JP2006274436A - Ferritic stainless sheet sheet and steel tube for bent tube having sectional shape for components - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To process a ferritic stainless steel sheet so as to have a sectional shape for components and to prevent a crack etc., from occurring even if the ferritic stainless steel sheet is used for applications where strict processing conditions are requested. <P>SOLUTION: The ferritic stainless steel having the tensile strength TS of ≤500 MPa, total elongation EL of ≥25%, and an average r value of ≥1.3 is made into a ferritic stainless steel tube by winding the steel sheet to a tubular form and welding its butt surfaces. The ferritic stainless steel tube is subjected to such bending that the radius of bending at the central axis of the tube is ≤1.2 times the diameter of the ferritic stainless steel tube and that the direction of the central axis of the tube at one end of the ferritic stainless steel tube forms an angle of ≥160° with the direction of the central axis of the tube at the other end. In addition, the processing is so performed that the ferritic stainless steel tube has the sectional shape for components. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a ferritic stainless steel plate and a steel pipe for curved pipes having a cross-sectional shape for parts, which are used as piping for automobiles, electrical equipment, etc., in addition to air conditioning equipment and various plants.

In addition to air-conditioning equipment and various plants, a bent metal pipe having a cross-sectional shape peculiar to the pipe may be used as a pipe for an automobile or an electric device. An example of a metal curved pipe having such a unique cross-sectional shape is shown in FIG. (A) is a top view, (b) is a side view, (c) is each sectional drawing.
For example, as shown in FIG. 11, two metal bent tubes having such a unique cross-sectional shape are conventionally prepared by press-molding the metal plate 10 and then trimming to form the flange 20 portion. The flange 20 is welded so that the front and back surfaces are opposed to each other so that the whole is substantially tubular. Although not shown in the drawing, there is a case where a portion of the overlap margin 30 formed by welding is trimmed and removed.

However, such a manufacturing method with many man-hours has a problem that it is very time-consuming and the cost is higher than anything.
Furthermore, since it is necessary to perform trimming after press forming, generation of trimming scraps (scrap) is unavoidable, and the yield of the metal sheet is poor, which also increases the cost.

  In addition, since there is a flange 20 for welding and an overlap 30, piping after forming into a metal curved pipe having such a unique cross-sectional shape (hereinafter, a curved pipe having a cross-sectional shape for parts) is necessary. In addition to the above-mentioned air conditioning equipment and various plants, it may affect the design of automobiles and electrical equipment, such as being heavier than the above and excessively large installation space. As described above, trimming and cutting off the flange 20 after welding has a problem that leads to high costs due to an increase in man-hours.

In addition, various characteristics are required for the various pipes described above. That is, in order not to be deformed or broken under the usage environment, in addition to strength, corrosion resistance, heat resistance, wear resistance, and the like may be required.
When designing various types of piping as described above, increase the thickness of the piping so that it can withstand even if there is an effect of thinning due to corrosion or wear, or a decrease in strength due to temperature rise. In order to ensure durability. That is, the material to be used is determined from the characteristics required for the piping, such as corrosion resistance and heat resistance, but once the material is determined, the strength at the highest temperature to be used is also determined, the above-mentioned corrosion and Even if there is an effect of thinning due to wear or an effect of strength reduction due to temperature rise, the lower limit of the plate thickness that can be sufficiently tolerated is also determined.

Then, it is most important that the various pipes designed as described above are molded so as not to fall below the lower limit of the plate thickness at the manufacturing stage. In other words, when thinning occurs due to molding, it is important to reduce the thinning as much as possible so that it does not fall below the lower limit of the plate thickness.
When manufacturing a curved pipe with a cross-sectional shape for parts, if the pipe material is used as the material, the number of parts is reduced and welding is not required, reducing the man-hours and improving the yield of the metal pipe. As a result, there is a possibility of cost reduction.

FIG. 12 shows a definition example of the axis and angle of a metal tube in which the direction X of the tube center axis at one end forms an angle θ of 60 degrees or more with the direction Y of the tube center axis at the other end. In so-called U-shaped bending, in which bending is performed at approximately 180 degrees, thinning occurs with such bending. And the thinning becomes so remarkable that the curvature radius of bending becomes small.
Nevertheless, recently, there is a need for parts that require a higher level of processing technology.

As for the needs of parts that require more advanced processing technology, what kind of shape characteristics and processing methods are required is based on the premise that bending is performed first. There are geometrical features such that “radius R / tube diameter D” is 1.2 or less and the bending angle is 160 degrees or more.
Examples of such metal tubes are shown in FIGS.

  The thing of FIG. 13 is one of the exhaust system components for motor vehicles. The diameter D of the tube of the material before bending is φ50, and when the bending is performed, the direction X of the central axis at one end of the tube is an angle θ of 160 degrees with the direction Y of the central axis at the other end. It can be bent to make Further, the bending radius R at the central axis of the tube is 60 mm and the diameter D is 50 mm, whereas the relationship of R / D = 1.2 is established.

  Furthermore, the cross-sectional shape of the curved pipe having the cross-sectional shape for parts may change in the direction of the central axis. Since there will also be a part where the cross-sectional shape deforms into a complicated shape due to hydroforming described below, the direction of the extension of the ferritic stainless steel curved pipe is more accurate than the central axis direction, but in any case, In FIG. 13, the one end aa portion of the tube is substantially semicircular, the bb portion is approximately triangular, the cc portion is circular with the same diameter as the tube, and the dd portion is circular, but the diameter is enlarged to φ60.

The thing of FIG. 14 is also one of the exhaust system parts for motor vehicles. The diameter of the material metal tube before bending is φ50 so that the direction X of the central axis at one end of the ferritic stainless steel tube after bending forms an angle of 180 degrees with the direction Y of the central axis at the other end. It is bent and is just U-shaped.
The bending radius R at the central axis of the tube is 50 mm, and the diameter D is 50 mm, whereas R / D = 1.0. Furthermore, the cross-sectional shape changes in the direction of the central axis. In the figure, the one end ee of the tube is almost semicircular, the ff is almost triangular, the gg is circular with the same diameter as the tube, and the hh is circular. However, its diameter is expanded to φ60.

In the case of FIG. 14, the E portion is further bent at an angle of 90 degrees to the upper side of the drawing, but this is a part different from that of FIG. It is only a three-dimensional shape as described above.
Further, as shown in FIG. 14, a three-dimensional shape is obtained by performing a hydroforming process described later after bending and further processing a three-dimensional shape after bending. It is also applicable to the one that has been processed into a three-dimensional shape and further subjected to hydroforming, or the one that has been subjected to hydroforming and subjected to bending or processing into a three-dimensional shape.

By the way, as described above, in many cases, thinning occurs with bending, but as the bending angle increases to 160 degrees or more, thinning becomes more prominent. Later, there is a processing method requirement that it is necessary not to fall below the design lower limit plate thickness that can ensure characteristics such as corrosion resistance and heat resistance.
In order to perform bending at 160 degrees or more while suppressing thinning, it is effective to divide the bending process into two times and three times instead of once, but if the number of bendings is increased too much This will increase man-hours and costs.

In addition, if the pipe is expanded by machining in order to obtain a curved pipe having a cross-sectional shape for a part, it is necessary to insert a core metal having a complicated shape to build the cross-sectional shape.
Due to such problems, the above-described parts are hardly mass-produced on an industrial scale.
By the way, as a technique of using a steel pipe as a material and performing hydroforming so as to have an irregular cross-sectional shape, a method of devising the shape of a mold as in Patent Document 1, Patent Document 2, and Patent Document 3 Thus, there has been a method for defining the mechanical properties and surface properties of the steel pipe used for hydroforming.
JP 2000-343141 A Japanese Patent Laid-Open No. 10-175027 JP 2000-17329 A

  However, in any case, it is assumed that hydroforming is performed on a straight pipe, and the axial direction X at one end of the metal bent pipe as shown in FIGS. For forming a metal curved pipe having an irregular cross-sectional shape that forms an angle θ of 160 degrees or more with the axial direction Y at the other end of the metal curved pipe, the structure of the tool mold is remarkably complicated. In addition, since it is difficult to manage and there is little substantial cost for improvement of moldability, it is difficult to say that it is advantageous and effective in an industrial mass production scale.

  Moreover, as a material for various pipes as described above, austenitic stainless steel pipes represented by SUS304 are often used for applications that require corrosion resistance or heat resistance. Recently, there has been a movement to use ferritic stainless steel pipe as a material, which is relatively inexpensive, but has been approaching austenitic stainless steel pipe due to various researches so far. It is coming.

Although the corrosion resistance and workability have approached SUS304, ferritic stainless steel still requires some improvement in workability such as elongation, and is used for applications that require high workability. difficult.
The present invention has been made to solve the above-described problems of the prior art, and provides a ferritic stainless steel sheet and a steel pipe for curved pipes having a cross-sectional shape for parts.

That is, the present invention
(1) Ferritic stainless steel sheet with a tensile strength TS of 500 MPa or less, total elongation El of 25% or more, and average r value of 1.3 or more are wound in a tube, and the butt surfaces are welded to ferritic stainless steel The steel pipe is bent and the bending radius at the pipe central axis is 1.2 times or less the diameter of the ferritic stainless steel pipe, and the direction of the pipe central axis at one end of the ferritic stainless steel pipe is the other end. For a curved pipe having a cross-sectional shape for a part, which is formed so as to form an angle of 160 degrees or more with the direction of the pipe central axis at Ferritic stainless steel sheet,
(2) The ferritic stainless steel sheet according to (1), in mass%, Cr: 10 to 20%, C: 0.05% or less, Mn: 0.5% or less, Ni: 1.0% or less, N: 0.05% or less , P: 0.04% or less, Si: 0.5% or less, Al: 0.2% or less, S: 0.005% or less, and for curved pipes with a cross-sectional shape for parts characterized by consisting of the remainder Fe and inevitable impurities Ferritic stainless steel sheet,
(3) In addition to the above, further contain one or two selected from Ti: 0.5% or less and Nb: 1.0% or less, and satisfy (2Ti + Nb) ≧ 16 (C + N) Ferritic stainless steel sheet for curved pipes having a cross-sectional shape for parts as described in (2) above,
(4) In addition to the above, further containing one or two kinds selected from Mo: 4.0% or less and Cu: 4.0% or less. For parts according to (2) or (3), Ferritic stainless steel sheet for curved pipes with a cross-sectional shape of
(5) A ferritic stainless steel pipe having a tensile strength TS of 510 MPa or less, a total elongation El of 20% or more and an average r value of 1.3 or more, and a bending radius at the pipe center axis of the ferritic stainless steel pipe Bending such that the diameter is not more than 1.2 times the diameter so that the direction of the tube center axis at one end of the ferritic stainless steel tube forms an angle of 160 degrees or more with the direction of the tube center axis at the other end And ferritic stainless steel pipes for curved pipes having a cross-sectional shape for parts, characterized in that they are processed to have a cross-sectional shape for parts.
(6) The ferritic stainless steel pipe of (5), in mass%, Cr: 10 to 25%, C: 0.05% or less, Mn: 0.5% or less, Ni: 1.0% or less, N ≦ 0.05% or less , P: 0.04% or less, Si: 0.5% or less, Al: 0.2% or less, S: 0.005% or less, and for curved pipes with a cross-sectional shape for parts characterized by consisting of the remainder Fe and inevitable impurities Ferritic stainless steel pipe,
(7) In addition to the above, further contain one or two selected from Ti: 0.5% or less and Nb: 1.0% or less, and satisfy (2Ti + Nb) ≧ 16 (C + N) Ferritic stainless steel pipes for curved pipes having a cross-sectional shape for parts as described in (6) above,
(8) In addition to the above, further containing one or two kinds selected from Mo: 4.0% or less and Cu: 4.0% or less. For parts according to (6) or (7), Ferritic stainless steel pipe for curved pipes with a cross-sectional shape of
It is.

  According to the present invention, even when a curved pipe having a cross-sectional shape for a part is formed using a ferritic stainless steel plate or a steel pipe, it is possible to achieve both reduction in thickness and realization of the cross-sectional shape for the part. The cost can be reduced by improving the yield of the metal pipe, and it also contributes to reducing the weight of the pipe after being formed into a curved pipe having a cross-sectional shape for parts and saving installation space.

(mechanical nature)
In order to describe the ferritic stainless steel sheet according to the present invention, first, the necessity for a tensile strength TS of 500 MPa or less, a total elongation El of 25% or more, and an average r value of 1.3 or more will be described.
Incidentally, the average r value is the r value measured in the length direction of the test piece cut in the direction in which the extending direction of the ferritic stainless steel tube is the length direction, r ₀ , with respect to the length direction. When the r value measured in the direction of 45 degrees in the surface is r ₄₅ and the r value measured in the direction of 90 degrees in the surface is r ₉₀ ,
Average r value = (r ₀ + 2r ₄₅ + r ₉₀ ) / 4
It is. Hereinafter, the average r value is simply referred to as r value.

FIG. 1 shows the total elongation El of a ferritic stainless steel sheet, which is a material for pipe making, and the limit bending angle in bending of the ferritic stainless steel pipe after pipe making. Table 1 shows the results and the respective test material components and mechanical properties.
The smaller the total elongation El of the ferritic stainless steel sheet, which is a material for pipe making, is the smaller the limit bending angle. When the total elongation El is smaller than 25%, cracking occurs before the bending angle reaches 180 degrees. Therefore, the total elongation El of the ferritic stainless steel sheet, which is a material for pipe making, is preferably 25% or more. Compared with the plate before pipe forming, the total elongation El of the ferritic stainless steel pipe after pipe forming is reduced by about 5%. Therefore, when converted into the ferritic stainless steel pipe after pipe forming, the total elongation El is 20%. It means that more than% is necessary.

  FIG. 2 shows the r value of the ferritic stainless steel plate, which is a material for pipe making, and the center of the bent outer periphery after bending (FIG. 7). The relationship between the plate thickness was investigated. The results and the respective test material components and mechanical properties are also shown in Table 1 above.

  The lower the r value of the ferritic stainless steel sheet, which is a material for pipe making, the greater the thickness loss in bending, particularly when the r value is less than 1.3. Therefore, the r value of the material needs to be 1.3 or more. Compared to the plate before pipe making, the r value hardly changes even after pipe making. Therefore, the r value of a ferritic stainless steel sheet as a material for pipe making is required to be 1.3 or more.

Fig. 3 shows the relationship between the tensile strength TS of ferritic stainless steel plate, which is a material for pipe making, and the occurrence of buckling in hydroforming, which will be described later, for what was bent 180 degrees in the bending process shown in Fig. 1. It is a thing. The results at this time and the respective test material components and mechanical properties are also shown in Table 1 above.
As the tensile strength TS of ferritic stainless steel sheet, which is a material for pipe making, increases, buckling occurs with a smaller amount of axial push, and this tendency is more pronounced when the tensile strength TS exceeds 500 MPa. Therefore, the tensile strength TS of the ferritic stainless steel sheet, which is a material for pipe making, needs to be 500 MPa or less. Compared with the plate before pipe forming, the tensile strength TS increases by about 10 MPa. Therefore, when converted into a ferritic stainless steel pipe after pipe forming, the tensile strength TS needs to be 510 MPa or less.

Fig. 4 shows the plate of the outer periphery of bending when 180-degree bending is performed in 60-degree three times with Hi-r material (r value = 1.3) and Lo-r material (r value = 1.0). The thickness distribution is shown. The components and mechanical properties of the respective test materials at this time are those of Steel 2 and Steel 10 shown in Table 1 above.
Hi-r material has a larger minimum thickness. That is, it can be seen that the higher the r value of the material, the smaller the thickness reduction due to the bending process of 180 degrees.
(Shape characteristics of curved pipes with cross-sectional shapes for parts)
In the present invention, bending is performed such that the bending radius is 1.2 times or less of the diameter of the ferritic stainless steel pipe, and the direction of the central axis at one end of the ferritic stainless steel pipe is the direction of the central axis at the other end And to make an angle of 160 degrees or more.

  As described above with reference to FIG. 12, this means that the angle θ between the direction X of the tube center axis at one end and the direction Y of the tube center axis at the other end is 160 ° or more. However, if the direction of the central axis at one end forms an angle of 160 degrees or more with the direction of the central axis at the other end of the metal tube, two-dimensional processing is performed, and the central axis at one end of the metal tube This is not limited to the case where the direction is in the same virtual plane as the direction of the central axis at the other end of the metal tube. Hydroforming is performed after two-dimensional bending, and then three-dimensional processing is performed, or hydroforming is performed after three-dimensional bending. As a result, as shown in FIG. 5, when the direction of the central axis at one end of the metal tube is not in the same virtual plane as the direction of the central axis at the other end of the metal tube, The angle formed by the direction of the central axis and the direction of the central axis at the other end of the metal tube is the angle θ formed by both axes when the central axes are virtually translated and intersected. Shall be pointed to.

  In the present invention, “having a cross-sectional shape for a part” means that when any part of a bent metal pipe takes a point on the central axis, the direction of the central axis at that point is calculated. This means that the cross section of the metal bent pipe cut by a virtual plane such as a line is a non-simple shape designed to satisfy the function as a part. Non-simple means that a metal tube having a cylindrical cross-section is simply bent or is not flattened.

Next, a preferable range is described as a component of the ferritic stainless steel as a raw material. In addition,% shall mean mass% hereafter.
Cr: 10-20%
If Cr is less than 10%, sufficient corrosion resistance cannot be ensured, so it is preferable that Cr be defined as 10% or more. The upper limit is preferably 20% or less because it may crack during production due to embrittlement. A more preferable range is 10 to 18%.

C: 0.05% or less
C is better as it is smaller because it forms a compound with Cr to lower the corrosion resistance and also adversely affects workability. Therefore, the upper limit is preferably 0.05%.
Mn: 0.5% or less
Mn is added to suppress the occurrence of cracks during deoxidation, desulfurization, or hot working, but if added excessively, sulfide is formed and corrosion resistance is reduced. For this reason, it is preferable that the Mn content is low, and the upper limit is preferably set to 0.5% in consideration of economics during production.

Ni: 1.0% or less
Ni is an element that improves corrosion resistance when added, and is an element that is particularly effective in improving rust resistance among corrosion resistance, but excessive addition impairs economic efficiency due to saturation of its effect, so the upper limit is 1.0% Is preferable.
N: 0.05% or less
N is preferably as small as possible because it forms a compound with Cr to lower the corrosion resistance and also adversely affects workability. Therefore, the upper limit is preferably 0.05%.

P: 0.04% or less
P is preferably as small as possible because it segregates at grain boundaries and lowers toughness. Therefore, the upper limit is preferably 0.04%.
Si: 0.5% or less
Si is an element added as a deoxidizer, but is also effective in improving corrosion resistance. However, on the other hand, in order to increase the strength by solid solution strengthening and to reduce the ductility, if added excessively, the workability is adversely affected. Therefore, the smaller one is preferable, and the upper limit is preferably 0.5%.

Al: 0.2% or less
Al is an element added as a deoxidizer, and when used in combination with Ti, it contributes to economic efficiency by increasing the yield of Ti, but if added excessively, the amount of inclusions increases and the surface quality increases. In order to reduce the content, the upper limit is preferably made 0.2%.
S: 0.005% or less S is present in steel as non-metallic inclusions such as MnS. If the amount of S increases, it adversely affects toughness and corrosion resistance. Therefore, the upper limit is preferably made 0.005% or less.

Ti: 0.5% or less, Nb: 1.0% or less
Ti and Nb react with C and N to form precipitates and make the crystal grains finer, but if added excessively, surface properties deteriorate due to an increase in precipitates, and formation of intermetallic compounds. Therefore, Ti may be added in the range of 0.5% or less and Nb: 1.0% or less.

2Ti + Nb ≧ 16 (C + N)
In addition, Ti and Nb fix C and N as precipitates, so that the orientation of cold-rolled recrystallized grains is improved and the r value is improved. To achieve this effect, 2Ti + Nb It is preferable to satisfy the condition of ≧ 16 (C + N).
Mo: 4.0% or less, Cu: 4.0% or less
Mo and Cu have the effect of improving corrosion resistance, but if added excessively, they cause embrittlement and cracks occur during hot working such as hot rolling, which significantly lowers the surface quality of the product. Mo: 4.0% or less, Cu: 4.0% or less may be added.

Specific examples of the standards suitable for the component range as described above include those having components such as SUS430 specified in JIS G 4304 and SUH409 specified in JIS G 4312 as heat-resistant steel, The ferritic stainless steel referred to in the present invention has a generic meaning including them, and the ferritic stainless steel of the present invention has specially adjusted mechanical properties according to the manufacturing conditions described below. Will be allocated.
(Production conditions)
The preferred ranges of the components are as described above. Next, manufacturing conditions for adjusting to the above-described mechanical properties will be described.

  Steels with the components shown in Table 2 were melted and hot-rolled, roughly rolled at a temperature of 1030 ° C and a reduction rate of 85%, and subsequently finished at a temperature of 770 ° C and a reduction rate of 85%. A hot-rolled sheet is wound at a temperature and wound in a coil shape. Further, annealing is performed at 810 ° C. for 6 hours and pickling scale removal is performed, cold rolling at a reduction rate of 56% is performed, and then finish annealing is performed at 890 ° C. for 60 seconds to obtain a cold-rolled annealed sheet. As shown in Table 2, the mechanical properties of the resulting product plate (cold-rolled annealed plate) are as follows. Tensile strength TS is 500 MPa or less, total elongation El is 25% or more, and average r value is 1.3 or more. Satisfy the requirements of the invention.

  However, the above is not limited to an example of manufacturing conditions, and the mechanical properties in the present invention are obtained as a result of a combination of steel components and manufacturing conditions, and are realized even when steels of the same component are used. There are various methods for the adjustment, and any known method may be used for the adjustment, and the method is not particularly limited. Can meet the requirements.

That is, the hot rolling is performed by hot rolling at a reduction rate of 60% or more in a temperature range of 1000 to 1150 ° C., and the subsequent finish rolling of the hot rolling has a reduction rate of 60% or more in a temperature range of 600 to 900 ° C., Winding in the temperature range of 400-700 ° C, making it a hot-rolled sheet wound in a coil shape, further annealing at 800-950 ° C for 20 seconds or longer, relatively short time annealing, or 700-850 ° C for 4 hours or longer After annealing at a relatively low temperature for a long time, the oxide scale is removed by pickling and cold rolling at a reduction rate of 50% or more is performed, followed by finish annealing at 800 to 950 ° C. for 20 seconds or more. It is a manufacturing condition which goes and makes it a cold-rolled annealing board.
(Other preferred conditions)
The preferred component ranges and production conditions are as described above. Another preferred condition is the roughness of the surface of the ferritic stainless steel sheet or steel pipe as the material, and will be described next.

  Roughness affects the slidability with the tool during the molding process, and when the roughness is small, the contact surface pressure with the tool increases and the lubricating oil film breaks, which tends to increase the friction coefficient. , The moldability is significantly reduced. In addition, the tool may be severely worn due to mold galling or seizure. In order to suppress these, it is preferable that the roughness of the surface of the ferritic stainless steel plate or steel pipe as the raw material is Ra ≧ 0.1 μm.

In order to adjust the roughness, it is preferable to adjust the roughness of the roll at the time of cold rolling, and to adjust the roughness of the roll, the count of the grindstone for polishing the roll is appropriate. It is preferable to do this.
In addition, Ra here is based on JIS B 0601-2001, JIS B 0651-2001, the stylus type surface roughness tester is applied to the surface of the ferritic stainless steel plate or steel pipe, and the length direction The reference length lr (λc) for the roughness curve is 0.8 mm, the reference length lw (λf) for the waviness curve is 8 mm, and the reference length lp for the cross-section curve, that is, the evaluation length ln is 40 mm. The measured value.
(Bent pipe manufacturing method)
FIG. 6 shows one embodiment of a method for manufacturing a curved pipe having a cross-sectional shape for parts, which is an application of the present invention.

  The material used was a ferritic stainless steel pipe having the components and mechanical properties shown in Table 3. The shape of the cross section of each part of the ferritic stainless steel pipe after completion of the final processing including hydroforming described in detail below was the same as that shown in FIG.

  First, bend the ferritic stainless steel pipe. The bending radius of curvature is 1D (bending radius of curvature = the diameter of the ferritic stainless steel pipe material), and after performing bending at 180 degrees, a die 40 having a transfer inner shape to be machined. And hydroforming to apply an axial pushing force to at least one end of the ferritic stainless steel pipe, and to apply the hydraulic pressure to the ferritic stainless steel pipe. Have a cross-sectional shape.

The series of manufacturing steps will be described in further detail below.
The goal of the shape after bending is the shape of the ferritic stainless steel pipe after final processing including hydroforming, and the direction X of the tube center axis at one end is the direction of the tube center axis at the other end. Since it is U-shaped at an angle of 180 degrees with Y, first bending is performed at 180 degrees. The bending process is performed by a method called rotational pulling bending. Although the rotary pulling machine used here is not shown, it can be bent up to 90 degrees at a time because of its mechanical specifications. The number of man-hours can be reduced by dividing the process into times. However, in theory, bending of a ferritic stainless steel pipe after final processing, including hydroforming, can be performed at a maximum of 180 degrees at a time, as long as the degree of thinning of the portion where bending was performed is allowed. It can also be done.

On the other hand, in FIG. 6 (b), the bending process of 180 degrees is performed in three steps of 60 degrees, but as the number of bending processes is increased in this way, the angle of one bending process becomes smaller. Further, it is possible to suppress the thinning of the portion where the bending of the ferritic stainless steel pipe after the final processing including the hydroforming is performed.
However, as the number of times of bending increases, the man-hour increases and the production efficiency decreases. From the viewpoint of suppressing this, it is preferable that bending of 180 degrees is completed in three times or less, and the angle is not necessarily equal. There is no need to bend each step, for example, 70 degrees, 55 degrees, and 55 degrees may be performed. For this reason, the bending process is preferably performed so that the bending angle at one time is 70 degrees or less.

After bending at 180 °, annealing may be performed prior to hydroforming. This is because the ductility is restored by annealing.
FIG. 6 (c) shows how to perform hydroforming. In hydroforming, a ferritic stainless steel pipe after bending at 180 degrees is set in a mold having a transfer inner shape of the outer shape to be processed, and hydraulic pressure is applied to the ferritic stainless steel pipe and the ferrite stainless steel pipe is applied. An axial pushing force is applied to at least one end of the stainless steel pipe. In order to be able to set the ferritic stainless steel pipe, which is the material, the mold 40 is divided as shown in the lower side (side view) of FIG. After the steel pipe is set, it is fastened so that it does not open due to the action of hydraulic pressure and axial pushing force.

By applying hydraulic pressure to the ferritic stainless steel pipe, the ferritic stainless steel pipe swells and comes into contact with the mold that has been pre-processed into the transferred inner shape of the outer shape to be processed. At the same time, axial pressing force is applied to both ends of the ferritic stainless steel pipe.
By applying the axial pushing force, the thinning that occurred during the 180 degree bending process is prevented from further progressing, and the compression by the axial pushing reduces the thinning during the 180 degree bending process. A portion of the ferritic stainless steel pipe that is the material is supplied to the portion where the occurrence of this occurs, whereby the thinning can be suppressed or compensated for the thinning.

However, excessive axial pushing causes buckling of ferritic stainless steel tubes. It is important to appropriately adjust the relationship between the hydraulic pressure and the axial pushing force.
Using the components and mechanical properties shown in Table 4, bending at 180 degrees was performed in three portions of 60 degrees, followed by hydroforming to produce a U-shaped tube shown in FIG. Table 4 also shows whether or not molding is possible. Further, when the molding was completed without any cracks or bucklings, the thickness at the center of the outer periphery of the bending was measured, and the results are also shown in Table 4. The plate thickness measurement location at the center of the outer periphery of the bending is shown in FIG. In the example of the present invention, it can be seen that the desired molding can be achieved and the thinning is suppressed to a small extent. Looking at the comparative example, when the material does not satisfy El ≧ 25%, the bending does not occur at 180 degrees, and a crack (unmoldable) occurs. When the material does not satisfy TS ≦ 500Mpa, hydroforming forms buckling (cannot be molded). Further, it can be seen that if the material does not satisfy the r value ≧ 1.3, the thickness reduction is large even if the material can be formed into the target shape. Therefore, it can be seen that the ferritic stainless steel pipe according to the present invention is suitable for manufacturing a metal bent pipe having a cross-sectional shape for parts without causing cracking or buckling.

As described above, the embodiments of the present invention are not limited to those described above.
For example, as a method of bending, rotary pull bending, which is widely used as a method with relatively high mass productivity, has been taken as an example, but press bending (also referred to as pressing bending), tensile bending (also referred to as pressing bending), roll Any known bending method such as bending or high-frequency local heating bending can be used, and an optimal bending method may be selected and used as appropriate according to the material, product dimensions, and shape.

  Furthermore, the shape of the cross-section of each part of the curved pipe having a cross-sectional shape for parts is not limited to that shown in FIG. 10 or FIGS. 12 to 14, and the corresponding part of the mold compared to the diameter of the raw pipe If the bent metal tube cannot be set in the mold as it is, such as where the cross-sectional dimension is smaller, place the bent metal tube in the die after appropriate crushing. Alternatively, it may be set while being crushed by the fastening force of the mold.

  Furthermore, in the above-described embodiment, the axial pushing force is applied to both ends of the ferritic stainless steel pipe, but depending on the target outer shape, it is not always necessary to apply the axial pushing force to both ends. The axial pushing force may be applied to at least one end.

The present invention is suitable, for example, for manufacturing a curved pipe having a cross-sectional shape for parts as described below.
FIG. 8 shows an example of a pipe used for an automobile exhaust pipe as an example of a ferritic stainless curved pipe having a cross-sectional shape for parts. Reference numeral 101 denotes an automobile engine. In this example, a V-type 6-cylinder engine is mounted horizontally on the vehicle body. Reference numeral 102 denotes an exhaust manifold that collects exhaust gas discharged from the three cylinders on the front side of the vehicle body among the six cylinders. Reference numeral 103 denotes another exhaust manifold that collects exhaust gas discharged from the rear three cylinders of the vehicle body.

Exhaust gases exhausted from both exhaust manifolds are further combined into one in the collecting pipe 106 via the exhaust pipes 104 and 105, and exhausted to the rear of the vehicle body.
At this time, the exhaust pipes 104 and 105 are required to be designed to have the same length in order to improve the engine performance by aligning the performance of the front three cylinders and the rear three cylinders of the engine 101 and eliminating the mutual interference of exhaust. .

However, considering the positional relationship between the front three cylinders and the rear three cylinders from the arrangement of the engine, the exhaust pipe 105 may be forced to have a shape bent nearly 180 degrees as shown in FIG. 10 and 12 to 14 described above are enlarged views of two typical examples of such an exhaust pipe 105. Usually, the required bending angle is 160 degrees or more.
Moreover, in order to improve engine performance, it is desirable that the cross-sectional area of the exhaust pipe (directly, the diameter D in FIGS. 10, 13 and 14) be as large as possible, and a very narrow space under the engine. In order to reduce the size of the component, it is desirable that the bending radius (the bending radius R in the central axis of the tube in FIGS. 10, 13, and 14) be as small as possible. That is, the R / D value is desired to be as small as possible, but for automobiles, the R / D is preferably 1.2 or less.

Also, even if it is made of a ferritic stainless steel pipe with the same diameter, the cross-sectional shape of the exhaust pipe is round or reduced due to problems such as reducing the resistance of gas passing through it and dealing with other parts. It is desirable that the cross-sectional shape change in the direction of the central axis as shown in FIGS.
As mentioned above, although the exhaust pipe for automobiles has been described, there are other parts 201 used for the radiator piping of the air conditioner parts shown in FIG.

  In the case of a ferritic stainless steel curved pipe having a cross-sectional shape for such a part, the piping needs to be arranged as densely as possible. Therefore, the bending angle of the ferritic stainless steel curved pipe having the cross-sectional shape for a part shown in FIG. It is 180 degrees. R / D is 1.0 or less.

It is a figure which shows the effect of this invention. It is a figure which shows the effect of this invention. It is a figure which shows the effect of this invention. It is a figure which compares and shows the plate | board thickness distribution of the bending process outer periphery at the time of carrying out the bending process of 180 degree | times with a Hi-r material and a Lo-r material divided into 3 times 60 degree | times. It is a figure which shows an example of the metal tube which makes the direction of the axis | shaft in one end the angle of the axis | shaft in the other end, and 60 degrees or more. FIG. 6 is a diagram showing a U-shaped tube manufactured by hydroforming after bending at 180 degrees. It is a figure which shows the position of the plate | board thickness measurement point of a bending outer periphery center part. It is a figure for demonstrating the industrial applicability of this invention. It is a figure for demonstrating the industrial applicability of this invention. It is a figure which shows the example of the curved pipe which has the cross-sectional shape for components. It is a figure which shows the manufacturing method of the curved pipe with the cross-sectional shape for the conventional parts. It is a figure which shows another example of the curved pipe with the cross-sectional shape for components. It is a figure which shows another example of the curved pipe with the cross-sectional shape for components. It is a figure which shows another example of the curved pipe with the cross-sectional shape for components.

Explanation of symbols

10 Metal plate 20 Flange 30 Overlay 40 Mold X Direction of tube center axis at one end Y Direction of tube center axis at other end
Z portion pressed against the mold W side line portion θ Angle formed by the direction of the tube center axis at one end of the metal tube and the direction of the tube center axis at the other end of the metal tube 101 Automotive engine 102, 103 Exhaust manifold 104 , 105 Exhaust pipe 106 Collecting pipe 201 Parts used for radiator piping

Claims (8)

Ferritic stainless steel plate with tensile strength TS of 500 MPa or less, total elongation El of 25% or more, and average r value of 1.3 or more.
Wrapped in a tube,
Weld the butt surface to make a ferritic stainless steel pipe,
Bending such that the bending radius at the tube central axis is 1.2 times or less the diameter of the ferritic stainless steel tube,
The direction of the tube center axis at one end of the ferritic stainless steel tube is at an angle of 160 degrees or more with the direction of the tube center axis at the other end,
A ferritic stainless steel sheet for bent pipes having a cross-sectional shape for parts, which is processed so as to have a cross-sectional shape for parts. The ferritic stainless steel sheet according to claim 1,
In mass%, Cr: 10-20%, C: 0.05% or less, Mn: 0.5% or less, Ni: 1.0% or less, N: 0.05% or less, P: 0.04% or less, Si: 0.5% or less, Al: 0.2 % Or less, S: 0.005% or less,
A ferritic stainless steel sheet for curved pipes having a cross-sectional shape for parts, comprising the remaining Fe and inevitable impurities.   In addition to the above, it contains one or two selected from Ti: 0.5% or less and Nb: 1.0% or less, and satisfies (2Ti + Nb) ≧ 16 (C + N) A ferritic stainless steel sheet for curved pipes having a cross-sectional shape for parts according to claim 2.   4. In addition to the above, it further contains one or two kinds selected from Mo: 4.0% or less and Cu: 4.0% or less, having the cross-sectional shape for parts according to claim 2 or 3, Ferritic stainless steel sheet for curved pipes. Ferritic stainless steel pipes with tensile strength TS of 510 MPa or less, total elongation El of 20% or more, and average r value of 1.3 or more.
Bending such that the bending radius at the tube central axis is 1.2 times or less the diameter of the ferritic stainless steel tube,
The direction of the tube center axis at one end of the ferritic stainless steel tube is at an angle of 160 degrees or more with the direction of the tube center axis at the other end,
A ferritic stainless steel pipe for a bent pipe having a cross-sectional shape for a part, which is processed so as to have a cross-sectional shape for the part. The ferritic stainless steel pipe according to claim 5,
In mass%, Cr: 10-20%, C: 0.05% or less, Mn: 0.5% or less, Ni: 1.0% or less, N: 0.05% or less, P: 0.04% or less, Si: 0.5% or less, Al: 0.2 % Or less, S: 0.005% or less,
A ferritic stainless steel pipe for a bent pipe having a cross-sectional shape for a part, characterized by comprising the balance Fe and inevitable impurities.   In addition to the above, it contains one or two selected from Ti: 0.5% or less and Nb: 1.0% or less, and satisfies (2Ti + Nb) ≧ 16 (C + N) A ferritic stainless steel pipe for a curved pipe having a cross-sectional shape for a part according to claim 6.   In addition to the above, it further contains one or two kinds selected from Mo: 4.0% or less and Cu: 4.0% or less. The cross-sectional shape for parts according to claim 6 or 7, Ferritic stainless steel pipe for curved pipes.

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