KR100537693B1 - Fe- base heat-resistant alloy having improved the high temperature oxidation resistance and the method of making the same - Google Patents

Abstract

The present invention relates to an alloy for an iron resistance heating element having improved high temperature oxidation resistance, and a method of manufacturing the same. In particular, an oxidation resistance resistance heating element alloy in which a small amount of Be is added to a conventional Fe-Cr-Al alloy and the Fe-Cr-Al-Be alloy The present invention relates to a manufacturing method of increasing cleanliness by applying ESR (Electro-Slag Remelting) method.

In the present invention, by weight percent Cr 14.0 ~ 24.0%, Al 4.0 ~ 6.0%, Be 0.01 ~ 0.1%, the balance provides an alloy for iron resistance heating element improved high temperature oxidation resistance, characterized in that the Fe.

Then, Be is added to the molten electrode of the electrode in the manufacturing method of the electrode by adding Be, using the slag of the component system of CaF ₂ -Al ₂ O ₃ -CaO, the secondary current is 28 ~ 35A / 1 cm ~ 14.0 ~ 24.0%, Al 4.0 ~ 6.0%, Be 0.01 ~ by weight by the ESR (Electro-Slag Remelting) process to maintain the electrode lowering speed of 2 ~ 10mm / min in a way to adjust the range / cm ² 0.1%, the remainder provides a method for producing an alloy for iron resistance heating element with improved high temperature oxidation resistance, characterized in that the alloy is Fe.

Description

Fe-base heat-resistant alloy having improved the high temperature oxidation resistance and the method of making the same

The present invention relates to an alloy for an iron resistance heating element having improved high temperature oxidation resistance, and a method of manufacturing the same. In particular, an oxidation resistance resistance heating element alloy in which a small amount of Be is added to a conventional Fe-Cr-Al alloy and the Fe-Cr-Al-Be alloy The present invention relates to a manufacturing method of increasing cleanliness by applying ESR (Electro-Slag Remelting) method.

Currently, metal resistance heating elements are widely used as heating elements for industrial purposes, and the main characteristics to be satisfied with metal resistance heating elements are high resistivity, excellent high temperature oxidation resistance, mechanical strength, heat resistance and corrosion resistance, thin wire processing and easy price Should also be cheap.

The properties and lifespan of the resistance heating element are determined by many factors such as resistivity, temperature coefficient of resistance, thermoelectric power, mechanical properties, chemical stability, processing and bonding properties, and each of these properties is characterized by alloy composition, segregation degree, and grain size. It is a key technology in the manufacture of metal resistance heating element with excellent control technology because it has the characteristics that are greatly changed by the surface treatment method.

Metal-based resistance heating elements include single-phase metals such as Pt, W, Mo, and Ta, and alloys such as Ni-Cr, Mo-Si, Fe-Cr-Al, and the like.

Particularly, Mo-Si alloys include high temperature resistant heating elements, MoSi ₂ systems for 1700 ° C and 1800 ° C, which are manufactured by demanding powder metallurgy such as raw material synthesis, molding, sintering, heat treatment, and post-treatment of non-oxides. Recently, interest in the development and improvement of ultra-high temperature heating elements by powder metallurgy technology has been greatly increased, but not only know-how about detailed process technologies is required, but also the supply and demand of raw materials and the reduction of process cost are competitive. It is still far from the production of the product.

On the other hand, Fe-Cr-Al-based metal resistive heating element with Fe- (13 ~ 33%) Cr- (3 ~ 8%) Al alloy component, which can be used from 1300 ℃ to 1400 ℃, has high resistivity and long life in air It has long, excellent corrosion resistance and low cost compared to Ni-Cr-based metal resistance heating element, and has the widest application field as metal-based resistance heating element in various industrial furnaces, home appliance parts, transportation machinery parts, chemical plant industry, etc. It is material.

Each alloy element of Fe-Cr-Al-based metal resistance heating element is added according to the characteristics required as resistance heating element such as oxidation resistance, heat resistance, and electrical resistance. In addition, a small amount of zirconium (Zr) is added for oxidation resistance. .

The problem with the Fe-Cr-Al based heating element is that as the Al content increases, the performance as a heating element improves, but the dissolution and processability deteriorate. In addition, the grain growth is made at a high speed by high temperature heating, and has a feature that is easily degraded by a gas such as nitrogen, the situation is required to improve.

The Fe-Cr-Al alloy, which is widely used as a resistance heating element, is a Fe-based alloy that has a basic alloying element of 14 to 24% Cr and 4 to 6% Al, and inhibits oxidation by forming an oxide film of Cr and Al. It is an alloy system that is preferred as a heating material because of its high resistivity. The advantages of this alloy are: ① high resistivity, ② long service life and excellent corrosion resistance, ③ very high operating temperature (1300 ℃ in air), and ④ low cost of raw metal compared to Ni-Cr alloy. However, with the development of the industry, as the use environment and temperature of the resistance heating element become severe, further improvement in oxidation resistance and processability are required.

To date, the electrical, thermal and mechanical properties of metal resistive heating elements have been used to control the amount of alloys in existing ingot-hot rolling (extrusion) -cold rolling (drawing) processes or to prevent the mixing of harmful elements and inclusions and segregation of alloy components. It depends greatly on the control technology that can be done.

It is an object of the present invention to provide an alloy for a metal resistance heating element having excellent performance by improving the high temperature quality such as high temperature oxidation resistance by adding a small amount of Be to the Fe-Cr-Al alloy billet.

In addition, the Fe-Cr-Al-Be-based alloy billet is controlled by using an ESR manufacturing process technology that controls process variables to control impurities and trace amounts of alloying elements and cleanliness so that alloying components are uniform, and impurities are removed to provide high temperature oxidation resistance. An object of the present invention is to provide a manufacturing method for producing the ingot for the improved Fe-Cr-Al-based metal resistance heating element.

The present invention provides a Fe-Cr-Al-Be-based alloy for a metal resistance heating element, characterized in that by weight% Cr 14.0 ~ 24.0%, Al 4.0 ~ 6.0%, Be 0.01 ~ 0.1%, the balance is Fe.

Fe-Cr-Al-Be alloy of the present invention by weight% Cr 14.0 ~ 24.0%, Al 4.0 ~ 6.0%, Be 0.01 ~ 0.1%, the balance is Fe, the numerical limitation reason of the composition range of the main alloying elements As follows.

(1) Cr forms oxidized Cr carbide on the material surface, so the higher the content, the higher the hardness and the higher the oxidation resistance. However, due to practical limitations such as formability, weldability and economy, the content is limited to 17% by weight in steel. However, steels with 17% by weight of Cr are not suitable for high temperature heat resistance because the temperature of 800 to 850 ° C. is the limit temperature for high temperature oxidation, and the oxidation resistance deteriorates rapidly at higher temperatures. In addition, ferritic Fe-Cr alloy is a material that is brittle and difficult to cold work compared with austenitic alloys, but is an optimal alloy for heating wire due to its high corrosion resistance to sulfide gas and high electrical resistance density. Therefore, in recent years, a small amount of elements are added to the Fe-Cr alloy to stabilize Cr and have been used for heat resistance. In particular, when Al is added to Fe-Cr, since Cr acts as a second oxidation resistant element, there is an advantage of forming a stable oxide film even in a small Al. This is because Cr reduces the depletion rate of Al by increasing the activation energy in forming the Al ₂ O ₃ oxide film.

Therefore, Cr is unsuitable as an alloy for resistance heating elements because the oxidation resistance is lowered and the electrical conductivity is increased at 14.0% or less by weight, and the brittleness is increased at 24% or more, which is undesirable because it is difficult to be wired by post processing.

(2) Al is added for the purpose of improving the oxidation resistance life of the Fe-Cr group alloy. This can be seen from the analysis of the amount of Al remaining in the metal matrix after the oxidation test, the higher the residual Al content of the alloy having a longer oxidation duration. This is because as the Al content increases, a large amount of Al for forming Al ₂ O ₃ is provided to the surface oxide layer at the base metal.

Therefore, Al is unsuitable at 4% or less by weight because the improvement of high temperature oxidation resistance cannot be expected greatly, and at 6% or more, it is inappropriate because post-processing becomes extremely difficult.

(3) Be is inadequate due to the weight% of 0.01% or less, which is not suitable for the high temperature oxidation resistance improvement, and in 1.0% or more, it is not suitable because the effect of improving the oxidation resistance is not significantly increased and harmful fumes are generated during dissolution. Do.

In the present invention, the slag of the component system of CaF ₂ -Al ₂ O ₃ -CaO is used, and the electrode lowering speed is maintained at 2 ~ 10mm / min by adjusting the secondary current to 28 ~ 35A / cm ² range Fe-Cr for metal resistance heating element, characterized in that the alloy is produced by weight of Cr 14.0 ~ 24.0%, Al 4.0 ~ 6.0%, Be 0.01 ~ 0.1%, the balance of Fe by the Electro-Slag Remelting (ESR) process Provided is a method for producing an Al-Be alloy.

ESR (Electro Slag Remelting) method used in the present invention is a special dissolution refining method with wide applicability, easy to control accurate chemical composition, harmful impurities (S, O, N, etc.) by the reaction technology of molten metal and slag, Incorporation of the inclusions can be minimized, and segregation of various alloying elements can be prevented and chemical refining is easy. In addition, it is an economical clean technology that minimizes the additional cost and maximizes the yield of ingot when using ESR of ingot material.

This ESR process prevents segregation due to continuous melting and resolidification, increases ingot cleanliness by removing impurities through the reaction of metal and slag, high ingot yield without shrink pipes or shrink holes in the ingot, and suppression of internal shrinkage holes. There is an advantage that can produce a healthy ingot through.

1 is a schematic diagram of an ESR operation.

High current flows when the consumed electrode 1 connected to the power source is settled in the molten slag 3 in the water-cooled copper mold 2 through which cold water flows. The molten slag 3 is overheated by the continuous flow of current, and the consumed electrode 1 settled in the molten slag 3 is melted by the molten alloy 4. As the remelting process progresses, a refined clean alloy 5 having a controlled solidification structure is gradually produced.

The chemical composition of the consumed electrode 1 of the present invention by weight% Cr 20.0 ~ 23.5%, Al 5.0 ~ 6.0% Si 0.15 ~ 0.30%, C 0.10 ~ 0.15%, S 0.010% or less, O 0.006% or less, N 0.02 % Or less, the rest is Fe.

The consumed electrode of the invention changes the chemical composition of Be by adding Al-30% Be alloy to the molten metal when dissolving the consumed electrode. The consumed electrode of the comparative material uses an alloy containing no trace element or Zr.

The preliminary test resulted in the fact that when the electrode was subjected to the ESR process, Al tended to decrease slightly and Si tended to increase slightly, but the remaining elements remained unchanged. Therefore, the chemistry of the electrode (1) was changed according to the chemical composition of the final target ESR material. The composition can be determined.

Slag used in the present invention is a _{CaF 2 -Al CaF 2 50 ~ 80} % by weight% in the composition of the ternary _{2 O 3 -CaO, Al 2 O} 3 10 ~ 30%, CaO 10 ~ 30%.

(1) CaF ₂ has a relatively low electric conductivity at 50% or less, which can speed up the dissolution. However, there is a risk of severe arcing during operation.

(2) Al ₂ O ₃ is added within the range of 10-30% for the purpose of lowering the conductivity and increasing the operation speed. If it is less than 10%, the effect of lowering the electric conductivity is not great and it is not suitable for the purpose of use, so more than 30% is not added.

(3) CaO mainly adds desulfurization as the main purpose, and the desulfurization effect is not large at 10% or less, and at 30% or more, the amount of addition is limited because there is a risk of severe arc generation during operation.

In the present invention, the electrode lowering speed is adjusted to 2 ~ 10mm / min to have a stable operating conditions. In particular, for stable operation, the secondary current is maintained in the 28 ~ 35A / cm ² range and the electrode lowering speed is controlled.

The reason for controlling the electrode falling speed and the secondary current is as follows.

If the gap between the electrode 1 and the molten alloy 4 in the molten slag 3 is large, the heat of resistance does not generate well, the operation is unstable, and if the gap is small, overcurrent occurs and equipment failure occurs. When the lowering the electrode 1 is lowered quickly, the gap may become smaller, and when the lowered electrode is lowered, the gap may become larger, so that the adjustment of the gap is directly connected to the lowering speed of the electrode 1. In addition, since the secondary current is generated according to the size of the gap, by adjusting the secondary current, the falling speed of the consumed electrode can be adjusted, and by adjusting the falling rate of the consumed electrode, the gap between the consumed electrode 1 and the molten alloy 4 is adjusted. Can be adjusted. Accordingly, in the present invention, stable operation is maintained by controlling the gap between the electrode 1 and the molten alloy 4 by maintaining the secondary current in the range of 28 to 35 A / cm ² and adjusting the electrode lowering speed to 2 to 10 mm / min. Be sure to

Hereinafter, the manufacturing method and the characteristics of the alloy of the present invention will be described with reference to Examples.

In the present invention, the alloy design was made by grasping the role of each element on the basis of the investigation results of the existing commercialized products, and the shape of 40mm × 500mm to make the consumed electrode for making the ESR material after calculating the loading amount considering the recovery rate Melt casting was carried out using a high frequency induction furnace. Herein, ESR material refers to a clean alloy obtained by melting and casting a consumed electrode.

Table 1 shows the composition of the chemical composition of the electrode for producing each ESR material.

Table 1. Composition (wt%) of chemical composition of the electrode

The chemical composition of the electrode was analyzed by wet analysis and the qualitative analysis of micro precipitates and inclusions in the matrix was analyzed for each condition using EDX attached to a scanning electron microscope (SEM).

Table 2 shows the composition of the chemical components of the ESR material.

Table 2. Composition of Chemical Composition (wt%) of ESR Material

This is the result of analyzing the composition of chemical components after the ESR operation using the consumption electrode.

KT-EBe1 to KT-EBe4 are the invention materials, which change the chemical composition of Be by adding Al-30% Be alloy to the molten metal when dissolving the electrode, and do not add trace elements as a comparative material to compare the characteristics of the final alloy. KT-E and ZT added as a trace element were prepared.

Be is an element added in the present invention to improve the high temperature oxidation resistance, Zr is a trace element added to improve the oxidation resistance of the current commercial alloy was prepared for the oxidation resistance comparative analysis.

As a result of the component analysis, Al slightly decreased and Si tended to increase slightly in the ESR material, which was probably due to the reduction of Al, which is used as an initial insulator. In addition, it can be seen that in all ESR materials, oxygen, nitrogen and sulfur as impurities are reduced after the ESR operation. As a result, it was found that the ingot can be cleaned by reducing the residual amount of impurities such as oxygen, nitrogen and sulfur due to the ESR operation.

The electrode thus prepared is attached to an arm attached to a column moving up and down, and the ESR process is performed in the water-cooled copper mold at the bottom. The ESR device used in the present invention is a cold start method using ignition for generating initial resistance heat. For cold start ESR operation, the position of the consuming electrode and ignition is very important. ESR operation can be started by passing the current after the consuming electrode and ignition are completely contacted.

The composition of slag in the ESR operation is an important dissolution parameter that determines the degree of cleanliness of the melt and the stability of the ESR operation. In the present invention, a slag based on a ternary composition of CaF ₂ -Al ₂ O ₃ -CaO, which is relatively electrically stable and expects a clean effect, is used. Slag used 700g of CaF ₂ , Al ₂ O ₃ and CaO in the component ratio of 60 ~ 80: 10 ~ 20: 10 ~ 20, respectively, and it was used after drying more than 8h at 800 ℃ for safety. . On the other hand, the consumed electrode was surface processed before use to remove impurities present on the surface through surface processing.

For stable ESR operation, the secondary current of the sample is 28 ~ 35A / cm ² and the falling speed of the consumed electrode is 4 ~ 8mm / min.

The primary voltage and the secondary voltage were the same at 440 V and 40 V in accordance with the ratio between the electrode and the mold.

Macro and microstructures were observed for the prepared samples, and macrostructures were observed after polishing using abrasive stones. The change of microstructures according to each condition was observed with an optical microscope after polishing using abrasive paper up to 3㎛. As the etching solution, HCl + 25HNO ₃ + 25H ₂ O was used as the etching solution.

Hardness measurements were compared by casting, ESR and condition. It was measured by B Scale using Rockwell hardness tester.

The high temperature oxidation experiments measured oxidation behavior at intervals of 10 seconds for 24 h at 1200 ° C. at an isothermal rate of 5 ° C./min using TGA (Thermogravimetric Analysis). At this time, in order to resemble the atmospheric atmosphere, O ₂ : N ₂ gas was flowed through the water removal device at a ratio of 1:34, and an inert gas was used to reduce the effects of oxidation generated during cooling after the experiment was completed. Filled with helium gas. The oxide layer of the obtained specimen was analyzed by XRD. In addition, the specific resistance was measured by contact using a resistance measuring device (YOKOGAWA 7563).

Hereinafter, the result of the characteristic evaluation is demonstrated.

2 is a macrostructure of (a) a consumed electrode and (b) a KT-EBe1 ingot.

As a result of observing the macro section of the ingot, it was observed that dendritic tissues were developed in both the electrode and the ESR ingot. In the case of the consumed electrode (a), which is a mold casting material, a large amount of defects appearing as shrinkage defects existed along the center of the ingot, but in the case of the ingot (b) obtained after the ESR process, no shrinkage defects were found.

3 is a microstructure after (a) consumption electrode (KT-A3) and (b) KT-EBe3 ingot grinding, Figure 4 is a second phase distribution in the electrode (KT-A3) and KT-EBe3 ingot. The second phase distribution diagram for KT-E and KT-EZr as comparative materials is also shown.

It is determined that the gray color in FIG. 3 is an oxide or a crystallized phase, and the area fraction of KT-EBe3 ash is significantly lower. In fact, the area fraction was measured to be 0.27% for the die-casting electrode (a) and 0.13% for the KT-EBe3 material.

The size distribution of the crystallographic phases present in the KT-EBe1 to KT-EBe4, KT-E, and KT-EZr ashes measured by the same method as above is shown as an area, and the average size and the aspect ratio measurement results of the crystallized phases are summarized in Table 3. It was.

Table 3. Average Size and Aspect Ratio of Second Phase

It can be seen that the size and aspect ratio of the crystallographic phase in the case of the ESR ingot added Be of the present invention is smaller than that of the KT-E and Zr added with no trace elements.

Above, the refining effect by ESR was analyzed through component analysis and microstructure observation, and it was confirmed that ESR is an effective refining process.

5 is a result of measuring the specific resistance of the ESR material.

It can be seen that the effect of trace elements on the specific resistance is not large because it has a similar specific resistance value. In particular, it was confirmed that the specific resistance was not lost by the addition of Be.

6 is an evaluation result of oxidation resistance at 1200 ° C. (a) KT-EBe1, (b) KT-EZr, (c) KT-E

In addition to the resistivity characteristics, an important characteristic to be provided as a metal-based resistance heating element is oxidation resistance, which is also a key point of the present invention. Since the material for the resistance heating element is used in a high temperature oxidizing atmosphere, the oxidation resistance is to determine whether or not the life of the resistance heating element can be relied on. 6 is a result of evaluating the oxidation characteristics of the Be-added alloy (KT-EBe1) and the comparative material KT-EZr and the trace element-free species (KT-E) developed in the present invention. Oxidation characteristics were evaluated for 24 hours at 1200 ℃ by a thermogravimetric measuring device, and the most excellent oxidation characteristics were observed in alloys containing trace amounts of Be. This is because a stable BeO oxide is formed on the surface when Be is added.

7 is an XRD analysis result of the oxidation test sample. (a) KT-EBe1, (b) KT-EZr, (c) KT-E

XRD analysis was performed on specimens subjected to 8 and 24 hour oxidation tests. Figure 7 (a) shows the addition of 0.011% of Be as a trace element. No oxide film was observed except for the formation of alumina layer on the surface until 8 hours, and the amount of alumina oxide film increased after 24 hours. It was observed that a CrO ₂ oxide film was formed. However, after 8 hours and 24 hours, the XRD peaks of the oxide films were almost identical, indicating that a large amount of oxide films were initially formed and they continued to grow. As a result, when Be is contained, the initial oxidation resistance of the Fe-Cr-Al alloy is greatly improved due to the strong oxide film of Be.

8 is a flow chart of the ingot production and wire refining process of Fe-Cr-Al-Be alloy.

The drawing shows an example of the wire refining process of Fe-Cr-Al-Be-based metal resistance heating elements through sound ingot production, swaging, rod mill, drawing and intermediate heat treatment.

By applying the ESR method to the production of Fe-Cr-Al-Be alloy ingot, it was possible to produce a clean ingot that greatly reduced the residual amount in the ingots of oxygen, nitrogen and sulfur as impurities, and no shrinkage defects were found inside the ingot. The yield was greatly improved.

1 is a schematic diagram of an ESR process,

Figure 2 is a macrostructure of (a) the electrode consumed and (b) KT-EBe1 ingot

Figure 3 shows the microstructure after polishing of the (a) consumption electrode and (b) KT-EBe3 ingot

4 is a second phase distribution of the electrode and the KT-EBe3 ingot

5 is the specific resistance of the ESR ingot

6 shows oxidation resistance evaluation at 1200 ° C .; (a) KT-EBe1, (b) KT-EZr, (c) KT-E

7 is an XRD analysis result of an oxidation test sample; (a) KT-EBe1, (b) KT-EZr, (c) KT-E

8 is a wire drawing process diagram of the Fe-Cr-Al-Be metal resistance heating element.

♣ Explanation of symbols for main part of drawing ♣

1 consumed electrode 2 mold

3: molten slag 4: molten alloy

5: clean alloy

Claims (2)

Cr 14.0 ~ 24.0% by weight, Al 4.0 ~ 6.0%, Be 0.01 ~ 0.1% for improving the high temperature oxidation resistance , the balance is Fe alloy with improved high temperature oxidation resistance, characterized in that the Fe. In the manufacturing of the consumed electrode, using the consumed electrode to which Be is added by adding Al-Be alloy to the melt of the consumed electrode , Using slag of the component system of CaF ₂ -Al ₂ O ₃ -CaO, By the ESR (Electro-Slag Remelting) process of maintaining the falling speed of the consumed electrode at 2 ~ 10mm / min in a manner to control the secondary current in the range 28 ~ 35A / cm ² , By weight% Cr 14.0 ~ 24.0%, Al 4.0 ~ 6.0%, Be 0.01 ~ 0.1% for improving the high temperature oxidation resistance , the balance of Fe is an alloy for iron resistance heating element with improved high temperature oxidation resistance, characterized in that for producing an alloy Manufacturing method.