KR101903403B1 - Austenitic stainless steel with improved pitting corrosion resistance - Google Patents

Abstract

The present invention relates to austenitic stainless steels having excellent resistance to formaldehyde and having excellent strength and ductility in austenitic stainless steels containing a large amount of both carbon and nitrogen, (Si): 15 to 19%, manganese (Mn): 9 to 11%, carbon (C): 0.18 to 0.3%, nitrogen (N): 0.25 to 0.3%, niobium (Nb) : 0.3% or less, and the balance of Fe and unavoidable impurities.

Description

TECHNICAL FIELD [0001] The present invention relates to austenitic stainless steels having improved pitting corrosion resistance,

The present invention relates to austenitic stainless steels, and more particularly to austenitic stainless steels having improved pitting resistance.

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

Of these, commercial austenitic stainless steels are composed mainly of Fe-Cr-Ni elements and are alloys and are excellent in workability and corrosion resistance. However, due to the high dependence of Ni, it has some disadvantages. Therefore, to cope with this problem, steel grades have been developed in which Ni is replaced by economical austenite forming elements such as Mn, N and C.

However, due to the high contents of C and N, the stainless steels containing both C and N are highly likely to precipitate chromium carbides or nitrides such as Cr ₂ N and Cr ₂₃ C ₆ . Such a chromium-based carbide or nitride is a factor that deteriorates the formability.

To solve this problem, there is a method of lowering the C content to about 0.05 wt%. However, in this case, there is a problem of reduction in strength, and excessive addition of Si, Cr, and Ni is required to compensate for the decrease. (Patent Document 1)

In the case of Patent Document 1, the steel has a composition of 0.06% or less of C, 0.5-3.0% of Si, 1.5% or less of Mn, 21-23% of Cr, 9-11% of Ni, 0.1 to 0.25% of N, 0.1 to 1.0% of W, and the balance of Fe and other unavoidable impurities. The present invention relates to austenitic stainless steels excellent in pitting corrosion resistance. In the case of this document, it shows high contents of Si, Cr and Ni together with low carbon and manganese content. Especially, a high nickel content significantly increases the price of austenitic stainless steel.

Patent Document 1: Korean Patent Laid-Open Publication No. 10-2016-0082376 (published on Jul.

An object of the present invention is to provide a stainless steel excellent in strength and ductility while having an excellent resistance to formaldehyde in austenitic stainless steels containing a cerium-containing element containing a large amount of both carbon and nitrogen.

To achieve the above object, the austenitic stainless steel according to an embodiment of the present invention includes 15 to 19% of chromium (Cr), 9 to 11% of manganese (Mn), 0.18 to 10% of carbon (C) 0.3 to 0.3% of nitrogen (N), 0.25 to 0.3% of niobium (Nb), and 0.3% or less of silicon (Si), and the balance of Fe and unavoidable impurities.

At this time, the total content of carbon (C) and nitrogen (N) may be 0.47 wt% or more.

In addition, the atomic number ratio (Nb / C) of the niobium and carbon may be 4 to 20.

The austenitic stainless steel may contain phosphorus (P) in an amount of 0.005 wt% or less and sulfur (S) in an amount of 0.005 wt% or less.

In addition, the ratio of free carbon (Free C) not bonded to niobium in the carbon may be 80% or more.

The austenitic stainless steels may have an average crystal grain diameter of 10 to 16 탆 and a product of tensile strength and elongation of 60,000 MPa ·% or more.

In addition, the austenitic stainless steel may exhibit an elongation of 58% or more and a formal dislocation of 0.36V _SCE or more.

In addition, the austenitic stainless steel may have a volume fraction of primary NbC of not more than 3 vol%.

The austenitic stainless steel according to the present invention is a stainless steel containing a cemented element containing both carbon and nitrogen, which not only has excellent pitting corrosion resistance, but also utilizes Nb to provide a stainless steel having finer crystal grains and improved strength and ductility .

On the other hand, as a result of the experiment, it has been found that the range of the maximum resistance and elongation is within the specific Nb range. This result can be regarded as an inherent characteristic of the austenitic stainless steel according to the present invention.

Figure 1 shows photographs and EDS mapping results of primary NbC.
Fig. 2 shows the change in the volume fraction of the primary NbC with increasing niobium content.
Fig. 3 shows changes in grain size, yield strength, tensile strength, elongation, elongation x tensile strength, and formal resistance according to the Nb content in Examples and Comparative Examples, respectively.

BRIEF DESCRIPTION OF THE DRAWINGS The advantages and features of the present invention, and the manner of achieving them, will be apparent from and elucidated with reference to the embodiments described hereinafter in conjunction with the accompanying drawings.

However, it should be understood that the present invention is not limited to the embodiments disclosed herein but may be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the concept of the invention to those skilled in the art. Is provided to fully convey the scope of the invention to those skilled in the art, and the invention is only defined by the scope of the claims.

Stainless steels with carbon and nitrogen compounds have high carbon and nitrogen content. As a result, there is a high risk of precipitation of chromium-based carbides such as Cr ₂ N and Cr ₂₃ C ₆ or chromium nitride. These chromium-based carbonitrides cause local depletion of chromium in the base material, resulting in deterioration of various physical properties such as corrosion resistance and mechanical properties.

Therefore, it is necessary to increase the homogenization heat treatment temperature in order to prevent formation of chromium-based carbonitride. However, in the case of this method, crystal grain coarsening is brought about. Grain coarsening can lead to deterioration of mechanical properties and surface quality after processing, and also has the disadvantage of lowering the formal resistance.

Therefore, in the present invention, a small amount of Nb, which is a strong carbonitride-forming element, is utilized in the austenitic stainless steels containing carbon and nitrogen composite, for grain refinement of steel.

As the Nb content increases, the grain is effectively refined. However, as can be seen from Fig. 3, the effect of grain refinement is not remarkable in the addition of 0.3 wt% or more of Nb. In addition, since the formation and the fraction of NbC increase with the increase of the Nb content, there is a fear of a decrease in the formal resistance due to the increase of the heterogeneity of the base material, and the linearity of the elongation decreases.

However, in the case of the austenitic stainless steel according to the present invention, as shown in FIG. 3, when the Nb content is increased up to 0.2 wt% as a result of the evaluation of the manufactured alloys, the effect of improving the resistance to formaldehyde (passivation film strengthening effect) , The reverse-V curve was increased when the Nb content increased to 0.3 wt%.

The Nb content, which can maximize grain refinement and physical properties, is 0.1 to 0.3% by weight in the austenitic stainless steel according to the present invention.

More specifically, the austenitic stainless steel according to the present invention comprises 15 to 19% of chromium (Cr), 9 to 11% of manganese (Mn), 0.18 to 0.3% of carbon (C) N: 0.25 to 0.3%, niobium (Nb): 0.1 to 0.3%, silicon (Si): 0.3% or less, and the balance of Fe and unavoidable impurities.

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

Cr (chromium)

Cr is an element that accelerates the formation of a passive film in stainless steel and is an indispensable element particularly for improving corrosion resistance. In order to improve the solubility of nitrogen in the steel steadily and to improve the corrosion resistance, it is effective to contain not less than 15% by weight, but when it exceeds 19% by weight excessively, Lowering in the degree of appearance and deterioration in hot workability.

Mn (manganese)

Mn is an economical element contributing to strength enhancement and austenite stabilization. In the present invention, manganese is contained in an amount of 9 to 11% by weight. When the Mn content is less than 9% by weight, it is difficult to obtain austenite single phase due to the lack of austenite phase forming ability, and it may cause a decrease in N-solubility. However, since Mn can form Mn oxide and the like, corrosion resistance is lowered, and Mn in the solid state is also known to deteriorate the passive property and reduce the formal resistance, so that the content thereof is limited to 11 wt%.

C (carbon)

C is effective for improving the strength of the austenitic stainless steel, and is an economical and strong austenite phase stabilizing element, and has the advantage of significantly increasing the strength by solid solution strengthening. In addition, C in the employment status has been reported to improve the official resistance. For this, in the present invention, the content of C is 0.18 wt% or more. However, when the content of C is more than 0.3% by weight, formation of Cr-carbide is facilitated, so that the solid Cr content effective for corrosion resistance can be reduced, the homogenization heat treatment temperature for preventing the formation of carbides must be increased, And the upper limit thereof is limited to 0.3% by weight.

N (nitrogen)

When N is contained in an amount of 0.25% by weight or more, Ni can be substituted as an element for stabilizing the austenite phase, and it can contribute to enhance strength and corrosion resistance. Nitrogen also has the advantage of increasing strength by solid solution strengthening and maintaining ductility at a high level. However, when the nitrogen content exceeds 0.3% by weight, the homogenization heat treatment temperature for preventing nitride formation must be increased, and the Cr nitride is formed, which may lead to reduction in toughness and deterioration in weldability.

Nb (niobium)

Nb is a strong carbide-forming element and can form MX-type carbide, contributing to improving the strength at room temperature and high temperature and improving the thermal fatigue characteristics. Also, the finely formed carbide of MX type inhibits the movement of grains and is effective for grain refinement. For this effect, 0.1 wt% or more of Nb is added in the present invention. However, when the Nb content exceeds 0.3% by weight, the formation of superficial NbC promotes and the size of NbC becomes large, which may cause problems such as deterioration of workability.

Si (silicon)

Si is mainly used as a deoxidizer in austenitic stainless steel. However, when the Si content exceeds 0.3% by weight, the mechanical properties associated with the toughness of the steel are greatly reduced and intermetallic compounds are formed, which may cause deterioration in weldability of the steel.

In the case of the austenitic stainless steel according to the present invention, substantially no nickel may be contained. As described above, since the cost of nickel is not included, the steel manufacturing cost can be greatly reduced.

In the case of the ozone-based stainless steel according to the present invention, a large amount of carbon and nitrogen are mixedly added. More specifically, the total content of carbon (C) and nitrogen (N) may be 0.47 wt% or more.

In addition, the atomic number ratio (Nb / C) of the niobium and carbon may be 4 to 20.

The austenitic stainless steel may contain phosphorus (P) in an amount of 0.005 wt% or less and sulfur (S) in an amount of 0.005 wt% or less.

In addition, the ratio of free carbon (Free C) not bonded to niobium in the carbon may be 80% or more.

The austenitic stainless steels may have an average crystal grain diameter of 10 to 16 탆 and a product of tensile strength and elongation of 60,000 MPa ·% or more.

In addition, the austenitic stainless steel may exhibit an elongation of 58% or more and a formal dislocation of 0.36 V or more.

In the case of the present invention, Nb is used in a stainless steel containing both carbon and nitrogen, and more specifically, NbC crystallization phases as in the example shown in FIG. 1 can be used to refine the crystal grains, The improved stainless steel can be provided. At this time, a region where the formal resistance and the elongation are maximized at a specific Nb range, that is, 0.1 to 0.3% by weight of Nb, is found, which is a phenomenon not reported in the related study results.

For example, the osteointegrated stainless steel according to the present invention may be subjected to a homogenization heat treatment at 1200 to 1250 占 폚 for at least 1 hour, a hot rolling and a primary cooling at a finish rolling temperature condition of 1000 占 폚 or more, a homogenization heat treatment temperature A secondary cooling such as water quenching for prevention of formation of a precipitate phase, but is not limited thereto, and can be produced by various known methods.

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.

Table 1 shows alloy compositions of the steel specimens according to Examples and Comparative Examples.

In Table 1, free carbon (Free C) means the content (wt%) of C that does not form NbC.

[Table 1] (unit:% by weight)

The castings of each specimen having the composition shown in Table 1 were subjected to homogenization heat treatment at 1200 ° C for 1 hour and then hot rolled at a reduction ratio of 40% or more at a temperature of 1000 ° C finish rolling temperature and then cooled to room temperature. Then, each specimen was annealed at a temperature of 1100 ℃ or higher for 1 hour, and water quenched at room temperature to prepare each specimen.

Figure 1 shows photographs and EDS mapping results of primary NbC.

Fig. 2 shows the change in the volume fraction of the primary NbC with increasing niobium content. The primary NBC volume fraction was obtained based on the EDS mapping results.

Referring to FIG. 2, when the amount of Nb added is 0.3 wt% or less, the volume fraction of primary NbC is 3 vol% or less. When the amount of Nb is more than 0.3 wt%, the volume fraction increases sharply. Since the rapid increase of the crude superfine NbC volume fraction leads to a decrease in the formal resistance and elongation, the addition amount of Nb is limited to 0.3 wt% in the present alloy system.

Table 2 shows the results of evaluating the physical properties of each specimen according to Examples and Comparative Examples. Fig. 3 shows changes in grain size, yield strength, tensile strength, elongation, elongation x tensile strength, and resistance to formaldehyde according to the Nb content in the examples and comparative examples, respectively.

[Table 2]

Referring to Table 2 and FIG. 3, it can be seen that as the Nb content increases, the grain size decreases, which can be seen as a result of the fine dispersion of the stable primary NbC. However, even when Nb is added in an amount exceeding 0.3 wt%, the effect does not increase any more. This is because Nb added in excess may be used to form coarse NbC rather than fine NbC which can fine grain.

In Examples 1 to 2 and 4 to 6 except for Example 3, it can be seen that C that is not combined with Nb, that is, the free carbon ratio is 80% or more. These high free carbon ratios also seem to contribute in part to strengthening strength and pitting resistance.

Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims. These changes and modifications may be made without departing from the scope of the present invention. Accordingly, the scope of the present invention should be determined by the following claims.

Claims (7)

(C): 0.18 to 0.3%, nitrogen (N): 0.25 to 0.3%, niobium (Nb): 0.1 To 0.29%, silicon (Si): more than 0% to 0.3%, and the balance of Fe and unavoidable impurities,
An elongation of 58% or more and a formal dislocation of 0.36 V _SCE or more.
The method according to claim 1,
Wherein the total content of carbon (C) and nitrogen (N) is 0.47 wt% or more.
The method according to claim 1,
Wherein the atomic number ratio (Nb / C) of the niobium and the carbon is 4 to 20. The austenitic stainless steel according to claim 1,
The method according to claim 1,
Wherein a ratio of free carbon (free C) not bonded to niobium in the carbon is 80% or more.
The method according to claim 1,
Wherein the austenitic stainless steel has an average crystal grain diameter of 10 to 16 占 퐉 and a product of a tensile strength and an elongation of 60,000 MPa% or more.
delete The method according to claim 1,
Wherein the austenitic stainless steel has a volume fraction of primary NbC of not more than 3 vol%.

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