A fine-grained 316l stainless steel and a method for producing the same

Abstract

The present application belongs to the technical field of stainless steel organization regulation, and particularly relates to a fine-grained 316L stainless steel and a preparation method thereof, wherein the preparation method comprises the following steps: S1, obtaining 316L stainless steel powder and 8YSZ powder; S2, alternately and positively and reversely ball-milling the 316L stainless steel powder and the 8YSZ powder to obtain mixed powder; S3, discharging plasma sintering of the mixed powder under vacuum condition, and obtaining dispersion strengthening fine-grained 316L stainless steel after cooling; the present application utilizes the grain boundary pinning effect of 8YSZ in the sintering process of 316L stainless steel, can significantly inhibit the grain growth, and realizes the stable refinement of the organization after SPS sintering. Through the alternately and positively and reversely ball-milling procedure, the agglomeration problem of the ceramic phase is effectively avoided, the dispersion uniformity of 8YSZ in the 316L powder is improved, the second phase is beneficial to the uniform pinning effect in the subsequent sintering process, and the comprehensive mechanical properties of the material are effectively improved.

Description

Technical Field

[0001] This invention belongs to the field of stainless steel microstructure control technology, specifically a fine-grained 316L stainless steel and its preparation method. Background Technology

[0002] 316L stainless steel possesses excellent corrosion resistance, formability, and comprehensive mechanical properties, making it widely used in medical devices, chemical equipment, and integrated structural and functional components. Spark plasma sintering (SPS), characterized by rapid heating, short sintering time, and high densification efficiency, is one of the important technical routes for preparing high-performance 316L stainless steel.

[0003] However, 316L stainless steel is still prone to grain growth at higher sintering temperatures, making it difficult to maintain a stable fine-grained structure, which in turn affects the material's strength and microstructure uniformity. Existing methods that simply adjust the sintering temperature, holding time, or pressure often fail to achieve a balance between densification and microstructure refinement; directly introducing a ceramic phase can easily lead to problems such as uneven dispersion, agglomeration, or insufficient action of the reinforcing phase.

[0004] 8YSZ possesses characteristics such as good high-temperature stability, high thermal stability, and the ability to form grain boundary pinning when combined with a metal matrix. The current technical challenge is to effectively integrate 8YSZ into the 316L stainless steel powder system at appropriate dosages and mixing methods, and to coordinate it with SPS process parameters to achieve significant grain refinement while ensuring sintering efficiency.

[0005] To address the issues of grain growth and unstable microstructure refinement in existing SPS-sintered 316L stainless steel, this invention proposes an 8YSZ dispersion-strengthened SPS-sintered fine-grained 316L stainless steel and its preparation method. By controlling the amount of 8YSZ added, the ball milling mixing program, and the SPS sintering parameters, significant microstructure refinement of 316L stainless steel can be achieved. Summary of the Invention

[0006] To address the aforementioned technical problems, this invention provides a method for preparing fine-grained 316L stainless steel, comprising the following steps: S1. Obtain 316L stainless steel powder and 8YSZ powder, wherein the mass of the 8YSZ powder is 0.5wt%~0.9wt% of the mass of the 316L stainless steel powder; S2. The 316L stainless steel powder and the 8YSZ powder are subjected to alternating forward and reverse ball milling to obtain a mixed powder. The ball milling speed of the alternating forward and reverse ball milling is 180~220 r / min. The alternating forward and reverse ball milling includes 3~5 single ball milling cycles. The single ball milling cycle includes 20~40 min forward rotation, 20~40 min first interval, 20~40 min reverse rotation, and 20~40 min second interval in sequence. S3. The mixed powder is subjected to discharge plasma sintering under vacuum conditions, and after cooling, a dispersion-strengthened fine-grained 316L stainless steel is obtained. The heating rate of the discharge plasma sintering is 90~110℃ / min, the sintering temperature is 1130~1170℃, the sintering pressure is 45~55MPa, and the holding time is 4~6min.

[0007] Furthermore, the mass of the 8YSZ powder is any one of 0.5wt%, 0.6wt%, 0.7wt%, 0.8wt%, or 0.9wt% of the mass of the 316L stainless steel powder, or a range between two of these.

[0008] Furthermore, the ball milling speed of the alternating forward and reverse ball milling is any one of 180 r / min, 190 r / min, 200 r / min, 210 r / min, 220 r / min or a range between two of them.

[0009] Furthermore, the heating rate of the discharge plasma sintering is any one of 90℃ / min, 100℃ / min, 110℃ / min, or a range between two of them.

[0010] Furthermore, the sintering temperature is 1130℃, 1140℃, 1150℃, 1160℃, or 1170℃.

[0011] Furthermore, the sintering pressure is any one of 45 MPa, 50 MPa, 55 MPa, or a range between two of them.

[0012] Furthermore, the heat preservation time is any one of 4 min, 5 min, 6 min, or a range between two of them.

[0013] The 8YSZ powder is a ZrO2 powder containing 8 mol% of the Y2O3 stable phase.

[0014] In step S1, the mass of the 8YSZ powder is 0.7 wt% of the mass of the 316L stainless steel powder. In step S2, the ball milling speed of the alternating forward and reverse ball milling is 200 r / min; In step S3, the heating rate of the discharge plasma sintering is 100℃ / min, the sintering temperature is 1150℃, the sintering pressure is 50MPa, and the holding time is 5min.

[0015] To address the aforementioned technical problems, the present invention also provides a dispersion-strengthened fine-grained 316L stainless steel, characterized in that it is prepared using the aforementioned method for preparing fine-grained 316L stainless steel.

[0016] The average equivalent circle diameter of the grains in the dispersion-strengthened fine-grained 316L stainless steel is less than or equal to 6.5 μm.

[0017] In the dispersed-strengthened fine-grained 316L stainless steel, the proportion of grains with a size of less than 10μm is greater than or equal to 85%.

[0018] This invention utilizes the grain boundary pinning effect of 8YSZ during the sintering process of 316L stainless steel to significantly inhibit grain growth and achieve stable and refined microstructure after SPS sintering. By alternating forward and reverse ball milling processes, the agglomeration of the ceramic phase is effectively avoided, improving the dispersion uniformity of 8YSZ in the 316L powder. This facilitates the uniform pinning effect of the second phase during subsequent sintering. The material prepared by this method exhibits a shift in grain size distribution towards a finer grain range, significantly increasing grain number density and effectively enhancing the overall mechanical properties of the material. Attached Figure Description

[0019] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the structures shown in these drawings without creative effort.

[0020] Figure 1 The image shows the EBSD grain structure of fine-grained 316L stainless steel after adding 8YSZ in Example 1. Figure 2 The EBSD grain diagram shows the 316L stainless steel without 8YSZ in Comparative Example 1.

[0021] The realization of the objective, functional features and advantages of the present invention will be further explained in conjunction with the embodiments and with reference to the accompanying drawings. Detailed Implementation

[0022] The technical solutions described below in conjunction with the embodiments will be clearly and completely described. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0023] This invention provides a method for preparing fine-grained 316L stainless steel, comprising the following steps: S1. Obtain 316L stainless steel powder and 8YSZ powder, wherein the mass of the 8YSZ powder is 0.5wt%~0.9wt% of the mass of the 316L stainless steel powder; The 8YSZ powder introduced as a reinforcing phase is specifically ZrO2 powder containing 8 mol% of the Y2O3 stabilizing phase. This proportion of 8YSZ was chosen because of its extremely high high-temperature and thermal stability, which allows it to act as an effective grain boundary pinning phase during subsequent sintering, significantly inhibiting grain growth in the 316L stainless steel matrix. The amount of 8YSZ added is controlled between 0.5 wt% and 0.9 wt%, a range that maximizes grain refinement while ensuring material density.

[0024] S2. The 316L stainless steel powder and the 8YSZ powder are subjected to alternating forward and reverse ball milling to obtain a mixed powder. The ball milling speed of the alternating forward and reverse ball milling is 180~220 r / min. The alternating forward and reverse ball milling includes 3~5 single ball milling cycles. The single ball milling cycle includes 20~40 min forward rotation, 20~40 min first interval, 20~40 min reverse rotation, and 20~40 min second interval in sequence. A specific alternating forward and reverse ball milling process was employed to improve the dispersion uniformity of nano-sized 8YSZ powder in 316L stainless steel matrix powder, effectively preventing agglomeration of the ceramic phase due to electrostatics or excessively high surface energy. A cyclical pattern of "forward rotation, first interval, reverse rotation, second interval" was repeated multiple times. This intermittent, directional mixing method ensures that the reinforcing phase particles uniformly coat or embed themselves on the surface of the matrix powder, laying the foundation for the formation of a uniformly distributed, dispersed reinforcing phase during subsequent sintering.

[0025] S3. The mixed powder is subjected to discharge plasma sintering under vacuum conditions, and after cooling, a dispersion-strengthened fine-grained 316L stainless steel is obtained. The heating rate of the discharge plasma sintering is 90~110℃ / min, the sintering temperature is 1130~1170℃, the sintering pressure is 45~55MPa, and the holding time is 4~6min.

[0026] By utilizing the rapid heating rate and short sintering time of spark plasma sintering (SPS), combined with pre-dispersed uniform 8YSZ pinned phase, the densification of the material can be completed in a very short time, while suppressing grain coarsening, thus achieving a high degree of microstructure refinement and homogenization.

[0027] Example 1

[0028] S1. Obtain 55g of 316L stainless steel powder and 0.385g of 8YSZ powder; S2. The 316L stainless steel powder and the 8YSZ powder are subjected to alternating forward and reverse ball milling to obtain mixed powder. The ball milling speed of the alternating forward and reverse ball milling is 200 r / min. The alternating forward and reverse ball milling includes 4 single ball milling cycles. The single ball milling cycle includes 30 min forward rotation, 30 min first interval, 30 min reverse rotation and 30 min second interval in sequence. S3. The mixed powder is subjected to discharge plasma sintering under vacuum conditions, and after cooling, a dispersion-strengthened fine-grained 316L stainless steel is obtained. The heating rate of the discharge plasma sintering is 100℃ / min, the sintering temperature is 1150℃, the sintering pressure is 50MPa, and the holding time is 5min.

[0029] Please see Figure 1 , Figure 1 This is a grain diagram of the fine-grained EBSD 316L stainless steel after adding 8YSZ in Example 1. Figure 1 The image shows the EBSD grain size distribution of the fine-grained 316L stainless steel after adding 8YSZ in Example 1. The 316L stainless steel obtained in Example 1 has relatively fine grains with a more uniform grain size distribution and a significantly reduced number of coarse grains. Under the same EBSD statistical field of view, the sample in Example 1 had 2040 grains, with an average equivalent circle diameter of 6.30 μm and a median equivalent circle diameter of 5.29 μm. Grains smaller than 10 μm accounted for 85.05%, while coarse grains of 15 μm and above accounted for 3.38%. This indicates that under the synergistic effect of 0.7 wt% 8YSZ addition, alternating forward and reverse ball milling, and SPS sintering conditions, 8YSZ can effectively inhibit grain growth in 316L stainless steel during sintering, causing the grain size distribution to concentrate in the fine-grained range.

[0030] Example 2

[0031] Unlike Example 1, the amount of 8YSZ added in step S1 is 0.275g; The ball milling speed in the alternating forward and reverse ball milling in step S2 is 180 r / min. The alternating forward and reverse ball milling includes 3 single ball milling cycles. The single ball milling cycle includes 20 min forward rotation, 20 min first interval, 20 min reverse rotation and 20 min second interval in sequence. The heating rate of the discharge plasma sintering in step S3 is 90℃ / min, the sintering temperature is 1130℃, the sintering pressure is 45MPa, and the holding time is 4min.

[0032] Example 3

[0033] Unlike Example 1, the amount of 8YSZ added in step S1 is 0.495g; The ball milling speed in the alternating forward and reverse ball milling in step S2 is 220 r / min. The alternating forward and reverse ball milling includes 5 single ball milling cycles. The single ball milling cycle includes 40 min forward rotation, 40 min first interval, 40 min reverse rotation and 40 min second interval in sequence. The heating rate of the discharge plasma sintering in step S3 is 110℃ / min, the sintering temperature is 1170℃, the sintering pressure is 55MPa, and the holding time is 6min.

[0034] Comparative Example 1 Unlike Example 1, 8YSZ powder is not added in step S1.

[0035] Please see Figure 2 , Figure 2 The image shows the EBSD grain structure of 316L stainless steel without 8YSZ in Comparative Example 1. The 316L stainless steel obtained in Comparative Example 1 has relatively large grain sizes, with many coarse grains in some areas, and the degree of microstructure refinement is significantly lower than that of Example 1. Under the same EBSD statistical field of view, the sample in Comparative Example 1 has 1092 statistically significant grains, with an average equivalent circle diameter of 8.35 μm and a median equivalent circle diameter of 6.63 μm; grains smaller than 10 μm account for 69.14%, and coarse grains of 15 μm and above account for 13.83%. Figure 1 In comparison, the sample without 8YSZ had a lower grain number density and a higher proportion of coarse grains, indicating that the addition of 8YSZ can significantly inhibit grain growth in 316L stainless steel during SPS sintering, thereby achieving fine grain strengthening.

[0036] Comparative Example 2 Unlike Example 1, in step S2, the 316L stainless steel powder and the 8YSZ powder are subjected to unidirectional continuous ball milling to obtain mixed powder. The rotation speed of the unidirectional continuous ball mill is 200 r / min, and the ball milling time is the same as the sum of the forward and reverse rotation time in Example 1.

[0037] Comparative Example 3 Unlike Example 1, in step S1, 0.165g of 8YSZ powder is obtained.

[0038] Comparative Example 4 Unlike Example 1, in step S1, 0.605g of 8YSZ powder is obtained.

[0039] Comparative Example 5 Unlike Example 1, in step S2, the ball milling speed of the alternating forward and reverse ball milling is 150 r / min.

[0040] Comparative Example 6 Unlike Example 1, in step S2, the ball milling speed of the alternating forward and reverse ball milling is 250 r / min.

[0041] Comparative Example 7 Unlike Example 1, in step S2, the alternating forward and reverse ball milling includes two single ball milling cycles.

[0042] Comparative Example 8 Unlike Example 1, in step S2, the single ball milling cycle only includes 30 minutes of forward rotation and 30 minutes of reverse rotation performed sequentially, excluding the first and second intervals.

[0043] The stainless steel samples prepared in Examples 1 to 3 and Comparative Examples 1 to 8 were subjected to performance tests. The relative density was determined by the Archimedes method; the yield strength, tensile strength and elongation after fracture were obtained by room temperature tensile property testing. Three parallel samples were taken from each group of samples and the average value was calculated. The test results are shown in Tables 1 and 2.

[0044] Table 1 Mechanical properties of stainless steel obtained in the examples and comparative examples

[0045] Table 2. Statistical analysis of density and grain size of stainless steel obtained in the examples and comparative examples.

[0046] This invention utilizes the grain boundary pinning effect of 8YSZ during the sintering process of 316L stainless steel to significantly inhibit grain growth and achieve stable and refined microstructure after SPS sintering. By alternating forward and reverse ball milling processes, the agglomeration of the ceramic phase is effectively avoided, improving the dispersion uniformity of 8YSZ in the 316L powder. This facilitates the uniform pinning effect of the second phase during subsequent sintering. The material prepared by this method exhibits a shift in grain size distribution towards a finer grain range, significantly increasing grain number density and effectively enhancing the overall mechanical properties of the material.

[0047] The above description is merely a preferred embodiment of the present invention and does not limit the patent scope of the present invention. Any equivalent structural transformations made using the contents of the present invention under the inventive concept of the present invention, or direct / indirect applications in other related technical fields, are included within the patent protection scope of the present invention.

Claims

1. A method for preparing fine-grained 316L stainless steel, characterized in that, Includes the following steps: S1. Obtain 316L stainless steel powder and 8YSZ powder, wherein the mass of the 8YSZ powder is 0.5wt%~0.9wt% of the mass of the 316L stainless steel powder; S2. The 316L stainless steel powder and the 8YSZ powder are subjected to alternating forward and reverse ball milling to obtain a mixed powder. The ball milling speed of the alternating forward and reverse ball milling is 180~220 r / min. The alternating forward and reverse ball milling includes 3~5 single ball milling cycles. The single ball milling cycle includes 20~40 min forward rotation, 20~40 min first interval, 20~40 min reverse rotation, and 20~40 min second interval in sequence. S3. The mixed powder is subjected to discharge plasma sintering under vacuum conditions, and after cooling, a dispersion-strengthened fine-grained 316L stainless steel is obtained. The heating rate of the discharge plasma sintering is 90~110℃ / min, the sintering temperature is 1130~1170℃, the sintering pressure is 45~55MPa, and the holding time is 4~6min.

2. The method for preparing fine-grained 316L stainless steel according to claim 1, characterized in that, The 8YSZ powder is a ZrO2 powder containing 8 mol% of the Y2O3 stable phase.

3. The method for preparing fine-grained 316L stainless steel according to claim 1, characterized in that, In step S1, the mass of the 8YSZ powder is 0.7 wt% of the mass of the 316L stainless steel powder. In step S2, the ball milling speed of the alternating forward and reverse ball milling is 200 r / min; In step S3, the heating rate of the discharge plasma sintering is 100℃ / min, the sintering temperature is 1150℃, the sintering pressure is 50MPa, and the holding time is 5min.

4. A dispersion-strengthened fine-grained 316L stainless steel, characterized in that, It is prepared by the method for preparing fine-grained 316L stainless steel according to any one of claims 1 to 3.

5. The dispersion-strengthened fine-grained 316L stainless steel according to claim 4, characterized in that, The average equivalent circle diameter of the grains in the dispersion-strengthened fine-grained 316L stainless steel is less than or equal to 6.5 μm.

6. The dispersion-strengthened fine-grained 316L stainless steel according to claim 4, characterized in that, In the dispersion-strengthened fine-grained 316L stainless steel, the proportion of grains with a size of less than 10μm is greater than or equal to 85%.

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