A method for reducing the hardness of round steel and slow cooling equipment thereof

Abstract

The present invention discloses a method for reducing the hardness of round steel and its slow cooling equipment. The method is based on the steel CCT curve as a theoretical basis. By adopting the process of adding a heat preservation cover to slow cooling after rolling, the temperature and cooling rate of each stage are accurately controlled during the rolling process. The heat preservation cover of the slow cooling equipment is used to control the slow cooling temperature and speed, so as to avoid the steel being completely exposed to the air. The air mobility around the round steel is reduced when the round steel is cooled. To ensure that the round steel is slowly cooled, the generation of abnormal bainite and martensite structures of the round steel is avoided, and P+F structure is obtained, thereby achieving the purpose of reducing the hardness of the round steel. It is possible to achieve a reduction in hardness without increasing the annealing cost. Through practice, the hardness of CrMo steel is reduced from 310HBW to 210-230HBW, meeting the target of less than 250HBW, and the core-surface hardness difference is less than 20HBW. For 45# steel, the core-surface hardness difference is 7-12HBW, and the organizational uniformity is good.

Description

Technical Field

[0001] The invention relates to a production method and equipment for round steel, and in particular to a method for reducing the hardness of round steel and slow cooling equipment thereof. Background Art

[0002] Round steel is an important industrial raw material for the engineering machinery equipment manufacturing industry, automobile manufacturing industry, etc. According to the different chemical composition and performance of round steel, round steel can be used in different industries to manufacture different equipment parts, mainly used for engineering machinery chain links, gears, stabilizer bars, etc.

[0003] When using round steel as raw material, equipment component manufacturers typically begin by cutting the relatively long steel bars to the required length for component manufacturing. Cutting techniques typically include band sawing, punch shearing, and plasma cutting. Shearing, compared to the other two methods, offers high efficiency and minimal material loss, making it the preferred method in the industry. Shearing fractures the steel at room temperature, requiring the round steel to have low hardness and uniform microstructure. High hardness and uneven microstructure can lead to shear blade wear and uneven cut surfaces.

[0004] Currently, there are two main production processes for controlling hardness: adding rolling and temperature control equipment, which requires the addition of water tanks and other equipment; and post-annealing. Both processes significantly increase production costs, resulting in low market competitiveness. Summary of the Invention

[0005] Purpose of the invention: In order to overcome the defects of the prior art, the present invention provides a method for reducing the hardness of round steel, which can reduce the hardness of round steel at a low cost.

[0006] Another object of the present invention is to provide a slow cooling device for implementing the above method.

[0007] Technical solution: The method for reducing the hardness of round steel described in the present invention comprises the following steps:

[0008] (1) After the billet is heated to 1228±10℃, rolling is started at 1089±5℃; the average cooling rate between heating and rolling is 2.17±0.2℃ / s;

[0009] (2) Rolling at 1066±5℃ in the tandem mill, with an average cooling rate of 0.15±0.2℃ / s between the start of rolling and the entry into the tandem mill;

[0010] (3) The temperature out of the continuous rolling mill is 1009±5℃, and the average cooling rate from the entry to the exit of the continuous rolling mill is 5.7±0.2℃ / s;

[0011] (4) The pre-finishing rolling inlet temperature is 904±5℃, and the average cooling rate from the exit of the continuous rolling mill to the pre-finishing rolling inlet is 0.94±0.2℃ / s;

[0012] (5) KOCKS inlet temperature 886±5℃, average cooling rate from pre-finishing rolling inlet to KOCKS inlet 0.33±0.2℃ / s;

[0013] (6) The upper cooling bed temperature was 829 ± 5 °C, and the average cooling rate from KOCKS to the cooling bed entrance was 1.21 ± 0.2 °C / s;

[0014] (7) The temperature of the cooling bed insulation cover is 655±5℃, and the average cooling rate from the cooling bed entrance to the insulation cover is 0.28±0.1℃ / s;

[0015] (8) The sawing temperature out of the cooling bed is 265±5℃, and the average cooling rate from the insulation cover to the cooling bed outlet is 0.12±0.1℃ / s.

[0016] Corresponding to the above-mentioned method of reducing the hardness of round steel, the slow cooling equipment for implementing the method provided by the present invention includes a cooling bed, a support base arranged on the inlet side of the cooling bed, a support arm arranged on the support base, and a thermal insulation cover installed on the end of the support arm; the thermal insulation cover covers the top of the cooling bed; the length direction of the cooling bed is from the cooling bed inlet to the cooling bed outlet, and the covering length of the thermal insulation cover accounts for 1 / 4-1 / 2 of the length of the cooling bed.

[0017] The end of the support arm is provided with a connector with multiple connection holes evenly distributed along its length. The heat preservation cover is selectively fixed in one of the connection holes by bolts. Selecting different connection holes can adjust the length of the heat preservation cover covering the cooling bed, thereby adjusting the cooling rate of the round steel on the cooling bed, achieving the purpose of auxiliary cooling control.

[0018] Specifically, the distance between the two connection holes that are farthest apart is more than 600 mm, thereby ensuring sufficient adjustment space.

[0019] The support arm is hinged to the support base. Furthermore, the slow cooling device also includes a drive device, one end of which is hinged to the support base and the other end is hinged to the support arm. The drive device is used to raise and lower the support arm, and the thermal insulation cover follows the raising and lowering of the support arm. This allows for adjustment of the distance between the thermal insulation cover and the cooling bed, further providing room for adjustment of the cooling rate.

[0020] In order to further ensure the heat preservation effect, the heat preservation cover is filled with heat insulation cotton.

[0021] Beneficial Effects: Compared with the existing technology, this method is based on the theoretical basis of the steel CCT curve. During production, the process of slow cooling with a heat preservation cover after rolling is adopted. The temperature and cooling rate at each stage of the rolling process are precisely controlled. In the slow cooling process of the cooling bed, the heat preservation cover of the slow cooling equipment is used to control the slow cooling temperature and speed, avoiding the steel from being completely exposed to the air. The air mobility around the round steel is reduced during cooling to ensure the slow cooling of the round steel, avoid the formation of abnormal bainite and martensite structures in the round steel, and obtain the P+F structure, thereby achieving the purpose of reducing the hardness of the round steel and improving the shear performance of the round steel. The hardness reduction can be achieved without increasing the annealing cost. In practice, the hardness of CrMo steel is reduced from 310HBW to 210-230HBW, meeting the target of less than 250HBW, and the core-surface hardness difference is less than 20HBW. For 45# steel, the core-surface hardness difference is 7-12HBW, and the structural uniformity is good. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 It is a partial structural schematic diagram of the slow cooling device of the present invention;

[0023] Figure 2 yes Figure 1 Schematic diagram of the lifting and lowering state of the support arm in the intermediate cooling equipment;

[0024] Figure 3 It is the full-line temperature variation curve of 90mm42CrMo round steel in Example 1;

[0025] Figure 4 The cooling rate of the 90mm42CrMo round steel from the start of rolling to the cooling bed outlet in Example 1;

[0026] Figure 5 This is a schematic diagram of the grain structure of the edge of 70mm size 45# round steel in Example 2;

[0027] Figure 6 This is a schematic diagram of the grain structure of 1 / 4 diameter of 70mm specification 45# round steel in Example 2;

[0028] Figure 7 This is a schematic diagram of the grain structure of the core of 70mm size 45# round steel in Example 2;

[0029] Description of the accompanying drawings: cooling bed 1, support base 2, support arm 3, heat preservation cover 4, connecting head 5, connecting hole 51, driving device 6. DETAILED DESCRIPTION

[0030] The following examples all adopt Figure 1-2 The slow cooling equipment shown in the figure has a cooling bed of a conventional long length. Due to space limitations, the structure thereof is not fully illustrated. The covering length of the heat preservation cover in the embodiment is 3 meters.

[0031] Example 1 Taking CrMo series steel grade 42CrMo as an example, the cooling rate of round steel after rolling is effectively controlled by using a heat preservation cover to cover and slow cooling. The temperature detection data during the rolling process are as follows: Figure 3 As shown in the figure, the cooling rate of round steel during rolling is as follows: Figure 4 After adding the insulation cover, the cooling speed of the round steel is reduced to the target speed.

[0032] Through the above method, the hardness of 42CrMo, ∮90mm round steel is reduced to 210-230HBW (as shown in Table 1), achieving the hardness control target (≤250HBW). This hardness is more suitable for users to cut and blank. The hardness of this steel using air cooling process is above 300HBW.

[0033] Table 1: 42CrMo, ∮90mm hardness test values

[0034]

[0035] It can be seen that the average hardness of Example 1 is only 222 HBW, and the hardness difference is 19 HBW.

[0036] In Example 2, the above method for 70mm, 45# high-quality carbon structural steel not only ensures the internal quality of the steel and achieves a uniform F+P, but also significantly reduces steel bending and saves finishing costs. The average hardness is 190 HBW, and the core-surface hardness difference for the same cross section is ≤12 HB.

[0037] Recombination Figure 5-7 As shown in the figure, the microstructures near the edge, at the quarter diameter, and in the core of the cross section are all normal pearlite + ferrite. The grain size at the edge is 6-7, the grain size at the quarter diameter is 5-6, and the grain size at the core is 4-5. The microstructure is well-uniform, so the hardness difference is controlled to be small.

Claims

1. A method for reducing the hardness of round steel, characterized in that: The steps include: (1) After the billet is heated to 1228±10℃, rolling is started at 1089±5℃; the average cooling rate between heating and rolling is 2.17±0.2℃ / s; (2) Rolling at 1066±5℃ in the tandem mill, with an average cooling rate of 0.15±0.2℃ / s between the start of rolling and the entry into the tandem mill; (3) The temperature out of the continuous rolling mill is 1009±5℃, and the average cooling rate from the entry to the exit of the continuous rolling mill is 5.7±0.2℃ / s; (4) The pre-finishing rolling inlet temperature is 904±5℃, and the average cooling rate from the exit of the continuous rolling mill to the pre-finishing rolling inlet is 0.94±0.2℃ / s; (5) KOCKS inlet temperature 886±5℃, average cooling rate from pre-finishing rolling inlet to KOCKS inlet 0.33±0.2℃ / s; (6) The upper cooling bed temperature was 829 ± 5 °C, and the average cooling rate from KOCKS to the cooling bed entrance was 1.21 ± 0.2 °C / s; (7) The temperature of the cooling bed insulation cover is 655±5℃, and the average cooling rate from the cooling bed entrance to the insulation cover is 0.28±0.1℃ / s; (8) The sawing temperature out of the cooling bed is 265±5℃, and the average cooling rate from the insulation cover to the cooling bed outlet is 0.12±0.1℃ / s; The round steel is 42CrMo steel; the diameter of the round steel is 70-90 mm.

2. A slow cooling device for implementing the method for reducing the hardness of round steel according to claim 1, characterized in that: The invention comprises a cooling bed (1), a support base (2) arranged at the inlet side of the cooling bed (1), a support arm (3) arranged on the support base (2), and a heat-insulating cover (4) installed at the end of the support arm (3); the heat-insulating cover (4) covers the upper part of the cooling bed (1); the length direction of the cooling bed (1) is from the inlet of the cooling bed (1) to the outlet of the cooling bed (1), and the covering length of the heat-insulating cover (4) accounts for 1 / 4 to 1 / 2 of the length of the cooling bed (1).

3. The slow cooling device according to claim 2, characterized in that: The end of the support arm (3) is provided with a connecting head (5), and the connecting head (5) has a plurality of connecting holes (51) evenly distributed in the length direction. The heat-insulating cover (4) is selectively fixed in one of the connecting holes (51) by means of bolts.

4. The slow cooling device according to claim 3, characterized in that: The distance between the two connection holes (51) that are farthest apart is more than 600 mm.

5. The slow cooling device according to claim 2, characterized in that: The support arm (3) is hinged on the support base (2).

6. The slow cooling device according to claim 5, characterized in that: The invention also comprises a driving device (6), one end of which is hinged on the support seat (2) and the other end is hinged on the support arm (3); the driving device (6) is used to lift and lower the support arm (3), and the heat-insulating cover (4) is lifted and lowered following the lifting and lowering of the support arm (3).

7. The slow cooling device according to claim 2, characterized in that: The heat-insulating cover (4) is filled with heat-insulating cotton.

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