KR101974815B1 - Austenitic steel excellent in high temperature strength using reduction of chromium content - Google Patents

Abstract

Austenite steel for a high temperature according to the present invention is characterized by comprising 0.35-0.5 wt% of carbon (C), 1.0-2.0 wt% of silicon (Si), 5.0-8.0 wt% of manganese (Mn), 13.5-16.5 wt% of nickel (Ni), 20-24 wt% of chromium (Cr), 0.5 to less than 1.5 wt% of molybdenum (Mo), and the remainder consisting of iron (Fe) and inevitable impurities, wherein the ratio of the chromium (Cr) content to the carbon (C) content, C_(Cr)/C_C, in alloy elements is 50-60.

Description

TECHNICAL FIELD [0001] The present invention relates to austenitic steel having excellent room temperature and high temperature strength through reduction of chromium (Cr)

The present invention relates to an austenitic steel excellent in high temperature strength, more specifically, a heat resistant stainless steel used for a high temperature such as a turbo charger or an automobile exhaust system, and is a high-priced alloy element nickel The present invention relates to austenitic steel capable of replacing Ni with a low-cost alloy element and reducing the content of chromium (Cr), realizing improved high-temperature properties as compared with conventional heat-resistant stainless steels, and improving cost competitiveness.

High temperature austenitic steels have been used for turbochargers and exhaust systems in automobiles, as they not only have excellent hardness, strength, thermal-mechanical fatigue life and fracture toughness but also have thermally stable microstructure.

The turbocharger improves the output of the engine by compressing and supplying a large amount of air into the cylinder of the engine. The turbocharger rotates the turbine wheel in the turbine housing using the exhaust gas discharged from the engine, And a structure in which a compressor wheel in a compressor housing that compresses air in the air by transmitting a rotational force generated when the wheel rotates is rotated and supplied to the engine.

Since the turbine housing accommodating such a turbine wheel is continuously brought into contact with the exhaust gas at 800 to 900 ° C discharged from the engine, the turbine housing receives a very high thermal shock according to the output of the engine. Therefore, the turbine housing has excellent strength at high temperature, As shown in Fig.

As a material for such a turbine housing, high temperature austenitic steels such as SCH 22 heat resistant stainless steels are currently used. In order to increase the stability of austenite structure at high temperature, such high heat resistant stainless steels include Ni %, Which is a major cause of increasing the manufacturing cost of the turbine housing.

In order to solve such a problem, Patent Document 1 discloses a method for manufacturing a semiconductor device, which comprises 0.4 to 0.5 wt% of carbon (C), 1.0 to 2.0 wt% of silicon (Si), 1.0 to 2.0 wt% of manganese (Mn) (Fe), and other unavoidable impurities, such as iron (Fe), iron (Fe) and other inevitable impurities, in an amount of 12.0 wt%, chromium (Cr) 21-24 wt%, niobium 1.0-2.5 wt%, tungsten (Nb) and tungsten (W) are added to a casting and an alloy to increase the castability and the high-temperature strength of the cast steel, while significantly reducing the content of nickel (Ni).

However, niobium (Nb) and tungsten (W), which are added to replace nickel (Ni), are expensive alloying elements. In particular, niobium (Nb) There is a problem that the brittleness of the alloy is increased.

Korean Patent Laid-Open Publication No. 2016-0091041

The present invention aims at providing a high temperature austenitic steel having improved room temperature and high temperature tensile properties as compared with conventional alloys while lowering the production cost by reducing the content of nickel (Ni) and chromium (Cr) We will do it.

In order to solve the above-described problems, the present invention provides a method of manufacturing a semiconductor device, comprising: 0.35 to 0.5 wt% of carbon; 1.0 to 2.0 wt% of silicon; 5.0 to 8.0 wt% of manganese; (C) content of the alloying elements, and the content of iron (Fe) and inevitable impurities is less than 16.5 wt%, chromium (Cr) 20-24 wt%, molybdenum Austenitic steel excellent in high-temperature strength, in which the ratio of Cr (Cr) content, C _Cr / C _C is 50 to 60, is provided.

The present invention relates to a method of manufacturing a nickel-metal hydride alloy, which is a method of replacing manganese (Mn), which is a relatively inexpensive alloying element, with nickel (Ni) at a predetermined ratio while maintaining the austenite structure at high temperature and adding niobium (Nb) and tungsten , The tensile strength at room temperature is 400 MPa or more (490 MPa or more in one embodiment), and the tensile strength at room temperature is controlled by controlling the content of chromium (Cr) while minimizing the formation of ferrite phase to maintain the ratio of the carbide phase at 1.5 to 4% Temperature strength at 900 占 폚 of 145 MPa or higher and excellent in shape retention performance and provides austenitic steel which can be suitably used for a turbine housing of a turbocharger.

In addition, the austenitic steel according to the present invention can obtain a significant cost reduction effect as compared with a conventional austenitic steel containing nickel (Ni) in an amount of 20% by weight or more, and at the same time, the strength at room temperature and high temperature strength can be improved.

1 is a microstructure image of austenite steel produced according to Example 2 and Comparative Example 1 of the present invention.
2 shows the XRD measurement results of the austenitic steel produced according to Example 2 and Comparative Example 1 of the present invention.
3 is a microstructure image of austenitic steel produced according to Example 2 of the present invention and Comparative Example 1 after high temperature tensile test at 900 ° C.
4 shows the tensile test results of the austenitic steels at room temperature according to Examples 1 and 2 and Comparative Examples 1 to 4 of the present invention.
5 shows the results of high-temperature tensile test at 900 DEG C of the austenitic steel produced according to Examples 1 and 2 and Comparative Examples 1 to 4 of the present invention.

The singular forms used to describe the embodiments of the present invention are meant to include plural forms unless the phrases expressly mean the opposite. And includes meaning of specific characteristics, regions, integers, steps, and actions. Elements and / or components, and other particular features, regions, integers, steps, acts. Quot; does not exclude the presence or addition of elements, elements and / or groups.

Although not defined differently, all terms including technical terms and scientific terms used herein have the same meaning as commonly understood by those of ordinary skill in the art to which the present invention belongs. Also, commonly used predefined terms are not to be construed as ideal or very formal meanings unless further defined and interpreted as having a meaning consistent with the relevant technical literature and the present disclosure.

The present inventors have studied alloys capable of achieving high temperature strength capable of withstanding high temperature environments of 900 ° C or higher while maintaining price competitiveness. As a result, the amount of nickel (Ni) added in large amounts to maintain the austenite structure at high temperatures is somewhat (Mn) is replaced at a predetermined ratio and at the same time molybdenum (Mo) is added in a predetermined amount without using an expensive carbide forming element such as niobium (Nb) or tungsten (W) ) Is lowered to increase the amount of carbon in the matrix of the austenitic steel and to decompose the formed carbide into a fine carbide at a high temperature, it is possible to improve the strength at room temperature and high temperature And reached the present invention.

In general, it is desirable to increase the ratio of carbides because the phases serve to increase the room temperature and high-temperature strength. However, the present invention can reduce the area ratio of carbide to 4% or less, It is characterized by the design.

The austenitic steel according to the present invention contains 0.35 to 0.5 wt% of carbon (C), 1.0 to 2.0 wt% of silicon (Si), 5.0 to 8.0 wt% of manganese (Mn), 13.5 to 16.5 (C) content of said alloying elements, wherein said chromium (Cr) content is from 20 to 24% by weight, said molybdenum (Mo) is from 0.5 to less than 1.5% (Cr) content, and C _Cr / C _C is 50 to 60.

Further, it is preferable that P: not more than 0.04% by weight, and S: not more than 0.04% by weight of the impurities.

The reason for limiting the components of the austenitic steel according to the present invention is as follows.

Carbon (C): 0.35-0.5 wt%

Carbon (C) is known as a strong austenite stabilizing element, and it also plays an important role in the strength at high temperature due to strengthening in the base structure. In addition, it forms carbides with alloy elements such as chromium (Cr) included in the present invention to improve the main composition of the liquid phase and improve the high temperature strength. In order to obtain the effect of carbon (C), 0.35 wt% or more of carbon is required. When it exceeds 0.5 wt%, the overall mechanical property and creep resistance may be deteriorated due to coarsening of carbide. Do.

Silicon (Si): 1.0 to 2.0 wt%

Silicon (Si) has the effect of improving the oxidation resistance at high temperature and serves as a reducing agent in the molten alloy. Silicon (Si) improves oxidation resistance by helping to prevent oxidation by chromium (Cr). Silicon oxide particles formed by silicon (Si) are precipitated under the coating formed on the alloy surface by chromium (Cr), which helps formation of a passive film and suppresses unnecessary escape of chromium (Cr) ions. This effect of silicon (Si) is further enhanced at high temperatures. If it is less than 1.0% by weight, it is difficult to sufficiently obtain the effect of silicon (Si). If silicon (Si) is added excessively, not only low temperature creep resistance is lowered but also silicon (Si) is a ferrite stabilizing element. It should be added in an amount of 2.0 wt% or less, since it destabilizes the knitted base structure.

Manganese (Mn): 5.0 to 8.0 wt%

Manganese (Mn) acts as an austenite stabilizing element and acts as a reducing agent in the melt, similar to silicon (Si). In the austenitic steel according to the present invention, the content of nickel (Ni), which is an austenite stabilizing element, is less than that of the conventional alloy. Therefore, when the content of manganese (Mn) is less than 5.0 wt%, the austenite base structure is made unstable A ferrite phase may be formed. When it exceeds 8.0 wt%, the oxidation resistance at high temperature and the high temperature moldability are deteriorated, so that it is kept at 8.0 wt% or less. The content of manganese (Mn) is more preferably 7.0 to 8.0% by weight.

Nickel (Ni): 13.5 to 16.5 wt%

Nickel (Ni) is an element that stabilizes austenite and is an essential element for improving all mechanical properties including toughness and corrosion resistance and oxidation resistance. When it is less than 13.5% by weight, the strength at high temperature is lowered. When it exceeds 16.5% The effect of reducing the manufacturing cost is reduced, which is not desirable. The content of nickel (Ni) is more preferably 13.5 to 14.5% by weight.

Chromium (Cr): 20 to 24 wt%

Cr (Cr) is the most important element of the excellent oxidation resistance and corrosion resistance of stainless steel. It forms a stable passive film of Cr ₂ O ₃ type on the surface of the alloy and improves the corrosion resistance. The higher the content of chromium (Cr), the higher the corrosion resistance and also contributes to the oxidation resistance and the corrosion resistance at high temperatures. In order to improve the corrosion resistance, it is preferable that chromium (Cr) is added by 20 wt% or more.

On the other hand, chromium (Cr) is excessively added as a ferrite stabilizing element, and a ferrite phase can be formed and an amount of carbon (C) in the austenite base is reduced due to an increase in the proportion of chromium carbide, And a large amount of carbide can be formed, so that it is limited to not more than 24% by weight. From the standpoint of improving the tensile strength at room temperature, the content of chromium (Cr) is more preferably 20 to 22 wt%.

Molybdenum (Mo): 0.5 to less than 1.5% by weight

Molybdenum (Mo) acts as a ferrite stabilizing element or carbon to promote the formation of M ₇ C ₃ phase, and it enhances the room temperature and high temperature strength simultaneously by generating solid solution strengthening effect in the austenite base.

If the content of molybdenum (Mo) is less than 0.5% by weight, the M ₇ C ₃ phase is less likely to be formed and the strengthening effect is difficult to obtain. When the content of molybdenum (Mo) is more than 1.5% by weight, the ferrite phase can be stabilized and formed in large amounts. Do.

The ratio of chromium (Cr) content to carbon (C) content, C _Cr / C _C  50 to 60

When the ratio of the chromium (Cr) content (wt%) to the carbon (C) content (wt%) in the alloy element is less than 50 or more than 60, the base strengthening effect due to carbon solubility is not sufficiently obtained, Is too small to reduce the strength, it is desirable to maintain the above range.

Phosphorus (P): 0.04% by weight or less

Phosphorus (P) is a component which is inevitably incorporated as an impurity. It may be segregated in the alloy and may adversely affect the physical properties of the alloy, so that it is preferable to maintain the phosphorus (P) at 0.04% by weight or less, more preferably 0.03% .

Sulfur (S): 0.04% by weight or less

The sulfur (S) forms a sulfide such as MnS in the alloy to improve the workability of the alloy, but the formed sulfide deteriorates the overall physical properties of the alloy.

If the content of carbon (C) in the matrix of the austenite steel is less than 0.35% by weight, the solid solution strengthening effect is not sufficient and it is difficult to increase the tensile strength at room temperature. When the carbon (C) content exceeds 0.45% It is not preferable because the high-temperature tensile strength may be lowered, and it is preferable to maintain 0.35 to 0.45 wt%.

The area fraction (%) occupied by the carbide phase in the microstructure of the austenite steel is preferably 1.5 to 4% or less, because it is not preferable to improve the room temperature and high temperature strength when the area fraction is less than 1.5% or exceeds 4%.

Further, the austenitic steel is meant an M ₇ C ₃ (where M is the mean and the metallic alloying elements), at least a portion of the shape of the carbide is M ₂₃ C ₆ (wherein M is a metal alloy elements at high temperature tensile at 900 ℃ ) To form a fine secondary carbide. When the M ₇ C ₃ type carbide is decomposed into a fine carbide of the M ₂₃ C ₆ type in a high temperature environment, the high temperature strength can be further improved, which is preferable.

The austenitic steel may have a tensile strength at room temperature of 400 MPa or higher (more preferably 480 MPa or higher) and a tensile strength at 900 占 폚 of 140 MPa or higher (more preferably 145 MPa or higher).

In the austenitic steel according to the present invention, when the ratio of the microstructure-based ferrite structure is 1% or more, the area fraction of the ferrite phase is preferably 1% or less since the stability is deteriorated at high temperature.

The austenitic steel according to the present invention can also be used for turbo housing.

[Example]

Table 1 below shows the compositions of Comparative Examples 1 to 4 in which the addition ratios of nickel (Ni) and chromium (Cr) are different for comparison with Examples 1, 2 and Examples of austenitic steel according to the present invention .

Steel grade Composition (% by weight) C Si Mn P S Ni Cr Mo Fe Comparative Example 1 0.40 1.2 7.9 0.04 0.04 14 25 One Honey. Comparative Example 2 0.40 1.2 7.9 0.04 0.04 14 19 One Honey. Example 1 0.40 1.2 7.9 0.04 0.04 14 23 One Honey. Example 2 0.40 1.2 7.9 0.04 0.04 14 21 One Honey. Comparative Example 3 0.40 1.2 7.9 0.04 0.04 14 21 0 Honey. Comparative Example 4 0.40 1.2 7.9 0.04 0.04 14 21 2 Honey.

Six types of raw materials were prepared so as to have the composition shown in Table 1, and then dissolved in a melting furnace and then taped at 1550 to 1600 ° C. and immediately injected into the mold at 1500 ° C. to 1550 ° C. to form a Y- Length: 180 mm, thickness: 50 mm, height: 110 mm).

The thus obtained test pieces were analyzed by optical microscope, electron microscope, XRD and EBSD (Electron Back-scatter Diffraction), and the phase fractions were measured. The specimens were measured at room temperature (25 ° C.) Tensile test was carried out.

Fig. 1 shows the results of an optical microscope analysis of austenitic steel according to Example 2 and Comparative Example 1 of the present invention.

As shown in Fig. 1, in the case of Comparative Example 1 in which the content of chrome (Cr) is large as compared with Example 2 of the present invention, many of the microstructure-based carbides are observed.

2 shows the results of XRD analysis of austenitic steel according to Example 2 and Comparative Example 1 of the present invention and austenitic steel after 900 ° C high temperature tensile test.

As can be seen from FIG. 2, no peak of ferrite phase was detected in Comparative Example 1 in which the chromium (Cr) content was the highest as the ferrite stabilizing element, and thus the ferrite phase did not appear even in Example 2 in which the Cr content was low. In addition, in Comparative Example 1, no peak change was observed after high-temperature stretching at 900 ° C, whereas in Example 2, a peak of carbide M ₂₃ C ₆ was further formed.

3 shows electron microscopic analysis results of the austenitic steel according to Example 2 and Comparative Example 1 after the high temperature tensile test at 900 ° C.

As shown in FIG. 3, no change in microstructure was observed in the case of Comparative Example 1 in the high-temperature stretching process at 900 ° C., whereas in Example 2, the initial carbides were decomposed and fine carbides were formed in the austenite matrix.

Table 2 below shows the results of measuring the fraction occupied by the ferrite phase and the M ₇ C ₃ phase in the microstructure of the steel as shown in Figs. 1 and 2, using EBSD.

Steel grade Area fraction (%) Inside the base organization
Carbon content
(weight%) At room temperature (25 ° C)
Longitude of base
(VHN) High temperature (900 ℃)
(VHN) ferrite Carbide
(M ₇ C ₃ + M ₂₃ C ₆ ) Comparative Example 1 - 6.3 ± 0.4 0.29 258 ± 4 61.6 ± 2.5 Comparative Example 2 - 0.5 ± 0.2 0.44 281 ± 5 - Example 1 - 3.6 ± 0.5 0.38 269 ± 6 - Example 2 - 2.0 ± 0.1 0.41 280 ± 6 75.8 ± 1.5 Comparative Example 3 - 0.7 ± 0.1 0.42 279 ± 5 - Comparative Example 4 - 6.0 ± 0.1 0.30 259 ± 3 -

As shown in Table 2 above, no ferrite phase was detected in Examples 1 and 2 of the present invention.

Compared with the comparative example 1, the carbide area fraction of Examples 1 and 2 was controlled to 1.5 to 4% through reduction of chromium (Cr) content. As a result, the carbon content in the austenitic matrix structure was increased and the hardness of the base was improved.

Steel grade Room temperature tensile properties High Temperature Tensile Properties (900 ℃) Yield strength
(MPa) The tensile strength
(MPa) Elongation
(%) Yield strength
(MPa) The tensile strength
(MPa) Elongation
(%) Comparative Example 1 305 ± 2 396 ± 1 3.3 ± 0.0 127 ± 1.3 144 ± 0.1 10.3 ± 1.8 Comparative Example 2 290 ± 3 360 ± 3 2.3 ± 0.1 124 ± 1.0 140 ± 0.6 12.1 ± 1.7 Example 1 314 ± 2 407 ± 4 4.4 ± 0.3 130 ± 0.8 151 ± 0.3 12.3 ± 1.6 Example 2 324 ± 2 493 ± 7 10.0 + - 0.4 133 ± 0.2 152 ± 0.6 10.9 ± 1.2 Comparative Example 3 289 ± 2 362 ± 4 3.6 ± 0.3 121 ± 0.9 134 ± 0.5 12.0 ± 1.1 Comparative Example 4 310 ± 3 400 ± 3 3.3 ± 0.1 125 ± 0.4 143 ± 0.1 11.3 ± 0.5

As shown in the above Table 3, Examples 1 and 2 of the present invention are comparative examples 1 and 2 including chromium (Cr) contents of 19 wt% and 25 wt%, respectively, and molybdenum (Mo) % And 2% by weight, respectively, as compared with Comparative Examples 3 and 4.

In the case of the tensile properties at room temperature, in Examples 1 and 2, the austenitic matrix structure was stabilized by reducing chromium (Cr), which is a ferrite stabilizing element, and the carbide fraction was adjusted to 1.5 to 4% As a result, the tensile strength at room temperature and the elongation were both increased.

On the other hand, in Comparative Examples 2 and 3, the carbide fraction was as low as less than 1%, and the base hardness was excellent, but the tensile strength at room temperature was low due to the low effect of strengthening with carbide.

Therefore, the carbide fraction should be adjusted to 1.5 to 4% in order to simultaneously enhance the solid solution strength and the strength improvement effect by the carbide.

On the other hand, in the case of the high temperature tensile properties, Examples 1 and 2 showed a decrease in the high temperature stability of the carbide due to the reduced chromium (Cr) content. Thus, as shown in FIG. 3, during the high-temperature tensile test, the initial M ₇ C ₃ carbide was decomposed into M ₂₃ C ₆ carbide, and fine M ₂₃ C ₆ carbide was further precipitated in the matrix.

When the carbide fraction is adjusted to 1.5 to 4%, the hardness of the base plays a major role in improving the strength. As shown in Table 2 above, in Examples 1 and 2, excellent base hardness was secured even at high temperature through precipitation of fine M ₂₃ C ₆ carbide in the matrix, thereby improving high temperature strength.

On the other hand, in the case of Comparative Example 1, the high temperature stability of the carbide is maintained due to the high chromium (Cr) content, and there is no effect of improving the strength by decomposition of carbide in a high temperature environment.

Further, in the case of Comparative Examples 2 and 3, the carbide fraction was less than 1% as in the room temperature tensile characteristic, so that the effect of strengthening with carbide was low and the high temperature tensile strength was low.

In Comparative Example 4, since the carbide fraction was as high as 6%, the effect of improving the strength by the carbide was large, but it was difficult to obtain the effect of increasing the strength by the known solid solution strengthening. Compared with Examples 1 and 2 utilized, Comparative Example 4 has a low high temperature strength.

Claims (8)

(Ni): 13.5 to 16.5 wt%, chromium (Cr): 20, carbon (C): 0.35 to 0.5 wt%, silicon (Si): 1.0 to 2.0 wt%, manganese To about 24 wt%, molybdenum (Mo): less than about 0.5 wt% to about 1.5 wt%, balance of iron (Fe) and unavoidable impurities,
Austenite steel excellent in high-temperature strength, in which the ratio of the chromium (Cr) content to the carbon (C) content in the alloying elements, and C _Cr / C _C is 50 to 60. The method according to claim 1,
Austenitic steels having a high temperature strength, wherein the carbon content in the matrix of the austenitic steels is less than 0.35 to 0.42 wt%. The method according to claim 1,
(%) Occupied by the carbide phase of the austenite steel is 1.5 to 4.0%, and the austenite steel excellent in high temperature strength. 4. The method according to any one of claims 1 to 3,
Austenite steel excellent in high-temperature strength, wherein 0.04 wt% or less of P and 0.04 wt% or less of S are included in the impurities. 4. The method according to any one of claims 1 to 3,
The austenitic steel has excellent high-temperature strength at high temperature strength, which decomposes at least part of the M ₇ C ₃ type carbide into M ₂₃ C ₆ type carbide to form a fine secondary carbide upon high temperature stretching at 900 ° C. 4. The method according to any one of claims 1 to 3,
The austenite steel excellent in high-temperature strength, wherein the area fraction of the ferrite phase in the microstructure of the austenite steel is 1% or less. 4. The method according to any one of claims 1 to 3,
The austenitic steel has a tensile strength at room temperature of 400 MPa or more and a tensile strength at 900 占 폚 of 140 MPa or more and excellent austenitic strength at high temperature. A turbo housing made of an austenitic steel according to any one of claims 1 to 3.

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