Production method for online spheroidizing annealing of high-strength B-containing cold forging steel

Abstract

The invention discloses an on-line spheroidizing annealing production method for high-strength B-containing cold forging steel, the method sequentially comprises the steps of converter smelting, LF + RH refining, continuous casting, casting blank heating, rolling, spinning, on-line isothermal treatment, hot coil collection, heat preservation gallery heating and slow cooling, the casting blank heating temperature is 1000-1050 DEG C, the rolling and finishing mill entering temperature and the reducing and sizing temperature are 800-850 DEG C, and the rolling and finishing mill entering temperature is 800-850 DEG C; the spinning temperature ranges from 800 DEG C to 850 DEG C, the temperature of online isothermal treatment ranges from 430 DEG C to 450 DEG C, and the heating temperature of the heat preservation gallery enables the wire rod to be heated to 680 DEG C to 700 DEG C; the spheroidizing grade of the wire rod reaches grade 6, light drawing and cold heading machining can be directly carried out, direct fastener forming is achieved, one-time large drawing and off-line annealing are omitted, the overall machining process is shortened by more than 40%, annealing energy consumption is reduced by more than 60%, and green carbon reduction is achieved.

Description

Technical Field

[0001] The present invention relates to the technical field of steel smelting, and in particular to a production method for online spheroidizing annealing of high-strength B-containing cold heading steel. Background Art

[0002] As the global economy tightens and the automotive industry intensifies its internal competition, competition in the fastener industry has become increasingly fierce. European companies represented by Anguote and Kamax have begun to use 30MnB4 / 10B30 (replacing ML40Cr) or 32CrB4 (replacing SCM435) in large quantities for automotive fasteners. By reducing alloy costs and saving annealing steps, they have implemented SAIP (one ball two pulls) instead of PASAIP (two balls two pulls) to reduce costs throughout the fastener process and improve the market competitiveness of their products.

[0003] At present, the annealing of B-containing high-strength cold heading steel is mainly carried out by offline annealing after a single drawing (reduction of more than 30%) to ensure that the spheroidization grade is greater than level 5, so as to achieve subsequent large deformation cold heading. This process has a high overall production cost and a long material flow cycle, and still does not have high market competitiveness. For example, CN115852106A discloses a spheroidizing annealing method for medium and low carbon alloy cold heading steel. The medium and low carbon alloy cold heading steel is subjected to pickling, phosphating and saponification treatments before being drawn and deformed, and the cold-drawn wire is subjected to spheroidizing annealing. For example, CN115161545A discloses a high-plasticity, low-strength medium carbon cold heading steel fine wire and its production method. During production, the rough drawing reduction rate is controlled at 30-40%, and an offline annealing method is adopted to achieve an effect of a spheroidization grade greater than level 5. At the same time, there are also CN116121512A, a cold heading steel wire rod for high-strength fasteners and its production method, which adopts a cooling process of first fast cooling and then slow cooling, so that the wire rod structure is mainly bainite + martensite, and there are high-density dislocations and twin substructures in the steel. In the subsequent offline annealing process, carbides are rapidly nucleated at dislocations and substructures, achieving the purpose of rapid spheroidization of the wire rod and shortening the spheroidizing annealing time. However, this process will cause brittle fracture during coiling, PF line transportation, packaging, and transportation after Stelmor cooling after rolling, affecting subsequent use, and the wire rod can only be advanced offline spheroidizing annealing treatment, with many restrictions on production and use, and a long overall cycle.

[0004] In summary, the traditional offline annealing process has a long production cycle and is difficult to achieve the required spheroidization grade of 5 or higher in a single annealing step. This leads to numerous production restrictions, hindering further improvement in the product's market competitiveness. Furthermore, the wire rod undergoes secondary heating after cooling to reach the spheroidizing annealing temperature, resulting in secondary energy waste and hindering the full-process carbon reduction requirements. Summary of the Invention

[0005] The purpose of the present invention is to provide a production method for online spheroidizing annealing of high-strength B-containing cold heading steel to solve the problems of long production cycle and difficulty in achieving a spheroidizing grade greater than level 5 in one annealing process using an offline annealing method.

[0006] To achieve the above object, the present invention provides the following technical solutions:

[0007] A production method for online spheroidizing annealing of high-strength B-containing cold heading steel, the method sequentially comprising the steps of converter smelting, LF+RH refining, continuous casting, billet heating, rolling, wire laying, online isothermal treatment, hot coiling, heating in a heat preservation gallery, and slow cooling. The billet heating temperature is 1000-1050°C, the rolling temperature into the finishing mill and the sizing reduction temperature are 800-850°C, the wire laying temperature is 800-850°C, the online isothermal treatment temperature is 430-450°C, and the heat preservation gallery heating temperature heats the wire rod to 680-700°C.

[0008] Preferably, after the spinning is completed, the wire rod is controlled to enter the isothermal salt bath within 20 seconds for online isothermal treatment.

[0009] Preferably, the temperature of the heat-coiling is above 400°C.

[0010] Preferably, the heat preservation corridor heats the wire rod to 680-700° C. and then keeps the temperature for 30 minutes.

[0011] Preferably, the wire rod is kept warm for 30 minutes and then passed through a slow cooling zone from 680-700°C, and then cooled to below 500°C after 3.5-4 hours.

[0012] The wire rod produced by the online spheroidizing annealing production method of the high-strength B-containing cold heading steel has the following chemical compositions, measured by mass percentage: C 0.32-0.38%, Si 0.15-0.30%, Mn 0.80-0.95%, Cr 0.10-0.35%, P≤0.015%, S≤0.015%, Al 0.015-0.045%, Ti 0.02-0.05%, B 0.0010-0.0030%, and the remainder being Fe and unavoidable impurities.

[0013] Preferably, the specification of the wire rod is 5.5mm-26mm.

[0014] Preferably, the lower bainite content of the wire rod is ≥90%, the remainder is a small amount of ferrite, and the spheroidization grade is level 6.

[0015] Compared with the prior art, the present invention has the following beneficial effects:

[0016] (1) Through low-temperature heating at 1000-1050℃, low-temperature rolling at 800-840℃ and wire drawing, the refinement of austenite grains before phase transformation and the precipitation of proeutectoid ferrite can be achieved, the nucleation center is increased, and the transformation rate of austenite to bainite is improved;

[0017] (2) After spinning, the wire rod quickly enters the isothermal salt bath with a molten salt temperature of 430-450℃, so that the austenite obtains a large degree of undercooling, avoids the pearlite transformation zone, and isothermally transforms into the lower bainite structure in the lower bainite zone of the steel grade. This can achieve the existence of cementite in the wire rod in the form of extremely small particles and the overall distribution is uniform, which is conducive to the subsequent spheroidization process. At the same time, the good plasticity of the lower bainite structure can also ensure that the wire rod does not suffer brittle fracture during the coiling and transportation process.

[0018] (3) When the wire rod is discharged from the salt tank, a uniform layer of molten salt is attached to the surface, which can form a protective film to prevent the wire rod from coming into contact with oxygen in the air in the heating zone of the insulation corridor and causing surface decarburization;

[0019] (4) The wire rod enters the heating zone of the insulation corridor for heating, which can be raised from 400°C to 680-700°C. Compared with the traditional online annealing process, it can achieve relatively low heating energy consumption (the traditional process is heated from room temperature to A1+60°C). Compared with the traditional process, the power consumption per ton of steel is reduced by more than 65%. Moreover, because the molten salt attached to the surface of the wire rod does not come into contact with the oxygen in the air, it is not necessary to carry out nitrogen filling, cracking gas protection, etc., and decarburization will not occur, further reducing the annealing cost by 8 yuan / ton;

[0020] (5) After the wire rod is heated in the heat preservation corridor and slowly cooled, the spheroidization grade can reach level 6, which ensures that no annealing treatment is required in the subsequent fastener manufacturing process, and direct 1 / 8 cold forging will not crack, reducing the overall production process by more than 40%; at the same time, due to the early low-temperature heating, low-temperature rolling and wire drawing, as well as the rapid cooling and isothermal transformation after rolling, the pearlite nodules of the wire rod after spheroidizing annealing are relatively uniformly distributed, and the ferrite grain size is smaller. Therefore, the wire rod has better plasticity than the traditional process, ensuring that the one-time drawing still has a high cold heading ratio;

[0021] (6) The spheroidization grade of the wire rod reaches level 6. Customers can then directly perform light drawing + cold heading processing to achieve direct fastener forming. Compared with the traditional one-ball two-drawing process, it saves one large drawing (surface reduction rate greater than 30%) and offline annealing, and the overall processing flow is shortened by more than 40%, and the annealing energy consumption is reduced by more than 60%, achieving green carbon reduction and improving the competitiveness of products in the entire industry chain. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 This is the bainite microstructure diagram of the wire rod after isothermal heat treatment in Example 1;

[0023] Figure 2This is the metallographic structure of the wire rod after online spheroidizing annealing in Example 1;

[0024] Figure 3 This is the bainite microstructure diagram of the wire rod after isothermal heat treatment in Example 2;

[0025] Figure 4 This is the metallographic structure of the wire rod after online spheroidizing annealing in Example 2;

[0026] Figure 5 This is the bainite microstructure diagram of the wire rod after isothermal heat treatment in Example 3;

[0027] Figure 6 This is the metallographic structure of the wire rod after online spheroidizing annealing in Example 3;

[0028] Figure 7 This is the metallographic structure picture of the wire rod of Comparative Example 1;

[0029] Figure 8 This is the metallographic structure picture of the wire rod of comparative example 2. DETAILED DESCRIPTION

[0030] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the specific implementation methods of the present invention are described in detail below through examples.

[0031] 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 different from those described herein. Those skilled in the art may make similar generalizations without violating the connotation of the present invention. Therefore, the present invention is not limited to the specific embodiments disclosed below.

[0032] Secondly, the term "one embodiment" or "embodiment" herein refers to a specific feature, structure, or characteristic that may be included in at least one implementation of the present invention. The phrase "in one embodiment" appearing in various places throughout this specification does not necessarily refer to the same embodiment, nor does it refer to a separate or selective embodiment that is mutually exclusive of other embodiments.

[0033] Example 1:

[0034] See also Figure 1-2 This embodiment provides a production method for online spheroidizing annealing of high-strength B-containing cold heading steel. The specific implementation steps include converter smelting, LF+RH refining, continuous casting, billet peeling, billet heating, rolling, wire laying, online isothermal treatment, hot coiling, insulation gallery heating and slow cooling.

[0035] The specific operation steps are as follows: (1) 100-ton converter smelting, 100-ton LF furnace refining, RH vacuum degassing, 6-machine 6-stream continuous casting machine casting, obtain 180mm×240mm ingot, carry out ingot full skinning treatment, and then heat the ingot. The soaking section temperature is 1000-1020℃, the total heating time is 140-160min, the rolling start temperature is 920-940℃, the temperature of the finishing mill and the sizing reduction is controlled at 820-840℃, the wire drawing temperature is 820-840℃, and the wire rod diameter is 1000-1020℃.

[0036] (3) After the wire rod is spun, the roller main speed is set at 0.5 m / s, so that the wire rod quickly enters the isothermal salt bath at 445±1℃ within 20 seconds, and a slow isothermal transformation is performed at a roller main speed of 0.05 m / s to ensure that the wire rod stays in the salt bath for more than 6.5 minutes.

[0037] (4) After the wire rod completes the isothermal transformation of the lower bainite in the salt bath, it is quickly hot-rolled in the range of 410-425℃, while ensuring that the surface is evenly adhered to the molten salt, and then enters the insulation corridor through the vertical coiling core rack.

[0038] (5) The wire rod is heated in the heating zone of the insulation corridor, and the temperature is rapidly raised to 690-700℃ in about 28 minutes, and kept warm for 30 minutes. It then enters the slow cooling zone and slowly runs through the insulation corridor from 690-700℃, and cools down to 465-485℃ in 3.5 hours. It leaves the insulation corridor and is transported to the PF line, sampled and tested, packaged, and offline.

[0039] (6) The chemical composition of the obtained wire rod is shown in Table 1, and the rest is Fe and inevitable impurities.

[0040] (7) The metallographic structure of the obtained product is as follows Figure 1 、 2 As shown, it can be seen that the metallographic structure of the wire rod after leaving the salt tank is 95% lower bainite + 5% ferrite, and the spheroidization grade after online annealing is level 6.

[0041] Example 2:

[0042] See also Figure 3-4 This embodiment provides a production method for online spheroidizing annealing of high-strength B-containing cold heading steel. The specific implementation steps include converter smelting, LF+RH refining, continuous casting, billet peeling, billet heating, rolling, wire laying, online isothermal treatment, hot coiling, insulation gallery heating and slow cooling.

[0043] The specific operation steps are as follows: (1) 100-ton converter smelting, 100-ton LF furnace refining, RH vacuum degassing, 6-machine 6-stream continuous casting machine casting, obtain 170mm×240mm ingot, carry out ingot full skinning treatment, and then heat the ingot. The soaking section temperature is 1020-1040℃, the total heating time is 140-160min, the rolling start temperature is 940-960℃, the temperature of the finishing mill and the sizing reduction is controlled at 830-850℃, the wire drawing temperature is 830-850℃, and the wire rod diameter is 0.

[0044] (3) After the wire rod is spun, the roller main speed is set at 0.4 m / s, so that the wire rod quickly enters the isothermal salt bath at 440±1℃ within 20 seconds, and a slow isothermal transformation is performed at a roller main speed of 0.05 m / s to ensure that the wire rod stays in the salt bath for more than 7 minutes.

[0045] (4) After the wire rod completes the isothermal transformation of the lower bainite in the salt bath, it is quickly hot-rolled in the range of 415-430℃, while ensuring that the surface is evenly adhered to the molten salt, and then enters the insulation corridor through the vertical coiling core rack.

[0046] (5) The wire rod is heated in the heating zone of the insulation corridor, and the temperature is rapidly raised to 680-690℃ in about 26 minutes, and kept warm for 30 minutes. Then it enters the slow cooling zone, and slowly runs through the insulation corridor from 680-690℃, and cools down to 475-495℃ in 3.8 hours. It leaves the insulation corridor and is transported online, sampled and tested, packaged, and taken off the line.

[0047] (6) The chemical composition of the obtained wire rod is shown in Table 1, and the rest is Fe and inevitable impurities.

[0048] (7) The metallographic structure of the obtained product is as follows Figure 3 、 4 As shown, it can be seen that the metallographic structure of the wire rod after leaving the salt tank is 92% lower bainite + 8% ferrite, and the spheroidization grade after online annealing is level 6.

[0049] Example 3:

[0050] See also Figure 5-6 This embodiment provides a production method for online spheroidizing annealing of high-strength B-containing cold heading steel. The specific implementation steps include converter smelting, LF+RH refining, continuous casting, billet peeling, billet heating, rolling, wire laying, online isothermal treatment, hot coiling, insulation gallery heating and slow cooling.

[0051] The specific operation steps are as follows: (1) 100-ton converter smelting, 100-ton LF furnace refining, RH vacuum degassing, 6-machine 6-stream continuous casting machine casting, obtain 180mm×240mm ingot, carry out ingot full skinning treatment, and then heat the ingot. The soaking section temperature is 1030-1050℃, the total heating time is 140-160min, the rolling start temperature is 910-920℃, the temperature of the finishing mill and the sizing reduction is controlled at 800-830℃, the wire drawing temperature is 800-830℃, and the wire rod diameter is 200-300℃.

[0052] (3) After the wire rod is spun, the roller main speed is set at 0.4 m / s, so that the wire rod quickly enters the isothermal salt bath at 435±1℃ within 20 seconds, and a slow isothermal transformation is performed at a roller main speed of 0.04 m / s to ensure that the wire rod stays in the salt bath for more than 7 minutes.

[0053] (4) After the wire rod completes the isothermal transformation of lower bainite in the salt bath, it is quickly hot-rolled in the range of 420-435℃, while ensuring that the surface is evenly adhered to the molten salt, and then enters the insulation corridor through the vertical coiling core rack.

[0054] (5) The wire rod is heated in the heating zone of the insulation corridor, and the temperature is rapidly raised to 680-690℃ in about 26 minutes, and kept warm for 30 minutes. It then enters the slow cooling zone, slowly runs from 680-690℃ through the insulation corridor, and cools down to 485-495℃ in 4 hours. It leaves the insulation corridor and is transported online, sampled and tested, packaged, and taken offline.

[0055] (6) The chemical composition of the obtained wire rod is shown in Table 1, and the rest is Fe and inevitable impurities.

[0056] (7) The metallographic structure of the obtained product is as follows Figure 5 、 6 As shown, it can be seen that the metallographic structure of the wire rod after leaving the salt tank is 90% lower bainite + 10% ferrite, and the spheroidization grade after online annealing is level 6.

[0057] Comparative Example 1

[0058] See also Figure 7 This embodiment provides a production method for online spheroidizing annealing of high-strength B-containing cold heading steel. The specific implementation steps include converter smelting, LF+RH refining, continuous casting, billet peeling, billet heating, rolling, wire laying, online isothermal treatment, hot coiling, insulation gallery heating and slow cooling.

[0059] The specific operation steps are as follows: (1) 100-ton converter smelting, 100-ton LF furnace refining, RH vacuum degassing, 6-machine 6-stream continuous casting machine casting, obtain 180mm×240mm ingot, carry out ingot full skinning treatment, and then heat the ingot. The soaking section temperature is 1080-1100℃, the total heating time is 150-170min, the rolling temperature is 1000-1020℃, the temperature of the finishing mill and the sizing reduction is controlled at 900-920℃, the wire drawing temperature is 880-900℃, and the wire rod diameter is 1000-1020℃.

[0060] (3) After the wire rod is spun, the main speed of the roller is set at 0.4 m / s, so that the wire rod quickly enters the isothermal salt bath at 550±1℃ within 20 seconds, and a slow isothermal transformation is performed at the main speed of the roller at 0.05 m / s to ensure that the wire rod stays in the salt bath for more than 7 minutes.

[0061] (4) After the wire rod completes the isothermal transformation of the upper bainite in the salt bath, it is quickly hot-rolled in the range of 510-525℃, while ensuring that the surface is evenly adhered to the molten salt, and then enters the insulation corridor through the vertical coiling core rack.

[0062] (5) The wire rod is heated in the heating zone of the insulation corridor, and the temperature is rapidly raised to 680-690℃ in about 22 minutes, and kept warm for 30 minutes. It then enters the slow cooling zone and slowly runs through the insulation corridor from 680-690℃, and cools down to 475-495℃ in 3 hours. It leaves the insulation corridor and is transported online, sampled and tested, packaged, and taken offline.

[0063] (6) The chemical composition of the obtained wire rod is shown in Table 1, and the rest is Fe and inevitable impurities.

[0064] (7) The metallographic structure of the obtained product is as follows Figure 7 As shown, it can be seen that the metallographic structure of the wire rod after leaving the salt tank is upper bainite, and the spheroidization level after online annealing is level 5.

[0065] Comparative Example 2

[0066] See also Figure 7 This embodiment provides a production method for online spheroidizing annealing of high-strength B-containing cold heading steel. The specific implementation steps include converter smelting, LF+RH refining, continuous casting, billet peeling, billet heating, rolling, wire laying, online isothermal treatment, hot coiling, insulation gallery heating and slow cooling.

[0067] The specific operation steps are as follows: (1) 100-ton converter smelting, 100-ton LF furnace refining, RH vacuum degassing, 6-machine 6-stream continuous casting machine casting, obtain 180mm×240mm ingot, carry out ingot full skinning treatment, and then heat the ingot. The soaking section temperature is 1080-1100℃, the total heating time is 150-170min, the rolling temperature is 1000-1020℃, the temperature of the finishing mill and the sizing reduction is controlled at 900-920℃, the wire drawing temperature is 880-900℃, and the wire rod diameter is 1000-1020℃.

[0068] (3) After the wire rod is spun, the roller main speed is set at 0.5 m / s, so that the wire rod quickly enters the isothermal salt bath at 560±1℃ within 20 seconds, and a slow isothermal transformation is performed at a roller main speed of 0.05 m / s to ensure that the wire rod stays in the salt bath for more than 6.7 minutes.

[0069] (4) After the wire rod completes the isothermal transformation of the upper bainite in the salt bath, it is quickly hot-rolled in the range of 510-530℃, while ensuring that the surface is evenly adhered to the molten salt, and then enters the insulation corridor through the vertical coiling core rack.

[0070] (5) The wire rod is heated in the heating zone of the insulation corridor, and the temperature is rapidly raised to 680-690℃ in about 21 minutes, and kept warm for 30 minutes. It then enters the slow cooling zone and slowly runs through the insulation corridor from 680-690℃. It is cooled to 475-495℃ in 3.8 hours, and then leaves the insulation corridor for line transportation, sampling and testing, packaging, and offline.

[0071] (6) The chemical composition of the obtained wire rod is shown in Table 1, and the rest is Fe and inevitable impurities.

[0072] (7) The metallographic structure of the obtained product is as follows Figure 8 As shown, it can be seen that the metallographic structure of the wire rod after leaving the salt tank is upper bainite, and the spheroidization level after online annealing is level 5.

[0073] Table 1 Chemical composition and mass percentage (%)

[0074] Serial number C Si Mn P S Cr Ti B Al Example 1 0.34 0.23 0.90 0.010 0.004 0.20 0.04 0.0023 0.024 Example 2 0.33 0.20 0.90 0.008 0.005 0.16 0.03 0.0022 0.025 Example 3 0.35 0.25 0.85 0.009 0.005 0.22 0.05 0.0024 0.030 Comparative Example 1 0.34 0.22 0.88 0.009 0.006 0.17 0.03 0.0024 0.024 Comparative Example 1 0.33 0.24 0.87 0.009 0.005 0.18 0.04 0.0025 0.023

[0075] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solutions of the present invention may be modified or replaced by equivalents without departing from the spirit and scope of the technical solutions of the present invention, which should all be included in the scope of the claims of the present invention.

Claims

1. A production method for online spheroidizing annealing of high-strength B-containing cold heading steel, characterized in that: The method sequentially comprises the steps of converter smelting, LF+RH refining, continuous casting, billet heating, rolling, wire laying, online isothermal treatment, hot coiling, insulation gallery heating and slow cooling, wherein the billet heating temperature is 1000-1050°C, the rolling temperature into the finishing mill and the sizing reduction temperature are 800-850°C, the wire laying temperature is 800-850°C, the online isothermal treatment temperature is 430-450°C, and the insulation gallery heating temperature heats the wire rod to 680-700°C.

2. The production method of online spheroidizing annealing of high-strength B-containing cold heading steel according to claim 1, characterized in that: After the spinning is completed, the wire rod is controlled to enter the isothermal salt bath within 20 seconds for online isothermal treatment.

3. The production method of online spheroidizing annealing of high-strength B-containing cold heading steel according to claim 1, characterized in that: The temperature of the heat-coiled product is above 400°C.

4. The production method of online spheroidizing annealing of high-strength B-containing cold heading steel according to claim 1, characterized in that: The heat preservation corridor heats the wire rod to 680-700° C. and then keeps the temperature for 30 minutes.

5. The production method of online spheroidizing annealing of high-strength B-containing cold heading steel according to claim 4, characterized in that: The wire rod is kept warm for 30 minutes and then passes through a slow cooling zone from 680-700°C, and is cooled to below 500°C after 3.5-4 hours.

6. A wire rod produced by the method according to any one of claims 1 to 5, characterized in that: The chemical composition of the wire rod, in percentage by mass, includes: C 0.32-0.38%, Si 0.15-0.30%, Mn 0.80-0.95%, Cr0.10-0.35%, P≤0.015%, S≤0.015%, Al 0.015-0.045%, Ti 0.02-0.05%, B 0.0010-0.0030%, and the rest is Fe and unavoidable impurities.

7. The wire rod according to claim 6, characterized in that The specification of the wire rod is 5.5mm-26mm.

8. The wire rod according to claim 6, characterized in that The lower bainite content of the wire rod is ≥90%, the remainder is a small amount of ferrite, and the spheroidization grade is level 6.

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