Method for manufacturing stainless steel and stainless steel

Abstract

To provide a method for manufacturing stainless steel having strength comparable to that of a nickel-based superalloy, and the stainless steel.SOLUTION: A method for manufacturing stainless steel comprises: cooling a liquid phase containing at least iron, carbon, chromium, and nickel at a cooling rate of 1,000°C / second or more; and forming a eutectic solidified structure by performing eutectic solidification on at least a part of the liquid phase.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for producing stainless steel, and to a stainless steel. [Background technology]

[0002] Currently, the use of hydrogen energy has been attracting attention as a way to create a low-carbon society, and one of the most representative uses of hydrogen energy is power generation using a hydrogen gas turbine.

[0003] High-strength nickel-based superalloys are often used as materials for hydrogen gas turbines, but nickel-based superalloys are expensive, which is an obstacle to the widespread use of hydrogen energy on a global scale, including in developing countries.

[0004] Therefore, in recent years, research has been progressing into hydrogen gas power generation using relatively inexpensive iron-based high-temperature materials (hereinafter sometimes referred to as "stainless steel"), and there is a desire for the development and widespread use of high-strength stainless steels comparable to nickel-based superalloys. In addition to hydrogen gas turbines, high-strength stainless steel is expected to be used in a wide range of fields, including aircraft and rocket engines, nuclear reactors, various plants, and electronic products.

[0005] Conventional stainless steels that have been proposed include stainless steels intended for use in a wide range of applications, such as shafts, gears, valves, turbine parts, and aircraft parts (see, for example, Patent Document 1). [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Publication No. 2023-122798 Summary of the Invention [Problem to be solved by the invention]

[0007] However, no stainless steel having strength comparable to that of nickel-based superalloys has yet been proposed.

[0008] The stainless steel of the present disclosure has been made in view of the above points, and aims to provide a method for producing stainless steel and stainless steel having strength comparable to that of nickel-based superalloys. [Means for solving the problem]

[0009] A method for producing stainless steel according to one embodiment of the present disclosure involves cooling a liquid phase containing at least iron, carbon, chromium, and nickel at a temperature drop rate of 1,000°C / second or more to eutectically solidify at least a portion of the liquid phase, thereby forming a eutectic solidification structure. A stainless steel according to one embodiment of the present disclosure has a eutectic solidification structure consisting of at least an iron-containing γ phase and a metal carbide phase as its main structure, and the distance between adjacent metal carbide phases is 2 μm or less. Another stainless steel according to an embodiment of the present disclosure has a eutectic solidification structure consisting of at least an iron-containing γ phase and a metal carbide phase as its main structure, and has a Vickers hardness of 350 HV or more. [Effects of the Invention]

[0010] According to the present disclosure, it is possible to provide a method for producing stainless steel and stainless steel having strength comparable to that of nickel-based superalloys. [Brief explanation of the drawings]

[0011] [Figure 1] 1 is a flowchart showing an example of a method for manufacturing stainless steel according to a first embodiment. [Figure 2] 1 is a schematic diagram showing an example of the structure of a stainless steel manufactured by a stainless steel manufacturing method according to a first embodiment. FIG. [Figure 3] 10 is a flowchart showing an example of a method for producing stainless steel according to a second embodiment. [Figure 4]FIG. 4 is a schematic diagram showing an example of the structure of a stainless steel manufactured by a stainless steel manufacturing method according to a second embodiment. [Figure 5] FIG. 2 is a schematic diagram showing an example of the structure of a TIG welding machine used in the examples. [Figure 6] FIG. 1 is a diagram showing a button-shaped ingot produced in an example. [Figure 7] 7 is an image of the structure observed using an optical microscope at cross section AA shown in FIG. 6 in the example. [Figure 8] 7 is an image of the structure observed using a transmission electron microscope at cross section AA shown in FIG. 6 in the example. DETAILED DESCRIPTION OF THE INVENTION

[0012] Hereinafter, embodiments of the present invention will be described in detail. Note that the embodiments are not limited by the following description and can be modified as appropriate within the scope of the present invention. Furthermore, in this specification, unless otherwise specified, the symbol "to" indicating a range of numerical values ​​means that the numerical values ​​before and after it are included as the lower limit and upper limit.

[0013] (Stainless steel manufacturing method) The method for producing stainless steel according to the present disclosure preferably includes a step of cooling a liquid phase containing at least iron, carbon, chromium, and nickel at a temperature drop rate of 1,000°C / second or more to eutectic solidify at least a portion of the liquid phase and form a eutectic solidification structure (hereinafter sometimes referred to as the "rapid eutectic solidification step"), and a step of performing heat treatment after eutectic solidification (hereinafter sometimes referred to as the "heat treatment step"). If necessary, the method may further include other steps such as a step of forming a liquid phase containing at least iron, carbon, chromium, and nickel (hereinafter sometimes referred to as the "melting step").

[0014] As described above, no stainless steel having strength comparable to that of nickel-based superalloys has been proposed to date. However, as a result of extensive research by the present inventors, they have discovered a stainless steel with higher strength based on the rule of mixtures, the Hall-Petch rule, and the Orowan mechanism, as well as a method for producing the same, as described below.

[0015] When stainless steel has a composite structure of the γ phase and the metal carbide phase, the strength σ of the stainless steel is expressed by the rule of mixtures in the following formula (1). c As the strength of the stainless steel increases, the strength of the stainless steel increases. [Number 1] σ=f c σ c +(1-f c )σ γ ...Equation (1) In equation (1), f c denotes the volume fraction of the metal carbide phase, and σ c indicates the strength of the metal carbide phase, and σ γ indicates the strength of the γ phase.

[0016] Furthermore, the strength σ of stainless steel satisfies the Hall-Petch law of the following formula (2), and increases as the distance D between adjacent metal carbide phases decreases. [Number 2] σ=k / D 1 / 2 ...Equation (2) In equation (2), k is a coefficient that depends on the matrix material, and D represents the distance between adjacent metal carbide phases.

[0017] Furthermore, the shear stress τ of stainless steel increases as the spacing x between adjacent fine metal carbide particles decreases, based on the Orowan mechanism of the following formula (3). [Number 3] τ=μb / x Equation (3) In formula (3), μ represents the modulus of rigidity of the matrix material, b represents the absolute value of the Burgers vector, and x represents the spacing between adjacent fine metal carbide particles.

[0018] This disclosure describes a method for producing higher strength stainless steel based on the rule of mixtures, the Hall-Petch rule, and the Orowan mechanism.

[0019] Below, an embodiment of the stainless steel manufacturing method of the present disclosure will be described in detail, but the embodiment is not limited to the following description and can be modified as appropriate within the scope that does not deviate from the gist of the present invention.

[0020] First Embodiment FIG. 1 is a flowchart showing an example of a method for producing stainless steel according to the first embodiment. The method for producing stainless steel according to the first embodiment includes a melting step S1 and a rapid eutectic solidification step S2.

[0021] The melting step S1 is a step of melting a metal or a master alloy to obtain a liquid phase L containing at least iron, carbon, chromium, and nickel.

[0022] The melting temperature in the melting step S1 is not particularly limited as long as it is a temperature at which the metal or mother alloy can be melted, and can be appropriately selected depending on the purpose.

[0023] The melting method in the melting step S1 is not particularly limited and can be appropriately selected depending on the purpose. For example, melting can be performed using arc discharge.

[0024] The rapid eutectic solidification step S2 is a step of eutectic solidification of at least a portion of the liquid phase containing at least iron, carbon, chromium, and nickel. That is, it is sufficient to eutectically solidify a portion of the liquid phase so that the main structure of the stainless steel becomes a eutectic solidification structure, and it is not necessary for all of the liquid phase to form a eutectic solidification structure.

[0025] The cooling rate in the rapid eutectic solidification step S2 is 1,000°C / second or higher. This allows the structure to be solidified rapidly, resulting in finer, denser distribution of the metal carbide phase, which in turn allows for the production of higher-strength stainless steel based on the Hall-Petch law. Furthermore, in the rapid eutectic solidification step S2, solute diffusion is insufficient, resulting in poor distribution of solute between the solid and liquid phases, resulting in excess solute being trapped in the solid phase, which leads to the precipitation of a large amount of fine carbide particles during the subsequent heat treatment process.

[0026] The liquid phase contains at least iron, carbon, chromium, and nickel, and may further contain other components as required.

[0027] The carbon content in the liquid phase is not particularly limited as long as it can form a eutectic solidification structure, and can be selected appropriately depending on the purpose, but it is preferably 2.0 mass % or more and 3.0 mass % or less with respect to the total amount of the liquid phase.

[0028] The chromium content in the liquid phase is not particularly limited as long as it can form a eutectic solidification structure, and can be appropriately selected depending on the purpose. However, it is preferably 25.0 mass % or more and 35.0 mass % or less relative to the total amount of the liquid phase.

[0029] The content of nickel contained in the liquid phase is not particularly limited as long as it can form a eutectic solidification structure, and can be selected appropriately depending on the purpose, but it is preferably 6.0 mass % or more and 9.0 mass % or less with respect to the total amount of the liquid phase.

[0030] The iron content in the liquid phase is not particularly limited as long as it can form a eutectic solidification structure, and can be appropriately selected depending on the purpose. For example, the iron content can be the amount remaining after subtracting the contents of carbon, chromium, nickel, and other components described below from the total amount of the liquid phase.

[0031] The other components are not particularly limited and can be appropriately selected depending on the purpose. Examples include silicon, manganese, niobium, molybdenum, tungsten, titanium, boron, and nitrogen.

[0032] The silicon content in the liquid phase is not particularly limited as long as it can form a eutectic solidification structure, and can be selected appropriately depending on the purpose, but it is preferably 0.5 mass % or more and 3.5 mass % or less relative to the total amount of the liquid phase.

[0033] The manganese content in the liquid phase is not particularly limited as long as it can form a eutectic solidification structure, and can be appropriately selected depending on the purpose, but it is preferably 0.5 mass % or more and 5.5 mass % or less relative to the total amount of the liquid phase.

[0034] The niobium content in the liquid phase is not particularly limited as long as it can form a eutectic solidification structure, and can be appropriately selected depending on the purpose, but it is preferably 0.1 mass % or more and 3.5 mass % or less relative to the total amount of the liquid phase.

[0035] The molybdenum content in the liquid phase is not particularly limited as long as it can form a eutectic solidification structure, and can be appropriately selected depending on the purpose, but it is preferably 0.1 mass % or more and 3.5 mass % or less relative to the total amount of the liquid phase.

[0036] The tungsten content in the liquid phase is not particularly limited as long as it can form a eutectic solidification structure, and can be appropriately selected depending on the purpose. However, it is preferably 0.1 mass % or more and 3.5 mass % or less relative to the total amount of the liquid phase.

[0037] The titanium content in the liquid phase is not particularly limited as long as it can form a eutectic solidification structure, and can be selected appropriately depending on the purpose, but it is preferably 0.1 mass % or more and 3.5 mass % or less relative to the total amount of the liquid phase.

[0038] The boron content in the liquid phase is not particularly limited as long as it can form a eutectic solidification structure, and can be appropriately selected depending on the purpose. However, it is preferably 0.001 mass % or more and 0.300 mass % or less relative to the total amount of the liquid phase.

[0039] The nitrogen content in the liquid phase is not particularly limited as long as it can form a eutectic solidification structure, and can be selected appropriately depending on the purpose. However, it is preferably 0.001 mass % or more and 0.300 mass % or less relative to the total amount of the liquid phase.

[0040] Fig. 2 is a schematic diagram showing an example of the structure of stainless steel produced by the stainless steel production method according to the first embodiment. As shown in Fig. 2, the stainless steel produced by the stainless steel production method according to the first embodiment has a γ phase 10 and a metal carbide phase 20, and further, the distance between the metal carbide phases 20 is 2 µm or less.

[0041] Second Embodiment The method for producing stainless steel according to the second embodiment is the same as that of the first embodiment, and further includes a step of performing heat treatment at 700° C. or higher after eutectic solidification. FIG. 3 is a flowchart showing an example of a method for producing stainless steel according to the second embodiment. The method for producing stainless steel according to the second embodiment includes a melting step S1, a rapid eutectic solidification step S2, and a heat treatment step S3. Note that the same components as those already described in the first embodiment are designated by the same reference numerals, and their description will be omitted.

[0042] The heat treatment step S3 is a step of performing heat treatment after the eutectic solidification. By performing the heat treatment step S3, the solute trapped in the solid phase in the rapid eutectic solidification step S2 can be precipitated by aging, and fine metal carbide particles can be formed in the structure.

[0043] The particle size of the metal carbide particles is preferably, for example, 100 nm or less. By including metal carbide particles with a particle size of 100 nm or less in the structure, it is possible to produce a stainless steel with higher strength based on the Orowan mechanism.

[0044] The heating temperature in the heat treatment step S3 is not particularly limited as long as it can cause aging precipitation of the structure, and can be appropriately selected depending on the purpose, but is preferably 700°C or higher and 900°C or lower.

[0045] The heating time in the heat treatment step S3 is not particularly limited and can be appropriately selected depending on the purpose, but is preferably 0.5 hours or more and 4 hours or less.

[0046] Fig. 4 is a schematic diagram showing an example of the structure of stainless steel produced by the method for producing stainless steel according to the second embodiment. As shown in Fig. 4, the stainless steel produced by the method for producing stainless steel according to the second embodiment further includes metal carbide particles 30 having a particle size of 100 nm or less in addition to the stainless steel shown in Fig. 2.

[0047] (stainless steel) A stainless steel according to one embodiment of the present disclosure has a eutectic solidification structure consisting of at least an iron-containing γ phase and a metal carbide phase as its main structure, and the distance between adjacent metal carbide phases is 2 μm or less. Furthermore, the stainless steel according to one embodiment of the present disclosure preferably has a layered structure in which a gamma phase and a metal carbide phase are layered one on top of the other, in order to enable the production of a stainless steel with higher strength, and more preferably has a layered structure in which the layer spacing is 100 nm or less.

[0048] In a stainless steel according to an embodiment of the present disclosure, the entire structure may be a eutectic structure, or may include both a eutectic structure and a primary crystal. Of these, it is preferable that the entire structure be a eutectic structure, since this allows for the production of a stainless steel with higher strength.

[0049] The Vickers hardness of the stainless steel according to an embodiment of the present disclosure is 350 HV or more, preferably 450 HV or more, and more preferably 500 HV or more. The stainless steel according to an embodiment of the present disclosure can have a Vickers hardness of 350 HV or more by eutectic solidification of at least a portion of a liquid phase containing at least iron, carbon, chromium, and nickel to form a eutectic solidification structure. Furthermore, the stainless steel according to an embodiment of the present disclosure can have a Vickers hardness of 450 HV or more by cooling the liquid phase containing at least iron, carbon, chromium, and nickel at a temperature drop rate of 1,000°C / s or more to form a lamellar eutectic structure, and the Vickers hardness can be increased to 500 HV or more by heat treating the resulting structure.

[0050] The method for measuring Vickers hardness is not particularly limited and can be appropriately selected depending on the purpose. For example, the method can be measured in accordance with JIS B 7735. [Example]

[0051] Examples of the present invention will be described below, but the present invention is not limited to these examples in any way.

[0052] Cast iron (Fe-3.35% by mass C-1.67% by mass Si), pure Cr, and pure Ni were used as raw materials and placed in a graphite mold of the TIG welding machine shown in Figure 5. Arc melting was performed under an Ar shielding gas atmosphere to cast a button-shaped ingot (Fe-2.0% by mass C-33.7% by mass Cr-6.6% by mass Ni-1.0% by mass Si) shown in Figure 6. The temperature drop rate during this process was 500°C / s or less. Thereafter, arc discharge was applied to arc discharge portion 40 of the button-shaped ingot shown in Fig. 6, and the ingot was partially melted and then left to cool. The cooling rate of arc discharge portion 40 at this time was 1,000°C / sec or more.

[0053] Thereafter, the structure was observed using an optical microscope at cross section AA shown in Fig. 6. The image is shown in Fig. 7. As shown in Fig. 7, a primary crystal structure consisting of the γ phase and a eutectic structure consisting of the γ phase and a metal carbide phase were observed at the arc discharge area 40 and the other area 50.

[0054] Furthermore, the eutectic structures in the arc discharge area 40 and the other area 50 (areas 40' and 50', respectively) were observed using a transmission electron microscope (TEM). The images are shown in FIG. 8. As shown in FIG. 8, the eutectic structure in the arc discharge area 40 is composed of a γ phase and a metal carbide phase (Cr 23 An ultrafine layered structure of metal carbide phase (Cr C6) was observed. The layer spacing of the metal carbide phase was several tens of nanometers. On the other hand, in the eutectic structure of the other part 50, metal carbide phase (Cr 23 C6) was observed. The particle spacing of the metal carbide phase was approximately 1 μm.

[0055] Furthermore, when the Vickers hardness of the ingot after arc discharge was measured, the Vickers hardness of the arc discharge portion 40 was 470 HV, and the Vickers hardness of the other portion 50 was 360 HV. The Vickers hardness was measured in accordance with JIS B 7735.

[0056] Next, the ingot after arc discharge was heat treated at 800°C for 1 hour to perform aging precipitation. When the Vickers hardness of the ingot after aging precipitation was measured, the Vickers hardness of the arc discharge portion 40 was 550 HV, and the Vickers hardness of the other portion 50 was 370 HV.

[0057] The present invention includes, for example, the following aspects. <1> A method for producing stainless steel, comprising cooling a liquid phase containing at least iron, carbon, chromium, and nickel at a temperature drop rate of 1,000°C / sec or more to cause at least a portion of the liquid phase to eutectic solidify, thereby forming a eutectic solidification structure. <2> A liquid phase containing at least iron, carbon, chromium, and nickel is cooled at a temperature decreasing rate of 1,000°C / second or more to form a eutectic solidification structure containing a primary crystal structure and metal carbides; The content of the primary crystal structure is 30% by volume or less relative to the total amount of the stainless steel. <1> A method for producing stainless steel according to claim 1. <3> After eutectic solidification, heat treatment is performed at 700°C or higher. <1> or <2> A method for producing stainless steel according to claim 1. <4> A stainless steel characterized in that its main structure is a eutectic solidification structure consisting of a γ phase containing at least iron and a metal carbide phase, and the distance between the metal carbide phases is 2 μm or less. <5> Further, the particle size of the metal carbide particles is 100 nm or less. <4> The stainless steel described in <6> Stainless steel characterized by having a eutectic solidification structure consisting of a gamma phase containing at least iron and a metal carbide phase as its main structure, and having a Vickers hardness of 350 HV or more. <7> It has a primary crystal structure, The content of the primary crystal structure is 30% by volume or less relative to the total amount of the stainless steel. <4> or <6> The stainless steel described in

[0058] As described above, the present invention has been described based on specific embodiments and examples, but these embodiments and examples are presented merely as examples, and the present invention is not limited to the above embodiments and examples. The above embodiments can be embodied in various other forms, and various combinations, omissions, substitutions, additions, modifications, etc. can be made without departing from the spirit of the invention. These embodiments and their modifications are included in the scope and spirit of the invention, and are also included in the inventions described in the claims and their equivalents. [Explanation of symbols]

[0059] 10 γ phase 20 Metal carbide phase 30 Metal carbide particles

Claims

1. A method for producing stainless steel, comprising cooling a liquid phase containing at least iron, carbon, chromium, and nickel at a temperature decreasing rate of 1,000°C / second or more to eutectic solidify at least a portion of the liquid phase, thereby forming a eutectic solidification structure.

2. a liquid phase containing at least iron, carbon, chromium, and nickel is cooled at a temperature decreasing rate of 1,000°C / second or more to form a eutectic solidification structure containing a primary crystal structure and metal carbides; 2. The method for producing stainless steel according to claim 1, wherein the content of the primary crystal structure is 30% by volume or less relative to the total amount of the stainless steel.

3. 3. The method for producing stainless steel according to claim 1, wherein heat treatment is carried out at 700°C or higher after eutectic solidification.

4. A stainless steel characterized in that its main structure is a eutectic solidification structure consisting of a γ phase containing at least iron and a metal carbide phase, and the distance between the metal carbide phases is 2 μm or less.

5. The stainless steel of claim 4, further comprising metal carbide particles having a particle size of 100 nm or less.

6. A stainless steel characterized by having a eutectic solidification structure consisting of a γ phase containing at least iron and a metal carbide phase as its main structure, and having a Vickers hardness of 350 HV or more.

7. It has a primary crystal structure, 7. The stainless steel according to claim 4, wherein the content of the primary crystal structure is 30% by volume or less relative to the total amount of the stainless steel.

Images