Steel for blade, steel for martensitic blade, and method for producing same

Abstract

Provided is steel for a blade having higher tensile strength and ductility than in the past, steel for a martensitic blade, and a method for producing the same.　The steel for a blade and the steel for a martensitic blade are characterized by: having a component composition comprising, in mass%, 0.35-0.51% of C, 0.20-0.95% of Si, 0.1-1.5% of Mn, 9.6-12.8% of Cr, Mo and W alone or in combination such that (Mo + W / 2) represents 1.5-2.1%, V and Nb alone or in combination such that (V + Nb) represents 0.5% or less, and 0.10% or less (excluding 0) of N, the balance including Fe and inevitable impurities; and satisfying the relational expression 30[C + N] - 2.16 ≤ [Cr + Mo + W / 2] ≤ 19.44[C + N] + 4.58. In the method for producing a steel for a martensitic blade, the quenching temperature during quenching is set to 1050-1120°C, the tempering temperature during tempering is set to 320-380°C, and there is obtained steel for a martensitic blade having a tensile strength equal to or greater than 2000 MPa and an elongation equal to or greater than 7.0%.

Description

Cutlery steel, martensitic cutlery steel and its manufacturing method

[0001] The present invention relates to a cutlery steel, a martensitic cutlery steel, and a method for producing the same.

[0002] Martensitic stainless steel is widely used for cutting tools such as razors and kitchen knives because it not only achieves high hardness through quenching and tempering, but also resists rust due to its 12-13% Cr content.

[0003] The sharpness of a blade is mainly determined by the hardness of the cutting edge, the angle at which the blade is sharpened, and the distribution of hard particles, but hardness is an essential characteristic for improving sharpness. For example, in Patent Document 1, the applicant of the present application describes a blade having a composition containing, by mass %, C: 0.45 to 0.55%, Si: 0.4 to 1.0%, Mn: 0.5 to 1.0%, Cr: 12 to 14%, Mo: 2.1 to 2.8%, and the balance being Fe and unavoidable impurities, and in a state after quenching and tempering, it is possible to measure M of 0.1 μm or more by a transmission electron microscope. ３ Patent Document 2 proposes a strip steel for cutlery having a hardness of 630 HV or more and no observed C carbides. Patent Document 2 also proposes a cutlery characterized in that at least the portion forming the cutting edge is composed of an Fe-based cutlery alloy containing Fe as the main component, 4 to 13 mass% of Cr, and 0.4 to 0.65 mass% of C, in which 90% or more by volume of the Fe-based main phase is a martensite phase, and in which, when the cross-sectional structure is observed, the dimensions of the (Fe, Cr)-based carbides formed in the Fe-based main phase are 5 μm or less. Patent Document 2 also proposes a cutlery that is less likely to chip due to the inclusion of 0.1 to 0.3% by mass of N and 0.2 to 1% by mass of Mo.

[0004] JP 2018-522139 A JP 2002-212679 A

[0005] In metallic materials, high hardness generally leads to low ductility, and the cutting edge will chip when subjected to a heavy load. Therefore, balancing hardness and ductility is important. It is known that hardness correlates with tensile strength, with higher hardness tending to result in higher tensile strength. Therefore, materials with high tensile strength and high ductility can be considered to be materials that can produce blades with excellent sharpness and chip resistance. The blade strip steel described in Patent Document 1 is a useful invention with a hardness of 630 HV or higher, but no investigation has been conducted into combining the aforementioned high tensile strength and ductility. Patent Document 2 also describes a blade with a maximum dimension of (Fe, Cr)-based carbides of 2 to 4 μm. However, the dimensions of the (Fe, Cr)-based carbides are large, exceeding 2 to 4 μm, suggesting the possibility that cracks originating from coarse carbides may prevent sufficient ductility. Therefore, the object of the present invention is to provide a blade steel that, after quenching and tempering, possesses both the high tensile strength and high ductility required for blades.

[0006] In order to solve the above problems, the present inventors investigated methods for improving tensile strength and ductility in a well-balanced manner, and as a result, they discovered that by strictly adjusting the contents of C and N, as well as Cr and Mo, among the steel components, it is possible to provide high levels of tensile strength and ductility in steel (martensitic steel for cutlery) after quenching and tempering, and thus arrived at the present invention. Specifically, one aspect of the present invention is a steel for cutlery, characterized by a composition, by mass%, of 0.35-0.51% C, 0.20-0.95% Si, 0.1-1.5% Mn, 9.6-12.8% Cr, 1.5-2.1% Mo and W, either alone or in combination (Mo + W / 2), 0.5% or less V and Nb, either alone or in combination (V + Nb), 0.10% or less N (excluding 0), the balance being Fe and unavoidable impurities, and satisfying the relationship: 30[C + N] - 2.16 ≤ [Cr + Mo + W / 2] ≤ 19.44[C + N] + 4.58. Preferably, the carbide area fraction in the cross-sectional structure is 14.0-18.5%, and the average circle-equivalent diameter of the carbides is 0.20-0.45 μm, with the maximum being less than 1.10 μm.

[0007] Another aspect of the present invention is a martensitic cutlery steel having the chemical composition of the cutlery steel described above, and having a tensile strength of 2000 MPa or more and an elongation of 7.0% or more. Preferably, the carbide area ratio in the cross-sectional structure is 0.5 to 6.3%, and the average equivalent circle diameter of the carbides is 0.20 to 0.45 μm, with the maximum being less than 1.10 μm.

[0008] Another aspect of the present invention is a method for producing a martensitic cutlery steel, which comprises quenching and tempering the cutlery steel at a quenching temperature of 1050 to 1120°C and a tempering temperature of 320 to 380°C, thereby obtaining a martensitic cutlery steel having a tensile strength of ≥ 2000 MPa and an elongation of ≥ 7.0%.

[0009] The cutlery steel of the present invention has high tensile strength and higher ductility than conventional steels after quenching and tempering, and can solve the problem of chipping during use.

[0010] 1 is a scanning electron microscope photograph showing the cross-sectional structure of a martensitic cutlery steel according to an example of the present invention; 2 is a scanning electron microscope photograph showing the cross-sectional structure of a martensitic cutlery steel according to a comparative example; 3 is a diagram showing the stroke-tensile stress curve of a martensitic cutlery steel according to an example of the present invention; and 4 is a diagram showing the stroke-tensile stress curve of a martensitic cutlery steel according to a comparative example.

[0011] An embodiment of the present invention will be described below. However, the present invention is not limited to the embodiment described herein, and appropriate combinations and improvements are possible within the scope of the technical concept of the invention. The martensitic cutlery steel of the present invention has a metal structure primarily composed of a ferrite phase before quenching. Therefore, the martensitic cutlery steel of the present invention before quenching will be referred to as the "cutlery steel" and the martensitic cutlery steel of the present invention after quenching will be referred to as the "martensitic cutlery steel." First, the reasons for limiting the chemical composition of the cutlery steel of the present invention will be explained. In the description of the chemical composition, the unit "%" used to represent the content of each element means "mass %." C: 0.35-0.51% C is an important element that dissolves from carbides into the matrix at the austenitizing temperature during quenching and determines the hardness of the martensite formed by quenching. Here, the carbon in the steel is divided into those that dissolve in the matrix and those that precipitate as carbides. The ratio is determined by the interaction between Cr and Mo, so it is important that the Cr and Mo contents also fall within the composition ranges and relationship formulas described below. To obtain a martensitic cutlery steel with high tensile strength and higher ductility than conventional steels, suitable for the present invention, the lower limit is set to 0.35%. A preferred lower limit is 0.39%, and a more preferred lower limit is 0.40%. On the other hand, too much carbon leads to excessive carbides and coarsening of the carbides (i.e., larger average and maximum equivalent circle diameters), which reduces ductility and may cause chipping. Therefore, the upper limit is set to 0.51%. A preferred upper limit is 0.49%, a more preferred upper limit is 0.47%, and a still more preferred upper limit is 0.45%.

[0012] Si: 0.20 to 0.95% Si is used as a deoxidizer during the refining of cutlery steel, and is also an element that dissolves in the steel and suppresses softening during low-temperature tempering, so the lower limit is set to 0.20%. On the other hand, excessive content reduces the toughness of the cutlery steel, which may reduce cold workability during cold rolling, for example. Therefore, the upper limit of Si is set to 0.95%. A preferred upper limit is 0.90%, a more preferred upper limit is 0.70%, an even more preferred upper limit is 0.50%, and a particularly preferred upper limit is 0.40%.

[0013] Mn: 0.1 to 1.5% Like Si, Mn also acts as a deoxidizer during refining, dissolving in the matrix of the cutlery steel and improving its hardenability. If the Mn content is too low, the hardenability of the cutlery steel will decrease, and there is a possibility that the cutlery steel will not be hardened, particularly in the center of the wall thickness. Therefore, the lower limit of Mn is set to 0.1%. On the other hand, an excessive Mn content will decrease hot workability, so the upper limit of Mn is set to 1.5%. The preferred upper limit is 1.2%, and the more preferred upper limit is 1.0%.

[0014] Cr: 9.6-12.8% Cr is an element necessary for forming a strong non-conductive film on martensitic cutlery steel and achieving excellent corrosion resistance. To achieve this corrosion resistance, the cutlery steel must contain at least 9.6% Cr. The preferred lower limit of Cr is 10.0%, more preferably 10.5%, and even more preferably 11.0%. On the other hand, excessive Cr content can lower the Ms point and increase retained austenite during quenching, resulting in a decrease in tensile strength. Therefore, in order to achieve both high tensile strength and good ductility, the upper limit of Cr is set to 12.8%. The preferred upper limit of Cr is 12.5%, and even more preferably 12.2%.

[0015] Mo and W alone or in combination (Mo+W / 2): 1.5-2.1% Mo and W have similar effects, and are defined as (Mo+W / 2) based on the relationship of atomic weight. Mo and W can be contained alone or in combination. Mo and W are highly effective in stabilizing the passivity of martensitic cutlery steel, and are effective elements in improving corrosion resistance by making the pitting potential more noble in chloride solutions. In addition, Mo and W are elements that form their own carbides, and M ３ In the tempering temperature range, only C diffuses into M. ３ However, when certain amounts of Mo and W are present in the matrix, M ３It is believed to inhibit the aggregation and coarsening of C carbides. To achieve these effects, at least 1.5% is required. On the other hand, excessive addition of Mo and W reduces workability during hot working and, for the same reason as Cr, can cause a decrease in tensile strength due to an increase in retained austenite during quenching. Therefore, the upper limit of the (Mo + W / 2) content is set to 2.1%. The preferred lower limit of the (Mo + W / 2) content is 1.6%, and the preferred upper limit of the (Mo + W / 2) content is 2.0%. In general, W is more expensive than Mo, so if one of them is to be added, it is better to select Mo.

[0016] N: 0.10% or less (excluding 0) N is an important element that dissolves in martensite and improves not only hardness and tensile strength but also corrosion resistance. N in steel is divided into those that dissolve in the matrix and those that dissolve in carbides. The ratio is determined by the interaction between Cr and Mo. Therefore, as with C, it is important to keep Cr and Mo within the composition ranges and relationship formulas described below. To ensure the effects of N, a content of 0.01% or more is preferable. However, too much N can cause bubbles to form during casting, significantly reducing manufacturability. Therefore, the N content should be less than 0.10% (excluding 0). The preferred upper limit is 0.08%, and the more preferred upper limit is 0.05%.

[0017] The cutlery steel of the present invention satisfies the above-mentioned chemical composition ranges while also satisfying the following formula: 30[C+N]-2.16≦[Cr+Mo+W / 2]≦19.44[C+N]+4.58 The balance between tensile strength and ductility of martensitic cutlery steel tends to be determined primarily by the contents of C, N, Cr, and Mo. In order to improve the sharpness of a blade and reduce the risk of chipping, it is necessary to increase the tensile strength of the cutlery steel after quenching and tempering and to precisely control the contents of C, N, Cr, and Mo. By satisfying the above formula, the cutlery steel of the present invention can achieve high levels of tensile strength and ductility of martensitic cutlery steel.

[0018] V and Nb, alone or in combination (V + Nb): 0.5% or less. Nb and V have similar effects and can be contained alone or in combination. Nb has a high affinity with carbon and forms thermally very stable MC carbides. Because these MC carbides are thermally very stable, they remain without dissolving in high-temperature austenite, suppressing austenite coarsening by pinning the carbides. Similarly, V finely disperses thermally very stable carbides, suppressing austenite coarsening and improving wear resistance. However, because MC carbides containing Nb and V are thermally very stable, they remain without dissolving in high-temperature austenite, reducing the amount of carbon dissolved in martensite and tending to reduce tensile strength. Furthermore, a high content increases the likelihood of cracking due to reduced cold workability. Therefore, even when V and Nb are contained, the upper limit of the (V + Nb) content in this embodiment is 0.5%. The upper limit of the (V + Nb) content is 0.4%, and more preferably, the upper limit of the (V + Nb) content is 0.3%. The more preferable upper limit of the (V + Nb) content is 0.14%, the even more preferable upper limit of the (V + Nb) content is 0.12%, and the particularly preferable upper limit of the (V + Nb) content is 0.10%. V and Nb may be contained as unavoidable impurities in a range of less than 0.05% each. In general, Nb is more expensive than V, so if one of them is to be contained, it is better to select V.

[0019] The cutlery steel according to the present invention may contain the following elements: Ni + Cu: 0.5% or less. Ni and Cu are effective elements for improving corrosion resistance against non-oxidizing acids such as sulfuric acid, and may be contained alone or in combination. However, they may lower the Ms point and increase retained austenite, resulting in a decrease in tensile strength. Therefore, even when contained, the upper limit of the (Ni + Cu) content is set to 0.5%. A preferred upper limit of the (Ni + Cu) content is 0.4%, and a more preferred upper limit of the (Ni + Cu) content is 0.3%. Ni and Cu may be contained as unavoidable impurities in a range of less than 0.15% each. Compared to Cu, Ni may increase hardness and deteriorate cold workability when dissolved in the cutlery steel. Therefore, if either one of them is to be contained, it is recommended to select Cu.

[0020] The cutlery steel according to the present invention may contain the following elements: Co: 0.5% or less Co is an element that dissolves in martensite and increases temper softening resistance. However, for applications where there is a possibility of contact with the human body, such as razor materials, Co may be a cause of metal allergies, so the cutlery steel according to this embodiment may contain Co in a range of 0.5% or less.

[0021] In this embodiment, the components other than those mentioned above are Fe and unavoidable impurities. Examples of unavoidable impurity elements include P, S, Al, Ti, and O. These may be contained within the following ranges that do not impair the effects of the present invention: P≦0.04%, S≦0.03%, Al≦0.1%, Ti≦0.1%, and O≦0.05%.

[0022] The cutlery steel of this embodiment contains carbides, and the carbide area ratio in the cross-sectional structure is preferably 14.0 to 18.5%. By setting the carbide area ratio within the above range, a martensitic cutlery steel with high tensile strength and high ductility, as described below, can be obtained. A more preferable upper limit for the carbide area ratio is 18.2%, an even more preferable upper limit for the carbide area ratio is 18.0%, and a particularly preferable upper limit for the carbide area ratio is 17.8%. A more preferable lower limit for the carbide area ratio is 14.5%, an even more preferable lower limit for the carbide area ratio is 15.0%, and a particularly preferable upper limit for the carbide area ratio is 15.3%. Furthermore, the average circle-equivalent diameter (area-equivalent circle diameter) of the carbides in the cross-sectional structure is preferably 0.20 to 0.45 μm. A more preferable upper limit for the average circle-equivalent diameter is 0.40 μm, and a particularly preferable upper limit for the average circle-equivalent diameter is 0.36 μm. The preferred lower limit of the average equivalent circle diameter is 0.23 μm, and the more preferred lower limit of the average equivalent circle diameter is 0.25 μm. The maximum equivalent circle diameter (area equivalent circle diameter) of carbides in the cross-sectional structure is preferably less than 1.1 μm. A more preferred upper limit of the maximum equivalent circle diameter is 1.05 μm, and an even more preferred upper limit of the average equivalent circle diameter is 1.00 μm. Note that the carbide area ratio and the average and maximum equivalent circle diameter in this embodiment are determined based on the field area of ​​100 μm photographed with a scanning electron microscope (magnification 10,000 times) in a cross-sectional structure parallel to the processing direction (stretching direction of rolling) of the cutlery steel. ２ The carbides can be observed in the above fields of view and calculated by image analysis. The carbides to be analyzed in the image analysis are limited to those with an equivalent circle diameter of 0.10 μm or more. By limiting the scope to those with an equivalent circle diameter of 0.10 μm or more, inclusions other than carbides can be excluded from the measurement targets. Furthermore, the identification of carbides can be confirmed by elemental mapping using an EPMA (electron probe microanalyzer) attached to a scanning electron microscope. By processing cutlery steel having the characteristics described above, it is possible to obtain martensitic cutlery steel with high tensile strength and good ductility.

[0023] Here, the above-mentioned cutlery steel is a steel mainly composed of a ferrite phase, or in other words, it can be expressed as a ferritic cutlery steel. When this cutlery steel mainly composed of a ferrite phase is quenched and tempered, the metal structure transforms into a martensite phase, and a martensitic cutlery steel, which will be described later, can be obtained. Here, "mainly composed of a ferrite phase" means that when the microstructure of the steel is observed at room temperature, the proportion of the ferrite phase in the matrix, excluding carbides, is 90% or more in terms of area ratio.

[0024] Next, an embodiment of the martensitic cutlery steel of the present invention will be described. By quenching and tempering a cutlery steel having the above-described chemical composition, a martensitic cutlery steel with high tensile strength and ductility can be obtained. The tensile strength of the martensitic cutlery steel of this embodiment, measured at room temperature (normal temperature), is 2000 MPa or more. It is preferably 2030 MPa or more, and more preferably 2050 MPa or more. There is no particular upper limit, but since higher tensile strength reduces ductility and contributes to edge chipping, it is set to 2300 MPa. Furthermore, the elongation of the martensitic cutlery steel of this embodiment, measured at room temperature, is 7.0% or more. It is preferably 7.3% or more. The cutlery steel before quenching can be produced by annealing a hot-rolled material having the above-described chemical composition through batch annealing, continuous annealing, or the like, and then subjecting the annealed cold-rolled material to one or more cold working processes (e.g., cold rolling, etc.).

[0025] The martensitic cutlery steel of this embodiment contains carbides, and the carbide area ratio in the cross-sectional structure is preferably 0.5 to 6.3%. By setting the carbide area ratio within the above range, a good cutlery with high ductility can be obtained. A more preferable upper limit of the carbide area ratio is 6.2%, an even more preferable upper limit of the carbide area ratio is 6.0%, and a particularly preferable upper limit of the carbide area ratio is 5.9%. Furthermore, a more preferable lower limit of the carbide area ratio is 0.8%, an even more preferable lower limit of the carbide area ratio is 1.0%, and a particularly preferable upper limit of the carbide area ratio is 1.5%. Furthermore, since coarse carbides reduce the ductility of cutlery as described above, it is preferable that the average circle-equivalent diameter (area-equivalent circle diameter) of the carbides in the cross-sectional structure be 0.20 to 0.45 μm, with a maximum of less than 1.10 μm. A more preferable upper limit of the average equivalent circle diameter is 0.43 μm, and an even more preferable upper limit of the average equivalent circle diameter is 0.41 μm. A preferable lower limit of the average equivalent circle diameter is 0.25 μm, and more preferably 0.30 μm. A more preferable maximum upper limit of the equivalent circle diameter is 1.05 μm, and an even more preferable upper limit of the average equivalent circle diameter is 1.00 μm. Note that the carbide area ratio and the average and maximum equivalent circle diameter in this embodiment are calculated based on the field area of ​​100 μm photographed with a scanning electron microscope (magnification 10,000 times) in a cross-sectional structure parallel to the processing direction (stretching direction of rolling) of the martensitic cutlery steel. ２ The carbides can be observed in the above fields of view and calculated by image analysis. The carbides to be analyzed in the image analysis are limited to those with an equivalent circle diameter of 0.10 μm or more. By limiting the scope to those with an equivalent circle diameter of 0.10 μm or more, inclusions other than carbides can be excluded from the measurement targets. Furthermore, the identification of carbides can be confirmed by elemental mapping using an EPMA (electron probe microanalyzer) attached to a scanning electron microscope. By processing martensitic cutlery steel with the characteristics described above, it is possible to obtain excellent cutlery that is sharp and less prone to chipping.

[0026] Next, a method for manufacturing the martensitic cutlery steel of the present invention will be described. In the manufacturing method of the present invention, the cutlery steel having the above-mentioned composition range is quenched and tempered. The quenching temperature is preferably set to 1050 to 1120°C, and the tempering temperature is preferably set to 320 to 380°C. In the cutlery steel of the present invention, if the quenching temperature is less than 1050°C, the carbides do not dissolve sufficiently in austenite, resulting in low tensile strength. Furthermore, if the quenching temperature exceeds 1120°C, the excessive dissolved C and N significantly reduce ductility. For this reason, the quenching temperature is set to 1050 to 1120°C. The preferred lower limit of the quenching temperature is 1070°C, and the more preferred lower limit is 1090°C.

[0027] The martensitic cutlery steel of the present invention may be subjected to sub-zero treatment after the quenching process. The temperature during sub-zero treatment is -50°C or lower. Adjusting the temperature to this level tends to facilitate obtaining the high tensile strength that is a feature of the present invention. Although no particular lower limit is set, the lower limit may be set to, for example, -196°C, assuming treatment with liquid nitrogen. In the sub-zero treatment of this embodiment, a mixture of dry ice and alcohol at -75°C is used, but liquefied carbon dioxide or liquid nitrogen may also be used. Furthermore, electric refrigeration equipment may be used, or a gas such as carbon dioxide may also be used.

[0028] In the manufacturing method of this embodiment, tempering is performed after the quenching (subzero treatment) step. By setting the tempering temperature to 320 to 380°C, a martensitic cutlery steel having a tensile strength of 2000 MPa or more and an elongation of 7.0% or more can be obtained. In the martensitic cutlery steel of the present invention, if the tempering temperature is less than 320°C, the ductility tends to be excessively low. On the other hand, if the tempering temperature exceeds 380°C, a large amount of carbides precipitates from the martensite structure, resulting in a decrease in tensile strength. The preferred upper limit of the tempering temperature is 370°C, and the preferred lower limit of the tempering temperature is 330°C.

[0029] Hot-rolled materials with a thickness of 2.0 mm and having the chemical composition shown in Table 1 were annealed in a batch annealing furnace, then cold-rolled and annealed to a thickness of 0.1 to 0.5 mm, to prepare examples of the present invention (Steel Nos. 1 to 8) and comparative examples (Steel Nos. 100 to 115). Table 1 also indicates whether the relationship: 30[C+N]-2.16≦[Cr+Mo+W / 2]≦19.44[C+N]+4.58 is satisfied, with a "Good" mark indicating that the relationship is satisfied, and an "Unsatisfactory" mark indicating that the relationship is not satisfied. Subsequently, the average and maximum circle-equivalent diameters of carbides and the carbide area ratio of the obtained cutlery steel (before quenching) were measured. The area ratio and circle-equivalent diameter of the cutlery steel were measured using a scanning electron microscope (magnification 10,000x) to measure the cross-sectional structure of the cutlery steel parallel to the elongation direction of the rolling process, with a field area of ​​100 μm 2 Carbides of 0.10 μm or larger in size in the above fields of view were measured using an image analyzer. The measurement results are shown in Table 2. In both the inventive and comparative cutlery steels, the proportion of ferrite phase in the matrix excluding carbides was 90% or more in terms of area ratio. In Table 1, the range of 0.15% or less for Ni + Cu is the level of impurities normally contained. In addition, 0.03% or less for V + Nb is the level of impurities normally contained.

[0030]

[0031]

[0032] As a result of the measurements, the average circle-equivalent diameter of carbides in the cross-sectional structure of the cutlery steel of the invention examples was 0.32 to 0.36 μm, the maximum was 1.01 μm or less, and the carbide area ratio was 15.3 to 18.4%. For Comparative Examples 101, 102, 104, and 105, the average circle-equivalent diameter, maximum carbide area ratio, and carbide area ratio were comparable to those of the invention examples. It was confirmed that Comparative Example 103 had a larger carbide area ratio than the invention examples, and Comparative Example 109 had a smaller carbide area ratio than the invention examples. Furthermore, it was confirmed that Comparative Examples 106 and 107 had larger average and maximum circle-equivalent diameters of carbides than the invention examples, Comparative Example 110 had a larger maximum circle-equivalent diameter of carbides than the invention examples, and Comparative Example 115 had larger maximum circle-equivalent diameters and carbide area ratios than the invention examples.

[0033] Next, the cutlery steel samples (cutlery steel before quenching) of the invention examples and comparative examples were quenched by heating to 1,070°C in an Ar atmosphere and then rapidly cooled, followed by tempering at the tempering temperatures listed in Table 3 to obtain martensitic cutlery steel samples. Note that martensitic cutlery steel samples Nos. 1 to 8 and 100 to 115 in Table 3 are martensitic cutlery steel samples obtained by quenching and tempering the corresponding steel Nos. 1 to 8 and 100 to 115 in Table 1. Only No. 108 underwent subzero treatment after quenching. Furthermore, sample No. 9 in Table 3 is a martensitic cutlery steel sample obtained by quenching sample No. 4 in Table 1 at the quenching temperature listed in Table 3. Sample No. 116 in Table 3 is a martensitic cutlery steel sample obtained by quenching steel No. 1 in Table 1. Samples Nos. 117 to 123 in Table 3 are martensitic cutlery steel samples obtained by tempering Steel No. 3 in Table 1 at the temperatures listed in Table 2, and Samples Nos. 117 to 123 in Table 3 are martensitic cutlery steel samples obtained by tempering Steel No. 4 in Table 1 at the temperatures listed in Table 2. Subsequently, the average and maximum circle-equivalent diameters of carbides and the carbide area ratio in the obtained martensitic cutlery steels after quenching and tempering were measured. The area ratio and circle-equivalent diameter of the martensitic cutlery steels were measured in a cross-sectional structure parallel to the elongation direction of the rolling process, with a field area of ​​100 μm as measured with a scanning electron microscope (magnification 10,000x). 2 The number of carbides of 0.10 μm or more in the above fields of view was measured using an image analyzer. The micrograph of Inventive Example 1 is shown in FIG. 1, the micrograph of Comparative Example 103 is shown in FIG. 2, and the measurement results are shown in Table 3.

[0034] To investigate the tensile strength and ductility of the martensitic cutlery steels after heat treatment, tensile test specimens were collected from each martensitic cutlery steel sample and subjected to tensile tests. Ductility was evaluated by the fracture elongation, calculated by dividing the gauge length of the fractured tensile test specimen by the gauge length. The tensile test was performed in accordance with JIS-Z2241:2022, using proportional test specimens with a gauge length of 25 mm, a parallel section width of 6.25±0.05 mm, and a parallel section thickness of 0.5 mm. The tensile strength and fracture elongation of the martensitic cutlery steels after quenching and tempering are shown in Table 3. Furthermore, as representative examples, the stroke-tensile stress curve of Inventive Example 1 (Sample No. 1) is shown in Figure 3, and the stroke-tensile stress curve of Comparative Example 103 (Sample No. 103) is shown in Figure 4.

[0035]

[0036] The results in Table 3 show that in Inventive Examples 1 to 9, the average equivalent circle diameter and area ratio of the components and carbides were well balanced, and the maximum equivalent circle diameter of the carbides was 0.95 μm or less. The tensile strength was 2006 to 2156 MPa, and the elongation at break was within the range of 7.5 to 9.7%, demonstrating good tensile strength and ductility. On the other hand, Comparative Examples 101, 103 to 105, 108, 110 to 111, 115, 118, and 121 to 123 had high tensile strengths of 2012 to 2255 MPa, but low elongations at break of 2.5 to 6.8%. Comparative Examples 100, 102, 106 to 107, 109, 112, 116, and 120 had high elongations at break of 7.0% or more, demonstrating high ductility, but low tensile strengths of 1759 to 1992 MPa. It was confirmed that Comparative Examples 113 to 114, 117, and 119 had tensile strengths of 1977 MPa or less and elongations at break of 6.5% or less, all of which were inferior to the Examples of the present invention. This confirmed that the Examples of the present invention were able to simultaneously achieve higher tensile strength and ductility than the Conventional Examples.

Claims

1. A steel for cutlery having a chemical composition, in mass%, of C: 0.35-0.51%, Si: 0.20-0.95%, Mn: 0.1-1.5%, Cr: 9.6-12.8%, Mo and W alone or in combination (Mo+W / 2): 1.5-2.1%, V and Nb alone or in combination (V+Nb): 0.5% or less, N: 0.10% or less (excluding 0), the balance being Fe and unavoidable impurities, and satisfying the relational expression 30[C+N] - 2.16 ≦ [Cr+Mo+W / 2] ≦ 19.44[C+N] + 4.

58.

2. A steel for cutlery as set forth in claim 1, in which the carbide area ratio in the cross-sectional structure is 14.0 to 18.5%, the average circle-equivalent diameter of the carbides is 0.20 to 0.45 μm, and the maximum is 1.10 μm.

3. A martensitic steel for cutlery having the chemical composition described in claim 1 and having a tensile strength of 2000 MPa or more and an elongation of 7.0% or more.

4. A martensitic cutlery steel as set forth in claim 3, wherein the carbide area ratio in the cross-sectional structure is 0.5 to 6.3%, the average circle-equivalent diameter of the carbides is 0.20 to 0.45 μm, and the maximum is 1.10 μm.

5. A method for producing martensitic cutlery steel, comprising quenching and tempering the cutlery steel according to claim 1 or 2, setting the quenching temperature during the quenching to 1050 to 1120°C and the tempering temperature during the tempering to 320 to 380°C, to obtain martensitic cutlery steel having a tensile strength of 2000 MPa or more and an elongation of 7.0% or more.

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