JPS6130624A - Manufacture of austenitic stainless steel tube - Google Patents

Abstract

PURPOSE:To obtain economically the titled stainless steel tube having controlled ultrafine grains of >=No 11 grain size, by cold working at high draft an element tube obtained by hot extruding tubing method, etc., then cold working suitably said tube through intermediate solution heat treatment. CONSTITUTION:To the element tube 1 obtained by e.g. hot extruding tubing method, cold working is applied by >=60% draft at the first cold working process 2 to control firstly whole structure to fine grain range. Next, the grain controlled tube 1 is charged into large heating furnace in an intermediate solution heat treating process 3, to heat whole of the tube to 900-1,050 deg.C and held thereat, then subjected to solution treatment. Next, cold working by >=20% draft is applied thereto in the second cold working process 4, further modification to smaller grain size is accelerated, then said tube is heated to 900-1,050 deg.C, held thereat then cooled rapidly at the last solution heat treating process 5. By this way, an austenitic stainless steel tube 6 having controlled ultrafine grains of >=No 11 grain size is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to a method for manufacturing an austenitic stainless steel pipe having an extremely fine austenite crystal grain size.

<Prior art> JIS-8US821TP, D engineering N17440-45
Seamless austenitic stainless steel pipe specified by 41 etc. has excellent corrosion resistance, heat resistance, and tightness, and has T
Bronze Award, which has completely improved intergranular corrosion resistance by stabilizing the ash by adding i, and is widely used in piping, heat exchangers, etc. that require corrosion resistance and heat resistance. Recently, the austenite grain size (hereinafter simply referred to as grain size) has been changed to l'&+11 (JIS) for the seamless stainless steel g used in places where impact resistance is required in cold regions. A request was made to make the grains extremely fine. However, the above-mentioned seamless steel pipes are generally produced using the roll perforation method (Mannesmann method) or the hot extrusion method (
However, the above-mentioned requirements cannot be satisfied at all if these pipe manufacturing methods are used.

That is, since all of the above-mentioned pipe-making methods are processed in a light temperature region of around 1200°C, it is impossible to make the particle f teeth into 11 or more fine particles until this pipe-making process.
In addition, due to slight variations in tube manufacturing conditions, particle size cracks exist in the tube group and in the longitudinal direction within the same tube.

One method for refining the grains is to cold-work the tube at an appropriate degree of working (indicated by area reduction rate; the same applies hereinafter), then apply solution heat treatment to heat and hold at a predetermined temperature and rapidly cool it to regenerate the structure. A method for crystallization is considered, but this method
In order to obtain one or more extremely fine grains, it is necessary to repeat the cold knee solution heat treatment cycle many times, which is not economically viable.

<Object of the invention> The present invention has a particle size of No. The present invention aims to provide a method for economically manufacturing an austenitic stainless steel pipe having extremely fine grains of 11 or more.

<Structure of the Invention> Based on the theory of grain refining by cold knee solution heat treatment described above, the present inventor utilizes this theory to create a seamless austenitic stainless steel pipe with extremely fine grains having a grain size of 1+11 or more. In order to find a method for next month, I conducted various miracles and research, and as a result, I obtained the following knowledge. In other words, in order to eliminate the variation in particle size of tubes immediately after manufacture by roll perforation method or hot extrusion method and to achieve a high level of fine particles that meet the target at a low cost, cold knee solution heat treatment is necessary. The process described above is repeated twice, and in the first step, a very large degree of cold working is applied to make the whole grain into a fine-grained region, and in the second time, a moderate cold-opening process is performed to further refine the grain. It is most advantageous to facilitate this.

The present invention has been made based on the above findings, and involves first applying cold working to a working degree of 60 (X or more) to a raw pipe obtained by a roll perforation pipe manufacturing method or a hot extrusion pipe manufacturing method.
After performing intermediate solution heat treatment by heating and holding at 050°C and rapidly cooling, then cold working with a working degree of 20% or more, 900°C
Aunutenite grain size is No. 1, which is characterized by performing final solution heat treatment by heating and holding at ~1050°C and then rapidly cooling.
．． The gist of the present invention is a method for manufacturing an austenitic stainless steel pipe with extremely fine grains of 11 or more.

FIG. 1 is a process diagram showing the steps of the method of the present invention, and a specific example of the method of the present invention will be explained with reference to the figure.

In the figure, (1) is a tube obtained, for example, by the hot extrusion tube manufacturing method, and this material is first subjected to cold working at a working degree of 60% in the first cold working step (2). After that, in the intermediate solution heat treatment step (3), the tube is charged into a large heating furnace, and the entire tube is heated to, for example, 1000° C., held at this temperature for, for example, 2 minutes, and then cooled with water for solution heat treatment.

Next, in the second cold working step (4), the working degree is, for example, 30%.
After the cold opening process, a final solution heat treatment step (5) is performed under the same conditions as the intermediate solution heat treatment to obtain a product (6).

Next, the reason why the working degree of cold working and the heating temperature of solution heat treatment are limited as described above in the present invention will be explained.

The reason why the degree of work in the first cold working was 60% or more was because
This is because if it is less than 60%, the degree of processing is insufficient and sufficient grain size cannot be obtained when intermediate solution heat treatment is performed.

The working degree of the second cold working was set to 20X or more because
This is because, in the case of steel pipes that are the object of the present invention, complete recrystallization cannot be obtained during solution treatment if the content is less than 20%.

Next, the heating temperature in the intermediate and final solution heat treatment was set to 9
The reason for setting the temperature to be 00 to 1050°C is because recrystallization becomes insufficient if it is less than 900°C, and if it exceeds <1050°C, the grain size will become coarser.

Note that the holding time after heating in the solution heat treatment is not particularly specified, but a short time of about 2 minutes is sufficient.

Further, water cooling is suitable for rapid cooling after heating and holding.

Furthermore, as the primary cold working means in the present invention, it is desirable to use, for example, so-called cold rolling by cold rolling mill/l'. The reason for this is that if the cold working with a working degree of 60% or more required in the first cold working of the present invention is performed in one so-called cold drawing process using a die and a blowfish, problems such as material breakage occur. Therefore, the processing limit in cold drawing processing is 50%.
It is a repeated drawing operation performed at least twice with the following working degree without softening treatment but with lubrication treatment, which requires a large number of work hours, but in the case of cold rolling, there is no problem at 60%. This is because machining with the above degree of machining can be performed in one go.

<Effect of the invention> Next, the present invention will be explained in detail with reference to some examples.

Outer diameter 55. Made by hot extrusion pipe making method. (Fll$
× 5US821'r with wall thickness 6.5 mm × length 4000
Using an austenitic Nutenrene steel pipe of P as a raw material, the working degree in cold working and the heating temperature in solution heat treatment were variously changed, and cold working (first time) - medium t[ J body heat treatment - cold working (second
1st) - Various tests were conducted using final solution heat treatment.

The results are shown in the graphs of FIGS. 2-4.

Figure 2 is a graph showing the relationship between particle size and solution treatment temperature for the first cold knee intermediate solution heat treatment material, and shows that the working degree of cold working (first time) was 28% and 49%, respectively.
% (comparative example) and 60% (inventive example).

Figure 3 shows the comparative example (2) shown in Figure 2 (intermediate solution temperature f
f: 1020°C), then the second cold working (working degree 29
%) → is a graph showing the relationship between particle size and final solution temperature in materials subjected to final solution heat treatment.

Figure 4 shows the material obtained by subjecting Example (3) of the present invention shown in Figure 2 (intermediate solution temperature: 1000'lU) to second cold working (working degree 30%) → final solution heat treatment. 2 is a graph showing the relationship between particle size and final solution temperature at .

In FIGS. 2 to 4 above, the heat treatment conditions for the intermediate and final solution heat treatments were water cooling after holding each heating temperature for 2 minutes. Further, in the same figure, the * mark is a symbol indicating material A, the ○ mark is a symbol indicating material B, and the Δ mark is a symbol indicating material C.

As shown in Figure 2, comparative example (1) is cold working (first time)
Because the degree of processing in K was too low at 28%, material A was reduced to 95%.
When heated at 0°C, 70% recrystallization was not completed, and even when heated at 1000°C, 10% recrystallization was not completed, and grain size refinement hardly progressed.

Comparative example (2) has a working degree of 4 in cold working (first time).
9% is still insufficient, 900t: 10% recrystallization is not completed in material A in heating, and material B in heating at 950 ° C.
Recrystallization was incomplete by 10%, and the grain size of Material A and Material B was only about 9 in the gap from 6 in the raw tube stage, and even in the final product stage in Figure 3, the grain size of Material A and Material B was the same. is NTIII
This was not achieved and was insufficient.

On the other hand, in Example (3) of the present invention, the degree of workability in the first cold working was as high as 60%, so that material C was already N011 or higher, material B, and material A at the stage after the intermediate solution heat treatment. The grain size is also regulated to a fine grain area with a diameter of approximately 10 or more, and the variation in grain size is extremely small, and in the final product, as shown in Figure 4, all of Material A, Material B, and Material C are fine.
Very fine grains with l'b11 or more were obtained.

As is clear from the above description, according to the present invention, it is possible to obtain seamless steel pipes of austenitic nutless steel with grains of yF+411 or more in an extremely economical manner by repeating the cold knee solution heat treatment process only twice. It is something that can be done.

[Brief explanation of the drawing]

Fig. 1 is a process diagram based on the manufacturing method of the present invention, and Fig. 2 is a process diagram based on the manufacturing method of the present invention.
Graphs showing the relationship between the particle size and the solution treatment temperature in the second cold knee intermediate solution heat treatment material, Figures 3 and 4
The figure shows the comparative example (2) of the treated material shown in Figure 2 and the inventive example (
3) is a graph showing the relationship between particle size and final solution temperature in materials (final products) subjected to cold drawing and final solution heat treatment, FIG. 8 is a comparative example, and FIG. Indicate the case.

Claims (1)

[Claims] (1) An intermediate solution obtained by first applying cold working to a degree of working of 60% or more to a raw pipe obtained by roll perforation pipe manufacturing method or hot extrusion pipe manufacturing method, then heating and holding at 900 to 1050°C, and then quenching. The austenite crystal grain size is No. 1, which is characterized by applying chemical heat treatment, then cold working with a degree of working of 20% or more, and then performing final solution heat treatment in which the product is heated and held at 900 to 1050°C and then rapidly cooled. A method for manufacturing an austenitic stainless steel pipe having extremely fine grains of 11 or more.