Patent Application
20240175111
The present application claims priority to Chinese Patent Application No. 202210781652.8, entitled “HIGH-STRENGTH AND HIGH DUCTILITY STAINLESS STEEL BY ADDITIVE MANUFACTURING AND METHOD OF PREPARING THE SAME,” filed Aug. 5, 2022, in the China National Intellectual Property Administration (CNIPA), the entire of which is hereby incorporated in its entirety by reference.
The invention relates to the technical field of additive manufacturing of metals, specifically, an additive manufacturing of high-strength and high-ductility stainless steel and its preparation process.
With the rapid development of aerospace and ocean engineering, equipment structural parts are more and more complex and the service environment is gradually severe, so that high-performance and high-strength stainless steel components more and more demand. The service performance of the high-strength martensitic stainless steel is closely related to the microstructure, and its higher strength is mainly contributed to ultrahigh-density dislocation and nano-scale precipitated phase of a martensitic matrix. The plasticity and ductility of the high-strength martensitic stainless steel are continuously optimized mainly by improving the distribution and the content of austenite. However, the increase of either strength and plasticity will lead to the decrease of another property, which is called the strength-ductility trade off. Therefore, it is necessary to optimize its microstructure and service performance with the help of a new preparation process.
Additive manufacturing technology, also known as 3D printing, is a rapidly developing emerging process in recent years. With the additive manufacturing technology, components with complex structures, which are difficult or impossible to be produced with traditional processes, can be manufactured quickly and accurately with simpler production process. At the same time, the raw materials can be saved and the development cycle can be significantly shortened. The additive manufacturing has the characteristics of high laser energy, rapid cooling, and multi-pass circulating heat treatment, etc. Therefore, the additive manufacturing stainless steel has an obvious molten pool structure, and the prepared structural part has the characteristics of uneven stress distribution, multiple interfaces and fine microstructure.
The high-strength stainless steel printed at present has a fine microstructure, nano-scale oxide inclusions, and obvious molten pool interface. Tensile experiments show that the mechanical properties of the martensitic stainless steel fabricated by the additive manufacturing are comparable to those of conventional fabrication, but the microstructure phase distribution is significantly different. Therefore, it is of great importance to develop a stainless steel with controlled microstructure and simultaneous enhancement of strength and ductility.
The invention mainly aims to provide an additively manufactured high-strength and high-ductility stainless steel and its preparation process to overcome the contradiction relationship of strength-ductility inversion. By optimizing the alloy composition, the phase diagram relationship between alloy composition and structure is established for the additively manufactured stainless steel. The multi-scale multiple heterostructure high-strength martensitic stainless steels are prepared by the fast cooling and high-energy laser features of additive manufacturing. The strength and elongation for the additively manufactured heterostructure martensitic stainless steel are significantly higher than those of conventionally fabricated stainless steels with similar compositions.
To solve the above technical problem, according to an aspect of the present invention, the present invention provides the following technical solutions:
An additively manufactured high-strength and high-ductility stainless steel with a composition of, by weight percentage, C≤0.05 wt %, Si≤1 wt %, Mn≤1 wt %, Cr 14.5-15.5 wt %, Ni 5.0-5.5 wt %, Cu 4-4.5 wt %, Nb 0.35-0.45 wt %, and the balance of Fe and unavoidable impurities.
The yield strength of the high-strength and high-ductility stainless steel ≥1270 MPa, the tensile strength ≥1380 MPa, and the elongation after fracture ≥15%.
The microstructure of high-strength and high-ductility stainless steel comprises bulk austenite distributed at the bottom of a molten pool and thin-film austenite formed among martensite laths, and a high-density nano-scale multiple precipitated phases is precipitated in a fine martensite lath matrix. The high content of austenite improves the ductility and ductility of the stainless steel, and the high-density nano-scale multiple precipitates in the fine martensite lath matrix to further improve the tensile strength of the stainless steel.
The preferable scheme for an additively manufactured high-strength and high-ductility stainless steel is as follows: the Cr equivalent Creq is 13.8-15.4 and Ni equivalent Nieq is 7.5-8.8. More preferably, the Cr equivalent Creq is 14.5-15.0, the Ni equivalent Nieq is 7.8-8.4.
The preferable scheme for an additively manufactured high-strength and high-ductility stainless steel is as follows: the yield strength of the high-strength and high-ductility stainless steel ≥1300 MPa, the tensile strength ≥1440 MPa, and the elongation after fracture ≥16%.
In order to solve the above technical problem, according to another aspect of the present invention, the present invention provides the following technical solutions:
A preparation process for manufacturing high-strength and high-ductility stainless steel by additive manufacturing comprises the following steps:
As a preferred scheme of the preparation process for the additive manufacturing of the high-strength and high-ductility stainless steel, the method comprises the following steps: in the step S1, the stainless-steel powder has a particle size of 15-45 μm.
As a preferred scheme of the preparation process for the additive manufacturing of the high-strength and high-ductility stainless steel, the method comprises the following steps: In step S2, the parameters of the 3D printing process are: the diameter of the laser spot is 100-300 μm, the laser power is 230-400 W, the scanning interval is 0.07-0.10 mm, the scanning speed is 550-900 mm/s, and the powder spreading thickness is 0.02-0.04 mm. the density of the printed product can reach more than 97%.
As a preferred scheme of the preparation process for the additive manufacturing the high-strength and high-ductility stainless steel, the method comprises the following steps: In the step S3, the heating rate of the heat treatment is 6-10° C./min until the temperature is increased to 450-500° C., and holding for 2-10 h.
The invention has the following beneficial effects:
The present invention proposes an additively manufactured high-strength and high-ductility stainless steel and its preparation process. Firstly, the phase diagram of the relationship between different alloy compositions and phase composition for additive manufacturing stainless steel is established. After that, the new alloy composition is optimized in the martensitic-austenitic duplex region by alloy composition design and optimization of austenite forming element content. Finally, the periodically distributed heterogeneous structured martensitic stainless steel with yield strength ≥1270 MPa, tensile strength ≥1380 MPa, and elongation after fracture ≥15% were prepared by means of additive manufacturing. Compared with traditional martensitic stainless-steel materials of similar composition, the high strength and high plasticity are simultaneously improved, breaking the trade-off of the strength and the plasticity of traditional martensitic stainless steel of the similar composition
In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the embodiments or the prior art will be briefly described below. It is obvious that the drawings in the following description are only some embodiments of the present invention, and for those skilled in the art, other drawings can be obtained according to the structures shown in the drawings without creative efforts.
The realization of the purpose, functional features and advantages of the present invention will be further described with reference to the accompanying drawings in conjunction with the embodiments.
The following will clearly and completely describe the technical solutions in the embodiments, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all embodiments. Based on the embodiments in the present invention, all other embodiments obtained without creative labor by a person of ordinary skill in the art fall within the scope of protection of the present invention.
The present invention provides an additive manufacturing high-strength and high-ductility stainless steel and its preparation process, which can break the inverse contradiction of the strength-ductility of the traditional martensitic stainless steel of the similar composition, and realize the simultaneous improvement of high strength and high ductility. Firstly, the phase diagram of the relationship between different alloy compositions and phase composition for additive manufacturing stainless steel is established. After that, the new alloy composition is optimized in the martensitic-austenitic duplex region by alloy composition design and optimization of austenite forming element content. Finally, the periodically distributed heterogeneous structured martensitic stainless steel with yield strength ≥1270 MPa, tensile strength ≥1380 MPa, and elongation after break ≥15% were prepared by means of additive manufacturing.
According to one aspect of the invention, the invention provides the following technical scheme:
An additively manufactured high-strength and high-ductility stainless steel with a composition of, by weight percentage, C≤0.05 wt %, Si≤1 wt %, Mn≤1 wt %, Cr 14.5-15.5 wt %, Ni 5.0-5.5 wt %, Cu 4-4.5 wt %, Nb 0.35-0.45 wt %, and the balance of Fe and unavoidable impurities.
The Cr equivalent Creq is 13.8-15.4 and Ni equivalent Nieq is 7.5-8.8. More preferably, the Cr equivalent Creq is 14.5-15.0, the Ni equivalent Nieq is 7.8-8.4.
Specifically, the Cr equivalent Creq is, for example, but not limited to, any one of 14.5, 14.6, 14.7, 14.8, 14.9, 15.0 or a range between any two. Specifically, the Ni equivalent Nieq is, for example, but not limited to, any one of 7.8, 7.9, 8.0, 8.1, 8.2, 8.3, 8.4 or a range between any two.
The present invention is based on the study of the formation mechanism of the phase composition of stainless steel and the optimization of austenite forming elements compared to the conventional similar composition 15-5PH stainless steel. The stainless steel of the present invention has a higher content of Ni and Cu elements, resulting in a higher Ni equivalent Nieq, which allows this alloy composition stainless steel to be shifted from the martensitic region to the martensitic-austenitic duplex region of the phase diagram of the additive manufactured stainless steel in order to obtain a periodically distributed martensitic stainless steel. Meanwhile, the main chemical composition of Cr, Ni, Nb and Cu ratios in this composition can be designed to meet the formation of high-density precipitation phase, thus simultaneously improving its strength and ductility.
The microstructure of high-strength and high-ductility stainless steel comprises bulk austenite distributed at the bottom of a molten pool and thin-film austenite formed among martensite laths, and a high-density nano-scale multiple precipitated phases is precipitated in the fine martensite lath matrix. The high content of austenite improves the ductility of the stainless steel, and the high-density nano-scale multiple precipitated phases is precipitated in the fine martensite lath matrix to further improve the tensile strength of the stainless steel.
The microstructure matrix of additively manufactured high-strength and high-ductility stainless steel is mainly martensite, which contains more than 20% austenite. The size of high-density nanoscale multiple precipitation phase is 1-3 nm, with an average size of about 1.5 nm.
The yield strength of the high-strength and high-ductility stainless steel ≥1270 MPa, the tensile strength ≥ to 1380 MPa, and the elongation after fracture ≥15%. Preferably, the yield strength of the high-strength and high-ductility stainless steel ≥1300 MPa, the tensile strength ≥1440 MPa, and the elongation after fracture ≥16%.
According to another aspect of the invention, the invention provides the following technical scheme:
A preparation process for high-strength and high-ductility stainless steel by additive manufacturing comprises the following steps:
The present invention establishes the phase diagram relationship between alloy composition and microstructure for additively manufactured stainless steel. By optimizing the alloy composition, the multi-scale multiple heterostructure high-strength stainless steel is prepared with the features of rapid cooling and high-energy laser in additive manufacturing. The strength and elongation after fracture of this additively manufactured heterostructure stainless steel are significantly higher than those of conventionally fabricated stainless steels with similar compositions.
The size of powders is 15-45 μm. Specifically, the size of powder particles is, for example, but not limited to, a range between any two of 15 μm, 20 μm, 25 μm, 30 μm, 35 μm, 40 μm, and 45 μm. The powders are free of hollow powder and the sphericity is greater than or equal to 95%. There were no inclusion and the particle size distribution of the powder is as follows: D10: 19.3%, D50: 30.9%, D90: 49.2%.
The parameters of the 3D printing process are as follows: the diameter of the laser spot is 100-300 μm, laser power 230-400 W, scanning pitch 0.07-0.10 mm, scanning speed 550-900 mm/s, powder thickness 0.02-0.04 mm, protective atmosphere is nitrogen. The densities of the printed products can reach more than 97%.
The 3D printing process parameters can be adjusted according to the particle size and composition of the raw material to be printed. And specifically, the spot diameter is, for example, but not limited to, any one of 100 μm, 150 μm, 200 μm, 250 μm, and 300 μm, or a range between any two of them. The laser power is, for example, but not limited to, any one of 230 W. 250 W, 300 W. 400 W, or a range between any two. The scan pitch is, for example, but not limited to, any one of 0.07 mm. 0.08 mm, 0.09 mm, 0.10 mm, or a range between any two. The scanning speed is, for example, but not limited to, any one of 550 mm/s, 600 mm/s, 650 mm/s, 700 mm/s, 750 mm/s, 800 mm/s, 850 mm/s, 900 mm/s or a range between any two. The powder thickness is, for example, but not limited to, any one of 0.02 mm, 0.025 mm, 0.03 mm, 0.035 mm, 0.04 mm, or a range between any two.
The heating rate of the heat treatment is 6-10° C./min until the temperature is increased to 450° C. and 500° C., and hold for 2-10 h. The heat treatment process parameters can be adjusted according to the composition of the printing raw materials, and specifically, the heating rate is, for example, but not limited to, any one of 6° C./min, 7° C./min, 8° C./min, 9° C./min, 10° C./min or a range between any two. The heat treatment temperature is, for example, but not limited to, any one of 450° C., 460° C., 470° C., 480° C., 490° C., 500° C. or a range between any two. The holding time is, for example, but not limited to, 2 h, 3 h, 4 h, 5 h, 6 h, 7 h, 8 h, 9 h, 10 h.
A high-strength and high-ductility stainless steel manufactured by additive manufacturing adopts the following preparation process:
A high-strength and high-ductility stainless steel manufactured by additive manufacturing adopts the following preparation process:
A high-strength and high-ductility stainless steel manufactured by additive manufacturing adopts the following preparation process:
A high-strength and high-ductility stainless steel manufactured by additive manufacturing adopts the following preparation process:
The difference from embodiment 1 is that comparative embodiment 1 does not perform heat treatment, and the specific process is as follows:
Comparative embodiment 2 was manufactured using a conventional smelting and manufacturing process to produce 15-5 PH martensitic stainless steel of similar composition to embodiment 1-4. The peak aging process is used: the heating rate is 8° C./min until the temperature rises to 500° C. and the holding time is 4 h.
The difference with comparative embodiment 1 is that comparative embodiment 3 has a slight reduction in the alloy composition, especially in Ni and Cu content. The specific process is as follows:
The powder components comprise 0.039 wt % of C, 0.42 wt % of Si, 0.53 wt % of Mn, 14.35 wt % of Cr, 4.39 wt % of Ni, 3.25 wt % of Cu and 0.485 wt % of Nb. The balance composition of the powder is Fe and inevitable impurity elements. The particle size of the powder was 15-45 μm, with an average diameter of 21.81 μm. There were no hollow powders, and the sphericity was more than 95%. No inclusions were detected in the powder and the oxygen content of the powder was 186 ppm;
The heat treatment is carried out in a muffle furnace, the heating rate of the heat
The stainless steels prepared in the embodiments and comparative embodiments were manufactured and the results of the tensile tests are shown in Table 1.
Table 1 The tensile tests results of embodiments and comparative embodiments of the invention.
The tensile strength and the elongation after fracture of the traditional martensitic stainless steel with similar compositions are subjected to statistical analysis. And as can be seen from
The above description is only a preferred embodiment of the present invention, and is not intended to limit the scope of the present invention, and all equivalent structural changes made by using the content of the present specification or other related technical fields within the spirit of the present invention are included in the scope of the present invention.
| Number | Date | Country | Kind |
|---|---|---|---|
| 202210781652.8 | Jul 2022 | CN | national |
