JP4379176B2 - Method of suppressing Mn removal from molten steel and RH degassing apparatus - Google Patents

Description

  The present invention relates to a method for suppressing deMn from molten steel when decarburizing from molten steel, and an RH degassing apparatus used for secondary refining of molten steel.

  Conventionally, in secondary refining of molten steel, for example, an RH (Ruhrstahl-Hausen) reflux-type vacuum degassing facility (hereinafter referred to as “RH degassing apparatus”) is used. In the RH degassing apparatus, the molten steel is recirculated between the inside of the vacuum chamber of the RH degassing apparatus and the ladle containing molten steel, and oxygen gas is injected from the top blowing lance to the molten steel surface in the vacuum tank. Decarburization is performed by blowing oxygen into the gas (see, for example, Patent Document 1).

The oxygen gas injected from the upper blow lance is normally blown at a constant lance height and an oxygen supply amount per unit time of the oxygen gas to be injected (hereinafter referred to as “acid feed rate”). ing.
JP-A-9-143546

By the way, in the secondary refining of molten steel with low carbon high Mn (manganese) steel containing Mn as a required component of steel products, when decarburizing from molten steel, Mn is evaporated and oxidized from the molten steel, Mn required as a component is lost to some extent (hereinafter referred to as “de-Mn”). Therefore, when decarburizing from molten steel, it is an important subject to suppress deMn from molten steel.
Here, when oxygen gas is injected onto the surface of the molten steel, a concave portion serving as a reaction interface is formed on the surface of the molten steel by the dynamic pressure of the oxygen gas (hereinafter referred to as “fire point”). If the supply amount of oxygen gas at the hot spot increases, the hot spot temperature rises. And, as the hot spot temperature rises, evaporation and oxidation of Mn also progress, so the amount of de-Mn increases.

However, if the acid feed rate is decreased to suppress de-Mn, the hot spot temperature is lowered, but the hot spot area is reduced and the refining ability for the molten steel is also relatively lowered. Therefore, in order to suppress deMn while efficiently performing decarburization, simply reducing the acid feed rate will extend the refining time, and thus is insufficient as a countermeasure.
The present invention has been made paying attention to such problems. When decarburizing from molten steel containing Mn (manganese), the present invention efficiently performs decarburization and suppresses deMn. An object of the present invention is to provide a de-Mn suppression method and an RH degassing apparatus that improve the yield of RH.

Normally, carbon reacts preferentially with oxygen over Mn at the beginning of refining (oxygen supply rate limiting). Therefore, at the initial stage of refining, the degree to which Mn is lost is relatively small compared to the end of refining. Therefore, at the initial stage of refining, it is effective to expel decarburization by giving a large amount of oxygen to the molten steel and preferentially reacting carbon and oxygen by the CO reaction.
However, after the middle stage of refining, the progress of de-Mn starts when the decarburization reaction proceeds to some extent. This is because the carbon concentration in the molten steel decreases and the carbon in the molten steel near the reaction interface decreases with respect to Mn, so the probability that the injected oxygen reacts with Mn increases (carbon transfer rate limiting). ). As described above, as the decarburization from the molten steel proceeds, the degree of transition from the oxygen supply rate limiting rate to the carbon transfer rate limiting rate gradually increases.

  Therefore, the inventors of the present application paid attention to the degree of transition from oxygen supply rate limiting to carbon transfer rate limiting, that is, a decrease in carbon concentration. A study of oxygen supply from the top blowing lance to maintain the decarburization capability while suppressing deMn was carried out. As a result, the hot spot temperature was lowered according to the decrease in the carbon concentration, and the fire at the end of refining. It was found that by maintaining the point area in the initial stage of refining, the decarburization ability can be maintained as much as possible while suppressing the conditions under which Mn preferentially reacts with oxygen over carbon. And based on this knowledge, it came to obtain the refining conditions aimed at by setting the height of the top blowing lance and the oxygen feed rate of the oxygen gas injected from the top blowing lance most effectively.

That is, in order to solve the above-mentioned problem, the first aspect of the present invention is the method in which the molten steel is decarburized from the molten steel by injecting oxygen gas from the top blowing lance onto the surface of the molten steel containing Mn. The method of suppressing de-Mn from the steel, wherein the oxygen feed rate of the oxygen gas from the top blowing lance is reduced based on the decrease in the carbon concentration in the molten steel, and the molten steel is injected by the oxygen gas injection. the height of the upper blowing lance is brought close to the surface of molten steel so as to suppress the reduction in the fire spot area is the area of the recesses formed on the surface, due to be brought close to the height of the upper blowing lance to the molten steel surface The suppression of the reduction of the hot spot area is carried out so as to keep the hot spot area constant.

The second aspect of the present invention is an RH degassing apparatus used for secondary refining, wherein an upper blowing lance for injecting oxygen gas onto the surface of the molten steel, and an oxygen gas injected from the upper blowing lance. Measures the carbon concentration in the molten steel from the exhaust gas discharged by the secondary refining, the lance height moving device that moves the height of the upper blowing lance, and the oxygen feeding rate adjusting device that adjusts the acid feeding rate. A carbon concentration measuring device, and based on the carbon concentration measured by the carbon concentration measuring device, the acid feeding rate adjusted by the acid feeding rate adjusting device is controlled to a desired acid feeding rate, and the lance height moving device An upper blowing lance control means for controlling the height of the upper blowing lance to be moved to a desired height, and the upper blowing lance control means controls the desired height and the desired acid feed rate to Injection of oxygen gas from the blowing lance Therefore, the height of the upper blowing lance is brought close to the molten steel surface so as to suppress the reduction of the hot spot area, which is the area of the recess formed on the surface of the molten steel, and the height of the upper blowing lance is set to the molten steel surface. The suppression of the reduction of the hot spot area by making it approach is characterized by controlling the hot spot area to be kept constant .
Here, in the RH degassing apparatus according to the second aspect of the present invention, the upper blowing lance control means includes a plurality of management carbons in a stepwise manner based on the degree of refining shifting from oxygen supply rate limiting to carbon transfer rate limiting. The concentration is set in advance, and when the current carbon concentration read sequentially reaches the control carbon concentration of each step, the process proceeds to the next control step in sequence, and the hot spot area is set to the control step before the transfer. While maintaining a constant level, if the height of the top lance is lowered and the oxygen gas feed rate is reduced compared to the management stage before the transition, decarburization is efficiently performed and de-Mn is suppressed. Thus, it is suitable for improving the yield of Mn.

According to the present invention, based on the decrease in the carbon concentration in the molten steel, the oxygen gas feeding rate can be reduced. As a result, the amount of oxygen supplied to the hot spot decreases, so the hot spot temperature can be lowered. Therefore, removal of Mn can be suppressed and the yield of Mn can be improved. Furthermore, since the hot spot area is kept constant so as to suppress the reduction of the hot spot area, which is the reaction interface, when reducing the oxygen gas feeding rate, the reduction in decarburization efficiency is also suppressed. Can do .

  In applying the present invention, the carbon concentration may be monitored either directly or indirectly (via a parameter). In other words, it is obvious that the carbon concentration usually decreases as the refining progresses, as well as directly monitoring the carbon concentration itself. For example, the relationship between the carbon concentration corresponding to the refining elapsed time is clarified from the past operation results. If it is made, based on the elapsed time of refining, that is, by using the elapsed time of refining as a parameter, it is possible to make a determination based on a decrease in carbon concentration indirectly.

  ADVANTAGE OF THE INVENTION According to this invention, decarburization can be performed efficiently and de-Mn can be suppressed and the yield of Mn can be improved.

Hereinafter, an embodiment of the present invention will be described with reference to the drawings as appropriate.
FIG. 1 is an explanatory view for explaining the configuration of an RH degassing apparatus according to the present invention, in which a part of the apparatus is shown in a block diagram and a vacuum tank of the RH degassing apparatus is shown in a longitudinal section. Yes.
As shown in the figure, the RH degassing apparatus 10 includes a vacuum chamber 13 and an upper blowing lance 11.
The upper blowing lance 11 is installed in the vacuum chamber 13 so as to hang down from the top of the vacuum chamber 13, and the oxygen gas 200 can be injected from an opening at the tip of the lance.

  The vacuum chamber 13 is a substantially cylindrical container, and an exhaust gas pipe 16 is connected to the upper portion of the vacuum chamber 13. The vacuum chamber 13 is connected to a vacuum exhaust device (not shown) via the exhaust gas pipe 16, and the inside of the vacuum chamber 13 can be evacuated by the vacuum exhaust device. Further, a suction pipe 14 and a discharge pipe 15 that are immersed in the molten steel 100 in the ladle 300 are connected to the lower part of the vacuum chamber 13. An argon gas blowing tube 12 is further connected to the suction pipe 14, and the molten steel in the suction pipe 14 is sucked up by an air lift pump action by the argon gas blown from the blowing pipe 12, so that the molten steel 100 is vacuumed. 13 can be raised. In the vacuum chamber 13, the gas component is removed from the molten steel 100 by reducing the equilibrium partial pressure of the molten steel 100 taken into the vacuum chamber 13 and injecting the oxygen gas 200 from the top blowing lance 11. 16 is exhausted. Then, the molten steel 100 taken into the vacuum chamber is returned to the ladle 300 from the discharge pipe 15 again. Thus, the RH degassing apparatus 10 can decarburize the molten steel 100 while refluxing the molten steel 100 between the inside of the vacuum chamber 13 and the ladle 300 containing the molten steel 100.

Here, as shown in FIG. 1, the RH degassing device 10 includes a carbon concentration measuring device 20, a lance height moving device 30, an acid feed rate adjusting device 40, and an upper blowing lance control unit 50. Further, the height of the top blowing lance 11 and the oxygen feed rate of the oxygen gas from the top blowing lance 11 can be controlled.
Specifically, the carbon concentration measuring device 20 includes a gas flow meter 21, a carbon concentration calculating unit 22, and a timer (not shown). The gas flow meter 21 is installed in the exhaust gas pipe 16 at the upper part of the vacuum chamber 13 so that the amount of exhaust gas can be measured. Moreover, the timer can measure the decarburization processing time of molten steel. Then, the carbon concentration calculation unit 22 calculates the timer from the exhaust gas flow rate difference which is the difference between the exhaust gas amount measured by the gas flow meter 21 and the latest actual exhaust gas amount measured by the RH degassing apparatus 10 in advance. It is possible to measure (estimate) the carbon concentration in the molten steel by calculating the estimated value of the carbon concentration at that time at the time of the decarburization treatment time being measured. Note that the carbon concentration calculation unit 22 performs a measurement (estimation) process of the carbon concentration in the molten steel based on a predetermined program, and the carbon concentration measurement process is a technique (already disclosed by the applicant of the present application). The detailed description is omitted because it can be implemented based on Japanese Patent No. 3293684).

The lance height moving device 30 includes a lance height moving unit 31 installed at the top portion of the vacuum chamber 13 and a lance height control unit 32 that controls the lance height moving unit 31.
The lance height moving part 31 includes a moving actuator part 31c and an airtight seal part 31f.
The movement actuator unit 31c includes a linear guide device 31a and a movement actuator 31b. The linear guide device 31a is operatively connected to the end of the top blowing lance 11 opposite to the opening, and supports the top blowing lance 11 on a predetermined track. The moving actuator 31b is operatively connected to the upper blowing lance 11 by, for example, a rack and pinion mechanism (not shown), and is set to a predetermined position by raising and lowering the upper blowing lance 11. A servo motor is used for the moving actuator 31b.

The airtight seal portion 31f includes an airtight seal 31d and an airtight seal pressure control valve 31e.
The hermetic seal 31d is attached to the iron shell 13a at the top of the vacuum chamber 13, and is an elastic member that can expand and contract and has a hollow portion into which high-pressure gas can be introduced. The hermetic seal pressure control valve 31e adjusts the pressure of the high-pressure gas introduced into the hermetic seal 31d. When a high-pressure gas is introduced into the airtight seal 31d from the airtight seal pressure control valve 31e, the airtight seal 31d expands in a direction to close the gap with the upper blowing lance 11, and the inside of the vacuum chamber 13 is airtight. The upper blowing lance 11 can be gripped. Further, when the high-pressure gas in the hermetic seal 31d is appropriately depressurized by the hermetic seal pressure control valve 31e, the gripping force of the upper blowing lance 11 is weakened. As a result, the hermetic seal portion 31 f can seal the inside of the vacuum chamber 13 and can hold the upper blowing lance 11.

  The lance height control unit 32 includes a control unit (not shown) that controls the opening and closing of the hermetic seal pressure control valve 31e that adjusts the flow rate of the high-pressure gas supplied into the hermetic seal 31d, and a driver (not shown) that drives the moving actuator 31b. The expansion amount of the hermetic seal 31d and the height of the upper blowing lance 11 can be controlled by a control signal from an upper blowing lance control unit 50, which will be described later. That is, the lance height control unit 32 first appropriately depressurizes the high-pressure gas in the hermetic seal 31d by the control signal from the upper blowing lance control unit 50, and then drives the moving actuator 31b to a desired position. The upper blowing lance 11 is moved, and then the moving actuator 31b is stopped and the pressure in the airtight seal 31d is increased again to hold the upper blowing lance 11 while airtight in the vacuum chamber 13.

The acid feed rate adjusting device 40 includes a flow rate control valve 41 and a throttle valve actuator 42.
The flow rate control valve 41 is installed on the oxygen gas supply line, and can adjust the oxygen gas feeding rate. The throttle valve actuator 42 is an actuator that opens and closes the throttle valve in the flow control valve 41. As a result, the acid feed rate adjusting device 40 drives the throttle valve in the flow rate control valve 41 in accordance with a control signal from the upper blow lance control unit 50 to be described later, and sends the oxygen gas delivered from the upper blow lance 11. The speed can be adjusted.

Next, the upper blowing lance control unit will be described in detail.
Based on a predetermined control program, the upper blow lance control unit 50 includes a CPU that controls the calculation and the entire system, a ROM that stores a CPU control program in a predetermined area, data read from the ROM, The RAM for storing the calculation results necessary for the calculation process of the CPU and the external device including the operation desk 60, such as the carbon concentration calculation unit 22, the lance height control unit 32, the throttle valve actuator 42, etc. It consists of an I / F that mediates input and output. These are connected to each other via a bus which is a signal line for transferring data so that data can be exchanged. As a result, the supervisor can input an execution command for the upper blowing lance control process from the operation desk 60 to the upper blowing lance control unit 50. Further, the upper blowing lance control unit 50 can output predetermined execution commands to the carbon concentration calculation unit 22, the lance height control unit 32, and the throttle valve actuator 42.

  FIG. 2 is a flowchart showing an upper blowing lance control process executed by the upper blowing lance control unit. In the following flowchart, “the management table data at the n-th stage” indicates that the “fire point area” is kept constant based on the height of the upper blow lance and the oxygen gas feeding rate measured in advance. The data of the top blowing lance height LHn and the oxygen gas acid feeding rate Jn are stored in advance as management table data for each stage so that they can be referred to as appropriate in a predetermined area of the ROM.

  Here, the “fire point area” refers to the area of a recess formed on the surface of the molten steel 100 by the dynamic pressure of the oxygen gas 200 injected from the top blowing lance 11 as shown in FIG. 3. In addition, since the shape seen from the axial direction of the upper blowing lance 11 is usually circular, the concave portion (fire point) has a size of “fire point area” as shown in FIG. The circle diameter Dk on the surface of the molten steel is represented.

  In the upper blowing lance control unit 50, when the supervisor inputs an execution command for the upper blowing lance control process from the operation desk 60 in the secondary refining by the RH degassing apparatus, the upper blowing lance control process shown in FIG. First, the process proceeds to step S100. The CPU is composed of a microprocessing unit MPU and the like, and activates a predetermined program stored in a predetermined area of the ROM so that the upper blowing lance control process shown in the flowchart of FIG. 2 can be executed according to the program. Yes.

  In step S100, the management stage n of the top blowing lance control process is set to “n = 1”, and the process proceeds to step S102. In step S102, the management carbon concentration Kn in the nth stage is read from the “nth stage management table data”, and the process proceeds to step S104. In step S104, the management lance height LHn at the nth stage is read from the “nth stage management table data”, and the current lance height is moved so as to become the read management lance height LHn. Is output to the lance height control unit 32, and the process proceeds to step S106. Further, in step S106, a predetermined oxygen feed rate Jn is read from the “nth stage management table data” and the current acid feed rate is adjusted so as to be the read managed acid feed rate Jn. Is output to the throttle valve actuator 42, and the process proceeds to step S108. In step S108, the current carbon concentration Kd measured by the carbon concentration measuring device 20 and stored in a predetermined area of the RAM is read from the carbon concentration calculation unit 22, and the process proceeds to step S110.

  In step S110, it is determined whether or not the current carbon concentration Kd read from the carbon concentration calculator 22 has reached a predetermined target carbon concentration K. That is, when it is determined that the current carbon concentration Kd exceeds the predetermined target carbon concentration K (the concentration is low) (Yes), the series of upper blowing lance control processing is terminated and the original processing is restored. However, when it determines with it not being so (No), it transfers to step S112.

In step S112, it is determined whether or not the current carbon concentration Kd has reached the management carbon concentration Kn in the n-th stage. That is, when it is determined that the current carbon concentration Kd exceeds the control carbon concentration Kn in the n-th stage (the concentration is low) (Yes), the process proceeds to step S114, but when it is determined that it is not ( No) returns the process to step S108.
In step S114, the management stage n of the upper blowing lance control process is newly set to “n = n + 1”, and the process returns to step S102.

  Here, in the flowchart, steps S100 to S114 correspond to the upper blowing lance control means. In addition, each signal from the upper blow lance control unit 50 is configured to be displayed on the monitor display of the operation desk 60 operated by the monitor at any time in a form that can be recognized by the monitor. The monitor can easily know the state of the upper blowing lance by the display on the display.

Next, a secondary refining method using the RH degassing apparatus having the above-described configuration will be described in detail with reference to FIG. 4 as appropriate. In this embodiment, as the “n-th stage management table data”, an example of managing each of the three stages (n = 1 to 3 in FIG. 2) of the initial stage of refining, the middle stage of refining, and the end stage of refining. explain.
In secondary refining by the RH degassing apparatus 10, decarburization is performed by injecting oxygen gas 200 from the top blowing lance 11 onto the surface of the molten steel 100 containing Mn in the vacuum chamber 13 and blowing oxygen into the molten steel 100. However, as described above, carbon preferentially reacts with oxygen over Mn at the initial stage of refining (oxygen supply rate limiting).

  Therefore, in the initial stage of refining, which is the first management stage (n = 1), the oxygen gas sending rate from the top blowing lance 11 is increased. At this time, if the height of the upper blow lance 11 is too low, there is a problem that the amount of metal that adheres to the inner wall of the vacuum chamber 13 increases due to the jumping (spitting) of the molten steel due to the dynamic pressure of oxygen gas. Therefore, the operation is started by determining the height of the upper blowing lance 11 so as to make the hot spot area as a reaction interface as large as possible while suppressing the spitting as much as possible. That is, the initial stage of refining is set to the first management stage (n = 1) (step S100), and the “first stage management table data” is referred to, as shown in FIG. Is set to LH1, the oxygen feed rate of oxygen gas corresponding to the lance height LH1 is set to J1, and the hot spot area formed by the lance height LH1 and the oxygen feed rate J1 is set to Dk1. Refining is started (steps S102 to S106).

Here, when the refining is started and the decarburization reaction proceeds to some extent, the progress of de-Mn starts. This is because the carbon concentration in the molten steel decreases, the carbon in the molten steel in the vicinity of the reaction interface decreases with respect to Mn, and the probability that the injected oxygen reacts with Mn increases (carbon transfer rate limiting). .
Therefore, in the secondary refining by the RH degassing device 10, when the secondary refining is started, the carbon concentration measuring device 20 measures (estimates) the carbon concentration in the molten steel as needed, and the top blowing lance control unit 50 The current carbon concentration Kd at that time is sequentially read and the changing carbon concentration is monitored (step S108).

  At this time, since the appropriate management carbon concentration K1 is set based on the past operation results based on the degree of shift from the oxygen supply-controlled rate to the carbon transfer-controlled rate, the current carbon concentration Kd read sequentially is managed as When the carbon concentration K1 is reached, it is considered that the refining initial stage which is the first management stage (n = 1) is completed (step S112), and the first management stage (n = 1) to the second management stage (n = 2) (Step S114).

  Next, in the refining middle stage which is the second management stage (n = 2), the “second stage management table data” is referred to, and as shown in FIG. While lowering to LH2, the oxygen gas feeding rate is reduced to J2. At this time, the lance height LH2 and the oxygen delivery rate J2 are set so that the formed hot spot area Dk2 is substantially constant with the hot spot area Dk1 in the first management stage (steps S102 to S102). S106). That is, in the second management stage (n = 2), the hot spot temperature is reduced by reducing the oxygen gas feed rate from the acid feed rate J1 to the acid feed rate J2 based on the current reduction in the carbon concentration Kd. Decreasing and decarbonizing capability while suppressing the condition where Mn preferentially reacts with oxygen over carbon by making the hot spot area Dk2 in the middle of refining almost the same as the state of fire point area Dk1 in the refining initial stage Is maintained as much as possible.

  In the middle of refining, an appropriate management carbon concentration K2 is set based on the past operation results based on the degree of shift from oxygen supply rate-limiting to carbon transfer rate-limiting. For this reason, when the current carbon concentration Kd read sequentially reaches the control carbon concentration K2, it is considered that the refining middle period, which is the second control stage (n = 2), has ended (step S112), and the second control The stage (n = 2) can be shifted to the third management stage (n = 3) (step S114).

  Next, in the same manner, at the final stage of refining, which is the third management stage (n = 3), the “third stage management table data” is referred to, and as shown in FIG. Is lowered to LH3, and the oxygen gas feeding rate is lowered to J3. At this time, the lance height LH3 and the oxygen delivery rate J3 are set so that the formed hot spot area Dk3 is substantially constant with the hot spot areas Dk1 and Dk2 in the first and second management stages. (Steps S102 to S106). That is, in the third management stage (n = 3), the hot spot temperature is reduced by reducing the oxygen gas feed rate from the acid feed rate J2 to the acid feed rate J3 based on the current reduction in the carbon concentration Kd. Further, by reducing the hot spot area Dk3 at the end of refining to the state of the hot spot area Dk2 at the middle of refining, the condition where Mn preferentially reacts with oxygen over carbon is suppressed while suppressing the conditions. Maintaining charcoal capacity as much as possible. When the current carbon concentration Kd read sequentially reaches the target carbon concentration oxygen K in the secondary refining, the upper blowing lance control unit 50 determines that the refining has been completed and ends the secondary refining ( Step S110).

Next, the operation and effect of secondary refining by the RH degassing apparatus according to the present invention will be described.
As described above, the RH degassing apparatus injects the oxygen gas 200 from the upper blowing lance 11 onto the surface of the molten steel 100 in the vacuum chamber 13 while refluxing the molten steel 100 between the vacuum chamber 13 and the ladle 300. Thus, decarburization is performed by blowing oxygen into the molten steel 100.
In particular, the RH degassing device 10 includes an acid feed rate adjusting device 40 that adjusts the acid feed rate of the oxygen gas 200 injected from the upper blowing lance 11, and a lance height that moves the height of the upper blowing lance 11. Based on the carbon concentration measuring device 20 for measuring the carbon concentration in the molten steel 100 from the exhaust gas discharged by the secondary refining, and the carbon concentration Kd measured by the carbon concentration measuring device 20. The acid feed rate J adjusted by the acid rate adjusting device 40 is controlled to a desired acid feed rate (J1, J2, J3), and the height LH of the upper blowing lance 11 moved by the lance height moving device 30 is set to a desired level. And an upper blow lance control unit 50 for controlling the height (LH1, LH2, LH3). Then, the upper blowing lance control unit 50 more specifically sets the desired height (LH1, LH2, LH3) and the desired acid feed rate (J1, J2, J3) so as to suppress the reduction of the hot spot area Dk. Specifically, control is performed so that the hot spot area Dk is substantially constant (that is, Dk1≈Dk2≈Dk3).

  With such a configuration, in the secondary refining by the RH degassing apparatus 10, when decarburization from the molten steel 100 is performed by injecting the oxygen gas 200 onto the surface of the molten steel 100 containing Mn, carbon in the molten steel 100 is used. Based on the decrease in the concentration Kd, the acid feed rate J of the oxygen gas 200 can be decreased while suppressing the decrease in the hot spot area Dk. More specifically, based on the decrease in the carbon concentration Kd in the molten steel 100, the oxygen feed rate J of the oxygen gas 200 from the top blowing lance 11 is decreased, and the decrease in the hot spot area Dk is suppressed. The height LH of the upper blowing lance 11 can be made to approach the molten steel surface.

  Thereby, according to the secondary refining by this RH degassing apparatus 10, according to the fall of the carbon concentration Kd in the molten steel 100, it becomes possible to lower a hot spot temperature. Therefore, removal of Mn can be suppressed and the yield of Mn can be improved. Furthermore, since the reduction of the hot spot area Dk is suppressed by making the hot spot area Dk constant, the decarburization efficiency can be maintained even if the acid feed rate J of the oxygen gas 200 is reduced. Therefore, decarburization can be performed efficiently and de-Mn can be suppressed to improve the yield of Mn.

Note that the RH degassing apparatus and the deMn prevention method according to the present invention are not limited to the above-described embodiments, and various modifications can be made without departing from the spirit of the present invention.
For example, in the above embodiment, the transition time of the management stage is determined by monitoring the carbon concentration itself, but the present invention is not limited to this. For example, the transition stage of the management stage can be managed by the refining elapsed time. In this case, it is obvious that the carbon concentration decreases as the refining time elapses. For example, if the relationship between the carbon concentration corresponding to the refining elapsed time is clarified from the past operation results, the refining elapsed time is based on the refining elapsed time. This is because even when the management stage is shifted, the top blowing lance can be indirectly controlled according to the decrease in the carbon concentration. In addition, if the management stage transition time by time management is changed according to the steel type and the initial carbon content, it can cope with various conditions. For example, in the case of the low-carbon high-Mn steel in the examples described later, the time to shift from the first management stage (n = 1) to the second management stage (n = 2) is about 9 minutes from the start of refining. What is necessary is just to transfer at the timing before and after. However, in order to more accurately determine the degree of shift from oxygen supply rate-controlled to carbon transfer rate-controlled and more appropriately shift each management stage, the actual carbon concentration is monitored and managed as in the above embodiment. It is desirable to determine the stage transition time. In the above embodiment, the carbon concentration itself is monitored based on the technology already disclosed by the applicant of the present application (Japanese Patent No. 3293684) to determine the transition time of the management stage. However, the present invention is not limited to this. Any configuration capable of measuring the carbon concentration is applicable to the present invention.

  Further, for example, in the above-described embodiment, the height of the lance and the acid feed rate are controlled by three management stages (n = 1 to 3), respectively, but the invention is not limited to this, and the management stage is, for example, two stages. Alternatively, the height of the lance and the acid feed rate may be controlled by more management steps (n = 4 or more). Furthermore, the height of the lance and the acid feed rate may be controlled continuously instead of stepwise. Here, when the upper blowing lance 11 is moved by the lance height moving device 30 described above, it is necessary to depressurize the inside of the airtight seal 31d to weaken the gripping force of the upper blowing lance 11, but at this time, the vacuum chamber 13 Gas leaks and the vacuum rate in the tank decreases. Therefore, in consideration of operation stability from such a viewpoint, it is desirable to set the number n of management stages of the top blowing lance control process to about three stages as in the above embodiment. It is needless to say that this is not a limitation as long as the configuration does not cause problems such as gas leakage in the vacuum chamber 13.

  Further, for example, in the above-described embodiment, the lance is controlled so that the hot spot area formed by the height of the lance and the acid delivery rate in each of the three management stages (n = 1 to 3) is substantially constant. However, the present invention is not limited to this, and even if the hot spot area is not always constant, the lance is controlled so as to reduce the acid feed rate while suppressing the reduction of the hot spot area. If it is done, the effect of the present invention is achieved.

  Specifically, for example, lance control as shown in FIG. 5 can be exemplified as an example of lowering the acid feed rate while suppressing the reduction of the hot spot area. That is, as shown in FIG. 5A, in this example, the acid feed rate J is continuously decreased as time t elapses. At this time, the lance height LH is controlled in three stages at a predetermined timing as in the above embodiment, as shown in FIG. As a result, the hot spot area Dk formed by the acid feed rate J and the lance height LH oscillates up and down as shown in FIG. However, since the hot spot area in the conventional lance control has been reduced as shown by the broken line in FIG. Therefore, even with the lance control as shown in this example, it is possible to suppress the decarburization efficiency and the Mn removal to some extent, and the effects of the present invention are exhibited.

Next, an example of secondary refining by the RH degassing apparatus according to the present invention will be described.
In this example, secondary refining by the RH degassing apparatus having the configuration of the above embodiment is applied when melting low carbon high Mn steel. In the RH degassing apparatus in this embodiment, the state of a fire point or the like in the vacuum chamber can be confirmed by a monitoring camera or the like.
This low-carbon high-Mn steel is an example in which the target component values of carbon and Mn are [C] 0.02 ± (allowable value) wt% and [Mn] 1.55 wt% or more, respectively. Then, the said management stage in the secondary refining was managed in three stages of the refining initial stage, the refining middle stage, and the refining final stage similarly to the said embodiment. In addition, since each said management step is automatically controlled by the upper blowing lance control part 50 of the RH degassing apparatus mentioned above, the description regarding control is abbreviate | omitted suitably here.

As shown in FIG. 6, in the refining initial stage C1 (first management stage (n = 1)), the oxygen gas feed rate J1 from the upper blowing lance 11 in the “first stage management table data” is 60 [Nm ³ / min], and the lance height LH1 was set to 2.0 [m]. The fire spot area at this time is Dk1.
Here, the management carbon concentration K1 that is a reference for the transition time from the refining initial stage C1 to the refining middle stage C2 (second management stage (n = 2)) was set to [C] 0.09 wt%. This is because, based on past refining results, it has been determined that the rate at which the carbon concentration in the molten steel shifts from oxygen supply-limited to carbon-transfer-limited at a boundary of [C] 0.09 wt% is particularly large. Refining started under these conditions. In addition, the carbon concentration of the molten steel in the initial stage of refining was [C] 0.15% by weight.

When about 9 minutes passed from the start of refining, the current carbon concentration Kd measured by the carbon concentration measuring device 20 reached [C] 0.09 wt%. As a result, the transition control from the refining initial stage C1 to the refining intermediate stage C2 is executed by the top blowing lance control unit 50. In the refining middle period C2, as shown in FIG. 6, the acid feed rate J2 in the “second stage management table data” was set to 45 [Nm ³ / min]. The lance height LH2 was controlled so that the hot spot area Dk2 at this time was substantially constant with the hot spot area Dk1 in the refining initial stage C1. In addition, the management carbon concentration K2 serving as a reference for the transition time to the refining end C3 (third management stage (n = 3)) was set to [C] 0.07% by weight.

Next, shortly after the transition to the refining mid-term C2, the current carbon concentration Kd reached [C] 0.07% by weight. Thereby, the transition control from the refining intermediate period C2 to the refining final stage C3 was executed by the upper blowing lance control unit 50. In the refining final stage C3, as shown in FIG. 6 , the acid feed rate J3 in the “third stage management table data” is set to 35 [Nm ³ / min], and the hot spot area Dk3 at this time is The lance height LH2 was set so as to be substantially constant with the hot spot areas Dk1 and Dk2 in the refining initial stage C1 and the refining intermediate stage C2. In addition, in the same figure, as a comparative example, a graph when the lance height and the oxygen feed rate of the oxygen gas to be injected are kept constant is shown together, as shown in FIG. In the comparative example, the acid feed rate J was set to 60 [Nm ³ / min], and the lance height LH at this time was set to 2.0 [m].

Next, after the transition to the refining end C3, the current carbon concentration Kd reached [C] 0.02 wt% (target carbon concentration oxygen K). Thereby, the scheduled decarburization process was judged by the upper blow lance control unit 50 to be finished, and the refining was finished.
FIG. 7 is a graph showing the relationship between the Mn concentration in the molten steel and the treatment time in the secondary refining in the above example. In addition, in the same figure, as a comparative example, a graph when the blowing is performed with the lance height and the acid feeding speed being fixed is also shown.

  As can be seen from the figure, the decrease in the Mn concentration is suppressed after the refining middle period C2 (when the processing time is about 9 minutes). Moreover, the refining treatment time is also completed in about 25 minutes, which is the same treatment time as the comparative example, and it is possible to decarburize to a predetermined carbon concentration without extending the treatment time. I understand that. Here, as shown by the arrow B in the figure, the Mn concentration at the end of the refining was suppressed from decreasing the concentration of [Mn] 0.10 wt% with respect to the comparative example.

FIG. 8 plots the ratio of the average Mn concentration to the average carbon concentration in the molten steel in the secondary refining in the above example. In addition, in the same figure, the example at the time of blowing in with each lance height and acid sending speed | velocity | rate being made constant as a comparative example is also shown in figure.
As can be seen from the figure, in the range where the average carbon concentration is lower than [C] 0.09% by weight (arrow E shown in the figure), the proportion of the average Mn concentration is greatly reduced in the examples as compared with the comparative example. I understand that

  FIG. 9 is a graph showing the results of summing up the amount of MetMn used for melting the low-carbon high-Mn steel by month for the MetMn basic unit (kg / t). In the figure, from June to September, the results of secondary refining (comparative example) using a conventional RH degassing apparatus are shown, and in October, secondary refining (implementation using the RH degassing apparatus of the present invention) Example) shows the result.

As can be seen from the figure, after September (October), when secondary refining using the RH degassing apparatus of the present invention was implemented, the amount of MetMn used was greatly suppressed and the yield of MetMn was improved. I understand.
As described above, according to the RH degassing apparatus of the present invention and the deMn suppressing method of the present invention, it is confirmed that decarburization can be efficiently performed and deMn can be suppressed to improve the yield of Mn. It was done.

It is explanatory drawing explaining one Embodiment of the RH degassing apparatus which concerns on this invention. It is the elements on larger scale which expand and show the A section in FIG. 1, In the same figure, the upper blowing lance is shown by the longitudinal cross-section. It is a flowchart which shows the upper blowing lance control process performed in an upper blowing lance control part. It is explanatory drawing explaining the management stage of the secondary refining which concerns on this invention. It is explanatory drawing explaining the other example of the control which reduces the acid sending speed | rate, suppressing the reduction | decrease of a hot spot area. It is explanatory drawing explaining one Example which employ | adopted the removal Mn suppression method which concerns on this invention. It is a graph which shows the relationship between the Mn density | concentration in molten steel, and processing time in an Example. It is explanatory drawing explaining the effect of the removal Mn suppression method which concerns on this invention. It is explanatory drawing explaining the effect of the removal Mn suppression method which concerns on this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Degassing apparatus 11 Top blowing lance 12 Blowing pipe 13 Vacuum tank 14 Suction pipe 15 Exhaust pipe 16 Exhaust pipe 20 Carbon concentration measuring device 21 Gas flow meter 22 Carbon concentration calculating part 30 Moving device 31 Moving part 32 Control part 40 Sending acid Speed control device 41 Flow control valve 42 Valve actuator 50 Upper blowing lance control unit 60 Operation desk 100 Molten steel 200 Oxygen gas 300 Ladle C1 Early refining C2 Middle refining C3 Final refining Dk Fire point diameter (fire point area)
Jn Managed acid feed rate K Target carbon concentration Kd Current carbon concentration Kn Managed carbon concentration

Claims (3)

A method of suppressing de-Mn from the molten steel when performing decarburization from the molten steel by injecting oxygen gas from an upper blowing lance onto the surface of the molten steel containing Mn,
Based on the decrease in the carbon concentration in the molten steel, the oxygen point from the upper blowing lance is decreased, and the fire point is the area of the recess formed on the surface of the molten steel by the injection of the oxygen gas. The suppression of the reduction of the hot spot area by bringing the height of the upper blowing lance closer to the surface of the molten steel so as to suppress the decrease of the area and bringing the height of the upper blowing lance closer to the molten steel surface A method for suppressing Mn removal, which is carried out so as to keep the point area constant. An RH degassing device used for secondary refining,
An upper blowing lance for injecting oxygen gas onto the surface of the molten steel, an acid feeding speed adjusting device for adjusting an acid feeding speed of oxygen gas injected from the upper blowing lance, and a lance height for moving the height of the upper blowing lance A carbon concentration measuring device for measuring the carbon concentration in the molten steel from the exhaust gas exhausted by the secondary refining, and the acid feed rate based on the carbon concentration measured by the carbon concentration measuring device. And an upper blowing lance control means for controlling the acid feeding speed adjusted by the adjusting device to a desired acid feeding speed and controlling the height of the upper blowing lance moved by the lance height moving device to a desired height. ,
The upper blowing lance control means reduces the hot spot area, which is the area of a recess formed on the surface of the molten steel by injecting oxygen gas from the upper blowing lance so that the desired height and the desired acid feed rate are obtained. In order to suppress the reduction of the hot spot area by bringing the height of the upper blowing lance closer to the surface of the molten steel and suppressing the height of the upper blowing lance closer to the molten steel surface, An RH degassing apparatus that is controlled so as to be kept constant .   The upper blowing lance control means has a plurality of control carbon concentrations set in advance step by step based on the degree of refining shifting from oxygen supply rate limiting to carbon transfer rate limiting. When the management carbon concentration of the stage is reached, the process proceeds to the next management stage in sequence, maintaining the hot spot area constant with respect to the management stage prior to the transition, while maintaining a higher blowing lance than the management stage prior to the transition. The RH degassing apparatus according to claim 2, wherein the RH degassing apparatus is configured to lower the height and reduce the oxygen gas feeding rate.

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