JPH01287249A - Austenitic stainless steel tube and its manufacture - Google Patents

Abstract

PURPOSE:To manufacture the title steel tube combining excellent steam oxidizing resistance and high temp. strength by specifying the coarse-grained structure and the fine-grained layer in an austenitic stainless steel tube consisting of specific compsn. CONSTITUTION:The austenitic stainless tube contg. 0.05-0.10% C, <=1.0% Si, <=2.0% Mn, 15-26% Cr, 10-35% Ni, <=0.02% S, <=0.05% N, <=0.04% P and 0.4-1.1% Nb, furthermore contg. one or more kinds among <=3.0% Mo, <=3.0% W, <=3.0% Cu, <=3.0% V, <=0.5% Al, <=0.15% Ti and <=0.15% Zr and the balance consisting of iron with inevitable impurities is prepd. The steel tube furthermore has the coarse-grained structure of No. 6 or below that in average crystal grain sized-number and the fine-grained layer of No. 7 or above that in average crystal grain sized-number having 50-300mu thickness on the side of its inner face; C+N in the fine grained-layer part is regulated to >=0.15%.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an austenitic stainless steel pipe and a method for manufacturing the same, and an object of the present invention is to provide an austenitic stainless steel pipe that has both excellent steam oxidation resistance and high-temperature strength, and a preferable method for manufacturing the same.

Nb-added SUS 347 stainless steel pipes have long been considered high-temperature steel pipes with excellent steam oxidation resistance because they have considerable high-temperature strength even when the grain size is fine. In order to improve thermal efficiency, the operating temperature and pressure of steel pipes are rising or increasing, so materials with higher strength are required. However, the above 5U
In S347 steel, a considerable increase in strength can be expected by coarsening the grain size to a certain extent, but in this case, the steam oxidation resistance decreases due to the coarse grain size. That is, in such stainless steel pipes, it is technically impossible to improve both high temperature strength properties and steam oxidation resistance.

The present invention was devised after repeated studies in view of the above-mentioned actual circumstances, and has been developed to improve high-temperature strength and also improve high-temperature steam oxidation resistance.
We succeeded in obtaining a doped stainless steel mesh pipe and its manufacturing method. That is, in order to improve the high-temperature strength as described above, the steel pipe according to the present invention has a composition in which the contents of C, Nb, and S are adjusted in particular, and most of the wall thickness of the steel pipe with such a composition (inner surface fineness) is adjusted. The crystal grain size (other than the grain layer) was adjusted to a relatively coarse grain size, and a fine grain layer was formed within a specified range on the inner surface exposed to high-temperature steam to maintain good steam oxidation resistance. It is designed to achieve comprehensive performance, which is different from stainless steel containing fine particles on the inner surface.

To further explain the present invention as described above, first, in order to ensure creep rupture strength, SUS 3, which is generally known as Nb-added austenitic stainless steel, is used.
The aim of the present invention is to make it equivalent to or higher than the coarse grained version of 47fi, more specifically, 650.
The creep rupture strength after 100,000 hours at ℃ is at least 9 kg/mu2 or more. Regarding this relationship, Figure 1 shows 18%Cr-12%Ni-0.01%S.
The relationship between the amount of C, the amount of Nb, and the creep rupture strength is shown for the heath of The range where am2 or more is inside the notched curve. Also, in Figure 2, the grain size N
The influence of the amount of S on the creep rupture strength is similarly shown for Q4 and positive 8.5 samples, and it is clear from FIG. 2 that the adverse effects of S become more pronounced as the grains become finer.

Furthermore, Fig. 3 shows the influence of the crystal grain size on the breaking strength when the above-mentioned steel is held at the solution temperature indicated on the upper horizontal axis for 10 minutes each. From the results shown in Figure 3, in order to obtain the above-mentioned creep rupture strength of 9 kg/in2 or more, the particle size number should be coarser than No. 6, and the composition should be 0.05 to 0.1% C and N.
It is understood that it is a necessary condition that b be 0.1% or more and S be 0.02% or less, and that this is a condition that should be met for SUS 347wA, which accounts for most of the wall thickness of the steel pipe. Knew.

On the other hand, it is necessary to make the grain near the inner surface of the steel pipe fine, and in order to make the grain fine, available precipitates are Nb
(C,N), so these Nb.

The amounts of C and N will also be regulated. However, among these, N
The amount of b and the amount of C are usually adjusted and determined when steel is melted, but the amount of N is not only adjusted during melting, but also is diffused into the gas in a solid state in the present invention. As shown in Figure 3, it is necessary to set the solution treatment temperature to 1170°C or higher due to high-temperature strength limitations, and even at such temperatures, crystal grains do not grow near the inner surface of the steel pipe. If sufficient Nb (C,N) force is not exerted to suppress the Nb (C,N), the required fine grain layer near the inner surface cannot be obtained after the solution treatment. Therefore, it is necessary to determine the required amount of Nb and C as well as the amount of N to be introduced as described above. That is, the SUS 34 shown in FIG.
The relationship between the amount of (C+N) and the grain size obtained after solution treatment at 1170°C in Steel No. 7 is shown. It has been experimentally confirmed that the high temperature steam oxidation resistance is improved when the particle size is present at 50N or more from the surface, preferably 100μ or more and 300μ or less. It can be seen from FIG. 4 that the amount of (C+N) is required to be at least 0.15% in the range of the fine grain layer required as described above. In the present invention, as described above, Nb carbonitride prevents the growth of crystal grains in the inner surface layer of the tube during high-temperature solution treatment, which is necessary to ensure high-temperature strength, thereby maintaining the fine grains in this area. It is something. Therefore, if the amounts of C and N required for the formation of these precipitates were adjusted in the molten state of the steel, after solution treatment, only a steel pipe with a -like grain size could be obtained over the entire wall thickness of the pipe. Therefore, the purpose of the present invention cannot be achieved. Therefore, in the present invention, N, which does not penetrate into the steel, is infiltrated in the solid state as described above within the thickness (depth) range as described above on the inner surface of the pipe where the fine grain layer is to be formed. In order to make the grain size into fine grains within the above range, it is basic that N is introduced in a fine grain state with a grain size number of 7 or higher. N that has entered the coarse-grained material only becomes precipitates within the grains, and there is no grain-refining effect unless there is a subsequent processing step (below 11.70° C.) and an accompanying recrystallization step.

The reason why the fine grain layer with an average grain size of 7 or more is set at 50N or more and 300μ or less from the tube inner surface is as follows.

The usage period of steel pipes is usually 100,000 to 300,000 hours, and during this time, if the grain size is 7 or higher, oxidation of about 40N will progress on the inner surface of the steel pipe due to water atrophy, so the fine grain layer should be at least 50μ
shall be.

Further, the upper limit of the fine grain layer is set to 300μ because even if it exceeds 300μ, there is almost no increase in the effect of reducing the oxide scale thickness, and furthermore, it has an adverse effect on the creep rupture strength.

The amount of (C+N) needs to be at least 0.15%, but as the amount of (C+N) increases, the ductility decreases, so the upper limit is preferably 0.5%.

Figure 5 shows the base component] 8Cr -12Ni 0
．． 01N, and the inner surface of the steel pipe made by varying C and Nb.
r N2 mixed gas (N2: 20-50%) 10-2
Intrusion nitrogen treatment was carried out while flowing for 0 minutes, and then 117
The figure shows the average grain size near the inner surface of the tube obtained when solution treatment was carried out at 0°C. In order to achieve the above, C should be 0.04% or more and Nb should be 0.04% or more.
It can be understood that it is necessary to set it to 4% or more.

As shown in Figs. 1 and 5 above, in order to maintain high-temperature strength above the above level and obtain appropriate high-temperature steam oxidation resistance, the C content of Nb-added Austevy 1 stainless steel should be 0.05 to 0.05. It must be 0.10%, and the lower limit of Nb is 0.4%. Furthermore, if the Nb content exceeds 1.1%, the high temperature strength begins to deteriorate and weldability also deteriorates, so the upper limit is set at 1.1%.

Cr is required to be at least 15% in order to ensure water atrophy oxidation resistance, but if the content exceeds 26%, austenite becomes unstable, so the content is set at 15 to 26%. Also, Ni is 1
If it is less than 0%, austenite becomes unstable and this Ni
As the amount increases, austenite becomes more stable, but if the content exceeds 35%, the effect will be saturated and the cost will be disadvantageous, but the content is set at 10 to 35%.

Among the components of the austenitic stainless steel of the present invention, the content ranges of C, Cr, Ni, S, and Nb and the reasons for numerical limitations have been described above, but other components will be described below.

Si: Added for deoxidation and oxidation resistance, but if it exceeds 1%, it promotes precipitation of brittle phases such as sigma phase, so its content is kept at 1% or less.

Mn: Effective in deoxidizing, desulfurizing, and stabilizing the austenite phase, but the effect is not significant when it exceeds 2%, so the content is set to 2% or less.

N: Effective for forming a fine grain layer, but excessive addition causes fine grains in the central part of the wall thickness and impairs creep strength, so the content should be 0.05% or less.

P: It is contained as an impurity, but if its amount is large, it reduces hot workability, weldability, etc.
．． Keep it below 0.04%.

Furthermore, one or more types of Mo, W, Cu, ■, Aff, Ti, and Zr are added.

Mo, W: Elements effective for high-temperature creep strength, but in order to promote the precipitation of sigma phase, the content of both should be reduced to 3%.
Do the following.

Cu, ■= an element effective in improving creep strength,
Excessive content significantly reduces hot workability, so the content should be kept at 3% or less.

Al: Effective as a deoxidizing agent, but since it is a ferrite phase-forming element, excessive addition impairs the stability of the austenite phase, so the content is set to 0.5% or less.

Ti, Zr: Forms carbides and increases creep rupture strength, but if added excessively, brittle phases such as Laves phase will precipitate, so their content should be 0.15% or less.

FIG. 6 shows typical processing conditions for obtaining cape grain layers with thicknesses of 50 μm and 100 μm on the inner surface of the steel pipe. The test piece is 0.06%C-0.01%N-0.7%N
It has a composition of b-17%Cr-12%Ni-0.01%S, the grain size is 8.5, and the depth of the fine grain layer is 1170℃ or higher after nitrogen invasion treatment. Measurements were made on those that had been subjected to a chemical treatment (that is, a treatment to coarsen grains other than the nitrogen-contaminated portion). N2 gas is used for the nitrogen invasive treatment, and the time it takes for the book temperature to rise to the specified nitrogen invasive holding temperature is approximately 2 minutes, and as the N2 content in the nitrogen invasive gas decreases, the processing time is as shown in Figure 6. Similarly, when the amount of oxygen in the gas is large, the required time also becomes slightly longer.

If lightning resistance is applied to the nitrogen invasive treatment time shown in Figure 6, the relationship between treatment temperature and time will be N as shown in the figure.
If the ratio of N2 gas and impurity 0□ is 0□:5% or less, it is the case that a gas with N2:25% or more is used,
0□: N if it is 1% or less, : holds true if it is 5% or more. For example, by creating a reducing atmosphere by significantly reducing oxygen to 0.1% or less and mixing NH3 decomposition gas, the results in this figure shift slightly to the left in the diagram. Needless to say, this effect is more remarkable at low temperatures where long-term treatment is required. Also 9
If nitrogen invasion is performed at a low temperature below 00°C, the N concentration near the surface will increase, but it will take time for the N to diffuse to the required depth.
It is preferable to perform a heat treatment at a temperature of less than 0° C. for diffusion. This diffusion time is the holding time at the high temperature shown (e.g. 110
If the temperature is 0°C, you only need to do it for 5 minutes).

As mentioned above, the grain size number before treatment must be 7 or higher; on the other hand, if it has been cold worked in advance or 11
When processed at a temperature below 70°C, the processed layer recrystallizes by the time it reaches the nitrogen invasive treatment temperature, so fine grain materials,
It can be used regardless of coarse grain material. In this case, the degree of cold working required is 10% or more, and since recrystallization does not occur if the degree of cold working is 10% or less, a degree of cold working of 10% or more is required. Further, the degree of cold working is the wall thickness reduction rate in the case of cold rolling, etc., and in the case of applying cold working only to the vicinity of the inner surface such as shot processing, it is converted by microhardness measurement.

Regarding the upper limit of the degree of cold working, (1) There is no need to specifically limit the degree of cold working as long as it can be carried out industrially. In the processing of steel pipes (tube drawing, rolling), the upper limit is usually about 60%, but even higher processing rates can be achieved.

(2) In processing near the surface such as shot processing and grinder processing, the surface has a hardness of 1
Exceeds 00% (extrapolation). However, the average processing degree difference is 50
% or less.

Also, its depth is 501! It is sufficient if the thickness is greater than that, and the entire thickness may be sufficient. If the nitrogen invasive treatment temperature exceeds 1170'C, the rate of temperature rise becomes a problem, and a rough guideline is 500'C/min; if the temperature is raised at a rate higher than that, the fine grain layer becomes shallow. The required fine grain layer thickness of 50μ cannot be obtained. It goes without saying that when rapid heating of 1000° C./min or more is performed by a method such as direct current heating, the heating temperature is limited to 1170° C. or less.

``The temperature increase rate limit when the nitrogen invasive treatment temperature exceeds 1] 70°C as described in ``1'', and the heating temperature limit during ultra-rapid heating, apply when the average grain size of the treatment tube is No. 7 or higher.'' This must be observed both in the case of fine grains and in the case of additional processing.

The results shown in FIG. 6 above were obtained by nitrogen invasive treatment and solution treatment for grain size numbers of +7 and above, but the results are not limited to the case of tobacco according to the present invention; A steel pipe having a coarse-grained/fine-grained two-phase MI weave can also be manufactured by such a process.

1) [Yes] Processing of 10% or more (processing temperature is 117°
C) -〇 Nitrogen invasive treatment as shown in Figure 6-■
Solution treatment at 1170°C or higher (■ ■ can be treated in the same process by appropriately selecting the heating rate. In other words, nitrogen invasive treatment can be performed during the process of raising the temperature to the solution treatment temperature) 2) −・■ −■ 3)” 2nd entry 1. ), the treatment process (■) in 2) is separated into nitrogen invasion treatment and diffusion treatment.

As a precautionary measure, if lightning resistance is provided, in the case of processing in step 3), if processing at a temperature of 1100'C or less is performed after the diffusion treatment, there is no need to set a limit on the temperature of the diffusion treatment; If processing steps such as
i.e., less than 1170'c). In addition to the basic pattern as described above, combinations of 1) to 3) above are also possible. For example, in 1), the ■ and ■
■ Process may be charged again in between.

To summarize the manufacturing method of the present invention as described above, the basics are as follows: (1) Final heat treatment (solution treatment) must be at 1170°C or higher.

■ At least at a stage before the material is exposed to the solution treatment temperature (this may be while the steel pipe is being heated to the solution treatment temperature, or in a completely different process). A fine grain state (or when exhibiting such a fine grain state) with a grain size number of No. 7 or higher in the thickness range of 50 to 300 μ from the inner surface of the steel pipe, and maintains the fine grain state even at solution treatment temperature. (i.e. C-1-N20.15%
) must be within the above thickness range.

However, various steps such as cold working, heat treatment, and pickling may be freely inserted before the final heat treatment, and as a matter of course, as long as the second heat treatment after the final heat treatment does not significantly exceed the solution treatment temperature, There are no problems with high temperature steam oxidation resistance and high temperature strength.

Nitrogen invasion treatment can also be performed by sealing gas inside the pipe, and the gas conditions are N of 0.1 cc/cnt or more per unit area converted to normal temperature and normal pressure, gas (gas to be decomposed is assumed to be decomposed) Calculate and if it is NH3, it is 0.2
ccZcIl), a gas containing 1 cc or less of 02 gas (calculated in 02 minutes) is a good guideline.

The following table summarizes the composition and performance of specific manufacturing examples according to the present invention, comparative examples thereof, and manufacturing methods thereof. Note that in all representative examples, the S content is 0.02% or less, and all are pickling materials.

] 8 According to the present invention as explained above, it is possible to further improve the high-temperature strength of a stainless steel pipe to which Nb has been added, and to maintain its high-temperature steam oxidation resistance at an appropriately high level. This is a highly effective invention.

[Brief explanation of the drawing]

The drawings show the technical contents of the present invention, and FIG. 1 shows 18%Cr-1,2%Ni-0,01%N-0,01
Figure 2 shows the effects of C and Nb on the creep rupture strength of %S steel at 650°C for 1.1 million hours during solution treatment at temperatures above 1170°C. Figure 3 is a diagram showing the effect of grain size on the creep rupture strength of the above steel, Figure 4 is a diagram showing the relationship between C+N content and 1170°C grain size, and Figure 5 is a diagram showing the relationship between the grain size of the above steel. 1050 in steel
Figure 6 shows the average grain size near the inner surface of a steel pipe after nitrogen invasion treatment for 110 minutes and solution treatment at 1170°C. It is a chart showing the relationship between required heat treatment time and temperature.

Claims (1)

[Claims] 1. C: 0.05-0.10%, Si≦1.0%, Mn
≦2.0%, Cr: 15-26%, Ni: 10-35%
, S≦0.02%, N≦0.05%, P≦0.04%,
Contains Nb: 0.4 to 1.1%, and further Mo≦3.0%
, W≦3.0%, Cu≦3.0%, V≦3.0%, A1
≦0.5%, Ti≦0.15%, Zr≦0.15% 1
The average grain size number of a steel pipe made of austenitic stainless steel containing one or more species and the remainder consisting of iron and unavoidable impurities is No. 6 or less, the thickness on the inner surface side is 50 to 300μ, and the average grain size number is No. An austenitic stainless steel pipe having 7 or more fine grain layers, the fine grain layer having a C+N content of 0.15% or more. 2, C: 0.05-0.10%, Si≦1.0%, Mn
≦2.0%, Cr: 15-26%, Ni: 10-35%
, 5≦0.02%, N≦0.05%, P≦0.04%,
Contains Nb: 0.4 to 1.1%, and further Mo≦3.0%
, W≦3.0%, Cu≦3.0%, V≦3.0%, Al
≦0.5%, Ti≦0.15%, Zr≦0.15% 1
117 steel pipes made of austenitic stainless steel containing one or more species and the remainder consisting of iron and unavoidable impurities.
When solution treatment is performed to form a coarse grain structure at 0°C or higher, the average grain size number at least within the thickness range of 50 to 300μ on the inner surface of the steel pipe is No. C+N within the above thickness range in a fine grain state of 7 or more is 0.15%
By infiltrating N as described above, even during the solution treatment, the average grain size number within the thickness range of 50 to 300μ on the inner surface side becomes No. A method for producing an austenitic stainless steel pipe characterized by maintaining a fine grain state of 7 or more. 3. The average grain size number is No. No. 7 or more fine-grained austenitic steel pipe without any processing that would change its grain size.
C+N within the range of 50 to 300μ on the inner surface of the steel pipe is 0.7 or more under the condition of exhibiting a fine grain state of 7 or more.
The method for manufacturing an austenitic stainless steel pipe according to claim 2, characterized in that the nitrogen invasion treatment is carried out so that the nitrogen content becomes 15% or more. 4. At least a step of processing 10% or more at a temperature of 1170° C. or lower within a thickness range of 50 to 300 μ on the inner surface side, and a nitrogen invasive treatment step, so that the average grain size in the thickness range is No. The method for manufacturing an austenitic stainless steel pipe according to claim 2, wherein C+N is 0.15% or more in a fine grain state of 7 or more.