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

Abstract

The present invention provides an iron-based alloy that exhibits high strength without the addition of expensive alloying elements and can achieve both high ductility and high strength. The iron-based alloy of the present invention 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 inevitable impurities, and has a solidification structure with an arithmetic mean grain size of 10 μm or less. The iron-based alloy of the present invention is a laminated molding alloy that 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 inevitable impurities. The iron-based alloy of the present invention has mechanical properties with a 0.2% proof stress or yield point of 700 MPa or more and an elongation of 20% or more.

Description

Technical Field

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

Background Art

[0002] Iron-based materials are widely used as structural members constituting various devices. Recently, such iron-based materials for structural members are required to be high-strength from the viewpoint of suppressing the increase in total weight accompanying the increase in size. In addition to this, high ductility and a large degree of freedom in shape are also required. As such iron-based materials, there are generally rolled steel materials, forged steel materials, and cast steel materials (cast alloys). Among these, cast steel materials are overwhelmingly excellent in the degree of freedom in shape. Patent Document 1 and Patent Document 2 disclose high-strength and high-ductility cast alloys having a tensile strength of about 1000 MPa and an elongation of about 12 to 16% and excellent in the degree of freedom in shape.

[0003] In addition, iron-based materials may also be required to have high toughness in addition to high strength and high ductility. As such iron-based materials, the above-described rolled steel materials, forged steel materials, and cast steel materials (cast alloys) are common. Patent Document 3 discloses an iron-based alloy obtained by sintering a powder of a carbon steel composition and having high strength and high ductility by controlling a harmonious structure and a method for producing the same. Similar techniques are also described in Non-Patent Document 1. Non-Patent Document 2 and Non-Patent Document 3 describe high-strength iron-based alloys obtained by three-dimensionally laminated manufacturing of a powder of a maraging steel composition.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

Patent Document 3

Non-Patent Documents

[0005] [Non-Patent Document 1] 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 [Non-Patent Document 2] https: / / www.tpc.toray / technology / additive-print / add_material / add_material_005.html, Toray Precision Co., Ltd., Metal 3D Printer Forming Material "Maraging Steel" [Non-Patent Document 3] https: / / www.sanyo-steel.co.jp / technology / images / pdf / 25 / 25_12.pdf "Co-Free Maraging Steel Powder for 3D Laminated Forming" (Sanyo Technical Report Vol. 25 (2018) No. 1) [Summary of the Invention] [Problems to be Solved by the Invention]

[0006] However, although the casting alloys described in Patent Document 1 and Patent Document 2 are supposed to achieve both high strength and high ductility, alloying elements such as Ni and Cr are essential to ensure high strength, and depending on the application, the ductility such as elongation and drawing with respect to strength may be insufficient.

[0007] In addition, although the iron-based materials described in Patent Document 3 and Non-Patent Documents 1 to 3 are supposed to have high toughness in addition to high strength and high ductility, they have the following problems. That is, in order to obtain high strength, the iron-based alloy described in Patent Document 3 requires mechanical milling, quenching, and tempering heat treatments. Also, although Patent Document 3 does not describe toughness in the Charpy impact test, according to Figure 3 of Non-Patent Document 1, which is considered to be the same technology, the impact value at -60 °C is about 13 J / cm 2and has low toughness. The iron-based alloy of Non-Patent Document 2 requires the addition of alloying elements such as Ni, Co, Mo, etc. and heat treatment to ensure high strength, and when the elongation after heat treatment is less than 10%, it cannot be said to be sufficient. The iron-based alloy of Non-Patent Document 3 requires the addition of alloying elements such as Ni, Mo, etc. and heat treatment to ensure high strength, and the Charpy impact value at room temperature after heat treatment is 30 J / cm 2 or less, and it cannot be said to have high toughness. Therefore, an object of the present invention is to provide an iron-based alloy having excellent mechanical properties that exhibits high strength without adding expensive alloying elements and can achieve both high ductility and high strength, and a method for producing the same.

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

[0009]

Means for Solving the Problems

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

[0011] (1) 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, containing with the balance being Fe and inevitable impurities, and having a solidification structure with an arithmetic average grain size of the crystal of 10 μm or less, an iron-based alloy having excellent mechanical properties.

[0012] (2) The iron-based alloy having excellent mechanical properties according to (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.

[0013] (3) An iron-based alloy having excellent mechanical properties as described in (1) or (2), with a yield strength of 700 MPa or more and an elongation of 20% or more, and a proof stress of 0.2%.

[0014] (4) By mass, C: 0.10 - 0.30%, Si: 0.1 - 1.0%, Mn: 0.2 - 2.0%, P: 0.030% or less, S: 0.030% or less, Containing, An iron-based alloy having excellent mechanical properties, which is a laminated molding alloy with the balance consisting of Fe and inevitable impurities.

[0015] (5) By mass, an iron-based alloy having excellent mechanical properties as described in (4), further containing at least one of Ni: 0.1 - 3%, Cr: 0.1 - 1.0%, and Mo: 0.01 - 0.5%.

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

[0017] (7) A method for producing an iron-based alloy having excellent mechanical properties as described in (6), which is left in the laminated molding state.

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

[0019] An iron-based alloy excellent in mechanical properties according to (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.

[0020] An iron-based alloy excellent in mechanical properties according to (8) or (9), having a drawing ratio of 60% or more.

[0021] An iron-based alloy excellent in mechanical properties according to (8) or (9), which is a laminated molding material.

[0022] A method for producing an iron-based alloy excellent in mechanical properties, comprising melting and solidifying an alloy material having the composition according to (8) or (9) by a laser or an electron beam to perform laminated molding.

[0023] A method for producing an iron-based alloy excellent in mechanical properties according to (12), wherein the laminated molding is left as it is.

[0024] A method for producing an iron-based alloy excellent in mechanical properties according to (12), wherein after the laminated molding, heating is performed at a temperature of 500°C or lower.

Advantages of the Invention

[0025] According to one aspect of the present invention, there are provided an iron-based alloy excellent in mechanical properties, which exhibits high strength without adding expensive alloying elements and can achieve both high ductility and high strength, and a method for producing the same. Further, by using it as a laminated molding material, the degree of freedom in shape is excellent as in the case of a cast steel material.

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

Brief Description of the Drawings

[0027]

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Embodiments for Carrying Out the Invention

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

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

[0030] As a result, a low-carbon iron-based alloy that does not rely on the addition of expensive alloying elements such as those added to the high-strength cast alloys of Patent Document 1 and Patent Document 2, for example, a powder of an iron-based alloy having a composition equivalent to S20C, which is a type of carbon steel for mechanical structures, is laminated and shaped to refine the structure, resulting in a high-strength iron-based alloy with an elongation of 20% or more and a 0.2% proof stress or yield point of 700 MPa or more, which could not be achieved by conventional methods, has been found.

[0031] Based on such findings, in a first aspect, there is provided an iron-based alloy having excellent mechanical properties, containing, in 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 inevitable impurities, and having a solidified structure with an arithmetic mean grain size of the crystal of 10 μm or less.

[0032] Such a fine structure is particularly obtained as-is (As print) after laminated shaping, and it has been found that excellent properties as described above can be obtained. In a structural member made of an iron-based alloy, it is common technical knowledge to perform tempering in order to impart appropriate ductility after hardening treatment such as quenching or annealing. However, it is a new finding that high strength and high ductility can be achieved by using a low-carbon iron-based alloy that does not rely on the addition of expensive alloying elements as-is (As print) without heat treatment. Also, the laminated shaped material is excellent in the degree of freedom of shape, similar to a cast steel material.

[0033] By laminated shaping, the cooling rate during solidification of the alloy can be made 3000 °C / sec. or more, which is unthinkable in general casting, and thereby the microstructure can be refined. It is considered that by refining the structure of the alloy having the above composition in this way, high strength can be obtained while maintaining high ductility. Also, it has been found that by laminated shaping the alloy having the above composition, a strength higher than the strength of the conventional knowledge actually obtained in the literature can be obtained due to the refinement of the structure. This is presumably because the structure has been refined by laminated shaping and the dislocation density has been increased, or the dislocation density is equivalent to that of the conventional knowledge but the high strength is achieved due to other physical properties.

[0034] Next, a second aspect will be described. The inventors conducted various studies with the aim of obtaining an iron-based alloy having excellent mechanical properties, which exhibits high strength, high ductility, and high toughness without adding expensive alloying elements.

[0035] As a result, without adding expensive alloying elements such as those added to the high-strength alloys of Non-Patent Document 2 and Non-Patent Document 3, and without mechanical milling or heat treatment, which are required in the technologies of Patent Document 3, Non-Patent Document 2, and Non-Patent Document 3, by laminating and forming (three-dimensional laminating and forming) powders of an iron-based alloy having a low-carbon iron-based alloy composition, for example, an iron-based alloy composition equivalent to S20C, which is a carbon steel for machine structural use, to refine the structure, 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, which could not be achieved by conventional methods, was found to be obtained. Moreover, it has high toughness (low transition temperature) with an absorbed energy of 100 J or more in the Charpy impact test at -60°C.

[0036] Based on such findings, in the second aspect, an iron-based alloy having excellent mechanical properties is provided, 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, has a solidified structure with an arithmetic mean grain size of the crystal of 10 μm or less, has a tensile strength of 800 MPa or more, an elongation of 15% or more, and an absorbed energy of 100 J or more in the Charpy impact test at -60°C.

[0037] In particular, it has been found that by leaving the component in the as-printed state, better mechanical properties can be obtained. In high-strength structural members made of iron-based alloys, it is common knowledge that after some heat treatment, such as quenching, tempering, or aging hardening treatment, a stress relief treatment may be performed as necessary to impart appropriate ductility. However, it is a new finding not previously known that by using a low-carbon iron-based alloy without adding expensive alloying elements in the as-printed state without heat treatment (excluding stress relief), better mechanical properties can be obtained.

[0038] In addition, from the perspective of SDGs, the manufacturing method of components is required to have a high degree of freedom in shape in order to enable resource savings, energy savings, and shortening of the construction period. Additive manufactured materials are excellent in terms of freedom of shape.

[0039] As described above, by additive manufacturing, the cooling rate during solidification of the alloy can be made 3000 °C / sec or higher, which is impossible to achieve by general manufacturing methods such as casting. As a result, the microstructure can be refined to obtain high strength. Non-Patent Document 2 also uses additive manufacturing, and there is a possibility that the microstructure is refined. However, in the present invention, different from Non-Patent Document 2, alloying elements such as Ni, Mo, and Co are not added, or if added, they are limited to levels that do not affect ductility and toughness. Only by adjusting the composition in this way can high strength, high ductility, and high toughness as described above be realized.

[0040] In iron-based alloy materials, it has been shown in the literature that the strength increases due to refinement of the structure. In the second aspect, by additive manufacturing of the alloy having the above composition, a strength higher than that obtained by refinement in the literature is actually obtained, and it has been newly found that the ductility and toughness do not decrease with the increase in strength. Also, it has been found that by additive manufacturing of the alloy having the above composition, the dislocation density is more than 10 times higher than that of materials obtained by conventional methods. From this, it is considered that the characteristics of high strength, high ductility, and high toughness are obtained because the structure is refined by additive manufacturing and the dislocation density is increased.

[0041] <First Embodiment> The first embodiment corresponds to the first aspect described above. Hereinafter, the reasons for limitation will be described separately in terms of chemical composition, structure, and manufacturing conditions. In the following description, unless otherwise specified, the % representation in the components is by mass %.

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

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

[0044] Mn: 0.2 - 2.0% Mn is an element effective for improving strength and also has a deoxidation effect. However, if its content is less than 0.2%, the effect is small, and if it exceeds 2.0%, the ductility will decrease. Therefore, the Mn content is in the range of 0.2 - 2.0%.

[0045] P: 0.030% or less S: 0.030% or less P and S are elements that have a great influence on toughness. If their contents exceed 0.030% respectively, the toughness will be significantly reduced. Therefore, the contents of P and S are 0.030% or less.

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

[0047] Since Ni is an element with a large austenite stabilizing effect and contributes to high strength, it may be added as necessary. Also, since Ni has a relatively small adverse effect on ductility compared to other elements, a relatively large amount can be added. However, if Ni is less than 0.1%, the effect of high strength increase is small, and if it is 3% or more, the adverse effect on ductility becomes large. Therefore, when adding Ni, its content should be in the range of 0.1 to 3%.

[0048] Since Cr is an element effective for high strength increase, it may be added as necessary. However, if it is less than 0.1%, the effect is small, and if it exceeds 1.0%, the adverse effect on ductility becomes large. Therefore, when adding Cr, 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 when heat treated, it may be added as necessary. However, if it is less than 0.01%, the effect is small, and if it exceeds 0.5%, the adverse effect on ductility becomes large. Therefore, when adding Mo, 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 average grain size of the crystal 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 with an elongation of 20% or more and a 0.2% proof stress or yield point of 700 MPa or more can be obtained.

[0052] In conventional knowledge, as shown in Fig. 1, the hardness of steel increases with the carbon content, the strength is proportional to the hardness, but the elongation decreases with the increase in the carbon content. Therefore, in the composition with a C content of 0.1 to 0.3% as described above, a relatively high elongation can be obtained, but it is difficult to obtain a high strength. Actually, in the steel with the S20C composition having a C content of about 0.2%, an elongation of 20% or more can be obtained, but the strength is 400 MPa or less (yield strength of 0.2%: about 300 MPa). On the other hand, in this embodiment, since the alloy having the composition equivalent to S20C is rapidly solidified during the laminated forming, a fine structure with a crystal grain size of 10 μm or less is obtained, and due to the "high strength due to the refinement of the structure" explained by the Hall-Petch relationship, the coexistence of high elongation and high strength is realized.

[0053] Fig. 2 shows the experimental values of the relationship between the crystal grain size of steel and the strength (yield strength of 0.2%) and hardness (Source: Yuji Kimura et al., Materials: Transactions of the Japan Institute of Metals 36 (11), 1062-, 1997-11; "Pursuit of Grain Refinement Strengthening"). According to this figure, in order to obtain a yield strength of 0.2% of 0.7 GPa (700 MPa) or more, for example, about 0.9 GPa (900 MPa), it is necessary to refine the crystal grain size until it becomes about 0.5 μm. However, in the case of rapid solidification by laminated forming, a yield strength of 0.2% of about 900 MPa can be obtained even when the crystal grain size is about 5 μm. In this regard, the dislocation density may be related. In conventional knowledge, as shown in Fig. 3, in the case of an iron-based alloy with a grain size of 5 μm, the dislocation density is on the order of 3 to 4×10 and, as shown in Fig. 4, the yield strength (yield strength of 0.2%) in that case is 0.4 GPa (400 MPa) (Source: both are Tomoki Tanaka et al., Iron and Steel: Vol. 104 (2018) No. 5: "Effect of Crystal Grain Size on Yield Stress of Cold-Worked Iron"). This time, the reason why a yield strength of 0.2% of about 900 MPa can be obtained even when the crystal grain size is about 5 μm may be that the dislocation density exists more than in conventional knowledge (4×10 14 / m 14 / m 2 or more). Or, the dislocation density may be the same as in conventional knowledge, but it may have been strengthened to a high strength due to other physical property changes.

[0054] [Manufacturing Conditions] The 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 crystal grain size of 10 μm or less can be obtained.

[0055] Specifically, alloy powder is prepared as the alloy material having the 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 set to 3000 °C / sec or more, and an alloy with a fine solidified structure having a crystal grain size of 10 μm or less can be obtained. In this case, as described above, since a higher strength is obtained by additive manufacturing than the relationship between the refinement of the crystal grain size and the strength in the conventional knowledge, it is presumed that additive manufacturing has brought about an increase in dislocation density or other physical property changes in addition to the refinement of the structure by rapid solidification. Since heat treatment after additive manufacturing may increase the crystal grain size and decrease the strength, it is preferable to leave it as it is after additive manufacturing. However, heat treatment that does not affect the crystal grain size, such as stress relief treatment performed at a temperature of about 150 to 250 °C, is allowed.

[0056] <Second Embodiment> The second embodiment corresponds to the above second aspect. Hereinafter, similar to the first embodiment, the reasons for limitation will be described separately in terms of chemical composition, structure, and manufacturing conditions. In the description of this embodiment as well, unless otherwise specified, the % representation in the components is mass %.

[0057] [Chemical Composition] C: 0.10 to 0.30% C is an element effective for improving strength. However, if its content is less than 0.10%, sufficient strength cannot be obtained, and if it exceeds 0.30%, ductility and toughness will decrease. Therefore, the C content is in 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 its 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 - 0.7%.

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

[0060] P: 0.030% or less S: 0.030% or less P and S are elements that have a great impact on toughness. If each exceeds 0.030%, toughness is significantly reduced. Therefore, the contents of P and S are set to 0.030% or less.

[0061] At least one of Ni: 0.1 - 3%, Cr: 0.1 - 1.0%, Mo: 0.01 - 0.5% Ni, Cr, and Mo are elements that increase strength and decrease ductility. However, as follows, in order to improve certain properties, they may be added in an amount that can maintain the desired ductility.

[0062] Ni is an element with a large austenite stabilizing effect and contributes to high strength and improved low-temperature toughness. Therefore, it may be added as necessary. Also, since Ni has a relatively small adverse effect on ductility compared to other elements, it can be added in a relatively large amount. However, if Ni is less than 0.1%, the effect of high strength is small, and if it is 3% or more, the effect of high strength saturates. Therefore, when adding Ni, its content is in the range of 0.1 - 3%.

[0063] Cr is an element effective for high strength and may be added as necessary. However, if it is less than 0.1%, the effect is small, and if it exceeds 1.0%, the adverse effects on ductility and toughness increase. Therefore, when adding Cr, its content is in the range of 0.1 - 1.0%.

[0064] Mo is an element that improves hardenability and suppresses temper embrittlement, and may be added as necessary when heat-treating. However, if the amount is less than 0.01%, the effect is small, and if it exceeds 0.5%, the adverse effect on ductility becomes large. Therefore, when adding Mo, its content should be in the range of 0.01 - 0.5%.

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

[0066] [Solidification Structure] Also in this embodiment, the structure is refined so that the arithmetic mean grain size of the crystal is 10 μm or less. By performing such refinement of the structure by additive manufacturing, high strength with a tensile strength of 800 MPa or more, high ductility with an elongation of 15% or more, and high toughness (low transition temperature) with an absorbed energy of 100 J or more in the Charpy impact test at -60 °C can be obtained. Also, regarding ductility, it can satisfy a drawing of 60% or more.

[0067] In conventional knowledge, as shown in FIG. 5, the strength of steel increases with the carbon content, while the elongation, drawing, and absorbed energy (toughness) decrease with the carbon content. Therefore, it is difficult to satisfy all of high strength, high ductility, and high toughness with a composition having a C content of 0.10 - 0.30%. On the other hand, it has been shown in the literature that the strength increases by refining the structure conventionally. Also in this embodiment, the structure is refined to a level that cannot be reached by normal processes by additive manufacturing. And this embodiment is based on the fact that when refining the structure by additive manufacturing of the alloy having the above composition, a strength higher than the strength obtained by the refinement in the literature is actually obtained, and it has been experimentally found that the dislocation density is more than 10 times higher than that of the material by the conventional method. That is, in this embodiment, it is considered that the characteristics of high strength, high ductility, and high toughness are obtained because the structure is refined by additive manufacturing and the dislocation density is increased. In particular, it is considered that high toughness (low transition temperature) is obtained because the crack propagation at fracture is effectively suppressed by increasing the dislocation density by additive manufacturing.

[0068] [Manufacturing Conditions] The alloy material having the above composition is melted and solidified by laser or electron beam for laminated manufacturing in the same manner as in the first embodiment. As a result, after the alloy material is melted, it is rapidly cooled, and a fine structure with an arithmetic average crystal grain size of 10 μm or less can be obtained.

[0069] Specifically, in the same manner as in the first embodiment, alloy powder is prepared as the alloy material having a composition within the above range, and is melted and solidified by laser or electron beam for laminated manufacturing. By using a laser or 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 an arithmetic average crystal grain size of 10 μm or less can be obtained. Further, it is presumed that such rapid solidification results in not only refinement of the structure but also a high dislocation density. In fact, in the as-printed (as-laminated) material, a high dislocation density and high toughness are obtained, whereas when heat treatment is performed after laminated manufacturing, a phenomenon is observed in which the dislocation density decreases and the toughness rapidly decreases. From this, it is considered that the high toughness in the laminated material is due to the high dislocation density. Therefore, the as-printed (as-laminated) material is most preferable because it has the highest dislocation density. However, heat treatment performed for the purpose of stress relief or strength adjustment at a temperature of 500 °C or less, for example, about 300 to 500 °C, is acceptable because the decrease in dislocation density is limited and hardly causes a decrease in toughness.

Examples

[0070] Hereinafter, examples of the present invention will be described. <First Example> Raw material powder of carbon steel corresponding to S20C, which is a type of carbon steel for machine structural use in JIS G 4051, was prepared. Table 1 shows the standard of the chemical composition of S20C and the composition of the raw material powder.

[0071] The raw material powder was granulated by the gas atomization method, and the -63 μm spherical powder obtained by sieving was used as the raw material for shaping. Then, using a laser-type laminated manufacturing apparatus, output: 300 W, energy density: 90 J / mm 3、Under the condition that the thickness per layer was 40 μm, a raw material for shaping was laminated and molded to produce a 3D molded object (Φ15×100 mm). Regarding this 3D molded object, as shown in Table 2, for the as-printed (As print) sample (No. 1), the samples (No. 2 to 4) that were subjected to heat treatment including annealing and tempering, and the sample (No. 5) that was subjected to quenching and tempering in the same manner as conventional heat treatment, the 0.2% proof stress, tensile strength, elongation, and reduction of area were measured. These are also listed in Table 2. Further, Fig. 6 is a microstructural photograph of No. 1 which is an As print material, Fig. 7 is the microstructural photograph after quenching of No. 5, and Fig. 8 is the microstructural photograph after tempering.

[0072] As shown in Fig. 6, the sample of No. 1 which is an As print material had a fine structure with an arithmetic mean grain size of crystals of about 5 μm, and had unprecedented mechanical properties with high ductility of 22% elongation and 82% reduction of area and high strength close to 1 GPa (0.2% proof stress and tensile strength). In particular, the reduction of area had a higher value than that of the heat-treated materials described below. On the other hand, as shown in Figs. 7 and 8, for the heat-treated materials, the arithmetic mean grain size of crystals coarsened to about 20 μm due to heat treatment. And as shown in Table 2, the samples of No. 2 to 5 which are heat-treated materials showed high elongation and very large reduction of area values regardless of the heat treatment conditions, but the 0.2% proof stress was 299 to 355 MPa, and the tensile strength was 439 to 475 MPa, which were values about half or less of those of No. 1 which is an as-printed material.

[0073]

Table 1

[0074]

Table 2

[0075] <The Second Embodiment> The alloy with the chemical composition shown in Table 3, Nos. 11 to 16, was granulated by the gas atomization method and sieved to obtain -63 μm spherical powder, which was used as the raw material for shaping. Then, using a laser type additive manufacturing apparatus, with the output: 300 W, energy density: 90 J / mm 3 , thickness per layer: 40 μm, the raw material for shaping was additively manufactured to produce a 3D object (Φ15 × 100 mm). For this 3D object, samples were prepared by leaving it as printed (heat treatment number 1 in Table 3), stress relieving (heat treatment numbers 2 to 4 in Table 3), or annealing and tempering (heat treatment number 5 in Table 3). Samples Nos. 11 to 16 are examples of the present invention within the scope of the second aspect (second embodiment) of the present invention, and samples Nos. 17 to 19 are comparative examples outside the scope of the second aspect (second embodiment) of the present invention. Also, sample No. 20 is a commercially available material of S20C, a type of carbon steel for machine structural use (JIS G4051), and is a reference example. Note that samples Nos. 13 to 15 and 17 have the same chemical composition as sample No. 12, but different heat treatments.

[0076] For these samples, mechanical properties (0.2% proof stress, tensile strength, elongation, reduction of area, absorbed energy by Charpy impact test) were measured respectively. The results are shown in Table 4. As shown in Table 4, for samples Nos. 11, 12, and 16, which are samples as printed having the composition of the present invention, all of them satisfied that the 0.2% proof stress was 700 MPa or more, the tensile strength was 800 MPa or more, the elongation was 15% or more, and the absorbed energy in the Charpy impact test at -60 °C was 100 J or more. In particular, for samples Nos. 11 and 12, the absorbed energy in the Charpy impact test at -60 °C was unchanged from the room temperature value and exceeded 200 J, and extremely high values were obtained. Also, as will be described later, it also satisfied that the arithmetic mean grain size of the crystal was 10 μm or less. For samples Nos. 13 to 15, which were obtained by heating the shaped object with the composition of No. 12 to a temperature of 500 °C or less to adjust the strength, values of elongation, reduction of area, and absorbed energy equivalent to those of as printed No. 12 were obtained.

[0077] In contrast, in Comparative Examples, Samples No. 17 to 19 did not satisfy the range defined by the present invention in one or more of tensile strength, elongation, absorbed energy in the Charpy impact test at -60°C, and crystal grain size. Sample No. 17 is a shaped article having the same chemical composition as No. 12, which was annealed by heating to 920°C and then air-cooled, and then tempered at 500°C. As described later, although the crystal grain size became coarser and high elongation was shown, the 0.2% proof stress was 322 MPa and the tensile strength was 479 MPa, which was less than about half of that of Sample No. 12 as an As print material. Further, in Sample No. 17 as a heat-treated material, the absorbed energy in the Charpy test was 125 J at room temperature, indicating high toughness, but less than 10 J at -60°C, showing significant embrittlement. Also, since the C content of Sample No. 18 was less than 0.10%, the tensile strength was low, and since the C content of Sample No. 19 exceeded 0.30%, the elongation and the absorbed energy in the Charpy impact test at -60°C were low. Note that Sample No. 20 as a reference example (commercially available material of S20C) had a low tensile strength of 470 MPa and a low absorbed energy of 22 J in the Charpy impact test at -60°C.

[0078] In addition, the microstructures of the above samples were observed, and the dislocation density and crystallite size were also measured. The dislocation density was determined from the XRD diffraction peaks in six directions ((110), (200), (211), (220), (310), (222)). The crystallite size was measured using XRD. Note that the crystallite size is the smallest unit that can be regarded as a single crystal within the crystal grains, and was measured to accurately evaluate the particle size. Fig. 9 is a microstructural photograph of Sample No. 12 (As print material) as an example of the present invention, Fig. 10 is a microstructural photograph of Sample No. 17 (annealed and tempered material) as a comparative example, and Table 5 shows the dislocation density and crystallite size of these samples.

[0079] Sample No. 12 (As print material) as an example of the present invention has a crystallite size of 114 nm (corresponding to an arithmetic average particle size of 10 μm or less), a fine structure, and a dislocation density of 8.4×10 14 / m2 was confirmed to be extremely high. The dislocation density of the cast block of SCW480, a general cast steel product for welding structures having the same C amount as the present invention, was 4.2×10 13 / m 2 . Therefore, it was confirmed that Sample No. 12, which is an example of the present invention, has a dislocation density more than 10 times that of the conventional material. Similar structures and dislocation densities were also obtained for Samples No. 11, 13 to 16.

[0080] On the other hand, Sample No. 17 (annealed and tempered material), which is a comparative example, has the same chemical composition as Sample No. 12, but the crystallite size was coarsened to 435 nm (equivalent to an arithmetic average particle size exceeding 10 μm) by heat treatment, and the dislocation density was 1.6×10 13 / m 2 , which became equivalent to the conventional material.

[0081] From the above results, it was confirmed that the alloy obtained by additive manufacturing with the chemical composition of the second aspect (second embodiment) has high strength, high ductility, and high toughness in addition to high strength due to refinement, with a high dislocation density.

[0082] [Table 3]

[0083] [Table 4]

[0084] [Table 5]

Claims

1. In mass%, C: 0.10-0.30%, Si: 0.1-1.0%, Mn: 0.2 to 2.0%, P: 0.030% or less, S: 0.030% or less, Contains An iron-based alloy having excellent mechanical properties, with the balance consisting of Fe and unavoidable impurities, a solidification structure in which the arithmetic mean grain size of the crystals is 10 μm or less, a 0.2% proof stress or yield point of 700 MPa or more, and an elongation of 20% or more.

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

3. An iron-based alloy having excellent mechanical properties as described in claim 1 or claim 2, which is an additive manufacturing material. Lamination

4. A method for producing an iron-based alloy with excellent mechanical properties, comprising melting and solidifying an alloy material having the composition defined in claim 1 or 2 using a laser or an electron beam to form an additive manufacturing process, and producing an iron-based alloy having a solidification structure with an arithmetic mean grain size of crystals of 10 μm or less, a 0.2% proof stress or yield point of 700 MPa or more, and an elongation of 20% or more.

5. The method for producing an iron-based alloy having excellent mechanical properties according to claim 4, wherein the alloy is left as is after additive manufacturing.

6. In mass%, C: 0.10-0.30%, Si: 0.1 to 0.7%, Mn: 0.2-1.0%, P: 0.030% or less, S: 0.030% or less, Contains An iron-based alloy with excellent mechanical properties, the balance of which is Fe and unavoidable impurities, having a solidification structure in which the arithmetic mean grain size of the crystals is 10 μm or less, having 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.

7. 7. The iron-based alloy having excellent mechanical properties according to claim 6, further containing, in mass%, at least one of Ni: 0.1 to 3%, Cr: 0.1 to 1.0%, and Mo: 0.01 to 0.5%.

8. 8. The iron-based alloy having excellent mechanical properties according to claim 6 or 7, wherein the reduction of area is 60% or more.

9. The iron-based alloy having excellent mechanical properties according to claim 6 or 7, which is an additive manufacturing material.

10. A method for producing an iron-based alloy with excellent mechanical properties, comprising melting and solidifying an alloy material having the composition according to claim 6 or 7 using a laser or an electron beam to form an additive manufacturing process, and producing an iron-based alloy 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.

11. The method for producing an iron-based alloy having excellent mechanical properties according to claim 10, wherein the alloy is left as is after additive manufacturing.

12. The method for producing an iron-based alloy having excellent mechanical properties according to claim 10, wherein after additive manufacturing, the alloy is heated at a temperature of 500°C or less.

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