A smelting method for controlling b-type inclusions in high-carbon chromium bearing steel

Abstract

The application discloses a smelting method for controlling B type inclusions in high-carbon chromium bearing steel, and belongs to the technical field of metallurgy. The smelting method comprises a converter, LF refining, RH refining and continuous casting processes. The sulfur content at the end of the converter smelting process is controlled to be S≤0.045%, the white slag with a reduction degree of 5-7 is used to remove sulfur in the LF refining process, and the Als content in the molten steel at the LF outlet is ≤0.015%. The Als content in the molten steel at the RH refining process outlet is ≤0.010%, and a casting blank is obtained through casting. The aluminum deoxidizer is added in advance, the time for the refining slag to absorb inclusions is prolonged, the desulfurization efficiency is improved, the Als content in the molten steel is controlled, and the Al2O3 inclusion content is reduced. The high vacuum time and the soft blowing time are prolonged in the RH, which is beneficial to the further collision and growth of the inclusions so as to float and remove the inclusions, the molten steel is more purified, the B type inclusions in the final casting blank are ≤4.8 / mm 2 , the B coarse is ≤0.5 grade, and the B fine is ≤1 grade.

Description

Technical Field

[0001] This invention belongs to the field of metallurgical technology, specifically relating to a smelting method for controlling Class B inclusions in high-carbon chromium bearing steel. Background Technology

[0002] The addition of aluminum as a deoxidizer leads to the formation of high-melting-point Al2O3 inclusions in the molten steel, reducing its purity and hindering product performance improvement. Furthermore, due to the stringent control of calcium levels in bearings, it is impossible to modify these high-melting-point Al2O3 inclusions. Consequently, bearing steel often experiences phenomena such as stopper rise and crystallizer level fluctuations during continuous casting. In severe cases, nozzle clogging during casting can cause interruptions in continuous casting, resulting in scrap being returned to the furnace. Therefore, improving the smelting process and reducing the amount of high-melting-point inclusions in the molten steel plays a crucial role in improving product quality and fluidity. Summary of the Invention

[0003] The technical problem to be solved by the present invention is to provide a smelting method for controlling Class B inclusions in high carbon chromium bearing steel. The present invention can reduce the amount of Al2O3 inclusions generated, prolong the time for inclusions to float and be removed, and clean the molten steel. The austenitic nickel-based alloy obtained by the method of the present invention has uniform composition, high purity and excellent performance.

[0004] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is: a smelting method for controlling Class B inclusions in high-carbon chromium bearing steel, wherein the smelting method includes converter, LF refining, RH refining and continuous casting processes, and the steps of each process are as follows:

[0005] (1) Converter process: The sulfur content at the end of smelting is controlled at S≤0.045%. During the tapping process, aluminum blocks are added at a ratio of 1.2 to 1.5 kg / ton of steel for deoxidation. The amount of aluminum blocks added for each batch of molten steel is adjusted according to the oxygen content at the end of the process. Slag addition is strictly prohibited.

[0006] (2) LF refining process: lime is added for submerged arc heating, no aluminum deoxidizer is added, and reducing white slag with an alkalinity of 5-7 is generated for desulfurization. The temperature is raised to 1575-1585℃ by submerged arc heating. 2.1-3.2 kg / t silicon carbide is added in the middle and late stages of smelting to continue deoxidation and maintain white slag. The Al content in the molten steel leaving the LF station is controlled at ≤0.015%.

[0007] (3) RH refining process: Degassing and deoxidation to remove inclusions under high vacuum, without adding aluminum products, high vacuum time ≥20min, and control the Al content in the RH molten steel ≤0.010%; after RH returns to atmospheric pressure, no calcium treatment is performed, and control the soft blowing time ≥20min.

[0008] (4) Continuous casting process: casting to obtain a billet.

[0009] In the LF refining process described in this invention, the amount of lime added is 10-12 kg / ton of steel, and the amount of silicon carbide added is 2.1-3.2 kg / ton of steel.

[0010] The RH refining process described in this invention controls the soft blowing gas volume to 5-10 Nm. 3 / h.

[0011] The continuous casting process described in this invention employs a ladle long nozzle and an immersion nozzle under micro-positive pressure for fully oxidation-free casting.

[0012] In the continuous casting process described in this invention, the superheat of the tundish is controlled within the range of 15 to 35°C, and the casting speed is controlled at 0.9 ± 0.1 m / min.

[0013] In the converter smelting process described in this invention, the purity of the aluminum blocks is ≥98%.

[0014] The B-type inclusions in the cast billet described in this invention are ≤4.8 per mm. 2 B coarse ≤ 0.5 grade, B fine ≤ 1 grade.

[0015] The design concept of this invention is as follows:

[0016] Aluminum deoxidation, a type of precipitation deoxidation, is a commonly used deoxidation method in the smelting of bearing steel. Aluminum is a deoxidizer with a strong affinity for oxygen, and it can directly react with oxygen in molten steel to form Al2O3. Al2O3 then floats from the molten steel to the steel-slag interface and is absorbed by the refining slag, thus achieving deoxidation. The advantages of using aluminum precipitation deoxidation are its simple operation and fast reaction speed. However, this method also has disadvantages, because after aluminum deoxidation, not all inclusions in the steel can float to the slag; a small portion remains in the molten steel.

[0017] Aluminum is a strong deoxidizer for bearing steel. When added to molten steel, most of the aluminum reacts with the oxygen in the steel to form Al2O3, a small portion of the aluminum combines with nitrogen to form aluminum nitride, and another portion forms metallic solid solution aluminum in the steel. Aluminum nitride and metallic solid solution aluminum are collectively referred to as acid-soluble aluminum.

[0018] The reaction equation between aluminum and dissolved oxygen in steel is:

[0019] 2[Al] + 3[O] = Al₂O₃(S)

[0020]

[0021] In the formula Therefore, we can conclude that:

[0022] Therefore, it can be concluded that increasing the amount of aluminum particles added during the converter tapping process and carrying out precipitation deoxidation in advance is beneficial to rapid desulfurization in refining and improves deoxidation efficiency.

[0023] During the RH process, based on the content of each element in the bearing steel and the interaction coefficients of the activity of each element, the activity coefficients of C, Al, and O can be calculated as lgfC=0.082, lgfAl=0.132, and lgfo=-0.747.

[0024] 2[Al]+3[O]=Al2O3(s) Δ Gθ=-1225000+393.8T (J / mol)

[0025] [C]+[O]=CO(g) Δ Gθ=-22200-38.34T (J / mol)

[0026] The inlet temperature of the RH furnace is generally between 1560-1570℃, and the outlet temperature is between 1515-1525℃. The vacuum degree is calculated based on 100Pa for inlet and outlet, with the inlet temperature calculated at 1560℃. ΔG(Al2O3) = -26575.3J / mol; ΔG(CO) = -68516.8J / mol; the outlet time is calculated based on 1515℃.

[0027] ΔG (Al2O3) = -55996.6J / mol; ΔG (CO) = -67379.8J / mol;

[0028] Therefore, within the temperature range of the RH refining process, ΔG(Al₂O₃) < ΔG(CO) always exists. Furthermore, in actual production, the high vacuum level is approximately 30-60 Pa, and the aluminum content in the molten steel is approximately 0.015%-0.025%. Consequently, the oxidation reaction of carbon is always much stronger than that of aluminum. Therefore, the reducing properties of carbon in the vacuum are utilized to produce CO bubbles, accelerating the removal of inclusions. Based on this principle, the RH vacuum furnace increases the high vacuum time to over 20 minutes, which can further reduce the total oxygen content in the steel.

[0029] This method reduces the Al content in molten steel by altering the timing and quantity of aluminum product addition, thereby promoting further deoxidation and inclusion removal during RH refining. This aims to reduce the number of Class B inclusions, extend the inclusion flotation and removal time, and ultimately improve the fluidity of the molten steel. Compared to the original process, which involves adding aluminum products to the LF furnace for aluminum deoxidation and desulfurization, controlling the Al content within the range of 0.020–0.04%, and using RH degassing to remove some inclusions, this method allows for further control over the number of Class B inclusions.

[0030] The beneficial effects of adopting the above technical solution are as follows: This invention, by adding aluminum deoxidizer in advance during the converter process, facilitates pre-precipitation deoxidation, which is beneficial for rapid desulfurization in refining and improves deoxidation efficiency. In the LF refining process, no aluminum deoxidizer is added, controlling the Al content in the molten steel and reducing the formation of Al2O3 inclusions. In the RH vacuum refining process, inclusions are removed by degassing and deoxidation under high vacuum. Extending the high vacuum time facilitates inclusion removal; after negative pressure is applied, the soft blowing gas volume is controlled at 5-10 Nm³. 3 / h, time ≥20min, further remove inclusions. In the production process, aluminum deoxidizer is added in advance in the converter process to carry out precipitation deoxidation in advance, which improves the deoxidation efficiency. In the LF refining process, no aluminum deoxidizer is added to control the Al content in the molten steel and reduce the formation of Al2O3 inclusions. The RH vacuum extends the high vacuum time, which is conducive to the removal of inclusions and makes the molten steel cleaner. Detailed Implementation

[0031] The present invention will now be described in further detail with reference to specific embodiments.

[0032] Examples 1-6

[0033] This invention employs the method of adding aluminum deoxidizer to the converter in advance for precipitation deoxidation, controlling the Al content in the refining LF to reduce Al2O3 inclusions, and extending the high vacuum time in the RH vacuum to facilitate the removal of inclusions, thus making the molten steel cleaner.

[0034] A smelting method for controlling Class B inclusions in high-carbon chromium bearing steel includes converter, LF refining, RH refining, and continuous casting processes, the steps of which are described below:

[0035] (1) Converter process: The sulfur content at the end of smelting is controlled at S≤0.045wt%. During the tapping process, aluminum blocks with a purity ≥98% are added at a ratio of 1.2~1.5kg / ton of steel for deoxidation. The amount of aluminum blocks added for each batch of molten steel is adjusted according to the oxygen content at the end of the process. Slag addition is strictly prohibited.

[0036] (2) LF refining process: Within 5 minutes of the start of refining heating, 10-12 kg / ton of steel is added to the lime-submerged arc furnace for heating, creating a reducing white slag with an alkalinity of 5-7 for desulfurization. No further aluminum products are added for deoxidation. After the white slag is formed, 2.1-3.2 kg / ton of steel silicon carbide is added continuously to maintain the white slag, and the temperature is raised to 1575-1585℃. Considering that Als loss is 40-70 ppm under RH high vacuum, the Als content in the molten steel leaving the LF station should be controlled at ≤0.015%.

[0037] (3) RH refining process: After the molten steel enters the RH refining process, it undergoes vacuum degassing. After the vacuum degree is lower than 100 Pa, it is maintained for more than 20 minutes to fully promote the flotation and removal of inclusions. The Al content in the molten steel leaving the RH station is ≤0.010%; no calcium treatment is performed after the RH returns to atmospheric pressure; the soft blowing air volume is controlled at 5-10 Nm. 3 / h, soft blowing time ≥20min.

[0038] (4) Continuous casting process: Molten steel from the RH furnace is hoisted to the continuous casting machine. Before casting, the tundish is purged with argon gas for at least 3 minutes. After purging, casting begins. The long nozzle and submerged entry nozzle of the ladle are protected by a slight positive pressure for oxidation-free casting throughout the process. This significantly reduces nitrogen addition to the molten steel and effectively inhibits secondary oxidation during casting. The billet cross-section is 200mm × 200mm, the tundish superheat is 15–35℃, and the casting speed is controlled at 0.9 ± 0.1 m / min. After adopting the above process, the liquid level in the crystallizer is stable, and casting is significantly more stable. The number of Class B inclusions in the billet is ≤4.8 per mm. 2 B coarse ≤ 0.5 grade, B fine ≤ 1 grade.

[0039] Taking the 120t converter and refining of GCr15 steel as an example, the parameters of each process in the smelting process of each embodiment are shown in Table 1; the changes in the composition of the ladle samples in the smelting process of embodiments 1-6 are shown in Tables 2-7, and the content of Class B inclusions is shown in Table 8.

[0040] Table 1. Parameters of each step in the smelting process of each embodiment.

[0041] Example Converter final S / wt% Aluminum block addition amount / kg / ton of steel LF Refined Lime Addition Amount / kg / ton of Steel Silicon carbide addition amount / kg / ton of steel Submerged arc heating temperature / ℃ White residue alkalinity 1 0.026 1.2 10.2 2.10 1575 5.0 2 0.029 1.4 11.1 2.32 1582 5.3 3 0.028 1.3 10.8 2.38 1579 6.0 4 0.031 1.3 12.0 2.41 1580 6.5 5 0.038 1.2 10.0 3.20 1585 7.0 6 0.040 1.5 11.3 2.50 1579 6.8

[0042] Table 1 (Continued) Parameters of each step in the smelting process of each embodiment

[0043] Example High vacuum time / min RH outgoing Als / content wt% <![CDATA[Soft blowing gas flow Nm 3 / h]]> Soft blow time / min Superheat of continuous casting tundish / °C Continuous casting speed / m / min 1 22.4 0.0084 8.2 20 28 0.90 2 20.7 0.0095 7.8 21 29 0.95 3 24.1 0.0083 5.0 22 32 1.00 4 22.6 0.0094 8.7 20 30 0.85 5 21.7 0.0071 9.0 23 35 0.80 6 22.9 0.0084 10 24 15 1.00

[0044] Table 2. Composition (wt%) of ladle samples from each step of the smelting process in Example 1.

[0045] Example 1 C Si Mn Cr P S Als w(TO) / ppm Ingredient requirements 0.95～1.00 0.20～0.30 0.30～0.45 1.40～1.55 ≤0.020 ≤0.015 ≤0.015 / Converter output composition 0.90 0.18 0.28 1.37 0.013 0.025 0.09 / LF outbound components 0.96 0.22 0.33 1.42 0.014 0.0021 0.018 / RH high vacuum 10min 0.95 0.22 0.33 1.43 0.015 0.0018 0.012 6.6 RH high vacuum 20min 0.96 0.21 0.32 1.41 0.016 0.0017 0.0098 5.5 RH soft blow 0.96 0.22 0.33 1.42 0.015 0.0018 0.0084 4.9 Billet composition 0.97 0.22 0.32 1.41 0.015 0.0014 0.0081 4.7

[0046] Table 3. Composition (wt%) of ladle samples from each step of the smelting process in Example 2.

[0047] Example 2 C Si Mn Cr P S Als w(TO) / ppm Ingredient requirements 0.95～1.00 0.20～0.30 0.30～0.45 1.40～1.55 ≤0.020 ≤0.015 ≤0.015 / Converter output composition 0.92 0.19 0.29 1.38 0.015 0.027 0.089 / LF outbound components 0.95 0.21 0.33 1.43 0.016 0.0029 0.017 / RH high vacuum 10min 0.95 0.22 0.33 1.43 0.015 0.0022 0.012 7.2 RH high vacuum 20min 0.95 0.21 0.34 1.42 0.016 0.0019 0.0095 6.6 RH soft blow 0.96 0.22 0.33 1.43 0.016 0.0018 0.0095 5.3 Billet composition 0.97 0.21 0.33 1.42 0.015 0.0017 0.0088 5.2

[0048] Table 4. Composition (wt%) of ladle samples from each step of the smelting process in Example 3.

[0049] Example 3 C Si Mn Cr P S Als w(TO) / ppm Ingredient requirements 0.95～1.00 0.20～0.30 0.30～0.45 1.40～1.55 ≤0.020 ≤0.015 ≤0.015 / Converter output composition 0.89 0.17 0.26 1.40 0.012 0.029 0.102 / LF outbound components 0.97 0.22 0.33 1.41 0.013 0.0032 0.021 / RH high vacuum 10min 0.96 0.21 0.33 1.42 0.013 0.0029 0.014 6.1 RH high vacuum 20min 0.96 0.21 0.32 1.41 0.014 0.0027 0.0092 5.4 RH soft blow 0.96 0.22 0.33 1.42 0.013 0.0025 0.0083 5.0 Billet composition 0.96 0.21 0.32 1.41 0.013 0.0023 0.0080 4.9

[0050] Table 5. Composition (wt%) of ladle samples from each step of the smelting process in Example 4.

[0051] Example 4 C Si Mn Cr P S Als w(TO) / ppm Ingredient requirements 0.95～1.00 0.20～0.30 0.30～0.45 1.40～1.55 ≤0.020 ≤0.015 ≤0.015 / Converter output composition 0.90 0.18 0.28 1.37 0.019 0.021 0.092 / LF outbound components 0.97 0.22 0.32 1.43 0.018 0.0016 0.019 / RH high vacuum 10min 0.95 0.22 0.33 1.43 0.019 0.0019 0.013 6.8 RH high vacuum 20min 0.96 0.21 0.32 1.42 0.019 0.0017 0.0096 5.7 RH soft blow 0.96 0.22 0.31 1.41 0.018 0.0018 0.0094 5.3 Billet composition 0.96 0.23 0.31 1.40 0.018 0.0019 0.0091 5.3

[0052] Table 6. Composition (wt%) of ladle samples from each step of the smelting process in Example 5.

[0053] Example 5 C Si Mn Cr P S Als w(TO) / ppm Ingredient requirements 0.95～1.00 0.20～0.30 0.30～0.45 1.40～1.55 ≤0.020 ≤0.015 ≤0.015 / Converter output composition 0.88 0.16 0.32 1.39 0.010 0.035 0.086 / LF outbound components 0.95 0.22 0.33 1.42 0.011 0.0027 0.020 / RH high vacuum 10min 0.95 0.22 0.33 1.43 0.010 0.0024 0.010 6.9 RH high vacuum 20min 0.95 0.21 0.32 1.43 0.011 0.0027 0.0081 6.5 RH soft blow 0.96 0.22 0.33 1.42 0.011 0.0024 0.0071 5.9 Billet composition 0.97 0.23 0.32 1.41 0.12 0.0025 0.0073 5.4

[0054] Table 7. Composition (wt%) of ladle samples from each step of the smelting process in Example 6.

[0055] Example 6 C Si Mn Cr P S Als w(TO) / ppm Ingredient requirements 0.95～1.00 0.20～0.30 0.30～0.45 1.40～1.55 ≤0.020 ≤0.015 ≤0.015 / Converter output composition 0.91 0.19 0.26 1.35 0.012 0.028 0.093 / LF outbound components 0.97 0.22 0.33 1.42 0.013 0.0026 0.016 / RH high vacuum 10min 0.96 0.22 0.33 1.43 0.013 0.0025 0.014 6.1 RH high vacuum 20min 0.96 0.23 0.32 1.43 0.012 0.0022 0.0092 5.7 RH soft blow 0.96 0.22 0.32 1.42 0.013 0.0027 0.0084 5.1 Billet composition 0.97 0.23 0.31 1.41 0.013 0.0024 0.0083 5.0

[0056] Tables 2 to 7 show that adding sufficient aluminum products in advance for precipitation and deoxidation in the converter process helps the LF process to form slag quickly and improves desulfurization efficiency. In the RH process, under high vacuum, the vigorous flow of molten steel in the vacuum tank helps inclusions grow, float, and be removed, further reducing the total oxygen content.

[0057] To compare the levels of micro-inclusions in molten steel under the two process conditions, cake samples were taken from Examples 1-6 for analysis, and metallographic specimens were prepared. After mounting and polishing, the metallographic specimens were analyzed for type B micro-inclusions under a spectroscopic microscope and a metallographic microscope at 500x magnification. The number of type B micro-inclusions in each specimen was counted, and the equivalent diameter was taken as 7.5 μm, as shown in Table 8.

[0058] Table 8. Content of Class B inclusions (number / mm) in Examples 1-6 2 )

[0059] Serial Number LF Exit RH high vacuum 10min RH high vacuum 20min RH soft blow Invention casting billet Original process casting billet Example 1 8.2 6.8 6.0 4.7 4.4 6.4 Example 2 7.6 6.2 5.6 3.8 3.7 5.9 Example 3 7.4 6.4 5.3 3.9 4.1 5.8 Example 4 7.9 7.2 6.4 5.1 4.6 6.6 Example 5 6.6 5.8 5.1 4.5 4.4 5.9 Example 6 8.7 8.1 6.4 5.4 4.8 7.2

[0060] Table 9. Class B inclusion levels (grades) in Examples 1-6

[0061] Serial Number B (rough) B Fine Example 1 0.5 1 Example 2 0 0.5 Example 3 0 0.5 Example 4 0.5 1 Example 5 0.5 0.5 Example 6 0.5 1

[0062] Table 8 shows that the present invention, by adding sufficient aluminum products in advance during the converter process for pre-precipitation and deoxidation, facilitates rapid slag formation in the LF process and adsorption of inclusions. In the LF process, no more aluminum products are added, and the Al content in the molten steel is controlled at 0.010–0.020%, reducing the formation of Al2O3 inclusions and lowering the inclusion content at the LF outlet. During the high-vacuum refining process in the RH process, the molten steel flows violently, which facilitates further collision and growth of inclusions, leading to their removal by flotation. Controlling the Al content at the RH outlet to ≤0.010% prevents the molten steel from being oxidized during casting to form Al2O3 inclusions, thus reducing the content of Class B inclusions in the billet. Tables 8 and 9 show that the billet produced by the present invention has higher cleanliness, reducing Class B inclusions by 30.4% compared to the previous process, with Class B inclusions in the billet ≤4.8 per mm. 2The B coarse grade is ≤0.5, and the B fine grade is ≤1. This indicates that the B-type inclusions in the high-carbon chromium bearing steel have been further controlled.

[0063] The above embodiments are only used to illustrate and not limit the technical solutions of the present invention. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the present invention without departing from the spirit and scope of the present invention. Any modifications or partial substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A smelting method for controlling Class B inclusions in high-carbon chromium bearing steel, characterized in that, The smelting method includes converter, LF refining, RH refining and continuous casting processes, and the steps of each process are as follows: (1) Converter process: The sulfur content at the end of smelting is controlled at S≤0.045%. During the tapping process, aluminum blocks are added at a ratio of 1.2 to 1.5 kg / ton of steel for deoxidation. The amount of aluminum blocks added for each batch of molten steel is adjusted according to the oxygen content at the end of the process. Slag addition is strictly prohibited. The purity of the aluminum blocks is ≥98%. (2) LF refining process: lime is added for submerged arc heating, no aluminum deoxidizer is added, and reducing white slag with an alkalinity of 5-7 is generated for desulfurization. The temperature is raised to 1575-1585℃ by submerged arc method. 2.1-3.2 kg / t silicon carbide is added in the middle and late stages of smelting to continue deoxidation and maintain white slag. The Al content in the molten steel leaving the LF station is controlled at ≤0.015%. The amount of lime added is 10-12 kg / ton of steel, and the amount of silicon carbide added is 2.1-3.2 kg / ton of steel. (3) RH refining process: Degassing and deoxidation to remove inclusions under high vacuum, without adding aluminum products, high vacuum time ≥20min, and control the Al content in the RH molten steel ≤0.010%; after RH returns to atmospheric pressure, no calcium treatment is performed, and control the soft blowing time ≥20min. (4) Continuous casting process: casting to obtain the billet; using micro-positive pressure protection for casting with long nozzles and submerged nozzles in the ladle for full-process oxidation-free casting; the superheat of the tundish is controlled within the range of 15 to 35℃, and the casting speed is controlled at 0.9±0.1m / min.

2. The smelting method for controlling Class B inclusions in high-carbon chromium bearing steel according to claim 1, characterized in that, In the RH refining process, the soft blowing volume is controlled at 5-10 Nm. 3 / h.

3. A smelting method for controlling Class B inclusions in high-carbon chromium bearing steel according to claim 1 or 2, characterized in that, The number of Class B inclusions in the cast billet is ≤4.8 per mm. 2 B coarse ≤ 0.5 grade, B fine ≤ 1 grade.

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