Method for reducing segregation of carbon element in center of medium-carbon axle steel extra-large round billet

Abstract

The invention discloses a method for reducing the carbon element segregation range of a medium carbon axle steel large round billet, which comprises the following steps: during continuous casting, controlling the electromagnetic stirring parameters of a crystallizer to be 80-100A and 2-4Hz, controlling the pulling speed of a large round billet continuous casting machine to be 0.30-0.32 m / min, carrying out online pressing by adopting a 6 # withdrawal and straightening machine and a 7 # withdrawal and straightening machine, respectively controlling the pressure to be 5-8MPa and 10-12MPa, and stopping tail end electromagnetic stirring equipment; according to the method, existing tool equipment of a large round billet continuous casting machine does not need to be adjusted, electromagnetic stirring parameters of a crystallizer and the pulling speed of the continuous casting machine are controlled, and a large round billet pressing technology is adopted, so that the carbon element segregation index of the medium-carbon axle steel large round billet is stably controlled within 1.05%.

Description

Technical Field

[0001] The invention belongs to the technical field of axle steel, and in particular relates to a method for reducing the central carbon element segregation of a super-large round billet of medium-carbon axle steel. Background Art

[0002] As an important load-bearing component in the transportation process of railway vehicles, the train axle bears almost all the loads generated and obtained by the vehicle during the operation of the vehicle. When the train is running and stopping, the vehicle must bear the large impact and braking force generated during the acceleration and deceleration of the train, and play the role of transmitting torque and alternating loads; and may be subjected to certain impacts, and the stress state is complex. Its main failure form is fatigue damage, and fatigue damage is always closely related to the uneven structure and performance of steel, especially the uneven mechanical properties caused by the element segregation of steel. Therefore, downstream manufacturing companies generally have strict requirements on the carbon segregation of the cross-section of the axle billet.

[0003] In addition, since axle steel products have very high requirements for the core density, most domestic steel companies use ultra-large round billets (round billet diameter ≥ 600mm) for continuous casting to produce axle steel, and use a large compression ratio to ensure the core density. It is well known that the serious location of carbon segregation in ultra-large round billets is mainly concentrated in the CET transformation zone of the billet (the intersection of columnar crystals and equiaxed crystals) and the solidification center of the billet. At present, there are relatively many domestic studies on the control methods of carbon segregation in the CET transformation zone of the billet, which are relatively mature, such as secondary cooling electromagnetic stirring technology, equal superheat casting technology, etc., but there are few reports on the control methods of central segregation of ultra-large round billets in China.

[0004] According to relevant literature, the control of carbon segregation in the center of extra-large round billets can be roughly divided into three types:

[0005] The first is the traditional metallurgical control method, which is to reduce the continuous casting superheat, adjust the casting speed, optimize the cooling water volume, etc. The core is to adjust the process parameters during continuous casting to reduce the temperature gradient at the solidification front, reduce the dendrite spacing, and reduce the central carbon element segregation. Its disadvantages are that the process window is narrow and the stability of the process control effect is poor. At the same time, due to the different levels of process control in various enterprises, the applicability of this method is low.

[0006] The second is multi-stage electromagnetic stirring technology, such as a liquid core flow control device and method for vertical semi-continuous casting of large round billets disclosed in Chinese patent CN116809886A. The core of this technology is to use multi-stage electromagnetic stirring technology to interfere with the solidification process of large round billets, and use electromagnetic force to promote the stirring of molten steel. On the one hand, it reduces the temperature gradient at the solidification front, reduces the length of columnar crystals, increases the area of ​​equiaxed crystals, and reduces the carbon enrichment caused by central shrinkage cavities; on the other hand, it promotes the uniform dispersion of elements through electromagnetic stirring, and inhibits selective crystallization to a certain extent. Its disadvantages are: 1) additional electromagnetic stirring equipment is required, which increases production costs, and the continuous casting conditions of large round billets in some steel companies do not meet the installation conditions for secondary cooling electromagnetic stirring, so its promotion and applicability are relatively narrow; 2) In general, electromagnetic stirring technology has strict requirements on the regional range of the solid phase ratio (fs) of the billet solidification process. Therefore, when the conditions affecting the solid phase ratio (pulling speed, superheat, molten steel composition, cooling intensity, etc.) change, electromagnetic stirring is likely to produce a "white bright band" inside the billet, forming serious negative segregation, thereby causing new quality problems for the billet. Summary of the invention

[0007] In order to solve the above technical problems, the present invention provides a method for reducing the extreme difference of carbon element segregation in extra-large round billets of medium-carbon axle steel. This method does not require adjustment of the existing tooling equipment of the large round billet continuous casting machine. By controlling the electromagnetic stirring parameters of the crystallizer, the end electromagnetic stirring parameters, and the continuous casting machine pulling speed, and adopting the large round billet pressing technology, the central carbon element segregation index of the extra-large round billet of medium-carbon axle steel can be stably controlled within 1.05%.

[0008] The technical solution adopted by the present invention is as follows:

[0009] A method for reducing the extreme difference of carbon segregation in large round billets of medium-carbon axle steel is characterized in that, during continuous casting, the electromagnetic stirring parameters of the crystallizer are controlled to be 80-100A and 2-4Hz, the pulling speed of the large round billet continuous casting machine is 0.30-0.32m / min, 6# and 7# straightening machines are used for online pressing, the pressures are 5-8MPa and 10-12MPa respectively, and the terminal electromagnetic stirring equipment is disabled.

[0010] Furthermore, when the casting speed of the large round billet continuous casting machine exceeds the range of 0.30 to 0.32 m / min, the corresponding pressing position moves back 1.5 m for every 0.02 m / min increase in the casting speed of the large round billet continuous casting machine. Specifically:

[0011] When 0.32m / min<pulling speed≤0.34m / min, 7# and 8# tensioning and straightening machines are used for online pressing, and the pressures are 5~8MPa and 10~12MPa respectively;

[0012] When the pulling speed is 0.34m / min<pulling speed≤0.36m / min, 8# and 9# straightening machines are used for online pressing, and the pressures are 5~8MPa and 10~12MPa respectively;

[0013] When 0.36m / min<pulling speed≤0.38m / min, 9# and 10# straightening machines are used for online pressing, with pressures of 5~8MPa and 10~12MPa respectively; the upper limit of the pulling speed of the continuous casting machine shall not exceed 0.38m / min, otherwise the continuous casting machine will have safety risks such as steel leakage.

[0014] When the casting speed of the large round billet continuous casting machine exceeds the range of 0.3-0.32m / min, the corresponding pressing position moves forward 1.5m for every 0.02m / min reduction in the casting speed of the large round billet continuous casting machine, and the end electromagnetic stirring equipment is disabled.

[0015] When the pulling speed is 0.28m / min≤<0.30m / min, 5# and 6# tensioning and straightening machines are used for online pressing, and the pressures are 5~8MPa and 10~12MPa respectively;

[0016] When the pulling speed is 0.26m / min≤<0.28m / min, 4# and 5# straightening machines are used for online pressing, and the pressures are 5~8MPa and 10~12MPa respectively;

[0017] When 0.24m / min≤pulling speed<0.26m / min, 3# and 4# straightening machines are used for online pressing, and the pressures are 5-8MPa and 10-12MPa respectively; the lower limit of the continuous casting machine pulling speed shall not be lower than 0.24m / min. If the pulling speed is too low, the surface temperature of the ingot will be too low, and surface cracks will be prone to occur.

[0018] The chemical composition and weight percentage of the medium carbon axle steel are: C 0.32-0.37%, Si 0.15-0.46%, Mn 0.39-1.10%, P≤0.020%, S≤0.015%, Cr≤0.30%, V≤0.05%, Mo≤0.05%, and the balance is iron and unavoidable impurities.

[0019] The central carbon element segregation index of the extra-large round billet of medium-carbon axle steel is controlled within 1.05%.

[0020] The core equipment required for the method for controlling the central carbon segregation of the extra-large round billet of medium-carbon axle steel is the electromagnetic stirring of the crystallizer and a straightening unit with a terminal pressing function, and no terminal electromagnetic stirring is required.

[0021] The method provided by the present invention for reducing the extreme difference of carbon segregation in large round billets of medium carbon axle steel utilizes the existing tooling equipment of large round billet continuous casting machine and solves the problem of carbon segregation in large round billets through process optimization. Specifically: 1) Adjusting the electromagnetic stirring parameters of the crystallizer to solve the problem of subcutaneous segregation in large round billets; the mechanism is that if the electromagnetic stirring parameters are too large, the fragile dendrite tips will be broken under the action of "necking" and continuous stirring and scouring by electromagnetic stirring, becoming free crystal nuclei, and the solutes enriched at the dendrite tips will "migrate" with the free crystal nuclei, converging to the solidification center of the ingot, causing the center segregation to increase; if the electromagnetic stirring parameters are too small, the electromagnetic stirring force will be weakened, the fluidity of the molten steel will be relatively weak, and the effect of electromagnetic stirring on uniform superheat and reducing the temperature gradient will be greatly reduced, thereby promoting the growth of columnar crystals. Under the action of "selective crystallization", the high-solute molten steel will converge to the solidification center of the ingot, causing the center segregation to increase.

[0022] 2) An innovative large round billet pressing technology was proposed to solve the problem of large round billet center segregation. The mechanism is that on the one hand, the solid-liquid two-phase region can be deformed and the solute-enriched liquid phase can be squeezed out to reduce center segregation; on the other hand, pressing before the solidification end of the billet can increase the equivalent strain near the solidification end, and achieve efficient transmission of the deformation to the core of the billet, achieving the effect of significantly reducing the center shrinkage cavity, thereby reducing the suction effect of the surrounding solute-enriched steel liquid caused by the center shrinkage cavity.

[0023] Compared with the prior art, the present invention has the following beneficial effects:

[0024] 1) No additional equipment is required, the applicability is relatively wide, and no additional process flow is required. At the same time, the end electromagnetic stirring equipment is disabled, which reduces production energy consumption and saves production costs;

[0025] 2) It avoids the shortcomings of the multi-stage electromagnetic stirring technology and can stably control the central carbon segregation index of the extra-large round billet medium-carbon axle steel within 1.05%. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 Medium carbon axle steel produced by current technology Schematic diagram of carbon segregation index at the center of round billet;

[0027] Figure 2 Medium carbon axle steel produced by current technology Low-power image of the cross section of a round billet;

[0028] Figure 3 This is a picture of the cross section of the A1 steel large round billet produced in Example 1 of the present invention. DETAILED DESCRIPTION

[0029] The composition and weight percentage of the axle steel involved in the present invention are shown in Table 1.

[0030] Table 1

[0031]

[0032] Under the current process conditions, the main process tooling information of the continuous casting machine in the production of axle steel is shown in Table 2.

[0033] Table 2

[0034]

[0035]

[0036] The detection and analysis method of the extreme difference of carbon element segregation in large round billets is as follows:

[0037] Take a point at the solidification center of the cross section of the ingot, the sampling depth should not be less than 5mm, the single point sampling weight should not be less than 3g, and the sampling drill bit should be 5mm. After sampling, send it to the carbon sulfur analyzer for testing and analysis of carbon content.

[0038] The calculation method of the central carbon segregation index is shown in Formula 1.

[0039]

[0040] Where η is the central carbon segregation index, C i is the carbon concentration at the corresponding point, C0 is the initial carbon concentration

[0041] The production process of axle steel round billets is as follows: converter smelting - refining - RH vacuum treatment - continuous casting - slow cooling in a slow cooling pit.

[0042] The present invention is described in detail below with reference to Examples and Comparative Examples.

[0043] Example 1

[0044] On July 10, 2023, A1 steel was produced on site. The number of experimental furnaces was 5. The electromagnetic stirring parameters of the crystallizer were adjusted from the previous 260A and 2Hz to 80A and 2Hz; the end electromagnetic stirring was disabled; the pulling speed of the large round billet continuous casting machine was adjusted from 0.24m / min to 0.32m / min; the casting was pressed online using 6# and 7# straightening machines, with pressures of 5MPa and 10MPa respectively. Other parameters were implemented according to the current process in Table 2.

[0045] Example 2

[0046] On July 28, 2023, A1 steel was produced on site with 10 furnaces produced. The electromagnetic stirring parameters of the crystallizer were adjusted from the previous 260A and 2Hz to 80A and 2Hz; the end electromagnetic stirring was disabled; the pulling speed of the large round billet continuous casting machine was adjusted from 0.24m / min to 0.34m / min; the castings were pressed online using 7# and 8# straightening machines, with pressures of 5MPa and 10MPa respectively. Other parameters were implemented according to the current process in Table 2.

[0047] Example 3

[0048] On August 9, 2023, A1 steel was produced on site with 10 furnaces produced. The electromagnetic stirring parameters of the crystallizer were adjusted from 260A and 2Hz to 80A and 2Hz; the end electromagnetic stirring was disabled; the pulling speed of the large round billet continuous casting machine was adjusted from 0.24m / min to 0.36m / min; the ingots were pressed online using 8# and 9# straightening machines, with pressures of 5MPa and 10MPa respectively. Other parameters were implemented according to the current process in Table 2.

[0049] Example 4

[0050] On August 30, 2023, A1 steel was produced on site with 3 furnaces. The electromagnetic stirring parameters of the crystallizer were adjusted from 260A and 2Hz to 80A and 2Hz; the end electromagnetic stirring was disabled; the pulling speed of the large round billet continuous casting machine was adjusted from 0.24m / min to 0.38m / min; the castings were pressed online using 9# and 10# straightening machines, with pressures of 5MPa and 10MPa respectively. Other parameters were implemented according to the current process in Table 2.

[0051] Example 5

[0052] On September 15, 2023, A1 steel was produced on site, with 8 furnaces produced. The electromagnetic stirring parameters of the crystallizer were adjusted from 260A and 2Hz to 80A and 2Hz; the end electromagnetic stirring was disabled; during the casting of 5 consecutive furnaces of molten steel, the pulling speeds of 2 flows of the continuous casting machine were adjusted from 0.32m / min to 0.28m / min and 0.26m / min respectively; when the pulling speed was 0.28m / min, the ingot was pressed online by 5# and 6# pulling and straightening machines, with pressures of 5MPa and 10MPa respectively; when the pulling speed was 0.26m / min, the ingot was pressed online by 4# and 5# pulling and straightening machines, with pressures of 5MPa and 10MPa respectively. Other parameters were implemented according to the current process in Table 2.

[0053] Comparative Example 1

[0054] On July 10, 2023, A1 steel was produced on site, of which 10 furnaces of steel had process parameters implemented according to the current process card, namely, the crystallizer electromagnetic stirring parameters were 260A, 2Hz; the end electromagnetic stirring parameters were 300A, 5Hz; the large round billet continuous casting machine pulling speed was 0.24m / min; the pressing process was not adopted; other parameters were implemented according to the current process in Table 2.

[0055] Comparative Example 2

[0056] On October 14, 2023, A1 steel was produced on site, with 10 furnaces and a pulling speed of 0.32m / min. The ingots were pressed online using 6# and 7# straightening machines, with pressures of 5MPa and 10MPa respectively. The end electromagnetic stirring was disabled and the electromagnetic stirring parameters of the crystallizer of one of the flows were controlled at 260A and 2Hz. Other parameters were implemented according to the current process in Table 2.

[0057] The carbon segregation extremes of the A1 steel round blooms produced in the above-mentioned embodiments and comparative examples are shown in Table 3:

[0058] Table 3

[0059]

[0060]

[0061]

[0062] From the above content, it can be seen that according to the method provided by the present invention, the carbon segregation index of the medium carbon axle steel round billet can be stably controlled within 1.05%.

[0063] The above-mentioned reference embodiment provides a detailed description of a method for reducing the central carbon element segregation index of a super-large round billet of medium-carbon axle steel. This is illustrative rather than restrictive, and several embodiments can be listed according to the limited scope. Therefore, changes and modifications without departing from the overall concept of the present invention should fall within the scope of protection of the present invention.

Claims

1. A method for reducing the extreme difference of carbon segregation in large round billets of medium carbon axle steel, characterized in that: During continuous casting, the electromagnetic stirring parameters of the crystallizer are controlled to 80-100A and 2-4Hz, the pulling speed of the large round billet continuous casting machine is 0.30-0.32m / min, and 6# and 7# straightening machines are used for online pressing, with pressures of 5-8MPa and 10-12MPa respectively, and the end electromagnetic stirring equipment is disabled.

2. The method for reducing the extreme difference of carbon segregation in large round blooms of medium carbon axle steel according to claim 1, characterized in that: The production method of the medium carbon axle steel round billet is: converter smelting - refining - RH vacuum treatment - continuous casting - slow cooling in a slow cooling pit.

3. The method for reducing the extreme difference of carbon segregation in large round blooms of medium carbon axle steel according to claim 1, characterized in that: When the pulling speed of the large round billet continuous casting machine exceeds the range of 0.30 to 0.32 m / min, the corresponding pressing position moves back 1.5 m for every 0.02 m / min increase in the pulling speed of the large round billet continuous casting machine.

4. The method for reducing the extreme difference of carbon segregation in large round blooms of medium carbon axle steel according to claim 3, characterized in that: When the pulling speed is 0.32m / min≤≤0.34m / min, 7# and 8# tensioning and straightening machines are used for online pressing, and the pressures are 5~8MPa and 10~12MPa respectively; When the pulling speed is 0.34m / min<pulling speed≤0.36m / min, 8# and 9# tensioning and straightening machines are used for online pressing, and the pressures are 5~8MPa and 10~12MPa respectively; When the pulling speed is 0.36m / min<pulling speed≤0.38m / min, 9# and 10# straightening machines are used for online pressing, and the pressures are 5~8MPa and 10~12MPa respectively.

5. The method for reducing the extreme difference of carbon segregation in large round blooms of medium carbon axle steel according to claim 1, characterized in that: When the pulling speed of the large round billet continuous casting machine exceeds the range of 0.30 to 0.32 m / min, the corresponding pressing position moves forward 1.5 m for every 0.02 m / min reduction in the pulling speed of the large round billet continuous casting machine.

6. The method for reducing the extreme difference of carbon segregation in large round blooms of medium carbon axle steel according to claim 5, characterized in that: When the pulling speed is 0.28m / min≤<0.30m / min, 5# and 6# tensioning and straightening machines are used for online pressing, and the pressures are 5~8MPa and 10~12MPa respectively; When the pulling speed is 0.26m / min≤<0.28m / min, 4# and 5# tensioning and straightening machines are used for online pressing, and the pressures are 5~8MPa and 10~12MPa respectively; When the pulling speed is 0.24m / min≤≤0.26m / min, 3# and 4# straightening machines are used for online pressing, and the pressures are 5~8MPa and 10~12MPa respectively.

7. The method for reducing the extreme difference of carbon segregation in large round blooms of medium carbon axle steel according to any one of claims 1 to 6, characterized in that: The casting speed of the large round billet continuous casting machine shall not be lower than 0.24m / min and not higher than 0.38m / min.

8. The method for reducing the extreme difference of carbon segregation in large round blooms of medium carbon axle steel according to any one of claims 1 to 6, characterized in that: The chemical composition and weight percentage of the medium carbon axle steel are: C 0.32-0.37%, Si 0.15-0.46%, Mn 0.39-1.10%, P≤0.020%, S≤0.015%, Cr≤0.30%, V≤0.05%, Mo≤0.05%, and the balance is iron and unavoidable impurities.

9. The method for reducing the extreme difference of carbon segregation in large round blooms of medium carbon axle steel according to any one of claims 1 to 6, characterized in that: The carbon segregation index of the medium carbon axle steel round billet is controlled within 1.05%.

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