Full-bainite medium-carbon-chromium cold forging steel manufacturing method capable of simplifying fine line machining

Abstract

The invention provides a manufacturing method of full-bainite medium-carbon-chromium cold forging steel with simplified fine wire machining, which comprises the following steps: firstly, manufacturing a hot-rolled wire rod of a full-bainite structure easy for downstream spheroidizing annealing, and then performing fine wire manufacturing by adopting a'one-ball and one-drawing 'process; the manufacturing method of the hot-rolled wire rod sequentially comprises the following steps of continuous casting billet heating, rough and medium rolling, pre-finish rolling, finish rolling, reducing and sizing, spinning and salt bath cooling line controlled cooling, the salt bath cooling line controlled cooling adopts a full-soaking mode, the wire rod runs along with a roller way, the salt bath temperature is 300-440 DEG C, the salt bath cooling time is 400-800 s, and the wire rod coiling temperature is lower than 300 DEG C. The spheroidizing level of the fine wire reaches the level of 6 under the condition that the medium-carbon-chromium cold forging steel adopts the one-ball and one-drawing process, and the manufacturing cost of the fine wire is reduced.

Description

Technical Field

[0001] The invention relates to the technical field of cold heading steel rolling, in particular to a method for manufacturing full bainite medium carbon chromium cold heading steel with simplified fine wire processing. Background Art

[0002] Medium carbon chromium cold heading steel is one of the most widely used steels in the machinery manufacturing industry. After quenching and tempering, it has good comprehensive mechanical properties, good low-temperature impact toughness and low notch sensitivity, especially in the field of automotive parts. It is widely used in the fastener industry to manufacture 10.9 grade high-strength bolts. Common grades include ML40Cr, 40Cr, 41Cr4, SCr440, SAE5140, etc.

[0003] The production of high-strength fasteners requires drawing and cold heading deformation processes, but due to the medium carbon Cr-containing component system, the wire rod structure is pearlite + ferrite structure. The strength of the pearlite + ferrite structure wire rod is ≥700MPa. After drawing, the strength further increases, and the plasticity decreases, which significantly reduces the life of the mold, increases the brittleness of the material, and easily causes insufficient plasticity and cracking during the cold heading process. Therefore, in the fine wire manufacturing process, spheroidizing annealing treatment is usually performed to reduce hardness and increase deformation capacity. The metallographic structure of medium carbon Cr cold heading steel wire produced by the traditional Stelmor process is a pearlite + ferrite equilibrium structure. The pearlite and ferrite structures are stable, and the cementite is distributed in lamellar layers. The primary spheroidization rate is lower than 60% (below grade 4.0), and the "two balls and two draws" process is required.

[0004] Comparing the effects of different original microstructures and states on the spheroidizing annealing of cold heading steel, in the bainite structure, there are solid-dissolved carbon atoms and carbides distributed in the form of dots or short rods, which are evenly dispersed. In the spheroidizing process, the carbide nucleation points are numerous and dispersed. In the pearlite + ferrite structure, the cementite is in the form of coarse flakes, and after annealing, the carbides are in an incompletely globalized state, with a low spheroidization rate. Under the same annealing process conditions, the carbides in the lower bainite structure are fine and dispersed, with high energy storage, and many nucleation points of divorced eutectoids, which are easier to spheroidize and grow particles. Therefore, the spheroidization rate of a single spheroidizing annealing is high.

[0005] Therefore, developing a manufacturing method for obtaining uniform bainite structure in hot-rolled wire rods to meet the requirements of obtaining a spheroidizing rate ≥ 95% and a grade 6 spheroidized structure for the medium carbon Cr cold heading steel fine wire produced by the one-ball-one-drawing process, meeting the downstream users' 10.9 grade and above flange face bolt large deformation (cold heading ratio > 1 / 4) cold heading processing requirements, and reducing the manufacturing cost of fine wires, is an urgent problem to be solved in the current fastener manufacturing industry.

[0006] In order to meet the large deformation (cold heading ratio > 1 / 4) cold heading requirements of flange bolts of grade 10.9 and above, the medium carbon Cr cold heading steel wire produced by the traditional Stelmor process has a pearlite + ferrite structure. Although the strength and hardness are low, the one-time spheroidization rate is less than 60% (below grade 4.0), and the "two balls and two withdrawals" process is required.

[0007] This patent discloses a method for obtaining a full bainite structure by rapid cooling through a salt bath cooling line. Since the bainite structure contains a large number of granular and rod-shaped carbides and the matrix structure has high energy storage and is easy to spheroidize, direct spheroidization of the wire can obtain a spheroidization rate ≥ 95%, reaching a 6-level spheroidized structure. After fine drawing, it still has the characteristics of low deformation resistance and good cold heading performance. That is, the "one ball and one draw" process can meet the cold heading processing requirements of large deformation (cold heading ratio > 1 / 4) of flange bolts of level 10.9 and above, replacing the "two balls and two draws" process, thereby simplifying the downstream fine wire processing technology and achieving energy saving and carbon reduction in the industrial chain.

[0008] Chinese patent CN117753777A: A method for producing ML40Cr hot-rolled wire rod. This patent mainly uses the high-temperature waste heat of the product after coiling in the Stelmor air-cooling line, and further insulates it through the insulation corridor to obtain fewer pearlite clusters and grains. This patent does not involve the design of the full bainite structure of medium-carbon Cr steel and the technology of rapid cooling through salt bath.

[0009] Chinese patent CN116334469A: ML40Cr spheroidizing-free annealing hot-rolled wire rod and its preparation method. This patent mainly realizes the spheroidization structure transformation of the wire rod on the Stelmor production line through the process control of the controlled rolling and controlled cooling process to forge bolts with small deformation. This patent does not involve the design of the full bainite structure of medium carbon Cr steel and the content of rapid cooling through salt bath.

[0010] Chinese patent CN201910402830.X: A spheroidizing annealing process for ML40Cr hot-rolled wire. This patent mainly studies the control method of the spheroidizing annealing process in the downstream annealing furnace, improves the spheroidizing grade, shortens the annealing process time, and achieves cost reduction. This patent does not involve the design of the full bainite structure of medium carbon Cr steel and the content of rapid cooling through salt bath. Summary of the invention

[0011] The present invention proposes a method for manufacturing a fully bainitic medium-carbon chromium cold heading steel with simplified fine wire processing, adopts salt bath cooling instead of the traditional Steyr cooling method, and replaces the traditional pearlite + ferrite organization design with a fully bainitic organization design, so as to achieve the spheroidization level of 6 for the fine wire of the medium-carbon chromium cold heading steel under the "one ball one draw" process condition, meet the needs of downstream users to simplify the manufacturing process of the medium-carbon Cr-containing cold heading steel fine wire, replace the original "one ball two draws" or "two balls two draws" process, and reduce the manufacturing cost of the fine wire.

[0012] The invention provides a method for manufacturing a full-bainite medium-carbon chromium cold heading steel with simplified fine wire processing. The method comprises the following steps in sequence: the method comprises: first manufacturing a hot-rolled wire rod with a full-bainite structure that is easy for downstream spheroidizing annealing, and then manufacturing the fine wire, wherein the fine wire manufacturing adopts a "one ball and one draw" process; the method for manufacturing the hot-rolled wire rod comprises the following steps in sequence: continuous casting billet heating - roughing and intermediate rolling - pre-finishing rolling - finishing rolling - sizing reduction - wire laying - salt bath cooling line controlled cooling, wherein the salt bath cooling line controlled cooling adopts a full immersion method, the wire rod follows the roller table to run, the salt bath temperature is 300-440°C, the salt bath cooling time is 400-800s, and the wire rod coiling temperature is less than 300°C.

[0013] Preferably, the continuous casting billet is heated by a regenerative heating furnace, and the cross section of the continuous casting billet is 180mm*240mm. 2 The heating furnace is divided into preheating section, heating section one, heating section two and soaking section; the temperature of the preheating section is 600-850℃, the temperature of the heating section one is 900-1050℃, the temperature of the heating section two is 1050-1100℃, and the temperature of the soaking section is 1110-1150℃.

[0014] Preferably, the roughing, intermediate rolling and pre-finishing rolling mill group adopts a horizontal and vertical alternating two-roll rolling mill to roll the heated steel billet into an intermediate rolled piece; the rolling temperature of the roughing, intermediate rolling and pre-finishing rolling mill group is 980-1050°C.

[0015] Preferably, the rolled piece is rolled to finished product specifications by the finishing mill and the sizing mill, the rolling temperature of the finishing mill is controlled at 880-920°C, the rolling temperature of the sizing mill is controlled at 850-900°C, and the spinning temperature is controlled at 880-920°C.

[0016] Preferably, the coil after spinning directly enters the salt bath cooling line, and the temperature entering the salt bath cooling line is about 830-850°C.

[0017] Preferably, it is characterized in that: the method is applicable to hot-rolled wire rods with specifications of Φ5.5 to 22 mm.

[0018] Compared with the prior art, the advantages and positive effects of the present invention are:

[0019] The present invention improves the controlled cooling method of wire rod rolling and innovatively utilizes the salt bath controlled cooling process to replace the traditional Stelmor process, so that the hot-rolled wire rod obtains a full bainite structure instead of the traditional P+F structure. The spheroidizing grade of the fine wire produced by the "one ball and one draw" process reaches 6.0, the qualified rate of 1 / 6 cold heading forging of the fine wire is 100%, the mechanical properties are stable (Rm: 490-530MPa, A≥35%, Z≥75%, HRB≤80), and a small drawing reduction (0.3-0.6mm) is adopted, which meets the needs of downstream users to simplify the manufacturing process of medium carbon Cr cold heading steel fine wire and reduces the manufacturing cost of the fine wire. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 The metallographic structure diagram of the ML40Cr cold heading steel wire rod produced in Example 1 (horizontally -500X);

[0021] Figure 2 The metallographic structure diagram of the ML40Cr cold heading steel wire rod produced in Comparative Example 1 (horizontally -500X);

[0022] Figure 3 The metallographic structure diagram of the ML40Cr fine wire produced in Example 1 (horizontally -500X);

[0023] Figure 4 The metallographic structure diagram of the ML40Cr fine wire produced in Comparative Example 1 (horizontally -500X);

[0024] Figure 5 The metallographic structure diagram of the ML40Cr cold heading steel wire rod produced in Example 2 (horizontally -500X);

[0025] Figure 6 The metallographic structure diagram of the ML40Cr cold heading steel wire rod produced in Comparative Example 2 (horizontally -500X);

[0026] Figure 7 The metallographic structure diagram of the ML40Cr fine wire produced in Example 2 (horizontally -500X);

[0027] Figure 8 This is the metallographic structure diagram of the ML40Cr fine wire produced in Comparative Example 2 (horizontal - 500X). DETAILED DESCRIPTION

[0028] In order to more clearly understand the above-mentioned purpose, features and advantages of the present invention, the present invention is further described below in conjunction with the accompanying drawings and embodiments. It should be noted that the embodiments of the present application and the features in the embodiments can be combined with each other without conflict.

[0029] In the following description, many specific details are set forth to facilitate a full understanding of the present invention. However, the present invention may also be implemented in other ways than those described herein. Therefore, the present invention is not limited to the specific embodiments of the following disclosure.

[0030] The embodiment takes the production of ML40Cr cold heading steel wire rod as an example to illustrate the manufacturing method of the all-bainitic medium carbon chromium cold heading steel with simplified fine wire processing of the present invention. The specific chemical composition of the ML40Cr cold heading steel is as follows: C: 0.38-0.43Wt / %, Si: 0.10-0.30Wt / %, Mn: 0.60-0.90Wt / %, P: ≤0.025Wt / %, S: ≤0.025Wt / %, Cr: 0.90-1.20Wt / %, Ni: ≤0.20Wt / %, B: 0.0030~0.0050Wt / %, Cu: ≤0.20Wt / %, and the remainder is Fe and unavoidable impurities.

[0031] Example 1

[0032] Reference Figure 1 This embodiment provides a method for manufacturing a fully bainitic medium carbon chromium cold heading steel with simplified fine wire processing. The chemical composition of the ML40Cr cold heading steel in this embodiment is as follows:

[0033] The chemical composition is designed in percentage by weight: C: 0.40%, Si: 0.15%, Mn: 0.72%, Cr: 0.98%, P: 0.012%, S: 0.005%, Ni: 0.012%, Cu: 0.022%, B: 0.0035%, and the rest is iron and unavoidable impurities.

[0034] The controlled rolling method of ML40Cr cold heading steel wire rod: the rolling process is continuous casting billet heating - roughing and intermediate rolling - pre-finishing rolling - finishing rolling - sizing reduction - wire laying - salt bath cooling line controlled cooling - surface washing - packaging.

[0035] Continuous casting billet heating: The continuous casting billet is heated by a regenerative heating furnace, and the cross section of the continuous casting billet is 180mm*240mm 2 The heating furnace is divided into preheating section, heating section one, heating section two and soaking section; the temperature of the heating section is 920-1145°C, of ​​which the temperature of the preheating section is 600-650°C, the temperature of the heating section one is 920-985°C, the temperature of the heating section two is 1060-1082°C, and the temperature of the soaking section is 1125-1145°C.

[0036] Roughing and intermediate rolling and pre-finishing rolling: The roughing and intermediate rolling and pre-finishing rolling units adopt horizontal and vertical alternating two-roller rolling mills to roll the heated steel billets into intermediate rolled products; the number of roughing and intermediate rolling stands is 15, the number of pre-finishing rolling stands is 6, and the rolling temperature of the roughing and intermediate rolling and pre-finishing rolling units is between 980 and 1032°C.

[0037] Finishing and sizing: The finishing mill and sizing mill use 4 rolling mills to roll the rolled piece to a finished product specification of 14mm. The finishing mill is empty, and the rolling temperature of the sizing mill is controlled at 878℃. The wire laying temperature is controlled at 880℃.

[0038] Controlled cooling of the salt bath cooling line: the coil after spinning directly enters the salt bath cooling line, the salt bath temperature is 843℃, the salt tank is fully immersed, and it runs along the roller table. The salt liquid temperature is 418℃, the salt bath cooling line runs for 620s, the salt tank outlet temperature is 415℃, and the coil collection temperature is 285℃.

[0039] Finally, the salt on the surface of the wire rods after controlled cooling is rinsed off and packaged.

[0040] The above ML40Cr cold heading steel wire rod is then processed into fine wire by the "one ball one draw" process. The manufacturing process of fine wire is hot rolled wire rod - spheroidizing annealing - pickling, phosphorus saponification - cold drawing. The metallographic structure of fine wire is shown in Figure 3 shown.

[0041] Example 2

[0042] Reference Figure 5 This embodiment provides a method for manufacturing a fully bainitic medium carbon chromium cold heading steel with simplified fine wire processing. The chemical composition of the ML40Cr cold heading steel in this embodiment is as follows:

[0043] The chemical composition is designed in percentage by weight: C: 0.41%, Si: 0.18%, Mn: 0.75%, Cr: 1.01%, P: 0.013%, S: 0.006%, Ni: 0.015%, Cu: 0.018%, B: 0.0038%, and the rest is iron and unavoidable impurities.

[0044] The controlled rolling method of ML40Cr cold heading steel wire rod: the rolling process is continuous casting billet heating - roughing and intermediate rolling - pre-finishing rolling - finishing rolling - sizing reduction - wire laying - salt bath cooling line controlled cooling - surface washing - packaging.

[0045] Continuous casting billet heating: The continuous casting billet is heated by a regenerative heating furnace, and the cross section of the continuous casting billet is 180mm*240mm 2 The heating furnace is divided into preheating section, heating section one, heating section two and soaking section; the temperature of the heating section is 924-1148°C, of ​​which the temperature of the preheating section is 620-658°C, the temperature of the heating section one is 924-965°C, the temperature of the heating section two is 1054-1072°C, and the temperature of the soaking section is 1123-1148°C.

[0046] Roughing and intermediate rolling and pre-finishing rolling: The roughing and intermediate rolling and pre-finishing rolling units adopt horizontal and vertical alternating two-roll rolling mills to roll the heated steel billets into intermediate rolled products; the number of roughing and intermediate rolling stands is 15, the number of pre-finishing rolling stands is 8, and the rolling temperature of the roughing and intermediate rolling and pre-finishing rolling units is between 985 and 1032°C.

[0047] Finishing mill and sizing: The steel passes through the finishing mill and the sizing mill adopts 4 rolling mills to roll the steel to the finished product specification of 16.0mm. The rolling temperature of the finishing mill is 883℃, the rolling temperature of the sizing mill is controlled at 882℃, and the wire laying temperature is controlled at 886℃.

[0048] Salt bath cooling line controlled cooling (liquid salt is used as cooling medium, and salt temperature is adjustable): After the wire rod enters the salt bath cooling line, the temperature of the salt tank is 838°C, and the salt tank is fully immersed. It runs along the roller table, the salt liquid temperature is 398°C, the running time of the salt bath cooling line is 684s, the temperature of the salt tank is 395°C, and the coiling temperature is 272°C.

[0049] Finally, the salt on the surface of the wire rods after controlled cooling is rinsed off and packaged.

[0050] The above ML40Cr cold heading steel wire rod is then processed into fine wire by the "one ball one draw" process. The manufacturing process of fine wire is hot rolled wire rod - spheroidizing annealing - pickling, phosphorus saponification - cold drawing. The metallographic structure of fine wire is shown in Figure 7 shown.

[0051] Comparative Example 1

[0052] Reference Figure 2 The difference between this comparative example and Example 1 is that the salt bath cooling line controlled cooling is replaced by the Stelmor air cooling line + insulation cover slow cooling method, the insulation cover temperature is 765℃, the insulation cover temperature is 652℃, and the cooling rate is 0.27℃ / s. The hot-rolled wire rod manufactured by the comparative example method is processed into a fine wire using the same "one ball one draw" process. The metallographic structure of the fine wire is shown in the figure. Figure 4 shown.

[0053] Comparative Example 2

[0054] Reference Figure 6 The difference between this comparative example and Example 2 is that the salt bath cooling line controlled cooling is replaced by the Stelmor air cooling line + insulation cover slow cooling method, the insulation cover temperature is 765℃, the insulation cover temperature is 652℃, and the cooling rate is 0.27℃ / s. The hot-rolled wire rod manufactured by the comparative example method is processed into a fine wire using the same "one ball one draw" process. The metallographic structure of the fine wire is shown in the figure. Figure 8 shown.

[0055] The metallographic structure, drawing strength, surface shrinkage and hardness of wire rods manufactured by different processes are compared as shown in the following table:

[0056] Table 1 Comparison of hot rolled wire rods

[0057] category Metallographic structure Tensile strength / Mpa Shrinkage / % hardness Example 1 bainite 963 53 29.2HRC Example 2 bainite 949 55 29HRC Comparative Example 1 Pearlite + Ferrite 710 45 96.1HRB Comparative Example 2 Pearlite + Ferrite 708 46 96HRB

[0058] The performance comparison of the wire rods made by different processes and the fine wire made by the "one draw one ball" process is as follows:

[0059] Table 2 Comparison of refined silk performance

[0060]

[0061] The above description is only a preferred embodiment of the present invention and does not limit the present invention in other forms. Any technician familiar with the profession may use the technical content disclosed above to change or modify it into an equivalent embodiment with equivalent changes and apply it to other fields. However, any simple modification, equivalent change and modification made to the above embodiment based on the technical essence of the present invention without departing from the content of the technical solution of the present invention still falls within the protection scope of the technical solution of the present invention.

Claims

1. A method for manufacturing a fully bainitic medium carbon chromium cold heading steel with simplified fine line processing, characterized in that: The method comprises the following steps: firstly manufacturing a hot-rolled wire rod with a full bainite structure that is easy for downstream spheroidizing annealing, and then manufacturing a fine wire rod, wherein the fine wire rod manufacturing adopts a "one ball and one draw" process; the manufacturing method of the hot-rolled wire rod comprises the following steps in sequence: continuous casting billet heating - roughing and intermediate rolling - pre-finishing rolling - finishing rolling - sizing reduction - wire laying - salt bath cooling line controlled cooling, wherein the salt bath cooling line controlled cooling adopts a full immersion method, the wire rod follows the roller table, the salt bath temperature is 300-440°C, the salt bath cooling time is 400-800s, and the wire rod coiling temperature is less than 300°C.

2. The method for manufacturing all-bainite medium carbon chromium cold heading steel with simplified fine line processing according to claim 1, characterized in that: The continuous casting billet is heated by a regenerative heating furnace, and the cross section of the continuous casting billet is 180mm*240mm 2 The heating furnace is divided into preheating section, heating section one, heating section two and soaking section; the temperature of the preheating section is 600-850℃, the temperature of the heating section one is 900-1050℃, the temperature of the heating section two is 1050-1100℃, and the temperature of the soaking section is 1110-1150℃.

3. The method for manufacturing all-bainite medium carbon chromium cold heading steel with simplified line finishing according to claim 1, characterized in that: The roughing, intermediate rolling and pre-finishing rolling mill group adopts a horizontal and vertical alternating two-roller rolling mill to roll the heated steel billet into an intermediate rolled piece; the rolling temperature of the roughing, intermediate rolling and pre-finishing rolling mill group is 980-1050°C.

4. The method for manufacturing all-bainite medium carbon chromium cold heading steel with simplified fine line processing according to claim 1, characterized in that: The finished product is rolled to finished product specifications by the finishing mill and the sizing mill, the rolling temperature of the finishing mill is controlled at 880-920°C, the rolling temperature of the sizing mill is controlled at 850-900°C, and the spinning temperature is controlled at 880-920°C.

5. The method for manufacturing all-bainite medium carbon chromium cold heading steel with simplified line finishing according to claim 4, characterized in that: After spinning, the coils directly enter the salt bath cooling line, where the temperature is about 830-850℃.

6. The method for manufacturing a fully bainitic medium carbon chromium cold heading steel with simplified line finishing according to any one of claims 1 to 5, characterized in that: The method is applicable to hot-rolled wire rods with specifications of Φ5.5-22 mm.

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