Method for reducing longitudinal cracks on surface of 45 round steel

Abstract

The invention belongs to the technical field of steel smelting, and relates to a method for reducing longitudinal cracks on the surface of 45 round steel. The method comprises the steps that 1, combined blowing is avoided in the later converter smelting period, and the total oxygen content of molten steel is controlled; 2, argon stirring is performed in the whole smelting process, white slag is quickly produced in the early smelting stage, and the soft blowing time is prolonged as much as possible; 3, the time for molten steel to enter the VD furnace is longer than or equal to 25 minutes, the vacuumizing target is 0.5 tor or below, the holding time is longer than or equal to 10 minutes, the soft blowing time before the molten steel is discharged from the VD furnace is longer than or equal to 10 minutes, the molten steel is discharged for hydrogen determination, the tundish superheat degree continuous casting furnace is smaller than or equal to 15 DEG C, and the casting furnace starting temperature is smaller than or equal to 30 DEG C; 4, a continuous casting long nozzle is sealed by adding a sealing gasket; 5, argon washing is conducted on the tundish before casting is started, and argon is closed when the argon pressure reaches 0.5 Mpa and the liquid level of the tundish reaches the casting liquid level; 6, covering a continuous casting tundish in a manner of firstly adding a covering agent and then adding carbonized rice husks; 7, sealing the continuous casting crystallizer and the cover plate; and 8, cleaning and steel casting work is well done. By adopting the method, the occurrence rate of longitudinal cracks on the surface of the 45 round steel is reduced from 1.28% to below 0.30%.

Description

Technical Field

[0001] The invention belongs to the technical field of steel smelting and relates to a method for reducing longitudinal cracks on the surface of 45 round steel. Background Art

[0002] The causes of pore defects on the surface of the ingot are generally: poor sealing of the immersion nozzle, high gas content in the molten steel, damp refractory in the tundish, excessive moisture in the protective slag, etc. High hydrogen and nitrogen content in the steel is the cause of pores on the surface of the ingot. During the solidification process of molten steel, bubbles will be generated when the partial pressure of CO, nitrogen, hydrogen and other gases produced from C and O in the steel is greater than the sum of the static pressure of the molten steel and the atmospheric pressure. The solubility of gas in steel decreases with decreasing temperature. If the gas content exceeds the solubility in steel or the [CO], [H], [N] and other enriched interdendritic cells in the steel reach or exceed the saturation amount, then gas will be precipitated at the solidification interface or interdendritic cells of the steel. Most of these gases will be discharged but a small amount will remain, thereby generating pores or causing microscopic pore defects. When W(O) in the molten steel is greater than 100×10%~4%~150×10%~4%, W(N) is greater than 60×10%~4%~100×10%~4%, or W(H) is greater than 6×10%~4%~8×10%~4%6, pores are very likely to form in the molten steel during the solidification process.

[0003] There are many types of surface defects on round bars, which can be divided into: cracks, scars, slag inclusions, ears, folds, warping and scratches according to the shape of the defects. There are pore defects on the surface of the continuous casting billet. When the billet is heated in the heating furnace, the periphery of the pores is eroded by the high-temperature oxidizing atmosphere, decarburization and oxidation spots appear, and it cannot be pressed during rolling, and finally intermittently distributed slender crack defects are formed on the surface of the round steel. After rolling, a large number of linear cracks distributed along the rolling direction appear on the surface of the 45 steel continuous casting billet produced by the steel mill, and the crack length varies from 10 to 20 mm. This defect seriously affects the normal production and on-time delivery of the product. It is necessary to analyze the causes of the longitudinal cracks on the surface of 45 round steel, find out the causes of the cracks on the surface of 45 round steel, and take corresponding measures to reduce the longitudinal cracks on the surface of 45 round steel.

[0004] Patent CN 116748479 A discloses a continuous casting production method for reducing longitudinal cracks in low-carbon steel thick slabs, and belongs to the field of continuous casting technology in the metallurgical industry. The continuous casting production method includes using a crystallizer copper plate with non-uniform bolt arrangement, a slow cooling system for the crystallizer, and using high-basicity, high-viscosity protective slag to reduce the generation of longitudinal cracks in low-carbon steel thick slabs. The present invention significantly improves the uniformity of the thickness of the primary slab shell by accurately controlling the core process link of the solidification of the primary slab shell in the continuous casting process of low-carbon steel thick slabs, thereby reducing the longitudinal cracks in the slab and improving the quality of the slab. The surface quality of the produced slab is good, and the rate of longitudinal cracks on the surface of the slab is ≤0.3%. However, this method cannot be used to reduce the generation of longitudinal cracks on the surface of 45 round steel. Summary of the invention

[0005] To achieve the above purpose, the present invention provides a method for reducing longitudinal cracks on the surface of 45 round steel, which solves the problems existing in the prior art.

[0006] The technical solution adopted by the present invention is:

[0007] A method for reducing longitudinal cracks on the surface of 45 round steel comprises the following steps:

[0008] Step 1: Avoid reblowing in the later stage of converter smelting, control the total oxygen content of molten steel, strictly control the amount of slag during steel tapping, strictly prohibit the use of damp or wet carbon powder alloys, and use low-nitrogen carbon powder to adjust the carbon composition;

[0009] Step 2: Refining ensures that the raw and auxiliary materials and carbon powder alloy used are dry, the arc is submerged during the smelting process, the arc effect is controlled during the smelting process, and the arc flashing phenomenon during power transmission is strictly prohibited; adjust the dust removal valve to ensure that the pressure in the dust removal hood is slightly positive during the power transmission smelting process, argon is stirred during the whole smelting process, white slag is quickly made in the early stage of smelting, and the soft blowing time is extended as much as possible;

[0010] Step 3: VD furnace operation. During the VD furnace operation stage, the molten steel enters the VD furnace for ≥25 minutes, the vacuum target is below 0.5tor, the holding time is not less than 10 minutes, the soft blowing time before leaving the VD furnace is not less than 10 minutes, the molten steel is hydrogenated when leaving the station, and the temperature of the molten steel on the continuous casting table when leaving the station is ≤15℃ for the continuous casting furnace and ≤30℃ for the open casting furnace;

[0011] Step 4: The continuous casting shroud is sealed with a sealing gasket to ensure that the shroud is not damaged during the shroud process. At the same time, the long shroud is sealed by double argon blowing;

[0012] Step 5: Before pouring, the tundish is purged with argon until the argon pressure reaches 0.5Mpa. When the liquid level in the tundish reaches the pouring liquid level, the argon is turned off.

[0013] Step 6: The continuous casting tundish is covered by first adding the covering agent and then adding the carbonized rice husk. It is strictly forbidden to add the carbonized rice husk directly without adding the covering agent. It is strictly forbidden to expose the molten steel. The tundish should be covered as soon as possible after the tundish sampling and tundish slag removal operations;

[0014] Step 7: Seal the continuous casting mold and the cover plate to prevent the secondary cooling water vapor from overflowing and causing the mold protection slag to become damp;

[0015] Step 8: Do a good job of cleaning the steel pouring and clean the powdered protective slag accumulated on the crystallizer cover in time to prevent it from flowing into the crystallizer.

[0016] Preferably, in step 1, the total oxygen content of the molten steel is ≤600ppm.

[0017] Preferably, the amount of slag added in step 1 is ≤200kg.

[0018] Preferably, the nitrogen content of the carbon powder in step 1 is ≤200 ppm.

[0019] Preferably, the number of power transmissions during the power transmission smelting process in step 2 is ≤ 3 times.

[0020] Preferably, in step 2, the white slag making time is ≥20 min, and the target soft blowing time is ≥15 min.

[0021] Furthermore, in step 3, hydrogen determination is controlled according to a target value [H]≤2ppm.

[0022] Preferably, in step 4, the argon pressure is ≥ 0.2 MPa.

[0023] Preferably, the sealing gasket in step 4 is a bowl-shaped sealing gasket and a flat gasket.

[0024] Furthermore, before the pouring starts in step 5, the tundish truck drives to the pouring position.

[0025] The beneficial effect of the present invention is that by adopting the technical solution of the present invention and implementing measures to reduce the gas content in 45 steel, the incidence of longitudinal cracks on the surface of 45 round steel is reduced from 1.28% to below 0.30%. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0027] Figure 1 This is the macroscopic morphology of the crack of 45 round steel;

[0028] Figure 2 is the microscopic morphology of the crack of sample 1;

[0029] Figure 3 Microscopic morphology of the crack of sample 2;

[0030] Figure 4 This is the pore morphology of the surface of 45 round steel continuous casting billet;

[0031] Figure 5 Surface image of continuous casting billet after vacuum treatment in VD furnace;

[0032] Figure 6 This is the surface diagram of the continuous casting billet without vacuum treatment in the VD furnace. DETAILED DESCRIPTION

[0033] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0034] Xianggang 45 round steel is rolled from 150mm×150mm and 240mm×240mm continuous casting billets. Its production process is: 80t converter → LF furnace refining → billet continuous casting machine → heating → rough and medium rolling → finishing rolling → cooling bed → sizing → inspection → finished product collection → storage. During the inspection, it was found that there were a large number of linear longitudinal cracks distributed along the rolling direction on the surface of 45 round steel. The macroscopic morphology of the cracks can be seen. Figure 1 .

[0035] 1. Inspection and testing:

[0036] Round steel crack defect inspection: Take a small sample from the linear longitudinal crack of 45 round steel, grind and polish, and acid-etch, and use a metallographic microscope to observe and analyze the morphology and microstructure of the crack. The microscopic morphology of the crack is as follows: Figure 2 and Figure 3 . The specimen is decarburized at the crack, there are traces of unrolled cracks at the end of the crack, and there are oxidation spots near the crack. Generally, decarburization requires a higher temperature (above 800°C) and stays at a higher temperature for a period of time. Carbon atoms diffuse from the inside to the outside, forming CO or CO2 gas with oxygen atoms in the air and escaping, resulting in decarburization around the crack. The oxygen that diffuses into the steel combines with the strong oxidizing elements silicon and manganese to form oxide particles enriched in silicon and manganese. The formation of point-like oxidation particles, that is, the occurrence of internal oxidation, requires higher temperatures and longer times. Based on this, it can be judged that this type of defect is not formed during the rolling process, but is caused by the continuous casting billet.

[0037] The macroscopic morphology and microstructure of the crack defect have obvious characteristics of the pores on the surface of the billet after rolling: the crack tail is rounded and blunt, there are iron oxide and oxidation spots, and the crack edge is decarburized. It is speculated that there are pore defects on the surface of the billet, which leads to the formation of longitudinal linear cracks on the surface of the bar during the rolling process. In order to find out whether there are pore defects on the surface of the billet, it is necessary to take samples of the continuous casting billet for acid immersion inspection.

[0038] Acid immersion test of continuous casting billet: In order to check whether there are defects on the surface of continuous casting billet, samples are cut from the continuous casting billet of the same batch as 45 round steel. Each sample is 600mm long and immersed in hot acid for more than 20min. After the iron oxide scale on the surface of the billet is removed, the hot acid is taken out and the surface of the billet is cleaned and inspected.

[0039] There are many pinhole-shaped pores on each surface of each sample. The pore morphology on the surface of the ingot is as follows: Figure 6 As shown. The diameter of the pores in the billet is 0.6-2.0mm. In order to check the depth of the surface pores, the billet with serious surface pore defects was selected and the defects were basically eliminated after milling 1.5mm. After further processing to 3.5mm, the defects were completely eliminated. The pore depth was measured to be 0.3-

[0040] Gas content detection in steel: After inspection, it was found that there were pores on the surface of the continuous casting billet, and it was inferred that the gas content in the steel exceeded the standard. The hydrogen content in the molten steel was detected with a hydrogen meter before the molten steel was poured on the platform. The hydrogen content in the molten steel was 4.9×10%~4%~8.5×10%~4%, and the average was 7.1×10%~4%. The continuous casting billet sample with a specification of 240mm×240mm×100mm was cut, and a rod sample of Φ5mm×100mm was cut at 1 / 4 of the diagonal of the cross-section of the continuous casting billet sample using a wire cutting instrument. The rod sample was polished and cut, and then tested with an oxygen and nitrogen analyzer. The gas nitrogen content in the steel was 38×10%~4%~65×10%~4%, with an average of 50.6×10%~4%, and the total oxygen content of the gas in the steel was 10×10%~4%~20×10%~4%, with an average of 14.8×10%~4%.

[0041] According to the test results, the hydrogen and nitrogen content in the steel is high. During the solidification process of steel, due to the high gas content, it is easy to produce pore defects on the surface of the continuous casting billet. There are pores on the surface of the continuous casting billet. When heated in the heating furnace, oxidation and decarburization occur around the pores, and it cannot be pressed during rolling. Finally, intermittent linear crack defects are formed on the surface of the round steel, and iron oxide exists at the crack defects.

[0042] In order to reduce the gas content in 45 steel and reduce the pores on the surface of 45 steel continuous casting billet, according to the actual situation of the steelmaking plant, the present invention adopts the following technical solution:

[0043] 1) Avoid reblowing in the later stage of converter smelting, the total oxygen content of converter steel is ≤600ppm, and the amount of slag is strictly controlled during steel tapping, and the amount of slag is ≤200kg. Use low-nitrogen carbon powder to adjust the carbon composition, and the nitrogen content of the carbon powder is ≤200ppm.

[0044] 2) Refining ensures that the raw and auxiliary materials and carbon powder alloy used are dry. The smelting process is submerged arc operation, and the pressure in the dust removal hood is slightly positive during the power supply smelting process. The number of power supply is ≤3 times, and argon is stirred during the whole smelting process. White slag is quickly made in the early stage of smelting, and the white slag time is guaranteed to be ≥20min. Try to extend the soft blowing time, and the target soft blowing time is ≥15min.

[0045] 3) VD furnace operation

[0046] During the VD furnace operation stage, the molten steel enters the VD furnace for ≥25 minutes, the vacuum target is below 0.5tor, and the holding time is not less than 10 minutes. After the molten steel composition is adjusted to meet the requirements, the soft blowing time before leaving the VD furnace is not less than 10 minutes, and the molten steel is hydrogenated at the exit station, and the target value [H] is controlled at ≤2ppm; the temperature of the continuous casting table on the exit station should ensure that the overheat of the tundish is controlled at ≤15℃ for the continuous casting furnace and ≤30℃ for the open casting furnace.

[0047] 4) The continuous casting long nozzle is sealed with a sealing gasket and a double argon blowing method is used to seal the long nozzle, and the argon pressure is ≥0.2MPa.

[0048] 5) Before pouring starts (when the tundish truck arrives at the pouring position), the tundish is purged with argon and the argon pressure reaches 0.5Mpa. When the liquid level in the tundish reaches the pouring level, the argon is turned off.

[0049] 6) The continuous casting tundish must be covered by adding the covering agent first and then the carbonized rice husk. It is strictly forbidden to add the carbonized rice husk directly without adding the covering agent. It is strictly forbidden to expose the molten steel, especially after the tundish sampling and tundish slag discharge operation, the tundish must be covered as soon as possible.

[0050] 7) Ensure a good seal between the continuous casting crystallizer and the cover plate to prevent the secondary cooling water vapor from overflowing and causing the crystallizer protective slag to become damp.

[0051] 8) Do a good job of cleaning the steel pouring and clean the powdered protective slag accumulated on the crystallizer cover in time to prevent it from flowing into the crystallizer.

[0052] In order to verify that high gas content in steel can easily lead to pores on the surface of continuous casting billets, a vacuum treatment test of 45 round steel in the first two furnaces of casting was arranged. The specific process data are shown in Table 1:

[0053] Table 1 Gas content of 45 steel in VD furnace and not in VD furnace

[0054]

[0055] As can be seen from the table above, the hydrogen and nitrogen content in the 45 steel treated in the VD furnace is significantly lower than that in the heats that are not treated in the VD furnace. Samples of the ingots were taken from the heats that were treated in the VD furnace and the heats that were not treated in the VD furnace for acid leaching inspection and comparison. Figure 5 The surface of the continuous casting billet that is not vacuum treated in the VD furnace is as follows Figure 6 .

[0056] After inspection, it was found that the number of pores on the surface of the continuous casting billet after vacuum treatment in the VD furnace was very small, and the pore diameter was small and shallow; while the number of pores on the surface of the continuous casting billet without vacuum treatment in the VD furnace was large, and some of the pores were larger and deeper.

[0057] Table 2 shows the comparison of surface crack grinding of round steel rolled from continuous casting billets with and without vacuum treatment in VD furnace.

[0058] Table 245 Comparison of surface crack grinding of round steel entering VD furnace and not entering VD furnace

[0059]

[0060] It can be seen from the above table that the surface of the round steel rolled from the continuous casting billet subjected to vacuum treatment in the VD furnace is normal without crack defects; while the surface of the round steel rolled from the continuous casting billet subjected to vacuum treatment in the non-VD furnace has cracks.

[0061] The above analysis shows that there are pore defects on the surface of the continuous casting billet. When the billet is heated in the heating furnace, the area around the pores is corroded by the high-temperature oxidizing atmosphere, resulting in decarburization and oxidation spots. It cannot be pressed during rolling, and eventually discontinuously distributed long and thin crack defects are formed on the surface of the round steel.

[0062] After taking the above improvement measures, the surface cracks of 45 round steel were counted and analyzed, and the results are shown in Table 3.

[0063] Table 3 Comparison of longitudinal cracks on the surface of 45 round steel before and after the implementation of improvement measures

[0064]

[0065] It can be seen from the above table that before and after the implementation of the improvement measures, the surface longitudinal cracks of 45 round steel were effectively controlled, and the surface longitudinal crack rate was reduced from 1.28% to below 0.30%.

[0066] The above description is only a preferred embodiment of the present invention and is not intended to limit the protection scope of the present invention. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention are included in the protection scope of the present invention.

Claims

1. A method for reducing longitudinal cracks on the surface of 45 round steel, characterized in that: The steps include: Step 1: Avoid reblowing in the later stage of converter smelting, control the total oxygen content of molten steel, strictly control the amount of slag during steel tapping, and use low-nitrogen carbon powder to adjust the carbon composition; Step 2: Refining ensures that the raw and auxiliary materials and carbon powder alloy used are dry, the smelting process is submerged arc operation, the pressure in the dust removal hood is slightly positive during the power supply smelting process, argon is stirred during the whole smelting process, white slag is quickly made in the early stage of smelting, and the soft blowing time is extended as much as possible; Step 3: VD furnace operation. During the VD furnace operation stage, the molten steel enters the VD furnace for ≥25 minutes, the vacuum target is below 0.5tor, the holding time is not less than 10 minutes, the soft blowing time before leaving the VD furnace is not less than 10 minutes, the molten steel is hydrogenated when leaving the station, and the temperature of the molten steel on the continuous casting table when leaving the station is ≤15℃ for the continuous casting furnace and ≤30℃ for the open casting furnace; Step 4: The continuous casting shroud is sealed by adding a sealing gasket and double argon blowing is used to seal the shroud; Step 5: Before pouring, the tundish is purged with argon until the argon pressure reaches 0.5Mpa. When the liquid level in the tundish reaches the pouring liquid level, the argon is turned off. Step 6: The continuous casting tundish is covered by first adding the covering agent and then adding the carbonized rice husk. It is strictly forbidden to add the carbonized rice husk directly without adding the covering agent. It is strictly forbidden to expose the molten steel. The tundish should be covered as soon as possible after the tundish sampling and tundish slag removal operations; Step 7: Seal the continuous casting mold and the cover plate to prevent the secondary cooling water vapor from overflowing and causing the mold protection slag to become damp; Step 8: Do a good job of cleaning the steel pouring and clean the powdered protective slag accumulated on the crystallizer cover in time to prevent it from flowing into the crystallizer.

2. A method for reducing longitudinal cracks on the surface of 45 round steel as claimed in claim 1, characterized in that: In step 1, the total oxygen content of the molten steel is ≤600ppm.

3. A method for reducing longitudinal cracks on the surface of 45 round steel as claimed in claim 1, characterized in that: The amount of slag added in step 1 is ≤200kg.

4. A method for reducing longitudinal cracks on the surface of 45 round steel as claimed in claim 1, characterized in that: The nitrogen content of the carbon powder in step 1 is ≤ 200 ppm.

5. A method for reducing longitudinal cracks on the surface of 45 round steel as claimed in claim 1, characterized in that: In step 2, the number of power supply times during the power supply smelting process is ≤ 3 times.

6. A method for reducing longitudinal cracks on the surface of 45 round steel as claimed in claim 1, characterized in that: In step 2, the white slag making time is ≥ 20 min, and the target soft blowing time is ≥ 15 min.

7. A method for reducing longitudinal cracks on the surface of 45 round steel as claimed in claim 1, characterized in that: In step 3, hydrogen determination is controlled according to the target value [H] ≤ 2ppm.

8. A method for reducing longitudinal cracks on the surface of 45 round steel as claimed in claim 1, characterized in that: In step 4, the argon pressure is ≥ 0.2 MPa.

9. A method for reducing longitudinal cracks on the surface of 45 round steel as claimed in claim 1, characterized in that: The gaskets in step 4 are bowl-shaped gaskets and flat gaskets.

10. A method for reducing longitudinal cracks on the surface of 45 round steel as claimed in claim 1, characterized in that: Before the pouring starts in step 5, the middle ladle truck drives to the pouring position.

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