JP2018168460A - Ferritic stainless steel pipes and ferritic stainless steel pipes for automotive exhaust system parts - Google Patents

Abstract

A ferritic stainless steel pipe excellent in formability and corrosion resistance and a ferritic stainless steel pipe for automobile exhaust system parts are provided. The steel base material has a steel base material formed into a tubular shape and a welded portion, and the steel base material contains, by mass %, C: 0.001 to 0.020% and Si: 0.01 to 1.00. %, Mn: 0.01 to 1.00%, P: 0.010 to 0.040%, S: 0.0003 to 0.0050%, Cr: 10.0 to 20.0%, N: 0. 001-0.020%, B: 0.0002-0.0020%, Al: 0.005-0.300%, Mg: 0.0002-0.0030% and Sn: 0.005-0.500% and further contains one or two of Ti: 0.05 to 0.70% and Nb: 0.05 to 0.70%, and the maximum Sn concentration in the Sn segregation part in the weld is 0 010% or more, and the n value of the region including the weld zone is 0.15 or more. [Selection figure] None

Description

  The present invention relates to a ferritic stainless steel pipe and a ferritic stainless steel pipe for automobile exhaust system parts.

  Ferritic stainless steel sheets are used in a wide range of fields such as home appliances, kitchen equipment, and electronic equipment. In recent years, the application of ferritic stainless steel plates as materials used for exhaust pipes, fuel tanks and pipes of automobiles and motorcycles has been studied. The material applied to these parts is required to have high workability for forming into a part shape, in addition to corrosion resistance and heat resistance in an exhaust environment and a fuel environment. However, although ferritic stainless steel sheets are less expensive than austenitic stainless steel sheets, they are inferior in formability, so their use and part shapes may be limited.

  In particular, in recent years, with the complexity of the component configuration corresponding to environmental regulations and weight reduction, a complicated shape of the component has been directed. Further, from the viewpoint of reducing the cost of parts and reducing the thickness and weight, various attempts have been made to reduce the number of moldings and the number of welding processes in part molding. Since exhaust pipes of automobiles and motorcycles are joined to various parts, a certain thickness is required to ensure the strength, rigidity and weldability of the welded parts. Thus, there are cases in which the overall exhaust system becomes thinner.

  On the other hand, there is a technique for securing strength, rigidity, and weldability by forming an exhaust pipe and increasing the thickness of a pipe end of a steel pipe that is welded and joined to other parts, thereby increasing the thickness of the welded portion. In this case, since only the pipe end to be welded can be thickened, the non-welded portion can be thinned, and the entire exhaust system can be thinned and lightened.

  Several techniques relating to thickening (thickening) the pipe end as described above have been disclosed. In Patent Document 1, for the purpose of securing the strength of the pipe end and reducing the weight of the pipe, the roller is pressed against the pipe end while rotating the pipe, and the pipe end is bent inward in the radial direction. A processing method is disclosed. Patent Document 2 discloses a construction method for preventing melting during welding by forming a pipe end into a double tubular shape and doubling the wall thickness of the pipe end. Patent Document 3 discloses a technique related to a raw pipe for folding the pipe end to increase the thickness, and the inner bead part of the welded part protrudes from the pipe inner face, and the protruding amount is 4 to 15 of the plate thickness. %.

JP 2010-234406 A JP2013-103250A JP 2004-255414 A

The present inventors have grasped the problems of the known techniques shown below.
Patent Document 1 and Patent Document 2 relate to a construction method for increasing the thickness of a pipe end, and perform spinning processing or forging processing. The shape of the raw tube is not necessarily uniform, and roundness and thickness distribution are different. Further, when the raw pipe is a welded pipe, the welded portion is different from the base material in structure and material characteristics. Therefore, in some cases, there is a problem that the pipe end is cracked or necked.
Patent Document 3 relates to a raw pipe, and in the shape of a steel pipe welded portion, an inner surface bead shape is defined. However, when the inner surface bead protrudes, cracking and necking occur from the protruding portion as a starting point during bending. There was a problem.

  The object of the present invention is to solve the above-mentioned problems of the known technology, and particularly to be used suitably as a material for automobile exhaust system parts. To provide a steel pipe.

  In order to solve the above problems, the present inventors have studied in detail the steel components and the material properties of the welded portion of the steel pipe with respect to the formability of the ferritic stainless steel pipe. As a result, for example, when the pipe end is thickened by using bending by spinning, or when the pipe end is thickened by forging, the ferritic stainless steel pipe is welded to suppress cracking during processing. It was found that the work hardening of the part is extremely important, and that the region including the welded part has an excellent n value, so that the degree of freedom in forming at the pipe end can be remarkably improved.

The gist of the present invention for solving the above problems is as follows.
(1) A steel base material formed into a tubular shape and a welded portion are provided, and the steel base material is in mass%, C: 0.001 to 0.020%, Si: 0.01 to 1.00%. , Mn: 0.01 to 1.00%, P: 0.010 to 0.040%, S: 0.0003 to 0.0050%, Cr: 10.0 to 20.0%, N: 0.001 -0.020%, B: 0.0002-0.0020%, Al: 0.005-0.300%, Mg: 0.0002-0.0030% and Sn: 0.005-0.500% And further containing one or two of Ti: 0.05-0.70% and Nb: 0.05-0.70%, the balance consisting of Fe and inevitable impurities, The maximum Sn concentration of the Sn segregation part is 0.010% or more, and the n value of the region including the weld part is 0.15 or more. A ferritic stainless steel pipe characterized by being.
(2) The steel base material is further mass%, Ni: 0.10 to 0.50%, Mo: 0.10 to 2.00%, Cu: 0.10 to 2.00%, V: 0 0.05 to 1.00%, Ca: 0.0002 to 0.0030%, Zr: 0.01 to 0.30%, W: 0.01 to 3.00%, Co: 0.01 to 0.30 %, Sb: 0.005 to 0.500%, REM: 0.001 to 0.200%, Ga: 0.0002 to 0.3000%, Ta: 0.001 to 1.000%, and Hf: 0.00. The ferritic stainless steel pipe according to (1), which contains one or more of 001 to 1.000%.
(3) The ferritic stainless steel pipe according to (1) or (2), which is for pipe end thickening.
(4) A ferrite system for automotive exhaust system parts, comprising the ferritic stainless steel pipe according to any one of (1) to (3), wherein the outer diameter of the pipe end is larger than the outer diameter of the material. Stainless steel pipe.
(5) A ferritic stainless steel pipe for automobile exhaust system parts, comprising the ferritic stainless steel pipe according to any one of (1) to (3), wherein the inner diameter of the pipe end is smaller than the inner diameter of the material. .

  According to the present invention, a ferritic stainless steel pipe excellent in formability and corrosion resistance and a ferritic stainless steel pipe for automobile exhaust system parts can be provided.

The reason for limitation of the present invention will be described below.
As an index of workability of a ferritic stainless steel pipe, strength and elongation can be cited. In general, these are properties obtained by tensile testing of steel pipes. On the other hand, methods for increasing the thickness of the pipe end of steel pipes include bending and forging processes by spinning, but these involve local deformation of the pipe end, and in a manner different from the characteristics of the entire steel pipe. is there.

The steel pipe of the present invention is manufactured by processing and welding using a steel plate as a steel base material. As the welding method, TIG (Tungsten Inert Gas) welding, laser welding, ERW (Electric Resistance Welding) welding or the like is used.
In processing the pipe end, the formability of the weld is extremely important. This is because the structure of the welded part is different from the structure of the base metal, so cracking and necking may occur starting from the welded part, and it may affect the butt shape and penetration of the welded part. Because there is.

In the present invention, the weld end cracking in the pipe end thickening process is suppressed, and the purpose is to provide a high quality pipe end thickened steel pipe free from cracking and necking. Was examined. As a result, in the pipe end thickening process, the present inventors need to improve the work hardening ability of the region including the welded part in order to suppress the cracking in the place where the crack occurs. I found out that there was. Specifically, an arc-shaped tensile test piece is taken from the welded part of the steel pipe so that tension can be applied in the welding direction of the steel pipe and the welded part is included in the width of the test piece, and a tensile test is performed. It was clarified that cracks generated during spinning or forging of the pipe end can be suppressed by setting the work hardening index (n value) to 0.15 or more.
In this specification, the pipe end thickening process refers to a process of increasing the pipe end by folding the end (pipe end) of the pipe inside or outside the pipe by spinning or forging. .

  Here, as for the tensile characteristics of the welded portion, an arc-shaped tensile test piece (JIS No. 13B) is collected and subjected to a tensile test so that tension can be imparted in the welding direction of the steel pipe. Steel pipe welds have various widths, but the specimens are taken to be included within the width of the tensile specimen. The work hardening index (n value) may be obtained by determining the slope of the true stress-true strain curve. In the present invention, the slope at 5% to 10% strain is defined as the n value.

  As described above, in the pipe end thickening process of the steel pipe, since the pipe end is locally spun or forged, the local large deformation behavior of the weld is important. In the welded portion, the coarsening of crystal grains and the shape factor act, and the work hardening ability (n value) decreases. When the n value of the region including the welded portion is less than 0.15, the local deformability is low, and cracking occurs during the tube end thickening process. By setting the n value of the region including the welded portion to 0.15 or more, even if strong processing such as spinning or forging is performed, strain is dispersed by work hardening of the welded portion, and the surrounding mother The region including the welded portion is uniformly deformed by the deformation of the material. In the case of spinning processing, bending deformation is generated, and in the case of forging processing, compression deformation is performed in the axial direction. In consideration of more severe processing conditions in pipe end thickening, the n value of the region including the welded portion is preferably 0.20 or more.

As main factors of the metal structure of n value in the region including the welded portion, solidified structure, inclusions and segregation are considered, and these are affected by the base material component.
Regarding the solidification structure, when deformation is applied to the steel pipe, the accumulation of dislocations at the grain boundaries increases, so the finer the crystal grains, the higher the n value.
In addition, with respect to inclusions, oxides and precipitates are generated in the weld, and particularly in the case of ERW welding, a large amount of oxide is generated in the weld surface layer when the discharge of oxide in the steel material is insufficient. . Around the oxides and precipitates, voids are likely to be generated when local deformation is applied, and the local deformability is reduced.
Furthermore, when there is a lot of solidification segregation in the welded portion, the deformation becomes non-uniform and cracks are likely to occur. Therefore, it is better that the solidification segregation in the welded portion is small.

Further, the present inventors have found that Sn segregates at the grain boundary of the welded portion and has an effect of improving the work hardening characteristics of the welded portion, so that it is effective for improving the n value of the region including the welded portion. did. Furthermore, as a result of examining the Sn segregation state of the welded portion and the work hardening property of the welded portion in detail, the present inventors satisfy the work hardening property defined by the present invention (n value is 0.15 or more). Therefore, it has been found that the maximum Sn concentration of the Sn segregation portion in the welded portion may be 0.010% or more. Regarding the upper limit of the maximum Sn concentration of the Sn segregation part in the welded part, it is preferably 0.80% or less from the viewpoint of the toughness of the welded part. From the viewpoint of improving the n value in the region including the welded portion and pipe expandability (tube end thickening workability), the maximum Sn concentration of the Sn segregated portion in the welded portion is more preferably 0.10 to 0.40%.
In addition, the maximum Sn concentration of the Sn segregation part in a welded part measures the Sn density | concentration of a welded part with an electron beam microanalyzer, and makes the maximum value the maximum Sn density | concentration of the Sn segregation part in a welded part.

  In order to improve these factors, the steel component range defined by the present invention will be described below. In the present specification, “%” means mass%.

(C: 0.001 to 0.020%)
C deteriorates moldability and corrosion resistance. In particular, if the content exceeds 0.020%, hard martensite and Cr carbide are generated in the welded portion, and the n value is decreased, so the upper limit of the C content is set to 0.020% or less. However, excessive reduction leads to an increase in refining costs, so the lower limit of the C content is set to 0.001% or more. Furthermore, considering the manufacturing cost and intergranular corrosion properties of the welded portion, the C content is preferably 0.002 to 0.009%.

(Si: 0.01-1.00%)
Si may be added as a deoxidizing element, and in order to improve the oxidation resistance, the lower limit is made 0.01% or more. On the other hand, since Si is a solid solution strengthening element, the n value of the weld zone is lowered, so the upper limit is made 1.00% or less. However, excessive reduction leads to an increase in refining cost, and considering the oxidation resistance and weldability in the exhaust gas atmosphere, the Si content is preferably 0.20 to 0.90%. Furthermore, considering the high temperature strength, high temperature and high cycle fatigue characteristics, and suppression of carburization by exhaust gas, the Si content is more preferably 0.40 to 0.90%.

(Mn: 0.01-1.00%)
Since Mn is a solid solution strengthening element like Si, the smaller the content, the better from the viewpoint of improving the n value in the weld zone. In addition, Mn combines with S to form MnS and suppress segregation of S, and is also an element that improves oxidation peelability. In consideration of corrosion resistance, the upper limit of the Mn content is 1.00% or less. On the other hand, since excessive reduction leads to an increase in refining costs, the lower limit of the Mn content is set to 0.01% or more. Furthermore, considering the material and manufacturing cost, the Mn content is preferably 0.05 to 0.90%.

(P: 0.010 to 0.040%)
Since P is a solid solution strengthening element like Mn and Si, the smaller the content, the better from the viewpoint of improving the n value of the weld. For the purpose of suppressing grain boundary segregation in the weld zone, the upper limit of the P content is set to 0.040% or less. However, excessive reduction leads to an increase in raw material cost, so the lower limit of the P content is 0.010% or more. Furthermore, considering the production cost and corrosion resistance, the P content is preferably 0.020 to 0.030%.

(S: 0.0003 to 0.0050%)
S needs to make S content 0.0050% or less in order to suppress the solidification crack of a welding part. Therefore, the upper limit of the S content is set to 0.0050% or less. S has an effect of suppressing segregation by bonding with Mn. In order to obtain this effect, the lower limit of the S content is set to 0.0003% or more. In consideration of refining costs and suppression of crevice corrosion at the pipe end thickening portion when used as a part, the S content is preferably 0.0005 to 0.0030%.

(Cr: 10.0-20.0%)
Cr is an element that improves corrosion resistance and oxidation resistance, and considering the environment to which the exhaust system parts are exposed, a content of 10.0% or more is necessary from the viewpoint of suppressing abnormal oxidation. Therefore, the lower limit of the Cr content is 10.0% or more. On the other hand, excessive content causes the steel plate to become hard and deteriorate the formability of the welded part, and also lowers the n value of the welded part, so the upper limit of the Cr content is made 20.0% or less. In consideration of plate breakage and bending cracking during steel plate production and steel pipe production due to toughness degradation, the Cr content is preferably less than 10.5 to 18.0%. Furthermore, considering the alloy cost, the Cr content is more preferably 10.5 to 15.0%.

(N: 0.001-0.020%)
N, like C, degrades moldability and corrosion resistance. In particular, if the content exceeds 0.020%, hard martensite and Cr carbide are generated in the welded portion, and the n value is lowered, so the upper limit of the N content is set to 0.020% or less. However, excessive reduction leads to an increase in refining costs, so the lower limit of the N content is set to 0.001% or more. Furthermore, considering the production cost and intergranular corrosion properties of the welded portion, the N content is preferably 0.002 to 0.009%.

(B: 0.0002 to 0.0020%)
B is an element that improves the secondary workability of the product by segregating at the grain boundaries. In the pipe end thickening process, a multi-step process is performed in a spinning process or a forging process. For example, when the outside of the tube end is increased by spinning, the tube end is inflated to the outside in the first step, and then the expanded tube end and the outside of the tube are brought into close contact by the spinning process of the second step. . Under the present circumstances, when the secondary workability of a steel pipe is bad, a crack may arise in a welding part in the 2nd process. In the present invention, it has been found that when the steel as a raw material contains 0.0002% or more of B, B segregates at the grain boundary of the weld and suppresses cracks after being strongly processed. Therefore, the lower limit of the B content is set to 0.0002% or more. On the other hand, if the content exceeds 0.0020%, the steel material becomes extremely hard, so the upper limit of the B content is set to 0.0020% or less. Furthermore, if the refining cost and intergranular corrosion properties are taken into consideration, the B content is preferably 0.0003 to 0.0015%.

(Mg: 0.0002 to 0.0030%)
Mg may be contained as a deoxidizing element, and is an element that refines the structure of the weld and improves formability. In the present invention, it has been found that when Mg is contained in an amount of 0.0002% or more, the n value of a welded part by TIG welding is improved, and spinning workability and forging workability are improved. Therefore, the lower limit of the Mg content is set to 0.0002% or more. However, if it exceeds 0.0030%, cracks due to inclusions occur due to coarse MgO generation, so the upper limit of Mg content is 0.0030% or less. Furthermore, considering the refining cost, the Mg content is preferably 0.0002 to 0.0010%.

(Al: 0.005 to 0.300%)
In addition to being contained as a deoxidizing element, Al has an effect of suppressing oxide scale peeling. Moreover, in this invention, it discovered that the cleanliness of a welding part improved by containing Al, and the crack resulting from an inclusion can be suppressed. Therefore, the lower limit of the Al content is set to 0.005% or more. On the other hand, if the content exceeds 0.300%, coarse AlN or Al ₂ O ₃ is generated and becomes the base point of the void, so the upper limit of the Al content is 0.300% or less. Furthermore, considering the refining cost and the pickling property at the time of manufacturing the steel sheet, the Al content is preferably 0.010 to 0.080%.

(Sn: 0.005-0.500%)
Sn contributes to improvement of corrosion resistance and high temperature strength. In particular, when a pipe end thickened steel pipe is used as an exhaust pipe, salt damage resistance on the outer surface of the exhaust pipe and corrosion resistance against condensed water on the inner surface of the exhaust pipe are improved. As described above, the present inventors have found that it is effective to contain a small amount of Sn for improving the n value of the region including the welded portion. In general, a stainless steel pipe is manufactured by rolling or bending a steel sheet and then welding by TIG welding, laser welding or ERW welding. At this time, the segregating element segregates at the weld, but Sn segregates at the grain boundary of the weld because it is a grain boundary segregating element. As a result, it was found that the grain boundaries in the welded portion are strengthened and the work hardening characteristics are improved. In order to acquire the said effect by containing Sn, the minimum of Sn content shall be 0.005% or more. On the other hand, when the content exceeds 0.500%, Sn segregation at the grain boundary of the welded portion becomes prominent, and the difference in deformation between the grain boundary and the inside of the grain becomes too large, so that the pipe end processed portion is likely to be cracked. . Therefore, the upper limit of Sn content is 0.500% or less. Furthermore, considering the refining cost and manufacturability, the Sn content is preferably 0.005 to 0.300%. Furthermore, considering the toughness when exposed to high temperature for a long time at high temperature, the Sn content is more preferably 0.005 to 0.150%.

  The ferritic stainless steel pipe according to the present invention contains one or two of Ti and Nb in a range of 0.05 to 0.70%. In the case where either one of Ti and Nb is contained within a range of 0.05 to 0.70%, the other may not be contained. Moreover, you may contain both Ti and Nb in 0.05 to 0.70% of range, respectively. Hereinafter, Ti and Nb will be described.

(Ti: 0.05-0.70%)
Ti is an element that is contained in combination with C, N, and S in order to improve corrosion resistance, intergranular corrosion resistance, deep drawability, and to refine the solidification structure of the weld. Since the fixing action of C and N by Ti is expressed from 0.05%, when Ti is contained, the lower limit of the Ti content is set to 0.05% or more. Moreover, since it will become a starting point of the crack at the time of a spinning process and a forge process by the production | generation of coarse TiN when it contains exceeding 0.70%, the upper limit of Ti content shall be 0.70% or less. Furthermore, the upper limit of the Ti content is preferably 0.30% or less in order to avoid breakage during steel plate and steel pipe production due to toughness deterioration.

(Nb: 0.05-0.70%)
Nb contains Cb, N, and S in combination with C, N, and S to improve the corrosion resistance, intergranular corrosion resistance, deep drawability, and refine the solidification structure of the weld. The lower limit of the Nb content is set to 0.05% or more. However, if it exceeds 0.70%, coarse Nb (C, N) precipitates and becomes the starting point of cracking during spinning and forging, so the upper limit of Nb content is 0.70% or less. To do. Furthermore, the upper limit of the Nb content is preferably 0.55% or less in order to avoid breakage during steel plate production and steel pipe production due to toughness deterioration.

  The ferritic stainless steel pipe according to the present invention contains the above-described elements, and the balance consists of Fe and inevitable impurities. Moreover, in addition to the element mentioned above, the ferritic stainless steel pipe according to the present invention may or may not contain the following elements selectively. The lower limit of each element content when not containing is 0% or more.

(Ni: 0.10 to 0.50%)
Ni is preferably added as necessary to promote crevice corrosion suppression and repassivation. Since the effect | action which improves the external surface corrosion property of the pipe end thickening steel pipe of this invention expresses at 0.10% or more, the minimum of Ni content shall be 0.10% or more. However, if the content exceeds 0.50%, the steel material becomes hard and cracks are likely to occur in the tube end processed portion, so the upper limit of the Ni content is 0.50% or less. In consideration of toughness, raw material costs and stress corrosion cracking, the Ni content is preferably 0.20 to 0.40%.

(Mo: 0.10 to 2.00%)
Mo is an element that improves the corrosion resistance, and is an effective element particularly in the case of a pipe end thickened steel pipe having a gap structure, which is manufactured by spinning, and therefore, it is preferably contained as necessary. Since this effect is manifested at 0.10% or more, the lower limit of the Mo content is set to 0.10% or more. Moreover, when it contains exceeding 2.00%, a moldability will deteriorate remarkably or manufacturability will worsen, Therefore The upper limit of Mo content shall be 2.00% or less. Considering alloy costs and productivity, the Mo content is preferably 0.10 to 1.80%. Furthermore, considering the tube forming property from a thin steel plate to a steel pipe, 0.10 to 1.20% is more preferable.

(Cu: 0.10 to 2.00%)
Cu is preferably contained as necessary in order to promote the suppression of crevice corrosion and the repassivation to improve the high temperature strength and thermal fatigue characteristics. Since this effect appears from 0.10% or more, the lower limit of the Cu content is set to 0.10% or more. However, excessive content makes the steel material hard and cracks are likely to occur in the tube end processed portion, so the upper limit of the Cu content is 2.00% or less. In consideration of manufacturability, the Cu content is preferably 0.10 to 1.30%.

(V: 0.05-1.00%)
V may be contained as necessary to suppress crevice corrosion. Since this effect appears from 0.05% or more, the lower limit of the V content is set to 0.05%. However, if the content exceeds 1.00%, coarse VN is generated, and cracks are likely to occur in the tube end processed portion, so the upper limit of V content is 1.00% or less. In consideration of the raw material cost, the V content is preferably 0.10 to 0.50%.

(Ca: 0.0002 to 0.0030%)
Ca may be contained as necessary for desulfurization. Since this effect does not appear at less than 0.0002%, the lower limit of the Ca content is set to 0.0002% or more. Further, if contained over 0.0030%, CaS which is a water-soluble inclusion is generated, and cracks are likely to occur in the tube end processed portion, so the upper limit of Ca content is 0.0030% or less. . Furthermore, from the viewpoint of surface quality, the Ca content is preferably 0.0002 to 0.0015%.

(Zr: 0.01-0.30%)
Zr is combined with C and N to improve workability and oxidation resistance, so it is preferable to contain Zr in an amount of 0.01% or more as necessary. However, if the content exceeds 0.30%, coarse ZrN is generated, and cracks are likely to occur in the tube end processed portion. Therefore, the upper limit of the Zr content is set to 0.30% or less. Furthermore, considering refining costs and manufacturability, the Zr content is preferably 0.01 to 0.10%.

(W: 0.01 to 3.00%)
W contributes to improvement of corrosion resistance and high-temperature strength, so it is preferable to contain 0.01% or more if necessary. On the other hand, if the content exceeds 3.00%, coarse WC is generated and cracks are likely to occur in the tube end processed portion. Therefore, the upper limit of the W content is set to 3.00% or less. Furthermore, considering the refining cost and manufacturability, the W content is preferably 0.01 to 1.00%.

(Co: 0.01-0.30%)
Co contributes to the improvement of the high temperature strength, so it is preferable to contain 0.01% or more as necessary. On the other hand, if the content exceeds 0.30%, coarse CoS ₂ is generated and cracks are likely to occur in the tube end processed portion, so the upper limit of the Co content is set to 0.30% or less. Furthermore, considering refining costs and manufacturability, the Co content is preferably 0.01 to 0.10%.

(Sb: 0.005-0.500%)
Sb is an element that segregates at the grain boundaries and contributes to the improvement of the high-temperature strength, so it is preferable to contain it as necessary. In order to obtain this effect, the lower limit of the Sb content is set to 0.005% or more. However, if the content exceeds 0.500%, Sb segregation at the grain boundaries becomes prominent and cracks are likely to occur in the tube end processed portion, so the upper limit of the Sb content is 0.500% or less. In consideration of high temperature characteristics, production cost and toughness, the Sb content is preferably 0.030 to 0.300%. More preferably, it is 0.050 to 0.200%.

(REM: 0.001 to 0.200%)
REM (rare earth element) is effective in improving oxidation resistance, and may be contained in an amount of 0.001% or more as necessary. Moreover, even if it contains exceeding 0.200%, the effect will be saturated, and development of {111} direction will be suppressed by formation of a coarse oxide. Furthermore, corrosion resistance is reduced by REM sulfide, and cracks are likely to occur in the tube end processed portion. Therefore, the upper limit of the REM content is 0.200% or less. Considering the workability and manufacturing cost of the product, it is preferable that the lower limit of the REM content is 0.002% or more and the upper limit is 0.100% or less.
Note that REM (rare earth element) follows a general definition. It is a generic term for two elements of scandium (Sc) and yttrium (Y) and 15 elements (lanthanoid) from lanthanum (La) to lutetium (Lu). These elements may be contained alone or in a mixture.

(Ga: 0.0002 to 0.3000%)
Ga may be included as necessary for improving corrosion resistance and suppressing hydrogen embrittlement. However, if the content exceeds 0.3000%, coarse sulfides are generated, and cracks are likely to occur in the tube end processed portion. Therefore, the upper limit of Ga content is set to 0.3000% or less. From the viewpoint of sulfide or hydride formation, the lower limit of the Ga content is set to 0.0002% or more. Furthermore, from the viewpoint of manufacturability and cost, the lower limit of the Ga content is preferably 0.0020% or more.

(Ta: 0.001-1.000%)
(Hf: 0.001-1.000%)
Ta and Hf may each be contained in an amount of 0.001% or more for improving the high temperature strength. However, if the content exceeds 1.000%, the toughness is deteriorated, so the upper limits of Ta and Hf are each 1.000% or less.

  Moreover, although it does not prescribe | regulate especially about another component in this invention, you may contain 0.001-0.020% of Bi as needed. Note that it is preferable to reduce general harmful elements and impurity elements such as As and Pb as much as possible.

Since the ferritic stainless steel pipe according to the present invention described above is excellent in formability at the welded portion, it can be suitably used as a ferritic stainless steel pipe for pipe end thickening.
In the case of subjecting the ferritic stainless steel pipe to thickening at the pipe end, there are a case where the pipe end is thickened outside the pipe and a case where the pipe end is thickened inside the pipe. When the pipe end is increased outside the pipe, the outer diameter of the increased thickness portion is larger than the outer diameter of the raw pipe. On the other hand, when increasing the thickness of the tube end to the inside of the tube, the inner diameter of the increased thickness portion is smaller than the inner diameter of the raw tube. Steel pipes with increased pipe ends have a slight gap between the outside or inside of the pipe and the end of the pipe that is in close contact, but from the viewpoint of suppressing crevice corrosion, the gap interval may be 50 μm or more. preferable.
As described above, the ferritic stainless steel pipe according to the present invention in which the pipe end is thickened inside or outside the pipe is excellent in corrosion resistance, and further excellent in strength, rigidity and weldability in the thickened portion at the pipe end. It can be suitably used as a ferritic stainless steel pipe for exhaust system parts of automobiles and motorcycles.

Next, a method for producing a ferritic stainless steel pipe according to the present invention will be described.
The ferritic stainless steel pipe according to the present invention is made of a stainless steel plate having a steel component defined by the present invention. The manufacturing method of a stainless steel plate consists of each process of steelmaking-hot rolling-annealing, pickling-cold rolling-annealing, and it does not prescribe | regulate especially the manufacturing conditions of each process.

In steelmaking, a method in which a steel containing the above essential components and components added as necessary is melted in a converter and subsequently subjected to secondary refining is preferable. The molten steel is made into a slab according to a known casting method (continuous casting). The slab is heated to a predetermined temperature and hot-rolled to a predetermined plate thickness with a continuous rolling mill.
The annealing process after hot rolling may be omitted, and cold rolling after pickling may be performed with either a normal Sendzimir mill or a tandem mill, but considering the bendability of the steel pipe, tandem mill rolling Is preferred. In cold rolling, roll roughness, roll diameter, rolling oil, number of rolling passes, rolling speed, rolling temperature, etc. may be appropriately selected within the scope of the present invention.

Intermediate annealing may be put in the middle of cold rolling, and the intermediate and final annealing may be batch annealing or continuous annealing. Further, if necessary, the annealing atmosphere may be bright annealing performed in a non-oxidizing atmosphere such as hydrogen gas or nitrogen gas, or may be performed in the air.
Furthermore, the product plate after the final annealing may be lubricated to further improve the press formability, and the type of the lubricating film may be appropriately selected. Although temper rolling or leveler may be applied for shape correction after the final annealing, it is preferable not to apply these because it causes a decrease in work hardening ability.

A ferritic stainless steel pipe according to the present invention is manufactured using the ferritic stainless steel sheet manufactured by the method described above. The manufacturing method of the ferritic stainless steel pipe according to the present invention may be selected as appropriate. For example, the steel sheet may be formed into a tubular shape by roll forming or bending forming and then manufactured by welding. The welding method is not particularly limited, and ERW welding, laser welding, TIG welding or the like may be selected, and a welding wire may be used.
Moreover, it does not specifically limit about the size of a steel pipe, What is necessary is just to determine suitably according to a use. However, when the thickness of the steel pipe is excessively thick, cracks and the like are likely to occur during the process of increasing the thickness of the pipe end. Therefore, the upper limit of the thickness of the steel pipe is preferably 1.5 mm or less. More preferably, it is 2 mm or less. On the other hand, since the thin material has a small merit for increasing the thickness, the lower limit of the thickness of the steel pipe is preferably 0.4 mm or more.

  As a method of thickening the pipe end of the ferritic stainless steel pipe manufactured by the above method, spinning or forging of the pipe end is preferable, but these methods are not particularly specified. In addition, in the pipe end thickening process, for example, a softening heat treatment may be performed between the first step and the second step.

Moreover, although the case where it increases to the outer side of a pipe | tube and the case where it increases to the inner side of a pipe | tube are considered, you may increase in thickness to any.
In addition, as a method for increasing the thickness of the pipe end, spinning is more preferable in consideration of work efficiency and dimensional accuracy. This spinning process is a method of bending the pipe end to the outside or the inside of the pipe and bringing the bent pipe end into close contact with the outside or the inside of the pipe in the next step. At this time, a gap is formed between the outer side or the inner side of the pipe and the end of the pipe that is brought into close contact, but the gap interval is preferably 50 μm or more from the viewpoint of suppressing crevice corrosion.

  On the other hand, forging is a method of forging a steel pipe in the axial direction to increase the pipe end. Also in the forging process, it is preferable that the gap interval between the outer side or the inner side of the pipe and the closely attached pipe end is 50 μm or more from the viewpoint of suppressing crevice corrosion.

  The ferritic stainless steel pipe according to the present invention described above is excellent in work hardening characteristics of the welded portion. Therefore, when the pipe end thickening process is performed using the ferritic stainless steel pipe of the present invention, it is possible to obtain a pipe end thickened steel pipe which is suppressed in cracking of the welded portion and excellent in dimensional accuracy. This pipe end thickened steel pipe can reduce the thickness of non-welded parts and has excellent corrosion resistance, so it can be made thinner and lighter, especially by using it as a steel pipe processed into exhaust system parts for automobiles and motorcycles. This contributes to improved fuel efficiency.

  Steels having the composition shown in Tables 1 and 2 are melted and cast to form slabs. After hot rolling, hot-rolled sheet annealing is omitted, pickling is performed, cold rolling, intermediate annealing, and final cooling. Hot rolling and final annealing were performed to obtain a steel plate having a thickness of 0.8 mm. Using this steel plate as a raw material, a φ35 electric resistance welded steel pipe (ERW steel pipe) was manufactured. The ERW welding was performed by heating the material by high frequency induction heating after roll forming.

For the manufactured ERW steel pipe, the Sn concentration in the welded portion was measured by an electron beam microanalyzer, and the maximum value was taken as the maximum Sn concentration in the Sn segregated portion in the welded portion.
In addition, with respect to the ERW steel pipe, an arc-shaped tensile test piece (JIS No. 13B) is collected so that tension can be imparted in the welding direction of the steel pipe and the welded portion is included within the width of the test piece. A tensile test was performed in accordance with JIS Z 2241. From the results of the tensile test, a true stress-true strain curve was obtained, and the slope at 5% to 10% strain was defined as an n value.
Table 2 shows the maximum Sn concentration and n value of the Sn segregated portion in the welded portion obtained by the above method.

Moreover, about the manufactured ERW steel pipe, the pipe end thickening process was performed inside or the outside by spinning process or forging process, and the presence or absence of the crack of a weld part was observed visually. The case where no crack was confirmed in the welded portion after the pipe end thickening processing was marked with ◯, and the case where crack was confirmed was marked with ×.
In the spinning process, the tube end was bent to the outside or the inside of the tube, and the bent tube end was brought into close contact with the outside or the inside of the tube in the next step. In the forging process, the pipe was forged in the axial direction, and the pipe end was increased inward or outward.

  In addition, as an evaluation of corrosion resistance, a corrosion test was performed on a sample whose pipe end was thickened by the above-described method. Prior to the corrosion test, a preheating treatment was performed at 400 ° C. for 24 hours in the air, 5% sodium chloride was sprayed at 35 ° C. for 4 hours, and then dried at 25% humidity and 60 ° C. for 2 hours. The test that was allowed to stand at 95% humidity and 50 ° C. for 2 hours was repeated 100 cycles. After this, the gap at the tube end thickening part is opened, and the maximum pitting depth on the surface of the released tube end that is in close contact with the outside or inside of the tube is determined by the depth of focus method using a microscope. The measured corrosion depth. A sample having a corrosion depth of less than 0.7 mm was marked with ◯, and a sample with a depth of 0.7 mm or more was marked with ×.

  Table 3 shows the manufacturing method and test results described above. In addition, about the thing which the crack generate | occur | produced at the time of the thickening process, the corrosion resistance test was not implemented. “-” In Table 3 indicates that the corrosion resistance test was not performed.

From Tables 1 to 3, the steel pipes (steel No. 1 to No. 18) of the present invention have a chemical composition within the range of the present invention, and the n value in the region including the welded portion is high, and Sn in the welded portion. Because the maximum Sn concentration in the segregated part is high, cracks do not occur in the pipe end thickening process, and the local work hardening characteristics of the welded part are excellent, and furthermore, the corrosion resistance test results in excellent corrosion resistance. I understand.
On the other hand, steel No. 1 in Table 1 was used. 19-No. In No. 39, at least one of the chemical composition, the maximum Sn concentration of the Sn segregation part in the welded part, or the n value of the region including the welded part is outside the scope of the present invention, cracking occurs during the thickening process, or the corrosion resistance is sufficient. It wasn't. Below, each comparative example is demonstrated.

Steel No. No. 19 is an example in which cracking occurred during tube end thickening because the C content was outside the range of the present invention and the n value was low.
Steel No. No. 20 is an example in which cracking occurred during tube end thickening because the N content was outside the range of the present invention and the n value was low.
Steel No. No. 21 is an example in which cracking occurred during tube end thickening because the Si content was outside the range of the present invention and the n value was low.
Steel No. No. 22 is an example in which cracks occurred during tube end thickening because the C, Mn, and S contents were outside the scope of the present invention and the n value was low.

Steel No. No. 23 is an example in which cracking occurred during pipe end thickening because the P content was outside the range of the present invention and the n value was low.
Steel No. No. 24 is an example in which cracking occurred during tube end thickening because the S content was outside the range of the present invention and the n value was low.
Steel No. No. 25 is an example in which cracking occurred during tube end thickening because the Cr content was outside the scope of the present invention.
Steel No. No. 26 is an example in which cracking occurred at the time of pipe end thickening because the Sn content was outside the range of the present invention, the maximum Sn concentration of the Sn segregation part in the welded part was low, and the n value was low.

Steel No. No. 27 is an example in which the corrosion resistance was inferior because the Ti content was outside the scope of the present invention.
Steel No. No. 28 is an example in which the corrosion resistance was inferior because the Nb content was outside the scope of the present invention.
Steel No. No. 29 is an example in which cracking occurred during tube end thickening because the B content was outside the range of the present invention and the n value was low.
Steel No. No. 30 is an example in which cracking occurred during pipe end thickening because the Al content was outside the range of the present invention and the n value was low.
Steel No. No. 31 is an example in which cracking occurred during tube end thickening because the Mg content was outside the scope of the present invention.

Steel No. No. 32 is an example in which cracking occurred during tube end thickening because the Zr content was outside the range of the present invention and the n value was low.
Steel No. No. 33 is an example in which neither Ti nor Nb is contained, the V content is outside the range of the present invention, and the n value was low, so that cracking occurred during the tube end thickening process.
Steel No. No. 34 is an example in which cracking occurred during tube end thickening because the Ni content was outside the range of the present invention and the n value was low.
Steel No. No. 35 is an example in which neither Ti nor Nb was contained, the Mo content was outside the range of the present invention, and the n value was low, so that cracking occurred during tube end thickening.

Steel No. No. 36 is an example in which cracks occurred during tube end thickening processing because Al was not contained, and the contents of Mn, P and W were outside the scope of the present invention and the n value was low.
Steel No. No. 37 is an example in which cracking occurred during tube end thickening because the Co content was outside the range of the present invention and the n value was low.
Steel No. No. 38 is an example in which cracks occurred during tube end thickening because the Mn, Cu, B and Al contents were outside the scope of the present invention and the n value was low.
Steel No. No. 39 is an example in which the chemical composition satisfies the scope of the present invention, but the maximum Sn concentration of the Sn segregation portion in the welded portion is low and the n value is low, so that cracking occurs during the tube end thickening processing.

  ADVANTAGE OF THE INVENTION According to this invention, it is possible to provide the ferritic stainless steel pipe excellent in the workability and corrosion resistance of a welding part. By using the ferritic stainless steel pipe according to the present invention as a material that is processed into exhaust system parts for automobiles and motorcycles in particular, the degree of freedom of molding is improved and the thickness can be reduced, and new equipment is introduced. Therefore, efficient parts manufacturing and fuel efficiency can be improved. That is, the present invention is extremely useful in industry.

Claims (5)

A steel base material formed into a tubular shape, and a welded portion;
The steel base material is mass%,
C: 0.001 to 0.020%,
Si: 0.01 to 1.00%,
Mn: 0.01 to 1.00%,
P: 0.010 to 0.040%,
S: 0.0003 to 0.0050%,
Cr: 10.0-20.0%,
N: 0.001 to 0.020%,
B: 0.0002 to 0.0020%,
Al: 0.005 to 0.300%,
Mg: 0.0002 to 0.0030% and Sn: 0.005 to 0.500%
Further,
Ti: 0.05-0.70% and Nb: 0.05-0.70%
1 type or 2 types of
The balance consists of Fe and inevitable impurities,
The ferritic stainless steel pipe, wherein the maximum Sn concentration of the Sn segregation part in the welded part is 0.010% or more, and the n value of the region including the welded part is 0.15 or more. The steel base material is further mass%,
Ni: 0.10 to 0.50%,
Mo: 0.10 to 2.00%,
Cu: 0.10 to 2.00%,
V: 0.05-1.00%,
Ca: 0.0002 to 0.0030%,
Zr: 0.01 to 0.30%,
W: 0.01 to 3.00%
Co: 0.01-0.30%
Sb: 0.005 to 0.500%,
REM: 0.001 to 0.200%,
Ga: 0.0002 to 0.3000%,
Ta: 0.001-1.000% and Hf: 0.001-1.000%
The ferritic stainless steel pipe according to claim 1, comprising one or more of the following.   The ferritic stainless steel pipe according to claim 1 or 2, which is used for pipe end thickening.   A ferritic stainless steel pipe for automobile exhaust system parts, comprising the ferritic stainless steel pipe according to any one of claims 1 to 3, wherein the outer diameter of the pipe end is larger than the outer diameter of the material.   A ferritic stainless steel pipe for automobile exhaust system parts, comprising the ferritic stainless steel pipe according to any one of claims 1 to 3, wherein the inner diameter of the pipe end is smaller than the inner diameter of the material.