KR20150023796A - Method for continuously casting steel - Google Patents

Abstract

The cast steel is intentionally bulged, and thereafter the cast steel having the un-solidified layer is continuously cast by pressing down the cast steel, the cast steel is not broken out and further the internal cracks are generated in the solidified shell And also reduces center segregation of the cast steel and positive segregation near the center of the thickness. Until the thickness of the solidification shell 11 of the cast steel 10 reaches 15 mm, the roll gap of the cast steel support roll 6 is set to be equal to the value immediately below the cast steel mold, Thereafter, the roll opening degree is increased stepwise, the casting is bulged with a total amount of 3 to 20 mm of bulging, and thereafter, the roll opening degree is set to be constant in a section of 0.5 to 5.0 m on the downstream side in the casting direction, The cast steel having a solid phase ratio of 0.2 to 0.9 at the center is subjected to rolling at least once by a rolling roll 7 under the condition that the product of the rolling speed and the casting speed is 0.3 to 1.0 mm · m / min ² .

Description

METHOD FOR CONTINUOUSLY CASTING STEEL [0002]

SUMMARY OF THE INVENTION The present invention relates to a method and apparatus for intentionally bulging without slitting a drawn slab out of a casting mold and without causing internal cracks, to a method for reducing the central segregation of a cast steel by bulging the cast steel in a continuous casting with an unsolidified layer.

In continuous casting of steel, molten steel is injected into a water-cooled mold and cooled (called "primary cooling") to form a solidified shell on the mold inner wall. The cast steel having the outer shell of the solidification shell is continuously pulled downward while being supported by a plurality of casting support rolls provided below the casting mold. During the drawing, the surface of the cast steel is cooled by spray cooling water or the like (called "secondary cooling") and completely coagulated to the central portion of the cast steel, and then the cast steel is cut to a predetermined length to produce a steel cast.

An internal defect called center segregation may occur in the thickness center portion of the steel strip produced in this way. This center segregation is generated by enrichment of solute components such as carbon (C), sulfur (S), phosphorus (P), and manganese (Mn) in the final solidified portion of the cast steel, The center segregation of the cast steel may be caused by hydrogen-induced cracking of a large-diameter welded steel pipe produced by welding the steel sheet after bending the steel sheet after lowering the toughness of the steel sheet, Is known.

The mechanism for generating the center segregation of cast steel is considered as follows. That is, along with the progress of solidification of the cast steel, the solute component is concentrated between the branches of the dendritic crystal, which is the solidification structure of the cast steel, according to the partition law. This is the microsegregation formed between the dendritic branches. When voids are formed in the center of the thickness of the cast steel or negative pressure is generated due to shrinkage of the cast steel during solidification or swelling of the cast steel called bulging, molten steel is sucked into the cast steel. However, since there is not a sufficient amount of molten steel in the solidification layer at the end of solidification, the molten steel in which the solute component is concentrated by the above micro segregation flows and accumulates in the center portion of the thickness of the cast steel, and solidifies in an integrated state. Since the molten steel in which the solute component is concentrated solidifies and solidifies, a thickening zone of the solute component is formed at the center of the cast steel thickness. This thickening zone is the center segregation, and the above micro segregation is called macrosegregation.

As countermeasures against the center segregation of the cast steel, it is effective to prevent the movement of molten steel (referred to as " concentrated molten steel ") present in the dendrite branches where the solute component is concentrated, and to prevent localized accumulation of molten steel , And several methods for preventing center segregation using these principles have been proposed.

Among them, in the continuous casting machine, the cast ends of the solidification stage having the non-solidified layer were subjected to a collection of reduction rolls (" light-rolling reduction ") with a total reduction amount and a reduction speed corresponding to the sum of the solidification shrinkage amount and the heat shrinkage amount (Referred to as " soft reduction ") in which the casting is carried out while gradually lowering the temperature by a soft reduction zone (see, for example, Patent Document 1). Here, the total reduction amount is a reduction amount from the start of pressing down to the end of pressing down.

This soft reduction method is a technique for preventing centered segregation by preventing the movement of molten steel present between dendrite branches. However, since the total amount of reduction is such that the amount of solidification shrinkage is slightly exceeded, the descending force is weak. That is, when the coagulation completion position of the cast steel is not at the same position in the width direction of the cast steel, the already solidified portion becomes the pressing resistance and the uncoagulated portion to be pressed down There is a case where the force is not given. In this case, the improvement effect of the center segregation is small in the portion where no reduction force is applied. Therefore, even in the case of the light-rolling method, the effect of improving the center segregation is limited.

Further, as a method for improving center segregation of cast steel, a method has been proposed in which the cast steel in the final stage of solidification is pressed down by a total amount of reduction in rolling much larger than the sum of the amount of coagulation shrinkage and the amount of heat shrinkage by a pair of roll- See Patent Document 2). This method is also referred to as " hard reduction "

In order to impart a large pressure to the cast steel, the coagulation-completed longer side of the cast steel, which is located at both ends of the cast steel, must also be pressed down, thus requiring a large depressing force. In other words, in the case of applying a large pressing force to a conventional continuous casting machine, the supporting frame body supporting the pair of pressing rolls is warped to generate a sufficient pressing effect. In addition, there is a case where the operation becomes difficult due to a trouble on a facility, such as bending down, breakage, or the like. In order to prevent bending of the support frame and bending of the pair of rollers, it is necessary to make the continuous casting equipment capable of withstanding a high load. The problem caused by such a high load occurs in the same way even when the total amount of the reduction in pressure is increased as in the case of the light pressure reduction method.

Therefore, in the light-hard pressing method, for the purpose of increasing the total amount of hot rolling and reducing the load on the continuous casting machine by pressing or by reducing the load on the continuous casting machine in the hot rolling method, Suggestions are being made.

For example, in Patent Document 3, a cast steel is intentionally bulged at a position where the solid phase fraction of the cast steel is 0.1 or less, so that the thickness of the central portion in the width direction of the cast steel is smaller than the thickness of the cast steel short- Immediately after the solidification completion position, a pressure of at least 20 mm per one pair of pressure drop rolls is given by at least one pressure roll pair to reduce the amount corresponding to the total amount of the bulging, A method for preventing such a problem is proposed. Here, the total amount of bulge is the amount of punishment from the deliberate start of the bullet to the end of the deliberate bullet.

Patent Document 4 discloses that the thickness of the central portion in the width direction of the cast steel is intentionally bulged by a thickness corresponding to 10% to 50% of the thickness of the cast steel short side portion until the thickness of the non-solidified layer of cast steel becomes 30 mm, Thereafter, a method of reducing the amount corresponding to the total amount of the bulging by applying a pressure drop of at least 80 mm / m per slab length by at least one pushing roll pair to the solidification completion position to prevent center segregation is proposed have.

Patent Literature 5 discloses a method for locally bulging 3 to 25% of the thickness of a cast steel at the start of the intended casting, There has been proposed a method of preventing center segregation by pressing down by a thickness corresponding to 30% or more and 70% or less of the total amount of the bulge by one pressing roll pair.

Japanese Unexamined Patent Publication No. 8-132203 Japanese Patent Application Laid-Open No. 6-218509 Japanese Patent Application Laid-Open No. 9-57410 Japanese Patent Application Laid-Open No. 9-206903 Japanese Patent Application Laid-Open No. 2000-288705

In the above Patent Documents 3 to 5, since the cast steel is pressed down within the amount corresponding to the total amount of the bulge or less, the short side of the cast steel is not pressed down, and the load on the continuous casting machine by the rolling is alleviated . However, Patent Documents 3 to 5 have the following problems.

That is, in Patent Documents 3 to 5, when the cast steel is intentionally bulged, the thickness of the casting shell before casting is not specified. Therefore, when the starting time of the casting is excessively fast, cracks or expansion of the solidified shell There is a risk that a breakout may occur. Further, in Patent Documents 3 to 5, the bulging rate at the time of intentionally bulging the cast steel is not specified, and therefore, when the cast steel is rapidly bulged, internal cracking occurs in the bulging solidifying shell, In severe cases, there is even a risk of breakout.

Further, in Patent Documents 3 to 5, since the bulging region and the push-down region are continuously provided in the continuous casting machine, the shape of the casted billet is not stabilized, Therefore, there is a possibility that the center segregation can not be improved because the descending force is not transmitted to the thickness center portion of the cast steel. In addition, in the case of Patent Documents 3 to 5, when the press-down time or the total amount of depressurization is not appropriate, center segregation of the solute component occurs at the center of the thickness of the cast steel, ) Positive segregation occurs or occurs.

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and it is a first object of the present invention to provide a casting method for casting a cast steel cast from a casting mold by intentionally bulging the cast steel cast, In the continuous casting of the cast steel, the cast steel is not broken out from the casting mold, and intentionally bulged without causing internal cracking in the casting shell of the cast steel. In addition, the center segregation of the cast steel, The present invention provides a continuous casting method of a steel capable of alleviating the segregation of segregation in the steel.

A second object of the present invention is to provide a casting method for continuously casting a cast steel having a non-solidified layer inside the casting die by pressing down a cast steel cast on the casting mold, , The total amount of the bulging at the time of intentionally bulging can be reduced and the total amount of the bulging in the intentional bulging and the time of the pressing under the light pressure, the total amount of the pressing down, the pressing speed and the like are appropriately adjusted, A method for continuously casting a steel capable of stably reducing center segregation of a steel plate.

The gist of the present invention to solve the above problems is as follows.

[1] The roll gap of the casting support rolls arranged in the casting direction is made equal to the roll opening below the casting mold until the coagulating shell thickness drawn from the continuous casting mold reaches at least 15 mm Setting,

Thereafter, the roll opening degree of the casting support roll was gradually increased toward the downstream side in the casting direction, the longer side of the casting jig was bulged with a total amount of 3 to 20 mm of bulging,

The roll opening degree of the casting support roll is set to be constant so that the thickness of the casting slab is not changed in the section of 0.5 to 5.0 m downstream from the casting direction,

Subsequently, the long side face of the cast steel having a solid phase ratio of 0.2 to 0.9 at the center of the cast steel is pressed at least once by a roll-down roll under the condition that the product of the rolling speed and the casting speed is 0.3 to 1.0 mm · m / min ² , Continuous casting method of steel.

[2] The continuous casting method of a steel according to [1], wherein the roll opening degree of the casting support rolls arranged in the casting direction is gradually increased to a slope of 4.0 mm or less per 1 m in the casting direction when the long- .

[3] The roll opening degree of a plurality of pairs of casting support rolls is gradually decreased toward the downstream side in the casting direction so that the product of the descending speed and the casting speed is 0.3 Min to ² mm / min ² , and the width of the short side of the cast strip is made 3 to 20 mm smaller than the shorter side width of the cast strip at the lower end of the cast by this pressing force,

The roll opening degree of the plural pairs of slab support rolls is gradually increased toward the downstream side in the casting direction and the long side face of the cast slab is bulged to a total amount of 3 to 20 mm,

Wherein the roll opening degree of the plurality of pairs of casting support rolls is gradually decreased toward the downstream side in the casting direction after the long side face of the casting is bulged and is 0.9 or more when the solidification rate at the center of the casting thickness is at least 0.2 Min to a point in time at which the product of the reduction rate and the casting speed is in the range of 0.3 to 1.0 mm · m / min ² is applied to the long side surface of the cast steel, and the total reduction amount equivalent to the total amount of the above- A continuous casting method of steel wherein the long side surface of the cast steel is pressed down by the total amount of the casting.

[4] The solidification completion position of the cast steel is detected on-line using the solidification completion position detecting device, and based on the detected information of the solidification completion position, at least 0.9 of the solidification rate at the center portion of the cast steel is at least 0.2 The amount of the secondary cooling water, the reduction of the width of the secondary cooling, and the casting speed so that the main shaft is positioned at the above-mentioned low-pressure lowering bed, Of the continuous casting of the steel according to the above [3].

According to the invention of [1], since the long side surface of the cast strip is intentionally bulged after the solidification shell thickness of the cast steel exceeds 15 mm, breakout of the cast steel can be prevented in advance. Further, since the roll opening degree is set so that the thickness of the cast steel is not changed in a section of 0.5 to 5.0 m downstream from the casting direction after the cast steel is bulged, the solidified shell becomes flat during this period. The coagulating shell has a flat shape so that the pressing down efficiency at the time of subsequent pressing is improved so that the center segregation of the cast steel and the segregation in the vicinity of the thickness center portion of the cast steel can be stably reduced.

According to the invention of [2], since the short width of the casting is smaller than the bottom dimension of the casting mold at the stage where the deformation strength of the solidification shell is low, the total amount of the bulging at the time of intentionally bulging can be suppressed to a small extent. As a result, break-out of the cast steel is prevented and internal breakage of cast steel is suppressed. In addition, when the temperature is lowered, the time of the lowering of the hardness and the product of the lowering speed and the casting speed are defined, so that the center segregation of the casting can be stably reduced. In addition, in the low-pressure drop zone, the short sides at both ends of the cast steel are not pressed down, but the cast steel can be pressed down with a small load, thereby reducing the load on the equipment constituting the low- and- Further, since the short span of the cast strip is not pressed down, even when the cast strip has an unsteady casting period at the beginning or end of the continuous casting operation in which the short side of the cast strip tends to be particularly low, the descent force is transferred to the inside of the cast, The center segregation of the cast steel can be remarkably improved as compared with the conventional one. Naturally, in the steady casting period, center segregation of the cast steel can be improved in the same way as or more than the conventional one.

1 is a schematic cross-sectional view of a slab continuous casting machine used in a first embodiment of the present invention.
2 is a side schematic view of a vertical bending type continuous slab continuous casting machine used in a second embodiment of the present invention.
Fig. 3 is a view showing an example of a profile of the roll opening degree of the cast strip supporting roll in the second embodiment. Fig.
4 is a view showing a sampling position of a cross-sectional sample in Example 1 and analysis positions of manganese (Mn) by EPMA.

(Mode for carrying out the invention)

Hereinafter, the present invention will be described in detail with reference to the accompanying drawings. First, the first embodiment of the present invention will be described. 1 is a schematic cross-sectional view of a continuous slab casting machine used in the first embodiment of the present invention.

1, molten steel 9 is injected into a slab continuous casting machine 1, the molten steel 9 is cooled and solidified to form a mold 5 for forming an outer shape of the cast steel 10 Is installed. A tundish 2 for relaying molten steel 9 supplied from a ladle (not shown) to the mold 5 is provided at a predetermined position above the mold 5. A sliding nozzle 3 for adjusting the flow rate of the molten steel 9 is provided at the bottom of the turn-dish 2, and an immersion nozzle 3 is provided on the lower surface of the sliding nozzle 3. [ 4) are installed. On the other hand, on the lower side of the mold 5, a plurality of pairs of slab support rolls each comprising a guide roll 6, a push-down roll 7, and a pinch roll 8 are disposed. Spray nozzles (spray nozzles) such as water spray nozzles or air mist spray nozzles are disposed in the gaps between these support rolls adjacent to each other in the casting direction to constitute a secondary cooling band. The cast piece 10 is configured to be cooled by cooling water (also referred to as " secondary cooling water ") sprayed from a spray nozzle of the secondary cooling zone while being pulled out.

The molten steel 9 injected into the mold 5 from the tundish 2 is cooled in the mold 5 to form the solidifying shell 11. [ The casting 10 having the solidification shell 11 as an outer shell and the inside as the non-solidification layer 12 is supported by the guide roll 6 and is pressed down by the pressure roll 7, (Not shown). In the meantime, the cast steel 10 is cooled by the secondary cooling water of the secondary cooling zone to increase the thickness of the solidification shell 11.

The roll opening degree of the guide roll 6 is controlled so that the roll 10 passes under the mold 5 until the thickness of the solidification shell 11 of the casting 10 reaches at least 15 mm. The same as the opening degree. The thickness of the solidifying shell 11 is obtained by a two-dimensional heat-transfer solidification calculation considering the casting condition or a sensor for measuring the thickness of the solidifying shell from the permeation time of the ultrasonic wave transmitting the casting . When the thickness of the solidification shell 11 differs from the long side face and the short side face of the cast steel 10 due to casting conditions or the like, the solidification shell 11 having a smaller thickness is targeted.

After the thickness of the solidification shell 11 of the cast steel 10 exceeds 15 mm, the roll opening degree of the guide rolls 6 arranged in the casting direction is gradually increased toward the downstream side in the casting direction, The long side face is intentionally bulged in the direction of the thickness of the billet with a total amount of 3 to 20 mm of the bulge. In this case, since the casting direction length of the continuous casting machine is limited, it is preferable to intentionally start the bulging until the thickness of the solidifying shell 11 reaches 30 mm. Here, the roll opening degree is a gap distance between the casting support rolls sandwiching the casting 10 therebetween, and is also called roll spacing. Also, the total amount of bulge is the amount of bulge 10 of the casting 10 from the deliberate start of bulging to the end of deliberate bulging.

If the cast steel 10 is intentionally bulged in a state where the thickness of the solidification shell 10 of the cast steel 10 is 15 mm or less, the strength of the solidification shell 11 is not sufficient and the solidification shell 11 is cracked Cracks occur. There is also a risk that cracking or cracking of the solidifying shell 11 may cause breakout. On the other hand, in the first embodiment of the present invention, since the solidification shell 11 is bulged after the thickness exceeds 15 mm, the strength of the solidification shell 11 is ensured, and cracking in the solidification shell 11 It is possible to prevent cracks from occurring. As a result, the break-out caused by cracking or cracking of the solidifying shell 11 is avoided by itself.

By increasing the roll opening degree of the guide roll 6 stepwise toward the downstream side in the casting direction, the long side face of the cast strip 10 is guided by the ferrostatic pressure of the molten steel in the interior of the uncracked layer 12, And bulges along the roll opening of the roll 6. On the other hand, the short side face of the cast steel 10 is narrower in width than the long side face of the cast steel, and is supported at a corner portion where the temperature is low and the rigidity is high (the intersection of the long- Even if the roll opening degree of the guide roll 6 is increased stepwise, the shape of the short side of the cast strip is hardly changed. Likewise, the minor side surface of the cast strip 10 does not change. On the long side surface of the cast strip 10, the bulge is generated in a range from the portion having the non-solidified layer 12 in the inside to the central portion of the long side surface, which is separated from the short-

The side of the short side face of the long side face of the cast strip 10 does not contact the guide roll 6 because the short side face of the cast strip 10 on the long side face of the cast strip 10 does not change when bulging occurs on the long side face of the cast strip 10 . Further, when the rate of increase of the billet thickness due to the bulging is faster than the rate of increase of the thickness of the solidification shell 11, that is, the solidification rate, the thickness of the unsolidified layer 12 is increased by the bulging.

In this case, in order to reduce the stress acting on the solidifying shell 11 due to the bulging, that is, to prevent breakage due to internal cracking of the solidifying shell 11 or breakage or cracking of the solidifying shell 11 , It is preferable to gradually increase the roll opening degree of the guide roll 6 toward the downstream side in the casting direction to 4.0 mm or less per 1 m in the casting direction and preferably to a gradient of 1.0 mm or less per 1 m in the casting direction. This is because if the roll opening degree of the guide roll exceeds 4.0 mm per 1 m of the casting direction (hereinafter referred to as " 4.0 mm / m ") at the time of bulging, the inclination is too large, However, if it is 4.0 mm / m or less, internal cracking is prevented.

After the casting 10 has been bulged, the roll opening degree of the guide roll 6 is set to be constant so that the thickness of the casted billet 10 is not changed in the section of 0.5 to 5.0 m toward the downstream side in the casting direction. Then, the slab thickness center is 0.2 to 0.9 and sangryul the slab 10 a, the reduction rate and the casting speed and multiplication 0.3~1.0㎜ · m / min ² The conditions in product thickness by reduction rolls 7 is of the Direction at least once or several times. In this case, the thickness of the cast steel 10 after being pressed by the pressure roll 7 is set to be equal to or larger than the thickness of the cast steel prior to the bulging. That is, even the short side face of the cast steel 10 is not pressed down as it is pressed down.

In the first embodiment of the present invention, after the cast steel 10 is bulged, in the section of 0.5 to 5.0 m toward the downstream side in the casting direction, the roll of the guide roll 6 The thickness of the solidification shell 11 at the bulged portion of the cast steel 10 is increased and the solidification shell 11 is formed in a flat shape so that the reduction efficiency at the time of the subsequent pressing is improved .

Since the length of the section in which the roll opening degree of the guide roll 6 is set to be constant is 0.5 m or more after the bulging, the growth of the solidification shell thickness of the bulged portion of the cast steel 10 is promoted, Shape. Thus, it is possible to uniformly lower the steel strip in the steel strip width direction during the subsequent pressing. When the length of the section is less than 0.5 m, the effect is too short to obtain this effect. On the other hand, since the length of the section in which the roll opening degree of the guide roll 6 is set constant after bulging is 5 m or less, the thickness of the solidification shell 11 in the bulged portion of the cast steel 10 is not excessively thick . As a result, the reduction efficiency at the time of the subsequent pressing is improved. When the length of the section exceeds 5 m, the thickness of the solidifying shell 11 is excessively increased, and the reduction efficiency is lowered.

In the first embodiment of the present invention, the reason why the total amount of deliberate bulging is set within the range of 3 to 20 mm is as follows. The thickness of the slab cast steel is generally from 200 to 300 mm, and therefore, when the cast steel is bulged more than 20 mm, an excessive downward pressing equipment is required, the equipment cost becomes high, More. In order to prevent this, the upper limit value of the total amount of the bullet is intentionally set to 20 mm. On the other hand, if the total amount of the intrinsic bulging is less than 3 mm, the amount of thickness that can be reduced is small and the effect of discharging the molten steel in which the solute component is concentrated to the upstream side in the casting direction is small. That is, the effect of improving center segregation of the cast steel 10 is small. Therefore, in order to obtain the effect of improving the center segregation of the cast steel 10, the intended total amount of the bulging is set to 3 mm or more.

Herein, the intentional total amount of the bulging refers specifically to the total thickness of the guide rolls 6 at the position of the guide roll 6 at the most downstream position where the roll opening degree of the guide roll 6 is set to be the same as the roll opening degree ₀ ) of the guide roll 6 and the cast steel thickness D ₁ at the position of the guide roll 6 closest to the uppermost descending roll 7 (= D ₁ -D ₀ ).

In the first embodiment of the present invention, after casting the cast steel 10, the cast steel 10 having a solidification rate of 0.2 to 0.9 at the center of the cast steel thickness is pressed down once or plural times by the press roll 7. That is, the cast steel 10 having a solid phase ratio of 0.2 to 0.9 at the center of the cast steel thickness is pressed down at least once by the rolling roll 7 (in FIG. 1, the cast steel 10 is pressed twice). Here, the solid phase rate is defined as the solid phase rate before the start of solidification = 0 and the solid phase rate at the completion of solidification = 1.0, and the solid phase rate at the center of the cast steel thickness can be calculated by two-dimensional electrothermal solidification calculation.

When the cast steel 10 having a solidification rate at the center of the cast steel thickness of less than 0.2 is lowered and thereafter is not pressed down because the thickness of the unsolidified layer 12 of the cast steel 10 at the lowered position immediately after being pressed down is thick, Center segregation occurs with the progress of the subsequent solidification. On the other hand, if the cast steel 10 having a solidification rate at the center of the cast steel thickness exceeding 0.9 is pressed down without pressing down until the solidification rate at the center of the cast steel thickness is 0.9, the thickness of the solidification shell 11 becomes thick, The molten steel in which the solute component is concentrated is hardly discharged and, as a result, the effect of improving center segregation is reduced. Further, when the cast steel 10 having a solid phase ratio of 0.9 or more at the center of the cast steel is pressed down, a solitary segregation of the solute component occurs near the center of the thickness of the cast steel 10.

In the first embodiment of the present invention, since the cast steel 10 having a solid phase ratio of 0.2 to 0.9 in the cast steel thickness center portion is pressed down by the rolling roll 7, the above problem does not occur, The center segregation can be stably prevented.

In the first embodiment of the present invention, the cast steel 10 is rolled down in the thickness direction by the rolling roll 7 in such a range that the product of the rolling speed and the casting speed is 0.3 to 1.0 mm · m / min ² .

When the product of the descending speed and the casting speed is less than 0.3 mm · m / min ² , the thickness of the unsolidified layer 12 of the billet 10 at the depressed position after pressing down becomes thick, and the dendrite number The molten steel is not sufficiently discharged from the dendrites water. As a result, center segregation occurs after pressing down. On the other hand, when the product of the reduction rate and the casting speed exceeds 1.0 mm · m / min ² , almost all of the molten steel in which the solute component of the dendritic water is concentrated is squeezed and discharged to the upstream side in the casting direction. However, since the thickness of the non-solidification layer 12 is thin, the squeezed concentrated molten steel is caught in the solidification shell 11 on both sides in the thickness direction of the casting slightly upstream of the squeeze position in the casting direction. As a result, a solitary segregation of the solute component occurs in the vicinity of the center of the thickness of the cast steel 10.

In the first embodiment of the present invention, the above problem does not occur because the pressing force is lowered in the range where the product of the pressing speed and the casting speed is 0.3 to 1.0 mm · m / min ² , It is possible to stably prevent segregation in the vicinity of the center portion.

The effect of the reduction in the center segregation of the cast steel 10 and the prevention of the generation of the chisel stones in the vicinity of the thickness center portion is also affected by the solidification structure of the cast steel 10. That is, when the solidification structure at the center of the thickness of the slab is an equiaxed crystal structure, the concentrated molten steel causing the simi-macro segregation exists in the equiaxed crystal, and the reduced load is transmitted to the center of the slab thickness It is difficult to improve the effect. Therefore, it is preferable to set the casting conditions so that the solidification structure of the cast steel 10 becomes a columnar crystal structure.

As described above, according to the first embodiment of the present invention, since the cast steel 10 is bulged after the solidification shell thickness of the cast steel 10 exceeds 15 mm, breakout of the cast steel 10 can be prevented . Further, since the roll opening degree is set so that the thickness of the cast steel 10 does not change in the section of 0.5 to 5.0 m downstream from the casting direction after the casting 10 is bulged, the solidifying shell 11 has a flat shape. The solidification shell 11 has a flat shape so that the pressing efficiency at the time of subsequent pressing is improved and the segregation of the center of the cast steel 10 and the segregation of the segregation in the vicinity of the center of thickness of the cast steel 10 can be stably It becomes possible to alleviate the problem.

Next, a second embodiment of the present invention will be described. 2 is a side schematic view of a vertical-bending continuous slab continuous casting machine used in a second embodiment of the present invention.

2, molten steel 31 is injected into the slab continuous casting machine 21, the molten steel 31 is cooled and solidified, and a mold for forming an outer shape of the cast steel 32 having a rectangular cross- (Not shown). At a predetermined position above the mold 25, a turn-dish 22 for relaying molten steel 31 supplied from a ladle (not shown) to the mold 25 is provided. A sliding nozzle 23 for adjusting the flow rate of the molten steel 31 is provided at the bottom of the turn desk 22. An immersion nozzle 24 is provided on the lower surface of the sliding nozzle 23. [

On the other hand, on the lower side of the mold 25, a plurality of pairs of slab support rolls including a support roll 26, a guide roll 27 and a pinch roll 28 are disposed. Among them, the pinch roll 28 is a drive roll for supporting the cast steel 32 and pulling out the cast steel 32. Spray nozzles (not shown) such as water spray nozzles or air mist spray nozzles are disposed in the gaps between the support rolls adjacent to each other in the casting direction to constitute a secondary cooling band. The cast steel 32 is cooled by the secondary cooling water sprayed from the spray nozzle of the secondary cooling zone while being pulled out.

On the downstream side of the casting support roll, a plurality of conveying rolls 29 for conveying the cast casting 32 are provided. A casting cutter 30 for cutting a casting piece 32a having a predetermined length from the casting casting 32 is disposed above the conveying roll 29. [

There is provided a light-hard fall-down bar 36 on the upstream side and the downstream side of the casting direction with sandwiching the solidification completion position 35 of the cast piece 32 therebetween. The small-diameter tumbler 36 is constituted by a plurality of pairs of guide rolls which are set so that the roll opening degree with respect to the guide rolls 27 facing each other is gradually narrowed toward the downstream side in the casting direction, that is, a roll gradient is performed. In the light-load-reduction bed 36, it is possible to perform a light-down operation on the cast steel 32 in the whole or a part of the selected area. A spray nozzle for cooling the cast steel 32 is disposed between guide rolls of the small-diameter tumbler 36. In Fig. 2, only the guide roll 27 is disposed on the small-diameter tumbler 36, but the pinch roll 28 may be disposed. The casting support roll disposed in the soft-reduction bed 36 is also referred to as a " push-down roll ".

The slab support roll disposed between the lower end of the casting mold 25 and the liquidus crater end position of the casting slab 32 is provided with a narrow width reduction reduction stage 37 ) And an intentional penalty zone (38). In each of the short-side-width narrowing down barriers 37, each of the slab support rolls is set so that the roll opening degree becomes narrower every one roll or several rolls toward the downstream side in the casting direction until the reduction amount of the roll opening becomes a predetermined value . In the intentional bulging zone 38, each of the slab support rolls is set so that the roll opening degree becomes wider per roll or every several rolls toward the downstream side in the casting direction until the amount of roll opening becomes a predetermined value. Here, the intentional bulge 38 is provided on the downstream side of the short side width narrowing downward descending bar 37.

The casting support roll provided on the downstream side of the intentional bulging barrel 38 is narrowed to a degree suitable for the constant value of the roll opening degree or the contraction amount accompanying the temperature drop of the casting 32, It is connected.

In the second embodiment of the present invention, the short-side-width coining lowering bar 37 and the intentional gathering band 38 are disposed between the lower end of the mold 25 and the liquid phase line crater end position of the casting 32 The reason is as follows. That is, on the upstream side of the liquid phase crater end position of the cast steel 32, all of the cast steel thickness center portions are liquid phase, and the solidification shell 33 of the cast steel 32 has a high temperature and a low deformation resistance. Therefore, by narrowing down the width of the short side of the cast strip, it is possible to easily make the width. In addition, when the cast steel 32 is intentionally bulged, if the uncoagulated layer 34 existing in the cast steel 32 is bulged at a small point, the center segregation is rather deteriorated. However, when the casting is performed at the upstream side of the liquid phase crater end position of the cast steel 32 in the casting direction, molten steel having an initial concentration in which the solute element is not concentrated is present in the cast steel in a rich state at this point, Molten steel flows easily. Segregation does not occur even when the molten steel flows, and therefore, the bulging at this point does not cause center segregation.

The liquidus of the cast steel 32 is a solidification starting temperature determined by the chemical composition of the cast steel 32 and can be obtained from the following equation (1), for example.

TL = 1536- (78 x [% C] + 7.6 x [% Si] + 4.9 x [% Mn] +34.4 x [% P] +38 x [% S] + 4.7 x [% Cu] +3 . 1 x [% Ni] + 1.3 x [% Cr] + 3.6 x [% Al] (One)

[% Mn] is the molar concentration of the molten steel (mass%), [% Si] is the molar concentration of silicon (% (% By mass), [% P] is the phosphorus concentration of the molten steel (mass%), [% S] is the sulfur concentration of the molten steel , [% Ni] is the nickel concentration (mass%) of molten steel, [% Cr] is the chromium concentration (mass%) of molten steel, and [% Al] is the aluminum concentration (mass%) of molten steel.

The liquid phase crater end position of the cast steel 32 can be obtained by comparing the temperature gradient of the inside of the cast steel obtained by the two-dimensional heat transfer coagulation calculation with the liquidus line temperature determined by the formula (1). Further, the position of the end of the liquid phase crater can be obtained also by inserting a metal pin having a melting point known to the center of the thickness of the cast steel 32 during casting and examining the melting state of the metal pin.

The liquid phase crater end position of the cast steel 32 can be accurately obtained by two-dimensional electrothermal solidification calculation as described above. However, if the distance from the meniscus of molten steel to the inlet side of the light-load-reduction platform 36 is L, if the solidification completion position 35 is the casting condition present in the light-load-reduction platform 36, From the results of the two-dimensional electro-thermal coagulation calculation, it is evident that the range from the inner molten steel bath face to the L × 2/3 region is on the upstream side of the liquid phase line crater end position. Therefore, for example, as shown in Fig. 2, the short-side width narrowing down load block 37 and the intentional gathering base 38 may be disposed on the upper side of the secondary cooling bed.

Since the short width side coercive force lowering bar 37 and the intentional pulverizing belt 38 are constructed only by adjusting the roll opening degree without any special mechanism, It can be installed at any position as long as it is within the range of the position. However, it is necessary to provide the short-side width coining down barriers 37 on the upstream side of the intentional gathering band 38 in the casting direction.

Fig. 3 shows an example of the profile of the roll opening degree of the cast strip supporting roll in the second embodiment. In the example shown in Fig. 3, a cast steel having a thickness of 250 mm at the lower end of the casting mold is reduced to 245.2 mm in the short side width coining lowering stage 37, that is, And the thickness of the central portion of the long side of the cast steel was set to 254.4 mm (the total amount of the bulging was 9.2 mm). Thereafter, the small-diameter side of the cast- 36), the thickness of the central portion of the long side of the cast steel is reduced to 245.4 mm (total amount of rolled down = 9.0 mm). In addition, the total reduction amount is the reduction amount of the cast piece 32 from the start of the reduction to the end of the reduction.

That is, in the short-side-width coining down load bar 37, the roll opening degree of the casting support rolls is gradually narrowed toward the downstream side in the casting direction so that the short side of the casting product 32 is pressed down, and the thickness of the short- That is, the span width is narrowed. In addition, in the intentional gathering zone 38, the roll opening degree of the casting support roll is sequentially widened toward the downstream side in the casting direction, so that the long side face except for the vicinity of the short side of the casting product 32, Lt; RTI ID = 0.0 > support roll. ≪ / RTI > The side near the short side of the long side of the cast slab is solidly held on the short side of the cast slab after the solidification has been completed, thereby maintaining the thickness at the time when the deliberate bulging is started. Therefore, in the cast steel 32, only the bulged portion of the long side surface of the cast steel comes into contact with the cast steel support roll by intentional bulging. In addition, only the bulged portion of the long side surface of the cast steel is pressed down by the small-diameter tumbler 36.

In the second embodiment of the present invention, in the short-width-coined downward descending / descending bar 37, the total amount of downsizing for short-side width coarsening is set to 3 mm or more and 20 mm or less. By setting the total reduction amount for short-side width narrowing to 3 mm or more and 20 mm or less, there is no need to increase the total amount of the subsequent intentionally bulging, and the surface cracking on the short side of the cast strip can be prevented. If the total reduction amount is less than 3 mm, it is necessary to reduce the amount of narrowing the width of the short side of the cast steel and to increase the total amount of the subsequent intentionally bulging. In this case, there is a risk of internal cracking in the cast steel 32 due to excessive bulging. On the other hand, if the total reduction amount for short-side width narrowing exceeds 20 mm, the compression strain on the short-side surface of the cast steel becomes large, and surface cracking occurs on the short-side surface of the cast steel, which may cause break-out.

In the short-side-width coining lowering bar 37, it is necessary to apply a descending force in which the product of the pressing speed and the casting speed is in the range of 0.3 to 1.0 mm · m / min ² . When the value of the product of the pressing speed and the casting speed is less than 0.3 mm · m / min ² , the distance until the completion of the pressing down of the predetermined amount is long, and the length of the short- So that the installation length of the intentional bulging jig 38 thereafter can not be ensured. On the other hand, if the internal value of the multiplication exceeds the 1.0㎜ · m / min ^2, is a rapid reduction, is applied at least within the (耐) weight load on the cast steel support roll, as well as result in equipment damage, the cast steel 32 Causes breakage.

The rolling speed in the short-side-width concave downward descending bar 37 is set so that the roll rate (mm / m) when the roll opening degree of the cast support rolls is gradually narrowed toward the downstream side in the casting direction, ). ≪ / RTI > The reduction rate in the low pressure lowering stage 36 is also a product of the roll gage (mm / m) of the low pressure lowering stage 36 and the casting speed (m / min).

In the intentional bulge 38, the total amount of bulge is set at 3 to 20 mm. Assuming that the total amount of the bulging is less than 3 mm and that the short side of the cast steel 32 is not pressed down in the next tough load bar 36, The improvement effect of center segregation may be insufficient. Therefore, in order to obtain the effect of improving the center segregation of the cast steel 32, the total amount of the deliberate bulging is set to 3 mm or more. On the other hand, if the total amount of deliberate bulging exceeds 20 mm, there is a possibility that internal cracking may occur in the cast steel 32 due to distortion caused by the bulging. Therefore, in order to prevent internal cracking of the cast steel 32, the total amount of the deliberate bulging is made 20 mm or less.

In addition, in the intentional pulverizer 38, the amount of increase in roll opening per roll is preferably 1.5 mm or less. This is to prevent cracks from occurring at the boundary position between the bulging portion of the long side of the cast steel and the non-bulging portion.

When the solidification rate at the center of the slab thickness is 0.2 or less, the slab 36 starts to be pressed down and continues to be pressed until the solidification rate at the center of the slab becomes 0.9 or more. This is because even if the solidification rate at the center of the thickness of the cast steel exceeds 0.2, the rolling of the molten steel may occur before the start of the rolling, and therefore the center segregation occurs, . The flow of molten steel is likely to occur until the solidification rate reaches 0.9, and if the rolling is stopped earlier than this, the flow of the molten steel is generated, resulting in center segregation, The effect can not be obtained sufficiently. The center segregation of the cast steel 32 can be reliably prevented by lowering the steel casting to a point of 0.9 at the time when the solidification rate at the center of the cast steel thickness is 0.2.

The solidification rate at the center of the slab thickness can be obtained by two-dimensional electrothermal solidification calculation as in the case of obtaining the liquid phase line crater end position. Here, the solid phase rate is defined as a solid phase ratio before the start of solidification and a solid phase ratio = 1.0 at the completion of solidification. Therefore, the position at which the solidification rate at the center of the slab thickness is 1.0 is the solidification completion position 35 (solidus crater end position), and the position of the liquid phase crater end is the position at which the solidification rate at the center of the slab thickness becomes zero And corresponds to the position on the downstream side.

The solidification rate at the center of the thickness of the slab can also be obtained by a solidification completed position detecting device using transverse ultrasonic waves or longitudinal ultrasonic waves, which can detect the solidification completion position 35 on-line. The solidified position detecting device is a device that transmits transverse ultrasonic waves or longitudinal ultrasonic waves to the cast piece 32 and detects the solidified position 35 on-line based on the propagation time in the cast piece 32 of these ultrasonic waves.

Specifically, an accurate position of the solidification completion position 35 is obtained by the solidification completion position detecting device, and a method such as two-dimensional electrothermal solidification calculation is used together with the obtained solidification completion position 35 as a reference, In the casting direction. The solidification rate at the center of the thickness of the steel strip can be obtained also by the solidified position detecting device for detecting the solidification completion position 35 by using the property that the transverse ultrasonic wave does not pass through the liquid phase. That is, based on the coagulation completion position 35 coinciding with the installation position of the transverse ultrasonic wave sensor, a method such as two-dimensional heat transfer coagulation calculation or the like is used together with the coagulation completion position 35 as a reference, To obtain the solidification rate in the casting direction.

Therefore, as shown in Fig. 2, in the vicinity of the exit (exit) of the slender continuous casting machine 21 according to the second embodiment of the present invention, It is preferable that the ultrasonic transmission sensor 39 and the ultrasonic reception sensor 40 are disposed. The coagulation completion position detecting device is a constituent device other than the coagulation completion position detecting device and is constituted by a sending section for sending a signal to the ultrasonic transmitting sensor 39 or a feeding section for feeding the cast 32 using a calculation formula or the like based on the receiving signal received by the ultrasonic receiving sensor 40. [ And a solidification completed position calculator for obtaining a solidification completion position 35 of the solidification completion position calculator 35. However, these are omitted in Fig. As the solidified position detecting device using ultrasonic waves, there are a device of a system of obtaining a solidifying completion position 35 from a propagation time in a casting piece 32 of a transverse ultrasonic wave or a longitudinal ultrasonic wave, or a device of a type in which a transverse ultrasonic wave does not pass through a liquid phase, There is an apparatus in which the completion position 35 is obtained and any type of solidified position detecting device may be used as long as the solidifying position 35 can be obtained.

In the second embodiment of the present invention, in order to prevent segregation in the center of the cast steel 32 and segregation in the vicinity of the thickness center portion, the product of the descending speed and the casting speed in the light- 1.0 mm · m / min ² is applied to the cast steel 32. When the product of the reduction speed in the light-pressure reduction bed 36 and the casting speed is less than 0.3 mm · m / min ² , the reduction amount per unit time is small relative to the solidification shrinkage amount, not. On the other hand, when the product of the reduction speed in the light-load-reduction bed 36 and the casting speed exceeds 1.0 mm · m / min ² , the amount of the reduction per unit time becomes excessively large and the non-solidified molten steel is squeezed to the upstream side, There is a fear that a negative segregation (a state in which the solute concentration is lower than the ambient) is generated.

The molten steel 31 injected into the mold 25 from the turn-dish 22 through the immersion nozzle 24 is cooled in the mold 25 to form the solidifying shell 33. The cast steel 32 having the solidified shell 33 as an outer shell and having the non-solidified layer 34 therein is wound around a support roll 26, a guide roll 27 and a pinch roll Is continuously drawn downwardly of the mold (25) while being supported on the mold (28). The cast steel 32 is cooled by the secondary cooling water in the secondary cooling zone while passing through these cast steel supporting rolls to increase the thickness of the solidification shell 33. [ The cast steel 32 reduces the thickness of the cast steel in the short side narrowing downward descending bar 37 while increasing the thickness of the portion of the long side of the cast steel except for the short side end in the intended gathering base 38, In the low pressure lowering stage (36), the solidification from the solidification completion position (35) to the inside is completed while being lightly pressed. After completion of solidification, the cast steel 32 is cut by the cast steel cutter 30 to become the cast steel 32a.

In the second embodiment of the present invention, the total amount of the pressure reduction in the light-load-reduction bed 36 is adjusted so as to be equal to or smaller than the total amount of the bulging in the intentional gathering band 38. The solidification rate at the center of the thickness of the cast steel at the time of entering the light-and-rolled bed 36 is 0.2 or less and the solidification rate at the center of the thickness of the cast steel at the time of exiting the light- The cooling rate of the secondary cooling, the reduction of the width of the secondary cooling, and the casting speed. The control of the solidification rate at the center of the cast steel thickness can be easily carried out by obtaining the thickness of the solidification shell 33 and the solidification rate at the center of the cast steel thickness in various casting conditions by using two-dimensional electrothermal solidification calculation or the like in advance have. Also, the solidification rate at the center of the thickness of the billet can be easily controlled by obtaining the solidification rate at the center of the billet thickness on-line using the coagulation complete position detecting device. Here, " reduction in the width of the secondary cooling " is to stop the spraying of the cooling water to both ends of the long side of the cast strip. By performing the reduction of the width of the secondary cooling, the secondary cooling is weakly cooled, and generally the solidified position 35 extends to the downstream side in the casting direction.

As described above, according to the second embodiment of the present invention, at the stage where the deformation strength of the solidification shell (33) is low, the short side of the cast strip (32) It is possible to suppress the total amount of the bulging at the time of intentionally bulging, thereby preventing the internal cracking of the cast steel 32. Further, in the case of the low-pressure reduction, since the product of the time of light-pressure reduction and the reduction rate and the casting speed is specified, the center segregation of the cast piece 32 can be stably reduced.

The continuous casting machine shown in FIG. 2 is a vertical bending continuous casting machine, but a second embodiment of the present invention can be applied to a curved continuous casting machine or a vertical continuous casting machine as described above . In addition, the effect of the light pressure on the center segregation of the cast steel 32 is affected by the solidification structure of the cast steel 32. Concretely, when the solidification structure at the center of the thickness of the cast steel 32 is equiaxed, the concentrated molten steel causing the semi-macro segregation exists in the equiaxed crystal, and the effect of improving the center segregation by the light hardening is deteriorated. Therefore, in the second embodiment of the present invention, it is preferable to set the casting conditions so that the solidification structure at the center of the thickness of the cast steel 32 is a columnar solid structure.

Example 1

A test applying the first embodiment of the present invention in which the cast steel was intentionally bulged and then rolled down was carried out by using the slab continuous casting machine shown in Fig. Cast width 2100㎜, the cast thickness, has a thickness (D ₀₎ immediately before the start of a bulging 250㎜. The casting speed was 0.85 to 1.2 m / min, and the secondary cooling ratio (water content) of the cast steel was 1.0 to 2.0 liters / steel-kg. The total amount of intentional bulging was 3.0 mm to 21.0 mm. The solidification shell thickness immediately before the bulging, the gradient of the roll opening degree at the time of the bulging, the casting direction length (the section in which the roll opening degree of the guide roll is set constant) for making the thickness of the post-bulge casting constant is fixed, Respectively. The cast steel is a steel grade for a steel sheet having a carbon concentration of 0.05 to 0.08 mass%.

In each test, a full width test specimen having a length of 1,000 mm in the casting direction was taken from the test piece corresponding to the normal casting condition, and one cross section sample having a thickness of 50 mm was cut out from the test piece. Analysis of manganese (Mn) was carried out using EPMA (Electron Probe Micro Analyzer) in the thickness direction of the cast steel at the center of the width of the cross section sample. Fig. 4 is a view showing sampling positions of cross-sectional samples and analysis positions of manganese by EPMA. Fig. Analysis of manganese by EPMA revealed that the center segregation at the center of the thickness of the steel strip and the segregation at the center of the thickness of the steel strip (the segregation where the concentration of the solute component becomes higher than the initial concentration) and the fragile seas (the segregation ).

The manganese analysis values measured at each position of the cross-sectional sample are divided by the manganese concentration of the analytical sample taken from the ladle into the manganese concentration (manganese concentration (% by mass) at each position of the cross-sectional sample / The manganese concentration (mass%) of the analytical sample collected from the molten steel was determined as the degree of segregation, and the distribution of the degree of segregation in the direction of the thickness of the slab was investigated. And the highest value among the center segregation and the segregation near the center of the thickness of the cast steel was defined as the degree of segregation of the cast steel. In addition, the presence or absence of internal cracking was examined from the sulfur print test of the cross-sectional samples.

Table 1 shows test conditions and survey results. In the remarks column of Table 1, the tests conducted under the conditions within the scope of the first embodiment of the present invention are referred to as "the present invention" and the others are referred to as "comparative example".

In the test examples Nos. 1 to 6 of the present invention, center segregation and gypsum near the center of the thickness of the cast steel did not occur and no internal cracks occurred. Test Nos. 1 to 6 were conducted under the following conditions: the solidification shell thickness immediately before the bulging, the total amount of the bulging, the length in the casting direction of the section in which the thickness of the cast steel after the bulging was made constant, the product of the pressing speed and the casting speed, All five conditions are within the scope of the first embodiment of the present invention and the gradient of the roll opening degree at the time of the bulging is also within the preferable range of the first embodiment of the present invention.

In Test No. 7, the gradient of the roll opening degree at the time of the bulging was out of the suitable range of the first embodiment of the present invention, and slight internal cracking occurred. However, the thickness of the solidification shell just before the bulging was 25 mm thick, and the internal cracking was slight. Other conditions were within the scope of the first embodiment of the present invention, and no center-segregation and gypsum stones near the center of the thickness of the cast steel occurred.

Test No. 8 showed that the casting direction length of the section for making the cast steel thickness after bulging was outside the range of the first embodiment of the present invention and that the segregation degree was 1.090 near the center of the cast steel thickness. This is because the casting direction length of the zone in which the thickness of the cast steel after bulging is constant is too short, so that it is not possible to uniformly reduce the cast steel in the casting width direction.

Test No. 9 indicates that the solidification shell thickness immediately before the bulging and the casting direction length of the section in which the thickness of the cast steel after bulging is constant are outside the range of the first embodiment of the present invention, And a segregation with a segregation degree of 1.085 occurred in the vicinity of the center. The occurrence of internal cracking is due to the fact that the thickness of the solidification shell immediately before the bulging is outside the scope of the first embodiment of the present invention. In addition, the generation of the slabs is too short in the casting direction length of the section where the thickness of the slab after the bulging is made constant, so that it is not possible to uniformly reduce the slab widthwise direction.

Test No. 10 indicates that the solidification shell thickness immediately before the bulging is out of the range of the first embodiment of the present invention and since the gradient of the roll opening degree at the time of the bulging is outside the preferable range of the first embodiment of the present invention, Internal breakage occurred. However, since the other conditions are within the range of the first embodiment of the present invention, center segregation and gypsum stones near the center of the thickness of the cast steel did not occur.

Test No. 11 had internal cracks in the cast steel in that the total amount of the bulges exceeded 20 mm, which is the range of the first embodiment of the present invention. Further, since the product of the descending speed and the casting speed was smaller than the range of the first embodiment of the present invention, the effect of the depressurization was not obtained, and a slab with a segregation degree of 1.158 occurred near the center of the casting thickness.

Test No. 12 is a test to be carried out when the solidification rate at the center of the cast steel thickness is 0.95. Since the pressing time is out of the range of the first embodiment of the present invention, the effect of the pressing down is not obtained. As a result, a slab with a segregation degree of 1.198 occurred near the center of the thickness of the slab. However, since the solidification shell thickness and the total amount of the bulging immediately before the bulging are within the range of the first embodiment of the present invention and the gradient of the roll opening degree at the time of the bulging is also within the suitable range of the first embodiment of the present invention, Did not occur.

Example 2

A test for casting by applying the second embodiment of the present invention was conducted using a slab continuous casting machine having the structure shown in Fig. 2 (Inventive Tests: Test Nos. 21 to 25). The width of the cast steel was 2100 mm, the thickness of the cast steel was 250 mm, and the cast steel slab cast steel with a carbon content of 0.05 to 0.08% by mass was cast. The casting speed was 0.85 ~ 1.42m / min and the secondary cooling rate was 1 ~ 2 liters / steel-kg.

The total amount of the bulging in the small-width narrowing lowering zone is 3.0 to 20.0 mm, the total amount of bulging in the intended bulging zone is 3.0 to 20.0 mm, and the total reduction amount in the light- Or less than that. In addition, the solidification rate at the center of the thickness of the cast steel at the time of termination under light pressure was 0.9 or more.

Further, for comparison, it is preferable that the total amount of the reduction in the short-side width narrowing downward bar, the total amount of the bulging in the intentional bulging, the product of the reduction speed and the casting speed in the light- (Comparative Examples: Test Nos. 26 to 30).

In each casting test, a full width test piece having a length in the casting direction of 1000 mm was taken from a cast piece corresponding to a normal casting condition, and one cross section sample having a thickness of 50 mm was cut out from the full width test piece , And manganese was analyzed using EPMA in the thickness direction of the cast steel at the center of the width of the cross section sample. Analysis of manganese by EPMA revealed that the center segregation at the center of the thickness of the steel strip and the segregation at the center of the thickness of the steel strip (the segregation where the concentration of the solute component becomes higher than the initial concentration) and the fragile seas (the segregation ).

Manganese analysis (mass% Mn) measured at each position of the cross-sectional sample was divided by the manganese concentration (mass% Mn ₀ ) of the analytical sample taken from ladle steels before casting (mass% Mn / mass% Mn ₀ ) was determined as a segregation degree, and the distribution of the segregation degree in the thickness direction of the cast steel was examined. And the highest value among the center segregation and the segregation near the center of the thickness of the cast steel was defined as the degree of segregation of the cast steel. In addition, the presence or absence of internal breakage was examined from the sulfur print test of the cross section sample. Table 2 shows the test conditions and investigation results.

In the test examples Nos. 21 to 25 of the present invention, center segregation at the center of the cast steel thickness and gypsum stones in the vicinity of the center of the cast steel thickness did not occur and no internal cracks occurred.

Test No. 26 is a test in which the total amount of reduction in the short-side width narrowing downward bar is 23.0 mm which is outside the range of the second embodiment of the present invention. The other conditions were within the scope of the second embodiment of the present invention, but the total amount of reduction in the short-side width coining downfalling zone was too large and internal cracking occurred in the casting.

Test No. 27 is a test in which the total intentional bulge amount is 30.0 mm, which is outside the scope of the second embodiment of the present invention. The other conditions were within the scope of the second embodiment of the present invention, but the total amount of the bulging was too large, and internal cracking occurred in the cast.

Test No. 28 is a test in which the product of the descending speed and the casting speed at the time of tough pressing is 0.2, which is outside the scope of the second embodiment of the present invention. Although the internal cracks did not occur in the cast, the manganese segregation degree was 1.080, and the center segregation was confirmed by the lack of hardness.

Test No. 29 is a test in which the total reduction amount in the short-width narrowing down-pressure zone is 2.0 mm outside the scope of the second embodiment of the present invention. Other conditions were made within the scope of the second embodiment of the present invention. As a result, internal cracking did not occur, but center segregation with a manganese segregation degree of 1.040 was confirmed although it was slight. This is presumed to be because the effect of reducing the thickness of the short side is weakened from the point that the total reduction amount in the short-side width narrowing downward bar is small, and the effect of light-

Test No. 30 is an example in which the hardness is lowered after the solidification rate at the center of the cast steel reaches 1.0 (completely solidified), which is outside the scope of the second embodiment of the present invention. After the completion of the solidification, no effect was observed even at a low temperature, and center segregation with manganese segregation degree of 1.080 was recognized.

1: Slab continuous casting machine
2: Turn Dish
3: Sliding nozzle
4: immersion nozzle
5: Mold
6: guide roll
7: Throw roll
8: Pinch roll
9: Molten steel
10: Casting
11: Solidification shell
12: Uncoagulated layer
21: Slab continuous casting machine
22: Turn Dish
23: Sliding nozzle
24: immersion nozzle
25: Mold
26: Support roll
27: guide roll
28: Pinch roll
29: conveying roll
30: slab cutting machine
31: molten steel
32: Casting
33: Solidification shell
34: Uncoagulated layer
35: Coagulation completion position
36: light pressure drop
37: Short-side width reduction
38: Intentional punishment
39: ultrasonic transmission sensor
40: Ultrasonic receiving sensor

Claims (4)

The roll gap of the casting support rolls arranged in the casting direction until the coagulating shell thickness of the casting pellets drawn from the casting mold for continuous casting reaches at least 15 mm, The same as the roll opening degree,
Thereafter, the roll opening degree of the casting support roll is gradually increased toward the downstream side in the casting direction to bulge the longer side of the casting slab with a total amount of bulging of 3 to 20 mm,
The roll opening degree of the casting support roll is set to be constant so that the thickness of the casting slab is not changed in the section of 0.5 to 5.0 m downstream from the casting direction,
Subsequently, the long side face of the cast steel having a solid phase ratio of 0.2 to 0.9 at the center of the cast steel is pressed at least once by a roll-down roll under the condition that the product of the rolling speed and the casting speed is 0.3 to 1.0 mm · m / min ² , Continuous casting method of steel. The method according to claim 1,
Wherein the roll opening degree of the cast strip supporting rolls arranged in the casting direction is gradually increased to a slope of 4.0 mm or less per 1 m in the casting direction when the long side face of the casting is bulged. Wherein a roll opening degree of a plurality of pairs of cast strip supporting rolls is gradually decreased toward the downstream side in the casting direction so that the product of the descending speed and the casting speed is 0.3 to 1.0 mm on the long side surface of the cast steel having a rectangular cross- , and give the smaller screw down force is m / min, and ^2, 3~20㎜ than cast byeonpok end (shorter side width) of the mold in the bottom of the short-side width of the slab by the rolling force,
The roll opening degree of the plural pairs of slab support rolls is gradually increased toward the downstream side in the casting direction and the long side face of the cast slab is bulged to a total amount of 3 to 20 mm,
In a tough reduction zone in which the roll opening degree of a plurality of pairs of slab support rolls is gradually decreased toward the downstream side in the casting direction after the long side surface of the cast slab is bulged, Min to a point at which the ratio of the reduction rate to the casting speed becomes 0.3 to 1.0 mm · m / min ² from the point of time when the ratio of the total amount of the extrudate to the total amount of the extrudate Or the long side surface of the cast steel is lowered by the total amount of the reduced load smaller than the total amount of the bulges. The method of claim 3,
The solidification completion position of the cast piece is detected online by using the solidification completion position detecting device and based on the detected information of the solidification completion position, when the solidification rate at the center portion of the casting is at least 0.2, , Wherein either one or two or more of the secondary cooling quantity, the width reduction of the secondary cooling, and the casting speed are adjusted such that the main blade is positioned on the above-mentioned hardly-depressed bed.

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