Martensitic stainless steel sheet, martensitic stainless steel member, and component

Abstract

A martensitic stainless steel sheet having a prescribed chemical composition, wherein: the total of C and N in the chemical composition is 0.050-0.110%; the Ms point represented by formula (1) is 50-100; and, where the sheet thickness is t (mm), the area ratio of retained austenite in a cross-section (L cross-section) parallel to the rolling direction and the sheet thickness direction is 1.5-5.0% at the 1 / 2t position and 1.0-5.0% at the 1 / 8t position. (1): Ms=550-361C-28Si-39Mn-10Cu-17Ni-20Cr-5Mo

Description

Martensitic stainless steel sheet, martensitic stainless steel member, and part

[0001] The present invention relates to a martensitic stainless steel sheet, a martensitic stainless steel member, and a part. The present application claims priority based on Japanese Patent Application No. 2025-050164 filed in Japan on March 25, 2025, the content of which is incorporated herein by reference.

[0002] For parts such as steel belts used in belt conveyors and the like, and press plates used for manufacturing printed wiring boards, a martensitic stainless steel sheet such as SUS630 is used as a material. By the way, high strength is required for parts such as steel belts and press plates and members before processing thereof. Therefore, the martensitic stainless steel sheet, which is the material for these, may be subjected to aging treatment to improve strength. On the other hand, flatness is also required for these parts and members before processing thereof. The flatness is adjusted by shape correction with a leveler before aging treatment. To improve flatness, the steel sheet as the material needs to be excellent in shape correctability. Therefore, shape correctability is also required for martensitic stainless steel sheets used in such applications.

[0003] Patent Document 1 discloses that, in mass%, after heating an ingot or steel slab of precipitation hardened stainless steel comprising C: 0.06% or less, Si: 0.25% or less, Mn: 0.25% or less, Cr: 10.5 to 13.0%, Ni: 10.0 to 12.0%, Mo: 1.5 to 2.5%, Al: 1.0 to 2.0%, Ti: 0.1 to 0.6%, with the balance being Fe and impurities, to 875 to 1050°C, a hot forging step is performed at a forging ratio of 20 to 95%. According to the method described in Patent Document 1, crystal grains can be effectively refined, and the strength of stainless steel can be increased.

[0004] Furthermore, Patent Document 2 states that C < 0.10 mass%, 0.01 ≤ Si ≤ 0.10 mass%, 0.01 ≤ Mn ≤ 0.10 mass%, P ≤ 0.010 mass%, S ≤ 0.010 mass%, 7.5 ≤ Ni ≤ 11.0 mass%, 10.0 ≤ Cr ≤ 14.0 mass%, 1.0 ≤ Mo ≤ 2.5 mass%, and 0.001 ≤ N ≤ 0.010 mass%. It contains 0.40 ≤ Al ≤ 1.40 mass%, Cu < 0.10 mass%, 0.30 ≤ Ti ≤ 1.40 mass%, and 0 ≤ Nb ≤ 0.50 mass%, with the remainder being Fe and unavoidable impurities, and 1.00 ≤ [Al] + [Ti] + [Nb] ≤ 2.00, 4.00 ≤ [Ni] / ([Al] + [Ti] + [Nb]) ≤ 8.00, 8.00 ≤ Ni eq ≤ 12.00 and ≤ 16.00 Cr eq A precipitation-hardening martensitic stainless steel satisfying ≤21.00 is disclosed. According to the precipitation-hardening martensitic stainless steel described in Patent Document 2, strength, toughness, and corrosion resistance can be improved.

[0005] Japanese Patent Publication No. 2017-066495 Japanese Patent Publication No. 2020-41208

[0006] However, the stainless steel obtained by the method described in Patent Document 1 has a low Cr content, and there is no mention of the Ms point. Furthermore, Patent Document 1 does not mention anything about the shape modifiability of the martensitic steel sheet used as the base material by a leveler.

[0007] Furthermore, the precipitation-hardening martensitic stainless steel described in Patent Document 2 also has a low Cr content, and there is no mention of its shape-correctability with a leveler.

[0008] As mentioned above, high strength is required for parts such as steel belts and press plates, as well as the materials before processing. On the other hand, during the manufacturing of these parts and their pre-processed components, the martensitic stainless steel sheet material is shaped using a leveler. However, if the strength of the steel sheet is too high, shaping can be difficult, and as a result, good flatness of the parts and their pre-processed components may not be achieved.

[0009] In other words, conventional technologies do not result in excessively high strength in steel plates before aging treatment, thus providing excellent shape correction capabilities. However, no technology has yet been considered that can achieve high flatness and strength in components after leveling adjustment and aging treatment.

[0010] The present invention was devised to solve the above-mentioned problems and provides a martensitic stainless steel sheet with excellent shape modification properties suitable as a material for parts such as steel belts and press plates, as well as a martensitic stainless steel member with excellent strength and flatness.

[0011] The gist of this invention is as follows: [1] The martensitic stainless steel sheet according to one embodiment of the present invention has the following composition in mass%, C: 0.010 to 0.070%, Si: 0.10 to 1.00%, Mn: 0.10 to 1.00%, P: 0.010 to 0.040%, S: 0.0001 to 0.0300%, Ni: 3.0 to 5.0%, Cr: 15.0 to 17.0%, Mo: 0.01 to 0.50%, Cu: 3.0 to 5.0%, N: 0.001 to 0.100%, Nb: 0.15 to 0.45%, Ti: 0 to 0.0090%, V: 0 to 0.500%, W: 0 to 0.500%, Co: 0 to 0.50%, B The chemical composition contains 0-0.0080%, Sn: 0-0.500%, Al: 0-0.0100%, Mg: 0-0.0100%, Ca: 0-0.0100%, Ta: 0-0.0500%, Ga: 0-0.0500%, Zr: 0-0.5000%, REM: 0-0.0500%, and O: 0-0.0100%, with the remainder being Fe and impurities, wherein the total of C and N is 0.050-0.110%, the Ms point represented by the following formula (1) is 50-100, and the area ratio of retained austenite in a cross section (L section) parallel to the rolling direction and the thickness direction is, when the thickness of the sheet is t (mm), The content is 1.5–5.0% at the 1 / 2t position and 1.0–5.0% at the 1 / 8t position. Ms = 550–361C–28Si–39Mn–10Cu–17Ni–20Cr–5Mo …(1) Note that each element in the above formula (1) represents the content (mass%).[2] The martensitic stainless steel sheet described in [1] above has the following chemical composition, in mass%, Ti: 0.0001 to 0.0090%, V: 0.0001 to 0.500%, W: 0.0001 to 0.500%, Co: 0.01 to 0.50%, B: 0.0001 to 0.0080%, Sn: 0.001 to 0.500%, Al: 0.0001 to 0.0100%, Mg: 0.0002 to 0.0100%, Ca: 0.0002 to 0.0100%, Ta: 0.0001 to 0.0500%, Ga: 0.0001 to 0.0500%, Zr: 0.01 to 0.50%, It may contain one or more of the following: REM: 0.0001 to 0.0500%, and O: 0.0010 to 0.0100%. [3] The martensitic stainless steel sheet of [1] or [2] above may have a surface hardness of 330 to 380 HV. [4] Any of the martensitic stainless steel sheets of [1] to [3] above may have a 0.2% yield strength of 900 MPa or less. [5] The martensitic stainless steel member according to one embodiment of the present invention has the following composition in mass%, C: 0.010 to 0.070%, Si: 0.10 to 1.00%, Mn: 0.10 to 1.00%, P: 0.010 to 0.040%, S: 0.0001 to 0.0300%, Ni: 3.0 to 5.0%, Cr: 15.0 to 17.0%, Mo: 0.01 to 0.50%, Cu: 3.0 to 5.0%, N: 0.001 to 0.100%, Nb: 0.15 to 0.45%, Ti: 0 to 0.0090%, V: 0 to 0.500%, W: 0 to 0.500%, Co: 0 to 0.50%, B The chemical composition contains 0-0.0080%, Sn: 0-0.500%, Al: 0-0.0100%, Mg: 0-0.0100%, Ca: 0-0.0100%, Ta: 0-0.0500%, Ga: 0-0.0500%, Zr: 0-0.5000%, REM: 0-0.0500%, and O: 0-0.0100%, with the remainder being Fe and impurities. In the above chemical composition, the total of C and N is 0.050-0.110%, the Ms point represented by the following formula (2) is 50-100, and the surface hardness is 450 HV or higher.Ms = 550 - 361C - 28Si - 39Mn - 10Cu - 17Ni - 20Cr - 5Mo ... (2) Note that each element in the above formula (2) represents the content (mass%). [6] The martensitic stainless steel member described in [5] above has the following chemical composition, in mass%, Ti: 0.0001 to 0.0090%, V: 0.0001 to 0.500%, W: 0.0001 to 0.500%, Co: 0.01 to 0.50%, B: 0.0001 to 0.0080%, Sn: 0.001 to 0.500%, Al: 0.0001 to 0.0100%, Mg: 0.0002 to 0.0100%, Ca: 0.0002 to 0.0100%, Ta: 0.0001 to 0.0500%, Ga: 0.0001 to 0.0500%, Zr: 0.01 to 0.5000% It may include one or more of the following: REM: 0.0001 to 0.0500%, and O: 0.0010 to 0.0100%. [7] A component according to one embodiment of the present invention includes the martensitic stainless steel member described in [5] or [6] above.

[0012] According to the present invention, it is possible to provide martensitic stainless steel sheets with excellent shape modifiability, suitable as materials for parts such as steel belts and press plates, as well as martensitic stainless steel members with excellent strength and flatness.

[0013] The following describes a martensitic stainless steel sheet and a martensitic stainless steel component according to one embodiment of the present invention. However, the present invention is not limited to the configuration disclosed in this embodiment, and various modifications are possible without departing from the spirit of the invention. Furthermore, the numerical limit ranges described below, separated by "~", include both a lower limit and an upper limit. Numerical values ​​indicated as "less than" or "greater than" are not included in the numerical range.

[0014] To solve the above problems, the inventors conducted various studies. The results of these studies are shown below.

[0015] <1. Investigative Study by the Inventors> First, the inventors focused on the metal structure in order to realize a martensitic stainless steel sheet with shape-modification properties suitable as a material for parts such as steel belts and press plates.

[0016] Generally, martensitic stainless steel is often used in components that require high strength, so retained austenite (also called retained gamma), which can lead to a decrease in strength, tends to be reduced. On the other hand, during the manufacturing of components, tensile deformation is often applied to correct the shape (shape correction), but if the strength of the martensitic stainless steel sheet is too high, the ductility may be insufficient, making it impossible to perform normal shape correction, and resulting in a lack of good flatness in the component.

[0017] Therefore, in order to ensure the flatness of the member after shape correction by a leveler and aging treatment, the inventors investigated a metallographic structure in the steel sheet material that can provide good shape correction properties. As a result, they obtained a new finding that by appropriately generating retained austenite, which has been reduced from the standpoint of strength until now, the shape correction properties of the steel sheet can be ensured, while the strength of the member can be ensured by precipitating precipitates through subsequent aging treatment.

[0018] Furthermore, since it is necessary to ensure shape modifiability while also requiring a certain level of material strength, we investigated chemical compositions that can strike a balance between these requirements.

[0019] As a result, we discovered that there is an optimal range for the chemical composition of the martensitic stainless steel sheet used as the raw material, particularly the total amount of carbon and nitrogen content, as well as the Ms point, which is the martensitic transformation initiation temperature. In other words, we newly discovered that by optimizing the chemical composition of the martensitic stainless steel sheet to ensure a certain amount of retained austenite and make it relatively soft, it is possible to achieve both shape modifiability and appropriate strength. Furthermore, by applying shape modification with a leveler and aging treatment to such a steel sheet, it is possible to obtain a component (martensitic stainless steel component) that is excellent in hardness and has high strength.

[0020] <2. Martensitic Stainless Steel Sheet> Based on the above findings, the present invention has found a desired product microstructure for martensitic stainless steel sheets and a method for controlling it. The martensitic stainless steel sheet according to this embodiment will be described below.

[0021] In this embodiment, the martensitic stainless steel sheet also includes steel strips and coils made by winding steel strips.

[0022] [Chemical Composition] The reasons for the limitations on each component of the martensitic stainless steel sheet of this embodiment are explained below. In the following explanation, the "%" indicating the content of each element refers to "mass%" unless otherwise specified.

[0023] (C: 0.010-0.070%) C is an element that greatly affects the stability of the austenite phase. It is also an essential element for obtaining hardness and is included in combination with N to achieve a predetermined hardness level. Furthermore, a certain amount of C is necessary to define the Ms point, which will be described later. Therefore, in order to enjoy these effects, the C content is 0.010% or more. Preferably, it is 0.015% or more, and more preferably 0.020% or more. On the other hand, if the C content is too high, ductility (workability) may decrease, or the precipitation of Cr carbides may be promoted, leading to intergranular corrosion. From the viewpoint of hardness control and ensuring ductility, the upper limit of the C content is 0.070% or less. Preferably, it is 0.060% or less, more preferably 0.050% or less, and even more preferably 0.045% or less.

[0024] (Si: 0.10-1.00%) Si is necessary for deoxidation during melting and refining, and is also a useful element for improving oxidation resistance. Since its effect is observed at concentrations of 0.10% or higher, the lower limit of the Si content is set to 0.10% or higher. Furthermore, Si may be introduced from raw materials such as molten iron, and excessive reductions lead to increased costs, so it is desirable to set the Si content to 0.20% or higher. More preferably, it is 0.50% or higher. On the other hand, if the Si content is too high, it hardens and reduces ductility, so the upper limit is set to 1.00% or lower. Preferably, it is 0.90% or lower, more preferably 0.80% or lower, and more preferably 0.60% or lower.

[0025] (Mn: 0.10-1.00%) Mn is an element included as a deoxidizing agent and also plays an important role in stabilizing the austenite phase by concentrating in it. To enjoy this effect, the lower limit of the Mn content is set to 0.10% or more. To ensure stable hardness, it is desirable that the Mn content be 0.20% or more. More preferably, it is 0.50% or more. However, if the Mn content is too high, not only ductility but also corrosion resistance and hot workability may decrease. For this reason, the upper limit of the Mn content is set to 1.00% or less. Considering the decrease in corrosion resistance due to granular material such as MnS, it is desirable that the Mn content be 0.90% or less.

[0026] (P: 0.010-0.040%) P is an element that is present as an impurity in the main raw materials such as molten iron and ferrochrome. If the P content is excessively high, the moldability may decrease. Therefore, the upper limit of the P content should be 0.040% or less. Preferably, it should be 0.035% or less. On the other hand, excessive reduction of P may lead to increased costs, such as requiring the use of high-purity raw materials, so the lower limit of the P content may be 0.010% or more, more preferably 0.025% or more.

[0027] (S: 0.0001 to 0.0300%) S forms sulfide inclusions and degrades the general corrosion resistance (overall corrosion and pitting corrosion) of steel plates, so it is preferable to keep the upper limit of its content low. For this reason, the upper limit of the S content is 0.0300% or less. More preferably, it is 0.0020% or less. In addition, the lower the S content, the better the corrosion resistance, but excessively reducing the S content increases the desulfurization load and leads to increased manufacturing costs. For this reason, the lower limit of the S content may be 0.0001% or more. The S content is preferably 0.0010% or more.

[0028] (Ni: 3.0-5.0%) Ni is an austenite-stabilizing element and has the effect of increasing the hardness of steel sheets. In addition, Ni is an element that effectively suppresses excessive increases in the hardness of steel sheets by lowering the Ms point, which will be discussed later, and increasing retained austenite. Ni also has the effect of suppressing the precipitation of nitrides and improving corrosion resistance. In order to enjoy these effects, the Ni content should be 3.0% or more. Preferably, it should be 3.5% or more. However, considering the excessive increase in alloy costs, it is desirable that the Ni content be 5.0% or less.

[0029] (Cr: 15.0-17.0%) Cr is an essential element in this embodiment for ensuring oxidation resistance and corrosion resistance. Cr also effectively suppresses excessive increases in the hardness of the steel sheet by lowering the Ms point, as described later, and increasing retained austenite. These effects may not be achieved if the Cr content is less than 15.0%, therefore the Cr content should be 15.0% or higher. Preferably, it is 15.2% or higher, more preferably 15.5% or higher. On the other hand, if the Cr content exceeds 17.0%, the austenite single-phase region may shrink, impairing hardenability. Therefore, the Cr content should be 17.0% or lower. Preferably, it is 16.7% or lower, more preferably 16.5% or lower.

[0030] (Mo: 0.01-0.50%) Mo is an essential element for ensuring corrosion resistance. It also effectively suppresses excessive increases in the hardness of the steel sheet by lowering the Ms point, as described later, and increasing retained austenite. If the Mo content is less than 0.01%, these effects may not be achieved, so the Mo content should be 0.01% or more. Preferably, it is 0.05% or more, more preferably 0.10% or more. On the other hand, Mo is a stabilizing element of the ferrite phase, and excessive content may impair the stability of the hardness of the steel sheet by narrowing the austenite single-phase temperature range. Therefore, the upper limit of Mo should be 0.50% or less. Preferably, it is 0.25% or less.

[0031] (Cu: 3.0-5.0%) Cu, like Mn and Ni, is an austenite-forming element and has the effect of suppressing nitride precipitation and improving corrosion resistance. To enjoy this effect, the lower limit of the Cu content is set to 3.0% or more. Preferably it is 3.1% or more, and more preferably 3.5% or more. On the other hand, excessive Cu content leads to a decrease in hot workability and an increase in raw material costs, so the Cu content is set to 5.0% or less. Preferably it is 4.0% or less.

[0032] (N: 0.010 to 0.100%) N is one of the important elements in this embodiment. Like C, N is an effective element for obtaining a predetermined hardness and is included in combination with C to achieve a predetermined hardness level. N is also an element that dissolves in steel to enhance corrosion resistance. In order to enjoy these effects, the N content is set to 0.010% or more. Preferably 0.015% or more, more preferably 0.020% or more, and even more preferably 0.025% or more. On the other hand, the above effects saturate at 0.100%, and from the viewpoint of suppressing an excessive increase in strength, the upper limit of the N content is set to 0.100% or less. Preferably 0.060% or less, more preferably 0.050% or less, and even more preferably 0.045% or less.

[0033] (Nb: 0.15-0.45%) Nb forms nitrides (NbN) and carbides (NbC), which have the effect of improving processability. In order to enjoy these effects, the Nb content should be 0.15% or more. Preferably, it should be 0.18% or more, and more preferably 0.20% or more. On the other hand, excessive Nb content may reduce ductility, so the upper limit of the Nb content should be 0.45% or less. Preferably, it should be 0.40% or less, more preferably 0.35% or less, and more preferably 0.30% or less.

[0034] (Total C and N: 0.050-0.110%) Both C and N are effective elements for obtaining the hardness of martensitic stainless steel sheets and martensitic stainless steel components obtained using these sheets as materials. Therefore, C and N are included in combination in both the steel sheets and steel components to achieve the desired hardness level. In addition, a certain amount or more of C and N is necessary to lower the Ms point, which will be described later. To enjoy these effects, the total content of C and N is 0.050% or more. Preferably 0.060% or more, more preferably 0.070% or more, and even more preferably 0.080% or more. On the other hand, if C and N are included in excess, the strength will become too high, leading to a decrease in ductility. Therefore, the total content of C and N is 0.110% or less. Preferably 0.100% or less, and more preferably 0.090% or less.

[0035] In the martensitic stainless steel sheet according to this embodiment, the remainder of the elements other than those mentioned above consists of Fe and impurities. However, other elements other than those mentioned above may also be included to the extent that they do not impair the effects of this embodiment. Here, impurities refer to components that are mixed in during the industrial production of the martensitic stainless steel according to the present invention due to various factors in the raw materials such as ore and scrap, and the manufacturing process, and are acceptable to the extent that they do not adversely affect the present invention.

[0036] Furthermore, the martensitic stainless steel sheet according to this embodiment may contain one or more of the following elements in addition to those mentioned above: Ti, V, W, Co, B, Sn, Al, Mg, Ca, Ta, Ga, Zr, REM, and O. In other words, the lower limit of any of these elements may be 0%.

[0037] (Ti: 0.0001 to 0.0090%) Like Nb, Ti forms nitrides (TiN) and carbides (TiC), which improve workability. To enjoy these effects, it is preferable to have a Ti content of 0.0001% or more. More preferably 0.0005% or more, and even more preferably 0.0020% or more. On the other hand, excessive Ti content may reduce ductility, so the upper limit of the Ti content should be 0.0090% or less. More preferably 0.0080% or less.

[0038] (V: 0.0001 to 0.500%) V has the effect of forming fine nitrides and improving workability. For this reason, it is sometimes included intentionally. This effect is stably exhibited at a content of 0.0001% or more, so it is preferable to set the lower limit of the V content to 0.0001% or more. More preferably 0.010% or more, and even more preferably 0.015% or more. On the other hand, if V is included in excess, it may lead to the coarsening of precipitates, which may reduce the ductility and hot workability of the steel sheet. For this reason, it is preferable to set the upper limit of the V content to 0.500% or less.

[0039] (W: 0.0001 to 0.500%) W has the effect of improving corrosion resistance. For this reason, it is sometimes included intentionally. This effect is stably exhibited at a content of 0.0001% or more, so it is preferable to set the lower limit of the W content to 0.0001% or more. More preferably, it is 0.010% or more. On the other hand, if W is included in excess, the ductility of the steel sheet may decrease, so it is preferable to set the upper limit of the W content to 0.500% or less.

[0040] (Co: 0.01-0.50%) Co is an effective element for increasing high-temperature strength and improving hot workability. For this reason, it is sometimes included intentionally. This effect is stably exhibited at a Co content of 0.01% or more, so it is preferable to set the lower limit of the Co content to 0.01% or more. More preferably, it is 0.10% or more. However, excessive Co content may reduce toughness, so it is preferable to set the upper limit of the Co content to 0.50% or less.

[0041] (B: 0.0001% to 0.0080%) Since B has the effect of increasing grain boundary strength through grain boundary segregation and improving hot workability, it may be contained as necessary. In order to exert this effect, the lower limit of the B content is preferably made 0.0001% or more, more preferably 0.0010% or more. However, since excessive content of B may impair corrosion resistance, the upper limit of the B content is preferably made 0.0080% or less, more preferably 0.0030% or less.

[0042] (Sn: 0.001% to 0.500%) Sn, similar to Mo and Cu, is an element that has the effect of improving corrosion resistance by suppressing the progression of pitting corrosion. Therefore, it is desirable to contain Sn as necessary. In order to achieve this effect, the Sn content is preferably 0.001% or more. However, since excessive content of Sn may reduce hot workability, the Sn content is preferably 0.500% or less.

[0043] (Al: 0.0001% to 0.0100%) In addition to being contained as a deoxidizing element, Al is an element that has the effect of improving oxidation resistance. In order to obtain these effects, the Al content may be made 0.0001% or more, more preferably 0.0020% or more. However, since excessive content of Al may cause an increase in manufacturing defects and an increase in raw material costs, the Al content is preferably 0.0100% or less, more preferably 0.0050% or less.

[0044] (Mg: 0.0002% to 0.0100%) In addition to being contained as a deoxidizing element, Mg is an element that has the effect of refining the solidification structure. In order to obtain these effects, the Mg content may be made 0.0002% or more, more preferably 0.0008% or more. However, since excessive content of Mg may cause a reduction in hot workability, the Mg content is preferably 0.0100% or less.

[0045] (Ca: 0.0002 to 0.0100%)  Ca is an element effective for desulfurization and deoxidation. To obtain these effects, the Ca content may be set to 0.0002% or more. More preferably, it is 0.0010% or more. However, excessive Ca content tends to cause hot working cracking and may lead to a decrease in corrosion resistance, so the Ca content is preferably 0.0100% or less. More preferably, it is 0.0035% or less.

[0046] (Ta: 0.0001 to 0.0500%)  Ta is an element that has the effect of improving corrosion resistance by modifying inclusions. To obtain this effect, the Ta content may be set to 0.0001% or more. More preferably, it is 0.0005% or more. However, excessive Ta content may cause a decrease in room-temperature ductility and a decrease in toughness, so the Ta content is preferably 0.0500% or less.

[0047] (Ga: 0.0001 to 0.0500%)  Ga is an element that contributes to improving corrosion resistance and suppressing hydrogen embrittlement. To obtain these effects, the Ga content may be set to 0.0001% or more. More preferably, it is 0.0005% or more. However, excessive Ga content may cause a decrease in workability, so the Ga content is preferably 0.0500% or less.

[0048] (Zr: 0.0001 to 0.5000%)  Zr is an element that improves oxidation resistance, and is preferably contained as necessary. To achieve this effect, the Zr content is preferably set to 0.0001% or more. On the other hand, excessive Zr content may cause not only a decrease in ductility but also an increase in raw material cost, so the upper limit of the Zr content is preferably set to 0.5000% or less.

[0049] (REM: 0.0001 to 0.0500%) REM (rare earth elements) are effective elements for improving hot workability, and it is desirable to include them as needed. To achieve this effect, it is preferable that the total REM content be 0.0001% or more. More preferably 0.0005% or more, and even more preferably 0.0010% or more. On the other hand, if the total REM content is too high, it may impair manufacturability and increase costs. For this reason, it is preferable that the total REM content be 0.0500% or less. More preferably 0.0450% or less, and even more preferably 0.0400% or less.

[0050] REM is a collective term for Sc, Y, and the 15 elements (lanthanides) from La to Lu. In this embodiment, these elements can be used individually or in combination of two or more as REM.

[0051] (O: 0.0010 to 0.0100%) O is an element that inevitably gets mixed in, and the lower its content, the better. In this embodiment, it is preferable that the O content be 0.0100% or less. On the other hand, excessive reduction of the O content may lead to increased costs, so it is preferable that the lower limit of the O content be 0.0010% or more.

[0052] (Ms point: 50-100) In the chemical composition of the martensitic stainless steel sheet of this embodiment and the martensitic stainless steel component described later, the amount of retained austenite produced in the steel sheet is adjusted by controlling the Ms point, which is the martensitic transformation initiation temperature. The Ms point in this embodiment is calculated by the following formula (1).

[0053] The Ms point is the temperature at which martensitic transformation begins. In other words, by designing a chemical composition with a relatively low Ms point, the start of martensitic transformation can be delayed to a lower temperature, making it possible to increase the amount of retained austenite, which has been targeted for reduction in conventional martensitic stainless steels. By ensuring a certain amount of retained austenite, an excessive increase in the hardness of the steel sheet can be suppressed, and the shape modifiability of the steel sheet can be improved. To obtain these effects, the Ms point in the chemical composition should be kept below 100. Preferably below 85, more preferably below 80. However, if the Ms point is lowered too much, there will be an excess of retained austenite and the material will become too soft, so the lower limit of the Ms point should be 50 or higher. Preferably above 55.

[0054] Ms = 550 - 361C - 28Si - 39Mn - 10Cu - 17Ni - 20Cr - 5Mo ... (1) Note that each element in equation (1) represents the content (mass%).

[0055] [Retained Austenite (Retained γ)] In the martensitic stainless steel sheet according to this embodiment, the area ratio of retained austenite in the cross section (L section) parallel to the rolling direction and the thickness direction is 1.5 to 5.0% at the 1 / 2t position and 1.0 to 5.0% at the 1 / 8t position, when the thickness of the sheet is t (mm).

[0056] As described above, high strength and flatness are required in the aged material. Therefore, a certain amount of retained austenite is secured in the raw material before aging to create a relatively soft steel sheet. This allows for a balance between shape modifiability and appropriate strength in the raw steel sheet material, and after shape modification with a leveler (hereinafter also referred to as leveler straightening) and aging treatment, a material (martensitic stainless steel material) with excellent flatness and high strength can be obtained.

[0057] Specifically, by controlling the chemical composition and the area ratio of retained austenite as described above, a martensitic stainless steel sheet with a hardness of 330 to 370 HV and a 0.2% yield strength of 900 MPa or less can be realized. However, if the area ratio of retained austenite in the L-section is less than 1.5% at the 1 / 2t position or less than 1.0% at the 1 / 8t position, these effects cannot be obtained. Therefore, the area ratio of retained austenite in the L-section is 1.5% or more at the 1 / 2t position and 1.0% or more at the 1 / 8t position. Preferably, it is 1.7% or more at the 1 / 2t position and 1.2% or more at the 1 / 8t position. However, if the area ratio of retained austenite is excessively high, it may not be possible to ensure appropriate strength as a material. In particular, if the area ratio of retained austenite is excessively high, there is a risk that the hardness will decrease too much. Therefore, the area ratio of retained austenite in the L-section is 5.0% or less at both the 1 / 2t position and the 1 / 8t position. Preferably, the percentage is 4.0% or less at any position.

[0058] The area ratio of retained austenite is determined by the following method. First, test pieces are taken from the steel plate so that the metallographic structure can be observed at the 1 / 2 position (the range from 3 / 8 of the plate thickness to 5 / 8 of the plate thickness in the thickness direction from the surface) and the 1 / 8 position (the range from 1 / 16 of the plate thickness to 3 / 16 of the plate thickness in the thickness direction from the surface). After the thickness cross section of each test piece is finished with mirror polishing, electropolishing is performed to suppress work-induced martensitic transformation due to polishing strain.

[0059] Next, at an arbitrary position in the longitudinal direction (rolling direction) of the cross-section of the polished specimen, the crystal orientation information of the region is obtained by electron backscatter diffraction, measuring a 100 μm × 100 μm region from the 3 / 8 position to the 5 / 8 position from the surface, with measurement intervals of 0.2 μm, with the center being at the position 1 / 2 of the thickness from the surface. The microstructure identified by the obtained crystal orientation information is defined as the metal structure at the center position in the thickness direction (1 / 2t position). Furthermore, the crystal orientation information is obtained by electron backscatter diffraction, measuring a 100 μm × 100 μm region from the 1 / 16 position to the 3 / 16 position from the surface, with measurement intervals of 0.2 μm, with the center being at the position 1 / 8 of the thickness from the surface. The microstructure identified by the obtained crystal orientation information is defined as the metal structure at the surface layer (1 / 8t position).

[0060] The above measurements are performed using an apparatus consisting of an FE-SEM (Thermal Field Emission Scanning Electron Microscope) (JEOL JSM7200F, manufactured by JEOL Ltd.) and an EBSD detector (Digview5, manufactured by AMETEK EDAX).

[0061] Next, the crystal orientation information obtained from the EBSD measurement is used to calculate the area fraction of retained austenite using the "PhaseMap" function included in the "OIMAnalysis®" software that comes with the EBSD analyzer. In calculating the area fraction, crystals with an fcc crystal structure are identified as retained austenite. Crystals with a bcc crystal structure are identified as either the ferrite phase or the martensite phase.

[0062] In this embodiment, it is preferable that the martensitic stainless steel sheet has a martensitic structure as the majority of the structure other than the retained austenite. Specifically, the area ratio of the martensitic structure is preferably 80% or more, and preferably 90% or more. Furthermore, in the metallic structure of the martensitic stainless steel sheet of this embodiment, it may consist of retained austenite and martensite, but the remaining structure other than retained austenite and martensite may include ferrite, carbides, nitrides, and inclusions, to the extent that they do not hinder the effects of the present invention. The area ratio of martensite is calculated by the following method: Among the bcc phase obtained by the EBSD measurement, the region with a KAM value of 0.60° or more is defined as martensite.

[0063] [Surface hardness of steel sheet: 330-380 HV] The surface hardness of the martensitic stainless steel sheet in this embodiment is preferably 330-380 HV. As described above, good flatness is required in the member after leveling and aging treatment. Therefore, at the stage of the steel sheet material, a certain amount of retained austenite is secured to make the steel sheet relatively soft, thereby achieving both shape correctability and appropriate strength. To obtain such an effect, the surface hardness of the steel sheet is preferably 330-380 HV. From the viewpoint of improving shape correctability, the upper limit of the surface hardness of the steel sheet is more preferably 375 HV or less, and even more preferably 365 HV or less. On the other hand, if the hardness is too low, the strength as a material may not be guaranteed, so the lower limit of the surface hardness of the steel sheet is preferably 340 HV or more, and even more preferably 350 HV or more. The surface hardness of the steel sheet can be determined in the same way as the surface hardness of the steel member described later.

[0064] [0.2% yield strength: 900 MPa or less] The 0.2% yield strength of the martensitic stainless steel sheet in this embodiment is preferably 900 MPa or less. Regarding the 0.2% yield strength, similar to the surface hardness described above, it is effective not to increase it excessively in order to achieve both shape modifiability and appropriate strength by using a relatively soft steel sheet as the raw material. From this viewpoint, the 0.2% yield strength of the steel sheet is preferably 900 MPa or less. More preferably, it is 880 MPa or less. The 0.2% yield strength of the steel sheet can be measured in accordance with JIS Z 2241:2011.

[0065] <3. Martensitic Stainless Steel Members> The martensitic stainless steel members of this embodiment are manufactured by applying leveling and aging treatment to the martensitic stainless steel sheets of this embodiment described above. Therefore, the martensitic stainless steel members of this embodiment have the same chemical composition as the martensitic stainless steel sheets described above and have a hardness of 450 HV or higher. Such martensitic stainless steel members can be suitably used as parts such as steel belts and press plates.

[0066] [Surface hardness of steel members: 450 HV or higher] High hardness is required for parts such as steel belts and press plates. Therefore, the surface hardness of steel members must be 450 HV or higher. Preferably it is 480 HV, and more preferably 490 HV or higher. On the other hand, if the surface hardness of the steel member becomes too high, the toughness may deteriorate. Therefore, it is preferable to keep the surface hardness of the steel member 550 HV or lower. More preferably 540 HV or lower.

[0067] The surface hardness of steel components is measured and calculated by the following method: A Vickers hardness test is performed on the surface of the steel component in accordance with JIS Z 2244:2020. In the Vickers hardness test, five measurements are taken on the surface of the polished steel component with a test force of 5 kgf and a load holding time of 10 seconds. The Vickers hardness is obtained by calculating the average of the three points excluding the maximum and minimum values. The surface to be measured for surface hardness should be a sound surface free from defects or other damage.

[0068] Herein, the martensitic stainless steel member according to this embodiment has excellent strength and flatness, and is therefore suitable for use in parts, particularly steel belts used in belt conveyors and press plates used in the manufacture of printed circuit boards. These parts may consist solely of the martensitic stainless steel member according to this embodiment, or they may be formed by joining the martensitic stainless steel member according to this embodiment with other steel members. In the parts according to this embodiment, if they have the same configuration as the martensitic stainless steel member according to this embodiment, they can exhibit the same properties as the martensitic stainless steel member according to this embodiment.

[0069] Parts manufactured using the martensitic stainless steel component according to this embodiment have the same chemical composition as the steel sheet described above. Furthermore, the component may contain a mixture of processed and unprocessed portions. The unprocessed portions have the same mechanical properties as the martensitic stainless steel component described above. While processed portions generally have the same mechanical properties as the martensitic stainless steel component described above, areas subjected to heavy processing or welding, the edges of the component, or areas where red rust has occurred may not possess the aforementioned mechanical properties, or it may be difficult to determine. Therefore, when performing various measurements such as mechanical properties on a component, these areas should be avoided, and measurements should be taken only on the unprocessed portions. If there are no unprocessed portions, measurements should be taken only on portions that have not undergone heavy processing. Unprocessed or heavily processed portions refer to, for example, flat portions of the component, portions where the increase or decrease in plate thickness due to processing is small, and portions that avoid punching, hole widening, or bending processes.

[0070] <4. Manufacturing Method> The following describes a preferred manufacturing method for the martensitic stainless steel sheet and martensitic stainless steel component of this embodiment.

[0071] <4.1 Method for Manufacturing Martensitic Stainless Steel Sheets> The martensitic stainless steel sheet of this embodiment is a cold-rolled steel sheet manufactured using a typical method for manufacturing martensitic stainless steel sheets. For example, a hot-rolled steel sheet is manufactured through the processes of melting and casting, hot-rolling, hot-rolled sheet annealing, and pickling, and then a cold-rolled steel sheet is manufactured by subsequently performing cold rolling.

[0072] The resulting cold-rolled steel sheets undergo finish annealing, pickling, and temper rolling. The main processes are described below.

[0073] First, steel adjusted to the specified components as described above is melted and a cast slab is obtained by a known method. Using the obtained cast slab, a hot-rolled steel sheet is manufactured according to a conventional method. The hot-rolled steel sheet is subjected to hot-rolled sheet annealing, preferably at an annealing temperature of 1000 to 1150°C and a cooling rate of 1 to 30°C / s after annealing. By performing hot-rolled sheet annealing under these conditions, a solution treatment can be achieved in which precipitates such as carbides are more sufficiently dissolved.

[0074] Next, the hot-rolled steel sheet, after annealing, is subjected to cold rolling by a conventional method, followed by finish annealing and pickling to obtain a cold-rolled steel sheet. The finish annealing temperature is preferably 900 to 1150°C, and the cooling rate after annealing is preferably 1 to 30°C / s. By performing finish annealing under these conditions, a quenched structure can be obtained in which the rolled structure has been recrystallized.

[0075] Next, the cold-rolled steel sheet, after finish annealing and pickling, is subjected to temper rolling. The reduction ratio (temper rolling rate) for temper rolling is preferably 1% or less. From the viewpoint of improving the strength and shape correction of the steel member, the reduction ratio for temper rolling is preferably 0.8% or less. Through the above steps, the martensitic stainless steel sheet of this embodiment is obtained. The thickness of the martensitic stainless steel sheet in this embodiment can be, for example, 0.3 mm to 6.0 mm.

[0076] <4.2 Method for Manufacturing Martensitic Stainless Steel Members> The martensitic stainless steel member of this embodiment can be obtained by applying shape correction (leveling) and aging treatment to a martensitic stainless steel sheet obtained by the above method.

[0077] The specific conditions for leveler straightening are not particularly limited, but for example, a cumulative strain of 0.1% or more may be applied to the martensitic stainless steel sheet, and the elongation may be 1.0% or less. In the aging process, the aging temperature can be in the range of 400 to 550°C, and the holding time can be 0.0014 to 30 hours. By carrying out the aging process within this range, the hardness of the steel member after aging can be made to a desired range. Furthermore, it is preferable to carry out the aging process under conditions where the LMP shown in the following formula (3) is 14000 to 16000. This "LMP" is a so-called tempering parameter, and is calculated by the following formula (3) when the processing temperature of the aging process is T (°C) and the holding time is t (h). log is a common logarithm with base 10.

[0078] LMP=(273+T)×(20+logt)...(3)

[0079] As described above, this embodiment provides a martensitic stainless steel sheet with excellent shape-modification properties because it does not have excessively high strength. In particular, the martensitic stainless steel sheet of this embodiment can be suitably used as a material for parts such as steel belts and press plates.

[0080] Furthermore, the martensitic stainless steel members produced using the martensitic stainless steel sheet of this embodiment, after leveling and aging treatment, possess high strength and excellent flatness, making them suitable for use as components such as steel belts and press plates.

[0081] The following are examples of the present invention. The conditions in these examples are merely examples of conditions adopted to confirm the feasibility and effectiveness of the present invention, and the present invention is not limited to the conditions used in the following examples. The present invention can adopt various conditions as long as they do not depart from the spirit of the invention and achieve the objectives of the present invention. In the table below, underlined items indicate that they fall outside the scope of the present invention or the preferred scope of the present invention.

[0082] Using slabs having the chemical composition shown in Table 1, hot rolling was performed using a conventional method, followed by hot-rolled sheet annealing under the conditions shown in Table 2 to obtain hot-rolled sheets. The average cooling rate after hot-rolled sheet annealing was controlled within the range of 1 to 30°C / s. Subsequently, cold rolling, finish annealing, and pickling were performed sequentially under the conditions shown in Table 2. Next, the cold-rolled steel sheets after pickling were subjected to temper rolling at a temper rolling rate of 0.2% to produce martensitic stainless steel sheets with a thickness of 1.4 mm (test material, finish-annealed material). The average cooling rate after finish annealing was controlled within the range of 1 to 30°C / s. For other manufacturing conditions not specifically mentioned, the preferred manufacturing conditions described in the specification were adopted. In addition, the oxygen (O) content was less than 0.0100% for all component Nos shown in Table 1.

[0083] By the above method, martensitic stainless steel sheets (test material, finish annealed material) for Examples 1 to 6 and Comparative Examples 7 to 10 of the present invention were obtained, as shown in Table 2.

[0084] The area percentage of retained austenite and the surface hardness at each location of the obtained test material were measured using the procedure described above.

[0085] The 0.2% proof stress was measured in accordance with JIS Z 2241:2011. Specifically, a No. 13B tensile test specimen, as specified in JIS Z 2241:2011, was prepared so that the tensile direction was parallel to the rolling direction of the test material, and a tensile test was performed at a strain rate of 4.8% / min to measure the 0.2% proof stress.

[0086] The results of these various measurements are shown in Table 2.

[0087] Next, the martensitic stainless steel sheets (test material, finish annealed material) of Examples 1 to 6 and Comparative Examples 7 to 10 of the present invention were subjected to the aging treatment shown in Table 2, and the surface hardness of the resulting aged material (sheet thickness 1.4 mm) was measured using the procedure described above.

[0088] As shown in Table 2, all of the Examples 1 to 6 of the present invention satisfy the requirements of this embodiment in terms of chemical composition, Ms point, and area ratio of retained austenite, and the surface hardness and 0.2% yield strength are also within the desired range. Therefore, it can be expected that all of the Examples 1 to 6 of the present invention have good shape modifiability. Furthermore, the aged material obtained using the martensitic stainless steel sheet of the Examples of the present invention as the material was able to obtain an excellent hardness of 450 HV or more. In other words, it was evaluated that by using the martensitic stainless steel sheet of the Examples of the present invention as the material, a martensitic stainless steel member with excellent strength and flatness can be obtained.

[0089] Therefore, all of the martensitic stainless steel sheets of the present invention that satisfy the requirements of this embodiment can be evaluated as martensitic stainless steel sheets with suitable shape-modification properties for use as materials for components such as steel belts and press plates.

[0090] On the other hand, in Comparative Example 7, the Ms point in the chemical composition was too high, resulting in insufficient retained austenite, and the hardness and 0.2% yield strength of the martensitic stainless steel sheet increased excessively. Therefore, it was evaluated that good shape correction properties could not be expected and the desired flatness could not be obtained. In Comparative Example 8, the Ms point in the chemical composition was too low, resulting in an excess of retained austenite, and the martensitic stainless steel sheet became too soft, failing to guarantee the strength of the material. Furthermore, the hardness of the aged material was also insufficient, and it was evaluated that the strength of the steel member could not be guaranteed. In Comparative Example 9, the total content of C and N in the chemical composition was too low, resulting in the martensitic stainless steel sheet becoming too soft, and failing to guarantee the strength of the material. Furthermore, the hardness of the aged material was also insufficient, and it was evaluated that the strength of the steel member could not be guaranteed. In Comparative Example 10, the total content of C and N in the chemical composition was too high, resulting in an excessive increase in the hardness and 0.2% yield strength of the martensitic stainless steel sheet. Therefore, it could be concluded that good shape correction was not expected, and the desired flatness could not be obtained.

[0091]

[0092]

[0093] According to the above-described embodiment of the present invention, it is possible to provide martensitic stainless steel sheets with excellent shape modifiability, martensitic stainless steel members with excellent strength and flatness, and parts. This makes it applicable to applications such as steel belts used in belt conveyors and parts such as press plates used in the manufacture of printed circuit boards.

Claims

1. In mass%, C: 0.010 to 0.070%, Si: 0.10 to 1.00%, Mn: 0.10 to 1.00%, P: 0.010 to 0.040%, S: 0.0001 to 0.0300%, Ni: 3.0 to 5.0%, Cr: 15.0-17.0%, Mo: 0.01-0.50%, Cu: 3.0-5.0%, N: 0.001-0.100%, Nb: 0.15-0.45%, Ti: 0-0.0090%, V: 0-0.500%, W: 0-0.500%, Co: 0 to 0.50%, B: 0 to 0.0080%, The chemical composition contains Sn: 0-0.500%, Al: 0-0.0100%, Mg: 0-0.0100%, Ca: 0-0.0100%, Ta: 0-0.0500%, Ga: 0-0.0500%, Zr: 0-0.5000%, REM: 0-0.0500%, and O: 0-0.0100%, with the remainder being Fe and impurities, wherein the total of C and N in the said chemical composition is 0.050-0.110%, the Ms point represented by the following formula (1) is 50-100, and the area ratio of retained austenite in a cross section (L section) parallel to the rolling direction and the thickness direction is 1.5-5.0% at the 1 / 2t position when the thickness is t (mm), A martensitic stainless steel sheet with a content of 1.0 to 5.0% at the 1 / 8t position. Ms = 550 - 361C - 28Si - 39Mn - 10Cu - 17Ni - 20Cr - 5Mo ... (1) Note that each element in the above formula (1) represents the content (mass%).

2. In the above chemical composition, further, in mass%, Ti: 0.0001 to 0.0090%, V: 0.0001 to 0.500%, W: 0.0001 to 0.500%, Co: 0.01 to 0.50%, B: 0.0001 to 0.0080%, Sn: 0.001 to 0.500%, Al: 0.0001 to 0.0100%, Mg: 0.0002 to 0.0100%, Ca: 0.0002 to 0.0100%, Ta: 0.0001 to 0.0500%, Ga: 0.0001 to 0.0500%, Zr: 0.01 to 0.5000%, A martensitic stainless steel sheet according to claim 1, comprising one or more of the following: REM: 0.0001 to 0.0500%, and O: 0.0010 to 0.0100%.

3. The martensitic stainless steel sheet according to claim 1 or 2, wherein the hardness of the sheet surface is 330 to 380 HV.

4. A martensitic stainless steel sheet according to claim 1 or 2, wherein the 0.2% yield strength is 900 MPa or less.

5. In mass %, C: 0.010-0.070%, Si: 0.10-1.00%, Mn: 0.10-1.00%, P: 0.010-0.040%, S: 0.0001-0.0300%, Ni: 3.0-5.0%, Cr: 15.0-17.0%, Mo: 0.01-0.50%, Cu: 3.0-5.0%, N: 0.001-0.100%, Nb: 0.15-0.45%, Ti: 0-0.0090%, V: 0-0.500%, W: 0-0.500%, Co: 0 to 0.50%, B: 0 to 0.0080%, A martensitic stainless steel component having a chemical composition containing Sn: 0-0.500%, Al: 0-0.0100%, Mg: 0-0.0100%, Ca: 0-0.0100%, Ta: 0-0.0500%, Ga: 0-0.0500%, Zr: 0-0.5000%, REM: 0-0.0500%, and O: 0-0.0100%, with the remainder being Fe and impurities, wherein the total of C and N in the said chemical composition is 0.050-0.110%, the Ms point represented by the following formula (2) is 50-100, and the surface hardness is 450 HV or higher. Ms = 550 - 361C - 28Si - 39Mn - 10Cu - 17Ni - 20Cr - 5Mo ... (2) Note that each element in the above formula (2) represents the content (mass%).

6. In the above chemical composition, further, in mass%, Ti: 0.0001 to 0.0090%, V: 0.0001 to 0.500%, W: 0.0001 to 0.500%, Co: 0.01 to 0.50%, B: 0.0001 to 0.0080%, Sn: 0.001 to 0.500%, Al: 0.0001 to 0.0100%, Mg: 0.0002 to 0.0100%, Ca: 0.0002 to 0.0100%, Ta: 0.0001 to 0.0500%, Ga: 0.0001 to 0.0500%, Zr: 0.01 to 0.5000%, A martensitic stainless steel component according to claim 5, comprising one or more of the following: REM: 0.0001 to 0.0500%, and O: 0.0010 to 0.0100%.

7. A component characterized by including a martensitic stainless steel member as described in claim 5 or 6.

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