JP4330518B2 - Continuous casting method - Google Patents

Description

  The present invention relates to a method for producing a slab slab by a continuous casting method while performing electromagnetic stirring in a mold, and particularly suppresses the generation of a white band in the slab slab and effectively prevents the occurrence of a breakout or the like. The present invention relates to a continuous casting method that can realize stable operation.

  In continuous casting of steel, molten steel in a ladle is poured into a mold, cooled inside the mold (primary cooling) to form a solidified shell, and then cooled while being guided by a guide roll into a water spray zone ( (Secondary cooling) The solidified shell is gradually thickened and then gradually pulled out by a pinch roll, and after solidification is completed, the solidified shell is sent to a subsequent process as a slab.

  In such continuous casting, so-called electromagnetic stirring is often performed to control the flow rate of molten steel by applying a magnetic field to the molten steel near the meniscus in the mold in order to improve the surface properties and internal quality of the slab.

  However, if the electromagnetic stirring strength is too strong, slab solidification delay may occur, and such solidification delay causes various problems in continuous casting operations. For example, it is known that a negative segregation band called a white band is generated at a portion where the solidification delay occurs (solidification interface). FIG. 1 schematically shows the occurrence of this white band. As the current applied to electromagnetic stirring is increased and the molten steel flow rate is increased, the non-uniformity of the solidified shell thickness increases and white bands are more likely to be generated. Further, when the segregation degree becomes larger than a certain value and a strong negative segregation zone is formed, a slab breakout (hereinafter sometimes abbreviated as “BO”) occurs. .

  The electromagnetic stirring in continuous casting is performed for the purpose of preventing the inclusion of these in the solidified shell by stirring the molten steel to wash away inclusions and bubbles in the molten steel. The stronger it is, the greater the quality improvement effect. However, when the stirring is too strong, a white band as described above is generated and B.I. O. There is a problem that it leads to operational troubles.

  The occurrence of such a white band varies depending on the casting speed and the mold narrow surface taper amount. Also, the casting speed and mold narrow taper amount not only affect the white band, but also appropriately set according to various steel grade components and product applications depending on factors such as the length of the continuous casting machine, internal cracks, and slab surface scratches. There is a need to. Therefore, B.B. O. It is necessary to appropriately set the electromagnetic stirring condition that does not generate.

  The occurrence of white bands as described above varies depending on the cross-sectional shape of the slab, and there are various causes, so the optimum electromagnetic stirring conditions for effectively suppressing the occurrence of white bands are the steel type. It was difficult to make a logical decision every time.

As a technique that suppresses the generation of white bands, for example, Patent Document 1 proposes a method of performing electromagnetic stirring while controlling the molten steel flow rate so as to satisfy a predetermined relational expression in accordance with the steel type component. However, as a practical problem, it is difficult to accurately measure the molten steel flow rate, and there is a problem that appropriate molten steel stirring conditions cannot be set.
Japanese Patent No. 3257546 Patent Claim etc.

  The present invention has been made paying attention to the above-mentioned circumstances, and its purpose is to suppress the occurrence of white bands in slab slabs and to effectively prevent the occurrence of breakouts and the like, thereby achieving stable operation. It is to provide a continuous casting method that can realize the above.

In the continuous casting method according to the present invention, when continuously casting slab slabs while performing electromagnetic stirring in the mold, when the carbon concentration of the ladle molten steel is [C] (mass%), each of the slab slab corners. To 70 mm × 70 mm within the range of the segregation degree Ke so that the following formulas (1) and (2) are satisfied and the segregation degree Ko within 70 mm from the slab surface is 1.05 or less. It has a gist in that it operates under controlled conditions.
Ke ≧ 1.15 × [C] ^(0.073) −0.12 (1)
Ke ≦ 1.15 × [C] ^(0.038) −0.12 (2)
However, the segregation degree Ke is the ratio ([Cmin] / [C]) of the carbon concentration [Cmin] (mass%) at the minimum to the carbon concentration [C], and the segregation degree Ko is the carbon at the maximum. concentration[
Cmax] (mass%) and the ratio of carbon concentration [C] ([Cmax] / [C]).

  In the continuous casting method of the present invention, the carbon concentration of the ladle molten steel is preferably [C] of 0.001 to 0.25%. In addition, the cross section perpendicular to the axis of the slab slab assumes a thickness of about 150 to 250 mm and a width of about 800 to 1800 mm.

  In the present invention, according to the carbon content of the slab slab, the segregation degree in a predetermined region of the slab slab corner satisfies a certain relational expression, and by controlling the segregation degree inside the slab, Suppress the generation of white bands O. A continuous casting method that does not cause such problems can be realized.

  The inventors of the present invention have made extensive studies to solve the above problems. As a result, it was found that the solidification delay causing white bands is likely to occur in the area of the corners of the mold corners, especially in the case of slab slabs with a relatively flat slab cross-sectional shape. It has been found that the above object can be achieved by controlling the magnetic stirring conditions so that the segregation degree Ke is within an appropriate range according to the carbon content of the slab (ie, the carbon concentration of the ladle molten steel). Completed the invention.

  FIG. 2 shows a region in the mold where the solidification delay of the slab occurs when the slab slab is continuously cast. In the case of a slab slab, when the electromagnetic stirring condition is not appropriate, the segregation degree Ke in the range A to D of 70 mm × 70 mm changes from each of the slab slab corners due to the influence of the molten steel flow velocity. And if electromagnetic stirring conditions are controlled so that the segregation degree Ke in this area | region may become an appropriate range according to the carbon concentration (namely, carbon concentration of ladle molten steel) of a slab slab, generation | occurrence | production of a white band can be suppressed. It is.

  In addition, if the above conditions are satisfied, the segregation degree Ko within the slab surface is inevitably 1.05 or less than 70 mm from the slab surface. It has also been found that bending workability at the weld is sometimes very good. The phrase “inner than 70 mm from the slab surface” means an internal region excluding the region from each surface of the slab to 70 mm, including the areas A to D of the slab slab corner.

  Specific examples of the electromagnetic stirring condition for carrying out the method of the present invention include the frequency and current intensity of the electromagnetic coil that constitutes the electromagnetic stirring device, and these change the thrust acting on the molten steel. It will be.

  The range of the carbon concentration [C] (that is, the carbon concentration of the ladle molten steel) of the slab cast slab targeted in the present invention is not particularly limited, but considering that it is used as a steel plate, it is 0. 0.001 to 0.25% is preferable.

  Further, the slab targeted in the present invention is intended for a slab whose cross-sectional shape perpendicular to the axis is flat, and the shape is assumed to have a thickness of about 150 to 250 mm and a width of about 800 to 1800 mm, for example. It is.

  In carrying out the method of the present invention, the conditions other than the electronic stirring conditions are not particularly limited and normal conditions may be followed. However, the casting speed and the mold narrow surface taper amount also affect the occurrence of white bands. In view of the above, it is preferable that these values are also in an appropriate range. For example, the casting speed is preferably set to about 1.2 to 2.0 m / min, and the mold narrow surface taper amount on one side is set to 0.5 to 0.75%. Note that the mold narrow surface taper amount means [(mold upper width−mould lower width) / (mould upper width)] × 100 (%), and is intended to compensate for thermal shrinkage in the mold. It affects the slab short side shape and surface cracks.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, but may be appropriately modified within a range that can meet the purpose described above and below. Of course, it is possible to implement them, and they are all included in the technical scope of the present invention.

  Using two continuous casters, slab casting was performed on a plurality of steel types having different carbon concentrations under various electromagnetic stirring conditions, and the segregation degree (Ke) at each corner (70 mm × 70 mm region from the slab corner) And B. The relationship between the O generation status and the slab surface quality (hege and sliver generation rate) was investigated. The chemical composition of each manufactured slab slab (ladder molten steel) is as shown in Table 1 below. At this time, the casting speed of continuous casting was arbitrarily changed in the range of 1.2 to 2.0 m / min and the slab size (width) in the range of 800 to 1700 mm. Further, the taper ratio of the narrow surface steel plate was changed in the range of 0.50 to 0.75% on one side according to the mold size. As for the segregation degree Ke, the carbon concentration was measured by collecting chips with a drill having a diameter of 5 mm or less at a pitch of 10 mm perpendicularly to the center (in the width direction of the slab) from the surface layer portion of the slab slab. The lowest value was [Cmin] (mass%), and the ratio ([Cmin] / [C]) with the carbon concentration [C] (mass%) of the ladle molten steel was obtained.

  The carbon concentration [C] and segregation degree Ke of each steel type are B. O. FIG. 3 shows the influence on the generation of slabs and the slab surface quality. As apparent from this result, the higher the carbon concentration [C] of the slab and the higher the segregation degree Ke, the more B. O. It can be seen that the range where this occurs is expanding. It can also be seen that at any carbon concentration [C], when the degree of segregation Ke is excessively increased, the molten steel flow rate becomes too slow, and the cleaning effect due to electromagnetic stirring is lost, leading to poor quality.

  Based on this result, as a result of examining the operation range in which operation troubles and surface quality defects do not occur, the range defined by the above formulas (1) and (2) can be derived.

  According to the method of the present invention, the segregation degree Ke of the slab is controlled by controlling the electromagnetic stirring conditions according to the carbon concentration of the slab, thereby obtaining a slab slab that does not cause operational troubles and surface quality defects. However, the specific conditions at this time were examined in detail.

  In the above experiment, the frequency during electromagnetic stirring was set to 3.0 Hz, but the segregation degree was changed by changing the thrust of the molten steel in the mold by changing the current intensity. Here, the “thrust force” is a propulsive force that causes the molten steel in the mold to be swirled and stirred by the electromagnetic force generated from the electromagnetic stirring coil, and this propulsive force is a position 15 mm away from the wide steel plate. The movement load of the brass plate when current was applied was measured by measuring with a spring balance.

  The relationship between the current value and hydraulic power in the electromagnetic stirring device used is as shown in Table 2 below. Moreover, the optimal electromagnetic stirring conditions (applied current value and thrust) according to the carbon concentration [C] of the ladle molten steel when electromagnetic stirring is performed by such an electromagnetic stirring device is as shown in Table 3 below.

  Therefore, the electromagnetic stirring conditions may be appropriately controlled so as to appropriately control within the range of Table 3 according to the carbon concentration in the steel and satisfy the relations of the expressions (1) and (2). .

  If continuous casting is carried out while performing electromagnetic stirring so as to satisfy the above formulas (1) and (2), the segregation degree Ko can be basically reduced to 1.05 or less inside the slab surface layer from 70 mm. However, the relationship between the degree of segregation Ko and cracking during bending was also investigated.

  At this time, when the slab having a carbon concentration [C] of the ladle molten steel of 0.14% is continuously cast in the same manner as described above, the segregation degree inside the slab slab obtained is more than 70 mm from the surface. The relationship between Ko ([Cmax] / [C]) and the weld crack during cold working was investigated. In addition, each carbon amount ([Cmax] / [C]) measuring method at this time was based on the above-mentioned method.

  Various slab slabs with different internal segregation degrees are produced by changing the electromagnetic stirring conditions during continuous casting, and a 300 × 300 × 2 (mm) steel plate is produced by rolling from this slab slab. A test piece [600 × 300 × 2 (mm)] was prepared by arc welding. At this time, the mutual segregation part was positioned at the welded part.

  The obtained test piece was subjected to bulge processing (a kind of bending process), and the workability was investigated by the occurrence rate when cracks due to segregation occurred in the weld. At this time, the presence or absence of occurrence of cracks is judged by visual observation, and the occurrence rate (ratio of the number of test pieces fractured to the number of test pieces obtained by bulging the welded portion) is less than 10 ppm (10 pieces per million). “O” was evaluated as “x” when 10 ppm or more.

  The results are shown in the following Table 4 in relation to the segregation degree Ko, and it is understood that good bending workability is achieved by setting the segregation degree Ko within 1.0 mm or less from the surface to 70 mm. .

It is explanatory drawing which showed typically the generation | occurrence | production state of a white band. It is explanatory drawing in the casting_mold | template which shows the area | region where solidification delay generate | occur | produces. The carbon concentration [C] and the degree of segregation Ke are O. It is the graph which showed the influence which it has on generation | occurrence | production and slab surface quality.

Claims (1)

In continuous casting of a slab slab having a thickness of 150 to 250 mm and a width of 800 to 1800 mm while performing electromagnetic stirring in the mold, the carbon concentration of the ladle molten steel is set to [C] (mass%) (however, , [C] is 0.001 to 0.25 mass%) , the segregation degree Ke in the range of 70 mm × 70 mm from each of the slab slab corners satisfies the following formulas (1) and (2): The electromagnetic stirring conditions are controlled so that the degree of segregation Ko within 70 mm from the slab surface is 1.05 or less , the casting speed is 1.2 to 2.0 m / min, and the mold narrow surface taper amount Is a continuous casting method characterized in that it is operated at a setting of 0.5 to 0.75% on one side .
Ke ≧ 1.15 × [C] ^(0.073) −0.12 (1)
Ke ≦ 1.15 × [C] ^(0.038) −0.12 (2)
However, the segregation degree Ke is the ratio ([Cmin] / [C]) of the carbon concentration [Cmin] (mass%) at the minimum to the carbon concentration [C], and the segregation degree Ko is the carbon at the maximum. It is represented by the ratio ([Cmax] / [C]) of the concentration [Cmax] (mass%) and the carbon concentration [C].

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