JP2005224852A - Continuous casting method of high-titanium-contained steel - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a continuous casting method of high-titanium-containing steel which makes a cast steel slab free from any surface defect and enables the high-titanium-containing steel slab to be rolled without care. <P>SOLUTION: In the continuous casting of high-titanium-containing steel containing 0.1-1.0 mass% Ti, the angle θ of the delivery hole of an immersion nozzle 1 is within a range between 2 and 8 degrees upward, the immersion depth H of the immersion nozzle is within a range between 80 and 130 mm, and the consumption of molding powder 11 is within a range between 0.3-0.6 kg/m×min. These casting conditions prevent a longitudinal crack and a slag residue, and can produce the cast steel slab excellent in surface properties which can be rolled without care. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a continuous casting method of a high Ti-containing steel containing 0.1 to 1.0% by mass of Ti, and more specifically, a high Ti-containing steel capable of stably casting a slab having few surface defects. The present invention relates to a continuous casting method.

A high Ti-containing steel containing 0.1 to 1.0% by mass of Ti, known as a representative of precipitation-type wear-resistant steel, oxidizes Ti in the molten steel due to the inevitable oxidation reaction during continuous casting. TiO ₂ is generated. This TiO ₂ is concentrated in the mold powder added on the molten steel surface in the mold (hereinafter referred to as “meniscus”) to precipitate a high melting point compound (CaO · TiO ₂ ), and the viscosity and melting point of the mold powder. Change the characteristics such as. For this reason, the amount of mold powder flowing into the mold and the solidified shell (hereinafter referred to as “powder consumption”) decreases, causing vertical cracks on the surface of the slab, and high melting point compounds are solidified shells. In other words, so-called roaring occurs and the quality becomes abnormal.

  In addition, titanium carbide, titanium nitride, and titanium carbonitride generated by the reaction of Ti, C, and N in the molten steel are absorbed into the mold powder, and promote the generation of deckle (also referred to as “skinning”). Due to the generation of deckle, the temperature of the meniscus is lowered, resulting in insufficient hatching of the mold powder. This also becomes a cause of roar and vertical cracks, leading to quality abnormalities.

  In the worst case, not only the quality abnormalities, but in the worst case, the mold and the solidified shell are seized due to insufficient powder consumption, causing operational accidents such as breakout. In order to solve these problems and stably cast a high Ti-containing steel, various countermeasures have been conventionally implemented.

  For example, in Patent Document 1, when the mold is vibrated with a sine curve with the slab drawing speed in the range of 0.8 to 1.2 m / min, the mold vibration stroke S is in the range of 2 to 5 mm, and the mold vibration is A method of continuously casting a high Ti-containing steel under vibration conditions satisfying the number f of 120 cycles / min or more and satisfying the mold frequency f × the mold vibration stroke S ≦ 600 is disclosed. According to Patent Document 1, since the vibration condition is a low stroke and a high frequency, even if a mold powder generally used for casting of ordinary steel is used, the oscillation mark is made shallow and It is said that surface defects such as can be prevented.

Patent Document 2 discloses a method of continuously casting a high Ti-containing steel with the discharge nozzle angle θ of the immersion nozzle being in the range of 2 to 8 degrees upward and the immersion depth H of the immersion nozzle being 80 to 130 mm. Has been. According to Patent Document 2, by casting in this way, the molten steel in the meniscus is constantly renewed, and the heat that is the melting heat of the mold powder is always supplied. Therefore, even mold powder that has absorbed TiO ₂ is consumed. The amount can be increased and surface defects of the slab are prevented.
JP 7-251251 A Japanese Patent Laid-Open No. 10-314892

  The continuous cast slab is desirably supplied to the next hot rolling step without care of the surface of the slab in order to improve yield and save energy. However, in order to supply without maintenance, it is necessary to prevent all surface defects such as vertical cracks of the slab and blades.

  As a result of studying the continuous casting method for making the high Ti-containing steel unmanaged, the present inventors have found that the above-mentioned prior art has the following problems. That is, in the technique of Patent Document 1, the oscillation mark depth is shallow and effective for lateral cracking, but the mold vibration condition is low stroke and high frequency, so the powder consumption is reduced, and vertical cracking occurs. And scabs may occur and cannot be stably maintained. Further, in the technique of Patent Document 2, only the control of the discharge hole angle and the immersion depth of the immersion nozzle is effective, and this is extremely effective for preventing the generation of deckle, but this is not sufficient, and at a certain frequency. It was found that vertical cracks and scabs occurred.

  As described above, it is difficult to say that the conventional method is sufficiently effective in preventing the surface defects of the slab for keeping the high Ti-containing steel unmaintained, and there is much room for improvement.

  The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a continuous casting method of high Ti-containing steel that can prevent surface defects of a slab and can be rolled without maintenance. It is.

  The inventors of the present invention have conducted intensive studies and studies to solve the above problems. The following describes the results of research and examination.

  From the investigation results of the present inventors, it has been found that surface defects that hinder the maintenance of high Ti-containing steel are represented by noro-kami and vertical cracks. Therefore, as a means for solving the above-mentioned problems, a continuous casting method capable of simultaneously preventing a wolf and a vertical crack was examined.

In the case of continuous casting of high Ti-containing steel, the mold powder absorbs TiO ₂ that floats from the molten steel, so high-melting point CaO · TiO ₂ is generated in the mold powder. The characteristic changes. Further, since the mold powder absorbs the generated titanium carbide, titanium nitride, and titanium carbonitride, deckle is generated in the meniscus using these compounds as nuclei. In order to prevent stagnation and to prevent vertical cracks, it is necessary to melt the produced high melting point CaO.TiO ₂ and at the same time suppress the production of the deckle or melt the produced deckle. For this purpose, the molten steel in the meniscus must be constantly renewed, and heat that is the melting heat of the mold powder must be constantly supplied.

  Therefore, in continuous casting of high Ti content steel, it is necessary to supply more discharge flow from the immersion nozzle to the meniscus than in the case of normal carbon steel. However, if the supply amount is increased too much and the molten steel flow velocity of the meniscus is increased too much, vortex and hot water will be generated in the meniscus, and unmelted mold powder will be involved, and on the contrary, mold powder-like scab will be generated. Therefore, the discharge flow supplied to the meniscus must be in an appropriate range.

  Based on the results of various tests, the present inventors have appropriately controlled the discharge hole angle θ of the immersion nozzle and the immersion depth H of the immersion nozzle, so that the molten steel can be produced without causing vortex or runaway in the meniscus. It was found that the surface defects of the high Ti-containing steel can be improved as a result.

  That is, when the discharge hole angle θ is in the range of 2 to 8 degrees upward and the immersion depth H is in the range of 80 to 130 mm, the molten metal surface fluctuation in the meniscus is small and the molten steel is renewed smoothly. It was found that longitudinal cracks can be suppressed simultaneously. In addition, the discharge hole angle θ of the immersion nozzle is represented by an angle formed by the direction of the discharge hole and the horizontal line, and when the direction of the discharge hole is above the horizontal, it is defined as upward, and when it is below the horizontal, it is defined as downward. The immersion depth H of the immersion nozzle is defined by the distance between the upper end position of the discharge hole of the immersion nozzle and the meniscus.

  When the discharge hole angle θ exceeds 8 degrees upward, the discharge flow that flows from the discharge hole toward the meniscus increases, and vortex and hot water fluctuation occur in the meniscus, resulting in severe fluctuations in the molten metal surface. Due to the fluctuation of the molten metal surface, unmelted mold powder is forcibly wound into the molten steel, and is trapped by the solidified shell and generates nox. In this case, when the immersion depth H is increased and deepened, fluctuations in the molten metal surface of the meniscus are suppressed and the swarf is reduced. However, the molten steel is not sufficiently renewed and the melting of the mold powder is prevented, and the powder consumption is reduced. As a result, vertical cracks increase. Thus, it has been found that when the discharge hole angle θ exceeds 8 degrees upward, even if the immersion depth of the immersion nozzle is changed, it is not possible to simultaneously prevent the throat and the vertical crack.

  On the other hand, when the discharge hole angle θ is less than 2 degrees upward, the discharge flow that flows in the horizontal direction or downward from the horizontal increases, and renewal of the molten steel due to the discharge flow in the meniscus decreases. Therefore, the molten steel temperature of the meniscus is lowered to inhibit the melting of the mold powder, the powder consumption is reduced, and vertical cracks are generated. In this case, when the immersion depth H is made shallow and the molten steel is renewed in the meniscus, the vertical cracks are reduced, but the molten metal surface fluctuation of the meniscus becomes intense, the mold powder is caught and the scab is increased. Thus, when the discharge hole angle θ is less than 2 degrees upward, it has been found that even if the immersion depth H of the immersion nozzle is changed, it is not possible to simultaneously prevent the throat and the vertical crack.

  Further, when the immersion depth H of the immersion nozzle is less than 80 mm, since the immersion depth H is too shallow, a fluctuation of the molten metal surface appears in the meniscus, and noxia is generated. Conversely, the immersion depth H of the immersion nozzle exceeds 130 mm. And, since the immersion depth H was too deep, it was found that the renewal of the molten steel in the meniscus was hindered and vertical cracking occurred.

However, in the case of continuous casting of high Ti-containing steel, the mold powder absorbs TiO ₂ that floats from the molten steel, so the properties of the mold powder, such as melting point and viscosity, change considerably. As a result of further research by the present inventors, when the characteristics of the mold powder deviate from the range of the predetermined characteristics by absorbing the floating TiO ₂ , the molten steel flow of the meniscus is as described above. It was found that vertical cracks and blades were generated even when the control was performed.

Therefore, an examination was made as to how much the amount of TiO ₂ picked up into the mold powder can be suppressed without excessively degrading the properties of the mold powder. Here, assuming that the mass of TiO ₂ floating from the molten steel is constant per unit mass of the molten steel, attention was paid to the amount of powder consumed as a factor for determining the amount of TiO ₂ picked up. This is because if the powder consumption increases, the renewal of the molten mold powder in contact with the molten steel is promoted, and the amount of TiO ₂ pick-up decreases. Conversely, if the powder consumption decreases, the mold powder This is based on the fact that the update is delayed and the amount of TiO ₂ pickup increases.

FIG. 1 shows the results of investigating the relationship between the powder consumption and the amount of TiO ₂ picked up into the mold powder. As shown in FIG. 1, a strong correlation was observed between the amount of powder consumed and the amount of TiO ₂ picked up, and it was found that the amount of TiO ₂ picked up decreased as the amount of powder consumed increased.

On the other hand, from the test results comparing the amount of TiO ₂ picked up into the mold powder and the change in the characteristics of the mold powder, when the TiO ₂ pickup index on the vertical axis shown in FIG. Was found to change rapidly. The pickup index of TiO ₂ becomes 0.4 or less when the powder consumption is 0.3 kg / m · min or more, as is apparent from FIG.

From these results, by securing a powder consumption of 0.3 kg / m · min or more, the pickup index of TiO ₂ is suppressed to 0.4 or less, and the characteristic change of the mold powder is suppressed to the minimum. It has been found that casting can be performed without excessively degrading the properties of the powder.

  On the other hand, when the powder consumption exceeded 0.6 kg / m · min, it was found that the amount of flow increased so much that the mold powder flowed unevenly and vertical cracks occurred. That is, it has been found that it is necessary to cast under conditions where the powder consumption is 0.3 to 0.6 kg / m · min. Specifically, the mold powder to be used is selected so that the powder consumption is 0.3 to 0.6 kg / m · min, or the casting conditions such as the slab drawing speed and mold vibration are selected. It is to cast. Of course, both may be selected. In addition, the powder consumption in this invention is shown with the mass of the mold powder which flows in per minute in the contact surface per 1 m length of a solidification shell and a mold wall surface.

  The present invention has been made on the basis of the above research and examination results, and the continuous casting method for high Ti content steel according to the present invention is a high Ti content steel containing 0.1 to 1.0% by mass of Ti. In the continuous casting method, the discharge nozzle angle θ of the immersion nozzle is in the range of 2 to 8 degrees upward, the immersion depth H of the immersion nozzle is in the range of 80 to 130 mm, and the consumption of the mold powder is Casting is performed under conditions that are within a range of 0.3 to 0.6 kg / m · min.

  According to the present invention, in continuous casting of high Ti-containing steel, the discharge hole angle θ of the immersion nozzle and the immersion depth H of the immersion nozzle are controlled to an optimal range, and at the same time, the powder consumption is controlled within a predetermined range. Therefore, a sound slab with very little rotting and vertical cracks can be obtained, and hot rolling can be performed without maintenance. As a result, an improvement in product yield, an improvement in energy saving, and the like are achieved, and an industrially beneficial effect is brought about.

  Hereinafter, the present invention will be described in detail with reference to the accompanying drawings. FIG. 2 is a schematic diagram of a front cross section of a mold part of a slab continuous casting machine embodying the present invention.

  In FIG. 2, a tundish 6 is disposed above a mold 3 composed of opposed mold long sides 4 and opposed mold short sides 5 housed inside the mold long sides 4. An upper nozzle 13 is disposed at the bottom of the tundish 6, and a sliding nozzle 7 including a fixed plate 14, a sliding plate 15, and a rectifying nozzle 16 is disposed in contact with the lower surface of the upper nozzle 13. Further, the sliding nozzle An immersion nozzle 1 having a pair of discharge holes 2 in the lower part is disposed in contact with the lower surface of 7, and a molten steel outflow hole 17 from the tundish 6 to the mold 3 is formed. The molten steel 8 injected into the tundish 6 from the ladle (not shown) was provided at the lower part of the immersion nozzle 1 via the molten steel outflow hole 17 and immersed in the molten steel 8 in the mold 3. From the discharge hole 2, the discharge flow 12 is injected into the mold 3 toward the mold short side 5. The molten steel 8 is cooled in the mold 3 to form a solidified shell 9, and is continuously drawn below the mold 3 to form a cast piece. In FIG. 2, the discharge flow 12 is shown only on one side.

An Ar gas introduction pipe (not shown) is connected to the upper nozzle 13, and Ar gas is supplied to the inner wall of the upper nozzle 13 in order to prevent adhesion of TiO ₂ and Al ₂ O ₃ to the inner wall of the immersion nozzle 1. The molten steel outflow hole 17 is blown through the fine pores. Mold powder 11 is added on the meniscus 10 in the mold 3.

  The immersion nozzle 1 has a circular or elliptical outer shape, and a concave hot water reservoir is formed on the bottom of the inner surface of the immersion nozzle 1. One discharge hole 2 is provided on each of the left and right with the center of the immersion nozzle 1 symmetrical, and the cross-sectional shape of the discharge hole 2 may be circular, elliptical, or square. Then, the discharge hole angle θ of the immersion nozzle 1 is set to an arbitrary value within the range of 2 degrees upward to 8 degrees upward. However, from the stability of the slab surface properties, the discharge hole angle θ of the immersion nozzle 1 is preferably about 5 degrees upward (4 to 6 degrees), which is the median value of the limited range.

  Then, casting is performed with the immersion depth H set to an arbitrary value in the range of 80 mm to 130 mm. If the immersion depth H is set to 80 mm or 130 mm, which is the boundary value of the limited range, the position of the meniscus 10 may change due to disturbance during casting, and the immersion depth H may be outside the limited range. This immersion depth H is also preferably set to the median side of the limited range of 100 mm to 110 mm in order to stably prevent surface defects. Here, in the present invention, the discharge hole angle θ of the submerged nozzle 1 is represented by an angle formed by the direction of the discharge hole 2 and a horizontal line, upward when the direction of the discharge hole 2 is above the horizontal, and below the horizontal. The immersion depth H of the immersion nozzle 1 is defined by the distance between the upper end position of the discharge hole 2 of the immersion nozzle 1 and the meniscus 10.

  Moreover, the mold powder and casting conditions from which powder consumption becomes arbitrary values in the range of 0.3 to 0.6 kg / m · min are selected, and casting is performed under the conditions. In this case, from the viewpoint of the stability of the slab surface properties, it is preferable to cast under conditions where the powder consumption is 0.45 to 0.50 kg / m · min. The amount of powder consumption is determined by the characteristics such as the viscosity of the mold powder 11 to be used, the slab drawing speed and the vibration conditions of the mold 3. It is preferable to conduct a test in which the vibration conditions are changed, grasp the powder consumption based on these casting conditions and mold powder characteristics, and select the mold powder 11 to be used according to the casting conditions. The mold powder 11 to be used may be one for high Ti content steel or ordinary carbon steel, but in order to stably prevent surface defects, high Ti content steel mold powder is preferable. .

The slab drawing speed is 0.7 to 1.8 m / min, and the slab shape slab having a slab thickness of 150 to 250 mm and a slab width of 650 to 2300 mm is an object. Moreover, the thickness of the immersion nozzle 1 may be a dimension of 10 mm to 50 mm, and the cross-sectional area of the discharge hole 2 may be ²⁰ to 120 cm ² .

Since the molten steel 8 in the meniscus 10 is constantly renewed by continuously casting the high Ti-containing steel containing 0.1 to 1.0% by mass of Ti in this way, the generation of deckle is prevented and the mold is prevented. Since heat as melting heat of the powder 11 is always supplied, melting of the mold powder 11 is promoted, and furthermore, the mold powder 11 in contact with the molten steel 8 is updated by controlling the powder consumption within a predetermined range. Since the casting can be performed while minimizing the characteristic change of the mold powder 11 caused by the pickup of TiO ₂ , it is possible to obtain a sound slab with extremely little slag and vertical cracks, and hot rolling without maintenance. It becomes possible to do. As a result, improvement in product yield and energy saving can be achieved.

The example which implemented this invention using the continuous casting machine of the structure shown in FIG. 2 is demonstrated below. Using a slab continuous casting machine having a slab cross-sectional thickness of 250 mm and a width of 1500 mm, a high Ti-containing steel having a Ti content of 0.4 mass% was cast at a slab drawing speed of 0.8 m / min. . The immersion nozzle used had an inner diameter of 80 mm, a wall thickness of 30 mm, a discharge hole cross-sectional area of 50 cm ² , a concave hot water reservoir with a depth of 15 mm was provided at the bottom, and the discharge hole angle θ was 5 degrees upward. Then, the immersion depth H was set to 100 mm, a mold powder for high Ti-containing steel was used, and Ar gas was blown into the immersion nozzle at a flow rate of 9 Nl / min for casting. As the mold powder used, a mold powder whose powder consumption was confirmed to be 0.4 to 0.5 kg / m · min in advance was used. For comparison, the discharge hole angle θ and the immersion depth H are the same as those of the present invention example, but the casting using the mold powder whose powder consumption is outside the limited range of the present invention (comparative example) is also included. Carried out.

  The surface of the obtained slab was examined by a penetrant testing method, and the length and number distribution of longitudinal cracks were measured. Moreover, after investigating the vertical crack by the penetration testing method, the slab surface was scarfed with a thickness of 2 mm, and then the scarf surface was examined by the penetration testing method, and the size and number distribution of the scab were measured. The surface properties of the slab were comprehensively evaluated including vertical cracks and blades.

  Table 1 summarizes the casting conditions and the evaluation results of the surface properties of the slabs of the present invention and Comparative Examples 1 to 3. In the column of slab surface property evaluation in Table 1, ○ mark indicates a surface state in which there are very few vertical cracks and scratches and it can be hot-rolled without maintenance, and × marks indicate vertical cracks or cracks that occur. This indicates that the surface condition requires surface care.

  As is apparent from Table 1, in the present invention example, both vertical cracks and blades were extremely small, and a slab having a good surface state could be cast. On the other hand, in Comparative Examples 1 to 3, vertical cracks or blades occurred, and it was confirmed that hot rolling without maintenance cannot be performed.

Is a diagram showing the relationship between TiO ₂ of the pickup of the powder consumption and in the mold powder. It is a schematic diagram of the front cross section of the casting_mold | template part of the slab continuous casting machine to which this invention is applied.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Immersion nozzle 2 Discharge hole 3 Mold 4 Mold long side 5 Mold short side 6 Tundish 7 Sliding nozzle 8 Molten steel 9 Solidified shell 10 Meniscus 11 Mold powder 12 Discharge flow 13 Upper nozzle 14 Fixed plate 15 Sliding plate 16 Rectification nozzle 17 Molten steel Outflow hole

Claims (1)

  In the continuous casting method of high Ti-containing steel containing 0.1 to 1.0% by mass of Ti, the immersion nozzle discharge hole angle θ is set in the range of 2 to 8 degrees upward, and the immersion nozzle immersion depth Continuous casting of high Ti-containing steel, characterized in that H is cast in a range of 80 to 130 mm and mold powder consumption is in a range of 0.3 to 0.6 kg / m · min. Casting method.

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