JP2012102375A - Method for producing austenitic alloy large-diameter pipe - Google Patents

Abstract

An object of the present invention is to provide a method for producing a large-diameter pipe made of an austenitic alloy that can suppress the formation of wrinkles on the outer surface of the raw pipe when the ingot is drilled into the raw pipe.
A method for producing an austenitic alloy large-diameter pipe comprising a step of hot drilling an ingot made of an alloy containing Cr: 21-31% and Ni: 43-60% by mass. Before the process, the ingot is hot-worked with a cross-section reduction degree R calculated by the following formula (1) of 20% or more, and is a method for producing an austenitic alloy large-diameter pipe.
R = (1-S2 / S1) × 100 (%) (1)
S1: Ingot cross-sectional area (mm ² ) before hot working,
S2: Ingot cross-sectional area after hot working (mm ² )
However, when performing hot working more than twice, the following equation (2) is applied.
R = R ₁ + R ₂ +... + R _n-1 + R _n (2)
[Selection figure] None

Description

  The present invention relates to a method for producing a large-diameter pipe made of a high Cr-high Ni austenitic alloy. More specifically, before the ingot is drilled into a blank tube, the outer surface of the blank tube is secured by securing a machining amount that reduces the cross section of the ingot by hot working (hereinafter referred to as “cross section reduction degree of machining”). The present invention relates to a method for manufacturing a large-diameter pipe capable of suppressing the formation of wrinkles.

In recent years, ultra super critical (USC) boilers that have been improved in efficiency by increasing the steam temperature and pressure of thermal power generation have been newly established all over the world. This is because reduction of CO ₂ gas emissions is one of the challenges for solving energy problems and is an important industrial policy. Therefore, power generation boilers that burn fossil fuels can reduce CO ₂ gas emissions with high efficiency. This is because an advantageous ultra-supercritical boiler is easily adopted. The steam temperature of current ultra-supercritical boilers is increased to about 600 ° C.

  In an ultra-supercritical boiler, a pipe made of fire STPA 28, fire STPA 29 or the like described in the interpretation of technical standards for thermal power generation facilities is usually provided. The fire STPA 28 and the fire STPA 29 are high Cr ferritic steels containing about 9% by mass of Cr.

  Among these alloy steel pipes, as a method for producing a large diameter pipe having an outer diameter of 250 to 1000 mm and a wall thickness of 20 to 150 mm, a forging pipe manufacturing method such as an Erhard push bench method or a mandrel forge method is used. Commercially established.

  1A and 1B are diagrams showing a method of manufacturing a large-diameter pipe by the Erhardt push bench method. FIG. 1A shows a state in which an ingot is inserted into a circular rigid rod, and FIG. 1B shows that an ingot is perforated. FIG. 6C shows a state where the bottomed hollow shell is taken out, and FIG. 10D shows a state where the bottomed hollow shell is pushed out.

The manufacture of large diameter tubes by the Erhard push bench method generally consists of the following steps:
(1) The cross-section of the surface perpendicular to the longitudinal direction is substantially quadrangular, and the ingot 1 as cast is heated to a predetermined temperature, and as shown in FIG. Insert the mold 2 (hereinafter, also simply referred to as “bamboo”).
(2) As shown in FIG. 2B, the mandrel 3 is faced from above at the center of the ingot 1 inserted into the cage using the guide 4, and is press-fitted to make a cup-shaped bottom. A hollow shell is used.
(3) As shown in FIG. 3C, the take-out bar 5 disposed at the bottom of the heel 2 is moved upward to take out the bottomed hollow shell 6 from the heel 2.

(4) As shown in FIG. 4 (d), the mandrel 3 is inserted into the bottomed hollow shell 6 and the inside of a plurality of dies 7 is pushed out hot to reduce the thickness of the bottomed hollow shell. .
(5) After the step (4) is repeated a plurality of times and finished to a predetermined size, the bottom of the bottomed hollow shell is cut to form a bottomless hollow shell.
(6) After heat-treating the bottomless hollow shell, its inner and outer surfaces are cut and finished to a predetermined surface property and dimensions to obtain a large-diameter tube.

Also, the manufacture of large diameter tubes by the mandrel-forge method generally consists of the following procedures:
(1) In a state where a tool with a sharp tip is rotated, the tool is pressed against the upper surface of an ingot arranged with the longitudinal direction vertical, and the ingot is perforated hot to form a hollow shell.
(2) The hollow shell is arranged with the longitudinal direction horizontal, and the mandrel is hot pressed against the inner surface of the hollow shell to reduce the thickness.
(3) After the step (2) is repeated a plurality of times and finished to a predetermined size, the hollow shell is subjected to heat treatment.
(4) The inner and outer surfaces of the hollow shell are cut and finished to a predetermined surface texture and dimensions to obtain a large-diameter tube.

As demand for CO ₂ emission reduction increases, there is an active movement to increase the steam temperature of thermal power generation to higher temperatures and pressures. Specifically, research and development for increasing the steam temperature to about 700 ° C. has been promoted. Large diameter pipes made of high Cr ferritic steel are used for main steam pipes and reheat steam pipes in 600 ° C class super supercritical pressure boilers, but 700 ° C class super super critical pressure boilers have high temperature strength. It cannot be used because it is insufficient. Therefore, it is considered to use a large-diameter pipe to which an alloy containing an austenite structure containing 20 mass% or more of Cr and 40 mass% or more of Ni is used in an ultra supercritical boiler of 700 ° C. class.

Various proposals have heretofore been made regarding this high Cr-high Ni austenitic alloy, for example, Patent Documents 1 and 2. Patent Document 1 discloses a high Cr—high Ni austenitic alloy containing Mo and Co. The austenitic alloy disclosed in Patent Document 1 is said to be able to increase the stress-breaking strength due to a special microstructure comprising M ₆ C carbide and M ₂₃ C ₆ carbide.

  Patent Document 2 discloses a high Cr—high Ni austenitic alloy having excellent creep strength in a high temperature range of 700 ° C. or higher by adding a large amount of W.

  However, in order to manufacture a large-diameter pipe made of a high Cr-high Ni austenitic alloy disclosed in Patent Document 1 and Patent Document 2, when an ingot made of a high Cr-high Ni austenitic alloy is hot-drilled, In some cases, wrinkles are formed on the outer surface of the obtained raw tube.

  FIG. 2 is a view showing a ridge formed on the outer surface of a raw tube obtained by perforating an ingot in manufacturing a large-diameter tube using the conventional Erhard push bench method. This figure is a photograph showing the external appearance of a bottomed hollow shell obtained by perforating an ingot in the production of a large-diameter pipe by the conventional Erhard push bench method described with reference to FIG. The bottomed hollow shell shown in the figure is a high Cr-high Ni austenitic alloy, and is formed by drilling an ingot made of alloy A shown in Table 1 of Examples described later. In the figure, the outer surface 6a of the bottomed hollow shell with the vertical direction in the figure as the longitudinal direction is shown, and wrinkles can be confirmed in a portion 6b surrounded by a two-dot chain line.

  When a crease is formed on the outer surface of the raw tube by the perforation, it is necessary to care for the outer surface by manual operation using a grinder or the like and to remove the crease from the outer surface before pushing out with a plurality of dies. However, the wrinkles formed on the outer surface of the base tube made of austenitic alloy have a depth of about 30 mm at the maximum, which requires a lot of labor for maintenance and increases the outer diameter cutting allowance for maintenance. Yield deteriorates. For this reason, in the manufacture of a large-diameter pipe made of a high Cr-high Ni austenitic alloy, there is a problem in that flaws are formed on the outer surface of the raw pipe by perforation.

Japanese Patent Application Laid-Open No. 2-1077736 JP 2004-3000 A

  As described above, in the production of a large-diameter pipe made of a high Cr-high Ni austenitic alloy, there is a problem in that wrinkles are formed on the outer surface of the raw pipe due to drilling. The present invention has been made in view of such a situation, and a method for manufacturing a large-diameter pipe that can suppress the formation of wrinkles on the outer surface of the raw pipe when the ingot is perforated to form the raw pipe. The purpose is to provide.

  In order to solve the above problems, the present inventors have studied the cause of the formation of wrinkles on the outer surface of the raw tube when the ingot is perforated into a raw tube.

  FIG. 3 is a top view showing the shape of the lumen of the heel used for drilling an ingot and the shape of the ingot in the manufacture of a large-diameter pipe by the Erhardt push bench method. In the figure, the lumen 2a of the heel and the outline (outer surface 1a) of the upper surface of the ingot inserted into the heel are schematically shown. As shown in the figure, the contour of the upper surface of the ingot is substantially rectangular and has a plurality of convex portions. For this reason, the outer surface 1a of the ingot and the inner cavity 2a of the heel are in a state in which some of the plurality of convex portions of the outer surface of the ingot are in contact with each other. For example, in FIG. Among the portions, the four convex portions 1b surrounded by the dotted line are in contact with each other.

  When the center of the ingot is perforated by the mandrel in a state where the outer surface 1a of the ingot and the inner cavity 2a of the ingot are partially in contact with each other, a part of the outer surface of the ingot that has a clearance with the inner cavity of the hemorrhoid is projected (same as above). The outer surface of the ingot becomes circular along the lumen shape of the heel. When a part of the outer surface of the ingot extends, a tensile stress acts in the direction indicated by the thick arrow in the figure near the outer surface of the ingot.

  Even ingot drilling made of high Cr ferritic steel (for example, fire STPA28) conventionally used for large diameter pipes, the same shape as the ingot and rod shown in FIG. 3 is used. For this reason, it is presumed that when an ingot made of a high Cr ferritic steel is drilled, tensile stress acts as in the case of drilling an ingot made of a high Cr-high Ni austenitic alloy. However, since no flaws are formed on the outer surface of the raw tube made of a high Cr ferritic steel ingot, the flaws formed on the outer surface of the raw tube when drilling an ingot made of a high Cr-high Ni austenitic alloy. Is presumed to be due to the characteristics unique to the high Cr-high Ni austenitic alloy.

  The greatest concern for the effects of the components of the high Cr-high Ni austenitic alloy is intergranular fusion cracking, in which cracks are generated on the surface due to melting near the grain boundaries of the material during drilling. In order to evaluate the intergranular melt cracking, a greeble test (tensile test by energization heating) was performed.

The test conditions for the greeble test are as follows.
Specimen: Dimensions Round bar with a parallel part diameter of 10 mm and a length of 130 mm Material High Cr-High Ni austenitic alloy, alloy A shown in Table 1 of Examples described later
Test method:
1) When the test temperature is 1200 ° C. or less: After heating at 1200 ° C. for 5 minutes, cool to a predetermined test temperature at 100 ° C./min, and then perform a tensile test at a 1 / s tensile speed to set the aperture value. It was measured.
2) When the test temperature is higher than 1200 ° C .: After heating for 5 minutes at the test temperature (for example, 1225 ° C. when testing at 1225 ° C.), a tensile test was performed at a 1 / s tensile speed, and the aperture value was measured.

  FIG. 4 is a diagram showing a result of a greeble test using a test piece made of a high Cr—high Ni austenitic alloy. From the test results shown in the figure, it is confirmed that when the test temperature exceeds 1275 ° C., the greeble aperture value is greatly reduced. For this reason, when the ingot is perforated to form a blank tube, it is estimated that ductility can be ensured by perforating the ingot at a heating temperature of 1275 ° C. or lower.

  Based on the result of the greeble test, a test was performed in which the ingot was heated to 1230 ° C. and punched, and a crease was formed on the outer surface of the punched tube similar to FIG. Therefore, when drilling an ingot with an actual machine, the test was performed with the heating temperature further lowered by 50 ° C. in consideration of the fact that the temperature of the ingot drilled may increase due to processing heat generation. That is, a test was conducted in which the ingot was perforated by heating to 1180 ° C., and ridges were formed on the outer surface of the perforated element tube as in FIG. Therefore, it is estimated that the soot formed on the outer surface of the raw tube is not caused by the grain boundary melt cracking.

  Next, the solidification structure of the ingot was investigated, and the possibility that the solidification structure of the ingot was changed by the high Cr-high Ni austenitic alloy and adversely affected the hot workability was investigated. The investigation of the solidified structure was performed by collecting a plate-like test material from a cross section perpendicular to the longitudinal direction of the ingot, corroding the collected test material with aqua regia, and observing the solidified structure on the surface of the test material.

  FIG. 5 is a view showing a solidification structure of a cross section perpendicular to the longitudinal direction in an ingot made of a high Cr—high Ni austenitic alloy. In the drawing, the outer surface 1a of the ingot is indicated by a white two-dot chain line, and the center 1c of the ingot is indicated by an x mark. From the figure, a solidified structure called a columnar crystal that is long in the direction from the outer surface 1a to the center 1c of the ingot exists in the vicinity of the outer surface of the ingot, and the length from the outer surface to the center of the columnar ingot is as large as several millimeters. It became clear that there was.

  Here, since the columnar crystal is long and narrow at most of the interface of the columnar crystals, it is confirmed that the angle is substantially perpendicular to the direction in which the tensile stress shown in FIG. 3 acts. In addition, since the interface of the columnar crystals is considered to have solidified in the later stage of the solidification process of the ingot, there is a possibility that P, S, etc. are segregated at the interface of the columnar crystals and the ductility of the material is lowered. For this reason, when a large tensile stress is applied in the direction perpendicular to the columnar crystal, it is considered that there is a possibility of cracking at the interface of the columnar crystal, and the solidification structure of the raw tube with the flaws formed on the outer surface shown in FIG. did.

  FIG. 6 is a view showing a solidification structure of a cross section perpendicular to the longitudinal direction in a base pipe in which an ingot made of a high Cr—high Ni austenitic alloy is perforated by a conventional large diameter pipe manufacturing method. This figure shows a solidified structure in the vicinity of the outer surface 6a of the blank tube, and shows a portion 6b where a wrinkle is formed on the outer surface of the blank tube surrounded by a dotted line. From the figure, the solidified structure of the raw tube in which the ingot is perforated has a long columnar crystal in the direction from the outer surface of the raw tube to the center, similar to the solidified structure of the ingot shown in FIG. It became clear that cracks occurred and ridges were formed.

  From the above results, it was presumed that the columnar crystals generated when the ingot solidifies were the cause of formation of wrinkles on the outer surface of the base tube in which the ingot made of a high Cr-high Ni austenitic alloy was perforated. As a method of suppressing wrinkles formed on the outer surface of the blank tube, the present invention was completed because it was considered effective to break the columnar crystals existing near the outer surface of the ingot by hot working before drilling. I let you.

  The gist of the present invention is the following (1) to (4) methods for producing austenitic alloy large diameter tubes.

(1) A method for producing an austenitic alloy large-diameter pipe comprising a step of hot drilling an ingot made of an alloy containing Cr: 21 to 31% and Ni: 43 to 60% by mass. Prior to the process, the ingot is hot-worked with a cross-section reduction degree R calculated by the following formula (1) of 20% or more, and a method for producing an austenitic alloy large diameter pipe.
R = (1-S2 / S1) × 100 (%) (1)
S1: Ingot cross-sectional area (mm ² ) before hot working,
S2: Ingot cross-sectional area after hot working (mm ² )
However, when performing hot working more than twice, the following equation (2) is applied.
R = R ₁ + R ₂ +... + R _n-1 + R _n (2)

(2) By mass%, C: 0.05 to 0.10%, Si: 0.05 to 0.4%, Mn: 0.01 to 1.3%, P: 0.030% Hereinafter, S: 0.010% or less, Cr: 21-31%, Ni: 43-60%, Al: 0.005-1.5%, B: 0.001-0.005%, N: 0.00. A large austenitic alloy according to (1) above, containing not more than 05% and at least one element belonging to each of groups 1 and 2 below, with the balance being Fe and impurities A manufacturing method of a diameter pipe.
First group: Mo: 10% or less, W: 9% or less, and Co: 13% or less.
Second group: Ti: 1% or less, Nb: 1% or less, and Zr: 0.5% or less.

(3) The alloy contains one or more of Ca: 0.03% or less, Mg: 0.03% or less, and rare earth elements: 0.1% or less in mass%, instead of a part of Fe. The method for producing an austenitic alloy large-diameter pipe as described in (2) above.

(4) The method for producing an austenitic alloy large-diameter pipe according to any one of (1) to (3) above, wherein the raw tube obtained by the drilling step is further hot-punched to have a predetermined dimension. The manufacturing method of the austenitic alloy large diameter pipe | tube characterized by including the process of finishing to.

The manufacturing method of the austenitic alloy large diameter pipe of the present invention has the following remarkable effects.
(1) By hot working the ingot at a cross-section reduction degree of 20% or more before the drilling step, coarse columnar crystals existing near the outer surface of the ingot are destroyed and segregation at the interface of the solidified structure Is reduced.
(2) According to the above (1), when an ingot is perforated to form a raw pipe, it is possible to suppress the formation of wrinkles on the outer surface of the raw pipe.

It is a figure which shows the method of manufacturing a large diameter pipe by the Erhard push bench method, The figure (a) is the state which inserted the ingot in the circular rigid rod, The figure (b) is the state which perforates the ingot, FIG. 2C shows a state where the bottomed hollow shell is taken out, and FIG. 4D shows a state where the bottomed hollow shell is pushed out. It is a figure which shows the scissors formed in the outer surface of the raw tube obtained by perforating an ingot in manufacture of the large diameter pipe | tube using the conventional Erhardt push bench method. It is a top view which shows the shape of the lumen | bore lumen | bore used when drilling an ingot, and the shape of an ingot in manufacture of the large diameter pipe | tube by the Erhard push bench method. It is a figure which shows the result of the greeble test by the test piece which consists of a high Cr-high Ni austenitic alloy. It is a figure which shows the solidification structure of a cross section perpendicular | vertical to the longitudinal direction in the ingot which consists of an austenitic alloy of high Cr-high Ni. It is a figure which shows the solidification structure | tissue of a cross section perpendicular | vertical to a longitudinal direction in the raw tube which perforated the ingot which consists of an austenitic alloy of high Cr-high Ni with the manufacturing method of the conventional large diameter pipe | tube.

  Below, the manufacturing method of the austenitic alloy large diameter pipe | tube of this invention is demonstrated.

  The method for producing a large-diameter pipe of the present invention is a method for producing an austenitic alloy large-diameter pipe including a step of hot drilling an ingot made of an alloy containing Cr: 21-31% and Ni: 43-60% by mass. In the manufacturing method, the ingot is hot-worked at a cross-section reduction degree R of 20% or more according to the formula (1) before the drilling step.

  In the present invention, it is defined that the ingot is made of an alloy containing Cr: 21 to 31% and Ni: 43 to 60% by mass when drilling an ingot made of a high Cr-high Ni austenitic alloy. This is because the present invention aims to suppress wrinkles formed on the outer surface of the raw tube. The reason for limiting the contents of Cr and Ni will be described later.

  The method for manufacturing a large-diameter pipe of the present invention reduces the cross-sectional area perpendicular to the longitudinal direction by adding hot forging to the surface, for example, hot working with a cross-section reduction degree of 20% or more before the drilling step. The ingot is subjected to hot working. In this hot working, the ingot receives a compressive stress in the center direction from the outer surface, so that coarse columnar crystals existing near the outer surface of the ingot are destroyed and P and S are segregated at the interface of the solidified structure. It is reduced.

  As a result, columnar crystals that are long from the outer surface to the center of the outer surface of the ingot are destroyed, the solidified structure can be refined, and a decrease in ductility due to segregation can be suppressed. For this reason, even when a force in the tensile direction acts near the outer surface when perforating the ingot after hot working, grain boundary melt cracking starting from the interface of coarse columnar crystals does not occur, as a result, It is possible to suppress the formation of wrinkles on the outer surface of the perforated element tube.

  When the degree of reduction in hot cross-section is less than 20%, coarse columnar crystals existing in the vicinity of the outer surface of the ingot are not sufficiently destroyed, and segregation at the interface of the solidified structure cannot be sufficiently reduced. For this reason, when the ingot is drilled to form a blank tube, wrinkles may be formed on the outer surface of the blank tube due to grain boundary melt cracking. 20% or more. Moreover, it is preferable that the upper limit of the cross-section reduction degree of hot working is 75%. In order to perform hot working with a cross-section reduction degree of over 75% on an ingot, it is necessary to prepare a steel ingot with a very large cross-sectional area compared to the steel ingot necessary for drilling, and the production yield of large-diameter pipes is reduced. It is because it falls remarkably.

  In the method for producing a large-diameter pipe of the present invention, hot working for ensuring the degree of cross-section reduction can be performed by forging with a conventionally used press. Specifically, hot forging (hereinafter referred to as “outside diameter forging”) in which the ingot is rotated around the longitudinal direction at predetermined angles while the outer surface of the ingot is pressed by a mold having a flat pressing surface. ) Can be hot worked.

  In the method for manufacturing a large-diameter pipe of the present invention, the hot working may be performed once or may be performed in a plurality of times as shown in the examples described later. If the ingot cross-sectional area is small, it is difficult to secure a cross-section reduction degree of 20% or more by one hot working. In such a case, for example, after forging the outer diameter of the ingot, upsetting forging to increase the cross-sectional area of the ingot, and then forging the outer diameter, the hot working can be performed in multiple times. .

  That is, upset forging is hot working that applies compressive stress in the longitudinal direction of the ingot to reduce the length of the ingot in the longitudinal direction. By applying upset forging to the ingot, the ingot cross-sectional area can be increased as the length of the ingot decreases.

When the hot working is performed in a plurality of times, the cross-section reducing degree R is calculated by the following equation (2).
R = R ₁ + R ₂ +... + R _n-1 + R _n (2)

As shown in the above (2), area reduction working ratio R is the sum of the working ratio R _m of each of the hot working. area reduction working ratio in the hot working of the m-th R _m is, m-th hot ingot sectional area before working (mm ²⁾ of S1, m-th ingot sectional area after hot working (mm ²⁾ of S2 Is calculated by the equation (1).

When S2 (cross-sectional area after the m-th hot working) ≧ S1 (cross-sectional area before the m-th hot working), the cross-sectional reduction processing when calculating the total working degree by the above equation (2). the degree R _m to 0%. This is because, in hot working where the ingot cross-sectional area increases, such as upsetting forging, the fracture effect of coarse columnar crystals existing in the vicinity of the outer surface of the ingot is small compared to outer diameter forging.

  When hot working at a cross-section reduction degree of 20% or more, even when securing the cross-section reduction degree by multiple hot workings, the cross-section reduction work degree calculated by the above equation (2) is 20% or more. If it exists, as shown in the Example mentioned later, it can suppress that a wrinkle is formed in the outer surface of the pierced raw tube.

  By subjecting the ingot to hot working, a slight wrinkle may be formed on the outer surface of the ingot. In this case, the wrinkle can be easily removed by taking care of the ingot before the drilling step. . The depth of the ridge formed on the outer surface of the ingot by hot working is about 3 to 5 mm at the maximum, and it is light compared to the ridge formed on the outer surface of the raw tube during drilling. The impact on is small.

  In the method for producing a large-diameter pipe of the present invention, the raw pipe obtained by the perforating process can be hot-punched and finished to a predetermined size. By applying the above hot working to the ingot, it is possible to suppress the formation of wrinkles on the outer surface of the raw tube obtained by the drilling step. Thereby, the manufacturing yield of a large diameter pipe made of a high Cr-high Ni austenitic alloy can be improved.

  Next, the reason for limiting the chemical composition of the alloy in the method for producing a large-diameter pipe of the present invention will be described. In the following description, “%” display of the content of each element means “mass%”.

Cr: 21-31%
Cr is an important element for ensuring oxidation resistance, steam oxidation resistance and corrosion resistance. In order to ensure corrosion resistance at high temperatures, a content of at least 21% is required. The corrosion resistance increases as the Cr content increases, but if the content exceeds 31%, the structural stability is lowered and the creep strength is impaired. Moreover, not only is it necessary to increase the expensive Ni content in order to stabilize the austenite structure, but also the weldability is reduced. Therefore, the Cr content is 21 to 31%.

Ni: 43-60%
Ni is an element that stabilizes the austenite structure, and is an alloy element that is also important for ensuring corrosion resistance. From the balance with the above Cr amount, Ni should be 43% or more. On the other hand, excessive Ni causes not only an increase in cost but also a decrease in creep strength, so the upper limit is made 60%.

The austenitic alloy used in the method for producing a large-diameter pipe of the present invention is mass%, C: 0.05 to 0.10%, Si: 0.05 to 0.4%, Mn: 0.01 to 1. .3%, P: 0.030% or less, S: 0.010% or less, Cr: 21-31%, Ni: 43-60%, Al: 0.005-1.5%, B: 0.001 It is preferable to contain ˜0.005%, N: 0.05% or less, and one or more elements belonging to each of the following groups 1 and 2, with the balance being Fe and impurities.
First group: Mo: 10% or less, W: 9% or less, and Co: 13% or less.
Second group: Ti: 1% or less, Nb: 1% or less, and Zr: 0.5% or less.

C: 0.05-0.10%
C is an important component for forming carbides and ensuring high-temperature tensile strength and high-temperature creep strength necessary for high-temperature austenitic stainless steel, and it is preferable to contain 0.05% or more. However, if the content exceeds 0.10%, undissolved carbides are produced, or Cr carbides increase and weldability is lowered, so the upper limit is preferably 0.10%.

Si: 0.05-0.4%
Si is added as a deoxidizer during steelmaking, but it is also an important element for enhancing the steam oxidation resistance of steel, and it is preferable to contain 0.05% or more. However, if the content is excessive, the workability of steel deteriorates, so the upper limit is preferably 0.4%.

Mn: 0.01 to 1.3%
Mn combines with the impurity S contained in the steel to form MnS and improves hot workability, but if its content is less than 0.01%, this effect decreases, so 0.01% It is preferable to contain the above. On the other hand, if the content is excessive, the steel becomes hard and brittle, and on the contrary, the workability and weldability are impaired, so the upper limit is preferably made 1.3%.

P: 0.030% or less Although P is inevitably mixed as an impurity, excessive P impairs weldability and workability, so the upper limit is preferably 0.030%. The smaller the P content, the better.

S: 0.010% or less S is inevitably mixed as an impurity as in the case of P described above. However, since excessive S impairs weldability and workability, the upper limit is preferably 0.010%.

Al: 0.005 to 1.5%
When Al is added as a deoxidizer, 0.005% or more is preferably contained in order to obtain a sufficient deoxidation effect. In addition, Al is combined with Ni and finely precipitated as an intermetallic compound to ensure the creep strength at high temperature. In that case, it is preferable to contain 0.1% or more. On the other hand, when the content of Al increases, especially exceeding 1.5%, the intermetallic compound phase rapidly becomes coarse during use at high temperatures, and there is a risk that the creep strength and toughness will be extremely reduced. . Therefore, the upper limit of the Al content is preferably 1.5%.

B: 0.001 to 0.005%
B is an element having a grain boundary slip creep inhibiting action, but if its content is less than 0.001%, this action effect is reduced, so it is preferable to contain 0.001% or more. On the other hand, if the content exceeds 0.005%, weldability is impaired, so the upper limit is preferably made 0.005%.

N: 0.05% or less N is an element effective for stabilizing the austenite phase. However, if the N content becomes excessive and exceeds 0.05%, a nitride of Cr is formed in addition to the nitride of Ti and Al, leading to a decrease in creep ductility and toughness. Therefore, the N content is preferably 0.05% or less.

First group: Mo: 10% or less, W: 9% or less, and Co: 13% or less.
Mo: 10% or less, W: 9% or less W and Mo are both dissolved in the matrix austenite structure and contribute to the improvement of creep strength at high temperatures. It may be added. However, when the content of W and Mo is increased, especially when Mo exceeds 10% or W exceeds 9%, the stability of the austenite phase is decreased and the creep strength is decreased. The susceptibility to embrittlement cracking of HAZ in use increases. For this reason, the contents of W and Mo are 10% or less and 9% or less, respectively. On the other hand, in order to reliably obtain the effects of W and Mo described above, it is preferable to contain 1% or more of W and Mo in total.

Co: 13% or less Co, like Ni, is an austenite-forming element and contributes to the improvement of creep strength by increasing the stability of the austenite phase. Therefore, Co may be added to obtain such an effect. However, since Co is an extremely expensive element, an increase in the content causes an increase in cost. In particular, when the content exceeds 13%, the increase in cost becomes significant. Accordingly, the Co content when added is set to 13% or less. On the other hand, in order to reliably obtain the effect of Co described above, the lower limit of the Co content is preferably 0.5%.

Second group: Ti: 1% or less, Nb: 1% or less, and Zr: 0.5% or less.
Ti: 1% or less Ti, when used in a high temperature range, contributes to the improvement of high temperature strength by precipitation of carbides, so Ti may be added to obtain such an effect. However, when the content of Ti increases, undissolved carbonitrides and oxides are formed to promote mixing of austenite crystal grains, and cause uneven creep deformation and ductility reduction. The content is 1% or less. On the other hand, if the content is less than 0.01%, it is difficult to obtain the effect of improving the high-temperature strength due to precipitation of carbides, so the Ti content is more preferably 0.01% or more. More preferred is 0.03 to 0.2%.

Nb: 1% or less Nb, which does not become a harmful oxide like Ti, may be added to improve the creep strength due to carbide. However, excessive Nb impairs weldability, so the upper limit is made 1%. On the other hand, if the content is less than 0.01%, it is difficult to obtain the effect of improving the creep strength by the carbide, so the Nb content is more preferably 0.01% or more. More preferred is 0.1 to 0.5%.

Zr: 0.5% or less Zr has the effect of strengthening grain boundaries and improving high-temperature strength. Therefore, when it is desired to obtain the effect, it may be actively added and contained. However, if its content exceeds 0.5%, not only Ti but also an insoluble oxide or nitride is formed, which not only promotes intergranular slip creep and non-uniform creep deformation, but also steel. It also deteriorates the quality and impairs the creep strength and ductility at high temperatures. Therefore, the upper limit of the Zr content is 0.5% or less. On the other hand, if the Zr content is less than 0.0005%, the effect of strengthening the grain boundaries and improving the high-temperature strength decreases, so the Zr content is more preferably 0.0005% or more. More preferred is 0.001 to 0.2%.

  In the method for producing a large-diameter pipe of the present invention, the alloy is in mass%, Ca: 0.03% or less, Mg: 0.03% or less, and rare earth elements: 0.1% or less, instead of part of Fe. It is preferable to contain 1 or more types of these.

Ca: 0.03% or less, Mg: 0.03% or less, and rare earth elements: 0.1% or less These elements preferably form harmless and stable oxides and sulfides, and are preferably O and S. It has the effect of reducing the effects of corrosion and improving the corrosion resistance, workability, creep strength and creep ductility. Therefore, when it is desired to obtain the effect, one or more kinds may be positively added and contained, and in this case, the above effect becomes remarkable at a content of 0.0005% or more. However, if the respective contents of Ca and Mg exceed 0.03%, and if the rare earth element (REM) exceeds 0.1%, inclusions such as oxides increase, which only impairs workability and weldability. Instead, it causes an increase in cost. Therefore, Ca: 0.03% or less, Mg: 0.03% or less, and rare earth elements: 0.1% or less.

  REM is a general term for 17 elements obtained by adding Y and Sc to 15 elements from La of atomic number 57 to Lu of 71, and can contain one or more selected from these elements. . The content of REM means the total amount of the above elements.

  Among REMs, Nd binds to S, which hinders high-temperature workability, to make it harmless, and greatly improve hot workability, toughness, and creep ductility. Therefore, when REM is contained, it is preferable to contain Nd. When Nd is used, the upper limit of the Nd content is preferably 0.1%. In addition, in order to acquire the effect by containing Nd stably, it is preferable to make it contain 0.01% or more, and 0.05% is more preferable.

  The “impurities” in “Fe and impurities” refers to those mixed due to various factors in the manufacturing process, including raw materials such as ore or scrap, when the alloy is manufactured industrially.

  In order to verify the effect of the method for producing a large-diameter pipe of the present invention, a test for obtaining a pipe was conducted by the method for producing a large-diameter pipe by the Erhard push bench method described with reference to FIG.

[Test method]
A tube made of a high Cr-high Ni austenitic alloy is obtained from an ingot made of a high Cr-high Ni austenitic alloy by the method of manufacturing a large-diameter pipe by the Erhard push bench method described with reference to FIG. It was. At this time, the ingot was hot-worked before the drilling step. Table 1 shows the chemical composition (mass%) of Alloy A used for the ingot in the present invention example and the comparative example.

  In both the inventive example and the comparative example, the hot working applied to the ingot before the drilling step was performed by one outer diameter forging. Punching using a die and a mandrel was performed by connecting two or three dies in series, and punching was performed a total of five times to finish a hollow hollow tube with a predetermined size.

  Table 2 shows test categories, dimensions of the ingot before hot working, dimensions of the ingot after hot working, the degree of cross-section reduction calculated by the above formula (1), the lumen diameter and mandrel used in the drilling process The outer diameter of the tube, the state of wrinkles on the outer surface of the raw tube, and the dimensions of the obtained tube are shown. The diagonal length dimensions of the ingot shown in Table 2 and Tables 3 and 5 to be described later are diagonal lengths when the ingot in the longitudinal vertical section is square, and the ingot length dimension is the longitudinal dimension of the ingot. Length. In this embodiment, the ingot cross-sectional area before or after hot working when calculating the degree of cross-section reduction by the above equation (1) is the diagonal length of the ingot. Calculation was made using the square area.

[Evaluation criteria]
The outer surface of the bottomed hollow shell with a perforated ingot was inspected to confirm the occurrence of soot. The meanings of the symbols in the column of “the occurrence of flaws on the outer surface of the tube” shown in Table 2 and Tables 3 and 5 described below are as follows:
○: Indicates that no wrinkles were found on the outer surface.
X: It shows that wrinkles were confirmed on the outer surface.

[Test results]
As shown in Table 2, in Comparative Example 1, the ingot was hot-worked with a cross-section reduction degree of 12.1% before the drilling process, and in Comparative Example 2, the ingot was reduced with a cross-section reduction degree of 16.1% before the drilling process. Hot working at 0%. Thus, in Comparative Examples 1 and 2 in which the degree of cross-section reduction in hot working was less than 20%, wrinkles were confirmed on the outer surface of the raw tube in which the ingot was perforated.

  On the other hand, in Example 1 of the present invention, the ingot was hot-worked with a cross-section reduction degree of 23.4% before the drilling process, and in Example 2 of the invention, the ingot was reduced with a cross-section reduction degree of 30.6% before the drilling process. With hot working. As described above, in Examples 1 and 2 of the present invention in which the degree of cross-section reduction in hot working was 20% or more, no wrinkles were observed on the outer surface of the raw tube in which the ingot was drilled.

  From these, in the method for manufacturing a large-diameter pipe according to the present invention, the ingot is hot-worked at a cross-section reduction degree of 20% or more before the drilling process, so that a ridge is formed on the outer surface of the drilled raw pipe. It became clear that can be suppressed.

  Next, a test was performed in which the hot working before the drilling step was performed in a plurality of times.

[Test method]
In Comparative Example 3, Invention Example 3 and Invention Example 4, after performing outer diameter forging, upsetting forging was performed, and the ingot was subjected to hot working in two portions. In Example 5 of the present invention, after performing outer diameter forging, upsetting forging was performed, and then outer diameter forging was performed, whereby the ingot was hot-worked in three portions.

  In Comparative Example 3 and Invention Examples 3 to 5, the ingots made of Alloy A shown in Table 1 were used. The hot-worked ingot was made into a hollow tube with a bottom using a punch having a lumen diameter of 670 mm and a mandrel having an outer diameter of 315 mm in the drilling step. At this time, the outer surface of the bottomed hollow shell was inspected to confirm the occurrence of soot. The bottomed hollow shell was punched using three connected dies, and punched five times in total to finish the tube with an outer diameter of 305 mm and a wall thickness of 25 mm.

  Table 3 shows test categories, ingot dimensions before hot working, ingot dimensions after each hot working and cross-section reduction degree in each hot working, total cross-section reducing work calculated by the above equation (2). The degree of wrinkles on the outer surface of the pipe is shown.

[Test results]
In Comparative Example 3, Invention Example 3 and Invention Example 4, hot working consisting of upsetting forging and outer diameter forging was performed before the drilling step. As shown in Table 3, in Comparative Example 3 in which the total cross-section reduction degree of processing was 14.8%, wrinkles were confirmed on the outer surface of the raw tube in which the ingot was perforated. On the other hand, in Invention Example 3 in which the total cross-section reduction degree is 26.5% and Inventive Example 4 in which the total cross-section reduction degree is 36.0%, wrinkles are formed on the outer surface of the raw tube in which the ingot is perforated. It was not confirmed.

  Further, in Example 5 of the present invention, after performing outer diameter forging, upsetting forging, and then performing hot working by performing outer diameter forging, the total degree of cross-sectional reduction was 30.8%. It was. In Example 5 of the present invention, no wrinkles were observed on the outer surface of the raw tube in which the ingot was perforated.

  From these, even when performing hot working on the ingot divided into a plurality of times, it is possible to suppress the formation of wrinkles on the outer surface of the perforated raw pipe by setting the total cross-section reduction degree to 20% or more. Became clear.

  Next, tests were performed using alloys having different chemical compositions.

[Test method]
In Inventive Example 6, an ingot made of Alloy B was used, and in Inventive Example 7, an ingot made of Alloy C was used. In Invention Examples 6 and 7, the other test conditions were the same as those in Invention Example 2 described above. Table 4 shows the chemical compositions of Alloy B and Alloy C, respectively. Table 5 also shows test categories, ingot alloys, dimensions of the ingot before hot working, dimensions of the ingot after hot working, the degree of cross-section reduction calculated by the above equation (1), and wrinkles on the outer surface of the raw tube Indicates the situation.

[Test results]
As shown in Table 5, in Example 6 of the present invention using an ingot made of alloy B and Example 7 of the present invention using an ingot made of alloy C, the ingot was hot worked at a cross-section reduction degree of 30.6%, No wrinkles were observed on the outer surface of the raw tube perforated with the ingot.

  As described above, the method for producing a large-diameter pipe according to the present invention, when producing a large-diameter pipe made of a high Cr-high Ni austenitic alloy, heats the ingot with a cross-section reduction degree of 20% or more before the drilling step. It has been clarified that it is possible to suppress the formation of wrinkles on the outer surface of the perforated blank tube by performing the interworking.

The manufacturing method of the austenitic alloy large diameter pipe of the present invention has the following remarkable effects.
(1) By hot working the ingot at a cross-section reduction degree of 20% or more before the drilling step, coarse columnar crystals existing near the outer surface of the ingot are destroyed and segregation at the interface of the solidified structure Is reduced.
(2) According to the above (1), it is possible to suppress the formation of wrinkles on the outer surface of the raw pipe when the ingot is perforated to form the raw pipe.

  Therefore, if the method for producing a large-diameter pipe of the present invention is applied to the production of a large-diameter pipe made of a high Cr-high Ni austenitic alloy, the production yield can be improved, and the steam temperature in the ultra supercritical pressure boiler can be improved. It can greatly contribute to the increase in temperature.

1: ingot, 1a: outer surface of ingot, 1b: contact portion with lumen of heel,
1c: the center of the ingot, 2: the round rigid rod, 2a: the lumen of the circular rigid rod,
3: mandrel, 4: guide, 5: take-out rod, 6: hollow shell with bottom,
6a: the outer surface of the blank tube, 6b: the buttocks of the outer surface of the blank tube, 7: the die

Claims (4)

A method for producing an austenitic alloy large-diameter pipe comprising a step of hot drilling an ingot made of an alloy containing Cr: 21-31% and Ni: 43-60% by mass%,
Prior to the perforating step, the ingot is hot-worked at a cross-section reduction degree R calculated by the following equation (1) of 20% or more, and a method for producing an austenitic alloy large-diameter pipe.
R = (1-S2 / S1) × 100 (%) (1)
S1: Ingot cross-sectional area (mm ² ) before hot working,
S2: Ingot cross-sectional area after hot working (mm ² )
However, when performing hot working more than twice, the following equation (2) is applied.
R = R ₁ + R ₂ +... + R _n-1 + R _n (2) The alloy is, by mass, C: 0.05 to 0.10%, Si: 0.05 to 0.4%, Mn: 0.01 to 1.3%, P: 0.030% or less, S : 0.010% or less, Cr: 21 to 31%, Ni: 43 to 60%, Al: 0.005 to 1.5%, B: 0.001 to 0.005%, N: 0.05% or less , And one or more elements belonging to each of the following groups 1 and 2, wherein the balance is made of Fe and impurities, and the austenitic alloy large-diameter pipe according to claim 1 Method.
First group: Mo: 10% or less, W: 9% or less, and Co: 13% or less.
Second group: Ti: 1% or less, Nb: 1% or less, and Zr: 0.5% or less.   The alloy contains one or more of Ca: 0.03% or less, Mg: 0.03% or less, and rare earth elements: 0.1% or less in mass%, instead of part of Fe. The manufacturing method of the austenitic alloy large diameter pipe | tube of Claim 2.   It is a manufacturing method of the austenitic alloy large diameter pipe in any one of Claims 1-3, Comprising: The process which further punches out the raw pipe | tube obtained by the said perforation process and finishes it to a predetermined dimension is included. A method for producing an austenitic alloy large diameter tube.