Iron and steel materials for additive manufacturing and manufacturing method of iron alloys

Abstract

To provide a lamination shaping steel material that has suppressed crack generation and can yield an iron alloy having excellent strength and resilience, and to provide a method for producing iron alloy using the lamination shaping steel material.SOLUTION: Provided are a lamination shaping steel material that contains C: 0.3 to 0.8%, Mn: 0.6 to 2%, Cr: 1 to 7%, V: 2% or less, Mo: 3% or less in mass%, and in which the martensitic transformation start temperature is 130 to 200°C, and the bainite transformation start time at martensitic transformation start temperature + 30°C in isothermal transformation curve is 200 seconds or more, a lamination shaping method using the powder metal material, and a method for producing iron alloy using the lamination shaping steel material.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a steel material for additive manufacturing and a method for manufacturing an iron alloy. [Background technology]

[0002] In recent years, a method for manufacturing iron alloy shaped products by metal additive manufacturing using steel materials has been attracting attention. For example, Patent Document 1 describes a method for manufacturing an iron alloy in which the temperature of the iron alloy layer from the surface layer of the formed iron alloy layer to a predetermined number of layers and the temperature of the base stand are controlled within a specific range during the forming process. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent Publication No. 2021-195578 Summary of the Invention [Problem to be solved by the invention]

[0004] In additive manufacturing of steel materials, cracks are more likely to occur during manufacturing if the amount of C is high. This is because parts that are locally melted by a laser beam or electron beam are transformed into martensite by rapid cooling, but then the heat received from the surrounding parts when they are locally melted transforms them again into austenite, and then they are rapidly cooled again and transformed back into martensite. As martensite transformation and austenite transformation are repeated many times, volume changes can cause cracks to occur.

[0005] In Patent Document 1, the occurrence of distortion and cracks is suppressed by controlling the temperature of the iron alloy layer from the surface layer of the iron alloy layer to a predetermined number of layers and the temperature of the base during the molding process, but this does not consider strength and toughness, and there is room for improvement. Maraging steel is also a material for additive manufacturing that has a strength of 1500 MPa or more. However, because maraging steel contains large amounts of rare elements such as Co, Ni, and Mo, it is expensive and poses a high risk of resource depletion.

[0006] The present invention provides a steel material for additive manufacturing that can produce an iron alloy that is suppressed from generating cracks and has excellent strength and toughness, and a method for producing an iron alloy that uses the steel material for additive manufacturing. [Means for solving the problem]

[0007] The steel material for additive manufacturing of the present invention is In mass%, C: 0.3~0.8%, Mn: 0.6-2% Cr: 1-7%, V: 2% or less, Mo: 3% or less Contains The martensitic transformation start temperature is 130 to 200°C, This is a steel material for additive manufacturing, which has a bainite transformation start time of 200 seconds or more at the martensite transformation start temperature + 30°C on the isothermal transformation curve. [Effects of the Invention]

[0008] According to the present invention, it is possible to provide a steel material for additive manufacturing that can suppress the occurrence of cracks and produce an iron alloy that has excellent strength and toughness, and a method for producing an iron alloy that uses the above-mentioned steel material for additive manufacturing. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 1 is a schematic diagram illustrating an example of an object being manufactured by an additive manufacturing method. [Figure 2] FIG. 1 is a schematic diagram illustrating an example of an object being manufactured by an additive manufacturing method. [Figure 3] 10 is a graph showing the relationship between the number of layers of an object during additive manufacturing and the temperature of point P on the object. [Figure 4] 1 is a graph showing the relationship between time and temperature after the maximum temperature value at point P in an object being fabricated by additive manufacturing falls below the austenite transformation temperature, and showing the isothermal transformation curve. DETAILED DESCRIPTION OF THE INVENTION

[0010] Hereinafter, embodiments of the present invention will be described in detail.

[0011] [Steel materials for additive manufacturing] The steel material for additive manufacturing of the present invention contains, by mass%, C: 0.3 to 0.8%, Mn: 0.6 to 2%, Cr: 1 to 7%, V: 2% or less, and Mo: 3% or less, and has a martensitic transformation start temperature of 130 to 200°C, and a bainite transformation start time of 200 seconds or more at the martensitic transformation start temperature + 30°C on the isothermal transformation curve. The steel material for additive manufacturing of the present invention can suppress the occurrence of cracks and can produce an iron alloy (shaped object) that is excellent in strength and toughness.

[0012] First, we will explain how the temperature of a particular part of a molded object changes during additive manufacturing using the steel material for additive manufacturing of the present invention (hereinafter also referred to as "molding material"). As an example, the temperature of point P shown in Figure 1 will be described. Figure 1 is a schematic diagram showing the state during additive manufacturing using powder bed fusion. In powder bed fusion, as shown in Figure 1, powdered building material is typically supplied to base plate 1 from a feed bed (not shown), and a laser beam (not shown, which may be an electron beam, is also applicable; the same applies below) is irradiated to sinter the building material and create a shape. In Figure 1, a layer L1 is being layered on object 2 being built. An arbitrary point on the building surface G1 is designated as point P. Note that base plate 1 is a component of the additive manufacturing machine that serves as the foundation for object 2. The temperature of base plate 1 can be controlled using a base heater (not shown) or the like. The building surface is the surface that is irradiated with the laser beam.

[0013] The state of FIG. 1 is set as the number of layers stacked, and many more layers are stacked on the layer L1. FIG. 2 shows the state in which the shaping has progressed and n layers (layers L1 to Ln) have been stacked (number of layers: n).

[0014] FIG. 3 shows how the temperature at point P changes with the number of layers when additive manufacturing is performed from the state in FIG. 1 to the state in FIG. 2 (for example, the case where n=10 is shown). FIG. 3 is a graph showing the relationship between the number of layers and the temperature when a modeling material having the chemical composition of Example 1 described below is used. The horizontal axis of FIG. 3 represents the number of layers, and the vertical axis represents the temperature at point P. In the case of the layer number 1 in Figure 3, the laser beam is irradiated at point P of the modeling material, and the temperature of point P rises to over 2000°C, indicating that the modeling material is melted by the laser beam. Then, as the laser beam irradiation position moves away from point P, the temperature of point P drops rapidly. At layer number 2, the temperature change is shown when a building material (powder) is laid on top of layer L1 and layer L2 is built. When the laser beam is irradiated near point P, the temperature at point P rises sharply and then drops sharply. This is the same for subsequent layers up to layer number 10.

[0015] (Martensitic transformation start temperature) The steel material for additive manufacturing of the present invention has a martensitic transformation start temperature of 130 to 200°C. In metal additive manufacturing, the area from the build surface of the object to several tens of layers toward the base plate generally undergoes rapid heating and cooling between a temperature higher than the austenite transformation temperature and the temperature of the base plate (base plate temperature) each time it is irradiated with laser (see Figure 3). Setting the base plate temperature higher than the martensitic transformation start temperature can avoid martensitic transformation, thereby reducing the risk of cracks. However, if the martensitic transformation start temperature is high, the heat from the base plate maintained at a higher temperature will soften the molded object, so a lower martensitic transformation start temperature is better. On the other hand, if the amount of C added is increased too much in an attempt to lower the martensitic transformation start temperature, the elongation will become extremely small. Therefore, in the present invention, the martensitic transformation start temperature of the steel material for additive manufacturing is set to 130 to 200°C. The martensitic transformation start temperature of the steel material for additive manufacturing of the present invention is preferably 150 to 200°C, more preferably 170 to 195°C. FIG. 3 also shows the martensitic transformation start temperature of the molding material of Example 1 and the base plate temperature.

[0016] (Bainite transformation start time) The steel material for additive manufacturing of the present invention has a bainite transformation start time (hereinafter simply referred to as "bainite transformation start time") of 200 seconds or more at a temperature 30°C above the martensitic transformation start temperature in the isothermal transformation curve. As mentioned above, even if the martensitic transformation start temperature of the steel material for additive manufacturing is set to 130 to 200°C to avoid martensitic transformation, if the steel material is held at the base plate temperature for a long time, it will transform into a ferrite-pearlite structure or a bainite structure. In the present invention, in order to obtain an iron alloy with high strength (preferably a tensile strength of 1500 MPa or more), the structure after transformation is preferably bainite. Furthermore, in order to minimize the number of bainite transformations in order to reduce strain due to phase transformation, the time until bainite transformation begins is set longer than the time until the next layer begins to be built in additive manufacturing. The time until the next build begins varies depending on the size of the object to be built and the performance of the additive manufacturing machine, but is usually 180 seconds or less, so it is sufficient if transformation does not begin for at least 200 seconds. Therefore, in the present invention, the bainite transformation start time at the martensitic transformation start temperature + 30°C in the isothermal transformation curve of the steel material for additive manufacturing is set to 200 seconds or more. The bainite transformation start time of the steel material for additive manufacturing of the present invention is preferably 500 seconds or more, and more preferably 800 seconds or more. In addition, the cooling rate in additive manufacturing is so high that it can be considered as an isothermal transformation at the base plate temperature.

[0017] Fig. 4 shows the relationship between time and temperature after the maximum temperature value at point P falls below the austenite transformation temperature in Example 1. In addition, Fig. 4 shows the isothermal transformation curve of the molding material in Example 1 with a dotted line. As shown in Figure 4, the steel material for additive manufacturing in Example 1 has a bainite transformation start time of 200 seconds or more at the martensitic transformation start temperature + 30°C on the isothermal transformation curve. Therefore, bainite transformation does not occur between the time one layer is formed and the time the next layer is formed, and distortion due to phase transformation is reduced.

[0018] (chemical composition) The steel material for additive manufacturing of the present invention contains, by mass%, C: 0.3 to 0.8%, Mn: 0.6 to 2%, Cr: 1 to 7%, V: 2% or less, and Mo: 3% or less. Unless otherwise specified, the content of each alloying element is a mass value with the entire steel material for additive manufacturing being 100%.

[0019] The steel material for additive manufacturing of the present invention is In mass%, C: 0.3~0.8%, Mn: 0.6-2% Cr: 1-7%, V: 2% or less, Mo: 3% or less Ti: 1% or less, Ni: 5% or less and the remainder is Fe and unavoidable impurities. The steel material for additive manufacturing having the above chemical composition can further improve toughness.

[0020] The steel material for additive manufacturing of the present invention contains, by mass %, 0.3 to 0.8% C. C is an element that can increase the strength of steel materials. Furthermore, the more C is added, the lower the martensitic transformation temperature. To keep the martensitic transformation start temperature below 200°C, the amount of C added must be 0.3% or more. On the other hand, if more than 0.8% is added, sufficient elongation cannot be obtained even if a bainite structure is obtained, so the amount must be below this. The C content in the steel material for additive manufacturing is preferably 0.3 to 0.5%, and more preferably 0.35 to 0.45%.

[0021] The steel material for additive manufacturing of the present invention contains, by mass %, 0.6 to 2% Mn and 1 to 7% Cr. Mn and Cr have the effect of lowering the martensite transformation temperature and increasing the time to start bainite transformation. Focusing particularly on the time to start bainite transformation, the addition of 0.6% or more of Mn and 1% or more of Cr is necessary. However, even if the addition of Mn exceeds 2% or Cr exceeds 7%, the effect of further suppressing transformation strain by suppressing the number of bainite transformations is no longer obtained, so the amounts are set below these limits. The Mn content in the steel material for additive manufacturing is preferably 1.0 to 2.0%, and more preferably 1.2 to 1.8%. The Cr content in the steel material for additive manufacturing is preferably 3.0 to 6.0%, more preferably 4.5 to 5.5%.

[0022] The steel material for additive manufacturing of the present invention contains, by mass%, 2% or less of V. It is desirable to add V because it improves strength and toughness by forming carbonitrides. However, if it is added in an amount of 2% or more, excessive nitrides are formed, reducing ductility, so the amount should be below this level. The V content in the steel material for additive manufacturing is preferably 0.05 to 1.0%, more preferably 0.10 to 0.50%, and even more preferably 0.20 to 0.50%.

[0023] The steel material for additive manufacturing of the present invention contains, by mass%, 3% or less of Mo. It is desirable to add Mo because it forms carbides to improve strength and toughness. However, adding Mo in excess of the amount of carbon present that can form carbides does not provide any benefits, so the amount added is limited to 3% or less. The content of Mo in the steel material for additive manufacturing is preferably 2% or less, more preferably 1% or less, and even more preferably 0.3 to 0.8%.

[0024] The steel material for additive manufacturing of the present invention may contain, by mass %, 1% or less of Ti. Adding Ti can form carbides to improve strength and toughness, but adding more than the amount of carbon present that can form carbides does not provide any effect, so the amount added is preferably 1% or less.

[0025] The steel material for additive manufacturing of the present invention may contain, by mass %, 5% or less of Ni. Adding Ni can improve toughness, but the effect saturates when added in an amount of 5% or more, so the amount added is preferably 5% or less.

[0026] The steel material for additive manufacturing of the present invention preferably has the above chemical composition, with the balance being Fe and unavoidable impurities. The content of Fe in the steel material for additive manufacturing of the present invention is preferably 70% by mass or more, more preferably 80% by mass or more, and even more preferably 90% by mass or more.

[0027] Inevitable impurities are components that can be unavoidably mixed in from raw materials or the environment when manufacturing steel materials for additive manufacturing, such as P, S, Cu, etc. Typically, the P and S contents are 0.1% by mass or less, and the Cu content is 0.5% by mass or less.

[0028] The form of the steel material for additive manufacturing of the present invention is not particularly limited, but is preferably in the form of a powder or sheet, more preferably in the form of a powder. When the steel material for additive manufacturing is in the form of a powder, additive manufacturing can be performed by powder bed fusion.

[0029] When the steel material for additive manufacturing of the present invention is in powder form, the particle size is not particularly limited, and may be any known particle size suitable for additive manufacturing (preferably for manufacturing using a 3D printer) (for example, a volume average particle size (D) measured with a laser diffraction particle size distribution analyzer). 50 ) can be 30 to 60 μm, etc.

[0030] The method for producing the steel material for additive manufacturing of the present invention is not particularly limited, and any known method can be used. When the steel material for additive manufacturing of the present invention is in powder form, it can be produced by employing a known method (for example, gas atomization, water atomization, plasma atomization, centrifugal atomization, etc.).

[0031] [Method of manufacturing iron alloys] The method for producing an iron alloy of the present invention comprises: A method for producing an iron alloy by a metal additive manufacturing method, In a manufacturing process of the present invention, a steel material for additive manufacturing is irradiated with at least one of a laser beam and an electron beam to melt the steel material for additive manufacturing, thereby manufacturing an iron alloy layer while stacking it on a base plate, This is a method for manufacturing an iron alloy, in which the temperature of at least an area within 1 mm from the forming surface toward the base plate is heated and maintained at or above the martensitic transformation start temperature.

[0032] According to the method for producing an iron alloy of the present invention, it is possible to obtain an iron alloy (shaped article) that is suppressed from generating cracks and has excellent strength and toughness.

[0033] The method for producing an iron alloy of the present invention involves heating and maintaining the temperature of at least a region within 1 mm from the build surface toward the base plate at or above the martensitic transformation start temperature, thereby avoiding martensitic transformation and reducing the risk of cracks occurring in the iron alloy. There are no particular limitations on the method for heating and maintaining the temperature of the region within 1 mm from the build surface toward the base plate at or above the martensitic transformation start temperature, but examples include using a base heater to control the base plate temperature at or above the martensitic transformation start temperature, or using an infrared heater to control the temperature of the region within 1 mm from the build surface toward the base plate at or above the martensitic transformation start temperature.

[0034] (base plate temperature) In the method for producing an iron alloy of the present invention, it is preferable to set the base plate temperature higher than the martensitic transformation temperature, thereby avoiding the martensitic transformation of the iron alloy. Even if martensitic transformation is avoided, if the iron alloy is held at the base plate temperature for a long time, it will transform into a ferrite-pearlite structure or a bainite structure. In the present invention, in order to obtain an iron alloy with high strength (preferably a tensile strength of 1500 MPa or more), the structure after transformation is preferably bainite. Therefore, the base plate temperature is preferably set to a temperature 0 to 50°C higher than the martensitic transformation temperature, more preferably 5 to 30°C higher, and even more preferably 10 to 30°C higher.

[0035] The metal additive manufacturing method in the method for producing an iron alloy of the present invention is preferably a metal additive manufacturing method using a powdered manufacturing material, and more preferably a manufacturing method using a 3D printer. A known 3D printer can be used. The method of additive manufacturing is not particularly limited, but for example, powder bed fusion, directed energy deposition, etc. are preferred, with powder bed fusion being particularly preferred.

[0036] The iron alloy (shaped object) produced by the manufacturing method of the present invention is free from cracking and has excellent strength and toughness (preferably a tensile strength of 1500 MPa or more and an elongation of 7% or more), making it suitable for use in a variety of structural components, such as automobile parts. It is particularly effective in lightweight components, taking advantage of the high degree of freedom in shape that is a feature of additive manufacturing. It can also be used in molds with internal cooling circuits. [Example]

[0037] EXAMPLES The present invention will be explained in more detail below with reference to examples and comparative examples, but the present invention is not limited to these.

[0038] Powders (average particle size 45 μm) were prepared with the composition shown in Table 1 below, with the balance being Fe and unavoidable impurities. The powders of each example and comparative example were used as steel materials for additive manufacturing (modeling materials).

[0039] The steel materials for additive manufacturing of each of the Examples and Comparative Examples in Table 1 were additively manufactured using a modeling machine (3D printer) to produce round bars with a diameter of 12 mm and a length of 80 mm. The modeling machine used was an EOS M290, and the modeling conditions were an output of 260 W, a layer thickness of 40 μm, a scan speed of 700 mm / s, a hatch distance of 0.1 mm, and a preheating temperature of 200°C. The obtained round bars were machined on all sides to form test pieces.

[0040] The martensitic transformation start temperature and bainite transformation start time of the steel materials for additive manufacturing of each example and comparative example are listed in Table 1. The martensitic transformation start temperature and bainite transformation start time were measured using a Formaster testing machine using steel ingot test pieces with a diameter of 3 mm and a length of 10 mm.

[0041] <Presence or absence of cracks> The resulting shaped object was observed visually and under an optical microscope to check for the presence or absence of cracks.

[0042] <Measurement of tensile strength> Test specimens were prepared by machining the entire surface of a 12 mm diameter, 80 mm long round bar manufactured using the steel materials for additive manufacturing listed in Table 1. Tensile tests were carried out at room temperature at a tension rate of 5 mm / min using an autograph manufactured by Shimadzu Corporation. The tensile strength (MPa) was measured in accordance with JIS Z 2241, and the test specimen shape was a JIS No. 14A round bar test specimen.

[0043] <Measurement of elongation> The elongation at break of the above test specimens was measured by measuring the change in the distance between the gauge marks during the above tensile strength measurement using an extensometer.

[0044] [Table 1]

[0045] It can be seen from Table 1 that the steel materials for additive manufacturing of Examples 1 to 4 can be used to produce crack-free shaped articles with a tensile strength of 1500 MPa or more and an elongation of 7% or more.

[0046] Example 1 represents a typical composition. Figures 3 and 4 show the temperature changes during additive manufacturing using the molding material of Example 1. As shown in Figure 3, by setting the base plate temperature to 200°C, higher than the martensitic transformation start temperature, the sample of Example 1 could be stably molded without cracking. Furthermore, as the molding progressed and the maximum temperature fell below the austenite transformation temperature, as shown in Figure 4, the temperature fluctuated between the temperature range in which the austenite structure remained, the temperature range in which transformation to a bainite structure progressed, and the temperature range in which transformation to a ferrite-pearlite structure progressed. However, the structure after molding was a mixed structure of bainite and retained austenite. Therefore, high elongation was obtained despite the high strength.

[0047] Example 2 is an example in which the amounts of Mn and Cr added are increased compared to Example 1, and a large elongation is obtained. Example 3 is an example in which the amount of C added was increased, and a large tensile strength was obtained. Example 4 is an example in which Mo was actively added, and a large elongation was obtained. Comparative Example 1 is an example in which the martensitic transformation start temperature was higher than 200° C. and the base plate temperature was set lower than the martensitic transformation start temperature. Cracks occurred during molding, making it impossible to perform a tensile test. Comparative Example 2 is an example in which the bainite transformation start time was shorter than 200 seconds. Although the martensite transformation start temperature also slightly exceeded 200°C, no cracks occurred. When a tensile test was performed on this sample, it broke at a very small elongation, so it was not possible to measure the strength. Comparative Example 3 is an example in which the amount of added C was too large. When a tensile test was carried out on this sample, it broke at a very small elongation, so that the strength could not be measured.

[0048] Although the embodiments of the present invention have been described above, the present invention is not limited to the above-described embodiments, and modifications, improvements, etc. are possible as appropriate.

[0049] Furthermore, this specification describes at least the following:

[0050] (1) In mass %, C: 0.3~0.8%, Mn: 0.6-2% Cr: 1-7%, V: 2% or less, Mo: 3% or less Contains The martensitic transformation start temperature is 130 to 200°C, A steel material for additive manufacturing, in which the bainite transformation start time at the martensite transformation start temperature +30°C on the isothermal transformation curve is 200 seconds or more.

[0051] According to (1), it is possible to obtain an iron alloy in which the occurrence of cracks is suppressed and which has excellent strength and toughness.

[0052] (2) In mass %, C: 0.3~0.8%, Mn: 0.6-2% Cr: 1-7%, V: 2% or less, Mo: 3% or less Ti: 1% or less, Ni: 5% or less The steel material for additive manufacturing according to (1), containing the above, with the remainder consisting of Fe and unavoidable impurities.

[0053] According to (2), the toughness can be further improved.

[0054] (3) The steel material for additive manufacturing according to (1) or (2) above, which is in powder form.

[0055] According to (3), additive manufacturing can be achieved using powder bed fusion. (4) A method for manufacturing an iron alloy by a metal additive manufacturing method, A manufacturing process in which the steel material for additive manufacturing according to any one of (1) to (3) is irradiated with at least one of a laser beam and an electron beam to melt the steel material for additive manufacturing, thereby manufacturing an iron alloy layer while stacking it on a base plate, A method for manufacturing an iron alloy, in which the temperature of at least an area within 1 mm from the forming surface toward the base plate is heated and maintained at or above the martensitic transformation start temperature.

[0056] According to (4), it is possible to obtain an iron alloy in which crack generation is suppressed and which has excellent strength and toughness. [Explanation of symbols]

[0057] 1 base plate 2 Sculptures G1, Gn modeling surface

Claims

1. In mass%, C: 0.3-0.8%, Mn: 0.6-2%, Cr: 1 to 7%, V: 2% or less, Mo: 3% or less and the balance being Fe and unavoidable impurities, The martensitic transformation start temperature is 130 to 200°C, A steel material for additive manufacturing, in which the bainite transformation start time at a temperature 30°C above the martensitic transformation start temperature in an isothermal transformation curve is 200 seconds or more.

2. The steel material for additive manufacturing according to claim 1, which is in powder form.

3. A method for producing an iron alloy by a metal additive manufacturing method, A manufacturing process in which the steel material for additive manufacturing according to claim 1 or 2 is irradiated with at least one of a laser beam and an electron beam to melt the steel material for additive manufacturing, thereby manufacturing an iron alloy layer while stacking it on a base plate, A method for manufacturing an iron alloy, in which the temperature of at least a region within 1 mm from the shaping surface toward the base plate is heated and maintained at a temperature 0 to 50°C higher than the martensitic transformation start temperature.

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