JP4364064B2 - Disk roll and manufacturing method thereof - Google Patents

Description

  The present invention relates to a disk roll and a manufacturing method thereof, and more specifically, a disk roll used in a piercing and rolling machine for manufacturing a medium-diameter seamless pipe (a seamless pipe having an outer diameter of 150 mm to 450 mm) and a manufacturing method thereof. About.

  Normally, the medium diameter seamless pipe is manufactured by the Mannesmann pipe manufacturing method. As shown in FIGS. 15 and 16, in the Mannesmann pipe manufacturing method, the piercing and rolling mill 70 opens a hole in the axial center portion of the round billet that is the material to be rolled 4 to form a hollow shell (hollow shell). The hollow shell becomes a seamless pipe through a drawing and rolling process and a shape correction process.

  The piercing and rolling machine 70 includes a pair of inclined rolls 1, a pair of disk rolls 2, and a plug 3 for piercing the material to be rolled 4. The pair of inclined rolls 1 are arranged to face each other across a center line (hereinafter referred to as a pass center L) of a line through which the material to be rolled 4 passes. The pair of disc rolls 2 are disposed at 90 ° positions with respect to the pair of inclined rolls 1 with the path center L as the center.

  The material 4 to be rolled is conveyed on the path center L and is bitten by the rotating inclined roll 1. The material to be rolled 4 is extruded in the X direction while being spirally rotated by the inclined roll 1. At this time, the material to be rolled 4 is pierced by the plug 3 and rolled into a hollow shell. As the hole is drilled, the outer diameter of the material to be rolled 4 (hollow shell) expands.

  As shown in FIG. 17, the disk roll 2 has a curvature on the surface 22. During piercing and rolling, the disk roll 2 rotates while bringing the surface 22 into contact with the material to be rolled 4 (hollow shell), and suppresses the expansion of the outer diameter of the material to be rolled 4.

  As described above, the disk roll 2 receives the material to be rolled 4 that is going to expand the outer diameter while spirally rotating. Therefore, high surface pressure is applied to the surface 22 and friction occurs. When the surface 22 of the disk roll 2 is worn by friction, the disk roll 2 cannot suppress the swelling of the outer diameter of the material 4 to be rolled. Further, when uneven wear or seizure occurs on the surface 22, soot (pipe scissors) is generated on the surface of the material to be rolled 4. For this reason, the anti-wear and anti-seizure measures of the disk roll 2 are important. In the following Patent Documents 1 and 2, measures for suppressing wear and seizure of the disk roll 2 are reported by adjusting the rolling method. However, no measures for improving the wear resistance and seizure resistance of the disk roll itself have been reported.

  By the way, as a countermeasure against wear and seizure resistance, a technique for forming a build-up layer (PTA build-up layer) on the surface of a tool that contacts the material to be rolled by a plasma powder build-up welding method (Plasma Transferred Arc: PTA method). Has been reported (see Patent Documents 3 and 4 below). Since the PTA overlay layer has high hardness, it has excellent wear resistance. Moreover, the surface of the PTA build-up layer is oxidized by repeatedly contacting with the hot rolled material. This oxide film has a lubricating action and prevents the occurrence of seizure. In short, the PTA overlay layer is also excellent in seizure resistance. Note that when a tool having a PTA build-up layer is used for the first time, the lubricant is applied to the PTA build-up surface, and the lubricant prevents seizure until an oxide film is formed. The lubricant is peeled off from the tool while coming into contact with the material to be rolled a plurality of times, and an oxide film is formed during that time. Therefore, after the lubricant is peeled off, the oxide film prevents the seizure from occurring.

  As described above, the PTA overlay layer is excellent in wear resistance and seizure resistance. In Patent Document 4, the wear resistance and seizure resistance of the tool are improved by including NbC in the PTA build-up layer.

  However, disc rolls having a PTA overlay layer containing NbC on the surface have not been produced so far. A disk roll 2 used in a piercing and rolling machine for manufacturing a medium diameter pipe has a large diameter of 3200 mm to 3500 mm. When overlay welding by the PTA method (hereinafter referred to as PTA overlay welding) is performed on such a large disk roll, thermal stress is easily applied to the PTA overlay layer, and a plurality of cracks are formed on the surface of the PTA overlay layer. Will occur. When cracks exist in the PTA overlay layer, the cracks may cause wrinkles on the surface of the material to be rolled. Moreover, the PTA build-up layer may peel off from the crack. Therefore, when forming a PTA build-up layer on a disk roll, it is necessary to suppress cracking of the PTA build-up layer to a practical level.

In general, in PTA build-up welding, beads are overlapped to form a PTA build-up layer, so that the surface of the PTA build-up layer is uneven. The presence of unevenness becomes a factor that causes wrinkles on the surface of the material to be rolled. This is because the material to be rolled comes into contact with the convex portion of the bead and stress concentrates on the convex portion. Therefore, as shown in Patent Documents 3 and 4 below, the surface of the PTA build-up layer is usually ground with a grinder or the like to eliminate irregularities. However, in a large disk roll, it takes a lot of time to grind the entire surface of the PTA build-up layer, and the manufacturing cost increases accordingly. Furthermore, since the PTA overlay layer on the surface is ground, the life of the disk roll is shortened by the amount of the ground PTA overlay layer. Therefore, in view of the manufacturing cost and the life resulting from the grinding amount, it is preferable that the surface of the PTA build-up layer can be used without polishing.
JP 2002-248506 A JP 2001-340904 A JP-A-9-85487 JP-A-6-315704

  An object of the present invention is to provide a disk roll having excellent wear resistance and seizure resistance by forming a PTA build-up layer containing NbC on the surface.

  Another object of the present invention is to provide a disc roll that can be used without grinding the surface of the PTA build-up layer.

Means for Solving the Problems and Effects of the Invention

  The present inventors formed a PTA build-up layer containing NbC on the disk roll, and investigated the cause of cracking of the PTA build-up layer after formation. As a result, the inventors considered that the temperature of the disk roll during PTA build-up welding was caused by cracks in the PTA build-up layer. The PTA overlay layer has high hardness and poor ductility. For this reason, when a normal temperature disk roll is used for PTA build-up welding, the formed PTA build-up layer cannot follow the thermal expansion of the disk roll due to welding heat, resulting in cracks. On the other hand, when a disc roll preheated in an appropriate temperature range is used, thermal expansion of the disc roll due to welding heat is suppressed. Therefore, the formed PTA build-up layer is less likely to crack.

  Based on the above idea, as a result of investigating the holding temperature of the disk roll during PTA build-up welding, if the temperature of the disk roll is kept at 250 ° C. to 350 ° C., a PTA built-up layer containing NbC is put into practical use It was found that it can be formed in a possible state. Below 250 ° C., the thermal expansion of the disk roll due to welding heat could not be sufficiently suppressed. Further, when the temperature exceeded 350 ° C., the surface of the disk roll subjected to the arc of PTA build-up welding was melted, and the disk roll melted into the PTA build-up layer, so that the hardness of the PTA build-up layer was lowered.

  Furthermore, the present inventors investigated the unevenness | corrugation of the PTA overlaying layer surface. In the present invention, as shown in FIG. 1, the difference Δh between the peaks of adjacent beads of the PTA build-up layer is measured, and Δh is set as the unevenness value indicating the unevenness state. The unevenness value Δh can be measured using a gauge 500 having the same curvature as the curvature of the surface 30C of the disk roll 50, for example, as shown in FIG.

  As a result of investigating the relationship between the unevenness value Δh and the occurrence of wrinkles of the material to be rolled, the present inventors have found that the generation of wrinkles can be suppressed if the unevenness value Δh of the PTA overlay layer is suppressed to within 2 mm. I found it. It is considered that the unevenness on the surface of the PTA build-up layer is reduced, so that the material to be rolled comes into contact with the entire PTA build-up layer, and the load generated on the convex portions on the surface is alleviated.

  Furthermore, it has also been found that generation of wrinkles cannot be prevented unless the unevenness value Δh is set to 0.1 mm or more. When using a dill scroll having a PTA overlay layer, a lubricant is applied to the surface in the initial stage of use. This is to prevent image sticking until an oxide film is formed on the PTA overlay layer. However, if there is no unevenness on the surface of the PTA build-up layer, the lubricant is peeled off before the oxide film is formed, and seizure occurs. Therefore, the unevenness value Δh needs to be 0.1 mm or more.

  From the above, if the irregularity value Δh on the surface of the PTA overlay layer of the disk roll is 0.1 to 2.0 mm, generation of wrinkles on the surface of the material to be rolled can be prevented at the initial stage of use of the disk roll. In addition, since it is not necessary to grind a PTA build-up layer after PTA build-up welding, the lifetime of a disk roll improves compared with the case where it grinds.

  In order to suppress the surface irregularity value Δh to 0.1 mm to 2 mm, the present inventors examined the conditions for PTA overlay welding. As a result of investigating the shape of the formed bead by performing PTA overlay welding on the disk roll, the present inventors have found that the shape is different for each bead.

As shown in FIG. 3, it is compared with the bead BD3 formed on the surface having a curvature and BD4, an angle alpha _BD3 <angle alpha _BD4. Here, as shown in FIG. 4, the angle α is an angle formed by a tangent line passing through the intersection of the vertical bisector of the oscillating width W1 and the surface of the disk roll and a horizontal line. Here, the oscillating width W1 refers to the moving width of the PTA torch during bead formation. Details will be described later.

Returning to FIG. 3, since the angle alpha _BD4 angle alpha _BD3 and BD4 bead BD3 is different, a different shape from the bead BD3 and BD4. Specifically, the peak of the convex portion of BD4 having a large angle α is shifted to the valley side (BD3 side) from the bisector L4. The larger the angle α, the larger the deviation of the peak of the bead with respect to the bisector. Therefore, if the inventors control the angle α at the time of PTA build-up welding within a predetermined range, the shape of the formed bead can be made substantially the same shape, and the surface unevenness of the PTA build-up can be adjusted. Thought. Based on the above examination, Formula (1) was derived.
W1 × tanα ≦ 1.0mm (1)

  Furthermore, in PTA build-up welding, adjacent beads are overlapped to form a PTA build-up layer. Referring to FIG. 5, the PTA torch moves in the direction perpendicular to the paper surface of FIG. 5 while moving between the oscillating widths W1, thereby forming a bead BD1. After forming the bead BD1, the PTA torch shifts by the bead transition width W2. After the transition, the PTA torch forms the bead BD2 while moving between the oscillation widths W1 around the transitioned position. By forming a plurality of beads in this way, a PTA overlay layer is completed.

  The inventors considered that it is necessary to adjust not only the control of the angle α but also the region SP in which the bead BD1 and the bead BD2 are overlapped in order to suppress the unevenness value Δh. If the overlapping region SP is small, the region SP has a concave shape, and if the overlapping region SP is large, the region SP has a convex shape. Therefore, in order to set the unevenness value Δh within an appropriate range, it is necessary to set the overlapping area SP to an appropriate size. In other words, it is necessary to properly define the ratio of the bead width W1 and the transition width W2.

Based on the above examination, Formula (2) was derived.
0.8 ≦ W2 / W1 ≦ 1.5 (2)

  From the above, the present inventors have found that the concavo-convex value Δh can be set to 0.1 mm to 2 mm if Expressions (1) and (2) are satisfied during PTA overlay welding. Based on these findings, the present inventors have completed the following present invention.

The disc roll according to the present invention is a disc roll having a build-up layer formed on the surface by a plasma powder build-up welding method, and the build-up layer occupies 30% to 60% in volume%, and the particle size is It contains a carbide mainly composed of NbC of 50 μm or more and a matrix alloy made of a Co-base alloy or a Ni-base alloy. The unevenness of the surface of the overlay layer is 0.1 to 2 mm. Here, the unevenness on the surface means the unevenness value Δh shown in FIG.

  A disc roll manufacturing method according to the present invention is a disc roll manufacturing method having a build-up layer on the surface, a step of preheating the disc roll to 250 to 350 ° C., and a plasma powder while maintaining the preheated temperature. Forming a build-up layer on the surface of the disk roll by the build-up welding method, and forming the build-up layer occupies 30% to 70% by volume% and NbC having a particle size of 50 to 250 μm. The main carbide and a matrix alloy powder made of a Co-based alloy or Ni-based alloy are used as raw materials for the PTA build-up layer.

Preferably, in the step of forming the build-up layer, the oscillate width W1 and the bead transition width W2 in the plasma powder build-up welding method satisfy the following expressions (1) and (2).
W1 × Tanα ≦ 1.0mm (1)
0.8 ≦ W2 / W1 ≦ 1.5 (2)

  Here, as shown in FIG. 4, α is an angle formed by a tangent line passing through the intersection of the vertical bisector of the oscillating width W1 and the disk roll surface and a horizontal line.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In the drawings, the same or corresponding parts are denoted by the same reference numerals and description thereof will not be repeated.

1. Chemical Composition The PTA overlay layer formed on the surface of the disk roll according to the embodiment of the present invention contains a carbide mainly composed of NbC and a matrix alloy.

[Carbides mainly composed of NbC]
NbC is superior in hardness and oxidation resistance at high temperatures to other carbides (WC, TiC, etc.). Therefore, the wear resistance and seizure resistance of the PTA built-up layer containing a carbide mainly composed of NbC are improved. The carbide mainly composed of NbC is a carbide containing, for example, 50% or more by volume of NbC. The particle size of NbC is 50 μm or more. The particle size referred to here is a value determined by (major axis + minor axis) / 2 after measuring the major axis and minor axis of the carbide particles present in the cross section of the PTA build-up layer. The major axis and the minor axis are measured using an optical microscope or a scanning electron microscope (SEM).

  If the NbC particle size is less than 50 μm, the contact between the material to be rolled and the matrix alloy in the PTA build-up layer cannot be sufficiently prevented, and seizure cannot be prevented. Desirably, the particle size is 100 μm or more. Further, the volume ratio of carbide particles mainly composed of NbC in the PTA build-up layer is 30% to 60%. The volume fraction of carbides mainly composed of NbC affects the wear resistance. When it is less than 30%, the wear resistance is lowered. On the other hand, when it exceeds 60%, the toughness of the PTA overlay layer is remarkably lowered. Therefore, a part of the PTA overlay layer is easily lost during use, and the life is shortened. Preferably, the volume ratio of the carbide mainly composed of NbC is 40% to 60%.

[Matrix alloy]
The matrix alloy is a Co-based alloy or Ni-based alloy that is excellent in oxidation resistance and corrosion resistance in consideration of the use environment of the disk roll.

The Co-based alloy refers to an alloy containing, for example, 30% or more by mass of Co. A stellite alloy generally used as a hardfacing material is a Co-based alloy. For example, Stellite 1, Stellite 6, Stellite 21, Stellite 32, etc. shown in Table 1 correspond to a Co-based alloy.

The Ni-based alloy refers to an alloy containing, for example, 30% or more by mass of Ni. For example, Alloy 600, Alloy 625, Alloy 718, C-276, 50Cr50Ni shown in Table 2 are listed.

  The disc roll that is the base material of the PTA overlay layer is preferably a soft material. This is to prevent the development of heat cracks. For example, STKM13A and S45C are good.

2. Manufacturing Method A method for manufacturing a disk roll having a PTA build-up layer on the surface according to this embodiment will be described below.

  Referring to FIG. 6, the disk roll 50 is divided into a shaft member 40 including a shaft 21 and an annular member 30. When replacing the disk roll 50 installed in the piercing and rolling mill, the annular member 30 is replaced as a consumable item. Specifically, the worn annular member 30 is removed from the shaft member 40, and the new annular member 30 is attached to the shaft member 40 by shrink fitting. In the present embodiment, PTA build-up welding is performed on the surface 30 c of the annular member 30. PTA overlay welding is performed using a PTA overlay welding apparatus.

  7 to 9, the PTA build-up welding apparatus 10 includes an annular fixing jig 11, a positioner 12, a PTA stand 14, and a main body 15.

  The annular fixing jig 11 includes a disk-shaped jig main body 110 and eight arm parts 11A. Each arm portion 11 </ b> A is arranged at equal intervals on the circumference of the jig main body portion 110. Each arm portion 11A sandwiches and fixes the annular member 30. Specifically, in the annular member 30, the side surface 30 a and the bottom surface 30 b other than the surface 30 c on which PTA build-up welding is performed are wrapped with the heat insulating sheet 20. The arm portion 11 </ b> A sandwiches the annular member 30 wrapped in the heat insulating sheet 20.

  An annular fixing jig 11 in FIG. 8 is a cross-sectional view taken along line VII-VII in FIG. A through hole 111 is formed in the arm portion 11 </ b> A, and a fixing jig 11 </ b> C is inserted into the through hole 111. The fixing jig 11C is, for example, a bolt and a nut. Each arm portion 11A sandwiching the annular member 30 is fastened by a fixing jig 11C to firmly sandwich the annular member 30.

  The heat insulating sheet 20 is constituted by a car wall. During PTA overlay welding, the annular member 30 is difficult to keep heat constant. Therefore, a device for making the temperature of the annular member constant is necessary. In the present embodiment, the heat insulating sheet 20 directly wraps the surface of the annular member 30 to suppress temperature fluctuations during PTA build-up welding.

  The positioner 12 includes a pedestal 120 installed on the ground and a rotating tilt member 121. As shown in FIG. 8, the gantry 120 has a U-shape, and the upper protrusion 120 </ b> A pivotally supports the rotating inclined member 121. The rotation tilt member 121 is mounted with the ring fixing jig 11 to which the ring member 30 is fixed, and rotates or tilts the ring member 30.

  The rotation inclination member 121 includes an inclination member 122 and a rotation mechanism 123. The inclined member 122 is a semicircular rack gear, and rotates about a horizontal axis 120B by driving a pinion gear (not shown) provided in the gantry 120. FIG. 10 shows a state in which the inclined member 122 is rotated and the annular member 30 is inclined. Thereby, angle (alpha) can be adjusted at the time of PTA overlay welding.

  The rotation mechanism 123 includes a mounting plate 124, spur gears 125 and 126, and a motor 127. The mounting plate 124 is a rectangular plate, and the annular fixing jig 11 is mounted on one surface. A spur gear 125 is fixed to the other surface. The spur gear 125 is rotatably supported on the inclined member 122. Spur gear 125 meshes with spur gear 126. When the spur gear 126 is rotated by the motor 127, the mounting plate 124 to which the spur gear 125 is fixed rotates. As a result, the annular fixing jig 11 and the annular member 30 are rotated clockwise (or counterclockwise) in FIG.

  The main body 15 includes a top plate 15A and two frames 151 and 152. As shown in FIG. 7, the frames 151 and 152 are arranged in parallel, and a part of the frames 151 and 152 is fixed to the top plate 15A as shown in FIG. A plurality of wheels 153 are provided below the frames 151 and 152, and rails 160 are installed below the plurality of wheels 153. The main body 15 moves on the rail 160 in the horizontal direction.

  The main body 15 further includes a PTA stand 14 and a PTA torch control panel 145. The PTA stand 14 and the PTA torch control panel 145 are installed on the top board 15A. The PTA stand 14 includes a PTA arm 141 and a PTA torch 142. The PTA arm 141 is fixed in the horizontal direction, and the PTA torch 142 is fixed in the vertical direction at the tip thereof. As shown in FIG. 10, even when the annular member 30 is inclined, the position of the PTA torch 142 relative to the surface 30c of the annular member 30 can be adjusted by moving the main body 15 in the horizontal direction.

  As shown in FIG. 7, the burners 16 </ b> A to 16 </ b> E are arranged at 3 o'clock, 5 o'clock, 7 o'clock, 9 o'clock, and 11 o'clock from the center of the jig main body 110, respectively.

  A method for manufacturing the disk roll 50 according to the present embodiment will be described.

  First, the annular member 30 is wrapped with the heat insulating sheet 20. Subsequently, the annular member 30 wrapped with the heat insulating sheet 20 is fixed to the annular fixing jig 11. Thereafter, the annular fixing jig 11 is mounted on the mounting plate 124. At this time, the positioner 12 is adjusted so that the central axis of the annular member 30 is horizontal (that is, in the state of FIG. 8).

  After adjusting the positioner 12, the annular member 30 is preheated. Specifically, the mounting plate 124 is rotated by the motor 127 and the annular member 30 is rotated. The annular member 30 is heated by the burner 16 during rotation. At this time, the temperature of the annular member 30 is maintained at 250 ° C. to 350 ° C. The temperature of the annular member 30 is measured every predetermined time with a thermometer (not shown), and the burner 16 is adjusted based on the measurement result. The fire power of the burners 16A to 16E can be changed in three levels: strong, medium, and weak. The heating power of the burner 16A is always set to “strong”. Further, the heating power of the burners 16C and 16D is set to “medium”. The heating power of the burners 16B and 16E is initially set to “medium”, but is varied based on the temperature of the disk roll. A thermometer is installed in the vicinity of the PTA torch 142, and the temperature is measured every predetermined period.

  After the temperature of the annular member 30 is stably maintained at 250 ° C. to 350 ° C., a PTA overlay layer is formed on the surface 30c of the annular member 30 by the PTA method. As a raw material for the PTA build-up layer, a carbide mainly composed of NbC having a particle size of 50 to 250 μm and a matrix alloy powder are used. The carbide mainly composed of NbC is 30% to 70% of the raw material by volume ratio. The matrix alloy powder is a Co-based alloy or Ni-based alloy.

  When forming a bead in PTA build-up welding, the angle α, the oscillating width W1, and the bead transition width W2 are determined so as to satisfy the expressions (1) and (2).

  After determining the angle α, the oscillation width W1, and the bead transition width W2, the annular member 30 is rotated to perform the first PTA build-up welding. When the annular member 30 makes one rotation, a bead BD is formed on the surface 30c of the annular member 30 as shown in FIG. At this time, the first PTA build-up welding is completed. Subsequently, as shown in FIG. 12, after the PTA torch 142 is shifted by the bead transition width W2, the second PTA build-up welding is performed. Similarly, when the (n-1) th time is completed, the PTA torch 142 is shifted by the bead transition width W2, and the nth PTA overlay welding is performed. Before performing the n-th PTA overlay welding, it is confirmed whether or not the angle αn exceeds the set angle α. If not, perform PTA overlay welding. When the angle αn exceeds the angle α, the positional relationship between the surface 30c of the annular member 30 and the PTA torch 142 is adjusted so that α = 0 °. Specifically, as shown in FIG. 10, the tilting member 122 of the positioner 12 is rotated to tilt the annular member 30 so that the angle α = 0 °. At this time, the main body 15 is moved in the horizontal direction, and the position of the PTA torch 142 with respect to the surface 30c of the annular member 30 is adjusted. After adjustment, PTA overlay welding is performed.

  As described above, PTA build-up welding is repeatedly performed so as to satisfy the expressions (1) and (2), and a PTA build-up layer containing NbC is formed on the surface 30 c of the annular member 30. The unevenness value Δh of the PTA build-up layer after formation can be suppressed to 0.1 mm to 2.0 mm.

  The disk roll 50 is completed by attaching the annular member 30 to the shaft member 40 by shrink fitting after PTA overlay welding.

  During PTA overlay welding, the temperature of the annular member 30 is prevented from fluctuating by the heat insulating sheet 20, and the temperature of the annular member 30 is arranged by arranging a plurality of burners substantially evenly in the circumferential direction of the disk roll. Can be stably maintained at 250 ° C to 350 ° C. Thereby, it can suppress that a crack generate | occur | produces in a PTA overlaying layer. Further, the positioner 12 can change the angle of the annular member 30 so that the angle αn at the n-th PTA build-up welding is within the set angle α. Thereby, the unevenness | corrugation value (DELTA) h of a PTA overlaying layer can be restrained to 0.1 mm-2 mm.

PTA build-up layers were formed on the surfaces of the disk rolls D1 to D4 and E1 to E4. At this time, NbC having a particle size of 75 to 150 μm and matrix alloy powder shown in Table 3 were used as raw materials. For the disk rolls D1 to D4, the matrix alloy D0 in Table 3 was used. For the disk rolls E1 to E4, the matrix alloy E02 in Table 3 was used. In addition, the volume ratio of NbC with respect to the whole raw material was 50%.

  After forming the PTA overlay layer, the state of the PTA overlay layer was investigated. The disk rolls D1 to D4 and E1 to E4 are both mass% and contain C: 0.2 to 2.0%, Si: 0.15 to 0.35%, and Cr: 3.1 to 3.5. A 0.6C-0.2Si-3.0Cr system was used.

  In plasma powder overlay welding, PTA overlay welding was performed at the preheating temperature and holding temperature of the disk roll shown in Table 4. The manufacturing conditions other than preheating and temperature during PTA build-up welding were the same.

After production, the composition of the PTA overlay layer of each of the disk rolls D1 to D4 and E1 to E4, the particle size and the volume ratio of NbC were investigated. Specifically, a plurality of regions were selected from the cross section of the overlay layer. At this time, the total area of the selected regions was selected to be larger than 3 mm × 3 mm. Using a scanning electron microscope (SEM), the particle size and area ratio of all carbides in the selected region were measured. The particle size was a value of (longer diameter of carbide + shorter diameter of carbide) / 2. The particle size and area ratio were measured by image analysis. The obtained area ratio was used as the volume ratio of the carbide. This is because it has been proved that the volume ratio of a certain phase is equal to the area ratio in the cross section (see, for example, Journal of the Japan Institute of Metals, Vol. 10, No. 5, pages 279 to 289). Table 4 shows the survey results. There was no significant difference in the composition of the PTA overlay layer of the disk rolls D1 to D4. Moreover, there was no big difference in the composition of the PTA overlay layer of the disk rolls E1 to E4.

[PTA overlay layer evaluation method]
About the PTA surfacing layer manufactured on the said conditions, the crack of the surface and the penetration depth were investigated. The penetration depth refers to the depth of the PTA overlay layer that has melted into the disk roll base material. Specifically, as shown in FIG. 13, the depth DEP of the PTA build-up layer 150 melted from the surface 30 </ b> C of the disk roll 50 is defined as the melt depth.

(1) Surface crack investigation The surface of the PTA surface was examined by visual observation of the PTA surface, and the crack condition of the surface generated on the PTA surface was investigated. Specifically, the size of cracks generated on the surface and the number of cracks generated on the surface were investigated.

  When a large crack is generated on the surface of the PTA overlay layer, the disk roll cannot be used in a piercing and rolling mill. This is because the shape of the crack is printed on the surface of the material to be rolled and wrinkles are generated on the surface of the material to be rolled. Then, the opening width of the crack which has generate | occur | produced on the disk roll surface was investigated. In each of the disk rolls D1 to D4 and E1 to E4, among the cracks occurring on the surface of the PTA build-up layer, if the maximum opening width is 0.5 mm or less, it is acceptable and if it is larger than 0.5 mm Rated as rejected.

  On the other hand, even if a large opening crack does not occur, when a large number of minute cracks are generated, there is a high possibility that the PTA build-up layer peels off from the disk roll during use. If the PTA build-up layer peels, the peeled portion generates wrinkles on the material to be rolled. Therefore, a disk roll in which many fine cracks are generated cannot be used. Therefore, the number of cracks generated on the disk roll surface was investigated. The number of cracks was investigated as follows. As shown in FIG. 14, at 10 arbitrary positions on the surface of the PTA build-up layer, a straight line of 10 cm was drawn in the circumferential direction, and the number of cracks crossing the straight line was counted. The number counted was averaged by the number of points where a straight line was drawn. When the number of cracks was 8 or less, it was evaluated as acceptable, and when it was 9 or more, it was evaluated as rejected.

(2) Penetration depth investigation During PTA overlay welding, it is necessary to melt the surface of the disk roll into the PTA overlay layer. This is for ensuring the adhesion strength between the disk roll and the overlay layer. When the penetration depth of the disk roll with respect to the PTA build-up layer is large, the wear resistance of the PTA build-up layer is lowered even if cracks are not generated on the surface of the PTA build-up layer. Specifically, since the PTA build-up layer is thinned by the material of the disc roll in which the PTA build-up layer is melted and the amount of carbide per unit volume is reduced, the hardness of the PTA build-up layer is lowered. Furthermore, since Fe in the material of the disk roll is inferior in corrosion resistance, the penetration of Fe causes corrosive wear in the PTA overlay layer. Therefore, the wear resistance of the PTA overlay layer is reduced.

  Then, the penetration depth was investigated about the PTA build-up layer of each disk roll D1-D4, E1-E4. Specifically, the disk roll body including the PTA build-up layer was cut at 10 locations in the radial direction. The cut surface was observed with an optical microscope, and the penetration depth from the surface of the base material before PTA overlay welding was measured. Those with a penetration depth of 0.5 mm or more and 3.0 mm or less were accepted and those other than that were rejected. This is because when the penetration depth is deeper than 3.0 mm, the wear resistance deteriorates, and when it is less than 0.5 mm, the PTA overlay layer may be peeled off during use due to insufficient adhesion strength.

[Evaluation results]
Table 5 shows the evaluation results of the number of cracks, the size of the cracks, and the penetration rate.

  Referring to Table 5, the number of cracks in the PTA overlay layers of the disk rolls D1 and E1 was unacceptable. Since the base material temperature was insufficient at the preheating of 100 ° C., fine cracks occurred in the PTA overlay layer. Further, the PTA build-up layers of the disk rolls D4 and E5 had a deep penetration depth and were rejected. Large cracks also occurred.

  On the other hand, the disk rolls D2, D3, E2, and E3 of the present invention passed both the number of cracks, the size of the cracks, and the penetration depth. That is, a usable PTA build-up layer could be formed even with a disk roll that is a large roll.

PTA build-up layers having different unevenness values Δh were formed on a plurality of disc rolls, and pipe wrinkles generated based on each disc roll and seizure generated on the disc roll were investigated.

  PTA overlay welding was carried out on the disk rolls D5 to D14 and E5 to E14 under the production conditions shown in Table 6. The matrix alloy used for the disk rolls D5 to D14 was Stellite 21. The matrix alloy used for the disk rolls E5 to E14 was Alloy 600. Further, NbC having a particle size of 75 to 125 μm was used as a raw material. In addition, the volume ratio of NbC with respect to the whole raw material was 50%.

  After carrying out PTA build-up welding, the unevenness value Δh of the PTA build-up layers of the respective disk rolls D5 to D14 and E5 to E14 was measured. In addition, the disk roll D5, D6, E5, E6 grind | polished and grind | polished the surface of the PTA overlaying layer for the comparison, and it was set as uneven | corrugated value (DELTA) h = 0.

  After measuring the unevenness value Δh, the respective disk rolls D5 to D14 and E5 to E14 were installed in a piercing and rolling mill. At the time of installation, a lubricant was applied to the surface of the disk roll. After the installation, five rolls of material to be rolled at 1230 ° C. were pierced and rolled, and the rolled material was used as a medium diameter pipe. Then, pipe wrinkles generated on the surface of the medium diameter pipe were examined. STKM13A steel and SUS304 were used as the material to be rolled.

  Pipe dredging was investigated by the following method. For STKM13A steel, the presence or absence of pipe flaws and the depth of the generated pipe flaws were investigated by a magnetic particle inspection method. At this time, the magnetic powder concentration was 0.2 g / L, and the current value was 5500 AT. For SUS304, pipe culling was investigated by pickling after shot blasting. In pickling, nitric acid and hydrofluoric acid were used, and the presence / absence of pipe rods and the depth of pipe rods were investigated by investigating the location of acid solution penetration after pickling.

  In addition, after piercing and rolling five materials to be rolled, it was investigated whether or not seizure occurred on the surface of the PTA overlay layer of each of the disk rolls D5 to D14 and E5 to E14. The presence or absence of seizure was visually observed, and it was judged that seizure occurred when the rolled material was transferred to the surface of the PTA build-up layer.

  With reference to Table 5, as for disc roll D7-D12 and E7-E12 which satisfy | fill Formula (1) and Formula (2), uneven | corrugated value (DELTA) h became 0.1 mm-2.0 mm. Furthermore, in the disk rolls D7 to D12 and E7 to E12, pipe flaws did not occur, or even if pipe flaws occurred, the depth was within an allowable range of 0.2 mm. Further, no burn-in occurred in the disk rolls D7 to D12 and E7 to E12.

  On the other hand, in the disk rolls D13, D14, E13, and E14 that did not satisfy the expressions (1) and (2), the unevenness value Δh exceeded 2.0 mm and the depth of the pipe trough exceeded 0.2 mm. . Since the unevenness value exceeded 2.0 mm, it is considered that when the material to be rolled comes into contact with the PTA build-up layer, stress is concentrated on the convex portion of the PTA build-up layer and pipe wrinkles are generated. Further, due to the occurrence of stress concentration, image sticking occurred in the disk rolls D14 and E14.

  In the disk rolls D5, D6, E5, and E6 in which the concavo-convex value Δh = 0 was obtained by surface grinding, no pipe wrinkles occurred. However, in the disk rolls D5 and E5, seizure occurred in the PTA overlay layer after rolling. When the unevenness value Δh = 0, it is considered that the lubricant applied in the initial stage of use before the oxide film was peeled off and seizure occurred.

  While the embodiments of the present invention have been described above, the above-described embodiments are merely examples for carrying out the present invention. Therefore, the present invention is not limited to the above-described embodiment, and can be implemented by appropriately modifying the above-described embodiment without departing from the spirit thereof.

It is the schematic for demonstrating the definition of the unevenness | corrugation value (DELTA) h which shows the unevenness | corrugation of the PTA overlaying layer formed in the surface of the disc roll of this invention. It is the schematic of the gauge for measuring unevenness | corrugation value (DELTA) h. It is the schematic for demonstrating the bead shape after PTA overlay welding. It is the schematic for demonstrating the definition of the angle (alpha) which is the conditions of PTA build-up welding in the disc roll of this invention. It is the schematic for demonstrating PTA overlay welding. It is a perspective view which shows the structure of the disk roll by embodiment of this invention. It is a side view of the PTA build-up welding apparatus for manufacturing the disk roll by embodiment of this invention. It is a front view of the PTA overlay welding apparatus shown in FIG. It is a figure which shows the structure of the annular member 30 periphery in the PTA build-up welding apparatus shown in FIG. It is a figure which shows an example of operation | movement of the PTA build-up welding apparatus shown in FIG. It is the schematic for demonstrating operation | movement of the PTA torch during PTA build-up welding. It is the schematic for demonstrating operation | movement of the PTA torch after FIG. It is the schematic for demonstrating the penetration depth. It is the schematic for demonstrating the investigation method of the surface crack of the disc roll after PTA build-up welding. It is the schematic which shows the structure at the time of seeing the conventional piercing rolling machine from the upper surface. It is the schematic which shows the structure at the time of seeing the piercing-rolling machine shown in FIG. 15 from the side surface. It is the schematic which shows the structure at the time of seeing from the front of the piercing-rolling machine shown in FIG.

Explanation of symbols

2,50 Disc roll 10 PTA build-up welding device 11 Ring fixing jig 12 Positioner 14 PTA stand 15 Body 16 Burner 20 Insulating sheet 30 Ring member 142 PTA torch

Claims (3)

A disk roll having a build-up layer formed on the surface by a plasma powder build-up welding method,
The overlay layer is
A carbide mainly composed of NbC having a volume percentage of 30% to 60% and a particle size of 50 μm or more;
Containing a matrix alloy composed of a Co-based alloy or a Ni-based alloy ,
The disc roll according to claim 1, wherein the unevenness of the surface of the build-up layer is 0.1 to 2 mm . A method for producing a disk roll having a build-up layer on the surface,
Preheating the disk roll to 250-350 ° C .;
Forming the build-up layer on the surface of the disc roll by a plasma powder build-up welding method while maintaining the preheated temperature,
The step of forming the overlay layer includes
It is characterized by using a carbide mainly composed of NbC having a volume percentage of 30% to 70% and a particle size of 50 to 250 μm and a matrix alloy made of a Co-base alloy or a Ni-base alloy as a raw material for the PTA build-up layer. A method for manufacturing a disc roll. It is a manufacturing method of the disk roll according to claim 2 ,
The step of forming the build-up layer is a disk roll manufacturing method characterized in that the oscillate width W1 and the bead transition width W2 in the plasma powder build-up welding method satisfy the following expressions (1) and (2).
W1 × tanα ≦ 1.0mm (1)
0.8 ≦ W2 / W1 ≦ 1.5 (2)
Here, α is an angle formed by a tangent line passing through the intersection of the vertical bisector of the oscillating width W1 and the surface of the disk roll and a horizontal line.

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