Direct conversion process for 200 and 300 series steels on a cold reduction, annealing and pickling line

Abstract

A method for direct conversion between 200 series and 300 series steel grades in a cold-rolled annealing and pickling line includes: Step 1, annealing and pickling the previous steel grade; before conversion, when the remaining quantity of the previous steel grade is 2-4 coils, a pre-preparation stage for process parameter adjustment is entered; Step 2, welding the tail of the previous steel grade strip to the head of the subsequent steel grade strip; Step 3, after the tail of the last coil of the previous steel grade strip enters the annealing furnace area, adjusting according to the process parameters of the subsequent steel grade. This invention uses overlapping intervals of process parameter ranges for different specifications, allowing direct conversion between 200 series and 300 series steel grades. By adjusting the operating process parameters during conversion between the previous and subsequent steel grades, this invention can reduce the consumption of auxiliary materials and media such as nitric acid, hydrofluoric acid, and natural gas, saving costs. After process optimization, the surface quality of the steel strip is stable, reducing quality defects such as roll marks and stains previously caused by operation and debugging, achieving a win-win situation for both output and quality.

Description

Technical Field

[0001] This invention relates to the technical field of metal material processing, and more specifically, to a method for direct conversion between 200 series and 300 series steel grades in a cold rolling annealing and pickling line. Background Technology

[0002] Please refer to Figure 1 , Figure 1 This is a schematic diagram illustrating the connection between steel grades when using the debug roll conversion steel grade conversion mode in the existing technology.

[0003] When changing steel grades in a cold-rolled annealing and pickling line, a test roll conversion mode is typically used (the previous steel grade a is connected to the next steel grade b using test roll c). In actual production, this mode takes an average of about 45 minutes per steel grade conversion. If the steel grade conversion takes too long, it will significantly impact output, thus failing to meet the company's production requirements for the cold pickling workshop. Furthermore, different steel grades have different control requirements for process parameters such as furnace plate temperature and pickling zone temperature and concentration. Excessive fluctuations in process parameters after steel grade conversion can easily lead to product quality degradation. Summary of the Invention

[0004] (I) Technical Issues

[0005] In summary, how to reduce the time spent switching steel grades, thereby minimizing the impact of steel grade switching on production output, has become an urgent problem to be solved by those skilled in the art.

[0006] (II) Technical Solution

[0007] To achieve the above objectives, the present invention provides the following technical solution:

[0008] This invention provides a method for direct conversion between 200 series and 300 series steel grades in a cold-rolled annealing and pickling line. The method includes:

[0009] Step 1: Anneal and pickle the previous steel grade. Before changing the steel grade, when the remaining amount of the previous steel grade is 2-4 coils, enter the pre-preparation stage for adjusting process parameters.

[0010] Step 2: Weld the tail of the strip of the previous steel grade to the head of the strip of the next steel grade.

[0011] Step 3: After the last coil of the previous steel grade enters the annealing furnace area, adjust the process parameters according to the next steel grade.

[0012] Preferably, in the method for direct conversion between 200 series and 300 series steel grades in the cold rolling annealing and pickling line provided by the present invention, in step three, after the last coil of the previous steel grade enters the annealing furnace area, the process production speed is reduced to a range of 50m / min-70m / min.

[0013] Preferably, the method for direct conversion between 200 series and 300 series steel grades in the cold rolling annealing and pickling line provided by the present invention further includes step four: after the first coil of the latter steel grade exits the furnace, the surface condition of the latter steel grade is inspected, and the production speed is restored after the quality inspection is qualified.

[0014] Preferably, in the method for direct conversion between 200 series and 300 series steel grades in the cold rolling annealing and pickling line provided by the present invention, a pre-selection step is included before step one. In the pre-selection step, the steel grade is selected according to the thickness of the steel grade, and the window thickness of the steel grade to be converted is 0.4mm≤H≤0.6mm.

[0015] Preferably, the method for direct conversion between 200 series and 300 series steel grades in a cold-rolled annealing and pickling line provided by the present invention further includes step five: pickling process adjustment. In step five, when 200 series steel grades need to be converted to 300 series steel grades, at least two coils are converted from production mode to test mode before the steel grade conversion. The 300 series steel grade mixed acid circulation pump is started and operates simultaneously with the 200 series steel grade mixed acid circulation pump. After the circulation pump pressure stabilizes, the 300 series steel grade mixed acid circulation pump is increased and the 200 series steel grade mixed acid circulation pump is turned off. After the circulation pump pressure stabilizes again, the circulation pump switching is completed.

[0016] Preferably, in the method for direct conversion between 200 series and 300 series steel grades in a cold-rolled annealing and pickling line provided by the present invention, the acid temperature is raised for the two coils before the steel grade change, and nitric acid and hydrofluoric acid are added to raise the acid concentration of the mixed acid to the process parameter range for 300 series steel grade operation; when the 300 series steel grade coil enters the pickling section, the acid temperature and acid concentration of the mixed acid are raised to the process control parameters for 300 series steel grade operation.

[0017] Preferably, the method for direct conversion between 200 series and 300 series steel grades in the cold rolling annealing pickling line provided by the present invention further includes step five: pickling process adjustment. In step five, when converting from 300 series steel grade to 200 series steel grade, two hours before the conversion, the chiller is started and its normal operation is confirmed. Five coils before the conversion, the parameter control circulation pump is switched to test mode, and the 200 series steel grade mixed acid circulation pump is started and operates simultaneously with the 300 series steel grade mixed acid circulation pump. After the circulation pump pressure stabilizes, the 200 series steel grade mixed acid circulation pump is increased and the 300 series steel grade mixed acid circulation pump is turned off. After the circulation pump pressure stabilizes again, the circulation pump switching is completed.

[0018] Preferably, in the method for direct conversion between 200 series and 300 series steel grades in the cold rolling annealing and pickling line provided by the present invention, in step five, the pickling temperature of the first three rolls of steel grade conversion is reduced to the intersection temperature of the pickling temperature ranges of the two steel grades.

[0019] Preferably, in the direct conversion method between 200 series and 300 series steel grades in the cold rolling annealing pickling line provided by the present invention, no new nitric acid and hydrofluoric acid are added two hours before the steel grade conversion, and the acid concentration is maintained at the lower limit of the acid concentration parameter range for 300 series steel grades; after the 200 series steel grade coils enter the mixed acid section, the pickling process parameters are adjusted to complete the conversion of 200 series steel grades.

[0020] Preferably, in the direct conversion method between 200 series and 300 series steel grades in the cold rolling annealing and pickling line provided by the present invention, when switching the mixed acid circulation pump, the starting pressure of the mixed acid circulation pump for the latter steel grade is 0.1 MPa; after the mixed acid circulation pump is switched, the operating pressure of the mixed acid circulation pump for the latter steel grade is 0.25 MPa.

[0021] (III) Beneficial Effects

[0022] As described above, this invention provides a method for direct conversion between 200 series and 300 series steel grades in a cold-rolled annealing and pickling line. This conversion method includes: Step 1, annealing and pickling the previous steel grade; before conversion, when the remaining quantity of the previous steel grade is 2-4 coils, a pre-preparation stage for process parameter adjustment is initiated; Step 2, welding the tail of the previous steel grade strip to the head of the next steel grade strip; Step 3, after the tail of the last coil of the previous steel grade strip enters the annealing furnace area, adjustments are made according to the process parameters of the next steel grade. This invention modifies the process by using overlapping ranges of process parameters for different specifications, enabling direct conversion between 200 series and 300 series steel grades. After optimizing the steel grade conversion method, this invention eliminates the need for running test coils during steel grade conversion. By adjusting the operating process parameters during both the conversion of the previous and next steel grades, the consumption of auxiliary materials and media such as nitric acid, hydrofluoric acid, and natural gas can be reduced, thus saving costs. After process optimization, the surface quality of the steel strip is stable, reducing quality defects such as roller marks and stains caused by previous operation and debugging, achieving a win-win situation in both output and quality. Attached Figure Description

[0023] The accompanying drawings, which form part of this application, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an undue limitation of the invention. Wherein:

[0024] Figure 1 This is a schematic diagram illustrating the connection between steel grades when using the debug roll conversion steel grade conversion mode in the existing technology.

[0025] exist Figure 1In the diagram, the correspondence between component names and reference numerals is as follows:

[0026] The first steel grade a, the second steel grade b, and the test roll c.

[0027] Figure 2 This is a schematic diagram illustrating the connection between steel grades when using the direct conversion method in an embodiment of the present invention.

[0028] exist Figure 2 In the diagram, the correspondence between component names and reference numerals is as follows:

[0029] The first steel grade is 1, and the second steel grade is 2.

[0030] Figure 3 This is a flowchart illustrating the method for directly converting 200 series and 300 series steel grades in a cold-rolled annealing and pickling line according to an embodiment of the present invention. Detailed Implementation

[0031] The present invention will now be described in detail with reference to the accompanying drawings and embodiments. Various examples are provided by way of explanation and not by way of limitation. Indeed, those skilled in the art will recognize that modifications and variations can be made to the invention without departing from its scope or spirit. For example, a feature shown or described as part of one embodiment may be used in another embodiment to produce yet another embodiment. Therefore, it is desirable that the invention encompass such modifications and variations falling within the scope of the appended claims and their equivalents.

[0032] In the description of this invention, the terms "longitudinal," "lateral," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," and "bottom," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are used only for the convenience of describing the invention and do not require the invention to be constructed and operated in a specific orientation; therefore, they should not be construed as limitations on the invention. The terms "connected" and "linked" used in this invention should be interpreted broadly. For example, they can refer to a fixed connection or a detachable connection; they can refer to a direct connection or an indirect connection through intermediate components. Those skilled in the art can understand the specific meaning of the above terms according to the specific circumstances.

[0033] Please refer to Figure 2 and Figure 3 ,in, Figure 2 This is a schematic diagram illustrating the connection between steel grades when using the direct conversion method in an embodiment of the present invention. Figure 3 This is a flowchart illustrating the method for directly converting 200 series and 300 series steel grades in a cold-rolled annealing and pickling line according to an embodiment of the present invention.

[0034] This invention provides a method for direct conversion between 200 series and 300 series steel grades in a cold-rolled annealing and pickling line. The method includes the following steps: a pre-selection step, where steel grades are selected based on their thickness, with a conversion window thickness of 0.4mm ≤ H ≤ 0.6mm; Step 1, annealing and pickling the previous steel grade 1, with a remaining 2-4 coils of the previous steel grade 1 before conversion, entering a pre-preparation stage for process parameter adjustment; Step 2, welding the strip tail of the previous steel grade 1 to the strip head of the subsequent steel grade 2; Step 3, after the last coil of the previous steel grade 1 enters the annealing furnace area, adjusting the process parameters according to the subsequent steel grade 2; Step 4, after the first coil of the subsequent steel grade 2 exits the furnace area, quality inspection of the plate surface of the subsequent steel grade 2 is performed, and production speed is resumed after passing the quality inspection; and Step 5. Step five includes two options:

[0035] Option 1: In step five, when switching from 200 series steel to 300 series steel, at least two coils must be switched from production mode to test mode before the switch. The 300 series steel mixed acid circulation pump should be started and operated simultaneously with the 200 series steel mixed acid circulation pump. After the circulation pump pressure stabilizes, the 300 series steel mixed acid circulation pump should be increased and the 200 series steel mixed acid circulation pump should be shut down. The circulation pump switchover should be completed after the circulation pump pressure stabilizes again. For the two coils before the steel grade change, the acid temperature should be increased, and nitric acid and hydrofluoric acid should be added to raise the acid concentration of the mixed acid to the process parameter range for 300 series steel operation. When the 300 series steel coils enter the pickling section, the acid temperature and concentration of the mixed acid should be increased to the process control parameters for 300 series steel operation.

[0036] Option 2, in step five, when converting from 300 series steel to 200 series steel, two hours before the conversion, start the chiller and confirm its normal operation. For the first five coils of steel, switch the parameter control circulation pump to test mode, start the 200 series steel mixed acid circulation pump and operate it simultaneously with the 300 series steel mixed acid circulation pump. After the circulation pump pressure stabilizes, increase the 200 series steel mixed acid circulation pump and shut down the 300 series steel mixed acid circulation pump. After the circulation pump pressure stabilizes again, complete the pump switching. Specifically, for the first three coils of steel conversion, lower the pickling temperature to the intersection of the acid temperature ranges of the two steel grades. Furthermore, two hours before the conversion, do not add new nitric acid or hydrofluoric acid; maintain the acid concentration at the lower limit of the 300 series steel operating acid concentration parameter range. After the 200 series steel coils enter the mixed acid section, adjust the pickling process parameters to complete the 200 series steel conversion.

[0037] In step five, when switching the mixed acid circulation pump, the starting pressure of the mixed acid circulation pump of the second steel grade 2 is 0.1 MPa; after the switching of the mixed acid circulation pump is completed, the operating pressure of the mixed acid circulation pump of the second steel grade 2 is 0.25 MPa.

[0038] The aforementioned "previous steel grade 1" refers to the steel grade that is processed first among the two steel grades to be converted, while the "later steel grade 2" is the steel grade that follows "previous steel grade 1" (the steel grade to be converted) in the production sequence.

[0039] Specifically, in step three, after the last coil of the previous steel grade 1 enters the annealing furnace area, the process production speed is reduced to 50m / min-70m / min.

[0040] In existing technologies, such as Figure 1 As shown, the conversion between different steel grades is achieved through trial rolls. Before the steel grade conversion, after the production line completes the operation of the previous steel grade 'a' coil, the trial roll is welded to the tail end and run. During the time the trial roll 'c' is running, operators adjust the process parameters according to the requirements of the subsequent steel grade. Once the process parameters are stable, the trial roll is welded to the tail end and run for the next steel grade 'b', thus completing the steel grade conversion. This method of steel grade conversion through trial rolls can lead to problems such as long conversion times.

[0041] This invention provides a method for direct conversion between 200 series and 300 series steel grades in a cold-rolled annealing and pickling line. In this method, the conversion between different steel grades is performed directly without adjusting the coils. The specific construction requirements for this method are as follows: Before converting steel grades, the production line needs to prepare a production schedule based on the on-site conditions of the hard coils (mainly meeting the welding thickness overlap conditions and furnace temperature control conditions). Three coils should be prepared in advance for process parameter adjustments before the steel grade conversion. After the previous steel grade 1 coil is completed, the tail of the coil is directly welded to the head of the next steel grade 2 coil. When the tail of the previous steel grade 1 coil enters the furnace area, the unit's process section speed is reduced to 60 m / min, and the operators adjust the process parameters of each section according to the steel grade to be processed. After the next steel grade 2 coil exits the furnace area, the production line can only increase its speed to complete the steel grade conversion after the quality control personnel determine that the plate surface condition is normal.

[0042] Specifically, such as Figure 2 As shown, the thickness of the steel grade conversion window is 0.4mm≤H≤0.6mm, which means that it is possible to freely convert between SUS304 steel coils with a thickness of 0.4mm≤H≤0.5mm and 200 series steel coils with a thickness of 0.5mm≤H≤0.6mm.

[0043] The above schemes describe the overlapping and temperature control adjustment methods for steel grade conversion. The pickling control scheme is as follows:

[0044] There are two common methods for converting steel grades in cold acid processing workshops: one is to convert 200 series steel grades to 300 series steel grades, and the other is to convert 300 series steel grades to 200 series steel grades.

[0045] During the production of 300 series steel, the mixed acid is heated by a No. 1 circulating pump through a steam heat exchanger before being pumped into each pickling tank. During the production of 200 series steel, the mixed acid is cooled by a No. 3 circulating pump through a refrigeration heat exchanger before being pumped into each pickling tank. Therefore, it is evident that the circulating pumps used during pickling differ between 200 series and 300 series steel, and the required acid temperature also differs.

[0046] When switching from 200 series steel to 300 series steel, operators must switch the production mode to test mode two coils before the switch. Start the No. 1 circulating pump in the first, second, and third stages of the mixed acid process at 0.1 MPa, operating it in conjunction with the No. 3 circulating pump. Once the pressures of both pumps stabilize, increase the pressure of pump No. 1 by adjusting the inlet and outlet manual valves. After reaching 0.2 MPa, shut down pump No. 3 and then adjust the pressure of pump No. 1 to stabilize at a working pressure of 0.25 MPa, completing the pump switching. Simultaneously, because the acid temperature and concentration differ between 200 series and 300 series steel, two coils before the switch, shut down the refrigeration unit to raise the acid temperature in the second and third stages of the mixed acid process to 45°C. At the same time, add nitric acid and hydrofluoric acid to raise the acid concentration in the second and third stages to the range of process parameters for 300 series steel operation. When welding 300 series steel coils at the inlet, the acid temperature and concentration of the first stage of mixed acid treatment are increased to the process control parameters for 300 series steel operation. When the 300 series steel strip enters the furnace area, the acid temperature of the second and third stages of mixed acid treatment are increased to the process control parameters for 300 series steel operation. The heat exchanger mode is changed to steam mode, and then the testing mode is switched back to normal production mode, completing the 300 series steel conversion.

[0047] When converting from 300 series steel to 200 series steel, the production process for 200 series steel involves using a No. 3 circulating pump to cool the mixed acid in each stage via a refrigeration heat exchanger before pumping it into the pickling tanks. However, for 300 series steel, the mixed acid in each stage uses a No. 1 circulating pump to heat the mixed acid in each stage via a steam heat exchanger before pumping it into the pickling tanks. Therefore, two hours before the steel conversion, the refrigeration unit should be started and its normal operation confirmed. Then, the bypass valve should be opened to circulate and cool the heat exchange pipes and heat exchangers. This cooling process is relatively long and requires operators to monitor the cooling of the refrigeration unit and the cooling of the circulating water in the pipes. During the first five coils of steel grade conversion, the parameter control circulation pump was switched to test mode. The No. 3 circulation pump for the first, second, and third stages of the mixed acid conversion was started at 0.1 MPa, working in conjunction with the No. 1 circulation pump. Once the pressures of both pumps stabilized, the pressure of pump No. 3 was increased by adjusting the inlet and outlet manual valves. After the pressure reached 0.2 MPa, pump No. 1 was shut off, and the pressure of pump No. 3 was then adjusted to stabilize at a working pressure of 0.25 MPa, completing the circulation pump switchover. Because the acid temperature and concentration differ between 200 series and 300 series steel grades, the acid temperature in the first three coils of steel grade conversion was cyclically cooled from 55°C (for 300 series steel) to 45°C (the intersection of the acid temperature ranges of the two steel grades). This ensured both the quality of the 300 series steel before conversion and rapid cooling to a suitable acid temperature during the subsequent conversion to 200 series steel.

[0048] Furthermore, two hours before the steel grade conversion, no new nitric acid or hydrofluoric acid is added. The acid concentration is maintained at the lower limit of the acid concentration parameter range for 300 series steel grades, i.e., 40 g / L nitric acid and 4 g / L hydrofluoric acid in the first stage of mixed acid treatment, and 60 g / L nitric acid and 6 g / L hydrofluoric acid in the second and third stages of mixed acid treatment. This acid concentration can also be used for 200 series steel coils. After the 200 series steel coils enter the mixed acid treatment stage, the parameter control mode is switched to cooling mode to complete the 200 series steel grade conversion.

[0049] In the conversion method provided by this invention, the direct influencing factor of the matching of hard coil specifications between steel coils is the furnace plate temperature, and the indirect influencing factor is the steel strip thickness. Considering this indirect factor (steel strip thickness), in this invention, the overlap range of thickness parameters in the steel conversion is selected as 0.4mm≤H≤0.6mm, and steel grades within this thickness range can be converted. If the difference in furnace plate temperature between different steel grades is small, it is convenient for operators to adjust the TV value (product of thickness and speed) and temperature. For example, when converting a steel coil of SUS304 (thickness 0.5mm) to another steel coil of CJ5L (thickness 0.6mm), the operating speed of both steel grades is 100m / min, the TV value of SUS304 is 50, the TV value of CJ5L is 60, and their plate temperatures are similar. Therefore, SUS304 steel coils with a thickness of 0.4mm≤H≤0.5mm and 200 series steel coils with a thickness of 0.5mm≤H≤0.6mm can be used as the overlapping range thickness of the conversion parameter.

[0050] Please refer to the table below, which is the furnace temperature reference table for SUS304 and 314L steel grades.

[0051]

[0052] Please refer to the table below, which is the furnace temperature reference table for CJ5L steel.

[0053]

[0054] Please refer to the table below, which shows the acid temperature control parameters for the mixed acid section.

[0055]

[0056] Please refer to the table below, which shows the acid concentration control parameters for the mixed acid section.

[0057]

[0058]

[0059] Please refer to the table below, which shows the acid temperature control parameters for the mixed acid section.

[0060]

[0061] Please refer to the table below, which shows the acid concentration control parameters for the mixed acid section.

[0062]

[0063] The design changes in this invention are based on the following: The original process required a trial run to switch steel grades, utilizing the trial run time to adjust process parameters. After optimization, steel grade switching no longer requires running the trial run, reducing the consumption of auxiliary materials and media such as nitric acid, hydrofluoric acid, and natural gas, thus saving costs. The optimized process also ensures stable steel strip surface quality, reducing quality defects such as roller marks and stains previously caused by trial run operations, achieving a win-win situation for both output and quality. The main principle of this change is that the original process only considered differences in process parameters for the same thickness, preventing direct steel grade conversion. Instead, it uses overlapping ranges of process parameters for different specifications, allowing direct conversion between 200 series and 300 series steel.

[0064] The above are merely preferred embodiments of the present invention and are not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for direct conversion between 200 series and 300 series steel grades in a cold-rolled annealing and pickling line, characterized in that, include: In the pre-selection step, the steel grade is selected based on its thickness, and the window thickness for steel grade conversion is 0.4mm≤H≤0.6mm; Step 1: Anneal and pickle the previous steel grade. Before switching steel grades, when the remaining amount of the previous steel grade is 2-4 coils, enter the pre-preparation stage for adjusting process parameters. Step 2: Weld the tail of the strip of the previous steel grade to the head of the strip of the next steel grade. Step 3: After the last coil of the previous steel grade enters the annealing furnace area, adjust the process parameters according to the next steel grade. Step 4: After the first coil of the next steel grade exits the furnace, conduct a quality inspection on the surface condition of the next steel grade. Once the quality inspection is passed, resume the production speed. Step 5, pickling process adjustment, in which: When 200 series steel needs to be converted to 300 series steel, at least two coils should be switched from production mode to test mode before the conversion. The 300 series steel mixed acid circulation pump should be started and operated simultaneously with the 200 series steel mixed acid circulation pump. After the circulation pump pressure stabilizes, the 300 series steel mixed acid circulation pump should be increased and the 200 series steel mixed acid circulation pump should be turned off. After the circulation pump pressure stabilizes again, the circulation pump switching should be completed. For the two coils before the steel grade change, the acid temperature should be increased, and nitric acid and hydrofluoric acid should be added to raise the acid concentration of the mixed acid to the process parameter range for 300 series steel operation. When the 300 series steel coils enter the pickling section, the acid temperature and acid concentration of the mixed acid should be increased to the process control parameters for 300 series steel operation. When converting from 300 series steel to 200 series steel, start the chiller two hours before the conversion and confirm that it is starting normally. For the first five coils of steel, switch the parameter control circulation pump to test mode, start the 200 series steel mixed acid circulation pump and operate it simultaneously with the 300 series steel mixed acid circulation pump. After the circulation pump pressure stabilizes, increase the 200 series steel mixed acid circulation pump and shut down the 300 series steel mixed acid circulation pump. After the circulation pump pressure stabilizes again, complete the circulation pump switching. For the first three coils of steel, lower the pickling temperature to the intersection temperature of the two steel grades' acid temperature ranges. Two hours before the conversion, do not add new nitric acid or hydrofluoric acid; maintain the acid concentration at the lower limit of the 300 series steel operating acid concentration parameter range. After the 200 series steel coils enter the mixed acid section, adjust the pickling process parameters to complete the 200 series steel conversion.

2. The method for direct conversion between 200 series and 300 series steel grades in a cold-rolled annealing and pickling line according to claim 1, characterized in that, In step three, after the last coil of the previous steel grade enters the annealing furnace area, the production speed is reduced to 50 m / min-70 m / min.

3. The method for direct conversion between 200 series and 300 series steel grades in a cold-rolled annealing and pickling line according to claim 1, characterized in that, When switching between mixed acid circulation pumps, the starting pressure of the mixed acid circulation pump for the next steel grade is 0.1 MPa. After the mixed acid circulation pump is switched over, the operating pressure of the mixed acid circulation pump for the next steel grade is 0.25 MPa.

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