Control method for B-type inclusions in ultra-low sulfur steel

Abstract

The invention belongs to the technical field of metallurgical steelmaking production, and particularly relates to a control method for B-type inclusions in ultralow-sulfur steel. The production process comprises the steps of molten iron treatment, converter smelting, LF refining, RH refining, calcium treatment and soft blowing, continuous casting and hot rolling, and is characterized in that in the calcium treatment and soft blowing process, after RH is finished, a calcium line is fed for calcium treatment, the content of calcium in molten steel is reduced, and the content of calcium in tundish molten steel is controlled to be 5-10 ppm; the total air volume of soft blowing is controlled to be 80-180 NL / min, and the soft blowing time is controlled to be 7-12 min. According to the method, large-size CaO-Al2O3 series and CaS-CaO series inclusions do not exist in the molten steel, the large-size strip-string-shaped inclusions are obviously improved after hot rolling, and the cleanliness of the molten steel and the product quality are obviously improved; meanwhile, the consumption of calcium lines in the production process is reduced, and the production cost is reduced.

Description

Technical Field

[0001] The present invention belongs to the technical field of metallurgical steelmaking production, and particularly relates to a method for controlling B-type inclusions in ultra-low sulfur steel. Background Technique

[0002] Ultra-low sulfur steel generally refers to steel grades with a sulfur content less than 50 ppm, which are widely used in various fields such as metallurgy, mining, railways, and military, such as oil and gas pipelines, steel for offshore oil production platforms, thick ship plates, and steel for aviation. With the progress of modern technology and the development of modern industry, the requirements for the strength, toughness, weldability, and other properties of ultra-low sulfur steel are getting higher and higher, and inclusions in steel are one of the main reasons affecting the properties of ultra-low sulfur steel.

[0003] For ultra-low sulfur steels such as pipeline steel and wear-resistant steel, the sulfur content in the steel has a significant impact on the transverse impact toughness of the steel plate. Therefore, high-grade steels require a sulfur content lower than 20 ppm. At the same time, MnS inclusions in the steel can cause anisotropy of the steel and lead to hydrogen-induced cracks during service. Therefore, generally, a calcium treatment process is adopted during the refining process to transform the MnS inclusions in the steel into spherical CaO-Al 2 O 3 -CaS composite inclusions, improving the impact toughness of the steel and eliminating the anisotropy of the steel. However, after the calcium treatment process, more large-sized CaO-Al 2 O 3 -CaS composite inclusions are formed in the molten steel; CaO-Al 2 O 3 system inclusions are low-melting-point inclusions, which can deform during the rolling process to form large-sized string-like inclusions, resulting in the over-standard of B-type inclusions in the steel and seriously affecting the performance of the steel.

[0004] The application number is 201910005354.8, and the name is "A production method for removing non-metallic inclusions in wear-resistant steel". This invention application conducts calcium treatment on the molten steel in two refining processes of LF and RH respectively to ensure that there is enough calcium in the molten steel and transform the inclusions into CaO-Al 2 O 3 system inclusions. However, sufficient calcium treatment will cause a significant increase in the number of inclusions in the molten steel. At the same time, CaO-Al 2 O 3 system inclusions are liquid inclusions, which deform into large-sized string-like inclusions during the rolling process, resulting in the over-standard of B-type inclusions in ultra-low sulfur steel.

[0005] The application number is 202310632964.7, and the name is "A Method for Controlling Class B Inclusions in Pipeline Steel". This method involves subjecting the molten steel to calcium treatment in the LF process and magnesium treatment in the RH process, transforming the inclusions in the molten steel into CaO-Al 2 O 3 -MgO-CaS. However, after the magnesium treatment in this method, the production cost increases; meanwhile, the increase in the magnesium content in the molten steel will cause spinel-type inclusions to accumulate at the stopper rod, leading to problems such as poor castability of the molten steel.

[0006] Therefore, there is an urgent need to develop a control process for Class B inclusions in ultra-low sulfur steel to solve the problem of excessive large-sized Class B inclusions during the production of ultra-low sulfur steel, and improve the cleanliness of the molten steel and the product quality. Summary of the Invention

[0007] The present invention provides a method for controlling Class B inclusions in ultra-low sulfur steel. By reducing the addition amount of calcium wire, extending the RH vacuum circulation time, etc., the type and quantity of inclusions in the steel are effectively improved, the large-sized string-like Class B inclusions in the steel are reduced, the cleanliness of the molten steel is increased, and the quality and performance of the product are enhanced.

[0008] To solve the above technical problems, the technical solution adopted by the present invention is: A method for controlling Class B inclusions in ultra-low sulfur steel, the production process includes: hot metal treatment → converter smelting → LF refining → RH refining → calcium treatment and soft blowing → continuous casting → hot rolling, and the improvement is: In the calcium treatment and soft blowing process, after the RH is completed, calcium wire is fed for calcium treatment to reduce the calcium content in the molten steel, and the calcium content in the tundish molten steel is controlled at 5-10 ppm; the total soft blowing gas volume is controlled at 80-180 NL / min, and the soft blowing time is controlled at 7-12 min.

[0009] For the above method for controlling Class B inclusions in ultra-low sulfur steel, in the RH refining process, the RH vacuum circulation time is extended to ensure that the RH vacuum circulation time ≥ 30 min.

[0010] For the above method for controlling Class B inclusions in ultra-low sulfur steel, in the converter smelting process, the converter tapping basicity is controlled at 2.5-4.5, and the temperature is controlled at 1610℃-1660℃; when the tapping amount reaches 1 / 4 of the total tapping amount, alloys are added to the ladle for deoxidation alloying, and then lime is added for preliminary refining; the bottom blowing gas volume is controlled to prevent the liquid surface from churning; slagging from the converter is strictly prohibited; after tapping, the sulfur content in the ladle molten steel is lower than 180 ppm; For the LF refining process, the heating time is concentrated to avoid nitrogen increase due to multiple electrode heating. The number of power-on times is 1 - 3 times, and the LF refining time is controlled within 45 minutes. Calcium treatment is not carried out. The sulfur content in the molten steel before leaving the LF is less than 15 ppm.

[0011] For the RH refining process, during the RH vacuum treatment, the vacuum degree is less than 1.2 mbar. Bottom blowing argon is prohibited from the ladle entering the station to the breaking of the vacuum. After the vacuum is broken and the ladle is lowered to a low position, bottom blowing argon operation is carried out, and strong stirring is strictly prohibited.

[0012] In the present invention, the RH vacuum circulation time is extended to ensure that the RH circulation time ≥ 30 min. The purpose is to ensure that high melting point Al 2 O 3 、Al 2 O 3 -CaO-MgO and other oxide inclusions float up and are removed sufficiently to reduce the number of inclusions after calcium treatment. Different from the traditional process of feeding a sufficient amount of calcium wire, the present invention reduces the amount of calcium wire fed. The purpose is to reduce CaO-Al 2 O 3 -based liquid inclusions and CaO-CaS-based inclusions, and control the inclusions to be CaS-Al 2 O 3 -based, reducing large-size chain-like inclusions on the rolled material. Different from the traditional process of large gas volume and long-time soft blowing process, this process reduces the RH soft blowing gas volume after calcium treatment and reduces the RH soft blowing time after calcium treatment. The purpose is to avoid the aggregation and growth of tiny CaO-Al 2 O 3 -based liquid inclusions in the molten steel while ensuring the sufficient floating up of solid inclusions.

[0013] In the present invention, there are no large-size CaO-Al 2 O 3 -based inclusions in the molten steel, and the inclusions are mainly Al 2 O 3 -CaS-based inclusions; after hot rolling, the inclusions are small particles of Al 2 O 3 -CaS distributed dispersedly.

[0014] Mechanism analysis of the present invention: Aluminum is added for deoxidation after the converter. Therefore, the inclusions before the LF enters the station are mainly Al 2 O 3 . After the LF refining is completed, due to the addition of alloys and a certain amount of calcium in the alloys, the inclusions approach the CaO-Al 2 O 3 region. After the RH vacuum treatment is completed, the number of inclusions decreases significantly. At the same time, due to the volatilization of calcium during the circulation process, the inclusions tend to Al2 O 3 Approach the direction. After the RH vacuum treatment, calcium treatment is carried out. Under the traditional process, the calcium treatment process provides sufficient calcium to modify MnS and Al 2 O 3 inclusions. Therefore, the inclusions after calcium treatment are mainly CaO-Al 2 O 3 -CaS, and the morphology is that CaS wraps or adheres to the outside of CaO-Al 2 O 3 inclusions. The inclusions in the continuous casting billet are still mainly CaO-Al 2 O 3 -CaS. However, due to secondary oxidation and calcium volatilization during the casting process, the content of CaO in the inclusions slightly decreases. The inclusions in the rolled steel plate are similar to those in the billet, mainly CaO-Al 2 O 3 -CaS.

[0015] The traditional process believes that sufficient calcium treatment can achieve the transformation of MnS to CaS, and at the same time, the high-melting-point Al 2 O 3 inclusions in the molten steel can be modified into liquid CaO-Al 2 O 3 inclusions, improving the castability of the molten steel; but the finally formed CaO-Al 2 O 3 -CaS inclusions, with a low-melting-point CaO-Al 2 O 3 inclusion at its core, is prone to deformation during rolling, forming large-sized string-like type B inclusions, resulting in unqualified steel plate properties.

[0016] The traditional process believes that after calcium treatment, sufficient soft blowing time should be ensured to ensure the full floating of inclusions; however, it is observed under a high-temperature microscope that compared with solid inclusions, liquid inclusions are less likely to aggregate. Therefore, too long soft stirring will cause the aggregation and growth of liquid inclusions in the molten steel. Therefore, too long soft stirring will cause the size of liquid CaO-Al 2 O 3 inclusions to increase. Large-sized liquid inclusions form large-sized string-like type B inclusions after rolling, resulting in unqualified steel plate properties.

[0017] The present invention subverts the traditional inclusion control concept: (1) In the traditional process, sufficient calcium is fed. In this process, the amount of calcium wire added is reduced, and the calcium content in the tundish molten steel is controlled within the range of 5-10 ppm, forming Al 2 O 3-CaS-based high-melting-point target inclusions. (2) The traditional process extends the soft blowing time after calcium treatment to remove inclusions after calcium treatment; this process focuses on the control points before calcium treatment, that is, increasing the RH vacuum circulation time before calcium treatment and reducing Al 2 O 3 oxide-like inclusions, aiming to reduce the nucleation cores of calcium sulfide precipitation and reduce the inclusion content in the steel grade. (3) The traditional process extends the soft blowing time after calcium treatment to remove inclusions after calcium treatment; this process appropriately reduces the pure circulation time and soft blowing gas volume of RH after calcium treatment to avoid the aggregation and growth of tiny CaO-Al 2 O 3 -based liquid inclusions in the molten steel.

[0018] The beneficial effects of the present invention are as follows: (1) There are no large-sized CaO-Al 2 O 3 -based and CaS-CaO-based inclusions in the molten steel of the present invention. The inclusions in the molten steel are mainly CaS-Al 2 O 3 -based inclusions, reducing the content of type B inclusions in the molten steel, which is beneficial to the control of type B inclusions in the steel.

[0019] (2) The large-sized strip-shaped inclusions after hot rolling of the present invention are significantly improved, and most type B inclusions are inspected as grade 0.

[0020] (3) Compared with the traditional process, the number density and average size of inclusions of the present invention are significantly reduced, improving the cleanliness of molten steel and product quality; at the same time, reducing the consumption of calcium wire in the production process and lowering the production cost. Description of the Drawings

[0021] Figure 1 shows the composition distribution of inclusions in the rolled material of Example 1; Figure 2 shows the composition distribution of inclusions in the rolled material of Comparative Example 1. Detailed Embodiments

[0022] The technical solution of the present invention will be further described below through specific examples. Example 1

[0023] The steel grade for smelting is NM400, and the furnace capacity is 260 tons, including the following steps: (1) Converter smelting: The basicity of the converter tapping is 3.61, and the temperature is 1645°C; after tapping, the sulfur content of the molten steel in the ladle is 150 ppm.

[0024] (2) LF refining: The LF refining time is 40 min, and the power supply is carried out 2 times; no calcium treatment is carried out; the sulfur content of the molten steel before leaving the LF is 15 ppm.

[0025] (3) RH refining: The RH vacuum time is 35 min.

[0026] (4) Calcium treatment: 120 m of calcium wire is fed in and argon blowing and stirring are carried out.

[0027] (5) Soft blowing: The total soft blowing gas volume is 180 L / min and the soft blowing time is 12 min.

[0028] (6) After soft blowing, the molten steel is lifted to the continuous casting process for casting, and the calcium content in the tundish molten steel is 10 ppm. Example 2

[0029] The steel grade to be smelted is X80, the furnace capacity is 270 tons, and it includes the following steps: (1) Converter smelting: The converter tapping basicity is 4.5 and the temperature is 1660 °C; after tapping, the sulfur content in the ladle molten steel is 180 ppm.

[0030] (2) LF refining: The LF refining time is 43 min, and the power supply is switched on 2 times; no calcium treatment is carried out; the sulfur content in the molten steel before LF tapping is 5 ppm.

[0031] (3) RH refining: The RH vacuum time is 32 min.

[0032] (4) Calcium treatment: 110 m of calcium wire is fed in and argon blowing and stirring are carried out.

[0033] (5) Soft blowing: The total soft blowing gas volume is 150 L / min and the soft blowing time is 10 min.

[0034] (6) After soft blowing, the molten steel is lifted to the continuous casting process for casting, and the calcium content in the tundish molten steel is 10 ppm. Example 3

[0035] The steel grade to be smelted is NM450, the furnace capacity is 268 tons, and it includes the following steps: (1) Converter smelting: The converter tapping basicity is 2.5 and the temperature is 1610 °C; after tapping, the sulfur content in the ladle molten steel is 136 ppm.

[0036] (2) LF refining: The LF refining time is 45 min, and the power supply is switched on 3 times; no calcium treatment is carried out; the sulfur content in the molten steel before LF tapping is 7 ppm.

[0037] (3) RH refining: The RH vacuum time is 37 min.

[0038] (4) Calcium treatment: 110 m of calcium wire is fed in and argon blowing and stirring are carried out.

[0039] (5) Soft blowing: The total soft blowing gas volume is 100 L / min and the soft blowing time is 8 min.

[0040] After the soft blowing is completed, the molten steel is lifted to the continuous casting process for casting, and the calcium content in the tundish molten steel is 6 ppm. Example 4

[0041] The steel grade to be smelted is X70, and the furnace capacity is 120 tons, including the following steps: (1) Converter smelting: The basicity of the converter tapping is 2.9, and the temperature is 1641 °C; after the tapping is completed, the sulfur content in the molten steel in the ladle is 145 ppm.

[0042] (2) LF refining: The LF refining time is 35 min, and the number of power-on times is 3; no calcium treatment is carried out; the sulfur content in the molten steel before leaving the LF is 5 ppm.

[0043] (3) RH refining: The RH vacuum time is 33 min.

[0044] (4) Calcium treatment: 80 meters of calcium wire is fed in, and argon blowing and stirring are carried out.

[0045] (5) Soft blowing: The total soft blowing gas volume is 80 L / min, and the soft blowing time is 7 min.

[0046] (6) After the soft blowing is completed, the molten steel is lifted to the continuous casting process for casting, and the calcium content in the tundish molten steel is 5 ppm. Example 5

[0047] The steel grade to be smelted is X65, and the furnace capacity is 130 tons, including the following steps: (1) Converter smelting: The basicity of the converter tapping is 3.13, and the temperature is 1650 °C; after the tapping is completed, the sulfur content in the molten steel in the ladle is 150 ppm.

[0048] (2) LF refining: The LF refining time is 35 min, and the number of power-on times is 3; no calcium treatment is carried out; the sulfur content in the molten steel before leaving the LF is 10 ppm.

[0049] (3) RH refining: The RH vacuum time is 35 min.

[0050] (4) Calcium treatment: 90 meters of calcium wire is fed in, and argon blowing and stirring are carried out.

[0051] (5) Soft blowing: The total soft blowing gas volume is 100 L / min, and the soft blowing time is 8 min.

[0052] (6) After the soft blowing is completed, the molten steel is lifted to the continuous casting process for casting, and the calcium content in the tundish molten steel is 7 ppm.

[0053] Comparative Example 1

[0054] The steel grade to be smelted is NM400, and the furnace capacity is 270 tons, including the following steps: (1)Converter smelting: The basicity of the converter tapping is 4.37, and the temperature is 1645 °C; after tapping, the sulfur content in the molten steel in the ladle is 145 ppm.

[0055] (2)LF refining: The LF refining time is 40 min, and the number of power-on times is 2; no calcium treatment is carried out; the sulfur content in the molten steel before the LF tapping is 9 ppm.

[0056] (3)RH refining: The RH vacuum time is 20 min.

[0057] (4)Calcium treatment: 300 meters of calcium wire is fed in, and argon blowing and stirring are carried out.

[0058] (5)Soft blowing: The total soft blowing gas volume is 200 L / min, and the soft blowing time is 23 min.

[0059] (6)After soft blowing, the molten steel is lifted to the continuous casting process for casting, and the calcium content in the molten steel in the tundish is 26 ppm.

[0060] Comparative Example 2

[0061] The steel grade to be smelted is X80, and the furnace capacity is 130 tons, including the following steps: (1)Converter smelting: The basicity of the converter tapping is 2.73, and the temperature is 1654 °C; after tapping, the sulfur content in the molten steel in the ladle is 159 ppm.

[0062] (2)LF refining: The LF refining time is 40 min, and the number of power-on times is 2; no calcium treatment is carried out; the sulfur content in the molten steel before the LF tapping is 7 ppm.

[0063] (3)RH refining: The RH vacuum time is 15 min.

[0064] (4)Calcium treatment: 200 meters of calcium wire is fed in, and argon blowing and stirring are carried out.

[0065] (5)Soft blowing: The total soft blowing gas volume is 200 L / min, and the soft blowing time is 20 min.

[0066] (6)After soft blowing, the molten steel is lifted to the continuous casting process for casting, and the calcium content in the molten steel in the tundish is 23 ppm.

[0067] As can be seen from Appendix Figure 1 、 2 In the present invention, the inclusions are mainly Al 2 O 3 -CaS system with high melting point, which is not easily deformed at the rolling temperature due to its high melting point during the rolling process; while in the original process, the inclusions are mainly liquid CaO-Al 2 O 3 inclusions and CaO-Al 2 O 3-CaS inclusions. Such inclusions are prone to deformation into large-sized string-like type B inclusions at the rolling temperature. Moreover, the number of inclusions in the present invention is significantly lower than that in the original process. After calculation, the inclusion number density in the present invention is 15 per mm 2 , and the inclusion number density under the original process is 24 per mm 2 .

[0068] Table 1 shows the comparison of inclusion rating results in the hot-rolled plates of the examples and the comparative examples.

[0069] Table 1 Inclusion Rating Results in Hot-Rolled Plates

[0070] As can be seen from Table 1, the type B rating results of the rolled plates in the examples are all grade 0, which is significantly better than the inclusion ratings in the comparative examples. Thus, compared with the conventional calcium treatment process, the control of string-like type B inclusions in the ultra-low sulfur steel products produced by the process of the present invention is good.

[0071] In summary, the present invention controls type B inclusions in ultra-low sulfur steel, so that there are no large-sized CaO-Al 2 O 3 inclusions and CaO-Al 2 O 3 -CaS inclusions in the molten steel. The inclusions are mainly Al 2 O 3 -CaS-based with high melting points. At the same time, the number of inclusions is also significantly reduced, and the cleanliness of the molten steel is significantly improved. The large-sized string-like type B inclusions are significantly improved, and the vast majority of the type B inclusion inspections are grade 0.

Claims

1. A method for controlling type B inclusions in ultra-low sulfur steel, the production process comprising: Hot metal treatment → converter smelting → LF refining → RH refining → calcium treatment and soft blowing → continuous casting → hot rolling, characterized by: In the calcium treatment and soft blowing process, after the RH is completed, the calcium line is fed for calcium treatment to reduce the calcium content in the molten steel, and the calcium content in the molten steel in the middle package is controlled to be 5-10ppm; the total gas volume of the soft blowing is controlled to be 80-180NL / min, and the soft blowing time is controlled to be 7-12min.

2. The method for controlling type B inclusions in ultra-low sulfur steel according to claim 1, characterized in that: In the RH refining process, the RH vacuum cycle time is ≥30 min.

3. The method for controlling type B inclusions in ultra-low sulfur steel according to claim 1, characterized in that: In the converter smelting process, the basicity of the converter steel is controlled at 2.5-4.5, and the temperature is controlled at 1610°C-1660°C; when the steel tapping reaches 1 / 4 of the total steel tapping, alloy is added to the ladle for deoxidation and alloying, and then lime is added for pre-refining; the bottom blowing volume is controlled to prevent the liquid surface from churning; slag discharge from the converter is strictly prohibited; after the steel tapping is completed, the sulfur content of the ladle steel liquid is lower than 180ppm.

4. The method for controlling type B inclusions in ultra-low sulfur steel according to claim 1, characterized in that: In the LF refining process, the power supply times of the LF refining process are 1-3 times, and the LF refining time is controlled within 45 minutes; no calcium treatment is performed; and the sulfur content of the molten steel before the LF leaves the station is less than 15ppm.

5. The method for controlling type B inclusions in ultra-low sulfur steel according to claim 1, characterized in that: In the RH refining process, the vacuum degree of the RH vacuum treatment process is lower than 1.2 mbar; bottom argon blowing is prohibited from the time when the ladle enters the station to the time when the air is broken. After the air is broken, the bottom argon blowing operation is carried out when the ladle drops to a low level, and strong stirring is strictly prohibited.

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