KR102020507B1 - Non-magnetic austenitic stainless steel improved in strength and surface conductivity - Google Patents

Abstract

A nonmagnetic austenitic stainless steel having improved strength and surface conductivity is disclosed. According to the austenitic stainless steel according to an embodiment of the present invention, in weight%, C: 0.07 to 0.2%, N: 0.15 to 0.4%, Si: 0.8 to 2%, Mn: 16 to 22%, S: 0.01% or less (excluding 0), Cr: 12.5 to 20%, Cu: 1 to 3%, balance Fe and other unavoidable impurities, and satisfy the following formula (1).
(1) Ni + 0.65Cr + 1.05Mn + 0.35Si + 12.6C + 33.6N ≥ 40
Where Ni, Cr, Mn, Si, C, and N are the weight percentages of each element.

Description

Non-magnetic austenitic stainless steel with improved strength and surface conductivity {NON-MAGNETIC AUSTENITIC STAINLESS STEEL IMPROVED IN STRENGTH AND SURFACE CONDUCTIVITY}

The present invention relates to a nonmagnetic austenitic stainless steel, and more particularly, to a nonmagnetic austenitic stainless steel having improved strength and surface conductivity, which can be applied to an environment requiring strength and surface conductivity as well as nonmagnetic austenitic stainless steel. .

Recently, according to industrial development in various fields, materials for electronic components require austenitic stainless steel having excellent surface conductivity in addition to high strength and nonmagnetic properties or high strength and nonmagnetic properties. In general, a material for an electronic component contains a large amount of expensive Ni, resulting in a high raw material cost.

The austenitic stainless steels represented by STS304 have good corrosion resistance, exhibit nonmagnetic austenitic structure in the annealing heat treatment state, and are used in various devices and devices as nonmagnetic steels. However, depending on the application, there are cases where processing is performed, and when deep drawing and press working are applied to STS304 steel, it is difficult to maintain nonmagnetic properties due to phase transformation into plastic organic martensite structure, and delay cracking occurs. there is a problem.

Therefore, in order to compensate for this, it is necessary to develop a new steel grade that can secure the strength and surface conductivity that is equal to or higher than that of general austenitic stainless steel while lowering the Ni content.

Embodiments of the present invention solve the problems described above, by controlling the content element without adding Ni, suppressing the plastic organic martensite, and controlling the δ-ferrite content during solidification, non-magnetic austenitic system improved in strength, surface conductivity It is intended to provide stainless steel.

According to the non-magnetic austenitic stainless steel with improved strength and surface conductivity according to an embodiment of the present invention, in weight%, C: 0.07 to 0.2%, N: 0.15 to 0.4%, Si: 0.8 to 2%, Mn : 16 to 22%, S: 0.01% or less (excluding 0), Cr: 12.5 to 20%, Cu: 1 to 3%, balance Fe and other unavoidable impurities, and satisfy the following formula (1).

(1) Ni + 0.65Cr + 1.05Mn + 0.35Si + 12.6C + 33.6N ≥ 40

Here, Ni, Cr, Mn, Si, C, and N are the weight% of each element.

In addition, according to an embodiment of the present invention, the austenitic stainless steel may have a yield strength of 450 MPa or more represented by the following formula (2).

(2) Yield strength (MPa) = 185 + 1977C + 605N + 3.65Cu-3.63Mn

Here, C, N, Cu and Mn are the weight% of each element.

In addition, according to an embodiment of the present invention, the austenitic stainless steel may have a ferrite content of 0.1% or less after cold processing of 70%.

In addition, according to an embodiment of the present invention, the austenitic stainless steel may have a permeability of 1.005 or less even in 70% cold working.

In addition, according to an embodiment of the present invention, the austenitic stainless steel may have a stacking defect energy (SFE) of 41 mJ / m ^{2 or} more represented by the following Equation (3).

(3) SFE (mJ / m 2) = 25.7 + 1.59 (Ni + Cu) -0.85Cr + 0.001Cr 2 + 38.2N 0 .5 -2.8Si-1.34Mn + 0.06Mn 2

Here, Ni, Cu, Cr, N, Si, and Mn are weight% of each element.

In addition, according to an embodiment of the present invention, the austenitic stainless steel may have a cold rolled material hardness (Hv) value of 215 or more.

In addition, according to an embodiment of the present invention, the austenitic stainless steel may have a Cu + Mn content of 0.2% or more in a region within 2 nm of the passivation film.

In addition, according to an embodiment of the present invention, the austenitic stainless steel may have a surface resistance of less than 10 mΩcm ² .

According to the disclosed embodiment it is possible to provide a non-magnetic austenitic stainless steel with improved strength, surface conductivity by controlling the content element without Ni content to suppress the plastic organic martensite, and control the δ-ferrite content during solidification.

In addition, the non-magnetic austenitic stainless steel having improved strength and surface conductivity according to the disclosed embodiments may be variously used for non-magnetic parts used in various devices or devices.

In addition, according to the disclosed embodiment there is no need for an additional process of heat-treating the material for a long time in order to remove the magnetism by δ-ferrite, it is possible to provide a non-magnetic austenitic stainless steel with a simple manufacturing process.

1 is a graph showing the correlation between Ni equivalent weight and permeability.
2 is a graph showing the correlation between Ni equivalent weight and yield strength prediction equation.

The following embodiments are presented to sufficiently convey the spirit of the present invention to those skilled in the art. The present invention is not limited to the embodiments presented herein but may be embodied in other forms. The drawings may omit illustrations of parts not related to the description in order to clarify the present invention, and may be exaggerated to some extent in order to facilitate understanding.

Throughout the specification, when a part is said to "include" a certain component, it means that it can further include other components, without excluding other components unless specifically stated otherwise.

Singular expressions include plural expressions unless the context clearly indicates an exception.

Hereinafter, by controlling the amount of δ-ferrite present in the microstructure of the steel, there is no need for an additional process for decomposing δ-ferrite, even if manufactured in a conventional process, it is possible to secure nonmagnetic properties, and the commonly used STS304 system A nonmagnetic austenitic stainless steel having improved strength and surface conductivity compared to stainless steel is described.

Specifically, the present invention provides an austenitic stainless steel that exhibits excellent nonmagnetic properties only by controlling alloy element components even without expensive Ni addition without undergoing an additional heat treatment.

Austenitic stainless steel according to an aspect of the present invention, in weight percent, C: 0.07 to 0.2%, N: 0.15 to 0.4%, Si: 0.8 to 2%, Mn: 16 to 22%, S: 0.01% Or less (excluding 0), Cr: 12.5 to 20%, Cu: 1 to 3%, balance Fe, and other unavoidable impurities, and satisfy the following formula (1).

(1) 0.65Cr + 1.05Mn + 0.35Si + 12.6C + 33.6N ≥ 40

Hereinafter, the reason for the numerical limitation of the content of the alloying component in the embodiment of the present invention will be described. In the following, the unit is% by weight unless otherwise specified.

The content of C is 0.07 to 0.2%.

Carbon (C) is a strong austenitic stabilizing element and is preferably added at least 0.07% in order to increase the material strength by solid solution strengthening. However, when the content is excessive, the upper limit can be limited to 0.2% by easily combining with a carbide forming element such as Cr, which is effective for corrosion resistance, thereby lowering the Cr content around the grain boundary and reducing the corrosion resistance.

The content of N is 0.15 to 0.4%.

Nitrogen (N) is a strong austenite phase stabilizing element, and is an element which is essentially added in steel without adding Ni, and preferably 0.15% or more in the present invention. However, if the content is excessive, the occurrence of surface defects due to nitride precipitation and nitrogen pores, the upper limit can be limited to 0.4%.

The content of Si is 0.8 to 2%.

Silicon (Si) is an element useful for deoxidation, and when Ni is not added, it is effective to improve corrosion resistance, and therefore it is preferably added at 0.8% or more. However, if the content is excessive, the mechanical properties related to impact toughness are reduced, and the upper limit thereof may be limited to 2%.

The content of Mn is 16 to 22%.

Manganese (Mn) is a key element that is essential for stabilizing austenite phase when Ni is not added. It is preferable to add at least 16%. However, if the content is excessive, the surface defect occurs, the upper limit can be limited to 22%.

The content of S is 0.01% or less.

Sulfur (S) forms MnS, and this MnS is the starting point of corrosion and reduces the corrosion resistance, it is preferable to limit to 0.01% or less.

The content of Cr is 12.5 to 20%.

Chromium (Cr) is the most basic element contained and contained among the corrosion resistance improving elements of stainless steel, and it is preferable to add 12.5% or more in order to express corrosion resistance. However, Cr is a ferrite stabilizing element, and as the Cr content increases, the ferrite fraction increases to inhibit austenite stabilization, so the upper limit thereof may be limited to 20%.

The content of Cu is 1-3%.

Copper (Cu) is an element essential in the present invention, such as Mn, increases austenite stability and improves corrosion resistance, as well as Mn is added together to increase the surface conductivity by solid solution in the passivation film. It is preferable to add more. However, when the content is excessive, the moldability is rather lowered, and the upper limit thereof may be limited to 3%.

Nickel (Ni) is rather reduced in elution and moldability when added in a small amount, and is managed as an impurity in the present invention.

The remaining component of the present invention is iron (Fe). However, in the conventional manufacturing process, impurities which are not intended from the raw material or the surrounding environment may be inevitably mixed, and thus cannot be excluded. Since these impurities are known to those skilled in the art, all of them are not specifically mentioned in the present specification.

In general, austenitic stainless steels used for electronic parts require plate forming, deep drawing, and the like, and a molded product having a deformation amount of about 50% or more is formed in the finished product, and non-magnetic properties are maintained even in such deformation parts. Should be.

In materials for electronic parts using the nonmagnetic properties of steel, the magnetic permeability (μ) of the steel applied to the part must be less than 1.005 for normal operation. In order to satisfy this, it is necessary to control the content of δ-ferrite formed during solidification of the steel.

In general, δ-ferrite in the microstructure of austenitic stainless steel becomes magnetic due to the characteristics of the structure having a body centered cubic structure (BCC), and austenite has a face center cubic structure (Face Center). Cubic Structure (FCC) will not be magnetic. Therefore, the magnetic properties of the desired size can be obtained by controlling the fraction of δ-ferrite, and in the case of nonmagnetic steel, it is necessary to lower or eliminate the fraction of δ-ferrite as much as possible.

In particular, by adding an austenite stabilizing element, the δ-ferrite fraction can be reduced, and generally, the formation of δ-ferrite can be suppressed by controlling the Ni content useful for stabilizing austenite without deteriorating other physical properties.

However, since Ni is an extremely expensive element, its use range may be limited. Therefore, the present inventors attempted to secure the nonmagnetic properties of the austenitic stainless steel by controlling the contents of Mn, Si, C, and N without adding Ni. The nonmagnetic property can be expressed as a Ni equivalent (Nieq) value representing austenite stabilization.

Ni equivalent refers to the minimum Ni content to prevent the δ-ferrite is formed in a given composition system, it can be expressed as follows.

Nieq = Ni + 0.65Cr + 1.05Mn + 0.35Si + 12.6C + 33.6N

Here, Ni, Cr, Mn, Si, C, and N are the weight% of each element.

The present inventors have found that when the Ni equivalent value is 40 or more, the ferrite content is 0.105 or less to satisfy the nonmagnetic properties when the ferrite content measured after 70% cold working is simulated by the actual harsh molded part to satisfy 0.1% or less. .

1 is a graph showing the magnetic permeability correlation according to Nieq. Referring to FIG. 1, when the Ni equivalent value is 40 or more, the permeability after 70% cold deformation of the austenitic stainless steel satisfies 1.005 or less.

According to one embodiment of the present invention, the cold-rolled annealing plate of the austenitic stainless steel can satisfy the yield strength of 450 Mpa or more and the hardness (Hv) value of 215 or more represented by the following formula (2).

(2) Yield strength (Mpa) prediction formula = 185 + 1977C + 605N + 3.65Cu-3.63Mn

Here, C, N, Cu and Mn are the weight% of each element.

In electronic component materials, it is necessary to secure strength for various processing environments. In the present invention, it is intended to realize the high strength of the austenitic stainless steel by controlling the content of C, N and Cu, which is effective in increasing yield strength, without adding Ni.

The inventors have found that the yield strength prediction formula including C, N and Cu contents, represented by Equation (2), well reflects the strength of the steel, and when the range of Equation (2) is 450 or more, the desired strength is secured. I found it possible.

2 is a graph showing the yield strength (MPa) correlation according to Nieq.

2, it can be seen that the yield strength of the cold rolled annealing plate of the austenitic stainless steel satisfies 450 Mpa or more when the Ni equivalent value is 40 or more.

According to an embodiment of the present invention, the austenitic stainless steel may satisfy a stacking defect energy of 41 mJ / m ² or more represented by the following Equation (3).

(3) SFE (mJ / m 2) = 25.7 + 1.59 (Ni + Cu) - 0.85Cr + 0.001Cr 2 + 38.2N 0 .5 - 2.8Si - 1.34Mn + 0.06Mn 2

In addition to high strength, it is necessary to secure the ductility of the austenitic stainless steel in consideration of process ease such as sheet forming and deep drawing.

The stacking defect energy (SFE, mJ / m ² ) of the austenite phase is known to control the deformation mechanism of the austenite phase. Usually, the stacking defect energy of the austenite phase represents the degree to which the plastic deformation energy added from the outside contributes to the deformation of the austenite phase in the case of the single phase austenitic stainless steel.

In general, the lower the lamination defect energy, the higher the degree of formation of the calcined organic martensite phase which forms the epsilon martensite phase in the austenite phase and contributes to the work hardening of the steel.

If the lamination defect energy is moderate, mechanical twins are formed on the austenite. In the case of moderate lamination defect energy, a plastic organic martensite phase is formed at the intersection of these twins, and the applied plastic strain energy mechanically causes a phase change, resulting in a transformation from the austenite phase to the martensite phase. Thus, in the case of stainless steel, it is known that a calcined organic martensite phase is formed except for the difference between the intermediate phase (epsilon martensite phase or mechanical twin) in a fairly wide range. Accordingly, when the lamination defect energy is less than 41 mJ / m ² , the calcined organic martensite phase is formed after the epsilon martensite phase is formed on the austenite phase, or the calcined organic martensite phase is formed after the mechanical twins are formed on the austenite phase.

However, when the lamination defect energy is 41 mJ / m ² or more, it is known that transformation from austenite phase to martensite phase is difficult because deformation proceeds by dislocation movement without formation of mechanical twin or epsilon martensite phase.

The inventors of the present invention confirmed that the lattice defect energy of the austenite phase represented by the formula (3) was 41 mJ / m ² or more, using a transmission electron microscope. As a result, the formation of the martensite phase was not observed after plastic deformation.

According to an embodiment of the present invention, the austenitic stainless steel may have a Cu + Mn content of 0.2% or more in a region within 2 nm from the surface layer.

Surface conductivity is an important factor in austenitic stainless steel used for electronic parts. In the present invention, by controlling the content of Cu and Mn, it was confirmed that the surface resistance is less than 10mΩcm ² when the Cu + Mn content is 0.2% or more in the region within 2nm thickness of the passivation film. This suggests that the partial mobility of Cu and Mn in the passivation film composed of the Cr oxide layer increases the electron mobility, thereby increasing the surface conductivity.

Hereinafter, the preferred embodiment of the present invention will be described in more detail.

Example

As shown in Table 1, stainless steels were produced through 50 kg ingot casting while changing the content of each component of the steel. After heating the ingot at 1250 ° C. for 3 hours, hot rolling was performed to produce a 4 mm thick hot rolled material. The hot rolled material was cold rolled, processed to a final thickness of 2.5 mm, annealed in the air at 1100 ° C. for 30 seconds, and then pickled.

Yield strengths (YS, Mpa) were measured through tensile tests on specimens prepared in this way and compared with the yield strength prediction equation. In addition, hardness (Hv) was measured through the Vickers hardness test.

C N Si Mn S Cr Cu Ni Ni eq Inventive Steel 1 0.095 0.3 1.16 18 0.0075 14.9 2.04 - 40.268 Inventive Steel 2 0.095 0.32 1.01 18.1 0.0066 13.5 1.03 - 40.0825 Inventive Steel 3 0.095 0.36 1.05 17.9 0.0068 13.1 1.03 - 40.9705 Inventive Steel 4 0.092 0.31 1.19 20 0.0067 17.6 2.08 - 44.4317 Inventive Steel 5 0.093 0.36 0.82 18.1 0.0068 15.3 1.49 - 42.5048 Comparative Steel 1 0.06 0.04 0.4 1.5 0.007 18.2 0.12 8.1 23.745 Comparative Steel 2 0.1 0.1 0.8 17 0.008 18 0.5 4 38.45 Comparative Steel 3 0.05 0.1 0.4 10 0.009 15 - 6 30.38

Invented steels and comparative steels according to Table 1 were used in the experiment.

2.5mm cold rolled specimens were cold rolled at 70% cold rolling rate to simulate the nonmagnetic and surface resistance characteristics of the molded parts of actual electronic parts. The ferrite content (%) of the cold rolled sheet manufactured using the ferrite scope equipment was measured, and the permeability was measured using the permeability measuring equipment (FERROMASTER).

In addition, using a GDS (Glow Discharge Spectrometer) analysis equipment was analyzed Mn + Cu (% by weight) in the passivation film at the 2nm point from the surface layer of the cold-rolled sheet material.

For surface resistance, the gold-plated Cu-plate (area 2cm ² ) was placed on the upper and lower surfaces of the cold rolled sheet, and the pressure was applied by applying 10N / cm ² to the DC 4 terminal method. . Surface resistance measurement criteria were evaluated that less than 10mΩcm ² was good, more than that.

The evaluation results of lamination defect energy (SFE), ferrite content, permeability, yield strength and actual value, hardness, Mn + Cu content and surface resistance of the austenitic stainless steel in each component are shown in Table 2 below. .

ferrite
content(%) Permeability YS (Mpa)
Prediction YS (Mpa)
Found Hardness
(Hv) SFE
(mJ / m ² ) Mn + Cu at 2nm of surface layer
(weight %) Surface resistance
(mΩcm ² ) Example 1 0 1.002 496.421 496 238 42.5 1.2 4.8 (good) Example 2 0 1.001 504.4715 498 234 43.3 0.8 6 (good) Example 3 0 1.001 529.3975 529 232 44.4 0.7 5.9 (good) Example 4 0 1.001 489.426 489 219 45.5 1.1 5 (good) Example 5 0.08 1.004 526.3965 526 229 44.4 0.5 6.8 (good) Comparative Example 1 50 or more 5.2 293.41 262 160 28.4 0.001 35 (not sufficient) Comparative Example 2 0.26 1.1 368.795 369 196 33.8 0.002 33 (Low) Comparative Example 3 0.26 2.5 286.27 370 180 30.3 0.0001 45 (Low)

1 is a graph showing the magnetic permeability correlation according to Nieq.

Referring to FIG. 1 and Table 2, the Nieq value expressed by Equation (1) satisfies 40 or more, and the permeability is 1.005 or less, compared to the comparative examples, and it can be confirmed that the nonmagnetic properties are satisfied. .

2 is a graph showing the yield strength (MPa) correlation according to Nieq.

1 and Table 2, the Nieq value represented by Equation (1) compared to the comparative examples in the case of the embodiment satisfies 40 or more, yield strength is 450 MPa or more, hardness is 215 Hv or more You can check it. In addition, referring to Table 2, it can be seen that, in the case of the inventive steels, the difference between the predicted yield strength yield value and the yield strength measured value is extremely small, and Equation (2) well reflects the strength of the austenitic stainless steel.

In addition, the stacking defect energy (SFE) value of 41 mJ / m ² or more compared to the comparative examples in the case of the embodiment can not only secure the ductility by inhibiting the formation of martensite phase after plastic deformation, but also 2nm from the surface layer The Cu + Mn content of 0.2% or more in the region within the concentration of Cu and Mn occurred, the surface resistance was measured to 10mΩcm ² or less. That is, it can be confirmed that the surface conductivity is improved.

On the contrary, in Comparative Example 1, Ni contained 8.1%, but the Mn content was excessively low as 1.5%, and the Nieq value was less than 40. Specifically, referring to Table 1 and Table 2, in the case of Comparative Example 1 Nieq value is 23.745 outside the scope of the present invention, magnetic permeability is 5.2, not only could not secure high strength and the desired surface conductivity of 450 MPa or more .

 Referring to Table 1 and Table 2, in the case of Comparative Example 2, the content of C, Si, Mn satisfies the scope of the present invention, but the Nieq value is 38.45, which is less than 40, and the magnetic permeability is 1.1 to achieve the desired nonmagnetic properties. Not only could not be secured, but also high strength of 450 MPa or more and desired surface conductivity could not be secured.

Referring to Table 1 and Table 2, even in the case of Comparative Example 3 Nieq value is 30.38, less than 40, the permeability is 2.5, not only can not secure the desired nonmagnetic properties, but also high strength properties of 450 MPa or more can be secured There was no.

In addition, in the case of Comparative Example 3, Cu and Mn dissolved in the passivation film were not added together with Mn, and the content of Cu + Mn was 0.0001% in the region within 2 nm from the surface layer. Accordingly, the surface resistance was measured to be 45 mΩcm ² , thereby providing the desired surface conductivity. Could not be secured.

Austenitic stainless steel according to an embodiment of the present invention is to control the content element without adding Ni to suppress the plastic organic martensite, and to control the δ-ferrite content during solidification to increase the strength, surface conductivity and non-magnetic properties It can be secured.

As described above, the exemplary embodiments of the present invention have been described, but the present invention is not limited thereto, and a person skilled in the art does not depart from the spirit and scope of the following claims. It will be understood that various changes and modifications are possible in the following.

Claims (8)

By weight, C: 0.07 to 0.2%, N: 0.15 to 0.4%, Si: 0.8 to 2%, Mn: 16 to 22%, S: 0.01% or less (excluding 0), Cr: 12.5 to 20%, Cu : 1 to 3%, balance Fe and other unavoidable impurities,
Satisfy the following formula (1),
Austenitic stainless steel having a lamination defect energy (SFE) of 41 mJ / m ² or more represented by the following formula (3).
(1) Ni + 0.65Cr + 1.05Mn + 0.35Si + 12.6C + 33.6N ≥ 40
Where Ni, Cr, Mn, Si, C, and N are the weight percentages of each element.
(3) SFE (mJ / m ² ) = 25.7 + 1.59 (Ni + Cu) -0.85Cr + 0.001Cr ² + 38.2N ^0.5 -2.8Si-1.34Mn + 0.06Mn ²
Where Ni, Cu, Cr, N, Si, and Mn are the weight percent of each element. The method of claim 1,
The austenitic stainless steel whose yield strength represented by following formula (2) is 450 Mpa or more.
(2) Yield strength (MPa) = 185 + 1977C + 605N + 3.65Cu-3.63Mn
(Where C, N, Cu, and Mn are the weight percent of each element.) The method of claim 1,
Austenitic stainless steel with a ferrite content of 0.1% or less after 70% cold working. The method of claim 1,
Austenitic stainless steel with a permeability of 1.005 or less, even at 70% cold working. delete The method of claim 1,
Austenitic stainless steel, characterized in that the cold-rolled material hardness (Hv) value of 215 or more. The method of claim 1,
Austenitic stainless steel with a Cu + Mn content of 0.2% or more in the region within 2 nm of the passivation film. The method of claim 7, wherein
Austenitic stainless steel with a surface resistance of less than 10 mΩcm ² .

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