JP5655731B2 - Manufacturing method of steel ingot for roll by electroslag melting method - Google Patents

Description

  The present invention relates to a method of manufacturing a steel ingot for a roll, which is a material such as a work roll of a roll rolling mill, by an electroslag melting method (hereinafter referred to as “ESR method”).

  For example, rolls such as work rolls and backup rolls are used for rolling cold-rolled steel sheets. The roll is obtained by using a cylindrical steel ingot as a raw material, and reducing the diameter of both ends of the steel ingot by forging, followed by heat treatment and various machining. Usually, a steel ingot for a roll is manufactured by ingot-forming a cylindrical consumable electrode from an electric furnace, and possibly a secondary refined molten steel, and melting the consumable electrode by an ESR method (for example, patents). Reference 1).

  FIG. 1 is a schematic diagram for explaining a method for producing a steel ingot for rolls by a conventional ESR method. As shown in the figure, in the ESR method, a cylindrical consumable electrode 2 that is a base material of a steel ingot 1 is connected to a stub 4 by welding at its upper end, and as the stub 4 is lowered by a lifting mechanism (not shown). Descend. At that time, a molten slag 7 is held in a mold (water-cooled copper mold) 6 in the chamber 5, and when the consumable electrode 2 is immersed in the molten slag 7, an electric current is applied to the molten slag 7. Flows and the molten slag 7 generates heat. The consumable electrode 2 is sequentially dissolved from the lower end by Joule heat of the molten slag 7. The dissolved consumable electrode 2 becomes a droplet and settles in the molten slag 7 and is stacked and solidified while being stored in the mold 6. In this way, the consumable electrode 2 is sequentially melted to the upper end, and the molten metal 3 is sequentially solidified in the mold 6 to obtain the steel ingot 1 for rolls.

  According to the ESR method, since the molten metal 3 stored in the mold 6 is solidified while maintaining a relatively shallow molten metal pool, a good solidified structure with little segregation can be obtained. Moreover, since the melted droplets of the consumable electrode 2 settle in the molten slag 7, an effect of removing inclusions such as oxides can be expected in the sedimentation process. For these reasons, the ESR method is suitable for the production of steel ingots for rolls that require strict inclusion standards and require a homogeneous solidified structure.

JP 2006-265621 A

  In recent years, there has been a strong demand for reduction in manufacturing costs in the production of steel ingots for rolls by the ESR method in order to cope with the rising prices of steel raw materials.

  By the way, in reality, when a roll of a product is produced from a steel ingot for a roll through a process such as forging, a waste roll that is considered to be defective may be generated due to defects such as dimensions and quality. Moreover, even if it is a roll actually used with the roll rolling machine as a product, there is also a disposal roll collected as used. Conventionally, these waste rolls are melted in an electric furnace as scrap materials, and after the adjustment of the components, they are agglomerated into cylindrical consumable electrodes as a base material for the steel ingot for rolls. That is, in the conventional technique, the disposal roll is reused as a scrap material in an electric furnace to reduce the manufacturing cost.

  However, since melting with an electric furnace originally requires enormous energy and time, even if the scrap roll is reused in the electric furnace, the reduction of the manufacturing cost cannot be expected so much.

  The present invention has been made in view of the above problems, and provides a method of manufacturing a steel ingot for rolls by an ESR method that can significantly reduce the manufacturing cost by effectively reusing a disposal roll. Objective.

  In order to achieve the above-mentioned object, the present inventors have paid attention to a waste roll that has been reused as a scrap raw material in the past, and have conducted extensive studies on techniques for effectively reusing this waste roll, and conducted various tests. Carried out. As a result, the following findings (a) to (h) were obtained.

  (A) Disposal rolls are those that are made defective at the stage of producing product rolls from steel ingots, that is, after forging, or are collected as used after being actually used in a roll mill. Therefore, the component composition is consistent with the steel ingot for rolls, and the internal quality is equal to or higher than that of a normal consumable electrode that is melted in an electric furnace and secondarily refined and then ingoted, and the ESR method Satisfy the characteristics required for consumable electrodes. For this reason, in the production of the steel ingot for rolls by the ESR method, it is possible to directly use the waste roll for the consumable electrode. If a disposal roll is used as a consumable electrode, all steps of melting, secondary refining and ingot making using an electric furnace can be omitted, so that the manufacturing cost can be greatly reduced.

  (B) The disposal roll is not a mere columnar shape, but a complex stepped columnar shape having a reduced diameter at both ends, similar to the product shape of the roll. Due to this shape, in the ESR method using a waste roll as a consumable electrode, some problems and inconveniences may occur as described below, and therefore countermeasures are desired.

  (C) In general, the dissolution process of the ESR method is divided into three stages: an initial stage, a steady dissolution stage, and a hot top stage after the completion of dissolution. The initial stage is roughly divided into a hot start method using molten slag at the start of melting and a cold start method using solid powder slag at the start of melting. In the hot start method, since it is not necessary to hatch the slag, the melting of the waste roll used as the consumable electrode can be easily started.

  However, the cold start method is different, and the powder slag needs to be hatched first. At this time, if a waste roll is used as a consumable electrode and the diameter of its lower end is too small with respect to the diameter of the mold, the powder slag spread on the bottom of the mold only receives heat in the central area. Therefore, normal initial hatching of the powder slag over the entire region becomes difficult. Regarding this problem, in order to achieve normal initial hatching of the powder slag, as will be described in detail later, if the diameter of the lower end of the consumable electrode composed of the disposal roll is 55% or more of the diameter of the mold, Good.

  (D) As described above, the disposal roll has a stepped cylindrical shape whose diameter is reduced at both ends. For this reason, in the ESR method using a waste roll as a consumable electrode, the electrode diameter varies at the beginning and end of the steady dissolution stage, and the dissolution rate varies accordingly. The situation will be described below.

  FIG. 2 is a diagram showing the behavior of the dissolution rate when a test by the ESR method is performed using the disposal roll as a consumable electrode. FIG. 3 is a schematic view of the disposal roll used in the test shown in FIG. As shown in FIG. 3, the disposal roll used as a consumable electrode in this ESR test had a diameter of the central portion excluding both ends with a reduced diameter, that is, a maximum diameter of 575 mm. For comparison, an ESR test was performed using a normal consumable electrode. This normal consumable electrode had a diameter of 560 mm almost the same as the maximum diameter of the disposal roll over the entire length. In both of these ESR tests, a mold having a diameter of 825 mm, which is usually applied in the ESR method using a consumable electrode having a diameter of 560 mm, was used, the hot start method was adopted, and the same dissolution conditions were used except that the consumable electrode was different. .

  As shown in FIG. 2 and FIG. 3, when a waste roll is used as a consumable electrode, the electrode diameter is reduced at both the upper and lower ends, so that in the initial and final stages of the steady dissolution stage corresponding to that part It can be seen that the electrode diameter fluctuates and the dissolution rate fluctuates remarkably. When the dissolution rate decreases due to fluctuations, Joule heat generated in the mold decreases, and the molten slag solidifies temporarily. Along with this, the thickness of the slag skin on the surface of the steel ingot varies, so that slag bites into the surface of the steel ingot, which may adversely affect the steel ingot skin and the solidified structure.

(E) As described in (d) above, among the changes in the electrode diameter accompanying the progress of dissolution of the consumable electrode (disposal roll), when the electrode diameter increases, that is, at the initial stage of the steady dissolution stage Since the gap between the consumable electrode and the mold is reduced, the current that flows linearly from the electrode to the mold increases, thereby reducing the current that flows through the molten slag near the inner surface of the mold. For this reason, since the Joule heat Q of the molten slag represented by the following formula (A) is lowered and the temperature of the molten slag is lowered, the slag skin thickness on the steel ingot surface varies. This is because a part of the molten slag solidifies in the vicinity of the inner surface of the mold.
Q = RI ² t (A)
Here, the meanings of the symbols in the above formula (A) are as follows.
R: resistance value of molten slag,
I: current value of the current flowing through the molten slag, and t: energization time

  Regarding this problem, in order to suppress fluctuations in the thickness of the slag skin at the beginning of the steady melting stage, the diameter of the consumable electrode increases with the progress of melting of the consumable electrode (disposal roll), as will be described in detail later. When shifting to dissolution of the portion, the current applied to the consumable electrode may be changed to a current value I represented by the following equation (1), 5 to 15 mm before the boundary of the large diameter portion. This is because the value of the current applied to the consumable electrode increases due to the change, and the current flowing in the molten slag near the inner surface of the mold can be stabilized.

ここで、上記（１）式中の記号の意味は下記の通りである。
Ｉ₁：移行前の電流値、
ｄ₁：移行前の消耗電極の直径、
ｄ：移行後の消耗電極の直径、および
Ｄ：鋳型の直径

Here, the meanings of the symbols in the above formula (1) are as follows.
I ₁ : current value before transition,
d ₁ : Consumable electrode diameter before transition,
d: Diameter of consumable electrode after transfer, and D: Diameter of mold

  (F) On the other hand, at the time of fluctuation when the electrode diameter (waste roll) becomes small, that is, at the end of the steady dissolution stage, the consumable electrode is detached from the molten slag unless the lowering speed of the electrode is changed to maintain the dissolution speed. There is no doubt. When the consumable electrode is detached from the molten slag, the Joule heat of the molten slag is lowered and the temperature of the molten slag is lowered, so that the slag skin thickness on the surface of the steel ingot is changed.

  Regarding this problem, in order to suppress the fluctuation of the slag skin thickness at the end of the steady melting stage, the diameter of the consumable electrode becomes smaller as the consumable electrode (disposal roll) melts, as will be described in detail later. When shifting to part melting, the feed rate of the consumable electrode descending may be changed to the feed rate s represented by the following equation (2) at least 5 mm before the boundary of the small diameter portion. This is because the feed rate of the consumable electrode is increased by the change, whereby the thickness of the electrode immersed in the molten slag is slightly increased, and the consumable electrode can be prevented from being detached from the molten slag.

ここで、上記（２）式中の記号の意味は下記の通りである。
ｓ₁：移行前の送り速度、
ｄ₁：移行前の消耗電極の直径、および
ｄ：移行後の消耗電極の直径

Here, the meanings of the symbols in the above formula (2) are as follows.
s ₁ : Feed speed before transition,
d ₁ : Consumable electrode diameter before transition, and d: Consumable electrode diameter after transition

  (G) In the hot top stage after the completion of dissolution, even if the diameter of the consumable electrode (disposal roll) at the end of the dissolution, that is, the diameter of the upper end of the consumable electrode is small, it can be dissolved to the end. However, if the diameter of the upper end of the consumable electrode is smaller than the diameter of the stub joined to the upper end, it will be difficult to check the remaining portion of the electrode that has not been melted, so it will be difficult to grasp the timing of completion of melting. . Concerning this problem, a consumable electrode whose upper end diameter is larger than the diameter of the stub may be used in order to accurately grasp the melting end timing.

  (H) In the case of the ESR method using a disposal roll as a consumable electrode, the product weight of the roll that can be produced from the steel ingot obtained is smaller than the original roll (disposition roll) in consideration of the yield. . Regarding this inconvenience, in order to produce a steel ingot capable of producing a roll having a weight equal to or higher than that of the original roll by the ESR method, two or more waste rolls of the same steel type are connected by welding. It is effective to use this as a consumable electrode.

  This invention is completed based on knowledge of said (a)-(h), The summary exists in the manufacturing method of the steel ingot for rolls shown below. That is, it is a method of manufacturing a steel ingot for a roll by melting a consumable electrode by the ESR method, and using a waste roll that has been made defective after forging or a used and recovered waste roll as a consumable electrode. It is the manufacturing method of the steel ingot for rolls characterized.

  In this steel ingot manufacturing method, the diameter of the lower end of the consumable electrode is preferably 55% or more of the diameter of the mold.

  In the steel ingot manufacturing method, the diameter of the upper end of the consumable electrode is preferably larger than the diameter of the stub joined to the upper end.

  In the method for manufacturing a steel ingot, when the consumable electrode is melted, when the transition is made to the melting of the large diameter portion, the application to the consumable electrode is 5 to 15 mm before the boundary of the large diameter portion. It is preferable to change the current to the current value I represented by the above formula (1).

  Further, in the method for manufacturing a steel ingot, when the melting of the consumable electrode proceeds, the transition speed of the consumable electrode is 5 mm or more before the boundary of the small diameter portion when the transition is made to the melting of the small diameter portion. Is preferably changed to a feed speed s represented by the following equation (2).

  Furthermore, the manufacturing method of said steel ingot can connect the said discard roll of the 2 or more same steel types by welding, and can be set as the said consumable electrode.

  According to the method of manufacturing a steel ingot for rolls according to the ESR method of the present invention, since a waste roll is used as a consumable electrode, all the steps of melting, secondary refining and ingot making by an electric furnace as in the past can be omitted. Manufacturing costs can be greatly reduced.

It is a schematic diagram for demonstrating the manufacturing method of the steel ingot for rolls by the conventional ESR method. It is a figure which shows the behavior of a melt | dissolution rate when the test by an ESR method is done using a disposal roll for a consumable electrode. It is the schematic of the discard roll used by the test shown in FIG. It is a schematic diagram for demonstrating the manufacturing method of the steel ingot for rolls by the ESR method of this invention, The figure (a) shows the state before melt | dissolution, and the figure (b) shows the state in melt | dissolution, respectively. It is the schematic of the consumable electrode comprised with a discard roll. It is a figure which shows the behavior of the slag skin thickness of the steel ingot part corresponding to the initial stage of a steady melt | dissolution stage. It is a figure which shows the behavior of the slag skin thickness of the steel ingot part corresponded to the last stage of a steady melting stage.

  Below, the embodiment is explained in full detail about the manufacturing method of the steel ingot for rolls by the ESR method of this invention.

  FIG. 4 is a schematic diagram for explaining a method for producing a steel ingot for a roll by the ESR method of the present invention, where FIG. 4 (a) shows a state before melting, and FIG. 4 (b) shows a state during melting. Each is shown. In the ESR method of the present invention shown in FIG. 4, a cold start method is adopted, and a waste roll is used as the consumable electrode 2 that is a base material of the steel ingot 1. The waste roll is one that has been made defective at the stage of producing a roll of product from the steel ingot 1, that is, after the forging, or has been collected as used after being actually used in a roll mill. . The consumable electrode 2 composed of a waste roll is a stepped columnar shape whose diameter is reduced at both ends. A stub 4 is connected to the upper end of the consumable electrode 2 by welding, and the consumable electrode 2 is lowered as the stub 4 is lowered by a lifting mechanism (not shown). To do.

  Here, when the cold start method is adopted, the diameter of the lower end of the consumable electrode 2 is preferably 55% or more of the diameter of the mold (water-cooled copper mold) 6 in the chamber 5. This is because the powder slag 8 spread on the bottom of the mold 6 is normally initially hatched over the entire area. In the cold start method, the powder slag 8 needs to be hatched first, but as will be demonstrated in the test described later, if the diameter of the lower end of the consumable electrode 2 is less than 55% of the diameter of the mold 6, the powder slag 8 Normal initial hatching becomes difficult. However, the both ends of the disposal roll are usually reduced in diameter in a plurality of stages as shown in FIG. 3, and if the diameter does not satisfy the condition of 55% or more of the diameter of the mold 6, the condition is satisfied. What is necessary is just to excise the step which does not satisfy | fill.

  In the ESR method of the present invention, the powder slag 8 is normally initially hatched by energization of the consumable electrode 2, and after the molten slag 7 is formed in the mold 6, the consumable electrode 2 is immersed in the molten slag 7. Continue energizing at. Thereby, a current flows through the molten slag 7 and the molten slag 7 generates heat, and the consumable electrode 2 is sequentially melted from the lower end by the Joule heat of the molten slag 7. The dissolved consumable electrode 2 becomes a droplet and settles in the molten slag 7 and is stacked and solidified while being stored in the mold 6. In this way, the consumable electrode 2 is sequentially melted to the upper end, and the molten metal 3 is sequentially solidified in the mold 6 so that inclusions such as oxides are removed, and the steel ingot 1 for rolls having a good solidified structure with little segregation. Is obtained.

  At that time, due to the shape of the consumable electrode 2, the electrode diameter varies at the initial stage of the steady dissolution stage, and conversely, the electrode diameter varies at the end of the steady dissolution stage.

  As in the former, when the electrode diameter increases, the gap between the consumable electrode 2 and the mold 6 is reduced. For this reason, as the current flowing linearly from the consumable electrode 2 to the mold 6 increases and the current flowing to the molten slag 7 near the inner surface of the mold 6 decreases, the Joule heat of the molten slag 7 decreases and the molten slag Since the temperature of 7 falls, there exists a possibility that the slag skin thickness in the surface of the steel ingot 1 may fluctuate. When the slag skin thickness varies, slag biting into the surface of the steel ingot 1 occurs, which may adversely affect the steel ingot skin and the solidified structure.

  Therefore, in the ESR method of the present invention, when shifting to the melting of the large diameter portion of the consumable electrode 2, the applied current to the consumable electrode 2 is 5-15 mm before the boundary of the large diameter portion as shown in (1) below. It is preferable to change to the current value I represented by the formula. As will be demonstrated in the test described later, the current value applied to the consumable electrode 2 is increased by the change, and the current flowing through the molten slag 7 in the vicinity of the inner surface of the mold 6 can be stabilized. As a result, the fluctuation of the slag skin thickness can be reduced. This is because it can be suppressed.

ここで、上記（１）式中の記号の意味は下記の通りである。
Ｉ₁：移行前の電流値、
ｄ₁：移行前の消耗電極の直径、
ｄ：移行後の消耗電極の直径、および
Ｄ：鋳型の直径

Here, the meanings of the symbols in the above formula (1) are as follows.
I ₁ : current value before transition,
d ₁ : Consumable electrode diameter before transition,
d: Diameter of consumable electrode after transfer, and D: Diameter of mold

  On the other hand, in the latter case, when the electrode diameter is changed, the consumable electrode 2 may be detached from the molten slag 7 unless the descending speed of the electrode is changed in order to maintain the dissolution rate. When the consumable electrode 2 is detached from the molten slag 7, the Joule heat of the molten slag 7 is decreased and the temperature of the molten slag 7 is decreased. The slag skin thickness on the surface may vary.

  Therefore, in the ESR method of the present invention, when the transition is made to dissolution of a portion having a small diameter of the consumable electrode 2, the lowering feed rate of the consumable electrode 2 is set to the following formula (2) at least 5 mm before the boundary of the small diameter portion. It is preferable to change to the feed speed s represented by As will be demonstrated in the test described later, the feed rate of the consumable electrode 2 is increased by the change, thereby preventing the consumable electrode 2 from being detached from the molten slag 7, and as a result, fluctuations in the slag skin thickness can be suppressed. It is.

ここで、上記（２）式中の記号の意味は下記の通りである。
ｓ₁：移行前の送り速度、
ｄ₁：移行前の消耗電極の直径、および
ｄ：移行後の消耗電極の直径

Here, the meanings of the symbols in the above formula (2) are as follows.
s ₁ : Feed speed before transition,
d ₁ : Consumable electrode diameter before transition, and d: Consumable electrode diameter after transition

  The timing for changing the applied current to the consumable electrode 2 and the timing for changing the feed rate of the consumable electrode 2 are obtained in advance by accurately grasping the shape and dimensions of the consumable electrode 2 constituted by the disposal roll. The amount of dissolution of the consumable electrode 2 and the remaining weight can be easily set by accurately managing the amount and the remaining weight by a weight sensor such as a load cell.

  In the ESR method of the present invention, even when the electrode diameter at the end of dissolution, that is, the diameter of the upper end of the consumable electrode 2 is small, it can be dissolved to the end. However, if the upper end diameter of the consumable electrode 2 is smaller than the diameter of the stub 4, it is difficult to check the remaining portion of the consumable electrode 2, and therefore it is preferable to use the consumable electrode 2 whose upper end diameter is larger than the diameter of the stub 4. . The lower limit of the upper end diameter of the consumable electrode 2 is not particularly limited, but is preferably 55% or more of the diameter of the mold 6 in the same manner as the lower end diameter in order to achieve more reliable melting.

  Furthermore, in the case of the ESR method using a waste roll as the consumable electrode 2, the product weight of the roll that can be produced from the steel ingot 1 obtained is smaller than the original roll in consideration of the yield. Therefore, when it is desired to produce a roll having a weight equal to or higher than that of the original roll in the production of a steel ingot for a roll by the ESR method of the present invention, two or more waste rolls of the same steel type are welded. It is effective that they are connected as a consumable electrode 2.

  Note that the ESR method of the present invention described above can also be applied to a hot start method. In the hot start method, since it is not necessary to hatch the slag, there is no restriction on the lower end diameter of the waste roll used as the consumable electrode 2.

<Diameter of consumable electrode in cold start method>
A cold start ESR test was performed using consumable electrodes with variously changed diameters at the lower end, and the hatching condition of the powder slag at the initial stage was investigated in relation to the lower end diameter of the consumable electrode and the mold diameter. Table 1 below shows the survey results.

  From the results shown in Table 1, it can be seen that if the ratio of the lower end diameter of the consumable electrode to the mold diameter is 55% or more, the powder slag can be reliably hatched over the entire area. Therefore, in order to realize normal initial hatching of the powder slag, it is sufficient that the diameter of the lower end of the consumable electrode constituted by the disposal roll is 55% or more of the diameter of the mold.

<Current applied to the consumable electrode in the initial stage of the steady dissolution stage, and feed rate of the consumable electrode in the final stage of the steady dissolution stage>
Using a waste roll as a consumable electrode, a cold start method ESR test was conducted using a mold having a diameter of 825 mm, and the thickness of the slag skin adhering to the surface of the obtained steel ingot was investigated over the entire length.

  FIG. 5 is a schematic view of a consumable electrode composed of a disposal roll. As shown in the figure, the waste roll used as a consumable electrode in this ESR test was originally reduced in diameter by three steps at both ends, but the condition from the end to the second step was 55% or more of the mold diameter. Because it does not satisfy, the part is excised.

  In this ESR test, at the beginning of the steady dissolution stage, when the transition is made from the lower end of the electrode having a small diameter of the consumable electrode to the dissolution of the central portion of the electrode having a large diameter of the consumable electrode, The applied current was changed to the current value I according to the above equation (1) and increased. Furthermore, at the end of the steady dissolution stage, when shifting from the center of the electrode having a large diameter of the consumable electrode to the dissolution of the upper end of the electrode having a small diameter of the consumable electrode, the feed rate of the consumable electrode is changed at the boundary between the diameter variations of the two The feed speed s was changed according to the equation (2) and increased.

  FIG. 6 is a diagram showing the behavior of the slag skin thickness of the steel ingot portion corresponding to the initial stage of the steady melting stage. FIG. 7 is a diagram showing the behavior of the slag skin thickness of the steel ingot portion corresponding to the end of the steady melting stage. 6 and 7 also show the thickness of the slag skin during normal operation.

  From the results shown in FIG. 6, when the electrode diameter changes from a small diameter to a large diameter in the initial stage of the steady dissolution stage, if the current applied to the consumable electrode is changed to the current value I according to the above equation (1), the slag It can be seen that the skin thickness is substantially constant, and fluctuations in the slag skin thickness can be suppressed. It can also be seen that there is a time delay of about 5 mm from when the applied current is changed until the slag skin thickness becomes substantially constant.

  Therefore, in order to suppress fluctuations in the thickness of the slag skin at the initial stage of the steady melting stage, when shifting to the melting of the large diameter portion of the consumable electrode, the consumable electrode is moved 5 to 15 mm before the boundary of the large diameter portion. It is effective to change the applied current to the current value I represented by the above equation (1). Here, if the applied current is changed too early, the slag skin thickness variation suppressing effect does not occur at an appropriate timing. Therefore, as described above, the applied current is changed at least 15 mm before the boundary of the large diameter portion. .

  On the other hand, from the results shown in FIG. 7, when the electrode diameter changes from a large diameter to a small diameter at the end of the steady dissolution stage, the feed speed of the consumable electrode is changed to the feed speed s according to the above equation (2). It can be seen that the thickness of the slag skin is substantially constant, and fluctuations in the thickness of the slag skin can be suppressed. It can also be seen that there is a time delay of about 5 mm from when the feed rate is changed until the slag skin thickness becomes substantially constant.

  Therefore, in order to suppress fluctuations in the thickness of the slag skin at the end of the steady melting stage, when shifting to the melting of a portion with a small diameter of the consumable electrode, the feed rate of the consumable electrode is at least 5 mm before the boundary of the small diameter portion. Is effectively changed to the feed speed s expressed by the above equation (2). Here, if the feed rate is changed too early, the slag skin thickness fluctuation suppressing effect does not occur at an appropriate timing. Therefore, it is preferable to change the feed rate at least 15 mm before the boundary of the small diameter portion.

  In order to confirm the effect by the manufacturing method of the steel ingot for rolls of this invention, the test of the following Example 1 and 2 was implemented.

<Example 1>
Using the waste roll shown in FIG. 5 as a consumable electrode, a cold start ESR test was performed using a mold having a diameter of 825 mm. At this time, when shifting from the lower end portion of the consumable electrode to the dissolution of the central portion, the current applied to the consumable electrode was changed to the current value I according to the above equation (1) 5 mm before the boundary between the two. Furthermore, when shifting from the central part of the consumable electrode to dissolution of the upper end part, the feed speed of the consumable electrode was changed to the feed speed s according to the above equation (2) 5 mm before the boundary between the two. For comparison, an ESR test was performed using a normal consumable electrode having a diameter of 560 mm over the entire length after melting and secondary refining in an electric furnace.

  In the example of the present invention in which the waste roll was used as a consumable electrode, the initial hatching of the powder slag took longer than in the comparative example using a normal consumable electrode, but normal hatching of the entire slag could be confirmed. It was. Moreover, when the thickness of the slag skin adhering to the surface of the obtained steel ingot was investigated over the full length, the slag skin thickness was substantially constant.

<Example 2>
When producing a roll of a product from a steel ingot manufactured by the ESR method, as described in Patent Document 1, the freckle position depth inside the steel ingot and the dendrite deflection angle of the steel ingot surface layer are large in product quality. affect. In particular, with respect to freckle, it is known that if freckle is present in the thickness used as a roll, the product to be rolled is adversely affected. Further, regarding the dendrite deflection angle, if the deflection angle is small, a dendrite pattern may be printed on the product to be rolled.

  Therefore, using a waste roll as a consumable electrode as an example of the present invention, using a normal consumable electrode as a comparative example, and using a mold having various diameters for each of the present invention example and the comparative example, ESR of the cold start method A test was conducted. And each steel ingot obtained by the example of the present invention and the comparative example was cut along the central axis, and after the cut surface was corroded, the freckle position depth and the dendrite deflection angle were measured at the corroded cut surface. The depth of the freckle position was obtained from the macro observation of the steel ingot cross section, and the dendrite deflection angle was obtained from the macro observation of the steel ingot longitudinal section. The results are shown in Table 2 below.

  From the results shown in Table 2, the example of the present invention using the waste roll as the consumable electrode and the comparative example using the normal consumable electrode are comparable to each other in terms of the freckle position depth and the dendrite deflection angle, and have the same quality. A steel ingot could be obtained.

  According to the method of manufacturing a steel ingot for rolls according to the ESR method of the present invention, since a waste roll is used as a consumable electrode, all the steps of melting, secondary refining and ingot making by an electric furnace as in the past can be omitted. Manufacturing costs can be greatly reduced. Therefore, the method for producing a steel ingot for rolls according to the present invention is extremely useful in that it can meet the demand caused by the rising price of steel raw materials.

1: steel ingot, 2: consumable electrode, 3: molten metal, 4: stub,
5: Chamber, 6: Mold, 7: Molten slag, 8: Powder slag

Claims (7)

A method for producing a steel ingot for a roll by melting a consumable electrode by an electroslag melting method,
As a consumable electrode, a waste roll made defective after forging or a used and recovered waste roll is used, and the diameter of the lower end of the consumable electrode is 55% or more of the diameter of the mold. Steel ingot manufacturing method. As the melting of the consumable electrode proceeds, the current applied to the consumable electrode is expressed by the following formula (1), 5 to 15 mm before the boundary of the large diameter portion when the transition is made to the large diameter portion. It changes to the electric current value I represented, The manufacturing method of the steel ingot for rolls of Claim 1 characterized by the above-mentioned.

Here, the meanings of the symbols in the above formula (1) are as follows.
I ₁ : current value before transition,
d ₁ : Consumable electrode diameter before transition,
d: Diameter of consumable electrode after transfer, and D: Diameter of mold A method for producing a steel ingot for a roll by melting a consumable electrode by an electroslag melting method,
As a consumable electrode, a waste roll that has been made defective after forging or a used and collected waste roll is used. A method for producing a steel ingot for rolls, characterized in that the current applied to the consumable electrode is changed to a current value I represented by the following formula (1), 5 to 15 mm before the boundary of the diameter portion.

Here, the meanings of the symbols in the above formula (1) are as follows.
I ₁ : current value before transition,
d ₁ : Consumable electrode diameter before transition,
d: Diameter of consumable electrode after transfer, and D: Diameter of mold As the melting of the consumable electrode proceeds, when the transition is made to the melting of the portion having a small diameter, the feed rate of the consumable electrode is expressed by the following equation (2) at a distance of 5 mm or more from the boundary of the small diameter portion. The method for producing a steel ingot for rolls according to any one of claims 1 to 3, wherein the feed speed s is changed.

Here, the meanings of the symbols in the above formula (2) are as follows.
s ₁ : Feed speed before transition,
d ₁ : Consumable electrode diameter before transition, and d: Consumable electrode diameter after transition A method for producing a steel ingot for a roll by melting a consumable electrode by an electroslag melting method,
As a consumable electrode, a waste roll that has been made defective after forging or a used and recovered waste roll is used, and when the transition of the consumable electrode proceeds to dissolution of a portion with a small diameter, its small diameter A method for producing a steel ingot for a roll, wherein the feed rate of the consumable electrode is changed to a feed rate s represented by the following equation (2) at least 5 mm before the boundary of the section.

Here, the meanings of the symbols in the above formula (2) are as follows.
s ₁ : Feed speed before transition,
d ₁ : Consumable electrode diameter before transition, and d: Consumable electrode diameter after transition   The diameter of the upper end of the consumable electrode is larger than the diameter of the stub joined to the upper end, The method for producing a steel ingot for rolls according to any one of claims 1 to 5.   The method for producing a steel ingot for a roll according to any one of claims 1 to 6, wherein the waste rolls of two or more same steel types are connected by welding to form the consumable electrode.

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