Ce-containing composite precipitation strengthening martensitic stainless steel and preparation method thereof

Abstract

The invention discloses Ce-containing composite precipitation strengthening martensitic stainless steel and a preparation method thereof, and belongs to the technical field of stainless steel new materials. The martensitic stainless steel comprises the following chemical components in percentage by weight: 0.02 to 0.04 percent of C, 8.10 to 8.80 percent of Ni, 12.50 to 13.00 percent of Cr, 2.00 to 2.50 percent of Mo, 0.02 to 0.08 percent of Ce, 1.20 to 1.50 percent of Al, 2.20 to 3.50 percent of Cu and the balance of Fe and impurities. The tensile strength is greater than or equal to 1820 MPa, the yield strength is greater than or equal to 1612 MPa, and the percentage elongation after fracture is greater than or equal to 19.5%. According to the martensitic stainless steel, through component design and heat treatment process optimization and through the synergistic effect of fine grain strengthening, composite precipitation strengthening, austenite toughening and the like, the strength and ductility of existing martensitic stainless steel are remarkably improved, and excellent strength and plasticity matching is obtained.

Description

Technical Field

[0001] The invention belongs to the technical field of new stainless steel materials, and in particular relates to a Ce-containing composite precipitation-strengthened martensitic stainless steel and a preparation method thereof. Background Art

[0002] Materials are the forerunners of scientific and technological development and an important factor affecting the world pattern. In the process of rapid development of any country, steel materials play a vital role and are an important basic industry of the national economy. Under the background of "carbon peak" and "carbon neutrality", the steel industry should develop towards innovation and green and low-carbon development. Ordinary low-strength steel materials can no longer meet the needs of national development, especially for high-tech manufacturing industries such as aerospace, marine engineering, energy engineering and automobile industry. High-strength martensitic stainless steel usually has excellent mechanical properties and corrosion resistance, and is widely used in the above fields as key structural parts and fasteners. With the continuous development of science and technology and the continuous improvement of service requirements, how to develop martensitic stainless steel with higher strength and higher elongation has become one of the main development directions in the field of advanced high-strength steel in the future. Precipitation strengthening is a common and most important strengthening method for high-strength steel. The precipitation strengthening effect brought by a single precipitate phase is general, and its contribution to the improvement of the strength of steel materials is limited. In addition, the strength-elongation of steel materials is usually a relationship of mutual checks and balances, which greatly limits the overall mechanical properties of the existing commercial grades of high-strength martensitic stainless steel.

[0003] CN115572804A discloses a high-strength and high-toughness precipitation-hardened martensitic stainless steel and a preparation method thereof. The method introduces two types of nano-precipitates and a specific content of austenite into the refined martensitic matrix by adding a specific amount of Cu to the traditional martensitic stainless steel and adjusting the heat treatment process at the same time, so that the prepared martensitic stainless steel can simultaneously improve strength and elongation under the synergistic effect of fine grain strengthening, composite precipitation strengthening and TRIP effect; the mass proportion of Cu is 3.20% to 5.00%; the nano-precipitates are Cu-rich precipitation phases and NiAl precipitation phases; the volume fraction of austenite is 18.5 to 25%. The tensile strength of the martensitic stainless steel prepared by this method is 1700MPa, the elongation is 18%, and the elongation after hydrogen charging is 8.3%. Although this method achieves a simultaneous improvement in strength and elongation, the overall strength and elongation are still relatively low. How to further improve the strength and elongation of martensitic stainless steel on the basis of the existing technology is a problem that needs to be solved urgently. Summary of the invention

[0004] The technical problem to be solved by the present invention is how to further improve the strength and elongation of martensitic stainless steel.

[0005] In order to achieve the above application purpose, the technical solution adopted in this application is as follows:

[0006] In a first aspect, the present invention provides a method for preparing a Ce-containing composite precipitation-strengthened martensitic stainless steel, which specifically comprises the following steps:

[0007] S1 is an ingot obtained by vacuum induction melting, and its chemical composition is as follows by mass percentage: C 0.02-0.04%, Ni 8.10-8.80%, Cr 12.50-13.00%, Mo 2.00-2.50%, Ce 0.02-0.08%, Al 1.20-1.50%, Cu 2.20-3.50%, and the balance is Fe and unavoidable impurities;

[0008] S2 high temperature homogenization treatment;

[0009] S3 multi-directional and multi-pass deformation treatment;

[0010] S4 heat treatment.

[0011] Among them, in the above step S1, vacuum induction melting specifically includes: first adding high-purity iron, starting refining after melting, adding alloy components such as Ni, Mo, Cr to adjust the composition, taking out of the furnace and casting to obtain an ingot.

[0012] Furthermore, in the above step S1, the melting time is not less than 5 hours, and the vacuum degree is 25±5Pa.

[0013] Furthermore, in the above step S1, the refining time is 30±3 min.

[0014] Furthermore, in the above step S1, the temperature of the furnace is 1480-1500°C.

[0015] Wherein, in the above step S2, the temperature of high temperature homogenization treatment is 1200-1300° C., and the time is 24-48 hours.

[0016] The specific steps of the above step S3 are:

[0017] S3-1: the ingot after the treatment in step S2 is cooled to a forging temperature, and then forged in at least three different directions to obtain a forging blank;

[0018] S3-2 The forging blank is heated to the rolling temperature, and then four rolling processes are performed, and air-cooled to room temperature to obtain a rolled plate.

[0019] Furthermore, in the above step S3-1, the ingot after homogenization is cooled to 1180±100°C, kept warm for 2±0.5h and then taken out of the furnace for forging, the initial forging temperature is not lower than 1130°C, and the final forging temperature is not lower than 950°C.

[0020] Furthermore, in the above step S3-1, the forging treatment in at least three different directions is: performing forging treatment in three directions including but not limited to X, Y and Z, so that the thickness of the forging blank is 50 mm.

[0021] Furthermore, in the above step S3-2, the forging billet is heated to 1200±100°C, kept at this temperature for 1±0.5h, and then taken out of the furnace for rolling. The starting rolling temperature is not lower than 1150°C, and the final rolling temperature is not lower than 980°C.

[0022] Furthermore, in the above step S3-2, the initial thickness of the four rolling passes is 50 mm, the first pass is rolled to 35 mm, the second pass is rolled to 25 mm, the third pass is rolled to 15 mm, and the fourth pass is rolled to 10 mm, obtaining a rolled plate of 10 mm×150 mm×1000 mm.

[0023] The specific steps of the above step S4 are:

[0024] S4-1 solution treatment: solution temperature is 850-900℃, solution time is 0.5-1h;

[0025] S4-2 Cryogenic treatment: Cryogenic treatment temperature is -196~-80℃, and the holding time is 60~90min;

[0026] S4-3 Aging treatment: The aging treatment temperature is 480℃~510℃, and the treatment time is 2h~4h.

[0027] Furthermore, in the above step S4-1, the rolled plate is heated to the solution temperature, kept at this temperature for a certain period of time, and then rapidly quenched and cooled, and the cooling medium is an ice-water mixture.

[0028] Furthermore, in the above step S4-2, the rolled plate treated in step S4-1 is subjected to a cryogenic treatment, wherein the cryogenic treatment medium is a mixture of liquid nitrogen and anhydrous ethanol, and after the cryogenic treatment, the rolled plate is allowed to naturally return to room temperature in the air.

[0029] In a second aspect, the present invention provides Ce-containing composite precipitation-strengthened martensitic stainless steel prepared by the above preparation method.

[0030] The tensile strength of the Ce-containing composite precipitation-strengthened martensitic stainless steel is ≥1820MPa, the yield strength is ≥1612MPa, and the elongation after fracture is ≥19.5%.

[0031] The original austenite grain size in the Ce-containing composite precipitation-strengthened martensitic stainless steel is 8.75±0.56 μm.

[0032] The Ce-containing composite precipitation-strengthened martensitic stainless steel contains body-centered cubic NiAl precipitates and face-centered cubic Cu precipitates. The total number density of the two precipitates is 8.17×10 23 m -3 .

[0033] Furthermore, the size of the NiAl precipitated phase is 2.88±0.15 nm, and the size of the Cu precipitated phase is 3.65±0.23 nm.

[0034] The beneficial effects of the present invention are as follows: the present invention significantly improves the strength and elongation of martensitic stainless steel through component design and heat treatment process optimization, through the synergistic effects of fine grain strengthening (rare earth-cryogenic treatment), composite precipitation strengthening, austenite toughening, etc., obtains excellent strength-plasticity matching, and breaks the bottleneck of the mutual balance between strength and elongation of traditional steel materials. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 is a tensile engineering stress-strain curve diagram of Example 1 of the present invention and Comparative Example 1;

[0036] Figure 2 It is a comparison of the metallographic microstructures of Example 1 of the present invention and Comparative Example 1 (comparison of the original austenite grain size);

[0037] Figure 3 This is a comparison diagram of non-metallic inclusions between Example 1 of the present invention and Comparative Example 1;

[0038] Figure 4 TEM analysis diagram of two precipitated phases in Example 1 of the present invention;

[0039] Figure 5 This is the XRD analysis diagram of Example 1 of the present invention (reflecting the relationship between cryogenic treatment and austenite). DETAILED DESCRIPTION

[0040] In order to make the technical problems, technical solutions and beneficial effects to be solved by the present application more clear, the present application is further described in detail below in combination with the implementation methods. Unless otherwise defined, all scientific and technological terms used herein have the same meanings as understood by ordinary technicians in the field.

[0041] The present invention provides a high-strength martensitic stainless steel containing Ce and a preparation method thereof. The method adds a certain content range of rare earth element Ce and alloy element Cu to the existing commercial grade martensitic stainless steel components, and adopts a suitable heat treatment process, so that the original austenite grain size of the newly designed martensitic stainless steel is relatively small, specifically 8.75±0.56μm, and the original austenite grain size of the original grade martensitic stainless steel is specifically 19.13±0.88μm. At the same time, two types of precipitation phases with a certain number density and dispersed distribution are precipitated in the material matrix, namely, a body-centered cubic NiAl precipitation phase (size is 2.88±0.15nm) and a face-centered cubic Cu precipitation phase (size is 3.65±0.23nm), and the total number density of the two precipitation phases is 8.17×10 23 m -3 The precipitate phase in the original martensitic stainless steel is only body-centered cubic NiAl precipitate phase (size is 4.22±0.17nm), and the number density of the precipitate phase is 5.23×10 23 m -3 .

[0042] The addition of rare earth element Ce can play two main roles. One is to purify the molten steel and reduce the number of non-metallic inclusions in the steel. The other is to refine the size of the precipitate phase in the steel and increase the number density of the precipitate phase, thereby improving the comprehensive mechanical properties of the material. The addition of Cu element mainly introduces the second type of precipitate phase to form a composite precipitate phase (i.e., Cu precipitate phase and NiAl precipitate phase), which plays a role in composite precipitation strengthening and can promote the formation of austenite in the steel. Austenite has a certain effect on the plasticity and toughness of the material. The synergistic effect of the above-mentioned Cu-Ce elements can simultaneously improve the strength and elongation of the newly designed martensitic stainless steel.

[0043] Specifically, a Ce-containing composite precipitation-strengthened martensitic stainless steel has the following chemical compositions by mass percentage: C 0.02-0.04%, Ni 8.10-8.80%, Cr 12.50-13.00%, Mo 2.00-2.50%, Ce 0.02-0.08%, Al 1.20-1.50%, Cu 2.20-3.50%, and the remainder is Fe and unavoidable impurities.

[0044] Too high a rare earth element content will increase the difficulty of steel smelting, increase the cost, and deteriorate the impact toughness of steel. Therefore, in the present invention, Ce is controlled within a range of 0.02 to 0.08%.

[0045] According to the Fe-Cu binary phase diagram, the solid solubility of Cu in Fe is about 0.025at% to 2.7at%. The content of Cu in steel must be at least 1.0wt.% to play a significant precipitation strengthening role; but excessive addition of Cu can easily cause the material to become hot brittle during heat treatment, and the cost increases. Therefore, in the present invention, Cu is controlled to 2.20-3.50%.

[0046] The preparation method of Ce-containing composite precipitation-strengthened martensitic stainless steel comprises the following steps: vacuum induction melting, high temperature homogenization treatment, multi-directional multi-pass deformation treatment and heat treatment process (solid solution treatment, cryogenic treatment, aging treatment). In the present invention, there is a significant synergistic effect between the chemical composition design, the preparation process (melting, homogenization, deformation treatment) and the subsequent heat treatment process (solid solution treatment, cryogenic treatment, aging), which jointly affect the precipitation phase, crystal phase change and final mechanical properties of the material.

[0047] Step 1, vacuum induction melting: The uniformity of the composition is the basis for subsequent homogenization, deformation and heat treatment. If the composition is uneven during the smelting process, local segregation may occur, affecting the distribution of the precipitated phase and the effect of grain refinement. Therefore, the present invention uses a vacuum induction smelting method to smelt the initial ingot according to the chemical composition ratio of the stainless steel. Set a suitable melting time according to the ingot size, first add high-purity iron, the melting time is greater than 5h, the vacuum degree is 25±5Pa, the refining time is 30±3min, and then add Ni, Mo, Cr and other alloy components to adjust the composition; then reduce the temperature to 1480~1500℃ and cast it into the initial ingot. If the temperature of the molten steel out of the furnace is too high, it is easy to increase the secondary oxidation of the molten steel and the subsequent pouring stability. If the temperature of the molten steel out of the furnace is too low, it is not conducive to the floating of inclusions and is easy to produce surface defects; then the initial ingot is treated accordingly, its surface is peeled, and the observable shrinkage cavities and macroscopic defects are removed. Folding, slag inclusions, inclusions, cracks, etc. are not allowed on the surface. The present invention achieves control of composition and purity through vacuum induction melting, and optimizes the precipitation strengthening potential of the material by precisely controlling the alloy composition (such as adding Ce and Cu). Ce, as a rare earth element, can refine grains, purify grain boundaries, and reduce impurity segregation; the addition of Cu provides conditions for the formation of Cu-rich precipitation phase and austenite in subsequent aging treatment.

[0048] Step 2, homogenization treatment: Uniform composition distribution can make the grain refinement more uniform during multi-pass forging / rolling, avoiding uneven deformation due to local composition differences. Uniform matrix composition is the prerequisite for uniform nucleation of precipitation phase. If segregation exists, local coarse precipitation phases or insufficient precipitation areas may be formed during aging treatment, reducing the strengthening effect. Therefore, in order to ensure uniform diffusion of alloying elements in the ingot, eliminate microsegregation in the ingot, and ensure composition uniformity, the above-mentioned ingot is subjected to high-temperature homogenization treatment. The temperature range is 1200-1300℃, and the time is 24-48h. If the homogenization temperature is too high and the time is too long, it is easy to cause decarburization of the material and coarse grains; if the homogenization temperature is too low and the time is too short, it is difficult to fully eliminate the composition segregation and structural inhomogeneity in the material.

[0049] Step 3, multi-directional multi-pass deformation treatment: cool the ingot after homogenization to 1180±100℃, keep it at 1180±100℃ for 2±0.5h, then take it out of the furnace and start forging. The initial forging temperature is above 1130℃, and the final forging temperature is above 950℃. If the cooling speed of the ingot is too fast, it should be put into the furnace in time for heating, and at least multiple directions (including but not limited to X, Y, Z directions) should be forged to finally obtain a forging billet of corresponding size with a thickness of about 50mm. Heat the above forging billet to 1200±100℃, keep it for 1±0.5h and roll it, ensure that the rolling starts at 1150℃, the initial rolling thickness is 50mm, the first rolling to 35mm, the second rolling to 25mm, and the third rolling to 15mm. The fourth pass was rolled to 10mm, the final forging temperature was above 980℃, and it was air-cooled to room temperature, and finally a rolled plate of 10mm×150mm×1000mm was obtained. Multi-directional and multi-pass deformation treatment can effectively eliminate defects such as pores and looseness inside the ingot, and significantly improve the density of the material. At the same time, the work hardening effect during multiple deformation processes will refine the grains of the ingot and increase the dislocation density, thereby improving the mechanical properties of the ingot. The addition of rare earth element Ce can improve the deformation uniformity of the ingot during multi-directional and multi-pass deformation treatment, which can affect the rheological behavior and deformation mechanism of the ingot, so that the ingot exhibits better deformation uniformity during thermal deformation. This helps to reduce the local stress and strain concentration of the ingot, reduce the risk of cracking of the ingot, improve the thermal cracking resistance of the ingot, and provide qualified ingots for subsequent heat treatment.

[0050] Step 4, a suitable heat treatment process, specifically comprising the following steps:

[0051] Solution treatment: heat the above martensitic stainless steel to the solution temperature, keep it at this temperature for a certain period of time, and then quickly quench and cool it. The cooling medium is an ice-water mixture. The solution temperature is 850-900°C and the solution time is 0.5-1h.

[0052] Cryogenic treatment: Cryogenic treatment is performed on the martensitic stainless steel after the above-mentioned solution treatment; a special cryogenic device is used, the cryogenic treatment medium is a mixture of liquid nitrogen and anhydrous ethanol, the cryogenic treatment temperature is -196 to -80°C, preferably -196°C, and the temperature is kept for 60 to 90 minutes (the cryogenic treatment time needs to be flexibly adjusted considering the size of the material), and then the martensitic stainless steel is taken out of the cryogenic treatment device and naturally restored to room temperature in the air;

[0053] Aging treatment: The martensitic stainless steel after the above-mentioned deep cryogenic treatment is subjected to aging treatment at a temperature of 480°C to 510°C for a treatment time of 2h to 4h, and then air-cooled to room temperature.

[0054] After the above-mentioned solid solution treatment step, a matrix structure with a relatively small original austenite grain size can be obtained, and the use of ice-water mixture for rapid quenching can reduce the probability of residual austenite appearing in the matrix; after the above-mentioned deep cryogenic treatment step, the original austenite grain size can be further refined, and the residual austenite can be further eliminated, so that the matrix is ​​a full martensite structure; after the above-mentioned aging treatment step, two precipitated phases with a certain number density and dispersed distribution can be precipitated in the matrix, and a certain content of austenite can appear in the matrix.

[0055] The process chain of the present invention has a synergistic effect: through composition design + smelting + homogenization + multi-directional multi-pass deformation treatment, the matrix composition is uniform, the grains are fine, and the dislocation density is high, providing ideal conditions for the subsequent nucleation and distribution of the precipitation phase. Through the synergistic effect of solid solution + deep cryogenic treatment, the grains are refined, the residual austenite is eliminated, and a full martensite matrix is ​​formed, laying a foundation for high strength. Through aging treatment, the microscopic defects (dislocations, subgrain boundaries) formed in the previous process are used to achieve high-density dispersed distribution of the precipitation phase, and the plasticity is improved by retaining austenite. The previous process (smelting to rolling) provides the material with a matrix with uniform composition, fine grains, and high dislocation density, which is the physical basis for the nucleation and distribution of the precipitation phase. The later heat treatment (solid solution + deep cryogenic + aging) transforms the microstructural advantages of the previous process into a comprehensive improvement of mechanical properties through phase change control and precipitation phase regulation. The synergistic effect is reflected in: the superposition of fine grain strengthening, composite precipitation strengthening, and austenite toughening breaks through the traditional "inverted relationship" of strength-elongation.

[0056] The tensile strength of the martensitic stainless steel can reach 1820MPa, and the elongation after fracture can reach 19.5%. Compared with the existing commercial grade of martensitic stainless steel, the tensile strength is increased by about 350MPa, and the elongation is increased by about 60%. If the previous process is omitted (such as directly heat treating the commercial steel), it is difficult to achieve the same level of precipitation phase distribution and fine grain effect due to the unoptimized composition, coarse grains and low dislocation density, and the final performance improvement will be significantly limited. Therefore, the full-process process design of the present invention is the key to achieving high strength and high plasticity.

[0057] The technical solution and effects of the present invention are further illustrated below through practical examples.

[0058] Example

[0059] This embodiment provides three groups of specific preparation processes and comparative examples of Ce-containing composite precipitation-strengthened high-strength martensitic stainless steels. The specific chemical composition of the ingot of the Ce-containing composite precipitation-strengthened high-strength martensitic stainless steel is shown in Table 1, and the remainder is Fe and unavoidable impurities.

[0060] Table 1 Chemical composition of ingot (in mass percentage, %)

[0061] C Ni Cr Mo Ce Al Cu Example 1 0.021 8.13 12.57 2.08 0.022 1.20 2.29 Example 2 0.028 8.55 12.85 2.31 0.061 1.37 2.84 Example 3 0.034 8.78 13.00 2.45 0.079 1.45 3.36 Comparative Example 1 0.036 8.25 12.20 2.20 / 1.10 / Comparative Example 2 0.041 8.30 12.59 2.26 / 1.18 3.20 Comparative Example 3 0.050 8.38 12.94 2.30 0.055 1.20 /

[0062] Example 1

[0063] (1) According to the chemical composition shown in Table 1, the initial ingot is smelted by vacuum induction smelting method; according to the ingot diameter of 440 mm, the melting time is appropriately set, high-purity iron is first added, the melting time is 8 hours, the vacuum degree is 25 Pa, and refining begins. The refining time is controlled at 30 minutes, and the alloy components are added in sequence to adjust the composition; then the temperature is lowered to 1500°C and the ingot is taken out of the furnace for casting; the initial ingot is treated accordingly, the surface is peeled, and the observable shrinkage cavities and macro defects are removed. The surface is not allowed to have folds, slag inclusions, inclusions, cracks, etc.

[0064] (2) The ingot is subjected to high temperature homogenization treatment at a temperature range of 1280° C. for 24 h.

[0065] (3) After homogenization, the ingot is cooled to 1180°C, kept warm for 2 hours, and then taken out of the furnace for forging. The initial forging temperature is 1150°C and the final forging temperature is 980°C. If the ingot cools too fast, it should be placed in the furnace for heating in time, and forging treatment is carried out in the X, Y, and Z directions to finally obtain a forging billet of corresponding size with a thickness of about 50 mm.

[0066] (4) The forging billet is heated to 1200°C, kept at this temperature for 1 hour and then rolled. The starting rolling temperature is 1150°C, the initial rolling thickness is 50 mm, the first rolling pass is to 35 mm, the second rolling pass is to 25 mm, the third rolling pass is to 15 mm, the fourth rolling pass is to 10 mm, the final rolling temperature is 1000°C, and the billet is air-cooled to room temperature to finally obtain a rolled plate of 10 mm×150 mm×1000 mm.

[0067] (5) The muffle furnace is heated to 850°C at a heating rate of 20°C / min. The temperature is kept at this temperature for 15 minutes to ensure uniform temperature in the furnace. The prepared martensitic stainless steel is then quickly placed in the furnace. The temperature is kept at 850°C for 1 hour, and then the martensitic stainless steel is quickly quenched by an ice-water mixture to complete the solution treatment.

[0068] (6) The martensitic stainless steel after the solution treatment is placed in a cryogenic treatment device whose cryogenic medium is a mixture of liquid nitrogen and anhydrous ethanol, and kept at -196°C for 90 minutes. The martensitic stainless steel is then taken out of the cryogenic treatment device and naturally restored to room temperature in the air to complete the cryogenic treatment.

[0069] (7) The muffle furnace was heated to 510°C at a heating rate of 20°C / min. The temperature was kept at this temperature for 15 min to ensure uniform temperature in the furnace. The martensitic stainless steel that had completed the cryogenic treatment was then quickly placed in the furnace. The temperature was kept at 510°C for 4 h and finally taken out and air-cooled to room temperature to complete the aging treatment.

[0070] Example 2

[0071] (1) According to the chemical composition shown in Table 1, the initial ingot is smelted by vacuum induction smelting method; according to the ingot diameter of 440 mm, the melting time is appropriately set, high-purity iron is first added, the melting time is 6 h, the vacuum degree is 25 Pa, and refining begins. The refining time is controlled at 30 min, and the alloy components are added in sequence to adjust the composition; then the temperature is lowered to 1480 ° C and the furnace is taken out for casting; the initial ingot is treated accordingly, the surface is peeled, and the observable shrinkage cavities and macro defects are removed. The surface is not allowed to have folds, slag inclusions, inclusions, cracks, etc.

[0072] (2) The ingot is subjected to high temperature homogenization treatment at a temperature range of 1200° C. for 36 h.

[0073] (3) After homogenization, the ingot is cooled to 1080°C, kept warm for 1.5 hours, and then taken out of the furnace for forging. The initial forging temperature is 1180°C and the final forging temperature is 1000°C. If the ingot cools too fast, it should be placed in the furnace for heating in time, and forging treatment is carried out in the X, Y, and Z directions to finally obtain a forging billet of corresponding size with a thickness of about 50 mm.

[0074] (4) The forging billet is heated to 1100°C and kept at this temperature for 0.5h for rolling treatment. The starting rolling temperature is 1180°C, the initial rolling thickness is 50mm, the first rolling pass is to 35mm, the second rolling pass is to 25mm, the third rolling pass is to 15mm, the fourth rolling pass is to 10mm, the final rolling temperature is 1030°C, and the billet is air-cooled to room temperature to finally obtain a rolled plate of 10mm×150mm×1000mm.

[0075] (5) The muffle furnace is heated to 870°C at a heating rate of 20°C / min. The temperature is kept at this temperature for 15 minutes to ensure uniform temperature in the furnace. The prepared martensitic stainless steel is then quickly placed in the furnace and kept at this temperature for 0.5 hours until the temperature stabilizes at 870°C. The martensitic stainless steel is then quickly quenched by an ice-water mixture to complete the solution treatment.

[0076] (6) The martensitic stainless steel after the solution treatment is placed in a cryogenic treatment device whose cryogenic medium is a mixture of liquid nitrogen and anhydrous ethanol, and kept at -120°C for 60 minutes. The martensitic stainless steel is then taken out of the cryogenic treatment device and naturally restored to room temperature in the air to complete the cryogenic treatment.

[0077] (7) The muffle furnace is heated to 480°C at a heating rate of 20°C / min. The temperature is kept at this temperature for 15 min to ensure uniform temperature in the furnace. The martensitic stainless steel that has completed the cryogenic treatment is then quickly placed in the furnace. The temperature is kept at 480°C for 2 h and finally taken out and air-cooled to room temperature to complete the aging treatment.

[0078] Example 3

[0079] (1) According to the chemical composition shown in Table 1, the initial ingot is smelted by vacuum induction smelting method; according to the ingot diameter of 440 mm, the melting time is appropriately set, high-purity iron is added first, the melting time is 10 h, the vacuum degree is 25 Pa, and refining begins. The refining time is controlled at 30 min, and the alloy components are added in sequence to adjust the composition; then the temperature is lowered to 1490 ° C and the furnace is taken out for casting; the initial ingot is treated accordingly, the surface is peeled, and the observable shrinkage cavities and macro defects are removed. Folding, slag inclusions, inclusions, cracks, etc. are not allowed on the surface.

[0080] (2) The ingot is subjected to high temperature homogenization treatment at a temperature range of 1300° C. for 48 h.

[0081] (3) After homogenization, the ingot is cooled to 1280°C, kept warm for 2.5 hours, and then taken out of the furnace for forging. The initial forging temperature is 1200°C and the final forging temperature is 1030°C. If the ingot cools too fast, it should be placed in the furnace for heating in time, and forging treatment is carried out in the X, Y, and Z directions to finally obtain a forging billet of corresponding size with a thickness of about 50 mm.

[0082] (4) The forging billet is heated to 1300°C, kept at this temperature for 1.5 hours and then rolled. The starting rolling temperature is 1200°C, the initial rolling thickness is 50 mm, the first rolling pass is to 35 mm, the second rolling pass is to 25 mm, the third rolling pass is to 15 mm, the fourth rolling pass is to 10 mm, the final rolling temperature is 1050°C, and the billet is air-cooled to room temperature to finally obtain a rolled plate of 10 mm × 150 mm × 1000 mm.

[0083] (5) The muffle furnace is heated to 900°C at a heating rate of 20°C / min. The temperature is kept at this temperature for 15 minutes to ensure uniform temperature in the furnace. The prepared martensitic stainless steel is then quickly placed in the furnace. The temperature is kept at 900°C for 0.75 hours, and then the martensitic stainless steel is quickly quenched by an ice-water mixture to complete the solution treatment.

[0084] (6) The martensitic stainless steel after the solution treatment is placed in a cryogenic treatment device whose cryogenic medium is a mixture of liquid nitrogen and anhydrous ethanol, and kept at -80°C for 75 minutes. The martensitic stainless steel is then taken out of the cryogenic treatment device and naturally restored to room temperature in the air to complete the cryogenic treatment.

[0085] (7) The muffle furnace is heated to 500°C at a heating rate of 20°C / min. The temperature is kept at this temperature for 15 min to ensure uniform temperature in the furnace. The martensitic stainless steel that has completed the cryogenic treatment is then quickly placed in the furnace. The temperature is kept at 500°C for 3 h and then taken out and air-cooled to room temperature to complete the aging treatment.

[0086] Comparative Example 1

[0087] The chemical composition of the ingot is mainly different from that of Example 1 in that it does not contain Ce and Cu. The specific chemical composition is shown in Table 1, and the remaining process steps are the same as Example 1.

[0088] Comparative Example 2

[0089] The chemical composition of the ingot is mainly different from that of Example 1 in that it does not contain Ce. The specific chemical composition is shown in Table 1, and the remaining process steps are the same as Example 1.

[0090] Comparative Example 3

[0091] The chemical composition of the ingot is mainly different from that of Example 1 in that it does not contain Cu. The specific chemical composition is shown in Table 1, and the remaining process steps are the same as Example 1.

[0092] Comparative Example 4

[0093] In step (3), only forging treatment is performed in the X and Y directions; the remaining process steps and the chemical composition of the ingot are consistent with those in Example 1.

[0094] Comparative Example 5

[0095] In step (4), the first pass is rolled to 35 mm, the second pass is rolled to 25 mm, and the third pass is rolled to 10 mm; the remaining process steps and the chemical composition of the ingot are consistent with Example 1.

[0096] Comparative Example 6

[0097] Step (6) is deleted, and step (7) aging treatment is directly performed after the solution treatment in step (5) of Example 1; the remaining process steps and the chemical composition of the ingot are consistent with those in Example 1.

[0098] The mechanical properties test results of the above embodiments and comparative examples are as follows: The mechanical properties test was conducted in three groups according to the national standard, and the average value was selected for display. The results were very consistent and had a small fluctuation range. Therefore, one group was selected for illustration, and the results are shown in Table 2.

[0099] Table 2 Tensile properties data

[0100] serial number Yield strength(MPa) Tensile strength(MPa) Elongation after break (%) Example 1 1648 1870 22.5 Example 2 1620 1825 19.5 Example 3 1630 1850 21.0 Comparative Example 1 1480 1520 9 Comparative Example 2 1530 1770 19.0 Comparative Example 3 1500 1550 11.2 Comparative Example 4 1528 1680 13.6 Comparative Example 5 1540 1700 15.1 Comparative Example 6 1490 1665 19.2

[0101] Compared with the mechanical properties of Examples 1-3 and Comparative Examples 1-6, there is a significant improvement. The present invention mainly uses Example 1 and the existing commercial martensitic stainless steel Comparative Example 1 to illustrate in detail. Compared with Comparative Example 1, the yield strength of Example 1 is increased by about 170MPa, the tensile strength is increased by about 350MPa, and the elongation is increased by about 60%. Figure 1 shown. Figure 2 , 3 3 is a comparison of the metallographic microstructure and non-metallic inclusions of Example 1 of the present invention and Comparative Example 1. It can be seen from the figure that the original austenite grain size of the martensitic stainless steel prepared by the present invention is relatively small, and the number of non-metallic inclusions in the steel is significantly reduced. Figure 4 It is a TEM analysis diagram of two precipitated phases in Example 1 of the present invention. It can be seen from the figure that the steel prepared by the present invention contains a composite precipitated phase (i.e., a Cu precipitated phase and a NiAl precipitated phase). Figure 5 This is the XRD analysis diagram of Example 1 of the present invention. It can be seen from the figure that the present invention eliminates residual austenite through deep cryogenic treatment, making the matrix a full martensite structure, and in the subsequent aging treatment process, the Cu element promotes the formation of austenite, providing favorable conditions for the material to obtain excellent plasticity and toughness.

[0102] The present invention proposes a method of strengthening and toughening the existing commercial stainless steel by fine grain strengthening, composite precipitation strengthening and austenite toughening, and combining the corresponding pre-treatment and subsequent heat treatment processes, which provides a reference for the development and design of a new generation of ultra-high strength and toughness stainless steel. Therefore, we believe that the technical solution and technical effect of the present invention have obvious practical significance.

Claims

1. A method for preparing Ce-containing composite precipitation-strengthened martensitic stainless steel, characterized in that: The steps include: S1 is an ingot obtained by vacuum induction melting, and its chemical composition is C 0.02-0.04%, Ni 8.10-8.80%, Cr 12.50-13.00%, Mo 2.00-2.50%, Ce 0.02-0.08%, Al 1.20-1.50%, Cu 2.20-3.50%, and the balance is Fe and unavoidable impurities; S2 high temperature homogenization treatment; S3 multi-directional and multi-pass deformation treatment; S4 heat treatment; Wherein, step S3 is specifically as follows: S3-1: the ingot after the treatment in step S2 is cooled to a forging temperature, and then forged in at least three different directions to obtain a forging blank; S3-2: heating the forging billet to the rolling temperature, then performing four rolling processes, and air cooling to room temperature to obtain a rolled plate; Wherein, step S4 is specifically as follows: S4-1 solution treatment: solution temperature is 850-900℃, solution time is 0.5-1h; S4-2 Cryogenic treatment: Cryogenic treatment temperature is -196~-80℃, and the holding time is 60~90min; S4-3 Aging treatment: The aging treatment temperature is 480℃~510℃, and the treatment time is 2h~4h.

2. The preparation method according to claim 1, characterized in that: In step S1, vacuum induction melting specifically includes: firstly adding high-purity iron, and starting refining after melting, adding alloy components such as Ni, Mo, Cr, etc. to adjust the composition, and then casting out of the furnace to obtain an ingot; The melting time is not less than 5 hours, the vacuum degree is 25±5Pa, the refining time is 30±3min, and the furnace temperature is 1480-1500°C.

3. The preparation method according to claim 1, characterized in that: In step S2, the temperature of the high temperature homogenization treatment is 1200-1300°C and the time is 24-48 hours.

4. The preparation method according to claim 1, characterized in that: In step S3-1, the following conditions are met: After homogenization, cool the ingot to 1180±100℃, keep it at this temperature for 2±0.5h, then take it out of the furnace and start forging. The initial forging temperature shall not be lower than 1130℃, and the final forging temperature shall not be lower than 950℃. The forging treatment in at least three different directions is: performing forging treatment in three directions including but not limited to X, Y and Z directions, so that the thickness of the forging blank is 50 mm.

5. The preparation method according to claim 1, characterized in that: In step S3-2, the following conditions are met: Heat the forging billet to 1200±100℃, keep it at this temperature for 1±0.5h, then take it out of the furnace and start rolling. The starting rolling temperature shall not be lower than 1150℃, and the final rolling temperature shall not be lower than 980℃. The initial thickness of the four rolling passes is 50 mm, the first rolling pass is to 35 mm, the second rolling pass is to 25 mm, the third rolling pass is to 15 mm, and the fourth rolling pass is to 10 mm, so as to obtain a rolled plate of 10 mm×150 mm×1000 mm.

6. The preparation method according to claim 1, characterized in that: In step S4, the following conditions are met: In step S4-1, the rolled plate is heated to the solution temperature, kept at the temperature for a certain period of time, and then rapidly quenched and cooled, and the cooling medium is an ice-water mixture; In step S4-2, the rolled plate treated in step S4-1 is subjected to a cryogenic treatment, wherein the cryogenic treatment medium is a mixture of liquid nitrogen and anhydrous ethanol. After the cryogenic treatment, the rolled plate is allowed to naturally return to room temperature in the air.

7. Ce-containing composite precipitation-strengthened martensitic stainless steel prepared by the preparation method of Ce-containing composite precipitation-strengthened martensitic stainless steel according to any one of claims 1 to 6.

8. The Ce-containing composite precipitation-strengthened martensitic stainless steel according to claim 7, characterized in that: Its tensile strength is ≥1820MPa, yield strength is ≥1612MPa, and elongation after fracture is ≥19.5%.

9. The Ce-containing composite precipitation-strengthened martensitic stainless steel according to claim 7, characterized in that: The original austenite grain size in the steel is 8.75±0.56μm.

10. The Ce-containing composite precipitation-strengthened martensitic stainless steel according to claim 7, characterized in that: The steel contains body-centered cubic NiAl precipitates and face-centered cubic Cu precipitates. The total number density of the two precipitates is 8.17×10 23 m -3 ; The size of the NiAl precipitated phase is 2.88±0.15 nm, and the size of the Cu precipitated phase is 3.65±0.23 nm.

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