Preparation method of ultra-pure stainless steel for automobile trim strip

Abstract

This invention discloses a method for preparing ultrapure stainless steel for automotive trim strips, comprising the following steps: (1) molten iron undergoes initial decarburization, reduction, desulfurization, and nitrogen control in an AOD furnace before tapping; (2) after slag removal, the iron enters a VOD furnace for vacuum deep decarburization and denitrification, FeSi alloy reduction, and low-carbon material slag formation; (3) the iron is fed into an LF refining furnace for alloying, and CaSiBa is introduced; (4) a magnesium + aluminum-calcium composite covering agent is used in continuous casting, and an aerosol baffle is used in the tundish. Low-carbon and nitrogen-rich ultrapure ferritic stainless steel is smelted using AOD+VOD. LF furnace deep deoxidation and modification treatment of magnesium-aluminum spinel inclusions are employed. Weak blowing and stirring, along with high-alkalinity slag, promote the flotation and adsorption removal of inclusions in the molten steel. Simultaneously, a ladle atomizing baffle device is used in continuous casting, along with a magnesium + aluminum-calcium low-melting-point composite ladle covering agent to fully absorb small-sized inclusion particles that float to the surface. This ensures the ultrapurity of the ultrapure ferritic stainless steel molten steel, resulting in a total oxygen content of <20ppm for continuous casting ladle samples and an inclusion rating of ≤1.0 for plate (Class B + Class D).

Description

Technical Field

[0001] This invention relates to the field of stainless steel smelting technology, and specifically to a method for preparing ultrapure stainless steel for automotive trim strips. Background Technology

[0002] Automotive trim refers to enhancing the aesthetics of a car's exterior and interior by adding accessories; these added accessories are called decorative items. Automotive trim strips are strip-shaped decorative pieces frequently used in car decoration. To increase the strength of these strips, manufacturers embed high-strength stainless steel within them, improving both vehicle safety and overall aesthetics. Ultra-pure stainless steel automotive trim strips are primarily used for the side panels and windows of passenger vehicles, and are known as "zero-defect" materials. The material used is high-gloss BA thin-gauge raw material, without any special surface treatments such as polishing or brushing.

[0003] Ultra-pure ferritic stainless steel has low carbon and nitrogen content and high Nb and Ti stabilizing elements. The ultra-pure ferritic stainless steel of grade SUS430J1L (J442D) has excellent properties such as high surface gloss, smoothness, corrosion resistance and good formability. It is mainly used in brand vehicles with high independent requirements. Its conventional smelting process is: hot metal pretreatment → AOD → VOD → LF → CCM. The following problems exist in the smelting process: (1) VOD uses AL2O3, a deoxidation product after aluminum particle reduction, which is a brittle inclusion and is easily combined with magnesium refractory to form large-sized inclusions of magnesium aluminum spinel, which in turn affects the surface quality of the plate; (2) The equilibrium oxygen content after reduction with ferrosilicon alloy is high, and AL2O3 inclusions will also be generated in the later stage of smelting, causing sand hole defects on the bright surface of thin plate; (3) Although the traditional magnesium covering agent in the continuous casting ladle has a good heat preservation effect, it has a high melting point, is easy to form a shell, and has a weak ability to adsorb inclusions and purify molten steel; (4) Ultra-low carbon stainless steel has a high equilibrium oxygen content after deep decarburization. Summary of the Invention

[0004] This invention provides a method for preparing ultrapure stainless steel for automotive trim strips, aiming to solve the problems of high oxygen content in molten steel and large inclusion size in existing preparation methods.

[0005] To achieve its purpose, the present invention adopts the following technical solution:

[0006] A method for preparing ultrapure stainless steel for automotive trim strips includes the following steps:

[0007] Step 1: Hot metal pretreatment; smelting the blast furnace hot metal to remove C, Si, Mn, P, and S elements. The weight percentage of each element in the hot metal after removal is: C > 2.5%, Si < 0.20%, Mn < 0.30%, P < 0.015%, S < 0.050%, with the balance being Fe; hot metal temperature > 1300℃;

[0008] Step Two: AOD Furnace Smelting; The molten iron obtained in Step One is added to the AOD furnace for sequential oxygen blowing decarburization, chromium / copper alloying, FeSi alloy reduction, and desulfurization, with argon blowing and nitrogen control throughout the process; The weight percentage of each element in the molten steel is: C 0.20-0.50%, Si <0.20%, Mn <0.30%, P <0.025%, S <0.0050%, Cr 19.00-20.00%, Cu 0.20-0.40%, N <0.0090%, with the balance being Fe; The tapping temperature of the molten steel is >1700℃;

[0009] Step 3: VOD Vacuum Smelting; After slag removal, the molten steel obtained in Step 2 is sent to a vacuum ladle with a steel temperature >1600℃; First, vacuum decarburization with oxygen blowing is performed to reduce the vacuum degree to 20-40 torr; then, deep vacuum stirring and dynamic decarburization is performed to reduce the vacuum degree to <1.0 torr; finally, FeSi alloy reduction is used, and low-carbon lime and fluorite are added to adjust the slag ratio so that the slag basicity is greater than >2.4; the weight percentage of each element in the molten steel exiting the ladle is: C <0.0085%, Si 0.20-0.50%, Mn <0.30%, P <0.025%, S <0.0020%, Cr 19.00-20.00%, Cu 0.20-0.40%, N <0.0085%, with the balance being Fe;

[0010] Step 4: Refining in an LF refining furnace; After electrolytic slag formation, the molten steel obtained in Step 3 is alloyed with FeNb, stirred with argon, and fed with 8-10 m / t of CaSiBa wire, with a weak blowing time > 25 min; The weight percentage of each element in the molten steel is: C < 0.0100%, Si 0.40-0.60%, Mn < 0.30%, P < 0.025%, S < 0.0015%, Cr 19.00-20.00%, N < 0.0100%, with the balance being Fe;

[0011] Step 5: Continuous casting; The molten steel obtained in Step 4 is hoisted to the continuous casting platform for casting to obtain the ultra-pure stainless steel billet product for automotive trim strips.

[0012] Preferably, the blast furnace iron smelting work in step one is completed by a dephosphorization converter or a "three-stage dephosphorization" station.

[0013] Preferably, in step three, the time for vacuum oxygen blowing decarburization is 25-40 min, the time for deep vacuum stirring dynamic decarburization is 10-20 min, and the time for FeSi alloy reduction is >20 min.

[0014] Preferably, the argon flow rate during argon blowing and stirring in step four is 200-500 L / min.

[0015] Preferably, the CaSiBa linear composition in step four is Ca≥10%, Ba≥10%, Si≥50% by weight percentage, with the balance being Fe.

[0016] Preferably, during the casting process in step five, argon gas is used to protect the long nozzle and seal the molten steel in the tundish.

[0017] Preferably, 80-100 kg of magnesium covering agent is added after the molten steel in the tundish reaches 15-20t, and 40-50 kg of aluminum-calcium low-melting-point covering agent is added after the molten steel in the tundish reaches 20-25t.

[0018] Preferably, the magnesium covering agent has the following composition by weight percentage: MgO ≥ 85%, Al2O3 + SiO2 ≤ 10%; the aluminum-calcium low-melting-point covering agent has the following composition: CaO ≥ 40%, Al2O3 25-35%, MgO + SiO2 ≤ 10%.

[0019] Preferably, the superheat of the molten steel in the tundish is 30-50℃, and the drawing speed is 0.8-1.0m / min.

[0020] The beneficial effects of this invention are as follows: Through the reduction of FeSi alloy by VOD smelting and the deep deoxidation of CaSiBa by LF refining, the amount of brittle inclusions Al2O3 in the molten steel is reduced, the T[O] content of the molten steel is lowered, and the inclusions are effectively modified into low-melting-point inclusions; at the same time, the tundish of continuous casting uses a magnesium + aluminum-calcium composite covering agent to fully adsorb small particulate inclusions floating with the mist, ensuring the ultra-purity of the molten steel. The T[O] of the continuous casting tundish sample is <20ppm, and the inclusion rating of the plate (B class + D class) is <1.0, which meets the "zero defect" requirement of no sand holes and no short scales in the processing and manufacturing of thin plate materials for automotive trim strips. Attached Figure Description

[0021] Figure 1 Photographs of inclusions in the cast billet and rolled sheet produced according to Embodiment 1 of the present invention;

[0022] Figure 2 Photographs of inclusions in the cast billet and rolled sheet produced in Embodiment 2 of the present invention;

[0023] Figure 3 Photographs of inclusions in the cast billet and rolled sheet produced in Embodiment 3 of the present invention;

[0024] Figure 4 A photograph of the ultrapure stainless steel produced in Example 1 of this invention used in automotive trim strips. Detailed Implementation

[0025] The present invention will be further described below with reference to embodiments:

[0026] Example 1

[0027] This invention discloses a method for preparing ultra-pure stainless steel for automotive trim strips, using SUS430J1L (J442D) ultra-pure ferritic stainless steel, and includes the following steps:

[0028] Step 1: Hot metal pretreatment; The 71t blast furnace hot metal produced by the "three-removal" station is smelted. The hot metal exit temperature is 1362℃. After removing C, Si, Mn, P and S elements, the weight percentage of each element in the hot metal is: C 3.55%, Si 0.022%, Mn 0.080%, P 0.013%, S 0.027%, with the balance being Fe.

[0029] Step Two: AOD Furnace Smelting; The molten iron obtained in Step One is added to the AOD furnace at a temperature of 1345℃ for oxygen blowing decarburization, with an oxygen blowing rate of 7420 Nm³. 3 Add 29630 kg of ferrochrome alloy and 336 kg of copper, and purge with 2300 Nm of argon throughout the process. 3 The steel was reduced using 1580 kg of FeSi alloy and tapped at a tapping temperature of 1708℃. The weight percentages of each element in the molten steel were: C 0.25%, Si 0.15%, Mn 0.18%, P 0.017%, S 0.0017%, Cr 19.60%, Cu 0.32%, N 0.0068%, with the balance being Fe. The tapping temperature of the molten steel was 1733℃.

[0030] Step 3: VOD Vacuum Smelting; After slag removal, the molten steel obtained in Step 2 is sent to a vacuum tank. Upon entering the vacuum tank, the slag thickness is measured to be 120 mm, and the molten steel temperature is 1620℃. Vacuum decarburization with oxygen blowing is then performed for 32 minutes, with an oxygen blowing rate of 595 Nm³. 3 The vacuum degree was reduced to 35 torr; then deep vacuum stirring and dynamic decarburization were performed for 14 minutes to reduce the vacuum degree to 0.5 torr; finally, 850 kg of FeSi alloy was used for reduction for 23 minutes, with 3000 kg of low-carbon lime and 200 kg of fluorite added to achieve a slag basicity of 2.60; the weight percentage of each element in the molten steel was: C 0.0073%, Si 0.31%, Mn 0.18%, P 0.020%, S 0.0015%, Cr 19.43%, Cu 0.32%, N 0.0084%, with the balance being Fe;

[0031] Step 4: Refining in an LF refining furnace; the molten steel obtained in Step 3 is slag-forming via electro-smelting. The molten steel temperature at the station is 1580℃. After slag-forming via electro-smelting for 10 minutes, 850 kg of FeNb alloy is added. Argon is blown and stirred at a rate of 450 L / min. 830 m of CaSiBa wire is fed in, and the weak blowing time is 27 minutes. The weight percentage of each element in the molten steel leaving the station is: C 0.0084%, Si 0.48%, Mn 0.18%, P 0.020%, S 0.0011%, Cr 19.27%, Cu 0.32%, N 0.0088%, with the balance being Fe.

[0032] Step 5: Continuous casting; The 102t of molten steel obtained in Step 4 is hoisted to the continuous casting platform and calmed for 15 minutes. Protective pouring is performed, and an air mist baffle is used in the tundish. When the molten steel in the tundish reaches 17t, 80kg of magnesium covering agent is added. When the molten steel in the tundish reaches 25t, 40kg of aluminum-calcium low-melting-point covering agent is added. The superheat of the tundish is 38℃, and the casting speed is 0.90m / min. The T[O] of the finished racket sample in the tundish is 17ppm, and the ultra-pure stainless steel billet product for automotive trim strips is obtained.

[0033] Figure 1 Metallographic microscope images of inclusions in the rolled sheet of SUS430J1L (J442D) billet produced in Example 1 (left side is billet, right side is sheet). Figure 4 The image shows a photograph of the ultra-pure stainless steel produced in Example 1 of this invention used in automotive trim strips. According to GB / T 10561 standard (Class B + Class D), the inclusion rating is <1.0, meeting the "zero-defect" requirement of no sand holes and no short-line scales in the processing of thin sheet materials for automotive trim strips. Photos of its application in automotive trim strips are shown below. Figure 4 As shown.

[0034] Example 2

[0035] This invention discloses a method for preparing ultra-pure stainless steel for automotive trim strips, using SUS430J1L (J442D) ultra-pure ferritic stainless steel, and includes the following steps:

[0036] Step 1: Hot metal pretreatment; 75t of blast furnace hot metal produced is smelted in a dephosphorization converter. The hot metal exit temperature is 1348℃. After removing C, Si, Mn, P, and S elements, the weight percentage of each element in the hot metal is: C 3.33%, Si 0.011%, Mn 0.11%, P 0.009%, S 0.044%, with the balance being Fe.

[0037] Step Two: AOD Furnace Smelting; The molten iron obtained in Step One is added to the AOD furnace at a temperature of 1322℃ for oxygen blowing decarburization, with an oxygen blowing rate of 8219 Nm³. 3Add 39066 kg of ferrochrome alloy and 300 kg of copper, and purge with 2423 Nm of argon throughout the process. 3 The steel was reduced using 1798 kg of FeSi alloy and tapped at a tapping temperature of 1734℃. The weight percentages of each element in the molten steel were: C 0.22%, Si 0.18%, Mn 0.17%, P 0.015%, S 0.0026%, Cr 19.31%, Cu 0.28%, N 0.0085%, with the balance being Fe. The tapping temperature of the molten steel was 1728℃.

[0038] Step 3: VOD Vacuum Smelting; After slag removal, the molten steel obtained in Step 2 is sent to a vacuum tank. Upon entering the vacuum tank, the slag thickness is measured to be 115 mm, and the molten steel temperature is 1613℃. Vacuum decarburization with oxygen blowing is then performed for 30 minutes, with an oxygen blowing rate of 605 Nm³. 3 The vacuum level was reduced to 38 torr; then, deep vacuum stirring and dynamic decarburization were performed for 12 minutes to reduce the vacuum level to 0.3 torr; finally, 900 kg of FeSi alloy was used for reduction for 25 minutes, with 2800 kg of low-carbon lime and 330 kg of fluorite added to achieve a slag basicity of 2.70; the weight percentage of each element in the molten steel was: C 0.0081%, Si 0.43%, Mn 0.18%, P 0.016%, S 0.0010%, Cr 19.28%, Cu 0.41%, N 0.0074%, with the balance being Fe;

[0039] Step 4: Refining in an LF refining furnace; the molten steel obtained in Step 3 is slag-forming via electro-smelting. The molten steel temperature at the station is 1567℃. After slag-forming via electro-smelting for 15 minutes, 880 kg of FeNb alloy is added. Argon is blown and stirred at a rate of 400 L / min. 890 m of CaSiBa wire is fed in and the weak blowing time is 30 minutes. The weight percentage of each element in the molten steel leaving the station is: C 0.0093%, Si 0.51%, Mn 0.18%, P 0.016%, S 0.0010%, Cr 19.18%, Cu 0.41%, N 0.0083%, with the balance being Fe.

[0040] Step 5: Continuous casting; The 99t of molten steel obtained in Step 4 is hoisted to the continuous casting platform and calmed for 18 minutes. Protective pouring and use of the tundish with a mist baffle wall are then performed. When the molten steel in the tundish reaches 20t, 90kg of magnesium covering agent is added. When the molten steel in the tundish reaches 26t, 50kg of aluminum-calcium low-melting-point covering agent is added. The superheat of the tundish is 42℃, and the casting speed is 0.90m / min. The T[O] of the finished racket sample in the tundish is 19ppm, resulting in an ultra-pure stainless steel billet product for automotive trim strips.

[0041] Figure 2The metallographic microscope images of the inclusions in the rolled sheet of SUS430J1L (J442D) billet produced in Example 2 (the billet is on the left and the sheet is on the right) show inclusions with a rating of <1.0 according to GB / T 10561 standard (Class B + Class D), which meets the "zero defect" requirement of no sand holes and no short scales in the processing of thin sheet materials for automotive trim strips.

[0042] Example 3

[0043] This invention discloses a method for preparing ultra-pure stainless steel for automotive trim strips, using SUS430J1L (J442D) ultra-pure ferritic stainless steel, and includes the following steps:

[0044] Step 1: Hot metal pretreatment; 69t of blast furnace hot metal produced is smelted in a dephosphorization converter. The hot metal exit temperature is 1333℃. After removing C, Si, Mn, P, and S elements, the weight percentage of each element in the hot metal is: C 3.45%, Si 0.013%, Mn 0.16%, P 0.010%, S 0.038%, with the balance being Fe.

[0045] Step Two: AOD Furnace Smelting; The molten iron obtained in Step One is added to the AOD furnace at a temperature of 1311℃ for oxygen blowing decarburization, with an oxygen blowing rate of 7350 Nm³. 3 Add 38403 kg of ferrochrome alloy and 320 kg of copper, and purge with 2213 Nm of argon throughout the process. 3 The steel was reduced using 1920 kg of FeSi alloy and tapped at a tapping temperature of 1740℃. The weight percentages of each element in the molten steel were: C 0.25%, Si 0.11%, Mn 0.14%, P 0.015%, S 0.0043%, Cr 19.70%, Cu 0.30%, N 0.0073%, with the balance being Fe. The tapping temperature of the molten steel was 1744℃.

[0046] Step 3: VOD Vacuum Smelting; After slag removal, the molten steel obtained in Step 2 is sent to a vacuum tank. Upon entering the vacuum tank, the slag thickness is measured to be 100 mm, and the molten steel temperature is 1641℃. Vacuum decarburization with oxygen blowing is then performed for 26 minutes, with an oxygen blowing rate of 585 Nm³. 3 The vacuum level was reduced to 33 torr; then, deep vacuum stirring and dynamic decarburization were performed for 15 minutes to reduce the vacuum level to 0.1 torr; finally, 927 kg of FeSi alloy was used for reduction for 21 minutes, with 3200 kg of low-carbon lime and 410 kg of fluorite added as slag to achieve a slag basicity of 2.80; the weight percentage of each element in the molten steel was: C 0.0076%, Si 0.47%, Mn 0.16%, P 0.016%, S 0.0012%, Cr 19.42%, Cu 0.36%, N 0.0082%, with the balance being Fe;

[0047] Step 4: Refining in an LF refining furnace; the molten steel obtained in Step 3 is slag-forming via electro-smelting. The molten steel temperature at the station is 1581℃. Electro-smelting is carried out for 18 minutes. 873 kg of FeNb alloy is added, and argon is blown and stirred at a rate of 350 L / min. 870 m of CaSiBa wire is fed in, and the weak blowing time is 26 minutes. The weight percentage of each element in the molten steel leaving the station is: C 0.0099%, Si 0.53%, Mn 0.16%, P 0.016%, S 0.0010%, Cr 19.30%, Cu 0.42%, N 0.0095%, with the balance being Fe.

[0048] Step 5: Continuous casting; The 103t of molten steel obtained in Step 4 is hoisted to the continuous casting platform and calmed for 21 minutes. Protective pouring and use of the tundish with an air mist baffle are performed. After 21t of molten steel in the tundish, 100kg of magnesium covering agent is added. After 26t of molten steel in the tundish, 60kg of aluminum-calcium low-melting-point covering agent is added. The superheat of the tundish is 38℃ and the casting speed is 0.95m / min. The T[O] of the finished racket sample in the tundish is 17ppm, and the ultra-pure stainless steel billet for automotive trim strips is obtained.

[0049] Figure 3 The metallographic microscope images of the inclusions in the rolled sheet of SUS430J1L (J442D) billet produced in Example 3 (the billet is on the left and the sheet is on the right) show inclusions with a rating of <1.0 according to GB / T 10561 standard (Class B + Class D), which meets the "zero defect" requirement of no sand holes and no short scales in the processing of thin sheet materials for automotive trim strips.

Claims

1. A method of producing an ultra-pure stainless steel for an automobile trim strip, characterized by, Includes the following steps: Step 1: Hot metal pretreatment; smelting the blast furnace hot metal to remove C, Si, Mn, P, and S elements. The weight percentage of each element in the hot metal after removal is: C > 2.5%, Si < 0.20%, Mn < 0.30%, P < 0.015%, S < 0.050%, with the balance being Fe; hot metal temperature > 1300℃; Step Two: AOD Furnace Smelting; The molten iron obtained in Step One is added to the AOD furnace for sequential oxygen blowing decarburization, chromium / copper alloying, FeSi alloy reduction, and desulfurization, with argon blowing and nitrogen control throughout the process; The weight percentage of each element in the molten steel is: C 0.20-0.50%, Si < 0.20%, Mn < 0.30%, P < 0.025%, S < 0.0050%, Cr 19.00-20.00%, Cu 0.20-0.40%, N < 0.0090%, with the balance being Fe; The tapping temperature of the molten steel is > 1700℃; Step 3: VOD Vacuum Smelting; After slag removal, the molten steel obtained in Step 2 is sent to a vacuum ladle with a steel temperature >1600℃; First, vacuum decarburization with oxygen blowing is performed to reduce the vacuum degree to 20-40 torr; then, deep vacuum stirring and dynamic decarburization is performed to reduce the vacuum degree to <1.0 torr; finally, FeSi alloy reduction is used, and low-carbon lime and fluorite are added to adjust the slag ratio so that the slag basicity is >2.4; The weight percentage of each element in the molten steel exiting the ladle is: C <0.0085%, Si 0.20-0.50%, Mn <0.30%, P <0.025%, S <0.0020%, Cr 19.00-20.00%, Cu 0.20-0.40%, N <0.0085%, with the balance being Fe; Step 4: Refining in an LF refining furnace; After electrolytic slag formation, FeNb alloying is added to the molten steel obtained in Step 3, followed by argon stirring and feeding with 8-10 m / t of CaSiBa wire, with a weak blowing time > 25 min; The weight percentage of each element in the molten steel is: C < 0.0100%, Si 0.40-0.60%, Mn < 0.30%, P < 0.025%, S < 0.0015%, Cr 19.00-20.00%, N < 0.0100%, with the balance being Fe; Step 5: Continuous casting; The molten steel obtained in Step 4 is hoisted to the continuous casting platform for casting to obtain the ultra-pure stainless steel billet product for automotive trim strips.

2. The method of producing ultra-pure stainless steel for an automobile trim strip according to claim 1, characterized by: The blast furnace iron smelting work in step one is completed by a dephosphorization converter or a "three-stage dephosphorization" station.

3. The method of producing ultra-pure stainless steel for an automobile trim strip according to claim 1, characterized by: In step three, the time for vacuum oxygen blowing decarburization is 25-40 min, the time for deep vacuum stirring dynamic decarburization is 10-20 min, and the time for FeSi alloy reduction is >20 min.

4. The method of producing ultra-pure stainless steel for an automobile trim according to claim 1, wherein: In step four, the argon gas flow rate during stirring is 200-500 L / min.

5. The method of producing ultra-pure stainless steel for an automobile trim strip according to claim 4, characterized by: The CaSiBa linear composition in step four is, by weight percentage, Ca≥10%, Ba≥10%, Si≥50%, with the balance being Fe.

6. The method of producing ultra-pure stainless steel for an automobile trim according to claim 1, wherein: During the casting process in step five, argon gas is used to protect the long nozzle and seal the molten steel in the tundish.

7. The method of producing ultra-pure stainless steel for an automobile trim strip according to claim 6, characterized by: After the molten steel in the intermediate ladle reaches 15-20t, 80-100kg of magnesium covering agent is added; after the molten steel in the intermediate ladle reaches 20-25t, 40-50kg of aluminum-calcium low-melting-point covering agent is added.

8. The method for preparing ultra-pure stainless steel for automotive trim strips as described in claim 7, characterized in that: The magnesium covering agent is composed of MgO ≥ 85% and Al2O3 + SiO2 ≤ 10% by weight; the aluminum-calcium low-melting-point covering agent is composed of CaO ≥ 40%, Al2O3 25-35%, and MgO + SiO2 ≤ 10%.

9. The method for preparing ultra-pure stainless steel for automotive trim strips as described in claim 7, characterized in that: The superheat of the molten steel in the tundish is 30-50℃, and the casting speed is 0.8-1.0m / min.

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