JP5716414B2 - Continuous casting equipment for round slabs for seamless steel pipe production - Google Patents

Description

  The present invention relates to a continuous casting facility for round slabs, and more particularly to a continuous casting facility for round slabs for seamless steel pipe production. The continuous casting equipment for seamless steel pipe round slabs according to the present invention is particularly used for pipe making of high chromium steel, particularly 13Cr seamless steel pipe (API-13Cr steel pipe) used for oil well excavation. It can be used for continuous casting of round slabs for pipe making.

  Seamless steel pipes are generally prepared with a round billet (round slab) as a starting material, drilled by the Mannesmann drilling method, then stretched by a rolling machine such as an elongator, plug mill or mandrel mill, and further by a sizer or stretch reducer. Manufactured by a series of steps to make the diameter constant.

As means for preparing such a round billet, means for directly continuously casting a round billet is known. For example, Patent Document 1 discloses a slab as a method for reducing the center porosity and center segregation of a continuously cast slab. A secondary cooling method using heat shrinkage during cooling has been proposed. Specifically, in continuous bloom or billet casting of steel, until the solid phase ratio at the center of the slab reaches 0.99 or more from a position 0.1 to 2.0 m before the most recent casting direction of the residual molten metal pool A method of water-cooling the slab surface at a water density of 100 to 300 liters / (min. · M ² ) in the forced cooling zone at the end of solidification is shown (see Patent Document 1: Claim 1). Patent Document 2 discloses a molten steel having a carbon content of 0.1% by mass or less, or a molten steel having a Cr content of 1% by mass or more and a carbon content of 0.15% by mass or less. After the secondary cooling of the slab from directly under the mold in the casting method, the secondary cooling of the slab is started at the end of solidification when the surface temperature of the slab reaches 1100 to 950 ° C. A continuous casting method of a round slab is disclosed in which the secondary cooling at the end of solidification is continued within a range until the center of the slab is completely solidified.

  On the other hand, as means for cooling the slab with the above-mentioned purpose, for example, Patent Document 3 discloses a secondary cooling facility for continuous casting in which the slab is secondarily cooled by spraying through a roll gap using a spray nozzle. Is disclosed.

JP 2001-62550 A JP 2004-330252 A JP-A-9-141408

  The means disclosed in Patent Documents 1 and 2 relate the solid phase ratio or the slab surface temperature at the center of the slab and the water volume density of the cooling water applied to the slab surface. However, even if the water density of the cooling water is related to the above relationship, the continuous casting equipment actually used for continuous casting of round slabs is suitable for the solidification state of the strands in the continuous casting process. It is not compatible.

  For example, a round cast slab for 13Cr seamless steel pipe used for oil well drilling contains a large amount of Cr, so the solidification section during continuous casting is very long. It is known that type, B type and C type axial center cracks are likely to occur. According to the knowledge of the present inventor, in order to reduce these axial cracks at a practical level, it is necessary to optimize the water cooling conditions over a wide range corresponding to the generation region. No continuous casting equipment for seamless steel pipe round cast slabs has been proposed. For example, the means described in Patent Document 3 merely discloses a spray nozzle used when the slab is secondarily cooled.

  The present invention corresponds to the continuous casting of the above-mentioned round cast slabs for seamless steel pipes, and particularly to the region where the axial center cracks occur in the round product slabs in the continuous casting of round casts for 13Cr seamless steel pipes. Then, it aims at proposing the continuous casting which has the forced cooling zone which can be water-cooled by appropriate water quantity density.

  The inventor conducted a detailed investigation on the cause of the occurrence of defects in the continuous casting process of high chrome steel round slabs for seamless steel pipes, in particular, 13Cr seamless steel pipes. And in the process of continuous casting, even if it reaches the stage of complete solidification on the assumption that the axial center part of the strand does not contain macro segregation from the stage containing unsolidified molten steel inside, it is compressed at a higher water density than in the past. By continuing forced cooling for applying stress, it is possible to effectively reduce axial center cracks that occur in the end of solidification and in the subsequent cooling process, and the water density of the cooling water is an indicator within the strand in the continuous casting process. It has been found that cracks can be effectively reduced by applying in relation to the solid phase rate fs of the strand axis, the axis temperature Tc, and the bulk solidus temperature Ts of continuously cast molten steel. The continuous casting equipment for seamless steel pipe round slabs according to the present invention is configured to embody such association.

The continuous casting equipment for seamless steel pipe round slabs according to the present invention is obtained by injecting molten steel into a mold and cooling the water to a circular cross-section including unsolidified molten steel inside to completely solidify it. , A continuous casting facility for round steel slabs for seamless steel pipe making to obtain round slabs by cutting,
The continuous casting facility supplies cooling water at a water density of at least 10 to 50 L / min / m ² with respect to the outer peripheral surface of the strand within a range from a position of 10 m to a position of 25 m from the meniscus in the continuous casting mold. obtaining polarization析抑system forced cooling zone, at least 10 to 100 L / min / ^m primary compressive stressing force cooling zone capable of supplying cooling water in the ^second water density and at least 0L / min / ^{m 2} ultra 60L / min / ^{m 2} And a secondary compressive stress-added forced cooling zone that can supply cooling water at a lower water density than the first compressive-stress-added forced cooling zone in order,
Each of the forced cooling zones includes the axial solid phase fraction fs in the longitudinal direction of the strand calculated from the composition and composition of the continuously cast steel and the continuous casting conditions, the axial temperature Tc, and the continuous cast steel. Based on the solidus temperature Ts of the bulk molten steel determined from the composition and components, the cooling water density density indicating means for each cooling zone for determining the cooling water volume density for each forced cooling zone,
A spray water supply device is provided for supplying cooling water to the outer surface of the strand in each forced cooling zone based on an instruction from the spray water amount indication means for each cooling zone.

  The continuous casting equipment for seamless steel pipe-made round cast slabs according to the invention is particularly suitable for continuous casting of 13Cr steel seamless steel pipe-made round cast pieces.

In the above invention, the segregation suppression forced cooling zone supplies cooling water at a rate of a water density of 10 to 50 L / min / m ² within a range where the axial center solid fraction fs is 0.3 or more and less than 0.5. In the first compressive stress-added forced cooling zone, the axial solid phase ratio is 0.5 or more and the axial temperature Tc is in the range of (Ts-145) ° C. The cooling water can be supplied at a water density of 10 to 100 L / min / m ² , and the second compressive stress-added forced cooling zone has an axial temperature Tc of (Ts-145). in ° C. than the range of (Ts-255) ℃, it is preferable to assume that can supply at least 0L / min / ^{m 2} ultra 60L / min / ^m coolant ² in the following proportions.

  In each of the above inventions, the water density is gradually increased or gradually decreased from the segregation suppression forced cooling zone to the first compression stress addition forced cooling zone between the segregation suppression forced cooling zone and the primary compression stress addition forced cooling zone. The amount of water from the primary compressive stress-added forced cooling zone to the secondary compressive-stressed forced cooling zone between the primary compressive stress-added forced cooling zone and the secondary compressive stress-added forced cooling zone It is desirable to provide a connection that gradually reduces the density.

  According to the present invention, it corresponds to the continuous casting of round cast slabs for seamless steel pipe pipes, especially the continuous casting of round cast slabs for 13Cr seamless steel pipes, corresponding to the region of axial center cracking that occurs in product round cast slabs. Therefore, it is possible to perform water cooling at an appropriate water density, thereby suppressing the occurrence of axial center cracks in the round cast pieces for seamless steel pipe production at a practical level.

It is a conceptual diagram which shows the forced cooling zone and its arrangement | positioning in the continuous casting installation which concerns on this invention. It is typical explanatory drawing of the internal defect which appears in the round slab for 13Cr steel seamless steel pipe pipe making. (A) A type crack, (b) B type crack, (c) C type crack It is a typical explanatory view of a segregation suppression forced cooling zone. It is a typical explanatory view of the primary compression stress addition forced cooling zone. It is a typical explanatory view of the secondary compression stress addition forced cooling zone. It is a conceptual diagram of the control system of the forced cooling zone of the continuous casting equipment which concerns on this invention. The water flow rate Q to be applied in the present invention _{(Q 1, Q 2, Q} 3) is a graph schematically illustrating relative fraction solid fs of the axis of the strands.

  FIG. 1 is a conceptual diagram showing a forced cooling zone and its overall arrangement in a continuous casting facility according to the present invention. As shown in FIG. 1, the continuous casting equipment according to the present invention includes a segregation suppression forced cooling zone 5, a primary compressive stress-added forced cooling zone 3, and a secondary compressive stress-added forced after the secondary cooling zone 2. A cooling zone 4 is provided. In this continuous casting equipment, the molten steel injected from the tundish (not shown) into the continuous casting mold 1 having a circular cross section passes through the secondary cooling zone 2 equipped with a spray nozzle in the same manner as the conventionally known continuous casting equipment. The solidified shell 10 grows to form a strand S having unsolidified molten steel 11 inside. After complete solidification, the strand S is straightened by the straightening strip 6 and then cut into a predetermined length by a cutting means (not shown). It is a round slab.

  For example, when a continuous casting facility is used to continuously cast a round cast piece for 13Cr seamless steel pipe, various internal defects are generated in the product round cast piece depending on the secondary cooling zone or the water density of the water after that. . These internal defects are typically classified into three types: (1) A-type crack, (2) B-type crack, and (3) C-type crack. Here, as shown in FIG. 2A, the A-type crack is a relatively small crack defect that occurs in the center of the cross section perpendicular to the casting direction of the strand, and shrinkage that occurs immediately after the end of solidification of the strand or immediately after solidification. It is a star-shaped crack generated from a hole, and is divided into a primary A type crack and a recuperated A type crack as described later. As shown in FIG. 2B, the B-type crack is a V-shaped crack that occurs in the cross section in the casting direction of the strand, and is a crack derived from a V-shaped segregation that occurs from the middle to the end of solidification. As shown in FIG. 2 (c), the C-type crack is a crack having a relatively large opening that appears in a cross section perpendicular to the casting direction of the strand, and is applied to the axial center after the strand is almost solidified. This is caused by the expansion of the shrinkage hole due to the tensile stress during recuperation.

  In the present invention, in the continuous casting apparatus shown in FIG. 1, following the secondary cooling zone 2, the segregation suppression forced cooling zone 5 is within a range in which the distance from the meniscus of the continuous casting mold 1 is 10 m to 25 m. In order to prevent the occurrence of the A-type crack, B-type crack and C-type crack by these forced cooling zones, the primary compressive stress-added forced cooling zone 3 and the secondary compressive stress-added forced cooling zone 4 are provided. It enables forced cooling with an appropriate water density.

  The segregation suppression forced cooling zone 5 is a forced cooling zone for suppressing the B-type crack, and as shown in FIG. 1, the distance from the meniscus position (A) of the continuous casting mold 1 is a position (B ) To a position (C) of about 13 m, and the segregation suppression forced cooling zone 5 is installed at least at a solid phase ratio fs of 0.3 at the axial center portion of the strand. From the position to the position of 0.5 is covered. That is, the range from the position (B) of the meniscus to the position (C) of about 13 m from the position (B) of 10 m is the continuous casting condition of the seamless steel pipe round slab, for example, the component / composition Even if the solidification state of the strand S fluctuates due to casting temperature, continuous casting speed, mold diameter, and cooling conditions (including primary cooling conditions and secondary cooling conditions), fs is 0.3 within the range. This corresponds to the range where the position and the position of 0.5 exist. In addition, fs means the mass ratio of [solid phase / (solid phase + liquid phase)] in the strand axial part, for example, 196 of Yasuo Ohnaka, Introduction to Computer Heat Transfer and Solidification Analysis, published by Maruzen in 1985. To 208, described in “4.3.2 Solidification analysis of alloy” and the like.

The function of the segregation suppression forced cooling zone 5 is to suppress the occurrence of B-type cracks, that is, V-shaped cracks that occur along the cross section in the casting direction of the strand shown in FIG. Therefore, in the present invention, the supply capacity (water density) of the cooling water to the outer peripheral surface of the strand S in the segregation suppression forced cooling zone 5 can be determined in association with the solid phase rate fs. Preferably, the cooling water supply capacity to the outer peripheral surface of the strand S in the segregation suppression forced cooling zone 5 is at least 10 from the position where the solid fraction fs is 0.3 to the position where 0.5. It is intended to allow a water density of ˜50 L / min / m ² to be provided.

  As shown in FIG. 3, in the segregation suppression forced cooling zone 5, the solidified shell 10 of the strand S grows and becomes thicker, and solidification of the unsolidified molten steel 11 proceeds inside. This unsolidified molten steel 11 is in a semi-solid state where the solid phase rate is high on the shell 10 side and the solid phase rate is low on the strand center side. When solidification proceeds and the range of the unsolidified molten steel 11 is narrowed within about 30 mm from the axial center of the strand, V-shaped segregation begins to form inside the strand. This V-shaped segregation is caused by a phenomenon in which the concentrated molten steel remaining in the center of the strand keeps a volume balance with the shrinkage of the surrounding shell as the solidification progresses, so that the concentrated molten steel is drawn downward. This is a cause of V-shaped internal defects (B-type cracks) in product round casts. The segregation suppression forced cooling zone according to the present invention is capable of forcibly cooling the strands in the region (section) where the V-shaped segregation is formed, thereby suppressing the generation thereof.

The V-shaped segregation part is an area where C, S, P, etc. are concentrated, and Cr is concentrated and is precipitated (crystallized) as hard Cr carbide. Become. According to the knowledge of the present inventor, suppression of the occurrence is possible by setting the water density Q ₃ to 10 L / min / m ² or more over the generation range of the V-shaped segregation, and particularly 15 L / min. It becomes more certain by setting it to / m ² or more. However, when the water density exceeds 50 L / min / m ² , the strands are bent, causing a problem that the continuous casting operation cannot be performed smoothly.

  It is desirable that the forced water cooling function in the segregation suppression forced cooling zone 5 be performed over a range where the solid phase ratio fs of the strand axial center portion is 0.3 to 0.5. Even if water cooling is strengthened from a location where fs is less than 0.3, the effect of reducing the B-type crack due to the occurrence of the V-shaped segregation portion cannot be obtained. On the other hand, if fs exceeds 0.5, the already generated V-shaped segregation cannot be eliminated, and the effect of reducing the B-type crack can no longer be obtained.

  In order to exhibit the said function, the segregation suppression forced cooling zone 5 is specifically comprised as shown in FIG. That is, a header 51 is provided so as to surround the strand S, and a spray nozzle 53 is attached to the header 51 via a flow rate control valve 52. Therefore, the flow rate of the cooling water sprayed from the spray nozzle 53 to the strand S can be controlled individually or by grouping two or several by the flow rate control valve 52. In addition, a necessary amount of cooling water over a range from the position where the solid phase ratio fs is 0.3 to the position where 0.5 is obtained, for example, with a uniform water amount density or, if necessary, the strand S It is also possible to increase the water density gradually toward the downstream side so that a smooth transition to the water density in the subsequent primary compression stress-added forced cooling zone can be achieved. As is well known, the strand S is guided downstream by a hook-shaped support roll having a cross section (not shown), but it is natural that the spray nozzle 53 does not interfere with the support roll.

  Subsequent to the segregation-suppressing forced cooling zone 5, a primary compression stress-added forced cooling zone 3 is provided. This primary compression stress-added forced cooling zone 3 is a zone in which the strand S is forcibly water-cooled in order to prevent the occurrence of A-type cracks that occur when the strand axial center solidifies, and its installation range is the range. The position where fs is 0.5 and the position where the temperature Tc at the axial center portion of the strand is (Ts-155 ° C.) are present. Specifically, as shown in FIG. 1, the distance from the meniscus of the continuous casting mold 1 from the outlet side of the segregation suppression forced cooling zone 5 ((B) of FIG. 1) reaches a position of about 15 m. It is good. As a result, the solidification state of the strand S fluctuates due to fluctuations in continuous casting conditions for seamless steel pipe round slabs, such as components / composition, casting temperature, continuous casting speed, mold diameter, and cooling conditions. However, the position where fs becomes 0.5 and the position where the temperature Tc in the axial center part of the strand becomes (Ts-155 ° C.) exist within the range. The temperature Tc at the axial center of the strand is, for example, “4.3.2 Solidification analysis of alloy” described in pages 196 to 208 of Yasuo Ohnaka, Introduction to Computer Heat Transfer and Solidification Analysis, published in 1985 by Maruzen. On the other hand, Ts can be calculated using commercially available phase diagram calculation software “Thermocalc” (Thermocalc software Inc.), for example.

The function of the primary compressive stress-added forced cooling zone 3 is to suppress the occurrence of A-type cracks, particularly primary A-type cracks that occur at the end of solidification of the strand axis. Therefore, the supply capacity to Q ₁ cooling water of the strand outer peripheral surface of the first primary compression stressing force cooling zone 3, it is necessary to be able to suppress the formation of primary A type cracking in a continuous casting process.

  As shown in FIG. 4, in the primary compression stress applied forced cooling zone 3, the solidified shell 10 of the strand S grows and becomes thicker, and solidification of the unsolidified molten steel 11 progresses in the inside. Further, when the center temperature Tc of the strand S reaches Ts, that is, the solidus temperature of the bulk molten steel subjected to continuous casting, the state of the solid phase ratio fs: 1, that is, the macroscopic state of the unsolidified molten steel 11 It is assumed that there is no remaining. The Tc and Ts can be calculated using, for example, commercially available phase diagram calculation software “Thermocalc” (Thermocalc software Inc.).

  In the primary A type crack, after the solidification of the strand axial part progresses to some extent in the continuous casting process, the tensile stress is applied to the axial part until the temperature of the axial central part decreases to a temperature at which hot ductility develops. It is presumed that it is caused by action. Therefore, the center temperature Tc of the strand S reaches the solidus temperature Ts of the bulk molten steel subjected to continuous casting, and the solid solid fraction fs: 1, that is, the unsolidified molten steel 11 remains macroscopically. Even if it is assumed that there is not, it is necessary to keep the shaft portion under compressive stress until the solidified product at the shaft portion reaches the hot ductility temperature. This region is a range from the location where the solid phase ratio fs is 0.5 to the state where the tensile stress generated in the shaft center member can withstand 8 MPa. In the upstream region where fs is less than 0.5, the fluidity of the molten steel in the shaft center is high, and the occurrence of primary A-type cracks is not observed. If the member becomes able to withstand the tensile stress of about 8 MPa, the primary A type crack will no longer occur. It has been confirmed that the axial temperature at which the tensile stress can withstand about 8 MPa is (Ts-145) ° C. in 13Cr steel, for example.

Note that the hot strength of the shaft center portion is measured by performing a low-temperature high-temperature hot tensile test of 1 × 10 ⁻³ / s in the solidification process. Taking the 13Cr steel as an example, the temperature at which a significant cross-sectional reduction rate is obtained in the solidification process is determined to be 1300 ° C., and the difference from Ts is 145 ° C. The application range of forced cooling is determined to be (Ts-145) ° C.

Based on the above knowledge, in the present invention, in order to suppress primary A-type cracking, the cooling water supply capacity of the primary compression stress applied forced cooling zone 3 is set to a water amount density of 10 to 100 L with respect to the outer surface of the strand S. / Min / m ² can be provided. This is because if the water density is less than 10 L / m ² / min, the temperature gradient between the strand surface and the shaft center is small and sufficient compressive stress cannot be applied to the strand shaft, while the water density is 100 L. When exceeding / m ² / min, in the case of uneven cooling in the circumferential direction, the tensile stress in the radial direction becomes large, and as a result, the tensile stress on the shaft center portion becomes too large, and the C type crack This is because warpage remains in the round product slab obtained by cutting the strand, and cooling at a higher water density is unnecessary.

  Specifically, the primary compressive stress-added forced cooling zone 3 is configured as shown in FIG. 4, and a header 31 is provided so as to surround the strand S. A spray nozzle 33 is attached to the header 31 via a flow control valve 32, and the flow rate of the cooling water sprayed from the spray nozzle 33 to the outer surface of the strand S corresponds to the region where the temporary A-type crack occurs. The flow rate control valve 32 can be controlled individually or in groups of several. By constructing in this way, from the exit side of the segregation suppression forced cooling zone 3 until the temperature Tc at the axial center portion of the strand becomes sufficiently high, the axial center of the 13Cr steel Cooling by the water density required for the primary compressive stress-added forced cooling zone 4 is possible until the part temperature Tc reaches (Ts-145 ° C.). In addition to performing the cooling at a uniform water amount density, if necessary, the water amount density is gradually reduced toward the downstream side of the strand S, and the water amount density of the subsequent secondary compression stress-added forced cooling zone is reduced. A smooth transition can also be possible.

As shown in FIG. 4, the point of the solid phase ratio fs: 0.5 in the strand S is in the segregation suppression forced cooling zone 5 described above. Therefore, in order to exert the forced water cooling function in the primary compression stress-added forced cooling zone 3 in the range where the axial center solid phase ratio fs of the strand S is 0.5 or more, the segregation suppression forced cooling zone 5 is It is desirable to enhance the water cooling function in the vicinity of the side and to enable cooling to a water density of 100 L / m ² / min.

  Subsequent to the primary compressive stress-added forced cooling zone 3, a secondary compressive stress-added forced cooling zone 4 is provided. The secondary compression stress-added forced cooling zone 4 is a zone in which the strand S is forcibly water-cooled to prevent the occurrence of A-type cracks particularly during recuperation, and the temperature Tc at the axial center of the strand within the installation range. Is included such that (Ts-255 ° C.). Specifically, as shown in FIG. 1, the distance from the meniscus of the continuous casting mold 1 to the position of 25 m from the outlet side ((D) of FIG. 1) of the primary compression stressed forced cooling zone 3 is reached. Take within range. As a result, even if the solidification state of the strand S fluctuates due to continuous casting conditions of the seamless steel pipe round slab, for example, composition / composition, casting temperature, continuous casting speed, mold diameter, cooling condition, etc. A position where the temperature Tc at the axial center portion of the strand becomes (Ts-255 ° C.) is included in the range.

The function of the secondary compressive stress-added forced cooling zone 4 is as follows: A-type crack, especially after recuperation that occurs when the strand S is left in an air-cooled state after the primary compressive stress-added forced cooling. It is in suppressing A type cracking. Therefore, the supply capacity of the cooling water in the secondary compression stressing forced cooling zone 4 must be assumed capable of providing a 0L / min / m ² Ultra 60L / min / m ² or less of water density with respect to the strand outer peripheral surface . In addition, for the reason described later, the secondary compressive stress-added forced cooling zone must be provided with a lower water density than the first compressive stress-added forced cooling zone.

  As shown in FIG. 5, the unsolidified molten steel 11 is hardly recognized in the installation section of the secondary compression stress application forced cooling zone 4. However, according to the knowledge of the present inventor, the axial center portion of the strand S has a sufficiently high tensile stress of the member, for example, a temperature exceeding 8 MPa, for 13Cr, (Ts-145) ° C. or lower. There are still shrinkage holes with a diameter of about several millimeters that are generated during the solidification of the concentrated molten steel, or shrinkage holes that are produced during the forced cooling with the first compression stress. It is presumed that the A-type crack at the time of recuperation is caused by an excessive tensile stress applied around the shrinkage hole.

  The installation section of the secondary compressive stress-added forced cooling zone 4 must cover the limit of occurrence of A-type cracks during recuperation. This occurrence limit is, for example, observed by EPMA for the state of Cr segregation in the axial center portion of the strand (round cast piece) obtained by continuous casting, and 95% of the Cr segregating portion in the axial center portion (number of observed pixels of EPMA). It can be determined by measuring the Cr concentration covering the ratio. Taking 13Cr steel as an example, the Cr concentration is 14.5 mass%, and the solidification temperature corresponding to the solidification temperature of this Cr concentration is 1190 ° C., that is, (Ts-255) ° C.

The water cooling function of the secondary compression stress-added forced cooling zone 4 must be capable of giving a water density of up to 60 L / min / m ² to the outer surface of the strand. The water density up to 60 L / min / m ² increases the compression stress-tensile stress transition point (distance from the molten steel meniscus in the continuous casting mold to the downstream side) by performing the secondary compression stress applied forced cooling. Although it gives an effect, if it exceeds that, the slab warpage occurring in the product round slab is greatly increased, so that the water cooling capacity exceeding this is not necessary.

Incidentally, water flow rate Q ₂ of the cooling water of the secondary compression stressing forced cooling zone 4 must be such that less than water cooling density to Q ₁ the primary compressive stressing forced cooling zone. If Q ₁ ≦ Q ₂ , the effect of lowering the compression stress-tensile stress transition point to the downstream side is recognized, but the temperature drop of the strand becomes excessive, so the straightening force in the straightening band (see FIG. 1) (Bending stress) becomes excessive. As a result, the straightening stand is damaged, or the slab warpage remains in the round product slab. Therefore, a control mechanism for supplying cooling water to the secondary compression stress-added forced cooling zone with cooling water density less than that of the first compressive stress-added forced cooling zone in relation to the first compressive stress-added forced cooling zone It is necessary to have.

  Specifically, the secondary compressive stress-added forced cooling zone 4 is configured as shown in FIG. 5 and can achieve the above functions. That is, a header 41 is provided so as to surround the strand S, and a spray nozzle 43 is attached to the header 41 via a flow rate control valve. Further, the flow rate of the cooling water sprayed from the spray nozzle 43 to the strand S can be controlled by the flow rate control valve 42 individually or by grouping several. By configuring in this way, 13Cr so that the temperature Tc at the axial center portion of the strand from the exit side of the above-mentioned primary compression stress applied forced cooling zone 3 is below the limit of occurrence of A-type cracks during recuperation, Cooling with a required water density is possible up to the position (Ts-255 ° C.).

  In addition, as shown in FIG. 5, the point of (Ts-145) ° C. in the strand S is in the above-described primary compression stress applied forced cooling zone 3. Therefore, in order to suppress the occurrence of A-type cracks at the time of recuperation, the secondary compressive stress-added forced cooling zone 4 that continues by gradually reducing the water cooling function in the vicinity of the outlet side in the primary compressive stress-added forced cooling zone 3. It is desirable to be able to smoothly shift to the cooling water volume density required in (1).

  Each forced cooling zone of the continuous casting facility for seamless steel pipe round slabs according to the present invention is configured as described above, and the operation thereof can suppress the occurrence of internal defects. In the operation, as shown in FIG. 6, the solidification state in the strand is estimated from the molten steel components and the continuous casting conditions, and based on the result, a water cooling pattern to be applied to each forced cooling zone is created. The water cooling valve is opened / closed and controlled so that it can be realized.

  Hereinafter, the operation of the segregation suppression forced cooling zone 5 will be described as an example. First, molten steel components, continuous casting conditions, and a basic water cooling pattern (water density distribution) in the segregation suppression forced cooling zone are input to the strand solidification state estimation calculation device. Thereby, the solid phase rate fs at each position in the longitudinal direction of the strand is calculated by heat transfer solidification calculation. The ladle analysis value is used as the molten steel component. For heat transfer solidification calculation, solidification of strands such as casting temperature, mold diameter (inner diameter), drawing speed (continuous casting speed), primary cooling (cooling water amount), secondary cooling condition (water density) as continuous casting conditions As the initial condition, the basic water cooling pattern (water density) of the segregation suppression forced cooling zone 5 is used as the initial condition.

  Based on the calculation result, the water density pattern in the longitudinal direction of the strand of the segregation suppression forced cooling zone 5 is determined as the operation water cooling pattern. Specifically, the range of the solid phase ratio fs: 0.3 to 0.5 is located in the segregation suppression forced cooling zone 5, and the desired V-shaped segregation can be suppressed within the above range. The opening degree of the flow rate control valve 52 of each spray nozzle 53 installed in the longitudinal direction of the strand of the segregation suppression forced cooling zone 5 is set so that a proper water amount density is given.

  The flow rate control valve 52 is controlled to be opened and closed by the opening degree set as described above, whereby it becomes possible to suppress the occurrence of V-shaped segregation in the segregation suppression forced cooling zone 5. The amount of water in each spray nozzle in the segregation suppression forced cooling zone 5 obtained as a result of this operation is fed back to the basic water cooling pattern setting device from the flow control valve or the flow meter associated therewith, and the correction is performed.

  The operation control in the primary compression stress-added forced cooling zone 3 and the secondary compressive stress-added forced cooling zone 4 can also be performed following the segregation-suppressed forced cooling zone 5. However, in the primary compression stress applied forced cooling zone 3, Ts (fs: 1) and the temperature of the shaft center portion are sufficiently lowered so that the shaft center member can withstand a tensile stress of about 8 MPa generated therein. The 13Cr steel is operated to give the required water density based on the point of Tc: Ts-145 ° C. In the secondary compression stressed forced cooling zone 4, the axial center Cr In the case of Cr concentration and 13Cr steel covering 95% of the segregated portion (EPMA observation pixel ratio), the secondary water is further added so that the necessary water density is given on the basis of Tc: Ts-255 ° C. The operation is performed so that the water amount density in the forced cooling zone 4 with compressive stress is smaller than that in the forced cooling zone 3 with the first compressive stress.

  In the operation control of each forced cooling zone, V-shaped segregation occurs between solid phase ratios fs: 0.3 to 0.5, and solid phase ratios 0.3 to Tc: Tc = (Ts-145) ° C. Then, it is necessary to provide cooling water necessary for suppressing the occurrence of A-type cracks during recuperation between primary A-type cracks and Tc: (Ts-145 ° C) to Tc: (Ts-255) ° C. . In order to ensure this, for example, in the segregation suppression forced cooling zone 5, the flow rate control valve 52 after the point of the solid phase ratio fs: 0.5 is controlled, and the amount of cooling water discharged from the nozzle 53 The density may be made to coincide with that given in the first compressive stress-added forced cooling zone 3. Similar measures can be taken in the primary compression stressed forced cooling zone 3.

  Further, the operation control in the segregation suppression forced cooling zone 5 is performed by gradually increasing the water density between the solid fraction fs: 0.3 to 0.5 and the primary compression at the solid fraction fs: 0.5. The water density required in the stress-added forced cooling zone 3 can be matched. A control system can be constructed so that such a gradual increase or decrease in the water density can be performed also in the first compressive stress-added forced cooling zone 3.

FIG. 7 is a schematic diagram showing the relationship between the temperature of the strand axial center, the solid phase rate, and the applied water density Q (Q ₁ , Q ₂ , Q ₃ ) in each zone in the strand when operating the continuous casting apparatus according to the present invention. This is schematically illustrated. As shown, first, the cooling due to the relatively small amount of water density Q ₃ is performed by the polarized析抑system cooling zone, the occurrence of B-type cracking is prevented, then the high water density Q in the primary compressive stressing cooling zone ₁ is cooled, and the occurrence of A-type cracks and C-type cracks at the end of solidification is prevented. Finally, the water density Q is less in the secondary compression stress-added forced cooling zone than in the first compression stress-added forced cooling zone. ₃ is performed, and generation of A-type cracks during recuperation is prevented.

1: Continuous casting mold
2: Secondary cooling zone
3: Primary compression stressed forced cooling zone
4: Secondary compression stress applied forced cooling zone
5: Segregation suppression forced cooling zone
6: Correction belt
10: Solidified shell
11: Unsolidified molten steel
31: Header
32: Flow control valve
33: Spray nozzle
41: Header
42: Flow control valve
43: Spray nozzle
51: Header
52: Flow control valve
53: Spray nozzle
S: Strand

Claims (6)

A round cast slab for seamless steel pipes is obtained by injecting molten steel into a mold and water-cooling a strand having a circular cross section containing unsolidified molten steel inside to obtain a round cast after cutting. Continuous casting equipment,
The continuous casting equipment, supplying cooling water at a water density of the continuous casting mold within a distance from the meniscus against the strand outer peripheral surface in a range leading to 25m from one location of 10m 1 0~50L / min / m ² polarized析抑system forced cooling zone may, 1 0~100L / min / ^m primary compression stress cooling water can be supplied at ² in water density additional forced cooling zone and, 0 L / min / ^{m 2} ultra 60L / min / a secondary compression stress-added forced cooling zone that is m ² or less and that can supply cooling water at a lower water density than the primary compressive stress-added forced cooling zone,
Each of the forced cooling zones includes the axial solid phase fraction fs in the longitudinal direction of the strand calculated from the composition and composition of the continuously cast steel and the continuous casting conditions, the axial temperature Tc, and the continuous cast steel. Based on the solidus temperature Ts of the bulk molten steel determined from the composition and components, the cooling water density density indicating means for each cooling zone for determining the cooling water volume density for each forced cooling zone,
A continuous casting facility for round slabs for seamless steel pipe pipes, comprising a spray water supply device for supplying cooling water to the outer surface of the strand in each forced cooling zone based on an instruction from the spray water amount instruction means for each cooling zone.   The continuous casting equipment for round slabs for seamless steel pipe production according to claim 1, wherein the molten steel is 13Cr steel. The segregation suppression forced cooling zone is capable of supplying cooling water at a rate of a water density of 10 to 50 L / min / m ² within a range where the axial center solid fraction fs is 0.3 or more and less than 0.5. The continuous casting equipment for round slabs for seamless steel pipe making according to claim 2. The primary compression stress-added forced cooling zone is capable of supplying cooling water at a rate of a water density of 10 to 100 L / min / m ² in a range where the axial center temperature Tc is (Ts-145) ° C. The continuous casting equipment for round cast pieces for seamless steel pipe making according to claim 2 or 3. The secondary compression stressing forced cooling zone, within the axial center temperature Tc of (Ts-255) ℃, the water density 0L / min / ^{m 2} Ultra 60L / min / ^{m 2} at a rate of less cooling water The continuous casting equipment for round slabs for seamless steel pipe production according to any one of claims 2 to 4, wherein the equipment can be supplied.   The segregation suppression forced cooling zone has a connection part that gradually increases or decreases the water density toward the primary compressive stress-added forced cooling zone, and the secondary compressive stress-added forced cooling zone has the secondary compressive stress-added forced cooling zone. 6. The continuous casting equipment for round slabs for seamless steel pipe production according to any one of claims 1 to 5, wherein a connection part for gradually decreasing the water density toward the cooling zone is provided.

Images