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

Abstract

The present invention provides a high-strength, high-ductility iron-based alloy containing, by mass%, 0.35 to 0.65% C, 0.1 to 1.0% Si, 0.2 to 2.0% Mn, 0.030% or less P, and 0.030% or less S, 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, a tensile strength of 1100 MPa or more, an elongation of 13% or more, and satisfying the equation [tensile strength (MPa)] × [elongation (%)] ≥ 16000.

Description

[Technical Field]

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

[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 are disclosed.

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

[0005] [Patent Document 1] Japanese Patent Application Publication No. 1-255647 [Non-patent literature]

[0006] [Non-Patent Document 1] https: / / www.tpc.toray / technology / additive-print / add_material / add_material_005.html Summary of the Invention [Problem to be solved by the invention]

[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. [Means for solving the problem]

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

[0010] (1) In mass%, C: 0.35~0.65%, Si: 0.1 to 1.0%, Mn: 0.2 to 2.0%, P:0.030% or less, S: 0.030% or less, Contains A high-strength, high-ductility iron-based alloy with a solidification structure in which the balance consists of Fe and unavoidable impurities, the arithmetic mean grain size of the crystals is 10 μm or less, the tensile strength is 1100 MPa or more, the elongation is 13% or more, and the relationship [tensile strength (MPa)] × [elongation (%)] ≥ 16000 is satisfied.

[0011] (2) A 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) In mass%, C: 0.35~0.65%, Si: 0.1 to 1.0%, Mn: 0.2 to 2.0%, P:0.030% or less, S: 0.030% or less, Contains A high-strength, high-ductility iron-based alloy that is an additive manufacturing alloy with the balance being Fe and unavoidable impurities.

[0014] (5) A 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, comprising melting and solidifying an alloy material having the composition described in (1) or (2) above using a laser or electron beam to form an additive manufacturing process.

[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. [Effects of the Invention]

[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. [Brief explanation of the drawings]

[0019] [Figure 1] 1 is a conceptual diagram showing the relationship between the carbon content of steel and hardness, tensile strength, and elongation. [Figure 2] FIG. 1 is a diagram showing the relationship between tensile strength and hardness of steel. [Figure 3] This figure shows the relationship between tensile strength and total elongation of various steels, and the heat treatment of Q&P steel. [Figure 4] FIG. 1 is a diagram showing the relationship between the grain size and dislocation density of steel. [Figure 5] FIG. 1 is a diagram showing the relationship between dislocation density and yield strength of steel. [Figure 6] FIG. 1 is a diagram showing the relationship between the tensile strength and the yield point (0.2% proof stress) of steel. [Figure 7] The upper part of FIG. 3 shows the relationship between the tensile strength and total elongation of steel materials, and the properties of the alloys Nos. 1 to 4 and 6 of the present invention are plotted in this figure. DETAILED DESCRIPTION OF THE INVENTION

[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 for machine structures commonly known as an SC material, and refining the structure to 10 μm or less, it is possible to obtain a high-strength, highly ductile iron-based alloy with a tensile strength of 1100 MPa or more, an elongation of 13% or more, and satisfying the relationship [tensile strength (MPa)] × [elongation (%)] ≥ 16000, without the addition of expensive alloy elements as described in Patent Document 1 and Non-Patent Document 1. They also found that the refinement of the structure that results in these properties can be achieved by additive manufacturing of powder with the above composition, and that using the powder as printed is particularly effective.

[0022] It is technically common knowledge that high-strength structural components 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-printing it, 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 and unprecedented finding.

[0023] The reason why the microstructure is refined by AM is that AM allows the cooling rate during solidification of the alloy to be 3000°C / sec or more, which is unthinkable with conventional casting. This refinement of the microstructure allows for high strength. Non-Patent Document 1 also uses AM, which may have refined the microstructure. However, in the present invention, by increasing the C content to a relatively high level of 0.35 to 0.65 mass%, and adding no alloying elements such as Ni, Mo, V, and Co, or limiting their addition to a level that does not affect ductility, an extremely high tensile strength of 1100 MPa or more is achieved, while high ductility of elongation of 13% or more is achieved, which 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, it is possible to satisfy [tensile strength (MPa)] × [elongation (%)] ≥ 20,000. In addition to elongation, reduction of area is also good as an indicator of high ductility, achieving a 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 description, unless otherwise specified, the percentages of components are mass %.

[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%, a high strength of 1300 MPa cannot be obtained, 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 that is effective in improving strength and also has a deoxidizing effect. However, if its 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 because they can improve certain properties as described below, 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%, it has a significant adverse effect on ductility. 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] [Coagulation tissue] 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 that 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 (%)] ≥ 16000. 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] Conventional wisdom shows that, 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 as tensile strength increases, and elongation decreases as the carbon content increases. For example, steel with an S20C composition, which is carbon steel with a C content of about 0.2%, has a tensile strength of approximately 450 MPa and an elongation of about 30%, while steel with an S50C composition, which is carbon steel with a C content of about 0.5%, has a tensile strength of approximately 700 MPa and an elongation of about 15%.

[0037] In contrast, in the present invention, the alloy with the above composition is rapidly solidified during additive manufacturing, resulting in a microstructure with an arithmetic mean grain size of 10 μm or less. A C content of 0.35%, equivalent to S35C, achieves a tensile strength of 1100 MPa or more. However, conventional wisdom has dictated that achieving an elongation of 13% or more with such high-strength materials requires advanced heat treatment, such as that used for Q&P steel, as 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, as shown in the lower part of Figure 3, is first quenched, where cooling is stopped at a temperature where some untransformed austenite remains, and then partitioned 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, including a range that satisfies 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 alloying elements that form precipitates and the like 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 furthermore, it is possible 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, the arithmetic mean grain size of the crystals is 10 μm or less due to rapid solidification by additive manufacturing. 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 roughly corresponds to a tensile strength of 390 MPa (Source: Tanaka Tomoki 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 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, when the dislocation density was 5 × 10 14 m -2 If the test is performed at or above this level, a tensile strength of 1100 MPa or more and an elongation of 13% or more can be simultaneously achieved. 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 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 using 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. 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. In this case, as described above, additive manufacturing achieves higher strength than previously believed, suggesting that additive manufacturing not only refines the structure through rapid solidification but also increases dislocation density or changes other physical properties. Because heat treatment after additive manufacturing can increase the grain size and reduce strength, it is preferable to leave the material as is. However, heat treatment that does not affect the grain size, such as stress relief at temperatures of around 150–250°C or strength adjustment at temperatures below 500°C, is permissible. In other words, even if heat treatment is performed after additive manufacturing, as long as the temperature is below 500°C, the strength of the material can be reduced by 5×10. 14 / m 2 It is possible to maintain a dislocation density of 1100 MPa or more, have a tensile strength of 1100 MPa or more, an elongation of 13% or more, and satisfy the condition [tensile strength (MPa)] × [elongation (%)] ≥ 16000. [Example]

[0041] Examples of the present invention will be described below. Carbon steel raw powder with the composition shown in Table 1 was prepared, granulated by gas atomization, and sieved to obtain spherical powder with a particle size of -63 μm. The resulting powder was used as the raw material for modeling. A laser-type additive manufacturing device was used, with an output of 300 W and an energy density of 90 J / mm. 3 The raw material for fabrication was additively fabricated under the condition of a thickness per layer of 40 μm to produce a 3D model (Φ15 × 100 mm). The tensile strength, elongation, reduction of area, and arithmetic mean grain size of the crystals (simply referred to as mean grain size in Table 1) of this 3D model were measured. The results of these measurements 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. Additionally, 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 as-formed, while invention example No. 5, comparative example No. 12, and reference example No. 22 were heat-treated after forming. Regarding the heat treatment, invention example No. 5 was heat-treated at 500°C after forming, and comparative example No. 12 was heat-treated at 580°C after forming. 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, Examples 1 to 6 of the present invention, 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 Examples 1 to 4 and 6 of the present invention alloys 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, Examples 1 to 4 and 6 of the present invention, which are as-printed materials, have properties comparable to those of Q&P steels that have undergone advanced heat treatment. Specifically, Nos. 1 to 3 of the present invention examples satisfy the condition [tensile strength (MPa)] × [elongation (%)] ≥ 20,000, and Nos. 4 and 6 have a high tensile strength of 1,400 MPa or more while satisfying an elongation of 13% or more, and at the same time maintain high values ​​of [tensile strength (MPa)] × [elongation (%)] of 19,348 and 18,853, respectively. In addition, No. 4, which is an as-printed material, has a dislocation density of 27.0 × 10 measured by the CMWP method. 14 / m 2 No. 5, an example of the present invention, which has 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 conditions of tensile strength 1100 MPa or more, elongation 13% or more, [tensile strength (MPa)] × [elongation (%)] ≥ 16000, and the dislocation density was also lower than No. 4, but was still 8.7 × 1014 / m 2 and 5×10 14 / m 2 It maintains a high value.

[0044] In contrast, in Comparative Examples Nos. 11 to 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] [Table 1]

Claims

1. In mass%, C: 0.35-0.65%, Si: 0.1-1.0%, Mn: 0.2 to 0.49%, P: 0.030% or less, S: 0.030% or less, Contains A high-strength, highly ductile iron-based alloy having a solidification structure in which the balance is Fe and unavoidable impurities, the arithmetic mean grain size of crystals being 10 μm or less, a tensile strength of 1100 MPa or more, an elongation of 13% or more, and satisfying the relationship [tensile strength (MPa)] × [elongation (%)] ≧ 16000.

2. 2. The high-strength, high-ductility iron-based alloy 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. 3. The high-strength, high-ductility iron-based alloy according to claim 1, wherein the reduction of area is 45% or more.

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

5. 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 claim 1 or 2, wherein

6. A method for producing a high-strength, highly ductile iron-based alloy, comprising melting and solidifying an alloy material having the composition according to 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 in which the arithmetic mean grain size of the crystals is 10 μm or less, a tensile strength of 1100 MPa or more, an elongation of 13% or more, and satisfying the relationship [tensile strength (MPa)] × [elongation (%)] ≧ 16000.

7. The method for producing a high-strength, high-ductility iron-based alloy according to claim 6, wherein the alloy is left as it is after additive manufacturing.

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