KR20220064618A - Cobalt-based alloy having high strength and high ductility through multi-pass thermomechanical processing and method of manufacturing the same - Google Patents

Abstract

Disclosed are cobalt-based alloy having high strength and high ductility through multi-pass thermo-mechanical treatment and a manufacturing method thereof, that can secure high strength and high ductility at the same time by applying a special thermo-mechanical treatment method to perform control for having a microstructure in which fine crystal grains and coarse crystal grains are mixed, or having a microstructure with uniform ultra-fine crystal grains. To this end, the cobalt-based alloy having high strength and high ductility through multi-pass thermo-mechanical treatment according to an embodiment of the present invention comprises the steps of: (a) performing solution heat treatment and cooling operations for the cobalt-based alloy containing 19 to 21 wt% of Cr, 14 to 16 wt% of W, 9 to 11 wt% of Ni, 1 to 2 wt% of Mn and the remainder of Co; and (b) performing the multi-pass thermo-mechanical treatment of the cooled cobalt-based alloy, wherein the multi-pass thermo-mechanical treatment is repeatedly performed at least twice.

Description

Cobalt-based alloy having high strength and ductility through multi-stage heat treatment and manufacturing method thereof

The present invention relates to a cobalt-based alloy having high strength and high ductility through multi-stage processing heat treatment and a method for manufacturing the same, and more particularly, to have a microstructure in which fine grains and coarse grains are mixed by applying a special processing heat treatment method, or , It relates to a cobalt-based alloy having high strength and high ductility through multi-stage heat treatment that can simultaneously secure high strength and high ductility by controlling to have a microstructure having uniform ultrafine grains, and a method for manufacturing the same.

Cobalt-based alloys have excellent corrosion resistance and wear resistance, and are widely used in biomedical parts such as artificial hip joints. For example, cobalt-based alloys have been used in dentistry and orthopedic surgery for several decades, and are recently used in stent fabrication.

With respect to mechanical properties, an improvement in strength may improve the reliability of a part, and ductility may improve the transmittance for delivering a part to a target point in the human body. However, there is a disadvantage in that it is difficult to simultaneously improve strength and ductility. For example, when the average grain size (d) is 200㎛ or more, low tensile strength (UTS < 1,000 MPa), low yield strength (YS < 500 MPa), and low elongation (EL < 50%).

On the other hand, when the average grain size is 50 ~ 100㎛ shows high ductility, but the strength level is low. In addition, when the average grain size is finely controlled to approximately 15 μm, strength (UTS: 1,180 MPa and YS: 550 MPa) can be improved, but there is a problem in that elongation (EL: 39%) is lowered.

As another example, a 50% cold worked cobalt-based alloy has a high tensile strength in excess of 1,800 MPa but a very low elongation of 5%. Accordingly, a cobalt-based alloy having high strength and high ductility through working heat treatment (TMP) including hot working or cold working and various types of heat treatment, and a method for manufacturing the same are essential.

As a related prior art, there is Korean Patent Laid-Open Publication No. 10-2009-0012144 (published on Feb. 2, 2009), which describes a soft magnetic iron-cobalt-chromium-based alloy and a method for manufacturing the same.

It is an object of the present invention to have a microstructure in which fine grains and coarse grains are mixed by applying a special processing heat treatment method, or to have a microstructure having uniform ultrafine grains, so that high strength and high ductility can be secured at the same time. It is to provide a cobalt-based alloy having high strength and high ductility through multi-stage heat treatment and a method for manufacturing the same.

A method for manufacturing a cobalt-based alloy having high strength and high ductility through multi-stage heat treatment according to an embodiment of the present invention for achieving the above object is (a) Cr: 19 to 21 wt%, W: 14 to 16 wt%, Ni : 9 to 11% by weight, Mn: 1 to 2% by weight, and solution treatment of a cobalt-based alloy containing Co and the remaining Co, followed by cooling; and (b) subjecting the cooled cobalt-based alloy to multi-stage heat treatment, wherein the multi-step heat treatment is repeated at least twice or more.

The cobalt-based alloy may further include one or more of Fe: 3 wt% or less, C: 0.15 wt% or less, Si: 0.4 wt% or less, P: 0.04 wt% or less, and S: 0.03 wt% or less.

In step (a), the solution treatment is preferably carried out at a temperature of 1,200 °C or higher to 1,250 °C or lower for 10 minutes or longer.

In the step (b), cold-rolling the cooled cobalt-based alloy at a cumulative reduction ratio of more than 5% to 90% or less, annealing heat treatment at a temperature of 950° C. or more to less than 1,250° C., followed by rapid cooling. Repeat 2 or more times.

The cold rolling is preferably performed at a cumulative reduction ratio of 30 to 70%.

When the cold rolling, annealing heat treatment and cooling are repeated two or more times, the last annealing heat treatment is preferably performed at a temperature of 950 to 1,100°C.

The annealing heat treatment is performed for 5 minutes or more.

After step (b), the cobalt-based alloy has a double grain structure in which fine grains having an average diameter of 5 μm or less and coarse grains having an average diameter of more than 5 μm are mixed.

A cobalt-based alloy having high strength and high ductility through multi-stage heat treatment according to an embodiment of the present invention for achieving the above object is Cr: 19 to 21 wt%, W: 14 to 16 wt%, Ni: 9 to 11 wt% %, Mn: 1 to 2% by weight and the remaining Co, and has a double grain structure composed of fine grains with an average diameter of 5 μm or less and coarse grains with an average diameter of more than 5 μm.

The cobalt-based alloy further includes at least one of Fe: 3 wt% or less, C: 0.15 wt% or less, Si: 0.4 wt% or less, P: 0.04 wt% or less, and S: 0.03 wt% or less.

The cobalt-based alloy has a yield strength (YS) of 590 to 740 MPa and a tensile strength (TS) of 1,190 to 1,310 MPa, and an elongation (EL) of 54 to 62%.

The cobalt-based alloy has a yield strength (YS) of 790 to 900 MPa and a tensile strength (TS) of 1,320 to 1,370 MPa, and an elongation (EL) of 49 to 65%.

High-strength and high-ductility cobalt-based alloy through multi-stage heat treatment according to the present invention and a method for manufacturing the same, by applying multi-step heat treatment in which cold rolling, annealing heat treatment and quenching process are repeated at least twice or more, an average diameter of 5 μm It has a double grain structure in which the following microcrystal grains and coarse grains having an average diameter of more than 5 μm are mixed. Thereby, high strength by fine grains and high ductility by coarse grains can be made possible.

Specifically, the present invention generates various deformation mechanisms related to multi-planar slip, thin deformable organic platelets and stacked defect energy. In addition, the fixing effect of nano-carbides generated during annealing heat treatment after cold rolling plays an important role in suppressing grain growth during subsequent iterative processes.

As such, cold rolling combined with an appropriate annealing heat treatment can easily produce a double-grained structure, providing a superior combination of strength and ductility compared to a single-grained structure.

As a result, the cobalt-based alloy having high strength and high ductility through the multi-stage heat treatment according to the present invention and the method for manufacturing the same have a yield strength (YS) of 590 ~ 740 MPa and a tensile strength (TS) of 1,190 ~ 1,310 MPa, An elongation (EL) of 54 to 62% can be obtained at the same time.

In addition, a cobalt-based alloy having high strength and high ductility through multi-stage heat treatment according to an embodiment of the present invention and a method for manufacturing the same are performed at least two times during the multi-step heat treatment step, by changing the final annealing heat treatment, the final The particle size can be further refined to about 2 μm, and the particle size distribution can be adjusted.

Due to this, it is possible to secure more excellent mechanical performance, and while having a yield strength (YS) of 790 to 900 MPa and a tensile strength (TS) of 1,320 to 1,370 MPa, an elongation (EL) of 49 to 65% can be obtained.

As such, in the present invention, by applying a multi-stage processing heat treatment that repeatedly performs repeated cold rolling and annealing heat treatment at least twice or more, a combination of strength and ductility through control of grain distribution in the final microstructure, high strength and high ductility can be secured at the same time, so it is very suitable for the manufacture of mini-tube for stents.

1 is a process flow chart showing a method for manufacturing a cobalt-based alloy having high strength and high ductility through multi-stage heat treatment according to the present invention.
FIG. 2 is a schematic diagram for explaining the multi-stage heat treatment step of FIG. 1 .
3 shows backscattered electron (BSE) images of the initial sample and the solution-treated (STed) sample.
4 shows a deformed microstructure after the first cold rolling of a solution heat treatment (STed) sample.
Figure 5 shows electron backscatter diffraction (EBSD) and BSE images of samples after 1st, 3rd and 5th thermomechanical machining (TMP).
6 shows the results of repeating the process of performing annealing heat treatment at 1,100° C. for 15 minutes at 1,100° C. for 15 minutes after solution heat treatment at 1,200° C., followed by 30% cold rolling of the initial sample, and repeating the cooling process 5 times.
7 shows the results of repeating the process of performing annealing heat treatment at 950°C for 15 minutes at 950°C after solution heat treatment of the initial sample at 1,200°C, then 70% cold rolling, and repeating the cooling process 5 times.
FIG. 8 shows the results of repeating the process of performing annealing heat treatment at 1,100° C. for 15 minutes at 1,100° C. for 15 minutes after solution treatment at 1,200° C., followed by 70% cold rolling of the initial sample, and repeating the cooling process 5 times.
9 shows the results of repeating the process of 5% cold rolling, annealing heat treatment at 1,100° C. for 15 minutes, and cooling of the initial sample after solution treatment at 1,200° C. 5 times.
FIG. 10 shows the results of repeating the process of performing annealing heat treatment at 1,250° C. for 15 minutes at 1,250° C. for 15 minutes after the initial sample is solution heat treated at 1,200° C., followed by 70% cold rolling, and repeated 5 times.
11 shows the microstructure of a sample subjected to annealing heat treatment at 950° C., 1,000° C., and 1050° C. for 15 minutes after the fifth cold rolling.
12 shows the results of measuring the mechanical properties of the initial sample, the STed sample, and the TMP sample, respectively.
13 summarizes the tensile properties of cobalt alloys and literature data.
14 is a graph showing the strength-ductility relationship for the average particle size (davg) of TMP samples.

Advantages and features of the present invention, and methods for achieving them, will become apparent with reference to the embodiments described below in detail in conjunction with the accompanying drawings. However, the present invention is not limited to the embodiments disclosed below, but will be implemented in a variety of different forms, only this embodiment allows the disclosure of the present invention to be complete, and common knowledge in the technical field to which the present invention belongs It is provided to fully inform the possessor of the scope of the invention, and the present invention is only defined by the scope of the claims. Like reference numerals refer to like elements throughout.

Hereinafter, with reference to the accompanying drawings, a cobalt-based alloy having high strength and high ductility through multi-stage processing heat treatment according to a preferred embodiment of the present invention and a method for manufacturing the same will be described in detail as follows.

In general, a method for producing a cobalt-based alloy having a single grain structure or a double grain structure by one-time cold rolling and subsequent annealing heat treatment is well known.

For example, the average grain size can be controlled from 200 μm to about 4 to 6 μm by cold swaging (reduction of section 58.3%) and annealing at 1,100° C. for a short time (3 to 5 minutes).

Therefore, cold working combined with appropriate annealing heat treatment is more efficient to achieve significant grain refinement via static recrystallization (SRX).

However, in order for the cobalt-based alloy to be used as a medical component such as a stent, a thin wire or tube is required. However, the effect of multi-stage cold working and annealing heat treatment on microstructure and mechanical properties has not yet been investigated, which is more important for making thin tubes and fine wires from a practical point of view.

Therefore, in the present invention, in order to reduce the cross section of the cobalt-based alloy from 12 mm in diameter to 5 mm, a multi-stage working heat treatment (TMP) method consisting of repeated cold rolling and annealing heat treatment was used. It is possible to control the microstructure composed of microstructure or ultrafine grains in which microcrystal grains and coarse grains are mixed through at least 2 or more, more preferably 5 repetitions.

Accordingly, the cobalt-based alloy according to the embodiment of the present invention exhibits an excellent strength-ductility combination.

In the present invention, the details of the microstructure change and mechanical properties of the cobalt-based alloy to which the multi-stage TMP is applied were investigated, and the results can be very usefully utilized for the production of cobalt-based alloy tubes, stents, and fine wires.

Cobalt-based alloy with high strength and high ductility

In the present invention, changes in the microstructure of the cobalt-based alloy during multi-stage processing heat treatment (TMP) were investigated, and the effect on the tensile properties was clarified.

It was confirmed that the multi-stage working heat treatment (TMP), which consists of repeated cold rolling and annealing heat treatment, showed an excellent effect in producing a cobalt-based alloy for manufacturing fine wires and mini tubes.

By repeatedly performing multi-stage heat treatment twice or more, it is possible to create a double grain structure composed of fine grains with an average grain size of 5 µm or less and coarse grains with an average grain size of more than 5 µm.

This bimodal grain structure provides an excellent combination of strength and ductility, yielding strength (YS) of 590 to 740 MPa, tensile strength (TS) of 1,190 to 1,310 MPa, and elongation (EL) of 54 to 62%. .

It was confirmed that, by changing the annealing heat treatment conditions in the final TMP step, the average grain size can be more finely controlled to 2 μm, and the particle size distribution, dispersion and fraction of nano-carbides can also be controlled.

Therefore, the cobalt-based alloy according to the embodiment of the present invention can further optimize the mechanical properties by controlling the multi-stage heat treatment process, so that the yield strength (YS) of 790 to 900 MPa, the tensile strength (TS) of 1,320 to 1,370 MPa and An elongation (EL) of 49 to 65% can be secured.

For this, the cobalt-based alloy having high strength and high ductility through processing heat treatment according to an embodiment of the present invention is Cr: 19 to 21 wt%, W: 14 to 16 wt%, Ni: 9 to 11 wt%, Mn: It contains 1 to 2% by weight and the remainder of Co, and has a double grain structure in which fine grains having an average diameter of 5 μm or less and coarse crystal grains having an average diameter of more than 5 μm are mixed.

In addition, the cobalt-based alloy according to an embodiment of the present invention is Fe: 3 wt% or less, C: 0.15 wt% or less, Si: 0.4 wt% or less, P: 0.04 wt% or less, and S: One of 0.03 wt% or less More may be included.

Here, the cobalt-based alloy according to the embodiment of the present invention has cobalt (Co) as a main component. Such cobalt (Co) is a non-ferrous alloy and has excellent oxidation resistance, corrosion resistance, abrasion resistance, and mechanical properties at high temperatures.

Chromium (Cr) has good corrosion resistance and is added to maintain stability in the oral environment as a passivation mechanism by the formation of a dense oxide film. Therefore, chromium is preferably added in a content ratio of 19 to 21% by weight of the total weight of the cobalt-based alloy in order to improve corrosion resistance.

Tungsten (W) and manganese (Mn) are added to improve mechanical strength. Tungsten (W) is added in a content ratio of 14 to 16 wt% of the total weight of the cobalt-based alloy, and manganese (Mn) is preferably added in a content ratio of 1 to 2 wt% of the total weight of the cobalt-based alloy.

Nickel (Ni) is a material with excellent corrosion resistance and discoloration resistance. It significantly increases the tensile strength and yield strength of the alloy, and is a biologically stable element by mixing with chromium (Cr). Therefore, in order to obtain the above properties, nickel (Ni) is preferably added in a content ratio of 9 to 11% by weight of the total weight of the cobalt-based alloy.

Carbon (C) has the advantage of increasing the strength by strengthening the grain boundary. Accordingly, carbon (C) is preferably added in a content ratio of 0.15 wt% or less of the total weight of the cobalt-based alloy.

In addition, iron (Fe) has the advantage of improving the bonding strength with the ceramic material by improving the properties of the oxide layer as well as improving the elongation. Such iron (Fe) is preferably added in a content ratio of 3 wt% or less of the total weight of the cobalt-based alloy.

Silicon (Si) improves strength and lowers the melting point, thereby improving the ease of alloy manufacturing. Such silicon (Si) has a high melting point, but when silicon (Si) is added as an additive element, it is used as a deoxidizer of the alloy to lower the melting point of the alloy and improve fluidity. Therefore, silicon (Si) is mainly used as a component of a heat-resistant alloy. The silicon (Si) is preferably added in a content ratio of 0.4 wt% or less based on the total weight of the cobalt-based alloy.

The cobalt-based alloy having high strength and high ductility through the multi-step heat treatment according to the embodiment of the present invention described above is subjected to multi-step heat treatment in which cold rolling, annealing heat treatment and rapid cooling are repeated at least twice or more. It becomes possible to produce a double grain structure composed of fine grains having a grain size of 5 µm or less and coarse grains having an average grain size of more than 5 µm.

In addition, the present invention generates a variety of deformation mechanisms related to multi-planar slip, thin deformable organic platelets and stacked defect energy. In addition, the fixation effect of nano-carbides generated during annealing after cold rolling plays an important role in suppressing grain growth during subsequent iterative processes.

Cold rolling combined with an appropriate annealing heat treatment can easily produce a double-grained structure, providing a superior combination of strength and ductility compared to typical single-grained structures.

As a result, the cobalt-based alloy having high strength and high ductility through multi-stage heat treatment according to an embodiment of the present invention has a yield strength (YS) of 590 to 740 MPa and a tensile strength (TS) of 1,190 to 1,310 MPa, 54 An elongation (EL) of ~62% can be achieved simultaneously.

In addition, by changing the annealing heat treatment of the final TMP, the cobalt-based alloy having high strength and high ductility through the multi-step heat treatment according to the embodiment of the present invention is performed at least twice or more during the multi-step heat treatment step, the final particle size can be further refined to about 2 μm.

Due to this, the tensile properties can be further optimized, and better mechanical performance can be secured through fine dispersion and particle refinement of nano carbides, resulting in a yield strength (YS) of 790 to 900 MPa and a tensile strength (TS) of 1,320 to 1,370 MPa. ), it is possible to obtain an elongation (EL) of 49 to 65%.

As such, in the present invention, by applying a multi-stage processing heat treatment that repeatedly performs repeated cold rolling and annealing heat treatment at least twice or more, a combination of strength and ductility through grain refinement and grain size distribution control of the final microstructure, It can secure high strength and high ductility at the same time, so it is very suitable for large-scale manufacturing processes related to mini-tube for stents.

Method for manufacturing cobalt-based alloy having high strength and high ductility

1 is a process flow chart showing a method for manufacturing a cobalt-based alloy having high strength and high ductility through multi-stage heat treatment according to the present invention.

Referring to FIG. 1 , the method for manufacturing a cobalt-based alloy having high strength and high ductility through multi-step heat treatment according to the present invention includes a solution heat treatment step (S110) and a multi-step heat treatment step (S120).

solution heat treatment

In the solution heat treatment step (S110), Cr: 19 to 21% by weight, W: 14 to 16% by weight, Ni: 9 to 11% by weight, Mn: 1 to 2% by weight and the remainder Cobalt-based alloy containing Co sieve and cool.

Here, the cobalt-based alloy may be used that is hot worked in the form of a rod, but is not limited thereto.

Such a cobalt-based alloy may further include at least one of Fe: 3 wt% or less, C: 0.15 wt% or less, Si: 0.4 wt% or less, P: 0.04 wt% or less, and S: 0.03 wt% or less.

In this step, the solution treatment is preferably carried out for at least 10 minutes at 1,200 °C or more and 1,250 °C or less. If the solution heat treatment temperature is less than 1,200 ° C or the solution heat treatment time is less than 10 minutes, the solution heat is not sufficiently achieved, there is a risk of having a non-uniform local composition and microstructure. Conversely, if the solution heat treatment temperature exceeds 1,250° C., the grains may become very coarse or incipient melting of the alloy may occur and mechanical properties may be greatly reduced.

Multi-stage heat treatment

In the multi-stage heat treatment step (S120), the cooled cobalt-based alloy is subjected to multi-step heat treatment.

FIG. 2 is a schematic diagram for explaining the multi-stage heat treatment step of FIG. 1 , and the multi-step heat treatment step will be described in more detail in connection with FIG. 1 .

1 and 2, in the multi-step heat treatment step (S120), the cobalt-based alloy is cold-rolled at a cumulative reduction ratio of more than 5% to 90% or less, and annealed at a temperature of 950 ° C. or more to less than 1,250 ° C. After heat treatment, the cooling process of rapid cooling or air cooling is repeated at least twice or more.

Here, when the cumulative reduction ratio is 5% or less, even if the multi-stage processing heat treatment is repeatedly performed two or more times, the generation of fine grains is insignificant, and thus the effect of refining the average particle size is almost insignificant. Therefore, the cumulative reduction ratio is preferably carried out in excess of at least 5%, and as a preferred range, the cumulative reduction ratio of 30% to 70% can be presented.

At this time, when cold rolling, annealing heat treatment and cooling are repeatedly performed two or more times, the final annealing heat treatment is preferably performed at a temperature of 950 to 1,100°C.

It is preferable to perform annealing heat processing for 5 minutes or more, respectively.

In this step, when the annealing heat treatment temperature is less than 950° C. or the annealing heat treatment time is less than 5 minutes, the generation of fine grains is insignificant, so that it may be difficult to refine the grains. Conversely, when the annealing heat treatment temperature is 1,250° C. or higher, even if the multi-stage heat treatment is repeatedly performed twice or more, rapid grain growth occurs and the average grain size becomes very large, which is not preferable.

The cobalt-based alloy having high strength and high ductility through the multi-step heat treatment produced by the above process (S110 to S120) is subjected to multi-step heat treatment that repeats the cold rolling, annealing heat treatment and cooling process at least twice or more. Accordingly, it has a double grain structure in which fine crystal grains with an average diameter of 5 μm or less and coarse crystal grains with an average diameter of more than 5 μm are mixed.

In addition, the present invention generates a variety of deformation mechanisms related to multi-planar slip, thin deformable organic platelets and stacked defect energy. In addition, the fixation effect of nano-carbides generated during annealing after cold rolling plays an important role in suppressing grain growth during subsequent iterative processes.

Cold rolling combined with an appropriate annealing heat treatment can easily produce a double-grained structure, providing a superior combination of strength and ductility compared to a typical single-grained structure.

As a result, the cobalt-based alloy having high strength and high ductility through multi-stage heat treatment produced by the method according to the embodiment of the present invention has a yield strength (YS) of 590 to 740 MPa and a tensile strength (TS) of 1,190 to 1,310 MPa. While having an elongation (EL) of 54 to 62% can be obtained at the same time.

In addition, the cobalt-based alloy having high strength and high ductility through the multi-step heat treatment produced by the method according to the embodiment of the present invention is performed at least twice or more during the multi-step heat treatment step, by changing the final annealing heat treatment, The final particle size can be further refined to about 2 μm, and the particle size distribution can be adjusted.

Due to this, the tensile properties can be further optimized, and better mechanical performance can be secured through fine dispersion and particle refinement of nano carbides, resulting in a yield strength (YS) of 790 to 900 MPa and a tensile strength (TS) of 1,320 to 1,370 MPa. ), it is possible to obtain an elongation (EL) of 49 to 65%.

As such, in the present invention, by applying a multi-stage processing heat treatment that repeatedly performs repeated cold rolling and annealing heat treatment at least twice or more, a combination of strength and ductility through control of grain distribution in the final microstructure, high strength and high ductility can be secured at the same time, so it is very suitable for the manufacture of mini-tube for stents.

Example

Hereinafter, the configuration and operation of the present invention will be described in more detail through preferred embodiments of the present invention. However, this is presented as a preferred example of the present invention and cannot be construed as limiting the present invention in any sense.

Content not described here will be omitted because it can be technically inferred sufficiently by a person skilled in the art.

1. Experimental method

A biomedical grade Co-20Cr-15W-10Ni alloy (ASTM F90) was prepared. At this time, the Co-20Cr-15W-10Ni alloy is in the form of a rod with a diameter of 12 mm.

The measured chemical composition of Co-20Cr-15W-10Ni alloy is Cr: 19.7wt%, W: 14.6wt%, Ni: 9.6wt%, Mn: 1.5wt%, Fe: 1.1wt%, Si: 0.25wt%, C: 0.06 wt%, P: 0.02 wt%, S: 0.001 wt% and the remainder of Co.

The initial sample was solution heat treated (STed) at 1,200 °C for 1 hour, and cooled to room temperature (15 °C) by quenching. The solution heat treatment (STed) sample was subjected to multi-stage processing heat treatment (TMP).

At this time, the multi-stage processing heat treatment (TMP) was repeated 5 times, and each step was reduced to 5%, 30%, and 70% at room temperature, respectively, cold rolling and annealing heat treatment at 1,100°C for 15 minutes, The annealing heat treatment included quenching in which the sample was quenched with water.

Here, the 1st to 5th samples are referred to respectively. The total area reduction of the sample rods after 5 step TMP was 82.5% (reduced from 12 mm to 5 mm). After the 5th cold rolling, the sample was subjected to annealing heat treatment at 950° C., 1,000° C., and 1,050° C. for 15 minutes in addition to annealing heat treatment at 1,100° C., respectively.

Regarding these, 5th-950 (30%), 5th-1,000 (30%), 5th-1,050 (30%), 5th-950 (70%), 5th-1,100 (70%), 5th-1,100 ( 5%) and 5th-1,250 (70%) samples.

2. Physical property evaluation

Specimens for microstructure observation were mechanically polished and then polished with colloidal silica suspension. A plane perpendicular to the rolling direction was prepared and observed. Backscattered electron (BSE) imaging and electron backscattered diffraction (EBSD) were used to characterize details in the microstructure of the sample. Electron channeling contrast imaging (ECCI) and EBSD were used to observe the deformed structure of the cold rolled samples.

BSE and ECCI observations were performed using a field emission scanning electron microscope (TESCAN MIRA 3) with a working distance of 8 to 9 mm at 15 kV.

EBSD observations were performed using a field emission scanning electron microscope (Hitachi SU6600) equipped with a TSL EBSD system operating at 20 kV. EBSD maps were taken with a working distance of 15 mm. The step size of the EBSD scan is 0.5 μm. All EBSD maps were marked as conditions scanned for points with confidence index (CI) > 0.1. The indexing ratios of the strained and annealed samples were greater than 81.8% and 99.0%, respectively.

The tensile specimen had a gauge length of 10 mm and a gauge diameter of 2.5 mm according to ASTM E8. Tensile tests were performed at room temperature with an initial strain of 0.001 s ⁻¹ measured by a contact extensometer.

For each condition, two specimens were tested for average tensile strength (UTS), yield strength (YS) and uniform elongation (uEL).

3 shows backscattered electron (BSE) images of the initial sample and the solution-treated (STed) sample. At this time, (a) of FIG. 3 is an image of the initial sample, and (b) of FIG. 3 is an image of the sample subjected to solution treatment (STed).

As shown in (a) of FIG. 3 , it can be seen that the initial sample bar has a microstructure composed of fcc γ-phase (grey) and carbide (white).

The average particle size (davg) of the γ phase is about 2 μm, and it can be seen that the initial microstructure has a unimodal FG (UMFG) structure. Carbide is observed at the triple point of the γ-grain boundary. Most of the carbides are on the sub-micron scale (about 500 nm), and it can be seen that there are only a few carbides with a size of 1 to 2 μm. It can be seen that two main carbides are present in the alloy, namely M ₆ C and M ₂₃ C ₆ , where M is one or several of the metal elements of the alloy. EBSD analysis confirmed that the carbide of the initial sample was M ₂₃ C ₆ .

As shown in (b) of FIG. 3 , it can be confirmed that the microstructure of the solution heat treated (STed) sample is a unimodal CG (UMCG) structure having an average particle size (davg) of approximately 100 μm. At this time, it is composed of a single γ phase without carbides. Both the initial sample rods and the solution heat treated (STed) samples exhibit annealing twins in the microstructure typically seen in annealed fcc metals.

4 shows a deformed microstructure after the first cold rolling of a solution heat treatment (STed) sample. 4 (a) and (b) show each EBSD phase map and KAM (Kernel Average Miorientation) map, FIG. 4 (c) is an ECC image, and FIG. 4 (d) shows a high magnification ECC image will be.

As shown in (a) and (b) of Fig. 4, the strain-induced phase transformation from fcc-γ to hcp-ε was observed to occur after 30% room temperature transformation. A small amount of phase (about 6%) formed together with the slip band SB and formed near the grain boundary GB or twin boundary TB is easily accumulated in the deformation position as shown in FIG. 4(b).

As shown in Fig. 4(c), details of the strain structure including hcp-ε phase shift and plane slip are shown. Multi-planar slips and plates resulted in grain fragmentation, sub-grain formation on the micrometer or sub-micrometer scale.

As can be seen from the high-magnification ECC image in the additional inspection, as shown in FIG. 4 , it can be confirmed that high-density stacking defects (SF) and individual dislocations between the SB and ε plates appear.

Figure 5 shows electron backscatter diffraction (EBSD) and BSE images of samples after 1st, 3rd and 5th thermomechanical machining (TMP). Figure 5 (a) is a sample after the first TMP, Figure 5 (b) is a sample after the third TMP, Figure 5 (c) is a sample after the fifth TMP.

As shown in (a), (b) and (c) of FIG. 5 , it was observed that the samples after the 1st, 3rd and 5th TMP showed a fully recrystallized structure.

Multi-TMP (multi-TMP) brought a significant grain refinement effect in that the average grain size decreased from 100 μm to 4.2 μm after the 5th TMP. More specifically, TMP also made a double grain structure (BMG) consisting of fine grains (FG) having an average grain size (dfg) of 3.0 µm and coarse grains (CG) having an average grain size (dcg) of 7.4 to 18.0 µm.

The properties of the BMG structure, such as the volume fraction and particle size of FG/CG, are highly dependent on the TMP.

6 shows the results of repeating the process of performing annealing heat treatment at 1,100° C. for 15 minutes at 1,100° C. for 15 minutes after solution heat treatment at 1,200° C., followed by 30% cold rolling of the initial sample, and repeating the cooling process 5 times.

As shown in FIG. 6 , quantitative results of overall average grain size (davg), fine grain fraction (volfg), coarse grain fraction (volcg), fine grain size (dfg) and coarse grain size (dcg) obtained from EBSD analysis represents

At this time, as the TMP increased, dcg and volcg decreased, and since the annealing heat treatment temperature (about 1,100° C.) was lower than the solvent temperature of the carbide (1,200° C.), carbide precipitation was observed in all samples. EBSD analysis confirmed that most of the carbides were M ₂₃ C ₆ .

A small amount of carbide is M ₇ C ₃ , which is a transition phase that appears along grain boundaries in the initial stage of annealing heat treatment. In the first TMP sample, the nanocarbide (M ₂₃ C ₆ ) can be observed at three positions: the original grain boundary (GB) or twin boundary (TB), the slipband (SB) and the new GB of the static recrystallization (SRX) grains.

Subsequent TMP disrupts the arrangement of carbides in these SBs, providing a more uniform carbide distribution in the γ-matrix. However, carbide fixation in the GB of FG ensures the formation and stability of the BMG structure, as shown in Fig. 5(e) and (f). In addition, the carbide fraction ( _{Vol_carbide} ) gradually increased according to the continuous TMP, and then, as shown in FIG. can confirm that

7 shows the results of repeating the process of performing annealing heat treatment at 950°C for 15 minutes at 950°C after solution heat treatment of the initial sample at 1,200°C, then 70% cold rolling, and repeating the cooling process 5 times. In addition, Figure 8 shows the results of repeating the process of performing annealing heat treatment at 1,100 °C for 15 minutes at 1,100 °C after solution heat treatment of the initial sample at 1,200 °C, cold rolling to 70%, and cooling five times.

7(a) and 8(a), in the case of 5th-950 (70%) and 5th-1,100 (70%) samples, when multi-stage heat treatment is repeatedly performed two or more times, It can be seen that the average particle size (davg) is rapidly refined.

In addition, as shown in FIGS. 7 (b) and 8 (b), in the case of 5th-950 (70%) and 5th-1,100 (70%) samples, multi-stage heat treatment is repeated two or more times. At the time, it can be seen that the coarse grain fraction decreases and the fine grain fraction increases.

Meanwhile, FIG. 9 shows the results of repeating the process of 5% cold rolling, annealing heat treatment at 1,100° C. for 15 minutes, and cooling of the initial sample after solution treatment at 1,200° C. 5 times. In addition, FIG. 10 shows the results of repeating the process of performing annealing heat treatment at 1,250°C for 15 minutes at 1,250°C after solution heat treatment of the initial sample at 1,200°C, then 70% cold rolling, and repeating the cooling process 5 times.

As shown in (a) and (b) of Figure 9, in the case of the 5th-1,100 (5%) sample, even if the multi-stage heat treatment is repeated two or more times, the generation of fine grains is insignificant, so the average particle size (davg) It was confirmed that there is almost no miniaturization effect. Therefore, it was found that the cumulative reduction amount should exceed at least 5%.

In addition, as shown in (a) and (b) of Figure 10, in the case of the 5th-1,250 (70%) sample, when the multi-stage processing heat treatment is repeatedly performed two or more times, grain growth occurs predominantly, resulting in average grain It was confirmed that the size (davg) was very large. Therefore, it was found that the annealing heat treatment temperature should be carried out below 1,250°C.

On the other hand, Figure 11 shows the microstructure of the sample subjected to annealing heat treatment at 950 ℃, 1,000 ℃, 1050 ℃ after the fifth cold rolling for 15 minutes.

As shown in FIG. 11 , it was observed that the lower annealing temperature resulted in more significant particle refinement and more uniform particle size distribution.

The davgs of the 5th-1,050 (30%), 5th-1,000 (30%), and 5th-950 (30%) samples were 3.1 μm, 2.3 μm, and 2.0 μm, respectively.

As can be seen from (b) of Figure 5, the volcg of the 5th-1,050 (30%), 5th-1,000 (30%) and 5th-950 (30%) samples are about 10.3%, 2.8%, and 1.9%, respectively. In this context, the 5th-1,050 (30%) samples have a double grain (BMG) structure, while the 5th-1,000 (30%) and 5th-950 (30%) samples have a very fine single grain (UMFG) structure. .

In addition, large and small carbides can be distinguished from (d) to (f) of FIG. 11 . The size of the large carbide was 500 nm, and the small carbide was 50 to 100 nm. Most of the larger ones were located in or very close to the GB or TB, while the smaller ones were distributed both in the GB and within the crystal.

12 shows the results of measuring the mechanical properties of the initial sample, the STed sample, and the TMP sample, respectively.

The stress-strain plots shown in Figs. 12(a) and (b) indicate that all samples showed a significant degree of strain hardening and no necking occurred. This resulted in a very large uniform elongation (uEl). The significant strain hardening is due to the deformation mechanism of this alloy, which can be deformed by SFs, and the deformation from γ to ε by strain in addition to dislocation slip (if strain ≥ 16%).

In addition, as shown in (c) and (d) of Figure 12, the initial sample (UMFG structure) showed high strength (UTS: 1,248 MPa and YS: 853 MPa) and low ductility (uEl: 36.1%), whereas the STed sample (UMCG structure) showed low strength (UTS: 1,000 MPa and YS: 453 MPa) and high ductility (uEl: 74.5%).

However, the second to fifth TMP samples simultaneously exhibited high strength (UTS: 1,190 to 1310 MPa and YS: 590 to 740 MPa) and high ductility (uEl: 54 to 62%) due to the BMG structure.

UTS and YS increased as the number of TMPs increased, but the ductility did not decrease, and a high elongation of about 60% was maintained.

Among the 2nd to 5th TMP samples, 5th-1,050 (30%) showed high strength (UTS: 1,328 MPa, YS: 795 MPa) and high ductility (uEl: 64.0%). Additional grain refinement improved strength and decreased ductility in the 5th-1,000 (30%) and 5th-950 (30%) samples, but both strength and ductility were still higher than the initial samples even with similar grain sizes (davg about 2.0 μm) ). This may be related to the influence of the size and distribution of carbides in the sample.

Comparing Fig. 3 (a) and Fig. 11 (e) and (f), the carbide of the 5th-1,000 (30%) and 5th-950 (30%) samples has a finer size (50 ~ 100nm) and γ - It was found to have a more uniform distribution in the matrix.

Based on the microstructure changes described above, we found that two independent mechanisms play an essential role in controlling the grain structure of cobalt-based alloys during cold rolling and annealing heat treatment.

First, the various deformation mechanisms of this alloy resulted in significant improvement of each microstructure during deformation. Therefore, it provided a large number of nucleation sites for SRX during the subsequent annealing heat treatment.

As can be seen in Fig. 4(c) and (d), the multiplanar slip, SF and the intersection of the plates provide the generation of a large number of sub-grain structures. The spacing of these SBs is sub-micrometer, and as shown in FIG. 4(d), a sub-crystal structure was formed.

13 summarizes the tensile properties of cobalt alloys and literature data.

As shown in FIG. 13 , the coarse single grain (UMCG) structural alloy sample shows low strength and high ductility, and the fine single grain (UMFG) structural alloy sample shows high strength and low ductility. However, the double crystal grain (BMG) structure sample according to the present invention exhibits both high strength (YS: 550 to 900 MPa and UTS: 1,200 to 1,300 MPa) and high ductility (uEl: 55 to 80%), which is superior to UMCG or UMFG - Represents ductility balance. The reason for the simultaneous improvement of strength and ductility related to the BMG structure is that when fine grains and coarse grains are mixed, fine grains improve strength and coarse grains improve ductility.

14 is a graph showing the strength-ductility relationship for the average particle size (davg) of TMP samples.

As shown in Fig. 14(a), it can be seen that all samples followed the Hall-Petch relationship. That is, as the average particle size (davg) is higher in YS, the Hall-Petch relationship for the UMG sample is consistent with the previous study where the relationship for the UMG sample follows YS = 421 + 689 d ^-1/2 . However, the current BMG sample exhibited a higher kHP of 769 MPa μm ^1/2 , 684 MPa μm ^1/2 than the UMG sample, indicating a more pronounced strengthening.

This also means that the BMG sample can have a higher YS than the UMG sample when the mean particle size (davg) is the same.

Moreover, as can be seen in Fig. 14(b), the BMG sample always has much higher ductility than the UMG sample when the average particle size (davg) is the same. The uEl of BMG samples can be as high as 55-80%.

As described so far, the cobalt-based alloy of the present invention is subjected to multi-stage processing heat treatment in which cold rolling, annealing heat treatment, and cooling processes are repeated at least twice or more, so that microcrystal grains with an average diameter of 5 μm or less and an average diameter of 5 It has a double grain structure in which coarse grains larger than μm are mixed.

In addition, the present invention generates various deformation mechanisms related to multi-planar slip, thin deformable organic platelets and stacked defect energy. In addition, the fixation effect of nano-carbides generated during annealing after cold rolling plays an important role in suppressing grain growth during subsequent iterative processes.

Cold rolling combined with appropriate annealing heat treatment can easily produce a double-grained structure, providing a superior combination of strength and ductility compared to single-grained structures.

As a result, the cobalt-based alloy of the present invention has a yield strength (YS) of 590 to 740 MPa and a tensile strength (TS) of 1,190 to 1,310 MPa, and an elongation (EL) of 54 to 62% can be obtained at the same time.

In addition, the cobalt-based alloy of the present invention can further refine the final grain size to about 2 μm by changing the final annealing heat treatment during the multi-stage heat treatment step performed at least twice or more, and the grain size distribution can be adjusted there will be

Due to this, it is possible to secure more excellent mechanical performance, and while having a yield strength (YS) of 790 to 900 MPa and a tensile strength (TS) of 1,320 to 1,370 MPa, an elongation (EL) of 49 to 65% can be obtained.

As described above, in the present invention, by applying a multi-stage processing heat treatment that repeatedly performs repeated cold rolling and annealing heat treatment at least twice or more, a combination of strength and ductility through control of grain distribution in the final microstructure, high strength and high ductility can be secured at the same time, so it is very suitable for the manufacture of mini-tube for stents.

In the above, the embodiments of the present invention have been mainly described, but various changes or modifications can be made at the level of those skilled in the art to which the present invention pertains. Such changes and modifications can be said to belong to the present invention without departing from the scope of the technical spirit provided by the present invention. Accordingly, the scope of the present invention should be judged by the claims described below.

S110: solution treatment step
S120: Multi-stage heat treatment step

Claims (11)

(a) Cr: 19 to 21% by weight, W: 14 to 16% by weight, Ni: 9 to 11% by weight, Mn: 1 to 2% by weight and the remaining Co. step; and
(b) subjecting the cooled cobalt-based alloy to multi-step heat treatment;
The multi-step heat treatment is a method for producing a cobalt-based alloy having high strength and high ductility through multi-step heat treatment, characterized in that repeated at least two times.
According to claim 1,
In step (a),
The cobalt-based alloy is
Fe: 3 wt% or less, C: 0.15 wt% or less, Si: 0.4 wt% or less, P: 0.04 wt% or less, and S: Multi-stage processing heat treatment, characterized in that it further comprises at least one of 0.03 wt% or less A method of manufacturing a cobalt-based alloy with high strength and high ductility through
According to claim 1,
In step (a),
The solution treatment is
A method of manufacturing a cobalt-based alloy having high strength and high ductility through multi-stage heat treatment, characterized in that it is carried out at 1,200° C. or more to 1,250° C. or less for 10 minutes or more.
According to claim 1,
The step (b) is,
Cold rolling the cooled cobalt-based alloy at a cumulative reduction ratio of more than 5% to 90% or less, annealing heat treatment at a temperature of 950 ° C. or more to less than 1,250 ° C., and then repeating the cooling process at least twice or more A method for producing a cobalt-based alloy having high strength and high ductility through multi-stage heat treatment, characterized in that it is
5. The method of claim 4,
The cold rolling
A method for manufacturing a cobalt-based alloy having high strength and high ductility through multi-stage heat treatment, characterized in that it is carried out at a cumulative reduction ratio of 30 to 70%.
5. The method of claim 4,
When the cold rolling, annealing heat treatment and cooling are repeated twice or more,
The final annealing heat treatment
A method for producing a cobalt-based alloy having high strength and high ductility through multi-stage heat treatment, characterized in that it is carried out under a temperature condition of 950 to 1,100°C.
According to claim 1,
After step (b),
The cobalt-based alloy is
A method for producing a cobalt-based alloy having high strength and ductility through multi-stage heat treatment, characterized in that it has a double grain structure in which fine grains with an average diameter of 5 μm or less and coarse grains with an average diameter of more than 5 μm are mixed.
Cr: 19 to 21 wt%, W: 14 to 16 wt%, Ni: 9 to 11 wt%, Mn: 1 to 2 wt% and the remainder of Co,
A cobalt-based alloy having high strength and high ductility through multi-stage heat treatment, characterized in that it has a double grain structure composed of fine grains with an average diameter of 5 μm or less and coarse grains with an average diameter of more than 5 μm.
9. The method of claim 8,
The cobalt-based alloy is
Fe: 3 wt% or less, C: 0.15 wt% or less, Si: 0.4 wt% or less, P: 0.04 wt% or less, and S: Multi-stage processing heat treatment, characterized in that it further comprises at least one of 0.03 wt% or less Cobalt-based alloy with high strength and high ductility through
9. The method of claim 8,
The cobalt-based alloy is
While having a yield strength (YS) of 590 ~ 740 MPa and a tensile strength (TS) of 1,190 ~ 1,310 MPa,
A cobalt-based alloy having high strength and ductility through multi-stage heat treatment, characterized in that it has an elongation (EL) of 54 to 62%.
9. The method of claim 8,
The cobalt-based alloy is
While having a yield strength (YS) of 790 ~ 900 MPa and a tensile strength (TS) of 1,320 ~ 1,370 MPa,
A cobalt-based alloy having high strength and ductility through multi-stage heat treatment, characterized in that it has an elongation (EL) of 49 to 65%.

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