JP2684037B2 - Thin plate continuous casting method - Google Patents

Description

TECHNICAL FIELD The present invention relates to a thin plate continuous casting method for producing a metal thin plate by using a twin roll type thin plate continuous casting apparatus. [Prior Art] A pair of internal cooling rolls rotating in opposite directions are arranged in parallel with each other with an appropriate gap, and a pool is formed on the circumferential surface of the roll above the gap. A twin roll type continuous casting machine is known in which hot water is cooled on a rotating roll circumferential surface and continuously cast into a thin plate through the gap. It has been proposed to apply such a twin-roll type continuous casting machine to continuous casting of steel to directly produce thin steel sheets from molten steel. The twin roll type continuous casting method obtains substantially the same solidification conditions on both sides by rotating two rolls at the same speed with each other, so that the casting structure is uniform and the gap between the rolls is the narrowest. Since the solidified shells (solidified shells) formed on the circumferential surface of the rolls are gradually pressed between the rolls, it is difficult to form shrinkage cavities and porosity inside the slab, and moreover, the surface of the slab It is considered as an effective thin plate casting method for molten steel because it has advantages such as good properties. In order to continuously feed the thin plate continuous casting product from the gap between the roll pairs, a molten steel pool is formed above the gap between the roll pairs.
It is necessary to continuously inject molten steel into this pool,
In order to form this pool on the circumferential surface of the roll, a dam is required to control the flow of the molten metal in the width direction of the thin plate to be cast. The dam usually serves to control the width of the slab, and thus consists of two fixed walls in pairs to control both edges of the slab. This dam is called a side dam. Further, in order to form a large-capacity basin on the circumferential surface of the roll, a fixed wall along the axis of each roll may be raised on each circumferential surface of the pair of rolls. However, when a roll with a large diameter is used, the roll circumferential surface itself can play the role of this wall without providing a wall along this roll axis, and it is possible to form a basin container with only the side dam. . In the former case, if the level of the molten metal surface is constantly maintained on the fixed wall along the roll axis, the area where the circumferential surface of the internal cooling roll contacts the molten metal can be kept substantially constant. In other words, the area of contact between the circumferential surface of the internal cooling roll and the molten metal changes depending on the height of the molten metal surface. Especially in the case of the latter method, the molten steel cools and solidifies in accordance with the contact length in the circumferential direction where the molten steel is in contact with the circumferential surface of the roll. It is necessary to lengthen the time that molten steel contacts the peripheral surface,
For this reason, it is essential to always form a sufficient pool (molten steel pool) on the circumferential surface. For this reason, at the start of casting, a method has been proposed in which molten metal is poured into a stopped roll to form a molten steel pool to some extent, and then rotation of the roll is started to start casting in earnest. It was [Problems to be Solved by the Invention] In a twin roll type continuous casting machine of a type in which a pool is formed by a side dam and a roll circumferential surface, at the start of casting, after forming a molten steel pool in the pool, When the rotation is started, molten steel is supplied between the stationary rolls, so the shell solidified between the rolls is thicker than the target slab thickness, that is, the set roll gap value or more. A solidified shell is formed on both roll surfaces,
Therefore, it has been found in the experiments conducted by the present inventors that the following problems occur. (A). The solidified shell, which is thicker than the narrowest gap between the rolls, is rolled between the rolls, and an excessive load acts between the rolls, which puts a load on the casting equipment and exceeds the capability of the roll rotation motor. It may cause trips and stop casting. In order to prevent this, it is necessary to increase the motor capacity and the rigidity of the rolling mill-like equipment.
The equipment becomes large-scale and becomes expensive equipment. (B). An excessive load is generated and the gap between the rolls is expanded, and a slab thicker than the target slab thickness is cast. (C). The excessively solidified shell is rolled between the rolls up to the closest contact point of the two rolls, and the slab spreads in the direction along the roll axis (the slab widens), which holds the molten steel pool. The side dam provided for this purpose expands outward in the direction along the roll axis, creating a gap between the roll and the side dam, and molten steel is inserted into this part, damaging the roll and the side dam. Further, the end of the slab is restrained by the soldering portion and is cut, and the slab is cut off. The present invention relates to continuous casting of molten steel by a twin roll type continuous casting machine, solves the problems as described above, and continuously produces a thin sheet casting while maintaining a stable casting state. It is intended to provide. [Means for Solving the Problems] As a method for achieving the above object, the present invention is an improvement in the casting method particularly at the start of casting. The method of pouring molten steel metal while preliminarily rotating the roll pair. Starting the process, keep the rotation speed low until the reduction load applied to the slab between rolls reaches the target load, and start controlling the roll rotation speed when the reduction load reaches the target load. Characterize. More specifically, a pair of internal cooling rolls that rotate in opposite directions are arranged in parallel to each other, side dams are arranged on both sides of the roll pair, and a space surrounded by the circumferential surfaces of the side dam and the roll pair. During continuous casting, the solidified shell formed on the circumferential surface of the roll pair is pressed directly in the thin plate while being pressed in the gap of the roll pair. Continue to measure the reduction load when the roll is crimped, control the rotation speed of the roll pair so that the measured reduction load is maintained at the target load, and Start the pouring while rotating the pair of rolls in advance, maintain the rotation speed of this roll until the reduction load reaches the target load, and when the target load is reached, execute the above-mentioned line speed control. There is provided a thin plate continuous casting method according to symptoms. When starting casting according to the method of the present invention, no dummy bar may be used. At the start of casting, there may be a gap between the pair of rolls that are already rotating (that is, both rolls may not be in contact with each other). The contents of the present invention will be described below in detail with reference to the drawings. FIG. 1 is a schematic side view showing an example of a twin roll type continuous casting machine for carrying out the method of the present invention, and FIG. 2 is a schematic plan view thereof. As shown in the figure, a pair of internal cooling rolls 1a, 1b rotating in opposite directions are arranged in parallel and opposite to each other, and side dams 2a, 2b are arranged on both sides of the rolls 1a, 1b. While continuously pouring into the space surrounded by the circumferential surface of the rolls 1a, 1b (called the pool 3), the solidified shells 4a, 4b formed by the circumferential surface of the rolls 1a, 1b The thin plate 5 is directly cast while being pressed in the gap. Now, let l be the distance in the circumferential direction where the molten metal in the pool 3 is in contact with the circumferential surfaces of the rolls 1a, 1b. This l is the length between the position H where the molten metal level contacts the circumferential surface and the position L of the narrowest gap 6 between the rolls 1a and 1b. When casting molten steel, the casting speed corresponds to the peripheral speed of the rolls 1a, 1b, that is, the number of revolutions of the rolls 1a, 1b. The proper value of this casting speed is the steel type, the thickness of the slab 5, and the molten metal. It can be obtained by substituting 1 obtained from the molten steel surface height of the molten steel in the pool 3 into the solidification rate formula obtained by measuring in advance. The specific calculation method is as follows. The casting speed V is calculated from the above l and the cooling time t as follows: V = 60 l / 1000t ... V: casting speed (m / min) l: contact length (mm) t: cooling time (sec). Here, the cooling time t represents the time during which the molten steel is in contact with the circumferential surface of the roll, specifically, the time until the point H in FIG. 1 reaches the point L. In order for the slab to be formed, solidification must be completed by the closest contact point of the roll. D: Shell thickness = 1/2 of target cast piece thickness (mm) A, B: Cooling time t is calculated from constants. From the formula, By substituting ′ into the equation, the target casting speed V is The basic casting condition is to cast at this casting speed. When the casting is started in carrying out the method of the present invention, the roll is rotated in advance at a speed equal to or lower than the calculated appropriate casting speed, and pouring into the basin 3 is started. From the experimental results, it was found that it is appropriate to set the speed of this pre-rotated roll to 60% to 90% of the calculated proper casting speed. If it is set to less than 60%, a solidified shell larger than the gap between the rolls (width of the narrowest gap part 6) may be formed when pouring is started, and the rolling load applied to the slab is reduced. It is impossible to completely eliminate the above-mentioned problems caused by the increase. On the other hand, if it is set to more than 90%, when the pouring is started, the molten steel in the initial stage may not be sufficiently solidified, a molten steel pool may be difficult to form, and a slab may not be formed. The molten steel begins to form a solidified shell on the roll surface when pouring is started. When the solidified shell is formed in the entire gap of the roll closest contact point, the molten steel pool gradually starts to be formed between the rolls by increasing the pouring amount to be larger than the casting amount. When the molten steel pool starts to be formed, the load applied between the rolls, that is, the reduction load applied to the slab increases. It was found that in order to form a good slab, the reduction load applied to this slab needs to be suppressed to 50 kgf / mm or less per unit length in the width direction of the slab. When the load is reached, control of the casting speed is started, and the target casting speed and molten steel pool amount are adjusted and controlled. If the load exceeds 50 kgf / mm, cracks may occur in the slab when the slab is rolled down between rolls, so it is recommended to set the value below this value. As a result, it is possible to suppress an excessive load between rolls that occurs in the initial stage of casting, and it is possible to perform casting without causing the above-mentioned operational problems. FIG. 3 shows a control operation and a flow when carrying out the method of the present invention. Chock 7a and 7 for fixing each axis of rolls 1a and 1b
Highly rigid load cells 8 and 8'are inserted between b and 7a 'and 7b'. When the slab 5 is cast,
The load corresponding to the reduction load P applied to the slab 5 is the rolls 1a, 1
Since it is applied to b as its reaction force, the load applied to the rolls 1a and 1b can be detected by the load cells 8 and 8 '. In the initial stage of casting, the casting speed is kept constant below the target casting speed until the load detected by the load cells 8 and 8 ′ reaches the target value Po, and when the Po value is reached, the casting speed is controlled. To start. This control can be performed by controlling the roll rotation speed by the casting speed control device 10 based on the information obtained from the load measuring device 9. As described above, in carrying out the method of the present invention, the device 9 for measuring the reduction load by the load cells 8, 8 ', etc. is installed to detect the change over time of the load and feed it back to the control device 10 for setting the roll rotation speed. As a result, the method of transmitting the timing for starting the control of the casting speed and introducing an automatic control system for automatically controlling the casting speed can be used as an effective means. Example A specific example of the method of stopping the roll and starting the pouring and the method of rotating the roll to start the pouring according to the present invention will be described below. (Comparison method) Twin roll continuous casting machine Roll diameter 800mm Roll width 600mm [Casting conditions] Steel grade SUS304, Roll gap 2.1mm Casting temperature 1500 ℃ Target casting speed 30m / min Target load 25ton 2 seconds after starting pouring Later, I started spinning the roll,
Accelerated to the target speed. FIG. 4 shows a load chart between rolls at the initial stage of casting, which was carried out under the above conditions by adopting the control flow as shown in FIG. As shown in Fig. 4, the load between the rolls was abnormally large at the start of roll rotation, and the load could not be stabilized by controlling the casting speed, and the slab was larger than the set roll gap. 2.4 mm ~
A 2.8 mm slab was cast. In addition, molten steel pouring between the roll and the side dam was observed, and the edge of the cast piece was cut off. In addition, the current value of the motor that rotates the roll was too large and tripped, and the casting was stopped at a casting length of 10 m. (Invention method) [Twin roll continuous casting machine] Roll diameter 800mm Roll width 600mm [Casting conditions] Steel grade SUS304 Roll gap 2.1mm Casting temperature 1500 ℃ Target casting speed 30m / min Rolling speed before casting start 25m / min Target load The 25 ton roll is rotated at the above rotation speed in advance and pouring is started. FIG. 5 shows a load chart between the rolls at the initial stage of casting, which was carried out under the above conditions by adopting the control flow as shown in FIG. Load between rolls is 0 to 25 tons
It has a smooth start-up until the abnormal load increase and stable casting is possible.
It was possible to completely cast molten steel of 0 kg. As for the slab, it was possible to obtain a slab with a thickness of 2.1 mm, which is the same as the roll gap, and no abnormalities were observed at the end of the slab as in the comparative method. As described above, according to the method of the present invention, it is possible to prevent the generation of an excessive rolling load in the initial stage of casting, and troubles of the casting apparatus, the pouring due to the generation of the gap between the roll and the side dam, the damage of the side dam due to this, and the formation of the slab It became possible to prevent slab defects such as edge cutting and cracking, and it was possible to bring about a great effect on the stability of the continuous casting of thin plates. In addition, since an excessive load does not act between the rolls, no load is applied to the casting equipment, troubles in the casting equipment such as trips of the motor that rotates the rolls do not occur, and stable operation for a long time is possible, increasing the load. As a result, the gap between the rolls did not expand and it was possible to cast a slab with the target slab thickness.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic side view of a twin roll type continuous casting machine for explaining the thin plate continuous casting method of the present invention, FIG. 2 is a schematic plan view of the same, and FIG. 3 is a method of the present invention. FIG. 4 is a control explanatory diagram showing a control operation and a control flow for carrying out the present invention, FIG. 4 is a chart for measuring a rolling load of a cast piece at the start of casting when the comparative method is carried out, and FIG. 5 is a start chart of casting when the present invention is carried out. It is a rolling load measurement chart of the slab at the time. 1a, 1b …… Roll, 2a, 2b …… Side dam, 3 …… Boiler pool, 4a, 4b …… Solid shell, 5 …… Slab, 6 …… Narrowest gap of rolls 1a, 1b, 7 …… Roll chock, 8,8 '... Load cell, 9 ... Load measuring device, 10 ... Casting speed control device, l ... Circumferential length with which molten metal contacts the roll circumferential surface.

   ────────────────────────────────────────────────── ─── Continuation of front page    (72) Inventor Morihiro Hasegawa               4976 Tomita, Shinnanyo City Nisshin Steel Works               Shunan Steel Works (72) Inventor Takashi Yamauchi               4976 Tomita, Shinnanyo City Nisshin Steel Works               Shunan Steel Works                (56) Reference JP-A-60-3951 (JP, A)                 JP-A-60-44164 (JP, A)                 JP-A-60-137562 (JP, A)                 JP-A-61-266159 (JP, A)

Claims (1)

(57) [Claims] A pair of internal cooling rolls rotating in opposite directions are arranged parallel to each other, side dams are arranged on both sides of the roll pair, and continuous pouring is performed in the space surrounded by the circumferential surfaces of the side dam and the roll pair. However, when the solidified shell formed on the circumferential surface of the roll pair is pressed directly in the thin plate while being pressed in the gap between the roll pairs, during the casting operation, when the solidified shell is pressed in the gap between the roll pairs, Continue to measure the load and control the rotation speed of the roll pair so that the measured reduction load is maintained at the target load. At the start of casting, the roll pair is pre-rotated below the target casting speed while pouring. A continuous casting method for thin plates, characterized in that hot water is started, the rotational speed of the roll is maintained until the reduction load reaches a target load, and when the target load is reached, the line speed is controlled. . 2. The thin plate continuous casting method according to claim 1, wherein the rotation speed of the pair of rolls which is rotated in advance at the start of pouring is not more than the target casting speed obtained by the following formula: However, V: Target casting speed (m / min) l: Circumferential contact length (mm) where molten metal contacts the roll circumferential surface D: Solidified shell thickness = 1/2 (mm) of target slab thickness A and B are constants, and this formula is a casting speed formula calculated by the following simultaneous equations. t = 60l / 1000V ...... t: Cooling time (sec) 3. The continuous thin sheet casting method according to claim 1 or 2, wherein the rotation speed of the roll that is rotated in advance at the start of casting is 60% to 90% of the target casting speed.