JPS6214628B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to an austenitic high-temperature steel having excellent oxidation resistance at high temperatures. Materials for parts used at high temperatures, such as parts for heating furnaces and heat exchangers, combustion parts for heating equipment,
Structural components of automobile exhaust gas treatment equipment are not only oxidized by low oxidation due to use at high temperatures, but also oxidation resistant, which includes the fact that scale does not peel off even after repeated high-temperature heating and cooling to room temperature. Many properties are required, such as cold workability, weldability, which is required for processing and forming parts into complex shapes, and sufficient mechanical strength at room temperature and high temperature. Furthermore, since the above-mentioned equipment is a so-called mass-produced product, it is desired that the materials thereof be as inexpensive as possible. Austenitic heat-resistant steel is a material that has been conventionally used for such purposes. As is well known, this steel
The main alloy components are Ni and Cr, and it has an austenitic structure that is stable at room temperature. Generally, many steel types are known due to the combination of Ni content in the range of 7 to 45% and Cr content in the range of 15 to 30%. There is. for example,
The 18Cr-10Ni system represented by SUS304, the 25Cr-20Ni system represented by SUS310, the 20Cr-32Ni system known as Incoloy 800, and improved versions of these containing Mo, Si, Ti, Nb, etc. are commercially available. Each type is selected and used according to its purpose. Now, if the high-temperature oxidation resistance of the austenitic steel described above could be further improved without impairing its basic characteristics, it would be possible to expect extremely large economic benefits in practical use. An object of the present invention is to provide an austenitic steel which has the basic characteristics and advantages inherent in austenitic steel, and which also has extremely excellent oxidation resistance at high temperatures. As a result of detailed experiments and studies on the influence of impurities that are inevitably mixed in austenitic steel on the properties of the steel, the inventors have found that S and O in steel have a significant adverse effect on oxidation resistance. I found out that there is. The present invention was made based on the above findings, and the present invention is based on the knowledge that S+O in austenitic steel containing 7 to 45% Ni and 15 to 30% Cr is 0.005% or less, and O is
The main feature is an austenitic steel with excellent high-temperature oxidation resistance, characterized by a content of 0.004% or less. The amount of S in this type of steel has been regulated in the past, but it has been said that it should be 0.03% or less for both heat-resistant steel and corrosion-resistant steel from the viewpoint of hot workability. The regulations have become nothing more than extremely lax. Furthermore, the current situation is that there are no particular regulations regarding the amount of O. In commercially available products, the S content is approximately 0.010% and the O content is approximately 0.008%. In other words, it was conventionally thought that this level of S and O would not have such a significant effect on the properties of steel. However, according to the inventor's many experimental results,
It has been found that these S and O significantly impair the oxidation resistance of steel when the content is lower than the above-mentioned usual case. Figure 1 plots the relationship between weight change and (O+S) amount when a 30 minute heating-cooling cycle is repeated 500 times at a heating temperature of 1000°C for 19Cr-9.5Ni-1.5Si austenitic steel. It is something. As is clear from the figure, the amount of weight change due to oxidation decreases as the amount of (O+S) decreases, and especially where the amount of (O+S) is less than 0.008%, the amount of weight change decreases to approximately 0.015%. %, which is less than 1/2 that of commercially available products, indicating extremely good oxidation resistance. The reason for the improvement in oxidation resistance based on the reduction in O and S contents is considered as follows. Normally, S, which is contained in steel at around 0.01%, combines with Mn, which is also present in steel, to form MnS. However, this MnS decomposes and changes into Cr-Mn-O oxides while the steel is used at high temperatures, and the free S concentrates on the steel surface and grain boundaries, and around inclusions. It creates a Cr-deficient layer and has a negative effect on the high-temperature oxidation resistance of steel. Cr or even more
The oxidation resistance of steel containing Si, Al, etc. is due to the protective film of stable oxides produced by the oxidation of these elements, but as mentioned above, free S is concentrated on the steel surface and grain boundaries. If this occurs, the diffusion and movement of Cr, Si, etc. to the surface layer of the steel (actively occurring in particular through grain boundaries) is inhibited, making it difficult to form and repair the protective oxide film quickly. In addition, free S present on the steel surface and grain boundaries itself combines with oxygen and becomes a starting point for oxidation, causing embrittlement of grain boundaries and promotion of peeling of oxide scale. On the other hand, O is normally contained in steel in an amount of 0.005 to 0.010%, but most of this exists in the form of oxides or oxysulfides in combination with active elements such as Cr, Si, Al, and Mn. Therefore, as described above, diffusion of Cr, Si, Al, etc. into the surface layer of the steel is inhibited, and the formation of oxides of these elements on the steel surface is accordingly suppressed. Also steel
At the oxide scale interface, oxygen exists as much as the oxygen partial pressure based on the dissociation equilibrium of, for example, Cr oxide, which is generated at the interface, and this diffuses into the steel and causes internal oxidation and grain boundary oxidation. . The undesirable effects of S and O as described above are
Although it is thought that S and O in steel can be completely removed, it is practically impossible to completely remove such impurities. However, if the S and O contents in the steel are kept low according to the provisions of the present invention, the above-mentioned adverse effects can be eliminated. That is, extremely small amounts of S and O based on the present invention combine with Ca, Mg, etc. mixed in from the refractories of the furnace material or slag during steel melting, and form Ca-Al-Mg. It exists in steel as an -O-S type compound. Since such sulfides are stable even at high temperatures and fix S and O, it is thought that the above-mentioned adverse effects of O and S are removed. Here, stable even at high temperatures refers to a state in which sulfide-containing inclusions decompose and are difficult to cause a reaction to liberate S, and the degree of stability needs to be more difficult than the ease with which MnS decomposes. . Even from this mechanism, it is clear that (O+
S) Content of 0.005% or less and O0.004% or less is
Acceptable in terms of practical effect. Considering the above phenomenon, in the steel of the present invention, the (S+O) amount is less than 0.008% and the O amount is 0.005%.
Even if it is less than %, if they are close to the upper limit,
It is more advantageous to actively utilize Ca, Mg, or a rare earth element having an effect equivalent to these, Y, to promote the production of the above-mentioned stable compound. Furthermore, during melting, refining or adding elements to reduce the O content in the steel as much as possible is also effective in improving oxidation resistance. Note that the steel of the present invention is used at about 700 to 1200°C. It is applicable to all types of austenitic steel, and it is of course possible to include alloy components normally contained in austenitic steel. The type and content of such alloying elements are determined by the amount of Ni, Cr, etc.
It may be selected according to the usage conditions of the steel, that is, the required service temperature, workability, weldability, mechanical properties, etc., taking into consideration the balance with the quantity. In any case, the steel of the present invention, in which S and O are suppressed to extremely low levels, has significantly better oxidation resistance than steel that contains normal amounts of S and O in the same space, and is at least inferior in other properties. Never. The preferred ranges and reasons for the basic components of the austenitic steel of the present invention will be explained below. C
When steel is used at high temperatures, or after welding, Cr ₂₃ C ₆ type carbides are formed in the welded part, which reduces the oxidation resistance improvement effect of Cr and deteriorates scale adhesion. Moreover, an excessive amount of C has a negative effect on the weldability and workability of steel, and the upper limit should be kept at 0.15%, especially from the viewpoint of preventing the precipitation of large amounts of carbides. From the viewpoint of oxidation resistance, it is better to keep the amount as low as possible, but if mechanical strength is important, it may be contained close to the upper limit. To eliminate the above-mentioned harmful effects of C, Ti, Nb, Zr and
It is preferable to use Ta. These ingredients are equivalent in terms of action and effect, so even one type or two of them are equivalent.
Combinations of more than one species can also be used, and a total content of 4 times or more of C (%) is effective. However, if added in too large an amount, the amount of intermetallic compounds precipitated will increase, impairing the cleanliness and workability of the steel, so the upper limit should be 1.5%. Si is usually used as a deoxidizing agent, and 0.1% or more is required to ensure its effectiveness. Si is also a component that exhibits an excellent effect on improving the oxidation resistance of steel. According to the inventor's experimental results,
The effect of improving oxidation resistance by suppressing O and S within the range of the present invention is particularly remarkable in steel containing 1% or more of Si, so it is recommended to contain up to 5% of Si. However, the content of Si exceeding 5% causes deterioration of the workability and weldability of the steel. Mn is used as a deoxidizing agent, but it is not a desirable component from the viewpoint of oxidation resistance, so it is preferably kept at 3% or less. Moreover, if it exceeds 3%, it impairs hot workability and also causes problems in that it corrodes the refractories of the furnace during steel manufacturing. Ni and Cr are necessary to ensure the basic properties of austenitic steel. If Ni is less than 7% and Cr is less than 15%, it becomes difficult to maintain the austenite structure itself. On the other hand, those containing more than 45% Ni are already in the category of Ni-based alloys and are difficult to put into practical use economically. The higher the amount of Cr, the better the oxidation resistance, but if it exceeds 30%,
It is difficult to maintain the austenite phase. As mentioned above, the steel of the present invention contains Ni, Cr,
Even though its content is the same as that of known austenitic steels, its oxidation resistance is much better, so it can withstand harsh conditions of use at higher temperatures than heat-resistant steels of the same group. In other words, under the same conditions of use, steel with lower Ni and Cr contents, that is, cheaper steel, can be used. The above components are the basic components of the austenitic steel according to the present invention, but various subcomponents can be added depending on the purpose of use of the steel and manufacturing needs. The main subcomponents and their preferred added contents are as follows. Al
is a component that is often required as a deoxidizing agent. In particular, when actively adding Ca, Mg, etc., which will be described later, or when making full use of the effects of slag, it is necessary to sufficiently lower the oxygen in the molten steel in advance, and a small amount of Al is added. . However, if the amount of Al remaining in the steel exceeds 5%, it may cause problems in the casting process after melting. A small amount of Cu is effective in improving the adhesion of oxide scale that forms on the steel surface. However, if the content exceeds 1.5%, the oxidation resistance will deteriorate. Mo is a component mainly effective in improving the high-temperature strength of steel. Therefore, when used in applications that require high temperatures and loads, it should be added, but 5.
If the content exceeds %, it has an unfavorable effect on oxidation resistance and at the same time causes an increase in material cost. The purpose of use of Ti, Nb, Zr, and Ta is as described above in relation to C. Ca, Mg, rare earth elements, Y, these elements are S
Combines with oxidation sulfides to form extremely stable sulfides or oxysulfides that do not decompose even at high temperatures, contributing to improving the oxidation resistance of steel. However, this effect also
It is a premise that S and S are suppressed within the range of the present invention, and if a large amount of the above elements is added to steel that contains normal amounts of these impurities, an excessive amount of compounds will be generated, which will deteriorate not only the oxidation resistance but also the oxidation resistance. This has the effect of adversely affecting mechanical properties. As long as O and S are within the range of the present invention, Ca,
Corrosion resistance can be improved even if Mg, rare earth elements, and Y are not substantially present. Normally, trace amounts of Ca, Mg, etc. enter into molten steel from furnace materials or slag without any active addition, and this combines with the trace amounts of O and S mentioned above. It is expected that harm will be removed. In any case, the scope of the present invention also includes a case where the O and S contents are further lowered by adding these elements. In addition to the above main components and subcomponents, there are impurities that are inevitably mixed into steel. Among these, O and S have an important relationship with oxidation resistance.
Its content is (O+S) 0.005% or less, and O
As mentioned earlier, the intended improvement in oxidation resistance cannot be achieved unless the content is suppressed to 0.004% or less. Next, embodiments of the present invention will be listed and specific examples thereof will be explained. In the following description, the oxidation resistance test was conducted under the following conditions. Test piece size: 25mm ^L x 20mm ^W x 1.5mm ^T plate Test method: Hold in a furnace heated to the indicated temperature for 30 minutes, then leave in the atmosphere for 30 minutes. After repeating the heating-cooling cycle 500 times, the weight of the test piece from which attached oxides have been removed is measured, and the weight loss relative to the weight before the test is determined, and the oxidation resistance is evaluated based on the weight loss. Table 1 shows the results of the above tests conducted on typical commercially available austenitic steels, and is listed for reference in the following explanation. [Embodiment 1] C0.15% or less, Si0.1-1.0%, Mn3.0% or less,
Ni7~15%, Cr15~20%, other as required
Al, Cu, Mo, Ti, Nb, Zr, Ta, Ca, Mg, Y,
High-temperature austenitic steel that contains rare earth elements, etc., and has O+S of 0.005% or less and O of 0.004% or less. This steel is compatible with commercially available austenitic steels such as SUS304, 316, 321, and 347, and is made of Ni and Cr.
It belongs to a relatively inexpensive steel type with a low content of Table 2 shows the composition and oxidation resistance results of the steel belonging to this embodiment and a comparative steel of the same type with high S and O contents and at a heating temperature of 850°C. In Table 2, when inventive steels 1 to 7 and 10 to 13 are compared with comparative steels 8 to 9 and 14 to 16, oxidation

[Embodiment 3]

C0.15% or less, Si3.0~5.0%, Mn3.0% or less,
Ni10~15%, Cr15~20%, Al if necessary,
High-temperature austenitic steel containing one or more of Cu, Mo, Ti, Nb, Zr, Ta, Ca, Mg, rare earth elements, and Y, with S+O being 0.005% or less and O being 0.004% or less. This steel has a Si content of 3.0 of the steel of embodiment 2.
~5.0% to further improve oxidation resistance. Table 4 shows the compositions and results of the oxidation resistance test at 1100° C. of the steels included in this embodiment and comparative steels of the same type as these steels but with S and O contents outside the range of the present invention. The steel of the present invention has an oxidation weight loss of about 1/3 to 1/8 that of comparative steel.
, indicating high oxidation resistance. [Embodiment 4] C0.15% or less, Si0.1-3.0%, Mn3.0% or less,
Ni10~15%, Cr20~25%, Al if necessary,
High-temperature austenitic steel containing one or more of Cu, Mo, Ti, Nb, Zr, Ta, Ca, Mg, rare earth elements, and Y, with S+O being 0.005% or less and O being 0.004% or less. This steel has a high Cr content of 20 to 25%, and is a commercially available steel type that corresponds to SUS309S. Table 5 shows the compositions and oxidation resistance test results at 1050° C. of the steel belonging to this embodiment and a comparative steel of the same steel type with high S and O contents. Again, the oxidation weight loss of the inventive steel is about half that of the comparative steel. [Embodiment 5] C0.15% or less, Si0.1-1.0%, Mn3% or less,
Ni30~35%, Cr20~25%, other as required
Contains one or more of Al, Cu, Mo, Ti, Nb, Zr, Ta, Ca, Mg, rare earth elements, Y, S + O is 0.005
% or less, and a high temperature austenitic steel with an O content of 0.004% or less. The commercially available steel Incoloy 800 (trade name) corresponds to this. Table 6 also shows the compositions of the steels belonging to this embodiment and specific mild steels of the same steel type with high contents of S and O, as well as the results of oxidation resistance tests at a heating temperature of 1000°C. The steel of the present invention has a weight loss due to oxidation of approximately 1/2 to 1/3 that of comparative steel.
It is clear that this steel type also exhibits excellent performance. [Embodiment 6] C0.15% or less, Si0.1-3.0%, Mn3% or less,
Ni12~25%, Cr20~30%, other as required
Contains one or more of Al, Cu, Mo, Ti, Nb, Zr, Ta, Ca, Mg, rare earth elements, Y, S + O is 0.005
% or less, and high temperature steel with O of 0.004% or less. This steel corresponds to the commercially available steel SUS310S. Similarly, Table 7 shows the compositions and oxidation resistance test results at 1100° C. of the steels belonging to the above-mentioned embodiments and comparative steels of the same steel type with high S and O contents. Here again, it is clear that the oxidation resistance is improved due to the reduction of S and O. The oxidation weight loss of the steel of the present invention is about 1/3 to 1/4 that of the comparative steel.

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[Table] As is clear from the above description, the present invention can greatly improve the high-temperature oxidation resistance of austenitic steel without impairing its basic characteristics. It has a remarkable effect on improving the economic efficiency of steel.

[Brief explanation of the drawing]

FIG. 1 is a graph showing the relationship between the amount of weight change of steel and the amount of (O+S) when the heating-cooling cycle is repeated.

Claims (1)

[Claims] 1 C: 0.15% or less, Si: 0.1 to 5%, Mn: 3%
Below, Ni: 7-45%, Cr: 15-30%, S and O as impurities are O: 0.004% or less, S+O:
0.005% or less, the remainder consists of Fe and other unavoidable impurities, and inclusions in the steel become MnS at high temperatures.
Austenitic steel with excellent high-temperature oxidation resistance, characterized by more stable inclusions. 2 C: 0.15% or less, Si: 0.1-5%, Mn: 3%
Below, Ni: 7-45%, Cr: 15-30%, and
One or more of Ti, Nb, Zr, Ta in total
The content of S and O as impurities satisfies O: 0.004% or less, S + O: 0.005% or less, and the remainder consists of Fe and other unavoidable impurities, and the inclusions in the steel are high temperature. An austenitic steel with excellent high-temperature oxidation resistance, characterized by inclusions that are more stable than MnS. 3 C: 0.15% or less, Si: 0.1-5%, Mn: 3%
Below, Ni: 7-45%, Cr: 15-30%, Ti,
Contains one or more of Nb, Zr, and Ta in total of 4 times or more and 1.5% or less of the amount of C, and one of the following: Al: 0.5% or less, Cu: 1.5% or less, Mo: 5% or less. Or it contains two or more kinds, and S and O as impurities satisfy O: 0.004% or less, S+O: 0.005% or less, and the remainder consists of Fe and other unavoidable impurities, and the inclusions in the steel are more intense than MnS at high temperatures. Austenitic steel with excellent high temperature oxidation resistance characterized by stable inclusions. 4 C: 0.15% or less, Si: 0.1-5%, Mn: 3%
Below, Ni: 7-45%, Cr: 15-30%, and
Contains one or more of the following: Al: 0.5% or less, Cu: 1.5% or less, Mo: 5% or less, and S and O as impurities are O: 0.004% or less, S+O: 0.005%.
An austenitic steel with excellent high-temperature oxidation resistance, which satisfies the following, is composed of the remainder Fe and other unavoidable impurities, and is characterized in that the inclusions in the steel are inclusions that are more stable than MnS at high temperatures. 5 C: 0.15% or less, Si: 0.1-5%, Mn: 3%
Below, Ni: 7-45%, Cr: 15-30%, Ti,
The total amount of one or more of Nb, Zr, and Ta is 4 times or more and 1.5% or less of the amount of C, Al: 0.5% or less, Cu: 1.5
% or less, Mo: 5% or less, and further contains Ca, Mg, rare earth elements, Y, one or more of 0.1% or less each, S as an impurity, O: 0.004% or less, S+O:
0.005% or less, the remainder consists of Fe and other unavoidable impurities, and inclusions in the steel become MnS at high temperatures.
Austenitic steel with excellent high-temperature oxidation resistance, characterized by more stable inclusions. 6 C: 0.15% or less, Si: 0.1-5%, Mn: 3%
Below, Ni: 7-45%, Cr: 15-30%, Ti, Nb,
Contains one or more of Zr and Ta in total at least 4 times the amount of C and 1.5% or less, and further contains one or more of Ca, Mg, rare earth elements, and Y at 0.1% or less each, and contains impurities. S, O as: 0.004% or less,
An austenitic steel with excellent high-temperature oxidation resistance, which satisfies S + O: 0.005% or less, consists of the remainder Fe and other unavoidable impurities, and is characterized by inclusions in the steel that are more stable than MnS at high temperatures. 7 C: 0.15% or less, Si: 0.1-5%, Mn: 3%
Below, Ni: 7-45%, Cr: 15-30%, Al: 0.5
% or less, Cu: 1.5% or less, Mo: 5% or less, and further contains one or more of Ca, Mg, rare earth elements, and Y at 0.1% or less each, S and O as impurities satisfy O: 0.004% or less, S + O: 0.005% or less, and the remainder consists of Fe and other unavoidable impurities, and the inclusions in steel are more stable inclusions than MnS at high temperatures. Austenitic steel with excellent high temperature oxidation resistance. 8 C: 0.15% or less, Si: 0.1-5%, Mn: 3%
Below, Ni: 7-45%, Cr: 15-30%, and
Contains 0.1% or less of one or more of Ca, Mg, rare earth elements, and Y, satisfies S and O impurities as O: 0.004% or less, S+O: 0.005% or less, and the remainder is Fe and other unavoidable substances. An austenitic steel with excellent high-temperature oxidation resistance, characterized by inclusions in the steel that are more stable than MnS at high temperatures.