JP2000288704A - Production of round cast billet with continuous casting - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce center segregation, to eliminate core cracking and porosity and to extend a critical casting length by forming the diameter of a casting mold to >=225 mm, in a producing method of the round cast billet after applying the rolling reduction to an unsolidified part with one set of horizontal rolls and then, applying the formation to an solidified billet with one set of vertical rolls. SOLUTION: After applying the rolling reduction to the unsolidified part with the one set of horizontal rolls, the formation is applied with one set of vertical rolls after solidifying, and further, thereafter, the formation is desirable to be applied after solidifying with at least the alternately one set of forming rolls such as the one set of horizontal rolls and the one set of vertical rolls. That is, if necessary, the vertical rolls and the horizontal rolls can alternately be arranged, such as the vertical roll pair 8, horizontal roll pair 9 and vertical roll pair 10. Further, the horizontal roll for holding the cast billet can be used with a flat roll or a caliver roll. In short, any roll arrangement so that the cast billet finally is formed as the shape having the roundness can be used.

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 a round billet slab by continuous casting, and more particularly to a method for producing a round billet slab for pipe making using an unsolidified rolling method and a post-solidification molding method.

[0002]

2. Description of the Related Art When manufacturing round billet slabs by continuous casting, when the material is low carbon steel, bearing steel, or high Cr steel, segregation (center segregation), axial core cracking, porosity at the last solidified center Is generated. If such a slab is used as it is in the production of a seamless pipe, internal flaws occur frequently and do not become a product. In particular, steels with a high [C] concentration, such as bearing steel, have a large degree of [C] segregation in the center and the segregated parts are easily melted during pipe production. It is very difficult to use.

[0003] In order to improve the inner surface quality of the round billet, there have been proposed many methods for reducing the cast slab. For example, JP-A-10-249490, JP-A-10-146651, JP-A-10-3288
Each publication of No. 00 discloses a method in which a central portion is subjected to a large pressure reduction at a position of an unsolidified portion and then formed into a perfect circle. This makes it possible to reduce center segregation, eliminate shaft center cracks and porosity, and produce a seamless pipe having good internal quality.

[0004] Japanese Patent Application Laid-Open No. Hei 5-318065 discloses that when the unsolidified end of a continuous cast slab is continuously subjected to a large pressure reduction using a forging device, in order to obtain a stable internal quality, the molten steel in the diffusion region is melted. A method is disclosed in which the forging pressure is stopped in the range of influence of the diffused impurity solute element and the concentrated portion of the impurity solute element is sealed. This is for avoiding the influence on the internal quality due to continuous forging, whereby it is possible to suppress the degree of segregation of the impurity elements P and S at the center of the slab to below the allowable limit. is there.

[0005]

However, when the above-mentioned JP-A-10-249490, JP-A-10-146651, and JP-A-10-328800 are used, a good internal quality can be obtained in the initial stage of the rolling, but it is difficult to obtain the internal quality. Thereafter, the solidification temperature of the molten steel in front of the unsolidified reduction roll is reduced by the effect of the concentrated molten steel discharged to the upstream side of the casting due to unsolidified reduction, and the unsolidified thickness is increased. Deviating (that is,
(Insufficient unsolidified rolling reduction) Internal cracks occur.

Further, when the method disclosed in Japanese Patent Application Laid-Open No. Hei 5-318065, in which the reduction is released halfway and the concentrated portion is sealed, is used, the concentrated portion does not become a product, so that the product yield is 12%. % Or more, which is not suitable for actual production.

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a method of manufacturing a round billet slab in which center segregation is reduced, shaft cracks and porosity are eliminated in order to manufacture a seamless pipe having excellent internal quality. It is. More specifically, an object of the present invention is to provide a method for producing a round billet slab that can exert the above-described effects even during long-term continuous operation.

[0008]

In order to solve this problem, the present inventors have extended the critical casting length at which internal cracks occur by appropriate means, and added a large draft to compensate for it. In the continuous operation of the period, based on the recognition that by releasing the rolling halfway on the way, it is possible to secure the product yield suitable for actual production, repeated studies, and the following Obtained knowledge.

In order to extend the critical casting length at which internal cracks occur, it is necessary to easily discharge the concentrated molten steel due to unsolidification pressure to the upstream side of the casting. For that purpose, it is more advantageous that the discharge volume is larger. That is, the larger the diameter of the casting mold, the more advantageous.

By increasing the diameter of the casting mold, which was conventionally at most about 200 mm, to 225 mm or more, it is possible to extend the critical casting length and reduce the product yield to within 5%. Become.

[0011] In order to enable actual production with an improvement in the index of continuous casting, it is effective to use a method in which the unsolidified pressure is temporarily released, the concentrated portion is sealed, and the corresponding portion is made defective. is there.

Here, the present invention is as follows. (1) A method of producing a round billet slab by applying unsolidification reduction by one horizontal roll and then performing solidification after solidification by one vertical roll during continuous casting of a round billet. A method for producing a round billet slab by continuous casting, wherein the critical casting length is extended by setting the diameter to 225 mm or more.

(2) In continuous casting of a round billet, a non-solidifying pressure is applied by one horizontal roll, and then the vertical roll 1
After solidification by a step, molding is added, and then horizontal roll 1
A method of producing a round billet slab by adding post-solidification molding by a plurality of molding rolls at least one of a step and a vertical roll one step alternately, wherein the casting mold diameter is 225 m
A method for producing a round billet slab by continuous casting, wherein the critical casting length is extended by setting the length to m or more.

(3) In the continuous casting of the round billet, a non-solidification pressure is applied by one vertical roll, and then the horizontal roll 1
A method for producing a round billet slab by adding post-solidification molding, wherein the critical casting length is extended by setting the casting mold diameter to 225 mm or more. How to make pieces.

(4) In continuous casting of a round billet, a non-solidifying pressure is applied by one vertical roll, and then the horizontal roll 1
After solidification by a step, molding is added, and then a vertical roll 1
A method for producing a round billet slab by adding post-solidification molding using at least one alternate forming roll of one step, a horizontal roll, and a critical casting length by setting the casting mold diameter to 225 mm or more. A method for producing a round billet slab by continuous casting.

(5) The round by continuous casting according to any one of the above (1) to (4), wherein the unsolidification reduction roll is once opened during casting and the unsolidification reduction is started again. A method for producing billet slabs. (6) The above-mentioned (1), characterized by performing electromagnetic stirring in the mold.
A method for producing a round billet slab by continuous casting according to any one of (5) to (5).

[0017]

FIG. 1 schematically shows an example of a production process of a round billet slab by continuous casting according to the present invention. In the figure, a molten steel 3 injected from a tundish 1 into a continuous casting mold having a circular cross section, that is, a casting mold 2.
Is cooled in the mold 2 and a solidified shell is formed on the outside. The slab 4 drawn from the mold 2 enters the pinch roll zone 6 via the spray cooling zone 5, and is largely reduced by the unsolidified reduction horizontal roll pair 7 while the central portion is unsolidified. At this time, the slab having an elliptical cross section is solidified and formed into a perfect circle by a post-solidification forming vertical roll 8 provided in a region after the slab solidification on the downstream side of the solidification completion position. It is a round billet slab for pipe production.

The pair of forming rolls after solidification may be a pair such as a pair of vertical rolls 8, but as shown in the drawing, if necessary, a pair of vertical rolls 8, a pair of horizontal rolls 9, and a pair of vertical rolls
Vertical rolls and horizontal rolls may be alternately provided in multiple stages, such as 10. The horizontal roll for holding the slab may be a flat roll or a caliber roll. in short,
Any roll arrangement may be used as long as the roll arrangement finally becomes a perfect circle.

The unsolidification draft in the present invention is such as to compensate for the critical casting length which increases with the increase of the casting mold diameter of 225 mm or more.
About 50%, preferably 20 to 50%. In addition,
Substantially no reduction is observed during molding after solidification.

FIG. 2 shows that when the casting mold diameter is 200 mm, the unsolidification reduction is continuously performed at a casting length of 50 m, from immediately below the unsolidification reduction roll to the casting upstream side (meniscus side). 6 is a graph showing a change in the degree of segregation of [C] at the center of the slab. According to this, the concentration zone where the degree of segregation exceeds 1
(This is defined as the “enrichment retention zone”), which is generated by a length of 5 m on the upstream side.

FIG. 3 is a schematic diagram at this time. That is, as shown, the slab 4 having the solidified shell and the molten steel pool is reduced at the position of the unsolidified reduction roll pair 7, and the unsolidified portion disappears. During this reduction, the molten steel constituting the molten steel pool is extruded to the upstream side, and the above-mentioned thickening retention zone 20 is formed upstream of the unsolidified reduction roll pair 7. In the illustrated example, the length of the concentrated stay area 20 is 5 m.

FIG. 4 is a graph showing the relationship between the casting length and the occurrence of internal cracks during casting when continuous unsolidification reduction is performed. Due to the effect of this concentrated retention area,
As shown in FIG. 4, when the casting mold diameter is 200 mm, an internal crack occurs when the casting length is ≧ 50 m. If an internal crack occurs, the inner surface of the pipe will be rash during pipe production, and it will not be a product.

FIG. 5 is a graph showing a critical casting length at which an internal crack occurs for each casting mold diameter. FIG. 5 shows that the larger the casting mold diameter, the longer the critical casting length. The reason for this is that the volume of the concentrated molten steel that can be discharged increases as the mold diameter increases, and the diffusion of the concentrated molten steel occurs easily.

FIG. 6 is a graph showing the product yield deterioration ratio for each casting mold diameter. In the case of actual production, this deterioration ratio needs to be 5% or less, and the diameter of the casting mold is preferably 225 mm or more, and more preferably 250 mm or more. The reason why the product yield deterioration ratio must be set to 5% or less is that the bloom loss is 5% when the bloom material is lumped and rolled into a round billet, and the advantage of as-cast material is enjoyed. That's why.

In order to improve the continuous casting index, the unsolidified reduction roll is once opened before reaching the critical casting length where internal cracking occurs as shown in FIG. After that, it is necessary to restart uncoagulation reduction again.

FIG. 7 shows such an embodiment. As shown in the figure, the thickening retention region is sealed in the open portion, and the portion where the reduction is started again can obtain good internal quality. The open part that encloses the thickening area is scrap instead of product. According to the present invention, the yield deterioration ratio can be suppressed to 5% or less.

As described above, according to the present invention, the diameter of the casting mold is set to 225 mm or more in order to increase the critical casting length. This is not for producing a large diameter round billet slab, but if necessary, the diameter of the round billet slab may be increased by changing cooling conditions. Usually, the final diameter of the round billet slab obtained according to the present invention will be about 120-216 mm or more.

On the other hand, considering the case of producing round billet cast pieces having a diameter of 120 to 216 mm, in the case of the present invention, a large-diameter casting mold which is increased by about 10 to 80% as compared with the conventional case. Is used.

[0029]

EXAMPLE In this example, a round billet slab was cast using equipment (machine R = 10 m) equivalent to the structure shown in FIG. 1 provided with a continuous casting mold having a circular cross section. The unsolidified rolling position by the roll pair 7 was 23 m from the molten metal mechanics, and the forming roll 8 for forming after solidification was disposed at a position 32 m from the molten metal meniscus. The diameter of the obtained round billet slab was 170 mm.

The cast steel type is a bearing steel, and its steel components are shown in Table 1. The casting speed was 1.2 to 2.2 m / min, and the spray cooling specific water volume was 0.05 to 0.8 liter / kg · steel. The results are summarized in Table 2.

(Example 1) Since the diameter of the casting mold was 225 mm, the yield deterioration rate was 5.0%, which is a level that can be actually produced, and the evaluation is good.

(Example 2) Since the casting mold diameter is 240 mm and the unsolidified rolling roll is opened every 110 m of casting length, the yield deterioration rate is 4.5%, which is a level that can be actually produced. In addition, the continuous casting index is ≧ 1.0, which is very good for evaluation.

(Example 3) Since the casting mold diameter is 260 mm, the yield deterioration rate is 4.2%, which is a level at which actual production is possible, and the evaluation is good.

Example 4 Since the unsolidified rolling roll is opened at a casting mold diameter of 280 mm and a casting length of 130 m, the yield deterioration rate is 3.7%, which is a practically usable level. In addition, the continuous casting index was ≧ 1.6, which is very good for evaluation. Furthermore, since electromagnetic stirring in the mold is applied, the occurrence rate of defects in parts that can be used as products is 0.30%
And very good.

(Example 5) Since the casting mold diameter is 300 mm, the yield deterioration rate is 3.6%, which is a level that can be actually produced, and the evaluation is good. Further, since the electromagnetic stirring in the mold is applied, the defect generation rate of a portion that can be made into a product is as good as 0.25%.

(Comparative Examples) Cast mold diameter 225 mm
Less than the appropriate conditions, the yield deterioration rate is> 5.0
%, Which is a level that cannot be actually produced, and is poor in evaluation.

[0037]

[Table 1]

[0038]

[Table 2]

[0039]

As described above, according to the present invention,
When manufacturing round billet slabs using the unsolidification rolling method and the post-solidification molding method by continuous casting, the casting length where internal quality deteriorates due to the effect of the concentrated stagnation zone is extended to a level suitable for actual production In addition, it is possible to continuously increase the index, and its practical significance is great.

[Brief description of the drawings]

FIG. 1 is a schematic explanatory view showing an outline of equipment for carrying out a method for producing a round billet according to the present invention.

FIG. 2 is a graph showing the degree of [C] segregation at the center of a slab from an unsolidified pressing roll to the upstream of casting.

FIG. 3 is a schematic diagram showing a thickening retention area.

FIG. 4 is a graph showing that internal cracks occur during casting when continuous solidification reduction is performed.

FIG. 5 is a graph showing a relationship between a casting mold diameter and a critical casting length at which an internal crack occurs.

FIG. 6 is a graph showing a relationship between a casting mold diameter and a product yield deterioration ratio.

FIG. 7 is a schematic diagram showing an uncoagulated rolling release portion.

Claims (6)

[Claims] 1. A method for producing a round billet slab by applying unsolidification reduction by one horizontal roll and then solidifying by one vertical roll during continuous casting of a round billet. A method for producing a round billet slab by continuous casting, characterized in that a critical casting length is extended by setting a diameter of a mold to 225 mm or more. 2. In the continuous casting of a round billet, a non-solidifying pressure is applied by one horizontal roll, a post-solidification molding is applied by one vertical roll, and then a horizontal roll 1 is formed.
This is a method of producing round billet slabs by adding post-solidification molding with at least one alternate forming roll of one step, a vertical roll, and a critical casting length by making the casting mold diameter 225 mm or more. A method for producing a round billet slab by continuous casting, characterized by extending. 3. A method for producing a round billet slab by applying unsolidification reduction by one step of a vertical roll and then performing post-solidification molding by one step of a horizontal roll during continuous casting of a round billet. A method for producing a round billet slab by continuous casting, wherein the critical casting length is extended by setting the mold diameter to 225 mm or more. 4. In the continuous casting of a round billet, a non-solidification pressure is applied by one vertical roll, a post-solidification molding is applied by one horizontal roll, and then a vertical roll 1 is formed.
A method for producing a round billet slab by adding post-solidification molding using at least one alternate forming roll of one step, a horizontal roll, and a critical casting length by setting the casting mold diameter to 225 mm or more. A method for producing a round billet slab by continuous casting. 5. The round billet cast piece by continuous casting according to any one of claims 1 to 4, wherein the unsolidified reduction roll is once opened during the casting and the unsolidification reduction is started again. Production method. 6. The method for producing a round billet slab by continuous casting according to claim 1, wherein electromagnetic stirring in the mold is performed.