JP7296705B2 - Ferritic stainless steel pipe, pipe end thickened structure and welded structure - Google Patents

Description

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

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

2. Description of the Related Art In recent years, with the aim of reducing the weight of automobiles, the number of cases where thinner materials used for parts are being considered is increasing. However, in order to ensure the strength, rigidity and weldability of the welded portion, a certain thickness may be required, and even the non-welded portion will be thick, which prevents the reduction of the thickness of the entire exhaust system. In response to this, there is a known technology that increases the thickness of the ends of the steel pipes that form the exhaust pipe and are welded to other parts to increase the strength of the welded parts, thereby ensuring rigidity and weldability. there is 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 have been disclosed regarding the tube end thickening as described above. In Patent Document 1, for the purpose of securing the strength of the end of the pipe and reducing the weight of the pipe, a roller is pressed against the end while rotating the pipe to bend it radially inward, and then it is brought into close contact with the roller. A processing method is disclosed. Patent Literature 2 discloses a construction method for preventing burn-through during welding by forming a pipe end into a double pipe shape and doubling the wall thickness. Patent Document 3 discloses a patent relating to a blank tube for increasing the wall thickness by folding back the tube end, and the inner bead portion of the weld portion protrudes from the inner surface of the tube, and the amount of protrusion is 4 to 15% of the plate thickness. is stipulated.

The pipes described in Patent Literatures 1 to 3 have pipe end portions that are bent radially inwardly or radially outwardly (hereinafter referred to as pipe end thickening portions). The pipe end thickened portions of Patent Documents 1 to 3 have a gap structure with a height of several to several hundred μm. Condensed water generated in the exhaust system tends to accumulate in this gap structure when it is bent inward as in Patent Documents 1 and 2, and when it is bent outward as in Patent Document 3. Salt water adhering from the outside of exhaust system parts tends to stay.

Corrosion that occurs in such an environment is not so-called crevice corrosion, but salt damage corrosion that is accelerated by the accumulation of salt water and exhaust gas condensed water inside the crevice structure. Furthermore, when the pipe end thickened portion is expanded or contracted for use, the gap structure in the pipe end thickened portion becomes extremely narrow locally, resulting in a very severe corrosive environment.

Since there is a possibility that corrosion in the gap of the pipe end thickened portion is accelerated in this way, the stainless steel to be used is required to be of a steel type that is excellent in salt damage resistance in the gap structure of the pipe end thickened portion. In particular, for exhaust system parts, it is important to use materials with high resistance to perforation, as perforations due to corrosion will lead to leakage of exhaust gas.

In Patent Document 4, 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%, one or two of Ti and Nb, Ti: 0.02 to 0.5%, Nb: 0.5%. Disclosed is a ferritic stainless steel having excellent resistance to crevice corrosion, characterized by containing Sn in the range of 0.005 to 2%, Sn: in the range of 0.005 to 2%, and the balance being Fe and unavoidable impurities. It is In the technique described in Patent Document 4, crevice corrosion resistance is improved by adding Sn, but there is no mention of the relationship between the crevice spacing in the crevice structure of the pipe end thickened portion and salt corrosion.

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: 0.020-0.100%, Cr: 10.5-13.05%, and Ti: 0.03-0.30% and Nb: 1 or 2 of 0.03 to 0.30%, Sn: 0.03 to 0.50% and Sb: 0.03 to 0.50%, the balance is composed of Fe and unavoidable impurities, and the A value defined by the formula (2) is 15.23 or more, and the alloy-saving ferritic stainless steel for automotive exhaust system members having excellent corrosion resistance after heating is provided. 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 spacing in the gap structure of the pipe end thickened portion and salt corrosion is not described. do not have.

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: 0.010-0.100%, Cr: 16.5-22.5%, and Ti: 0.03-0.30% and Nb: 0.03 to 0.30% of one or two kinds, Sn: 0.05 to 1.00%, and the balance consisting of Fe and unavoidable impurities Mo-saving type ferritic stainless steel for automobile exhaust system members which is excellent in corrosion resistance after heating is disclosed. In the technique described in Patent Document 6, the corrosion resistance after heating is improved by adding Sn, but the relationship between the gap spacing in the gap structure of the pipe end thickened portion and salt corrosion 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: 0.010-0.100%, Cr: 16.5-22.5%, Ni: 0.5-2.0%, Sn: 0.01% 01 to 0.50%, further contains one or two of Ti: 0.03 to 0.30% and Nb: 0.03 to 0.30%, the balance being Fe and unavoidable impurities A ferritic stainless steel for automotive exhaust system components is disclosed which is characterized by comprising: The technique described in Patent Document 7 discloses the corrosion resistance of exhaust system parts after heating, but does not describe the relationship between the gap spacing in the gap structure of the pipe end thickened portion and salt corrosion.

In Patent Document 8, in mass %, 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 further contains 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. There is disclosed a ferritic stainless steel for automobile exhaust system members which satisfies the above and has excellent oxidation resistance and corrosion resistance, wherein the balance is composed of Fe and unavoidable impurities.
A = [Si] x [Sn] + 0.014 [Si] (1)
Here, [Si] and [Sn] are the contents of Si and Sn in mass %, respectively.
The technique described in Patent Document 8 discloses the corrosion resistance of exhaust system parts after heating, but does not describe the relationship between the gap spacing in the gap structure of the pipe end thickened portion and salt corrosion.

In Patent Document 9, in mass%, 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: 0.010-0.20%, Cr: 10.5-11.5%, Mo: 0.02-0.20%, and Sn: Contains 0.005 to 0.050%, further contains one or two of Ti: 0.03 to 0.30% and Nb: 0.03 to 0.50%, and in formula (1) Disclosed is a ferritic stainless steel for exhaust system members having excellent corrosion resistance, which satisfies the defined A value of 0.00065% 2 or more, and the balance is composed of Fe and unavoidable impurities.
A = [Mo] x [Sn] ---- (1)
The technique described in Patent Document 9 discloses the corrosion resistance of exhaust system parts after heating, but does not describe the relationship between the gap spacing in the gap structure of the pipe end thickened portion and salt corrosion.

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. 6006660 JP 2014-169491 A

As described above, in the prior art, no method has yet been proposed for improving the corrosion resistance of the gap structure formed in the pipe end thickened portion of the pipe end thickened. In particular, no method has yet been proposed for improving the corrosion resistance of pipes with expanded or contracted pipe end thickened portions.

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

Means for solving the above problems has the following configuration.
[1] Having a steel pipe portion consisting of a steel base metal portion and a welded portion,
The steel base material portion, 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: 0.521-5.000 %,
N: 0.001 to 0.050%,
Furthermore, Ti: 0.01 to 1.00% and Nb: 0.001 to 1.000% Any one or two,
the balance being Fe and impurities,
And the Si content (mass%) and the Al content (mass%) satisfy 2.5Al + Si ≥ 0.100,
At one end in the longitudinal direction of the steel pipe portion, a pipe end thickened portion is provided by bending the end portion of the steel pipe portion radially outward or radially inward,
The relationship between the outer diameter d ₁ of the steel pipe portion, the outer diameter d ₂ of the increased pipe end portion, the amount of Si (% by mass), and the amount of Al (% by mass) is 2.5Al+Si≧|{(d ₂ − satisfies the relationship d ₁ )/d ₁ }|+0.1,
A ferritic stainless steel pipe, wherein the pipe end thickened portion is expanded or contracted with respect to the steel pipe portion.
[2] Furthermore, in % 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: 0.0001-0.0050%,
Zr: 0.0001 to 0.0300%,
Ga: 0.0001 to 0.0100%,
Ta: 0.001 to 0.050%,
REM: 0.001-0.100%
The ferritic stainless steel pipe according to [1], containing one or more of
[3] A pipe end thickened structure comprising the ferritic stainless steel pipe according to [1] or [2].
[4] The pipe end thickened portion of the pipe end thickened structure according to [3] and a steel pipe member are joined by a lap fillet weld,
Welding characterized in that the maximum penetration depth of the lap fillet weld on the side of the pipe end thickened portion is in the range of 0.3t to 2.0t with respect to the thickness t of the steel pipe portion. Structure.

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

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic cross-sectional view showing an example of a welded structure composed of a stainless steel pipe (pipe end thickening structure) of an embodiment and another steel pipe (steel pipe member). FIG. 4 is a schematic cross-sectional view showing another example of a welded structure composed of the stainless steel pipe (pipe end thickening structure) of the embodiment and another steel pipe (steel pipe member). FIG. 4 is a schematic cross-sectional view showing another example of a welded structure composed of the stainless steel pipe (pipe end thickening structure) of the embodiment and another steel pipe (steel pipe member). BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic cross-sectional view showing a main part of a welded structure composed of a stainless steel pipe (pipe end thickening structure) of an embodiment and another steel pipe (steel pipe member).

The pipe end thickened portion is formed at one end of the steel pipe in the longitudinal direction by folding the end portion of the steel pipe radially outward or radially inward. Although the end of the folded steel pipe is processed so as to be in close contact with the outer or inner peripheral surface of the steel pipe, there is a slight gap between the folded end and the outer or inner peripheral surface of the steel pipe. occur. The inventors of the present invention have extensively studied the corrosion resistance of the gap between the pipe end thickened portions formed in the ferritic stainless steel pipe. As a result, the inventors have found that the pitting corrosion depth decreases in stainless steels containing more Al or Si at the thickened portion at the pipe end. This knowledge is the same when the tube end thickened portion is further subjected to tube expansion or tube contraction. Furthermore, there is a relationship between the threshold value of the amount of Al+Si that suppresses the growth of pitting corrosion in the gap of the pipe end thickened portion, the outer diameter _d1 of the steel pipe, and the outer diameter _d2 of the pipe end thickened portion. I found out.

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 amount and Si amount in the steel (both are mass%) ) is preferably satisfied. Both Al and Si have the effect of reducing the corrosion resistance of gaps, particularly the depth of pitting that occurs. In particular, the effect of Al is greater than that of Si, and it was confirmed that the pitting corrosion depth is reduced by two to three times that of Si when compared with the same additive amount, so the coefficient of Al is set to 2.5. Furthermore, when the tube end thickened portion is expanded or contracted for use, the gap at the pipe end thickened portion becomes very narrow locally, and the corrosive environment becomes very severe. It has been found that the addition of Si can further improve the corrosion resistance. In addition, between the Al content and Si content, the outer diameter _d1 of the steel pipe, and the outer diameter _d2 of the thickened portion at the end of the pipe, 2.5Al+Si≧|{(d ₂ −d ₁ )/d ₁ } It was found that the corrosion resistance is further improved when the relationship |+0.1 is satisfied. In the present invention, when the pitting corrosion depth of the pipe end thickening structure expanded or contracted to various diameters was investigated, the change in the severity of the corrosive environment inside the gap due to the expansion or contraction was |{(d ₂ −d ₁ )/d ₁ }| term.

It should be noted that the corrosion phenomenon in the gap of the pipe end thickened portion of the present embodiment is a corrosion phenomenon different from the conventional crevice corrosion, and the corrosion phenomenon that occurs in the pipe end thickened portion gap of the present embodiment is similar to that of the conventional crevice. It is a corrosion phenomenon that occurs when the space between the crevices is wider than the "crevice" of corrosion, and the occurrence mechanism is different from that of conventional crevice corrosion.

Embodiments will be described below.

The ferritic stainless steel sheet of the present embodiment, which is excellent in corrosion resistance in the gap between the pipe end thickened portions, has a mass % of 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%, further contains one or two of Ti: 0.01 to 1.00% and Nb: 0.001 to 1.000%, the balance being Fe and impurities, and the amount of Si (% by mass) and the amount of Al (% by mass) satisfy 2.5Al+Si≧0.100.
In addition, the ferritic stainless steel sheet of the present embodiment further has Ni: 0.01 to 3.00%, Mo: 0.01 to 3.00%, Sn: 0.01 to 3.00%, Cu: 0.01-3.00%, B: 0.0001-0.0100%, W: 0.001-1.000%, V: 0.001-1.000%, Sb: 0.001- 0.100%, Co: 0.001-0.500%, Ca: 0.0001-0.0050%, Mg: 0.0001-0.0050%, Zr: 0.0001-0.0300%, 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, which is excellent in corrosion resistance in the gap between the pipe end thickened portions, has a steel pipe portion composed of a steel base metal portion and a welded portion, and the steel base metal portion has a mass % of At, 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 Ti: 0.01 to 1.00% and Nb: Contains any one or two of 0.001 to 1.000%, the balance being Fe and impurities, and the above Si amount (% by mass) and Al amount (% by mass) are 2.5 Al + Si A ferritic stainless steel pipe satisfying ≧0.100.
In addition, the ferritic stainless steel pipe of the present embodiment further includes Ni: 0.01 to 3.00%, Mo: 0.01 to 3.00%, Sn: 0.001 to 3.00%, Cu: 0.01-3.00%, B: 0.0001-0.0100%, W: 0.001-1.00%, V: 0.001-1.00%, Sb: 0.001- 0.100%, Co: 0.001-0.500%, Ca: 0.0001-0.0050%, Mg: 0.0001-0.0050%, Zr: 0.0001-0.0300%, Ga : 0.0001 to 0.0100%, Ta: 0.001 to 0.050%, and REM: 0.001 to 0.100%.

The chemical composition of the steel specified for the ferritic stainless steel plate and ferritic stainless steel pipe of the present embodiment will be described below. In addition, % means the mass %.

C: 0.001 to 0.100%
Since C lowers intergranular corrosion resistance and workability, its content must be kept low. Therefore, the upper limit of the C content is made 0.100% or less. However, excessively lowering the C content increases the scouring cost, so the lower limit of the C content is made 0.001% or more. A preferable range of C content is 0.002 to 0.010%.

Si: 0.01-5.00%
Si is an important element in this embodiment. Si is a very useful element that not only suppresses the occurrence of corrosion by concentrating on the surface, but also reduces the corrosion rate of the base material. Therefore, the lower limit of the Si content is made 0.01% or more. However, since an excessive Si content causes a decrease in elongation of the steel and deteriorates workability, the upper limit of the Si content is made 5.00% or less. A preferred range of Si content is 0.30 to 3.00%, and a more preferred range is 0.70 to 1.20%.

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

P: 0.050% or less P is an element that deteriorates workability and weldability, so it is necessary to limit its content. Therefore, the amount of P is made 0.050% or less. A preferable range for the amount of P is 0.030% or less.

S: 0.0100% or less S is an element that deteriorates corrosion resistance, so it is necessary to limit its content. Therefore, the amount of S is made 0.0100% or less. A preferable range for 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 ensure corrosion resistance in a salt-damaged environment. As the Cr content increases, corrosion resistance improves, but workability and manufacturability decrease. Therefore, the upper limit of the Cr content is made 30.0% or less. A preferable range of Cr content is 9.5 to 25.0%, and a more preferable range is 10.0 to 15.0%.

Ti: 0.01 to 1.00% and Nb: 1 or 2 of 0.001 to 1.000% Ti: 0.01 to 1.00%
Ti should be contained in an amount of 0.01% or more in order to prevent sensitization of stainless steel. However, a large amount of Ti leads to an increase in alloy cost, a decrease in toughness, a decrease in corrosion resistance due to an increase in inclusions in the steel, and a decrease in manufacturability, so the upper limit of the Ti amount is made 1.00% or less. A preferred range of Ti content is 0.03 to 0.50%, and a more preferred range is 0.10 to 0.25%.

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

Al: 0.010-5.000%
Al is an important element in this embodiment. Al is a very useful element that not only suppresses the occurrence of corrosion by concentrating on the steel surface, but also reduces the corrosion rate of the base material. Therefore, the lower limit of the Al content is made 0.010% or more. However, excessive Al content causes a decrease in the elongation of the material and deteriorates workability, so the upper limit of the Al content is made 5.000% or less. A preferable range of Al content is 0.050 to 3.000%, and a 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 made 0.050% or less. However, excessively reducing the N content increases the refining cost, so the lower limit of the N content is made 0.001% or more. A preferable range of N content is 0.002 to 0.020%.

2.5Al+Si≧0.100%
In this embodiment, it is preferable that the Al amount and the Si amount in the steel satisfy the relationship of 2.5Al+Si≧0.100%. As a result, the corrosion resistance of the gap at 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.

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

Mo: 0.01-3.00%
Mo can be contained in an amount of 0.01% or more in order to improve corrosion resistance. However, an excessive content deteriorates the workability and leads to an increase in cost because it is expensive. Therefore, the upper limit of the amount of Mo is made 3.00% or less. A preferred range of Mo content is 0.05 to 1.00%.

Sn: 0.001-3.00%
Sn can be contained in an amount of 0.001% or more in order to improve corrosion resistance. However, excessive content leads to an increase in cost. Therefore, the upper limit of the amount of Sn is made 3.00% or less. A preferred range of Sn content is 0.005 to 1.00%, more preferably 0.010 to 1.00%.

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

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

W: 0.001 to 1.00%
W can contain 1.00% or less in order to improve corrosion resistance. In order to obtain a stable effect, the lower limit of the W amount is made 0.001% or more. A preferable range of W content is 0.005 to 0.80%.

V: 0.001 to 1.00%
V can be contained in an amount of 1.00% or less in order to improve corrosion resistance. In order to obtain a stable effect, the lower limit of the V amount is made 0.001% or more. A preferable range of V content is 0.005 to 0.50%.

Sb: 0.001-0.100%
Sb can be contained in an amount of 0.100% or less in order to improve general corrosion resistance. In order to obtain a stable effect, the lower limit of the amount of Sb is made 0.001% or more. A preferred range of Sb content is 0.010 to 0.080%.

Co: 0.001-0.500%
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 amount of Co is made 0.001% or more. A preferable range of Co amount is 0.010 to 0.300%.

Ca: 0.0001-0.0050%
Ca is contained for desulfurization, but if it is contained excessively, water-soluble inclusions CaS are formed and the corrosion resistance is lowered. Therefore, Ca can be contained in the range of 0.0001 to 0.0050%. A preferable range of Ca content is 0.0005 to 0.0030%.

Mg: 0.0001-0.0050%
Mg refines the structure and is useful for improving workability and toughness. Therefore, Mg can be contained in the range of 0.0050% or less. In order to obtain a stable effect, the lower limit of the Mg amount is made 0.0001% or more. A preferred range of 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 content is made 0.0001% or more. A preferred range of Zr content 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 Ga content is made 0.0001% or more. A preferable range of Ga content is 0.0005 to 0.0050%.

Ta: 0.001-0.050%
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 Ta amount is made 0.001% or more. A preferable range of Ta content is 0.005 to 0.030%.

REM: 0.001-0.100%
Since REM has a deoxidizing effect and the like, and is a useful element in scouring, it can be contained in an amount of 0.100% or less. In order to obtain a stable effect, the lower limit of the amount of REM is made 0.001% or more. A preferred range of REM amounts 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 (lanthanides) from lanthanum (La) to lutetium (Lu), according to a general definition. . REM is one or more selected from these rare earth elements, and the amount of REM is the total amount of rare earth elements.

The ferritic stainless steel sheet and 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. In addition to the elements described above, it can be contained within a range that does not impair the effects of the present invention. In this embodiment, for example, Bi, Pb, Se, H, etc. may be contained, but in that case, it is preferable to reduce them as much as possible. On the other hand, the content ratio of these elements is controlled as long as the problems of the present invention are solved. % or less, and H may contain 0.01% or less.

The ferritic stainless steel pipe of the present embodiment has a steel pipe portion made up of a steel base material portion having the chemical composition described above and a welded portion. The steel pipe portion is formed by forming the ferritic stainless steel plate of the present embodiment into a tubular shape. The welded portion is formed by welding end portions of tubularly formed steel plates to each other by ERW (resistance welding), laser welding, TIG welding (tungsten inert gas welding), or the like. The welding method may be selected as appropriate. Also, the size of the steel pipe may be determined according to the application.

Next, in the ferritic stainless steel pipe of the present embodiment, a pipe end thickening portion is provided at one end in the longitudinal direction of the steel pipe portion, which is formed by folding the end portion of the steel pipe portion radially outward or radially inward. good too. A ferritic stainless steel pipe provided with a pipe end thickened portion may be referred to as a pipe end thickened structure. 1 to 3 show a pipe end thickening portion formed at one longitudinal end of a steel pipe portion of a ferritic stainless steel pipe.

FIG. 1 shows an example in which a pipe end thickening portion 1b is provided at one end of a steel pipe portion 1a of a ferritic stainless steel pipe 1. As shown in FIG. At one end of the steel pipe portion 1a, a portion of the steel pipe portion is folded radially inward. The folded portion 1c is bent so as to come into contact with the inner peripheral surface of the steel pipe portion 1a, and the folded portion 1c forms a thickened pipe end portion 1b. The thickness of the pipe end thickened portion 1b is approximately twice the thickness of the steel pipe portion 1a. A gap 1d is formed between the steel pipe portion 1a and the folded portion 1c in the pipe end thickened portion 1b. In this embodiment, it is important to improve the corrosion resistance of the gap 1d.

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

FIG. 2 shows a welded structure B of another example. The welded structure B shown in FIG. 2 has a ferritic stainless steel pipe 1 (pipe end thickening structure) joined to another steel pipe 2 via a lap fillet weld 3, as in the case of FIG. 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.

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

In the ferritic stainless steel pipe 1 (pipe _end thickened structure) _shown in FIGS. %) and the amount of Al (% by mass) satisfies the relationship 2.5Al+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.

The welded structure shown in FIGS. 1 to 3 shows an example in which the lap fillet weld 3 is formed between the outer peripheral surface of the pipe end thickened portion 1b and another steel pipe 2. 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 inside the pipe end thickened portion 1b, and the inner peripheral surface of the pipe end thickened portion 1b and other parts are inserted. A lap fillet weld 3 may be formed between the steel pipe 2 of

In addition, in the welded structures A to C shown in FIGS. 1 to 3, the maximum penetration depth of the lap fillet weld 3 on the pipe end thickened portion 1b side is 0 with respect to the wall thickness t of the steel pipe portion 1. It is preferably in the range of 0.3t to 2.0t. By setting the maximum penetration depth to 0.3t or more, the strength of the lap fillet welded portion 3 can be ensured, and the corrosion resistance in the gap 1d can be ensured. However, if the maximum welding depth exceeds 2.0t, the shape of the welded part becomes uneven, which may lead to various problems such as a decrease in strength, deterioration of corrosion resistance, and leakage of exhaust gas, so the upper limit is It should 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. Furthermore, if the penetration depth exceeds 1.0 t, the gap 1d at the pipe end thickened portion 1b is closed, and the gap structure that can become a corrosion starting point is further reduced. In addition to this, by adding Al and Si to the steel, even if corrosion should occur, the eluted Al ions will be adsorbed on the dissolved surface, and Si oxides will be generated on the steel surface. It is thought that further elution can be suppressed and deterioration of the corrosion resistance of the weld zone can be avoided.

As shown in FIG. 4, the maximum penetration depth is the distance _d3 between the outer peripheral surface of the pipe end thickened portion 1b and the deepest portion of the lap fillet weld 3 toward the pipe end thickened portion side.

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 tube 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, it is desirable to make it 5% or less.

The ferritic stainless steel pipe of the present embodiment is made of a stainless steel plate having the steel composition specified in the present embodiment. - It consists of each process of annealing, and the manufacturing conditions of each process are not particularly specified.

In steelmaking, a method of smelting steel containing the above-mentioned essential components and optionally added components in a converter and then secondary refining is suitable. The melted 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 plate thickness by continuous rolling. The annealing process after hot rolling may be omitted, and the cold rolling after pickling may be performed by either a normal Sendzimir mill or a tandem mill. is preferable.

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

Further, when the stainless steel plate is formed into a tubular shape, the stainless steel plate may be coated with a lubricant to improve press forming. The type of lubricating coating film may be appropriately selected. Although temper rolling or leveling may be applied for shape correction after the final annealing, it is desirable not to apply these because they cause a decrease in work hardening ability.

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

The process of forming the pipe end thickened portion at the end of the stainless steel pipe is preferably a pipe end spinning process or a forging process, but these methods are not particularly specified either. Spinning is preferable in consideration of work efficiency and dimensional accuracy.
In addition, it is conceivable to increase the thickness by bending the end portion of the steel pipe portion radially outward, or to increase the thickness by bending it radially inward. has the same inner diameter as the steel pipe portion 1a of the blank pipe. On the other hand, when the thickness is increased by bending inward in the radial direction, the outer diameter of the pipe end thickened portion 1b becomes the same as the outer diameter of the steel pipe portion which is the blank pipe.
Furthermore, after forming the pipe end thickened portion 1b, a method of expanding or shrinking the pipe in the next step may be adopted.

According to the ferritic stainless steel plate, the ferritic stainless steel pipe, the pipe end thickened structure, and the welded structure of the present embodiment, the corrosion resistance in the gap of the pipe end thickened portion is excellent. As a result, the thickness of the steel pipe can be reduced, and in particular, the thickness of the steel pipe can be reduced when it is applied to automobile parts and motorcycle parts. It is possible to improve the fuel efficiency of automobiles and motorcycles.

In the examples, a test was conducted to understand the corrosion resistance in the gap of the pipe end thickened part, and the effect of the elements constituting the ferritic stainless steel plate, the ferritic stainless steel pipe, the pipe end thickened structure, and the welded structure. The effect of the outer diameter of the tube end thickening structure before and after tube expansion or tube contraction was tested.
The present invention will be described in more detail below based on examples.

Steels having compositions shown in Tables 1A and 1B were melted and hot-rolled to a plate thickness of 4 mm under the conditions satisfying the above conditions. Then shot and pickling were applied. Then, it was cold-rolled to a plate thickness of 1.0 mm, annealed at 880° C. for 1 minute, and then pickled. Thus, a ferritic stainless steel sheet was produced.

Next, the ferritic stainless steel plate was formed into a tubular shape by UO molding and welded by TIG welding to produce a ferritic stainless steel pipe. Next, the end portion of the ferritic stainless steel pipe was folded radially inward to form a pipe end thickened portion as shown in FIGS. The length of the folded portion was 50 mm. A spinning process was applied to fabricate the thickened portion of the tube end. And it cut|disconnected by the length of 60 mm of bending parts of a return. After that, the tube end thickened portion was expanded or contracted. Thus, a pipe end thickened portion structure was manufactured.

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

This CCT test piece was evaluated in accordance with JASO-M610-92 automobile parts appearance corrosion test method. The number of cycles was set to 100 cycles, and after the test, the welded portion was cut to separate the two plates of the tube end thickening portion so that the maximum pitting corrosion depth in the gap could be evaluated. After rust removal, the pitting corrosion depth of the test pieces above and below the gap was measured at 10 points, respectively, and the deepest pitting corrosion value was taken as the maximum pitting corrosion depth of the steel type. The condition that the maximum pitting corrosion depth is less than 300 μm is evaluated as ⊚, the condition that the maximum pitting corrosion depth is 300 μm or more and less than 500 μm is evaluated as ◯, and the condition that the maximum pitting corrosion depth is 500 μm or more is evaluated as X. Results are shown in Tables 2A and 2B.

From the results of Tables 1A to 2B, it can be seen that when 2.5Al+Si≧0.100 is satisfied, the maximum pitting corrosion depth is small and the evaluation is ⊚ or ◯. Furthermore, the maximum pitting corrosion depth of the expanded or contracted pipe end thickening structure becomes very small when 2.5Al+Si≧|{(d ₂ −d ₁ )/d ₁ }|+0.1 is satisfied. I understand. A1, A2, A7, A8, A9, A10, A11, A12, A13, A15, A17, A18, A25 , A27 , A31, A32, A33, A35, A36, A38 , A39, A42, A44, A46 , A48, A49 and A50 are reference examples.

Observation of the surface of the steel sheet after the test revealed that pitting corrosion did not grow so much in the steel type with high Al and Si concentrations, and the rate of pitting corrosion progression was slow. From this, it was found that Al and Si in steel suppress the growth of pitting corrosion. In particular, Al is considered to promote suppression of pitting corrosion growth and re-passivation by eluting as ions inside the pitting corrosion at the initial stage and adsorbing to the surface. It is believed that Si forms an oxide inside the pitting corrosion and promotes suppression of pitting corrosion growth and re-passivation.

On the other hand, in Comparative Examples B1 to B16, the corrosion resistance of the thickened portion at the pipe end decreased because the steel components were outside the range of the present invention.

According to the present invention, it is possible to provide a ferritic stainless steel pipe having excellent corrosion resistance in the gap of the increased wall thickness at the end of the pipe. In addition, by using the steel pipe to which the present invention is applied, particularly as parts for automobiles and motorcycles, it becomes possible to reduce the thickness of the parts, and it becomes possible to efficiently manufacture parts and improve fuel efficiency.
That is, the present invention is industrially very useful.

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

Claims (4)

Having a steel pipe portion consisting of a steel base material portion and a welded portion,
The steel base material portion, 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: 0.521-5.000 %,
N: 0.001 to 0.050%,
Furthermore, Ti: 0.01 to 1.00% and Nb: 0.001 to 1.000% Any one or two,
the balance being Fe and impurities,
And the Si content (mass%) and the Al content (mass%) satisfy 2.5Al + Si ≥ 0.100,
At one end in the longitudinal direction of the steel pipe portion, a pipe end thickened portion is provided by bending the end portion of the steel pipe portion radially outward or radially inward,
The relationship between the outer diameter d ₁ of the steel pipe portion, the outer diameter d ₂ of the increased pipe end portion, the amount of Si (% by mass), and the amount of Al (% by mass) is 2.5Al+Si≧|{(d ₂ − satisfies the relationship d ₁ )/d ₁ }|+0.1,
A ferritic stainless steel pipe, wherein the pipe end thickened portion is expanded or contracted with respect to the steel pipe portion. Furthermore, 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: 0.0001-0.0050%,
Zr: 0.0001 to 0.0300%,
Ga: 0.0001 to 0.0100%,
Ta: 0.001 to 0.050%,
REM: 0.001-0.100%
The ferritic stainless steel pipe according to claim 1, characterized by containing one or more of A pipe end thickened structure comprising the ferritic stainless steel pipe according to claim 1 or 2. The pipe end thickened portion of the pipe end thickened structure according to claim 3 and a steel pipe member are joined by a lap fillet weld,
Welding characterized in that the maximum penetration depth of the lap fillet weld on the side of the pipe end thickened portion is in the range of 0.3t to 2.0t with respect to the thickness t of the steel pipe portion. Structure.

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