High-toughness austenitic stainless steel with high expansion and method for manufacturing the same

Abstract

The application relates to the technical field of austenitic stainless steel, in particular to high-strength and high-toughness austenitic stainless steel with large expansion and a preparation method thereof, alloy components of which are as follows in percentage by weight: C: 0.45-0.65 wt.%, Mn: 6.0-9.0 wt.%, Ni: 6.5-10.0 wt.%, Cr: 8.0-12.0 wt.%, Mo: 1.5-3.0 wt.%, V: 0.8-2.0 wt.%, Cu: 1.5-4.0 wt.%, Nb: 0.015-0.045 wt.% and Ti: 0.035-0.100 wt.%, and the balance is Fe. The thermal expansion coefficient of the obtained steel is superior to that of the prior art, the yield strength is greater than 950 MPa, the elongation after fracture is greater than 15%, no sensitization characteristics caused by grain boundary Cr carbide are generated, the steel is composed of pure FCC austenite with stable thermal mechanical properties, and no martensite and deformed austenite or austenite with high dislocation density appears.

Description

Technical Field

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

[0002] Austenitic stainless steel is a widely used alloy material with excellent oxidation and corrosion resistance, playing a vital role in numerous sectors, such as automotive, shipping, energy, and chemical industries. In some high-temperature environments, high-temperature resistant (e.g., 700°C) austenitic stainless steel is crucial for equipment that needs to operate under these extreme conditions.

[0003] Chinese patent CN112143984A discloses stainless steel for heat-shrink toolholders and its preparation method. Its chemical composition, by mass percentage, includes: 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 balance being iron. The resulting stainless steel, a special stainless steel with a high coefficient of thermal expansion, is then processed through a series of processing techniques. This special steel is capable of low-temperature shrink-fitting and exhibits high rigidity, durability, and compressive strength, making it ideal for heat-shrink toolholders. When used on high-end CNC machine tools, it ensures precision and safety.

[0004] However, through SEM experiments and statistical analysis, our technicians discovered that the temperature and duration of the solution treatment process in this process could not dissolve most of the Cr carbides into the matrix. Even changing the temperature and time did not achieve an excellent solution effect. In fact, after the temperature reached 1200°C, the grains grew significantly after a long holding time. They also found that if the solution treatment was not cooled promptly, Cr carbides would continue to form at the grain boundaries below 1100°C, and the solution treatment would not be effective. Moreover, other similar processes using furnace cooling also failed to eliminate grain boundary carbides. In addition, if the aging treatment temperature was too low, even for a long time, it would not achieve the precipitation strengthening effect.

[0005] Furthermore, referring to a number of existing technologies, we experimented with adding a certain amount of Al. Following this smelting method, we discovered the presence of numerous massive, large AlN inclusions in the steel, which severely impacted the steel's plasticity, toughness, and service life. Furthermore, Al reduced the steel's Young's modulus. Furthermore, adding >1wt% Al, perhaps with the intention of precipitating NiAl for strengthening, occurs in the BCC phase, which is difficult to achieve in austenite. Furthermore, even low Al contents make it difficult to achieve lightweighting.

[0006] Generally speaking, pure austenitic stainless steel that is hot-rolled (i.e., without quenching or deforming martensite) has very low yield strength and hardness. This can only be improved after significant cold deformation. However, there will be some martensite or dislocation defects within the steel, and its performance will degrade during actual service (repeated heating). Therefore, pure austenitic stainless steel must be used. The key question is how to improve its yield strength and hardness.

[0007] Based on the above-mentioned defects, we have conducted extensive research and attempted to provide a high-strength, tough, 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 950 MPa, the elongation after fracture is greater than 15%, and there is no sensitization characteristics caused by grain boundary Cr carbides. It 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] Disclosed is a high-strength, tough, and large-expansion austenitic stainless steel. The alloy composition by weight is: C: 0.45-0.65 wt.%, Mn: 6.0-9.0 wt.%, Ni: 6.5-10.0 wt.%, Cr: 8.0-12.0 wt.%, Mo: 1.5-3.0 wt.%, V: 0.8-2.0 wt.%, Cu: 1.5-4.0 wt.%, Nb: 0.015-0.045 wt.%, and Ti: 0.035-0.100 wt.%, with the balance being 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 solution treatment, water quenching to room temperature after the first heat preservation treatment; then subjecting it to heat treatment, aging treatment, 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 the temperature at 1150-1200° C. for 2.0-2.5 hours, and then hot rolling at 1100-1150° C. with a total reduction ratio 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 a temperature of 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., the phonon scattering mechanism). Through innovative design of the combination of steel components, a design scheme is finally obtained that significantly improves the thermal expansion coefficient of austenitic stainless steel. The resulting thermal expansion coefficient of the steel (room temperature - 600°C) is greater than 22ppm / °C, which is superior to the existing technology.

[0025] 2. Based on the large thermal expansion coefficient, the present invention has a yield strength greater than 950 MPa and an elongation after fracture greater than 15%. Through the solid solution + aging treatment process of the present invention combined with the early hot forging and hot rolling process parameter design, each process stage cooperates with each other to form a complete manufacturing process, so that the final austenitic stainless steel of corresponding size is free of sensitization characteristics caused by grain boundary Cr carbides, and is composed of pure FCC austenite with stable thermomechanical properties, without the appearance of martensite and deformed or high dislocation density austenite. In addition, the hot rolling reduction rate is designed according to the final bar size. The interdependence of each link makes the comprehensive performance of the final product better.

[0026] 3. The present invention incorporates Nb and Ti, achieving an optimal dosage range through extensive calculations. Nb significantly increases the recrystallization temperature, allowing some hot rolling deformation to occur below the recrystallization temperature. Combined with the hot forging process design of this solution, this significantly refines grains. Furthermore, within the dosage range of the present invention, Nb precipitates during the subsequent aging process, contributing to precipitation strengthening. Furthermore, the addition of Ti, when the liquid steel is nearing solidification, reacts with N to form TiN and with C to form TiC, significantly refining 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 is without Al, the right figure is with Al);

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

[0030] Figure 4 Comparison of SEM images of carbides before and after solution treatment in Example 4 of the present invention;

[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 understanding by those skilled in the art, the present invention will be further described below with reference to embodiments and drawings. The contents mentioned in the embodiments are not intended to limit the present invention.

[0033] Example 1:

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

[0035] 1) Weighing the alloy components, according to the weight percentage of the alloy components, weighing 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, and the balance Fe;

[0036] 2) Smelting, casting and forging: the alloy components weighed in step 1) are vacuum smelted and cast, and then forged 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 hours, and then hot rolled at 1150° C. with a total reduction of 50% to obtain a hot-rolled bar with a diameter of 100 mm;

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

[0039] Example 2:

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

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

[0042] 2) Smelting, casting and forging: the alloy components weighed in step 1) are vacuum smelted and cast, and then forged 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) was kept at 1200° C. for 2.5 h, and then hot rolled at 1150° C. with a total reduction of 56% to obtain a hot-rolled bar with a diameter of 75 mm;

[0044] 4) Solution treatment: The hot-rolled bar was kept at 1150°C for 5 hours and water quenched to room temperature; heat treatment was then performed, followed by keeping at 680°C for 10 hours and air cooling to room temperature to obtain a high-strength, tough, and large-expansion austenitic stainless steel of the present invention; the properties are shown in Table 2.

[0045] Example 3:

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

[0047] 1) Weighing the alloy components, according to the weight percentage of the alloy components, weighing 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, and the balance Fe;

[0048] 2) Smelting, casting and forging: the alloy components weighed in step 1) are vacuum smelted and cast, and then forged 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 of 60% to obtain a hot-rolled bar with a diameter of 55 mm;

[0050] 4) Solution treatment: The obtained hot-rolled bar is kept at 1160°C for 6 hours and water quenched to room temperature; heat treatment, then kept at 720°C for 12 hours, and air-cooled to room temperature to obtain a high-strength, toughness, and large-expansion austenitic stainless steel of the present invention; the properties are shown in Table 2.

[0051] Example 4:

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

[0053] 1) Weighing the alloy components, according to the weight percentage of the alloy components, weighing 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, and the balance Fe;

[0054] 2) Smelting, casting and forging: the alloy components weighed in step 1) are vacuum smelted and cast, and then forged 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 hours, and then hot rolled at 1125° C. with a total reduction of 70% to obtain a hot-rolled bar with a diameter of 33 mm;

[0056] 4) Solution treatment: The hot-rolled bar obtained was kept at 1180°C for 7 hours and water quenched to room temperature; heat treatment, then kept at 750°C for 13 hours, and air-cooled to room temperature to obtain a high-strength, toughness, and large-expansion austenitic stainless steel of the present invention; the properties are shown in Table 2.

[0057] Example 5:

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

[0059] 1) Weighing the alloy components, according to the weight percentage of the alloy components, weighing 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, and the balance Fe;

[0060] 2) Smelting, casting and forging: the alloy components weighed in step 1) are vacuum smelted and cast, and then forged 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) was kept at 1150° C. for 2.0 h, and then hot rolled at 1100° C. with a total reduction of 75% to obtain a hot-rolled bar with a diameter of 20 mm;

[0062] 4) Solution treatment: The hot-rolled bar was kept at 1200°C for 8 hours and water quenched to room temperature; heat treated, then kept at 780°C for 15 hours and air-cooled to room temperature to obtain 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 steels prepared in Examples 1-5 were tested. The test results are shown in Table 2.

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

[0069]

[0070] Comparative Example 1

[0071] According to the existing patent CN112143984A which discloses a stainless steel for heat-shrinkable knife handles and a preparation method, an attempt was made to add Al on the basis of Example 4 and smelt according to the process of Example 4 to 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 many large pieces of 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 sharply reduce the Young's modulus of steel, such as Figure 2 shown.

[0076] Therefore, it is proved that the technical effect of this 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 role in strengthening and hardening.

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

[0081] The test results are as follows Figure 4 As shown in Figure 3, the SEM images of carbides before and after solution treatment are compared. The treated steel does not have the sensitization characteristics caused by grain boundary Cr carbides.

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

[0083] The test results are as follows 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 can also be implemented in other ways. Any obvious replacement without departing from the concept of the present technical solution is within the scope of protection of the present invention.

Claims

1. A high-strength, high-toughness, high-expansion austenitic stainless steel, characterized by: The alloy composition by weight percentage is: C: 0.45~0.65 wt.%, Mn: 6.0~9.0 wt.%, Ni: 6.5~10.0 wt.%, Cr: 8.0~12.0 wt.%, Mo: 1.5~3.0 wt.%, V: 0.8~2.0 wt.%, Cu: 1.5~4.0 wt.%, Nb: 0.015~0.045 wt.% and Ti: 0.035~0.100 wt.%, and the balance is Fe; The preparation method of the austenitic stainless steel, 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, followed by a first holding treatment and water quenching to room temperature; then subjecting the hot-rolled bar to an aging treatment, followed by a second holding treatment and air cooling to room temperature to obtain austenitic stainless steel; The first heat preservation treatment in step S4 is carried out at 1150-1200° C. for 4-8 hours. The second heat preservation treatment in step S4 is carried out at 650-780°C for 8-15 hours; The austenitic stainless steel has a yield strength greater than 950 MPa and an elongation after fracture greater than 15%. The austenitic stainless steel does not produce sensitization characteristics caused 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.

2. The high-strength, toughness, and large-expansion austenitic stainless steel according to claim 1, 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.

3. The high-strength, toughness, and large-expansion austenitic stainless steel according to claim 1, characterized in that: The diameter of the forging blank obtained by forging in step S2 is 80-200 mm.

4. The high-strength, toughness, and large-expansion austenitic stainless steel according to claim 1, characterized in that: The specific operation of hot rolling in step S2 is: keeping the temperature 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%.

5. The high-strength, toughness, and large-expansion austenitic stainless steel according to claim 1, characterized in that: The hot-rolled bar obtained in step S3 has a diameter of 20 to 100 mm.

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