Iron-based alloy excellent in mechanical characteristics and method for producing same

Abstract

The present invention addresses the problem of providing an iron-based alloy which exhibits high strength even when an expensive alloy element is not added, and can achieve both high ductility and high strength.　This iron-based alloy contains, in mass%, 0.10-0.30% C, 0.1-1.0% Si, 0.2-2.0% Mn, no more than 0.030% P, and no more than 0.030% S, with the remainder comprising Fe and unavoidable impurities, and has a solidified structure in which the arithmetic average particle diameter of the crystals is 10 μm or less.　This iron-based alloy is an alloy for additive manufacturing and contains, in mass%, 0.10-0.30% C, 0.1-1.0% Si, 0.2-2.0% Mn, no more than 0.030% P, and no more than 0.030% S, with the remainder comprising Fe and unavoidable impurities.　This iron-based alloy has the mechanical characteristics of a 0.2% proof stress or a yield point of 700 MPa or more and an elongation of 20% or more.

Description

Iron-based alloy with excellent mechanical properties and manufacturing method thereof

[0001] The present invention relates to an iron-based alloy having excellent mechanical properties and a method for producing the same.

[0002] Iron-based materials are widely used in structural members that make up various types of equipment, and recently, iron-based materials for such structural members are required to have high strength in order to suppress the increase in total weight that accompanies the increase in size. In addition, they are also required to have high ductility and a high degree of freedom in shape. Such iron-based materials generally include rolled steel, forged steel, and cast steel (cast alloy), and among these, cast steel has an overwhelmingly superior degree of freedom in shape. Patent Documents 1 and 2 disclose high-strength, high-ductility cast alloys that have a tensile strength of about 1000 MPa and an elongation of about 12 to 16%, and that have an excellent degree of freedom in shape.

[0003] Furthermore, iron-based materials are sometimes required to have high toughness in addition to high strength and high ductility. While rolled steel, forged steel, and cast steel (cast alloy) are common examples of such iron-based materials, Patent Document 3 discloses an iron-based alloy with high strength and high ductility obtained by sintering powder with a carbon steel composition through harmonic structure control, and a method for manufacturing the same. Non-Patent Document 1 also describes a similar technique. Non-Patent Documents 2 and 3 describe high-strength iron-based alloys obtained by three-dimensional additive manufacturing of powder with a maraging steel composition.

[0004] JP 2008-007820 A JP 2014-181394 A JP 2020-029607 A

[0005] https: / / www.ritsumei.ac.jp / research / file / seeds / 22-13.pdf, Ritsumeikan University Research Seeds Collection: Creation of Steel Materials with Excellent Low-Temperature Brittleness Resistance https: / / www.tpc.toray / technology / additive-print / add_material / add_material_005.html, Toray Precision Co., Ltd., Metal 3D Printer Modeling Material "Maraging Steel" https: / / www.sanyo-steel.co.jp / technology / images / pdf / 25 / 25_12.pdf "Co-less Maraging Steel Powder for 3D Additive Manufacturing" (Sanyo Technical Report Vol. 25 (2018) No. 1)

[0006] However, although the casting alloys described in Patent Documents 1 and 2 are said to be capable of achieving both high strength and high ductility, alloy elements such as Ni and Cr are essential to ensure high strength, and depending on the application, ductility such as elongation and reduction of area may be insufficient relative to strength.

[0007] Furthermore, although the iron-based materials described in Patent Document 3 and Non-Patent Documents 1 to 3 are said to have high toughness in addition to high strength and high ductility, they have the following problems. That is, the iron-based alloy described in Patent Document 3 requires heat treatment such as mechanical milling, quenching, and tempering to achieve high strength. Furthermore, Patent Document 3 does not mention toughness in a Charpy impact test, but according to Figure 3 of Non-Patent Document 1, which is thought to be based on the same technology, the impact value at -60°C is approximately 13 J / cm. 2 The iron-based alloy of Non-Patent Document 2 requires the addition of alloying elements such as Ni, Co, and Mo and heat treatment to ensure high strength, and in some cases the elongation after heat treatment is less than 10%, which is not sufficient. The iron-based alloy of Non-Patent Document 3 requires the addition of alloying elements such as Ni and Mo and heat treatment to ensure high strength, and the Charpy impact value at room temperature after heat treatment is 30 J / cm. ２ Therefore, the toughness cannot be said to be high.

[0008] Therefore, an object of the present invention is to provide an iron-based alloy that exhibits high strength without the addition of expensive alloying elements, and that is capable of achieving both high ductility and high strength, and that has excellent mechanical properties, and a method for producing the same.

[0009] Another object of the present invention is to provide an iron-based alloy that exhibits high strength, high ductility, and high toughness without the addition of expensive alloying elements and has excellent mechanical properties, and a method for producing the same.

[0010] According to the present invention, the following means (1) to (14) are provided.

[0011] (1) An iron-based alloy having excellent mechanical properties, containing, by mass%, C: 0.10 to 0.30%, Si: 0.1 to 1.0%, Mn: 0.2 to 2.0%, P: 0.030% or less, S: 0.030% or less, with the balance being Fe and unavoidable impurities, and having a solidification structure in which the arithmetic mean grain size of the crystals is 10 μm or less.

[0012] (2) The iron-based alloy having excellent mechanical properties according to (1), further containing, by mass%, at least one of Ni: 0.1 to 3%, Cr: 0.1 to 1.0%, and Mo: 0.01 to 0.5%.

[0013] (3) An iron-based alloy having excellent mechanical properties according to (1) or (2), which has a 0.2% proof stress or yield point of 700 MPa or more and an elongation of 20% or more.

[0014] (4) An iron-based alloy having excellent mechanical properties, which is an additive manufacturing alloy containing, by mass%, C: 0.10 to 0.30%, Si: 0.1 to 1.0%, Mn: 0.2 to 2.0%, P: 0.030% or less, S: 0.030% or less, with the balance being Fe and unavoidable impurities.

[0015] (5) An iron-based alloy having excellent mechanical properties according to (4), further containing, by mass%, at least one of Ni: 0.1 to 3%, Cr: 0.1 to 1.0%, and Mo: 0.01 to 0.5%.

[0016] (6) A method for producing an iron-based alloy having excellent mechanical properties, in which an alloy material having the composition described in (1) or (2) above is melted and solidified by a laser or an electron beam to be layer-by-layer manufactured.

[0017] (7) The method for producing an iron-based alloy having excellent mechanical properties according to (6), in which the alloy is left as it is after additive manufacturing.

[0018] (8) An iron-based alloy with excellent mechanical properties, containing, by mass%, C: 0.10 to 0.30%, Si: 0.1 to 0.7%, Mn: 0.2 to 1.0%, P: 0.030% or less, S: 0.030% or less, with the balance being Fe and unavoidable impurities, having a solidification structure with an arithmetic mean grain size of crystals of 10 μm or less, a tensile strength of 800 MPa or more, an elongation of 15% or more, and an absorbed energy of 100 J or more in a Charpy impact test at -60°C.

[0019] (9) An iron-based alloy having excellent mechanical properties according to (8), further containing, by mass%, at least one of Ni: 0.1 to 3%, Cr: 0.1 to 1.0%, and Mo: 0.01 to 0.5%.

[0020] (10) An iron-based alloy having excellent mechanical properties according to (8) or (9), which has a reduction of area of ​​60% or more.

[0021] (11) The iron-based alloy having excellent mechanical properties according to (8) or (9), which is an additive manufacturing material.

[0022] (12) A method for producing an iron-based alloy having excellent mechanical properties, in which an alloy material having the composition described in (8) or (9) above is melted and solidified by a laser or an electron beam to be layer-by-layer manufactured.

[0023] (13) The method for producing an iron-based alloy having excellent mechanical properties according to (12), in which the alloy is left as it is after additive manufacturing.

[0024] (14) A method for producing an iron-based alloy having excellent mechanical properties according to (12), wherein after additive manufacturing, the alloy is heated at a temperature of 500°C or less.

[0025] According to one aspect of the present invention, there is provided an iron-based alloy having excellent mechanical properties, which exhibits high strength without the addition of expensive alloying elements and can achieve both high ductility and high strength, and a method for producing the same. Furthermore, by using the alloy as an additive manufacturing material, the alloy has excellent shape freedom, similar to cast steel.

[0026] According to another aspect of the present invention, there is provided an iron-based alloy having excellent mechanical properties, which can combine high strength with high ductility and high toughness without the addition of expensive alloying elements or special heat treatment, and a method for producing the same.

[0027] 1 is a conceptual diagram showing the relationship between the carbon content of steel and hardness, tensile strength, and elongation. It is an experimental value showing the relationship between the grain size of steel and strength (0.2% proof stress) and hardness. It is a diagram showing the relationship between the grain size of steel and dislocation density. It is a diagram showing the relationship between the dislocation density and yield strength of steel. It is a conceptual diagram showing the relationship between the carbon content of steel and mechanical properties. It is a microstructure photograph of Sample No. 1, which is an As-print material in the first example. It is a microstructure photograph of Sample No. 5 in the first example after quenching. It is a microstructure photograph of Sample No. 5 in the first example after quenching and tempering. It is a microstructure photograph of Sample No. 12 (As-print material), which is an example of the present invention in the second example. It is a microstructure photograph of Sample No. 17 (normalized and tempered material), which is a comparative example in the second example.

[0028] Hereinafter, an embodiment of the present invention will be described with reference to the accompanying drawings.

[0029] <Overview> First, a first aspect will be described. The present inventors have conducted various studies with the aim of obtaining an iron-based alloy that exhibits high strength without the addition of expensive alloying elements and that can achieve both high ductility and high strength.

[0030] As a result, it was discovered that by additively manufacturing powder of a low-carbon iron-based alloy that does not contain expensive alloying elements such as those added to the high-strength cast alloys of Patent Documents 1 and 2, for example, an iron-based alloy with a composition equivalent to S20C, a type of carbon steel for mechanical structures, to thereby refine the structure, it is possible to obtain a high-strength iron-based alloy that has a high elongation of 20% or more and a 0.2% proof stress or yield point of 700 MPa or more, something that could not be achieved by conventional methods.

[0031] Based on this finding, in a first aspect, there is provided an iron-based alloy having excellent mechanical properties, which contains, by mass%, C: 0.10 to 0.30%, Si: 0.1 to 1.0%, Mn: 0.2 to 2.0%, P: 0.030% or less, S: 0.030% or less, with the balance being Fe and unavoidable impurities, and which has a solidification structure in which the arithmetic mean grain size of the crystals is 10 μm or less.

[0032] It has been discovered that such a microstructure can be obtained, particularly in the as-printed state after additive manufacturing, and that the above-mentioned excellent properties can be obtained. It is common technical knowledge that structural members manufactured from iron-based alloys are tempered after hardening treatments such as quenching or normalizing to impart appropriate ductility. However, it is a new and unprecedented finding that high strength and high ductility can be achieved by using a low-carbon iron-based alloy that does not contain expensive alloying elements in the as-printed state without heat treatment. Furthermore, additive manufacturing materials, like cast steel materials, have excellent shape freedom.

[0033] Additive manufacturing allows the cooling rate during solidification of the alloy to be 3000°C / sec or more, which is unthinkable with conventional casting, thereby enabling the microstructure to be refined. It is believed that by refining the structure of the alloy with the above composition in this way, high strength can be obtained while maintaining high ductility. It has also been found that additive manufacturing of the alloy with the above composition can actually achieve higher strength than the strengths previously reported in the literature due to the refined structure. This is presumably because additive manufacturing not only refines the structure but also increases the dislocation density, or because the dislocation density is equivalent to that previously reported but the high strength is due to other physical properties.

[0034] Next, the second aspect will be described. The present inventors have conducted extensive research with the aim of obtaining an iron-based alloy that is excellent in mechanical properties, exhibiting high strength, high ductility, and high toughness without the addition of expensive alloying elements.

[0035] As a result, it was discovered that, without relying on the addition of expensive alloying elements as added to the high-strength alloys of Non-Patent Documents 2 and 3, and without the mechanical milling or heat treatment required in the techniques of Patent Document 3, Non-Patent Documents 2, and 3, it is possible to obtain an iron-based alloy having high strength and high ductility, with a tensile strength of 800 MPa or more and an elongation of 15% or more, and high toughness (low transition temperature), with an absorbed energy of 100 J or more in a Charpy impact test at -60°C, something that could not be achieved by conventional methods, by additive manufacturing (three-dimensional additive manufacturing) of powder of an iron-based alloy having a low-carbon iron-based alloy composition, for example, an iron-based alloy having a composition equivalent to S20C, which is a carbon steel material for mechanical structures, to thereby refine the structure.

[0036] Based on this finding, in a second aspect, there is provided an iron-based alloy having excellent mechanical properties, which contains, by mass%, C: 0.10 to 0.30%, Si: 0.1 to 0.7%, Mn: 0.2 to 1.0%, P: 0.030% or less, S: 0.030% or less, with the balance being Fe and unavoidable impurities, and which has a solidification structure in which the arithmetic mean grain size of the crystals is 10 μm or less, and which satisfies the following: a tensile strength of 800 MPa or more, an elongation of 15% or more, and an absorbed energy of 100 J or more in a Charpy impact test at −60° C.

[0037] In particular, it has been discovered that superior mechanical properties can be obtained by using the as-additive manufacturing (as-print) method. It is common technical knowledge that high-strength structural members manufactured from iron-based alloys undergo some kind of heat treatment, for example, hardening or normalizing, or aging treatment, followed by tempering as necessary to impart appropriate ductility. However, it is a new and previously undiscovered finding that superior mechanical properties can be obtained by using a low-carbon iron-based alloy that does not contain expensive alloying elements as-additive manufacturing (as-print) without heat treatment (except for stress relief).

[0038] Furthermore, from the perspective of the SDGs, manufacturing methods for components are required to have a high degree of freedom in terms of shape in order to enable resource and energy conservation, shorten construction periods, etc., and additive manufacturing materials offer excellent freedom in terms of shape.

[0039] As described above, additive manufacturing allows the cooling rate during solidification of the alloy to be 3000°C / sec or more, which is impossible to achieve with general manufacturing methods such as casting, thereby refining the microstructure and achieving high strength. Non-Patent Document 2 also uses additive manufacturing, which may have refined the microstructure, but in the present invention, unlike Non-Patent Document 2, alloy elements such as Ni, Mo, and Co are not added, or if added, are limited to a level that does not affect ductility or toughness. It is only by adjusting the composition in this way that the high strength, high ductility, and high toughness described above are achieved.

[0040] While literature has previously shown that the strength of iron-based alloy materials increases with the refinement of their structure, in the second aspect, it has been newly discovered that additive manufacturing of an alloy with the above composition can achieve a strength higher than that achieved by the refinement actually achieved in the literature, without the reduction in ductility or toughness that accompanies high strength. It has also been discovered that additive manufacturing of an alloy with the above composition results in a dislocation density that is 10 times higher than that of materials produced by conventional methods. From this, it is believed that the high strength, high ductility, and high toughness properties are achieved by the refinement of the structure and the increased dislocation density achieved by additive manufacturing.

[0041] The first embodiment corresponds to the first aspect described above, and the reasons for limitations will be explained below, dividing them into chemical components, structure, and manufacturing conditions. In the following explanation, percentages for components are mass % unless otherwise specified.

[0042] [Chemical Composition] C: 0.10 to 0.30% C is an element effective in improving strength. However, if the C content is less than 0.10%, sufficient strength cannot be obtained, and if it exceeds 0.30%, ductility decreases. Therefore, the C content is set in the range of 0.10 to 0.30%.

[0043] Si: 0.1 to 1.0% Si is an element added for the purpose of deoxidation. However, if the content is less than 0.1%, deoxidation is insufficient, and if it exceeds 1.0%, ductility decreases. Therefore, the Si content is set to 0.1 to 1.0%.

[0044] Mn: 0.2 to 2.0% Mn is an element effective in improving strength and also has a deoxidizing effect. However, if the Mn content is less than 0.2%, this effect is small, and if it exceeds 2.0%, ductility decreases. Therefore, the Mn content is set to the range of 0.2 to 2.0%.

[0045] P: 0.030% or less S: 0.030% or less P and S are elements that have a significant effect on toughness. If the content of each exceeds 0.030%, toughness will decrease significantly. Therefore, the P and S contents are set to 0.030% or less.

[0046] At least one of Ni: 0.1 to 3%, Cr: 0.1 to 1.0%, and Mo: 0.01 to 0.5% Although Ni, Cr, and Mo are elements that reduce ductility, they may be added in amounts sufficient to maintain the desired ductility, as described below.

[0047] Ni is an element that has a strong austenite stabilizing effect and contributes to high strength, so it may be added as needed. Furthermore, Ni has a smaller adverse effect on ductility compared to other elements, so it can be added in relatively large amounts. However, if Ni is less than 0.1%, the effect of increasing strength is small, and if it is 3% or more, the adverse effect on ductility becomes significant. Therefore, if Ni is added, its content should be in the range of 0.1 to 3%.

[0048] Cr is an element that is effective in increasing strength, so it may be added as needed. However, if it is less than 0.1%, the effect is small, and if it exceeds 1.0%, it has a significant adverse effect on ductility. Therefore, if Cr is added, its content should be in the range of 0.1 to 1.0%.

[0049] Mo is an element that improves hardenability and suppresses temper embrittlement, and may be added as needed during heat treatment. However, if it is less than 0.01%, the effect is small, and if it exceeds 0.5%, the adverse effect on ductility increases. Therefore, if Mo is added, its content should be in the range of 0.01 to 0.5%.

[0050] The balance of the alloy components having the above composition is Fe and inevitable impurities.

[0051] [Solidification structure] In this embodiment, the solidification structure of the alloy having the above composition is refined so that the arithmetic mean grain size of the crystals is 10 μm or less. By refining the solidification structure of the alloy having the above composition in this way, a high-strength iron-based alloy having a 0.2% proof stress or yield point of 700 MPa or more while having a high elongation of 20% or more can be obtained.

[0052] According to conventional knowledge, as shown in Figure 1, the hardness of steel increases with carbon content, and strength is proportional to hardness, but elongation decreases with increasing carbon content. Therefore, with a composition with a carbon content of 0.1 to 0.3%, as described above, relatively high elongation can be achieved, but high strength is difficult to achieve. In fact, steel with an S20C composition and a carbon content of approximately 0.2% can achieve elongation of 20% or more, but strength is 400 MPa or less (0.2% yield strength: approximately 300 MPa). In contrast, in this embodiment, an alloy with a composition equivalent to S20C is rapidly solidified during additive manufacturing, resulting in a fine structure with a grain size of 10 μm or less. This achieves both high elongation and high strength through "high strength due to microstructural refinement," as explained by the Hall-Petch relationship.

[0053] Figure 2 shows experimental values ​​showing the relationship between the grain size of steel and its strength (0.2% yield strength) and hardness (Source: Yuji Kimura et al., Material: Bulletin of the Japan Institute of Metals 36 (11), 1062-, 1997-11; "Pursuit of Strengthening by Grain Refinement"). According to this figure, to achieve a 0.2% yield strength of 0.7 GPa (700 MPa) or more, for example, approximately 0.9 GPa (900 MPa), the grain size must be refined to approximately 0.5 μm. However, in the case of rapid solidification using additive manufacturing, a 0.2% yield strength of approximately 900 MPa can be achieved even with a grain size of approximately 5 μm. Dislocation density may be related to this point. Previous knowledge suggests that, as shown in Figure 3, in an iron-based alloy with a grain size of 5 μm, the dislocation density is 3-4 × 10 １４As shown in Figure 4, the yield strength (0.2% proof stress) in this case is 0.4 GPa (400 MPa) (Source: Tanaka et al., Iron and Steel: Vol. 104 (2018) No. 5: "Effect of grain size on the yield stress of cold-worked iron"). In this study, a 0.2% proof stress of about 900 MPa was obtained even with a grain size of about 5 μm because the dislocation density was higher than previously known (4 × 10 １４ / m ２ Alternatively, it is possible that the dislocation density is the same as previously reported, but the strength has increased due to changes in other physical properties.

[0054] [Manufacturing conditions] The alloy material having the above composition is melted and solidified by a laser or electron beam to form an additive manufacturing process. After the alloy material is melted, it is rapidly cooled to form a fine structure with a crystal grain size of 10 μm or less.

[0055] Specifically, an alloy powder having a composition within the above range is prepared as an alloy material, and then melted and solidified using a laser or electron beam for additive manufacturing. Using a laser or electron beam allows the cooling rate during solidification of the alloy to be 3000°C / sec or higher, resulting in an alloy with a fine solidification structure with a crystal grain size of 10 μm or less. In this case, as described above, additive manufacturing achieves a strength higher than the relationship between crystal grain size refinement and strength as previously known. Therefore, it is presumed that additive manufacturing not only refines the structure through rapid solidification, but also increases dislocation density or causes other physical property changes. Since heat treatment after additive manufacturing may increase the crystal grain size and reduce strength, it is preferable to leave the additive manufacturing process as is. However, heat treatment that does not affect the crystal grain size, such as stress relief treatment at temperatures of approximately 150 to 250°C, is permissible.

[0056] Second Embodiment The second embodiment corresponds to the second aspect described above, and as in the first embodiment, the reasons for limitations will be explained below, dividing them into chemical components, structure, and manufacturing conditions. Note that, in the explanation of this embodiment, percentages for components are by mass unless otherwise specified.

[0057] [Chemical Composition] C: 0.10 to 0.30% C is an element effective in improving strength. However, if the C content is less than 0.10%, sufficient strength cannot be obtained, and if it exceeds 0.30%, ductility and toughness decrease. Therefore, the C content is set to the range of 0.10 to 0.30%.

[0058] Si: 0.1 to 0.7% Si is an element added for the purpose of deoxidation. However, if the content is less than 0.1%, deoxidation is insufficient, and if it exceeds 0.7%, ductility and toughness decrease. Therefore, the Si content is set to 0.1 to 0.7%.

[0059] Mn: 0.2 to 1.0% Mn is an element effective in improving strength and also has a deoxidizing effect. However, if the Mn content is less than 0.2%, this effect is small, and if it exceeds 1.0%, ductility decreases. Therefore, the Mn content is set to the range of 0.2 to 1.0%.

[0060] P: 0.030% or less S: 0.030% or less P and S are elements that have a significant effect on toughness. If the content of each exceeds 0.030%, toughness will decrease significantly. Therefore, the P and S contents are set to 0.030% or less.

[0061] At least one of Ni: 0.1 to 3%, Cr: 0.1 to 1.0%, and Mo: 0.01 to 0.5% Ni, Cr, and Mo are elements that increase strength and decrease ductility, but because they can improve predetermined properties as described below, they may be added in an amount sufficient to maintain the desired ductility.

[0062] Ni is an element that has a strong austenite stabilizing effect and contributes to increasing strength and low-temperature toughness, so it may be added as needed. Furthermore, Ni has a smaller adverse effect on ductility than other elements, so it can be added in relatively large amounts. However, if Ni is less than 0.1%, the effect of increasing strength is small, and if it is 3% or more, the effect of increasing strength saturates. Therefore, if Ni is added, its content should be in the range of 0.1 to 3%.

[0063] Cr is an element that is effective in increasing strength, so it may be added as needed. However, if it is less than 0.1%, the effect is small, and if it exceeds 1.0%, it has a significant adverse effect on ductility and toughness. Therefore, if Cr is added, its content should be in the range of 0.1 to 1.0%.

[0064] Mo is an element that improves hardenability and suppresses temper embrittlement, and may be added as needed during heat treatment. However, if it is less than 0.01%, the effect is small, and if it exceeds 0.5%, the adverse effect on ductility increases. Therefore, if Mo is added, its content should be in the range of 0.01 to 0.5%.

[0065] The balance of the alloy components having the above composition is Fe and inevitable impurities.

[0066] [Solidification structure] In this embodiment, the structure is refined so that the arithmetic mean grain size of the crystals is 10 μm or less. By performing such refinement of the structure by additive manufacturing, high strength of tensile strength of 800 MPa or more, high ductility of elongation of 15% or more, and high toughness (low transition temperature) of absorbed energy of 100 J or more in a Charpy impact test at −60°C can be obtained. Furthermore, with regard to ductility, a reduction of area of ​​60% or more can be achieved.

[0067] According to conventional knowledge, as shown in Figure 5, the strength of steel increases with carbon content, while elongation, reduction of area, and absorbed energy (toughness) decrease with carbon content. Therefore, it is difficult to achieve high strength, high ductility, and high toughness with a composition containing 0.10 to 0.30% C. On the other hand, literature has previously shown that strength increases with microstructural refinement, and in this embodiment, AM also refines the microstructural structure to a level not achievable with conventional processes. This embodiment is based on the experimental finding that when the microstructural refinement is achieved by AM of an alloy with the above composition, not only is a strength higher than that achieved by the microstructural refinement in the literature actually obtained, but the dislocation density is more than 10 times higher than that of materials produced by conventional methods. In other words, the high strength, high ductility, and high toughness properties achieved in this embodiment are believed to be due to the microstructural refinement and increased dislocation density achieved by AM. In particular, the high toughness (low transition temperature) is believed to be due to the increased dislocation density achieved by AM, which effectively suppresses crack propagation at fracture.

[0068] [Manufacturing conditions] The alloy material having the above composition is melted and solidified by a laser or electron beam to form an additive manufacturing process, as in the first embodiment. As a result, the alloy material is melted and then rapidly cooled, resulting in a fine structure with an arithmetic mean grain size of 10 μm or less.

[0069] Specifically, as in the first embodiment, an alloy powder having a composition within the above range is prepared as an alloy material, and then melted and solidified using a laser or electron beam for additive manufacturing. Using a laser or electron beam allows the cooling rate during solidification of the alloy to be 3000°C / sec or higher, resulting in an alloy with a fine solidification structure with an arithmetic mean grain size of 10 μm or less. It is also believed that this rapid solidification not only results in a finer structure but also in a high dislocation density. In fact, while Asprint (as-additively manufactured) materials exhibit high dislocation density and high toughness, heat treatment after additive manufacturing has been observed to result in a decrease in dislocation density and a rapid decrease in toughness. From this, it is believed that the high toughness of additively manufactured materials is due to their high dislocation density. For this reason, as-printed (as-additively manufactured) material has the highest dislocation density and is therefore preferred, but if heat treatment is carried out for the purpose of removing distortion or adjusting strength at a temperature of 500°C or less, for example, around 300 to 500°C, the reduction in dislocation density is limited and there is almost no reduction in toughness, so this is acceptable.

[0070] Examples of the present invention will be described below. <First Example> Raw powder of carbon steel corresponding to S20C, a type of carbon steel for machine structures specified in JIS G 4051, was prepared. Table 1 shows the chemical component standards of S20C and the composition of the raw powder.

[0071] The raw material powder was granulated by gas atomization and sieved to obtain spherical powder of -63 μm. The resulting powder was used as the raw material for modeling. Then, a laser-type layered modeling device was used with an output of 300 W and an energy density of 90 J / mm. ３The building materials were additively manufactured under the conditions of a thickness per layer of 40 μm to produce 3D-built objects (Φ15 × 100 mm). As shown in Table 2, the 0.2% yield strength, tensile strength, elongation, and reduction of area of ​​these 3D-built objects were measured for the as-built (as-print) sample (No. 1), samples that underwent normalizing and tempering as heat treatment (No. 2-4), and sample that underwent conventional quenching and tempering as heat treatment (No. 5). These results are also shown in Table 2. Figure 6 shows a microstructure photograph of No. 1, which is an as-print material, while Figure 7 and Figure 8 show microstructure photographs of No. 5 after quenching and tempering, respectively.

[0072] As shown in Figure 6, Sample No. 1, an as-printed material, had a fine structure with an arithmetic mean grain size of approximately 5 μm. It possessed unprecedented mechanical properties, combining high ductility (elongation of 22% and reduction of area of ​​82%) with high strength (0.2% proof stress and tensile strength) of nearly 1 GPa. In particular, the reduction of area was higher than that of the heat-treated material described below. On the other hand, as shown in Figures 7 and 8, the heat treatment of the heat-treated material coarsened the arithmetic mean grain size of the crystals to approximately 20 μm. Furthermore, as shown in Table 2, Samples No. 2 to 5, which were heat-treated materials, exhibited high elongation and very large reduction of area regardless of the heat treatment conditions. However, their 0.2% proof stress was 299 to 355 MPa and their tensile strength was 439 to 475 MPa, approximately half or less than that of the as-printed material No. 1.

[0073]

[0074]

[0075] Second Example Alloys having chemical compositions Nos. 11 to 16 shown in Table 3 were granulated by gas atomization, and the resulting sieved spherical powder of -63 μm was used as the raw material for modeling. Then, using a laser-type additive manufacturing device, output: 300 W, energy density: 90 J / mm ３The raw materials for fabrication were additively fabricated under the conditions of a thickness per layer of 40 μm to produce a 3D-printed object (Φ15 × 100 mm). This 3D-printed object was then either printed as is (heat treatment number 1 in Table 3), stress-relieved (heat treatment numbers 2 to 4 in Table 3), or normalized and tempered (heat treatment number 5 in Table 3) to produce samples. Samples 11 to 16 are examples of the present invention within the scope of the second aspect (second embodiment) of the present invention, while samples 17 to 19 are comparative examples outside the scope of the second aspect (second embodiment) of the present invention. Sample 20 is a commercially available S20C material, a type of carbon steel for machine structures (JIS G4051), and serves as a reference example. Samples 13 to 15 and 17 have the same chemical composition as sample 12 but were subjected to different heat treatments.

[0076] These samples were measured for mechanical properties (0.2% yield strength, tensile strength, elongation, reduction of area, and absorbed energy in a Charpy impact test). The results are shown in Table 4. As shown in Table 4, As-print samples Nos. 11, 12, and 16, which are samples having the composition of the present invention, all satisfied the following requirements: 0.2% yield strength of 700 MPa or more, tensile strength of 800 MPa or more, elongation of 15% or more, and absorbed energy in a Charpy impact test at -60°C of 100 J or more. In particular, samples Nos. 11 and 12 exhibited extremely high absorbed energy in a Charpy impact test at -60°C, exceeding 200 J, the same as the room temperature value. Furthermore, as described below, the arithmetic mean grain size of the crystals was also 10 μm or less. Samples Nos. 13 to 15, which were obtained by heating a shaped article having the composition of No. 12 to a temperature of 500°C or less to adjust the strength, exhibited the same absorbed energy as As-print samples No. 11 and 12. The same elongation, contraction and absorbed energy values ​​as those of 12 were obtained.

[0077] In contrast, the comparative samples Nos. 17 to 19 did not satisfy the ranges specified in the present invention in terms of tensile strength, elongation, absorbed energy in a Charpy impact test at -60°C, and / or crystal grain size. Sample No. 17 was a molded product having the same chemical composition as No. 12, which was normalized by heating to 920°C and air-cooling, followed by tempering at 500°C. As described below, the crystal grain size was coarsened, and although it exhibited high elongation, its 0.2% yield strength was 322 MPa and its tensile strength was 479 MPa, less than half that of Sample No. 12, which is an Asprint material. Furthermore, Sample No. 17, which is a heat-treated material, exhibited high toughness, with an absorbed energy of 125 J at room temperature in a Charpy impact test, but was significantly embrittled at -60°C, with an absorbed energy of less than 10 J. Furthermore, Sample No. 18 had low tensile strength due to its C content of less than 0.10%, and Sample No. Sample No. 19 had a C content exceeding 0.30%, and therefore exhibited low values ​​for elongation and absorbed energy in a Charpy impact test at -60°C. Sample No. 20, which is a reference example (a commercially available S20C material), exhibited a low tensile strength of 470 MPa and a low absorbed energy of 22 J in a Charpy impact test at -60°C.

[0078] The microstructures of the samples were also observed, and dislocation density and crystallite size were measured. Dislocation density was determined from XRD diffraction peaks in six directions ((110), (200), (211), (220), (310), and (222)). Crystallite size was measured using XRD. Note that crystallite size is the smallest unit of a crystal grain that can be considered a single crystal, and was measured to accurately evaluate grain size. Figure 9 shows a microstructure photograph of Sample No. 12 (As-print material), an example of the present invention, and Figure 10 shows a microstructure photograph of Sample No. 17 (normalized and tempered material), a comparative example. Table 5 shows the dislocation density and crystallite size of these samples.

[0079] Sample No. 12 (As-print material), which is an example of the present invention, has a crystallite size of 114 nm (corresponding to an arithmetic mean grain size of 10 μm or less), a fine structure, and a dislocation density of 8.4 × 10 １４ / m ２It was confirmed that the dislocation density of a cast block of SCW480, a general cast steel for welded structures having a C content equivalent to that of the present invention, was 4.2 × 10 １３ / m ２ Therefore, it was confirmed that Sample No. 12, an example of the present invention, had a dislocation density 10 times or more that of the conventional material. Similar structures and dislocation densities were also obtained for Samples Nos. 11 and 13 to 16.

[0080] On the other hand, Sample No. 17 (normalized and tempered material), which is a comparative example, has the same chemical composition as Sample No. 12, but the crystallite size is coarsened to 435 nm (corresponding to an arithmetic mean grain size of more than 10 μm) by heat treatment, and the dislocation density is 1.6 × 10 １３ / m ２ This was equivalent to that of conventional materials.

[0081] From the above results, it was confirmed that the alloy having the chemical composition of the second aspect (second embodiment) and obtained by additive manufacturing not only has high strength due to fine graining, but also has a high dislocation density, thereby achieving high strength, high ductility, and high toughness.

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Claims

1. An iron-based alloy excellent in mechanical properties, containing, by mass%, C: 0.10 to 0.30%, Si: 0.1 to 1.0%, Mn: 0.2 to 2.0%, P: 0.030% or less, S: 0.030% or less, with the balance being composed of Fe and inevitable impurities, and having a solidified structure with an arithmetic mean crystal grain size of 10 μm or less.

2. The iron-based alloy excellent in mechanical properties according to claim 1, further containing at least one of Ni: 0.1 to 3%, Cr: 0.1 to 1.0%, and Mo: 0.01 to 0.5% by mass%.

3. The iron-based alloy excellent in mechanical properties according to claim 1 or 2, having a 0.2% proof stress or yield point of 700 MPa or more and an elongation of 20% or more.

4. An iron-based alloy excellent in mechanical properties, which is a laminated molding alloy containing, by mass%, C: 0.10 to 0.30%, Si: 0.1 to 1.0%, Mn: 0.2 to 2.0%, P: 0.030% or less, S: 0.030% or less, with the balance being composed of Fe and inevitable impurities.

5. The iron-based alloy excellent in mechanical properties according to claim 4, further containing at least one of Ni: 0.1 to 3%, Cr: 0.1 to 1.0%, and Mo: 0.01 to 0.5% by mass%.

6. A method for producing an iron-based alloy excellent in mechanical properties, wherein an alloy material having the composition according to claim 1 or 2 is melted and solidified by laser or electron beam to perform laminated molding.

7. The method for producing an iron-based alloy excellent in mechanical properties according to claim 6, wherein the laminated molding is left as it is.

8. An iron-based alloy excellent in mechanical properties, containing, by mass%, C: 0.10 to 0.30%, Si: 0.1 to 0.7%, Mn: 0.2 to 1.0%, P: 0.030% or less, S: 0.030% or less, with the balance being composed of Fe and inevitable impurities, having a solidified structure with an arithmetic mean crystal grain size of 10 μm or less, having a tensile strength of 800 MPa or more, an elongation of 15% or more, and an absorbed energy in a Charpy impact test at -60°C of 100 J or more.

9. The iron-based alloy excellent in mechanical properties according to claim 8, further containing at least one of Ni: 0.1 to 3%, Cr: 0.1 to 1.0%, and Mo: 0.01 to 0.5% by mass%.

10. The iron-based alloy excellent in mechanical properties according to claim 8 or 9, having a reduction of area of 60% or more.

11. An iron-based alloy with excellent mechanical properties according to claim 8 or claim 9, which is a laminated formed material.

12. A method for manufacturing an iron-based alloy with excellent mechanical properties, comprising melting and solidifying an alloy material having the composition according to claim 8 or claim 9 by laser or electron beam and performing laminated forming.

13. A method for manufacturing an iron-based alloy with excellent mechanical properties according to claim 12, which leaves the laminated formed state as it is.

14. A method for manufacturing an iron-based alloy with excellent mechanical properties according to claim 12, which heats at a temperature of 500 °C or lower after laminated forming.

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