A duplex precipitation-strengthened ultra-high strength and toughness stainless steel

Abstract

A duplex precipitation-strengthened ultra-high strength and toughness stainless steel, wherein the duplex precipitates include NiAl nano-scale precipitates and M2C nano-scale precipitates. M in M2C is mainly Mo element, and also contains metal elements such as Cr and Ni. The NiAl nano-scale precipitates and M2C nano-scale precipitates are both promoted by Co to precipitate and are diffusely distributed in the lath martensite microstructure with a high dislocation density. The ultra-high strength and toughness means that the test results according to national standards GB / T 228.1-2018 and GB / T 21143-2014 are as follows: tensile strength ≥ 1900 MPa, elongation ≥ 10.0%, fracture toughness ≥ 70 MPa·m1 / 2. It is characterized in that it includes the following chemical element components and their wt% contents: C = 0.12 - 0.40%, Cr = 8.0 - 14.0, Ni = 7.0 - 17.0, Mo = 0.80 - 1.60, Al = 0.6 - 3.1, Co = 7.0 - 15.5, W = 0.35 - 0.75, Si ≤ 0.10%, Mn ≤ 0.10%, P ≤ 0.0050%, S ≤ 0.0015%, Fe = the balance.

Description

Technical Field

[0001] The present invention relates to the technical field of stainless steel, in particular to a duplex precipitation-strengthened ultra-high strength and toughness stainless steel. The duplex precipitates include NiAl nano-scale precipitates and M2C nano-scale precipitates. M in M2C is mainly composed of Mo element and also contains metal elements such as Cr and Ni. The NiAl nano-scale precipitates and M2C nano-scale precipitates are both Co-promoted precipitates and are dispersedly distributed in the lath martensite microstructure with a high dislocation density. The ultra-high strength and toughness means that the test results according to national standards GB / T228.1-2018 and GB / T21143-2014 are as follows: tensile strength ≥ 1900 MPa, elongation ≥ 10.0%, and fracture toughness ≥ 70 MPa·m 1 / 2 。 Background Art

[0002] At present, some key load-bearing components of civil aircraft, such as flap slide rails, landing gear load-bearing shear pins, and horizontal tail fuselage wing box connection bolts, are made of ultra-high strength steels such as 300M, AerMet100, 17-4PH, 15-5PH, PH13-8Mo, and Custom465. Their chemical compositions are shown in Table 1, and their mechanical properties and main strengthening phases are shown in Table 2. 300M and AerMet100 have relatively high strength but poor corrosion resistance. Stainless steels such as 17-4PH, 15-5PH, PH13-8Mo, and Custom465 have a single main precipitation strengthening phase and low strength, and cannot meet the ultra-high strength and toughness requirements in harsh environments. Under the condition of meeting the requirements of damage tolerance performance, it is necessary to design a stainless steel material with higher specific strength and specific stiffness to achieve the goal of weight reduction and cost reduction.

[0003] Table 1 Chemical Compositions of Several Ultra-High Strength Steels (wt%)

[0004] Grade C Si Mn Cr Ni Mo Al Co Cu Ti Fe 300M 0.31 1.51 0.01 1.44 3.52 0.25 — — — — Bal. AerMet100 0.24 0.03 0.01 2.99 11.2 1.18 — 13.4 — — Bal. 17-4PH 0.07 — ≤1.0 16.0 4.0 — — — 4.0 — Bal. 15-5PH 0.04 — ≤1.0 15.0 4.7 — — — 3.0 — Bal. PH13-8Mo 0.03 — — 12.6 7.9 1.7 1.0 — — — Bal. Custom465 0.02 — — 11.6 11.0 1.0 — — — 1.5 Bal.

[0005] Table 2 Mechanical Properties and Main Strengthening Phases of Several Ultra-High Strength Steels

[0006] Summary of the Invention

[0007] In view of the deficiencies existing in the prior art, the present invention provides a duplex precipitation-strengthened ultra-high strength and toughness stainless steel. The duplex precipitates include NiAl nano-scale precipitates and M2C nano-scale precipitates. M in M2C is mainly composed of Mo element and also contains metal elements such as Cr and Ni. The NiAl nano-scale precipitates and M2C nano-scale precipitates are both Co-promoted precipitates and are dispersedly distributed in the lath martensite microstructure with a high dislocation density. The ultra-high strength and toughness means that the test results according to national standards GB / T 228.1-2018 and GB / T 21143-2014 are as follows: tensile strength ≥ 1900 MPa, elongation ≥ 10.0%, and fracture toughness ≥ 70 MPa·m1 / 2.

[0008] The technical solution of the present invention is as follows:

[0009] A duplex precipitation-strengthened ultra-high strength and toughness stainless steel, characterized by including the following chemical element components and their wt% contents: C = 0.12 - 0.40%, Cr = 8.0 - 14.0, Ni = 7.0 - 17.0, Mo = 0.80 - 1.60, Al = 0.6 - 3.1, Co = 7.0 - 15.5, W = 0.35 - 0.75, Si ≤ 0.10%, Mn ≤ 0.10%, P ≤ 0.0050%, S ≤ 0.0015%, and Fe = the balance.

[0010] The ultra-high strength and toughness means that the test results according to national standards GB / T 228.1-2018 and GB / T 21143-2014 are as follows: tensile strength ≥ 1900 MPa, elongation ≥ 10.0%, and fracture toughness ≥ 70 MPa·m1 / 2.

[0011] The duplex precipitates include NiAl nano-scale precipitates and M2C nano-scale precipitates. M in M2C is mainly composed of Mo element and also contains metal elements Cr and Ni. The NiAl nano-scale precipitates and M2C nano-scale precipitates are both Co-promoted precipitates and are dispersedly distributed in the lath martensite microstructure with a high dislocation density.

[0012] The duplex precipitation-strengthened ultra-high strength and toughness stainless steel realizes its ultra-high strength and toughness through the following processes in sequence: smelting process, homogenization annealing process, forging process, and heat treatment process.

[0013] The smelting process includes using vacuum induction + vacuum consumable melting to form an ingot.

[0014] The homogenization annealing process includes holding the ingot at 600 ± 10 °C for 2 h, heating to 650 ± 10 °C and holding for 8 h, then heating to 1220 ± 10 °C and holding for 40 h, cooling in the furnace to 650 ± 10 °C, and finally taking out of the furnace and air-cooling to room temperature.

[0015] The forging process includes forging the ingot after heating it in the furnace. The initial forging temperature is 1050 - 1150°C, and the final forging temperature is ≥800°C. The three-upsetting and three-drawing process is adopted to forge the bar or slab into the preset size and specification, and the forging ratio is ≥20.

[0016] The heat treatment process includes the following systems implemented in sequence for the annealed bar or slab: solution treatment system, cryogenic treatment system, and aging treatment system.

[0017] The solution treatment system includes heating to 1050 - 1100°C and holding for a period of time, then oil cooling to room temperature; the cryogenic treatment system includes cryogenic treatment at -73 - -196°C for a period of time; the aging treatment system includes holding at 450 - 550°C for a period of time, and then taking it out and air cooling to room temperature.

[0018] The technical effects of the present invention are as follows: A duplex precipitation-strengthened ultra-high strength and toughness stainless steel of the present invention is alloyed by adding Cr, Ni, Mo, Al, Co, and W. Cr plays a corrosion-resistant role. Ni and Al form nano-scale NiAl precipitation phases, and Mo and C form M2C carbides to play a duplex precipitation strengthening role. Co promotes the dispersed precipitation of the duplex precipitation phases. Through the martensite matrix and the dispersed precipitation strengthening of the duplex precipitation phases, the steel of the present invention has both corrosion resistance and excellent strength and toughness. The mechanical properties of the steel of the present invention are respectively subjected to quasi-static tensile testing and quasi-static fracture toughness testing in accordance with national standards (GB / T 228.1 - 2018, GB / T 21143 - 2014) and meet the following requirements: the tensile strength reaches above 1900 MPa, the elongation rate reaches above 10.0%, and the fracture toughness reaches 70 MPa·m 1 / 2 and above. The steel of the present invention is equivalent to Custom475 with the highest strength level having stainless characteristics, while its plasticity is significantly better than that of Custom475, and the elongation rate reaches above 10.0%. Compared with several common stainless steels in Table 2, the steel of the present invention exhibits the most excellent comprehensive performance. Specific Embodiments

[0019] The present invention will be described below in conjunction with embodiments.

[0020] A duplex precipitation-strengthened ultra-high strength and toughness stainless steel, comprising the following chemical elements and their wt% contents: C = 0.12 - 0.40%, Cr = 8.0 - 14.0, Ni = 7.0 - 17.0, Mo = 0.80 - 1.60, Al = 0.6 - 3.1, Co = 7.0 - 15.5, W = 0.35 - 0.75, Si ≤ 0.10%, Mn ≤ 0.10%, P ≤ 0.0050%, S ≤ 0.0015%, Fe = the balance. The ultra-high strength and toughness means that the test results according to national standards GB / T 228.1-2018 and GB / T 21143-2014 are as follows: tensile strength ≥ 1900 MPa, elongation at break ≥ 10.0%, fracture toughness ≥ 70 MPa·m1 / 2. The duplex precipitation phases include NiAl nanoscale precipitation phases and M2C nanoscale precipitation phases. M in M2C is mainly Mo element, and also contains metal elements such as Cr and Ni. The NiAl nanoscale precipitation phases and M2C nanoscale precipitation phases are both Co-promoted precipitations and are dispersedly distributed in the lath martensite microstructure with high dislocation density.

[0021] The duplex precipitation-strengthened ultra-high strength and toughness stainless steel realizes its ultra-high strength and toughness through the following processes in sequence: smelting process, homogenization annealing process, forging process, and heat treatment process. The smelting process includes using vacuum induction + vacuum consumable electrode melting to form an ingot. The homogenization annealing process includes holding the ingot at 600 ± 10 °C for 2 h, heating to 650 ± 10 °C and holding for 8 h, then heating to 1220 ± 10 °C and holding for 40 h, furnace cooling to 650 ± 10 °C, and finally taking out and air cooling to room temperature. The forging process includes heating the ingot with the furnace and then forging. The initial forging temperature is 1050 - 1150 °C, the final forging temperature ≥ 800 °C, and the three-upsetting and three-drawing process is used to forge into bars or slabs with preset size specifications, and the forging ratio ≥ 20.

[0022] The heat treatment process includes implementing the following systems for the annealed bars or slabs in sequence: solution treatment system, cryogenic treatment system, and aging treatment system. The solution treatment system includes heating to 1050 - 1100 °C and holding for a period of time, and oil cooling to room temperature; the cryogenic treatment system includes cryogenic treatment at -73 - -196 °C for a period of time; the aging treatment system holds at 450 - 550 °C for a period of time, takes out and air cools to room temperature.

[0023] The object of the present invention is to meet the requirements of weight reduction and cost reduction of key load-bearing components of civil aircraft, and to provide a duplex precipitation-strengthened ultra-high strength and toughness stainless steel and its preparation process. The object of the present invention is achieved by the following technical solutions: A duplex precipitation-strengthened ultra-high strength and toughness stainless steel of the present invention, the mass percentage of chemical components: C: 0.12 - 0.40%, Cr: 8.0 - 14.0%, Ni: 7.0 - 17.0%, Mo: 0.80 - 1.60%, Al: 0.6 - 3.1%, Co: 7.0 - 15.5%, W: 0.35 - 0.75%, and the balance is Fe and inevitable impurities.

[0024] C: As the main solid-solution strengthening element of the steel of the present invention, an appropriate amount of C can ensure that the matrix structure of the steel is a low-carbon lath martensite matrix with a high dislocation density. However, if the C content is too high, large-sized eutectic carbides will precipitate at the grain boundaries, reducing the toughness and corrosion resistance of the steel. Considering comprehensively, the C content of the present invention is between 0.12 - 0.40%.

[0025] Cr: As an important element playing a role in corrosion resistance in the steel of the present invention, Cr combines with O to form a dense passivation film to protect the steel from atmospheric corrosion. With the increase of Cr content, the stainlessness and oxidation resistance of the steel are significantly improved. However, too high a Cr content will promote the formation of harmful phases, reducing the hot working performance of the stainless steel, and at the same time, it is also easy to cause segregation during smelting. Considering comprehensively, the Cr content of the present invention is between 8.0 - 14.0%.

[0026] Ni: As an alloying element expanding the γ-phase region, it can make screw dislocations not easily decompose, thus enabling cross-slip to proceed smoothly, significantly improving the toughness of the steel. In addition, Ni can improve the composition, structure and performance of the Cr oxide film, enhancing the corrosion resistance and oxidation resistance of the stainless steel. In the steel of the present invention, Ni is also used to form nano-scale NiAl phases with Al to increase the strength of the steel. However, too high a Ni content will lead to an increase in the content of retained austenite in the steel, reducing the strength of the steel. Considering comprehensively, the Ni content of the present invention is controlled between 7.0 - 17.0%.

[0027] Mo: As one of the important alloying elements in the steel of the present invention, the fine and dispersed M2C phases formed by Mo and C increase the strength of the steel, and Mo can also hinder the precipitation of precipitation phases at the prior austenite grain boundaries, avoiding intergranular fracture and improving the fracture toughness of the steel. In addition, in reducing media, Mo can also promote the passivation of Cr. Therefore, Mo can improve the corrosion resistance of chromium-nickel stainless steel in some reducing acids and some organic acids, and can effectively inhibit the pitting corrosion tendency of chloride ions on the steel, improving the intergranular corrosion resistance. However, Mo is also an alloying element expanding the γ-phase region, and too high a Mo content will also lead to an increase in the content of retained austenite in the steel. Considering comprehensively, the Mo content of the present invention is controlled between 0.80 - 1.60%.

[0028] Al: As one of the important alloying elements in the steel of the present invention, the main role of Al in the steel of the present invention is age hardening. Through age hardening heat treatment, fine and dispersed NiAl phases precipitate in the martensite matrix to produce precipitation hardening and improve the strength of the steel. In addition, Al can also form a dense oxide film on the steel surface to improve the oxidation resistance of the stainless steel. However, Al is an element that expands the α region, and excessive Al will lead to the formation of harmful phases and reduce the toughness of the steel. Considering comprehensively, the Al content in the present invention is controlled between 0.6% and 3.1%.

[0029] Co: As one of the important alloying elements in the steel of the present invention, Co can maintain a high dislocation density in the martensite laths, provide more nucleation sites for the precipitation of precipitates, and promote the dispersed distribution of the precipitates. That is, through the appropriate Co content, it forms a coordination with the NiAl and M2C double precipitation strengthening phases, where M is mainly Mo metal and also includes metal elements such as Cr and Ni. However, excessive Co will affect the corrosion resistance and the cost is very high. Considering comprehensively, the Co content in the present invention is controlled between 7.0% and 15.5%.

[0030] W: Adding a small amount of W to the steel of the present invention improves the hardenability of the steel. W and Mo are prone to segregation at the grain boundaries, which can enhance the grain boundary bonding force and improve the toughness of the steel. However, excessive W will form M6C carbides, whose remelting temperature is relatively high, resulting in coarse grains and reduced plasticity and toughness. And W will significantly increase the difficulty of hot working and is prone to cracking. Considering comprehensively, the W content in the present invention is controlled between 0.35% and 0.75%.

[0031] The control requirements for the content of harmful impurities in the steel of the present invention are: Si ≤ 0.10%, Mn ≤ 0.10%, P ≤ 0.0050%, S ≤ 0.0015%.

[0032] A preparation process for processing a low-alloy high-toughness ultra-high-strength steel using the above formula in the present invention is as follows:

[0033] 1) Smelting process: According to the designed composition ratio of the martensitic stainless steel, use vacuum induction + vacuum consumable melting to produce an ingot.

[0034] 2) Homogenization annealing process: Keep the smelted ingot at 600 ± 10 °C for 2 h, heat it up to 650 ± 10 °C and keep it for 8 h, then heat it up to 1220 ± 10 °C and keep it for 40 h, cool it in the furnace to 650 ± 10 °C, and finally take it out of the furnace and air-cool it to room temperature.

[0035] 3) Forging process: Heat the ingot with the furnace and then forge it. The initial forging temperature is 1050 - 1150 °C, the final forging temperature is greater than 800 °C, and use the three-upsetting and three-drawing process to forge it into bars or slabs of corresponding dimensions, and the forging ratio is not less than 20.

[0036] 4) Heat treatment process: After annealing the forgings, they undergo a solution treatment regime, a cryogenic treatment regime, and an aging treatment regime. The solution treatment regime involves heating to 1050 - 1100 °C, holding for a period of time, and then oil quenching to room temperature. The cryogenic treatment regime is carried out at -73 to -196 °C for a period of time. The aging treatment regime is to hold at 450 - 550 °C for a period of time, and then take out and air cool to room temperature.

[0037] Example 1: The double-precipitation-phase-strengthened high-strength and tough stainless steel prepared by vacuum induction melting and vacuum consumable remelting has the following chemical composition by mass percentage: C: 0.12%, Cr: 8.2%, Ni: 15.9%, Mo: 1.48%, Co: 9.5%, W: 0.6%, Al: 1.4%, and the balance is iron and unavoidable impurities. The initial forging temperature of the ingot is 1130 °C, the final forging temperature is controlled at 850 °C, and the forging ratio is about 20. The solution temperature is 1080 °C, holding for 1.5 h, and then oil quenching to room temperature. The cryogenic treatment temperature is -73 °C, holding for 3 h, and then air cooling to room temperature. The aging temperature is 480 °C, holding for 10 h, and then air cooling to room temperature. The ultra-high-strength and tough stainless steel prepared in this example, after mechanical property testing, has a tensile strength of 1925 MPa, an elongation of 12.5%, and a fracture toughness of 88 MPa·m 1 / 2 .

[0038] Example 2: The double-precipitation-phase-strengthened high-strength and tough stainless steel prepared by vacuum induction melting and vacuum consumable remelting has the following chemical composition by mass percentage: C: 0.21%, Cr: 12.5%, Ni: 10.5%, Mo: 0.88%, Co: 13.5%, W: 0.65%, Al: 1.8%, and the balance is iron and unavoidable impurities. The initial forging temperature of the ingot is 1130 °C, the final forging temperature is controlled at 850 °C, and the forging ratio is about 20. The solution temperature is 1080 °C, holding for 1.5 h, and then oil quenching to room temperature. The cryogenic treatment temperature is -196 °C, holding for 3 h, and then air cooling to room temperature. The aging temperature is 480 °C, holding for 10 h, and then air cooling to room temperature. The ultra-high-strength and tough stainless steel prepared in this example, after mechanical property testing, has a tensile strength of 1975 MPa, an elongation of 11.0%, and a fracture toughness of 80 MPa·m 1 / 2 .

[0039] Example 3: The composition of the dual-precipitation-phase strengthened high-strength and tough stainless steel prepared by vacuum induction melting and vacuum consumable remelting is as follows by mass percentage: C: 0.32%, Cr: 13.8%, Ni: 16.6%, Mo: 1.52%, Co: 14.8%, W: 0.70%, Al: 1.1%, and the balance is iron and inevitable impurities. The initial forging temperature of the ingot is 1130 °C, the final forging temperature is controlled at 850 °C, and the forging ratio is about 20. The solution temperature is 1080 °C, holding for 1.5 h, oil cooling to room temperature, the cryogenic treatment temperature is -196 °C, holding for 3 h, air cooling to room temperature, and the aging temperature is 480 °C, holding for 10 h, air cooling to room temperature. The ultra-high-strength and tough stainless steel prepared in this example, after mechanical property testing, has a tensile strength of 2015 MPa, an elongation of 10.0%, and a fracture toughness of 72 MPa·m 1 / 2 .

[0040] The content not described in detail in the specification of the present invention belongs to the prior art well-known to those skilled in the art. It is hereby indicated that the above description helps those skilled in the art to understand the present invention, but does not limit the protection scope of the present invention. Any implementation carried out by equivalent replacement, modification and improvement and / or simplification of the above description without departing from the essence of the present invention falls within the protection scope of the present invention.

Claims

1. A duplex precipitation-strengthened ultra-high strength and toughness stainless steel, characterized in that, It includes the following chemical element components and their wt% contents: C = 0.12 - 0.40%, Cr = 8.0 - 14.0%, Ni = 10.5 - 17.0%, Mo = 0.80 - 1.60%, Al = 1.1 - 3.1%, Co = 9.5 - 15.5%, W = 0.6 - 0.75%, Si ≤ 0.10%, Mn ≤ 0.10%, P ≤ 0.0050%, S ≤ 0.0015%, and Fe is the balance; The double precipitation phases include NiAl nanoscale precipitation phases and M2C nanoscale precipitation phases. M in M2C is mainly composed of the Mo element and also contains the metal elements Cr and Ni. The NiAl nanoscale precipitation phases and M2C nanoscale precipitation phases are both promoted by Co for precipitation and are in a dispersed distribution state in the lath martensite microstructure with a high dislocation density.

2. The duplex precipitation phase-strengthened ultra-high strength and toughness stainless steel according to claim 1, characterized in that, The so-called ultra-high strength and toughness means that the test results according to national standards GB / T 228.1-2018 and GB / T 21143-2014 are as follows: tensile strength ≥ 1900 MPa, elongation at break ≥ 10.0%, fracture toughness ≥ 70 MPa·m 1 / 2 .

3. The duplex precipitation-strengthened ultra-high strength and toughness stainless steel according to claim 1, wherein, The double precipitation phase strengthened ultra-high strength and toughness stainless steel realizes its ultra-high strength and toughness through the following processes in sequence: Smelting process, homogenization annealing process, forging process, and heat treatment process.

4. The duplex precipitation-strengthened ultra-high strength and toughness stainless steel according to claim 3, characterized in that, The smelting process includes using vacuum induction + vacuum consumable electrode melting to form an ingot.

5. The duplex precipitation-strengthened ultra-high strength and toughness stainless steel according to claim 4, wherein The homogenization annealing process includes holding the ingot at 600 ± 10°C for 2 h, heating up to 650 ± 10°C and holding for 8 h, then heating up to 1220 ± 10°C and holding for 40 h, furnace cooling to 650 ± 10°C, and finally taking it out of the furnace and air cooling to room temperature.

6. The duplex precipitation phase-strengthened ultra-high strength and toughness stainless steel according to claim 5, characterized in that, The forging process includes heating the ingot in the furnace and then forging it. The initial forging temperature is 1050 - 1150°C, the final forging temperature ≥ 800°C, and it is forged into a bar or slab with a preset size specification using the three-upsetting and three-drawing process, and the forging ratio ≥ 20.

7. The duplex precipitation-strengthened ultra-high strength and toughness stainless steel according to claim 6, characterized in that, The heat treatment process includes implementing the following systems in sequence for the annealed bar or slab: solution treatment system, cold treatment system, and aging system.

8. The duplex precipitation phase-strengthened ultra-high strength and toughness stainless steel according to claim 7, characterized in that, The solution treatment system includes heating to 1050 - 1100°C and holding for a period of time, then oil cooling to room temperature; the cold treatment system includes cold treatment at -73 - -196°C for a period of time; the aging system includes holding at 450 - 550°C for a period of time, taking it out and air cooling to room temperature.

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