JP2006176824A - Welded ferritic stainless steel tube having excellent tube expanding workability - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a welded ferritic stainless steel tube which is free from tube expanding working crack and constriction occurring in a base material part at eccentric tube expanding working to at least two times the diameter of a tube stock and has excellent tube expanding workability. <P>SOLUTION: The welded ferritic stainless steel tube is: (1) a welded ferritic stainless steel tube in which elongation in a circumferential direction of the base material part of the welded tube is ≥15%; (2) a welded ferritic stainless steel tube which has a composition containing, by mass, 0.001 to 0.015% C, 0.001 to 0.020% N, 11 to 25% Cr, 0.01 to 2.0% Mo, 0.05 to 0.6% of either or both of Ti and Nb and 0.0003 to 0.0030% B and also has ≥30% elongation in a direction to be a circumferential direction of the tube stock for the welded tube and ≥1.5 average Lankford value. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention is a ferritic stainless steel that is used in applications in which pipe expansion processing such as fuel supply pipes of automobile exhaust system parts and automobile fuel system parts is severe, and corrosion due to adhesion of high temperature oxidation or snow melting agent or high temperature salt damage corrosion is severe The present invention relates to a welded pipe, and is intended to prevent a pipe expansion crack that occurs in a base material in a pipe expansion process including an eccentric process that is twice or more the diameter of the raw pipe.

  Ferritic stainless steel has been used for automobile exhaust system parts that are repeatedly heated and cooled because of its low thermal expansion coefficient. Recently, the stress corrosion cracking susceptibility at the time of snow melting agent adhesion in a cold region is low, so that application to automobile fuel supply pipes for fuel fuel parts has also begun. Many of these parts are thin-walled welded pipes that have been subjected to bending, bulging, pipe expansion, and the like. In particular, in the fuel supply pipe, as shown in Patent Document 1, it is shown that the pipes are expanded integrally and eccentrically in order to achieve rust prevention performance, and processing that is twice or more the diameter of the raw pipe may be performed. In addition, excellent pipe expandability is required for the pipes used.

  In order to meet these requirements, there is an example in which the rankford value (r value) in the rolling direction and the direction perpendicular to the rolling direction is specified as a ferritic stainless steel pipe excellent in workability as disclosed in Patent Document 2. It is difficult to withstand a very severe tube expansion process that is more than twice the tube and accompanied by eccentric tube expansion.

  On the other hand, with regard to the materials, due to the fact that the usage environment has become severe and the demand for improvement in durability such as life, etc., improvement in corrosion resistance, high temperature salt damage resistance, oxidation resistance, high temperature strength and thermal fatigue is required, and Cr, Mo , Nb and other alloy element contents tend to be high, and there is a demand for a welded pipe material that has both performance and workability.

Japanese Patent Laid-Open No. 2002-242779 JP-A-6-41689

  In view of the present situation, the present invention provides a ferritic stainless steel welded pipe excellent in pipe expansion workability, which prevents pipe expansion cracks occurring in a base material in pipe expansion processing including eccentric processing that is more than twice as much as that of a base pipe. The purpose is to provide.

In order to achieve the above object, the present inventors studied the deformation behavior of the welded pipe in the pipe expansion process, the relation between the pipe expandability and the base material characteristics, improved the pipe expandability by the multistage process, and included the eccentric pipe expansion process. In order to obtain tube expandability more than 2 times, the knowledge that the characteristics of the base plate and the characteristics of the base material after pipe forming are very important has been obtained. It is as follows.
(1) In ferritic stainless steel welded pipes, ferritic stainless steel with excellent pipe workability characterized by a circumferential elongation of the welded pipe base material after forming, welding and straightening being 15% or more Welded pipe.
(2) In mass%, C: 0.001 to 0.015%, Si: 0.01 to 1.0%, Mn: 0.01 to 1.0%, P: 0.01 to 0.03 %, S: 0.0005-0.010%, N: 0.001-0.020%, Cr: 11-25%, Mo: 0.01-2.0%, one or two of Ti or Nb Contains 0.1-0.5% seed, B: 0.0003-0.0030%, the balance is made of Fe and inevitable impurities, and the elongation in the direction to be the circumferential direction of the welded pipe blank is 30. The ferritic stainless steel welded pipe with excellent tube expansion workability according to claim 1, wherein the average rankford value (r value) is 1.5 or more.

  The present invention is described in detail below. Normally, in the eccentric tube expansion process in a multistage process using a punch, as shown in FIG. 1, the welded pipe is compressed and deformed in the tube axis direction due to the stress in the tube axis direction due to friction with the punch in each process. The pipe is expanded in the circumferential direction while being subjected to tensile deformation. Also, when accompanied by eccentric tube expansion, the eccentric part is overhanged and locally subjected to tensile deformation in the axial direction and the circumferential direction, and in particular, the circumferential strain is large due to the tensile strain. It is considered that securing the ductility in the circumferential direction is important for eccentric tube expansion molding.

  In such processing, if the ductility of the welded pipe base material portion is insufficient, as shown in FIG. 2, constrictions and cracks along the axial direction occur in the overhanging portion of the eccentric expanded pipe portion. Such necking or cracking is a defect that is not permitted in the fuel supply pipe of the automobile fuel system part as an important safety part. As a result of examining the relationship between cracks in the eccentric expanded pipe and ductility of the welded pipe base material, there is a relationship between cracks in the eccentric expanded pipe and the circumferential extension of the welded pipe, as shown in Fig. 3. In order to suppress cracks and cracks, 15% or more is required in the circumferential direction of the welded pipe. Necking occurs in welded pipes with a circumferential elongation of 15% or less and 10% or more. It became clear that it would crack. In order to ensure a circumferential elongation of 15% or more, when forming into an open pipe shape by roll forming or gauge forming, the tube forming strain is reduced by a method of forming with as low strain as possible or heat treatment after tube forming. It is necessary to use means such as ensuring ductility. In addition, the circumferential direction elongation of the welded pipe base material part is cut and expanded in the circumferential direction, cut out the parallel part of the tensile test piece according to JIS No. 13B, weld the grip part at both ends, and perform a tensile test, Total elongation was measured.

  For welded pipe material plates, the elongation in the direction that should be in the circumferential direction is 30% or more in order to suppress the cracking of the base metal against the overall strain and local strain, and the average rankford value. It is desirable to use a ferritic stainless steel strip that is 1.5 or more. Moreover, it is desirable to use high-purity ferritic stainless steel added with Ti and Nb as the stainless steel plate material plate used for them, and the chemical composition defined in the present invention is described in detail below to ensure the formability of the material. Explained.

  C, N: When C and N are added in a large amount, the moldability and corrosion resistance are deteriorated. Further, since the amount of Ti required to fix them increases, the upper limit is set to 0.015% for C and 0.020% for N. The lower limit is 0.001% for both C and N in consideration of refining costs.

  Si: Si is an element used as a deoxidizing element, but if it exceeds 1.0%, the formability is remarkably deteriorated, so the upper limit was made 1.0%. Considering the refining process cost, 0.01% is unavoidably mixed in, so this is the lower limit.

  Mn: When a large amount of Mn is added, the upper limit is made 1.0% in order to reduce the moldability. Considering the refining process cost, 0.01% is unavoidably mixed in, so this is the lower limit.

  P: P lowers formability by solid solution strengthening, so the upper limit was made 0.03%. The lower limit is 0.01% because the steelmaking cost is increased by selecting raw materials and the like.

  S: When S is added in a large amount, corrosion resistance deteriorates due to inclusions and the like. Therefore, the upper limit is set to 0.010%, and the lower limit is set to 0.0005% in consideration of steelmaking costs.

  Cr: Cr is an element necessary for ensuring corrosion resistance, which is a basic characteristic of stainless steel. The lower limit of Cr required for automobile fuel system parts is 11% when coated, and the upper limit is 25% in order to reduce formability, increase product costs, and deteriorate manufacturability.

  Mo: Mo is also an element that improves the corrosion resistance of stainless steel. Similar to Cr, the upper limit was set to 2.0% from the viewpoint of formability and cost, and the lower limit was set to 0.01% as an inevitable level.

  Ti, Nb: Ti and Nb are elements that combine with C and N to form precipitates and improve formability. Moreover, it is an element which improves corrosion resistance, especially the corrosion resistance of a welded part. The level necessary for improving the formability is 0.1% or more, and if added excessively, the formability is lowered, or wrinkles due to Ti inclusions are caused. Therefore, the upper limit is set to 0.5%.

  B: B is an element that improves secondary workability, and suppresses secondary work cracks in various processes after pipe expansion. In order to obtain the effect, 0.0003% or more is necessary, so the lower limit was set. The upper limit is set to 0.0030% in order to deteriorate the formability such as the elongation and r value of the material.

  Using a steel plate having a thickness of 1.0 mm having the components shown in Table 1, 25.4 mmφ TIG welded pipes and ERW welded pipes were produced by changing the molding conditions, post-pipe forming correction amount, and heat treatment temperature as shown in Table 2. . Multi-stage expansion of the manufactured welded pipe with multi-stage punch 5-step eccentric expansion (25.4 mmφ welded pipe is 30φ, 38φ, 45φ, 51φ, eccentric expansion 51φ (13 mm offset)). The tube expandability was evaluated. In addition, in such a strict pipe expansion process, cracks originating from the welded portion also occur, so that the weld bead strength and the like were appropriately controlled and manufactured.

  As shown in Table 2, as a result of evaluating the eccentric pipe expandability of various welded pipes, For high-purity ferritic steels having components and material properties within the scope of the present invention such as 1 to 14, the roll interval of the staggered perforated rolls as disclosed in JP-A-2002-239626, etc. is one time the outer diameter of the perforated roll. By disposing it so that it is more than 2 times, it is possible to reduce molding distortion caused by bending and unbending in the molding process. Specifically, ten perforated rolls having a roll outer diameter of 50 mm and a roll interval of 52 mm were arranged in a staggered manner in the tube forming direction. With regard to the sizing amount after pipe making at such a low strain, by setting the sizing rate to 1.5% or less in the circumferential length ratio, the circumferential elongation can be secured by 15% or more, and in the eccentric expanded pipe portion. Necking and cracking are not seen, and sufficient eccentric tube expansion is obtained. No. As for tube forming materials by ordinary roll forming as in 15-21, ductility is recovered by heat treatment at 700-850 ° C. after tube forming and sizing, and the circumferential elongation becomes 15% or more. No constriction or cracking was observed, and sufficient tube expansion was obtained. On the other hand, no. When a low strain tube, sizing amount, heat treatment, etc., such as 22 to 28, are not performed, or when the heat treatment temperature is low, a welded tube having a circumferential elongation of 15 to 10% may cause constriction in the eccentric expanded portion. It can be seen that the material with a circumferential elongation of 10% or less is cracked and cannot be used for important safety parts such as automobiles.

  On the other hand, no. If steel grade H (SUS430) of 29 to 31 is used as the raw material, the material elongation and r value are low, and the properties of the welded part are inferior. It cannot be used as a material.

  As described above, the present invention provides a ferritic stainless steel welded pipe that can withstand severe use environments such as automobile exhaust systems and fuel system parts, and has excellent pipe expandability without cracking of the base material in eccentric pipe expansion processing. Is possible, and industrial value is great.

Processing strain path by eccentric tube expansion processing Forms of cracks and constrictions found in the eccentric tube expansion part Relationship between circumferential elongation and necking / cracking in eccentric tube expansion

Claims (2)

  A ferritic stainless steel welded pipe excellent in pipe expansion workability, characterized in that in the ferritic stainless steel welded pipe, the circumferential direction elongation of the welded pipe preform after forming, welding and straightening is 15% or more.   In mass%, C: 0.001 to 0.015%, Si: 0.01 to 1.0%, Mn: 0.01 to 1.0%, P: 0.01 to 0.03%, S : 0.0005 to 0.010%, N: 0.001 to 0.020%, Cr: 11 to 25%, Mo: 0.01 to 2.0%, 1 or 2 of Ti or Nb is 0 0.05 to 0.6%, B: 0.0003 to 0.0030%, the balance is made of Fe and inevitable impurities, and the elongation in the direction to be the circumferential direction of the welded pipe blank is 30% or more. 2. A ferritic stainless steel welded pipe excellent in pipe expansion workability according to claim 1, wherein a material plate having an average Rankford value (r value) of 1.5 or more is used.

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