JP2002035895A - Continuous casting method of steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a fast continuous casting method capable of reducing surface cracking of a casted slab and avoiding abnormal occurrence of blow holes. SOLUTION: In this continuous casting method in which molten steel is supplied through a nozzle into a mold, s straight nozzle is used for the nozzle. A horizontal torque is given to the molten steel in a mold by electromagnetic agitation and the molten steel containing C of 0.3-2.0 mass % and S of 0.15-3.5 mass % is cast in a 1.0 m/min. or higher casting speed. Equiaxed crystal ratio within 10 mm from the surface of the casting slab is made 30% or higher.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a continuous casting method for steel, and more particularly to a high-speed continuous casting method for high carbon steel.

[0002]

2. Description of the Related Art The following techniques are known as techniques for improving the quality of cast slabs in a continuous casting method.

[0003] Japanese Patent Application No. 11-247096 (not disclosed): Reduces surface cracks by controlling the thickness of a solidified shell in a mold.

Japanese Patent Application Laid-Open No. 57-64458: The casting structure is made to be equiaxed by controlling the temperature at which the molten metal is injected to reduce cracks and prevent breakout.

[0005] JP-A-57-75275: By introducing a coolant into a mold and performing electromagnetic stirring, an equiaxed crystal is formed at the axis of the slab to obtain steel of excellent quality.

JP 9-285855 A: Superheat degree of molten steel
By controlling (superheat) and secondary cooling capacity, the thickness of the columnar crystal layer on the surface layer of the slab is reduced, and as a result, surface cracks are reduced.

Japanese Patent Application Laid-Open No. 2000-61599: An upward force is applied by electromagnetic force to a molten steel discharge flow from an immersion nozzle to obtain excellent surface quality and internal quality.

[0008]

However, each of the above techniques has the following problems. As a method of suppressing surface flaws, it is effective to suppress the unevenness of the thickness of the solidified shell, but it is not perfect.

This is because, even if the thickness of the solidified shell is increased and the unevenness of the shell thickness is suppressed, if the solidified structure of the surface layer is mainly composed of columnar crystals, there is a high possibility that the solidified shell is easily broken between the columnar crystals. .

[0010] Although reducing the superheat of molten steel or introducing a cold material into a mold is one method of preferentially generating equiaxed crystals, the structure of the slab surface is columnar. It is very difficult to change from a crystal to an equiaxed crystal.

In order to preferentially generate equiaxed crystals on the slab surface, it is necessary to sufficiently supply effective crystal nuclei near the meniscus and to increase the degree of supercooling. However, it is difficult to satisfy the above condition only by reducing the degree of superheat of the molten steel flowing from the immersion nozzle.

The mechanism of forming an equiaxed crystal is significantly different between that generated at the center and that generated near the surface. The equiaxed crystal at the center is formed mainly by the agitation of the molten steel, the tip of the columnar crystal growing in the mold being divided into crystal nuclei, and gathering at the center. Therefore, when the temperature of the molten steel near the meniscus is relatively high and the stirring force of the molten steel in the meniscus and the mold is strong, it is easily generated. On the other hand, equiaxed crystals near the surface are generated by trapping in the solidified shell immediately when the crystal nuclei generated by the generation of foreign nuclei on the water-cooled copper wall (mold) at the meniscus portion are released from the wall surfaces. Therefore, it is easily generated when the temperature of the molten steel near the meniscus is low, and when the flow of the molten steel near the meniscus is somewhat small. Therefore, it is very difficult to satisfy such contradictory conditions at the same time.

Applying an upward force to the molten steel flow from the immersion nozzle improves inclusion capture, but increases casting defects such as surface cracks.

The molten steel stream flowing from the dip tube has the highest temperature in the mold, and how the molten steel stream is flowed largely determines the temperature distribution in the mold. Surface cracks are greatly affected by the temperature distribution near the meniscus, that is, substantially by the flow of molten steel. The higher the temperature, the more the columnar crystal structure develops and the easier it is to crack. Therefore, if the molten steel flow is stirred so as to give an upward force, the temperature of the molten steel in the mold tends to be uniform, and the temperature near the meniscus cannot be lowered.

It is difficult to promote the formation of equiaxed crystals on the surface of the slab by electromagnetic stirring.

It is possible to preferentially generate an equiaxed crystal at the center of the slab by electromagnetic stirring of the mold. But,
In order to increase the number of equiaxed crystals effective for suppressing cracks on the surface of the slab, it is difficult only by conventional electromagnetic stirring, and it is actually necessary to control the temperature and flow of molten steel near the meniscus.

Accordingly, it is a primary object of the present invention to provide a continuous casting method capable of reducing surface cracks of a slab and preventing abnormal occurrence of blow holes.

[0018]

The object of the present invention is to increase the degree of supercooling of molten steel by limiting the shape and immersion depth of the nozzle and by combining electromagnetic stirring to obtain an appropriate molten steel flow. To achieve.

That is, the present invention is a continuous casting method for supplying molten steel from a nozzle into a mold, wherein the nozzle is a straight nozzle, and a horizontal rotating force is applied to the molten steel in the mold by electromagnetic stirring to obtain a C. : 0.3 to 2.0% by mass,
Si: characterized in that molten steel containing 0.15 to 3.5 mass% is cast at a casting speed of 1.0 m / min or more.

Here, the use of the straight nozzle is as follows.
This is to reduce the temperature of the molten steel near the meniscus as much as possible to improve surface cracking. The higher the temperature of the molten steel near the meniscus, the more the columnar crystal structure develops and the more likely it is to crack.If a straight nozzle is used, the high-temperature molten steel flow discharged from the nozzle can be directed downward as much as possible. Can be lowered.

The immersion depth of the straight nozzle is 40 mm to 200 m
m is preferred. Below this lower limit, the mold powder may be entangled in the slab. Above the upper limit, blowholes increase.

The stirring of the molten steel in the mold by electromagnetic stirring is performed in a horizontal rotation. If the stirring involves stirring in directions other than the horizontal direction (especially in the vertical direction), the molten steel with a high temperature from the nozzle becomes uniform throughout the mold, and a large degree of supercooling near the meniscus is obtained. Becomes impossible. Conversely, if the rotation is only horizontal,
The degree of supercooling near the meniscus can be kept large. The horizontal rotation refers to a rotation about an axis substantially parallel to the axis of the nozzle (mold). Applying a horizontal rotation to the molten steel can be realized, for example, by disposing an electromagnetic coil capable of generating a rotating magnetic field outside the mold.

The magnetic field of the electromagnetic stirring is preferably not less than 20 gauss and less than 300 gauss in the range of 10 mm cube (10 mm × 10 mm × 10 mm) at the corner of the meniscus portion. Below this lower limit, blow holes near the meniscus tend not to be removed. Conversely, if the upper limit is exceeded, the flow of molten steel in the mold will be excessively uniform, and the degree of supercooling near the meniscus will not be able to be maintained large, and the entrainment of the mold powder will increase, and the surface of the slab will become large. May deteriorate the properties.

C: 0.3 to 2.0% by mass: C is less than 0.3 or 2.0
%, The form of the surface cracks changes, so it is difficult to reduce the surface cracks of the slab and to suppress the blow holes.

Si: 0.15 to 3.5% by mass: The amount of Si greatly affects surface cracking. When the content of Si is less than 0.15%, the problem of surface cracking becomes less significant, and a high-quality slab can be obtained. On the other hand, if it exceeds 3.5%, it is difficult to suppress surface cracks.

The casting speed is at least 1.0 m / min. If the casting speed is less than 1.0 m / min., No problems such as surface cracks and blowholes will occur, so there is no need to use the means of the present invention. The reason is that when the casting speed slows down,
This is due to the fact that the strain rate applied to the slab solidified shell is reduced, the solidified shell growing in the mold is thickened, and the discharge speed of molten steel from the nozzle is reduced, so that blow holes are easily removed.

By combining the above manufacturing conditions, a high quality cast slab is obtained with an equiaxed crystal ratio of 30% or more in a range of 10 mm inward from the slab surface. The equiaxed crystal ratio is
Samples are taken at two points, the corner and the center of the long side of the slab section, and after confirming the cast structure by etching, the area ratio of equiaxed crystals in the observed area is calculated by taking the average of the two points . The area where the area ratio is counted shall be the area from the slab surface to 10 mm.

[0028]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The embodiments of the present invention will be described below, including the circumstances leading up to the present invention. 1. Principle of generation of equiaxed crystals The internal structure of high carbon steel bloom slabs produced by the continuous casting method was investigated. As a result, as shown schematically in FIG. 1, the formation sites of the equiaxed crystals were broadly divided into two types. One is an equiaxed crystal 3A formed subsequently to the columnar crystal 2 near the center of the slab 1, and the other is an equiaxed crystal generated between the chilled crystal 4 and the columnar crystal 2 near the surface.
3B.

In order to elucidate the principle of formation of each of them in detail, as shown in Table 1, in a mold that does not use electromagnetic stirring, the molten steel flow in the mold (and indirectly the temperature distribution of molten steel in the mold) is changed. A test was conducted to determine how biaxial crystals are affected. In the test, the chemical components are as follows: C: 0.55, Si: 1.30, Mn: 0.67, C
r: 0.70, P: 0.009, S: 0.014, Fe: The remaining steel materials were used. The molten steel was supplied to the mold using the straight nozzle shown in FIG. 4A and the two-hole nozzle shown in FIG. 4C (details of the nozzle shape will be described later).

It is expected from this test that when a straight nozzle is used, the molten steel flow takes a long time to reach the vicinity of the meniscus because the molten steel flow has a strong downward flow after the nozzle discharge. This means that the degree of supercooling near the meniscus increases. Here, the degree of subcooling is
When the theoretical liquidus temperature determined by the metal composition is T1, and the temperature of the molten steel at the time of actual solidification is T0, the temperature difference is represented by ΔT = T1−T0.

On the other hand, two holes having two holes on the sides of the nozzle
When a hole nozzle is used, contrary to a straight nozzle,
Since the molten steel flow has a strong upward flow after the nozzle discharge, the molten steel having a high superheat reaches the meniscus, and as a result, the degree of supercooling near the meniscus is reduced. The molten steel flow velocity in the vicinity of the meniscus shown in Table 1 was not measured during the actual test, but was measured using a full-scale water model, and was relatively consistent with the actual value. It is considered to be. As can be seen from this value, it can be seen from the difference in the discharge flow pattern between the straight nozzle and the two-hole nozzle that the straight nozzle has a lower molten steel flow velocity near the meniscus than the two-hole nozzle.

In the test, the equiaxed crystal area ratio at the center and the equiaxed crystal ratio at the surface were measured. The area ratio of the equiaxed crystal at the center is a ratio obtained by dividing the area of the equiaxed crystal at the central portion in the cross section of the slab by the entire cross sectional area. The equiaxed crystal ratio of the surface was determined by a calculation method different from that of the center. That is, from the cross section of the obtained bloom cast slab 1, two points (a square portion surrounded by a broken line) of a corner portion and a long side center portion as shown in FIG.
After a sample is taken in and the casting structure is confirmed by etching, the area ratio of the equiaxed crystal in the observed area is calculated by averaging two places. The area where the area ratio is counted is the area up to 10 mm from the surface. Schematically, as shown in FIG. 3, the dendrites are arranged in random directions and the equiaxed crystals 3 are distinguished from the columnar crystals 2 in which the dendrites are arranged almost parallel to the solidification progression direction, and the equiaxed crystal ratio was determined. . The frequency of blowhole generation is 300 mm in diameter per unit area in the depth region up to 5 mm from the slab surface.
The number of blow holes having a size of μm or more was counted, and this was evaluated by converting the number into blows per 1 cm ² .

[0033]

[Table 1]

As shown in Table 1, equiaxed crystals near the surface are easily formed when the degree of supercooling of the molten steel near the meniscus is large, and particularly when the degree of supercooling near the meniscus is large and the flow rate of the molten steel is small. Can produce 90% or more of equiaxed crystals. On the other hand, the equiaxed crystal in the center is easily formed when the degree of supercooling near the meniscus is small, and particularly when the degree of supercooling near the meniscus is small and the flow rate of molten steel is large, the equiaxed crystal ratio in the center is significantly large. Was found to increase.

It is considered that the difference between these generation mechanisms is caused by the following reasons. The equiaxed crystals near the surface are generated by trapping in the solidified shell immediately when the crystal nuclei generated by the heterogeneous nucleation on the water-cooled copper wall of the meniscus part are released from the wall surfaces. On the other hand, the equiaxed crystal near the center is mainly generated at the center because the tip of the columnar crystal growing in the mold is divided by the stirring of the molten steel to become a crystal nucleus.

One of the most important facts in Table 1 is the effect of the surface cast structure on surface cracking. Surface cracks are reduced when the surface structure is more equiaxed than columnar crystals, and it is thought that surface cracks can be reduced by making the structure itself equiaxed. This is because the dendrite between columnar crystals extends long perpendicular to the surface,
This is because the dendrite growth direction of the equiaxed crystal is random, and when strain is applied between the dendrites, the columnar crystal is more likely to crack.

2. Deterioration of surface properties due to formation of equiaxed crystals (increase of blowholes) It is clear from the above results that surface cracks are significantly reduced by preferentially generating equiaxed crystals near the surface. However, it can also have a negative effect. That is, as shown in Table 1, the blowhole generation rate is increased by increasing the number of equiaxed crystals near the surface. Excessive blowholes deteriorate the surface properties, and this survey also revealed that the final product performance was adversely affected. As is evident from Table 1, the formation of blowholes is greatly affected by the temperature of the molten steel near the meniscus, and the higher the degree of supercooling of the molten steel, the more easily it is generated. Since the surface cracking is reduced as the degree of supercooling of the molten steel increases, it can be seen that the relationship between blowhole generation and surface cracking is just a trade-off relationship.

3. Test with controlled electromagnetic stirring and immersion nozzle shape In order to reduce surface cracking without causing blowholes, other measures must be taken while maintaining the supercooling degree of the molten steel surface large. Must be used at the same time. It has been found that blowholes are removed by stirring molten steel, and performing electromagnetic stirring is one effective means.
However, when electromagnetic stirring is performed in the mold, the temperature of molten steel generally tends to be uniform, and it is difficult to maintain a large degree of supercooling near the meniscus necessary for suppressing surface cracking.

Therefore, the nozzle shape, immersion depth, and the amount of electromagnetic stirring were changed to simultaneously reduce the surface cracks and blowholes by simultaneously achieving a large degree of supercooling near the meniscus and stirring the molten steel near the meniscus. Then, a test was performed using a mass production continuous casting machine. The conditions are shown below.

Mold section: large section; 256 mm × 164 mm, small section; 164 mm × 164 mm Casting speed: 1.5 m / min. Steel type: (mass%) C: 0.80 to 1.10 Si: 1.0 to 1.1 Mn: 0.5 to 0.6 P: 0.015 or less S: 0.015 or less Fe: Remaining

Nozzle shape Straight nozzle 10: As shown in FIG. 4A, a cylindrical nozzle 10 having a uniform thickness over the entire length. Molten steel is discharged downward from the hole 11 at the nozzle end.

Straight nozzle 20: As shown in FIG. 4B, a cylindrical nozzle having a tapered surface 21 whose inner diameter increases toward the opening side. The molten steel is discharged downward from the hole 22 at the nozzle end.

Two-hole nozzle 30: As shown in FIG.
The nozzle 30 has an end sealed and two holes 31 on the side surface of the nozzle. Molten steel is discharged laterally from these two holes 31.

Immersion Depth For straight nozzles 10 and 20: As shown in FIG. 5A, the distance L from the meniscus to the tip of the nozzle is defined as the immersion depth. FIG. 5 shows a state in which molten steel 50 is supplied from the nozzles 10, 20, 30 into the mold 40, and shows that a solidified shell 60 is formed on the inner surface of the mold.

In the case of a two-hole nozzle 30, as shown in FIG. 5B, the distance L from the meniscus to the upper part of the hole is set.

Electromagnetic stirring force Weak: 2 to less than 20 Gauss at the meniscus corner portion Medium: 20 to less than 300 Gauss at the meniscus corner portion Strong: 300 Gauss or more at the meniscus corner portion Except for those with no electromagnetic stirring force of 0 The agitation is such that a horizontal rotational force acts on the molten steel.

Surface crack index The surface crack index was determined as follows. 110mm × 110mm by blooming or forging each bloom of large section and small section
Prepare 20 1t billets with corners and check the largest crack depth for each billet. The following points are assigned according to the depth. Then, a value obtained by adding these scores for 20 pieces is defined as a surface crack index.

[0048] Less than 1mm → 0 1mm or more to less than 2mm → 0.3 2mm or more to less than 3mm → 0.8 3mm or more to less than 4mm → 24mm or more to less than 5mm → 45mm or more → 10

Blow hole generation frequency The blow hole generation frequency is determined by counting the number of blow holes having a diameter of 300 μm or more per unit area in a depth region up to 5 mm from the surface of the slab, and converting this to the number per cm ^2. It was evaluated by conversion.

Table 2 shows the results of tests performed by changing the nozzle shape, the electromagnetic stirring force, and the like. From this table,
By selecting the nozzle shape to be a straight nozzle and setting the electromagnetic stirring force at an appropriate level, it is possible to simultaneously reduce the generation of surface cracks and blowholes.

[0051]

[Table 2]

The ratio of the equiaxed crystal occupied by the surface near the surface (equiaxed crystal ratio of the surface) greatly affects the surface cracking. Has great influence. FIG. 6 shows the relationship between the equiaxed crystal ratio of the slab surface and the billet surface flaws after slab rolling. From this figure, it was found that when the equiaxed crystal ratio of the surface exceeded 30%, surface cracks were significantly reduced. A surface crack index of 2.0 or less is considered good.

In Japanese Patent Application No. 11-247096, it has been shown that by controlling the thickness and unevenness of the solidified shell on the surface, it is possible to prevent surface cracking. The results showed that controlling the microstructure near the surface (columnar and equiaxed) further improved surface cracking.

Although the size of the mold section has some influence on the flow of molten steel and the temperature of molten steel, no remarkable difference was confirmed from the results shown in Table 2. If the immersion depth is smaller than 40 mm, the mold powder may be entangled in some cases, and the lower limit of the immersion depth is preferably set to 40 mm. Conversely, when the immersion depth was increased to 220 mm, an increase in blowholes was confirmed, and it was clear that excessive immersion was not preferable. The immersion depth of the nozzle must be changed in actual operation if continuous casting increases, but as can be seen from Table 2, the immersion depth is 40 to 200
By maintaining the distance between mm, the surface quality can be kept higher.

In the test, the electromagnetic stirring force used three strength ranges in the test, but it was found that there was an appropriate range for maintaining not only surface cracks but also better surface properties. When the strength is low, there is a tendency that the blowhole near the meniscus cannot be completely removed. On the other hand, when the strength is strong, the surface texture is excessively uniform because the flow of molten steel in the mold is excessively uniform, and the degree of supercooling near the meniscus cannot be kept large, and the entrainment of the mold powder is large. May worsen. What can be said from the test results is that it is more preferable to adjust the mold stirring force to an appropriate range (medium strength).

[0056]

As described above, according to the continuous casting method of the present invention, surface cracks can be reduced by controlling the structure of the solidified shell, not the non-uniformity of the solidified shell.

Further, abnormal occurrence of blowholes can be avoided, and extremely high quality cast slabs can be obtained.

[Brief description of the drawings]

FIG. 1 is a structural diagram of a cross section of a slab.

FIG. 2 is an explanatory diagram showing an observation region in a cross section of a slab when obtaining an equiaxed crystal ratio.

FIG. 3 is a structural explanatory view showing an equiaxed crystal and a columnar crystal.

FIG. 4 shows nozzles having different forms, (A) is a vertical nozzle, (B) is a straight nozzle having a tapered surface, and (C) is a longitudinal sectional view of a two-hole nozzle.

5A and 5B are views illustrating the immersion state of a nozzle in molten steel, FIG. 5A is an explanatory view illustrating a immersion depth of a straight nozzle, and FIG.

FIG. 6 is a graph showing the relationship between the equiaxed crystal ratio and surface flaws.

[Explanation of symbols]

 1 Slab 2 Columnar crystal 3, 3A, 3B equiaxed crystal 4 Chill crystal 10, 20 Straight nozzle 11 hole 21 Tapered surface 22 hole 30 2-hole nozzle 31 hole 40 Mold 50 Molten steel 60 Solidified shell

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) B22D 11/20 B22D 11/20 A 27/02 27/02 W 41/50 520 41/50 520

Claims (3)

[Claims] 1. A continuous casting method for supplying molten steel from a nozzle into a mold, wherein the nozzle is a straight nozzle, and a horizontal rotating force is applied to the molten steel in the mold by electromagnetic stirring to obtain C: 0.3 to
Casting speed of molten steel containing 2.0 mass%, Si: 0.15 to 3.5 mass%
A continuous casting method characterized by casting at a rate of 1.0 m / min or more and making the equiaxed crystal ratio in the range of 10 mm inward from the slab surface 30% or more. 2. The immersion depth of the straight nozzle is 40 mm to 20 mm.
The continuous casting method according to claim 1, wherein the diameter is 0 mm. 3. The continuous casting method according to claim 1, wherein the magnetic field of the electromagnetic stirring is at least 20 gauss and less than 300 gauss at the corner of the meniscus.