KR20190074873A - Non-magnetic austenitic stainless steel improved in strength and surface conductivity - Google Patents

Abstract

Disclosed is a non-magnetic austenitic stainless steel with improved strength and surface conductivity. According to an embodiment of the present invention, the non-magnetic austenitic stainless steel with improved strength and surface conductivity comprises: 0.07-0.2 wt% of C; 0.15-0.4 wt% of N; 0.8-2 wt% of Si; 16-22 wt% of Mn; 0.01 wt% or less (excluding 0 wt%) of S; 12.5-20 wt% of Cr; 1-3 wt% of Cu; and the remainder consisting of Fe and other inevitable impurities. Moreover, the non-magnetic austenitic stainless steel with improved strength and surface conductivity satisfies a formula (1) represented by (1) Ni + 0.65Cr + 1.05Mn + 0.35Si + 12.6C + 33.6N >= 40 (Ni, Cr, Mn, Si, C, and N are wt% of each element).

Description

NON-MAGNETIC AUSTENITIC STAINLESS STEEL IMPROVED IN STRENGTH AND SURFACE CONDUCTIVITY WITH IMPROVED STRENGTH AND SURFACE CONDUCTION [0002]

More particularly, the present invention relates to a non-magnetic austenitic stainless steel having improved strength and surface conductivity applicable to environments requiring strength and surface conductivity as well as non-magnetic properties .

Recently, in accordance with various industry developments, materials for electronic parts require austenitic stainless steel having high strength and non-magnetic characteristics, high strength and non-magnetic characteristics, and excellent surface conductivity. Generally, a material for electronic parts contains a large amount of high-priced Ni, which raises a problem that raw material costs are increased.

The austenitic stainless steel represented by STS304 has good corrosion resistance and exhibits a non-magnetic austenite structure in the annealing heat treatment, and is used as a non-magnetic steel in various devices and devices. However, when deep drawing and press working are applied to STS304 steel, it is difficult to maintain non-magnetic properties due to phase transformation into sintered organic martensite structure, there is a problem.

Therefore, it is necessary to develop a new steel type which can maintain the strength and surface conductivity equal to or more than that of general austenitic stainless steel while lowering the Ni content.

The embodiments of the present invention solve the above problems and provide a non-magnetic austenitic stainless steel having improved strength and surface conductivity by controlling the content element without addition of Ni to control fired organic martensite and controlling the content of δ- Stainless steel.

According to the non-magnetic austenitic stainless steel having improved strength and surface conductivity according to an embodiment of the present invention, it is preferable that 0.07 to 0.2% of C, 0.15 to 0.4% of N, 0.8 to 2% of Si, Mn : 16 to 22%, S: not more than 0.01% (excluding 0), Cr: 12.5 to 20%, Cu: 1 to 3%, balance Fe and other unavoidable impurities.

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

Here, Ni, Cr, Mn, Si, C, and N are weight percentages of the respective elements.

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

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

Here, C, N, Cu, and Mn are weight percentages of the respective elements.

According to an embodiment of the present invention, the austenitic stainless steel may have a ferrite content of less than 0.1% measured after 70% cold working.

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.

According to an embodiment of the present invention, the austenitic stainless steel may have a stacking fault energy (SFE) expressed by the following formula (3): 41 mJ / m ^{2 or} more.

(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 percentages of the respective elements.

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

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 passive film.

Also, according to an embodiment of the present invention, the austenitic stainless steel may have a surface resistance of less than 10 m? Cm ² .

According to the disclosed embodiment, it is possible to provide a non-magnetic austenitic stainless steel having improved strength and surface conductivity by controlling content elements without addition of Ni to suppress sintered organic martensite and controlling δ-ferrite content during solidification.

Further, the non-magnetic austenitic stainless steel having improved strength and surface conductivity according to the disclosed embodiments can be applied to various non-magnetic parts used in various devices or devices.

In addition, according to the disclosed embodiments, it is possible to provide a non-magnetic austenitic stainless steel which is simple in the manufacturing process since an additional step of heat-treating the material for a long time is not necessary in order to remove magnetism by? -Ferrite.

1 is a graph showing a correlation between Ni equivalent and permeability.
2 is a graph showing the correlation between the Ni equivalent and the yield strength predicting formula.

The following embodiments are provided to fully convey the spirit of the present invention to a person having ordinary skill in the art to which the present invention belongs. The present invention is not limited to the embodiments shown herein but may be embodied in other forms. For the sake of clarity, the drawings are not drawn to scale, and the size of the elements may be slightly exaggerated to facilitate understanding.

Throughout the specification, when an element is referred to as "comprising ", it means that it can include other elements as well, without excluding other elements unless specifically stated otherwise.

The singular forms " a " include plural referents unless the context clearly dictates otherwise.

Hereinafter, the non-magnetic property can be secured even when the ferrite is produced by a conventional process without the need for an additional process for decomposing the δ-ferrite by controlling the content of δ-ferrite present in the microstructure of the steel, Non-magnetic austenitic stainless steels having improved strength and surface conductivity compared to stainless steels are described.

Specifically, the present invention provides an austenitic stainless steel which exhibits excellent non-magnetic properties only by controlling an alloy element system without addition of expensive Ni even without adding a heat treatment.

The austenitic stainless steel according to one aspect of the present invention comprises 0.07 to 0.2% of C, 0.15 to 0.4% of N, 0.8 to 2% of Si, 16 to 22% of Mn, 0.01 to 0.01% of S, (Excluding 0), Cr: 12.5 to 20%, Cu: 1 to 3%, the balance Fe and other unavoidable impurities, and satisfies the following formula (1).

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

Hereinafter, the reason for limiting the numerical value of the content of the component component in the embodiment of the present invention will be described. Unless otherwise stated, the unit is wt%.

The content of C is 0.07 to 0.2%.

Carbon (C) is a strong austenite phase stabilizing element, and it is preferable to add carbon (C) in an amount of 0.07% or more for increasing the strength of the material by solid solution strengthening. However, when the content is excessive, the Cr content of the grain boundaries is lowered by easily bonding with a carbide forming element such as Cr effective for corrosion resistance to lower the corrosion resistance, so that the upper limit can be limited to 0.2%.

The content of N is 0.15 to 0.4%.

Nitrogen (N) is a strong austenite phase stabilizing element, and it is preferably added in an amount of 0.15% or more in the present invention as an element which is essentially added in a steel to which Ni is not added. However, when the content is excessive, surface dislocations due to nitride precipitation and nitrogen pore are generated, and 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 deoxidization. When Ni is not added, it contributes to improvement in corrosion resistance, and it is preferable to add Si at a content of 0.8% or more. However, if the content is excessive, the mechanical properties related to the impact toughness are lowered, and the upper limit can be limited to 2%.

The content of Mn is 16 to 22%.

Manganese (Mn) is a core element which is essentially added to stabilize the austenite phase when no Ni is added, and it is preferable to add Mn of 16% or more. However, if the content is excessive, surface defects may occur, and the upper limit may be limited to 22%.

The content of S is 0.01% or less.

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

The Cr content is 12.5 to 20%.

Chromium (Cr) is the most element among the elements improving the corrosion resistance of stainless steel and is a basic element, and it is preferable to add at least 12.5% for the expression of corrosion resistance. However, Cr is a ferrite stabilizing element, and when the Cr content is increased, the ferrite fraction increases to inhibit the austenite stabilization, and the upper limit can be limited to 20%.

The content of Cu is 1 to 3%.

Copper (Cu) is an element added to the present invention such as Mn, which increases stability of the austenite phase and improves the corrosion resistance. In addition, Mn is added together to be dissolved in the passive film to increase the surface conductivity, Or more. However, when the content is excessive, the moldability is lowered, and the upper limit can be limited to 3%.

When nickel (Ni) is added in a very small amount, elution and formability are deteriorated, so that it is managed as an impurity in the present invention.

The remainder of the present invention is iron (Fe). However, in the ordinary manufacturing process, impurities which are not intended from the raw material or the surrounding environment may be inevitably incorporated, so that it can not be excluded. These impurities are not specifically mentioned in this specification, as they are known to any person skilled in the art of manufacturing.

Generally, the austenitic stainless steels used for electronic parts require processes such as sheet metal forming and deep drawing, and a deformed structure having a deformation amount of about 50% or more is formed in a finished product, and non-magnetic properties .

For materials for electronic components that use the non-magnetic properties of steel, the magnetic permeability (μ) of the steel applied to the part should be less than 1.005 for normal operation. To satisfy this requirement, the content of δ-ferrite formed during solidification of steel should be controlled.

Generally, δ-ferrite existing in the microstructure of the austenitic stainless steel is magnetized due to the characteristic of the structure having the body centered cubic structure (BCC), and the austenite is the face center structure Cubic Structure, FCC). Therefore, it is necessary to control the fraction of delta-ferrite to obtain a desired magnetic characteristic, and in the case of non-magnetic steel, it is necessary to minimize or eliminate the fraction of delta-ferrite.

In particular, by adding an austenite stabilizing element, it is possible to reduce the delta -f ferrite fraction. In general, the formation of delta-ferrite can be suppressed by controlling the Ni content, which is useful for stabilizing austenite without deteriorating other physical properties.

However, since Ni is a very expensive element, its use range may be limited. Therefore, the present inventors tried to secure the non-magnetic properties of austenitic stainless steel by controlling the content of Mn, Si, C, and N without addition of Ni. The non-magnetic property can be expressed by a Ni equivalent (Nieq) value indicating an austenite stabilization degree.

The Ni equivalent means the minimum Ni content that prevents? -Ferrite from being formed in a given composition system, and 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 weight percentages of the respective elements.

The present inventors have found that when the Ni equivalent value is 40 or more, the actual harsh forming portion is simulated and the ferrite content measured after 70% cold working satisfies 0.1% or less so that the permeability is 1.005 or less so that the non-magnetic property can be satisfied .

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

According to one embodiment of the present invention, the cold-rolled annealed sheet of the austenitic stainless steel can satisfy a yield strength of 450 MPa or more and a hardness (Hv) value of 215 or more expressed 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 weight percentages of the respective elements.

In materials for electronic parts, it is necessary to secure strength against various processing environments. In the present invention, the intensities of austenitic stainless steels were achieved by controlling the contents of C, N and Cu effective for increasing the yield strength without adding Ni.

The present inventors have found that the predicted yield strength including the C, N and Cu contents expressed by the formula (2) reflects the strength of the steel well. When the range of the formula (2) is 450 or more, .

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

Referring to FIG. 2, when the Ni equivalent value is 40 or more, the yield strength of the cold-rolled annealed sheet of the austenitic stainless steel satisfies 450 MPa or more.

According to one embodiment of the present invention, the austenitic stainless steel can satisfy the lamination defect energy represented by the following formula (3) to be 41 mJ / m ² or more.

(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

It is necessary to secure the ductility of the austenitic stainless steel in consideration of easiness of process such as plate material forming and deep drawing together with high strength.

The stacked defect energy (SFE, mJ / m ² ) on the austenite phase is known to control the austenite phase transformation mechanism. Typically, the stacked defect energy of austenitic phase represents the extent to which the plastic strain energy added externally contributes to the austenite phase transformation in the case of a single-phase austenitic stainless steel.

Generally, the lower the stacking defect energy, the greater the degree of formation of fired organic martensite phase that contributes to the work hardening of the steel after formation of the epsilon martensite phase on the austenite.

If the stacking fault energy is moderate, a mechanical twin is formed on the austenite. In the case of medium stacking fault energies, a fired organic martensite phase is formed at the intersection of these twinning, and the applied plastic strain energy causes a mechanical phase change, resulting in a transformation from the austenite to the martensite phase. Thus, stainless steels are known to form fired organic martensite phases in a very broad range, except for differences in the mesophase (epsilon martensite phase or mechanical twin). Therefore, when the stacking defect energy is less than 41 mJ / m ² , a fired organic martensite phase formed on the austenite on the epsilon martensite is formed, or a fired organic martensite phase is formed after the mechanical twinning is formed on the austenite.

However, it is known that when the stacking defect energy is 41 mJ / m ² or more, the transformation from the austenite to the martensite phase can not be performed because the deformation is progressed by dislocation movement without forming mechanical twinning or epsilon martensite phase.

The inventors of the present invention have confirmed that formation of a martensite phase after the plastic deformation is not observed when the stacking fault energy of the austenite phase represented by the formula (3) is 41 mJ / m ² or more and examined using a transmission electron microscope.

According to one 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 conductance is an important factor in austenitic stainless steels used for electronic components. In the present invention, it was confirmed that the content of Cu and Mn was controlled so that the surface resistance was found to be 10 m? Cm ² or less when the Cu + Mn content was 0.2% or more within the passive film thickness within 2 nm. It was found that Cu and Mn were partially substituted by a passive film composed of a Cr oxide layer, thereby increasing electron mobility and increasing surface conductivity.

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

Example

As shown in Table 1, stainless steel was produced through ingot casting of 50 kg while varying the content of each component of the steel. The ingot was heated at 1250 占 폚 for 3 hours and then hot rolled to produce a thermal laminate having a thickness of 4 mm. The hot rolled steel sheet was cold rolled to a final thickness of 2.5 mm, annealed at 1100 ° C for 30 seconds in the air, and pickled.

The yield strength (YS, Mpa) was measured by tensile test on the specimens produced by this method and compared with the predicted yield strength. The hardness (Hv) was also measured by 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 Invention river 2 0.095 0.32 1.01 18.1 0.0066 13.5 1.03 - 40.0825 Invention 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 Invention steel 5 0.093 0.36 0.82 18.1 0.0068 15.3 1.49 - 42.5048 Comparative River 1 0.06 0.04 0.4 1.5 0.007 18.2 0.12 8.1 23.745 Comparative River 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

The inventive and comparative steels according to Table 1 were used in the experiments.

The cold-rolled specimens were cold-rolled at a cold reduction of 70% in order to simulate the non-magnetic and surface resistance characteristics of molded parts of actual electronic components. The ferrite content (%) of the cold rolled sheet produced by using the ferrite scope equipment was measured, and the permeability was measured using a permeability meter (FERROMASTER).

In addition, Mn + Cu (wt.%) In the passive film at 2 nm from the surface portion of the cold rolled sheet was analyzed using a GDS (Glow Discharge Spectrometer) analyzer.

The surface resistance was measured by a DC 4-terminal method in which a gold-plated Cu-plate (area 2 cm ² ) was placed on the upper and lower surfaces of a cold-rolled plate and a pressure of 10 N / cm ^{2 was} applied . The surface resistance measurement standard was evaluated as being good when less than 10 m? Cm ^{2, and} less than 10 m? Cm ² being good.

The stacking fault energy (SFE), ferrite content, permeability, predicted value and actual value of hardness, Mn + Cu content at 2 nm of surface layer and surface resistance evaluation results of the austenitic stainless steels are shown in Table 2 below .

ferrite
content(%) Investment ratio YS (Mpa)
Prediction equation YS (Mpa)
Found Hardness
(Hv) SFE
(mJ / m ² ) Mn + Cu in the surface layer 2 nm
(weight %) Surface resistance
(m? cm 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 Over 50 5.2 293.41 262 160 28.4 0.001 35 (Poor) Comparative Example 2 0.26 1.1 368.795 369 196 33.8 0.002 33 (poor) Comparative Example 3 0.26 2.5 286.27 370 180 30.3 0.0001 45 (Poor)

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

Referring to FIGS. 1 and 2, it can be confirmed that the Nieq value expressed by the formula (1) satisfies 40 or more and the permeability is 1.005 or less as compared with the comparative examples in the examples, thereby satisfying the non-magnetic property .

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

Referring to FIGS. 1 and 2, the Nieq value expressed by Equation (1) satisfies 40 or more, and the yield strength is 450 MPa or more and the hardness is 215 Hv or more in comparison with the comparative examples Can be confirmed. Also, referring to Table 2, it can be seen that the difference between the predicted yield strength and the actual yield strength is extremely small in the inventive steels, and the equation (2) reflects the strength of the austenitic stainless steel well.

In addition, in the examples, the stacking defect energy (SFE) value is 41 mJ / m ² or more as compared with the comparative examples, so that formation of the martensite phase after plastic deformation is suppressed to secure ductility, The concentration of Cu and Mn was generated with a Cu + Mn content of 0.2% or more in the region within the region, and the surface resistance was measured to be 10 m? Cm ² or less. That is, it can be confirmed that the surface conductivity is improved.

In contrast, Comparative Example 1 contained 8.1% of Ni, but the Mn content was excessively low at 1.5%, and the Nieq value was less than 40%. Specifically, referring to Tables 1 and 2, in the case of Comparative Example 1, the Nieq value was 23.745, which was outside the range of the present invention, and the magnetic permeability was 5.2 and the high surface strength of 450 MPa or more and the desired surface conductivity could not be secured .

 Referring to Tables 1 and 2, in the case of Comparative Example 2, the C, Si, and Mn contents satisfy the range of the present invention, but the Nieq value is less than 40 at 38.45 and the permeability is 1.1, It is impossible to secure high strength of 450 MPa or more and desired surface conductivity.

Referring to Tables 1 and 2, even in the case of Comparative Example 3, the Nieq value is less than 40 at 30.38 and the permeability is 2.5, which means that the desired nonmagnetic property can not be secured and a high strength property of 450 MPa or more can be secured There was no.

In the case of Comparative Example 3, the Cu dissolved in the passive film was not added together with Mn, and the Cu + Mn content was 0.0001% in the region within 2 nm from the surface layer. Thus, the surface resistance was measured to be 45 mΩcm ² , Could not be secured.

The austenitic stainless steel according to one embodiment of the present invention controls the content element without addition of Ni to suppress the fired organic martensite and controls the δ-ferrite content during solidification to increase the strength and surface conductivity, .

While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited thereto. Those skilled in the art will readily obviate modifications and variations within the spirit and scope of the appended claims. It will be understood that various changes and modifications may be made therein without departing from the spirit and scope of the invention.

Claims (8)

0.1 to 0.4% of N, 0.8 to 2% of Si, 16 to 22% of Mn, 0.01% or less of S (excluding 0) of Cr, 12.5 to 20% of Cr, : 1 to 3%, balance Fe and other unavoidable impurities,
An austenitic stainless steel satisfying 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 weight percentages of each element). The method according to claim 1,
Austenitic stainless steel having 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
(Where C, N, Cu, and Mn are weight percentages of each element). The method according to claim 1,
An austenitic stainless steel having a ferrite content of not more than 0.1% measured after 70% cold working. The method according to claim 1,
Austenitic stainless steel having a permeability of 1.005 or less even in 70% cold working. The method according to claim 1,
Austenitic stainless steel having a stacking fault energy (SFE) of 41 mJ / m ² or more represented by the following formula (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
(Where Ni, Cu, Cr, N, Si, and Mn are weight percentages of each element). The method according to claim 1,
And a cold rolled steel material hardness (Hv) value of 215 or more. The method according to claim 1,
An austenitic stainless steel having a Cu + Mn content of 0.2% or more in a region within 2 nm of a passive film. 8. The method of claim 7,
An austenitic stainless steel having a surface resistance of less than 10 m? Cm ² .

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