JP3395749B2 - Steel continuous casting method - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a continuous casting method of steel capable of preventing clogging of an immersion nozzle and obtaining a slab of good internal quality. [0002] Molten steel continuously cast is usually deoxidized with Al or the like. The molten steel in the ladle is once injected into the tundish and then into the mold via the immersion nozzle. [0003] When casting molten steel deoxidized with Al, the immersion nozzle may be clogged. In extreme cases, it is difficult to continue the casting operation. The reason why the immersion nozzle is closed is that Al oxide or the like in the molten steel adheres to and deposits on the inner surface of the immersion nozzle. [0004] When the immersion nozzle is closed, a lance is inserted into the vicinity of the immersion nozzle from above the tundish containing molten steel, and oxygen gas is blown into the lance, thereby causing Al oxide or the like adhering to the inner surface of the immersion nozzle. Is performed. However, in this method, since oxygen is blown into the molten steel, an oxide is generated in the molten steel, so that nonmetallic inclusions remaining in the slab increase. Therefore, a cast piece with good internal quality cannot be obtained. As a method for preventing the clogging of the immersion nozzle,
Inert gas, such as Ar gas, is introduced into the molten steel passing through the immersion nozzle from the upper nozzle (the part to which the immersion nozzle is attached) arranged at the lower part of the tundish, or the sliding gate that controls the supply of molten steel to the immersion nozzle. The method of blowing is adopted. However, since the blown Ar gas becomes bubbles in the molten steel, the bubbles are trapped in the solidified shell, and bubble defects occur under the surface of the slab, or in extreme cases, the slab is used as a material. Surface defects may occur on the hot-rolled product surface. As a method for preventing clogging of the immersion nozzle without blowing Ar gas or the like into the molten steel passing through the immersion nozzle, a method of containing Ca in the molten steel has been adopted. For example, in Japanese Patent Application Laid-Open No. 9-192799, A
A method has been proposed in which 1 to 5 ppm of Ca is contained in molten steel deoxidized by 1. This method is a method for preventing the formation of agglomerates of Al oxides having a high melting point in molten steel by the addition of Ca, thereby preventing clogging of a dipping nozzle. In Japanese Patent Application Laid-Open No. 11-90597, when the degree of superheat of molten steel in a tundish becomes 20 ° C. or less, Ca is added to the molten steel in the tundish,
A method has been proposed in which the Ca content in molten steel is adjusted to 10 to 50 ppm. Superheat degree of molten steel in tundish is 20 ℃
It is stated below that the metal that adheres to the inner surface of the immersion nozzle when the temperature of the molten steel becomes low is dissolved by adding Ca. However, even if the methods proposed in JP-A-9-192799 and JP-A-11-90597 are used, depending on conditions such as the degree of superheat of the molten steel in the tundish and the casting speed, the molten steel cannot be used. Even if Ca is contained, the immersion nozzle may be blocked. SUMMARY OF THE INVENTION The present invention is directed to a steel capable of preventing clogging of an immersion nozzle when casting molten steel deoxidized with Al or the like and obtaining a cast piece of good internal quality. It is an object of the present invention to provide a continuous casting method. [0010] The gist of the present invention is a continuous casting method in which molten steel containing Al is injected from a tundish into a mold using a submerged nozzle.
Is contained in a range of 0.001 to 0.005% by mass and cast under a condition satisfying the following formula (A). [0011] Here, Q: the amount of molten steel passing through one immersion nozzle per unit time (t / min) ΔT: 10 to 4 degrees of superheat of molten steel in a tundish
A value in the range of 5 ° C. α: Ca content, a value in the range of 0.001 to 0.005% by mass The degree of superheat of molten steel in the tundish is the temperature of the molten steel in the tundish and the melting point of the steel to be cast. Means the temperature of the difference. The present inventors have solved the above-mentioned problem as follows. That is, according to the Ca content contained in the molten steel and the degree of superheat ΔT of the molten steel in the tundish, by setting the amount of molten steel Q passing through the immersion nozzle to an appropriate range, the oxides of Al and the like in the molten steel are immersed. The blockage of the immersion nozzle caused by the adhesion to the inner surface of the nozzle is prevented. The blockage of the immersion nozzle is caused by the fact that oxides of Al and the like in the molten steel adhere to and deposit on the inner surface of the immersion nozzle. The likelihood of the occurrence of the blockage depends on the flow rate of the molten steel passing through the immersion nozzle. However, the inner diameter of a commonly used immersion nozzle is about 70 to 90 mm, and in such an immersion nozzle, the likelihood of occurrence of blockage of the immersion nozzle depends on the amount of molten steel Q passing through the immersion nozzle. No problem. Therefore, in the method of the present invention,
An appropriate range of the amount of molten steel Q passing through the immersion nozzle was selected. FIG. 1 is a diagram showing the effects of the Ca content in molten steel, the degree of superheat of molten steel in a tundish, and the amount of molten steel passing through the immersion nozzle on the blockage of the immersion nozzle and the quality of the slab. Of the two curves shown in FIG. 1, the upper curve shows the following equation (B), and the lower curve shows the following equation (C). Further, of the two straight lines indicated by broken lines, the left straight line shows the following formula (D), and the right straight line shows the following formula (E). Q ₁ = −0.6 × ln (ΔT × α) +3.0 (B) Q ₂ = −0.6 × ln (ΔT × α) -0.5 (C) ΔT × α = 0. 01... (D) ΔT × α = 0.225 (E) FIG. 1 shows C deoxidized using a vertical bending type continuous casting machine having a vertical length of 1 m and a machine length of 20 m and a single strand. It is a summary of test results obtained by casting low-carbon steel having a content of 0.05% by mass at a speed of 0.3 to 10 m / min. The slab size is 100 mm in thickness and 1000 mm in width. The conditions of the immersion nozzle used in this test were two discharge holes,
The size of each discharge hole is 100 mm long and 30 m wide
At m, the orientation of the nozzle is 45 ° downward. The inner diameter of the part of the immersion nozzle through which the molten steel passes is 80 mm. The Ca content is 0.0008 to 0.005.
2% by mass, superheat degree of molten steel in tundish is 5-50 ° C
Is the condition. The condition of the clogging of the immersion nozzle was evaluated by determining the degree of clogging of the immersion nozzle described below and the internal quality of the slab by obtaining the number of nonmetallic inclusions under the surface of the slab by the method described below. As shown in FIG. 1, the area surrounded by the curves shown by the above-mentioned equations (B) and (C) and the straight line shown by the equations (D) and (E), that is, the above-mentioned equation (A) In the area that satisfies the condition of, the clogging of the immersion nozzle does not occur,
In addition, it was found that a slab of good quality was obtained. In other areas, clogging of the immersion nozzle, increase of nonmetallic inclusions in the slab, melting of refractory such as the immersion nozzle, breakout, etc. occurred. In the region satisfying the above condition (A), the Ca content in the molten steel is 0.0010 to 0.0050.
If it is mass%, when casting the molten steel deoxidized with Al, the oxide of Al in the molten steel causing the clogging of the immersion nozzle,
May be lower CaO-Al ₂ O ₃ composite oxide of melting point. The degree of superheat of molten steel in the tundish is 1
0-45 ° C, quantity of molten steel Q passing through immersion nozzle per unit time
When the value of (t / min) is in an appropriate range, it is apparent that adhesion of the oxide of Al in the molten steel, the complex oxide of the CaO-Al ₂ O ₃ system, etc. on the inner surface of the immersion nozzle can be prevented. The present invention has been completed based on the above results. DETAILED DESCRIPTION OF THE INVENTION The method of the present invention is directed to a steel containing Al and 0.001 to 0.005% by mass of Ca. By adding Ca to the molten steel, the oxide of Al in the molten steel, which causes blockage of the immersion nozzle,
May be lower CaO-Al ₂ O ₃ composite oxide of melting point. In order to obtain such an effect of Ca, Ca
The content is set to 0.0010% by mass or more. However, Ca
When the content exceeds 0.0050% by mass, the refractory of the tundish itself, the upper nozzle, the sliding gate, the immersion nozzle, and the like are easily melted. Further, since CaS or the like is easily formed, non-metallic inclusions in the cast piece may increase. In the method of the present invention, the degree of superheat of molten steel in the tundish is set to 10 to 45 ° C. If the temperature is lower than 10 ° C., the temperature of the molten steel is too low, so that the molten steel solidifies from the tundish in the immersion nozzle, and the nozzle may become slightly clogged. That is, casting may not be performed at the target casting speed. In extreme cases, the immersion nozzle will block during casting. In order to secure the casting speed, a process of inserting a lance into the molten steel from above the tundish and blowing oxygen gas is adopted, but oxides are generated in the molten steel and nonmetallic inclusions in the slab increase. Therefore, the cleanliness of the steel deteriorates. On the other hand, when the degree of superheat exceeds 45 ° C., particularly when the casting speed is high, the solidified shell in the mold tends to be re-dissolved by the discharge flow of the immersion nozzle, so that breakout may occur. In the method of the present invention, a commonly used nozzle may be used as the immersion nozzle. For example, an immersion nozzle having two ejection holes is used, with the ejection holes having a downward angle of about 10 to 30 °. The diameter of the inside of the immersion nozzle through which the molten steel passes may be about 70 to 90 mm. The material of the immersion nozzle may be ordinary alumina graphite or the like. In the method of the present invention, casting is performed under the condition that the amount of molten steel Q (t / min) passing through the immersion nozzle per unit time satisfies the above-mentioned formula (A). The reason will be described with reference to FIG. With immersion nozzles having different inner diameters, the flow speed of molten steel passing through the immersion nozzle differs even if the amount of molten steel passes through the immersion nozzle in the same unit time. FIG. 1 shows a slab having a thickness of 100 mm and a width of 1000 mm as described above and an inner diameter of 80 mm.
It is the result of casting using an immersion nozzle of mm. However, prior casting tests have shown that
With an immersion nozzle having an inner diameter of about 70 to 90 mm, it was confirmed that the occurrence of blockage of the immersion nozzle can be predicted by the amount of molten steel passing through the immersion nozzle in a unit time. In the region surrounded by the curve shown by the formula (C), the straight line shown by the formula (D), the straight line shown by the formula (E), and the horizontal axis, the Ca content and the tan Even if the degree of superheat of the molten steel in the dish is within an appropriate range, the amount of molten steel Q (t / min) passing through the immersion nozzle per unit time is too small, so that the immersion nozzle is likely to be blocked. When oxygen gas is blown into the molten steel near the inner surface of the immersion nozzle to eliminate the blockage of the immersion nozzle, the cleanliness of the molten steel deteriorates,
The quality of the slab under the skin deteriorates. The area shown in the figure surrounded by the curve shown by the equation (C), the straight line shown by the equation (D), and the vertical axis of the figure,
In the region surrounded by the curves shown by the formulas (B) and (C), the straight line shown by the formula (D), and the vertical axis of the drawing, the Ca content is too low, or , The degree of superheat of molten steel in the tundish is too low. Therefore, when oxygen gas is blown into the molten steel near the inner surface of the immersion nozzle, the cleanliness of the molten steel deteriorates, and the quality under the surface of the slab deteriorates, when the oxygen gas is blown into the vicinity of the inner surface of the immersion nozzle in order to easily block the immersion nozzle. Become. In the region surrounded by the curve shown by the expression (B), the straight line shown by the expression (D), and the vertical axis of the figure, the region indicated by the symbol のIn addition to the fact that blockage is likely to occur and the quality of the surface of the slab under the skin deteriorates, the amount of molten steel that passes through the immersion nozzle per unit time is too large, so the solidified shell in the mold causes the discharge flow of the immersion nozzle. May cause re-dissolution, and in extreme cases, breakout occurs. In the region surrounded by the curve shown by the formula (B) and the straight lines shown by the formulas (D) and (E), the Ca content and the degree of superheat of the molten steel in the tundish are appropriate. Even in the above range, the amount of molten steel passing through the immersion nozzle per unit time is too large, so the solidified shell in the mold is easily re-dissolved by the discharge flow of the immersion nozzle, and in extreme cases breakout occurs I do. The area surrounded by the curve shown by the equation (B) and the straight line shown by the equation (E), and the curve shown by the equations (B) and (C) and the area shown by the equation (E) The area shown in the figure surrounded by a straight line, and the curve shown by the equation (C),
In the region surrounded by the straight line represented by the formula (E) and the horizontal axis in the figure, the Ca content is too large. Therefore, the refractory of the tundish, the immersion nozzle, and the like are easily melted or CaS or the like is easily formed, so that the nonmetallic inclusions in the cast slab tend to increase. In addition, since the degree of superheating of the molten steel in the tundish is too high, the solidified shell in the mold is easily re-dissolved by the discharge flow of the immersion nozzle, and in extreme cases, breakout occurs. Therefore, in the method of the present invention, the condition of the region surrounded by the curves shown by the equations (B) and (C) and the straight line shown by the equations (D) and (E), that is, the above-mentioned (A) ) Cast under conditions that satisfy the formula. The procedure for carrying out the method of the present invention is, for example, a method in which a Ca content in molten steel containing Al is 0.001 to 0.001.
It is added so as to be in the range of 0.005% by mass. Then, the Ca content in the molten steel is measured. As a method for analyzing the Ca content in the molten steel, for example, the following method can be used. The molten steel in the ladle is collected with a commonly used paper tube sample or the like and analyzed by ordinary discharge emission spectroscopy. Next, the degree of superheat of the molten steel in the tundish is determined.
From the obtained Ca content in the molten steel and the degree of superheat of the molten steel,
The amount of molten steel Q that passes through one immersion nozzle per unit time Q
However, the casting speed is selected so as to satisfy the above expression (A). This is because if the casting speed is selected, the molten steel amount Q can be obtained. EXAMPLE A low-carbon steel (Al deoxidized steel) having a C content of 0.05% by mass was produced using a vertical bending type continuous casting machine having a vertical portion length of 1 m, a machine length of 20 m and a single strand at a speed of 1. 1-7.1m
At a rate of / min. To a slab having a thickness of 100 mm and a width of 1000 mm. There are two discharge holes, and the size of each discharge hole is
An immersion nozzle having a length of 100 mm and a width of 30 mm and a downward angle of 45 ° was used. The inner diameter of the immersion nozzle is 80 mm. Approximately 150 tons of molten steel in one heat was cast continuously for 5 heats. 0.0008 to 0.0052% by mass of Ca
And cast while changing the degree of superheat of molten steel in the tundish within the range of 5 to 50 ° C, the degree of clogging of the immersion nozzle, the occurrence of nonmetallic inclusions under the surface of the slab, and The quality status of product coils made from these slabs was investigated. The degree of blockage of the immersion nozzle was investigated as follows. First, the inner diameter of the immersion nozzle was measured before casting.
Next, the immersion nozzle was recovered after the casting was completed, and the nozzle was cut at the center in the height direction of the immersion nozzle, and the inner diameter was measured.
When there was a deposit inside the immersion nozzle and the inside shape was not circular, the long diameter and the short diameter were determined, and the average diameter was defined as the inner diameter. Therefore, the value obtained by dividing the inner diameter of the immersion nozzle after casting by the inner diameter before casting was defined as the immersion nozzle closing degree. Therefore,
If the immersion nozzle occlusion degree is less than 1.0, it is occluded,
If it exceeds 1.0, it means that the inside of the immersion nozzle is melted. The occurrence of non-metallic inclusions under the slab surface is as follows:
The measurement was performed as follows. A slab sample having a length of 200 mm in the casting direction was cut out from the obtained slab, and the slab skin was 1 m in length.
m was removed, and the number of nonmetallic inclusions having a size of 50 μm or more was measured on a polished surface 1 mm from the surface using a 100-fold optical microscope, and the number generated per 1 cm ² was obtained. Using the obtained cast slab as a raw material, a steel strip having a thickness of 2.5 mm was manufactured by hot rolling and wound around a coil. After pickling this product coil, the occurrence of surface flaws was investigated. If the product coil cannot be used as a product coil even after the surface of the product coil has been cleaned, it is determined that the product coil has been downgraded. A value obtained by dividing the product coil having the downgraded by the total number of product coils obtained by hot rolling the cast pieces obtained in the target test was defined as a product coil downgrade index. Table 1 shows test conditions and test results. [Table 1] Test No. of the present invention example. No. 1 to No. In No. 5, the casting was performed at a casting speed that resulted in the amount of molten steel passing through the immersion nozzle within the range of the conditions specified in the present invention. In these tests, the degree of clogging of the immersion nozzle was 0.85 to 1.00, and there was a slight amount of deposits inside the immersion nozzle, but this was not a problem.
The number of non-metallic inclusions under the slab surface is 1 to 3 / c
m ² was a cast of good quality. Therefore, the product coil made of the slab was not degraded, and a good quality product coil was obtained. Test No. of Comparative Example In No. 6, the test was performed at a value of 0.8 t / min, which is smaller than the lower limit of the molten steel amount condition specified in the present invention. The degree of clogging of the immersion nozzle was 0.35, and the clogging was considerably advanced. Oxygen gas was blown into molten steel near the inner surface of the immersion nozzle in order to secure the casting speed during casting and continue casting. As a result, oxides in the molten steel were generated, and 14 nonmetallic inclusions under the surface of the cast slab resulted in a cast of considerably poor quality. The degraded index of the product coil using the slab was 0.3, which was bad. Test No. of Comparative Example In No. 7, the test was performed with a value of the molten steel amount of 4.5 t / min which was larger than the range of the molten steel amount specified in the present invention. The immersion nozzle clogging degree was 0.97, which was no problem. However, a breakout occurred during the third heat casting. This is because the casting speed was increased and the amount of molten steel passing through the immersion nozzle was increased, so that the solidified shell in the mold was re-melted by the discharge flow. Test No. of Comparative Example In No. 8, the test was performed at a low superheat degree of 5 ° C. of the molten steel outside the range specified in the present invention. The degree of clogging of the immersion nozzle was 0.25, and the clogging was remarkably advanced. Oxygen gas was frequently blown into the molten steel near the inner surface of the immersion nozzle to secure the casting speed during casting. As a result, oxides in the molten steel were remarkably generated, and 26 nonmetallic inclusions under the surface of the slab resulted in a slab of extremely poor quality.
The degraded index of the product coil using the slab is 0.6
Was remarkably bad. Test No. of Comparative Example In No. 9, the test was performed at 50 ° C. which was higher than the degree of superheat of the molten steel specified in the present invention. A breakout occurred during the first heat casting. The degree of clogging of the immersion nozzle was 1.00, and the inner diameter remained at the initial value. Test No. of Comparative Example In the test No. 10, the Ca content was reduced to 0.0008% by mass outside the range defined by the present invention. The effect of Ca content is small, the immersion nozzle is slightly closed, and the degree of immersion nozzle closure after casting is 0.
65. Oxygen gas was blown into the vicinity of the inner surface of the immersion nozzle in the molten steel to secure the casting speed during casting. As a result, oxides in the molten steel were generated, and the non-metallic inclusions under the surface of the slab were 9 pieces, resulting in a slab of poor quality. The degraded index of the product coil using the slab as a material was also poor at 0.2. Test No. of Comparative Example In No. 11, the test was performed by increasing the Ca content to 0.0052% by mass outside the range defined by the present invention. The immersion nozzle was slightly melted, and the degree of clogging of the immersion nozzle after casting was 1.40. By applying the method of the present invention, it is possible to prevent clogging of the immersion nozzle when casting molten steel deoxidized with Al.
Good quality slabs can be obtained.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram showing the effects of the Ca content, the degree of superheat of molten steel in a tundish, and the amount of molten steel passing through an immersion nozzle, on the clogging of the immersion nozzle and the quality of slab.

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-7-116792 (JP, A) JP-A-11-90597 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) B22D 11/108 B22D 11/10 B22D 11/18

Claims (1)

(57) Claims 1. A continuous casting method in which molten steel containing Al is injected from a tundish into a mold by using an immersion nozzle, wherein 0.001 to 0 Ca is contained in the molten steel. A continuous casting method for steel, characterized by containing 0.005% by mass and casting under conditions satisfying the following formula (A). −0.5−0.6 × ln (ΔT × α) ≦ Q ≦ 3.0−0.6 × ln (ΔT × α) (A) where Q: amount of molten steel passing through the immersion nozzle per unit time ( t / min) ΔT: 10 to 4 in the degree of superheat of molten steel in the tundish
Value α in the range of 5 ° C .: Ca content, value in the range of 0.001 to 0.005% by mass