US20140294662A1 - Stainless steel for cutlery and method of manufacturing the same - Google Patents

Stainless steel for cutlery and method of manufacturing the same Download PDF

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US20140294662A1
US20140294662A1 US14/346,740 US201214346740A US2014294662A1 US 20140294662 A1 US20140294662 A1 US 20140294662A1 US 201214346740 A US201214346740 A US 201214346740A US 2014294662 A1 US2014294662 A1 US 2014294662A1
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cutlery
stainless steel
diffraction peak
plane
peak area
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Tomonori Ueno
Goh Yoshiyama
Ichiro Kishigami
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Proterial Ltd
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Hitachi Metals Ltd
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    • 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
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21B—ROLLING OF METAL
    • B21B3/00—Rolling materials of special alloys so far as the composition of the alloy requires or permits special rolling methods or sequences ; Rolling of aluminium, copper, zinc or other non-ferrous metals
    • B21B3/02—Rolling special iron alloys, e.g. stainless steel
    • C—CHEMISTRY; METALLURGY
    • 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
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C—CHEMISTRY; METALLURGY
    • 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
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—Modifying the physical properties 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 by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling
    • C—CHEMISTRY; METALLURGY
    • 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
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/18—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for knives, scythes, scissors, or like hand cutting tools
    • C—CHEMISTRY; METALLURGY
    • 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
    • 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
    • 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
    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
    • 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
    • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
    • 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
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • 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
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • 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
    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
    • 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
    • C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium 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
    • C22C38/24—Ferrous alloys, e.g. steel alloys containing chromium 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
    • C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium 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
    • C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium with boron
    • 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
    • C22C38/44—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
    • C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium

Definitions

  • the present invention relates to stainless steel for cutlery such as razors, cutters, kitchen knives, knives, and a method of manufacturing the same.
  • Martensitic stainless steel has been widely used as the material for cutlery such as razors, cutters, kitchen knives, and knives.
  • a strip of high carbon martensitic stainless steel containing about 13% by mass of Cr and about 0.65% by mass of C is known to be an optimum material for a razor.
  • the high carbon martensitic stainless steel (hereinafter referred to as “stainless steel for cutlery”) for these applications is usually used after being subjected to hardening and tempering, and is required to have high hardness and high toughness during use.
  • Stainless steel for cutlery is usually manufactured through the following manufacturing process.
  • a raw material is melted and cast to make a material.
  • the material is hot-rolled to make an intermediate material.
  • the material may be subjected to a blooming process such as hot forging or hot rolling.
  • the intermediate material is subjected to a first annealing to make an annealed material.
  • the annealed material is subjected to cold rolling followed by annealing for removing strain, which are repeated if necessary, thereby manufacturing a cold-rolled steel strip having the intended thickness.
  • the cold-rolled steel strip is subjected to hardening and tempering, thus obtaining stainless steel for cutlery.
  • stainless steel for cutlery is subjected to processing processes such as sharpening and cutting, and thus making an end product.
  • processing processes such as sharpening and cutting
  • stainless steel for cutlery is dealt on the market in the form of either an annealed material or a cold-rolled steel strip.
  • Patent Document 1 Japanese Patent Application Laid-Open (JP-A) No. H05-039547
  • Patent Document 1 proposes the increase in the carbide density of the cold-rolled steel strip before hardening and tempering for stainless steel for cutlery. According to this proposal, short time hardenability of the cold-rolled steel strip is markedly improved, and the stainless steel for cutlery after hardening improves hardness, and marks sharpness of a razor.
  • the invention has a final purpose of efficiently obtaining cutlery having marked characteristics such as high hardness and high toughness by optimizing the intermediate material which influences the structure of stainless steel for cutlery before hardening, and provides an intermediate material, an annealed material, and a cold-rolled steel strip for stainless steel for cutlery, and a method of manufacture them to achieve the purpose.
  • the inventors studied the properties of stainless steel for cutlery, focusing on the form of carbides as the factor influencing the hardness and toughness.
  • the cold-rolled steel strip has lower hardness and toughness after hardening and tempering in comparison with that containing uniformly distributed carbides.
  • the composition and the amount of an fcc phase influences the distribution of carbides in the structure of the cold-rolled steel strip obtained from the intermediate material.
  • an aspect of the invention is an intermediate material for stainless steel for cutlery, the intermediate material being a material after hot rolling but before annealing, having a composition, in terms of % by mass, of from 0.46 to 0.72% C, from 0.15 to 0.55% Si, from 0.45 to 1.00% Mn, from 12.5 to 13.9% Cr, from 0 to 1.5% Mo, from 0 to 0.012% B, and the balance being Fe and impurities; and wherein a ratio of a diffraction peak area from an fcc phase (a sum total of diffraction peak areas from (200) plane, (220) plane, and (311) plane) to a diffraction peak area from a bcc phase (a sum total of diffraction peak areas from (200) plane and (211) plane) (the diffraction peak area from the fcc phase/the diffraction peak area from the bcc phase) is 30 or less in an X-ray diffraction of a vertical section.
  • the content of B is preferably from 0.0005 to 0.0050%.
  • Another aspect of the invention is a method of manufacturing an intermediate material for stainless steel for cutlery, the intermediate material being a material after hot rolling but before annealing, the method including: heating a material, having a composition, in terms of % by mass, of from 0.46 to 0.72% C, from 0.15 to 0.55% Si, from 0.45 to 1.00% Mn, from 12.5 to 13.9% Cr, from 0 to 1.5% Mo, from 0 to 0.012% B, and the balance being Fe and impurities, to from 1100 to 1250° C., and hot rolling the material at a hot rolling end temperature of from 700 to 1000° C.; thereby manufacturing the intermediate material, wherein a ratio of a diffraction peak area from an fcc phase (a sum total of diffraction peak areas from (200) plane, (220) plane, and (311) plane) to a diffraction peak area from a bcc phase (a sum total of diffraction peak areas from (200) plane and (211) plane) (the
  • Another aspect of the invention is a method of manufacturing an annealed material for stainless steel for cutlery, including the above-described hot rolling, followed by annealing at from 800 to 860° C. for from 1 to 100 hours.
  • Another aspect of the invention is a method of manufacturing a cold-rolled steel strip for stainless steel for cutlery, including subjecting the above-described annealed material to cold rolling and annealing, thereby manufacturing a cold-rolled steel strip with a thickness less than 1.0 mm.
  • the cutlery manufactured from the stainless steel for cutlery of the invention has high hardness and high toughness, the cutlery is particularly suitable for razors having a small thickness.
  • the invention also allows quality control not in the stage of the end product but in the stage of the intermediate material, whereby the occurrence of defects is prevented, and the cost of manufacturing is reduced.
  • FIG. 1 is a schematic view showing the position from which the test piece is sampled, and the evaluated plane.
  • FIG. 2 is a photograph as a substitute of a drawing showing an example of the metal structure of the intermediate material for the stainless steel for cutlery of the invention.
  • FIG. 3 is a photograph as a substitute of a drawing showing an example of the metal structure of the intermediate material for the stainless steel for cutlery of a comparative example.
  • FIG. 4 is a photograph as a substitute of a drawing showing an example of the metal structure of the intermediate material for the stainless steel for cutlery of the invention.
  • FIG. 5 is a photograph as a substitute of a drawing showing an example of the metal structure of the intermediate material for the stainless steel for cutlery of a comparative example.
  • FIG. 6 is a photograph as a substitute of a drawing showing an example of the metal structure of the intermediate material for the stainless steel for cutlery of the invention.
  • FIG. 7 is a photograph as a substitute of a drawing showing an example of the metal structure of the intermediate material for the stainless steel for cutlery of the invention.
  • FIG. 8 is a photograph as a substitute of a drawing showing an example of the metal structure of the annealed material for the stainless steel for cutlery of the invention after hardening-subzero-tempering.
  • FIG. 9 is a photograph as a substitute of a drawing showing an example of the metal structure of the annealed material for the stainless steel for cutlery of a comparative example after hardening-subzero-tempering.
  • FIG. 10 is a photograph as a substitute of a drawing showing an example of the metal structure of the annealed material for the stainless steel for cutlery of the invention after hardening-subzero-tempering.
  • FIG. 11 is a photograph as a substitute of a drawing showing an example of the metal structure of the annealed material for the stainless steel for cutlery of a comparative example after hardening-subzero-tempering.
  • FIG. 12 is a photograph as a substitute of a drawing showing an example of the metal structure of the annealed material for the stainless steel for cutlery of the invention after hardening-subzero-tempering.
  • FIG. 13 is a photograph as a substitute of a drawing showing an example of the metal structure of the annealed material for the stainless steel for cutlery of the invention after hardening-subzero-tempering.
  • FIG. 14 is a photograph as a substitute of a drawing showing an example of the metal structure of the cold-rolled steel strip for the stainless steel for cutlery of the invention.
  • FIG. 15 is a photograph as a substitute of a drawing showing an example of the metal structure of the cold-rolled steel strip for the stainless steel for cutlery of the invention after hardening-subzero-tempering.
  • FIG. 16 is a photograph as a substitute of a drawing showing an example of the metal structure of the cold-rolled steel strip for the stainless steel for cutlery of the invention.
  • FIG. 17 is a photograph as a substitute of a drawing showing an example of the metal structure of the cold-rolled steel strip for the stainless steel for cutlery of the invention after hardening-subzero-tempering.
  • the invention is characterized in that it optimizes the alloy composition which influences the form of carbides, and controls the amount of the fcc phase in the intermediate material before annealing, thereby achieving both of high hardness and high toughness in the cutlery as the end product.
  • alloy composition contributing to the basic properties defined herein is described below.
  • the content of the respective elements are based on % by mass.
  • the C content is from 0.46 to 0.72%, thereby achieving sufficient hardness of cutlery, and minimizing the crystallization of the eutectic carbide during casting and solidification.
  • the C content is less than 0.46%, sufficient hardness of cutlery cannot be achieved.
  • the C content is more than 0.72%, the amount of crystallization of the eutectic carbide increases because of the balance with the Cr amount, which can cause blade chipping during sharpening.
  • the lower limit of the C content is preferably 0.50%, and more preferably, 0.65%.
  • the upper limit of the C content is preferably 0.70%.
  • Si is added as a deoxidizing agent for smelting. In order to achieve sufficient deoxidizing effect, the residual amount of Si would be 0.15% or more. When the Si content is more than 0.55%, the amount of inclusion increases, and can cause blade chipping during sharpening. Therefore, the Si content is from 0.15 to 0.55%. In addition, Si increases the tempering softening resistance. When Si is added at a proportion of 0.20% or more, sufficient hardness of cutlery will be achieved. Therefore, the lower limit of the Si content is preferably 0.20%, and the upper limit of the Si content is 0.35%.
  • Mn is also added as a deoxidizing agent for smelting. In order to achieve sufficient deoxidizing effect, the residual amount of Mn would be 0.45% or more. When the Mn content is more than 1.00%, hot workability decreases. Therefore, the Mn content is from 0.45 to 1.00%.
  • the lower limit of the Mn content is preferably 0.65%, and the upper limit of the Mn content is preferably 0.85%.
  • the Cr content is from 12.5 to 13.9%, thereby achieving sufficient corrosion resistance, and minimizing crystallization of the eutectic carbide during casting and solidification.
  • the lower limit of the Cr amount is preferably 13.0%.
  • the upper limit of the Cr amount is preferably 13.6%.
  • Mo is an element which improves corrosion resistance, so that it may be added if necessary at a proportion of up to 1.5%.
  • the Mo content is more than 1.5%, solid-solution strengthening is increased, whereby deformation resistance is increased, and hot workability is deteriorated. Therefore, the Mo content is from 0 to 1.5%.
  • B is an element effective at improving hardness and toughness.
  • the toughness can be improved by adjusting the below-described intensity ratio of X-ray, and the improvement in the toughness is more reliably achieved by adding B in advance. Therefore, B may be added at a proportion of up to 0.012%.
  • the upper limit of B is 0.012%.
  • the B content is preferably from 0.0005 to 0.0050%, because the effect of B addition will be insufficient when the B content is less than 0.0005%.
  • Typical impurity elements are P, S, Ni, W, V, Cu, Al, Ti, N, and O.
  • the contents of these elements are preferably controlled in the following ranges. P ⁇ 0.03%, S ⁇ 0.005%, Ni ⁇ 0.15%, W ⁇ 0.05%, V ⁇ 0.2%, Cu ⁇ 0.1%, Al ⁇ 0.01%, Ti ⁇ 0.01%, N ⁇ 0.05%, and O ⁇ 0.05%.
  • a ratio of a diffraction peak area from an fcc phase (a sum total of diffraction peak areas from (200) plane, (220) plane, and (311) plane) to a diffraction peak area from a bcc phase (a sum total of diffraction peak areas from (200) plane and (211) plane) (the diffraction peak area from the fcc phase/the diffraction peak area from the bcc phase) in an X-ray diffraction of the intermediate material for stainless steel for cutlery having the above-described composition is correlated with the toughness of a stainless steel for cutlery obtained using the intermediate material through a manufacturing process including annealing, cold rolling, hardening, and tempering.
  • the ratio of the diffraction peak area from the fcc phase to the diffraction peak area from the bcc phase is more than 30 in the X-ray diffraction of the intermediate material for stainless steel for cutlery, the amount of the fcc phase becomes too much.
  • carbides having a high aspect ratio tend to precipitate at the grain boundaries.
  • toughness markedly decreases after hardening and tempering performed before used as cutlery. Therefore, the ratio of the diffraction peak area from the fcc phase to the diffraction peak area from the bcc phase must be 30 or less.
  • the ratio is preferably 20 or less, and more preferably 5 or less.
  • the lower limit may be 0.
  • (200) plane, (220) plane, and (311) plane of an fcc phase and (200) plane and (211) plane of a bcc phase are chosen in the invention is that the above-described direction is a major peak in an X-ray diffraction of the alloy system having the composition defined in the invention.
  • the peaks other than the major peak has a low peak intensity, so that the ratio of the diffraction peak area from the fcc phase to the diffraction peak area from the bcc phase is not so influential. Therefore, the measurement of the major peak will suffice.
  • the ratio of the diffraction peak area from the fcc phase to the diffraction peak area from the bcc phase is correlated with the actual volume ratio between these phases, it can be evaluated as the value corresponding to the proportion of the constituting phases.
  • a vertical section of the intermediate material is irradiated with an X-ray.
  • the term “vertical section” herein means, as shown in FIG. 1 , in the surfaces of the test piece sampled in the vicinity of the center of the width of the intermediate material for stainless steel for cutlery 1 , the cross section corresponding to the evaluated plane 2 shown in FIG. 1 , more specifically the cross section perpendicular to the width direction of the intermediate material.
  • the reason why the vertical section is used for evaluation is that the rolled material has anisotropy that is dependent on the rolling direction, so that the fixity of the evaluated plane allows evaluation under the same conditions.
  • the vertical section most often corresponds to the blade edge which is most required to have both of high strength and toughness.
  • the test piece used for the X-ray diffraction is subjected to mirror plane polishing on the vertical section, and further subjected to electrolytic polishing so as to be adjusted to a test piece for the X-ray diffraction. Subsequently, the electrolytically polished plane is subjected to the X-ray diffraction, and then the ratio of the sum total of diffraction peak areas of (200) plane, (220) plane, and (311) plane of an fcc phase to the sum total of diffraction peak areas of (200) plane and (211) plane of a bcc phase is calculated. In a case in which the area of each diffraction peak is determined, the area of the diffraction peak from which the background strength is subtracted is determined.
  • the following section describes the methods of the invention for manufacturing the intermediate material for stainless steel for cutlery, the annealed material for stainless steel for cutlery, and the cold-rolled steel strip for stainless steel for cutlery.
  • a material for stainless steel for cutlery is manufactured by melting and casting.
  • the melting include a vacuum melting, an atmospheric melting, a vacuum arc remelting, and an electroslag remelting.
  • the casting include die casting and continuous casting. If necessary, the material thus obtained may be subjected to homogenization heat treatment. Furthermore, the material may be subjected to a blooming process by hot forging or hot rolling.
  • the material is hot-rolled, thereby manufacturing the intermediate material for stainless steel for cutlery.
  • the intermediate material for stainless steel for cutlery is manufactured by hot rolling at a heating temperature of from 1100 to 1250° C., wherein the end temperature of hot rolling is from 700 to 1000° C.
  • the reason why the heating temperature is from 1100 to 1250° C. is that this temperature range has relatively low deformation resistance and marked hot workability.
  • the temperature range When the temperature is higher than 1250° C., the temperature range extremely decreases ductility, and tends to cause cracking during hot working.
  • the temperature When the temperature is lower than 1100° C., deformation resistance of the material during hot rolling is high, working with a high reduction ratio is difficult, so that reheating must be repeated during hot working.
  • the lower limit of the heating temperature is preferably 1150° C.
  • the reason why the end temperature of hot rolling is from 700 to 1000° C. is that the phase and metal structure are controlled in consideration of the hot workability during manufacturing the intermediate material for stainless steel for cutlery. If the end temperature of hot rolling is higher than 1000° C., the ratio of the diffraction peak area from the fcc phase to the diffraction peak area from the bcc phase is more than 30 in X-ray diffraction, so that the amount of the fcc phase becomes too much. If the amount of the fcc phase is too much, carbides tend to precipitate at the grain boundaries of the fcc phase during annealing performed thereafter. In addition, if the end temperature is too high, residual strain is low, and the crystal grain diameter tends to increase, so that carbides which precipitate at the grain boundaries of the fcc phase during annealing tend to form networks.
  • the heating temperature in the hot rolling process is from 1100 to 1250° C.
  • the end temperature of hot rolling is from 700 to 1000° C.
  • the lower limit of the heating temperature is preferably 1150° C.
  • the upper limit of the end temperature is preferably 950° C.
  • the upper limit of the end temperature is more preferably 900° C.
  • the lower limit of the end temperature is preferably 750° C.
  • the intermediate material for stainless steel for cutlery obtained by the hot rolling wherein the end temperature of hot working is from 700 to 1000° C. is preferably cooled with water during passing the runout table, and/or during winding with a winding device.
  • the cooling is preferably performed with a water flow which can cool the wound coil to 600° C. or lower within 5 minutes from the finishing point of the last pass of hot rolling.
  • the wound coil is once cooled on the surface exposed to atmosphere, but the temperature of the once cooled coil surface is increased again by the heat retained by the coil, so that the metal structures can differ between the front end, center, and rear end of the coil.
  • the temperature of a part of the wound coil exceeds 600° C. within 5 minutes from the finishing point of the final pass of hot rolling, toughness of cutlery may decrease.
  • the wound coil be cooled with water to 600° C. or lower within 5 minutes from the finishing point of the final pass of hot rolling.
  • the intermediate material for stainless steel for cutlery manufactured by the above-described manufacture method is subjected to the first annealing at from 700° C. to 860° C. for from 1 to 100 hours, and thus manufacturing an annealed material for stainless steel for cutlery in which carbides are precipitated.
  • a cold-rolled steel strip for stainless steel for cutlery having a thickness of 1.0 mm or less is obtained.
  • subzero treatment may be performed after hardening, and surface coating may be performed after tempering.
  • the materials to be hot-rolled having a width of 45 mm, a length of 1000 mm, and a thickness of 20 mm were made from the steel ingots A to D. These materials to be hot-rolled are subjected to hot rolling under the following three types of conditions, and the intermediate materials of stainless steel for cutlery were obtained.
  • the alloy C which had been hot-rolled under the conditions (2) caused cracking during hot rolling, so that the operation was suspended.
  • test piece was sampled in the vicinity of the center of the width of each of the intermediate material for stainless steel for cutlery.
  • the sampling position of the test piece is the position shown in FIG. 1 , and the vertical section as the evaluated plane 2 is subjected to an observation of a metal structure, an X-ray diffraction, and an evaluation of hardness.
  • test piece used for the X-ray diffraction was subjected to mirror polishing on the vertical section, and further subjected to electrolytic polishing to be suitable as a test piece for the X-ray diffraction.
  • FIGS. 2 to 7 show the photographs as substitutes of drawings showing the metal structure of the materials Nos. 1 to 6 whose metal structures were observed.
  • Table 2 shows hardness and the ratio of the diffraction peak area from the fcc phase to the diffraction peak area from the bcc phase in X-ray diffraction.
  • Table 3 shows the diffraction peak area from the fcc and bcc phases for each plane index.
  • the observation of the metal structure was performed by mirror-polishing the vertical section of the test piece, corroding it in a ferric chloride aqueous solution, and then observing it using a scanning electron microscope.
  • the ratio of the diffraction peak area from the fcc phase to the diffraction peak area from the bcc phase in the X-ray diffraction was determined by a ratio of a diffraction peak area from an fcc phase (a sum total of diffraction peak areas from (200) plane, (220) plane, and (311) plane) to a diffraction peak area from a bcc phase (a sum total of diffraction peak areas from (200) plane and (211) plane) (the diffraction peak area from the fcc phase/the diffraction peak area from the bcc phase) in an X-ray diffraction.
  • the X-ray diffraction measurement used RINT2500V manufactured by Rigaku Corporation, the radiation source used Co.
  • the hardness was determined by polishing the sampled test piece with #1200 abrasive paper, and then measuring a hardness using Vickers hardness tester under a load of 98.1 (N). The hardness is the average of five points.
  • An intermediate material for sampling a test piece having a width of 40 mm, a length of 100 mm, and a thickness of 1 mm was cut out from each of the intermediate material for stainless steel for cutlery. At this time, the intermediate material for sampling a test piece was cut out so as to contain the vicinity of the center of the width of the rolled material shown in FIG. 1 .
  • the intermediate material for sampling a test piece was annealed at 840° C. for 5 hours to make an annealed material, and then, for hardening, maintained at 1100° C. for 3 minutes and water-cooled. Furthermore, after the hardening, the material was maintained at ⁇ 75° C. for 30 minutes for subzero treatment, and then tempered at 150° C. for 3 minutes.
  • test pieces for three point bending test having a width of 5 mm, a length of 70 mm, and a thickness of 0.5 mm were made from the intermediate materials for sampling a test piece after the above-described heat treatment.
  • FIGS. 8 to 13 show the photographs of the metal structure (Nos. 1 to 6).
  • Table 4 shows hardness, absorption energy by the three point bending test, bending strength by the three point bending test, and deflection by the three point bending test.
  • the observation of the metal structure was performed by mirror-polishing the vertical section of the test piece, corroding it in a ferric chloride aqueous solution, and then observing it using a scanning electron microscope.
  • a test piece having a width of 5 mm, a length of 70 mm, and a thickness of 0.5 mm was measured over a span of 50 mm.
  • the data of the hardness and three point bending test are the average of five points.
  • the material in a case in which the ratio of the diffraction peak area from the fcc phase to the diffraction peak area from the bcc phase is 30 or less in the X-ray diffraction of the intermediate material for stainless steel for cutlery after heat treatment including annealing-hardening-subzero treatment-tempering, the material has high toughness, which is evaluated by the absorption energy in the three point bending test, while maintaining high hardness even after heat treatment including annealing-hardening-subzero treatment-tempering.
  • the reason for this is likely due to the difference in the metal structure after heat treatment including annealing-hardening-subzero treatment-tempering.
  • the ratio of the diffraction peak area from the fcc phase to the diffraction peak area from the bcc phase is more than 30 in the X-ray diffraction of the rolled material for stainless steel for cutlery, as shown in FIGS. 9 and 11 , the carbides having a high aspect ratio are linked at the boundaries in the rolled material for stainless steel for cutlery after heat treatment, which likely causes the decrease in toughness.
  • the carbides after heat treatment are more dispersed, so that the structure is likely more preferable from the viewpoint of toughness.
  • cutlery having high hardness and high toughness is produced from the intermediate material for stainless steel for cutlery of the invention by subjecting it to heat treatment including annealing-hardening-subzero treatment-tempering.
  • the inclusion of B in the proportion of 0.0050% or less further improves the balance between hardness and toughness.
  • an intermediate material for sampling a test piece having a width of 40 mm, a length of 100 mm, and a thickness of 1.5 mm was cut out from each of the intermediate materials of stainless steel for cutlery shown in the above-described Nos. 1 and 3.
  • the intermediate material for sampling a test piece so as to contain the vicinity of the center of the width of the intermediate material shown in FIG. 1 .
  • the intermediate material for sampling a test piece was annealed at 840° C. for 10 hours, and then cold-rolled using a compact cold rolling machine; a cold-rolled material was obtained without rupture until the thickness became 0.075 mm. As a result of this, it was confirmed that a cold-rolled steel strip can also be obtained.
  • the cold-rolled material was, for hardening, maintained at 1100° C. for 40 seconds and then water-cooled. Furthermore, after the above-described hardening, the material was maintained at ⁇ 75° C. for 30 minutes for subzero treatment, and then maintained at 150° C. for 30 seconds for tempering.
  • test pieces were sampled from the position corresponding to the position shown in FIG. 1 (in the vicinity of the center of the width of the rolled material), and the test pieces were sampled in such a manner that the vertical section as the evaluated plane 2 is used for the observation of the metal structure and the evaluation of hardness.
  • FIGS. 14 to 17 show the photographs as substitutes of drawings showing the metal structure before and after the above-described heat treatment (Nos. 1 and 3), and Table 5 show the hardness.
  • results shown in FIGS. 14 to 17 and Table 5 indicate that the cold-rolled material of the invention, and the stainless steel for cutlery after hardening, subzero treatment, and tempering contained no carbide having a high aspect ratio which can deteriorate toughness, and spherical carbides contributing to good toughness are finely dispersed in them.
  • the hardness after hardening, subzero treatment, and tempering was 790 HV or more.
  • the cutlery made from the intermediate material and cold-rolled steel strip for stainless steel for cutlery of the invention has marked hardness and toughness, and thus can be used for razors and other applications.

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