JP5111000B2 - Spinning method for ferritic stainless steel welded pipe - Google Patents

Description

  The present invention relates to a method for forming a reduced diameter portion at the end of a ferritic stainless steel welded tube by spinning.

Large-diameter pipes are used as materials in the case of exhaust gas purifying catalysts (converters) or silencers (mufflers) that are installed in the exhaust system of automobiles because of the need to increase the capacity. And the case member edge part is provided with the taper part for the connection with the member before and behind, and also the small diameter parallel part which continued to the taper part as needed.
FIG. 1 shows that such a case 1 has a large-diameter pipe portion (1a) as a raw material, a tapered portion (1b) provided to connect to the front and rear members, and a small-diameter parallel portion continuous to the tapered portion ( The example comprised from 1c) is shown. The tapered portion (1b) is connected to the raw tube portion (1a) via the shoulder portion (2), and the tapered portion (1b) is connected to the small-diameter parallel portion (1c) via the neck portion (3). ing.

Such shaped products are often formed by applying a spinning method to the pipe end.
For example, as introduced in Patent Document 1, the spinning processing method is performed by bringing a processing roller, which is a forming tool, into contact with the surface of the processing tube, and relatively revolving the processing roller around the processing tube. In this method, the roller is driven in the radial direction and the axial direction of the tube to be processed, and a tapered portion that gradually decreases in diameter toward the end portion of the tube to be processed and a small-diameter parallel portion that continues to the tapered portion as necessary. .

As a background that the spinning method is often used as a method of forming a member of such a shape, compared to the method of welding and joining in a separate step after processing the plate material divided into a plurality of parts by press molding, 1) High material yield, 2) High molding strength due to integral molding, reducing the number of parts, 3) No need for welding, no deterioration of member reliability due to welds, etc. Can be mentioned.
However, compared to the press forming method, the spinning method repetitively moves the processing roller in the axial direction of the tube to be processed to advance the forming, so that the material is easily moved in the tube end direction due to plastic deformation, particularly the tapered portion. There is a tendency that the thickness of the steel sheet locally decreases than the material thickness of the material to be processed. Further, when the processed end of the reduced base metal part or the processed pipe is a welded pipe, breakage is likely to occur starting from the processed end of the welded part of the pipe end.

On the other hand, in the case of exhaust gas purifying catalyst (converter) or silencer (muffler) case equipped in the exhaust system of automobiles, stainless steel is used as the material from the viewpoint of corrosion resistance, especially corrosion resistance against high temperature exhaust gas. What is used is used.
And, for example, in Patent Documents 2 and 3, when the material is a ferritic stainless steel plate, by adjusting the component composition of the steel or by defining the shape of the butt weld, There has been proposed a technique that makes it difficult to cause a crack starting from the processed end of the welded portion when the processed end or the processed pipe is a welded pipe.
Japanese Patent Publication No. 4-46647 JP 2003-342694 A JP 2004-243354 A

However, the methods proposed in Patent Documents 2 and 3 are all intended to improve the processed pipe so as to be easily processed, and are not methods for improving the processing means themselves. Whichever method is employed, when the processed end of the base metal part or the processed pipe is a welded pipe, the break starting from the processed end of the welded part cannot be completely eliminated.
By the way, in order to increase the forming efficiency, it is necessary to increase the relative rotational speed between the forming roller, which is a forming tool, and the processing pipe, or to increase the processing amount per pass. There is.

Therefore, if it is attempted to increase the production efficiency by increasing the relative rotational speed or feed rate, cracks occur at the end of the tube, so there is a limit to increasing the relative rotational speed or feed rate. In particular, when the pipe to be processed is a welded steel pipe, cracks are likely to occur at the welded portion at the end of the pipe, so that the rotational speed and feed rate cannot be increased.
The present invention has been devised in order to solve such a problem. When forming a reduced diameter portion by contacting a processing roller to the end of a ferritic stainless steel welded tube, the tube end welding is performed. It aims at providing the processing method which can suppress a crack of a part.

In order to achieve the object of the spinning method for ferritic stainless steel welded pipes of the present invention, a processing roller made of ferritic stainless steel is arranged on the outer periphery of a fixed welded pipe and revolves around it. And when forming the reduced diameter portion that gradually reduces the diameter toward the end of the welded pipe to be processed by reciprocating in the axial direction while moving the working roller in the radial direction of the welded pipe to be processed. The reduced diameter portion is formed while cooling the welded portion of the processed welded pipe.
Moreover, it is preferable to heat the welded portion of the welded pipe to be processed to 100 ° C. or higher before forming the reduced diameter portion.

  In the present invention, when the reduced diameter portion is formed at the end of the work tube by reciprocating in the axial direction while moving the work roller in the radial direction of the work tube, the welded ferritic stainless steel welded pipe is welded. Since the part is cooled, it is removed even if the processing heat accompanying the processing is generated, and the progress of strain aging due to the processing heat is suppressed, and the hardness increase of the welded part is suppressed. As a result, it was possible to prevent cracks in the welded part of the pipe end during machining even when spinning at high speed, leading to improved productivity.

The inventors of the present invention use a ferritic stainless steel welded pipe as a work pipe, and use the work pipe and a work roller that is disposed around the work pipe and relatively revolves around the work pipe. When forming a diameter-reduced part gradually reducing the diameter toward the end of the pipe to be processed by reciprocating in the axial direction while moving in the radial direction of the pipe, and a reduced-diameter part when forming a continuous parallel part with a small diameter. Various investigations have been made on the cause of cracks that occur easily at the edges of the steel, particularly at the edges of the welds, and countermeasures.
As a result, it was predicted that the cracks were caused by the height of the weld being increased due to the progress of strain aging due to the processing heat associated with spinning. Then, it has been found that it is effective to perform spinning while cooling the welded portion in order to suppress the progress of strain aging to prevent the elevation of the altitude and suppress the occurrence of cracks in the welded portion.
Hereinafter, the background to the present invention will be described.

First, the mechanism by which strain aging progresses during spinning and the weld is hardened was investigated.
First, C: 0.013 mass%, Si: 0.41 mass%, Mn: 0.23 mass%, P: 0.027 mass%, S: 0.002 mass%, Ni: 0.11 mass%, Cr: 17.16% by mass, Mo: 0.02% by mass, Cu: 0.03% by mass, Ti: 0.22% by mass, Al: 0.041% by mass and N: 0.012% by mass, A test piece as shown in FIG. 2 was prepared using a stainless steel plate with the balance being Fe. In addition, the weld part shown in FIG. 2 forms a bead by plasma irradiation on a board | plate material.

(1) No strain applied, (2) 15% strain applied at room temperature, and (3) 15% strain applied at 400 ° C. It was measured. As a result, as shown in FIG. 3, it can be seen that the hardness of the welded portion is increased due to the warm distortion.
Then, the cross-sectional hardness of the welded part was measured for the test piece to which 15% strain was applied with various temperatures changed. As a result, as shown in FIG. 4, it can be seen that the higher the strain application temperature, the higher the hardness is due to strain aging. From the results of FIGS. 3 and 4, it is understood that the hardness has increased due to the progress of strain aging.

  The fact that the increase in hardness is accompanied by the progress of strain aging is supported by the following points. That is, for ferritic stainless steels with different amounts of C and N, similarly, after applying 15% strain at 400 ° C., the hardness of the weld is measured, and the relationship between the amount of C + N and the increase in hardness due to strain aging is organized. As shown in FIG. When C + N increases, the hardness increases due to heat reception. Considering the results shown in FIGS. 3 to 5 together, it is determined that the hardness increase of the welded portion of the ferritic stainless steel is due to strain aging.

When ferritic stainless steel is used as a raw material, it is effective to reduce the contents of C and N in the steel in order to suppress the occurrence of cracks due to strain aging. Sometimes upper and lower limits are set, causing variations within the range, and there is a problem in that steelmaking costs increase when trying to control everything to the lower limit.
The upper and lower limits are set for components such as C and N during steelmaking, and C and N are set to be low as target values, but it is difficult to make all within the target values, and within the set values There is variation. In order to cope with the spinning processing of the material in the vicinity of the upper limit (high C, high N, etc.) in the set value range, the strain aging suppression study was performed.
Therefore, even if it is made of ferritic stainless steel with a high C and N content, which is prone to cracking due to increased hardness of the weld due to strain aging, strain aging is possible even if the pipe end of the welded pipe is subjected to spinning processing. We studied measures to suppress the temperature rise of the welded parts that were subjected to spinning.

As a result, the welded part was cooled to prevent the temperature of the processed part from rising.
As a mode of cooling the welded portion to be subjected to the spinning process, it is sufficient to spray the refrigerant because it is only necessary to prevent the temperature of the portion from rising. Although liquefied gas may be sprayed, it is preferable to spray a relatively fast flow rate such as factory air. You can simply blow air with a fan.

Usually, as shown in FIG. 6, the spinning process performed on the pipe end is arranged on the outer periphery of the pipe to be processed while fixing the pipe 11 to be processed and rotating the pipe to be processed together with the fixture 12. This is performed in such a manner that the relatively revolving processing roller 13 is pressed.
Of course, the method of the present invention can also be applied to the spinning process performed in this mode. What is necessary is just to cool the whole by spraying a cooling medium. By using the inside of the pipe to be processed as a passage for the cooling medium, the whole can be cooled.

However, cooling the whole is not economically advantageous, such as an increase in the amount of cooling medium used.
Therefore, as shown in FIG. 7, the other end of the processed tube 21 is fixed, and the processing roller 23 arranged on the outer periphery of the processed tube 21 is processed without rotating the fixing bracket 22 and the processed tube 21. It is preferable to revolve around the tube 21. If this aspect is employ | adopted, it will become possible to suppress the temperature rise of the welding part 24 effectively by spraying the cooling media 25 and 26 along a welding line.

  By the way, the temperature rise in the processed part is due to a total of heat generation due to deformation of the work material itself and heat generation due to friction between the processing roller and the material. As discussed in FIG. 4, the strain aging is more likely to proceed as the temperature of the processed portion increases. When the temperature of the processed part exceeds 250 ° C., strain aging tends to proceed. Therefore, it is preferable to spray the cooling medium so that the temperature of the processed part does not exceed 250 ° C. More preferably, it is desirable to keep the temperature at 200 ° C. or lower.

However, the temperature of the processed part is not necessarily low. When trying to machine a machined part, particularly a welded part, brittle cracks are likely to occur if the initial temperature during machining is too low. When the temperature of the processing environment is low, it is considered that cracking is likely to occur due to a decrease in toughness accompanying a decrease in the absorbed energy of the welded portion at the initial stage during spinning processing. Therefore, it came to the idea that the crack generation | occurrence | production of a pipe end welded part can be suppressed by heating the welded part of a steel pipe before a spinning process start.
Although details are shown in the examples, it is preferable to carry out the spinning process while maintaining the welded portion in the processing region within a temperature range of 100 to 250 ° C.

Example 1;
A steel pipe of φ150 × 1.2t obtained by TIG welding the steel plate having the composition shown above was used as a work steel pipe.
As shown in FIG. 7, while fixing a steel pipe to be processed, a nozzle having a tip diameter of φ10 is fixedly arranged on the outer side of the steel pipe, and the length in the longitudinal direction of the spinning processing range with a width of about 4 mm, which is the same as the width of the welded part A slit-processed tube having a length corresponding to is fixedly arranged. The nozzle and the slit-processed tube can be supplied with air up to a pressure of 5 kg / cm ² at room temperature.
While cooling the welded portion from the inside and outside of the steel pipe with air, adjusting the air supply amount so that the welded portion temperature becomes 200 to 300 ° C., pressing the revolving roller to perform spinning processing, as shown in FIG. Molded into an end shape.

The processing conditions were as follows: rotational speed: 600 rpm, feed rate: 5000 mm / min, and 13-pass spinning processing to reduce the diameter to φ64.
As a comparative example, spinning processing was performed under the same conditions except that the welded portion was not cooled, and the hardness state of the welded portion and its peripheral portion and the occurrence of cracks in the welded portion were investigated. Without cooling, the weld rose to 350 ° C.
The results are shown in FIG.

Example 2;
For the φ150 × 1.2t steel pipe with TIG welded steel plate to the composition shown above, the temperature of the welded portion at the time of starting machining is variously changed by heating the welded portion in the working region with a heater. Spinning was performed in the same form and conditions.
In addition, the nozzle similar to Example 1 was used for cooling of a welding part, and the welding part temperature at the time of a spinning process was set to 200 degreeC.
Then, the occurrence of cracks at the tube end was observed. The reason for observing the occurrence of cracks at the pipe end in Example 2 is that the welded part temperature at the time of spinning is 200 ° C., and therefore in the processed product having the shape shown in FIG. ) Or the neck (3), the parallel portion (1c) is not cracked, but only the tip welded portion of the parallel portion (1c) tends to crack. It is because it assumed that the crack which generate | occur | produces only in a welding part originates in brittle fracture.
The results are shown in Table 2.

From the results in Table 2, it can be seen that low temperature brittle cracks can be reliably prevented by preheating the weld zone in the processed region to 100 ° C. or higher.
In order to prevent both cracks in the weld due to strain aging and brittle cracks in the low-temperature region, preheating is performed in advance, and a cooling medium is sprayed during the processing. As shown in FIG. It is preferable to carry out the spinning process while maintaining a temperature of 100 to 250 ° C.
It should be noted that even if there is no preheating of the welded portion of the processed region and a crack occurs at the tip after processing, it can be used without any problem if the cracked portion is ground and deleted.

Diagram explaining the shape of general spinning products Diagram explaining the shape of strain imparted specimen A graph showing the relationship between warm strain and hardness Graph showing the relationship between strain application temperature and hardness increase due to strain aging A graph showing the relationship between the amount of C + N and the increase in hardness due to strain aging Diagram explaining general spinning method The figure explaining the spinning processing method of this invention The figure which shows the relation between the processing part temperature and the welding part hardness at the time of spinning processing

Claims (2)

Using a processing roller that is made of ferritic stainless steel and is placed on the outer periphery of a fixed welded tube and revolves around it, reciprocating in the axial direction while moving the processing roller in the radial direction of the welded tube to be processed The reduced diameter portion is formed while the welded portion of the welded pipe to be processed is cooled when the reduced diameter portion is gradually reduced toward the end of the welded pipe to be processed by moving the welded pipe. Spinning method for welded stainless steel welded pipe. The method of spinning a ferritic stainless steel welded pipe according to claim 1, wherein the welded part of the welded pipe to be processed is heated to 100 ° C or higher before forming the reduced diameter part.

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