KR20180072930A - Austenitic stainless steel with excellent anti-oxidation and method of manufacturing the same - Google Patents

Abstract

The present invention provides an austenitic stainless steel having excellent anti-oxidation and a method of manufacturing the same. The austenitic stainless steel according to the present invention includes 17 to 18 wt% Cr, 8 to 9 wt% Ni, 0.25 to 0.8 wt% Mn, 0.25 to 0.4 wt% Si, 0.13 to 0.16 wt% C, 0.1 to 0.13 wt% N, 0.5 to 0.53 wt% Nb, 0.005 wt% or less P, 0.004 wt% or less S, 0.011 wt% or less Al, 0.007 wt% or less O, and the remainder Fe and unavoidable impurities, and the size of an average grain of the austenitic stainless steel is 12 to 18μm. In this case, it is preferred that the Mn content is 0.015 or more and Cr content is 0.045 or less. As the amount of added Mn increases, anti-oxidation normally decreases, but an austenitic stainless steel with excellent anti-oxidation can be manufactured by the control of Mn and Cr contents as mentioned above.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to austenitic stainless steel having excellent oxidation resistance and a method of manufacturing the same. 2. Description of the Related Art Austenitic stainless steels,

More particularly, the present invention relates to austenitic stainless steel having excellent oxidation resistance and a method for producing the same.

Stainless steels are classified into austenitic stainless steels, ferritic stainless steels, and two-phase stainless steels depending on the microstructure.

Of these, austenitic stainless steels are superior in creep resistance at high temperatures to ferritic stainless steels or duplex stainless steels, and are used at high temperatures of about 650 ° C.

However, if austenitic stainless steels are used for a long time in a higher temperature range, austenitic stainless steels may react with the atmosphere to form oxides The resulting oxidation problem occurs. The oxides generated in the steel can concentrate on the oxides and stress acting on the steel can act as a source of mechanical defects. In addition, oxides formed on the outside of the steel reduce the effective thickness of the steel to shorten the life of the structure, and if oxide peeling occurs, it may cause structural damage and pipeline closure due to scattering of oxide during the operation of the structure. Therefore, it is important to secure the oxidation resistance of the austenitic stainless steel.

On the other hand, Japanese Patent Application Laid-Open No. H05-337704 discloses a steel sheet having a composition of C: 0.05% or less, Si: 1.0-3.0%, Mn: 1.0-1.5%, P: 0.05% or less, S: 0.002 or less, Ni: 13-15% To about 25%, N: about 0.2 to about 0.3%, Nb: about 0.01% or less, the balance of Fe and inevitable impurities, and austenitic system Stainless steel is disclosed.

However, in the case of Patent Document 1, it is judged that the C content is 0.05 wt% or less and the Nb content is 0.01 wt% or less, thereby making it difficult to ensure strength and fine grain refinement. In order to compensate for this, in the case of Patent Document 1, 1.0 wt% or more of Mn and 23 wt% or more of Cr are required.

Patent Document 1: JP-A-10-2013-0014901 (published on Feb. 12, 2013)

An object of the present invention is to provide an austenitic stainless steel excellent in oxidation resistance through control of alloy components such as Mn, Cr C and Nb.

Another object of the present invention is to provide a method for producing an austenitic stainless steel excellent in oxidation resistance.

The austenitic stainless steel according to an embodiment of the present invention includes 17 to 18% of Cr, 8 to 9% of Ni, 0.25 to 0.8% of Mn, 0.25 to 0.8% of Si, 0.4 to 0.13% of C, 0.1 to 0.13% of N, 0.1 to 0.13% of N, 0.5 to 0.53% of Nb, 0.005% or less of P, 0.004% or less of S, 0.011% or less of Al and 0.007% or less of O , The balance iron and unavoidable impurities, and has an average grain size of 12 to 18 mu m.

At this time, the austenitic stainless steel preferably has 0.015? Mn content / Cr content (wt%)? 0.045.

In addition, the austenitic stainless steel may further contain 3.3 to 3.7% by weight of Cu.

According to another aspect of the present invention, there is provided a method of manufacturing an austenitic stainless steel comprising: 17 to 18% of Cr, 8 to 9% of Ni, 0.25 to 0.8% of Mn, 0.25 to 0.8% of Si, 0.10 to 0.13% of N, 0.5 to 0.53% of Nb, 0.005% or less of P, 0.004% or less of S, 0.011% or less of Al and 0.007% or less of O Heating the semi-finished steel consisting of the remaining iron and unavoidable impurities to perform a first heat treatment; Hot rolling the primary heat treated steel at a finish rolling temperature condition of 1000 占 폚 or higher; Firstly cooling the hot-rolled steel; Heating the primary cooled steel to perform a secondary heat treatment; Secondary cooling the secondary heat treated steel; First cold rolling the secondary cooled steel; Subjecting the primary cold-rolled steel to a tertiary heat treatment; And tertiary cooling the tertiary heat treated steel.

At this time, the third cold-rolled steel may be further subjected to secondary cold rolling at a reduction ratio of 10% or more.

Also, the first heat treatment and the second heat treatment may be performed at 1200 to 1250 ° C, and the third heat treatment may be performed at 1000 to 1050 ° C.

Further, the secondary cooling and the tertiary cooling may be performed by a water quenching method, respectively.

Further, the reduction rate in the primary cold rolling can be performed by a method of 50% or more.

Further, it is preferable that the steel has 0.015? Mn content / Cr content (wt%)? 0.045.

In addition, the steel may further contain 3.3 to 3.7% by weight of Cu.

The austenitic stainless steel according to the present invention can exhibit excellent oxidation resistance through precise control of the content of Mn and Cr.

In addition, the austenitic stainless steel according to the present invention can have a fine average grain size of about 15 mu m and is advantageous in exhibiting oxidation resistance.

In addition, the austenitic stainless steel according to the present invention can contribute to improvement of high temperature mechanical properties such as creep resistance through addition of Cu.

1 is a flowchart schematically showing a method of manufacturing austenitic stainless steel according to an embodiment of the present invention.
Fig. 2 shows the results of oxidation resistance measurement of the specimens of the examples and comparative examples.
3 shows the results of FIG. 2 rearranged according to manganese content.
4 is a photograph showing the thicknesses of oxide layers of the specimens of Examples and Comparative Examples.
5 is a photograph showing the grain refinement of the specimen of Example 2 through heat treatment.
6 compares the oxide layer thicknesses of Example 2 and Comparative Example 3. Fig.

Advantages and features of the present invention and methods of achieving them will become apparent with reference to the embodiments described in detail below. It should be understood, however, that the invention is not limited to the disclosed embodiments, but is capable of many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, To fully disclose the scope of the invention to those skilled in the art, and the invention is only defined by the scope of the claims.

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, austenitic stainless steel having excellent oxidation resistance according to the present invention and a method for producing the same will be described in detail with reference to the accompanying drawings.

Austenitic system  Stainless steel

The austenitic stainless steel according to an embodiment of the present invention may contain 17 to 18% of Cr, 8 to 9% of Ni, 0.25 to 0.8% of Mn, 0.25 to 0.4% of Si, 0.13 to 0.16 of C, % Of N, 0.1 to 0.13% of N, 0.5 to 0.53% of Nb, 0.005% or less of P, 0.004% or less of S, 0.011% or less of Al and 0.007% or less of O and the balance being iron and unavoidable impurities And an average grain size of 12 to 18 mu m.

Hereinafter, the role and content of each component included in the austenitic stainless steel according to the present invention will be described.

Cr (chrome)

Cr is an element which accelerates the formation of a passive film of stainless steel, and is an effective element particularly for suppressing formation of a high-temperature scale. And also has an effect of preventing coarsening of austenite grains at a high temperature of 1000 ° C or higher. In order to improve the corrosion resistance and high-temperature characteristics, the content is effectively contained in an amount of 17 wt% or more. When the content is over 18 wt%, the formation of δ-ferrite may lead to deterioration of corrosion resistance and deterioration of hot workability.

Ni (nickel)

Ni is added as an austenite stabilizing element to obtain austenite structure. In order to obtain such an effect, the content of nickel is more than 8 wt%, but when it exceeds 9 wt%, the steel manufacturing cost can be increased without any further effect.

Mn (manganese)

Mn is an element contributing to strength increase and austenite stabilization. In the present invention, manganese is contained in an amount of 0.25 to 0.8 wt%. When the Mn content is less than 0.25% by weight, there is no grain boundary decohesion effect, so it behaves similarly to a steel having a large grain size, making it difficult to exhibit oxidation resistance characteristics. When the Mn content exceeds 0.8% by weight, Mn partially substitutes for Cr in the Cr-enriched layer at the interface between the base material and the oxide layer, thereby lowering the protective property of the high chromium content layer and deteriorating the oxidation resistance .

Such manganese content can be controlled by controlling the purity of the parent alloy such as a purity of Mn of 99.7% or more and a purity of Fe electrolytic iron of 99.9% or more, an inert gas such as N2 or Ar at the time of casting, and S and O contents of 0.004% , 0.007% by weight or less and 0.11% by weight or less of Al in order to control the low O content. When all of these conditions are satisfied, the Mn content in the steel can be almost the same as that of the initially added Mn, which is most preferable.

On the other hand, it is preferable that the austenitic stainless steel has 0.015? Mn content / Cr content (wt%)? 0.045. When the Mn content with respect to the Cr content is satisfied, excellent oxidation resistance can be exhibited. According to the experiment, when the Cr-to-Mn content is lower or higher than the above-mentioned range, sufficient oxidation resistance can not be exhibited.

Si (silicon)

Si is an element contributing to improvement of oxidation resistance at high temperature in austenitic stainless steel. In order to exhibit this effect, Si is preferably contained in an amount of 0.25% by weight or more. However, if the Si content exceeds 0.4% by weight, the weldability of the steel may be deteriorated.

C (carbon)

C is effective for improving the high temperature strength of the austenitic stainless steel. For this, in the present invention, the content of C is 0.13 wt% or more. However, when the content of C exceeds 0.16% by weight, the effective Cr content effective for oxidation resistance can be reduced.

N (nitrogen)

When N is contained in an amount of 0.1% by weight or more, Ni can be substituted for an element that stabilizes the austenite phase, thereby contributing to improvement of strength and corrosion resistance. However, when the nitrogen content exceeds 0.13% by weight, toughness degradation and weldability deterioration may occur due to formation of Cr nitride.

Nb (Niobium)

Nb forms carbide and contributes to improvement of high-temperature strength and thermal fatigue characteristics. For this, Nb was added in an amount of 0.5 wt% or more in the present invention. However, when the Nb content exceeds 0.53% by weight, problems such as deterioration of workability may occur.

Cu (copper)

In addition, the austenitic stainless steel may further contain 3.3 to 3.7% by weight of Cu. By the addition of Cu, high-temperature mechanical properties such as creep resistance and the like can be further improved by formation of fine? -Cre precipitates.

Other

In the present invention, P (phosphorus), S (sulfur) and O (oxygen) are elements that lower the hot workability and oxidation resistance of the austenitic stainless steels. % Or less, and O: 0.007% or less. Al was able to act as a deoxidizer and was controlled to be 0.11 wt% or less for low O content control.

The austenitic stainless steel according to the present invention may have an average grain size of 12 to 18 탆. This can be achieved by the alloy composition as described above and the heat treatment described below. In the present invention, the superfine NbC can be precipitated through the C content of 0.13% by weight or more and the Nb content of 0.5% by weight or more, whereby the crystal grain refining effect can be increased. However, precipitation of crude NbC leads to deterioration of high-temperature mechanical properties, so that it is further controlled by subsequent heat treatment.

In addition, Mn loss such as inclusion formation can be prevented through extremely low-level control of S and O content, which is advantageous for grain refinement and oxidation resistance.

However, since the coarsening of NbC in the primary phase leads to deterioration of the high-temperature properties, it is necessary to control the Nb & C content and the subsequent heat treatment process in order to suppress the primary NbC coarsening.

Such fine average crystal grains can also contribute to improvement of oxidation resistance.

Austenitic system  Stainless steel manufacturing method

1 is a flowchart schematically showing a method of manufacturing austenitic stainless steel according to an embodiment of the present invention.

Referring to FIG. 1, a method of manufacturing an austenitic stainless steel according to an embodiment of the present invention includes a first heat treatment step S110, a hot rolling step S120, a first cooling step S130, a second heat treatment step S140 ), A secondary cooling step (S150), a primary cold rolling step (S160), a third heat treatment step (S170), and a third cooling step (S180). In addition, it may further include a secondary cold rolling step (S190) preferably after the third cooling step.

Hereinafter, the respective heat treatment conditions and cooling conditions were determined by calculating the state diagram through thermodynamic calculation.

In the first heat treatment step (S110), the semi-finished steel having the above-mentioned alloy composition is heated and subjected to a first heat treatment. Through the first heat treatment, the fraction of crude superfine NbC can be lowered and the composition of the steel can be homogenized. The first heat treatment is preferably performed at 1200 to 1250 DEG C for about 1 hour or more. When the primary heat treatment temperature is less than 1200 ° C, it may be difficult to obtain a homogenization effect, and when it exceeds 1250 ° C, crystal grain coordination problems may occur.

In the hot rolling step (S120), the primary heat-treated steel is hot-rolled at a finish rolling temperature of 1000 ° C or higher to break the cast structure. By controlling the finish hot rolling temperature at 1000 ° C or higher, it is possible to prevent formation of precipitates such as Cr ₂ N and M ₂₃ C ₆ .

In the primary cooling step (S130), the hot-rolled steel is first cooled. The primary cooling may be carried out to a temperature of 200 ° C or less at an average cooling rate of about 10 ° C / sec or more to prevent formation of precipitates such as Cr ₂ N and M ₂₃ C ₆ . The primary cooling may be performed by air cooling or water cooling.

In the secondary heat treatment step (S140), the primary cooled steel is heated again and subjected to the secondary heat treatment. Through the secondary heat treatment, it is possible to lower the fraction of coarse primary NbC and homogenize the composition of the steel as in the first heat treatment. In addition, recrystallization of the rolled structure occurs as the texture is unwound through the secondary heat treatment. The second heat treatment is preferably performed at 1200 to 1250 占 폚 for about 1 hour or more as in the first heat treatment. When the second heat treatment temperature is less than 1200 ° C, it may be difficult to obtain a homogenizing effect, and when the second heat treatment temperature is higher than 1250 ° C, crystal grain alignment problems may occur.

In the second cooling step (S150), the secondary heat treated steel is secondarily cooled. Secondary cooling plays a role to prevent microstructure retention and precipitation phases such as Cr ₂ N and M ₂₃ C ₆ . To achieve this effect, the secondary cooling may be performed to a temperature of 200 DEG C or less at an average cooling rate of 30 DEG C / sec or higher. If the cooling rate of the secondary cooling is not sufficient, the secondary cooling effect as described above may be insufficient. Preferably, the secondary cooling and the tertiary cooling described below can be applied to water quenching.

In the primary cold rolling step (S160), the secondary cooled steel is first cold-rolled. The rolling reduction during the primary cold rolling can be about 50% or more. The primary cold rolling is a pretreatment step for NbC precipitation dispersion and grain refinement, which can be considered as a preliminary step that plays an important role in grain refinement through dynamic recrystallization.

In the third heat treatment step (S170), the first cold-rolled steel is subjected to a third heat treatment. The tertiary heat treatment contributes to grain refinement by dynamic recrystallization as a solidification heat treatment. For this, the third heat treatment is performed at 1000 ~ 1050 ° C for about 1 hour or more. If the tertiary heat treatment temperature is less than 1000 ° C, the effect of solidification may be inadequate, and if it exceeds 1050 ° C, the grain refinement is disadvantageous.

In the third cooling step (S180), the third heat treated steel is cooled third. Through tertiary cooling, it is possible to prevent microstructure retention and formation of precipitates such as Cr ₂ N and M ₂₃ C _6, as in the secondary cooling, and the cooling conditions can be applied as they are in the secondary cooling.

In the second cold rolling step (S190), the third cooled steel is secondarily cold-rolled at a reduction ratio of 10% or more. Pre-strain can be imparted through the secondary cold rolling, and it is possible to increase the dislocation density, to promote the formation of fine strengthened stones such as NbC and ε-Cu in the creep deformation, and to form sub-crystal grains. This can improve creep resistance.

Example

Hereinafter, the configuration and operation of the present invention will be described in more detail with reference to preferred embodiments of the present invention. It is to be understood, however, that the same is by way of illustration and example only and is not to be construed in a limiting sense.

The contents not described here are sufficiently technically inferior to those skilled in the art, and a description thereof will be omitted.

1. Manufacture of steel specimens

The casting specimens composed of the composition and the balance iron described in Example 1-3 and Comparative Examples 1, 2, 4 and 5 of Table 1 were heat-treated at 1200 ° C for 1 hour, hot rolled at a temperature of 1000 ° C finish rolling temperature, The water was quenched to room temperature. Thereafter, the specimens were further heat-treated at 1200 ° C for 1 hour, water quenched to room temperature, and then cold-rolled under a 60% reduction ratio. Thereafter, after the solution was heat-treated at 1020 占 폚 for 1 hour, water quenched to room temperature again. Thereafter, it was cold-rolled again at a reduction rate of 12%.

In the case of Comparative Example 3, however, it was manufactured only through the first heat treatment, the hot rolling, the first cooling, the second heat treatment and the second cooling.

[Table 1]

2. Property evaluation

Fig. 2 shows the results of oxidation resistance measurement of the specimens of Examples and Comparative Examples. 3 shows the results of FIG. 2 rearranged according to manganese content. Table 2 shows the results of oxidation resistance measurement according to manganese content.

The oxidation resistance was evaluated by exposing the specimens to oxygen at 700 ° C for 8 weeks and measuring the weight gain (g) by oxidation. The smaller the increase in weight, the better the oxidation resistance.

[Table 2]

Referring to FIGS. 2, 3 and 2, it can be seen that the oxidation resistance of Examples 1 to 3 is relatively superior to that of Comparative Examples 1 to 5. In particular, it is generally known that the oxidation resistance tends to decrease as the content of Mn increases. However, except for the specimen according to Examples 1 to 3, which satisfies the alloy composition of the present invention, that is, the manganese content is 0.25 to 0.8 wt% The oxidation resistance was rather superior.

4 is a photograph showing the thicknesses of oxide layers of the specimens of Examples and Comparative Examples. Referring to FIG. 4, it can be seen that, in the case of the test pieces according to Examples 1 to 3, the thickness of the oxide layer is relatively thin and the penetration of the oxide layer into the base material is reduced compared to the test pieces according to Comparative Examples 1 to 5.

5 is a photograph showing the grain refinement of the specimen of Example 2 through heat treatment. Referring to FIG. 5, it can be confirmed that the crystal grains are refined by the cold rolling-heat treatment, and the grain refinement as described above can contribute to the oxidation resistance improvement.

6 compares the oxide layer thicknesses of Example 2 and Comparative Example 3. Fig. Referring to FIG. 6, it can be seen that the thickness of the oxide layer is relatively thick in the case of Comparative Example 3 in which crystal grains are coarse even though the contents of Mn and Cr are similar. Therefore, it can be confirmed that grain refinement plays an important role in improving oxidation resistance. Also, in Comparative Example 3, the S and O contents are relatively high, which is much lower than the Mn content added due to the high S and O content in the case of the specimen of Comparative Example 3, so that the desired oxidation resistance can not be exerted Can be seen as having an impact.

While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims. Accordingly, the true scope of protection of the present invention should be defined by the claims.

S110: Primary heat treatment step
S120: Hot rolling step
S130: primary cooling step
S140: Second heat treatment step
S150: Secondary cooling step
S160: primary cold rolling step
S170: Third heat treatment step
S180: Third cooling step
S190: Secondary cold rolling step

Claims (9)

The steel sheet according to any one of claims 1 to 5, wherein the steel sheet comprises, by weight, 17 to 18% of Cr, 8 to 9% of Ni, 0.25 to 0.8% of Mn, 0.25 to 0.4% of Si, 0.13 to 0.16% of C, 0.1 to 0.13% 0.53%, P: not more than 0.005%, S: not more than 0.004%, Al: not more than 0.011%, and O: not more than 0.007%, the balance being iron and unavoidable impurities, Austenitic stainless steel.
The method according to claim 1,
The austenitic stainless steel
0.015? Mn content / Cr content (% by weight)? 0.045.
The method according to claim 1,
The austenitic stainless steel
And Cu: 3.3 to 3.7% by weight based on the total weight of the austenitic stainless steel.
The steel sheet according to any one of claims 1 to 5, wherein the steel sheet comprises, by weight, 17 to 18% of Cr, 8 to 9% of Ni, 0.25 to 0.8% of Mn, 0.25 to 0.4% of Si, 0.13 to 0.16% of C, 0.1 to 0.13% 0.53%, P: not more than 0.005%, S: not more than 0.004%, Al: not more than 0.011%, and O: not more than 0.007%, the balance being iron and unavoidable impurities;
Hot rolling the primary heat treated steel at a finish rolling temperature condition of 1000 占 폚 or higher;
Firstly cooling the hot-rolled steel;
Heating the primary cooled steel to perform a secondary heat treatment;
Secondary cooling the secondary heat treated steel;
First cold rolling the secondary cooled steel;
Subjecting the primary cold-rolled steel to a tertiary heat treatment; And
And thirdly cooling the tertiary-heat-treated steel. The method of manufacturing austenitic stainless steel according to claim 1,
5. The method of claim 4,
Further comprising the step of: secondarily cold-rolling the tertiary-cooled steel at a reduction ratio of 10% or more.
5. The method of claim 4,
The primary heat treatment and the secondary heat treatment are performed at 1200 to 1250 占 폚,
Wherein the third heat treatment is performed at 1000 to 1050 占 폚.
5. The method of claim 4,
Wherein the secondary cooling and the tertiary cooling are performed by a water quenching method, respectively.
5. The method of claim 4,
Wherein the steel has 0.015? Mn content / Cr content (wt%)? 0.045.
5. The method of claim 4,
Wherein the steel further comprises 3.3 to 3.7% by weight of Cu.

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