A method for preparing high-strength and high-plasticity austenitic stainless steel with medium nitrogen content

Abstract

The present invention discloses a method for preparing high-strength and high-plasticity austenitic stainless steel with a medium nitrogen content, comprising the following steps: step 1, placing the austenitic stainless steel in a smelting device and vacuum heating it until it melts, introducing nitrogen into the smelting device by bottom blowing to perform nitrogen alloying treatment on the melted austenitic stainless steel; step 2, homogenizing and annealing the austenitic stainless steel after the nitrogen alloying treatment in step 1 and then cold deforming it; step 3, performing insufficient annealing treatment on the deformed austenitic stainless steel in step 2, and finally water quenching it to room temperature; wherein, the austenitic stainless steel described in the step 1 includes nitrogen with a mass percentage of 0.1%-0.2%. The present invention can effectively improve the yield strength of the material while maintaining good plasticity by performing cold deformation and insufficient annealing treatment on the austenitic stainless steel after nitrogen alloying treatment, and the whole preparation process of the present invention is relatively simple and does not require additional process costs.

Description

Technical Field

[0001] The invention belongs to the technical field of stainless steel smelting and processing, and particularly relates to a method for preparing high-strength and high-plasticity austenitic stainless steel with a medium nitrogen content. Background Art

[0002] Austenitic stainless steel refers to stainless steel with an austenitic structure at room temperature. Steel containing approximately 18% Cr, 8% to 25% Ni, and 0.1% C exhibits a stable austenitic structure. Austenitic chromium-nickel stainless steels include the well-known 18Cr-8Ni steel and the high-Cr-Ni series of steels developed by increasing the Cr and Ni content and adding elements such as Mo, Cu, Si, Nb, and Ti. Austenitic stainless steel is a widely used alloy due to its excellent corrosion resistance and good mechanical properties. Type 304 austenitic stainless steel is the most widely used of these. However, in practice, the relatively low yield strength of the austenitic matrix of 304 steel limits its use in structural components such as automotive chassis and load-bearing structures. Consequently, extensive research has been conducted in the prior art to improve the yield strength of 304 austenitic stainless steel. For example, Chinese invention patent No. 201410753437.2, entitled "A Method for Preparing 304 Austenitic Stainless Steel with an Average Grain Size Less than 100nm," discloses a method for preparing 304 austenitic stainless steel with a grain size less than 100nm. This method involves hot-rolling 304 austenitic stainless steel into a 4-5mm thick plate, resulting in an austenitic microstructure with a relatively uniform grain size of 18-20μm. This plate is then subjected to a three-stage cold rolling-annealing process to produce 304 austenitic stainless steel with a grain size less than 100nm. The patent states that mechanical properties of the experimental steel produced using this method were tested, ultimately yielding a yield strength of 1100-1200 MPa and a tensile strength of 1250-1350 MPa. Whether this method can actually produce stainless steel with such high yield and tensile strengths in actual applications remains to be seen. Even if stainless steel with such high yield strength and tensile strength could be achieved, the manufacturing method requires a three-stage cold rolling-annealing process after hot rolling, i.e., three cooling-heating-cooling cycles. This makes the manufacturing process extremely complex and, in practice, limited to laboratory use rather than practical production applications. Summary of the Invention

[0003] In order to solve the technical problem that the preparation process of austenitic stainless steel with both high strength and high plasticity in the prior art is relatively complicated, the present invention proposes a preparation method of high-strength and high-plasticity austenitic stainless steel with medium nitrogen content, so as to achieve the technical effect of improving the strength and plasticity of austenitic stainless steel without increasing the difficulty and complexity of the preparation process.

[0004] In order to achieve the above-mentioned object, the present invention adopts the following technical solutions:

[0005] A method for preparing high-strength and high-ductility austenitic stainless steel with a medium nitrogen content comprises the following steps:

[0006] Step 1: placing austenitic stainless steel in a melting device and heating it in vacuum until it melts, and introducing nitrogen into the melting device by bottom blowing to perform nitrogen alloying treatment on the melted austenitic stainless steel;

[0007] Step 2: Cooling the austenitic stainless steel after the nitrogen alloying treatment in step 1 and then performing cold rolling or cold drawing deformation;

[0008] Step 3: performing insufficient annealing on the austenitic stainless steel deformed in step 2, and finally quenching it with water to room temperature;

[0009] Wherein, the austenitic stainless steel in step 1 contains nitrogen in an amount of 0.1% to 0.2% by mass.

[0010] Furthermore, the austenitic stainless steel in step 1 further includes the following components in mass percentage: carbon ≤ 0.07%, silicon 0.2-0.4%, manganese 1.0-1.3%, chromium 18-20.5%, nickel 5-8%, aluminum ≤ 0.006%, copper 0.2-0.3%, molybdenum 0.1-0.2%, phosphorus ≤ 0.035%, and the balance iron.

[0011] Furthermore, the heating temperature in the smelting device in step 1 is 1500°C-1550°C.

[0012] Furthermore, in the process of introducing nitrogen into the smelting device in step 1 and the process of performing nitrogen alloying treatment, the temperature in the smelting device is maintained at 1500° C.-1550° C.

[0013] Furthermore, in the step 1, after nitrogen is introduced into the smelting device, when the pressure in the smelting device reaches 0.1-0.12 MPa, the introduction of nitrogen is stopped and the pressure in the smelting device is kept unchanged.

[0014] Furthermore, the treatment time of the nitrogen alloying treatment in step 1 is 15 min-30 min.

[0015] Furthermore, the deformation amount of the cold rolling deformation or cold drawing deformation in step 2 is 50-90%.

[0016] Furthermore, the annealing temperature of the insufficient annealing treatment in step 3 is 800-850°C.

[0017] Furthermore, the annealing time of the insufficient annealing treatment in step 3 is 1 min to 3 min.

[0018] Furthermore, the smelting device is a smelting furnace.

[0019] The beneficial effects of the present invention are:

[0020] The present invention nitrogen-alloys austenitic stainless steel containing 0.1% to 0.2% nitrogen by mass, followed by cold deformation and insufficient annealing. This effectively improves the material's yield strength while maintaining good plasticity (elongation), inhibits stress / strain-induced martensitic transformation, and avoids the resulting stress concentration, dimensional effects caused by volume expansion, and hydrogen embrittlement. The resulting material can be widely used in load-bearing structural parts requiring high dimensional accuracy and in harsh environmental conditions. Furthermore, the addition of nitrogen in the present invention can appropriately reduce the content of the expensive element Ni in the alloy, thereby lowering costs. The entire preparation process is relatively simple, requiring no additional processing costs. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 1 is a strain-stress curve diagram of sample 304N of Example 1 of the present invention and sample 304 of comparative example 1;

[0022] Figure 2 EBSD phase images of sample 304N in Example 1 of the present invention and sample 304 in Comparative Example 1 before and after deformation;

[0023] Figure 3 KAM phase diagrams of sample 304N in Example 1 of the present invention and sample 304 in Comparative Example 1 before and after deformation;

[0024] Figure 4 TEM bright field image, weak beam dark field image and SAED image of sample 304N of Example 1 of the present invention and sample 304 of Comparative Example 1 before deformation;

[0025] Figure 5 TEM bright field images of sample 304N of Example 1 of the present invention and sample 304 of comparative example 1 after deformation. DETAILED DESCRIPTION

[0026] The embodiments of the present invention provide a method for preparing high-strength and high-plasticity austenitic stainless steel with a medium nitrogen content, thereby solving the technical problem that the preparation process of austenitic stainless steel with both high strength and high plasticity in the prior art is relatively complicated.

[0027] The general idea adopted by the present invention is as follows:

[0028] An embodiment of the present invention provides a method for preparing high-strength and high-plasticity austenitic stainless steel with a medium nitrogen content, comprising the following steps: step 1, placing the austenitic stainless steel in a smelting device and vacuum heating it until it melts, and introducing nitrogen into the smelting device to perform nitrogen alloying treatment on the melted austenitic stainless steel; step 2, cooling the austenitic stainless steel after the nitrogen alloying treatment in step 1 and then performing cold rolling deformation or cold drawing deformation; step 3, performing insufficient annealing treatment on the deformed austenitic stainless steel in step 2, and finally water quenching to room temperature; wherein the austenitic stainless steel in step 1 contains 0.1% to 0.2% by mass of nitrogen.

[0029] In the above-mentioned preparation method, nitrogen can be dissolved into austenite through nitrogen alloying treatment, thereby effectively improving the yield strength of stainless steel. At the same time, it can also improve the stability of austenite, avoiding the situation where austenite undergoes martensitic transformation in the initial stage of deformation or under stress conditions, which causes the stainless steel material to yield prematurely. In addition, the dissolved nitrogen can also increase the stacking fault energy of austenitic stainless steel, promote the plane slip of dislocations, and increase the dislocation accommodation density within the grains, thereby maintaining its rheological stress under high strain conditions, so that the stainless steel material as a whole exhibits high strength and high plasticity. Because austenitic stainless steel contains 0.1%-0.2% nitrogen, it can effectively promote the recrystallization process, greatly shortening the time required for insufficient annealing, and obtaining an incompletely recrystallized austenite matrix structure within 1-3 minutes. That is, the grain distribution states of the obtained austenite matrix structure include three types: fully recrystallized, partially recrystallized, and unrecrystallized mixed structure. In this austenite phase, heterogeneous structures with different hardness are formed, that is, in-situ composite materials. This soft and hard combined grain distribution can effectively improve the yield strength of stainless steel materials. At the same time, the back stress strengthening between soft and hard grains can further improve the tensile strength of stainless steel materials and maintain good plasticity.

[0030] In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings of the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the described embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention. Unless otherwise defined, the technical terms or scientific terms used herein should be the common meanings understood by people with ordinary skills in the field to which the present invention belongs.

[0031] The words “first”, “second” and similar words used in the patent application specification and claims of the present invention do not indicate any order, quantity or importance, but are only used to distinguish different components. Similarly, unless the context clearly indicates otherwise, the singular forms of “a”, “an” or “the” and similar words do not indicate a quantity limitation, but rather indicate the presence of at least one. Words such as “include” or “comprise” mean that the elements or objects appearing before “include” or “comprises” cover the features, wholes, steps, operations, elements and / or components listed after “include” or “comprises”, and do not exclude the existence or addition of one or more other features, wholes, steps, operations, elements, components and / or their collections. “Up”, “down”, “left”, “right” and the like are only used to indicate relative positional relationships. When the absolute position of the described object changes, the relative positional relationship may also change accordingly.

[0032] Example 1

[0033] A method for preparing high-strength and high-ductility austenitic stainless steel with a medium nitrogen content comprises the following steps:

[0034] Step 1: Nitrogen alloying treatment

[0035] Place 304 austenitic stainless steel in the melting crucible of the melting furnace, close the melting furnace, start the vacuum device to evacuate the melting furnace to a pressure of 100Pa (because it is impossible to actually evacuate the melting furnace to a complete vacuum during operation, so when the pressure in the melting furnace reaches 100Pa, it can be considered as a vacuum). Then, turn on the heating device of the melting furnace to heat the 304 austenitic stainless steel to melt, wherein the heating temperature is 1550℃. Then, introduce industrial nitrogen (nitrogen purity is above 99%) into the melting furnace by bottom blowing until the pressure in the melting furnace reaches 0.1MPa. Then stop introducing nitrogen and close the melting furnace. Let the melting furnace maintain the nitrogen pressure of 0.1MPa and the temperature of the 304 austenitic stainless steel melt at 1550℃ for 15 minutes to ensure that the nitrogen can be fully dissolved in the 304 austenitic stainless steel melt.

[0036] The 304 austenitic stainless steel includes the following components in mass percentage: 0.074% carbon, 0.25% silicon, 1.11% manganese, 18.2% chromium, 8.07% nickel, 0.15% nitrogen, 0.006% aluminum, 0.26% copper, 0.14% molybdenum, 0.035% phosphorus, and the balance is iron.

[0037] Step 2: Cold deformation treatment

[0038] The austenitic stainless steel subjected to the nitrogen alloying treatment in step 1 is homogenized, annealed, and cooled, and then cold-drawn to a deformation amount of 50%, and the obtained semi-finished product has a diameter of 6 mm.

[0039] Step 3: Insufficient annealing

[0040] After the semi-finished austenitic stainless steel deformed in step 2 is cut into the required shape and size, it is annealed at 800°C for 3 minutes and finally water quenched to room temperature to obtain a high-strength and high-ductility austenitic stainless steel sample. The sample obtained in this example is named 304N sample.

[0041] Example 2

[0042] A method for preparing high-strength and high-ductility austenitic stainless steel with a medium nitrogen content comprises the following steps:

[0043] Step 1: Nitrogen alloying treatment

[0044] Place 304 austenitic stainless steel in the melting crucible of the melting furnace, close the melting furnace, start the vacuum device to pump the melting furnace to a pressure of 100Pa, and then turn on the heating device of the melting furnace to heat the 304 austenitic stainless steel to melt, wherein the heating temperature is 1500℃. Then, introduce industrial nitrogen (nitrogen purity is above 99%) into the melting furnace until the pressure in the melting furnace reaches 0.11MPa, then stop introducing nitrogen and close the melting furnace. Keep the melting furnace at a nitrogen pressure of 0.11MPa and a temperature of 1500℃ for 20 minutes to ensure that the nitrogen can be fully dissolved in the 304 austenitic stainless steel melt.

[0045] The 304 austenitic stainless steel includes the following components in mass percentage: 0.03% carbon, 0.3% silicon, 1.3% manganese, 19.1% chromium, 6.6% nickel, 0.17% nitrogen, 0.005% aluminum, 0.24% copper, 0.15% molybdenum, 0.02% phosphorus, and the balance is iron.

[0046] Step 2: Cold deformation treatment

[0047] After the austenitic stainless steel subjected to the nitrogen alloying treatment in step 1 is annealed and cooled, it is cold-drawn to a deformation amount of 50%, and the obtained semi-finished product has a diameter of 6 mm.

[0048] Step 3: Insufficient annealing

[0049] After the semi-finished austenitic stainless steel deformed in step 2 is cut into the required shape and size, it is annealed at 830°C for 2 minutes and finally water quenched to room temperature to obtain a high-strength and high-plasticity austenitic stainless steel sample.

[0050] Example 3

[0051] A method for preparing high-strength and high-ductility austenitic stainless steel with a medium nitrogen content comprises the following steps:

[0052] Step 1: Nitrogen alloying treatment

[0053] Place 304 austenitic stainless steel in the melting crucible of the melting furnace, close the melting furnace, start the vacuum device to pump the melting furnace to a pressure of 100Pa, and then turn on the heating device of the melting furnace to heat the 304 austenitic stainless steel to melt, wherein the heating temperature is 1530℃. Then, introduce industrial nitrogen (nitrogen purity is above 99%) into the melting furnace until the pressure in the melting furnace reaches 0.12MPa, then stop introducing nitrogen and close the melting furnace. Let the melting furnace maintain a nitrogen pressure of 0.12MPa and a temperature of 1530℃ for 30 minutes to ensure that the nitrogen can be fully dissolved in the 304 austenitic stainless steel melt.

[0054] The 304 austenitic stainless steel includes the following components in mass percentage: 0.04% carbon, 0.40% silicon, 1.21% manganese, 18.5% chromium, 5.3% nickel, 0.19% nitrogen, 0.006% aluminum, 0.21% copper, 0.12% molybdenum, 0.02% phosphorus, and the balance is iron.

[0055] Step 2: Cold deformation treatment

[0056] After the austenitic stainless steel subjected to the nitrogen alloying treatment in step 1 is annealed and cooled, it is cold-drawn to a deformation amount of 50%, and the obtained semi-finished product has a diameter of 6 mm.

[0057] Step 3: Insufficient annealing

[0058] After the semi-finished austenitic stainless steel deformed in step 2 is cut into the required shape and size, it is annealed at 850°C for 1 minute and finally water quenched to room temperature to obtain a high-strength and high-plasticity austenitic stainless steel sample.

[0059] Comparative Example 1

[0060] Commercially available ordinary 304 austenitic stainless steel. The sample of this comparative example is named 304 sample.

[0061] Performance test 1

[0062] The austenitic stainless steel samples obtained in Examples 1-3 and Comparative Example 1 were subjected to tensile testing according to GB / T228.1-2010, and the test results were as follows:

[0063] Table 1 Tensile test results of Examples 1-3 and Comparative Example 1

[0064]

[0065]

[0066] As can be seen from the above table, the austenitic stainless steel prepared by the embodiment of the present invention can have both excellent strength and plasticity compared with the commercially available 304 stainless steel. Figure 1 As shown, compared with the sample 304 of the comparative example 1, the yield ratio of the sample 304N of the embodiment 1 is significantly improved to 0.76, which is almost twice the yield ratio of the sample 304 of the comparative example 1, and the total elongation of 304N can be maintained at a high level of about 50%.

[0067] Performance test 2

[0068] In order to further verify the beneficial effects of the embodiments of the present invention, the sample 304N obtained in Example 1 of the present invention and the sample 304 of Comparative Example 1 are further subjected to microstructure detection and comparison, and the following results are obtained:

[0069] like Figure 2 As shown in the figure, parts (a) and (c) are the EBSD phase diagrams of the sample 304 of the comparative example before and after tensile deformation. Parts (b) and (d) are the EBSD phase diagrams of the sample 304N of Example 1 before and after tensile deformation. Comparing parts (a) and (b) in the figure, it can be seen that before tensile deformation, the 304 sample of the comparative example is composed of a majority of austenite and a small amount of ferrite, while the microstructure of the 304N sample of Example 1 is almost completely austenite. Comparing parts (c) and (d) in the figure again, it can be seen that after tensile deformation, more than 60% of the austenite in the 304 sample of the comparative example is transformed into martensite, while the proportion of austenite transformed into martensite in the 304N sample of Example 1 is less than 4%. This shows that the austenitic stainless steel matrix prepared by the method of the present invention not only has a high austenite content, but also has good austenite stability after tensile deformation, thereby greatly improving the yield strength of the stainless steel.

[0070] like Figure 3As shown in the figure, parts (e) and (g) are respectively the austenite-like contrast KAM diagram of sample 304 of the comparative example and the banded contrast KAM diagram of martensite. Parts (f) and (h) in the figure are respectively the austenite-like contrast KAM diagram of sample 304N of Example 1 and the banded contrast KAM diagram of martensite. It can be seen from the two parts (e) and (f) in the comparison figure that the dislocation density in the 304 sample of the comparative example is relatively low and unevenly distributed, while the dislocation density in the 304N sample of Example 1 is high and evenly distributed. It can be seen from the two parts (g) and (h) in the comparison figure that, relative to the 304 sample of the comparative example, there is a dense slip band in the austenite of the 304N sample of Example 1. It can be seen from this that the preparation method according to the embodiment of the present invention can improve the stacking fault energy of austenitic stainless steel, promote the plane slip of dislocations, and increase the dislocation accommodation density within the grains, thereby maintaining its flow stress under high strain conditions, so that the stainless steel material as a whole exhibits high strength and high plasticity.

[0071] like Figure 4 As shown in the figure, parts (a) and (c) are TEM bright field images of the comparative example 304 sample and the 304N sample of Example 1 before deformation. Parts (b) and (d) are weak beam dark field images and SAED images of the comparative example 304 sample and the 304N sample of Example 1 before deformation. It can be seen from parts (a) and (b) in the figure that many stacking faults (SFs) in the comparative example 304 sample are arranged along two (111) planes, and many short dislocations are randomly distributed between the two planes. It can be seen from parts (c) and (d) in the figure that the dislocations in the 304N sample of Example 1 are regularly arranged along the (111) planes, and almost no stacking faults (SFs) are found. It can be seen that the addition of nitrogen has a significant effect on the microstructure of the material and changes the configuration of crystal defects in the structure.

[0072] like Figure 5 As shown in Figure 1, parts (a) and (b) are TEM bright-field images of the 304 sample from the comparative example and the 304N sample from Example 1 after deformation, respectively. Comparing the two parts in the figure, it can be seen that the 304 sample undergoes a significant strain-induced martensitic transformation during deformation, while the 304N sample does not undergo martensitic transformation, only forming deformation twins in the austenite. This avoids the failure problem caused by martensitic transformation under stress or strain conditions during service.

[0073] Finally, it should be noted that these embodiments are intended only to illustrate the present invention and do not limit its scope. Furthermore, those skilled in the art will readily appreciate that other variations or modifications based on the above description are possible. It is not necessary and impossible to enumerate all embodiments here. Obvious variations or modifications arising therefrom remain within the scope of protection of the present invention.

Claims

1. A method for preparing high-strength and high-ductility austenitic stainless steel with medium nitrogen content, characterized in that: The steps include: Step 1: placing austenitic stainless steel in a melting device and heating it in vacuum until it melts, and introducing nitrogen into the melting device by bottom blowing to perform nitrogen alloying treatment on the melted austenitic stainless steel; Step 2: homogenizing annealing and cooling the austenitic stainless steel after the nitrogen alloying treatment in step 1, and then cold rolling or cold drawing the same; Step 3: performing insufficient annealing on the austenitic stainless steel deformed in step 2, and finally quenching it with water to room temperature; Wherein, the austenitic stainless steel in step 1 comprises nitrogen in an amount of 0.1% to 0.2% by mass; The annealing temperature of the insufficient annealing treatment in step 3 is 800-850°C; The annealing time of the insufficient annealing treatment in step 3 is 1 min to 3 min.

2. The method for preparing a high-strength and high-ductility austenitic stainless steel with a medium nitrogen content according to claim 1, characterized in that: The austenitic stainless steel in step 1 further includes the following components in mass percentage: carbon ≤ 0.07%, silicon 0.2-0.4%, manganese 1.0-1.3%, chromium 18-20.5%, nickel 5-8%, aluminum ≤ 0.006%, copper 0.2-0.3%, molybdenum 0.1-0.2%, phosphorus ≤ 0.035%, and the balance iron.

3. The method for preparing a high-strength and high-ductility austenitic stainless steel with a medium nitrogen content according to claim 1, characterized in that: The heating temperature in the smelting device in step 1 is 1500°C-1550°C.

4. The method for preparing a high-strength and high-ductility austenitic stainless steel with a medium nitrogen content according to claim 1, characterized in that: In the process of introducing nitrogen into the smelting device in step 1 and performing nitrogen alloying treatment, the temperature in the smelting device is maintained at 1500° C.-1550° C.

5. The method for preparing a high-strength and high-ductility austenitic stainless steel with a medium nitrogen content according to claim 1, characterized in that: In the step 1, after nitrogen is introduced into the smelting device, when the pressure in the smelting device reaches 0.1-0.12 MPa, the introduction of nitrogen is stopped and the pressure in the smelting device is kept unchanged.

6. The method for preparing a high-strength and high-ductility austenitic stainless steel with a medium nitrogen content according to claim 1, characterized in that: The treatment time of the nitrogen alloying treatment in step 1 is 15 min to 30 min.

7. The method for preparing a high-strength and high-ductility austenitic stainless steel with a medium nitrogen content according to claim 1, characterized in that: The deformation amount of the cold rolling deformation or cold drawing deformation in step 2 is 50-90%.

8. The method for preparing austenitic stainless steel with high strength and high plasticity and medium nitrogen content according to claim 1, characterized in that: The smelting device is a smelting furnace.

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