JP4265603B2 - Method for producing Fe-Cr alloy billet for seamless steel pipe - Google Patents

Description

The present invention relates to a method of manufacturing a steel billet (simply "Fe-Cr alloy" hereinafter in this specification) iron-base alloys containing Cr 5 to 17%, more particularly, Mu seam produced by slabbing a method for producing a significantly reduced can seamless steel pipe for Fe-Cr alloy billet surface care before pipe producing steel pipe for billet.

  In recent years, the demand for steel pipes made of Fe-Cr alloys for oil wells and chemical industries has increased, and in order to efficiently produce them with high quality, production by a hot seamless steel pipe production method has increased. However, in the manufacture of a seamless steel pipe made of Fe—Cr alloy, surface defects such as scale wrinkles may occur on the outer surface of the obtained steel pipe.

  Such surface defects are often caused by scale defects on the billet surface before pipe making. That is, a scale is left behind due to a descale defect in the billet manufacturing process, and a scale defect is generated when the scale is pushed in or rolled in, and a surface defect occurs when the pipe is formed while remaining on the billet surface.

  For this reason, although the descaling method in the billet manufacturing process has been improved, it is difficult to reliably eliminate the remaining scale at present. Therefore, in order to prevent surface defects occurring in the steel pipe after hot pipe making, most billets are subjected to surface inspection before pipe making, and surface care is performed based on the result.

  Usually, the billet used for manufacture of the seamless steel pipe of an Fe-Cr alloy is manufactured by block rolling using the steel piece made from the alloy as a raw material, as shown in FIGS. The steel slab is heated to about 1200 ° C., and then processed by split rolling with a box-shaped or hole-shaped roll. At this time, the roll is a multi-stage roll, and is gradually reduced to finish the billet shape while reducing the material diameter.

  In batch rolling, high-pressure water descaling is performed to remove the scale generated in the steel slab by heating, but the descaling failure often occurs, and the remaining scale is pushed or entangled on the steel slab surface, Scale defects on the billet surface.

  In order to reduce scale defects, the descaling capacity has been enhanced, for example, the flow rate of the descaling water and the injection pressure have been increased. However, the base material temperature decreases with the descaling, which may cause problems in billet manufacturing itself. For this reason, there is a restriction on the enhancement of the descaling ability. For these reasons, it is difficult to eliminate the remaining scale of the billet surface with certainty.

  In order to cope with the above-mentioned problems, various countermeasures have been proposed for heating equipment. In Patent Document 1, when continuously heating steel pieces such as slabs and billets with a combustion burner, generation of oxidized scale during heating is suppressed, and the steel pieces are oxidized after heating to produce a scale having good peelability. A continuous heating method for removing surface defects has been proposed. However, if the proposed method is to be implemented, the continuous heating furnace needs to be significantly improved and modified.

  Furthermore, Patent Document 2 discloses a method of improving the scale peelability by applying SiC before imparting rolling and imparting oxidizing properties. However, according to the method disclosed here, a coating facility is required for SiC coating, and the coating is also performed offline, so that the production efficiency is lowered.

  Therefore, any of the countermeasures proposed in Patent Document 1 and Patent Document 2 cannot be applied to actual operation as it is, and it is difficult to completely descale from the viewpoint of capability. For this reason, after billet manufacture, the surface maintenance before pipe making has not been omitted.

  As a method for cleaning the billet before pipe production, a method is commonly used in which a buttock is detected by ultrasonic flaw detection or the like and the corresponding portion is cut off with a grinder or a peeler. However, since the location and frequency of wrinkles differ from billet to billet, automation is difficult and is usually an off-line operation. For this reason, the production of seamless steel pipes using the billet has low production efficiency and the work environment for billet care is also poor.

  When automating billet care, all billet surfaces may be uniformly ground and removed regardless of the location and rate of wrinkles and, in some cases, the presence or absence of wrinkles. In this case, the billet yield is significantly deteriorated.

  For example, in Patent Document 3, after marking a steel piece, the image data of the steel piece is obtained and the image data of the steel piece is obtained from this image data. A surface wrinkle processing method for extracting surface wrinkle data is proposed. However, the surface flaw treatment method proposed here has a problem that it is not suitable as a billet care method because it requires a large amount of equipment and cost.

  As described above, various proposals have been made to prevent scale defects occurring on the surface of a billet for the production of seamless steel pipes, but it is difficult to completely descale the surface after the billet is manufactured. Care has not been omitted.

  In addition, when the surface of the billet is cleaned, the work is usually performed offline, resulting in low production efficiency and poor working environment. Even in the case of maintenance automation, a reduction in yield and a large equipment cost are required.

  For this reason, it is desired to develop a manufacturing method that can omit or reduce the surface treatment of the billet, particularly a manufacturing method that can significantly reduce the surface maintenance after billet rolling of a billet for manufacturing a seamless steel pipe of an Fe-Cr alloy. Has been.

JP 7-258740 A JP-A-57-2831 JP-A-10-277912

  The present invention has been made in response to the above-mentioned problems of the prior art and the demands for development of manufacturing methods. When manufacturing billets for seamless steel pipes from steel pieces of Fe-Cr alloy by split rolling, An object of the present invention is to provide an Fe—Cr alloy billet and a method of manufacturing the billet that can greatly reduce the care before the tube.

  In view of the fact that the conventional scale removal methods that have been used or proposed cannot completely eliminate the scale defects generated on the billet surface, the present inventors have not actively removed the scale, The idea was to suppress surface defects by coating the billet surface with a scale.

  In order to embody this idea for the Fe—Cr alloy billet, a detailed study was conducted on the ingot rolling of the steel pieces employed in the production process of the Fe—Cr alloy billet.

  FIG. 1 is a diagram for explaining a steel piece ingot rolling process in a billet manufacturing process and a change state of a steel piece cross section accompanying the process. The figure (a) shows the steel slab cross section before the partial rolling, the same (b) shows the steel slab cross section during the partial rolling, and (c) shows the billet cross section after the partial rolling. The lump rolling is performed in two stands, the first stand and the second stand, and the first stand uses a hole-type roll, for example, a box-type roll, and the second stand performs a lever roll using a hole-shaped roll.

  The steel slab 1 used for the block rolling is heated to about 1200 ° C. and then gradually reduced by the first stand for each squeezed surface, and processed into a steel slab 1 having a rectangular cross section as shown in FIG. Is done. Next, the steel piece 1 having a rectangular cross section is inserted into the second stand, and gradually rolled so that the cross sectional area of the steel piece becomes small, and as shown in FIG. Finished.

  FIG. 2 is a diagram for explaining in detail the situation in which the cross-sectional shape of the steel slab changes in the ingot rolling process for billet manufacture. In the block rolling process shown in FIG. 2, the cross-sectional area of the steel slab 1 is gradually reduced, and the final billet 2 is finished by rolling for 10 passes. In the rolling process, the steel slab 1 before split rolling is arranged in the longitudinal direction (corresponding to FIG. 1 (a)), subjected to 7 passes of rolling in the first stand, and processed into a steel slab 1 having a rectangular cross section. (Corresponding to FIG. 1B). Next, the steel pieces having a rectangular cross section are rolled by 8 passes to 10 passes at the second stand, and finished to the final billet 2 (corresponding to FIG. 1C).

  In the page shown in FIG. Each pass of 1, 2, 4, 6, 8, and 10 is rolling from the up and down direction. Each pass of 3, 5, 7 and 9 indicates rolling from the left and right directions. In actual operational rolling, the steel slab is turned over to switch the rolling direction.

  The steel slab 1 shown in FIG. 1 (a) is divided into a high-pressure reduction rate surface 3 and a low-pressure reduction rate surface 4. The high-pressure reduction rate surface 3 shows a surface with a high reduction rate in the above-described ingot rolling. The low-pressure lower rate surface 4 indicates the other surface. In normal segment rolling, as shown in FIG. 2, the steel slab before the segment rolling is arranged in the longitudinal direction, so the high-pressure lowering surface 3 becomes a short side surface in the slab-shaped steel slab, The rate surface 4 is a long side surface.

  However, the slab 1 is squeezed by the first stand by the first stand by the partial rolling process shown in FIGS. 1 (a) to 1 (c) and FIG. 2, and further rolled by the second stand to finish the billet 2. In this case, in the steel piece 1 occupying the outer surface of the billet 2, the area ratio of the portion that is the high-pressure lower-rate surface 3 and the portion that is the low-pressure lower-rate surface 4 is the same.

  That is, the billet 2 cross section after the partial rolling shown in FIG. 1 (c) has a high-pressure lowering surface 3 ′ (the portion of the steel piece 1 that was the high-pressure lowering surface) and a low-pressure lowering surface 4 ′ (the steel piece 1). The portion of the low-pressure lower-rate surface) is divided into four equal parts, and the central angle θ (the angle occupied by the surface portion of the billet 2) of the high-pressure lower-rate surface 3 ′ shown in FIG.

  FIG. 3 is a perspective view showing the entire structure of the billet after ingot rolling. In the rolling using the perforated roll in the first stand, the center portion of the low-pressure reduction rate surface 4 is not directly restrained by the reduction roller, or even if restrained, it is a little compared with other portions. . Therefore, as shown in FIG. 3, wrinkles 5 are generated in the billet length direction in the billet 2 after the block rolling.

  Examples of the hole-type roll used for the block rolling include a box-type roll, a diamond-type roll, and an oval-type roll. The box-type roll is effective for preventing the steel piece from collapsing. For this reason, many box rolls are adopted in consideration of the stability of the partial rolling.

  Therefore, based on the wrinkle 5 of the billet 2 after the block rolling, the high-pressure lowering ratio surface 3 ′ has a center angle of ± 45 ° (centering on a plane h perpendicular to the wrinkle 5 with respect to the center of the billet 2). It can be specified as a range of θ / 2).

  Based on the above-mentioned recognition of the steel slab and billet under the high-pressure ratio, the manufacturing process of the Fe—Cr alloy billet was further studied in detail. As a result, the following findings (a) to (e) were obtained.

(A) In order to prevent scale defects generated on the surface of the Fe—Cr alloy billet, it is difficult to completely remove the scale generated in the steel slab before the block rolling.
(B) Abandoning the complete removal of the scale generated on the steel slab, the generation pattern of the scale that is difficult to be pushed or caught during the batch rolling was studied. As a result, it has been found that the scale formed in close contact with the steel slab with a large covering area is pushed in at the time of the ingot rolling and is not easily entangled.
(C) Specifically, in the billet manufacturing process, it is not necessary to carry out descaling using a high-pressure water descaler or the like.
(D) Furthermore, the first scale rolling (first reduction) in the partial rolling (first stand) is started from the high-pressure reduction surface of the steel slab, so that the generated scale is in close contact with the steel slab. Can be made.
(E) Moreover, by adjusting the heating conditions (atmosphere, heating temperature and holding time) of the steel slab, it is difficult to drop off during the block rolling, and the scale can be generated on the steel slab with a wider covering area.

The present invention has been completed on the basis of the above findings, and the gist of the following (1) and (2) is a method for producing an Fe—Cr alloy billet for seamless steel pipes.

(1 ) In the method for producing a billet for seamless steel pipes in which a billet is produced by rolling a billet, a billet is formed without generating a scale of 1000 μm or more in the billet, and then performing a descaling. To produce an Fe-Cr alloy billet for a seamless steel pipe, in which an area ratio of 70% or more of the high-pressure area is covered with a scale layer. Is the method.
( 2 ) In the method for producing an Fe—Cr alloy billet for seamless steel pipe according to ( 1 ) above, it is desirable to first reduce the high-pressure reduction surface of the steel slab. In addition, it is desirable to generate the scale by holding the steel piece in an atmosphere containing 2.5% by volume or more of water vapor at a heating temperature of 1200 ° C. or more for 2 hours or more.

  In the present invention, the “Fe—Cr alloy” is an iron-based alloy containing 5 to 17% of Cr, and can contain other alloy elements such as Ni and Mo as required.

  In the present invention, the “high-pressure reduction surface” means a surface having a high reduction rate when the steel slab is rolled into a billet shape, and the billet is a high-pressure reduction surface in the steel slab before rolling. Say part. Usually, in a slab-shaped steel slab, the high-pressure lower rate surface is a short side surface.

  As shown in FIG. 3, the “high-pressure lower-rate surface” in the billet is, with a wrinkle as a reference, a center angle of ± 45 ° (centering on a plane perpendicular to the wrinkle with respect to the center of the billet). It can be specified as a range of θ / 2). The billet cross-sectional macro observation result can be used to more accurately specify the “high-pressure lowering plane” in the billet.

  FIG. 4 is a diagram illustrating an example of a photograph observation result of a billet cross-section macro. As shown by the elliptical broken line, segregation having a correlation with the cross-sectional direction of the steel slab before the bulk rolling is observed at the center of the macro observation. That is, since the position where segregation occurs coincides with the final solidification position of the slab, this final solidification position depends on the cross-sectional shape formed by the long side surface 4 and the short side surface 3 of the slab.

  From the result of photographic observation of the cross-sectional macro shown in FIG. 4, the surface parallel to the elliptical broken line is the long side surface 4 and becomes the “low-pressure lowering rate surface”, and the surface orthogonal to the elliptical broken line is the short side surface 3 Surface ". Therefore, even after rolling into the billet, segregation that correlates with the cross-sectional direction of the steel slab before partial rolling remains. it can.

  As described above, the area ratio between the high-pressure lower rate surface and the low-pressure lower rate surface on the outer surface of the billet after manufacture becomes the same, and the billet cross section is divided into four equal parts by the high-pressure lower rate surface and the low-pressure lower rate surface. For this reason, the value of “area ratio of the high-pressure lower-rate surface” (ratio of scale area on the high-pressure lower-rate surface) defined in the present invention is “(billet) total area ratio” (billet total area) if 1/2 is applied. Can be replaced with the ratio of the scale area).

  That is, in the present invention, “the area ratio 70% or more of the high-pressure ratio surface” is “total area ratio 35% or more”, and “the area ratio 80% or more of the high-pressure ratio surface” is “total area ratio 40% or more”. , “The area ratio of 90% or more under the high-pressure ratio surface” can be defined in other words as “total area ratio of 45% or more”.

  The Fe—Cr alloy billet of the present invention is characterized in that its high-pressure low-rate surface is covered with a scale layer having an area ratio of 70% or more, 80% or more, or 90% or more. In other words, it is characterized by being covered with a scale layer having a total area ratio of 35% or more, 40% or more, or 45% or more.

  As shown in the examples described later, when the high-pressure lower-rate surface is covered with a scale layer having an area ratio of 70% or more, the maintenance rate can be reduced by about 50% as compared with the comparative example in which the descaling is performed.

  In the Fe—Cr alloy billet of the present invention, the care rate of the billet tends to be lower as the area ratio of the high-pressure lowering plane is higher. For example, when the high-pressure ratio surface is covered with a scale layer having an area ratio of 80% or more, the maintenance rate is about 30% of the comparative example, and similarly, the scale layer having an area ratio of 90% or more is covered. In this case, the maintenance rate is about 20% of the comparative example. Therefore, the area ratio of the high-pressure lower-rate surface covered by the scale correlates well with the defect occurrence rate on the billet surface.

  The production method of the present invention is characterized in that, in the bulk rolling of a steel slab, the descaling is not performed using a high-pressure water descaler or the like in order to remove the scale generated when the steel slab is heated. As described above, since the technology for completely removing the scale has not been established, the scale remains incompletely or non-uniformly, and scale wrinkles are prevented from being generated due to the pushing or entrainment of the scale. Because.

  In the production method of the present invention, it is not specified whether the ingot rolling is started from the high-pressure lower surface or the low-pressure lower surface of the steel slab, but it is desirable to start from the high-pressure lower surface of the steel slab. This is because the scale formed on the steel slab can be sufficiently pressure-bonded to the high-pressure lower-rate surface by rolling the first pass of the block rolling at the high-pressure lower-rate surface.

  Also, the scale is pressure-bonded to the high-pressure reduction ratio surface because if it is pushed in a state where the scale remains halfway on the surface where the reduction ratio is large, a scale wrinkle is likely to occur. In the present invention, when the scale is brought into close contact with an area ratio of 70% or more, it is difficult for the scale to be pushed into the steel slab base material in the subsequent batch rolling process. This tendency becomes more prominent as the area ratio covered by the scale increases.

  The production method of the present invention is characterized in that a scale having a thickness of 1000 μm or more is formed which is difficult to cause defects in the steel slab and is difficult to cause defects on the billet surface after production. This scale thickness can be obtained by adjusting the heating conditions (atmosphere, heating temperature and holding time) of the steel slab.

  5-7 is a figure which shows the relationship between the defect incidence rate of the Fe-Cr alloy billet surface when not descaling, and the scale thickness of a steel piece. As test materials, 5 to 17% Cr-containing alloys A, B and C shown in Table 1 were used. FIG. 5 shows the relationship with the test material A, FIG. 6 shows the relationship with the test material B, and FIG. The relationship by the sample material C is shown, respectively.

  Specific conditions include changing the holding time when specimens A, B, and C are heated to 1200 ° C. in an atmospheric heating furnace, and the scale thicknesses of the high-pressure and low-pressure ratio surfaces of the steel slab. The defect occurrence rate on the billet surface was measured when the thickness was changed. The reason for heating to 1200 ° C. in the atmospheric heating furnace is that the heating temperature is appropriate for reducing the deformation resistance in the partial rolling.

  In addition, the billet surface defect rate is measured by removing the scale of the billet surface by shot blasting, then detecting the surface defect by the leakage magnetic flux flaw detection method, and calculating the defect rate (number of defects / total number) It showed in.

  From the results shown in FIGS. 5 to 7, it can be seen that the defect occurrence rate decreases as the scale becomes thicker. When the scale thickness on the high-pressure lower-rate surface is 1000 μm, the defect occurrence rate is 35% or less, and when it is 1200 μm, the defect occurrence rate is 25% or less. As will be described in the examples described later, this result shows that the defect rate is halved and further reduced to about 1/3 compared to the comparative example in which the conventional method is reproduced.

  For this reason, in the present invention, the scale thickness of the steel slab needs to be 1000 μm or more and more preferably 1200 μm or more before the batch rolling.

  Although the detailed mechanism is not clear, in order to suppress the defect occurrence rate on the billet surface, the billet surface stretched by the partial rolling is covered with a scale layer having as large an area ratio as possible. It is predicted that it is effective to ensure this.

  FIG. 8 is a diagram showing the relationship between the scale thickness of the steel slab and the holding temperature when the amount of water vapor in the atmosphere of the heating furnace is changed. In the figure, the amount of water vapor contained in the atmospheric gas was changed to 0%, 2.5%, 10% and 20% by volume%.

Using 13% Cr-containing alloy B shown in Table 1 as a test material, 10% CO ₂ -5% O _2- Bal. Using N ₂ as the base of the atmospheric gas, the water vapor concentration contained in the atmospheric gas was varied in the range of 0 to 20%. At this time, the steel piece was heated to 1200 ° C. to change the holding time, and the scale thickness generated in the steel piece was measured.

  The scale thickness was measured by oxidizing the steel piece with a holding time of 1 to 6 hours, cutting out the test piece, processing it into a micro sample, and performing cross-sectional observation. The scale structure at this time is shown in Table 2.

  From the results shown in FIG. 8, heating for about 6 hours is required to obtain a scale of 1000 μm or more in an atmosphere not containing water vapor. This atmosphere not containing water vapor is almost equivalent to the air atmosphere.

  On the other hand, the oxidation rate can be remarkably increased by including 2.5% or more of water vapor in the atmosphere. In order to efficiently obtain a scale thickness of 1200 μm or more, the steel slab may be heated to 1200 ° C. and held for 2 hours or more in an atmosphere containing 2.5% or more of water vapor.

  As shown in Table 2, the scale structure is a two-layer structure of an outer layer scale and an inner layer scale. In the present invention, the outer layer scale is a scale generated outside the original steel slab surface, and the inner layer scale is a scale generated inside the original steel slab surface.

As for the scale formed in the atmosphere containing 2.5% or more of water vapor, the outer scale consists of Fe ₂ O ₃ , Fe ₃ O ₄ and FeO, and the inner scale consists of FeCr ₂ O ₄ and FeO. In contrast, the scale formed in an atmosphere containing no water vapor, the outer layer scale composed of Fe ₂ O ₃ and Fe ₃ O _4, the inner layer scale consists of FeCr ₂ O ₄ and Fe ₃ O _4.

  The scale structure may be any of the above-mentioned forms, but a scale structure in which scale defects are less likely to occur is preferably one in which FeO is present. This is because FeO itself has a high deformability, so that it is difficult to cause breakage such as cracking even when subjected to a large reduction, and indentation flaws are unlikely to occur because the high-temperature hardness is lower than steel.

For example, Fe ₂ O ₃ has almost no deformation performance, and Fe ₃ O ₄ produces elongation when heated at a temperature of 800 ° C. or higher and is subjected to tensile deformation at a very low speed in the laboratory. In this case, it cannot be handled, and cracks will be generated and peeled off. On the other hand, FeO deforms following the deformation speed during rolling and does not cause cracks.

In the case where FeO is present, the thickness in the outer layer scale when the cross-sectional micro observation is performed is preferably 30% or more. The thickness of FeO can be measured by mapping the color tone by cross-sectional microscopic observation, mapping of O ₂ (oxygen) by EPMA, and identifying the structure of the entire scale from X-ray diffraction in advance.

  Further, when the water vapor concentration exceeds 20%, the effects of increasing the scale generation rate and increasing the FeO ratio are gradually saturated. For this reason, it is desirable that the upper limit of the water vapor concentration be about 25% in consideration of damage to the furnace wall of the heating furnace.

  In the present invention, in order to ensure a scale thickness of the steel slab of 1000 μm or more, it is desirable that the heating temperature of the steel slab is 1200 ° C. or more. Moreover, it is desirable that the heating temperature is 1200 ° C. or higher from the viewpoint of not only scale generation but also workability during partial rolling. On the other hand, it is desirable that the upper limit of the heating temperature be 1300 ° C. or lower in consideration of equipment damage and the like.

  In the present invention, in order to secure a scale thickness of the steel slab of 1000 μm or more, it is desirable that the holding time is 2 hours or more when the heating temperature of the steel slab is 1200 ° C. or more.

The effect which the manufacturing method of the Fe-Cr alloy billet prescribed by the present invention exhibits is explained based on concrete (Example 1) and (Example 2). The test materials were 5 to 17% Cr-containing alloys A, B, and C shown in Table 1 above, and a bloom CC material having a short side of 280 mm, a long side of 600 mm, and a length of 7400 mm was used as a steel piece material. This steel piece was heated at 1200 ° C. for 6 hours in an atmospheric heating furnace (not including water vapor). Furthermore, after the steel slab was heated, the production was performed under two conditions, with or without the descaling using a high-pressure water descaler with a pressure of 100 kg / cm ² .

  The piece rolling of the steel slab was performed by the first and second two stands, and each of them was subjected to lever rolling. The first pass rolling at the first stand was classified according to whether the reduction of the high-pressure reduction surface was performed or the reduction of the low-pressure reduction surface. Thereafter, the first stand was crushed to a cross-sectional shape having a short side of 250 mm and a long side of 400 mm, and then finished into a billet having a final diameter of 225φ using a second stand.

  After the billet was manufactured, the surface scale was removed by shot blasting, and wrinkle inspection was performed with an NDI flaw detector using a leakage magnetic flux flaw detection method. Here, the target was a ridge having a depth of 0.5 mm or more. This is because a flaw with a defect depth of 0.5 mm or more becomes a surface flaw of a steel pipe when it is pipe-rolled without maintenance, so that surface care is required. Although no standard was set for the defect length, a defect having a length of, for example, several tens of millimeters was also considered in consideration of being extended to the final product.

  The defect generation rate was evaluated by the number ratio of (number of defects generated / total number). Finally, the area ratio of the scale covering the billet surface was investigated. The area ratio of the scale is measured by taking a sample for cross-sectional observation every 1 m from the billet from the high-pressure lower surface, measuring the scale peeling length by micro observation, {(average scale peeling length in the vertical direction x horizontal It was evaluated by the area ratio of the average scale peeling length in the direction) / total area}. As the area ratio of the scale, the average value of the area ratios of all the samples in each billet was used.

  Table 3 to Table 5 show the defect occurrence rate and the scale area ratio covering the high-pressure lower rate surface of the billet. Table 3 shows the results of using 5% Cr-containing alloy A as a test material, Table 4 shows results of using 13% Cr-containing alloy B as a test material, and Table 5 shows results of using 17% Cr-containing alloy C. The results for the test material are shown.

In (Example 1), in any of the test materials, the scale thickness formed on the steel slab immediately after the heating furnace is about 1000 μm, and the scale structure has Fe ₂ O ₃ and Fe outer scales. ₃ O ₄ and the inner layer scale was FeCr ₂ O ₄ and Fe ₃ O ₄ . The thickness of the scale covering the billet surface immediately after production was 150 μm or more.

  As shown in Tables 3 to 5, when descale is performed by split rolling as a comparative example, the coating of the scale is 45 to 50% in terms of the area ratio of the high-pressure lowering surface (22.5 to 25 in total area) The defect occurrence rate was almost close to the total number, and surface care was required at a number ratio of 92 to 98%.

  On the other hand, in the example of the present invention, in the first pass, the rolling process of the low pressure area was carried out, the scale coating was 70 to 73% in terms of the area ratio of the high pressure area and the total area ratio was 35 to 36.5. %), And the defect occurrence rate was 44 to 47%, half that of the comparative example. Further, in the example of the present invention, in the first pass, rolling of the high pressure lowering surface was performed, and the scale coating was 80 to 83% in terms of the area ratio of the high pressure lowering surface (40 to 41.5% in the total area ratio). At the same time, the defect occurrence rate was about 1/3 of the comparative example, and was reduced to 32 to 35%.

  From the results shown in Tables 3 to 5, when the scale coating is about 70% (35% in total area ratio) in terms of the area ratio of the high-pressure area, the defect occurrence rate is about 50 compared to the comparative example in which the descaling was performed. If the scale coating is about 80% in terms of the area ratio of the low-pressure surface (40% in total area ratio), the defect rate can be reduced to about 1/3 compared to the comparative example. .

  Although the detailed mechanism is unclear, it is presumed that the occurrence of non-uniform scale that causes indentation and entrainment can be suppressed by bringing the scale into close contact with a certain area ratio close to the entire surface. Is done.

  Using the test material and billet material under the same conditions as in Example 1, the obtained billet was heated in a heating furnace. At this time, a steam addition apparatus was connected to the atmospheric furnace, and heating was performed at 1200 ° C. for 6 hours while changing the furnace atmosphere.

  The conditions of the ingot rolling after heating, the defect occurrence rate after billet manufacture, and the measurement conditions of the area ratio covered by the scale are the same as in (Example 1), and the influence of the heating atmosphere on the defect occurrence rate of the billet investigated. The survey results are shown in Tables 6-8.

  Of the survey results, Table 6 shows a case where 5% Cr-containing alloy A was used as a test material, Table 7 shows a case where 13% Cr-containing alloy B was used as a test material, and Table 8 shows 17%. The case where Cr-containing alloy C is used as a test material is shown. In any of the test materials used in Example 2, the thickness of the scale covering the billet surface was 150 μm or more.

  As shown in Tables 6 to 8, in the example of the present invention, as the water vapor concentration in the atmosphere increases, the area ratio of the high-pressure lower surface area covered by the scale increases, and at the same time, the billet defect occurrence rate increases. It turns out that it falls. This is because when the water vapor content is increased, a thick scale is formed in the steel slab, and at the same time, more FeO that is difficult to be pushed into the base material during partial rolling is generated.

  Among the examples of the present invention using each test material, the test No. As shown in A8-9, B8-9, and C8-9, the scale is obtained by holding the steel slab before the batch rolling for 2 hours or more at a heating temperature of 1200 ° C. or more in an atmosphere containing water vapor having a concentration of 10% or more. As a result of the generation, the area ratio of the high-pressure lower surface area covered by the scale can be further increased to 93% or more, and at the same time the billet defect occurrence rate can be reduced to 22% or less.

  According to the method for producing an Fe—Cr alloy billet according to the present invention, the high-pressure low-rate surface of a steel slab is covered with a scale layer having a large area ratio and subjected to ingot rolling, so scale intrusion and entrainment can be reduced. Thereby, when manufacturing the billet for seamless steel pipes from the steel piece of Fe-Cr alloy, the surface care before pipe making can be reduced significantly.

  Therefore, if this Fe-Cr alloy billet is adopted for seamless steel pipe production, even a relatively difficult-to-machine Fe-Cr alloy steel pipe can be produced at low cost and efficiently. It can be widely applied in the manufacturing field of hot seamless steel pipe.

It is a figure explaining the change situation of the billet rolling process and the billet cross section accompanying it in the manufacturing process of a billet. It is a figure explaining in detail the situation where the section shape of the steel slab changes in the ingot rolling process of billet manufacture. It is a perspective view which shows the whole billet structure after partial rolling. It is a figure which shows an example of the photograph observation result of a billet cross section macro. It is a figure which shows the relationship between the defect incidence rate of the billet surface using the test material A, and the scale thickness of a steel slab when not descaling. Similarly, it is a figure which shows the relationship between the defect incidence rate of the billet surface using the test material B, and the scale thickness of a steel piece. Similarly, it is a figure which shows the relationship between the defect incidence of the billet surface using the test material C, and the scale thickness of a steel piece. It is a figure which shows the relationship between the scale thickness of a steel slab at the time of changing the water vapor | steam amount in the atmosphere of a heating furnace, and holding temperature.

Claims (3)

In the method for producing a billet for seamless steel pipes in which a billet is produced by split-rolling a steel slab, a scale having a thickness of 1000 μm or more is generated on the steel slab, and then the steel-slab is rolled without performing descaling. To produce an Fe—Cr alloy billet for a seamless steel pipe, in which an area ratio of 70% or more of the high-pressure lower-rate surface is covered with a scale layer . In the method for producing an Fe-Cr alloy billet for seamless steel pipes, in which a billet is produced by split-rolling a billet, a scale with a thickness of 1000 μm or more is generated on the billet, and then the steel piece is subjected to high pressure without descale. Fe-Cr alloy billet for seamless steel pipe, in which an area ratio of 70% or more of the high- pressure lowering face is covered with a scale layer by first rolling the rate face, Fe- for seamless steel pipe, Manufacturing method of Cr alloy billet. The billet manufacturing method according to claim 1 or 2 , wherein the scale is generated by holding the steel slab for 2 hours or more at a heating temperature of 1200 ° C or more in an atmosphere containing 2.5% by volume or more of water vapor. The manufacturing method of the Fe-Cr alloy billet for seamless steel pipes.

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