Precipitation hardening stainless steel and preparation method and application thereof

Abstract

The invention relates to precipitation-hardening stainless steel and a preparation method and application thereof. The stainless steel comprises the following chemical components: 0.05%-0.25% of C, 0-0.15% of Si, 0.2%-0.5% of Mn, 9%-18% of Cr, 0.2%-5% of Ni, 0.5%-5% of Mo, 0-0.5% of V, 5%-13% of Co and the balance of Fe and inevitable impurities. During preparation, alloy raw materials are forged or rolled after being smelted and cast, or prepared into spherical alloy powder through an atomization method / rotating electrode method and then subjected to 3D printing, a formed component is prepared, and then the product is obtained through aftertreatment. By controlling the proportion of alloy elements and cooperating with an optimized post-treatment process, the dispersion degree and strengthening effect of second-phase precipitation are improved, impact toughness is improved, 3D printing cracking is avoided, excellent comprehensive mechanical properties, especially high-temperature strength, are obtained, the fatigue cracking risk is reduced, the service life is prolonged, and the production cost is reduced. And the corrosion resistance of polishing, machining and complex environments is improved, and the mold can be used in the field of molds.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of metallurgy, in particular to a precipitation hardening stainless steel and a preparation method and application thereof. BACKGROUND

[0002] Currently, in the field of 3D printing, alloy steels that can be used as hot working dies include H13 steel and 18Ni300 martensitic age hardening steel. H13 steel undergoes rapid melting and solidification during additive manufacturing, resulting in non-equilibrium martensitic phase transformation. The lattice distortion caused thereby results in very high thermal stress, and the high C content causes the hardness after printing to exceed 50HRC, thereby causing deformation or even cracking. Due to good printability, low-C martensitic age hardening steel represented by 18Ni300 is commonly used as a die casting die material, which not only avoids the generation of printing defects, but also can obtain high hardness after simple aging treatment. However, during die casting, the die surface repeatedly undergoes rapid heating and cooling. Due to its poor high-temperature performance and weak fatigue resistance, 18Ni300 is prone to thermal cracking, which reduces the fatigue life, and its poor corrosion resistance to aluminum liquid easily forms corrosion points, which affects the surface quality of the castings.

[0003] Therefore, there is an urgent need for a hot working die steel material that has good printability, superior high-temperature strength, high thermal fatigue resistance, and corrosion resistance. CN108356263B discloses a new martensitic heat-resistant steel for additive manufacturing, which also adopts a composition strategy of low C and high Cr, and is alloyed with Mo and V, and further adds a certain amount of W, a small amount of Ta, and rare earth elements to form oxide dispersion strengthening. However, the room temperature yield strength of the new martensitic heat-resistant steel is less than 1000MPa, and the high-temperature yield strength at 600℃ is only about 300MPa, which cannot meet the strength requirements of the die. In addition, due to the addition of rare earth elements and W, the cost of the new martensitic heat-resistant steel is high. CN116855852A discloses an additive manufacturing precipitation strengthened high-temperature martensitic steel containing relatively high Ni, Co, Mo, and W, and further alloyed with Cu and Al to form multiple second phases for composite strengthening. The strength of the steel meets the requirements of the die, but due to the large proportion of the contribution of precipitation strengthening to the strength, according to the principle of the strength and toughness contradiction of the material, the brittleness increases, and there is a risk of cracking during printing and heat treatment. Therefore, the steel cannot be applied on a large scale at present. CN111500936A discloses a precipitation hardening stainless steel material. Although the printability and mechanical properties of the steel are not reported, the addition of Cu, Al, and Ti also sacrifices the printability to improve the strength, which still cannot meet the requirements of 3D printing dies. SUMMARY

[0004] To solve the above problems, the application provides a precipitation hardening stainless steel and a preparation method and application thereof.

[0005] In a first aspect, the application provides a precipitation hardening stainless steel, which adopts the following technical scheme: The precipitation hardening stainless steel comprises the following chemical components in percentage by weight: C 0.05%-0.25%, Si 0-0.15%, Mn 0.2-0.5%, Cr 9-18%, Ni 0.2-5%, Mo 0.5-5%, V 0-0.5%, Co 5-13%, and the balance of Fe and inevitable impurities.

[0006] By adopting the above technical scheme, for the traditional Cr-Mo-V precipitation hardening stainless steel, only carbide and Laves phase are used for strengthening, Co is used to control the precipitation process of different types of carbides or Laves phases during heat treatment, especially for the Mo and V containing alloy carbides with good high temperature stability, the relative ideal morphology, size and distribution are obtained, and then the required strengthening effect is achieved, and the solid solution strengthening of Co, Ni and other alloy elements is used to meet the mechanical strength and fatigue resistance required by the mold. The upper limit of the C content is controlled to be within 0.25%, which can form enough alloy carbides to ensure the strength of the stainless steel, and can also avoid strong lattice distortion to improve the additive manufacturing process and reduce the cracking risk; the upper limit of the Si content is lower than 0.15%, which can prevent the excessive reduction of thermal conductivity and improve the toughness of the stainless steel; The addition of Cr forms Cr 23 C6 carbide, which can not only stabilize the matrix martensite lath structure, but also improve the corrosion resistance of the stainless steel; the addition of Ni can improve the plasticity and toughness of the stainless steel and reduce the cracking risk; Mo mainly produces precipitation strengthening, and under the action of Co, the precipitation hardening peak is higher, and the diffusion is slow, the carbide has limited growth or coarsening at high temperature, which ensures the strengthening effect and high temperature strength; V can not only form VC, but also form a composite carbide with smaller size and higher dispersion with Mo, and the strengthening effect is more obvious; When the Cr content reaches 18%, there is a tendency to form delta ferrite, Co can not only directly produce solid solution strengthening, but also reduce the solubility of Mo in the matrix to indirectly enhance the precipitation strengthening effect of carbide, and can also balance the tendency of delta ferrite formation caused by the existence of Cr and Mo; reducing the contents of Mn, P and S can further increase the toughness of the stainless steel and reduce the tendency of embrittlement.

[0007] In a second aspect, the application provides a preparation method of a precipitation hardening stainless steel, which adopts the following technical scheme: A method for preparing a precipitation hardening stainless steel, comprising the following preparation steps: S1, after alloy raw materials are smelted and cast, a shaped component is obtained through forging or rolling; or, the alloy raw materials are prepared into spherical alloy powder through atomization method or rotating electrode method, and then 3D printing is performed to obtain a shaped component; S2, the shaped component obtained in S1 is post-processed to obtain a precipitation hardening stainless steel.

[0008] Preferably, a practical powder forming method is used, 1) spherical alloy powder with fine-grained martensite structure is prepared according to the chemical component proportion; 2) the spherical alloy powder in step 1) is printed and formed by using selective laser melting (SLM) method; 3) heat treatment is performed, and the precipitation hardening stainless steel is obtained.

[0009] By adopting the above technical solution, the precipitation hardening stainless steel provided by the present application not only fully considers the synergistic effect between elements in the chemical component design to achieve excellent comprehensive performance, but also combines advanced 3D printing technology and heat treatment process in the preparation method, which can prepare a precipitation hardening stainless steel with good printing performance, thermal fatigue resistance, high temperature resistance and corrosion resistance, effectively solving the problem that the application of 3D printing manufacturing mold materials is limited in the die casting field, and providing new material selection and technical support for the development of the die casting industry.

[0010] Further, the preparation process of the spherical alloy powder is as follows: according to the above chemical component proportion, the high-temperature molten alloy is cooled to room temperature by using atomization method, and experiences liquid-solid phase change (solidification) and solid phase change (austenite-martensite phase change) to obtain spherical powder with fine-grained martensite structure, the particle size of which is 5-85 μm, normally distributed, D10 is 23 μm, D50 is 40 μm, and D90 is 70 μm; The impurity element content of the spherical alloy powder is: O is 179 ppm, N is 147 ppm, S is less than 0.003 wt%, and P is less than 0.021 wt%; the packaging density of the spherical alloy powder is 3.8-4.4 g / cm 3 The tap density of the spherical alloy powder is 4.6-5.0 g / cm 3 The Hall flow rate of the spherical alloy powder is ≤15 s / 50 g.

[0011] By adopting the above technical solution, the preparation process and various performance indicators of the spherical alloy powder are strictly controlled, which can ensure that the prepared powder has good fluidity and filling property, which is crucial for the subsequent 3D printing process.

[0012] Suitable particle size distribution and normal distribution characteristics help to ensure uniform spreading of the powder during printing, improve printing accuracy and forming quality. Strict limitation of impurity element content can avoid adverse effects of impurities on material properties, such as reducing the strength, toughness and corrosion resistance of the material. Reasonable ranges of packing density and tap density ensure the stability of the powder during storage and transportation, and also facilitate uniform supply of the powder during printing.

[0013] The control of the Hall flow rate is directly related to the printing efficiency and quality, and a lower Hall flow rate means that the powder can flow more smoothly, reducing the printing process and defects, so as to prepare a precipitation hardened stainless steel with fine-grained martensite structure and excellent performance, which can meet the high requirements of die casting field on hot work die steel materials in printing performance, thermal fatigue resistance, high temperature resistance and corrosion resistance.

[0014] Further, the SLM process parameters in S1 are: laser power is 200-450W, scanning speed is 0.5-1.0m / s, scanning interval is 50-150μm, powder layer thickness is 20-100μm, interlayer scanning path is at an angle of 65-70°, and substrate preheating temperature is 0-200℃.

[0015] By adopting the above technical scheme, the printing quality and material performance can be significantly improved.

[0016] Optionally, the post-processing in S2 includes at least one of austenitizing quenching treatment, low-temperature cryogenic treatment and tempering treatment.

[0017] Further, the austenitizing quenching treatment has a temperature of 1000-1070℃, a time of 0.5-5h and a cooling rate of 2-50℃ / s.

[0018] Further, the cryogenic treatment has a temperature of -196~-120℃ and a time of 0.5-3h.

[0019] Further, the tempering treatment has a temperature of 480-650℃, 2-3 times of tempering, and a time of 3-7h.

[0020] In the present application, the preferred heat treatment is aging treatment, the material is heated to 480-650℃ after printing, and then 2-3 times of tempering is performed to obtain the final strength, hardness and toughness.

[0021] By adopting the above technical scheme, carbides and Laves phases are fully analyzed, and significant precipitation strengthening effect is produced, so that the material finally obtains higher hardness and strength, meeting the stringent requirements of specific application scenarios on material performance.

[0022] Further, the microstructure of the precipitation hardening stainless steel in S2 presents a lamellar structure of interlaced cellular grains and columnar grains, or a lath martensite hierarchical structure, and the martensite matrix is distributed with dispersed Laves phase, alloy carbide and a small amount of residual austenite phase.

[0023] By adopting the above technical solution, when the microstructure presents a lamellar structure of interlaced cellular grains and columnar grains, this unique grain morphology helps to hinder the propagation of cracks. When the material is subjected to external force, the crack will deflect when encountering grain boundaries with different orientations, thereby consuming more energy, significantly improving the fracture toughness of the material, reducing the risk of brittle fracture of the material, and enabling the material to maintain good integrity under complex stress environment; The lath martensite hierarchical structure endows the material with high strength and high hardness. The lath martensite itself has high dislocation density and fine substructure, which can effectively resist external force. At the same time, the dispersed distribution of Laves phase, alloy carbide and a small amount of residual austenite phase in the martensite matrix plays a crucial role. Laves phase and alloy carbide as hard second phase can bear part of the load when the material is stressed, hinder the movement of dislocations, and produce significant precipitation strengthening effect, further improving the strength and hardness of the material. A small amount of residual austenite phase can relieve stress concentration in the material to some extent, improve the plasticity and toughness of the material, and avoid sudden fracture of the material due to excessive brittleness and hardness; This specific microstructure structure enables the precipitation hardening stainless steel to have high strength and high hardness while also having good toughness and plasticity, which can meet the use requirements under various complex working conditions, and has broad application prospects in the fields of aerospace, automobile manufacturing, die industry and other fields with high requirements on material performance.

[0024] In a third aspect, the application provides an application of the precipitation hardening stainless steel. The precipitation hardening stainless steel or the precipitation hardening stainless steel prepared by the preparation method of the precipitation hardening stainless steel is applied in a die.

[0025] In summary, the application has the following beneficial effects: 1) Compared with traditional die steel used in selective laser melting (SLM) process, the precipitation hardening stainless steel provided by the application realizes the perfect combination of printability and mechanical properties by precisely controlling the proportions of alloying elements such as C, Si, Ni and Co in the chemical composition design; The stainless steel not only has excellent printing performance and can adapt to 3D printing manufacturing of complex die structure, but also has high strength, high hardness, good toughness and plasticity in mechanical properties, which can meet the requirements of high pressure, high wear and complex stress environment of the die in the die casting process; At the same time, its excellent corrosion resistance and fatigue resistance effectively resist the corrosion of corrosive media and long-term cyclic load in the die casting process, significantly prolonging the service life of the mold and reducing the production cost. 1. The present application promotes the precipitation of carbides and Laves phase through optimized post-processing, especially aging treatment at 480-650℃, producing significant precipitation strengthening effect; this strengthening mechanism not only improves the hardness and strength of the material, but also improves the impact toughness, avoiding the risk of cracking due to stress concentration in the 3D printing process; in addition, deep cryogenic treatment can be introduced, further improving the dispersion degree and strengthening effect of the second phase precipitation, so that the material can still maintain excellent mechanical properties at high temperature, greatly reducing the risk of fatigue cracking and increasing the service life; 2. The precipitation hardening stainless steel of the present application combines 3D printing technology and heat treatment process in the preparation method, by strictly controlling the preparation process and various performance indicators of spherical alloy powder, ensuring good flowability and filling property of the powder, thereby improving the printing precision and forming quality, and the optimized SLM process parameters further improve the printing quality and material performance, so that the stainless steel can meet the high requirements of die steel materials in the die casting field. BRIEF DESCRIPTION OF DRAWINGS

[0026] Figure 1 The figure is the heat treatment curve of the precipitation hardening stainless steel material in the embodiment of the present application at different temperatures; Figure 2 The figure is the anti-tempering stability curve of the precipitation hardening stainless steel material in the embodiment of the present application and EM213, Dievar. DETAILED DESCRIPTION

[0027] In order to make the present application more obvious and easy to understand, the preferred embodiment is described in detail below in combination with the drawings.

[0028] EMBODIMENT A precipitation hardening stainless steel material for 3D printing, consisting of the following chemical components by mass percentage: C 0.13%, Si 0.09%, Mn 0.44%, Cr 11.16%, Ni 2.74%, Mo 4.75%, V 0.43%, Co 10.28%, P <0.01%, S <0.01%, the balance being Fe and unavoidable impurities.

[0029] And it is prepared by the following preparation method: According to the above chemical component allocation ratio, the high-temperature molten alloy is cooled to room temperature by using atomization method, and the spherical powder with fine-grained martensite structure is obtained through liquid-solid phase change (solidification) and solid phase change (austenite-martensite phase change), the particle size is between 5-85 μm and presents normal distribution, D10 is about 23 μm, D50 is about 40 μm, and D90 is about 70 μm; the impurity element content is: O is about 179 ppm, N is about 147 ppm, S is less than 0.003 wt%, and P is less than 0.021 wt%; the packaging density is 3.8-4.4 g / cm 3 , the tap density is 4.6-5.0 g / cm 3 , and the Hall flow rate is less than or equal to 15 s / 50 g. Then the spherical alloy powder is printed by using selective laser melting (SLM) method, and the specific SLM process parameters are: laser power is 200-450 W, scanning speed is 0.5-1.0 m / s, scanning interval is 50-150 μm, powder layer thickness is 20-100 μm, the scanning path between layers presents an angle of 65-70°, and the substrate preheating temperature is 0-200 ℃. Finally, the printed material is heat treated: heated to 550 ℃, then tempered twice, and air-cooled after tempering, to obtain the precipitation-hardened stainless steel material with a hardness of 48 HRC after printing, a hardness of 52 HRC after heat treatment, a tensile strength of 1927 MPa, a yield strength of 1544 MPa, and an impact toughness of 26.2 Akv.

[0030] To verify the temperature influence in the heat treatment process, the hardness of the precipitation-hardened stainless steel material after two times of tempering at different temperatures is tested according to the heat treatment curve of Figure 1 , and the specific mechanical properties are shown in the following table.

[0031]

[0032] In addition, the mechanical property test is also carried out on some metal materials of different brands on the market, and the test results are compared and analyzed with the mechanical properties of the precipitation-hardened stainless steel material prepared in the embodiment. The specific comparison is shown in the following table (it should be noted that the test of the tensile strength, yield strength and elongation is carried out under the condition that the hardness of the sample is 46±2 HRC).

[0033]

[0034] To verify the temper resistance characteristics of the precipitation hardening stainless steel material prepared in the embodiments of the present application, commercially available materials EM213 and Dievar are selected as control samples for comparative experimental study. The hardness reduction rate of each material is quantitatively determined under the condition of 100 h holding time, and the test results can be referred to the following table. Figure 2 and the following table.

[0035] EM213 Dievar Examples Heat treatment temperature of the test pieces 575℃ 615℃ 550℃ Tempering soak temperature 550℃ 550℃ 550℃ 100 h hardness drop 6.3 5.7 5 As can be seen from the above, the precipitation hardening stainless steel material prepared in the embodiments of the present application exhibits significant advantages in multiple key performance indicators.

[0036] In terms of mechanical properties, compared with commercially available materials EM213, EM181, H13, Dievar and AM-Dievar, the yield strength and tensile strength of the material of the present application are at a high level under different temperature conditions of room temperature, 300°C and 600°C. At room temperature, the yield strength of the material of the present application is 1138 MPa, and the tensile strength is 1561 MPa, although it is similar to EM213 and other materials in some room temperature tensile strength indicators, but at high temperature environment (300°C and 600°C), the strength retention ability of the material of the present application is more outstanding, the yield strength is 1062 MPa at 300°C, and the tensile strength is 1343 MPa; the yield strength is 602 MPa at 600°C, and the tensile strength is 822 MPa. Compared with other materials, it can still maintain a high strength at high temperature, which shows that the material of the present application has better carrying capacity in high temperature application scene, and can meet more stringent engineering requirements; In terms of elongation, the elongation of the material of the present application at room temperature, 300°C and 600°C is relatively balanced, and reaches 23% at 600°C, which is higher than that of some commercially available materials, showing good plastic deformation ability. This helps the material to deform plastically when subjected to external force without immediate fracture, improving the safety and reliability of the material, especially in some applications where the material toughness is required; In terms of toughness, the toughness index of the material of the present application is 33 (ISO V), which is higher than that of EM213, H13, Dievar and other materials, indicating that it can absorb more energy without brittle fracture when subjected to impact load, and has better impact resistance, which is of great significance for the application of the material under complex stress state, and can effectively reduce the risk of material failure caused by impact load; In terms of tempering stability, the hardness of the material of the application only decreased by 5HRC after 100h of holding time, which is lower than that of commercially available materials EM213 (6.3HRC) and Dievar (5.7HRC). This fully demonstrates that the material of the application can better maintain its hardness during long-term tempering treatment and has more excellent tempering stability. The tempering stability is crucial for the heat treatment process and long-term use performance of the material. The excellent tempering stability of the material of the application enables it to more stably obtain the required performance during heat treatment and maintain the stability of the performance during long-term use, thereby reducing the performance decline problem caused by tempering softening.

[0037] In summary, the precipitation hardening stainless steel material prepared in the embodiments of the application performs well in terms of mechanical properties, toughness and tempering stability, avoids the cracking risk during 3D printing, has excellent comprehensive mechanical properties, has broad application prospects, and can play an important role in the mold field.

[0038] The above are modifications or obvious technical solutions without creative contribution that can be made by those skilled in the art after reading the specification, but as long as they are within the scope of the claims of the application, they should be protected by the Patent Law.

Claims

1. A precipitation hardening stainless steel, characterized in that: Calculated by weight percentage, it includes the following chemical components: C 0.05%-0.25%, Si 0-0.15%, Mn 0.2-0.5%, Cr 9-18%, Ni 0.2-5%, Mo 0.5-5%, V 0-0.5%, Co 5-13%, and the balance is Fe and unavoidable impurities.

2. A method for preparing precipitation hardened stainless steel according to claim 1, characterized in that: The method comprises the following preparation steps: S1. Melting the alloy raw material and casting it, and then forging or rolling it to obtain a molded component; alternatively, preparing the alloy raw material into spherical alloy powder by atomization or rotating electrode method, and then 3D printing it to obtain a molded component; S2. Post-processing the formed component obtained in S1 to obtain precipitation hardened stainless steel.

3. The method for preparing precipitation hardened stainless steel according to claim 2, wherein: The particle size of the spherical alloy powder in S1 is 5-85 μm, showing a normal distribution, with D10 of 23 μm, D50 of 40 μm, and D90 of 70 μm; The impurity element content of the spherical alloy powder is: O is 179ppm, N is 147ppm, S is less than 0.003wt%, P is less than 0.021wt%; the packaging density of the spherical alloy powder is 3.8-4.4g / cm 3 The tap density of spherical alloy powder is 4.6-5.0g / cm 3 , the Hall flow rate of spherical alloy powder is ≤15s / 50g.

4. The method for preparing precipitation hardened stainless steel according to claim 2, wherein: The additive manufacturing process parameters of the 3D printing in S1 are: laser power of 200-450W, scanning rate of 0.5-1.0m / s, scanning spacing of 50-150μm, powder layer thickness of 20-100μm, interlayer scanning path angle of 65-70°, and substrate preheating temperature of 0-200℃.

5. The method for preparing precipitation hardened stainless steel according to claim 2, wherein: The post-treatment in S2 includes at least one of austenitizing quenching treatment, low-temperature deep freezing treatment and tempering treatment.

6. The method for preparing precipitation hardened stainless steel according to claim 5, characterized in that: The temperature of the austenitizing quenching treatment is 1000-1070° C., the time of the austenitizing quenching treatment is 0.5-5 hours, and the cooling rate of the austenitizing quenching treatment is 2-50° C. / s.

7. The method for preparing precipitation hardened stainless steel according to claim 5, characterized in that: The temperature of the cryogenic treatment is -196 to -120° C., and the time of the cryogenic treatment is 0.5 to 3 hours.

8. The method for preparing precipitation hardened stainless steel according to claim 5, wherein: The tempering treatment temperature is 480-650° C., the tempering is performed 2-3 times, and the tempering treatment time is 3-7 hours.

9. The method for preparing precipitation hardened stainless steel according to claim 2, wherein: The microstructure of the precipitation hardened stainless steel in S2 presents a layered structure of cellular grains and columnar grains, or a lath martensite hierarchical structure, and dispersed Laves phase, alloy carbides and a small amount of retained austenite phase are distributed in the martensite matrix.

10. Use of the precipitation hardening stainless steel according to claim 1 or the precipitation hardening stainless steel prepared by the method for preparing precipitation hardening stainless steel according to any one of claims 2 to 9 in a mold.

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