High-strength and high-toughness iron-based alloy and method for producing the same

Abstract

To provide a high-strength and high-toughness iron-based alloy suitable for various structural components / members, particularly members for aerospace-related equipment which has a specific strength equivalent to maraging steel, in which toughness higher than that of maraging steel is obtained at -100°C, a material cost can be also reduced, and which has a high degree of freedom of a shape, and a method for manufacturing the alloy.SOLUTION: A high-strength and high-toughness iron-based alloy that contains C:0.12 to 0.3%, Si:0.15 to 0.7%, Mn:0.4 to 1.2%, P:0.015% or less, S:0.015% or less, Ni:2 to 4%, Cr:0.1 to 0.5%, and Mo:0.3 to 0.5% by mass%, in which the remainder consists of Fe and inevitable impurity, and which has a solidified structure with a dendrite secondary arm spacing of 5 μm or less.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present invention relates to a high-strength and high-toughness iron-based alloy having excellent specific strength and toughness suitable for various structural parts and members, particularly members for aerospace-related equipment, and a method for producing the same.

Background Art

[0002] In recent years, structural members constituting various devices are often required to be lightweight in order to prevent an increase in total weight. This is a very important requirement particularly for aerospace-related members. Since steel materials have a higher specific gravity than aluminum alloys and titanium alloys, their application range is limited. However, maraging steels having a specific strength comparable to that of aluminum alloys and titanium alloys are applied as aerospace-related members (for example, Non-Patent Document 1).

[0003] Among aerospace-related members, those used in regions close to the outside air may be exposed to low temperatures during operation. For example, an aircraft flying at an altitude of 10,000 m is said to be exposed to an environment of -50°C, and an artificial satellite orbiting the earth is exposed to an environment of -100°C. For structural members used in such low-temperature environments, it is essential to prevent major accidents by not undergoing low-temperature embrittlement. Austenitic stainless steels (specific strength 130) and nickel steels (same 100) are examples of steel materials that do not embrittle or have little embrittlement at low temperatures, but their specific strength is about half that of aluminum alloys and titanium alloys.

[0004] In addition, Patent Document 1 and Patent Document 2 disclose cast alloys having high strength and high toughness, but their specific strength is about half that of aluminum alloys and titanium alloys, and it is difficult to reduce the weight of cast products, so their application has not progressed.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Patent Document 2

Non-Patent Document

[0006]

Non-Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0007] An object of the present invention is to provide a high-strength and high-toughness iron-based alloy suitable for various structural parts and members, particularly members for aerospace-related equipment, which has a specific strength equivalent to that of maraging steel, can obtain higher toughness than maraging steel at -100°C, can suppress the material cost low, and has a high degree of freedom in shape, and a method for producing the same.

Means for Solving the Problems

[0008] The present inventors have made various studies for the purpose of obtaining an iron-based alloy that has a high degree of freedom in shape, has a specific strength equivalent to that of maraging steel, can obtain higher toughness at -100°C, and can suppress the material cost lower than that of maraging steel.

[0009] As a result, for example, by laminating and shaping powders having a specific composition with a lower content of expensive alloying elements compared to those disclosed in Patent Document 1 and Patent Document 2 to significantly refine the structure, it has been found that high specific strength and high toughness that could not be achieved conventionally can be obtained.

[0010] The present invention has been completed based on these findings and provides the following (1) to (7).

[0011] (1) By mass%, C: 0.12 to 0.3%, Si: 0.15 to 0.7%, Mn: 0.4 to 1.2%, P: 0.015% or less, S: 0.015% or less, Ni: 2 to 4%, Cr: 0.1 to 0.5%, Mo: 0.3 to 0.5% containing, the balance being Fe and inevitable impurities, and having a solidification structure with a dendrite secondary arm spacing of 5 μm or less, a high-strength and high-toughness iron-based alloy.

[0012] (2) By mass, further containing V: 0.05 to 0.15%, and the contents of C and V being by mass C × 0.3 ≤ V ≤ C × 0.6 satisfying the above, the high-strength and high-toughness iron-based alloy according to (1) above.

[0013] (3) By mass, further containing Ca: 0.01 to 0.05%, and the contents of Ca and S being by mass S × 2 ≤ Ca ≤ S × 4.5 satisfying the above, the high-strength and high-toughness iron-based alloy according to (1) or (2) above.

[0014] (4) The value of the carbon equivalent represented by C + Si / 24 + Mn / 6 + Ni / 40 + Mo / 4 + Cr / 5 + V / 14 is 0.6% or less by mass, the high-strength and high-toughness iron-based alloy according to any one of (1) to (3) above.

[0015] (5) Specific strength ≥ 200 kN·m / kg, impact absorption energy at -100°C ≥ 60 J (2 mm V-notch Charpy impact test specimen), simultaneously satisfying the above, the high-strength and high-toughness iron-based alloy according to any one of (1) to (4) above.

[0016] (6) A method for manufacturing a high-strength and high-toughness iron-based alloy, characterized by melting and solidifying an alloy material having the composition according to any one of (1) to (4) above by laser or electron beam for additive manufacturing And produce a high-strength and high-toughness iron-based alloy having a solidified structure with a dendrite secondary arm spacing of 5 μm or less this.

[0017] (7) The manufacturing method of the high-strength and high-toughness iron-based alloy according to (6) above, characterized in that the alloy material is powder.

Advantages of the Invention

[0018] According to the present invention, there is provided a high-strength and high-toughness iron-based alloy having a specific strength equivalent to that of maraging steel, having higher toughness than maraging steel at -100 °C, capable of suppressing the material cost low, and having a high degree of freedom in shape, suitable for various structural components and members, particularly members for aerospace-related equipment, and a manufacturing method thereof.

Brief Description of the Drawings

[0019]

Figure 1

Figure 2

Figure 3

Modes for Carrying Out the Invention

[0020] Hereinafter, the reasons for limiting the present invention will be described separately in terms of chemical composition and manufacturing conditions. In the following description, unless otherwise specified, the % representation in the components is mass %.

[0021] [Chemical Composition] C: 0.12 to 0.3% C is an element effective for improving strength and hardenability. However, if the content is less than 0.12%, a specific strength of 200 kN·m / kg or more cannot be obtained, and if it exceeds 0.3%, ductility and toughness decrease, and cracks are likely to occur during manufacturing. Therefore, the C content is in the range of 0.12 to 0.3%.

[0022] Si: 0.15 to 0.7% Silicon is an element added for the purpose of deoxidation. However, if the content is less than 0.15%, deoxidation is insufficient and voids are generated, and if the content exceeds 0.7%, ductility and toughness decrease. Therefore, the Si content is set to 0.15 to 0.7%.

[0023] Mn: 0.4-1.2% Mn is an element that is effective in improving strength and hardenability, and also has a deoxidizing effect. However, if its content is less than 0.4%, its effect is small, and if its content exceeds 1.2%, it reduces ductility and toughness and deteriorates weldability. Therefore, the Mn content is set to the range of 0.4 to 1.2%.

[0024] Ni: 2-4% Ni is the most important element in the present invention, which aims to simultaneously obtain high strength, high ductility, and high low-temperature toughness. Ni is an element that is effective for strength and hardenability, but unlike other elements with similar effects, it has little adverse effect on ductility, toughness, and weldability. Ni is also an element with a large austenite stabilizing effect, and by shifting the ferrite transformation region to the long-term side and expanding the bainite transformation region, ferrite precipitation is suppressed even in thick parts with slow cooling rates, and high strength is obtained. However, if Ni is less than 2%, a specific strength of 200 kN·m / kg or more cannot be obtained, and if it exceeds 4%, heat embrittlement becomes noticeable. Therefore, the Ni content is set to the range of 2 to 4%.

[0025] Cr: 0.1-0.5% Cr is an effective element for improving specific strength. However, if it is less than 0.1%, the effect is small, and if it exceeds 0.5%, the effect is not obtained and the carbon equivalent increases, deteriorating weldability. Therefore, the Cr content is set to the range of 0.1 to 0.5%.

[0026] Mo: 0.3-0.5% Mo is added to improve hardenability and suppress temper embrittlement. However, if it is less than 0.3%, the effect is small, and if it exceeds 0.5%, the effect cannot be obtained and the carbon equivalent increases, reducing weldability. Therefore, the Mo content is in the range of 0.3 - 0.5%.

[0027] P: 0.015% or less S: 0.015% or less P and S are elements that have a great influence on toughness. If each is contained in an amount exceeding 0.015%, the toughness of the base material will be significantly reduced. Therefore, the contents of P and S are set to 0.015% or less.

[0028] V: 0.05 - 0.15%, C×0.3 ≤ V ≤ C×0.6 V combines with C to form carbides and is an element effective for maintaining specific strength during temper heat treatment by enhancing the temper softening resistance. In addition to facilitating the management of temper heat treatment conditions, it can prevent a decrease in the mechanical properties of the heat-affected zone of welding, so it may be added as necessary. If it is less than 0.05%, the effect is insufficient, and if it exceeds 0.15%, ductility and toughness decrease. Therefore, when adding V, the V content is in the range of 0.05 - 0.15%. Also, even if V satisfies the above range, if it is less than C×0.3, sufficient specific strength cannot be obtained, and if it exceeds C×0.6, the effect of improving specific strength saturates, leading to an increase in material cost. Therefore, when adding V, it is in the range of C×0.3 ≤ V ≤ C×0.6.

[0029] Ca: 0.01 - 0.05%, S×2 ≤ Ca ≤ S×4.5 Ca combines with S to form a high-melting sulfide, preventing the formation of low-melting FeS and MnS at the grain boundaries and having the effect of improving ductility. Therefore, it may be added as needed. When the S content is 0.015% or less, if the Ca content is less than 0.01%, almost no such effect can be obtained. If it exceeds 0.05%, it becomes excessive relative to the S content, only resulting in an increase in material cost. Therefore, when adding Ca, the Ca content should be in the range of 0.01 - 0.05%. Also, it is preferable to adjust the Ca content according to the S content. When Ca < S×2, the effect of Ca is small. When Ca > S×4.5, the effect saturates and it causes an increase in material cost. Therefore, when adding Ca, the Ca content should be in the range of S×2 ≤ Ca ≤ S×4.5.

[0030] C + Si / 24 + Mn / 6 + Ni / 40 + Mo / 4 + Cr / 5 + V / 14 ≤ 0.6% The carbon equivalent represented by C + Si / 24 + Mn / 6 + Ni / 40 + Mo / 4 + Cr / 5 + V / 14 is a value for evaluating weldability. When the carbon equivalent exceeds 0.6%, the hardenability increases, so special heat management is required to prevent the occurrence of cold cracking and the reduction of ductility in the welded part. Therefore, the carbon equivalent is preferably 0.6% or less. Incidentally, if necessary, it is also possible to adjust the carbon equivalent to be equivalent to that of the steel type for welded structural cast steel products (JIS G5102) within the composition range of the present invention.

[0031] The balance of the alloy components having the above composition is Fe and unavoidable impurities.

[0032] [Solidification Structure] In the high specific-strength and high-toughness alloy according to the present invention, by refining the solidification structure of the alloy having the above composition so that the dendrite secondary arm spacing (hereinafter sometimes referred to as DAS) is 5 μm or less, it is possible to simultaneously obtain a specific strength of 200 kN·m / kg or more and an impact absorption energy of 60 J or more at -100°C.

[0033] The reason is considered to be that by rapidly solidifying an appropriate compositional alloy, in addition to "strengthening due to refinement of the structure" explained by the Hall-Petch relationship, improvement in toughness due to refinement of the microstructure and reduction of microsegregation of alloying elements has been achieved.

[0034] [Manufacturing Conditions] An alloy material having the above composition is melted and solidified by a laser or an electron beam to perform additive manufacturing. As a result, after the alloy material is melted, it is rapidly cooled, and a fine structure with a DAS interval of 5 μm or less can be obtained.

[0035] Specifically, alloy powder is prepared as an alloy material having a composition within the above range, and is melted and solidified by a laser or an electron beam to perform additive manufacturing. By using a laser or an electron beam, the cooling rate during solidification of the alloy can be made 3000 °C / sec or more, and an alloy with a fine solidified structure having a DAS interval of 5 μm or less can be obtained.

[0036] On the other hand, as is clear from FIG. 2 described later, in the case of the copper alloy mold casting method in which a high melting point iron-based alloy such as the alloy of the present invention can be industrially cast, it is impossible to make the DAS 5 μm or less. Also, in the die casting method for non-ferrous alloys, although the cooling rate is the highest during the casting process, even with this, the cooling rate is insufficient to make the DAS 5 μm or less, and it is impossible to obtain the desired properties.

Examples

[0037] Hereinafter, examples of the present invention will be described. Test specimens were prepared for the alloys having the chemical components and compositions shown in Table 1. Among Table 1, Nos. 1 to 15 are examples within the scope of the present invention, and Nos. 16 to 27 are comparative examples outside the scope of the present invention. The test specimens of Nos. 1 to 26 were prepared by additive manufacturing, and the test specimen of No. 27 was prepared by sand casting.

[0038] In the case of the specimens formed by additive manufacturing numbered from No. 1 to No. 26, first, an alloy with the chemical composition of No. 1 to No. 26 in Table 1 was melted in a high-frequency induction furnace. Using the atomizing device shown in Fig. 1, the molten metal at 1700 °C was dropped, and an inert gas (argon gas in this example) was sprayed from the nozzle to divide the liquid into droplets and rapidly solidify them, obtaining spherical powder as the raw material. Next, the spherical powder was sieved to obtain shaping powder with a particle size of 10 - 45 μm. Then, using a laser additive manufacturing apparatus, the shaping powder was additively manufactured under the conditions of an output of 300 W, a laser moving speed of 1000 mm / second, a laser scanning pitch of 0.1 mm, and a powder layer thickness of 0.04 mm, and specimens compliant with Fig. 1b) of JIS G0307 were produced.

[0039] In the case of the specimen formed by sand casting numbered No. 27, the alloy of No. 27 was melted in a high-frequency induction furnace and cast into a sand mold compliant with Fig. 1b) of JIS G0307 to produce the specimen.

[0040] Fig. 2 estimates the cooling rate of the specimen from the measured DAS by optical microscope observation of the sample of the present invention and the extrapolation line of the relationship between DAS and cooling rate described in the following Reference 1, and also shows the cooling rates of various molds obtained from the information in the following References 2 - 4. R=(DAS / 709) 1 / -0.386 ···(1) R: Cooling rate (°C / min.), DAS: Dendrite secondary arm spacing (μm) Reference 1: "Production Technology of Cast Steel", P378, Shaped Material Center Reference 2: "Casting", Vol. 63 (1991), No. 11, P915 Reference 3: "Casting Engineering", Vol. 68 (1996), No. 12, P1076 Reference 4: "Shaped Material", Vol. 54 (2013), No. 1, P13

[0041] After the specimens formed by additive manufacturing were separated from the shaping base plate by wire electrical discharge cutting, and after the specimens formed by sand casting had the mold sand removed, they were held at 880 °C for 1 hour and then water-cooled, and annealing treatment at 600 °C for 2 hours was performed.

[0042] From these test specimens, JIS-Z2241, No. 14A test pieces were prepared for tensile tests, and JIS-Z2242, 2mm V-notch test pieces were prepared for Charpy impact tests. The tensile tests were carried out at 20°C, and the Charpy impact tests were carried out at -100°C. The test results are shown in Table 2.

[0043] As shown in Table 2, in all of the inventive examples Nos. 1 to 15, the DAS became a fine structure of 5 μm or less, and they had a specific strength of 200 kN·m / kg or more and an impact absorption energy at -100°C of 60 J or more. Fig. 3 shows an example of a microstructural photograph of the inventive material.

[0044] On the other hand, among the comparative examples Nos. 16 to 27, Nos. 16 to 26 were manufactured by laminated modeling, so the value of DAS was 5 μm or less, but since the composition deviated from the scope of the present invention, either the specific strength or the impact absorption energy became a low value. Also, in No. 27, the composition range was within the scope of the present invention, but the value of DAS was extremely large, and both the specific strength and the impact absorption energy became low values.

[0045]

Table 1

[0046]

Table 2

Claims

1. By mass percentage, C: 0.12 to 0.3%, Si: 0.15 to 0.7%, Mn: 0.4 to 1.2%, P: 0.015% or less, S: 0.015% or less, Ni: 2 to 4%, Cr: 0.1 to 0.5%, Mo: 0.3 to 0.5% are contained, the balance consisting of Fe and inevitable impurities, and having a solidification structure with a dendrite secondary arm spacing of 5 μm or less. A high-strength and high-toughness iron-based alloy characterized by this.

2. By mass percentage, further containing V: 0.05 to 0.15%, and the contents of C and V are by mass percentage C × 0.3 ≤ V ≤ C × 0.6 The high-strength and high-toughness iron-based alloy according to Claim 1, characterized by satisfying this.

3. By mass percentage, further containing Ca: 0.01 to 0.05%, and the contents of Ca and S are by mass percentage S × 2 ≤ Ca ≤ S × 4.5 The high-strength and high-toughness iron-based alloy according to Claim 1 or Claim 2, characterized by satisfying this.

4. The value of the carbon equivalent represented by C + Si / 24 + Mn / 6 + Ni / 40 + Mo / 4 + Cr / 5 + V / 14 is 0.6% or less by mass percentage. The high-strength and high-toughness iron-based alloy according to any one of Claims 1 to 3, characterized by this.

5. Specific strength ≥ 200 kN·m / kg, impact absorption energy at -100°C ≥ 60 J (2 mm V-notch Charpy impact test specimen). The high-strength and high-toughness iron-based alloy according to any one of Claims 1 to 4, characterized by satisfying this simultaneously.

6. A method for manufacturing a high-strength and high-toughness iron-based alloy, characterized by melting and solidifying an alloy material having the composition according to any one of Claims 1 to 4 by laser or electron beam and performing layered manufacturing, and having a solidification structure with a dendrite secondary arm spacing of 5 μm or less.

7. The method for manufacturing a high-strength and high-toughness iron-based alloy according to Claim 6, characterized in that the alloy material is powder.

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