JP5343746B2 - Continuous casting method of round slabs for seamless steel pipes - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a continuous casting method for a round slab for a seamless steel pipe. <P>SOLUTION: A round slab 9 during continuous casting by a circular mold is, before the completion of its solidification, subjected to rolling reduction with a pair of rolling reduction rolls 6a, to obtain a slab. At this time, as a pair of the rolling reduction rolls, saddle type rolls in which the opening angle &delta; of a caliber bottom is 75 to 105&deg; and also having projections 13a at the parts in contact with the round slab are used. Regarding the projection, at least one projecting protrusion continuous to the roll circumferential direction or a plurality of projections at least in one line discretely distributed to the roll circumferential direction are preferably used. In this way, the generation of porosities of the axial center part and axial center cracks easy to occur in a Cr-containing steel or the like can be suppressed at a reduced rolling draft and also without damaging the cross-sectional shape of the round slab, and the method can contribute to the reduction of production cost, the improvement of the quality of a seamless steel pipe or the like. <P>COPYRIGHT: (C)2010,JPO&amp;INPIT

Description

  The present invention relates to a continuous casting method of seamless steel pipe round cast slabs, and more specifically, round casting of carbon steel and alloy steel having a high carbon concentration, which has been problematic as a material for seamless steel pipes in the internal quality as continuously cast. The present invention relates to a continuous casting method for improving the internal quality of a Cr-containing steel round cast slab, which has been inferior in hot workability in a piece or as-cast, and could not be used as a material for seamless steel pipes.

  As described in Non-Patent Document 1, seamless steel pipes are generally manufactured by round or square steel pieces produced by processing a cast steel ingot (ingot), or by continuous casting. Round or square slabs are used as the material for seamless steel pipes, and these steel slabs or slabs are hollow by using the Mannesmann drilling method, the press drilling method, or the hot extrusion method. And then stretched by a rolling mill such as an elongator, a plug mill or a mandrel mill, and is manufactured through a process of making the diameter constant by a sizer or a stretch reducer as a finishing process.

  As a material for this seamless steel pipe, in the case of a steel type that can produce a round cast slab with excellent internal quality and good hot workability by continuous casting, like general low carbon steel, An as-cast round slab is used. However, in the production by continuous casting, such as stainless steel containing Cr, porosity and segregation are likely to occur in the shaft core, and in the case of a steel type that is inferior in hot workability, an as-cast round slab is used. If used, flaws are generated on the inner surface of the raw pipe (seamless steel pipe). For this reason, square-shaped slabs are manufactured by continuous casting, etc., and then the slabs are processed into round steel pieces or square steel pieces of a predetermined size, which are used as raw materials for seamless steel pipes. It was.

  The main cause of the poor hot workability of Cr-containing steels such as stainless steel is due to the increased Cr content added to improve corrosion resistance, causing segregation and porosity in the core part of the slab during continuous casting. This is because it tends to occur and becomes a round slab with inferior quality. Porosity, which has a particularly large adverse effect on hot workability, is caused by the fact that molten steel is difficult to be supplied due to the high viscosity of molten steel in the voids generated in the final solidified portion of the round cast slab.

  FIG. 7 shows the relationship between the Cr concentration in the molten steel and the viscosity of the molten steel. FIG. 7 shows that the viscosity of the molten steel increases as the Cr concentration in the molten steel increases, and that the viscosity of the molten steel has a peak at a Cr concentration of about 13% by mass. FIG. 8 shows the relationship between the Cr concentration in the molten steel and the viscosity of the molten steel in a region where the Cr concentration is low. FIG. 8 shows that when the Cr concentration exceeds 0.5% by mass, the viscosity of the molten steel increases significantly.

  When the Mannesmann drilling method, which is a severe processing method, is performed on the round slab having the internal defects in this way, soot is generated on the inner surface of the obtained raw tube due to porosity and segregation. For this reason, the steel grades that are called difficult-to-work materials are of course used, and round steel slabs manufactured through the rolling process are used as the material for seamless steel pipes for steel grades with a high carbon content and those with addition of Cr. It has been essential. For example, as described in Non-Patent Document 2, in the case of a steel type that is concerned about the occurrence of inner surface flaws in the raw pipe when using a round cast piece as a raw material, such as high Cr steel, Manufacturing steel ingots or continuous cast slabs with large cross-sections, heating them, and then rolling them into pieces to mechanically press the porosity to obtain round steel slabs with excellent internal quality as raw material for seamless steel pipes It was. The “steel slab” here is obtained through a rolling process such as ingot rolling, and the “slab slab” is continuously cast.

In recent years, in addition to porosity, it is said that radial cracks (hereinafter referred to as “axial core cracks”) generated around the porosity become an obstacle to pipe production. Several causes for the occurrence of this axial core crack have been proposed, but the factor having the greatest influence is said to be the thermal stress at the axial core portion that occurs when the slab is cooled.
In this way, if there is a concern about the occurrence of flaws when pipe production is performed with an as-cast material, the cast material is subjected to ingot rolling to mechanically pressure-bond the porosity, and there is porosity in the slab. However, the effect was not generated during pipe making.

  However, if the continuous cast slab is subjected to partial rolling, the end face of the steel slab after rolling becomes an uneven shape, and if it is drilled as it is as a raw material for seamless steel pipes, the convex part is involved and becomes the inner surface flaw of the raw pipe . Therefore, in order to use the rolled steel slab as a material for a seamless steel pipe, a cutting process for adjusting the shape of the end face of the steel slab is essential. That is, there is a problem that cropping is inevitably generated by cutting the end portion, and the product yield is lowered. In addition, of course, there is a problem that reheating for performing the ingot rolling also increases the product cost.

Therefore, a technique for improving the quality of the round slab has been proposed so that the round slab can be used as a raw material for a seamless steel pipe without going through the block rolling process.
For example, Patent Document 1 discloses that a slab during continuous casting has a solid-state ratio fs at the center of the slab at a position where the solid phase ratio fs is 0. A technique is disclosed in which casting is performed while applying a large reduction by continuous forging so that d is 0.5 times or more, that is, δ / d ≧ 0.5. This technique provides a reduction force during continuous casting, does not require heating for reduction, and is an excellent technique for reduction of porosity, but has a problem of high equipment costs. Moreover, since the burden of an installation cost will be applied also to slab which does not require reduction, such as general carbon steel, it is not realistic.

  Further, Patent Document 2 discloses a technique for continuously casting a round slab while stirring a molten steel by a magnetic stirrer arranged immediately below the mold and the mold in order to improve the quality of the slab during continuous casting. Yes. In this technology, solidified nuclei are generated in an unsolidified layer by electromagnetically stirring molten steel in the mold and directly under the mold, and the axial core portion of the slab is filled with equiaxed crystals by this solidified nuclei. This is a technique for suppressing the porosity and segregation of the shaft core. However, although this technique is widely implemented, the effect is not so great as to prevent the occurrence of porosity.

  Further, as another means for improving the quality of the continuous cast slab, for example, as shown in Patent Document 3, while applying a reduction corresponding to the solidification shrinkage to the slab at the end of solidification during continuous casting Casting technology is implemented. This technology reduces the porosity of the slab at the end of solidification with a roll by the amount of solidification shrinkage and reduces the flow of concentrated molten steel to prevent center segregation. It is well known as an internal quality improvement method and is called “light reduction technology”. In this technique, only reduction is applied during casting, equipment costs are small, reheating for reduction is unnecessary, and the manufacturing cost can be reduced.

As an example of this light reduction technique, Non-Patent Document 3 discloses an example in which the light reduction technique is applied to a round bloom slab of SUS304, which is a high Cr steel. According to the density measurement result of the core part of the slab shaft described in Non-Patent Document 3, the density when no porosity is generated is 7.8 g / cm ³ , whereas the slab shaft when reduction is applied The density of the core is 7.7 g / cm ³ , and a slight porosity remains in the shaft core from a photograph of the solidified structure, and the porosity is not completely crushed. However, the improvement effect is great compared to the case where light reduction is not performed.

The biggest problem when this light rolling technology is applied to continuous casting of round slabs is that the shape of the slab is deteriorated or flattened due to rolling by rolls, and the possibility of occurrence increases as the rolling amount increases. It is a crack near the solidification interface.
That is, when rolling is applied to a round slab by a flat roll having a rectangular cross-sectional shape perpendicular to the slab transfer direction, such as used for rolling a plate-shaped slab, The contacted portion is flattened, while the portion not in contact with the roll swells, the cross-sectional shape of the round cast slab is flattened, and further approaches a square shape. When such a slab is drilled into a seamless steel pipe, uneven thickness often occurs. Moreover, due to such a reduction, a tensile stress is generated in the direction perpendicular to the reduction direction in the cross section of the slab, so that cracking is likely to occur. In addition, if the amount of reduction is increased in order to enhance the pressure bonding effect of porosity, the cross-sectional shape is further away from the perfect circle, and as a result, the uneven thickness of the seamless steel pipe is increased and there is a high possibility that it will deviate from the desired standard. The occurrence rate of cracks increases, and furthermore, when used as a material for seamless steel pipes, it becomes impossible to convey by rolling round cast pieces, and the biting becomes unstable during drilling, etc. A serious problem occurs.

In order to solve the problem of flattening, Patent Document 4 describes casting an slab having an elliptical cross section with an elliptical mold and rolling it down in a major axis direction with a round hole type roll. A technique for obtaining a cross-section slab is disclosed.
Although the method described in Patent Document 4 solves the problem of the slab shape after the reduction, judging from the described examples, it exceeds 10% with respect to the desired slab diameter. Even if it is applied, a porosity of 10 mm or more in diameter remains and the effect of reducing the porosity at the slab stage is recognized, but the effect of suppressing flaws in the pipe making process is doubtful, and the effect is small. Also, in order to increase the amount of reduction, it is necessary to increase the difference between the major axis and the minor axis of the elliptical mold. In that case, the mold flow during casting is a mold with a round cross section (circular mold). Compared to the case of using, non-uniformity is caused, and fluctuations in the molten metal surface and entrainment of mold powder caused by this cause new defects. In addition, it is necessary to prepare a large number of molds corresponding to the required amount of reduction, and in the case of a steel type that does not have a problem in internal quality, reduction is also required, resulting in an increase in cost.

  In order to solve these problems, the present inventors, for example, as shown in Patent Document 5, are cast by a circular mold using a vertical roll having a caliber bottom opening angle δ of 70 ° to 115 °. We have proposed a technique for casting while rolling round slabs during continuous casting, and confirmed that a certain effect can be obtained.

Japanese Unexamined Patent Publication No. 63-183765 Japanese Unexamined Patent Publication No. 1-180762 JP 49-121738 A JP 7-108358 A Japanese Patent Laid-Open No. 10-34304

3rd edition Steel Handbook III (2) (1980), p.952, p.971 3rd Edition Steel Handbook III (2) (1980), p.107-170 Materials and processes, vo1.7 (1994), No.1, p.195

  In order to obtain a desired effect by the technique described in Patent Document 5, it is necessary to increase a parameter called volume reduction rate, that is, although there is a certain degree of correlation between casting speeds, It has been found necessary to increase the amount of reduction. In this case, the larger the slab diameter, the greater the reduction load. Therefore, it is necessary to select whether to introduce a large facility or abandon the suppression of the porosity of the shaft core part. It was recognized that even the technique described in Patent Document 5 does not lead to a complete solution.

In addition, in the conventional technique described above, the porosity generation cannot be completely suppressed in continuous casting of steel round cast slabs, particularly Cr-containing steel round cast slabs. When light reduction was applied to the slab, the cross-sectional shape of the slab was flattened, resulting in problems.
In other words, in the method of manufacturing round slabs for seamless steel pipes containing a large amount of alloy components, it is almost impossible to improve the internal quality without performing light reduction. Loss in the pipe making process caused by moving away from the pipe is large, and therefore it is not possible to perform a light reduction with a desired reduction amount, and as a result, the inner casting has a good quality and is suitable for pipe making. There was a problem that it was difficult to obtain a piece.

  The present invention has been made in view of the above-described problems of the prior art, and round cast slabs as cast can be obtained even for steels with poor hot workability, particularly steels with many alloying elements such as Cr-containing steels. A round cast slab for seamless steel pipe that can be used as a raw material for seamless steel pipe as it is, and can be manufactured stably and economically without deterioration of the cross-sectional shape, with excellent internal quality. An object is to provide a continuous casting method.

  In order to achieve the above-mentioned object, the present inventors diligently studied various factors that affect the internal quality of the round slab during continuous casting. As a result, in the vicinity of the solidification completion position of the round slab, there is a predetermined caliber bottom opening angle δ, and a continuous ridge in the circumferential direction at a portion in contact with the round slab, or discrete in the circumferential direction. It was concluded that it is effective to arrange a pair of vertical rolls with protrusions distributed in the area and to reduce the round slab. As a result, it has been found that the internal quality of the round slab is greatly improved.

The present invention has been completed based on such findings and further studies. That is, the gist of the present invention is as follows.
(1) Before the solidification of the round slab, the round slab during continuous casting with a circular mold is subjected to reduction by a pair of reduction rolls, and then the round slab is cut to obtain a round slab for seamless steel pipes. When the caliber bottom opening angle δ is not less than 75 ° and not more than 105 °, and the portion facing the round cast piece has a protrusion that contacts the round cast piece. A continuous casting method for round slabs for seamless steel pipes, characterized by using a mold roll.

(2) The continuous casting method for a round cast piece for a seamless steel pipe according to (1), wherein the protrusion is a protrusion that is continuous in at least one roll circumferential direction.
(3) A continuous casting method of a round cast piece for a seamless steel pipe according to (1), wherein the protrusions are a plurality of protrusions distributed discretely in a roll circumferential direction.
(4) In any one of (1) to (3), the protrusion has an arc shape in a cross section in the axial direction of the saddle roll, and a radius R of the arc is
R = 0.20D ~ 0.50D
(Where R: arc radius of the ridge cross section (mm), D: round slab diameter (mm))
A continuous casting method of round slabs for seamless steel pipes characterized by satisfying

(5) In (3), in the plurality of protrusions, the ratio of the bottom surface length B in the roll circumferential direction to the bottom surface length A in the roll axis direction, A / B, is 0.2 to 1. A continuous casting method for round cast slabs for seamless steel pipes.
(6) In (5), the interval between the end portions of the plurality of protrusions adjacent to each other among the plurality of protrusions is a projection length in the circumferential direction of the saddle roll, and includes zero. A continuous casting method of a round cast piece for a seamless steel pipe, characterized by being provided so as to be less than a contact length between the vertical roll and the round cast piece.

(7) In (5) or (6), the plurality of protrusions are such that the bottom surface length B in the circumferential direction of the plurality of protrusions is the contact between the saddle roll and the round cast piece. A continuous casting method for a round slab for a seamless steel pipe, characterized in that the protrusion is a half or more of the length.
(8) In any one of (1) to (7), the reduction is performed when the solid phase ratio fs at the shaft core portion of the round cast slab is 0.3 to 0.85. = {1- (Cross sectional area of round slab after reduction) / (Cross sectional area of round slab before reduction)} × 100
A continuous casting method of round slabs for seamless steel pipes, characterized in that the area reduction rate defined by is in the range of 1 to 5%.

  (9) In any one of (1) to (8), the round cast slab is made of Cr-containing steel containing 0.5% by mass or more of Cr. Casting method.

  According to the present invention, since the round slab is squeezed through the protrusions installed at the positions in contact with the round slab of the vertical roll having a predetermined caliber bottom opening angle, it is effective for the shaft core part of the round slab. It is possible to produce round slabs with reduced rolling force and with reduced reduction amount and without damaging the cross-sectional shape of round slabs, which can suppress the occurrence of shaft core porosity and shaft core cracks that are likely to occur in Cr-containing steel. As a material for seamless steel pipes, it can contribute to reducing manufacturing costs and improving the quality of seamless steel pipes, and has a remarkable industrial effect. In addition, according to the present invention, it is possible to suppress the occurrence of porosity and shaft core cracks in the shaft portion of the round cast slab even with a small reduction amount, and there is also an effect that expensive equipment investment is not required.

  In addition, according to the present invention, in a round cast piece for seamless steel pipes such as carbon steel, which is continuously cast, a significant improvement in internal quality can be obtained at a low cost, thereby improving product yield and producing efficiency. There is also a great effect of improving.

It is a figure which shows one example of embodiment of this invention, and is the schematic which shows the condition which manufactures the round cast piece for seamless steel pipes by continuous casting. It is explanatory drawing which shows typically the state which is rolling down the round slab during casting using the vertical roll with a protrusion. It is explanatory drawing which shows typically the state which is rolling down the round cast piece under casting using the vertical roll with a protrusion with the cross section containing a roll axial center. It is explanatory drawing which shows typically an example of the saddle type roll which has a 2 row | line | column protrusion. It is explanatory drawing which shows typically an example of the cross-sectional shape of a protrusion. It is explanatory drawing which shows typically the state which is rolling down the round slab during casting using the vertical roll with a some protrusion. It is the figure which showed an example of the relationship between Cr density | concentration in molten steel, and the viscosity of molten steel. It is the figure which showed an example of the relationship between Cr density | concentration (low concentration side) in molten steel, and the viscosity of molten steel. It is explanatory drawing which shows typically the state which is rolling down the round slab during continuous casting using a flat type roll as a reduction roll.

  The present invention is a method for producing a round cast slab for a seamless steel pipe. In the present invention, a continuous casting machine 1 is used, and a molten steel 8 accommodated in a tundish 2 is passed through an immersion nozzle 3 through an internal space cross section. Is poured into a continuous casting mold (circular mold) 4 that is a perfect circle (circular) and continuously casts a round slab 9, a pair of reductions that are provided with a hydraulic cylinder 7 and can apply a pressing force to the round slab The rolls 6 and 6 are arranged at appropriate positions upstream of the solidification completion position of the round slab, and the round slab 9 is pressed by the pair of reduction rolls 6 and 6 before the solidification of the round slab is completed. . And a round cast piece is cut | disconnected by the cast piece cutting machine (not shown) installed in the appropriate position downstream from the solidification completion position of a round cast piece, and the raw material for seamless steel pipes is obtained. This situation is shown in FIG.

  A large number of slab support rolls 5 for supporting the round slabs being cast are arranged on the downstream side of the circular mold 4. Further, a spray nozzle (not shown) for forcibly cooling the round slab 9 being cast is disposed in a range where the slab support roll 5 is disposed, thereby constituting a secondary cooling zone. In FIG. 1, the rolling rolls 6 and 6 are arranged at the same position as the position where the slab support roll 5 is to be installed, but may be arranged at a position different from the position where the slab support roll 5 is to be installed. Good. 1 shows an example in which the rolling rolls 6 and 6 are installed at positions where the round slab 9 is transferred in the horizontal direction, it is needless to say that the present invention is not limited to this. It is also possible to install the rolling rolls 6 and 6 at a position where the round cast slab 9 is transferred vertically or obliquely.

Further, the production method of the present invention will be described in more detail by taking as an example the case of producing round cast pieces for seamless steel pipes using the continuous casting machine 1 shown in FIG.
The molten steel 8 injected into the circular mold 4 from the tundish 2 via the immersion nozzle 3 is cooled by contacting the inner wall of the circular mold 4 to form a circular solidified shell 10 at the contact portion with the circular mold 4. . Then, the round slab 9 having the solidified shell 10 as an outer shell and having an unsolidified layer 11 therein is pulled out from the circular mold 4 by the pinch roll of the slab support roll 5, and the slab support roll 5 It is cooled in the secondary cooling zone while being supported by. Due to the cooling in the secondary cooling zone, the thickness of the solidified shell 10 increases, and eventually solidification to the shaft core is completed. The round slab 9 that has been solidified up to the shaft core is cut into a predetermined length by a slab cutting machine (not shown) to form a seamless steel pipe round slab. Although not shown in FIG. 1, in order to improve the quality of the shaft core part of the round cast piece 9, an electromagnetic stirring device may be arranged in the secondary cooling zone immediately below the circular mold 4 or the circular mold. . With this electromagnetic stirrer, the unsolidified layer 11 is forcibly stirred to form equiaxed crystals, and the porosity and segregation of the shaft core portion are improved.

In the present invention, rolling is applied to the round slab 9 during casting using a pair of rolling rolls 6 and 6 at an appropriate position before solidification is completed. As a result, the porosity of the shaft core portion and radial shaft core cracking are improved, and the internal quality of the round cast piece 9 is improved.
In the present invention, a pair of the rolling rolls 6 and 6 to be used has a caliber bottom opening angle δ of 75 ° or more and 105 ° or less, and a protrusion 13 that contacts the round cast piece at a portion facing the round cast piece. A pair of saddle type rolls 6a and 6a having

  As the protrusions 13 provided in the portion that contacts the round cast piece of the vertical roll, at least one protrusion 13a that is continuous in the roll circumferential direction, or a plurality of protrusions 13b that are discretely distributed in the roll circumferential direction. It is preferable that 2 shows an example having at least one protrusion 13a continuous in the roll circumferential direction, and FIG. 6 shows an example having a plurality of protrusions 13b distributed discretely in the roll circumferential direction.

  By using a pair of vertical rolls 6a and 6a as the pair of rolling rolls 6 and 6, there are four contact points between the rolling rolls 6a and 6a and the round cast piece 9, and the rolls 6a and 6a are round cast pieces. In addition to restraining 9, the round cast piece 9 can be rolled down from four different directions as shown in the cross-sectional view of FIG. 3. As a result, the flatness ε of the round slab 9 after the reduction can be reduced as compared with the reduction using the flat roll. The “flattening ratio ε” here is “ε (%) = 1 one (the shortest diameter portion length in a certain cross section of a round cast piece) / (the longest diameter portion length in the same cross section) / × 100. "Means the value defined by.

  The vertical roll 6a used as the rolling roll has a caliber bottom opening angle δ of 75 ° or more and 105 ° or less. As a result, the stress / strain generated by the reduction in the shaft core portion of the round slab can be made the stress / strain in the compression direction, and the porosity can be reduced. If the opening angle δ of the caliber bottom deviates from this range, a tensile stress acts in the width direction (direction perpendicular to the rolling direction) of the round slab at the time of rolling, which causes a problem. In order to reduce the porosity, it is considered effective to add hydrostatic pressure to the location, but it is difficult to simply apply to the shaft core portion of the round cast slab. In addition, if the reduction to the round slab is performed using a flat roll, it is possible to obtain a large compression effect in the reduction direction, but the slab width direction perpendicular to the reduction direction (the slab reduction direction) The stress in the direction perpendicular to the direction becomes tensile stress, and the effect of reducing the porosity is reduced.

  The vertical roll 6a used in the present invention has a caliber bottom opening angle δ as described above, and further has a protrusion 13 in contact with the round cast piece on the surface facing the round cast piece 9. . For example, as shown in FIG. 3, the protrusion 13 is brought into contact with the round slab 9 to apply the rolling force of the vertical roll 6 a to the round slab 9. In addition, the arrangement | positioning position of the processus | protrusion 13 should just be a position which can contact with the round slab 9, and it is not necessary to specifically limit, but from a viewpoint of raising the rolling efficiency to the axial center part of the round slab 9 further. The cross-section perpendicular to the axial direction (conveying direction) of the round slab 9 including the roll shafts of the vertical rolls 6a and 6a is connected to the center O of the round cast slab 9 and the center in the width direction of the vertical rolls 6a and 6a. The vertical roll surface has an angle of approximately 45 ° with respect to the straight line and is positioned near the intersection of the two straight lines passing through the center O of the round slab 9 and the vertical roll surface. It is preferable to arrange.

  In addition, it is preferable that the protrusion 13 to arrange | position is the protrusion 13a which continues in the roll circumferential direction as shown in FIG. By making the protrusion 13 into a protrusion 13a continuous in the roll circumferential direction, a rolling force can be uniformly applied over the entire length of the round cast piece 9, which is advantageous for improving the internal quality. The protrusions 13a may be one on one side of the roll width or two (a plurality) as shown in FIG. Moreover, it is good also as several protrusion 13b discretely distributed in the roll circumferential direction as shown to Fig.6 (a) instead of the protrusion continuous in a roll circumferential direction. The plurality of protrusions 13b distributed discretely in the roll circumferential direction may be formed in one row on one side of the roll width or in multiple rows on one side in the roll axis direction. When a plurality of protrusions 13b distributed discretely in the roll circumferential direction are formed in a plurality of rows in the roll axis direction, the protrusions 13b may be arranged in a staggered manner as shown in FIG. 6C, for example. This is preferable from the viewpoint of uniformly applying a rolling force in the longitudinal direction of the slab. Also, if the number of protrusions 13 (installation strips, installation rows) is increased, there will be no significant difference from the reduction by a normal vertical roll without the projections. It is preferable to make it below the row.

  In the case where a plurality of lines or a plurality of rows of protrusions are provided on one side of the roll width, the cross section perpendicular to the axial direction (conveying direction) of the round cast piece 9 includes the roll shafts of the saddle type rolls 6a and 6a. An angle of about 45 ° is formed with respect to a straight line connecting the center O of the round slab 9 and the center in the width direction of the saddle type rolls 6a, 6a, and 2 perpendicular to the center O of the round slab 9 It is preferable to arrange the protrusions or protrusions on the surface of the saddle roll so that they are positioned near the intersection of the two straight lines and the saddle roll surface.

  In addition, the cross-sectional shape of the protrusion 13 in the axial cross section of the saddle type roll 6a is preferably a shape whose width is narrower in the height direction than at least the bottom. However, in the case of a protrusion having an acute cross-sectional shape in which the change in the width with respect to the height direction of the protrusion is large, the round cast piece is sharply recessed at the contact portion with the protrusion, and the portion is shaped during a round shape or There is a risk that it may become the starting point for external defects during pipe making. Also, in the case of a plurality of protrusions 13b that are discretely distributed in the roll circumferential direction, particularly in a shape in which the cross-sectional shape of the protrusion is larger in the height direction than the bottom portion, when the round cast slab is reduced by the reduction roll, Protrusions may bite into the slab and hinder operation. From the viewpoint of effectively transmitting such a problem and the rolling force by the roll to the shaft core portion of the round cast slab, in the present invention, the sectional shape of the protrusion in the axial section of the saddle type roll 6a is as shown in FIG. It is preferable to use an arc shape as shown in FIG. If the cross-sectional shape of the protrusion is an arc, the processing is relatively simple, and there is an advantage that there is little problem that the dent caused by the rolling becomes the starting point of the outer surface flaw. There is no problem even if the cross-sectional shape of the protrusion is rectangular or trapezoidal. Moreover, when installing multiple protrusions and multiple rows of protrusions, it is not necessary that they all have the same shape.

When the cross-sectional shape of the protrusion is an arc shape, the radius R of the arc is related to the diameter D of the round cast slab, and R = 0.20D to 0.50D
(Where R: arc radius of the ridge cross section (mm), D: round slab diameter (mm))
It is preferable to make the range satisfy. When the radius R of the arc is less than 0.20 of the diameter D of the round slab, the projection has a sharp cross-sectional shape, and the dent of the round slab due to the projection may become a defect after pipe making. On the other hand, when the radius R of the circular arc exceeds 0.50 of the diameter D of the round slab, it becomes difficult to install in the limited area of the saddle type roll 6a, and the effect of installing the projection becomes too small. It makes no sense to install protrusions. For this reason, the radius R of the arc when the cross-sectional shape of the cross section in the roll axis direction of the protrusion 13 is an arc is in the range of 0.20D to 0.50D in relation to the diameter D of the round cast slab. It is preferable to limit. The center point of the arc may be on the roll surface or an area below the roll surface.

Further, when the protrusion 13 is a plurality of protrusions 13b distributed discretely in the roll circumferential direction, the ratio between the roll circumferential direction length B and the roll axial direction length A of the bottom surface of the protrusion, A / B is preferably 0.2 to 1. As shown in FIG. 6 (b), the roll circumferential length B is a cross section perpendicular to the roll axial direction and including the center of the projection 13b in the roll axial direction, and refers to the length along the roll surface. To do. Moreover, as shown to the AA arrow figure of FIG.6 (b), the roll axial direction length A of the bottom face of a processus | protrusion shall say the length along a roll surface in a roll axial direction cross section.
When A / B is less than 0.2, the range of the round slab is narrow, and it becomes difficult to effectively transmit the rolling force of the vertical roll to the shaft core portion of the round slab. Moreover, a dent tends to occur on the surface of the slab due to the protrusion. When A / B exceeds 1 and the width of the protrusion becomes too large, it becomes difficult to effectively transmit the rolling force of the saddle type roll to the shaft core portion of the round cast slab. For this reason, the ratio between the roll circumferential direction length B and the roll axial direction length A, A / B, is 0.2 or more and 1 or less, in the plurality of projections 13b distributed discretely in the roll circumferential direction. It is preferable that

  When the protrusion 13 is a plurality of protrusions 13b distributed discretely in the roll circumferential direction, the roll circumferential length B of the bottom surface of the protrusion is the circumferential direction of the saddle-shaped roll 6a. It is preferable that the contact length between the vertical roll 6a and the round cast piece 9 is equal to or less than ½ of the contact length. If the bottom circumferential length B of the roll is less than 1/2 of the contact length, the compressive stress field is maintained in the contact length range of the vertical roll and the slab, and the effect of discharging the fluid molten steel is effective. It becomes difficult to produce.

  In this case, the distance L between the protrusion ends, which is the distance between the ends of the adjacent protrusions, is at least the distance in the circumferential direction of the roll and less than the contact length between the vertical roll 6a and the round cast piece 9 (zero). A plurality of protrusions 13b is preferably provided. If the distance L between the projecting ends exceeds the contact length, there may be a case where the compressive stress field cannot be uniformly applied over the length direction of the round cast slab. When providing a plurality of projections 13b, it is preferable to provide a plurality of rows of projections 13b in a staggered manner as shown in FIG. In this case, the distance L between the end portions of the adjacent protrusions 13b is less than the contact length, preferably zero, at the distance on the projection surface on the roll circumferential cross section, that is, in FIG. As shown, it is preferably arranged so as to overlap in a staggered manner. Thereby, a compressive-stress field can be uniformly provided to an axial center part over the length direction of a round cast piece.

In the present invention, a vertical roll with projections is used as the reduction roll, and the reduction is preferably performed at a time when the solid phase ratio fs at the shaft core portion of the round cast slab is 0.3 to 0.85. . The “solid phase ratio fs at the shaft core portion of the round slab” here means that the temperature T of the shaft core portion of the round slab obtained by solidification / heat transfer calculation in the cross section is determined by the steel type. an index indicating how in any position between the liquidus temperature T _L and solidus temperature T _S, which shall be calculated using the following equation.

_{fs = (T L -T) /} (T L -T S)
The completely solidified state is fs: 1.0, and the unsolidified state is fs: 0.
When the solid phase ratio fs of the slab at the reduction position is less than 0.3, solidification does not proceed so much and porosity is further generated in the subsequent solidification process, so that the reduction effect on the shaft core is insufficient. On the other hand, when the solid phase ratio fs exceeds 0.85, the fluidity of the molten metal remaining as an unsolidified layer is lowered, the temperature of the slab is also lowered, and the reduction effect on the shaft core portion of the round slab is lowered. For this reason, it is preferable to perform the reduction by the reduction roll at a time when fs is 0.3 to 0.85. In addition, even if the rolling is applied to the round cast piece after the completion of solidification, although there is a slight reduction effect on the shaft core portion, there is a possibility that the shaft core portion of the cast piece may be cracked.

Furthermore, at the above-described position, when the reduction is performed using the pair of saddle type rolls 6a and 6a, it is preferable to reduce the reduction amount so that the area reduction rate is in the range of 1 to 5%. In addition, "area reduction rate" is the following formula
Area reduction rate (%) = {1− (cross-sectional area of round cast slab after reduction) / (cross-sectional area of round cast slab before reduction)} × 100
The value defined in is used.

  If the area reduction rate is less than 1%, a desired reduction effect on the round cast slab core cannot be expected. On the other hand, if the area reduction rate is increased beyond 5%, a rounded slab may need to be rounded thereafter, which complicates the process. For this reason, the reduction amount (area reduction rate) is preferably in the range of 1% to 5%. When a round slab is squeezed using a pair of vertical rolls 6a, 6a, the shaft core portion is increased without increasing the squeezing amount (area reduction rate) compared to the case where the round slab is squeezed with a flat roll. An effective reduction to can be applied. For this reason, when the round cast slab using the pair of vertical rolls 6a and 6a is pressed, the porosity can be sufficiently crimped if the area reduction rate is about 1% or more and 5% or less.

The present invention is effective for improving the quality of the round cast slabs 9 regardless of the steel type, particularly when applied to a steel type in which the molten steel has a high viscosity and is susceptible to porosity and segregation during casting. The effect is remarkable. Examples of such steel types include Cr-containing steels containing Cr exceeding 0.5% by mass.
As described above, according to the present invention, the production of round slabs for seamless steel pipes by continuous casting of difficult-to-process steel types such as Cr-containing steel, which has been difficult in the past, greatly changes the continuous casting equipment. It is realized without. Then, since it becomes possible to manufacture a round cast slab having greatly improved internal quality without impairing the cross-sectional shape of the round cast slab 9, the production cost of seamless steel pipes such as Cr-containing steel can be reduced.

In order to verify the effects of the present invention, the effects of various types of vertical rolls obtained by finite element analysis and experiments were investigated by casting round slabs in the billet continuous casting machine shown in FIG.
(Example 1)
[Test casting 1]
Using the billet continuous casting machine shown in FIG. 1, a pair of saddle-shaped rolls 6a and 6a having various shapes and different caliber bottom angles δ shown in Table 1 are used as the rolling rolls 6 and 6, as shown in Table 1. At the position of the solid phase ratio (0.4 to 0.6), the round slab 9 was subjected to reduction by 2% in terms of area reduction rate to produce a round slab. The value calculated using the slab temperature calculated by solidification / heat transfer calculation was used for the solid phase ratio at the reduction position.

The target round slab was a Cr-containing steel round slab containing 13% by mass of Cr, and the diameter D before reduction was 210 mm. The vertical roll 6a used is a case in which the vertical roll 13a, which is one continuous protrusion 13a in the circumferential direction of the roll, is used at the position shown in FIG. The case where the saddle type roll 6a which is not used was used, or the case where a pair of flat type rolls 6c and 6c as shown in FIG. In addition, the cross-sectional shape in the roll-axis direction cross section of the installed protrusion 13a was made into the circular arc shape of radius R: 50mm (R / D: 0.24).
About the obtained round cast piece, the area of the porosity and the shaft core crack were measured, and the properties of the shaft core portion were evaluated. The evaluation method was as follows.

From the obtained round cast slab, a test material for observing the shaft core part is collected, the cross section of the test material is polished, and the macrostructure of the cross section and the vicinity of the shaft core part using an optical microscope (magnification: 100 times) The microstructure of was measured, and the area of porosity in the cross section was measured by image analysis. The area ratio of porosity was calculated by the following formula.
Porosity area ratio (%) = (Porosity area) / (Round slab cross-sectional area) x 100
The obtained porosity area ratio (%) was used as an index, and the evaluation was made in 5 grades of 1-5. If the porosity area ratio exceeds 0.3%, the rating is 1, the porosity area ratio is 0.3% or less, and the rating is 2; the porosity area ratio is 0.15% or less, the rating is 0.1%; the rating is 3, and the porosity area ratio is 0.1% or less, and the porosity is more than 0.025%. Score 4 and a porosity area ratio of 0.025% or less were rated 5. The higher the score, the lower the porosity remaining.

Also, from the microstructure of the shaft core part, whether or not cracks occurred in the shaft core part, and if cracks are occurring, measure the length, find the total length of each crack, the test material The degree of shaft core cracking was evaluated as the shaft core crack length.
The total length of the obtained shaft core cracks was used as an index and evaluated in five stages. Score 1 if the total crack length exceeds 50 mm, Grade 2 if the length is 50 mm or less and 15 mm or more, Grade 3 if the length is 15 mm or less and 5 mm or more, Grade 4 if the length is 5 mm or less and 1 mm or more, Grade 5 , And. The higher the score, the lower the degree of shaft center cracking.

The obtained results are shown in Table 1.
Furthermore, a round cast slab (outer diameter: 210 mmφ) having the above-described inner core internal properties is used as a material (seamless steel pipe material), and seamless steel pipe (outside Diameter 177.8 mmφ × wall thickness 12 mm). About each obtained steel pipe, the inner surface was inspected visually and by the ultrasonic flaw detection method over the full length, and the presence or absence of the generation | occurrence | production of an inner surface flaw was investigated. And the ratio with respect to the total number of steel pipes in which inner surface defects occurred was defined and calculated as an inner surface defect rate (%).

  In addition, based on the obtained inner surface flaw occurrence rate, it evaluated in five steps and set it as the pipe making result. Note that the rate of internal flaws is 15% or more, grade 1; 15% or less, 10% or more; grade 2, 10% or less, 5% or more; grade 3, 5% or less, 3% or more. A rating of 4 was given when the rating was 4, 3% or less. A case where the pipe making result was a score of 3 or more was evaluated as acceptable. The results of the obtained pipe making are also shown in Table 1.

  Also, using finite element method analysis, the stress acting on the round slab during the rolling of the slab is analyzed, the presence or absence of tensile stress in the shaft part of the round slab is investigated, and the results are also shown in Table 1. did.

  In all of the examples of the present invention, there was no generation of tensile stress in the round slabs, the internal properties of the shaft part of the round slabs became good, the rate of occurrence of internal flaws after pipe making decreased, and the pipe making results were also good. It was. On the other hand, in the comparative example outside the scope of the present invention, tensile stress is generated in the shaft core portion of the round cast slab, and the porosity score and the shaft core crack score are low, so that the rate of occurrence of internal flaws after pipe making is high and the pipe making score. Is also 2 or less. In addition, by using a saddle type roll as the reduction roll, a certain degree of reduction effect can be expected as compared with a flat roll by appropriately selecting the opening angle δ of the caliber bottom without a protrusion.

In test No. 1 (comparative example), which is rolled using a flat roll and falls outside the scope of the present invention, tensile stress is generated in the slab during rolling, so the porosity score and shaft core crack score are low at 2, The tube-making score is 1. In addition, test No. 2 (comparative example) using a caliber bottom angle δ of 70 ° and a saddle-shaped roll having no protrusions and out of the scope of the present invention has a small caliber bottom opening angle δ. Since the tensile stress is generated in the slab and there is no protrusion, the transmission of the rolling force to the core part of the slab is not sufficient, the porosity score and the shaft core crack score are low at 2, and the pipe making The score is 1. In addition, test No. 3 (comparative example), which has a caliber bottom angle δ of 80 ° and is rolled down using a saddle type roll having no protrusions and deviates from the scope of the present invention, has no protrusions. The rolling force was not sufficiently transmitted from the slab to the core part of the slab, the porosity score in the round slab was 3, the shaft core crack score was 2, and the pipe making score was 2 low. In addition, test No. 4 (comparative example), which has a caliber bottom angle δ of 90 ° and is rolled down using a saddle type roll having no protrusions and deviates from the scope of the present invention, has no protrusions. The rolling force was not sufficiently transmitted from the core to the slab shaft core, the porosity score in the round slab was 3, the shaft core crack score was 3, and the pipe making score was 2. In addition, test No. 2 (comparative example) using a saddle type roll having a caliber bottom angle δ of 110 ° and having no protrusion and deviating from the scope of the present invention has an excessively large caliber bottom opening angle δ. Sometimes tensile stress is generated in the slab, and because it does not have protrusions, the transmission of the rolling force to the slab shaft core is not sufficient, and the porosity score and shaft core crack score are 2 low, The tube score is 1. In addition, test No. 11 (comparative example) in which the caliber bottom opening angle δ is 110 °, which is outside the range of the present invention, is because the caliber bottom opening angle δ is too large, and tensile stress is generated in the slab during rolling. Despite having protrusions, the transmission of the rolling force to the slab shaft core is not sufficient, the porosity score is 3, the shaft crack score is 2, and the pipe making score is 2. Yes.
[Test casting 2]
Similarly to the test casting 1, the billet continuous casting machine shown in FIG. 1 is used, and as the rolling rolls 6 and 6, the caliber bottom angle δ is 90 °, and one ridge 13a is continuous in the roll circumferential direction. As shown in Table 2, a pair of vertical rolls 6a and 6a were used, and as shown in Table 2, at each position where the solid phase ratio fs was in the range of 0.2 to 1.0, the round cast piece 9 had an area reduction rate of 3%. A round cast slab was produced. The target round cast slab was made of Cr-containing steel containing 13% by mass of Cr, as in the test casting 1, and the diameter D before reduction was 210 mm. Moreover, the installation position of the protrusion (protrusion) was as shown in FIG. In addition, the installed ridge was a ridge (R / D: 0.33) having a cross-sectional shape in the roll axis direction of an arc (radius R: 70 mm).

About the obtained round cast piece, similarly to the test casting 1, the porosity area ratio and the shaft core crack length were measured, and the porosity score and the shaft core crack score were obtained.
Furthermore, in the same manner as in the test casting 1, the obtained round cast slab was used to produce a seamless steel pipe (size: outer diameter 177.8 mmφ × thickness 12 mm). The inner surface of each of the obtained steel pipes was inspected, and the rate of occurrence of inner surface flaws (%) was calculated in the same manner as in test casting 1 to obtain a pipe making score. In addition, the outer surface after pipe making was also observed visually, and the presence or absence of an outer surface flaw was investigated.

  The obtained results are shown in Table 2.

  In all of the examples of the present invention, there was no generation of tensile stress in the round slabs, the internal properties of the shaft part of the round slabs became good, the rate of occurrence of internal flaws after pipe making decreased, and the pipe making results were also good. It was. On the other hand, test No. 17 (comparative example) out of the scope of the present invention is a reduction after solidification is completed with a solid phase ratio fs of 1.0, and a porosity score of 3 and a shaft core cracking score of 3 are low. The tube-making score is 2. In Test No. 12 (example of the present invention) in which the solid phase ratio during reduction falls outside the preferable range of the present invention, the porosity score is 3, the shaft core crack score is 4, and the pipe making score is 3. When the solid phase ratio during reduction falls outside the preferred range of the present invention, the effect of reduction is small, the degree of porosity reduction effect is small, and the pipe making performance is also lower than in the preferred range. On the other hand, when the solid fraction at the time of rolling is higher than the preferred range, the effect of reducing the porosity is greater than when the solid fraction is low, but due to the fluidity of the molten steel, the degree of improvement in porosity and axial core cracking is reduced. However, compared with the reduction at a preferable solid phase ratio, the pipe making performance is reduced. It can also be seen that the effect of the reduction after completion of solidification is very small.

Test No. 18 and No. 20 (examples of the present invention) in which the area reduction rate during rolling falls out of the preferred range of the present invention has a porosity score of 3, an axial core crack score of 3, and a pipe making score of 3 The reduction effect is lower than the preferred range of the area reduction rate during reduction. Test No. 19 and No. 21 (examples of the present invention) in which the area reduction rate during rolling greatly deviates from the preferred range of the present invention has a porosity score of 5, an axis core crack score of 4, and a pipe making score of 4 Or it was as high as 5, but the depression of the outer surface of the slab due to the protrusion was deep, and the cross-sectional shape of the slab became slightly poor, and generation of flaws was observed on the outer surface of the tube after pipe making. The increase in the area reduction rate during reduction increases the load during reduction, and it is necessary to consider the increase in the scale of the equipment. There is concern that it will be a source of
[Test casting 3]
Similarly to the test casting 1, the billet continuous casting machine shown in FIG. 1 is used, and as the rolling rolls 6 and 6, the caliber bottom angle δ is 90 °, and one ridge 13a is continuous in the roll circumferential direction. Using the installed pair of vertical rolls 6a and 6a, as shown in Table 3, the round cast slab 9 is subjected to 2% reduction in area reduction rate at a position where the solid phase rate fs is 0.4 to 0.6. A round slab was produced. The target round cast slab was made of Cr-containing steel containing 13% by mass of Cr, as in the test casting 1, and the diameter D before reduction was 210 mm. Moreover, the installation position of the protrusion (protrusion) was as shown in FIG. In addition, the installed ridges are ridges in which the cross-sectional shape of the cross section in the roll axis direction is an arc shape, the radius R of the arc is variously changed, and as shown in Table 3, the R / D is changed in the range of 0.10 to 0.65. Articles were used.

About the obtained round cast piece, similarly to the test casting 1, the porosity area ratio and the shaft core crack length were measured, and the porosity score and the shaft core crack score were obtained.
Furthermore, in the same manner as in the test casting 1, the obtained round cast slab was used to produce a seamless steel pipe (size: outer diameter 177.8 mmφ × thickness 12 mm). The inner surface of each of the obtained steel pipes was inspected, and the rate of occurrence of inner surface flaws (%) was calculated in the same manner as in test casting 1 to obtain a pipe making score. In addition, the pipe outer surface after pipe making was also observed visually, and the presence or absence of an outer surface flaw was investigated.

  The obtained results are shown in Table 3.

In the example of the present invention in which the cross-sectional shape of the protrusion satisfies the preferred range of the present invention, there is no generation of tensile stress in the round cast piece, the internal property of the round cast piece core is good, and the rate of occurrence of internal flaws after pipe making The pipe making results were also good.
In Test No. 22 (example of the present invention) in which the cross-sectional shape of the protrusion with R / D is 0.10 deviates from the preferred range of the present invention, the porosity score is 3, the shaft core crack score is 3, and the internal quality is slightly The pipe production score was 3, and the occurrence of defects on the outer surface of the pipe due to the depression of the cast slab due to the protrusion was observed. In Test No. 26 (invention example) in which the R / D is 0.65 and the cross-sectional shape of the protrusion deviates from the preferred range of the present invention, the rolling effect is slightly insufficient, the porosity score is 3, the shaft core crack score is The internal quality was slightly reduced at 3 and the pipe production score was 3, but no occurrence of external flaws on the pipe was observed.

Thus, if the radius R of the arc-shaped cross section of the protrusion (projection) is not too large, the reduction effect is enhanced, the pipe production score is increased, and the pipe production performance is improved. On the other hand, if the radius R is too small, the round cast slab is locally crushed, so that portion may become the base point of the outer surface flaw.
[Test casting 4]
Similarly to the test casting 1, the billet continuous casting machine shown in FIG. 1 is used, and as the rolling rolls 6 and 6, the caliber bottom angle δ is 90 °, and one ridge 13a is continuous in the roll circumferential direction. Using a pair of vertical rolls 6a, 6a installed, as shown in Table 4, the round cast slab 9 is subjected to 2% reduction in area reduction rate at a position where the solid phase rate fs is 0.4 to 0.6. A round slab was produced. In addition, the installation position of the protrusion (projection) was set to the position shown in FIG. In addition, the installed ridges have a circular cross section in the roll axial direction cross section, and the radius R of the arc is 50 mm (R / D: 0.24). As a comparative example, the case of no reduction was also performed.

The target round cast slab was made of steel containing 0% (carbon steel), 1%, and 13% by mass of Cr, and the diameter D before rolling was 210 mm.
About the obtained round cast piece, similarly to the test casting 1, the porosity area ratio and the shaft core crack length were measured, and the porosity score and the shaft core crack score were obtained.
Furthermore, in the same manner as in the test casting 1, the obtained round cast slab was used to produce a seamless steel pipe (size: outer diameter 177.8 mmφ × thickness 12 mm). The inner surface of each of the obtained steel pipes was inspected, and the rate of occurrence of inner surface flaws (%) was calculated in the same manner as in test casting 1 to obtain a pipe making score.

  Table 4 shows the obtained results.

In the case of general carbon steel, the porosity area is as small as 0 to several mm ² even without reduction, and when the present invention is applied, the pipe production score is 3 or more without applying the present invention. Tube performance is good. On the other hand, with 1% Cr steel, the porosity area is about several tens to 100 mm ² without reduction, but it varies at the cutting position. By applying the present invention, the porosity area is reduced to about several mm ² which is the same as carbon steel. The pipe making score is 4 and improved. Furthermore, the 13% Cr steel, is without pressure observed porosity of more than about 100 mm ² is regardless of the cutting position, the pipe-score is 1, by applying the present invention, greatly reduces the porosity area The pipe making score is improved to 3, and the pipe making performance is good.
(Example 2)
In addition to Example 1, the influence of the protrusion shape on the internal properties of the round cast slab was performed using a saddle type roll provided with a plurality of protrusions distributed in the circumferential direction of the roll.
[Test casting 2-1]
Similarly to [Test Casting-1] in (Example 1), using the billet continuous casting machine shown in FIG. The round cast slabs 6a, 6a were used, and the round cast slab 9 was subjected to reduction with the area reduction rate shown in Table 5 at the position of the solid phase rate fs shown in Table 5 to obtain the round cast slab (diameter D before reduction). : 210 mmφ). In addition, the value calculated using the slab temperature calculated by solidification and heat transfer calculation was used for the solid phase ratio in the reduction position. Moreover, the used saddle type roll 6a has a plurality of protrusions 13b distributed in the circumferential direction of the roll as shown in FIG. 6 (c) at positions as shown in FIG. It was set as the roll installed in 2 rows so that it might become. As shown in FIG. 6 (b), the shape of the protrusion is an arc shape (radius R) in the cross section in the roll axis direction, and the length B along the roll surface and the length along the roll surface in the roll circumferential direction. The ratio of A and A / B is 0.50, and the interval L between the ends of adjacent protrusions is made to be zero between adjacent rows on the projection plane on the circumferential surface of the roll.

The target round cast slab was made of Cr-containing steel containing 13% by mass of Cr. About the obtained round cast slab, the area of a porosity and a shaft core crack were measured similarly to Example 1, and the property of the shaft core part was evaluated. The evaluation method was the same as in Example 1.
Furthermore, using the obtained round cast slab (outer diameter: 210 mmφ) as the raw material (seamless steel pipe material), seamless steel pipe (size: outer diameter 177.8 mmφ x wall thickness) is produced by the usual Mannesmann drilling method. 12 mm). In the same manner as in Example 1, the entire length of each steel pipe obtained was inspected, and the presence / absence of inner surface flaws and the rate of inner surface flaws were determined. Based on the inner surface flaw occurrence rate obtained, the evaluation was made in five stages in the same manner as in Example 1 to obtain the pipe making result.

In addition, using finite element method analysis, the stress acting on the round slab when the round slab is reduced is analyzed, the presence or absence of tensile stress in the shaft part of the round slab is investigated, and the result is also shown in Table 5. did.
The results obtained are shown in Table 5.

In the present invention example using a vertical roll in which a plurality of protrusions 13b distributed in the roll circumferential direction are arranged in two rows, the internal properties of the obtained round cast slab are continuous protrusions in the roll circumferential direction. Similar to the case of using a saddle type roll with a single strip, round cast slabs with excellent porosity score and axial core crack score can be obtained without generating any tensile stress on round cast slabs. The internal properties of the shaft core portion were good, the incidence of internal flaws after pipe making was reduced, and the pipe making result was also good. Test No. 2-1 (comparative example) in which the opening angle δ of the caliber bottom is outside the range of the present invention of 70 ° and 110 ° (Comparative Example) is that the opening angle δ of the caliber bottom is too small. In Test No. 2-5 (Comparative Example) where tensile stress occurs and the caliber bottom opening angle δ is 110 °, which is outside the range of the present invention, the caliber bottom opening angle δ is too large. Even though the slab sometimes had tensile stress and had protrusions, the transmission of the rolling force to the slab shaft core was not sufficient, and in all cases, the porosity score was 3 and the shaft core crack score was 2 The pipe production score is 2.
[Test casting 2-2]
Similarly to [Test Casting 2-1], using the billet continuous casting machine shown in FIG. 1, a pair of vertical rolls 6a having the caliber bottom angle δ and the projection shape shown in Table 6 as the rolling rolls 6 and 6 are shown. , 6a is used, and the round slab 9 is subjected to reduction with the area reduction rate shown in Table 6 at the position of the solid fraction fs shown in Table 6 to produce a round slab (diameter before reduction D: 210 mmφ). did. In addition, the target round cast slab is made of Cr-containing steel containing 13% by mass of Cr. The value calculated using the slab temperature calculated by solidification / heat transfer calculation was used for the solid phase ratio at the reduction position.

  Moreover, the used vertical roll 6a has a plurality of discretely distributed in the roll circumferential direction as shown in FIG. 6C at the position as shown in FIG. 3 as in [Test Casting 2-1]. The protrusion 13b was a roll installed in two rows so as to form a staggered pattern between adjacent rows. As shown in FIG. 6 (b), the shape of the protrusion is an arc shape (radius R) in the cross section in the roll axis direction, and the length B along the roll surface and the length along the roll surface in the roll circumferential direction. The ratio of A and A / B is 0.50, and the interval L between the ends of adjacent protrusions is made zero between adjacent rows. The target round cast slab was made of Cr-containing steel containing 13% by mass of Cr. In addition, using finite element method analysis, the stress acting on the round slab when the round slab is being reduced is analyzed, the presence or absence of tensile stress in the shaft part of the round slab is investigated, and the results are also shown in Table 6. did.

About the obtained round cast piece, the porosity area rate and the axial core crack length were measured similarly to [Test casting 2-1], and the porosity score and the axial core crack score were obtained. The evaluation method was the same as [Test casting 2-1].
Furthermore, similarly to [Test casting 2-1], the obtained round cast slab (outer diameter: 210 mmφ) was produced as a raw material (seamless steel pipe material), and the seamless steel pipe (size: outer diameter 177.8 mmφ) × wall thickness 12 mm). In the same manner as in Example 1, the entire length of each steel pipe obtained was inspected, and the presence / absence of inner surface flaws and the rate of inner surface flaws were determined. Based on the inner surface flaw occurrence rate obtained, the evaluation was made in five stages in the same manner as in Example 1 to obtain the pipe making result. Moreover, the outer surface after pipe making was also observed visually to investigate the presence or absence of outer surface defects.

  The results obtained are shown in Table 6.

  In the present invention example using a vertical roll in which a plurality of protrusions 13b distributed in the roll circumferential direction are arranged in two rows, the internal properties of the obtained round cast slab are continuous protrusions in the roll circumferential direction. Similar to the case of using a saddle type roll with a single strip, round cast slabs with excellent porosity score and axial core crack score can be obtained without generating any tensile stress on round cast slabs. The internal properties of the shaft core portion were good, the incidence of internal flaws after pipe making was reduced, and the pipe making result was also good.

  On the other hand, test No. 2-11 (comparative example) outside the scope of the present invention was a reduction after solidification was completed with a solid fraction fs of 1.0 at the time of reduction, a porosity score of 3, and a core crack score of 3 The pipe production score is 2. In Test 2-6 (example of the present invention) in which the solid phase ratio during reduction falls outside the preferable range of the present invention, the porosity score is 3 and the shaft core crack score is 4, and the tube production score is 3. ing. When the solid phase ratio during the reduction falls outside the preferred range of the present invention, the effect of reduction is small, the effect of improving the porosity and axial core cracking is small, and the pipe making results are also low. It can also be seen that the effect of the reduction after completion of solidification is very small.

Test Nos. 2-12 and No. 2-14 (examples of the present invention) in which the area reduction rate during rolling falls out of the preferred range of the present invention, the porosity score is 3, the shaft core crack score is 3, The tube score is slightly lower at 3. When the area reduction rate during reduction is out of the preferred range of the present invention, the reduction effect is small, the effect of improving the porosity and shaft core cracking is small, and the area reduction rate during reduction is less than the preferred range. Pipe-making performance is reduced. Test No. 2-13 and No. 2-15 (examples of the present invention) in which the area reduction rate during rolling greatly deviates from the preferred range of the present invention, the porosity score is 5, the shaft core crack score is 4, Although the pipe score was as high as 4 or 5, the dent on the outer surface of the slab was deep due to the protrusions, and the cross-sectional shape of the slab was slightly poor. The increase in the area reduction rate during reduction increases the load during reduction, and it is necessary to consider the increase in the scale of the equipment. There is concern that it will be a source of
[Test casting 2-3]
Similarly to [Test Casting 2-1], using the billet continuous casting machine shown in FIG. 1, the caliber bottom angle δ is 90 ° as the rolling rolls 6 and 6, and the caliber bottom angle δ shown in Table 7 is: Using a pair of vertical rolls 6a, 6a having a protrusion shape, the round cast slab 9 is subjected to reduction at the solid phase rate fs shown in Table 7 at the area reduction rate shown in Table 7, and round cast slab ( A diameter D before the reduction: 210 mmφ was produced. In addition, the value calculated using the slab temperature calculated by solidification and heat transfer calculation was used for the solid phase ratio in the reduction position. Moreover, the used saddle type roll 6a has a plurality of protrusions 13b distributed in the circumferential direction of the roll as shown in FIG. 6 (c) at positions as shown in FIG. It was set as the roll which installed two rows so that it might become. As shown in FIG. 6 (b), the shape of the protrusion is an arc shape (radius R) in the cross section in the roll axis direction, and the length B along the roll surface and the length along the roll surface in the roll circumferential direction. The ratio of A and A / B was changed to 0.10 to 1.50. Note that B is the maximum value of the contact arc length. Further, the interval L between the ends of the adjacent protrusions was made to be zero between the adjacent rows on the projection plane onto the roll circumferential cross section.

In addition, the target round cast slab was similarly made of Cr-containing steel containing 13% by mass of Cr. About the obtained round cast slab, the area of a porosity and a shaft core crack were measured similarly to [Test casting 2-1], and the property of the shaft core part was evaluated. The evaluation method was the same as [Test casting 2-1].
Furthermore, the obtained round cast slab (outer diameter: 210 mmφ) was used as the raw material (seamless steel pipe material), and pipes were produced in the same manner as in [Test casting 2-1], and seamless steel pipe (size: outer diameter 177.8 mmφ) × wall thickness 12 mm). In the same manner as in Example 1, the entire length of each steel pipe obtained was inspected, and the presence / absence of inner surface flaws and the rate of inner surface flaws were determined. Based on the inner surface flaw occurrence rate obtained, it was evaluated in five stages in the same manner as in [Test Casting 2-1], and the pipe production result was obtained. Moreover, the outer surface after pipe making was also observed visually to investigate the presence or absence of outer surface defects.

In addition, the stress which acts on a round cast piece at the time of rolling down of a round cast piece was analyzed using the finite element method analysis, and the presence or absence of the tensile stress in a round cast piece core part was investigated.
The results obtained are shown in Table 7.

  In the present invention example in which the protrusion shape A / B satisfies the preferred range of the present invention, there is no generation of tensile stress in the round cast slab, the internal property of the shaft part of the round cast slab is good, and internal flaws are generated after pipe production The rate was also reduced and the pipe making results were good. In Test No. 2-16 (example of the present invention) where the protrusion shape A / B is 0.10 and the protrusion shape A / B falls outside the preferred range of the present invention, the porosity score is 3, and the shaft core crack score is 3. The internal quality was slightly deteriorated, the pipe making score was 3, and the occurrence of flaws on the outer surface of the pipe due to the slab dent due to the protrusion was observed. Further, in test No. 2-20 (invention example) in which the protrusion shape A / B is 1.50 and the protrusion shape A / B deviates from the preferred range of the present invention, the reduction effect is slightly insufficient, and the porosity score is 3 The inner core crack rating was 3 and the internal quality was slightly reduced, and the pipe making score was 2. However, the occurrence of flaws on the outer surface of the pipe was not observed.

Thus, if projection shape A / B is not too large, a rolling-down effect will increase, a pipe making score will also become high, and a pipe making result will improve. On the other hand, if the protrusion shape A / B is too small, the round cast slab is locally crushed, so that portion may become the base point of the outer surface defect.
[Test casting 2-4]
Similarly to [Test Casting 2-1], using the billet continuous casting machine shown in FIG. , 6a is used to produce a round slab (diameter D before being reduced: 210 mmφ) at the position of the solid phase rate fs shown in Table 8 with a reduction in the area reduction rate shown in Table 8 on the round slab 9 did. In addition, the value calculated using the slab temperature calculated by solidification and heat transfer calculation was used for the solid phase ratio in the reduction position.

  Moreover, the used saddle type roll 6a has a plurality of protrusions 13b distributed in the circumferential direction of the roll as shown in FIG. 6 (c) at positions as shown in FIG. It was set as the roll which installed two rows so that it might become. As shown in FIG. 6 (b), the shape of the protrusion is an arc shape (radius R) in the cross section in the roll axis direction, and the length B along the roll surface and the length along the roll surface in the roll circumferential direction. The ratio of A and A / B is 0.50, and the length (projection in the roll circumferential direction) L between the ends of adjacent protrusions is made zero between adjacent rows.

The target round slabs were made of carbon steel not containing Cr, 1% Cr steel containing 1.0% by mass of Cr, and 13% Cr steel containing 13% by mass of Cr. About the obtained round cast slab, the area of a porosity and a shaft core crack were measured similarly to [Test casting 2-1], and the property of the shaft core part was evaluated. The evaluation method was the same as in Example 1.
Further, the obtained round cast slab (outer diameter: 210 mmφ) was produced as a raw material (seamless steel pipe material) to obtain a seamless steel pipe (size: outer diameter 177.8 mmφ × wall thickness 12 mm). Similarly to [Test Casting 2-1], the entire length of each steel pipe obtained was inspected, and the presence / absence of internal flaws and the rate of internal flaws were determined. Based on the inner surface flaw occurrence rate obtained, it was evaluated in five stages in the same manner as in [Test Casting 2-1], and the pipe production result was obtained.

In addition, the finite element method analysis was used to analyze the stress acting on the round slab when the round slab was rolled down, to investigate the presence or absence of tensile stress in the core part of the round slab, and the results are also shown in Table 8. did.
Table 8 shows the obtained results.

An example of the present invention that uses a saddle type roll having a plurality of protrusions distributed in the circumferential direction of the roll to reduce the cross-section more than a predetermined cross-sectional reduction rate is a saddle type having a ridge that continuously extends in the circumferential direction. A reduction effect similar to that in the case of the test casting 4 in which a roll is used to reduce a predetermined amount or more at a position in a predetermined solid phase ratio range is obtained.
In the case of general carbon steel, the porosity area is as small as 0 to several mm ² even without reduction, and when the present invention is applied, the pipe production score is 3 or more without applying the present invention. Tube performance is good. On the other hand, with 1% Cr steel, the porosity area is about several tens to 100 mm ² without reduction, but it varies at the cutting position. By applying the present invention, the porosity area is reduced to about several mm ² which is the same as carbon steel. The pipe making score is 4 and improved. Furthermore, the 13% Cr steel, porosity of more than about 100 mm ² is without pressure is observed regardless of the cutting position, by applying the present invention, it is possible to significantly reduce the porosity area, the pipe producing The score is also improved to 3, and the pipe making performance is good.

1 Billet continuous casting machine
2 Tundish
3 Immersion nozzle
4 Circular mold
5 slab support roll
6, Rolling roll
6a, 6b Rolling roll (Roll type roll)
6c Rolling roll (flat roll)
7 Hydraulic cylinder
8 Molten steel
9 Round slab
10 Solidified shell
11 Unsolidified layer
12 Vertical roll
13, 13a, 13b Protrusion

Claims (9)

Before the solidification of the round slab, the round slab during continuous casting using a circular mold is subjected to reduction by a pair of reduction rolls, and then the round slab is cut to produce a round slab for seamless steel pipes. In this case, as the pair of rolling rolls, a caliber bottom opening angle δ is 75 ° or more and 105 ° or less, and a vertical roll having a protrusion that contacts the round cast piece at a portion facing the round cast piece. A continuous casting method for round slabs for seamless steel pipes, characterized by being used. 2. The continuous casting method for a round cast piece for a seamless steel pipe according to claim 1, wherein the protrusion is a protrusion that is continuous in at least one roll circumferential direction. 2. The continuous casting method for a round cast piece for a seamless steel pipe according to claim 1, wherein the protrusions are a plurality of protrusions distributed discretely in a roll circumferential direction. The protrusion has a circular cross section in the axial cross section of the saddle-shaped roll, and the radius R of the circular arc is:
R = 0.20D ~ 0.50D
(Here, R: radius of arc of protrusion cross section (mm), D: diameter of round cast slab (mm))
The continuous casting method for a round slab for a seamless steel pipe according to any one of claims 1 to 3, wherein: The ratio of the bottom surface length B in the roll circumferential direction to the bottom surface length A in the roll axis direction of the plurality of protrusions, A / B is 0.2 to 1, wherein the plurality of protrusions are defined in claim 3. The continuous casting method of the round cast piece for seamless steel pipes of description. In the plurality of protrusions, an interval between adjacent protrusions among the plurality of protrusions is a projection length in a circumferential direction of the saddle type roll, and includes zero, and the vertical roll and the round The continuous casting method for a round slab for a seamless steel pipe according to claim 5, wherein the continuous slab is provided so as to be less than a contact length with the slab. The plurality of protrusions are protrusions having a bottom surface length B of the plurality of protrusions in the circumferential direction of the saddle type roll that is not less than 1/2 of a contact length between the saddle type roll and the round cast piece. The continuous casting method of the round slab for seamless steel pipes according to claim 5 or 6. When the reduction is performed at a time when the solid phase ratio fs at the shaft core portion of the round slab is 0.3 to 0.85, the area reduction rate (%) = {1− (cross-sectional area of the round slab after reduction) / (Cross-sectional area of round slab before rolling)} × 100
8. The continuous casting method for round slabs for seamless steel pipes according to claim 1, wherein the area reduction rate defined by is reduced to a range of 1 to 5%. 9. The continuous casting method of a round slab for seamless steel pipes according to claim 1, wherein the round slab is made of Cr-containing steel containing 0.5 mass% or more of Cr.

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