EP4265784A1 - Acier inoxydable martensitique ayant une résistance et une résistance à la corrosion améliorées, et son procédé de fabrication - Google Patents

Acier inoxydable martensitique ayant une résistance et une résistance à la corrosion améliorées, et son procédé de fabrication Download PDF

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EP4265784A1
EP4265784A1 EP21911281.0A EP21911281A EP4265784A1 EP 4265784 A1 EP4265784 A1 EP 4265784A1 EP 21911281 A EP21911281 A EP 21911281A EP 4265784 A1 EP4265784 A1 EP 4265784A1
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hot
stainless steel
martensitic stainless
carbide
rolled
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Byoung-Jun Song
Youngjin Kwon
Gyujin JO
Nayeon CHU
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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
    • C22C38/40—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
    • 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
    • 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
    • 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
    • C21D8/0236—Cold 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
    • 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
    • C22C38/02—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
    • C22C38/18—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
    • 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
    • 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 martensitic stainless steel and a method of manufacturing the same, and more particularly, to a martensitic stainless steel applicable to various parts such as home appliances, automobile compressor parts, and doctor blades, and a method of manufacturing the same.
  • stainless steels are classified according to their chemical composition or metal structure. Based on metal structure, stainless steel may be classified into an austenite system, a ferrite system, a martensite system, and a dual-phase system.
  • Martensitic stainless steel which has excellent hardness and wear resistance but is very brittle and has low elongation, has a different carbon content depending on the application.
  • a brake disc and an anchor which do not require high wear resistance, use 0.1% or less of carbon
  • type 1 cutlery use 0.1 to 0.3% of carbon
  • kitchen knives, scissors, and surgical knives which require high wear resistance, use 0.3 to 0.7% of carbon
  • industrial knives use 1% or more of carbon.
  • STS 420 a representative martensitic stainless steel with 12 to 15% of chromium, is the most widely used because of its excellent strength, hardness, and corrosion resistance.
  • martensitic stainless steels utilize a tempered martensitic structure in which the austenitic phase, which is a high-temperature stable phase, is formed by introducing a hardening heat treatment into a microstructure in which chromium carbide is dispersed in a ferrite base after annealing, and then generated by rapid cooling.
  • the tempered martensite is a very hard structure and its hardness increases as the content of dissolved carbon increases.
  • wear resistance of the martensitic stainless steel may be ensured by a certain fraction of carbides remaining or precipitating after heat treatment. Carbon reacts with chromium to precipitate in the form of chromium carbide, so as the concentration of Cr in a base decreases, corrosion resistance decreases.
  • martensitic stainless steel with high brittleness needs to be softened to facilitate machining and is therefore subjected to a batch annealing furnace (BAF) process for easy heat treatment workability.
  • BAF batch annealing furnace
  • a thermal history deviation occurs in a longitudinal direction thereof.
  • the heating and cooling rates are the slowest, so that the size of carbides becomes coarse, the deviation is maintained after cold rolling, causing the deviation in the physical properties of a final material.
  • the present disclosure provides a martensitic stainless steel with improved strength and corrosion resistance while ensuring hardness by optimizing a content of Mo and V, and a method of manufacturing the same.
  • One aspect of the present disclosure provides a hot-rolled annealed martensitic stainless steel sheet with improved strength and corrosion resistance comprising, in percent by weight (wt%), 0.3 to 0.5% of C, 0.01 to 0.025% of N, 0.3 to 0.5% of Si, 0.4 to 0.6 of Mn, 13.1 to 14.5% of Cr, 0.95 to 1.10% of Mo, 0.05 to 0.3% of V, 0.3 to 0.5% of Ni, 0.001 to 0.5% of Cu, and the reminder of Fe and inevitable impurities, and satisfying Formula (1) below: 16.4 ⁇ Cr + 3.3 Mo + 16 N * Mo+V ⁇ 23.3 wherein Cr, N, Mo, and V denote contents (wt%) of elements, respectively.
  • Formula (2) below may be satisfied: ⁇ 14 ⁇ ⁇ 36442 + 248 C + 365 Cr + 373 Mo + 530 V + 365 Fe + 350 Si + 312 Mn + 331 Ni + 506 C ⁇ 50 wherein C, Cr, Mo, V, Fe, Si, Mn, Ni, and Cu denote wt% of the respective elements.
  • Formula (3) below may be satisfied: 0.37 ⁇ C + N ⁇ 0.43 .
  • Formula (4) below may be satisfied: 1.0 ⁇ Mo + V ⁇ 1.35 .
  • the hot-rolled annealed martensitic stainless steel sheet may further include a ferrite as a base structure, a primary carbide represented by (Cr, Fe, Mo, V) 7 C 3 , and a secondary carbide represented by (Cr, Fe, Mo, V) 23 C 6 .
  • the wt% of (Mo+V) in the primary carbide may be 2.93 to 5.67%.
  • the wt% of (Mo+V) in the secondary carbide may be 12.2 to 14.8%.
  • the particle size of the primary carbide may be 10 ⁇ m or less.
  • carbide deviation in a longitudinal direction may be 10 pieces/100 ⁇ m 2 or less.
  • the distribution density of carbide may be 42 to 58 pieces/100 ⁇ m 2 .
  • Another aspect of the present disclosure provides a method of manufacturing a martensitic stainless steel with improved strength and corrosion resistance.
  • the method includes hot-rolling a slab including, in percent by weight (wt%), 0.3 to 0.5% of C, 0.01 to 0.025% of N, 0.3 to 0.5% of Si, 0.4 to 0.6% of Mn, 13.1 to 14.5% of Cr, 0.95 to 1.10% of Mo, 0.05 to 0.3% of V, 0.3 to 0.5% of Ni, 0.001 to 0.5% of Cu, and the reminder of Fe and inevitable impurities, and satisfying the Formula (1) below; batch annealing in a temperature range of 600 to 900°C immediately after hot-rolled; cold rolling the hot-rolled annealed material; and hardening heat treatment the cold-rolled material; wherein Formula (1): 16.4 ⁇ (Cr+3.3Mo+16N)*(Mo+V) ⁇ 23.3 in Formula (1), Cr, N, Mo, and V denote contents (wt%) of elements, respectively.
  • the hot-rolled annealed material may further include a ferrite as a base structure, a primary carbide represented by (Cr, Fe, Mo, V) 7 C 3 , and a secondary carbide represented by (Cr, Fe, Mo, V) 23 C 6 .
  • the wt% of (Mo+V) in the primary carbide may be 2.93 to 5.67%.
  • the wt% of (Mo+V) in the secondary carbide may be 12.2 to 14.8%.
  • the particle size of the primary carbide may be 10 ⁇ m or less.
  • the hardening heat treatment may further include quenching at a temperature range of 980 to 1,050°C and tempering at a temperature of 400 to 600°C for 1 minute to 1 hour.
  • the Vickers hardness may be 520 to 650 Hv after the hardening.
  • the method may further include satisfying Formula (2) below: ⁇ 14 ⁇ ⁇ 36442 + 248 C + 365 Cr + 373 Mo + 530 V + 365 Fe + 350 Si + 312 Mn + 331 Ni + 506 Cu ⁇ 50 wherein C, Cr, Mo, V, Fe, Si, Mn, Ni, and Cu denote wt% of the respective elements.
  • the method may further include satisfying Formula (3) and (4) below: 0.37 ⁇ C + N ⁇ 0.43 , and 1.0 ⁇ Mo + V ⁇ 1.35 .
  • Various embodiments of the present disclosure may provide a martensitic stainless steel with improved strength and corrosion resistance while ensuring hardness, and a method of manufacturing the same.
  • a hot-rolled annealed martensitic stainless steel sheet with improved strength and corrosion resistance includes, in percent by weight (wt%), 0.3 to 0.5% of C, 0.01 to 0.025% of N, 0.3 to 0.5% of Si, 0.4 to 0.6 of Mn, 13.1 to 14.5% of Cr, 0.95 to 1.10% of Mo, 0.05 to 0.3% of V, 0.3 to 0.5% of Ni, 0.001 to 0.5% of Cu, and the reminder of Fe and inevitable impurities, and satisfies Formula (1) below: 16.4 ⁇ Cr + 3.3 Mo + 16 N * Mo+V ⁇ 23.3 , wherein Cr, N, Mo, and V denote contents (wt%) of elements, respectively.
  • the present inventors have made various studies to improve the corrosion resistance of high carbon martensitic stainless steel and to minimize material deviation, and then have found those described below.
  • a hot-rolled annealed material produced by typical continuous casting, hot-rolling, and batch annealing processes, has a ferrite as its base structure and contains chromium carbides.
  • the primary chromium carbide remains without being decomposed after hot-rolling and batch annealing.
  • cold rolling is performed by applying a certain degree of rolling reduction, it is difficult to segment and consequently remains coarse carbides of 3 ⁇ m or more.
  • cold rolling is performed by applying a certain degree of rolling reduction, it is difficult to segment and consequently remains coarse carbides of 3 ⁇ m or more.
  • Residual carbides reduce a re-solubility rate to an austenitic phase during hardening heat treatment, thereby lowering the hardness and corrosion resistance of martensitic stainless steel, which is the final material, and also causing local material imbalance.
  • the present inventors found that above a certain amount of Mo and V content allows for preventing coarsening of chromium carbide, securing uniform physical properties (e.g., corrosion resistance, hardness) by diversifying the precipitation sites of chromium carbide, and enabling rapid re-dissolution of chromium and carbon into a high-temperature austenite phase in a subsequent hardening heat treatment step, thereby improving corrosion resistance and strength.
  • uniform physical properties e.g., corrosion resistance, hardness
  • martensitic stainless steel will be described, and then a method of producing martensitic stainless steel will be described.
  • a hot-rolled annealed martensitic stainless steel sheet having improved strength and corrosion resistance includes, in percent by weight (wt%), 0.3 to 0.5% of C, 0.01 to 0.025% of N, 0.3 to 0.5% of Si, 0.4 to 0.6% of Mn, 13.1 to 14.5% of Cr, 0.95 to 1.10% of Mo, 0.05 to 0.3% of V, 0.3 to 0.5% of Ni, 0.001 to 0.5% of Cu, and the reminder of Fe and inevitable impurities.
  • the content of carbon (C) is 0.3 to 0.5%.
  • C is an essential element to ensure the hardness of martensitic stainless steel and is added in an amount of 0.3% or more to secure hardness after quenching/tempering heat treatment.
  • the upper limit may be limited to 0.5% and the C content is preferably 0.36 to 0.4%.
  • the content of nitrogen (N) is 0.01 to 0.025%.
  • N an element added to improve corrosion resistance and hardness at the same time, does not cause local fine segregation although N is added instead of C, which has the advantage of not forming coarse precipitates in the product.
  • 0.01% or more of N is added in the present disclosure.
  • the upper limit may be limited to 0.025% to ensure fatigue properties.
  • the C+N content is 0.37 to 0.43%.
  • the hardness of the martensitic stainless steel may be ensured by controlling the contents of C and N, which are interstitial elements, to 0.37% or more.
  • C and N which are interstitial elements
  • the rolling force increases during the hot-rolling process as C+N increases, leading to a decrease in manufacturability and a reduction in toughness. Therefore, the range of the C+N value may be controlled to 0.37 to 0.43%, considering the hardness and manufacturability of the final material.
  • the content of silicon (Si) is 0.3 to 0.5%.
  • Si an element added essentially for deoxidation, serves to improve strength. 0.3% or more of Si is added in the present disclosure. However, if the Si content is excessive, there is a risk of forming a scale on the surface of the steel sheet during hot-rolling, thereby degrading the surface quality. Therefore, the upper limit may be limited to 0.5%.
  • the content of manganese (Mn) is 0.4 to 0.6%.
  • Mn an element added to improve strength and hardenability, combines with sulfur (S), which is inevitably contained during the manufacturing process, to form MnS, thereby suppressing cracks caused by S. 0.4% or more of Mn is added in the present disclosure. However, if the Mn content is excessive, there is a risk of impairing the surface quality and toughness of the steel. Therefore, the upper limit may be limited to 0.6%.
  • the content of chromium (Cr) is 13.1 to 14.5%.
  • Cr a basic element enhancing corrosion resistance, serves to improve hardness and wear resistance by forming chromium carbide. 13.1% or more of Cr is added in the present disclosure. However, if the Cr content is excessive, the manufacturing cost increases, and the hardenability increases. Therefore, the upper limit may be limited to 14.5%.
  • the content of molybdenum (Mo) is 0.95 to 1.10%.
  • Mo an element improving corrosion resistance, suppresses decarburization, and improving hardenability, and serves to fine carbide by replacing Cr in chromium carbide.
  • 0.95% or more of Mo is added.
  • the upper limit may be limited to 1.10%.
  • the content of vanadium (V) is 0.05 to 0.3%.
  • V is an element effective in suppressing coarsening of chromium carbide by forming carbides, preventing coarsening of crystal grains during heat treatment, and improving wear resistance.
  • 0.05% or more of V is added.
  • the upper limit may be limited to 0.3%.
  • the Mo+V content is 1.0 to 1.35%.
  • Ni nickel
  • Ni an essential element added to ensure an austenitic structure in a hot working region of martensitic stainless steel, serves to improve corrosion resistance and hardenability.
  • 0.3% or more of Ni is added.
  • the upper limit may be limited to 0.5%.
  • the content of copper (Cu) is 0.001 to 0.5%.
  • Cu an element for forming an austenite phase
  • an element for forming an austenite phase serves to improve strength, hardness, and corrosion resistance.
  • 0.001% or more of Cu is added.
  • the upper limit may be limited to 0.5%.
  • 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.
  • the impurities are not specifically mentioned in the present disclosure, as they are known to any person skilled in the art of manufacturing.
  • the hot-rolled annealed martensitic stainless steel sheet having improved strength and corrosion resistance satisfies the following formula (1). 16.4 ⁇ Cr + 3.3 Mo + 16 N * Mo+V ⁇ 23.3 (wherein Cr, N, Mo, and V denote contents (wt%) of elements, respectively).
  • the Pitting Resistance Equivalent Number is represented as Cr+3.3Mo+16N.
  • the present disclosure attempts to ensure corrosion resistance even in a humid environment, such as a compressor, by controlling the PREN value to 16.4 or more in Formula (1), in addition to limiting the content of the alloying elements to the above conditions.
  • the hot-rolled annealed material produced through a batch annealing process has a ferrite as its base structure and contains chromium carbide.
  • the primary carbide formed during slab cooling has limitations in controlling its size and distribution during hot-rolling and cold-rolling processes.
  • FIG. 1 is a graph illustrating a relationship between (Cr+3.3Mo+16N)*(Mo+V) values and the Mo+V content in a carbide of a martensitic stainless steel according to an embodiment of the present disclosure.
  • FIG. 2 is a graph illustrating a relationship between (Cr+3.3Mo+16N)*(Mo+V) values and the size of a primary carbide represented by (Cr, Fe, Mo, V) 7 C 3 of a martensitic stainless steel according to an embodiment of the present disclosure.
  • Formula (2) is derived in consideration of the change in the properties of carbides during hardening heat treatment.
  • the present inventors derive Formula (2) by considering a relationship between the fact whether or not a Z phase (where M is 44V+41Cr) and vanadium nitride (where M is 74.2V+5Cr) are formed and the added components.
  • the Z phase is represented by the contents of C, Cr, and N, which are affected by the addition of Mo and V, which changes the properties of the precipitated carbide, and the Mo+V content in the chromium carbide
  • the vanadium nitride is represented by M-N.
  • C, Cr, Mo, V, Fe, Si, Mn, Ni, and Cu denote wt% of the respective elements.
  • Martensitic stainless steels are typically machined to their final shape and then subjected to a hardening heat treatment process to secure corrosion resistance and hardness.
  • the hardening heat treatment process is a process in which the material is held at a high temperature of about 1,000 to 1,200°C for a short time and then rapidly cooled to room temperature, which increases the chromium concentration of the base to about 12% by re-dissolving chromium carbide in the high-temperature austenite phase. As a result, this creates a dense layer of chromium oxide, which is a thin passivation film, on a surface of the material, thereby improving the corrosion resistance of the material.
  • the austenite phase containing re-dissolved carbon or nitrogen is transformed into the martensite phase during rapid cooling, the hardness of the material is improved.
  • the size of the spheroidized chromium carbide distributed in the base structure is large, it is difficult to re-dissolve chromium carbide in the high-temperature austenite phase, so that the concentration of chromium and carbon present in the base structure decreases, resulting in a lowering of the corrosion resistance and hardness of the material.
  • the addition of Mo and V suppresses the growth of carbides due to the substitution of Cr in the primary and secondary chromium carbides, and preferentially combines with C to form fine carbides, so that the precipitation sites of the primary and secondary chromium carbides is preoccupied., resulting in uniform micronization and distribution of the carbides.
  • a hot-rolled annealed martensitic stainless steel sheet having improved strength and corrosion resistance has the ferrite as its base structure and includes the primary carbide represented by (Cr, Fe, Mo, V) 7 C 3 and the secondary carbide represented by (Cr, Fe, Mo, V) 23 C 6 .
  • Mo and V form carbides in a compound with Cr, so the Cr content in the carbides may be reduced, and the concentration of chromium in the base structure may be increased by forming fine carbides.
  • the wt% of (Mo+V) in the primary carbide represented by (Cr, Fe, Mo, V) 7 C 3 is 2.93 to 5.67%, and the particle size of the primary carbide is 10 ⁇ m or less.
  • the wt% of (Mo+V) in the secondary carbide represented by (Cr, Fe, Mo, V) 23 C 6 is 12.2 to 14.8%.
  • the Z phase represented by M(C, N) (where M is 44V+41Cr) and the vanadium nitride represented by M-N (where M is 74.2V+5Cr) are formed.
  • the Z phase and the vanadium nitride themself then act as precipitation sites of the secondary carbide, so that the carbides may be finely and uniformly distributed.
  • the hot-rolled annealed martensitic stainless steel sheet having improved strength and corrosion resistance has a carbide deviation of 10 pieces/100 ⁇ m 2 or less in a longitudinal direction thereof.
  • the hot-rolled annealed martensitic stainless steel sheet of the present disclosure has, after cold-rolled, 42 to 58 pieces/100 ⁇ m 2 of chromium carbides distributed in the microstructure.
  • a method of manufacturing martensitic stainless steel with improved strength and corrosion resistance includes hot-rolling a slab that includes, in percent by weight (wt%), 0.3 to 0.5% of C, 0.01 to 0.025% of N, 0.3 to 0.5% of Si, 0.4 to 0.6% of Mn, 13.1 to 14.5% of Cr, 0.95 to 1.10% of Mo, 0.05 to 0.3% of V, 0.3 to 0.5% of Ni, 0.001 to 0.5% of Cu, and the reminder of Fe and inevitable impurities and satisfies the following formula (1), performing a batch annealing heat treatment in a temperature range of 600 to 900°C immediately after hot-rolled, cold rolling the hot-rolled annealed material, and performing a hardening heat treatment of the cold-rolled material. 16.4 ⁇ Cr + 3.3 Mo + 16 N * Mo + V ⁇ 23.3
  • the stainless steel including the above compositions is manufactured into a slab by continuous casting or ingot casting, and manufactured into a hot-rolled steel sheet capable of being processed by hot-rolling treatment.
  • the manufactured hot-rolled steel sheet is softened by batch annealing heat treatment to ensure good workability before further processing, such as precision rolling to a thickness suitable for crafting (e.g., swords, tools, etc.)
  • a thermal history deviation occurs during the cooling/reheating processes, causing a variation in the physical properties of a final material. More specifically, immediately after hot-rolling, as the wound coil comes into contact with the outside air, a partial cooling deviation occurs in the wound coil, and accordingly, a martensitic structure is formed in an area where the cooling rate is high, resulting in deriving a non-uniform microstructure.
  • an embodiment of the present disclosure is configured to prevent phase transformation to martensite by introducing a batch annealing heat treatment immediately after hot-rolling.
  • the batch annealing may be performed at a temperature ranging from 600 to 900°C to ensure uniform distribution of carbides. If the annealing temperature is low, the martensite phase may be remained due to insufficient driving force for annealing to the ferrite and carbide phases. If the annealing temperature is too high, reverse transformation to the austenite phase may occur, resulting in coarsening of the grain boundaries, and intensive formation of coarse chromium carbides at the grain boundaries during the cooling process. Considering such an annealing temperature, the temperature range of the batch annealing heat treatment is limited to 600 to 900°C.
  • the hot-rolled annealed martensitic stainless material which has been subjected to the batch annealing heat treatment, may be manufactured into martensitic stainless steel through a process of hardening heat treatment after machining to a final shape.
  • the hardening heat treatment may further include an austenizing treatment, a quenching treatment, and a tempering treatment.
  • the austenizing treatment is a process of transforming the base structure of the steel material from ferrite to austenite.
  • the austenizing treatment may include a heat treatment at a temperature of 1,000°C or more for 1 minute or more.
  • chromium carbide is re-dissolved into the base structure in the form of chromium and carbon, thereby increasing the hardness of the martensitic stainless steel after the subsequent quenching process.
  • the quenching treatment is a process of transforming the austenite structure into martensite with high hardness by rapidly cooling from a temperature range of 980 to 1,050°C to room temperature after the austenizing treatment. If the cooling rate is ensured at 0.2 °C/s or higher, a martensitic structure may be secured.
  • the tempering treatment is a process of imparting toughness to the brittle martensitic structure due to the high hardness achieved by the quenching treatment.
  • the tempering treatment may be performed at a temperature of 400 to 600°C for 1 minute to 1 hour depending on the thickness.
  • the ferrite structure may be finally transformed into the martensite structure, which achieves a desired hardness and corrosion resistance.
  • the Vickers hardness of the material re-dissolved by the hardening heat treatment may be 520 to 650 Hv.
  • Formula (1) is (Cr+3.3Mo+16N)*(Mo+V).
  • Table 1 Example C Si Mn Ni Cu Cr Mo V N C+N Mo+V Formula(1)
  • Example 1 0.38 0.4 0.45 0.4 0.05 13.5 1.05 0.07 0.02 0.40 1.12 19.4
  • Example 2 0.38 0.4 0.45 0.4 0.05 13.5 0.95 0.05 0.02 0.40 1.00 17.0
  • Example 3 0.38 0.4 0.45 0.4 0.05 13.5 1.1 0.15 0.02 0.40 1.25 21.8
  • Example 4 0.38 0.4 0.45 0.4 0.05 14.5 1.05 0.07 0.02 0.40 1.12 20.5
  • Example 5 0.4 0.5 0.6 0.5 0.5 13.9 1.1 0.15 0.025 0.43 1.25 22.4
  • Example 6 0.36 0.3 0.4 0.3 0.001 13.1 0.95 0.05 0.01 0.37 1.00 16.4
  • Example 7 0.38 0.4 0.45 0.4 0.05 13.5 1.05 0.3 0.02 0.40 1.35 23.3 Comparative Example 1 0.38 0.4 0.45 0.4
  • the hardening heat treatment was performed on the cold-rolled annealed material. More specifically, the cold-rolled annealed material was quenched at a cooling rate of 233°C/s to 300°C after heat treatment at 1,000°C for 420 seconds, followed by tempering at 350°C for 350 seconds. The martensitic stainless steel was finally prepared and the Vickers hardness was measured, and the results were shown in Table 2 below.
  • FIG. 1 to FIG. 3 are graphs illustrating a relationship between (Cr+3.3Mo+16N)*(Mo+V) values and the content of Mo+V in the carbide, the size of primary carbides represented by (Cr, Fe, Mo, V) 7 C 3 , and the hot-rolled annealed carbide distribution of the martensitic stainless steel according to an embodiment of the present disclosure.
  • Table 1, Table 2, and FIG. 1 to FIG. 3 in Examples 1 to 7 in which the Mo and V content, and the values of Formula (1) satisfy the range of 16.4 to 23.3, it can be seen that Cr in chromium carbide is replaced by Mo and V and the carbide is finely derived.
  • the wt% of (Mo+V) in the primary carbide represented by (Cr, Fe, Mo, V) 7 C 3 is 2.93 to 5.67%, and the particle size of the primary carbide is 10 ⁇ m or less, and the wt% of (Mo+V) in the secondary carbide represented by (Cr, Fe, Mo, V) 23 C 6 is 12.2 to 14.8%.
  • the content of Mo and V is added in a certain amount or more, but the range of Formula (1) was not satisfied, so that securing of the wt% of (Mo+V) in the secondary carbide is not possible. Accordingly, the chromium carbide may not be finely and uniformly distributed.
  • FIG. 4 and FIG. 5 are photographs of scanning electron micrographs (SEM) showing chromium carbides in the microstructure after the hardening heat treatment of Comparative Example 4 and Example 1, respectively, after tempering.
  • Example 4 it can be seen that the coarsened and segregated carbides remain without being re-dissolved after the hardening heat treatment. Whereas, in Example 1, it can be seen that most of the carbonaceous materials are re-dissolved after the hardening heat treatment and a martensitic structure having a low area fraction of residual carbonitrides is derived.
  • the corrosion resistance of the high carbon martensitic stainless steel can be improved and the material deviation can be minimized.
  • the hot-rolled annealed martensitic stainless steel sheet according to an embodiment of the present disclosure has improved strength and corrosion resistance while ensuring hardness and is therefore industrially usable.

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EP21911281.0A 2020-12-21 2021-11-29 Acier inoxydable martensitique ayant une résistance et une résistance à la corrosion améliorées, et son procédé de fabrication Pending EP4265784A1 (fr)

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