JP6673397B2 - Steel continuous casting method - Google Patents

Description

  The present invention relates to a continuous casting method for steel, and more particularly to a continuous casting method in which light reduction is performed in the latter half of continuous casting to reduce center segregation.

In continuous casting of steel, in order to reduce the center segregation of the slab, light reduction with a larger reduction gradient than the upstream side has been conventionally performed near the solidification completion position.
Then, in order to reduce center segregation effectively, it is necessary to reduce the slab lightly with an appropriate reduction gradient (amount of slab reduction per unit casting length) (for example, see Patent Document 1).

As shown in FIG. 6, the slab reduction in continuous casting is performed in the light reduction zone and the upstream side thereof, and the slab reduction amount is determined by the thickness of the slab at the exit of the mold and the thickness of the slab at the exit of the continuous casting machine. Therefore, the amount of reduction in the light reduction zone is also limited. In other words, when the rolling reduction in the light rolling zone is excessive, the slab thickness becomes too thin and the unit weight of the product becomes small, and it becomes impossible to produce a large unit weight product with a large thickness, so the slab thickness becomes too thin. The amount of light reduction is restricted to such an extent.
As described above, the amount of reduction in the light reduction zone is limited, but in order to reduce center segregation, it is necessary to perform light reduction with an appropriate reduction gradient under the above restrictions even when casting conditions and the like are changed. .

In this regard, the reduction of the rolling gradient with the change of the casting conditions and the like can be relatively easily performed by reducing the difference in the roll interval between the entrance and the exit in each segment of the light reduction zone. Here, “roll interval” refers to the shortest distance between the surfaces of a pair of rolls facing each other across a cavity through which a slab passes in a continuous casting machine.
On the other hand, in the case of increasing the rolling gradient, since the rolling amount is limited as described above, some contrivance is required. Conventionally, as this method, a method of shortening the range of light rolling and increasing the thickness of the mold are used. That is, a method of increasing the distance between opposing mold long sides has been performed.
Among these methods, examples of the method of shortening the range of light reduction include, in order to fit the unsolidified portion of the slab at the final stage of solidification within the reduction range disclosed in Patent Document 2, the reduction according to the casting speed. There is a way to move the range.

Japanese Patent Publication No. 62-34461 JP-A-5-15956

However, in the method disclosed in Patent Document 2 for moving the rolling range by shortening the light rolling range, equipment for moving the rolling range during casting in accordance with the final solidification position of the slab, that is, a roll during casting. Equipment that can change the interval, specifically, a special equipment such as a light pressure roll segment that raises and lowers the roll hydraulically instead of an electric worm jack is required.
In addition, in the method of increasing the thickness of the mold, it is necessary to remodel the molding equipment, for example, to increase the length of a tie rod for assembling the platform on which the lower roll group is mounted and the upper roll platform.

  The present invention has been made in order to solve such problems, and when it is necessary to increase the rolling down gradient to reduce center segregation, without adding or modifying special equipment, existing equipment can be used. It is an object of the present invention to provide a continuous casting method of steel capable of obtaining a center segregation reducing effect equivalent to increasing the rolling gradient by using steel.

(1) A continuous casting method for steel according to the present invention is a continuous casting method for steel in which a plurality of roll segments are installed and light reduction is performed for the purpose of reducing center segregation of a slab.
The roll intervals are set such that each roll segment has a predetermined rolling gradient, and in relation to the roll interval between adjacent roll segments, the roll interval on the outlet side of the roll segment arranged on the upstream side is downstream. The roll segment on the entry side of the roll segment arranged on the side is set to be large.

(2) In the roll described in the above (1), in the relationship of the roll interval between the adjacent roll segments, the roll on the outlet side of the roll segment on the upstream side from the roll interval on the inlet side of the roll segment on the downstream side. The value obtained by subtracting the interval is not less than 0.1 mm and not more than 20 mm.

  In the present invention, with respect to a plurality of roll segments in the light reduction zone, the roll interval is set so that each roll segment has a predetermined reduction gradient, and is arranged on the upstream side in relation to the roll interval between adjacent roll segments. When the roll gap on the inlet side of the roll segment arranged downstream is set to be larger than the roll gap on the outlet side of the roll segment that has been set, when it is desired to increase the reduction gradient in the light reduction zone The center segregation can be reduced by the same effect as increasing the reduction gradient in the segment without changing the range of the reduction band, and therefore without modifying the conventional equipment.

It is explanatory drawing explaining the rolling gradient of the light rolling zone in embodiment of this invention. BRIEF DESCRIPTION OF THE DRAWINGS It is explanatory drawing of the continuous casting machine in embodiment of this invention. It is explanatory drawing of the rolling-down gradient in the light pressure zone in a prior art example. 7 is a graph of an experimental result for explaining an effect of the present invention in an example (part 1). 10 is a graph of an experimental result for explaining the effect of the present invention in an example (part 2). It is explanatory drawing of the general rolling gradient in continuous casting.

A continuous casting machine used in the continuous casting method according to the present embodiment will be described with reference to FIGS.
As shown in FIG. 2, the continuous casting machine 1 supplies the molten steel 5 in the tundish 3 to the mold 11 via the sliding nozzle 7 and the immersion nozzle 9, and continuously draws the molten steel 5 from the mold 11. Thus, the slab 13 is manufactured. A slab support roll 15 is provided below the mold 11, and a secondary cooling zone 17 provided with a spray nozzle or an air mist spray nozzle (not shown) is provided between the slab support rolls 15. The slab 13 is cooled while being pulled out.
At a final position in the casting direction of the continuous casting machine 1, a slab transport machine 19 and a slab cutter 21 for cutting the slab 13 to a predetermined length are provided.

  On the upstream side of the solidification completion position T of the slab 13 in the casting direction, there is provided a light pressure reduction zone 23 for lightly reducing the slab 13 in order to suppress segregation of the slab 13 at the center. The light pressure lower zone 23 is configured by installing a plurality of roll segments 27 each including a plurality of pairs of slab support rolls 25 opposed to each other across the slab 13 in the casting direction. Although two roll segments 27 are shown in FIG. 2, the number of roll segments 27 is determined by casting conditions and the like.

  In each roll segment 27, the roll interval is set so that the roll interval gradually decreases from the entrance side to the exit side so that the inside of the roll segment has a predetermined reduction gradient, and in the relationship between adjacent roll segments 27, The roll interval on the entry side of the roll segment 27 arranged on the downstream side is set to be larger than the roll interval on the exit side of the roll segment 27 arranged on the upstream side. By setting the roll interval in this manner, the rolling gradient of the light rolling zone 23 does not become a continuous and constant slope as shown in FIG. 3 showing a conventional example, but as shown in FIG. The reduction gradient of 27 is steeper than the conventional example, and is intermittent in the light reduction zone 23.

By setting the reduction gradient of each roll segment 27 of the light reduction zone 23 as described above, when the reduction gradient needs to be increased, the modification of the special equipment or equipment is not required (the range of the light reduction is reduced). Without change), it is possible to obtain the same center segregation reduction effect as when the reduction gradient is increased using existing equipment. This point is demonstrated in the examples described later.
Note that the relationship between the roll intervals between adjacent roll segments 27 is such that the value obtained by subtracting the roll interval on the output side of the upstream roll segment 27 from the roll interval on the input side of the downstream roll segment 27 is 0.1 mm or more and 20 mm or less. It is preferable that The reason for setting the value in this way is as follows.
If it is less than 0.1 mm, the downstream roll interval is made smaller than the upstream roll segment outlet roll interval, but the degree of increase in the rolling gradient in the roll segment is insufficient, while if it exceeds 20 mm, the downstream This is because the solidification interface strain at the segment entry side becomes excessive, and there is a concern about the occurrence of internal cracks.

The most downstream position of the light pressure lowering zone 23 is set at a position where the solid phase ratio at the center of the thickness of the slab 13 becomes 0.9. This is because if the solid phase ratio exceeds 0.9, the rigidity of the slab 13 increases and the rolling resistance increases, so that the rolling load of the light rolling zone 23 is reduced.
Here, the solid phase ratio at the center of the thickness of the slab 13 is defined as 0 before the start of solidification and 1.0 when the solidification is completed. The position where the solid phase ratio is 1.0 corresponds to the solidification completion position T.

The continuous casting method using the continuous casting machine 1 configured as described above is outlined below. The molten steel 5 is injected from the tundish 3 into the mold 11 through the immersion nozzle 9. The molten steel 5 injected into the mold 11 is cooled by the mold 11 to form a solid phase portion, and is continuously drawn down as a slab 13 having a liquid phase portion while being supported by the slab support roll 15. . The slab 13 is cooled in the secondary cooling zone 17, the thickness of the solid phase portion is increased, an appropriate amount of light reduction is added by the light reduction zone 23, and after that, solidification is completed to the center, It is conveyed downstream by the one-conveyance roll 19 and cut by the slab cutter 21 into a predetermined length.
In the light reduction zone 23, light reduction is performed by the roll segments 27 having the roll intervals set as described above, and the center segregation is reduced.

In order to confirm the effect of the present invention, an experimental operation was performed, which will be described below.
The cast slabs manufactured in this example are those applied to a sour resistant UOE steel pipe thick plate for shipbuilding, and the steel grade is carbon 0.03 mass% to 0.06 mass%, silicon 0.10 mass% to 0.30 mass%, It has a composition of 1.00% by mass to 2.00% by mass of manganese, 0.010% by mass or less of phosphorus, and 0.0010% by mass or less of sulfur.
The outline of the continuous casting machine is as follows: machine length: 49 m, mold size: 220, 250, 300 mm in thickness, 675 to 2100 mm in width, roll segment under light pressure: 14.8 to 31.8 m from the meniscus.

  The slab thickness of the example was 250 mm, the slab width was 2100 mm, and the molten steel was superheated (difference between molten steel temperature and liquidus temperature in tundish) in the range of 25 ° C to 40 ° C.

Regarding the setting of the roll interval of the light pressure lowering zone, in the section 18.8 m to 27.4 m from the meniscus (the solid phase ratio in the center of the slab section is in the range of 0.3 to 0.9), the roll interval at the section exit and the roll interval at the section exit Is reduced by 6.3 mm, and with respect to the four roll segments 27 constituting this section, each roll segment has a constant reduction gradient, and in relation to adjacent roll segments, the roll segment disposed on the upstream side The roll interval on the entrance side of the roll segment arranged downstream was set to be larger by 0.8 mm than the roll interval on the exit side (see FIG. 1).
Note that, in the conventional method to be compared, the roll interval was set such that the roll interval monotonously decreased at a constant rate in this section (see FIG. 3).

  The rolling speed when casting at a casting speed of 1.1 m / min was 0.35 mm / min in the prior art, whereas the rolling speed could be increased to 0.58 mm / min in the present invention. That is, substantially the same state as that in which the reduction gradient was increased could be realized without changing the range of the light reduction band.

The center segregation degree of the slab was evaluated by the carbon concentration index in the slab thickness direction and the number of segregated grains.
The carbon concentration index was defined as follows.
In the plane perpendicular to the drawing direction of the slab, that is, in the center of the cross section of the slab, in the range from the center of the width of the slab to 930 mm toward the short side on one side, ±± in the slab thickness direction from the thickness center of the slab. In the range of 10 mm, the carbon concentration is analyzed by an electron probe microanalyzer (EPMA) having an electron beam diameter of 100 μm at a pitch of 0.2 mm in the slab thickness direction and a pitch of 0.2 mm in the slab width direction. Let the value be C _MAX .
The average carbon concentration measured at a pitch of 0.2 mm in the width direction within a range of ± 5 mm from the center of the thickness of the slab to 930 mm from the center of the width of the slab to the short side on one side is defined as C ₀ .
And the carbon concentration index was defined as C _MAX / C ₀ . This definition means that the center segregation decreases as C _MAX / C ₀ approaches 1.

FIG. 4 shows the carbon concentration index of the slab to which the present invention is applied and the slab of the unapplied material. In the case where the present invention is applied, the carbon concentration index is 1.050, whereas in the case of the material to which the present invention is not applied, it is 1.059. It can be seen that the center segregation is reduced in the case where the present invention is applied.
Regarding the number of segregated grains, in the cross section of the slab, the number of segregated grains whose carbon concentration index C _MAX / C ₀ is 1.333 or more and the size is 500 μm or more is counted. FIG. 5 shows the segregation particle number index of the present invention example when is set to 1.
In FIG. 5, the vertical axis represents the segregation particle number index, and the horizontal axis represents the value obtained by subtracting the exit roll interval of the immediately upstream roll segment from the entry roll interval of the downstream roll segment (hereinafter abbreviated as “Δd _DU ”). ) Are shown. In FIG. 5, the range of the horizontal axis is from 0 mm to 25 mm. The segregated particle number index is an index obtained by calculating the number of segregated particles at each Δd _DU in the numerator, using the number of segregated particles when Δd _DU is 0 as a denominator.

  The specific numerical values plotted in FIG. 5 are as shown in the table below.

According to FIG. 5 and Table 1, in the example of the present invention, when the Δd _DU is in the range of 0.1 mm or more and 20 mm or less, the number index of segregated grains is smaller than that in the conventional art. Further, more preferable effects can be obtained when Δd _DU is in the range of 0.1 mm or more and 10 mm or less.
In addition, the segregation particle number index obtained in the most preferable operation range in the present invention example was 0.63, and it was confirmed that the index was significantly reduced as compared with the prior art.

  As described above, it has been proved that the effect of reducing center segregation can be obtained by applying the present invention. Furthermore, in the example of the present invention, even if the rolling gradient of each roll segment constituting the light rolling zone is set to an appropriate range suitable for preventing the center segregation, the rolling gradient of the entire roll segment is still smaller for each roll segment. Since the rolling gradient is smaller than the rolling gradient of the segment, the rolling reduction of the entire roll segment is unlikely to be excessive. For this reason, center segregation is well suppressed and casting can be performed without reducing the thickness of the slab at the exit under light pressure. It is possible to perform in an environment where segregation is small.

  In addition, there is a concern that the fact that the roll opening degree on the entrance side of the next roll segment on the downstream side is wider than the exit side of the roll segment on the upstream side has an adverse effect on the center segregation of the slab. However, since the time required for the slab to pass through the position is shorter than the time required to be reduced in the segment, the solidified shell of the slab that has come to the portion of the entry-side roll reaches the position. Also, it is considered that the effect of improving the center segregation by reducing the roll segments individually is larger.

Although the preferred range of Δd _DU has been described above, the preferred range is also with respect to the rolling gradient of the entire roll segment of the light rolling zone. That is, the preferable range of the rolling gradient at the solid phase ratio of 0.3 to 0.9 is 0.3 to 3.0 mm / m.
Here, the rolling gradient of the entire roll segment of the light pressure lowering zone is defined as the roll interval between the rolls at the entrance of the light pressure lowering band indicated by the double-headed arrow of the "light pressure lowering band" in FIG. It refers to a value obtained by dividing the difference between the roll and the roll interval by the distance along the slab drawing path line from the inlet to the outlet.
If the rolling gradient of the entire roll segment in the light rolling zone is less than 0.3 mm / m, the flow of the concentrated molten steel cannot be stopped in the final solidification portion where the solid fraction is close to 1, and a large diameter of several hundred μm cannot be obtained. Segregated grains are generated. On the other hand, if the rolling gradient of the entire roll segment is greater than 3.0 mm / m, segregation will worsen due to the occurrence of internal cracks and the like.

REFERENCE SIGNS LIST 1 continuous casting machine 3 tundish 5 molten steel 7 sliding nozzle 9 immersion nozzle 11 mold 13 slab 15 slab support roll 17 secondary cooling zone 19 slab transport roll 21 slab cut machine 23 light pressure reduction zone 25 slab support roll ( Roll segment)
27 Roll segment T Solidification completion position

Claims (1)

A continuous casting method of steel in which a plurality of roll segments are installed for the purpose of reducing center segregation of a slab and light reduction is performed,
The roll intervals are set such that each roll segment has a predetermined rolling gradient, and in relation to the roll interval between adjacent roll segments, the roll interval on the outlet side of the roll segment arranged on the upstream side is downstream. It is set so that the roll interval on the entrance side of the roll segment arranged on the side is large ,
In the relationship of the roll interval between the adjacent roll segments, a value obtained by subtracting the roll interval on the output side of the upstream roll segment from the roll interval on the input side of the downstream roll segment is 0.1 mm or more and 20 mm or less. Characteristic continuous casting method of steel.