JP5907334B2 - Continuous casting method for cast slabs - Google Patents

Description

  The present invention relates to a continuous casting method of a cast slab, and in particular, continuous casting in which a light squeezing is performed on a cast slab in a drawing movement in a light squeezing zone provided downstream of a secondary cooling zone. Suggest a method.

  In the solidification process of steel, solute elements such as carbon, phosphorus and sulfur are concentrated on the unsolidified liquid phase side by redistribution during solidification. This is the so-called microsegregation formed between dendrite trees.

  In a cast slab that is being cast by a continuous casting machine, a void is formed in the central part of the cast slab due to solidification shrinkage or bulging of a solidified shell that occurs between rolls of the continuous caster, or negative pressure is generated. Cheap.

  Since a sufficient amount of molten steel does not exist in the unsolidified layer at the end of solidification of the slab, the molten steel concentrated by the above microsegregation accumulates in the central portion and solidifies.

  In the segregation spot formed by such solidification, the concentration of the solute element is remarkably higher than the initial concentration of the molten steel involved in casting. This is generally referred to as macro segregation, and is also referred to as central segregation because of its existence site.

  Steel materials applied to transportation line pipes such as crude oil and natural gas are likely to deteriorate in quality due to the above-mentioned center segregation. That is, when MnS and Nb carbides are generated in the central segregation part, hydrogen that has penetrated into the steel due to the corrosion reaction diffuses and accumulates around the MnS and Nb carbides in the steel, and cracks occur due to the internal pressure. Because.

  Since the center segregation part has high hardness, once a crack occurs, the crack easily propagates, and this is a so-called hydrogen-induced crack (hereinafter abbreviated as “HIC”).

  Therefore, the importance of reducing the center segregation of the cast slab cast by continuous casting is very high.

  Conventionally, in order to cope with this, various measures for reducing center segregation and detoxification have been taken from the continuous casting process to the rolling process, and many proposals have been made.

  Among them, as a means for effectively reducing the center segregation of cast slabs, for example, Patent Document 1 and Patent Document 2 disclose that cast cast slabs having an unsolidified layer are cast in a continuous casting machine. A continuous casting method is disclosed in which a single support roll is gradually reduced by a reduction amount corresponding to the sum of the solidification shrinkage amount and the heat shrinkage amount.

  In the above, gradually reducing the cast slab by a reduction amount corresponding to the sum of the solidification shrinkage amount and the heat shrinkage amount is a technique generally referred to as “light reduction” or “light reduction method”.

  Specifically, the technology called light reduction or light reduction method is based on the sum of solidification shrinkage and heat shrinkage by using multiple pairs of rolls arranged along the casting direction (the direction of drawing the cast slab). The cast slab is gradually squeezed with a reduced amount to reduce the volume of the unsolidified layer, preventing the formation of voids or negative pressure parts in the center of the cast slab, and at the same time the thickening formed between dendrite trees This prevents the molten steel from flowing, thereby reducing the center segregation of the cast slab.

  In recent continuous casting machines, a segment type continuous casting machine in which a plurality of segments each having a plurality of rolls are arranged as a set along the drawing direction of the cast slab is mainly used. The rolling roll group used in the area to be implemented (light rolling belt) is also composed of segments. In this case, the roll opening of the pair of rolls is the roll opening on the entry side of the segment, It is adjusted to be larger than the roll opening.

  When the cast slab is subjected to light reduction by applying the continuous casting machine, generally, the solidification completion position of the cast slab (also referred to as “crater end position”) is within the range of the light reduction belt. It is usual to perform control to be positioned at the position.

  By the way, the conventional light reduction technique has the following problems.

  In other words, when controlling the crater end position within the range of the light pressure lowering zone, the reduction applied to the cast slab at the end of the solidification stage depends on the position of the crater end position in the casting direction of the light pressure lowering zone. The amount may change.

  That is, when the crater end position is on the exit side of the segment of the light pressure lower belt, the cast slab in the segment has a large amount of unsolidified phase in its thickness direction, so the deformation resistance of the cast slab is small, The reduction can be performed without requiring a very large load.

  However, when the crater end position is on the entry side of the segment of the light pressure zone, most of the cast slab in the segment has been solidified. An excessive load may be applied to the four columns of the segment.

  And when the load applied to the support | pillar of a segment exceeds the withstand load set beforehand, the disk spring installed in the support | pillar upper part for the protection of a segment will bend.

  The segment is a rigid body, and when a load is applied to the segment, the roll opening degree is somewhat expanded. However, when the load until the disc spring is bent is reached, the roll opening degree is greatly expanded. Will end up.

  Moreover, since the rigidity of a segment will become low if a disc spring begins to bend, a roll opening degree will spread further.

  Therefore, in such a situation, in some cases, the roll opening on the exit side of the segment becomes equal to the roll opening on the entry side, and the cast slab at the end of solidification may not be reduced at all.

  In actual operation, since the water temperature, molten steel temperature, and casting speed in the secondary cooling zone vary, the crater end position of the cast slab during casting always varies.

  Therefore, when the cast slab is lightly reduced, the amount of reduction in the drawing direction of the cast slab may always vary greatly.

  According to Non-Patent Document 1 and Non-Patent Document 2, there is an optimum value for the reduction amount. If the reduction is insufficient, the flow of the concentrated molten steel occurs, and V-shaped segregation may be observed, but the reduction is excessive. In some cases, the backflow of the concentrated molten steel occurs, and negative segregation or reverse V-shaped segregation may occur. In any case, it has been reported that the center segregation is worse than when the optimum reduction amount is applied. Yes.

  Therefore, the degree of central segregation varies in the drawing direction of the cast slab, and it is assumed that the internal quality of the cast slab is remarkably deteriorated.

JP-A-8-132203 JP-A-8-192256

Iron and steel Vol. 80 (1994) No. 1. p42 Iron and steel Vol. 87 (2001) No. 2. p72

  In recent years, quality requirements for steel products have become more stringent than ever, and an increasing number of products require light reduction.

Along with this, the opportunities for light reduction of cast slabs are increasing, and it is inevitable to shorten the life of the segments that make up the light reduction zone. Therefore, it is desired to extend the life of the segment in the light pressure zone.

  As described above, the level of quality requirements for continuous cast slabs is currently increasing, and cast slabs with less center segregation than ever before are required. Therefore, as a measure for reducing center segregation, cast slabs are used in lightly pressed belt segments. However, in actuality, it is applied to the cast slab at the end of solidification due to fluctuations in the crater end position in the drawing direction of the cast slab. There are many cases where the amount of reduction is greatly deviated from the optimum value under light pressure.

  The present invention has been made in view of the above circumstances. That is, the object of the present invention is to always maintain a predetermined position in the light pressure belt even if the operating conditions that can cause the crater end position of the cast slab to fluctuate during the drawing process of the cast slab in continuous casting. By controlling so that the crater end position exists, light reduction is applied under the optimal reduction amount, and the center segregation is reduced over the entire length of the cast cast slab, and the segment constituting the light reduction belt The present invention proposes a continuous casting method for cast slabs that can reduce the load and extend the service life.

The present invention relates to a continuous casting method in which light reduction is performed on a cast slab drawn from a continuous casting mold in a light pressure lower zone located downstream of a secondary cooling zone, in accordance with the casting speed of the cast slab. Thus, by changing the specific amount of cooling water in the secondary cooling zone, the position where the solid phase rate at the center of the cast slab becomes the flow limit solid phase rate is adjusted so that the final segment of the light pressure lower zone is exposed. It is a continuous casting method of a cast slab characterized by performing control to be within a range from the side to the upstream side of 0.5 m ( excluding the position of the exit side of the final segment of the light pressure lower belt) .

In the continuous casting method of the cast slab having the above structure,
1) The position at which the solid fraction of the cast slab becomes the flow limit solidity is obtained based on on-line detection information on the solidification completion position of the cast slab, and the position is determined by the final segment of the light pressure lower zone. Adjusting the temperature of the cooling water in the secondary cooling zone so as to be within the range from the outlet side to the upstream side of 0.5 m,
2) The adjustment range of the temperature of the cooling water in the secondary cooling zone is 15 to 45 ° C.
3) The position at which the solid fraction of the cast slab becomes the flow limit solid fraction is the width position where the solidification completion position exists on the most downstream side in the width direction of the cast slab,
further,
4) The flow limit solid phase ratio of the cast slab is 0.6 to 0.8.
However, this is desirable as a specific solution of the present invention.

  According to the present invention having the above-described configuration, the specific water amount of the cooling water in the secondary cooling zone is changed according to the casting speed of the cast slab, and the solid phase ratio at the center of the cast slab flows. Since the position where the critical solid fraction is reached falls within the range from the exit side of the last segment of the light reduction zone to the upstream side of 0.5 m, light reduction is performed with a constant reduction amount over the entire length of the cast slab. As a result, the quality of the cast slab can be stabilized. In addition, in the segment, since an excessive load is not applied, the lifetime can be extended.

  Further, according to the continuous casting method of the cast slab according to the present invention, the position at which the solid fraction of the cast slab becomes the flow limit solidity is obtained based on the on-line detection information on the solidification completion position of the cast slab. Since the temperature of the cooling water in the secondary cooling zone is adjusted so that the position falls within the range from the exit side of the last segment of the light pressure lower zone to the upstream side of 0.5 m, the casting conditions vary greatly. Even in such a case, the cast slab can always be lightly reduced with a constant reduction amount.

  According to the present invention having the above-described configuration, since the adjustment range of the temperature of the cooling water in the secondary cooling zone is 15 to 45 ° C., the solid phase of the cast slab can be obtained under relatively easy temperature control. It is possible to accurately control the position where the rate becomes the flow limit solid phase rate.

  Further, according to the continuous casting method of the cast slab according to the present invention, the position where the solid fraction of the cast slab becomes the flow limit solid fraction, and the solidification completion position exists at the most downstream side in the width direction of the cast slab. Therefore, light reduction can be performed under a stable amount of reduction, and central segregation can be reduced over the entire length of the cast slab.

  Furthermore, according to the present invention, since the flow limit solid phase ratio of the cast slab is set to 0.6 to 0.8 and light reduction is performed, the center segregation can be surely reduced.

It is the figure which showed typically the suitable continuous casting machine used for implementing this invention. It is the figure which showed typically the segment which comprises a light pressure lower belt. It is the graph which showed the relationship between the displacement from the meniscus of the displacement of a disc spring, and the flow limit solid-phase rate of the thickness center part of a cast slab. It is the graph which showed the relationship between the position of the flow limit solid-phase rate of the thickness center part of a cast slab, and center segregation. It is the graph which showed the relationship between the specific water quantity of secondary cooling water, and a casting speed. It is the graph which showed the relationship between the crater end position by heat-transfer calculation, and a casting speed. It is the graph which showed the relationship between the specific water quantity of secondary cooling water, and a casting speed. It is the graph which showed the relationship between the position of the flow limit solid phase rate in the case of continuous casting according to this invention, and elapsed time. It is the graph which showed the relationship between the position of the flow limit solid phase rate in the case of continuous casting as a comparative example, and elapsed time. It is the graph which showed the result of having investigated the amount of displacement of a disc spring. It is the graph which showed the result of having investigated the degree (Mn) of center segregation.

Hereinafter, the present invention will be described more specifically with reference to the drawings.
FIG. 1 is a diagram schematically showing a continuous casting machine suitable for carrying out the present invention.

  The continuous casting machine is provided with a mold 1 for injecting and solidifying molten steel to form an outer shell shape of a slab. A tundish 2 for relaying and supplying molten steel supplied from a ladle to the mold 1 is installed at a predetermined position above the mold 1.

  A sliding nozzle 3 for adjusting the flow rate of the molten steel is installed on the bottom of the tundish 2, and an immersion nozzle 4 is installed on the lower surface of the sliding nozzle 3.

  On the other hand, a plurality of pairs of slab support rolls 5 including a support roll, a guide roll, and a pinch roll are disposed below the mold 2 along the drawing direction of the cast slab S.

  In addition, a secondary cooling zone 6 composed of a spray nozzle such as a water spray nozzle or an air mist nozzle for blowing cooling water or air through the slab support roll 5 is disposed below the mold 1. .

  The cast slab S drawn from the mold 1 is cooled by cooling water sprayed from the spray nozzle of the secondary cooling zone 6 (also referred to as “secondary cooling water”) or air during the drawing movement. Is done.

The secondary cooling zone 6 is usually divided into several cooling zones. In the case of applying secondary cooling water as a cooling medium, the pump for sending it out is common to each cooling zone, and by installing a heater or cooler for adjusting the temperature before and after the pump, The cast slab S can be cooled by adjusting the temperature of the secondary cooling water.

  A plurality of transport rolls 7 for transporting the cast slab S are installed on the downstream side of the final cast slab support roll 5 in the casting direction. A slab cutting machine 8 for cutting a slab of a predetermined length from S is disposed.

  A light pressure lower belt 9 is disposed in the vicinity of the solidification completion scheduled position of the cast slab S and on the upstream side (downstream of the secondary cooling zone 6).

  The light pressure lower belt 9 has an interval between slab support rolls facing each other with the cast slab S sandwiched in the thickness direction (this interval is referred to as “roll opening”) toward the downstream in the drawing direction of the cast slab S. It is composed of a plurality of segments (a plurality of pairs of slab support rolls are incorporated in the frame of the segments) set so as to be gradually narrowed.

  Here, the degree of opening of the roll opening that is set so as to gradually narrow toward the downstream in the drawing direction of the cast slab S is referred to as “rolling gradient”.

  Usually, the rolling gradient is displayed as a narrowing amount of the roll opening per 1 m in the casting direction, that is, “mm / m”. Accordingly, the reduction speed “mm / min” of the cast slab S in the light reduction zone 9 is obtained by multiplying this reduction gradient by the casting speed “m / min”.

  Further, on the downstream side from the light pressure lower belt 9, a transverse wave ultrasonic wave or a longitudinal wave ultrasonic wave is transmitted through the slab, and a solidification completion position detection device for detecting the solidification completion position of the cast slab S from the propagation time of these ultrasonic waves. 10 is installed so that the crater end position can be measured online.

  The solidification completion position detector 10 can also measure along the width direction of the cast slab S, and can also measure the shape of the crater end.

  Hereinafter, a case where the cast slab S is subjected to light reduction while casting the cast slab S using the above continuous casting machine will be described.

  The crater end position varies in the drawing direction (casting direction) of the cast slab S due to variations in the casting speed, the temperature of the secondary cooling water, and the like.

  In the light pressure lower belt 9, light pressure reduction is performed including the cast slab S at the end of solidification.

  As described above, the amount of reduction applied to the cast slab S at the end of solidification may vary depending on the variation of the crater end position.

  In the present invention, first, a low carbon steel material having a width of 2100 mm and a thickness of 250 mm is cast by the continuous casting machine having the above-described configuration, and at that time, in the segment of the light pressure lower belt 9 where the crater end is located, And the displacement situation of the disc spring of the segment was investigated.

  However, considering that the flow of the concentrated molten steel is prevented by light reduction, the pressure is actually lightly reduced from the crater end position to the flow limit of the liquid phase at the central portion (axial center) of the cast slab S. This is very important.

  Therefore, the position where the solid phase rate at the thickness center of the cast slab S becomes the flow limit solid phase rate is more important than the crater end position. For this reason, the position where the solid phase rate at the center of the thickness of the cast slab S becomes the flow limit solid phase rate and the displacement state of the disc spring of the segment of the light pressure lower belt 9 at that position were measured.

  In addition, said disc spring is shown with the code | symbol 12 arrange | positioned at the upper part of the support | pillar 11 which comprises this segment, as shown in FIG. The disc spring 12 is itself displaced when an excessive load is applied to the segment through the reduction roll 13 during the reduction of the cast slab S. This reduces the load on the segment.

  The position of the flow limit solid phase ratio at the center of the thickness of the cast slab S is determined by the heat transfer calculation, with the solidification completion position that can be measured by the solidification completion position detector 10 described above as the solid phase ratio of 1.0. The distance at which the phase ratio becomes 1.0 from the solid phase ratio at the flow limit was calculated for each casting condition, and the distance was calculated by subtracting the distance from the solidification completion position measured by the solidification completion position detector 10.

  The displacement of the disc spring 12 was calculated by averaging eddy current type distance meters by installing eddy current type distance meters on the disc springs 12 installed on the four struts 11 of the segment.

  FIG. 3 is a diagram showing the displacement of the disc spring 12 of the segment in which the position of the flow limit solid fraction varies and the flow limit solid fraction is located.

  In addition, the position of the flow limit solid phase ratio in FIG. 3 is a result of measurement at a width position where the crater end is located on the most downstream side in the width direction.

  From FIG. 3, the position of the flow limit solid phase ratio at the center of the thickness of the cast slab S varies along the drawing direction of the cast slab S, so that the displacement of the disc spring 12 varies greatly. it is obvious.

  That is, this indicates that the amount of rolling applied to the cast slab S near the flow limit varies greatly as the position of the flow limit solid phase ratio at the center of the thickness of the cast slab S varies. As described above, this shows that the degree of center segregation varies in the drawing direction of the cast slab S, and the internal quality of the cast slab S is also significantly reduced.

  From the above results, in order to prevent the deterioration of the internal quality of the cast slab S, the position of the flow limit solid phase ratio at the center of the thickness of the cast slab S is always constant, and the final segment of the light pressure lower zone 9 It is particularly effective to control from the exit side to 0.5 m upstream side.

  As shown in FIG. 4, the position of the flow limit solid phase ratio in the center portion in the thickness direction of the cast slab S is set to 0.5 m upstream from the exit side of the final segment of the light pressure lower belt 9. This is because the light pressure effect can be most effectively exhibited (central segregation degree of 1.03 or less). In FIG. 4, the center segregation degree of the cast slab S is indicated by the Mn segregation degree, but sufficient quality is ensured when the Mn segregation degree is 1.03 or less.

  In order to always control the position of the flow limit solid phase rate to a constant position even if operating conditions that may cause the position of the flow limit solid phase rate to fluctuate, first, in the thickness direction of the cast slab S, We investigated the effect of casting speed, which is one of the casting conditions affecting the position of the flow limit solid phase ratio in the center.

  As a result, in practice, the specific amount of secondary cooling water is reduced within the range of casting speeds that may be set in the casting, except during the transition period when the casting speed at the beginning of casting is increased and when the casting is decelerated at the end of casting. It has been found that the position of the flow limit solid phase ratio at the thickness center portion of the cast slab S can be controlled to a predetermined position by adjusting.

  FIG. 5 assumes a condition when the casting speed may be reduced to 1.2 m / min due to a casting speed of 1.4 m / min, waiting for a pan, and the like, in a continuous casting machine, a width of 2100 mm and a thickness In casting 250mm low carbon steel, it shows the specific water content of the secondary cooling water when the position of the flow limit solid phase ratio is controlled to be constant within the casting speed range of 1.2 to 1.4m / min. is there.

  The above FIG. 5 assumes a casting speed range of 1.2 to 1.4 m / min. Of course, in other steel types and continuous casting machines, there is a possibility of casting at a process casting speed or casting. Since the speed changes, it is necessary to determine the secondary cooling water amount in consideration of the range.

  FIG. 6 shows the result of calculating the position of the flow limit solid phase ratio in the case of continuous casting under the specific water amount shown in FIG. 5 by heat transfer calculation.

  In addition, the light pressure lower belt 9 of the continuous casting machine is located at a position 21.1 to 27.6 m from the molten metal surface in the mold 1 and the exit side of the last segment of the light pressure lower belt 9 is located at a position 27.6 m. It is assumed.

  As shown in FIG. 6, in the heat transfer calculation, even if the casting speed fluctuates, the position of the flow limit solid phase ratio is controlled from the outlet side of the last segment of the light pressure lower belt 9 to the upstream side of 0.5 m. Yes.

  However, the above heat transfer calculation is a case where the water temperature of the secondary cooling water is 30 ° C. and the molten steel superheat degree in the tundish (difference between the temperature of the molten steel and the solidus temperature) is 30 ° C. However, it is difficult to control the degree of superheated molten steel in the tundish.

  Also, considering the influence of the temperature of the atmosphere that is not taken into account in the heat transfer calculation and the heat removal by the rolls, even though the position of the flow limit solid phase ratio is constant in the heat transfer calculation, the cast slab S actually It is expected to vary in the drawing direction (casting direction).

  Therefore, in the present invention, in order to suppress the fluctuation in the position of the flow limit solid phase ratio, the position of the flow limit solid phase ratio is further controlled by the temperature of the secondary cooling water.

Specifically, the position of the flow limit solid fraction of the cast slab S which is thus cast in the ratio amount of water determined by the heat transfer calculations, using the coagulation completion position detecting device 10 for detecting the solidification completion position The position of the flow limit solid fraction measured in the heat transfer calculation and the position of the flow limit solid fraction measured online are adjusted by the temperature of the secondary cooling water, and the solidification completion position detection device 10 The position of the flow limit solid phase ratio is controlled from the exit side of the last segment of the light pressure lower zone 9 to 0.5 m upstream.

  In order to calculate the position at which the solid phase rate at the thickness center of the cast slab S becomes the flow limit solid phase rate using the solidification completion position detection device 10, it can be measured with the solidification completion position detection device 10. Assuming that the solidification completion position is the solid phase ratio of 1.0, the distance from the flow limit solid phase ratio to the solid phase ratio of 1.0 at the central part in the thickness direction of the cast slab S is calculated by heat transfer calculation. The actual flow limit position can be calculated by calculating for each casting condition and subtracting it from the solidification completion position measured online.

  The amount of fluctuation in the position of the flow limit solid phase ratio due to the change in the temperature of the secondary cooling water is preferably measured in advance by the solidification completion position detector 10 or predicted by heat transfer calculation.

  The position of the flow limit solid phase ratio may not be constant in the width direction, and the position of the flow limit solid phase ratio is the most downstream in the width direction. Also, by controlling the position to the upstream side, all the flow limit positions in the width direction can be put in the light pressure lowering zone, and light pressure lowering can be performed.

  In the width direction of the cast slab S, the width position where the flow limit solid phase ratio becomes the most downstream side is measured by the solidification completion position detector 10 described above, or the surface temperature of the cast slab S is set in the width direction. You can know by measuring.

  In the present invention, the adjustment range of the temperature of the cooling water in the secondary cooling zone is 15 to 45 ° C., but the flow limit solid phase ratio of the cast slab S is controlled by controlling the temperature of the cooling water within this range. This position can be reliably controlled to a predetermined position.

  As described above, according to the present invention, first, the fluctuation of the position of the flow limit solid phase ratio at the central portion of the thickness of the cast slab S due to the fluctuation of the casting speed is determined. Furthermore, the thickness of the cast slab S can be controlled regardless of variations in casting conditions by controlling the change in the position of the flow limit solid phase ratio due to other factors by the temperature of the secondary cooling water. The position where the solid phase rate at the center of the direction becomes the flow limit solid phase rate can be controlled from the exit side of the final segment of the light pressure lower zone 9 to 0.5 m upstream.

  Therefore, according to the present invention, it becomes possible to always apply the same reduction amount to the cast slab at the end of solidification regardless of the change in casting conditions, and the variation in center deflection in the drawing direction of the cast slab S is reduced. Therefore, it is possible to cast a cast slab having a low degree of center segregation at all times, and the segment constituting the light pressure lower belt is not subjected to an excessive load, so that the life of the segment is extended. The

  In the present invention, it is preferable to set the flow limit solid phase ratio at the center of the thickness of the cast slab to 0.6 to 0.8. This is because the effect of improving segregation can be enhanced.

  In accordance with the present invention, C: 0.039 mass%, Mn: 1.10 mass%, Si: 0.18 mass%, S: 0.0012 mass%, P: 0.012 mass using the continuous casting machine shown in FIG. %, SolAl: 0.020 mass%, a low carbon steel was continuously cast, and the quality improvement status of the obtained cast slab and the loading status of the load applied to the reduction of the light reduction zone were investigated.

  In the secondary cooling zone 6, the cast slab S was cooled with the specific water amount shown in FIG. 5, and the casting speed was varied at 1.25 to 1.4 m / min during casting.

  When the solidification completion position information is acquired by using the solidification completion position detection device 10, in this embodiment, the position of the flow limit solid phase ratio at the width position that is 800 mm from the width center is the most downstream side in the width direction. It was.

  For this reason, during continuous casting, the solidification completion position is such that the position of the flow limit solid phase ratio at the width position of 800 mm from the center of the width enters 0.5 m upstream from the exit side of the final segment of the light pressure lower belt 9. Based on the solidification completion position information acquired by the detection device 10, cooling was performed by adjusting the temperature of the secondary cooling water.

  In the casting conditions in this example, according to the heat transfer calculation, the flow limit solid phase ratio changes by 0.1 m when the water temperature of the secondary cooling water changes by 1 ° C. When the position is located upstream of the predetermined range, the water temperature of the secondary cooling water is raised, while when the position of the flow limit solid phase ratio is located downstream of the predetermined range, the secondary cooling water is increased. The water temperature was adjusted to decrease.

  For comparison, C: 0.042 mass%, Mn: 0.99 mass%, Si: 0.11 mass%, S: 0.0015 mass%, P: 0.015 mass%, solAl: 0.033 mass% The case where carbon steel continuous casting was carried out under the conditions shown in FIG. 7 (without adjusting the position of the flow limit solid phase ratio) was also investigated (comparative example).

  In all tests, the mold size was 250 mm in thickness and 2100 mm in width, and the rolling gradient in the segment in the light rolling belt 9 was 0.90 mm / m.

  FIG. 8 is a graph showing the transition of the position of the flow limit solid phase ratio and the transition of the casting speed when continuously cast according to the present invention (conforming example). The value of the secondary cooling water temperature adjustment is also shown in FIG.

  On the other hand, FIG. 9 is a graph showing the transition of the position of the flow limit solid phase ratio and the transition of the casting speed in continuous casting (comparative example) performed for comparison.

  As apparent from FIG. 8, in the continuous casting according to the present invention, the solid phase at the center in the thickness direction of the cast slab is always at the width position where the solidification completion position is the most downstream in the width direction during casting. It was confirmed that the position where the rate becomes the flow limit solid phase rate is controlled from the exit side of the last segment of the light pressure zone to the upstream side of 0.5 m.

  However, in the comparative example, as shown in FIG. 9, the position of the flow limit solid phase ratio also changed due to the fluctuation of the casting speed, and further the casting speed fluctuated within a certain range.

  FIG. 10 is a diagram comparing the displacement of the disc spring in the final segment of the light pressure belt when continuously cast according to the present invention and the displacement of the disc spring in the final segment of the light pressure belt in the comparative example. .

  In the present invention, the disc spring has a low displacement, and unlike the comparative example, the variation is small, and the cast slab at the end of solidification can be lightly reduced with a constant reduction amount, and the light reduction belt It has become clear that the load applied to the segments constituting the is reduced.

  FIG. 11 is a diagram comparing the degree of center segregation of a slab obtained by continuous casting according to the present invention and the degree of center segregation of a slab obtained in a comparative example.

  The degree of center segregation is calculated by analyzing the center of the cross section of the slab with EPMA (electron beam microanalyzer), calculating the Mn concentration, and multiplying by the Mn concentration of the raw steel component before casting. Calculated. Moreover, the sample of 10 pieces (full width sample) taken at random along the casting direction was used for the sample of the slab.

  As a result, in the slab continuously cast according to the present invention, the degree of center segregation was stable at a low level, whereas in the comparative sample, it was confirmed that the degree of center segregation greatly varied.

  According to the present invention, the center segregation of the cast slab is reduced, and it is constant in the longitudinal direction, so that the quality can be stabilized.

  Further, according to the present invention, since appropriate light reduction can be performed on the continuous cast slab, the load load (load due to reduction) in the segment of the light reduction belt can be reduced, and the life of the segment can be extended.

DESCRIPTION OF SYMBOLS 1 Casting casting mold 2 Tandille 3 Sliding nozzle 4 Immersion nozzle 5 Support roll 6 Secondary cooling zone 7 Conveying roll 8 Cutting device 9 Light pressure lower zone 10 Detector 11 Strut 12 Disc spring S Casting slab

Claims (5)

In a continuous casting method in which light reduction is performed on a cast slab drawn from a continuous casting mold in a light pressure lower zone located downstream of the secondary cooling zone,
According to the casting speed of the cast slab, by changing the specific amount of cooling water in the secondary cooling zone, the position where the solid fraction at the center of the cast slab becomes the flow limit solid fraction, The cast slab is controlled to be within a range from the exit side of the final segment of the light pressure lower belt to the upstream side of 0.5 m (excluding the position of the exit side of the final segment of the light pressure belt) . Continuous casting method. The position solid fraction of the cast slab becomes flow limit solid fraction, with determined based on-line by sensing information coagulation completion position of the cast slab, the position of the last segment of the soft reduction zone The continuous casting method for a cast slab according to claim 1, wherein the temperature of the cooling water in the secondary cooling zone is adjusted so as to be within a range from the outlet side to the upstream side of 0.5 m.   The method for continuously casting a cast slab according to claim 2, wherein a temperature adjustment range of the cooling water in the secondary cooling zone is 15 to 45 ° C.   The position at which the solid fraction of the cast slab becomes the flow limit solid fraction is at the width position where the solidification completion position exists on the most downstream side in the width direction of the cast slab. The continuous casting method of the cast slab described in any one of Items 1 to 3   The continuous casting method for a cast slab according to any one of claims 1 to 4, wherein a flow limit solid phase ratio of the cast slab is 0.6 to 0.8.

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