Martensitic stainless steel and method for producing same, and member and method for producing same

Abstract

A stainless steel having a chemical composition containing, in mass%, 0.01-0.20% of C, 0.01-1.00% of Si, 0.01-1.00% of Mn, 0.001-0.050% of P, 0.0001-0.0100% of S, 0.01-0.10% of Cu, 0.01-0.30% of Ni, 10.0-20.0% of Cr, 0.01-0.30% of Mo, 0.001-0.020% of N, 0.001-0.200% of Al, and 0.005-0.100% in total of one or both of Ti and Nb, with the remainder comprising Fe and impurities, wherein the area ratio of the ferrite phase is 95% or more, and the area ratio of Cr carbides having a particle size of 200 nm or more is 3% or less.

Description

Martensitic stainless steel and its manufacturing method, and component and its manufacturing method

[0001] The present invention relates to a martensitic stainless steel having excellent wear resistance and corrosion resistance, a method for producing the same, and a quenched member and a method for producing the same.

[0002] Martensitic stainless steel is a type of steel with a high carbon concentration to improve quench hardness, and is used for cutting tools such as table knives and scissors, tools such as loom parts and calipers, and structural members such as motorcycle disc brakes and reinforcing bars. In these applications, it is difficult to form a plating layer for rust prevention, apply paint, or apply rust-preventive oil.

[0003] Disc brakes are a key component in motorcycle safety. Disc brakes consist of a disc rotor sandwiched between disc pads, and the friction between them converts kinetic energy into thermal energy, controlling the vehicle's speed.

[0004] Recently, to improve vehicle braking performance, the pressing force between the disc rotor and disc pad may be increased, and the disc rotor material may be required to have improved hardness to enhance wear resistance. Furthermore, since the disc rotor is used as is, unpainted, if its corrosion resistance is low, corrosion may occur, resulting in a decrease in strength and a decrease in adhesion between the disc rotor and disc pad, which may result in reduced braking performance. Furthermore, as vehicle lifespans increase, unpainted vehicles are exposed to corrosive environments for extended periods, so improved corrosion resistance may be required. Therefore, evaluations based on corrosion assessments, such as CASS tests, which simulate environments more severe than salt spray tests, are required. Therefore, martensitic stainless steels with superior hardness and corrosion resistance after quenching are in demand.

[0005] Martensitic stainless steels have a ferrite phase metal structure before shipping, i.e., before quenching, and are hardened to a martensite structure after processing by the manufacturer. The hardness of the martensite structure increases as the amount of solute C after quenching increases. Conventionally, a large amount of chromium carbide precipitates before quenching, which prevents sufficient chromium carbide from dissolving during quenching. This reduces the amount of solute carbon and chromium after quenching, resulting in insufficient hardness and corrosion resistance after quenching.

[0006] Countermeasures include increasing the quenching temperature and increasing the C content, but increasing the quenching temperature not only increases costs but is also undesirable from the viewpoint of energy conservation. Furthermore, increasing the C content is not a desirable measure because it reduces material properties such as toughness and weldability. Therefore, an effective countermeasure is to reduce the amount of Cr carbide before quenching and ensure the amount of C and Cr solid solution after quenching.

[0007] For example, the following patent documents have been disclosed so far regarding the hardness and corrosion resistance of martensitic stainless steel.

[0008] Patent Document 1 discloses a martensitic stainless steel for motorcycle brake discs containing 0.025 to 0.080% C and 11.0 to 13.5% Cr, in which the DFE value defined by formula (1) is 5 to 30, and the delta ferrite fraction observed in the cross-sectional structure is 5 to 30% in area fraction: DFE=12(Cr+Si)-430C-460N-20Ni-7Mn-89... (1)

[0009] Patent Document 2 discloses a stainless steel sheet for brake disc rotors in which, in a martensitic stainless steel containing 0.001 to 0.5% of C and 10.0 to 35.0% of Cr, the area ratio of sulfides present in the matrix and having a particle size of 0.1 μm to 100 μm is 1% to 50%, and of these sulfides, the proportion of which having an aspect ratio of the longitudinal length to the length perpendicular thereto of 1.1 or more is 50% or more.

[0010] JP 2016-65301 A JP 2022-68891 A

[0011] However, although Patent Documents 1 and 2 both describe Cr carbides, they do not control their diameter or precipitation amount. Furthermore, they make no mention of the relationship between the diameter and area ratio of Cr carbides and manufacturing conditions. Therefore, in a stainless steel sheet that has a martensite structure as its main structure after quenching, they do not consider the possibility of achieving both hardness and corrosion resistance after quenching by controlling the chemical composition and precipitates by reducing the Cr carbides before quenching and increasing the amounts of solute C and solute Cr.

[0012] An object of the present invention is to provide a member that has both excellent hardness and corrosion resistance after quenching and a method for manufacturing the same, as well as a martensitic stainless steel that serves as the raw material before quenching and a method for manufacturing the same.

[0013] (1) Chemical composition, in mass%, C: 0.01 to 0.20%, Si: 0.01 to 1.00%, Mn: 0.01 to 1.00%, P: 0.001 to 0.050%, S: 0.0001 to 0.0100%, Cu: 0.01 to 0.10%, Ni: 0.01 to 0.30%, Cr: 10.0 to 20.0%, Mo: 0.01 to 0.30%, N: 0.0 (2) A martensitic stainless steel according to (1), characterized in that the chemical composition contains, in mass %, one or more elements selected from Zr: 1.0% or less, V: 0.10% or less, REM: 0.05% or less, and B: 0.0050% or less, in place of a portion of the Fe.

[0014] (3) A method for producing martensitic stainless steel according to (1) or (2), comprising: a slab heating step of heating a slab having the chemical composition according to (1) or (2); a hot rolling step of hot-rolling the heated slab to form a hot-rolled steel sheet; a cold rolling step of cold-rolling the hot-rolled steel sheet to form a cold-rolled steel sheet; and a cold-rolled sheet annealing step of annealing the cold-rolled steel sheet to form a cold-rolled annealed steel sheet, wherein the holding temperature in the cold-rolled sheet annealing step is higher than 700°C and not more than 1000°C, the holding time is 0 seconds or more and not more than 120 seconds, and after holding, the method for producing martensitic stainless steel is cooled in the temperature range from the holding temperature to 300°C at an average cooling rate of 4°C / s or more.

[0015] (4) Chemical composition, in mass%, C: 0.01 to 0.20%, Si: 0.01 to 1.00%, Mn: 0.01 to 1.00%, P: 0.001 to 0.050%, S: 0.0001 to 0.0100%, Cu: 0.01 to 0.10%, Ni: 0.01 to 0.30%, Cr: 10.0 to 20.0%, Mo: 0.01 to 0.30%, N: 0.001 to 0.20 (5) A member according to (4), characterized in that the chemical composition contains, in mass %, one or more elements selected from Zr: 1.0% or less, V: 0.10% or less, REM: 0.05% or less, and B: 0.01% or less, in place of a portion of the Fe.

[0016] (6) A manufacturing method for a component according to (4) or (5), which includes a quenching process in which the martensitic stainless steel according to (1) or (2) is quenched to form a component, characterized in that the holding temperature for the quenching process is 950°C or higher and 1200°C or lower, the holding time for the quenching process is 5 to 20 minutes, and after holding, the temperature range from the holding temperature to 300°C is cooled at an average cooling rate of 10°C / s or higher.

[0017] According to the present invention, it is possible to obtain a member that has both excellent hardness and corrosion resistance after quenching, as well as a martensitic stainless steel that serves as the raw material for the member before quenching.

[0018] FIG. 1 is a diagram showing the relationship between the Cr carbide area ratio and Vickers hardness of a member after quenching.

[0019] The inventors have conducted research into increasing the hardness of a member after quenching while also providing corrosion resistance, and have obtained the following findings (a) to (c).

[0020] (a) By adding a small amount of either or both of Ti and Nb as long as it does not impair the hardness of the martensitic structure, C can be immobilized, the formation of Cr carbides can be suppressed, and the amount of solute Cr can be increased. Furthermore, by adding a small amount of Al as a required element and forming AlN, the decrease in the amount of solute Cr due to Cr nitrides and / or Cr carbonitrides can be suppressed. These measures avoid the formation of Cr nitrides and / or Cr carbonitrides, ensure the amount of solute Cr before and after quenching, and improve corrosion resistance.

[0021] (b) Furthermore, in order to suppress the formation of Cr carbides before quenching, the holding temperature for annealing the cold-rolled sheet is set to more than 700°C and not more than 1200°C, the holding time is set to 0 seconds or more and not more than 120 seconds, and after holding, the cooling rate from the holding temperature to 300°C is set to 4°C / s or more. This prevents the formation of Cr carbides during cooling, and provides a martensitic stainless steel having increased amounts of solute C and solute Cr and primarily composed of a ferrite phase.

[0022] (c) The quenching conditions are a holding temperature of 950°C or higher and 1200°C or lower, a holding time of 0 to 20 minutes, and a cooling rate of 10°C / s or higher from the holding temperature to 300°C after holding, thereby avoiding the formation of Cr carbides during cooling and obtaining a member having an increased amount of dissolved C and Cr and mainly composed of a martensite structure.

[0023] The present invention has been made based on the above findings, and provides a member having a hardness of 400 Hv or more and a rust occurrence area of ​​10% or less in a CASS test in accordance with JIS H8502. Each of the requirements of the present invention will be described in detail below.

[0024] 1. Chemical Composition The chemical composition of the member after quenching according to this embodiment and the martensitic stainless steel that is the raw material for the member before quenching will be described.

[0025] Examples of steels that can be used for the martensitic stainless steel that serves as the material for the quenched member according to this embodiment and the material before quenching include steels whose chemical components, in mass %, contain C: 0.01 to 0.20%, Si: 0.01 to 1.00%, Mn: 0.01 to 1.00%, P: 0.001 to 0.050%, S: 0.0001 to 0.0100%, Cu: 0.01 to 0.10%, Ni: 0.01 to 0.30%, Cr: 10.0 to 20.0%, Mo: 0.01 to 0.30%, N: 0.001 to 0.200%, Al: 0.001 to 0.200%, and one or both of Ti and Nb in total in an amount of 0.005 to 0.100%, with the balance being Fe and impurities.

[0026] The reasons for limiting the content of each element are as follows. In the following description, "%" in the content means "mass %." Furthermore, the expression "A to B %" means "A % or more and B % or less."

[0027] C: 0.01 to 0.20% C is an essential element for obtaining hardness after quenching, and is added to achieve a predetermined hardness level. For this reason, the lower limit of the C content is set to 0.01%. To further increase hardness, it is preferable to set it to 0.03% or more. On the other hand, excessive C leads to the formation of Cr carbides, which reduces the amount of Cr solid solution in the base material and reduces corrosion resistance. It also leads to deterioration of toughness and weldability, so the upper limit is set to 0.20%. To suppress a decrease in corrosion resistance, the upper limit is preferably set to 0.18%.

[0028] Si: 0.01 to 1.00% Si is an essential element for deoxidation during melting and refining, and is also useful for suppressing excessive oxide scale formation during quenching, so 0.01% or more is added. To obtain a sufficient effect of suppressing oxide scale formation, the content is preferably 0.03% or more. On the other hand, excessive addition of Si reduces the austenite single-phase region, making it difficult to obtain a stable martensite structure after quenching, so the upper limit is set to 1.00%. Preferably, it is 0.80% or less.

[0029] Mn: 0.01 to 1.00% Mn is an essential element that has the same effect as Si as a deoxidizing element. It also has the effect of expanding the austenite single phase region and improving the stability of quenching, so the lower limit is set to 0.01%. Preferably, it is set to 0.03% or more. On the other hand, excessive addition promotes the precipitation of water-soluble MnS and reduces corrosion resistance, so the upper limit is set to 1.00%. Preferably, it is set to 0.80% or less.

[0030] P: 0.001 to 0.050% P is an element contained as an impurity in raw materials such as molten iron and ferrochrome. Since it reduces toughness after quenching, the lower the upper limit, the better, and it is set to 0.050% or less. It is preferably set to 0.040% or less. On the other hand, excessive reduction of P content leads to a significant increase in refining costs, so the lower limit is set to 0.001%. To suppress the increase in refining costs, it is preferable to set it to 0.005% or more.

[0031] S: 0.0001 to 0.0100% S combines with Mn to form water-soluble sulfides, which act as starting points for rusting and reduce corrosion resistance. To prevent a decrease in corrosion resistance, the upper limit of the content is set to 0.0100%, preferably 0.0050% or less. On the other hand, excessive reduction of S content leads to an increase in refining costs, so the lower limit is set to 0.0001%, preferably 0.0003% or more.

[0032] Al: 0.001 to 0.200% Al has the same effect as Si as a deoxidizing element. It is also an essential element that, when added in small amounts, fixes N as AlN and suppresses the formation of Cr nitrides. To obtain the effect of suppressing the formation of Cr nitrides, the lower limit is set to 0.001%, and preferably 0.003% or more. On the other hand, excessive addition of Al reduces the austenite single-phase region and reduces the stability of hardening, so the upper limit is set to 0.200%, and preferably 0.150% or less.

[0033] N: 0.001 to 0.200% N is an essential additive element that, like C, has the effect of increasing hardness after quenching. It also has the effect of increasing corrosion resistance, so the lower limit is set to 0.001%. Preferably, it is set to 0.003% or more. On the other hand, excessive N addition can cause sensitization due to the formation of Cr nitrides, so the upper limit is set to 0.200%. In order to sufficiently reduce the formation of Cr nitrides, it is preferably set to 0.150% or less.

[0034] Cr: 10.0 to 20.0% Cr is an essential additive element for improving corrosion resistance, with the lower limit set to 10.0%, preferably 10.8% or more. On the other hand, excessive Cr addition reduces the austenite single-phase region and reduces hardenability stability. It also leads to an increase in alloy costs, so the upper limit is set to 20.0%, preferably 18.0% or less.

[0035] Ni: 0.01 to 0.30% Ni is an essential additive element for improving corrosion resistance. It also has the effect of expanding the austenite single phase region and improving hardenability stability, so the lower limit is set to 0.01%. Preferably, it is set to 0.02% or more. On the other hand, excessive Ni addition leads to an increase in alloy costs, so the upper limit is set to 0.30%. Preferably, it is set to 0.25% or less.

[0036] Mo: 0.01 to 0.30% Mo has the effect of increasing corrosion resistance, similar to Ni, so the lower limit is set to 0.01%. Preferably, it is set to 0.04% or more. On the other hand, excessive addition of Mo leads to a significant increase in alloy costs, so the upper limit is set to 0.30%. Preferably, it is set to 0.25% or less.

[0037] Cu: 0.01 to 0.10% Like Mo, Cu is an element that improves corrosion resistance, and the lower limit is set to 0.01%, preferably 0.02% or more. On the other hand, excessive Cu addition reduces hot workability, so the upper limit is set to 0.10%, preferably 0.08% or less.

[0038] One or both of Ti and Nb, total 0.005 to 0.100% Ti and Nb are important essential additive elements in the present invention, and these elements have the effect of immobilizing C, thereby suppressing the formation of Cr carbides and increasing the amount of soluble Cr in the material before quenching. In order to immobilize C, the lower limit of the total content of Ti and Nb is set to 0.005%, preferably 0.030% or more. On the other hand, excessive addition leads to a decrease in the amount of solute C, which is important for obtaining hardness. In order to suppress a decrease in hardness, the upper limit of the total content of Ti and Nb is set to 0.100%, preferably 0.090% or less.

[0039] The quenched member according to this embodiment and the stainless steel that is the raw material for the quenched member before quenching have the above chemical composition, with the balance being Fe and impurities. Furthermore, as optional elements, they may contain one or more elements selected from the group consisting of Zr: 1.0% or less, V: 0.10% or less, REM: 0.05% or less, and B: 0.0050% or less, in place of a portion of the Fe.

[0040] Zr: 1.0% or less Zr has the effect of improving toughness and cold forgeability, so it may be added as needed. However, since adding excessive Zr leads to an increase in costs, the upper limit is set to 1.0%.

[0041] V: 0.10% or less Like Ti and Nb, V binds with carbon and has the effect of suppressing the formation of Cr carbides, so it may be added as needed. However, since excessive V addition leads to a decrease in solute C due to the formation of VC, the upper limit is set to 0.10%.

[0042] REM: 0.05% or less REM acts as a deoxidizing element and has the effect of reducing impurities. Therefore, it may be added as needed. However, excessive addition of REM leads to an increase in alloy cost, so the upper limit is set to 0.05%.

[0043] B: 0.0050% or less B is an element effective in improving hot workability and may be added as needed. However, excessive addition of B reduces hardenability due to the complex precipitation of borides and Cr carbides, so the upper limit is set to 0.0050%.

[0044] In the chemical composition of the present invention, the balance is preferably Fe and impurities. Here, "impurities" refer to components that are mixed in during industrial steel production due to various factors in raw materials such as ore and scrap, and in the manufacturing process, and are acceptable within a range that does not affect the present invention.

[0045] <Observation Surface> The observation surface for evaluating the metal structure, Cr carbides, and hardness of the martensitic stainless steel before quenching and the member after quenching according to the present invention is defined as follows. The observation surface is a surface that includes both the rolling direction (RD) as the horizontal direction and the plate thickness direction (ND) as the vertical direction, in other words, a surface perpendicular to the rolling width direction (TD). Hereinafter, this observation surface will be referred to as the "TD surface."

[0046] 2. Martensitic Stainless Steel <Area Ratio of Metallic Structure> The martensitic stainless steel according to the present invention before quenching is 95% or more of a ferrite phase. The remainder, excluding the ferrite phase, is a martensite structure. Because the ferrite phase is softer than the martensite structure, by making the ferrite phase 95% or more, press workability and punching workability can be improved when processing into a part shape. If the ferrite phase is less than 95%, cracks are likely to occur at the interface between different phases during processing, which reduces press workability, etc.

[0047] These phase fraction measurements were performed by EBSD measurement of the entire thickness of the TD plane, and regions with a KAM value of 1.0 or more in the obtained KAM map were calculated as martensite structures, and regions with a KAM value of less than 1.0 were calculated as ferrite phases. The phase fraction of martensite structures calculated in this manner may also include the area fraction of carbides, etc. The EBSD measurement conditions were as follows: Fe (alpha, ferrite, bcc) was selected as the phase to be measured, the magnification was 100x, the step size was 0.8 μm, and the acceleration voltage was 15 kV. Note that the EBSD measurement conditions are not particularly limited as long as they are similar to these measurement conditions.

[0048] <Area ratio of Cr carbides> In the martensitic stainless steel before quenching according to the present invention, the area ratio of Cr carbides having a circle-equivalent diameter of 200 nm or more is controlled in order to obtain good hardness and corrosion resistance after quenching. Precipitates with a diameter of less than 200 nm have little effect on the amount of solute Cr.

[0049] The area ratio of Cr carbide with a particle size of 200 nm or more in equivalent circle diameter before quenching is 3% or less. The area ratio of Cr carbide before quenching is preferably 2% or less. By setting the area ratio of Cr carbide before quenching to 3% or less, solid solution of Cr carbide is promoted during quenching retention, and the area ratio of Cr carbide after quenching can be reduced, resulting in a member that combines hardness and corrosion resistance. If the area ratio of Cr carbide before quenching exceeds 3%, Cr carbide is less likely to be solid-dissolved during quenching, resulting in a high area ratio of Cr carbide after quenching, making it impossible to obtain a member with excellent hardness and corrosion resistance.

[0050] Here, we will explain the area ratio of Cr carbides with a diameter of 200 nm or more in equivalent circle diameter. The area ratio of Cr carbides with a diameter of 200 nm or more is obtained by etching the TD surface of a test steel with Murakami's reagent, described in "Metal Structure Science" (by Hajime Sudo, Imao Tamura, and Yasuji Nishizawa) published by Maruzen Co., Ltd., observing the specimen with an optical microscope, and binarizing the observed image with NIS-ELEMENTS manufactured by NICON Corporation. Since Murakami's reagent only colors areas with high Cr concentrations, binarization can selectively extract only Cr carbides. This method allows for the measurement of precipitates with a diameter of 200 nm or more in equivalent circle diameter by observing at 500x magnification. The observation field used for binarization was approximately 185 μm wide x 140 μm long.

[0051] <Cold-rolled annealed sheet> The martensitic stainless steel according to the present invention is a cold-rolled annealed sheet. The reason for using a cold-rolled annealed sheet rather than a hot-rolled sheet or a hot-rolled annealed sheet is that the sheet thickness is reduced by cold rolling, thereby reducing the area ratio of coarse carbides with a short annealing time for the cold-rolled sheet, and reducing the arithmetic mean roughness Ra of the steel sheet surface to improve adhesion to the brake pad. Whether a stainless steel is a cold-rolled annealed sheet can be determined by the grain size number (GSN) in the metal structure (grain size) as viewed on the TD plane. Generally, the GSN of a hot-rolled annealed sheet is less than 6.0, and the GSN of a cold-rolled annealed sheet is 6.0 or more. Alternatively, it can be determined by the surface roughness of the target product. Generally, in terms of the arithmetic mean roughness Ra in the sheet width direction, the Ra of a hot-rolled annealed sheet is greater than 2.0 μm, and the Ra of a cold-rolled annealed sheet is 2.0 μm or less.

[0052] <Thickness> The martensitic stainless steel according to the present invention preferably has a thickness of 3.0 mm or less, more preferably 2.5 mm or less, from the viewpoint of ensuring accuracy in press working and punching.

[0053] 3. Member <Area fraction of metal structure> The quenched member according to the present invention has a martensite structure of 95% or more after quenching. By making the martensite structure 95% or more, a Vickers hardness of 400 Hv or more can be obtained regardless of the part shape. If the martensite structure is less than 95%, it becomes difficult to stably obtain a Vickers hardness of 400 Hv or more. The phase fraction measurement method for martensitic stainless steel described above can be used directly to evaluate the metal structure of the member.

[0054] <Area Ratio of Cr Carbide> In order to obtain good hardness and corrosion resistance, the member according to the present invention controls the area ratio of Cr carbide having a particle size of 200 nm or more after quenching. The area ratio of Cr carbide having a particle size of 200 nm or more after quenching is 11.5% or less. Preferably, the area ratio of Cr carbide after quenching is 10.5% or less. By setting the area ratio of Cr carbide after quenching to 11.5% or less, the amount of solute C and Cr can be increased, resulting in a martensitic structure with a hardness of 400 Hv or more and corrosion resistance with a rust occurrence area of ​​15% or less after CASS testing. If the area ratio of Cr carbide after quenching exceeds 11.5%, the amount of solute C and Cr decreases, making it difficult to obtain a martensitic structure with a hardness of 400 Hv or more and corrosion resistance with a rust occurrence area of ​​15% or less after CASS testing. For the evaluation of Cr carbides in members, the above-mentioned method for evaluating Cr carbides in martensitic stainless steel can be used as is.

[0055] <Hardness> The member according to the present invention has an average Vickers hardness of 400 Hv or more after quenching. By making the average Vickers hardness 400 Hv or more, the wear resistance can be improved when used as a brake disc component. By making the Vickers hardness 400 Hv or more, the wear resistance can be maintained. On the other hand, if the Vickers hardness is less than 400 Hv, the wear resistance decreases and thickness reduction easily occurs, making it difficult to use the brake disc for a long period of time.

[0056] The hardness is measured using a micro Vickers tester at the center of the plate thickness on the TD surface under a load of HV5 and calculated as the n5 average. The measurement interval and other conditions may be in accordance with JIS Z 2244.

[0057] <Corrosion resistance> The member according to the present invention has good corrosion resistance after quenching in an unpainted state, and the area where rust occurred after 24 hours in a CASS test is 15% or less. By adding trace amounts of Al, Ti, and / or Nb, it is possible to suppress the formation of Cr carbonitrides and obtain a martensitic structure with an increased amount of Cr solid solution in the matrix.

[0058] The corrosion resistance is evaluated by a CASS test, and the test piece is subjected to a #600 wet polishing on the entire surface after quenching, and the test is carried out in accordance with JIS H 8502. The test is carried out in an atmosphere of 50°C for 24 hours, and the evaluation is carried out by determining the rust occurrence area ratio. The CASS test is carried out by dissolving CuCl in a 5% NaCl aqueous solution. ２ ・2H ２ This is a more stringent evaluation method than the salt spray test, in which corrosion resistance is evaluated in an atmosphere sprayed with a solution containing 0.0275% O.

[0059] 4. Manufacturing Method The martensitic stainless steel of the present invention, which is mainly composed of a ferrite phase, is manufactured, for example, by the steps of melting, hot rolling, annealing the hot-rolled sheet, cold rolling, and annealing the cold-rolled sheet.

[0060] The member of the present invention, which is mainly composed of a martensite structure, is produced by quenching and tempering a cold-rolled annealed sheet. Before quenching, the sheet may be subjected to appropriate processing.

[0061] As long as the above chemical composition is satisfied, the target hardness and corrosion resistance can be ensured even if the manufacturing process is carried out under normal process conditions. However, it is preferable to carry out the manufacturing process under the conditions shown below.

[0062] <Hot rolling> Hot rolling is a process of thinly rolling a sheet softened by slab heating. The slab heating temperature is not particularly limited, but is preferably 1100°C or higher to avoid the formation of unnecessary precipitates. On the other hand, an excessive slab heating temperature leads to reduced manufacturability, such as the occurrence of slab sagging, so it is preferably 1250°C or lower. Conditions for rough hot rolling, finish hot rolling, and subsequent cooling may be selected appropriately within a general range. After hot rolling, coiling is preferably performed at 450°C or lower to suppress 475 embrittlement. The lower limit of the coiling temperature is not particularly limited, but is preferably 50°C or higher to ensure the toughness of the hot-rolled sheet.

[0063] <Hot-rolled sheet annealing> Hot-rolled sheet annealing is sufficient to recrystallize the steel sheet, and is preferably 700°C or higher. On the other hand, if the temperature exceeds 1000°C, sagging due to a decrease in the strength of the steel sheet is likely to occur, making production difficult, so the temperature is preferably 1000°C or lower. Furthermore, hot-rolled sheet annealing may be omitted to reduce production costs.

[0064] <Cold Rolling> Cold rolling is a process in which a hot-rolled sheet or a hot-rolled annealed sheet is rolled to the thickness of the final product. The rolling mill may be either a Sendzimir mill or a tandem mill. In cold rolling, the roll roughness, roll diameter, rolling oil, number of rolling passes, rolling speed, rolling temperature, cold reduction, etc. may be appropriately selected within typical ranges. In addition, intermediate annealing may be performed during cold rolling, and the number of cold rolling passes is not limited to one.

[0065] <Cold-rolled sheet annealing> Cold-rolled sheet annealing is a process for recrystallizing the cold-rolled sheet by heat treatment, and is also an important process for making 95% or more of the metal structure a ferrite phase while setting the area ratio of Cr carbides having a circle equivalent diameter of 200 nm or more to 3% or less.

[0066] In the martensitic stainless steel of the present invention, cold-rolled sheet annealing is carried out at a holding temperature of more than 700°C and not more than 1000°C, a holding time of 0 seconds or more and not more than 120 seconds, and an average cooling rate of 4°C / s or more in the temperature range from the holding temperature to 300°C.

[0067] When the holding temperature is 700°C or lower, recrystallization does not proceed, and Cr carbides of 200 nm or larger do not dissolve, resulting in an area ratio of Cr carbides of 200 nm or larger in equivalent circle diameter exceeding 3%. On the other hand, when the temperature exceeds 1000°C, although the area ratio of Cr carbides is 3% or less, a martensite structure is formed during cooling, resulting in an area ratio of ferrite phase of less than 95%, which significantly reduces workability in press forming and the like.

[0068] The lower limit of the holding time is set to 0 seconds in order to dissolve Cr carbide and to make the area ratio of Cr carbide 3% or less. The upper limit of the holding time is set to 120 seconds in order to suppress the diffusion of carbon to the grain boundaries and to suppress the precipitation of Cr carbide at the grain boundaries during cooling. This makes it possible to make the area ratio of Cr carbide 3% or less.

[0069] After holding at the above temperature, the cooling rate from the holding temperature to 300°C is set to 4°C / s or more in order to suppress reprecipitation of Cr carbide during cooling and to make the area ratio of Cr carbide with an equivalent circle diameter of 200 nm or more 3% or less.

[0070] <Quenching Treatment> Quenching treatment is an important process in which martensitic stainless steel, which is a cold-rolled annealed sheet mainly composed of ferrite phase, is held at high temperatures to convert it to an austenite phase, and then rapidly cooled to transform it into a hard martensite structure.

[0071] In the member of the present invention, the holding temperature of the quenching treatment is 950°C or higher and 1200°C or lower, the holding time is 5 to 20 minutes, and after holding, the material is cooled from the holding temperature to 300°C at an average cooling rate of 10°C / s or higher.

[0072] The holding temperature for the quenching treatment is set to 950°C or higher to promote the solid solution of Cr carbide and to make the area ratio of Cr carbide of 200 nm or more 11.5% or less. On the other hand, the holding temperature for the quenching treatment is set to 1200°C or lower to prevent the formation of a ferrite phase during holding and the martensite structure after cooling from becoming less than 95%.

[0073] The holding time is not particularly limited, but the lower limit is set to 5 minutes in order to promote the solid solution of Cr carbide and to make the area ratio 11.5% or less, and the upper limit is set to 20 minutes in order to prevent the diffusion of Cr to the grain boundaries and suppress grain boundary precipitation, thereby making the area ratio of Cr carbide 11.5% or less.

[0074] The average cooling rate in the section from the holding temperature to 300° C. is set to 10° C. / s or more in order to convert 95% or more of the metal structure into a martensite structure. The faster the cooling rate, the better, and there is no particular upper limit.

[0075] The above-mentioned hot-rolled sheet annealing, cold-rolled sheet annealing, and quenching treatments may be performed by batch annealing or continuous annealing. Furthermore, the atmosphere for each annealing may be air, or, if necessary, bright annealing using an inert gas atmosphere such as hydrogen gas or nitrogen gas. Alternatively, annealing in a vacuum may be performed. After the hot-rolled sheet annealing and the cold-rolled sheet annealing, shot blasting, salt treatment, pickling, electrolytic pickling, and the like may be performed. Furthermore, within the scope of the present invention, for example, a tension leveler process for shape correction may be performed after the cold-rolled sheet annealing, and a tempering process for hardness adjustment may be performed after the quenching process.

[0076] The present invention will be described in detail based on the following examples, but the present invention is not limited to the examples shown below.

[0077] Slabs having the chemical compositions shown in Tables 1 and 2 were heated to 1200°C, then hot rolled at a finishing temperature of 900°C, coiled at 450°C, and then hot-rolled sheet annealed at 850°C. In Table 2 and Tables 3, 5, 6, and 8 described below, values ​​outside the range of the present invention are underlined.

[0078]

[0079]

[0080] Example 1 Martensitic Stainless Steel After hot-rolled annealing, the steel sheet was cold-rolled and then annealed under the cold-rolled annealing conditions shown in Table 3, and the area ratio of the ferrite phase and the area ratio of Cr carbide were measured. The results are shown in Tables 4 and 5.

[0081]

[0082] [Evaluation Method and Criteria] (Area Ratio of Ferrite Phase) The area ratio of the ferrite phase was evaluated by electron backscatter diffraction (EBSD) on the TD surface of the steel sheet and determined from the KAM (Kernel Average Misorientation) value. The measurement conditions for the EBSD method were a measurement step of 1.0 μm, an acceleration voltage of 15 kV, and a measurement area of ​​204,000 μm. ２ The area ratio was calculated by assuming that the region where the KAM value in the KAM map was 1.0 or less was the ferrite phase. The above analysis was performed using the OIM-system.

[0083] (Area ratio of Cr carbide) The area ratio of Cr carbide was determined by etching the TD surface of the steel sheet with Murakami's reagent and binarizing an optical microscope image of the center of the sheet thickness using NIS-ELEMENTS manufactured by NICON CORPORATION. The optical microscope image used for the binarization was an image of the center of the sheet thickness observed at 500x magnification. The area ratio of Cr carbide was a value automatically calculated by the binarization. Observation at 500x magnification allowed the measurement of precipitates with a circle equivalent diameter of 200 nm or more.

[0084]

[0085]

[0086] Test specimen Nos. D1 to D35 in Table 5 are invention examples, and Test specimen Nos. d1 to d31 in Table 6 are comparative examples. Test specimen Nos. D1 to D35 were within the range of the present invention and the preferred range of the present invention in terms of both chemical composition and manufacturing method, and were able to obtain good quality. Test specimen Nos. d1 to d31 were out of the range of the present invention and the preferred range of the present invention in terms of either chemical composition or manufacturing method, and were unable to obtain good quality.

[0087] The martensitic stainless steel prepared in Example 1 was subjected to the quenching treatment shown in Table 6, and the area ratio of martensite structure, the area ratio of Cr carbide, Vickers hardness, and CASS test were evaluated. The results are shown in Tables 7 and 8.

[0088]

[0089] [Evaluation Method and Criteria] (Area Ratio of Martensite Structure) The area ratio of the martensite structure was evaluated by electron backscatter diffraction (EBSD) on the TD surface of the steel sheet and determined from the KAM (Kernel Average Misorientation) value. The measurement conditions for the EBSD method were an observation magnification of 100x, a measurement step of 0.8 μm, and an acceleration voltage of 15 kV, and the area ratio was calculated by regarding the region in the KAM map where the KAM value exceeded 1.0 as the martensite structure. The above analysis was performed using an OIM-system. In this case, the area ratio of the martensite structure includes the area ratio of precipitates. Examples of precipitates include Cr carbide, Ti carbide, and Nb carbide.

[0090] (Area ratio of Cr carbide) The area ratio of Cr carbide was determined by etching the TD surface of the steel sheet with Murakami's reagent and binarizing an optical microscope image of the center of the sheet thickness using NIS-ELEMENTS manufactured by NICON CORPORATION. The optical microscope image used for the binarization was an image of the center of the sheet thickness observed at 500x magnification. The area ratio of Cr carbide was a value automatically calculated by the binarization. Observation at 500x magnification allowed the measurement of precipitates with a circle equivalent diameter of 200 nm or more.

[0091] (Vickers Hardness) Vickers hardness was measured using a micro Vickers tester at the center of the plate thickness on the TD surface under a load of HV5, and calculated as the n5 average. The measurement interval and other conditions were in accordance with JIS Z 2244.

[0092] (CASS Test) The CASS test was carried out in accordance with JIS H 8502 on test pieces whose entire surfaces were wet-polished to #600 after quenching. The test was carried out in an atmosphere of 50°C for 24 hours, and the evaluation was carried out by determining the area of ​​rust generated from the appearance. The CASS test was carried out by adding CuCl ２ ・2H ２ This is a more stringent evaluation method than the salt spray test, in which corrosion resistance is evaluated in an atmosphere sprayed with a solution containing 0.0275% O. Corrosion resistance was evaluated based on the rust occurrence area rate in accordance with JIS G 0595, with a rating of 10% or less being "A," over 10% to 15% being "B," and over 15% being "X."

[0093]

[0094]

[0095] Test specimen Nos. E1 to E40 in Table 7 are invention examples, and test specimen Nos. e1 to e36 in Table 8 are comparative examples. Test specimen Nos. E1 to E40 were within the range of the present invention and the preferred range of the present invention in terms of both chemical composition and manufacturing method, and were able to obtain good quality. Test specimen Nos. e1 to e36 were out of the range of the present invention and the preferred range of the present invention in terms of either chemical composition or manufacturing method, and were unable to obtain good quality.

[0096] FIG. 1 shows the relationship between the Cr carbide area ratio and Vickers hardness of quenched components. The "open circles" represent examples of the present invention. The "X" represents some comparative examples, and the manufacturing methods shown in Tables 3 and 6 are outside the scope of the present invention. The "open circle" examples of the present invention ensure a Vickers hardness of 400 HV or more by reducing the Cr carbide area ratio, while also achieving good corrosion resistance in the CASS test. The "X" comparative examples had a high Cr carbide area ratio, resulting in a Vickers hardness of less than 400 HV and poor corrosion resistance in the CASS test. Therefore, it is clear that controlling the Cr carbide area ratio is important for component performance in the composition and manufacturing method of the present invention.

Claims

1. The chemical composition, in mass%, is C: 0.01 to 0.20%, Si: 0.01 to 1.00%, Mn: 0.01 to 1.00%, P: 0.001 to 0.050%, S: 0.0001 to 0.0100%, Cu: 0.01 to 0.10%, Ni: 0.01 to 0.30%, Cr: 10.0 to 20.0%, Mo: 0.01 to 0.30%, N: 0.001 to 0.200%, Al: 0.001 to 0.200%, Ti and / or Nb in a total amount of 0.005 to 0.100%, with the remainder being Fe and impurities, and the ferrite phase accounts for 95% or more in area ratio. A martensitic stainless steel characterized in that the area ratio of Cr carbides having a circle-equivalent diameter of 200 nm or more is 3% or less.

2. A martensitic stainless steel as described in claim 1, characterized in that the chemical composition contains, in mass %, one or more elements selected from Zr: 1.0% or less, V: 0.10% or less, REM: 0.05% or less, and B: 0.0050% or less, in place of a portion of the Fe.

3. A method for producing martensitic stainless steel as set forth in claim 1 or 2, comprising: a slab heating step of heating a slab having the chemical composition set forth in claim 1 or 2; a hot rolling step of hot-rolling the heated slab to form a hot-rolled steel sheet; a cold rolling step of cold-rolling the hot-rolled steel sheet to form a cold-rolled steel sheet; and a cold-rolled sheet annealing step of annealing the cold-rolled steel sheet to form a cold-rolled annealed steel sheet, wherein the holding temperature in the cold-rolled sheet annealing step is greater than 700°C and less than 1000°C, the holding time is 0 seconds or more and 120 seconds or less, and after holding, the temperature is cooled from the holding temperature to 300°C at an average cooling rate of 4°C / s or more.

4. The chemical composition, in mass%, is C: 0.01 to 0.20%, Si: 0.01 to 1.00%, Mn: 0.01 to 1.00%, P: 0.001 to 0.050%, S: 0.0001 to 0.0100%, Cu: 0.01 to 0.10%, Ni: 0.01 to 0.30%, Cr: 10.0 to 20.0%, Mo: 0.01 to 0.30%, N: 0.001 to 0.200%, Al: 0.001 to 0.200%, Ti and / or Nb in a total amount of 0.005 to 0.100%, with the balance being Fe and impurities, and the area ratio of the martensite structure being 95% or more. A member characterized in that the area ratio of Cr carbides having a circle-equivalent diameter of 200 nm or more is 11.5% or less, and the average Vickers hardness is 400 Hv or more.

5. A component as described in claim 4, characterized in that the chemical composition contains, in mass %, one or more elements selected from Zr: 1.0% or less, V: 0.10% or less, REM: 0.05% or less, and B: 0.0050% or less, in place of a portion of the Fe.

6. A manufacturing method for a component according to claim 4 or 5, which includes a quenching treatment step in which the martensitic stainless steel according to claim 1 or 2 is quenched to form a component, characterized in that the holding temperature for the quenching treatment is 950°C or higher and 1200°C or lower, the holding time for the quenching treatment is 5 to 20 minutes, and after holding, the component is cooled in the temperature range from the holding temperature to 300°C at an average cooling rate of 10°C / s or higher.

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