COMPOSITE TUBE AND WELDED JOINT

Abstract

A composite tube is provided that includes a first tube and a second tube. A chemical composition of the first tube includes, in mass %, C: more than 0.060% to 0.400% or less, Si: 0.01 to 1.00%, Mn: 0.01 to 1.20%, P: 0.0350% or less, S: 0.0150% or less, Sn: 0.0005 to 0.0400%, Al: 0.040% or less, N: 0.050% or less, O: 0.030% or less, and the balance: Fe and impurities. A chemical composition of the second tube includes, in mass %, C: 0.003 to 0.100%, Si: 0.01 to 1.50%, Mn: 0.01 to 2.20%, P: 0.0400% or less, S: 0.0100% or less, Sn: 0.0005 to 0.0300%, Ni: 7.0 to 52.0%, Cr: 15.0 to 27.0%, Al: 0.001 to 0.600%, N: 0.001 to 0.150%, O: 0.030% or less, and the balance: Fe and impurities. The composite tube satisfies formulas [Si.sub.ave+6?P.sub.ave+20?S.sub.ave+2?Sn.sub.ave?1.1000] and [0.0015?4?S.sub.ave+Sn.sub.ave].

Claims

1. A composite tube comprising a first tube and a second tube, wherein: a chemical composition of the first tube comprises, in mass %, C: more than 0.060% to 0.400% or less, Si: 0.01 to 1.00%, Mn: 0.01 to 1.20%, P: 0.0350% or less, S: 0.0150% or less, Sn: 0.0005 to 0.0400%, Al: 0.040% or less, N: 0.050% or less, O: 0.030% or less, and the balance: Fe and impurities; a chemical composition of the second tube comprises, in mass %, C: 0.003 to 0.100%, Si: 0.01 to 1.50%, Mn: 0.01 to 2.20%, P: 0.0400% or less, S: 0.0100% or less, Sn: 0.0005 to 0.0300%, Ni: 7.0 to 52.0%, Cr: 15.0 to 27.0%, Al: 0.001 to 0.600%, N: 0.001 to 0.150%, O: 0.030% or less, and the balance: Fe and impurities; and the composite tube satisfies Formulas (i) and (ii) below: $\begin{array}{cc}{\mathrm{Si}}_{\mathrm{ave}}+6?P_{\mathrm{ave}}+20?S_{\mathrm{ave}}+2?{\mathrm{Sn}}_{\mathrm{ave}}?1.1& \left(i\right)\end{array}$ $\begin{array}{cc}0.0015?4?S_{\mathrm{ave}}+{\mathrm{Sn}}_{\mathrm{ave}}& \left(\mathrm{ii}\right)\end{array}$ where, the meaning of each symbol in the above formulas is as follows: Si.sub.ave: average value (mass %) of contents of Si of first tube and second tube P.sub.ave: average value (mass %) of contents of P of first tube and second tube S.sub.ave: average value (mass %) of contents of S of first tube and second tube Sn.sub.ave: average value (mass %) of contents of Sn of first tube and second tube.

2. The composite tube according to claim 1, wherein: the chemical composition of the first tube contains, in lieu of a part of Fe, one or more elements selected from a group of, in mass %, Cr: 9.50% or less, a total of one or more elements selected from Cu, Ni and Co: 1.00% or less, a total of Mo and/or W: 4.00% or less, a total of one or more elements selected from V, Nb, Ti and Ta: 1.00% or less, B: 0.0200% or less, a total of Ca and/or Mg: 0.0100% or less, and REM: 0.0500% or less.

3. The composite tube according to claim 1 or claim 2, wherein: the chemical composition of the second tube contains, in lieu of a part of Fe, one or more elements selected from, in mass %, a total of Cu and/or Co: 6.00% or less, a total of Mo and/or W: 8.00% or less, a total of one or more elements selected from V, Nb, Ti and Ta: 2.00% or less, B: 0.0200% or less, a total of Ca and/or Mg: 0.0100% or less, and REM: 0.0500% or less.

4. A welded joint comprising the composite tube according to any one of claim 1 to claim 3.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0067] FIG. 1 is a schematic cross-sectional view illustrating the shape of a test material subjected to beveling in the Examples.

[0068] FIG. 2 is a schematic diagram illustrating the shape of a restraint weld test body.

DESCRIPTION OF EMBODIMENTS

[0069] To solve the aforementioned problem, the present inventors conducted detailed studies regarding cracking that occurs in weld metal when a composite tube which is composed of a low alloy steel and a high alloy steel that contain 0.0005 to 0.0400% of Sn and 0.0005 to 0.0300% of Sn, respectively with the objective of improving corrosion resistance, is subjected to butt welding using a Ni-based alloy welding consumables. As a result, the findings described hereunder were revealed.

[0070] (a) Cracking that occurred during welding occurred in the weld metal in the vicinity of a fusion line near the boundary between the inner tube and the outer tube, that is, in a region in which the inner tube and the outer tube fuse and mix at approximately the same ratio and are particularly susceptible to the influence of the components of the base metals (inner tube and outer tube). The region where cracking occurred exhibited a structure solidified with austenite single phase.

[0071] (b) Further, cracking was more liable to occur as the contents of Si, P, S and Sn of the inner tube and the outer tube increased. In addition, cracking occurred at columnar crystal boundaries of the weld metal near the fusion boundary, and concentration of Si, P, S and Sn was confirmed there.

[0072] Based on these results, the present inventors considered that cracking occurred due to the following reason.

[0073] (c) Si, P, S and Sn are distributed between the liquid phase and the solid phase (austenite phase) during solidification of the weld metal, and concentrate at columnar crystal boundaries which are places where the solid phases meet. Because these elements are all elements that lower the solidus temperature, the present inventors considered that the liquid phase remained at the columnar crystal boundaries until the last stage of solidification, and consequently cracking occurred due to shrinkage stress during solidification.

[0074] (d) The present inventors found that in order to stably prevent such cracking, it is necessary to control the average values of the amounts of Si, P, S and Sn contained in the outer tube and the inner tube to be not greater than a range that satisfies a predetermined relationship. As the amounts of these elements are reduced, cracking is less likely to occur in the weld metal.

[0075] (e) On the other hand, it has also been clarified that if S and Sn among these elements are extremely reduced, lack of fusion or lack of penetration is liable to occur during multi-pass welding.

[0076] (f) S is a surface active element and has an action of strengthening inward convection in the molten pool during welding. Therefore, heat from the arc is easily transmitted in the depth direction, and the weld penetration depth is deepened. Further, Sn evaporates from the molten pool surface during welding and forms a weld energizing path for the arc and increases the current density of the arc, and thus similarly has an effect of deepening the weld penetration depth.

[0077] (g) Therefore, it is considered that when the contents of S and Sn are extremely low, the aforementioned effect cannot be sufficiently obtained, and lack of fusion or lack of penetration easily occurs during multi-pass welding.

[0078] (h) It has been found that in order to prevent lack of fusion or lack of penetration, it is necessary to control the average values of the contents of S and Sn in the outer tube and the inner tube to be equal to or greater than a range that satisfies a predetermined relationship.

[0079] The present invention has been made based on the findings described above. The respective requirements of the present invention are described in detail hereunder.

(A) Overall Structure

[0080] A composite tube has a structure in which an outer tube and an inner tube are metallurgically bonded to each other, and is sometimes referred to as a clad tube. The composite tube according to the present invention includes a first tube and a second tube. In the present invention, depending on the intended use, the first tube may be used as an outer tube and the second tube may be used as an inner tube, or the second tube may be used as an outer tube and the first tube may be used as an inner tube.

[0081] Further, the composite tube of the present invention is a seamless tube. In addition, regarding the dimensions of the composite tube, although no particular limitations are set, preferably the outer diameter is 25.4 to 114.3 mm, the thickness is 2.0 to 15.0 mm, and a proportion that the second tube which is composed of high alloy steel that is described hereunder occupies with respect to the thickness of the overall tube is 0.10 to 0.50.

[0082] As described hereunder, the first tube is composed of low alloy steel, and the second tube is composed of high alloy steel. The respective chemical compositions of the first tube and the second tube will now be described in detail.

(B) Chemical Composition of First Tube

[0083] The reasons for limiting each element are as follows. Note that, the symbol % with respect to content in the following description means mass %.

C: more than 0.060% to 0.400% or less

[0084] C dissolves in the matrix or precipitates as a carbide during use at high temperatures, and contributes to securing the strength at room temperature and high temperatures. To obtain this effect, an amount of C that is more than 0.060% is to be contained. However, if C is excessively contained, it will lead to hardening of heat affected zones during butt welding, and will increase low-temperature cracking susceptibility. Therefore, the content of C is to be 0.400% or less. The content of C is preferably more than 0.100%, and more preferably 0.110% or more. Further, the content of C is preferably 0.380% or less, and more preferably 0.350% or less.

Si: 0.01 to 1.00%

[0085] Si has a deoxidizing action, and is also an effective element for improving corrosion resistance and oxidation resistance at high temperatures. To obtain these effects, the content of Si is to be 0.01% or more. However, if Si is excessively contained, Si will mix into the weld metal during welding, and will increase the solidification cracking susceptibility. Therefore, it is necessary to make the content of Si 1.00% or less, and also to satisfy a relationship with the contents of P, S and Sn that is described later. The content of Si is preferably 0.03% or more, and more preferably 0.05% or more. Further, the content of Si is preferably 0.90% or less, and more preferably 0.80% or less.

Mn: 0.01 to 1.20%

[0086] Mn has a deoxidizing action, similarly to Si, and also contributes to improving the strength by increasing hardenability. To obtain these effects, the content of Mn is to be 0.01% or more. However, if Mn is excessively contained, it will lead to embrittlement during use at high temperatures. Therefore, the content of Mn is to be 1.20% or less. The content of Mn is preferably 0.03% or more, and more preferably 0.05% or more. Further, the content of Mn is preferably 1.10% or less, and more preferably 1.00% or less.

P: 0.0350% or less

[0087] P mixes into the weld metal during welding, and thereby increases the solidification cracking susceptibility. Therefore, the content of P is to be 0.0350% or less. In addition, it is necessary for the content of P to satisfy a relationship with the contents of Si, S and Sn that is described later. The content of P is preferably 0.0330% or less, and more preferably 0.0300% or less. Note that, although it is not necessary to particularly set a lower limit of the content of P, and the content of P may be 0 (zero), extremely reducing the content of P will lead to an increase in the steel production cost. In addition, P has a not insignificant effect on increasing the strength. When it is desired to obtain this effect, the content of P is preferably made 0.0015% or more, and more preferably 0.0030% or more.

S: 0.0150% or less

[0088] Similarly to P, S mixes into the weld metal during welding, and markedly increases the solidification cracking susceptibility. Therefore, the content of S is to be 0.0150% or less. In addition, it is necessary for the content of S to satisfy a relationship with the contents of Si, P and Sn that is described later. The content of S is preferably 0.0130% or less, and more preferably 0.0100% or less. Note that, although it is not necessary to particularly set a lower limit of the content of S, and the content of S may be 0 (zero), if the content of S is extremely reduced, the weld penetration depth during welding will be small and a lack of fusion is liable to occur. Therefore, the content of S satisfies a relationship with Sn that is described later, and preferably is made 0.0001% or more, and more preferably made 0.0002% or more.

Sn: 0.0005 to 0.0400%

[0089] Sn concentrates under scale on the surface of the steel, and has an effect of improving corrosion resistance. Further, Sn mixes into the weld metal during welding and increases the weld penetration depth and thereby suppresses the occurrence of a lack of fusion. To obtain this effect, the content of Sn is to be 0.0005% or more, and must also satisfy a relationship with the content of S that is described later. On the other hand, if excessively contained, Sn will increase the solidification cracking susceptibility during welding. Therefore, the content of Sn is to be 0.0400% or less, and must also satisfy a relationship with the contents of Si, P and Sn described later. The content of Sn is preferably 0.0008% or more, and more preferably 0.0010% or more. Further, the content of Sn is preferably 0.0380% or less, and more preferably 0.0350% or less.

Al: 0.040% or less

[0090] Al is contained for the purpose of deoxidation. However, if Al is excessively contained, it will lead to a decrease in toughness. Therefore, the content of Al is to be 0.040% or less. The content of Al is preferably 0.035% or less, and more preferably 0.030% or less. Note that, although it is not necessary to particularly set a lower limit of the content of Al, and the content of Al may be 0 (zero), if the content of Al is extremely reduced, the deoxidation effect will not be sufficiently obtained and the cleanliness of the steel will decrease, and it will also lead to an increase in the production cost. Therefore, the content of Al is preferably made 0.001% or more, and more preferably 0.002% or more.

N: 0.050% or less

[0091] If N is excessively contained, it will lead to a decrease in toughness. Therefore, the content of N is to be 0.050% or less. The content of N is preferably 0.045% or less, and more preferably 0.040% or less. Note that, although it is not necessary to particularly set a lower limit of the content of N, and the content of N may be 0 (zero), extremely reducing the content of N will lead to an increase in the steel production cost. In addition, N forms nitrides and has a not insignificant effect on increasing the strength. When it is desired to obtain this effect, the content of N is preferably made 0.001% or more, and more preferably 0.003% or more.

O: 0.030% or less

[0092] If O is excessively contained, it will lead to a decrease in workability and ductility. Therefore, the content of O is to be 0.030% or less. The content of O is preferably 0.025% or less, and more preferably 0.020% or less. Note that, although it is not necessary to particularly set a lower limit of the content of O, and the content of O may be 0 (zero), extremely reducing the content of O will lead to an increase in the steel production cost. Therefore, the content of O is preferably made 0.001% or more, and more preferably 0.003% or more.

[0093] In the chemical composition of the first tube, the balance is Fe and impurities. Note that, the term impurity refers to components which are mixed in due to various factors during the production process when industrially producing a ferrous metal material, including those from raw material such as ore or scrap or the like.

[0094] The chemical composition of the first tube may contain one or more elements selected from the following group in lieu of a part of Fe. The reasons are described hereunder.

[0095] Cr: 9.50% or less

[0096] Total of one or more elements selected from Cu, Ni and Co: 1.00% or less

[0097] Total of Mo and/or W: 4.00% or less

[0098] Total of one or more elements selected from V, Nb, Ti and Ta: 1.00% or less

[0099] B: 0.0200% or less

[0100] Total of Ca and/or Mg: 0.0100% or less

[0101] REM: 0.0500% or less

[0102] Cr: 9.50% or less

[0103] Cr is effective for improving corrosion resistance and strength at high temperatures, and therefore may be contained as necessary. However, if Cr is excessively contained, the toughness will decrease. Consequently, when contained, the content of Cr is to be 9.50% or less. The content of Cr is preferably 9.40% or less, and more preferably 9.20% or less. Note that, when it is desired to reliably obtain the aforementioned effect, the content of Cr is preferably 0.01% or more, and more preferably 0.02% or more.

Total of one or more elements selected from Cu, Ni and Co: 1.00% or less

[0104] Cu, Ni and Co each increase the hardenability and are effective for improving the strength, and therefore may be contained as necessary. However, since each of these elements is an expensive element, when contained, the total content of one or more elements of element selected from these elements is to be 1.00% or less. The aforementioned total content is preferably 0.90% or less, and more preferably 0.80% or less. Note that, when it is desired to reliably obtain the aforementioned effect, the aforementioned total content is preferably 0.01% or more, and more preferably 0.02% or more.

Total of Mo and/or W: 4.00% or less

[0105] Mo and W each dissolve in the matrix and contribute to improving the high temperature strength, and hence each of these elements may be contained as necessary. However, if excessively contained, these elements form coarse intermetallic compounds and/or carbides during use at high temperatures, which leads to a decrease in toughness. Therefore, when contained, the total content of Mo and/or W is to be 4.00% or less. The aforementioned total content is preferably 3.80% or less, and more preferably 3.50% or less. Note that, when it is desired to reliably obtain the aforementioned effect, the aforementioned total content is preferably 0.01% or more, and more preferably 0.02% or more.

Total of one or more elements selected from V, Nb, Ti and Ta: 1.00% or less

[0106] V, Nb, Ti and Ta each form fine carbo-nitrides during use at high temperatures and contribute to improving the high temperature strength, and hence each of these elements may be contained as necessary. However, if excessively contained, a large amount of coarse carbo-nitrides will form, which will lead to a decrease in toughness. Therefore, when contained, the total content of one or more elements selected from these elements is to be 1.00% or less. The aforementioned total content is preferably 0.90% or less, and more preferably 0.80% or less. Note that, when it is desired to reliably obtain the aforementioned effect, the aforementioned total content is preferably 0.01% or more, and more preferably 0.02% or more.

B: 0.0200% or less

[0107] B increases hardenability and thereby contributes to improving the strength, and therefore may be contained as necessary. However, if B is excessively contained, B will mix into the weld metal during welding and thereby increase the solidification cracking susceptibility. Therefore, when contained, the content of B is to be 0.0200% or less. The content of B is preferably 0.0180% or less, and more preferably 0.0150% or less. Note that, when it is desired to reliably obtain the aforementioned effect, the content of B is preferably 0.0005% or more, and more preferably 0.0010% or more.

Total of Ca and/or Mg: 0.0100% or less

[0108] Ca and Mg each improve hot workability, and therefore may be contained as necessary. However, if excessively contained, the Ca and/or Mg will cause the cleanliness to markedly decrease and, on the contrary, will impair the hot workability. Therefore, when contained, the total content of Ca and/or Mg is to be 0.0100% or less. The aforementioned total content is preferably 0.0080% or less, and more preferably 0.0060% or less. Note that, when it is desired to reliably obtain the aforementioned effect, the aforementioned total content is preferably 0.0005% or more, and more preferably 0.0010% or more.

REM: 0.0500% or less

[0109] Similarly to Ca and Mg, REM improves hot workability, and therefore may be contained as necessary. However, if excessively contained, the REM will cause the cleanliness to markedly decrease and, on the contrary, will impair the hot workability. Therefore, when contained, the content of REM is to be 0.0500% or less. The content of REM is preferably 0.0400% or less, and more preferably 0.0300% or less. Note that, when it is desired to reliably obtain the aforementioned effect, the content of REM is preferably 0.0005% or more, and more preferably 0.0010% or more.

[0110] The term REM is a collective term for a total of 17 elements which include Sc, Y, and the lanthanoids, and the content of REM refers to the total content of one or more elements of the REM elements. Further, REM elements are usually contained in misch metal. Therefore, for example, misch metal may be added to an alloy so as to make the content of REM fall within the aforementioned range.

(C) Chemical Composition of Second Tube

[0111] The reasons for limiting each element are as follows. Note that, the symbol % with respect to content in the following description means mass %.

C: 0.003 to 0.100%

[0112] C stabilizes the austenitic structure and thereby contributes to ensuring high temperature strength. To obtain this effect, the content of C is to be 0.003% or more. However, if C is excessively contained, C forms carbides during welding or during use at high temperatures, which leads to a decrease in corrosion resistance. Therefore, the content of C is to be 0.100% or less. The content of C is preferably 0.005% or more, and more preferably 0.008% or more. Further, the content of C is preferably 0.090% or less, and more preferably 0.080% or less.

Si: 0.01 to 1.50%

[0113] Si has a deoxidizing action, and is also an effective element for improving corrosion resistance and oxidation resistance at high temperatures. To obtain these effects, the content of Si is to be 0.01% or more. However, if Si is excessively contained, Si will mix into the weld metal during welding, and will increase the solidification cracking susceptibility, and will also impair the stability of the austenitic structure, leading to a decrease in high temperature strength. Therefore, it is necessary to make the content of Si 1.50% or less, and also for the content of Si to satisfy a relationship with P, S and Sn that is described later. The content of Si is preferably 0.03% or more, and more preferably 0.05% or more. Further, the content of Si is preferably 1.30% or less, and more preferably 1.00% or less.

Mn: 0.01 to 2.20%

[0114] Mn has a deoxidizing action, and also increases the stability of the austenitic structure and thereby contributes to ensuring high temperature strength. To obtain these effects, the content of Mn is to be 0.01% or more. However, if Mn is excessively contained, it will lead to embrittlement during use at high temperatures. Therefore, the content of Mn is to be 2.20% or less. The content of Mn is preferably 0.03% or more, and more preferably 0.05% or more. Further, the content of Mn is preferably 2.00% or less, and more preferably 1.80% or less.

P: 0.0400% or less

[0115] P mixes into the weld metal during welding, and thereby increases the solidification cracking susceptibility. Therefore, the content of P is to be 0.0400% or less. In addition, it is necessary for the content of P to satisfy a relationship with the contents of Si, S and Sn described later. The content of P is preferably 0.0380% or less, and more preferably 0.0350% or less. Note that, although it is not necessary to particularly set a lower limit of the content of P, and the content of P may be 0 (zero), extremely reducing the content of P will lead to an increase in the steel production cost. In addition, P has a not insignificant effect on increasing the strength. When it is desired to obtain this effect, the content of P is preferably made 0.0030% or more, and more preferably 0.0050% or more.

S: 0.0100% or less

[0116] Similarly to P, S mixes into the weld metal during welding, and thereby markedly increases the solidification cracking susceptibility. Further, S increases the liquation cracking susceptibility of heat affected zones. Therefore, the content of S is to be 0.0100% or less. In addition, it is necessary for the content of S to satisfy a relationship with the contents of Si, P and Sn described later. The content of S is preferably 0.0090% or less, and more preferably 0.0080% or less. Note that, although it is not necessary to particularly set a lower limit of the content of S, and the content of S may be 0 (zero), if the content of S is extremely reduced, the weld penetration depth during welding will be small and a lack of fusion is liable to occur. Therefore, the content of S satisfies a relationship with Sn that is described later, and preferably is made 0.0001% or more, and more preferably made 0.0002% or more.

Sn: 0.0005 to 0.0300%

[0117] Sn has an effect of improving corrosion resistance. Further, Sn mixes into the weld metal during welding and increases the weld penetration depth and thereby suppresses the occurrence of a lack of fusion. To obtain this effect, the content of Sn is to be 0.0005% or more, and must also satisfy a relationship with the content of S that is described later. On the other hand, if excessively contained, Sn will increase the solidification cracking susceptibility during welding, and also increase the liquation cracking susceptibility of heat affected zones. Therefore, the content of Sn is to be 0.0300% or less, and must also satisfy a relationship with the contents of Si, P and Sn described later. The content of Sn is preferably 0.0008% or more, and more preferably 0.0010% or more. Further, the content of Sn is preferably 0.0280% or less, and more preferably 0.0250% or less.

Ni: 7.0 to 52.0%

[0118] Ni stabilizes the austenitic structure and contributes to high temperature strength. In addition, Ni increases corrosion resistance under an environment in which chloride ions are present. To obtain this effect, the content of Ni is to be 7.0% or more. However, since Ni is an expensive element, if Ni is excessively contained, it will lead to an increase in cost. Therefore, the content of Ni is to be 52.0% or less. The content of Ni is preferably 7.2% or more, and more preferably 7.5% or more. Further, the content of Ni is preferably 48.0% or less, and more preferably 45.0% or less.

Cr: 15.0 to 27.0%

[0119] Cr contributes to improving oxidation resistance and corrosion resistance at high temperatures. To obtain this effect, the content of Cristo be 15.0% or more. However, if Cr is excessively contained, Cr will impair the stability of the austenitic structure, leading to a decrease in high temperature strength. Therefore, the content of Cr is to be 27.0% or less. The content of Cr is preferably 15.2% or more, and more preferably 15.5% or more. Further, the content of Cr is preferably 26.8% or less, and more preferably 26.5% or less.

Al: 0.001 to 0.600%

[0120] Al is contained for the purpose of deoxidation. In addition, Al combines with Ni during use at high temperatures and precipitates as an intermetallic compound, and thereby contributes to improving high temperature strength. To obtain this effect, the content of Al is to be 0.001% or more. However, if Al is excessively contained, it will lead to a decrease in toughness. Therefore, the content of Al is to be 0.600% or less. The content of Al is preferably 0.002% or more, and more preferably 0.003% or more. Further, the content of Al is preferably 0.550% or less, and more preferably 0.500% or less.

N: 0.001 to 0.150%

[0121] N stabilizes the austenite phase, and thereby contributes to improving high temperature strength. To obtain this effect, the content of N is to be 0.001% or more. However, if Nis excessively contained, it will lead to a decrease in ductility. Therefore, the content of N is to be 0.150% or less. The content of N is preferably 0.002% or more, and more preferably 0.003% or more. Further, the content of N is preferably 0.130% or less, and more preferably 0.100% or less.

O: 0.030% or less

[0122] If O is excessively contained, it will lead to a decrease in workability and ductility. Therefore, the content of O is to be 0.030% or less. The content of O is preferably 0.025% or less, and more preferably 0.020% or less. Note that, although it is not necessary to particularly set a lower limit of the content of O, and the content of O may be 0 (zero), extremely reducing the content of O will lead to an increase in the steel production cost. Therefore, the content of O is preferably made 0.001% or more, and more preferably 0.003% or more.

[0123] In the chemical composition of the second tube, the balance is Fe and impurities. Note that, the term impurity refers to components which, when industrially producing a ferrous metal material, are mixed in due to various factors during the production process that include being mixed in from raw material such as ore or scrap or the like.

[0124] The chemical composition of the second tube may contain one or more kinds selected from the following group in lieu of a part of Fe. The reasons are described hereunder. [0125] Total of Cu and/or Co: 6.00% or less [0126] Total of Mo and/or W: 8.00% or less [0127] Total of one or more elements selected from V, Nb, Ti and Ta: 2.00% or less [0128] B: 0.0200% or less [0129] Total of Ca and/or Mg: 0.0100% or less [0130] REM: 0.0500% or less
Total of Cu and/or Co: 6.00% or less

[0131] Cu and Co each increase the stability of the austenitic structure and are effective for improving high temperature strength, and therefore may be contained as necessary. However, each of these elements is an expensive element, and furthermore, excessively containing these elements will lead to a decrease in ductility. Therefore, when contained, the total content of Cu and/or Co is to be 6.00% or less. The aforementioned total content is preferably 5.50% or less, and more preferably 5.00% or less. Note that, when it is desired to reliably obtain the aforementioned effect, the aforementioned total content is preferably 0.01% or more, and more preferably 0.02% or more.

Total of Mo and/or W: 8.00% or less

[0132] Mo and W each dissolve in the matrix and contribute to improving high temperature strength, and therefore these elements may be contained as necessary. However, if excessively contained, these elements will impair the stability of the austenitic structure, and will also form coarse intermetallic compounds and/or carbides during use at high temperatures, which will lead to a decrease in toughness. Therefore, when contained, the total content of Mo and/or W is to be 8.00% or less. The aforementioned total content is preferably 7.50% or less, and more preferably 7.00% or less. Note that, when it is desired to reliably obtain the aforementioned effect, the aforementioned total content is preferably 0.01% or more, and more preferably 0.02% or more.

Total of one or more elements selected from V, Nb, Ti and Ta: 2.00% or less

[0133] V, Nb, Ti and Ta each form fine carbo-nitrides during use at high temperatures and contribute to improving the high temperature strength, and hence each of these elements may be contained as necessary. However, if excessively contained, a large amount of coarse carbo-nitrides will form, which will lead to a decrease in toughness. Therefore, when contained, the total content of one or more elements selected from these elements is to be 2.00% or less. The aforementioned total content is preferably 1.90% or less, and more preferably 1.80% or less. Note that, when it is desired to reliably obtain the aforementioned effect, the aforementioned total content is preferably 0.01% or more, and more preferably 0.02% or more.

B: 0.0200% or less

[0134] B dissolves in carbides during use at high temperatures and is finely dispersed and thereby contributes to improvement of the high temperature strength, and therefore may be contained as necessary. However, if B is excessively contained, B will mix into the weld metal during welding and thereby increase the solidification cracking susceptibility. Therefore, when contained, the content of B is to be 0.0200% or less. The content of B is preferably 0.0180% or less, and more preferably 0.0150% or less. Note that, when it is desired to reliably obtain the aforementioned effect, the content of B is preferably 0.0005% or more, and more preferably 0.0010% or more.

Total of Ca and/or Mg: 0.0100% or less

[0135] Ca and Mg each improve hot workability, and therefore may be contained as necessary. However, if excessively contained, the Ca and/or Mg will cause the cleanliness to markedly decrease and, on the contrary, will impair the hot workability. Therefore, when contained, the total content of Ca and/or Mg is to be 0.0100% or less. The aforementioned total content is preferably 0.0080% or less, and more preferably 0.0060% or less. Note that, when it is desired to reliably obtain the aforementioned effect, the aforementioned total content is preferably 0.0005% or more, and more preferably 0.0010% or more.

REM: 0.0500% or less

[0136] Similarly to Ca and Mg, REM improves hot workability, and therefore may be contained as necessary. However, if excessively contained, the REM will cause the cleanliness to markedly decrease and, on the contrary, will impair the hot workability. Therefore, when contained, the content of REM is to be 0.0500% or less. The content of REM is preferably 0.0400% or less, and more preferably 0.0300% or less. Note that, when it is desired to reliably obtain the aforementioned effect, the content of REM is preferably 0.0005% or more, and more preferably 0.0010% or more.

[0137] The term REM is a collective term for a total of 17 elements which include Sc, Y, and the lanthanoids, and the content of REM refers to the total content of one or more elements of the REM elements. Further, REM elements are usually contained in misch metal. Therefore, for example, misch metal may be added to an alloy so as to make the content of REM fall within the aforementioned range.

(D) Average Chemical Composition of First Tube and Second Tube

[0138] In addition to the necessity for the first tube and the second tube according to the present invention to have the chemical compositions described above, respectively, it is also necessary for the average chemical composition of the first tube and the second tube to satisfy a predetermined relational expression. The reasons are as follows.

[0139] In a case where the composite tube according to the present invention is welded using welding consumables composed of austenitic stainless steel or a Ni alloy, the composite tube (base metal) will melt and the contained Si, P, S and Sn will mix into the weld metal. Each of these elements causes the solidus temperature to decrease, and thus increases the solidification cracking susceptibility of the weld metal. In particular, in weld metal near the fusion boundary, mixing during welding is insufficient, and the chemical composition of the weld metal is dominated by the influence of the base metal.

[0140] In addition, because the weld metal becomes a solidified structure of austenite single phase in the vicinity of the boundary between the inner tube and the outer tube, the vicinity of the boundary is influenced by these elements, and therefore solidification cracking is liable to occur. In order to stably prevent solidification cracking occurring in that region, it is necessary to control the average values of the contents of Si, P, S and Sn in the first tube and the second tube so that a relational expression that takes into account the degree of influence of the respective elements is not more than a predetermined range.

[0141] Specifically, it is necessary to satisfy the following Formula (i). The left-hand value in Formula (i) below is preferably 1.0500 or less, and more preferably 1.0000 or less.

[00002]$\begin{array}{cc}{\mathrm{Si}}_{\mathrm{ave}}+6?P_{\mathrm{ave}}+20?S_{\mathrm{ave}}+2?{\mathrm{Sn}}_{\mathrm{ave}}?1.1& \left(i\right)\end{array}$ [0142] Where, the meaning of each symbol in the above formula is as follows: [0143] Si.sub.ave: average value (mass %) of content of Si of first tube and second tube [0144] P.sub.ave: average value (mass %) of content of P of first tube and second tube [0145] S.sub.ave: average value (mass %) of content of S of first tube and second tube [0146] Sn.sub.ave: average value (mass %) of content of Sn of first tube and second tube

[0147] As mentioned above, the composite tube (base metal) melts during welding, and the contained S and Sn mix into the weld metal. While on the one hand these elements increase weld cracking susceptibility, on the other hand, if these elements are reduced too much, the weld penetration depth during welding will be small and lack of fusion or lack of penetration will be liable to occur. In order to stably prevent lack of fusion or lack of penetration, it is necessary to control the average amounts of the contents of S and Sn of the first tube and the second tube so that a relational expression that takes into account the degree of influence of the respective elements is not less than a predetermined range.

[0148] Specifically, it is necessary to satisfy the following Formula (ii). The right-hand value in Formula (ii) below is preferably 0.0020 or more, and more preferably 0.0025 or more.

[00003]$\begin{array}{cc}0.0015?4?S_{\mathrm{ave}}+{\mathrm{Sn}}_{\mathrm{ave}}& \left(\mathrm{ii}\right)\end{array}$ [0149] Where, the meaning of each symbol in the above formula is as follows: [0150] S.sub.ave: average value (mass %) of content of S of first tube and second tube [0151] Sn.sub.ave: average value (mass %) of content of Sn of first tube and second tube

(E) Welded Joint

[0152] A welded joint according to the present invention is a joint that includes the aforementioned composite tube. In other words, the welded joint is a joint in which a plurality of composite tubes are joined together by welding. In the case of assembling composite tubes by butt welding, it suffices to select an appropriate welding consumables according to the intended use.

[0153] For example, when welding a composite tube in which the outer tube is the first tube (low alloy steel) and the inner tube is the second tube (high alloy steel), a welded joint having the necessary performance can be obtained by, as is generally performed, welding a high alloy steel portion using welding consumables for austenitic stainless steel or for a Ni-based alloy, and thereafter welding using a pure Ni welding consumables in the vicinity of a boundary portion, and welding using welding consumables for carbon steel for the remaining low alloy steel portion. Further, in a case where the materials constituting the outer tube and the inner tube are the opposite to the above case, welding can be carried out by the reverse method to the method described above.

[0154] In addition, in the composite tube according to the present embodiment, even in a case where a plurality of elements of welding consumables as described above are not used and only welding consumables for austenitic stainless steel that solidifies austenite or for a Ni-based alloy is used, a welded joint having the necessary performance can be obtained. Therefore, preferably a welded joint according to one embodiment of the present invention is welded using welding consumables either for austenitic stainless steel or for a Ni-based alloy. Note that, in this case, the welding consumables that is used, and the weld metal that is formed preferably have the following chemical composition.

[0155] That is, the chemical composition of the welding consumables and the weld metal is, in mass %, [0156] C: 0.003 to 0.100%, [0157] Si: 0.01 to 1.50%, [0158] Mn: 0.01 to 2.50%, [0159] P: 0.0400% or less, [0160] S: 0.0100% or less, [0161] Sn: 0.0300% or less, [0162] a total of Cu and/or Co: 0 to 15.00%, [0163] Ni: 12.0 to 75.0%, [0164] Cr: 18.0% to 27.0%, [0165] a total of Mo and/or W: 0 to 10.00%, [0166] a total of one or more elements selected from V, Nb, Ti and Ta: 0 to 4.00%, [0167] B: 0 to 0.0200%, [0168] a total of Ca and/or Mg: 0 to 0.0100%, [0169] Al: 0.001 to 1.500%, [0170] N: 0.001 to 0.150%, [0171] O: 0.030% or less, and [0172] the balance: Fe and impurities, and [0173] preferably satisfies Formulas (iii) and (iv) below:

[00004]$\begin{array}{cc}{\mathrm{Si}}_w+6?P_w+20?S_w+2?{\mathrm{Sn}}_w?1.1\%& \left(\mathrm{iii}\right)\end{array}$$\begin{array}{cc}0.0015?4?S_w+{\mathrm{Sn}}_w.& \left(\mathrm{iv}\right)\end{array}$ [0174] Where, the meaning of each symbol in the above formulas is as follows: [0175] Si.sub.w: content (mass %) of Si of the welding consumables or the weld metal [0176] P.sub.w: content (mass %) of P of the welding consumables or the weld metal [0177] S.sub.w: content (mass %) of S of the welding consumables or the weld metal [0178] Sn.sub.w: content (mass %) of Sn of the welding consumables or the weld metal.

(F) Production Method

[0179] Although the method for producing the composite tube is not particularly limited, for example, the composite tube can be produced by subjecting a starting material assembled by inserting a solid billet of high alloy steel or low alloy steel which constitutes the inner tube into a hollow billet of low alloy steel or high alloy steel which constitutes the outer tube to so-called hot rolling such as hot extrusion and roll rolling, and integrating the outer tube and the inner tube to make the tube. By this means, a composite tube in which the outer tube is the first tube and the inner tube is the second tube, or a composite tube in which the outer tube is the second tube and the inner tube is the first tube can be obtained.

[0180] Note that, normally, in order to ensure the cleanliness of the mating surfaces, assembly of the aforementioned billets is performed in a vacuum or in an inert gas atmosphere. Thereafter, the composite tube subjected to the aforementioned hot rolling may be subjected to cold working such as rolling or drawing, and in addition, a heat treatment may be performed to obtain a composite tube having a required shape.

[0181] Hereunder, the present invention is described more specifically by way of Examples. Note that the present invention is not limited to these Examples.

EXAMPLES

[0182] Low alloy steels L1 to L7 and high alloy steels H1 to H7 having the chemical compositions shown in Table 1 were combined to prepare, by a hot rolling method, composite tubes having a thickness of 6.5 mm and an outer diameter of 63 mm that each included a first tube composed of low alloy steel and a second tube composed of high alloy steel, and these composite tubes were adopted as test specimen tubes. Note that, in a case where the outer tube was the first tube and the inner tube was the second tube, the thickness of the outer tube was set to 4.2 mm and the thickness of the inner tube was set to 2.3 mm, in other words, the proportion that the thickness of the second tube occupied in the tube overall was made 0.35. On the other hand, in a case where the outer tube was the second tube and the inner tube was the first tube, the thickness of the outer tube was set to 1.6 mm and the thickness of the inner tube was set to 4.9 mm, in other words, the proportion that the thickness of the second tube occupied in the tube overall was made 0.25.

TABLE-US-00001 TABLE 1 Chemical Composition (mass %; balance: Fe and impurities) Steel C Si Mn P S Sn Cr Cu Co Ni Mo L1 0.188 0.87 0.85 0.0280 0.0043 0.0240 L2 0.250 0.93 0.78 0.0330 0.0098 0.0350 L3 0.185 0.87 0.92 0.0340 0.0001 0.0010 0.02 L4 0.194 0.76 0.60 0.0310 0.0025 0.0340 0.03 L5 0.112 0.78 0.45 0.0220 0.0012 0.0080 1.27 0.05 0.52 L6 0.120 0.43 0.40 0.0220 0.0009 0.0005 1.02 0.05 0.04 0.55 L7 0.110 0.40 0.43 0.0230 0.0006 0.0010 2.30 0.03 0.02 0.08 1.12 H1 0.023 0.32 1.59 0.0290 0.0010 0.0130 18.15 8.12 H2 0.030 0.58 1.60 0.0320 0.0025 0.0250 17.90 7.95 H3 0.026 0.64 1.55 0.0300 0.0001 0.0008 18.20 8.09 H4 0.019 0.29 1.71 0.0340 0.0004 0.0140 16.36 10.13 2.55 H5 0.065 0.72 1.78 0.0160 0.0001 0.0010 25.08 0.12 19.95 H6 0.071 0.41 1.26 0.0250 0.0003 0.0020 19.75 0.80 30.49 0.16 H7 0.007 0.35 0.62 0.0180 0.0002 0.0009 21.20 1.78 0.66 40.20 2.95 Chemical Composition (mass %; balance: Fe and impurities) Steel W Ti V Nb Ta Al B N O Others L1 0.015 0.006 0.007 L2 0.013 0.0048 0.017 0.008 L3 0.010 0.005 0.006 L4 0.03 0.04 0.013 0.015 0.007 Mg: 0.0010 L5 0.04 0.02 0.009 0.014 0.004 Ca: 0.0012 L6 0.05 0.008 0.012 0.005 L7 0.04 0.05 0.03 0.01 0.008 0.0008 0.009 0.004 REM: 0.0015 H1 0.007 0.086 0.006 H2 0.009 0.078 0.006 H3 0.04 0.008 0.0052 0.096 0.006 H4 0.03 0.008 0.087 0.005 H5 0.05 0.06 0.008 0.006 0.006 Ca: 0.0016 H6 0.46 0.04 0.516 0.007 0.005 Mg: 0.0012 H7 0.03 0.69 0.03 0.02 0.095 0.0010 0.005 0.004 REM: 0.0020

[0183] After cutting a test material having a length of 100 mm from each test specimen tube, a bevel illustrated in FIG. 1 was machined at one end of the test material. The bevels of the respective test materials were butted, and a solid rod having an outer diameter of 48 mm and a length of 250 mm which was manufactured by machining from a commercially available steel plate equivalent to SM400B defined in JIS G 3106 (2008) was inserted into the tube. Thereafter, both ends were welded using a covered electrode defined in AWS A5.11-2005 ENiCrMo-3, and two restraint weld test bodies illustrated in FIG. 2 were prepared for each test number.

[0184] A filler wire defined in AWS A.5.14-2009 ERNiFeCr-1 having the chemical composition shown in Table 2 was used to perform multi-layer welding by TIG welding with a heat input of 8 to 12 J/cm in the bevel of the restraint weld test body.

TABLE-US-00002 TABLE 2 Chemical Composition (mass %; balance: Fe and impurities) C Si Mn P S Sn Cr Cu Ni Mo Ti Al N O 0.012 0.20 0.25 0.0200 0.0015 0.0008 21.25 1.95 41.80 3.10 0.82 0.032 0.004 0.005

[0185] Further, from each of the restraint weld test body after multi-layer welding, four test specimens were cut out so that a transverse cross section of the welded joint was the observation surface, and were mirror-polished.

[0186] Thereafter, after the test specimens were etched, the presence or absence of defects was examined in a weld zone transverse cross section of a total of eight cross-sections (two test bodies?four transverse cross sections) for each test number using an optical microscope. Test bodies in which neither solidification cracking nor lack of fusion or lack of penetration was observed in all the cross sections were judged to have been accepted or passed as A, and test bodies in which solidification cracking, or lack of fusion or lack of penetration was observed in only one cross section were judged to have been accepted or passed as B, while test bodies in which solidification cracking, or lack of fusion or lack of penetration was observed in two or more cross sections were judged to have been unaccepted or failed as F.

[0187] The evaluation results are shown in Table 3.

TABLE-US-00003 TABLE 3 Result of cross-sectional observation Left-hand Right-hand <Number of defects/Number of observations> Test Outer Inner value in value in Solidification Lack of fusion Number tube tube Formula (i) Formula (ii) cracking or penetration Evaluation T1-1 H1 L1 0.8560 0.0291 0/8 0/8 A Inventive example T1-2 H1 L2 0.9670 0.0456 0/8 0/8 A Inventive example T1-3 H1 L3 0.8090 0.0092 0/8 0/8 A Inventive example T1-4 H1 L4 0.8020 0.0305 0/8 0/8 A Inventive example T1-5 H1 L5 0.7460 0.0149 0/8 0/8 A Inventive example T1-6 H1 L6 0.5605 0.0106 0/8 0/8 A Inventive example T1-7 H1 L7 0.5460 0.0102 0/8 0/8 A Inventive example T1-8 L1 H1 0.8560 0.0291 0/8 0/8 A Inventive example T1-9 L2 H1 0.9670 0.0456 0/8 0/8 A Inventive example T1-10 L3 H1 0.8090 0.0092 0/8 0/8 A Inventive example T1-11 L4 H1 0.8020 0.0305 0/8 0/8 A Inventive example T1-12 L5 H1 0.7460 0.0149 0/8 0/8 A Inventive example T1-13 L6 H 0.5605 0.0106 0/8 0/8 A Inventive example T1-14 L7 H1 0.5460 0.0102 0/8 0/8 A Inventive example T2-1 H2 L1 1.0220 0.0381 1/8 0/8 B Inventive example T2-2 H2 L2 1.1330 0.0546 4/8 0/8 F Comparative example T2-3 H2 L3 0.9750 0.0182 0/8 0/8 A Inventive example T3-1 H3 L1 0.9978 0.0212 0/8 0/8 A Inventive example T3-2 H3 L2 1.1088 0.0377 4/8 0/8 F Comparative example T3-3 H3 L3 0.9508 0.0013 0/8 2/8 F Comparative example T3-4 H3 L4 0.9438 0.0226 0/8 0/8 A Inventive example T3-5 H3 L5 0.8878 0.0070 0/8 0/8 A Inventive example T3-6 H3 L6 0.7023 0.0027 0/8 0/8 A Inventive example T3-7 H3 L7 0.6878 0.0023 0/8 0/8 A Inventive example T3-8 L1 H3 0.9978 0.0212 0/8 0/8 A Inventive example T3-9 L2 H3 1.1088 0.0377 3/8 0/8 F Comparative example T3-10 L3 H3 0.9508 0.0013 0/8 4/8 F Comparative example T4-1 H4 L1 0.8510 0.0284 0/8 0/8 A Inventive example T4-2 H4 L2 0.9620 0.0449 0/8 0/8 A Inventive example T4-3 H4 L3 0.8040 0.0085 0/8 0/8 A Inventive example T5-1 H5 L1 0.9960 0.0213 0/8 0/8 A Inventive example T5-2 H5 L2 1.1070 0.0378 4/8 0/8 F Comparative example T5-3 H5 L3 0.9490 0.0014 0/8 2/8 F Comparative example T5-4 L1 H5 0.9960 0.0213 0/8 0/8 A Inventive example T5-5 L2 H5 1.1070 0.0378 3/8 0/8 F Comparative example T5-6 L3 H5 0.9490 0.0014 0/8 3/8 F Comparative example T6-1 H6 L1 0.8710 0.0222 0/8 0/8 A Inventive example T6-2 H6 L2 0.9820 0.0387 0/8 0/8 A Inventive example T6-3 H6 L3 0.8240 0.0023 0/8 0/8 A Inventive example T7-1 H7 L1 0.8179 0.0215 0/8 0/8 A Inventive example T7-2 H7 L2 0.9289 0.0380 0/8 0/8 A Inventive example T7-3 H7 L3 0.7709 0.0016 0/8 1/8 B Inventive example T7-4 H7 L4 0.7639 0.0229 0/8 0/8 A Inventive example T7-5 H7 L5 0.7079 0.0073 0/8 0/8 A Inventive example T7-6 H7 L6 0.5224 0.0029 0/8 0/8 A Inventive example T7-7 H7 L7 0.5079 0.0026 0/8 0/8 A Inventive example Si.sub.ave + 6 ? P.sub.ave + 20 ? S.sub.ave + 2 ? Sn.sub.ave ? 1.100 (i) 0.0015 ? 4 ? S.sub.ave + Sn.sub.ave (ii) indicates data missing or illegible when filed

[0188] As will be understood from Table 3, with the composite tubes satisfying all the provisions of the present invention, a defect such as solidification cracking or lack of fusion or lack of penetration did not occur in the weld metal, and a sound welded joint was obtained. Note that, it was confirmed that the chemical composition of the obtained weld metal satisfied the conditions regarding the preferable chemical composition described above.

[0189] In contrast, in Test Numbers T2-2, T3-2, T3-9, T5-2 and T5-5, because Formula (i) was not satisfied, solidification cracking occurred in the weld metal in the vicinity of the fusion line near the boundary between the inner tube and the outer tube. Further, in Test Numbers T3-3, T3-10, T5-3 and T5-6, because Formula (ii) was not satisfied, the bevel face did not sufficiently melt, and a so-called lack of fusion or lack of penetration occurred.

[0190] Although the chemical compositions of weld metals of these welded joints that did not satisfy the present invention did, on average, satisfy the aforementioned preferable chemical composition range, it was determined that Formula (i) or Formula (ii) was not satisfied in a local region of the weld metals near the fusion boundary of the boundary between the outer tube and the inner tube, and consequently solidification cracking, or lack of fusion or lack of penetration occurred.

[0191] As is clear from the foregoing, it can be seen that a sound welded joint can be obtained only when a composite tube satisfying all the provisions of the present invention is used.

INDUSTRIAL APPLICABILITY

[0192] According to the present invention, a composite tube can be obtained that prevents cracking occurring in weld metal during butt welding of the tube and with which a sound welded joint can be stably obtained.