KR102602916B1 - Ferritic stainless steel excellent in corrosion resistance and impact resistance - Google Patents

Abstract

The present invention relates to a ferritic stainless steel with excellent corrosion resistance and impact resistance that improves brittleness and corrosion resistance at high temperatures by suppressing the formation of sigma phases.
Ferritic stainless steel with excellent corrosion resistance and impact resistance according to an embodiment of the present invention has, in weight percent, C: 0.015% or less (excluding 0%), Si: 0.17% or less (excluding 0%), Mn: 1.35% or less. (excluding 0%), Cr: 17 ~ 20%, Ti: 0.5% or less (excluding 0%), Al: 3 ~ 5%, including the remaining Fe and other inevitable impurities, satisfying [Equation 1] below It is desirable.
(20Si+Mn)/Al < 0.7... … … … … … [Equation 1]
In [Formula 1], Si, Mn, and Al refer to the content (%) of each component.

Description

Ferritic stainless steel excellent in corrosion resistance and impact resistance}

The present invention relates to a ferritic stainless steel with excellent corrosion resistance and impact resistance, and more specifically, to a ferritic stainless steel with excellent corrosion resistance and impact resistance that improves brittleness and corrosion resistance at high temperatures by suppressing the formation of sigma phases. .

Exhaust system components such as automobile exhaust pipes and mufflers require thermal fatigue characteristics, high-temperature fatigue characteristics, and oxidation resistance.

In general, ferritic stainless steel, which has increased corrosion resistance by adding chromium (Cr) as an alloy component, is mainly used for exhaust system parts.

Ferritic stainless steel used for exhaust system parts has about 15 to 20% of chromium (Cr) added to maintain corrosion resistance at high temperatures. In order to improve corrosion resistance in this way, the content of alloy elements that improve corrosion resistance, such as chromium (Cr) and molybdenum (Mo), can be increased. However, in this case, there is a problem of increased cost due to the increase in the content of expensive alloy elements, especially sigma A Sigma phase was formed, and its fraction increased, causing a problem in which durability (impact resistance) deteriorated and corrosion resistance also deteriorated, contrary to expectations. Accordingly, it is important to suppress the formation of sigma phase in order to maintain excellent corrosion resistance and impact resistance, which are in a trade-off relationship.

The components of the sigma phase are composed of Fe, Si, Mn, Mo, and Cr in the order of content. In general ferritic stainless steel, the sigma phase is contained in a volume fraction of about 5 to 20%, resulting in ferrite. Research on technologies to reduce the volume fraction of the sigma phase in stainless steel is continuously being conducted.

Public Patent Publication No. 10-2012-0108785 (2012.10.05)

The present invention provides a ferritic stainless steel with excellent corrosion resistance and impact resistance that improves brittleness and corrosion resistance at high temperatures by suppressing the formation of sigma phase so that it can be used as exhaust system parts exposed to high temperature environments.

Ferritic stainless steel with excellent corrosion resistance and impact resistance according to an embodiment of the present invention has, in weight percent, C: 0.015% or less (excluding 0%), Si: 0.17% or less (excluding 0%), Mn: 1.35% or less. (excluding 0%), Cr: 17 ~ 20%, Ti: 0.5% or less (excluding 0%), Al: 3 ~ 5%, including the remaining Fe and other inevitable impurities, satisfying [Equation 1] below It is desirable.

The stainless steel preferably contains Ti: 0.1 to 0.5%.

(20Si+Mn)/Al < 0.7... … … … … … [Equation 1]

In [Formula 1], Si, Mn, and Al refer to the content (%) of each component.

The stainless steel further contains O, Zr, Ca, and Mg, and preferably satisfies [Formula 2] below.

0.001 ≤ Zr+Ca+Mg ≤ 0.01 … … … … … … [Equation 2]

In [Formula 2], Zr, Ca, and Mg refer to the content (%) of each component.

The stainless steel may further contain O: less than 0.001% and N: less than 0.02%.

The stainless steel is characterized in that the volume fraction of the sigma phase formed in the temperature range of 300 to 900°C is less than 5%.

The stainless steel is characterized in that the volume fraction of the sigma phase formed in the temperature range of 300 to 900 ° C is less than 0.5% or does not precipitate.

The stainless steel is characterized in that the volume fraction of Cr ₃ Si formed in the temperature range of 300 to 900° C. is 0.5% or less.

The stainless steel is characterized in that the volume fraction of AlN and Al ₂ O ₃ formed in the temperature range of 300 to 900 ° C is each 0.0001% or less.

The stainless steel is characterized in that the volume fraction of M ₂₃ C ₆ formed in the temperature range of 300 to 900 ° C is less than 0.02%.

The stainless steel is characterized in that the volume fraction of the Laves phase formed in the temperature range of 300 to 900°C is 0.2% or less.

The stainless steel is characterized by a pitting potential (Ept) of 300 mV _SCE or more in 3.5% sodium chloride (NaCl) at a temperature of 25°C.

The stainless steel is characterized in that it takes more than 250 days for rusting to occur due to 5% sodium chloride after spraying with 3% salt water.

The stainless steel is characterized by a toughness of 55J or more as measured by the Charpy keyhole-notch impact test.

According to an embodiment of the present invention, the formation of the sigma phase is suppressed by controlling the content of alloy elements that affect the formation of the sigma phase and Laves phase, and thus the ferritic stainless steel has excellent corrosion resistance and impact resistance at high temperatures where exhaust system components are used. You can get a river.

In addition, excellent corrosion resistance and impact resistance at high temperatures can be maintained without adding expensive alloy elements, which can be expected to reduce costs compared to other steel types that achieve similar performance.

1 is a table showing the components of examples and comparative examples,
Figures 2 and 3 are tables showing phases generated in examples and comparative examples;
Figure 4 is a table showing the measured physical properties of stainless steel according to examples and comparative examples,
Figure 5 is a graph showing the results of measuring the corrosion amount of Examples and Comparative Examples;
Figure 6 is a graph showing the calculation results of phase transformation by temperature according to the content of Al, and Figure 7 is a graph showing the calculation results of phase transformation by temperature according to the content of N.
Figure 8 is a graph showing the results of calculating phase transformation at temperature according to the total amount of Zr, Ca, and Mg;
Figure 9 is a graph showing the results of calculating phase transformation at temperature according to the content of C.

Hereinafter, embodiments of the present invention will be described in more detail with reference to the attached drawings. However, the present invention is not limited to the embodiments disclosed below and will be implemented in various different forms. These embodiments only serve to ensure that the disclosure of the present invention is complete and to those skilled in the art to fully convey the scope of the invention. This is provided to inform you.

Figure 1 is a table showing components of examples and comparative examples, and Figure 2 is a table showing phases generated in examples and comparative examples.

The ferritic stainless steel according to an embodiment of the present invention suppresses the formation of structures that negatively affect the physical properties of stainless steel, such as sigma phase, by optimizing the content of major alloy components. Specifically, in weight percentage, C: 0.015% or less (excluding 0%), Si: 0.17% or less (excluding 0%), Mn: 1.35% or less (excluding 0%), Cr: 17 to 20%, Ti: 0.5 % or less (excluding 0%), Al: 3 to 5%, including the remaining Fe and other inevitable impurities. Additionally, ferritic stainless steel may further contain O, N, Zr, Ca, and Mg.

The reason for limiting the alloy components and composition range in the present invention is as follows. Hereinafter, unless otherwise specified, % written in units of composition range means weight %.

It is preferable that carbon (C) contains 0.015% or less. Carbon (C) is an element that is effective in improving the strength of stainless steel. It can be expected to have a precipitation strengthening effect by forming Ti(C,N) and suppresses high-temperature structure growth (grain growth). Therefore, an increase in creep strength and improvement in tempering properties can be expected. However, if it exceeds 0.015%, M23C6 carbide is formed, causing a problem in which thermal shock properties are deteriorated.

It is desirable to contain 0.17% or less of silicon (Si). Silicon (Si) is an element that acts as a deoxidizer and increases oxidation resistance and castability. However, if the silicon (Si) content exceeds 0.17%, a sigma phase is formed, which causes the problem of reduced impact resistance and corrosion resistance.

It is desirable to contain 1.35% or less of manganese (Mn). Manganese (Mn) has the effect of improving the hardenability and yield strength of steel. However, if the manganese (Mn) content exceeds 1.35%, a sigma phase is formed, which causes the problem of reduced impact resistance and corrosion resistance.

It is preferable to contain 17 to 20% of chromium (Cr). Chromium (Cr) is the most important element added to ensure corrosion resistance of stainless steel. It can be expected to have a solid solution strengthening effect and is an element that stabilizes the austenite phase. However, if the chromium (Cr) content is less than 17%, oxidation resistance deteriorates, and if it exceeds 20%, the austenite phase is stabilized, causing the problem of the matrix structure changing to an austenitic or duplex type.

It is preferable to contain 3 to 5% of aluminum (Al). Aluminum (Al) is an element that acts as a solid solution strengthener, increases oxidation resistance, and refines and homogenizes the structure. If the aluminum (Al) content is less than 3%, a sigma phase is formed, which reduces tissue uniformity, and if it exceeds 5%, a problem occurs in which negative phases such as Cr3Si are formed.

It is preferable that the titanium (Ti) content is 0.5% or less. Titanium (Ti) forms carbides to improve precipitation strengthening and high-temperature strength. If the titanium (Ti) content exceeds 0.5%, a Laves phase is formed, which causes the problem of reduced impact resistance and corrosion resistance. Additionally, it is desirable to limit the minimum titanium (Ti) content to 0.1%. If the titanium (Ti) content is less than 0.1%, the production of AlN, which reduces strength and weldability at high temperatures, may increase.
It is preferable that the nitrogen (N) content is less than 0.02%. Nitrogen (N) is an element that determines the formation of carbonitride. By forming Ti(C,N), a precipitation strengthening effect can be expected and it suppresses high-temperature structure growth (grain growth). Therefore, an increase in creep strength and improvement in tempering properties can be expected. However, if it exceeds 0.02%, a problem occurs in which the production of AlN increases.

It is preferable that the oxygen (O) content is less than 0.001%. Oxygen (O) is an element that reduces impact resistance by forming inclusions, and it is desirable to control it as low as possible. Considering the removal process, the upper limit of oxygen (O) is limited to 0.001%.

It is desirable to adjust the sum of the contents of zirconium (Zr), calcium (Ca), and magnesium (Mg) to 0.001% to 0.01%. Zirconium (Zr), calcium (Ca), and magnesium (Mg) are elements that act as deoxidizers. If the total content of zirconium (Zr), calcium (Ca), and magnesium (Mg) is less than 0.001%, Al ₂ O ₃ is generated. If it exceeds 0.01%, the problem of deteriorating castability occurs.

Meanwhile, the remainder other than the above components is Fe and inevitably contained impurities.

In order to manufacture ferritic stainless steel with excellent corrosion resistance and impact resistance, the present invention produces a slab by casting molten steel having the above composition by a conventional method, reheating the slab, and then hot rolling, hot rolling annealing, cold rolling, and cold rolling annealing. Perform the same post-processing as follows.

In particular, when adjusting the content of each component of molten steel, Si, which affects the formation of negative phases, is used to suppress the formation of phases that have a negative effect on physical properties such as sigma phase in the temperature range of 300 to 900°C in which exhaust system components are used. It is desirable to more specifically limit the amounts of Mn and Al. For example, it is desirable to satisfy the following [Equation 1].

(20Si+Mn)/Al < 0.7... … … … … … [Equation 1]

In [Formula 1], Si, Mn, and Al refer to the content (%) of each component.

In addition, it is desirable to suppress the formation of AlN and Al ₂ O ₃ by further limiting the contents of Zr, Ca, and Mg along with limiting the amounts of Si, Mn, and Al. For example, it is desirable to satisfy the following [Equation 2].

0.001 ≤ Zr+Ca+Mg ≤ 0.01 … … … … … … [Equation 2]

In [Formula 2], Zr, Ca, and Mg refer to the content (%) of each component.

Hereinafter, the present invention will be explained using examples and comparative examples.

An experiment was conducted to produce the final product according to the production conditions of commercially produced ferritic stainless steel, and a hot-rolled plate was hot-rolled from a continuously cast slab using molten steel produced while changing the content of each component as shown in Figure 1. Annealing, cold rolling and cold rolling annealing were performed.

Specifically, each slab continuously cast using molten steel with adjusted component content as shown in Figure 1 is reheated and hot rolled in a temperature range of 1050 to 1100°C, and then hot rolled in a continuous annealing furnace in the temperature range of 900 to 950°C. It was annealed by holding it for 90 seconds. Next, the structure of the cold rolled and cold rolled annealed specimens was observed at 300 to 900°C, the temperature range in which exhaust system parts are used, and the results are shown in Figures 2 and 3.

As can be seen from the results shown in Figures 2 and 3, in the case of specimens No. 18, which is an example that satisfies the type and content of the alloy components presented in the present invention, Ti(C,N) and Laves phases were formed using ferrite as the base structure. It was confirmed that the Laves phase was formed, and the sigma phase was not formed. However, although the Laves phase, a phase that negatively affects physical properties, was formed, it was confirmed that it was formed in a very small amount with a maximum volume fraction of 0.13%.

On the other hand, Comparative Example No. 12, which does not satisfy the type or content of the alloy components presented in the present invention, In the case of specimens 1 to 4, Ti(C,N), sigma phase, Laves phase, and G phase were formed using ferrite as the base structure. In particular, it was confirmed that the sigma phase, a phase that negatively affects physical properties, was formed by more than 5% based on the volume fraction, and the Laves phase was also confirmed to be formed by more than 0.2% based on the volume fraction. .

In addition, the comparative example NO. In the case of specimens 5 and 6, the conditions of [Equation 1] were not satisfied, and it was confirmed that approximately 17% and 18% of the sigma phase was formed.

And, NO. Specimen 7 is a comparative example in which the C content exceeds the content presented in the present invention, and it was confirmed that about 0.2% of M ₂₃ C ₆ , a negative phase, was formed. Here, M refers to metallic elements, and in the present invention, Fe, Cr, Mn, and Ti correspond to M.

NO. 8 Specimens are comparative examples exceeding the Si content and conditions of [Equation 1] presented in the present invention, and NO. Specimen 9 is a comparative example in which the Mn content exceeds the content suggested in the invention, and it was confirmed that sigma phase was formed by about 10% and 8%.

NO. Specimen 10 is a specimen that satisfies most of the alloy element contents presented in the invention, and negative phases such as sigma phase and Laves phase were not formed. However, as a specimen with a lower Ti content than the desirable content presented in the present invention, AlN, which reduces strength and weldability at high temperatures, was formed by about 0.0002% because the Ti content was less than the limited range.

No. Specimen 11 is a comparative example in which the Ti content exceeds the content presented in the invention, and it was confirmed that the Laves phase was formed by about 2.1% because the Ti content was more than the limit range.

NO. 12 and NO. 13 Specimen is a comparative example that does not satisfy the Al content presented in the invention, and is NO. In specimen 12, the Al content was less than the limit, so a sigma phase was formed at about 7%, and NO. 13 In the specimen, the Al content was higher than the limit, so it was confirmed that about 0.8% of Cr ₃ Si was formed.

NO. Specimen 14 is a comparative example that does not satisfy the N content presented in the invention. It was confirmed that AlN was formed by about 0.001% because the N content was more than the limit range, and NO. Specimen 15 is a comparative example that does not satisfy the O content suggested in the invention, and it was confirmed that Al ₂ O ₃ was formed at about 0.0002% because the O content was greater than the limit range.

NO. Sample 16 is a comparative example that does not satisfy the conditions of [Equation 1] presented in the invention, and sigma phase was formed by about 8% in excess of the conditions of [Equation 1], and NO. Specimen 17 is a comparative example that does not satisfy the conditions of [Equation 2] presented in the invention. It can be confirmed that Al ₂ O ₃ was formed by about 0.0002% due to the lower contents of Zr, Ca, and Mg than the conditions of [Equation 2]. there was.

Therefore, when the type and content of the alloy components presented in the present invention are satisfied, the negative phases, sigma phase and Laves phase, as well as Cr ₃ Si AlN, Al ₂ O ₃ and M ₂₃ C ₆ are formed. It was confirmed that this was suppressed.

In addition, the produced products were evaluated to measure pitting potential, corrosion resistance, impact resistance, thermal expansion coefficient, and corrosion amount, and the results are shown in Figures 4 and 5. The pitting potential (Ept) was measured in 3.5% sodium chloride (NaCl) at a temperature of 25°C. The formal potential was measured using a type of measuring device with a Saturated calomel electrode (SCE) as the reference point.

Corrosion resistance was measured by measuring the time required for rusting to occur due to 5% sodium chloride after spraying 3% salt water.

Impact resistance was measured by the Charpy keyhole-notch impact test, and the amount of corrosion was measured when exposed to the air at 600 to 900°C, the temperature range in which exhaust system components are used.

As can be seen in Figure 4, NO. 4 is an example that satisfies the type and content of the alloy components presented in the present invention. In the case of specimen 18, the pitting potential (Ept) in 3.5% sodium chloride (NaCl) was measured to be 300 mV SCE at a temperature of 25°C, and the pitting potential (Ept) of sample No. 18 was measured as 300 mV _SCE . Specimens 1 to 4 all measured less than 300 mV _SCE . The higher the value of the pitting potential, the better the pitting resistance, and it was confirmed that the pitting potential was improved when the type and content of the alloy components presented in the present invention were satisfied.

In addition, No. 1, which is an example that satisfies the type and content of the alloy components presented in the present invention. In the case of specimen 18, the time required for rusting to occur due to 5% sodium chloride after spraying 3% salt water was measured to be 294 days, which is more than 250 days, and for comparative example No. It was confirmed that rust occurred earlier than 250 days in all specimens 1 to 4.

And, No. 1, which is an example that satisfies the type and content of the alloy components presented in the present invention. In the case of specimen 18, as a result of the impact resistance test, the toughness was measured to be 62J, which is more than 55J, and the comparative example No. Specimens 1 to 4 all had toughness measurements of less than 55J.

In addition, No. 1, which is an example that satisfies the type and content of the alloy components presented in the present invention. In the case of specimen 18, the thermal expansion coefficient was measured to be 11.5Х10 ^{- 6} , and the comparative example No. Specimens 1 to 4 measured 12.3Х10 ^-6 , 12.1Х10 ^-6 , 12.0Х10 ^-6 and 11.6Х10 ^{- 6} , respectively.

And, as can be seen in Figure 5, No. 5, which is an example that satisfies the type and content of the alloy components presented in the present invention. In the case of specimen 18, it was confirmed that the amount of corrosion was significantly reduced in the range of 600 to 900°C compared to specimens 1 to 4, which are the remaining comparative examples.

Next, in order to determine the phase formed according to the change in Al content in the temperature range of 300 ~ 900℃, which is the temperature range in which exhaust system components are used, Al was tested in an alloy of 0.01C-0.1Si-0.1Mn-18Cr-0.2Ti-0.01N. The phases formed by changing the content were investigated, and the results are shown in Figure 6.

As can be seen in Figure 6, it was confirmed that as the Al content increases, the precipitation temperature range of the sigma phase narrows and the volume fraction decreases until the Al content reaches about 3%. In particular, when 3% Al is added, the sigma phase decreases. It was confirmed that the stability of the phase was drastically reduced and that it was not produced. Additionally, it was confirmed that Cr ₃ Si was generated when the Al content exceeded 5%.

On the other hand, even if more than 3% of Al is added, a sigma phase may be formed if the Si and Mn contents are high. In particular, the formation of sigma phase affects the alloy components in that order: Si, Mn, and Cr, and the Si content is about 20 times more sensitive than the Mn content. Accordingly, it was confirmed that the formation of the sigma phase was suppressed when the relationship described in [Equation 1] was satisfied. This phenomenon is no. This can also be confirmed in the results of specimens 12 and 17.

Next, in order to find out the area where AlN is formed according to the change in N content, the area where AlN is formed was investigated by changing the N content in the alloy of 0.01C-0.1Si-0.1Mn-4Al-18Cr-0.2Ti. The results are shown in Figure 7.

As can be seen in Figure 7, it was confirmed that AlN was formed when the N content was 0.03% or more. Accordingly, it is desirable to limit the N content to less than 0.02%.

Next, in order to determine the area where Al ₂ O ₃ is formed depending on the total amount of Zr, Ca, and Mg, Zr, The area where Al ₂ O ₃ was formed was investigated by changing the total amount of Ca and Mg, and the results are shown in FIG. 8.

As can be seen in Figure 8, it was confirmed that Al ₂ O ₃ was formed when the total amount of Zr, Ca, and Mg was less than 0.001%, and that Al ₂ O ₃ was not formed when it exceeded 0.01%.

Next, in order to find out the area where M ₂₃ C ₆ is formed according to the change in C content, M ₂₃ C ₆ was formed by changing the C content in the 18Cr-0.1Mn-0.1Si-0.2Ti-0.01N-4Al alloy. The region in which this occurred was identified, and the phase transformation trend by temperature was examined with the C content fixed at 0.02%, and the results are shown in Figure 9.

As can be seen in Figure 9, it was confirmed that M ₂₃ C ₆ was formed when the C content was greater than 0.017, and when the C content was 0.02%, it was confirmed that about 0.2% of M ₂₃ C ₆ was formed. .

Therefore, it would be advantageous to improve the physical properties of stainless steel by containing C at less than 0.015% and maintaining the volume fraction of M ₂₃ C ₆ at less than 0.2%.

Although the present invention has been described with reference to the accompanying drawings and the above-described preferred embodiments, the present invention is not limited thereto and is limited by the claims described below. Accordingly, those skilled in the art can make various changes and modifications to the present invention without departing from the technical spirit of the claims described later.

Claims (13)

By weight percent, C: 0.015% or less (excluding 0%), Si: 0.17% or less (excluding 0%), Mn: 1.35% or less (excluding 0%), Cr: 17 to 20%, Ti: 0.1 to 0.5% , Al: 3 to 5%, O: less than 0.001% (excluding 0%), N: less than 0.02% (excluding 0%), the remaining Fe and other inevitable impurities are included, and satisfies the following [Equation 1],
(20Si+Mn)/Al < 0.7... … … … … … [Equation 1]
It further contains Zr, Ca, and Mg, but satisfies [Formula 2] below,
0.001 ≤ Zr+Ca+Mg ≤ 0.01 … … … … … … [Equation 2]
A ferritic stainless steel characterized in that the volume fraction of the Laves phase formed in the temperature range of 300 to 900°C is 0.2% or less.
In [Formula 1], Si, Mn, and Al refer to the content (%) of each component.
In [Formula 2], Zr, Ca, and Mg refer to the content (%) of each component.
delete delete delete In claim 1,
The stainless steel is a ferritic stainless steel, characterized in that the volume fraction of the sigma phase formed in the temperature range of 300 to 900 ° C is less than 5%.
In claim 5,
The stainless steel is a ferritic stainless steel, characterized in that the volume fraction of the sigma phase formed in the temperature range of 300 to 900 ° C is less than 0.5% or does not precipitate.
In claim 1,
The stainless steel is a ferritic stainless steel, characterized in that the volume fraction of Cr ₃ Si formed in the temperature range of 300 to 900 ° C is 0.5% or less.
In claim 1,
The stainless steel is a ferritic stainless steel, characterized in that the volume fraction of AlN and Al ₂ O ₃ formed in the temperature range of 300 to 900 ° C is each 0.0001% or less.
In claim 1,
The stainless steel is a ferritic stainless steel, characterized in that the volume fraction of M ₂₃ C ₆ formed in the temperature range of 300 to 900 ° C is less than 0.2%.
delete In claim 1,
The stainless steel is a ferritic stainless steel, characterized in that the pitting potential (Ept) in 3.5% sodium chloride (NaCl) is 300 mV _SCE or more at a temperature of 25°C.
In claim 1,
The stainless steel is a ferritic stainless steel, characterized in that the time required for rusting by 5% sodium chloride after spraying with 3% salt water is more than 250 days.
In claim 1,
The stainless steel is a ferritic stainless steel, characterized in that the toughness measured by the Charpy keyhole-notch impact test is 55J or more.