JP2019171447A - Method for continuously casting steel - Google Patents

Abstract

To provide a method for manufacturing a continuously cast slab which enables a sufficient light rolling draft to be provided under a wider range of manufacturing requirements without modifying light draft facilities and generates little center segregation.SOLUTION: A method is provided for casting by bulging a cast slab 10 by providing a light draft to a cast slab 10, when drafted thereafter in a light rolling draft zone 14, at a bulging amount of 2-15 mm in a segment 14B configured of a plurality of cast slab support roll pairs, at a draft gradient of 1.0-4.0 mm, including cast slab pulling-out direction position where a minimum solid phase rate of a cast slab cross section is 0.7, and by expanding the gap between a first pair of cast slab support rolls from the inlet side of the segment wider than the gap between a roll pair just ahead to satisfy a predetermined relational expression.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a continuous casting method for producing a steel slab such as a line pipe material in which segregation reduction at a thickness center portion is particularly demanded.

  In general, when steel is continuously cast, solute elements such as carbon, phosphorus, sulfur, and manganese are concentrated in the solidified liquid-liquid side during solidification, and this is concentrated between dendritic trees. It is known to form and result in microsegregation. On the other hand, a slab being cast by a continuous casting machine has a gap or a negative pressure at the center of its thickness due to solidification shrinkage, heat shrinkage, bulging of a solidified shell generated between rolls of the continuous caster, etc. Since this occurs, molten steel is easily sucked into this portion. In addition, since the liquid phase amount itself is small in the unsolidified layer at the end of solidification, the liquid phase causing concentration of the solute due to the microsegregation flows from the upstream side to the slab center and accumulates. Become. The segregation spot formed in the final solidified zone in this way has a much higher concentration of solute element than the initial concentration of molten steel, which is generally called macrosegregation, It is called.

  In recent years, as a line pipe material for transportation of crude oil, natural gas, etc., a material having higher strength has been demanded, but there has been a problem that the quality is deteriorated due to center segregation or the like. If MnS or Nb carbide is generated at the center segregation part of the line pipe material, hydrogen that has entered the steel due to corrosion reaction by hydrogen sulfide or the like diffuses and accumulates around the MnS or Nb carbide in the steel. Cracks may occur due to internal pressure. Furthermore, the center segregation portion not only has a high density of precipitates, but also hardens due to the concentration of the solute, so that the propagation and expansion of cracks are likely to occur. This is so-called hydrogen induced cracking (HIC). Therefore, it is important to reduce the center segregation of the slab.

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

  For example, in a continuous casting machine as disclosed in Patent Document 1 and Patent Document 2, an end-solidified slab having an unsolidified layer is equivalent to the sum of solidification shrinkage and heat shrinkage by a slab support roll. There has been proposed a method of continuous casting while gradually reducing the amount of reduction. These proposals are methods of gradually reducing the slab by a reduction amount corresponding to the sum of the solidification shrinkage amount and the heat shrinkage amount, and are called a light reduction method or a light reduction method. This light reduction technique uses a plurality of pairs of rolls arranged in the casting direction (also referred to as the drawing direction of the slab), and the slab is reduced with a reduction amount that is commensurate with the sum of solidification shrinkage and thermal shrinkage. By gradually reducing the volume of the unsolidified layer and preventing the formation of voids or negative pressure in the center of the slab, the flow of the concentrated molten steel formed between the dendritic trees is blocked, thereby This is a technique for reducing the center segregation of slabs.

  However, these techniques have the following problems. When a slab is subjected to light reduction, the reaction force applied to the segment used for the light reduction is the length of the segment, the size of the slab, the rolling gradient, the drawing speed, and the deformation of the slab. It is determined by resistance. By the way, when the slab is lightly reduced at the end of solidification, the proportion of unsolidified parts contained in the segment decreases, so the deformation resistance of the slab increases significantly, so the segment load must be increased greatly. There is a problem. Generally, a segment incorporating a slab support roll has an overload-preventing disc spring installed on each support (tie rod) that connects the upper and lower frames, and the upper and lower frames are fastened with a predetermined fastening force. Yes. If the load exceeds the set value of the fastening force, the excess load is absorbed by the constriction of the disc spring. At this time, the disc spring is greatly contracted so that the frame is displaced, so that it greatly changes from the set rolling gradient. On the other hand, increasing the set load (fastening force) of the disc spring in order to maintain the rolling gradient increases the load on the segment, and mechanical parts such as slab support rolls and bearings constituting the segment. There is a problem because it causes trouble. Therefore, depending on conditions such as the slab size, the reduction reaction force may be excessive, and light reduction under appropriate conditions may not be applied.

  In order to solve such problems, conventionally, by increasing the thickness of the central portion in the width direction of the slab before light reduction, the short side of the slab that has been solidified is not reduced during light reduction. The so-called IB method (forced bulging method) is considered (see Patent Document 3). However, even if this IB method is adopted, the load becomes too large for the segment that slabs the slab slab at the end of solidification where the solid phase ratio in the center of the slab thickness direction is close to the flow limit, and the disc spring is There is a case in which the frame is displaced due to contraction, and the rolling gradient becomes smaller than a predetermined value, which adversely affects the center segregation. In order to solve this problem, it is necessary to modify the segment itself to increase the rigidity, but in this case, the cost is enormous.

JP-A-8-132203 JP-A-8-192256 JP-A-2015-202510

  In recent years, quality requirements for continuous cast slabs are increasing, and slabs with less central segregation than ever before have been demanded. The present invention has been developed in view of the circumstances described above, and the object of the present invention is to provide a sufficient light reduction amount in a wider range of manufacturing conditions without modifying the conventional light reduction equipment. It is to propose an advantageous method that makes it possible to produce a continuous cast slab with less center segregation by applying it to the slab.

The continuous casting method of steel according to the present invention developed to solve the above problems is as follows.
In the continuous casting method of steel that solidifies the molten steel inside the slab by performing secondary cooling while supporting the slab slab drawn from the continuous casting mold with a plurality of pairs of slab support rolls,
The interval between the pair of slab support rolls is gradually expanded along the drawing direction of the slab to cause bulging in the thickness direction of the slab slab, and then the interval between the pair of slab support rolls is set as the slab. When applying a light reduction to the slab slab by providing a light reduction belt that narrows sequentially along the drawing direction of
The maximum increase in the thickness of the slab that causes the bulging is 2 to 15 mm,
The light pressure lower belt is composed of a frame in which a plurality of slab support rolls facing each other with a slab interposed therebetween, and a segment formed by connecting the frame in the slab thickness direction with a plurality of tie rods,
In the lightly reduced segment including the position in the slab drawing direction where the minimum solid phase ratio in the cross section of the slab slab of the lightly reduced zone is 0.7, 1.0 to 4.0 mm with respect to the slab slab The distance between the first slab support roll pair from the entry side of the lightly reduced segment is set so as to satisfy the relationship shown in the following mathematical formula 1 while applying a light reduction with a reduction gradient R of / m. It is a continuous casting method of steel characterized in that it is wider than the interval between the slab support roll pair immediately before the roll pair.

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である。

here,
D (mm): The distance between the first pair of slab support rolls from the entry side in the light rolling segment including the slab drawing direction position where the minimum solid fraction in the cross section of the slab becomes 0.7 and the immediately preceding Slab support difference between roll pair spacing,
K (−): a constant in the range of 0.3 to 0.8, and a correction coefficient for correcting the calculated value of the bulging amount of the slab long side surface between the slab support rolls before the lightly pressed segment ,
P (kgf / mm ² ): static iron pressure of unsolidified molten steel on the entry side of the lightly pressed segment,
L _{n-1 / n} (mm): distance between the first slab support roll and the immediately preceding slab support roll from the entry side of the lightly pressed segment,
L _n (mm): Roll pitch of the slab support roll of the lightly pressed segment,
d (mm): the solidified shell thickness at the center of the long side surface width of the slab at the first slab support roll position from the entry side of the lightly pressed segment,
R (mm / m): the rolling gradient of the slab in the light rolling segment.

In addition, the continuous casting method of steel according to the present invention configured as described above,
a. The lightly reduced segment including the slab drawing direction position where the minimum solid fraction in the transverse cross section of the slab slab in the lightly reduced zone is 0.7 is provided with 5 or more pairs of slab support rolls;
b. In the light reduction segment including the slab extraction direction position where the minimum solid phase ratio in the cross section of the slab slab of the light reduction zone is 0.7, the slab slab slab has a 1 in the slab extraction direction from the exit side of the light reduction segment. The minimum solid phase ratio in the cross section of the slab slab at the center position of the first slab support roll is 0.7 or more and 0.9 or less,
Is considered to be a more preferred embodiment.

  According to the continuous casting method of steel according to the present invention configured as described above, casting is performed in a lightly pressed segment including a slab drawing direction position where a minimum solid phase ratio in a cross section of a slab slab is 0.7. When applying light reduction to a piece, the load applied to the light reduction segment can be reduced even if light reduction is applied under the same conditions such as the reduction gradient, so a sufficient amount of light reduction can be cast using the conventional light reduction segment. Since the range of conditions that can be imparted to the piece can be expanded, it is possible to reduce the center segregation in a wider range of manufacturing conditions without incurring enormous facility modification costs.

It is a side schematic diagram showing an example of a slab continuous casting machine used when carrying out the present invention. It is the expansion schematic which looked at the cross section orthogonal to the slab drawing direction of the roll segment which forms a light pressure lower belt. It is a schematic diagram which shows transition of the roll opening degree in the light pressure lower zone in this invention. It is a figure which shows the influence which the space | interval difference D (mm) of a slab support roll pair and the rolling-down gradient R (mm / m) of a slab have on segregation improvement.

Embodiments of the present invention will be specifically described below with reference to the drawings.
FIG. 1 shows a very general configuration example of a slab continuous casting machine 1. A sliding nozzle 3, an immersion nozzle 4, a mold 5, and slab support rolls 6 and 6 ′ are sequentially arranged below the tundish 2. ing. The molten steel 9 in the tundish 2 finally becomes a continuous cast slab (slab) 10 having a rectangular cross-sectional shape.

  A portion of the slab support rolls 6, 6 ′ disposed below the mold 5 is sprayed with water in the gaps of the slab support rolls 6, 6 ′ disposed adjacent to each other in the casting direction. Spray nozzles (not shown) such as nozzles or air mist nozzles are arranged, and the inside is configured as a secondary cooling zone for supplying cooling water to the surface of the unsolidified cast piece 10 while adjusting the water density. Has been. The slab 10 is continuously drawn in the casting direction by a pinch roll (not shown) while being cooled by cooling water (also referred to as secondary cooling water) sprayed or sprayed from the spray nozzle of the secondary cooling zone. It has become.

  In the continuous bending machine of the vertical bending type or the curved type, a plurality of transport rolls 7 for transporting the cast slab 10 are installed on the downstream side of the final slab support rolls 6 and 6 ′ in the casting direction. Above the transport roll 7, a slab cutting machine 8 for cutting the continuously cast slab 10 into a slab 10a of a predetermined length is disposed.

  On the upstream side in the casting direction of the solidification completion position 13 of the slab 10, there is an interval between the slab support rolls 6 and 6 'facing each other with the slab 10 interposed therebetween (this interval is referred to as "roll opening"). A plurality of pairs of slabs set so as to become narrower sequentially toward the downstream side in the casting direction, that is, with a rolling gradient (the roll opening state set so as to become narrower toward the downstream side in the casting direction). A light pressure lower belt 14 composed of the support rolls 6 and 6 'is provided.

  In FIG. 1, the forced bulging band 15 disposed between the lower end of the mold 5 and the light pressure lower band 14 is directed toward the downstream side in the casting direction until the amount of expansion of the roll opening reaches a predetermined value of 2 to 15 mm. It consists of slab support rolls 6 and 6 'in which the roll opening degree is expanded sequentially for each roll or every several rolls. In this forced bulging band 15, the slab 10 having the unsolidified portion 12 inside is forcibly bulged (IB) by sequentially increasing the roll opening of the slab support rolls 6, 6 ′. Increase the thickness of the central portion in the width direction. In the subsequent light pressure lowering zone 14, light pressure reduction is applied within the range of the increase in the slab thickness increased in the forced bulging band 15. By applying such a light reduction, the solidified shell that grows from the short side surface where the reduction load is large is not reduced, and the solidified portion of the central portion in the width direction of the slab 10 at the end of solidification is not solidified. Effective light reduction is applied to the part with a relatively small load. In addition, if the amount of increase in roll opening, that is, the amount of increase in slab thickness (IB amount) by forced bulging is less than 2 mm, a sufficient load reduction effect cannot be obtained. On the other hand, if the amount of IB exceeds 15 mm, the risk of internal cracking increases. Therefore, the amount of IB applied to the slab before light pressure is desirably 2 to 15 mm.

The light pressure lower belt 14 is formed of a roll segment (hereinafter also simply referred to as “segment”) in which a plurality of pairs of slab support rolls 6 and 6 ′ are arranged in a row in the casting direction. The light pressure lower belt 14 shown in FIG. 1 is divided into two in the casting direction by two roll segments 14A and 14B. Here, the roll segment 14A on the upstream side in the casting direction forming the light reduction zone 14 is referred to as a first reduction zone, and the roll segment 14B on the downstream side in the casting direction is referred to as a second reduction zone. Although FIG. 1 shows an example in which the light pressure lower belt 14 is divided into two (14A, 14B) in the casting direction, the light pressure lower belt 14 may be divided into three or more, or may be a single segment. Also good.
Here, in order to suppress the center segregation, as described later, the light reduction of the slab slab is performed at a portion of the slab slab where the solid phase ratio is the lowest in the cross section (cross section perpendicular to the casting direction). The phase ratio (usually the solid phase ratio at the center of the slab thickness direction, hereinafter also referred to as “central solid phase ratio”) is applied with a predetermined rolling gradient over a range of solidification sections of 0.4 to 0.7. It is important that the rolling gradient in other central solid fraction regions is not important. In the continuous casting of steel according to the present invention, light reduction of the slab slab in the range of the solidification section with a central solid fraction of 0.4 to 0.7 is applied with a predetermined reduction gradient within the range of one segment. Hereinafter, this segment is referred to as a light reduction segment that reduces the slab at the end of solidification.

  In the light reduction segment that rolls down the slab at the end of solidification, at the slab extraction speed in the steady casting zone, the first slab from the segment exit side (side from which the slab exits) in the slab extraction direction The first one from the entrance side of the segment in the slab drawing direction so that the center solid phase ratio is 0.7 or more at the center position of the support roll (the slab support roll on the most downstream side in the casting direction of the segment) The slab drawing speed, secondary cooling water density, etc., so that the central solid fraction is at most less than 0.4 at the center position of the slab support roll (the slab support roll on the most upstream side in the casting direction of the segment). Set continuous casting conditions. At this time, the casting direction range of the light reduction segment for rolling down the slab at the end of solidification includes a casting direction position where the minimum solid phase ratio in the cross section of the slab slab is 0.7.

  Also, in the light reduction segment that reduces the slab at the end of solidification, from the viewpoint of reducing the load load due to reduction in order to maintain the desired reduction gradient, the first from the outlet side of the light reduction segment in the casting direction. The central solid fraction at the center position of the slab support roll (hereinafter, also referred to as “central solid fraction on the exit side of the lightly pressed segment”) is desirably 0.7 or more and as low as possible. However, even if there is some fluctuation in the central solid fraction due to fluctuations in continuous casting conditions, etc., the light reduction in the range including the solidification zone with a central solid fraction of 0.4 to 0.7 in this light reduction segment. In order to carry out the process stably, there is a problem in that the target value of the central solid phase ratio on the exit side of the lightly pressed segment is too low. In order to reduce the applied load, it is desirable that the central solid fraction on the exit side of the lightly pressed segment is at most 0.9 or less, so the target value is about 0.8 and there is some variation. Is preferably in the range of 0.7 to 0.9. In this way, by adjusting the central solid fraction (or the lowest solid fraction in the cross section of the slab slab) on the exit side of the lightly pressed segment within the range of 0.7 to 0.9. It is a more advantageous condition for reducing the rolling load in the light rolling segment without suppressing the center segregation and imparting unnecessary rolling to the slab having a high rolling resistance and a high central solid phase ratio.

  The light reduction segment that reduces the slab at the end of solidification is usually the segment in the most downstream reduction section, but is not necessarily limited to this, and the solid phase ratio is 0.4 to 0.7. Light reduction may be further applied to a segment subsequent to the light reduction segment that performs light reduction in a range including the section.

  In FIG. 1, the light rolling roll segment 14B for rolling down the slab at the end of solidification is composed of three pairs of slab support rolls 6 and 6 ′, but this simplifies the drawing. The light reduction roll segment for rolling down the slab at the end of solidification is composed of 5 pairs or more, more preferably 7 pairs or more of slab support rolls 6 and 6 ', as will be described later.

  Usually, the rolling gradient in the light rolling belt 14 is displayed as a roll opening degree narrowing amount (mm) per 1 m in the casting direction, that is, “mm / m”. Therefore, the reduction speed (mm / min) of the slab 10 in the light reduction zone 14 is obtained by the product of this reduction gradient (mm / m) and the slab drawing speed (m / min). In addition, the amount of reduction in each reduction section is determined by the difference between the roll opening at the inlet and the roll opening at the outlet in the reduction section, in other words, the reduction gradient in the reduction section and the casting in the reduction section. Calculated as the product of the direction length.

  According to the knowledge of the inventors, it is desirable that the light reduction applied in the light reduction segment that reduces the slab at the end of solidification is about 1.0 to 4.0 mm / m in terms of a reduction gradient. When the rolling gradient is less than 1.0 mm / m, the load acting on the segment is not likely to be excessive, but the rolling amount is insufficient and a sufficient center segregation suppressing effect cannot be obtained. On the other hand, when the rolling gradient exceeds 4.0 mm / m, the rolling amount is excessive and the risk of the occurrence of reverse V segregation increases, and conversely, the segregation at the center may be deteriorated.

  FIG. 2 is an enlarged schematic view of the roll segment 14B as an example of the roll segment forming the light pressure lower belt 14, and is a cross-sectional view seen from the slab drawing direction.

  As shown in FIG. 2, the roll segment 14 </ b> B includes a pair of an upper frame 16 and a lower frame 17 that hold a plurality of slab support rolls 6, 6 ′ via a roll chock 22. The illustrated example uses a roll in which the slab support rolls 6 and 6 'are divided into three in the slab width direction. The roll segment 14B is provided with a total of four tie rods 18 (both upstream and downstream sides) penetrating the upper frame 16 and the lower frame 17, and the upper frame 16 and the lower frame. 17 is connected in the thickness direction of the slab and plays a role of maintaining the roll opening degree of the slab support rolls 6 and 6 ′ against the molten steel static pressure received from the slab. The distance between the upper frame 16 and the lower frame 17 can be adjusted via a worm jack 20 (driven by a motor) installed on the lower frame 17 with the tie rod 18. Adjustment of the rolling gradient at 14B is performed.

  Further, the upper frame 16 is pressed downward with a predetermined set load by being pressed against the seat 21 fixed to the tie rod 18 from above through a disc spring module 19 contracted to a predetermined set length. It is energized. The disc spring module 19 is configured by stacking a plurality of disc springs, and does not contract when an upward load exceeding the set load does not act on the upper frame 16. Although it has a certain thickness, it begins to shrink when a load exceeding the set load is applied, shrinks in proportion to the excess of the load, and the upper frame 16 is displaced upward to reduce the load. It is configured to reduce the reaction force. For example, when the slab 10 has been solidified at the center of the roll segment 14B, an excessive load is applied to the roll segment 14B by reducing the solidified slab 10, and such an excessive load is applied. When the load is loaded, the disc spring module 19 contracts, so that the upper frame 16 is displaced upward, that is, the roll opening is enlarged, and an excessive load is not applied to the roll segment 14B. Yes. However, when the roll opening degree is increased in this way, the desired rolling gradient cannot be obtained and there is a possibility that the center segregation and thus the HIC may be adversely affected. Therefore, when a load is applied, it is desirable to apply light pressure in a range in which the frame is not displaced by the contraction of the disc spring module 19 as much as possible. The lower frame 17 on the lower surface side is fixed to the foundation of the continuous casting machine and is configured not to move during casting. In the example shown in FIG. 2, the disc spring module 19 is installed on the upper frame 16 side, and the worm jack 20 is installed on the lower frame 17 side. Conversely, the disc spring module 19 is installed on the lower frame 17. On the side, the worm jack 20 may be installed on the upper frame 16 side.

  The schematic diagram which shows the example of transition of the roll opening degree in the light pressure lower zone in this invention is shown in FIG. The light reduction treatment for the slab 10 is applied at the position of the light reduction segment 14B for reducing the slab at the end of solidification including at least the casting direction position where the central solid phase ratio is 0.7, and the light reduction is performed. It is also possible to apply light pressure reduction in the segment 14A upstream of the segment 14B. In FIG. 3, the transition of the position in the height direction (slab thickness direction) of the slab support roll 6 on the upper frame 16 side is shown in the casting direction (horizontal right side in FIG. 3) along with the thickness of the slab surface and the unsolidified portion. The slab support roll 6 'on the lower frame 17 side is not shown in the drawing, but is arranged along the horizontal roll path line similar to the case where light reduction is not applied. ing. That is, the difference in height of the slab support roll 6 in FIG. 3 indicates the difference in roll opening itself. However, the actual rolling gradient of the slab is in the range of 1.0 to 4.0 mm / m, and FIG. 3 shows an enlarged change in the height direction of the boundary between the slab surface and the unsolidified portion. It is a schematic diagram.

In order to suppress the HIC, it is effective to reduce the center segregation by applying a light reduction that compensates for the heat shrinkage of the solidified shell of the slab and the solidification shrinkage of the unsolidified portion. It is not necessary to apply light reduction over the entire coagulation section in the center. At the stage where the solid phase ratio in the central part is relatively low as f _S = 0.4, the concentration due to the distribution of the solute to the liquid phase is a relatively slight level. At this stage, the light pressure is not sufficiently reduced. Even if the liquid phase is sucked and macrosegregation occurs, it is difficult to achieve a segregation level that causes solute concentration and HIC.

In addition, at the stage where solidification has progressed to f _S = 0.7 or more at the center, the liquid phase is discontinuous or the flow resistance is remarkably high. Even if the light pressure is insufficient at the stage and the liquid phase is sucked, the liquid phase flow that causes macro segregation hardly occurs. Therefore, in order to effectively suppress the HIC, it is desirable to apply a light reduction over the entire solidification zone where the solid phase ratio at the thickness center portion is f _S = 0.4 to 0.7, and less than 0.4 And light reduction in the range of the central solid fraction exceeding 0.7 is not always necessary. However, as described above, in order to be able to cope with the fluctuation of the central solid fraction to some extent, the central solid fraction on the outlet side of the light pressure segment varies within the range of 0.7 to 0.9. It is desirable to allow this, and in this case, the light reduction segment may impart light reduction over the entire coagulation zone with a central solid fraction of 0.4 to 0.9.

  The length in the casting direction corresponding to the solidification section having the above-mentioned solid fraction in the central portion varies depending on the composition of the molten steel to be continuously cast, the drawing speed of the slab, the secondary cooling conditions, and the like. For example, the component composition of molten steel is C: 0.02 to 0.06 mass%, Mn: 0.8 to 1.6 mass%, Si: ≦ 0.5 mass%, Cu: ≦ 0.5 mass%, Ni: ≦ 0 0.5 mass%, Cr: ≦ 0.5 mass%, Mo: ≦ 0.5 mass%, V: ≦ 0.1 mass%, Ti: ≦ 0.02 mass%, Nb: ≦ 0.02 mass%, Ca: ≦ 0.004 mass %, Al: 0.01 to 0.1 mass%, with the balance being inevitable impurities such as Fe and P ≦ 0.01 mass%, S ≦ 0.005 mass%, N ≦ 0.005 mass%, and the thickness is In the case of standard continuous casting conditions in which a slab of 0.2 to 0.3 m is drawn out at a speed of 0.6 to 1.4 m / min and subjected to secondary cooling at a specific water amount of 1 to 2 L / kg. Then, the central solid phase ratio is 0.4 to 0.9. The casting direction length corresponding to the solidification section to be formed is less than 1 m, and the roll direction in the light rolling roll segment 14B for rolling down the slab at the end of solidification has a casting direction length that is within the two roll pitches. Is normal. In addition, the normal roll pitch of the slab support roll in each segment is set to an interval that does not cause bulging that causes a problem in the slab under normal continuous casting conditions. Therefore, in a general continuous casting machine, the roll pitch in the vicinity of the position at the end of solidification with a central solid fraction of 0.4 to 0.9 under the standard continuous casting conditions as described above is 0.3. It is about -0.5m.

However, the position of the slab drawing direction in which the solid phase rate of the slab thickness central part where solidification proceeds most late in the slab thickness direction is a specific value is not necessarily constant depending on the position in the width direction of the slab, When the central solid phase ratio on the light pressure segment outlet side is 0.9 at the maximum, the position where the solid phase ratio of the center part of the slab thickness located at the most downstream side in the slab drawing direction is 0.9, It means being in the center position of the first slab support roll from the exit side of the lightly reduced segment in the slab drawing direction. In this case, depending on the position in the width direction of the slab, the solid phase at the center of the slab thickness is located at the position in the slab drawing direction slightly upstream from the position of the first slab support roll from the exit side of the lightly pressed segment. The rate may be 0.9. Therefore, for the position in the width direction of the slab such that the solid phase rate at the center of the slab thickness is 0.9 on the upstream side, the solid phase rate at the center of the thickness f _S = 0.4-0. The solidification section that needs to be lightly reduced to 7 may also be displaced upstream in the slab drawing direction. Therefore, the length in the casting direction that requires light reduction is about 1 m or more upstream from the position in the slab drawing direction where the central solid fraction is 0.9, or about two or more in terms of roll pitch. It is desirable to ensure the length, and more desirably, it is desirable to ensure a length having a margin of about twice this (that is, 2 m or more, or four roll pitches or more).

  In the present invention, as shown in FIG. 3, from the entry side (the side where the slab enters the segment) of the light reduction segment that squeezes the slab at the end of solidification including the position in the casting direction where the central solid fraction is 0.7. The opening degree of the first slab support roll pair is made larger than the opening degree of the immediately preceding slab support roll pair (the opening degree of the first slab support roll pair from the exit side of the previous segment). Propose that. That is, the slab support roll group on the upper frame side of the segment is displaced in a parallel upward direction from a roll pass line continuous from the immediately preceding slab support roll while maintaining a predetermined rolling gradient. By setting, a roll that does not intentionally come into contact with the slab is generated. At the time of applying light reduction, the solidified shell thickness is sufficient, so that the bulging amount is not large, and the slab surface does not immediately follow the concave portion of the roll pass line. Also, if the opening difference (offset amount) between adjacent pair of support rolls when expanding the opening of the pair of slab support rolls is large to some extent, the slab support roll on the upper frame side of the entry side of the segment is cast. It is possible not to contact one surface. In this case, since a roll that does not come into contact with the slab is generated, even if the rolling gradient applied to the slab at the end of solidification is the same, the load on the segment is reduced.

  From another point of view, in the conventional light reduction method, the former segment exit side so that a part of the reduction load borne by the light reduction segment that squeezes the slab at the end of solidification is borne by the previous segment. It can be said that the opening degree of the pair of support rolls is set smaller. In other words, in the present invention, since the amount of reduction per lightly reduced segment that reduces the slab at the end of solidification is reduced, the segment load is reduced, and the tie rod load does not exceed the set load of the disc spring module. The range of conditions is relaxed accordingly.

If only the end of solidification where the solid fraction at the center of the slab thickness is f _S = 0.4 or more is lightly reduced, the center segregation is sufficiently improved. If the segment is located downstream from the first slab support roll from the exit side of the segment and the segment has a certain length in the slab pulling direction, immediately press down lightly from the segment entry side. Even if it doesn't start, it will be enough.

  Next, the condition of the offset amount D (mm) for preventing the slab support roll on the first upper frame side from the entry side of the segment from coming into contact with the slab surface will be described.

L _n (mm) is the roll pitch (distance between adjacent roll centers in the slab travel direction) of the lightly squeezed segment that squeezes the slab at the end of solidification including the position in the casting direction where the central solid fraction is 0.7. If the distance between the first roll of the lightly reduced segment and the immediately preceding roll is L _{n-1 / n} (mm), the distance between the second roll of the lightly reduced segment and the roll immediately before the first roll is It is separated by “L _{n−1 / n} + L _n ”. The left side of the following formula 2 “K · P · (L _{n−1 / n} + L _n ) ^3.5 / 220 / d ³ ” is the distance between the rolls separated by “L _{n−1 / n} + L _n ” in the casting direction. Indicates the amount of bulging. This mathematical formula 2 is based on the analysis result of the finite element method under the high-speed casting conditions with a relatively thin solidified shell thickness described in Sumitomo Metal Technical Report (Vol. 35 (1983), No. 3, p. 1). Based on the derived bulging amount calculation formula, considering the influence of the elastic modulus and plasticity coefficient depending on the slab temperature distribution and steel type, the final solidification phase including the casting direction position where the central solid fraction becomes 0.7 Correction factor for correcting the deviation between the slab bulging amount measured between the first slab support roll and the slab support roll immediately before it from the entry side of the light reduction segment for rolling down the slab Multiply by K.

Here, K is a constant of 0.3 to 0.8 that varies depending on the continuous casting conditions such as the steel type and the secondary cooling water density density distribution, and bulging between rolls of the slab is performed in an actual continuous casting machine. It can be quantified by measuring with an ultrasonic distance meter or the like and investigating the influence of the solidified shell thickness at the center of the long side surface width of the slab and the distance between the rolls on the bulging amount between rolls. Further, P represents the static iron pressure (kgf / mm ² ) of the unsolidified molten steel on the entry side of the light reduction segment for rolling down the slab at the end of solidification including the position in the casting direction where the central solid fraction is 0.7.

  As shown in Equation 2 above, the roll opening of the first slab support roll from the entry side of the light reduction segment that squeezes the slab at the end of solidification including the position in the casting direction where the center solid phase ratio becomes 0.7 is opened. The bulging amount of the slab between the immediately preceding roll upstream of the first roll of the light reduction segment and the second roll of the light reduction segment By setting to be larger than the first roll, it is possible to prevent the first roll on the upper frame side of the lightly pressed segment from coming into contact with the slab, thereby reducing the load acting on the lightly pressed segment.

  However, if the offset amount D is excessively increased, the range in the casting direction in which no reduction is applied to the slab becomes wide, so care must be taken.

Further, under the light pressure, in order to effectively suppress the center segregation and HIC, the solid phase rate f _S of the coagulation site that progresses most late in the central portion in the thickness direction ranges from 0.4 to 0.7. It is desirable to apply light reduction with a predetermined reduction gradient in the range of the coagulation section. The casting direction length of the light rolling application range required for that is about 1 m or more upstream from the casting direction position where the central solid fraction becomes 0.9 as described above, or about two or more at the roll pitch. More preferably, it is preferable to secure a length in the casting direction with a margin of more than about twice (that is, 2 m or more, or four roll pitches or more).

Further, as described above, the position in the casting direction where the central solid fraction is 0.9 is usually solidified so as not to unnecessarily reduce the slab having a high rolling resistance and a high central solid fraction. It is desirable to adjust so that it may be located in the downstream from the 1st slab support roll from the exit side of the light reduction segment which squeezes the final slab. Therefore, in general, since the roll segment is composed of at least 5 pairs of slab support rolls, as described above, in order to ensure a light rolling application range of two or more casting direction lengths at the roll pitch. In the third and subsequent slab support rolls from the entry side of the light reduction segment that squeezes the slab at the end of solidification including the position in the casting direction where the central solid fraction is 0.7, light reduction is applied to the third and subsequent slab support rolls. It is desirable to set the roll opening. This means that the roll interval of at least the third roll pair of the lightly pressed segment needs to be smaller than the roll interval of the immediately preceding roll pair upstream of the lightly pressed segment. In order to satisfy this condition geometrically, the offset amount D (mm), the roll pitch L _n (mm), and the rolling gradient R (mm / m) need to satisfy the relationship of the following Equation 3.

  When the offset amount D is set in the range of the above equation (3), since at least the third and subsequent cast slab support rolls 6 and 6 'are subjected to reduction, usually five or more pairs of slab support rolls 6 and 6 In the roll segments 14A and 14B of the light pressure lower belt 14 composed of 6 ', the roll pitch is two or more, or the effective length of the segment (distance between the most upstream roll center and the most downstream roll center). It is possible to ensure a range of light reduction of 1/2 or more in the casting direction length. In addition, if there are more slab support rolls 6 and 6 'pairs constituting the segments of the light reduction belt 14, the length range in the casting direction in which the light reduction can be applied can be expanded. The range that can cope with continuous casting conditions where the casting direction section corresponding to is expanded, or the variation of manufacturing conditions such that the casting direction position where solidification is completed varies in the slab width direction Is preferable. For example, as a light reduction segment for rolling down the slab at the end of solidification including the position in the casting direction where the central solid phase ratio is 0.7, the roll pitch is configured by including seven or more pairs of slab support rolls 6 and 6 ′. This is preferable because a light reduction application range having a length in the casting direction with a margin of four or more can be secured.

  Further, from the viewpoint of sufficiently securing the length in the casting direction for imparting light reduction, the first roll from the outlet side of the light reduction segment is at least the casting direction position where the central solid fraction is 0.7 in combination with the above. It is desirable to position it in a range downstream of the second roll from the position of.

  As is clear from the above description, the offset amount D (mm) is eventually set so as to satisfy the relationship of the following expression (1).

FIG. 4 shows the inside of the mold from the unsolidified portion of the horizontal slab where there is a lightly reduced segment that squeezes the slab at the end of solidification including the casting direction position where the slab thickness is 250 mm and the central solid phase ratio is 0.7 height of about 14.2m to molten steel surface, the soft reduction rolls pitch segment _{L n-1 / n = L} n = 300mm, roll pairs 8 pairs of the soft reduction segment, the entry side of the soft reduction segment 3 shows the relationship between the offset amount D and the rolling gradient R when the solidified shell thickness d at the central portion of the long side surface width of the slab 10a at the position of the first slab support roll 6, 6 ′ is 115 mm. is there. Here, in the above equation (1), the static iron pressure P is calculated as 0.0994 kgf / mm ² from the height of the molten steel surface, assuming that the density of the molten steel is about 7000 kg / m ³ . K was calculated as 0.5 based on the actual measurement value of the bulging amount. From FIG. 4, the hatched area is a range in which the load under light pressure applied to the segment is reduced and sufficient segregation improvement effect can be obtained. In other areas, the effect of reducing the load on the segment is insufficient or sufficient This is a case where the segregation improvement effect cannot be obtained.

  As an example, using a vertical bending slab continuous casting machine in which a forced bulging band and a light pressure lower band composed of a plurality of segments as shown in FIG. 1 are used, a low carbon aluminum killed steel (C: 0) for a line pipe material is used. 0.040-0.045 mass%, Si: 0.29-0.31 mass%, Mn: 1.28-1.32 mass%, P: 0.003-0.004 mass%, S: 0.0004-0.0005 mass %) Was continuously cast to produce a slab having a slab thickness of 250 mm and a slab width of 2100 mm. A plurality of segments arranged 19 to 32 m from the meniscus can be applied under light pressure, and the downstream side of the lower correction position at a position of about 20 m from the meniscus is a horizontal portion. Moreover, the height from the unsolidified part of the slab of the horizontal part to the molten steel surface in a mold is about 14.2 m.

  The casting direction in which the solid phase ratio is 0.7 is calculated in advance by calculating the progress of solidification accompanying the position in the slab drawing direction by heat transfer / solidification calculation as disclosed in JP-A-4-231158. The secondary cooling is applied so that light reduction is applied to the segment including the position, and the calculation result of the central solid phase ratio at the center position of the first slab support roll from the exit side of the segment is about 0.8. The conditions and the slab drawing speed were finely adjusted, and the slab drawing speed was finely adjusted within a range of 1.05 to 1.15 m / min. Further, the long side surface of the slab at the position of the first slab support roll from the entry side of the light reduction segment that squeezes the slab at the end of solidification including the position in the casting direction where the central solid fraction is 0.7 Similarly, the solidified shell thickness d (mm) at the center of the width was determined by heat transfer / solidification calculation. Here, the solidified shell thickness d is the solidified shell thickness at the position above the unsolidified portion of the slab, and was about 115 mm in any of the present invention examples and comparative examples.

  The adjustment of the rolling gradient in the segment of the lightly rolling belt is the sum of connecting the upper frame 16 and the lower frame 17 of each segment having the structure shown in FIG. 2 at both ends in the width direction of the segment and both ends in the slab drawing direction. The vertical positions of the four tie rods were adjusted with a worm jack fixed to the lower frame 17.

  The upper frame 16 of each segment is urged downward with a predetermined load by a disc spring module with respect to a seat fixed to the tie rod, and when the upper frame 16 receives a reaction force exceeding the load, the disc spring Is further contracted, and the upper frame 16 is displaced upward to prevent receiving an excessive reaction force. In the light reduction segment for rolling down the slab at the end of solidification used in the present invention example and the comparative example, the set load of each disc spring module provided on each of the four tie rods was adjusted to 150 tons.

  Whether or not light reduction was applied to the slab with a set-up gradient as set, whether or not each disc spring module of the light reduction segment that squeezes the slab at the end of solidification including the position in the casting direction where the central solid fraction is 0.7 is determined. The displacement was measured by a non-contact sensor, and from the result, when a load exceeding the set load by the disc spring module was applied to any tie rod, it was determined that the load applied to the segment was excessive. When the disc spring module is displaced, the substantial reduction gradient is smaller than a predetermined value.

  The interval between the first pair of slab support rolls from the entry side and the immediately preceding slab in the light reduction segment that squeezes the slab at the end of solidification including the rolling gradient R and the center direction solid fraction is 0.7. A slab cast produced by carrying out the continuous casting method of the present invention and the comparative example by variously changing the difference (offset amount) D with the interval between the pair of support rolls and the amount of increase in slab thickness (IB amount) by forced bulging. Evaluation was made on hydrogen-induced cracking (HIC) of a thick steel plate obtained by further hot-rolling the center segregation, internal cracking, and slab slab of the slab.

In any of the examples of the invention and the comparative example, the roll pitch value of the lightly reduced segment for rolling down the slab at the end of solidification of the lightly reduced zone and the roll pitch value of the slab support roll immediately before the segment are the same. , L _{n−1 / n} = L _n = 300 mm. The distance between the centers of the slab support rolls at both ends of the segment and the distance between the tie rods in the slab drawing direction are about 3 m. The segment has 11 pairs of slab support rolls.

  Except for the offset amount D and the amount of IB, the examples of the invention and the comparative example are set to the same continuous casting conditions. Under the same continuous casting conditions, a slab at the end of solidification including a casting direction position at which the central solid fraction is 0.7. Based on the result of actual measurement of the bulging amount of the long side surface (upper surface) of the slab at the intermediate position between the first slab support roll and the immediately preceding slab support roll from the entry side of the light reduction segment to be reduced, The correction coefficient K in the above equation (1) was set to 0.5.

For Invention Examples 1 to 3, the IB amount, the rolling gradient R, and the offset amount D were set within the ranges satisfying the conditions of the present invention.
For Comparative Example 1, the amount of IB was set to 1 mm, which is less than the range of the present invention.
For Comparative Example 2, the amount of IB was set to 18 mm exceeding the range of the present invention.
For Comparative Example 3, the IB amount was 5 mm, and the set rolling gradient was 0.5 mm / m, which was less than the range of the present invention. At that time, the offset amount D was set to 0 mm.
For Comparative Example 4, the IB amount was 15 mm, and the set rolling gradient was 4.5 mm / m, which exceeded the range of the present invention. At that time, the offset amount D was 1.5 mm.
For Comparative Example 5, the IB amount was 5 mm, and the set rolling gradient was 2.0 mm / m. The offset amount D was 0.5 mm, which is less than the range of the present invention.
For Comparative Example 6, the IB amount was 5 mm, and the set rolling gradient was 2.0 mm / m. The offset amount D was set to 2.0 mm exceeding the range of the present invention.

  After continuous casting, samples were cut out from the slabs of the inventive examples and comparative examples, and segregation investigation by EPMA was conducted. Moreover, the cross section of the full width perpendicular to the slab casting direction was corroded with warm hydrochloric acid to investigate the presence of internal cracks. Furthermore, a thick steel plate sample taken from a thick steel plate having a thickness of 20.6 mm manufactured by hot rolling a plurality of slab slabs at the steady casting portion in the continuous casting charge of the inventive example and the comparative example was subjected to the HIC test.

Table 1 shows the IB amount of the inventive example and the comparative example, the set reduction gradient R, the presence or absence of overload, and the light reduction segment for reducing the final solidification slab including the casting direction position where the central solid fraction is 0.7 D, K · P · (L _{n−1 / n} + L _n ) ^3.5 / 220 / d ³ , which is the amount by which the roll opening of the most upstream roll pair is expanded relative to the roll opening of the immediately preceding roll pair. Calculated values, calculated values of 2 · L _n · R / 1000, the number of segregation spot portions, HIC test results (CAR), and presence / absence of internal cracks are shown.

  The number of segregation spot portions in the thickness center portion is obtained by mapping the thickness center portion of the slab cross-section sample by EPMA, and segregation portions having a segregation degree of 1.33 or more are connected in the width direction by 500 μm or more from the quantitative analysis value of Mn. It was calculated by counting the number of locations. Specifically, from the cross section perpendicular to the casting direction of the slab slab for investigation, the height is 25 mm, the central segregation part is included in the center part, and the width is about 930 mm (from the width center to one side triple point (short side side and A sample corresponding to the half length of the slab up to the point where the solidified shells on both long sides grew and met was taken and subdivided. Thereafter, the distribution of Mn segregation degree is obtained by mapping analysis of Mn concentration over the entire surface with an electron beam diameter of 100 μm using EPMA, and Mn spots whose Mn segregation degree is 1.33 or more are 500 μm in the slab width direction. The number of locations (heavy segregation spots) connected over the above was counted. Here, the Mn segregation degree of the Mn spot means the ratio (B / A) of the Mn concentration B of the Mn spot to the Mn concentration A at a position 10 mm away from the center of the thickness.

Next, the result of the HIC test was evaluated based on the presence or absence of a sample having a crack area ratio (CAR; Crack Area Ratio) of 2% or more. Here, the test piece for the HIC test has dimensions of casting direction length L: 100 mm × width direction length W: 20 mm × sheet thickness t: 20.6 mm, and is manufactured as described above. Three thick plates were randomly extracted from a plurality of thick plates produced by hot rolling each of the plurality of slabs of Examples and Comparative Examples, and sampled from a position corresponding to 1/4 of the slab width. . This HIC test is based on NACE STANDARD TM-0284, pH 3.0 (when H ₂ S is saturated), and a temperature of 25 ° C. NACE test solution (5% NaCl, 0.5% CH ₃ COOH, H ₂ S : 2480 ppm (at the time of H ₂ S saturation)) After 96 hours of immersion, C scan was performed by ultrasonic flaw detection, and the area ratio (CAR) where cracking occurred was measured. The CAR having the highest value was the representative value of the casting conditions. When even one of the three test pieces had a CAR of 2% or more, all the planks manufactured from the charge from which the test piece was collected were rejected.

As a result, in Invention Examples 1 to 3, there was no overload in the lightly reduced segment, and lightly reduced pressure was applied to the slab according to the set value. In particular, the segregation and HIC results were good because the set rolling gradient was an appropriate value. Moreover, internal cracks were not confirmed.
On the other hand, in Comparative Example 1, since the amount of IB was small, the load on the segment was excessive, the actual rolling gradient greatly deviated from the set value, and the improvement in segregation was slight. The HIC test was also rejected.
In Comparative Example 2, because the amount of IB was too much, segregation and HIC results were good, but internal cracks occurred.
In Comparative Example 3, since the set rolling gradient was low, the effect of improving segregation was small and the HIC test was also rejected.
In Comparative Example 4, since the set rolling gradient was too large, a reverse flow of the concentrated molten steel occurred and segregation increased. The HIC test also failed.
In Comparative Example 5, since the offset amount D = 0.5 mm and the condition is smaller than the calculated value of K · P · (L _{n−1 / n} + L _n ) ^3.5 / 220 / d ³ , the segment load becomes excessive. Therefore, it was not possible to give the set reduction as set. As a result, the segregation improvement effect was reduced and the HIC test was also rejected.
In Comparative Example 6, a reduction close to the set value was applied, but the offset amount D was 2.0 mm, which was larger than the calculated value of 2 · L _n · R / 1000, and the timing of the application of reduction was delayed. Became insufficient and the HIC test was also rejected.

DESCRIPTION OF SYMBOLS 1 Slab continuous casting machine 2 Tundish 3 Sliding nozzle 4 Immersion nozzle 5 Mold 6, 6 'Cast piece support roll 7 Conveying roll 8 Cast piece cutting machine 9 Molten steel 10 Cast piece 10a Cut cast 11 Solidified shell 12 Unsolidified part 13 Solidified Completion position 14 Light pressure lower belt 14A Roll segment 14B Roll segment 15 Forced bulging belt 16 Upper frame 17 Lower frame 18 Tie rod 19 Belleville spring module 20 Warm jack 21 Seat 22 Roll chock

Claims (3)

In the continuous casting method of steel that solidifies the molten steel inside the slab by performing secondary cooling while supporting the slab slab drawn from the continuous casting mold with a plurality of pairs of slab support rolls,
The interval between the pair of slab support rolls is gradually expanded along the drawing direction of the slab to cause bulging in the thickness direction of the slab slab, and then the interval between the pair of slab support rolls is set as the slab. When applying a light reduction to the slab slab by providing a light reduction belt that narrows sequentially along the drawing direction of
The maximum increase in the thickness of the slab that causes the bulging is 2 to 15 mm,
The light pressure lower belt is composed of a frame in which a plurality of slab support rolls facing each other with a slab interposed therebetween, and a segment formed by connecting the frame in the slab thickness direction with a plurality of tie rods,
In the lightly reduced segment including the position in the slab drawing direction where the minimum solid phase ratio in the cross section of the slab slab of the lightly reduced zone is 0.7, 1.0 to 4.0 mm with respect to the slab slab The distance between the first slab support roll pair from the entry side of the lightly reduced segment is set so as to satisfy the relationship shown in the following mathematical formula 1 while applying a light reduction with a reduction gradient R of / m. A continuous casting method for steel, characterized in that it is wider than the interval between the slab support roll pair immediately before the roll pair.
Record

here,
D (mm): The distance between the first pair of slab support rolls from the entry side in the light rolling segment including the slab drawing direction position where the minimum solid fraction in the cross section of the slab becomes 0.7 and the immediately preceding Slab support difference between roll pair spacing,
K (−): a constant in the range of 0.3 to 0.8, and a correction coefficient for correcting the calculated value of the bulging amount of the slab long side surface between the slab support rolls before the lightly pressed segment ,
P (kgf / mm ² ): static iron pressure of unsolidified molten steel on the entry side of the lightly pressed segment,
L _{n-1 / n} (mm): distance between the first slab support roll and the immediately preceding slab support roll from the entry side of the lightly pressed segment,
L _n (mm): Roll pitch of the slab support roll of the lightly pressed segment,
d (mm): the solidified shell thickness at the center of the long side surface width of the slab at the first slab support roll position from the entry side of the lightly pressed segment,
R (mm / m): the rolling gradient of the slab in the light rolling segment.   The light-pressing segment including the slab drawing direction position where the minimum solid phase ratio in the cross section of the slab slab in the light-pressing zone is 0.7 includes five or more pairs of slab-supporting rolls. The continuous casting method for steel according to claim 1, wherein the steel is continuously cast.   In the light reduction segment including the slab extraction direction position where the minimum solid phase ratio in the cross section of the slab slab of the light reduction zone is 0.7, the slab slab slab has a 1 in the slab extraction direction from the exit side of the light reduction segment. 3. The steel according to claim 1, wherein a minimum solid phase ratio in a cross section of the slab slab at a center position of the first slab support roll is 0.7 or more and 0.9 or less. Continuous casting method.

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