Martensitic stainless steel, martensitic stainless steel sheet, and manufacturing method thereof

Abstract

To provide ferritic stainless steel having a wide temperature range of adequate hardening and both predetermined hardness and corrosion resistance.SOLUTION: Ferritic stainless steel of the present invention contains, by mass%, C: 0.14% or more and 0.45% or less, Si: 0.01% or more and 1.00% or less, Mn: 0.01% or more and 1.00% or less, Cr: 11.5% or more and 15.0% or less, Ni: 0% or more and 0.80% or less, N: 0.002% or more and 0.070% or less, P: 0.040% or less, and S: 0.0300% or less, and the balance Fe with impurities. An average crystal grain size of the ferrite is 10.0 μm or less, and 0.8 pieces / μm2 or more of carbide with a diameter of 1.0 μm or less is present in a ferrite phase.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention Martensite This relates to stainless steels, especially for cutting tools such as razors and knives. Suitable for manufacturing It is suitable. [Background technology]

[0002] High-carbon martensitic stainless steels, such as SUS420J1 and SUS420J2, are used for cutting tools such as razor blades and knives, which require high hardness and excellent corrosion resistance. SUS420J1 and SUS420J2 are rapidly cooled, using water or oil, from a high-temperature austenite phase where high carbon is soluble, to a hard martensitic phase where supersaturated carbon is dissolved at room temperature. The hardness of this martensitic phase corresponds to the amount of dissolved carbon in the austenitic phase during high-temperature heating, and it is known that the appropriate hardening temperature range for achieving the target hardness is affected by the size of the carbides before hardening.

[0003] On the other hand, when martensitic stainless steel is cooled relatively slowly from a high-temperature austenite phase, or when it is heated and held in a ferrite phase at a lower temperature, it decomposes into a soft ferrite phase and carbides. For product manufacturing Suitable for ferrite and carbide dispersion Martensite This applies to stainless steels.

[0004] Martensite Stainless steel SteelIn the manufacturing process, ingots obtained by continuous casting or ingot casting are typically hot-worked, cooled to room temperature, and then reheated to decompose into ferrite and carbides, resulting in softening (Non-Patent Document 1). This reheating typically requires a long time, typically several hours, for the decomposition, which tends to cause the carbides dispersed in the ferrite to become coarse. When ferritic stainless steel intermediate materials containing dispersed coarse carbides are quenched, they often become softer than the target hardness. To achieve the target hardness proportional to the amount of solute C, it is necessary to increase the quenching temperature and time and prolong the quenching time to dissolve (re-dissolve) the coarse carbides and ensure the required amount of solute C. If coarse carbides remain, there is a problem of unstable hardness after quenching.

[0005] As a means for solving the above problem, for example, Patent Document 1 discloses a method for optimizing the amounts of added C and N, and Nofu A method for limiting the number density of carbides in the ferrite matrix is ​​disclosed. This broadens the appropriate quenching temperature range for achieving the target hardness, and ensures the required hardness after quenching. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2007-224405 [Non-patent literature]

[0007] [Non-Patent Document 1] Stainless Steel Handbook, 3rd Edition, Japan Stainless Steel Association (1995), p. 829 Summary of the Invention [Problem to be solved by the invention]

[0008] Patent Document 1 steel for cutleryIn the material obtained by quenching and heat treatment, a thick Cr-depleted layer occurs due to oxidation during heating, and when it is made into a blade, depending on the grinding, there is an issue that the corrosion resistance required for the blade may not be secured. It is possible to remove the Cr-depleted layer by increasing the amount of surface grinding, but there is also the issue of increasing the manufacturing cost of the blade product due to a decrease in yield (and an increase in labor costs).

[0009] The present invention has a wide range of suitable quenching temperatures and has both the necessary hardness and corrosion resistance after quenching. Martensite Stainless steel and Martensite The present invention aims to provide a stainless steel sheet having a Cr-type alloy and a method for producing the same in an industrially stable manner. [Means for solving the problem]

[0010] The present inventors have developed a method for manufacturing a hard martensitic stainless steel product for cutting tools. To make Suitable, Martensite The metal structure of the stainless steel was investigated in detail, and the quenching temperature range in which the required hardness and corrosion resistance could be obtained was clarified.

[0011] As a result, it was revealed that the decrease in corrosion resistance after quenching is caused by Cr deficiency immediately below the Cr-containing oxides resulting from grain boundary oxidation. In addition, it was found that by making the crystal grain size finer and dissolving the fine carbides on the grain boundaries, the outward diffusion of Cr is promoted, and the Cr deficiency can be eliminated quickly. ,centre It was discovered that by refining the average grain size of the ferrite phase and controlling the distribution of carbides, the appropriate quenching temperature range for consistently achieving the desired hardness and corrosion resistance could be expanded. The present invention was completed by clarifying the characteristics of the steel composition and metal structure that would achieve this effect.

[0012] Based on the above findings, the present invention discloses the following techniques.

[0013] (1) By mass%, it contains C: 0.14% or more and 0.45% or less, Si: 0.01% or more and 1.00% or less, Mn: 1.00% or less, Cr: 11.5% or more and 15.0% or less, Ni: 0% or more and 0.80% or less, N: 0.002% or more and 0.070% or less, P: 0.040% or less, and S: 0.0300% or less, with the balance being Fe and impurities, and the average grain size of ferrite is 10.0 μm or less, and carbides with a diameter of 1.0 μm or less are present in the ferrite phase at a density of 0.8 particles / μm 2 characterized by the presence of Martensite Stainless steel.

[0014] (2) The above-mentioned (1), characterized in that the proportion of carbides having a diameter of 1.0 μm or less in the grain boundaries of the ferrite phase is 3.0% or more. Martensite Stainless steel.

[0015] (3) The alloy according to (1) or (2), characterized in that a part of the Fe is replaced with one or more of, by mass%, Al: 0.30% or less, Nb: 0.07% or less, B: 0.0030% or less, Ti: 0.07% or less, Mo: 0.75% or less, V: 0.30% or less, Sn: 0.12% or less, Cu: 0.40% or less, W: 1.0% or less, Co: 0.5% or less, Zr: 0.500% or less, Ca: 0.0050% or less, Mg: 0.0050% or less, Y: 0.10% or less, Hf: 0.20% or less, REM: 0.10% or less, and Sb: 0.15% or less. Martensite Stainless steel.

[0016] (4) Any of the above (1) to (3) Martensite Made of stainless steel, plate thickness 0.4 to 6.0 mm Martensite Stainless steel plate.

[0017] (5) Any of the above (1) to (3) Martensitea method for producing a stainless steel comprising the steps of: heating a steel containing the components described in (1) or (3) above to 1150°C or higher and then hot working it; finishing the hot working at a temperature of 850°C or higher and 950°C or lower; cooling the hot-worked steel at a cooling rate of 0.05°C / s or higher without lowering it below 700°C; and holding the cooled steel at a temperature of 700°C or higher and 800°C or lower for 30 minutes to 24 hours. Martensite A method for producing stainless steel.

[0018] (6) (4) above Martensite a method for producing a stainless steel sheet, the method comprising the steps of: heating a steel containing the components according to (1) or (3) to 1150°C or higher and then hot-rolling it; finishing the hot-rolling at a temperature of 850°C or higher and 950°C or lower; cooling the hot-rolled steel at a cooling rate of 0.05°C / s or higher without lowering the temperature below 700°C; holding the cooled steel at a temperature of 700°C or higher and 800°C or lower for 30 minutes to 24 hours; pickling the steel after heating and holding; cold-rolling the pickled steel to obtain a cold-rolled steel sheet; and heat-treating the cold-rolled steel sheet at a temperature of 700°C or higher and 800°C or lower. Martensite A method for manufacturing stainless steel sheets. [Effects of the Invention]

[0019] The present invention provides a wide range of suitable quenching temperatures and provides both the required hardness and corrosion resistance. Martensite We can provide stainless steels. [Brief explanation of the drawings]

[0020] [Figure 1] FIG. 1 is a diagram that schematically shows the criteria for determining whether a carbide is "carbide on a grain boundary" and the "length of a line segment occupying a grain boundary." DETAILED DESCRIPTION OF THE INVENTION

[0021] 1. Martensite Stainless steel Hereinafter, the present invention Martensite The stainless steels will be described in detail below.

[0022] (Chemical composition) First, the present invention Martensite The components contained in stainless steel are explained below. The "%" content of each element means mass %.

[0023] C is an important element for ensuring the hardness of martensite. It also forms Cr carbides, which affect the corrosion resistance of the base material. If the C content is less than 0.14%, the required hardening hardness for cutlery applications cannot be obtained. The number density of carbides of 1.0 μm or less, which contributes to stable hardening, is insufficient, narrowing the appropriate hardening temperature range. Furthermore, the pinning effect of the carbides is ineffective, and the average grain size of the ferrite phase coarsens during furnace heating after hot working. On the other hand, if the C content exceeds 0.45%, the carbides coarsen, resulting in an insufficient number density and a narrower appropriate hardening temperature range. Furthermore, the required corrosion resistance cannot be achieved. For these reasons, the C content is set to 0.14% to 0.45%. Preferably, it is set to 0.17% to 0.38%, and more preferably, to 0.20% to 0.35%.

[0024] Si is an element that improves oxidation resistance. If the Si content is less than 0.01%, sufficient oxidation resistance may not be obtained. Furthermore, an excessive decrease in the Si content increases manufacturing costs. If the Si content exceeds 1.00%, cracking during manufacturing is promoted. Therefore, the Si content is set to 0.01% or more and 1.00% or less. Preferably, it is 0.05% or more and 0.85% or less, and more preferably, it is 0.15% or more and 0.60% or less.

[0025] Mn is used as a deoxidizing element during refining, and Martensite The Cr content also remains in the stainless steel. Martensite Although it is not necessary for stainless steels to contain Mn, from the viewpoint of stable production, MartensiteIt is preferable to add Mn as a deoxidizer during refining so that the Mn content in the stainless steel is 0.01% or more. Martensite If the Mn content in a stainless steel exceeds 1.00%, it may form compounds such as sulfides, which may result in a decrease in corrosion resistance. Therefore, the Mn content is set to 1.00% or less. It is preferably 0.10% or more and 0.85% or less, and more preferably 0.20% or more and 0.70% or less.

[0026] Cr is an element that improves corrosion resistance. If the Cr content is less than 11.5%, sufficient corrosion resistance cannot be obtained. On the other hand, if the Cr content exceeds 15.0%, manufacturability will decrease. Therefore, the Cr content is set to 11.5% or more and 15.0% or less. Preferably, it is 12.0% or more and 14.5% or less, and more preferably, it is 12.5% ​​or more and 14.0% or less.

[0027] Ni is an element that improves toughness when the martensite phase is formed. The inclusion of Ni is not essential, and the lower limit of the Ni content is 0, but it may be added as needed. However, if the Ni content exceeds 0.8%, formability will be reduced. In addition, Ni is a rare and expensive element, which may lead to an increase in alloy costs and impede manufacturability. Therefore, the Ni content is set to 0.8% or less. It is preferably 0.01% or more and 0.6% or less, and more preferably 0.05% or more and 0.5% or less.

[0028] Like C, N is an element that ensures the hardness of martensite. To ensure sufficient hardness, the N content is set to 0.002% or more. On the other hand, if the N content is too high, hot workability deteriorates significantly. Therefore, the N content is set to 0.070% or less. Preferably, it is 0.009% or more and 0.06% or less, and more preferably, it is 0.011% or more and 0.050% or less.

[0029] P is an unavoidable impurity element that reduces formability and corrosion resistance. The lower its content, the better. Therefore, the P content is set to 0.040% or less. Since P does not need to be contained, the lower limit is 0. However, if the P content is reduced too much, the manufacturing cost increases. Therefore, the P content is preferably 0.005% or more and 0.030% or less. More preferably, it is 0.007% or more and 0.025% or less.

[0030] S is an unavoidable impurity element that promotes cracking during manufacturing. Therefore, the S content is set to 0.0300% or less, preferably 0.0100% or less, and more preferably 0.0030% or less. Since S does not need to be contained, the lower limit is 0. However, if the S content is reduced too much, manufacturing costs will increase. From this perspective, the S content is preferably 0.0003% or more.

[0031] The present invention Martensite In addition to the elements mentioned above, the stainless steel contains Fe and impurities (including unavoidable impurities).

[0032] The present disclosure Martensite In addition to the above basic composition, the stainless steel may selectively contain, in mass %, one or more of Al: 0.30% or less, Nb: 0.07% or less, B: 0.0030% or less, Ti: 0.07% or less, Mo: 0.75% or less, V: 0.30% or less, Sn: 0.12% or less, Cu: 0.40% or less, W: 1.0% or less, Co: 0.5% or less, Zr: 0.500% or less, Ca: 0.0050% or less, Mg: 0.0050% or less, Y: 0.10% or less, Hf: 0.20% or less, REM: 0.10% or less, and Sb: 0% to 0.15%.

[0033] (Al: 0.30% or less, Nb: 0.07% or less, B: 0.0030% or less, Ti: 0.07% or less) The elements Al, Nb, B and Ti do not need to be added. MartensiteThese elements improve the formability of stainless steels and have the effect of suppressing defects during hot working. When added, the Al content should be 0.30% or less, the Nb content 0.07% or less, the B content 0.030% or less, and the Ti content 0.07% or less. To ensure the above effects, it is preferable that the Al, Nb, and Ti contents be 0.01% or more, and the B content be 0.001% or more.

[0034] (Mo: 0.75% or less, V: 0.30% or less, Sn: 0.12% or less, Cu: 0.40% or less, W: 1.0% or less, Co: 0.5% or less, Zr: 0.500% or less) The elements Mo, V, Sn, Cu, W, Co, and Zr do not necessarily need to be added. These elements have the effect of improving corrosion resistance. When added, the Mo content should be 0.75% or less, the V content should be 0.30% or less, the Sn content should be 0.12% or less, the Cu content should be 0.40% or less, the W content should be 1.0% or less, the Co content should be 0.50% or less, and the Zr content should be 0.50% or less. To ensure the above effects, it is preferable that the Mo, V, Sn, Cu, Co, and Zr contents be 0.01% or more, and the W content be 0.1% or more.

[0035] (Ca: 0.0050% or less, Mg: 0.0050% or less, Y: 0.10% or less, Hf: 0.20% or less, REM: 0.10% or less, Sb: 0.15% or less) The elements Ca, Mg, Y, Hf, REM, and Sb do not necessarily need to be added. These elements have the effect of modifying inclusions such as oxides and sulfides to suppress hot working defects. When added, the Ca content should be 0.0050% or less, the Mg content should be 0.0050% or less, the Y content should be 0.10% or less, the Hf content should be 0.20% or less, the REM content should be 0.10% or less, and the Sb content should be 0.15% or less. To ensure the above effects, it is preferable that the Ca and Mg contents be 0.0001% or more, and the Y, Hf, and REM contents be 0.01% or more.

[0036] In the present application, "REM" refers to elements (lanthanoids) having atomic numbers 57 to 71, such as Ce, Pr, and Nd, but does not include Y.

[0037] The present disclosure Martensite In addition to the elements described above, the stainless steel may contain elements other than those described above in place of a portion of the Fe, to the extent that the above-mentioned problems can be solved. For example, Bi, Pb, Se, H, Ta, etc. may be contained, but it is preferable to reduce the contents of these elements as much as possible. The content ratios of these elements are controlled to the extent that the above-mentioned problems can be solved, and for example, one or more of Bi≦100 ppm, Pb≦100 ppm, Se≦100 ppm, H≦100 ppm, and Ta≦500 ppm may be contained.

[0038] (Average grain size of ferrite phase and carbide precipitation state) The present invention Martensite In ferrite-based stainless steels, good corrosion resistance after quenching is ensured by refining the average grain size of the ferrite phase and specifying the size and number density of carbides. Refining the average grain size increases the number of carbides located on the grain boundaries of the ferrite phase. During high-temperature heating, the carbides on the grain boundaries act as nuclei for transformation into the austenite phase, increasing the grain boundary area of ​​the austenite phase. Therefore, as the redissolution of carbides progresses, the outward diffusion of the redissolved Cr is promoted, allowing Cr deficiency to be resolved quickly.

[0039] To obtain good corrosion resistance, the average grain size of the ferrite phase must be 10.0 μm or less. The average grain size is preferably 9.0 μm or less, and more preferably 8.0 μm or less. On the other hand, the lower limit of the average grain size is not particularly limited, but based on past experience, it is set to 1.0 μm or more. On the other hand, if the average grain size exceeds 10.0 μm, the amount of carbides located at the grain boundaries decreases, the phenomenon of eliminating Cr deficiency does not occur, and good corrosion resistance cannot be ensured. The average grain size of the ferrite phase is specified as follows.

[0040] The L-section of a steel plate sample prepared by electrolytic polishing was measured using EBSD. The measurement area was 300 μm x 300 μm at 1 / 4 of the plate thickness, with a measurement step size of 0.1 μm. If the crystal orientation difference between adjacent plot data was less than 15°, they were considered to be the same crystal grain; if there was an orientation difference of 15° or more, they were treated as different crystal grains and the average crystal grain size was calculated. Note that if the measurement area contained phases other than ferrite, only the ferrite phase was extracted and the average crystal grain size was calculated.

[0041] The carbides have a size that allows them to be redissolved in the austenite phase during high-temperature heating, and the higher the number density, the better. The number density of the carbides is 0.8 particles / μm with a diameter of 1.0 μm or less. 2 It is necessary that the number of particles is 1.0 or more per μm. 2 More preferably, 1.2 particles / μm 2 That is all. There is no particular upper limit to the number density. Martensite The upper limit considered from the chemical composition of stainless steel is 30 particles / μm 2 In order to ensure stable hardness after quenching, the number of carbides of 1.0 μm or less should be 0.8 / μm. 2 It is sufficient if the size and number density of the carbides are equal to or greater than 1.0 μm, and there is no problem even if coarse carbides exceeding 1.0 μm are present. The presence of coarse carbides is not necessary. If the size and number density of the carbides satisfy the above conditions, the amount of dissolved C required for the target hardness can be sufficiently secured, and the appropriate hardening temperature range is also expanded. The size and number density of the carbides are specified using the following method.

[0042] After mirror polishing the L-section of the steel plate, it is etched with aqua regia to reveal the grain boundaries and carbides, and the size and number density of the carbides are measured by SEM observation. The measurement area is at 1 / 4 of the plate thickness, with a total area of ​​200 μm x 200 μm, and the SEM observation is performed at a magnification of 5000x. The size of the carbides is calculated by converting the observed carbides into a circle equivalent diameter. The number density of carbides [pieces / μm 2The area of ​​the measurement region is calculated as the number of carbides with a diameter of 1.0 μm or less confirmed in the measurement region relative to the area of ​​the measurement region. Here, the carbides to be measured, which will be described later, are carbides with a diameter of 0.05 μm or more. Furthermore, the "measurement region" is defined as including only the ferrite phase and not other phases (austenite phase or martensite phase).

[0043] Furthermore, in addition to the above-mentioned requirements, if the proportion (occupancy) of carbides in the grain boundaries of the ferrite phase is equal to or greater than a certain level, the effect of eliminating Cr deficiency is further enhanced, and corrosion resistance is significantly improved.

[0044] To significantly improve corrosion resistance, the proportion of carbides with a diameter of 1.0 μm or less in the grain boundaries is preferably 3.0% or more, more preferably 5.0% or more, and even more preferably 8.0% or more, with the upper limit being 15.0%.

[0045] In the present invention, the occupancy rate (P [%]) is defined as the proportion of carbides with a diameter of 1.0 μm or less that occupy the grain boundary. The occupancy rate is calculated as the ratio of the sum of the lengths of the line segments (b [μm]) along which the carbides occupy the grain boundary to the total grain boundary length (a [μm]) in the measurement area. The calculation formula is shown in the following formula (2). Figure 1 also shows the criteria for determining whether a carbide is on a grain boundary and the length of the line segments occupying the grain boundary.

[0046] P=b / a×100 … (2)

[0047] In addition, the present invention Martensite Carbides found in stainless steels are (Cr,Fe) 23 The majority of the carbide is C6, but it may contain some (Cr,Fe)7C3. Carbides can be confirmed by EDX.

[0048] (Presence of phases other than ferrite and carbides) The present invention MartensiteThe metal structure of the stainless steel is composed of a ferrite phase and a small amount of fine carbides at room temperature. However, the presence of other phases can be tolerated to a certain extent. Martensite At room temperature, there is no problem if the stainless steel contains phases other than the main ferrite phase, such as austenite and martensite, in an area ratio of 5% or less in total.

[0049] The presence or absence of the austenite phase can be determined using the data measured by the above-mentioned EBSD.

[0050] Since the austenite phase has an FCC structure and the ferrite phase has a BCC structure, the percentage of the FCC structure in the measurement area (γ [%]) is calculated for judgment. If the value calculated using the following formula (1) is 5% or less, it is determined that there is no austenite phase. Here, γ is the area ratio of the austenite phase (unit: [%]), and F and B represent the number of plots of the FCC structure and BCC structure obtained by EBSD measurement, respectively (unit: [pieces]).

[0051] γ=F / (F+B)×100 …(1)

[0052] The presence or absence of martensite phase is determined by Vickers hardness. If the martensite phase is 5% or more, the hardness will exceed 300HV. Using a Vickers hardness tester, measurements are taken 10 times with a load of 500g, and if the average value is 300HV or less, it is determined that there is no martensite phase.

[0053] 2. Martensite stainless steel manufacturing method (Manufacturing method) The present invention Martensite A method for producing stainless steel will now be described.

[0054] Steel having the above composition is melted and cast to produce an ingot. The ingot is then heated. If the heating temperature is less than 1150°C, the carbides cannot be sufficiently solid-dissolved, resulting in variations in properties depending on the part, and coarse carbides remain, narrowing the appropriate quenching temperature range of the product. Therefore, the heating temperature is set to 1150°C or higher, preferably 1180°C or higher.

[0055] Next, the heated ingot is hot-worked. If the end temperature of the hot working is less than 850°C, the deformation load is too high, which increases the load on the equipment performing the hot working, making it impossible to work it into the desired shape. On the other hand, if the end temperature exceeds 950°C, coarse carbides remain without being crushed, narrowing the appropriate quenching temperature range for the product. Therefore, the end temperature of the hot working is set to 850°C or higher and 950°C or lower. Preferably, it is set to 860°C or higher and 940°C or lower.

[0056] Immediately after the hot working is completed, the cooling rate is controlled to cool the material to a temperature of 700°C or higher and 800°C or lower for the subsequent heating and holding process. At this time, it is necessary to manage the thermal history so that the cooling rate is 0.05°C / s or higher and the temperature does not drop below 700°C during cooling. The cooling rate is preferably 0.10°C / s or higher. Since the processing strain accumulated by the hot working is maintained until just before heating and holding, the average crystal grain size of the ferrite phase after heating and holding becomes 10 μm or less.

[0057] On the other hand, if the cooling rate is slower than 0.05°C / s, the processing strain is recovered during cooling, and the ferrite phase precipitation nuclei decrease, causing the ferrite to coarsen during heating. Martensite Stainless steel Steel In the case of a typical manufacturing process, i.e., a thermal history in which the material is cooled to room temperature while controlling the cooling rate after the hot working, and then heated again and held at that temperature, the crystal grains of the ferrite phase become coarse as the strain in the martensite phase disappears.

[0058] In the present invention, the cooling rate after completion of hot working is defined as the average cooling rate from the completion of hot working until the temperature reaches the heating and holding temperature, and the temperature history is measured using a radiation thermometer.

[0059] Thus, the present invention Martensite In the manufacturing process of stainless steels, it is very important to control the cooling rate and temperature history from immediately after hot working to immediately before heating and holding. Martensite Stainless steel Steel This is a control that was not implemented in the manufacturing process.

[0060] The cooling rate is maintained, and the steel is cooled to the required temperature for heating and holding, followed by further heating and holding. If the heating and holding temperature is less than 700°C, the number density of carbides of 1.0 μm or less becomes significantly low, and ferrite transformation does not proceed sufficiently. When the steel is cooled to room temperature after the heating and holding, the resulting metal structure contains a large amount of hard martensite. As a result, subsequent processes such as cold rolling become difficult, resulting in increased manufacturing costs and reduced yields. On the other hand, if the temperature exceeds 800°C, the carbides aggregate and coarsen, narrowing the appropriate quenching temperature range. Furthermore, the average grain size of the ferrite phase also becomes coarse.

[0061] If the heating time is less than 30 minutes, the number density of carbides with a diameter of 1.0 μm or less significantly decreases, resulting in a metal structure containing a large amount of martensite phase after cooling at room temperature. As a result, subsequent processes such as cold rolling become difficult, increasing manufacturing costs and reducing yield. On the other hand, if the heating time exceeds 24 hours, the carbides aggregate and coarsen, narrowing the appropriate quenching temperature range. Furthermore, the ferrite phase grains also coarsen. Therefore, the heating time is preferably at a temperature of 700°C to 800°C for 30 minutes to 24 hours. Preferably, the heating time is at a temperature of 710°C to 790°C for 75 minutes to 20 hours.

[0062] The cooling rate after heating is not particularly limited. For example, the cooling rate may be 0.05° C. / s or more, or air cooling may be used.

[0063] The aforementioned Martensite In the manufacturing process of stainless steel, hot rolling is performed as hot processing, and after the heating and holding is completed, pickling, cold rolling, and final heat treatment are repeated as necessary to produce a plate of the specified thickness. Martensite A stainless steel sheet may be obtained.

[0064] Pickling is a process for removing surface oxide scale, cold rolling is a process for obtaining the specified plate thickness, and final heat treatment is a process for releasing the strain introduced by the cold rolling and softening the material through recrystallization. These are common methods used in the production of stainless steel and are acceptable.

[0065] If the temperature of the final heat treatment is less than 700°C, recrystallization will be insufficient, resulting in a hard material, making it difficult to thread the material to the next process or for material processing at the customer's site. On the other hand, if the temperature is higher than 800°C, the austenite phase will reach a stable temperature range, and after cooling, the metal structure will become one containing a large amount of martensite phase, making it hard, making it difficult to thread the material to the next process or for material processing at the customer's site. Therefore, the temperature of the final heat treatment is set to 700°C or higher and 800°C or lower. Preferably, it is set to 710°C or higher and 790°C or lower.

[0066] The thickness of the sheet may be, for example, 4.0 mm or more and 6.0 mm or less after hot rolling, and 0.4 mm or more and less than 4.0 mm after subsequent cold rolling. [Example]

[0067] The present invention will be described with reference to examples. Martensite The effect of the stainless steel will be explained below. Steel having the composition shown in Table 1 was melted and cast into an ingot with a thickness of 100 mm.

[0068] [Table 1]

[0069] This ingot was heated at the temperature shown in Table 2 for 120 minutes and hot-rolled to obtain a hot-rolled sheet with a thickness of 5.0 mm. Subsequently, the hot-rolled sheet was cooled at a cooling rate of 0.03 to 0.7°C / s to the heating and holding temperature shown in Table 2. After reaching the heating and holding temperature, the hot-rolled sheet was heated and held for the time shown in Table 2. After the heating and holding temperature was reached, the hot-rolled sheet was air-cooled to room temperature. These were then subjected to sulfuric acid pickling, cold rolling at a reduction ratio of 60%, and further heat treatment at 700 to 800°C for 2 minutes to obtain cold-rolled and annealed steel sheets with a thickness of 2.0 mm. In addition, the cold-rolled and annealed sheets were again subjected to cold rolling, cold-rolled annealing, and pickling to produce steel sheets with a thickness of 0.8 mm.

[0070] [Table 2]

[0071] In this study, as a comparison with the manufacturing method of the present invention described above, a steel sheet was also manufactured by a thermal history process in which the sheet was cooled to room temperature after hot rolling, then heated again and held at that temperature (No. 22).

[0072] (Steel plate structure) The average grain size of the ferrite phase, the presence or absence of the austenite and martensite phases, the size and number density of carbides, and the carbide occupancy rate at the grain boundaries were measured for the obtained steel sheets using the methods described above. The results are shown in Table 2. The austenite and martensite phases were 5% or less in all steel sheets.

[0073] (Evaluation test of hardness stability after quenching (appropriate quenching temperature range)) The present invention as shown in Table 2 Martensite The test specimens of stainless steel were subjected to quenching heat treatment at a heating temperature of 900 to 1100°C for 10 minutes, followed by air cooling, and their hardness was investigated. The heating temperature was changed in 10°C increments. The quenching stability (ΔT [°C]) was evaluated using the following formula (3).

[0074] ΔT=Tmax-Tmin …(3)

[0075] Tmin [°C] and Tmax [°C] respectively represent the minimum and maximum temperatures at which the hardness after quenching is H [HV] or higher. The larger ΔT, the wider the quenching temperature range at which H or higher can be achieved, indicating excellent hardness stability. On the other hand, when ΔT is 0, the minimum temperature Tmin and the maximum temperature Tmax are the same, indicating poor hardness stability. Here, ΔT of 30°C or higher was judged as pass (◯), and ΔT below 30°C was judged as fail (×). It is known that hardness after quenching varies depending on the amount of carbon added to the steel (Ct [mass%]), and the relationship between H and Ct can be expressed by the following equation (4).

[0076] H=1150×Ct+240 … (4)

[0077] (Corrosion resistance evaluation test) The present invention as shown in Table 2 Martensite The stainless steel test specimens were subjected to a quenching heat treatment at a heating temperature of Tmin [°C] for 10 minutes, followed by air cooling, and their corrosion resistance was evaluated. The test specimens were given a pickling immersion finish as a surface finish. Pickling immersion finish refers to a surface finish obtained by immersing the quenched material in a fluoronitric acid solution for 10 minutes, rinsing with water, and then removing any surface deposits and scale with a sponge.

[0078] In this study, evaluation tests were conducted using salt spray tests in accordance with JIS Z2371, and the pass / fail corrosion resistance was judged based on whether or not red rust was visible on the surface of the test piece. If no red rust was visible with the naked eye four hours after the start of the test, the test piece was rated as pass (〇), and was deemed to meet the corrosion resistance required for a blade. On the other hand, if red rust was visible with the naked eye, the test piece was rated as fail (×).

[0079] Furthermore, only for those judged as passing (◯), the evaluation test was extended until the total test time reached 24 hours. If no red rust was visually observed after the evaluation test, the corrosion resistance was deemed even better and the test was judged as passing (◎).

[0080] Table 2 shows the evaluation results of quenched hardness stability and corrosion resistance. For Nos. 1 to 18, in which the average grain size of the ferrite phase and the number density of carbides were both above the specified level, the appropriate quenching temperature range for achieving the target hardness was wide, and even with a small amount of surface grinding, the corrosion resistance required for cutting tools was ensured. In particular, for Examples 5 to 18, in which the carbide occupancy rate at the grain boundaries was also above the specified level, the corrosion resistance was significantly improved.

[0081] In contrast to the above, No. 19 had a low carbide number density and a coarse average crystal grain size of the ferrite phase, resulting in insufficient quenched hardness stability and poor corrosion resistance. No. 20 had a low carbide number density and insufficient quenched hardness stability. Furthermore, due to the insufficient amount of Cr added, its corrosion resistance was insufficient. No. 21 had poor corrosion resistance due to the presence of excessive coarse carbides. No. 22, which had a thermal history of being cooled to room temperature after hot rolling and then heated again and held at that temperature, had a coarse average crystal grain size and poor corrosion resistance. No. 23, which had a slow cooling rate after hot rolling, had a coarse average crystal grain size and poor corrosion resistance. [Industrial Applicability]

[0082] The present invention Martensite Stainless steels have a wide range of suitable hardening temperatures and combine the necessary hardness with excellent corrosion resistance, making them suitable for the efficient production of cutlery products that require both hardness and corrosion resistance. [Explanation of symbols]

[0083] 1 Grain boundary 2 Length of the line segment occupying the grain boundary 3 Carbides on grain boundaries 4 Carbides not on grain boundaries

Claims

1. In mass%, C: 0.14% or more and 0.45% or less, Si: 0.01% or more and 1.00% or less, Mn: 1.00% or less, Cr: 11.5% or more and 15.0% or less, Ni: 0% or more and 0.80% or less, N: 0.002% or more and 0.070% or less, P: 0.040% or less, and S: 0.0300% or less and the balance being Fe and impurities, The metal structure at room temperature is composed of a ferrite phase and phases other than the ferrite phase in an area ratio of 5% or less in total, The average grain size of ferrite is 10.0 μm or less, and carbides with a diameter of 1.0 μm or less are present in the ferrite phase at 0.8 particles / μm. 2 There are more than A martensitic stainless steel characterized by:

2. 2. The martensitic stainless steel according to claim 1, wherein the proportion of carbides having a diameter of 1.0 μm or less in the grain boundaries of the ferrite phase is 3.0% or more.

3. Instead of a part of the Fe, In mass%, Al: 0.30% or less, Nb: 0.07% or less, B: 0.0030% or less, Ti: 0.07% or less, Mo: 0.75% or less, V: 0.30% or less, Sn: 0.12% or less, Cu: 0.40% or less, W: 1.0% or less, Co: 0.5% or less, Zr: 0.500% or less, Ca: 0.0050% or less, Mg: 0.0050% or less, Y: 0.10% or less, Hf: 0.20% or less, REM: 0.10% or less, and Sb: 0.15% or less 3. The martensitic stainless steel according to claim 1, further comprising one or more of the following:

4. A martensitic stainless steel plate having a plate thickness of 0.4 to 6.0 mm, which is made of the martensitic stainless steel according to any one of claims 1 to 3.

5. A method for producing the martensitic stainless steel according to any one of claims 1 to 3, comprising: A steel containing the components according to claim 1 or 3 is heated to 1150°C or higher and then hot worked, The finishing process in the hot working is completed at 850°C or higher and 950°C or lower, The steel after hot working is cooled at a cooling rate of 0.05 ° C. / s or more without being cooled to less than 700 ° C., The cooled steel is heated and held at a temperature of 700°C or higher and 800°C or lower for 30 minutes or longer and 24 hours or shorter. A method for producing martensitic stainless steel, comprising the steps of:

6. 5. A method for producing the martensitic stainless steel sheet according to claim 4, comprising: A steel containing the components according to claim 1 or 3 is heated to 1150°C or higher and then hot-rolled, The finish rolling in the hot rolling is completed at 850°C or higher and 950°C or lower, The steel after hot rolling is cooled at a cooling rate of 0.05 ° C. / s or more without being cooled to less than 700 ° C., The cooled steel is heated and held at a temperature of 700°C or higher and 800°C or lower for 30 minutes to 24 hours, moreover, After heating, the steel is pickled. The pickled steel is cold-rolled to form a cold-rolled steel sheet. The cold-rolled steel sheet is heat-treated at 700°C or more and 800°C or less. A method for manufacturing a martensitic stainless steel sheet, comprising the steps of:

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