JP5636913B2 - Continuous casting method for slabs - Google Patents

Description

  The present invention relates to a continuous casting method of a slab, and more particularly to a continuous casting method capable of obtaining a slab having no center segregation over the entire width.

  When a slab is manufactured using a continuous casting method, the occurrence of internal defects often called center segregation becomes a problem. This center segregation is a phenomenon in which molten steel components such as C, S, P, and Mn are positively segregated at the center part (final solidified part) in the thickness direction of the slab. This phenomenon is a particularly serious problem in a thick plate material, and it is known that it causes a decrease in toughness and hydrogen-induced cracking in a portion where center segregation occurs.

  Central segregation is caused by unsolidified molten steel (hereinafter referred to as “residual molten steel”) between dendrites (resin-like crystals) at the end of solidification of the molten steel due to solidification shrinkage of the molten steel or bulging of the solidified shell. It is found that this occurs because the molten steel that has moved macroscopically toward the solidification completion point of C and concentrated in C, S, P, Mn, etc. (hereinafter referred to as “concentrated molten steel”) is locally accumulated. ing.

  As measures to prevent the occurrence of center segregation, the movement of residual molten steel and the accumulation of concentrated molten steel can be achieved by rolling down the vicinity of the solidification completion point with a roll or by using a mold or the like to reduce the concentration of molten steel. There are methods for preventing this, and methods based on various technical ideas have been proposed.

  For example, in Patent Document 1, by increasing the amount of secondary cooling water sprayed on the surface of the slab, the surface temperature of the final solidified portion of the slab is set in the range of 700 to 800 ° C., and the thickness of the solidified shell is increased. By suppressing the bulging between rolls, and further applying a rolling force with a strain rate of 0.2 to 0.4% per minute to the slab in the group of lightly rolling rolls, the flow of the concentrated molten steel is prevented, and the center segregation A method for preventing this problem has been proposed. Such light rolling by the group of rolling rolls has a problem in that solidification shrinkage and bulging between rolls cannot be sufficiently suppressed because rolling can be performed only in a point shape with respect to the longitudinal direction of the slab.

  Patent Document 2 discloses a method of continuously forging the vicinity of a solidification completion point of a slab with a mold having a flat work surface. However, this method has a drawback that the equipment cost is very high because it is necessary to provide a forging machine using a mold.

  Patent Document 3 discloses a method for preventing central segregation by temporarily bulging a slab including an unsolidified portion and reducing the amount corresponding to the bulging amount immediately before completion of solidification. However, in this method, when there is a large area of the unsolidified layer due to the solidification delay and the reduction in the vicinity of both ends in the slab width direction is insufficient, the center segregation in the vicinity of both ends in the slab width direction is insufficient. This may not be sufficient, and further improvements to uniform solidification in the width direction are desired.

  In Patent Document 4, after bulging a slab having an unsolidified portion, the slab is squeezed with a squeezing roll. A method is disclosed in which the thickness of the unsolidified portion is made uniform in the slab width direction by cooling with the increased secondary cooling water. However, in steel with high crack sensitivity, even if the amount of secondary cooling water is positively increased, insufficient cooling due to non-contact with the roll cannot be compensated. Furthermore, a slight internal crack generated when the above-mentioned unsolidified slab is bulged may become a product defect.

  As a countermeasure against uneven solidification in the width direction of the slab as described above, the applicant of Patent Document 5 controls the discharge flow of the molten steel by disposing two or more immersion nozzles in the mold. We proposed a method to prevent center segregation by eliminating uneven solidification in the width direction of the slab and continuously rolling down a slab having an unsolidified region of optimal shape.

JP 63-252655 A Japanese Patent No. 2915544 JP-A-9-57410 Japanese Patent No. 3960249 Japanese Patent No. 3077572

  Although the method proposed by the present applicant in Patent Document 5 can improve the center segregation, it is difficult to control the discharge flow rate from each immersion nozzle, and the fluctuation of the discharge flow rate causes fluctuations in the molten steel downward flow ( Variations in the flow velocity) occurred, and there were sporadic cases in which uneven solidification in the width direction of the slab could not be resolved, and there was room for improvement in its stabilization.

  As described above, in the conventional continuous casting method, the level of center segregation can be improved in the width direction of the slab, but it is difficult to always make it uniform uniformly in the width direction.

  Therefore, when continuously casting high-strength steels with high cracking sensitivity, such as those manufactured by ingots, and thick-walled materials that generate large bending strains, such as those for UOE pipes, the center segregation level is However, even if it is relatively minor, there remains a problem that a strong segregation of a part of the center causes a crack at the time of pipe making.

  This invention is made | formed in view of said subject, The objective is to provide the continuous casting method which can eliminate center segregation over the slab full width.

  In order to solve the above-mentioned problems, the inventors have ensured that the thickness of the unsolidified portion in the slab width direction is uniform, and the concentrated molten steel remains when the shape of the crater end (tip of the unsolidified portion) is reduced. We investigated the control to a shape that is difficult to perform.

  As a result, two immersion nozzles for supplying molten steel to the mold are arranged in parallel in the width direction of the mold, and the discharge speed of the molten steel from each immersion nozzle faces and collides with each other, so that the speed in the width direction of the molten steel mold It has been found that it is effective to control the distribution so that it is convex downward in the casting direction (U-shaped state). By controlling the discharge flow of the molten steel in this way, the thickness of the unsolidified part in the slab width direction can be made uniform, and the shape of the crater end in the longitudinal section including the center in the slab thickness direction This is because when the cast slab is reduced, the concentrated molten steel is less likely to remain in a U shape. Furthermore, it has been found that by reducing the slab to the completion of solidification, inflow of concentrated molten steel at the final solidification position of the slab can be prevented, and central segregation can be eliminated.

  Furthermore, it is possible to form a uniform downward flow in the mold width direction compared to the case where all of the opposed discharge flows from each immersion nozzle do not collide, and a part of them is allowed to escape slightly. In the cast slab, no center segregation was observed, the segregation degree was reduced, and the variation in the segregation degree was found to be small.

This invention is made | formed based on the said knowledge, The summary exists in the continuous casting method of the slab shown to following ( 1)- (3) .

(1) In the continuous casting method in which the molten steel is supplied from the immersion nozzle into the mold, the supplied molten steel is pulled out while solidifying, and the slab including the unsolidified portion is reduced using a reduction roll until solidification is completed, the immersion nozzle As described above, two immersion nozzles having one discharge hole for molten steel on the side surface are used, and each immersion nozzle is arranged in the width direction of the mold so that the molten steel discharged from the discharge hole collides with each other. The discharge direction of the molten steel from the hole is inclined at an angle of 5 ° to 25 ° downward in the casting direction with respect to the molten steel surface in the mold, and at the same time, the components in the direction parallel to the molten steel surface in the mold are parallel to each other. There, and are deflected at an angle of 5 ° to 15 ° with respect to the width direction of the mold, the while blowing with the following flow rates each immersion nozzle Ar gas per one of 5 NL / min or more 15 NL / min, the Continuous casting method of the slab, characterized in that Ar gas Deana is ejected molten steel mixed.

(2) As the reduction roll, a large diameter roll having a diameter 1.5 times or more that of the largest diameter among the roll groups that support and guide the slab, and the amount of reduction is 5 to 5 of the slab thickness. The slab continuous casting method according to (1 ), wherein the reduction is performed as 20%.

(3) As the reduction roll, a plurality of pairs of small-diameter rolls having a diameter less than 1.5 times that of the largest diameter among the group of rolls that support and guide the slab, and the reduction amount is 5 of the slab thickness. The method for continuous casting of a slab according to (1 ), wherein the reduction is performed as% or less.

  According to the continuous casting method of the slab of the present invention, the thickness of the unsolidified portion in the slab width direction is controlled by controlling the flow of the molten steel in the slab formed by the molten steel discharge flow from the immersion nozzle. While ensuring uniform, the shape of the crater end can be controlled so that the concentrated molten steel does not remain when reduced, and the amount of reduction sufficient to prevent inflow of the concentrated molten steel at the final solidification position of the slab Therefore, it is possible to obtain a slab with no center segregation in the entire width.

BRIEF DESCRIPTION OF THE DRAWINGS It is a figure which shows the outline of the longitudinal cross-section of the vertical bending type continuous casting machine which can be used for the continuous casting method of the slab of this invention, The figure (a) shows a large diameter roll in order to greatly squeeze a slab. When employed, FIG. 4B shows a case where a small-diameter roll is employed to lightly reduce the slab. It is the schematic of the immersion nozzle and mold periphery seen from the front direction of the casting_mold | template, The figure (a) shows an example at the time of applying the conventional continuous casting method, The figure (b) is the slab of this invention. An example when the continuous casting method is applied is shown. It is the schematic of the slab cross section which has an unsolidified part, The figure (a) shows the case where the method demonstrated in the said FIG. 2 (a) is used, The same figure (b) is the said FIG.2 (b). A case where the method described in the above is used is shown. It is a figure which shows the arrangement | positioning relationship of the casting_mold | template and two immersion nozzles seen from the casting direction, The figure (a) shows the case where the discharge hole is not deflected, The figure (b) shows that the deflection angle of the discharge hole is 5 degrees. (C) in the figure shows a case where the deflection angle of the ejection hole is 10 °.

As described above, the continuous casting method of the slab of the present invention supplies molten steel from the immersion nozzle into the mold, pulls out the supplied molten steel while solidifying it, and rolls the slab including the unsolidified part until the solidification is completed. In the continuous casting method that uses and presses down, as the immersion nozzle, two immersion nozzles each having one discharge hole for molten steel are used on the side surface, and each immersion nozzle is arranged so that the molten steel discharged from the discharge hole collides with each other. The molten steel is disposed in the width direction of the mold, and the discharge direction of the molten steel from the discharge holes is inclined at an angle of 5 ° to 25 ° downward in the casting direction with respect to the molten steel surface in the mold, and at the same time, the molten steel in the mold The components in the direction parallel to the molten metal surface are parallel to each other and are deflected at an angle of 5 ° to 15 ° with respect to the width direction of the mold, and Ar gas is supplied to each immersion nozzle at a rate of 5 NL / min. 15 NL / m While blowing in the following flow rate n, Ar gas from the discharge hole is a method of ejecting a molten steel mixed. Hereinafter, the contents of the present invention will be described in comparison with a conventional example.

  FIG. 1 is a diagram showing an outline of a vertical section of a vertical bending die continuous casting machine that can be used in the continuous casting method of a slab according to the present invention. FIG. When a large-diameter roll is employed, FIG. 5B shows a case where a small-diameter roll is employed in order to lightly reduce the slab. As shown in FIGS. 2A and 2B, molten steel 1 supplied from a ladle and a tundish (both not shown) is injected into a mold 4 from an immersion nozzle 3. An inert gas such as Ar is blown into the immersion nozzle 3 so as to be mixed into the molten steel 1. The molten steel 1 is solidified by primary cooling in the mold 4 to form a solidified shell 2 a that constitutes the surface of the slab 2. The slab 2 holding the unsolidified portion (molten steel 1) inside is supported and guided by the following support roll group 5, and solidification is promoted by secondary cooling by spraying cooling water or the like. And then completely solidified and pulled out by a pinch roll (not shown).

  Moreover, the slab 2 is rolled down by completion of solidification. The slab 2 is reduced by a pair of upper and lower large-diameter rolls 6 in the continuous casting machine shown in FIG. 5A, and by a plurality of small-diameter rolls 7 in the continuous casting machine shown in FIG. . The amount of reduction is preferably 5 to 20% of the slab thickness when a large diameter roll is used, and is preferably 5% or less of the slab thickness when a small diameter roll is used. Here, the large-diameter roll means a roll having a diameter 1.5 times or more that of the largest diameter among the rolls constituting the support roll group 5. The small diameter roll means a roll having a diameter less than 1.5 times that of the largest diameter among the rolls constituting the support roll group 5.

  FIG. 2 is a schematic view of the immersion nozzle and the periphery of the mold as viewed from the front direction of the mold. FIG. 2 (a) shows an example in which a conventional continuous casting method is applied, and FIG. 2 (b) shows the present invention. An example in the case of applying the continuous casting method of slabs is shown. FIG. 3 is a schematic view of a cross section of a slab having an unsolidified portion. FIG. 3A shows a case where the method described in FIG. 2A is used, and FIG. The case where the method demonstrated in 2 (b) is used is shown.

  FIG. 2A shows a case where one immersion nozzle 3 having two discharge holes 3 a on the side surface is arranged in the mold 4 as a conventional continuous casting method. The two discharge holes 3a are respectively opposed to the short side surface (surface perpendicular to the mold width direction) of the mold 4, and the discharge direction of the molten steel is a predetermined angle downward with respect to the molten steel surface in the mold in the casting direction. It is provided so as to be inclined. As shown in the figure, the discharge flow of molten steel from each discharge hole 3a is separated into a downward flow a2 downward in the casting direction and an upward flow a1 upward in the casting direction, and the center in the width direction of the slab 2 by the downward flow a2. A downward flow a <b> 3 toward the intermediate portion between the portion and the end portion is formed. Therefore, the velocity distribution Va in the mold width direction of the unsolidified portion (molten steel 1) in the solidified shell 2a is minimized at the central portion and a portion in contact with the solidified shell 2a, and is maximized at an intermediate portion between the central portion and the end portion. , W-shaped non-uniform distribution.

  As a result, also in the crater end of the slab 2, the longitudinal section including the center in the slab thickness direction is W-shaped. Further, due to the molten steel flow, the solidified shell 2a of the slab 2 has a thick cross-section at the center in the width direction of the slab 2 as shown in FIG. The middle part is thin. In this case, if the amount of reduction of the slab 2 is about the thickness of the unsolidified portion at the center in the width direction of the slab 2, the solidified shell 2a is sufficiently reduced at the center of the slab 2 in the width direction. Although the solidified molten steel 1 is discharged, since the reduction is insufficient at an intermediate portion between the center portion and the end portion, the unsolidified molten steel 1 that has not been discharged remains as segregation after solidification.

  FIG. 2B shows a case where two immersion nozzles 3 are arranged in the mold 4, which is a continuous casting method of a slab according to the present invention. As shown in FIG. 4 described later, the two immersion nozzles 3 are preferably arranged symmetrically with respect to the center line in the mold width direction on the center line in the mold thickness direction.

  Each immersion nozzle 3 is provided with one discharge hole 3a on the side surface so that the discharge direction of the molten steel is inclined at an angle of 5 ° to 25 ° downward in the casting direction with respect to the molten steel surface in the mold. . If the inclination angle is less than 5 °, the downward speed of the discharge flow in the casting direction cannot be obtained sufficiently, and if it exceeds 25 °, the discharge flow becomes unstable with fluctuations, and the solidification of the slab becomes uneven. .

  The discharge holes 3a of the respective immersion nozzles 3 are arranged so that the discharge directions of the molten steel face each other. Hereinafter, a state in which only one discharge hole 3 a is provided on the side surface of the immersion nozzle 3 is referred to as “one hole”.

  By disposing two single-hole immersion nozzles 3 so that the discharge holes 3a face each other, the molten steel discharge flow b1 from each discharge hole 3a is cast as shown in FIG. The downward flow b3 in the direction downward and the upward flow b2 in the casting direction upward are separated. When the upward flow b <b> 2 comes into contact with the liquid surface of the molten steel, the upward flow b <b> 2 reverses in the molten steel direction, and forms a downward flow b <b> 4 along the side wall of the mold 4. The downward flow b3 downward in the casting direction merges at the center in the width direction of the slab 2 to form a flow having a high speed. In the vicinity of the solidified shell 2a, an upward flow b5 having a small upward speed in the casting direction is generated. In this way, the velocity distribution Vb in the mold width direction of the unsolidified portion (molten steel 1) in the solidified shell 2a formed by the molten steel discharged from each discharge hole 3a is minimal and central at the portion in contact with the solidified shell 2a. The distribution is maximally flat and has a substantially flat bottom (U-shaped) distribution.

  As a result, even in the crater end of the slab 2, the longitudinal section including the center in the slab thickness direction is U-shaped. Further, due to the molten steel flow, the solidified shell 2a of the slab 2 has a substantially uniform thickness at the center as shown in FIG. 3 (b), and becomes thicker toward the end. When the crater end and the solidified shell have such a shape, the solidified shell over the entire width direction of the entire slab 2 in the vicinity of the casting direction upstream of the solidification completion position of the slab 2 regardless of the reduction position. Since 2a is sufficiently and uniformly reduced and the unsolidified molten steel 1 is discharged, segregation does not occur.

  The inclination angle from the casting direction of the molten steel discharge direction from the two discharge holes 3a is preferably the same for both immersion nozzles 3. However, if the center of the cross section of the mold 4 and the midpoint of the two immersion nozzles 3 cannot be matched due to the arrangement of the immersion nozzle 3 and the mold 4 in the continuous casting machine, each immersion nozzle 3 In order to make the molten steel flow from symmetric with respect to the center line in the mold width direction, the immersion nozzle 3 may have a different angle.

  Furthermore, in the continuous casting method of the slab of the present invention, the inert gas is blown into the immersion nozzle 3 so that the inert gas is mixed into the molten steel discharged from the discharge hole 3a. Thereby, since the flow of the molten steel can be stabilized without causing the immersion nozzle 3 to be blocked by inclusions, the shape of the crater end is controlled to be U-shaped with high accuracy, as shown in FIG. 3 (b). It is possible to stably form the solidified shell 2a having a shape and suppress the occurrence of segregation.

  Ar is used as the inert gas blown into the immersion nozzle 3. The flow rate of Ar gas blown into the immersion nozzle 3 is 5 to 15 NL / min per immersion nozzle. If it is less than 5 NL / min, there is a concern that the molten steel surface in the mold will be covered, and if it exceeds 15 NL / min, the disturbance will increase and the state of the molten metal will become unstable, which may cause powder entrainment. Because there is. The flow rate of Ar gas is preferably 8 to 10 NL / min.

  FIG. 4 is a diagram showing the positional relationship between the casting mold and the two immersion nozzles as seen from the casting direction. FIG. 4A shows the case where the discharge holes are not deflected, and FIG. 4B shows the deflection of the discharge holes. When the angle is 5 °, FIG. 7C shows the case where the deflection angle of the ejection hole is 10 °. As shown in the figure, the immersion nozzle 3 is preferably arranged symmetrically with respect to the center line in the mold width direction on the center line in the mold thickness direction when viewed from the casting direction.

Moreover, in the continuous casting method of this invention, the discharge direction of the molten steel from the discharge hole 3a of each immersion nozzle 3 has a component which goes to the center direction of the casting_mold | template 4, and the component of a direction parallel to the molten steel surface in a casting_mold | template is included. They are parallel to each other, and that are deflected at a predetermined angle with respect to the width direction of the mold. Therefore, as in the discharge direction of the molten steel is such an angle, to set the deflection angle of the discharge hole 3a. Here, the deflection angle refers to the discharge direction of the molten steel and the angle of the discharge hole 3a with respect to the mold width direction in the cross section of the mold 4. As described above, FIG. 4A shows a case where the ejection hole is not deflected, that is, when the deflection angle is 0 °, and FIG. 4B shows a case where the deflection angle is 5 °. Indicates a case where the deflection angle is 10 °.

  By deflecting the discharge hole 3a of the immersion nozzle 3 in this way, all of the discharge flows of the molten steel from the opposing immersion nozzle 3 do not collide with each other, and some of them are allowed to escape slightly. In this case, it is possible to form a downward flow having a velocity distribution in the width direction of the downwardly convex mold, which is more uniform than when the molten steel flow is caused to collide (when the molten steel flow is not deflected). Therefore, the shape of the longitudinal section of the crater end of the slab can be controlled to be U-shaped, and the solidified shell can be controlled to have a substantially uniform thickness at the center and thicker toward the end.

Deflection angle of the discharge direction of the molten steel is 5 ° to 15 °. This is because if the deflection angle exceeds 15 °, the uniformity of the downflow of the molten steel is disturbed, and the shapes of the crater end and the solidified shell cannot be accurately controlled.

  The continuous casting apparatus to which the method of the present invention can be applied is not limited to a vertical mold, but can be applied to any mold such as a curved mold and a vertical bending mold.

  In order to confirm the effect of the continuous casting method of the slab according to the present invention, the following continuous slab casting test was performed and the result was evaluated.

1. Test conditions The vertical bending type continuous casting machine shown in FIG. 1 was used as the continuous casting machine. The continuous casting machine had a vertical part of 2.5 m, a bending radius of 9.4 m, and a machine length of 28 m. The reduction of the slab was performed using a pair of upper and lower large-diameter rolls shown in (a) or a plurality of small-diameter rolls shown in (b). In the reduction using a large-diameter roll, 5 to 20% of the slab thickness was reduced before the completion of solidification. In the reduction using a small-diameter roll, 5% or less of the slab thickness was reduced until the completion of solidification. The casting size of the slab was 250 mm thick and 2300 mm wide, and the casting speed was 0.80 m / min. Further, Ar was used as the inert gas blown together with the molten steel from the immersion nozzle.

  The steel used for casting is mass%, C: 0.02-0.50%, Si: 0.04-0.60%, Mn: 0.50-2.50%, P: 0.050% Hereinafter, S was a composition (slab state) containing 0.01% or less, the balance being Fe and inevitable impurities. This is a steel type used as a thick steel plate.

  Further, the degree of superheated molten steel, the immersion nozzle used and the inclination (discharge angle) of the discharge flow of molten steel from the discharge hole to the molten steel surface in the mold, the reduction method and the length of the reduction zone are as shown in Table 1. . “Large reduction” described in the column of the reduction method in Table 1 is reduction performed using a large diameter roll, and “light reduction” is reduction performed using a small diameter roll. Further, “−” in the column of the reduction zone indicates that the reduction is performed at one place.

  Tests A, B and G are examples of the present invention that satisfy the provisions of the present invention. Tests C to F and H are comparative examples that do not satisfy the provisions of the present invention. Test C has an Ar blowing amount, Test D has an Ar blowing amount and molten steel discharge angle, Test E has a number of dipping nozzles, Test F has a number of dipping nozzles and a reduction amount, and Test H has a dipping nozzle number. Each inclination angle does not satisfy the provisions of the present invention.

  In tests A to F, the components in the direction parallel to the molten steel surface in the mold of the discharge flow from the opposed immersion nozzles are parallel to each other, and the deflection angle with respect to the width direction of the mold is 0 °. And H have deflection angles of 10 ° and 20 °, respectively. That is, in the tests A to F, the discharge flow from the opposed immersion nozzle was not deflected, but was deflected in the tests G and H. The deflection angle is the angle shown in FIG.

  For each test, the distribution of carbon concentration in the cross section perpendicular to the casting direction of the slab after complete solidification was measured, and the following items were evaluated. The carbon concentration was measured using a mapping analyzer.

2. Evaluation items Table 2 shows the evaluation items and the results. The evaluation items were “central segregation distribution”, “segregation area”, “maximum segregation degree of carbon”, and “variation of carbon segregation degree” observed visually. The degree of segregation is a value C / C ₀ obtained by dividing the carbon concentration C at the measurement location by the analysis value C ₀ of the carbon concentration of the molten steel in the tundish. The maximum segregation degree of carbon is the segregation degree with respect to the maximum carbon concentration C _{M at} the center segregation portion, that is, C _M / C ₀ . The segregation area is an area of a region where C / C ₀ > 1.0. The variation in the degree of carbon segregation is the difference C _M -C _m between the maximum carbon concentration C _M and the minimum carbon concentration C _m .

2-1. Comparison of Tests A to F First, the results of tests C to F, which are comparative examples, will be described. As shown in Table 2, in tests C and D using two single-hole immersion nozzles, compared to tests E and F using one two-hole immersion nozzle, the segregation area and the maximum degree of carbon segregation Although all were small and good values, the center segregation remained slightly, and the center segregation was not completely eliminated.

  The reason why the center segregation remained slightly in Test C and Test D was that Ar gas was not flowed together with the molten steel in Test C, Ar gas was not flowed together with the molten steel in Test D, and the vertical flow of the molten steel discharge flow. Since the downward inclination was excessive, the discharge flow of the molten steel from the immersion nozzle was accompanied by fluctuations, and it was considered that the solidification of the molten steel was uneven.

  In contrast to these comparative examples, in the tests A and B which are examples of the present invention, no central segregation was confirmed, and the maximum segregation degree of carbon and the variation of the carbon segregation degree were small.

2-2. Comparison of Tests A, G, and H Further, a case where the discharge flow from the opposed immersion nozzle is deflected with respect to the width direction of the mold will be described. When test A and test G, which are examples of the present invention, were compared, no center segregation was confirmed, and in the deflected test G, the maximum degree of segregation of carbon compared to test A which was not deflected. And the variation in the degree of segregation of carbon was small. This is because a uniform downflow of the molten steel can be formed in the width direction of the slab by causing the discharge flows from the opposed immersion nozzles not to collide with each other and causing them to escape slightly.

  On the other hand, center segregation was present in Test H, which is a comparative example. This is because the uniformity of the downward flow of the molten steel is disturbed because the deflection angle is too large.

  According to the continuous casting method of the slab of the present invention, by controlling the flow in the slab formed by the molten steel discharge flow from the immersion nozzle, the thickness of the unsolidified portion in the slab width direction is made uniform, and the crater Because it is possible to control the shape of the end to a shape in which the concentrated molten steel hardly remains at the time of rolling down, and furthermore, it is possible to provide a rolling amount sufficient to prevent the flow of the concentrated molten steel at the final solidification position of the slab, It is possible to obtain a slab having no center segregation in the entire width. The present invention is very valuable industrially in that a slab having good internal quality can be easily obtained.

1: molten steel, 2: slab, 2a: solidified shell, 3: immersion nozzle, 3a: discharge hole,
4: Mold, 5: Support roll group, 6: Large diameter roll, 7: Small diameter roll,
a1: Upflow, a2: Downflow, a3: Downflow, b1: Discharge flow, b2: Upflow,
b3: Downflow, b4: Downflow, b5: Upflow

Claims (3)

In the continuous casting method of supplying molten steel from the immersion nozzle into the mold, drawing it out while solidifying the supplied molten steel, and rolling down the slab containing the unsolidified part using a reduction roll until solidification is completed.
As the immersion nozzle, two immersion nozzles having one molten steel discharge hole on the side surface are used, and each immersion nozzle is arranged in the width direction of the mold so that the molten steel discharged from the discharge hole collides with each other. ,
The discharge direction of the molten steel from the discharge hole is inclined at an angle of 5 ° to 25 ° downward in the casting direction with respect to the molten steel surface in the mold, and at the same time, the components in the direction parallel to the molten steel surface in the mold are mutually Parallel and deflected at an angle of 5 ° to 15 ° with respect to the width direction of the mold,
A continuous casting method for a slab, characterized in that molten steel mixed with Ar gas is discharged from the discharge hole while Ar gas is blown into each of the immersion nozzles at a flow rate of 5 NL / min to 15 NL / min. As the reduction roll, a large-diameter roll having a diameter 1.5 times or more that of the largest diameter among the roll group that supports and guides the slab, and the reduction amount is 5 to 20% of the slab thickness. 2. The method for continuously casting a slab according to claim 1 , wherein the reduction is performed. As the reduction roll, a plurality of pairs of small-diameter rolls having a diameter less than 1.5 times the maximum diameter of the roll group that supports and guides the slab, and the reduction amount is 5% or less of the slab thickness. 2. The method for continuously casting a slab according to claim 1 , wherein the reduction is performed.

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