KR101809853B1 - Austenitic steel excellent in high temperature strength - Google Patents

Abstract

The present invention has been made to solve the above-mentioned problems by replacing nickel (Ni), which is a high-priced alloy element contained in large amounts in heat-resistant stainless steel used at a very high temperature such as turbocharger, And can realize high-temperature properties.
The austenitic steel according to the present invention comprises 0.4 to 0.5 wt% of C, 1.0 to 2.0 wt% of Si, 5.0 to 8.0 wt% of Mn, 13.5 to 16.5 wt% of Ni, 23 to 26 wt% of Cr, Fe and other unavoidable impurities, and the ratio of the Mn content to the Ni content in the alloy elements, C _Mn / C _Ni, is maintained in the range of 0.3 to 0.9.

Description

AUSTENITIC STEEL EXCELLENT IN HIGH TEMPERATURE STRENGTH "

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 (Ni) is replaced by a low-cost alloy element, and high-temperature properties equal to or higher than those of conventional heat-resistant stainless steels can be realized.

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 % Or more, which is one cause of increasing the manufacturing cost of the turbine housing.

In order to solve such a problem, Patent Literature discloses a method for producing a steel sheet which comprises 0.4 to 0.5 wt% of C, 1.0 to 2.0 wt% of Si, 1.0 to 2.0 wt% of Mn, 9.0 to 12.0 wt% of Ni, 21 to 24 wt% By adding Nb and W while greatly reducing the content of Ni through an alloy containing 1.0 to 2.5% by weight of Nb, 0.5 to 3.5% by weight of W, and the balance of Fe and other unavoidable impurities, Is disclosed.

However, Nb and W added to replace Ni are not only high-priced alloying elements but also can improve the main composition in case of Nb, but they have a problem of increasing the brittleness of the alloy when Nb carbide is formed.

Korean Patent Laid-Open Publication No. 2016-0091041

The present invention provides an austenitic steel which can be produced at low cost by reducing the content of Ni, while at the same time minimizing the formation of a microstructural ferrite phase and keeping the ratio of M ₇ C ₃ phase at a certain level or higher, And to solve the problem.

One aspect of the present invention for solving the above-mentioned problems is a method for manufacturing a semiconductor device, comprising: 0.4 to 0.5 wt% of C, 1.0 to 2.0 wt% of Si, 5.0 to 8.0 wt% of Mn, 13.5 to 16.5 wt% of Ni, And a balance of Fe and other unavoidable impurities, the ratio of the Mn content to the Ni content in the alloying elements, and C _Mn / C _Ni in the range of 0.3 to 0.9.

While the austenitic steel according to the present invention maintain the austenite at high temperature, and relatively inexpensive alternative to the alloy elements Mn with and remove the Nb and W such that Ni is a predetermined ratio, while minimizing the formation of the ferrite M ₇ C The high-temperature strength at 900 ° C is as high as 125 MPa or more, and the shape-retaining performance is excellent through the alloy design which keeps the ratio of the _three phases at a certain level or more, and can be suitably used for the turbine housing of the turbocharger.

In addition, the austenitic steel according to the present invention can achieve a cost reduction of 20% or more as compared with austenitic steel containing 20% or more of Ni.

1 shows XRD analysis results of austenitic steels according to Examples 1 and 2 and Comparative Examples 1 to 3 of the present invention.
2 is a microstructure photograph of Example 2 and Comparative Example 3 of the present invention.
Fig. 3 shows a photograph of a microstructure and the results of EDS mapping of carbon (C) and chromium (Cr) in Example 2 of the present invention.
Fig. 4 shows microstructure photographs of Comparative Example 1 of the present invention and EDS mapping results of carbon (C) and chromium (Cr).
5 shows tensile test results of austenitic steels at 25 캜 according to Examples 1 and 2 and Comparative Examples 1 to 3 of the present invention.
6 shows the results of a high temperature tensile test of austenitic steels at 900 DEG C according to Examples 1 and 2 and Comparative Examples 1 to 3 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 reducing the content of Ni while realizing a high temperature strength capable of withstanding a high temperature environment of 900 ° C or more while excluding expensive carbide-forming elements such as Nb and W. As a result, , It is possible to realize excellent high-temperature strength when Mn is substituted at a predetermined ratio without using alloying elements such as Nb and W, while slightly reducing the amount of Ni added to maintain austenite structure at high temperature. And came to the present invention.

The austenitic steel according to the present invention comprises 0.4 to 0.5 wt% of C, 1.0 to 2.0 wt% of Si, 5.0 to 8.0 wt% of Mn, 13.5 to 16.5 wt% of Ni, 23 to 26 wt% of Cr, Fe and other unavoidable impurities, and the ratio of the Mn content to the Ni content in the alloy elements, C _Mn / C _Ni, is maintained in the range of 0.3 to 0.9.

The reason for selecting the composition of the austenitic steel according to the present invention is as follows.

C: 0.4 ~ 0.5 wt%

C is known as a strong austenite stabilizing element, and it also plays an important role in the strength at high temperature by being strengthened in the base structure. In addition, by forming an alloy element and a carbide such as Cr included in the present invention, the casting of the liquid phase is improved and the high temperature strength is improved. In order to obtain such effect of C, at least 0.4% by weight of carbon is required. When it exceeds 0.5% by weight, the above range is preferable because coarsening of carbide may cause deterioration of overall mechanical property and creep resistance.

Si : 1.0 ~ 2.0 wt%

Si has the effect of improving the oxidation resistance at high temperature and serves as a reducing agent in the molten alloy. Si improves oxidation resistance by helping to prevent oxidation by Cr. The silicon oxide particles formed by Si precipitate under the film formed on the alloy surface by Cr to help form a passive film and suppress the unnecessary escape of Cr ions. This effect of Si is further enhanced at high temperatures. If the Si content is less than 1.0% by weight, it is difficult to sufficiently obtain the Si effect, and when Si is added excessively, the high temperature creep resistance is lowered and the ferrite stabilizing element becomes unstable in the austenite matrix structure. Should be added.

Mn: 5.0 ~ 9.0 wt%

Mn acts as an austenite stabilizing element and acts as a reducing agent in the melt, similar to Si. Since the austenitic steel according to the present invention reduces the Ni content, it is difficult to maintain the austenite structure when the content of Mn is less than 5.0% by weight, and when it exceeds 9.0% by weight, the oxidation resistance at high temperature and the deterioration , It is kept at 9.0 wt% or less. More preferably, the content of Mn is 5.5 to 8.0 wt%.

Ni : 13.5 ~ 16. 5 wt%

Ni is an element for stabilizing austenite and is an essential element for improving mechanical properties including corrosion resistance and oxidation resistance and oxidation resistance. If it is less than 13.5% by weight, the strength at high temperature is lowered, and if it exceeds 16.5% by weight, This is because it is not desirable to reduce the effect.

Cr : 23 ~ 26 wt%

Cr is the most important element of the excellent oxidation resistance and corrosion resistance of stainless steel, and it forms a stable passive film of Cr ₂ O ₃ type on the surface of the alloy to improve the corrosion resistance. The higher the Cr content, 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, Cr is preferably added in an amount of 23 wt% or more, and Cr is a ferrite stabilizing element, which can form a ferrite phase when added in an excessive amount, so that a large amount of carbide can be formed.

Ni  The ratio of the Mn content to the content, C _Mn / C _Ni  0.3 to 0.9

When the ratio of the Mn content to the Ni content in the alloy element is less than 0.3, the substitution amount of Ni is not sufficient and the economical efficiency is not high. When the ratio is more than 0.9, excellent high temperature strength can be obtained. A more preferred range is 0.3 to 0.6.

The austenitic steel according to the present invention may have a tensile strength at 900 占 폚 of 125 MPa or higher, preferably 128 MPa or higher, more preferably 130 MPa or higher.

P: 0.04 wt%  Below

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

S: 0.04 wt%  Below

S forms a sulfide such as MnS in the alloy to improve the workability of the alloy, but it is preferable to maintain the sulfide at 0.04 wt% or less because the sulfide lowers the overall physical properties of the alloy.

In order to stably maintain the austenite structure at a high temperature, the austenitic steel according to the present invention preferably has a Ni _eq of 31 to 32 represented by the following formula 1 and Cr _eq of the following formula 2: 25 to 28. < / RTI >

[Formula 1]

Ni _eq =% Ni + 30% C + 0.87% Mn - 0.33% Cu + 30 (% N-0.045)

[Formula 2]

Cr _eq =% Cr +% Mo +% W + 1.5% Si + 0.5% Nb + 5% V + 3% Al

In the austenitic steel according to the present invention, when the ratio of the microstructure to the ferrite structure is 1% or more in area ratio, the stability at high temperature is lowered, which is undesirable. The M ₇ C ₃ phase serves to increase the strength at room temperature and high temperature , It is preferably 2% or more. However, if the content of Mn is increased to increase the area ratio of the M ₇ C ₃ phase, the area ratio of the ferrite phase increases. Therefore, the area ratio of the M ₇ C ₃ phase is preferably 2 to 3%.

[Example]

Table 1 below shows compositions of Comparative Example 1, Comparative Example 2 and Comparative Example 3 in which the ratios of Ni and Mn are different for comparison with Example 1, Example 2 and Examples of the austenitic steel according to the present invention will be.

Steel grade Composition (% by weight) C Si Mn P S Ni Cr Fe Comparative Example 1 0.40 1.2 1.0 0.04
Below 0.04
Below 20 25 Honey. Comparative Example 2 0.40 1.2 3.3 0.04
Below 0.04
Below 18 25 Honey. Example 1 0.40 1.2 5.6 0.04
Below 0.04
Below 16 25 Honey. Example 2 0.40 1.2 7.9 0.04
Below 0.04
Below 14 25 Honey. Comparative Example 3 0.40 1.2 10.2 0.04
Below 0.04
Below 12 25 Honey.

Five kinds of raw materials were prepared so as to have the compositions shown in the above Table 1, dissolved in a melting furnace, and then boiled at 1550 to 1600 占 폚 and immediately injected into a mold for a cylindrical test piece at 1500 占 폚 to 1550 占 폚 to obtain test pieces.

The thus obtained test specimens were analyzed by XRD and EBSD (Electron Back-scatter Diffraction), and their microstructures were analyzed by using a microscope and EDS. At room temperature (25 ° C.) and high temperature (900 ° C.) , And the phase fraction was measured.

Microstructure

1 shows XRD analysis results of austenitic steels according to Examples 1 and 2 and Comparative Examples 1 to 3 of the present invention. FIG. 2 is a microstructure photograph of Example 2 and Comparative Example 3 of the present invention, FIG. 3 is a photograph of the microstructure of Example 2 of the present invention, and EDS mapping results of carbon (C) and chromium (Cr) Fig. 4 shows microstructure photographs of Comparative Example 1 of the present invention and EDS mapping results of carbon (C) and chromium (Cr).

As can be seen from Fig. 1, peaks of the ferrite phase were not detected in Comparative Example 1, Comparative Example 2, Example 1 and Example 2, and in the case of Comparative Example 3, some peaks of the ferrite phase were detected.

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. 3 and 4, using EBSD.

Steel grade Fraction (%) ferrite M ₇ C ₃ Comparative Example 1 - 1.5 ± 0.1 Example 1 - 2.2 ± 0.2 Example 2 ~ 0.1 2.5 ± 0.5 Comparative Example 3 9.8 3.6 ± 0.4

As shown in Table 2, Example 1 of the present invention did not show a ferrite phase. In Example 2, an extremely small amount of ferrite of 0.1% or less was detected, as shown in Fig.

Ni and Mn do not function as a carbide forming agent that directly forms a carbide. However, as shown in Table 2, as the substitution amount of Mn increases, M ₇ C ₃ The fraction of phase showed a tendency to increase. M ₇ C ₃ The phase acts to increase the room temperature and high temperature strength, so that it is advantageous to increase the high temperature strength. However, when the content of Mn is increased, as shown in Comparative Example 3, the ferrite phase is increased and the stability of the austenite phase is lowered and the high-temperature properties are rapidly lowered. Thus, M ₇ C ₃ It is preferable to increase the phase fraction while suppressing the ferrite phase to 1% or less (preferably 0.1% or less).

Room temperature and high temperature tensile strength

5 shows results of tensile tests of austenitic steels at 25 ° C according to Examples 1 and 2 and Comparative Examples 1 to 3 of the present invention, Shows a result of a high-temperature tensile test of an austenitic steel at 900 deg.

Table 3 below shows the tensile test results of Figs. 5 and 6.

Steel grade Room temperature tensile properties High Temperature Tensile Properties Yield strength
(MPa) The tensile strength
(MPa) Elongation
(%) Yield strength
(MPa) The tensile strength
(MPa) Elongation
(%) Comparative Example 1 307 ± 4 612 ± 7 52.0 ± 2.1 108 ± 2 129 ± 1 44.7 ± 8.6 Comparative Example 2 338 ± 4 629 ± 2 60.2 ± 5.1 113 ± 6 132 ± 7 50.4 ± 3.4 Example 1 340 ± 9 628 ± 7 62.0 ± 3.7 109 ± 1 130 ± 6 50.0 ± 2.0 Example 2 317 ± 5 634 ± 3 65.5 ± 1.8 111 ± 4 128 ± 4 47.7 ± 7.8 Comparative Example 3 347 ± 7 660 ± 2 50.0 + 4.1 99 ± 1 106 ± 3 54.5 ± 5.1

As shown in Table 3, in Examples 1 and 2 of the present invention, compared with Comparative Example 1 containing 20% by weight of Ni and Comparative Example 2 containing 18% by weight of Ni, Or better, and exhibits equivalent properties at high temperature tensile properties.

In contrast, in Comparative Example 3 in which the Ni content is 12% by weight, the tensile strength at room temperature is very high, but the tensile strength at high temperature is remarkably low. Therefore, it is not suitable for a turbocharger housing requiring durability at 900 ° C or higher .

The austenitic steel according to the embodiment of the present invention can maintain the high temperature tensile characteristics at the same or higher level while reducing the Ni content by 4 to 6% in comparison with the comparative example 1, thereby realizing excellent high temperature characteristics at low cost.

Claims (7)

Wherein the alloy contains 0.4 to 0.5% by weight of C, 1.0 to 2.0% by weight of Si, 5.0 to 8.0% by weight of Mn, 13.5 to 16.5% by weight of Ni, 23 to 26% by weight of Cr, balance Fe and other unavoidable impurities,
The ratio of the Mn content to the Ni content in the alloying elements, and the C _Mn / C _Ni ratio in the range of 0.3 to 0.9. The method according to claim 1,
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. The method according to claim 1,
Austenitic steel excellent in high temperature strength with a tensile strength of at least 125 MPa at 900 캜. The method according to claim 1,
Wherein the austenite has a Ni _eq of 31 to 32 expressed by the following formula 1,
Austenite steel excellent in high-temperature strength, having Cr _eq of 25 to 28 represented by the following formula (2).
[Formula 1]
Ni _eq =% Ni + 30% C + 0.87% Mn - 0.33% Cu + 30 (% N-0.045)
[Formula 2]
Cr _eq =% Cr +% Mo +% W + 1.5% Si + 0.5% Nb + 5% V + 3% Al The method according to claim 1,
The ferritic structure contained in the austenitic steel is less than 1% in area ratio, and the austenitic steel excellent in high temperature strength. 6. The method of claim 5,
The austenitic steel having an excellent high-temperature strength with an area ratio of 2 to 3% of the M ₇ C ₃ phase contained in the austenite steel. A turbine housing made of an austenitic steel according to any one of claims 1 to 6.

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