Method for reducing mixed casting quantity of different steel types in same tundish in continuous casting

Abstract

The invention relates to a method for reducing the mixed casting quantity of different types of steel in the same tundish in continuous casting, which comprises the following steps: step 1, transporting a steel ladle filled with molten steel in different types of steel to a continuous casting steel ladle rotary table by a travelling crane to enable the steel ladle to be at a casting waiting station, step 2, tightly adjusting a steel ladle roughing slag alarm value, step 3, controlling the residual molten steel quantity in the tundish, and step 4, controlling the molten steel quantity in the tundish. 4, when the step 3 is implemented, the opening degree of a tundish stopper rod is controlled, so that the remaining molten steel amount of the tundish continues to be reduced; and 5, when the tonnage (namely the mixed pouring amount) of the tundish is controlled to meet the requirement of the required mixed pouring amount of the different steel grades according to the steps, the steel ladle of the last heat is rotated to a non-working station, the steel ladle of the waiting molten steel of the different steel grades is rotated to a casting station, and a steel ladle sliding plate is opened for mixed pouring, so that the operation of reducing the mixed pouring amount is completed.

Description

Technical Field

[0001] The invention relates to a method, in particular to a method for reducing the mixed pouring amount of different steel grades in the same tundish for continuous casting, and belongs to the technical field of continuous casting control. Background Art

[0002] In continuous casting production, in order to improve the operating rate of the continuous casting machine, mixed casting of different steel grades is inevitable. However, in actual production, two steel grades with very different compositions are often cast continuously, so that the two compositions cross-penetrate, resulting in an increase in mixed casting billets (if mixed casting is not performed, it will affect the continuity of production and the utilization of production capacity). Since the mixed casting billets cannot be used as the original steel grade, they can only be downgraded or directly downgraded to the agreed product, causing great losses to the enterprise. In addition, due to the uneven composition and strength of the head and tail of the mixed casting billets, it brings inconvenience to the hot rolling mill.

[0003] At present, the mixed pouring volume of peers is controlled at 60 tons / tundish, while the mixed pouring volume of our factory is 70 tons / tundish (the mixed pouring volume is composed of the remaining molten steel in the original tundish and the molten steel of different steel grades to be cast). The mixed pouring volume is large, so it is necessary to reduce the mixed pouring volume.

[0004] In order to organize continuous production and reduce the amount of mixed casting of different steel grades, similar steel mills at home and abroad have made the following improvements:

[0005] Reference 1: Lu Yunwei et al. Transition time and influencing factors of continuous casting and continuous pouring mixed billet. Anshan Iron and Steel Technology. 1995, (3): 12-16;

[0006] Reference 2: Lu Yunwei et al. Transition time and influencing factors of continuous casting mixed billet. Steelmaking. 1995, (4): 30-33, 12;

[0007] Reference 3: Hou Jilin. Research on the determination of mixed steel zone in continuous casting of multiple furnaces. Baosteel Science and Technology. 2000, 26(1): 43-46. This paper introduces the research on the mixed steel ingot problem in continuous casting by Baosteel through mathematical calculation, water model experiment and on-site sampling. Finally, the determination of mixed steel zone was completed, the reasonable range of mixed steel zone was proposed, and the length cutting table of mixed steel ingot was formulated.

[0008] Document 4: Liang Lei et al. Shanghai Meishan Iron and Steel Co., Ltd. Automatic insertion device for continuous casting of different steel grades in the same tundish. Chinese patent. Application number: 200720041903, 2008-10-08.

[0009] Document 5: Chen Jikang et al. Baosteel Co., Ltd. Automatic iron plate insertion device for isolating different steel grades during continuous casting. Chinese patent. Application number: 99205873, 2000-03-01.

[0010] References 1 to 3 respectively introduce the rapid initial changes in the chemical composition of the molten steel and the ingot in the tundish during continuous casting of different steel grades, the slow transition of the chemical composition to the continuous casting furnace (tank) in the later stage, and the lag of the side flow behind the middle flow, as well as the formation and influencing factors of the transition time and the amount of mixed ingot during continuous casting, such as the shape and structure of the tundish, the amount of residual molten steel, the composition difference and the mass flow rate.

[0011] Document 4 provides an automatic insertion device for continuous casting of different steel grades in the same tundish with a quick-change mechanism installed at the bottom of the tundish, which can solve the problem of continuous casting of different steel grades in the same tundish in a slab continuous casting machine.

[0012] Reference 5 provides an automatic iron plate insertion device for isolating continuous casting of different steel grades, which can ensure the quality of the joints between different steel grades.

[0013] The present invention utilizes the control means to prevent the molten steel in the tundish from falling into the crystallizer during the continuous casting process (i.e., the means to control the falling slag during the continuous casting process) to achieve the purpose of reducing the mixed casting amount, which is different from the invention method of the above-mentioned related patents retrieved.

[0014] The mixing amount is determined by the amount of molten steel remaining in the tundish before mixing. The less the amount is, the less mixing amount is. Therefore, a new solution is urgently needed to solve the above technical problems. Summary of the invention

[0015] The present invention is aimed at the problems existing in the prior art and provides a method for reducing the mixed pouring amount of different steel types in the same tundish during continuous casting. The technical scheme utilizes the control means of preventing the molten steel in the tundish from slagging into the crystallizer during the continuous casting process (i.e. the means of controlling slag dropping during the continuous casting process) to achieve the purpose of reducing the mixed pouring amount.

[0016] In order to achieve the above object, the technical solution of the present invention is as follows: a method for reducing the amount of mixed casting of different steel grades in the same tundish for continuous casting, the method comprising the following steps:

[0017] Step 1: The ladle filled with molten steel of different steel grades is transported by a crane to the ladle turntable of the continuous casting, so that it is placed in the waiting casting position, because at this time the molten steel in the ladle originally being cast (which is of a different steel grade from the molten steel in the ladle at the waiting position) is being cast into the tundish through the long water inlet at the bottom of the ladle.

[0018] Step 2: Tighten the ladle slag alarm value to prevent slag mixed in the ladle molten steel from flowing into the crystallizer, improve the cleanliness of the molten steel in the tundish, and thus achieve the purpose of reducing the amount of mixed casting (because the cleanliness of the molten steel in the tundish is high, too much molten steel in the tundish can flow into the crystallizer to produce slabs). When the ladle molten steel is about to be cast as mentioned above, the molten steel slag detection and forecasting device at the bottom of the ladle will sound. Once it sounds, it means that molten steel slag has flowed into the tundish with the steel flow. At this time, the slide plate on the upper part of the ladle long nozzle must be turned off immediately, which means that this ladle of molten steel has been cast and the next ladle of molten steel is ready to continue to be cast into the tundish.

[0019] Step 2 Description: The alarm setting of the molten steel slag detection and prediction device is related to the slag area of ​​the steel flow cross section of the ladle long nozzle, that is, when the slag area reaches a certain percentage of the long nozzle cross-sectional area (the forecast setting value is the percentage of the slag area to the nozzle inner diameter cross-sectional area, which is called the slag alarm value in production), the slag detection and prediction alarm will sound. Obviously, the larger the slag alarm value, the greater the slag amount. However, in conventional production, the slag detection alarm value is always a fixed constant (M).

[0020] Step 3: Control the amount of molten steel remaining in the tundish (i.e., reduce the amount of mixed casting). The method is that before casting the next ladle of molten steel of a different steel grade, when the slag detection device alarms at the end of casting, the alarm value M is adjusted to a new alarm value (Y) required, and is linked to the amount of molten steel (T) in the ladle:

[0021] T=(MY) / 5

[0022] Where: T-the amount of molten steel without slag, tons;

[0023] P- daily fixed slag alarm value (do not add percentage sign %).

[0024] Y- adjusted slag alarm value (do not add percentage sign %)

[0025] Step 3 Description: In terms of reducing the mixed pouring amount in the tundish, if the alarm value decreases by 5 (do not add the percentage sign here), 1 ton of molten steel can be reduced from being cast into the tundish, that is, before the mixed pouring of molten steel of different steel grades in the lower ladle, the remaining molten steel in the tundish can be reduced by 1 ton.

[0026] Compared with the original step 3 before improvement, when the molten steel in the tundish is reduced to the normal tonnage (about 30 tons), it can continue to reduce, while in the original step, when it is reduced to the normal tonnage, it will not continue to reduce (otherwise slag will be deposited), but the next package of molten steel of different steel grade will be cast into the tundish, so the mixed pouring amount is relatively large. Now the mixed pouring amount is controlled by the above relationship, which is a control method for approximately reducing the mixed pouring amount of molten steel in the tundish developed by the present invention in production.

[0027] Step 4: While implementing step 3, control the opening of the tundish stopper rod to continue to reduce the amount of molten steel remaining in the tundish (i.e., continue to reduce the amount of mixed casting). The method is to establish the relationship between the opening of the tundish stopper rod and the reduction of the mixed casting amount according to the mixed casting steel grades with different quality requirements.

[0028] Step 4 description: When the ladle slide is closed, the amount of molten steel in the tundish is controlled by the stopper rod opening. The larger the opening of the tundish stopper rod, the more molten steel will flow into the crystallizer, the faster the amount of molten steel in the tundish will decrease, but the degree of slag entanglement will be greater. Conversely, the smaller the opening, the less molten steel will flow into the crystallizer, the slower the amount of molten steel in the tundish will decrease, the less slag entanglement will be, or even no slag entanglement (the slag is blocked by the stopper rod). Based on this feature, in practice, the following relationship has been established between the stopper rod opening and the reduction in the amount of molten steel in the tundish (i.e., molten steel waiting for mixed pouring):

[0029] T=(100-K) / 25

[0030] Where:

[0031] Q-reduction amount of molten steel waiting for mixed pouring, tons;

[0032] R-stopper opening value, substituted with a number without a percentage sign;

[0033] The "100" in the formula refers to the full opening value of the stopper rod.

[0034] Step 5: When the tundish tonnage (i.e., mixed pouring volume) is controlled to the required mixed pouring volume of different steel grades according to the above steps, the ladle of the previous furnace is transferred to the non-working position, and the ladle of the waiting different steel grade molten steel is rotated to the casting position, and the ladle slide is opened for mixed pouring. At this point, the operation of reducing the mixed pouring volume is completed.

[0035] Compared with the prior art, the present invention has the following advantages: (1) The ladle slag detection technology is used to control the slag at the end of ladle casting. It is a control technology for the slag of molten steel in the ladle during normal casting. The present invention uses this technology to reduce the amount of mixed pouring of molten steel in the tundish. It is an innovative technology that has not been reported. Its core technical means is to strictly control the slag amount to improve the cleanliness of the molten steel in the tundish to achieve the purpose of reducing the amount of mixed pouring of molten steel in the tundish (because the cleanliness of the molten steel in the tundish is high, too much molten steel in the tundish can flow into the crystallizer to produce slabs). When applying the slag detection, the parameters are corrected instead of being used directly, which reflects creativity. (2) The stopper rod is a technical means that must be used to control the steel flow rate of almost every furnace of molten steel in the tundish during the continuous casting process to control the stability of the amount of molten steel in the tundish during the casting process. However, in the mixed pouring process, peers have never used the method of controlling the stopper rod opening to reduce the mixed pouring amount. The general situation is to use baffles and other technologies to reduce the mixed pouring amount. The present invention uses the stopper rod opening control method to reduce the mixing amount, which has not been reported before. In addition, when using it, it does not directly control the stopper rod opening by conventional means, but establishes a mixing control mode of the stopper rod opening to reduce the mixing amount, which reflects creativity. (3) The technology used in the present invention to reduce the mixing amount is simple and easy, highly practical, does not require investment, is highly feasible, and does not pollute, reflecting the concept of green steelmaking production. DETAILED DESCRIPTION

[0036] In order to deepen the understanding of the present invention, a detailed description is given below in conjunction with embodiments.

[0037] Embodiment 1: A method for reducing the amount of mixed pouring of different steel grades in the same tundish for continuous casting, the method comprising the following steps:

[0038] Step 1: The ladle filled with molten steel of different steel grades is transported by a crane to the ladle turntable of the continuous casting, so that it is placed in the waiting casting position, because at this time the molten steel in the ladle originally being cast (which is of a different steel grade from the molten steel in the ladle at the waiting position) is being cast into the tundish through the long water inlet at the bottom of the ladle.

[0039] Step 2: Tighten the ladle slag alarm value to prevent slag mixed in the ladle molten steel from flowing into the crystallizer, improve the cleanliness of the molten steel in the tundish, and thus achieve the purpose of reducing the amount of mixed casting (because the cleanliness of the molten steel in the tundish is high, too much molten steel in the tundish can flow into the crystallizer to produce slabs). When the ladle molten steel is about to be cast as mentioned above, the molten steel slag detection and forecasting device at the bottom of the ladle will sound. Once it sounds, it means that molten steel slag has flowed into the tundish with the steel flow. At this time, the slide plate on the upper part of the ladle long nozzle must be turned off immediately, which means that this ladle of molten steel has been cast and the next ladle of molten steel is ready to continue to be cast into the tundish.

[0040] Step 2 Description: The alarm setting of the molten steel slag detection and prediction device is related to the slag area of ​​the steel flow cross section of the ladle long nozzle, that is, when the slag area reaches a certain percentage of the long nozzle cross-sectional area (the forecast setting value is the percentage of the slag area to the nozzle inner diameter cross-sectional area, which is called the slag alarm value in production), the slag detection and prediction alarm will sound. Obviously, the larger the slag alarm value, the greater the slag amount. However, in conventional production, the slag detection alarm value is always a fixed constant (M).

[0041] Step 3: Control the amount of molten steel remaining in the tundish (i.e., reduce the amount of mixed casting). The method is that before casting the next ladle of molten steel of a different steel grade, when the slag detection device alarms at the end of casting, the alarm value M is adjusted to a new alarm value (Y) required, and is linked to the amount of molten steel (T) in the ladle:

[0042] T=(MY) / 5

[0043] Where: T-the amount of molten steel without slag, tons;

[0044] S- daily fixed slag alarm value (do not add percentage sign %).

[0045] Y- adjusted slag alarm value (do not add percentage sign %)

[0046] Step 3 Description: In terms of reducing the mixed pouring amount in the tundish, if the alarm value decreases by 5 (do not add the percentage sign here), 1 ton of molten steel can be reduced from being cast into the tundish, that is, before the mixed pouring of molten steel of different steel grades in the lower ladle, the remaining molten steel in the tundish can be reduced by 1 ton.

[0047] Compared with the original step 3 before improvement, when the molten steel in the tundish is reduced to the normal tonnage (about 30 tons), it can continue to reduce, while in the original step, when it is reduced to the normal tonnage, it will not continue to reduce (otherwise slag will be deposited), but the next package of molten steel of different steel grade will be cast into the tundish, so the mixed pouring amount is relatively large. Now the mixed pouring amount is controlled by the above relationship, which is a control method for approximately reducing the mixed pouring amount of molten steel in the tundish developed by the present invention in production.

[0048] Step 4: While implementing step 3, control the opening of the tundish stopper rod to continue to reduce the amount of molten steel remaining in the tundish (i.e., continue to reduce the amount of mixed casting). The method is to establish the relationship between the opening of the tundish stopper rod and the reduction of the mixed casting amount according to the mixed casting steel grades with different quality requirements.

[0049] Step 4 description: When the ladle slide is closed, the amount of molten steel in the tundish is controlled by the stopper rod opening. The larger the opening of the tundish stopper rod, the more molten steel will flow into the crystallizer, the faster the amount of molten steel in the tundish will decrease, but the degree of slag entanglement will be greater. Conversely, the smaller the opening, the less molten steel will flow into the crystallizer, the slower the amount of molten steel in the tundish will decrease, the less slag entanglement will be, or even no slag entanglement (the slag is blocked by the stopper rod). Based on this feature, in practice, the following relationship has been established between the stopper rod opening and the reduction in the amount of molten steel in the tundish (i.e., molten steel waiting for mixed pouring):

[0050] T=(100-K) / 25

[0051] Where:

[0052] T-reduction amount of molten steel waiting for mixed pouring, tons;

[0053] U- plug opening value, substituted with a number without a percentage sign;

[0054] The "100" in the formula refers to the full opening value of the stopper rod.

[0055] Step 5: When the tundish tonnage (i.e., mixed pouring volume) is controlled to the required mixed pouring volume of different steel grades according to the above steps, the ladle of the previous furnace is transferred to the non-working position, and the ladle of the waiting different steel grade molten steel is rotated to the casting position, and the ladle slide is opened for mixed pouring. At this point, the operation of reducing the mixed pouring volume is completed.

[0056] Embodiment 2:

[0057] The highlight of the present invention is to establish the above relationship for reducing the amount of molten steel remaining in the tundish by means of controlling the slag during the continuous casting process, so as to achieve that when the amount of molten steel in the tundish is reduced (i.e., the amount of molten steel waiting for mixed casting), as the amount of molten steel is reduced, if the molten steel level is lower than the critical vortex height (i.e., the critical slag roll height, under normal circumstances, slag will roll into the crystallizer if it is lower than this height), it will not cause slag roll into the crystallizer, thereby reducing the mixed casting amount as much as possible. In short, by reducing the amount of molten steel in the tundish (i.e., the amount waiting for mixed casting) according to the present invention, when the molten steel level in the tundish is reduced to below the critical slag roll height, it will not cause slag roll into the crystallizer. However, the normal control means for reducing the mixed casting amount cannot reduce it to below the critical height of the molten steel in the tundish.

[0058] The specific implementation of this invention patent is as follows:

[0059] Step 1: The molten steel in the ladle of different steel grades that needs to be mixed and poured enters the ladle turntable of the continuous casting platform and is in the waiting position. At this time, when the molten steel in the ladle being cast is about to be finished, the predicted value of the ladle slag detection is 25%, and it is adjusted according to the relationship: T = (MY) / 5, so as to close the bottom slide of the ladle in advance, the purpose is to reduce the amount of slag (that is, to prevent or reduce the slag in the ladle from flowing into the molten steel in the tundish below the ladle with the steel flow). In this way, in the follow-up, due to the high cleanliness of the molten steel in the tundish, more can flow into the crystallizer to produce slabs, that is, the amount of mixed pouring of molten steel in the tundish is reduced. There are several situations in which the amount of mixed pouring of molten steel in the tundish can be reduced by controlling the slag detection, as shown in Table 1.

[0060] Table 1. Implementation method for reducing the amount of mixed steel pouring in the tundish by setting the slag detection prediction value

[0061]

[0062] Step 2: When the slag detection alarm is heard during step 1, the slide plate at the bottom of the ladle is immediately closed to stop the molten steel from flowing down. At this time, the molten steel in the tundish below the ladle continues to flow into the crystallizer. In order to allow too much molten steel to flow into the crystallizer to produce slabs, that is, to reduce the amount of mixed casting of molten steel in the tundish, the stopper rod opening is adjusted at this time, which can allow the molten steel in the tundish to continue to flow into the crystallizer and prevent slag from flowing into the crystallizer. The implementation method is to reduce the stopper rod opening, and the reduction range is implemented according to the relationship: T = (100-K) / 25, see Table 2.

[0063] Table 2. Implementation method of reducing the amount of mixed pouring by adjusting the stopper opening

[0064]

[0065]

[0066] Step 3: When step 2 is completed, the molten steel in the ladle of different steel grades in the waiting position is transferred back to the working position (or it can be transferred back in advance in step 2), the slide plate at the bottom of the ladle is opened, and the molten steel of different steel grades flows into the tundish, completing the mixed casting task.

[0067] Through the implementation of the above measures, the average number of times that the tonnage of the tundish needs to be controlled for mixed pouring is 1,500 times per year. The proportion of general mixed pouring steel types is 15%, and the mixed pouring amount is reduced by 2 tons / tundish, of which step 1 in the implementation (i.e. reducing the mixed pouring amount of molten steel in the tundish by setting the slag detection prediction value) reduces the mixed pouring amount by 1 ton / tundish, and step 2 in the implementation (i.e. reducing the mixed pouring amount by adjusting the stopper rod opening) reduces the mixed pouring amount by 1 ton / tundish; the proportion of steel types with high requirements for mixed pouring degradation is 85%, and the mixed pouring amount is reduced by 4 tons / tundish, of which step 1 in the implementation (i.e. reducing the mixed pouring amount of molten steel in the tundish by setting the slag detection prediction value) reduces the mixed pouring amount by 2 tons / tundish, and step 2 in the implementation (i.e. reducing the mixed pouring amount by adjusting the stopper rod opening) reduces the mixed pouring amount by 2 tons / tundish. The annual reduction in mixed pouring amount is calculated as:

[0068] 1500*15%*(1+1)+1500*85%*(2+2)=450+5100=5550 tons, calculated on an annual basis, the amount of mixed cast steel liquid is reduced by 5550 tons / year. According to actual production, 1 ton of residual steel liquid in the tundish can produce 2 tons of mixed cast steel slabs, and the total amount of mixed cast steel is reduced by 5550*2=11,100 tons per year. The actual annual economic benefits = loss of steel downgrade per ton (200 yuan / ton) * downgraded mixed cast steel slab converted steel amount = 200 yuan / ton * 11,100 tons = 2.22 million yuan.

[0069] It should be noted that the above embodiments are not intended to limit the protection scope of the present invention, and equivalent changes or substitutions made on the basis of the above technical solutions all fall within the protection scope of the claims of the present invention.

Claims

1. A method for reducing the amount of mixed casting of different steel grades in the same tundish for continuous casting, characterized in that: The method comprises the following steps: Step 1: The ladle filled with molten steel of different steel grades is transported by a crane to the ladle turntable of the continuous casting machine, where it is placed in the waiting position for casting. Step 2: Adjust the ladle slag alarm value. Step 3: Control the amount of molten steel remaining in the tundish. Step 4: While implementing step 3, control the opening of the tundish stopper rod to continue to reduce the amount of residual molten steel in the tundish; Step 5: When the tonnage of the tundish (i.e., the mixed pouring amount) is controlled to the required mixed pouring amount of different steel grades according to the above steps, the ladle of the previous furnace is transferred to the non-working position, and the ladle of the waiting different steel grade molten steel is rotated to the casting station, and the ladle slide is opened for mixed pouring. At this point, the operation of reducing the mixed pouring amount is completed.

2. The method for reducing the amount of mixed casting of different steel grades in the same tundish of continuous casting according to claim 1 is characterized in that: Step 2: strictly adjust the ladle slag discharge alarm value, as follows: the alarm setting of the molten steel slag discharge detection and prediction device is related to the slag area of ​​the steel flow cross-section of the ladle long nozzle, that is, when the slag area reaches a certain percentage of the long nozzle cross-sectional area (the forecast setting value is the percentage of the slag area to the nozzle inner diameter cross-sectional area, which is called the slag discharge alarm value in production), the slag discharge detection and prediction alarm will sound. Obviously, the larger the slag discharge alarm value, the greater the slag discharge amount. However, in conventional production, the slag discharge detection alarm value is always a fixed constant (M).

3. The method for reducing the amount of mixed casting of different steel grades in the same tundish of continuous casting according to claim 2 is characterized in that: Step 3: Control the remaining molten steel in the tundish (i.e., reduce the amount of mixed casting). Before pouring the next ladle of molten steel of a different steel grade, when the slag detection device alarms at the end of the casting, adjust the alarm value M to a new alarm value (Y) required, and link it with the slag molten steel amount (T) of the ladle: T=(MY) / 5 Where: T-the amount of molten steel without slag, tons; M- daily fixed slag alarm value; Y- adjusted slag alarm value; In terms of reducing the mixed pouring amount in the tundish, if the alarm value decreases by 5, 1 ton of molten steel can be reduced from being cast into the tundish, that is, before the mixed pouring of molten steel of different steel grades in the lower ladle, the remaining molten steel in the tundish can be reduced by 1 ton.

4. The method for reducing the amount of mixed pouring of different steel grades in the same tundish for continuous casting according to claim 3 is characterized in that: Step 4: While implementing step 3, control the opening of the tundish stopper rod to continue to reduce the amount of remaining molten steel in the tundish (i.e., continue to reduce the amount of mixed casting). When the ladle slide is closed, the amount of molten steel in the tundish is controlled by the opening of the stopper rod. The larger the opening of the tundish stopper rod, the more molten steel will flow into the crystallizer, the faster the amount of molten steel in the tundish will decrease, but the degree of slag entanglement will be greater. Conversely, the smaller the opening, the less molten steel will flow into the crystallizer, the slower the amount of molten steel in the tundish will decrease, the less slag entanglement will be, or even no slag entanglement (the slag is blocked by the stopper rod). Based on this feature, in practice, the following relationship is established between the stopper rod opening and the reduction of the amount of molten steel in the tundish (i.e., molten steel waiting for mixed casting): T = (100-K) / 25, Where: N-reduction amount of molten steel waiting for mixed pouring, tons; O-stopper opening value, substituted with a number without a percentage sign; The "100" in the formula refers to the full opening value of the stopper rod.

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