Super duplex stainless steel and manufacturing method therefor, and nickel-plated super duplex stainless steel and manufacturing method therefor

Abstract

The present invention relates to a super duplex stainless steel and a manufacturing method therefor, and a nickel-plated super duplex stainless steel and a manufacturing method therefor. Particularly, the present invention relates to a super duplex stainless steel and a manufacturing method therefor, and a nickel-plated super duplex stainless steel and a manufacturing method therefor, in which a super duplex stainless steel having excellent abrasion resistance and strength and a ferrite phase fraction of 70% or more is provided, and an electroless nickel plating process is performed on the super duplex stainless steel to plate same with a nickel plating layer, thereby providing a nickel-plated super duplex stainless steel with excellent corrosion resistance and electrochemical properties.

Description

Super duplex stainless steel and method for manufacturing the same, nickel-plated super duplex stainless steel and method for manufacturing the same

[0001] The present invention relates to super duplex stainless steel and a method for producing the same, and nickel-plated super duplex stainless steel and a method for producing the same, and more particularly, to providing super duplex stainless steel having a fraction of a ferrite phase of 70% or more, which is excellent in wear resistance and strength, and performing an electroless nickel plating process thereon to plate it with a nickel plating layer, thereby providing super duplex stainless steel having excellent corrosion resistance and electrochemical properties, and a method for producing the same, and nickel-plated super duplex stainless steel and a method for producing the same.

[0002]

[0003] Stainless steel is an alloy widely used in industrial fields due to its excellent corrosion resistance and mechanical properties, and is classified into four types: austenitic (high corrosion resistance), ferritic (high temperature strength), martensitic (high strength), and duplex (high corrosion resistance and strength).

[0004] Duplex stainless steel is an alloy (dual phase) composed of austenite and ferrite in a volume ratio of nearly 1:1, and has high corrosion resistance against localized corrosion and stress corrosion cracking, as well as excellent mechanical properties (ultimate tensile strength: 780 MPa, elongation: 30%). When the volume ratio of duplex stainless steel is 1:1, the chemical composition of austenite and ferrite has the best conditions for preventing pitting corrosion, and the localized corrosion resistance is evaluated based on the PREN index (Pitting Resistance Equivalent Number, PREN = wt.% Cr + 3.3 wt.% Mo + 16 wt.% N). The PREN index quantifies the resistance to localized corrosion in a chloride-containing environment, with a higher number indicating a higher resistance to localized corrosion.

[0005] Duplex stainless steels are classified into four grades (Lean, Standard, Super, and Hyper) based on their PREN index. A PREN index exceeding 40 is considered excellent corrosion resistance, and super duplex stainless steel (SDSS) has a PREN index of 40 or higher.

[0006] The application of materials with excellent properties, such as super duplex stainless steel, is becoming increasingly important, and research on them is necessary.

[0007]

[0008] The technical problem to be solved by the present invention is to provide a super duplex stainless steel having excellent wear resistance and strength and a method for manufacturing the same.

[0009] The technical problem to be achieved by the present invention is to provide a nickel-plated super duplex stainless steel having excellent corrosion resistance and electrochemical properties and a method for manufacturing the same by performing an electroless nickel plating process on super duplex stainless steel having excellent wear resistance and strength to form a nickel plating layer.

[0010] However, the problems to be solved by the present invention are not limited to the problems mentioned above, and other problems not mentioned can be clearly understood by those skilled in the art from the description below.

[0011]

[0012] One embodiment of the present invention for achieving the above-described purpose provides a super duplex stainless steel having a fraction of ferrite phase of 70% or more.

[0013] One embodiment of the present invention provides a method for manufacturing a super duplex stainless steel having a ferrite phase fraction of 70% or more, comprising: a step of casting a super duplex stainless steel; a homogenization heat treatment step of heat treating the cast super duplex stainless steel to homogenize austenite and ferrite into a uniform particle form; and a ferritic heat treatment step of heat treating the homogenized super duplex stainless steel to ferrite austenite.

[0014] One embodiment of the present invention provides a nickel-plated super duplex stainless steel comprising: a super duplex stainless steel; and a nickel-plated layer formed on the surface of the super duplex stainless steel.

[0015] One embodiment of the present invention provides a method for manufacturing nickel-plated super duplex stainless steel, comprising: a step of manufacturing super duplex stainless steel having a fraction of a ferrite phase of 70% or more; and an electroless nickel plating step of nickel-plating the manufactured super duplex stainless steel by placing it in a nickel chloride electrolyte solution.

[0016]

[0017] A super duplex stainless steel and a method for manufacturing the same according to one embodiment of the present invention can provide a super duplex stainless steel having a fraction of a ferrite phase of 70% or more and having excellent wear resistance and strength by performing a homogenization heat treatment step and a ferritic heat treatment step on the super duplex stainless steel.

[0018] According to one embodiment of the present invention, a nickel-plated super duplex stainless steel and a method for manufacturing the same are capable of providing a nickel-plated super duplex stainless steel having excellent corrosion resistance and electrochemical properties and a method for manufacturing the same by performing an electroless nickel plating process on a super duplex stainless steel having excellent wear resistance and strength to form a nickel plating layer.

[0019]

[0020] Figure 1 shows the results of backscatter electron diffraction (EBSD) analysis of super duplex stainless steel having a ferrite phase fraction of 70% or more according to one embodiment of the present invention.

[0021] Figure 2a schematically illustrates heat treatment conditions for manufacturing super duplex stainless steel having a ferrite phase fraction of 70% or more according to one embodiment of the present invention.

[0022] Figure 2b shows a graph of the shear tensile test results for each heat treatment condition of Figure 2a.

[0023] Figure 2c shows FE-SEM images for each heat treatment condition of Figure 2a.

[0024] Figure 2d shows the volume fractions of austenite and ferrite for each heat treatment condition of Figure 2a.

[0025] FIG. 3 shows FE-SEM photographs of nickel-plated super duplex stainless steel surfaces according to electroless nickel plating time according to Examples 1 to 3 and Comparative Examples 1 to 3 of the present invention.

[0026] FIG. 4 is an FE-SEM image of nickel-plated super duplex stainless steel according to Example 1 of the present invention.

[0027] Figure 5 shows the surface roughness of nickel-plated super duplex stainless steel according to the nickel plating time according to one embodiment of the present invention.

[0028] FIG. 6 shows an X-ray pattern according to electroless nickel plating time of nickel-plated super duplex stainless steel according to one embodiment of the present invention.

[0029] Figure 7 shows an open circuit curve according to electroless nickel plating time of nickel-plated super duplex stainless steel according to one embodiment of the present invention.

[0030] Figure 8 shows a potential polarization curve according to electroless nickel (Ni) plating time of super duplex stainless steel according to one embodiment of the present invention.

[0031] FIG. 9 is an FE-SEM photograph of the surface of a super duplex stainless steel according to an embodiment of the present invention, with respect to the electroless nickel (Ni) plating time.

[0032] FIG. 10a is a bode plot of IZI (ohm) versus frequency (Hz) of super duplex stainless steel according to one embodiment of the present invention.

[0033] FIG. 10b is a bode plot of phase Z (ohm) versus frequency (Hz) of super duplex stainless steel according to one embodiment of the present invention.

[0034] FIG. 10c is a Nyquist plot of Zim (ohm) versus Zre (ohm) of super duplex stainless steel according to one embodiment of the present invention.

[0035] Figure 11 is a schematic diagram of an electrical equivalent circuit during electroless nickel plating.

[0036]

[0037] In this specification, when a part is said to "include" a certain component, this does not mean that it excludes other components, but rather that it may include other components, unless otherwise specifically stated.

[0038] In this specification, “A and / or B” means “A and B, or A or B.”

[0039] The numerical range indicated by the term "to" in this specification refers to a numerical range that includes the values ​​described before and after the term as the lower limit and the upper limit, respectively. When multiple numerical values ​​are disclosed as the upper and lower limits of an arbitrary numerical range, the numerical range disclosed in this specification can be understood as any numerical range that has any one of the multiple lower limit values ​​and any one of the multiple upper limit values ​​as the lower limit and the upper limit, respectively. For example, when a to b, or c to d is described in the specification, it can be understood that a or more and b or less, a or more and d or less, c or more and d or less, or c or more and b or less is described.

[0040] In the present specification, super duplex stainless steel having a fraction of ferrite phase of 70% or more may mean super duplex stainless steel having a fraction of ferrite phase of 70% or more but not more than 80%, 70% or more but not more than 78%, 70% or more but not more than 76%, 70% or more but not more than 74%, 72% or more but not more than 80%, 72% or more but not more than 78%, 72% or more but not more than 76%, or 72% or more but not more than 74%.

[0041] In this specification, super duplex stainless steel may mean a super duplex stainless steel having a fraction of ferrite phase of 70% or more and 80% or less, 70% or more and 78% or less, 70% or more and 76% or less, 70% or more and 74% or less, 72% or more and 80% or less, 72% or more and 78% or less, 72% or more and 76% or less, or 72% or more and 74% or less.

[0042]

[0043] Hereinafter, the present invention will be described in more detail.

[0044] One embodiment of the present invention provides a super duplex stainless steel having a ferrite phase fraction of 70% or more. Specifically, the super duplex stainless steel may have a ferrite phase fraction of 70% or more and 80% or less, 70% or more and 78% or less, 70% or more and 76% or less, 70% or more and 74% or less, 72% or more and 80% or less, 72% or more and 78% or less, 72% or more and 76% or less, or 72% or more and 74% or less.

[0045] Super duplex stainless steel having a ferrite fraction of 70% or more according to one embodiment of the present invention exhibits excellent wear resistance and strength. Specifically, as the ferrite fraction of the super duplex stainless steel increases, wear resistance and strength are improved, while ductility is reduced.

[0046] According to one embodiment of the present invention, the tensile strength of the super duplex stainless steel may be 780 MPa to 820 MPa, 780 MPa to 810 MPa, 790 MPa to 820 MPa, or 790 MPa to 810 MPa. Specifically, the super duplex stainless steel having the tensile strength in the above-described range can be utilized in a wire saw that can be used for precision processing and / or a battery case for weight reduction and improved stability.

[0047]

[0048] One embodiment of the present invention provides a method for manufacturing a super duplex stainless steel having a ferrite phase fraction of 70% or more, comprising: a step of casting a super duplex stainless steel; a homogenization heat treatment step of heat-treating the cast super duplex stainless steel to homogenize austenite and ferrite into a uniform particle form; and a ferritic heat treatment step of heat-treating the homogenized cast super duplex stainless steel to ferrite austenite.

[0049] Through a method for manufacturing super duplex stainless steel according to one embodiment of the present invention, super duplex stainless steel having a fraction of ferrite phase of 70% or more and excellent wear resistance and strength can be manufactured.

[0050] A method for manufacturing super duplex stainless steel according to one embodiment of the present invention includes a step of casting super duplex stainless steel. By including the step of casting super duplex stainless steel, the method for manufacturing super duplex stainless steel can cast super duplex stainless steel having an austenite volume fraction of 51 to 53% and a ferrite volume fraction of 47 to 49%.

[0051] According to one embodiment of the present invention, based on 100 parts by weight of super duplex stainless steel, the content of chromium (Cr) may be 22 parts by weight or more and 26 parts by weight or less. Specifically, the content of chromium may be 22 parts by weight or more and 25 parts by weight or less, 24 parts by weight or more and 26 parts by weight or less, or 25 parts by weight or more and 26 parts by weight or less, based on 100 parts by weight of super duplex stainless steel. In addition, based on the total weight of the super duplex stainless steel, the content of chromium may be 22% by weight or more and 26% by weight or less, 22% by weight or more and 25% by weight or less, 24% by weight or more and 26% by weight or less, or 25% by weight or more and 26% by weight or less. Chromium (Cr) is a ferrite stabilizing element that not only plays a main role in securing a ferrite phase, but is also an essential element for securing corrosion resistance. When the content of chromium (Cr) is within the aforementioned range, corrosion resistance can be increased.

[0052] According to one embodiment of the present invention, based on 100 parts by weight of super duplex stainless steel, the content of nickel (Ni) may be 4 parts by weight or more and 10 parts by weight or less. Specifically, the content of nickel may be 4 parts by weight or more and 8 parts by weight or less, 4 parts by weight or more and 7 parts by weight or less, 6 parts by weight or more and 10 parts by weight or less, 6 parts by weight or more and 8 parts by weight or less, or 6 parts by weight or more and 7 parts by weight or less. In addition, based on the total weight of the super duplex stainless steel, the content of nickel may be 4% by weight or more and 10% by weight or less, 4% by weight or more and 8% by weight or less, 4% by weight or more and 7% by weight or less, 6% by weight or more and 10% by weight or less, 6% by weight or more and 8% by weight or less, or 6% by weight or more and 7% by weight. Nickel (Ni), along with manganese (Mn), copper (Cu), and nitrogen (N), is an austenite stabilizing element, and plays a major role in increasing the stability of the austenite phase. When the content of nickel is within the above-mentioned range, the phase fractions of the ferrite phase and the austenite phase can be maintained.

[0053] According to one embodiment of the present invention, based on 100 parts by weight of super duplex stainless steel, the content of molybdenum (Mo) may be 2.5 parts by weight or more and 5 parts by weight or less. Specifically, the content of molybdenum may be 2.5 parts by weight or more and 4 parts by weight or less, 3 parts by weight or more and 5 parts by weight or less, 3 parts by weight or more and 4 parts by weight or less, 3.5 parts by weight or more and 5 parts by weight or less, or 3.5 parts by weight or more and 4 parts by weight or less. In addition, based on the total weight of the super duplex stainless steel, the content of molybdenum may be 2.5 wt% or more and 5 wt% or less, 2.5 wt% or more and 4 wt% or less, 3 wt% or more and 5 wt% or less, 3 wt% or more and 4 wt% or less, 3.5 wt% or more and 5 wt% or less, or 3.5 wt% or more and 4 wt%. When the content of the above molybdenum is within the above-mentioned range, it can be very effective in improving corrosion resistance while stabilizing ferrite together with chromium (Cr).

[0054] According to one embodiment of the present invention, based on 100 parts by weight of super duplex stainless steel, the content of manganese (Mn) may be 0.1 parts by weight or more and 1 part by weight or less. Specifically, the content of manganese may be 0.1 parts by weight or more and 0.8 parts by weight or less, 0.2 parts by weight or more and 1 part by weight or less, 0.2 parts by weight or more and 0.8 parts by weight or less, 0.5 parts by weight or more and 1 part by weight or less, 0.5 parts by weight or more and 0.8 parts by weight or less, or 0.8 parts by weight or more and 1 part by weight or less. In addition, based on the total weight of the super duplex stainless steel, the manganese content may be 0.1 wt% or more and 1 wt% or less, 0.1 wt% or more and 0.8 wt% or less, 0.2 wt% or more and 1 wt% or less, 0.2 wt% or more and 0.8 wt% or less, 0.5 wt% or more and 1 wt% or less, 0.5 wt% or more and 0.8 wt% or less, or 0.8 wt% or more and 1 wt% or less. When the manganese content is within the above-mentioned range, the phase fraction of the ferrite phase and the austenite phase is maintained, thereby improving the stability of the austenite phase.

[0055] According to one embodiment of the present invention, based on 100 parts by weight of super duplex stainless steel, the nitrogen (N) content may be 0.1 part by weight or more and 1 part by weight or less. Specifically, the nitrogen content may be 0.1 part by weight or more and 0.8 part by weight or less, 0.1 part by weight or more and 0.6 part by weight or less, 0.1 part by weight or more and 0.4 part by weight or less, 0.2 part by weight or more and 1 part by weight or less, 0.2 part by weight or more and 0.8 part by weight or less, 0.2 part by weight or more and 0.6 part by weight or less, 0.2 part by weight or more and 0.4 part by weight or less, or 0.24 part by weight or more and 0.32 part by weight or less. In addition, based on the total weight of the super duplex stainless steel, the nitrogen content may be 0.1 wt% or more and 1 wt% or less, 0.1 wt% or more and 0.8 wt% or less, 0.1 wt% or more and 0.6 wt% or less, 0.1 wt% or more and 0.4 wt% or less, 0.2 wt% or more and 1 wt% or less, 0.2 wt% or more and 0.8 wt% or less, 0.2 wt% or more and 0.6 wt% or less, 0.2 wt% or more and 0.4 wt% or less, or 0.24 wt% or more and 0.32 wt% or less. The above nitrogen, together with carbon (C) and nickel (Ni), is an element that greatly contributes to the stabilization of the austenite phase, and is one of the elements that becomes concentrated in the austenite phase during annealing heat treatment. When the nitrogen (N) content is within the above-mentioned range, corrosion resistance can be increased and high strength can be achieved as a side effect, and surface defects caused by the generation of nitrogen pores during casting can be prevented.

[0056] According to one embodiment of the present invention, based on 100 parts by weight of super duplex stainless steel, the carbon (C) content may be 0.001 parts by weight or more and 0.1 parts by weight or less. Specifically, the carbon content may be 0.001 parts by weight or more and 0.05 parts by weight or less, 0.001 parts by weight or more and 0.01 parts by weight or less, 0.005 parts by weight or more and 0.1 parts by weight or less, 0.005 parts by weight or more and 0.05 parts by weight or less, 0.005 parts by weight or more and 0.01 parts by weight or less, 0.01 parts by weight or more and 0.1 parts by weight or less, or 0.01 parts by weight or more and 0.05 parts by weight or less. In addition, based on the total weight of the super duplex stainless steel, the carbon content may be 0.001 wt% or more and 0.1 wt% or less, 0.001 wt% or more and 0.05 wt% or less, 0.001 wt% or more and 0.01 wt% or less, 0.005 wt% or more and 0.1 wt% or less, 0.005 wt% or more and 0.05 wt% or less, 0.005 wt% or more and 0.01 wt% or less, 0.01 wt% or more and 0.1 wt% or less, or 0.01 wt% or more and 0.05 wt% or less. When the carbon content is within the above-mentioned range, the stability of the austenite phase can be improved.

[0057] According to one embodiment of the present invention, the super duplex stainless steel may include 22 wt% or more and 26 wt% or less of Cr, 4 wt% or more and 10 wt% or less of Ni, 2.5 wt% or more and 5 wt% or less of Mo, 0.1 wt% or more and 1 wt% or less of Mn, 0.1 wt% or more and 1 wt% or less of N, 0.001 wt% or more and 0.1 wt% or less of C, and the remainder being Fe and impurities. Specifically, the super duplex stainless steel may include, in wt%, 24% to 26% of Cr, 6% to 8% of Ni, 3.5% to 5.0% of Mo, 0.8% of Mn, 0.24% to 0.32% of N, 0.01% of C, and the remainder being Fe and impurities.

[0058] According to one embodiment of the present invention, the super duplex stainless steel may have a pitting resistance index (PREN) expressed by the following formula (1) of 40 or more and 45 or less.

[0059] Equation (1)

[0060] Pitting Resistance Equivalent Number (PREN) = wt% Cr + 3.3 wt% Mo + 16 wt% N

[0061] Specifically, if the PREN index exceeds 40, it is considered to have localized corrosion resistance against seawater, and if the pitting resistance index of the super duplex stainless steel is within the aforementioned range, it has very high localized corrosion resistance and high strength, so it can be used for high-performance materials that can be exposed to a corrosive environment.

[0062] A method for manufacturing super duplex stainless steel according to one embodiment of the present invention includes a homogenization heat treatment step of heat-treating the cast super duplex stainless steel to homogenize austenite and ferrite into a uniform particle form.

[0063] According to one embodiment of the present invention, the homogenization heat treatment step may be performed at a temperature of 1075°C or more and 1125°C or less for a time of 30 minutes or more and 3 hours or less. Specifically, the homogenization heat treatment step may be performed at a temperature of 1090°C or more and 1110°C or less, 1075°C or more and 1110°C or less, or 1090°C or more and 1125°C or less for a time of 30 minutes or more and 2 hours or less, 30 minutes or more and 1 hour or less, 45 minutes or more and 3 hours or less, 45 minutes or more and 2 hours or less, 45 minutes or more and 1 hour or less, 1 hour or more and 3 hours or less, or 1 hour or more and 2 hours or less. By performing a homogenization heat treatment step in the temperature and time ranges described above, a homogenized super duplex stainless steel having a uniform particle shape and excellent corrosion resistance with a volume fraction of both austenite and ferrite of 49 to 51% can be manufactured.

[0064] A method for manufacturing super duplex stainless steel according to one embodiment of the present invention includes a ferritic heat treatment step of heat-treating the homogenized super duplex stainless steel to convert austenite into ferrite.

[0065] According to one embodiment of the present invention, the temperature at which the ferritic heat treatment step is performed may be different from the temperature at which the homogenizing heat treatment step is performed. As described above, by setting the temperature at which the ferritic heat treatment step is performed to be different from the temperature at which the homogenizing heat treatment step is performed, the fraction of ferrite increases more than the fraction of austenite, thereby controlling the strength of the super duplex stainless steel.

[0066] According to one embodiment of the present invention, the temperature at which the ferritic heat treatment step is performed may be higher than the temperature at which the homogenizing heat treatment step is performed. As described above, by setting the temperature at which the ferritic heat treatment step is performed higher than the temperature at which the homogenizing heat treatment step is performed, the fraction of ferrite increases more than the fraction of austenite, thereby controlling the strength of the super duplex stainless steel.

[0067] According to one embodiment of the present invention, the ferritic heat treatment step may be performed at a temperature of 1275°C or more and 1325°C or less for a time of 30 minutes or more and 3 hours or less. Specifically, the ferritic heat treatment step may be performed at a temperature of 1290°C or more and 1310°C or less, 1275°C or more and 1310°C or less, or 1290°C or more and 1325°C or less for a time of 30 minutes or more and 2 hours or less, 30 minutes or more and 1 hour or less, 45 minutes or more and 3 hours or less, 45 minutes or more and 2 hours or less, 45 minutes or more and 1 hour or less, 1 hour or more and 3 hours or less, or 1 hour or more and 2 hours or less. By performing the ferritic heat treatment step in the temperature and time ranges described above, a ferritic super duplex stainless steel having excellent wear resistance and strength can be manufactured, with a volume fraction of austenite of 24 to 30% and a volume fraction of ferrite of 70 to 76%.

[0068]

[0069] One embodiment of the present invention provides a nickel-plated super duplex stainless steel comprising: super duplex stainless steel; and a nickel-plated layer formed on the surface of the super duplex stainless steel.

[0070] Nickel-plated super duplex stainless steel according to one embodiment of the present invention has excellent wear resistance, strength and corrosion resistance, and can be used for high-performance materials that may be exposed to environments where they may be worn or corroded, such as wire saws and battery cases.

[0071] According to one embodiment of the present invention, the super duplex stainless steel may have a fraction of a ferrite phase of 70% or more. Specifically, the super duplex stainless steel may have a fraction of a ferrite phase of 70% or more and 80% or less, 70% or more and 78% or less, 70% or more and 76% or less, 70% or more and 74% or less, 72% or more and 80% or less, 72% or more and 78% or less, 72% or more and 76% or less, or 72% or more and 74% or less. By forming a nickel plating layer on the super duplex stainless steel having a fraction of a ferrite phase having excellent wear resistance and strength within the above-described range, not only wear resistance and strength but also corrosion resistance and electrochemical properties can be improved.

[0072]

[0073] According to one embodiment of the present invention, the nickel plating layer formed on the surface of the super duplex stainless steel may have a thickness of 10 nm to 100 nm. Specifically, the thickness may be 10 nm to 70 nm, 10 nm to 50 nm, 10 nm to 30 nm, 30 nm to 100 nm, 30 nm to 70 nm, 30 nm to 50 nm, or 50 nm to 100 nm. By controlling the thickness of the nickel plating layer within the above-described range, the nickel-plated super duplex stainless steel can have stable electrochemical characteristics and excellent corrosion resistance.

[0074] According to one embodiment of the present invention, the nickel-plated super duplex stainless steel may have a surface roughness of 10 nm or more and 30 nm or less. Specifically, it may be 10 nm or more and 20 nm or less, 15 nm or more and 30 nm or less, or 15 nm to 20 nm. By controlling the surface roughness within the above-described range, the thickness of the nickel-plated layer increases and becomes uniform, and the nickel-plated super duplex stainless steel may have stable electrochemical properties and excellent corrosion resistance, and its adhesion to the diamond paste may be improved.

[0075]

[0076] One embodiment of the present invention provides a method for producing nickel-plated super duplex stainless steel, comprising: a step of producing super duplex stainless steel having a fraction of a ferrite phase of 70% or more; and an electroless nickel plating step of nickel-plating the produced super duplex stainless steel by placing it in a nickel chloride electrolyte solution.

[0077] A method for manufacturing nickel-plated super duplex stainless steel according to one embodiment of the present invention forms a nickel-plated layer on the surface of super duplex stainless steel having a ferrite phase fraction of 70% or more through an electroless nickel plating process, thereby manufacturing nickel-plated super duplex stainless steel having improved wear resistance and strength as well as corrosion resistance.

[0078] According to one embodiment of the present invention, a method for manufacturing nickel-plated super duplex stainless steel includes an electroless nickel plating step of nickel plating super duplex stainless steel by placing it in a nickel chloride electrolyte solution.

[0079] According to one embodiment of the present invention, the electroless nickel plating step may be performed for a time of 150 seconds or more and 600 seconds or less. Specifically, the electroless nickel plating step may be 150 seconds or more and 500 seconds or less, 150 seconds or more and 400 seconds or less, 150 seconds or more and 300 seconds or less, 180 seconds or more and 600 seconds or less, 180 seconds or more and 500 seconds or less, 180 seconds or more and 400 seconds or less, or 180 seconds or more and 300 seconds or less. Since the electroless nickel plating step does not supply external energy (electricity), the plating performance varies depending on the difference in reactivity, and therefore, by controlling the plating time within the above-described range, a nickel-plated super duplex stainless steel having an optimal surface state and electrochemical properties can be manufactured.

[0080]

[0081] Hereinafter, the present invention will be described in detail using examples. However, the examples according to the present invention may be modified in various ways, and the scope of the present invention is not limited to the examples described below. The examples in this specification are provided to more fully explain the present invention to those of ordinary skill in the art.

[0082]

[0083] <Manufacturing Example>

[0084] ingredient

[0085] Super duplex stainless steel SDSS2507 was used, and its chemical composition is shown in Table 1 below.

[0086] CNMnNiCrMoCuWFePRESDSS25070.010.270.86.825.03.80.20.02Bal42

[0087]

[0088] Manufacturing Example 1: Manufacturing of a super duplex stainless steel specimen having a ferrite phase fraction of 70% or more

[0089] Super duplex stainless steel SDSS2507 was cast, then heat treated for solution treatment, and further homogenization heat treated at 1100°C for 1 hour, cooled by immersion in water at a cooling rate of 50°C / s, and then ferritic heat treated at 1300°C for 1 hour, cooled by immersion in water at a cooling rate of 50°C / s, to produce specimens measuring 10 mm × 10 mm × 10 mm.

[0090]

[0091] Figure 1 shows the results of backscatter electron diffraction (EBSD) analysis of super duplex stainless steel having a ferrite phase fraction of 70% or more according to one embodiment of the present invention. Specifically, backscatter electron diffraction is analyzed using a backscatter electron diffraction pattern analyzer (OXFORD Instruments, AZtecHKL), and backscatter electron diffraction (EBSD) analysis is a method of measuring crystallographic phase and crystallographic orientation using a diffraction pattern of a sample and analyzing crystallographic information of the sample based on this.

[0092] Figure 1 (a) shows an IPF-IQ (Inverse Pole figure map-Image Quality) image of EBSD analysis. Figure 1 (b) shows a Phase-IQ (Phase-Image Quality) image of EBSD analysis. Figure 1 (c) shows a particle size curve of diameter. Figure 1 (d) shows an IPF color map.

[0093] Referring to the above Figure 1, it can be confirmed that the super duplex stainless steel having a ferrite phase fraction of 70% or more manufactured in Manufacturing Example 1 is composed of austenite and ferrite having volume fractions of 27% and 73%, respectively, and is composed of coarse ferrite having a particle size of 50 μm or more and austenite having a particle size of 20 to 30 μm.

[0094] Figures 2a to 2d illustrate heat treatment conditions and results for changing the ferrite phase fraction of super duplex stainless steel according to one embodiment of the present invention. These were confirmed using FE-SEM (Hitachi, Ltd., Tokyo, Japan).

[0095] Figure 2a schematically illustrates heat treatment conditions for manufacturing super duplex stainless steel having a ferrite phase fraction of 70% or more according to one embodiment of the present invention.

[0096] Figure 2b shows a graph of the shear tensile test results for each heat treatment condition of Figure 2a.

[0097] Figure 2c shows FE-SEM images for each heat treatment condition of Figure 2a.

[0098] Figure 2d shows the volume fractions of austenite and ferrite for each heat treatment condition of Figure 2a.

[0099] Referring to the above FIGS. 2a to 2d, it can be confirmed that a super duplex stainless steel having a fraction of a ferrite phase of 70% or more, which is an embodiment of the present invention, and having a fraction of a ferrite phase of 70% or more, and having excellent wear resistance and strength, a tensile strength of 780 MPa to 820 MPa, a ductility of 10 to 15 mm, and an elongation of 20% to 30%, was manufactured through a method for manufacturing a super duplex stainless steel having a fraction of a ferrite phase of 70% or more.

[0100]

[0101] <Example>

[0102] Example 1

[0103] The surface of the super duplex stainless steel specimen having a ferrite phase fraction of 70% or more manufactured in Manufacturing Example 1 was cleaned using nitric acid, the surface roughness was maintained at 50 nm, and nickel electroless plating was performed using a nickel chloride electrolyte solution. Nickel was supplied as nickel chloride in the plating solution, and the acidity was maintained at pH 3.25 ± 0.25 using hydrochloric acid. The solution temperature and stirring speed were maintained at 50 °C and 350 rpm, respectively, and the plating time was set to 180 s to manufacture a specimen plated with a nickel plating layer having a thickness of 50 nm.

[0104] Example 2

[0105] A specimen was manufactured in the same manner as in Example 1, except that the plating time was set to 240 seconds.

[0106] Example 3

[0107] A specimen was manufactured in the same manner as in Example 1, except that the plating time was set to 300 seconds.

[0108]

[0109] <Comparative Example>

[0110] Comparative Example 1

[0111] A specimen was manufactured in the same manner as in Example 1, except that the plating time was set to 0 seconds.

[0112] Comparative Example 2

[0113] A specimen was manufactured in the same manner as in Example 1, except that the plating time was set to 60 seconds.

[0114] Comparative Example 3

[0115] A specimen was manufactured in the same manner as in Example 1, except that the plating time was set to 120 seconds.

[0116]

[0117] <Example of an exam>

[0118] a) Surface analysis of nickel-plated super duplex stainless steel according to electroless nickel plating time

[0119] To analyze the surface condition of the specimens obtained according to Examples 1 to 3 and Comparative Examples 1 to 3, the specimens were polished using diamond paste.

[0120] The surface microstructure was characterized using field-effect scanning electron microscopy (FE-SEM) (Hitachi, Ltd., Tokyo, Japan) after etching the specimens in a 5 wt.% KOH electrolyte solution for 1 min. The surface roughness was calculated using an atomic force microscope (AFM: NX100). X-ray diffraction (XRD; Rigaku, Co., Ltd., Tokyo, Japan) was used to distinguish the phases based on the X-ray patterns of the surface.

[0121]

[0122] FIG. 3 shows FE-SEM photographs of nickel-plated super duplex stainless steel surfaces according to electroless nickel plating times according to Examples 1 to 3 and Comparative Examples 1 to 3 of the present invention. Specifically, (a) of FIG. 3 is an FE-SEM photograph of the nickel-plated super duplex stainless steel surfaces manufactured in Comparative Example 1, (b) in Comparative Example 2, (c) in Comparative Example 3, (d) in Example 1, (e) in Example 2, and (f) in Example 3.

[0123] Referring to Figure 3, the effect of plating time on the surface structure of super duplex stainless steel can be observed. Ferritic patterns and polishing lines, which account for approximately 73% of the surface volume fraction, can be observed, and nickel (Ni) plating reduces surface roughness, thereby reducing surface indentations.

[0124]

[0125] Fig. 4 is an FE-SEM image of nickel-plated super duplex stainless steel according to Example 1 of the present invention. Referring to Fig. 4, a recessed portion on the surface due to electroless Ni plating can be confirmed.

[0126] To calculate the surface roughness of nickel-plated super duplex stainless steel, the surface roughness of five points around the austenite-infiltrated area was measured seven times at 10 μm intervals using an atomic force microscope (AFM: NX100), and the average value was obtained to evaluate the contrast between austenite and ferrite.

[0127]

[0128] FIG. 5 shows the surface roughness according to the nickel plating time of nickel-plated super duplex stainless steel according to one embodiment of the present invention. As shown in FIG. 5, Comparative Examples 1 to 3 showed a tendency for the surface roughness to decrease as the grooves of the initial surface were filled with nickel (Ni). On the other hand, Examples 1 to 3 showed a tendency for the surface roughness to increase and showed a difference in the nickel (Ni) layer thickness due to the reduced influence of the base structure by the nickel coating layer. Since electroless nickel plating does not supply external energy (electricity), the plating performance varies depending on the difference in reactivity, which leads to a difference in the plating performance depending on the surface. Since nickel (Ni) has an affinity for the austenite structure, it reacts more easily with austenite than with ferrite. Therefore, a ferritic super duplex stainless steel with a low austenite fraction exhibits a slower reactivity than a super duplex stainless steel with a high austenite fraction.

[0129] Surface analysis of nickel-plated super duplex stainless steel over electroless nickel plating time revealed that electroless Ni plating was more reactive toward austenite than toward ferrite.

[0130] In addition, it was confirmed that the nickel-plated super duplex stainless steel of Example 2 formed a more uniform plating layer because the reactivity of electroless Ni plating was low due to the ferriticization process to achieve a ferrite fraction of over 70% in super duplex stainless steel (SDSS). Example 3 showed an increase in surface roughness, which affected the adhesion between the nickel plating layer and the diamond paste required for manufacturing a wire saw.

[0131]

[0132] FIG. 6 shows an X-ray pattern according to the electroless nickel plating time of nickel-plated super duplex stainless steel according to one embodiment of the present invention. Referring to FIG. 6, changes on the surface according to the electroless Ni plating time can be confirmed. The peak intensity of the body-centered cubic lattice (BCC) was the highest in Comparative Example 1 and decreased as the plating time increased. On the other hand, the peak intensity of the face-centered cubic lattice (FCC) was the highest in Example 3. This result is because the increase in the thickness of the nickel plating layer caused changes in the intensities of the FCC and BCC phases according to the growth of the nickel (Ni) layer.

[0133] As described above, surface analysis of nickel-plated super duplex stainless steel according to electroless nickel plating time confirmed that nickel has a face-centered cubic (FCC) lattice structure and that the surface of super duplex stainless steel shows excellent reactivity when it is FCC. From this, it was confirmed that the ferrite ratio can affect the nickel plating of super duplex stainless steel. Therefore, when manufacturing a wire saw using nickel-plated super duplex stainless steel, the electroless nickel plating time can be controlled to adhere the diamond paste.

[0134]

[0135] b) a) Electrochemical properties of nickel-plated super duplex stainless steel according to electroless nickel plating time

[0136] To analyze the electrochemical properties of the specimens obtained according to Examples 1 to 3 and Comparative Examples 1 to 3, open-circuit potential (OCP) measurements, potentiodynamic polarization tests, and electrochemical impedance spectroscopy (EIS) were performed. A three-electrode method was used, including a working electrode (WE: specimen), a reference electrode (RE: saturated calomel electrode (SCE), KCl), and a counter electrode (CE: Pt mesh), and 3.5 wt% NaCl was used as the electrolyte.

[0137]

[0138] Figure 7 illustrates an open-circuit potential (OCP) curve for electroless nickel plating time of nickel-plated super duplex stainless steel according to one embodiment of the present invention. By measuring voltage changes through the open-circuit potential (OCP), the reactivity of a material can be determined based on its potential.

[0139] Figure 7 (a) shows an open circuit potential versus time curve according to the electroless nickel (Ni) plating time of a super duplex stainless steel having a ferrite phase fraction of 70% or more in a 3.5 wt% NaCl electrolyte solution. In Figure 7 (a), the surface potential decreased after nickel (Ni) plating, and the potential increased from -170 mV and -350 mV to 0 mV and -50 mV when the plating time was 60 seconds (Comparative Example 2) and 120 seconds (Comparative Example 3), respectively, and it can be confirmed that the potential is stabilized due to the uniform nickel (Ni) layer. In the cases of Examples 1 to 3, it can be confirmed that the potential gradually increases.

[0140] (b) of Fig. 7 shows the potential gap according to the open circuit result. gap = Potential Max -Potential Min ), the difference in potential observed in the open-circuit potential (OCP) indicates the ease of reaction, and the lower the potential, the more easily the oxidation reaction occurs. The presence of an uneven nickel (Ni) layer increased galvanic corrosion, resulting in a decrease in potential, and the uneven nickel (Ni) plating caused a greater change in potential. Comparing Example 1 and Comparative Example 3, the potential difference decreased from 320 mV to 100 mV, indicating that the nickel (Ni) layer was stabilized.

[0141] As shown in Fig. 7 (b), it was confirmed that electroless nickel (Ni) plating on the surface of ferritic super duplex stainless steel required a plating time of more than 120 seconds to achieve a uniform plating layer, as observed in open-circuit potential (OCP) measurements.

[0142]

[0143] Figure 8 shows a potential polarization curve according to electroless nickel (Ni) plating time of super duplex stainless steel according to one embodiment of the present invention.

[0144] Specifically, the change in current density according to potential was measured through a potentiodynamic polarization test in a 3.5 wt% NaCl electrolyte solution, and the corrosion behavior according to the electroless nickel (Ni) plating time was analyzed. The scan speed was 0.167 mV / s.

[0145] The reactivity of the nickel (Ni) layer is consistent with the reactivity observed in the activation polarization during the potentiodynamic polarization test, which can provide insight into the corrosion behavior. As shown in Figure 8, as the electroless nickel (Ni) plating time increased, the voltage of the activation polarization decreased and the current density increased. The E obtained from Figure 8 corr , I corr and E pit The values ​​are shown in Table 2 below.

[0146] In this specification, "E corr " means corrosion potential, and "I corr " means corrosion current, and "E pit " can mean the potential for occurrence.

[0147] Plating time (s) Electrode polarization test value E corr (mV)I corr (A / cm 2 )E Pit (mV)0352 × 10 -7 116060-962 × 10 -6 1140120-1003 × 10 -6 1120180-1254 × 10 -6 1120240-1437 × 10 -6 1120300-1952 × 10 -5 920

[0148]

[0149] As shown in Table 2, when the plating time is 0 to 240 seconds, the generation potential (E) is about 1100 mV or more.pit ) exhibited uniform corrosion characteristics, and in the case of Example 3 with a plating time of 300 seconds, the plating layer exhibited non-uniform corrosion, and the generation potential decreased to 920 mV.

[0150] Uneven corrosion can lead to the formation of uneven plating layers and can affect processes such as electroplating and post-treatment.

[0151]

[0152] FIG. 9 is an FE-SEM photograph of the surface of a super duplex stainless steel according to an embodiment of the present invention, depending on the electroless nickel (Ni) plating time. Specifically, FIG. 9 (a) is an FE-SEM photograph of the corroded surface after a potentiodynamic polarization test of the super duplex stainless steel manufactured in Comparative Example 1, (b) in Comparative Example 2, (c) in Comparative Example 3, (d) in Example 1, (e) in Example 2, and (f) in Example 3.

[0153] As shown in Fig. 9, localized pitting corrosion growth is observed at the interface between the austenite and ferrite phases. The corrosion pattern was observed to propagate along the grain boundaries of the ferrite due to its low pitting resistance (PRE). Referring to Fig. 9, it was confirmed that the nickel layer was destroyed by uniform corrosion, resulting in a decrease in pitting in Comparative Examples 1 to 3 and Examples 1 and 2, while pitting growth occurred more in Example 3.

[0154]

[0155] FIGS. 10A to 10C illustrate the results of electrochemical impedance spectroscopy (EIS) measurements according to electroless nickel (Ni) plating time of super duplex stainless steel according to an embodiment of the present invention. Specifically, FIG. 10A is a Bode plot of IZI (ohm) versus frequency (Hz) of super duplex stainless steel according to an embodiment of the present invention, and FIG. 10B is a Bode plot of phase Z (ohm) versus frequency (Hz) of super duplex stainless steel according to an embodiment of the present invention. FIG. 10C is a Nyquist plot of Zim (ohm) versus Zre (ohm) of super duplex stainless steel according to an embodiment of the present invention.

[0156] Figure 11 is a schematic diagram of an electrical equivalent circuit during electroless nickel plating.

[0157] In Figs. 10a to 10c, the thickness of the passivation layer and the nickel (Ni) plating layer were measured by electrochemical impedance spectroscopy (EIS), and the state of the plating layer was checked. 0 ~10 6 Resistance was measured at a frequency of Hz. The results obtained through the EIS measurement are shown in Table 3 below.

[0158] Plating time (s) Electrolyte resistance (R) s , Ω)Constant Phase Element (CPE)Passive layer resistance (R1, kΩ)Plating layer resistance (R2, kΩ)C1n106.16.5 × 10 -5 0.743250606.16.5 × 10 -5 0.743405711206.16.6 × 10 -5 0.743456121806.16.7 × 10 -5 0.743556352406.26.8 × 10 -5 0.763687803006.26.9 × 10 -50.79401914

[0159]

[0160] Referring to Figures 10a to 10c and Table 3, an increase in the plating layer resistance was observed with increasing plating time, which indicates an increase in resistance due to the presence of the plating layer. A significant increase in the passive layer resistance was observed after a plating time of 300 seconds, which indicates an increase in the thickness of the nickel (Ni) plating layer and an acceleration of the plating speed, and the thickness of the nickel (Ni) plating layer increased over time. The Bode plot is 10 0 ~10 6 High phase shift in the frequency range of Hz, 10 0 A linear relationship between impedance (IZI) and frequency with a slope close to , and a phase decrease near 180° at low frequencies. The phase decrease indicates advanced passivation, which is commonly observed in super duplex stainless steels.

[0161] The passivation behavior of the super duplex stainless steel is due to its low passivation ratio, and the EIS data were applied to the equivalent circuit shown in Fig. 11 to quantify the electrochemical parameters. The nickel (Ni) plating layer of the super duplex stainless steel is combined with the passivation layer of the super duplex stainless steel. Applying the EIS data to the equivalent circuit shown in Fig. 11 revealed that the non-uniform corrosion behavior of Example 3 was due to accelerated uniform corrosion caused by Ni, which led to a decrease in the corrosion potential. The thickness of the nickel plating layer increased as the plating time increased, and the uniformity of the nickel plating layer was confirmed to increase after 240 seconds.

Claims

1. Super duplex stainless steel with a ferrite phase fraction of 70% or more.

2. In paragraph 1, Super duplex stainless steel with a tensile strength of 780 MPa to 820 MPa.

3. Step of casting super duplex stainless steel; and A homogenization heat treatment step of heat treating the cast super duplex stainless steel to homogenize austenite and ferrite into a uniform particle form; and A method for manufacturing a super duplex stainless steel having a fraction of a ferrite phase of 70% or more, comprising a ferritic heat treatment step of heat-treating the homogenized super duplex stainless steel to convert austenite into ferrite.

4. In paragraph 3, The above super duplex stainless steel is, A method for manufacturing a super duplex stainless steel comprising 22 wt% or more and 26 wt% or less of Cr, 4 wt% or more and 10 wt% or less of Ni, 2.5 wt% or more and 5 wt% or less of Mo, 0.1 wt% or more and 1 wt% or less of Mn, 0.1 wt% or more and 1 wt% or less of N, 0.001 wt% or more and 0.1 wt% or less of C, and the remainder being Fe and impurities.

5. In paragraph 3, The above super duplex stainless steel is, A method for manufacturing a super duplex stainless steel having a pitting resistance index (PREN) of 40 or more, represented by the following formula (1): Equation (1) Pitting Resistance Equivalent Number (PREN) = wt% Cr + 3.3 wt% Mo + 16 wt% N.

6. In paragraph 3, The above homogenization heat treatment step is, A method for manufacturing super duplex stainless steel, the method comprising: performing a process at a temperature of 1075°C or higher and 1125°C or lower for a time of 30 minutes or higher and 3 hours or lower.

7. In paragraph 3, The above ferritic heat treatment step is, A method for manufacturing super duplex stainless steel, the method comprising: performing a process at a temperature of 1275°C or higher and 1325°C or lower for a time of 30 minutes or higher and 3 hours or lower.

8. Super duplex stainless steel; and Nickel-plated super duplex stainless steel comprising a nickel plating layer formed on the surface of the above super duplex stainless steel.

9. In paragraph 8, The above super duplex stainless steel is, Nickel-plated super duplex stainless steel with a ferrite phase fraction of 70% or more.

10. In paragraph 8, The nickel-plated super duplex stainless steel has a nickel-plated layer having a thickness of 10 nm or more and 100 nm or less.

11. In paragraph 8, Nickel-plated super duplex stainless steel with a surface roughness of 10 nm or more and 30 nm or less.

12. A step for manufacturing a super duplex stainless steel having a ferrite phase fraction of 70% or more; and A method for manufacturing nickel-plated super duplex stainless steel, comprising an electroless nickel plating step of nickel plating the manufactured super duplex stainless steel by placing it in a nickel chloride electrolyte solution.

13. In paragraph 12, A method for manufacturing nickel-plated super duplex stainless steel, wherein the above electroless nickel plating step is performed for a time of 150 seconds or more and 600 seconds or less.

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