Martensitic stainless steel sheet and cutting tool

Abstract

To provide a high C martensitic stainless steel sheet that has excellent manufacturability.SOLUTION: Provided is a martensitic stainless steel sheet that has a steel composition containing, in mass%, C: 0.40 to 0.60%, Si: 0.05 to 0.60%, Mn: 0.10 to 1.50%, P: 0.020% or less, S: 0.005% or less, Cr: 11.0 to 15.0%, Ni: 0.01 to 2.00%, Cu: 0.01 to 0.50%, Al: 0.05% or less, N: 0.01 to 0.09%, O: 0.0030% or less, Ca: 0.0001 to 0.0010%, REM: 0.010 to 0.20%, and the balance being Fe and impurities. Mo, V, Sn, and Bi may be added by an appropriate amount.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a martensitic stainless steel sheet. Furthermore, it relates to a cutting tool made from the martensitic stainless steel sheet.

Background Art

[0002] Hitherto, as metals used for kitchen knives and the like, there are ordinary steel and stainless steel. Stainless steel, which has excellent corrosion resistance, is used mainly for household use because maintenance is simple. SUS420J2 is the most commonly used. However, in recent years, the requirements for hardness and corrosion resistance of metals used for kitchen knives and the like have become more stringent, and the demand for high-C martensitic stainless steel has been increasing. As a steel type with higher hardness, there is SUS440A steel, but since it is very difficult to manufacture, it is expensive and has some difficulty in corrosion resistance. Compatibility between hardness and corrosion resistance is required.

[0003] For example, Patent Document 1 discloses a steel in which N and Cu are added to improve hardness and corrosion resistance. However, it is difficult to contain a large amount of nitrogen, and in some cases, additional equipment is required, leading to an increase in cost.

[0004] Furthermore, Patent Document 2 discloses a steel excellent in hardness and corrosion resistance by making it low C, high N, and low Cr. Still, a high nitrogen content leads to an increase in cost.

[0005] Among these, since EN1.4034 steel of 0.45C-13Cr type and EN1.14116 steel obtained by adding Mo and V thereto, as disclosed in Non-Patent Document 1, can be manufactured with general-purpose equipment, they have been increasingly used.

[0006] Also, Patent Document 3 discloses a martensitic stainless steel for cutting tools containing a trace amount of tin and 0.40 to 0.50% of C.

Prior Art Documents

Patent Documents

[0007]

Patent Document 1

Patent Document 2

Patent Document 3

Non-Patent Document

[0008]

Non-Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0009] High-C martensitic stainless steels such as the 0.45C-13Cr alloy steel disclosed in Non-Patent Document 1 have a good balance between strength and corrosion resistance. However, due to their high C content, there are many problems in productivity when manufacturing with general-purpose equipment, especially equipment using continuous casting. In particular, they tend to have low hot ductility, so cracks are likely to occur during continuous casting, and in some cases, production may even be stopped. Furthermore, defects are likely to occur in the products, reducing the product yield. Therefore, in many cases, they are manufactured by ingot casting rather than continuous casting equipment, leading to increased costs. Thus, the inventor recognized that there was no high-C martensitic stainless steel with excellent productivity. Therefore, the present invention aims to improve the productivity of high-C martensitic stainless steels.

Means for Solving the Problems

[0010] In order to solve the above problems, the inventors first investigated the occurrence situation of defects in the steel plate. The defects in the steel plate are so-called hege defects, which are considered to occur during hot rolling. However, it was found that the actual situation is that fine cracks due to insufficient hot ductility during casting are manifested as defects stretched during hot rolling.

[0011] As a result of intensive studies on improving this hot ductility, it was found that the cause of microcracks is liquid-phase embrittlement at grain boundaries. In order to prevent this, it was found that it is good to reduce P and S, add Ca, and further contain an appropriate amount of REM. In particular, the effect of REM is large, and a remarkable effect was obtained by containing a larger amount. It was also found that when P and S are not reduced, the effects of containing REM and Ca are poor. It was also found that it is necessary to limit O in order to suppress the formation of REM-based inclusions.

[0012] The present invention has been achieved based on these findings, and the gist thereof is as follows.

[0013] (1) By mass, C: 0.40 to 0.60%, Si: 0.05 to 0.60%, Mn: 0.10 to 1.50%, P: 0.020% or less, S: 0.0050% or less, Cr: 11.0 to 15.0%, Ni: 0.01 to 2.00%, Cu: 0.01 to 0.50%, Al: 0.05% or less, N: 0.010 to 0.090%, O: 0.0050% or less, Ca: 0.0001 to 0.0010%, REM: 0.01 to 0.20% A martensitic stainless steel sheet characterized by having a steel composition containing the above and the balance being Fe and impurities. (2) On the surface of the martensitic stainless steel sheet, the presence rate R of surface defects obtained by dividing the total length of surface defects with a length of 10 mm or more by the total area of the steel sheet is 0.100 / m or less. The martensitic stainless steel sheet according to (1). (3) Furthermore, by mass, Mo: 1.00% or less, V: 0.30% or less The martensitic stainless steel sheet according to (1) or (2), characterized by containing one or two of the following. (4) Furthermore, in terms of mass%, Sn: 0.100% or less, Bi: 0.20% or less The martensitic stainless steel sheet according to any one of (1) to (3), characterized by containing one or two of the following. (5) A cutting tool made of the martensitic stainless steel according to any one of (1) to (4). [Effect of the Invention]

[0014] The high-C martensitic stainless steel of the present invention can stably perform continuous casting by improving hot ductility, and as a result of suppressing minute cracks formed during casting, surface roughness during hot rolling can be reduced. As a result, a martensitic stainless steel sheet for cutting tools excellent in manufacturability can be obtained. [Embodiments for Carrying Out the Invention]

[0015] Hereinafter, an example of an embodiment of the present invention will be described. Unless otherwise specified, "%" regarding the content of an element means mass%. Further, when the lower limit is not particularly defined, it may include the case of not containing (0%).

[0016] [Chemical Composition of Steel Sheet] C: 0.40 to 0.60% C is an essential element for obtaining a predetermined hardness after quenching. It is preferably contained at 0.40% or more in order to stably obtain a hardness of 54 HRC or more required for high-C martensitic stainless steel. On the other hand, if added excessively, it becomes prone to cracking during casting or hot rolling, the surface roughness increases, and furthermore, sensitization during quenching is promoted, impairing corrosion resistance and also reducing the toughness after quenching due to undissolved carbonitrides. Therefore, it is preferably contained at 0.60% or less. From the viewpoint of ensuring a certain strength and hardness, the lower limit of C is preferably 0.42%, 0.44%, 0.46%, 0.48%, or 0.50%.

[0017] Si: 0.05% - 0.60% Si is necessary for deoxidation during melting refining and is also useful for suppressing the formation of oxide scale during quenching heat treatment. Also, when Si is low, deoxidation is likely to be insufficient, inclusions increase, and rusting may occur starting from there, resulting in poor corrosion resistance. From these viewpoints, it is preferable to contain 0.05% or more. From the viewpoint of obtaining a stable deoxidation effect, the lower limit of the Si content may be 0.08%, 0.10%, 0.13%, 0.15%, 0.17%, or 0.20%. Also, since Si narrows the austenite single-phase temperature range and impairs quenching stability, it may be 0.60%, preferably 0.58%, 0.55%, 0.52%, 0.50%, 0.45%, or 0.40%.

[0018] Mn: 0.10 - 1.50% Mn is an element added as a deoxidizer and also contributes to expanding the austenite single-phase region and improving hardenability. If Mn is not added sufficiently, the two-phase region expands and the α-phase increases. As a result, Cr carbonitrides also increase, and a Cr-deficient layer forms around them, making it easy to become a rusting starting point and reducing corrosion resistance. From these viewpoints, it is considered to contain 0.10% or more. To stably ensure hardenability, the lower limit of the Mn content may be 0.20%, preferably 0.30%, 0.40%, 0.50%, or 0.60%. However, excessive Mn reduces corrosion resistance, promotes the formation of oxide scale during quenching heating, and increases the subsequent polishing load, so the upper limit may be 1.50%. Considering the reduction in corrosion resistance due to particulate matter such as MnS, the upper limit of the Mn content may be 1.45%, 1.40%, 1.35%, or 1.30%.

[0019] P: 0.020% or less P is an element contained as an impurity in the main raw materials such as hot metal and ferrochrome which are the raw materials. Since P is an element that mainly causes a decrease in hot ductility, its content should be reduced as much as possible. From this perspective, the P content is preferably 0.020% or less, more preferably 0.015% or less, 0.013% or less, 0.010% or less, or 0.008% or less. However, excessive reduction will lead to an extreme increase in cost, so the lower limit of the P content is preferably 0.001%, 0.002%, 0.003%, 0.004%, or 0.005%.

[0020] S: 0.0010% or less S is an element that forms sulfide-based inclusions and deteriorates the general corrosion resistance (general corrosion and pitting corrosion) of steel materials. Also, since it reduces hot ductility and increases the ear cracking susceptibility of hot-rolled steel sheets, its content is preferably as low as possible. Further, when S and P coexist, it has been found that it particularly reduces hot ductility and increases crack susceptibility, so S is particularly restricted and the upper limit of its content is set at 0.0010%. From these perspectives, since the S content is preferably as low as possible, the upper limit is preferably 0.0009%, 0.0008%, 0.0007%, 0.0006%, 0.0005%, 0.0004%, 0.0003%, or 0.0002%. On the other hand, the lower the S content, the better the hot workability and corrosion resistance, but desulfurization load increases for low-S and manufacturing cost increases, so the lower limit of its content may be 0.0001%.

[0021] Cr: 11.0 - 15.0% In order to ensure the corrosion resistance required in martensitic stainless steel, the Cr content is preferably 11.0% or more. On the other hand, in order to prevent the formation of retained austenite after quenching, the Si content is preferably 15.0% or less. In order for these effects to be more effectively exerted, the lower limit of the Cr content is preferably 11.3%, 11.5% or 12.0%, and the upper limit is preferably 14.7%, 14.5%, or 14.0%.

[0022] Ni: 0.01 - 2.00% Ni is an austenite stabilizing element like Mn. It also has the effect of improving toughness after quenching and suppressing the progress of pitting corrosion. From this perspective, the lower limit of the Mn content is preferably 0.01%, 0.02%, 0.03%, 0.04%, 0.05%, 0.06%, 0.07%, 0.08%, 0.09%, or 0.10%. On the other hand, excessive addition makes the austenite phase too stable and suppresses the martensite transformation. Therefore, the upper limit of the Ni content is set at 2.00%. There is a risk of reducing the press formability due to solid solution strengthening in hot-rolled annealed steel sheets, and since it is an expensive element, the upper limit of the Ni content is preferably 1.70%, 1.40%, 1.10%, 0.80%, 0.60%, 0.50%, 0.40%, or 0.30%.

[0023] Cu: 0.01 - 0.50% Cu is effective in improving the corrosion resistance of a martensite structure containing δ-ferrite. From the perspective of obtaining this effect, the lower limit of the Cu content is preferably 0.01%, 0.02%, 0.03%, 0.04%, or 0.05% or more. Also, in some cases, it may be actively added to improve hardenability as an austenite stabilizing element. However, excessive addition leads to a decrease in hot workability and an increase in raw material costs. Therefore, the upper limit of the Cu content is preferably 0.50%, 0.45%, 0.40%, 0.35%, or 0.30%.

[0024] Al: 0.05% or less Al is added as a deoxidizing element and is also an element that improves oxidation resistance. From the perspective of obtaining this effect, the Al content is preferably 0.001% or more. On the other hand, the inclusion of Al easily forms large oxide-based inclusions, and Cr segregates at the interface with the matrix phase, deteriorating the rust resistance of the matrix phase. Therefore, the upper limit of the Al content is preferably 0.05%. The lower the Al content, the more preferable it is, and its upper limit is desirably 0.04%, 0.03%, 0.02%, or 0.01%. Of course, it is also possible not to contain Al. Here, the Al content is the T.Al (total Al) content.

[0025] N: 0.010 - 0.090% N, when dissolved, has an excellent effect on corrosion resistance. To clearly manifest this effect, from the perspective of reliably obtaining this effect, the lower limit of the N content is preferably 0.010%, 0.015%, 0.020%, 0.025%, or 0.030%. However, when Cr-based nitrides are formed, a Cr-deficient layer may be generated, and in that case, the corrosion resistance may be reduced. Also, if added in excess, it becomes difficult to control during the steelmaking stage, and bubble system defects are likely to be formed. If bubble system defects are formed, they are likely to become the starting points of rusting, not only reducing the corrosion resistance but also feared to cause a decrease in product yield. Therefore, the upper limit of the N content is preferably 0.090%, 0.080%, 0.070%, 0.060%, or 0.050%.

[0026] O: 0.0050% or less In the present invention, since REM and Ca are added, it is preferable that O be less in order to reduce REM-based inclusions that cause nozzle clogging, etc. When O is contained in an amount exceeding 0.0050%, the number of large REM-based inclusions remaining in the steel increases, which has an adverse effect on productivity and corrosion resistance. Therefore, the upper limit of the O content is preferably 0.0050%, 0.0040%, or 0.0030%. Although it is preferable to reduce it as much as possible, excessive reduction results in an increase in cost. Therefore, the lower limit of the O content in practical use may be 0.001%. Here, the O content is the T.O (total oxygen) content.

[0027] Ca: 0.0001% - 0.0010% Ca is added for component adjustment during the steelmaking stage. Since it acts as a strong deoxidizer and has the effect of promoting deoxidation, it is added. However, since it is a strong deoxidizing element, most of it floats in the molten steel as inclusions and hardly remains in the steel. Also, although it is said that Ca alone has the effect of improving hot workability, it has been found that when added in combination with REM, the hot workability is improved more significantly. Therefore, the lower limit of the Ca content is preferably 0.0001%, 0.0002%, 0.0003%, 0.0004%, 0.0005%, or 0.0006%. On the other hand, since there is a concern that Ca may reduce the corrosion resistance, the upper limit of its content is 0.0010%.

[0028] REM: 0.01 - 0.20% It is an important element in the present invention. By containing an appropriate amount of REM, a remarkable improvement in hot ductility can be observed. It has been confirmed that this effect is more significantly improved in hot ductility by reducing the contents of P and S and further containing an appropriate amount of Ca and REM. Although the reason is not clear, P and S in the molten metal are distributed in the molten metal, so they are concentrated in the molten metal during solidification, which has the effect of lowering the melting point of the molten metal and forms a so-called low melting point phase. The existence of the low melting point phase at the grain boundaries in the final stage of solidification reduces the hot ductility. It is considered that the hot ductility has been improved because the presence of Ca and REM preferentially forms their sulfides, resulting in the non-formation of the low melting point phase. To obtain this effect, the lower limit of the REM content is preferably 0.01%, 0.02%, 0.03%, 0.04%, 0.05%, or 0.06%. On the other hand, excessive addition is not preferable because large REM-based oxides are likely to form, causing nozzle clogging during casting, etc., so the upper limit of the REM content is preferably 0.20%, 0.19%, 0.18%, 0.17%, 0.16%, or 0.15%. REM is usually added in the form of mischmetal, which is a composite, but the addition of single elements such as La, Ce, Pr, Nd, etc. also shows the same effect. Here, REM (rare earth element) refers to the general term for two elements, scandium (Sc) and yttrium (Y), and 15 elements (lanthanoids) from lanthanum (La) to lutetium (Lu) according to the general definition. These REM elements may be contained alone or in combination of multiple REM elements. When containing multiple REM elements, the total amount thereof is preferably within the range of the above lower and upper limits.

[0029] One embodiment of the present invention is, in addition to the above elements, Fe and impurities as the balance. Here, the impurities refer to elements that are inevitably and unintentionally mixed in during the manufacturing process, such as raw materials like ore and scrap, etc., when steel is industrially manufactured, and are allowed within a range that does not adversely affect the present invention.

[0030] By containing the above elements in specified amounts, it is possible to obtain a high-C martensitic stainless steel in which the balance between strength and corrosion resistance is achieved, and which has excellent hot workability with no microcracks during solidification, resulting in significantly improved manufacturability. Furthermore, in addition to these elements, the stainless steel of the present embodiment may contain one or more of Mo, V, Sn, and Bi in place of a part of Fe. These elements do not necessarily have to be contained, but additional effects can be obtained by containing them. These elements will be described below.

[0031] Mo: ~1.00% Mo is effective in improving the corrosion resistance of a martensitic structure containing δ-ferrite, and also has the function of increasing the tempering softening resistance, so it may be contained. However, Mo is an element that stabilizes the ferrite phase, and excessive addition may impair the hardening characteristics by narrowing the austenite single-phase temperature range. Therefore, the upper limit is preferably 1.00%, more preferably 0.90% or 0.80%. The lower limit is not particularly limited, but from the viewpoint of reliably obtaining the effect, it is preferably 0.01%, 0.02% or 0.03%.

[0032] V: ~0.30% V is an element that is inevitably mixed as an impurity in the alloy raw material of ferritic stainless steel and is difficult to remove in the refining process. However, it forms fine carbonitrides, improves wear resistance, and also has an effect on improving corrosion resistance, so it may be contained. On the other hand, excessive content may cause coarsening of the precipitates, resulting in a decrease in toughness after hardening. Therefore, the upper limit of the V content is preferably 0.30%, more preferably 0.20%. The lower limit of the V content is not particularly limited, but considering manufacturing cost and manufacturability, it is preferably 0.01%, 0.03%, 0.05%, 0.08% or 0.10%.

[0033] Sn: 0.100% or less Sn is an element effective in improving the corrosion resistance after quenching, but excessive addition promotes ear cracking during hot rolling. Therefore, the upper limit of the Sn content is preferably 0.100%, more preferably 0.090% or 0.080%. The lower limit of the Sn content is not particularly limited, but from the viewpoint of reliably obtaining the effect, it is preferably 0.002%, more preferably 0.005%, 0.010%, 0.015%, or 0.020%.

[0034] Bi: ~0.20% or less Bi is an element that improves corrosion resistance. Although the mechanism is not clear, it is considered that the probability of rusting initiation is reduced because Bi addition has the effect of refining MnS, which is likely to be a rusting initiation point. On the other hand, even if it is contained excessively, the effect only saturates, so the upper limit of the Bi content is preferably 0.20%, more preferably 0.15%. The lower limit of the Bi content is not particularly limited, but from the viewpoint of reliably obtaining the effect, it is preferably 0.01%, more preferably 0.02%, 0.03%, 0.04%, or 0.05%.

[0035] In addition to the elements described above, it may be contained within a range that does not impair the effects of the present invention. In particular, Ti, Nb, and B have the effect of improving the tempering characteristics, and may contain one or more of Ti: 0.05% or less, Nb: 0.05% or less, and B: 0.0050% or less.

[0036] Also, Zn, Pb, Se, Sb, H, Ga, Ta, Mg, Zr, etc. are preferably reduced as much as possible. On the other hand, within the limit of solving the problems of the present invention, these elements may contain one or more of Zn: 100 ppm or less, Pb: 100 ppm or less, Se: 100 ppm or less, Sb: 500 ppm or less, H: 100 ppm or less, Ga: 500 ppm or less, Ta: 500 ppm or less, Mg: 120 ppm or less, and Zr: 120 ppm or less as necessary.

[0037] <Surface defect> Surface defects (scratches) found on steel plates are problematic in terms of their number, length, and depth, but the important factor affecting quality is their presence rate. The inventors of the present invention measured the number and length of defects on a steel plate coil visually, and evaluated the presence rate R ( / m) of surface defects by dividing the total length (m) of defects with a length of 10 mm or more by the total area (m 2 ) of the coil. The lower the presence rate R, the higher the qualified rate in terms of quality during tool manufacturing. However, since defects with a length of less than 10 mm are mostly shallow and do not affect product quality, they were not counted as defects. Usually, it is preferable that the presence rate R of these surface defects is 0.010 m / m 2 ( / m) or less. Also, regarding the number of defects, the fewer the better, and it is evaluated by the number per unit area (number density), and preferably 0.10 pieces / m 2 or less. Note that the inspection method is not limited to visual inspection, and methods capable of detecting the number and length of defects (for example, an optical defect inspection device) can also be used.

[0038] <Manufacturing Method> An embodiment of the manufacturing method of the steel plate according to the present invention will be described. The steel plate of this embodiment is a hot-rolled steel plate or a cold-rolled steel plate manufactured using the manufacturing method of a normal martensitic stainless steel plate. That is, in the case of a hot-rolled steel plate, it is manufactured through the processes of melting & casting - hot rolling - annealing & pickling of the hot-rolled plate, and the cold-rolled steel plate is manufactured by cold rolling - annealing & pickling of the cold-rolled plate following the hot-rolled steel plate.

[0039] It is possible to improve the hot workability during casting and reduce cracks and defects during casting and hot rolling. That is, in the presence of Ca and REM under low P and low S contents, the enrichment of P and S in the liquid phase is suppressed, and sulfides of Ca and REM are preferentially formed, suppressing the formation of low melting point phases. As a result, the hot ductility is improved and the hot workability during casting is improved. However, as an adverse effect of REM addition, large inclusions (REM oxides) are likely to be formed. Therefore, an appropriate amount of Al is contained to reduce O as much as possible and suppress the presence of large inclusions. Therefore, in the melting process, although not particularly specified, stirring should be reduced as much as possible, within 1 hour, and then, for the inclusions to float, it is preferable to take a standing time of 3 minutes or more. The longer the standing time, the easier the inclusions float and are removed, but it is not preferable because the temperature of the molten steel decreases, and within 30 minutes is preferable. Furthermore, in the continuous casting process, by casting as slowly as possible, the time for inclusions to float can be ensured. The drawing speed in the continuous casting is preferably 2 m / min or less. By controlling the operating conditions in this way, the adverse effects of REM addition can be eliminated, the improvement of hot ductility during casting can be achieved, and cracks can be reduced.

[0040] The hot rolling and hot rolled sheet annealing processes are not particularly limited, and the conventional manufacturing methods for martensitic stainless steel can be applied. For example, in the hot rolling process, the slab is heated at 1100 - 1300 °C and then finished to a thickness of 2 - 8 mm by rough rolling and finish rolling. The hot rolled sheet annealing process uses normal box annealing and is annealed at 750 - 900 °C. Then, the scale on the surface is removed by pickling to obtain a hot rolled steel sheet. Subsequently, the obtained hot rolled steel sheet is cold rolled, and finally annealed and pickled to obtain a cold rolled steel sheet.

[0041] When the obtained steel plate is used for knives, quenching and tempering treatments are usually performed. The quenching condition preferably has a heating temperature of 1000 - 1100 °C. As a result, the hardness becomes 56 HRC or more. Also, as the tempering condition, holding at 150 - 250 °C for 5 min - 1 h is preferable.

Examples

[0042] Further, specific description will be given with reference to examples. It should be noted that the present invention is not limited to the conditions used in the following examples. Steel containing the components shown in Table 1 was melted and cast into a slab with a thickness of 200 mm using a continuous casting machine. After heating this slab to 1150 - 1250 °C, it was subjected to rough hot rolling and finish hot rolling to obtain a hot-rolled steel sheet with a thickness of 5 mm. Subsequently, annealing of the hot-rolled steel sheet was carried out in a box-type annealing furnace. The temperature range was set to 800 °C or higher and 900 °C or lower as the maximum heating temperature. Then, after pickling to remove the scale, it was cold-rolled into a steel sheet with a thickness of 2 mm. Finally, final annealing and pickling were performed to obtain a cold-rolled steel sheet as the product sheet. A part was left as the hot-rolled steel sheet without performing the processes after cold rolling.

[0043] <Evaluation of hot ductility> From the melted slab, a gleeble test piece (10φ×120 mm) was taken, and a molten gleeble test was conducted for the evaluation of hot ductility. The molten gleeble test is to heat the central part of the test piece to just above the melting point to make it semi-molten, and then lower the temperature to the tensile temperature and perform a tensile test. From the results, as an index of hot ductility, in the region of 850 - 1150 °C corresponding to the surface temperature range of continuous casting, those with a reduction value of 70% or more were designated as A, those with 60% or more and less than 70% as B, those with 50% or more and less than 60% as C, and those with less than 50% as D. A, B, and C are considered qualified.

[0044] <Evaluation of surface defects> Also, the surface defects of the hot-rolled steel sheet were investigated. The entire length and width of the coil were visually inspected. Those with a length of 10 mm or more were targeted, and according to the above, the existence rate R ( / m) and the number density (pieces / m2) were determined. And those with a surface defect existence rate R of 0.010 / m or less were considered qualified (○), and those exceeding it were considered unqualified (×). Furthermore, those with an existence rate R of 0.0050 / m or less were considered excellent (◎). Also, for the number density, those of 0.10 pieces / m2 or less were considered excellent (○), and those exceeding it were considered inferior (×).

[0045] <Properties of the steel sheet after heat treatment> To investigate the properties after heat treatment, both the hot-rolled steel sheet and the cold-rolled steel sheet were quenched at 1000 - 1100 °C and then subjected to tempering heat treatment at 150 - 250 °C, and the hardness and corrosion resistance were evaluated as follows. The results are shown in Table 2.

[0046] [Evaluation of Hardness] After polishing the surface to #80 finish, the hardness was measured on the Rockwell C scale in accordance with JIS Z 2245. A hardness of 54 HRC or more was considered qualified (〇), and less than that was considered unqualified (×). Among the qualified ones, those with 58 HRC or more were considered excellent (◎).

[0047] [Evaluation of Corrosion Resistance] For the evaluation of corrosion resistance, after polishing to #600 finish, a salt spray test was carried out for 24 hours or 96 hours in accordance with JIS Z 2371, and then the rust area ratio was measured. A rust area ratio of less than 10% was considered qualified (〇), and 10% or more was considered unqualified (×). Among the qualified ones, those that did not rust were considered excellent (◎).

[0048]

Table 1

[0049]

Table 2

Industrial Applicability

[0050] The present invention can be used throughout the industry as martensitic stainless steel. In particular, it can be used for cutting tools.

Claims

1. by mass%, C: 0.40 to 0.60%, Si: 0.05 to 0.60%, Mn: 0.10 to 1.50%, P: 0.020% or less, S: 0.0050% or less, Cr: 11.0 to 15.0%, Ni: 0.01 to 1.70%, Cu: 0.01 to 0.50%, Al: 0.05% or less, N: 0.010 to 0.090%, O: 0.0050% or less, Ca: 0.0001 to 0.0010%, REM: 0.01 to 0.20% A martensitic stainless steel sheet characterized by having a steel composition containing the above and the balance being Fe and impurities.

2. On the surface of the martensitic stainless steel sheet, the presence rate R of surface defects obtained by dividing the total length of surface defects with a length of 10 mm or more by the total area of the steel sheet is 0.100 / m or less. The martensitic stainless steel sheet according to Claim 1.

3. Furthermore, by mass%, Mo: 1.00% or less, V: 0.30% or less The martensitic stainless steel sheet according to Claim 1 or 2, characterized by containing one or two of the above.

4. Furthermore, by mass%, Sn: 0.100% or less, Bi: 0.20% or less The martensitic stainless steel sheet according to any one of Claims 1 to 3, characterized by containing one or two of the above.

5. A cutting tool made of the martensitic stainless steel sheet according to any one of Claims 1 to 4.

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