JP2008188637A - Method of manufacturing steel bar excellent in forgeability - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method of manufacturing a steel bar excellent in forgeability while preventing occurrence of crack during forging. <P>SOLUTION: When the steel bar obtained by hot-rolling a billet is inspected by combining the surface inspection and the inspection with an ultrasonic flaw detecting device, flaws caused by inclusions which are present in the part under the surface of the steel bar and press-bonding-like flaws are detected by angle beam flaw inspection method of the ultrasonic flaw detecting device. When these flaws are detected, the steel bar with these flaws is discarded, so that the steel bar with less flaws (defects) is obtained. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a method for manufacturing a steel bar for forging used in the manufacture of machine structural steel parts, and more particularly to a method for manufacturing a steel bar for forging that does not generate cracks during forging.

  Machine structural parts such as constant velocity joints are manufactured by hot forging from hot-rolled steel bars (rolled material). Recently, gears used in automobile differentials are manufactured by cold forging (or by performing cold forging after hot forging) from the viewpoint of improving yield. However, if there are defects on the surface or inside of the steel bar (rolled material) after hot rolling, cracks are likely to occur during forging, and therefore there is a need for a bar steel (rolled material) free from defects.

  The steel bars are inspected both at the slab stage (material stage) before hot rolling (ie before being processed into steel bars) and at the steel bar stage after hot rolling (ie after being processed into steel bars) When only surface flaws found by these inspections are removed and internal flaws (internal defects) are found, the steel pieces or steel bars are discarded.

  For surface wrinkles, for example, a relatively large wrinkle is detected by automatic inspection with an automatic inspection device using a magnetic particle inspection method, and after this is automatically removed by grinding or the like, it is visually checked again using a magnetic particle inspection method. A small wrinkle is found and removed at (see, for example, Patent Documents 1 and 2).

  On the other hand, internal flaws (internal defects) are generally detected using an ultrasonic flaw detector, and the vertical flaw detection method is used for internal flaws present at the center and middle of a steel piece. An oblique flaw detection method is used for wrinkles caused by inclusions present in the subcutaneous part of a steel piece (see, for example, Patent Document 3).

  Furthermore, in order to detect the surface defects of the steel bar after hot rolling with high efficiency and high accuracy, an inspection method using a leakage magnetic flux flaw detection method has been proposed and is in practical use (for example, see Patent Document 4).

  And, by using a combination of the above-mentioned flaw detection methods as appropriate, it is possible to efficiently detect and remove the flaws existing on the surface and inside at both the steel slab (material) stage and the bar steel stage as accurately as possible. Has been done.

As described above, the ultrasonic oblique angle flaw detection method is a very effective means for detecting wrinkles caused by inclusions existing in the subcutaneous part of a steel piece, and this method should be used in combination with other flaw detection methods. However, in recent years, the rapid progress of steel cleaning technology in the steelmaking and casting stages, which is the previous stage of slab processing, Inclusions have been reduced in size, and defects (wrinkles) resulting from the inclusions have been greatly reduced. Nevertheless, there is a certain percentage of steel bars that are cracked when forging for manufacturing parts, and further improvements in the quality of the steel bars have been demanded.
JP 2003-136135 A JP 2005-138168 A JP 59-148860 A JP 2006-159260 A

  Therefore, an object of the present invention is to provide a method for producing a steel bar having excellent forgeability in which cracking does not occur during forging.

  The present inventors consider that there are wrinkles that are not sufficiently detected by conventional ultrasonic flaws among the wrinkles present inside the steel bar, and first, by observing the cross section of the bar steel visually and with a microscope, Investigated the state of internal soot generation. As a result, a large number of crimped wrinkles (hereinafter referred to as “crimped wrinkles”) different from wrinkles caused by inclusions as shown in FIG. . This crimped wrinkle is assumed to be formed in a linear shape in the subcutaneous part, with the thermal stress cracks generated during continuous casting of the slab (bloom) being crimped during the partial rolling. Since this crimped scissors are not open on the surface of the steel slab and steel bar, it cannot be detected by magnetic particle flaw detection. Also, in ultrasonic flaw detection, the occurrence location is in the subcutaneous part. Although it should be detected by the oblique flaw detection method, in the past, an echo height threshold was set focusing on wrinkles caused by inclusions, and an appropriate threshold for detecting crimped wrinkles. It is thought that it was not.

  Therefore, the inventors have appropriately set not only the wrinkles caused by inclusions but also crimped wrinkles by appropriately setting the echo height threshold value in the ultrasonic oblique flaw detection method, and are caused by the inclusions. When a wrinkle or a crimped wrinkle (hereinafter collectively referred to as “crimped wrinkle” or the like) is detected, it is possible to more reliably prevent the wrinkle from remaining in the steel bar by discarding the steel bar. We have considered and further studied, and have completed the following invention.

  The invention described in claim 1 includes (1) a bundling process in which a slab is rolled into a steel slab, and (2) a steel slab inspection process for inspecting and scraping the slab that has been subjected to the batch rolling. And (3) a steel bar rolling process for hot rolling the steel pieces after cutting and processing into steel bars, and (4) a steel bar inspection process for inspecting the steel bars after hot rolling and scooping. A method of manufacturing a wire, wherein the steel slab inspection step of (2) includes (2A) a steel slab descaling step of removing the oxide scale on the surface of the slab after being rolled into pieces by shot blasting, and (2B ) The steel slab surface automatic inspection process that automatically inspects the steel slab after the scale removal with the automatic surface inspection device, and (2C) The steel slab internal automatic that automatically inspects the flaw inside the steel slab with the ultrasonic flaw detector. In the inspection process and (2D) the surface flaws detected in the steel piece surface automatic inspection process of (2B) above to the automatic scraping device A steel strip automatic scraping process for performing automatic scraping, and the steel bar inspection process of (4) above includes (4A) a steel bar straightening process for correcting the bending of hot-rolled steel bars, and (4B) bending. Steel bar surface inspection process in which the corrected steel bar is inspected with a leakage magnetic flux flaw detector, and (4C) manual steel bar scraping in which the surface defects detected in the steel bar surface inspection process in (4B) above are manually removed. And (4D) a steel bar internal automatic inspection process for automatically inspecting a bar in the steel bar with an ultrasonic flaw detector different from the ultrasonic flaw detector used in the steel piece internal automatic inspection process of (2C) above, Furthermore, in the steel bar internal automatic inspection process of (4D) above, wrinkles caused by inclusions existing in the subcutaneous part of the steel bar by an ultrasonic flaw detection method using an ultrasonic flaw detector and a crimped wrinkle (hereinafter referred to as a bar) , Collectively referred to as “crimp-shaped scissors” .) Detects, when the crimping form scratches or the like is detected is an excellent method for producing a steel bar forgeability, characterized by discarding the steel bars.

  The invention according to claim 2 further includes a crimped scissors or the like present in the subcutaneous part of the steel piece by an ultrasonic oblique flaw detection method using an ultrasonic flaw detector in the steel piece internal automatic inspection step of (2C) above. 2. The detection according to claim 1, wherein, when a crimped wrinkle or the like is detected, the steel piece is discarded, or the crimped wrinkle or the like is removed together with the surface flaw in the steel piece automatic scraping process of (2D). This is a method of manufacturing a steel bar having excellent forgeability.

  The invention according to claim 3 is the manufacturing of the steel bar with excellent forgeability according to claim 1 or 2, wherein, in the ultrasonic oblique flaw detection method, the one having an echo height of 15 dB or more is used as the crimped barb or the like. Is the method.

  The invention according to claim 4 is the steel bar of (4C) above (4C) after the steel bar manual scraping process of (4C) and before the steel bar internal automatic inspection process of (4D). A bar steel scooping part inspection / scraping process is provided, in which the part that has been scraped in the manual scraping process is visually inspected and if residual scrap is detected, the remaining scrap is manually scraped. It is a manufacturing method of the steel bar excellent in forgeability of any one of Claims 1-3.

  The invention according to claim 5 is the above-described (2) billet inspection step, after the (2D) billet automatic scraping step, (2E) the surface of the billet is visually inspected by a magnetic particle flaw detector, The forgeability according to any one of claims 1 to 4, wherein when a flaw is detected, a steel piece surface visual inspection / manual fraying step for manually removing the surface flaw is performed. It is an excellent method for producing steel bars.

  According to the manufacturing method of the present invention, not only wrinkles caused by inclusions existing in the subcutaneous portion of the steel bar but also crimped wrinkles are detected with high sensitivity by ultrasonic oblique flaw detection. By disposing of the steel bars, steel bars with fewer defects (defects) can be reliably obtained. As a result, the occurrence of cracks in the steel bar during forging is further suppressed, and a steel bar having excellent forgeability can be provided.

Embodiment
FIG. 1 is a flow diagram showing steps from manufacturing a steel bar through a steel piece to a steel bar according to an embodiment of the present invention. Hereinafter, each process is demonstrated in detail along the flow.

〔Manufacturing process〕
(1) Bundling process A slab (bloom or ingot) cast from a component-adjusted molten steel with a continuous casting machine or ingot casting mold is rolled into a shape that can be used in the next process to produce a billet. To do.

(2) Billet inspection step Next, the billet (billet) that has been rolled in pieces is inspected and scraped (care) in the following steps (2A) to (2D). In FIG. 1, the notation of “steel XX process” in each of the following steps (2A) to (2D) is simply abbreviated as “XX process”.

(2A) Steel slab descaling step First, in order to make it easy to detect surface wrinkles, the surface oxidized scale is removed by shot blasting for the steel slab that has been rolled in pieces.

(2B) Steel slab surface automatic inspection process And the steel slab after scale removal is automatically inspected with a surface automatic inspection device. Specifically, the steel piece after scale removal is magnetized, and the surface is sprinkled with fluorescent magnetic powder, and the magnetic powder pattern on the surface of the steel piece is photographed with a camera and analyzed for images, resulting in grain boundary cracking during casting. For example, surface flaws having a depth of 0.3 mm or more and a length of 50 mm or more can be automatically detected, such as the generated flaws and the bald wrinkles generated during the batch rolling.

(2C) Steel piece inside automatic inspection process Subsequently, the flaw inside the steel piece is automatically inspected by an ultrasonic flaw detector. Specifically, the ultrasonic flaw detection apparatus uses the vertical flaw detection method to adjust the threshold of the echo height so as to detect harmful wrinkles of, for example, 500 μm or more. If harmful flaws are detected inside the billet, the billet is discarded.

(2D) Steel piece automatic picking process The surface picking detected in the steel piece surface automatic inspection process of the above (2B) is automatically picked up by an automatic picking device. A milling cutter or a grinder may be used as the automatic harvesting device.

(3) Steel bar rolling step The steel piece after being scraped as described above is hot-rolled and processed into a steel bar. Specifically, after the steel slab is heated to a predetermined temperature in a heating furnace, the steel slab is processed into a target steel bar by sequentially reducing and reducing the surface to a predetermined diameter in a plurality of rolling mill rows.

(4) Steel Bar Inspection Step Next, the hot-rolled steel bar is inspected and scraped (care) in the following steps (4A) to (4D). In FIG. 1, the notation of “steel bar XX process” in each of the following steps (4A) to (4D) is simply abbreviated as “XX process”.

(4A) Steel bar straightening process The curvature of the hot rolled steel bar is corrected to ensure a predetermined dimensional accuracy, and the surface scale of the steel bar in the subsequent inspection is removed by using the fact that the surface scale of the steel bar is peeled off by the correction. Discovery can be facilitated.

(4B) Steel Bar Surface Inspection Step The steel bar whose curvature has been corrected and whose surface scale has been removed is inspected for the presence of surface defects using a leakage magnetic flux flaw detector (see, for example, paragraph [0004] of Patent Document 4 above).

(4C) Steel bar manual scraping process The surface crack detected in the steel bar surface inspection process of (4B) above is manually scraped, for example, by chipping, grinder, hot scarfing or the like.

(4D) Bar steel internal automatic inspection process Subsequently, the bar in the bar steel is automatically inspected by an ultrasonic flaw detection apparatus different from the ultrasonic flaw detection apparatus used in the steel piece internal automatic inspection process of (2C). Specifically, the ultrasonic flaw detection apparatus in this process combines the vertical flaw detection method and the oblique flaw detection method, and inspects the central part and the middle part of the steel piece in the former and the subcutaneous part of the steel piece in the latter, respectively. By comprising in this way, the whole inside of a steel piece can be test | inspected without a leak. In the vertical flaw detection method, the echo height threshold is adjusted so that harmful flaws of, for example, 500 μm or more can be detected in the same manner as in the ultrasonic flaw detection device used in the steel piece internal automatic inspection step (2C). . On the other hand, in the oblique flaw detection method, the threshold of the echo height is set to 15 dB, for example, so that the crimped wrinkle can be detected with high sensitivity, and an echo of 15 dB or more may be used as the crimped wrinkle or the like (see an example described later, FIG. 3) . If crimped wrinkles etc. are detected, the steel bar will be disposed of.

  Since the steel bar manufactured in this way has sufficiently removed the surface defects and internal defects (including crimped defects), it can effectively prevent cracks from occurring during forging of parts. It is possible to manufacture a steel bar having excellent properties.

(Modification)
In the said embodiment, although the example which uses only a vertical flaw detection method was shown in the ultrasonic flaw detection apparatus in the steel piece inside automatic inspection process of said (2C), the ultrasonic wave in the said steel bar internal automatic inspection process of said (4D) Similarly to the flaw detection apparatus, the vertical flaw detection method and the oblique flaw detection method may be combined to inspect the center portion and the middle portion of the steel piece with the former and the subcutaneous portion of the steel piece with the latter. Thereby, the whole inside of a steel piece can be test | inspected more precisely. Also in the oblique angle flaw detection method in this step, the echo height threshold is set to 15 dB, for example, so that the crimped wrinkle can be detected with high sensitivity in the same manner as the inspection process in the steel bar stage of (4C) above. Is preferably a crimped ridge or the like. If a crimped wrinkle or the like is detected, the steel slab is disposed of in principle, but if it can be removed, the crimped wrinkle or the like is removed together with the surface flaw in the above-described (2D) steel slab automatic cutting process. It may be removed and included in the material for bar steel processing. As a result, the hot-rolled steel slab has been sufficiently removed not only the surface defects but also the internal defects (including the crimped defects) in advance, producing a steel bar with higher yield and superior forgeability. It will be possible.

  Moreover, in the said embodiment, although the example which performs the steel bar internal automatic inspection process of said (4D) immediately after the steel bar manual scraping process of said (4C) was shown, the steel bar manual scraping process of said (4C) is shown. After the above (4D) steel bar internal automatic inspection process, when the steel bar manual scraping process of (4C) above is visually inspected and residual scissors are detected May be provided with a steel bar scraping part inspection / scoring step [(4C-1) step] in which the residual scrap is manually scraped.

  In the steel piece inspection step (2) above, the surface of the steel piece is visually inspected by a magnetic particle flaw detector after the steel piece automatic scraping step (2D), and if surface flaws are detected, You may provide the steel piece surface visual inspection and manual scooping process [(2E) process] which scissors a scissors manually. That is, shallow and short surface defects that cannot be detected by the surface automatic inspection device in the steel piece surface automatic inspection process of (2B) above are detected by visually inspecting the surface of the steel piece using another magnetic particle flaw detector. Recommended. Specifically, in the step (2E), after magnetizing the steel slab, sprinkle fluorescent magnetic powder on the surface, visually observe the magnetic powder pattern in the dark room, and mark the detected surface defects, The marked surface defects may be manually removed, that is, chipped, grindered, hot scarfed, or the like.

[Component composition of steel bars]
The component composition of the steel bar to which the present invention is applied includes, for example, mass%, C: 1.5% or less (0% is not included), Si: 3.0% or less (0% is not included), Mn : Iron alloy consisting of 0.2 to 3.0%, P: 0.030% or less (excluding 0%), S: 0.06% or less (not including 0%), balance Fe and inevitable impurities Is recommended. The reasons for limiting the above components will be described below.

C: 1.5% or less (excluding 0%)
C is an element necessary for ensuring the hardness as a machine structural component, but if it is too much, the toughness decreases, so it is preferable to keep it to 1.5% or less.

Si: 3.0% or less (excluding 0%)
Si acts effectively as a deoxidizing element when steel is melted, and also acts effectively on wear resistance and chipping resistance, but if it is too much, it will increase the deformation resistance during forging and reduce the die life. Therefore, it is preferable to suppress it to 3.0% or less.

Mn: 0.2 to 3.0%
Since Mn works as a deoxidizing / desulfurizing agent and a hardenability improving element, it should be contained in an amount of 0.2% or more. However, if it is too much, the deformation resistance during forging is increased and the die life is shortened, so 3.0% or less. It is preferable to suppress to

P: 0.030% or less (excluding 0%)
P causes grain boundary segregation and center segregation and lowers toughness. Therefore, P is preferably suppressed to 0.030% or less.

S: 0.06% or less (excluding 0%)
S is an element that improves machinability and may be added. However, excessive addition increases MnS inclusions formed by reaction with Mn and decreases toughness, so it is suppressed to 0.06% or less. Is preferred.

  Moreover, in addition to the said component, 1 or more types in the component shown to following (1)-(5) can be contained in the component composition of the steel bar to which this invention is applied.

(1) Al: 0.001 to 0.10%, Nb: 0.001 to 0.05%, Ti: 0.001 to 0.20%, V: 0.01 to 0.35%, N: 0 One or more of 0.001 to 0.025% Al, Nb, Ti, and V may be added because they have the effect of forming fine nitrides and refining crystal grains. However, since the nitride is coarsened by excessive addition, the addition amount is preferably within the upper and lower limits.

(2) O: 0.0020% or less (excluding 0%)
Since O reacts with Al to form Al ₂ O ₃ inclusions and lowers toughness, it is preferably suppressed to 0.0020% or less.

(3) One or more of Cu: 2.5% or less, Ni: 2.5% or less, Cr: 3.0% or less, Mo: 1.0% or less (however, each element includes 0%) )
Cu, Ni, Cr, Mo are mechanical structural parts, for example, may be added because of the effect of improving the strength of gears, but excessive addition generates a large amount of inclusions and decreases toughness. It is preferable to keep it below the upper limit.

(4) B: 0.001 to 0.03%
B, like Cu, Ni, Cr, and Mo, may be added in an amount of 0.001% or more because it has an effect of improving the strength of mechanical structural parts such as gears. However, excessive addition causes a large amount of inclusions. Since it produces | generates and toughness falls, it is preferable to restrain to 0.03% or less.

(5) Pb: 0.20% or less, Ca: 0.02% or less, Mg: 0.050% or less, Bi: 0.10% or less, Li: 0.10% or less (however, (Neither element contains 0%)
Pb, Ca, Mg, Bi, and Li may be added because they are elements that improve machinability. However, excessive addition generates a large amount of inclusions and lowers the toughness. Is preferred.

  In order to confirm the effect of the present invention, the following verification test was conducted.

Each steel type A to F having the composition shown in Table 1 was cast on a slab having a cross section of 300 mm × 430 mm with a Bloom continuous casting machine, and 10 steel slabs having a cross section of 155 mm square were produced by ingot rolling. And after performing the steel piece inspection for each steel type under the test conditions shown in Table 2, a steel bar having a diameter of 50 mm was manufactured by hot rolling, and further the steel bar inspection was performed.

  Here, when performing ultrasonic oblique flaw detection in steel slab inspection or steel bar inspection, the G-type standard test specimen is first used as a condition for ultrasonic oblique flaw detection in both inspection stages of steel slab inspection and steel bar inspection. The echo height of a defect with a diameter of 2 mm was calibrated to 30 dB using a slab, and then each steel slab or each steel bar was measured. The echo height threshold was set to 15 dB, and an echo height of 15 dB or more was generated. The part was determined to be a crimped wrinkle or the like. And in the billet inspection stage, billets that have been detected to have crimped defects etc. are basically disposed of and scraped in the billet automatic scraping process (2D), which is the next step only when possible. . On the other hand, in the steel bar inspection stage, all the steel slabs in which crimped defects were detected were discarded.

  And about steel types A, B, and C, the bar steel after a bar steel test | inspection was cut | disconnected to predetermined length, and it hot-forged in the manufacturing process shown in FIG. 1, and manufactured 500 constant velocity joints, respectively. On the other hand, for steel types D, E, and F, after heat-treating the steel bar after the steel bar inspection, cutting it to a predetermined length, and further performing pickling and film treatment, the same as steel types A, B, and C described above, 500 constant velocity joints were manufactured by hot forging in the processing step shown in FIG. Then, all constant velocity joints were examined for cracks, and those with cracks were counted as defective products, and the defect rate (number of defective products / total number × 100%) was determined.

The survey results are shown in Table 2 together with the test conditions. In the same table, the description of shot blasting and bending correction was omitted, but in all tests, shot blasting was performed on the slab before the surface automatic inspection at the slab inspection stage, and at the steel bar inspection stage. Prior to surface inspection, the steel bars were bent and straightened.

  Test No. 1, 2, 5, 6, 9, 10, 13, 14, 17, 18, 21, 22 (invention examples) satisfy the requirements of the present invention (the invention according to claim 1), and The defect rates were all less than 0.5%.

  In contrast, test no. 3, 4, 7, 8, 11, 12, 15, 16, 19, 20, 23, 24 (comparative examples) are not subjected to ultrasonic oblique flaw detection at the stage of steel bar inspection. Forging cracks resulting from the above were observed, and the defect rate was 1.0% or more.

  Here, in all invention examples in which ultrasonic oblique flaw detection was carried out at the bar inspection stage, the defect height of 15 dB or more was found and the total number of steel bars that were discarded was subjected to a cross-sectional observation of each defect site to form a crimp. FIG. 3 shows the relationship between the height of the defect echo and the frequency of occurrence of each wrinkle by identifying whether it is a wrinkle or a wrinkle caused by inclusions.

  As is clear from the figure, only one wrinkle caused by inclusions was found at an echo height of 31 dB or higher, but the other 11 cases were all crimped wrinkles, and the echo height threshold was set to 15 dB. It has been confirmed that the cracking of the steel bar due to the crimped rod can be effectively prevented by discarding 15 dB or more.

It is a flowchart which shows the process until it manufactures a steel bar from a slab through a steel slab according to an embodiment of the present invention. It is a figure which shows the manufacturing process which manufactures a constant velocity joint from steel bar. It is a graph which shows the relationship between the defect echo height and the generation | occurrence | production frequency of a flaw in the ultrasonic oblique angle flaw detection of a steel bar. It is sectional drawing which shows the mode of the crimp-shaped wrinkle which exists in the subcutaneous part of steel bar.

Claims (5)

(1) A bundling process in which the slab is rolled into a steel piece;
(2) A steel piece inspection process for inspecting and scraping the rolled steel pieces.
(3) a steel bar rolling process in which the steel pieces after shave are hot-rolled into a steel bar;
(4) A steel bar inspection process in which the steel bar after hot rolling is inspected and scraped,
A method of manufacturing a wire rod comprising:
The billet inspection process of (2) above
(2A) A steel piece descaling step of removing the oxide scale on the surface of the steel pieces that have been rolled in pieces by shot blasting;
(2B) a steel slab surface automatic inspection step of automatically inspecting the steel slab after the scale removal by a surface automatic inspection device;
(2C) Subsequently, a steel slab internal automatic inspection process for automatically inspecting wrinkles inside the steel slab with an ultrasonic flaw detector,
(2D) a steel slab automatic scraping step of automatically scraping the surface defects detected in the steel slab surface automatic inspection step of (2B) with an automatic scraping device;
And having
The steel bar inspection process of (4) above is
(4A) a steel bar straightening process for correcting the bending of the hot-rolled steel bar;
(4B) a steel bar surface inspection process for inspecting a steel bar whose curvature has been corrected with a leakage magnetic flux flaw detector;
(4C) A steel bar manual scraping process for manually scraping the surface defects detected in the steel bar surface inspection process of (4B) above;
(4D) Subsequently, the steel bar internal automatic inspection step of automatically inspecting the bar inside the steel bar with an ultrasonic flaw detection device different from the ultrasonic flaw detection device used in the steel piece internal automatic inspection step of (2C) above,
Have
Furthermore, in the steel bar internal automatic inspection process of (4D) above, wrinkles and crimped wrinkles (hereinafter referred to as “crimped shapes” caused by inclusions present in the subcutaneous portion of the steel bar by the ultrasonic oblique flaw detection method using an ultrasonic flaw detector. A method for producing a steel bar having excellent forgeability, characterized in that the steel bar is discarded when a crimped wrinkle or the like is detected.   Furthermore, in the steel piece internal automatic inspection process of (2C) above, a crimped wrinkle or the like existing in the subcutaneous part of the steel piece is detected by an ultrasonic flaw detection method using an ultrasonic flaw detector, and the crimped wrinkle is detected. 2. The method for producing a steel bar with excellent forgeability according to claim 1, wherein the steel slab is discarded, or the crimped slag and the like are removed together with the surface slag in the steel slab automatic scraping process of (2D). .   The method for producing a steel bar with excellent forgeability according to claim 1 or 2, wherein an ultrasonic wave having an echo height of 15 dB or more in the ultrasonic oblique flaw detection method is used as the crimped barb or the like. After the steel bar manual scraping process of (4C) above and before the steel bar internal automatic inspection process of (4D) above,
(4C-1) Visual inspection of the part that has been scraped in the steel bar manual scraping process of (4C) above, and if residual scrap is detected, the remaining scrap is manually scraped. The manufacturing method of the steel bar excellent in forgeability of any one of Claims 1-3 which provided the steel bar scraping part inspection and scraping process. In the billet inspection process of (2) above,
After the above-described (2D) billet automatic scraping process,
(2E) When the surface of the steel slab is visually inspected by a magnetic particle flaw detector, and surface flaws are detected, the steel piece surface visual inspection / manual chamfering process for manually scoring the surface flaws;
The manufacturing method of the steel bar excellent in forgeability of any one of Claims 1-4 which provided.

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