JP2021178995A - Stainless steel pipe and weld joint - Google Patents

Abstract

To provide a stainless steel pipe that allows a bead to be stably formed during butt welding, and can prevent cracking in a weld heat-affected zone, specifically liquefaction cracking, and provide a weld joint.SOLUTION: A stainless steel pipe has a chemical composition consisting of, in mass%, C: 0.003-0.020%, Si: 0.02-0.35%, Mn: 0.30-1.00%, P: 0.030% or less, S: 0.0001-0.0012%, Ni: 17.0-19.0%, Cu: 0.50-1.00%, Co: 0.05-1.00%, Cr: 19.0-21.0%, Mo: 5.50-7.00%, N: 0.150-0.250%, Al: 0.005-0.060%, O: 0.0004-0.0150%, and the balance being Fe and impurities, with 0.0010≤S+2O≤0.0280. The ratio of SNi, calculated by (Ni+Cu+Co)+30 (C+N)+0.5Mn, to SCr, calculated by (Cr+Mo)+1.5Si, is 0.90 or more and 1.10 or less.SELECTED DRAWING: None

Description

The present invention relates to stainless steel pipes and welded joints.

Chemical plants, power plants and energy transport equipment use stainless steel, which is relatively inexpensive and has good high temperature strength and corrosion resistance. For example, Patent Documents 1 to 6 disclose stainless steel having enhanced high-temperature strength and corrosion resistance.

Japanese Unexamined Patent Publication No. 7-109548 Japanese Patent Application Laid-Open No. 2003-160839 Japanese Patent Application Laid-Open No. 2003-221653 Japanese Unexamined Patent Publication No. 2004-149830 Japanese Unexamined Patent Publication No. 2018-172709 Japanese Unexamined Patent Publication No. 2019-123895

Some plant equipment is manufactured by assembling members by heat welding. For example, stainless steel pipes used in heat exchangers are assembled by butt welding pipes to each other. Then, during this welding, cracks may occur in the weld heat affected zone. On the other hand, in order to suppress the cracking, it is effective to reduce the amount of heat input during welding, but by reducing the amount of heat input, on the contrary, the butt surface is not completely melted and the bead is stable. May not form. Therefore, welding defects may occur.

The present invention solves the above-mentioned problems, and stainless steel capable of stably forming beads on the inner surface side during butt welding of pipes and suppressing cracks generated in weld heat-affected zones, specifically, liquefaction cracks. It is an object of the present invention to provide steel pipes and welded joints.

The present invention has been made to solve the above problems, and the gist of the present invention is the following stainless steel pipes and welded joints.

(1) The chemical composition is mass%.
C: 0.003 to 0.020%,
Si: 0.02 to 0.35%,
Mn: 0.30 to 1.00%,
P: 0.030% or less,
S: 0.0001 to 0.0012%,
Ni: 17.0 to 19.0%,
Cu: 0.50 to 1.00%,
Co: 0.05 to 1.00%,
Cr: 19.0 to 21.0%,
Mo: 5.50 to 7.00%,
N: 0.150 to 0.250%,
Al: 0.005-0.060%,
O: 0.0004 to 0.0150%,
Remaining: Fe and impurities,
Satisfy the following (i),
A stainless steel pipe in which the ratio of _{S Ni} calculated by the following formula (ii) _{to S Cr} calculated by the following formula (iii) satisfies the following formula (iv).
0.0010 ≤ S + 2O ≤ 0.0280 ... (i)
S _Ni = (Ni + Cu + Co) +30 (C + N) +0.5Mn ... (ii)
S _Cr = (Cr + Mo) + 1.5Si ... (iii)
0.90 ≤ S _Ni / S _Cr ≤ 1.10 ... (iv)
However, the element symbol in the above formula represents the content (mass%) of each element contained in the stainless steel, and if it is not contained, it is set to zero.

(2) In the region from the pipe end of the stainless steel pipe to the position of 50 mm in the axial direction, the maximum wall thickness difference Dw calculated by the following formula (v) satisfies the following formulas (vi) and (vii). , The stainless steel pipe according to (1) above.
Dw = D _MAX- D _min ... (v)
Dw / D _MAX × 100 ≦ 20 ・ ・ ・ (vi)
Dw ≤ 1.6 ・ ・ ・ (vii)
However, each symbol in the above formula is defined as follows.
D _MAX (mm): Maximum wall thickness D _min (mm): Minimum wall thickness

(3) The chemical composition is, instead of a part of the Fe, by mass%.
The stainless steel pipe according to (1) or (2) above, which contains Sn: 0.030% or less.

(4) The chemical composition is, instead of a part of the Fe, by mass%.
W: 1.00% or less,
Ti: 0.40% or less,
V: 0.40% or less,
Nb: 0.40% or less,
Ta: 0.40% or less,
Ca: 0.0100% or less,
B: 0.0100% or less, and REM: 0.0800% or less,
The stainless steel pipe according to any one of (1) to (3) above, which contains one or more selected from the above.

(5) A welded joint using the stainless steel pipe according to any one of (1) to (4) above.

According to the present invention, a stainless steel pipe and a welded joint capable of stably forming beads on the inner surface side during butt welding of pipes and suppressing cracks generated in a weld heat-affected zone, specifically, liquefaction cracks, are provided. Obtainable.

FIG. 1 is a diagram showing a groove shape in an embodiment.

The present inventor investigated cracks generated in stainless steel pipes during welding, and obtained the following findings (a) to (c).

(A) Cracks that occur during butt welding of pipes are likely to occur near the toe of the bead on the inner surface side of the butt weld, that is, at the grain boundaries at the weld heat-affected zone, and as P and S increase, the cracks are likely to occur. The tendency became remarkable. When the present inventors observed the fracture surface where cracks occurred, traces of liquefaction were observed on the fracture surface, and concentration of P and S was confirmed.

Cracks generated in the vicinity of the inner bead, that is, in the weld heat affected zone, are generated by the following mechanism. P and S are elements that significantly lower the solid phase line in the phase diagram, and the solid phase tends to melt at a relatively low temperature. These elements are segregated at the grain boundaries by the welding thermal cycle to melt the vicinity of the grain boundaries at a relatively low temperature. It is probable that, as a result of the thermal stress applied to the vicinity of the grain boundaries due to the volume shrinkage caused by heating, the vicinity of the grain boundaries was separated and opened, and cracks occurred. Here, when the stability of the austenite phase is increased, the grain boundary segregation becomes easier. Therefore, cracks are likely to occur. Therefore, from the viewpoint of suppressing cracking, it is desirable to reduce the contents of P and S.

(B) Further, the crack occurs when the shape of the bead on the inner surface side becomes a convex shape (hereinafter, simply referred to as “convex shape”) in which the shape of the bead on the inner surface side is excessively raised, and the surplus height becomes high. Cheap. The shape of the bead is affected by the content of S and O contained in the alloy. The higher the content of S and O, the higher the extra height of the bead shape, and the more likely it is to become a convex shape. On the other hand, if the contents of S and O are too small, the inner bead is not stably formed, and an unmelted butt surface remains.

Further, S and O are surface active elements and have an action of lowering the surface tension of the molten metal. As a result, during welding, the inward convection is strengthened in the welding pond. As a result, the welding heat is easily transferred in the depth direction, and the bead can be stably formed. Therefore, when the S content is reduced from the viewpoint of suppressing cracking, the bead forming ability may decrease. On the other hand, by controlling the O content having the same effect as S, the bead forming ability can be ensured. Therefore, it is necessary to adjust the contents of S and O within a predetermined range.

(C) In addition, cracks are more likely to occur when the left-right step (hereinafter referred to as "irregular bead") of the butt portion of the bead becomes large. In this case, the distribution of thermal stress due to welding becomes non-uniform, and local stress concentration tends to occur. Then, the stress generated at the toe of the bead becomes large, and cracks are likely to occur.

By the way, in the stainless steel pipe of the present application, it is industrially difficult to eliminate the variation in wall thickness. Therefore, in order to suppress cracking due to irregularity of the bead, it is desirable to reduce the difference in wall thickness, specifically, the difference between the maximum wall thickness and the minimum wall thickness in the pipe.

The present invention has been made based on the above findings. Hereinafter, each requirement of the present invention will be described in detail.

1. 1. Chemical composition of stainless steel pipe The reasons for limiting each element are as follows. In the following description, "%" for the content means "mass%".

C: 0.003 to 0.020%
C has the effect of enhancing the stability of the austenite phase. Therefore, the C content is set to 0.003% or more. The C content is preferably 0.005% or more, and more preferably 0.008% or more. However, when C is excessively contained, it is combined with Cr by the welding heat cycle to form carbides at the grain boundaries at the weld heat affected zone. As a result, a Cr-deficient layer is formed in the vicinity of the grain boundaries, and the corrosion resistance is lowered. Therefore, the C content is set to 0.020% or less. The C content is preferably 0.018% or less, more preferably 0.016% or less.

Si: 0.02-0.35%
Si has a deoxidizing effect at the time of manufacture. Therefore, the Si content is 0.02% or more. The Si content is preferably 0.05% or more, more preferably 0.08% or more. However, if Si is contained in an excessive amount, the stability of the austenite phase is lowered and the liquefaction cracking sensitivity is increased. In addition, it may be difficult to stably form the bead on the inner surface side. Therefore, the Si content is set to 0.35% or less. The Si content is preferably 0.30% or less, and more preferably 0.25% or less.

Mn: 0.30 to 1.00%
Mn has a deoxidizing effect like Si. It also has the effect of enhancing the stability of the austenite phase and contributes not a little to the stable formation of beads on the inner surface side. Therefore, the Mn content is set to 0.30% or more. The Mn content is preferably 0.35% or more, and more preferably 0.40% or more. However, if Mn is contained in an excessive amount, the hot workability is deteriorated. Therefore, the Mn content is set to 1.00% or less. The Mn content is preferably 0.80% or less, more preferably 0.60% or less.

P: 0.030% or less P is contained in steel as an impurity and segregates at grain boundaries during welding, significantly increasing the sensitivity of liquefaction cracks generated in the weld heat affected zone near the inner bead. Therefore, the P content is set to 0.030% or less. The P content is preferably 0.028% or less, more preferably 0.026% or less. The P content is preferably reduced as much as possible, but excessive reduction increases the manufacturing cost. Therefore, the P content is preferably 0.001% or more, and more preferably 0.002% or more.

S: 0.0001 to 0.0012%
Similar to P, S segregates at the grain boundaries during welding and remarkably increases the sensitivity of liquefaction cracks generated in the weld heat-affected zone near the inner bead. Therefore, the S content is set to 0.0012% or less. The S content is preferably 0.0010% or less, more preferably 0.0008% or less. The lower the S content, the more preferable, but the excessive reduction of the S content significantly increases the manufacturing cost. In addition, in the alloy pipe of the present invention, S, together with O, contributes to enhancing the ability to form the inner bead during welding. Therefore, the S content is set to 0.0001% or more. The S content is preferably 0.0002% or more, and more preferably 0.0003% or more. It is necessary that S satisfies the equation (i) with O described later.

Ni: 17.0 to 19.0%
Ni has the effect of enhancing the stability of the austenite phase. It is also effective in improving corrosion resistance in seawater and stress corrosion cracking resistance in chloride environments. Therefore, the Ni content is set to 17.0% or more. The Ni content is preferably 17.2% or more, more preferably 17.5% or more. However, since it is an expensive element, if Ni is contained in an excessive amount, the production cost increases. Therefore, the Ni content is set to 19.0% or less. The Ni content is preferably 18.8% or less, more preferably 18.5% or less.

Cu: 0.50 to 1.00%
Cu has the effect of increasing the stability of the austenite phase and improving the corrosion resistance in seawater. Therefore, the Cu content is set to 0.50% or more. The Cu content is preferably 0.55% or more, more preferably 0.60% or more. However, if Cu is contained in an excessive amount, the hot workability is deteriorated. Therefore, the Cu content is set to 1.00% or less. The Cu content is preferably 0.95% or less, more preferably 0.90% or less.

Co: 0.05 to 1.00%
Co has the effect of enhancing the stability of the austenite phase. Therefore, the Co content is set to 0.05% or more. The Co content is preferably 0.08% or more, more preferably 0.10% or more. However, since Co is a very expensive element, if it is contained in an excessive amount, the production cost will increase significantly. Therefore, the Co content is set to 1.00% or less. The Co content is preferably 0.90% or less, more preferably 0.80% or less.

Cr: 19.0 to 21.0%
Cr is an element that forms a passivation film to enhance corrosion resistance and is also effective in improving pitting corrosion resistance. Therefore, the Cr content is set to 19.0% or more. The Cr content is preferably 19.2% or more, more preferably 19.5% or more. However, excessive inclusion of Cr reduces the stability of the austenite phase. Therefore, the Cr content is set to 21.0% or less. The Cr content is preferably 20.8% or less, more preferably 20.5% or less.

Mo: 5.50 to 7.00%
Mo has the effect of enhancing pitting corrosion resistance. Therefore, the Mo content is set to 5.50% or more. The Mo content is preferably 5.70% or more, and more preferably 6.00% or more. However, excessive inclusion of Mo reduces the stability of the austenite phase. Further, since Mo is an expensive element, the manufacturing cost increases. Therefore, the Mo content is set to 7.00% or less. The Mo content is preferably 6.80% or less, more preferably 6.50% or less.

N: 0.150 to 0.250%
N has the effect of enhancing the stability of the austenite phase and enhancing the pitting corrosion resistance. Therefore, the N content is set to 0.150% or more. The N content is preferably 0.160% or more, and more preferably 0.180% or more. However, if N is contained in an excessive amount, a nitride is precipitated and the ductility is lowered. Therefore, the N content is set to 0.250% or less. The N content is preferably 0.230% or less, more preferably 0.220% or less.

Al: 0.005-0.060%
Al has a deoxidizing effect. It also contributes to the improvement of oxidation resistance at high temperatures. Therefore, the Al content is set to 0.005% or more. The Al content is preferably 0.007% or more, more preferably 0.010% or more. However, if Al is contained in an excessive amount, Al binds to oxygen and the cleanliness is lowered. As a result, the hot workability is lowered. In addition, it may be difficult to stably form the bead on the inner surface side. Therefore, the Al content is set to 0.060% or less. The Al content is preferably 0.050% or less, more preferably 0.040% or less.

O: 0.0004 to 0.0150%
O is generally contained in steel as an impurity, but in the stainless steel pipe of the present invention, it has an effect of enhancing the ability to form an inner bead at the time of welding together with S. Therefore, the O content is set to 0.0004% or more. The O content is preferably 0.0006% or more, and more preferably 0.0008% or more. However, when O is excessively contained, the bead on the inner surface side of the pipe hangs down and tends to have a convex shape, which increases the sensitivity of cracks generated in the weld heat affected zone. In addition, hot workability is also reduced. Therefore, the O content is set to 0.0150% or less. The O content is preferably 0.0120% or less, more preferably 0.0100% or less.

As described above, since S and O effectively contribute to the formation of beads on the inner surface side of the pipe, in the stainless steel pipe according to the present invention, the following (i) is a relational expression between the S content and the O content. ) It is necessary to satisfy the formula.

0.0010 ≤ S + 2O ≤ 0.0280 ... (i)
However, the element symbol in the above formula represents the content (mass%) of each element contained in the stainless steel, and if it is not contained, it is set to zero.

S and O are surface active elements and have an action of strengthening inward convection in the molten pool during welding. By transporting the welding heat in the depth direction, the bead on the inner surface side is stably formed, and this effect is obtained when the middle value of the formula (i) is less than 0.0010%. I can't. Therefore, the middle value of the formula (i) is set to 0.0010% or more. The middle value of the formula (i) is preferably 0.0012% or more, and more preferably 0.0015% or more.

On the other hand, when the middle value of the formula (i) exceeds 0.0280%, the surface tension of the molten metal becomes small and sagging occurs. As a result, the bead becomes convex and the extra height becomes high. Then, stress is easily concentrated on the toe, and the crack sensitivity of the stainless steel pipe is increased when used at a high temperature. Therefore, the middle value of the formula (i) is set to 0.0280% or less. The middle value of the formula (i) is preferably 0.0260% or less, and more preferably 0.0240% or less.

In the chemical composition, in addition to the above elements, Sn may be further contained in the range shown below.

Sn: 0.030% or less Sn has the effect of increasing the penetration depth and enhancing the ability to form the inner bead. Therefore, it may be contained if necessary. However, if Sn is contained in an excessive amount, the sensitivity of liquefied cracks generated in the weld heat affected zone is increased and the hot workability is lowered. Therefore, the Sn content is 0.030% or less. The Sn content is preferably 0.020% or less, more preferably 0.010% or less. On the other hand, in order to obtain the above effect, the Sn content is preferably 0.001% or more, more preferably 0.002% or more, and further preferably 0.003% or more.

In the chemical composition, in addition to the above elements, W, Ti, V, Nb, Ta, Ca, B, and one or more selected from REM may be further contained in the range shown below. The reason for limiting each element will be described.

W: 1.00% or less W has an effect of enhancing pitting corrosion resistance. Therefore, it may be contained as needed. However, excessive inclusion of W reduces the stability of the austenite phase. Further, since W is an expensive element, the manufacturing cost increases. Therefore, the W content is set to 1.00% or less. The W content is preferably 0.80% or less, more preferably 0.60% or less. On the other hand, in order to obtain the above effect, the W content is preferably 0.01% or more, more preferably 0.02% or more.

Ti: 0.40% or less Ti combines with carbon to form carbides, suppresses the formation of Cr carbides, and reduces the deterioration of corrosion resistance at grain boundaries. Therefore, it may be contained as needed. However, if Ti is contained in an excessive amount, a large amount of carbides and carbonitrides of Ti are precipitated, and the ductility is lowered. Therefore, the Ti content is set to 0.40% or less. The Ti content is preferably 0.35% or less, more preferably 0.30% or less. On the other hand, in order to obtain the above effect, the Ti content is preferably 0.01% or more, more preferably 0.02% or more.

V: 0.40% or less V, like Ti, combines with carbon to form carbides and suppresses the formation of Cr carbides. As a result, deterioration of corrosion resistance is reduced at the grain boundaries. Therefore, it may be contained as needed. However, if V is contained in an excessive amount, a large amount of carbides and carbonitrides of V are precipitated, and the ductility is lowered. Therefore, the V content is set to 0.40% or less. The V content is preferably 0.35% or less, and more preferably 0.30% or less. On the other hand, in order to obtain the above effect, the V content is preferably 0.01% or more, more preferably 0.02% or more.

Nb: 0.40% or less Nb, like Ti and V, combines with carbon to form carbides and suppresses the formation of Cr carbides. As a result, deterioration of corrosion resistance is reduced at the grain boundaries. Therefore, it may be contained if necessary. However, if Nb is contained in an excessive amount, a large amount of carbides and carbonitrides of Nb are precipitated, and the ductility is lowered. Therefore, the Nb content is set to 0.40% or less. The Nb content is preferably 0.35% or less, and more preferably 0.30% or less. On the other hand, in order to obtain the above effect, the Nb content is preferably 0.01% or more, and more preferably 0.02% or more.

Ta: 0.40% or less Ta has the effect of promoting the formation of a passivation film of Cr and improving the corrosion resistance. Therefore, it may be contained as needed. However, if Ta is contained in an excessive amount, a large amount of carbides of Ta are precipitated and the ductility is lowered. Therefore, the Ta content is set to 0.40% or less. The Ta content is preferably 0.35% or less, more preferably 0.30% or less. On the other hand, in order to obtain the above effect, the Ta content is preferably 0.01% or more, and more preferably 0.02% or more.

Ca: 0.0100% or less Ca has the effect of improving hot workability. Therefore, it may be contained as needed. However, if Ca is contained in an excessive amount, Ca binds to oxygen and the cleanliness is significantly reduced. As a result, the hot workability is rather lowered. Therefore, the Ca content is set to 0.0100% or less. The Ca content is preferably 0.0080% or less, more preferably 0.0060% or less. On the other hand, in order to obtain the above effect, the Ca content is preferably 0.0010% or more, and more preferably 0.0020% or more.

B: 0.0100% or less B has the effect of segregating at the grain boundaries at high temperatures, strengthening the grain boundaries, and enhancing hot workability. Therefore, it may be contained as needed. However, if B is excessively contained, the sensitivity of liquefaction cracking to occur in the weld heat affected zone increases. Therefore, the B content is 0.0100% or less. The B content is preferably 0.0080% or less, more preferably 0.0060% or less. On the other hand, in order to obtain the above effect, the B content is preferably 0.0002% or more, more preferably 0.0005% or more.

REM: 0.0800% or less REM, like Ca, has the effect of improving hot workability during manufacturing. Therefore, it may be contained as needed. However, if REM is contained in excess, it will combine with oxygen and significantly reduce the cleanliness. As a result, the hot workability is rather lowered. Therefore, the REM content is 0.0800% or less. The REM content is preferably 0.0600% or less, more preferably 0.0500% or less. On the other hand, in order to obtain the above effect, the REM content is preferably 0.0005% or more, and more preferably 0.0010% or more. Here, REM refers to Sc, Y and lanthanoids, and the REM content indicates the total content of these elements.

In the stainless steel pipe according to the present invention, in order to control the stability of the austenite structure, the content of not only the elements such as S and O but also the elements such as Ni and Cu which are the elements constituting the stainless steel pipe are controlled. .. Specifically, it is an index showing the stability of the austenite structure, and the ratio of _{S Ni calculated from the following formula (ii) to S Cr} calculated from the following formula (iii) is the following formula (iv). I am satisfied.

S _Ni = (Ni + Cu + Co) +30 (C + N) +0.5Mn ... (ii)
S _Cr = (Cr + Mo) + 1.5Si ... (iii)
0.90 ≤ S _Ni / S _Cr ≤ 1.10 ... (iv)
However, the element symbol in the above formula represents the content (mass%) of each element contained in the stainless steel pipe.

When the middle value of equation (iv) is less than 0.90, the stability of the austenite phase is lowered, and the structure is changed by processing and heating, and the performance is deteriorated. Therefore, the middle value of equation (iv) is set to 0.90 or more. The middle value of equation (iv) is preferably 0.92 or more, and more preferably 0.95 or more. On the other hand, when the middle value of Eq. (Iv) exceeds 1.10, the stability of the austenite phase is enhanced, and P and S are easily segregated at the grain boundary in the weld heat affected zone due to the thermal cycle during welding. As a result, cracks are likely to occur. Therefore, the middle value in equation (iv) shall be 1.10 or less. The middle value of equation (iv) is preferably 1.08 or less, and more preferably 1.05 or less.

In the chemical composition of the present invention, the balance is Fe and impurities. Here, the "impurity" is a component mixed with raw materials such as ore and scrap, and various factors in the manufacturing process when the stainless steel pipe is industrially manufactured, and is permitted as long as it does not adversely affect the present invention. Means what is done.

2. 2. Thickness of Stainless Steel Pipe In the stainless steel pipe according to the present invention, for example, as described later, a mandrel or the like is inserted into a round billet and hot-extruded to produce a hollow raw pipe, which is manufactured. However, not limited to this method, when manufacturing a pipe, it is difficult to make the same wall thickness in all parts of the pipe due to factors such as processing accuracy, and the wall thickness tends to vary. Therefore, even in one alloy pipe, a difference in wall thickness occurs depending on the portion.

If there is a difference in wall thickness, even if pipes of the same nominal size are butt-welded to manufacture a welded joint, cracks are likely to occur when the pipe is used at a high temperature. This is due to the formation of a step at the weld toe due to the difference in wall thickness at the end of the pipe to be welded, that is, the irregularity of the bead. When the irregularity of the bead becomes large, the thermal stress distribution due to welding becomes non-uniform, local stress concentration occurs, and the stress remaining at one bead toe becomes large. As a result, cracks are likely to occur.

In order to suppress irregularity of the bead, when the pipes are butted against each other, the pipes may be rotated to adjust the butting position so that the difference in wall thickness between the pipes becomes small. However, it is not easy to rotate a long pipe, and the construction efficiency is significantly reduced. Similarly, adjusting the groove shape and welding conditions may also suppress bead irregularities, but this is not desirable from the viewpoint of productivity.

Therefore, in order to reduce the difference in wall thickness between pipes and suppress irregularity of beads, it is desirable to reduce the variation in wall thickness in one pipe. Specifically, the maximum wall thickness difference Dw calculated by the following equation (v) satisfies the following equations (vi) and (vii) in the region from the pipe end of the stainless steel pipe to the position of 50 mm in the axial direction. It is preferable to do.

Dw = D _MAX- D _min ... (v)
Dw / D _MAX × 100 ≦ 20 ・ ・ ・ (vi)
Dw ≤ 1.6 ・ ・ ・ (vii)
However, each symbol in the above formula is defined as follows.
D _MAX (mm): Maximum wall thickness D _min (mm): Minimum wall thickness

As described above, when the wall thickness of the pipe is measured, the measured thickest wall thickness is defined as the maximum wall thickness D _MAX, and the thinnest wall thickness is defined as the minimum wall thickness D _min . Then, the relationship between the maximum wall thickness difference Dw and the maximum wall thickness D _{MAX , which is the difference between the maximum wall thickness D MAX} and the minimum wall thickness D _min , calculated by the formula (v), is the above formula (vi). It is preferable to satisfy. This is because when the lvalue in equation (vi) exceeds 20%, irregularities in the beads are likely to occur. The smaller the rvalue in equation (vi), the more preferable.

In addition, it is preferable that the maximum wall thickness difference Dw satisfies the equation (vii). This is because if the maximum wall thickness difference Dw exceeds 1.6 mm, it becomes difficult to butt the groove surfaces and welding work becomes difficult. The smaller the maximum wall thickness difference Dw, the more preferable.

The stainless steel pipe according to the present invention can be used for a welded joint, for example. In stainless steel pipes for welded joints, pipe ends of alloy pipes cut for the purpose of size adjustment may be butted against each other for welding. In this case, the pipe end portion of the cut steel pipe, that is, the portion abutted at the time of welding is the "pipe end portion" defined in the present invention.

If at least only the end of the pipe satisfies the equations (iii) and (iv), the desired effect in the present invention can be obtained, but (iii) and (iv) over the entire length and circumference of the alloy pipe. ) Satisfies the provisions of the present invention regardless of where the alloy tube is cut. Therefore, it is more preferable to satisfy the equations (iii) and (iv) over the entire length and circumference of the alloy tube.

Here, the wall thickness of the tube may be measured by using ultrasonic flaw detection. Alternatively, measurement may be performed using a measuring instrument such as a wall thickness gauge, but the measuring method is not limited to these.

3. 3. Welded joint A welded joint of a stainless steel pipe can be obtained by welding the pipe end of the above stainless steel pipe under predetermined conditions. The welded joint of the stainless steel pipe has a welded metal formed by solidifying the molten metal and forming a joint portion, and a base metal portion. The base metal portion includes a welding heat-affected zone that is affected by heat input due to welding. The base metal portion excluding the weld heat-affected zone inherits the chemical composition, metal structure, and other characteristics of the stainless steel pipe described in items 1 and 2 above.

4. Manufacturing Method A preferred manufacturing method for the stainless steel pipe according to the present invention will be described. The stainless steel pipe according to the present invention can obtain the effect as long as it has the above-mentioned configuration regardless of the manufacturing method, but can be stably manufactured by, for example, the following manufacturing method.

4-1. Stainless steel pipe First, the stainless steel ingot that is the material of the stainless steel pipe is manufactured, or the bloom is manufactured by continuous casting. The stainless steel ingot is preferably produced by melting steel having the above-mentioned chemical composition in an electric furnace or the like, removing impurities by refining, and then casting. Subsequently, it is preferable that the obtained ingot is hot forged to form a columnar billet. Then, the obtained billet is processed to form a tube shape.

Specifically, it is preferable to hot-extrude the billet and then perform cold rolling or cold drawing. At the time of processing, softening heat treatment and intermediate pickling may be performed on the way, if necessary.

After that, it is preferable to perform a solution treatment on the tube as a heat treatment. In order for the above-mentioned maximum wall thickness difference Dw to satisfy the equations (iii) and (iv), the solution treatment is performed by heating in a temperature range of 950 ° C to 1230 ° C for 1 to 15 minutes. , It is preferable to carry out under the condition of water cooling. In addition, after the solution treatment, pickling may be performed if necessary. Further, the entire length, the entire circumference, etc. of the pipe may be machined by grinding or grinding.

4-2. Welded joint of stainless steel pipe A welded joint can be obtained by welding the end of an alloy pipe using the stainless steel pipe according to the present invention as a material. The welding method is not particularly limited, but may be welded by, for example, arc welding. Further, as a condition for arc welding, for example, it is preferable to set the heat input amount in the range of 4 to 20 kJ / cm and to manufacture the welded joint of the stainless steel pipe.

Hereinafter, the present invention will be described in more detail with reference to Examples, but the present invention is not limited to these Examples.

Steels having the chemical compositions shown in Table 1 were melted to produce ingots. After that, hot forging and hot rolling were performed to obtain two types of plate thicknesses of 12 mm and 6 mm for steel types A and B, and 6 mm for other steel types. This material was held at 1160 ° C. for 10 minutes and then subjected to a solution treatment by cooling with water. Then, in order to simulate the difference in wall thickness of the stainless steel pipe, these materials were reduced to various thicknesses by machining and then cut into a width of 50 mm and a length of 100 mm to obtain a test material.

Subsequently, the end face of the produced test material in the rolling direction was grooved as shown in FIG. Then, the front side of the test material 1 simulating the portion having the maximum wall thickness (also referred to as "maximum wall thickness portion") and the test material 2 simulating the minimum wall thickness portion (also referred to as "minimum wall thickness portion"). The heights of the front surfaces were adjusted, butted so that there was a step on the back surface side, and the first layer was welded by automatic gas tungsten arc welding. The test materials 1 and 2 are both of the same steel type.

At the time of welding, AWS A5.14-2009 ERNiCrMo-3 having an outer diameter of 1.2 mm was used as a filler material, and the heat input was about 9 kJ / cm. Ar was used as the shield gas and the back shield gas, and the flow rate was set to 10 L / min.

Regarding the obtained welded joint, the one in which the bead on the back surface side was formed over the entire length of the weld line was judged to have no problem in the forming ability of the bead on the inner surface side of the steel pipe, and was judged as "passed". Among them, the one in which the width of the back surface side bead is 2 mm or more over the entire length of the weld line is "excellent", and the one in which the width is less than 2 mm but the back surface side bead of 1 mm or more is formed is "acceptable".

For welded joints for which the ability to form the inner bead was judged to be acceptable, A5.11 was placed on a commercially available steel plate (thickness 20 mm, width 150 mm, length 150 mm) equivalent to SM400B specified in JIS G 3106: 2008. -2005 EniCrMo-3 was restrained and welded around four circumferences using a specified shielded metal arc welding rod. Then, laminated welding was performed in the groove by automatic gas tungsten arc welding. For welding, AWS A5.14-2009 ERNiCrMo-3 with an outer diameter of 1.2 mm was used as a filler material, and the heat input was about 9 to 12 kJ / cm. Ar was used as the shield gas and the back shield gas, and the flow rate was set to 10 L / min.

Five cross sections were exposed from the obtained welded joint, mirror-polished, and corroded, and then microscopically examined with an optical microscope to investigate the presence or absence of liquefaction cracking in the weld heat-affected zone. Welded joints in which cracks were not observed in all five samples were evaluated as "excellent", and welded joints in which cracks were observed in one sample were evaluated as "acceptable" and judged as "passed". Welded joints in which cracks were observed in two or more samples were judged to be "impossible".

From Table 2, the test pieces obtained by using the symbols A to H satisfying the chemical components specified in the present invention have excellent crack resistance of the weld heat-affected zone after aging heat treatment, can suppress cracking, and have an inner surface. It can be seen that the ability to form side beads is also good.

Further, from the comparison of the test pieces A9 and A10, A16 and A17, B9 and B10 and B16 and B17, when the maximum wall thickness difference satisfies the equations (vi) and (vii), the crack resistance and the inner surface of the weld heat affected zone are satisfied. It can be seen that it is superior to the ability to form side beads.

On the other hand, since the S and P contents of the test bodies I1 and J1 using the symbols I and J exceeded the range of the present invention, respectively, liquefied cracks were observed in two or more cross sections in the cross-section observation test of the welded joint.

Further, the test body K1 using the reference numeral K did not satisfy the equation (i) and was higher than the specified range. As a result, the molten metal droops significantly and the convex shape of the bead on the back surface side becomes remarkable, so that stress concentration occurs and liquefaction cracking occurs in the weld heat-affected zone.

The test piece L1 using the symbol L did not satisfy the formula (i) and was lower than the specified range. Therefore, the melting in the plate thickness direction was not sufficient, and the target ability to form the inner bead on the inner surface side could not be obtained.

Further, in the test piece M1 using the reference numeral M, the middle value of the formula (iv), which is an index showing the stability of the austenite phase, exceeds the range specified in the present invention. Therefore, the grain boundary segregation of P and S was promoted, and liquefaction cracking was observed in the weld heat-affected zone. Further, in the test piece N1 using the reference numeral N, the grain boundaries were deeply corroded in the cross-sectional observation of the welded portion, and it was not possible to sufficiently distinguish them from cracks. It is considered that this is because the middle value of Eq. (Iv) is below the range specified in the present invention, the stability of the austenite phase is lowered, and the microstructure is changed. As described above, it can be seen that the crack resistance of the welded portion and the ability to form the inner bead can be compatible only when the requirements of the present invention are satisfied.

According to the present invention, it is possible to obtain a stainless steel pipe and a welded joint in which beads on the inner surface side are stably formed during butt welding of pipes, and cracks are less likely to occur in the heat-affected zone of welding, specifically, liquefaction cracking is unlikely to occur. Can be done.

Claims (5)

The chemical composition is by mass%,
C: 0.003 to 0.020%,
Si: 0.02 to 0.35%,
Mn: 0.30 to 1.00%,
P: 0.030% or less,
S: 0.0001 to 0.0012%,
Ni: 17.0 to 19.0%,
Cu: 0.50 to 1.00%,
Co: 0.05 to 1.00%,
Cr: 19.0 to 21.0%,
Mo: 5.50 to 7.00%,
N: 0.150 to 0.250%,
Al: 0.005-0.060%,
O: 0.0004 to 0.0150%,
Remaining: Fe and impurities,
Satisfy the following (i),
A stainless steel pipe in which the ratio of _{S Ni} calculated by the following formula (ii) _{to S Cr} calculated by the following formula (iii) satisfies the following formula (iv).
0.0010 ≤ S + 2O ≤ 0.0280 ... (i)
S _Ni = (Ni + Cu + Co) +30 (C + N) +0.5Mn ... (ii)
S _Cr = (Cr + Mo) + 1.5Si ... (iii)
0.90 ≤ S _Ni / S _Cr ≤ 1.10 ... (iv)
However, the element symbol in the above formula represents the content (mass%) of each element contained in the stainless steel, and if it is not contained, it is set to zero. The claim that the maximum wall thickness difference Dw calculated by the following equation (v) satisfies the following equations (vi) and (vii) in the region from the pipe end portion of the stainless steel pipe to the position of 50 mm in the axial direction. The stainless steel pipe according to 1.
Dw = D _MAX- D _min ... (v)
Dw / D _MAX × 100 ≦ 20 ・ ・ ・ (vi)
Dw ≤ 1.6 ・ ・ ・ (vii)
However, each symbol in the above formula is defined as follows.
D _MAX (mm): Maximum wall thickness D _min (mm): Minimum wall thickness The chemical composition is, in mass%, instead of a portion of the Fe.
The stainless steel pipe according to claim 1 or 2, which contains Sn: 0.030% or less. The chemical composition is, in mass%, instead of a portion of the Fe.
W: 1.00% or less,
Ti: 0.40% or less,
V: 0.40% or less,
Nb: 0.40% or less,
Ta: 0.40% or less,
Ca: 0.0100% or less,
B: 0.0100% or less, and REM: 0.0800% or less,
The stainless steel pipe according to any one of claims 1 to 3, which contains one or more selected from. A welded joint using the stainless steel pipe according to any one of claims 1 to 4.

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