JP6983321B2 - Austenitic stainless steel - Google Patents

Description

The present invention relates to non-magnetic austenitic stainless steel, and more specifically, non-magnetic austenitic stainless steel having improved strength and surface conductivity that can be applied to environments where strength and surface conductivity are required as well as non-magnetic properties. Regarding steel.

With the recent industrial development in various fields, austenitic stainless steel having excellent surface conductivity in addition to high strength and non-magnetic properties or high strength and non-magnetic properties is required as a material for electronic parts. Generally, a material for electronic parts contains a large amount of expensive Ni, so that there is a problem that the raw material cost rises.
Austenitic stainless steel typified by STS304 has good corrosion resistance, constitutes a non-magnetic austenitic structure by annealing heat treatment, and is used as non-magnetic steel in various devices and devices. However, processing may be performed depending on the application, and when dip drawing processing and press processing are applied to STS304 steel, it is difficult to maintain non-magnetic properties due to phase transformation to a plastic-induced martensite structure, and it is also difficult to maintain non-magnetic properties. Delayed cracks may occur.

Therefore, in order to supplement this, it is necessary to develop a new steel grade capable of ensuring strength and surface conductivity equal to or higher than that of general austenitic stainless steel while reducing the Ni content.

An object of the present invention is to solve the above-mentioned problems by controlling the constituent elements without adding Ni to suppress plastic-induced martensite, and controlling the δ-ferrite content during solidification to control the strength and surface. It is an object of the present invention to provide a non-magnetic austenitic stainless steel having improved conductivity.

The austenitic stainless steel of the present invention has C: 0.07 to 0.2%, N: 0.15 to 0.4%, Si: 0.8 to 2%, Mn: 16 to 22% in weight%. , S: 0.01% or less (excluding 0), Cr: 12.5 to 20%, Cu: 1-3%, balance Fe and other unavoidable impurities, and is characterized by satisfying the following formula (1). And.
(1) Ni + 0.65Cr + 1.05Mn + 0.35Si + 12.6C + 33.6N ≧ 40
Here, Ni, Cr, Mn, Si, C, and N are% by weight of each element.

The austenitic stainless steel preferably has a yield strength represented by the following formula (2) of 450 MPa or more.
(2) Yield strength (MPa) = 185 + 1977C + 605N + 3.65Cu-3.63Mn
Here, C, N, Cu, and Mn are% by weight of each element.
Further, the austenitic stainless steel preferably has a ferrite content of 0.1% or less as measured after 70% cold working.

The austenitic stainless steel can have a magnetic permeability of 1.005 or less even in 70% cold working.
Further, the austenitic stainless steel preferably has a laminated defect energy (SFE) represented by the following formula (3) of 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% by weight of each element.

The austenitic stainless steel can have a hardness (Hv) value of 215 or more for the cold-rolled material.
Further, the austenitic stainless steel preferably has a Cu + Mn content of 0.2% or more in the passivation film within 2 nm.
Further, the austenitic stainless steel preferably has a surface resistance of less than ^{10 mΩcm 2.}

According to the present invention, non-magnetic austenitic stainless steel with improved strength and surface conductivity by controlling the content element without adding Ni to suppress plastic-induced martensite and controlling the δ-ferrite content during solidification. Can be provided.
Further, the non-magnetic austenitic stainless steel having improved strength and surface conductivity of the present invention can be applied in various ways to non-magnetic parts used in various devices or devices.
Further, since the additional step of heat-treating the material for a long time is not required to remove the magnetism due to δ-ferrite, it is possible to manufacture a non-magnetic austenitic stainless steel having a simple manufacturing step.

It is a graph which shows the correlation of Ni equivalent and magnetic permeability. It is a graph which shows the correlation between the Ni equivalent and the prediction formula of a yield strength.

The non-magnetic austenitic stainless steel with improved strength and surface conductivity according to one embodiment of the present invention has a weight% of C: 0.07 to 0.2%, N: 0.15 to 0.4%, and 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 inevitable It is composed of impurities and is characterized by satisfying 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% by weight of each element.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. The following examples are presented in order to fully convey the idea of the present invention to a person having ordinary knowledge in the technical field to which the present invention belongs. The present invention is not limited to the examples presented here, and can be embodied in other embodiments. The drawings may omit parts unrelated to the description to clarify the present invention and may exaggerate the size of the components to aid understanding.
When any part of the specification "contains" a component, this does not exclude other components unless otherwise stated to be the opposite, but may further include other components. Means that you can.
A singular expression includes multiple expressions, unless there are explicit exceptions in the context.

In the following, the content of δ-ferrite present in the fine structure of steel is controlled, and no additional step for decomposing δ-ferrite is required. Therefore, non-magnetic properties are ensured even when manufactured by a normal step. A non-magnetic austenitic stainless steel having improved strength and surface conductivity as compared with the commonly used STS304 stainless steel will be described.

Specifically, the present invention can provide an austenitic stainless steel exhibiting excellent non-magnetic properties only by controlling the alloy element component system without adding expensive Ni, without going through an additional heat treatment step.
The austenitic stainless steel according to one aspect of the present invention has C: 0.07 to 0.2%, N: 0.15 to 0.4%, Si: 0.8 to 2%, Mn: 16 in weight%. ~ 22%, S: 0.01% or less (excluding 0), Cr: 12.5 ~ 20%, Cu: 1-3%, balance Fe and other unavoidable impurities, satisfying the following formula (1). ..
(1) 0.65Cr + 1.05Mn + 0.35Si + 12.6C + 33.6N ≧ 40
Here, Ni, Cr, Mn, Si, C, and N are% by weight of each element.

Hereinafter, the reason for limiting the numerical value of the alloy component content in the examples of the present invention will be described. In the following, unless otherwise specified, the unit is% by weight.
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 0.07% or more in order to increase the material strength by solid solution strengthening. However, if the content is too large, it easily binds to a carbide-forming element such as Cr, which is effective for corrosion resistance, and reduces the Cr content around the grain boundaries to reduce the corrosion resistance. Therefore, the upper limit is set to 0. Limited to 2%.

The content of N is 0.15 to 0.4%.
Nitrogen (N) is a strong austenite phase stabilizing element and is an element that is indispensably added to steel to which Ni is not added. In the present invention, it is preferable to add 0.15% or more. However, if the content is too high, surface defects due to nitride precipitation and nitrogen pores will occur, so the upper limit is 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 has an effect of contributing to improvement of corrosion resistance, so it is preferable to add 0.8% or more. However, if the content is too high, the upper limit is limited to 2% because it reduces the mechanical properties associated with impact toughness.

The content of Mn is 16 to 22%.
Manganese (Mn) is an important element that is essential for stabilizing the austenite phase when Ni is not added, and it is preferable to add 16% or more. However, if the content is too large, surface defects will occur, so the upper limit is limited to 22%.

The content of S is 0.01% or less.
Sulfur (S) forms MnS, which serves as a starting point for corrosion and reduces corrosion resistance, and is therefore limited to 0.01% or less.

The Cr content is 12.5 to 20%.
Chromium (Cr) is the most contained and basic element among the elements for improving the corrosion resistance of stainless steel, and it is preferable to add 12.5% or more in order to develop the corrosion resistance. However, Cr is a ferrite stabilizing element, and if the Cr content is high, the ferrite fraction increases and austenite stabilization is hindered, so the upper limit is limited to 20%.

The content of Cu is 1 to 3%.
Copper (Cu) is an element essentially added in the present invention, such as Mn, which not only increases the stability of the austenite phase and improves corrosion resistance, but is also added together with Mn in the passivation film. It is preferably added in an amount of 1% or more because it can be dissolved in a solid solution to increase surface conductivity. However, if the content is too large, the upper limit is limited to 3% in order to reduce moldability.

Nickel (Ni) is controlled as an impurity in the present invention because its elution and moldability deteriorate when a small amount is added.
The remaining component of the present invention is iron (Fe). However, in the normal manufacturing process, unintended impurities may be unavoidably mixed from the raw materials or the surrounding environment, and this cannot be excluded. Since these impurities can be known to any engineer in a normal manufacturing process, all the contents thereof are not specifically mentioned in the present specification.

Generally, austenitic stainless steel used for electronic parts requires processes such as plate forming and dip drawing, and in the finished molded product, a deformed structure having a deformation amount of about 50% or more is formed. Non-magnetic properties must be maintained even in such deformed parts.
In a material for an electronic component using the non-magnetic properties of steel, the magnetic permeability (μ) of the steel applied to the component must be 1.005 or less for normal operation. In order to satisfy this, the content of δ-ferrite formed during solidification of steel must be controlled.

In general, δ-ferrite present in the microstructure of austenitic stainless steel will exhibit magnetism due to the characteristics of the structure having a body-centered cubic structure (BCC), and austenite is a surface. It has a face center cubic structure (FCC) and does not exhibit magnetism. Therefore, it is possible to control the fraction of δ-ferrite to obtain the desired magnitude of magnetic properties, and in the case of non-magnetic steel, it is necessary to minimize or eliminate the fraction of δ-ferrite. Is.
In particular, the addition of an austenite stabilizing element can reduce the δ-ferrite fraction, but generally controls the Ni content useful for stabilizing austenite without degrading other physical characteristics. , Δ-Suppresses the formation of ferrite.

However, since Ni is a very expensive element, its range of use may be limited. Therefore, the present inventors aimed to secure the non-magnetic properties of austenitic stainless steel by controlling the contents of Mn, Si, C, and N without adding Ni. The non-magnetic property can be expressed by a Ni equivalent (Nieq) value indicating the degree of austenite stabilization.
The Ni equivalent (Nieq) means the minimum Ni content that prevents the formation of δ-ferrite in a given compositional component 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% by weight of each element.
The present inventors have a magnetic permeability of 1 when the Ni equivalent value is 40 or more and the ferrite content measured after 70% cold working by actually copying the severely molded portion satisfies 0.1% or less. Shown as .005 or less, it was discovered that the non-magnetic properties can be satisfied.

FIG. 1 is a graph showing the correlation between Ni equivalent and magnetic permeability. As shown in FIG. 1, it can be seen that when the Ni equivalent value is 40 or more, the magnetic permeability of the austenitic stainless steel satisfies 1.005 or less after 70% cold deformation.
According to one embodiment of the present invention, the cold-spread annealed plate of 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) Prediction formula for yield strength (Mpa) = 185 + 1977C + 605N + 3.65Cu-3.63Mn
Here, C, N, Cu, and Mn are% by weight of each element.

It is necessary to ensure the strength of materials for electronic components against various processing environments. The present invention realizes high strength of austenitic stainless steel by controlling the contents of C, N and Cu, which are effective for increasing the yield strength, without adding Ni.
The present inventors have noticed that the yield strength prediction formula including the C, N and Cu contents expressed by the formula (2) reflects the strength of the steel well, and the range of the formula (2) is expanded. It was discovered that when it is 450 or more, the desired strength can be secured.

FIG. 2 is a graph showing the correlation between the Ni equivalent and the prediction formula of the yield strength.
As shown in FIG. 2, it can be seen that when the Ni equivalent value is 40 or more, the yield strength of the cold-spread annealed plate of austenitic stainless steel satisfies 450 MPa or more.
According to one embodiment of the present invention, the austenitic stainless steel can satisfy the stacking defect energy represented by the following formula (3) of 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% by weight of each element.

In addition to high strength, it is necessary to ensure the softness of austenitic stainless steel in consideration of process easiness such as plate forming and dip drawing.
It is known that the stacking defect energy (SFE, mJ / m ² ) of the austenite phase controls the deformation mechanism of the austenite phase. Usually, the stacking defect energy of the austenite phase indicates the degree to which the externally applied plastic deformation energy contributes to the deformation of the austenitic phase in the case of a single-phase austenitic stainless steel.

In general, the lower the stacking defect energy, the greater the degree to which a plastic-induced martensite phase that contributes to work hardening of steel is formed in the austenite phase after the formation of the epsilon martensite phase.
When the stacking defect energy is in the middle, mechanical twins are formed in the austenite phase. In the case of intermediate stacking defect energy, a plastic-induced martensite phase is formed at the intersection of these twins, and the applied plastic deformation energy mechanically causes a phase change to transform from the austenite phase to the martensite phase. Wake up. Therefore, in the case of stainless steel, it is known that a plastic-induced martensite phase is formed in a very wide range except for the difference of the intermediate phase (epsilon martensite phase or mechanical twin). Therefore, when the stacking defect energy is ^{less than 41 mJ / m 2} , after the epsilon martensite phase is formed in the austenite phase, the plastic-induced martensite phase is formed, or after the mechanical twin crystals are formed in the austenite phase. , A plastic-induced martensite phase is formed.

However, when the stacking defect energy is 41 mJ / m ² or more, the deformation proceeds by potential transfer without the formation of mechanical twins or epsilon martensite phase, so that the transformation from the austenite phase to the martensite phase occurs. It is known to be suppressed.
When the stacking defect energy of the austenite phase represented by the formula (3) is 41 mJ / m ² or more, the present inventors investigated using a transmission electron microscope and found that the martensite phase was formed after plastic deformation. It was possible to confirm that it was not observed.

According to one embodiment of the present invention, the austenitic stainless steel preferably has 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 steels used in electronic component applications. In the present invention, the contents of Cu and Mn are controlled, and it is confirmed that the ^{surface resistance is 10 mΩcm 2} or less when the Cu + Mn content is 0.2% or more in the region where the thickness of the passivation film is within 2 nm. Was done. It is considered that this is because Cu and Mn are partially substituted and solid-solved in the passivation film composed of the Cr oxide layer, so that the electron mobility is increased and the surface conductivity is increased.

Hereinafter, it will be described in more detail through preferred embodiments of the present invention.
Example As shown in Table 1, the content of each component of the steel was changed to produce a stainless steel material through 50 kg ingot casting. After heating the ingot at 1250 ° C. for 3 hours, hot rolling was carried out to produce a hot-rolled material having a thickness of 4 mm. The hot-rolled material was cold-rolled to a final thickness of 2.5 mm, annealed in the air at 1100 ° C. for 30 seconds, and then pickled.
The yield strength (YS, Mpa) was measured for the test piece manufactured by such a method through a tensile test and compared with the prediction formula of the yield strength. In addition, the hardness (Hv) was measured through the Vickers hardness test.

The invention steels and comparative steels shown in Table 1 were used in the experiment.
The test piece subjected to 2.5 mm cold rolling is actually cold rolled at a cold rolling reduction rate of 70% in order to replicate the non-magnetic and surface resistance characteristics in the molded product of the electronic component material, and the thickness is increased. A 0.75 mm cold rolled plate material was manufactured. The ferrite content (%) of the cold-rolled plate material manufactured by utilizing the ferrite scope equipment was measured, and the magnetic permeability was measured by utilizing the magnetic permeability measuring equipment (FERROMASTER).
In addition, Mn + Cu (% by weight) in the passivation film at a point 2 nm from the surface layer of the cold-rolled plate was analyzed by utilizing the GDS (Glow Discharge Spectrometer) analysis equipment.

For the surface resistance, a Cu-plate (area 2 cm ² ) plated with gold is placed on the upper / lower surface of the cold-rolled plate material, a pressure of 10 N / cm ^{2 is} applied, and the resistance is measured by the DC4 terminal method to measure the surface. It is shown by the resistance value. The measurement standard of surface resistance was evaluated as good when it was less than ^{10 mΩcm 2 and insufficient when it was more than 10 mΩcm.}
Table 2 below shows the evaluation results of the laminated defect energy (SFE), ferrite content, magnetic permeability, expected and actual values of yield strength, hardness, Mn + Cu content and surface resistance at the surface layer 2 nm point for each component. It was shown to.

FIG. 1 is a graph showing the correlation between Ni equivalent and magnetic permeability.
As shown in FIGS. 1 and 2, in the case of the example, the Nieq value represented by the formula (1) satisfies 40 or more and the magnetic permeability is shown as 1.005 or less as compared with the comparative example. It can be confirmed that the non-magnetic properties are satisfied.

FIG. 2 is a graph showing the correlation between the Ni equivalent and the predicted value of the yield intensity (MPa).
As shown in FIGS. 1 and 2, in the case of the example, the Nieq value represented by the formula (1) satisfies 40 or more, the yield strength satisfies 450 MPa or more, and the hardness satisfies 215 Hv or more as compared with the comparative example. Can be confirmed. Further, as shown in Table 2, in the case of the invention steel, the difference between the yield strength prediction formula and the measured yield strength value is extremely small, and the formula (2) well reflects the strength of the austenitic stainless steel. I understand.

Further, in the case of the example, the stacking defect energy (SFE) value is ^{shown to be 41 mJ / m 2} or more as compared with the comparative example, and the formation of the martensite phase can be suppressed after the plastic deformation to secure the softness. The Cu + Mn content was 0.2% or more in the region within 2 nm from the surface layer, and the concentration of Cu and Mn occurred, and the surface resistance was measured to be 10 ^{mΩcm 2 or less.} That is, it was confirmed that the surface conductivity was improved.
In comparison with this, Comparative Example 1 contained 8.1% of Ni, but the Mn content was extremely low at 1.5%, and the Nieq value did not reach 40. Specifically, as shown in Tables 1 and 2, in the case of Comparative Example 1, the Nieq value is 23.745, which is outside the scope of the present invention, the magnetic permeability is 5.2, and the magnetism is high. Not only that, high strength of 450 MPa or more and the desired surface conductivity could not be ensured.

As shown in 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 38.45 and does not reach 40. The magnetic permeability was 1.1, the desired non-magnetic properties could not be secured, and the high strength of 450 MPa or more and the desired surface conductivity could not be secured.
As shown in Tables 1 and 2, even in the case of Comparative Example 3, the Nieq value was 30.38, did not reach 40, the magnetic permeability was 2.5, and the desired non-magnetic characteristics were secured. It was not possible to secure high strength characteristics of 450 MPa or more.
Further, in the case of Comparative Example 3, Cu to be solid-solved in the passivation 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, and the surface resistance was accompanied by this. Was measured to be 45 mΩcm ^2, and the desired surface conductivity could not be secured.

The austenitic stainless steel according to one embodiment of the present invention controls the content element without adding Ni to suppress plastic-induced martensite, and controls the δ-ferrite content during solidification to improve strength and surface conductivity. However, non-magnetic properties can be ensured.

Although the exemplary embodiments of the present invention have been described above, the present invention is not limited to this, and any person having ordinary knowledge in the relevant technical field does not deviate from the concept and scope of the claims. It can be understood that various changes and variations are possible within the range.

The non-magnetic austenitic stainless steel having improved surface conductivity according to the embodiment of the present invention can be applied as a material for electronic parts.

Claims (7)

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 ( An austenitic stainless steel comprising 0), Cr: 12.5 to 20%, Cu: 1-3%, balance Fe and other unavoidable impurities, and satisfying 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% by weight of each element. The austenitic stainless steel according to claim 1, wherein the yield strength represented by the following formula (2) is 450 MPa or more.
(2) Yield strength (MPa) = 185 + 1977C + 605N + 3.65Cu-3.63Mn
Here, C, N, Cu, and Mn are% by weight of each element. The austenitic stainless steel according to claim 1, wherein the ferrite content measured after 70% cold working is 0.1% or less. The austenitic stainless steel according to claim 1, wherein the magnetic permeability is 1.005 or less even in 70% cold working. The austenitic stainless steel according to claim 1, wherein the cold rolled material has a hardness (Hv) value of 215 or more. The austenitic stainless steel according to claim 1, wherein the Cu + Mn content in the passivation film within 2 nm is 0.2% or more. The austenitic stainless steel according to claim 1 , wherein the surface resistance is ^{less than 10 mΩcm 2.}

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