JP3277873B2 - 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., and more particularly, to the production of slabs having few internal defects such as center segregation, center porosity and internal cracks. The present invention relates to a method for producing a continuous cast slab that enables the production of a slab.

[0002]

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

[0003] For example, center segregation and center porosity generated in a slab for a wire rod cause a cut-off in wire drawing, and in a steel bar, cause a chevron crack in extrusion. 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.

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

The center porosity is generated at the center of the slab, which is the final solidified portion, due to solidification shrinkage. Therefore, in order to prevent the movement and accumulation of the residual molten steel, which is concentrated molten steel, and to suppress the generation of center porosity due to solidification shrinkage, the area near the solidification completion point is rolled down by means such as a roll or a mold as a countermeasure. Unsolidification rolling methods have been proposed.

[0006] For example, Japanese Patent Publication No. 59-16682 discloses that the last stage of solidification of a slab is subjected to light pressure reduction by a roll to suppress the occurrence of a negative pressure portion caused by solidification shrinkage and bulging between rolls, so that solid-liquid coexistence is achieved. A method is disclosed in which the liquid phase flow of a phase is suppressed as much as possible to reduce center segregation and porosity.

[0007] Japanese Patent Application Laid-Open No. 3-124352 discloses a method of rolling down the end of solidification with a roll having a diameter of 2 to 5 times the thickness of the slab.

[0008]

However, the conventional unsolidification rolling method as described in the above publication has the following problems.

(A) In a steel type having a high C content used for a section steel or the like, internal cracks are easily generated due to unsolidification pressure, and a required quality level cannot be secured. (b) If the amount of reduction is reduced to prevent the occurrence of internal cracks, reduction of center segregation and porosity becomes insufficient. (c) Due to reduction at the end of solidification, deformation resistance increases,
The rolling equipment becomes large.

An object of the present invention is to provide a method for producing a continuous cast slab which reduces center segregation and center porosity and suppresses the occurrence of internal cracks in continuous casting.

[0011]

Means for Solving the Problems In order to solve the above-mentioned problems, the present inventors have prepared a pair of horizontal casters for rolling a cast slab having an unsolidified portion before solidification is completed. Various experiments and examinations were conducted by incorporating a roll, and the internal quality of the slab was investigated.

FIG. 1 is a graph showing the influence of the rolling shape ratio on center segregation and center porosity. Here, the reduction shape ratio is arranged by the ratio of the roll contact length to the unsolidified thickness, and when the unsolidified reduction amount, the unsolidified reduction roll diameter, and the unsolidified thickness are R, D, and L, respectively, (DR) ^0.5 /
It is represented by L. The roll contact length, which is the contact length between the reduction roll and the slab during unsolidification reduction, is (D · R / 2)
^{Expressed as 0.5} .

FIG. 2 is a graph showing the effect of the reduction shape ratio on internal cracks. FIG. 3 is a graph illustrating an appropriate reduction shape ratio. From the experimental results shown in FIGS. 1 to 3 and other experimental results, the following findings (a) to (d) were obtained.

(A) The presence / absence of center segregation and the magnitude of the center porosity can be determined by the reduction shape ratio, and are significantly improved when the reduction shape ratio is 3.0 or more. (b) The length of the internal crack is maximized when the reduction shape ratio is in a specific range, and is significantly reduced when the reduction shape ratio is 3.0 or more. When the rolling shape ratio is 4.0 or more, no internal crack occurs.

(C) If the rolling reduction ratio exceeds 7.0, the roll load increases, and it becomes difficult to pull out the slab. (d) Unsolidified rolling roll diameter is 300mm or more and 1000m
m or less is preferable.

The present invention is based on the above findings, and the gist is as follows (1) and (2).

(1) In continuous casting of steel, a slab having an unsolidified portion is prepared by using a slab having a center solid phase ratio of 0.3 or more and 0.8 or less and a reduction shape ratio α of 3.0 or more and 7 or less. A method for producing a continuous cast slab, wherein the reduction is performed at 0.0 or less.

Α = (D · R) ^0.5 / L where L: unsolidified thickness D: unsolidified reduction roll diameter R: unsolidification reduction amount (2) The unsolidified reduction roll diameter is 300 mm or more and 100
The method for producing a continuous cast slab according to the above (1), wherein the diameter is 0 mm or less.

[0019]

FIG. 4 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. 4, 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, becomes a slab 28 having an unsolidified portion 27, and passes through an unsolidified rolling device 29 and a post-solidifying rolling device 30 to a pinch roll 31. Is pulled out by

In the method of the present invention, in the continuous casting apparatus having the above-described apparatus configuration, the slab 28 having an unsolidified portion before solidification is reduced by the unsolidification reduction device 29. FIG. 5 is a schematic diagram illustrating unsolidification reduction by the unsolidification reduction device according to the present invention. As shown in FIG.
Is provided with a pair of unsolidified rolling rolls 32 for rolling down a slab 28 having an unsolidified portion 27, and lowers a slab having an unsolidified portion thickness L by an unsolidified rolling amount R. Here, the unsolidified thickness L is the thickness of the unsolidified portion having a solid phase ratio immediately before the reduction of 0.99 or less, and reference numeral 33 is a line having a solid phase ratio of 0.99.

In FIG. 5, the unsolidified pressing roll 32 is
Although the horizontal roll is shown, the roll may be a vertical roll, and the shape thereof is a flat shape but may be a hole shape. In the case of a roll having a hole shape, the average roll diameter may be calculated as the unsolidified reduction roll diameter.

Here, according to the method of the present invention, a slab having an unsolidified portion has a center solid fraction of the slab of 0.3 or more and 0.8 or less, and the following reduction ratio α of 3.0 or more. Reduce the pressure below 7.0. α = (D · R) ^0.5 / L where L: unsolidified thickness D: unsolidified reduction roll diameter R: unsolidification reduction amount When the center solid phase ratio is less than 0.3, the unsolidified rolling reduction required to prevent the occurrence of internal cracks becomes large, and the cross-sectional reduction due to the rolling is large, so that the continuous casting is not performed. Productivity drops significantly. If the central solid phase ratio exceeds 0.8, the internal permeability under rolling becomes extremely poor, so that the amount of unsolidified rolling increases, and the deformation resistance also increases significantly. Preferably, the center solid fraction is 0.5 or more and 0.7 or less.

Next, the reason why the rolling shape ratio α is set to 3.0 or more and 7.0 or less will be described. As shown in FIG. 3, when the rolling shape ratio is 3.0 or more, the occurrence of internal defects such as center segregation, center porosity, and internal cracks is significantly suppressed. Under the pressure under these conditions, the solidification interface is in a compressive stress state, and because the flow of molten steel is large, the concentrated molten steel sucked between dendrites near the solidification interface is also squeezed out to the upstream side of casting. Conceivable.

If the rolling shape ratio is less than 3.0, the suppression of the occurrence of internal defects such as center segregation, center porosity and internal cracks is insufficient. If the reduction shape ratio exceeds 7.0, the load of the reduction roll increases, and it becomes difficult to extract the slab.

Preferably, the rolling shape ratio is 4.0 or more and 6.
6 or less. Next, the reason why the unsolidified rolling roll diameter is set to 300 mm or more and 1000 mm or less in a preferred embodiment of the present invention will be described.

When the diameter of the unsolidified rolling roll is less than 300 mm, the internal permeability under the rolling is deteriorated, and the suppression of the generation of internal defects becomes insufficient. If the diameter of the unsolidified rolling roll exceeds 1000 mm, the rolling force increases and the size of the unsolidified rolling device becomes large, which is not practical.

[0027]

EXAMPLE A high-carbon steel (C: 1.0% by weight) was cast in a 250 mm diameter mold using a round cast slab continuous casting apparatus having the equipment specifications shown in Table 1 with the basic structure shown in FIG. Then, unsolidification reduction and reduction after solidification were performed to produce a slab having a circular cross section with a diameter of 190 mm. In addition, unsolidification reduction was performed in one horizontal stand using a flat roll. The post-coagulation reduction was carried out on two vertical and horizontal stands using a hole-type roll.

[0028]

[Table 1]

Table 2 shows the casting conditions. In addition, the casting speed was adjusted between 1.7 and 2.2 m / min so that a predetermined center solid phase ratio was obtained immediately before the unsolidification rolling device. The measurement of the unsolidified thickness was performed by a slab temperature analysis and a solidified thickness measurement by adding Fe-S.

[0030]

[Table 2]

100 mm in the casting direction from the steady casting part
21 cross-sectional samples were taken at intervals of
Center segregation and center porosity were investigated. The internal cracks were subjected to sulfur printing of the cross section, and evaluated by the crack length. For the center segregation, the C concentration at the center of the cross section sample was analyzed, and the result was arranged as a ratio to the average concentration C ₀ of the cross section, 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 expressing the ratio as the center porosity area ratio with the total area of the slab. Table 3 shows the results of the quality survey. The values in the table are average values of 21 cross-sectional samples.

[0032]

[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 little internal cracks. On the other hand, Comparative Example 1 was poor in all of center porosity, center segregation and internal cracks. Comparative Example 2 had good internal quality,
The load on the reduction roll was large, and the slab could not be pulled out.

[0034]

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

[Brief description of the drawings]

FIG. 1 is a graph showing the effect of the reduction shape ratio on center segregation and center porosity.

FIG. 2 is a graph showing the effect of the reduction shape ratio on internal cracks.

FIG. 3 is a graph illustrating an appropriate reduction shape ratio.

FIG. 4 is a schematic view showing a configuration example of a continuous casting apparatus for realizing the method of the present invention.

FIG. 5 is a schematic diagram for explaining unsolidification reduction by the unsolidification reduction device according to the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 21 Immersion nozzle 22 Mold 23 Molten steel 24 Cooling roll group 25 Guide roll group 26 Solidified shell 27 Non-solidified part 28 Cast piece 29 Non-solidified pressing device 30 Post-solidification pressing device 31 Pinch roll 32 Non-solidified pressing roll 33 Solid phase ratio 0.99 Line L Unsolidified thickness R Unsolidified reduction

Continuation of the front page (56) References JP-A-11-129060 (JP, A) JP-A-9-99349 (JP, A) JP-A-7-204812 (JP, A) JP-A-11-179509 (JP, A) JP-A-63-108955 (JP, A) JP-A-7-227657 (JP, A) JP-A-7-132355 (JP, A) JP-A-3-124352 (JP, A) 59-16862 (JP, B2) (58) Fields investigated (Int. Cl. ⁷ , DB name) B22D 11/128 350 B22D 11/20

Claims (2)

(57) [Claims] In a continuous casting of steel, a slab having an unsolidified portion is made to have a slab having a center solid phase ratio of 0.3 to 0.8,
A method for producing a continuous cast slab, wherein the reduction is performed at the following reduction shape ratio α of 3.0 or more and 7.0 or less. α = (DR) ^0.5 / L where L: unsolidified thickness D: unsolidified reduction roll diameter R: unsolidification reduction 2. The unsolidified rolling roll diameter is 300 mm.
2. The structure according to claim 1, wherein the length is not less than 1000 mm.
3. The method for producing a continuous cast slab according to item 1.