JP2016035104A - Method for manufacturing stainless steel pipe, and stainless steel pipe - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for manufacturing a stainless steel pipe containing 20 mass% or more Cr, in a cooling step after distortion removal annealing, a Cr-depleted layer is hardly generated on a grain boundary, and as a result, the stainless steel pipe shows excellent intergranular corrosion resistance against acid dew condensation water generated on the outer surface of a steel pipe in an actual machine use environment.SOLUTION: A method for manufacturing a stainless pipe includes: a step of manufacturing a steel pipe using a steel having a component composition of, by mass%, C:0.01-0.10%, Si:0.10 -2.0%, Mn:0.10-2.0%, P:more than 0% and 0.045% or less, S: more than 0% and 0.01% or less, Cr:20-30%, Ni:15-25%, N:0.10-0.35%; B:0.0005-0.05%; Nb:0.10-0.60%, and Ta:0.20-1.00%, and the balance Fe with inevitable impurities; a step of subjecting the steel pipe to a solid solution heat treatment; and a step of subjecting the steel pipe to a re-heating treatment at 800-1,050°C for 5 minutes or more, in this order.SELECTED DRAWING: None

Description

  The present invention relates to a method for manufacturing a stainless steel pipe and a stainless steel pipe. In particular, the present invention relates to an austenitic stainless steel pipe excellent in intergranular corrosion resistance and a method for producing the same.

  In a boiler for thermal power generation using coal or the like as a fuel or a waste boiler for incineration of municipal waste or the like, stainless steel is generally used for a heat transfer tube such as a superheater tube. When stainless steel is oxidized at a high temperature, an oxide scale mainly composed of Cr is generated on the surface, and this acts as a protective film, thereby exhibiting excellent corrosion resistance at a high temperature.

  As an austenitic stainless steel suitable for a heat transfer tube of a boiler for thermal power generation, for example, Patent Document 1 is excellent over a long period of time in a high-temperature environment without adding an expensive metal such as W, Mo, or Cu. An austenitic stainless steel that can exhibit high creep strength has been proposed. Specifically, C: 0.01 to 0.10% by mass, Si: 0.10 to 1.00% by mass, Mn: 0.10 to 2.50% by mass, Ni: 15.0 to 25.0 Mass%, Cr: 20.0-30.0 mass%, Nb: 0.10-0.60 mass%, Ta: 0.20-1.00 mass%, B: 0.0005-0.0050 mass% N: 0.10 to 0.30 mass%, S: 0.0050 mass% or less (excluding 0 mass%) and P: 0.050 mass% or less (not including 0 mass%), An austenitic stainless steel has been proposed in which the balance is iron and inevitable impurities, and the Ta / Nb ratio is in the range of 0.8 to 4.0.

  Patent Document 2 discloses an austenitic stainless steel having excellent toughness after aging heat treatment that simulates an environment exposed to a high temperature environment for a long time by adding a specified amount of Ta. Specifically, Ta: 0.25 to 0.8 mass%, C: 0.01 to 0.15 mass%, Si: 0.1 to 1.0 mass%, Mn: 0.1 to 2.5 mass%, P: 0.05 mass% or less (Excluding 0% by mass), S: 0.005% by mass or less (excluding 0% by mass), Ni: 15-25% by mass, Cr: 20-30% by mass, Nb: 0.1-0. Austenitic stainless steel characterized by containing 8% by mass, B: 0.0005-0.005% by mass, and N: 0.10-0.35% by mass, the balance being iron and inevitable impurities Has been proposed.

  Further, Patent Document 3 discloses a high-strength austenitic heat-resistant steel that is excellent in resistance to embrittlement cracking of welds. Specifically, C: 0.04 to 0.18%, Si ≦ 1.5%, Mn ≦ 2.0%, Ni: 6 to 30%, Cr: 15 to 30%, N: 0.03 to 0 .35%, sol. In addition to Al ≦ 0.03%, Nb ≦ 1.0%, V ≦ 0.5%, and Ti ≦ 0.5%, and the balance is Fe and impurities. ≦ 0.04%, S ≦ 0.03%, Sn ≦ 0.1%, As ≦ 0.01%, Zn ≦ 0.01%, Pb ≦ 0.01% and Sb ≦ 0.01%, P1 = S + {(P + Sn) / 2} + {(As + Zn + Pb + Sb) / 5} ≦ 0.06 and 0.2 ≦ Nb + 2 (V + Ti) ≦ 1.7−10 × P1 has high strength. Yes, it has excellent resistance to embrittlement cracking of welds during use at high temperatures, and the austenitic stainless heat-resisting steel is suitable as a material for equipment used for a long time at high temperatures such as boilers for power generation Is described.

  By the way, the austenitic stainless steel pipe used as the heat transfer pipe for the thermal power generation boiler is bent and assembled into a panel shape when used in an actual machine. After the bending process, strain relief annealing is often performed in order to remove the residual stress generated by the bending process. Hereinafter, the strain relief annealing after bending may be simply referred to as “strain relief annealing”. Since the cooling after the strain relief annealing is slow such as furnace cooling, the stainless steel pipe can stay in a temperature range of 500 to 750 ° C. for a long time. However, in this temperature range, Cr carbide tends to precipitate at the grain boundaries. If a lot of Cr carbide precipitates at the grain boundaries during the cooling process after this strain relief annealing, the metallographic structure of the outer surface of the stainless steel pipe is the amount of Cr carbide precipitated at the grain boundaries. In this state, Cr is deficient in contributing to the state, that is, a state of advanced sensitization.

In an actual machine such as the above-mentioned thermal power generation boiler, the start and stop of equipment, that is, heating and cooling are repeated in an atmosphere of corrosive gas such as HCl gas. When stopped and cooled, condensation occurs on the outer surface of the stainless steel pipe, and the condensed water such as HCl gas or SO ₂ gas dissolves in the condensed water, so that acid condensation water such as hydrochloric acid or sulfuric acid adheres to the outer surface of the stainless steel pipe It becomes. When the metal structure of the stainless steel pipe is a state in which many Cr-deficient layers are formed as described above, that is, when the sensitization is advanced, the Cr-deficient layer part is easily corroded and the intergranular corrosion resistance is reduced. is there. Conventionally, in an austenitic stainless steel having a Cr content of 20% by mass or more, intergranular corrosion resistance has been ensured by including the amount of Cr. However, in recent years, an austenitic stainless steel having a Cr content of 20% by mass or more has been required to have superior intergranular corrosion resistance.

Japanese Patent No. 5547825 JP 2004-88593 A Japanese Patent No. 4258678

  The present invention has been made paying attention to the above-mentioned circumstances, and its purpose is to contain Cr in an amount of 20% by mass or more, and a Cr-deficient layer is formed at the grain boundary in the cooling process after performing strain relief annealing. As a result, it is an object of the present invention to provide a stainless steel pipe that exhibits superior intergranular corrosion resistance to acidic dew condensation water generated on the outer surface of the steel pipe in an actual machine use environment, and a method for producing the same.

  The manufacturing method of the stainless steel pipe of the present invention that has solved the above problems is that the component composition is mass%, C: 0.01 to 0.10%, Si: 0.10 to 2.0%, Mn: 0 .10 to 2.0%, P: more than 0% to 0.045% or less, S: more than 0% to 0.01% or less, Cr: 20 to 30%, Ni: 15 to 25%, N: 0.10 Steel that satisfies 0.35%, B: 0.0005 to 0.05%, Nb: 0.10 to 0.60%, and Ta: 0.20 to 1.00%, with the balance being iron and inevitable impurities And a step of performing a solution heat treatment; and a step of performing a reheat treatment at 800 to 1050 ° C. for 5 minutes or more in this order. Hereinafter, “%” in the component composition means “% by mass”.

  The manufacturing method may further include a step of bending after the step of performing the reheat treatment; and a step of performing strain relief annealing at 800 to 1050 ° C. for 5 minutes or more in this order.

  Another method for producing the stainless steel pipe of the present invention that has solved the above problems is that the component composition is mass%, C: 0.01 to 0.10%, Si: 0.10 to 2.0%, Mn : 0.10 to 2.0%, P: more than 0% to 0.045% or less, S: more than 0% to 0.01% or less, Cr: 20 to 30%, Ni: 15 to 25%, N: 0.00. 10 to 0.35%, B: 0.0005 to 0.05%, Nb: 0.10 to 0.60%, and Ta: 0.20 to 1.00%, with the balance being iron and inevitable impurities A feature is that it includes a step of manufacturing a steel pipe using a certain steel; a step of performing a solution heat treatment; a step of bending; and a step of performing strain relief annealing at 800 to 1050 ° C. for 5 minutes or more in this order. Have.

  As the steel, it is preferable to use steel satisfying a Ta / Nb mass ratio Ta / Nb of 0.8 or more and 4.0 or less.

  Further, as the steel, a steel containing at least one of Ca: more than 0% and 0.005% or less and Mg: more than 0% and 0.005% or less may be used.

  As the steel, steel containing at least one of Mo: more than 0% and 4.0% or less and W: more than 0% and 4.0% or less may be used.

  In the step of manufacturing the steel pipe, a seamless steel pipe may be manufactured.

In addition, the stainless steel pipe of the present invention that can solve the above-mentioned problems has a component composition of mass%, C: 0.01 to 0.10%, Si: 0.10 to 2.0%, Mn: 0.10. -2.0%, P: more than 0% and 0.045% or less, S: more than 0% and 0.01% or less, Cr: 20-30%, Ni: 15-25%, N: 0.10-0. 35%, B: 0.0005 to 0.05%, Nb: 0.10 to 0.60%, and Ta: 0.20 to 1.00%, with the balance being iron and inevitable impurities, and It is characterized in that there are 5 or more precipitates having a circle equivalent diameter of 10 nm or more containing at least one of Nb and Ta together with C per 1 μm ² .

  In the stainless steel pipe, the mass ratio Ta / Nb between Ta and Nb preferably satisfies 0.8 or more and 4.0 or less.

  The stainless steel pipe may further include at least one of Ca: more than 0% and 0.005% or less, and Mg: more than 0% and 0.005% or less.

  The stainless steel pipe may further include at least one of Mo: more than 0% and 4.0% or less and W: more than 0% and 4.0% or less.

  The stainless steel pipe may be a seamless steel pipe.

  The stainless steel pipe is preferably used for a heat transfer pipe of a thermal power generation boiler, a waste boiler or a biomass boiler.

  In the stainless steel pipe having a Cr content of 20 to 30% obtained by the production method of the present invention, the formation of Cr carbide, in other words, the formation of a Cr-deficient layer in the vicinity of the grain boundary, is performed in the cooling process after the strain relief annealing. It is suppressed. As a result, it exhibits superior intergranular corrosion resistance to acidic dew condensation water generated on the outer surface of stainless steel pipes in the actual machine environment.

FIG. 1 is a diagram illustrating a sampling position for measuring a grain boundary erosion depth in an example. 2 shows the steel No. of Table 2. 2 is an optical micrograph of 2. 3 shows the steel material No. 9 is an optical micrograph of 9.

  The inventors of the present invention have made extensive studies to solve the above problems. Specifically, for the purpose of improving the intergranular corrosion resistance of an austenitic stainless steel pipe having a Cr content of 20 to 30%, as described above, a steel pipe capable of suppressing the formation of Cr carbide during the cooling process after strain relief annealing. Earnestly researched to obtain

  As a result, as the component composition, steel having a Cr content of 20 to 30% and containing a certain amount of Ta and Nb was used, and in the stainless steel pipe manufacturing process, steel pipe manufacturing and solution heat treatment were sequentially performed. It was found that additional heat treatment under certain conditions is very effective in improving intergranular corrosion resistance. Hereinafter, this additional heat treatment is referred to as “reheat treatment”. In the following, reheat treatment is mainly described as this additional heat treatment, but as described in detail later, in the present invention, instead of the reheat treatment or together with the reheat treatment, strain relief annealing after bending is performed. The aspect implemented on the conditions similar to the said reheat processing is also contained.

  Although the effect of the additional heat treatment performed under certain conditions such as the above-mentioned reheat treatment has not been fully elucidated, it can be considered as follows. That is, it is considered that by performing the reheat treatment or the like, C was fixed as a carbon-containing precipitate by the Ta and Nb, and the formation of Cr carbide was suppressed in the cooling process after the strain relief annealing. As described in detail later, the carbon-containing precipitate is a precipitate having a circle equivalent diameter of 10 nm or more including C and at least one of Nb and Ta. Hereinafter, this precipitate is sometimes referred to as “(Ta, Nb) carbon-containing precipitate”.

  According to this mechanism of action, (Ta, Nb) carbon-containing precipitates are pre-deposited before use in an actual machine, so that fine Nb that contributes to an improvement in creep strength when used in an actual machine, that is, at a high temperature. It appears that the amount of carbide precipitation decreases. However, since the steel used in the present invention contains Ta that contributes to the improvement of the creep strength together with Nb, even if the (Ta, Nb) carbon-containing precipitates are precipitated in advance, the influence on the creep characteristics is small. Therefore, according to the present invention, the intergranular corrosion resistance can be improved while ensuring excellent creep characteristics.

  The reason why the production method is defined in the present invention will be described from the component composition of steel used for steel pipe production. The following component composition is also a component composition of the stainless steel pipe obtained by this manufacturing method.

C: 0.01 to 0.10%
C is an element that precipitates carbides such as M ₂₃ C ₆ in a high temperature environment and improves the creep strength. In order to obtain this effect, the C content needs to be 0.01% or more. The amount of C is preferably 0.02% or more, more preferably 0.03% or more. On the other hand, if the amount of C exceeds 0.10%, the carbon-containing precipitates tend to aggregate due to long-term use of the steel pipe, resulting in a decrease in strength. Therefore, the C amount is 0.10% or less. The amount of C is preferably 0.09% or less, and more preferably 0.08% or less.

Si: 0.10 to 2.0%
Si is an element that combines with oxygen in steel and improves the cleanliness of the steel. In order to obtain this effect, 0.10% or more of Si is necessary. The amount of Si is preferably 0.20% or more, more preferably 0.30% or more. However, Si is an element that promotes precipitation of the σ phase that causes embrittlement, and when the amount of Si exceeds 2.0%, toughness is reduced. Therefore, the Si amount is 2.0% or less. The amount of Si is preferably 1.60% or less, more preferably 1.40% or less.

Mn: 0.10 to 2.0%
Mn is an element that exhibits a desulfurization action and contributes to improvement of hot workability. It is also an element that exhibits the effect of stabilizing the austenite phase. In order to exhibit these effects, the amount of Mn is made 0.10% or more. The amount of Mn is preferably 0.50% or more, more preferably 0.80% or more. However, if the amount of Mn exceeds 2.0% and is contained excessively, the formation of intermetallic compounds is promoted to cause embrittlement. Therefore, the Mn content is 2.0% or less. The amount of Mn is preferably 1.90% or less, more preferably 1.80% or less.

P: more than 0% and 0.045% or less P is an element mixed as an inevitable impurity, and if it is contained excessively, weldability deteriorates. Therefore, the P content is suppressed to 0.045% or less. The amount of P is preferably 0.035% or less, more preferably 0.030% or less. The smaller the amount of P, the better, but it is difficult to make it zero, so the lower limit is more than 0%.

S: more than 0% and 0.01% or less S, like P, is an element mixed as an inevitable impurity. When S is contained excessively, hot workability deteriorates. Therefore, the upper limit of the S amount is set to 0.01% or less. The amount of S is preferably 0.008% or less, more preferably 0.005% or less, and still more preferably 0.003% or less. The smaller the amount of S, the better. However, since it is difficult to make it zero, the lower limit is more than 0%.

Cr: 20-30%
Cr is an element that forms a dense Cr ₂ O ₃ film on the surface of a steel pipe at a high temperature to improve the steam oxidation resistance and the high temperature corrosion resistance. In order to obtain this effect, it is necessary to contain 20% or more of Cr. The amount of Cr is preferably 22% or more, more preferably 24% or more. However, if the Cr content exceeds 30% and is excessively contained, precipitation of the σ phase that causes embrittlement is promoted, leading to a decrease in toughness. Therefore, the Cr amount is 30% or less. The Cr amount is preferably 28% or less, more preferably 26% or less.

Ni: 15-25%
Ni is an element having an effect of stabilizing the austenite phase. In order to maintain the austenite phase, 15% or more of Ni is necessary. The amount of Ni is preferably 17% or more, more preferably 19.0% or more. On the other hand, if the Ni amount is excessive, the cost is increased, so the Ni amount is 25% or less. The amount of Ni is preferably 24% or less, more preferably 23% or less.

N: 0.10 to 0.35%
N is an element that improves the high-temperature strength by dissolving in the matrix. In addition, it is an element that combines with nitride-forming elements such as Nb and Ta to form fine precipitates and improves creep strength during use at high temperatures in an actual machine. In order to sufficiently exhibit these effects, the N content is set to 0.10% or more. The N amount is preferably 0.15% or more, more preferably 0.20% or more. However, if the N content exceeds 0.35%, the hot workability deteriorates, so the N content is set to 0.35% or less. The N amount is preferably 0.33% or less, more preferably 0.30% or less.

B: 0.0005 to 0.05%
B is an element that improves the high-temperature strength by solid solution strengthening. In order to sufficiently exhibit the effect, the B content is set to 0.0005% or more. The amount of B is preferably 0.0010% or more, more preferably 0.0015% or more. On the other hand, if the amount of B exceeds 0.05%, weldability is impaired. Therefore, the B amount is 0.05% or less. The amount of B is preferably 0.03% or less, more preferably 0.02% or less.

Nb: 0.10 to 0.60%
Nb is an element necessary for the formation of (Ta, Nb) carbon-containing precipitates in the later-described reheat treatment and prescribed strain relief annealing. Nb is an element that forms a carbide, nitride, or Z phase (CrNbN) during use at a high temperature in an actual machine to improve the creep strength. In order to sufficiently precipitate the carbides and improve the creep strength, the Nb amount needs to be 0.10% or more. The Nb amount is preferably 0.15% or more, more preferably 0.20% or more. On the other hand, if the Nb content exceeds 0.60% and is excessively contained, precipitates aggregate when used for a long time in an actual machine, and the strength tends to decrease. Therefore, the Nb amount is 0.60% or less. The Nb amount is preferably 0.50% or less, more preferably 0.40% or less.

Ta: 0.20 to 1.00%
Ta, like Nb, is an element necessary for the formation of (Ta, Nb) carbon-containing precipitates in later-described reheat treatment and prescribed strain relief annealing. Ta, like Nb, is an element that improves the creep strength by forming carbides and nitrides during use at high temperatures. Furthermore, Ta has the effect of increasing the creep strength by dissolving in the Z phase (CrNbN). In order to sufficiently exhibit these effects, the Ta amount is set to 0.20% or more. The amount of Ta is preferably 0.23% or more, more preferably 0.25% or more. On the other hand, if the amount of Ta exceeds 1.00% and is excessively contained, precipitates aggregate when used for a long time in an actual machine, and the strength tends to decrease. Therefore, the Ta amount is 1.00% or less. The amount of Ta is preferably 0.80% or less, more preferably 0.60% or less.

Ta / Nb: 0.8 to 4.0
By controlling Ta / Nb, which is the mass ratio of Ta and Nb in the steel, within a predetermined range, the amount of Ta solid solution in the Z phase is optimized, and the creep strength can be improved. In order to secure the Ta solid solution amount and improve the creep strength, the Ta / Nb is preferably 0.8 or more, more preferably 0.85 or more. On the other hand, if Ta / Nb exceeds 4.0, the amount of Ta solid solution becomes excessive, the ductility is lowered, and the economy is impaired. Therefore, the Ta / Nb is preferably 4.0 or less, and more preferably 3.5 or less.

  The stainless steel used in the production method of the present invention satisfies the above component composition, and the balance consists of iron and inevitable impurities. In addition to the above elements, the high temperature strength and the like can be increased by adding an appropriate amount of the following Ca and the like. Hereinafter, these elements will be described in detail.

Ca: more than 0% to 0.005% or less, and Mg: more than 0% to 0.005% or less Ca and Mg act as desulfurization elements and contribute to improving hot workability by fixing S It is. One or two of these elements can be used. In order to obtain this effect, the lower limit of each content is preferably 0.0002% or more, and more preferably 0.0005% or more, when Ca or Mg is contained. However, when the amount of Ca or Mg is excessive, the hot workability tends to decrease. Therefore, it is preferable that the upper limit of Ca content and Mg content is 0.005% or less. A more preferable upper limit is 0.0020% or less.

Mo: more than 0% to 4.0% or less, and W: more than 0% to 4.0% or less Mo and W are elements that improve high temperature strength by solid solution strengthening. One or two of these elements can be used. In order to exhibit this effect, when Mo is contained, the amount of Mo is preferably 0.4% or more, and more preferably 0.8% or more. Moreover, when it contains W, it is preferable to make W amount into 0.6% or more, More preferably, it is 0.8% or more. However, since Mo and W are expensive elements, excessive addition causes high cost. Therefore, the upper limit of the Mo amount and the W amount is set to 4.0% or less. Preferably, both are 3.0% or less, and more preferably, both are 2.0% or less.

  The production method of the present invention includes a step of producing a steel pipe using steel satisfying the above component composition; a step of performing a solution heat treatment; and a step of performing a reheat treatment at 800 to 1050 ° C. for 5 minutes or more in this order. Hereinafter, each step will be described. In the production method of the present invention, instead of the above-mentioned reheat treatment, a mode in which strain relief annealing is performed under the same conditions as the above-mentioned reheat treatment after passing through a solution heat treatment step and a bending step in this order; And an embodiment in which strain relief annealing is performed under the same conditions as in the reheat treatment after the bending process steps are performed in this order. This point will be described in detail later.

[Process for manufacturing steel pipes]
For example, in the melting furnace such as an electric furnace, the alloy having the component composition described above is melted and cast into a steel ingot. A solution heat treatment may be performed on the steel ingot, or hot working or the like may be performed. Next, a steel pipe is manufactured using the obtained steel ingot. Examples of steel pipes include seamless steel pipes, ERW steel pipes, arc-welded steel pipes such as UOE steel pipes and spiral steel pipes, and forged steel pipes. A seamless steel pipe is preferable. These steel pipes can be manufactured by a generally used method. The seamless steel pipe can be manufactured by a hot extrusion method or a Mannesmann method. For the purpose of adjusting the size, for example, the steel pipe may be further subjected to cold working such as drawing or rolling, and heat treatment for softening in performing the cold working.

[Step of solution heat treatment]
A solution heat treatment is performed on the steel pipe. As a result, precipitates such as carbides and nitrides can be sufficiently dissolved once to make the structure uniform. The conditions for the solution heat treatment depend on the steel component composition. For example, the solution heat treatment temperature is preferably 1100 ° C. or higher, more preferably 1150 ° C. or higher, preferably 1250 ° C. or lower, more preferably 1230 ° C. or lower. can do. Further, the holding time at the solution heat treatment temperature may be, for example, 2 minutes or more, although it depends on the size of the steel pipe. Note that the upper limit of the holding time is approximately 1 hour or less from the viewpoint of equipment and productivity. After the solution heat treatment, rapid cooling to room temperature by a method such as water cooling can be mentioned.

[Step of performing reheat treatment at 800 to 1050 ° C. for 5 minutes or more]
The purpose of this heat treatment is, as described above, by depositing part of Ta and Nb in the steel as (Ta, Nb) carbon-containing precipitates and fixing C, so that Cr forms carbides in the subsequent process. The purpose is to suppress a Cr-deficient layer from being formed in the vicinity of the crystal grain boundary, thereby reducing the intergranular corrosion resistance. In order to acquire this effect, in this invention, it heats at the reheat processing temperature of 800 degreeC or more. The reheat treatment temperature is preferably 810 ° C. or higher, more preferably 820 ° C. or higher, and further preferably 830 ° C. or higher. On the other hand, even if the reheat treatment temperature is too high, the (Ta, Nb) carbon-containing precipitates are not easily formed, so the upper limit of the reheat treatment temperature is 1050 ° C. or less. The reheat treatment temperature is preferably 1000 ° C. or lower, more preferably 950 ° C. or lower.

  If the holding time at the reheat treatment temperature, that is, the reheat time is too short, the formation of the (Ta, Nb) carbon-containing precipitates is insufficient, and the intergranular corrosion resistance cannot be sufficiently improved. Therefore, the reheating time is 5 minutes or more. The reheat treatment time is preferably 10 minutes or more, more preferably 15 minutes or more. The upper limit of the reheat treatment time is not particularly limited from the viewpoint of forming the (Ta, Nb) carbon-containing precipitate. However, in consideration of productivity and the like, the upper limit of the reheat treatment time is preferably 90 minutes or less, more preferably 70 minutes or less, and still more preferably 60 minutes or less.

  The following method is mentioned as the implementation pattern of the reheat treatment. First, after performing the solution heat treatment, after cooling to near room temperature, it is heated from room temperature to the reheat temperature and held at the reheat temperature. Considering that Cr carbide is formed in the vicinity of about 700 ° C., the average heating rate between at least 500 ° C. and 800 ° C. is 0.2 ° C./s or more in the heating step from the room temperature to the reheat treatment temperature. It is preferable. As another method, there is a method in which reheat treatment is performed subsequent to the solution heat treatment, that is, after the solution heat treatment, the solution is cooled from the solution heat treatment temperature to the reheat treatment temperature and then held at the reheat treatment temperature. The cooling means from the solution heat treatment temperature to the reheat treatment temperature is not particularly limited.

  “Holding at the reheat treatment temperature” in this reheat treatment may include not only soaking, but also a case where the temperature fluctuates within the reheat treatment temperature range, that is, the temperature rises or falls.

  In the reheat treatment, it is important to precipitate a (Ta, Nb) carbon-containing precipitate, and the cooling method to room temperature after heating under the above conditions is not particularly specified. After the reheat treatment, it may be cooled to room temperature by a method such as water cooling or air cooling.

  In the production method of the present invention, instead of the above-mentioned reheat treatment, after undergoing a solution heat treatment step and a bending step in this order, an embodiment of performing strain relief annealing by heating at 800 to 1050 ° C. for 5 minutes or more is performed in the same manner as in the above reheat treatment Or, after passing through the reheat treatment and the bending process in this order, a strain relief annealing is performed in which heating is performed at 800 to 1050 ° C. for 5 minutes or more in the same manner as the reheat treatment. Even when the strain relief annealing is performed under these conditions, a part of Ta and Nb in the steel is precipitated as (Ta, Nb) carbon-containing precipitates and the C is fixed in the same manner as in the reheat treatment. Precipitation of Cr carbide at grain boundaries can be suppressed in the cooling process after the annealing.

  The explanation and preferred conditions of the temperature for this strain relief annealing, the holding time at that temperature, the execution pattern, etc. are the same as those for the above-mentioned re-heat treatment. In addition, it may include a case where the temperature fluctuates within a specified temperature range, that is, the temperature rises or falls. In this strain relief annealing, it is important to deposit (Ta, Nb) carbon-containing precipitates, and the method for cooling to room temperature after heating under specified conditions is not particularly specified. What is necessary is just to cool by methods, such as water cooling and air cooling, to room temperature after heating on regular conditions.

As described above, the following methods (I) to (III) are exemplified as the method for producing the stainless steel pipe of the present invention.
(I) A step of manufacturing a steel pipe; a step of performing a solution heat treatment; and a step of performing a reheat treatment at 800 to 1050 ° C. for 5 minutes or more; in this order (II) a step of manufacturing a steel pipe; a solution heat treatment A step of performing bending processing; and a step of performing strain relief annealing at 800 to 1050 ° C. for 5 minutes or more; a step of producing an aspect (III) steel pipe in this order; a step of performing solution heat treatment; 800 to 1050 An embodiment comprising: a step of performing a reheat treatment at 5 ° C. for 5 minutes or more; a step of bending; and a step of performing strain relief annealing at 800 to 1050 ° C. for 5 minutes or more.

  In the above aspect (I), the steps after the reheat treatment are not limited. Therefore, even when bending and strain relief annealing are performed after the reheat treatment, general conditions may be adopted for the strain relief annealing. Moreover, when performing a bending process, performing correction, pickling, etc. is mentioned as needed. In the above (III) mode, reheating is performed and heating is performed under specified conditions even in strain relief annealing after bending, but both heating conditions may be the same or different as long as they are within a predetermined range. Also good. In any of the above aspects (II) and (III), correction, pickling, and the like may be further performed as necessary when subjected to bending.

  As described above, the stainless steel pipe of the present invention is heated at 800 to 1050 ° C. for 5 minutes or more in at least one of the above-described reheat treatment and strain relief annealing. Is deposited as a (Ta, Nb) carbon-containing precipitate. As a form of the stainless steel pipe of the present invention, the one manufactured in the above aspect (I) includes a straight pipe before bending, and a steel pipe which has been bent and has a bent portion. A steel pipe having a bent portion can be cited as one produced in the mode (III).

The stainless steel pipe of the present invention produced by the above method satisfies the above-described component composition, and is a precipitate having a circle equivalent diameter of 10 nm or more containing C and at least one of Nb and Ta, that is, (Ta, Nb ) It is characterized in that there are 5 or more carbon-containing precipitates per 1 μm ² . As described above, the precipitate is formed and C is fixed, so that the formation of Cr carbide is suppressed in the cooling process after the strain relief annealing, that is, the Cr-deficient layer is hardly generated. It is considered that the intergranular corrosion resistance is superior to that of acid condensation on the outer surface of steel pipe.

In order to obtain the above effect, it is necessary that five or more (Ta, Nb) carbon-containing precipitates exist per 1 μm ² . The (Ta, Nb) carbon-containing precipitates are preferably 8.0 or more per 1 μm ² , more preferably 10.0 or more per 1 μm ² . The larger the number, the better. However, the upper limit is about 100 per 1 μm ² in consideration of the component composition and production conditions specified in the present invention.

  The observation can be performed using a transmission electron microscope as shown in the examples described later. The (Ta, Nb) carbon-containing precipitates have a circle equivalent diameter of 10 nm or more.

  The (Ta, Nb) carbon-containing precipitates include carbides containing Ta and Nb, carbides containing Ta and Nb, and carbon / nitrides containing at least one of these elements. Preferably, the precipitate contains both Ta and Nb. The (Ta, Nb) carbon-containing precipitates include composite precipitates that further include carbides, nitrides, and carbonitride-forming elements such as B and Al in addition to the carbides or carbonitrides.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, but may be appropriately modified within a range that can meet the purpose described above and below. Of course, it is possible to implement them, and they are all included in the technical scope of the present invention.

(1) Preparation of sample for reheat treatment In 1-3, 9, 15, and 16, the steel ingot of the component composition shown in the following Table 1 was melted using VOD (Vacuum Oxygen Decarburization) to produce an ingot of 530 mm × 530 mm × L 1500 mm. Thereafter, solution heat treatment was performed at 1250 ° C. or more for 24 hours or more, and a bloom having a diameter of 160 mm was produced by hot rolling. Using this bloom, a hollow billet having a wall thickness of 48 mm, an outer diameter of 155 mm, and a length of 570 mm was produced, and then using this hollow billet, the wall thickness was about 8.5 mm and the outer diameter was 71. A 5 mm steel pipe was produced. This steel pipe was heat treated at 1230 ° C. for 5 minutes for the purpose of softening, and then cold worked to obtain a steel pipe having a wall thickness of 7.6 mm and an outer diameter of 63.5 mm. Thereafter, a solution heat treatment was performed at 1220 ° C. for 5 minutes, followed by water cooling to obtain a sample. In Table 1, “-” means that no element is added.

  In Table 2 below, the steel material No. In 4-8, 10-14, and 17, 20 kg small ingots having the component compositions shown in Table 1 below were produced by vacuum induction melting, and samples were produced by a thermomechanical process simulating an actual machine. Specifically, the ingot obtained by the above-described vacuum induction melting was subjected to heat treatment at 1250 ° C. for 24 hours or more, and then formed into a plate shape by hot forging while simulating hot extrusion in the pipe making process. Thereafter, heat treatment was performed at 1230 ° C. for 5 minutes or more for the purpose of softening. Next, it was cut into a size of approximately W80 mm × L200 mm × t17.5 mm and cold-rolled at a processing rate of 20% to obtain a size of W80 mm × L250 mm × t14 mm. Thereafter, a solution heat treatment was performed at 1220 ° C. for 5 minutes, followed by water cooling to obtain a sample.

(2) Reheat treatment A plate material for reheat treatment of W15 mm × L70 mm × t3 mm was cut out from each sample so that the rolling direction was the longitudinal direction.

  The plate for reheat treatment was reheated in an electric furnace under the conditions shown in Table 2 (reheat treatment temperature, reheat treatment time), and cooled to room temperature to obtain a plate sample. In addition, the steel material No. Nos. 9 to 14 are comparative examples in which re-heat treatment is not performed.

  Using the plate-like sample obtained by the above method, an evaluation test was conducted as follows to evaluate the intergranular corrosion resistance. Moreover, the precipitate in steel was also evaluated.

(3) Intergranular corrosion resistance evaluation test Intergranular corrosion resistance was evaluated by a sulfuric acid / copper sulfate corrosion test method for stainless steel specified in JIS G 0575. Specifically, first, the plate-like sample was subjected to a heat treatment that was held at 700 ° C. for 1 hour as a sensitizing heat treatment corresponding to the cooling process after the strain relief annealing described above, and then cooled with water. In addition, since the bending process performed when using with an actual machine does not affect especially intergranular corrosion resistance, it was not performed in the present Example. Next, the oxide scale formed on the plate-like sample surface was removed by pickling and mirror-finished by mechanical polishing. As an accelerated test simulating deterioration due to acidic dew condensation water on the outer surface of a stainless steel pipe in an actual machine environment, the mirror-finished plate sample was immersed in a boiling sulfuric acid-copper sulfate solution for 16 hours. Thereafter, the central part of the plate-like sample was bent at 90 °, and the outer apex of the bent surface was observed with a stereomicroscope at a magnification of 15 times. And it evaluated that the thing which a crack generate | occur | produced is inferior to intergranular corrosion resistance, and the thing which did not generate | occur | produce a crack evaluated that it was excellent in intergranular corrosion resistance. The reason why the central portion of the plate-like sample is bent at 90 ° is that stress is applied to the corroded grain boundary for easy observation, and does not simulate bending in actual operation. The evaluation results are shown in Table 2.

  Furthermore, the grain boundary erosion depth was measured as follows using the plate-like sample bent at 90 °. First, as shown in FIG. 1, the steel piece in the hatched portion of FIG. 1 is cut out by a cutting machine from the end face portion to the position of 5 to 10 mm, which is not affected by the bending of the plate-like sample at 90 °. After being embedded in a resin so that the cross section A could be observed, it was polished to a mirror surface. Thereafter, the cross section A was observed at a magnification of 50 times with an optical microscope. And the depth (corrosion depth) from the said boundary line to the corroding grain boundary of a total of 10 points | pieces at 100 micrometer intervals in the boundary line of the base material which is a sample, and resin was measured. The average of the 10 corrosion depths was defined as “grain boundary erosion depth”.

  An example of the optical micrograph is shown in FIGS. FIG. 2 shows a steel material No. in Table 2 below which is an example of the present invention. 2 is an optical micrograph of No. 2 and FIG. 9 is an optical micrograph of 9. When these photographs are compared, the steel No. shown in FIG. No. 2 shows no corrosion of grain boundaries, whereas steel No. 2 shown in FIG. 9 shows that the grain boundary is corroded.

(4) Evaluation of precipitate No. in Table 2 For 2, 7, 8, 9, 13, 14, 16 and 17, the (Ta, Nb) carbon-containing precipitates were also evaluated. Details are as follows. Using a test piece prepared by a metal thin film method, a transmission electron microscope (TEM) is used to observe an observation magnification of 150,000 times, an observation field size of 800 nm × 680 nm, an observation field of five fields, and by image analysis. The number of precipitates having a circle equivalent diameter of 10 nm or more containing C and at least one of Nb and Ta together with C was determined. And the number of the said precipitate which exists per 1 micrometer ² was computed. The inclusion of C and at least one of Nb and Ta was determined by EDX (Energy Dispersive X-ray spectroscopy). The results are shown in Table 3.

  The following can be understood from Tables 1 to 3. Steel No. Since Nos. 1 to 8 were manufactured under specified conditions using steel having a specified component composition in the present invention, stainless steel pipes having excellent intergranular corrosion resistance were obtained. As described above, if the steel having the specified composition is used in the present invention and manufactured under the specified conditions, the steel Nos. As shown in 2, 7 and 8, the prescribed precipitates could be secured above a certain level. When these precipitates are formed and C is fixed, the formation of Cr carbide is suppressed, that is, a Cr-deficient layer is hardly formed, and it is considered that the intergranular corrosion resistance superior to that of the prior art is obtained.

On the other hand, steel material No. Nos. 9 to 16 used steel having a specified component composition in the present invention, but were not manufactured under specified conditions, so that the obtained stainless steel pipes were inferior in intergranular corrosion resistance. Specifically, the steel material No. Nos. 9 to 14 were not subjected to the re-heat treatment specified in the present invention after the solution heat treatment and were not subjected to the specified strain relief annealing, resulting in poor intergranular corrosion resistance. Steel No. in Table 3 As shown in 9, 13, and 14, when the above-mentioned reheat treatment was not performed and the specified strain relief annealing was not performed, the specified precipitate was zero. Steel No. Nos. 15 and 16 were subjected to the reheat treatment specified in the present invention. No. 15 has a low reheat treatment temperature, and steel no. In No. 16, the re-heat treatment time was too short, so that the prescribed precipitates were insufficient as shown in Table 3, and the formation of Cr carbide could not be suppressed. As a result, in these examples, the intergranular corrosion resistance was inferior. Furthermore, the steel material No. No. 17 is an example that does not satisfy the prescribed component composition in the present invention. Specifically, the steel material contains only Nb of Nb and Ta and does not contain Ta. In the case of this example, the number of the precipitates per 1 μm ² was insufficient, resulting in inferior intergranular corrosion resistance.

  The stainless steel pipe obtained by the production method of the present invention is a heat transfer pipe such as a superheater pipe or a reheater pipe in a boiler for thermal power generation, a waste boiler, or a biomass boiler that requires excellent intergranular corrosion resistance. Is preferably used.

Claims (13)

Ingredient composition is mass%,
C: 0.01-0.10%,
Si: 0.10 to 2.0%,
Mn: 0.10 to 2.0%,
P: more than 0% and 0.045% or less,
S: more than 0% and 0.01% or less,
Cr: 20-30%,
Ni: 15-25%,
N: 0.10 to 0.35%,
B: 0.0005 to 0.05%,
Nb: 0.10-0.60% and Ta: 0.20-1.00%
A steel pipe is manufactured using steel satisfying the above and the balance being iron and inevitable impurities;
A step of performing a solution heat treatment; and a step of performing a reheat treatment at 800 to 1050 ° C. for 5 minutes or more;
In this order, a method for producing a stainless steel pipe. After the step of performing the reheat treatment,
A step of bending; and a step of performing strain relief annealing at 800 to 1050 ° C. for 5 minutes or more;
The manufacturing method of Claim 1 which contains these in this order. Ingredient composition is mass%,
C: 0.01-0.10%,
Si: 0.10 to 2.0%,
Mn: 0.10 to 2.0%,
P: more than 0% and 0.045% or less,
S: more than 0% and 0.01% or less,
Cr: 20-30%,
Ni: 15-25%,
N: 0.10 to 0.35%,
B: 0.0005 to 0.05%,
Nb: 0.10-0.60% and Ta: 0.20-1.00%
A steel pipe is manufactured using steel satisfying the above and the balance being iron and inevitable impurities;
Performing a solution heat treatment;
A step of bending; and a step of performing strain relief annealing at 800 to 1050 ° C. for 5 minutes or more;
In this order, a method for producing a stainless steel pipe.   The manufacturing method in any one of Claims 1-3 using the steel with which mass ratio Ta / Nb of said Ta and said Nb satisfy | fills 0.8 or more and 4.0 or less.   Furthermore, the manufacturing method in any one of Claims 1-4 which uses the steel containing at least 1 sort (s) of Ca: more than 0% 0.005% or less and Mg: more than 0% 0.005% or less by mass%. .   Furthermore, the manufacturing method in any one of Claims 1-5 using the steel containing at least 1 sort (s) of Mo: more than 0% and 4.0% or less and W: more than 0% and 4.0% or less. .   The manufacturing method according to claim 1, wherein a seamless steel pipe is manufactured in the process of manufacturing the steel pipe. Ingredient composition is mass%,
C: 0.01-0.10%,
Si: 0.10 to 2.0%,
Mn: 0.10 to 2.0%,
P: more than 0% and 0.045% or less,
S: more than 0% and 0.01% or less,
Cr: 20-30%,
Ni: 15-25%,
N: 0.10 to 0.35%,
B: 0.0005 to 0.05%,
Nb: 0.10-0.60% and Ta: 0.20-1.00%
And the balance is iron and inevitable impurities, and there are 5 or more precipitates per 1 μm ² having a circle equivalent diameter of 10 nm or more including C and at least one of Nb and Ta. Stainless steel pipe to do.   The stainless steel pipe according to claim 8, wherein a mass ratio Ta / Nb of the Ta and the Nb satisfies 0.8 or more and 4.0 or less.   The stainless steel pipe according to claim 8 or 9, further comprising at least one of Ca: more than 0% and not more than 0.005% and Mg: more than 0% and not more than 0.005%.   The stainless steel pipe according to any one of claims 8 to 10, further comprising at least one of Mo: more than 0% and not more than 4.0% and W: more than 0% and not more than 4.0%.   The stainless steel pipe according to any one of claims 8 to 11, which is a seamless steel pipe.   The stainless steel pipe according to any one of claims 8 to 12, which is used for a heat transfer pipe of a thermal power generation boiler, a waste boiler or a biomass boiler.