JPS6147210B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for manufacturing heat-resistant cast steel pipes, and in particular to a long, small-diameter, seamless heat-resistant cast steel pipe that is expanded and reduced in diameter by applying cold working to a centrifugally cast heat-resistant cast steel pipe as a raw pipe. It manufactures. Centrifugally cast steel pipes are usually obtained by rotating a horizontal centrifugal casting mold at high speed (gravity multiplier: approximately 60 to 250G) and casting a specified amount of molten metal, forming a cylinder of molten metal in the mold and allowing it to solidify. The inner and outer diameters of the cast body are then machined to create the desired tube body. The pipe manufacturing method using centrifugal casting uses billet material as a plastic processing method, such as hot extrusion processing, and involves heating, piercing,
The process is simpler and more economical than methods that require reheating and extrusion. However, although the centrifugal casting method is suitable for manufacturing relatively large diameter tubes, it is not easy to cast small diameter tubes. This is because the smaller the diameter, the thinner the casting gutter that looks into the mold, and the more likely the inner hole of the casting gutter will be clogged with molten metal and slag. Since the amount of molten metal poured per unit time is smaller, solidification progresses sequentially from the point where the molten metal reaches the solidification temperature due to contact with the inner wall of the mold, before a cylinder of molten metal is formed in the mold, resulting in shrinkage cavities and cracks ( Casting defects such as gas holes) are more likely to occur. These problems become more noticeable as the cast pipe becomes longer. For this reason, it is extremely difficult to manufacture small-diameter, long heat-resistant cast steel pipes with an outer diameter of about 50 mm or less and a single pipe length of about 5 m or more using the centrifugal casting method. In order to solve the above-mentioned problems, the present invention attempts to obtain a small-diameter, long heat-resistant cast steel pipe by plastically working a heat-resistant cast steel pipe that has been centrifugally cast as a blank pipe, and the processing If cold rolling diameter reduction is applied as a method, cracks are less likely to occur during plastic working, and even centrifugally cast pipes with comparatively poor cold workability can be freed from cracks if appropriate processing conditions are applied. This was completed as a result of various studies from the viewpoint of making it possible to reduce and expand the diameter of the tube to the desired size. The method of the present invention will be described in detail below. The present invention has C0.05 to 0.25%, Si2.0% or less,
Mn2.0% or less, Cr19.0~27.0%, Ni30.0~40.0
%, Nb0.2-2.0%, N0.1% or less, the balance is Fe and unavoidable impurities.The base tube is a centrifugally cast tube made of heat-resistant cast steel, and the tube is machined to the appropriate inner and outer diameters by cutting. Afterwards, the diameter is reduced by cold rolling. In cold rolling diameter reduction processing, the diameter reduction rate is set to 30% or less when the area reduction rate is 40 to 60%. The area reduction rate and diameter reduction rate are determined by the following formula. Area reduction rate (%) = (1-A/A ₀ )×100 [where A ₀ : pipe cross-sectional area before plastic working, A: pipe cross-sectional area after plastic working]. Diameter reduction rate (%) = (1-D/D ₀ ) × 100 [where, D ₀ : outer diameter (or inner diameter) of the tube before plastic working, D: outer diameter (or inner diameter) of the tube after plastic working]. In the above cold rolling diameter reduction process, the diameter reduction rate is 30%.
The following is defined as if the area reduction rate exceeds 40~
This is because even if it is within the 60% range, many cracks will occur in the tube, especially in the tube end surface, during plastic working, making diameter reduction processing difficult. By limiting the diameter reduction rate to 30% or less, it is possible to obtain a healthy small-diameter long tube that has been expanded and contracted to a desired size without causing problems such as cracks. A more preferable processing condition for performing the cold rolling diameter reduction process more smoothly is to limit the diameter reduction rate to 30% or less and 10% or more when the area reduction rate is 40 to 60%. The casting size of the centrifugally cast tube used as the raw tube may be an appropriate size that is not unnecessarily large in diameter and does not pose difficulties in casting technology, depending on the intended tube size. For example, outer diameter 38.1mm, wall thickness
If the purpose is a tube body of 3.2 mm and length of 5000 mm, a cast tube finished by cutting to an outer diameter of 52 mm, wall thickness of 6 mm, and length of 2000 mm is used as a raw tube and subjected to cold rolling diameter reduction under the above processing conditions. By doing this, you can easily obtain the desired tube size. The chemical composition of the heat-resistant cast steel pipe of the present invention was specified in consideration of cold workability in diameter reduction processing, heat resistance characteristics, and other various properties, and the reasons for limiting each component are as follows. . C: 0.05-0.25% Effective for improving strength, but if it exceeds 0.25%,
It is difficult to obtain a sound tube due to the occurrence of cracks during cold rolling and diameter reduction processing, and on the other hand, if it is less than 0.05%, the high temperature strength required for heat-resistant applications is insufficient. Si: 2.0% or less It is necessary to obtain the fluidity of the molten metal during casting of the raw pipe, but if it exceeds 2.0%, the weldability of the pipe will deteriorate. Mn: 2.0% or less Mn is added for the purpose of improving slag fluidity during deoxidation, desulfurization, melting, and refining of molten metal, but if it exceeds 2.0%, the effect is almost saturated. Cr: 19.0-27.0% Necessary to obtain the oxidation resistance necessary for high-temperature materials, but if it is less than 19.0%, the effect will be insufficient,
On the other hand, if it exceeds 27.0%, diameter reduction processing due to hardening of the material becomes difficult, and weldability also deteriorates. Ni: 30.0 to 40.0% Necessary for improving high temperature strength. If it is less than 30.0%, the high-temperature strength will be insufficient even in combination with other elements, while if it exceeds 40.0%, the effect of improving high-temperature strength commensurate with the increase in Ni content will not be obtained, which will be uneconomical. Nb: 0.2-2.0% This is an important element for ensuring high-temperature strength and other high-temperature properties. If the content is less than 0.2%, the effect will not be sufficient, while if it exceeds 2.0%, the oxidation resistance will deteriorate significantly at high temperatures. N: 0.1% or less Has the effect of improving strength, but if it exceeds 0.1%,
Cold workability is poor, diameter reduction processing is difficult, and a sound tube cannot be obtained. P, S, and other impurities are allowed to be unavoidably mixed in by ordinary melting techniques. For example, P is 0.03%
Below, S is acceptable as long as it is 0.03% or less. Table 1 shows centrifugally cast pipes (outer diameter 50~ after machining).
60mm, wall thickness 4-6mm, length 2000mm) as a raw pipe by cold rolling diameter reduction (area reduction rate 40-60%, diameter reduction rate 25-
30%) is shown below. Trial number 1~
10 is heat-resistant cast steel having the composition specified by the present invention; 11
and 12 are heat-resistant cast steels in which C or N deviates from the upper limit stipulations of the present invention; the upper row of each trial number is an example that satisfies the processing conditions of the present invention, and the lower row is a comparative example in which the diameter reduction rate deviates from the stipulation of the present invention. It is. In the table, "〇" in the "Properties of plastically processed pipe" column indicates that the plastically processed pipe has no cracks on either the outer surface or both end surfaces, and no microscopic cracks are observed on the inner surface, indicating sound pipe properties. An "x" mark means that a crack has occurred on either the inner or outer surface of the tube or the end surface of the tube.

[Table] As shown in the examples in the upper row of each trial number 1 to 10 in the above table, by satisfying the chemical composition and processing conditions specified by the present invention, it is possible to expand and reduce the diameter without causing cracks. This results in a healthy small diameter and long pipe. On the other hand, as shown in the lower examples of trial numbers 1 to 10, even if the chemical composition is appropriate, cracks occur if the processing conditions deviate from the specifications of the present invention. Furthermore, trial number 10
If the chemical composition is not appropriate, as shown in 11 and 11, cracks will occur regardless of the diameter reduction rate, and especially in cases where the diameter reduction rate exceeds 30%, the cracks on the pipe end face will be significant and will result in poor pipe manufacturing. It was possible. Also, trial number
In the plastically processed tubes Nos. 10 and 11, microscopic cracks were found to have occurred on the inner surface of the tubes by liquid penetrant inspection after cutting. As described above, according to the present invention, it is possible to manufacture a desired pipe size using a centrifugal heat-resistant cast steel pipe as a raw pipe, especially a small diameter and long heat-resistant cast steel pipe that is impossible with centrifugal casting. In addition, since the present invention uses a centrifugally cast pipe as the raw pipe, the manufacturing cost is lower than when the raw pipe is a pipe that requires a complicated processing process, such as a hot extruded pipe. In addition, it has excellent high-temperature properties in heat-resistant applications, especially high-temperature strength. Heat-resistant cast steel pipes generally have larger crystal grains than heat-resistant steel pipes formed by hot plastic working, and the bonding strength between primary carbides that precipitate at grain boundaries during the casting solidification process and the metal matrix is strong. It must be strong, and the selection of alloying elements required to improve high-temperature strength must be relatively free (in the latter case, hot plastic working is performed, so the alloying elements that can be added must be able to improve hot workability). (Limited to items that do not cause damage) etc. However, when the plastically worked pipe obtained by the method of the present invention is subjected to solution treatment to recover strain hardening due to plastic deformation as necessary, the creep rupture strength may decrease due to the recrystallization phenomenon. In the heat treatment, the temperature is relatively high (e.g. 1250℃) depending on the chemical composition.
If the above) is set to increase the grain size of the crystal grains, it is fully possible to prevent a significant decrease in high temperature strength and maintain good high temperature strength. The heat-resistant cast steel pipe obtained by the present invention is used for various heat-resistant applications, and is useful as a substitute for drawn pipes such as pigtails, hair-pinch tubes, steam superheater tubes, and seamless steel tubes for various other heat exchangers.

Claims (1)

[Claims] 1 C0.05~0.25%, Si2.0% or less, Mn2.0% or less, Cr19.0~27.0%, Ni30.0~40.0%, Nb0.2~
A heat-resistant cast steel centrifugally cast tube consisting of 2.0% N0.1 or less, the balance Fe and unavoidable impurities is used as the raw tube, and after processing the inner and outer diameters by cutting, the area reduction rate is 40 to 60.
% and a diameter reduction rate of 30% or less.