JP6816523B2 - Continuous steel casting method - Google Patents

Description

The present invention relates to a continuous steel casting method, and specifically, it is possible to improve production efficiency by constantly applying the machine length limit speed without causing bulging after the machine end of the continuous casting machine. Regarding the continuous casting method of steel that can be made.

FIG. 2 is an explanatory diagram showing an outline of a general continuous casting machine 100 that continuously casts steel. Continuous casting of steel will be described with reference to FIG.

Inside a water-cooled mold 102 via a dipping nozzle 101 from a tundish (not shown), which is a molten steel container of a continuous casting machine 100 arranged below a ladle (not shown) having a function of holding and transporting molten steel. While the molten steel is supplied to the steel and moved downstream in the casting direction, the molten steel is solidified by cooling with a cooling device (not shown) such as a water spray from the outside to produce the slab 107. The slab 107 is also referred to as a "steel piece", a "slab", or the like.

In the vicinity of the mold 102 of the continuous casting machine 100, only the surface layer of the slab 107 is solidified (initial solidification), and the solidified layer (solid phase) develops toward the center as it goes downstream in the casting direction along the support roll 103. As a result, the unsolidified molten steel 106 (liquid phase) becomes smaller. After the slab 107 passes through the mold 102, secondary cooling with cooling water or the like is generally performed before the slab 107 goes out of the machine end of the continuous casting machine 100. The slab 107 that has been solidified is adjusted to a predetermined length by the cutting device 108, so that a plurality of slabs are continuously produced.

In continuous casting, it is necessary to complete solidification of the slab 107 at the machine end 110, which is the installation position of the final support roll 105 in the casting direction of the continuous casting machine 100 and the position where the final support roll 105 and the slab 107 are in contact with each other. There is. This is because when the solidification completion position moves from the machine end 110 to the downstream side in the casting direction, bulging occurs in which the slab 107 of the portion not supported by the support roll 103 swells due to the action of the static iron pressure acting on the unsolidified molten steel. However, this is because the internal quality of the slab 107 deteriorates and the continuous casting operation is hindered such as stopping casting.

Further, if the solidification of the slab 107 is not completed at the time of cutting, the unsolidified molten steel may be ejected from the inside of the slab 107, and the damage may be expanded due to the burning of peripheral equipment. Therefore, in general, the casting rate is determined in consideration of the solidification rate according to the cooling capacity of the continuous casting machine.

The "machine length limit speed" means the maximum casting speed at which stable casting is possible, which is determined according to the cooling capacity of the mold 102 and the secondary cooling zone of the continuous casting machine 100 and the arrangement of the support roll 103. It is calculated by the equation (1).
In equation (1), Vc _MAX indicates the machine length limit speed, L indicates the distance (machine length) along the casting path from the molten steel surface of the mold to the machine end, k indicates the solidification coefficient, and T indicates the slab thickness. Shown.

Equation (1) uses the machine length limit speed Vc _MAX as the casting speed of the basic formula (for example, 3rd Edition Steel Handbook II Ironmaking / Steelmaking Maruzen, p. 624, 1979), which expresses the relationship of general continuous casting, and the casting path length. It is obtained by substituting the captain L for each. In the ideal case where the cooling in continuous casting is uniform throughout the slab and there is no operational fluctuation, equation (1) can be applied smoothly.

However, in the actual continuous casting operation, the non-uniform solidification caused by the initial solidification and the secondary cooling remains as it is as the final solidification profile. Further, if an unintended excess or deficiency of cooling of the slab occurs, the expected solidification completion position may not be achieved.

In addition, when the degree of superheat of molten steel or the casting speed fluctuates, it is difficult to accurately control the solidification completion position. Therefore, it is rare that the casting speed in actual casting can be set at the captain's limit speed on the premise of complete solidification with the full captain, and in consideration of safety costs, continuous casting operations such as deterioration of internal quality and casting stoppage are performed. It has been set to be smaller than the captain's limit speed so as to completely solidify on the upstream side of the machine edge so as not to cause hindrance. That is, in reality, in the equation (1), the casting speed is made smaller than the machine length limit speed Vc _MAX in order to make the casting path length until solidification smaller than the machine length L.

On the other hand, in order to increase the production efficiency as much as possible, it is desirable that the casting speed can be constantly cast at the machine length limit speed on the premise that the continuous casting operation is not hindered.

Patent Documents 1 and 2 disclose an estimation of the solidification completion position of a continuously cast slab and a method of adjusting the casting speed and setting and adjusting the secondary cooling according to the estimation result.

Patent Document 3 discloses an invention in which the casting speed is improved by improving the cooling capacity of the roll of the continuous casting machine.

In Patent Document 4, the casting upstream is described by producing a slab from which unsolidified molten steel is separated at the solidification completion point by creating a mechanical solidification completion point by intermittently pressing during continuous casting. Since the static pressure from the side is not transmitted to the downstream side of the casting from the solidification completion point, a casting method that prevents bulging without using a slab guide or the like and is not affected by the casting speed or cooling conditions is disclosed.

Further, Patent Document 5 discloses a method for producing a slab for an extra-thick steel sheet having excellent internal quality by significantly reducing the slab when the central solid phase ratio of the slab is 0.8 or more. Document 6 discloses a casting method capable of continuously performing slab reduction without lowering the casting speed even in continuous casting during unsolidified large reduction, and makes it possible to produce slabs of stable quality. There is.

Japanese Patent No. 4474772 Japanese Patent No. 4453556 Japanese Unexamined Patent Publication No. 2011-041960 Japanese Unexamined Patent Publication No. 8-224646 Patent No. 4296985 Japanese Patent No. 5177067

However, in the inventions disclosed in Patent Documents 1 and 2, since the maximum casting speed cannot be stably maintained, a decrease in production efficiency is unavoidable.

When the invention disclosed in Patent Document 3 is carried out, the equipment cost increases due to the deployment of a special roll and the addition of peripheral equipment associated therewith.

Patent Document 4 does not specify specific specifications of the reduction equipment required to complete solidification of the slab, and at least in order to reduce a wide slab such as a slab, a large diameter that moves up and down at high speed. It is necessary to arrange the rolls in the continuous casting machine, which increases the equipment cost. Further, the shape of the slab sandwiched between the solidification completion points can vary extremely in thickness at least at the front end and the rear end, so that the slab shape may become a problem in the rolling process.

Further, in all the inventions disclosed in Patent Documents 5 and 6, the casting speed is limited to about 0.7 to 0.8 m / min, and the arrangement of large reduction equipment and the utilization of large reduction operation in consideration of production efficiency can be utilized. There is no disclosure of how to fully enjoy it.

The present invention has been made in view of such problems of the prior art, and it prevents the operation hindrance due to bulging after the machine end of the continuous casting machine, which is a problem when casting at the machine length limit speed in continuous casting. The purpose is to provide technology for prevention and stable operation with high production efficiency.

The present invention is as listed below.

(1) From the position where the thickness center solid phase ratio of the slab is 0.7 when casting at the machine length limit speed, which is the maximum casting speed at which the continuous casting machine can perform stable casting, the final casting direction of the continuous casting machine. Casting using a pair of reduction rolls having a roll diameter of 450 to 600 mm, which are arranged within the range up to the machine edge, which is the installation position of the support roll and the position where the final support roll and the slab are in contact with each other. A method for continuous casting of steel, which comprises pressing a piece and casting at a casting speed of 95% or more and 106% or less of the machine length limit speed.

Hereinafter, a pair of reduction rolls having a roll diameter of 450 to 600 mm may be referred to as a "large reduction roll".

In the present invention, the "central solid phase ratio" means the ratio of the solid phase at the center of the slab thickness to the total amount of the liquid phase.

(2) The method for continuously casting steel according to item 1, wherein the specific water amount at the time of secondary cooling of the slab is cast from 0.6 to 1.6 L / kg-steel.

(3) The method for continuously casting steel according to item 1 or 2, wherein the casting speed of the continuous casting machine is 95% or more and 106% or less of the machine length limit speed.

In addition, the present invention can be listed as follows.

(4) A method for continuously casting steel according to the method for continuously casting steel according to item 1, wherein the porosity defect evaluation index at the center of the slab thickness defined by the equation (3) is set to 50% or more.

Porosity defect evaluation index (%) = {(V0-V) / V0} × 100 ... (3)
Here, V0 (cm ³ / g) indicates the porosity volume per unit mass of the average slab thickness center in the slab width direction when the slab is not reduced by the reduction roll, and is V (cm ^3). / G) represents the porosity volume per unit mass of the average slab thickness center in the slab width direction.

Here, "porosity" refers to pores generated by solidification shrinkage at the center of the slab thickness and heat shrinkage after solidification. Generally, it is also called "Center Porosity".

(5) A continuously cast slab produced by the continuous steel casting method described in paragraph 1 and characterized in that the porosity defect evaluation index at the center of the slab thickness defined by the formula (3) is 50% or more. ..

Porosity defect evaluation index (%) = {(V0-V) / V0} × 100 ... (3)
Here, V0 (cm ³ / g) indicates the porosity volume per unit mass of the average slab thickness center in the slab width direction when the slab is not reduced by the reduction roll, and is V (cm ^3). / G) represents the porosity volume per unit mass of the average slab thickness center in the slab width direction.

According to the continuous steel casting method according to the present invention, stable high production efficiency is prevented while preventing operation obstruction due to bulging after the machine end of the continuous casting machine, which is a problem when casting at the machine length limit speed in continuous casting. The operation can be carried out.

Further, according to the continuous casting method of the present invention, it is possible to provide steel in which deterioration of internal quality is suppressed.

FIG. 1 is an explanatory diagram showing an outline of arrangement of a continuous casting machine and a large reduction roll according to the present invention. FIG. 2 is an explanatory diagram showing an outline of a general continuous casting machine that continuously casts steel.

Hereinafter, embodiments for carrying out the present invention will be described with reference to the accompanying drawings.

FIG. 1 is an explanatory diagram showing an outline of arrangement of a continuous casting machine 0 and a reduction roll 4 (large reduction roll 4) having a roll diameter of 450 to 600 mm. Reference numeral 1 is a dipping nozzle, reference numeral 2 is a mold, reference numeral 3 is a support roll, reference numeral 4 is a large reduction roll, reference numeral 5 is a machine end (final support roll), reference numeral 6 is unsolidified molten steel in a slab, and reference numeral 7 is solidification. Subsequent slabs, reference numeral 8, indicate a cutting device (cutter). The description of the content that overlaps with the background technology column may be omitted.

The continuous casting machine 0 is not limited to the one shown in FIG. 1, and may be applied to any generally used continuous casting machine. For example, a vertical type continuous casting machine, a vertical bending type continuous casting machine, a fully curved type continuous casting machine and the like can be mentioned. Further, as a general continuous casting machine, there are various continuous casting machines used for manufacturing slabs, blooms, billets, round steel pieces, beam blanks, etc. depending on the shape of the mold cross section, and the present invention applies to any of them. You may.

INDUSTRIAL APPLICABILITY The present invention enables stable casting at a machine length limit speed against changes in the superheat degree of molten steel including a continuous casting boundary and fluctuations in the casting speed due to operation. In addition, "continuous casting" is based on the premise that the same tundish is used, and the ladle whose molten steel has been emptied due to the progress of casting is put into the next ladle filled with molten steel during continuous casting. It refers to an operation mode in which molten steel in a plurality of ladle is continuously cast by exchanging.

According to the continuous steel casting method of the present invention, the slabs 7 to be cast are reduced by using the large reduction rolls 4 and 4, and the solid-liquid interfaces above and below the unsolidified molten steel 6 in the slabs 7 are crimped. Since solidification can be forcibly completed, it is possible to prevent bulging that occurs after the end of the continuous casting machine, which occurs due to residual unsolidified part due to non-uniform solidification in the width direction due to initial solidification and secondary cooling. It is possible to continuously cast at a speed of 95% or more and 106% or less of the machine length limit speed of the continuous casting machine.

The large reduction roll 4 used in the present invention is formed from a position where the thickness center solid phase ratio of the slab is 0.7 when the continuous casting machine is cast at the machine length limit speed, which is the maximum casting speed capable of stable casting. A pair of rolls and rolls sandwich the slab within the range up to the machine edge, which is the installation position of the final support roll in the casting direction of the continuous casting machine and the position where the final support roll and the slab are in contact with each other. Be placed.

In other words, for example, when the machine length L of the continuous casting machine 0 according to the present invention is 28.3 m, the thickness of the slab is 300 mm, and the molten steel superheat degree is 45 ° C., the upstream side in the casting direction from the machine end 5 of the continuous casting machine 0. Place the large reduction roll 4 within 1.5 m.

The position of the large reduction roll 4 means a position where the large reduction roll 4 and the slab 7 are in contact with each other, and is a position connecting the centers of the pair of large reduction rolls 4 and 4.

The roll diameter of the large reduction roll 4 is preferably 450 mm or more and 600 mm or less.

The reason why the roll diameter of the large reduction roll 4 is 450 mm or more is that if the roll diameter is small, a sufficient reduction amount cannot be secured. If the reduction is insufficient in the present invention, there is a concern that bulging may occur due to the unsolidified molten steel, so it is extremely important to secure the reduction amount. In addition, there is a concern that porosity defects in the slab may remain.

On the other hand, the maximum roll diameter of the large reduction roll 4 is 600 mm. If the roll diameter of the large reduction roll 4 is larger than 600 mm, equipment obstacles such as an increase in the reduction reaction force occur, which is impractical for installation in a continuous casting machine.

Further, depending on the large phase rule roll 4, it is more preferable that the core solid phase ratio is less than 1.0. By performing a large reduction with a central solid phase ratio of less than 1.0, the temperature at the center of the thickness of the slab is high and the deformation resistance is low compared to the surface of the slab due to the large reduction at the casting stage. Since the penetration is improved, the center porosity can be effectively reduced. For more effective porosity reduction, it is desirable that the central solid phase ratio is 0.8 or more.

Further, if the reduction amount is less than 3 mm, the effect of reducing the porosity of the slab cannot be expected, so it is preferable to secure a reduction amount of 3 mm or more. More preferably, the amount of reduction by the pair of large reduction rolls 4 is 10 mm or more.

It is exemplified that the reduction and support of the slab by the large reduction roll 4 is carried out by hydraulically raising and lowering the large reduction roll 4 with the position sensor. At that time, the large reduction roll 4 is forcibly crimped to the unsolidified interface of the slab by reducing the amount of reduction corresponding to crimping the unsolidified portion. The position of the large reduction roll 4 in the slab thickness direction can be grasped by calculating from the position information of the initial state and the current state by the position sensor. Therefore, the continuous casting operation can be carried out after grasping the roll intervals of the large reduction rolls 4 and 4 arranged vertically with the slab sandwiched between them.

The central solid phase ratio fs can be obtained by fs = (TL-T) / (TL-Ts) from the liquidus phase line temperature TL of the molten steel, the solid phase line temperature Ts, and the temperature T at the center of the thickness. When the temperature T at the center of the thickness of the slab is equal to or higher than the liquidus temperature TL of the molten steel, fs = 0, and when the temperature T at the center of the thickness is smaller than the solidus temperature Ts of the molten steel, fs = It is assumed to be 1.0. Further, the temperature T at the center of the thickness of the slab can be obtained by a general one-dimensional heat transfer analysis calculation in the slab thickness direction in consideration of the casting speed, surface cooling of the slab, physical properties of the cast steel type, and the like. ..

When the machine length L is 28.3 m, when casting at the same casting speed as the machine length limit speed assumed in the present invention, if the large reduction roll 4 is installed on the casting upstream side of 1.5 m from the machine end, the reduction occurs. The central solid phase ratio of the slab is less than 0.7, the amount of slab reduction is increased, and the slab thickness is reduced, which is not preferable. Since the machine length limit speed calculated by the equation (1) with respect to the slab thickness is substantially increased, the effect of the present invention that the operation can be efficiently performed at the machine length limit speed cannot be obtained. In addition, the porosity reduction effect is reduced, which may lead to deterioration of internal quality, which is not preferable.

On the other hand, in the present invention in which the large reduction roll 4 is installed, the roll of the machine end 5 does not have to be limited to the support roll and may be the large reduction roll 4, so in that case, from the machine end 5 to the large reduction roll 4. The lower limit of the distance is 0 m.

Further, in the continuous casting method of the present invention, the specific water content when the slab is secondarily cooled is preferably in the range of 0.6 to 1.6 L / kg-steel. This is because if it is less than 0.6 L / kg-steel, it is difficult to obtain the central solid phase ratio at the time of reduction, which is the premise of the casting method of the present invention. That is, it is not preferable because the central solid phase ratio decreases due to insufficient cooling, the amount of slab reduction increases, and the slab thickness decreases. Further, if it exceeds 1.6 L / kg-steel, the cooling strength of the slab becomes excessively large locally. In particular, at the corners of the slab where cooling tends to proceed, the amount of reduction may not be secured due to the increase in deformation resistance due to supercooling. The secondary cooling ratio water amount is more preferably 0.8 to 1.4 L / kg-steel.

In the above description, the case of the segment type reduction equipment has been illustrated as shown in FIG. 1, but the present invention is not limited to this aspect.

The "segment type reduction equipment" refers to a reduction equipment having a structure in which roll pairs called segments constituting a continuous casting machine are arranged in the casting direction. In the present invention, the large reduction roll 4 is contained in the roll pairs. It is a reduction equipment with a structure in which. Needless to say, the present invention can also be applied to, for example, "stand-type reduction equipment". The "stand-type reduction equipment" is a reduction equipment having a structure in which a roll pair is independently arranged between segments or on the downstream side of casting at the machine end, and the roll itself can be raised and lowered. It is desirable that the stand-type reduction equipment itself is provided with a driving force for drawing slabs, or a drawing roll for drawing slabs is arranged on the downstream side of the casting.

Further, for example, in order to obtain the maximum casting speed, it is possible to further arrange a stand-type reduction facility on the downstream side of casting from the machine end (final roll) 5.

In this case, if the backup roll method is adopted, the lower limit of the reduction roll diameter due to the increase in the reduction reaction force is not particularly limited, but for example, the lower limit of the roll diameter can be set to about 200 mm. Regarding the upper limit of the rolling roll, a roll diameter of 600 mm at the maximum is appropriate because there is a concern about bulging between rolls due to an increase in the roll pitch.

The casting speed in the present invention is limited only to the machine length according to the equation (1) for obtaining the machine length limit speed. That is, since solidification is forcibly completed by the machine end reduction equipment, it is possible to increase the maximum casting speed at which stable casting is possible up to 106% of the machine length limit speed, for example, without increasing the cooling capacity.

Further, according to the continuous casting method of the present invention, it is possible to provide steel in which deterioration of internal quality is suppressed even during continuous casting at such a casting speed of about the machine length limit speed.

Specifically, it is possible to provide a continuously cast slab having a porosity defect evaluation index of 50% or more at the center of the slab thickness defined by the following equation (3).
Porosity defect evaluation index (%) = {(V0-V) / V0} × 100 ... (3)
Here, V0 (cm 3 / ^g) represents the porosity volume per unit mass of the slab thickness center average of slab width direction when no pressure the slab at a reduction roll, V (cm ³ / g) represents the porosity volume per unit mass of the average slab thickness center in the slab width direction. That is, the porosity defect evaluation index represents the porosity reduction rate due to the operation of reducing the slab by the reduction roll, and is an index of 100 (%) in the ideal case where the porosity disappears (= 0).

[Example 1]
<Creation of slabs>
Using a vertical bending type continuous casting machine with a machine length L: 28.3 m, the present inventors have a diameter of 470 mm at a casting length position 27.2 m from the molten steel surface of the mold and 1.1 m upstream from the machine end. A pair of large reduction rolls was arranged to carry out a continuous casting operation.

The solidification coefficient of this example is 26.0, and as can be calculated from Eq. (1), the machine length limit speed is 1.22 m / min for a slab thickness of 250 mm and 0.85 m for a slab thickness of 300 mm. / Minute.

The components of the steel used for continuous casting are C: 0.15 to 0.16%, Si: 0.17 to 0.22%, Mn: 0.99 to 0.98%, P: 0.020% or less. , S: 0.0060% or less, the balance of which is composed of Fe and impurities, and was carried out with a steel grade used as a thick steel sheet. In addition, "%" indicating the composition of steel is mass%.

The molten steel having the above components was cast into a slab having a thickness of 250,300 mm and a width of 2300 mm by the above-mentioned continuous casting machine. The degree of superheat of molten steel in the tundish, the casting speed, and the specific amount of water for secondary cooling were operated under the conditions shown in Table 1.

The “molten steel superheat degree” means a temperature difference obtained by subtracting the liquidus temperature obtained from an equilibrium state diagram or the like from the molten steel temperature actually measured in the tundish.

"Secondary cooling specific water amount" is the total amount of cooling water used from the time the slab passes through the mold to the outside of the end of the continuous casting machine, per unit weight of the slab. It is expressed as an index obtained by dividing the amount of secondary cooling water (L / min) per unit time by [weight of slab (kg / m) x casting speed (m / min) per unit length]. is there.

The solid phase ratio at each center of the arrangement position of the large phase rule was as shown in Table 1. The central solid phase ratio was determined by the above method.

The reduction by the large reduction roll was started after the tip of the slab passed the large reduction roll position at each reduction amount shown in Table 1. The length of the tip of the slab was set within the range of the cast portion below the machine length limit speed at the initial stage of casting. The amount of reduction of the slab was calculated from the difference between the roll interval of the large reduction roll and the roll interval of the support roll immediately before the large reduction by the position sensor.

<Evaluation of slab internal quality>
For the cast slabs, samples of 50 mm in the vertical direction × 100 mm in the width direction × 7 mm in the thickness direction were collected evenly from 16 locations in the width direction based on the slab thickness center from the slab cross section. Was measured according to JIS-Z8807: 2012 (method for measuring the density and specific gravity of a solid). By averaging the densities of 16 points, the slab density ρ (g / cm ³ ) of 1/2 thick part was determined. Similarly, samples of the same shape were evenly collected at 5 locations in the width direction from the 1/4 thickness position of the slab, and their densities were measured.

In the 1/4 thick part, there is usually no porosity defect and it is sound. Therefore, from the average value of 5 points, the reference density of the slab without porosity ρ _std. (G / cm ³ ) was determined. At this time, the porosity volume V per unit weight of the central portion of the slab slab thickness is the reciprocal of the density ρ of the sample of 1/2 thickness and the reference of the sample of 1/4 thickness as shown in equation (2). Density ρ _std. It was calculated from the difference of the reciprocals of.

V = 1 / ρ-1 / ρ _std.・ ・ ・ ・ (2)
Based on the porosity volume V0 (cm ³ / g) per unit weight at the center of the thickness of the slab when the continuously cast slab is not reduced with a large reduction roll, the reduction rate of the porosity volume according to equation (3) is calculated. It was used as an evaluation index for porosity defects.

Porosity defect evaluation index (%) = {(V0-V) / V0} × 100 ... (3)
In calculating the porosity defect evaluation index, the porosity volume of 2.6 in Comparative Example 2 in which reduction by a large reduction roll was not performed was set to V0.

The above results are shown in Table 1. The value in parentheses in the casting speed column indicates the ratio (percentage) to the machine length limit speed. Further, in Comparative Examples 1 and 2, since the reduction was not performed by the large reduction roll, their porosity volumes are described in the column of V0.

In Examples 1 to 4 of the present invention, the casting speed was 96 to 106% of the machine length limit speed, the central solid phase ratio was in the range of 0.75 to 0.90, and the reduction amount in the large reduction roll was 10 to 12 mm. .. In particular, Invention Examples 1 and 2 had a porosity defect evaluation index of 58%.

On the other hand, in Comparative Example 1, the large reduction was not carried out as the reduction amount of the large reduction roll was 0 mm. The casting speed was 92% of the machine length limit speed, which was the level of the conventional technology in consideration of the safety allowance, but the porosity volume, which is an internal quality index, was 2.1.

In Comparative Example 2, the casting speed was set to 96% of the machine length limit speed within the range specified in the present invention without performing a large reduction as in Comparative Example 1, but the porosity volume, which is an internal quality index, was 2. It was 0.6.

According to Examples 1 to 3 of the present invention, the central solid phase ratio is less than 1.00 (0.75 to 0.) while the casting speed is 95% or more of the machine length limit speed (range of 96% to 106%). When the reduction amount is 3 mm or more (range of 10 mm to 12 mm), bulging does not occur near the machine edge, and the porosity volume is relative to the porosity volume V0 when the slab is not reduced. A decrease in V was confirmed.

In particular, the porosity defect evaluation index of Examples 1 and 2 of the present invention is 58%, and the porosity volume can be further reduced by performing reduction in the range where the central solid phase ratio is 0.85 or more and less than 1.00. You can check. Further, in Examples 3 and 4 of the present invention, the porosity defect evaluation index is 42%, and an improvement effect can be seen when the slab is not pressed down.

[Example 2]
Using the same continuous casting machine as in Example 1, molten steel in the same composition range as in Example 1 was continuously cast.

The results are shown in Table 2.

An example of the present invention is an example in continuous casting, in which Example 4 of the present invention is before the continuous casting of Example 5 of the present invention, Example 6 of the present invention is after the continuous casting of Example 5 of the present invention, and Example 4 of the present invention. ~ 6 It shows that a series of continuous casting is carried out as a whole. Similarly, Examples 7-9 of the present invention show another series of continuous castings.

Although the casting speed including before and after continuous casting is 95% or more of the machine length limit speed (range from 100% to 106%), bulging does not occur near the machine edge and the central solid phase ratio is less than 1.0 (the central solid phase ratio is less than 1.0). It was in the range of 0.71 to 0.90), and the reduction amount was 3 mm or more (range of 10 mm to 13 mm). The porosity defect evaluation index was 50% or more except for Invention Examples 7 and 8. Also in Invention Examples 7 and 8, the percentages are 42% and 31%, respectively, and the improvement effect can be seen by the reduction by the large reduction roll.

The temperature of the molten steel in the ladle tends to decrease as the weight of the molten steel decreases as the casting progresses and the heat is removed over time. On the other hand, when continuous casting is carried out, the molten steel in the pre-charged ladle is stopped from being supplied into the tundish, and then the molten steel in the post-charged ladle is supplied into the tundish. The molten steel temperature during casting may rise to reflect the temperature change due to the charge change. According to Example 2, it was confirmed that the present invention can be applied even in such an operation mode in which the molten steel temperature changes, and that the operation can be performed at 95% or more of the machine length limit speed.

1: Immersion nozzle 2: Mold 3: Support roll 4: Large reduction roll 5: Machine end (final support roll)
6: Unsolidified molten steel in the slab 7: Shard after solidification 8: Cutting device (cutter)

Claims (3)

From the position where the central solid phase ratio of the thickness of the slab is 0.7 when casting at the machine length limit speed, which is the maximum casting speed at which the continuous casting machine can perform stable casting, the final support roll in the casting direction of the continuous casting machine The slab is crushed using a pair of reduction rolls having a roll diameter of 450 to 600 mm arranged within the range up to the machine edge, which is the installation position where the final support roll and the slab are in contact with each other. A method for continuous casting of steel. The continuous casting method for steel according to claim 1, wherein the specific water amount at the time of secondary cooling of the slab is cast with 0.6 to 1.6 L / kg-steel. The method for continuously casting steel according to claim 1 or 2, wherein the casting speed of the continuous casting machine is 95% or more and 106% or less of the machine length limit speed.