JPS60262945A - Oxidation resistant austenitic steel and its manufacture - Google Patents

Abstract

PURPOSE:To improve the oxidation resistance and strength of a steel at high temp. by hot rolling the steel contg. prescribed percentages of C, Si, Mn, Cr, Ni, Mo, Al, REM, etc. at a prescribed temp. CONSTITUTION:The steel consisting of, by weight, <=0.1% C, 1-2% Si, <=2% Mn, 18-24% Cr, 20-32% Ni, 0.2-1.5% Mo, 3-5% Al, 0.01-0.08% REM, <=1% one or more among Ti, Nb and Zr, and the balance Fe is melted and refined and formed into an ingot. This ingot is hot rolled at 1,000-1,200 deg.C.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a heat-resistant steel that has excellent oxidation resistance at high temperatures, and more specifically to an AI steel suitable for applications where heating and cooling are repeated in a high-temperature oxidizing atmosphere or combustion exhaust gas.
The present invention relates to austenitic oxidation-resistant steel.

In general, parts such as the inner case and matsuru of industrial heating furnaces are repeatedly heated and cooled in a high-temperature atmospheric atmosphere or combustion exhaust gas atmosphere, so the materials for these parts must have good heat resistance, especially high-temperature oxidation resistance. required.

Conventionally, high Cr-Ni austenitic stainless steels such as SUS 302B steel or 5US310S steel were often used as materials for this type of parts.
With the development of industrial technology in recent years, the usage conditions for such parts are becoming more severe than before, and as a result, the conventional high Cr-Ni austenitic steels mentioned above are often insufficient. ing. In other words, the operating temperature of these types of parts tends to be higher than before.
In addition, although it is now required to withstand longer use, conventional high Cr-Ni austenitic steels still do not have sufficient high temperature oxidation resistance, and due to oxidative wear and tear, There is a problem that cannot be tolerated.

Therefore, recently, a high AA' content austenitic heat-resistant steel has been proposed as a steel with improved high-temperature oxidation resistance than conventional high-Cr-N1 stainless steel. For example, in Japanese Patent Publication No. 54-12887, a dense Al2O film is formed in a high-temperature oxidizing atmosphere by adding a composite of 2.0 to 4.0% Si and 0.5 to 3.0% AA'. Austenitic heat-resistant steels have been proposed that have improved high-temperature oxidation resistance by producing . Although these AII austenitic heat-resistant steels have significantly improved high-temperature oxidation resistance compared to conventional high-Cr-Ni stainless steels, they are still insufficient in terms of high-temperature strength. .

In other words, the inner case of the industrial furnace as mentioned above,
Parts such as Tuffles are subject to large thermal stress due to repeated high-temperature heating and cooling, and are often subject to collisions with heavy materials being treated, so the lifespan of these parts depends solely on their high-temperature oxidation resistance. However, the proposed AII austenitic heat-resistant steels do not have sufficient high-temperature strength, and therefore are not practical in practice. However, there was a problem in that it was not possible to sufficiently extend the life of this type of parts.

This invention was made in view of the above circumstances, and it improves not only high-temperature oxidation resistance but also high-temperature strength, which makes it much more suitable for parts such as inner cases of industrial heating furnaces and matsuru than before. A new AA with K that can extend its lifespan? The object of the present invention is to provide an austenitic heat-resistant steel and a method for producing the same.

In order to overcome the drawbacks of the already proposed AI-containing austite heat-resistant steel, the present inventors have conducted extensive experiments and studies, and have found that Al2O, A for film formation.
It has been found that the high temperature strength can be significantly improved by adding an appropriate amount of Mo in conjunction with the combined addition of I and Si. In addition, when high-temperature strength is increased in this way, hot workability generally decreases and cracks are more likely to occur during hot working, but by keeping the hot working temperature within an appropriate range, It has been found that it is possible to suppress the occurrence of cracks during processing and to produce an Al-containing austenitic heat-resistant steel with high high-temperature strength as described above on a mass production scale with good yield. This invention was made based on these findings, and its gist is as follows.

That is, the austenitic oxidation-resistant steel of the first invention has C0
, 1% or less, Si 1.0-2.0%, Mn 2
．． 0 or less, Cr 18-24%, Ni 20-3
2%.

Mo 0.2-1.5%, AA! 3.0-5.
0%, REM (rare earth element) 0.01 to 0.08 Ti, and contains Ti and Nb.

2" in a total amount of 1.0 or less, and the remainder substantially consists of Fe.

Further, the austenitic oxidation-resistant steel of the second invention is
The invention is characterized in that the ingot of the component is hot rolled in a temperature range of 100°C or higher and 1200°C or lower. Note that in the case of the second invention, it is permissible to perform cold rolling as necessary following hot rolling.

Hereinafter, the austenitic oxidation-resistant steel of the present invention and the method for producing the same will be explained in more detail.

The steel of this invention is basically a high-Ni-high-Cr austenitic heat-resistant steel with the combined addition of AA' and Si.
The steel is based on a steel that forms an oxide film mainly composed of 2O3, and 0.2 to 1.5 inches of M is added to it.

The high temperature strength is improved by adding Ti, Nb.

One or more types of Zr and a small amount of rare earth elements such as Ce and La are added to improve hot workability.The reason for limiting the steel components will be explained below.

C: C is an austenite-forming element and an element that improves high-temperature strength, but if excessively contained, it impairs hot workability, so the upper limit is o, i.

It was hot.

si: st improves the high-temperature oxidation resistance of steel by itself, and at the same time, in coexistence with Alt, makes the improvement in high-temperature oxidation resistance by AI even more remarkable.

In order to obtain such a combined effect with Al, Si1.0
-The above is necessary. However, if it is contained in excess, the amount of δ ferrite produced will increase, impairing hot workability and lowering high-temperature strength. Therefore, it is necessary to keep the content at least 2.0 inches, and therefore the SI is 1.0 to 2. ．． It was set within the range of 0%.

Mn: Mn is an element that impairs high temperature oxidation resistance, so it is preferable that the Mn content is as low as possible,
The amount contained in ordinary stainless steel and heat-resistant steel is not harmful, and up to about 2% is permissible, so the upper limit was set at 20 inches.

Cr: Cr forms a strong oxide film by itself and is a basic element necessary to improve the high-temperature oxidation resistance of steel.
d. By suppressing the oxidation of Fe, it helps to form a dense oxide film mainly composed of Alt203, contributing to further improvement of high-temperature oxidation resistance. This effect is small in the range of 18 to 24 Cr, since adding a large amount of Cr exceeding 24% promotes the formation of δ ferrite and deteriorates hot workability and cold workability. It was inside.

Ni: Ni is a basic element that makes the matrix of the steel of this invention have an austenitic structure and provides high high-temperature strength.

Like the steel of this invention, C
When a large amount of ferrite-forming elements such as r, Si, and kl are added, if the Ni content is less than 20%, the formation of a large amount of δ ferrite cannot be suppressed, resulting in a decrease in high-temperature strength, so the lower limit is set to 20%. %. The higher the amount of Ni, the more stable the austenite phase becomes, which is preferable. However, if a large amount of Ni is contained, the cost increases, so the upper limit was set at 32% in consideration of cost.

Al:AA! is the most basic element that gives the steel of this invention excellent oxidation resistance. That is, Al forms a dense oxide film mainly composed of Al2O3 in the coexistence of Cr, Si, and REM in a high-temperature oxidizing atmosphere, thereby significantly improving high-temperature oxidation resistance. In order to fully obtain the effect, A13.0% or more is required, but A
Since I is a ferrite-forming element, if it is added in excess, it becomes necessary to add a larger amount of Ni to maintain the austenitic structure of the steel matrix, leading to an increase in cost.

Therefore, the range of A/ was set to 3.0 to 5.0 inches. .

Mo: Mo is an essential element for obtaining excellent high temperature strength in the steel of this invention. In other words, No is high C
It forms a solid solution in the austenite matrix of r-Ni and improves the deformation resistance of the matrix at high temperatures. Its effect is exhibited above 0.2 cm, but since MO is a ferrite-forming element like kl, Cr, 8i, etc.

If Ni is added in excess, it becomes necessary to add a larger amount of Ni in order to maintain the austenitic structure of the steel matrix, leading to an increase in cost. Therefore, the MO range is 02-1.
The range was set at 5%.

Ti, Nb, Zra These elements are effective in fixing C and N and suppressing cracking during hot working, but if added in excess they will embrittle the material, so the upper limit should be 1.0%. And so. Note that any one of these elements may be added singly, or two or more types may be added in combination, as long as the upper limit of the total amount is 1.0'S or less.

Further, in order to sufficiently obtain the effect of preventing hot working cracking of these elements, it is preferable that the total amount is 0.15% or more.

REM: Rare earth elements such as La and Ce improve oxidation resistance by adding small amounts, and at the same time improve hot workability by controlling the form of S. Since the effect of improving hot workability is expressed at REM 0101% or more, 0.0
It is necessary to add 1% or more, but if you add 0.08% or more, the thermal and stoichiometric workability will deteriorate, so 0.01
-0.08 inch. In addition, as REM, L
Any one of a, Ce, etc. may be used, or two or more types may be used.

Steel having the above components exhibits excellent high-temperature oxidation resistance and high-temperature strength. However, when manufacturing the steel plate, it has been found that edge cracking may occur during the hot rolling process. Therefore, the present inventors investigated the hot rolling conditions, and as a result, the hot rolling temperature was increased to 1000.
It has been found that the occurrence of edge cracking can be prevented by keeping the temperature within the range of -1200°C. Next, the experiment that served as the basis for finding such an optimal hot rolling temperature will be explained.

A 50 kg steel ingot having the two types of chemical compositions shown in Table 1 was melted in a vacuum, and from the surface layer of the steel ingot, the diameter was 64mm and the length was 12mm.
A round bar-shaped Greeple test piece of 0101 was cut out and
Grieple tests were carried out at various test temperatures within the range of 0 to 1250°C. In addition, in this Grieple test, the temperature is raised from room temperature to the test temperature in 20 seconds, held at the test temperature for 50 seconds, and then immediately subjected to a tensile test at that test temperature. the law of nature. The cross-sectional area at the time of fracture is Df, and the area reduction rate expressed as (I Dr/Do ) and the maximum stress, that is, the maximum stress expressed as the ratio of the maximum load to fracture and the initial cross-sectional area, were investigated.

The results are shown in FIG. 1 in correspondence with each test temperature.

As is clear from FIG. 1, the test temperature was ioo.

Within the temperature range of ~1200°C, the area reduction rate is 60 inches or more, and if the test temperature is outside this range, the area reduction rate is 60 inches or more.
It was found that the percentage was not reached. According to experiments conducted by the present invention, it is difficult to obtain a sound hot-rolled sheet when the cross-section reduction rate in the Greeple test is less than about 60 inches due to occurrence of edge cracks during hot rolling. It has been found that a sound hot-rolled sheet can only be obtained when the ratio is 60% or more, and therefore, in this invention, in order to prevent defects such as edge cracks during hot rolling, the hot-rolling temperature is set to 60% or more. 1200℃
The temperature must be within the range of
It is not possible to obtain a sound hot rolled sheet.

Examples and comparative examples of the present invention will be described below.

A 30 kg steel ingot having the chemical components shown in Samples 1 to 9 in Table 2 was melted and subjected to hot rolling-annealing, cold rolling, and annealing to obtain a cold-rolled annealed plate with a thickness of 4IIll.

In hot rolling, after heating at 1200°C for 1 hour, the 1-pass reduction amount is 10%, and reheating is repeated multiple times to 1000°C.
The hot-rolled sheet was finished at a temperature of -1200° C. and had a thickness of 8 m, but no edge cracking occurred in any of the hot-rolled sheets. After polishing the surface of the cold-rolled annealed product obtained as described above with #320 emery abrasive paper,
A test (oxidation resistance test) was conducted in which a heating-cooling cycle of holding at ℃ for 50 minutes and cooling for 10 minutes was repeated 200 times to examine the oxidation loss.
A high temperature tensile test specified in O567 was carried out at 1100℃.
The tensile strength was investigated. The results are shown in Table 3.

Among the samples, Samples 1 to 6 are steels within the composition range of this invention, Samples 7 and 8 are comparison materials of Al-containing austenitic heat-resistant steel outside the scope of this invention, and Sample 9 is SUS 3
10S standard steel, but Table 3 shows that the high-temperature oxidation resistance of the steel of this invention is much superior to that of 5US310S steel (Sample 9), and that it is much better than the existing Al-containing austenitic heat-resistant steel (Sample 7). , 8). On the other hand, the high temperature strength of the steel of this invention is 1100.
The tensile strength at °C is 57-62-i, which is much better than the 5.2-5.3-d of existing AJ-containing austenitic heat-resistant steels, and also slightly better than SUS 310 S. That is clear.

As is clear from the above examples, the austenitic oxidation-resistant steel of the present invention has excellent high-temperature oxidation resistance and high high-temperature strength. It is possible to significantly extend the service life in applications such as the following. Further, according to the manufacturing method of the present invention, it is possible to effectively prevent the occurrence of edge cracks, etc. during hot rolling, and therefore, a steel plate having both high temperature oxidation resistance and high temperature strength can be produced without causing a decrease in yield or an increase in cost. can be manufactured on a mass production scale.

Table 3

[Brief explanation of the drawing]

FIG. 1 is a correlation diagram showing the relationship between test temperature, maximum stress, and area reduction rate in the Greeple test. Applicant: Kawasaki Steel Corporation Agent: Taketo Toyota, patent attorney (1 person)

Claims (2)

[Claims] (1) CO, IO (weight%, same below), Si
1. O~2. O%, Mn 2.0'4 or less, Cr
18~24chi, Ni 20~32chi, Mo 0.2
~1.5 inches, A13.0~5.0 inches, REM 0.
01 to 0.08%, and contains Ti, Nb,
A total of 1 or more of Zr. O%
An austenitic oxidation-resistant steel comprising the following, with the remainder consisting of Fe and unavoidable impurities. (2) C0.10% or less, Si 1.0-2.0%
, Mn2, 0% or less, Cr18-24%, Ni
20-32f6. A method for producing austenitic oxidation-resistant steel, which comprises hot rolling an ingot made of a blunt material in a temperature range of 1200°C or higher.