High-strength, high-ductility iron-base alloy and method for producing same

Abstract

The present invention provides a high-strength, high-ductility iron-base alloy containing, in terms of mass%, 0.35%-0.65% of C, 0.1-1.0% of Si, 0.2%-2.0% of Mn, 0.030% or less of P, and 0.030% or less of S, with the balance being Fe and inevitable impurities, wherein the alloy has a solidified structure in which the arithmetic average crystal grain size is 10 μm or less, has a tensile strength of 1100 MPa or more and an elongation of 13% or more, and satisfies the inequation: [tensile strength (MPa) ] × [elongation (%) ] ≥ 16000.

Description

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

[0001] The present invention relates to a high-strength, high-ductility iron-based alloy and a method for producing the same.

[0002] Recently, there has been a demand for reducing the total weight of structural components that make up various types of equipment. Iron-based materials are widely used in structural components, but due to their high specific gravity, reducing their weight has become an urgent issue. Therefore, studies are being conducted to increase the specific strength of materials. Furthermore, such structural components are required to have high ductility, which is a property that contradicts high strength. Furthermore, a large degree of freedom in shape is desired to reduce weight.

[0003] A common iron-based material that has excellent freedom of shape is cast steel (steel-based cast alloy), and Patent Document 1 states that the strength of the material is 110.1 to 129.3 kgf / mm ２ A high strength, high ductility cast alloy having a tensile strength of (1079-1267 MPa) and an elongation of 11.8-18.5% and a method for producing the same is disclosed.

[0004] Recently, attention has been focused on iron-based materials that are produced by three-dimensional additive manufacturing, as they offer excellent freedom in shape. Non-Patent Document 1 discloses a high-strength steel material for additive manufacturing, which is obtained by additive manufacturing of powder with a maraging steel composition, and has a tensile strength of 1000 to 2150 MPa and an elongation of 2 to 16%.

[0005] Japanese Patent Application Publication No. 1-255647

[0006] https: / / www.tpc.toray / technology / additive-print / add_material / add_material_005.html

[0007] The iron-based alloy of Patent Document 1 requires the addition of alloying elements such as Ni, Mo, and V to achieve high strength while maintaining high ductility, and undergoes complex heat treatment. Furthermore, the iron-based alloy of Non-Patent Document 1 contains 9% Co, which is designated as a specified chemical substance, in addition to alloying elements such as Ni and Mo to ensure high strength and ductility, and is subject to legal restrictions. Furthermore, aging treatment is essential to achieve high strength, but aging significantly reduces ductility, such as elongation and reduction of area. Furthermore, both Patent Document 1 and Non-Patent Document 1 add relatively large amounts of expensive alloying elements, resulting in relatively high alloy costs.

[0008] Therefore, an object of the present invention is to provide a high-strength, high-ductility iron-based alloy that has a high degree of freedom in shape and can achieve both high strength and high ductility without the addition of expensive alloying elements or special heat treatment, and a method for producing the same.

[0009] According to the present invention, the following (1) to (8) are provided.

[0010] (1) A high-strength, highly ductile iron-based alloy containing, by mass%, C: 0.35 to 0.65%, Si: 0.1 to 1.0%, Mn: 0.2 to 2.0%, P: 0.030% or less, and S: 0.030% or less, with the balance being Fe and unavoidable impurities, having a solidification structure in which the arithmetic mean grain size of crystals is 10 μm or less, having a tensile strength of 1100 MPa or more, an elongation of 13% or more, and satisfying the equation [tensile strength (MPa)] × [elongation (%)] ≥ 16000.

[0011] (2) The high-strength, high-ductility iron-based alloy 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%.

[0012] (3) A high-strength, high-ductility iron-based alloy according to (1) or (2), having a reduction of area of ​​45% or more.

[0013] (4) A high-strength, high-ductility iron-based alloy that is an additive manufacturing alloy containing, by mass%, C: 0.35 to 0.65%, 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.

[0014] (5) The high-strength, high-ductility iron-based alloy 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%.

[0015] (6) The dislocation density measured by the Convolutional Multiple Whole Profile method is 5×10 １４ m -２ The high-strength, high-ductility iron-based alloy according to (1) or (2) above.

[0016] (7) A method for producing a high-strength, high-ductility iron-based alloy, 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] (8) A method for producing a high-strength, high-ductility iron-based alloy according to (7), characterized in that the alloy is left as it is after additive manufacturing.

[0018] According to the present invention, there are provided a high-strength, high-ductility iron-based alloy that has a high degree of freedom in shape and can achieve both high strength and high ductility without the addition of expensive alloying elements or special heat treatment, and a method for producing the same.

[0019] 1 is a conceptual diagram showing the relationship between the carbon content of steel and hardness, tensile strength, and elongation. 2 is a diagram showing the relationship between the tensile strength and hardness of steel. 3 is a diagram showing the relationship between the tensile strength and total elongation of various steels, and the heat treatment of Q&P steel. 4 is a diagram showing the relationship between the crystal grain size of steel and dislocation density. 5 is a diagram showing the relationship between the dislocation density and yield strength of steel. 6 is a diagram showing the relationship between the tensile strength and yield point (0.2% proof stress) of steel. 7 is a diagram in which the properties of Nos. 1 to 4 and 6 of the present invention alloys are plotted on the diagram showing the relationship between the tensile strength and total elongation of steel materials at the top of FIG. 3.

[0020] The present inventors have conducted various investigations with the aim of obtaining an iron-based alloy that has a high degree of freedom in shape and can achieve both high strength and high ductility without the addition of expensive alloying elements or special heat treatment.

[0021] As a result, they found that by using an iron-based alloy composition containing 0.35 to 0.65 mass% carbon, for example, a composition equivalent to a carbon steel material commonly known as S-C material for mechanical structures, and refining the structure to 10 μm or less, it is possible to obtain a high-strength, highly ductile iron-based alloy having a tensile strength of 1100 MPa or more, an elongation of 13% or more, and satisfying the formula [tensile strength (MPa)] × [elongation (%)] ≧ 16000, without the addition of expensive alloy elements such as those added in Patent Document 1 and Non-Patent Document 1. They also found that the refinement of the structure that achieves these properties can be achieved by a method of additive manufacturing of powder of the above composition, and that using the powder as-printed (as-printed) is particularly effective.

[0022] It is technically common knowledge that high-strength structural members manufactured from iron-based alloys are subjected to some kind of heat treatment, for example, a hardening treatment such as quenching or normalizing, followed by a tempering treatment to impart appropriate ductility. However, as in the present invention, by using a carbon-containing iron-based alloy with a C content of 0.35 to 0.65% without the addition of expensive alloying elements and as-printed by additive manufacturing, it is possible to achieve high strength and high ductility with a tensile strength of 1100 MPa or more and an elongation of 13% or more, which is a new finding not previously known.

[0023] The reason why the microstructure is refined by additive manufacturing is that additive manufacturing allows the cooling rate during solidification of the alloy to be 3000°C / sec or more, which is unthinkable with general casting. This refinement of the microstructure allows for high strength. Non-Patent Document 1 also uses additive manufacturing, which may have refined the microstructure. However, in the present invention, the C content is relatively high at 0.35 to 0.65 mass%, and alloy elements such as Ni, Mo, V, and Co are not added, or if added, are limited to a level that does not affect ductility. This achieves an extremely high tensile strength of 1100 MPa or more, while also achieving high ductility with an elongation of 13% or more, and is believed to satisfy [tensile strength (MPa)] × [elongation (%)] ≧ 16,000. Furthermore, by adjusting the conditions, for example, by limiting the C content to 0.52% or less, [tensile strength (MPa)] × [elongation (%)] ≧ 20,000 can be satisfied. Indicators of high ductility include good elongation and reduction of area, and can achieve reduction of area of ​​45% or more.

[0024] Although literature has shown that the strength of iron-based alloy materials increases with the refinement of their structure, it was discovered that additive manufacturing of the alloy with the above composition actually achieves a strength higher than that achieved by the refinement reported in the literature. This is thought to be because additive manufacturing not only refines the structure but also increases the dislocation density.

[0025] The reasons for the limitations of the present invention 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.

[0026] [Chemical Composition] C: 0.35 to 0.65% C is an element effective in improving strength. However, if the C content is less than 0.35%, it is not possible to obtain a high strength of 1300 MPa, and if it exceeds 0.65%, the strength saturates and ductility decreases. Therefore, the C content is set to the range of 0.35 to 0.65%.

[0027] 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%.

[0028] 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%.

[0029] 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.

[0030] 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 reduce ductility, but they can improve predetermined properties as described below, so they may be added in amounts sufficient to maintain the desired ductility.

[0031] 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%.

[0032] 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%.

[0033] 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%.

[0034] The balance of the alloy components having the above composition is Fe and inevitable impurities.

[0035] [Solidification structure] In the present invention, the solidification structure of an 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 manner, a high-strength, high-ductility iron-based alloy can be obtained, which has a high tensile strength of 1100 MPa or more, a high elongation of 13% or more, and satisfies the formula [tensile strength (MPa)] × [elongation (%)] ≥ 16,000. Furthermore, with regard to ductility, not only can the elongation be 13% or more, but also the reduction of area can be 45% or more.

[0036] According to conventional knowledge, as shown in Figure 1 (Source: https: / / hitopedia.net / %7%82%AD%E7%B4%A0%E9%8%BC#gsc.tab=0) and Figure 2 (Source: "Materials," Vol. 39, No. 442, Fig. 1), the hardness of steel increases with carbon content, and tensile strength is proportional to hardness, but elongation tends to decrease with increasing tensile strength, and elongation decreases with increasing carbon content. For example, S20C steel, which is carbon steel with a carbon content of about 0.2%, has a tensile strength of approximately 450 MPa and an elongation of about 30%, while S50C steel, which is carbon steel with a carbon content of about 0.5%, has a tensile strength of approximately 700 MPa and an elongation of about 15%.

[0037] On the other hand, in the present invention, the alloy with the above composition is rapidly solidified during additive manufacturing, resulting in a fine structure with an arithmetic mean grain size of 10 μm or less, and a tensile strength of 1100 MPa or more can be obtained with a C content of 0.35%, for example, equivalent to S35C. However, conventional technical knowledge required advanced heat treatment, such as that of Q&P steel shown in Figure 3 (Source: Journal of the Japan Society of Mechanical Engineers, Vol. 125, February 2022, https: / / www.jsme.or.jp / kaisi / 1239-20 / ). The upper part of Figure 3 shows the relationship between tensile strength and total elongation for each steel type. Among these, Q&P steel is first subjected to quenching, as shown in the lower part of Figure 3, where cooling is stopped at a temperature where some untransformed austenite remains, and then partitioning is performed to maintain the appropriate temperature. In Q&P steel, the solute carbon in the partially transformed martensite is distributed to the untransformed austenite and stabilized by this heat treatment, increasing the amount of retained austenite, thereby achieving both high tensile strength and elongation, and the range includes a tensile strength of 1100 MPa or more and an elongation of 13% or more. In contrast, in the present invention, by refining the structure and limiting the alloy elements that form precipitates that hinder elongation, it is possible to achieve both a high strength of 1100 MPa or more and an elongation of 13% or more without performing advanced heat treatment, and further to satisfy the condition [tensile strength (MPa)] × [elongation (%)] ≥ 16,000 or even 20,000 or more, thereby contributing to weight reduction of structural members.

[0038] In the present invention, rapid solidification by additive manufacturing results in an arithmetic mean grain size of 10 μm or less. Figure 4 shows experimental values ​​showing the relationship between the grain size and dislocation density of steel. According to this figure, when the grain size is 10 μm, the dislocation density is 2 to 3 × 10 １４ / m 2As shown in Figure 5, the yield point (0.2% proof stress) in this case is 0.35 GPa (350 MPa), which corresponds to a tensile strength of roughly 390 MPa (Source: Tanaka et al., Iron and Steel: Vol. 104 (2018) No. 5: "Effect of Grain Size on Yield Stress of Cold-Worked Iron"). In this study, a tensile strength of 1100 MPa or more was obtained even with a grain size of about 10 μm because the dislocation density was higher than previously known. Specifically, the dislocation density determined by the CMWP (Convolutional Multiple Whole Profile) method was 5 × 10 １４ m -２ If the temperature is above 1000 MPa, a tensile strength of 1100 MPa or more and an elongation of 13% or more can be simultaneously obtained. The CMWP method is a well-known technique and is described, for example, at https: / / support.spring8.or.jp / Doc_workshop / iuss / 2020 / metal_mate_eval-16 / yoshida.pdf. The conversion of the yield point (0.2% proof stress) to the tensile strength was performed with reference to Figure 6 (source: "Materials," Vol. 39, No. 442, Fig. 3).

[0039] [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, resulting in a fine structure with an arithmetic mean grain size of 10 μm or less.

[0040] 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. By using a laser or electron beam, the cooling rate during solidification of the alloy can be increased to 3000°C / sec or more, resulting in an alloy with a fine solidification structure with an arithmetic mean grain size of 10 μm or less. In this case, as described above, additive manufacturing achieves a strength higher than the relationship between grain size refinement and strength as previously known. It is therefore 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 grain size and reduce strength, it is preferable to leave the additive manufacturing process as is. However, heat treatments that do not affect the grain size, such as stress relief at temperatures of approximately 150-250°C or strength adjustment at temperatures below 500°C, are permissible. In other words, even if heat treatment is performed after additive manufacturing, as long as the temperature is below 500°C, the grain size will be 5×10 １４ / m ２ This allows the dislocation density to be maintained at or above 1000 MPa, the tensile strength to be 1100 MPa or more, the elongation to be 13% or more, and the condition [tensile strength (MPa)] × [elongation (%)] ≥ 16000 to be satisfied.

[0041] Examples of the present invention will be described below. Raw material powder of carbon steel having the composition shown in Table 1 was prepared, and the raw material powder was granulated by gas atomization. The resulting sieved spherical powder of -63 μm was used as the raw material for modeling. Then, using a laser-type layered modeling device, output: 300 W, energy density: 90 J / mm ３ The raw materials for fabrication were subjected to additive manufacturing under the conditions of a thickness per layer of 40 μm to produce a 3D-shaped object (Φ15 × 100 mm). The tensile strength, elongation, reduction of area, and arithmetic mean grain size of the crystals (simply referred to as the mean grain size in Table 1) of this 3D-shaped object were measured. The measurement results are also shown in Table 1.

[0042] In Table 1, Nos. 1 to 6 are invention examples that satisfy the scope of the present invention, while Nos. 11 to 13 are comparative examples that fall outside the scope of the present invention. Furthermore, Nos. 21 and 22 are reference examples with maraging steel compositions. Note that invention examples Nos. 1 to 4 and 6, comparative examples Nos. 11 and 13, and reference example No. 21 were all formed as-cast, while invention example No. 5, comparative example No. 12, and reference example No. 22 were heat-treated after forming. Regarding heat treatment, invention example No. 5 was heat-treated at 500°C after forming, comparative example No. 12 was heat-treated at 580°C after forming, and reference example No. 22 was heated at 820°C for 0.5 hours, air-cooled, and then aged at 490°C for 6 hours.

[0043] As shown in Table 1, Invention Examples Nos. 1 to 6, which are within the scope of the present invention, have an arithmetic mean grain size of 10 μm or less, a tensile strength of 1100 MPa or more, and an elongation of 13% or more, resulting in high-strength, high-ductility iron-based alloys that satisfy the relationship [tensile strength (MPa)] × [elongation (%)] ≥ 16,000. Furthermore, the reduction of area is 45% or more. Figure 7 is a plot of the properties of Invention Examples Nos. 1 to 4 and 6 on the graph showing the relationship between tensile strength and elongation for various steels in the upper part of Figure 3. As shown in Figure 7, Invention Examples Nos. 1 to 4 and 6, which are as-printed materials, have properties comparable to those of Q&P steels obtained by advanced heat treatment. Specifically, Invention Examples Nos. 1 to 3 satisfy the relationship [tensile strength (MPa)] × [elongation (%)] ≥ 20,000, and No. No. 4 and No. 6 have a high tensile strength of 1400 MPa or more, satisfy an elongation of 13% or more, and at the same time, maintain high values ​​of [tensile strength (MPa)] × [elongation (%)] of 19348 and 18853, respectively. In addition, No. 4, which is an as-print material, has a dislocation density of 27.0 × 10 measured by the CMWP method. １４ / m ２ Inventive example No. 5, which had the same chemical composition as No. 4 but was heat-treated at 500°C, had a lower tensile strength than No. 4, but still satisfied the requirements of tensile strength of 1100 MPa or more, elongation of 13% or more, and [tensile strength (MPa)] × [elongation (%)] ≥ 16000. The dislocation density was also lower than No. 4, but was still 8.7 × 10 １４ / m２ and 5 x 10 １４ / m ２ It maintains a high value.

[0044] In contrast, in Comparative Examples No. 11 to No. 13, which are outside the scope of the present invention, at least one of tensile strength, elongation, and [tensile strength (MPa)] × [elongation (%)] is outside the above range. In Comparative Example No. 12, the arithmetic mean grain size of the crystals is 15 μm or more, and the dislocation density is 5 × 10 １４ m -２ Furthermore, the comparative example No. 12 had an reduction in area of ​​less than 45%.

[0045]

Claims

1. A high-strength, highly ductile iron-based alloy containing, by mass%, C: 0.35 to 0.65%, 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, having a solidification structure in which the arithmetic mean grain size of the crystals is 10 μm or less, having a tensile strength of 1100 MPa or more, an elongation of 13% or more, and satisfying the formula [tensile strength (MPa)] × [elongation (%)] ≧ 16000.

2. The high-strength, high-ductility iron-based alloy according to claim 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%.

3. A high-strength, high-ductility iron-based alloy according to claim 1 or 2, having a reduction of area of ​​45% or more.

4. A high-strength, high-ductility iron-based alloy that is an additive manufacturing alloy containing, by mass%, C: 0.35 to 0.65%, Si: 0.1 to 1.0%, Mn: 0.2 to 2.0%, P: 0.030% or less, S: 0.030% or less, with the remainder being Fe and unavoidable impurities.

5. The high-strength, high-ductility iron-based alloy according to claim 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%.

6. The dislocation density measured by the Convolutional Multiple Whole Profile method is 5 x 10 １４ m -２ The high-strength, high-ductility iron-based alloy according to claim 1 or 2, wherein:

7. A method for producing a high-strength, high-ductility iron-based alloy, in which an alloy material having the composition described in claim 1 or 2 is melted and solidified by a laser or electron beam to be layer-by-layer manufactured.

8. The method for producing a high-strength, high-ductility iron-based alloy according to claim 7, wherein the alloy is left as is after additive manufacturing.

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