Ferritic stainless steel and its manufacturing method

Abstract

The present invention addresses the problem of providing: a ferritic stainless steel which has a wide adequate quenching temperature range, while having high hardness and excellent corrosion resistance after quenching, and which serves as a material for beautiful martensitic stainless steel products; and an industrially stable production method therefor. A ferritic stainless steel according to the present invention is characterized by having a steel composition that contains, in mass%, 0.45% to 0.55% of C, 0.10% to 1.00% of Si, 0.1% to 1.0% of Mn, 12.0% to 15.0% of Cr, 0% to 1.0% of Ni, 0.50% to 0.80% of Mo, 0.10% to 0.20% of V, 0.015% to 0.100% of N, 0% to 0.040% of P and 0% to 0.030% of S, with the balance being made up of Fe and impurities. This ferritic stainless steel is also characterized in that: the ferrite phase has an average crystal grain size of 10 µm or less; and carbides having a diameter of 1.5 µm or less are present at a density of 0.8 per µm2 or more.

Description

[Technical field]

[0001] The present invention relates to a ferritic stainless steel, and in particular to a ferritic stainless steel suitable as an intermediate material for martensitic stainless steel products suitable for blades such as razors and kitchen knives. [Background technology]

[0002] Carbon-containing martensitic stainless steels such as SUS420J1, SUS420J2, and EN1.4116 (Non-Patent Document 1) are used for cutlery applications such as razor blades and kitchen knives that require high hardness and corrosion resistance. These are steels also described in JIS G43034 and G43035. SUS420J1 and SUS420J2, which contain 0.40% or less C, are used for general-purpose cutlery. On the other hand, EN1.4116, which has a high Cr content and is further enhanced in corrosion resistance by adding V and Mo, is used for high-end cutlery that requires even higher hardness and excellent corrosion resistance.

[0003] Stainless steel is produced by rapid cooling, such as by water or oil cooling, from a high-temperature austenite phase where a relatively high concentration of carbon can be dissolved to a hard martensite phase where carbon is supersaturated at room temperature. In other words, it becomes martensitic stainless steel. The hardness of this martensite phase corresponds to the amount of dissolved carbon in the austenite phase when heated at high temperatures, and it is known that the appropriate quenching temperature range for obtaining the target hardness is affected by the size of the carbides before quenching.

[0004] In addition, the carbides that exist before and after hardening are mainly composed of Cr, and are thought to also contain V and Mo, which are intended to improve corrosion resistance, and have a significant effect on corrosion resistance. In other words, if coarse carbides exist, the corrosion resistance in their vicinity will deteriorate.

[0005] On the other hand, when stainless steel is cooled relatively slowly from a high-temperature austenite phase, or when it is heated and held in the ferrite phase, which is lower in temperature than the austenite phase, the carbon that is dissolved in the matrix precipitates, causing the stainless steel to decompose into a soft ferrite phase and carbides.

[0006] Therefore, in the manufacture of typical martensitic stainless steel products, the intermediate material that serves as the raw material is soft during the manufacturing stage, and various shapes such as plates, bars, and wires are manufactured in the form of ferritic stainless steel, which generally has excellent workability, and then the material is processed into products, or quenched to convert it into martensitic stainless steel either simultaneously with or after processing.

[0007] The present invention is premised on application to high-grade cutlery made of martensitic stainless steel, which requires particularly high hardness and excellent corrosion resistance, and targets ferritic stainless steel with 0.45% or more C added as an intermediate material suitable for the manufacture of such cutlery. Note that the application is not limited to high-grade cutlery, and the invention can be applied to other uses requiring excellent properties and processing. Furthermore, for high-grade cutlery, it is preferable that the product surface is beautiful. Beautiful means excellent surface shape, excellent corrosion resistance, and excellent surface properties that do not rust for a longer period of time than before or even in a severe corrosive environment.

[0008] In the manufacturing process of ferritic stainless steel, an intermediate material, the ingot obtained by continuous casting or ingot casting is generally hot worked, cooled to room temperature, and then reheated to decompose it into ferrite and carbides and soften it (Non-Patent Document 2).

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

[0010] Furthermore, if the carbides present before and after quenching contain Cr, Mo, and V, which are necessary for obtaining excellent corrosion resistance, the corrosion resistance around the carbides often deteriorates.

[0011] To obtain excellent properties due to the solid solution of each element, it is necessary to increase the hardening temperature and time to dissolve (re-solid solution) the coarse carbides and ensure a certain amount of solid solution. If coarse carbides remain, there is an issue that the properties after hardening deteriorate and become unstable.

[0012] As a means for solving this problem, for example, Patent Document 1 discloses a method of optimizing the amount of added C and N and limiting the number density of carbides in a ferritic stainless steel intermediate material before quenching. This widens the appropriate quenching temperature range for obtaining the target properties, and ensures the necessary properties after quenching. [Prior art documents] [Patent documents]

[0013] [Patent Document 1] JP 2007-224405 A [Non-patent literature]

[0014] [Non-Patent Document 1] Stainless Steel European Standard EN10088-2 [Non-Patent Document 2] Stainless Steel Handbook, 3rd Edition, Japan Stainless Steel Association (1995), p. 829 Summary of the Invention [Problem to be solved by the invention]

[0015] In the material obtained by heat treating the ferritic stainless steel intermediate material of Patent Document 1, thick Cr deficiency occurs in some parts due to oxidation during heating, and when the material is made into a blade product, uneven patterns may appear, marring the surface appearance of the blade product.

[0016] The present invention aims to provide a ferritic stainless steel which has a wide appropriate quenching temperature range, has high hardness and excellent corrosion resistance after quenching, and can be used as a raw material for beautiful martensitic stainless steel products, and to provide an industrially stable manufacturing method thereof. [Means for solving the problem]

[0017] The inventors conducted a detailed investigation into the metal structure of ferritic stainless steel with 0.45% or more C added, which is suitable as an intermediate material for martensitic stainless steel products for cutlery, having high hardness and excellent corrosion resistance, and clarified the hardening conditions that result in the desired hardness, corrosion resistance, and beautiful surface.

[0018] As a result, it was revealed that the uneven patterns that appear on the surface of cutlery products, which deteriorate the corrosion resistance and mar the beauty of the product, are caused by Cr deficiency directly below the oxides containing Cr due to grain boundary oxidation, etc. In addition, it was found that by making the crystal grain size fine and increasing the grain boundary density in the material, the carbides on the grain boundaries dissolve early, promoting the outward diffusion of Cr, Mo, and V, and quickly eliminating the Cr deficiency on the surface and near coarse carbides.

[0019] Furthermore, it was discovered that by refining the average crystal grain size and controlling the distribution of carbides, the appropriate quenching temperature range in which high hardness, excellent corrosion resistance and beautiful surface appearance can be stably obtained can be expanded.

[0020] The inventors have clarified the characteristics of the steel composition and metal structure that provide such effects, and have completed the present invention. The gist of the present invention is as follows.

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

[0022] (2) The ferritic stainless steel according to (1) above, characterized in that the proportion of carbides having a length of 1.5 μm or less in the ferritic grain boundary length is 5.0% or more.

[0023] (3) The ferritic stainless steel according to (1) or (2), characterized in that, in place of a portion of the Fe, one or more of the following are contained, by mass%, in the alloy: 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 sheet having the characteristics described in any one of (1) to (3) above and having a thickness of 0.4 to 6.0 mm.

[0025] (5) A method for producing a ferritic stainless steel according to any one of (1) to (3), comprising the steps of hot rolling a steel having a composition according to (1) or (3) at a starting temperature of 1,150°C or more and an end temperature of 850°C to 900°C to obtain a hot-rolled steel sheet, subsequently cooling the hot-rolled steel sheet at a cooling rate of 0.07°C / s or more to a temperature of 700°C to 800°C, and after cooling, heating and holding the hot-rolled steel sheet at a temperature of 700°C to 800°C for 20 minutes to 20 hours.

[0026] (6) A method for producing the ferritic stainless steel sheet according to (4) above, comprising the steps of hot rolling a steel having a composition according to (1) or (3) above at a starting temperature of 1150°C or more and an end temperature of 850°C to 900°C to obtain a hot-rolled steel sheet, subsequently cooling the hot-rolled steel sheet at a cooling rate of 0.07°C / s or more to a temperature of 700°C to 800°C, after cooling, heating and holding the hot-rolled steel sheet at a temperature of 700°C to 800°C for 20 minutes to 20 hours, pickling the hot-rolled steel sheet that has been heated and held, cold rolling the hot-rolled steel sheet after pickling to obtain a cold-rolled steel sheet, and heat treating the cold-rolled steel sheet at 700°C to 800°C. Effect of the Invention

[0027] According to the present invention, it is possible to provide an attractive ferritic stainless steel that has a wide range of suitable quenching temperatures, high hardness, and excellent corrosion resistance. [Brief description of the drawings]

[0028] [Figure 1] FIG. 1 is a diagram showing the criteria for determining whether a carbide is "carbide located on a grain boundary" and the "length of a line segment occupying a grain boundary." DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0029] 1. Ferritic stainless steel The ferritic stainless steel of the present invention will be described in detail below.

[0030] (Chemical Composition) First, the components contained in the ferritic stainless steel of the present invention will be described. Note that the "%" for the content of each element means % by mass.

[0031] C is an important element for ensuring the hardness of martensite. It also acts as an element that generates Cr carbides and affects the corrosion resistance of the base material. If the C content is less than 0.45%, the hardening hardness required for blade applications cannot be obtained. In addition, the number density of carbides of 1.5 μm or less, which contributes to stable hardening hardness, becomes insufficient, and the appropriate hardening temperature range is narrowed. Furthermore, pinning by the carbides does not work effectively, and the average crystal grain size of the ferrite phase becomes coarse during heating in the furnace after hot rolling. On the other hand, if the C content exceeds 0.55%, the carbides become coarse, the number density becomes insufficient, and the appropriate hardening temperature range is narrowed. In addition, the required corrosion resistance cannot be satisfied. Therefore, the C content is set to 0.45% or more and 0.55% or less. 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%.

[0032] Si is an element that improves oxidation resistance. If the Si content is less than 0.10%, sufficient oxidation resistance cannot be obtained. Moreover, if the Si content is excessively decreased, the manufacturing cost increases. On the other hand, if the Si content exceeds 1.00%, cracking during manufacturing is promoted. Therefore, the Si content is set to 0.10% or more and 1.00% or less. 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%.

[0033] Mn is used as a deoxidizing element. In addition, the amount of solute C increases through interaction with C, which is thought to contribute to improving hardness after quenching. From the viewpoint of stable production and the expression of the effect of increasing solute C through interaction with C, the Mn content is set to 0.1% or more. On the other hand, if the Mn content exceeds 1.0%, it may form compounds such as sulfides, which may lead to a decrease in corrosion resistance. In addition, it is thought that the effect of increasing solute C through interaction with C becomes saturated, and no effect commensurate with the amount added is obtained. Therefore, the Mn content is set to 0.1% or more and 1.0% or less. The lower limit of the Mn content is preferably 0.2%, more preferably 0.3%. The upper limit of the Mn content is preferably 0.9%, more preferably 0.8%.

[0034] Cr is an element that improves corrosion resistance. Cr is also an element that improves hardenability, and is an element that causes diffusion transformation and suppresses the decrease in hardness after quenching. It is also an element that constitutes carbides, and affects the carbide density in the metal structure before quenching. If the Cr content is less than 12.0%, sufficient corrosion resistance, the effect of suppressing diffusion transformation, and the carbide density cannot be obtained. On the other hand, if the Cr content exceeds 15.0%, manufacturability is reduced. Furthermore, corrosion resistance commensurate with the cost of the added alloy cannot be obtained. Furthermore, the generation of residual γ increases due to the decrease in the quenching transformation temperature (Ms point), resulting in a decrease in hardness. Therefore, the Cr content is set to 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%. Furthermore, 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%, and more preferably 14.7%.

[0035] Ni is an element that improves toughness when the martensite phase is formed, and may be added as necessary. However, if the Ni content exceeds 1.0%, it leads to a decrease in formability. In addition, Ni is a rare element and is expensive, which may lead to an increase in alloy costs and an impairment of manufacturability. Therefore, the Ni content is set to 1.0% or less. It is preferably 0.60% or less, and more preferably 0.05% or more and 0.50% or less. When Ni is contained, its content may be a small amount, but the lower limit is preferably 0.05%, more preferably 0.10%. The upper limit of the Ni content is preferably 0.60%, more preferably 0.50%.

[0036] Mo is an element that improves corrosion resistance. It is also an element that improves hardness by solid solution strengthening. If the Mo content is less than 0.50%, sufficient corrosion resistance and the effect of improving hardness by solid solution strengthening cannot be obtained. On the other hand, even if the Mo content exceeds 0.80%, the effect on corrosion resistance and solid solution strengthening saturates, and an effect commensurate with the cost of adding cannot be obtained. Therefore, the Mo content is set to 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%.

[0037] V is an element that improves corrosion resistance. It also acts as an element that finely precipitates carbides, increasing the number density of the carbides. If the V content is less than 0.10%, sufficient corrosion resistance cannot be obtained. In addition, the effect of increasing the number density of the carbides cannot be sufficiently obtained. On the other hand, even if the V content exceeds 0.20%, the effect on corrosion resistance and the effect of increasing the number density of the carbides are saturated, and an effect commensurate with the cost of addition cannot be obtained. Therefore, the V content is set to 0.10% or more and 0.20% or less. The lower limit of the V content is preferably 0.11%, more preferably 0.13%. The upper limit of the V content is preferably 0.19%, more preferably 0.17%.

[0038] 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%, the hot workability is significantly deteriorated. Therefore, the N content is set to 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 further preferably 0.040%. The upper limit of the N content is preferably 0.090%, and more preferably 0.080%.

[0039] P is an element that reduces formability and corrosion resistance. The lower the content, the better. Therefore, the P content is set to 0.040% or less. There is no particular lower limit.

[0040] S is an inevitable impurity element that promotes cracking during manufacturing. Therefore, the S content is set to 0.030% or less. There is no particular lower limit.

[0041] The ferritic stainless steel of the present invention contains Fe and impurities (including unavoidable impurities) in addition to the above-mentioned elements.

[0042] In addition to the basic composition described above, the ferritic stainless steel of the present disclosure may selectively contain, in place of a portion of Fe, one or more of the following, by mass%, 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: up to 0.10% or less, and Sb: 0.15% or less.

[0043] (Al: 0.30% or less, Nb: 0.070% or less, B: 0.0030% or less, Ti: 0.070% or less) The elements Al, Nb, B, and Ti do not have to be added. If these elements are added, they have the effect of improving the formability of ferritic stainless steel and suppressing defects during hot working. When added, the Al content is 0.30% or less, the Nb content is 0.070% or less, the B content is 0.0030% or less, and the Ti content is 0.070% or less. In order to reliably obtain the above effects, it is preferable that the Al, Nb, and Ti contents are 0.01% or more, and the B content is 0.001% or more.

[0044] (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) The elements Sn, Cu, W, Co, and Zr do not have 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. In order to reliably obtain the above effects, it is preferable that the Sn, Cu, Co, and Zr contents are 0.01% or more, and the W content is 0.1% or more.

[0045] (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) The elements Ca, Mg, Y, REM and Sb do not have to be added. These elements have the effect of changing inclusions such as oxides and sulfides to suppress hot working defects. 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. In order to reliably obtain the above effects, it is preferable that the Ca and Mg contents are 0.0001% or more, and the Y, Hf and REM contents are 0.01% or more.

[0046] In the present application, "REM" refers to elements having atomic numbers of 57 to 71 (lanthanoids), such as La, Ce, Pr, and Nd, but does not include Y.

[0047] In addition to the above elements, the ferritic stainless steel of the present disclosure may further contain elements other than the above elements in place of a portion of Fe, to the extent that the above problems can be solved. For example, Bi, Pb, Se, H, Ta, etc. may be contained, but the content ratios are controlled to the extent that the above problems can be solved, and for example, one or more of Bi≦100 ppm, Pb≦100 ppm, Se≦100 ppm, H≦100 ppm, and Ta≦500 ppm may be contained.

[0048] (Average grain size of ferrite phase and carbide precipitation state) In the ferritic stainless steel of the present invention, excellent properties including a beautiful surface are ensured by refining the average crystal grain size of the ferrite phase and defining the size and number density of carbides.

[0049] By making the average crystal grain size finer, the number of carbides located on the grain boundaries of the ferrite phase increases. During high-temperature heating, the carbides on the grain boundaries act as nuclei for transformation into the austenite phase, increasing the grain boundary area of ​​the austenite phase. As a result, the re-dissolution of the carbides progresses, and the outward diffusion of the re-dissolved Cr, M, and V is promoted, allowing Cr deficiency to be eliminated at an early stage. In addition to the above, if the ratio (occupancy rate) of carbides to the length of the grain boundaries of the ferrite phase is equal to or greater than a certain level, the effect of eliminating Cr deficiency is further enhanced, and corrosion resistance is significantly improved.

[0050] The average grain size of the ferrite phase must be 10 μm or less. The average grain size is preferably 9 μm or less, and more preferably 8 μm or less. On the other hand, the lower limit of the average grain size is not particularly limited, but is set to 1 μm or more based on past experience. On the other hand, if the average grain size exceeds 10 μm, the amount of carbides located at the grain boundaries decreases, and the phenomenon of eliminating Cr deficiency does not occur, making it impossible to ensure excellent properties.

[0051] (Method of measuring the average grain size of the ferrite phase) The average grain size of the ferrite phase is determined as follows. The L-section of a steel plate sample prepared by electrolytic polishing is measured by EBSD. The measurement area is 300 μm x 300 μm at the 1 / 4t position of the plate thickness, and the measurement step size is 0.1 μm. If the crystal orientation difference between adjacent plot data is less than 15°, they are considered to be the same grain, and if there is an orientation difference of 15° or more, they are treated as different grains to determine the average grain size. Note that if the measurement area contains phases other than the ferrite phase, only the ferrite phase is extracted and the average grain size is determined.

[0052] The carbides should be of a size that allows them to be redissolved in the austenite phase during high-temperature heating, and the higher the number density, the better. The size of the carbides must not include coarse carbides with a diameter exceeding 1.5 μm. The diameter is preferably 1.0 μm or less.

[0053] The number density of carbides with a diameter of 1.5 μm or less is 0.8 / μm 2 Existence of more than It is necessary that the number of particles is 1.0 / μm. 2 More preferably, 1.2μ m 2 The upper limit of the number density is not particularly limited. When the number density satisfies the above conditions, the amount of dissolved C required for the target hardness can be sufficiently secured, and the appropriate hardening temperature range is also expanded.

[0054] In order to ensure a beautiful surface appearance after quenching and to significantly improve corrosion resistance, the percentage of carbides with a diameter of 1.5 μm or less in the grain boundaries is preferably 5.0% or more. The percentage is preferably 7.0% or more, and more preferably 10.0% or more. There is no upper limit, but it is preferably 17.0% or less.

[0055] (Method of measuring the size and number density of carbides, and the percentage of carbides in grain boundaries) The size and number density of carbides are specified by the following method.

[0056] The L-section of the steel plate is mirror-polished, then etched with aqua regia to reveal the grain boundaries and carbides, and the size and number density of the carbides are measured by SEM observation. The measurement area is 200μm x 200μm in total area at 1 / 4t of the plate thickness, and the SEM observation is performed at a magnification of 5000x. The size of the carbides is calculated by converting the observed carbides into circle-equivalent diameters. The number density of carbides [pieces / μm2] is calculated as the area of ​​the measurement area relative to the number of carbides with a diameter of 1.5μm or less confirmed in the measurement area.

[0057] In the present invention, the occupancy rate (P [%]) is defined as the proportion 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 sum of the lengths of the line segments in which the carbides occupy the grain boundary (b [μm]) to the total grain boundary length (a [μm]) in the measurement area. The calculation formula is shown in formula (2). Figure 1 also shows the criteria for determining that a carbide is on a grain boundary and the "length of the line segments occupying the grain boundary" in schematic form.

[0058] P = b / a × 100 … (1)

[0059] The carbides observed in the ferritic stainless steel of the present invention are (Cr,Fe) 23 The majority is C6, but it may contain some (Cr,Fe)7C3. Carbides were identified by EDX. It can be acknowledged.

[0060] (Presence of phases other than ferrite and carbides) The metal structure of the ferritic stainless steel of the present invention is composed of a ferrite phase and numerous fine carbides at room temperature, which account for only a small proportion of the carbides. However, the presence of other phases can be tolerated to a certain extent. For example, the ferritic stainless steel of the present invention can be treated without any problem if it contains phases other than the main ferrite phase, such as austenite and martensite phases, in an area ratio of 5% or less in total at room temperature.

[0061] (Method to determine the presence or absence of austenite and martensite phases) The presence or absence of austenite phase is judged using the data measured by EBSD described in the previous paragraph. Since the austenite phase has an FCC structure and the ferrite phase has a BCC structure, the percentage of the FCC structure in the measurement area (γ [%]) is calculated to determine the presence or absence of austenite phase. If the value calculated by formula (1) is 5% or less, it is determined that there is no austenite phase. Here, γ is the area ratio of the austenite phase (unit: [%]), and F and B represent the number of plots of the FCC structure and the BCC structure obtained by EBSD measurement, respectively (unit: [pieces]).

[0062] γ = F / (F + B) × 100 … (2)

[0063] The presence or absence of martensite phase is judged by Vickers hardness. If the martensite phase is 5% or more, the hardness exceeds 300HV. Using a Vickers hardness tester, measurements are taken 10 times with a load of 500g. If the average value is 300HV or less, it is determined that there is no martensite phase.

[0064] (Thickness) The thickness after hot rolling is 4.0 mm to 6.0 mm, and the thickness at the subsequent cold rolling stage is 0.4 mm to less than 4.0 mm. The thickness of the ferritic stainless steel of the present invention is 0.4 mm to 6 mm from the stage after hot rolling to the stage of cold rolling, including the product thickness.

[0065] 2. Manufacturing method of ferritic stainless steel

[0066] (Manufacturing method) A method for producing the ferritic stainless steel of the present invention will now be described.

[0067] The steel having the above-mentioned composition is melted, cast into an ingot, and heated. If the heating temperature (the starting temperature of hot rolling, described later) is less than 1150°C, the carbides cannot be sufficiently dissolved, the properties vary depending on the part, and coarse carbides remain, narrowing the appropriate quenching temperature range of the product. Therefore, the heating temperature is set to 1150°C or higher. It is preferably 1180°C or higher.

[0068] Next, the heated ingot is subjected to hot rolling. If the end temperature of hot rolling is less than 850°C, the deformation load is too high, so the load on the equipment performing hot rolling is high and it is not possible to process it into the desired shape. On the other hand, if the temperature exceeds 950°C, coarse carbides remain without being crushed, narrowing the appropriate quenching temperature range of the product. Therefore, the end temperature of hot rolling is 850°C or more and 950°C or less. It is preferably 860°C or more and 940°C or less.

[0069] Immediately after the hot rolling is completed, the cooling rate is controlled to cool the steel to a temperature of 700°C or more and 800°C or less for the subsequent heating and holding process. At this time, it is necessary to set the cooling rate to 0.07°C / s or more, and to manage the thermal history so as not to lower the temperature below 700°C during cooling. The cooling rate is preferably 0.20°C / s or more. The processing strain accumulated by hot rolling is maintained until just before the heating and holding process, so that the average grain size of the ferrite phase becomes 10μm 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 strain is restored during cooling, and the precipitation nuclei of the ferrite phase are reduced, so that the ferrite becomes coarse during the heating and holding process.

[0070] In the conventional manufacturing method of a ferritic stainless steel intermediate, a typical manufacturing process is performed in which the material is cooled to room temperature while controlling the cooling rate after hot rolling, and then heated and held at the same temperature. In the thermal history of this process, the grains of the ferritic phase become coarse as the strain in the martensite phase disappears. In contrast, in the manufacturing method of the ferritic stainless steel of the present invention, it is very important to control the cooling rate and temperature history from immediately after hot rolling to immediately before holding at the same temperature. In the conventional manufacturing method of a ferritic stainless steel intermediate, such control of the cooling rate and temperature history is not performed.

[0071] After cooling to a heating and holding temperature of 700°C or more and 800°C or less, the heating and holding is continued. If the heating and holding temperature is less than 700°C, the number density of carbides of 1.5 μm or less is significantly reduced, and the ferrite transformation does not proceed sufficiently, and when the heating and holding is performed and the material is cooled to room temperature, the metal structure becomes one containing a large amount of hard martensite phase. As a result, it becomes difficult to pass the sheet in the subsequent process such as cold rolling, which increases the manufacturing cost and reduces the yield. On the other hand, if the temperature exceeds 800°C, the carbides aggregate and coarsen, narrowing the appropriate quenching temperature range. In addition, the average crystal grain size of the ferrite phase also becomes coarse.

[0072] The heating time is 20 minutes or more and 20 hours or less. If the heating time is less than 20 minutes, the number density of carbides with a diameter of 1.5 μm or less is significantly reduced, and the metal structure contains a large amount of martensite phase after cooling at room temperature. As a result, it becomes difficult to pass the sheet in the post-process such as cold rolling, which increases the manufacturing cost and reduces the yield. On the other hand, if the heating time exceeds 20 hours, the carbides aggregate and coarsen, narrowing the appropriate quenching temperature range. In addition, the crystal grains of the ferrite phase also become coarse. Therefore, the heating time is 700°C to 800°C for 20 minutes to 20 hours. Preferably, the heating time is 710°C to 790°C for 75 minutes to 15 hours.

[0073] The cooling rate after the heating and holding is not particularly limited. For example, the cooling rate may be 0.05° C. / s or more, or air cooling may be performed.

[0074] After the heating and holding and cooling are completed, pickling, cold rolling, and final heat treatment can be repeatedly performed as necessary to obtain a steel plate having a predetermined plate thickness.

[0075] Pickling is a process for removing oxide scale from the surface, cold rolling is a process for obtaining the specified plate thickness, and final heat treatment is a process for releasing the strain introduced by the cold rolling and softening the plate through recrystallization. These are generally the methods used in the production of stainless steel and there is no problem with them.

[0076] If the temperature of the final heat treatment is less than 700°C, recrystallization is insufficient and the sheet becomes hard, making it difficult to pass it to the next process or to process it at the customer's site. 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 hard due to the large amount of martensite phase, making it difficult to pass it to the next process or to process it at the customer's site. Therefore, the temperature of the final heat treatment is set to 700°C or higher and 800°C or lower. Preferably, it is set to 710°C or higher and 790°C or lower. EXAMPLES

[0077] The effects of the ferritic stainless steel of the present invention will be described with reference to examples.

[0078] Steels having the compositions shown in Table 1 were smelted in a laboratory to obtain ingots with a thickness of 100 mm. These ingots were heated at the temperatures shown in Table 2 for 120 minutes and then hot-rolled to obtain hot-rolled plates with a thickness of 5.0 mm.

[0079] Subsequently, the hot-rolled sheets were cooled at a cooling rate in the range of 0.05 to 2.00°C / s to the heating and holding temperature shown in Table 2. After reaching the heating and holding temperature, the sheets were heated and held for the time shown in Table 2. After the heating and holding, the sheets were air-cooled to room temperature. Steel sheets Nos. 1 to 16 and 18 to 34 were pickled with sulfuric acid, cold-rolled at a reduction rate of 60%, and further subjected to a heat treatment at 700 to 800°C for 2 minutes to obtain steel sheets with a thickness of 2.0 mm.

[0080] Further, the steel sheets Nos. 1 to 15 and 18 to 34 were subjected to cold rolling, cold-rolled sheet annealing, and pickling again to obtain steel sheets having a thickness of 0.8 mm.

[0081] In this study, in order to compare with the manufacturing process of the present invention, No. 33 was cooled to room temperature after hot rolling, and then heated again and held at that temperature.

[0082] [Table 1]

[0083] [Table 2]

[0084] The average crystal grain size of the ferrite phase, the presence or absence of the austenite phase and the martensite phase, the size and number density of the carbides, and the occupancy rate of the grain boundaries of the carbides were measured by the methods described above.

[0085] (Method of measuring cooling rate after completion of hot rolling) The cooling rate after the completion of hot rolling is defined as the average cooling rate from the completion of hot rolling until the temperature reaches the heating and holding temperature. The temperature history was measured using a radiation thermometer.

[0086] (Evaluation test of hardness stability after quenching) The test materials shown in Table 2 were heated at 900 to 1150°C for 5 minutes and then air-cooled. The quenched hardness of the samples was then investigated. The heating temperature was changed in 10°C increments. The quenching stability (ΔT [°C]) was evaluated using formula (3).

[0087] ΔT = Tmax - Tmin … (3)

[0088] Tmin [°C] and Tmax [°C] respectively indicate the minimum and maximum temperatures at which the hardness is 550HV or more. The larger ΔT is, the wider the quenching temperature range at which the hardness can be obtained is, and the better the hardness stability is. On the other hand, when ΔT is 0, the minimum temperature Tmin and the maximum temperature Tmax are the same, which means that the hardness stability is poor. Here, ΔT of 30°C or more is evaluated as pass, and ΔT of less than 30°C is evaluated as fail.

[0089] (Surface appearance evaluation test) The test materials shown in Table 2 were heated at temperatures Tmin and Tmax for 5 minutes, then cooled in air to obtain hardened samples. The oxide scale of the hardened samples was removed to expose the metal surface, and the surface was finished by wet polishing with #600. The unevenness pattern was visually confirmed. The total area of ​​the observation field was 1 m2. 2 The ratio of the total area of ​​unevenness patterns to the total area of ​​the entire product (hereinafter and in Table 2, this is referred to as the "defect rate"). If the defect rate is 5.0% or less for both heating temperatures Tmin and Tmax, the blade is deemed to have passed the test and satisfied the beautiful surface appearance required for a high-quality blade. Otherwise, the blade is deemed to have failed the test. The defect rate in Table 2 is the larger of the defect rates for the heating temperatures Tmin and Tmax.

[0090] (Corrosion resistance evaluation test) Test pieces prepared using the same quenching heat treatment and polishing method as in the surface appearance evaluation test were subjected to a salt spray test using a 7% NaCl solution at a test temperature of 50°C. The pass / fail corrosion resistance was judged based on whether or not red rust was observed on the surface of the test piece. If no red rust was observed visually at both heating temperatures Tmin and Tmax after 4 hours from the start of the test, the test piece was deemed to have passed, and was deemed to have satisfied the corrosion resistance required for a blade. In all other cases, the test piece was deemed to have failed. Only for those judged to have passed, the evaluation test was extended until the total test time reached 24 hours. If no red rust was observed visually after the evaluation test at both heating temperatures Tmin and Tmax, the corrosion resistance was deemed to have been even better.

[0091] Table 2 shows the evaluation results of the quenched hardness stability, defect rate, and corrosion resistance. Nos. 1 to 26, in which the average crystal grain size of the ferrite phase and the number density of carbides are both above the specified level, had a wide appropriate quenching temperature range for obtaining high hardness and excellent corrosion resistance, and also had a beautiful surface appearance. In particular, in Examples 2 to 26, in which the occupancy rate of carbides at the grain boundaries was also above the specified level, the corrosion resistance was significantly improved.

[0092] On the other hand, in No. 31, the number density of carbides was small and the average crystal grain size of the ferrite phase was coarse, so the quenched hardness stability was insufficient and the defect rate was also poor.

[0093] No. 32 had a low density of carbides and insufficient hardness stability after quenching. In addition, the amount of added Cr was insufficient, so the corrosion resistance was insufficient. No. 32 had an excessive amount of added C, so there were excessive coarse carbides, and the hardness stability after quenching was poor. In addition, the amount of added Cr was insufficient, so the corrosion resistance was poor.

[0094] No. 33, which had a thermal history of being cooled to room temperature after hot rolling and then heated again and held at that temperature, had a coarse average crystal grain size and a higher defect rate than the specified level.

[0095] In No. 34, in which the cooling rate after hot rolling was slow, the average crystal grain size became coarse and the defect rate in the surface appearance was higher than the specified value. [Industrial Applicability]

[0096] The ferritic stainless steel disclosed herein has a wide range of suitable quenching temperatures and exhibits high hardness after quenching, excellent corrosion resistance, and a beautiful surface. In other words, it is suitable as an intermediate material for martensitic stainless steel, and can be used to efficiently produce, for example, high-grade cutlery products that require hardness, excellent corrosion resistance, and a beautiful surface. [Explanation of symbols]

[0097] 1 Grain boundary 2 Length of the line segment occupying the grain boundary 3 Carbide on grain boundaries 4. Carbides not on grain boundaries

Claims

1. In mass percent, 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, Remainder: Fe and impurities The steel composition comprises: The average grain size of the ferrite phase is 10 μm or less, Carbides with a diameter of 1.5 μm or less are 0.8 pieces / μm 2 More than 1. A ferritic stainless steel comprising:

2. 2. The ferritic stainless steel according to claim 1, characterized in that the percentage of carbides having a diameter of 1.5 μm or less in the ferritic grain boundary length is 5.0% or more.

3. Instead of a part of the Fe, In mass percent, 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 The ferritic stainless steel according to claim 1 or 2, characterized in that it contains one or more of the following:

4. A ferritic stainless steel sheet having the characteristics according to claim 1 or 2 and a thickness of 0.4 to 6.0 mm.

5. A ferritic stainless steel sheet having the features of claim 3 and a thickness of 0.4 to 6.0 mm.

6. A method for producing the ferritic stainless steel according to claim 1 or 2, comprising the steps of: A steel having the composition according to claim 1 is hot-rolled at a starting temperature of 1150°C or more and a finishing temperature of 850°C or more and 900°C or less to obtain a hot-rolled steel sheet; Next, the hot-rolled steel sheet is cooled at a cooling rate of 0.07°C / s or more to a temperature of 700°C or more and 800°C or less, After cooling, the hot-rolled steel sheet is heated and held at a temperature of 700° C. to 800° C. for 20 minutes to 20 hours. A method for producing ferritic stainless steel, comprising the steps of:

7. A method for producing the ferritic stainless steel of claim 3, comprising the steps of: The steel having the composition according to claim 3 is hot-rolled at a starting temperature of 1150°C or more and a finishing temperature of 850°C or more and 900°C or less to obtain a hot-rolled steel sheet; Next, the hot-rolled steel sheet is cooled at a cooling rate of 0.07°C / s or more to a temperature of 700°C or more and 800°C or less, After cooling, the hot-rolled steel sheet is heated and held at a temperature of 700° C. to 800° C. for 20 minutes to 20 hours. A method for producing ferritic stainless steel, comprising the steps of:

8. A method for producing the ferritic stainless steel sheet according to claim 4, comprising the steps of: A steel having the composition according to claim 1 is hot-rolled at a starting temperature of 1150°C or more and a finishing temperature of 850°C or more and 900°C or less to obtain a hot-rolled steel sheet; Next, the hot-rolled steel sheet is cooled at a cooling rate of 0.07°C / s or more to a temperature of 700°C or more and 800°C or less, After cooling, the hot-rolled steel sheet is heated and held at a temperature of 700° C. or more and 800° C. or less for 20 minutes to 20 hours or less, The hot-rolled steel sheet that has been heated and held is pickled with an acid. The hot-rolled steel sheet after pickling is cold-rolled to obtain a cold-rolled steel sheet; The cold-rolled steel sheet is heat-treated at 700°C or more and 800°C or less. A method for producing a ferritic stainless steel sheet, comprising the steps of:

9. A method for producing the ferritic stainless steel sheet according to claim 5, comprising the steps of: The steel having the composition according to claim 3 is hot-rolled at a starting temperature of 1150°C or more and a finishing temperature of 850°C or more and 900°C or less to obtain a hot-rolled steel sheet; Next, the hot-rolled steel sheet is cooled at a cooling rate of 0.07°C / s or more to a temperature of 700°C or more and 800°C or less, After cooling, the hot-rolled steel sheet is heated and held at a temperature of 700° C. or more and 800° C. or less for 20 minutes to 20 hours or less, The hot-rolled steel sheet that has been heated and held is pickled with an acid. The hot-rolled steel sheet after pickling is cold-rolled to obtain a cold-rolled steel sheet; The cold-rolled steel sheet is heat-treated at 700°C or more and 800°C or less. A method for producing a ferritic stainless steel sheet, comprising the steps of:

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