Classifications

• • 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
  • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
• • 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
  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/004—Heat treatment of ferrous alloys containing Cr and Ni
• • 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
  • 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
  • C22C38/002—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
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • 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
• • 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
• • 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
  • C21D2211/001—Austenite
• • 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 material appropriate for armored vehicles, explosion-proof structures, and the like, and more particularly to armored steel having excellent low-temperature impact toughness and having high hardness, and a manufacturing method therefor.
• Armored steel is a material of which a surface is made very hard for its main function of blocking bullets, and is used where protection is required, such as for an exterior of armored vehicles used on the battlefield. Since bulletproof performance is directly related to human life, research to improve the performance of bulletproof materials has been actively conducted in the past, and recently, a non-ferrous material such as titanium and aluminum has been developed.
• the non-ferrous material has an advantage of weight reduction compared to a steel material, but is relatively expensive and has poor workability. As compared to the non-ferrous material, since the steel material is relatively inexpensive and may change physical properties such as harness relatively easily, the steel material is widely used as a material for self-propelled artillery, wheeled armored vehicles, and the like.
• Hardness is one of the most important physical properties for securing the performance of armor steel, but simple high hardness does not guarantee bulletproof performance.
• a high hardness characteristic is a factor that increases resistance to bullets penetrating through a material, but the material having high hardness may be relatively easily broken, so the high hardness characteristic cannot necessarily guarantee excellent bulletproof performance. Therefore, there is a need to develop a material that can simultaneously secure brittle fracture resistance to external impact as well as high hardness characteristics rather than simply promoting high hardness of the material.
• Patent Document Korean Patent Publication No. 10-2018-0043788 (published on April 30, 2018 )
• An aspect of the present disclosure is to provide armored steel having high hardness characteristics and excellent low-temperature impact toughness and a manufacturing method therefor.
• An object of the present disclosure is not limited to the above description.
• the object of the present disclosure will be understood from the entire content of the present specification, and a person skilled in the art to which the present disclosure pertains will understand an additional object of the present disclosure without difficulty.
• Mn manga
• [C] and [Si] refer to contents (weight %) of carbon (C) and silicon (Si) included in the steel sheet, and 0 is substituted when the corresponding element is not intentionally added.
• the armored steel may further include, by weight: at least one of 0.005 to 0.025% of titanium (Ti) and 0.2% or less of vanadium (V).
• a fraction of the tempered martensite may be 90% or more by area, and a fraction of the retained austenite may be 1% by area to 10% by area.
• the armored steel may have a surface hardness of 460 to 540 HB, and an impact absorption energy of 19 J or more at -40°C.
• the armored steel may have a thickness of 5 to 40 mm.
• a manufacturing method for a high-hardness armored steel having excellent low-temperature impact toughness includes operations of: preparing a steel slab including, by weight: 0.29 to 0.37% of carbon (C), 1.0 to 2.0% of silicon (Si), 0.5 to 1.6% of manganese (Mn), 0.5 to 1.2% of nickel (Ni), 0.4 to 1.5% of chromium (Cr), 0.05% or less of phosphorus (P), 0.02% or less of sulfur (S), 0.006% or less of nitrogen (N), 0.07% or less of aluminum (Al) (excluding 0%), 0.1 to 0.5% of molybdenum (Mo), 0.01 to 0.05% of niobium (Nb), 0.0002 to 0.005% of boron (B), 0.0005 to 0.004% of calcium (Ca), with a balance of iron (Fe) and inevitable impurities, satisfying the following [Relational Expression 1]; heating the steel slab in a temperature range of 1050 to 12
• [C] and [Si] refer to contents (weight %) of carbon (C) and silicon (Si) included in the steel slab, and 0 is substituted when the corresponding element is not intentionally added.
• the steel slab may further include, by weight: at least one of 0.005 to 0.025% of titanium (Ti) and 0.2% or less of vanadium (V).
• a residence time during the primary heat treatment may be 1.3t * 10 minutes (t: plate thickness (mm)) or more.
• a holding time during the secondary heat treatment may be 1.9t+10 minutes (t: plate thickness (mm)) or more.
• armored steel having excellent low-temperature toughness while having ultra-high hardness may be provided.
• the present disclosure may provide armored steel having a target level of physical properties without performing a further heat treatment from optimization of alloy compositions and manufacturing conditions, and thus, is economically favorable.
• the present disclosure relates to a high-hardness armored steel having excellent low-temperature impact toughness and a manufacturing method therefor.
• preferred embodiments of the present disclosure will be described.
• Embodiments of the present disclosure may be modified in various forms, and the scope of the present disclosure should not be construed as being limited to the embodiments described below.
• the present embodiments are provided to those skilled in the art to further elaborate the present disclosure.
• the present inventors have studied in depth, in order to provide a steel material having excellent physical properties such as high hardness characteristics, low-temperature impact toughness, and the like, which are essentially required physical properties, as a material which may be appropriately applied to wheeled armored vehicles, explosion-proof structures, and the like.
• the bulletproof performance of the steel material was intended to be improved by an economically favorable method, and thus, the present disclosure was provided.
• % represents a content of each element based on weight, unless otherwise particularly specified.
• Carbon (C) is an element which is effective for improving strength and hardness in steel having a low-temperature transformation phase such as a martensite or bainite phase, and is effective for improving hardenability. In order to sufficiently obtain the effect described above, 0.29% or more of carbon (C) may be included.
• a lower limit of a content of carbon (C) may be 0.30%, and more preferably, the lower limit of the content of carbon (C) may be 0.31%.
• an upper limit of the content of carbon (C) may be limited to 0.37%.
• the upper limit of the content of carbon (C) may be 0.36%.
• Silicon (Si) is an element which is effective for improving strength due to solid solution strengthening together with a deoxidation effect, and is also an element suppressing formation of carbides such as cementite in a steel material containing a certain amount or more of C to promote production of residual austenite.
• carbides such as cementite
• a steel material containing a certain amount or more of C may be included.
• silicon (Si) may be included.
• a lower limit of a content of silicon (Si) may be 1.1%, and more preferably, the lower limit of the content of silicon (Si) may be 1.2%.
• an upper limit of the content of silicon (Si) may be limited to 2.0%.
• the upper limit of the content of silicon (Si) may be 1.9%, and more preferably, the upper limit of the content of silicon (Si) may be 1.8%.
• Manganese (Mn) is an element favorable to suppress production of ferrite and lower an Ar3 temperature, thereby improving quenching properties of steel to increase strength and toughness.
• 0.5% or more of manganese (Mn) may be included.
• a lower limit of a content of manganese (Mn) may be 0.6%, and more preferably, the lower limit of the content of manganese (Mn) may be 0.7%.
• an upper limit of the content of manganese (Mn) may be limited to 1.6%.
• the upper limit of the content of manganese (Mn) may be 1.5%, and more preferably, the upper limit of the content of manganese (Mn) may be 1.45%.
• Nickel (Ni) is an element favorable to improve both strength and toughness of steel. In order to obtain the effect described above, in the present disclosure, 0.5% or more of nickel (Ni) may be included. Preferably, a lower limit of a content of nickel (Ni) may be 0.6%, and more preferably, the lower limit of the content of nickel (Ni) may be 0.7%. However, nickel (Ni) is an expensive element, when nickel (Ni) is excessively added, manufacturing costs may be greatly increased, so in the present disclosure, an upper limit of the content of nickel (Ni) may be limited to 1.2%. Preferably, the upper limit of the content of nickel (Ni) may be 1.17%, and more preferably, the upper limit of the content of nickel (Ni) may be 1.15%.
• Chromium (Cr) is an element of increasing quenching properties of steel to improve strength, and effectively contributing to securing hardness in a surface part and a center part of steel.
• Cr chromium
• Cr is also an element for economically securing hardness and toughness.
• 0.4% or more of chromium (Cr) may be included.
• a lower limit of a content of chromium (Cr) may be 0.5%.
• an upper limit of the content of chromium (Cr) may be limited to 1.5%.
• the upper limit of the content of chromium (Cr) may be 1.4%, and more preferably, the upper limit of the content of chromium (Cr) may be 1.3%.
• Phosphorous (P) 0.05% or less
• Phosphorus (P) is an element which is inevitably contained in steel, and is also an element which deteriorates toughness of the steel. Thus, it is preferred to lower a content of P as much as possible.
• an upper limit of the content of phosphorus (P) may be limited to 0.05%. More favorably, the content thereof may be limited to 0.03% or less. However, 0% may be excluded considering an inevitably contained level.
• Sulfur (S) is an element which is inevitably contained in steel, and is also an element forming MnS inclusions to deteriorate toughness of steel. Thus, it is preferred to lower a content of S as much as possible.
• an upper limit of the content of sulfur (S) may be limited to 0.02%. More favorably, the content thereof may be limited to 0.01% or less. However, 0% may be excluded considering an inevitably contained level.
• Nitrogen (N) is an element which is favorable to improve strength of steel by forming precipitates in steel, but when a content of nitrogen (N) is more than a certain level, which may rather cause deterioration in toughness of steel.
• an upper limit of a content of nitrogen (N) may be limited to 0.006%. However, 0% may be excluded considering an inevitably contained level.
• Aluminum (Al) is an element effective for lowering an oxygen content in molten steel as a deoxidizing agent of steel. However, when aluminum (Al) is excessively added, cleanliness of steel may be impaired, so in the present disclosure, an upper limit of a content of aluminum (Al) may be limited to 0.07%.
• Molybdenum (Mo) is an element favorable to increase quenching properties of steel, and in particular, to improve hardness of a thick material having a certain thickness or more. In order to obtain the effect described above, in the present disclosure, 0.1% or more of molybdenum (Mo) may be included. Preferably, a lower limit of a content of molybdenum (Mo) may be 0.15%, and more preferably, the lower limit of the content of molybdenum (Mo) may be 0.2%.
• an upper limit of the content of molybdenum (Mo) may be limited to 0.5%.
• the upper limit of the content of molybdenum (Mo) may be 0.48%, and more preferably, the upper limit of the content of molybdenum (Mo) may be 0.45%.
• Niobium (Nb) is an element which is effective for increasing hardenability of austenite by being dissolved in austenite, and increasing strength of steel and suppressing growth of austenite crystal grains by forming carbonitrides such as Nb(C,N).
• 0.01% or more of niobium (Ni) may be included.
• an upper limit of a content of niobium (Nb) may be limited to 0.05%.
• the upper limit of the content of niobium (Nb) content may be 0.04%.
• Boron (B) is an element effectively contributing to strength improvement by increasing quenching properties of steel even with a small addition amount thereof.
• 0.0002% or more of boron (B) may be contained.
• a lower limit of a content of boron (B) may be 0.0005%, and more preferably, the lower limit of the content of boron (B) may be 0.001%.
• an upper limit of the content of boron (B) may be limited to 0.005%.
• the upper limit of the content of boron (B) may be 0.004%, and more preferably, the upper limit of the content of boron (B) may be 0.003%.
• Ca an element having a good binding force with sulfur (S) and producing CaS on the periphery (around) MnS, thereby suppressing elongation of MnS to improve toughness in a direction perpendicular to a rolling direction.
• CaS produced by adding Ca has an effect of increasing corrosion resistance under a humid external environment.
• 0.0005% or more of Ca may be included.
• a lower limit of a content of calcium (Ca) may be 0.001%.
• an upper limit of the content of calcium (Ca) may be limited to 0.004%.
• the upper limit of the content of calcium (Ca) may be 0.003%.
• armored steel of the present disclosure may further include the following elements for the purpose of favorably securing target physical properties.
• the armored steel of the present disclosure may further include at least one of titanium (Ti) and vanadium (V).
• Titanium (Ti) is an element which maximizes the effect of boron (B), which is an element favorable to improve quenching properties of steel. That is, titanium (Ti) is bonded to nitrogen (N) in steel to be precipitated into TiN to reduce the content of solid-solubilized N, while suppressing formation of BN of B therefrom to increase solid-solubilized B, thereby maximizing improvement of quenching properties. In order to sufficiently obtain the effect described above, 0.005% or more of titanium (Ti) may be contained. However, when titanium (Ti) is excessively added, coarse TiN precipitates may be formed and toughness of steel may be deteriorated, so in the present disclosure, an upper limit of the content of titanium (Ti) may be limited to 0.025%.
• V Vanadium (V): 0.2% or less (including 0%)
• Vanadium (V) is an element favorable to form a VC carbide when reheating after hot rolling, thereby suppressing growth of austenite crystal grains and improving quenching properties of steel to secure strength and toughness.
• vanadium (V) is a relatively expensive element, an upper limit of a content of vanadium (V) may be limited to 0.2% in consideration of manufacturing costs.
• the armored steel according to an aspect of the present disclosure may include a remainder of Fe and other inevitable impurities in addition to the components described above.
• the component since in the common manufacturing process, unintended impurities may be inevitably incorporated from raw materials or the surrounding environment, the component may not be excluded. Since these impurities are known to any person skilled in the common manufacturing process, the entire contents thereof are not particularly mentioned in the present specification. In addition, further addition of effective ingredients other than the above-mentioned ingredients is not entirely excluded.
• the armored steel according to an aspect of the present disclosure may satisfy the following [Relational Expression 1]. 10 * C * Si ⁇ 4
• [C] and [Si] refer to contents (weight %) of carbon (C) and silicon (Si) included in the steel sheet, and 0 is substituted if the corresponding element is not intentionally added.
• the inventors of the present disclosure have conducted in-depth research on a method capable of securing high-hardness characteristics and excellent low-temperature impact toughness of a steel sheet at the same time, and have derived that it is effective to control not only a content range of each respective alloy composition, but also a relative content range of the specific alloy composition included in the steel sheet.
• not only the content range of each respective alloy composition included in the steel sheet is controlled to be within a certain range, but also the relative content range of carbon (C) and silicon (Si) is controlled to be within a certain range, as illustrated in [Relational Expression 1], so that high hard characteristics and excellent low-temperature impact toughness may be effectively compatible.
• 10*[C]*[Si] defined by the Relational Expression 1 may be 4.3 or more.
• the armored steel of the present disclosure having the alloy composition described above may have a tempered martensite base structure including retained austenite as a microstructure, and may further include other inevitable structures.
• a fraction of retained austenite may be 1% by area to 10% by area, and a fraction of tempered martensite may be 90% or more by area.
• Retained austenite is a structure remaining without being completely phase transformed into martensite during a rapid cooling heat treatment, and has relatively low hardness but excellent toughness as compared to martensite.
• the armor steel of the present disclosure may include 1% or more by area of retained austenite, more preferably 2% or more by area of retained austenite.
• an upper limit of the fraction of the retained austenite may be set to be 10% by area.
• the upper limit of the fraction of the retained austenite may be 5% by area, and a lower limit of the fraction of the tempered martensite may be 95% by area.
• the armor steel of the present disclosure may have the above-described microstructural configuration over the entire thickness.
• the armor steel of the present disclosure having the above-described alloy composition and the proposed microstructure may have a thickness of 5 to 40 mm, and have a surface hardness of 460 to 540HB, exhibiting ultra-high hardness, and an impact absorption energy of 19 J or more at -40°C, exhibiting excellent low-temperature toughness.
• the surface hardness refers to an average value of three measurements after milling a surface of the armored steel at 2 mm in a thickness direction using a Brinell hardness tester (load: 3000 kgf, 10 mm tungsten injection port).
• a steel slab having a predetermined component is prepared. Since the steel slab of the present disclosure has an alloy composition corresponding to the alloy composition of the hot-rolled steel sheet described above (including [Relational Expression 1]), a description of the alloy composition of the steel slab is substituted for the description of the alloy composition of the above-described hot-rolled steel sheet.
• the armored steel may be manufactured by preparing a steel slab satisfying the alloy composition described above, and then subjecting the steel slab to the processes of [heating - rolling - primary heat treatment (quenching) - secondary heat treatment (tempering)].
• the processes of [heating - rolling - primary heat treatment (quenching) - secondary heat treatment (tempering)] will be described in detail.
• a steel slab having the alloy composition proposed in the present disclosure is prepared, which may be then heated in a temperature range of 1050 to 1250°C.
• the steel slab may be heated in a temperature range of 1050 to 1250°C.
• the steel slab heated as described above may be rolled, and then may be subjected to rough rolling and finish hot rolling to manufacture a hot-rolled steel sheet.
• the heated steel slab is roughly rolled in a temperature range of 950 to 1150°C to be manufactured into a bar, which may be then subjected to finish hot rolling in a temperature range of 850 to 950°C.
• the hot-rolled steel sheet manufactured through the rolling process described above is air cooled to room temperature, and then heated to a residence time of 1.3t + 10 minutes (t: plate thickness (mm)) or more in a temperature range of 880 to 930°C.
• the heating is performed for reverse transformation of a hot-rolled steel sheet composed of ferrite and pearlite into an austenite single phase.
• a heating temperature is lower than 880°C, austenitization is not sufficiently achieved, and coarse soft ferrite is mixed, and thus hardness of a final product may be deteriorated.
• the temperature thereof is higher than 930°C, there is an effect of increasing quenching properties due to coarse austenite crystal grains, but there is a disadvantage in terms of thermal efficiency during mass production. Therefore, heating during a primary heat treatment is preferably performed in a range of 880 to 930°C.
• a lower limit of the heating temperature is more preferably 885°C, even more preferably 890°C, and most preferably 895°C.
• an upper limit of the heating temperature is more preferably 925°C, even more preferably 920°C, and most preferably 915°C.
• a residence time during the heating is preferably 1.3t + 10 minutes (t: plate thickness (mm)) or more.
• the residence time during the heating is more preferably 1.3t + 12 minutes (t: plate thickness (mm)) or more, even more preferably 1.3t + 13 minutes (t: plate thickness (mm)) or more, and most preferably 1.5t + 15 minutes (t: plate thickness (mm)) or more.
• an upper limit of the residence time during the primary heat treatment is not particularly limited.
• the residence time during the heating exceeds 1.3 t + 60 minutes (t: plate thickness (mm))
• austenite crystal grains become coarse, so that quenching properties may increase, but there may be a disadvantage in that productivity is relatively lowered. Therefore, the residence time during the heating is preferably 1.3t + 60 minutes (t: plate thickness (mm)) or less.
• the residence time during the heating is more preferably 1.3t + 50 minutes (t: plate thickness (mm)) or less, even more preferably 1.3t + 40 minutes (t: plate thickness (mm)) or less, and most preferably 1.3t + 30 minutes (t: plate thickness (mm)) or less.
• the heated hot-rolled steel sheet may be cooled to 120°C or lower at a cooling rate of 10°C/s or more with respect to a plate thickness center portion(e.g., 1/2t point, t: plate thickness (mm)).
• the cooling is preferably rapid cooling through water cooling.
• the cooling rate is less than 10°C/s or a cooling end temperature is higher than 120°C, there is a concern that a ferrite phase may be formed or a bainite phase may be excessively formed during cooling. Therefore, the cooling is preferably performed to 120°C or lower at a cooling rate of 10°C/s or more. The faster the cooling rate, the more favorable it is to form the microstructure to be obtained in the present disclosure.
• an upper limit of the cooling rate is not particularly limited, and it may be appropriately set in consideration of facility limitations by any person skilled in the art.
• the cooling end temperature is more preferably 100°C or lower, even more preferably 80°C or lower, and most preferably 50°C or lower.
• the hot-rolled steel sheet manufactured through the above-described primary heat treatment may be heated to a temperature range of 350°C or lower and maintained for 1.9t+10 minutes (t: plate thickness (mm)) or more.
• the reheating is performed to release internal stress of a hot-rolled steel sheet fully composed of martensite after quenching.
• a tempering heat treatment As dislocation density inside a material thereof decreases through such a tempering heat treatment, hardness may decrease somewhat, but toughness may be secured.
• a subsequent tempering heat treatment is essential if no special element is added.
• a tempering temperature exceeds 350°C, the dislocation density inside the martensite is excessively reduced, and thus hardness of a final product may be lowered.
• a lower limit of the tempering temperature is not separately specified, but is preferably 100°C or higher in order to obtain the effect described above. It is preferable to perform tempering, more preferably at a temperature of 125°C or higher, and even more preferably 150°C or higher.
• a lower limit of the holding time during the tempering is not particularly limited.
• the holding time during the tempering is less than 1.9t + 10 minutes (t: plate thickness (mm))
• a thickness center portion compared to the surface thereof may not be sufficiently heated, as the heat treatment is performed at a relatively low temperature for a short time. Therefore, the holding time during the tempering is preferably 1.9t + 10 minutes (t: plate thickness (mm)) or more.
• the holding time during the tempering is more preferably 1.9t + 12 minutes (t: plate thickness (mm)) or more, more preferably 1.9t + 15 minutes (t: plate thickness (mm)) or less, and most preferably 1.9t + 20 minutes (t: plate thickness (mm)) or more.
• the hot-rolled steel sheet according to an aspect of the present disclosure may have softened tempered martensite as a base structure.
• the hot-rolled steel sheet obtained through the above-described series of manufacturing processes is a steel material having a thickness of 5 to 40 mm, and can provide excellent bullet-proof resistance by securing high hardness and high toughness.
• Cooli ng end tempe ratur e (°C) Cooli ng rate (°C/s ) Heatin 9 temper ature (°C) Holdin g time (min.) 1 A 12 1171 1034 886 908 31 23 50.6 224 46 2 A 18 1125 1018 924 910 37 29 38.8 206 57 3 A 40 1159 1046 911 905 68 22 25.7 217 100 4 B 40 1159 1046 911 905 25 19 63.4 432 40 5 B 10 1165 1040 887 909 47 23 45.5 561 58 6 B 20 1154 1035 916 915 74 24 26.6 306 97 7 C 5 1180 1065 871 902 31 17 71.1 214 38 8 C 15 1176 1063 899 900 45 26 47.5 421 52 9 C 25 1165 1025 923 910 54 23 44.6 223 68 10 D 12 1159 1059 890 911 38 25 64.7 212 44 11 D 25 1163 1024 915 904 55 21 49.8 230 59 12 D
• microstructure of each hot-rolled steel sheet was cut into an arbitrary size as a specimen to manufacture a mirror surface, a Nital etching solution was used to corrode the specimen, and then an optical microscope and a scanning electron microscope (SEM) were used to observe a 1/2t point which was a thickness center portion. In this case, a fraction of the microstructure was measured by electron back-scattered diffraction (EBSD) analysis.
• EBSD electron back-scattered diffraction
• specimens satisfying the alloy composition and process conditions of the present disclosure have a surface harness of 460 to 540 HB and an impact absorption energy of 19 J or more at -40°C, but specimens not satisfying at least one of the alloy compositions or process conditions of the present disclosure do not have a surface hardness of 460 to 540 HB or an impact absorption energy of 19 J or more at -40°C at the same time.

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• 2020
  • 2020-12-18 KR KR1020200179082A patent/KR102498149B1/ko active IP Right Grant
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