JPH0453943B2 - - Google Patents

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 thereof 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 austenitic stainless steel pipes have high high-temperature strength and have been used as high-temperature steel pipes. In these steel pipes, a considerable increase in high temperature altitude can be expected by coarsening the grain size to some 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 created after repeated studies in view of the above-mentioned circumstances, and provides an Nb-added stainless steel pipe with improved high-temperature strength and improved high-temperature steam oxidation resistance, and a method for manufacturing the same. It was very successful. That is, in order to improve the high-temperature strength as described above, the steel pipe according to the present invention adjusts the amounts of C, Nb, N, Mo, Cu, W, etc. in the composition, and also adjusts the wall thickness of the steel pipe with such a composition. Most of the grain size (other than the inner fine-grained layer) was adjusted to a relatively coarse grain size, and a fine-grained layer was identified on the inner surface, which is exposed to high-temperature steam, to maintain good steam oxidation resistance. It is formed within this range, and is designed to achieve comprehensive performance unlike stainless steel with fine grained Ti added to the inner surface. To further explain the present invention as described above, first of all, regarding ensuring creep rupture strength,
The goal of the present invention is to achieve a steel that is equivalent to or better than that of coarse-grained SUS347 steel, which is generally known as Nb-added austenitic stainless steel. The creep rupture strength is at least 11 Kg/mm ² or more. Figure 1 shows this relationship.
18%Cr-12%Ni-0.01%S base and 17%Cr-15%N:-2.5%Mo-2.5%Cu-0.01
%S according to the present invention (hereinafter referred to as MC steel), the relationship between C content, Nb content, and creep rupture strength is also shown. The range where the creep rupture strength is 9Kg/mm ² or more for the base steel shown in parentheses is inside the hatched thin line curve, but according to the present invention, the creep rupture strength is 11Kg/mm 2 or more.
The range of mm ² or more is the range of the thick line. Figure 2 also shows the influence of the amount of S on the creep rupture strength for base steels with grain sizes No. 4 and No. 8.5 and steel of the present invention with grain size No. 4. It is clear that the adverse effects of S become more pronounced as the temperature increases. Furthermore, Fig. 3 shows the relationship between the influence of grain size and the creep rupture strength when the base steel and the invention steel as described above are held at the solution temperature indicated on the upper horizontal axis for 10 minutes each. However, based on the results shown in Figures 1 to 3, the above 11
In order to obtain creep rupture strength of Kg/ ^mm2 or more, the grain size number should be coarser than No. 6, and the composition should be C.
It is understood that the necessary conditions are 0.05-0.1%, Nb 0.1-1.1%, S 0.02% or less, and the addition of reinforcing elements Mo, Cu, W, V, Ti, and Zr. I learned that this is a condition that must be met for most of the steel pipe wall thickness. On the other hand, it is necessary to make the grains near the inner surface of the steel pipe fine, and the precipitates that can be used to make the grains fine are Nb (C, N), so these Nb,
The amounts of C and N will also be regulated. Of these, the amount of Nb and the amount of C are usually adjusted and determined during melting of steel, but in addition to adjusting the amount of N during melting, in the present invention, the amount of N is not only adjusted at the time of melting, but also diffused in from the gas in the solid state. It was assumed that In other words, as shown in Figure 3, it is necessary to set the solution treatment temperature to 1170°C or higher due to the limitations on high-temperature strength, and even at such temperatures, the growth of crystal grains near the inner surface of the steel pipe is suppressed. If sufficient Nb (C, N) is not precipitated, the required fine grain layer near the inner surface cannot be obtained after the solution treatment. Therefore, it is necessary to determine the amount of N to be introduced as described above as well as the required amount of Nb and C. That is, (C
The relationship between the amount of +N) and the grain size obtained after solution treatment at 1170°C is shown for the base steel shown by the thin line and the invention steel shown by the thick line. It has been experimentally confirmed that high-temperature steam oxidation resistance is improved when a fine particle layer of 50μ or more, preferably 100μ or more, and 300μ or less exists from the inner surface of the pipe, and the particle size number is No. 7 or 3. In order to achieve more than that, within the range of the fine grain layer required as described above,
It can be seen from FIG. 4 that the (C+N) amount needs to be at least 0.15%. In the present invention, as mentioned above, Nb carbonitride prevents the growth of crystal grains in the inner surface layer of the pipe during high-temperature solution treatment, which is necessary to ensure high-temperature strength, thereby making this part fine-grained. It is to be maintained. 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 uniform grain size over the entire wall thickness of the pipe would be produced. Therefore, the object of the present invention cannot be achieved. Therefore, in the present invention, N, which easily penetrates into steel, is infiltrated in a solid state as described above within the thickness (depth) range described above on the inner surface of the tube where a fine grain layer is to be formed. In order to make the grain size into fine grains within the above range, it is essential that N is introduced in a fine grain state with a grain size number of No. 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 1170° C.) and an accompanying recrystallization step. The reason why the fine grain layer with an average grain size No. 7 or more is set at 50 μm or more and 300 μm or less from the inner surface of the tube 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 No. 7 or more, the inner surface of the steel pipes will be damaged by water vapor.
Since oxidation of about 40μ is thought to proceed, the fine grain layer should be at least 50μ. Further, the upper limit of the fine grain layer is set at 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 a negative effect on the creep rupture strength. The amount of (C+N) must 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 a steel pipe with a base composition of 18Cr-12Ni-0.01N and a material of the MC steel according to the present invention with a grain size number of 8.5, and the inner surface of a steel pipe made by varying C and Nb. _N2 at 1100℃
gas or Ar- _N2 mixed gas ( _N2 :20~50%) at 10%
The average grain size near the inner surface of the tube obtained by performing intrusion nitrogen treatment while flowing the pipe for ~20 minutes and then solution treatment at 1170°C is shown for the base steel (thin line) and the invention steel (thick line). According to this figure, in order to obtain a grain size number of 7 or more to obtain excellent high-temperature steam oxidation resistance, C is
It can be understood that Nb needs to be 0.04% or more, and Nb needs to be 0.4% or more. Note that the range shown in FIG. 5 does not change even if the solution treatment temperature is increased to 1170° C. or higher. The upper limit of the solution treatment temperature is practically 1250°C, and while increasing the temperature above 1250°C has a slight effect of increasing strength, there are disadvantages such as a decrease in ductility and the generation of a large amount of scale due to heat treatment. It becomes noticeable. As shown in Figures 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 austenitic stainless steel must be 0.05 to 0.10%. Naturally, the lower limit of Nb is 0.4%. Furthermore, if the Nb content exceeds 1.1%, high temperature strength begins to deteriorate and weldability also deteriorates, so 1.1% is set as the upper limit.
Cr is at least 15% to ensure steam oxidation resistance
is necessary, but if the content exceeds 26%, the austenite becomes unstable, so the content is set at 15 to 26%.
Also, if Ni is less than 10%, austenite becomes unstable, and as the amount of Ni increases, austenite becomes stable, but if it is contained in more than 35%, the effect will be saturated and it will only be disadvantageous in terms of price. Ten~
It shall be 35%. Among the components of the austenitic stainless steel of the present invention, the content ranges of C, Cr, Ni, S, and Nb and reasons for numerical limitations have been described above, but other components will be described below. Si: Added for deoxidation and oxidation resistance, 1%
If the content exceeds 1%, the precipitation of embrittling phases such as sigma phase will be promoted, so the content should be 1% or less. Mn: Effective for deoxidizing, desulfurizing, and stabilizing the austenite phase, but the effect is not significant even if 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, impairing creep strength, so the content should be 0.05% or less. P: Although contained as an impurity, it has a reinforcing effect. However, if the amount is large, it will reduce hot workability, weldability, etc., so the content should be reduced to 0.04%.
Do the following. Mo, W, Cu, V, Ti and Zr as reinforcing elements
For these, one or more types may be added. Mo and W: Both are effective elements for high-temperature creep strength, but since excessive addition promotes precipitation of sigma phase, the content of each is set to 3% or less. Cu, V: These elements are also effective in improving creep strength, but excessive content significantly reduces hot workability, so the content should be reduced to 3.
% or less. That is, as shown in FIG. 6, creep rupture strength is improved by adding a few percent of each. However, its effect tends to be saturated at around 3%, and addition of a large amount leads to () decrease in toughness due to sigma phase precipitation (Mo, W, V), () decrease in hot workability (Cu,
V), () causes a decrease in high temperature corrosion resistance (V), so the upper limit is set to 3%. Ti, Zr: These elements also form carbides and increase creep rupture strength, but excessive addition has no effect and embrittlement phases such as Laves phase precipitate, so their content should be 0.15% or less. . Further, in the present invention, Al is added as an element that increases oxidation resistance. This Al is effective as a deoxidizing agent and improves oxidation resistance, but it is also a ferrite phase forming element and excessive addition impairs the stability of the austenite phase, so its content must be kept at 0.5% or less. Figure 7 shows typical processing conditions for obtaining fine grain layers with thicknesses of 50μ and 100μ on the inner surface of the steel pipe. The test piece was made of 0.09% C- as the base steel.
0.01%N-0.7%Nb-17%Cr-12%Ni-0.01%S
and has a grain size No. of 8.5, and as the steel of the present invention, 0.05%C-0.01%N-0.7%Nb-
17%Cr-15%Ni-2.5%Mo-2.5%Cu-0.01%S
The crystal grain size No. with the same composition is 8.5, and the depth of the fine grain layer is determined by solution treatment at 1170℃ or higher (i.e., coarse graining treatment for areas other than the nitrogen invaded portion) after the nitrogen invaded treatment. Measurements were taken on those that had been carried out. N ₂ gas is used for the nitrogen invasive treatment, and the temperature rise time to the specified nitrogen invasive holding temperature is approximately 2 minutes, and as the N ₂ minutes in the nitrogen invasive gas decreases, the treatment time will be as shown in Figure 7. Similarly, when the amount of oxygen in the gas is large, the required time also becomes slightly longer. Regarding the nitrogen invasion treatment time shown in Figure 7, the relationship between treatment _temperature and time is as _follows :
If O ₂ : 5% or less, it means that a gas with N ₂ : 25% or more is used, and if O ₂ : 1% or less, N ₂ :
This holds true for gases with a content of 5% or more. For example, _O2
By creating a reducing atmosphere with a low oxygen content, such as 0.1% or less, and by mixing NH ₃ 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 900℃
If nitrogen immersion is performed at a low temperature as shown below, the N concentration near the surface will increase, but since it takes time for the N to diffuse to the required depth, heat treatment at a high temperature (below 1170℃) is performed after the low temperature nitrogen immersion treatment. It is preferable to diffuse it. The diffusion time may be as long as the holding time at the high temperature shown in the figure (for example, 5 minutes at 1100° C.). As mentioned above, the grain size number before treatment must be No. 7 or higher; on the other hand, if it has been previously cold worked or worked at a temperature below 1170°C, the worked layer must be subjected to nitrogen invasive treatment. Since it recrystallizes before reaching temperature, it can be used regardless of whether it is a fine-grained material or a coarse-grained material. In this case, the working degree required is 10% or more, and since recrystallization does not occur if the working degree is less than 10%, a working degree of 10% or more is required. In the case of processing such as rolling, it is the wall thickness reduction rate, and in the case of processing only near the inner surface, such as shot processing, it is converted by microhardness measurement. Regarding the upper limit on the degree of processing: (1) There is no need to limit the processing 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 machining near the surface such as shot machining and grinder machining, the hardness of the surface exceeds 100% (extrapolation). However, the average processing degree difference is about 50% or less.
The lower limit of processing temperature does not need to be specified, but it is between room temperature and 1170℃, and usually between room temperature and 500℃.
That's about it. The depth may be 50μ or more, 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 below and the temperature is increased at a rate higher than that, the fine grain layer becomes shallow and the required fine grain layer thickness of 50μ cannot be obtained. In addition, the temperature can be increased to 1000℃/
The heating temperature when performing rapid heating over min is 1170
Needless to say, the temperature is limited to below ℃. As mentioned above, the temperature increase rate limit when the nitrogen invasion treatment temperature exceeds 1170℃, and the heating temperature limit during ultra-rapid heating, are applied when the average grain size of the treatment tube is No. 7 or finer grains. This must be observed in any case where processing is added. The results shown in FIG. 7 above were obtained by nitrogen invasion treatment and solution treatment with grain size numbers of No. 7 or higher, but the results according to the present invention are not limited to this case, and are as follows. A steel pipe having a coarse-grained-fine-grained two-phase structure can also be produced by the following steps. (1) Processing of 10% or more (processing temperature is 1170℃ or less)
→Nitrogen invasive treatment as shown in Figure 7 →1170
Solution treatment at temperatures above ℃ ( 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) As mentioned above →→ ( 3) The treatment processes in (1) and (2) above are separated into nitrogen invasion treatment and diffusion treatment. Just to be sure, in the case of processing in step (3), if there is a process of processing at a temperature below 1170℃ after the diffusion treatment, there is no need to set a limit on the temperature of the diffusion treatment; If such a processing step is not included, the diffusion treatment must also be carried out at a temperature range in which crystal grains do not become coarse (ie, below 1170° C.). In addition to the above basic patterns, combinations of the above (1) to (3) are also possible. For example, in (1), a process may be reintroduced between the two. To summarize the manufacturing method of the present invention as described above,
The following are the basics: The final heat treatment (solution treatment) is 1170℃ or higher.
Must be below 1250℃. 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), at least the inner surface of the steel pipe is from 50 to 300μ
In a fine-grained state (or when exhibiting such a fine-grained state) with a grain size number of No. 7 or higher in the thickness range of (C+N0.15%) penetrates into the above thickness range. However, various processes such as processing, heat treatment, pickling, etc. may be freely inserted before the final heat treatment, and of course, as long as the heat treatment after the final heat treatment does not significantly exceed the solution treatment temperature, it is resistant to high temperature steam. There are no problems with oxidation and high temperature strength. Nitrogen invasion treatment can also be performed by sealing gas inside the pipe, and the gas conditions are N ₂ gas of 0.1 cc/cm ² or more per unit area at room temperature and normal pressure (the gas to be decomposed is the decomposed gas). For NH ₃ , it is calculated as 0.2 cc/cm ² ), and a gas containing 1 c.c. or less of O ₂ gas (calculated as O ₂ min) is a rough guideline. The compositions and performances of specific manufacturing examples according to the present invention, comparative examples thereof, and manufacturing methods thereof are summarized as shown in Tables 1 to 5 below. In addition, in these items, what is appended to the product number is the crystal grain size number of the material, and anything without an appendix is a material with grain size number 8.0. In other words, Table 1 shows the base component as a reinforcing element.
The graph shows cases in which Mo and Cu are added singly and in combination, and Ti is added in some cases, and comparative materials are also shown. Table 2 shows Mo and W as reinforcing elements in the base component.
This figure shows cases in which these are added singly and cases in which they are added in combination. Also, Table 3 shows the case where Ti and Zr are added alone,
The case where they are added in combination with Mo and Cu is shown. Furthermore, Table 4 shows the case where Al is added, and this Al is added in combination with a reinforcing element, typically Ti,
This shows the case where Mo and Cu are added in combination. Table 5 summarizes the manufacturing method of the present invention. The component composition of the raw pipe is the same as that of manufacture No. 10 in Table 2, and the manufacturing conditions and nitrogen invasive treatment for the raw pipe using this material are shown. The product characteristics under different conditions are shown in the same manner as in Tables 1 to 4 above.

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[Table] When according to Tables 1 to 5 as described above, steel pipes No. 1 to 23 according to the present invention are in any case
The creep rupture strength at 650℃ for 100,000 hours is 11Kg/mm ² or more, and the steam oxidation resistance is low at 15μm or less at 600℃ for 1000 hours. On the other hand, the comparative example has a temperature of 650℃ and 100,000 yen.
The high temperature strength of hr is 10Kg/ ^mm2 or less, or if it is higher, the amount of oxide scale is 40μ
It was confirmed that the sample exhibited a high value of m or more, and was considerably inferior to that of the present invention in at least one aspect. According to the present invention as described above, the high temperature strength of Nb-added stainless steel pipes can be further improved, and the high temperature steam oxidation resistance can be maintained at an appropriately high level. This is a great invention.

[Brief explanation of drawings]

The drawings show the technical content of the present invention, and Figure 1 shows 18%Cr-12%Ni-0.01%N-0.01%
S base steel and 17%Cr-15%Ni-2.5%Mo-
A chart showing the influence of solution treatment of C and Nb at 1170°C or higher on the creep rupture strength of 2.5%Cu-0.01%N-0.01%S steel according to the present invention at 650°C for 100,000 hours. Figure 2 is a diagram showing the influence of S on the creep rupture strength of each of the above steels, Figure 3 is a diagram showing the effect of grain size on the creep rupture strength of each of the above steels, and Figure 4 is the diagram shown above. A chart showing the relationship between the amount of C+N and the grain size at 1170°C for each steel. Figure 5 shows the steel pipes of each of the above steels after being subjected to nitrogen invasion treatment at 1050°C for 10 minutes and then solution treatment at 1170°C. Figure 6 is a diagram showing the average grain size near the inner surface, Figure 6 is a diagram showing the influence of alloying elements on creep rupture strength, and Figure 7 is the heat treatment required to obtain a fine grain layer depth of 50μ or more. It is a chart showing the relationship between time and temperature.

Claims (1)

[Claims] 1 C: 0.05 to 0.10%, Si≦1.0%, Mn≦2.0%,
Cr: 15-26%, Ni: 10-35%, S≦0.02%, N
Contains ≦0.05%, P≦0.04%, Nb: 0.4-1.1%,
Furthermore, Mo≦3.0%, W≦3.0%, Cu≦3.0%, V≦3.0
%, Ti≦0.15%, Zr≦0.15% or both.
A steel pipe made of austenitic stainless steel containing at least 100% of the grain size, with the remainder consisting of iron and unavoidable impurities, has a coarse grain structure with an average grain size number of No. 6 or less, and a thickness of 50 to 300 μ on the inner surface side and has an average grain size number of an austenitic stainless steel having a fine grain layer of No. 7 or higher, and a C+N content of the fine grain layer portion being 0.15% or more. 2 C: 0.05-0.10%, Si≦1.0%, Mn≦2.0%,
Cr: 15-26%, Ni: 10-35%, S≦0.02%, N
Contains ≦0.05%, P≦0.04%, Nb: 0.4-1.1%,
Furthermore, Mo≦3.0%, W≦3.0%, Cu≦3.0%, V≦3.0
%, Ti≦0.15%, Zr≦0.15% or both.
A steel pipe made of austenitic stainless steel that contains more than 100% Al and 0.5% Al, with the remainder being iron and unavoidable impurities, has a coarse grain structure with an average grain size number of No. 6 or less, and a thickness of 50% on the inner surface side. ~300μ and has a fine grain layer with an average grain size number of No. 7 or higher, and the C+N of the fine grain layer portion
Austenitic stainless steel characterized by having a content of 0.15% or more. 3 C: 0.05-0.10%, Si≦1.0%, Mn≦2.0%,
Cr: 15-26%, Ni: 10-35%, S≦0.02%, N
Contains ≦0.05%, P≦0.04%, Nb: 0.4-1.1%,
Furthermore, Mo≦3.0%, W≦3.0%, Cu≦3.0%, V≦3.0
%, Ti≦0.15%, Zr≦0.15% or both.
A steel pipe made of austenitic stainless steel, which contains more than 100% of carbon dioxide, with the remainder consisting of iron and unavoidable impurities.
During solution treatment to form a coarse grain structure at 1170 to 1250°C, the average grain size number within the thickness range of at least 50 to 300 μ on the inner surface of the steel pipe is No. By infiltrating N so that C+N within the thickness range is 0.15% or more in a fine grain state of . A method for producing an austenitic stainless steel pipe characterized by maintaining a fine grain state with a grain size number of No. 7 or higher.