Steel for cutting tools, martensitic steel for cutting tools, cutting tools, and method for producing martensitic steel for cutting tools

Abstract

Provided are: steel for knives, having a higher hardness and better corrosion resistance than conventional steel for knives; a knife; steel for martensitic knives; and a production method for same.　The steel for knives comprises a component composition containing, in mass%, 0.45%–1.00% C, 0.1%–1.5% Si, 0.1%–1.5% Mn, 7.5%–11.0% Cr, and 0.5%–3.0% of either Mo or W or a complex of both (Mo + W / 2), with the remainder being Fe and unavoidable impurities. Also provided are steel for martensitic knives and a knife. A production method for steel for martensitic knives is also provided that includes a quenching temperature at quenching of 1,050–1,250°C, a processing temperature for subzero processing of no more than –50°C, and a tempering temperature at tempering of 100–400°C, and obtains steel for martensitic knives that has a hardness of at least 700 HV.

Description

Technical Field

[0001] The present invention relates to steel for cutting tools, martensitic steel for cutting tools, cutting tools, and a method for manufacturing martensitic steel for cutting tools.

Background Art

[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 not only obtains high hardness by quenching and tempering, but also has excellent corrosion resistance, so it is not easily rusted and is generally widely used.

[0003] The sharpness of a cutting tool is mainly determined by the hardness of the cutting edge, the angle at the time of edge grinding, and the distribution state of hard particles. Among them, hardness is an essential characteristic for improving sharpness. On the other hand, the corrosion resistance of a cutting tool is mainly determined by the contents of Cr and Mo. Therefore, in order to improve the sharpness of a cutting tool and enhance its corrosion resistance, it is essential to increase the hardness of the cutting tool after quenching and tempering and increase the contents of Cr and Mo. However, the method of increasing the contents of Cr and Mo has a problem that the amount of retained austenite remaining during quenching increases, so the hardness of the cutting tool after quenching and tempering decreases. To solve this problem, for example, the applicant has disclosed in Patent Document 1 a method for improving the short-time hardenability of martensitic stainless steel and obtaining high hardness. In terms of mass%, C: 0.55 to 0.73%, Si: 1.0% or less, Mn: 1.0% or less, Cr: 12 to 14%, and the balance Fe and impurities, and the carbide density in the normalized state by a continuous furnace is 140 to 600 pieces / 100 μm 2Patent Document 2 also proposes stainless steel for razors with high heat treatment hardness, which contains, by mass%, 0.55-0.85% C, 2.0% or less Si, 1.0% or less Mn, 8-15% Cr, 0.03% or less N, and further contains either one or two of a group consisting of one or more of W, V, Mo, and Co at 3.0% or less, and one or two of Ni and Cu at 2.0% or less, with the balance being Fe and a small amount of impurities. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 5-39547 [Patent Document 2] Japanese Patent Publication No. 53-114719 Summary of the Invention [Problem to be solved by the invention]

[0005] In recent years, in order to meet the demand for improved sharpness and shaving quality, blades with higher hardness and corrosion resistance than before are required. 2 A razor steel is disclosed that has a high hardness of 660 to 720 HV after tempering and has good corrosion resistance, and is obtained by subjecting an annealed material in which carbides are finely dispersed, to quenching, subzero treatment, and tempering. Patent Document 2 also describes a stainless steel razor steel with a tempered hardness of 620 to 716 HV, but the steels described in Patent Documents 1 and 2 are insufficient to meet the demand for even higher hardness and higher corrosion resistance, and there is still room for further study. In view of the above-mentioned problems, the object of the present invention is to provide a steel for blades that is harder and more corrosion-resistant than ever before. Another object of the present invention is to provide a manufacturing method that can obtain a steel for blades that is hard and more corrosion-resistant without adding a process for increasing the carbide number density. [Means for solving the problem]

[0006] The present invention has been made in view of the above problems. That is, one aspect of the present invention is a steel for cutting tools having a component composition of, by mass%, C: 0.45 to 1.00%, Si: 0.1 to 1.5%, Mn: 0.1 to 1.5%, Cr: 7.5 to 11.0%, and Mo and W alone or in combination (Mo + W / 2): 0.5 to 3.0%, with the balance being Fe and inevitable impurities. Preferably, further, it contains V and Nb alone or in combination (V + Nb): 0.5% or less, or Ni and Cu alone or in combination (Ni + Cu): 0.5% or less.

[0007] Another aspect of the present invention is a martensitic steel for cutting tools having the component composition of the above steel for cutting tools and a hardness of 700 HV or more. Preferably, the carbide area ratio in the cross-sectional structure is 8.0% or less, and the average equivalent circle diameter of the carbides is 0.2 to 0.8 μm. Another aspect of the present invention is a cutting tool using the above martensitic steel for cutting tools.

[0008] Another aspect of the present invention is a method for manufacturing a martensitic steel for cutting tools, which comprises performing quenching, sub-zero treatment, and tempering on the steel for cutting tools having the above component composition, setting the quenching temperature during the quenching to 1050 to 1250°C, the treatment temperature during the sub-zero treatment to -50°C or lower, and the tempering temperature during the tempering to 100 to 400°C, to obtain a martensitic steel for cutting tools having a hardness of 700 HV or more. Preferably, the tempering temperature is set to 100 to 160°C to obtain a martensitic steel for cutting tools having a hardness of 800 HV or more.

Effects of the Invention

[0009] According to the present invention, a steel for cutting tools with higher hardness and better corrosion resistance than before can be obtained more efficiently.

Brief Description of the Drawings

[0010]

Figure 1

Figure 2

Figure 3

Figure 4

Embodiments for Carrying Out the Invention

[0011] Hereinafter, an embodiment of the present invention will be described. However, the present invention is not limited to the embodiments listed here, and appropriate combinations and improvements are possible without departing from the technical idea of the invention. First, the reasons for limiting the component composition of the tool steel according to the present invention will be explained. C: 0.45 to 1.00% C is an important element that dissolves from carbide into the matrix at the austenitizing temperature during quenching and determines the hardness of the martensite formed by quenching. Here, C in the steel is divided into that which dissolves in the matrix and that which precipitates as carbide, but the ratio is determined by the interaction with Cr, so it is important to keep Cr within the composition range described later. In order to obtain a martensitic tool steel with higher hardness suitable for the present invention, the lower limit of C is set to 0.45%. The preferred lower limit value of C is 0.50%, the more preferred lower limit value is 0.55%, the further preferred lower limit value is 0.58%, and the particularly preferred lower limit value is 0.60%. On the other hand, if the amount of C is too large, there is a possibility of generating large eutectic carbides that cause tool chipping. Also, if the amount of C is too large, the amount of carbide generated will also be excessive, reducing the amount of Cr and Mo dissolved in the martensite and also being a factor in reducing the corrosion resistance. Therefore, the upper limit of C is set to 1.00%. The preferred upper limit value of C is 0.95%, the more preferred upper limit value is 0.90%, the further preferred upper limit value is 0.85%, and the particularly preferred upper limit value is 0.79%.

[0012] Si: 0.1 to 1.5% Si is an element that not only is used as a deoxidizer during the refining of tool steel but also dissolves in the steel to suppress softening during low-temperature tempering. Therefore, the lower limit is set at 0.1%. On the other hand, excessive inclusion reduces the toughness of tool steel and may, for example, reduce the cold workability during cold rolling. Therefore, the upper limit of the Si content is set at 1.5%. The preferred upper limit is 1.2%, the more preferred upper limit is 1.0%, the even more preferred upper limit is 0.98%, and the particularly preferred upper limit is 0.95.

[0013] Mn: 0.1 - 1.5% Similar to Si, Mn also has the role of a deoxidizer during refining, dissolves in the matrix, and is an element that enhances hardenability. If the amount of Mn is too small, the hardenability of the steel decreases, and there is a possibility that the steel may not harden, especially in the center of the steel thickness. Therefore, the lower limit is set at 0.1%. On the other hand, excessive inclusion of Mn reduces the hot workability, so the upper limit is set at 1.5%. The preferred upper limit is 1.2%, and the more preferred upper limit is 1.0%.

[0014] Cr: 7.5 - 11.0% Cr is an important element for forming a strong passive film on the steel and obtaining excellent corrosion resistance. In order to exhibit this corrosion resistance, it is necessary for the steel to contain at least 7.5% of Cr. The preferred lower limit of Cr is 8.0%, the more preferred lower limit of Cr is 8.5%, and the even more preferred lower limit of Cr is 9.0%. On the other hand, an excessive amount of Cr causes a decrease in the martensite transformation start temperature (Ms point) and becomes a factor for hardness reduction due to an increase in retained austenite. In order to achieve both high hardness and good corrosion resistance, the upper limit of Cr is set at 11.0%. The preferred upper limit of Cr is 10.5%, and the more preferred upper limit of Cr is 10.2%.

[0015] Mo + W / 2: 0.5 - 3.0% Mo and W have similar effects and are defined as (Mo + W / 2) due to the relationship of atomic weights. Mo and W can be contained alone or in combination. Mo and W have a high effect of stabilizing passivation, and are elements that ennoble the pitting potential in a chloride solution and are effective for improving corrosion resistance. They are also elements that suppress softening during low-temperature annealing. To obtain these effects, at least 0.5% is required. On the other hand, excessive addition of Mo and W significantly reduces the workability during hot working, so the upper limit is set at 3.0%. The lower limit of the preferred amount of (Mo + W / 2) is 0.8%, and the upper limit of the preferred amount of (Mo + W / 2) is 2.0%.

[0016] Preferably, Nb + V: 0.5% or less Nb and V have similar effects and can be contained alone or in combination. Nb has a high affinity for carbon and forms a thermally stable carbide. Since this carbide is very thermally stable, it does not dissolve in high-temperature austenite and remains, suppressing the coarsening of austenite by pinning of the carbide. Similarly, V is also an element that finely disperses thermally stable carbides, suppresses the coarsening of austenite, and improves wear resistance. However, since the carbides containing Nb and V are thermally stable, they do not dissolve in high-temperature austenite and remain, tending to reduce the amount of carbon dissolved in martensite and cause a decrease in hardness. Also, when the content is high, the possibility of cracks due to decreased cold workability increases. For this reason, in this embodiment, even when V and Nb are contained, the upper limit of the amount of (V + Nb) is set at 0.5%. The upper limit of the preferred amount of (V + Nb) is 0.4%, and the upper limit of the more preferred amount of (V + Nb) is 0.3%.

[0017] Preferably, Ni + Cu: 0.5% or less Ni and Cu are elements effective in improving the corrosion resistance against non-oxidizing acids such as sulfuric acid, and can be contained singly or in combination. However, they cause a decrease in the Ms point and are also a factor in the hardness reduction due to an increase in retained austenite. Therefore, even when contained, the upper limit of the (Ni + Cu) amount is set to 0.5%. The preferable upper limit of the (Ni + Cu) amount is 0.4%, and the more preferable upper limit of the (Ni + Cu) amount is 0.3%.

[0018] The steel for cutting tools according to the present invention can contain the following elements. Co: 0.5% or less Co dissolves in martensite and is an element that increases the tempering softening resistance. On the other hand, for applications that may come into contact with the human body such as razor materials, since it may also cause metal allergies, it may be contained in the steel of this embodiment in the range of 0.5% or less.

[0019] N dissolves in the martensite structure and is an element that improves corrosion resistance. However, it causes a decrease in the Ms point and is also a factor in the hardness reduction due to an increase in retained austenite. Therefore, it may be contained in the steel of this embodiment in the range of 0.1% or less. The preferable upper limit is 0.07%, and the more preferable upper limit is 0.05%.

[0020] In this embodiment, components other than the above are Fe and inevitable impurities. Examples of inevitable impurity elements include P, S, Al, Ti, N, and O. However, they may be contained as long as they are within the following ranges that do not inhibit the effects of the present invention. P ≦ 0.04%, S ≦ 0.03%, Al ≦ 0.1%, Ti ≦ 0.1%, and O ≦ 0.05%.

[0021] Subsequently, embodiments of the martensitic steel for cutting tools of the present invention will be described. By quenching, sub-zero treatment, and tempering the steel for cutting tools having the above-described component composition, a martensitic steel for cutting tools with extremely high hardness can be obtained. The hardness of the martensitic steel for cutting tools of the present embodiment is a value measured at room temperature (normal temperature) and is 700 HV or more. Preferably it is 720 HV or more, more preferably 735 HV or more, still more preferably 770 HV or more, and particularly preferably 800 HV or more. The upper limit is not particularly limited, but can be about 950 HV due to manufacturing constraints. Incidentally, the steel for cutting tools before quenching can be produced by annealing a hot-rolled material having the above-described component composition, such as batch annealing or continuous annealing, and performing cold working (for example, cold rolling, etc.) one or more times on the material for cold rolling after annealing.

[0022] The martensitic steel for cutting tools of the present embodiment contains carbides, and preferably the carbide area ratio in the cross-sectional structure is 8.0% or less. By setting the carbide area ratio within the above range, excellent corrosion resistance can be obtained. The upper limit of the more preferable carbide area ratio is 6.0%, more preferably 4.0%, even more preferably 2.0%, particularly preferably 1.0%, and most preferably 0.8%. Also, as described above, coarse carbides cause a decrease in the strength of the cutting tool, so the average of the equivalent circle diameters (equivalent area circle diameters) of the carbides in the cross-sectional structure is preferably 0.2 to 0.8 μm. The upper limit of the average of the more preferable equivalent circle diameter is 0.6 μm, and the upper limit of the average of the even more preferable equivalent circle diameter is 0.5 μm. Incidentally, the carbide area ratio and the average of the equivalent circle diameter in the present embodiment are in a cross-sectional structure parallel to the processing direction (drawing direction of rolling processing) of the martensitic steel for cutting tools, and the field area photographed with a scanning electron microscope (magnification 5000 times) is 500 μm 2The carbides in the above-mentioned visual field can be observed and calculated by image analysis. The carbides targeted in image analysis are limited to those with an equivalent circle diameter of 0.1 μm or more, and those less than that are not targeted. Also, the identification of carbides can be confirmed by elemental mapping using EPMA (electron probe microanalyzer) attached to a scanning electron microscope. By processing the martensitic tool steel having the characteristics as described above, it is possible to obtain a tool with good sharpness and excellent corrosion resistance.

[0023] Subsequently, a method for manufacturing the martensitic tool steel of the present invention will be described. In the present invention, quenching, sub-zero treatment, and tempering are performed on the tool steel having the above-described component range. The quenching temperature is 1050 to 1250 °C, the treatment temperature during sub-zero treatment is -50 °C or lower, and the tempering temperature during tempering is 100 to 400 °C. In this component system, when the quenching temperature is less than 1050 °C, the carbides do not dissolve sufficiently in austenite, resulting in low hardness. Also, when the quenching temperature exceeds 1250 °C, cracking occurs during quenching or sub-zero treatment due to the excessively dissolved carbon. Therefore, the quenching temperature is set to 1050 to 1250 °C. The preferable lower limit of the quenching temperature is 1100 °C, and the more preferable lower limit is 1150 °C. Also, the preferable upper limit of the quenching temperature is 1230 °C, and the more preferable upper limit is 1210 °C.

[0024] The temperature during sub-zero treatment performed after the quenching step is set to -50 °C or lower. By adjusting to this temperature, it becomes easier to obtain the high hardness characteristic which is a feature of the present invention. Although the lower limit is not particularly set, assuming treatment with liquid nitrogen, for example, the lower limit may be set to -196 °C. In the sub-zero treatment of this embodiment, a mixture of dry ice and alcohol at -75 °C is used, but liquefied carbon dioxide gas or liquid nitrogen may also be used. Also, an electric refrigeration facility may be used, or a gas such as carbon dioxide gas may be used.

[0025] In the manufacturing method of the present embodiment, tempering is performed after the sub-zero treatment step. By setting the tempering temperature to 100 to 400 °C, martensitic tool steel with a hardness of 700 HV or more can be obtained. In this component system, when the tempering temperature is less than 100 °C, the toughness tends to be excessively low. On the other hand, when the tempering temperature exceeds 400 °C, a large amount of carbides precipitate from the martensite structure, leading to a decrease in hardness. The upper limit of the preferable tempering temperature is 350 °C. Further, when obtaining martensitic tool steel with higher hardness, it is preferable to set the tempering temperature to 100 °C to 160 °C. The upper limit of the more preferable tempering temperature is 150 °C. Thereby, the precipitation of carbides can be further suppressed, and martensitic tool steel with a high hardness of 800 HV or more can be obtained.

Example

[0026] Hot-rolled materials with a thickness of 2.0 mm having the component compositions shown in Table 1 (the balance being Fe and unavoidable impurities) were annealed in a batch annealing furnace, and then cold rolling and annealing were repeated to finish at a thickness of 0.1 mm, and Invention Examples 1 to 16 and Comparative Examples 1 to 13 were prepared.

[0027] Subsequently, the hardness and corrosion resistance after the heat treatment were investigated. Regarding the hardness, the samples of the invention examples and comparative examples were heated to 1100 to 1200 °C in an Ar atmosphere and then quenched by quenching treatment, then sub-zero treatment was performed at -75 °C for 15 minutes, and tempering was performed at temperatures of 150 °C and 350 °C. The hardness was measured at three types: during quenching, during tempering at 150 °C, and during tempering at 350 °C. Regarding the corrosion resistance, the samples tempered at 350 °C were subjected to a salt spray test (based on JIS-Z-2371:2015) using 35 °C, 5% neutral saline, and the rusting state after 1 h was evaluated by the rusted area ratio. In this example, an area ratio of rust less than 1% was judged as ○ (no rust), and 1% or more was judged as × (rust present). Table 2 shows the respective hardnesses. Further, as representative examples, the salt spray test results of Invention Example 1 are shown in FIG. 3, and the salt spray test results of Comparative Example 1 are shown in FIG. 4.

[0028]

Table 1

[0029]

Table 2

[0030] From the results in Table 2, in Invention Examples 1 to 16 of the present invention, the hardening hardness was 800 HV or more, the tempering hardness at 350°C was 700 HV or more, the tempering hardness at 150°C was 800 HV or more, and the rust generation area ratio was 1% or less. Both the hardness and corrosion resistance were good. On the other hand, in Comparative Examples 1 and 5, the corrosion resistance was low, and the hardening hardness and tempering hardness were also lower than those of the invention examples. It was confirmed that in all of Comparative Examples 2, 4, 6, and 7, the rust generation area ratio was high and the corrosion resistance was low. In Comparative Examples 3 and 11 to 13, the rust generation area ratio was less than 1%, and although the corrosion resistance was high, the tempering hardness at 350°C was respectively less than 700 HV, which was a low value. Thereby, it was confirmed that the invention examples of the present invention can simultaneously obtain high hardness and excellent corrosion resistance as compared with the conventional examples. Regarding Comparative Examples 8 to 10 in which V + Nb was 0.6% or more, since a plurality of cracks entered the sample end face and the inside of the sample from the early stage of the cold rolling process, the evaluation was aborted. Subsequently, observation samples were collected from the prepared Invention Examples 1, 15, and 16, and Comparative Example 1, and the average equivalent circle diameter of the carbides and the carbide area ratio were measured. The area ratio and the equivalent circle diameter were measured for carbides with an equivalent circle diameter of 0.1 μm or more in a cross-sectional structure parallel to the rolling direction of the martensitic cutting tool steel in a field of view photographed with a scanning electron microscope (magnification: 5000 times) having a field area of 500 μm 2 The carbides with an equivalent circle diameter of 0.1 μm or more in the above field of view were measured using an image analyzer. The micrograph of Invention Example 1 is shown in FIG. 1, the micrograph of Comparative Example 1 is shown in FIG. 2, and the measurement results are shown in Table 3.

[0031]

Table 3

[0032] As a result of the measurement, the average equivalent circle diameter of the carbides in the example of the present invention was 0.4 to 0.5 μm, and the carbide area ratio was 5.5% or less. On the other hand, although the average equivalent circle diameter of the carbides in Comparative Example 1 was 0.5 μm, which was at the same level as that of the example of the present invention, it was confirmed that the carbide area ratio was 8.5%, which was larger than that of the sample of the present invention.

Claims

1. A method for manufacturing a martensitic steel for cutting tools, comprising: quenching, sub-zero treatment, and tempering a cutting tool steel having a component composition of C: 0.50 to 0.95%, Si: 0.1 to 0.49%, Mn: 0.1 to 1.5%, Cr: 8.0 to 10.2%, Mo and W alone or in combination (Mo + W / 2): 0.5 to 3.0% by mass, with the balance being Fe and inevitable impurities, wherein the quenching temperature during quenching is 1050 to 1250 °C, the treatment temperature during sub-zero treatment is -50 °C or lower, the tempering temperature is 100 to 160 °C, to obtain a martensitic steel for cutting tools having a hardness of 800 HV or higher.

2. The method for manufacturing a martensitic steel for cutting tools according to Claim 1, wherein the cutting tool steel further contains V and Nb alone or in combination (V + Nb): 0.5% or less by mass.

3. The method for manufacturing a martensitic steel for cutting tools according to Claim 1 or 2, wherein the cutting tool steel further contains Ni and Cu alone or in combination (Ni + Cu): 0.5% or less by mass.

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