JP2018075609A - Method for thermally processing two-phase stainless steel pipe - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for thermally processing a two-phase stainless steel by which generation of warping in a gravity direction is suppressed.SOLUTION: A method for thermally processing a two-phase stainless steel pipe comprises a process in which a two-phase stainless steel pipe having an outer diameter of 50 mm or less, a pipe thickness of 3 mm or less, and a length of 600 mm or less is substantially horizontally point-supported at two or more points, and a process in which the two-phase stainless steel pipe is thermally processed at 1,000°C or higher while maintaining the point-support state. The two-phase stainless steel contains Mn: 3 mass% or less, Cr: 18 mas% or more and 28 mass% or less; Ni: 2 mass% or more and 9 mass% or less, Mo: 5 mass% or less, and N:0.10 mass% or more and 0.35 mass% or less, the remainder has a component composition comprising Fe and inevitable impurities, and a holding time at 1,000°C or higher in the thermal processing process is so made as to be within five minutes.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a thermal processing method for a duplex stainless steel pipe.

  A duplex stainless steel composed of a ferrite phase and an austenite phase is a material having both strength and corrosion resistance. Duplex stainless steel pipes are used, for example, as heat exchanger pipes, oil well pipes, chemical plant pipes, and the like. In recent years, use as a high-pressure fuel pipe such as a gasoline engine has been studied.

  This duplex stainless steel pipe is greatly deformed when heated compared to other general austenitic stainless steel pipes. Therefore, in order to prevent such deformation during heating, a method has been proposed in which a heat-treated stainless steel tube is inserted into the outer tube and heat-treated in a double tube state (see Patent Document 1). Further, as a method for reducing the residual stress of the stainless steel pipe, there has also been proposed a method in which cooling after heating is performed by furnace cooling while rotating the steel pipe in a heating furnace (see Patent Document 2).

JP-A-5-78749 JP 2001-262228 A

  On the other hand, in a duplex stainless steel pipe or the like, heat processing for attaching various metal members to the outer surface by brazing or other welding may be performed depending on the application. As a method for attaching a metal member to a stainless steel pipe by brazing, a furnace heating method or the like is generally used. As a specific method, brazing is performed by placing a metal member together with a brazing material on the upper surface of the steel pipe while supporting both ends of the steel pipe and heating in this state in a furnace.

  During such heat processing, the duplex stainless steel pipe is not supported between the support points. For this reason, in heat processing, there exists a problem that the curvature to the gravity direction of a duplex stainless steel pipe arises between support points. This warpage is particularly likely to occur when the diameter of the steel pipe is small and the plate thickness is small. In order to suppress the occurrence of such warpage, it is conceivable to perform heating in a double tube state as in the prior art described above, but this method cannot braze a member of any shape. In addition, it is not preferable to perform cooling or the like while rotating because it affects brazing.

  This invention is made | formed based on the above situations, The objective is to provide the heat processing method of the duplex stainless steel pipe by which generation | occurrence | production of the curvature to a gravitational direction is suppressed.

  The inventor has found that the occurrence of warping during the heat processing is mainly caused by creep deformation rather than the influence of residual stress. Furthermore, if the heating temperature is less than 1,000 ° C, and even if the heating temperature is 1,000 ° C or more, if the holding time of 1,000 ° C or more is less than 5 minutes, the occurrence of warpage is reduced. As a result, the present invention has been completed.

  That is, the invention made to solve the above problems is a step of supporting a duplex stainless steel pipe having an outer diameter of 50 mm or less, a pipe thickness of 3 mm or less, and a length of 600 mm or less substantially horizontally at two or more points. And the step of heat-treating the duplex stainless steel pipe at 1,000 ° C. or higher while maintaining the point support state, the duplex stainless steel pipe being Mn: 3 mass% or less, Cr: 18 mass% 28% by mass or less, Ni: 2% by mass or more and 9% by mass or less, Mo: 5% by mass or less, and N: 0.10% by mass or more and 0.35% by mass or less, with the balance being Fe and inevitable impurities And a heat processing method for a duplex stainless steel pipe in which the holding time at 1,000 ° C. or higher in the heat processing step is within 5 minutes.

  According to the said heat processing method, generation | occurrence | production of the curvature to the gravity direction in the time of the heat processing of the duplex stainless steel pipe performed in the state of being point-supported by two or more points can be suppressed. Note that “substantially horizontal” means within a range of ± 10 ° with respect to the horizontal direction. “Point support” means that only a part of the duplex stainless steel pipe is supported and there is an unsupported part. In “point support”, it is not necessary to support at a strict point, and it may be supported by a line or a surface having an appropriate region.

  In the heat processing step, it is preferable to perform cooling from the maximum temperature to 1,000 ° C. at 20 ° C./s or more. By rapidly cooling at such a cooling rate, the holding time of 1,000 ° C. or more can be shortened while performing heat processing at a sufficient temperature, and the occurrence of warpage can be further suppressed.

  The component composition preferably further contains C: 0.04% by mass or less, P: 0.03% by mass or less, and S: 0.03% by mass or less.

  The component composition is at least one selected from the group consisting of Al: 0.03% by mass or less, Si: 1.0% by mass or less, Mg: 0.005% by mass or less, and Ca: 0.005% by mass or less. Preferably it contains seeds.

  The component composition is at least 1 selected from the group consisting of Nb: 0.3% by mass or less, Ti: 0.1% by mass or less, V: 0.3% by mass or less, and Ta: 0.1% by mass or less. Preferably it contains seeds.

  The heating process is preferably brazing. The said heat processing method can be used suitably in the brazing to a duplex stainless steel pipe.

  ADVANTAGE OF THE INVENTION According to this invention, the heat processing method of the duplex stainless steel pipe by which generation | occurrence | production of the curvature to a gravity direction is suppressed can be provided.

Drawing 1 is an explanatory view showing the state at the time of performing the heat processing method of the duplex stainless steel pipe concerning one embodiment of the present invention. FIG. 2 is a graph showing a heating pattern in the thermal processing method for a duplex stainless steel pipe according to an embodiment of the present invention. FIG. 3 is a graph showing the yield stress of each steel pipe in Experimental Example 1 under a high temperature environment. FIG. 4 is a graph showing the warpage amount of each steel pipe in Experimental Example 2. FIG. 5 is a graph showing the amount of warp of the steel pipe A in Experimental Example 3. FIG. 6 is a graph showing the amount of warpage of the steel pipe B in Experimental Example 3. FIG. 7 is a graph showing the warpage amount of the steel pipe C in Experimental Example 3. FIG. 8 is a graph showing the amount of warpage of the steel pipe A in Experimental Example 4.

  Hereinafter, a thermal processing method for a duplex stainless steel pipe according to an embodiment of the present invention will be described in detail with reference to the drawings as appropriate.

  The thermal processing method according to an embodiment of the present invention includes a step of supporting a duplex stainless steel tube at two or more points substantially horizontally (supporting step), and the duplex stainless steel tube while maintaining the point support state. A step of heat processing (heat processing step). Hereinafter, the case where this heat processing is brazing is demonstrated.

(Support process)
In the supporting step, as shown in FIG. 1, both end portions of the duplex stainless steel pipe 10 are point-supported by the respective supports 11. This support is performed so that the duplex stainless steel pipe 10 is substantially horizontal. The duplex stainless steel pipe 10 is supported substantially horizontally within a range of ± 10 ° with respect to the horizontal direction, but is preferably within a range of ± 5 °. The support 11 is not particularly limited as long as it is a member that can withstand heating at 1,000 ° C. or higher. In addition, in FIG. 1, although it supports with the two support bodies 11, you may support a duplex stainless steel pipe | tube with three or more support bodies. However, when supported by three or more supports, warpage in the horizontal direction of the duplex stainless steel pipe 10 may easily occur due to a slight difference in the height of the supports, a shift in the arrangement position, or the like.

  A plurality of members 12 to be brazed are disposed above the outer surface of the duplex stainless steel tube 10. This member 12 is usually a metal member, and may be stainless steel such as duplex stainless steel or other metal member. Moreover, the number of members 12 may be one, and in the case of a plurality of members, they may have the same shape or material, or may have different shapes or materials.

  A brazing material (not shown) for brazing is disposed between the duplex stainless steel tube 10 and the member 12 or in a position close to them. Further, a member in which a brazing material is laminated may be used as the member 12. The brazing material is not particularly limited, and may be a brazing material having a melting temperature of 1,000 ° C. or higher, or a brazing material having a melting temperature of less than 1,000 ° C. Examples of the brazing material include copper brazing, brass brazing, phosphor bronze brazing, nickel brazing and the like.

  The outer diameter of the duplex stainless steel pipe 10 is 50 mm or less, and may be 30 mm or less. The lower limit of the outer diameter may be 5 mm or 10 mm, for example. The tube thickness of the duplex stainless steel tube 10 is 3 mm or less, and may be 2.5 mm or less. As a minimum of this tube thickness, it is 0.5 mm, for example, and may be 1 mm. The length of the duplex stainless steel pipe 10 is 600 mm or less. The lower limit of this length is, for example, 100 mm and may be 300 mm. Thus, the duplex stainless steel pipe having a small diameter and thickness is likely to warp in the direction of gravity during heat processing. Therefore, by applying the thermal processing method to a steel pipe of such a size, the advantage of suppressing warpage is sufficiently exhibited, and a steel pipe subjected to thermal processing (such as brazing) with small warpage is applied. Can be obtained.

  In addition, the duplex stainless steel pipe 10 to be subjected to the heat processing step may be already subjected to processing such as formation of a hole or may not be processed.

  The duplex stainless steel tube 10 is made of a duplex stainless steel mainly composed of a ferrite phase and an austenite phase. In addition, the duplex stainless steel pipe 10 may be present to such an extent that phases other than the ferrite phase and the austenite phase do not affect various properties such as corrosion resistance and strength. The lower limit of the total area ratio of the ferrite phase and the austenite phase is preferably 95% and more preferably 97% with respect to the total phase (total structure).

  The component composition of the duplex stainless steel pipe 10 contains a predetermined amount of Mn, Cr, Ni, Mo and N, with the balance being Fe and inevitable impurities. Moreover, as for the duplex stainless steel pipe 10, it is preferable that content of C, P, and S is below predetermined amount about a component composition. Moreover, it is preferable that the duplex stainless steel pipe 10 further contains at least one selected from the group consisting of a predetermined amount of Al, Si, Mg and Ca in the component composition. Moreover, it is preferable that the duplex stainless steel pipe 10 further contains at least one selected from the group consisting of a predetermined amount of Nb, Ti, V, and Ta. Hereinafter, the numerical range of the component composition and the reason for limitation will be described.

(Mn: 3% by mass or less)
Mn is an element that has a deoxidizing effect and is contained for securing the strength and stabilizing the austenite phase. The lower limit of the Mn content may be 0% by mass, but in order to sufficiently obtain the above effect, 0.1% by mass is preferable, 0.2% by mass is more preferable, and 0.5% by mass is preferable. Further preferred. On the other hand, the upper limit of the Mn content is 3% by mass, and preferably 2.5% by mass. By making Mn content below the said upper limit, it can suppress that coarse MnS etc. are formed and corrosion resistance deteriorates.

(Cr: 18% to 28% by mass)
Cr is a main component of the passive film, and is a basic element for developing the corrosion resistance of the stainless steel material. Cr is an element that stabilizes the ferrite phase. Therefore, in order to maintain the two-phase structure of ferrite and austenite and to achieve both corrosion resistance and strength, the lower limit of the Cr content is 18% by mass, preferably 20% by mass, and more preferably 22% by mass. On the other hand, the upper limit of the Cr content is 28% by mass, and preferably 26% by mass. Sufficient hot workability can be exhibited by making Cr content below the said upper limit.

(Ni: 2 mass% or more and 9 mass% or less)
Ni is an element necessary for improving corrosion resistance, and is particularly effective in suppressing local corrosion. Ni is also an element that is effective for improving low-temperature toughness and is also necessary for stabilizing the austenite phase. In order to obtain such effects, the lower limit of the Ni content is 2% by mass, preferably 3% by mass, and more preferably 4% by mass. On the other hand, the upper limit of the Ni content is 9% by mass, preferably 8% by mass, and more preferably 7% by mass. By making Ni content below the said upper limit, it can suppress that an austenite phase increases too much and an intensity | strength falls.

(Mo: 5 mass% or less)
Mo is an element that improves the corrosion resistance. Mo is an element that stabilizes the ferrite phase, and has the effect of improving the pitting corrosion resistance and crack resistance of the steel material. The lower limit of the Mo content may be 0% by mass, but in order to obtain the above effect, 0.1% by mass is preferable, and 0.3% by mass is more preferable. On the other hand, the upper limit of the Mo content is 5% by mass, preferably 4% by mass. By making Mo content below the said upper limit, production | generation of intermetallic compounds, such as (sigma) phase, can be suppressed, and corrosion resistance and hot workability can be improved.

(N: 0.10% by mass to 0.35% by mass)
N is an element that stabilizes a strong austenite phase, and has an effect of improving corrosion resistance without increasing the formation sensitivity of the σ phase. Furthermore, N is an element effective for increasing the strength of steel. In order to obtain such an effect, the lower limit of the N content is 0.10% by mass, and preferably 0.12% by mass. On the other hand, the upper limit of the N content is 0.35% by mass, and preferably 0.30% by mass. By making N content below the said upper limit, it can suppress that toughness and corrosion resistance fall by formation of nitride. In addition, good hot workability can be maintained, and the occurrence of ear cracks and surface defects during forging and rolling can be suppressed.

(C: 0.04 mass% or less)
C is an undesirable element that forms a carbide with Cr or the like in the steel material to lower the corrosion resistance. Therefore, the upper limit of the C content is preferably 0.04% by mass, more preferably 0.03% by mass, and still more preferably 0.02% by mass. C may not be contained in the steel material, that is, 0% by mass.

(P: 0.03 mass% or less)
P is an element that is inevitably mixed as an impurity and adversely affects the corrosion resistance, and also deteriorates weldability and workability. Therefore, the upper limit of the P content is preferably 0.03% by mass or less. In addition, although P may not be contained in the steel material, that is, 0% by mass, excessive reduction of the P content causes an increase in manufacturing cost. The lower limit is about 0.001% by mass.

(S: 0.03 mass% or less)
S, like P, is inevitably mixed as an impurity, and combines with Mn to form sulfide inclusions (MnS), thereby deteriorating corrosion resistance and hot workability. Therefore, the upper limit of the S content is preferably 0.03% by mass or less. In addition, S may not be contained in the steel material, that is, it may be 0% by mass. However, excessive reduction of the S content causes an increase in manufacturing cost, so the lower limit of the S content is, for example, About 0.001 mass% may be sufficient.

(Al: 0.03 mass% or less, Si: 1.0 mass% or less, Mg: 0.005 mass% or less, and Ca: at least one selected from the group consisting of 0.005 mass% or less)
Al, Si, Mg, and Ca are deoxidizing elements, and are elements that can reduce the amount of O and S during melting. Al, Si, Mg and Ca are not contained in the steel material, that is, each may be 0% by mass, but in order to achieve the above effect, the lower limit of the Al content is 0.001% by mass. Preferably, the lower limit of the Si content is preferably 0.1% by mass. The lower limit of Mg content and Ca content is preferably 0.0005% by mass, respectively. On the other hand, the upper limit of the Al content is preferably 0.03% by mass. The upper limit of the Si content is preferably 1.0% by mass. The upper limit of the Mg content is preferably 0.005% by mass. The upper limit of the Ca content is preferably 0.005% by mass or less. By making the Al content, the Si content, the Mg content, and the Ca content equal to or less than the above upper limit, it is possible to suppress deterioration of corrosion resistance and workability due to an increase in oxide inclusions.

(Nb: 0.3 mass% or less, Ti: 0.1 mass% or less, V: 0.3 mass% or less, and Ta: at least one selected from the group consisting of 0.1 mass% or less)
Nb, Ti, V, and Ta are elements that improve corrosion resistance and improve strength characteristics and hot workability. In order to obtain such an effect, when these elements are contained, the Nb content, the Ti content, the V content, and the lower limit of Ta are each preferably 0.01% by mass. On the other hand, the upper limit of Nb content is preferably 0.3% by mass, the upper limit of T content is preferably 0.1% by mass, the upper limit of V content is preferably 0.3% by mass, and the upper limit of Ta content is 0.1 mass% is preferable. By making the Nb content, Ti content, V content and Ta content not more than the above upper limits, deterioration of toughness due to the formation of coarse carbides and nitrides can be suppressed.

(Fe and inevitable impurities)
The basic components of the component composition constituting the duplex stainless steel pipe are as described above, and the remaining components are Fe and inevitable impurities. Inevitable impurities are impurities that are inevitably mixed during melting, and are contained within a range that does not impair various properties of the steel pipe. Further, the component composition of the steel pipe may further contain other elements in addition to the above components as long as it does not adversely affect the duplex stainless steel pipe.

(Heating process)
A heat processing process is a process of heat-processing the duplex stainless steel pipe | tube 10 at 1000 degreeC or more, maintaining the point support state shown in FIG. That is, the duplex stainless steel pipe 10 is heat-processed while being point-supported at two or more points substantially horizontally. This heat processing method is not particularly limited, and can be performed by an in-furnace heating method, an induction heating method, or the like, but among these, it can be suitably performed by an in-furnace heating method.

Specifically, the duplex stainless steel pipe 10 in the state shown in FIG. 1 is placed in a furnace heated to 1,000 ° C. or higher. The heating pattern (temperature change) of the duplex stainless steel pipe 10 at this time is shown in FIG. The duplex stainless steel tube 10 placed in the heated furnace is gradually heated and maintained at a predetermined temperature (maximum temperature: T _MAX ). The maximum temperature (T _MAX ) is not particularly limited as long as it is 1,000 ° C. or higher, and is appropriately set according to the melting temperature of the brazing material. The lower limit of the maximum temperature is, for example, 1,050 ° C., 1,100 ° C., 1,150 ° C., or 1,200 ° C. On the other hand, the upper limit of the maximum temperature is about 1,250 ° C. in consideration of the structure balance of the duplex stainless steel.

  In the heat processing step, the upper limit of the holding time (ht) at 1,000 ° C. or higher is 5 minutes, 4 minutes, 3 minutes, or 2 minutes. . When the holding time at 1,000 ° C. or higher exceeds 5 minutes, the duplex stainless steel pipe 10 is likely to warp in the direction of gravity. On the other hand, the lower limit of the holding time at 1,000 ° C. or higher may be appropriately set according to, for example, the type and amount of the brazing material, and may be 0.5 minutes or 1 minute. It may be 2 minutes or 3 minutes. By setting the holding time at 1,000 ° C. or more to the above lower limit or more, sufficient heat processing, that is, brazing can be performed. Note that the holding time (ht) above 1,000 ° C. does not indicate only the time held at the maximum temperature (TMAX), but exceeds 1,000 ° C. until reaching the maximum temperature. And the time after starting to lower the temperature from the maximum temperature and exceeding 1,000 ° C. are also included.

In the heating process, it is preferable to perform cooling from the maximum temperature (T _MAX ) to 1,000 ° C. at 20 ° C./s or more. In this way, by performing the cooling after the heat treatment in a short period, the holding time (ht) at 1,000 ° C. or more is shortened while performing the heat processing for a sufficient time at the maximum temperature (T _MAX ). Can do. The lower limit of this cooling rate is preferably 30 ° C./s, more preferably 40 ° C./s. On the other hand, the upper limit of the cooling rate is not particularly limited, but is 200 ° C./s, for example, and may be 100 ° C./s. The cooling method at 20 ° C./s or higher is not particularly limited. For example, by removing the duplex stainless steel tube 10 with the member 12 brazed from the furnace, liquid cooling, etc. Cooling can be performed at a high cooling rate. If cooling at the above cooling rate is possible, cooling may be performed by air cooling or the like.

  When the temperature of the duplex stainless steel tube 10 is below 1,000 ° C., almost no warpage in the direction of gravity occurs. Therefore, the cooling rate after cooling to 1,000 ° C. may be slow.

  The duplex stainless steel pipe 10 obtained by brazing the member 12 thus obtained has high straightness, and can be suitably used as a high-pressure fuel pipe for automobile gasoline engines and the like. In addition, you may use as piping for heat exchangers.

(Other embodiments)
The present invention is not limited to the above-described embodiment, and can be implemented in a mode in which various changes and improvements are made in addition to the above-described mode. For example, in the above-described embodiment, the heating process is described as brazing, but heating process other than brazing may be performed. Examples of such heat processing include welding other than brazing, heat treatment after welding, and the like.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited to the following examples.

  The composition of the components shown in Table 1 (the balance is Fe and inevitable impurities), the outer diameter, the inner diameter and the tube thickness shown in Table 2, and a duplex stainless steel pipe having a length of 400 mm is used in the following experimental examples, etc. Using.

[Experimental Example 1]
Using each steel pipe, a high-temperature tensile test was performed while maintaining a predetermined temperature in the range of 600 ° C. to 1000 ° C., 0.2% proof stress was measured, and the measured 0.2% proof stress was taken as the yield stress. The measurement results are shown in FIG.

[Experiment 2]
The amount of warpage in the direction of gravity of each steel pipe accompanying heating was measured by the following method. Each steel pipe was put into a furnace heated to 1,100 ° C. with both ends supported. It heated until the steel pipe temperature became 1,100 degreeC, and after the steel pipe temperature became 1,080 degreeC or more, it hold | maintained for 3 minutes, 10 minutes, or 60 minutes. Thereafter, air cooling was performed, and water cooling was performed when the temperature was below 700 ° C. The amount of warpage in the direction of gravity of the central portion of each steel pipe after heating was measured. The measurement results are shown in FIG.

  As shown in FIG. 3, it can be seen that the yield stress of the duplex stainless steel pipes (steel pipes A to C) decreases to the same level as the SUS304 pipe at high temperatures. Although the yield stress at high temperature is similar to that of SUS304 and duplex stainless steel, as shown in FIG. 4, the amount of warping in the direction of gravity during heating is as follows. It can be seen that it occurs greatly in steel pipes. It can also be seen that the amount of warpage in the direction of gravity increases depending on the holding time in the high temperature state. From these, it is surmised that the warpage in the gravitational direction is less affected by the residual stress and is mainly caused by creep deformation. Moreover, when compared with a duplex stainless steel pipe, it can be seen that a steel pipe having a high strength at a high temperature has a relatively small amount of warpage.

[Experiment 3]
The steel pipes A to C were put into a furnace heated with both end portions supported. It heated until the steel pipe temperature became 850 degreeC, 950 degreeC, or 1,050 degreeC, and was hold | maintained for 3 minutes after the steel pipe temperature became 850 degreeC, 950 degreeC, or 1,050 degreeC. Thereafter, it was air cooled and water cooled when it was below 700 ° C. The amount of warpage in the direction of gravity for each distance from the pipe end of each steel pipe after the heat treatment was measured. The measurement result of the steel pipe A is shown in FIG. 5, the measurement result of the steel pipe B is shown in FIG. 6, and the measurement result of the steel pipe C is shown in FIG.

  As shown in FIGS. 5 to 7, the duplex stainless steel pipe has a small amount of warpage at 950 ° C. or less, but the amount of warpage suddenly increases at 1,050 ° C. It should be noted that the amount of warpage at 850 ° C. and 950 ° C. is almost the same as the amount of warpage before heating, and it can be said that substantially no warpage occurs when heating at 850 ° C. and 950 ° C.

[Experimental Example 4]
The steel pipe A was put into a furnace heated with both end portions supported. It heated until the steel pipe temperature became 1,100 degreeC, and after the steel pipe temperature became 1,100 degreeC, it hold | maintained for 1 minute. Thereafter, the steel pipe A was taken out of the furnace and cooled with water.

  On the other hand, the steel pipe A was put into a furnace heated with both ends supported. It heated until the steel pipe temperature became 1,100 degreeC, and after the steel pipe temperature became 1,100 degreeC, it hold | maintained for 1 minute. Then, it cooled in the furnace (slow cooling) and cooled with water when the temperature was below 700 ° C.

  The amount of warpage in the direction of gravity of the central portion of each steel pipe after the heat treatment was measured. Two samples each were tested by water cooling and air cooling. The measurement results are shown in FIG. It can be seen from FIG. 8 that the amount of warpage can be greatly reduced by water cooling. The cooling rate up to 1000 ° C. during water cooling was about 50 ° C./s, and the cooling rate during air cooling was about 10 ° C./s.

  From these results, the occurrence of warpage in the direction of gravity when heating a duplex stainless steel tube is noticeably generated when heated to 1,000 ° C or higher, and the warpage at less than 1,000 ° C is small. (Figs. 5-7 etc.). Further, it can be seen that the warpage amount increases as the holding time of 1,000 ° C. or higher is increased, and particularly when the holding time of 1,000 ° C. or higher exceeds 5 minutes (FIG. 4 and the like). From these, it can be seen that it is effective to set the holding time of 1,000 ° C. or more to 5 minutes or less in order to suppress the warping in the direction of gravity during the heat processing of the duplex stainless steel pipe. Further, it can be seen that rapid cooling after heating is effective as a means of shortening the time of heating to 1,000 ° C. or more while performing sufficient heat processing and suppressing the occurrence of warping (FIG. 8, etc.). ).

  The thermal processing method of the duplex stainless steel pipe of the present invention can be suitably used as a member brazing to the duplex stainless steel pipe and other thermal processing methods.

10 Duplex stainless steel pipe 11 Support 12 Member

Claims (6)

A step of supporting a duplex stainless steel pipe having an outer diameter of 50 mm or less, a pipe thickness of 3 mm or less, and a length of 600 mm or less substantially horizontally at two or more points;
A process of heating the duplex stainless steel pipe at 1,000 ° C. or higher while maintaining the point support state,
The above duplex stainless steel pipe
Mn: 3% by mass or less,
Cr: 18% by mass or more and 28% by mass or less,
Ni: 2 mass% or more and 9 mass% or less,
Mo: 5% by mass or less, and N: 0.10% by mass to 0.35% by mass, with the balance being composed of Fe and inevitable impurities,
A heat processing method for a duplex stainless steel pipe, wherein the holding time at 1,000 ° C. or higher in the heat processing step is within 5 minutes.   The method for heat-processing a duplex stainless steel pipe according to claim 1, wherein in the heat-processing step, cooling from the maximum temperature to 1,000 ° C is performed at 20 ° C / s or more. The above component composition is further C: 0.04 mass% or less,
The heat processing method of the duplex stainless steel pipe according to claim 1 or 2, comprising P: 0.03 mass% or less, and S: 0.03 mass% or less. The above component composition is further Al: 0.03% by mass or less,
Si: 1.0 mass% or less,
The heat | fever of the duplex stainless steel pipe of Claim 1, 2 or 3 containing at least 1 sort (s) chosen from the group which consists of Mg: 0.005 mass% or less and Ca: 0.005 mass% or less. Processing method. The above component composition is further Nb: 0.3% by mass or less,
Ti: 0.1% by mass or less,
The duplex stainless steel pipe according to any one of claims 1 to 4, comprising at least one selected from the group consisting of V: 0.3% by mass or less, and Ta: 0.1% by mass or less. Thermal processing method.   The said heat processing is brazing, The heat processing method of the duplex stainless steel pipe of any one of Claims 1-5.

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