JPS63242453A - Method for casting by light rolling reduction - Google Patents

Abstract

PURPOSE:To reduce impurity segregated at center part in a continuously cast slab and to produce the cast slab having excellent quality by impressing the static magnetic field to molten metal at crater end part in the cast slab at the time of executing light rolling reduction to the continuously cast slab and preventing flowing of the molten metal. CONSTITUTION:In the cast slab 30 drawn from a mold in continuous casting apparatus, non-solidified molten metal 34 exists in inner part of solidified shell 32, and the rolling-reduction is executed to the cast slab by many light reducing rolls 16 in the light rolling reduction device 10, and the cast slab is cooled by injection of cooling water from spray nozzles and bent toward horizontal direction while gradually increasing thickness of solidified shell 32, and transferred. In this case, a magnetic field generator 22 is arranged at the part of crater end 36 in the cast slab 30, to generate static magnetic field crossing to thickness direction of the cast slab 30. By this magnetic field, movement of the non-solidified molten metal concentrating the impurity is hindered and the segregation of impurities, such as C, P, S, at the center part, which solidifies at last time, is prevented and the continuously cast slab having quality can be produced.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a light reduction casting method for preventing center segregation of slabs in continuous casting.

[Prior art] Normally, in continuous casting, when molten steel is cooled by a water-cooled mold, a thin solidified shell is formed at a portion corresponding to the outer periphery of the slab, and the slab that has passed through the mold has unsolidified inside. While holding the molten steel, it is guided by a group of support guide rolls and pulled out by pinch rolls. During the slab drawing process, spray water is injected onto the slab to promote solidification inside the slab, and the thickness of the solidified shell is thick enough to withstand deformation.

When the slab has grown to a larger size, the slab is bent at a predetermined curvature approximately 90 degrees and a light reduction blade is applied to the slab in the middle of solidification using a light reduction device. The slab is completely solidified in this light reduction zone, and then
At the end position (straightening point) of the light reduction band, the rolls of the straightening device correct the bends in the slab, and the straight slab is cut into a predetermined length by a cutting machine.

In the final solidification part of the slab (crater end), carbon (C),
Component elements such as sulfur (S), manganese (Mn), and phosphorus (P) are concentrated in the unsolidified molten steel.

Since this concentrated molten steel has a low melting point, it does not precipitate when the molten steel is stationary in the solidified shell, but when the concentrated molten steel flows in the solidified shell, it precipitates, resulting in so-called central segregation. Normally, solidification shrinkage of molten steel causes the molten steel to be sucked and flow toward the bottom of the slab (in the direction of slab withdrawal), so a light reduction blade commensurate with the amount of solidification shrinkage is added to the unsolidified slab to prevent center segregation. There is. On the other hand, if the casting speed becomes relatively high,
The static pressure of the molten steel increases, and so-called bulging occurs in which the solidified shell partially expands in areas where the reduction amount is insufficient.

When bulging occurs, the expanded solidified shell is compressed by the reduction roll, causing molten steel to flow toward the top of the slab (in the opposite direction to the direction in which the slab is pulled out), and concentrated molten steel precipitates at the end of the crater, forming a wide center. Segregation occurs. In particular, bulging is likely to occur when the pitch interval of the reduction roll is large or when the reduction roll is deflected or worn. For this reason, one layer of small-diameter rolls is usually arranged in the light rolling zone to reduce the pitch between the rolls, thereby preventing the occurrence of bulging and reducing center segregation.

In the conventional light reduction casting technology, center segregation of the slab cannot be sufficiently reduced only by reduction using the light reduction device. For this reason, a rotating magnetic field or a moving magnetic field is applied to the crater end using an electromagnetic stirring device (EMS), and the concentrated molten steel at the crater end is stirred by electromagnetic induction to prevent center segregation.

[Problems to be solved by the invention] However, in the conventional light reduction casting technology, concentrated components such as [C] and [P] concentrated at the crater end are simply diffused within a certain area by electromagnetic stirring. Although it is possible to lower the peak value of the concentration of the segregated components, it is not possible to eliminate the segregation itself. For this reason, a segregation zone with an average concentration level is generated within a certain area at the center of the slab. This segregation zone contains small island-like segregation (semi-macro segregation), which causes various problems as shown below.

In recent years, demands from customers regarding the quality of steel materials have become more sophisticated and diversified, and it is necessary to further reduce the segregation of impurity elements and non-metallic inclusions that inevitably exist in manufactured steel materials. desired.

In other words, in pipe materials for oil and natural gas transportation, hydrogen-induced cracking (HIC) occurs along the central segregation zone due to the action of sour gas containing hydrogen sulfide.
There is a strong desire to prevent HIC by improving materials. It is known that HIC is likely to occur in areas where the P concentration is high in the central segregation zone, and generally, when the peak concentration of P is about 0.04% or more, the cracking occurrence rate due to HIC increases.

With conventional light reduction casting technology, so-called macro-segregation occurs in the center of the slab, where the P concentration is about 10 times that of the sound part. The concentration is reduced to a level of 50 ppm or less, and the solidified slab is soaked at about 1300° C. to diffuse the segregated P and lower the maximum value of P /a degrees.

Figure 7 shows the semi-macro segregated particles of various steel materials having a central segregation zone, with the horizontal axis representing the segregated particle diameter and the vertical axis representing the number of segregated particles present per 100 mm of slab length. It is a graph diagram which investigated the influence on HIC. In the figure, black circles indicate those in which cracks occurred due to I (IC), and white circles indicate those in which cracks did not occur.

As shown in this figure, for HIC generation, the particle size is approximately 0.
．． Large segregated particles of 5+ nm or larger (semi-macro segregation) are particularly harmful, and reduction of this semi-macro segregation is HIC.
It is known to be effective in preventing outbreaks. That is, in order to improve the HIC resistance of steel materials, it is necessary to reduce both the above-mentioned macro segregation and semi-macro segregation.

On the other hand, with respect to steel sheet materials for offshore structures, customers primarily desire improvements in the welding characteristics of the steel materials. In other words, if there is a central segregation zone in the steel material,
It is necessary to preheat the steel material during welding to prevent weld cracking, and after welding, the toughness of the weld heat-affected zone (HAZ) deteriorates and lamellar tear (step-like appearance that occurs along the lamination of the steel plates) occurs in the welded joint. cracks) occur. Therefore, from the viewpoint of improving the reliability of structures, it is strongly desired to reduce the central segregation zone of steel materials.

Furthermore, since segregation has a significant effect on the workability of deep-drawn steel such as beer cans, the development of segregation-free materials is strongly desired.

This invention was made in view of such circumstances, and
It is an object of the present invention to provide a light reduction casting method capable of suppressing the flow of molten metal at the final stage of solidification and reducing the absolute amount of impurities and the like segregated in the center of a slab.

[Means for Solving the Problems] 'The light reduction casting method according to the present invention is a light reduction casting method in which a continuous cast unsolidified slab is lightly rolled down by a number of rolls and the slab is completely solidified. It is characterized by applying a static magnetic field to the molten metal in the final stage of solidification to prevent the molten metal from flowing.

[Function] FIG. 3 is a schematic diagram illustrating the flow of molten metal at the final stage of solidification. When the molten metal solidifies and shrinks, the molten metal is sucked in the slab drawing direction and flows in the direction of arrow a in the figure. In the final solidification region, both solidified shells meet at the axial center of the slab, and the main axes of the dendrites that have developed toward the axial center come to communicate with each other, and the concentrated molten metal flowing in the direction of arrow a Obstructed by the dendrites, the flow direction changes in the direction indicated by the arrow in the figure (towards the center of the casting pan). Then, the molten metal trapped between the dendrites flows toward the axial center of the slab, and the molten metal solidifies to form a highly concentrated central segregation.

However, in the light reduction casting method according to the present invention, a static magnetic field is applied to the molten metal at the final stage of solidification, so when the molten metal flows in the direction of arrow a, an electromotive force is generated and a current flows through the molten metal, which causes the molten metal to flow. An induced force in the opposite direction acts on the molten metal. Therefore, when the concentrated molten metal flows, a repulsive force acts immediately to suppress the flow of the molten metal and prevent precipitation of the concentrated molten metal.

[Embodiments] Hereinafter, embodiments of the present invention will be specifically described with reference to the accompanying drawings.

First, the basic concept of center segregation reduction will be explained.

FIG. 4 is a schematic diagram showing the generation mechanism of center segregation at the final stage of solidification. In the figure, the shaded area indicates the solid phase, and the area other than the shaded area indicates the liquid phase. In addition, the symbol g in the figure indicates the limit solid phase ratio at which residual molten steel can flow, the symbol LD indicates the thickness of the solid-liquid coexistence region, and the symbol LL indicates the thickness of the liquid phase region (100% liquid phase region). . In the figure, assuming that central segregation is formed by the flowing and mixing of concentrated molten steel existing in the region from the thickness position where the solid fraction is zero to the thickness position where the solid fraction is gl. , the component concentration ratio CL/C of the molten steel at the final stage of solidification is calculated by the following equation (1).

...(1) However, each symbol in the above formula (1) is as follows.

α-LL/LD (ratio of liquid phase region to solid-liquid coexistence region at the end of solidification) LL+ LD, thickness of liquid phase region at the end of solidification, thickness of solid-liquid coexistence region g; limit of flow of residual molten steel Solid phase ratio Ko; [
Figure 5 shows the equilibrium distribution coefficient for each component such as [C], [P], etc., with the component concentration C1,/CO of [C] determined by the above equation (1) taken on the vertical axis, and the solid-liquid coexistence at the final stage of solidification. This study examines the relationship between molten steel flow and segregation degree at the final stage of solidification, with α, the ratio of the liquid phase region to the region, as the horizontal axis. In the figure, the numerical values written on each curve indicate the respective solid fraction g. Note that the component l decay rate CL/Co was calculated by setting the equilibrium distribution coefficient KO to 0.141. As is clear from this figure, as the flow limit solid fraction g of concentrated molten steel increases, the component concentration CL/C increases, and as the ratio α of the completely liquid phase region decreases, the component concentration increases. In other words, when the flow of concentrated molten steel becomes active, the concentration rate of components increases and the degree of segregation increases. Therefore, in order to reduce center segregation, it is necessary to increase the flow of concentrated molten steel. need to be suppressed.

As a means of suppressing the flow of this concentrated molten steel, firstly, the pitch between the rolls is narrowed to suppress the occurrence of bulging between the rolls of the slab, and secondly, the concentrated molten steel is moved toward the bottom side by solidification shrinkage. In order to prevent the slab from being sucked and flowing toward the downstream side of the slab being drawn, an appropriate amount of light reduction blade corresponding to the amount of solidification shrinkage may be applied to the slab. For this reason, it is necessary to reduce the diameter of the light reduction roll and to divide the roll into shorter lengths to apply fine reduction to the slab. In addition, according to the results of actual casting tests conducted under various conditions, the effect of roll reduction of the light reduction zone is maximized when the crater end is positioned approximately 3/4 from the starting position of the light reduction zone. It turned out to be.

By the way, the inventors found that the thickness of the unsolidified region was approximately 3011 mm.
1 [Tracer (F e S) on the slab in the middle of solidification of 11
As a result of investigating the flow condition of concentrated molten steel at the final stage of solidification by driving a stud with encapsulated steel and varying the amount of light reduction, the following findings were obtained.

Figure 6 shows the amount of light reduction (the amount of roll spacing reduced per 1 m of slab drawing length) on the horizontal axis, and the flow length of molten steel on the vertical axis, and the amount of light reduction and molten steel based on the above survey results. It is a graph diagram showing the relationship with flow length. As is clear from this figure, the flow length of molten steel decreases as the light reduction amount increases, and it is confirmed that the flow length of molten steel becomes the minimum at a light reduction amount of approximately 1.201111/il+. Ta.

Furthermore, when examining slabs cast under the above-mentioned light reduction conditions, center segregation is observed, although very slight. This is presumed to be due to the occurrence of bulging and the limits of equipment accuracy, and it has been found that it is extremely difficult to completely prevent the flow of molten steel in the final solidification region only by mechanical means (light reduction). Therefore, the inventors applied a static magnetic field to the final solidification region of the slab in order to prevent the minute flow of molten steel, and investigated the effect of preventing center segregation. As a result, when casting aluminum, applying a static magnetic field with a magnetic flux density of approximately 2250 Gauss to the molten metal at the final stage of solidification was able to confirm the effect of stopping the flow of the molten aluminum. Based on this result, when casting molten steel under light reduction, applying a static magnetic field with a magnetic flux density of about 10,000 Gauss to molten steel in the final stage of solidification is sufficient to stop the flow of molten steel.

FIG. 1 is a schematic cross-sectional side view of a light reduction device used in a light reduction casting method according to an embodiment of the present invention, and FIG. 2 is a perspective view of the same light reduction device. A mold (not shown) is provided in the upper part of the vertical bending continuous casting machine, and the unsolidified slab 30, which becomes a slab with a predetermined cross-sectional shape, is pulled out from the mold by pinch rolls. A layer of support guide rolls is arranged below the mold to surround the slab, thereby forming a vertical draw-out.

A plurality of pairs of bending rolls 24 are provided below the vertical drawing section, and the bending rolls 24 bend the slab 30 during solidification so that the direction of drawing the slab is changed to horizontal. Furthermore, a light rolling device 10 is provided below the bending roll 24.
is provided to apply a predetermined amount of reduction to the slab. The light rolling device 10 has two segments, and each segment is provided with a plurality of pairs of small diameter light rolling rolls 16, respectively. A hydraulic cylinder (not shown) is provided for each pair of these light reduction rolls 16, so that the slab 30 is reduced approximately evenly and overload is removed from the rolls 1.
This is to prevent it from being added to 6. Further, a group of spray nozzles (not shown) are arranged along the width of the slab 30, and each row of spray nozzles is arranged between the light reduction rolls 16, respectively. The group of spray nozzles is connected to a cooling water supply source (not shown) having a water volume adjustment function, and by controlling the spray pattern of each zone, the slab 30 is cooled at a predetermined rate.

As shown in FIG. 2, the light reduction roll 16 has a length of 3
It is divided. On the other hand, in the segment on the downstream side, a magnetic field generator 22 is arranged along the width of the slab. The magnetic field generator 22 has a pair of electromagnetic coils, each coil is connected to a DC power source (not shown), and when energized, generates a static magnetic field that crosses the slab 30 in the thickness direction. There is.

The diameter of the light reduction roll 16 is approximately 375 m compared to the conventional diameter.
m to about 210ffII11, the pitch between the rolls from about 420vn to about 235+no+, and the number of rolls in each segment from the conventional 5 pairs to 8 pairs. In addition, the length of the vertical drawing part in this continuous casting machine is about 4 m, the radius of curvature of the bent part of the slab is about 8 m, and the height difference from the meniscus in the mold to the light reduction device 10 is about 10.4 to 14 m. .1m.

Next, a case in which slabs are manufactured by the method of the present invention will be specifically explained. The casting steel type is Nb, V-based high tensile strength steel for in-pipe (API X-, 65), and the width of the slab is about 1950+ nm.

Molten steel whose composition has been adjusted by RH degassing treatment and ladle refining treatment is cast into a mold from a tundish. At this time, the temperature of the molten steel in the tundish is about 1552°C, and the molten steel is in a superheated state of about 33°C (the solidification temperature of this steel type is about 1519°C). When molten steel is injected into the mold, a solidified shell 32 is formed in contact with the mold wall. At this time, while drawing out the unsolidified slab 30 at a casting speed of about 0.75 m/min, the cooling rate and reduction amount of the slab are appropriately controlled to position the crater end 36 at the magnetic field generator 22. .

In addition, in the light reduction device 10, a group of light reduction rolls 16 apply reduction blades to the slab 30 at a rate of about 1.21 per meter of slab pulling length.

As shown in FIG. 3, when the reduction blade of the light reduction roll 16 is applied, the solidified shell 32 around the crater end 36 is pushed toward the center of the slab, and the concentrated molten steel within the crater end 36 is transferred to the top of the slab. A push-back force is applied to push back the concentrated molten steel in the direction, and the solidification contraction force that draws the concentrated molten steel toward the bottom side of the crater end 36 and the push-back force are substantially balanced, and the flow of the concentrated molten steel is prevented.

Furthermore, since a static magnetic field is applied to the molten steel at the final stage of solidification by the magnetic field generator 22, an electromagnetic induction force acts in the opposite direction to the direction in which the molten steel attempts to flow due to bulging, and even the slightest flow of the molten steel is blocked. . In other words, when a static magnetic field with a magnetic flux density B that crosses from the right side to the left side of the figure is applied, and if the flow velocity of the molten steel to the bottom side is ■, and the molten steel flow length is L, then the electromotive force moving from the front side to the back side of the paper is E (=BLv
) occurs. When a current I with an electromotive force E flows through the molten steel, an induced force F (-BI) directed towards the top side acts on the molten steel due to electromagnetic induction, which acts as a deterrent force and suppresses the flow in the direction of the arrow a in the figure. . This prevents the flow of molten steel toward the bottom and prevents the concentrated molten steel from colliding with the main axes of the dendrites connected at the axial center of the slab, so that the concentrated molten steel solidifies and precipitates between the dendrites. It is prevented from doing so. In this case, by applying a magnetic field with a magnetic flux density in the range of approximately 2,000 to 12,000 Gauss to the molten steel, it is possible to cast various metals.

According to the above embodiment, it is possible to apply a static magnetic field directly above the crater end to prevent even the slightest flow of molten steel, thereby effectively preventing the segregation of impurity elements such as P and S. .

In addition, since the diameter of the light reduction rolls 16 and the pitch between the rolls are smaller than those of the prior art, the occurrence of bulging can be effectively prevented.

Furthermore, since the length of the light reduction roll 16 is divided into three,
The driving force of the hydraulic system is reliably transmitted to each divided roll without deflection of the rolls, and roll reduction control can be made even more precise, reducing the flow limit solid fraction g of molten steel to 0.70
It was possible to reduce it from 0.15 to 0.15.

In the above embodiments, the case of Nb, V-based high-strength steel was shown, but the method of the present invention is not limited to this, and can be used for continuous casting of other steel types.

Furthermore, although the above embodiments have been described with reference to the case of manufacturing slabs, the method of the present invention is not limited to this, and can also be used in the case of manufacturing blooms, round billets, and the like. In this case, the effect can be further enhanced by surrounding the bloom or round billet with two pairs of coils.

[Effects of the Invention] According to the present invention, it is possible to apply a static magnetic field to the molten metal at the final stage of solidification to prevent even the slightest flow of the concentrated molten metal. The amount can be significantly reduced.

[Brief explanation of drawings]

Fig. 1 is a schematic sectional view from the side of a light reduction device in which a light reduction casting method according to an embodiment of the present invention is used, Fig. 2 is a perspective view of the light reduction device, and Fig. 3 is a solidification A schematic diagram explaining the flow of molten metal at the final stage, Figure 4 is a schematic diagram showing the solidification mechanism of concentrated molten steel, Figure 5 is a schematic diagram showing the relationship between the flow of molten steel and the degree of segregation, and Figure 6 is a diagram showing the relationship between the flow of molten steel and the degree of segregation. Fig. 7 is a graph showing the influence of semi-macro analysis on hydrogen-induced cracking (HIC). 10; Light reduction device, 12. 14; Segment, 16.1
8; Light reduction roll, 20; Spray nozzle, 22; Magnetic field generator, 30; Slab, 36; Crater End Applicant's representative Patent attorney Takehiko Suzue Figure 1 Figure 2 Figure 3 Figure 4 Figure 0 0. 5 1.0 1.5 20 α Figure 5

Claims (1)

[Claims] A light reduction casting method in which continuously cast unsolidified slabs are lightly rolled down by a number of rolls to completely solidify the slabs, and is characterized by applying a static magnetic field to the molten metal in the final stage of solidification to prevent the molten metal from flowing. Light reduction casting method.