Maraging steel

Abstract

The present invention relates to a maraging steel suitable for hot working tools, the steel comprising, in weight percent (wt%), C≦0.08, Si 0.1-0.9, Mn≦2, Cr 4.0-6.5, Ni 2.0-5.0, Mo 3.5-6.5, Co 2.0-5.5, Cu≦4.0, Nb≦0.1, V≦0.1, Ti≦0.1, with the balance being Fe and impurities.

Description

Technical Field

[0001] The present invention relates to a new type of maraging steel having properties suitable for tools such as hot working tools and plastic forming tools, and its composition is also suitable for use in additive manufacturing methods.

Background Art

[0002] The term "hot working tool" applies to tools for processing and shaping metals at relatively high temperatures, such as die casting, hot pressing, plastic molding dies, and various types of tools intended for use in high-temperature operations. Conventional hot working tool steels have been developed with an emphasis on strength and hardness when exposed to high temperatures for long periods, and generally a significant amount of carbide forming alloys are used.

[0003] In hot working applications, it has conventionally been common to use various types of hot working tool steels, particularly 5% Cr steels such as H11 and H13. Uddeholm DIEVAR® is a high-grade hot working tool of this type. It is a high-performance chromium-molybdenum-vanadium steel manufactured by ESR. As described in Patent Document 1, it contains a balanced carbon and vanadium content. This type of steel precipitates nanometer-sized carbides to inhibit dislocation movement and achieve particle strengthening, and is called a so-called secondary hardening steel.

[0004] Furthermore, maraging steel is also known to be used for hot working applications. Maraging steel hardens not by carbon hardening, but by the precipitation of intermetallic compounds in a high-alloy matrix of low-carbon martensite. Commercially available maraging steel often contains 18% Ni and large amounts of Mo, Co, Ti, and Al. One of the most common 18% Ni steels is Grade 300 maraging steel, also known as 1.2709. Another class of maraging steel is stainless steel, which includes 17-7PH, 17-4PH, 15-5PH, PH 15-7Mo, PH 14-8Mo, and PH 13-8Mo.

[0005] Maraging steel possesses both ultra-high strength and ductility, but it has the drawback of containing large amounts of expensive alloying elements. Furthermore, many maraging steels have the disadvantage of partially reverting from martensite to austenite during the aging process. This type of austenite is called reverted austenite and is distinguished from retained austenite, which can also be present in maraging steel after hardening and aging. Because austenite and martensite have different densities, transformation stresses and strains occur due to microstructural changes that occur during heat treatment and use. The transformation from austenite to martensite results in volume increase, and the transformation from austenite to martensite results in shrinkage of tool steel. Therefore, these unwanted deformations can lead to harmful dimensional changes, which is a difficult problem in high-precision molds, tools, and dies.

[0006] Furthermore, the presence of highly reactive elements can lead to the formation of hard inclusions such as Al2O3, TiN, and VN, which may impair the polishability.

[0007] Maraging steel powder is increasingly used in additive manufacturing (AM) because its low carbon content prevents cracking during cooling. However, AM powder is expensive, making its recycling a focus of attention. But since the oxygen concentration in a selective laser melting (SLM) chamber is generally around 1000 ppm, AM powder will pick up oxygen during processing. Therefore, especially when the steel contains high levels of oxygen-reactive elements such as Al and Ti, repeated heating of the powder during reuse could effectively absorb oxygen. [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] WO9950468A1 [Overview of the project]

[0009] The present invention aims to overcome the aforementioned drawbacks of conventionally known materials.

[0010] A general objective of the present invention is to provide a new type of universal, economical, and viable maraging steel with improved properties for hot working and plastic molding tools.

[0011] In particular, the present invention is aimed at steels that possess high temper resistance, combining high dimensional stability with high toughness. Temper resistance refers to the ability of steel to maintain its hardness at high temperatures for extended periods. Toughness refers to the ability of steel to absorb energy and undergo plastic deformation without fracture.

[0012] Furthermore, the objective is to provide maraging steel that has good polishability as well as good dimensional stability.

[0013] Another objective is to provide a maraging steel having a low content of reactive elements, which can be melt-atomized using nitrogen gas and whose powder has improved properties for recirculation in laser-based AM.

[0014] The present invention is defined in the claims. [Brief explanation of the drawing]

[0015] [Figure 1] Figure 1 shows a laser microscope image (×200) of the comparative steel in Example 3. [Figure 2] Figure 2 shows a laser microscope (×200) of the steel of the present invention in Example 3. [Modes for carrying out the invention]

[0016] Detailed description The importance of individual elements and their interactions with one another, as well as the limitations on the chemical composition of the claimed alloy, are briefly described below. All percentages for the chemical composition of steel are given herein in weight percent (wt.%). Phase amounts are given in volume percent (vol.%). The upper and lower limits for individual elements can be freely combined within the limits set forth in the claims.

[0017] The importance of individual elements and their interactions with each other, as well as limitations on the chemical composition of the claimed alloy, are briefly described below. All percentages for the chemical composition of steel are given herein in weight percent (wt.%). The upper and lower limits of individual elements can be freely combined within the ranges defined in the claims. For all numerical values ​​given herein, the arithmetic precision of the numbers can be increased by one or two orders of magnitude. Thus, a value reported as, for example, 0.1%, can also be expressed as 0.10% or 0.100%. The amounts of microstructural components are given in volume percent (vol.%).

[0018] Carbon (≤0.08%) Carbon is an undesirable impurity element in maraging steel. The upper limit of carbon is 0.08%. The upper limit may also be 0.07, 0.06, 0.05, 0.04, 0.03 or 0.02%.

[0019] Silicon (0.1~0.9%) Silicon is used for deoxidation. Also, Si is a strong ferrite former. Therefore, Si is limited to 0.9%. The upper limit may also be 0.8, 0.7, 0.6, 0.5 or 0.4%. The lower limit may also be 0.1%, 0.2% or 0.3%.

[0020] Manganese (≦2%) Manganese contributes to the deoxidation property and the improvement of hardenability of steel. The content of Mn is not important but is limited to 2%. The upper limit may also be 1.5%, 1.0%, 0.6%, 0.5% or or 0.4%.

[0021] Chromium (4.0~6.5%) Chromium is present at a content rate of at least 4.0% in order to provide good hardenability and corrosion resistance. If the chromium content is too high, it may lead to the formation of an undesirable phase such as delta ferrite. Therefore, the upper limit is 6.5%. The upper limit may also be 6.0% or 5.5%. The lower limit value may also be 4.0, 4.1, 4.2, 4.3, 4.4 or 4.5%.

[0022] Nickel (2.0~5.0%) Nickel is an austenite stabilizer and suppresses the formation of delta ferrite. Nickel gives good hardenability and toughness to the steel. Ni promotes the precipitation of Mo as the μ-phase. The lower limit may also be 2.0%, 2.5% or 3%. The upper limit may also be 5.0, 4.5, 4.3, 4.1 or 4.0%.

[0023] Molybdenum (3.5~6.5%) Solid-solution Mo is known to have a very favorable effect on hardenability. In this invention, Mo is necessary for precipitation hardening during aging. Mo appears to form the intermetallic compound μ-phase (Fe,Ni,Co)7Mo6 during aging. For this reason, the amount of Mo should be 3.5 to 6.5%. The lower limit may be 3.7, 3.8, 3.9, 4.0, 4.1, 4.2, 4.3, 4.4, or 4.5%. The upper limit may also be 6.4, 6.3, 6.2, 6.2, 6.0, 5.9, 5.8, 5.7, 5.6, or 5.5%.

[0024] Cobalt (2.0-5.5%) Cobalt dissolves in the matrix of maraging steel and reduces the solubility of molybdenum, so Co promotes the precipitation of Mo as a μ-phase. Furthermore, Co increases the MS temperature and Ac1 temperature, thereby reducing the risk of reverse-transformation austenite formation.

[0025] Copper (≦4%) Cu may be optionally added to increase the strength of the steel through the precipitation of ε-Cu. The upper limit is 4%, and it may be set to 3.5%, 3.0%, 2.5%, or 2.0%.

[0026] V, Nb, Ti, Al V, Nb, and Ti are strong carbide, nitride, and / or oxide-forming agents. Therefore, the content of these elements should be limited to avoid the formation of undesirable carbides and nitrides. Thus, the maximum content of each of these elements is 0.1%. Preferably, these elements are limited to 0.05%, 0.03%, 0.02%, 0.01%, or 0.005%. In particular, the content of Nb and Ti is preferably limited to impurity levels.

[0027] Impurity elements P and S are the main impurities and can adversely affect the mechanical properties of steel. Therefore, P may be limited to 0.1, 0.05, 0.04, 0.03, 0.02, or 0.01%. If sulfur is not intentionally added, the sulfur (S) impurity content may be limited to 0.05, 0.04, 0.003, 0.001, 0.0008, 0.0005, or even 0.0001%. However, sulfur may be intentionally added in amounts up to 0.35% to improve the machinability of the steel. The upper limit for sulfur may be reduced to 0.30, 0.25, 0.15, or 0.10%.

[0028] The steel composition of the present invention is by weight % (wt.%) C ≤ 0.08, Si 0.1~0.9, Mn ≤ 2, Cr 4.0~6.5, Ni 2.0~5.0, Mo 3.5~6.5, CO 2.0~5.5, Cu ≤ 4.0, Nb ≤ 0.1, V ≤ 0.1, Ti ≤ 0.1, The remainder consists of iron and impurities. It consists of.

[0029] The steel of the present invention is extremely stable against the formation of reverse austenite during heating. The martensitic-austenite transformation temperature (Ac1) of the steel of the present invention is preferably higher than 680°C, which is a typical temperature for Al die castings. Ac1 can be easily determined with a dilatometer and can be defined as the temperature at which thermal expansion first deviates from a linear relationship. The Ac1 temperature is preferably at least 690°C, preferably ≥700°C, more preferably ≥710°C, and most preferably ≥720°C.

[0030] The steel preferably satisfies at least one of the following requirements: C ≤ 0.07, Si 0.2~0.8, Mn ≤ 1, Cr 4.1~6.0, Ni 2.5~4.5, Mo 4.0~6.0, CO 2.5~5.0, Cu ≤ 3.0, V ≤ 0.05, Nb ≤ 0.05, Ti ≤ 0.05, And / or, the transformation temperature Ac1 from martensite to austenite is higher than 680°C. and / or, the steel is in an aged state and contains intermetallic compound precipitates, at least 50 vol.% of which are of the (Fe,Ni,Co)7Mo6 type.

[0031] The steel preferably satisfies at least one of the following compositional requirements: C ≤ 0.06, Si 0.2~0.7, Mn ≤ 0.6, Cr 4.3~5.7, Ni 2.7~4.3, Mo 4.2~5.8, Co 2.7~4.7 Cu ≤ 2.5, V ≤ 0.03, Nb ≤ 0.03, Ti ≤ 0.03, And / or, the transformation temperature Ac1 from martensite to austenite is higher than 700°C. and / or, the steel is aged and contains intermetallic compound precipitates, and at least 70 vol.% of the precipitates are of the (Fe, Ni, Co)7Mo6 type. and / or, the cleanliness of fine slag conforming to ASTM E45-97, Method A, meets the following maximum requirements. [Table A] It satisfies the condition.

[0032] According to a further preferred embodiment, the steel satisfies the following requirements: C ≤ 0.06, Si 0.3~0.6, Mn ≤ 0.4, Cr 4.5~5.5, Ni 3.0~4.0, Mo 4.5~5.5, Co 3.0~4.5, V ≤ 0.05, Nb ≤ 0.05, Ti ≤ 0.05, Al ≤ 0.01.

[0033] According to yet another preferred embodiment, the steel satisfies the following requirements: C ≤ 0.06, Si 0.2~0.7, Mn ≤ 0.6, Cr 4.3~5.7, Ni 2.7~4.3, Mo 4.2~5.8, Co 2.7~4.7 V ≤ 0.03, Ti ≤ 0.03, Al ≤ 0.01.

[0034] The alloy of the present invention may be in the form of a pre-alloy powder produced by molten atomization, and this powder has the composition specified above.

[0035] The pre-alloy powder is produced by gas atomization, and at least 80% of the powder particles have a particle size in the range of 5 to 150 μm, and the powder satisfies at least one of the following requirements: Powder particle size distribution (μm): 5≦D10≦35, 20 ≤ D50 ≤ 55, D90≦80, Average sphericity, SPHT ≧0.85, Average aspect ratio, b / l ≥ 0.85, (Here, SPHT = 4πA / P 2 (In the formula, A is the measured area covered by the particle projection, and P is the measured outer circumference / periphery of the particle projection.) Sphericity (SPHT) is measured using a camsizer in accordance with ISO 9276-6, where b is the shortest width of the particle projection and l is the longest diameter.)

[0036] The pre-alloy powder particles preferably have a particle size distribution in which at least 90% of the powder particles have a particle size in the range of 10 to 100 μm, and the powder satisfies at least one of the following requirements: Powder particle size distribution (μm): 10≦D10≦30, 25≦D50≦45, D90≦70, Average sphericity, SPHT ≧0.90, Average aspect ratio, b / l ≥ 0.88.

[0037] The present invention also includes articles formed by additive manufacturing, wherein the articles contain the alloy of the present invention.

[0038] The alloy of the present invention may be used in the manufacture of any tool, such as hot working tools, plastic molding tools, and small molds. These products may be manufactured by any suitable method. Preferred manufacturing methods include heat-injection (HIP) or additive manufacturing (PM), which includes AM. Steel powder, in particular, is suitable for selective laser melting with recirculation of the alloy powder because of its low reactivity with oxygen and nitrogen.

[0039] The alloy of the present invention can be manufactured by powder metallurgy (PM).

[0040] PM powder can be produced by conventional gas or water atomization of pre-alloy steel.

[0041] When the aforementioned powder is used for AM (atomization), it is important to use a technique that produces a powder with high roundness and low satellite content; therefore, gas atomization is a preferred atomization method. In particular, the close-coupled gas atomization method can be used for this purpose.

[0042] The maximum particle size of powder particles for AM is 150 μm, the preferred particle size range is 10 to 100 μm, and the average particle size is approximately 25 to 45 μm.

[0043] Liquid metal deposition (AM) methods such as liquid metal deposition (LMD), selective laser melting (SLM), and electron beam (EB) melting are attracting attention. Furthermore, powder properties are also important in AM. The powder particle size distribution measured with a Camsizer compliant with ISO 4497 ​​must meet the following requirements (μm): 5≦D10≦35, 20 ≤ D50 ≤ 55, D90 ≤ 80.

[0044] Preferably, the powder should satisfy the following particle size requirements (unit: μm): 10 ≤ D10 ≤ 30, 25≦D50≦45, D90 ≤ 70.

[0045] Even more preferable is that the coarse particle size fraction D90 is limited to ≤60 μm, and even more preferably to ≤55 μm.

[0046] It is desirable that the aforementioned powder has high sphericity. Sphericity (SPHT) can be measured with a Camsizer and is defined by ISO 9276-6. SPHT = 4πA / P 2 In the formula, A is a measured area covered by the particle projection, and P is a measured outer / periphery of the particle projection. The average SPHT should be at least 0.80, preferably at least 0.85, 0.90, 0.91, 0.92, 0.93, 0.94, or even 0.95. Furthermore, it is desirable that 5% or less of the particles have an SPHT ≤ 0.70. Preferably, the value should be 0.70, 0.65, 0.55, or even less than 0.50. In addition to SPHT, aspect ratio can also be used for classifying powder particles. The aspect ratio is defined as b / l, where b is the shortest width of the particle projection and l is the longest diameter. The average aspect ratio should preferably be at least 0.85, more preferably 0.86, 0.87, 0.88, 0.89, or 0.90. [Examples]

[0047] Example 1 In this example, one of the alloys of the present invention is compared with the high-grade steel Uddeholm Dievar®. The nominal composition of the alloy is as follows (wt%): The present invention Uddeholm Dievar (registered trademark) C 0.03 0.36 Si 0.43 0.20 Mn 0.03 0.5 Cr 5.25 5 Ni 3.43 - Mo 5.51 2.3 V 0.07 0.55 Co 3.82 - Cu 0.04 - Remainder: Iron and impurities

[0048] The steel of the present invention is formed by gas atomization and HIP. After cooling to room temperature over a period of 600 seconds at a temperature interval of 800-500°C (t 8 / 5 (600 seconds), the steel was tempered twice at 605°C for 3 hours, resulting in a hardness of 45 HRC.

[0049] The comparative steel was conventionally manufactured using ESR after ingot casting. The remelted steel was austenitized at 1020°C in a vacuum furnace, followed by gas quenching at intervals of 800°C to 500°C for 600 seconds (t8 / 5 = 600s). After cooling to room temperature, the comparative steel was tempered twice at 615°C for 2 hours each (2x2) to achieve a hardness of 45HRC.

[0050] Subsequently, the temper resistance of the alloy was investigated at a temperature of 600°C. The results are shown in Table 1.

[0051] [Table 1]

[0052] Furthermore, when the toughness after tempering was examined, the Charpy V toughness of the steel of the present invention was 25 J, while the Charpy V toughness of the comparative steel was 22 J. This indicates that the steel of the present invention has significantly higher temper resistance compared to the comparative steel.

[0053] Upon examining the microstructure of the steel after aging treatment, it was found that the precipitate causing the hardening of the steel of the present invention is (Fe,Ni,Co)7Mo6, i.e., the intermetallic μ-phase. The precipitation module (TC-Prisma, Thermo Calc Version 2021b) was used as a calculation tool to simulate the precipitation process of the steel of the present invention. The precipitation calculation results for the steel of the present invention used in this example showed that the particle size of the intermetallic μ-phase after 10 hours was approximately 20 nm, and the amount of the μ-phase was slightly less than 5 vol.%.

[0054] Example 2 In this example, one of the alloys of the present invention is compared to Grade 300 maraging steel (1.2709).

[0055] The nominal composition of the alloy is as follows (wt%). Invention 1.2709 C 0.03 0.005 Si 0.43 0.04 Mn 0.03 0.05 Cr 5.25 0.02 Ni 3.43 18.1 Mo 5.51 5.1 V 0.07 0.03 Co 3.82 8.8 Cu 0.04 0.01 Ti - 0.95 Remainder: Iron and impurities

[0056] These alloy gas atomized powders were subjected to selective laser melting (SLM) using the EOS M290 system.

[0057] As a result, it was found that the steel of the present invention contained no retained austenite (<2 vol.%) in its as-built condition, while the comparative steel contained 11 vol.% retained austenite. The amount of retained austenite was determined by X-ray diffraction according to the standard ASTM E975-13.

[0058] The property of steel to form reverse austenite during aging was investigated by holding it at an aging temperature of 540°C for 1 hour. As a result, no reverse austenite was formed in the steel of the present invention, whereas the amount of austenite increased to 17 vol.% in the comparative steel. Therefore, aging was carried out at a temperature above the transformation temperature (Ac1) of the comparative steel 1.2709 from martensite to austenite. Consequently, the thermal expansion of steel 1.2709 is affected by the transformation from martensite to austenite during reheating.

[0059] The absence of austenite-to-martensite transformation in the steel of the present invention is thought to be due to the austenite transformation initiation temperature being higher than the aging temperature. This was confirmed by dilatation testing, which revealed that the Ac1 temperature of the steel of the present invention is 730°C and the Ac3 temperature is 925°C. Therefore, reheating to the Ac1 temperature does not result in the formation of reverse-transformed austenite. From this, it can be seen that the steel of the present invention can be used in aluminum die casting, which is generally performed at a melting temperature of approximately 680°C.

[0060] Upon investigating the microstructure of the steel after aging treatment, it was found that the precipitate causing the hardening of the steel in the present invention is (Fe,Ni,Co)7Mo6, i.e., an intermetallic μ-phase.

[0061] As expected, the precipitate causing the hardening of comparative steel 1.2709 was (Fe,Ni,Co)3(Ti,Mo), and only trace amounts of the μ-phase were found.

[0062] Example 3 The sensitivity of nonmetallic inclusion formation in an oxygen-containing atmosphere was qualitatively investigated. The steel of the present invention, having the same composition as in Example 2, was compared with Uddeholm Dievar® steel. The nominal composition of the comparative steel is C 0.03%, Si 0.3%, Mn 0.3%, Cr 12.0%, Ni 9.2%, Mo 1.4%, and Al 1.6%.

[0063] Both steels were melted in a high-frequency furnace under an argon protective atmosphere, and then cast in the open air using an inclined copper chute.

[0064] The same sample was taken and examined with a laser microscope (LOM) at 200x magnification. The examination results are shown in Figures 1 and 2, and a comparison reveals that the steel of the present invention has significantly lower susceptibility to oxygen compared to the comparative steel. [Industrial applicability]

[0065] The alloy of the present invention is useful for a wide range of applications. In particular, it is useful for tools and molds for hot working and plastic molding, and for additive manufacturing (AM) applications.

Claims

1. Maraging steel for hot work tools, The steel is, in weight % (wt.%), C ≤ 0.08, Si 0.1~0.9, Mn ≤ 2, Cr 4.0-6.5, Ni 2.0-5.0, Mo 3.5-6.5, Co 2.0-5.5, Cu ≤ 4.0, Nb ≤ 0.1, V ≤ 0.1, Ti ≤ 0.1, The remainder consists of iron and impurities. Steel composed of [this material].

2. The maraging steel according to claim 1 that satisfies at least one of the following requirements: C ≤ 0.07, Si 0.2-0.8, Mn ≤ 1, Cr 4.1~6.0, Ni 2.5-4.5, Mo 4.0-6.0, Co 2.5-5.0, Cu ≤ 3.0, V ≤ 0.05, Nb ≤ 0.05, Ti ≤ 0.05, And / or, the transformation temperature Ac1 from martensite to austenite is higher than 680°C. and / or, the maraging steel is in an aged state and contains intermetallic compound precipitates, the precipitates of which at least 50 vol.% are (Fe, Ni, Co) ７ Mo ６ It is a type.

3. The maraging steel according to claim 1 or 2, satisfying at least one of the following compositional requirements: C ≤ 0.06, Si 0.2-0.7, Mn ≤ 0.6, Cr 4.3~5.7, Ni 2.7-4.3, Mo 4.2-5.8, Co 2.7-4.7, Cu ≤ 2.5, V ≤ 0.03, Nb ≤ 0.03, Ti ≤ 0.03, And / or, the transformation temperature Ac1 from martensite to austenite is higher than 700°C. and / or, the maraging steel is in an aged state and contains intermetallic compound precipitates, the precipitates of which at least 70 vol.% are (Fe, Ni, Co) ７ Mo ６ It is a type, and / or the following maximum requirements for fine slag with cleanliness in accordance with ASTM E45-97, Method A Table A It satisfies the condition.

4. The maraging steel according to claim 1 or 2, which satisfies the following requirements: C ≤ 0.06, Si 0.3~0.6, Mn ≤ 0.4, Cr 4.5-5.5, Ni 3.0~4.0, Mo 4.5-5.5, Co 3.0-4.5, V ≤ 0.05, Nb ≤ 0.05, Ti ≤ 0.05, Al ≤ 0.01, And, arbitrarily, the transformation temperature Ac1 from martensite to austenite is higher than 710°C.

5. The maraging steel according to claim 1 or 2, which satisfies the following requirements: C ≤ 0.06, Si 0.2-0.7, Mn ≤ 0.6, Cr 4.3~5.7, Ni 2.7-4.3, Mo 4.2-5.8, Co 2.7-4.7, V ≤ 0.03, Ti ≤ 0.03, Al ≤ 0.01, And, optionally, the transformation temperature Ac1 from martensite to austenite is higher than 720°C.

6. A prealloy powder having the composition described in any one of Claims 1 to 5.

7. The prealloy powder according to claim 6, wherein at least 80% of the powder particles have a particle size in the range of 5 to 150 μm, and the powder satisfies at least one of the following requirements: Average sphericity, SPHT ≧0.85, Average aspect ratio, b / l ≥ 0.85, (Here, SPHT = 4πA / P2 (wherein A is the measured area covered by the particle projection, and P is the measured outer circumference / periphery of the particle projection), and the sphericity (SPHT) is measured with a camsizer in accordance with ISO 9276-6, where b is the shortest width of the particle projection and l is the longest diameter.)

8. The prealloy powder according to claim 6, wherein at least 90% of the powder particles have a particle size in the range of 10 to 100 μm, and the powder satisfies at least one of the following requirements: Average sphericity, SPHT ≧0.90, Average aspect ratio, b / l ≥ 0.88.

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