JP2020050931A - Ferritic stainless steel, ferritic stainless steel pipe, pipe end thickening structure, and weldment structure - Google Patents

Abstract

To provide a ferritic stainless steel having enhanced corrosion resistance in a gap structure at a pipe end thickening part, a ferritic stainless steel pipe, a pipe end thickening structure, and a weldment structure.SOLUTION: There is adopted a ferritic stainless steel in which a steel part contains, by mass%, C:0.001 to 0.100%, Si:0.01 to 5.00%, Mn:0.01 to 2.00%, P:≤0.050%, S:≤0.0100%, Cr:9.0 to 30.0%, Sn:0.001 to 3.00%, one or two kind of Ti:0.01 to 1.00%, and Nb:0.01 to 1.00%, Al:0.010 to 5.000%, N:0.001 to 0.050% and the balance Fe with impurities, a thickening part consisting of a folding flexure part is arranged at an end of the steel part, and gap interval d (μm) formed at the thickening part satisfies d≥Cr/{1000(Al+Si+Sn)}, where Cr, Al, Si and Sn represent content of each element (mass%).SELECTED DRAWING: None

Description

  The present invention relates to a ferritic stainless steel, a ferritic stainless steel pipe, a pipe end thickened structure, and a welded structure.

  Ferritic stainless steels are used in a wide range of fields such as home appliances, electronic devices, and automobiles. Particularly in the field of automobiles, since it is used for various parts from exhaust manifolds to mufflers, the stainless steel used is required to have heat resistance, corrosion resistance, and the like. In addition, since these parts are mostly welded, strength, rigidity and corrosion resistance of the welded part are also required.

  2. Description of the Related Art In recent years, in order to reduce the weight of automobiles, the number of cases where the thickness of materials used for each component is being studied has been increasing. However, in order to ensure the strength, rigidity, and weldability of the welded portion, a certain thickness may be required, and the thickness of the non-welded portion is also large, which hinders a thin exhaust system as a whole. On the other hand, there is known a technique of increasing the thickness of a welded portion by increasing the thickness of a steel pipe end that is welded to another part by forming an exhaust pipe, increasing the strength of the welded portion, and securing rigidity and weldability. ing. This is referred to as pipe-end thickening (to increase the pipe end of a steel pipe). In this case, the thickness of the non-welded portion can be reduced, and the thickness and weight of the entire exhaust system can be reduced.

  Several techniques relating to the above-mentioned pipe end thickening are disclosed. Patent Document 1 discloses a process in which a roller is pressed against an end portion while rotating the pipe to bend radially inward, and then adhered by a roller in order to secure the strength of the end portion of the pipe and reduce the weight of the pipe. A method is disclosed. Patent Literature 2 discloses a method for preventing a burn-through during welding by forming a pipe end into a double tubular shape and doubling the wall thickness. Patent Literature 3 discloses a patent relating to a raw pipe in order to turn up the pipe end to increase the wall thickness, wherein an inner bead portion of a welded portion protrudes from the inner surface of the pipe, and the amount of protrusion is 4 to 15 of the plate thickness. %.

The pipes with increased pipe ends described in Patent Literatures 1 to 3 have a gap structure with a height of several μm to several hundred μm at the bent portion. When this gap is bent inward as in Patent Documents 1 and 2, exhaust gas condensed water generated inside the exhaust system components tends to stay in the gap. When bent outward as in Patent Literature 3, salt water adhering from the outside of the exhaust system component tends to stay in the gap.
The corrosion that occurs in this environment is not crevice corrosion, but is salt damage corrosion that is promoted by salt water and exhaust gas condensed water being more likely to stay in the crevice environment. As described above, corrosion in the gap may be promoted, and therefore, a stainless steel having excellent salt damage resistance in the gap is required as the stainless steel to be used. Particularly, in exhaust system components, perforation due to corrosion leads to leakage of exhaust gas, so it is important to apply a material having high resistance to perforation.

  Patent Document 4 discloses that, in mass%, C: 0.001 to 0.02%, N: 0.001 to 0.02%, Si: 0.01 to 0.5%, Mn: 0.05 to 1 %, P: 0.04% or less, S: 0.01% or less, Cr: 12 to 25%, and Ti: 0.02 to 0.5% and Nb: 0.02 to 1% A ferritic stainless steel excellent in crevice corrosion resistance, comprising one or both of them, further containing 0.005 to 2% of Sn, and the balance being Fe and unavoidable impurities is disclosed. . In the technique described in Patent Literature 4, crevice corrosion resistance is improved by adding Sn, but the relationship between the gap interval and the salt damage corrosion in the gap structure of the pipe end thickened portion is not described.

In Patent Document 5, in mass%, C: ≤ 0.015%, Si: 0.10 to 0.50%, Mn: 0.05 to 0.50%, P ≤ 0.050%, S: ≤ 0.0100%, N: ≤ 0.015%, Al: 0.020 to 0.100%, Cr: 10.5 to 13.05%, and further Ti: 0.03 to 0.30% And Nb: one or both of 0.03 to 0.30%, Sn: 0.03 to 0.50% and Sb: one or both of 0.03 to 0.50% The balance being Fe and unavoidable impurities, wherein the A value defined by the formula (2) is not less than 15.23, and which is excellent in corrosion resistance after heating and is low in alloy type for automobile exhaust system members. A series stainless steel is disclosed.
A = [Cr] + [Si] +0.5 [Mn] +10 [Al] +15 ([Sn] + [Sb]) Equation (2)
In the technique described in Patent Document 5, the corrosion resistance after heating is improved by adding Sn and Sb, but the relationship between the gap interval and the salt damage corrosion in the gap structure of the pipe end thickened portion is described. Absent.

  In Patent Document 6, in mass%, C: ≤ 0.015%, Si: 0.01 to 0.50%, Mn: 0.01 to 0.50%, P ≤ 0.050%, S: ≤ 0.010%, N: ≤ 0.015%, Al: 0.010 to 0.100%, Cr: 16.5 to 22.5%, and Ti: 0.03 to 0.30% And Nb: one or both of 0.03 to 0.30%, Sn: 0.05 to 1.00%, the balance being Fe and unavoidable impurities. Mo-saving ferritic stainless steel for automobile exhaust system members having excellent post-heating corrosion resistance is disclosed. In the technique described in Patent Document 6, the corrosion resistance after heating is improved by adding Sn. However, the relationship between the gap interval and the salt damage corrosion in the gap structure of the pipe end thickened portion is not described.

  In Patent Document 7, in mass%, C: ≤ 0.015%, Si: 0.01 to 0.50%, Mn: 0.01 to 0.50%, P ≤ 0.050%, S: ≤ 0.010%, N: 0.015%, Al: 0.010 to 0.100%, Cr: 16.5 to 22.5%, Ni: 0.5 to 2.0%, Sn: 0. 0.01 to 0.50%, and further contains one or both of Ti: 0.03 to 0.30% and Nb: 0.03 to 0.30%, with the balance being Fe and unavoidable. A ferritic stainless steel for automobile exhaust system members characterized by being composed of impurities is disclosed. The technique described in Patent Document 7 discloses the corrosion resistance of exhaust system components after heating, but does not describe the relationship between the gap interval and the salt damage corrosion in the gap structure of the pipe end thickened portion.

In Patent Document 8, C: 0.0150% or less, Si: 1.0 to 1.5%, Mn: 0.15 to 1.0%, P: 0.050% or less, S: 0.0100% or less, N: 0.0150% or less, Al: 0.010 to 0.200%, Cr: 13.0 to 16.0%, and Sn: 0.002 to 0.050% And any one or both of Ti: 0.03 to 0.30% and Nb: 0.03 to 0.50%, and the A value defined by the formula (1) is 0.024 or more. A ferritic stainless steel for automobile exhaust system members excellent in oxidation resistance and corrosion resistance, which satisfies certain requirements and has a balance of Fe and inevitable impurities, is disclosed.
A = [Si] × [Sn] +0.014 [Si] (1)
Here, [Si] and [Sn] are the contents of Si and Sn as mass%, respectively.
The technique described in Patent Document 8 discloses the corrosion resistance of exhaust system components after heating, but does not describe the relationship between the gap distance and the salt damage corrosion in the gap structure of the pipe end thickening portion.

In Patent Document 9, C: 0.0150% or less, Si: 0.2 to 0.7%, Mn: 0.2 to 0.6%, P: 0.050% or less, S: 0.0100% or less, N: 0.0150% or less, Al: 0.010 to 0.20%, Cr: 10.5 to 11.5%, Mo: 0.02 to 0.20%, and Sn: 0.005 to 0.050%, and further contains one or both of Ti: 0.03 to 0.30% and Nb: 0.03 to 0.50%, and the following (1) The present invention discloses a ferritic stainless steel for an exhaust system member having excellent corrosion resistance, which satisfies that the A value defined by the formula is not less than 0.00065% ² and the balance consists of Fe and unavoidable impurities. ing.
A = [Mo] × [Sn] (1)
The technique described in Patent Document 9 discloses the corrosion resistance of exhaust system components after heating, but does not describe the relationship between the gap interval and the salt damage corrosion in the gap structure of the pipe end thickened portion.

  As described above, the prior art has not yet proposed a method of improving the corrosion resistance of the gap structure formed in the pipe end thickened portion of the pipe having the increased pipe end.

JP 2010-234406 A JP 2013-103250 A JP 2004-255414 A Japanese Patent No. 4727601 Japanese Patent No. 5297713 Japanese Patent No. 5320034 Japanese Patent No. 5586279 Japanese Patent No. 606660 JP 2014-169492 A

  The present invention has been made in view of the above circumstances, and provides a ferritic stainless steel, a ferritic stainless steel pipe, a pipe end thickened structure, and a welded structure having improved corrosion resistance in a gap structure of a pipe end thickened portion. The task is to provide.

  In order to solve the above-mentioned problems, the present inventors have made intensive studies on the corrosion resistance of the gaps in ferritic stainless steel pipes. As a result, it was found that in a gap environment, the pitting depth increases as the stainless steel content increases. Then, they have found that there is a relationship between the amounts of Cr, Al, Si, and Sn and the critical gap interval where pitting corrosion grows deeply.

Means for solving the above problem has the following configuration.
[1] Steel part is mass%
C: 0.001 to 0.100%,
Si: 0.01-5.00%,
Mn: 0.01-2.00%,
P: ≦ 0.05%
S: ≦ 0.0100%,
Cr: 9.0-30.0%,
Sn: 0.001 to 3.00%,
One or two of Ti: 0.01 to 1.00% and Nb: 0.01 to 1.00%,
Al: 0.010 to 5.000%,
N: 0.001 to 0.050%, the balance being Fe and impurities,
A thickened portion including a folded portion is provided at an end of the steel portion, and a gap d (μm) formed in the thickened portion is d ≧ Cr ² / {1000 (Al + Si + Sn)} (Cr in the formula) , Al, Si and Sn indicate the contents (% by mass) of the respective elements).
[2] Further, in mass%,
Ni: 0.01 to 3.00%,
Mo: 0.01 to 3.00%,
Cu: 0.01 to 3.00%,
B: 0.0001 to 0.0100%,
W: 0.001 to 1.000%,
V: 0.001 to 1.000%,
Sb: 0.001 to 0.100%,
Co: 0.001 to 0.500%,
Ca: 0.0001 to 0.0050%,
Mg: 0.0001 to 0.0050%,
Zr: 0.0001-0.0300%,
Ga: 0.0001-0.0100%,
Ta: 0.001 to 0.050%,
REM: 0.001 to 0.100%
The ferritic stainless steel according to [1], wherein the ferritic stainless steel contains one or more of the following.
[3] having a steel pipe portion composed of a steel base material portion and a welded portion,
The steel base material portion is represented by mass%
C: 0.001 to 0.100%,
Si: 0.01-5.00%,
Mn: 0.01-2.00%,
P: ≦ 0.05%
S: ≦ 0.0100%,
Cr: 9.0-30.0%,
Sn: 0.001 to 3.00%,
One or two of Ti: 0.01 to 1.00% and Nb: 0.01 to 1.00%,
Al: 0.010 to 5.000%,
N: 0.001 to 0.050%, the balance being Fe and impurities,
A tube end thickening portion formed by a folded portion is provided at a tube end of the steel tube portion, and a gap d (μm) formed in the tube end thickening portion is d ≧ Cr ² / {1000 (Al + Si + Sn)}. (Cr, Al, Si, and Sn in the formulas indicate the content (% by mass) of each element).
[4] Further, in mass%,
Ni: 0.01 to 3.00%,
Mo: 0.01 to 3.00%,
Cu: 0.01 to 3.00%,
B: 0.0001 to 0.0100%,
W: 0.001 to 1.000%,
V: 0.001 to 1.000%,
Sb: 0.001 to 0.100%,
Co: 0.001 to 0.500%,
Ca: 0.0001 to 0.0050%,
Mg: 0.0001 to 0.0050%,
Zr: 0.0001-0.0300%,
Ga: 0.0001-0.0100%,
Ta: 0.001 to 0.050%,
REM: 0.001 to 0.100%
The ferrite-based stainless steel pipe according to [3], wherein the ferrite-based stainless steel pipe contains at least one of the following.
[5] The ferritic stainless steel pipe according to [3] or [4], wherein the pipe end thickened part is expanded or contracted with respect to the steel pipe part.
[6] A tube end thickening structure comprising the stainless steel tube according to any one of [3] to [5].
[7] A welded structure, wherein the pipe end thickened portion of the pipe end thickened structure according to [6] and a steel pipe member are joined by overlapping fillet welds.
[8] The maximum penetration depth of the lap fillet welded part on the side of the pipe end thickened part is in the range of 0.3 t to 2.0 t with respect to the wall thickness t of the steel pipe part. [7] The welding structure according to [7].

  According to the present invention, it is possible to provide a ferritic stainless steel, a ferritic stainless steel pipe, a tube end thickened structure, and a welded structure having improved corrosion resistance in the gap structure of the tube end thickened portion.

FIG. 1 is a schematic cross-sectional view showing an example of a welded structure including a stainless steel pipe (a pipe-end thickened structure) and another steel pipe (a steel pipe member) according to the embodiment. FIG. 2 is a schematic cross-sectional view showing another example of a welded structure including the stainless steel tube (tube end thickened structure) of the embodiment and another steel tube (steel tube member). FIG. 3 is a schematic cross-sectional view showing another example of a welded structure including the stainless steel pipe (tube end thickened structure) of the embodiment and another steel pipe (steel pipe member). Drawing 4 is a figure showing the important section of the welding structure consisting of the stainless steel pipe (tube end thickening structure) of the embodiment, and another steel pipe (steel pipe member), and is a cross section showing the maximum penetration depth. FIG. FIG. 5 is a schematic cross-sectional view showing a main part of a welded structure including a stainless steel pipe (a pipe end thickened structure) and another steel pipe (a steel pipe member) according to the embodiment.

  At one end in the longitudinal direction of the steel pipe, the end of the steel pipe is turned radially outward or radially inward to form a folded portion. At the folded portion, the thickness of the steel pipe is increased. Therefore, the folded portion formed at the end of the steel pipe is called a pipe end thickened part. When forming the pipe end thickened part, although processing is performed so that the folded end is closely attached to the outer peripheral surface or the inner peripheral surface of the steel pipe, the folded end and the outer peripheral surface or the inner peripheral surface of the steel pipe are There is a slight gap between them.

The present inventors produced steel sheets of various compositions in order to evaluate the corrosion resistance by simulating the gap environment of the pipe end thickened portion of a ferritic stainless steel pipe. Then, test pieces having various gaps simulating the gaps of the pipe-end thickened pipe were produced from these steel sheets by spot welding. According to the corrosion test method for the appearance of automotive parts of JASO-M610-92, a corrosion test was performed 100 cycles to evaluate the salt damage corrosion of the gap. The maximum pit depth was used for evaluation. Samples with a maximum pit depth of less than 500 μm were evaluated as ““ ”(good), and samples with a maximum pit depth of 500 μm or more were evaluated as“ x ”(poor). evaluated. As a result, as shown in Table 1, Table 2A, and Table 2B described later, it was found that when d ≧ Cr ² / {1000 (Al + Si + Sn)}, the maximum pitting depth was reduced.

  Observation of the form of corrosion in the gaps of the steel type with a high Cr content revealed that a small number of pits grew deeply. On the other hand, it was found that the pitting corrosion form of the low Cr content steel type had a large number of pits, but the depth of each pit was shallower than that of the high Cr content steel type. .

  It is considered that, in the steel type having a high Cr content, the number of occurrences of pitting corrosion decreased because the Cr concentration in the passive film was high and the corrosion resistance was high. Therefore, it is considered that the oxygen reduction reaction, which is a cathode reaction, contributes only to the growth of a small number of pits, and each pit has grown deeply. On the other hand, in the case of a steel type having a low Cr content, the cathode reaction contributes to the generation of a large number of pits.

In addition, it was found from the above-mentioned test that Al, Si and Sn are effective in generating pitting corrosion in a gap environment. It has been known that Sn suppresses active dissolution of stainless steel and improves crevice corrosion resistance. However, the fact that Sn suppresses the occurrence of pitting corrosion in the gap environment and reduces the critical gap distance is new knowledge based on the results of this test. It is considered that Al ^elutes as Al ³⁺ ions inside the pit at the beginning of generation and is adsorbed on the surface, thereby promoting the suppression of the pit growth and the re-passivation. It is considered that Si forms an oxide inside the pit and promotes suppression of pit growth and re-passivation.

  The corrosion phenomenon in the gap between the thickened part and the pipe end thickened part of the present embodiment is a corrosion phenomenon different from the conventional crevice corrosion, and the corrosion phenomenon in the gap between the thickened part and the pipe end thickened part of the present embodiment. The corrosion phenomenon that occurs is a corrosion phenomenon in the case where the gap is wider than the “gap” of the conventional crevice corrosion, and its occurrence mechanism is different from that of the conventional crevice corrosion.

Hereinafter, the present embodiment will be described in detail.
In the ferritic stainless steel of the present embodiment, the steel portion is represented by mass%, C: 0.001 to 0.100%, Si: 0.01 to 5.00%, Mn: 0.01 to 2.00%. , P: ≤ 0.050%, S: ≤ 0.0100%, Cr: 9.0 to 30.0%, Sn: 0.001 to 3.00%, Ti: 0.01 to 1.00%, and Nb: one or two kinds of 0.01 to 1.00%, Al: 0.010 to 5,000%, N: 0.001 to 0.050%, the balance being Fe and impurities, A thickened portion comprising a folded portion is provided at the pipe end of the steel portion, and a gap d (μm) formed in the thickened portion is d ≧ Cr ² / {1000 (Al + Si + Sn)} (wherein (Cr, Al, Si, and Sn indicate the contents (% by mass) of the respective elements.) It is steel.
Further, the ferritic stainless steel of the present embodiment further includes Ni: 0.01 to 3.00%, Mo: 0.01 to 3.00%, Cu: 0.01 to 3.00% by mass%. B: 0.0001 to 0.0100%, W: 0.001 to 1.000%, V: 0.001 to 1.000%, Sb: 0.001 to 0.100%, Co: 0.001 to 0.500%, Ca: 0.0001 to 0.0050%, Mg: 0.0001 to 0.0050%, Zr: 0.0001 to 0.0300%, Ga: 0.0001 to 0.0100%, Ta : 0.001 to 0.050%, and REM: 0.001 to 0.100%.

Further, the ferritic stainless steel pipe of the present embodiment has a steel pipe part composed of a steel base material part and a welded part, and the steel base material part is represented by mass%, C: 0.001 to 0.100%, Si : 0.01 to 5.00%, Mn: 0.01 to 2.00%, P: ≤ 0.050%, S: ≤ 0.0100%, Cr: 9.0 to 30.0%, Sn: 0.001 to 3.00%, one or two of Ti: 0.01 to 1.00% and Nb: 0.01 to 1.00%, Al: 0.010 to 5,000%, N: 0.001 to 0.050%, the balance being Fe and impurities, a pipe end thickening portion comprising a folded portion is provided at the pipe end of the steel pipe portion, and formed at the pipe end thickening portion. that the gap distance d ([mu] m) ^{is, d ≧ Cr 2 / {1000} (Al + Si + Sn)} (Cr in the formula, Al, Si and Sn are each original A ferritic stainless steel which satisfies the relationship of the content of shows (mass%)).
Further, the ferritic stainless steel pipe of the present embodiment further includes Ni: 0.01 to 3.00%, Mo: 0.01 to 3.00%, Cu: 0.01 to 3.00% by mass%. B: 0.0001 to 0.0100%, W: 0.001 to 1.000%, V: 0.001 to 1.000%, Sb: 0.001 to 0.100%, Co: 0.001 to 0.500%, Ca: 0.0001 to 0.0050%, Mg: 0.0001 to 0.0050%, Zr: 0.0001 to 0.0300%, Ga: 0.0001 to 0.0100%, Ta : 0.001 to 0.050%, and REM: 0.001 to 0.100%.

  Hereinafter, the chemical composition of the steel part of the ferritic stainless steel and the steel base material part of the ferritic stainless steel pipe specified in the present embodiment will be described in more detail. In addition,% means mass%.

  C reduces the intergranular corrosion resistance and workability, so its content must be kept low. Therefore, the upper limit of the content of C is set to 0.100% or less. However, excessively lowering the amount of C increases the refining cost, so the lower limit of the amount of C is set to 0.001% or more. A preferred range of the C content is 0.002 to 0.010%.

  Si is a very useful element that not only concentrates on the surface to suppress corrosion but also reduces the corrosion rate of the base material. Therefore, the lower limit of the content of Si is set to 0.01% or more. However, excessive Si content causes a reduction in elongation of the steel and lowers workability, so the upper limit of the Si content is set to 5.00% or less. A preferable range of the Si amount is 0.30 to 3.00%, and a more preferable range is 0.70 to 1.20%.

  Mn is useful as a deoxidizing element, but if an excessive amount of Mn is contained, corrosion resistance is degraded. Therefore, the Mn content is set to 0.01 to 2.00%. A preferred range of the Mn content is 0.05 to 1.00%, and a more preferred range is 0.02 to 0.50%.

  Since P is an element that deteriorates workability and weldability, its content must be limited. Therefore, the P content is set to 0.050% or less. A preferred range of the P content is 0.030% or less.

  Since S is an element that deteriorates corrosion resistance, its content needs to be limited. Therefore, the amount of S is set to 0.0100% or less. The preferable range of the amount of S is 0.0070% or less.

  Cr must be contained in an amount of 9.0% or more in order to secure corrosion resistance in a salt damage environment. As the Cr content increases, the corrosion resistance improves, but the workability and manufacturability decrease. Therefore, the upper limit of the amount of Cr is set to 30.0% or less. A preferable range of the Cr content is 9.5 to 25.0%, and a more preferable range is 10.0 to 15.0%.

  Sn can be contained in an amount of 0.001% or more to improve corrosion resistance. However, an excessive content leads to an increase in cost. Therefore, the upper limit of the amount of Sn is set to 3.00% or less. The preferred range of the Sn amount is 0.005 to 1.00%.

  Ti and / or Nb must contain at least 0.01% of either or both in order to prevent sensitization of stainless steel. When the content is less than 0.01%, the corrosion resistance is deteriorated due to sensitization. However, a large amount of Ti leads to an increase in alloy cost and toughness, a decrease in corrosion resistance due to an increase in inclusions in steel, and a decrease in manufacturability. A large amount of Nb decreases workability and manufacturability. The upper limits of the amounts are each 1.00%. The preferable ranges of the Ti and Nb contents are respectively 0.03 to 0.50%, and the more preferable ranges are respectively 0.10 to 0.25%.

  Al is a very useful element that not only concentrates on the surface to suppress the occurrence of corrosion but also reduces the corrosion rate of the base material. Therefore, the lower limit of the Al content is set to 0.010% or more. However, an excessive content of Al causes a reduction in elongation of the material and lowers workability. Therefore, the upper limit of the content of Al is set to 5.000% or less. The preferable range of the Al amount is 0.050 to 3.000%, and the more preferable range is 0.800 to 2.500%.

  N is an element useful for pitting corrosion resistance, but lowers intergranular corrosion resistance and workability. Therefore, it is necessary to keep the N content low. Therefore, the upper limit of the amount of N is set to 0.050% or less. However, excessively lowering the amount of N increases the refining cost, so the lower limit of the amount of N is set to 0.001% or more. A preferable range of the amount of N is 0.002 to 0.020%.

  The above is the basic chemical composition of the ferritic stainless steel and the ferritic stainless steel pipe of the present embodiment. In the present embodiment, the following elements can be further contained as necessary.

  Ni can be contained in an amount of 0.01% or more to improve corrosion resistance. However, since a large content leads to an increase in alloy cost, the upper limit of the Ni content is set to 3.00% or less. A preferred range of the Ni content is 0.02 to 1.00%.

  Mo can be contained in an amount of 0.01% or more to improve corrosion resistance. However, an excessive content deteriorates processability and leads to an increase in cost due to high cost. Therefore, the upper limit of the amount of Mo is set to 3.00% or less. The preferred range of the Mo amount is 0.05 to 1.00%.

  Cu can be contained in an amount of 0.01% or more to improve corrosion resistance. However, an excessive content leads to an increase in cost. Therefore, the upper limit of the amount of Cu is set to 3.00% or less. A preferable range of the Cu content is 0.02 to 1.00%, and a more preferable range is 0.05 to 0.09%.

  B is an element useful for improving the secondary workability, and can be contained in an amount of 0.0100% or less. The lower limit of the amount of B is set to 0.0001% or more at which a stable effect is obtained. The preferable range of the B content is 0.0005 to 0.0050%.

  W can be contained in an amount of 1.000% or less in order to improve corrosion resistance. In order to obtain a stable effect, the lower limit of the W amount is set to 0.001% or more. A preferable range of the W amount is 0.005 to 0.800%.

  V can be contained at 1.000% or less in order to improve corrosion resistance. In order to obtain a stable effect, the lower limit of the amount of V is set to 0.001% or more. A preferable range of the V amount is 0.005 to 0.500%.

  Sb can be contained in an amount of 0.100% or less in order to improve the overall corrosion resistance. In order to obtain a stable effect, the lower limit of the amount of Sb is set to 0.001% or more. A preferable range of the Sb amount is 0.010 to 0.080%.

  Co can be contained in an amount of 0.500% or less in order to improve secondary workability and toughness. In order to obtain a stable effect, the lower limit of the Co content is set to 0.001% or more. A preferred range of the Co amount is 0.010 to 0.300%.

  Ca is contained for desulfurization. However, if Ca is contained excessively, water-soluble inclusions CaS are generated to reduce the corrosion resistance. Therefore, Ca can be contained in the range of 0.0001 to 0.0050%. A preferable range of the Ca amount is 0.0005 to 0.0030%.

  Mg is also useful for refining the structure and improving workability and toughness. Therefore, Mg can be contained in a range of 0.0050% or less. In order to obtain a stable effect, the lower limit of the amount of Mg is set to 0.0001% or more. A preferable range of the Mg content is 0.0005 to 0.0030%.

  Zr can be contained in an amount of 0.0300% or less in order to improve corrosion resistance. In order to obtain a stable effect, the lower limit of the Zr amount is set to 0.0001% or more. A preferred range of the Zr amount is 0.0010 to 0.0100%.

  Ga can be contained in an amount of 0.0100% or less in order to improve corrosion resistance and hydrogen embrittlement resistance. In order to obtain a stable effect, the lower limit of the amount of Ga is set to 0.0001% or more. A preferable range of the Ga amount is 0.0005 to 0.0050%.

  Ta can be contained in an amount of 0.050% or less in order to improve corrosion resistance. In order to obtain a stable effect, the lower limit of the amount of Ta is set to 0.001% or more. A preferable range of the Ta amount is 0.005 to 0.030%.

Since REM has a deoxidizing effect and the like, and is a useful element in scouring, REM can be contained in 0.100% or less. In order to obtain a stable effect, the lower limit of the amount of REM is set to 0.001% or more. A preferable range of the REM amount is 0.003 to 0.050%.
Here, REM (rare earth element) is a general term for two elements, scandium (Sc) and yttrium (Y), and 15 elements (lanthanoids) from lanthanum (La) to lutetium (Lu) according to a general definition. . REM is at least one selected from these rare earth elements, and the amount of REM is the total amount of rare earth elements.

  The ferritic stainless steel of the present embodiment is composed of Fe and impurities (impurities include unavoidable impurities) other than the elements described above. Further, in addition to the above-described elements, they can be contained in a range that does not impair the effects of the present invention. In the present embodiment, for example, Bi, Pb, Se, H or the like may be contained, but in that case, it is preferable to reduce as much as possible. On the other hand, the content ratio of these elements is controlled to the extent that the object of the present invention is solved. If necessary, Bi is 0.01% or less, Pb is 0.01% or less, and Se is 0.01% or less. % Or less, and H may contain one or more of 0.01% or less.

Further, the ferritic stainless steel of the present embodiment is provided with a thickened portion including a folded portion at an end of the steel portion. The folded part is formed by folding the end of the steel part. At the folded portion, the thickness of the steel is increased. Therefore, the folded portion is referred to as a thickened portion. When forming the thickened portion, the folded end is worked so as to be in close contact with one surface or the other surface of the steel, but there is a slight gap between the folded end and one surface or the other surface of the steel. A gap is formed. The gap d (μm) of the gap existing in the thickened portion is d ≧ Cr ² / {1000 (Al + Si + Sn)} (where Cr, Al, Si and Sn are the contents of the respective elements in the steel material portion ( Mass%) is preferably satisfied.

  Next, the ferritic stainless steel pipe of the present embodiment (hereinafter, may be referred to as a stainless steel pipe) has a steel pipe part including a steel base material part having the above-described chemical components and a welded part. The steel base member is formed by forming a steel plate made of stainless steel having a steel component according to the present embodiment into a tubular shape. The welded portion is formed by welding the ends of a steel sheet formed into a tubular shape by ERW (resistance welding), laser welding, TIG welding (tungsten inert gas welding), or the like. The welding method may be appropriately selected. Also, the size of the steel pipe may be determined according to the application.

  The ferritic stainless steel pipe of the present embodiment is provided with a pipe end thickening portion formed of a folded portion at an end of the steel pipe portion. The pipe end thickening portion may be provided at one end of the stainless steel pipe, or may be provided at both ends. The folded part is formed by folding the end of the steel pipe part radially outward or radially inward. At the folded portion, the thickness of the steel pipe is increased. For this reason, the folded portion is referred to as a tube end thickened portion. When forming the pipe end thickened part, although processing is performed so that the folded end is closely attached to the outer peripheral surface or the inner peripheral surface of the steel pipe, the folded end and the outer peripheral surface or the inner peripheral surface of the steel pipe are A slight gap is formed between them.

  A stainless steel pipe provided with a pipe end thickening portion may be referred to as a pipe end thickening structure. 1 to 3 show a pipe end thickening portion formed at one end in the longitudinal direction of a steel pipe portion of a stainless steel pipe.

  FIG. 1 is an example in which a tube end thickening portion 1b is provided at one end of a steel tube portion 1a of a stainless steel tube 1. At one end of the steel pipe portion 1a, a part of the steel pipe portion is bent approximately 180 degrees inward in the radial direction to form a bent portion 1c. The folded portion 1c is bent so as to be in contact with the inner peripheral surface of the steel pipe portion 1a, and a tube end thickened portion 1b is formed by the folded portion 1c. The wall thickness of the pipe end thickened portion 1b is increased by the thickness of the folded back portion 1c with respect to the thickness of the steel pipe portion 1a, and is approximately twice the thickness of the steel pipe portion 1a. . A gap 1d is formed in the pipe end thickened portion 1b between the steel pipe portion 1a and the folded portion 1c. In the present embodiment, it is important to improve the corrosion resistance in the gap 1d.

  Further, another steel pipe 2 is joined to the stainless steel pipe 1 (tube end thickened structure) shown in FIG. A welded structure A is formed by the stainless steel pipe 1 (tube end thickened structure) and another steel pipe 2 (steel pipe member). As shown in FIG. 1, the pipe end thickened portion 1 b of the stainless steel pipe 1 is set to the male side, the end 2 a of the steel pipe 2 is set to the female side, and the pipe end thickened portion 1 b is inserted into the end 2 a of the steel pipe 2. I have. A fillet weld 3 is formed between the outer surface of the pipe end thickened portion 1b and the end 2a of the steel pipe 2.

  FIG. 2 shows another example of the welded structure B. In the welded structure B shown in FIG. 2, similarly to the case of FIG. 1, another steel pipe 2 is joined to the stainless steel pipe 1 (pipe-end thickened structure) via a fillet weld 3. The difference from FIG. 1 is that the tube end thickened portion 1b of the stainless steel tube 1 is expanded with respect to the steel tube portion 1a.

  FIG. 3 shows another example of the welded structure C. In the welded structure C shown in FIG. 3, similarly to the case of FIG. 1, another steel pipe 2 is joined to the stainless steel pipe 1 (tube end thickened structure) via a fillet weld 3. The difference from FIG. 1 is that the tube end thickened portion 1b of the stainless steel tube 1 is contracted with respect to the steel tube portion 1a.

  In addition, in the welded structures A to C shown in FIGS. 1 to 3, an example is shown in which the overlap fillet weld 3 is formed between the outer peripheral surface of the pipe end thickened portion 1 b and another steel pipe 2. However, the present embodiment is not limited to this, and a steel pipe having an outer diameter slightly smaller than the inner diameter of the pipe end thickening portion 1b is inserted into the inside of the tube end thickening portion 1b, and the inner periphery of the tube end thickening portion 1b is inserted. A fillet weld 3 may be formed between the surface and another steel pipe 2.

In the ferritic stainless steel pipe 1 ((tube end thickened structure) of the present embodiment, the interval d (μm) of the gap 1d existing in the tube end thickened portion 1b is d ≧ Cr ² / {1000 (Al + Si + Sn) } (Cr, Al, Si, and Sn in the formula preferably represent the content (mass%) of each element in the steel base material). The interval d (μm) of the gap 1d refers to the maximum value of the interval of the gap 1d between the steel pipe portion 1a and the folded portion 1c.

  In addition, in the welded structures A to C shown in FIGS. 1 to 3, the maximum penetration depth of the overlap fillet welded portion 3 on the side of the pipe end thickened portion 1 b is 0 with respect to the wall thickness t of the steel pipe portion 1. It is preferable to be in the range of 0.3 to 2.0 t. By setting the maximum penetration depth to 0.3 t or more, the strength of the overlap fillet weld 3 is ensured, and the corrosion resistance in the gap 1d can be further improved. However, if the maximum welding depth exceeds 2.0 t, the shape of the welded portion becomes uneven, which may lead to various problems such as a decrease in strength, deterioration of corrosion resistance, and leakage of exhaust gas. It is good to be 2.0t or less.

Note that the maximum penetration depth, as shown in FIG. 4, to the outer peripheral surface of the pipe end thickening unit 1b, a distance d ₃ between the deepest portion of the fillet welds 3 to the pipe end thickening portion.

The reason why the corrosion resistance in the gap 1d can be further improved by setting the maximum penetration depth to 0.3t or more is that the shape of the welded portion of the pipe end thickened portion 1b is stabilized, and a gap structure that can serve as a corrosion starting point is not formed. it is conceivable that. Further, when the maximum penetration depth is more than 1.0 t, the gap 1d in the pipe end thickened portion 1b is closed, and the gap structure that can be a corrosion starting point is further reduced. In addition to this, the ferritic stainless steel pipe (tube end thickened structure) of the present embodiment contains 0.010 to 5,000% of Al and 0.01 to 5.00% of Si in steel. And Sn in an amount of 0.001 to 3.00%. Therefore, even in the event of corrosion, the eluted Al ³⁺ ions and Sn ²⁺ ions are adsorbed on the dissolution surface to suppress further elution of the steel base material, and furthermore, Si forms an oxide inside the pit corrosion. In addition, it is considered that deterioration of the corrosion resistance of the welded portion can be avoided by suppressing pit growth and promoting re-passivation.

  FIG. 5 shows an enlarged view around the overlap fillet weld 3. Assuming that the thickness of the steel pipe portion 1a of the stainless steel pipe is t, FIG. 5 (a) shows a case where the maximum penetration depth is 0.3t, and FIG. 5 (b) shows a case where the maximum penetration depth is 1.0t. 5 (c) shows the case where the maximum penetration depth is 2.0t, and FIG. 5 (d) shows the case where the maximum penetration depth exceeds 2.0t.

  FIG. 5 shows a case where the fillet weld 3 is formed by welding by bringing the electrode / arc closer to the outer peripheral surface side of the tube end thickened portion 1b. Therefore, the outer peripheral surface of the tube end thickened portion 1b becomes a surface on the electrode / arc side, and the inner peripheral surface of the tube end thickened portion 1b becomes a surface (back surface) on the opposite side of the electrode / arc side surface. . The distance (depth) from the outer peripheral surface of the pipe end thickened portion 1b to the maximum penetration portion is the maximum penetration depth.

  As shown in FIG. 5, when the overlap fillet weld 3 has not reached the inner peripheral surface of the pipe end thickened portion 1b, the maximum penetration depth is less than 2.0t. When the overlap fillet weld 3 has just reached the inner peripheral surface of the pipe end thickened portion 1b, the maximum penetration depth is 2.0t. When the overlap fillet welded portion 3 reaches the inner peripheral surface of the pipe end thickened portion 1b and a molten portion is also present on the inner peripheral surface, the maximum penetration depth is more than 2.0 t. That is, the case where the maximum penetration depth exceeds 2.0 t is the case where a fusion portion is present on the surface (back surface) opposite to the electrode / arc side surface during welding.

In order to obtain such a lap fillet weld, a selected shielding gas is required, especially in welding where a shielding gas is required. In particular, since the pipe end thickened portion 1b has the gap 1d, it is essential to appropriately shield with an inert gas. Specifically, Ar is most desirable. When CO ₂ or O ₂ is mixed, the content is desirably 5% or less.

  The stainless steel pipe of the present embodiment is made of a stainless steel sheet having a steel component defined in the present embodiment, and the method of manufacturing the stainless steel sheet is as follows: steelmaking-hot rolling-annealing / pickling-cold rolling-annealing. The manufacturing conditions in each step are not particularly specified.

  In steelmaking, a method is preferred in which a steel containing the essential components and components added as necessary is melted in a converter and then subjected to secondary refining. The smelted molten steel is made into a slab by casting (continuous casting). The slab is heated to a predetermined temperature and hot-rolled to a predetermined thickness by continuous rolling. The annealing step after the hot rolling may be omitted, and the cold rolling after the pickling may be performed by any of a normal Sendzimir mill and a tandem mill. Is more desirable.

  In cold rolling, roll roughness, roll diameter, rolling oil, number of rolling passes, rolling speed, rolling temperature, etc. may be appropriately selected within a general range. Intermediate annealing may be performed during the cold rolling, and the intermediate and final annealing may be batch annealing or continuous annealing. If necessary, the annealing atmosphere may be bright annealing in which an annealing is performed in a non-oxidizing atmosphere such as hydrogen gas or nitrogen gas, or annealing may be performed in the air.

  Further, when the stainless steel plate is formed into a tubular shape, the stainless steel plate may be subjected to lubrication coating to improve press forming. The type of the lubricating coating film may be appropriately selected. Temper rolling or leveling may be applied after the final annealing to correct the shape, but it is desirable not to add them because it causes a reduction in work hardening ability.

  The method of manufacturing the steel pipe may be appropriately selected, and is not limited to the welding method, and may be appropriately selected such as ERW (resistance welding), laser welding, TIG welding (tungsten inert gas welding). Also, the size of the steel pipe may be determined according to the application.

As a process for forming the pipe end thickening portion at the end of the stainless steel pipe, spinning or forging of the pipe end is preferable, but these methods are not particularly specified. In consideration of work efficiency and dimensional accuracy, spinning is more preferable.
In addition, there is a case where the end of the steel pipe portion is bent outward in the radial direction to increase the wall thickness, and a case where the wall is bent radially inward to increase the wall thickness. Is the same as the inner diameter of the steel pipe portion 1a of the raw tube. On the other hand, when the wall is bent inward in the radial direction to increase the wall thickness, the outer diameter of the pipe end thickened portion 1b becomes the same as the outer diameter of the steel pipe portion which is a raw tube.
Furthermore, after forming the pipe end thickened portion 1b, a method of expanding or contracting the pipe in the next step may be adopted.

Next, in order to manufacture a welded structure using a stainless steel pipe having a pipe end thickened portion (tube end thickened structure) as a material, the pipe end thickened portion of the stainless steel pipe is welded to another steel pipe member. To join. In the joining step by welding, it is preferable to perform welding while supplying a shielding gas to the welded portion. Examples of the shielding gas include an inert gas such as Ar, and a mixed gas of one or both of CO _{2 and} O ₂ and an inert gas. The amount of CO ₂ and O _{2 in} the mixed gas is preferably 5.0% by volume or less. In particular, when the welding method is TIG welding, MIG welding, or mag welding, it is preferable to perform welding while supplying a shielding gas to the welded portion. On the other hand, when the welding method is laser welding, it is not necessary to supply the shielding gas.

  In the above description, the thickened portion of the ferrite stainless steel pipe is mainly described, and the description of the thickened portion of the ferritic stainless steel is omitted. It has almost the same configuration and effect as the pipe end thickening part of the stainless steel pipe.

  According to the ferritic stainless steel, the ferritic stainless steel pipe, the pipe end thickened structure and the welded structure of the present embodiment, the corrosion resistance in the gap between the pipe end thickened portions is excellent. This makes it possible to reduce the wall thickness of the steel pipe part, and in particular, it is possible to make the steel pipe part thinner when applied as an automobile part or a motorcycle part, and to reduce the weight of the part while preventing corrosion. As a result, the fuel efficiency of automobiles and motorcycles can be improved.

Hereinafter, the present invention will be described in more detail based on examples.
(Example 1)
Steel having the composition shown in Table 1 was melted. In particular, Sn was set at five levels of 0.005, 0.010, 0.030, 0.100%, and 0.300% to examine its effect. The melted steel was hot-rolled to a thickness of 4 mm, annealed at 1050 ° C. for 1 minute, and pickled. Thereafter, cold rolling was performed to a sheet thickness of 0.8 mm.

  Then, a test piece of 70 mm × 70 mm and 40 mm × 40 mm was cut out from a steel sheet having each composition shown in Table 1, and test pieces having the same component composition were overlapped and spot-welded to form a gap portion of the pipe end thickening structure. A simulated CCT specimen was prepared. By adjusting the spot welding conditions, CCT test pieces with various gap intervals were produced.

  The CCT test piece was evaluated by JASO-M610-92 according to the corrosion test method for the appearance of automobile parts. The number of cycles was set to 100, and after the test, the spot weld was hollowed out to separate the two plates, so that the maximum pit depth in the gap could be evaluated. After rust removal, the pit depth of each of the test pieces above and below the gap was measured at 10 points, and the value of the deepest pit was defined as the maximum pit depth of the steel type. A sample having a maximum pit depth of less than 500 μm was evaluated as “○” (good), and a sample having a maximum pit depth of 500 μm or more was evaluated as “x” (poor).

Table 1 shows the calculation results of the critical gap distance (Cr ² / {1000 (Al + Si + Sn))} of each composition stainless steel (Cr, Al, Si and Sn are the contents (% by mass) of the respective elements). Table 1 also shows the value of the gap d (μm), the maximum pit depth (μm) according to the corrosion test method (JASO-M610-92) for the appearance of automobile parts, and the results of the determination. An underline in the middle indicates that it is out of the range of the present invention or out of the range of preferable characteristics.

Sample No. of the present invention example. In samples A1 to A25, the maximum pitting corrosion depth was less than 500 μm. In B1 to B14, the maximum pit depth was 500 μm or more.
Therefore, from the results shown in Table 1, in the pipe end thickened structure made of the ferritic stainless steel pipe of the present embodiment, the gap distance d (μm) is d ≧ Cr ² / {1000 (Al + Si + Sn)} (Cr , Al, Si and Sn indicate the content (% by mass) of each element), it can be understood that a pipe end thickened structure having a small maximum pit depth can be provided.

  In addition, Cr in the base material improves the corrosion resistance in a general environment. However, according to the relational expression shown in Table 1, as the amount of Cr in the base material increases, the pitting depth in the gap environment increases. I understand. Then, it was found that by increasing the amount of Al + Si + Sn added to the steel sheet (as the amount of Al + Si + Sn in the base material increased), the critical gap interval became smaller.

  Observation of the form of corrosion in the gaps of the steel type with a high Cr content revealed that a small number of pits grew deeply. On the other hand, it was found that the pitting corrosion form of the low Cr content steel type had a large number of pits, but the depth of each pit was shallower than that of the high Cr content steel type. .

  It is considered that, in the steel type having a high Cr content, the number of occurrences of pitting corrosion decreased because the Cr concentration in the passive film was high and the corrosion resistance was high. Therefore, it is considered that the oxygen reduction reaction, which is a cathode reaction, contributes only to the growth of a small number of pits, and each pit has grown deeply. On the other hand, in the case of a steel type having a low Cr content, the cathode reaction contributes to the generation of a large number of pits.

Further, it was found from the above test that Al, Si and Sn are effective for the occurrence of pitting corrosion in a gap environment. It has been known that Sn suppresses active dissolution of stainless steel and improves crevice corrosion resistance. However, the fact that Sn suppresses the occurrence of pitting corrosion in the gap environment and reduces the critical gap distance is new knowledge based on the results of this test. It is considered that Al ^elutes as Al ³⁺ ions inside the pit at the beginning of generation and is adsorbed on the surface, thereby promoting the suppression of the pit growth and the re-passivation. It is considered that Si forms an oxide inside the pit and promotes suppression of pit growth and re-passivation.

(Example 2)
Using a steel sheet having the composition shown in Table 2A, a ferritic stainless steel pipe having a diameter of 60 mm was produced by TIG welding. Next, the end of the ferritic stainless steel pipe was folded 180 ° inward in the radial direction by spinning to produce a tube end thickened part having a length of 50 mm. As described above, a tube end thickened structure having a diameter of 60 mm and an inwardly turned end (tube end thickened portion) having a length of 50 mm was produced. Then, the pipe end thickened structure was cut at a length of 60 mm from the folded portion.
In addition, the gap interval of the gap portion in the pipe end thickening portion was set to various values by adjusting the conditions of the spinning process.

  Next, a steel pipe (steel pipe member) having a diameter of 62 mm was manufactured using a steel sheet having the same chemical composition as each of the various pipe end thickening structures. A steel pipe (steel pipe member) having the same chemical composition is superimposed on the outside of the pipe end thickening portion of the pipe end thickening structure, and the end (pipe end thickening portion) is folded back inside the pipe end thickening structure. Welding was performed by various methods (TIG welding, MIG welding, mag welding, or laser welding) so as to form a weld. As described above, a CCT test piece having a total length of 100 mm and a welded portion between the steel pipe member and the pipe end thickened structure positioned at the center was produced.

  In various types of welding, the amount of current was adjusted to adjust the penetration depth of the weld, and the effect of the penetration depth on corrosion resistance was examined. In the case of welding using a shielding gas, welding was performed using various shielding gases, and the effect of the shielding gas on corrosion resistance was also examined.

  The maximum penetration depth was measured by the following method. Welding was performed under the same conditions to separately produce a CCT test piece. The cross section of the welded portion was observed, and the portion of the welded portion that had melted to the deepest was defined as the maximum penetration portion, and the depth was defined as the maximum penetration depth. In detail, the outer peripheral surface of the end portion (tube end thickened portion) of the pipe end thickened structure and the steel pipe member are overlapped, and the outer peripheral surface of the end portion (tube end thickened portion) of the tube end thickened structure is overlapped. The welding was performed by bringing the electrode / arc closer to the side. For this reason, the outer peripheral surface of the end portion (tube end thickened portion) of the tube end thickened structure becomes the surface on the electrode / arc side, and the inside of the end portion (tube end thickened portion) of the tube end thickened structure. The peripheral surface is a surface (back surface) on the opposite side of the electrode / arc side surface. The distance (depth) from the outer peripheral surface of the end portion of the pipe end thickened structure (tube end thickened portion) to the maximum penetration portion is the maximum penetration depth.

  The CCT test piece was evaluated by JASO-M610-92 according to the corrosion test method for the appearance of automobile parts. The number of cycles was set to 100, and the weld was cut after the test to separate the two plates of the pipe end thickened portion so that the maximum pitting depth in the gap could be evaluated. After rust removal, the pit depth of each of the test pieces above and below the gap was measured at 10 points, and the value of the deepest pit was defined as the maximum pit depth of the steel type. A sample having a maximum pit depth of less than 500 μm was evaluated as “○” (good), and a sample having a maximum pit depth of 500 μm or more was evaluated as “x” (poor).

  In Table 2B, the penetration depth of the welded portion of the test piece prepared using the stainless steel having each composition shown in Table 2A, the welding shield gas, and the maximum value according to the corrosion test method (JASO-M610-92) for the appearance of automobile parts. The pit depth (μm) and the results of the determination are also shown. In addition, the underline in Table 2B shows that it is out of the range of the present invention or the range of preferable characteristics.

  Assuming that the thickness of the steel pipe portion of the pipe end thickened structure is t, from the results in Table 2B, the penetration depth of the welded portion is in the range of 0.3 t or more and 2.0 t or less, and the maximum pit depth is less than 500 μm. It turns out that it becomes.

According to the present embodiment, it is possible to provide a ferritic stainless steel pipe excellent in salt damage resistance in the gap. In addition, by using the steel pipe to which the present embodiment is applied, in particular, as parts for automobiles and motorcycles, it is possible to reduce the wall thickness, and it is possible to efficiently manufacture parts and improve fuel efficiency.
That is, this embodiment is extremely useful industrially.

  A to C: welded structure, 1: ferritic stainless steel tube (tube end thickened structure), 1a: steel tube portion, 1b: tube end thickened portion, 1d: gap, 2: steel tube (steel tube member), 3: Fillet fillet weld.

Claims (8)

Steel part is mass%
C: 0.001 to 0.100%,
Si: 0.01-5.00%,
Mn: 0.01-2.00%,
P: ≦ 0.05%
S: ≦ 0.0100%,
Cr: 9.0-30.0%,
Sn: 0.001 to 3.00%,
One or two of Ti: 0.01 to 1.00% and Nb: 0.01 to 1.00%,
Al: 0.010 to 5.000%,
N: 0.001 to 0.050%, the balance being Fe and impurities,
A thickened portion including a folded portion is provided at an end of the steel portion, and a gap d (μm) formed in the thickened portion is d ≧ Cr ² / {1000 (Al + Si + Sn)} (Cr in the formula) , Al, Si and Sn indicate the contents (% by mass) of the respective elements). In further mass%,
Ni: 0.01 to 3.00%,
Mo: 0.01 to 3.00%,
Cu: 0.01 to 3.00%,
B: 0.0001 to 0.0100%,
W: 0.001 to 1.000%,
V: 0.001 to 1.000%,
Sb: 0.001 to 0.100%,
Co: 0.001 to 0.500%,
Ca: 0.0001 to 0.0050%,
Mg: 0.0001 to 0.0050%,
Zr: 0.0001-0.0300%,
Ga: 0.0001-0.0100%,
Ta: 0.001 to 0.050%,
REM: 0.001 to 0.100%
The ferritic stainless steel according to claim 1, wherein the ferritic stainless steel comprises one or more of the following. Having a steel pipe part consisting of a steel base material part and a welded part,
The steel base material portion is represented by mass%
C: 0.001 to 0.100%,
Si: 0.01-5.00%,
Mn: 0.01-2.00%,
P: ≦ 0.05%
S: ≦ 0.0100%,
Cr: 9.0-30.0%,
Sn: 0.001 to 3.00%,
One or two of Ti: 0.01 to 1.00% and Nb: 0.01 to 1.00%,
Al: 0.010 to 5.000%,
N: 0.001 to 0.050%, the balance being Fe and impurities,
A pipe end thickening portion formed of a folded portion is provided at the pipe end of the steel pipe portion, and a gap d (μm) formed in the pipe end thickening portion is d ≧ Cr ² / {1000 (Al + Si + Sn)}. (Cr, Al, Si, and Sn in the formulas indicate the content (% by mass) of each element). In further mass%,
Ni: 0.01 to 3.00%,
Mo: 0.01 to 3.00%,
Cu: 0.01 to 3.00%,
B: 0.0001 to 0.0100%,
W: 0.001 to 1.000%,
V: 0.001 to 1.000%,
Sb: 0.001 to 0.100%,
Co: 0.001 to 0.500%,
Ca: 0.0001 to 0.0050%,
Mg: 0.0001 to 0.0050%,
Zr: 0.0001-0.0300%,
Ga: 0.0001-0.0100%,
Ta: 0.001 to 0.050%,
REM: 0.001 to 0.100%
The ferritic stainless steel pipe according to claim 3, wherein the ferrite-based stainless steel pipe contains at least one of the following.   5. The ferritic stainless steel pipe according to claim 3, wherein the pipe end thickened part is expanded or contracted with respect to the steel pipe part. 6.   A pipe end thickening structure comprising the ferritic stainless steel pipe according to any one of claims 3 to 5.   A welded structure, wherein the pipe end thickened portion of the pipe end thickened structure according to claim 6 and a steel pipe member are joined by overlapping fillet welds.   The maximum penetration depth of the lap fillet welded part on the side of the pipe end thickening part is in the range of 0.3 t to 2.0 t with respect to the wall thickness t of the steel pipe part. Item 8. A welded structure according to Item 7.

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