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  • 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
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  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/02—Hardening articles or materials formed by forging or rolling, with no further heating beyond that required for the formation
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  • C21D1/18—Hardening; Quenching with or without subsequent tempering
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  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
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  • 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
  • C21D1/19—Hardening; Quenching with or without subsequent tempering by interrupted quenching
  • C21D1/22—Martempering
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  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/56—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering characterised by the quenching agents
  • C21D1/60—Aqueous agents
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  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/004—Heat treatment of ferrous alloys containing Cr and Ni
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  • C21D6/00—Heat treatment of ferrous alloys
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  • 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
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  • 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
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  • 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
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  • 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
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  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
  • C21D9/42—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for armour plate
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  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
  • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
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  • C22C—ALLOYS
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  • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
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  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
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  • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
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  • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
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  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/34—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of silicon
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  • 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
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  • 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
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  • 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
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  • 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
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  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron
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  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/001—Austenite
• • C—CHEMISTRY; METALLURGY
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  • 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 suitable 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.
• Armor 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 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, non-ferrous materials such as titanium and aluminum have been developed.
• the non-ferrous materials have an advantage of weight reduction compared to steel materials, but are relatively expensive and have poor workability. As compared to the non-ferrous material, since the steel material is relatively inexpensive and may change physical properties such as hardness 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 important physical properties for securing the performance of armored 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 1 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 for the same.
• 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.
• a high-hardness armored steel having excellent low-temperature impact toughness includes, 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], wherein a tempered martensite base structure including retained austenite is included as a microstructure.
• A refers to a value calculated by the following Relational Expression 2.
• A 539 ⁇ 423 * C ⁇ 30.4 * Mn ⁇ 17.7 * Ni ⁇ 12.1 * Cr ⁇ 7.5 * Mo
• [C], [Mn], [Ni], [Cr], and [Mo] refer to contents (weight %)of carbon (C), manganese (Mn), nickel (Ni), chromium (Cr) and molybdenum (Mo) 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: one or more 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 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 greater than 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: 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), and a balance of iron (Fe) and inevitable impurities, satisfying the following [Relational Expression 1]; heating the steel
• A 200 / 100 ⁇ 1.5 in Relational Expression 1 , A refers to a value calculated by the following Relational Expression 2 .
• A 539 ⁇ 423 * C ⁇ 30.4 * Mn ⁇ 17.7 * Ni ⁇ 12.1 * Cr ⁇ 7.5 * Mo
• [C], [Mn], [Ni], [Cr], and [Mo] refer to contents (weight %) of carbon (C), manganese (Mn), nickel (Ni), chromium (Cr), and molybdenum (Mo) 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 0.25% of titanium (Ti) and 0.2% or less of vanadium (V).
• the hot-rolled steel sheet may have a thickness greater than 40 mm.
• 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 an alloy composition and manufacturing conditions, and thus, is economically favorable.
• the present disclosure relates to high-hardness armored steel having excellent low-temperature impact toughness and a manufacturing method therefor.
• preferred embodiments of the present disclosure will be described.
• 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 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 hardenability improvement. 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 1.4%.
• 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 1.15%, and more preferably, the upper limit of the content of nickel (Ni) may be 1.1%.
• 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.
• 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 0.13%, and more preferably, the lower limit of the content of molybdenum (Mo) may be 0.15%.
• 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%.
• Calcium (Ca) is 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.
• inevitable impurities may be inevitably added from raw materials or an ambient environment, and thus, impurities may not be excluded.
• a person skilled in the art of a general manufacturing process may be aware of the impurities, and thus, the descriptions of the impurities may not be provided in the present disclosure.
• additional 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].
• A refers to a value calculated by the following [Relational Expression 2].
• A 539 ⁇ 423 * C ⁇ 30.4 * Mn ⁇ 17.7 * Ni ⁇ 12.1 * Cr ⁇ 7.5 * Mo
• [C], [Mn], [Ni], [Cr], and [Mo] refer to contents (weight %) of carbon (C), manganese (Mn), nickel (Ni), chromium (Cr), and molybdenum (Mo) included in a 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 a specific alloy composition included in the steel sheet.
• each respective alloy compositions included in the steel sheet is controlled to be within a certain range, but also the relative content range of carbon (C), manganese (Mn), chromium (Cr), nickel (Ni), and molybdenum (Mo) is controlled to be within a certain range, as illustrated in [Relational Expression 1] and [Relational Expression 2], so that high hard characteristics and excellent low-temperature impact toughness may be effectively compatible.
• (A - 200)/100 defined by Relational Expression 1 may be 1.45 or less.
• the armor steel of the present disclosure having the above-described alloy composition 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 bullet-proof 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 armored steel of the present disclosure may have the above-described microstructural configuration over the entire thickness.
• the armored steel of the present disclosure having the above-described alloy composition and the proposed microstructure may have a thickness greater than 40 mm.
• An upper limit of the thickness thereof is not particularly limited, but a preferred thickness thereof may be 100 mm, and a more preferred thickness thereof may be 80 mm or less.
• the armored steel according to an aspect of the present disclosure may have a surface hardness of 460 to 540HB, exhibiting ultra-high hardness, and may have 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 armor 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] and [Relational Expression 2]), 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 - cooling - self- tempering] .
• each process condition 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 cooled to 50 to 250°C at a rate of 3°C/s or more and then air-cooled to room temperature.
• the cooling is performed to obtain a martensite base structure to satisfy high hardness, and when a cooling end temperature is higher than 250°C, phase transformation from austenite particles produced by hot rolling to martensite may not be completed, and thus hardness of a final product may be deteriorated.
• the cooling end temperature is lower than 50°C, the phase transformation therefrom to martensite is completely completed, which is favorable in terms of securing hardness, but latent heat in a material decreases, so that a self-tempering effect cannot be obtained.
• Self-tempering is a method that can produce an effect similar to that of normal tempering through latent heat of rapidly cooled material without a separate subsequent process.
• the cooling of the hot-rolled steel sheet is preferably ended in a range of 50 to 250°C.
• a lower limit of the cooling end temperature is more preferably 60°C, even more preferably 70°C, and most preferably 80°C.
• an upper limit of the cooling end temperature is more preferably 220°C, even more preferably 200°C, and most preferably 180°C.
• the cooling rate is preferably 3°C/s or more.
• the cooling rate is more preferably 3.2°C/s or more, more preferably 3.5°C/s or more, and most preferably 4°C/s or more.
• the thickness of the hot-rolled steel sheet manufactured by the series of manufacturing processes is not limited, but a lower limit of the thickness thereof may be limited to be more than 40 mm in terms of securing a self-tempering effect.
• the hot-rolled steel sheet may have a thickness of 41 mm or more, and an upper limit of the thickness thereof is also not limited, but the hot-rolled steel sheet may preferably have a thickness of 100 mm or less, more preferably 80 mm or less.
• a steel slab having alloy compositions shown in the following Table 1 was prepared, and then was subjected to [heating - rolling - cooling and self-tempering] according to process conditions shown in the following Table 2 to manufacture each hot rolled steel sheet.
• water cooling was performed to a cooling end temperature, and then air cooling was performed to room temperature.
• Alloy compositions not listed in Table 1 refers to inevitable impurities and iron (Fe).
• a part marked with"-" in Table 1 means that the corresponding element was not intentionally added, which is preferable to be interpreted as 0% by weight within an error range.
• 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 part. In this case, a fraction of the microstructure was measured by electron back-scattered diffraction (EBSD) analysis.
• EBSD electron back-scattered diffraction
• hardness and toughness of each hot-rolled steel sheet were measured using a Brinell hardness tester (load: 3000 kgf, 10 mm tungsten injection port) and a Charpy impact tester.
• specimens satisfying both the alloy compositions and the process conditions of the present disclosure have a surface hardness of 460 to 540 HB and 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 impact absorption energy of 19 J or more at -40°C at the same time.

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