JP2020050929A - Ferritic stainless steel sheet, ferritic stainless steel pipe, pipe end thickening structure, and weldment structure - Google Patents

Abstract

To provide a ferritic stainless steel pipe having enhanced corrosion resistance in a gap structure at a pipe end thickening part.SOLUTION: There is adopted a ferritic stainless steel pipe excellent in corrosion resistance at a gap in a pipe end thickening part and having a steel pipe part consisting of a steel base material part and a weldment part, in which the steel base material 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%, Al:0.010 to 5.000%, N:0.001 to 0.050%, and further one or two kind of Ti:0.01 to 1.00% and Nb:0.001 to 1.000%, and the balance Fe with impurities, and Si content (mass%) and Al content (mass%) satisfy 2.5Al+Si≥0.100.SELECTED DRAWING: None

Description

  The present invention relates to a ferritic stainless steel sheet, 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. In particular, in the automotive field, since it is used for various parts from an exhaust manifold to a muffler, the stainless steel used is required to have heat resistance and corrosion resistance. In addition, since these parts are mostly welded, the strength, rigidity and corrosion resistance of the welded part are also required.

  2. Description of the Related Art In recent years, there has been an increasing number of studies to reduce the thickness of materials used for components for the purpose of reducing the weight of automobiles. 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 for forming an exhaust pipe, increasing the thickness of a steel pipe end to be welded and joined to other parts, increasing the thickness of a welded portion to increase strength, and securing rigidity and weldability. I have. This is called pipe end thickening. 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. In Patent Literature 1, in order to secure the strength of the end of the pipe and reduce the weight of the pipe, a roller is pressed against the end while rotating the pipe to bend inward in the radial direction, and then adhered by the roller. A processing 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 Document 3 discloses a patent relating to a base tube in order to increase the wall thickness by folding the tube end, and an inner bead portion of a welded portion protrudes to the tube inner surface, and the protrusion amount is 4 to 15% of the plate thickness. It is prescribed.

  The pipes described in Patent Literatures 1 to 3 have a portion where the pipe end is turned radially inward or radially outward (hereinafter, referred to as a pipe end thickening portion). The pipe end thickening portions of Patent Documents 1 to 3 have a gap structure with a height of several to several hundred μm. In this gap structure, when bent inward as in Patent Literatures 1 and 2, exhaust gas condensed water generated inside the exhaust system components tends to stay, and when bent outward as in Patent Literature 3, Salt water adhering from the outside of the exhaust system components tends to stay.

  Corrosion occurring in such an environment is not so-called crevice corrosion, but is salt damage corrosion accelerated by salt water or exhaust gas condensed water easily remaining in the gap structure. Furthermore, when the pipe end thickening part is used by expanding or contracting the pipe end, the gap structure in the pipe end thickening part locally becomes extremely narrow, and the corrosive environment becomes extremely severe.

  As described above, corrosion in the gap between the pipe end thickening portions may be accelerated. Therefore, the stainless steel used is required to be a steel type having excellent salt damage resistance in the gap structure of the pipe end thickening portions. In particular, in the case of exhaust system components, the occurrence of perforations due to corrosion leads to the leakage of exhaust gas, so it is important to apply a material having high perforation resistance.

  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 one or two types of Ti and Nb: Ti: 0.02 to 0.5%, Nb: 0. Disclosed is a ferritic stainless steel excellent in crevice corrosion resistance, characterized in that it is contained in the range of 02 to 1% and Sn is contained in the range of 0.005 to 2%, with the balance being Fe and inevitable impurities. Have been. 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 two kinds of 0.03 to 0.30%, Sn: one or two kinds of 0.03 to 0.50% and Sb: 0.03 to 0.50%, and the balance Is composed of Fe and unavoidable impurities, and the A value defined by the formula (2) is 15.23 or more. The alloy-saving ferritic stainless steel for automotive exhaust system members having excellent corrosion resistance after heating is provided. It has been disclosed.
A = [Cr] + [Si] +0.5 [Mn] +10 [Al] +15 ([Sn] + [Sb
]) Expression (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 two of 0.03 to 0.30%, Sn: 0.05 to 1.00%, and the balance being Fe and unavoidable impurities. A Mo-saving ferritic stainless steel for automotive 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. 0.1 to 0.50%, and further contains one or two of Ti: 0.03 to 0.30% and Nb: 0.03 to 0.30%, with the balance being Fe and unavoidable impurities. A ferritic stainless steel for automobile exhaust system members, characterized by comprising: 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 one or two 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, characterized in that the balance is made up of Fe and unavoidable impurities, is disclosed.
A = [Si] x [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%, further contains one or two kinds of Ti: 0.03 to 0.30% and Nb: 0.03 to 0.50%, and in the formula (1) A ferritic stainless steel for exhaust system members excellent in corrosion resistance, characterized by satisfying that the defined A value is not less than 0.00065% 2 and the balance being Fe and unavoidable impurities, is disclosed.
A = [Mo] × [Sn] ----- (1)
The technique described in Patent Document 9 discloses 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.

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

  As described above, the prior art has not yet proposed a method for improving the corrosion resistance of a gap structure formed in a pipe end thickened portion of a pipe having a thickened pipe end. In particular, no method has been proposed yet for improving the corrosion resistance of a pipe whose pipe end wall thickness is expanded or contracted.

  The present invention has been made in view of the above circumstances, and provides a ferritic stainless steel sheet, 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.

Means for solving the above problem has the following configuration.
[1] In 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%,
Al: 0.010 to 5.000%,
N: 0.001 to 0.050%,
Further, it contains any one or two of Ti: 0.01 to 1.00% and Nb: 0.001 to 1.000%,
The balance is Fe and impurities,
A ferritic stainless steel sheet characterized in that the Si content (% by mass) and the Al content (% by mass) satisfy 2.5Al + Si ≧ 0.100.
[2] Further, in mass%,
Ni: 0.01 to 3.00%,
Mo: 0.01 to 3.00%,
Sn: 0.001 to 3.00%,
Cu: 0.01 to 3.00%,
B: 0.0001 to 0.0100%,
W: 0.001 to 1.00%,
V: 0.001 to 1.00%,
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 sheet according to [1], comprising 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%,
Al: 0.010 to 5.000%,
N: 0.001 to 0.050%,
Further, it contains any one or two of Ti: 0.01 to 1.00% and Nb: 0.001 to 1.000%,
The balance is Fe and impurities,
A ferritic stainless steel pipe characterized in that the Si content (% by mass) and the Al content (% by mass) satisfy 2.5Al + Si ≧ 0.100.
[4] Further, in mass%,
Ni: 0.01 to 3.00%,
Mo: 0.01 to 3.00%,
Sn: 0.001 to 3.00%,
Cu: 0.01 to 3.00%,
B: 0.0001 to 0.0100%,
W: 0.001 to 1.00%,
V: 0.001 to 1.00%,
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 [3], which comprises one or more of the following.
[5] At one end in the longitudinal direction of the steel pipe portion, a pipe end thickening portion is provided in which an end portion of the steel pipe portion is folded radially outward or radially inward,
The outer diameter _{d 1} of the steel pipe section, the outer diameter _{d 2} of the pipe end thickening unit, the relationship of the Si content (mass%) and the Al content (wt%) is, 2.5Al + Si ≧ | {( d 2 - The ferritic stainless steel tube according to [3] or [4], wherein the relationship d ₁ ) / d ₁ } | +0.1 is satisfied.
[6] The ferritic stainless steel pipe according to [5], wherein the pipe end thickened part is expanded or contracted with respect to the steel pipe part.
[7] A tube end thickening structure comprising the ferritic stainless steel tube according to [5] or [6].
[8] The pipe end thickened portion of the pipe end thickened structure according to [7], and a steel pipe member are joined by overlapping fillet welds,
The welding wherein a maximum penetration depth of the overlapped fillet welded portion on the side of the pipe end thickened portion is in a range of 0.3 t to 2.0 t with respect to a thickness t of the steel pipe portion. Structure.

  According to the present invention, it is possible to provide a ferritic stainless steel sheet, 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. 4 is a schematic cross-sectional view showing an example of a welded structure including the stainless steel pipe (tube end thickened structure) of the embodiment and another steel pipe (steel pipe member). FIG. 6 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). Sectional drawing which shows the other example of the welded structure which consists of the stainless steel pipe (tube end thickening structure) of embodiment, and another steel pipe (steel pipe member). Sectional drawing which shows the principal part of the welded structure which consists of the stainless steel pipe (tube end thickening structure) of embodiment, and another steel pipe (steel pipe member).

The pipe end thickening portion is formed at one end in the longitudinal direction of the steel pipe by bending the end of the steel pipe radially outward or radially inward. The end of the folded steel pipe is processed so as to be in close contact with the outer peripheral surface or inner peripheral surface of the steel pipe, but there is a slight gap between the folded end and the outer peripheral surface or inner peripheral surface of the steel pipe. Occurs. The present inventors have conducted intensive studies on the corrosion resistance in the gap between the pipe end thickening portions formed in the ferritic stainless steel pipe. As a result, it was found that in the pipe end thickened portion, the pitting corrosion depth decreased as the stainless steel contained more Al or Si. This finding is the same when the pipe-end thickening portion is further expanded or contracted. Furthermore, relationship between the threshold suppressing Al + Si content growth pitting in the gap of the pipe end thickening portion, the outer diameter d ₁ of the steel pipe, the outer diameter d ₂ of the pipe end thickening unit I found that.

Specifically, first, in order to ensure the corrosion resistance of the gap at the pipe end thickened portion, 2.5Al + Si ≧ 0.100% (Al and Si are the Al content and the Si content in steel, respectively (both are mass%). Is preferable. Both Al and Si have the effect of reducing the corrosion resistance of the gap, particularly the depth of the generated pitting corrosion. In particular, the effect of Al is greater than the effect of Si, and it has been confirmed that, when compared at the same addition amount, the pit depth is reduced by a factor of two to three times that of Si. Further, when the pipe end thickening portion is used by expanding or contracting the pipe end, the gap in the pipe end thickening portion is locally very narrow, and becomes extremely severe as a corrosive environment. It has been found that the corrosion resistance can be further improved by adding Si. Also, the Al content and the Si content, the outer diameter _{d 1} of the steel pipe, between the outer diameter _{d 2} of the pipe end thickening _{unit, 2.5Al + Si ≧ | {(} d 2 -d 1) / d 1} It was found that when the relationship of | +0.1 was satisfied, the corrosion resistance was further improved. In the present invention, when the pit depth of the pipe end thickening structure expanded or contracted with various diameters was examined, the change in severity as a corrosive environment inside the gap due to the expansion or contraction was | {(d _2- d ₁ ) / d ₁ } |.

  It should be noted that the corrosion phenomenon in the gap of the pipe end thickening portion of the present embodiment is a corrosion phenomenon different from the conventional crevice corrosion, and the corrosion phenomenon occurring in the gap of the pipe end thickening portion of the present embodiment is the conventional crevice. This is a corrosion phenomenon in the case where the gap interval is wider than the "gap" of corrosion, and the mechanism of occurrence is different from that of conventional crevice corrosion.

  Hereinafter, embodiments will be described.

The ferritic stainless steel sheet having excellent corrosion resistance in the gap between the pipe end thickened portions of the present embodiment is, in 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%, Al: 0.010 to 5.000%, N: 0.001 to 0.050%, and further contains any one or two of Ti: 0.01 to 1.00% and Nb: 0.001 to 1.000%, with the balance being Fe And a ferrite stainless steel sheet which is an impurity and has the above-mentioned Si amount (% by mass) and Al amount (% by mass) satisfying 2.5Al + Si ≧ 0.100.
Further, the ferritic stainless steel sheet of the present embodiment further includes Ni: 0.01 to 3.00%, Mo: 0.01 to 3.00%, Sn: 0.01 to 3.00% by mass%. 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 0.001% 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%.

The ferritic stainless steel pipe according to the present embodiment, which has excellent corrosion resistance in the gap between the pipe end thickened parts, has a steel pipe part composed of a steel base material part and a welded part, and the steel base material part has a mass%. And 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%, Al: 0.010 to 5,000%, N: 0.001 to 0.050%, and further, Ti: 0.01 to 1.00% and Nb: contains any one or two of 0.001 to 1.000%, the balance is Fe and impurities, and the Si content (% by mass) and the Al content (% by mass) are 2.5 Al + Si It is a ferritic stainless steel pipe satisfying ≧ 0.100.
Further, the ferritic stainless steel pipe of the present embodiment further includes, by mass%, Ni: 0.01 to 3.00%, Mo: 0.01 to 3.00%, Sn: 0.001 to 3.00%, Cu: 0.01 to 3.00%, B: 0.0001 to 0.0100%, W: 0.001 to 1.00%, V: 0.001 to 1.00%, 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 specified in the ferritic stainless steel sheet and the ferritic stainless steel pipe of the present embodiment will be described. In addition,% means mass%.

C: 0.001 to 0.100%
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: 0.01 to 5.00%
Si is an important element in the present embodiment. 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: 0.01-2.00%
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%.

P: 0.050% or less 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.

S: 0.0100% 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: 9.0-30.0%
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%.

One or two kinds of Ti: 0.01 to 1.00% and Nb: 0.001 to 1.000% Ti: 0.01 to 1.00%
Ti must be contained at 0.01% or more in order to prevent sensitization of stainless steel. However, a large content 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 productivity, so the upper limit of the Ti content is set to 1.00% or less. A preferable range of the Ti amount is 0.03 to 0.50%, and a more preferable range is 0.10 to 0.25%.

Nb: 0.001 to 1.000%
Nb is useful for improving the high-temperature strength and the intergranular corrosion resistance of the welded portion, but an excessive content lowers the workability and manufacturability. Therefore, the amount of Nb is set to 0.001 to 1.000%. A preferable range of the Nb amount is 0.005 to 0.500%.

Al: 0.010 to 5.000%
Al is an important element in the present embodiment. Al is a very useful element that not only concentrates on the steel 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: 0.001 to 0.050%
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%.

2.5Al + Si ≧ 0.100%
In the present embodiment, it is preferable that the Al content and the Si content in the steel satisfy the relationship of 2.5Al + Si ≧ 0.100%. Thereby, the corrosion resistance of the gap of the pipe end thickened portion can be improved. More preferably, 2.5Al + Si is 0.500% or more, 1.500% or more, 3.000% or more, or 7.000% or more.

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

  Depending on the purpose, Ni, Mo, Sn, Cu, B, W, V, Sb, Co, Ca, Mg, Zr, Ga, Ta, and REM may contain one or more of these, depending on the purpose. Good. The lower limit of these elements is 0% or more, preferably more than 0%.

Ni: 0.01 to 3.00%
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: 0.01 to 3.00%
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%.

Sn: 0.001 to 3.00%
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%, more preferably 0.010 to 1.00%.

Cu: 0.01 to 3.00%
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: 0.0001 to 0.0100%
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: 0.001-1.00%
W can contain 1.00% or less 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 0.80%.

V: 0.001 to 1.00%
V can contain 1.00% or less 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 0.50%.

Sb: 0.001 to 0.100%
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: 0.001 to 0.500%
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: 0.0001 to 0.0050%
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: 0.0001-0.0050%
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: 0.0001-0.0300%
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: 0.0001 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: 0.001 to 0.050%
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%.

REM: 0.001 to 0.100%
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 plate and the ferritic stainless steel pipe of the present embodiment are composed of Fe and impurities (impurities include unavoidable impurities), in addition to 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. % And H may contain 0.01% or less.

  The ferritic stainless steel pipe of the present embodiment has a steel pipe part including a steel base material part having the above chemical components and a welded part. The steel pipe portion is formed by forming the ferritic stainless steel sheet of 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.

  Next, the ferritic stainless steel pipe of the present embodiment is provided with a pipe end thickening portion in which the end of the steel pipe is bent radially outward or radially inward at one end in the longitudinal direction of the steel pipe. Is also good. A ferritic 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 tube end thickening portion formed at one longitudinal end of a steel tube portion of a ferritic stainless steel tube.

  FIG. 1 shows an example in which a tube end thickening portion 1b is provided at one end of a steel tube portion 1a of a ferritic stainless steel tube 1. At one end of the steel pipe part 1a, a part of the steel pipe part is folded inward in the radial direction. The folded portion 1c is bent so as to be in contact with the inner peripheral surface of the steel pipe portion 1a, and the folded portion 1c forms a tube end thickened portion 1b. The wall thickness of the pipe end thickened portion 1b is almost twice the wall 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 ferritic stainless steel pipe 1 (tube end thickened structure) shown in FIG. A welded structure A is formed by a ferritic stainless steel tube 1 (tube end thickened structure) and another steel tube 2 (steel tube member). As shown in FIG. 1, the pipe end thickened portion 1b of the ferritic stainless steel pipe 1 is a male side, the end 2a of the steel pipe 2 is a female side, and the tube end thickened portion 1b is inserted into the end 2a of the steel pipe 2. Have been. 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. As in the case of FIG. 1, the welded structure B shown in FIG. 2 is obtained by joining another steel pipe 2 to a ferritic stainless steel pipe 1 (tube end thickened structure) via a fillet weld 3. However, the difference from FIG. 1 is that the pipe end thickened portion 1b of the ferritic stainless steel pipe 1 is expanded with respect to the steel pipe portion 1a.

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

In FIGS. 1 to 3 in ferritic stainless steel tube 1 shown (pipe end thickening structure), an outer diameter d _2, Si content in steel of the outer diameter d _1, the tube end thickening portion 1b of the steel pipe section 1a ( Mass%) and the amount of Al (mass%) preferably satisfy the relationship of 2.5 Al + Si ≧ | {(d ₂ −d ₁ ) / d ₁ } | +0.1. When this relationship is satisfied, the corrosion resistance in the gap 1d of the pipe end thickened portion 1b can be further improved.

  In addition, in the welding structures shown in FIGS. 1 to 3, an example is shown in which the overlap fillet weld portion 3 is formed between the outer peripheral surface of the pipe end thickened portion 1 b and another steel pipe 2, but this embodiment The form is not limited to this, and a steel pipe having an outer diameter slightly smaller than the inner diameter of the pipe end thickened portion 1b is inserted into the inside of the tube end thickened portion 1b, and the inner peripheral surface of the pipe end thickened portion 1b and other portions are inserted. May be formed between the steel pipe 2 and the overlapped fillet welded portion 3.

  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 welded portion 3 is ensured, and corrosion resistance in the gap 1d can be secured. 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.

  The reason why the corrosion resistance in the gap 1d can be further improved by setting the 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 be a corrosion starting point is not formed. Conceivable. Further, if the penetration depth exceeds 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, by adding Al and Si to steel, even in the unlikely event of corrosion, the eluted Al ions are adsorbed on the dissolution surface, and Si oxides are generated on the steel surface, so that the steel base metal It is considered that further elution can be suppressed and deterioration of the corrosion resistance of the welded portion can be avoided.

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

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, the pipe end thickened portion 1b has a large gap 1d, and proper shielding with an inert gas is indispensable. Specifically, Ar is most desirable. When CO ₂ or O ₂ is mixed, the content is desirably 5% or less.

  The ferritic stainless steel pipe of the present embodiment is made of a stainless steel sheet having a steel component specified in the present embodiment, and the method of manufacturing the stainless steel sheet is as follows: steelmaking-hot rolling-annealing / pickling-cold rolling -It consists of each step of annealing, and the manufacturing conditions of 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 pipe 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.

  According to the ferritic stainless steel sheet and 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.

In the examples, a test was performed to determine the corrosion resistance in the gap between the pipe end thickening portions, and the effects of the elements constituting the ferrite stainless steel sheet, the ferritic stainless steel pipe, the pipe end thickening structure, and the welded structure. The effect of the outer diameter before and after the tube expansion or contraction of the pipe end thickening structure was tested.
Hereinafter, the present invention will be described in more detail based on examples.

  Steels having the compositions shown in Tables 1A and 1B were melted and hot-rolled to a thickness of 4 mm under the above-mentioned conditions. Next, shot and pickling were performed. Thereafter, cold rolling was performed to a thickness of 1.0 mm, annealing was performed at 880 ° C. for 1 minute, and then pickling was performed. Thus, a ferritic stainless steel sheet was manufactured.

  Next, a ferritic stainless steel sheet was formed into a tubular shape by UO molding and welded by TIG welding to produce a ferritic stainless steel pipe. Next, the end of the ferritic stainless steel pipe was folded inward in the radial direction to form a pipe end thickened portion as shown in FIGS. The length of the folded portion was 50 mm. The spinning process was applied to the production of the pipe end thickened portion. And it cut | disconnected by the length of 60 mm of the bending part of a return. Thereafter, the pipe end thickened portion was expanded or contracted. In this way, a pipe end thickened portion structure was manufactured.

  Further, various steel pipes made of a ferritic stainless steel sheet having the same chemical composition are superimposed on the inner peripheral surface side of the pipe end thickened portion, and a TIG is formed so that a folded portion inside the pipe end thickened portion becomes a welded portion. Welding was performed to produce a CCT test piece having a total length of 100 mm and a fillet weld between a steel pipe portion and a pipe end thickened portion located at the center. At that time, welding conditions were adjusted to make the penetration depth various.

  The CCT test piece was evaluated by the automotive part appearance corrosion test method of JASO-M610-92. The number of cycles was set to 100, and after the test, the welded portion was cut to separate the two plates of the pipe end thickened portion 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. The condition where the maximum pit depth was less than 300 μm was rated as ◎, the condition between 300 μm and less than 500 μm was rated as ○, and the condition with 500 μm or greater was rated as x. The results are shown in Table 2A and Table 2B.

From the results in Tables 1A and 2B, it can be seen that when 2.5Al + Si ≧ 0.100 is satisfied, the maximum pit depth is small, and the evaluation is ◎ or ○. Further, when the pipe end thickening structure expanded or contracted satisfies 2.5 Al + Si ≧ | {(d ₂ −d ₁ ) / d ₁ } | +0.1, the maximum pit depth becomes very small. I understand.

  When the steel sheet surface after the test was observed, it was found that the pitting corrosion did not grow much and the pitting propagation speed was slow in the steel type having a high Al and Si concentration. From this, it was found that Al and Si in the steel suppress the growth of pitting corrosion. In particular, it is considered that Al elutes as ions inside the pit at the beginning of generation and is adsorbed on the surface, thereby promoting the suppression of pit growth and promoting the re-passivation. It is considered that Si forms an oxide inside the pit and promotes suppression of pit growth and re-passivation.

  On the other hand, in Comparative Examples B1 to B16, since the steel component was out of the range of the present invention, the corrosion resistance of the pipe end thickened portion was reduced.

ADVANTAGE OF THE INVENTION According to this invention, it is possible to provide the ferritic stainless steel pipe excellent in the corrosion resistance of the clearance gap of a pipe end thickening part. In addition, by using the steel pipe to which the present invention is applied, in particular, as parts for automobiles and motorcycles, it is possible to reduce the wall thickness, and it is possible to manufacture parts efficiently and improve fuel efficiency.
That is, the present invention is extremely useful in industry.

  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)

In 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%,
Al: 0.010 to 5.000%,
N: 0.001 to 0.050%,
Further, it contains any one or two of Ti: 0.01 to 1.00% and Nb: 0.001 to 1.000%,
The balance is Fe and impurities,
A ferritic stainless steel sheet characterized in that the Si content (% by mass) and the Al content (% by mass) satisfy 2.5Al + Si ≧ 0.100. In further mass%,
Ni: 0.01 to 3.00%,
Mo: 0.01 to 3.00%,
Sn: 0.001 to 3.00%,
Cu: 0.01 to 3.00%,
B: 0.0001 to 0.0100%,
W: 0.001 to 1.00%,
V: 0.001 to 1.00%,
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 sheet according to claim 1, comprising 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%,
Al: 0.010 to 5.000%,
N: 0.001 to 0.050%,
Further, it contains any one or two of Ti: 0.01 to 1.00% and Nb: 0.001 to 1.000%,
The balance is Fe and impurities,
A ferritic stainless steel pipe characterized in that the Si content (% by mass) and the Al content (% by mass) satisfy 2.5Al + Si ≧ 0.100. In further mass%,
Ni: 0.01 to 3.00%,
Mo: 0.01 to 3.00%,
Sn: 0.001 to 3.00%,
Cu: 0.01 to 3.00%,
B: 0.0001 to 0.0100%,
W: 0.001 to 1.00%,
V: 0.001 to 1.00%,
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, comprising one or more of the following. At one end in the longitudinal direction of the steel pipe portion, a pipe end thickening portion is provided in which an end portion of the steel pipe portion is folded radially outward or radially inward,
The outer diameter _{d 1} of the steel pipe section, the outer diameter _{d 2} of the pipe end thickening unit, the relationship of the Si content (mass%) and the Al content (wt%) is, 2.5Al + Si ≧ | {( d 2 - _{d 1)} / d _1} | ferritic stainless steel according to claim 3 or claim 4, characterized in that to satisfy the relation of +0.1.   The ferritic stainless steel pipe according to claim 5, wherein the pipe end thickened part is expanded or contracted with respect to the steel pipe part.   A tube end thickening structure comprising the ferritic stainless steel tube according to claim 5. The pipe end thickened portion of the pipe end thickened structure according to claim 7, and a steel pipe member are joined by overlapping fillet welds,
The welding wherein the maximum penetration depth of the overlapped fillet weld on the side of the pipe end thickened portion is in the range of 0.3 t to 2.0 t with respect to the thickness t of the steel pipe. Structure.

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