Method for improving structure stability and special grain boundary density of 316L stainless steel

Abstract

The invention provides a method for improving the structure stability and special grain boundary density of 316L stainless steel, which comprises the following steps: S1, preparing a 316L stainless steel ingot blank by adopting an additive manufacturing process, and outputting a stainless steel plate; s2, the stainless steel plate is polished, and the polished stainless steel plate is subjected to rolling treatment; s3, the rolled stainless steel plate is subjected to annealing treatment, abrasive paper is adopted for polishing the stainless steel plate to be bright, and the surface of the stainless steel plate is cleaned and dried; and S4, the stainless steel plate is subjected to vacuum tube sealing treatment, and a tube sealing sample is subjected to secondary annealing. The invention provides a processing method for simultaneously obtaining high-density special grain boundary and excellent structure stability in additive manufacturing of 316L stainless steel, and the processing method has the advantages of simple equipment, convenience in operation, reliable technology, high efficiency, good repeatability and the like.

Description

Technical Field

[0001] This invention relates to the technical field of metal material processing, specifically to a method for improving the microstructure stability and special grain boundary density of 316L stainless steel. Background Technology

[0002] 316L stainless steel, due to its strong corrosion resistance and excellent biocompatibility, has important engineering applications in marine engineering, chemical engineering, and biomedicine. However, in its application, when 316L stainless steel operates at certain high temperatures for extended periods, Cr-rich M23C6 type compounds tend to form at the grain boundaries. This results in excessively low Cr content at the grain boundaries, drastically reducing its corrosion resistance. Therefore, researchers have improved its resistance to intergranular corrosion by introducing a large number of coincidence site lattice (CSL) grain boundaries into the interior of 316L stainless steel.

[0003] CSL grain boundaries are generally represented by ΣCSL, where Σ is the reciprocal of the lattice symmetry ratio of the two crystals. CSL grain boundaries are a special type of grain boundary with a certain orientation difference that appears in FCC metals with low to medium stacking fault energies. Σ3 grain boundaries are low CSL grain boundaries, belonging to annealed twin boundaries, and possess excellent resistance to intergranular corrosion and grain boundary slip. The main cause of annealed twin formation is the "growth accident" that occurs during recrystallization nucleation when large-angle grain boundaries migrate to the {111} close-packed plane. The probability of a "growth accident" is directly proportional to the speed and distance of large-angle migration during recrystallization nucleation. In the mechanism of annealed twin formation, higher annealing temperatures and larger residual strain jointly increase the migration speed of large-angle grain boundaries, while lower nucleation sites facilitate longer migration distances. Therefore, 316 stainless steel with medium to low rolling yields is more likely to form a large number of annealed twins after high-temperature, short-time annealing heat treatment. Meanwhile, due to their extremely low interfacial energy (approximately 1% of that of ordinary high-angle interfaces), twin boundaries exhibit high stability. Compared to ordinary grain boundaries, annealed twin boundaries can hinder crack propagation in the alloy and improve its resistance to interfacial decomposition, resulting in superior crack resistance and tensile properties. Therefore, the presence of Σ3 grain boundaries is crucial for the corrosion resistance and mechanical properties of 316L stainless steel. Numerous studies have shown that although 316L stainless steel can achieve a high proportion of Σ3 grain boundaries through appropriate rolling and annealing treatment, its microstructure exhibits poor stability at high temperatures, leading to coarse grains. This results in a significant reduction in the Σ3 grain boundary density and mechanical properties of 316L stainless steel, severely limiting its engineering applications. Summary of the Invention

[0004] To address the problems existing in the prior art, this invention provides a method for improving the microstructure stability and special grain boundary density of 316L stainless steel. This method solves the technical problem that while 316L stainless steel can achieve a high proportion of Σ3 grain boundaries after appropriate rolling and annealing treatment, its microstructure has poor stability at high temperatures, resulting in coarse grains. This leads to a significant reduction in the Σ3 grain boundary density and mechanical properties of 316L stainless steel, severely limiting its engineering applications.

[0005] This invention provides a method for improving the microstructure stability and special grain boundary density of 316L stainless steel, comprising:

[0006] S1. 316L stainless steel ingots are prepared using additive manufacturing process, and stainless steel sheets are output.

[0007] S2. Grind the stainless steel sheet and then roll the ground stainless steel sheet.

[0008] S3. Anneal the rolled stainless steel sheet, polish it with sandpaper until it is bright, and clean and dry the surface of the stainless steel sheet.

[0009] S4. Vacuum seal the stainless steel sheet and then perform a second annealing on the sealed sample.

[0010] Optionally, the rolling process includes:

[0011] Rolling strain rate 10-20s -1 The single-pass reduction is 0.1 mm, the total reduction is 30%, and the rolling temperature is 25℃.

[0012] Optionally, the preparation of 316L stainless steel ingots using additive manufacturing includes:

[0013] 316L stainless steel ingots were prepared using selective laser melting technology with a strip scanning strategy. The laser melting technology included substrate preheating of 100°C, laser power of 280W, line spacing of 100μm, scanning speed of 900mm / s, strip spacing of 10mm, layer thickness of 60μm, interlayer rotation of 67°, and spot diameter of 100μm. The 316L stainless steel ingots were then cut into plates.

[0014] Optionally, the secondary annealing includes:

[0015] The sealed sample was placed in a high-temperature resistance furnace at a heating temperature of 1000℃ or 1200℃ and kept at that temperature for 0.5 hours. The sealed sample was then removed and water-cooled.

[0016] Optionally, the high-temperature resistance furnace includes:

[0017] The high-temperature resistance furnace is model SX-G07103.

[0018] Optionally, the step of polishing the surface of the stainless steel sheet to a bright shine with sandpaper and cleaning and drying it includes:

[0019] The stainless steel sheet was polished to a bright finish using 400#, 800#, 1000#, 1200#, 2000# and 3000# sandpaper in sequence. After polishing, it was cleaned with anhydrous ethanol and its surface was dried.

[0020] Compared with the prior art, the present invention:

[0021] This invention provides a processing method for additive manufacturing of 316L stainless steel that simultaneously achieves high-density special grain boundaries and excellent microstructure stability. The method has the advantages of simple equipment, convenient operation, reliable technology, high efficiency and good repeatability. Attached Figure Description

[0022] The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments consistent with the invention and, together with the description, serve to explain the principles of the invention.

[0023] 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, for those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0024] Figure 1 This is a schematic diagram of the method flow of the present invention;

[0025] Figure 2 This is a schematic diagram of the selective laser melting strip scanning strategy used in this invention;

[0026] Figure 3 This is a schematic diagram comparing physical images of 316L stainless steel prepared by the method of this invention and 316L stainless steel samples with high Σ3 grain boundary density produced by conventional methods.

[0027] Figure 4 This is a schematic diagram showing the specific grain boundary density changes of the 316L stainless steel processed in this invention and the control group material (commercial 316L stainless steel sheets manufactured in a conventional manner and subjected to the same rolling and annealing treatment).

[0028] Figure 5 This is a color-coded inverse pole figure of the 316L stainless steel prepared by this method in this invention and the control group material (316L stainless steel sheet manufactured by conventional methods and subjected to the same rolling and annealing treatment).

[0029] Figure 6This is a special grain boundary distribution diagram of the 316L stainless steel prepared by this method in this invention and the control group material (316L stainless steel sheet manufactured by conventional methods and subjected to the same rolling and annealing treatment). Detailed Implementation

[0030] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments of this application, all other implementation cases obtained by those skilled in the art without creative effort are within the scope of protection of this application. Functional units with the same reference numerals in the examples of this invention have the same and similar structures and functions.

[0031] See Figure 1 This invention provides a method for improving the microstructure stability and special grain boundary density of 316L stainless steel, comprising:

[0032] S1. 316L stainless steel ingots are prepared using additive manufacturing process, and stainless steel sheets are output.

[0033] S2. The stainless steel sheet is ground, and then the ground stainless steel sheet is rolled. The rolling strain rate of the rolling process is 10-20 s. -1 The single-pass reduction is 0.1 mm, the total reduction is 30%, and the rolling temperature is 25°C.

[0034] S3. Anneal the rolled stainless steel sheet, polish it with sandpaper until it is bright, and clean and dry the surface of the stainless steel sheet.

[0035] S4. Vacuum seal the stainless steel sheet and then perform a second annealing on the sealed sample.

[0036] In this embodiment, see Figure 2 Workpiece preparation: 316L stainless steel ingots were prepared using selective laser melting technology with a strip scanning strategy. The main process parameters were as follows: substrate preheating 100℃, laser power 280W, line spacing 100μm, scanning speed 900mm / s, strip spacing 10mm, layer thickness 60μm, interlayer rotation 67°, and spot diameter 100μm. Subsequently, four 90×50×2mm pieces were cut from the ingot. 3 The board material.

[0037] The additively manufactured stainless steel sheet is ground and placed on a rolling mill. The roll spacing is adjusted for rolling. The rolling deformation process parameters are: rolling strain rate 10-20 s. -1The single-pass reduction was 0.1 mm, the total reduction was 30%, and the rolling temperature was 25℃. After rolling, samples were taken for subsequent annealing. The rolled samples were sequentially polished to a glossy finish using 400#, 800#, 1000#, 1200#, 2000#, and 3000# sandpaper. After polishing, the samples were cleaned with anhydrous ethanol and dried. The samples were then placed in a high-temperature resistance furnace along with the crucible and held for a period of time. Finally, the samples were removed and rapidly water-cooled. The annealing process parameters were controlled as follows: heating temperature 1000℃, holding time 0.5 h, and water cooling.

[0038] In another embodiment, see Figure 3 From left to right, the samples are in their printed state (AM-30%-1000℃, AM-30%-1200℃). Additively manufactured stainless steel sheets and conventionally manufactured (rolled and annealed) stainless steel sheets (control group) were ground and placed on a rolling mill. The roll spacing was adjusted during rolling. The rolling deformation process parameters were: rolling strain rate 10-20 s. -1 The single-pass reduction was 0.1 mm, the total reduction was 30%, and the rolling temperature was 25℃. Samples were then taken from both types of rolled plates for subsequent annealing. The rolled samples were polished to a glossy finish using 400#, 800#, 1000#, 1200#, 2000#, and 3000# sandpaper, respectively. After polishing, the samples were cleaned with anhydrous ethanol and dried. The samples were then placed in a high-temperature resistance furnace with the crucible and held for a period of time. Finally, the samples were removed and rapidly water-cooled. The annealing process parameters were controlled as follows: heating temperature 1000℃, holding time 0.5 h, and water cooling.

[0039] See Figures 3 to 5 ,in Figure 4 In the middle: (a) conventionally processed (CP) material, (b) CP material 30% rolled and annealed at 1000℃ (CP-30%-1000℃), (c) CP material 30% rolled and annealed at 1200℃ (CP-30%-1200℃), (d) laser additively manufactured (AM) material, (e) AM material 30% rolled and annealed at 1000℃ (AM-30%-1000℃), (f) AM material 30% rolled and annealed at 1000℃ (AM-30%-1200℃); Figure 5The samples obtained using the above methods are: (a) conventionally processed (CP) materials, (b) CP materials rolled at 30% and annealed at 1000℃ (CP-30%-1000℃), (c) CP materials rolled at 30% and annealed at 1200℃ (CP-30%-1200℃), (d) laser-added manufactured (AM) materials, (e) AM materials rolled at 30% and annealed at 1000℃ (AM-30%-1000℃), and (f) AM materials rolled at 30% and annealed at 1000℃ (AM-30%-1200℃). The Σ3 grain boundary density of the AM-30%-1000℃ sample obtained using the above methods is as high as 17.3 × 10⁻⁶. 4 m -1 The Σ3 grain boundary density was significantly higher than that of the control group sample (5.09 × 10⁻⁶). 4 m -1 Meanwhile, its average grain size was only 3.7 μm, which was significantly lower than the average grain size of the control group sample (13.4 μm).

[0040] This invention utilizes an additive manufacturing-rolling-annealing composite process to prepare 316L stainless steel with high Σ3 grain boundary density and high microstructural stability. First, the 316L stainless steel prepared by laser melting additive manufacturing exhibits significant residual strain and abundant silicon-oxygen precipitates. The substantial residual strain provides ample driving energy for recrystallization nucleation, increasing the probability of "growth accidents." Simultaneously, the abundant precipitates inhibit grain growth, improving the microstructural stability of the stainless steel in high-temperature environments. Second, combining rolling and annealing processes yields stainless steel with high Σ3 grain boundary density and good microstructural stability, effectively enhancing its resistance to intergranular corrosion, crack resistance, and tensile properties. Test results show that the additive manufacturing-rolling-annealing composite process provided by this invention effectively improves the microstructure of 316L stainless steel, resulting in a fine equiaxed crystal morphology and a high density of Σ3 grain boundaries. Furthermore, the preparation process of high-density Σ3 grain boundary microstructure is convenient to operate, requires simple equipment, is economical and practical, technically reliable, highly efficient, and produces stable quality, thus achieving good economic benefits.

[0041] It should be noted that, in this document, relational terms such as "first" and "second" are used merely to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitations, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element.

[0042] The above description is merely a specific embodiment of the present invention, enabling those skilled in the art to understand or implement the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the present invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features claimed herein.

Claims

1. A method for improving the microstructure stability and special grain boundary density of 316L stainless steel, characterized in that, include: S1. 316L stainless steel ingots are prepared using additive manufacturing process, and stainless steel sheets are output. S2. Grind the stainless steel sheet and then roll the ground stainless steel sheet. S3. Anneal the rolled stainless steel sheet, polish it with sandpaper until it is bright, and clean and dry the surface of the stainless steel sheet. S4. Vacuum seal the stainless steel sheet and then perform a second annealing on the sealed sample.

2. The method for improving the microstructure stability and special grain boundary density of 316L stainless steel as described in claim 1, characterized in that, The rolling process includes: Rolling strain rate 10-20s -1 The single-pass reduction is 0.1 mm, the total reduction is 30%, and the rolling temperature is 25℃.

3. The method for improving the microstructure stability and special grain boundary density of 316L stainless steel as described in claim 1, characterized in that, The preparation of 316L stainless steel ingots using additive manufacturing process includes: 316L stainless steel ingots were prepared using selective laser melting technology with a strip scanning strategy. The laser melting technology included substrate preheating of 100°C, laser power of 280W, line spacing of 100μm, scanning speed of 900mm / s, strip spacing of 10mm, layer thickness of 60μm, interlayer rotation of 67°, and spot diameter of 100μm. The 316L stainless steel ingots were then cut into plates.

4. The method for improving the microstructure stability and special grain boundary density of 316L stainless steel as described in claim 1, characterized in that, The secondary annealing includes: The sealed sample was placed in a high-temperature resistance furnace at a heating temperature of 1000℃ or 1200℃ and kept at that temperature for 0.5 hours. The sealed sample was then removed and water-cooled.

5. The method for improving the microstructure stability and special grain boundary density of 316L stainless steel as described in claim 4, characterized in that, The high-temperature resistance furnace includes: The high-temperature resistance furnace is model SX-G07103.

6. The method for improving the microstructure stability and special grain boundary density of 316L stainless steel as described in claim 1, characterized in that, The process of polishing the stainless steel sheet to a bright finish with sandpaper and cleaning and drying the surface includes: The stainless steel sheet was polished to a bright finish using 400#, 800#, 1000#, 1200#, 2000# and 3000# sandpaper in sequence. After polishing, it was cleaned with anhydrous ethanol and its surface was dried.

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