JP2007245178A - Method for continuously casting steel - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a continuous casting method by which in a method for continuously casting a steel, cast while pressing corner parts of a cast slab, the development of transverse crack can further be prevented by limiting the surface temperature of the cast slab at the time point when the corner parts are pressed. <P>SOLUTION: When molten steel is continuously cast while pressing the corner parts of the cast slab 12 with a spinning wheel state roll 18 or rolling-reduction rolls 19 at least an upstream side from a correcting part of a continuous caster, the corner parts are pressed under state of being ≥900°C the surface temperature at the corner parts of the cast slab. Then, oscillation mark is pressed so as to become the flat, and the transverse crack is prevented. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a continuous casting method of steel, and more particularly to a continuous casting method of steel in which a corner portion of a slab is cast while being reduced.

  In continuous casting of steel, various surface cracks are known to occur in the slab. This surface cracking, especially when performing direct feed rolling or hot charge rolling, requires defects to be detected and surface care in the hot state, thereby causing trouble in the process and increasing productivity. Remarkably inhibits. In addition, if the surface cracks cannot be removed at the slab stage, maintenance is required after rolling.

  Surface cracks in slabs occur in the mold starting from solidification cracks in the vicinity of the freezing point, and so-called longitudinal cracks (cracks in the casting direction) that expand in the secondary cooling zone, and occur in the embrittlement temperature range of steel There are so-called transverse cracks along the grain boundaries, but the latter transverse cracks are mainly caused by the stress associated with the bending deformation and straightening deformation of the slab, and at a low strain rate as in the continuous casting process. It is known that it is likely to occur at the time of deformation and extremely easily in the embrittlement temperature range near 800 ° C. High temperature embrittlement of steel during deformation at such a low strain rate is mainly caused by precipitation of carbonitrides such as AlN, NbC, Nb [C · N], VC, V [C · N], [ It is thought to be caused by precipitation of sulfides such as Fe · Mn] S. Lateral cracks are called lateral cracks, corner cracks, key cracks, and the like because of their occurrence position and shape in the slab, but the generation mechanism is the same. Here, these are collectively referred to as transverse cracks.

  The conventional countermeasures for reducing transverse cracks are to avoid the slab surface temperature at the bending part and the straightening part of the slab from the above-mentioned embrittlement temperature range, which is the position where the most strain or stress acts on the slab surface. In order to improve the high temperature ductility by setting the cooling pattern or reducing the above-mentioned harmful precipitates, the content of Al, Nb, V, B, N, etc. is reduced, or the addition of Ti is harmless In this method, TiN is preferentially precipitated.

  However, in order to avoid the embrittlement temperature range in the bent part or straightened part of the slab, the so-called “width cutting” is performed to reduce the secondary cooling water or to cut the cooling water at the corner of the slab where the temperature drop is particularly large. If the slow cooling is strengthened too much, the solidified shell becomes thin and bulging occurs between rolls, resulting in surface cracks and internal cracks, and other defects such as molten metal surface fluctuations. Will be generated. Because of these limitations, in particular, in steels containing alloy elements such as Nb and V, which have a wide embrittlement temperature range, it is actually possible to avoid the embrittlement temperature region only by controlling the surface temperature of the slab. It was impossible.

  In addition, the addition of elements such as Nb and V is indispensable due to the demands on the material properties of steel, so it is impossible to reduce the content, or depending on the steel type, the addition of Ti causes a decrease in low-temperature toughness. The measures from the component system as described above have not been drastic measures. In particular, at the corner of the slab, the occurrence of transverse cracks was prominent due to the fact that the temperature tends to decrease compared to other parts of the slab and the effect of the notch action by the oscillation mark.

For the purpose of reducing this transverse crack, Patent Document 1 discloses that the convex portion of the oscillation mark generated at the corner portion of the slab when the slab is continuously cast through the bending portion and the correction portion is the bending portion. There has been proposed a method of rolling down and flattening at an upper position. According to Patent Document 1, the oscillation mark is flattened, thereby reducing the stress concentration at the bent portion and the correcting portion and preventing lateral cracking.
JP 2000-197953 A

  Although patent document 1 is effective as a means for preventing lateral cracking, specific implementation conditions are not clear. That is, the slab temperature when rolling down the corner portion of the slab is preferably 500 ° C. or more. In the examples, the slab corner portion is reduced in a range of 750 to 800 ° C. Therefore, the embrittlement temperature range of steel or the temperature lower than the embrittlement temperature range is used. According to the examination results of the present inventors, it has been confirmed that even when the surface temperature of the slab corner portion is 750 to 800 ° C., the effect of preventing transverse cracks is not sufficiently exhibited even if the slab corner portion is crushed. Yes.

  The present invention has been made in view of the above circumstances, and the object of the present invention is to provide a slab surface temperature at the time of rolling down the corner portion in a continuous casting method of steel cast while rolling down the corner portion of the slab. The present invention is to provide a continuous casting method of steel which can be limited and can further prevent the occurrence of transverse cracks.

  In the continuous casting method of steel according to the first invention for solving the above-mentioned problem, at the time of continuous casting of molten steel while reducing the corner portion of the slab slab, at least upstream of the straightening portion of the continuous casting machine, The corner slab surface temperature is set to 900 ° C. or more, and the corner is pressed down.

  The steel continuous casting method according to the second invention is the method according to the first invention, wherein the slab surface temperature of the corner portion is controlled to 900 ° C. or higher without cooling the corner portion in the secondary cooling zone of the continuous casting machine. It is characterized by this.

  The continuous casting method for steel according to a third invention is characterized in that, in the first invention, after the corner portion is heated to 900 ° C. or higher in a continuous casting machine, the corner portion is reduced.

  According to the present invention, since the corner portion is squeezed under the condition that the slab surface temperature of the corner portion is 900 ° C. or more, it is possible to efficiently avoid the lateral cracking of the slab corner portion, and to detect and care for the lateral cracking. Thus, the slab can be made to go straight from the continuous casting process to the hot rolling process without requiring such an auxiliary process. As a result, it greatly contributes to productivity improvement, energy saving, and cost reduction, and has an industrially beneficial effect.

  The present invention will be specifically described below.

  First, the slab continuous casting machine used when implementing this invention using FIG. 1 is demonstrated. FIG. 1 is a schematic side view of a vertical bending slab continuous casting machine used in carrying out the present invention. Although FIG. 1 shows a vertical bending slab continuous casting machine, the present invention can be applied even to a curved slab continuous casting.

  The vertical bending slab continuous casting machine 1 is provided with a mold 5 for cooling and solidifying the molten steel 11 to form the outer shell shape of the slab cast piece 12. A tundish 2 for relaying and supplying molten steel 11 supplied from a pan (not shown) to the mold 5 is installed. On the other hand, a plurality of pairs of slab support rolls comprising a support roll 6, a guide roll 7 and a pinch roll 8 are arranged below the mold 5, and the slab slab 12 pulled out from the mold 5 is made of these slabs. While being supported by the support roll, it is pulled out downward in the casting direction. Among these, the pinch roll 8 is a drive roll for pulling out the slab cast 12 while supporting the slab cast 12.

  A plurality of pairs of guide rolls 7 disposed at a position about 1 to 5 m away from the exit of the mold 5 constitutes a bending portion 16 in which the support / guide direction of the slab cast piece 12 changes the direction from the vertical direction to the bending direction. ing. That is, the slab slab 12 on the flat plate drawn out from the mold 5 in the vertical direction is gradually bent into an arc shape by the bending portion 16 and is corrected to a curved portion having a constant radius. Similarly, the plurality of pairs of guide rolls 7 arranged in the vicinity of the position where the curved portion comes into contact with the horizontal line constitutes a correcting portion 17 in which the support / guide direction of the slab slab 12 changes from the curved direction to the horizontal direction. ing. In other words, the arc-shaped slab slab 12 is gradually bent back on the flat plate by the correcting portion 17 and is corrected to the horizontal portion. In FIG. 1, both the bending portion 16 and the correction portion 17 are configured by a plurality of pairs of guide rolls 7, but may be configured by only a pair of guide rolls. The bending part 16 and the correction | amendment part 17 of this invention shall include the case where it corrects with a pair of guide roll.

  A secondary cooling zone in which a spray nozzle (not shown) such as a water spray nozzle or an air mist spray nozzle is arranged in the gap between the slab support rolls adjacent in the casting direction is configured. The slab cast slab 12 is cooled while being drawn out by cooling water sprayed from (also referred to as “secondary cooling water”).

  A sliding nozzle 3 for adjusting the flow rate of the molten steel 11 injected from the tundish 2 into the mold 5 is installed at the bottom of the tundish 2, and the molten steel 11 is injected into the mold 5 at the lower surface of the sliding nozzle 3. An immersion nozzle 4 made of a refractory material is installed. A plurality of transport rolls 9 for transporting the cast slab cast slab 12 are installed on the downstream side of the slab support roll. Above the transport roll 9, a slab cast slab to be cast is provided. A slab cutting machine 10 for cutting a slab slab 12a having a predetermined length from 12 is disposed.

  The molten steel 11 injected into the tundish 2 is injected into the mold 5 from the tundish 2 through the immersion nozzle 4, and the molten steel 11 cast into the mold 5 is cooled by the mold 5 to form a solidified shell 13. The slab slab 12 having the unsolidified layer 14 is continuously drawn downward while being supported by a plurality of pairs of slab support rolls including a support roll 6, a guide roll 7 and a pinch roll 8. Mold powder (not shown) is added on the molten steel surface of the mold 5. While the slab slab 12 is pulled out, it is cooled by the secondary cooling zone. The cooled slab slab 12 increases the thickness of the solidified shell 13 and eventually completes the solidification to the center at the solidification completion position 15. The slab slab 12 thus cast is cut by the slab cutting machine 10 to obtain a slab slab 12a. The slab cast slab 12a is conveyed to the next hot rolling step. The curved slab continuous casting machine has the same structure as the vertical bending slab continuous casting machine 1 except that the bending portion 16 is not installed.

  The present invention performs continuous casting of the molten steel 11 using such a slab continuous casting machine while reducing the corner portion of the slab slab 12 being cast.

  Oscillation marks remain deep in the corners of the slab slab 12, stresses tend to concentrate on the portions of the slab slabs during bending and correction, and compared to other parts of the slab slabs. Lateral cracks, corner cracks, key cracks, etc. are often caused because the surface temperature tends to decrease and the slabs tend to become brittle when correcting slabs in curved continuous casters and vertical bend continuous casters. Is likely to occur.

  In the present invention, slab bending refers to bending a flat slab in a vertical portion into an arc shape in a vertical bending type continuous casting machine, and slab correction refers to a curved continuous casting machine and a vertical slab. In a bending type continuous casting machine, an arc-shaped slab is bent into a flat plate shape along a horizontal plane. The bending of the slab is performed at the bending portion of the vertical continuous casting machine, and the correction of the slab is performed at the correcting portion of the curved continuous casting machine and the vertical bending continuous casting machine.

  As a means for rolling down the corner of the slab, for example, as shown in FIG. 2 (A), two places of the corner portion of the slab slab 12 can be simultaneously applied by a spinning wheel 18 having tapered portions that are outwardly spread at both ends. A method of rolling down, or a method of rolling down each corner portion with an individual rolling roll 19 as shown in FIG. FIG. 2 is a schematic plan view showing a rolling method of the slab corner. In FIG. 1, the spinning wheel roll 18 and the reduction roll 19 are not shown.

  In the present invention, it is an indispensable condition to reduce the corner portion under the condition that the surface temperature of the slab at the corner portion is 900 ° C. or higher. In general, the corner portion is cooled from both the long side and the short side of the slab, so that the temperature is likely to drop compared to the other parts of the slab. For this reason, in order to ensure a temperature of 900 ° C. or higher, the corner portion cooling by secondary cooling is stopped or reduced, and the corner portion is actively heated by high-frequency heating or the like in the continuous casting machine. It is also effective to do. Note that the temperature condition of 900 ° C. or higher is a temperature that is 100 ° C. to 150 ° C. higher than the temperature range performed in the embodiment of Patent Document 1 described above.

  Through experiments and examinations by the inventors, it has been confirmed that high-temperature embrittlement of steel near 800 ° C. is significantly reduced by imparting work strain to the slab at a temperature range of 900 ° C. or higher. FIG. 3 shows the test results by the present inventors. FIG. 3 shows a drawing value (= RA) when a test piece is heated to a temperature of 800 to 1000 ° C. and processed, and then held at a temperature of 800 ° C. to conduct a tensile test. The higher the aperture value, the better the ductility. As shown in FIG. 3, it has been confirmed that even if processing strain is applied to a test piece of less than 900 ° C., sufficient improvement in ductility is not observed and embrittlement cannot be avoided. Thus, it is considered that the recovery of ductility by applying work strain to steel having a high temperature of 900 ° C. or higher is an effect due to refinement of the structure and control of precipitation of carbonitride.

  Note that the reduction of the slab at a high temperature range of 900 ° C. or higher is basically processing at a higher temperature side than the embrittlement temperature range, and not a low strain rate at which ductility deterioration due to embrittlement is significant. Since the processing is performed at a high strain rate that is relatively difficult to cause a reduction in ductility, it can be said that there is almost no risk of causing cracks or the like in the slab due to the reduction processing of the corner portion.

  In general, corner cracks (lateral cracks at the slab corner portion) are likely to occur at the corner portion on the upper surface side of the slab cast by the curved continuous casting machine and the vertical bending die continuous casting. This is because the tensile stress in the casting direction acts on the upper surface side of the slab when the slab is corrected in the continuous casting machine, and the surface temperature during the slab correction is around 800 ° C. especially at the corner. This is because the susceptibility to cracking is high because it almost corresponds to the embrittlement temperature region of steel.

  For this reason, lateral cracks such as lateral cracks and corner cracks often occur in the correction part. Therefore, it is necessary to apply a reduction for mitigating embrittlement before the slab reaches the correction part. However, in the vicinity of the correction portion, the surface temperature of the corner portion is often lowered to about 800 ° C. as described above, and even if the reduction is applied in this state, the ductility is observed as in the data of FIG. 3 described above. Recovery is not expected. Therefore, in particular, it is effective to secure a temperature of 900 ° C. or higher by suppressing the cooling of the corner portion or actively heating the corner portion.

  Such an effect of adding strain by rolling is not limited to the corner portion, but performing the rolling process on a slab whose state is not solidified involves risks such as occurrence of internal cracks, In some cases, an unexpected shape change may be given to the slab, which may lead to operational troubles. In this respect, the corner portion of the slab is relatively tough against reduction due to its shape, and the present inventors, from the experimental results, will not cause any problems in operation or quality even if reduction is performed. Have confirmed.

  In addition, the surface temperature of 900 ° C. or higher, which is the condition of the present invention, can be relatively easily secured directly under the mold of the continuous casting machine, but on the other hand, it is not possible to perform a reduction process on the slab of the secondary cooling zone directly under the mold. However, since the solidified shell is still thin and fragile, there is a risk of breakout. In that respect, the reduction in the secondary cooling zone away from the mold, where a solid shell having a certain thickness is formed, is ideal conditions except that the temperature of the corner is lowered. I can say that. For this reason, it is possible to reduce the temperature of the corner portion or to apply distortion to the corner portion by performing rolling reduction in the secondary cooling zone away from the mold after applying positive heating. It is an effective implementation method.

  As described above, according to the present invention, since the corner portion of the slab slab is squeezed under the condition that the surface temperature of the slab of the corner portion is 900 ° C. or more, it is possible to avoid lateral cracking of the slab corner portion. In addition, the slab can be made to go straight from the continuous casting process to the hot rolling process without requiring auxiliary processes such as detection and care of transverse cracks.

  Using a vertical bending type continuous casting machine shown in FIG. 1, a slab slab of medium carbon steel having a thickness of 250 mm and a width of 1600 mm was cast at a casting speed of 1.4 m / min. The vertical bending type continuous casting machine has a vertical portion length of 3 m and a bending radius of 10 m. At this time, on the short side of the slab at a position 2.8 m away from the molten steel surface position in the mold on the downstream side in the casting direction (above 0.2 m of the bent portion), the spinning wheel roll shown in FIG. The corner portion of the slab was reduced. The spinning wheel roll was operated by a hydraulic cylinder, and the operation of the hydraulic cylinder was controlled using a hydraulic unit.

The rolling amount of the slab corner was 5 mm, the average strain of the slab corner surface layer 5 mm was 20%, and the strain rate was 0.25 sec ⁻¹ . The surface temperature of the slab corner at the time of slab pressure reduction was 900 to 940 ° C. (Invention Example 1). After casting, corner cracks of the slab were investigated by the penetrant flaw detection method. For comparison, the occurrence of corner cracks in a slab (Comparative Example 1) cast with the other casting conditions the same as in Example 1 of the present invention without reducing the corners was also investigated. Table 1 shows the results of the investigation of these casting conditions and the occurrence of corner cracks in the slab.

  In Comparative Example 1 in which the corner portion was not crushed above the bent portion during continuous casting, a number of corner cracks occurred in the slab corner portion. There were no cracks.

  Using a vertical bending type continuous casting machine having a vertical portion length of 3 m and a bending radius of 10 m shown in FIG. 1, a medium carbon steel slab having a cross-sectional thickness of 250 mm and a width of 1600 mm is cast at a casting speed of 1.5 m / min. did. At this time, the spur roll shown in FIG. 2 is installed on the short side of the slab at a position 12 m away from the molten steel surface position in the mold downstream in the casting direction (in the middle of the curved portion), and the corner portion of the slab Was reduced. The spinning wheel roll was operated by a hydraulic cylinder, and the operation of the hydraulic cylinder was controlled using a hydraulic unit.

The rolling amount of the slab corner was 5 mm, the average strain of the slab corner surface layer 5 mm was 20%, and the strain rate was 0.25 sec ⁻¹ . At this time, the secondary cooling water on the short side and corner of the slab was stopped, and the surface temperature of the slab corner was increased. As a result, the surface temperature of the slab corner at the reduced position was 900 ° C. (Invention Example 2). After casting, corner cracks of the slab were investigated by the penetrant flaw detection method.

  For comparison, casting was also performed to reduce the corner of the slab without stopping the secondary cooling water on the short side and corner of the slab (Comparative Example 2). In the comparative example 2, it carried out on the casting conditions same as the example 2 of this invention except not stopping the secondary cooling water of the short side and corner part of a slab. In Comparative Example 2, the surface temperature of the slab corner at the time of rolling down the corner was 800 ° C.

  Table 1 described above shows the results of the investigation of these casting conditions and the occurrence of corner cracks in the slab. As shown in Table 1, in Comparative Example 2, which was reduced when the temperature of the slab corner was 800 ° C., many surface cracks occurred in the slab corner, but the temperature of the slab corner was 900 ° C. In Example 2 of the present invention that was sometimes crushed, there was no surface crack at the slab corner.

  Using a vertical bending type continuous casting machine having a vertical portion length of 3 m and a bending radius of 10 m shown in FIG. 1, a slab of medium carbon steel having a cross-sectional thickness of 250 mm and a width of 1600 mm is cast at a casting speed of 1.3 m / min. did. At this time, the spur roll shown in FIG. 2 is installed on the short side of the slab at a position 12 m away from the molten steel surface position in the mold downstream in the casting direction (in the middle of the curved portion), and the corner portion of the slab Was reduced. The spinning wheel roll was operated by a hydraulic cylinder, and the operation of the hydraulic cylinder was controlled using a hydraulic unit.

The rolling amount of the slab corner was 5 mm, the average strain of the slab corner surface layer 5 mm was 20%, and the strain rate was 0.25 sec ⁻¹ . At this time, an induction heating device was installed near the corner of the slab, and the corner portion was heated to increase the surface temperature of the slab corner. As a result, the surface temperature of the slab corner at the reduced position was 930 ° C. (Example 3 of the present invention). After casting, corner cracks of the slab were investigated by the penetrant flaw detection method.

  For comparison, casting was also performed without heating the slab, and without stopping the secondary cooling water on the short side and corner part of the slab (Comparative Example). 3). In Comparative Example 3, the casting conditions were the same as in Invention Example 3 except that the slab was not heated. In Comparative Example 3, the surface temperature of the slab corner at the time of rolling down the corner was 780 ° C.

  Table 1 described above shows the results of the investigation of these casting conditions and the occurrence of corner cracks in the slab. As shown in Table 1, in Comparative Example 3, which was reduced when the temperature of the slab corner portion was 780 ° C., many surface cracks occurred in the slab corner portion, but the temperature of the slab corner portion was 930 ° C. In Example 3 of the present invention, which was sometimes crushed, there was no surface crack at the slab corner.

It is the schematic of the vertical bending type continuous casting machine used by this invention. It is a plane schematic diagram which shows the rolling-down method of a slab corner part. It is a figure which shows the relationship between a drawing value and processing temperature when a tensile test is implemented at 800 degreeC after adding process distortion to a slab.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Vertical bending type slab continuous casting machine 2 Tundish 3 Sliding nozzle 4 Immersion nozzle 5 Mold 6 Support roll 7 Guide roll 8 Pinch roll 9 Conveyance roll 10 Cast slab cutting machine 11 Molten steel 12 Slab slab 13 Solidified shell 14 Unsolidified layer 15 Solidification completion position 16 Bending part 17 Correction part 18 Spindle-shaped roll 19 Rolling-down roll

Claims (3)

  When continuously casting molten steel while rolling down the corner portion of the slab slab, at least upstream from the straightening portion of the continuous casting machine, the corner portion is crushed by setting the slab surface temperature of the corner portion to 900 ° C or higher. A continuous casting method of steel.   2. The continuous casting method for steel according to claim 1, wherein the surface temperature of the slab of the corner portion is controlled to 900 ° C. or higher without cooling the corner portion in a secondary cooling zone of a continuous casting machine.   The steel continuous casting method according to claim 1, wherein the corner portion is crushed after the corner portion is heated to 900 ° C or higher in a continuous casting machine.

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