temperature is 850±5℃ and the salt bath temperature is 390℃.

[0047] Comparative Example 3-12

[0048] The high-carbon free-cutting steel wire rods of Comparative Examples 3-12 have the same composition and production process as Example 2, except that the salt quenching temperature and salt bath temperature are different from those of Example 2.

[0049] Table 2. Cracking of quenched wire rods in Comparative Example 3-15 and their surface and core microstructure.

[0050]

[0051] As shown in Table 2, the results indicate that both salt quenching temperature and salt bath temperature have a significant impact on the microstructure and toughness of the resulting wire rod. When the salt quenching temperature is A... c3 -30℃~A c3 Or A ccm -30~A ccm When the salt bath temperature is 390~450℃, the wire rod microstructure can be made into ferrite + carbon-free bainite, effectively reducing the wire rod hardness, improving the wire rod toughness, preventing the wire rod from cracking, and also effectively ensuring the uniformity of the resulting wire rod microstructure. However, when the salt bath temperature exceeds 450℃, a large amount of nitrate will evaporate, causing environmental pollution; when the salt bath temperature is below 330℃, the nitrate begins to solidify and crystallize, resulting in poor salt quenching effect.

Claims

1. A process for the production of high carbon free cutting steel wire rod characterized in that, The wire rod is composed of the following components in percentage by mass C: 0.60%~1.0%, Si: 1.60%~2.20%, Mn: 0.15%~0.35%, Al: 0.03%~0.50%, Si+Al: ≥2.10%, P: ≤0.035%, S: ≤0.015%, N: ≤0.008%, the balance being iron and inevitable impurities; the metallographic structure of the wire rod is composed of ferrite and carbon-free bainite; the production method comprises the following production procedures: hot rolling, wire drawing, Stelmor air cooling line slow cooling and salt quenching; The salt bath adopts a mixed solution of KNO3 and NaNO3, and the mass ratio of KNO3 to NaNO3 is 1:

1. Wherein, the hot state wire rod is cooled to A c3 -30℃ ~ A c3 ℃ or A ccm -30℃ ~ A ccm ℃, and is subjected to isothermal salt bath treatment, the salt bath temperature is 390℃ ~ 450℃, and the salt bath time is 3 min ~ 5 min.

2. The production method of high carbon free cutting steel wire rod according to claim 1, characterized in that, The laying temperature is controlled at A c3 + 30°C ~ A c3 + 60°C or A ccm + 30°C ~ A ccm + 60°C, while the cooling of the rod after laying is controlled at a cooling rate of ≤ 5°C / s.

3. The production process of high carbon free cutting steel wire rod as claimed in claim 1, wherein, After the wire rod leaves the salt bath line and is naturally cooled for 1 min, high-pressure water is used to remove the residual nitrate on the surface of the wire rod.

4. The production method of high carbon free cutting steel wire rod according to claim 3, characterized in that, The diameter of the wire rod is 5.5~13 mm.

5. The production method of high carbon free cutting steel rod as claimed in any one of claims 1 to 4, characterized in that, ​

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