Method of producing steel

Abstract

To provide a method of producing steel allowing for enhancing cleanliness of in-tundish molten steel in unsteady time so as to suppress the cleanliness of the molten steel in unsteady time from having effects during steadiness.SOLUTION: A method of producing steel includes continuous-continuous casting satisfying expressions [1] and [2] at a start of injecting molten steel from a ladle into a tundish, a≤h / h0≤0.45...[1] a=[VD2 / 2.05{(ρS-ρP)g / ρS}1 / 2]2 / 5 / h0...[2]; in which when expression [1] is satisfied, a molten-steel injection flow from the ladle has at a tundish bottom a vertical flow velocity vb satisfying expressions [3] and [4], vb≤9.91...[3] and vb=[2g{H0+H-(h-h1)}]1 / 2...[4], and a molten-steel flow along a tundish bottom surface at a tundish side or weir nearest to the molten-steel injection flow from the ladle has a velocity vw satisfying expressions [5] and [6], vw≤0.62...[5] vw=3.5{(Hr0)2 / r}(2 gH0)1 / 2...[6].SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to a method for producing steel. [Background technology]

[0002] In continuous casting of steel, molten steel from multiple ladles is cast continuously, a process known as "continuous casting." In this process, molten steel from a ladle is poured into a tundish, and when the ladle is empty, it is replaced with the next ladle, and the molten steel from the next ladle is poured into the tundish. This process is repeated.

[0003] It has been known that in continuous casting, the cleanliness of molten steel in a tundish decreases before and after ladle exchange. The decrease in the cleanliness of molten steel during non-steady conditions, such as before and after ladle exchange, can affect the cleanliness of molten steel during steady conditions.

[0004] Patent Documents 1 and 2 describe methods for increasing the cleanliness of molten steel in a tundish before and after ladle replacement. When the ladle is replaced, molten steel is not poured into the tundish, but the molten steel continues to be poured from the tundish into the mold. This reduces the amount of molten steel in the tundish. In the method described in Patent Document 1, the "molten steel pouring rate from the ladle to the tundish" after the ladle is replaced is made higher than the "molten steel pouring rate from the tundish to the mold" before and after the ladle is replaced. In addition, in the method described in Patent Document 2, the "molten steel supply amount to the tundish" after the ladle is replaced is made higher. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Patent No. 5760801 [Patent Document 2] Japanese Patent Application Publication No. 7-236947 Summary of the Invention [Problem to be solved by the invention]

[0006] It is desirable to be able to increase the cleanliness of molten steel in a tundish by a method other than the methods described in Patent Documents 1 and 2. For example, it is desirable to be able to increase the cleanliness of molten steel in a tundish by a method other than controlling the molten steel pouring rate from the ladle (the amount of molten steel poured from the ladle) and the amount of molten steel supplied from the tundish. Furthermore, it is desirable to be able to suppress the effect of the cleanliness of molten steel in an unsteady state on the cleanliness of molten steel in a steady state.

[0007] The present invention aims to increase the cleanliness of molten steel in a tundish during unsteady conditions by using a method different from conventional methods, and to suppress the effect of the cleanliness of the molten steel during unsteady conditions on the cleanliness of the molten steel during steady conditions. [Means for solving the problem]

[0008] The method for producing steel disclosed in the present specification is a continuous casting method for continuously casting molten steel from a plurality of ladles, comprising the steps of: After the ladle is changed, when the next ladle starts pouring molten steel into the tundish, the following equations [1] and [2] are satisfied. a ≦ h / h0≦ 0.45 [1] a = [VD 2 / 2.05{(ρ S -ρ P )g / ρ S} 1 / 2 ] 2 / 5 / h0···[2] When the above formula [1] is satisfied, At the bottom of the tundish, the vertical flow velocity vb (m / s) of the molten steel poured from the ladle satisfies the following equations [3] and [4], and vb≦9.91 [3] vb = [2g{H0+H-(h-h1)}] 1 / 2 [4] The flow velocity vw (m / s) of the molten steel flow along the bottom of the tundish at the side of the tundish closest to the molten steel pouring flow from the ladle or at a weir located within the tundish satisfies the following equations [5] and [6]. vw ≦ 0.62 [5] vw = 3.5{(Hr0) 2 / r}(2gH0) 1 / 2 [6] Where: h: Height of molten steel in the tundish (m) h0: Height of molten steel in the tundish at steady state (m) V: Speed ​​of molten steel poured from the tundish to the mold (m / s) D: Diameter of the outlet for pouring molten steel from the tundish to the mold (m) ρ S : Molten steel density (ton / m 3 ) ρ P : Density of tundish powder (ton / m 3 ) g: Gravitational acceleration (m / s 2 ) H: Vertical distance (m) from the discharge outlet of the molten steel in the ladle to the first bottom surface part that vertically overlaps with the discharge outlet at the bottom of the tundish H0: Height of molten steel from the discharge outlet of the molten steel in the ladle to the surface of the molten steel in the ladle (m) h1: Vertical distance (m) between the first bottom surface portion of the bottom of the tundish and the second bottom surface portion of the bottom of the tundish that is at the same height as the upper end of the casting hole r0: Radius of the outlet of the molten steel in the ladle (m) r: The horizontal distance (m) between the position P at the bottom of the tundish that vertically overlaps with the center of the discharge outlet of the molten steel in the ladle and the side of the tundish or the weir located inside the tundish that is closest to the position P It is. Effect of the Invention

[0009] According to the present invention, the cleanliness of molten steel in a tundish during an unsteady state can be increased by a method different from conventional methods, and the cleanliness of the molten steel during an unsteady state can be prevented from affecting the cleanliness of the molten steel during a steady state. [Brief description of the drawings]

[0010] [Figure 1]FIG. 2 is a schematic diagram of a portion of a continuous casting facility. [Diagram 2] FIG. 2 is a schematic diagram showing dimensions of the continuous casting equipment shown in FIG. [Diagram 3] FIG. 13 is a schematic diagram of a portion of a continuous casting facility according to a modified example. [Figure 4] FIG. 13 is a schematic diagram of a portion of a continuous casting facility according to another modified example. [Diagram 5] FIG. 2 is an enlarged schematic view of the bottom of the ladle and the tundish. [Figure 6] FIG. 1 shows the results of Experiment 1. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] Preferred embodiments of the present invention will now be described.

[0012] Fig. 1 shows a part of a continuous casting equipment 1. The continuous casting equipment 1 includes a ladle 2, a tundish 3, a mold 4, etc. Fig. 1 shows a state before molten steel in the ladle 2 is poured into the tundish 3. Hatching showing cross sections is omitted in Fig. 1. Hatching showing cross sections is also omitted in Figs. 2 to 4 described later.

[0013] As shown in Fig. 1, a discharge hole 2A is formed in the bottom of a ladle 2. Here, an example is shown in which a sliding nozzle 5 is disposed below the discharge hole 2A. Fig. 1 shows a simplified view of the sliding nozzle 5. In Fig. 1, the sliding nozzle 5 blocks the lower end of the discharge hole 2A, thereby stopping the discharge of molten steel in the ladle 2. When the sliding nozzle 5 is open as shown in Fig. 2, the molten steel in the ladle 2 is discharged.

[0014] In the configuration shown in Figures 1 and 2, the size of the discharge outlet of the molten steel in the ladle 2 is determined by the sliding amount or opening degree of the sliding nozzle 5. As shown in Figure 2, when the sliding nozzle 5 does not block the lower end of the discharge hole 2A of the ladle 2 (when the sliding nozzle 5 does not overlap the lower end of the discharge hole 2A of the ladle 2 in the vertical direction), the discharge outlet of the molten steel in the ladle 2 is the lower end of the discharge hole 2A of the ladle 2. When the sliding nozzle 5 blocks a part of the lower end of the discharge hole 2A of the ladle 2 (when a part of the lower end of the discharge hole 2A of the ladle 2 overlaps with the sliding nozzle 5 in the vertical direction), the discharge outlet of the molten steel in the ladle 2 is the part of the lower end of the discharge hole 2A of the ladle 2 that is not blocked by the sliding nozzle 5 (the part of the lower end of the discharge hole 2A of the ladle 2 that does not overlap with the sliding nozzle 5 in the vertical direction). A nozzle or the like that is long in the vertical direction may be disposed below the sliding nozzle 5. The configuration of the sliding nozzle 5 is not limited to the above configuration.

[0015] As shown in Fig. 1, the tundish 3 has a bottom 3P and a side 3Q. Fig. 1 shows the tundish 3 in which the bottom 3P is stepped. The bottom 3P and the side 3Q are made of a refractory material or the like.

[0016] The bottom portion 3P has an upper bottom portion 3P1, a lower bottom portion 3P2, and an inclined portion 3P3 connecting the upper bottom portion 3P1 and the lower bottom portion 3P2. The upper bottom portion 3P1 and the lower bottom portion 3P2 are disposed substantially horizontally.

[0017] The upper bottom portion 3P1 is located at a higher position than the lower bottom portion 3P2. The ladle 2 is disposed above the upper bottom portion 3P1.

[0018] The lower bottom portion 3P2 is located at the lowest position of the bottom portion 3P. As shown in FIG. 1, when the bottom portion 3P of the tundish 3 is stepped, a casting hole 3A is formed in the lowest lower bottom portion 3P2. Here, an example is shown in which a sliding nozzle 6 is disposed below the casting hole 3A. In FIG. 1, the sliding nozzle 6 is illustrated in a simplified form. In the state shown in FIG. 1, the sliding nozzle 6 is open. When the sliding nozzle 6 is open, molten steel in the tundish 3 is cast into the mold 4 through the casting hole 3A.

[0019] In the case of the configuration shown in FIG. 1, the size of the molten steel injection outlet from the tundish 3 to the mold 4 is determined by the amount of sliding or the opening degree of the sliding nozzle 6. As shown in FIG. 1, when the sliding nozzle 6 does not block the lower end of the injection hole 3A of the tundish 3 (when the sliding nozzle 6 does not overlap the lower end of the injection hole 3A of the tundish 3 in the vertical direction), the molten steel injection outlet from the tundish 3 to the mold 4 is the lower end of the injection hole 3A of the tundish 3. When the sliding nozzle 6 blocks a part of the lower end of the injection hole 3A of the tundish 3 (when a part of the lower end of the injection hole 3A of the tundish 3 overlaps with the sliding nozzle 6 in the vertical direction), the molten steel injection outlet from the tundish 3 to the mold 4 is the part of the lower end of the injection hole 3A of the tundish 3 that is not blocked by the sliding nozzle 6 (the part of the lower end of the injection hole 3A of the tundish 3 that does not overlap with the sliding nozzle 6 in the vertical direction). A nozzle or the like that is long in the vertical direction may be disposed below the sliding nozzle 6. The configuration of the sliding nozzle 6 is not limited to the above configuration.

[0020] As shown in Fig. 1, tundish powder 20 is present in the tundish 3 so as to cover the molten steel. The tundish powder 20 keeps the molten steel in the tundish 3 warm and captures inclusions mixed in the molten steel. The tundish powder 20 contains, for example, Al2O3, CaO, SiO2, and the like.

[0021] The tundish 3 is not limited to the tundish shown in Fig. 1 and Fig. 2. For example, as shown in Fig. 3, the bottom 103P of the tundish 103 may be flat. Also, as shown in Fig. 4, a weir 130 may be arranged in the tundish 3. The weir 130 has a hole 130A through which the molten steel passes. A refractory material is used for the weir 130. The weir may be arranged in the tundish 103 shown in Fig. 3.

[0022] In addition to the above, the continuous casting equipment 1 is equipped with, for example, a control device (not shown). The control device can adjust, for example, the height of the ladle 2. By adjusting the height of the ladle 2, "H" shown in Figs. 2 and 3 changes. The control device can adjust the timing and degree of opening and closing of the sliding nozzle 5 below the discharge hole 2A of the ladle 2. For example, the timing of opening and closing of the sliding nozzle 5 may be adjusted based on the height h of the molten steel surface in the tundish 3. The control device can also adjust the timing and degree of opening and closing of the sliding nozzle 6 below the casting hole 3A of the tundish 3.

[0023] Here, an explanation will be given of the parameters shown in Figures 2 and 3. Figures 2 and 3 show, as an example, a state immediately after the start of pouring molten steel from the ladle (2) into the tundish (3, 103).

[0024] "H0" is the height of the molten steel (m) from the discharge outlet of the molten steel in the ladle (2) to the surface of the molten steel in the ladle (2). "H" is the vertical distance (m) from the "discharge outlet of molten steel in the ladle (2)" to the "portion of the bottom (3P, 103P) of the tundish (3, 103) that vertically overlaps with the discharge outlet of molten steel in the ladle (2)" (the first bottom portion (3X, 103X)). Figure 5 shows a view of the bottom 3P of the tundish 3 shown in Figure 2 as seen from the inside. The "first bottom portion 3X" shown in Figure 5 is the "portion of the bottom 3P of the tundish (3) that vertically overlaps with the discharge outlet of molten steel in the ladle" (the first bottom portion 3X) in Figure 2.

[0025] "r0" is the radius of the molten steel discharge outlet in the ladle (2). Here, when the molten steel discharge outlet in the ladle (2) is circular, "r0" is the radius of the molten steel discharge outlet in the ladle (2). When the molten steel discharge outlet in the ladle (2) is a shape other than circular, "r0" is 1 / 2 the circle-equivalent diameter of the molten steel discharge outlet in the ladle (2). In the structure shown in Figures 2 and 3, r0 can be changed by adjusting the sliding amount or opening degree of the sliding nozzle 5.

[0026] "h" is the height (m) of molten steel in the tundish (3, 103). "h" is the height of molten steel in the tundish 3 from the lowest part of the tundish (3, 103) (in the case of the tundish 3 shown in FIG. 2, the "lower bottom part 3P2"). Note that, as shown in FIG. 3, when the bottom part 103P of the tundish 103 is flat, the lowest part of the tundish 103 may be any part of the bottom part 103P except for the casting hole 3A.

[0027] "h0" is the height (m) of molten steel in the tundish (3, 103) in a steady state. Like "h", "h0" is the height of molten steel in the tundish 3 in a steady state from the lowest part of the tundish (3, 103) (in the case of the tundish 3 shown in FIG. 2, "lower bottom part 3P2"). "h0" may be a value calculated from, for example, the tundish capacity and the size of the tundish.

[0028] "Steady state" refers to a state in which molten steel is present in the tundish 3 at or near the tundish capacity. In Figures 2 and 3, the surface of molten steel and tundish powder 20 in the tundish 3 during steady state are shown by phantom lines. Any time other than "steady state" is referred to as "unsteady state". "Unsteady state" refers to, for example, the period after ladle 2 is emptied, when ladle 2 is replaced, molten steel is poured from the next ladle 2 into tundish 3, and until steady state is reached. During "steady state", "h" is "h0". During "unsteady state", "h" changes. During "unsteady state", "h" is smaller than "h0".

[0029] “h1” is the vertical distance (m) between “the portion (first bottom surface portion (3X, 103X)) of the bottom (3P, 103P) of the tundish (3, 103) that vertically overlaps with the discharge outlet of the molten steel in the ladle (2)” and “the portion of the bottom (3P, 103P) of the tundish (3, 103) that is at the same height as the upper end of the pouring hole (3A).” In the case of the tundish 3 shown in Figure 2, the "portion of the bottom (3P) of the tundish (3) that is at the same height as the upper end of the casting hole (3A)" is the lower bottom portion 3P2 of the tundish 3. In the case of the tundish 3 shown in Figure 2, "h1" is the vertical distance between the first bottom surface portion 3X of the tundish 3 and the lower bottom portion 3P2. In the case of the tundish 3 shown in Figure 2, h1>0. In the case of the tundish 103 shown in Fig. 3, the bottom 103P is flat, so that the "portion of the bottom (103P) of the tundish (103) that vertically overlaps with the discharge outlet of the molten steel in the ladle" (the "first bottom surface portion 103X" in Fig. 3) and the "portion of the bottom (103P) of the tundish (103) that is at the same height as the upper end of the casting hole 3A" are at the same height. In the case of the tundish 103 shown in Fig. 3, h1 = 0.

[0030] "V" is the speed (m / s) of molten steel pouring from the tundish (3, 103) to the mold (4). "D" is the diameter of the outlet for pouring molten steel from the tundish (3, 103) into the mold (4). Here, when the outlet for pouring molten steel from the tundish (3, 103) into the mold (4) is circular, "D" is the diameter of the outlet for pouring molten steel from the tundish (3, 103) into the mold (4). When the outlet for pouring molten steel from the tundish (3, 103) into the mold (4) is a shape other than circular, "D" is the equivalent circle diameter of the outlet for pouring molten steel from the tundish (3, 103) into the mold (4). In the structure shown in Figures 2 and 3, D can be changed by adjusting the sliding amount or opening degree of the sliding nozzle 6.

[0031] Although the above description has been given mainly with reference to FIG. 2 and FIG. 3, the configuration shown in FIG. 4 also has parameters similar to those described above. There is no particular limitation on the method of obtaining the above parameters. The above parameters are measured, calculated, or detected in the usual manner by those skilled in the art. For example, "h" may be detected by a sensor attached to the tundish 3 or may be predicted from the measured value of the weight of the molten steel in the tundish. "H0" may be detected by a sensor attached to the ladle or may be predicted from the measured value of the weight of the molten steel in the ladle. Incidentally, "vb", "vw" and "r" shown in FIG. 2 and FIG. 3 will be described later.

[0032] The steel is produced by the following method. Molten steel in a ladle 2 shown in FIG. 2 is poured into a tundish 3 from a discharge hole 2A. The molten steel in the tundish 3 is poured into a mold 4 from a pouring hole 3A and withdrawn downward, thereby continuously casting steel. In this embodiment, continuous casting is performed in which molten steel in a plurality of ladles 2 is continuously cast. Specifically, the molten steel in the ladle 2 is poured into the tundish 3, and when the ladle 2 becomes empty, the ladle 2 is replaced, and the molten steel in the next ladle 2 is poured into the tundish 3, and this process is continued. After casting, a product is obtained through a rolling process and the like. The above is also true for the production of steel using the continuous casting equipment shown in Figures 3 and 4. In the following, the reference symbols in Figures 1 and 2 will be mainly used for explanation.

[0033] In continuous casting, it is known that the cleanliness of the molten steel in the tundish 3 decreases during "unsteady" conditions. "Unsteady" conditions refer to, for example, the period from when the ladle 2 is emptied, when the ladle 2 is replaced, and when molten steel is poured from the next ladle 2 into the tundish 3, until the steady state is reached. The cleanliness of the molten steel during unsteady conditions can affect the cleanliness of the molten steel during steady conditions.

[0034] Therefore, the inventors of the present application have (1) studied a method for suppressing the influence of the cleanliness of the molten steel in the tundish during unsteady operation on the cleanliness during steady operation. In order to achieve the above (1), it is necessary that the cleanliness of the molten steel in the tundish during unsteady operation does not become too low. Therefore, (2) they have also studied a method for increasing the cleanliness of the molten steel in the tundish during unsteady operation (for example, after changing the ladle) by a method different from the conventional method.

[0035] (1) A method to suppress the influence of the unsteady cleanliness of the molten steel in the tundish on the steady state Before and after the replacement of the ladle 2, molten steel is not poured into the tundish 3. However, molten steel continues to be poured from the tundish 3 into the mold 4. As a result, the molten steel level in the tundish 3 drops. As shown in Figure 2, the molten steel level in the tundish 3 during an unsteady state (solid line in Figure 2) is lower than the molten steel level during a steady state (phantom line in Figure 2).

[0036] After the ladle 2 is replaced, a large amount of molten steel is poured from the next ladle 2 into the tundish 3 in a short period of time. This causes the molten steel level in the tundish 3 to rise rapidly. However, pouring a large amount of molten steel into the tundish 3 in a short period of time applies strong pressure to the tundish 3, which makes the refractory of the tundish 3 prone to peeling. The peeled refractory gets mixed into the molten steel and becomes inclusions. In addition, during steady operation, the tundish 3 is covered with a lid (not shown), but during non-steady operation, the lid may be opened. When the lid is opened, the molten steel in the tundish 3 is oxidized. The cleanliness of the molten steel in the tundish 3 during an unsteady state decreases due to inclusions caused by the refractories, oxidation of the molten steel, and the like.

[0037] The inventors of the present application considered that if the amount of molten steel in the tundish 3 is large when molten steel pouring from the next ladle 2 begins after the ladle 2 is replaced, inclusions caused by the refractories will spread throughout the molten steel in the tundish 3, and the molten steel will remain in the steady state, degrading the quality of the product in the steady state. They therefore considered that it would be better if the amount of molten steel in the tundish 3 was small when molten steel pouring from the next ladle 2 began.

[0038] Based on the above findings, attention was focused on the timing to start pouring molten steel from the next ladle 2. In the following experiment 1, an experiment was conducted to investigate the timing to start pouring molten steel from the next ladle 2 and its effect on the steady state.

[0039] In addition, in the first charge of continuous casting, minute chips of refractory material generated during construction may remain in the tundish 3. In addition, in the first charge of continuous casting, the oxygen concentration in the tundish 3 is high, so oxidation of the molten steel is likely to progress. Therefore, in the first charge of continuous casting, the cleanliness of the molten steel in the tundish 3 is low. Therefore, the second charge of continuous casting and onward are the targets.

[0040] (Experiment 1) After the refining process in the converter, a ladle refining process (LF process) was carried out. These refining processes were carried out according to the usual methods of those skilled in the art. The molten steel temperature in the ladle was 1450-1700°C. The amount of molten steel in the ladle was 225-280 tons. High carbon steel was continuously cast according to the usual methods of those skilled in the art. In the continuous casting, successive casting was carried out. Table 1 shows the implementation conditions at the start of molten steel pouring into the target ladle (at the start of molten steel pouring into the target charge). The target ladle (target charge) is a ladle (charge) in the second or subsequent consecutive casting.

[0041] In this experiment, a tundish (103) having a flat bottom (103P) as shown in FIG. 3 was used. A weir was installed in the tundish. The refractories used in the tundish and the weir in the tundish were rosestone bricks, high alumina bricks, MgO-based spraying material, etc. The refractories were installed according to the usual method known to those skilled in the art.

[0042] The density of molten steel is 7 (ton / m 3 The density of the tundish powder is 2.5 (ton / m 3 ).

[0043] After casting, the steel was rolled and other processes were carried out to produce 155mm square billets. Table 1 shows the percentage of products that were used as second-grade products after the charge boundary ("second-grade amount"). "After the charge boundary" refers to the time from when molten steel started to be poured into the target charge (from when molten steel started to be poured into the target ladle). The results of Table 1 are shown in Figure 6.

[0044] "h / h0" in Table 1 is the ratio of "molten steel surface height h" in the tundish when molten steel starts pouring from the target ladle into the tundish after ladle exchange to "molten steel surface height h0 under steady state." Using "h / h0," the amount of molten steel in the tundish at the start of molten steel pouring can be estimated.

[0045] The operating condition "a" in Table 1 is a value calculated using the following formula [2]. a=[VD 2 / 2.05{(ρ S -ρ P )g / ρ S} 1 / 2 ] 2 / 5 / h0···[2] In this experiment, "h / h0" at the start of pouring molten steel from the ladle into the tundish was set to "a" or more for the following reasons.

[0046] Even during unsteady operation, molten steel continues to be poured from the tundish into the mold. When the amount of molten steel in the tundish is small, a depression occurs in the molten steel above the pouring hole formed in the bottom of the tundish, and the tundish powder on the molten steel flows out of the pouring hole into the mold together with the molten steel. This causes product defects caused by the tundish powder.

[0047] The following formula [A-1] is the limiting condition for intake of a vertical intake pipe (Nobuyuki Tamai, "New System of Civil Engineering: Hydraulics of Density Flow", Japan Society of Civil Engineers). V / {(ρ S -ρ P )gh / ρ S} 1 / 2 =3.20K(h / d) 2 [A-1] where K is 0.64. From the above formula [A-1], h is expressed by the following formula. h=[VD 2 / 2.05{(ρ S -ρ P )g / ρ S} 1 / 2 ] 2 / 5 [A-2] The "water intake limit of the vertical water intake pipe" can be considered to be the point just before a depression in the molten steel appears above the injection hole at the bottom of the tundish. In this case, "h" in the above formula [A-2] corresponds to the molten steel surface height "h" in the tundish (see Figures 2 and 3). When the molten steel surface height "h" in the tundish is lower than "h" in the above formula [A-2], it is thought that the tundish powder above the molten steel will flow into the mold together with the molten steel. In order to prevent the tundish powder from flowing into the mold, it is necessary to start pouring molten steel from the ladle when the molten steel surface height "h" in the tundish is equal to or higher than "h" in the above formula [A-2].

[0048] From the above, the ratio "h / h0" of "h" shown in formula [A-2] to "height of the molten steel surface in the tundish at steady state h0" is set to "a" (see formula [2] above), and "h / h0" at the start of pouring molten steel from the ladle to the tundish is set to be greater than "a".

[0049] [Table 1]

[0050] From Table 1 and Fig. 6, when the "h / h0" ratio at the start of pouring molten steel from the ladle to the tundish is 0.55 or more, the amount of second-grade products is large. These second-grade products are the products during the unsteady state and the products during the steady state. On the other hand, when "h / h0" is 0.45 or less, the amount of second-grade products is small. When "h / h0" is 0.45 or less, the amount of second-grade products during steady state is reduced by 3.2% compared to when "h / h0" is 0.55 or more.

[0051] From this, it was found that by starting molten steel pouring from the ladle when "h / h0" is 0.45 or less, it is possible to prevent the cleanliness of the molten steel in the tundish during unsteady conditions from affecting the steady state.

[0052] From the above, after changing the ladle, when starting to pour molten steel from the next ladle into the tundish, the following formulas [1] and [2] should be satisfied. a ≦ h / h0≦ 0.45 [1] a = [VD 2 / 2.05{(ρ S -ρ P )g / ρ S} 1 / 2 ] 2 / 5 / h0···[2] In other words, when the above formulas [1] and [2] are satisfied, molten steel begins to be poured from the ladle into the tundish. This makes it possible to suppress the cleanliness of the molten steel in the tundish during an unsteady state from affecting the steady state.

[0053] (2) Methods for improving the cleanliness of molten steel in a tundish during unsteady operation As described above, the molten steel level in the tundish 3 drops before and after the replacement of the ladle 2. Even after the start of pouring molten steel from the ladle 2, the amount of molten steel in the tundish 3 remains small for a while. When molten steel is poured from the ladle 2 into an area with a small amount of molten steel, the flow of molten steel poured from the ladle 2 collides with the bottom 3P of the tundish 3. A large pressure is applied to the part where the molten steel collides. In addition, after the flow of molten steel collides with the bottom 3P of the tundish 3, a very fast molten steel flow is generated along the bottom surface of the tundish 3. This molten steel flow applies a large pressure to the side 3Q of the tundish 3 and other parts. The part that is most affected by this is the side 3Q of the tundish 3 that is closest to the flow of molten steel poured from the ladle 2 or the weir 130 (see Figure 4).

[0054] In areas where a large pressure is applied, the refractory is prone to peeling off. The peeled off refractory gets mixed into the molten steel and becomes inclusions. The inclusions caused by the refractory reduce the cleanliness of the molten steel in the tundish 3 during unsteady operation.

[0055] The flow of molten steel poured from the ladle 2 is fastest just below the discharge outlet of the molten steel in the ladle 2 (see "vb" in Fig. 2). This fast flow of molten steel collides with the part of the bottom 3P of the tundish 3 that vertically overlaps with the discharge outlet of the molten steel in the ladle 2 (the "first bottom part 3X" in Fig. 5).

[0056] After the molten steel pouring flow collides with the first bottom surface portion 3X, a molten steel flow is generated along the bottom surface of the tundish 3 (see "vw" in FIG. 2). This molten steel flow collides with the side portion 3Q of the tundish 3 closest to the first bottom surface portion 3X or the weir 130 (see FIG. 4) closest to the first bottom surface portion 3X.

[0057] From the above, the inventors of the present application have focused on the following two flow velocities which are considered to have a large effect on the spalling of the refractory of the tundish 3. (A) The flow velocity "vb" of the molten steel poured from the ladle 2 at the first bottom portion 3X (see Figures 2 and 5) (B) The flow velocity "vw" of the molten steel flow along the bottom surface of the tundish 3 at the side 3Q or weir 130 (see FIG. 4) of the tundish 3 closest to the first bottom portion 3X (see FIG. 2 and FIG. 5).

[0058] By controlling the flow velocity "vb" of the above (A), it is possible to suppress the peeling of the refractory of the first bottom surface portion 3X to which a large pressure is applied. This also makes it possible to suppress the peeling of the refractory of the entire bottom portion 3P of the tundish 3. By controlling the flow velocity "vw" of (B) above, it is possible to suppress the peeling of the refractory of the side portion 3Q or the weir 130 closest to the first bottom surface portion 3X, to which a large pressure is applied. This makes it possible to suppress the peeling of the refractory of all the side portions 3Q and the weirs of the tundish 3. As a result of the above, the amount of inclusions caused by the refractories is reduced, so that the cleanliness of the molten steel in the tundish 3 during an unsteady state can be improved.

[0059] The "flow velocity vb of the molten steel pouring flow" in the above (A) is the flow velocity in the vertical direction toward the first bottom surface portion 3X, as shown in FIG. The "flow velocity vw of the molten steel flow" in (B) above is the flow velocity vw of the molten steel flow that occurs after the molten steel poured from the ladle 2 collides with the bottom 3P of the tundish 3. The "flow velocity vw of the molten steel" in (B) above is the flow velocity vw of the molten steel approximately parallel to the bottom surface of the tundish 3 toward the side 3Q or the weir closest to the first bottom portion 3X.

[0060] The "flow velocity vb of the molten steel pouring flow" in (A) above is a flow velocity calculated by the following formula. vb = [2g{H0+H-(h-h1)}] 1 / 2 [4] Here, "g" is the gravitational acceleration (m / s 2 ) where "H", "H0", "h" and "h1" are the parameters shown in Figures 2 and 3 above. The "flow velocity vb of the molten steel pouring flow" can be controlled, for example, by "H" and / or "h." H can be controlled, for example, by adjusting the height of the ladle 2. h can be controlled, for example, by adjusting the start time of pouring molten steel from the ladle 2 into the tundish 3.

[0061] The above equation [4] is derived from the following equations [4-1] and [4-2]. Assuming that the flow inside the tundish is laminar, the following equation is obtained from Bernoulli's law. [The discharge outlet of the molten steel in the ladle 2 and the first bottom part 3X of the tundish 3] From Bernoulli's equation, P0+ρ S ×g×H+(1 / 2)×ρ S ×v0 2 =Pb+(1 / 2) × ρ S ×vb 2 [4-1] Where: P0: Atmospheric pressure ρ S : Molten steel density g: Gravitational acceleration (m / s 2 ) v0: Molten steel flow velocity at the discharge outlet of the molten steel in ladle 2 (m / s) Pb: pressure at the first bottom portion 3X of the tundish 3 vb: flow velocity of molten steel poured from ladle 2 at first bottom portion 3X vb It is. v0 was calculated from the following formula: v0 = (2gH0) 1 / 2 [First bottom surface 3X of tundish 3 and molten steel surface in tundish 3 when there is no flow of molten steel in tundish 3] From Bernoulli's equation, Pb = P0 + ρ S ×g×(h-h1) [4-2] Here, "h" and "h1" are parameters shown in FIG. 2 and the like.

[0062] The "flow velocity vw of the molten steel flow" in (B) above is calculated by the following formula. vw = 3.5{(Hr0) 2 / r}(2gH0) 1 / 2 [6] Here, "H", "r0" and "H0" are the parameters shown in FIG. 2 and FIG. As shown in Figure 5, "r" is the horizontal distance (m) between a position P at the bottom 3P of the tundish 3 that vertically overlaps with the center C of the discharge outlet of the ladle 2 and the side 3Q of the tundish 3 or the weir (see "weir 130" in Figure 4) that is closest to position P. "g" is the acceleration due to gravity (m / s 2 ). The "flow velocity vw of the molten steel flow" can be controlled, for example, by "H" and / or "r0". As described above, "H" can be controlled, for example, by adjusting the height of the ladle 2. "r0" can be controlled, for example, by adjusting the opening of the sliding nozzle 5.

[0063] The above formula [6] is the formula for the maximum flow velocity of the wall jet region of a three-dimensional circular impinging jet (Toshiko Shagouchi, "Jet Engineering - Fundamentals and Applications", Morikita Publishing Co., Ltd.). When the molten steel poured from the ladle 2 impinges on the bottom surface of the tundish 3, a molten steel flow is generated along the bottom surface of the tundish 3. This is considered to be the three-dimensional circular impinging jet. In addition, the wall jet region of the three-dimensional circular impinging jet is considered to be the region up to the tundish wall surface or weir, and the maximum flow velocity obtained from the formula for the maximum flow velocity of the wall jet region of the three-dimensional circular impinging jet is taken as the "molten steel flow velocity vw" in (B) above.

[0064] It is believed that by controlling the "flow velocity vb of the molten steel pouring flow" in (A) above and the "flow velocity vw of the molten steel flow" in (B) above, the cleanliness of the molten steel during unsteady operation can be improved. The inventors of the present application focused on the following equation [1] obtained from the above Experiment 1, and conducted an experiment to investigate the relationship between the "flow velocity vb of the molten steel pouring flow" and the "flow velocity vw of the molten steel flow" when equation [1] is satisfied, and the cleanliness of molten steel during an unsteady state. a ≦ h / h0≦ 0.45 [1]

[0065] (Experiment 2) High carbon steel was continuously cast under the same conditions as in Experiment 1. Sequential casting was performed in the continuous casting. After casting, the steel billets were produced in a size of 155 mm square through processes such as rolling. Table 2 shows the implementation conditions and results. Experiment numbers 1 to 7 in Table 2 correspond to Experiment numbers 1 to 7 in Table 1. Table 2 also shows the results of Experiment 1. Note that "vb" in Table 2 is the maximum value of "vb" when the above formula [1] is satisfied in the target charge. "vw" in Table 2 is the maximum value of "vw" when the above formula [1] is satisfied in the target charge. "h" and "H0" in Table 2 are "h" and "H0" at the time of "vb" (maximum value) and "vw" (maximum value) shown in Table 2. The other implementation conditions shown in Table 2 are the conditions at the start of pouring molten steel from the target ladle into the tundish.

[0066] The cleanliness of the molten steel was evaluated by the following method.

[0067] As described in JP 2014-189895 A (

[0017] etc.), it is known that fatigue spalling (rolling fatigue damage) occurs in steel starting from nonmetallic inclusions in the steel. For such fatigue spalling, it is effective to control the size of oxide-based inclusions. In JP 2014-189895 A,

[0017] ,

[0059] etc., it is described that when a round steel bar having a diameter of 65 mm is manufactured by rolling the cast piece after casting, if the predicted value of the square root (√area max) of the projected area of ​​the largest oxide-based inclusion in the evaluation area (area) based on the extreme value statistics method (hereinafter, sometimes referred to as the "predicted value of √area max of oxide-based inclusions") exceeds 40 μm, it becomes difficult to improve the rolling fatigue properties. From these descriptions, it can be seen that when a round steel bar having a diameter of 65 mm is manufactured, the threshold value of the "predicted value of √area max of oxide-based inclusions" for evaluating the rolling fatigue properties is 40 μm. The above-mentioned inclusions also reduce the cleanliness of the molten steel in the tundish, and it is therefore believed that the cleanliness of the molten steel can also be evaluated in a manner similar to that used to evaluate the rolling fatigue properties.

[0068] In this experiment, 155 mm square billets were produced. The reduction amount is smaller when producing 155 mm square billets than when producing 65 mm diameter round bars. Therefore, the inclusions present in 155 mm square billets are larger than those present in 65 mm diameter round bars. In addition, the threshold value of the "predicted value of √area max of oxide-based inclusions" used to evaluate the cleanliness of molten steel is also higher for 155 mm square billets than for 65 mm diameter round bars. Calculating backwards from the reduction amount when rolling 155 mm square billets to 65 mm diameter round bars, the threshold value of 40 μm for 65 mm diameter round bars is equivalent to 95.4 μm for 155 mm square billets. From the above, the threshold for evaluating the cleanliness of molten steel was set to 95.4 μm.

[0069] A steel billet was produced from the cast slab during the unsteady state, and the short diameter of the inclusions in the 155 mm square billet was measured using the slime dissolution method. The largest short diameter was taken as the "maximum short diameter." If the "maximum short diameter" was 95.4 μm or less, the cleanliness of the molten steel during the unsteady state was determined to be high, and the "evaluation" of slime dissolution in Table 2 was given as "◯." If the "maximum short diameter" exceeded 95.4 μm, the cleanliness of the molten steel during the unsteady state was determined to be low, and the "evaluation" of slime dissolution in Table 2 was given as "×."

[0070] In addition, in Table 2, when the "second-grade amount" in Experiment 1 was as low as 9.6% and the evaluation of slime dissolution in Experiment 2 was "good", the "overall evaluation" was marked as "good". Otherwise, the "overall evaluation" was marked as "bad".

[0071] [Table 2]

[0072] From Table 2, in Experiments 1 to 5, the evaluation of slime dissolution was "good". The maximum value of "vb" in Experiments 1 to 5 was 9.91 (m / s), and the maximum value of "vw" was 0.62 (m / s). In Experiment 6, in which the evaluation of slime dissolution was "poor", "vb" was greater than the maximum value of "vb" (9.91 (m / s)). In Experiment 7, in which the evaluation of slime dissolution was "poor", "vb" was greater than the maximum value of "vb" (9.91 (m / s)), and "vw" was greater than the maximum value of "vw" (0.62 (m / s)). From this, it was found that when the following formula [1] is satisfied, the cleanliness of molten steel during unsteady state can be increased by controlling "vb" and "vw" so that "vb" satisfies the following formula [3] and "vw" satisfies the following formula [5]. a ≦ h / h0≦ 0.45 [1] vb≦9.91 [3] vw ≦ 0.62 [5]

[0073] In addition, in the overall evaluation of Table 2, the test numbers 1 to 3 were given a "good" rating. From this, it was found that the following method can increase the cleanliness of the molten steel in the tundish during unsteady operation and suppress the influence of the cleanliness of the molten steel in the tundish during unsteady operation on the steady operation.

[0074] In continuous casting of steel, molten steel from multiple ladles is cast continuously. After changing the ladle, when starting to pour molten steel from the next ladle into the tundish, the following equations [1] and [2] must be satisfied. a ≦ h / h0≦ 0.45 [1] a = [VD 2 / 2.05{(ρ S -ρ P )g / ρ S} 1 / 2 ] 2 / 5 / h0···[2] In addition, when the above formula [1] is satisfied, At the bottom of the tundish, the vertical flow velocity vb (m / s) of the molten steel poured from the ladle satisfies the following equations [3] and [4], and vb≦9.91 [3] vb = [2g{H0+H-(h-h1)}] 1 / 2 [4] The flow velocity vw (m / s) of the molten steel flow along the bottom of the tundish at the side of the tundish closest to the molten steel pouring flow from the ladle or at a weir located within the tundish should satisfy the following equations [5] and [6]. vw ≦ 0.62 [5] vw = 3.5{(Hr0) 2 / r}(2gH0) 1 / 2 [6]

[0075] The above method can suppress the influence of the cleanliness of molten steel during unsteady operation on the cleanliness during steady operation. In the conventional method, the cleanliness of molten steel in the tundish during unsteady operation is increased by controlling the "amount of molten steel poured from the ladle to the tundish" and the "amount of molten steel supplied from the tundish to the ladle". However, the above method, unlike the conventional method, can increase the cleanliness of molten steel in the tundish during unsteady operation by controlling the "flow velocity vb of the molten steel pouring flow" and the "flow velocity vw of the molten steel flow".

[0076] The lower limit of vb is not particularly limited. The smaller vb, the less likely the refractory at the bottom of the tundish will peel off. For example, vb may be set to 9.26 (m / s) or more in order to prevent the tundish and the ladle from interfering with each other.

[0077] In addition, there is no particular lower limit for vw. The smaller vw is, the less likely the refractory material on the side and gate of the tundish is to peel off. For example, vw may be set to 0.19 (m / s) or more in order to prevent the time during the unsteady state from becoming too long.

[0078] Although the embodiments of the present invention have been described above with reference to the drawings, the specific configuration should not be considered to be limited to these embodiments. The scope of the present invention is indicated by the claims rather than the above description, and includes all modifications within the meaning and scope of the claims.

[0079] For example, in Fig. 1, a sliding nozzle 5 is disposed below the discharge hole 2A of the ladle 2. By adjusting the sliding or opening / closing of the sliding nozzle 5, the discharge of molten steel from the ladle 2 can be started. By adjusting the sliding amount or opening degree of the sliding nozzle 5, the size of the discharge outlet for the molten steel in the ladle 2 can be adjusted. However, the method and configuration for controlling the start of discharge of molten steel from the ladle 2 and the size of the discharge outlet for the molten steel in the ladle 2 are not limited to the above-mentioned method and configuration and can be changed. The method and configuration for controlling the size of the outlet for pouring molten steel from the tundish into the mold can also be changed in the same manner as described above.

[0080] In the above-described embodiment, the steel type, operating conditions, dimensions of components constituting the continuous casting equipment, and methods of determining various parameters can be within the scope of what a person skilled in the art would normally do, or can be easily imagined by a person skilled in the art. [Explanation of symbols]

[0081] 1. Continuous casting equipment 2 ladle 2A discharge hole 3, 103 Tundish 3A injection hole 3P, 103P bottom 3P1 top bottom 3P2 lower bottom 3P3 slope 3Q Side 3X, 103X 1st bottom part 4. Mold 5, 6 Sliding nozzle 20 Tundish Powder 130 Weir P position

Claims

[Claim 1] In continuous casting of steel, molten steel from multiple ladles is cast continuously. After changing the ladle, when starting to pour molten steel from the next ladle into the tundish, the following formulas [1] and [2] must be satisfied. a ≦ h / h 0 ≦ 0.45 ・・・[1] a = [VD 2 / 2.05{(r) S -r P )g / p S } 1 / 2 ] 2 / 5 / h 0 ・・・[2] When the above formula [1] is satisfied, At the bottom of the tundish, the vertical flow velocity vb (m / s) of the molten steel poured from the ladle satisfies the following equations [3] and [4], and vb≦9.91...[3] vb = [2g{H 0 +H-(h-h 1 )}] 1 / 2 ・・・[4] The flow velocity vw (m / s) of the molten steel flow along the bottom of the tundish at the side of the tundish closest to the molten steel pouring flow from the ladle or at the weir located inside the tundish satisfies the following equations [5] and [6]. vw≦0.62...[5] vw = 3.5{(Hr 0 ) 2 / r}(2gH 0 ) 1 / 2 ・・・[6] A method for producing steel, comprising: Where: h: Height of molten steel in the tundish (m) h 0 :Height of molten steel in the tundish at steady state (m) V: Speed ​​of pouring molten steel from the tundish to the mold (m / s) D: Diameter of the outlet for pouring molten steel from the tundish to the mold (m) ρ S :Molten steel density (ton / m 3 ) ρ P : Density of tundish powder (ton / m 3 ) g: acceleration due to gravity (m / s 2 ) H: Vertical distance (m) from the discharge outlet of the molten steel in the ladle to the first bottom surface portion that vertically overlaps with the discharge outlet at the bottom of the tundish H 0 :Height of molten steel from the discharge outlet of the molten steel in the ladle to the surface of the molten steel in the ladle (m) h 1 : Vertical distance (m) between the first bottom surface portion of the bottom of the tundish and the second bottom surface portion of the bottom of the tundish that is at the same height as the upper end of the casting hole r 0 :Radius of the outlet of the molten steel in the ladle (m) r: The horizontal distance (m) between a position P at the bottom of the tundish that vertically overlaps with the center of the discharge outlet of the molten steel in the ladle and the side of the tundish or the weir located in the tundish that is closest to the position P

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