JP5045258B2 - Continuous casting method and continuous casting machine - Google Patents

Description

  The present invention relates to a continuous casting method and a continuous casting machine that continuously draw slabs such as slabs, blooms, and billets from a mold, and more particularly to a continuous casting method and a continuous casting machine that can reduce cracks on the surface of the slab. .

  The continuous casting machine includes a vertical type in which a slab is pulled out from a mold in a vertical direction and a curved type or a vertical bending type in which a mold is bent in the vertical direction and then bent in the middle and pulled out in a horizontal direction. If the mold is pulled out in the vertical direction, the building becomes taller, so a curved type or vertical bending type continuous casting machine that curves the mold has become mainstream.

  FIG. 11 is a schematic diagram of a vertical bending type continuous casting machine. The slab 1 is bent in the upper correction band 1a, kept in a curved state in the bending band 1b, and then bent back into a flat plate shape in the lower correction band 1c. Bending stress is applied to the slab 1 in the upper straightening band 1a and the lower straightening band 1c. Due to this bending stress, a tensile strain 2 is applied to the lower surface side of the slab 1 in the upper correction band 1a, and a tensile strain 3 is applied to the upper surface side of the slab 1 in the lower correction band 1c. Due to the tensile strains 2 and 3 on the upper surface side or the lower surface side of the slab 1, as shown in FIG. 12, the upper surface side or the lower surface side (mainly the corner portion) of the slab 1 (slab) Crack 4 may occur. Since the transverse crack 4 leads to a defect of the final product, it is necessary to reduce the transverse crack 4 or to care for the surface of the slab 1.

  Currently, in steel types with high cracking sensitivity such as thin / thick high tensile steel (high-tensile steel plate), Nb, V-added steel, B, N-added tin, The company has been forced to take measures such as setting restrictions and setting restrictions on the continuous casting machines it produces. The productivity of these steel types is lower than that of general steel types, and countermeasures against transverse cracking are a great burden on operations and operations.

As a measure for preventing lateral cracking of the slab, a method of controlling secondary cooling of the slab is known. In this secondary cooling control method, for example, the high temperature ductility of a material is investigated by a high temperature tensile test or the like, and the high temperature embrittlement temperature region is determined from the relationship between the temperature and the reduction of area (hereinafter referred to as RA). Ask. And secondary cooling is controlled so that the temperature of the surface of the slab in the upper straightening zone and the lower straightening zone does not enter the high temperature embrittlement temperature range, and correction in the high temperature embrittlement temperature range is avoided. As an example of a method for controlling the secondary cooling of the slab, Patent Document 1 discloses that the slab is strongly cooled below the mold outlet, and the surface temperature of the slab is once lowered below the Ar ₃ transformation point, and then the high temperature brittleness is obtained. There has been proposed a continuous casting method that prevents the occurrence of transverse cracks in the slab by reheating it to a higher temperature side than the crystallization temperature range and correcting the slab.

In addition to a method for controlling secondary cooling of the slab as a technique for preventing lateral cracking of the slab, many studies have been conducted on a method for imparting strain to the slab. For example, in Patent Document 2, a processing strain of 5% or more is imparted to a surface layer portion depth of 2 mm or more at the time of continuous casting, and thus hot cracking of the slab at the time of continuous casting, as well as direct feed rolling and hot charging. A technique for preventing hot cracking of a slab in rolling has been disclosed. Non-Patent Document 1 discloses a technique for reducing the austenite grain size by dynamic recrystallization by applying a strain of 20% or more to a slab.
Japanese Patent Laid-Open No. 9-253814 (refer to the claims) Japanese Patent Publication No. 4-68069 (refer to claims) Materials and Processes, 14 (2001), 1151 Ouchi et al.

  However, only some steel types can prevent transverse cracking by controlling secondary cooling. In addition, the casting speed has to be reduced (in the case of high speed casting, the surface of the slab is not sufficiently cooled due to insufficient secondary cooling), which may hinder productivity.

  In the method of imparting strain to the slab, the amount of strain required to exhibit the effect is a relatively large value in the cases reported so far. For example, Patent Document 2 describes that in order to prevent hot cracking of a slab during continuous casting, it is necessary to apply a work strain of 5% or more. Non-Patent Document 1 describes that the amount of strain necessary for dynamic recrystallization is approximately 15 to 40%.

  Since it is difficult to give a large strain of 5% or more to a slab in a continuous casting machine (for example, a large facility such as a die-pressing press is required), the present situation is that it cannot be applied to an actual machine. In addition, if the slab is subjected to a strain of 5% or more in front of the lower straightening zone of the continuous casting machine, the slab breaks due to the strain.

  Then, an object of this invention is to provide the new continuous casting method and continuous casting machine which can prevent the lateral crack of the slab in a continuous casting machine.

In order to solve the above problems, the invention according to claim 1, the slab is pulled out from the mold in a continuous casting process bent back in the lower straightening zone, until the slab is pulled out from the mold reaches arrives to the lower straightening zone In the meantime , the slab surface temperature range of 700 to 900 ° C. is subjected to a plurality of strains with a strain amount of 5% or less, and the strain rate of the slab strain is set to be equal to that of the slab of the lower straightening strip. It is characterized by being larger than the strain rate .

The invention according to claim 2 is characterized in that, in the continuous casting method according to claim 1 , a plurality of rolling rolls imparts strain to the slab a plurality of times by rolling the slab a plurality of times.

According to a third aspect of the invention, in a continuous casting method according to claim 1, bend the slab until the slab to the lower straightening zone is reached, by or bent back, the cast It is characterized in that a piece is subjected to distortion several times.

In the invention according to claim 4 , the slab drawn from the mold can be bent back at the lower correction band, and a large number of rolls arranged facing both side surfaces of the slab and the cast drawn from the mold. until piece reaches arrives to the lower straightening zone, the billet surface temperature range and a distortion imparting means for providing a plurality of distortion of the slab to the strain amount is within the 5% 700 to 900 ° C., In the continuous casting machine, the strain rate of the slab is set to be larger than the strain rate of the slab of the lower straightening band .

  In order to prevent lateral cracking of the slab during continuous casting by imparting strain, it has been common knowledge to impart a strain of 5% or more to the slab. In other words, it was said that there was no effect at a strain of 5% or less. However, as a result of the experiment, it was found that by giving a strain of 5% or less multiple times, the crystal grains of the slab are refined and the ductility is improved. It is practically possible to apply a strain of 5% or less to the slab in the continuous casting machine. Since ductility is improved by preliminarily applying a strain of 5% or less to the slab in front of the lower straightening zone, it is possible to prevent lateral cracking of the slab in the lower straightening zone.

  An embodiment of a continuous casting method of the present invention will be described below with reference to the accompanying drawings. FIG. 1 shows a schematic view of a two-strand type vertical bending type continuous casting machine which is an example of a continuous casting machine. Molten steel is poured into the mold 7 through the tundish 6. Cooling water is sprayed from the spray nozzle 8 to the slab 9 drawn out from the mold 7. In order to guide the slab 9 drawn out from the mold 7 from the vertical direction to the horizontal direction, a large number of rolls 10 are arranged along a curve such as an arc or a parabola. On the downstream side of the many rolls 10, a drawing device is continuously provided. On the downstream side of the drawing device, a cutting device 11 such as a gas torch for cutting the solidified slab and hydraulic cutting is provided. The slab 9 cut | disconnected by the cutting device 11 is discharged | emitted from a continuous casting machine, and is conveyed to a rolling apparatus. In the figure, 12 is an electromagnetic stirring device.

  In this embodiment, as shown in FIG. 2, the lower straightening is used as a crack prevention measure in a continuous casting machine of steel types having high cracking sensitivity such as thin plate / thick plate high-tensile steel, Nb, V-added steel, B, N-added tin, A strain applying means 14 is provided for preliminarily applying strain to the slab 9 of the curved strip 9b in front of the strip 9c. In a continuous casting machine, there is a limit to applying a large strain to the slab 9 at a time. However, the slab 9 (slab) is supported by a plurality of rolls 10 in the continuous casting machine. A small strain is applied to the surface layer of the slab 9 a plurality of times using the plurality of rolls 10. The strain imparting means 14 imparts a plurality of strains with a strain amount within 5% to the slab in the curved band 9b. By imparting strain at this position, the crystal grains are refined to improve ductility.

  FIG. 3 shows a specific example of the strain applying means 14. This example is a roll reduction type strain applying means. In the continuous casting machine, a plurality of, for example, three rolling rolls 10 a to 10 c are provided in the conveying direction of the cast piece 9. By rolling the slab 9 a plurality of times with a plurality of rolling rolls 10a to 10c, the slab 9 can be strained a plurality of times, for example, three times. A pair of rolling rolls 10a to 10c provided on both side surfaces of the slab 9 imparts one strain. The amount of distortion at one time is within 5%. Since roll reaction force is applied to the rolling rolls 10a to 10c when imparting strain, a backup roll 16 is provided on the back side of the rolling rolls 10a to 10c.

  FIG. 4 shows another example of the strain applying means. In this example, a plurality of pass line changing rolls 10d to 10f are arranged by shifting the pass line of the slab 9 so that the slab can be bent. The slab 9 is bent a plurality of times by further bending or bending back the slab 9 in a curved state in the bending band 9b. Each time the slab 9 is bent, a tensile strain is applied to one surface of both side surfaces of the slab 9, and a compressive strain is applied to the other surface. The maximum compressive or tensile strain applied to the slab 9 is set within 5%. In FIG. 4, the slab is shown linearly so that the strain application method can be easily understood, but the strain is also applied in the same manner in the curved band.

  The strain rate of the strain that is preliminarily applied to the slab 9 by the strain applying means 14 (hereinafter referred to as pre-strain) is set to be larger than the strain rate of straight bending of the continuous casting machine. This prevents cracking due to pre-strain. The strain rate will be described later.

  In the following, regarding the mechanism of embrittlement of steel: 1. Relationship between embrittlement behavior and strain rate The explanation will be divided into items to be examined by the prediction formula of embrittlement.

1. Relationship between embrittlement behavior and strain rate FIG. 5 is a graph showing the embrittlement behavior of steel in each temperature range ((Source) Suzuki et al., Iron and Steel, 65, 2038 (1979)).
The embrittlement between 1200 ° C. and solidus is called I region embrittlement, and it is said that the cause is liquid film embrittlement due to residual molten steel between dendrites, and liquid film embrittlement due to grain boundary melting. Is independent of strain rate.
The embrittlement at 900 ° C. to 1200 ° C. is called II region embrittlement, which is said to be due to grain boundary precipitation of oxides, sulfides, phosphides, etc., and becomes brittle as the strain rate increases.
Embrittlement at 600 ° C to 900 ° C is called III region embrittlement, which is caused by grain boundary precipitation of oxides, sulfides, charcoal / nitrides, etc., and proeutectoid ferrite formed along austenite grain boundaries. It is said that the smaller the strain rate, the more brittle.

  Table 1 shows the characteristics of the embrittlement behavior of the steel in each temperature range.

Table 2 shows the results of classifying the correction cracks in the continuous casting machine according to the mechanism of FIG. Straightening cracks in the continuous casting machine occur when the surface temperature of the slab (slab) is approximately 700 ° C to 900 ° C. This temperature region is a region III of embrittlement, and is a region that becomes embrittled as the strain rate decreases. Since the strain rate of the straightening strain in the continuous casting machine is as small as about 10 ⁻⁴ to 10 ⁻³ , it can be said that it is easier to promote cracking.

2. Examination by embrittlement prediction formula Here, the effects of strain rate and crystal grain size on RA are discussed with reference to the study of Mintz et al. Mintz et al. Investigated the ductility of several types of steels and also looked at data from other researchers, and as a result, derived the RA estimation formula shown below.

（（出典）B.Mintz,etc, International Materials Reviews,36,187(1991)）

(Source: B. Mintz, etc, International Materials Reviews, 36, 187 (1991))

The range in which the formula (1) can be applied is that the crystal grain size d is 500 μm or less, the strain rate ε ′ is 10 ⁻⁴ or more and 10 ⁻¹ or less, the component is 1.0% <Mn <1.4%, 005% <S, and within this range, it can be predicted with an error of ± 12% (confidence interval 95%). The predictable temperature is 700 ° C. to 1000 ° C., which corresponds to the embrittlement of the III region in Table 1.

FIG. 6 shows the relationship between Minimum RA (%) calculated from this prediction formula and strain rate. Here, for convenience, the calculation was performed with s = 1000 nm. From FIG. 6, it can be said that when the strain rate increases, RA increases, and it becomes difficult to break against deformation with a large strain rate. When prestraining the slab in this temperature range, it is possible to avoid processing with a low RA by increasing the strain rate. For example, when the crystal grain size is 500 μm, the minimum RA is about 17% at a strain rate of 10 ⁻³ / s corresponding to straightening bending in a continuous casting machine, but when the strain rate is increased to 10 ⁻¹ / s, the minimum RA is about Increase to 57%.

From the above examination, it is understood that when prestraining is performed in the slab surface temperature range of about 700 ° C. to 900 ° C., it is possible to avoid the occurrence of cracking due to the prestraining itself by increasing the strain rate. Since the strain rate corresponding to the straightening bending in the continuous casting machine is about 10 ⁻³ / s, the strain rate when performing pre-straining in the curved band is set to 10 ⁻³ / s to 1 / s. This is because if the strain rate exceeds 1 / s, cracks may occur beyond the predicted range of the equation. Furthermore, if the crystal grain size of the slab surface layer can be refined from 500 μm to 100 μm by this pre-straining process, RA is improved to 50% due to the refinement effect, and therefore the continuous casting machine having a strain rate of 10 ⁻³ / s. It is possible to have a certain level of resistance to cracking even against straightening distortion.

FIG. 7 shows the relationship between the Minimum RA (%) calculated from the prediction formula and the crystal grain size. As in FIG. 6, it can be seen that the minimum RA (%) is improved when the crystal grain size is reduced, that is, when the crystal grain size is refined. For example, if the strain rate is 10 ^-3 / s, which corresponds to straightening bending in a continuous casting machine, the minimum RA is about 17% when the crystal grain size is 500 μm, but the crystal grain size is reduced to 100 μm. It can be seen that Minimum RA rises to about 50% if possible. Thereby, the effect of the ductility recovery by making the crystal grains fine can be roughly calculated.

  In addition, this invention is not restricted to the said embodiment, In the range which does not change the summary of this invention, it can change variously. For example, in the above-described embodiment, an example in which the present invention is applied to a vertical bending type continuous casting machine has been described. However, the present invention can also be applied to a curved type continuous casting machine shown in FIG. In the vertical bending type continuous casting machine, a flat plate mold is used to feed the slab vertically, but in the case of a curved type continuous casting machine, the curved mold 21 is used to send the slab in an arc shape. Since the inner surface of the mold is curved, a curved slab is fed out, and bending back is corrected at the lower correction band 22 to obtain a slab. It differs from the vertical bending type continuous casting machine in that there is no bending process in the upper straightening belt.

An experiment was conducted to confirm the recovery effect of embrittlement by applying pre-strain. The experiment uses a machining formaster. The sample is pre-strained and then subjected to a tensile test to measure the cross-sectional shrinkage ratio RA. FIG. 8 shows the history of the cyclic straining experimental temperature. The strain rate of pre-strain is 0.01 / s. The strain rate at the time of fracture is 2 × 10 ⁻³ / s assuming the strain rate of the lower straightening zone of the continuous casting machine.

  Table 3 shows a list of repeated strain imparting experiments-experimental levels.

  Table 4 shows the sample components.

  As a comparative example, a sample that was not subjected to pre-strain processing, a sample that was given strain once, and a sample that was given strain twice was finally broken at a test temperature of 850 ° C.

  FIG. 10 shows the measurement result of RA. Compared to RA = 45.6% in the comparative example in which the pre-strain processing was not performed, it can be seen that in the sample to which the strain was applied once, the RA increased according to the strain amount and the embrittlement was recovered. In addition, the RA is increased in accordance with the total strain amount for the sample to which the strain is applied twice. RA is increased in accordance with the total strain amount in both the case where the strain is applied once and the case where the strain is applied twice. In other words, it can be seen that even if the strain is divided and applied in small amounts, the same effect as the one-time strain application is obtained. Even in the continuous casting machine, it was suggested that the amount of strain per one time required for embrittlement recovery can be reduced by applying a plurality of times instead of applying a large strain at once.

In this experiment, it was found that RA was highest when 3% strain was applied twice. In addition, although the strain rate of the pre-straining process is as small as 10 ⁻² / s, here, there was no reduction in RA due to the pre-straining process (the occurrence of cracks and micro-cracks due to the pre-strain itself) was not caused.

Schematic diagram of vertical bending type continuous casting machine Schematic diagram showing distortion imparting means Schematic diagram showing roll-rolling type strain applying means Conceptual diagram showing distortion applying means of the pass line changing method Graph showing the embrittlement behavior of steel in each temperature range Graph showing the effect of strain rate on RA Graph showing the effect of crystal grain size on RA Figure showing the history of cyclic straining experimental temperature Schematic diagram of curved continuous casting machine Graph showing ductility recovery status due to multiple grants Schematic diagram of vertical bending type continuous casting machine The perspective view which shows the crack which occurs in the corner part of slab

Explanation of symbols

DESCRIPTION OF SYMBOLS 9 ... Slab 9b ... Curve zone 9c ... Lower straightening zone 10 ... Rolls 10a-10c ... Roll rolls 10d-10f ... Pass line change roll 14 ... Strain imparting means

Claims (4)

In a continuous casting method in which the slab drawn from the mold is bent back at the lower straightening band,
Until the slab is pulled out from the mold reaches arrives to the lower straightening zone, the billet surface temperature range gives a multiple of strain of the slab to the strain amount is within the 5% 700 to 900 ° C., the cast A continuous casting method , wherein the strain rate of the strain of the piece is made larger than the strain rate of the cast piece of the lower straightening band . The continuous casting method according to claim 1, wherein the plurality of rolling rolls rolls the slab a plurality of times to give the slab a plurality of strains. 2. The continuous casting according to claim 1, wherein the slab is subjected to a plurality of distortions by bending or bending back the slab until the slab reaches the lower straightening band. Casting method. A large number of rolls arranged facing both side surfaces of the slab so that the slab drawn from the mold can be bent back in the lower correction band;
Until the slab is pulled out from the mold reaches arrives to the lower straightening band, the distortion imparting means billet surface temperature range to provide a plurality of distortion of the strain amount in the cast piece within the 5% 700 to 900 ° C. and, with a,
A continuous casting machine in which the strain rate of the slab is larger than the strain rate of the slab of the lower straightening strip .

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