JPS6159382B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention is a method for manufacturing a small-diameter long duplex cast stainless steel pipe, specifically, a duplex stainless steel cast steel pipe with excellent strength, toughness, and stress corrosion cracking resistance by a combination of centrifugal force casting and cold working of the cast pipe. The present invention relates to a method of manufacturing a small diameter long cast steel pipe. Corrosion-resistant pipe materials are required to have excellent strength, toughness, stress corrosion cracking resistance, etc. depending on the usage conditions. Furthermore, there are many applications in which small diameter long tubes with such performance are required. Among stainless steel cast steels, which are corrosion-resistant alloys, cast steels have a two-phase structure in which a δ-ferrite phase and an austenite phase coexist.
High toughness, especially 5-30% δ-ferrite phase
(area ratio) is a preferable material that also has excellent stress corrosion cracking resistance. Further, as a method for manufacturing a tube body, the centrifugal force casting method has the advantage that there are almost no restrictions in terms of materials that can be cast, and the tube body can be manufactured at a low cost. Therefore, if duplex stainless steel is centrifugally cast, a cast pipe having the above performance can be supplied at low cost. 〓〓〓〓
However, the centrifugal force casting method has a relatively large diameter,
Although it is easy to manufacture short tubes, it is not suitable for casting long tubes with small diameters, and for example, it is extremely difficult to manufacture cast tubes with an outer diameter of 100 mm or less and a length of 5 m or more. As a countermeasure, it is possible to centrifugally cast a cast pipe of an appropriate size and then hot-process it into the desired long pipe with a small diameter. A tube body having a two-phase structure containing . However, the structure should be made into a single austenite phase, or the amount of δ-ferrite should be reduced.
If it is 50% or more, the problem of hot workability can be solved to some extent, but then it cannot meet the demands for corrosion cracking resistance, high strength, and high toughness. The present invention has been made in view of the above, and provides a method for manufacturing a small-diameter long tube that satisfies the above-mentioned performance, which is a combination of centrifugal force casting and cold working. The method of the present invention is a method for manufacturing a small-diameter long corrosion-resistant cast steel pipe made of duplex stainless steel cast steel containing 5 to 30% (area ratio) of ferrite phase, in which molten cast steel is subjected to centrifugal casting, and after solidification, The average cooling rate until the temperature reaches 600°C is controlled to be 20°C/min or more, and the resulting cast pipe is subjected to solution treatment and cold working to obtain the desired small diameter. It is characterized by being processed into long tubes. In the present invention, the cooling rate is controlled to an average of 20°C/min or more until the temperature reaches 600°C after casting, which makes the crystal grains of the cast tube fine enough to withstand the subsequent cold working. This is to provide good workability. The size of crystal grains depends on the cooling rate after solidification, and by increasing the cooling rate, the growth of crystal grains during the cooling process is suppressed, resulting in a fine structure. By making the crystal grains finer, the cold workability of the cast pipe is improved. The refinement effect due to cooling is large in the temperature range from the solidification point to the recrystallization temperature (approximately 1000°C in the case of cast steel, which is the object of the present invention). Therefore, it is sufficient to cool at 20°C/min or more after solidification until the recrystallization temperature is reached, but it is difficult to measure the cooling rate at high temperatures in industrial production, so this is not practical. Considering the convenience of temperature control during operation, it was decided to cool at a rate of 20°C/min or more until the temperature reached 600°C. The higher the cooling rate, the finer the crystal grains will be, and the cooling will not cause any quality problems, but a cooling rate of up to about 34°C/min is sufficient to achieve the purpose; There is no need to increase the cooling rate, and increasing the cooling rate further increases the difficulty in actual work, so
The upper limit is preferably 34°C/min. With this control, as will be shown in the Examples below, it is possible to easily and smoothly process a small diameter long tube with an outer diameter of 100 mm or less and a length of more than 5 m. The present invention also provides a method for controlling the cooling rate and performing recrystallization refinement treatment prior to cold working of the cast pipe obtained through the subsequent solution treatment or during the cold working process. The present invention provides a method for manufacturing small-diameter long cast steel pipes. This recrystallization refinement can be achieved by plastic working and heat treatment, which further improves the cold workability of the cast tube and makes it easier to process it into small diameter long tubes. The duplex stainless cast steel used in the present invention is basically a Cr--Ni--Fe system, and one having a composition depending on the intended use and usage conditions may be selected as appropriate. For example, C0.08% or less, Si0.2
~2.0%, Mn0.2~2.0%, Cr16~30%, Ni8~24
%, the balance essentially consisting of Fe, or further containing Mo, Cu, W, etc. as an alloying element. Centrifugal force casting in the present invention may be performed under normal conditions, and the casting size may be appropriately set depending on the size of the intended small-diameter long tube. The cast tube obtained through centrifugal casting and controlling the cooling rate described above is produced by dissolving carbides (mainly chromium carbide) precipitated in the austenite phase and improving the material quality by uniformly dispersing alloying elements. Then, it is subjected to solution treatment (solid solution heat treatment) in which it is heated and maintained at a constant temperature using a conventional method and then rapidly cooled. The appropriate heating temperature depends on the composition of the pipe material, but is approximately 1100℃.
may be used as a guideline. The reason for rapid cooling after heating and holding in solution treatment is to prevent the precipitation of carbides during the cooling process.If slow cooling is performed, chromium will precipitate as carbides at grain boundaries, reducing the effectiveness of chromium. This is because the deterioration of the material due to the loss of properties, particularly the intergranular corrosion resistance, is greatly reduced. The rapid cooling is performed by blast cooling, fountain cooling, air cooling, or the like. Through this solution treatment, carbide
A cast pipe is obtained which has a highly homogeneous microstructure of two phases of austenite and 5 to 30% ferrite in which contamination of the austenite phase and grain boundaries has been eliminated.
As is well known, the amount of ferrite in the structure is adjusted to a desired value by changing the chemical composition and heat treatment conditions. After the heat treatment, the inner and outer surfaces of the cast tube are machined and subjected to cold working to obtain the desired small diameter and elongated size. For the cold working, a method such as press drawing may be applied, and there are no particular restrictions on the working conditions. Further, the recrystallization refinement treatment for further enhancing the cold workability of the cast pipe is achieved by plastic working, heating to a recrystallization temperature range after the plastic working, and rapid cooling after heating. For example, press forming and drawing can be applied to the plastic working. The appropriate degree of processing is about 20%. The appropriate heating temperature after plastic working depends on the composition of the tube material, but is approximately 1100℃.
This heat treatment eliminates work hardening and promotes recrystallization, resulting in a fine crystal grain structure with uniform grain size. Note that the reason why the cooling after heating to the recrystallization temperature range is performed as rapid cooling is to prevent precipitation of carbides during the cooling process. This recrystallization refinement treatment may be performed before cold working or during the cold working process, and the objective is achieved by repeating cold working and recovery of workability through recrystallization refinement. Alternatively, a long tube with a small diameter may be obtained. Next, examples of the method of the present invention will be described. Example 1 Molten stainless steel was centrifugally cast at a temperature of 600
A cast tube was obtained by controlling the cooling rate to 20°C/min to 34°C/min. Its casting size is the outer diameter
It is 120mm, wall thickness 28mm, and length 4m. The chemical composition and δ-ferrite content (%) of each cast tube are shown in Table 1. For comparison, the same cast tube as above was obtained, except that it was cooled under normal cooling conditions and at a cooling rate of less than 20° C./min. Each cast tube was subjected to solution heat treatment at a temperature of 1100℃ x 2 hours for rapid cooling, then machined on the inner and outer surfaces of the tube, and then subjected to press drawing, with an outer diameter of 82 mm and a wall thickness of 21 mm.
A small diameter long tube with a length of 8 m and a diameter of 8 m was manufactured. Table 2 shows the presence or absence of cracks after cold working.

[Table] 〓〓〓〓

[Table] As shown in the above results, all cast pipes obtained under normal cooling conditions crack during cold working, making it impossible to process them into small diameter long pipes. According to the invention, a small diameter long tube having an outer diameter of 100 mm or less and a length of 5 m or more can be obtained without cracking. Example 2 Small diameter long tubes No. 3 and 4 obtained in Example 1
For 5 and 6, plastic working by press drawing (area reduction rate 20
%) and heat treatment (temperature 1100℃ rapid cooling), press drawing is performed again to reduce the outer diameter.
A tube body of 61 mm, wall thickness of 18 mm, and length of 16 m was manufactured. Even in this reprocessing, no cracks were observed at all. As described above, the present invention makes it possible to manufacture small-diameter long tubes by combining centrifugal force casting and cold working.
It is possible to obtain a duplex cast stainless steel pipe having a small diameter and long size with a length of 5 m or more. In addition, since the raw tube is a centrifugally cast tube, it is relatively inexpensive.
Moreover, there are almost no restrictions in terms of applicable materials. The cast steel pipe obtained by the method of the present invention not only has good corrosion resistance but also contains 5 to 30% of δ-ferrite.
Due to its two-phase weave, it has excellent strength and toughness as well as stress corrosion cracking resistance, making it suitable for various corrosion-resistant piping materials, oil well tubing materials, casing materials, heat exchanger pipe materials, etc. . 〓〓〓〓

Claims (1)

[Claims] 1. A method for manufacturing a small-diameter long cast steel pipe made of duplex stainless steel cast steel having a δ-ferrite content of 5 to 30%, wherein molten cast steel is subjected to centrifugal casting, and after solidification, the temperature
Average cooling rate up to 600℃ from 20℃/min to 34
℃/min to obtain a cast tube, which is heated to dissolve the precipitated carbide in the austenite phase and then subjected to solution treatment, in which it is rapidly cooled, and then subjected to cold working to form a small-diameter long length. A method for producing a small-diameter long duplex cast stainless steel pipe characterized by processing it into a pipe. 2. The method for manufacturing a small-diameter long duplex cast stainless steel pipe according to item 1 above, which is cold-worked into a pipe having an outer diameter of 100 mm or less and a length of 5 m or more. 3. A method for manufacturing a small-diameter long cast steel pipe made of duplex stainless steel cast steel with a δ-ferrite content of 5 to 30%, in which molten steel is subjected to centrifugal casting, and after solidification, the temperature
Average cooling rate up to 600℃ from 20℃/min to 34
℃/min to obtain a cast tube, which is heated to dissolve the precipitated carbide in the austenite phase, and then subjected to solution treatment in which it is rapidly cooled, followed by plastic working.
After the processing, a recrystallization refinement process consisting of a heat treatment of heating to a recrystallization temperature range and then rapid cooling is performed, followed by cold working, or by performing the recrystallization refinement treatment in the middle of the cold working process to obtain a small diameter. A method for producing a small-diameter long duplex cast stainless steel pipe characterized by processing it into a long pipe. 4. The method for producing a small-diameter long duplex cast stainless steel pipe according to item 3 above, which is cold-worked into a pipe having an outer diameter of 100 mm or less and a length of 5 m or more.