Method for increasing surface hardness of steel billet by using continuous casting withdrawal and straightening machine

Abstract

The invention relates to the technical field of production for improving the surface hardness of steel billets, in particular to a method for improving the surface hardness of steel billets through a continuous casting withdrawal and straightening machine. A CCM process flow is adopted, molten steel passes through a crystallizer to obtain a solidified blank shell, and the solidified blank shell is cut into a finished product by a flame cutting machine after passing through a foot roller, secondary cooling and a withdrawal and straightening machine. The method comprises the following steps: aiming at steel types of which the surface hardness of steel billets needs to be increased, controlling the temperature of the steel billets entering a first withdrawal and straightening machine to be 30-50 DEG C below Ar3 and the temperature of the steel billets exiting a last withdrawal and straightening machine to be 0-50 DEG C below Ar1 by adjusting the pulling speed and the secondary cooling strength. By controlling the pulling speed and the specific water flow, the phase transition temperature of the steel billet is adjusted to be in the pulling and straightening process of the pulling and straightening machine, and the surface hardness of the steel billet is improved through the cooling capacity, higher than that of air cooling, of a straightening roller of the pulling and straightening machine. Under the condition that new equipment is not added, the hardness of the steel billet can be stably improved to be larger than or equal to 10 HRB.

Description

Technical Field

[0001] The invention relates to the technical field of production for increasing the surface hardness of a steel billet, and in particular to a method for increasing the surface hardness of a steel billet by utilizing a continuous casting and straightening machine. Background Art

[0002] Due to their own characteristics, some steel products are more likely to have surface cracks on the billet after fire cutting. There are two reasons for the occurrence of such surface cracks. One is that the strength of the billet itself is insufficient, and its typical indicator is the low surface hardness of the billet; the other is that the corners of the billet shrink faster than the face, resulting in tensile stress at the center of the billet surface. Under the combined effect, the billet will have surface cracks. In order to improve the surface defects of such billets, it is a practical solution to optimize the billet surface hardness.

[0003] There are many ways to improve the surface hardness of steel, such as carburizing and nitriding the surface of steel to make carbon and nitrogen atoms penetrate into the surface of the billet to form a carbide layer or a nitride layer; or cold extrusion of steel to make it plastically deformed, thereby increasing the hardness of the billet. These are very effective methods for the final steel product parts, but for continuous casting billets, cold processing during the continuous casting process is an impossible solution; in terms of increasing the carbon and nitrogen content, its effect is also very limited. On the other hand, controlling the billet to obtain a large cooling rate during the cooling phase change can also improve the surface hardness of the billet. Therefore, some companies will add three cooling (CN115194114A), that is, spray water or water gas cooling again after the billet is completed after the fire cutting. However, adding three coolings not only requires a large investment, but also has a great impact on the site occupation and environment. Therefore, most domestic companies do not have three coolings.

[0004] The cooling of the billet during the continuous casting process is usually divided into: primary cooling (i.e., mold cooling), secondary cooling (i.e., full water or water vapor cooling after exiting the crystallizer) and air cooling. Although the cooling capacity of primary cooling and secondary cooling is large (cooling rate > 5°C / s), the surface temperature of the billet is in the complete austenite temperature range during primary and secondary cooling. Since no phase change occurs, the strong cooling rate in this area will not have a significant effect on the surface hardness of the billet. In the subsequent air cooling process, since the air cooling rate is very small (cooling rate < 0.1°C / s), no more attention is paid to the cooling of the billet after exiting the secondary cooling chamber.

[0005] At present, if the temperature of the billet entering and leaving the straightening machine is not deliberately controlled to the phase transition temperature, taking Q345B as an example, the surface hardness of the billet is ≤85HRB. The billet surface has many defects and cannot meet the requirements of large-scale on-site production. Summary of the invention

[0006] In view of the shortcomings of the existing methods, the purpose of the present invention is to provide a method for increasing the surface hardness of a steel billet by using a continuous casting straightening machine, so as to solve the problem of increasing the surface hardness of the steel billet from ≤85HRB to ≥95HRB.

[0007] The technical solution adopted by the present invention is: A method for increasing the surface hardness of a steel billet using a continuous casting straightening machine includes the following requirements: (1) The continuous casting machine is an arc continuous casting machine, including 5 to 8 straightening machines, each of which is spaced 1.5 m to 2.5 m apart; the straightening machines are provided with water-cooled straightening rollers.

[0008] Furthermore, the straightening machine used in the present invention is a Danieli FTSC product, flat roller straightening, and there are 7 straightening machines in total. The upper and lower clamping rolls of each straightening machine are both water-cooled, and the hot billet pressure is 3MPa.

[0009] (2) Comprehensively adjust the billet drawing speed and specific water content so that the billet temperature entering the first straightening machine is 30℃~50℃ below Ar3, and the billet temperature leaving the last straightening machine is 0~50℃ below Ar1.

[0010] Furthermore, the adjustment amount of the casting speed and the specific water content shall not exceed ±20% of the target casting speed and the target specific water content.

[0011] Further, the pulling speed is between target pulling speed×80% and target pulling speed×120%; preferably, the pulling speed is between target pulling speed×85% and target pulling speed×110%.

[0012] Furthermore, the water specific volume is between target water specific volume × 80% and target water specific volume × 120%. Preferably, the water specific volume is between target water specific volume × 87.5% and target water specific volume × 115%. More preferably, the water specific volume is between target water specific volume × 112% and target water specific volume × 113%.

[0013] Furthermore, the pulling speed is 85% of the target pulling speed, and the water content is 112-113% of the target water content.

[0014] Further, the Ar3 temperature here refers to the temperature when austenite begins to precipitate ferrite; the Ar1 temperature refers to the temperature when austenite is completely transformed into ferrite and pearlite. Ar3 and Ar1 temperatures are the start and end temperatures of the phase transformation obtained by cooling at 0.05℃ / s ~0.15℃ / s.

[0015] (3) Adjust the cooling water flow of the straightening roller of the straightening machine to ensure that the inlet and outlet water temperature difference is 5℃~10℃.

[0016] Furthermore, the water inlet temperature of the straightening rollers of the tension leveler is room temperature.

[0017] Among them, the number of straightening machines is 5 to 8, and the interval between each machine is 1.5m to 2.5m. The number and distance of straightening machines determine the cooling capacity and cooling temperature range of the straightening rollers in the entire straightening area. When the number of straightening machines is less than 5 and the interval is greater than 2.5m, the cooling of the straightening rollers cannot be effectively utilized, thereby reducing the effect of the high cooling rate of the straightening rollers; when the number of straightening machines is greater than 8 and the interval is less than 1.5m, the overall investment cost is greatly increased, and the difficulty of maintenance is increased when the interval is too small, which is not conducive to equipment maintenance.

[0018] Among them, the reason for adjusting the billet drawing speed and specific water content so that the temperature of the billet entering the first straightening machine is 30℃~50℃ below Ar3 and the temperature of the billet leaving the last straightening machine is 0~50℃ below Ar1 is that the billet gradually cools down and solidifies from top to bottom. The key to determining the surface hardness of the billet lies in the cooling rate corresponding to the phase change. The greater the phase change cooling rate, the higher the hardness after the phase change, and vice versa. The billet is in a temperature range without phase change in the second cooling section and the previous area. Therefore, the cooling at this time mainly accelerates the solidification of the billet and cannot increase the hardness of the billet. When the billet leaves the second cooling section, the temperature of the billet gradually decreases to the temperature at which the phase change begins. At this time, if the billet is placed under a larger cooling rate during the phase change, the billet will obtain a higher hardness. The largest cooling equipment after the billet exits the second cooling section is the water-cooled straightening roller of the straightening machine. Considering the stability of the phase change, the temperature when entering the first straightening machine is set to 30℃~50℃ below the starting phase change temperature Ar3. When the temperature exceeds (Ar3-30)℃ or is lower than (Ar3-50)℃, each straightening machine cannot be effectively utilized to ensure that the phase change is completed within the phase change temperature range. When the temperature of the last straightening machine is 0~50℃ below Ar1, it can ensure that the entire phase change is completed in the straightening machine area from the beginning to the end, ensuring a high cooling rate during the phase change process, thereby obtaining high hardness; the temperature when exiting the last straightening machine is set to 0℃~50℃ below the ending phase change temperature Ar1. When the temperature exceeds Ar1, it indicates that the phase change has not yet ended, and the maximum hardness cannot be obtained at this time; when the temperature is lower than (Ar1-50)℃, it is difficult to match the number of straightening machines, the pulling speed, and the specific water volume, which is inconvenient to operate and will not continue to increase the hardness.

[0019] As one of the preferred technical solutions for the above-mentioned method of increasing the surface hardness of the steel billet by using a continuous casting straightening machine, the adjustment of the steel billet drawing speed and specific water content does not exceed 20% of the target drawing speed and target specific water content. The reason is that each type of steel has a theoretically optimal drawing speed and specific water content to obtain the best crystal structure, surface quality and internal quality. The corresponding theoretical optimal drawing speed and specific water content (target value) will be given in industrial production. When the drawing speed and specific water content vary greatly, the best structure, surface or internal quality cannot be obtained. Therefore, it is required that the adjustment of the drawing speed and specific water content does not exceed 20% of the target drawing speed and target specific water content.

[0020] As one of the preferred technical solutions of the method of increasing the surface hardness of the steel billet by using the continuous casting straightening machine, the Ar3 and Ar1 temperatures are the phase change start and end temperatures obtained by cooling under the conditions of 0.05℃ / s~0.15℃ / s. The actual cooling intensity of the continuous casting steel billet after the second cooling section is 0.05℃ / s~0.15℃ / s. Since the corresponding start and end phase change temperatures are different under different cooling intensities, in order to obtain the start and end phase change temperatures more accurately, it is necessary to specify the cooling rate range.

[0021] As one of the preferred technical solutions for the method of increasing the surface hardness of the steel billet by using the continuous casting straightening machine, the cooling water flow rate of the straightening roller of the straightening machine is adjusted to ensure that the inlet and outlet water temperature difference is 5℃~10℃ and the inlet water temperature of the straightening roller of the straightening machine is at room temperature. The reason is that ensuring the inlet and outlet water temperature difference can ensure the cooling capacity of the straightening roller is stable and meet the cooling requirements. When the inlet and outlet water temperature difference is less than 5℃, the required water flow rate will be very large, and the equipment requirements are relatively high; when the inlet and outlet water temperature difference is greater than 10℃, the cooling capacity of the straightening roller is reduced and cannot meet the strong cooling requirements brought by the straightening roller to the steel billet. Therefore, the optimal water temperature difference is 5℃~10℃. The requirement for the inlet water temperature to be controlled at room temperature is mainly to facilitate on-site operation and ensure the inlet and outlet water temperature difference is stable. Too high water temperature or too low water temperature is not conducive to operation.

[0022] In some exemplary specific embodiments of the present invention, taking Q345B continuous casting billet as an example, the continuous casting straightening machine control method of the present invention is adopted to increase the surface hardness of the billet to above 95HRB.

[0023] Beneficial effects of the present invention: Without adding three cooling cycles, the present invention comprehensively adjusts the billet drawing speed and specific water content, so that the billet surface temperature is 30°C to 50°C below Ar3 when entering the first straightening machine, and 0 to 50°C below Ar1 when leaving the last straightening machine. The cooling capacity (cooling rate 1°C / s to 1.5°C / s) brought by the straightening rollers of the straightening machine is fully utilized, which is stronger than air cooling. By controlling the phase change of the billet in the interval of entering and leaving the straightening machine, a higher cooling rate than air cooling can be obtained, thereby improving the surface hardness of the billet. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 It is a schematic diagram of the continuous casting cooling area distribution of the present invention.

[0025] Figure 2 This is the center crack of Comparative Example 4.

[0026] Figure 3 The middle and surface cracks of Comparative Example 5. DETAILED DESCRIPTION

[0027] The present invention is further described below in conjunction with the accompanying drawings and embodiments. This figure is a simplified schematic diagram, which only illustrates the basic structure of the present invention in a schematic manner, and therefore it only shows the components related to the present invention.

[0028] The raw materials or process methods not particularly described in the following examples are conventional raw materials and methods in the art.

[0029] Taking Q345B as an example for production, its standard composition requirements are shown in Table 1 below; other relevant parameters are shown in Table 2, where the measurement of Ar3 and Ar1 temperatures is carried out using Gleeble 3800 thermal / mechanical simulation testing machine equipment, and the sample is prepared using Q345B continuous casting billet, the sample diameter is φ10mm, the sample size is 150mm, the sample is heated to 1200℃ and then kept warm for 180s, and then cooled at a cooling rate of 0.10 and 1.00℃ / s. At the same time, a volume expansion meter is used to measure the volume change of the sample, and the Ar3 temperature at the beginning of the corresponding phase change and the Ar1 temperature at the end are read.

[0030] Table 1 Standard composition of Q345B steel, %

[0031] Table 2 Process parameters and physical property parameters

[0032] The main adjustment parameters are as follows: The pulling speed is controlled as: 1m / min, 0.85m / min, 1.1m / min, 0.7m / min, 1.3m / min.

[0033] Specific water volume control level: 0.4L / kg, 0.35L / kg, 0.45L / kg, 0.6L / kg.

[0034] Number of tension and leveling machines pressed down: all 7 machines pressed down; only the 1st, 3rd, 5th to 7th machines pressed down; only the 1st to 4th machines pressed down.

[0035] The temperature difference between the inlet and outlet of the cooling water of the straightening roller of the straightening machine is: 8℃ and 30℃.

[0036] Statistics: Take the continuous casting billet samples for hardness testing, 3 samples per group, and test two hardness data near the center of the surface of each sample. At the same time, pickling is carried out at a low magnification for internal and external quality inspection.

[0037] Performance requirements: Stable surface hardness ≥ 95HRB; no obvious surface or internal defects are allowed on the ingot. Example 1

[0038] The pulling speed is controlled at 0.85m / min; the specific water volume is 0.45L / kg; the 1st to 7th straightening machines are put into use; the temperature difference of the cooling water of the straightening rollers of the straightening machine is controlled at 8℃.

[0039] The surface temperature of the ingot entering the first straightening machine was measured to be 788°C, and the temperature of the ingot entering the last straightening machine was measured to be 691°C.

[0040] The final sampling and testing hardness values ​​are: 95, 96, 95, 97, 98, 95 (unit HRB).

[0041] There is no obvious abnormality in the internal and external quality of the ingot. Comparative Example 1

[0042] Control the pulling speed to 0.85m / min; reduce the specific water volume to 0.35L / kg; put the 1st to 7th straightening machines into use; control the temperature difference of the cooling water of the straightening rollers of the straightening machine to 8℃.

[0043] The surface temperature of the ingot entering the first straightening machine was measured to be 878°C, and the temperature of the last straightening machine was measured to be 793°C.

[0044] The final sampling and testing hardness values ​​are: 94, 93, 94, 92, 94, 93 (unit HRB).

[0045] There is no obvious abnormality in the internal and external quality of the ingot. Comparative Example 2

[0046] Increase the pulling speed to 1.10m / min; specific water volume 0.45L / kg; put the 1st to 7th straightening machines into use; the temperature difference of the cooling water of the straightening rollers of the straightening machine is controlled to 8℃.

[0047] The surface temperature of the ingot entering the first straightening machine was measured to be 894°C, and the temperature of the last straightening machine was measured to be 789°C.

[0048] The final sampling and testing hardness values ​​are: 92, 93, 92, 93, 94, 91 (unit HRB).

[0049] There is no obvious abnormality in the internal and external quality of the ingot. Comparative Example 3

[0050] The target pulling speed and specific water volume are adopted to control the pulling speed at 1.00m / min; the specific water volume is 0.40L / kg; the 1st to 7th straightening machines are put into use; the temperature difference of the cooling water of the straightening rollers of the straightening machine is controlled at 8℃.

[0051] The surface temperature of the ingot entering the first straightening machine was measured to be 907°C, and the temperature of the last straightening machine was measured to be 803°C.

[0052] The final sampling and testing hardness values ​​are: 85, 84, 83, 85, 84, 84 (unit HRB).

[0053] There is no obvious abnormality in the internal and external quality of the ingot. Comparative Example 4

[0054] The pulling speed is controlled at 0.70m / min (20% lower than the target pulling speed); the specific water volume is 0.40L / kg; the 1st to 7th straightening machines are put into use; the temperature difference of the cooling water of the straightening rollers of the straightening machine is controlled at 8℃.

[0055] The surface temperature of the ingot entering the first straightening machine was measured to be 781°C, and the temperature of the last straightening machine was measured to be 680°C.

[0056] The final sampling and testing hardness values ​​are: 95, 96, 96, 96, 97, 95 (unit HRB).

[0057] The central crack defect can be found by low magnification inspection of the ingot, such as Figure 2 shown. Comparative Example 5

[0058] The pulling speed is controlled at 1.30m / min (exceeding the target pulling speed by 20%); the specific water volume is 0.60L / kg; the 1st to 7th straightening machines are put into use; the temperature difference of the cooling water of the straightening rollers of the straightening machine is controlled at 8℃.

[0059] The surface temperature of the ingot entering the first straightening machine was measured to be 799°C, and the temperature of the last straightening machine was measured to be 693°C.

[0060] The final sampling and testing hardness values ​​are: 96, 95, 96, 97, 95, 96 (unit HRB).

[0061] Low magnification detection of ingots reveals cracks in the middle and on the surface, such as Figure 3 shown. Comparative Example 6

[0062] The pulling speed is controlled at 0.85m / min; the specific water volume is 0.45L / kg; the 1st, 3rd, 5th to 7th straightening machines are put into use (the interval between straightening machines is more than 2.5m); the temperature difference of the cooling water of the straightening rollers of the straightening machine is controlled at 8℃.

[0063] The surface temperature of the ingot entering the first straightening machine was measured to be 789°C, and the temperature of the last straightening machine was measured to be 748°C.

[0064] The final sampling and testing hardness values ​​are: 94, 95, 94, 93, 94, 95 (unit HRB).

[0065] There is no obvious abnormality in the internal and external quality of the ingot. Comparative Example 7

[0066] The pulling speed is controlled at 0.85m / min; the specific water volume is 0.45L / kg; the 1st to 4th straightening machines are put into use; the temperature difference of the cooling water of the straightening rollers of the straightening machine is controlled at 8℃.

[0067] The surface temperature of the ingot entering the first straightening machine was measured to be 786°C, and the temperature of the last straightening machine was measured to be 759°C.

[0068] The final sampling and testing hardness values ​​are: 93, 93, 94, 93, 92, 94 (unit HRB).

[0069] There is no obvious abnormality in the internal and external quality of the ingot. Comparative Example 8

[0070] The pulling speed is controlled at 0.85m / min; the specific water volume is 0.45L / kg; the 1st to 4th straightening machines are put into use; the temperature difference of the cooling water of the straightening rollers of the straightening machine is controlled at 30℃.

[0071] The surface temperature of the ingot entering the first straightening machine was measured to be 787°C, and the temperature of the last straightening machine was measured to be 751°C.

[0072] The final sampling and testing hardness values ​​are: 94, 94, 93, 94, 91, 93 (unit HRB).

[0073] There is no obvious abnormality in the internal and external quality of the ingot.

[0074] Based on the above ideal embodiments of the present invention, the relevant staff can make various changes and modifications without departing from the technical concept of the present invention through the above description. The technical scope of the present invention is not limited to the contents of the specification, and its technical scope must be determined according to the scope of the claims.

Claims

1. A method for increasing the surface hardness of a steel billet using a continuous casting straightening machine, characterized in that: These include the following requirements: (1) The continuous casting machine is an arc continuous casting machine, including 5 to 8 straightening machines, each of which is 1.5 to 2.5 meters apart; the straightening machine has a water-cooled straightening roller; (2) Comprehensively adjust the billet drawing speed and specific water content so that the billet temperature entering the first straightening machine is 30℃~50℃ below Ar3, and the billet temperature leaving the last straightening machine is 0~50℃ below Ar1; (3) Adjust the cooling water flow of the straightening roller of the straightening machine to ensure that the inlet and outlet water temperature difference is 5℃~10℃.

2. The method for increasing the surface hardness of a steel billet by using a continuous casting and straightening machine according to claim 1, characterized in that: The adjustment of pulling speed and water content shall not exceed plus or minus 20% of the target pulling speed and target water content.

3. The method for increasing the surface hardness of a steel billet by using a continuous casting and straightening machine according to claim 1, characterized in that: Ar3 is the temperature at which ferrite begins to precipitate from austenite obtained by cooling at 0.05°C / s ~ 0.15°C / s.

4. The method for increasing the surface hardness of a steel billet by using a continuous casting and straightening machine according to claim 1, characterized in that: Ar1 is the temperature at which all austenite obtained by cooling under the conditions of 0.05℃ / s ~0.15℃ / s is transformed into ferrite and pearlite.

5. The method for increasing the surface hardness of a steel billet by using a continuous casting and straightening machine according to claim 1, characterized in that: The water inlet temperature of the straightening roller of the straightening machine is room temperature.

6. The method for increasing the surface hardness of a steel billet by using a continuous casting and straightening machine according to claim 1, characterized in that: The straightening machine has two upper and lower straightening rollers, both of which are water-cooled.

7. The method for increasing the surface hardness of a steel billet by using a continuous casting and straightening machine according to claim 1, characterized in that: The number of the straightening machines is 7.

8. The method for increasing the surface hardness of a steel billet by using a continuous casting and straightening machine according to claim 1, characterized in that: The tension leveler is a Danieli FTSC product.

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