JP3240978B2 - Manufacturing method of continuous cast slab - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing continuously cast slabs of carbon steel, stainless steel, high alloy steel, etc. The present invention relates to a method for producing a continuous cast slab which enables production of a simple cast slab.

[0002]

2. Description of the Related Art In continuous casting, particularly continuous casting of steel, internal defects such as center segregation and center porosity occur at the center of a slab, which is problematic.

[0003] For example, center segregation and center porosity generated in a slab for a wire rod cause cut-off of a wire during wire drawing, and also cause chevron cracks during extrusion in a steel bar. In addition, the center segregation and center porosity of the slab for a thick plate cause a decrease in toughness of a joint part for large heat input and cause hydrogen-induced cracking of a large diameter steel pipe for sour gas transport.

[0004] The center segregation is caused by C,
This is a phenomenon in which the molten steel components such as S, P and Mn are concentrated, and the residual molten steel between the dendrites that has been concentrated as the solidification progresses is a slab that is the final solidified part due to solidification shrinkage of the molten steel and bulging between rolls. It is thought to be caused by moving to the center and accumulating.

The center porosity is generated at the center of the slab, which is the final solidified portion, due to solidification shrinkage. Therefore, as a countermeasure against these problems, in order to prevent the movement and accumulation of the residual molten steel, which is concentrated molten steel, and to suppress the occurrence of center porosity due to solidification shrinkage, the area near the solidification completion point should be rolled or dies. There have been proposals for a method of rolling down.

For example, Japanese Patent Application Laid-Open No. 63-252655 discloses that the surface temperature of a final solidified portion is set to a range of 700 to 800 ° C. by increasing the amount of secondary cooling water injected to the surface of a cast slab. The bulging generated between the rolls is suppressed by increasing the thickness, and the flow of the residual molten steel is reduced by applying a reduction of 0.2 to 0.4% per minute to the slab by a light reduction roll group. A method has been proposed to block and prevent center segregation.

Japanese Patent Application Laid-Open No. 61-42460 discloses that dendrite is cut by molten steel flow using an electromagnetic stirrer or an ultrasonic wave applicator installed upstream of a solidification completion point, and an equiaxed crystal is formed near the solidification completion point. Is formed, and a large reduction of 3 mm or more is applied by a reduction roll placed immediately before the solidification completion point to forcibly form the solidification completion point, and a method of preventing center segregation without generating internal cracks is presented. Have been.

Further, Japanese Patent Application Laid-Open No. 3-124352 discloses a method of reducing the center segregation and center porosity by rolling down the end of solidification with a roll having a diameter 2 to 5 times the thickness of the slab. I have.

[0009]

However, according to the method disclosed in Japanese Patent Application Laid-Open No. 63-252655, the rolling can be reduced only in a pointwise manner in the longitudinal direction of the slab, so that solidification shrinkage and bulging cannot be sufficiently prevented. The problem of center segregation and center porosity remains. Furthermore, since a large number of reduction rolls are required, there is also a problem that equipment costs are increased.

The method disclosed in JP-A-61-42460 or JP-A-3-124352 reduces a slab having an unsolidified portion before solidification is completed. In the production of a rectangular slab having a thick section (hereinafter, also referred to as a round slab) or a thick slab, internal slabs of the slab tend to occur.

FIG. 1 is a schematic view showing the occurrence of internal cracks when a round slab is reduced by a conventional method. As shown in FIG. 1, when a round cast piece 13 having an unsolidified portion 12 is rolled down by a roll 11, a tensile stress acts on a solidified interface 14 to generate an internal crack 15. This internal crack 15 is observed as a positive segregation line by sucking the concentrated molten steel into the crack. In particular, in steel types for steel bars with a high C concentration or pipes with a high alloy component concentration, internal cracks are likely to occur, leading to quality defects.

Further, when the round slab 13 is reduced, as shown in FIG. 1, the slab after the reduction has an elliptical shape, and there is a transportation trouble or a defective pipe shape in a pipe manufacturing process for manufacturing a pipe or the like from the slab. And other problems occur.

[0013] Therefore, in continuous casting, it is required to produce a slab with reduced center segregation and center porosity, no internal cracks, and a good cross-sectional shape. An object of the present invention is to provide a method of manufacturing a continuous cast slab that reduces center segregation and center porosity, suppresses internal cracks, and enables the manufacture of a slab having a good cross-sectional shape.

[0014]

Means for Solving the Problems In order to solve the above-mentioned problems, the inventors of the present invention provided a continuous casting apparatus for round cast slabs with a horizontal roll for rolling down a cast slab having an unsolidified portion before solidification was completed. A pair and a pair of vertical and horizontal rolls for rolling down a completely solidified slab were incorporated, and various experiments and examinations were conducted to investigate the internal quality and cross-sectional shape of the slab. In addition,
In the present specification, the reduction of the slab having an unsolidified portion is the unsolidification reduction, the amount of reduction is also referred to as the unsolidification reduction amount, the reduction of the completely solidified slab is the reduction after solidification, and the reduction amount is the reduction after solidification. Also called quantity. The solid fraction is the solid fraction immediately before the reduction.

FIG. 2 shows the effect of the unsolidified rolling reduction ratio of the solid phase ratio of 0.99 or less (the unsolidified rolling reduction / the thickness of the unsolidified portion having the solid phase ratio of 0.99 or less) on center porosity and center segregation. It is a graph shown. FIG. 3 is a graph showing the effect of the solid fraction at the center of the slab on internal cracks.

FIG. 4 is a graph showing the effect of the unsolidified draft ratio at a solid fraction of 0.8 or less (unsolidified draft amount / thickness of the unsolidified portion at a solid fraction of 0.8 or less) on internal cracking. is there. Figures 2-4
From the experimental results shown in and the experiment under pressure after coagulation,
The findings in (a) to (d) were obtained.

(A) When the unsolidification reduction ratio with a solid fraction of 0.99 or less is reduced at a specific value or more, the center porosity and center segregation are remarkably improved. (b) When the solid fraction at the center of the slab (hereinafter, also referred to as the solid fraction at the center) is larger than 0.8, no internal cracking occurs due to unsolidification pressure.

(C) In the case where the solid fraction at the center is 0.8 or less, the internal crack does not occur if the unsolidified draft ratio at the solid fraction of 0.8 or less is reduced to a specific value or more. (d) A slab having a good cross-sectional shape is obtained by reduction after solidification,
There are no internal cracks.

The present invention is based on the above findings, and the gist is as follows (1) to (4). (1) The continuous casting apparatus is provided with an unsolidified rolling device for rolling down a slab having an unsolidified portion, and the solid phase ratio is 0.4 times or more and 1.2 times or less the thickness of the unsolidified portion where the solid phase ratio becomes 0.99 or less. With the amount of reduction, and
When the solid fraction at the center of the slab is 0.8 or less, the unsolidified slab is reduced by a reduction amount of 1.5 times or more the unsolidified portion thickness at which the solid fraction is 0.8 or less. A method for producing continuous cast slabs.

(2) The method for producing a continuous cast slab according to the above item (1), wherein the solid fraction of the central part of the unsolidified slab is 0.4 to 0.8. (3) The continuous cast casting as described in (1) or (2) above, wherein the unsolidified rolling device is a roll-type unsolidified rolling device in which two to four rolls are arranged in the same plane. How to make pieces.

(4) A post-solidification drafting device is provided downstream of the unsolidified drafting device in the pouring direction, and the slab pressed down by the unsolidified drafting device is rolled down after solidification is completed.
The method for producing a continuous cast slab according to any one of (1) to (3). The solid phase ratios described in the above items (1) and (2) are solid phase ratios immediately before reduction.

[0022]

FIG. 5 is a schematic view showing an example of the configuration of a continuous casting apparatus for realizing the method of the present invention. As shown in FIG. 5, molten steel 2 injected into mold 22 from immersion nozzle 21
3 is cooled through a cooling roll group 24 and a guide roll group 25 to form a solidified shell 26 and becomes a slab 28 having an unsolidified portion 27, and a pinch roll 31 through an unsolidified rolling device 29 and a post-solidifying rolling device 30. Is pulled out by

In the method of the present invention, in the continuous casting machine having the above-described apparatus configuration, the slab 28 having an unsolidified portion before solidification is reduced by the unsolidification rolling device 29. FIG. 6 is a schematic diagram for explaining unsolidification reduction by the unsolidification reduction device according to the present invention.

As shown in FIG.
Is a roll 3 for rolling down a slab 28 having an unsolidified portion 27.
2 or a mold (not shown) with a solid phase ratio of 0.99
The slab 28 having an unsolidified portion thickness M and a solid phase ratio N of 0.8 or less is reduced. Note that reference numeral 33
Is a line having a solid phase ratio of 0.99 in the vertical section of the slab, and 34 is a line having a solid phase ratio of 0.9.
Line 8.

Here, according to the method of the present invention, the solid phase ratio is 0.4 times or more and 1.2 times or more the thickness M of the unsolidified portion where the solid fraction is 0.99 or less.
The amount of reduction is not more than twice and the solid phase ratio at the center of the slab is 0.
When it is 8 or less, the unsolidified slab is reduced by a reduction amount of 1.5 times or more of the unsolidified portion thickness N at which the solid fraction is 0.8 or less.

Next, the reason why the reduction amount is set to be 0.4 times or more and 1.2 times or less the thickness M of the unsolidified portion where the solid fraction is 0.99 or less will be described. As shown in FIG. 2, when the reduction is performed at a reduction amount of 0.4 times or more of the unsolidified portion thickness M at which the solid fraction is 0.99 or less, the reduction penetrates into the inside, and the concentrated molten steel flows upstream of the casting. The center porosity and center segregation are significantly improved. On the other hand, when the reduction amount is less than 0.4 times, the reduction in the reduction is small, and the improvement of the center porosity and the center segregation is insufficient. When the rolling reduction is more than 1.2 times, the center porosity and the center segregation are improved, but the rolling force is remarkably increased. Further, there is a problem that a roll or a mold as a pressing tool is easily damaged.

Next, the reason why the reduction amount is 1.5 times or more the thickness N of the unsolidified portion when the solid fraction at the center of the slab is 0.8 or less will be described. As shown in FIG. 3, when a slab having a solid fraction of 0.8 or less at the center is rolled down, a tensile force acts on the solidification interface to cause internal cracks. However, as shown in FIG. 4, the unsolidified portion thickness N of 1.
When the rolling reduction is more than 5 times, the solidification interface is in the state of compressive stress, and the flow of molten steel increases, so the concentrated molten steel sucked between dendrites near the solidification interface is also squeezed out to the upstream side of casting. As a result, the occurrence of internal cracks is greatly suppressed. When the solid fraction at the center is 0.8 or less, if the amount of reduction is less than 1.5 times N, the suppression of internal cracking is insufficient.

Preferably, when the reduction amount is 0.6 times or more and 1.0 times or less of M, and when the solid phase ratio at the center is 0.8 or less, the reduction amount is 1.6 times or more of N. is there. When a plurality of unsolidified rolling devices are provided in the casting direction, the above-described reduction may be performed by at least the last unsolidified rolling device.

Next, as a preferred embodiment of the present invention, the solid phase ratio at the center of the unsolidified slab when the unsolidifying reduction is performed is set to 0.1%.
The reason why the value is 4 to 0.8 will be described. If the solid phase ratio at the center exceeds 0.8, the deformation resistance of the slab increases, so a large rolling force is required, and the size of the unsolidified rolling device is increased, and the equipment cost is increased.

If the solid phase ratio at the center is less than 0.4, the unsolidified rolling reduction required to prevent the occurrence of internal cracks becomes large, so that the reduction in cross-section due to rolling is large and the productivity of continuous casting is significantly reduced. I do. Further, in the case of a round slab or the like, a change in the cross-sectional shape due to the reduction is large, and it is difficult to correct the cross-sectional shape by the post-solidification reduction described later, and a defective shape is likely to occur. Preferably, the solid fraction at the center is in the range of 0.6 to 0.8.

Next, a preferred embodiment of the present invention, in which two or four rolls are arranged in the same plane and a roll-type unsolidification rolling device, will be described with reference to a 4-roll type non-solidification rolling device as an example. .

It is necessary to perform unsolidification reduction in synchronization with the movement of the slab. In the case of reduction by a die, a reduction device incorporating the die is controlled in running in the casting direction in synchronization with the slab. And a sophisticated speed control technology for that. On the other hand, under rolling by a roll, the above mechanism is unnecessary, and furthermore, high speed control is not required, so that stable casting can be performed with inexpensive equipment.

FIG. 7 is a schematic diagram for explaining the reduction in a four-roll unsolidified reduction device. As shown in the figure, the four-roll unsolidification rolling device 35 includes a pair of grooved rolls 36 in the same plane and a pair of grooved rolls 37 arranged at right angles thereto.
The cast slab 39 having the unsolidified portion 38 is reduced by one or both of the grooved rolls 36 and 37. The unsolidified rolling reduction is set so that the rolling reduction of at least one of the roll pairs satisfies the above-described condition.
Although FIG. 2 shows a case where the roll has a hole shape, a flat roll may be used.

In the above, "in the same plane" means that the contact area formed by each roll and the cast piece at least partially overlaps, and the roll axis is inclined or the roll is cast. It also includes shifting in the direction.

Next, as a preferred embodiment of the present invention, the reason why a post-solidification rolling device is provided downstream of the unsolidification rolling device in the casting direction and the unsolidified rolling slab is lowered after solidification is completed will be described.

As shown in FIG. 5, between the unsolidified rolling device 29 and the pinch roll 31, a post-solidifying rolling device 30 provided with a roll or a mold for rolling the unsolidified rolling and the solidified slab is provided. .

In the unsolidified rolling, the unsolidified rolling amount is restricted from the viewpoint of ensuring the internal quality of the slab as described above. Is difficult to secure.

In the reduction by the reduction device 30 after solidification, there is no restriction on the reduction amount from the viewpoint of ensuring the internal quality of the slab.
From the viewpoint of ensuring dimensional accuracy, an appropriate amount of reduction can be selected, and the cross-sectional shape of the unsolidified and reduced slab can be adjusted to form a slab with high dimensional accuracy. Further, if necessary, the slabs of several kinds of dimensions to be supplied to the pipe making process or the like by adjusting the amount of reduction thereof can be separately formed.

As shown in FIG. 5, usually, the post-solidification pressing device 30 is provided with a plurality of vertical pressing devices 30a and horizontal pressing devices 30b alternately. In the case of manufacturing round cast slabs or the like, a one-stage post-solidification rolling device in which three or more rolls or dies are arranged in the same plane may be provided.

Further, the quality of the slab can be further improved by providing an electromagnetic stirrer upstream of the unsolidifying rolling device and making the structure of the slab before the unsolidifying rolling equiaxed.

[0041]

【Example】

(Example 1) High-carbon steel (C: 1.0% by weight) was cast with a 250 mm diameter mold using a continuous casting device for round cast slabs having the basic configuration shown in FIG. Then, unsolidification reduction and reduction after solidification were performed to produce a slab having a diameter of 190 mm.

[0042]

[Table 1]

Table 2 shows the casting conditions. In addition, the casting speed was adjusted between 1.7 and 2.3 m / min so that a predetermined unsolidified portion thickness was obtained immediately before the unsolidification rolling device. In addition, the measurement of the unsolidified portion thickness was performed by a slab temperature analysis, a solidified thickness measurement by adding Fe-S, and the like.

[0044]

[Table 2]

100 mm in the casting direction from the steady casting part
21 cross-sectional samples were taken at intervals of
Center segregation, center porosity and roundness were investigated. The internal cracks were evaluated by the length of the cracks by sulfur printing of the cross section. Center segregation analyzes the C concentration at the center of a cross-sectional sample and calculates the average C concentration of the cross-section.
_It was arranged as a ratio to ₀ , that is, the degree of segregation (C / C ₀ ). The center porosity was obtained by measuring the total area of the center porosity in the cross-sectional sample, and expressed as a center porosity area ratio by a ratio to the slab cross-sectional area. The roundness was evaluated by defining the value obtained by dividing the difference between the maximum diameter and the minimum diameter of the cross-sectional sample by the average diameter as the roundness deviation rate. Table 3 shows the results of the quality survey. The values in the table are average values of 21 cross-sectional samples.

[0046]

[Table 3]

In the examples of the present invention, the center porosity area ratio and the degree of segregation were small and good, and there was no problem with no or slight internal cracks. In addition, in Invention Examples 2 to 6 in which the reduction after solidification was performed, the roundness deviation rate was good, and the roundness deviation rate required from the pipe making process was within 3%.

On the other hand, Comparative Example 1 had good center porosity and center segregation, but had large internal cracks. Comparative Example 2 had no internal cracks, but had a low center porosity area ratio and degree of segregation. It was big bad.

[0049]

According to the present invention, it is possible to produce a slab having a good sectional shape with little internal cracks, center porosity and center segregation.

[Brief description of the drawings]

FIG. 1 is a schematic view showing the occurrence of internal cracks when a round slab is reduced by a conventional method.

FIG. 2 is a graph showing the effect of the unsolidified rolling ratio at a solid phase ratio of 0.99 or less (the amount of unsolidified rolling reduction / the thickness of an unsolidified portion having a solid phase ratio of 0.99 or less) on center porosity and center segregation. is there.

FIG. 3 is a graph showing the effect of the solid fraction at the center on internal cracks.

FIG. 4 is a graph showing the effect of an unsolidified rolling reduction ratio of a solid phase ratio of 0.8 or less (unsolidified rolling reduction / thickness of an unsolidified portion having a solid phase ratio of 0.8 or less) on internal cracking.

FIG. 5 is a schematic view showing a configuration example of a continuous casting apparatus for realizing the method of the present invention.

FIG. 6 is a schematic diagram for explaining unsolidification reduction by the unsolidification reduction device according to the present invention.

FIG. 7 is a schematic diagram for explaining the reduction in a four-roll unsolidified reduction device.

[Explanation of symbols]

 11, 32 Roll 12, 27, 38 Unsolidified part 13 Round cast piece 14 Solidified interface 15 Internal crack 21 Immersion nozzle 22 Mold 23 Molten steel 24 Cooling roll group 25 Guide roll group 26 Solidified shell 28, 39 Cast piece 29 Unsolidified rolling device Reference Signs List 30 Post-solidification drafting device 30a Vertical drafting device 30b Horizontal drafting device 31 Pinch roll 33 Line of solid phase ratio 0.99 in vertical section of slab 34 Line of solid phase ratio 0.8 in vertical section of slab 35 4-roll unsolidified Roll-down device 36 Perforated roll 37 Perforated roll arranged perpendicular to the perforated roll 72 M Non-solidified portion thickness with solid phase ratio of 0.99 or less N Non-solidified portion thickness with solid phase ratio of 0.8 or less

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-9-24448 (JP, A) JP-A-5-293618 (JP, A) JP-A-1-273658 (JP, A) JP-A-7- 132355 (JP, A) JP-A-63-183765 (JP, A) JP-A-8-206804 (JP, A) JP-A-8-164460 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) B22D 11/128 350 B22D 11/20

Claims (4)

(57) [Claims] An unsolidified rolling device for rolling a slab having an unsolidified portion is provided in a continuous casting device, wherein the thickness of the unsolidified portion is 0.4 times or more and 1.2 times the thickness of the unsolidified portion where the solid phase ratio is 0.99 or less. When the solidification ratio at the center of the slab is 0.8 or less, the reduction amount is 1.5 times or more the thickness of the unsolidified portion where the solidification ratio is 0.8 or less. A method for producing a continuous cast slab, comprising reducing a solidified slab. 2. The method according to claim 1, wherein the unsolidified slab has a solid fraction of 0.
The method for producing a continuous cast slab according to claim 1, wherein the ratio is from 4 to 0.8. 3. The continuous cast slab according to claim 1, wherein the unsolidified rolling device is a roll-type unsolidified rolling device in which two to four rolls are arranged in the same plane. Production method. 4. A post-solidification drafting device is provided downstream of the unsolidified drafting device in the casting direction, and the slab reduced by the unsolidified drafting device is reduced after solidification is completed. 3. The method for producing a continuously cast slab according to any one of the above items 3.