Stainless steel for cutting tool and method for manufacturing stainless steel strip for cutting tool

Abstract

Provided is stainless steel for a cutting tool, the stainless steel for a cutting tool having superior workability than in the past.　The stainless steel for a cutting tool has a composition comprising, by mass, 0.41-0.50% of C, 0.10-0.40% of Si, 0.1-1.5% of Mn, Ni and Cu alone or in combination such that Ni + Cu = 0-0.50%, 11.5-12.3% of Cr, Mo and W alone or in combination such that Mo + W / 2 = 0-0.30%, and 0.04-0.12% of N, with the balance made up by Fe and unavoidable impurities, the stainless steel for a cutting tool being characterized by having a hardness of 190 HV or less at room temperature.

Description

Manufacturing Method of Stainless Steel for Cutting Tools and Stainless Steel Strip for Cutting Tools

[0001] The present invention relates to a manufacturing method of stainless steel for cutting tools and stainless steel strip for cutting tools.

[0002] Conventionally, as steel for cutting tools such as cutters and razors, high-carbon steel equivalent to SK1 and martensitic stainless steel containing 12 to 13% Cr have been used. The former can obtain high hardness by heat treatment of quenching and tempering, but due to poor corrosion resistance, it is only used for portable purposes. On the other hand, the latter martensitic stainless steel can obtain high hardness by quenching and tempering, and also has excellent corrosion resistance, so it is not easily rusted and is generally widely used.

[0003] For example, in Patent Document 1, a high-corrosion-resistant martensitic stainless steel is proposed, which contains, in mass%, C: 0.45 to 0.60%, N: 0.02 to 0.08%, Si: 0.2 to 0.4%, Mn: 0.3 to 0.6%, Cr: 12 to 15%, Mo: 0.1 to 1.5%, and the balance contains Fe and unavoidable impurities. Also, in Patent Document 2, a high-hardness martensitic stainless steel excellent in corrosion resistance and cold workability is proposed, which contains, in mass%, C: 0.10 to 0.40%, Si: less than 2.0%, Mn: less than 2%, S: less than 0.010%, Cu: 0.01 to 3.0%, Ni: more than 1.0 to 3.0%, Cr: 11.0 to 15.0%, one or two of Mo and W with Mo + 1 / 2W: 0.01 to 1.0%, N: 0.13 to 0.18%, Al: less than 0.02%, O: less than 0.010%, and the balance is substantially composed of Fe.

[0004] Japanese Patent Publication No. 2014-504332, Japanese Unexamined Patent Application Publication No. 2000-239805

[0005] Martensitic stainless steels require high levels of both Cr and C to maintain high hardness and corrosion resistance, and tend to have inferior hot and cold workability compared to high-carbon steels when manufacturing stainless steel for cutlery. The highly corrosion-resistant martensitic stainless steel described in Patent Document 1 is a useful invention possessing hardness and corrosion resistance, but as mentioned above, it has high levels of both C and Cr, as well as high levels of Mo and W, which further worsen workability, leaving room for further improvement in cold workability before quenching and tempering. Furthermore, the high-hardness martensitic stainless steel described in Patent Document 2 has an appropriate C content, but its Ni content is very high, ranging from over 1.0% to 3.0%, requiring a two-stage annealing process of heating to 750°C and then cooling, followed by heating to 650°C and then cooling, in order to improve cold workability. Further optimization of the alloy composition is necessary to achieve manufacturing efficiency. Therefore, the object of the present invention is to provide stainless steel for cutlery with good cold workability.

[0006] To solve the above problems, the inventors investigated a method to simultaneously improve the hot and cold workability of stainless steel for cutting tools. As a result, they discovered that by precisely adjusting the content of C and N, as well as Cr and Mo, among the components of stainless steel for cutting tools, it is possible to simultaneously improve the hot and cold workability of stainless steel for cutting tools before quenching and tempering, leading to the present invention. That is, one aspect of the present invention is a stainless steel for cutting tools having a composition of mass%, C: 0.41-0.50%, Si: 0.10-0.40%, Mn: 0.10-1.50%, Ni and Cu individually or in combination Ni+Cu: 0-0.50%, Cr: 11.5-12.3%, Mo and W individually or in combination Mo+W / 2: 0-0.30%, N: 0.04-0.12%, with the remainder being Fe and unavoidable impurities, characterized in that the hardness at room temperature is 190 HV or less. Furthermore, in the present invention, in addition to the above composition, V and Nb individually or in combination (V+Nb): 0.50% or less may also be included. Preferably, the reduction of area is 80% or more in high-speed tensile stress at a strain rate of 40 / s or more at 1050°C. Another aspect of the present invention is a method for producing a stainless steel strip for cutlery, comprising a rolling step of performing at least one of hot rolling and cold rolling on the stainless steel for cutlery described above to obtain a stainless steel strip for cutlery. Another aspect of the present invention is a stainless steel for cutlery having a composition by mass% of C: 0.41 to 0.50%, Si: 0.10 to 0.40%, Mn: 0.1 to 1.5%, Ni and Cu individually or in combination Ni+Cu: 0 to 0.50%, Cr: 11.5 to 12.3%, Mo and W individually or in combination Mo+W / 2: 0 to 0.30%, N: 0.04 to 0.12%, with the remainder being Fe and unavoidable impurities, wherein the reduction of area is 80% or more in high-speed tensile stress at a strain rate of 40 / s or more at 1050°C.

[0007] According to the present invention, it is possible to provide stainless steel for cutting tools that has better machinability than conventional stainless steel in the cold working process before quenching and tempering. Furthermore, by appropriately adjusting the alloy composition, excellent hot workability can also be improved.

[0008] This is a photograph of the appearance of the stainless steel for cutlery according to the present invention after cold rolling. This is a photograph of the appearance of the stainless steel for cutlery according to the comparative example after cold rolling. This is a photograph of the appearance of the test piece of the stainless steel for cutlery according to the present invention after simulated hot working. This is a photograph of the appearance of the test piece of the stainless steel for cutlery according to the comparative example after simulated hot working.

[0009] One embodiment of the present invention will be described below. However, the present invention is not limited to the embodiments described herein, and can be appropriately combined and improved without departing from the technical spirit of the invention. First, the reason for limiting the component composition of the stainless steel for cutlery according to the present invention (hereinafter also simply referred to as stainless steel or steel) will be explained. The stainless steel for cutlery according to the present invention has a metal structure mainly consisting of a ferrite phase before quenching. C: 0.41 to 0.50% C is an important element that dissolves from carbides into the matrix at the austenitization temperature during quenching and determines the hardness of the martensite produced by quenching. Here, C in steel is divided into C that dissolves into the matrix and C that precipitates as carbides, but the ratio is determined by the interaction with carbide-forming elements such as Cr and Mo, which can be selectively contained, so it is important to keep Cr and other carbide-forming elements within the composition range defined in the present invention. In order to obtain stainless steel for cutlery that has even better cold workability than conventional stainless steel and is suitable for the present invention, the lower limit is set to 0.41%. The preferred lower limit is 0.42%, and the more preferred lower limit is 0.43%. On the other hand, if there is too much carbon, there will be an excessive amount of carbides, and the carbides will also become coarser (i.e., the average and maximum equivalent diameter of the circle will increase), which will reduce workability during cold forming, potentially causing cracks to form at the edges of the product during manufacturing and reducing the yield. Therefore, the upper limit is set at 0.50%. The preferred upper limit is 0.48%, the more preferred upper limit is 0.46%, and the even more preferred upper limit is 0.45%.

[0010] Si: 0.10–0.40% Si is used as a deoxidizing agent during the refining of stainless steel for cutlery, and also dissolves in the steel, suppressing softening during low-temperature tempering. Therefore, the lower limit is set at 0.10%. A preferred lower limit is 0.15%. On the other hand, excessive Si content drastically reduces the hot and cold workability of stainless steel for cutlery. For this reason, the upper limit of Si content is set at 0.40%. A preferred upper limit is 0.37%, a more preferred upper limit is 0.35%, and an even more preferred upper limit is 0.33%.

[0011] Mn: 0.10–1.50% Like Si, Mn acts as a deoxidizing agent during refining, dissolves in the matrix, and enhances hardenability. If the amount of Mn is too low, the hardenability of stainless steel for cutlery decreases, and there is a possibility that hardening will not occur, especially in the thickest part of the stainless steel for cutlery; therefore, the lower limit is set at 0.10%. The preferred lower limit is 0.30%, and more preferably above 0.60%. On the other hand, excessive Mn content reduces hot workability, so the upper limit is set at 1.50%. The preferred upper limit is 1.20%, and the more preferred upper limit is 1.00%.

[0012] Ni and Cu alone or in combination (Ni + Cu): 0-0.50% Ni and Cu are effective elements for improving corrosion resistance to non-oxidizing acids such as sulfuric acid, and can be included alone or in combination. To reliably obtain the above effect, it is best to use Ni alone. However, because it increases the hardness of stainless steel for cutlery before quenching and tempering and drastically reduces ductility during cold working, the upper limit of the (Ni + Cu) amount should be 0.50% even when included. The preferred upper limit of the (Ni + Cu) amount is 0.40%, and the more preferred upper limit of the (Ni + Cu) amount is 0.30%. If the effect of improving corrosion resistance can be compensated for by other additive elements, it is acceptable to have no Ni or Cu added (0%). Note that Ni, compared to Cu, may increase hardness and worsen cold workability when dissolved in stainless steel for cutlery, so if one of them is to be included, it is better to choose Cu.

[0013] Cr: 11.5–12.3% Cr is an element necessary to form a strong non-conductive film on stainless steel for cutlery after quenching or quenching and tempering, and to obtain excellent corrosion resistance. In order to exhibit this corrosion resistance, it is necessary for the stainless steel for cutlery to contain at least 11.5% Cr. The preferred lower limit of Cr is 11.6%, the more preferred lower limit of Cr is 11.7%, and the still preferred lower limit of Cr is 11.8%. On the other hand, since Cr forms carbides together with C, if there is an excessive amount of Cr, the carbides will also become coarser (i.e., the average and maximum equivalent diameter of the circle will increase), which can reduce workability during cold forming, potentially causing cracks to form at the edges of the product during manufacturing and reducing the yield. Therefore, in order to improve cold workability, the upper limit of Cr should be 12.3%. The preferred upper limit of Cr is 12.1%, and the more preferred upper limit of Cr is 11.9%.

[0014] Mo and W, either alone or in combination, can be present in amounts of (Mo + W / 2): 0 to 0.30%. Mo and W have similar effects and are defined by (Mo + W / 2) based on their atomic weight relationship. Mo and W can be included alone or in combination. Mo and W have a high effect in stabilizing passivation and are effective elements in improving corrosion resistance by increasing the pitting potential in chloride solutions. The lower limit for obtaining this effect is 0.01%. Furthermore, to reliably obtain the above effect, it is best to use W alone. On the other hand, excessive addition of Mo and W can drastically reduce workability during hot working and, for similar reasons as Cr, reduce workability during cold forming, potentially lowering the yield. Therefore, the upper limit for the (Mo + W / 2) amount is 0.30%. The lower limit of the preferred (Mo + W / 2) amount is 0.20%, the upper limit of the more preferred (Mo + W / 2) amount is 0.10%, and the upper limit of the even more preferred (Mo + W / 2) amount is 0.08%. Furthermore, if the effect of stabilizing the passivation can be compensated for by other additive elements, it is acceptable to have no Mo or W added (0%). Also, since W is generally more expensive than Mo, if one is to be included, it is preferable to choose Mo.

[0015] N: 0.04-0.12% N is an important element that dissolves in the martensitic structure and improves hardness and corrosion resistance. To obtain the effect of N, it is preferable to include 0.04% or more. However, if there is too much N, in addition to generating bubbles during casting, it dissolves in the carbides and makes the carbides coarser (i.e., the average and maximum equivalent circle diameters increase), which can reduce workability during cold forming, cause cracks to form at the edges of the product during manufacturing, and significantly worsen manufacturability. Therefore, the N content should be 0.04-0.12%. The preferred upper limit is 0.11%, a more preferred upper limit is 0.10%, an even more preferred upper limit is 0.09%, the preferred lower limit is 0.05%, a more preferred lower limit is 0.06%, and an even more preferred lower limit is 0.07%.

[0016] V + Nb: 0.50% or less. Nb and V have similar effects and can be included individually or in combination. Nb has a high affinity for carbon and forms thermally very stable MC carbides. Because these MC carbides are thermally very stable, they remain in the austenite at high temperatures without dissolving, and the pinning of the carbides suppresses the coarsening of the austenite. Similarly, V also finely disperses thermally very stable carbides, suppressing the coarsening of the austenite and is an effective element for improving wear resistance, and can be selectively included. However, Nb and V form fine MC carbides during annealing, increasing the temper hardness and increasing the possibility of cracks occurring due to reduced cold workability. For this reason, even when V and Nb are included in this embodiment, it is preferable to set the upper limit of the (V + Nb) amount to 0.50%. A more preferable upper limit for the (V + Nb) amount is 0.40%, and an even more preferable upper limit for the (V + Nb) amount is 0.30%. Generally, Nb is more expensive than V, so if you must include either one, it is better to choose V. Also, V and Nb may be present as unavoidable impurities in amounts of less than 0.05% each.

[0017] The stainless steel for blades 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 the resistance to tempering and softening. On the other hand, for applications that may come into contact with the human body, such as razor blades, it may be included in the stainless steel for blades of this embodiment in an amount of 0.5% or less, as it may cause metal allergies.

[0018] In this embodiment, components other than those mentioned above are Fe and unavoidable impurities. Examples of unavoidable impurity elements include P, S, Al, Ti, and O, but they may be included as long as they do not hinder the effects of the present invention, within the ranges shown below: P ≤ 0.04%, S ≤ 0.03%, Al ≤ 0.1%, Ti ≤ 0.1%, and O ≤ 0.05%.

[0019] In this invention, by adjusting the component composition as described above, both excellent cold workability and excellent hot workability can be provided. The manufacturing conditions for achieving this can be standard manufacturing conditions. For example, simply optimizing the alloy composition eliminates the need to apply the complex manufacturing conditions described in Patent Document 2.

[0020] The stainless steel for cutlery of this embodiment has the alloy composition described above, while having a hardness of 190 HV or less at room temperature. By keeping the hardness within the above range, the present invention exhibits excellent cold workability and contributes to reducing manufacturing costs, such as by reducing the number of annealing cycles. For this reason, it is more preferable that the stainless steel for cutlery of this embodiment, having the hardness described above, is a cold work material, such as a material for cold rolling or cold forging. There is no particular lower limit, but if the hardness is extremely low, the surface is easily scratched during handling, requiring processes such as scratch removal, which leads to increased manufacturing costs. For example, it is possible to set it to 150 HV. The hardness specified in this invention is measured after annealing and before quenching. For example, the annealing process before quenching can be carried out by holding the material at a temperature of 840°C for 30 minutes or more, and then cooling it from 840°C to 690°C at a rate of 50°C / h or less, and then measuring the hardness of the stainless steel for cutlery after annealing. The hardness in this invention can be measured in accordance with JIS-Z2244-1:2024.

[0021] The stainless steel for cutting tools of this embodiment can also possess good hot workability by achieving a reduction in area of ​​area of ​​80% or more in high-speed tensile testing at a strain rate of 40 / s or more at 1050°C. High-speed tensile testing at a very large strain rate of 40 / s or more simulates hot working, and it is known that when high-speed tensile testing is performed under these conditions, the mechanical properties exhibited are significantly different from those obtained when tested at a static strain rate as specified in JIS-Z2241:2022. The higher the strain rate, the greater the tensile strength and the smaller the reduction in area of ​​area tends to be. The stainless steel for cutting tools of this embodiment, having the reduction in area of ​​area described above, exhibits superior hot workability, suppressing excessive cracking during hot working and thus reducing manufacturing costs. Therefore, it is more preferable that the stainless steel for cutting tools of this embodiment, having the reduction in area of ​​area described above, be used as a material for hot working, such as for hot rolling or hot forging. The strain rate of 40 / s or more was chosen as a test condition applicable to all hot working processes requiring strong processing, such as hot forging and hot rolling. Here, in order to simulate a material used for hot working, it is preferable to apply the high-speed tensile test to an annealed material that has been annealed for 4 hours at a holding temperature of, for example, 750°C to 860°C (preferably 780°C) for the test specimen used for the high-speed tensile test of strain rate.

[0022] Carbides can be observed in the metal structure of the stainless steel for cutting tools of this embodiment. When the area ratio of carbides in the cross-sectional structure exceeds 20.0% (hereinafter referred to as the carbide area ratio), both cold and hot workability decreases. Therefore, it is preferable to perform hot and cold working within a range of 20.0% or less for the carbide area ratio. Carbides with an equivalent circle diameter of less than 0.10 μm are unlikely to contribute to a decrease in workability and may be excluded when measuring the area ratio. Furthermore, the average equivalent circle diameter (area equivalent circle diameter) of carbides in the cross-sectional structure is preferably 0.30 to 0.60 μm, and the maximum equivalent circle diameter (area equivalent circle diameter) of carbides in the cross-sectional structure is preferably less than 1.40 μm. The identification of the above-mentioned carbides can be confirmed by elemental mapping using SEM (scanning electron microscope) or EPMA (electron beam microanalyzer), and the carbide area ratio should be at least 1000 μm. ２The above observations are recommended. In this case, for example, a randomly selected field of view area of ​​100 μm. ２ Observing five fields of view is preferable because it averages out the areas with sparse and dense carbides. Note that the carbides targeted in the image analysis are limited to those with an equivalent circle diameter of 0.10 μm or larger. Limiting the target to those with an equivalent circle diameter of 0.10 μm or larger also makes it possible to exclude inclusions other than carbides from the measurement. Furthermore, the identification of carbides can be confirmed by elemental mapping using an EPMA (electron beam microanalyzer) attached to the scanning electron microscope.

[0023] Here, the stainless steel for cutlery mentioned above is steel mainly composed of the ferrite phase, or in other words, it can also be described as ferritic stainless steel for cutlery. By performing quenching and tempering on this stainless steel for cutlery, a martensitic stainless steel for cutlery can be obtained in which the metal structure has transformed into the martensitic phase. Here, "mainly composed of the 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 by area ratio.

[0024] In this embodiment, stainless steel for cutlery is subjected to at least one of hot rolling and cold rolling to obtain stainless steel strip for cutlery. As described above, the stainless steel for cutlery of the present invention has excellent cold workability, which tends to suppress the occurrence of cracks during cold rolling and yield high-quality stainless steel strip for cutlery. In the case of cold-rolled steel strip, the preferred thickness is 0.50 mm or less, the more preferred thickness is 0.20 mm or less, and the even more preferred thickness is 0.10 mm or less.

[0025] Ten kg of molten metal, melted in a high-frequency induction melting furnace, was cast to produce stainless steel ingots having the component compositions shown in Table 1. In Table 1, "Example of the Invention" refers to stainless steel within the composition range specified in the Invention, while "Comparative Example" refers to stainless steel outside the composition range specified in the Invention. Next, these ingots were hot-forged with a forging ratio (cross-sectional area before forging / cross-sectional area after forging) of about 10, cooled, and then annealed in a batch-type annealing furnace at a holding temperature of 780°C for 4 hours to obtain intermediate annealed materials No. 1-5 and No. 101-116. Notably, no surface defects such as cracks were observed in the hot-forged intermediate annealed materials adjusted to the component composition specified in the Invention. The obtained intermediate annealed material was then held at a holding temperature of 1150°C for 30 minutes, followed by hot rolling with a rolling ratio (cross-sectional area before rolling / cross-sectional area after rolling) of approximately 5. After cooling to room temperature, it was held in a batch-type annealing furnace at a holding temperature of 840°C for 30 minutes or more, and then cooled from 840°C to 690°C at a rate of 8°C / h to obtain the annealed material (stainless steel for cutting tools) of the present invention example and comparative example. Note that a V+Nb content of 0.03% or less is a normal level of impurities.

[0026]

[0027] The hardness and cold workability of the stainless steel (annealed material) obtained as examples of the present invention and comparative examples were measured. Hardness was measured on the surface after grinding the surface hardened phase, in accordance with JIS-Z2244-1:2024. The load was set to 1 kgf, and the average value was calculated from three measurement points. Cold workability was evaluated by cutting a 40 × 50 × 2.0 mm test piece from the above-mentioned annealed material and rolling it at room temperature to a reduction ratio of 95%, and checking for the presence or absence of cracks in the rolled material. The evaluation criteria were as follows: "A" for materials where no cracks were visually observed at the ends after rolling to a reduction ratio of 95%, or where very minor cracks of less than 0.5 mm occurred; and "B" for materials where cracks of 0.5 mm or more occurred at the ends. These results are shown in Table 3. Also, the test piece of sample No. 3 is shown in Figure 1, and the test piece of sample No. 104 is shown in Figure 2. Furthermore, the stainless steel used for cutlery in both the present invention example and the comparative example has not undergone annealing and quenching treatment, and is primarily composed of a ferrite phase.

[0028]

[0029] For hot workability, tensile test specimens (parallel section length: 24 mm, parallel section diameter: 8 mm) were prepared from the intermediate annealed material. High-speed tensile tests were performed under conditions simulating actual hot working (held at 1200°C, then cooled and evaluated at 1050°C, strain rate 42 / s), and hot workability was evaluated from the reduction of area. The evaluation criteria were "A" for specimens with a reduction of area of ​​80% or more and excellent hot workability, and "B" for specimens with a reduction of area of ​​area of ​​less than 79%. The evaluation results are shown in Table 3. In addition, the specimen of sample No. 3 after testing is shown in Figure 3, and the specimen of sample No. 112 is shown in Figure 4.

[0030]

[0031] From the results in Tables 2 and 3 and Figures 1 to 4, it was confirmed that in Examples 1 to 5 of the present invention, both cold workability and hot workability were very good. On the other hand, in Comparative Examples 101 to 116, the hot workability was poor. The reason for the reduced hot workability is thought to be that the C and Mo were too high. From these results, it was confirmed that the stainless steel for cutlery in the examples of the present invention exhibits good cold workability and also superior hot workability compared to conventional examples.

Claims

1. Stainless steel for cutting tools having a composition of mass%, C: 0.41-0.50%, Si: 0.10-0.40%, Mn: 0.1-1.5%, Ni and Cu individually or in combination: Ni+Cu: 0-0.50%, Cr: 11.5-12.3%, Mo and W individually or in combination: Mo+W / 2: 0-0.30%, N: 0.04-0.12%, with the remainder being Fe and unavoidable impurities, characterized in that its hardness at room temperature is 190 HV or less.

2. The stainless steel for cutting tools according to claim 1, further containing V and Nb individually or in combination in mass% of (V + Nb): 0.50% or less.

3. The stainless steel for cutting tools according to claim 1, wherein the reduction of area is 80% or more in high-speed tensile stress at a strain rate of 40 / s or more at 1050°C.

4. A method for manufacturing stainless steel strip for cutlery, comprising a rolling step of performing at least one of hot rolling and cold rolling on the stainless steel for cutlery described in claims 1 to 3 to obtain stainless steel strip for cutlery.

5. Stainless steel for cutting tools having a composition in mass%, of C: 0.41-0.50%, Si: 0.10-0.40%, Mn: 0.1-1.5%, Ni and Cu individually or in combination as Ni+Cu: 0-0.50%, Cr: 11.5-12.3%, Mo and W individually or in combination as Mo+W / 2: 0-0.30%, N: 0.04-0.12%, with the remainder being Fe and unavoidable impurities, wherein the reduction of area is 80% or more in high-speed tensile stress at a strain rate of 40 / s or more at 1050°C.

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