EP4098765A1 - Acier inoxydable martensitique à haut pouvoir anti-corrosif et son procédé de fabrication - Google Patents

Acier inoxydable martensitique à haut pouvoir anti-corrosif et son procédé de fabrication Download PDF

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EP4098765A1
EP4098765A1 EP20925160.2A EP20925160A EP4098765A1 EP 4098765 A1 EP4098765 A1 EP 4098765A1 EP 20925160 A EP20925160 A EP 20925160A EP 4098765 A1 EP4098765 A1 EP 4098765A1
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Prior art keywords
stainless steel
martensitic stainless
chromium
temperature
hot
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German (de)
English (en)
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EP4098765A4 (fr
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Byoung-Jun Song
Junghyun KONG
Yongho Kim
Seongin JEONG
Gyujin JO
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Posco Holdings Inc
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Posco Co 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
    • C22C38/22—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
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • 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
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/18—Hardening; Quenching with or without subsequent tempering
    • 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
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/18—Hardening; Quenching with or without subsequent tempering
    • C21D1/25—Hardening, combined with annealing between 300 degrees Celsius and 600 degrees Celsius, i.e. heat refining ("Vergüten")
    • 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
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/26—Methods of annealing
    • 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
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/78—Combined heat-treatments not provided for above
    • 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
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/002—Heat treatment of ferrous alloys containing Cr
    • 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
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/005—Heat treatment of ferrous alloys containing Mn
    • 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
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/008—Heat treatment of ferrous alloys containing Si
    • 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
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/02—Hardening by precipitation
    • 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
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/04—Hardening by cooling below 0 degrees Celsius
    • 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
    • C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips of ferrous alloys
    • 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
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0226—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
    • 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
    • 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
    • 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
    • C21D8/0273—Final recrystallisation annealing
    • 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
    • C22C38/04—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
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • 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
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/004—Dispersions; Precipitations
    • 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
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/005—Ferrite
    • 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
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/008—Martensite

Definitions

  • the present disclosure relates to a highly anticorrosive martensitic stainless steel and a manufacturing method therefor, and more particularly, to a highly anticorrosive martensitic stainless steel applicable as a material for tableware and a manufacturing method therefor.
  • edge tools In general, materials for edge tools widely used in our daily lives such as knives, scissors, razors, and scalpels, which are medical instruments, require high hardness in order to maintain cutting performance and abrasion resistance and require excellent corrosion resistance because they are used in contact with moisture or stored in a humid atmosphere. Accordingly, high carbon martensitic stainless steels having high hardness are widely used as the materials for edge tools.
  • edge tools Materials for edge tools that require high hardness are highly brittle. Thus, the materials for edge tools need to be softened to a certain level or more for easy processing. To this end, edge tools are manufactured by using a batch annealing furnace (BAF) or a high-temperature continuous annealing process to facilitate heat treatment of a brittle material.
  • BAF batch annealing furnace
  • a high-temperature continuous annealing process to facilitate heat treatment of a brittle material.
  • fine particles of chromium carbide are distributed and precipitated in a ferrite matrix as a result of reactions between carbon and chromium.
  • the resultant material may be easily applied to a stainless steel manufacturing process such as rolling and acid pickling.
  • the fine chromium carbide particles uniformly distributed in the ferrite matrix enable rapid resolidification of chromium and carbon to an austenite phase of high temperature during a hardening heat treatment process performed by an edge tool manufacturer and improve hardness and corrosion resistance of the martensitic stainless steel after quenching.
  • a hot-rolled, annealed martensitic stainless steel sheet having improved corrosion resistance by uniformly distributing fine chromium carbonitride in a matrix and having an appropriate hardness during hardening heat treatment, a highly anticorrosive martensitic stainless steel using the same, and a manufacturing method therefor.
  • a hot-rolled, annealed highly anticorrosive martensitic stainless steel sheet includes, in percent by weight (wt%), 0.14 to 0.21% of C, 0.05 to 0.11% of N, 0.1 to 0.6% of Si, 0.4 to 1.2% of Mn, 14.0 to 17.0% of Cr, 0.2 to 0.32% of C+N, and the balance of Fe and inevitable impurities, wherein chromium carbide or chromium nitride is distributed in a microstructure at a density of 25 particles/100 ⁇ m 2 , a precipitation temperature of chromium carbide is 950°C or lower, and a PREN value, represented by Formula (1) below, is 16 or more: Cr + 3.3 Mo + 16 N (wherein Cr, Mo, and N represent contents (wt%) of each alloying element).
  • the hot-rolled, annealed highly anticorrosive martensitic stainless steel sheet may have an elongation of 20% or more.
  • a highly anticorrosive martensitic stainless steel includes, in percent by weight (wt%), 0.14 to 0.21% of C, 0.05 to 0.11% of N, 0.1 to 0.6% of Si, 0.4 to 1.2% of Mn, 14.0 to 17.0% of Cr, 0.2 to 0.32% of C+N, and the balance of Fe and inevitable impurities, wherein a PREN value, represented by Formula (1) below, is 16 or more, a value, represented by Formula (2) below, is 950 or less: Cr + 3.3 Mo + 16 N 674 + 569 C ⁇ 4.17 Si + 0.46 Mn + 10.3 Cr + 193 N (wherein Cr, Mo, N, C, Si, and Mn represent contents (wt%) of each alloying element).
  • the highly anticorrosive martensitic stainless steel may have a Rockwell hardness of 47 to 53 HRC.
  • the highly anticorrosive martensitic stainless steel may have a pitting potential is 180 mV or more in a 3.5% NaCl aqueous solution at 25°C.
  • a method for manufacturing a highly anticorrosive martensitic stainless steel includes: hot rolling a slab including, in percent by weight (wt%), 0.14 to 0.21% of C, 0.05 to 0.11% of N, 0.1 to 0.6% of Si, 0.4 to 1.2% of Mn, 14.0 to 17.0% of Cr, 0.2 to 0.32% of C+N, and the balance of Fe and inevitable impurities; batch annealing the hot-rolled steel material; and hardening heat-treating the hot-rolled, annealed steel material, wherein the batch annealing includes s a first cracking process performed in a temperature range of 720 to 900°C for 5 to 25 hours and a second cracking process performed in a temperature range of 500 to 700°C for 5 to 15 hours, and the hot-rolled, annealed steel material includes ferrite as a matrix in which chromium carbide or chromium nitride is distributed at a density of 25 particles/100
  • the batch annealing may further include a pre-cracking process performed in a temperature range of 400 to 600°C for 5 to 10 hours before the first cracking process.
  • the temperature may be raised at a rate of 40 to 200°C/h after the pre-cracking process until the first cracking process.
  • the temperature may be lowered at a rate of 10°C/h or more after the first cracking process until the second cracking process.
  • the hardening heat-treating may include an austenitizing treatment process performed at a temperature of 1,000°C or higher for 1 minute or more, and a quenching process to room temperature at a rate of 0.15°C/s or more.
  • the hardening heat-treating may further include a deep freezing process performed in a temperature range of -150 to -50°C for 10 seconds to 5 minutes and a tempering process performed in a temperature range of 400 to 600°C for 30 minutes to 2 hours, after the quenching process.
  • the hot-rolled, annealed martensitic stainless steel sheet according to the present disclosure may have enhanced workability by uniformly distributing fine chromium carbide in a microstructure.
  • a chromium carbide may not be retained after hardening heat treatment by lowering the precipitation temperature of a carbide, and thus excellent corrosion resistance may be obtained even without containing relatively high contents of chromium and carbon.
  • a martensitic stainless steel having a hardness suitable for tableware may be provided.
  • FIG. 1 is a scanning electron microscope (SEM) image of chromium carbide of a microstructure of a hot-rolled, annealed steel sheet of Steel Type F.
  • FIG. 2 is an SEM image of chromium carbide of a microstructure of a hot-rolled, annealed steel sheet of Steel Type B after hardening heat treatment.
  • FIG. 3 is an SEM image of chromium carbide of a microstructure of a hot-rolled, annealed steel sheet of Steel Type F after hardening heat treatment.
  • a hot-rolled, annealed highly anticorrosive martensitic stainless steel sheet includes, in percent by weight (wt%), 0.14 to 0.21% of C, 0.05 to 0.11% of N, 0.1 to 0.6% of Si, 0.4 to 1.2% of Mn, 14.0 to 17.0% of Cr, 0.2 to 0.32% of C+N, and the balance of Fe and inevitable impurities, wherein chromium carbide or chromium nitride is distributed in a microstructure at a density of 25 particles/100 ⁇ m 2 , a precipitation temperature of chromium carbide is 950°C or lower, and a PREN value, represented by Formula (1) below, is 16 or more: Cr + 3.3 Mo + 16 N (wherein Cr, Mo, and N represent contents (wt%) of each alloying element).
  • fine chromium carbide and/or chromium nitride hereinafter, referred to as chromium carbonitride
  • chromium carbonitride fine chromium carbide and/or chromium nitride
  • hardening heat treatment is performed for rapid resolidification into a high-temperature austenite phase.
  • chromium carbonitride is easily resolidified and the following conditions are required to obtain a martensite structure having excellent corrosion resistance.
  • fine chromium carbonitride should be formed in a ferrite matrix of a hot-rolled, annealed steel material, and then a precipitation temperature thereof should be low.
  • a precipitation temperature of chromium carbonitride is high due to a high C content of 0.3% or more and coarse chromium carbonitride is locally formed because the chromium carbonitride are precipitated and grow preferentially in grain boundaries, and thus a resolidification rate into an austenite phase decreases during hardening heat treatment, thereby causing deterioration of hardness and corrosion resistance.
  • the precipitation temperature of chromium carbonitride is high even when carbon is contained in an amount of 0.2 to 0.3%, a higher temperature should be applied to completely decompose chromium carbonitride during hardening heat treatment, and thus a lot of energy is consumed by a final manufacturer to increase a hardening heat treatment temperature, thereby increasing energy costs, or the chromium carbonitride is retained due to limited heating capacity of a heat treatment furnace.
  • the carbide acts as an origin of corrosion, and thus expected enhancement of corrosion resistance may not be obtained even by adding a high content of chromium.
  • the present disclosure provides an alloy composition of a highly anticorrosive martensitic stainless steel having enhanced corrosion resistance and appropriate hardness when hardening heat treatment is performed by uniformly distributing fine chromium carbonitride in a matrix by size regulation of a batch annealing pattern and by controlling a precipitation temperature of the chromium carbonitride at a low level for complete decomposition during hardening heat treatment.
  • a hot-rolled, annealed martensitic stainless steel sheet includes, in percent by weight (wt%), 0.14 to 0.21% of C, 0.05 to 0.11% of N, 0.1 to 0.6% of Si, 0.4 to 1.2% of Mn, 14.0 to 17.0% of Cr, 0.2 to 0.32% of C+N, and the balance of Fe and inevitable impurities.
  • the content of carbon (C) is from 0.14 to 0.21%.
  • C When the C content is low, hardness decreases after hardening heat treatment, so that cutting performance and abrasion resistance may be not obtained. Therefore, in the present disclosure, C may be added in an amount of 0.14% or more. However, an excess of C may cause excessive formation of the chromium carbonitride and increase the precipitation temperature, so that the chromium carbonitride retained after hardening heat treatment deteriorates corrosion resistance and increases a risk of formation of coarse carbide in an annealed structure due to carbon segregation. Therefore, an upper limit of the C content is controlled to 0.21% in the present disclosure. More preferably, the C content may be in the range of 0.145 to 0.17%.
  • the content of nitrogen (N) is from 0.05 to 0.11%.
  • N does not cause local fine segregation so as not to form coarse precipitates in a product when added thereto instead of C.
  • N may be added in an amount of 0.05% or more, preferably 0.08% or more.
  • the N content is excessive out of melting capacity of a molten steel during casting, and thus it may be difficult to control the alloying elements and pin hole defects may occur on the surface.
  • the martensitic stainless steel for tableware according to the present disclosure does not require a high hardness exceeding a Rockwell hardness of 53 HRC but requires a high gloss for aesthetic properties. Therefore, an upper limit of the N content may be controlled to 0.11%.
  • the content of silicon (Si) is from 0.1 to 0.6%.
  • Si is an element essentially added for deoxidation.
  • Si may be added in an amount of 0.1% or more in the present disclosure.
  • an excess of Si deteriorates acid pickling performance, thereby increasing embrittlement. Therefore, an upper limit of the Si content may be controlled to 0.6% in the present disclosure.
  • the content of manganese (Mn) is from 0.4 to 1.2%.
  • Mn is an element essentially added for deoxidation.
  • Mn is added in an amount of 0.4% or more to compensate for stability of austenite decreased by the lowered contents of C and Ni and to obtain solid solubility of N.
  • an excess of Mn may deteriorate the surface quality of the steel and form retained austenite in a finally heat-treated material and thus it may be difficult to obtain hardness. Therefore, an upper limit of the Mn content may be controlled to 1.2%. More preferably, the Mn content may be in the range of 0.8 to 1.1%.
  • the content of chromium (Cr) is from 14.0 to 17.0%.
  • Cr is a representative element enhancing corrosion resistance of a stainless steel and increasing solid solubility of N.
  • Cr is added in an amount of 14.0% or more to obtain sufficient corrosion resistance.
  • an excess of Cr may increase manufacturing costs and increase fine segregation of the Cr component in the structure to cause local coarsening of chromium carbonitride, thereby reducing corrosion resistance and hardness of the hardening heat-treated material. Therefore, an upper limit of the Cr content is controlled to 17.0% in the present disclosure.
  • the Cr content may be controlled to be greater than 14.5% and less than 15.5%.
  • a sum of the C content and the N content is from 0.2 to 0.32%.
  • C and N may be added in an amount of 0.2% or more to obtain hardness of a steel after hardening heat treatment, preferably, in an amount of 0.23% or more to obtain the number of carbonitride particles.
  • an upper limit of the C+N content is controlled to 0.32%.
  • a high hardness excessing a hardness of 53 HRC which is required for general-use edge tools, is not required and a high gloss is required for aesthetic properties.
  • the upper limit of the C+N content may be controlled to 0.28% to prevent excessive hardening and adjust hardness to an appropriate range.
  • the remaining component of the composition of the present disclosure is iron (Fe).
  • the composition may include unintended impurities inevitably incorporated from raw materials or surrounding environments, and thus addition of other alloy components is not excluded. These impurities are known to any person skilled in the art of manufacturing and details thereof are not specifically mentioned in the present disclosure.
  • the hot-rolled, annealed martensitic stainless steel sheet and the hardening heat-treated martensitic stainless steel according to the present disclosure may have a pitting resistance equivalent number (PREN), represented by Formula (1) below, of 16 or more. Cr + 3.3 Mo + 16 N
  • a method for manufacturing a hot-rolled, annealed martensitic stainless steel sheet in which fine chromium carbonitride is distributed before hardening heat treatment will be described.
  • a hot-rolled martensitic stainless steel material having the above-described alloy composition is subjected to continuous casting or steel ingot casting to prepare a slab and the slab is hot-rolled to prepare a hot-rolled steel sheet ready for processing. Subsequently, the prepared hot-rolled steel sheet is batch-annealed, to obtain excellent workability, before starting processing such as fine rolling to a thickness applicable to edge tools. After the batch annealing, the microstructure may include ferrite as a matrix, and fine chromium carbide may be uniformly distributed therein.
  • the hot-rolled, annealed martensitic stainless steel material is manufactured into a martensitic stainless steel by subsequent hardening heat treatment.
  • the batch annealing includes a first cracking process and a second cracking process. Also, the batch annealing may optionally include a pre-cracking process before the first cracking process.
  • the pre-cracking process is a cracking step, previously performed before the first cracking process, to uniformly raise the temperature over the entire material.
  • the pre-cracking process may be performed in a temperature range of 400 to 600°C for 5 to 10 hours.
  • the heating temperature When the heating temperature is below 400°C or exceeds 600°C, the temperature cannot be uniformly raised over the entire material. In addition, when the heating time is less than 5 hours or exceeds 10 hours, the temperature cannot be uniformly raised over the entire material.
  • the first cracking process is a step of uniformly distributing chromium carbonitride in the microstructure of the hot-rolled steel sheet.
  • the first cracking process may be performed at a constant temperature in a temperature range of 720 to 900°C for 5 to 25 hours.
  • the heating temperature is below 720°C, agglomerates of chromium carbonitride may be formed locally in grain boundaries.
  • the heating temperature exceeds 900°C, coarse chromium carbonitride is formed in grain boundaries.
  • the heating time is less than 5 hours, the size of chromium carbonitride may be decreased, but chromium carbonitride may be distributed intensively in a portion.
  • the heating time exceeds 25 hours, chromium carbonitride particles adjacent to each other are combined to be locally coarsened.
  • Chromium carbide agglomerates or coarse chromium carbide may cause non-uniformity of a material, thereby deteriorating ductility and deteriorating rigidity ductility, and corrosion resistance of a final product.
  • the heating temperature of the first cracking process is controlled in the range of 720 to 900°C, and the heating time is controlled in the range of 5 to 25 hours.
  • the second cracking process is a step of spheroidizing chromium carbonitride.
  • spheroidizing chromium carbonitride workability of a subsequent processing operation may be improved.
  • the second cracking process may be performed at a constant temperature in a temperature range of 500 to 700°C for 5 to 15 hours.
  • a heating temperature of 500°C or higher is required for spheroidization of chromium carbonitride.
  • the heating temperature exceeds 700°C, spheroidized chromium carbonitride excessively grow and the number thereof decreases, thereby deteriorating ductility.
  • the heating time is less than 5 hours, chromium carbonitride is not spheroidized.
  • the heating time exceeds 15 hours, chromium carbonitride excessively grows, thereby deteriorating ductility.
  • the temperature may be raised at a rate of 40 to 200°C/h after the pre-cracking process until the first cracking process.
  • the heating rate is less than 40°C/h, a time required for passing through a temperature range of 700 to 750°C in which chromium carbonitride coarsens increases, and thus the chromium carbonitride coarsen and the number of the chromium carbonitride particles distributed in the microstructure decreases, thereby deteriorating ductility.
  • the heating rate exceeds 200°C/h, a time required for passing through the temperature range in which chromium carbonitride coarsens decreases, and thus fine chromium carbonitride may be obtained.
  • chromium carbonitride is non-uniformly distributed due to insufficient time for distribution of chromium carbonitride.
  • the temperature may be lowered at a rate of 10°C/h or more until the second cracking process.
  • air cooling may be performed.
  • the fine chromium carbonitride uniformly distributed in the microstructure by the above-described batch annealing process may enable rapid resolidification of carbon, nitrogen, and chromium to a high-temperature austenite phase during the subsequent hardening heat-treating process and improve hardness and corrosion resistance of the martensite structure after rapid cooling.
  • fine chromium carbonitride may be uniformly distributed in the microstructure of the hot-rolled, annealed martensitic stainless steel sheet by the above-described batch annealing process, and chromium carbonitride may be distributed in the microstructure at a density of 25 particles/100 ⁇ m 2 or more.
  • chromium carbonitride When the chromium carbonitride is distributed in the microstructure at a density less than 25 particles/100 ⁇ m 2 , ductility deteriorates due to the small number and the large size of the chromium carbonitride, and resolidification of chromium and carbon is difficult in the subsequent hardening heat treatment, and thus a desired hardness cannot be obtained.
  • the batch-annealed hot-rolled, annealed martensitic stainless steel material is subjected to the hardening heat treatment to prepare a martensitic stainless steel.
  • the hardening heat treatment may include an austenitizing treatment process, and a quenching process, and may further include a deep freezing process and a tempering process, if required.
  • the austenitizing treatment process is a step of transforming the matrix of the steel material from ferrite to austenite.
  • the chromium carbonitride is resolidified in the matrix in the form of chromium, carbon, and nitrogen, and thus hardness of the martensitic stainless steel may be enhanced after the subsequent quenching or deep freezing process.
  • the austenitizing treatment process may be performed by heat treatment at a temperature of 1,000°C or higher for 1 minute or more.
  • both of chromium and carbon may be resolidified during the austenitizing treatment in accordance with a precipitation temperature of the chromium carbide (Cr 23 C 6 ).
  • a desired precipitation temperature of the chromium carbide of the present disclosure is 950°C or lower.
  • the precipitation temperature of the chromium carbide may vary according to the composition of the alloying elements and may be expressed by Formula (2) below. As shown in Formula (2), as the contents of chromium and carbon increase, the precipitation temperature of the chromium carbide increases. 674 + 569 C ⁇ 4.17 Si + 0.46 Mn + 10.3 Cr + 193 N
  • the precipitation temperature of the chromium carbide increases, and thus there are limitations on the temperature range of austenitizing.
  • the chromium carbide may not be completely resolidified but may be retained in actual hardening heat treatment due to facility problems caused by limited heating capacity or an increase in energy costs. In this case, corrosion resistance may deteriorate. Therefore, in the present disclosure, all of the chromium and carbon added thereto may contribute to corrosion resistance by controlling the precipitation temperature of the chromium carbide to 950°C or lower as well as by adjusting the alloy composition.
  • the austenitizing treatment temperature is below 1,000°C, it may be difficult to completely decompose the chromium carbide and a treatment time may increase, thereby deteriorating economic feasibility. Meanwhile, when the austenitizing treatment temperature is too high, energy costs increase to deteriorate economic feasibility, the amount of resolidified carbide increases to excessively form retained austenite, thereby deteriorating hardness, and the grains grow excessively. Therefore, it is preferable to control the austenitizing treatment temperature to 1,200°C or lower.
  • the austenitizing treatment time is less than 1 minute, it is difficult to completely decompose chromium carbide making it difficult to obtain a desired hardness.
  • the austenitizing treatment time increases, grains grow excessively, thereby causing retained austenite. Therefore, it is preferable to control the austenitizing treatment time to 30 minutes or less.
  • the quenching process is a step of transforming the austenite structure to a martensite having a high hardness via rapid cooling to room temperature at a cooling rate of 0.15°C/s or more after the austenitizing treatment. By cooling at a cooling rate of 0.2°C/s or more, a higher martensite hardness may be obtained.
  • the deep freezing process is a step of additionally transforming the retained austenite structure to the martensite structure by further cooling the steel material quenched to room temperature to an extremely low temperature.
  • the hardness of the steel material may further be increased.
  • the deep freezing process may be performed by subzero heat treatment performed at a temperature of -150 to -50°C for 10 seconds to 5 minutes.
  • the tempering process is a step of imparting toughness to a martensite structure, which has high embrittlement due to high hardness, after the deep freezing process.
  • the tempering process may be performed in a temperature range of 400 to 600°C for 30 minutes to 2 hours.
  • the ferrite structure may be finally transformed into the martensite structure and desired hardness and corrosion resistance may be obtained.
  • an area fraction of chromium carbonitride retained in a cross-section of the material after resolidification by hardening heat-treating may be 2% or less.
  • the highly anticorrosive martensitic stainless steel according to an embodiment of the present disclosure may have a pitting potential of 180 mV or more in a 3.5% NaCl aqueous solution at 25°C. This may be obtained by completely resolidifying the carbide by controlling the PREN value, represented by Formula (1), to 16.0 or more and controlling the precipitation temperature of chromium carbide to 950°C or lower.
  • the highly anticorrosive martensitic stainless steel according to an embodiment of the present disclosure may have a Rockwell hardness of 47 to 53 HRC.
  • martensitic stainless steels for edge tools those for tableware do not require a high hardness and a high hardness exceeding 53 HRC is not required therefor because there may be a problem in working productivity during polishing to obtain gloss.
  • a hardness of 49 to 53 HRC is suitable for a blade and a hardness of 47 to 51 HRC is suitable for a handle for tableware knives. Therefore, in the present disclosure, the upper limit of the C+N content is controlled to 0.32%, and the contents of the alloying elements are limited to the above-described ranges to obtain an appropriate hardness even when they are completely resolidified by controlling the precipitation temperature of chromium carbide. Accordingly, the highly anticorrosive martensitic stainless steel of the present disclosure may have a Rockwell hardness of 47 to 53 HRC.
  • the batch annealing was conducted by performing a pre-cracking process at 500°C for 7 hours, raising the temperature at a rate of about 100°C/h, performing a first cracking process at 840°C for 10 hours, lowering the temperature at a rate of 15°C/h, maintaining at 580°C for 10 hours, and performing an air cooling process.
  • Table 1 shows precipitation temperature (°C) of chromium carbonitride and occurrence of pine holes on the surface of the steel materials caused by nitrogen gas as ⁇ and ⁇ .
  • Pine holes were formed on the surface of steel type B because a large amount of N, out of the range of the present disclosure, was added. Although the N content was appropriate, pin holes were formed in steel type E because the N content exceeds the solid solubility of N due to the low content of Cr, which affects solid solubility of nitrogen, and relatively low contents of C and Mn, which are austenite-stabilizing elements, thereby generating nitrogen gas. No pin holes were formed in steel type F having the alloy composition within the range of the present disclosure, and the precipitation temperature of the chromium carbide was low as 937°C and thus steel type F may be efficiently applied to hardening heat treatment which will be described below.
  • the precipitation temperature of the chromium carbide was 990°C or higher in the case where the contents of C and Cr were high, it was confirmed that the precipitation temperature was 950°C or lower when the alloy composition including C and Cr was within the range of the present disclosure.
  • steel type A included the C content of 0.6% or more, a large number of chromium carbonitride particles were observed at a density of 60/100 ⁇ m 2 or more, but a very low elongation of 17.6% was observed.
  • steel type B and C had higher C contents of about 0.25%, the N contents thereof were different.
  • steel type B had a higher C+N content than that of steel type C, the number of carbonitride particles was 21 particles/100 ⁇ m 2 which is smaller than that of steel type C. This is estimated because the chromium carbonitride coarsened due to a too high fraction of the precipitated chromium carbonitride.
  • steel type B had a slightly low elongation of 19.6% due to the high C+N content.
  • Steel type C had a large number of chromium carbonitride particles of 32 particles/100 ⁇ m 2 and a high elongation of 29.3%, there is a high possibility of residual chromium carbonitride after hardening heat treatment due to the high precipitation temperature of chromium carbide of 991°C.
  • FIG. 1 is a scanning electron microscope (SEM) image of chromium carbonitride of a microstructure of a hot-rolled, annealed steel sheet of Steel Type F. It was confirmed that fine chromium carbonitride are uniformly distributed in the ferrite matrix of steel type F corresponding to the hot-rolled, annealed steel material according to the inventive steel of the present disclosure. As shown in Table 2, as well as the chromium carbide distribution at a density of about 30/100 ⁇ m 2 , a high elongation of 30.2% was measured.
  • the hot-rolled, annealed martensitic stainless steel material was subjected to austenitizing treatment at 1,050°C and quenching at a cooling rate of 0.27°C/s to prepare a martensitic stainless steel.
  • Table 3 PREN and pitting potential values are shown to evaluate corrosion resistance, and Rockwell hardness is shown to evaluate hardness.
  • the PREN value was derived by substituting the content (wt%) of each alloying element into Formula (1) and the pitting potential was measured in a 3.5% NaCl aqueous solution at 25°C.
  • steel type B in which nitrogen gas pin holes were formed by adding N exceeding the solid solubility thereof exhibited the highest PREN value and pitting potential due to the influence of N, steel type B could not be applied to products due to pin holes formed on the surface.
  • steel type C had a PREN value of 17.21 and a high pitting potential of 212 mV, a hardness of 54.7 HRC, which exceeds an appropriate range of 47 to 53 HRC required to prevent occurrence of surface defects during a polishing process to obtain gloss, was obtained due to the high C content.
  • Steel type F corresponding to the inventive steel according to the present exhibited a PREN value of 16.52, which is greater than 16.0, a high pitting potential value of 199 mV, and an appropriate hardness value of 51.4 HRC.
  • FIGS. 2 and 3 are SEM images of chromium carbide of microstructures of hot-rolled steel sheets of steel type B and steel type F after hardening heat treatment. Because steel type B shown in FIG. 2 had a high C+N content, the chromium carbonitride could not be uniformly distributed in the hot-rolled, annealed steel material but coarsen to be segregated. Also, it was confirmed that the chromium carbide could not be resolidified but retained even after hardening heat treatment due to the high precipitation temperature of chromium carbide.
  • the martensitic stainless steel according to the present disclosure has improved corrosion resistance and appropriate hardness by hardening heat treatment and thus applicable to a material for tableware.

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