JP4699001B2 - Continuous casting method - Google Patents

Description

  The present invention relates to a continuous casting method in which casting is performed while applying vibration to a slab by continuously striking a short side surface of the slab including an unsolidified portion.

  Internal defects, which are macro segregation called center segregation or V segregation, are likely to occur at the central portion in the thickness direction of the slab cast by continuous casting and in the vicinity thereof. Center segregation is an internal defect that occurs due to concentration of solute components such as C, S, P, and Mn that are easily segregated in the final solidified portion of the slab (hereinafter also referred to as “segregation component”). In the vicinity of the final solidified portion of the slab, these segregation components are internal defects generated by concentrating in a V shape.

  Products that have been hot-worked using slabs with macrosegregation as a raw material are prone to toughness reduction and hydrogen-induced cracking, and when these products are processed into final products in the cold. , Cracking is likely to occur.

  The generation mechanism of segregation in the slab is considered as follows. That is, as solidification progresses, segregation components are concentrated between columnar dendritic branches that are solidified structures. The molten steel enriched in the segregation component flows out between the columns of the columnar crystals due to shrinkage of the slab during solidification or swelling of the slab called bulging. The concentrated molten steel that has flowed out flows toward the solidification completion point of the final solidified portion and solidifies as it is to form a concentrated band of segregation components. The concentrated band of the segregation component thus formed is segregation.

  Measures for preventing segregation of slabs include preventing the movement of molten steel enriched in segregation components remaining between columnar dendrites, and preventing these concentrated molten steels from accumulating locally. It is effective. The inventors of the present invention have repeated research to achieve the above-mentioned problems, and it has been found that applying a blow from the slab surface to a slab containing an unsolidified portion is extremely effective in reducing segregation, The continuous casting method described in Patent Document 1 and Patent Document 2 below was proposed.

  The continuous casting method proposed in Patent Document 1 is the following first to third casting methods. That is, in the first method, when casting a slab having a rectangular cross-sectional shape, a striking vibration device arranged on the short side surface side of the slab including the unsolidified part is used to perform the slab including the unsolidified part. This is a continuous casting method of steel in which a short side surface is continuously hit and cast while giving vibration to a slab. Further, the second method is the first method, wherein the central solid phase ratio is 0.2 to 0.95 further downstream in the casting direction from the position where the short side surface of the slab was first hit. This is a casting method in which a slab including a certain unsolidified portion is reduced at a rate of 0.5 mm to 2.5 mm per 1 m in the casting direction by using a pair of guide rolls. And the 3rd method bulges the slab containing an unsolidified part in the 1st method in the upstream or the downstream of the position which hits the short side surface of the slab containing an unsolidified part continuously. First, the bulging slab is represented by R = D1 / D2 until the thickness center portion on the downstream side in the casting direction is completely solidified from the position where the short side surface of the slab was first hit. This is a casting method in which the slab is squeezed by a pair of squeezing rolls under a condition that the reduction ratio R is 0.8 to 1.1. Here, D1 represents the amount of reduction (mm) at the center of the width of the slab including the unsolidified portion, and D2 represents the thickness (mm) of the unsolidified portion having a solid phase ratio of 0.8 or less at the start of the reduction. .

  The method proposed in Patent Document 2 is the following first and second casting methods. That is, in the first method, when the position in the casting direction including the unsolidified portion of the slab having a rectangular cross-sectional shape is reduced by the pair of guide rolls for reduction, within the range of the reduction region in the casting direction, This is a steel continuous casting method in which casting is performed while applying vibration to the slab by continuously striking the slab surface. Furthermore, in the second method, when casting a slab having a rectangular cross-sectional shape, the slab at a position including an unsolidified portion is bulged, and the squeezed slab is solidified at the center in the thickness direction. Is a continuous casting method in which rolling is performed by a pair of rolling rolls until completion of slab, in the range of the casting direction area after the start of bulging, or within the range of the rolling area in the casting direction. This is a method of casting while applying vibration to the slab by continuously hitting the surface.

After proposing the above continuous casting method, as a result of research on the segregation prevention method, when the slab width is large, even when the above casting method is used, the component segregation cannot be sufficiently reduced. It turned out to be. The reason why the segregation cannot be sufficiently reduced is as follows. That is,
(1) When the slab is struck from the short side, if the slab width is increased, vibration due to the impact will not be sufficiently propagated inside the slab. In other words, if the vibration due to the impact does not propagate sufficiently inside the slab, the columnar crystal in the middle of growth cannot be broken, and therefore the columnar crystal continues to grow, so that the solidified structure becomes a fine crystal structure. Can not be.
(2) Furthermore, even when striking and vibrating near the final solidified part, vibration cannot be transmitted to the generated equiaxed crystal, and the equiaxed crystal bridges.

  As described above, in order to prevent the occurrence of segregation such as center segregation and V segregation in continuous casting, and to obtain a slab having good internal quality, there still remains a problem to be solved.

JP 2002-273554 A (claims, paragraph [0010] and paragraph [0011]) JP 2003-334461 A (claims, paragraphs [0016] and [0017])

  As described above, the method of casting while giving a blow to the slab has the following problems. That is, when the slab is struck from the short side, if the slab width becomes large, the vibration due to the impact does not sufficiently propagate to the inside of the slab, so the columnar crystals during the growth cannot be broken. In addition, columnar crystals grow. Therefore, the solidified structure cannot be made a fine crystal structure, and a sufficient segregation reducing effect cannot be obtained. Furthermore, even when striking and vibrating in the vicinity of the final solidified part, vibration cannot be transmitted to the generated equiaxed crystal, and the equiaxed crystal bridges, thereby obtaining a sufficient segregation reducing effect. Absent.

  The present invention has been made in view of the above-mentioned problems, and its problem is to prevent the occurrence of segregation such as center segregation and V segregation by imparting impact to the slab and to improve casting quality with good internal quality. The object is to provide a continuous casting method of steel from which pieces can be obtained.

  In order to solve the above-mentioned problems, the present inventor has repeatedly studied on a method for sufficiently propagating the impact vibration imparted to the slab to the inside of the slab based on the conventional problems. The knowledge of (c) was obtained and the present invention was completed.

  (A) In order to obtain the segregation reduction effect, it is necessary to strike the slab with an energy equal to or higher than the minimum impact energy for obtaining the segregation reduction effect.

  Here, the impact energy per impact is defined by the following equation (2).

E = (1/2) × m × V ² (2)
Here, m represents the mass (kg) of the striking mold, and V represents the collision speed (m / s) of the striking mold with the slab.

  (B) The minimum impact energy per impact of (a) is proportional to the slab width and is expressed by the following formula (3).

Emin = 0.0065 × W (3)
Here, Emin represents the minimum impact energy (J) per impact at which a segregation reduction effect is obtained, and W represents the long side width (mm) of the slab.

  (C) Furthermore, by giving an impact higher than the minimum impact energy given by the above formula (3), and by reducing the position in the casting direction including the unsolidified portion of the slab by a plurality of reduction guide roll pairs, A further effect of reducing segregation can be obtained.

  The present invention has been completed based on the above findings, and the gist thereof is the continuous casting method shown in the following (1) and (2).

  (1) When casting a slab having a rectangular cross-sectional shape, a short side surface of the slab is formed using a striking vibration device disposed at least at one short side of the slab including an unsolidified portion. A continuous casting method of steel in which casting is performed while applying vibration to the slab by continuously striking, and the striking energy satisfying the relationship expressed by the following formula (1) is given to the slab.

E ≧ 0.0065 × W (1)
Here, E represents the impact energy (J) per impact given to the slab, and W represents the long side width (mm) of the slab.

  (2) In the continuous casting method according to the above (1), the long side surface of the slab including the unsolidified portion is continuously reduced using a plurality of reduction guide roll pairs. Steel continuous casting method.

  In the present invention, “continuously hitting” means a suitable unit for hitting the short side surface of a slab by a striking vibration device arranged at a certain location where a slab containing an unsolidified portion exists. In the range of the number of hits per hour, this means that the hitting die described later is vibrated to hit the short side surface of the slab, and the number of hits is preferably in the range of 60 to 480 times / min.

  “Continuous reduction” refers to a slab formed by a plurality of guide roll pairs that are continuously arranged toward the downstream side in the casting direction and are sequentially provided with a reduction gradient to reduce the thickness of the slab. The rolling gradient is preferably in the range of 0.5 to 2.5 mm per meter of length in the casting direction.

  According to the continuous casting method of the present invention, it is possible to produce a slab having good internal quality in which macro-segregation such as center segregation and V segregation does not occur. Therefore, the present invention can greatly contribute to the improvement of the internal quality of hot rolled wire rods, steel bars, steel pipes, thick plates and the like made from these slabs.

  As described above, when casting a slab having a rectangular cross-sectional shape, the method of the present invention uses a striking vibration device disposed at least at one location on the short side surface side of the slab including the unsolidified portion. This is a continuous casting method in which the short side surface of the slab is continuously struck to give the slab a striking energy of a predetermined value or more and cast while applying vibration. Furthermore, in the above continuous casting method, the long side surface of the slab including the unsolidified portion may be continuously reduced using a plurality of reduction guide pairs. Below, the continuous casting method of this invention is demonstrated in detail.

(A) Basic configuration of the invention
FIG. 1 is a schematic view showing an example of a continuous casting machine in which a device for continuously hitting a slab is provided in a continuous casting machine to carry out the method of the present invention. FIG. It is a longitudinal cross-sectional view which shows the whole outline typically, and the figure (b) is a top view which shows typically the A1-A2 cross section of the figure (a).

  The molten steel 3 injected into the mold 2 through the immersion nozzle 1 is cooled by the mold 2 and forms a solidified shell 5 from the contact portion with the mold 2. The solidified shell 5 gradually increases in thickness while being guided by the plurality of guide roll pairs 4, and becomes a slab 7 including the unsolidified portion 6. The slab including the unsolidified portion and the slab 7 having been solidified are pulled out to the downstream side in the casting direction by the pinch roll 8.

  In the continuous casting machine illustrated in FIG. 1, the five guide roll pairs indicated by reference numeral 4 a indicate guide roll pairs for lightly reducing the slab. These five guide roll pairs can adjust the rolling gradient and can prevent light rolling. In addition, the guide roll pair shown with the code | symbol 4 in FIG. 1 means the normal guide roll pair which does not reduce a slab.

  In the method of the present invention, when casting a slab having a rectangular cross-sectional shape, a casting vibrator including an unsolidified portion is formed by a striking vibration device disposed at least at one position on the short side surface side of the slab including the unsolidified portion. By casting the short side surface of the piece continuously, casting is performed while applying vibration to the slab. The slab is continuously struck by a striking vibration device disposed at least at one position on the short side surface side of the slab including the unsolidified portion. The impact vibration device disposed at two locations on the short side surface side of the slab including the unsolidified portion, for example, at the same position of the slab in the casting direction and on the two short side surfaces on both sides of the slab. Thus, it may be hit continuously, or may be hit continuously by a striking vibration device arranged on the short side surface side of the slab including two or more unsolidified portions in the casting direction.

  As a device for continuously striking a slab, as shown in FIG. 1B, a striking vibration device 10 having a striking die 11 at the tip can be used. Using such an apparatus, vibration can be applied to the slab by continuously striking the short side surface 9 of the slab including the unsolidified portion.

  The reason for hitting the short side surface of the slab is as follows. That is, when striking the long side surface, in order to obtain a striking effect over the entire width direction of the long side, it is necessary to arrange a large number of striking dies 11 of the striking device in the long side width direction of the slab. This is because the equipment cost increases. Moreover, it is because there is little margin space for arrange | positioning the impact vibration apparatus 10 in the long side surface side of a slab, Therefore, in order to arrange | position the impact vibration apparatus 10, a large installation modification is required.

  From the viewpoint of durability, heat resistance, etc., it is desirable that the hitting die placed at the tip of the hitting vibration device be a casting hitting die. The length in the casting direction of the striking mold depends on the slab size, but is preferably about 100 to 500 mm. The cross-sectional shape of the mold in the casting direction parallel to the short side surface of the slab is preferably rectangular or elliptical. It is desirable that the hitting mold be of a replaceable type. For example, it is desirable to adopt a method in which a die is attached to the tip of the impact vibration device with a bolt or the like. Further, as a mechanism for vibrating the striking mold, for example, an air cylinder, an electric hammer, or the like can be used.

(B) Reason for limitation and preferred range of the present invention It is necessary to continuously apply impact energy satisfying the relationship represented by the following formula (1) to the slab.

E ≧ 0.0065 × W (1)
Here, E represents impact energy (J) per impact given to the slab, and W represents the long side width (mm) of the slab.
In addition, the impact energy of said (1) Formula is energy defined by the following (2) Formula.

E = (1/2) × m × V ² (2)
Here, m represents the mass (kg) of the striking mold, and V represents the collision speed (m / s) of the striking mold with the slab.

  If the striking energy satisfying the above formula (1) is applied to the slab, vibration due to striking sufficiently propagates to the inside of the slab, so that the columnar crystals in the middle of growth are broken and the solidified structure becomes a fine crystal structure. Therefore, a sufficient segregation reduction effect can be obtained. In addition, even when striking and vibrating in the vicinity of the final solidified part, vibration can be sufficiently transmitted to the generated equiaxed crystal, thus preventing bridging of the equiaxed crystal and sufficient segregation reduction effect Can be obtained.

  In order to adjust the striking energy, the mass of the striking mold may be adjusted, or the collision speed of the striking mold to the slab may be adjusted. The upper limit value of the striking energy applied to the slab is not particularly defined, but it is desirable to adjust it within a range not exceeding 500 J from the viewpoint of making the value within a range that does not damage the slab or the continuous casting machine.

  The number of hits per unit time is preferably 60 to 480 times / minute. If the number of hits is less than 60 times / minute, the segregation reduction effect of the slab tends to be non-uniform in the longitudinal direction of the slab, and if it exceeds 480 times / minute, the slab and the continuous casting machine are damaged. This is because it is not desirable because of fear.

  When the striking vibration device is disposed on the short side surface side of the slab including the unsolidified portion, it is desirable to dispose the striking vibration device at a position where the solid fraction at the center of the slab corresponds to 0.1 to 0.9. Here, the central part solid phase ratio means the fraction of the solid phase amount with respect to the total amount of the liquid phase and the solid phase in the central part in the thickness direction of the slab. Since bridging of equiaxed crystals occurs in the region where the solid fraction of the central part is 0.1 or more, the formation of equiaxed crystals is produced at the position where the solid fraction of the central part of the slab is less than 0.1 This is because the effect of hitting the slab is small. Also, in the region where the solid fraction in the central part exceeds 0.9, the unsolidified molten steel is less likely to vibrate and flow, so that the space formed by bridging or bridging such as equiaxed crystals is struck by the slab. This is because it becomes difficult to destroy.

  Furthermore, the method of the present invention includes a position at which the surface of the slab is hit, and includes an unsolidified portion of the slab whose center solid phase ratio from the upstream side to the downstream side is 0.1 to 0.95. It is desirable to reduce the range in the slab longitudinal direction at a rate of 0.5 to 2.5 mm per 1 m in the length in the casting direction by using a plurality of reduction roll pairs. This is because when the slab is rolled down, if the solid fraction in the central part is less than 0.1, the amount of segregation-concentrated concentrated molten steel discharged from between the branches of the columnar crystals decreases, and the reduction effect is small. . On the other hand, when the solid fraction in the central part exceeds 0.95, the unsolidified molten steel is difficult to flow, and thus it is difficult to discharge the concentrated molten steel upstream.

  Further, when the rolling is less than 0.5 mm per 1 m in the length in the casting direction, the effect of preventing the concentrated molten steel from accumulating in the vicinity of the final solidified portion is small. This is because the support frame that supports the pair is bent and it becomes difficult to obtain a sufficient reduction effect.

  Even when the slab is squeezed, it is necessary to give the slab impact energy that satisfies the relationship expressed by the formula (1). This is because by giving impact energy satisfying the relationship of the expression (1) to the slab, vibration due to impact can be sufficiently propagated inside the slab, and a further segregation reducing effect can be obtained.

(Test method)
In order to confirm the effect of the continuous casting method of the present invention, the following casting test was conducted and the result was evaluated. Using a continuous casting apparatus having the configuration shown in FIG. 1 having five pairs of reduction guide rolls for reducing the slab, a medium carbon steel and a high carbon steel are made into a bloom or slab having a thickness of 300 mm and a width of 400 to 2300 mm. Cast into. The casting speed was 0.75 m / min.

  Table 1 shows the component compositions of the test steels of medium carbon steel and high carbon steel used for casting.

  The solid fraction in the center at the time of slab reduction was in the range of 0.1 to 0.9, and the slab was reduced at a rate of 1.0 mm per 1 m length in the casting direction. In the secondary cooling, the specific water amount was constant under the condition of 0.8 liter / kg-steel.

  One portion of the short side surface at a position including the unsolidified portion of the slab was continuously struck by a striking vibration device to impart vibration to the slab. The distance from the meniscus when placing the striking vibration device was fixed at 10 m, and the test was performed by changing the mass of the striking die of the striking vibration device and the collision speed with the slab. When vibration was applied to the slab, the slab was continuously struck with the short side surface as the reference plane so that the amplitude of vibration on the short side surface was ± 3 mm. Moreover, the vibration frequency (the number of hits) of the hitting vibration device of the hitting vibration device was 120 times / minute, and the hitting mold was vibrated by an air cylinder method to give the hitting vibration. The striking conditions were such that the mass of the striking mold was 15 to 300 kg and the collision speed was 0.2 to 0.5 m / sec.

  The shape of the contact surface of the hammering die placed on the tip of the hammering vibration device with the bloom is 200 mm in the thickness direction of the slab and 400 mm in the length of the casting direction. Shaped.

In each casting test, a sample of a slab is taken, and from the position corresponding to the central portion in the thickness direction and the width direction of the cross section of the sample, the thickness direction is 10 mm across the central portion in the thickness direction, in the width direction. Test specimens of about 200 mm and about 15 mm in the casting direction were collected. Using these test pieces, chips were collected from 26 locations corresponding to the center of the slab in the thickness direction with a drill blade having a diameter of 2 mm at a pitch of 7 mm, and the C content was determined by analysis. By dividing the C content (C) (mass%) by the C content (C ₀ ) (mass%) of the molten steel in the ladle, the value of the ratio (C / C ₀ ) is calculated, and the maximum value of the ratio ( (Hereinafter referred to as “maximum center segregation rate”). In the evaluation of the test results, the case where the maximum center segregation rate was 1.15 or less was judged good, and the case where it exceeded 1.15 was judged as bad.

(Test results)
Table 2 shows test conditions and test results.

  In Table 2, test numbers 1 to 22 are tests for the examples of the present invention, and test numbers 23 to 44 are tests for the comparative examples.

  In the example of the present invention, as shown in the table, the impact energy is equal to or greater than the value on the right side of the formula (1), that is, the relationship represented by the formula (1) is satisfied. Regardless of the presence or absence, good results were obtained in which the maximum segregation rate of the slab was less than 1.15. In contrast, in the comparative example, the impact energy is less than the value on the right side of the formula (1), that is, the relationship of the formula (1) is not satisfied, and the maximum segregation rate of the slab exceeds 1.15. It became.

  FIG. 2 is a diagram showing the relationship between the long side width of the slab and the impact energy that affects the segregation situation of the slab. In the figure, ◯ indicates a test result in which the maximum segregation rate of the slab is less than 1.15, and ● indicates a test result in which the maximum segregation rate of the slab exceeds 1.15.

  In the region satisfying the relationship of the above expression (1) (the region on the upper side of the straight line E = 0.005 × W in FIG. 2), a good result is obtained in which the maximum segregation rate of the slab is less than 1.15. This is because the impact energy is sufficiently propagated to the inside of the slab and the effect of reducing segregation is exhibited. However, in the region that does not satisfy the relationship of the formula (1) (the region below the straight line in the figure), the impact energy does not sufficiently propagate to the inside of the slab, and as a result, the maximum segregation rate of the slab is increased. It is inferred that the result was inferior above 1.15.

  FIG. 3 is a diagram comparing the maximum center segregation rate of slabs according to the present invention example and the comparative example according to the presence or absence of reduction. According to the results shown in the figure, by applying the reduction to the slab including the unsolidified portion, the maximum center segregation rate of the slab is lowered in both the inventive example and the comparative example, and the segregation state is improved.

  From the above results, it is possible to obtain a further segregation reduction effect by applying a reduction to the unsolidified portion of the slab while giving a blow to the slab under the condition satisfying the relationship of the above formula (1). confirmed.

  FIG. 4 is a diagram showing a comparison of the quality degradation rate investigated in the thick plate factory before and after the continuous casting method of the present invention. Here, the internal quality failure rate (%) is the percentage obtained by dividing the number of inspection results determined to be internal quality defects by the total number of inspections in the ultrasonic test (UST) for the internal quality of the plate product. Means rate. The internal quality defect is a defect near the center of the plate thickness, and is an internal defect caused by macrosegregation such as center segregation or V segregation of the slab.

  By implementing the continuous casting method according to the present invention, the quality degradation rate in the plank factory is reduced to about ¼ compared to the quality degradation rate before the implementation of the method of the present invention. The effect of the method of the present invention on the improvement of the internal quality of the product was confirmed.

  According to the continuous casting method of the present invention, the short side surface of a slab including an unsolidified portion is subjected to casting having energy exceeding the minimum level for giving a segregation improvement effect, and the slab is vibrated while being vibrated. By doing this, it is possible to manufacture a slab of good internal quality that prevents the occurrence of macrosegregation such as center segregation and V segregation. Therefore, the method of the present invention is a casting method that can be widely applied in the continuous casting field for supplying raw material slabs for producing hot rolled wire rods, steel bars, steel pipes, thick plates and the like having good internal quality.

It is a schematic diagram which shows the example of the continuous casting machine provided with the apparatus which hits a slab continuously, The same figure (a) is a longitudinal cross-sectional view which shows the whole outline of a continuous casting machine typically, (B) is a top view which shows typically the A1-A2 cross section of the figure (a). It is a figure which shows the relationship between the long side width | variety of a slab and impact energy which affects the segregation condition of a slab. It is the figure which compared the maximum center segregation rate of the slab about the example of this invention, and the comparative example according to the presence or absence of reduction. It is the figure which compared the internal quality degradation rate in the plate factory before and behind implementation of this invention method.

Explanation of symbols

1: immersion nozzle, 2: mold, 3: molten steel, 4: guide roll pair,
4a: a pair of guide rolls for rolling down the slab, 5: a solidified shell, 6: an unsolidified part,
7: slab, 8: pinch roll, 9: short side surface of slab, 10: impact vibration device,
11: Die for hitting.

Claims (2)

When casting a slab having a rectangular cross-sectional shape, the short side surface of the slab is continuously formed by using a striking vibration device disposed at least at one short side of the slab including the unsolidified portion. A continuous casting method for steel, which is a method of casting while applying vibration to a slab by striking, and applying impact energy satisfying the relationship represented by the following formula (1) to the slab.
E ≧ 0.0065 × W (1)
Here, E represents the impact energy (J) per impact given to the slab, and W represents the long side width (mm) of the slab. The continuous casting method of steel according to claim 1, wherein the long side surface of the slab including the unsolidified portion is continuously reduced using a plurality of reduction guide pairs.

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