High-toughness large-expansion austenitic stainless steel and preparation method thereof

Abstract

The invention relates to the technical field of austenitic stainless steel, in particular to high-toughness large-expansion austenitic stainless steel and a preparation method thereof, and the high-toughness large-expansion austenitic stainless steel comprises the following alloy components in percentage by weight: 0.45 to 0.65 weight percent of C, 6.0 to 9.0 weight percent of Mn, 6.5 to 10.0 weight percent of Ni, 8.0 to 12.0 weight percent of Cr, 1.5 to 3.0 weight percent of Mo, 0.8 to 2.0 weight percent of V, 1.5 to 4.0 weight percent of Cu, 0.015 to 0.045 weight percent of Nb, 0.035 to 0.100 weight percent of Ti and the balance of Fe. The thermal expansion coefficient of the obtained steel is superior to that of the prior art, the yield strength is larger than 950 MPa, the percentage elongation after fracture is larger than 15%, sensitization characteristics caused by grain boundary Cr carbides are avoided, the steel is composed of pure FCC austenite with stable thermal mechanical performance, and martensite and deformed or high-dislocation-density austenite are avoided.

Description

Technical Field

[0001] The invention relates to the technical field of austenitic stainless steel, in particular to a high-strength, tough and large-expansion austenitic stainless steel and a preparation method thereof. Background Art

[0002] Austenitic stainless steel is a widely used alloy material with excellent oxidation and corrosion resistance, playing an important role in many fields (such as automobiles, ships, energy, and chemicals). In some high-temperature scenarios, high-temperature resistant (such as 700°C) austenitic stainless steel is essential for equipment that needs to operate under extreme high temperature conditions.

[0003] Chinese patent CN112143984A discloses a stainless steel for heat shrinkable tool handle and its preparation method. Its chemical composition includes, by mass percentage: carbon 0.5%-0.6%; silicon ≤0.5%; manganese 7%-8%; phosphorus ≤0.025%; sulfur ≤0.01%; nickel 7%-8%; chromium 9%-10%; molybdenum 1.8%-2%; vanadium 1%-1.5%; copper 2.3%-2.5%; aluminum 0.9%-1.35%; the remainder is iron. Then, according to a series of processing techniques, the obtained stainless steel material belongs to a special stainless steel with a large thermal expansion coefficient. This special steel can realize low-temperature hot loading, and also has the properties of high rigidity, high durability, high pressure resistance, etc. It is very suitable for heat shrinkable tool handles. When applied to high-end CNC machine tools, it can ensure the accuracy and safety of high-end CNC machine tool processing.

[0004] However, our technicians found through SEM experiments and statistical analysis that according to the temperature and duration of the solution treatment process, most of the Cr carbides cannot be dissolved into the matrix. Even if the temperature and time are changed, it is impossible to achieve an excellent solution effect. Even after the temperature reaches 1200°C, the grains will grow significantly if the temperature is kept for a long time. At the same time, it was found that if the solution treatment is not cooled in time, Cr carbides will continue to form at the grain boundaries below 1100°C, and the solution treatment will not be effective. Moreover, other similar processes that use furnace cooling cannot eliminate grain boundary carbides. In addition, the aging treatment temperature is too low, and even if the time is long, it is impossible to achieve the effect of precipitation strengthening.

[0005] In addition, we refer to a lot of existing technologies and try to add a certain amount of Al. According to this method, we found that there are a lot of blocky and large AlN inclusions in the steel, which seriously affect the plasticity, toughness and service life of the steel. In addition, Al will reduce the Young's modulus of the steel. Moreover, adding more than 1wt% of Al may be intended to precipitate NiAl for strengthening, but that is in the BCC phase, which is difficult to achieve in austenite. Moreover, it is difficult to achieve lightweight effect with low Al content.

[0006] Generally speaking, pure austenitic stainless steel is hot-rolled, that is, produced without quenching or deforming martensite, and its yield strength and hardness are very low. It can only be improved after large cold deformation, but there will be some martensite or dislocation defects inside. When this steel is actually in service (repeated heating process), its performance will also degrade. Therefore, pure austenitic stainless steel must be selected. Then how to improve its yield strength and hardness is the key.

[0007] Based on the above-mentioned defects, we have conducted a lot of research and tried to provide a high-strength, toughness and large-expansion austenitic stainless steel and a preparation method thereof to overcome the various deficiencies in the prior art. Summary of the invention

[0008] The technical problem to be solved by the present invention is to provide a high-strength, tough, and large-expansion austenitic stainless steel and a preparation method thereof. The thermal expansion coefficient of the obtained steel is superior to that of the prior art, and the yield strength is greater than 950MPa, the elongation after fracture is greater than 15%, and there is no sensitization characteristic caused by grain boundary Cr carbides. The steel is composed of pure FCC austenite with stable thermomechanical properties, and there is no martensite and deformed or high dislocation density austenite.

[0009] In order to solve the above technical problems, the present invention adopts the following technical solutions:

[0010] A high-strength, tough and large-expansion austenitic stainless steel, the alloy components are as follows by weight: C: 0.45-0.65wt.%, Mn: 6.0-9.0wt.%, Ni: 6.5-10.0wt.%, Cr: 8.0-12.0wt.%, Mo: 1.5-3.0wt.%, V: 0.8-2.0wt.%, Cu: 1.5-4.0wt.%, Nb: 0.015-0.045wt.%, Ti: 0.035-0.100wt.%, and the balance is Fe.

[0011] According to the above-mentioned method for preparing austenitic stainless steel with high strength, toughness and large expansion, the method comprises the following steps:

[0012] S1, weighing the alloy components, and weighing the corresponding C, Mn, Ni, Cr, Mo, V, Cu, Nb, Ti and Fe according to the weight percentage of the alloy components;

[0013] S2, vacuum smelting and casting the alloy components weighed in step S1, and then forging to obtain a forging blank;

[0014] S3, subjecting the forging blank obtained in step S2 to heat preservation treatment and then hot rolling to obtain a hot-rolled bar;

[0015] S4, subjecting the hot-rolled bar obtained in step S3 to a solution treatment, and water quenching to room temperature after the first heat preservation treatment; and then subjecting the hot-rolled bar to a heat treatment, an aging treatment, a second heat preservation treatment, and air cooling to room temperature to obtain austenitic stainless steel.

[0016] Preferably, the forging temperature in step S2 is controlled to be 1150-1200° C., and the forging ratio is 1.5-2.0.

[0017] Preferably, the diameter of the forging blank obtained by forging in step S2 is 80 to 200 mm.

[0018] Preferably, the specific operation of hot rolling in step S3 is: keeping at 1150-1200° C. for 2.0-2.5 hours, and then hot rolling at 1100-1150° C., with a total reduction rate of 50-75%.

[0019] Preferably, the hot-rolled bar obtained in step S3 has a diameter of 20 to 100 mm.

[0020] Preferably, the first heat preservation treatment in step S4 is carried out at 1150-1200° C. for 4-8 hours.

[0021] Preferably, the temperature of the heat treatment in step S4 is controlled to be 650-780°C.

[0022] Preferably, the second heat preservation treatment in step S4 is carried out at 650-780° C. for 8-15 hours.

[0023] Beneficial effects of the present invention:

[0024] 1. The present invention conducts in-depth research on the fundamental mechanism of the influence of various elements in steel on thermal expansion (i.e., phonon scattering mechanism), and through innovative design of the combination of steel components, ultimately obtains a design scheme that greatly improves the thermal expansion coefficient of austenitic stainless steel. The thermal expansion coefficient of the obtained steel (room temperature - 600°C) is greater than 22ppm / °C, which is better than the prior art.

[0025] 2. Based on the large thermal expansion coefficient, the yield strength of the present invention is greater than 950MPa, and the elongation after fracture is greater than 15%. Through the solid solution + aging treatment process of the present invention and the design of the previous hot forging and hot rolling process parameters, each process section cooperates with each other to form a complete manufacturing process, so that the final austenitic stainless steel of the corresponding size has no sensitization characteristics brought by grain boundary Cr carbides, and is composed of pure FCC austenite with stable thermomechanical properties, without martensite and deformed or high dislocation density austenite, and the hot rolling reduction rate is designed according to the final bar size. The interdependence of each link makes the final product more comprehensive.

[0026] 3. The present invention obtains the optimal dosage range by adding Nb and Ti through a large number of calculations, and utilizes Nb to greatly increase the recrystallization temperature, so that part of the deformation during hot rolling is performed below the recrystallization temperature. The hot forging process design of this scheme can significantly refine the grains, and Nb under the dosage range of the present invention can precipitate in the subsequent aging process to play a precipitation strengthening role. At the same time, the addition of Ti in the liquid molten steel at the temperature of solidification can precipitate TiN with N and precipitate TiC with C, which can significantly refine the high-temperature austenite grains. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 This is a SEM scan image after Al is added to the components of Example 4 of the present invention;

[0028] Figure 2 The effect of adding Al to the components of Example 4 of the present invention on the Young's modulus of the steel (the left figure does not add Al, the right figure adds Al);

[0029] Figure 3 TEM scanning image of austenitic stainless steel of Example 4 of the present invention;

[0030] Figure 4 The SEM images of carbides before and after solution treatment of Example 4 of the present invention are compared;

[0031] Figure 5 This is the EBSD diagram of Example 4 after solution treatment of the present invention. DETAILED DESCRIPTION

[0032] In order to facilitate the understanding of those skilled in the art, the present invention is further described below in conjunction with embodiments and drawings. The contents mentioned in the implementation modes are not intended to limit the present invention.

[0033] Embodiment 1:

[0034] A high-strength, high-toughness, large-expansion austenitic stainless steel and a preparation method thereof, comprising the following steps;

[0035] 1) Weigh the alloy components, and weigh 0.45wt.% C, 9.0wt.% Mn, 10.0wt.% Ni, 12.0wt.% Cr, 3.0wt.% Mo, 2.0wt.% V, 4.0wt.% Cu, 0.045wt.% Nb, 0.100wt.% Ti according to the weight percentage of the alloy components, and the balance is Fe;

[0036] 2) Smelting, casting and forging: the alloy components weighed in step 1) are subjected to vacuum smelting and casting, and then forging at a temperature of 1200° C. with a forging ratio of 1.5 to obtain a forging billet with a diameter of 200 mm;

[0037] 3) hot rolling: the 200 mm thick forging billet obtained in 2) is kept at 1200° C. for 2.5 h, and then hot rolled at 1150° C. with a total reduction rate of 50% to obtain a hot rolled bar with a diameter of 100 mm;

[0038] 4) Solution treatment: keep the hot-rolled bar at 1150°C for 4 hours and water quench it to room temperature; heat treatment, then keep it at 650°C for 8 hours and air cool it to room temperature to obtain austenitic stainless steel with high strength, toughness and large expansion; the properties are shown in Table 2.

[0039] Embodiment 2:

[0040] A high-strength, high-toughness, large-expansion austenitic stainless steel and a preparation method thereof, comprising the following steps;

[0041] 1) Weigh the alloy components, and weigh 0.50wt.% C, 8.0wt.% Mn, 9.0wt.% Ni, 11.0wt.% Cr, 2.7wt.% Mo, 1.7wt.% V, 3.5wt.% Cu, 0.040wt.% Nb, 0.080wt.% Ti according to the weight percentage of the alloy components, and the balance is Fe;

[0042] 2) Smelting, casting and forging: the alloy components weighed in step 1) are subjected to vacuum smelting and casting, and then forging at a temperature of 1200° C. with a forging ratio of 1.7 to obtain a forging billet with a diameter of 170 mm;

[0043] 3) hot rolling: the 170 mm thick forging billet obtained in 2) is kept at 1200° C. for 2.5 h, and then hot rolled at 1150° C. with a total reduction rate of 56%, to obtain a hot rolled bar with a diameter of 75 mm;

[0044] 4) Solution treatment, the obtained hot-rolled bar is kept at 1150°C for 5 hours, and water quenched to room temperature; heat treatment, then kept at 680°C for 10 hours, and air-cooled to room temperature, thus obtaining a high-strength, tough, and large-expansion austenitic stainless steel of the present invention; the properties are shown in Table 2.

[0045] Embodiment 3:

[0046] A high-strength, high-toughness, large-expansion austenitic stainless steel and a preparation method thereof, comprising the following steps;

[0047] 1) Weigh the alloy components, and weigh 0.55wt.% C, 7.0wt.% Mn, 8.0wt.% Ni, 10.0wt.% Cr, 2.3wt.% Mo, 1.4wt.% V, 3.0wt.% Cu, 0.035wt.% Nb, 0.060wt.% Ti according to the weight percentage of the alloy components, and the balance is Fe;

[0048] 2) Smelting, casting and forging: the alloy components weighed in step 1) are subjected to vacuum smelting and casting, and then forging at a temperature of 1180° C. with a forging ratio of 1.8 to obtain a forging billet with a diameter of 140 mm;

[0049] 3) hot rolling: the 140 mm thick forging billet obtained in 2) is kept at 1180° C. for 2.3 h, and then hot rolled at 1130° C. with a total reduction rate of 60% to obtain a hot rolled bar with a diameter of 55 mm;

[0050] 4) Solution treatment, keeping the obtained hot-rolled bar at 1160°C for 6 hours, water quenching to room temperature; heat treatment, then keeping at 720°C for 12 hours, air cooling to room temperature, thus obtaining a high-strength, tough and large-expansion austenitic stainless steel of the present invention; the properties are shown in Table 2.

[0051] Embodiment 4:

[0052] A high-strength, high-toughness, large-expansion austenitic stainless steel and a preparation method thereof, comprising the following steps;

[0053] 1) Weigh the alloy components, and weigh 0.60wt.% C, 6.5wt.% Mn, 7.0wt.% Ni, 9.0wt.% Cr, 2.0wt.% Mo, 1.0wt.% V, 2.5wt.% Cu, 0.020wt.% Nb, 0.045wt.% Ti according to the weight percentage of the alloy components, and the balance is Fe;

[0054] 2) Smelting, casting and forging: the alloy components weighed in step 1) are subjected to vacuum smelting and casting, and then forging at a temperature of 1180° C. with a forging ratio of 1.9 to obtain a forging billet with a diameter of 110 mm;

[0055] 3) hot rolling: the 110 mm thick forging billet obtained in 2) is kept at 1170° C. for 2.1 h, and then hot rolled at 1125° C. with a total reduction rate of 70% to obtain a hot rolled bar with a diameter of 33 mm;

[0056] 4) Solution treatment, keeping the obtained hot-rolled bar at 1180°C for 7 hours, water quenching to room temperature; heat treatment, then keeping at 750°C for 13 hours, air cooling to room temperature, thus obtaining a high-strength, tough and large-expansion austenitic stainless steel of the present invention; the properties are shown in Table 2.

[0057] Embodiment 5:

[0058] A high-strength, high-toughness, large-expansion austenitic stainless steel and a preparation method thereof, comprising the following steps;

[0059] 1) Weigh the alloy components, and weigh 0.65wt.% C, 6.0wt.% Mn, 6.5wt.% Ni, 8.0wt.% Cr, 1.5wt.% Mo, 0.8wt.% V, 1.5wt.% Cu, 0.015wt.% Nb, 0.035wt.% Ti according to the weight percentage of the alloy components, and the balance is Fe;

[0060] 2) Smelting, casting and forging: the alloy components weighed in step 1) are subjected to vacuum smelting and casting, and then forging at a temperature of 1150° C. with a forging ratio of 2.0 to obtain a forging billet with a diameter of 80 mm;

[0061] 3) hot rolling: the 80 mm thick forging billet obtained in 2) is kept at 1150° C. for 2.0 h, and then hot rolled at 1100° C. with a total reduction rate of 75% to obtain a hot rolled bar with a diameter of 20 mm;

[0062] 4) Solution treatment, the obtained hot-rolled bar is kept at 1200°C for 8 hours, and water quenched to room temperature; heat treatment, then kept at 780°C for 15 hours, and air-cooled to room temperature, thus obtaining a high-strength, tough, and large-expansion austenitic stainless steel of the present invention; the properties are shown in Table 2.

[0063] Table 1 Ingredients of each component in Examples 1-5

[0064]

[0065]

[0066] Test Example 1

[0067] The tensile strength, yield strength, elongation after fracture and thermal expansion coefficient of the austenitic stainless steel obtained in Examples 1-5 were tested. The test results are shown in Table 2.

[0068] Table 2 Performance test results of austenitic stainless steel in Examples 1-5

[0069]

[0070] Comparative Example 1

[0071] According to the existing patent CN112143984A, a stainless steel for heat shrinkable knife handle and a preparation method thereof are disclosed. It is attempted to add Al on the basis of Example 4, and smelt according to the process of Example 4, and finally obtain austenitic stainless steel.

[0072] Test Example 2

[0073] The austenitic stainless steel obtained in Comparative Example 1 was subjected to SEM scanning.

[0074] SEM scanning revealed that the steel contained a lot of large AlN, strip-shaped MnS and Cr23C6 inclusions and many cracks, such as Figure 1 As shown, it seriously affects the plasticity, toughness and service life of steel.

[0075] It is also found that Al can dramatically reduce the Young's modulus of steel. Figure 2 shown.

[0076] Therefore, it is proved that the technical effect of the present technical solution cannot be achieved simply by adding Al.

[0077] Test Example 3

[0078] A TEM scanning test was performed on Example 4.

[0079] Test as Figure 3 As shown, it can be clearly seen that there are a large number of precipitates, which can effectively play a strengthening and hardening effect.

[0080] The SEM scanning test was performed on Example 4.

[0081] Test results such as Figure 4 As shown in the figure, the SEM images of carbides before and after solution treatment are compared. The treated steel has no sensitization characteristics caused by grain boundary Cr carbides.

[0082] The EBSD test analysis was performed on Example 4.

[0083] Test results such as Figure 5 As shown in the figure, the grain size of the steel after hot working and heat treatment is finer, only 35μm.

[0084] All technical features in this embodiment can be modified in appearance according to actual needs.

[0085] The above embodiments are preferred implementation schemes of the present invention. In addition, the present invention may also be implemented in other ways. Any obvious replacement without departing from the concept of the present technical solution is within the protection scope of the present invention.

Claims

1. A high-strength, high-toughness, high-expansion austenitic stainless steel, characterized by: The weight percentages of the alloy components are: C: 0.45-0.65wt.%, Mn: 6.0-9.0wt.%, Ni: 6.5-10.0wt.%, Cr: 8.0-12.0wt.%, Mo: 1.5-3.0wt.%, V: 0.8-2.0wt.%, Cu: 1.5-4.0wt.%, Nb: 0.015-0.045wt.% and Ti: 0.035-0.100wt.%, and the balance is Fe.

2. The method for preparing austenitic stainless steel with high strength, toughness and large expansion according to claim 1, characterized in that: The following steps are involved: S1, weighing the alloy components, and weighing the corresponding C, Mn, Ni, Cr, Mo, V, Cu, Nb, Ti and Fe according to the weight percentage of the alloy components; S2, vacuum smelting and casting the alloy components weighed in step S1, and then forging to obtain a forging blank; S3, subjecting the forging blank obtained in step S2 to heat preservation treatment and then hot rolling to obtain a hot-rolled bar; S4, subjecting the hot-rolled bar obtained in step S3 to a solution treatment, and water quenching to room temperature after the first heat preservation treatment; then subjecting the hot-rolled bar to an aging treatment, and air cooling to room temperature after the second heat preservation treatment after the aging treatment, to obtain austenitic stainless steel.

3. The method for preparing austenitic stainless steel with high strength, toughness and large expansion according to claim 2, characterized in that: The forging temperature in step S2 is controlled to be 1150-1200° C., and the forging ratio is 1.5-2.

0.

4. The method for preparing austenitic stainless steel with high strength, toughness and large expansion according to claim 2, characterized in that: The diameter of the forging blank obtained by forging in step S2 is 80 to 200 mm.

5. The method for preparing austenitic stainless steel with high strength, toughness and large expansion according to claim 2, characterized in that: The specific operation of hot rolling in step S3 is: keeping at 1150-1200° C. for 2.0-2.5 hours, and then hot rolling at 1100-1150° C. with a total reduction rate of 50-75%.

6. The method for preparing austenitic stainless steel with high strength, toughness and large expansion according to claim 2, characterized in that: The hot-rolled bar obtained in step S3 has a diameter of 20 to 100 mm.

7. The method for preparing austenitic stainless steel with high strength, toughness and large expansion according to claim 2, characterized in that: The first heat preservation treatment in step S4 is carried out at 1150-1200° C. for 4-8 hours.

8. The method for preparing austenitic stainless steel with high strength, toughness and large expansion according to claim 2, characterized in that: The temperature of the heat treatment in step S4 is controlled to be 650-780°C.

9. The method for preparing austenitic stainless steel with high strength, toughness and large expansion according to claim 2, characterized in that: The second heat preservation treatment in step S4 is carried out at 650-780° C. for 8-15 hours.

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