JPH0581657B2 - - Google Patents

Abstract

A heat-formable, ferritic steel alloy having 20 to 25% of chromium, 5 to 8% of aluminium, not more than 0.01% of phosphorus, not more than 0.01% of magnesium, not more than 0.5% of manganese and not more than 0.005% of sulphur, the remainder being iron, including unavoidable impurities, is proposed, according to the invention the said alloy furthermore containing 0.03 to 0.08% of yttrium, 0.004 to 0.008% of nitrogen, 0.02 to 0.040% of carbon and about equal amounts of 0.035 to 0.07% of titanium and 0.035 to 0.07% of zirconium, with the proviso that the sum of the contents of titanium and zirconium in percent is 1.75 to 3.5 times as large as the sum of the contents of carbon and nitrogen in percent. The alloy has substantially better properties in use than the base alloy.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention provides hot workable
Regarding ferritic alloy steel. More specifically, the present invention comprises 20-25% chromium, 5-8% aluminum
%, phosphorus 0.01% or less, magnesium 0.01% or less,
It relates to ferritic alloy steel containing 0.5% or less manganese, 0.005% sulfur, and the balance iron, and containing unavoidable impurities.

[Problems to be solved by conventional technology and invention]

Alloy steels of the general types described above are known in the art and are utilized in the manufacture of electrical heating elements and high temperature strength catalyst supports. These alloy steels are
It forms a very adherent oxide layer and has very good anti-scaling strength. Several attempts have been made to further improve the basic composition by adding further elements or reducing unavoidable impurities that arise during the manufacturing process.

For example, British Patent Application Publication No. GB-A-2
In Publication No. 070 642, yttrium of 2% or less,
It has been proposed to add 0.1 to 2% titanium as well as hafnium, zirconium, cerium, or lanthanum to form a particulate cast structure and improve hot workability. Yztrium, hafnium,
The amount of zirconium and mixed metal (Ce+La) added is, for example, 1, respectively, as described in claims 7, 8, 10, and 11.
% or less. In Example B, titanium is 0.34
The best results were obtained with 0.46% niobium. However, there are several points in the teachings of this publication.

The above-mentioned amounts of titanium improve the mechanical properties, however, since some of the proposed mixtures are very expensive and the properties of the alloy steel are significantly affected even at proportions below 1%. , at the same time, titanium leads to a deterioration of the property behavior under periodically varying oxidation conditions. The service life is
In the VIW test, the sample with titanium content of 0.47% reduces to 2800 variation cycles, compared to 5000 variation cycles for the sample without titanium. This is related to the concentration of titanium oxide in the outer oxide layer, which
The oxide layer is removed [Corrosion Science, Vol. 24, No. 7,
1984, pp. 613-627].

During the VIW test described above, a small wire-like test coil with a thickness of 0.4 mm is heated by passing an electric current directly through it. Regarding the supply of current, supply (energization) and stop are continuously repeated at 2 minute intervals. The maximum temperature obtained is measured optically and the applied voltage is varied to maintain the same switching frequency constant during the test. This is discussed in KEVolk, Nickel und Nickellegierungen, Vol.
145 pages, Springer Verlag
Verlag), 1970.

Moreover, for the sample with titanium content of 0.47%, strong and unevenly distributed titanium carbide defects were observed during the hot deformation stage. As a result, the mechanical properties differ significantly, making uniform cold working difficult or impossible.

By adding niobium, ferrite type
Fe-Cr alloy steel has slightly increased brittleness at 475°C (Iron and Steel).
Steel), p. 199, John Whalley &
John Wiley & Sons, New York
(see Boron, Calcium, Columbium and Zirconium in 1966), but also its strength deteriorates under periodically changing oxidizing conditions.
Higher temperatures result in the formation of various niobium oxides (Nb, NbO ₂ , Nb ₂ O ₅ ) which are associated with volume increases, growth stresses and consequent oxide spalling. Furthermore, niobium oxide has a melting point of e.g.
Since the temperature is relatively low, such as 1500℃, it is not stable at high temperatures [P. Kofstad (P.
Kofstad), high temperature oxidation of metals (High
Temperature Oxidation of Metals), page 215,
John Whalley & Sons
Wiley & Sons), New York, 1966].

The basic alloy steel according to European Patent No. EP-B-0 091 526 contains 0.002-0.06% rare earths, up to 0.04% phosphorus and 0.03% sulfur, as well as zirconium for stabilization and improved fatigue strength. It also contains niobium. Zirconium and niobium have a low carbon content (up to 0.05%) and nitrogen content (up to
0.05%), and niobium is always added in an amount of 0.364 to 1.209% or less when carbon and nitrogen are completely absent [see claim 1 of the publication].

The alloy steel disclosed in this cited patent also has drawbacks. As long as rare earth elements are added, it is necessary to consider the formation of oxides with relatively low melting points.
Alloy steels can only be used at certain maximum temperatures.
In addition, phosphorus content is 0.04% or less and sulfur content is 0.03%.
% or less is necessary in the applicant's limitations. In particular, significant drawbacks must be taken into account when the zirconium content is below about 1% and the niobium content is below about 2%. Regarding niobium, it has the same drawbacks as German Patent Application No. GB-A-2 070 642. When the zirconium content is reduced further, the oxidation resistance improves very quickly, and the opposite is true [H.
Pfeiffer) and H.Thomas,
Oxidation Resistant Alloys,
Page 260, Springer Verlag
- Verlag), 1963]. Furthermore, when the zirconium content is much higher than the solubility of the ferritic iron (Fe) matrix, zirconium nitride,
A coarse dispersion (precipitation) of zirconium carbide and zirconium carbonitride occurs, with no sustained hindrance to grain growth and no significant strength increase.

[Summary of the invention]

It is therefore an object of the present invention to provide a hot-deformable ferritic alloy steel which further improves the known basic alloy steels and avoids the disadvantages of the prior art.

More particularly, it is an object of the present invention to provide an alloy steel as described above, in which grain growth is significantly limited and the service life in repeated oxidation tests is significantly improved.

In view of the above object and other objects that will become clear from the following description, one feature of the present invention is that: Chromium 20-25% Aluminum 5-8% Phosphorus 0.01% or less Magnesium 0.01% or less Manganese 0.5% Alloy steel containing sulfur 0.005% or less iron, the balance of unavoidable impurities, and further containing yttrium 0.03-0.08%, nitrogen 0.004-0.008%, carbon 0.020-0.040%, titanium 0.035-0.07%, and zirconium 0.035-0.07%. The hot-deformable ferritic alloy steel is characterized in that the total content percentage of titanium and zirconium is 1.75 to 3.5 times the total content percentage of carbon and nitrogen.

Building alloy steels according to these characteristics improves on basic alloy steels and eliminates the drawbacks of the prior art.

According to another feature of the invention, the ratio of the titanium to zirconium mixture is between 0.6 and 1.4.

According to yet another feature of the invention, at least one of hafnium, tantalum and vanadium is further added to the alloy steel.

Chromium is required at least 20% in combination with at least 5% aluminum to form a single aluminum oxide layer. More than 25% chromium and more than 8% aluminum reduce hot and cold workability.

Yttrium is required in an amount of 0.03% for oxidation resistance, but if it exceeds 0.08%, segregation will be promoted at grain boundaries.

Carbon 0.020 to obtain high temperature creep strength
%, but if it exceeds 0.04%, the oxidation resistance will decrease.

Nitrogen is 0.004 to 0.008 for the same reason as carbon mentioned above.
limited to %.

Titanium and zirconium are
Improves creep strength in combination with carbon and nitrogen in the range of 0.035% to 0.07%.

Titanium + zirconium: carbon + nitrogen = 1.75 ~
A ratio of 3.5 is determined according to stoichiometric relationships based on the atomic weights of each. When the Ti+Zr:C+N ratio is less than 1.75, the amount of carbide or carbonitride precipitation is so small that effective suppression of grain growth cannot be achieved. If this ratio is greater than 3.5, the maximum achievable precipitation amount will be extreme and the amount of titanium and zirconium present in solution will result in a periodic deterioration of the oxidation resistance of the material.

A ratio of Ti:Zr=0.6 to 1.4 is preferably determined in order to ensure the thermodynamic stability of the titanium and zirconium carbides defining the nitride or carbonitride. Although the titanium:zirconium ratio cannot be determined simply, both of these in a mixture are added to bind carbon and nitrogen and result in fine precipitation of carbides and carbonitrides to inhibit crystal growth. The upper and lower limits of the sum of titanium and zirconium are determined by the upper and lower limits of carbon and nitrogen in the present invention.

Novel features considered unique to the present invention include:
In particular, it is stated in the claims. The invention itself, both as to its construction and method of operation, may, however, be best understood, as well as further objects and advantages, from the following description of specific embodiments, taken in conjunction with the accompanying drawings, in which: FIG.

[Example of implementation]

According to the invention, chromium 20-25%, aluminum 5-8%, phosphorus 0.01% or less, magnesium 0.01%
% or less, manganese 0.5% or less, sulfur 0.005% or less, the balance being iron, including unavoidable impurities, and yttrium 0.03~0.08%, nitrogen 0.004~
0.008%, carbon 0.020~0.040%, titanium 0.035~0.07
% and zirconium 0.035 to 0.07%, characterized in that the total content percentage of titanium and zirconium is 1.75 to 3.5 times the sum of the content percentages of carbon and nitrogen. A hot-deformable ferritic alloy steel is proposed.

The mixing ratio of titanium and zirconium is 0.6 to 1.4
It is acceptable within the range of .

Furthermore, at least one of hafnium, tantalum, and vanadium may be further added.

An apparatus for testing the service life of a heating conductor 1 arranged horizontally and bent in a helical manner is shown in FIG. 1a. A heating conductor 1 is fixed on one side to a holder 2 and connected to a power source 3. In this case, the heating conductor 1 consists of a 12-turn coil with a length of 50 mm and an inner diameter of 3 mm. The diameter of the heating conductor wire is 0.4mm. The heating conductor is switched on and off every 2 minutes (current is applied to the heating conductor for 2 minutes, de-energized for 2 minutes, and this process is repeated). A radiation (light) pyrometer measured the temperature reached during the heating step in a non-contact manner, and temperature adjustment was carried out by varying the applied voltage to a constant value.

The above test was conducted in a normal air atmosphere until the heating conductor passed through and burned (broken). The number of cycles is a direct value of service life. As with all materials, the unavoidable presence of some degree of strong oxidation reduces the effective metal cross section for carrying current over time. Therefore, the electrical resistance increases correspondingly, and when increasing the voltage, the predetermined test temperature can only be maintained without changing the switching rhythm. The test equipment utilized is an automatically operated temperature control device. Therefore, the test temperature of the heating stage can be maintained throughout the test period, regardless of the progress of oxidation of the heating conductor, until burnout (breakage).

In the apparatus for testing the service life of a vertically suspended heating conductor wire 4 as shown in FIG. 1b, the length of the heating conductor is 1 meter. Its upper end is fixed to a holder 5, a variable weight load 6 is applied, and it is connected to a power source 7.

In this device, a 0.4 mm thick heating conductor wire is turned on and off alternately at two-minute intervals. Here too, as in the apparatus of FIG. 1a, the temperature reached during the heating phase was measured in a non-contact manner and adjusted to a constant value.

The results shown in FIGS. 2-5 are for a comparative alloy steel (Sample 1) and an improved alloy steel of the present invention (Sample 2) having the following composition.

Sample 1 Sample 2 Cr 20.10 20.45 Al 4.91 5.05 P 0.009 0.007 Mg 0.01 0.01 Mn 0.22 0.15 S 0.003 0.002 Y − 0.04 N 0.010 0.007 C 0.045 0.037 Ti0 − 0. 07 Zr 0.16 0.06 Fe Balance Balance Figure 2 is Figure 1a The service life value obtained with the device is expressed as the number of cycles (number of energizations) until combustion (wire breakage) occurs. sample,
Turn the power switch on and off every 2 minutes,
The temperature reached during the heating phase was then measured in a non-contact manner and the applied voltage was varied such that a constant test temperature of 1200 was maintained during the total test time in each cycle. Sample 1 lasted up to 5343 cycles, while sample 2 burnt out after 6213 cycles. This represents an increase of more than 15%.

FIG. 3 shows the relationship between test temperature and measured tensile strength. From this figure, it can be seen that the improved alloy steel of the present invention has higher tensile strength over the test temperature range.

FIG. 4 shows the number of cycles obtained in the test according to FIG. 1b in relation to the applied load. The improved alloy steel showed significantly higher service life at all loads. That is, at a load of 2N/mm ² it is 6 times, at a load of 3N/mm ² it is about 5 times, and
A load of 4 N/mm ² always resulted in a 3.5-fold increase in service life.

The ductility of the material during long-term use at high temperatures is also an important structural feature. Ferrite type
The decrease in ductility of Fe-Cr-Al alloy steels is related to strong grain growth at high temperatures. In Figure 5a,
The particle size (particle size value) is expressed in μm for sample 1 after storage for 6.5 days at 950-1050°C (upper curve 1). Furthermore, 13 at 950℃, 1050℃ and 1150℃
The grain size of the improved alloy steel after holding for days is shown (bottom dashed line 2). As is evident from these curves, the improved alloy steel with twice the holding time has a significantly finer grain structure than the comparative alloy steel.

Therefore, the number of bends shown in Figure 5b (the number of bends at 180° before breaking) is 950°C and 1075°C.
It is not surprising that for samples kept at a temperature of 1175° C. for 13 days or 6.5 days, it is significantly higher for the improved alloy steel of the invention than for the comparative alloy steel due to the fine grain structure. In this comparison,
It shows that the improved alloy steel has significantly higher ductility than the comparative alloy steel.

Furthermore, sample 3 was prepared by adding 0.10% hafnium to the improved alloy steel according to the present invention (sample 2).
Sample 4 containing 0.10% tantalum was tested in the same manner as in FIG. 3, and the results shown in FIG. 6 were obtained. As can be seen in Figure 6, the addition of hafnium or tantalum further increases the high temperature tensile strength.

It will be understood that the above-described elements can be used alone or in combination of two or more for useful purposes in configurations different from those described above.

Although the present invention has been described above using specific examples of hot-deformable ferritic alloy steel, various improvements and structural changes are possible without departing from the spirit of the present invention. is not limited to the above.

Moreover, since the gist of the present invention is sufficiently clear from the above description, one skilled in the art can understand the present invention in general or specific embodiments from the perspective of the prior art by applying current knowledge. It can be easily adapted to various applications without omitting components that constitute essential features. Matters that are new and should be protected by a patent are as described in the claims.

〔Effect of the invention〕

As explained above, the present invention improves the shortcomings of the prior art, including high cost, poor behavior under oxidizing conditions, and difficulty in uniform low-temperature deformation, and significantly limits grain growth. and a hot-deformable ferritic alloy steel with significantly improved service life in repeated oxidation tests is provided.

[Brief explanation of the drawing]

Figures 1a and 1b are schematic diagrams of the equipment used in the test, Figure 2 is a graph showing actual measured values of service life, and Figure 3 is a graph showing the relationship between temperature and tensile strength. Figure 4 is a graph showing actual measured values of service life under a condition where a pull-down load is applied, Figures 5a and 5b are graphs showing values related to particle growth or the number of bends, and Figure 6 is a graph showing the high temperature strength of the alloy according to the present invention. 1... Heating conductor, 2, 5... Holder, 3, 7
...Power source, 4...Heating conductor wire, 6...Variable load.

Claims (1)

[Claims] 1 Contains 20 to 25% chromium, 5 to 8% aluminum, 0.01% or less phosphorus, 0.01% or less magnesium, 0.5% or less manganese, 0.005% or less sulfur, and the balance of iron and unavoidable impurities, In addition, yztrium
0.03~0.08%, Nitrogen 0.004~0.008%, Carbon 0.020~
0.040%, titanium 0.035-0.07% and zirconium
A hot workable ferritic alloy steel containing 0.035 to 0.07%, characterized in that the total content percentage of titanium and zirconium is 1.75 to 3.5 times the sum of the content percentages of carbon and nitrogen. Deformable ferritic alloy steel. 2 Mixing ratio of titanium and zirconium is 0.6 to 1.4
The ferritic alloy steel according to claim 1, characterized in that: 3. The ferritic alloy steel according to claim 1, wherein the alloy steel contains at least one element of hafnium, tantalum, and vanadium.