WO2023027129A1 - フェライト系ステンレス鋼及びその製造方法 - Google Patents

フェライト系ステンレス鋼及びその製造方法 Download PDF

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WO2023027129A1
WO2023027129A1 PCT/JP2022/031950 JP2022031950W WO2023027129A1 WO 2023027129 A1 WO2023027129 A1 WO 2023027129A1 JP 2022031950 W JP2022031950 W JP 2022031950W WO 2023027129 A1 WO2023027129 A1 WO 2023027129A1
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stainless steel
ferritic stainless
temperature
carbides
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French (fr)
Japanese (ja)
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耕一 坪井
一成 今川
慎一 寺岡
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Nippon Steel Stainless Steel Corp
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Nippon Steel Stainless Steel Corp
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Priority to CN202280049525.7A priority Critical patent/CN117642521A/zh
Priority to KR1020247005254A priority patent/KR20240036621A/ko
Priority to EP22861417.8A priority patent/EP4394055A4/en
Priority to JP2023543967A priority patent/JP7672496B2/ja
Priority to US18/287,114 priority patent/US12601039B2/en
Publication of WO2023027129A1 publication Critical patent/WO2023027129A1/ja
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    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
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    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
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    • C21D6/00—Heat treatment of ferrous alloys
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    • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
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    • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
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    • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0236—Cold rolling
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    • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • C21D8/0263—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
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    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/005—Ferrous alloys, e.g. steel alloys containing rare earths, i.e. Sc, Y, Lanthanides
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    • C22C38/008—Ferrous alloys, e.g. steel alloys containing tin
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    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/24—Ferrous alloys, e.g. steel alloys containing chromium with vanadium
    • C—CHEMISTRY; METALLURGY
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/52—Ferrous alloys, e.g. steel alloys containing chromium with nickel with cobalt
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron
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    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/005—Ferrite

Definitions

  • the present invention relates to ferritic stainless steel.
  • a ferritic stainless steel that is suitable as an intermediate material for martensitic stainless steel products that are particularly suitable for cutlery such as razors and kitchen knives.
  • Carbon-containing martensitic stainless steels represented by SUS420J1, SUS420J2, and EN1.4116 are used for blades 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 containing 0.40% or less of C are used for general-purpose blades.
  • EN1.4116 which has a high Cr content and further adds V and Mo to improve corrosion resistance, is used for high-grade cutlery that requires higher hardness and excellent corrosion resistance.
  • Stainless steel goes from austenite phase at high temperature, where relatively high concentrations of carbon can be dissolved, to a hard martensite phase with supersaturated carbon dissolved at room temperature by rapid cooling such as water or oil cooling. can get. That is, it becomes martensitic stainless steel.
  • the hardness of the martensite phase corresponds to the amount of solid solution C in the austenite phase during high-temperature heating, and it is known that the appropriate quenching temperature range for obtaining the target hardness is affected by the size of the carbide before quenching. It is
  • the carbides that exist before and after quenching are mainly composed of Cr, and are thought to contain V and Mo for the purpose of improving corrosion resistance, which greatly affects corrosion resistance. That is, when coarse carbides are present, corrosion resistance deteriorates in the vicinity thereof.
  • the intermediate materials that are the raw materials are soft at the manufacturing stage, and generally excellent in workability. After the shape is manufactured, it is processed into a product, or simultaneously or after processing, it is quenched into a martensitic stainless steel.
  • the present invention is premised on application to martensitic stainless steel high-grade cutlery that requires particularly high hardness and excellent corrosion resistance, and 0.45% or more of C is added as an intermediate material suitable for its production. Intended for ferritic stainless steel. It should be noted that the application is not limited to high-grade cutlery, but can also be applied to other uses that require excellent characteristics and require processing. Moreover, it is preferable that the surface of the product is beautiful in the case of a high-grade cutlery. Beauty means excellent surface shape, excellent corrosion resistance, and excellent surface properties such that rust does not occur even for a longer time than before or in a severe corrosive environment.
  • Non-Patent Document 2 In the manufacturing process of ferritic stainless steel, which is an intermediate material, an ingot obtained by continuous casting or ingot casting is hot worked, cooled to room temperature, and then reheated to decompose into a ferrite phase and carbides to soften it. is common (Non-Patent Document 2).
  • This reheating usually requires a long time of several hours due to the decomposition described above, and the carbides dispersed in the ferrite phase tend to become coarse.
  • a ferritic stainless steel intermediate material in which coarse carbides are dispersed is quenched, it often becomes softer than the target hardness.
  • 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.
  • Patent Document 1 discloses a method of optimizing the amount of added C and N and limiting the number density of carbides in the ferritic stainless steel intermediate material before quenching. As a result, the appropriate quenching temperature range for obtaining target properties is widened, and the required properties can be stably secured after quenching.
  • the present invention provides a ferritic stainless steel that has a wide appropriate quenching temperature range, high hardness and excellent corrosion resistance after quenching, and is a material for martensitic stainless steel products that are beautiful, and an industrially stable manufacturing method.
  • the task is to provide
  • the present inventors conducted a detailed investigation of the metal structure of ferritic stainless steel containing 0.45% or more of C, which is suitable as an intermediate material for martensitic stainless steel products for cutlery having high hardness and excellent corrosion resistance. Then, we clarified the quenching conditions to obtain the desired hardness, corrosion resistance, and beautiful surface.
  • the present inventors have clarified the characteristics of the steel composition and metallographic structure that provide such effects, and completed the present invention.
  • the gist of the present invention is as follows.
  • a ferritic stainless steel sheet having a plate thickness of 0.4 to 6.0 mm and having the characteristics described in any one of (1) to (3) above.
  • a method for producing a ferritic stainless steel according to any one of (1) to (3) above comprising: Hot rolling is performed at a finishing temperature of 850° C. or higher and 900° C. or lower to obtain a hot rolled steel sheet, and then 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 higher, A method for producing a ferritic stainless steel sheet, comprising, after cooling, heating and holding 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.
  • a hot-rolled steel sheet is obtained by hot rolling as follows, and then 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 higher, and after cooling, the hot-rolled steel plate is heated and held at a temperature of 700 ° C. or higher and 800 ° C.
  • a method for producing a ferritic stainless steel sheet comprising a step of heat-treating the cold-rolled steel sheet at 700°C or higher and 800°C or lower.
  • FIG. 1 is a diagram schematically showing criteria for judging carbides as “carbides on grain boundaries" and “line segment lengths occupying grain boundaries”.
  • C is an important element for ensuring the hardness of martensite. It also acts as an element that produces Cr carbide and affects the corrosion resistance of the base material. If the C content is less than 0.45%, the quenching hardness required for cutlery cannot be obtained. In addition, since the number density of carbides of 1.5 ⁇ m or less, which contributes to stable quenching hardness, becomes insufficient, the appropriate quenching temperature range also becomes narrow. Furthermore, the pinning by the carbide does not work effectively, and the average 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 and the number density becomes insufficient, narrowing the proper quenching temperature range.
  • the C content should be 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%.
  • Si is an element that improves oxidation resistance. If the Si content is less than 0.10%, sufficient oxidation resistance cannot be obtained. In addition, an excessive decrease leads to an increase in manufacturing cost. On the other hand, when the Si content exceeds 1.00%, cracking during production is promoted. Therefore, the Si content should be 0.10% or more and 1.00% or less.
  • the lower limit of Si content is preferably 0.20%, more preferably 0.30%.
  • the upper limit of Si content is preferably 0.90%, more preferably 0.80%.
  • Mn is used as a deoxidizing element. It is also believed that the interaction with C increases the amount of dissolved C and contributes to the improvement of hardness after quenching.
  • the Mn content is set to 0.1% or more from the viewpoint of stable production and expression of the effect of increasing solid solution C through interaction with C. On the other hand, if the Mn content exceeds 1.0%, there is a possibility that compounds such as sulfides are formed, resulting in deterioration of corrosion resistance. Also, it is considered that the effect of increasing the amount of dissolved C is saturated due to the interaction with C, and the effect commensurate with the amount added cannot be obtained. Therefore, the Mn content should be 0.1% or more and 1.0% or less.
  • the lower limit of Mn content is preferably 0.2%, more preferably 0.3%.
  • the upper limit of Mn content is preferably 0.9%, more preferably 0.8%.
  • Cr is an element that improves corrosion resistance. Further, Cr is an element that improves hardenability, and is an element that causes diffusion transformation and suppresses a decrease in hardness after hardening. Furthermore, it is also an element that constitutes carbides and affects the density of carbides in the metallographic structure before quenching. If the Cr content is less than 12.0%, sufficient corrosion resistance, diffusion transformation suppressing effect, and carbide density cannot be obtained. On the other hand, when the Cr content exceeds 15.0%, the manufacturability is lowered. Also, the corrosion resistance corresponding to the added alloy cost cannot be obtained. In addition, a decrease in the quenching transformation temperature (Ms point) causes a large amount of residual ⁇ to be generated, leading to a decrease in hardness.
  • Ms point quenching transformation temperature
  • the Cr content is set to 12.0% or more and 15.0% or less.
  • the lower limit of Cr content is preferably 12.5%, more preferably 13.0%, still more preferably 14.0%.
  • the lower limit of the Cr content may be 14.1% or 14.3%.
  • the upper limit of Cr content is preferably 14.9%, more preferably 14.7%.
  • Ni is an element that improves the toughness of the martensite phase, and may be added as necessary. However, when the Ni content exceeds 1.0%, the moldability is deteriorated. In addition, it is a rare element and expensive, which may lead to an increase in alloy cost and impede manufacturability. Therefore, the Ni content is set to 1.0% or less. It is preferably 0.60% or less, more preferably 0.05% or more and 0.50% or less. When Ni is contained, its content may be very small, 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%.
  • 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 hardness improvement effects due to solid solution strengthening cannot be obtained. On the other hand, even if the Mo content exceeds 0.80%, the effects of corrosion resistance and solid-solution strengthening become saturated, and the effect corresponding to the cost of addition cannot be obtained. Therefore, the Mo content is set to 0.50% or more and 0.80% or less.
  • the lower limit of Mo content is preferably 0.55%, more preferably 0.60%.
  • the upper limit of Mo content is preferably 0.75%, more preferably 0.70%.
  • V is an element that improves corrosion resistance. It also acts as an element that finely precipitates carbides and increases the number density of 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 carbides cannot be obtained sufficiently. 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 carbides are saturated, and the effect corresponding to 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%.
  • N is an element for ensuring the hardness of martensite. If the N content is less than 0.015%, sufficient hardness cannot be secured. On the other hand, when the N content exceeds 0.100%, the hot workability is remarkably 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%, still more preferably 0.040%.
  • the upper limit of the N content is preferably 0.090%, more preferably 0.080%.
  • P is an element that reduces formability and corrosion resistance.
  • the lower limit is not particularly limited.
  • the S content should be 0.030% or less.
  • the lower limit is not particularly limited.
  • the ferritic stainless steel of the present invention consists of Fe and impurities (including unavoidable impurities) in addition to the above elements.
  • the ferritic stainless steel of the present disclosure has Al: 0.30% or less, Nb: 0.070% or less, and B: 0.0030% by mass in place of part of Fe. % 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, Sb: 0.15% or less, one or more of may optionally include
  • 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. These elements have the effect of improving the formability of the ferritic stainless steel and suppressing flaws 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.
  • the Al, Nb, and Ti contents be 0.01% or more and the B content be 0.001% or more.
  • 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 may not be added. These elements have the effect of improving corrosion resistance.
  • 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 Zr The content should be 0.500% or less.
  • the Sn, Cu, Co, and Zr contents are preferably 0.01% or more, and the W content is preferably 0.1% or more.
  • the elements Ca, Mg, Y, REM and Sb may not be added. These elements have the effect of changing inclusions such as oxides and sulfides to suppress hot working flaws.
  • 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
  • REM The content is 0.10% or less
  • the Sb content is 0.15% or less.
  • REM refers to elements (lanthanoids) belonging to atomic numbers 57 to 71, such as La, Ce, Pr, Nd, etc., and does not include Y.
  • the ferritic stainless steel of the present disclosure may further contain elements other than the above-described elements in place of a part of Fe within a range capable of solving the above-described problems.
  • elements other than the above-described elements for example, Bi, Pb, Se, H, Ta, etc. may be contained, but the content ratio is controlled to the extent that the above problems can be solved. It may contain one or more of ⁇ 100 ppm and Ta ⁇ 500 ppm.
  • the number of carbides located on the grain boundaries of the ferrite phase increases.
  • the carbides on the grain boundaries act as nuclei for transformation into the austenite phase, increasing the grain boundary area of the austenite phase. Therefore, as the redissolution of carbides progresses, the outward diffusion of redissolved Cr, M, and V is promoted, and Cr deficiency can be quickly eliminated.
  • the proportion (occupancy) of carbides in the grain boundary length of the ferrite phase is at least a certain value, the effect of eliminating Cr deficiency is further enhanced, and the corrosion resistance is remarkably improved.
  • the average grain size of the ferrite phase must be 10 ⁇ m or less.
  • the average grain size is preferably 9 ⁇ m or less, more preferably 8 ⁇ m or less.
  • the lower limit of the average crystal grain size is not particularly limited, it is set to 1 ⁇ m or more according to actual results.
  • the average crystal grain size exceeds 10 ⁇ m, the amount of carbides located at the grain boundaries is reduced, and the phenomenon of resolving Cr deficiency does not occur, failing to ensure excellent properties.
  • the average grain size of the ferrite phase is specified as follows.
  • the L cross section of the steel sheet prepared by electropolishing is measured by EBSD.
  • the measurement area is 300 ⁇ m ⁇ 300 ⁇ m at the position of 1/4 t of plate thickness, and the measurement step size is 0.1 ⁇ m. If the crystal orientation difference between adjacent plot data was less than 15°, they were regarded as the same crystal grains.
  • the phases other than the ferrite phase are included in the measurement region, only the ferrite phase is extracted and then the average crystal grain size is obtained.
  • the carbides have a size that can redissolve in the austenite phase when heated to a high temperature, and the higher the number density, the better.
  • the carbide size should not include coarse carbides with a diameter greater than 1.5 ⁇ m. Preferably, the diameter is 1.0 ⁇ m or less.
  • the number density of carbides is required to be 0.8 pieces/ ⁇ m 2 or more of carbides having a diameter of 1.5 ⁇ m or less. It is preferably 1.0/ ⁇ m 2 or more, more preferably 1.2 ⁇ m 2 or more.
  • the upper limit of the number density is not particularly limited. When the size and number density of the carbides satisfy the above conditions, a sufficient amount of solid solution C required for the target hardness can be secured, so that the appropriate quenching temperature range is also expanded.
  • the occupancy rate of carbides with a diameter of 1.5 ⁇ m or less in grain boundaries is 5.0% or more.
  • the occupation rate is preferably 7.0% or more, more preferably 10.0% or more.
  • the upper limit is not specified, it is preferably 17.0% or less.
  • the size and number density of carbides are specified by the following method.
  • the L cross section of the steel plate is mirror-polished, it is etched with aqua regia to expose the grain boundaries and carbides, and the size and number density of the carbides are measured by SEM observation.
  • the measurement area is 1/4 t of the plate thickness and the total area is 200 ⁇ m ⁇ 200 ⁇ m, and the observation magnification is 5000 times for SEM observation.
  • the size of the carbide is obtained by converting the observed carbide into a circle-equivalent diameter.
  • the number density of carbides [pieces/ ⁇ m 2 ] is calculated as the area of the measurement region with respect to the number of carbides with a diameter of 1.5 ⁇ m or less confirmed in the measurement region.
  • the occupation rate (P [%]) in the present invention is defined as the proportion of carbides with a diameter of 1.5 ⁇ m or less in the grain boundary.
  • the occupancy ratio is obtained as a ratio of the sum of the line segment lengths (b [ ⁇ m]) occupied by the carbides in the grain boundaries to the total grain boundary length (a [ ⁇ m]) in the measurement region.
  • a calculation formula is shown in Formula (2).
  • FIG. 1 schematically shows the criteria for judging "carbide on grain boundary" and "line segment length occupying grain boundary".
  • carbides found in the ferritic stainless steel of the present invention are (Cr, Fe) 23 C 6 , but may partially contain (Cr, Fe) 7 C 3 . Carbide can be confirmed by EDX.
  • the metallographic structure of the ferritic stainless steel of the present invention is composed of a large number of fine carbides with a ferrite phase and only a very small proportion at room temperature.
  • the ferritic stainless steel of the present invention may contain phases other than the ferrite phase, which is the main phase, such as an austenite phase and a martensite phase, at a total area ratio of 5% or less at room temperature. .
  • the presence or absence of the austenite phase is determined using the data measured by EBSD as described in the previous paragraph. Since the austenite phase has an FCC structure and the ferrite phase has a BCC structure, the ratio ( ⁇ [%]) of the FCC structure in the measurement region is determined and determined. If the value obtained by the formula (1) is 5% or less, it is determined that there is no austenite phase.
  • ⁇ represents the area ratio of the austenite phase (unit: [%])
  • F and B represent the number of plots of the FCC structure and BCC structure obtained when measured by EBSD (unit: [number]).
  • the presence or absence of the martensite phase is determined by the Vickers hardness.
  • the presence of 5% or more of the martensite phase results in a hardness exceeding 300 HV.
  • measurements are performed 10 times with a load of 500 g, and if the average value is 300 HV or less, it is determined that there is no martensite phase.
  • the sheet thickness after hot rolling is 4.0 mm or more and 6.0 mm or less, and the sheet thickness at the subsequent cold rolling stage is 0.4 mm or more and less than 4.0 mm.
  • the sheet 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 sheet thickness.
  • the steel with the composition described above is melted, cast to produce an ingot, and heated. If the heating temperature (starting temperature of hot rolling described later) is less than 1150° C., the carbides cannot be dissolved sufficiently, the properties vary depending on the part, and coarse carbides remain, making the product unsuitable. Quenching temperature range narrows. Therefore, the heating temperature is set to 1150° C. or higher. It is preferably 1180° C. or higher.
  • the finishing temperature of hot rolling is set to 850° C. or more and 950° C. or less.
  • the temperature is preferably 860°C or higher and 940°C or lower.
  • the cooling rate is controlled to cool to a temperature of 700°C or higher and 800°C or lower in the subsequent heating and holding process. At that time, it is necessary to set the cooling rate to 0.07° C./s or higher and manage the thermal history so that the temperature does not drop below 700° C. during cooling.
  • the cooling rate is preferably 0.20° C./s or higher. Since the working strain accumulated by hot rolling is maintained until immediately before heating and holding, the average grain size of the ferrite phase becomes 10 ⁇ m or less after heating and holding. On the other hand, if the cooling rate is slower than 0.07° C./s, the working strain is recovered during cooling, and the ferrite phase precipitation nuclei are reduced, so that ferrite coarsens during heating and holding.
  • the heating and holding temperature After cooling to the heating and holding temperature of 700°C or higher and 800°C or lower, the heating and holding is continued. If the heating and holding temperature is lower than 700° C., the number density of carbides of 1.5 ⁇ m or less is remarkably low, and ferrite transformation does not proceed sufficiently. It becomes a metal structure containing a lot of As a result, it becomes difficult to pass sheets in post-processes such as cold rolling, resulting in an increase in manufacturing costs and a decrease in yield. On the other hand, if the temperature exceeds 800° C., the carbides agglomerate and coarsen, narrowing the proper quenching temperature range. Moreover, the average crystal grain size of the ferrite phase is also coarsened.
  • the heating and holding time should be 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 ⁇ m or less is significantly reduced, resulting in a metal structure containing a large amount of martensite phase after cooling to room temperature. As a result, it becomes difficult to pass sheets in post-processes such as cold rolling, resulting in an increase in manufacturing costs and a decrease in yield.
  • the heating is held for more than 20 hours, the carbides agglomerate and coarsen, narrowing the appropriate quenching temperature range. Moreover, the crystal grains of the ferrite phase are also coarsened. Therefore, the heating and holding is performed at a temperature of 700° C. or higher and 800° C. or lower for 20 minutes or longer and 20 hours or shorter. Preferably, the temperature is 710° C. or higher and 790° C. or lower for 75 minutes or longer and 15 hours or shorter.
  • the cooling rate after heating and holding is not particularly limited.
  • the cooling rate may be 0.05° C./s or more, or air cooling may be used.
  • pickling, cold rolling, and final heat treatment can be repeated as necessary to obtain a steel sheet with a predetermined thickness.
  • Pickling is a process of removing oxide scale on the surface
  • cold rolling is a process of obtaining a predetermined plate thickness
  • final heat treatment is a process of releasing the strain introduced by the cold rolling and softening by recrystallization
  • the temperature of the final heat treatment should be 700° C. or higher and 800° C. or lower. Preferably, it is 710°C or higher and 790°C or lower.
  • the hot-rolled sheet was cooled to the heating and holding temperature shown in Table 2 at a cooling rate in the range of 0.05 to 2.00°C/s. After reaching the heating and holding temperature, heating and holding was performed for the time shown in Table 2. After heating and holding, it was cooled to room temperature by air cooling. No.
  • the steel sheets of 1 to 16 and 18 to 34 were subjected to sulfuric acid pickling, cold rolling with a rolling reduction of 60%, and heat treatment at 700 to 800° C. for 2 minutes to obtain steel sheets with a thickness of 2.0 mm. .
  • No. No. 33 was once cooled to room temperature after hot rolling, then heated again and kept under heating.
  • the average grain size of ferrite phase, the presence or absence of austenite phase and martensite phase, the size and number density of carbides, and the occupation rate of carbides in grain boundaries were measured by the methods described above.
  • the cooling rate after the completion of hot rolling is defined as the average cooling rate during the period from the completion of hot rolling to the temperature for heating and holding. Temperature history was measured using a radiation thermometer.
  • Tmin [° C.] and Tmax [° C.] respectively indicate the minimum temperature and maximum temperature at which the quenching hardness becomes 550 HV or more. It means excellent hardness stability.
  • ⁇ T the lowest temperature Tmin and the highest temperature Tmax match, which means that the quenching stability is poor.
  • a ⁇ T of 30°C or more was evaluated as acceptable, and a ⁇ T of less than 30°C was evaluated as unacceptable.
  • Corrosion resistance evaluation test A test piece prepared by the same quenching heat treatment and polishing method as in the surface appearance evaluation test was subjected to a test temperature of 50° C. and a 7% NaCl solution salt spray test. Corrosion resistance was determined by whether or not red rust was observed on the surface of the test piece. After 4 hours from the start of the test, if no red rust is visually observed under the conditions of the heating temperatures Tmin and Tmax, the cutlery is judged to have passed the required corrosion resistance. Otherwise, the test was rejected. The evaluation test was extended until the total test time was 24 hours only for those judged to pass. When no red rust was visually observed after the evaluation test for both the heating temperatures Tmin and Tmax, it was determined that the corrosion resistance was further excellent.
  • Table 2 shows the evaluation results of quenching hardness stability, defect rate, and corrosion resistance.
  • No. 4 in which both the average crystal grain size of the ferrite phase and the number density of carbides are at least predetermined values.
  • the appropriate quenching temperature range for obtaining high hardness and excellent corrosion resistance was wide, and the surface appearance was beautiful.
  • the occupancy ratio of carbides at grain boundaries was a predetermined value or more, the corrosion resistance was remarkably improved.
  • the cooling rate after hot rolling is slow No. In No. 34, the average crystal grain size was coarse, and the defect rate of the surface appearance was higher than specified.
  • the ferritic stainless steel of the present disclosure has a wide appropriate quenching temperature range, 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 as an example, it can efficiently produce high-grade cutlery products that require hardness, excellent corrosion resistance, and beauty.

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CN116904868B (zh) * 2023-07-12 2025-09-23 成都先进金属材料产业技术研究院股份有限公司 900MPa级低锰铁素体易切削钢及其制备方法
CN117960829B (zh) * 2024-03-29 2024-07-05 攀钢集团研究院有限公司 热冲压构件的制备方法及热冲压构件

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