JPH0999349A - Method for continuously casting round cast billet for bar steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To establish a continuous casting method for improving the center segregation and porosity without developing the inner crack in a continuously cast billet of a high carbon steel round billet. SOLUTION: Molten metal having >=0.6wt.% C is electromagnetically stirred in a mold, and at the position in the range of 0.3 0.7 solid phase ratio in the axial part, rolling reduction is applied with one set of rolls so that reduction in area of the round cast billet in C direction (the direction at the vertical to the longitudinal direction of the cast billet) becomes in the range of 0.1-3%. By this method, the development of a wire cutting-off or breakage caused by the internal defect at the time of wire-drawing or at the time of rolling a straight bar in the rolling process of the bar steel product.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a continuous casting method for high carbon steel slabs, and more particularly to a slab shaft by applying a reduction to a slab with a reduction roll installed near the solidification completion point of the slab. The present invention relates to a method for improving the inner quality such as segregation of the core and porosity.

[0002]

Steel bar products are generally produced by heating a slab produced by a square-shaped bloom continuous casting machine having a large cross-sectional area, slabbing, and then feeding the wire rod, or It is rolled into a straight rod and supplied as a material for bearings and tire cords. However, in wire rod rolling, problems such as segregation of components formed in the final solidified portion of the continuous cast slab and disconnection due to porosity have occurred. The component segregation and porosity are the center portion of the cross section (hereinafter, "C direction") perpendicular to the longitudinal direction (hereinafter, "L direction") of the slab, which is the final solidification portion of the continuous cast slab. , "Axis center part") is formed because the molten steel in which the chemical constituent elements are concentrated is finally solidified during the solidification process of the molten steel.

Various techniques have been developed in the past in order to improve such internal defects in the axial center of the cast slab. For example,
"Materials and Processes" 1994 vol. 7 No. 1
p179-182 has a carbon content of 0.4-0.6 wt.%
In order to improve the segregation of the axial center of the high carbon steel continuous casting bloom (hereinafter referred to as "center segregation"), the continuous casting of a continuous cast slab by pushing the slab into a die reduces large The forging method (hereinafter referred to as "Prior Art 1") is disclosed. This is to push the slab into the die with a proper taper so that the cross-sectional area in the C direction decreases uniformly from the inlet to the outlet of the slab.

Further, Japanese Patent Publication No. 59-16862 discloses that
In order to improve the center segregation of the continuous cast slab, at the end of solidification of the continuous cast slab, many pairs of rolls are installed between the tip of the liquidus crater and the tip of the solidus crater. Disclosed is a method (hereinafter, referred to as "Prior Art 2") of suppressing the flow of a component-enriched molten steel in a crater by rolling down by a roll an amount corresponding to the volumetric shrinkage accompanying solidification.

Furthermore, "Materials and Processes" 1994 vo
l. 7 No. In order to improve center segregation and cavities of continuous casting blooms, 1 p194 to 197 are cast by two pairs of rolls at a solid phase ratio of the axial center portion in the final stage of solidification of 0.2 to 1.0. Disclosed is a method (hereinafter, referred to as "Prior Art 3") in which a piece is pressed and a reduction in area of a cross section in the C direction is within a range of 0 to 3%.

[0006]

However, the above-mentioned prior art has the following problems. Prior art 1 exerts an excellent effect in eliminating porosity at the center of the continuous cast slab, but has a problem of enormous equipment cost.

The prior art 2 is effective in improving the center segregation of the continuous cast slab, but has a problem that the reduction roll must be installed over a long section of the casting line, and the equipment cost and the operating cost are high.

Prior art 3 is effective in improving the solidification structure of the core of the cast slab. However, there is a problem that the porosity disappearance in the axial center portion and the accompanying increase in density are not yet perfect.

As described above, in the prior art, porosity cannot be completely eliminated,
Further, in order to manufacture an excellent steel bar product with few internal defects, it has not been possible to manufacture a slab with improved internal quality with inexpensive equipment. Further, in the conventional technique of applying a reduction to an unsolidified slab, if porosity is sufficiently eliminated to improve the internal quality, there is a tendency that too much reduction is applied, and the solidification progress interface inside the slab becomes fragile. There is a problem that the tensile stress generated in the part causes cracks inside the slab, which causes defects in the rolling process of the steel bar product. For this reason, in many cases, the amount of reduction has to be limited.

In view of the above situation, by improving the center segregation and porosity of the continuously cast round billet slab supplied to the bar rolling, a material with few defects can be supplied at a low cost during wire drawing and straight bar rolling. A continuous casting process capable of achieving this has been demanded. Therefore, the object of the present invention is to inexpensively manufacture a round cast product in which the component segregation and porosity of the shaft center portion are improved by solving the above-mentioned problems, and moreover, cracking does not occur inside the cast product due to rolling. It is to provide a continuous casting method of a round cast slab for steel bars which can be performed.

[0011]

Means for Solving the Problems The present inventors have obtained the following findings as a result of earnest studies to solve the above-mentioned problems. That is, as a condition for applying the reduction to the slab at the final stage of solidification of the continuous slab, the molten steel at the axial center portion at that position can flow due to the reduction, and, further, after the reduction is once performed, The conditions for obtaining a sufficient slab cross-section reduction rate to completely eliminate porosity without causing the concentrated molten steel to flow, and further, with a pair of rolls, have a proper axial solid fraction. It was found that this can be achieved by adding an appropriate amount of reduction.

The present invention has been made on the basis of the above-mentioned findings, and the continuous casting method for round bar slabs for bar steel according to the present invention uses a molten steel having a carbon content of 0.6 wt. In the continuous casting method of continuously casting a round slab by casting, the round-shaped mold has a diameter of 340 mm or less in the continuous casting method in which the round slab is rolled by a rolling roll installed near the solidification completion point of the round slab. Yes, the molten steel poured into the round mold is subjected to electromagnetic stirring in the round mold, and
At a position where the solid fraction in the axial center portion of the round slab is within the range of 0.3 to 0.7, the round slab is rolled by one set and the reduction rate of the cross-sectional area in the C direction of the round slab is 0. It is characterized in that the round cast piece is rolled down so as to fall within the range of 1 to 3%.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION In the present invention, in order to obtain a desired slab, the molten steel injected into the mold is subjected to electromagnetic stirring in the mold so that the non-metallic inclusions in the molten steel are slabs. By reducing the content remaining in the
The quality level of the slab should be improved.

At the final stage of solidification of the slab, the axial center of the slab crater is rolled down to prevent center segregation and porosity formation. At this time, the position where the solid fraction in the axial center portion of the cast slab is within the range of 0.3 to 0.7 is only within the range of the cross sectional area reduction rate in the C direction of 0.1 to 3%. Reduction should be applied. The reason is as follows.

If the solid phase ratio of the axial center portion of the cast slab to which the reduction is applied is less than 0.3, the solid-liquid coexisting phase region of the axial center portion once reduced is still lower than the solidus temperature. Since this is not the case, molten steel flow again occurs in the crater thereafter, and as a result, center segregation and porosity are formed. On the other hand, when the solid fraction of the axial center exceeds 0.7, the solid fraction exceeds the critical solid fraction in which the molten steel can flow. as a result,
Even if reduction is applied to the slab, the effect of reducing center segregation is not sufficiently exerted. FIG. 1 shows the solid fraction of the slab core,
It is a graph which shows an example of the relation with the score showing the grade of quality of a cast piece. However, the reduction rate of the cross-sectional area in the C direction by the reduction roll is 1.2%. Also in the figure, the solid fraction of the shaft center portion suitable for applying reduction to the cast slab is
It can be seen that it is in the range of 0.3 to 0.7.

Next, if the reduction rate of the cross-sectional area in the C direction due to the reduction of the cast piece at the final stage of solidification is less than 0.1%, the formation of center segregation can be suppressed, but the porosity cannot be suppressed or pressure-bonded. On the other hand, if the C-direction cross-sectional area reduction rate exceeds 3%, porosity can be pressure-bonded, but internal cracks occur in the slab due to tensile stress acting on the solid-liquid interface. FIG. 2 is a graph showing an example of the relationship between the C-direction cross-sectional area reduction rate of the axial portion of the cast slab due to the reduction and the score indicating the quality of the cast slab. However, this is the case where the solid fraction of the shaft center is 0.57. Also in this figure, it can be seen that the appropriate reduction amount to be added to the slab at the final stage of solidification is such that the C-direction cross-sectional area reduction rate is within the range of 0.1 to 3%.

Further, in the present invention, the rolling under the above conditions should be carried out by one pass of the roll. This is because it is more effective to press the porosity by pressing it down in one pass rather than pressing down in two or more passes.

With respect to the size of the round mold, the diameter is 34
When the thickness is 0 mm or less, the cooling rate of the final solidified portion of the axial center of the cast slab becomes particularly high, so that porosity is easily formed and axial center cracking is induced by thermal stress. Therefore, when the diameter is 340 mm or less, the above-mentioned internal quality improvement method should be adopted.

Next, the present invention will be described with reference to the drawings. FIG. 3 is a schematic vertical cross-sectional view in the length direction of the continuous casting machine for explaining one embodiment of the present invention. 1 is a round mold, 2 is a molten steel electromagnetic stirrer, 3 is an unsolidified slab,
4 is a reduction roll, 5 is a slab crater, 6 is a guide roll group, and 7 is a drawing roll. The unsolidified cast slab 3 is pulled out while stirring the high carbon molten steel having a carbon content of 0.6 wt.% Or more injected into the round-shaped mold 1 by the molten steel in-mold electromagnetic stirring device 2. On the other hand, one set of reduction rolls 4 for applying a predetermined reduction to the final solidification stage slab 3 is installed at a predetermined position near the final stage of solidification of the slab.

The installation position of the reduction roll 4 is such that the solid fraction of the slab axial center is 0.3 to 0 ..% with respect to the entire range of the slab drawing speed (casting speed) carried out in the continuous casting operation. A section that falls within the range of 7 should be obtained and installed within this section. The unsolidified slab 3 is reduced by a reduction roll 4. For the reduction amount, the roll load is adjusted so that the reduction rate of the C-direction cross-sectional area of the unsolidified slab 3 due to the reduction falls within the range of 0.1 to 3%. The main body of the reduction roll 4 may be a flat roll.

[0021]

Next, the present invention will be described in more detail with reference to examples. A round billet slab was manufactured by casting a high-carbon molten steel having a carbon content of 0.6 wt.% Or more in a round mold in a curved billet continuous casting machine having a radius of 11.5 m.
Table 1 shows examples manufactured under conditions within the scope of the present invention (casting Nos. 2, 3 and 5 to 7) and comparative examples manufactured under conditions outside the scope of the present invention (casting No. 1). 4 and 8)
The method will be shown.

[0022]

[Table 1]

The chemical composition of the cast molten steel was 2 with carbon contents of 0.79 wt.% And 1.02 wt.% For steel bars.
It is a steel grade that is easy to form center segregation.
There are two mold sizes, 170 mm and 330 mm in diameter, and an electromagnetic stirrer for molten steel in the mold is installed. The casting temperature is the temperature during the ordinary billet continuous casting according to the chemical composition of the molten steel, and the molten steel adjusted to a temperature as constant as possible between the casting Nos. The molten steel in the mold was stirred by. Also,
The amount of slab cooling water was also set to an appropriate constant amount between casting Nos. According to the chemical composition of molten steel.

To reduce the unsolidified slab in the final stage of solidification, 1
A reduction device with a set of reduction rolls was installed in place on the continuous casting line. The rolling system is a hydraulic system, the roll diameter is 380 mm, and the type is a flat roll. The installation position of the pressing roll is 16. from the molten steel meniscus in the mold.
It is located at a distance of 7 m. At this position, the solid phase ratio of the core of the cast slab, the molten steel composition, the casting speed, and other operating conditions are the casting N
It is within the scope of the present invention even if the difference between the o.
It is decided so as to satisfy the condition of being within the range of 0.7. In addition, in Table 1, the distance from the meniscus at which the solid fraction of the axial portion of the cast slab is in the range of 0.3 to 0.7 during steady casting during casting is also shown. As described above, in the example, the solid fraction was positioned within the range of 0.3 to 0.7, while in the comparative example, the solid fraction was less than 0.3 or more than 0.7. The casting speed was adjusted so that

After starting the casting of the molten steel and starting the drawing of the slab, a predetermined amount of reduction was applied to the slab in the final stage of solidification with the above-mentioned reduction roll. The amount of reduction was adjusted so that the reduction rate of the cross-sectional area of the slab in the C direction was within the range of the present invention (within the range of 0.1 to 3%) and outside the range. The amount of reduction of the slab was adjusted by quantifying the relationship among the hydraulic load of the reduction roll, the displacement of the reduction roll, and the reduction rate of the cross-sectional area of the slab in the C direction in advance, and controlling the computer based on the information.

A predetermined test piece was sampled from the round billet cast piece cast as described above, and an evaluation test of the internal properties of the center portion of the cast piece was performed. The evaluation test method is as follows.

[Measurement of Density] In the L direction including the shaft center of the slab,
A block of 10 mm square and 40 mm length was cut out, subjected to replica treatment, and then immersed in phthalic acid to measure the density by the Archimedes method. On the other hand, a block having the same shape and dimension as above was cut out from the surface of the cast slab at a position having a radius of 1/4 and the density was measured by the same method. The density is evaluated by calculating the ratio (ρ _1/4 / ρ _C ) of the density (ρ _C ) of the shaft center to the density at the _1/4 position (ρ _1/4 ), and the magnitude of this calculated value is calculated. It was divided into predetermined ranges and given a score.
The smaller the score, the greater the density ratio (ρ _1/4 / ρ _C ) is. Scores 1 and 2 are passed.

[Internal cracking test] Three macro-corrosion photographs of the cross section of the slab in the C direction, and L having a length of 500 mm in the L direction.
By observing one macro-corrosion photograph of the directional cross-section, a score was assigned according to the degree of internal cracking. A rating of 1 indicates that no internal crack could be found. The smaller the score, the better.

[Center segregation test] Diameter 5 from the axial center of the slab
The ratio (C _i / C ₀ ) of the component analysis value (C _i ) of the cutting powder sample collected with the mm drill to the component analysis value (C ₀ ) of the molten steel sample in the tundish was calculated, and the calculated value was large. The size was divided into a predetermined range and a score was given. The smaller the score, the smaller the degree of segregation and the better.
In the case of negative segregation, a negative sign-is added. Scores 1 and 2 are passed.

[Product Internal Quality Test] The internal quality of the product manufactured from the continuous cast slab is evaluated based on the fracture surface observation in the tensile test and the defects generated during wire drawing and straight bar rolling. did. Ratings 1 and 2 are when no defects occur,
A rating of 3 indicates that the wire was broken or broken during wire drawing and straight bar rolling.

The above test results are also shown in Table 1. As is apparent from the test results, at least one of the internal quality evaluation tests was inferior in the casting Nos. 1, 4 and 8 of Comparative Examples manufactured by the method outside the scope of the present invention.

On the other hand, the cast Nos. 2, 3 and 5 to 7 manufactured by the method within the scope of the present invention are excellent in all the scores of each internal quality evaluation test.

[0033]

EFFECTS OF THE INVENTION Since the present invention is constructed as described above, in order to prevent the occurrence of disconnection or breakage due to internal defects during wire drawing or straight bar rolling in the rolling process of steel bar products. It is possible to provide a continuous casting method of round cast slabs for steel bars in which the center segregation is improved, the axial center porosity disappears, and internal cracking does not occur, and an excellent industrial effect is brought about. .

[Brief description of drawings]

FIG. 1 is a graph showing an example of the relationship between the solid fraction of the slab axial center and the score indicating the quality level of the slab.

FIG. 2 is a relationship between the rate of reduction of the cross-sectional area in the C direction of the axial portion of the cast slab due to the reduction and the score indicating the quality level of the internal quality of the cast slab.
It is a graph which shows an example.

FIG. 3 is a schematic longitudinal cross-sectional view in the length direction of the continuous casting machine for explaining one embodiment of the present invention.

[Explanation of symbols]

 1 Round Shaped Mold 2 Molten Steel Electromagnetic Stirrer 3 Unsolidified Slab 4 Rolling Roll 5 Slab Crater 6 Guide Roll Group 7 Drawing Roll

Continuation of front page (51) Int.Cl. ⁶ Identification number Office reference number FI technical display location // B21J 1/02 B21J 1/02 Z (72) Inventor Takashi Itakura 1-2-1, Marunouchi, Chiyoda-ku, Tokyo No. Nihon Steel Pipe Co., Ltd. (72) Inventor Yoichi Tanmura 1-2-2 Marunouchi, Chiyoda-ku, Tokyo Nihon Steel Pipe Co., Ltd. (72) Inventor Tsuji Shinde 1-2-1 Marunouchi, Chiyoda-ku, Tokyo Nippon Steel Tube Co., Ltd.

Claims (1)

[Claims] 1. A reduction roll installed near a solidification completion point of the round slab when continuously casting a round slab by casting molten steel having a carbon content of 0.6 wt.% Or more into a round mold. In the continuous casting method of applying a reduction to the round slab, the round mold has a diameter of 340 mm or less, the molten steel injected into the round mold is subjected to electromagnetic stirring in the round mold, and, At the position where the solid phase ratio in the axial center portion of the round slab is within the range of 0.3 to 0.7, the round slab is rolled by a set of rolls, and the C-direction cross-sectional area reduction rate of the round slab is set. The method for continuous casting of round slabs for bar steel, characterized in that the round slabs are reduced so that the ratio falls within the range of 0.1 to 3%.