CN114622145B

CN114622145B - Cobalt-free maraging steel with dual-phase structure and preparation method thereof

Info

Publication number: CN114622145B
Application number: CN202210263342.7A
Authority: CN
Inventors: 刘咏; 李虎; 刘彬
Original assignee: Central South University
Current assignee: Central South University
Priority date: 2022-03-17
Filing date: 2022-03-17
Publication date: 2023-03-17
Anticipated expiration: 2042-03-17
Also published as: CN114622145A

Abstract

The invention discloses cobalt-free maraging steel with a dual-phase structure and a preparation method thereof, wherein the cobalt-free maraging steel has a dual-phase structure of martensite and austenite, and comprises the following components in percentage by mass: 16 to 20 percent of Ni, 3 to 7 percent of Mo, 1 to 3 percent of Cr, 0.5 to 2 percent of Ti, 0 to 2 percent of Al, 0 to 1 percent of V, 0 to 1 percent of Si and the balance of Fe. The preparation method comprises the following steps: the method comprises the following steps: the method comprises the following steps of preparing raw materials according to a designed proportion, smelting, casting and forming to obtain an alloy block, carrying out homogenization treatment on the alloy block, carrying out cold rolling treatment to obtain a cold-rolled block, and carrying out solid solution treatment and aging treatment on the cold-rolled block in sequence to obtain the dual-phase structure maraging steel. The cobalt-free maraging steel provided by the invention has the tensile strength of 1850-2100 MPa, the yield strength of 1730-2000 MPa and the elongation of 9-11%, and is low-cost, high-strength and high-toughness maraging steel.

Description

Cobalt-free maraging steel with dual-phase structure and preparation method thereof

Technical Field

The invention relates to the technical field of high-strength steel structure materials, in particular to cobalt-free maraging steel with a dual-phase structure and a preparation method thereof.

Background

The maraging steel is used as advanced high-strength steel, and the application of the maraging steel mainly faces the technical fields of aviation, atomic energy, high-end tools and dies and the like. Since the first generation of Fe-Ni maraging steel was developed by INCO corporation in the early 60 th of the 20 th century, maraging steel with different strength grades is successively developed by adjusting the contents of Co, mo and Ti, the grades are respectively 18Ni200, 18Ni250, 18Ni300 and 18Ni350, and the yield strengths respectively reach 1400 MPa, 1700 MPa, 1900MPa and 2400MPa. Among them, 18Ni200 and 18Ni250 steels were first applied to rocket motor casings.

However, the alloy of maraging steel is expensive to produce due to the addition of precious metal elements such as Co, which greatly limits the range of applications of maraging steel containing Co. Is composed ofTo solve the above problems, a Co-free maraging steel excellent in performance has been developed. The specific method comprises the following steps: removing Co element, increasing Ni and Ti content, and promoting semi-coherent nanophase such as Ni ₃ And (4) Ti precipitation. In addition, researchers have also added trace amounts of Al elements to Co-free maraging steels to promote Ni ₃ The precipitation of Al and B2-NiAl nano-phases achieves the purpose of strengthening the martensite matrix. For example, the Co-free maraging steel with the mark T250 comprises the alloy components of Fe-19Ni-3Mo-1.5Ti-0.3Cr-0.1Al-0.007C. The alloy has excellent mechanical property, the ultimate tensile strength is 1800MPa, the yield strength is 1600MPa, and the elongation percentage is 8 percent.

The application environment of the martensitic steel is extreme, which puts higher requirements on the mechanical properties of the material. In order to meet the requirement of using the material in special environment, the mechanical properties of the alloy need to be further optimized. Therefore, medium-carbon or high-carbon steels with higher mechanical properties, such as Fe-0.19C-1.01Mn-1.46Si steel and Fe-0.66C-1.42Cr-0.4Si-0.42Mn-0.07V steel, have been developed, and the ultimate tensile strength of the above carbon steels is higher than 2GPa. However, the harsh preparation method and cumbersome heat treatment process limit its wide application; secondly, the addition of high amounts of carbon leads to a reduction in the weldability of the material, which has a negative effect on the long-term service of the structural component.

Patent specification of publication No. CN 112322991A discloses a manganese component high strength steel in a Fe-C-Si-Mn-V low alloy, the yield strength of which is 2GPa. But the chemical composition of the material contains 0.2 to 0.4 mass percent of C, which can affect the welding performance of the material.

Disclosure of Invention

Aiming at the defects of the prior art, the invention aims to provide the cobalt-free maraging steel with excellent mechanical property and a dual-phase structure and the preparation method thereof.

In order to achieve the purpose, the invention adopts the following technical scheme:

the invention relates to cobalt-free maraging steel with a dual-phase structure, which has a dual-phase structure of martensite and austenite, and comprises the following components in percentage by mass: 16 to 20 percent of Ni, 3 to 7 percent of Mo, 1 to 3 percent of Cr, 0.5 to 2 percent of Ti, 0 to 2 percent of Al, 0 to 1 percent of V, 0 to 1 percent of Si and the balance of Fe.

In the process of tensile deformation, martensite bears higher external stress, and austenite synergistically deforms to improve toughness; in addition, the stress induces the transformation of martensite phase, and the tensile strength and the elongation of the material are improved.

In the invention, ni is enabled to be in the aging treatment process by adding Al and Mo with higher contents ₃ And (Al, mo), niAl and Mo-rich phases are separated out to increase the strength, and trace V elements are added to improve the solid solubility of the martensitic steel, refine crystal grains and form dispersed nano-precipitates to increase the strength of the material.

In addition, the hardenability of the steel can be improved by adding Cr and Si elements, and in the heat treatment process, si (0-1%) and Cr (1-3%) can effectively inhibit the formation of carbides, so that a proper volume fraction of stable austenite is formed in the final structure to form a dual-phase structure; the alloy disclosed by the invention does not contain noble metal elements and Co and C elements influencing the welding performance, so that the preparation cost of the alloy is reduced, and the welding performance is improved.

It is to be noted, however, that the additions of Mo and Ti in the cobalt-free maraging steel of the dual-phase structure according to the invention are not higher than 5% and 2%, respectively, since the addition of higher Mo and Ti contents in the maraging steel results in oversaturation of the precipitated phases and consumption of large amounts of Ni and Fe, reducing the contribution of the martensitic structure to strength and toughness.

Preferably, in the cobalt-free maraging steel, the volume fraction of martensite is 75 to 95%, and the volume fraction of austenite is 5 to 25%.

The inventors have found that controlling the volume fraction of austenite in cobalt-free maraging steel within the above range results in the final cobalt-free maraging steel having the best overall properties.

More preferably, in the cobalt-free maraging steel, the volume fraction of martensite is 85 to 95%, and the volume fraction of austenite is 5 to 15%.

Preferably, the martensite contains high-density dislocation, and the dislocation density is in the range of 2-30 × 10 ¹⁴ m ^-2 。

In the process of transforming austenite into martensite, the cobalt-free maraging steel provided by the invention forms a large amount of dislocation in lath martensite and a martensite boundary. Although some dislocations recover during aging, a large number of dislocations remain because aging is performed at a low temperature, and a higher dislocation density can increase tensile strength, i.e., dislocation strengthening.

Preferably, the cobalt-free maraging steel contains acicular Ni ₃ Ti nano precipitated phase, mo-rich phase, B2-NiAl precipitated phase, and the acicular Ni ₃ The Ti nanometer precipitated phase is non-uniformly distributed in space, the width is 1-8 nm, the length is 5-25 nm, the Mo-rich phase is spherical and is distributed in a dispersion manner, the size range of the Mo-rich phase is 14-16 nm, and the size of the B2-NiAl precipitated phase is 1-10 nm.

In addition, in the invention, the size and distribution of the precipitated phase are controlled by component regulation and heat treatment process; according to the invention, the lath martensite matrix and the dislocation can provide Ni during the aging process ₃ Nucleation point of Ti. According to the invention, the formation of the B2-NiAl ordered phase and the Mo-rich phase can be promoted by adding Al and Mo with higher contents, and a finer precipitated phase is obtained.

The invention controls Ni ₃ The Ti and B2-NiAl nanometer precipitated phases are in the size range, so that a cutting interaction mechanism is generated between the Ti and B2-NiAl nanometer precipitated phases and dislocation continues to move after the dislocation cuts the precipitated phases, local stress concentration is relieved, and premature failure of the material is avoided. Furthermore, ni according to the invention ₃ The Ti nano precipitated phase and the martensite matrix are in a semi-coherent orientation relationship, and the B2-NiAl phase and the martensite matrix are completely coherent and uniformly distributed in the space, thereby playing the roles of strengthening the matrix and toughening.

In a preferred scheme, the cobalt-free maraging steel comprises the following components in percentage by mass: 17 to 19 percent of Ni, 3 to 6 percent of Mo, 1 to 3 percent of Cr, 0.5 to 2 percent of Ti, 0 to 1.5 percent of Al, 0.3 to 0.6 percent of V, 0 to 0.8 percent of Si, and the balance of Fe.

Further preferably, the cobalt-free maraging steel comprises the following components in percentage by mass: 17 to 19 percent of Ni, 4 to 6 percent of Mo, 1 to 2 percent of Cr, 1 to 1.2 percent of Ti, 0.6 to 1.5 percent of Al, 0.3 to 0.4 percent of V, 0 to 0.2 percent of Si, and the balance of Fe.

Preferably, the cobalt-free maraging steel has a tensile strength of 1100 to 2200MPa, an elongation of 5 to 30% and a Vickers hardness of 400 to 650HV.

More preferably, the cobalt-free maraging steel has a tensile strength of 1850 to 2100MPa, a yield strength of 1730 to 2000MPa and an elongation of 9 to 11%.

The invention also provides a preparation method of the cobalt-free maraging steel with the dual-phase structure, which comprises the following steps: preparing raw materials according to a designed proportion, smelting, casting and molding to obtain an alloy block, homogenizing the alloy block, then carrying out cold rolling treatment to obtain a cold rolled block, and then carrying out solution treatment and aging treatment on the cold rolled block in sequence to obtain the maraging steel with the dual-phase structure.

The raw materials in the invention are selected from pure metals with the purity of more than or equal to 99.5 percent.

In a preferable scheme, the smelting mode is vacuum arc smelting under the protection of inert atmosphere, and the absolute vacuum degree of the smelting is 0.05-0.08 MPa.

Alloy smelting is carried out under the protection of inert gas and high vacuum degree, so as to avoid the oxidation of the molten liquid and the volatilization of elements. And after the smelting is finished, casting the melt obtained by the smelting into a copper mold to obtain an alloy block body with a certain size and shape.

Preferably, the temperature of the homogenization treatment is 1100-1300 ℃, and the time of the homogenization treatment is 15-25 h, preferably 20-24 h.

By carrying out the homogenization treatment at the above temperature, the homogenization of the microstructure of the martensitic steel can be ensured.

More preferably, the homogenization treatment is performed under a vacuum environment, and the absolute vacuum degree of the homogenization treatment is 0.02 to 0.05MPa.

The homogenization annealing is carried out in the vacuum environment, so that the alloy block can be prevented from being oxidized in the heat treatment process, and the adverse effect on the mechanical property of the alloy is avoided.

Preferably, the cold rolling has a strain amount of 60 to 65%.

In the present invention, work hardening can be caused in the material by cold rolling, and the strength and hardness of the maraging steel can be increased by cold hardening due to continuous cold deformation. However, if hot rolling is used, the performance is rather degraded.

In a preferable scheme, the temperature of the solution treatment is 950-1150 ℃, the time of the solution treatment is 2-30 min, preferably 5-15 min, and after the solution treatment, the water quenching is carried out to the room temperature.

More preferably, the temperature of the solution treatment is 950 to 1100 ℃, and the time of the solution treatment is 5 to 15min.

The inventors found that the volume fraction of austenite is too high and the crystal grains are coarsened to reduce the tensile strength of the material when the solution treatment is carried out for a long time, and the high-temperature short-time solution treatment method of the invention has the following characteristics: in one aspect, high temperature quenching can increase the martensite start temperature (M) _s ) This is due to the increased cooling rate which increases M _s And a large amount of austenite is converted into martensite, and a part of residual austenite is still remained in the matrix. On the other hand, the solution treatment in a short time avoids the occurrence of austenite recovery and recrystallization.

Preferably, the temperature of the aging treatment is 400-500 ℃, and the time of the aging treatment is 0.5-20 h.

Further preferably, the temperature rise rate of the aging treatment is 10-15 ℃/min, the temperature of the aging treatment is 480-500 ℃, and the time of the aging treatment is 3-4 h.

Through aging treatment, the nano precipitated phase Ni is promoted ₃ Ti andand (5) forming NiAl.

According to the aging treatment, the high heating rate can inhibit a large amount of dislocation from recovering, the grain size is reduced, the nanosheet austenite is formed at the martensite boundary after the high-temperature short-time solution treatment and the aging, the nanosheet austenite is completely different from the columnar or rod-shaped austenite obtained in the prior art, the nanosheet austenite can be used as a repairing layer in the deformation process, cracks are prevented from piercing martensite grains, the local stress concentration is effectively avoided, and the elongation is improved. In addition, the invention can effectively control the volume fraction of austenite to be between 5 and 25 percent by adopting high-temperature short-time solution treatment, thereby optimizing the performance of the cobalt-free maraging steel.

Principles and advantages

The invention provides a cobalt-free maraging steel with a dual-phase structure, which has the following alloy components: (1) The Al and Mo contents in the alloy components are higher than those of the traditional maraging steel, and the alloy is more beneficial to Ni in the aging treatment process ₃ (Al,Mo)、NiAl、FeMo、Fe ₂ Mo and Fe ₇ Mo ₆ Separating out an equienriched Mo phase; the addition of the trace V element can improve the solid solubility of the martensitic steel, refine crystal grains and form dispersion precipitation so as to increase the strength of the material; (2) The Cr and Si elements added into the alloy components can improve the hardenability of steel, and in the heat treatment process, si (0-1%) and Cr (1-3%) can effectively inhibit the formation of carbides, so that a stable austenite and martensite dual-phase structure with a proper volume fraction is formed in a final structure; (3) The alloy composition of the invention does not contain noble metal elements and Co and C elements which influence the welding performance, thereby reducing the preparation cost of the alloy and improving the welding performance; (4) Compared with the traditional single-phase cobalt-containing maraging steel, the cobalt-free maraging steel with the dual-phase structure has the advantages that the volume fraction of martensite is 85-95%, and the volume fraction of austenite is 5-15%. In the stretching deformation process, martensite bears higher external stress, and austenite cooperates with deformation to improve toughness; in addition, stress induces martensitic phase transformation, increasing the elongation of the material.

Specifically, in the invention, the Co element is removed, so that the contents of Al and Mo are improved, and the preparation cost of the alloy is greatly reduced.

The main role of Mo in maraging steel is to form a supersaturated solid solution by solid solution into the matrix during austenitization. During subsequent ageing treatment, ni ₃ Mo、FeMo、Fe ₂ Mo and Fe ₇ Mo ₆ And intermetallic compounds are precipitated to achieve the purpose of strengthening the matrix.

The main role of Ti in maraging steel is to form a supersaturated solid solution by solid solution into the matrix during austenitization. In the subsequent ageing treatment, with Ni ₃ And intermetallic compounds such as Ti are precipitated to achieve the purpose of strengthening the matrix.

The main role of Al in maraging steels is to dissolve into the matrix during austenitization, forming a supersaturated solid solution. In the subsequent ageing treatment with Ni ₃ Intermetallic compounds such as Al are precipitated to achieve the purpose of strengthening the matrix. In addition, the addition of Al has the following advantages: on the one hand, al can increase the M of maraging steel _s The formation of martensite structure is facilitated; on the other hand, in the alloy smelting process, al can also be used as a deoxidizer to remove the residual oxygen content in the atmosphere.

The main function of Cr and Si elements in maraging steel is to improve the hardenability of steel, but the addition of a large amount of Cr elements causes internal defects of steel parts, and the service life of the maraging steel is reduced. According to the invention, si (0-1%) and Cr (1-3%) are added, so that the formation of carbide can be inhibited in the heat treatment process, and the formation of stable austenite with a proper volume fraction in the final structure is ensured.

The V element can also improve the solid solubility of the martensitic steel, and mainly plays a role in refining the prior austenite grains and simultaneously forming dispersed nano precipitation to increase the strength of the material.

The dual-phase structure maraging steel obtained by the invention has reasonable component design, does not contain elements which influence welding performance and noble metal elements such as Co and C, has excellent mechanical property, low alloy preparation cost and good application prospect, and can be widely applied to the technical fields of industrial manufacturing, aviation, automobile industry and the like.

Compared with the prior art, the invention has the main advantages that: the heat treatment process is simple, the maraging steel with the dual-phase structure can be obtained only by simple solid solution and aging treatment, and the method is more suitable for batch production.

Drawings

FIG. 1 is a Transmission Electron Microscope (TEM) bright field image of a maraging steel of a comparative example;

FIG. 2 is a Transmission Electron Microscope (TEM) bright field image of a maraging steel of example 1;

FIG. 3 is a Transmission Electron Microscope (TEM) bright field image of a maraging steel of example 2;

FIG. 4 is a Transmission Electron Microscope (TEM) bright field image of a maraging steel of example 3;

FIG. 5 is an electron back-scattered diffraction (EBSD) antipole map of the maraging steel of example 1;

figure 6 is an electron back-scattered diffraction (EBSD) antipole diagram for the maraging steel of example 2.

Detailed Description

The invention is further described with reference to the following drawings and specific examples. It should be understood that these examples are for illustrative purposes only and are not intended to limit the scope of the present invention. The experimental procedures, in which specific conditions are not noted in the following examples, are generally carried out under conventional conditions or conditions recommended by the manufacturers.

Example 1

The cobalt-free maraging steel of the embodiment comprises the following components in percentage by mass: 18% of Ni, 5% of Mo, 2% of Cr, 1.2% of Ti1, 0.6% of V and the balance of Fe.

Step 1: fe, ni, mo, cr, ti and V with purity higher than 99.95% are mixed according to the components in the table 1.

Step 2: putting the alloy raw materials prepared in the step 1 into a copper crucible of an arc melting furnace, and then starting to vacuumize until the vacuum degree is not more than 5 multiplied by 10 ^-3 Until MPa. Argon is filled to maintain the absolute vacuum degree to be 0.3MPa, then smelting is started, and each alloy is repeatedly smelted for 6 times to ensure that the components are uniform. The molten alloy was then liquid cast onto copper with dimensions of 5mm by 12mm by 100mmAnd cooling in the die for more than 20min to obtain an alloy block.

And step 3: placing the alloy block obtained by smelting in the step 2 into a high vacuum sintering furnace for carrying out homogenization heat treatment, wherein the temperature is 1250 ℃, and the heat preservation time is 24 hours; the absolute vacuum degree is not higher than 0.02MPa, and an alloy block with more uniform microstructure is obtained.

And 4, step 4: and (3) carrying out cold rolling treatment on the uniform alloy block obtained in the step (3), wherein the total deformation is 68%, and the deformation of each pass is 0.5mm, so that a cold-rolled block with the thickness of 2mm is obtained.

And 5: and (4) putting the cold-rolled block obtained in the step (4) into a box furnace for high-temperature solution treatment at 990 ℃ for 5min, then performing water quenching, and cooling to room temperature to obtain the alloy block with the microstructure of lath martensite.

Step 6: and (3) continuing low-temperature aging treatment on the solid solution block obtained in the step (5), wherein the temperature rise speed of the aging treatment is 10-15 ℃/min, the temperature of the aging treatment is 480 ℃, the time of the aging treatment is 3 hours, and then air cooling is carried out to room temperature to obtain the dual-phase structure maraging steel. The tensile mechanical test of example 1 showed 1852.9. + -. 10.9MPa of tensile strength, 1730.3. + -. 36.7 of yield strength and 10.6% of elongation.

Example 2

The cobalt-free maraging steel of the embodiment comprises the following components in percentage by mass: 18% of Ni, 5% of Mo, 2% of Cr, 1% of Ti, 0.6% of Al, 0.4% of V and the balance of Fe.

Step 1: fe, ni, mo, cr, ti, al and V with purity higher than 99.95% are mixed according to the components in the table 1.

Step 2: putting the alloy raw materials prepared in the step 1 into a copper crucible of an arc melting furnace, and then starting to vacuumize until the vacuum degree is not more than 5 multiplied by 10 ^-3 Up to MPa. Argon gas is filled to keep the absolute vacuum degree at 0.3MPa, then smelting is started, and each alloy is repeatedly smelted for 6 times to ensure that the components are uniform. Then casting the molten alloy liquid into a copper mould with the size of 5mm multiplied by 12mm multiplied by 100mm, and cooling for more than 20min to obtain an alloy block.

And 5: and (5) putting the cold-rolled block obtained in the step (4) into a box furnace for high-temperature solution treatment at 990 ℃ for 5min, then performing water quenching, and cooling to room temperature to obtain the alloy block with the microstructure of lath martensite.

And 6: and (4) continuing low-temperature aging treatment on the solid solution block obtained in the step (5), wherein the temperature rise speed of the aging treatment is 10-15 ℃/min, the temperature of the aging treatment is 490 ℃, the time of the aging treatment is 3 hours, and then performing air cooling to room temperature to obtain the dual-phase structure maraging steel. The tensile mechanical test of example 2 showed a tensile strength of 1967. + -. 12MPa, a yield strength of 1896. + -.11 and an elongation of 11.3%.

Example 3

The cobalt-free maraging steel of the embodiment comprises the following components in percentage by mass: 18% of Ni, 5% of Mo, 2% of Cr, 1% of Ti, 0.3% of V, 0.2% of Si, 1.5% of All and the balance of Fe.

Step 1: fe, ni, mo, cr, ti, V, si and Al with the purity higher than 99.95 percent are mixed according to the components in the table 1.

And 2, step: putting the alloy raw materials prepared in the step 1 into a copper crucible of an arc melting furnace, and then starting to vacuumize until the vacuum degree is not more than 5 multiplied by 10 ^-3 Until MPa. Argon gas is filled to keep the absolute vacuum degree at 0.3MPa, then smelting is started, and each alloy is repeatedly smelted for 6 times to ensure that the components are uniform. Then casting the molten alloy liquid into a copper mould with the size of 5mm multiplied by 12mm multiplied by 100mm, and cooling for more than 20min to obtain an alloy block.

And 3, step 3: placing the alloy block obtained by smelting in the step 2 into a high vacuum sintering furnace for carrying out homogenization heat treatment, wherein the temperature is 1250 ℃, and the heat preservation time is 24 hours; the absolute vacuum degree is not higher than 0.02MPa, and an alloy block with more uniform microstructure is obtained.

And 4, step 4: and (4) carrying out cold rolling treatment on the uniform alloy block obtained in the step (3), wherein the total deformation is 68%, and the deformation of each pass is 0.5mm, so that a cold-rolled block with the thickness of 2mm is obtained.

Step 6: and (4) continuing low-temperature aging treatment on the solid solution block obtained in the step (5), wherein the temperature rise speed of the aging treatment is 10-15 ℃/min, the temperature of the aging treatment is 490 ℃, the time of the aging treatment is 3 hours, and then performing air cooling to room temperature to obtain the dual-phase structure maraging steel. Tensile mechanical testing was performed on example 3, with tensile strength of 2084 + -23 MPa, yield strength of 1996 + -21, and elongation of 9.1%.

Comparative example 1

The cobalt-free maraging steel of the comparative example comprises the following components in percentage by mass: 18% of Ni, 5% of Mo, 2% of Cr, 0.6% of Al, 0.4% of V and the balance of Fe.

Step 1: fe, ni, mo, cr, al and V with purity higher than 99.95% are mixed according to the components in the table 1.

And 2, step: putting the alloy raw materials prepared in the step 1 into a copper crucible of an arc melting furnace, and then starting to vacuumize until the vacuum degree is not more than 5 multiplied by 10 ^-3 Until MPa. Argon is filled to maintain the absolute vacuum degree to be 0.3MPa, then smelting is started, and each alloy is repeatedly smelted for 6 times to ensure that the components are uniform. Then casting the molten alloy liquid into a copper mould with the size of 5mm multiplied by 12mm multiplied by 100mm, and cooling for more than 20min to obtain an alloy block.

And step 3: placing the alloy block obtained by smelting in the step 2 into a high vacuum sintering furnace for carrying out homogenization heat treatment, wherein the temperature is 1250 ℃, and the heat preservation time is 24 hours; the absolute vacuum degree is not higher than 0.02MPa, and the alloy block with more uniform microstructure is obtained.

Step 6: and (3) continuing low-temperature aging treatment on the solid solution block obtained in the step (5), wherein the temperature of the aging treatment is 490 ℃, the time of the aging treatment is 3 hours, and then performing air cooling to room temperature to obtain the dual-phase structure maraging steel. The tensile mechanical test of example 3 showed that the tensile strength was 1531. + -. 12MPa, the yield strength was 1442. + -. 15MPa, and the elongation was 11.6%.

Comparative example 2

The temperature for solid solution was 825 ℃ and the time for solid solution was 120min, all other conditions being the same as in example 1.

Comparative example 3

The other conditions were the same as in example 1 except that the temperature increase rate in aging was 5 ℃/min.

Table 1 alloy compositions and mechanical properties (mass% in wt.% and the balance Fe) of examples 1 to 3 and comparative examples 1 to 3

Test example 1 alloy thermophysical property test

Measuring thermal expansion curve with thermal expansion instrument to obtain martensite phase transformation starting temperature (M) _s ) Austenite transformation initiation temperature (A) _s ) And austenite transformation end temperature (A) _f ) 211 ℃, 602 ℃ and 721 ℃ respectively. The thermal expansion sample size was 5mm × 5mm × 25mm,the heating rate is 10 ℃/min, and the temperature is kept for 5min at 1000 ℃. The cooling rate is divided into two stages, in the first stage, the cooling rate from 1000 ℃ to 500 ℃ is 10 ℃/min, and the temperature is kept at 500 ℃ for 5min. And in the second stage, the sample is cooled from 500 ℃ to room temperature at a cooling rate of 5 ℃/min, and the experimental environment is protected by argon.

Test example 2 mechanical Property test of alloy

And cutting a tensile test sample from the alloy block after heat treatment, wherein the length of a gauge length of the tensile test sample is 9.5mm, the width of the tensile test sample is 2mm, and the thickness of the tensile test sample is 1.8mm. Strain rate of 10 ^-3 /s ^-1 The strain variable is obtained by a digital video extensometer, the tensile strength is 1177-2111 MPa, and the elongation at break is 5-27%. To investigate the effect of the heat treatment on the mechanical properties, the samples were annealed at 400-520 ℃ for 0.5-19h, respectively. The Vickers hardness test was performed on a micro hardness tester with an applied load of 3kg and a retention time of 10 seconds. The test result shows that the Vickers hardness is 400-610 HV.

Test example 3 microstructural test of alloy

The fracture morphology is observed under a scanning electron microscope, and the fracture is shown to be a dimple structure. And (3) observing lath martensite and restored austenite by using an optical microscope, and grinding and polishing to remove an oxide layer on the surface before microscopic observation. Before observing the sample with an optical microscope, HNO with a volume fraction of 5% ₃ And 95% ethanol solution (5 vol.% HNO) ₃ +95vol.％C ₂ H ₅ OH) was etched on the sample surface. The electropolished sample alloy samples were analyzed for electron back-scattered diffraction (EBSD) using a 5% perchloric acid and 95% ethanol solution (5 vol.% HClO) as the corrosion electrolyte ₄ +95vol.％C ₂ H ₅ OH), the temperature was maintained at-20 ℃ using liquid nitrogen. The test results show that the microstructure of the maraging steel of the dual-phase structure is lath-shaped martensite and austenite, wherein the volume fraction of the austenite of the embodiment 1 is 10-20 vol.%. A3 mm diameter disk-shaped sheet was prepared and observed by transmission electron microscopy on a sample prepared from 10% perchloric acid in 90% alcohol (10 vol.% HClO) ₄ +90vol.％C ₂ H ₅ OH) is subjected to double-spraying treatment.The test result shows that the martensite matrix contains high-density nanometer precipitated phase and dislocation accumulation. Ni ₃ The average sizes of the Ti nano-particles and the Mo-rich phase are 2-3 nm and 14-16 nm respectively.

Test example 4 alloy phase transition test and analysis

The deformation behavior of the maraging steel in a solid solution state and an aging state is analyzed by adopting an in-situ synchronous X-ray diffraction technology. The length of the gauge length of the in-situ synchronous X-ray diffraction test sample is 20mm, the width of the gauge length is 4mm, and the thickness of the gauge length is 0.5mm. And integrating the obtained two-dimensional diffraction patterns along the stretching axis direction, the direction vertical to the stretching axis and the whole direction respectively to obtain a one-dimensional diffraction pattern. All diffraction patterns are shown in the bragg angle range of 8-24 °. The lattice strain of the crystal plane under load, and the full width at half maximum (FWHM) of each diffraction peak in the one-dimensional diffraction pattern were calculated. Calculating the dislocation density under given stress by using a modified Williamson-Hall model based on the FWHM value, wherein the calculation result of the dislocation density is 2-30 multiplied by 10 ¹⁴ m ^-2 。

The Transmission Electron Microscope (TEM) picture results of the comparative examples show that the nanoparticles formed in the microstructure of the comparative examples are Mo-rich phases having an average size of 15 to 16nm. The lamellar reversed austenite is formed in the lath-shaped martensite matrix. The austenite size is 280-290 nm, as shown in FIG. 1.

The TEM image of example 1 shows that high density of dislocations and Ni are formed in the alloy specimens of example ₃ Ti nanometer precipitated phase, the average size of the precipitated phase is 2-3 nm. Furthermore, strip-shaped reversed austenite having an average size of 0.1 to 0.5 μm is formed in the martensite matrix, as shown in FIG. 2.

Electron Back Scattering Diffraction (EBSD) of example 1 shows that a maraging steel with a dual-phase structure is obtained by solution and aging treatment. The dual-phase structure is mainly martensite matrix, and austenite with volume fraction of 10-20% is formed in the matrix, as shown in fig. 5.

TEM image of the alloy sample of example 1 shows that a high density of Ni is formed in the martensitic matrix ₃ Nano precipitated phase of Ti, ni ₃ Ti is atThe space is non-uniformly distributed, and the width and the length of the space are respectively 1-3 nm and 15-25 nm, as shown in figure 3. This high density nano-precipitation contributes significantly to the yield strength of the maraging steel of said dual phase structure.

The EBSD map results of example 2 show that example 2 forms austenite with a volume fraction of 8 to 12% by solution and aging treatment, as shown in fig. 6.

TEM image results of the alloy sample of example 3 show that high density Ni is formed in the martensitic matrix ₃ Ti and NiAl nanophase as shown in FIG. 4.

In conclusion, the dual-phase structure maraging steel has excellent mechanical property and reasonable alloy component design, can obtain a certain content of austenite and martensite dual-phase structures through simple solid solution and aging treatment, forms high-density dislocation and nano precipitated phases in a martensite matrix, has great improvement effect on the mechanical property, and has important significance for developing the preparation of the alloy of the high-strength steel.

Furthermore, it should be understood that various changes and modifications can be made by one skilled in the art after reading the above description of the present invention, and equivalents also fall within the scope of the invention as defined by the appended claims.

Claims

1. A cobalt-free maraging steel having a dual-phase structure, characterized in that: the cobalt-free maraging steel has a dual-phase structure of martensite and austenite, and comprises the following components in percentage by mass: 16 to 20 percent of Ni, 3 to 7 percent of Mo, 1 to 3 percent of Cr, 0.5 to 2 percent of Ti, 0.6 to 2 percent of Al, 0 to 1 percent of V, 0 to 1 percent of Si and the balance of Fe;

in the cobalt-free maraging steel, the volume fraction of martensite is 75-95%, and the volume fraction of austenite is 5-25%;

the cobalt-free maraging steel contains acicular Ni ₃ Ti nano precipitated phase, mo-rich phase and B2-NiAl precipitated phase;

the preparation method of the cobalt-free maraging steel with the dual-phase structure comprises the following steps of: preparing raw materials according to a design proportion, smelting, casting and forming to obtain an alloy block, homogenizing the alloy block, then carrying out cold rolling to obtain a cold-rolled block, and then carrying out solid solution treatment and aging treatment on the cold-rolled block in sequence to obtain the dual-phase structure maraging steel,

the temperature of the solution treatment is 950 to 1150 ℃, the time of the solution treatment is 2 to 30min, and after the solution treatment, water quenching is carried out to the room temperature;

the temperature of the aging treatment is 400-510 ℃, and the time of the aging treatment is 0.5-20h.

2. A cobalt-free maraging steel with a dual-phase structure according to claim 1, characterized in that:

the martensite contains high-density dislocation, and the dislocation density range is 2 to 30 multiplied by 10 ¹⁴ m ^-2 ；

The acicular Ni ₃ The Ti nanometer precipitated phase is in nonuniform distribution in space, the width is 1 to 8nm, the length is 5 to 25nm, the Mo-rich phase is spherical and is in dispersion distribution, the size range of the Mo-rich phase is 14 to 1690 nm, and the size of the B2-NiAl precipitated phase is 1 to 10nm.

3. A cobalt-free maraging steel with a dual-phase structure according to claim 1, characterized in that: the cobalt-free maraging steel has tensile strength of 1100 to 2200MPa, elongation of 5 to 30 percent and Vickers hardness of 400 to 650HV.

4. A cobalt-free maraging steel with a dual-phase structure according to claim 1, characterized in that: the smelting mode is carried out in a high vacuum arc smelting furnace under the protection of inert atmosphere, and the absolute vacuum degree of smelting is 0.05 to 0.08MPa.

5. A cobalt-free maraging steel with a dual-phase structure according to claim 1, characterized in that: the temperature of the homogenization treatment is 1100 to 1300 ℃, and the time of the homogenization treatment is 15 to 25h.

6. A cobalt-free maraging steel with a dual-phase structure according to claim 1, characterized in that: the cold rolling deformation is 60 to 65 percent.

7. A cobalt-free maraging steel with a dual-phase structure according to claim 1, characterized in that: the temperature of the solution treatment is 950 to 990 ℃, and the time of the solution treatment is 5 to 15 min;

the temperature rise rate of the aging treatment is 10 to 15 ℃/min, the temperature of the aging treatment is 480 to 500 ℃, and the time of the aging treatment is 3 to 4h.