Smelting method for controlling cracks on outer surface of large-section round billet of seamless pipeline pipe with low carbon and high V content

Abstract

The invention discloses a smelting method for controlling cracks on the outer surface of a large-section round billet of a low-carbon high-V-content seamless pipeline pipe. The smelting method mainly comprises the following production steps: smelting in a converter; performing external refining and vacuum treatment; related smelting process parameters are disclosed; the low-carbon high-V-content seamless line pipe large-section round billet material comprises the following chemical components in percentage by mass: 0.06 to 0.12 percent of C, 0.17 to 0.37 percent of Si, 1.40 to 1.60 percent of Mn, 0.05 to 0.10 percent of V, 0.005 to 0.045 percent of Ti, 0.005 to 0.050 percent of Al, less than or equal to 0.015 percent of P, less than or equal to 0.003 percent of S and the balance of Fe and trace impurities, and the total mass fraction is 100 percent. According to the smelting method for controlling the cracks on the outer surface of the large-section round billet of the seamless pipeline pipe with low carbon and high V content, the quality of the outer surface of the casting billet is obviously improved.

Description

Technical Field

[0001] The invention belongs to the technical field of material metallurgy, and in particular relates to a method for controlling outer surface cracks of a large-section round billet of a seamless pipeline for low-carbon and high-V content. Background Art

[0002] Pipelines for oil and natural gas transportation are required to be economical, safe, efficient, uninterrupted and pollution-free. In addition, pipelines can also be used as a means of transporting solid materials. Currently, oil transportation by pipeline accounts for more than 80% of the entire transportation method, and natural gas pipeline transportation accounts for 100%. According to my country's national conditions, it is expected that pipeline transportation of oil and gas will still be the main transportation method in the oil and gas industry in the future. The use conditions of seamless steel pipes for pipeline transportation are complex, and they are required to have good performance under working conditions such as high mountains, rivers, deserts, cold, and corrosive transportation media to ensure the safety of oil and gas transportation. The round billet for large-section pipeline pipes of the present invention requires the C content to be controlled below 0.12%, and is equipped with an appropriate amount of Mn elements. At the same time, it is strengthened by the micro-alloying element V. The steel has good comprehensive performance matching. At the same time, due to the high prices of alloying elements such as Cr, Mo, and Nb, the alloying of V can greatly reduce the production cost of steel pipes. However, in the process of smelting large-section continuous casting billets with low C and V content, cracks are easily generated on the outer surface of the round billet, resulting in a large number of external scars in the subsequent rolled steel pipes, affecting the yield rate of hot-rolled steel pipes. The present invention improves the outer surface quality of large-section continuous casting billets by optimizing the smelting parameters of low-C and V-containing steels, and meets the technical requirements of hot-rolled seamless pipelines. Summary of the invention

[0003] The purpose of the present invention is to provide a method for controlling the outer surface cracks of a large-section round billet of a seamless pipeline for low-carbon and high-V content, and to solve the problem that cracks are easily generated on the outer surface of a continuous casting round billet and that the subsequent hot-rolled steel pipe is prone to external scarring and low yield by optimizing and controlling the smelting parameters.

[0004] In order to solve the above technical problems, the present invention adopts the following technical solutions:

[0005] The present invention discloses a method for controlling cracks on the outer surface of a large-section round billet of a seamless pipeline for low-carbon and high-V content, and the production steps thereof mainly include:

[0006] For converter smelting, the incoming molten iron composition requirements are P≤0.012%, S≤0.050%. The converter molten iron adopts pre-desulfurized molten iron, and the S is required to be ≤0.010% after pre-desulfurization. All alloy raw materials in the converter smelting process must meet the standard requirements. The single slag smelting process is adopted, and the final slag basicity is controlled as ≥3.0. The end point carbon must be well controlled, and overoxidation and ultra-low carbon steel cannot be tapped. The converter end point control target C≥0.06%, P≤0.010%, and the tapping temperature is controlled at 1610-1630℃. Ferrosilicon and ferromanganese are used for deoxidation and alloying. The final deoxidation adopts aluminum deoxidation. The amount of slag is strictly controlled. Slag must be blocked when tapping. If the slag blocking fails, the slag must be removed.

[0007] Refining and vacuum treatment outside the furnace, slag desulfurization, composition adjustment and temperature increase operations are carried out according to the composition and temperature of the molten steel in the converter. Argon is blown normally throughout the whole process, and the blowing intensity is controlled to keep the molten steel creeping and not expose the molten steel. The deoxidation operation is strengthened to make good white slag. The white slag holding time is ≥20min, and the slag condition is adjusted according to the situation to ensure white slag tapping. V iron is added in the middle and late stages of refining; vacuum degassing requires vacuum degree ≤0.10KPa, target value ≤0.06KPa, deep vacuum time ≥13min, and attention should be paid to the boiling of molten steel at any time during the vacuum process and the flow rate of argon should be adjusted in time. The soft blowing time after vacuum is ≥13min, and attention should be paid to the weak stirring effect. The molten steel should not be exposed, and the non-metallic inclusions in the molten steel should be removed well.

[0008] During the continuous casting process, the baking of the tundish should be strengthened, the tundish covering agent and the mold protective slag must be dry, and the protective casting should be adopted in the whole continuous casting process. In particular, the drawing speed should be increased to ensure constant drawing speed casting. During the continuous casting process, the coverage of the tundish and the crystallizer liquid surface should be observed at any time to prevent the tundish and the crystallizer from poorly covering the molten steel or the steel metallurgy from being exposed. The pouring liquid surface should not be less than 500mm, and the process should adopt full ladle casting. The superheat of the molten steel should be controlled at 15℃-35℃, and the crystallizer electromagnetic stirring and terminal electromagnetic stirring processes should be adopted. Pay special attention to the ratio of the secondary cooling water and adopt the weak cooling process. The secondary cooling water should be sprayed evenly, and no interruption or water column spraying is allowed;

[0009] The chemical composition mass percentage of the low-carbon and high-V-content seamless pipeline large-section round billet material is: C0.06-0.12, Si 0.17-0.37, Mn 1.40-1.60, V 0.05-0.10, Ti 0.005-0.045, Al 0.005-0.050, P≤0.015, S≤0.003, and the rest is Fe and trace impurities, with a total mass fraction of 100%.

[0010] Furthermore, the vacuum degree of vacuum degassing is 0.06 KPa, and the deep vacuum time is 13 min. During the vacuuming process, attention should be paid to the boiling of the molten steel at any time and the flow rate of argon should be adjusted in time.

[0011] Furthermore, the soft blowing time after vacuum is 13 minutes. At the same time, attention should be paid to the weak stirring effect, the molten steel should not be exposed, and the non-metallic inclusions in the molten steel should be removed.

[0012] Furthermore, the water volume of the crystallizer is 2400 L / min, and the deviation is less than 50 L / min.

[0013] Furthermore, the continuous casting adopts speed-up production, and the continuous casting speed is 0.58m / min.

[0014] Furthermore, the cooling intensity of the secondary cooling zone of the ingot was reduced by reducing the secondary cooling water ratio, and the secondary cooling water ratio was reduced to 0.18L / kg.

[0015] Furthermore, the continuous casting production carries out slag line operation. The first slag line can be controlled at 95-105mm, and the second slag line can be controlled at 125-135mm. The electromagnetic stirring of the crystallizer and the end electromagnetic stirring are controlled well. The door of the billet discharge workshop is closed. The billet must be taken offline in advance at the cooling bed receiving position to reduce the influence of air temperature, increase the billet offline temperature and slow cooling temperature, avoid cracks, and the billet exit pit temperature is ≤200℃.

[0016] Compared with the prior art, the beneficial technical effects of the present invention are:

[0017] The round tube billet smelted by the outer surface crack control smelting method of the large-section round billet for low-carbon and high-V content seamless pipeline of the present invention solves the problem that cracks are easy to occur on the outer surface of the tube billet. The steel tube has good outer surface quality after hot rolling. The carbon content of the steel is as low as ≤0.012%, and the V content is high. During the solidification process of the crystallizer billet shell, the linear contraction coefficient at the billet shell is about 5 times that of ordinary steel. Therefore, the solidification of the billet shell is prone to unevenness, stress concentration and poor bonding strength. During the process of entering the drawing and leveling machine, if the temperature is too low, cracks are easily generated on the outer surface of the billet. The steel quality is strengthened and high-clean steel is used for smelting, especially by controlling the ratio of secondary cooling water and the drawing speed during the continuous casting process, the outer surface quality of the ingot is improved to meet the needs of subsequent hot rolling. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] The present invention will be further described below in conjunction with the accompanying drawings.

[0019] Figure 1 Adjust the outer surface quality of the round billet before smelting process;

[0020] Figure 2 Adjust the outer surface quality of the round billet before smelting process. DETAILED DESCRIPTION

[0021] The present invention is described in detail below through specific examples.

[0022] The low-carbon and high-V-content seamless pipeline billet of the present invention is produced by adopting a 390-section round billet, and the chemical composition range of the round billet is as follows: C 0.06-0.12, Si 0.17-0.37, Mn 1.40-1.60, V 0.05-0.10, Ti0.005-0.045, Al 0.005-0.050, P≤0.015, S≤0.003, and the rest is Fe and trace impurities, and the total mass fraction is 100%.

[0023] The C content is designed to be 0.06% to 0.12%. The low C design is used together with an appropriate amount of Mn to maintain the strength of the steel. The low C design can make the pipeline have better low-temperature toughness and weldability. The high V content design replaces expensive alloying elements such as Cr, Mo, and Nb to reduce the production cost of the steel. At the same time, V can also improve the hardenability of the steel and improve the strength of the steel through low-temperature precipitation at 650°C.

[0024] The main production steps of round tube billets are as follows:

[0025] For converter smelting, the incoming molten iron has P: 0.010%, S: 0.040%, and the S content after pre-desulfurization is 0.008%. During the converter smelting process, all alloy raw materials must meet the standard requirements. The single slag smelting process is adopted, and the final slag basicity is controlled to 3.2. The terminal carbon must be well controlled, and overoxidation and ultra-low carbon steel cannot be tapped. The converter terminal C is controlled to 0.08%, P to 0.008%, and the tapping temperature is controlled to 1625℃. Ferrosilicon and ferromanganese are used for deoxidation and alloying, and aluminum deoxidation is used for final deoxidation. The amount of slag is strictly controlled, slag must be blocked when tapping, and the slag must be removed if the slag blocking fails.

[0026] Refining outside the furnace and vacuum treatment: slag desulfurization, composition adjustment and temperature raising operations are carried out according to the composition and temperature of the molten steel in the converter. Argon is blown normally during the whole process, and the blowing intensity is controlled to keep the molten steel creeping and not expose the molten steel. The deoxidation operation is strengthened to make good white slag. The white slag holding time is 22 minutes, and the slag condition is adjusted according to the situation to ensure the white slag tapping. V iron is added in the middle and late stages of refining. The vacuum degree of vacuum degassing is 0.06KPa, and the deep vacuum time is 13 minutes. During the vacuuming process, attention should be paid to the boiling of the molten steel at any time and the flow rate of argon should be adjusted in time. The soft blowing time after vacuum is 13 minutes. At the same time, attention should be paid to the weak stirring effect. The molten steel should not be exposed, and the non-metallic inclusions in the molten steel should be removed well.

[0027] The most critical control process of the smelting method for controlling the outer surface cracks of large-section round billets of low-carbon and high-V seamless steel pipes is continuous casting. In order to reduce the crack defects on the outer surface of the billet, the continuous casting process parameters are adjusted. The water volume of the crystallizer is 2400L / min, and the deviation value is less than 50L / min. The continuous casting adopts speed-up production. The comparison before and after the adjustment of the continuous casting speed is shown in Table 1:

[0028] Table 1 Comparison of continuous casting speed before and after adjustment

[0029] Before speed adjustment After speed adjustment 0.52m / min 0.58m / min

[0030] Adjust the secondary cooling water distribution to increase the straightening temperature of the casting machine. For low-carbon, high-V content, and highly crack-sensitive steels, in order to avoid straightening at the brittle temperature (700-900°C) of the ingot, reduce the cooling intensity of the secondary cooling zone of the ingot by reducing the secondary cooling water ratio, reduce the temperature loss during the ingot casting process, and increase the straightening temperature. Parameter adjustment is shown in Table 2:

[0031] Table 2 Comparison of secondary cooling water before and after adjustment

[0032] Before secondary cooling water adjustment After the second cold water adjustment 0.20L / kg 0.18L / kg

[0033] The tensioning and straightening machine is not the root cause of the cracks, but the cracks will further expand in the tensioning and straightening machine area. Adjust the hot billet pressure parameters to slow down the crack expansion. The tensioning and straightening machine adjustment is shown in Table 3:

[0034] Table 3 Comparison of tension and straightening machine pressure (Pa) before and after adjustment

[0035] plan Two Three Four Five Before adjusting the tensioning machine 1 1.3 4.2 5.0 After adjustment of the tensioning machine 0.8 1.0 4.0 4.8

[0036] Continuous casting production carries out slag line operation. The first slag line can be controlled at 95-105mm, and the second slag line can be controlled at 125-135mm. The electromagnetic stirring of the crystallizer and the end electromagnetic stirring are controlled well. The door of the billet discharge workshop is closed. The billet must be taken offline in advance at the cooling bed receiving position to reduce the influence of air temperature, increase the billet offline temperature and slow cooling temperature to avoid cracks. The billet outlet temperature is ≤200℃.

[0037] Through the above adjustment of smelting process parameters, the outer surface quality of the ingot is significantly improved. Figure 1 To adjust the crack photo of the outer surface of the large section round billet, Figure 2 The outer surface quality of large-section round billet after smelting parameters are adjusted.

[0038] The embodiments described above are only descriptions of the preferred modes of the present invention, and are not intended to limit the scope of the present invention. Without departing from the design spirit of the present invention, various modifications and improvements made to the technical solutions of the present invention by ordinary technicians in this field should all fall within the protection scope determined by the claims of the present invention.

Claims

1. A method for controlling the outer surface cracks of a large-section round billet of a seamless line pipe for low carbon and high V content, characterized in that: The production steps mainly include: For converter smelting, the incoming molten iron composition requirements are P≤0.012%, S≤0.050%. The converter molten iron adopts pre-desulfurized molten iron, and the S is required to be ≤0.010% after pre-desulfurization. All alloy raw materials in the converter smelting process must meet the standard requirements. The single slag smelting process is adopted, and the final slag basicity is controlled as ≥3.

0. The end point carbon must be well controlled, and overoxidation and ultra-low carbon steel cannot be tapped. The converter end point control target C≥0.06%, P≤0.010%, and the tapping temperature is controlled at 1610-1630℃. Ferrosilicon and ferromanganese are used for deoxidation and alloying. The final deoxidation adopts aluminum deoxidation. The amount of slag is strictly controlled. Slag must be blocked when tapping. If the slag blocking fails, the slag must be removed. Refining and vacuum treatment outside the furnace, slag desulfurization, composition adjustment and temperature increase operations are carried out according to the composition and temperature of the molten steel in the converter. Argon is blown normally throughout the whole process, and the blowing intensity is controlled to keep the molten steel creeping and not expose the molten steel. The deoxidation operation is strengthened to make good white slag. The white slag holding time is ≥20min, and the slag condition is adjusted according to the situation to ensure white slag tapping. V iron is added in the middle and late stages of refining; vacuum degassing requires vacuum degree ≤0.10KPa, target value ≤0.06KPa, deep vacuum time ≥13min, and attention should be paid to the boiling of molten steel at any time during the vacuum process and the flow rate of argon should be adjusted in time. The soft blowing time after vacuum is ≥13min, and attention should be paid to the weak stirring effect. The molten steel should not be exposed, and the non-metallic inclusions in the molten steel should be removed well. During the continuous casting process, the baking of the tundish should be strengthened, the tundish covering agent and the mold protective slag must be dry, and the protective casting should be adopted in the whole continuous casting process. In particular, the drawing speed should be increased to ensure constant drawing speed casting. During the continuous casting process, the coverage of the tundish and the crystallizer liquid surface should be observed at any time to prevent the tundish and the crystallizer from poorly covering the molten steel or the steel metallurgy from being exposed. The pouring liquid surface should not be less than 500mm, and the process should adopt full ladle casting. The superheat of the molten steel should be controlled at 15℃-35℃, and the crystallizer electromagnetic stirring and terminal electromagnetic stirring processes should be adopted. Pay special attention to the ratio of the secondary cooling water and adopt the weak cooling process. The secondary cooling water should be sprayed evenly, and no interruption or water column spraying is allowed; The chemical composition mass percentage of the low-carbon and high-V-content seamless pipeline large-section round billet material is: C 0.06-0.12, Si 0.17-0.37, Mn 1.40-1.60, V 0.05-0.10, Ti 0.005-0.045, Al 0.005-0.050, P≤0.015, S≤0.003, and the rest is Fe and trace impurities, with a total mass fraction of 100%.

2. The method for controlling outer surface cracks of a large-section round billet of a seamless line pipe for low carbon and high V content according to claim 1, characterized in that: The vacuum degree of vacuum degassing is 0.06KPa, and the deep vacuum time is 13min. During the vacuuming process, pay attention to the boiling of molten steel at any time and adjust the flow rate of argon in time.

3. The method for controlling outer surface cracks of a large-section round billet of a seamless line pipe for low carbon and high V content according to claim 2, characterized in that: The soft blowing time after vacuum is 13 minutes. At the same time, pay attention to the weak stirring effect, do not expose the molten steel, and do a good job of removing non-metallic inclusions in the molten steel.

4. The method for controlling outer surface cracks of a large-section round billet of a seamless line pipe for low carbon and high V content according to claim 1, characterized in that: The water volume of the crystallizer is 2400L / min, and the deviation is less than 50L / min.

5. The method for controlling outer surface cracks of a large-section round billet of a seamless line pipe for low carbon and high V content according to claim 1, characterized in that: The continuous casting adopts speed-up production, and the continuous casting speed is 0.58m / min.

6. The method for controlling outer surface cracks of a large-section round billet of a seamless line pipe for low carbon and high V content according to claim 1, characterized in that: By reducing the secondary cooling water ratio, the cooling intensity of the secondary cooling zone of the ingot is reduced, and the secondary cooling water is reduced to 0.18L / kg.

7. The method for controlling outer surface cracks of a large-section round billet of a seamless line pipe for low carbon and high V content according to claim 1, characterized in that: Continuous casting production carries out slag line operation. The first slag line can be controlled at 95-105mm, and the second slag line can be controlled at 125-135mm. The electromagnetic stirring of the crystallizer and the end electromagnetic stirring are controlled well. The door of the billet discharge workshop is closed. The billet must be taken offline in advance at the cooling bed receiving position to reduce the influence of air temperature, increase the billet offline temperature and slow cooling temperature to avoid cracks. The billet outlet temperature is ≤200℃.

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