TW202200290A - Continuous casting method can prevent corner of continuation casting piece from cracking and provide high quality of billet steel - Google Patents

Abstract

A continuous casting method disclosed by the invention can reliably suppress surface cracks on the casting piece, especially for the manufacturing of the high quality billet steel without corner crack. The continuous casting method according to the invention is a method of continuously casting steel, wherein a casting mold in which a chamfer shape of a casting mold corner portion satisfies the relationship of 0.09 ≤ C/L ≤ 0.20 (in the formula, C: a corner chamfer amount (mm), L: a short edge length (mm) of the casted piece) is used, and the average secondary cooling water amount density corrected from directly under the casting mold at the casting mold corner portion to the lower portion is set as 22 to 60L/( min.m2). Particularly, the compound composition of steel, according to calculation of mass%, preferably contains C: 0.05 to 0.25% and Mn: 1.0 to 4.0%, and randomly contains more than one selected from Nb: 0.01 o 0.1%, V: 0.01 to 0.1%, and Mo: 0.01 to 0.1%.

Description

Continuous casting method

The present invention relates to a continuous casting method of steel which can suppress the occurrence of cracks on the slab surface during continuous casting.

In recent years, the required specifications of high-tensile steels tend to be stricter. In order to improve the mechanical properties of steel plates, the content of alloying elements such as Cu, Ni, Nb, V, and Ti has been increased. When such alloy steel is cast using, for example, a vertical bending type continuous casting machine, the four corners of the rectangular cross-section perpendicular to the casting direction of the slab (hereinafter also referred to as "casting Slab corner ") load stress, resulting in easy occurrence of surface cracks, especially in the corners of the slab more prone to cracks. The corner cracks are likely to cause surface flaws in the thick steel sheet, and this causes a decrease in the yield of steel sheet products.

That is to say, the hot ductility of the cast alloy steel is significantly reduced at the temperature near the Ar ₃ transformation point where the solidification structure of the alloy steel is transformed from the Vostian iron phase to the fertile iron phase.

Therefore, in order to prevent the above-mentioned corner cracks in the continuous casting step, it is generally adopted: use secondary cooling to control the surface temperature of the slab, and correct it at the temperature above the metamorphosis point, or control the solidified structure of the slab so that it is not easy to crack. cracked tissue.

In order to keep the surface temperature of the slab at a high temperature, generally, the shower piping near the corner of the slab is closed, and the shower width cut without cooling is performed.

Furthermore, as a method of controlling the solidification structure, for example, Patent Document 1 discloses that the secondary cooling of the cast slab is started immediately after pulling out the cast slab from the rectangular casting mold, and the surface temperature of the cast slab is first cooled to a temperature lower than After the temperature of Ar ₃ transformation point, reheat to the temperature exceeding Ar ₃ transformation point, and then when straightening the slab, by keeping the surface temperature of the slab at a temperature lower than the Ar ₃ transformation point for a time, and casting The minimum temperature reached by the surface temperature of the slab is set to an appropriate range, and the solidified structure at a depth of at least 2 mm from the surface of the casting slab is formed into a mixed structure of ferritic iron and bleed iron with unclear grain boundaries of the Wostian iron. [Prior Art Literature] [Patent Literature]

[Patent Document 1] Japanese Patent Laid-Open No. 2002-307149

(The problem that the invention intends to solve)

However, the above-mentioned conventional technique has the following problems. That is, the technique of the spray slitting stops spraying from the shower near the corner of the slab to prevent the temperature of the corner from being lowered. However, in response to various demands in recent years, the widths of the cast slabs are also diversified, and there is a problem that a huge investment in equipment is required in order to properly perform spray slitting for all sizes of slab corners. In addition, if the casting speed becomes slow, since the corner portion of the slab is cooled from both the long side and the short side of the billet, it is easy to overcool. In addition, since the residence time in the continuous casting machine is increased, even if the cooling spray is not sprayed, there is a problem that the corner temperature is lowered due to radiative cooling.

Furthermore, in the technique described in Patent Document 1, after spraying from the secondary cooling shower to the slab, there is a concern that it will be affected by dripping water that flows down the slab. In particular, when the casting speed becomes slow, the cooling of the slab surface is affected by dripping water, so it may be difficult to quantitatively control the slab surface temperature, for example, by heat transfer calculation.

The present invention has been made in view of such circumstances, and an object of the present invention is to propose a continuous casting method capable of reliably suppressing cracks on the surface of the slab that cannot be sufficiently resolved when the slab temperature is controlled only by the conventional secondary cooling, and in particular Manufacture of high-quality steel billets without corner cracks. (Technical means to solve problems)

The inventors of the present invention discovered that, by using a casting mold having a casting space of an appropriate shape, by suppressing the temperature drop of the corner portion of the cast slab during secondary cooling, cracks on the slab surface can be suppressed, and completed the present invention. .

The continuous casting method of the present invention, which contributes to solving the above-mentioned problems, is a method of continuously casting steel, characterized in that a casting mold having a chamfered shape of the corner portion of the casting mold satisfying the following formula (1) is used. The average secondary cooling water density corrected from directly below the casting mold to the lower part is set to 20~60L/(min・m ² ); 0.09≦C/L≦0.20 ・・・(1) Among them, C: amount of corner chamfering (mm), L: the length of the short side of the cast piece (mm).

In addition, the continuous casting method of the present invention is based on the composition of the above-mentioned steel, in terms of mass %, C: 0.05 to 0.25% and Mn: 1.0 to 4.0%, and optionally from Nb: 0.01 to 0.1%, V: One or more selected from 0.01 to 0.1% and Mo: 0.01 to 0.1% can be considered as a better solution. (Compared to the efficacy of the prior art)

According to the present invention, since the temperature of the corner portion of the slab is controlled by the secondary cooling in the case of using the casting mold in which the casting space of the appropriate shape is divided, the corner cracking of the continuous casting slab can be prevented, and high Quality steel billet.

A continuous casting method for steel (a method for producing a steel sheet) according to an embodiment of the present invention includes the step of supporting a cast sheet drawn out from a continuous casting mold while being supported by a plurality of pairs of rolls facing each other. casting steps. First, the molten steel is initially cooled by a casting mold. Then, the cast piece is drawn out from the casting mold at a predetermined drawing speed, and the cast piece is subjected to secondary cooling while being supported by a plurality of pairs of rolls arranged in the casting direction to obtain a steel sheet. For example, in the case of a curved continuous casting machine, there are one pair or a plurality of pairs of rolls for correcting the curved cast slab near the exit side, and the rolls are used to correct the curvature and draw the sheet in the horizontal direction. In this case, in order not to induce surface cracks at the corners of the slab during correction, it is important to use a casting mold with a casting space of an appropriate shape, and to use a casting mold from directly below the casting mold to the point where the bending is corrected (lower correction). ) on the cooling belt through an appropriate cooling mode. The continuous casting machine used in the present embodiment is not particularly limited as long as it includes the premise of bending or correcting the bending recovery from directly below the casting mold to the unloading of the slab.

Here, the present inventors observed surface cracks with respect to the slab cast by the curved continuous casting machine. The surface cracks of the slab are concentrated in the upper corner and its vicinity. This phenomenon is due to the tensile stress that occurs during the recovery of the rectified bending. In addition, the "slab upper surface side" refers to the inner side of the bending of the bending belt of the bending type continuous casting machine, that is, the long side surface side which becomes the upper surface in the horizontal belt.

When the cracked portion is etched, the crack propagates along the grain boundaries of the old Worthian iron, so it is considered that the cracking occurs in the temperature range (generally referred to as the "embrittlement temperature") where the fat iron metamorphism starts from the Worthian iron. Therefore, experiments with various changes were carried out for the secondary cooling conditions.

That is, from the results of experiments using heat transfer analysis under various secondary cooling conditions, it was found that when the secondary cooling spray near the corner of the slab is injected between directly below the casting mold and entering the lower (bending) straightening portion. The average water density of the shower is controlled to be less than 20L/(min·m ² ), and the surface temperature before bending correction is controlled so as not to be below the Ar ₃ point, so that the cracks in the corners of the cast slab can be reduced.

However, as described above, since the temperature of the corner portion of the slab is more likely to drop than that of the surroundings, it is necessary to greatly reduce the amount of cooling spray, resulting in insufficient cooling of the surface of the slab other than the corner portion. Following this phenomenon, the slab bulge (the phenomenon that the slab bulges between the backup rolls due to the static pressing of molten steel) will occur due to insufficient thickness of the solidified shell, and cracks will occur inside the solidified shell.

The reason is that the inventors of the present invention paid attention to the shape of the slab. Conventionally, the slab is rectangular and the corners are cooled from both sides, so the corners of the slab are prone to overcooling. It was considered whether the cooling structure was changed due to the change of the shape of the slab and the overcooling could not be suppressed, and the appropriate shape of the slab was examined by thermal stress analysis.

As a result of the review by thermal stress analysis, it was found that the overcooling of the corners of the slab can be reduced by setting the slab into a chamfered shape in which the corners of the four corners of the rectangular cross-section perpendicular to the casting direction are removed. Further, the stress load can be reduced. However, in order to form the four corners of the slab into a chamfered shape, it is important to remove the four corners (right-angled parts) belonging to the rectangular casting space like the rectangular cross-section casting mold into a right-angled triangle shape to form a chamfered shape, and then use The obtained mold was subjected to casting. Hereinafter, a mold having a casting space in which such a chamfered shape is formed is also referred to as a "chamfering mold".

In order to fully understand the shape of the mold chamfer suitable for the purpose of the present invention, as a result of intensive research, it was found that the following shape specification is required. The chamfered portion 4 of the chamfering mold is a plan view of the chamfering mold shown in FIG. 1 . When chamfering is performed to remove the right-angled portions of the corners of the rectangular casting space into a right-angled triangle shape, the right-angled triangle is based on the ratio b/a of the length b of the short side 3 of the mold to the length a of the long side 2 of the mold It is specified that thermal analysis is performed with respect to the influence of the ratio b/a on the subcooling of the corner portion of the slab. The calculation results are shown in Figure 2 after normalizing the temperature of the rectangular casting mold (b=a=0 in Figure 1) to 750 before chamfering. Here, the a system is fixed in the range of 2 to 20 mm, and the b system is fixed at 20 mm for investigation. The temperature of the corner portion of the cast piece of the chamfering mold was set to the lowest temperature between two points of the corner formed by the chamfering. As shown in FIG. 2 , first, it was found that by forming the chamfered mold, the temperature of the corner portion of the slab was higher than that of the rectangular mold. In particular, when the ratio b/a=1, the temperature of the corner portion of the slab becomes the maximum. In this embodiment, the chamfering amount C (=a=b) is set according to the condition of b/a=1 to obtain the maximum effect, and the continuous casting mold 1 is designed.

This embodiment is as described above, and is preferably applied to steels with high embrittlement susceptibility from Worth iron to ferrite. For example, it can be preferably applied to the composition of steel, in terms of mass %, containing C: 0.05~0.25% and Mn: 1.0~4.0%, and optionally containing from Nb: 0.01~0.1%, V: 0.01~0.1% and Mo: One or more selected from 0.01 to 0.1%. Hereinafter, unless otherwise stated, "mass %" is only described as "%" unless otherwise stated.

C: 0.05~0.25% When the C content is 0.05 to 0.25%, the iron grains in the Vostian area are easily coarsened. Therefore, the present embodiment is preferably applied to a steel composition with high embrittlement sensitivity and a C content of 0.05 to 0.25%.

Mn: 1.0~4.0% When the Mn content is less than 1.0%, MnS, which is an embrittlement factor, is not easily generated, so it does not pose a problem. When more than 1.0%, the embrittlement sensitivity increases, but when more than 4.0%, the product becomes excessively high strength, which is not expected. Therefore, the present embodiment is preferably applied to a steel composition with high embrittlement sensitivity and a Mn content of 1.0 to 4.0%.

One or more selected from Nb: 0.01 to 0.1%, V: 0.01 to 0.1%, and Mo: 0.01 to 0.1% Nb, V, and Mo are elements that contribute to the improvement of the strength of steel, and when their contents are less than 0.01%, nitrocarbides, which are embrittlement factors, are not easily formed, so they do not pose a problem. On the other hand, if it exceeds 0.1%, the price of the alloy will increase, the cost will increase, and the excess performance will become more than necessary. Therefore, adding more than 0.1% is not desirable. [Example]

(Example 1) Using a bending type continuous casting machine, casting was carried out to contain C: 0.18%, Si: 1.4%, Mn: 2.8%, P: 0.020% or less, S: 0.003% or less, and Ti: 0.020% steel of a given composition. The Ar ₃ transformation point of this steel is 805°C. The casting conditions were in the range of a casting thickness of 220 mm, a casting width of 1000 to 1600 mm, and a casting speed of 1.20 to 1.80 m/min. In addition, the slab temperature at the time of bending (lower part correction) was confirmed by measuring using a thermocouple or a radiation thermometer. After casting, in order to easily observe the cracks on the surface of the slab, bead blasting is used to remove the oxides on the surface of the slab, and then a coloring inspection (dye penetrant inspection test) is performed to investigate whether the corners of the slab are present or not. cracked. On the other hand, the corner cracking occurrence rate was evaluated by the number of corner cracked slabs/the number of investigated slabs×100%. For the investigation of internal cracks, a cross-sectional sample perpendicular to the casting direction of the slab was cut, and after milling and finishing, macroscopic etching was performed with warm hydrochloric acid. The presence or absence of internal cracks was checked from the photo etched by the macroscopic view.

First, an investigation is conducted to determine the chamfering dimension (chamfering amount) C [mm] which is effective. Here, the average secondary cooling water volume density corrected from directly below to the lower part of the casting mold at the corner of the slab was fixed at 60 L/(min·m ² ). The results are shown in Table 1. If the length of the short side of the slab is set to L [mm], in the case of Test Nos. 1 and 2 where C/L is less than 0.09, the distances from the long side and the short side are almost the same as those of the rectangular corners, and it is almost impossible to Gain the effect of suppressing overcooling. On the other hand, in the case of Test Nos. 8 and 9 in which the C/L was greater than 0.20, two-sided cooling occurred at the connection between the chamfered portion and the short side, or the chamfered portion and the long side, resulting in the formation of the corner portion of the slab. temperature decreases. That is, it was found that the amount of chamfering of the chamfering mold must be set in the range of 0.09≦C/L≦0.20.

[Table 1] No. Thickness L Chamfer C C/L Secondary cooling water density corner temperature Corner Cracking Incidence Whether there are internal cracks Remark [mm] [mm] [-] [L/(min・m ² )] [°C] [%] 1 220 15 0.068 60 780 0.7 none Comparative example 2 220 18 0.082 60 800 0.4 none Comparative example 3 220 20 0.091 60 825 0 none Invention example 4 220 25 0.114 60 880 0 none Invention example 5 220 30 0.136 60 850 0 none Invention example 6 220 40 0.182 60 820 0 none Invention example 7 220 42 0.191 60 810 0 none Invention example 8 220 45 0.205 60 765 1.5 none Comparative example 9 220 50 0.227 60 763 1.6 none Comparative example

(Example 2) Next, tests were carried out under the same steel type and continuous casting conditions as in Example 1 to determine the average secondary cooling water density at the corners of the cast slab, and the ratios of corner cracks and internal cracks until passing through the bent portion (lower part correction). relation. The results are shown in Table 2.

In the case of rectangular molds (test Nos. 10 to 16), it was found that the corner temperature can be increased by setting the average secondary cooling water density to less than 20 L/(min·m ² ) (test Nos. 10 and 11). Ar ₃ or more, reducing corner cracking. However, it is not possible to gradually cool only the corners, so that the solidified shell thickness near the corners is insufficient, and internal cracks may occur due to bulge. From this, it was found that a general rectangular casting mold cannot achieve both the suppression of corner cracks and the suppression of internal cracks. In addition, when using the chamfering molds other than the present embodiment (Test Nos. 17 to 23), as shown in Example 1, the effect of suppressing the corner supercooling was almost not obtained. When the secondary cooling water density is reduced to less than 20L/(min·m ² ), corner cracks cannot be suppressed, and internal cracks caused by bulge cannot be avoided. When the chamfering mold of the present embodiment (Test Nos. 24 to 31) was applied, internal cracks occurred similarly because it was less than 20 L/(min·m ² ) (Test Nos. 24 and 25). On the other hand, by the effect of changing the shape of the slab, in the average secondary cooling water density range (test No. 24 to 30) of 60 L/(min·m ² ) or less, the overcooling of the corner portion of the slab is suppressed , it can prevent corner cracking. That is, by setting the average secondary cooling water volume density corrected from directly below to the lower part of the casting mold at the corner portion in the range of 20 to 60 L/(min·m ² ) (test No. 26 to 30), it was possible to manufacture A slab that can suppress both corner cracking and internal cracking.

[Table 2] No. Thickness L Chamfer C C/L Secondary cooling water density corner temperature Corner Cracking Incidence Whether there are internal cracks Remark [mm] [mm] [-] [L/(min・m ² )] [°C] [%] 10 220 0 0 10 820 0 have Comparative example 11 220 0 0 15 805 0.2 have Comparative example 12 220 0 0 20 800 0.4 none Comparative example 13 220 0 0 30 790 0.9 none Comparative example 14 220 0 0 40 780 1.2 none Comparative example 15 220 0 0 50 770 1.1 none Comparative example 16 220 0 0 60 740 1.8 none Comparative example 17 220 18 0.082 10 830 0 have Comparative example 18 220 18 0.082 15 810 0.1 have Comparative example 19 220 18 0.082 20 805 0.3 none Comparative example 20 220 18 0.082 30 795 0.8 none Comparative example twenty one 220 18 0.082 40 790 1 none Comparative example twenty two 220 18 0.082 50 785 1.3 none Comparative example twenty three 220 18 0.082 60 780 1.2 none Comparative example twenty four 220 20 0.091 10 980 0 have Comparative example 25 220 20 0.091 15 940 0 have Comparative example 26 220 20 0.091 20 920 0 none Invention example 27 220 20 0.091 30 900 0 none Invention example 28 220 20 0.091 40 870 0 none Invention example 29 220 20 0.091 50 840 0 none Invention example 30 220 20 0.091 60 810 0 none Invention example 31 220 20 0.091 65 800 0.5 none Comparative example

1: Continuous casting mold 2: Long side 3: Short side 4: Chamfered part

FIG. 1 is a schematic plan view showing a casting mold according to an embodiment of the present invention. Fig. 2 is a graph showing the effect of the shape of the chamfer on the temperature at the corner of the slab.

Claims (2)

A continuous casting method, which is a method for continuously casting steel, characterized in that a casting mold whose chamfered shape of the corner portion of the casting mold satisfies the following formula (1) is used, and a casting mold is corrected from directly below to the lower part of the casting mold at the corner portion of the casting sheet. The average secondary cooling water density is set to 20~60L/(min・m ² ); 0.09≦C/L≦0.20 ・・・(1) Among them, C: corner chamfering amount (mm), L: short slab Side length (mm). The continuous casting method according to claim 1, wherein the composition of the steel is based on mass %, contains C: 0.05-0.25% and Mn: 1.0-4.0%, and optionally contains from Nb: 0.01-0.1%, V : 0.01 to 0.1% and Mo: 1 or more selected from 0.01 to 0.1%.

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