JP6482075B2 - Welded duplex stainless steel pipe and its manufacturing method - Google Patents

Description

  The present invention relates to a welded duplex stainless steel pipe used in an environment that requires extremely excellent intergranular corrosion resistance, such as a chemical plant, and a method for producing the same.

  Duplex stainless steel is an excellent material that combines strength and corrosion resistance, even in environments where chlorides are present at high concentrations such as seawater, or in environments subject to severe corrosion such as oil wells or chemical plants. It is known as a material having excellent corrosion resistance. Therefore, duplex stainless steel is also used in urea plants that are subjected to particularly severe corrosion among chemical plants, and the demand tends to increase year by year as the production amount of urea increases.

  By the way, the duplex stainless steel pipe used in the urea plant or the like can be manufactured by equipment and a manufacturing method used for manufacturing a normal stainless steel pipe represented by SUS304 and SUS316L. However, currently, the hot extrusion method represented by the Eugene method and the bush bench method, or the inclined rolling method represented by the plug mill method and the mandrel mill method, which are described in Patent Document 1 and Patent Document 2, etc. Seamless steel pipes (seamless steel pipes) manufactured by the (Mannesmann method) are the mainstream. However, this seamless steel pipe has a problem that the manufacturing cost is high from the viewpoint of manufacturing equipment, and only a steel pipe having a relatively small diameter can be manufactured.

  On the other hand, a welded pipe manufactured by welding a seam after bending a plate material (steel plate) has an advantage that the manufacturing cost is lower than that of a seamless steel pipe and a large diameter steel pipe can be manufactured. However, in the case of a welded steel pipe, it is known that there are problems in various characteristics including corrosion resistance in a weld heat affected zone (HAZ). The main reason for the decrease in corrosion resistance is that Cr, which greatly contributes to improving the corrosion resistance of stainless steel due to heat input during welding, combines with C contained in the alloy to form chromium carbide, and the Cr concentration around it A low region, a so-called “Cr deficient layer” is generated, and the corrosion resistance of the portion is locally reduced. This phenomenon is generally referred to as “sensitization” and can be avoided by reducing the amount of C contained in the alloy.

  In addition, in the case of stainless steel containing a large amount of Cr and Mo for the purpose of improving corrosion resistance, an intermetallic compound (σ phase) mainly composed of Cr and Mo precipitates due to heat input during welding, which is similar to sensitization. It is known to cause a decrease in local corrosion resistance. As a countermeasure, for example, in Patent Document 3, in order to prevent a local decrease in the pitting corrosion index (PREW) due to precipitation of the σ phase, the heat-affected zone is water-cooled after welding of the duplex stainless steel. A technique for preventing the precipitation of the σ phase has been proposed.

JP 2005-014032 A JP 2004-122137 A Japanese Patent Laid-Open No. 06-116684

  By the way, as for the welded steel pipe of duplex stainless steel, TIG welding or plasma welding which does not use a welding material is used for welding at the time of pipe making, but the structure of the weld bead (welded metal) is composed of α phase and γ phase. Since the phase ratio is different from that of the base material and is a solidified structure and solidified segregation exists, the corrosion resistance is poor. Similarly, also in the heat affected zone adjacent to the weld metal, the phase ratio between the α phase and the γ phase changes from the base material due to heat input during welding, and the α phase has a large phase ratio and the γ phase has a low phase ratio. Therefore, it is inferior to corrosion resistance. Furthermore, stress corrosion cracking and the like are promoted in the welded steel pipe by strain introduced by bending during pipe making and welding residual stress. Therefore, in addition to the purpose of solidifying harmful precipitates in welded steel pipes, in order to return the phase ratio of the α phase and γ phase in the weld zone to the same phase ratio as that of the base metal and to eliminate strain, A solution heat treatment is generally performed.

  However, according to the investigations by the inventors, welding after solution heat treatment was made using a duplex stainless steel sheet having the same component composition and manufactured so that precipitates such as σ phase, carbide, and nitride were not observed. In steel pipes, it has been found that large differences may occur in the intergranular corrosion resistance of welds, particularly weld heat-affected zones. However, the techniques disclosed in Patent Documents 1 to 3 are all techniques for improving pitting corrosion and stress corrosion cracking, and improving intergranular corrosion resistance in the heat-affected zone of a steel pipe manufactured by welding. Did not consider anything.

  The present invention has been made in view of the above-mentioned problems of the prior art, and its purpose is to provide a welded duplex stainless steel pipe excellent in intergranular corrosion resistance of the weld heat-affected zone and a method for producing the same. It is to propose.

  Inventors repeated earnest examination in order to solve the said subject. As a result, the difference in intergranular corrosion resistance in the heat affected zone is due to the difference in Cr concentration distribution in the γ phase near the α / γ phase interface (grain boundary), that is, intergranular corrosion resistance. In steel pipes with inferior properties, there is a low Cr concentration region in the γ phase in the vicinity of the α / γ phase interface of the weld heat affected zone, which is a cause of reducing intergranular corrosion resistance, and In order to prevent the formation of the low Cr concentration region, it has been found effective to optimize the solution heat treatment conditions, and the present invention has been developed.

  That is, the present invention is C: 0.05 mass% or less, Si: 0.10 to 1.00 mass%, Mn: 0.3 to 2.0 mass%, P: 0.010 to 0.050 mass%, S: 0 0.0001-0.02 mass%, Al: 0.001-0.05 mass%, N: 0.05-0.4 mass%, Ni: 4-9 mass%, Cr: 20-27 mass%, Mo: 2-5 mass% Cu: 0.01-0.30 mass%, W: 0.01-0.4 mass%, B: 0.0001-0.001 mass%, and Ca: 0.0006-0.01 mass%, the balance being It has a composition composed of Fe and unavoidable impurities, has no σ phase in the weld heat affected zone, and has a minimum area of 0.5 μm from the α / γ phase interface to the γ phase side in the weld heat affected zone. Cr concentration; In the same γ phase, the welded duplex stainless steel pipe is characterized in that the difference in average Cr concentration in the region separated from 0.5 μm from the α / γ phase interface to the γ phase side is less than 1.5 mass%.

  In addition to the above component composition, the welded duplex stainless steel pipe of the present invention is further one or two selected from V: 0.003 to 0.5 mass% and Nb: 0.003 to 0.5 mass%. It is characterized by containing.

Further, the welded duplex stainless steel pipe of the present invention is characterized in that the corrosion rate by 70 mass% boiling nitric acid is less than 0.30 g / m ² · hr.

  Moreover, when manufacturing the welded duplex stainless steel pipe according to any of the above, the present invention heats the welded duplex stainless steel pipe manufactured according to a conventional method to a temperature of 1000 to 1100 ° C, and then a temperature of 800 ° C or higher. We propose a method for producing a welded duplex stainless steel pipe, characterized in that it is cooled at a cooling rate of 3 ° C./s or higher, and then subjected to solution heat treatment immediately cooled at 6 ° C./s or higher.

  According to the present invention, a welded duplex stainless steel pipe having excellent intergranular corrosion resistance can be provided at a low cost. Therefore, as a corrosion resistant steel pipe used in an environment where the occurrence of intergranular corrosion is a concern, such as a chemical plant. It can be used suitably.

It is a graph which shows the result of having measured the Cr density | concentration distribution of the alpha / gamma phase interface vicinity of the welded duplex stainless steel pipe with favorable intergranular corrosion resistance by TEM-EDX. It is a graph which shows the result of having measured Cr density distribution near the alpha / gamma phase interface of a welded duplex stainless steel pipe inferior to intergranular corrosion resistance by TEM-EDX.

First, the basic technical idea of the present invention will be described.
As described above, even in welded steel pipes made of a duplex stainless steel sheet having the same component composition, and no precipitates such as σ phase, carbides, and nitrides are observed at the grain boundaries and in the grains, welding heat It was often observed that large differences occurred in the intergranular corrosion resistance of the affected area.
Therefore, the inventors considered that the difference in intergranular corrosion resistance in the weld heat affected zone is due to a difference in some component concentration in the vicinity of the grain boundary, and conducted component analysis in the vicinity of the α / γ phase interface. Specifically, samples were taken from welded duplex stainless steel pipes with poor intergranular corrosion resistance and welded duplex stainless steel pipes with good intergranular corrosion resistance, and the cross-section perpendicular to the pipe forming direction was used as the observation surface. Prepare a sample for an electron microscope (TEM), and use the energy dispersive X-ray spectroscopic analysis (EDX analysis) apparatus attached to the TEM, respectively, on the α phase side and the γ phase side with reference to the α / γ phase interface. Point analysis was performed at 60 nm intervals.

  As a result, in a welded duplex stainless steel pipe with good intergranular corrosion resistance in the weld heat affected zone, as shown in FIG. 1, the α / γ phase interface is near the α / γ phase interface as shown in FIG. As shown in FIG. 2, in the welded duplex stainless steel pipe having poor intergranular corrosion resistance, as shown in FIG. 2, the α / γ phase side α / In the vicinity of the γ-phase interface, there is a region where the Cr concentration is lower than the inside of the γ-phase (hereinafter, this region is referred to as “Cr-deficient region” in the present invention), which reduces the intergranular corrosion resistance. In particular, the difference between the minimum Cr concentration in the Cr-deficient region near the α / γ phase interface and the average Cr concentration in the γ-phase in which the Cr-deficient region exists is 1.5 mass% or more. It became clear that the intergranular corrosion resistance was greatly reduced. The “Cr-deficient region” is completely different from the conventionally known “Cr-deficient layer” because it does not involve the formation of chromium carbide.

  Therefore, in order to investigate the cause of the formation of the Cr-deficient region, the production history of welded duplex stainless steel pipes with poor intergranular corrosion resistance and welded duplex stainless steel pipes with good intergranular corrosion resistance was investigated in detail. . As a result, there is a difference in the solution heat treatment temperature and the cooling rate between the two welded steel pipes. And found that the cooling rate tends to be slow.

From the above results, the inventors estimate the mechanism by which the Cr-depleted region is formed as follows.
When the duplex stainless steel heated to a temperature in the two-phase region is cooled in the solution heat treatment, the ferrite phase (α phase) and the austenite phase (γ phase) become α / γ phase interfaces in an attempt to reach the phase ratio in the equilibrium state. Simultaneously with the movement of the (grain boundaries), Cr diffusion in the α phase and the γ phase tends to reach an equilibrium concentration.
In the case of the duplex stainless steel having the composition of the present invention, on the phase diagram, the proportion of the γ phase increases and the equilibrium Cr concentration decreases as the temperature decreases from the solution heat treatment temperature. Therefore, in the welded duplex stainless steel pipe of the present invention, during cooling from the solution heat treatment temperature, the α / γ phase interface moves to the α phase side, and the region of the γ phase expands and becomes a new γ phase. In this region, the Cr concentration is lower than that of the conventional γ phase.

Here, when the cooling rate is slow and a sufficient Cr diffusion time can be secured, Cr is supplied to a region having a low Cr concentration, and the region having a low Cr concentration is eliminated. However, in the slow cooling that is normally performed in the solution heat treatment, it is not possible to expect the effect of uniforming the Cr concentration by the Cr diffusion as described above, so a local “Cr deficient region” remains. End up. Therefore, in the present invention, the rapid cooling is performed from the solution heat treatment temperature to prevent the movement of the α / γ phase interface during the cooling, thereby suppressing the formation of the “Cr deficient region” in the weld heat affected zone.
The present invention has been made by further studying the above-described novel findings.

Next, the component composition that the duplex stainless steel sheet as the material of the welded duplex stainless steel pipe of the present invention should have will be described.
C: 0.05 mass% or less C is an element that forms Cr and chromium carbide, lowers the concentration of Cr necessary for improving corrosion resistance, and generates a so-called “Cr-deficient layer”. Is desirable. However, excessive reduction of C causes a decrease in steel strength or an increase in manufacturing cost. Therefore, the C content is 0.05 mass% or less. Preferably it is 0.005-0.040 mass%, More preferably, it is the range of 0.010-0.035 mass%.

Si: 0.10 to 1.00 mass%
Si is an element added as a deoxidizing material and needs to be contained in an amount of 0.10 mass% or more. However, excessive addition of Si not only saturates the above effects, but also reduces ductility and strength, and further promotes precipitation of intermetallic compounds such as σ phase and χ phase, thereby reducing corrosion resistance. Therefore, the Si content is in the range of 0.10 to 1.00 mass%. Preferably it is 0.15-0.75 mass%, More preferably, it is the range of 0.20-0.60 mass%.

Mn: 0.3 to 2.0 mass%
Mn is an austenite-forming element and can be an alternative to expensive Ni, which is the same austenite-forming element, so 0.3 mass% or more is added. However, excessive addition promotes the precipitation of intermetallic compounds such as σ phase and χ phase that lower the corrosion resistance, so the upper limit is made 2.0 mass%. Preferably it is 0.4-1.5 mass%, More preferably, it is the range of 0.5-1.2 mass%.

P: 0.010-0.050 mass%
P is an impurity element that is inevitably mixed in, and is easily segregated at the crystal grain boundaries, and lowers corrosion resistance and hot workability, so it is desirable to reduce it as much as possible. However, P competes with B, which greatly reduces intergranular corrosion resistance, and segregates at the grain boundary. Therefore, by containing a certain amount of P, segregation of B to the grain boundary can be reduced. In addition, since a certain amount of P is segregated at the grain boundaries, the movement of the α / γ phase interface due to the change in the phase ratio during cooling is suppressed, so there is also an effect of suppressing the formation of the Cr-deficient region. Said effect is acquired by addition of 0.010 mass% or more. However, addition exceeding 0.050 mass% significantly reduces the intergranular corrosion resistance. Therefore, the P content is in the range of 0.010 to 0.050 mass%. Preferably it is the range of 0.015-0.040 mass%.

S: 0.0001 to 0.02 mass%
S, like P, is an impurity element that is inevitably mixed in, and is easily segregated at the crystal grain boundaries, and lowers corrosion resistance and hot workability. Therefore, the upper limit is set to 0.02 mass%. However, S, like P, has the effect of suppressing the movement of the α / γ phase interface during cooling and suppressing the generation of Cr-deficient regions by containing a certain amount. Therefore, in the present invention, the S content is in the range of 0.0001 to 0.02 mass%. Preferably it is the range of 0.0003-0.01 mass%.

Al: 0.001 to 0.05 mass%
Al is a strong deoxidizing material, and it is necessary to add 0.001 mass% or more. However, even if added over 0.05 mass%, the effect is not only saturated, but also promotes the formation of giant inclusions that adversely affect the surface quality (appearance) and corrosion resistance of the steel sheet. Bonds to form AlN and reduces N effective for corrosion resistance. Therefore, Al is set to a range of 0.001 to 0.05 mass%. Preferably it is the range of 0.005-0.04 mass%.

N: 0.05 to 0.4 mass%
N is a strong austenite-forming element and, like Cr and Mo, is an element effective for improving corrosion resistance and suppressing the precipitation of intermetallic compounds, so it is necessary to contain 0.05 mass% or more. There is. However, if added over 0.4 mass%, the hot deformation resistance is increased and the hot workability is impaired, and it is difficult to maintain a two-phase structure. Therefore, N is set to a range of 0.05 to 0.4 mass%. Preferably it is 0.06-0.3 mass%, More preferably, it is the range of 0.08-0.25 mass%.

Ni: 4-9 mass%
Ni is an austenite-forming element and needs to be contained in an amount of 4 mass% or more in order to maintain a two-phase structure with the ferrite structure. However, the addition exceeding 9 mass% causes an austenite structure to be excessive and an acceleration factor for overpassive corrosion, thereby reducing the corrosion resistance. Therefore, the Ni content is in the range of 4-9 mass%. Preferably it is the range of 5.0-8.5 mass%, More preferably, it is the range of 5.5-8.0 mass%.

Cr: 20-27 mass%
Cr is an element that improves the corrosion resistance. In order to obtain the effect, it is necessary to contain 20 mass% or more. However, when added in excess of 27 mass%, formation of intermetallic compounds such as σ phase and χ phase is promoted, and the corrosion resistance is reduced. Further, Cr is a ferrite-forming element, and excessive addition makes it difficult to maintain a two-phase structure. Therefore, the Cr content is in the range of 20 to 27 mass%. Preferably it is 22-26.5 mass%, More preferably, it is the range of 23-26 mass%.

Mo: 2-5 mass%
Mo is an element effective for improving the corrosion resistance. In order to acquire the effect, it is necessary to add 2 mass% or more. However, if added over 5 mass%, the precipitation of intermetallic compounds is promoted and the corrosion resistance is lowered, so the upper limit is made 5 mass%. Preferably it is 2.5-4.3 mass%, More preferably, it is the range of 3.0-4.0 mass%.

Cu: 0.01-0.30 mass%
Cu is an element effective for improving general corrosion resistance. In order to acquire the said effect, it is necessary to contain 0.01 mass% or more. On the other hand, in a specific corrosive environment such as a urea plant, it becomes an element that causes corrosion to proceed. Therefore, the upper limit needs to be limited to 0.30 mass%. Therefore, Cu is set to a range of 0.01 to 0.30 mass%. Preferably it is 0.05-0.25 mass%, More preferably, it is the range of 0.08-0.20 mass%.

W: 0.01-0.4 mass%
W is an element that improves the corrosion resistance of the duplex stainless steel in the presence of Mo and needs to be added in an amount of 0.01 mass% or more. However, addition exceeding 0.4 mass% promotes precipitation of intermetallic compounds such as σ phase and χ phase, and lowers corrosion resistance. Therefore, W is in the range of 0.01 to 0.4 mass%. Preferably it is 0.05-0.3 mass%, More preferably, it is the range of 0.08-0.2 mass%.

B: 0.0001 to 0.001 mass%
B is an extremely effective element for improving hot workability, and the above effect can be obtained even by adding a very small amount. However, B is also an element that segregates at grain boundaries and greatly reduces intergranular corrosion resistance. Therefore, the B content is in the range of 0.0001 to 0.001 mass%.

Ca: 0.0006 to 0.01 mass%
Ca is an element effective to improve hot workability by combining with S harmful to hot workability to form CaS. In order to obtain the above effect, it is necessary to contain 0.0006 mass% or more. There is. However, addition exceeding 0.01 mass% forms inclusions containing CaO, and rather reduces the corrosion resistance. Therefore, Ca is set to a range of 0.0006 to 0.01 mass%. Preferably it is the range of 0.0007-0.005 mass%.

In the duplex stainless steel sheet used in the present invention, the balance other than the essential components is Fe and inevitable impurities. However, you may contain 1 type or 2 types chosen from V and Nb in the following range as needed.
V: 0.003-0.5 mass%, Nb: 0.003-0.5 mass%
V and Nb are effective elements for improving the corrosion resistance. In order to obtain the effect, V and Nb are each preferably added in an amount of 0.003 mass% or more. However, if added in excess of 0.5 mass%, precipitation of intermetallic compounds such as σ phase and χ phase is promoted, corrosion resistance is reduced, and hot workability is impaired. Therefore, when adding the said element, it is preferable to add in each said range. More preferably, it is the range of 0.003-0.2 mass%, respectively.

Next, the manufacturing method of the welded duplex stainless steel pipe of this invention is demonstrated.
The welded duplex stainless steel pipe of the present invention is a welded steel pipe manufactured by bending and welding a duplex stainless steel sheet as a raw material. The duplex stainless steel sheet (steel pipe material) used as the material of the steel pipe may be either a hot-rolled steel sheet or a cold-rolled steel sheet, and the manufacturing method may be any conventionally known method / condition, There is no limit. For example, steel is melted in an electric furnace, a converter, or the like, subjected to secondary refining and adjusted to the steel having the above-described composition, and then made into a steel material (slab) by a continuous casting method or an ingot-decomposition rolling method. Next, the hot-rolled steel sheet is obtained by reheating the slab, hot-rolling, heat-treating it under appropriate conditions, and pickling it to obtain a steel pipe material. On the other hand, the cold-rolled steel sheet is re-heated, hot-rolled, heat-treated under appropriate conditions, pickled to form a hot-rolled annealed sheet, and further cold-rolled the hot-rolled annealed sheet as appropriate. After heat treatment under the above conditions, pickling is performed to obtain a steel pipe material.

  Next, the steel pipe material obtained as described above is bent into a cylindrical shape by roll forming or the like, and then welded to form a welded steel pipe, and then subjected to solution heat treatment to obtain a product steel pipe. In addition, the welding method at the time of manufacturing a steel pipe from the above steel pipe material may be any method as long as it does not use a welding material. For example, TIG welding, MIG welding, laser welding, electron beam welding, plasma welding, etc. If it is, it can use suitably.

Here, the most important thing in the present invention is that the solution heat treatment after the pipe making needs to be performed under the following conditions.
Solution heat treatment temperature: 1000-1100 ° C
As described above, the solution heat treatment adjusts the metal structure of the welded part (welded metal + welding heat affected zone) to the same level as the base metal, and removes processing strain and residual welding stress introduced during pipe making. It is necessary to make the heat treatment temperature within the range of 1000 to 1100 ° C. If the temperature is lower than 1000 ° C., the Cr concentration in the γ phase that is in equilibrium with the solution treatment temperature decreases, and therefore the difference from the minimum Cr concentration in the Cr-deficient region formed during cooling becomes small. However, if the temperature is lower than 1000 ° C., the welding residual stress in the weld heat affected zone is not sufficiently removed, so the precipitation of the σ phase in the weld heat affected zone is promoted, leading to a decrease in corrosion resistance and toughness, and the residual stress as a driving force. Movement of the α / γ interface is promoted, and formation of a Cr-deficient region is promoted. Also, in steel pipe materials, if the temperature is lower than 1000 ° C., processing strain during pipe forming remains, and σ phase is precipitated, or σ phase precipitated during hot rolling does not disappear even by solution heat treatment. Or the corrosion resistance and toughness are reduced. On the other hand, when the heat treatment temperature exceeds 1100 ° C., element diffusion becomes active, so that it becomes easy to form a Cr-deficient region during the primary cooling described later, resulting in a decrease in corrosion resistance. Therefore, the solution heat treatment temperature is in the range of 1000 to 1100 ° C. Preferably it is the range of 1030-1080 degreeC.
In addition, it is preferable that the time for maintaining soaking at the above temperature is 30 to 600 seconds. If the time is less than 30 seconds, the effect of solid solution cannot be obtained sufficiently. On the other hand, if the time exceeds 600 seconds, the above effect is saturated and productivity is inhibited.

The cooling after the heat treatment is performed at a cooling rate of 3 ° C./s or higher to a temperature of 800 ° C. or higher (hereinafter, this cooling is referred to as “primary cooling”), and then immediately cooled at 6 ° C./s or higher (hereinafter referred to as “cooling”). This cooling is called “secondary cooling”).
Primary cooling: Cooling at a cooling rate of 3 ° C./s or higher to a temperature of 800 ° C. or higher If the primary cooling rate after the solution heat treatment is less than 3 ° C./s, the time for the element to diffuse during cooling is secured, and welding In the heat-affected zone and the base material, formation of Cr-deficient regions proceeds, leading to a decrease in corrosion resistance. In addition, the σ phase, nitrides, and carbides may precipitate, leading to a decrease in corrosion resistance. The cooling rate of the primary cooling is preferably 4 ° C./s or more, more preferably 5 ° C./s or more. The upper limit of the cooling rate of primary cooling is not particularly limited, but is preferably 20 ° C./s or less. The primary cooling method is not particularly limited as long as the cooling rate can be secured, but gas cooling is preferable.

  The reason for setting the end temperature of the primary cooling to 800 ° C. or higher is that the equilibrium concentration of Cr is higher in the α phase and lower in the γ phase as the solution temperature is lower than the solution heat treatment temperature (1000 to 1100 ° C.). Become. In particular, the tendency becomes remarkable at a temperature of less than 800 ° C. Since the Cr-deficient region corresponds to the γ phase generated during cooling, the Cr-deficient region formed when cooled to less than 800 ° C. has a low Cr concentration and greatly deteriorates the corrosion resistance. Accordingly, the cooling end temperature of the primary cooling is set to 800 ° C. or higher in order to suppress generation of a Cr-deficient region inferior in corrosion resistance. Preferably it is 900 degreeC or more. In addition, the upper limit of cooling completion temperature should just be below the said solution heat treatment temperature (1000-1100 degreeC), and there is no restriction | limiting in particular.

Secondary cooling rate: 6 ° C./s or more The secondary cooling following the primary cooling needs to be performed at a cooling rate of 6 ° C./s or more. If the secondary cooling rate is less than 6 ° C./s, the formation of a Cr-deficient region is promoted in the weld heat affected zone and the base metal, and the σ phase and carbides are precipitated, resulting in a decrease in corrosion resistance. In addition, if the temperature near 475 ° C. is slowly cooled, 475 ° C. embrittlement occurs, resulting in a decrease in manufacturability. Preferably it is 7 degrees C / s or more, More preferably, it is 8 degrees C / s or more. The cooling method of the secondary cooling is not particularly limited as long as the cooling rate can be ensured. For example, a gas jet cooling method in which a cooling gas is blown onto the surface of a traveling steel pipe, a water injection cooling method in which cooling water is blown, a gas and cooling Any method such as a mist cooling method in which water is mixed and sprayed or a water cooling method in which a steel pipe is immersed in water for cooling may be used.

  In the above description, the cooling process from the solution heat treatment temperature is divided into primary cooling and secondary cooling. However, if the above cooling rate can be ensured, there is no need to divide the cooling process into two. Of course, the process may be cooled at 6 ° C./s or more.

  The heat treatment applied to hot-rolled steel sheets and cold-rolled steel sheets in the process of manufacturing a duplex stainless steel sheet, which is the material of the welded duplex stainless steel pipe, is for the purpose of softening by solidifying carbides and removing strain. The conditions are not particularly limited as long as the above object can be achieved. For example, the temperature is maintained at 1000 to 1150 ° C. for 30 to 300 seconds, and then cooled at a cooling rate of 6 ° C./s or more. Is preferred.

Next, the welded duplex stainless steel pipe of the present invention will be described.
The welded duplex stainless steel pipe of the present invention obtained by subjecting a steel pipe made of a steel plate having the above-described composition to a solution heat treatment under the above-mentioned conditions, has no σ phase in the metal structure of the weld heat affected zone. is necessary. Since the σ phase is mainly composed of Cr and Mo, a local Cr and Mo deficient region is formed in the vicinity of the σ phase and the corrosion resistance is lowered, or the toughness is lowered because the σ phase is hard and brittle. Because it does. The existence of this σ phase can be easily confirmed by observation with a transmission electron microscope or an optical microscope after etching.

  Moreover, the welded duplex stainless steel pipe of the present invention has a σ phase not present in the metal structure of the weld heat affected zone, and is further reduced from the α / γ phase interface to the γ phase side in the weld heat affected zone. The difference between the minimum Cr concentration in the region up to 5 μm and the average Cr concentration in the region away from 0.5 μm from the α / γ phase interface to the γ phase side within the same γ phase must be less than 1.5 mass%. It is. This is because when the concentration difference is 1.5 mass% or more, a local battery is formed between the low Cr concentration region and the high Cr concentration region, and intergranular corrosion becomes remarkable. The reason why the measurement region of the minimum Cr concentration is in the range of 0.5 μm from the α / γ phase interface to the γ phase side is that the low Cr formed by the movement of the α / γ phase interface, as can be seen from FIG. This is because the region extends from the α / γ phase interface to about 0.5 μm. The Cr concentration difference is preferably 1.3 mass% or less, more preferably 1.2 mass% or less.

Here, the reason why the present invention prescribes the Cr concentration difference in the weld heat affected zone is that if the residual stress due to welding is not completely removed by the solution heat treatment, α / This is because the movement of the γ-phase interface is promoted and the formation of a Cr-deficient region is promoted more than the base material.
In addition, although there is no restriction | limiting in particular about the measuring method of said Cr density | concentration, From a viewpoint which measures the Cr density | concentration of a micro part accurately, the energy dispersive X-ray-spectral-analysis (EDX analysis) apparatus attached to TEM, SEM, etc. is used. In this case, it is preferable to perform point analysis from line analysis from the viewpoint of increasing measurement accuracy. The α / γ phase interface (grain boundary) for measuring the Cr concentration is preferably an interface perpendicular to the sample surface (observation surface). This is because if the interface is inclined, the distance from the α / γ phase interface becomes inaccurate.

The minimum Cr concentration in the region from the α / γ phase interface to 0.5 μm on the γ phase side and the average Cr in the region separated from the α / γ phase interface to the γ phase side by 0.5 μm in the same γ phase. A welded duplex stainless steel pipe having a concentration difference of less than 1.5 mass% is extremely excellent in intergranular corrosion resistance. Therefore, for example, in accordance with ASTM A262 Practice C (Huey Test), intergranular corrosion in which 70 mass% boiling nitric acid is used as a corrosive solution and an immersion test with 48 batches as one batch is performed for 5 batches while renewing the corrosive solution in each batch. The corrosion rate in the test can be less than 0.30 g / m ² · hr.

Fe-Ni-Cr-Mo type duplex stainless steels having various composition shown in Table 1 are melted by a conventional refining process, made into a slab by a continuous casting method, and reheated to a temperature of 1100 to 1200 ° C. Then, hot rolling was performed to obtain a hot rolled steel sheet having a thickness of 4 to 6 mm.
Subsequently, the hot-rolled steel plate and the cold-rolled steel plate used as the raw material of a welded steel pipe were manufactured from the said hot-rolled steel plate with the following method.
First, the hot-rolled steel sheet was manufactured by subjecting the hot-rolled steel sheet to a heat treatment at a temperature of 1000 to 1150 ° C. and then pickling.
The cold-rolled steel sheet is heat-treated at a temperature of 1000 to 1150 ° C., pickled, cold-rolled to obtain a cold-rolled steel sheet having a thickness of 3 to 4 mm, Heat treatment was performed at a temperature of 1150 ° C., and pickling was performed.
Next, using the hot-rolled steel sheet and the cold-rolled steel sheet as a raw material, after bending into a cylindrical shape, a TIG welding or plasma welding is used to form a welded steel pipe having an outer diameter of 30 to 400 mmφ with the rolling direction as the length direction. In an air atmosphere furnace, solution heat treatment was performed at various solution heat treatment temperatures and cooling rates shown in Table 2 to obtain a welded duplex stainless steel pipe.

The various welded duplex stainless steel pipes obtained as described above were subjected to the following evaluation tests.
<Measurement of Cr concentration distribution near α / γ phase interface>
A sample having a cross section perpendicular to the welding direction is taken from the region where the particle size is most coarsened in the vicinity of the bond portion of the welded heat affected zone (HAZ) of the welded duplex stainless steel pipe, and the sample is electrolyzed. By polishing, a sample for TEM-EDX analysis having the cross section as an observation surface and a region having a thickness of about 50 nm was produced.
Next, the TEM-EDX analysis sample is observed with a TEM to find an α / γ phase interface parallel to the electron beam, that is, substantially perpendicular to the observation surface, and the γ phase side of the α / γ phase interface is Point analysis was performed at intervals of 60 nm in the vertical direction from the α / γ phase interface. In order to ensure analysis accuracy, the beam irradiation time per point was set to 200 seconds.
Next, from the measurement results, a difference ΔCr between the lowest Cr concentration in the region within 0.5 μm in the vertical direction from the α / γ phase interface and the average Cr concentration in the region separated from 0.5 μm was obtained.
<Existence of sigma phase>
Using the sample used for the Cr concentration distribution measurement, the presence or absence of the σ phase was confirmed by TEM.
<Intergranular corrosion test>
A test piece having a thickness of 20 mm × 25 mm including the welded portion was collected from the welded duplex stainless steel pipe, and the surface of the test piece was wet-polished to # 120 to obtain a corrosion test piece.
In accordance with ASTM A262 Practice C (Huey Test), an immersion test using 70 mass% boiling nitric acid as a corrosive solution and 48 batches as one batch is performed while updating the corrosive solution in each batch. Perform intergranular corrosion test in batches, calculate the corrosion weight loss from the mass difference before and after the test, calculate the corrosion rate, and those with the above corrosion rate less than 0.30 g / m ² · hr have excellent intergranular corrosion resistance. (◯), 0.30 g / m ² · hr or more and less than 0.33 g / m ² · hr with good intergranular corrosion resistance (Δ), 0.33 g / m ² · hr or more with grain resistance The interfacial corrosion was evaluated as inferior (x).

The results of the evaluation test are also shown in Table 2.
From this result, the welded duplex stainless steel pipe subjected to the solution heat treatment under the conditions suitable for the present invention with respect to the steel pipe made of the steel sheet having the composition composition suitable for the present invention is all α of the weld heat affected zone. The difference in Cr concentration at the / γ phase interface is less than 1.5 mass%, the intergranular corrosion rate is less than 0.30 g / m ² · hr, and the intergranular corrosion resistance is excellent.
On the other hand, the steel pipe subjected to the solution heat treatment that does not conform to the present invention even if it satisfies the component composition of the present invention has a Cr concentration difference of 1.5 mass% or more at the α / γ phase interface, and the intergranular corrosion rate. Is 0.30 g / m ² · hr or more, and the intergranular corrosion resistance is inferior.
Moreover, even if the composition of the present invention is satisfied, a steel pipe that has been subjected to a solution heat treatment at a temperature lower than that of the present invention produces a σ phase in the metal structure of the heat-affected zone, resulting in decreased intergranular corrosion resistance. There is a tendency.
In addition, a steel pipe not satisfying the composition of the present invention is inferior in corrosion resistance of the steel pipe itself even if it is subjected to a solution heat treatment under conditions suitable for the present invention and the Cr concentration difference ΔCr is less than 1.5 mass%. The field corrosion rate is 0.30 g / m ² · hr or more.

  Since the welded duplex stainless steel pipe of the present invention is excellent in intergranular corrosion resistance, it can be suitably used as a high corrosion resistance steel pipe used in severe corrosive environments other than chemical plants.

Claims (4)

C: 0.05 mass% or less, Si: 0.10 to 1.00 mass%, Mn: 0.3 to 2.0 mass%, P: 0.010 to 0.050 mass%, S: 0.0001 to 0.02 mass %, Al: 0.001 to 0.05 mass%, N: 0.05 to 0.4 mass%, Ni: 4.0 to 9 mass%, Cr: 20.0 to 27 mass%, Mo: 2.0 to 5 mass% Cu: 0.01-0.30 mass%, W: 0.01-0.4 mass%, B: 0.0001-0.001 mass%, and Ca: 0.0006-0.01 mass%, the balance being It has a composition composed of Fe and unavoidable impurities, has no σ phase in the weld heat affected zone, and has a minimum area of 0.5 μm from the α / γ phase interface to the γ phase side in the weld heat affected zone. Same as Cr concentration A welded duplex stainless steel pipe characterized in that the difference in average Cr concentration in the region separated from 0.5 μm from the α / γ phase interface to the γ phase side in the γ phase is less than 1.5 mass%. 2. In addition to the above component composition, the composition further comprises one or two selected from V: 0.003 to 0.5 mass% and Nb: 0.003 to 0.5 mass%. The welded duplex stainless steel pipe described in 1. The welded duplex stainless steel pipe according to claim 1 or 2, wherein a corrosion rate by 70 mass% boiling nitric acid is less than 0.30 g / m ² · hr. When manufacturing the welded duplex stainless steel pipe according to any one of claims 1 to 3, after heating the welded duplex stainless steel pipe manufactured according to a conventional method to a temperature of 1000 to 1100 ° C, a temperature of 800 ° C or higher. A method for producing a welded duplex stainless steel pipe, wherein the solution is cooled to a cooling rate of 3 ° C./s or more and then subjected to a solution heat treatment immediately cooled to 6 ° C./s or more.

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