JP6384447B2 - Continuous casting method - Google Patents

Description

  The present invention relates to a continuous casting method of steel effective in reducing the center segregation of a slab.

  In continuous casting of steel, molten steel injected into a mold discharges solute elements such as P, S, and Mn into the molten steel in the process of solidification. These solute elements are concentrated in the remaining molten steel and so-called segregation occurs. The degree of segregation is maximized at the center of a continuous cast slab (hereinafter sometimes referred to as a slab) that solidifies. In addition, molten steel causes volume shrinkage of several percent during the solidification process. This volume shrinkage generates a negative pressure void in a solid / liquid coexistence region containing a large amount of equiaxed crystals near the end of solidification of the slab. As a result, the molten steel enriched in the solute element (hereinafter sometimes referred to as concentrated molten steel) passes through the narrow passage in the solid / liquid coexistence region and is sucked into the negative pressure gap to enter the center of the slab. Central segregation is formed.

  Central segregation adversely affects product quality. Therefore, various techniques have been proposed and implemented in order to reduce center segregation. In Patent Document 1, the molten steel superheat degree in the tundish is set to 50 ° C. or less, poured into a mold, electromagnetic force is applied to the molten steel in the strand, and the solidified structure in the center of the slab is finely equiaxed. Crystallization is disclosed. Furthermore, in the range where the solid phase ratio in the central part of the strand cross-section is in the range of 10 to 80%, the flow of the concentrated molten steel at the end of solidification is suppressed by lightly reducing the unsolidified part by 5 to 50 mm to compensate for the solidification shrinkage. Is disclosed.

  Patent Document 2 discloses that the solidification interface is made uniform by performing flow control by applying a static magnetic field at the lower end of the mold to make the solidified interface uniform, and in addition, the center segregation is improved by lightly reducing.

  In Patent Document 3, the degree of superheating of molten steel is set to 50 ° C. to 80 ° C. so that the solidified structure becomes columnar crystals, and the center segregation is performed by applying a static magnetic field at a position where the solid phase ratio of the slab is 30% to 75%. Improvements are disclosed.

JP-A-6-126405 JP-A-7-1000060 JP 2008-212278 A

  However, the technique using both electromagnetic stirring and light pressure disclosed in Patent Document 1 makes the solidification structure of the slab center part a fine equiaxed crystal by the electromagnetic force stirring, and increases the flow resistance of the center part. This reduces the flow and accumulation of concentrated molten steel in the center of the slab. Furthermore, the technique reduces the flow driving force of the concentrated molten steel and suppresses the flow by compensating for the solidification shrinkage due to the light pressure at the end of solidification. Thereby, a high center segregation reduction effect can be expected. However, in order to meet strict quality requirements, the technique disclosed in Patent Document 1 is insufficient, and it is necessary to further improve the center segregation in the equiaxed crystal structure.

  In addition, in the structure control technique using electromagnetic force, for example, the technique disclosed in Patent Document 2 has a slab part to which a magnetic field is applied at the lower end of the mold, so that even if a magnetic field is applied to this part, the center segregation is affected. There is no effect at the end of solidification and columnar crystallization cannot be achieved.

  Moreover, the technique described in patent document 3 can completely crystallize a structure | tissue by making molten steel superheat degree into 50-80 degreeC. However, in this technique, the superheat degree of the molten steel is set to 50 ° C. or higher, so that the risk of breakout due to insufficient shell thickness is extremely high. As a countermeasure, it is necessary to reduce the casting speed, so that productivity is deteriorated.

  The present invention solves these problems of the prior art, and its purpose is to produce a slab free of center segregation that can meet strict requirements for internal quality in recent years. The purpose is to propose a continuous casting method.

The features of the present invention for solving such problems are as follows.
[1] A static magnetic field generator is installed in a continuous casting machine, and the solid phase ratio of the slab drawn from the mold is at least part of the range represented by the following mathematical formula (1), and the slab is drawn out. Applying a static magnetic field to the slab so that the static magnetic field in a direction orthogonal to the direction has a magnetic field strength of 0.15 T or more and an application time rate represented by the following formula (2) is 10% or more. A continuous casting method characterized by the above.


Here, fs represents the central solid phase ratio of the slab.
[2] The continuous casting method according to [1], wherein the value of the following mathematical formula (3) is 0.27 or more when the center solid phase ratio of the slab is 0.3.


However, G is a temperature gradient (° C.) at which the solid phase ratio on the straight line perpendicular to the casting direction at that point passes through a point where the central solid phase ratio of the slab is 0.3 and is 0.99. / Mm), and V represents the moving speed (mm / min) of the solid-liquid interface of the slab.

  According to the present invention, it is possible to make the solidification interface uniform by reducing the average segregation particle size of the solidified structure by applying an electromagnetic force to the concentrated molten steel. Thereby, the center segregation of solute elements, such as P, S, or Mn, of the slab cast by the continuous casting machine can be reduced.

It is a cross-sectional schematic diagram which shows an example of the continuous casting machine 10 in which the continuous casting method which concerns on embodiment of this invention is used. It is the graph which showed the relationship between an average segregation particle size and an application time rate for every magnetic field intensity. It is the graph which showed the relationship between average segregation particle size and magnetic field intensity for every application time rate.

  FIG. 1 is a schematic cross-sectional view showing an example of a continuous casting machine 10 in which a continuous casting method according to an embodiment of the present invention is used. In FIG. 1, 12 indicates a mold, 14 indicates molten steel, 16 indicates a solidified shell, and 18 and 20 indicate static magnetic field generators. Moreover, the continuous casting machine 10 is provided with a segment (not shown) having a plurality of roll groups provided on the side surface in the thickness direction for pulling out a cast piece.

  The static magnetic field generators 18 and 20 are, for example, provided in segments at positions where the central solid phase ratio fs of the molten steel 14 is 0.24 to 0.30, which is a magnetic field application coil. The static magnetic field generators 18 and 20 apply a static magnetic field in a direction orthogonal to the drawing direction of the slab to the molten steel 14. When a static magnetic field is applied to the molten steel 14 from the magnetic field application coil, thermal convection in a direction orthogonal to the drawing direction of the slab is suppressed, and the temperature gradient of the molten steel 14 in that direction increases. By increasing the temperature gradient of the molten steel, the solidification interface can be made uniform by columnar crystallization of the solidified structure at the center in the thickness direction of the molten steel 14, so that the generation of voids at the end of solidification can be suppressed. Thereby, the center segregation of the slab continuously cast by the continuous casting machine 10 can be reduced.

  The static magnetic field generators 18 and 20 are installed so as to apply a static magnetic field in a direction orthogonal to the drawing direction of the slab at a position where the central solid phase ratio fs of the molten steel 14 is greater than 0 and equal to or less than 0.3. It only has to be done. Thermal convection of the molten steel 14 occurs when the central solid fraction fs is low and the fluidity of the molten steel 14 is high, and does not occur when the central solid fraction fs of the molten steel 14 is high and the fluidity of the molten steel 14 is low. Therefore, the thermal convection is effectively suppressed by applying a static magnetic field to a position where the central solid phase ratio fs is greater than 0 and less than or equal to 0.3, and as a result, the average solidification structure at the center in the thickness direction of the molten steel 14 The segregated particle size can be reduced.

  In addition, the center solid phase rate fs of the molten steel 14 means the solid phase rate of the center point in the cross section in the direction perpendicular to the drawing direction of the slab. The central solid fraction fs of the molten steel 14 can be calculated from the molten steel temperature at the central point of the molten steel 14. That is, the relational expression between the molten steel temperature and the solid fraction can be calculated from the correspondence between the solid fraction difference and the temperature difference between the molten steel temperature with a solid fraction of 0 and the molten steel temperature with a solid fraction of 1.0. If the molten steel temperature at the center point of the molten steel 14 can be calculated, the solid fraction corresponding to the molten steel temperature can be calculated.

  Moreover, the temperature of the center point of the molten steel 14 is the surface temperature of the solidified shell 16, the Japan Iron and Steel Institute, “Heat transfer experiment and calculation method in a continuous slab superheated furnace”, Japan Iron and Steel Association, 1971 It can be calculated using the heat transfer calculation formula described in “Issued on March 10”. By providing a thermocouple in the solidified shell 16 and acquiring the temperature change of the surface temperature of the solidified shell 16, the temperature profile of the solidified shell surface in the drawing direction can be acquired. A temperature profile along the drawing direction of the center point of the molten steel 14 can be calculated using the obtained surface temperature profile of the solidified shell and the heat transfer calculation formula. The profile of the central solid fraction fs along the drawing direction of the molten steel 14 can be calculated using the temperature profile of the central point and the relational expression between the molten steel temperature and the solid fraction calculated in advance. Based on the calculated profile of the central solid fraction fs of the molten steel 14, the installation positions of the static magnetic field generators 18 and 20 in the continuous casting machine 10 are determined.

  Moreover, the magnetic field strength applied to the molten steel 14 is 0.15 T or more. If the applied magnetic field strength is smaller than 0.15 T, the average segregation grain size of the solidified structure at the center in the thickness direction of the molten steel 14 cannot be reduced, and the center segregation of the slab cannot be suppressed.

  Moreover, the application time rate which applies the static magnetic field of the magnetic field strength of 0.15T or more to the molten steel 14 is 10% or more. If the application time rate is shorter than 10%, the solidified structure at the center in the thickness direction of the molten steel 14 cannot be made columnar crystals, and the center segregation of the slab cannot be suppressed. The application time rate is a ratio (%) calculated by the following mathematical formula (2).

  Further, in order to further suppress the center segregation of the slab, it is more preferable to control the temperature gradient and solidification rate of the molten steel 14 to make the solidified structure uniform columnar crystals. The temperature gradient (° C./mm) at the point where the solid phase ratio on the straight line orthogonal to the casting direction at that point passes through the point where the central solid fraction of the slab is 0.3 and the temperature gradient G is 0.99. ), And the solidification rate V is the moving speed (mm / min) of the solid-liquid interface of the slab, the following formula consisting of the temperature gradient G and the solidification rate V when the central solid phase ratio fs is 0.3 ( 3) is preferably 0.27 or more. Thereby, the solidification structure | tissue in the thickness direction center of the molten steel 14 can be made into a uniform columnar crystal, and the center segregation of the slab continuously cast by the continuous casting machine 10 can be further suppressed.

  On the other hand, if the numerical formula (3) is smaller than 0.27, the solidified structure at the center in the thickness direction of the molten steel 14 cannot be made into a uniform columnar crystal, and the slab continuously cast by the continuous casting machine 10 Central segregation cannot be further suppressed.

  Confirmation of the center segregation of the slab can be evaluated using a sample obtained by cutting the center part of the slab into a size having a thickness of 50 mm, a width of 410 mm, and a length of 80 mm. Specifically, a cross section parallel to the casting direction of the cut sample is etched with saturated picric acid to reveal a macro structure, and a macro particle having a segregation particle size of about 5 mm observed at the center of the thickness of the slab. A semi-macrosegregated grain having a segregation and a segregated grain size of about 1 mm is photographed. And the image | photographed photograph was image-analyzed, the average area of the segregated grain was measured, and the magnitude | size of the segregated grain was evaluated by calculating the average particle diameter (average segregated grain size) equivalent to a circle from this average area.

  The segregated grain is the final solidified structure in the central part in the thickness direction where the columnar crystals grown from the front and back surfaces of the slab collide with the progress of solidification of the molten steel. The size of the segregated grain (segregated grain size) is It is known that the larger the center segregation, the larger the process, and the workability and the like are reduced accordingly. That is, reducing the segregation particle size means reducing the center segregation, so that the center segregation of the slab can be evaluated by measuring the segregation particle size.

  The same configuration as the continuous casting machine 10 shown in FIG. 1, the length of the cast slab is 19.9 m, the radius of curvature is 15 m, and the slab to be cast has a sectional size of 410 × 250 mm. Continuous casting. The molten steel component injected into the mold 12 includes C: 0.7% by mass, Si: 0.2% by mass, Mn: 0.9% by mass, and the drawing rate of the slab is 0.8 m / min. The degree of superheat of the molten steel (molten steel temperature in the tundish−liquidus temperature) was set to 20 ° C.

  Static magnetic field generators 18 and 20 are installed at positions where the center solid phase ratio fs of the slab is 0.24 to 0.30, and the application time ratio is 2%, 5%, 8%, 10%, 15% and Continuous casting was performed while varying the application time rate and the magnetic field strength so that the magnetic field strength was 0.05T, 0.1T, 0.15T, 0.2T, and 0.3T. Table 1 shows the solidification structure at the center of each slab and the measured average segregation particle size. As described above, the solidified structure in the central part was confirmed by etching the cross section of the sample with saturated picric acid to reveal a macrostructure and visually observing the structure to confirm the type of the solidified structure. Further, as described above, the average segregated particle diameter was also measured as the average area of the segregated grains, and the average particle diameter corresponding to the circle calculated from the average area was defined as the average segregated particle diameter.

  FIG. 2 is a graph showing the relationship between the average segregation particle size and the application time rate for each magnetic field strength, and FIG. 3 shows the measurement result shown in Table 1 with the application time. It is the graph which showed the relationship between average segregation particle size and magnetic field intensity for every rate.

  From FIG. 2, it can be seen that when the magnetic field strength is 0.1 T or less, the average segregation particle diameter hardly changes even when the application time rate is increased. On the other hand, it can be seen that when the magnetic field strength is 0.15 T or more, the average segregation particle size can be reduced by setting the application time rate to 8% or more.

  From FIG. 3, it can be seen that when the application time rate is 5% or less, the average segregation particle diameter hardly changes even when the magnetic field strength is increased. On the other hand, when the application time rate is 8% or more, it can be seen that the average segregation particle size can be reduced by setting the magnetic field strength to 0.15 T or more.

  Further, it can be seen from Table 1 that if the magnetic field strength is 0.05 T or more, the solidification structure at the center can be made columnar crystals by setting the application time rate to 10% or more. From these results, the continuous casting machine 10 is provided with the static magnetic field generators 18 and 20 in at least a part of the range in which the central solid phase ratio fs of the slab is greater than 0 and 0.3 or less, and the static magnetic field The generators 18 and 20 perform continuous casting while applying a static magnetic field with a magnetic field application time rate of 10% or more and a magnetic field application strength of 0.15 T or more to the molten steel 14. The static magnetic field generators 18 and 20 suppress the thermal convection of the molten steel 14 and increase the temperature gradient of the molten steel 14. Thereby, the solidified structure of the center part of the slab can be converted into columnar crystals, and a slab having improved center segregation with a small average segregation particle size of the solidified structure of the center part can be continuously cast.

  In order to crystallize the solidified structure, it is preferable to control the temperature gradient and the solidification rate. Specifically, when the temperature gradient is small, the solidification rate is slowed down, and when the temperature gradient is large, it is predicted that a uniform columnar crystal structure will be formed even if the solidification rate is increased. The relationship between the gradient G and the solidification rate V was investigated. A static magnetic field was applied to the molten steel 14 having a central solid phase ratio fs of 0.3, and water cooling was performed only on the long side surface of the mold.

  As described above, the temperature gradient G passes through the point where the central solid fraction of the slab is 0.3 and the solid fraction on the straight line perpendicular to the casting direction at that point is 0.99. Represents a temperature gradient (° C./mm). The solidification speed V represents the moving speed (mm / min) of the solid-liquid interface of the slab. The temperature gradient (° C./mm) at the point at which the solid phase ratio becomes 0.99 is determined as follows: two R thermocouples (long side 1/2 short side 1/2 position, long side 1/2 short side 1 / 4 positions), and is calculated from the temperature profile along the direction toward the center of the molten steel 14 from the temperature data output from the thermocouple and the heat transfer calculation formula. The temperature gradient was calculated using the temperature before and after the point where the solid phase ratio calculated from the temperature profile was 0.99, and the distance before and after the temperature.

  The position of the solid-liquid interface of the slab can be calculated from the temperature profile in the molten steel 14 calculated from the temperature data output from the thermocouple and the heat transfer calculation formula. The moving speed (mm / min) of the solid-liquid interface of the slab was calculated using the amount of change per time of the temperature profile.

  The results of investigating the relationship between the temperature gradient G and the solidification rate V are shown in Table 2. When the value of the following formula (3) is smaller than 0.19 from Table 2, an equiaxed crystal structure in which the dendrite growth direction varies in the center portion of the slab was observed. On the other hand, when the value of the following formula (3) is 0.19 or more, a columnar crystal structure is formed, and when the value of the following formula (3) is 0.27 or more, a uniform columnar crystal is formed. It was observed.

  From Table 2, by controlling the temperature gradient G and the solidification rate V so that the value of the above formula (3) is 0.27 or more, the average segregation particle size in the solidified structure of the center portion of the slab is reduced, The solidified structure at the center can be made into uniform columnar crystals. Thereby, it turns out that the center segregation of the solute element of the slab cast by the continuous casting machine can be further reduced.

DESCRIPTION OF SYMBOLS 10 Continuous casting machine 12 Mold 14 Molten steel 16 Solidified shell 18 Static magnetic field generator 20 Static magnetic field generator

Claims (2)

A static magnetic field generator is installed in the continuous casting machine, and the central solid phase ratio of the slab drawn from the mold is at least part of the range represented by the following formula (1), and is orthogonal to the drawing direction of the slab. The static magnetic field is applied to the slab so that the static magnetic field has a magnetic field strength of 0.15 T or more and an application time rate represented by the following formula (2) is 10% or more. Continuous casting method.


However, fs is the central solid fraction of the slab, and the central solid fraction is the solid fraction of the central point in the cross section in the direction perpendicular to the drawing direction of the slab. . 2. The continuous casting method according to claim 1, wherein when the central solid phase ratio of the slab is 0.3, the value of the following mathematical formula (3) is 0.27 or more.

However, G is a temperature gradient (° C.) at which the solid phase ratio on the straight line perpendicular to the casting direction at that point passes through a point where the central solid phase ratio of the slab is 0.3 and is 0.99. / Mm), and V represents the moving speed (mm / min) of the solid-liquid interface of the slab.