A precipitation-strengthened high-temperature-resistant martensitic steel, its preparation method and application

Abstract

The present invention provides a precipitation-strengthened high-temperature martensitic steel, its preparation method and application, belonging to the technical field of high-performance alloy steels. The precipitation-strengthened high-temperature martensitic steel provided by the present invention, by mass percentage, includes: C 0-0.04%, Cr 8.75-11.82%, Ni 6.25-7.50%, Co 7.50-8.50%, Mo 2.25-3.00%, V 0-0.50%, Al 0.55-2.00%, W 1.50-2.75%, Cu 0.50-1.80%, Re 0-0.05%, P <0.015%, S <0.010% and the balance of Fe. The present invention improves the strength through the co-precipitation strengthening effect of multiple nano-phases; forms Laves phases by Cr and W to ensure the stability and high-temperature strength of the martensitic hierarchical structure, while improving the corrosion resistance and friction and wear performance.

Description

Technical Field

[0001] The present invention relates to the technical field of high-performance alloy steel, and particularly relates to a precipitation-strengthened high-temperature martensitic steel, a preparation method thereof, and an application thereof. Background Art

[0002] Die steel is used to manufacture injection molds, cold stamping dies, hot forging dies, die-casting dies and other dies. Dies are the main processing tools for machinery manufacturing, radio instruments, motors, electrical appliances, industrial production, etc. At present, there are still many deficiencies in die steel, such as unstable microstructure and mechanical properties, short service life, poor reliability, high cost, etc., so most high-end dies still rely on imports. The emerging additive manufacturing technology has changed the traditional die manufacturing method, greatly reducing or even avoiding the adverse effects of impurity elements, composition segregation, etc., and improving the comprehensive mechanical properties and quality of the die; at the same time, the "free design" conformal water channels have greatly improved the cooling capacity of the die, thereby improving the surface quality and production efficiency of the product. These advantages given by additive manufacturing to the die can greatly reduce the overall cost of die production, and further widen the window of material composition design, that is, elements such as Co, Ni, W, Cu, etc. that cannot be used in traditional die steel can be utilized, and the addition of these elements combined with an optimized preparation process further enhances the advantages of the die in terms of mechanical properties and service life, forming a virtuous cycle. This idea or concept makes the quality of additive manufacturing die steel close to or even exceed the foreign level, with high cost performance and market competitiveness.

[0003] However, under the action of repeated thermal fatigue loads, compared with the traditional manufacturing method, the die steel manufactured by additive manufacturing may induce the initiation and propagation of local cracks due to potential printing defects, resulting in premature failure of the die. Improving the plasticity and toughness of the die steel, or increasing its high-temperature strength, can make up for the deficiencies brought by printing defects. At present, in the field of injection molds, the additive manufacturing 18Ni300 steel has many application cases due to its good formability, and it belongs to the category of maraging steel. Maraging steel usually consists of a relatively soft but ductile martensite matrix with a very low C content + nanoscale intermetallic compounds with excellent precipitation strengthening effect dispersed therein, and often exhibits a perfect combination of ultra-high strength and good toughness. The 18Ni300 steel contains 18 wt% Ni, a high content of Co (8 - 13 wt%) and Mo (3 - 5 wt%), and a small amount of Ti and Al. The high Ni content ensures that the alloy steel can obtain a complete martensite structure even at medium or even slow cooling rates, that is, it improves hardenability; but at the same time, Ni also limits the high-temperature tissue stability and mechanical properties of maraging steel because of the lower martensite-austenite reverse transformation temperature (A C1It makes it possible to form a "softer" austenite phase at a lower temperature, which cannot meet the requirements of some die steels, especially die-casting dies, for high-temperature strength. In addition, the 18Ni300 steel has poor corrosion resistance and friction and wear resistance, and has no advantage over AISI420 stainless steel, etc. In the field of die-casting dies used at higher temperatures, the additively manufactured H13 steel is prone to defects such as porosity and lack of fusion, and coarse chain-type carbides appear, so its wide use is also limited. In summary, there is an urgent need for a corrosion-resistant alloy steel with both ultra-high strength and good plasticity and toughness to make up for the deficiencies of existing injection mold steels. Summary of the Invention

[0004] The purpose of the present invention is to provide a precipitation-strengthened high-temperature-resistant martensitic steel, its preparation method and application. The precipitation-strengthened high-temperature-resistant martensitic steel provided by the present invention has good yield strength and tensile strength at both normal temperature and high temperature, and excellent high-temperature performance.

[0005] In order to achieve the above-mentioned invention purpose, the present invention provides the following technical solutions:

[0006] The present invention provides a precipitation-strengthened high-temperature-resistant martensitic steel, which includes the following chemical components by mass percentage: C 0-0.04%, Cr 8.75-11.82%, Ni 6.25-7.50%, Co 7.50-8.50%, Mo 2.25-3.00%, V 0-0.50%, Al 0.55-2.00%, W 1.50-2.75%, Cu 0.50-1.80%, Re 0-0.05%, P < 0.015%, S < 0.010% and the balance Fe.

[0007] The present invention provides a preparation method of the precipitation-strengthened high-temperature-resistant martensitic steel described in the above technical solution, including the following steps:

[0008] (1) Melting and casting the alloy raw materials, and then obtaining a formed component through forging or rolling;

[0009] Alternatively, preparing the alloy raw materials into spherical alloy powder by atomization method or rotating electrode method, and then performing additive manufacturing to obtain a formed component;

[0010] (2) Post-treating the formed component obtained in the step (1) to obtain a precipitation-strengthened high-temperature-resistant martensitic steel.

[0011] Preferably, in the step (1), the particle size of the spherical alloy powder is 15-75 μm, the packing density of the spherical alloy powder is 3.8-4.1 g / cm 3 , the tap density of the spherical alloy powder is 4.6-4.8 g / cm 3 , and the Hall flow rate of the spherical alloy powder ≤ 15 s / 50 g.

[0012] Preferably, the process parameters of additive manufacturing in the step (1) include: laser power of 120 - 380 W, scanning speed of 0.2 - 1.2 m / s, powder spreading layer thickness of 42 - 110 μm, and the included angle between the interlayer scanning paths is 65 - 70°.

[0013] Preferably, the post - treatment in the step (2) includes one, two or a combination of three of austenitizing quenching treatment, cryogenic treatment and tempering treatment.

[0014] Preferably, the temperature of the austenitizing quenching treatment is 1000 - 1070 °C, the time of the austenitizing quenching treatment is 0.5 - 5 h, and the cooling rate of the austenitizing quenching treatment is 2 - 50 °C / s.

[0015] Preferably, the temperature of the cryogenic treatment is - 196 - 120 °C, and the time of the cryogenic treatment is 0.5 - 3 h.

[0016] Preferably, the temperature of the tempering treatment is 400 - 670 °C, and the time of the tempering treatment is 3 - 7 h.

[0017] Preferably, the microstructure of the precipitation - strengthened high - temperature - resistant martensitic steel in the step (2) presents a layered structure with cellular grains and columnar grains alternating, or a lath martensite hierarchical structure, and there are dispersed Cu - rich phases, Cr - rich phases, NiAl phases, Laves phases and retained austenite phases distributed in the martensite matrix.

[0018] The present invention provides the application of the precipitation - strengthened high - temperature - resistant martensitic steel described in the above technical solution or the precipitation - strengthened high - temperature - resistant martensitic steel prepared by the preparation method described in the above technical solution in a mold.

[0019] The present invention provides a precipitation-strengthened high-temperature martensitic steel, which, by mass percentage, includes the following chemical components: C 0 to 0.04%, Cr 8.75 to 11.82%, Ni 6.25 to 7.50%, Co 7.50 to 8.50%, Mo 2.25 to 3.00%, V 0 to 0.50%, Al 0.55 to 2.00%, W 1.50 to 2.75%, Cu 0.50 to 1.80%, Re 0 to 0.05%, P < 0.015%, S < 0.010%, and the balance Fe. On the basis of 18Ni300 steel, on the one hand, by significantly reducing the Ni content and adjusting the Co content, while abandoning the addition of Ti and the precipitation strengthening of Ni3Ti, instead, by maintaining the Mo and Al contents, adding an appropriate amount of Cu, and adding trace amounts of carbide-forming elements such as V under low C conditions, a composite nano-phase of potential nano-β-NiAl phase and / or Cu-rich phase and / or VC carbide is formed; the β-NiAl phase or Cu-rich phase is mainly distributed in the martensite matrix, with a size between 1.2 and 6 nm and a number density of 10 24 m -3 level, relatively uniformly dispersed, and often co-precipitated or adjacent-precipitated, generating precipitation strengthening through the cutting mechanism of dislocations to improve the room-temperature and high-temperature strength; the VC carbide is introduced as an auxiliary. Under the condition of low C, the presence of a small amount of V can induce the dispersed precipitation of VC, and the precipitation reaction is mutually promoted with the precipitation of the β-NiAl phase and Cu-rich phase, further improving the strengthening effect of the nano-precipitation phase and the comprehensive mechanical properties; on the other hand, by adding Cr and an appropriate amount of W to form (Fe,Cr)2(W,Mo) Laves phase, which is mainly divided into two types: one is the primary Laves phase formed during the additive manufacturing process or austenitization process, mainly distributed at the martensite lath interface and the original austenite grain boundary, with a size between 6 and 150 nm and a number density between 10 21 ~10 22 m -3 level, unevenly distributed, mainly used to stabilize the hierarchical structure of martensite tissue to ensure the stability and high-temperature strength of the high-temperature martensite tissue, while improving the corrosion resistance and friction and wear properties; the other is the secondary Laves phase formed during the tempering process, mainly distributed in the martensite matrix, with a size between 1.5 and 12 nm and a number density of 10 23 m -3 ~10 24 m -3They are at a [certain] level, dispersedly distributed, and often precipitate adjacent to the above-mentioned β-NiAl phase or Cu-rich phase. Precipitation strengthening is generated through the cutting mechanism of dislocations, which can greatly improve the strength at room temperature and high temperature. The results of the examples show that the room-temperature yield strength of the precipitation-strengthened high-temperature-resistant martensitic steel provided by the present invention is 1364-1886 MPa, the tensile strength is 1581-2007 MPa, the elongation is 4.3-15.7%, and the V-notch room-temperature impact energy is 2.6-25.0 J; the yield strength at 300 °C is 1256.3 ± 12.1 MPa, the tensile strength is 1387.7 ± 15.3 MPa, and the elongation is 10.5 ± 0.5%; the yield strength at 600 °C is 496.3 ± 10.2 MPa, the tensile strength is 566.3 ± 3.2 MPa, and the elongation is 20.0 ± 1.3%. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 It is a graph showing the change of tempering hardness of the precipitation-strengthened high-temperature-resistant martensitic steel prepared in Examples 1-3;

[0021] Figure 2 It is a SEM impact fracture morphology diagram of the precipitation-strengthened high-temperature-resistant martensitic steel prepared in Example 2;

[0022] Figure 3 It is a tensile fracture morphology diagram of the precipitation-strengthened high-temperature-resistant martensitic steel prepared in Example 2;

[0023] Figure 4 It is a SEM impact fracture morphology diagram of the precipitation-strengthened high-temperature-resistant martensitic steel prepared in Example 3;

[0024] Figure 5 It is a tensile fracture morphology diagram of the precipitation-strengthened high-temperature-resistant martensitic steel prepared in Example 3;

[0025] Figure 6 It is an OM microstructure diagram of the precipitation-strengthened high-temperature-resistant martensitic steel prepared in Example 3;

[0026] Figure 7 It is a SEM microstructure diagram of the precipitation-strengthened high-temperature-resistant martensitic steel prepared in Example 3;

[0027] Figure 8 It is a TEM diagram of the distribution characteristics of the second phase in the precipitation-strengthened high-temperature-resistant martensitic steel prepared in Example 2;

[0028] Figure 9 It is a TEM diagram of the distribution characteristics of the second phase in the precipitation-strengthened high-temperature-resistant martensitic steel prepared in Example 3;

[0029] Figure 10STEM-HAADF images and EDS surface distribution maps of the precipitation-strengthened high-temperature resistant martensitic steel prepared in Example 3;

[0030] Figure 11 TEM analysis images of the NiAl phase in the precipitation-strengthened high-temperature resistant martensitic steel prepared in Example 3;

[0031] Figure 12 TEM analysis images of the Laves phase in the precipitation-strengthened high-temperature resistant martensitic steel prepared in Example 3;

[0032] Figure 13 APT analysis images of the β-NiAl phase and Laves phase in the precipitation-strengthened high-temperature resistant martensitic steel prepared in Example 3;

[0033] Figure 14 Composition analysis images of the NiAl phase in the precipitation-strengthened high-temperature resistant martensitic steel prepared in Example 3;

[0034] Figure 15 Composition analysis images of the Laves phase in the precipitation-strengthened high-temperature resistant martensitic steel prepared in Example 3. Detailed implementation manners

[0035] The present invention provides a precipitation-strengthened high-temperature resistant martensitic steel, which, by mass percentage, comprises the following chemical components: C 0 to 0.04%, Cr 8.75 to 11.82%, Ni 6.25 to 7.50%, Co 7.50 to 8.50%, Mo 2.25 to 3.00%, V 0 to 0.50%, Al 0.55 to 2.00%, W 1.50 to 2.75%, Cu 0.50 to 1.80%, Re 0 to 0.05%, P < 0.015%, S < 0.010% and the balance of Fe.

[0036] By mass percentage, the precipitation-strengthened high-temperature resistant martensitic steel provided by the present invention comprises C 0 to 0.04%, preferably 0 to 0.03%, and more preferably 0 to 0.01%. By controlling the content of the C element, the carbon content in the martensitic steel is maintained within a very low range, thereby promoting the formation of potential composite nano-phases of nano β-NiAl phase and / or Cu-rich phase and / or VC carbides.

[0037] By mass percentage, the precipitation-strengthened high-temperature resistant martensitic steel provided by the present invention comprises 8.75 - 11.82% of Cr, preferably 9.80 - 11.50%, more preferably 9.80 - 10.50%. By adding Cr element and an appropriate amount of W, the present invention forms (Fe,Cr)2(W,Mo) Laves phase, which is mainly divided into two types: one is the primary Laves phase formed during the additive manufacturing process or austenitization process, mainly distributed at the martensite lath interface and the original austenite grain boundary, with a size between 6 - 150 nm and a number density of 10 21 ~10 22 m -3 level, with uneven distribution, mainly used to stabilize the hierarchical structure of martensite tissue to ensure the stability and high-temperature strength of the high-temperature martensite tissue, while improving corrosion resistance and friction and wear performance; the other is the secondary Laves phase formed during the tempering process, mainly distributed in the martensite matrix, with a size between 1.5 - 12 nm and a number density of 10 23 m -3 ~10 24 m -3 level, with a dispersed distribution, and often precipitated adjacent to the above-mentioned β-NiAl phase or Cu-rich phase, and precipitation strengthening is generated through the cutting mechanism of dislocations, which can greatly improve the strength at room temperature and high temperature.

[0038] By mass percentage, the precipitation-strengthened high-temperature resistant martensitic steel provided by the present invention comprises 6.25 - 7.50% of Ni, preferably 6.50 - 7.20%, more preferably 6.80 - 7.20%. By reducing the content of Ni element in the martensitic steel, it is beneficial to form β-NiAl phase in the martensite matrix, thereby generating precipitation strengthening through the cutting mechanism of dislocations to improve the strength.

[0039] By mass percentage, the precipitation-strengthened high-temperature resistant martensitic steel provided by the present invention comprises 7.50 - 8.50% of Co, preferably 7.60 - 8.30%, more preferably 7.80 - 8.00%. By controlling the content of Co element, it can promote the formation of composite nano-phases of nano β-NiAl phase and / or Cu-rich phase and / or VC carbide.

[0040] By mass percentage, the precipitation-strengthened high-temperature resistant martensitic steel provided by the present invention comprises 2.25 - 3.00% of Mo, preferably 2.30 - 2.80%, more preferably 2.50 - 2.60%. By controlling the content of Mo element, it can further promote the formation of composite nano-phases of nano β-NiAl phase, Cu-rich phase and / or VC-type carbide.

[0041] By mass percentage, the precipitation-strengthened high-temperature resistant martensitic steel provided by the present invention comprises V 0 to 0.50%, preferably 0.05 to 0.45%, more preferably 0.25 to 0.45%. By adding a small amount of V element, the present invention forms a composite nano-phase of potential nano β-NiAl phase and / or Cu-rich phase and / or VC carbide. At the same time, the V element forms VC carbide with the C element. The VC carbide is introduced as an auxiliary. In the case of lower C, the presence of a small amount of V can induce the dispersion precipitation of VC, and it promotes the precipitation reaction with the β-NiAl phase and the Cu-rich phase, further improving the strengthening effect of the nano-precipitation phase and the comprehensive mechanical properties.

[0042] By mass percentage, the precipitation-strengthened high-temperature resistant martensitic steel provided by the present invention comprises Al 0.55 to 2.00%, preferably 0.70 to 1.80%, more preferably 1.20 to 1.50%. By controlling the content of the Al element, the present invention can further promote the formation of a composite nano-phase of nano β-NiAl phase, Cu-rich phase and / or VC-type carbide.

[0043] By mass percentage, the precipitation-strengthened high-temperature resistant martensitic steel provided by the present invention comprises W 1.50 to 2.75%, preferably 1.70 to 2.55%, more preferably 1.90 to 2.40%. By adding the W element and controlling its content, the present invention can form (Fe,Cr)2(W,Mo) Laves phase with the Cr element, thereby playing a role in stabilizing the martensite structure of the hierarchical structure to ensure the stability of the martensite hierarchical structure and the high-temperature strength at high temperature, and at the same time further improving the corrosion resistance and friction and wear performance.

[0044] By mass percentage, the precipitation-strengthened high-temperature resistant martensitic steel provided by the present invention comprises Cu 0.50 to 1.80%, preferably 0.60 to 1.50%, more preferably 0.80 to 1.20%. By adding the Cu element and controlling its dosage, a Cu-rich phase can be formed in the case of low C. The Cu-rich phase promotes the dispersion precipitation of the β-NiAl phase and VC, further improving the strengthening effect of the nano-precipitation phase and the comprehensive mechanical properties.

[0045] By mass percentage, the precipitation-strengthened high-temperature resistant martensitic steel provided by the present invention comprises Re 0 to 0.05%, preferably 0.01 to 0.04%, more preferably 0.02 to 0.03%. By adding the Re element, the present invention can improve the creep resistance, oxidation resistance and corrosion resistance of the alloy, and at the same time can also improve the plasticity and toughness.

[0046] By mass percentage, the precipitation-strengthened high-temperature resistant martensitic steel provided by the present invention comprises P < 0.015%. In the present invention, the P element is an impurity element.

[0047] By mass percentage, the precipitation-strengthened high-temperature resistant martensitic steel provided by the present invention includes S < 0.010%. In the present invention, the S element is an impurity element.

[0048] By mass percentage, the precipitation-strengthened high-temperature resistant martensitic steel provided by the present invention includes the balance of Fe. In the present invention, the Fe element serves as a matrix element.

[0049] Based on the 18Ni300 steel, on the one hand, by significantly reducing the Ni content and adjusting the Co content, while abandoning the Ti addition and the precipitation strengthening of Ni3Ti, but by maintaining the Mo and Al contents, adding an appropriate amount of Cu, and adding trace amounts of carbide-forming elements such as V under low C conditions, a composite nanophase of potential nano-β-NiAl phase and / or Cu-rich phase and / or VC carbide is formed; the β-NiAl phase or the Cu-rich phase is mainly distributed in the martensite matrix, with a size between 1.2 and 6 nm, and a number density of 10 24 m -3 level, relatively uniformly dispersed, and often co-precipitated or adjacent-precipitated, generating precipitation strengthening through the cutting mechanism of dislocations to improve the room-temperature and high-temperature strength; the VC carbide is introduced as an auxiliary. Under the condition of low C, the presence of a small amount of V can induce the dispersed precipitation of VC, and it promotes the precipitation reaction with the β-NiAl phase and the Cu-rich phase, further improving the strengthening effect of the nano-precipitation phase and the comprehensive mechanical properties; on the other hand, by adding Cr and an appropriate amount of W to form (Fe,Cr)2(W,Mo) Laves phase, which is mainly divided into two types: one is the primary Laves phase formed during the additive manufacturing process or the austenitizing process, mainly distributed at the martensite lath interfaces and the original austenite grain boundaries, with a size between 6 and 150 nm, and a number density between 10 21 ~10 22 m -3 level, unevenly distributed, mainly used to stabilize the hierarchical structure of the martensite tissue to ensure the stability and high-temperature strength of the high-temperature martensite tissue, while improving the corrosion resistance and friction and wear properties; the other is the secondary Laves phase formed during the tempering process, mainly distributed in the martensite matrix, with a size between 1.5 and 12 nm, and a number density of 10 23 m -3 ~10 24 m -3 level, dispersed distribution, and often adjacent-precipitated with the above-mentioned β-NiAl phase or Cu-rich phase, generating precipitation strengthening through the cutting mechanism of dislocations, which can significantly improve the room-temperature and high-temperature strength.

[0050] The present invention also provides a preparation method for the precipitation-strengthened high-temperature resistant martensitic steel described in the above technical solution, including the following steps:

[0051] (1) The alloy raw materials are melted and then cast, and then a formed component is obtained through forging or rolling;

[0052] Alternatively, the alloy raw materials are prepared into spherical alloy powder by atomization method or rotating electrode method, and then additive manufacturing is carried out to obtain a formed component;

[0053] (2) The formed component obtained in the step (1) is post-treated to obtain a precipitation-strengthened high-temperature resistant martensitic steel.

[0054] In a technical solution of the present invention, the alloy raw materials are melted and then cast, and then a formed component is obtained through forging or rolling. The present invention has no special limitation on the specific process parameters and operations of the melting and casting, and it is only necessary to completely melt and mix the alloy raw materials evenly.

[0055] After casting, the present invention preferably performs homogenization heat treatment on the cast product. In the present invention, the holding temperature of the homogenization heat treatment is preferably 1150 - 1250 °C; the holding time of the homogenization heat treatment is preferably 2 - 6 h. Through the homogenization heat treatment, the present invention can make the composition distribution of the ingot more uniform, and at the same time improve the plasticity of the ingot, so as to facilitate subsequent forging or rolling.

[0056] In the present invention, the rolling preferably includes a total of not less than 12 passes of rough rolling and finish rolling; the final rolling temperature of the rolling is preferably not higher than 750 °C; the cumulative reduction ratio of the rolling is preferably 4 - 7; the cooling method of the rolling is preferably air cooling or water cooling. In the present invention, the cumulative reduction ratio of the forging is preferably 4 - 7; the final forging temperature of the forging is preferably not higher than 750 °C. Through forging or rolling, the present invention can eliminate most of the casting defects such as porosity and segregation, and optimize the microstructure and mechanical properties of the final formed component.

[0057] In another technical solution of the present invention, the alloy raw materials are prepared into spherical alloy powder by atomization method or rotating electrode method, and then additive manufacturing is carried out to obtain a formed component.

[0058] The present invention has no special limitation on the specific operations of the atomization method or the rotating electrode method, and it is only necessary to make the parameters of the spherical alloy powder meet the requirements.

[0059] In the present invention, the particle size of the spherical alloy powder is preferably 15 - 75 μm; the particle size of the spherical alloy powder preferably shows a normal distribution; the packing density of the spherical alloy powder is preferably 3.8 - 4.1 g / cm 3 , more preferably 3.9 - 4.0 g / cm 3 ; the tapped density of the spherical alloy powder is preferably 4.6 - 4.8 g / cm 3 , more preferably 4.7 g / cm 3; The Hall flow rate of the spherical alloy powder is preferably ≤15 s / 50 g; the structure of the spherical alloy powder is preferably a fine-grained martensite structure. By controlling the parameters of the spherical alloy powder, the present invention is beneficial to further improving the performance of the formed component in the additive manufacturing process.

[0060] In the present invention, the process parameters of the additive manufacturing preferably include: the laser power is 120 - 380 W, the scanning rate is 0.2 - 1.2 m / s, the powder spreading layer thickness is 42 - 110 μm, and the interlayer scanning paths form an angle of 65 - 70°. More preferably: the laser power is 150 - 360 W, the scanning rate is 0.4 - 0.9 m / s, the powder spreading layer thickness is 50 - 100 μm, and the interlayer scanning paths form an angle of 65 - 70°. By controlling the parameters of the additive manufacturing process, the present invention can further improve the strength of the martensitic steel.

[0061] The present invention has no special limitation on the specific types and amounts of the alloy raw materials, as long as the chemical composition of the precipitation-strengthened high-temperature resistant martensitic steel meets the requirements.

[0062] In the present invention, the chemical composition of the formed component preferably is the same as that of the precipitation-strengthened high-temperature resistant martensitic steel.

[0063] After obtaining the formed component, the present invention performs post-treatment on the formed component to obtain the precipitation-strengthened high-temperature resistant martensitic steel.

[0064] In the present invention, the post-treatment preferably includes one, two or a combination of three of austenitizing quenching treatment, cryogenic deep cooling treatment and tempering treatment, and more preferably any one of tempering treatment, austenitizing quenching treatment + tempering treatment, cryogenic deep cooling treatment + tempering treatment, austenitizing quenching treatment + cryogenic deep cooling treatment + tempering treatment.

[0065] In the present invention, the temperature of the austenitizing quenching treatment is preferably 1000 - 1070 °C, more preferably 1020 - 1060 °C, and further preferably 1030 - 1050 °C; the time of the austenitizing quenching treatment is preferably 0.5 - 5 h, more preferably 1 - 4 h, and further preferably 2 - 3 h; the cooling rate of the austenitizing quenching treatment is preferably 2 - 50 °C / s, more preferably 5 - 40 °C / s, and further preferably 10 - 30 °C / s. Through the austenitizing quenching treatment of the martensitic steel in the present invention, the room-temperature structure is equiaxed grains (original austenite grains), and the grain memory is in a hierarchical martensite lath structure, similar to the as-cast microstructure.

[0066] In the present invention, the temperature of cryogenic treatment is preferably -196 to -120 °C, more preferably -180 to -130 °C, and further preferably -160 to -140 °C; the time of cryogenic treatment is preferably 0.5 to 3 h, more preferably 1 to 2.5 h, and further preferably 1.5 to 2 h; the warming-up mode of cryogenic treatment is preferably to place at room temperature for 10 to 60 min, more preferably 20 to 50 min, and further preferably 30 to 40 min. By adding cryogenic treatment, the present invention still makes the microstructure of martensitic steel a layered structure with cellular + columnar grains alternating; in the tempered structure, in addition to the α-phase matrix with a body-centered cubic structure, there is also a certain amount of γ-phase (austenite phase) with a face-centered cubic structure, mainly distributed at the martensite lath interfaces or the original austenite grain boundaries. This retained austenite mostly appears as irregular blocks and is unevenly distributed, becoming an unfavorable structure for inducing crack initiation, reducing the plasticity and toughness of alloy steel. Cryogenic treatment can promote the transformation of this retained austenite into lath martensite, increasing crystal defects (such as dislocation density), so as to better control the content and distribution morphology of retained austenite in the tempered structure; and during the high-temperature tempering process, nano-austenite or reverse-transformed austenite is formed at the martensite lath interfaces. These fine γ-phases are evenly distributed, Ni atoms diffuse and enrich therein, increasing its volume fraction and stability, thereby inhibiting the initiation and rapid propagation of cracks, and playing a role in improving the plasticity and toughness of alloy steel.

[0067] In the present invention, the temperature of tempering treatment is preferably 400 to 670 °C, more preferably 460 to 640 °C, further preferably 500 to 600 °C, and most preferably 550 to 570 °C; the time of tempering treatment is preferably 3 to 7 h, more preferably 4 to 6 h, and further preferably 5 h. By tempering treatment, the present invention can make the structure of martensitic steel a layered structure with cellular + columnar grains alternating, and at the same time form a reverse-transformed austenite phase with an FCC crystal structure in the martensite matrix. Mainly through the transformation-induced plasticity effect, it hinders the initiation and propagation of cracks, and thus achieves a better strength-ductility ratio.

[0068] In the present invention, the microstructure of the precipitation-strengthened high-temperature-resistant martensitic steel presents a layered structure with cellular grains + columnar grains alternating, or a lath martensite hierarchical structure, and preferably there are dispersed Cu-rich phases, Cr-rich phases, NiAl phases, Laves phases and retained austenite phases distributed in the martensite matrix.

[0069] Based on 18Ni300 steel and considering the service temperature of H13 steel for die-casting molds, the present invention optimizes the chemical composition of the precipitation-strengthened martensitic steel, clarifies the corresponding preparation method and subsequent heat treatment process, and regulates a flexible strength-ductility ratio, providing technical support for the stable manufacturing and large-scale application of additive manufacturing molds.

[0070] The present invention provides an application of the precipitation-strengthened high-temperature resistant martensitic steel described in the above technical solution or the precipitation-strengthened high-temperature resistant martensitic steel prepared by the preparation method described in the above technical solution in a mold.

[0071] In the present invention, the mold preferably includes an injection mold and / or a die-casting mold, and more preferably a die-casting mold. The present invention has no special limitation on the specific manner of the application, and those skilled in the art can make a conventional selection according to different heat treatment processes and target mechanical properties. By using an additive manufacturing process to prepare the alloy, compared with other processes, the alloy can be applied to a die-casting mold, expanding the application field of the martensitic steel.

[0072] The technical solutions in the present invention will be clearly and completely described below with reference to the embodiments in the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments in the present invention without making creative efforts shall fall within the protection scope of the present invention.

[0073] Example 1

[0074] A precipitation-strengthened high-temperature resistant martensitic steel, by mass percentage, consists of the following chemical components: C 0.04%, Cr 9.8%, Ni 7.20%, Co 8.0%, Mo 2.6%, V 0.45%, Al 1.5%, W 2.4%, Cu 0.8%, Re 0.02%, P < 0.015%, S < 0.010% and the balance of Fe;

[0075] The preparation method of the precipitation-strengthened high-temperature resistant martensitic steel comprises the following steps:

[0076] (1) The alloy raw materials are prepared into spherical alloy powder by atomization method, and then additive manufacturing is carried out to obtain a formed component; the particle size of the spherical alloy powder is 45 ± 12 μm, the particle size of the spherical alloy powder shows a normal distribution, the packaging density of the spherical alloy powder is 4.0 g / cm 3 , the tap density of the spherical alloy powder is 4.8 g / cm 3 , the Hall flow rate of the spherical alloy powder is 15 s / 50 g, and the structure of the spherical alloy powder is a fine-grained martensite structure; the process parameters of the additive manufacturing are: laser power is 360 W, scanning rate is 0.9 m / s, powder laying layer thickness is 100 μm, and the interlayer scanning path forms a 67° angle;

[0077] (2) The formed component obtained in step (1) is post-treated to obtain a precipitation-strengthened high-temperature resistant martensitic steel; the post-treatment is tempering treatment, the tempering treatment temperature is 560 °C, and the tempering treatment time is 4 h.

[0078] The density of the precipitation-strengthened high-temperature resistant martensitic steel prepared in Example 1 was tested, and the results were as follows: the porosity was 0.13%, and the density was 99.39%.

[0079] Example 2

[0080] The tempering temperature was 460 °C; other conditions were the same as those in Example 1.

[0081] Example 3

[0082] The tempering temperature was 600 °C; other conditions were the same as those in Example 1.

[0083] Example 4

[0084] A precipitation-strengthened high-temperature resistant martensitic steel, by mass percentage, consists of the following chemical components: Cr 9.8%, Ni 7.20%, Co 8.0%, Mo 2.6%, Al 1.5%, W 2.4%, Cu 0.8%, Re 0.02%, P < 0.015%, S < 0.010% and the balance Fe;

[0085] In the preparation method of the precipitation-strengthened high-temperature resistant martensitic steel, the post-treatment process is austenitizing quenching treatment + cryogenic deep cooling treatment + tempering treatment. Among them, the austenitizing temperature is 920 °C, the holding time is 1 h, and the quenching method is water cooling; the temperature of the cryogenic deep cooling treatment is -120 °C, the time of the cryogenic deep cooling treatment is 2 h, and the warming-up method of the cryogenic deep cooling treatment is to directly place it in a room temperature environment; the tempering temperature is 600 °C; other conditions are the same as those in Example 1.

[0086] Example 5

[0087] A precipitation-strengthened high-temperature resistant martensitic steel, by mass percentage, consists of the following chemical components: Cr 9.8%, Ni 7.20%, Co 8.0%, Mo 2.6%, Al 1.5%, W 2.4%, Cu 0.8%, Re 0.02%, P < 0.015%, S < 0.010% and the balance Fe;

[0088] In the preparation method of the precipitation-strengthened high-temperature resistant martensitic steel, the post-treatment process is austenitizing quenching treatment + tempering treatment. Among them, the austenitizing temperature is 920 °C, the holding time is 1 h, and the quenching method is water cooling; the tempering temperature is 640 °C; other conditions are the same as those in Example 1.

[0089] Comparative Example 1

[0090] A commercial CX steel, by mass percentage, consists of the following chemical components: C 0.03%, Si 0.3%, Mn 0.3%, Ni 9.2%, Cr 12%, Al 1.6%, Mo 1.4% and the balance Fe;

[0091] In the preparation method of the CX steel, the tempering temperature is 525 °C; other conditions are the same as those in Example 5.

[0092] The mechanical properties of the martensitic steels prepared in Examples 1 to 5 and the CX steel prepared in Comparative Example 1 were tested, and the results are shown in Table 1:

[0093] Table 1 Mechanical properties of the martensitic steels prepared in Examples 1 to 5 and the CX steel prepared in Comparative Example 1

[0094]

[0095] Specification

[0096]

[0097] It can be seen from Table 1 that for the currently mainstream commercially available additive manufacturing CX die steel, after being treated at the peak strengthening temperature of 525 °C, although it has a relatively high tensile strength and yield strength at room temperature, at high temperatures (300 °C and 600 °C), the tensile strength and yield strength will decrease significantly, and are inferior to the precipitation-strengthened high-temperature-resistant martensitic steel prepared in Example 5 of the present invention, indicating that the technical solution of the present invention improves the comprehensive performance of the martensitic steel.

[0098] Figure 1 It is a graph showing the change of tempering hardness of the precipitation-strengthened high-temperature-resistant martensitic steel prepared in Examples 1 to 3. From Figure 1 It can be seen that the hardness of the precipitation-strengthened high-temperature-resistant martensitic steel in the initial additive manufacturing state is very low, indicating that the cellular grain + columnar grain layered fine-grained structure formed during the additive manufacturing process and the possible formation of primary Laves phase cannot produce an effective strengthening effect. As the tempering temperature increases, the hardness increases sharply and reaches the hardness peak at about 560 °C, and then the hardness begins to gradually decrease. When tempering before the peak strengthening temperature of 560 °C, the precipitation strengthening of nano-second-phase dominates, while during the subsequent high-temperature tempering, the softening of the lath martensite matrix due to recovery and recrystallization begins to take effect, thus resulting in the above-mentioned change law of the tempering curve. Considering the requirements of dies and superalloys for strength and toughness, the tempering temperature is controlled at 400 - 670 °C.

[0099] Figure 2 It is the SEM impact fracture morphology diagram of the precipitation-strengthened high-temperature-resistant martensitic steel prepared in Example 2; Figure 3Tensile fracture morphology diagram of the precipitation-strengthened high-temperature resistant martensitic steel prepared in Example 2;

[0100] Figure 4 SEM impact fracture morphology diagram of the precipitation-strengthened high-temperature resistant martensitic steel prepared in Example 3;

[0101] Figure 5 Tensile fracture morphology diagram of the precipitation-strengthened high-temperature resistant martensitic steel prepared in Example 3. It can be seen that Figures 2 - 5 although the martensitic steels prepared in Example 2 and Example 3 have similar hardness, there are obvious differences in their impact and tensile fracture morphologies: the samples involved in Example 2 (tempered at 460 °C) show cleavage fracture characterized by river patterns, and some areas of the tensile fracture have dimple morphologies; the samples involved in Example 3 (tempered at 600 °C) show plastic fracture characterized by dimples, and the final impact energy value and elongation are both greater than those of Example 2. This shows that under the same strength requirements, selecting high-temperature tempering can improve toughness and plasticity.

[0102] Figure 6 OM microstructure diagram of the precipitation-strengthened high-temperature resistant martensitic steel prepared in Example 3; Figure 7 SEM microstructure diagram of the precipitation-strengthened high-temperature resistant martensitic steel prepared in Example 3. It can be seen from Figure 6 and Figure 7 that the precipitation-strengthened high-temperature resistant martensitic steel prepared in Example 3 shows a layered structure with equiaxed grains + columnar grains alternating, and there are no obvious printing defects, indicating that the additive manufacturing process used in the present invention is suitable for the said martensitic steel; during tempering at a relatively high temperature of 600 °C, reverse transformation of austenite occurs, forming a duplex structure of martensite matrix + reverse-transformed austenite. Among them, dispersed nano-precipitates are distributed in the martensite matrix, and there are no precipitates in the reverse-transformed austenite. This duplex structure is beneficial to improving toughness and plasticity.

[0103] Figure 8 TEM diagram of the distribution characteristics of the second phase in the precipitation-strengthened high-temperature resistant martensitic steel prepared in Example 2; Figure 9 TEM diagram of the distribution characteristics of the second phase in the precipitation-strengthened high-temperature resistant martensitic steel prepared in Example 3. It can be seen from Figure 8 and Figure 9 that although Example 2 and Example 3 have similar hardness, there are obvious differences in the distribution characteristics of the second phase: there are almost no visible second phases in the martensitic steel prepared in Example 2 (tempered at 460 °C), and only a small amount of Al2O3 particles formed during the printing process may exist. It may also be that due to the limited resolution of TEM, finer nano-precipitates cannot be detected; there are dispersed precipitates in the martensitic steel prepared in Example 3 (tempered at 600 °C).

[0104] Figure 10 STEM-HAADF images and EDS surface distribution maps of the precipitation-strengthened high-temperature resistant martensitic steel prepared in Example 3. It can be seen from Figure 10 that the precipitates in the precipitation-strengthened high-temperature resistant martensitic steel prepared in Example 3 (tempered at 600 °C) ( Figure 9 as shown) are mainly Laves phases containing Mo, W, and Cr, and all three elements show different degrees of enrichment. Al, Ni, and Cu also seem to form fine atomic clusters, corresponding to the appearance of the β-NiAl phase, while other alloy elements are basically evenly distributed.

[0105] Figure 11 TEM analysis images of the NiAl phase in the precipitation-strengthened high-temperature resistant martensitic steel prepared in Example 3. It can be seen from Figure 11 that through the confirmation of the selected area diffraction pattern (SADP), there are nano-sized β-NiAl phases with the same crystal orientation in the precipitation-strengthened high-temperature resistant martensitic steel prepared in Example 3 (tempered at 600 °C), which are clearly visible in the corresponding TEM dark field (DF) image, while the bright field (BF) image shows a high density of dislocations around these second phases.

[0106] Figure 12 TEM analysis images of the Laves phase in the precipitation-strengthened high-temperature resistant martensitic steel prepared in Example 3. It can be seen from Figure 12 that through the confirmation of the selected area diffraction pattern (SADP), there are two types of Laves phases in the precipitation-strengthened high-temperature resistant martensitic steel prepared in Example 3 (tempered at 600 °C). One is the nano-sized secondary Laves phase formed during tempering, and the other is the larger-sized primary Laves phase formed during printing. Both have the same crystal orientation and are clearly visible in the corresponding TEM dark field (DF) image.

[0107] Figure 13 APT analysis images of the β-NiAl phase and the Laves phase in the precipitation-strengthened high-temperature resistant martensitic steel prepared in Example 3. It can be seen from Figure 13 that the APT reconstruction data of the precipitation-strengthened high-temperature resistant martensitic steel prepared in Example 3 (tempered at 600 °C) are divided into two regions. One is the inverse transformation austenite region (γ) without precipitates in the upper part, and the other is the tempered martensite matrix region (α) rich in nano-precipitates, which is consistent with Figure 6 , Figure 7 and Figure 10The results are consistent. In the α matrix, there are β-NiAl phases rich in Cu and Laves phases containing Mo and W, adjacent to each other. Other alloying elements such as C, Mn, Si, and Re are enriched to varying degrees in the Laves phases. This α+γ duplex microstructure and the co-precipitation of β-NiAl phases and Laves phases jointly improve the comprehensive mechanical properties of the precipitation-strengthened high-temperature martensitic steel.

[0108] Figure 14 It is the composition analysis diagram of the NiAl phase in the precipitation-strengthened high-temperature martensitic steel prepared in Example 3. Figure 15 It is the composition analysis diagram of the Laves phase in the precipitation-strengthened high-temperature martensitic steel prepared in Example 3. From Figure 14 and Figure 15 it can be seen that the β-NiAl phase and the Laves phase have different chemical compositions, and their characteristics confirm the rationality and advancement of the composition design of the precipitation-strengthened high-temperature martensitic steel, which is beneficial to the formation of nano-precipitation phases and the maximization of the strengthening effect.

[0109] The above are only the preferred embodiments of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the principle of the present invention, several improvements and refinements can be made, and these improvements and refinements should also be regarded as the protection scope of the present invention.

Claims

1. A precipitation-strengthened high-temperature resistant martensitic steel, by mass percentage, comprises the following chemical components: C 0-0.04%, Cr 8.75-11.82%, Ni 6.25-7.50%, Co 7.50-8.50%, Mo 2.25-3.00%, V 0-0.50%, Al 0.55-2.00%, W 1.50-2.75%, Cu 0.50-1.80%, Re 0-0.05%, P < 0.015%, S < 0.010% and the balance Fe.

2. The preparation method of the precipitation-strengthened high-temperature resistant martensitic steel according to claim 1, comprising the following steps: (1) Melting and casting the alloy raw materials, and then obtaining a formed component by forging or rolling; Or, preparing the alloy raw materials into spherical alloy powder by atomization method or rotating electrode method, and then performing additive manufacturing to obtain a formed component; (2) Post-treating the formed component obtained in the step (1) to obtain the precipitation-strengthened high-temperature resistant martensitic steel.

3. The preparation method according to claim 2, wherein, In the step (1), the particle size of the spherical alloy powder is 15 - 75 μm, the packing density of the spherical alloy powder is 3.8 - 4.1 g / cm 3 , the tapped density of the spherical alloy powder is 4.6 - 4.8 g / cm 3 , and the Hall flow rate of the spherical alloy powder is ≤ 15 s / 50 g.

4. The preparation method according to claim 2, characterized in that, The process parameters of the additive manufacturing in the step (1) include: laser power of 120-380 W, scanning rate of 0.2-1.2 m / s, powder laying layer thickness of 42-110 μm, and the angle between the interlayer scanning paths being 65-70°.

5. The preparation method according to claim 2, characterized in that, The post-treatment in the step (2) includes one, two or a combination of three of austenitizing quenching treatment, cryogenic deep cooling treatment and tempering treatment.

6. The preparation method according to claim 5, characterized in that, The temperature of the austenitizing quenching treatment is 1000-1070 °C, the time of the austenitizing quenching treatment is 0.5-5 h, and the cooling rate of the austenitizing quenching treatment is 2-50 °C / s.

7. The preparation method according to claim 5, wherein The temperature of the cryogenic deep cooling treatment is -196 to -120 °C, and the time of the cryogenic deep cooling treatment is 0.5-3 h.

8. The preparation method according to claim 5, characterized in that, The temperature of the tempering treatment is 400-670 °C, and the time of the tempering treatment is 3-7 h.

9. The preparation method according to claim 2, characterized in that, The microstructure of the precipitation-strengthened high-temperature resistant martensitic steel in the step (2) presents a layered structure with alternating cellular grains and columnar grains, or a lath martensite hierarchical structure, and dispersed Cu-rich phases, Cr-rich phases, NiAl phases, Laves phases and retained austenite phases are distributed in the martensite matrix.

10. The application of the precipitation-strengthened high-temperature resistant martensitic steel according to claim 1 or the precipitation-strengthened high-temperature resistant martensitic steel prepared by the preparation method according to any one of claims 2-9 in a mold.

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