A preparation method of high-strength corrosion-resistant 316L stainless steel

Abstract

The present invention discloses a method for preparing high-strength, corrosion-resistant 316L stainless steel, comprising the steps of: preheating an ECAP mold; placing a sample in the ECAP mold for heat preservation; extruding in an equal channel angle mold; stress-relief annealing the extruded sample; rotating the sample by 90° and then placing it in the ECAP mold for repeated extrusion and annealing; repeating the extrusion for 4 to 6 times to obtain high-strength, corrosion-resistant 316L steel. The present invention utilizes the equal channel angular extrusion method ECAP to process 316L austenitic stainless steel. During the ECAP treatment process, a large amount of plastic deformation is accumulated through multiple extrusion deformations, and an annealing heat treatment process is added between the extrusion passes, which can refine the grain size of the 316L steel, optimize the grain boundary distribution of the material, especially increase the proportion of small-angle grain boundaries, and effectively improve the mechanical strength and corrosion resistance of the material.

Description

Technical Field

[0001] The invention relates to a method for preparing high-strength corrosion-resistant 316L stainless steel. Background Art

[0002] 316L stainless steel is widely used in petrochemical, shipbuilding steel and nuclear power due to its excellent corrosion resistance and toughness. In the third generation pressurized water reactor AP1000 nuclear power plant, integrally forged 316L austenitic stainless steel has been used as the main pipeline material; the main chemical composition of the alloy is: Cr: 16-18%, Ni: 10-14%, Mo: 2-3%, Mn: ≤2%, Si: ≤1%, P: ≤0.045%, C: ≤0.03%; the Mo content of 316L makes this steel have excellent corrosion resistance and can be safely used in halogen ion environments such as chloride ions. However, corrosion cracking may still occur in harsh service environments. Intergranular corrosion is one of the most dangerous forms of damage. It is a local corrosion damage that occurs and develops along or close to the grain boundary of the metal in a specific corrosive environment, and it may also turn into intergranular stress corrosion cracking; intergranular corrosion starts from the surface of the metal material and develops along the grain boundary to the inside, causing a great loss of the bonding force between the grains, which leads to the almost complete disappearance of the strength of the material. Therefore, although some stainless steel materials that have intergranular corrosion have not shown obvious damage on the surface, they are actually damaged inside and are very fragile. Intergranular corrosion cracks often occur at high-angle grain boundaries. This is because high-angle grain boundaries are prone to produce Ni-rich and Cr-poor element segregation and local stress concentration, which are the main nucleation points for stress corrosion cracking, while low-angle grain boundaries are not prone to stress corrosion cracking. Therefore, increasing the content of low-angle grain boundaries in austenitic steel and reducing the content of high-angle grain boundaries at the same time are the key to reducing or delaying the occurrence of cracks. 316L stainless steel has a large sensitivity to stress corrosion cracking (SCC) in boiling MgCl2 solution, which is a method for quickly evaluating the SCC sensitivity of stainless steel.

[0003] On the other hand, during the deformation of austenitic stainless steel, the grain size and yield strength show a Hall-Petch relationship. When the grain size decreases, the yield strength increases. When the grain size decreases, the number of grains in the same space increases. At this time, the energy here is higher, and it is difficult for dislocations to pass through the grain boundaries when they slip. Therefore, they are easily blocked at the grain boundaries, which means that the dislocation sources inside the grains are also reduced. In mechanical processing, the influence of grain size and dislocation density is very large. In order to improve the mechanical properties of stainless steel, it is particularly important to control the size of grains. Among the methods of large plastic deformation of metals at room temperature currently studied, commonly used methods include high pressure torsion, cold rolling and equal channel angular pressing (ECAP), etc. These methods are considered to be effective methods for refining grains and significantly improving the strength of metals and alloys. Among them, equal channel angular pressing and high pressure torsion (HPT) are currently the most commonly used methods. Compared with the HPT process, which is relatively complex and only suitable for grain refinement of materials within a shallow thickness on the surface of small samples, the ECAP method can process larger samples and is a relatively simple process used for various metal materials. It can refine their grains as a whole and improve their mechanical properties. It is an effective method for obtaining submicron or nanometer-level materials, superimposing a large amount of plastic deformation in multiple extrusion deformations to obtain ultrafine grain structure to improve the strength of the material.

[0004] Patent CN 114250421 A discloses a high nitrogen austenitic stainless steel with better intergranular corrosion and pitting resistance than 316L after welding and a manufacturing method thereof; the process includes smelting, hot rolling, coiling, hot rolling annealing and pickling, cold rolling, cold rolling annealing and pickling; by increasing the C solid solubility of the material and controlling the C content < C solid solubility, Cr precipitation in the material during welding is avoided 23 C6, improves the material's resistance to intergranular corrosion after welding, but this method only reduces the possibility of grain boundary segregation and fails to effectively increase the ratio of small-angle grain boundaries to improve corrosion resistance.

[0005] Patent CN 104017967 A discloses a high-strength and high-plasticity stainless steel and its preparation method and application. The preparation method includes: placing an austenitic stainless steel sample into a preheated ECAP mold, keeping it warm, extruding it multiple times, taking out the sample and placing it into a heat treatment furnace, annealing it, and air cooling it to room temperature to obtain high-strength and high-plasticity stainless steel; the invention extrudes the sample 6 to 8 times at an extrusion speed of 100 to 200 mm / min, and obtains ultrafine grains to improve the strength and plasticity of austenitic stainless steel, but ignores the influence of the extrusion path, and when there are too many extrusion passes, it is easy to get stuck or the sample breaks, resulting in waste.

[0006] Patent CN 107779563 A discloses a method for preparing high-strength and high-plasticity stainless steel for furnace supports; the steps are as follows: placing an austenitic stainless steel sample into a preheated ECAP mold, keeping it warm, and after multiple extrusions, taking out the sample and placing it into a heat treatment furnace for annealing, and air cooling it to room temperature; the number of extrusion passes is 5 to 10 times, the extrusion speed is 120 to 280 mm / min, and the extrusion path is bc. High-strength and high-plasticity austenitic stainless steel is also obtained through equal-pass angular extrusion treatment, but the ECAP technology is not effectively utilized, the appropriate extrusion path and internal and external angles are not calculated, and the small-angle grain boundary in the sample may have reached the critical value but continued to be extruded, resulting in fracture, increasing the test cost.

[0007] It can be seen that although the equal channel angular extrusion method can achieve the effective superposition of plastic deformation of certain metal materials, obtain ultrafine grain structure, and refine grains to improve the strength of metal materials, for 316L stainless steel materials, its elemental composition is complex. While obtaining ultrafine grain structure (ultrafine grains), a large number of high-angle grain boundaries may also be formed. Intergranular corrosion cracks often occur at high-angle grain boundaries. This is because high-angle grain boundaries are prone to produce Ni-rich and Cr-poor element segregation and local stress concentration, which is the main nucleation point of stress corrosion cracks, while low-angle grain boundaries are not easy to produce stress corrosion cracks. Therefore, increasing the content of low-angle grain boundaries in austenitic steel and reducing the content of high-angle grain boundaries at the same time are the key to reducing or delaying the occurrence of cracks.

[0008] Annealing is a metal heat treatment process that refers to slowly heating the metal to a certain temperature, maintaining it for a sufficient time, and then cooling it at an appropriate rate. The purpose is to reduce the hardness, help reduce the stress level inside the metal material, and improve machinability; reduce residual stress, stabilize size, reduce deformation and crack tendency; refine grains, adjust structure, and eliminate structural defects. Summary of the invention

[0009] In order to realize that 316L stainless steel material can have both mechanical strength and corrosion resistance, the present invention combines appropriate equal channel angular extrusion technology and appropriate annealing process intervention control, thereby achieving both grain refinement (high strength) of 316L stainless steel material and a high proportion of small-angle grain boundaries (corrosion resistance).

[0010] Specifically, a method for preparing high-strength corrosion-resistant 316L stainless steel of the present invention comprises the following steps:

[0011] Step 1: Preheat the ECAP mold at a temperature of 25 to 120°C. The role of preheating the mold is to prevent the low-temperature mold from rapidly cooling the preheated sample, causing the preheated sample to drop rapidly. If the preheating temperature is too high, the mold itself will soften to a certain extent and will also cause reverse temperature interference to the sample. Studies have found that the preheating temperature is preferably 90 to 110°C, and 100°C is the best.

[0012] Step 2: Place the sample into the ECAP mold and keep it warm for 25 to 60 minutes;

[0013] Step 3: Extrusion is performed in an equal channel angular extrusion die at an extrusion speed of 150-250 mm / min; the extrusion path is path A, B, or Bc mode, and the inner and outer angles of the extrusion channel are 80°-100° and 35°-40°, respectively. The preheated die does not need to be cooled to room temperature, and the sample is extruded at the preheated temperature. The study found that the material grains obtained when the inner angle of the extrusion channel is 90° and the outer angle is 37° are the best. Under the same extrusion conditions, the grain refinement effect of the 90° inner angle die is more obvious than that of the 120° die. For example, a die with an inner angle of 90° can refine the average grain size to a saturated size of 300nm after 4-6 extrusions, while a die with an inner angle of 120° requires more than 6 passes to approach the saturated average grain size of 300nm. Extrusion of too many passes may cause the small angle grain boundaries in the sample to reach the critical value but continue to be extruded, which is easy to cause fracture.

[0014] Step 4: Stress relief annealing of the extruded sample at 200℃~450℃, keeping warm for 1~3 hours. The inventors found that the best comprehensive effect in terms of grain and small-angle grain boundary content of the material obtained when the extruded sample is kept warm at 300℃~350℃, especially at 300℃ for 2 hours for stress relief annealing. Too short a holding time will result in insufficient annealing of the sample and uneven internal microstructure. On the other hand, annealing after each extrusion can reduce dislocation entanglement, which is beneficial to the next extrusion.

[0015] Step 5: Rotate the sample 90° and put it into the ECAP mold to repeat steps 3 and 4; repeat the extrusion for 4 to 6 times. The inventors found that the more extrusion times, the better. Generally, the microstructure change is saturated after 6 times and it is difficult to change further.

[0016] Step 6: After the last extrusion-annealing treatment, the sample is taken out of the heat treatment furnace and cooled to room temperature to obtain high-strength and corrosion-resistant 316L steel.

[0017] The present invention can partially eliminate the structural defects inside the grains through appropriate annealing process in combination with ECAP, which is beneficial to improving the mechanical strength and corrosion resistance of metal materials, and can simultaneously maintain the original grain size and grain boundary type. When the original low-angle grain boundary type accounts for a high proportion, the high corrosion resistance of the low-angle grain boundaries can be maintained.

[0018] The present invention uses a large plastic deformation method and an equal channel angular extrusion method (ECAP) to process 316L stainless steel. During the ECAP treatment process, a large amount of plastic deformation is accumulated through multiple extrusion deformations, and an annealing heat treatment process is added between each extrusion pass. Through the combination of appropriate ECAP conditions such as internal and external angles and appropriate annealing processes, the grain size of 316L steel is refined to a range of 300 nanometers to 3 microns, the grain boundary distribution of the material is optimized, and especially the proportion of small-angle grain boundaries is increased (up to 60%), thereby effectively improving the mechanical strength (yield strength can reach more than 250MPa) and corrosion resistance (increased by about 10% to 15%) of the material. DETAILED DESCRIPTION

[0019] The following examples are further descriptions of the content of the present invention as an explanation of the technical content of the present invention, but the essence of the present invention is not limited to the following examples. A person skilled in the art can and should know that any simple changes or substitutions based on the essence of the present invention should fall within the scope of protection required by the present invention. Electron backscatter diffraction (EBSD) is used to quantitatively evaluate the proportion of small-angle grain boundaries with an angle of less than 10 degrees between adjacent grains, and conventional tensile test experiments are used to measure the yield strength σ 0.2， The corrosion condition is measured by the length of time from the beginning of a significant decrease in material strength after stress and MgCl2 boiling water immersion corrosion.

[0020] Embodiment 1:

[0021] 1. Preheat the prepared ECAP mold at 100° for one hour;

[0022] 2. Draw a scale line on the cross-section surface of the 316L austenitic stainless steel sample as the starting 0° scale, place the rod-shaped sample into the manufactured ECAP mold, align the 0° scale line with the corner scale line inside the ECAP mold, and prepare for the first extrusion process of the sample;

[0023] 3. Start the first pass of equal channel angular extrusion, with an extrusion speed of 200 mm / min, an extrusion path of path bc, and inner and outer angles of the extrusion channel of 90° and 37° respectively. Extrude the sample until it is completely horizontal, and then take out the sample;

[0024] 4. After the first extrusion, the sample is subjected to stress relief annealing at 300°C for 2 hours and then furnace cooled;

[0025] 5. Preheat the mold at 100°C for one hour; rotate the 0° scale line of the sample after one extrusion clockwise by 90°, align it with the angle scale line in the ECAP mold and put it in for the second extrusion; stress relief anneal the sample after the second extrusion at 300°C for 2 hours, and furnace cool; repeat the above extrusion and stress relief annealing treatment, and perform 6 ECAP extrusion-annealing treatments in total. After the last extrusion-annealing treatment, take the sample out of the heat treatment furnace and cool it to room temperature to obtain high-strength and corrosion-resistant 316L steel. The average grain size of the obtained 316L steel is refined to the range of 300nm to 1μm, and the proportion of small-angle grain boundaries can be increased from 20% to 30% (without extrusion deformation) to 50% to 60% (after extrusion). At the same time, the yield strength increases from about 180MPa (without extrusion deformation) to more than 250MPa (after extrusion). Under stress and MgCl2 boiling water corrosion conditions, the corrosion resistance of the extruded samples is about 12% higher than that of the unextruded samples.

[0026] Example 2

[0027] In step 5, the above extrusion and stress relief annealing treatment are repeated, and a total of 4 ECAP extrusion-annealing treatments are performed, and the other processes are the same as in Example 1. The average grain size of the obtained 316L steel is refined to the range of 500nm to 2μm, and the proportion of small-angle grain boundaries can be increased from 20% to 30% (without extrusion deformation) to 40% to 50%. At the same time, the yield strength is increased from about 180MPa (without extrusion deformation) to more than 250MPa. Under stress and MgCl2 boiling water corrosion conditions, the corrosion resistance of the extruded sample is about 10% higher than that of the unextruded sample.

[0028] Comparative Example 1:

[0029] In step 3, the inner angle of the extrusion channel is 120°, and the other processes are the same as those in Example 2. Under the same extrusion pass (4 passes), the average grain size of the obtained 316L steel is about 700nm-5μm, the small angle grain boundary accounts for 32%-36%, the yield strength is 200-220MPa, and the corrosion resistance is improved by about 6%.

[0030] Comparative Example 2:

[0031] In step 3, the inner corner of the extrusion channel is 60°, and the other processes are the same as those in Example 1. This leads to a sharp increase in the difficulty of the equal channel angle extrusion experiment, which is not suitable for practical application.

[0032] Comparative Example 3:

[0033] The annealing treatment described in steps 4 and 5 was not performed at all, and a total of 6 ECAP extrusions were performed. The other processes were the same as those in Example 1. The average grain size of the obtained 316L steel was 30-100 μm, and the grain shape was irregular and disorderly. The small-angle grain boundaries accounted for 22% to 28%, the yield strength was 200 to 210 MPa, and the corrosion resistance was almost not improved.

[0034] Comparative Example 4:

[0035] Only one ECAP extrusion-annealing treatment is performed, and the other processes are the same as Example 1. The average grain size of the obtained 316L steel can be refined to a range of 2μm to 20μm, and the grain shape is long and strip-shaped. The proportion of small-angle grain boundaries is increased by about 5% compared with 20% to 30% without extrusion deformation. The yield strength is 200 to 220MPa, and the corrosion resistance is improved by about 2%. Comparative Example 5:

[0036] Only two passes of ECAP extrusion and annealing were performed, and the other processes were the same as in Example 1. Due to the small number of extrusion passes and the low degree of plastic deformation, the average grain size of the obtained 316L steel can be refined to a range of 1 μm to 10 μm, and the grain shape gradually changes from a long strip after one pass of extrusion to a nearly equiaxed shape, and the proportion of small-angle grain boundaries increases by 20% to 30% (about 8%) compared to those without extrusion deformation. The yield strength is 200 to 240 MPa, and the corrosion resistance is improved by about 4%.

[0037] Comparative Example 6:

[0038] A total of 8 ECAP extrusion-annealing treatments were performed, and the other processes were the same as in Example 1. Compared with the 6-pass extrusion in Example 1, the degree of plastic deformation was close to saturation, and the average grain size no longer increased, which was about 500nm to 2μm. The small-angle grain boundaries accounted for 50% to 60%, the yield strength was above 250MPa, and the corrosion resistance was increased by about 10%, which was similar to the results of Example 1.

[0039] Comparative Example 7:

[0040] In steps 4 and 5, stress relief annealing is performed at 500°C for 2 hours, and the other processes are the same as in Example 1. Due to the high annealing temperature, the refined material structure shows partial recovery and recrystallization. As a result, the material structure and performance after 6 extrusions in this comparative example are only equivalent to the results after 2 to 3 extrusions in Example 1, that is, the average grain size of the obtained 316L steel is about 700nm to 5μm, the small angle grain boundary accounts for 30% to 45%, the yield strength is only increased to 210 to 230MPa, and the corrosion resistance is increased by about 6%.

[0041] Comparative Example 8:

[0042] In steps 4 and 5, stress relief annealing is performed at 180°C for 2 hours, and the other processes are the same as in Example 1. Since the annealing temperature is too low and the corresponding extrusion passes are insufficient, the degree of plastic deformation and annealing effect are small. The proportion of small-angle grain boundaries is about 40% to 45%, the yield strength is about 200 to 210 MPa, and the corrosion resistance is improved by about 5%.

[0043] The present invention introduces the equal channel angular extrusion method, which can perform multi-pass extrusion deformation. By selecting a suitable stress relief annealing process between extrusion passes, the obstacles caused by dislocation winding to the next extrusion between extrusion passes are reduced, so that a large amount of plastic deformation can be superimposed, and ultrafine grain structure can be obtained, and the strength and corrosion resistance of the material can be improved. In order to effectively utilize the equal channel angular extrusion and reduce the sample cost, the optimal equal channel angular extrusion process parameters of 316L steel that can achieve the smallest grain size, the highest mechanical strength and other properties are determined by calculation, including determining the number of extrusions, paths, extrusion speeds, and internal and external angles, reducing the number of invalid tests, and avoiding the problem of material fracture after multiple extrusions; saving processing time and reducing costs.

[0044] It should be noted that the above-mentioned technical contents of the present invention are only for explanation and clarification to enable those skilled in the art to understand the technical essence of the present invention, so the technical contents are not used to limit the substantial protection scope of the present invention. The substantial protection scope of the present invention shall be subject to the claims. Those skilled in the art should know that any modification, equivalent substitution and improvement based on the substantial spirit of the present invention shall be within the substantial protection scope of the present invention.

Claims

1. A method for preparing high-strength, corrosion-resistant 316L stainless steel, comprising the following steps: Step 1: Preheat the ECAP mold at a temperature of 25-120°C. Step 2: Place the sample into the ECAP mold and keep it warm for 25 to 60 minutes; Step 3: Extrusion is performed in an equal channel angle die at an extrusion speed of 150-250 mm / min; the extrusion path is path bc, and the inner and outer angles of the extrusion channel are 80°-100° and 35°-40° respectively; Step 4: Stress relief annealing of the extruded sample at 200°C~450°C for 1~3 hours; Step 5: Rotate the sample 90° and put it into the ECAP mold to repeat steps 3 and 4. Repeat the extrusion process 4 to 6 times. Step 6: After the last extrusion and annealing treatment, take the sample out of the heat treatment furnace and cool it to room temperature to obtain high-strength and corrosion-resistant 316L stainless steel.

2. The preparation method according to claim 1, characterized in that Step 1: The preheating temperature is 90~110℃.

3. The preparation method according to claim 1, characterized in that: The insulation time for step 2 is 30~40 min.

4. The preparation method according to claim 1, characterized in that: Step 3: The extrusion speed is 200~250 mm / min.

5. The preparation method according to claim 1, characterized in that: Step 3: The inner and outer corners of the extrusion channel are 90° and 37° respectively.

6. The preparation method according to claim 1, characterized in that: Step 4: Stress relief annealing is performed on the extruded sample at 300°C~350°C for 2 hours.

7. The preparation method according to claim 1, characterized in that: Step 5: Repeat the extrusion 6 times.