JPH08311620A - Stainless steel excellent in hot workability and molten salt corrosion resistance - Google Patents

Abstract

PURPOSE: To produce a stainless steel excellent in hot workability and molten salt corrosion resistance. CONSTITUTION: This stainless steel has a compsn. contg. 0.03 to 0.1% C, <=2.0% Mn, <=0.02% N, 10 to 15% Ni, 14.0 to 20.0% Cr, 0.2 to 1.5% Si, 0.5 to 4.0% Al and 0.0005 to 0.05% B, and the balance substantial Fe. If required, it may contain <=0.5% Ti and <=0.5% La as well. Furthermore, the relationship of, by atomic ratio, Al%/N%>=4 is preferably valid between the Al content and the N content. Thus, it is used as the material suitable for structural materials, parts or the like exposed to a severe high temp. corrosive atmosphere such as a separator for a molten carbonate type fuel battery.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to stainless steel used as a material for a separator for molten carbonate fuel cells and having excellent hot workability and molten salt corrosion resistance.

[0002]

2. Description of the Related Art When Al is added to stainless steel, a stainless steel exhibiting excellent oxidation resistance and high temperature corrosion resistance is obtained. For example, Fe-Cr-Al-based stainless steel utilizes its excellent high-temperature characteristics to make members for combustion appliances,
It is used as an exhaust gas member for automobiles. But,
The ferritic stainless steel has a lower high temperature strength than the austenitic stainless steel and is not used as a high temperature structural material or the like in reality. Austenitic stainless steel is exclusively used for applications used in an atmosphere exposed to a high temperature molten salt. For example, molten carbonate is another type of fuel cell separator material that is required to have corrosion resistance so that molten carbonate, which is an electrolyte, does not cause molten salt corrosion. High-grade stainless steels such as SUS316L and SUS310S have been used as materials satisfying such requirements. Corrosion resistance to molten carbonate can be improved by adding Al, as disclosed in JP-A-62-294153.

However, when a large amount of Al is added to austenitic stainless steel, Ni-Al based intermetallic compounds are precipitated in the austenitic matrix and the hot workability is remarkably reduced. Therefore, cracks are likely to occur during continuous casting, hot rolling, hot forging, etc., and in some cases, the steel material may break during manufacturing. In particular, when it is attempted to manufacture a hot-rolled steel sheet of Al-containing austenitic stainless steel with a normal continuous casting facility, the surface temperature of the slab is 1000.
Since the temperature is below ℃, surface cracking occurs and the product yield is significantly reduced. The hot workability of Al-containing austenitic stainless steel is described in JP-A-55-443498.
Precipitating a small amount of δ-ferrite during solidification, La, C
It is said to be improved by adding a rare earth element such as e. In JP-A-60-262945, 1000-
Cracking is prevented by hot rolling in the temperature range of 1200 ° C. In Japanese Patent Publication No. 64-4580, hot workability is improved by extremely lowering S and suppressing oxide forming elements.

[0004]

SUS31 conventionally used as a separator for molten carbonate fuel cells
0S stainless steel lacks corrosion resistance and the cost of steel materials is high. In particular, the material disclosed in JP-A-62-294153 is
Since it has a basic composition of 25% Cr-20% Ni steel, it is a very expensive steel material. On the other hand, 1000-120
Suppressing cracking by hot rolling at 0 ° C
The 62945 gazette requires not only a special equipment structure because the surface temperature of the slab becomes 1000 ° C. or less when it is manufactured by a normal continuous casting equipment, but also the product yield is low. The present invention has been devised to solve such a problem, and adjusts the content of alloying elements in a correlative manner, and among them, suppresses the N content and regulates the B content to improve heat The object of the present invention is to obtain an Al-containing stainless steel which prevents cracking particularly at 800 to 1000 ° C. during hot working, exhibits excellent molten salt corrosion resistance, and is inexpensive.

[0005]

In order to achieve the object, the stainless steel of the present invention has C: 0.03 to 0.1% by weight, Mn: 2.0% by weight or less, N: 0.02% by weight. % Or less, Ni: 10 to 15% by weight, Cr: 14.0 to 20.
0% by weight, Si: 0.2 to 1.5% by weight, Al: 0.5
~ 4.0 wt% and B: 0.0005-0.05 wt%
And the balance is substantially Fe. The stainless steel of the present invention has a T
i: 0.5 wt% or less, and La: 0.5 wt% or less can be included. Further, it is preferable to establish a relationship of Al% / N% ≧ 4 in atomic ratio between the Al content and the N content. Further, it is preferable that the content of each alloy component is adjusted so that the value of δ (%) defined by the formula (1) is in the range of 0.5 to 4.0%. δ (%) = 1.57 × Cr% + 0.7 × Si% + 3.89 × Al% −1.57 × Ni% −0.16 × Mn% −38.5 × C% −7.65 ... (1)

In the stainless steel of the present invention, AlN, which is harmful to hot workability, is found in the grain boundaries from a number of experimental results in which the characteristics of the structural members that come into contact with the molten salt at high temperature and the occurrence of cracks due to hot working are investigated. Each alloy component and its content are determined in order to suppress precipitation in the alloy. The molten salt corrosion resistance is secured by setting a high Al content as in the above-mentioned prior art documents. On the other hand, the hot workability that tends to deteriorate with increasing Al content is improved by controlling the N content and adding B, Ti, La and the like. The alloy components contained in the stainless steel of the present invention and their contents will be described. C: 0.03 to 0.1% by weight It is an alloy element effective for high temperature strength, and a remarkable effect is exhibited by adding 0.03% by weight or more of C. However, the addition of a large amount of C exceeding 0.1% by weight deteriorates the hot workability of the steel material, and cracks easily occur during hot rolling or hot forging. Mn: 2.0 wt% or less Since it is an austenite forming element and is an element effective for deoxidation and desulfurization during melting, M is usually about 1 wt%.
n is included. However, a large amount of Mn content exceeding 2.0% by weight deteriorates the corrosion resistance to molten salt.

N: 0.02% by weight or less It is an austenite forming element and is an alloying element that also effectively works to improve high temperature strength. As a result of various studies on the hot workability of the Al-containing austenitic stainless steel, the inventors have found that the decrease in hot workability at 800 to 1000 ° C. is due to AlN segregated at the grain boundaries. Found out. AlN reduces hot workability only when segregated at grain boundaries. It was found that the N content should be 0.02 wt% or less in order to suppress the adverse effect of AlN. Then, it was found that when the concentration of Al and N in the grain boundary was set to Al / N ≧ 4 in atomic ratio, the adverse effect of AlN was suppressed. Ni: 10 to 15 wt% It is an austenite forming element, and 10 wt% or more is necessary to maintain the austenite phase. But,
The addition of a large amount of Ni increases the price of the product, so in the present invention, the upper limit of the Ni content is defined as 15% by weight.

Cr: 14.0 to 20.0% by weight It is an alloying element which plays an important role in improving the corrosion resistance to molten salt. The improvement of the corrosion resistance becomes remarkable when the content of Cr is 14.0% by weight or more. However, an excess of Cr exceeding 20.0% by weight makes the austenite phase unstable and deteriorates the hot workability. Si: 0.2 to 1.5% by weight It is an alloy element effective for improving the high temperature strength and corrosion resistance of steel materials, and 0.2% by weight or more of Si is required for improving the corrosion resistance. However, a large amount of Si content exceeding 1.5% by weight becomes a cause of impairing manufacturability and hot workability. Al: 0.5 to 4.0% by weight It is an alloying element that exhibits a deoxidizing action during melting and is effective in improving the corrosion resistance against molten salt. The improvement in corrosion resistance is due to the formation of a dense oxide film of Al, which becomes remarkable when Al is added in an amount of 0.5% by weight or more. However, the inclusion of a large amount of Al exceeding 4.0% by weight causes deterioration of manufacturability and hot workability.

B: 0.0005 to 0.05% by weight It is an alloying element which segregates preferentially at the grain boundaries and prevents the deterioration of the hot workability due to the segregation of S, P, etc. at the grain boundaries. . In addition, the precipitation of AlN at the grain boundaries, which is a cause of lowering the ductility during hot working at 800 to 1000 ° C., is N
Is fixed in the steel by the addition of B. Such an effect of adding B becomes remarkable at 0.0005% by weight or more. However, the addition of a large amount of B exceeding 0.05% by weight rather causes a reduction in hot workability. Ti: 0.5% by weight or less if necessary N and C are fixed and are alloy elements effective in improving hot workability and improving high temperature strength. Also,
Ti densifies an oxide film formed on the surface of the steel material and improves molten salt corrosion resistance. Further, like La, it is taken into the Al-based oxide film and improves the thermal conductivity of the scale. However, if a large amount of Ti exceeding 0.5% by weight is contained, the hot workability deteriorates.

La: 0.5% by weight or less as required Rare earth elements have the effect of improving hot workability, and mixtures of La, Ce and the like, Y and the like are generally used.
However, Ce and Y are likely to form clusters, and particularly during continuous casting, the production of shavings and vertical cracks is promoted, the yield of the product is significantly reduced, and the product may not be provided in some cases. In this respect, La effectively acts on the improvement of hot workability without generating clusters, whiskers and the like. Also,
It is preferable that the steel material used as the collector plate of the molten carbonate fuel cell separator has electrical conductivity. However, the Al-based oxide film has high electric resistance, and the scale formed in the environment of molten carbonate also shows low electric conductivity. On the other hand, the oxide film formed on the surface of the steel material to which La is added exhibits high electrical conductivity because it incorporates the oxide of La. However, the inclusion of a large amount of La exceeding 0.5% by weight promotes cracking during hot working.

Δ (%): 0.5-4.0% δ appearing in the ingot or slab of stainless steel according to the present invention
The amount of ferrite can be predicted by the above-mentioned formula (1). Expression (1) is an expression obtained by the present inventors. When each alloy component is adjusted so that δ (%) is in the range of 0.5 to 4.0, excellent hot workability is exhibited, and cracking occurs during hot rolling or hot forging. To be prevented.
As a result, it becomes possible to manufacture a high-quality hot-rolled steel strip with a high yield by using an ordinary hot-rolling equipment.

[0012]

EXAMPLE Various stainless steels having the components and compositions shown in Table 1 were melted in a 30 kg vacuum melting furnace and then continuously cast into ingots. In Table 1, Group A steels are stainless steels according to the invention and Group B steels are comparative stainless steels. B1 does not satisfy the requirements of the present invention in that it does not contain B, and B2 does not satisfy the requirements of the present invention in that it contains Ce. The obtained ingot was cut to prepare a hot tensile test piece having a diameter of 10 mm.
In the hot tensile test, the test temperature was set to 800 to 1000 ° C., and the tensile speed was set to 10 ⁻² / sec assuming continuous casting.

[0013]

[Table 1]

Each test piece was subjected to a hot tensile test at a test temperature of 1000 ° C., and the cross-sectional reduction rate before and after the test was obtained. When the obtained cross-section reduction rate was arranged in relation to the N content, as shown in FIG. 1, A1 in which the N content was reduced and B was added
It was found that stainless steel No. 3 exhibited high ductility. Further, it was found that the A4 and A5 stainless steels added with Ti and the A6 and A7 stainless steels added with La show higher ductility. In particular, La-added A6 and A7 had excellent ductility, even though the N content was relatively high. On the other hand, the test piece B1 containing no B has a cross-section reduction rate of 40.
%, The ductility was remarkably lower than that of the group A. Further, when the relationship between the test temperature and the cross-section reduction rate was investigated, as shown in FIG. 2, all the test pieces of the group A showed high ductility in the temperature range of 800 to 1000 ° C. Among them, it is understood that the test piece A7 to which B and Ti are added in combination has a high area reduction rate of more than 70% at any test temperature and is excellent in ductility.

Then, a hot tensile test was carried out at 800 ° C. at which the ductility was the lowest, and after breaking the test piece at the grain boundary,
The fracture surface was subjected to Auger analysis. The hot tensile test is performed in a vacuum atmosphere with a vacuum degree of 10 ^-6 Torr in order to prevent the fracture surface after the test from being contaminated by the atmosphere, and the Auger analysis is performed without exposing the test piece after fracture to the atmosphere. It was transferred to the device. As a result of SEM observation of the fractured surface after the test, granular precipitates were observed on the entire fractured surface in the test specimens of group B. The granular precipitate was assumed to be AlN as a result of Auger analysis. On the other hand, in the test piece of group A, almost no granular precipitate was observed on the fracture surface. FIG. 3 shows the relationship between the Al concentration and the N concentration on the fracture surface of each test piece. From FIG. 3, the atomic ratio between the Al concentration and the N concentration at the grain boundary is A
It was confirmed that better ductility can be obtained when the relationship of 1% / N% ≧ 4 is established.

Further, when the relationship between the amount of B added and the cross-sectional reduction rate after the hot tensile test was investigated, it was found that the relationship shown in FIG. 4 was established between them. From FIG. 4, 800
It was confirmed that the ductility in the temperature range of up to 1000 ° C. was improved with the addition of B. Further, when Ti and B were added together, the ductility was further improved, and the effect was remarkable at 800 ° C and 1000 ° C. The above shows the results of an investigation of hot workability. The stainless steel used in this example was a material having excellent molten salt corrosion resistance because of its high Al content. In addition, the test pieces A5 to 7 containing Ti, La, etc., which improve the thermal conductivity of the Al-based oxide film, were materials sufficiently provided with the characteristics required for the molten carbonate fuel cell separator. Further, in order to compare the addition effect of La and Ce, respective steel materials having compositions of A6, A7 and B2 were melted in a vacuum melting furnace of 15 kg. After obtaining the ingot, the ingot was cut from the center, the cut surface was mirror-polished, and the cross section of the steel ingot L was observed. As shown in Table 2 showing the observation results, the steel B2 added with Ce is the steel A added with La.
6, remarkable formation of clusters was observed as compared with 6, A7. From this, it was confirmed that the addition of La was effective.

[0017]

[Table 2]

[0018]

As described above, in the stainless steel of the present invention, the hot workability of stainless steel having an increased Al content in order to improve the molten salt corrosion resistance is regulated and regulated with respect to the N content. The amount is improved by adding B. Although this stainless steel is a steel material containing a relatively large amount of Al, it can be manufactured into a hot rolled steel strip with a high yield. The obtained hot rolled steel strip is used as a material suitable for structural materials, parts and the like exposed to a severe high temperature corrosive atmosphere such as a molten carbonate fuel cell separator.

[Brief description of drawings]

FIG. 1 Influence of N content on cross-section reduction rate by hot tensile test at 1000 ° C.

[Fig. 2] Relationship between test temperature and cross-section reduction rate in hot tensile test

FIG. 3 Relationship between Al concentration and N concentration on the fracture surface of a test piece fractured in a hot tensile test

FIG. 4 Effect of B content on reduction rate of cross section after hot tensile test

Claims (5)

[Claims] 1. C: 0.03 to 0.1% by weight, Mn:
2.0 wt% or less, N: 0.02 wt% or less, Ni: 1
0-15% by weight, Cr: 14.0-20.0% by weight, S
i: 0.2 to 1.5% by weight, Al: 0.5 to 4.0% by weight, and B: 0.0005 to 0.05% by weight, with the balance being essentially Fe. And stainless steel with excellent molten salt corrosion resistance. 2. A Ti content of 0.5% by weight or less.
Stainless steel listed. 3. La: 0.5 wt% or less is included.
Alternatively, the stainless steel described in 2. 4. The stainless steel according to claim 1, wherein the relationship of Al% / N% ≧ 4 in atomic ratio is established between the Al content and the N content. 5. The content of each alloy component is adjusted so that the value of δ (%) defined by the formula (1) is in the range of 0.5 to 4.0%. The stainless steel according to any one of 1. δ (%) = 1.57 × Cr% + 0.7 × Si% + 3.89 × Al% −1.57 × Ni% −0.16 × Mn% −38.5 × C% −7.65 ... (1)