Method for continuous-casting slab

Abstract

A primary object of this invention is to provide a continuous casting method by which a slab of excellent internal quality can be obtained even if the casting speed is changed. In this invention, upon continuous casting with two pairs of the reduction rolls arranged along a casting direction and support rolls arranged between the reduction rolls, when a casting speed is reduced compared to a state where combination of reduction with reduction rolls at a first stage on an unsolidified portion of the slab and reduction with reduction rolls at a second stage on a solidified portion thereof is employed, the combination is switched to combination of reduction with the reduction rolls at the first stage on a portion of the slab at an end of solidification and the reduction with the reduction rolls at the second stage on the solidified portion thereof.

Claims

1. A method for continuous-casting a slab while reduction is carried out on the slab using a continuous casting machine with two stages of reduction rolls, each of the two stages consisting of a pair of the reduction rolls, and being arranged along a casting direction, a diameter of each of the reduction rolls being 1.2 to 2.0 times as much as thickness of the slab just before reduction with corresponding reduction rolls, the continuous casting machine including the reduction rolls and support rolls, the support rolls being arranged between said two stages of the reduction rolls, the method comprising; increasing a casting speed compared to a first state where the slab is cast at a constant speed while reduction is carried out on the slab with reduction rolls at a first stage, and switching the reduction with the reduction rolls at the first stage to reduction with reduction rolls at a second stage, the reduction rolls at the second stage being arranged more downstream than the reduction rolls at the first stage in a casting direction, in such a manner that a reduction amount of the reduction rolls at the second stage is same as that of the reduction rolls at the first stage, which are used for the reduction on the slab in the first state, accompanying movement of a place of the slab where solidification is ended downstream in the casting direction due to the increase of the casting speed.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 shows a structure of a continuous casting machine to which the method for continuous-casting a slab of this invention can be applied, in a state where reduction is not carried out on a slab.

(2) FIG. 2 shows a structure of a continuous casting machine to which the method for continuous-casting a slab of this invention can be applied, in a state where reduction is carried out on a slab with the large diameter reduction rolls in both upstream side and downstream side in the casting direction.

(3) FIG. 3 shows a structure of a continuous casting machine to which the method for continuous-casting a slab of this invention can be applied, in a state where reduction is carried out on a slab with the large diameter reduction rolls in both upstream side and downstream side in the casting direction.

(4) FIG. 4 shows a structure of a continuous casting machine to which the method for continuous-casting a slab of this invention can be applied, in a state where reduction is carried out on a slab only with the large diameter reduction rolls in the downstream side in the casting direction.

(5) FIG. 5 shows a structure of a continuous casting machine to which the method for continuous-casting a slab of this invention can be applied, in a state where reduction is carried out on a slab only with the large diameter reduction rolls in the upstream side in the casting direction.

DESCRIPTION OF EMBODIMENTS

(6) The embodiments of this invention will be described hereinafter. The embodiments described below are examples of this invention but do not limit this invention.

1.Basic Structure of Continuous Casting Machine

(7) FIG. 1 shows a structure of a continuous casting machine 10 to which the method for continuous-casting a slab of this invention can be applied, in a state where reduction is not carried out on a slab. Molten steel 3 poured into a mold 1 so as to form a molten steel bath surface (meniscus) 2 is cooled by water spray (secondary cooling water) jetting out from the mold 1 and a group of secondary cooling spray nozzles that was not shown and was under the mold 1, to form a solidified shell 4, to be a slab 5. The slab 5 is withdrawn as keeping the molten steel 3 that is unsolidified in its inside, and reduction is appropriately carried out on the slab 5 with plural pairs of large diameter reduction rolls 6. The slab 5 on which the reduction has been carried out with the large diameter reduction rolls 6 passes through support rolls 7 that are arranged between the large diameter reduction rolls 6 and also more downstream than the large diameter reduction rolls 6 in the casting direction, and is withdrawn with pinch rolls that are not shown. Fig. 1 shows the casting direction using an arrow.

(8) The large diameter reduction rolls 6 shown in Fig. 1 are constituted by two stages arranged along the casting direction: each stage consists of a pair of large diameter reduction rolls. Hereinafter each pair of the large diameter reduction rolls is referred to as first large diameter reduction rolls 6a and second large diameter reduction rolls 6b in order from the upstream side in the casting direction. Diameter of each first large diameter reduction rolls 6a is 1.2 to 2.0 times as much as the thickness of the slab 5 just before reduction is carried out thereon with the first large diameter reduction rolls 6a. Diameter of each second large diameter reduction rolls 6b is 1.2 to 2.0 times as much as the thickness of the slab 5 just before reduction is carried out thereon with the second large diameter reduction rolls 6b. Here, the reason why the lower limit of each diameter of the first large diameter reduction rolls 6a and the second large diameter reduction rolls 6b is 1.2 times as much as the thickness of the slab just before the reduction with corresponding large diameter reduction rolls is to maintain reduction force necessary to obtain the slab of excellent internal quality. On the other hand, the reason why the upper limit of each diameter of the first large diameter reduction rolls 6a and the second large diameter reduction rolls 6b is twice as much as the thickness of the slab just before the reduction with corresponding large diameter reduction rolls is to check the increase of the cost of equipment and bulging between rolls.

(9) The support rolls 7 are arranged between the large diameter reduction rolls 6 in the continuous casting machine 10. Therefore, even if intervals between the large diameter reduction rolls 6 are long, bulging is hard to occur in the slab 5, and it is possible to check deterioration of the internal quality of the slab 5.

2. Method for Continuous-Casting Slab of this Invention

(10) In the method for continuous-casting a slab of this invention, reduction is carried out on the slab 5 with two stages of the large diameter reduction rolls 6 that are arranged along the casting direction within the continuous casting machine 10. The large diameter reduction rolls 6 are large diameter reduction rolls 1.2 to 2.0 times as much as the thickness of the slab 5 just before each case of reduction therewith. Here, the reduction on the slab with the large diameter reduction rolls is referred to as heavy reduction.

(11) FIGS. 2 to 5 show structures of a continuous casting machine to which the method for continuous-casting a slab of this invention can be applied. FIGS. 2 and 3 show the state where the reduction is carried out on the slab with the large diameter reduction rolls in both upstream side and downstream side in the casting direction. FIG. 4 shows the state where the reduction is carried out on the slab only with the large diameter reduction rolls in the downstream side in the casting direction. FIG. 5 shows the state where the reduction is carried out on the slab only with the large diameter reduction rolls in the upstream side in the casting direction.

(12) As shown in Figs. 1 to 5, when two stages of the large diameter reduction rolls 6 are used, Cases 1 to 5 presented in Table 1 below show the combinations of a state of solidification of the slab at the position where heavy reduction is carried out and whether or not the reduction is carried out.

(13) TABLE-US-00001 TABLE 1 First Large Second Large Diameter Diameter Corresponding Case Reduction Rolls Reduction Rolls Drawing 1 No Reduction No Reduction FIG. 1 2 Reduction on Reduction after FIG. 2 Unsolidified Solidified 3 Reduction at End Reduction after FIG. 3 of Solidification Solidified 4 No Reduction Reduction at End of FIG. 4 Solidification 5 Reduction at End No Reduction FIG. 5 of Solidification 6 Reduction at End Reduction after FIG. 3 of Solidification Solidified 7 Reduction on Reduction after FIG. 2 Unsolidified Solidified 8 of Reduction at End No Reduction FIG. 5 Solidification 9 No Reduction Reduction at End of FIG. 4 Solidification

(14) In the above Table 1, Reduction on Unsolidified, Reduction at End of Solidification and Reduction after Solidified mean, as presented in Table 2 below, reduction with the large diameter reduction rolls 6 on places of the slab 5 where the solid-phase ratio at the center of the slab 5 in the thickness direction (hereinafter also referred to as center solid-phase ratio) is less than 0.8, no less than 0.8 and less than 1.0 and 1.0 respectively.

(15) TABLE-US-00002 TABLE 2 Type of Reduction Center Solid-phase Ratio Reduction on Unsolidified less than 0.8 Reduction at End of Solidification no less than 0.8 and less than 1.0 Reduction after Solidified 1.0

(16) Case 1 corresponds to FIG. 1, which is a case where no reduction is carried out on the slab 5 with any of the first large diameter reduction rolls 6a and the second large diameter reduction rolls 6b.

(17) Cases 2, 3, 6 and 7 correspond to FIGS. 2 and 3, which are cases where the reduction is carried out on the slab 5 with the first large diameter reduction rolls 6a but no reduction is carried out with the second large diameter reduction rolls 6b. In Cases 2 and 7 among these cases, the reduction is carried out on a place of an unsolidified portion of the slab 5 (a portion where the center solid-phase ratio is less than 0.8) with the first large diameter reduction rolls 6a. On the other hand, in Cases 3 and 6, the reduction is carried out on a place of a portion at the end of solidification of the slab 5 (a portion where the center solid-phase ratio is no less than 0.8 and less than 1.0) with the first large diameter reduction rolls 6a.

(18) Cases 4 and 9 correspond to FIG. 4, which are cases where the reduction is carried out on the place of the portion at the end of solidification of the slab 5 (the portion where the center solid-phase ratio is no less than 0.8 and less than 1.0) with the second large diameter reduction rolls 6b while no reduction is carried out on the slab 5 with the first large diameter reduction rolls 6a.

(19) Cases 5 and 8 correspond to FIG. 5, which are cases where the reduction is carried out on the place of the portion at the end of solidification of the slab 5 (the portion where the center solid-phase ratio is no less than 0.8 and less than 1.0) with the first large diameter reduction rolls 6a while no reduction is carried out on the slab 5 with the second large diameter reduction rolls 6b.

(20) The method for continuous-casting a slab of this invention includes the following two embodiments: (1) A method for continuous-casting a slab wherein upon continuous-casting the slab 5 using the continuous casting machine 10 as carrying out the reduction on the slab 5 with the large diameter reduction rolls 6, a manner of the reduction on the slab 5 is switched from Case 2 to Case 3 when the casting speed is reduced compared to the state where the slab 5 is cast at a constant speed as carrying out the reduction on the slab 5 in Case 2. accompanying the movement of the place of the slab 5 where solidification is ended, to the upstream side in the casting direction, due to the reduction of the casting speed; and (2) A method for continuous-casting a slab wherein upon continuous-casting the slab 5 using the continuous casting machine 10 as carrying out the reduction on the slab 5 with the large diameter reduction rolls 6, a manner of the reduction on the slab 5 is switched from Case 4 to Case 5 at the casting speed where the reduction amount of the first large diameter reduction rolls 6a is same as that of the second large diameter reduction rolls 6b in Case 4 when the casting speed is reduced compared to the state where the slab is cast at a constant speed as carrying out the reduction on the slab 5 in Case 4, accompanying the movement of the place of the slab 5 where solidification is ended, to the upstream side in the casting direction, due to the reduction of the casting speed.

(21) In the method for continuous-casting a slab of this invention, the heavy reduction is carried out on plural places in combination according to a state of solidification of the slab. Therefore, the slab of excellent internal quality can be stably obtained even if heavy reduction operation accompanied by the reduction of the casting speed is carried out.

2-1. Preferred Embodiment of Method for Continuous-Casting Slab of This Invention (1)

(22) In the method for continuous-casting a slab of this invention, when continuous casting is carried out in the embodiment shown in FIG. 2, preferably, the reduction is carried out on an unsolidified portion of the slab 5 where the center solid-phase ratio is no less than 0.2, with the first large diameter reduction rolls 6a by 5 to 30 mm, and the reduction is carried out on the solidified portion of the slab 5 with the second large diameter reduction rolls 6b by 1 to 15 mm.

2-2. Preferred Embodiment of Method for Continuous-Casting Slab of This Invention (2)

(23) In the method for continuous-casting a stab of this invention, when continuous casting is carried out in the embodiment shown in FIG. 3, preferably, the reduction is carried out on the portion of the slab 5 at the end of solidification with the first large diameter reduction rolls 6a by 5 to 20 mm, and the reduction is carried out on the solidified portion of the slab 5 with the second large diameter reduction rolls 6b by 1 to 15 mm.

(24) The above description concerning this invention mentions the embodiments of: (1) the manner of the reduction on the slab 5 is switched from Case 2 to Case 3; and (2) the manner of the reduction on the slab 5 is switched from Case 4 to Case 5, accompanying the movement of the place of the slab 5 where solidification is ended, to the upstream side in the casting direction, due to the reduction of the casting speed. Other than these embodiments, this invention can include embodiments of: (3) a manner of the reduction on the slab 5 is switched from Case 6 to Case 7; and (4) a manner of the reduction on the slab 5 is switched from Case 8 to Case 9, accompanying the movement of the place of the slab 5 where solidification is ended, to the downstream side in the casting direction, due to the increase of the casting speed. Even in these embodiments, heavy reduction is carried out on plural places in combination according to a state of solidification of the slab. Thus, the slab of excellent internal quality can be stably obtained even if heavy reduction operation accompanied by increase of the casting speed is carried out.

EXAMPLES

(25) The following continuous casting tests were carried out in order to confirm the effect of the method for continuous-casting a slab of this invention, and their results were evaluated.

(26) A vertical bending-type continuous casting machine shown in FIGS. 1 to 5 was used as a continuous casting machine. A slab continuous-cast was made of steel of 0.16 mass% C content, 280 to 300 mm in thickness and 2300 mm in width. The casting speed was 0.58 to 0.80 m/min. Secondary cooling was carried out under the condition of 0.78 to 0.94 L/kg-steel in specific water amount.

(27) The first large diameter reduction rolls were arranged at a position 21.2 m away from the molten steel bath surface in the mold downstream in the casting direction. The second large diameter reduction rolls were arranged at a position 27.0 m away from the molten steel bath surface in the mold downstream in the casting direction. Each diameter of the first large diameter reduction rolls and the second large diameter reduction rolls was 1.2 to 2.0 times as much as the thickness of the slab just before corresponding reduction.

(28) The reduction on the slab was started after the tip of the slab had passed through a position of the large diameter reduction rolls.

(29) Evaluation categories included Index of Internal Quality of Slab and Evaluation of Internal Quality of Slab.

(30) Index of Internal Quality of Slab was a ratio of the central porosity volume of a slab that was used as the basis (hereinafter may be referred to as base material) to the central porosity volume of the slab cast in each test.

(31) Here, the central porosity volume of a slab was a specific volume of the central porosity calculated from a specific gravity at the central part in the thickness direction on the basis of the average specific gravity of the slab at its position of in thickness where it was assumed to be almost no central porosity formed. That is, the central porosity volume was defined by the following formula (1):
Vp=1/1/.sub.o. . . (1)
wherein Vp (cm.sup.3g) represented the central porosity volume, (g/cm.sup.3) represented the average specific gravity at the center of the thickness of a slab, and .sub.0 (g/cm.sup.3) represented the average specific gravity of a slab at its position of in thickness.

(32) Evaluation of Internal Quality of Slab was evaluation of the index of the internal quality of a slab (the index of the base material was 1.0, which was the basis), and was represented by symbols of and . The meaning of each symbol was as follows:

(33) (excellent): the index of the internal quality of a slab was large, which exceeded 3.0.

(34) (good) : the index of the internal quality of a slab was more than 1.0 and less than 3.0.

(35) Tests were carried out on the following kinds of steel presented in Table 3 under the conditions presented in the following Table 4. In Table 4, Case means the combinations of a state of solidification of the slab at the position where heavy reduction was carried out and whether or not the reduction was carried out, which are presented in the above Table 1. Table 4 also presents the reduction amount of the slab with the large diameter reduction rolls and the casting speed. The reduction amount of the slab was calculated from difference between an interval of the large diameter reduction rolls and an interval of a support roll that was adjacent to a large diameter reduction roll upstream in the casting direction.

(36) TABLE-US-00003 TABLE 3 Chemical Component (mass %) C Si Mn P S Ni Al 0.16 0.18 0.93 0.016 0.003 0.01 0.026

(37) TABLE-US-00004 TABLE 4 Reduction Amount with Rolls [mm] (Center Solid-phase Ratio upon Reduction) 1st Large 2nd Large Idx. of Evaluation Diameter Diameter Internal of Internal Casting Reduction Reduction Quality Quality of Speed Item Case Rolls Roils of Slab Slab [m/min] Com- 1 1.0 Basis 0.80 parison Ex. 1 Ex. of 2 32 5 3.2 0.80 This (0.05) (1.0) Invention 3 12 5 3.8 0.58 1 (0.9) (1.0) Ex. of 4 12 1.7 0.80 This (0.9) Invention 5 12 2.5 0.58 2 (0.9) Ex. of 6 12 5 3.8 0.58 This (0.9) (1.0) Invention 7 32 5 3.2 0.80 3 (0.05) (1.0) Ex. of 8 12 2.5 0.58 This (0.9) Invention 9 12 1.7 0.80 4 (0.9)

(38) The above Table 4 presents the index of internal quality of a slab and evaluation of internal quality of a slab together with the test conditions. For the index of internal quality of a slab, the slab of Comparative Example 1 was used as the base material. In Comparative Example 1, any of the first large diameter reduction rolls and the second large diameter reduction rolls were not used for the reduction on the slab (Case 1).

(39) In Example 1 of this invention, both two stages of the large diameter reduction rolls were used for the reduction on the slab. While the casting speed was constant at 0.80 m/min, reduction on the unsolidified was carried out with the first large diameter reduction rolls and reduction after solidified was carried out with the second large diameter reduction rolls (Case 2). The index of internal quality of a slab as a result was 3.2. The slab of excellent internal quality was able to be obtained.

(40) In Example 1 of this invention, after that, the place where solidification was ended moved upstream in the casting direction due to reduction of the casting speed, so that the reduction with the first large diameter reduction rolls became reduction at the end of solidification (Case 3). Accompanying this, the reduction amount of the first large diameter reduction rolls decreased from 32 mm to 12 mm. After the casting speed reduced to 0.58 m/min, both two stages of the large diameter reduction rolls were used for the reduction on the slab as well, and the reduction at the end of solidification was carried out with the first large diameter reduction rolls and the reduction after solidified was carried out with the second large diameter reduction rolls (Case 3). As a result, the index of internal quality of a slab was 3.8, which was the maximum level. Even when the casting speed reduced, the slab of very excellent internal quality was able to be obtained.

(41) In Example 2 of this invention, only the second large diameter reduction rolls among two stages of the large diameter reduction rolls were used for the reduction on the slab. While the casting speed was constant at 0.80 m/min, the reduction at the end of solidification was carried out (Case 4). The index of internal quality of a slab as a result was 1.7, which was good.

(42) In Example 2 of this invention, after that, the place where solidification was ended moved upstream in the casting direction due to reduction of the casting speed. After the casting speed reduced to 0.58 m/min, only the first large diameter reduction rolls were used for the reduction on the slab, and the reduction at the end of solidification was carried out (Case 5). The reduction amounts of both Cases 4 and 5 were same, which was 12 mm. As a result, the index of internal quality of a slab was 2.5. Even when the casting speed reduced, the slab of very excellent internal quality was able to be obtained.

(43) In Example 3 of this invention, both two stages of the large diameter reduction rolls were used for the reduction on the slab. While the casting speed was constant at 0.58 m/min, the reduction at the end of solidification was carried out with the first large diameter reduction rolls and the reduction after solidified was carried out with the second large diameter reduction rolls (Case 6). The index of internal quality of a slab as a result was 3.8. The slab of excellent internal quality was able to be obtained.

(44) In Example 3 of this invention, after that, the place where solidification was ended moved downstream in the casting direction due to the increase of the casting speed, so that the reduction with the first large diameter reduction rolls became the reduction on the unsolidified (Case 7). Accompanying this, the reduction amount of the first large diameter reduction rolls increased from 12 mm to 32 mm. After the casting speed increased to 0.80 m/min, both two stages of the large diameter reduction rolls were used for the reduction on the slab as well, and the reduction on the unsolidified was carried out with the first large diameter reduction rolls, and the reduction after solidified was carried out with the second large diameter reduction rolls (Case 7). As a result, the index of internal quality of a slab was 3.2. Even when the casting speed increased, the slab of excellent internal quality was able to be obtained.

(45) In Example 4 of this invention, only the first large diameter reduction rolls among two stages of the large diameter reduction rolls were used for the reduction on the slab. While the casting speed was constant at 0.58 m/min, the reduction at the end of solidification was carried out (Case 8). The index of internal quality of a slab as a result was 2.5, which was good.

(46) In Example 4 of this invention, after that, the place where solidification was ended moved downstream in the casting direction due to increase of the casting speed. After the casting speed increased to 0.80 m/min, only the second large diameter reduction rolls were used for the reduction on the slab, and the reduction at the end of solidification was carried out (Case 9). The reduction amounts of both Cases 8 and 9 were same, which was 12 mm. As a result, the index of internal quality of a slab was 1.7. Even when the casting speed increased, the slab of very excellent internal quality was able to be obtained.

(47) Industrial Applicability

(48) According to the method for continuous-casting a slab of this invention, a slab of good internal quality can be obtained even if the casting speed changes. Therefore. even if slabs of different materials and for different purposes are cast in the same continuous casting machine, the slabs of good internal quality can be obtained. In addition, the cost of equipment can be held down because large diameter reduction rolls that are arranged in a continuous casting machine are used.

REFERENCE SIGNS LIST

(49) 1: mold, 2: molten steel bath surface (meniscus), 3: molten steel, 4: solidified shell, 5: slab, 6: large diameter reduction rolls, 6a: first large diameter reduction rolls, 6b: second large diameter reduction rolls, 7: support rolls, 10: continuous casting machine