JPS6164860A - Austenite compound heat resisting steel containing nb excellent in corrosion resistance and strength or the like and its manufacture - Google Patents

Abstract

PURPOSE:To improve intergranular corrosion resistance and strength at a room temperature and a high temperature by reducing a C content of an Ni-Cr heat resisting steel, also controlling correctly its N content, and also executing cold working and heat treatment under specified conditions. CONSTITUTION:An Ni-Cr heat resisting steel containing 0.02-0.05% C, <2% Si, <2% Mn, 9-35% Ni, 15-26% Cr, 0.05-0.15% N, and <2.0% Nb, or also one kind or two kinds or more of <3% Mo, <1% Al and <3% Cr, as necessary, and having a quantitative relation of Nb/N>=100[c]+5 among Nb, N and C is heated and held at >=1,150 deg.C and softened on the way of cold working after hot working. The subsequent cold working is executed by >=15% draft, and the final solution treatment is executed at a temperature of >=1,150 deg.C. In this way, an austenite heat-resisting steel which has set a crystal particle size number to <=6 in order to strengthen high temperature strength, and also has set a particle size number to >=6 in order to improve vapor oxidation resistance is manufactured.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an Nb-containing austonitic heat-resistant steel having excellent intergranular corrosion resistance, strength, etc., and a method for producing the same.

[Conventional technology and its problems]

5US347H steel is known as an austenitic heat-resistant steel pipe material containing Nbi.

The properties of this steel are determined by the final heat treatment temperature (solution treatment temperature)
be greatly affected. In other words, when this steel is solution-treated at a high temperature, i.e., at a high temperature where most of the Nb carbides melt into the matrix, the creep rupture strength increases, but the presence of Nb carbides that prevents grain coarsening is present. As a result, the grain size becomes large, creep rupture ductility, and steam oxidation resistance decrease. Furthermore, since the amount of solid solute carbon increases, sensitization tends to occur, making it impossible to obtain sufficient intergranular corrosion resistance. - If solution treatment is performed at a low temperature, that is, at a low temperature where Nb carbide hardly forms a solid solution in the matrix, the crystal grains will be aggregated, improving creep rupture ductility and steam oxidation resistance, and increasing sensitization. (Although intergranular corrosion resistance is improved, creep rupture strength is slightly lower.) 5US347I (for the characteristics of steel, there is a method of obtaining a fine-grained structure with a certain amount of carbide infiltration, but the intergranular corrosion resistance does not necessarily improve greatly. As mentioned above, it is possible to obtain fine-grained high-strength steel even if solution treatment is performed at a high temperature such that Nb carbides dissolve in solid solution but some or all of Nb nitrides do not dissolve in solid solution. In this case, the intergranular corrosion resistance is further reduced.

The present invention was made in view of such conventional problems,
Excellent intergranular corrosion resistance, and moderate strength (about 53 Kff/- at room temperature) at relatively low temperatures, that is, at room temperature and at temperatures below 600°C where creep deformation does not occur.
It is an object of the present invention to provide an Nb-containing austenitic heat-resistant steel having either one of the following properties: excellent high-temperature strength in a high-temperature region where creep deformation occurs; and high-temperature steam oxidation resistance. The present invention aims to provide a method for producing heat-resistant steel of this type that has all of these properties.

[A precautionary measure to solve the problem]

Therefore, the present invention reduces the amount of C compared to the conventional 5US347H steel, and also improves the strength by reducing the amount of C.
In other words, a decrease in strength at room temperature and a temperature range below 600°C where creep deformation does not occur (hereinafter simply referred to as room temperature strength) and high temperature strength in a temperature range where creep deformation occurs (hereinafter simply referred to as high temperature strength). By containing all N, this can be prevented and further improved. Moreover, in the present invention, the amounts of C, N, and N'b can be adjusted under the most severe final solution treatment conditions for intergranular corrosion resistance, that is, at high temperatures where the calm carbides and Nb nitrides are completely dissolved. By adjusting the corrosion so that it does not become a problem even when solution treatment is performed, it is possible to improve the intergranular corrosion resistance, which is impossible to achieve with conventional technology. The present invention has succeeded in obtaining a Nb-containing austenitic heat-resistant steel that has both excellent high-temperature strength and excellent high-temperature strength.
Intergranular corrosion resistance and excellent strength can be obtained regardless of the grain size, whether fine or relatively coarse. The feature of the present invention is that C: 0.02
~0.05 wt%, Si: 2 wt% or less, Mn:
2 wt% or less, Nl: 9 to 35 wt%, cr:
15-26wl, %, N: 0.05-0.15wt
H, Nb: 2 wt% or less, further MO: 3 as necessary
wt% or less, Al: 1wt% or less, Cu: 3wt% or less, the balance consists of Fe and unavoidable impurities, and Nbft, the amount of N, and the amount of C are Nb/N≧loo[ c] + 5, and the grain size number is less than 6.
Appropriate intergranular corrosion properties, room temperature strength, and high temperature strength can be obtained.

Another feature of the present invention is that C: 0.02
~0.05 wt%, Si: 2 wt% or less, Mn: 2w
t% or less, Ni: 9 to 35 wt%, Cr: l
5-26wt Chi, N: 0.05-0.15wt%
, Nb: 2wt% or less, further Mo as necessary
: 3 wt% or less, A4: 1 wt% or less, C'u: 3
Contains one or more of the following (wt. The advantage is that the grain size number is set to be 6 or more, and this also provides excellent intergranular corrosion resistance and appropriate normal temperature strength, as well as a 1.
Provides excellent steam oxidation resistance.

Furthermore, other features of the present invention include the above-mentioned objects,
That is, it relates to a method for manufacturing this type of heat-resistant steel that can highly satisfy all of intergranular corrosion resistance, strength, and steam oxidation resistance, and is characterized by C: 0.02.
~0.05wt%, Si: 2wt% or less, MH: 2
wt% or less, Ni: 9 to 35 wt%, Cr: 15
~26wt%, N:o, os~0.15wt%, Nb
: 2wt% or less, further Mo: 3wt% if necessary
Hereinafter, it contains one or more of Al: 1 wt% or less, Cu: 3 wt% or less, and the balance consists of Fe and unavoidable impurities, and the amount of Nbi, the amount of N, and the amount of C are Nb/N≧xoo[c ) + s The tuning is carefully adjusted to satisfy the relationship, softening treatment is performed at a temperature of 1150℃ or higher during cold working, and the subsequent cold working is performed at a cold working degree of t-15% or higher. 1150℃
The reason is that the final solution treatment was carried out at a temperature higher than that.

The details of the present invention and the reasons for its limitations will be explained below.

A further object of the steel of the present invention is to have excellent overall ratio properties when used in boilers and the like. Even in materials used at high temperatures, intergranular corrosion tends to occur), so intergranular corrosion resistance is one of the characteristics that should be given top priority. Intergranular corrosion tends to increase as the C content increases, and this tendency becomes more pronounced when subsequent high-temperature solution treatment is performed to increase the high-temperature adhesive IJ-build (rupture) strength. Therefore, in the present invention, the substitution with CiN is harmful to intergranular corrosion, and from this point of view, the upper limit of C is limited to 0.05 wt%. However, in order to ensure high temperature creep (rupture) strength, the lower limit of C is set to O', 02wt.

N for Cl substitution is 0.0 to ensure normal and high temperature strength.
5wt% or more is required, but the effect is saturated even if it exceeds 0.15wt%, so 0.05 to 0.15wt% is required.
% range. Figure 2 shows the effect of N amount on high temperature strength: 0.03wt%C-x7wt%Cr 12wt%Ni
-0.85wt%Nbfi4 (the figures in the figure indicate the sample steel Nα in Table 1).The final solution treatment temperature was to dissolve not only Nb carbides but also Nb nitrides. 11190℃lc is selected. To jfL,
It is shown that the creep rupture time is 1000 hours or more under a load of 20 Kg/1han at 650°C, indicating that the high temperature strength is improved by containing 05wt or more of NkO. Figure 3 shows the effect of N on room temperature strength at the same temperature (0.03wt%C17wt%Cr).
-12wt%Ni -0.85wt%Nb steel (the ugliness in the figure indicates sample steel m'6 in Table 1) was investigated, and shows the results of a tensile test at room temperature. In the figure, the mark ○ indicates the final solution treatment at 1220°C and the mark e indicates the final solution treatment at 1050°C, but both are almost on the same line, and in order to obtain a strength of 53 Kg/arm2 or more, it is necessary to It can be seen that N of .05 wt% or more is required. When solution treatment is performed at a temperature where Nb carbides and nitrides are completely dissolved, the strength is improved mainly due to solid solution strengthening (precipitation strengthening), and at a temperature where carbonitrides are completely dissolved, the strength is improved. In the case of solution treatment and fc, it is thought that the strength is improved mainly due to the fine grain effect, but it can be seen that the strengthening effect is almost the same. Therefore, the strength in a temperature range where creep deformation does not occur can be improved by adding N in any case.

In order to prevent intergranular corrosion, Nb is added while its amount is completely regulated in relation to C1N. That is, in the present invention, Nb
The amount of N, the amount of N, and the amount of C are adjusted so that they satisfy the relationship t-Nb/N≧100 CC]+5. Figure 1 is 17wt
%Cr-12wt%Ni-N: 0.05-0.1
5wt1 steel with varying amounts of C and N was solution-treated at 1220°C to completely dissolve Nb nitrides, and then subjected to sensitization treatment at 650°C for 2 hours.
Strauss test i according to JIS G O575
11 and investigated the occurrence of %n (in addition,
Items marked with n in the figure correspond to steels of the same ra in Examples described later regarding C-view and Nb/H). According to the same figure, steel with a C content exceeding 0.05 wt% causes intergranular corrosion regardless of the Nb/N value, and conversely, in a range of less than 0.02 wt%, grain boundary corrosion occurs regardless of the Nb/H value. No interfacial corrosion occurred. And C which is the range of the present invention: 0.02
In the range of ~0.05wt%, Nb/N28 or higher for C: 0.03wt%, Nb for C: 0.05wt%
/N: The larger the Cm is, such as 10 or more, the more Nb and t'e must be contained relative to the same -Ni. Specifically, Nb, It is clear that each content of C and H must be adjusted.As mentioned above, intergranular corrosion is most likely to occur when carbonitrides are completely dissolved in solid solution.
By adjusting the quantitative relationship between Nb and N1C so that intergranular corrosion does not occur in this state, the possibility of intergranular corrosion will be eliminated regardless of the grain size or final solution treatment. . Incidentally, in a region where C: is less than 0.02wt1, intergranular corrosion does not occur, but as described above, the strength (creep strength and room temperature strength) is inferior, and therefore it is outside the scope of the present invention.

Further, even if more than 2 wt % of ferrite is added, the corresponding effect will not be obtained, and there is a problem that weldability will deteriorate, and for this reason, the upper limit is limited to 2 wt %.

Ni is required to be at least 9 wt % to obtain an austenite single phase structure, but if it exceeds 35 wt %, it will be uneconomical, so it is set at 9 to a s wt %.

Cr is required in an amount of 15 wt% or more in order to obtain the desired corrosion resistance, but if it is added in an amount exceeding 26 wt%, an austenite single phase structure cannot be obtained).
Let it be wt%.

To explain the other components, first Mn is an element that has a deoxidizing effect and is normally contained in austenitic heat-resistant steel, but if it is contained in excess of 2 wt%, weldability deteriorates, so the total upper limit of 2 wt% is do. St is also an element contained in austenitic heat-resistant steel as a deoxidizing element, and it can be contained up to 2 wt so long as an austenite single phase structure is obtained.

In the present invention, in addition to the above components, Mo:
3 wt% or less, Cu2 3 wt% or less, Al: 1 wt%
One or more of the following may be included.

MO and Cu have the effect of improving the strength at room temperature and high temperature, but their effect is almost saturated at n (J-n awt%), so the upper limit for each is set at fL 3 wt%. Although it improves the flow of the pipe, if it exceeds 1 wt%, it becomes difficult to melt copper.

In addition to regulating the component composition as described above, the present invention also regulates the crystal grain size depending on the desired characteristics. In other words, in addition to the basic properties of intergranular corrosion resistance and room-temperature strength, if you want a steel that is particularly excellent in high-temperature strength, in addition to the above-mentioned basic properties, it is necessary to When NFC steel has excellent steam oxidation properties, the grain size number is regulated to be No. 6 or higher.

The grain size is basically controlled by the final solution treatment conditions.If the final solution treatment temperature is higher than 1150'C'lr, the solid solution of Nb nitride is likely to occur, and therefore the crystal grain size is The effect of preventing grain coarsening is weakened, and the crystal grain size tends to be less than No. 6. Such steel can provide particularly excellent high-temperature strength, and this is the first invention of the present application.

10,000, Final solution treatment i If performed at a temperature below 1150° C. at which almost no Nb nitride is dissolved in solid solution, grains with grain numbers 6 or higher can be obtained due to the grain coarsening prevention effect of the Nb nitride. Such steel has excellent steam oxidation resistance due to its fine grain size. Steam oxidation resistance is governed by grain size. In other words, scale exfoliation due to steam oxidation begins when the scale exceeds 70 μm, and when the scale exceeds 100 μm
It becomes noticeable when exceeding nl.

In austenitic stainless steels with a Cr content of 20% or less, scale growth during long-term use often stops at about twice the grain size at 550 to 600°C; The scale thickness in the case of No. 6 is approximately 80 to 100 μm, and the scale thickness in the case of No. 8 is 40 to 50 μm. For this reason, in order to keep the scale thickness to 70 μm or less without peeling, it is necessary to adjust the particle size to T-6 or higher), and this is especially resistant to steam oxidation. This is the second invention of the present invention, which is defined as a product of sexual excellence.

Next, the manufacturing method of the present invention will be explained.

As mentioned above, the grain size is basically controlled by the final solution treatment conditions, but more specifically, the final solution treatment conditions, the degree of cold working immediately before that, and the softening before that. Affected by processing conditions. On the other hand, high temperature creep (rupture strength) is influenced by the dissolution of carbides and is substantially determined by the final solution treatment temperature.

Figure 4 shows 0.03 wt%C-17wt%cr-izv
t*Ni-0,095wt%N-0,85wt%Wb
After softening at 50°C and 1175°C, cold working was carried out at a working degree of 15%, resulting in 10 (0
This is an illustration of the grain size obtained when the final solution treatment is carried out at 0 to 1200°C. Also, for reference, 119
For sample K, which was softened at 0°C and processed in the same way, the final solution treatment was performed at 1190°C. This figure shows that even if the final solution treatment temperature and cold working degree are the same, the grain size is not the same, and the higher the softening treatment temperature, the finer the grains after the final solution treatment. I understand. This means that sufficient transitions are introduced into the steel material during cold working.
If the temperature is increased during the final solution treatment, Nb nitride, which had been dissolved in the previous softening treatment, will precipitate in the lattice defects. This is thought to be due to an increase in the number of solid-dissolved Nbide compounds. During the softening process, undissolved Nb nitrides form austenite because the space between the final annealing heat raising process mentioned above and the number of screws is very small. There is also a significant difference in the effect of preventing coarsening of crystal grains.

Figure 5 shows the softening temperature of the same steel as in Figure 4:
Processing was performed at 1190°C, final solution treatment temperature: 1190°C, and the relationship between the degree of cold work after softening treatment and the particle size after solution treatment was investigated. According to this nlc, as the degree of cold working increases, the crystal grain size becomes finer, and a sufficient refining effect is obtained at %15'J or more. Further, the improvement in the effect becomes gradual when the degree of processing exceeds 15%. As explained earlier, K.

In the final annealing process, the grains become finer as the dislocation density, which becomes the precipitation site of Nb nitrides, increases.As mentioned above, the improvement in the refinement effect becomes gradual due to the amount of solute Nb. Although there may be a relationship between
It means.

Here, as can be seen from Figure 4, softening treatment - cold working -
When producing Nb-containing austenitic steel through a process called grain final solution treatment, grains with the same grain size after final solution treatment (for example, grain size number 6) are processed under different conditions (softening treatment temperature - final solution treatment For example, the particle size number for both the softening treatment at 1150°C and solution treatment at 1170°C and the softening treatment at 1175°C and solution treatment at 1190°C has a particle size number of 6. However, the latter Since the final solution treatment temperature is higher, solid solution of Nb nitride progresses, and the high-temperature strength should be higher.

From this point of view, FIG. 6 shows the relationship between the grain size number and the rupture time in a creep test at 650° C. and a load of 20 to 9/−, depending on the softening treatment temperature. The steel sample was the same as that shown in FIGS. 4 and 5, and the degree of cold working was 15%.

According to the figure, it can be seen that when looking at the same particle size, the higher the softening temperature, the higher the high temperature strength can be obtained. That is,
As mentioned above, the steel of the present invention (first invention of the present application) has a grain size of less than No. 6 and high high temperature strength (1 at 650°C, 20K, 47mm).
However, in the process of softening treatment - cold working - final solution treatment, if the softening treatment is carried out at 11150 °C or more, even if the grain size is No. 6 or more, 650℃, 20 Kg/lt
J for 1000. A creep rupture time of hr or more can be obtained. As already explained, if the final solution treatment is performed at a temperature of 1150°C or higher, solid solution of Nb nitride will start and the high temperature strength will be improved. It is rather natural that even fine grains with a grain size of 6 or more have high high temperature strength.

In view of the above, in the present invention, the softening treatment, which was conventionally performed at a temperature range of less than 1150°C, is particularly performed at a temperature of 1150°C or higher, and the softening treatment at 1150°C or higher - cold processing with a working degree of 15% or higher. By specifying a series of conditions including final solution treatment at a temperature of 1150°C or higher, we can manufacture heat-resistant steel containing aqueous steel that is excellent in intergranular corrosion resistance, room-temperature strength, steam oxidation resistance, and high-temperature strength. This made it possible. In the present invention, by setting the softening treatment temperature and the final solution treatment temperature to 11,175° C. or higher, and by setting the degree of cold working to 20% or higher, it is possible to obtain much better properties.

〔Example〕

Table 1 shows examples of the present invention. Steel materials were manufactured using the ingredients and treatment conditions shown in the table, and the high temperature strength, room temperature strength, and intergranular corrosion properties of each sample material were examined. In the table, 凰1
Steel ~ N15 steel is the first invention steel of the present application, NILG steel and No. 7 steel are the second invention steel of the present application, II steel with a grain size of less than No. 6 ~
Na,5 steel has excellent intergranular corrosion resistance and room temperature strength.
In particular, N11Ls steel, which has been subjected to final solution treatment at a temperature at which nitrides are completely dissolved, has particularly excellent high-temperature strength. On the other hand, high-temperature steel with a grain size of No. 6 or higher and No. 7 steel have a high-temperature, strong malleability surface μH with just the addition of N; It can be seen that oxidizing properties are obtained. Taki 1 for this
0 steel and Na1l steel have KNb/N≧ as shown in Part 1A.
These are comparative materials under the condition of 1oo[C]+5, and all of these are inferior in intergranular corrosion resistance.

In addition, all steels such as No. 8 steel, Pa9 steel, N112 steel and N1114 steel are obtained by the manufacturing method of the present invention, and these have excellent intergranular corrosion resistance, room temperature strength, steam oxidation resistance, and high temperature It can be seen that excellent properties have been obtained in all aspects of strength.

[Brief explanation of drawings]

Figure 1 shows 17wt%Cr-12wt%Ni-N:
0.05-0.15. , which shows the influence of Nb/N and C on intergranular corrosion in wt% steel. Second
The figure shows 0.03wt%C17wt%Cr-12wt%Ni
This figure shows the influence of the amount of N on the high-temperature strength of -0.85 wt% Nb steel. Figure 3 is the same (o, o3wt
%C-17wt%Cr-12wt%Ni -0,85w
This figure shows the influence of the amount of N on the room temperature strength of t%Nb steel. Figure 4 shows 0.03 wt%C-17wt%
Cr-12wt%Ni-0, 85wt%Nb-0,
Relationship between heat treatment conditions and grain size in 095wt SN steel. Figure 5 is the same (0.03wt%C-x7wt%Cr-
12wt%Ni-0,85wt%Nb-0,095w
This figure shows the relationship between the degree of cold work and the grain size after final solution treatment in t%N steel. Figure 6 is the same (0,0
3wt% C-17wt%Cr-12wt%Ni −
This figure shows the relationship between grain size and creep rupture strength in 0.85wt%Nb-0.095wt%N steel. Patent applicant: Nippon Doppan Co., Ltd. Inventor: Toyama Mikami Minegishi First time
Kei Hattori Intercostal space
Kazuhiro Kanero, Patent Attorney, Sho Yoshihara, Sando
1st Tsukasa Seido Takahashi
Attorney Hiroko Yoshihara ('ON)
Maru pipe collection

Claims (3)

[Claims] (1) C: 0.02-0.05wt%, Si: 2wt%
Below, Mn: 2 wt% or less, Ni: 9 to 35 wt%, C
r: 15-26wt%, N: 0.05-0.15wt%
, Nb: 2wt% or less, further Mo: 3w as necessary
t% or less, Al: 1wt% or less, Cu: 3wt% or less, the balance consists of Fe and unavoidable impurities, and the amount of Nb, the amount of N, and the amount of C are Nb/N≧100 [C]+5 Nb-containing austenitic heat-resistant steel having a grain size number of less than 6 and having excellent corrosion resistance and strength. (2) C: 0.02-0.05wt%, Si: 2wt%
Below, Mn: 2 wt% or less, Ni: 9 to 35 wt%, C
r: 15-26wt%, N: 0.05-0.15wt%
, Nb: 2wt% or less, further Mo: 3w as necessary
t% or less, Al: 1wt% or less, Cu: 3wt% or less, the balance consists of Fe and unavoidable impurities, and the amount of Nb, the amount of N, and the amount of C are Nb/N≧100 An austenitic heat-resistant steel containing Nb that is adjusted to satisfy the relationship of [C]+5 and has a grain size number of 6 or more and has excellent corrosion resistance and strength. (3) C: 0.02-0.05wt%, Si: 2wt%
Below, Mn: 2 wt% or less, Ni: 9 to 35 wt%, C
r: 15-26wt%, N: 0.05-0.15wt%
, Nb: 2wt% or less, further Mo: 3w as necessary
t% or less, Al: 1wt% or less, Cu: 3wt% or less, the balance consists of Fe and unavoidable impurities, and the amount of Nb, the amount of N, and the amount of C are Nb/N≧100 Steel adjusted to satisfy the relationship [C] + 5 is softened at a temperature of 1150°C or higher during cold working, and the subsequent cold working is performed at a degree of cold working of 15% or higher. A method for producing Nb-containing austenitic heat-resistant steel having excellent corrosion resistance and strength, the method comprising final solution treatment at a temperature of 1150° C. or higher.