HIGH-NITROGEN NICKEL-FREE AUSTENITIC STAINLESS STEEL SEAMLESS THIN-WALLED TUBE, A HIGH-SAFETY NICKEL-FREE METAL DRUG-ELUTING VASCULAR STENT MANUFACTURED THEREFROM, AND MANUFACTURING METHODS THEREFOR

Abstract

A high-nitrogen nickel-free austenitic stainless steel seamless thin-walled tube, a high-safety nickel-free metal-based drug-eluting vascular stent manufactured therefrom, and manufacturing methods therefor. In the process of manufacturing a stent tube, the nitrogen content of a material is further increased by means of stage-by-stage nitriding, so as to obtain a high-nitrogen nickel-free austenitic stainless steel seamless thin-walled tube having the nitrogen content of 0.8-1.2% as a metal stent platform material. By using rolling line contact type electrochemical polishing, the surface of the stent forms a micron-scale protrusion-recess structure by means of crystal grains having different orientations, thus improving a binding force between a metal stent material and a drug coating. The vascular stent has the characteristics of high fatigue life, high biological safety, and a high binding force between the drug coating and a substrate.

Claims

1. A high-nitrogen nickel-free austenitic stainless steel seamless thin-walled tube comprising: N content is 0.7-1.3% by weight, and the tube is a single austenite structure in a solid solution state and in a cold deformation state of 66% or less, with a grain size of ≥grade 7, said tube has a wall thickness of 60-200 μm, an outer diameter size deviation of ±0.03 mm, a wall thickness size deviation of ±0.02 mm, a yield strength of ≥600 MPa, a tensile strength of ≥1000 MPa, an axial elongation rate of ≥50%, and a pitting potential of ≥1000 mV.

2. The high-nitrogen nickel-free austenitic stainless steel seamless thin-walled tube according to claim 1, wherein in % by weight, said tube has the following composition: Cr: 17-20%, Mn: 14-18%, Mo: 1-4%, N: 0.7-1.3%, Si: ≤0.75%, Cu: ≤0.25%, C: ≤0.03%, Si: ≤0.01%, P: ≤0.025%, Ni: ≤0.05%, and Fe: balance.

3. The high-nitrogen nickel-free austenitic stainless steel seamless thin-walled tube according to claim 1, wherein said tube is used in the fields of medical devices, food and drug devices, jewelries, and instrumentations.

4. The high-nitrogen nickel-free austenitic stainless steel seamless thin-walled tube according to claim 1, wherein said tube is used for surgical implants.

5. The high-nitrogen nickel-free austenitic stainless steel seamless thin-walled tube according to claim 4, wherein said surgical implants are human lumen stents.

6. The high-nitrogen nickel-free austenitic stainless steel seamless thin-walled tube according to claim 5, wherein said human lumen stents are vascular stents.

7. A preparation method of the high-nitrogen nickel-free austenitic stainless steel seamless thin-walled tube according to claim 1, which comprises: molding the tube and controlling size accuracy, through a combination of cold deformation and heat treatment of the high-nitrogen nickel-free austenitic stainless steel tube blank with a nitrogen content of <0.7% by weight, with no manganese being volatilized on the surface layer while molding the tube and controlling size accuracy, and the nitrogen content of the tube is increased, performing cold deformation 2 to 3 times in a single pass with a gradient decreasing, wherein the cumulative deformation amount of the pass is ≤50%, and the cold deformation amount of a single time is ≤30%, and performing heat treatment after the 2 to 3 times of cold deformation with the gradient decreasing in each pass, wherein the temperature of said heat treatment is 1000-1150° C., and the treatment time is 5-90 minutes.

8. The preparation method according to claim 7, wherein during said heat treatment, a positive pressure atmosphere of a mixed gas of argon and nitrogen is applied, the total gas pressure in a cold state is 0.12-0.30 MPa, and the nitrogen partial pressure is 5%-30%.

9. The preparation method according to claim 7 wherein when the outer diameter of the tube is ≥3.0 mm, the cold deformation is performed 3 times in each pass, and the deformation amount of each time is sequentially 45-50%, 30-35% and 20-25% of the deformation amount of the pass; when the outer diameter of the tube is <3.0 mm, the cold deformation is performed 2 times in each pass, and the deformation amount of each time is sequentially 55-60% and 40-45% of the deformation amount of the pass.

10. The preparation method according to claim 7, wherein the tube is subjected to the cold deformation of next pass after mechanical removal of a nitrogen-rich hard layer on the inner and outer surfaces after the heat treatment.

11. A nickel-free metal drug-eluting vascular stent comprising: the metal platform material of said stent is high-nitrogen nickel-free austenitic stainless steel, and in % by weight, the composition of the metal platform material is: Cr: 17-20%, Mn: 14-18%, Mo: 1-3%, N: 0.8-1.2%, Si: ≤0.75%, Cu: ≤0.25%, C: ≤0.03%, Si: ≤0.01%, P: ≤0.025%, Ni: ≤0.05%, Fe: balance, said metal platform material has a tensile strength of 1100 MPa or more, a fatigue strength in the solid solution state of 570 MPa or more, and a fatigue strength at 20% cold deformation of 750 MPa or more, the pitting potential of said metal platform material in physiological saline and PBS buffer is 1000 mV or more, and when the cold deformation amount reaches 50%, said metal platform material still has a single austenite structure, with a grain size of ≤grade 7.

12. The nickel-free metal drug-eluting vascular stent according to claim 11, wherein, the deformation amount of all deformation points of the said stent during crimping and expanding deformation is 15-25%, and the fatigue strength of the deformation part of the said stent is 750 MPa or more.

13. The nickel-free metal drug-eluting vascular stent according to claim 11, wherein, on the surface of the said stent metal platform, grains with different orientations form a micron-scale protrusion-recess structure, and the height difference between grains is 0.1-0.5 μm.

14. The nickel-free metal drug-eluting vascular stent according to claim 11, which is used in heart or cerebral vessels.

15. The nickel-free metal drug-eluting vascular stent according to claim 14, configured for implantation in one or more coronary arteries.

16. A manufacturing method of the nickel-free metal drug-eluting vascular stent, which comprises: during the preparation of the stent tube, through a combination of cold deformation and heat treatment of a high-nitrogen nickel-free austenitic stainless steel tube blank with a nitrogen content of <0.7% by weight, the nitrogen content of the tube is increased to 0.8-1.2% and no manganese is volatilized on the surface layer while molding the tube and controlling size accuracy, in a single pass, 2 to 3 times of cold deformation with gradient decreasing, the cumulative deformation amount of the pass is ≤50%, and the cold deformation amount of a single time is ≤30%, and heat treatment after the 2 to 3 times of cold deformation with the gradient decreasing in each pass, the temperature of said heat treatment is 1000-1150° C., and the treatment time is 5-90 minutes.

17. The manufacturing method of the nickel-free metal drug-eluting vascular stent according to claim 16, wherein, the temperature of said heat treatment is 1045-1055° C., the nitrogen partial pressure in the applied atmosphere is 5-30%, the balance is inert gas, and the pressure in the furnace is 1.5-3 atm.

18. The manufacturing method of the nickel-free metal drug-eluting vascular stent according to claim 16, wherein, when the outer diameter of the tube is ≤3.0 mm, the cold deformation is performed 3 times in each pass, and the deformation amount of each time is sequentially 45-50%, 30-35% and 20-25% of the deformation amount of the pass; when the outer diameter of the tube is <3.0 mm, the cold deformation is performed 2 times in each pass, and the deformation amount of each time is sequentially 55-60% and 40-45% of the deformation amount of the pass.

19. The manufacturing method of the nickel-free metal drug-eluting vascular stent according to claim 16, wherein, the tube is cut into a stent metal platform using a laser, and rolling line contact type electrochemical polishing is used so that said stent metal platform and a metal electrode are continuously in rolling line contact, the surface finishing of the stent metal platform is performed by controlling the rolling speed so as to control the thinning and breaking speed of the polishing solution film at the protrusions on the surface of the stent metal platform, and said metal electrode is selected from a dissimilar inert metal material to that of said stent metal platform, so that said metal electrode and said stent metal platform are conducted in a continuous rolling line contact mode, the surface of the stent metal platform forms a micron-scale protrusion-recess structure by means of grains with different orientations through the micro-potential difference between said metal electrode and said stent metal platform, and the height difference between grains is 0.1-0.5 μm.

20. The manufacturing method of the nickel-free metal drug-eluting vascular stent according to claim 19, wherein, during rolling line contact type electrochemical polishing, current density is controlled at 0.8-1.0 A/cm.sup.2.

21. The manufacturing method of the nickel-free metal drug-eluting vascular stent according to claim 19, wherein, during rolling line contact type electrochemical polishing, the electrochemical treatment temperature is controlled at 10-40° C.

22. The manufacturing method of the nickel-free metal drug-eluting vascular stent according to claim 19, wherein, during rolling line contact type electrochemical polishing, the composition of the electrochemical polishing solution includes perchloric acid, glacial acetic acid and corrosion inhibitor, and the volume ratio of perchloric acid and glacial acetic acid, that is, perchloric acid/glacial acetic acid, is 1:4 to 1:20, and the volume ratio of the corrosion inhibitor in the polishing solution is 2-8%.

23. The manufacturing method of the nickel-free metal drug-eluting vascular stent according to claim 19, wherein, during rolling line contact type electrochemical polishing, the polishing rolling speed is controlled at 2-2.5 cm/s.

24. The manufacturing method of the nickel-free metal drug-eluting vascular stent according to claim 19, wherein, said dissimilar inert metal material is platinum or tantalum.

25. The manufacturing method of the nickel-free metal drug-eluting vascular stent according to claim 19, wherein, said dissimilar inert metal material is platinum.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0063] FIG. 1 is a schematic diagram showing a conventional polishing method for a small-caliber tubular mesh sample.

[0064] FIG. 2 is a metallographic photograph of a manganese-depleted layer formed on the surface of the tube.

[0065] FIG. 3 is a metallographic photograph of cracking on the surface of the tube with severe manganese depletion.

[0066] FIG. 4 is a metallographic structure photograph showing the axial cross section of a Φ3.0×0.11 mm tube of Example 1. It is a metallographic structure photograph with a magnification of 100 times taken with a Zeiss Observer Z1M metallographic microscope according to the GB/T 6397-2017 metal average grain size measurement method.

[0067] FIG. 5 is a metallographic structure photograph showing the axial cross section of a Φ1.8×0.09 mm tube of Example 2. It is a metallographic structure photograph with a magnification of 100 times taken with a Zeiss Observer Z1M metallographic microscope according to the GB/T 6397-2017 metal average grain size measurement method.

[0068] FIG. 6 is a metallographic structure photograph showing the axial cross section of a Φ4.5×0.19 mm tube of Example 3. It is a metallographic structure photograph with a magnification of 100 times taken with a Zeiss Observer Z1M metallographic microscope according to the GB/T 6397-2017 metal average grain size measurement method.

[0069] FIG. 7 is a diagram showing the structure of a stent having a 2.5 mm nominal diameter of Example 4.

[0070] FIG. 8 is a schematic diagram showing a rolling line contact type electrochemical polishing apparatus of the present invention. FIG. 8A is a front view of the polishing apparatus. FIG. 8B is a top view of the polishing apparatus.

[0071] FIG. 9 is a diagram showing the macroscopic and microscopic morphology of the surface of the high-nitrogen nickel-free stainless steel vascular stent after surface modification by the method of the present invention in Example 4. FIG. 9A and FIG. 9B are macroscopic morphology of the surface of the high-nitrogen nickel-free stainless steel vascular stent, wherein FIG. 9B is a partial enlarged view of FIG. 9A. FIG. 9C is microscopic morphology of the surface of the high-nitrogen nickel-free stainless steel vascular stent.

[0072] FIG. 10 is a graph showing the morphology of a 316 L stainless steel vascular stent after surface finishing, precise molding, and surface micro-patterning in Example 5. FIG. 10A is morphology of the surface of the 316 L stainless steel vascular stent under the metallographic microscope.

[0073] FIG. 10B is microscopic morphology of the surface of the 316 L stainless steel vascular stent.

[0074] FIG. 11 is a diagram showing the structure of a stent having a 2.5 mm nominal diameter of Example 6.

[0075] FIG. 12 is a diagram showing the structure of a stent having a 3.0 mm nominal diameter of Example 7.

[0076] FIG. 13 is a diagram showing a scanning electron microscope photograph of the surface of the roughened stent metal platform of Example 7.

[0077] FIG. 14 is a diagram showing a laser confocal photograph of the surface of the roughened stent metal platform of Example 7.

[0078] FIG. 15 is a diagram showing a scanning electron microscope photograph of the stent surface coating after fatigue in Example 7.

[0079] FIG. 16 shows the X-ray diffraction spectrums of the Φ12×1.1 mm high-nitrogen nickel-free stainless steel solid solution tube (N: 0.92% by weight) obtained after the cold deformation and heat treatment of the seventh pass in Example 3, and the high-nitrogen nickel-free stainless steel tubes after 21%, 43%, and 66% cold deformation thereof.

[0080] FIG. 17 is a diagram showing the comparison results of the coating firmness of the stent metal platform surface after different surface treatments.

[0081] FIG. 17A shows the surface morphology of the drug coating on the stent metal platform surface after stent crimping and expanding, wherein the drug coating is sprayed on the surface after the roughening modification of the present invention. FIG. 17B shows the surface morphology of the drug coating on the stent metal platform surface after stent crimping and expanding, wherein the drug coating is directly sprayed on the surface after conventional electrochemical polishing without roughening modification.

DETAILED EMBODIMENTS

[0082] The present invention provides a high-nitrogen nickel-free austenitic stainless steel seamless thin-walled tube, characterized in that, the N content is 0.7-1.3% by weight, and the tube is a single austenite structure in the solid solution state and in a cold deformation state of 66% or less, with a grain size of grade 7 or more (including grade 7) (measured according to GB/T 6394-2002 metal average grain size measurement method), the wall thickness is 60-200 μm, the outer diameter size deviation is ±0.03 mm, the wall thickness size deviation is ±0.02 mm, the yield strength is ≥600 MPa, the tensile strength is ≥1000 MPa, the axial elongation rate is ≥50%, and the pitting potential is ≥1000 mV.

[0083] The above-mentioned high-nitrogen nickel-free austenitic stainless steel seamless thin-walled tube of the present invention preferably, in % by weight, has the following composition: Cr: 17-20%, Mn: 14-18%, Mo: 1-4%, N: 0.7-1.3%, Si: ≤0.75%, Cu: ≤0.25%, C: ≤0.03%, Si: ≤0.01%, P: ≤0.025%, Ni: ≤0.05%, and Fe: balance.

[0084] The above-mentioned high-nitrogen nickel-free austenitic stainless steel seamless thin-walled tube of the present invention is suitable for use in the fields such as medical devices, food and drug devices, jewelry, and instrumentations, preferably used for surgical implants. Said surgical implants are human lumen stents, more preferably vascular stents.

[0085] The present invention also provides a preparation method of the above-mentioned high-nitrogen nickel-free austenitic stainless steel seamless thin-walled tube, characterized in that, through a combination of cold deformation and heat treatment of the high-nitrogen nickel-free austenitic stainless steel tube blank with a nitrogen content of <0.7% by weight, no manganese is volatilized on the surface layer while molding the tube and controlling size accuracy, and the nitrogen content of the tube is increased. In this preparation method, according to material properties, in a single pass, 2 to 3 times of cold deformation with gradient decreasing are performed, the cumulative deformation amount of the pass is ≤50%, and the cold deformation amount of a single time is ≤30%, thereby controlling size accuracy of the tube. Heat treatment is performed after the 2 to 3 times of cold deformation with the gradient decreasing in each pass, the temperature of said heat treatment is 1000-1150° C., and the treatment time is between 5-90 minutes depending on the charging amount of the furnace and the wall thickness of the tube.

[0086] In the above-mentioned preparation method of the present invention, preferably, during said heat treatment, a positive pressure atmosphere of a mixed gas of argon and nitrogen is applied, the total pressure in a cold state is 0.12-0.30 MPa, and the nitrogen partial pressure is 5%-30%. By adjusting the total pressure and nitrogen partial pressure of the protective atmosphere, the surface manganese can be prevented from volatilizing while the nitrogen content of the tube is controllable within the range of 0.7-1.3 wt %.

[0087] In the above-mentioned preparation method of the present invention, preferably, when the outer diameter of the tube is ≥3.0 mm, the cold deformation is performed 3 times in each pass, and the deformation amount of each time is sequentially 45-50%, 30-35% and 20-25% of the deformation amount of the pass; when the outer diameter of the tube is <3.0 mm, the cold deformation is performed 2 times in each pass, and the deformation amount of each time is sequentially 55-60% and 40-45% of the deformation amount of the pass.

[0088] In the above-mentioned preparation method of the present invention, preferably, the tube is subjected to the cold deformation of the next pass after mechanical removal of a nitrogen-rich hard layer on the inner and outer surfaces after the heat treatment. Thereby, it is possible to prevent cracking of the tube and introduction of foreign substances during the next cold deformation.

[0089] The metal platform material of the high-safety nickel-free metal drug-eluting vascular stent of the present invention adopts high-nitrogen nickel-free austenitic stainless steel with high strength, high fatigue strength and high corrosion resistance, in % by weight, the composition thereof is: Cr: 17-20%, Mn: 14-18%, Mo: 1-3%, N: 0.8-1.2%, Si: ≤0.75%, Cu: ≤0.25%, C: ≤0.03%, Si: ≤0.01%, P: ≤0.025%, Ni: ≤0.05%, Fe: balance.

[0090] In order to obtain the above-mentioned stent metal platform material of the present invention where the nitrogen content is high and the volatilization of manganese in the material has been suppressed, in the preparation process of the stent tube, while heat treatment eliminates cold deformation stress to achieve solid solution, stage-by-stage pressure nitriding increases the nitrogen content in the tube. Thus, the obtained stent metal platform material of the present invention can have a tensile strength of 1100 MPa or more and a fatigue strength of 570 MPa or more, much higher than the fatigue strength of the mainstream stent materials currently used clinically.

[0091] Through the design of the stent structure, the deformation amount of all the mesh wires of the stent during crimping and expanding deformation is 15-25%, so that the fatigue strength of the deformation part of the stent (long-term fatigue) is increased to 750 MPa or more. As a result, the risk of late fracture and collapse of the stent is reduced, the long-term safety and effectiveness of the stent in vivo is maximized, thereby the stent has a longer fatigue life, and the safe service period of the stent is prolonged.

[0092] In addition, the corrosion potential of the stent metal platform material of the present invention in physiological saline and PBS buffer solution can reach 1000 mV or more, and no passivation treatment is required to improve its surface corrosion resistance. Because the stent metal platform material of the present invention has excellent corrosion resistance and no harmful nickel element with allergenic and carcinogenic effects are added to the material, after the drug coating on the stent surface is degraded, the metal material has high biological safety, and the risk of late restenosis of the stent segment is reduced.

[0093] The tube is shaped by a laser cutting, and the surface finishing and size control of the stent metal platform are realized by means of rolling line contact type electrochemical polishing. In the rolling line contact type electrochemical polishing suitable for small-caliber tubular mesh metal samples of the present invention, stent metal platform and the metal electrode are continuously in rolling line contact. The surface finishing of the stent metal platform is performed quickly and uniformly by controlling the rolling speed so as to control the thinning and breaking speed of the polishing solution film at the protrusions on the surface of the stent metal platform. More preferably, in the rolling line contact type electrochemical polishing of the present invention, said metal electrode is selected to be a dissimilar inert metal material to that of said stent metal platform, so that said metal electrode and said stent metal platform to be polished are conducted in a continuous rolling line contact mode. Through the micro-potential difference between said metal electrode and said stent metal platform, the surface of the stent metal platform has a difference in polishing amount between grains, and thus the inner surface of the stent metal platform is micro-patterned.

[0094] The invention changes the surface treatment (surface polishing and surface roughening) technology of the existing small-caliber tubular mesh metal samples, from the traditional single-point and partial clamping to rolling line contact type electrochemical polishing. First, the contact type rolling polishing is used to accelerate the thinning and breaking speed of the polishing solution film at the protrusions on the surface of the sample, so as to accelerate the smoothing and polishing purpose of the protrusion parts on the surface, thus realizing the rapid surface finishing of the metal sample. Second, the rolling line contact type electrochemical polishing is used to avoid the unevenness of the metal sample caused by single-point and partial clamping polishing, and to avoid the phenomenon that the structure of the metal sample deviates from the target sample structure after polishing.

[0095] In addition, the metal electrode is selected to be dissimilar inert metal materials and conducts with the small-caliber tubular mesh metal sample to be polished in a continuous rolling line contact mode. Combined with an appropriate polishing voltage, through the micro-potential difference between the electrode and the metal piece to be polished, the surface of the small-caliber tubular mesh metal sample has a difference in polishing amount between grains, and thus the inner surface of the tubular mesh metal sample is micro-patterned, which increases the coating binding strength for the subsequent coating processing.

[0096] The rolling line contact type electrochemical polishing of the present invention is suitable for hollowed-out small-caliber tubular mesh metal samples including stents, the length of which is less than 80 mm, the tube diameter is less than 5 mm, and the metal coverage rate is less than 50%. In addition, the metal sample material includes: stainless steel, titanium alloy, cobalt-based alloy, magnesium alloy, iron alloy, zinc alloy, but not limited to the above alloys.

[0097] In the rolling line contact type electrochemical polishing of the present invention, the dissimilar inert metal material may be platinum or tantalum, preferably platinum.

[0098] In the rolling line contact type electrochemical polishing of the present invention, it is preferable to control the current density at 0.8-1.0 A/cm.sup.2 and the polishing temperature at 10-40° C. By polishing at a low temperature of 10-40° C., the reaction speed can be reduced, and the controllability of the precise structure size is improved, which is beneficial to the uniform polishing. In addition, in the rolling line contact type electrochemical polishing of the present invention, it is preferable to use a polishing solution whose components include perchloric acid, glacial acetic acid, and a corrosion inhibitor. In the polishing solution, the volume ratio of perchloric acid and glacial acetic acid, that is, perchloric acid/glacial acetic acid, is preferably 1:4 to 1:20. In addition, the volume ratio of the corrosion inhibitor in the polishing solution is preferably 2-8%, more preferably 5%. The surface of the stent metal platform is made smooth by controlling the composition of the polishing solution, the polishing current density and the reaction temperature. Meanwhile, by controlling the electrode potential, the grains with different orientations have different polishing amounts, so as to realize the micro-roughening of the surface of the stent metal platform, forming a height difference of 0.1-0.5 μm, thereby increasing the binding force of the stent metal platform and the drug coating.

[0099] In addition, in the rolling line contact type electrochemical polishing of the present invention, it is preferable to control the rolling speed at 2-2.5 cm/s.

[0100] The rolling line contact type electrochemical polishing of the present invention can be applied not only to the surface modification of vascular stents, but also to the surface modification of peripheral stents, the surface modification of digestive tract stents, the surface modification of urinary system stents, the surface modification of large-sized metal conduits, and the surface modification of bone cages for bone filling, etc.

[0101] The drug coating that inhibits the proliferation of smooth muscle cells is prepared on the stent surface by ultrasonic atomization spraying. By controlling the spraying process and the surface roughening of the stent metal platform, the drug coating on the stent surface binds with the substrate in high strength. The drug is preferably rapamycin and its derivatives. Therefore, the coating will not be damaged or fall off during the mounting, delivery and expansion of the stent, and the coating will not be severely damaged due to fatigue of the stent and flushing of blood flow before the stent is wrapped in the endothelium, thereby reducing the risk of thrombosis in the initial stage after stent implantation.

[0102] The high-safety nickel-free metal drug-eluting vascular stent of the present invention can be used for heart and cerebral vessels and other arterial and venous vessels, and is preferably used for coronary arteries.

[0103] Hereinafter, the present invention will be described in detail based on examples. However, the examples are merely illustrative of the present invention, and do not limit the scope of the present invention.

Example 1 High-Nitrogen Nickel-Free Austenitic Stainless Steel Seamless Thin-Walled Tube 1

[0104] A high-nitrogen nickel-free stainless steel as-forged bar with a nitrogen content of 0.62% by weight and a manganese content of 15.4% by weight was taken and processed by a deep hole drilling machine to obtain a tube blank with a size of Φ30×6 mm. The size of the designed finished tube was Φ3.0×0.11 mm. The number of cold deformation passes was 17, and the deformation amount of each pass was 40-50%. Each pass included three times of cold deformations, and the deformation amount of each time was sequentially 45-50%, 30-35% and 20-25% of the deformation amount of the pass. After cold deformations of each pass, ultrasonic cleaning was performed on the surface of the tube to remove the surface lubricant. After drying, it was put into a heat treatment furnace tank that can be evacuated and pressurized. The furnace tank material was 2520 high-temperature alloy, and there were three temperature measuring thermocouples inside to monitor the temperature in real time. After the furnace tank was evacuated to 10.sup.−1 Pa, pumping was continuously performed for 10 minutes or more, and the valve of the vacuum system was closed. The furnace tank was filled with a mixed gas of nitrogen and argon, such that the total pressure was 0.15 MPa, and the ratio of nitrogen and argon was 1:9, that is, the nitrogen partial pressure was 10%. When the heating furnace temperature reached 1100° C., the furnace tank was sent into the tubular heating furnace, and timing was started when the furnace tank temperature reached 1100° C. and became stable. The temperature holding time was determined depending on the charging amount of the furnace and the wall thickness of the tube, ranging between 5-60 minutes. After heat treatment of each pass, the inner and outer surfaces of the tube were mechanically ground and polished.

[0105] The inspection results of the finished tube are as follows: the outer diameter was 3.0±0.02 mm, the wall thickness was 0.11±0.01 mm, the nitrogen content was 0.81% by weight, the manganese content was 15.42% by weight, the yield strength was 608 MPa, the tensile strength was 1019 MPa, the axial elongation rate was 51%, and the pitting potential was 1000 mV. Among them, the measurement methods of the yield strength, the tensile strength and the elongation rate are as follows: according to GB/T 228.1-2010 Tensile Test of Metallic Materials Part 1: Test Method at Room Temperature, Z150 mechanical testing machine was used to conduct tensile test on metal tubes. The metallographic structure of the axial cross section of the tube is shown in FIG. 4, which is a single austenite structure with a grain size of ≥grade 7. And, according to the “GB/T3505-2009, GB/T1031-2009, GB/T10610-2009” standards, the Alpha-Step IQ contact surface morphology instrument was used to measure the inner and outer surface roughness of the tube, and the measurement results were Ra.sub.inner=0.046 μm, Ra.sub.outer=0.039 μm, respectively.

Example 2 High-Nitrogen Nickel-Free Austenitic Stainless Steel Seamless Thin-Walled Tube 2

[0106] A high-nitrogen nickel-free stainless steel as-forged bar with a nitrogen content of 0.62% by weight and a manganese content of 15.4% by weight was taken and processed by a deep hole drilling machine to obtain a tube blank with a size of 130×6 mm. The size of the designed finished tube was Φ1.8×0.09 mm. The number of cold deformation passes was 21, and the deformation amount of each pass was 40-50%. When the outer diameter of the tube is ≥3.0 mm, the cold deformation was performed 3 times in each pass, and the deformation amount of each time was sequentially 45-50%, 30-35% and 20-25% of the deformation amount of the pass; when the outer diameter of the tube was <3.0 mm, the cold deformation was performed 2 times in each pass, and the deformation amount of each time was sequentially 55-60% and 40-45% of the deformation amount of the pass. After cold deformations of each pass, ultrasonic cleaning was performed on the surface of the tube to remove the surface lubricant. After drying, it was put into a heat treatment furnace tank that can be evacuated and pressurized. The furnace tank material was 2520 high-temperature alloy, and there were three temperature measuring thermocouples inside to monitor the temperature in real time. After the furnace tank was evacuated to 10.sup.−1 Pa, pumping was continuously performed for 10 minutes or more, and the valve of the vacuum system was closed. The furnace tank was filled with a mixed gas of nitrogen and argon, such that the total pressure was 0.25 MPa, and the ratio of nitrogen and argon was 1:4, that is, the nitrogen partial pressure was 20%. When the heating furnace temperature reached 1050° C., the furnace tank was sent into the tubular heating furnace, and timing was started when the furnace tank temperature reached 1050° C. and became stable. The temperature holding time was determined depending on the charging amount of the furnace and the wall thickness of the tube, ranging between 5-60 minutes. After heat treatment of each pass, the inner and outer surfaces of the tube were mechanically ground and polished.

[0107] The inspection results of the finished tube are as follows: the outer diameter was 1.8±0.02 mm, the wall thickness was 0.09±0.01 mm, the nitrogen content was 1.15% by weight, the manganese content was 15.45% by weight, the yield strength was 781 MPa, the tensile strength was 1215 MPa, the axial elongation rate was 56%, and the pitting potential was 1090 mV. Among them, the measurement methods of the yield strength, the tensile strength and the elongation rate were the same as in Example 1. The metallographic structure of the axial cross section of the tube is shown in FIG. 5, which is a single austenite structure with a grain size of ≥grade 7. And, according to the roughness measurement method described in Example 1, the inner surface roughness of the tube Ra.sub.inner=0.07 μm, and outer surface roughness of the tube Ra.sub.outer=0.05 μm.

Example 3 High-Nitrogen Nickel-Free Austenitic Stainless Steel Seamless Thin-Walled Tube 3

[0108] A high-nitrogen nickel-free stainless steel as-forged bar with a nitrogen content of 0.62% by weight and a manganese content of 15.4% by weight was taken and processed by a deep hole drilling machine to obtain a tube blank with a size of 130×6 mm. The size of the designed finished tube was Φ4.5×0.19 mm. The number of cold deformation passes was 15, and the deformation amount of each pass was 40-50%. Each pass included three times of cold deformations, and the deformation amount of each time was sequentially 45-50%, 30-35% and 20-25% of the deformation amount of the pass. After cold deformations of each pass, ultrasonic cleaning was performed on the surface of the tube to remove the surface lubricant. After drying, it was put into a heat treatment furnace tank that can be evacuated and pressurized.

[0109] The furnace tank material was 2520 high-temperature alloy, and there were three temperature measuring thermocouples inside to monitor the temperature in real time. After the furnace tank was evacuated to 10.sup.−1 Pa, pumping was continuously performed for 10 minutes or more, and the valve of the vacuum system was closed. The furnace tank was filled with a mixed gas of nitrogen and argon, such that the total pressure was 0.30 MPa, and the ratio of nitrogen and argon was 1:3, that is, the nitrogen partial pressure was 25%. When the heating furnace temperature reached 1100° C., the furnace tank was sent into the tubular heating furnace, and timing was started when the furnace tank temperature reached 1100° C. and became stable. The temperature holding time was determined depending on the charging amount of the furnace and the wall thickness of the tube, ranging between 15-60 minutes. After heat treatment of each pass, the inner and outer surfaces of the tube were mechanically ground and polished.

[0110] The inspection results of the finished tube are as follows: the outer diameter was 4.5±0.02 mm, the wall thickness was 0.19±0.01 mm, the nitrogen content was 1.08% by weight, the manganese content was 15.41% by weight, the yield strength was 711 MPa, the tensile strength was 1112 MPa, the axial elongation rate was 55%, and the pitting potential was 1040 mV. Among them, the measurement methods of the yield strength, the tensile strength and the elongation rate were the same as in Example 1. The metallographic structure of the axial cross section of the tube is shown in FIG. 6, which is a single austenite structure with a grain size of ≥grade 7. And, according to the roughness measurement method described in Example 1, the inner surface roughness of the tube Ra.sub.inner=0.058 μm, and outer surface roughness of the tube Ra.sub.outer=0.053 μm.

Example 4 Surface Finishing and Precise Molding of the High-Nitrogen Nickel-Free Stainless Steel Vascular Stent

[0111] (1) Surface pretreatment of the high-nitrogen nickel-free stainless steel vascular stent before polishing

[0112] The high-nitrogen nickel-free austenitic stainless steel seamless thin-walled tube of Example 2 was cut into coronary stents by laser. The stent structure is shown in FIG. 7. The crimped diameter of the stent on the balloon was 0.9 mm, and the expanded diameter of the stent was 2.5 mm. The stent needs to be pretreated by pickling before polishing to remove the oxide layer produced by laser processing on the stent surface. The goal of pretreatment is to completely remove the oxide layer on the stent surface, so as to avoid the barrier of the oxide layer to the exchange of polishing solution during the electrochemical polishing. The pickling solution was a solution mainly composed of sulfuric acid and hydrogen peroxide. During the pickling process, the temperature of the pickling solution was controlled at 10-50° C. The stent was rinsed with plenty of running water after pickling to remove the residual pickling solution on the stent surface.

[0113] (2) Surface finishing and precise molding of the high-nitrogen nickel-free stainless steel vascular stent

[0114] The surface finishing and precise molding of the stent are realized by the electrochemical polishing of the present invention. The schematic diagram of the electrochemical polishing apparatus is shown in FIG. 8. As shown in the figure, the electrochemical polishing apparatus of the present invention is represented by FIG. 8A (a front view of the polishing apparatus) and FIG. 8B (a top view of the polishing apparatus). The apparatus is mainly composed of five parts, (1) the polishing tank, which is made of polypropylene or glass, and the size can be adjusted according to the size of the piece to be polished;

[0115] (2) the cathode plate, which is made of a stainless steel disc, located at the bottom of the polishing tank; (3) Limiting sponge, made of polypropylene sponge or melamine sponge, etc., used to limit the distance between the piece to be polished and the cathode; (4) polishing hanger or polishing fixture, made of platinum filament, with a size of 0.8-1.2 mm depending on the inner diameter of the tube to be polished; (5) polishing solution, which immerses the cathode plate, the limiting sponge and the polishing hanger, and the piece to be polished remains completely immersed in the polishing solution during the rolling polishing of the piece to be polished.

[0116] In the electrochemical polishing of the present invention, the polishing solution was a mixture of perchloric acid, glacial acetic acid and a corrosion inhibitor. Among them, the volume ratio of perchloric acid to glacial acetic acid was 1:4, the corrosion inhibitor accounted for 2%-8% of the total volume of the polishing solution, the polishing temperature was 15° C., the cathode plate material was stainless steel, the metal electrode material was platinum, and the polishing voltage was 15 V depending on the size of the stent.

[0117] The specific polishing operation is as follows: the cathode was placed at the bottom of the container containing the electrochemical polishing solution, a porous sponge-like limiting plate was placed on the cathode, a platinum wire with a diameter of 0.9 mm passed through the stent, and the stent rolled at a constant speed with a linear speed of 20 mm/s on the limiting plate by moving the platinum wire, and stopped after reaching the preset polishing effect. After cleaning the stent with pure water, the residual acidic polishing solution on the stent surface was neutralized with NaOH solution. The polished stent should have a smooth surface, a uniform stent mesh wire structure, and meet the nominal weight requirements of the stent, so as to achieve the surface finishing and precise molding of the stent.

[0118] (3) Morphology of high-nitrogen nickel-free stainless steel vascular stent after surface finishing and precise molding

[0119] FIG. 9 shows the surface morphology of the high-nitrogen nickel-free stainless steel vascular stent after surface finishing and precise molding of this Example. It can be seen from the Figure that the surface modification method of the present invention can make the surface of the high-nitrogen nickel-free stainless steel vascular stent uniformly polished, thus it is suitable for surface finishing and precise molding of the vascular stent.

Example 5 Surface Finishing, Precise Molding and Surface Micro-Patterning of the 316 L Stainless Steel Vascular Stent

[0120] (1) Surface pretreatment of the 316 L stainless steel vascular stent before polishing

[0121] The 316 L stainless steel vascular stent needs to be pretreated by pickling before polishing to remove the oxide layer produced by laser processing on the stent surface. The goal of pretreatment is to completely remove the oxide layer on the stent surface, so as to avoid the barrier of the oxide layer to the exchange of polishing solution during the electrochemical polishing. The pickling solution was a solution mainly composed of nitric acid and hydrofluoric acid. During the pickling process, the temperature of the pickling solution was controlled at 10-50° C. The stent was rinsed with plenty of running water after pickling to remove the residual pickling solution on the stent surface.

[0122] (2) Surface finishing, precise molding and surface micro-patterning of the 316 L stainless steel vascular stent

[0123] The surface finishing and precise molding of the stent were realized by electrochemical polishing, and the specific electrochemical polishing method was the same as Example 4. The polishing conditions such as the composition of the polishing solution, the rolling speed, and the polishing time were the same as those in Example 4. The polishing temperature was 15° C., the cathode plate material was stainless steel, the metal electrode material was platinum, and the polishing voltage was 25 V, depending on the size of the stent. The polishing process adopted the mode of continuous line contact rolling polishing. The polished stent should meet the nominal weight requirements of the stent, and a grain oriented micro-pattern should appear on the stent surface, so as to realize the surface finishing, precise molding and surface micro-patterning of the stent.

[0124] (3) Morphology of the 316 L stainless steel vascular stent after surface finishing and precise molding

[0125] FIG. 10 shows the surface morphology of the 316 L stainless steel vascular stent after surface finishing and precise molding of this Example. It can be seen from the figure that the surface modification method of the present invention can make the surface of the 316 L stainless steel vascular stent uniformly polished and meanwhile realize micro-patterning, thus it is suitable for surface finishing, precise molding and micro-patterning of the vascular stent.

Example 6

[0126] Using the high-nitrogen steel bar with the composition shown in Table 1, the stent tube was prepared by the nitriding method described in Example 2, and the tube blank size was Φ30×6 mm. The number of cold deformation passes was 21. The furnace pressure was 0.25 MPa, the nitrogen partial pressure was 20%, the heat treatment temperature was 1050° C., and the temperature holding time was 30-5 minutes during heat treatment. After heat treatment of each pass, the inner and outer surfaces of the tube were mechanically ground and polished. According to GB/T 20124 Steel —Determination of nitrogen content—inert gas fusion thermal conductivity method (conventional method), the nitrogen content in the tube was measured with a TCH600 nitrogen, hydrogen and oxygen analyzer, and the nitrogen content of the finished tube was measured as 1.10% by weight. According to GB/T 228.1-2010 Tensile Test of Metallic Materials Part 1: Test Method at Room Temperature, Z150 mechanical testing machine was used to conduct tensile test on the finished tube, and the yield strength of the finished tube was measured as 761 MPa, the tensile strength was 1215 MPa and the axial elongation rate was 56%. According to YY/T 1074 the pitting potential of stainless steel surgical implants, electrochemical corrosion analysis was performed with GAMRY Reference 600 electrochemical work station and the pitting potential of tube was measured as 1090 mV.

TABLE-US-00001 TABLE 1 Chemical composition of bar used in stent tube preparation Elements Cr Mn Mo N C Si S P Cu Ni Fe Content (%, by 18.48 15.30 2.58 0.62 0.029 0.31 0.006 0.008 <0.01 0.022 balance weight)

[0127] The tube was cut into a coronary stent with a laser, and its structure was shown in FIG. 11. The crimped diameter of the stent on the balloon was 0.9 mm, and the expanded diameter of the stent was 2.5 mm. The mesh wire deformation amount of the stent during crimping and expanding was 15-25%. Through finite element analysis, the fatigue safety factor of stent was calculated as 3.77. According to “YY/T 0808-2010 Standard test methods for in vitro pulsatile durability of vascular stents”, the fatigue performance of stent was tested with RDTL-0200 stent fatigue test system. The stent was released in a semi-compliant silicone tube matching the size of the stent. The working medium in the tube was PBS buffer at 37±2° C., and a pressure was applied inside the compliant tube in a pulsatile way. The minimum pressure was 75-80 mmHg, and the maximum pressure was 160-165 mmHg, the pulsation frequency was 45 Hz. After 570 million fatigue cycles (15 years of service life), no stent fracture and collapse were found.

Example 7

[0128] Using the high-nitrogen steel bar with the composition shown in Table 2, the stent tube was prepared by the nitriding method under the following conditions, and the tube blank size was Φ30×6 mm. The number of cold deformation passes was 21. The furnace pressure was 0.25 MPa, the nitrogen partial pressure was 20%, the heat treatment temperature was 1050° C., and the temperature holding time was 30-5 minutes during heat treatment. After heat treatment of each pass, the inner and outer surfaces of the tube were mechanically ground and polished. The nitrogen content of the finished tube was 1.12% by weight, and according to the same method in Example 6, the yield strength was measured as 782 MPa, the tensile strength was measured as 1190 MPa, the axial elongation rate was measured as 54%, and the pitting potential of tube was measured as 1060 mV.

TABLE-US-00002 TABLE 2 Chemical composition of bar used in stent tube preparation Elements Cr Mn Mo N C Si S P Cu Ni Fe Content (%, 18.03 16.00 2.33 0.64 0.028 0.28 0.004 0.005 <0.018 0.020 balance by weight)

[0129] The tube was cut into a coronary stent metal platform with a laser, and its structure is shown in FIG. 12.

[0130] The obtained coronary stent metal platform was electrochemically modified as follows: the cathode was placed at the bottom of the container containing the electrochemical polishing solution, a porous sponge-like limiting plate was placed on the cathode, a platinum wire with a diameter of 0.9 mm passed through the stent, and the stent rolled at a constant speed with a linear speed of 20 mm/s on the limiting plate by moving the platinum wire, and stopped after reaching the preset polishing effect. After cleaning the stent with pure water, the residual acidic polishing solution on the stent surface was neutralized with NaOH solution to make the surface of the stent metal platform micro-roughened. The composition of the electrochemical polishing solution was perchloric acid and glacial acetic acid, the composition ratio was 1:10, the temperature of the electrochemical polishing solution was 35±2° C., and the electrochemical current density was 2.3 A/cm.sup.2. FIG. 13 and FIG. 14 show the scanning electron microscope photograph and the laser confocal photograph of the surface of the stent metal platform after roughening treatment, respectively. According to the measurement, the surface height difference of the roughened stent metal platform was about 0.2 μm. Then, the rapamycin drug coating was sprayed on the surface of the stent by ultrasonic atomization. According to “YY/T 0808-2010 Standard test methods for in vitro pulsatile durability of vascular stents”, the fatigue performance of the stent was tested with the RDTL-0200 stent fatigue test system. The stent was released in a semi-compliant silicone tube matching the size of the stent. The working medium in the tube was PBS buffer at 37±2° C., and a pressure was applied inside the compliant tube in a pulsatile way. The minimum pressure was 75-80 mmHg, and the maximum pressure was 160-165 mmHg, and the pulsation frequency was 1.2 Hz. FIG. 15 shows the coating morphology of the coated stent after simulating pulsation and blood flow flushing in PBS buffer for 90 days. It can be seen from this result that the stent coating of the present invention does not fall off and is not damaged in a large area, and the coating has a high binding strength with the substrate.

Experimental Example 1 Changes in Mechanical Properties Before and After Nitrogen Increase

[0131] The mechanical properties of the high-nitrogen nickel-free stainless steel as-forged bar used in Example 1-3 and the finished tube of Example 1-3 obtained by high-temperature nitriding to further increase the nitrogen content of the material were measured, the measurement methods of yield strength, tensile strength, and elongation rate are as follows. According to GB/T 228.1-2010 Tensile Test of Metallic Materials Part 1: Test Method at Room Temperature, Z150 mechanical testing machine was used to conduct tensile test on metal tubes.

[0132] Table 3 summarizes the mechanical properties of the tube under different nitrogen contents. From the results, it can be seen that with the increase of nitrogen content, the strength of the material increases, and there is no substantial change in plasticity. That is, Examples 1-3 of the present invention obtain high manganese high-nitrogen nickel-free austenitic stainless steel thin-walled tubes with high size accuracy, high surface quality and excellent comprehensive properties.

TABLE-US-00003 TABLE 3 Nitrogen Yield Tensile Elongation content (%, by strength strength rate weight) (MPa) (MPa) (%) High-nitrogen 0.62 480 900 54 nickel-free stainless steel as-forged bar Finished tube of 0.81 608 1019 51 Example 1 Finished tube of 1.08 711 1112 55 Example 3 Finished tube of 1.15 781 1215 56 Example 2

Experimental Example 2 Structural Changes Before and After Cold Deformation

[0133] X-ray diffraction spectrums of the Φ12×1.1 mm high-nitrogen nickel-free stainless steel solid solution tube (N: 0.92% by weight) obtained after the cold deformation and heat treatment of the seventh pass in Example 3, and the high-nitrogen nickel-free stainless steel tubes after 21%, 43%, and 66% cold deformation thereof were measured. The specific measurement method is according to JY/T 009-1996 General rules for rotating target polycrystal X-ray diffractometry, Rigaku D/max 2500PC X-ray Diffractometer was used to measure the metal tube samples.

[0134] FIG. 16 shows the X-ray diffraction spectrums of the high-nitrogen nickel-free stainless steel (N: 0.92% by weight) tubes in the solid solution state and in the above three cold deformation states, where X-ray diffraction spectrums of the (111) crystal plane, (200) crystal plane, and (220) crystal plane are standard austenite X-ray diffraction spectrums, and there is no shift in all diffraction peaks, indicating that the material remains a stable austenitic structure in the solid solution state and in a cold deformation state of less than 66%. That is, the austenite structure of the high-nitrogen nickel-free austenitic stainless steel thin-walled tube obtained by the present invention will not be affected in term of stability when used in a cold deformation state of less than 66%.

Experimental Example 3 Comparison of Coating Firmness on the Surface of Stent Metal Platform after Different Surface Treatments

[0135] The coating firmness index on the stent surface is an important evaluation index for drug coatings. The coating firmness evaluation method adopted in the present invention is as follows:

[0136] (1) Experimental grouping: Group I was the stent after conventional electrochemical polishing (that is, the high-nitrogen nickel-free stainless steel vascular stent after surface pretreatment before polishing in Example 4 after conventional electrochemical polishing), which was directly sprayed to form a drug coating, conventional electrochemical polishing referring to: the sample was partially clamped with an anode, and the polishing of the sample was realized by reciprocating motion; Group II was the stent after the surface roughening by the rolling line contact type electrochemical polishing of the present invention (that is, the high-nitrogen nickel-free stainless steel vascular stent after surface finishing and precise molding obtained in Example 4), which was directly sprayed to form a drug coating. After drying the stent for 1 day, it was sterilized by ethylene oxide and aerated for 7 days.

[0137] (2) The drug stents in Groups I and II were crimped and mounted using the special mounting equipment for stent system—vascular stent crimper to form a stent system.

[0138] (3) A pressure pump was used to expand the stent system mounted above respectively, with a nominal pressure of 12 atm. After the stent was unloaded, a scanning electron microscope observation was performed to observe particularly the coating morphology at the part with the largest deformation of the stent mesh wire.

[0139] FIG. 17 shows the surface morphology of the stent metal platforms subjected to different surface treatments after being sprayed to form drug coatings followed by stent crimping and expanding. As shown in FIG. 17B, when the drug coating is directly sprayed after conventional electrochemical polishing, the stent will partially fall off after expansion. On the other hand, as shown in FIG. 17A, after the surface modification of the present invention, the coating has very good binding properties, and even after relatively large reciprocating deformations, the coating still maintains good shape characteristics.