Production method of high-cleanliness ultra-low carbon series steel

Abstract

The invention discloses a production method of high-cleanliness ultra-low carbon series steel. The production method comprises the steps of deep desulfurization of KR molten iron, converter smelting, LF refining, RH furnace and continuous casting. In order to improve the control level of the inclusions in the steel, analysis is carried out by combining generation and removal of the inclusions in actual production processes such as a converter process, an RH refining process and a casting machine process, a removal process for reducing generation of the inclusions in the production process and optimizing the inclusions is formulated, and secondary oxidation in the casting process of a casting machine is prevented. And an ultra-low-carbon high-cleanliness converter end point control process, an RH refining ultra-low-carbon deoxidation process, an RH refining top slag modification process, casting machine castability research and casting blank quality control are researched. And high-cleanliness stable control over the low inclusions of the ultra-low-carbon high-cleanliness steel is achieved.

Description

Technical Field

[0001] The present invention belongs to the technical field of steelmaking processes in iron and steel metallurgy, and particularly relates to a production method for high-purity ultra-low-carbon series steel. Background Art

[0002] In recent years, China has made remarkable progress in the field of high-purity ultra-low-carbon series steel. Large domestic steel enterprises have continuously improved production processes and product quality through technology introduction, independent research and development, and innovation. Some enterprises have been able to produce high-purity industrial pure iron that meets the needs of domestic high-end manufacturing, and occupy a certain share in the domestic market. At the same time, China has also increased investment in related technology research and development, such as pure steel smelting technology and continuous casting process optimization, to improve the purity and quality stability of products. High-purity ultra-low-carbon steel has excellent soft magnetic properties, electrical conductivity, thermal properties, and corrosion resistance. With the rapid development of high-end manufacturing, such as the aerospace, precision instrument, and electronic information industries, the demand for high-purity industrial pure iron is increasing.

[0003] The research purpose of this project is to fill the market gap, provide high-performance materials that meet the special needs of these fields, and improve the self-sufficiency rate of domestic high-end materials. Adapt to the diverse needs of different industries for the specifications and shapes of industrial pure iron, develop products with various sizes and cross-sectional shapes, and provide customized solutions for customers. Summary of the Invention

[0004] The purpose of the present invention is to provide a production method for high-purity ultra-low-carbon series steel, aiming to produce products with extremely low impurity content and high purity. By optimizing the steelmaking continuous casting production process, precisely controlling the contents of components C, Mn, P, S, inclusions, and gases, improving the uniformity and stability of products, and meeting the strict requirements of high-end manufacturing for raw material quality.

[0005] To solve the above technical problems, the present invention adopts the following technical solutions:

[0006] The production process of high-purity ultra-low-carbon steel grades has relatively high requirements for the control of inclusions in steel, the contents of main elements in steel, and castability. The high-purity control of products is often affected by the control of inclusions in steel and the deoxidation process. Therefore, the control of inclusions and the contents of main factors in steel becomes the key to restricting the improvement of the finished product qualification rate. To improve the control level of inclusions inside the steel quality, combined with the generation and removal of inclusions in the actual production processes such as the converter process, RH refining process, and continuous casting machine process, an analysis is carried out, and a process for reducing the generation of inclusions during production and optimizing the removal process of inclusions is formulated to prevent secondary oxidation during the casting process of the continuous casting machine. Research on the end-point control process of ultra-low-carbon high-purity converters, the ultra-low-carbon deoxidation process of RH refining, the top slag modification process of RH refining, the castability research of continuous casting machines, and the quality control of continuous casting billets. Realize the stable control of high purity with lower inclusions for ultra-low-carbon high-purity steel.

[0007] A production method of steel for high cleanliness ultra-low carbon series of the present invention includes KR deep desulfurization of hot metal → converter smelting → LF refining → RH furnace → continuous casting; it is characterized in that the control of each process is as follows:

[0008] KR process:

[0009] Study the KR deep desulfurization process to ensure that the S content of the hot metal entering the furnace is low enough, and avoid excessive sulfur brought in by scrap steel and furnace charges. The hot metal for ironmaking needs to be treated by KR desulfurization. It is required that the sulfur content of the hot metal entering the converter is ≤0.001%, and the cleaning area of the desulfurization slag is greater than 95%;

[0010] Converter process:

[0011] The converter end-point temperature is controlled at ≥1600 °C, and the end-point oxygen content is controlled within 600 ppm;

[0012] LF refining process:

[0013] During the tapping process of the converter, no deoxidation is carried out, and the oxygen content in the LF in-place is between 380 - 420 ppm; due to the low in-place temperature, the total heating time is greater than 25 minutes. To promote the dephosphorization efficiency during the refining process and improve the submerged arc heating effect, the addition amount of quicklime during the LF refining process is between 1.5 - 1.7 t, and the addition amount of the modifier is between 1.0 - 1.2 t;

[0014] RH process:

[0015] Deep decarburization treatment is carried out in the RH furnace. The oxygen content at the end of decarburization is controlled within 300 ppm. After decarburization, aluminum is added for deoxidation and alloying to ensure that the pure degassing time of the molten steel is more than 6 minutes; through the top slag modification treatment of the converter, LF furnace and RH, the content of TFe + MnO in the RH off-position slag is controlled within 15%, which is beneficial to the floating of inclusions and further improves the cleanliness of the molten steel;

[0016] Continuous casting process:

[0017] The continuous casting machine adopts constant casting speed control, and the casting speed range is 1.0 - 1.5 m / min. The casting superheat of the continuous casting machine is controlled between 20 and 35 °C, and protective casting is carried out throughout the continuous casting process.

[0018] Furthermore, the converter achieves successful first-time carbon pulling. To ensure the control of low sulfur content at the converter end-point, all the scrap steel in the converter is used as the casting residue in the tundish. During the converter smelting process, to control the process temperature and prevent the phenomenon of high-temperature manganese reversion, the converter end-point temperature is controlled at 1600 - 1618 °C. Through the converter end-point low-temperature control, the content of residual manganese at the end-point is effectively controlled, and the manganese content supplied by the converter for LF refining meets the control requirements.

[0019] Furthermore, the oxygen content at the end of the converter is controlled at 511 - 558 ppm. Since dephosphorization occurs during the LF refining process, the oxygen content at the end of the converter needs to be controlled slightly higher to maximize the dephosphorization efficiency.

[0020] Furthermore, the converter effectively controls the phenomenon of manganese reversion in the later stage of smelting through end-point low-temperature control, large slag volume control during the process, and high oxidability control of the slag. The MnO content in the final slag is stably controlled at 4.9 - 5.2%, and the TFe content in the final slag is controlled between 24 - 29%. During the tapping process of the converter, quicklime and modifier are added to modify the top slag.

[0021] Furthermore, the dephosphorization rate reaches 56 - 72% from the tapping of the converter to the end of LF treatment, and the off-position phosphorus content in LF is ≤ 0.0040%.

[0022] Furthermore, the RH treatment process is stable, the in-position oxygen content meets the decarburization requirements, and no oxygen blowing operation is carried out during the whole smelting process.

[0023] Furthermore, quicklime and modifier are added during the LF furnace refining process to modify the top slag and reduce the TFe content in the slag; the basicity of the steel slag is controlled between 5 - 7 during the whole refining process. After the RH furnace is recompressed, 40 - 60 kg of aluminum powder and 80 - 120 kg of modifier are added to the surface of the slag to modify the top slag, and the TFe + MnO content in the off-position slag of RH is in the range of 13.80 - 14.54%.

[0024] Compared with the prior art, the beneficial technical effects of the present invention are:

[0025] Combined with the current market situation, the profit of the finished product of high-purity ultra-low-carbon series steel is about 300 yuan per ton of steel. Thus, the net profit of the finished product of high-purity ultra-low-carbon series steel is 350 yuan / ton. Calculated based on the trial production volume of 10,000 tons of the developed product, the annual net profit range = profit per ton of steel * annual output = 350 yuan / ton * 10,000 tons / year = 3.5 million / year, and the annual net profit is over 3.5 million yuan.

[0026] The implementation of this project enriches the product structure, significantly improves the profit level of the company's products, and enhances the market competitiveness of the company. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] The present invention will be further described below in conjunction with the drawings.

[0028] Figure 1 It is a pouring trend chart;

[0029] Figure 2 It is the control of Mn content during the smelting process;

[0030] Figure 3 It is the control of P content during the smelting process. DETAILED DESCRIPTION OF THE INVENTION

[0031] The following embodiments are used to specifically illustrate the content of the present invention. These embodiments are only general descriptions of the content of the present invention and do not limit the content of the present invention.

[0032] Embodiment 1:

[0033] In order to ensure that the continuously cast slabs of high-purity extra-low-carbon series steel for production can meet the user requirements, the steelmaking continuous casting process route and process control requirements are formulated. High-grade industrial pure iron has high requirements for the purity of molten steel. The requirements for the contents of carbon, manganese, phosphorus, and sulfur in the molten steel smelted this time need to meet the requirements of the quality plan.

[0034] I. KR hot metal desulfurization

[0035] For the finished products of high-purity extra-low-carbon series steel produced in this casting, a low sulfur content is required (the finished product requirement is ≤0.0050%). The KR desulfurization process adopts deep desulfurization operation. The S content of the hot metal entering the converter is all 0.001% (the requirement for the sulfur content entering the furnace is ≤0.0020%), as shown in Tables 1.1 and 1.2.

[0036] Table 1.1 Conditions of hot metal before KR desulfurization

[0037] Melting number Si (%) Mn (%) P(％) S(％) Temperature (°C) 1413 0.43 0.56 0.138 0.048 1309 1415 0.55 0.64 0.141 0.031 1344 1416 0.54 0.59 0.138 0.044 1328 1417 0.44 0.60 0.134 0.042 1279

[0038] Table 1.2 KR desulfurization process and addition amount of desulfurizer

[0039] Melting number Workstation Stirring time (min) Desulfurizer dosage (kg) KR treatment cycle (min) 1413 KR3 25 4034 65 1415 KR3 25 4116 69 1416 KR2 25 4126 54 1417 KR3 25 4004 61

[0040] Table 1.3 Conditions of hot metal entering the furnace

[0041] Melting number Si (%) Mn (%) P(％) S(％) Temperature (°C) 1413 0.37 0.57 0.141 0.001 1247 1415 0.52 0.65 0.142 0.001 1280 1416 0.50 0.60 0.137 0.001 1271 1417 0.40 0.61 0.137 0.001 1276

[0042] As can be seen from Table 1.3, for the hot metal entering the furnace in this casting, the Si content of the hot metal is in the range of 0.37 - 0.52%, and the Si content of the hot metal entering the furnace is suitable for smelting; the P content of the hot metal entering the furnace in this casting is in the range of 0.137 - 0.142%; the manganese content of the hot metal entering the furnace is relatively high, in the range of 0.57 - 0.65% (the requirement for the manganese content of the finished product is less than 0.08%).

[0043] II. Converter process treatment

[0044] The converter's first carbon tapping is successful. To ensure the control of low sulfur content at the converter end, all the scrap steel in the converter uses the casting residue from the tundish. During the converter smelting process, in order to control the process temperature and prevent the phenomenon of high-temperature manganese reversion, the converter end temperature is controlled at 1600 - 1618°C. Through the low-temperature control at the converter end, the residual manganese content at the end is effectively controlled, and the manganese content supplied to the LF refining by the converter meets the control requirements; the oxygen content at the converter end is controlled at 511 - 558 ppm. Since dephosphorization occurs during the LF refining process, the oxygen content at the converter end needs to be controlled slightly higher to ensure the maximization of dephosphorization efficiency (as shown in the following table).

[0045] Table 2.1 Temperature Drop during Tapping with Converter Endpoint Control

[0046]

[0047] Table 2.2 Chemical Composition of Tapped Steel from Converter Endpoint (%)

[0048] Melting number C Mn P S 1413 0.035 0.10 0.011 0.004 1415 0.033 0.09 0.010 0.004 1416 0.034 0.06 0.012 0.003 1417 0.033 0.06 0.011 0.003

[0049] Table 2.3 Test Results of Converter Tapped Steel Slag Samples (%)

[0050] Melting number <![CDATA[SiO2]]> CaO TFe MgO <![CDATA[Al203]]> MnO <![CDATA[P205]]> R 1413 11.1 36.3 24.19 7.88 1.95 5.2 2.32 3.3 1415 10.68 32.36 28.01 8.45 1.42 5.11 2.22 3.0 1416 10.55 35.67 25.11 7.58 1.72 5.18 2.34 3.4 1417 10.91 31.97 28.74 8.88 1.73 4.94 2.09 2.9

[0051] From Table 2.2, the composition control at the converter endpoint is relatively stable, meeting the requirements of the quality plan control; from Table 2.3, through the low-temperature control at the converter endpoint, large slag volume control during the process, and high oxidizability control of the slag, the phenomenon of manganese reversion in the later stage of smelting is effectively controlled. The MnO content in the final slag is stably controlled between 4.9% and 5.2%, and the TFe content in the final slag is controlled between 24% and 29%. During the tapping process of the converter, quicklime and modifier are added to modify the top slag.

[0052] Table 2.4 Chemical Composition of Samples after Ladle Furnace (LF) Treatment (%)

[0053] Melting number C Si Mn P S 1413 0.030 0.01 0.06 0.0076 0.0033 1415 0.030 0.01 0.06 0.0074 0.0035 1416 0.028 0.01 0.05 0.0092 0.0030 1417 0.034 0.02 0.05 0.0081 0.0029

[0054] From Table 2.4, the chemical composition of the converter steel supplied to the LF refining for the entire casting heat meets the requirements. By comparing the manganese content at the converter endpoint and in the ladle samples, when not deoxidized during the tapping process, the manganese content drops by 100 - 400 ppm during the tapping process; the P content in the ladle samples is between 74 and 92 ppm, and the drop value of the P content during the tapping process is between 25 and 32 ppm. The sulfur content of all ladle samples after the converter for the entire casting heat is less than 35 ppm.

[0055] III. LF Refining Treatment Process

[0056] 1. LF Smelting Process

[0057] During the tapping process of the converter, no deoxidation is carried out, and the oxygen content when the LF is in place is between 380 - 420 ppm. Due to the relatively low in-place temperature and the total heating time being greater than 25 minutes, to promote the dephosphorization efficiency during the refining process and improve the submerged arc heating effect, the addition amount of quicklime during the LF refining process is between 1.5 - 1.7 t, and the addition amount of modifier is between 1.0 - 1.2 t (as shown in the following table).

[0058] Table 3.1 Process Parameters during Treatment

[0059]

[0060] 2. Chemical Composition Control

[0061] Due to the low tapping temperature and the un-deoxidized molten steel, the LF heating time is relatively long. As can be seen from Table 3.2, the dephosphorization rate reaches 56 - 72% from the tapping of the converter to the end of LF treatment, and the off-position phosphorus content in LF is ≤0.0040%. The increase in sulfur content during the refining process is mainly related to the fact that the molten steel is not deoxidized, and as the treatment time extends, the oxidizability of the steel slag continuously rises, resulting in the occurrence of sulfur pick-up in the molten steel.

[0062] Table 3.2 Chemical composition of molten steel during treatment (%)

[0063]

[0064] IV. RH Vacuum Treatment Process

[0065] 1. RH Smelting Process

[0066] The RH treatment process of this casting heat is stable. Since the in-position oxygen content meets the decarburization requirement, no oxygen blowing operation is carried out during the whole smelting process. The parameters of the RH furnace vacuum treatment process are shown in Table 4.1.

[0067] Table 4.1 Parameters of Vacuum Treatment Process

[0068]

[0069] 2. RH Off-position Composition

[0070] During the RH smelting process, only deoxidized aluminum and component aluminum are added according to the requirements of chemical composition. The chemical composition of the off-position molten steel in RH meets the requirements of the quality plan (as shown in the following table).

[0071] Table 4.2 Chemical composition of off-position molten steel in RH (%)

[0072] Melting number C Si Mn P S Alt Als N 1413 0.0007 0.003 0.06 0.0030 0.0046 0.022 0.020 0.0020 1415 0.0005 0.001 0.05 0.0031 0.0048 0.013 0.012 0.0019 1416 0.0008 0.002 0.05 0.0044 0.0041 0.019 0.017 0.0023 1417 0.0005 0.001 0.05 0.0032 0.0043 0.014 0.014 0.0023

[0073] 3. Composition of Refined Steel Slag

[0074] During the refining process of the LF furnace, quicklime and modifier are added to modify the top slag and reduce the TFe content in the slag. The basicity of the steel slag is controlled between 5 - 7 during the whole refining process. After the RH furnace is repressurized, 50 kg of aluminum powder and 100 kg of modifier are added to the surface of the slag for top slag modification. The content of TFe + MnO in the off-position slag of the RH furnace is in the range of 13.80 - 14.54%.

[0075] Table 4.3 Chemical composition of steel slag during treatment (%)

[0076]

[0077] V. Continuous Casting Process

[0078] During continuous casting, argon is blown through the hollow stopper rod in the tundish. The casting process of the continuous caster is stable, and no flocculent flow phenomenon occurs. The casting speed of the continuous caster is 1.2 m / min, and the superheat during casting is controlled between 26 and 34 °C. The slag surface in the mold is stable (as Figure 1 ), and the tundish composition meets the requirements of the quality plan as shown in Table 5.1.

[0079] Table 5.1 Chemical composition of tundish (%)

[0080] Melting number C Si Mn P S Alt Als N O Superheat (°C) 1413 0.0014 0.002 0.06 0.0024 0.0040 0.014 0.012 0.0025 0.0046 30 1415 0.0009 0.003 0.05 0.0040 0.0050 0.007 0.004 0.0020 0.0044 26 1416 0.0013 0.003 0.05 0.0049 0.0047 0.011 0.009 0.0020 0.0038 34 1417 0.0015 0.001 0.05 0.0036 0.0040 0.010 0.008 0.0020 0.0040 31

[0081] Take the macrostructure samples of the continuous casting billets with furnace numbers 1413 and 1417. The macrostructure determination results of the continuous casting billets show that the central porosity is 0.5, and the central segregation is class C 0.5, meeting the requirements of the quality plan. The internal quality of the continuous casting billets is well controlled (as shown in Table 5.2).

[0082] Table 5.2 Macrostructure results of continuous casting billets

[0083]

[0084] VI. Research on composition control during smelting process

[0085] During the smelting treatment process of the casting heat, the change of manganese content is shown in Figure 2 . It can be directly seen from the figure that during the tapping process of the converter without deoxidation, due to the decrease in the molten steel temperature during the tapping process, the manganese content has a certain decrease, and the decrease value is between 100 and 400 ppm; during the heating and temperature rising process of LF refining without deoxidation, the manganese content does not change; during the RH vacuum decarburization treatment process, there is a phenomenon of manganese recovery in the first ladle of the casting heat, and the increase value of manganese content is 100 ppm. The analysis is that the melting of the residual cold steel in the first ladle leads to an increase in manganese content.

[0086] The change trend of P content during the smelting treatment process of the casting heat can be seen (as Figure 3 ). During the tapping process of the converter for the whole casting heat, the decrease value of P content is between 25 and 32 ppm; during the heating and temperature rising process of LF refining furnace without deoxidation, the decrease value of P content is between 50 and 56 ppm; from the tapping of the converter to the off-position curve of LF, the change trend of P content is relatively stable, effectively solving the operation control of low-P steel smelting; during the RH treatment process and the casting process of the continuous caster, due to the decrease in the oxidability of the ladle top slag, there is a phenomenon of P reversion to different degrees, and the increase value of P content is between 3 and 16 ppm.

[0087] By optimizing the steelmaking continuous casting process control, multi-hearth continuous casting of high cleanliness ultra-low carbon series steel is realized. The extremely low requirements for the contents of carbon, manganese, phosphorus, and sulfur in high cleanliness ultra-low carbon series steel are achieved. The target requirements of Al ≤ 0.015% in the molten steel composition of high cleanliness ultra-low carbon series steel and O content ≤ 0.0050% in the molten steel gas are realized. Ensure that the qualification rate of high cleanliness ultra-low carbon series steel billets reaches more than 99.5%.

[0088] The embodiments described above are only descriptions of the preferred embodiments of the present invention and do not limit the scope of the present invention. Without departing from the design spirit of the present invention, various deformations and improvements made by those of ordinary skill in the art to the technical solutions of the present invention shall fall within the protection scope determined by the claims of the present invention.

Claims

1. A method for producing high-cleanliness ultra-low carbon series steel, comprising KR molten iron deep desulfurization → converter smelting → LF refining → RH furnace → continuous casting; characterized in that: Each process is controlled as follows: KR process: Study the KR deep desulfurization process to ensure that the sulfur content of the hot metal entering the furnace is low enough and avoid excessive sulfur brought into the furnace by scrap steel and charge. The hot metal needs to be desulfurized by KR, and the sulfur content of the hot metal entering the converter is required to be ≤0.001%, and the desulfurization slag cleaning area is greater than 95%; Converter process: The final temperature of the converter is controlled at ≥1600℃, and the final oxygen content is controlled within 600ppm; LF refining process: There is no deoxidation during the converter steel-making process, and the LF in-situ oxygen content is between 380-420ppm; due to the low in-situ temperature, the total heating time is greater than 25 minutes. In order to promote the dephosphorization efficiency and submerged arc heating effect during the refining process, the amount of lime added during the LF refining process is between 1.5-1.7t, and the amount of modifier added is between 1.0-1.2t; RH process: The RH furnace carries out deep decarburization treatment, and the oxygen content at the end of decarburization is controlled within 300ppm. After decarburization, aluminum is added for deoxidation and alloying to ensure that the pure degassing time of molten steel is more than 6min. Through the converter, LF furnace and RH top slag modification treatment, the TFe+MnO content of RH off-site slag is controlled within 15%, which is conducive to the floating of inclusions and further improves the cleanliness of molten steel. Continuous casting process: The casting machine adopts constant pulling speed control, the pulling speed range is 1.0-1.5m / min, the casting superheat of the casting machine is controlled between 20 and 35℃, and protective pouring is carried out throughout the continuous casting process.

2. The method for producing high-cleanliness ultra-low carbon steel according to claim 1, characterized in that: The converter successfully pulled carbon in one time. In order to ensure the low sulfur content control of the converter end point, all the scrap steel in the converter was cast in the middle package. In order to control the process temperature in the converter smelting process and prevent the high-temperature manganese return phenomenon, the converter end point temperature was controlled at 1600-1618℃. The converter effectively controlled the end point residual manganese content through the end point low temperature control. The manganese content supplied by the converter for LF refining met the control requirements.

3. The method for producing high-cleanliness ultra-low carbon steel according to claim 1, characterized in that: The oxygen content at the converter end point is controlled at 511-558ppm. Due to dephosphorization during the LF refining process, the oxygen content at the converter end point needs to be controlled slightly higher to ensure maximum dephosphorization efficiency.

4. The method for producing high-cleanliness ultra-low carbon steel according to claim 1, characterized in that: The converter effectively controls the manganese return phenomenon in the later stage of smelting by controlling the final low temperature, the large slag amount during the process, and the high oxidation property of the slag. The MnO content in the final slag is stably controlled at 4.9-5.2%, and the TFe content in the final slag is controlled between 24 and 29%. Lime and modifiers are added during the converter tapping process to modify the top slag.

5. The method for producing high-cleanliness ultra-low carbon steel according to claim 1, characterized in that: The dephosphorization rate from tapping of converter to the end of LF treatment reaches 56-72%, and the LF off-site phosphorus content is ≤0.0040%.

6. The method for producing high-cleanliness ultra-low carbon steel according to claim 1, characterized in that: The RH treatment process is stable, the in-situ oxygen content meets the decarburization requirements, and no oxygen blowing operation is performed during the entire smelting process.

7. The method for producing high-cleanliness ultra-low carbon steel according to claim 1, characterized in that: During the LF furnace refining process, lime and modifiers are added to modify the top slag to reduce the TFe content in the slag; during the entire refining process, the slag basicity is controlled between 5-7, and after the RH furnace is re-pressed, 40-60kg of aluminum powder and 80-120kg of modifier are added to the slag surface to modify the top slag. The TFe+MnO content of the RH off-site slag is in the range of 13.80-14.54%.

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