JP2015178120A - Continuous casting method for round casting piece - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a continuous casting method capable of simply and easily producing a round casting piece having high cross-sectional circularity from the molten steel having high total content of Cr and Ni.SOLUTION: A continuous casting method includes using a mold flux satisfying 25≤I/I×100, and giving an agitation flow in which the maximum circumferential flow rate u (mm/s) on a molten steel surface satisfies 200≤u≤570 to the molten steel in a casting mold having a circular cross section. Here, I is the strength value of a highest one of the spectral peaks by a crystal phase obtained by the X-ray diffraction measurement of the powder which is obtained by crushing at room temperature, the specimen prepared by cooling an aggregate-removed mold flux from 1,450°C to 1,000°C at 2K/min. Iis the strength value of the peak by (111) plane obtained by the same X-ray diffraction measurement as that of I.

Description

  The present invention relates to a continuous casting method for steel, and more particularly to a method for continuously casting a round cast piece having a circular cross section.

  Seamless steel pipes used for applications such as oil well pipes for extracting oil or natural gas and line pipes for transporting them are manufactured by the Eugene Sejurne pipe manufacturing method, the Mannesmann pipe manufacturing method, or the like. More specifically, a cast slab having a circular cross section (hereinafter also referred to as “round cast slab”) is processed into a hollow shell by these pipe making methods, and then, if necessary, In addition, a seamless steel pipe is manufactured by performing processes such as drawing and heat treatment.

  As a round slab that can be used in these pipe making methods, a continuous casting slab having a rectangular cross section is manufactured using a mold having a rectangular inner wall cross section (hereinafter referred to as a “rectangular cross section mold”). In addition, there can be mentioned those obtained by performing a piece rolling of this rectangular cross-section slab, and those obtained by continuous casting using a mold whose inner wall has a circular cross section (hereinafter referred to as “circular cross-section mold”). .

  When using a rectangular cross-section mold, the obtained rectangular cross-section slab needs to be made into a round slab by partial rolling. Unsteady portions are formed at both ends of the round cast slab. It is necessary to cut and remove the unsteady portion as a crop, which is one factor that increases the manufacturing cost of the round cast slab. On the other hand, when a circular cross-section mold is used, since no ingot rolling is performed, a round slab can be manufactured at a lower cost than when a rectangular cross-section mold is used. Therefore, if this inexpensive round slab can be used in a continuously cast state and a seamless steel pipe can be produced without causing defects, the production cost can be greatly reduced.

  However, a round cast slab continuously cast using a circular cross-section mold is also referred to as a roundness (hereinafter simply referred to as “roundness”) of a cross section as compared with a round cast obtained by ingot rolling. ) Is often low. This is due to the following reason.

  In the case of continuous casting with a rectangular cross-section mold, since the cross section of the solidified shell formed on the slab is rectangular, bulging is more likely to occur than when the cross section of the solidified shell is circular. Therefore, when a rectangular cross-section mold is used, even if the solidified shell is deformed due to solidification shrinkage and the mold and the solidified shell are separated from each other, the solidified shell is bulged by the static pressure of the molten steel. Immediately reattaches Due to the ease of bulging, the slab is less likely to be delayed in cooling. For this reason, when a rectangular cross-section mold is used, a slab having a higher roundness can be obtained than when a circular cross-section mold is used.

  In other words, the roundness of continuously cast round slabs is reduced when a circular cross-section mold is used. This is because the cross-section of the solidified shell formed on the slab is circular and bulging is unlikely to occur. . If the solidified shell does not bulge, the mold and the solidified shell are likely to be separated from each other, and cooling is delayed at a portion of the slab that is separated from the mold. In the part where cooling is delayed, the thickness of the solidified shell becomes thinner than the others, and the amount of shrinkage due to solidification becomes small, so the thickness of the solidified shell becomes uneven in the circumferential direction, and the roundness of the cross section of the slab Decreases.

  In particular, when continuous casting is performed from a molten steel having a high Cr content using a circular cross-sectional mold, the roundness of the resulting slab tends to be lower. This is because the higher the Cr content, the higher the high-temperature strength of the steel, so that the solidified shell deformed by solidification shrinkage becomes more difficult to bulge.

  Furthermore, even if continuous casting is performed from molten steel having a high Ni content using a circular cross-sectional mold, the roundness of the resulting slab tends to be lower. This is due to the fact that the higher the Ni content, the easier the austenite phase to crystallize as primary crystals or during the solidification process. Since the austenite phase has a higher density than the ferrite phase, when the austenite phase is crystallized, the solidification shrinkage is larger than when the ferrite phase is crystallized, and the mold and the solidified shell are easily separated. .

  Generally, when continuous casting is performed using a circular cross-section mold from molten steel having a total content of Cr and Ni of 20% by mass or more, a slab having high roundness is difficult to obtain.

  The decrease in roundness is caused by the separation of the mold and the solidified shell as the solidified shell contracts in the initial stage of solidification. Therefore, in order to suppress the decrease in roundness, it is important to suppress the detachment between the mold and the solidified shell in the initial stage of solidification.

  In general, when a circular cross-section mold is used, it is difficult to bulge the molten steel once solidified, and it is difficult to make the mold and the solidified shell adhere to each other. However, if the molten steel is solidified while being pressed against the mold at the solidification start point near the meniscus, it is possible to suppress the separation of the mold and the solidified shell due to the solidification shrinkage of the solidified shell and to bring the mold and the solidified shell into close contact with each other. It becomes easy.

  As a method for bringing the mold and the solidified shell into close contact with each other, Patent Document 1 discloses a method of pressing the solidified shell at the initial stage of solidification against a mold with a roller installed in molten steel in the mold. However, in this method, when the support rod of the roller passes through the molten steel and the mold flux at the top of the molten steel, the mold flux is caught in the vortex generated at the rear portion of the support rod, resulting in an internal defect of the slab. In addition, there is a possibility that problems such as that heat is removed from the molten steel by the support rod and the molten steel surface is covered and the operation becomes difficult.

  Conventionally, electromagnetic stirring that stirs molten steel in a mold by electromagnetic force has been applied to continuous casting. In Patent Document 2, formation of defects such as slab surface undercutting is performed by electromagnetic stirring, in Patent Document 3, formation of a white band (negative segregation band of billet surface layer) is performed, and in Patent Document 4, macrosegregation and internal defects are formed. It is said that the formation of each can be suppressed. However, it has not been studied so far to bring the mold and the solidified shell into close contact by electromagnetic stirring.

  Patent Documents 5 to 8 report mold fluxes that crystallize various crystal phases. In any of these documents, the effective crystallization amount of crystal phases and the growth of solidified shells are reported. The relationship has not been studied.

Japanese Patent Laid-Open No. 3-8536 JP-A-1-180762 Japanese Patent Laid-Open No. 2001-25848 Japanese Patent Laid-Open No. 60-44157 JP 2002-103008 A JP 2003-326342 A JP 2004-223599 A Japanese Patent Laid-Open No. 2005-40835

  As described above, when a continuous casting is performed from a molten steel having a total content of Cr and Ni of 20% by mass or more using a circular cross-sectional mold, a slab having a high roundness cannot be obtained. This is because the solidified shell has a high temperature strength and a large amount of solidification shrinkage, so that the mold and the solidified shell are easily separated. Since round slabs with low roundness cannot be used as they are, pipe cutting is necessary to increase roundness, which increases work costs and decreases yield. Cause problems.

  The present invention has been made in view of these problems. When continuously casting steel from molten steel having a high total content of Cr and Ni, a round slab having a high roundness of a cross section is simply obtained. It aims at providing the continuous casting method which can be manufactured.

The gist of the present invention is the continuous casting method (I) below.
(I) Molten steel having a total content of Cr and Ni of 20% by mass or more and crystallizing an austenite phase in the solidification process is poured into a mold having a circular cross section through a single-hole downward dipping nozzle. A continuous casting method of casting a round cast piece having a circular cross section while giving a stirring flow having a velocity component along the circumferential direction of the mold to the molten steel,
Using a mold flux in which the X-ray diffraction intensity ratio I / I _{0 of the} crystal phase satisfies the following formula (A), the maximum circumferential flow velocity u (mm / s) on the molten metal surface is expressed by the following formula ( A continuous casting method of steel, characterized by performing continuous casting while applying a stirring flow that satisfies B).
25 ≦ I / I ₀ × 100 (A)
200 ≦ u ≦ 570 (B)
I: The mold flux from which the aggregate has been removed is cooled from 1450 ° C. to 1000 ° C. at 2 K / min to obtain a sample, and the crystal phase obtained by X-ray diffraction measurement of the powder obtained by crushing this sample at room temperature Intensity value of the highest spectrum peak due to (count per second)
I ₀ : For polycrystalline Si, the intensity value of spectrum peak (count per second) by (111) plane obtained by the same X-ray diffraction measurement as I

The solidification point of the mold flux is 1140 ° C. or higher, the X-ray diffraction intensity ratio I / I ₀ satisfies the following formula (C) instead of the formula (A), and the circumferential maximum flow velocity u is the formula It is preferable to satisfy the following formula (D) instead of (B).
27 ≦ I / I ₀ × 100 (C)
300 ≦ u ≦ 570 (D)

  According to the continuous casting method of the present invention, when the total content of Cr and Ni in the steel is high, a round cast slab having a high roundness, for example, the longest thickness and the shortest in the cross section is obtained by a simple method. It is possible to produce a round slab having a difference from the thickness of 10 mm or less. Here, the thickness means an interval between the two parallel lines when the cross section is sandwiched between the two parallel lines.

It is a figure which shows the relationship between the X-ray diffraction intensity ratio of mold flux, the maximum flow velocity of the circumferential direction of molten steel, and the roundness of a slab.

  As a means for bringing the mold and the solidified shell into close contact with each other, the inventors focused attention on the centrifugal force generated by rotating and flowing the molten steel in the horizontal direction and the mold flux that is easily crystallized, and repeated studies. With regard to the mold flux, it is expected that a slow cooling action can be obtained and a solidified shell can be uniformly grown by using one that is easily crystallized during casting. And when the present inventors continuously cast a round slab from a molten steel with a high total content of Cr and Ni (20% by mass or more) using a circular cross-sectional mold, the amount of crystallization of the mold flux It has been found that there is a range in which round cast slabs with high roundness can be obtained with respect to the flow rate of molten steel.

  The present invention is based on this finding. As described above, in this continuous casting method, a molten steel having a total content of Cr and Ni of 20% by mass or more and crystallizing an austenite phase in the solidification process is obtained. A round slab having a circular cross section is cast while being poured into a mold having a circular cross section through a submerged nozzle facing the hole and giving a stirring flow having a velocity component along the circumferential direction of the mold to the molten steel in the mold.

When continuous casting is performed, a mold flux in which the X-ray diffraction intensity ratio I / I ₀ of the crystal phase satisfies the above formula (A) is used, and the maximum circumferential flow velocity u (mm / s on the molten metal surface) is applied to the molten steel in the mold. ) Gives a stirring force satisfying the above formula (B).

By applying a stirring flow having a velocity component along the circumferential direction of the mold to the molten steel in the mold, the molten steel is given a centrifugal force and is pressed against the inner wall of the mold. Further, when I / I ₀ satisfies the above formula (A), a sufficient amount of crystals are crystallized from the mold flux, and the radiant heat from the molten steel and the solidified shell is reduced, that is, the effect of slow cooling is achieved. It is fully obtained and the solidified shell grows uniformly. By these actions, the mold and the solidified shell can be brought into close contact with each other, and a slab having high roundness can be obtained.

  Giving a stirring flow having a velocity component along the circumferential direction of the mold to the molten steel by an electromagnetic stirring device or the like is widely performed, and a mold satisfying the above formula (A) (or formula (B)) The flux is readily available. Therefore, a round cast slab having a high roundness can be easily obtained by the method of the present invention.

  As a means for giving a stirring flow to molten steel, for example, a known electromagnetic stirring device can be used. Since the molten steel is injected into the mold by a single-hole downward dipping nozzle, a stirring flow having a velocity component along the circumferential direction of the mold is not substantially generated by this nozzle. For this reason, the speed component along the circumferential direction of the mold can be accurately controlled for the stirring flow by means for giving the stirring flow to the molten steel.

Regarding I ₀ , “in the same X-ray diffraction measurement as I” means that the same X-ray diffraction measurement apparatus as that used for the X-ray diffraction measurement for obtaining I is used and the X-ray diffraction measurement for obtaining I is used. It means that the measurement is performed under the same conditions.

When I / I ₀ × 100 <25, the amount of crystals crystallized from the mold flux is small, and at the time of casting, the radiant heat from the molten steel and the solidified shell cannot be sufficiently reduced due to this crystal. Deviations from are likely to occur. In order to further increase the amount of reduction in radiant heat from the molten steel and the solidified shell, it is preferable that 27 ≦ I / I ₀ × 100 (the above formula (C) is satisfied).

Type of crystal phase crystallizes from the melt in the mold flux is not particularly limited, for _{example, akermanite (Ca 2 MgSi 2 O} 7), or, complete solid solution between akermanite and _{gehlenite (Ca 2 Al 2 SiO 7} ) It can be melite. The highest spectral peak in the X-ray diffraction measurement may be due to these crystalline phases. akermanite and melilite usually have crystallization temperatures suitable for slow cooling of molten steel and solidified shells. Therefore, if the highest spectral peak in the X-ray diffraction measurement is due to akermanite or melilite, it is easy to obtain the effect of slowly cooling the molten steel and the solidified shell.

Further, the second highest spectral peak in the X-ray diffraction measurement can be caused by, for example, cuspidine (Ca ₄ Si ₂ O ₇ F ₂ ). Cuspidine has a crystallization temperature suitable for slow cooling of molten steel and solidified shell, similar to akermanite and melilite. On the other hand, since cuspidine has a high basicity, if a large amount of cuspidine is crystallized, crystallization of akermanite or melilite may be suppressed. For this reason, it is preferable that the highest spectral peak in the X-ray diffraction measurement is not due to cuspidine.

  If u <200, sufficient centrifugal force is not generated in the molten steel. Therefore, the force for pressing the molten steel against the mold at the solidification start position near the meniscus in the mold becomes insufficient, the mold and the solidified shell cannot be brought into close contact with each other, and the mold and the solidified shell are easily separated. In order to sufficiently bring the mold and the solidified shell into close contact with each other and sufficiently suppress the deviation between the mold and the solidified shell, 300 ≦ u is preferable.

  On the other hand, if 570 <u, the centrifugal force generated in the molten steel becomes excessive, and the molten steel surface is low in the vicinity of the immersion nozzle and high in the vicinity of the inner wall of the mold, and the difference in height between the two becomes remarkable. Along with this, the thickness of the mold flux on the molten steel surface becomes thinner as it approaches the inner wall of the mold. Therefore, the mold flux hardly flows between the mold and the solidified shell, and poor lubrication is likely to occur between the mold and the solidified shell.

  The freezing point of the mold flux is preferably 1100 ° C. or higher and 1250 ° C. or lower. When the freezing point is lower than 1100 ° C., the crystal does not crystallize until the mold flux reaches a lower temperature region in the mold, and a sufficient amount of crystals to suppress the radiant heat from the molten steel and the solidified shell. Cannot be obtained. In order to sufficiently suppress radiant heat, the freezing point of the mold flux is more preferably 1140 ° C. or higher. Further, when the freezing point is higher than 1250 ° C., the crystal is crystallized as soon as the mold flux flows between the mold and the solidified shell, so that the mold flux does not flow sufficiently between the mold and the solidified shell. There is a fear.

  The viscosity of the mold flux at 1300 ° C. (hereinafter referred to as “high temperature viscosity”) is preferably 0.2 Pa · s or more and 0.8 Pa · s. If the high-temperature viscosity is less than 0.2 Pa · s, the mold flux may flow excessively between the mold and the solidified shell. In order to sufficiently suppress such excessive inflow of mold flux, the high temperature viscosity is more preferably 0.3 Pa · s or more. On the other hand, when the high-temperature viscosity is higher than 0.8 Pa · s, there is a possibility that the mold flux cannot sufficiently flow between the mold and the solidified shell. In order to allow the mold flux to sufficiently flow between the mold and the solidified shell, the high temperature viscosity is more preferably 0.7 Pa · s or less.

  The casting speed is preferably 0.3 m / min or more and 2.0 m / min or less. If the casting speed is less than 0.3 m / min, the solidified shell becomes too thick as a whole, and the effect of bringing the mold and the solidified shell into close contact with each other due to the centrifugal force generated in the molten steel cannot be obtained. On the other hand, when the casting speed is higher than 2.0 m / min, the solidified shell becomes too thin, which promotes non-uniform growth of the solidified shell.

  The diameter of the mold is preferably 160 mm or more and 380 mm or less. The smaller the inner diameter of the mold, the greater the proportion of heat dissipated through the mold relative to the heat supplied by the molten steel injected into the mold, and if the mold diameter is 160 mm or less, the molten steel in the mold Is difficult to maintain at a sufficiently high temperature. On the other hand, if the velocity component in the circumferential direction of the molten steel is the same in the portion adjacent to the inner wall of the mold, the centrifugal force acting on the molten steel decreases as the mold diameter increases, and if the mold diameter is greater than 380 mm, it acts on the molten steel. The above-mentioned effect due to centrifugal force cannot be obtained sufficiently.

In order to confirm the effect of the continuous casting method of the present invention, the following tests were conducted and the results were evaluated.
1. Test Method For casting, a circular cross-section mold having a diameter of 180 to 360 mm was used, and a single-hole downward dipping nozzle was used to supply molten steel to the mold.

  Table 1 shows the contents of C, Si, Mn, Cr, and Ni in steel corresponding to the molten steel. In any steel type, the total content of Cr and Ni is 20% by mass or more.

Table 2 shows the chemical composition of the mold flux used, the X-ray diffraction intensity ratio I / I ₀ , the freezing point, and the crystal phase showing the maximum peak in the X-ray diffraction measurement. As shown in Table 2, the above formula (A) is satisfied by the mold fluxes A to D, but not by the mold flux E.

  While using any of the mold fluxes A to E in Table 2, an electromagnetic stirrer applies a stirring force to the molten steel in the mold to produce a flow having a velocity component along the circumferential direction of the mold, Was continuously cast. Table 3 shows test conditions and evaluation results. As the test conditions, the maximum flow velocity in the circumferential direction of the molten steel was difficult to measure, and was calculated by numerical analysis simulation of the flow in the mold in consideration of electromagnetic force.

2. Test Results The obtained round cast slabs were evaluated by measuring the roundness. FIG. 1 shows the relationship between the X-ray diffraction intensity ratio I / I ₀ , the circumferential maximum flow velocity u, and the roundness as a test result of the example. In Table 3 and FIG. 1, the meanings of the symbols are as follows.
○: It has high roundness that can be sent to the pipe making process without any external cutting.
(Triangle | delta): It has roundness of the grade which requires the external cutting within 10 mm regarding a radial direction.
X: It has roundness of the grade which requires an external cutting more than 10 mm regarding a radial direction.
◇: The roundness is low, the solidification unevenness is large, and the continuous casting operation itself is difficult.

  In the case where the evaluation result is ◯ or Δ, the difference between the longest diameter and the shortest diameter in the cross section of the round slab is 10 mm or less. Although depending on the manufacturing conditions, performing an external cut within 10 mm in the radial direction has a cost merit, while performing an external cut greater than 10 mm in the radial direction has no cost merit.

From FIG. 1, when the X-ray diffraction intensity ratio I / I _{0 of} the crystal phase and the circumferential maximum flow velocity u satisfy the requirements of the present invention (the above formulas (A) and (B)), the evaluation result is It turns out that it is (circle) or (triangle | delta) and a slab with high roundness is obtained. When at least one of the X-ray diffraction intensity ratio I / I ₀ and the circumferential maximum flow velocity u does not satisfy the requirements of the present invention, the evaluation result is x or ◇, and a high roundness slab is obtained. It is not done.

In the range of 27 ≦ I / I ₀ and 300 ≦ u ≦ 570, the evaluation results of most slabs are “◯”, and round slabs with higher roundness are obtained.

  Solidified shells produced from any molten steel of steel types I to III are considered to have high strength at high temperatures and a large amount of solidification shrinkage. It can be seen that such a deviation between the solidified shell and the mold can be suppressed by the method of the present invention.

  The present invention can be applied to manufacturing a round slab having high roundness from molten steel having a total content of Cr and Ni of 20% by mass or more.

Claims (2)

A molten steel having a total content of Cr and Ni of 20% by mass or more and crystallizing an austenite phase during the solidification process is poured into a mold having a circular cross section through a single-hole downward dipping nozzle, and the molten steel in the mold is injected into the mold. A continuous casting method for casting a round cast piece having a circular cross section while giving a stirring flow having a velocity component along the circumferential direction of the mold,
Using a mold flux in which the X-ray diffraction intensity ratio I / I _{0 of the} crystal phase satisfies the following formula (A), the maximum circumferential flow velocity u (mm / s) on the molten metal surface is expressed by the following formula ( A continuous casting method of steel, characterized by performing continuous casting while applying a stirring flow that satisfies B).
25 ≦ I / I ₀ × 100 (A)
200 ≦ u ≦ 570 (B)
I: The mold flux from which the aggregate has been removed is cooled from 1450 ° C. to 1000 ° C. at 2 K / min to obtain a sample, and the crystal phase obtained by X-ray diffraction measurement of the powder obtained by crushing this sample at room temperature Intensity value of the highest spectrum peak due to (count per second)
I ₀ : For polycrystalline Si, the intensity value of spectrum peak (count per second) by (111) plane obtained by the same X-ray diffraction measurement as I The continuous casting method according to claim 1,
The freezing point of the mold flux is 1140 ° C. or higher,
The X-ray diffraction intensity ratio I / I ₀ satisfies the following formula (C) instead of the formula (A),
The steel continuous casting method in which the circumferential maximum flow velocity u satisfies the following formula (D) instead of the formula (B).
27 ≦ I / I ₀ × 100 (C)
300 ≦ u ≦ 570 (D)

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