TWI784570B - continuous casting method - Google Patents

Abstract

The continuous casting method proposed by the present invention can reliably suppress cracks on the surface of cast slabs, and can especially produce high-quality steel blanks without corner cracks. The continuous casting method of the present invention is a method for continuously casting steel, wherein the chamfered shape of the corners of the casting mold satisfies 0.09≦C/L≦0.20 (wherein, C: corner chamfering amount (mm), L: casting The relationship between the length of the short side of the slab (mm)), the average secondary cooling water density from the corner of the slab to the bottom of the mold is set to 20~60L/(min·m ² ). In particular, the composition of steel is preferably composed of C: 0.05-0.25% and Mn: 1.0-4.0%, and optionally contains Nb: 0.01-0.1%, V: 0.01-0.1%, and Mo : 1 or more types selected from 0.01 to 0.1%.

Description

continuous casting method

The present invention relates to a method for continuous casting of steel capable of suppressing surface cracking of slabs during continuous casting.

In recent years, the requirements and specifications of high-tensile steel tend to be stricter. For the purpose of improving the mechanical properties of the steel plate, the content of alloying elements such as Cu, Ni, Nb, V, and Ti has been increased. When such an alloy steel is cast using, for example, a vertical bending continuous casting machine, the four corners of the rectangular section perpendicular to the casting direction of the cast slab (hereinafter also referred to as "casting") will be formed in the straightening and bending parts of the slab. The corners of the sheet) are loaded with stress, resulting in surface cracks, especially in the corners of the cast sheet. These corner cracks are likely to cause surface flaws on the thick steel plate, which cause a decrease in the yield of steel plate products.

That is to say, the casting sheet of alloy steel has a temperature near the Ar ₃ transformation point where the solidification structure transforms from the Wostian iron phase to the fertile iron phase, and the hot ductility is significantly reduced.

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

In order to keep the surface temperature of the cast slab at a high temperature, the spray piping near the corner of the cast slab is generally closed, and the spray width cutting 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 slab is started immediately after the slab is pulled out from the rectangular mold, and the surface temperature of the slab is cooled to below After the temperature of Ar ₃ transformation point, reheat to a temperature exceeding Ar ₃ transformation point, and then when rectifying the cast sheet, by keeping the surface temperature of the cast sheet at a temperature lower than the Ar ₃ transformation point for the time, and the cast The minimum temperature reached by the sheet surface temperature is set in an appropriate range, and the solidification structure at least 2mm deep from the cast sheet surface is formed to form a mixed structure of ferrite and pleite with unclear grain boundaries of waustian iron. [Prior Art Document] [Patent Document]

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

(Problem to be solved by the invention)

However, the above conventional technology has the following problems. That is, the technique of spraying the width cut is to stop the spraying from the shower near the corner of the slab to prevent the decrease of the temperature of the corner. However, in response to various needs in recent years, the width of cast slabs has also diversified, and there is a problem that a large investment in equipment is required in order to properly spray the corners of cast slabs of all sizes. In addition, if the casting speed is slowed down, the corners of the slab are cooled from both the long side and the short side of the slab, so it is easy to overcool. In addition, since the residence time in the continuous casting machine increases, there is also a problem that the corner temperature decreases due to radiative cooling even if the cooling spray is not sprayed.

Furthermore, in the technique described in Patent Document 1, after the secondary cooling spray is sprayed toward the slab, there is a possibility of being affected by dripping water flowing down the slab. In particular, when the casting speed becomes slow, dripping water affects the cooling of the surface of the slab, so it may be difficult to quantitatively control the surface temperature of the slab, for example, by heat transfer calculation.

The present invention has been made in view of this fact, and its object is to propose a continuous casting method that can reliably suppress cracks on the surface of the slab that cannot be sufficiently resolved by conventionally controlling the temperature of the slab only by secondary cooling. Manufactures high-quality billets without corner cracks. (technical means to solve the problem)

The inventors of the present invention have found that cracking of the surface of the slab can be suppressed by suppressing the temperature drop at the corner of the slab during secondary cooling by using a mold having a casting space of an appropriate shape, and thus completed the present invention. .

The continuous casting method of the present invention, which is advantageous for solving the above-mentioned problems, is a method for continuously casting steel, and is characterized in that a mold whose corner chamfer shape satisfies the following formula (1) is used, The average secondary cooling water density from directly below the casting mold to the lower part of the correction 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 sheet (mm).

In addition, the continuous casting method of the present invention is the composition of the above-mentioned steel, which contains C: 0.05-0.25% and Mn: 1.0-4.0%, and optionally contains Nb: 0.01-0.1%, V: 0.01~0.1% and Mo: 0.01~0.1% can be considered as a better solution. (compared to the effect of previous technology)

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

The continuous casting method of steel (manufacturing method of steel sheet) according to one embodiment of the present invention includes: the cast sheet drawn from the continuous casting mold is carried out under the condition of using a plurality of pairs of rollers facing each other to support it. Casting steps. First, the molten steel is first cooled using a casting mold. Then, the cast piece is pulled out from the casting mold at a predetermined pulling speed, and the cast piece is subjected to secondary cooling under the support of a plurality of pairs of rollers arranged in the casting direction to obtain a steel piece. For example, in the case of a bending type continuous casting machine, one or more pairs of rolls for correcting a bent slab exist near the exit side, and these rolls are used for bending correction and drawing in the horizontal direction. At this time, in order not to induce surface cracks at the corners of the slab during straightening, it is important to use a mold that has a casting space of an appropriate shape, and to use a mold that has a casting space of an appropriate shape, and to correct the bending recovery point from directly below the mold (lower straightening) ) through the appropriate cooling pattern on the cooling belt. The continuous casting machine used in this embodiment is not particularly limited as long as it includes bending or correction of bending recovery from directly below the casting mold to unloading of the slab.

Here, the inventors of the present invention observed surface cracks in slabs cast by a curved continuous casting machine. The surface cracks of the cast sheet concentrated on the upper corner and its vicinity. This phenomenon is caused by the tensile stress that occurs when the bending is corrected and restored. In addition, "the top side of the cast slab" refers to the inside of the curve of the bending belt of the bending type continuous casting machine, that is, the side of the long side that becomes the upper surface of the horizontal belt.

If the cracks are etched, the cracks will propagate along the grain boundaries of the old Worthite iron, so it is believed that the cracks will occur in the temperature range (generally called "brittle temperature") where the fertile iron metamorphosis begins from the Worthite iron. Therefore, experiments with various changes were carried out for the secondary cooling conditions.

That is, as a result of experiments using heat transfer analysis under various secondary cooling conditions, it was found that if the secondary cooling spray near the corner of the cast sheet The average water density of showering is controlled to less than 20L/(min·m ² ), and the surface temperature before bending correction is controlled so that it will not fall below the Ar ₃ point, so that cracks at the corners of the cast slab can be reduced.

However, as mentioned above, since the temperature at the corner of the slab is easier to drop than the surrounding area, the amount of cooling spray must be greatly reduced, resulting in insufficient cooling of the surface of the slab other than the corner. Along with this phenomenon, slab swelling (a phenomenon in which the cast slab bulges between support rolls due to static pressure of molten steel) occurs due to insufficient thickness of the solidified shell, and cracks occur inside the solidified shell.

The reason is that the present inventors paid attention to the shape of the slab. It is known that the cast slab is rectangular and the corners are cooled from two sides, so overcooling of the corners of the cast slab is prone to occur. Considering whether the change of the cooling structure due to the change of the shape of the slab could not suppress the supercooling, the appropriate shape of the slab was checked by thermal stress analysis.

As a result of examination by thermal stress analysis, it was found that the overcooling of the corners of the cast slab can be reduced by making the cast slab into a chamfered shape without the corners of the four corners of the rectangular cross section perpendicular to the casting direction. Furthermore, the stress load can be reduced. However, in order to chamfer the four corners of the slab, it is important to remove the four corners (right-angled parts) of the casting space belonging to the rectangle like the rectangular cross-section mold to form a right-angled triangle to form a chamfered shape, and then use Casting is performed on the obtained mold. Hereinafter, a mold having a casting space in which such a chamfered shape is formed is also referred to as a "chamfered mold".

In order to fully understand the mold chamfer shape suitable for the purpose of the present invention, as a result of intensive research, it was found that the following shape regulation is required. The chamfering part 4 of the chamfering mold is a plan view of the chamfering mold shown in FIG. 1 . When performing chamfering that removes the right-angled parts of each corner 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 sides of the mold to the length a of the long side 2 sides of the mold It is stipulated that thermal analysis is performed on the effect of the ratio b/a on the supercooling of the corner of the slab. The calculation results are shown in Figure 2 after normalizing the temperature of the rectangular mold (b=a=0 in Figure 1) at 750 before chamfering. Here, a is fixed at the range of 2~20mm, and b is fixed at 20mm for investigation. The corner temperature of the slab in the chamfering mold is set to the lowest temperature between two points of the corner formed by the chamfering. As shown in Fig. 2, firstly, it is known that by forming a chamfered casting mold, the temperature at the corner of the cast sheet is higher than that of the rectangular casting mold. In particular, when the ratio b/a=1, the temperature at the corner of the slab becomes the maximum. In this embodiment, the chamfer 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 sensitivity, ranging from wasted iron to ferrite. For example, the composition that can be preferably applied to steel contains C: 0.05~0.25% and Mn: 1.0~4.0%, and optionally contains Nb: 0.01~0.1%, V: 0.01~0.1% and Mo: one or more of 0.01 to 0.1%. Hereinafter, "mass %" is simply expressed as "%" unless otherwise stated.

C: 0.05~0.25% When the C content is 0.05~0.25%, the iron grains of Wostian are easy to coarsen. Therefore, this embodiment is preferably applied to a steel composition with high embrittlement sensitivity and a C content of 0.05-0.25%.

Mn: 1.0~4.0% When the Mn content is less than 1.0%, MnS, which is an embrittlement factor, is not easily formed, so there is no problem. When the content is more than 1.0%, the embrittlement sensitivity will increase, but if it exceeds 4.0%, the product will become too high in strength, so it is not satisfactory. Therefore, this embodiment is preferably applied to a steel composition with a high embrittlement sensitivity and a Mn content of 1.0 to 4.0%.

One or more selected from Nb: 0.01~0.1%, V: 0.01~0.1%, and Mo: 0.01~0.1% Nb, V, and Mo are elements that contribute to the improvement of the strength of steel. When the content of each of them is less than 0.01%, nitrogen carbides that are embrittlement factors are difficult to form, so they do not pose a problem. On the other hand, if it exceeds 0.1%, the price of the alloy becomes high, the cost increases, and the excess performance becomes more than necessary, so adding more than 0.1% is not desirable. [Example]

(Example 1) Using a curved continuous casting machine, casting has, in terms of mass%, C: 0.18%, Si: 1.4%, Mn: 2.8%, P: 0.020% or less, S: 0.003% or less, and Ti: Steel with a given composition of 0.020%. The Ar ₃ transformation point of this steel is 805°C. The casting conditions are in the range of casting thickness 220mm, casting width 1000~1600mm and casting speed 1.20~1.80m/min. In addition, the slab temperature at the time of passing the bend (lower part straightening) is confirmed by measuring with a thermocouple or a radiation thermometer. After casting, in order to easily observe the cracks on the surface of the cast piece, it is convenient to use bead blasting to remove the oxides on the surface of the cast piece, and then perform a coloring inspection (dye penetration flaw detection test) to investigate whether there are cracks in the corners of the cast piece. Cracked. On the other hand, the incidence rate of corner cracks was evaluated by the number of corner cracked slabs/the number of investigated slabs×100%. The investigation of internal cracks is to cut a cross-sectional sample perpendicular to the casting direction of the cast slab, perform milling and finishing, and perform macroscopic etching with warm hydrochloric acid. Check for internal cracks from macro-etched photographs.

First, conduct research to determine the size of the chamfer size (chamfer amount) C [mm] that exerts the effect. Here, the average secondary cooling water volume density from directly below the casting mold at the corner of the slab to the lower rectification was fixed at 60L/(min·m ² ). The results are shown in Table 1. If the length of the short side of the cast piece is set to L [mm], when the C/L is less than 0.09 in the case of test No. 1 and 2, the distance from the long side and short side is almost the same as the corner of the rectangle, and it is almost impossible to Gain the effect of suppressing supercooling. On the other hand, in the case of Test Nos. 8 and 9 where the C/L was greater than 0.20, two-sided cooling occurred at the connection between the chamfer and the short side, or the chamfer and the long side, resulting in cracking at the corner of the slab. The temperature is lowered. That is, it is found that the chamfering amount 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 Incidence of corner cracking 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, a test was 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 corner of the cast slab and the ratio of corner cracks and internal cracks when passing through the bend (lower part straightening). relation. The results are shown in Table 2.

In the case of rectangular molds (Test No.10~16), it was found that by setting the average secondary cooling water density to less than 20L/(min·m ² ) (Test No.10 and 11), the corner temperature can be lowered. Up to Ar ₃ or more, reducing corner cracks. However, it is impossible to only gradually cool the corners, so the thickness of the solidified shell near the corners is insufficient, and internal cracks will occur due to swelling. From this, it can be seen that the general rectangular casting mold cannot suppress corner cracks and internal cracks at the same time. Also, when using chamfered molds (Test Nos. 17 to 23) other than this embodiment, as shown in Example 1, there is almost no effect of suppressing corner supercooling, so similar to the rectangular mold, if the average If the density of the secondary cooling water is lowered to less than 20L/(min·m ² ), cracks at the corners cannot be suppressed, and internal cracks caused by swelling cannot be avoided. When the chamfered molds (Test Nos. 24 to 31) of this embodiment were applied, similarly, internal cracks occurred due to less than 20 L/(min·m ² ) (Test Nos. 24 and 25). On the other hand, through the effect of changing the shape of the slab, the supercooling of the corner of the slab is suppressed within the average secondary cooling water density range (Test No. 24~30) below 60L/(min·m ² ) , can prevent corner cracking. That is, by setting the average secondary cooling water volume density from directly below the corner part of the mold to the lower part of the correction in the range of 20~60L/(min·m ² ) (Test No. 26~30), it is possible to manufacture A cast sheet that suppresses both corner cracks and internal cracks.

[Table 2] No. ThicknessL Chamfer C C/L Secondary cooling water density Corner temperature Incidence of corner cracking 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: Chamfering

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 corner temperature of the slab.

Claims (2)

A continuous casting method, which is a continuous casting method for steel, is characterized in that the chamfered shape of the corner of the mold satisfies the following formula (1), and the corners of the casting mold are corrected from the bottom to the bottom of the casting mold. 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: casting length Side length (mm). Such as the continuous casting method of claim 1, wherein the composition of the above-mentioned steel contains C: 0.05-0.25% and Mn: 1.0-4.0%, and optionally contains Nb: 0.01-0.1%, V : 0.01~0.1% and Mo: 0.01~0.1% at least one selected.

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