Ferritic stainless steel and method of manufacturing the same

Abstract

The present invention aims to provide a ferritic stainless steel that has a wide appropriate quenching temperature range, high hardness and excellent corrosion resistance after quenching, and serves as a material for beautiful martensitic stainless steel products, as well as an industrially stable manufacturing method. The ferritic stainless steel of the present invention has a steel composition in mass% consisting of C: 0.45% or more and 0.55% or less, Si: 0.10% or more and 1.00% or less, Mn: 0.1% or more and 1.0% or less, Cr: 12.0% or more and 15.0% or less, Ni: 0% or more and 1.0% or less, Mo: 0.50% or more and 0.80% or less, V: 0.10% or more and 0.20% or less, N: 0.015% or more and 0.100% or less, P: 0% or more and 0.040% or less, S: 0% or more and 0.030% or less, and the remainder being Fe and impurities, and is characterized by having an average crystal grain size of the ferritic phase of 10㎛ or less and having 0.8 carbides / ㎛2 or more with a diameter of 1.5㎛ or less.

Description

Technology Field

[0001] The present invention relates to ferritic stainless steel. In particular, the invention discloses ferritic stainless steel suitable as an intermediate material for martensitic stainless steel products suitable for blades such as razors or knives. Background Technology

[0002] For blade applications such as razor blades and knives requiring high hardness and corrosion resistance, carbon-containing martensitic stainless steels, represented by SUS420J1, SUS420J2, and EN1.4116 (Non-patent Literature 1), are used. These are steels also described in JIS G43034 and G43035. For general-purpose blades, SUS420J1 and SUS420J2 containing 0.40% or less of C are used. On the other hand, for high-grade blades requiring higher hardness and excellent corrosion resistance, EN1.4116 is used, which has a high Cr content and also has V and Mo added to enhance corrosion resistance.

[0003] Stainless steel is obtained by rapidly cooling, such as by water or oil cooling, from a high-temperature austenite phase in which a relatively high concentration of carbon can be dissolved, to a hard martensite phase in which supersaturated carbon is dissolved at room temperature. In other words, it becomes martensitic stainless steel. It is known that the hardness of this martensite phase corresponds to the amount of dissolved carbon in the austenite phase during high-temperature heating, and that the appropriate quenching temperature range to obtain the target hardness is influenced by the size of the carbides before quenching.

[0004] In addition, carbides present before and after quenching are thought to be mainly composed of Cr and also contain V and Mo for the purpose of improving corrosion resistance, and thus have a significant effect on corrosion resistance. That is, if coarse carbides are present, corrosion resistance in the vicinity deteriorates.

[0005] On the other hand, when stainless steel is cooled relatively slowly from a high-temperature austenite phase or heated and maintained in a ferrite phase at a temperature lower than the austenite phase, it decomposes into soft ferrite and carbides to precipitate carbon dissolved in the matrix phase.

[0006] Therefore, when manufacturing general martensitic stainless steel products, various shapes such as plates, rods, and wires are manufactured in the state of ferritic stainless steel, which is soft and generally has excellent workability, during the manufacturing stage of the intermediate material, and then processed into products, or quenched into martensitic stainless steel simultaneously with or after processing.

[0007] The present invention is intended for application to high-grade blades made of martensitic stainless steel, which require particularly high hardness and excellent corrosion resistance, and targets ferritic stainless steel with 0.45% or more of C added as an intermediate material suitable for manufacturing such blades. Furthermore, the application is not limited to high-grade blades but can also be applied to other uses requiring excellent properties and processing. Additionally, for high-grade blades, it is desirable for the product surface to be aesthetically pleasing. Aesthetics are defined as an excellent surface condition that, along with excellent surface shape, exhibits excellent corrosion resistance and prevents rust from forming even after longer periods or in harsh corrosive environments compared to conventional methods.

[0008] In the manufacturing process of intermediate ferritic stainless steel, it is common practice to hot work an ingot obtained from continuous casting or ingot casting, cool it to room temperature, and then reheat it to decompose into a ferritic phase and carbides to soften it (Non-patent Literature 2).

[0009] In this reheating process, a long time, typically several hours, is required for the aforementioned decomposition, and the carbides dispersed in the ferrite phase tend to become coarse. When a ferritic stainless steel intermediate material containing these coarse carbides is quenched, it often becomes softer than the target hardness.

[0010] In addition, if the carbides present before and after quenching contain Cr, Mo, and V, which are necessary to obtain excellent corrosion resistance, the corrosion resistance around the carbides often deteriorates.

[0011] In order to obtain excellent properties through the solid solution of each element, it is necessary to increase the quenching temperature and time to a high temperature and a long time, and to dissolve (re-dissolve) coarse carbides to secure a specified solid solution capacity. When coarse carbides remain, there was a problem in that the properties deteriorated after quenching and were unstable.

[0012] As a means of solving this problem, for example, Patent Document 1 discloses a method of optimizing the amounts of added C and N and limiting the number density of carbides in the ferritic stainless steel intermediate material before quenching. By doing so, the appropriate quenching temperature range for obtaining target characteristics is widened, and the necessary characteristics after quenching can be stably secured. Prior art literature

[0013] Japanese Patent Publication No. 2007-224405

[0014] European Standard for Stainless Steel EN10088-2, Stainless Steel Handbook, 3rd Edition, compiled by the Stainless Steel Association (1995), page 829 The problem to be solved

[0015] In the material obtained by heat-treating the ferritic stainless steel intermediate of Patent Document 1, a thick Cr deficiency occurs partially due to oxidation during heating, and when the material is made into a blade product, an uneven shape appears, which damages the surface appearance of the blade product.

[0016] The present invention aims to provide a ferritic stainless steel that has a wide appropriate quenching temperature range, high hardness and excellent corrosion resistance after quenching, and serves as a material for beautiful martensitic stainless steel products, as well as an industrially stable manufacturing method. means of solving the problem

[0017] The inventors have investigated in detail the metal structure of a ferritic stainless steel with 0.45% or more of C added, which is suitable as an intermediate material for a martensitic stainless steel product for a blade having high hardness and excellent corrosion resistance, and have revealed quenching conditions that obtain a predetermined hardness, corrosion resistance, and a beautiful surface.

[0018] As a result, it was revealed that the non-uniformity in shape appearing on the surface of blade products, which degrades corrosion resistance and impairs aesthetics, is caused by a Cr deficiency immediately beneath Cr-containing oxides resulting from grain boundary oxidation. Furthermore, it was discovered that by finer crystal grain size and increasing grain boundary density within the material, carbides on the grain boundaries dissolve early, thereby promoting the outward diffusion of Cr, Mo, and V, and allowing the Cr deficiency on the surface or near coarse carbides to be resolved early.

[0019] In addition, it was discovered that by refining the average crystal grain size and controlling the distribution of carbides, the appropriate quenching temperature range for stably obtaining high hardness, excellent corrosion resistance, and a beautiful surface appearance is expanded.

[0020] The inventors have clarified the characteristics of the steel composition and metal structure in which these effects are obtained, and have completed the present invention. The gist of the present invention is as follows.

[0021] (1) A steel composition having, in mass%, C: 0.45% or more and 0.55% or less, Si: 0.10% or more and 1.00% or less, Mn: 0.1% or more and 1.0% or less, Cr: 12.0% or more and 15.0% or less, Ni: 0% or more and 1.0% or less, Mo: 0.50% or more and 0.80% or less, V: 0.10% or more and 0.20% or less, N: 0.015% or more and 0.100% or less, P: 0% or more and 0.040% or less, S: 0% or more and 0.030% or less, and remainder: Fe and impurities, having an average crystal grain size of the ferrite phase of 10㎛ or less and 0.8 carbides / ㎛ with a diameter of 1.5㎛ or less 2 Ferritic stainless steel characterized by the presence of an abnormality.

[0022] (2) The ferritic stainless steel of (1) characterized by having a carbide content of 1.5 μm or less in the ferrite grain boundary length of 5.0% or more.

[0023] (3) A ferritic stainless steel of (1) or (2) characterized by comprising, in mass%, one or two or more of Al: 0.30% or less, Nb: 0.070% or less, B: 0.0030% or less, Ti: 0.070% or less, Sn: 0.12% or less, Cu: 0.40% or less, W: 1.000% or less, Co: 0.500% or less, Zr: 0.500% or less, Ca: 0.0050% or less, Mg: 0.0050% or less, Y: 0.1000% or less, REM: 0.10% or less, and Sb: 0.15% or less.

[0024] (4) A ferritic stainless steel plate with a thickness of 0.4 to 6.0 mm having the features described in any one of (1) to (3) above.

[0025] (5) A method for manufacturing a ferritic stainless steel of any one of (1) to (3) above, wherein the method comprises a process of hot rolling a steel having the composition described in (1) or (3) at a starting temperature of 1150°C or higher and a finishing temperature of 850°C or higher and 900°C or lower to produce a hot-rolled steel sheet, and subsequently cooling the hot-rolled steel sheet to a temperature of 700°C or higher and 800°C or lower at a cooling rate of 0.07°C / s or higher, and after cooling, maintaining the hot-rolled steel sheet at a temperature of 700°C or higher and 800°C or lower for 20 minutes or more and 20 hours or less.

[0026] (6) A method for manufacturing a ferritic stainless steel sheet according to (4) above, wherein the method comprises the steps of: hot rolling a steel having the composition described in (1) or (3) at a starting temperature of 1150°C or higher and a finishing temperature of 850°C or higher and 900°C or lower to produce a hot-rolled steel sheet; subsequently, cooling the hot-rolled steel sheet to a temperature of 700°C or higher and 800°C or lower at a cooling rate of 0.07°C / s or higher; after cooling, maintaining the hot-rolled steel sheet at a temperature of 700°C or higher and 800°C or lower for 20 minutes or more and 20 hours or less; pickling the hot-rolled steel sheet maintained at the temperature; cold rolling the hot-rolled steel sheet after pickling to produce a cold-rolled steel sheet; and heat treating the cold-rolled steel sheet at a temperature of 700°C or higher and 800°C or lower. Effects of the invention

[0027] According to the present invention, a beautiful ferritic stainless steel can be provided that has a wide appropriate quenching temperature range, high hardness, and excellent corrosion resistance. Brief explanation of the drawing

[0028] FIG. 1 is a diagram schematically illustrating the criteria for determining a carbide as a “carbide on the grain boundary” and the “length of the line segment occupying the grain boundary.” Specific details for implementing the invention

[0029] 1. Ferritic stainless steel

[0030] Hereinafter, the ferritic stainless steel of the present invention will be described in detail.

[0031] (Chemical composition)

[0032] First, the components included in the ferritic stainless steel of the present invention will be explained. Additionally, the "%" notation for the content of each element refers to mass percent.

[0033] C is an important element for ensuring the hardness of martensite. It also acts as an element that affects the corrosion resistance of the base material by forming Cr carbides. If the C content is less than 0.45%, the quenching hardness required for knife blade applications cannot be obtained. Furthermore, since the number density of carbides smaller than 1.5 μm, which contributes to stable quenching hardness, becomes insufficient, the appropriate quenching temperature range is also narrowed. Additionally, peening by carbides does not function effectively, and the average crystal grain size of the ferrite phase coarsens during furnace heating after hot rolling. On the other hand, if the C content exceeds 0.55%, the carbides coarsen, the aforementioned number density becomes insufficient, and the appropriate quenching temperature range is narrowed. Furthermore, the required corrosion resistance cannot be satisfied. Therefore, the C content is set to be between 0.45% and 0.55%. The lower limit of the C content is preferably 0.46%, more preferably 0.47%. The upper limit of the C content is preferably 0.54%, more preferably 0.53%.

[0034] Si is an element that improves oxidation resistance. If the Si content is less than 0.10%, sufficient oxidation resistance is not obtained. Furthermore, if it is excessively reduced, it leads to an increase in manufacturing costs. On the other hand, if the Si content exceeds 1.00%, it promotes cracking during manufacturing. Therefore, the Si content is set to be between 0.10% and 1.00%. The lower limit of the Si content is preferably 0.20%, more preferably 0.30%. The upper limit of the Si content is preferably 0.90%, more preferably 0.80%.

[0035] Mn is used as a deoxidizing element. Additionally, it is thought that the amount of dissolved carbon increases through interaction with carbon, contributing to the improvement of hardness after quenching. From the perspective of stable manufacturability and the manifestation of the effect of increasing dissolved carbon through interaction with carbon, the Mn content should be 0.1% or more. On the other hand, if the Mn content exceeds 1.0%, there is a risk that compounds such as sulfides will be formed, leading to a decrease in corrosion resistance. Furthermore, it is thought that the effect of increasing dissolved carbon through interaction with carbon becomes saturated, and an effect corresponding to the amount added is not obtained. Therefore, the Mn content should be 0.1% or more and 1.0% or less. The lower limit of the Mn content is preferably 0.2%, and more preferably 0.3%. The upper limit of the Mn content is preferably 0.9%, and more preferably 0.8%.

[0036] Cr is an element that improves corrosion resistance. Additionally, Cr is an element that improves quenchability and suppresses the decrease in hardness after quenching caused by the occurrence of diffusion transformation. It is also an element that constitutes carbides and affects the carbide density in the metal structure prior to quenching. If the Cr content is less than 12.0%, sufficient corrosion resistance, the effect of suppressing diffusion transformation, and carbide density are not obtained. On the other hand, if the Cr content exceeds 15.0%, it leads to a decrease in manufacturability. Furthermore, corrosion resistance commensurate with the cost of the additive alloy is not obtained. In addition, the formation of a large amount of residual γ due to the lowering of the quenching transformation temperature (Ms point) leads to a decrease in hardness. Therefore, the Cr content is set to be 12.0% or more and 15.0% or less. The lower limit of the Cr content is preferably 12.5%, more preferably 13.0%, and even more preferably 14.0%. In addition, the lower limit of the Cr content may be 14.1% or 14.3%. The upper limit of the Cr content is preferably 14.9%, more preferably 14.7%.

[0037] Ni is an element that improves toughness when in the martensite phase and may be added as necessary. However, if the Ni content exceeds 1.0%, it leads to a decrease in formability. In addition, rare elements are also expensive, and there is a concern that this may lead to an increase in alloy costs or hinder manufacturability. Therefore, the Ni content is kept at 1.0% or less. Preferably, it is 0.60% or less, and more preferably 0.05% or more and 0.50% or less. When Ni is included, the content may be in a small amount, but the lower limit is preferably 0.05% and more preferably 0.10%. The upper limit of the Ni content is preferably 0.60% and more preferably 0.50%.

[0038] Mo is an element that improves corrosion resistance. It is also an element that improves hardness through solid solution strengthening. If the Mo content is less than 0.50%, sufficient corrosion resistance and the effect of improving hardness through solid solution strengthening are not obtained. On the other hand, even if the Mo content exceeds 0.80%, the effects on corrosion resistance and solid solution strengthening become saturated, and the effect is not obtained in proportion to the cost of addition. Therefore, the Mo content is set to be 0.50% or more and 0.80% or less. The lower limit of the Mo content is preferably 0.55%, more preferably 0.60%. The upper limit of the Mo content is preferably 0.75%, more preferably 0.70%.

[0039] V is an element that improves corrosion resistance. It also acts as an element that causes fine precipitation of carbides and increases the number density of carbides. If the V content is less than 0.10%, sufficient corrosion resistance is not obtained. Furthermore, the effect of increasing the number density of carbides is not sufficiently obtained. On the other hand, even if V is added in excess of 0.20%, the effect on corrosion resistance and the effect of increasing the number density of carbides become saturated, and the effect is not obtained in proportion to the cost of addition. Therefore, the V content is set to be 0.10% or more and 0.20% or less. The lower limit of the V content is preferably 0.11%, and more preferably 0.13%. The upper limit of the V content is preferably 0.19%, and more preferably 0.17%.

[0040] N, like C, is an element for ensuring the hardness of martensite. If the N content is less than 0.015%, sufficient hardness cannot be ensured. On the other hand, if the N content exceeds 0.100%, hot workability deteriorates significantly. Therefore, the N content is set to be 0.015% or more and 0.100% or less. The lower limit of the N content is preferably 0.020%, more preferably 0.030%, and even more preferably 0.040%. The upper limit of the N content is preferably 0.090%, and more preferably 0.080%.

[0041] P is an element that reduces formability and corrosion resistance. It is desirable for its content to be low. Therefore, the P content is 0.040% or less. There is no specific lower limit.

[0042] S is an unavoidable impurity element and promotes cracking during manufacturing. Therefore, the S content is 0.030% or less. There is no specific lower limit.

[0043] The ferritic stainless steel of the present invention comprises, in addition to each of the elements described above, Fe and impurities (including unavoidable impurities).

[0044] The ferritic stainless steel of the present disclosure may, in addition to the basic composition above, optionally include one or two or more of the following in mass% instead of a portion of Fe: Al: 0.30% or less, Nb: 0.070% or less, B: 0.0030% or less, Ti: 0.070% or less, Sn: 0.12% or less, Cu: 0.40% or less, W: 1.000% or less, Co: 0.500% or less, Zr: 0.500% or less, Ca: 0.0050% or less, Mg: 0.0050% or less, Y: 0.1000% or less, REM: 0.10% or less, and Sb: 0.15% or less.

[0045] (Al: 0.30% or less, Nb: 0.070% or less, B: 0.0030% or less, Ti: 0.070% or less)

[0046] The elements Al, Nb, B, and Ti do not need to be added. When these elements are added, they improve the formability of ferritic stainless steel and have the effect of suppressing scratches during hot working. When added, the Al content is set to 0.30% or less, the Nb content to 0.070% or less, the B content to 0.0030% or less, and the Ti content to 0.070% or less. To reliably obtain the above effects, it is preferable that the Al, Nb, and Ti contents be 0.01% or more, and the B content be 0.001% or more.

[0047] (Sn: 0.12% or less, Cu: 0.40% or less, W: 1.000% or less, Co: 0.500% or less, Zr: 0.500% or less)

[0048] The elements Sn, Cu, W, Co, and Zr do not need to be added. These elements have the effect of improving corrosion resistance. When added, the Sn content is 0.12% or less, the Cu content is 0.40% or less, the W content is 1.000% or less, the Co content is 0.500% or less, and the Zr content is 0.500% or less. To reliably obtain the above effects, it is preferable that the Sn, Cu, Co, and Zr content be 0.01% or more, and the W content be 0.1% or more.

[0049] (Ca: 0.0050% or less, Mg: 0.0050% or less, Y: 0.1000% or less, Hf: 0.20% or less, REM: 0.10% or less, Sb: 0.15% or less)

[0050] The elements Ca, Mg, Y, REM, and Sb do not need to be added. These elements have the effect of suppressing hot working defects by altering inclusions such as oxides or sulfides. When added, the Ca content is 0.0050% or less, the Mg content is 0.0050% or less, the Y content is 0.1000% or less, the Hf content is 0.20% or less, the REM content is 0.10% or less, and the Sb content is 0.15% or less. To reliably obtain the above effects, it is preferable that the Ca and Mg content be 0.0001% or more, and the Y, Hf, and REM content be 0.01% or more.

[0051] In addition, in this invention, "REM" refers to elements (lanthanoids) belonging to atomic numbers 57 to 71, such as La, Ce, Pr, Nd, etc., and Y is not included.

[0052] The ferritic stainless steel of the present disclosure may contain elements other than the elements described above, in addition to the elements described above, and in place of some of Fe, to the extent that the above problem can be solved. For example, it may contain Bi, Pb, Se, H, Ta, etc., but the content ratio thereof may be controlled to the extent that the above problem can be solved, and it may contain, for example, one or more of Bi≤100ppm, Pb≤100ppm, Se≤100ppm, H≤100ppm, and Ta≤500ppm.

[0053] (Average crystal grain size of ferrite phase and carbide precipitation state)

[0054] In the ferritic stainless steel of the present invention, excellent properties including a beautiful surface are secured by refining the average crystal grain size of the ferrite phase and also by defining the size and number density of carbides.

[0055] By making the average crystal grain size finer, the amount of carbides located on the grain boundaries of the ferrite phase increases. When heated to a high temperature, the carbides on the grain boundaries act as nuclei for transformation into the austenite phase, increasing the grain boundary area of ​​the austenite phase. Therefore, as the redissolution of carbides proceeds, the outward diffusion of the redissolved Cr, M, and V is promoted, allowing for the early resolution of Cr deficiency. In addition to the above, if the ratio (occupancy rate) of carbides to the length of the grain boundaries of the ferrite phase is above a certain level, the effect of resolving Cr deficiency is further enhanced, and corrosion resistance is significantly improved.

[0056] The average crystal grain size of the ferrite phase needs to be 10 µm or less. The average crystal grain size is preferably 9 µm or less, and more preferably 8 µm or less. Meanwhile, although the lower limit of the average crystal grain size is not specifically limited, it should be 1 µm or more than the actual value. On the other hand, if the average crystal grain size exceeds 10 µm, the carbides located at the grain boundaries decrease, and the phenomenon of resolving Cr deficiency does not occur, so excellent properties cannot be secured.

[0057] (Method for measuring the average crystal grain size of the ferrite phase)

[0058] The average grain size of the ferrite phase is determined as follows. The L cross-section of a steel plate, prepared by electrolytic polishing, is measured by EBSD. The measurement area is set to 300 µm × 300 µm at a position of 1 / 4 t thickness, and the measurement step size is set to 0.1 µm. If the difference in crystal orientation between adjacent plot data is less than 15°, they are considered to be the same grain, and if the difference in orientation is 15° or more, they are treated as different grains to determine the average grain size. In addition, if a phase other than the ferrite phase is included in the measurement area, the average grain size is determined after extracting only the ferrite phase.

[0059] The carbides are of a size that can be redissolved into the austenite phase upon high-temperature heating, and a higher number density is preferable. The size of the carbides must not include coarse carbides with a diameter exceeding 1.5 μm. Preferably, the diameter is 1.0 μm or less.

[0060] In addition, the number density of carbides is 0.8 carbides / µm with a diameter of 1.5 µm or less. 2 It is necessary for at least one to be present. Preferably 1.0 individual / µm 2 Above, more preferably 1.2 μm 2 That is all. The upper limit of the number density is not specifically limited. In addition, when the size and number density of the carbides satisfy the above conditions, the amount of dissolved carbon required for the target hardness can be sufficiently secured, so the appropriate quenching temperature range is also expanded.

[0061] In order to secure a beautiful surface appearance after quenching and to significantly improve corrosion resistance, it is desirable that the proportion of carbides with a diameter of 1.5 μm or less occupying the grain boundaries be 5.0% or more. The proportion is preferably 7.0% or more, and more preferably 10.0% or more. Although no upper limit is specified, it is desirable that it be 17.0% or less.

[0062] (Method for measuring carbide size and number density, and the carbide occupancy rate at grain boundaries)

[0063] The size and number density of the carbides are determined by the following method.

[0064] After mirror-polishing the L cross-section of the steel plate, grain boundaries and carbides are exposed by etching with aqua regia, and the size and number density of the carbides are measured by SEM observation. The measurement area is set to a total area of ​​200㎛ × 200㎛ at the 1 / 4t position of the plate thickness, and SEM observation is performed at a magnification of 5000x. The size of the carbides is determined by converting the observed carbides into their equivalent diameters. Carbide number density [pieces / ㎛ 2] is calculated as the area of ​​the measurement area relative to the number of carbons with a diameter of 1.5㎛ or less confirmed in the measurement area.

[0065] In the present invention, the occupancy rate (P[%]) is defined as the ratio of carbides with a diameter of 1.5 μm or less that occupy the grain boundary. The occupancy rate is calculated as the ratio of the total length of the line segments (b[μm]) where the carbides occupy the grain boundary to the total length of the grain boundary (a[μm]) in the measurement area. The calculation formula is shown in Equation (2). In addition, FIG. 1 schematically illustrates the criteria for determining "carbides on the grain boundary" and the "length of the line segments occupying the grain boundary."

[0066]

[0067] In addition, the carbide identified as the ferritic stainless steel of the present invention is (Cr, Fe) 23 It is predominantly C6, but may contain some (Cr, Fe)7C3. Carbides can be identified by EDX.

[0068] (Presence of phases other than ferrite and carbides)

[0069] The metal structure of the ferritic stainless steel of the present invention is composed of a ferrite phase and a large number of fine carbides, which remain in a very small proportion at room temperature. However, the presence of phases other than the above may be permitted to some extent. For example, in the ferritic stainless steel of the present invention, there is no problem even if phases other than the main ferrite phase, such as the austenite phase or the martensite phase, are included in a total area percentage of 5% or less at room temperature.

[0070] (Method for determining the presence or absence of austenite and martensite phases)

[0071] The presence or absence of the austenite phase is determined using data measured by EBSD as described in the paragraph above. Since the austenite phase has an FCC structure and the ferrite phase has a BCC structure, the determination is made by calculating the proportion (γ[%]) of the FCC structure in the measurement area. If the value calculated by Equation (1) is 5% or less, it is determined that there is no austenite phase. Here, γ represents the area percentage of the austenite phase (unit: [%]), and F and B represent the number of plots of the FCC structure and BCC structure obtained when measured by EBSD, respectively (unit: [number]).

[0072]

[0073] The presence or absence of a martensite phase is determined by Vickers hardness. If 5% or more of the martensite phase is present, the hardness exceeds 300 HV. Using a Vickers hardness tester, 10 measurements are taken at a load of 500 g, and if the average value is 300 HV or less, it is determined that there is no martensite phase.

[0074] (Plate thickness)

[0075] The plate thickness after hot rolling is 4.0 mm or more and 6.0 mm or less, and the plate thickness in the subsequent cold rolling stage is 0.4 mm or more and less than 4.0 mm. The plate thickness of the ferritic stainless steel of the present invention is 0.4 mm or more and 6 mm or less from the hot rolling stage to the cold rolling stage, including the product plate thickness.

[0076] 2. Method for manufacturing ferritic stainless steel

[0077] (Manufacturing method)

[0078] The method for manufacturing ferritic stainless steel according to the present invention will be described.

[0079] Steel composed of the aforementioned composition is melted, cast to produce an ingot, and heated. If the heating temperature (the starting temperature for hot rolling described later) is less than 1150°C, carbides cannot be sufficiently dissolved, and while properties vary depending on the area, coarse carbides remain, narrowing the appropriate quenching temperature range of the product. Therefore, the heating temperature is set to 1150°C or higher. Preferably, it is 1180°C or higher.

[0080] Next, hot rolling is performed on the heated ingot. If the end temperature of the hot rolling is less than 850°C, the deformation load is too high, so the load on the substrate undergoing hot rolling increases, making it impossible to process it into a desired shape. On the other hand, if it exceeds 950°C, coarse carbides remain without being crushed, and the appropriate quenching temperature range of the product narrows. Therefore, the end temperature of the hot rolling is set to 850°C or higher and 950°C or lower. Preferably, it is 860°C or higher and 940°C or lower.

[0081] Immediately after the hot rolling is completed, the cooling rate is controlled to cool the material to a temperature of 700°C or higher and 800°C or lower, which is the subsequent heating and holding process. At that time, the cooling rate must be 0.07°C / s or higher, and it is also necessary to manage the thermal history so that the temperature is not lowered below 700°C during cooling. The cooling rate is preferably 0.20°C / s or higher. As the processing deformation accumulated by hot rolling is maintained until just before the heating and holding process, the average crystal grain size of the ferrite phase becomes 10 μm or less after the heating and holding process. On the other hand, if the cooling rate is slower than 0.07°C / s, the processing deformation recovers during cooling, and the precipitation nucleation of the ferrite phase decreases, causing the ferrite to coarsen during the heating and holding process.

[0082] In conventional methods for manufacturing ferritic stainless steel intermediates, under the general manufacturing process—specifically, a thermal history in which the material is cooled to room temperature while controlling the cooling rate after hot rolling, and then heated and maintained—the grain size of the ferrite phase coarsens during the process in which deformation within the martensite phase is lost. In contrast, in the method for manufacturing ferritic stainless steel according to the present invention, it is crucial to control the cooling rate and temperature history during the period immediately following hot rolling and immediately before maintaining the heat. In conventional methods for manufacturing ferritic stainless steel intermediates, such control of the cooling rate and temperature history is not performed.

[0083] After cooling to a heating holding temperature between 700°C and 800°C, heating and holding are continued. If the heating and holding temperature is below 700°C, the number density of carbides of 1.5 μm or less is significantly reduced, and the ferrite transformation does not proceed sufficiently. When the above heating and holding is performed and the material is cooled to room temperature, a metal structure containing a large amount of hard martensite phase is formed. As a result, passing the material through subsequent processes such as cold rolling becomes difficult, leading to increased manufacturing costs and reduced yield. On the other hand, if the temperature exceeds 800°C, the above carbides aggregate and coarsen, and the appropriate quenching temperature range narrows. Additionally, the average crystal grain size of the ferrite phase also coarsen.

[0084] The heating and holding time is set to 20 minutes or more and 20 hours or less. If the heating and holding time is less than 20 minutes, the number density of carbides with a diameter of 1.5 μm or less decreases significantly, and after cooling to room temperature, the metal structure becomes one containing a large amount of martensite phase. As a result, passing the material through subsequent processes such as cold rolling becomes difficult, leading to increased manufacturing costs and reduced yield. On the other hand, if the heating and holding time exceeds 20 hours, the aforementioned carbides aggregate and coarsen, narrowing the appropriate quenching temperature range. Additionally, the ferrite phase crystal grains also coarsen. Therefore, the heating and holding is performed at a temperature of 700°C or higher and 800°C or lower for 20 minutes or more and 20 hours or less. Preferably, it is at a temperature of 710°C or higher and 790°C or lower for 75 minutes or more and 15 hours or less.

[0085] The cooling rate after heating is not specifically limited. For example, it may be cooled at a rate of 0.05℃ / s or higher, or air cooling may be used.

[0086] After heating, holding, and cooling are completed, pickling, cold rolling, and final heat treatment are repeated as needed to obtain a steel plate of a predetermined thickness.

[0087] Pickling is a process for removing surface oxide scale, cold rolling is a process for obtaining a predetermined plate thickness, and final heat treatment is a process for releasing the deformation introduced by the cold rolling and softening through recrystallization; this is a standard method for manufacturing stainless steel and poses no problem.

[0088] If the temperature of the final heat treatment is below 700°C, recrystallization is insufficient, resulting in a hardened material, which makes it difficult to pass the material to the next process or process it by the customer. On the other hand, if the temperature is higher than 800°C, the austenite phase reaches a stable temperature range, and after cooling, the metal structure becomes hardened by containing a large amount of martensite phase, which makes it difficult to pass the material to the next process or process it by the customer. Therefore, the temperature of the final heat treatment is set to 700°C or higher and 800°C or lower. Preferably, it is 710°C or higher and 790°C or lower.

[0089] Examples

[0090] The effects of the ferritic stainless steel of the present invention are explained while illustrating examples.

[0091] Steel having the composition shown in Table 1 was fluxed in the laboratory, and an ingot with a thickness of 100 mm was obtained. After heating this ingot at the temperature shown in Table 2 for 120 minutes, it was hot-rolled to obtain a hot-rolled plate with a thickness of 5.0 mm.

[0092] Next, the hot-rolled plates were cooled at a cooling rate in the range of 0.05 to 2.00°C / s to the heating holding temperature shown in Table 2. After reaching the heating holding temperature, heating was maintained for the time shown in Table 2. After heating, the plates were air-cooled to room temperature. For steel plates No. 1 to 16 and 18 to 34, sulfuric acid pickling, cold rolling with a reduction rate of 60%, and heat treatment at 700 to 800°C for 2 minutes were performed to obtain steel plates with a thickness of 2.0 mm.

[0093] In addition, for steel plates No. 1 to 15 and 18 to 34, cold rolling, cold rolling plate annealing, and pickling were performed again to make steel plates with a thickness of 0.8 mm.

[0094] In this review, for comparison with the manufacturing process of the present invention, No. 33 was cooled to room temperature after hot rolling, then heated again and maintained.

[0095]

[0096]

[0097] The average crystal grain size of the ferrite phase, the presence or absence of the austenite and martensite phases, the size and number density of carbides, and the occupancy rate of carbides at the grain boundaries were measured by the aforementioned method.

[0098] (Method for measuring cooling rate after completion of hot rolling)

[0099] The cooling rate after the completion of hot rolling is defined as the average cooling rate between the completion of hot rolling and reaching the temperature of the heat holding process. The temperature history was measured using a radiation thermometer.

[0100] (Evaluation test of quenching hardness stability)

[0101] The test specimens shown in Table 2 were heated at a temperature of 900 to 1150°C for 5 minutes and then air-cooled. The quenching hardness of the samples was then investigated. Additionally, the heating temperature was varied in increments of 10°C. Quenching stability (ΔT[°C]) was evaluated by Equation (3).

[0102]

[0103] Tmin[°C] and Tmax[°C] represent the minimum and maximum temperatures at which the quenching hardness is 550 HV or higher, respectively. A larger ΔT indicates a wider quenching temperature range at which the above quenching hardness is obtained, and indicates superior quenching hardness stability. On the other hand, when ΔT is 0, it indicates that the minimum temperature Tmin and the maximum temperature Tmax are equal, and that quenching stability is poor. Here, a ΔT of 30°C or higher was evaluated as passing, and a ΔT of less than 30°C was evaluated as failing.

[0104] (Evaluation test of surface appearance)

[0105] The test material shown in Table 2 was heated at temperatures Tmin and Tmax for 5 minutes, then air-cooled to obtain a quenched sample. After removing the oxide scale from the quenched sample to expose the metal surface, the surface was finished by wet polishing with #600 grit. Non-uniform shapes were visually inspected. If the ratio of the total area of ​​non-uniform shapes to the total area of ​​the observation field (hereinafter referred to as the "defect rate" in Table 2) is 5.0% or less for both heating temperatures Tmin and Tmax, it is considered acceptable and evaluated as satisfying the beautiful surface appearance required for high-quality blades. Otherwise, it is considered unacceptable. The defect rates in Table 2 are the higher of the respective defect rates for heating temperatures Tmin and Tmax.

[0106] (Corrosion resistance evaluation test)

[0107] Test specimens prepared using the same quenching heat treatment and polishing methods as those used in the surface appearance evaluation test were subjected to a salt spray test with a 7% NaCl solution at a test temperature of 50°C. The pass / fail status of corrosion resistance was determined by whether red rust was observed on the surface of the test specimen. After 4 hours from the start of the test, if no red rust was visually observed under both heating temperature conditions Tmin and Tmax, the specimen was deemed to have passed and was judged to have satisfied the corrosion resistance required for a blade. Otherwise, it was deemed to have failed. Only those judged to have passed were extended until the total test time reached 24 hours. If no red rust was visually observed under both heating temperature conditions Tmin and Tmax after the evaluation test, the corrosion resistance was judged to be superior.

[0108] Table 2 shows the evaluation results of quenching hardness stability, defect rate, and corrosion resistance. In No. 1 to 26, where the average crystal grain size of the ferrite phase and the number density of carbides were both above a predetermined level, a wide range of appropriate quenching temperatures was obtained to achieve high hardness and excellent corrosion resistance, and furthermore, a beautiful surface appearance was also achieved. In particular, in Examples 2 to 26, where the proportion of carbides at the grain boundaries was also above a predetermined level, corrosion resistance was significantly improved.

[0109] On the other hand, No. 31 had a low number density of carbides and a coarse average crystal grain size of the ferrite phase, so the quenching hardness stability was insufficient and the defect rate was poor.

[0110] No. 32 has a low number density of carbides and insufficient quenching hardness stability. In addition, corrosion resistance was poor because the added Cr was insufficient. No. 32 had an excessive amount of added C, so coarse carbides were present in excess and quenching hardness stability was poor. In addition, corrosion resistance was poor because the added Cr was insufficient.

[0111] No. 33, which had a thermal history of being cooled to room temperature after hot rolling and then heated and maintained, had a coarse average crystal grain size and a defect rate higher than the specified amount.

[0112] No. 34, which had a slow cooling rate after hot rolling, had a coarsened average crystal grain size, and the defect rate of the surface appearance was higher than the specified amount. Industrial applicability

[0113] The ferritic stainless steel of the present disclosure has a wide appropriate quenching temperature range and combines high hardness after quenching, excellent corrosion resistance, and a beautiful surface. That is, it is suitable as an intermediate material for martensitic stainless steel, and, for example, can efficiently produce high-grade blade products that require hardness, excellent corrosion resistance, and a beautiful surface. Explanation of the symbols

[0114] 1: Grain boundary 2: Length of the line segment occupying the grain boundary 3: Carbides on the grain boundaries 4: Carbides not present at grain boundaries

Claims

Claim 1 The steel has a composition consisting of, in mass%, C: 0.45% or more and 0.55% or less, Si: 0.10% or more and 1.00% or less, Mn: 0.1% or more and 1.0% or less, Cr: 12.0% or more and 15.0% or less, Ni: 0% or more and 1.0% or less, Mo: 0.50% or more and 0.80% or less, V: 0.10% or more and 0.20% or less, N: 0.015% or more and 0.100% or less, P: 0% or more and 0.040% or less, S: 0% or more and 0.030% or less, and the remainder being Fe and impurities; the average crystal grain size of the ferrite phase is 10㎛ or less, and there are 0.8 carbides / ㎛ with a diameter of 1.5㎛ or less. 2 Ferritic stainless steel characterized by the presence of an abnormality. Claim 2 A ferritic stainless steel according to claim 1, characterized in that the proportion of carbides with a diameter of 1.5 μm or less occupying the ferrite grain boundary length is 5.0% or more. Claim 3 A ferritic stainless steel according to claim 1 or 2, characterized in that, instead of a portion of Fe, it comprises, in mass%, one or two or more of Al: 0.30% or less, Nb: 0.070% or less, B: 0.0030% or less, Ti: 0.070% or less, Sn: 0.12% or less, Cu: 0.40% or less, W: 1.000% or less, Co: 0.500% or less, Zr: 0.500% or less, Ca: 0.0050% or less, Mg: 0.0050% or less, Y: 0.1000% or less, REM: 0.10% or less, and Sb: 0.15% or less. Claim 4 A ferritic stainless steel sheet having the features described in claim 1 or 2, with a thickness of 0.4 to 6.0 mm. Claim 5 A ferritic stainless steel sheet having the characteristics described in paragraph 3, with a thickness of 0.4 to 6.0 mm. Claim 6 A method for manufacturing a ferritic stainless steel according to claim 1 or 2, characterized by comprising the steps of: hot rolling a steel having the composition described in claim 1 at a starting temperature of 1150°C or higher and a finishing temperature of 850°C or higher and 900°C or lower to produce a hot-rolled steel sheet; subsequently, cooling the hot-rolled steel sheet to a temperature of 700°C or higher and 800°C or lower at a cooling rate of 0.07°C / s or higher; and after cooling, maintaining the hot-rolled steel sheet at a temperature of 700°C or higher and 800°C or lower for 20 minutes or more and 20 hours or less. Claim 7 A method for manufacturing ferritic stainless steel as described in Paragraph 3, wherein the composition, in mass%, is C: 0.45% or more and 0.55% or less, Si: 0.10% or more and 1.00% or less, Mn: 0.1% or more and 1.0% or less, Cr: 12.0% or more and 15.0% or less, Ni: 0% or more and 1.0% or less, Mo: 0.50% or more and 0.80% or less, V: 0.10% or more and 0.20% or less, N: 0.015% or more and 0.100% or less, P: 0% or more and 0.040% or less, S: 0% or more and 0.030% or less, Al: 0.30% or less, Nb: 0.070% or less, B: 0.0030% or less, Ti: 0.070% or less, Sn: 0.12% or less, Cu: A method for manufacturing ferritic stainless steel, characterized by comprising the following steps: hot rolling a steel having 0.40% or less, W: 1.000% or less, Co: 0.500% or less, Zr: 0.500% or less, Ca: 0.0050% or less, Mg: 0.0050% or less, Y: 0.1000% or less, REM: 0.10% or less, and Sb: 0.15% or less, and remainder: Fe and impurities, at a starting temperature of 1150°C or higher and a finishing temperature of 850°C or higher and 900°C or lower to produce a hot-rolled steel sheet; subsequently, cooling the hot-rolled steel sheet to a temperature of 700°C or higher and 800°C or lower at a cooling rate of 0.07°C / s or more; and after cooling, maintaining the hot-rolled steel sheet at a temperature of 700°C or higher and 800°C or lower for 20 minutes or more and 20 hours or less. Claim 8 A method for manufacturing a ferritic stainless steel sheet as described in claim 4, comprising the steps of: hot rolling a steel having the composition described in claim 1 at a starting temperature of 1150°C or higher and a finishing temperature of 850°C or higher and 900°C or lower to produce a hot-rolled steel sheet; subsequently, cooling the hot-rolled steel sheet to a temperature of 700°C or higher and 800°C or lower at a cooling rate of 0.07°C / s or higher; after cooling, heating and maintaining the hot-rolled steel sheet at a temperature of 700°C or higher and 800°C or lower for 20 minutes or more and 20 hours or less; pickling the hot-rolled steel sheet that has been heated and maintained; cold rolling the hot-rolled steel sheet after pickling to produce a cold-rolled steel sheet; and heat treating the cold-rolled steel sheet at a temperature of 700°C or higher and 800°C or lower. Claim 9 A method for manufacturing a ferritic stainless steel sheet as described in claim 5, wherein the composition, in mass%, is C: 0.45% or more and 0.55% or less, Si: 0.10% or more and 1.00% or less, Mn: 0.1% or more and 1.0% or less, Cr: 12.0% or more and 15.0% or less, Ni: 0% or more and 1.0% or less, Mo: 0.50% or more and 0.80% or less, V: 0.10% or more and 0.20% or less, N: 0.015% or more and 0.100% or less, P: 0% or more and 0.040% or less, S: 0% or more and 0.030% or less, Al: 0.30% or less, Nb: 0.070% or less, B: 0.0030% or less, Ti: 0.070% or less, Sn: 0.12% or less, Cu: A steel having 0.40% or less, W: 1.000% or less, Co: 0.500% or less, Zr: 0.500% or less, Ca: 0.0050% or less, Mg: 0.0050% or less, Y: 0.1000% or less, REM: 0.10% or less, and Sb: 0.15% or less, with the remainder being Fe and impurities, is hot-rolled at a starting temperature of 1150°C or higher and a finishing temperature of 850°C or higher and 900°C or lower to produce a hot-rolled steel sheet; subsequently, the hot-rolled steel sheet is cooled to a temperature of 700°C or higher and 800°C or lower at a cooling rate of 0.07°C / s or more; after cooling, the hot-rolled steel sheet is heated and maintained at a temperature of 700°C or higher and 800°C or lower for 20 minutes or more and 20 hours or less; the hot-rolled steel sheet maintained at the temperature is pickled; and after pickling, the A method for manufacturing a ferritic stainless steel sheet characterized by comprising a process of cold-rolling a hot-rolled steel sheet to form a cold-rolled steel sheet, and heat-treating the cold-rolled steel sheet at a temperature of 700°C or higher and 800°C or lower.

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