Martensitic stainless steel material and method for producing the same, and quenched or quenched and tempered martensitic stainless steel material

Abstract

A martensitic stainless steel material having a composition containing, on a mass basis, C: 0.305% to 0.600%, Si: 0.05% to 1.00%, Mn: 0.10% to 1.50%, P: 0.0085% to 0.0400%, S: 0.0300% or less, Cr: 13.0% to 18.0%, Ni: 0.01% to 0.30%, Mo: 0.01% to 1.00%, Al: 0.300% or less, N: 0.010% to 0.350%, Ca: 0.0001% to 0.0030%, O: 0.001% to 0.010%, with the balance being Fe and impurities, and 2.5C+N (C and N refer to the contents of C and N) being 0.85% or more. In the cross section of the martensitic stainless steel material, the average particle diameter of carbides is more than 0.50 μm to 2.00 μm or less, and carbides having a size of 10 μm or more are 0.20 pieces / cm 2 Hereinafter, the area ratio of carbides is 15.0% to 20.0%.

Description

Technical Field

[0001] This invention relates to martensitic stainless steel materials and their manufacturing methods, as well as quenched or tempered materials of martensitic stainless steel. Background Technology

[0002] Among the stainless steel materials used in various knives such as razors, scissors, and kitchen knives, martensitic stainless steel with a higher carbon content is used due to the requirement of high hardness (for example, see Patent Document 1).

[0003] However, when the carbon content is excessive, it can form carbides with alloying elements such as chromium, and these carbides tend to precipitate as coarse eutectic carbides during manufacturing. These eutectic carbides are difficult to completely dissolve even through annealing, leading to excessive softening during quenching or tempering due to reduced carbon solubility. Furthermore, besides becoming corrosion initiation points and reducing corrosion resistance, these eutectic carbides can also cause defects or irregular streak-like or island-like patterns during machining. Moreover, since carbon is a strong austenite stabilizing element, when a large amount of residual austenite remains after quenching or tempering due to carbon segregation, it can also lead to reduced hardness, deteriorated sharpness, and the formation of irregular streak-like or island-like patterns.

[0004] Therefore, Patent Document 2 proposes a martensitic stainless steel material for cutting tools, characterized by containing, by mass%, 0.40% to 0.50% C, 0.05% to 0.60% Si, 0.5% to 1.5% Mn, less than 0.035% P, less than 0.010% S, 11.0% to 15.5% Cr, 0.01% to 0.30% Ni, 0.01% to 0.30% Cu, and 0.01% Mo. The composition is as follows: Cu to Ni to Mo = 0.30%, V: 0.01% to 0.10%, Al: less than 0.02%, Sn: 0.002% to 0.10%, N: 0.010% to 0.035%, Ca: 0.0001% to 0.0010%, O: 0.001% to 0.01%, with the balance being Fe and unavoidable impurities, satisfying Cu + Ni + Mo = 0.05% to 0.30%, and inclusions larger than 10 μm being 0.2 per cm. 2 the following.

[0005] Furthermore, Patent Document 3 discloses a method for manufacturing a grain-refined martensitic stainless steel, characterized by comprising: a step of preparing a substrate having a composition of Cr: 13.0 wt% to 14.0 wt%, Mo: 1.15 wt% to 1.35 wt%, C: 0.35 wt% to 0.55 wt%, Si: 0.20 wt% to 0.50 wt%, Mn: 0.20 wt% to 0.50 wt%, P: less than 0.025 wt%, S: less than 0.020 wt%, balance: Fe and unavoidable impurity elements; a step of subjecting the substrate to at least one of a high-density dislocation introduction method and an ultra-rapid solidification method, followed by annealing to obtain a fine-grained ferritic steel; and a step of cold rolling, annealing, plastically shaping as needed, and quenching the aforementioned ferritic steel to obtain a grain-refined martensitic stainless steel.

[0006] Furthermore, Patent Document 4 discloses a method for manufacturing martensitic stainless steel (martensitic stainless steel sheet). For a stainless steel sheet blank with a composition of C: 0.25% to 0.45% by mass%, Si: less than 1.0%, Mn: 0.1% to 1.5%, Cr: 12.0% to 15.0%, Mo: 0.5% to 3.0%, N: 0.30% to 0.45%, with the balance being Fe and impurities, and a thickness of 0.1 mm or less, a heat treatment is performed by heating in a nitrogen atmosphere at a temperature exceeding 1000°C for 1 to 10 minutes followed by cooling. This heat treatment controls the number density of carbides with an equivalent circular diameter of 0.5 μm or more to 0 per 1000 μm. 2 Up to 50 per 1000μm 2 The paper describes how martensitic stainless steel manufactured using this method achieves high hardness throughout the entire thickness of the plate, from the surface to the center, during quenching and tempering, and also exhibits good corrosion resistance.

[0007] Furthermore, Patent Document 5 proposes a method for manufacturing martensitic stainless steel (stainless steel sheet), comprising: a first step of taking a steel slab having a composition comprising, by mass percent, C: 0.45% to 0.60%, Si: 0.05% to 1.00%, Mn: 0.05% to 1.00%, P: less than 0.05%, S: less than 0.020%, Cr: more than 13.0% to less than 16.0%, Ni: 0.10% to 1.00%, and N: 0.010% to 0.200%, with the balance being Fe and unavoidable impurities, and then... The process involves three steps: first, holding the steel slab at 1200°C to 1350°C for at least 30 minutes; second, hot-rolling the steel slab to obtain a hot-rolled steel sheet and coiling the hot-rolled steel sheet; and third, hot-rolled annealing the hot-rolled steel sheet to obtain a hot-rolled annealed steel sheet. In the second step, the hot rolling passes have an ending temperature of 1050°C or higher and a reduction rate of 20% or higher, with at least three passes, and the coiling temperature of the hot-rolled steel sheet is 600°C or higher. In the third step, the holding temperature during hot-rolled annealing is 750°C to 900°C, and the holding time is at least 10 minutes. This method describes the manufacture of martensitic stainless steel with high hardness and good surface quality.

[0008] However, the martensitic stainless steel described in Patent Document 2 sometimes produces irregular patterns because the average grain size of inclusions (especially carbides) is not controlled.

[0009] The martensitic stainless steel described in Patent Document 3 is unsuitable for mass production due to the introduction of special steps such as high-density dislocation introduction or super-rapid solidification. Furthermore, this martensitic stainless steel has a higher cost due to its higher Mo content.

[0010] The martensitic stainless steel described in Patent Document 4 has a higher cost because it involves nitrogen absorption treatment in a nitrogen atmosphere for heat treatment in order to achieve high nitriding and dissolution of coarse carbides.

[0011] The martensitic stainless steel described in Patent Document 5, while dissolving coarse carbides by heating the steel slab at high temperatures of 1200°C to 1350°C, cannot suppress the segregation of carbon (C), the cause of coarse carbides, solely through this heating method. Therefore, the re-precipitation of coarse carbides and the segregation of carbon lead to the formation or softening of retained austenite, resulting in reduced hardness or irregular texture. Furthermore, excessively high temperatures in heating the slab increase the likelihood of deformation due to its own weight.

[0012] In addition to sharpness, knives such as kitchen knives also require a long lifespan (durability of sharpness). The sharpness of a knife depends on the hardness of the material, while its lifespan depends on the hardness and wear resistance of the material. The martensitic stainless steel materials described in Patent Documents 2 to 5 do not consider the lifespan of knives as a particular issue, nor do they mention any means of improvement.

[0013] Furthermore, Patent Document 6 discloses a martensitic stainless steel material for cutting tools, characterized by containing, by mass%, 0.50% to 1.20% C, 0.10% to 1.00% Si, 0.10% to 1.00% Mn, 0.008% or less P, 0.010% or less S, 13.00% to 18.00% Cr, and one or more of Ni, 0.01% to 1.20%, Mo, and 0.01% to 1.20% V, with the balance being Fe and unavoidable impurities. This martensitic stainless steel material, by reducing the P content to below 0.008%, can maintain the tool's lifespan for a long period.

[0014] Existing technical documents

[0015] Patent documents

[0016] Patent Document 1: Japanese Patent Application Publication No. 2000-273587

[0017] Patent Document 2: Japanese Patent Application Publication No. 2018-9231

[0018] Patent Document 3: Japanese Patent Application Publication No. 2003-313612

[0019] Patent Document 4: International Publication No. 2019 / 146743

[0020] Patent Document 5: International Publication No. 2021 / 220754

[0021] Patent Document 6: Japanese Patent Application Publication No. 2020-45511 Summary of the Invention

[0022] The problem the invention aims to solve

[0023] The martensitic stainless steel described in Patent Document 6 requires the use of special raw materials with extremely low P content in order to reduce the P content to below 0.008%, thus resulting in higher costs. Furthermore, this martensitic stainless steel does not specifically address the generation of irregular patterns as a problem.

[0024] The present invention was made in view of the above-mentioned problems, and its purpose is to provide a martensitic stainless steel material with high hardness and high corrosion resistance after quenching or quenching and tempering, excellent wear resistance, and the ability to suppress the occurrence of irregular patterns, as well as a method for manufacturing the same.

[0025] In addition, the purpose of this invention is to provide quenching or tempering materials for martensitic stainless steel that have high hardness and high corrosion resistance, excellent wear resistance, and can suppress the occurrence of irregular patterns.

[0026] Solution for solving the problem

[0027] The inventors conducted in-depth research on martensitic stainless steel and found that although carbides in martensitic stainless steel cause reduced corrosion resistance, hardenability (hardness), and irregular texture, they also contribute to improved wear resistance. Furthermore, they discovered that by controlling the composition of martensitic stainless steel, the average particle size of carbides, the number of carbides with a size of 10 μm or larger, and the area ratio of carbides, all the above problems can be solved, and the present invention was completed.

[0028] That is, the present invention relates to a martensitic stainless steel material having the following composition: by weight, containing C: 0.305% to 0.600%, Si: 0.05% to 1.00%, Mn: 0.10% to 1.50%, P: 0.0085% to 0.0400%, S: less than 0.0300%, Cr: 13.0% to 18.0%, Ni: 0.01% to 0.30%, Mo: 0.01% to 1.00%, Al: less than 0.300%, N: 0.010% to 0.350%, Ca: 0.0001% to 0.0030%, O: 0.001% to 0.010%, with the balance being Fe and impurities, and 2.5C+N (C and N being the content of C and N) being 0.85% or more;

[0029] In the cross-section of the aforementioned martensitic stainless steel, the average particle size of the carbides is between 0.50 μm and 2.00 μm, and the number of carbides with a size of 10 μm or larger is 0.20 per cm. 2 The area fraction of the aforementioned carbides is 15.0% to 20.0%.

[0030] In addition, the present invention relates to a quenched or quenched and tempered material of martensitic stainless steel, which has the following composition: by weight, it contains C: 0.305% to 0.600%, Si: 0.05% to 1.00%, Mn: 0.10% to 1.50%, P: 0.0085% to 0.0400%, S: less than 0.0300%, Cr: 13.0% to 18.0%, Ni: 0.01% to 0.30%, Mo: 0.01% to 1.00%, Al: less than 0.300%, N: 0.010% to 0.350%, Ca: 0.0001% to 0.0030%, O: 0.001% to 0.010%, with the balance being Fe and impurities, and 2.5C+N (C and N are the contents of C and N) is 0.85% or more;

[0031] In the cross section of the aforementioned quenched or quenched and tempered material, the average particle size of the carbides is 0.10 μm to 0.50 μm, the area fraction of the aforementioned carbides is 2.0% to 15.0%, and the area fraction of retained austenite is less than 15.0%.

[0032] Furthermore, the present invention relates to a method for manufacturing martensitic stainless steel, comprising:

[0033] The breakdown rolling step involves rough rolling a slab with the following composition to obtain a breakdown product: by weight, containing C: 0.305% to 0.600%, Si: 0.05% to 1.00%, Mn: 0.10% to 1.50%, P: 0.0085% to 0.0400%, S: less than 0.0300%, Cr: 13.0% to 18.0%, Ni: 0.01% to 0.30%, Mo: 0.01% to 1.00%, Al: less than 0.300%, N: 0.010% to 0.350%, Ca: 0.0001% to 0.0030%, O: 0.001% to 0.010%, with the balance being Fe and impurities, and 2.5C+N (C and N being the content of C and N) being 0.85% or more;

[0034] The hot rolling step involves hot rolling the aforementioned billet to obtain a hot-rolled product; and

[0035] The softening step involves heating the aforementioned hot-rolled material at a temperature exceeding Ac1 point to below (Ac1 point + 100°C) for 2 to 10 hours, followed by cooling at a cooling rate of 0.100°C / second or less.

[0036] The effects of the invention

[0037] This invention provides a method for manufacturing martensitic stainless steel with high hardness and corrosion resistance after quenching or quenching and tempering, excellent wear resistance, and the ability to suppress irregular patterns.

[0038] In addition, the present invention provides quenching or tempering materials for martensitic stainless steel with high hardness, high corrosion resistance, excellent wear resistance, and the ability to suppress irregular textures. Detailed Implementation

[0039] The embodiments of the present invention will be described in detail below. The present invention is not limited to the following embodiments. Without departing from the spirit of the present invention, modifications and improvements can be made to the following embodiments based on the common knowledge of those skilled in the art. These modifications should also be included within the scope of the present invention. This point should be understood.

[0040] In addition, the percentages for ingredients in this specification, unless otherwise specified, refer to "mass %".

[0041] (1. Martensitic stainless steel)

[0042] The martensitic stainless steel material in the embodiments of the present invention has the following composition: containing C: 0.305% to 0.600%, Si: 0.05% to 1.00%, Mn: 0.10% to 1.50%, P: 0.0085% to 0.0400%, S: less than 0.0300%, Cr: 13.0% to 18.0%, Ni: 0.01% to 0.30%, Mo: 0.01% to 1.00%, Al: less than 0.300%, N: 0.010% to 0.350%, Ca: 0.0001% to 0.0030%, O: 0.001% to 0.010%, with the balance being Fe and impurities, and 2.5C+N (C and N are the contents of C and N) is 0.85% or more.

[0043] Here, "steel" in this specification refers to materials in various forms, such as steel plates. Furthermore, "steel plate" includes the concept of steel strip. Further, "impurity" refers to components introduced during the industrial manufacturing of stainless steel due to various factors from raw materials such as ores and waste, and from various manufacturing processes, and is permitted to exist within a range that does not adversely affect the present invention. Examples of impurities include Zn, Pb, Se, Sb, H, Ga, Ta, Mg, and Zr. When these elements are present as impurities, Zn ≤ 100 ppm, Pb ≤ 100 ppm, Se ≤ 100 ppm, Sb ≤ 500 ppm, H ≤ 100 ppm, Ga ≤ 500 ppm, Ta ≤ 500 ppm, Mg ≤ 120 ppm, and Zr ≤ 120 ppm.

[0044] Furthermore, the martensitic stainless steel material in the embodiments of the present invention may further contain one or more of the following: Ti: less than 0.30%, Nb: less than 0.50%, Cu: less than 4.0%, B: less than 0.005%, V: less than 0.50%, Sn: less than 0.10%, and Co: less than 0.30%.

[0045] Furthermore, regarding the content of each element, "below xx%" refers to amounts containing less than xx% but exceeding 0% (especially exceeding the impurity level). Additionally, the numerical ranges indicated by "to" in this specification refer to the range including the values ​​listed before and after "to" as both the lower and upper limits.

[0046] The following will describe each ingredient in detail.

[0047] <C: 0.305% to 0.600%>

[0048] Carbon (C) is an essential element for achieving the desired hardness (Vickers hardness) after quenching or tempering. Specifically, to consistently achieve a hardness of 500 HV or higher, the C content must be 0.305% or more. Excessive C addition promotes sensitization during quenching, impairing corrosion resistance, and also reduces toughness after quenching or tempering through undissolved carbonitrides. Therefore, the C content must be 0.600% or less. Considering the reduction in hardness and toughness caused by variations in heating conditions during quenching or tempering, the lower limit of the C content is preferably 0.320% or more, more preferably 0.330% or more, and the upper limit is preferably 0.590% or less, more preferably 0.580% or less.

[0049] Furthermore, the numerical range of C content mentioned above can be any combination within that range. Therefore, for example, the C content can be 0.305% to 0.600%, 0.305% to 0.590%, 0.305% to 0.580%, 0.320% to 0.600%, 0.320% to 0.590%, 0.320% to 0.580%, 0.330% to 0.600%, 0.330% to 0.590%, or 0.330% to 0.580%. The numerical ranges for the contents of the other elements listed below are also the same.

[0050] <Si: 0.05% to 1.00%>

[0051] Si is an element required for deoxidation during dissolution refining and also helps suppress the formation of oxide scale during quenching. However, when the Si content is too low, it easily leads to insufficient deoxidation, increasing carbides and making these areas prone to rust initiation points, thus reducing corrosion resistance. Therefore, the Si content should be set at 0.05% or higher. Si narrows the single-phase temperature range of austenite, impairing quenching stability; therefore, the Si content should be set at 1.00% or lower. From the viewpoint of consistently obtaining the above-mentioned effects of Si, the lower limit of the Si content is preferably 0.07% or higher, more preferably 0.10% or higher, and the upper limit is preferably 0.98% or lower, more preferably 0.90% or lower.

[0052] <Mn: 0.10% to 1.50%>

[0053] Mn is added as a deoxidizer and helps improve hardenability by expanding the single-phase temperature range of austenite. When Mn is not added sufficiently, the two-phase region expands, increasing the α phase. As a result, Cr carbonitrides also increase, forming a Cr-deficient layer around them, which easily becomes a rust initiation point and reduces corrosion resistance. Therefore, the Mn content should be set to 0.10% or more. From the viewpoint of stably obtaining the above-mentioned effects of Mn, the Mn content is preferably 0.15% or more, more preferably 0.20% or more. Excessive Mn will reduce corrosion resistance, promote the formation of oxide scale during quenching, increase the subsequent grinding load, and may also increase the amount of retained austenite. Therefore, the Mn content should be set to 1.50% or less. Considering the reduction in corrosion resistance caused by MnS and other granules, the Mn content is preferably 1.30% or less, more preferably 1.00% or less.

[0054] <P: 0.0085% to 0.0400%>

[0055] Phosphorus (P) is an element present as an impurity in molten iron and ferrochrome, which are major raw materials. Furthermore, P is detrimental to the toughness and corrosion resistance of quenched or tempered materials. Therefore, the P content should be set to 0.0400% or less, preferably 0.0380% or less, and more preferably 0.0350% or less. However, excessively reducing P content would necessitate the use of high-purity raw materials, leading to increased costs. Therefore, the P content is 0.0085% or more, preferably 0.0090% or more, and more preferably 0.0100% or more.

[0056] <S: below 0.0300%>

[0057] Sulfide (S) forms sulfide inclusions, which deteriorates the general corrosion resistance (general corrosion and pitting corrosion) of steel. Furthermore, S reduces hot workability and increases the susceptibility of hot-rolled sheets to edge cracking. Therefore, the S content should be set to 0.0300% or less, preferably 0.0200% or less, and more preferably 0.0150% or less. While there is no particular limitation on the lower limit of S content, lower S content generally results in better corrosion resistance, but on the other hand, it increases the desulfurization load and manufacturing costs. Therefore, the lower limit of S content is preferably 0.0001% or more.

[0058] <Cr: 13.0% to 18.0%>

[0059] Cr is an essential element for maintaining the required corrosion resistance in martensitic stainless steel. Therefore, the Cr content should be set at 13.0% or more. However, Cr easily forms carbides, and excessive addition of Cr not only contributes to the formation of coarse carbides but also increases the amount of residual austenite after quenching or tempering. Therefore, to suppress these problems, the Cr content should be set at 18.0% or less. From the viewpoint of consistently obtaining the above-mentioned effects of Cr, the lower limit of the Cr content is preferably 13.1% or more, more preferably 13.2% or more, and the upper limit is preferably 17.8% or less, more preferably 17.5% or less.

[0060] <Ni: 0.01% to 0.30%>

[0061] Both Ni and Mn are austenite stabilizing elements and improve toughness after quenching or tempering. However, excessive Ni content can lead to reduced stamping formability in hot-rolled annealed materials due to solid solution strengthening, and may increase the amount of retained austenite after quenching or tempering. Furthermore, as a high-valence element, it increases manufacturing costs. Therefore, the Ni content should be set below 0.30%. Since Ni is effective in inhibiting pitting corrosion, its content should be above 0.01%. From the viewpoint of consistently achieving the aforementioned effects of Ni, the lower limit of the Ni content is preferably 0.02% or more, more preferably 0.03% or more, and the upper limit is preferably 0.28% or less, more preferably 0.25% or less.

[0062] <Mo:0.01% to 1.00%>

[0063] Mo is an effective element for improving the corrosion resistance of martensitic structures containing δ-ferrite. To achieve this effect, the Mo content should be 0.01% or more. However, Mo is a stabilizing element for the ferrite phase, and excessive addition will narrow the temperature range of the austenite single phase and impair its quenching properties. Therefore, the Mo content should be 1.00% or less. From the viewpoint of stably obtaining the above-mentioned effects of Mo, the lower limit of the Mo content is preferably 0.02% or more, more preferably 0.03% or more, and the upper limit is preferably 0.80% or less, more preferably 0.70% or less.

[0064] <Al: Below 0.300%>

[0065] Al is added as a deoxidizing element and also to improve oxidation resistance. However, when Al is present in excess, carbides tend to grow larger. Furthermore, Al is a ferrite stabilizing element, thus hindering austenite transformation and causing the Ac1 line to rise. Therefore, the Al content should be set to 0.300% or less, preferably 0.250% or less, and more preferably 0.200% or less. On the other hand, the lower limit of the Al content is not particularly limited, but from the viewpoint of obtaining the aforementioned effects of Al, it is preferable to be 0.001% or more, and more preferably 0.002% or more. Here, Al refers to T.Al.

[0066] <N: 0.010% to 0.350%>

[0067] Both nitrogen (N) and carbon (C) are essential elements for obtaining the desired hardness (Vickers hardness) after quenching or tempering. In particular, in embodiments of the present invention, since the content of C is reduced, N is required as a substitute. Furthermore, N also has the effect of improving corrosion resistance in the solution state. To obtain these effects, the N content needs to be set to 0.010% or more. However, N forms Cr-based nitrides, resulting in a Cr-deficient layer, which reduces corrosion resistance. In addition, excessive addition of N makes control during the steelmaking process difficult and easily leads to the formation of bubbly defects. When bubbly defects form, not only do these defects easily become rust initiation points, reducing corrosion resistance, but they may also lead to a decrease in yield. Therefore, the N content needs to be set to 0.350% or less. From the viewpoint of consistently obtaining the above-mentioned effects of N, the lower limit of the N content is preferably 0.012% or more, more preferably 0.013% or more, and the upper limit is preferably 0.300% or less, more preferably 0.250% or less, and even more preferably 0.150% or less.

[0068] <Ca: 0.0001% to 0.0030%>

[0069] Ca is added during the steelmaking process for composition adjustment. In particular, Ca acts as a powerful deoxidizer, promoting deoxidation. However, because Ca is a strong deoxidizing element, it almost always floats to the surface of the molten steel as an inclusion, leaving almost no residue. However, when excessive Ca is added, the steel inclusions may contain CaO, which can potentially become the starting point for rusting and reduce corrosion resistance. Therefore, the Ca content should be set to 0.0030% or less, preferably 0.0029% or less, and more preferably 0.0028% or less. Since fine inclusions cannot be completely removed, it is difficult to set the Ca content below 0.0001% from a manufacturing perspective. Therefore, the Ca content is set to 0.0001% or more.

[0070] <O: 0.001% to 0.010%>

[0071] To reduce inclusions, oxygen (O), like Al and Ca, is an important element. Excessive O addition increases the number of large inclusions (especially carbides) remaining in the steel, negatively impacting corrosion resistance. Therefore, the O content should be set below 0.010%. Furthermore, while lower O content is better, excessive reduction leads to increased costs; therefore, the O content is set above 0.001%. From the perspective of balancing cost and corrosion resistance, the lower limit of the O content is preferably above 0.002%, and the upper limit is preferably below 0.009%.

[0072] <2.5C+N: 0.85% or more>

[0073] As previously mentioned, C and N are essential elements for obtaining a predetermined hardness (Vickers hardness) after quenching or quenching and tempering. In embodiments of the present invention, N is substituted to reduce the C content, and C contributes 2.5 times more to the hardness than N. Therefore, from the viewpoint of obtaining the predetermined hardness, 2.5C+N should be 0.85% or more, preferably 0.90% or more. Furthermore, excessively increasing 2.5C+N will result in excessively high hardness before quenching, potentially reducing machinability and grindability. Therefore, the upper limit of 2.5C+N is preferably 1.60% or less, more preferably 1.50% or less.

[0074] <Ti: below 0.30%>

[0075] Ti is an element that forms carbonitrides and can suppress sensitization and reduced corrosion resistance caused by the precipitation of chromium carbonitrides; it is added as needed. However, excessive addition of Ti will form coarse TiN, leading to hot-rolling defects and reduced toughness. Therefore, the Ti content should be set to 0.30% or less, preferably 0.25% or less. In addition, although the lower limit of the Ti content is not particularly limited, from the viewpoint of obtaining the above-mentioned effects, it is preferred to be 0.01% or more, more preferably 0.06% or more, and even more preferably 0.10% or more.

[0076] <Nb: below 0.50%>

[0077] Nitrogen oxides (Nb) are elements that form carbonitrides and can suppress sensitization and reduced corrosion resistance caused by the precipitation of chromium carbonitrides. They are added as needed. However, excessive addition of Nb can destabilize the martensitic phase and reduce hardness. Therefore, the Nb content should be set to 0.50% or less, preferably 0.35% or less, more preferably 0.30% or less, and even more preferably 0.25% or less. While the lower limit of the Nb content is not particularly limited, from the viewpoint of achieving the aforementioned effects, it is preferably 0.01% or more, more preferably 0.05% or more.

[0078] <Cu: below 4.0%>

[0079] Cu is an effective element for improving the corrosion resistance of martensitic structures containing δ-ferrite, and as an austenite stabilizing element, it also helps improve hardenability; therefore, it should be added as needed. However, excessive addition of Cu leads to a decrease in hot workability and an increase in raw material costs. Therefore, the Cu content should be set to 4.0% or less, preferably 3.8% or less, and more preferably 3.5% or less. In addition, although the lower limit of the Cu content is not particularly limited, from the viewpoint of obtaining the above-mentioned effects, it is preferable to be 1.0% or more, more preferably 1.3% or more, and even more preferably 1.5% or more.

[0080] <B: Below 0.005%>

[0081] Boron (B) is an effective element for improving hot workability and should be added as needed. However, excessive addition of B may reduce hardenability due to the combined precipitation of borides and carbides. Therefore, the B content should be set to 0.005% or less, preferably 0.004% or less. While there is no particular limitation on the lower limit of the B content, from the viewpoint of achieving the aforementioned effects, 0.0002% or more is preferred.

[0082] <V: below 0.50%>

[0083] V is an element that forms fine carbonitrides, which helps improve corrosion resistance, and is added as needed. However, excessive addition of V may lead to coarsening of precipitates, resulting in reduced toughness after quenching or tempering. Therefore, the V content should be set to 0.50% or less, preferably 0.30% or less, and more preferably 0.20% or less. While there is no particular limitation on the lower limit of V content, V can be mixed into the alloy raw materials as an impurity and is sometimes difficult to remove during the refining process. Furthermore, from the viewpoint of achieving the above-mentioned effects, the lower limit of V content is preferably 0.01% or more, more preferably 0.02% or more, and even more preferably 0.03% or more.

[0084] <Sn: below 0.10%>

[0085] Sn is an effective element for improving corrosion resistance after quenching or quenching and tempering, and should be added as needed. However, excessive addition of Sn can promote edge cracking during hot rolling. Therefore, the Sn content should be set to 0.10% or less, preferably 0.09% or less. In addition, although the lower limit of the Sn content is not particularly limited, from the viewpoint of obtaining the above-mentioned effects, it is preferable to be 0.002% or more, and more preferably 0.05% or more.

[0086] <Co: less than 0.30%>

[0087] Co is an element that improves heat resistance and is added as needed. However, since Co is a high-valence element, excessive Co content will lead to increased manufacturing costs. Therefore, the Co content should be set to 0.30% or less, preferably 0.10% or less, and more preferably 0.05% or less. Furthermore, while there is no particular limitation on the lower limit of the Co content, from the viewpoint of achieving the aforementioned effects, 0.01% or more is preferred.

[0088] In the martensitic stainless steel of the embodiments of the present invention, the average grain size of carbides in its cross-section is greater than 0.50 μm and less than 2.00 μm. By controlling the average grain size of carbides to be greater than 0.50 μm, wear resistance is improved, thus extending the life (sharpness durability) of cutting tools. Furthermore, by controlling the average grain size of carbides to be less than 2.00 μm, the reduction in corrosion resistance and the formation of irregular patterns can be suppressed. From the viewpoint of stably ensuring these effects, the average grain size of carbides is preferably 0.55 μm to 1.80 μm, more preferably 0.60 μm to 1.50 μm.

[0089] Here, the carbides with average particle size are defined as both eutectic carbides generated during casting and precipitated carbides generated during the rolling process.

[0090] In addition, the average particle size of the carbides was calculated as follows: by observing the cross-section of the martensitic stainless steel with SEM, measuring the equivalent circle diameter of each carbide in the field of view, and calculating its average value.

[0091] In the embodiments of the present invention, the martensitic stainless steel has a cross-section containing 0.20 carbides with a size of 10 μm or larger. 2 The preferred value is 0.15 pieces / cm. 2 The following is more preferably 0.10 per cm. 2 The following points are relevant. Since carbides larger than 10 μm are prone to becoming rust initiation sites, controlling the number of carbides larger than 10 μm within this range can suppress rusting and improve corrosion resistance. Furthermore, carbides larger than 10 μm also contribute to irregular textures; therefore, controlling the number of these carbides can also suppress the formation of irregular textures. Additionally, the fewer carbides larger than 10 μm, the better; an ideal number is 0.00 per cm. 2 .

[0092] Here, the number of carbides with a size of 10 μm or larger is defined primarily as eutectic carbides formed during casting. Furthermore, the size of the carbide is defined as (major axis + minor axis) / 2.

[0093] In addition, the number of carbides with a size of 10 μm or more is calculated as follows: the number of carbides with a size of 10 μm or more is determined by observing the cross-section of the martensitic stainless steel with an optical microscope, and the number is then divided by the area of ​​the measurement region.

[0094] In the martensitic stainless steel of the embodiments of the present invention, the area fraction of carbides in its cross-section is 15.0% to 20.0%, more preferably 15.5% to 19.8%, and particularly preferably 16.0% to 19.5%. By controlling the area fraction of carbides to 15.0% or more, hardness and wear resistance are improved, thus extending the life (sharpness durability) of cutting tools while improving initial sharpness. In addition, by controlling the area fraction of carbides to 20.0% or less, the reduction in corrosion resistance and the formation of irregular patterns can be suppressed.

[0095] Here, the carbides for which the area ratio is specified are the eutectic carbides generated during casting and the precipitated carbides generated during the rolling process.

[0096] In addition, the area ratio of carbides is calculated as follows: by observing the cross-section of martensitic stainless steel with SEM, measuring the area of ​​each carbide in the field of view, dividing it by the area of ​​the field of view, and multiplying by 100.

[0097] Although the martensitic stainless steel material used in the embodiments of the present invention is not particularly limited, it is preferably hot-rolled plate, hot-rolled annealed plate, cold-rolled plate or cold-rolled annealed plate.

[0098] The martensitic stainless steel in the embodiments of the present invention exhibits high hardness and corrosion resistance after quenching or quenching and tempering, as well as excellent wear resistance and the ability to suppress the formation of irregular patterns. Therefore, it can be used in various applications requiring these properties. In particular, this martensitic stainless steel is well-suited for use in cutting tools. When this martensitic stainless steel is used in cutting tools, in addition to improving corrosion resistance and suppressing the formation of irregular patterns, it can also improve the initial sharpness of the tool while extending its lifespan (sharpness durability).

[0099] (2. Manufacturing method of martensitic stainless steel)

[0100] The martensitic stainless steel material in the embodiments of the present invention can be manufactured using a slab having the same composition as the martensitic stainless steel material described above, by a method including a billet rolling step, a hot rolling step, and a softening step (hot rolling annealing step).

[0101] The rough rolling step is the process of rough rolling a slab to obtain a rough billet.

[0102] While the conditions for the roughing rolling step are not particularly limited, it is preferable to perform roughing rolling at a temperature of 1000°C or higher but less than 1200°C for 1 to 10 hours, followed by a total rolling yield of 30% to 70%. Furthermore, it is preferable to perform roughing rolling in two or more passes. By performing the roughing rolling step under these conditions, the eutectic carbides generated during casting can be dissolved during the high-temperature heat treatment (holding) of the slab. In addition, roughing rolling reduces the width of macroscopic segregation and promotes carbon diffusion by introducing dislocations that affect the center of the roughing stock, thus eliminating carbon segregation generated during casting. Furthermore, the formation of retained austenite caused by a decrease in the Ms point due to carbon segregation can be suppressed. As a result, it is easy to control the average particle size of carbides, the number of carbides with a size of 10 μm or more, and the area fraction of carbides within the aforementioned ranges. These effects can be effectively obtained by combining the subsequent hot rolling step and, if necessary, a soaking heat treatment step.

[0103] When the heat treatment time for the initial rolling step is less than 1 hour or the heat treatment temperature is less than 1000℃, it is difficult to achieve the desired effect of solid solution of eutectic carbides and reduction of carbon segregation. In addition, when the heat treatment time for the initial rolling step exceeds 10 hours or the heat treatment temperature is above 1200℃, sagging deformation may occur due to the weight of the slab, which may make it difficult to carry out subsequent steps.

[0104] Furthermore, when the total rolling ratio of the initial rolling step is less than 30%, dislocation introduction is insufficient, and the width of macroscopic segregation is difficult to reduce, thus failing to fully achieve the effects of eutectic carbide solid solution and carbon segregation reduction. Additionally, when the total rolling ratio of the initial rolling step exceeds 70%, it may lead to insufficient rolling ratio in subsequent hot rolling.

[0105] The hot rolling step is the process of hot rolling a billet to obtain a hot-rolled product.

[0106] While the conditions for hot rolling are not particularly limited, it is preferable to heat-treat the billet at a temperature of 1000°C to 1200°C for 1 to 5 hours before hot rolling. By performing heat treatment under such conditions, the eutectic carbides generated during casting can be completely dissolved, thus making it easier to control the average particle size of the carbides, the number of carbides with a size of 10 μm or more, and the area ratio of the carbides within the aforementioned ranges.

[0107] Although the conditions for hot rolling are not particularly limited, it is preferred to finish the plate to a thickness of 2mm to 8mm.

[0108] When the heat treatment time for the hot rolling step is less than 1 hour or the heat treatment temperature is less than 1000°C, the effects of solid solution of eutectic carbides and reduction of carbon segregation cannot be fully achieved. Furthermore, when the heat treatment time for the hot rolling step exceeds 5 hours or the heat treatment temperature is above 1200°C, sagging deformation will occur due to the weight of the billet, potentially making subsequent steps difficult. From the viewpoint of consistently ensuring the above effects, the heat treatment time for the hot rolling step is preferably 1.5 to 3 hours.

[0109] The softening step (hot rolling annealing step) is a step in which the hot rolled material is heated at a temperature above Ac1 point to below (Ac1 point + 100°C) for 2 to 10 hours, and then cooled at a cooling rate of less than 0.100°C / second.

[0110] Heating for 2 to 10 hours at a temperature exceeding Ac1 point to below (Ac1 point + 100°C) will cause the martensitic structure to undergo an austenitic transformation, allowing carbides to disperse in the austenitic phase.

[0111] In addition, when heating above the Ac1 point, the austenite phase generated during heating usually undergoes martensitic transformation and hardens. However, by controlling the cooling rate to below 0.100℃ / second, the austenite phase decomposes into soft ferrite phase and carbides, thus achieving softening while precipitating a large amount of carbides of a certain quantity and size.

[0112] Preferably, a softening step is performed in a batch annealing furnace after the hot-rolled material is coiled. That is, the heating is preferably carried out as follows: the coiled hot-rolled sheet is placed in a batch annealing furnace and heated at a temperature exceeding point Ac1 to below (point Ac1 + 100°C) for 2 to 10 hours.

[0113] Here, point Ac1 is calculated by the following formula (1).

[0114]

[0115] In the formula, the symbol for each element refers to the mass of that element.

[0116] In addition, the softened material obtained from the softening process can also be pickled if necessary.

[0117] A homogenization process may also be performed between the billet rolling step and the hot rolling step, if necessary.

[0118] The homogenization heat treatment step is to hold the billet at a temperature of 1000°C to 1200°C for 1 hour to 48 hours.

[0119] By performing the homogenization treatment under such conditions, the effects of solid solution of eutectic carbides and reduction of carbon segregation can be improved.

[0120] When the heat treatment time in the soaking heat treatment step is less than 1 hour or the heat treatment temperature is less than 1000°C, the effects of solid solution of eutectic carbides and reduction of carbon segregation cannot be fully obtained. Furthermore, when the heat treatment time in the soaking heat treatment step exceeds 48 hours or the heat treatment temperature exceeds 1200°C, sagging deformation may occur due to the weight of the billet, potentially making subsequent steps difficult. From the viewpoint of consistently ensuring the above-mentioned effects, the heat treatment time in the soaking heat treatment step is preferably 3 to 30 hours, more preferably 3 to 24 hours.

[0121] After the softening step, cold rolling and annealing steps may also be performed as needed.

[0122] The cold rolling step is a step of cold rolling the softened material obtained in the softening step to obtain cold-rolled material.

[0123] The conditions for cold rolling are not particularly limited and can be adjusted appropriately according to the requirements of the cold-rolled material.

[0124] The annealing step is a process of heating the cold-rolled material to a temperature below point Ac1. By performing this annealing step, cold-rolled annealed material can be obtained. Although the lower limit of the heating temperature is not particularly limited, it is preferably (Ac1 point - 50°C). In addition, the annealing of cold-rolled material can be carried out from a temperature of 25°C to 100°C.

[0125] By performing the annealing step under the above conditions, the coarsening of carbides can be suppressed, and the average particle size of carbides, the number of carbides with a size of 10 μm or more, and the area fraction of carbides can be stably controlled within the above range.

[0126] (3. Quenched or tempered materials for martensitic stainless steel)

[0127] The martensitic stainless steel quenched or tempered material (hereinafter referred to as "quenched or tempered material") in the embodiments of the present invention is obtained by quenching or tempering the above-mentioned martensitic stainless steel material.

[0128] While the quenching conditions are not particularly limited, it is preferably performed at 1000°C to 1200°C. Similarly, the tempering conditions are not particularly limited, but it is preferably performed at 100°C to 400°C. Furthermore, after quenching, it is preferable to perform cryogenic treatment at -200°C to -50°C.

[0129] The quenched or tempered materials in the embodiments of the present invention have the same composition as the martensitic stainless steel materials described above. Therefore, the description of the composition is omitted.

[0130] In the embodiments of the present invention, the quenched or tempered material has an average carbide particle size of 0.10 μm to 0.50 μm in its cross-section. By controlling the average carbide particle size to 0.10 μm or more, wear resistance is improved, thus extending the life (sharpness durability) of cutting tools during manufacturing. Furthermore, by controlling the average carbide particle size to 0.50 μm or less, the reduction in corrosion resistance and the formation of irregular patterns can be suppressed. From the viewpoint of stably ensuring these effects, the average carbide particle size is preferably 0.11 μm to 0.49 μm, more preferably 0.11 μm to 0.48 μm.

[0131] The average particle size of the carbides is calculated as follows: by observing the cross-section of the quenched or tempered material with SEM, measuring the equivalent circle diameter of each carbide in the field of view, and calculating its average value.

[0132] In the embodiments of the present invention, the quenched or tempered material has a carbide area fraction of 2.0% to 15.0% in its cross-section, more preferably 2.5% to 14.5%, and particularly preferably 3.0% to 14.0%. By controlling the carbide area fraction to 2.0% or more, hardness and wear resistance are improved, thus extending the life (sharpness durability) of the tool while improving its initial sharpness during manufacturing. Furthermore, by controlling the carbide area fraction to 15.0% or less, the reduction in corrosion resistance and the formation of irregular patterns can be suppressed.

[0133] The area ratio of carbides is calculated as follows: by observing the cross-section of quenched or tempered material with SEM, measuring the area of ​​each carbide in the field of view, dividing it by the area of ​​the field of view, and multiplying by 100.

[0134] In embodiments of the present invention, the quenched or tempered material has a retained austenite area fraction of 15.0% or less, preferably 14.5% or less, and more preferably 14.0% or less in its cross-section. The retained austenite is formed when the austenite generated during quenching does not undergo martensitic transformation in subsequent treatments (e.g., cryogenic treatment, tempering). Retained austenite is particularly prevalent in the segregated portions of carbon, a strong austenite stabilizing element. Since retained austenite is softer than martensite, the desired hardness cannot be obtained when it is present in large quantities.

[0135] In addition, the lower the area ratio of retained austenite, the better. Therefore, although not specifically limited, it is typically 1.0% or more, and preferably 1.5% or more.

[0136] Carbon segregation mainly occurs during casting as central segregation. Therefore, in most cases, retained austenite is generated in large quantities in the central part of the thickness direction of quenched or tempered materials. Since this central part of the thickness direction of quenched or tempered materials becomes the cutting edge during tool forming, the presence of soft retained austenite at the cutting edge may cause edge defects or deteriorate the sharpness of the tool.

[0137] The area ratio of retained austenite can be obtained as follows: First, by observing the cross-section of the quenched or tempered material using EBSD, the phases with BCC and FCC crystal structures are distinguished, and their respective areas are calculated. Next, based on these areas, the proportion of the FCC crystal structure phase area in the sum of the areas of the BCC and FCC crystal structures is calculated (i.e., the area ratio of the FCC crystal structure phase). This calculated area ratio of the FCC crystal structure phase is taken as the area ratio of retained austenite.

[0138] In addition, in the central segregation region of carbon mentioned above, coarse carbides tend to form in rows, which is the reason for irregular patterns (stripes or islands). Therefore, reducing central segregation is also effective in reducing these irregular patterns.

[0139] In embodiments of the present invention, the quenched or tempered material preferably has a Vickers hardness of 500 HV to 900 HV, more preferably 520 HV to 850 HV, and particularly preferably 540 HV to 800 HV. A Vickers hardness within this range particularly ensures strength and initial sharpness for use as a cutting tool.

[0140] Here, Vickers hardness in this specification refers to the value measured using a Vickers hardness tester at room temperature (25°C).

[0141] In the embodiments of the present invention, the quenched or tempered material preferably has a specific wear rate of 0.0020 mm. 3 / N•m or less, more preferably 0.0019mm 3 / N•m or less. If the specific wear rate is within this range, wear resistance can be ensured, especially the sharpness and durability when used as a cutting tool.

[0142] Here, the specific wear rate in this specification refers to that obtained by means of the pin-disc friction wear test described in the examples described later.

[0143] The quenched or tempered material in the embodiments of the present invention possesses high hardness, high corrosion resistance, and excellent wear resistance, and can suppress the formation of irregular patterns, thus making it suitable for various applications requiring these properties. This quenched or tempered material is particularly suitable for cutting tools. When this quenched or tempered material is used in cutting tools, in addition to improving corrosion resistance and suppressing the formation of irregular patterns, it can also improve the tool's initial sharpness while extending its lifespan (sharpness durability).

[0144] (4. Manufacturing methods of cutting tools)

[0145] The manufacturing method of the cutting tool in the embodiments of the present invention includes a quenching step, wherein the above-mentioned martensitic stainless steel material is processed into a predetermined shape, heated at a temperature of 1000°C to 1200°C for 5 minutes to 60 minutes, and cooled at a cooling rate of 3°C / second or higher. The cooling rate is preferably 10°C / second or higher, more preferably 20°C / second or higher.

[0146] By performing the quenching step under the above conditions, precipitates such as carbides and nitrides are fully dissolved, increasing the amount of dissolved C and dissolved N, thus achieving high hardness. Furthermore, by controlling the cooling rate within the above range, the re-precipitation of Cr carbonitrides during the quenching step can be suppressed, thereby inhibiting sensitization and achieving high corrosion resistance.

[0147] There are no particular limitations on the processing methods for martensitic stainless steel; well-known methods such as forging and grinding can be used.

[0148] After the quenching step, a tempering step or a cryogenic treatment step may be performed as needed. The tempering step is preferably performed at a temperature of 100°C to 400°C, and the cryogenic treatment step is preferably performed at a temperature of -200°C to -50°C.

[0149] The cutting tools manufactured by the above method can not only improve corrosion resistance and suppress the formation of irregular patterns, but also improve the sharpness (initial) of the cutting tool and extend its life (sharpness durability).

[0150] Example

[0151] The present invention will now be described in more detail with reference to the embodiments, but the present invention is not limited to these explanations.

[0152] (Examples 1 to 21 and Comparative Examples 1 to 16)

[0153] The steel composition shown in Table 1 was melted and cast into 200 mm thick slabs. These slabs were then subjected to a series of steps: initial rolling, hot rolling, softening, cold rolling, and annealing. The conditions for these steps are shown in Table 2. In Example 20, a soaking heat treatment step was performed between the initial rolling and hot rolling steps. In Comparative Example 14, the initial rolling step was not performed.

[0154] In the initial rolling step, the total rolling rate is set to 40%.

[0155] In the hot rolling process, a 3mm thick hot-rolled plate is finally produced.

[0156] In the softening step, the hot-rolled plate obtained in the hot rolling step is rolled into a coil and then transferred to a batch annealing furnace for softening under the conditions shown in Table 2.

[0157] In the cold rolling step, the softened sheet obtained in the softening step is cold rolled to a thickness of 2.0 mm.

[0158] Pickling is performed after the annealing step.

[0159] The cold-rolled annealed sheet (martensitic stainless steel) obtained by the above method is evaluated as follows. Furthermore, regarding Comparative Example 16, since quenching occurred during cooling in the softening step, cold rolling was impossible, making it unmanufacturable. Therefore, the following evaluation cannot be performed.

[0160] [Table 1]

[0161]

[0162] [Table 2]

[0163]

[0164] (Number of carbides larger than 10 μm)

[0165] For the cross-section of the obtained cold-rolled annealed sheet parallel to the rolling direction and the thickness direction, 20 visual observations were made in a 50mm×50mm area using a 50x optical microscope. The average number of carbides was calculated and divided by the area of ​​the observation area.

[0166] (Average particle size and area fraction of carbides)

[0167] Cross-sections of cold-rolled annealed sheets parallel to the rolling and thickness directions, obtained by SEM observation, were used. Carbides observed within the field of view were excluded, including those with an equivalent circle diameter less than 0.10 μm and some carbides that partially spilled out of the field of view. The equivalent circle diameter (μm) was measured, and the sum of the equivalent circle diameters of the measured carbides was divided by the total number of measured carbides to obtain the average particle size (μm). Multiple randomly selected, non-repeating observation fields were used to ensure that the total number of measured carbides was at least 100. The equivalent circle diameter of the carbides was calculated by processing the SEM images using image processing software to obtain the area of ​​the carbides.

[0168] In addition, regarding the area ratio of carbides, in the above observations, the area of ​​the carbides was measured, and the value obtained by dividing it by the area of ​​the field of view and multiplying by 100 was taken as the area ratio of the carbides.

[0169] The evaluation results are shown in Table 3.

[0170] [Table 3]

[0171]

[0172] As shown in Table 3, the cold-rolled annealed sheets (martensitic stainless steel) of Examples 1 to 21 have a predetermined composition, and the average particle size of carbides, the number of carbides with a size of 10 μm or more, and the area ratio of carbides can be controlled within a predetermined range.

[0173] In contrast, the cold-rolled annealed sheets (martensitic stainless steel) of Comparative Examples 4, 7 and 11 to 12, although they can control the average particle size of carbides, the size of carbides of 10 μm or more and the area ratio of carbides within a predetermined range, do not have a predetermined composition.

[0174] In addition, the cold-rolled annealed sheets (martensitic stainless steel) of Comparative Examples 1 to 3, 5, 6, 8 to 10 not only lacked the predetermined composition, but also failed to control more than one of the following within the predetermined range: the average particle size of the carbides, the number of carbides with a size of 10 μm or more, and the area ratio of the carbides.

[0175] Furthermore, although the cold-rolled annealed sheets (martensitic stainless steel) of Comparative Examples 13 to 15 have a predetermined composition, due to inappropriate manufacturing conditions, it is impossible to control more than one of the following within the predetermined range: the average particle size of the carbides, the number of carbides with a size of 10 μm or more, and the area ratio of the carbides.

[0176] Next, the cold-rolled annealed sheet is heated to 1050°C for 10 minutes, then air-cooled (cooling rate of 3°C / second or higher) for quenching, followed by tempering at 200°C for 30 minutes to obtain a quenched and tempered sheet. Furthermore, for tool evaluation, the cold-rolled annealed sheet is punched, ground into a tool shape, and then quenched and tempered as described above. Afterwards, the portion to be used as the tool tip is rough-ground and fine-ground to form the tool tip, thus completing the tool.

[0177] The quenched and tempered plates and cutting tools obtained above are evaluated as follows.

[0178] (Vickers hardness)

[0179] The Vickers hardness of the quenched and tempered plate was determined using a Vickers hardness tester. The measurement temperature was set to room temperature (25°C).

[0180] (Corrosion resistance)

[0181] For quenched and tempered plates, the surface is ground to #600 and subjected to JIS Z2371:2015 "Salt spray test". In this evaluation, a rust area rate of less than 10% is considered acceptable (○), and more than 10% is considered unacceptable (×). In addition, among those with a rust area rate of less than 10%, those with a rust area rate of 0% are considered particularly excellent (◎).

[0182] (Average particle size and area fraction of carbides)

[0183] Except for using quenched and tempered plates instead of cold-rolled annealed plates, the average particle size and area fraction of carbides are determined using the same method as described above.

[0184] (Area ratio of retained austenite (γ))

[0185] For the cross-section of the quenched and tempered plate parallel to the rolling direction and the thickness direction, after EBSD measurement, the phases with BCC and FCC crystal structures were distinguished, and their respective areas were calculated. Then, based on these areas, the proportion of the FCC crystal structure phase area in the total area of ​​the BCC and FCC crystal structure phases was calculated (i.e., the area ratio (%) of the FCC crystal structure phase), and the calculated area ratio of the FCC crystal structure phase was used as the area ratio of the retained austenite.

[0186] (Specific wear)

[0187] A 5mm diameter test piece was cut from a quenched and tempered plate and attached to the tip of a 5mm diameter base pin to create a pin. The specific wear was then evaluated using a pin-disc friction and wear testing machine. The disc, made of sandpaper (#400) with hard SiC particles, was used as the abrasive material. The wear test was conducted as follows: the pin was fixed to the sample holder, and the surface of the test piece was pressed against a rotating disc with a test load of 20N. The test was performed at a sliding speed of 0.66m / s for 5 minutes. The volume of material lost due to wear was calculated from the difference in thickness of the test piece before and after the test, and this was taken as the wear reduction (mm). 3 Then, the specific wear rate is calculated using the following formula.

[0188] Specific wear [mm] 3 / N•m] = Wear loss [mm] 3 ] / (Test load [20N] × Friction distance [200m])

[0189] (Initial sharpness of the knife)

[0190] This multi-mode sharpness testing machine is used to evaluate the sharpness of cutting tools.

[0191] The sharpness test is as follows: With the blade fixed in place, a load of approximately 750g is applied while stacking 7.5mm wide sheets of paper (approximately 70μm thick), roughly the width of newspaper, and the blade moves back and forth for 20mm. One back-and-forth motion constitutes one cycle, and the number of sheets of paper completely cut in one cycle is counted. In this evaluation, if the number of sheets of paper cut is 10 or more, the blade's initial sharpness can be considered good.

[0192] (Knife sharpness and durability)

[0193] In the aforementioned sharpness test, 500 cycles were performed, and the number of sheets of paper that were completely cut was counted. In this evaluation, if the number of sheets of paper cut is 3 or more, the sharpness and durability of the blade can be judged to be good.

[0194] (Surface texture of the knife)

[0195] For knives, the irregular patterns on the surface of the knife are observed and evaluated with the naked eye. In this evaluation, those with no observed irregular patterns are considered excellent (◎), those with very slight irregular patterns are considered acceptable (○), and those with observed irregular patterns are considered unacceptable (×).

[0196] The evaluation results are shown in Table 4.

[0197] [Table 4]

[0198]

[0199] As shown in Table 4, the quenched and tempered plates (quenched and tempered materials) of Examples 1 to 21 have a predetermined composition, and the average particle size of carbides, the area ratio of carbides, and the area ratio of retained austenite can be controlled within a predetermined range. Therefore, the quenched and tempered plates of Examples 1 to 21 have high hardness and high corrosion resistance, and low specific wear (exhibiting excellent wear resistance). Furthermore, the cutting tools of Examples 1 to 21 have good initial sharpness and sharpness durability, and can suppress the generation of irregular patterns.

[0200] In contrast, the quenched and tempered plate of Comparative Example 1 has a smaller carbide area ratio due to insufficient C content and 2.5C+N content. In addition, the quenched and tempered plate has reduced hardness, resulting in insufficient initial sharpness of the tool, and the tool's sharpness durability is also insufficient due to increased specific wear.

[0201] In Comparative Example 2, the quenched and tempered plate, due to its excessive carbon content, exhibits higher hardness but also a larger area ratio of carbides and retained austenite. Therefore, the retained austenite and carbides become the starting point for edge defects, resulting in insufficient tool sharpness and durability, and failing to suppress the formation of irregular patterns. Furthermore, this quenched and tempered plate also has lower corrosion resistance.

[0202] In Comparative Example 3, the quenched and tempered plate had excessive C, Si, and Ni content, resulting in larger average particle size and area fraction of carbides. Consequently, carbides became the starting point for edge defects, leading to insufficient sharpness and durability of the tool, and failing to suppress the formation of irregular patterns.

[0203] In Comparative Example 4, the quenched and tempered plate had a higher residual austenite area ratio due to its excessive Mn content. Furthermore, this quenched and tempered plate had lower hardness and higher specific wear. Therefore, the tool's initial sharpness and sharpness durability were insufficient, and the generation of irregular patterns could not be suppressed.

[0204] In Comparative Example 5, the quenched and tempered plate had a low Cr content, resulting in a smaller carbide area ratio and a larger retained austenite area ratio. Furthermore, this quenched and tempered plate exhibited lower hardness and corrosion resistance, and higher specific wear. Consequently, the tool's initial sharpness and sharpness durability were insufficient, and the formation of irregular patterns could not be suppressed.

[0205] In Comparative Example 6, the quenched and tempered plate had an excessive Cr content, resulting in a larger average grain size and area fraction of carbides. Furthermore, this quenched and tempered plate had lower hardness. Therefore, although the specific wear was sufficient, the tool became dull quickly, leading to insufficient initial sharpness and sharpness durability, and the inability to suppress the formation of irregular patterns.

[0206] In Comparative Example 7, the quenched and tempered plate had a higher residual austenite area ratio due to its excessive Ni content. Furthermore, this quenched and tempered plate had lower hardness and higher specific wear. Therefore, the tool's initial sharpness and sharpness durability were insufficient, and the formation of irregular patterns could not be suppressed.

[0207] The quenched and tempered plate in Comparative Example 8 has a low hardness and a high specific wear due to its excessive Mo content. As a result, the initial sharpness and sharpness durability of the tool are insufficient, and the generation of irregular patterns cannot be suppressed.

[0208] The quenched and tempered plate in Comparative Example 9 has a low hardness and a high specific wear due to its excessive Al content. Therefore, the initial sharpness and sharpness durability of the tool are insufficient, and the generation of irregular patterns cannot be suppressed.

[0209] In Comparative Example 10, the quenched and tempered plate had a high residual austenite area ratio due to its insufficient nitrogen content. Furthermore, this quenched and tempered plate exhibited lower hardness and corrosion resistance, and higher specific wear. Consequently, the tool's initial sharpness and sharpness durability were insufficient, and the formation of irregular patterns could not be suppressed.

[0210] The quenched and tempered plate of Comparative Example 11 has reduced corrosion resistance due to excessive Ca content.

[0211] The quenched and tempered plate of Comparative Example 12 has reduced corrosion resistance due to excessive O content.

[0212] In Comparative Example 13, the quenched and tempered plate had a higher specific wear due to the excessively small average particle size of the carbides. Consequently, the tool's sharpness and durability were insufficient.

[0213] The quenched and tempered plate of Comparative Example 14 has a higher specific wear due to the excessively large average particle size of the carbides. As a result, the sharpness and durability of the tool are insufficient, and the generation of irregular patterns cannot be suppressed.

[0214] The quenched and tempered plate of Comparative Example 15 has a higher specific wear due to the excessively large average grain size and area fraction of carbides. Therefore, the sharpness and durability of the cutting tool are insufficient.

[0215] The results above demonstrate that this invention provides a method for manufacturing martensitic stainless steel with high hardness, high corrosion resistance, excellent wear resistance, and the ability to suppress irregular textures after quenching or tempering. Furthermore, this invention provides quenched or tempered martensitic stainless steel with high hardness, high corrosion resistance, excellent wear resistance, and the ability to suppress irregular textures.

[0216] Therefore, by employing the structures [1] to [3] below, the present invention provides martensitic stainless steel with high hardness and high corrosion resistance after quenching or quenching and tempering, and excellent wear resistance, which can suppress the formation of irregular patterns. Furthermore, by employing the structures [4] to [8] below, quenched or quenched and tempered materials of martensitic stainless steel with high hardness and high corrosion resistance, and excellent wear resistance, which can suppress the formation of irregular patterns, can be provided. Further, by employing the structures [9] to

[12] below, the present invention provides a method for manufacturing martensitic stainless steel with high hardness and high corrosion resistance after quenching or quenching and tempering, and excellent wear resistance, which can suppress the formation of irregular patterns.

[0217] [1] A martensitic stainless steel material having the following composition: by weight, containing C: 0.305% to 0.600%, Si: 0.05% to 1.00%, Mn: 0.10% to 1.50%, P: 0.0085% to 0.0400%, S: less than 0.0300%, Cr: 13.0% to 18.0%, Ni: 0.01% to 0.30%, Mo: 0.01% to 1.00%, Al: less than 0.300%, N: 0.010% to 0.350%, Ca: 0.0001% to 0.0030%, O: 0.001% to 0.010%, with the balance being Fe and impurities, and 2.5C+N (C and N refer to the content of C and N) is more than 0.85%;

[0218] In the cross-section of the aforementioned martensitic stainless steel, the average particle size of the carbides is between 0.50 μm and 2.00 μm, and the number of carbides with a size of 10 μm or larger is 0.20 per cm. 2 The area fraction of the aforementioned carbides is 15.0% to 20.0%.

[0219] [2] The martensitic stainless steel described in [1] further contains, by weight, one or more of the following: Ti: less than 0.30%, Nb: less than 0.50%, Cu: less than 4.0%, B: less than 0.005%, V: less than 0.50%, Sn: less than 0.10%, and Co: less than 0.30%.

[0220] [3] Martensitic stainless steel as described in [1] or [2], wherein the aforementioned martensitic stainless steel is intended for use as a cutting tool.

[0221] [4] A quenched or tempered material of martensitic stainless steel, having the following composition: by mass, containing C: 0.305% to 0.600%, Si: 0.05% to 1.00%, Mn: 0.10% to 1.50%, P: 0.0085% to 0.0400%, S: less than 0.0300%, Cr: 13.0% to 18.0%, Ni: 0.01% to 0.30%, Mo: 0.01% to 1.00%, Al: less than 0.300%, N: 0.010% to 0.350%, Ca: 0.0001% to 0.0030%, O: 0.001% to 0.010%, with the balance being Fe and impurities, and 2.5C+N (C and N refer to the content of C and N) is more than 0.85%;

[0222] In the cross section of the aforementioned quenched or quenched and tempered material, the average particle size of the carbides is 0.10 μm to 0.50 μm, the area fraction of the aforementioned carbides is 2.0% to 15.0%, and the area fraction of retained austenite is less than 15.0%.

[0223] [5] The quenched or quenched and tempered stainless steel as described in [4] further contains, by weight, one or more of the following: Ti: less than 0.30%, Nb: less than 0.50%, Cu: less than 4.0%, B: less than 0.005%, V: less than 0.50%, Sn: less than 0.10%, and Co: less than 0.30%.

[0224] [6] Quenched or tempered martensitic stainless steels as described in [4] or [5] have a Vickers hardness of 500 HV to 900 HV.

[0225] [7] The specific wear of the martensitic stainless steel quenched or quenched and tempered material described in any of [4] to [6] is 0.0020 mm. 3 / N·m or less.

[0226] [8] Quenched or tempered materials of martensitic stainless steel as described in any of [4] to [7], wherein the aforementioned quenched or tempered materials of martensitic stainless steel are intended for use as cutting tools.

[0227] [9] A method for manufacturing martensitic stainless steel, comprising:

[0228] The roughing rolling step involves rough rolling a slab having the following composition to obtain a roughing stock: based on mass, containing C: 0.305% to 0.600%, Si: 0.05% to 1.00%, Mn: 0.10% to 1.50%, P: 0.0085% to 0.0400%, S: less than 0.0300%, Cr: 13.0% to 18.0%, Ni: 0.01% to 0.30%, Mo: 0.01% to 1.00%, Al: less than 0.300%, N: 0.010% to 0.350%, Ca: 0.0001% to 0.0030%, O: 0.001% to 0.010%, with the balance being Fe and impurities, and 2.5C+N (C and N refer to the content of C and N) being 0.85% or more;

[0229] The hot rolling step involves hot rolling the aforementioned billet to obtain a hot-rolled product; and

[0230] The softening step involves heating the aforementioned hot-rolled material at a temperature exceeding Ac1 point to below (Ac1 point + 100°C) for 2 to 10 hours, followed by cooling at a cooling rate of 0.100°C / second or less.

[0231]

[10] The method for manufacturing martensitic stainless steel as described in [9], wherein the aforementioned slab further contains, by weight, one or more of the following: Ti: 0.30% or less, Nb: 0.50% or less, Cu: 4.0% or less, B: 0.005% or less, V: 0.50% or less, Sn: 0.10% or less, and Co: 0.30% or less.

[0232]

[11] The method for manufacturing martensitic stainless steel as described in [9] or

[10] further includes, between the aforementioned billet rolling step and the aforementioned hot rolling step, a heat treatment step in which the aforementioned billet is held at a temperature of 1000°C to 1200°C for 1 hour to 48 hours.

[0233]

[12] The method for manufacturing martensitic stainless steel as described in any of [9] to

[11] further includes:

[0234] The cold rolling step involves cold rolling the softened material obtained in the aforementioned softening step to obtain a cold-rolled material; and

[0235] The annealing step involves heating the aforementioned cold-rolled material to a temperature below point Ac1.

Claims

1. A martensitic stainless steel material having the following composition: based on mass, containing C: 0.305% to 0.600%, Si: 0.05% to 1.00%, Mn: 0.10% to 1.50%, P: 0.0085% to 0.0400%, S: less than 0.0300%, Cr: 13.0% to 18.0%, Ni: 0.01% to 0.30%, Mo: 0.01% to 1.00%, Al: less than 0.300%, N: 0.010% to 0.350%, Ca: 0.0001% to 0.0030%, O: 0.001% to 0.010%, with the balance being Fe and impurities, and 2.5C+N (C and N refer to the content of C and N) being 0.85% or more; In the cross-section of the martensitic stainless steel, the average grain size of the carbides is greater than 0.50 μm but less than 2.00 μm, and the number of carbides with a size greater than 10 μm is 0.20 per cm. 2 The area fraction of the carbides is 15.0% to 20.0%.

2. The martensitic stainless steel according to claim 1, further comprising, by weight, one or more of the following: Ti: less than 0.30%, Nb: less than 0.50%, Cu: less than 4.0%, B: less than 0.005%, V: less than 0.50%, Sn: less than 0.10%, and Co: less than 0.30%.

3. The martensitic stainless steel material according to claim 1 or 2, wherein, The martensitic stainless steel is supplied for use in cutting tools.

4. A quenched or tempered material of martensitic stainless steel, having the following composition: by weight, containing C: 0.305% to 0.600%, Si: 0.05% to 1.00%, Mn: 0.10% to 1.50%, P: 0.0085% to 0.0400%, S: less than 0.0300%, Cr: 13.0% to 18.0%, Ni: 0.01% to 0.30%, Mo: 0.01% to 1.00%, Al: less than 0.300%, N: 0.010% to 0.350%, Ca: 0.0001% to 0.0030%, O: 0.001% to 0.010%, with the balance being Fe and impurities, and 2.5C+N (C and N refer to the content of C and N) being more than 0.85%; In the cross section of the quenched or tempered material, the average particle size of the carbides is 0.10 μm to 0.50 μm, the area fraction of the carbides is 2.0% to 15.0%, and the area fraction of the retained austenite is less than 15.0%.

5. The quenched or tempered material of martensitic stainless steel according to claim 4, further comprising, by weight, one or more of the following: Ti: less than 0.30%, Nb: less than 0.50%, Cu: less than 4.0%, B: less than 0.005%, V: less than 0.50%, Sn: less than 0.10%, and Co: less than 0.30%.

6. The quenched or tempered material of martensitic stainless steel according to claim 4 or 5, wherein the Vickers hardness is 500 HV to 900 HV.

7. The martensitic stainless steel quenched or tempered material according to any one of claims 4 to 6, wherein the specific wear loss is 0.0020 mm. 3 / N·m and below.

8. The martensitic stainless steel quenched or tempered material according to any one of claims 4 to 7, wherein, The martensitic stainless steel quenching or tempering materials are supplied for use in cutting tools.

9. A method for manufacturing martensitic stainless steel, comprising: The roughing rolling step involves rough rolling a slab having the following composition to obtain a roughing stock: based on mass, containing C: 0.305% to 0.600%, Si: 0.05% to 1.00%, Mn: 0.10% to 1.50%, P: 0.0085% to 0.0400%, S: less than 0.0300%, Cr: 13.0% to 18.0%, Ni: 0.01% to 0.30%, Mo: 0.01% to 1.00%, Al: less than 0.300%, N: 0.010% to 0.350%, Ca: 0.0001% to 0.0030%, O: 0.001% to 0.010%, with the balance being Fe and impurities, and 2.5C+N (C and N refer to the content of C and N) being 0.85% or more; The hot rolling step involves hot rolling the billet to obtain a hot-rolled material. as well as The softening step involves heating the hot-rolled material at a temperature exceeding Ac1 point to below (Ac1 point + 100°C) for 2 to 10 hours, followed by cooling at a rate of 0.100°C / second or less.

10. The method for manufacturing martensitic stainless steel according to claim 9, wherein, The slab further contains, by weight, one or more of the following: Ti: less than 0.30%, Nb: less than 0.50%, Cu: less than 4.0%, B: less than 0.005%, V: less than 0.50%, Sn: less than 0.10%, and Co: less than 0.30%.

11. The method for manufacturing martensitic stainless steel according to claim 9 or 10, further comprising, between the billet rolling step and the hot rolling step: The heat treatment step involves holding the billet at a temperature of 1000°C to 1200°C for 1 hour to 48 hours.

12. The method for manufacturing martensitic stainless steel according to any one of claims 9 to 11, further comprising: The cold rolling step involves cold rolling the softened material obtained in the softening step to obtain a cold-rolled material. as well as The annealing step involves heating the cold-rolled material to a temperature below point Ac1.

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