EP4386104A1

EP4386104A1 - High-hardness bulletproof steel having excellent low-temperature toughness, and manufacturing method therefor

Info

Publication number: EP4386104A1
Application number: EP22856100.7A
Authority: EP
Inventors: Yong-Woo Kim; Hyun-Kwan Cho; Young-Sub BYUN; Nam-Young Cho; In-Ho Kim
Original assignee: Posco Co Ltd
Current assignee: Posco Holdings Inc
Priority date: 2021-08-11
Filing date: 2022-08-03
Publication date: 2024-06-19
Also published as: WO2023018101A1; KR20230024090A

Abstract

The present invention relates to high-hardness bulletproof steel having excellent low-temperature toughness, and a manufacturing method therefor. More specifically, the present invention relates to high-hardness bulletproof steel having excellent low-temperature toughness, and a manufacturing method therefor, the high-hardness bulletproof steel being preferably usable in an armored vehicle, an explosion-proof structure and the like.

Description

Technical Field

The present disclosure relates to high-hardness bulletproof steel having excellent low-temperature toughness, and a method of manufacturing the same. More specifically, the present disclosure relates to high-hardness bulletproof steel having excellent low-temperature toughness that may be preferably used for an armored vehicle, an explosion-proof structure, or the like, and a method of manufacturing the same.

Background Art

In order to stably prevent the risk of human casualties by blocking a bullet on the battlefield, high hardness of a bulletproof steel material providing protection such as in an armored vehicle, an explosion-proof structure, or the like, is required. This may be because such high hardness is a factor increasing resistance to prevent the bullet from penetrating the material. However, since materials with high hardness may be broken relatively easily, in order to secure more stable protection, it is necessary to develop materials that may simultaneously secure fracture resistance to external impact, in addition to such high hardness.
Patent Document 1 discloses a steel material used for crushing of industrial waste, a wear component of a grinder, or the like, and seeking to secure surface hardness by actively utilizing Nb together with large amounts of Cr and Mo, but there is a limit to secure low-temperature toughness due to an excessive amount thereof.
Patent Document 2 discloses a technique for securing high hardness by causing a retained austenite structure to have a plastic-induced machining hardening phenomenon after forming a steel material or when the steel material is applied to a product and in use, but since, in bulletproof steel, deformation/impact speed increase greatly by a bullet, it is difficult to obtain an effect by the above phenomenon.

[Prior Art Literature]

(Patent Document 1) Japanese Unexamined Patent Publication No. Hei 10-102185
(Patent Document 2) Japanese Unexamined Patent Publication No. Hei 07-173571

Summary of Invention

Technical Problem

An aspect of the present disclosure is to provide high-hardness bulletproof steel having excellent low-temperature toughness, and a method of manufacturing the same.

Solution to Problem

According to an aspect of the present disclosure, high-hardness bulletproof steel having excellent low-temperature toughness, includes by weight, carbon (C): 0.28 to 0.32%, silicon (Si): 0.5% or less (excluding 0%), manganese (Mn): 0.2 to 1.1%, nickel (Ni): 0.7 to 1.2%, chromium (Cr): 0.2 to 1.1%, phosphorus (P): 0.03% or less (excluding 0%), sulfur (S): 0.015% or less (excluding 0%), nitrogen (N): 0.006% or less (0% excluding), aluminum (Al): 0.05% or less (excluding 0%), molybdenum (Mo): 0.2 to 1.0%, vanadium (V): 0.02 to 0.5%, calcium (Ca): 0.0005 to 0.004%, and a remainder of Fe and unavoidable impurities, the unavoidable impurities including Nb: 0.0015% or less and B: 0.0008% or less, satisfying the following relationship 1, including a microstructure with tempered martensite at least 90% (including 100%), by area, wherein an average effective grain size of the tempered martensite is 20 um or less, including a V(C,N)-based precipitate having an average size of 30 nm or less in a grain of the tempered martensite:
$225 [C] - 11.3 [Mn] + 37.5 [Cr] - 2.5 [Ni] + 228.6 [V] + 338.8 [Mo] \geq 220$
in relationship 1, amounts of elements are expressed in wt%.
According to another embodiment of the present disclosure, a method for manufacturing a high-hardness bulletproof steel having excellent low-temperature toughness, includes heating a slab including, by weight, carbon (C): 0.28 to 0.32%, silicon (Si): 0.5% or less (excluding 0%), manganese (Mn): 0.2 to 1.1%, nickel (Ni): 0.7 to 1.2%, chromium (Cr): 0.2 to 1.1%, phosphorus (P): 0.03% or less (excluding 0%), sulfur (S): 0.015% or less (excluding 0%), nitrogen (N): 0.006% or less (0% excluding), aluminum (Al): 0.05% or less (excluding 0%), molybdenum (Mo): 0.2 to 1.0%, vanadium (V): 0.02 to 0.5%, calcium (Ca): 0.0005 to 0.004%, and a remainder of Fe and unavoidable impurities, and satisfying the following relationship 1, at 1050 to 1250°C; rough-rolling the heated slab at 950 to 1150°C to obtain a bar; finish hot-rolling the bar at 850 to 950°C to obtain a hot-rolled steel sheet, and cooling the same to room temperature; performing a first heat treatment operation of heating the cooled hot-rolled steel sheet to 880 to 930°C for 1.3 t + 30 minutes or more (t: sheet thickness (mm)) and then cooling the same to 150°C or less at a cooling rate of 10°C/s or more; and performing a second heat treatment operation of tempering the first heat-treated hot-rolled steel sheet for 1.5 t + 32 minutes (t: sheet thickness (mm)) to 1.5 t + 60 minutes to satisfy the following relationship 2:
$225 [C] - 11.3 [Mn] + 37.5 [Cr] - 2.5 [Ni] + 228.6 [V] + 338.8 [Mo] \geq 220$

$200 \leq tempering heat treatment temperature \leq T_{\max}$
in relationship 2, T_max is 112.5[C]-5.7[Mn]+18.8[Cr]-1.3[Ni]+114.3[V]+169.4[Mo]+200, and in relationship 1 and T_max, amounts of elements are expressed in wt%.

Advantageous Effects of Invention

According to an aspect of the present disclosure, high-hardness bulletproof steel having excellent low-temperature toughness, and a method of manufacturing the same may be provided.

Best Mode for Invention

Hereinafter, high-hardness bulletproof steel having excellent low-temperature toughness according to an embodiment of the present disclosure will be described. First, an alloy composition of the present disclosure will be described. An amount of the alloy composition described below is in wt%.

Carbon (C): 0.28 to 0.32%

Carbon (C) may be an element effective in improving strength and hardness in steel having a low-temperature transformation phase such as martensite or bainite, and effective in improving hardenability. When an amount thereof is less than 0.28%, it may be difficult to obtain the above-mentioned effects, and when an amount thereof exceeds 0.32%, there may be a concern that weldability and toughness of the steel may be impaired. Therefore, an amount of C may be 0.28 to 0.32%. A lower limit of the amount of C is preferably 0.29%. An upper limit of the amount of C is preferably 0.31%.

Silicon (Si): 0.5% or less (excluding 0%)

Silicon (Si) may be an element effective in improving hardness due to solid solution strengthening along with a deoxidation effect, and particularly advantageous for securing low-temperature toughness by suppressing formation of coarse cementite in tempered martensite. When an amount thereof is excessive, surface quality and weldability may be rapidly deteriorated. Therefore, in the present disclosure, an amount of Si may be limited to 0.5% or less. The amount of Si is preferably 0.48% or less, even more preferably 0.45% or less, and most preferably 0.4% or less.

Manganese (Mn): 0.2 to 1.1%

Manganese (Mn) may be an element advantageous for securing hardness by improving quenchability of steel to suppress formation of ferrite and promote formation of martensite. When an amount thereof is less than 0.2%, it may be difficult to obtain the above-mentioned effects, and when an amount thereof exceeds 1.1%, weldability of the steel may be deteriorated and center segregation may be promoted, to have a risk of lowering toughness of a central portion of the steel. Therefore, an amount of Mn may be 0.2 to 1.1%. A lower limit of the amount of Mn is preferably 0.22%, even more preferably 0.25%, and most preferably 0.3%. An upper limit of the amount of Mn is preferably 1.08%, even more preferably 1.05%, and most preferably 1.0%.

Nickel (Ni): 0.7 to 1.2%

Nickel (Ni) may be an element advantageous for simultaneously improving strength and toughness of steel. When an amount thereof is less than 0.7%, it may be difficult to obtain the above-mentioned effects, and when an amount thereof exceeds 1.2%, since Ni may be an expensive element, economic efficiency may be reduced and weldability may be deteriorated. Therefore, an amount of Ni may be 0.7 to 1.2%. A lower limit of the amount of Ni is preferably 0.72%, even more preferably 0.75%, and most preferably 0.8%. An upper limit of the amount of Ni is preferably 1.18%, even more preferably 1.15%, and most preferably 1.1%.

Chromium (Cr): 0.2 to 1.1%

Chromium (Cr) may be an element improving strength by increasing quenchability of steel and effectively contributing to securing hardness of surface and central portion of steel. In addition, since Cr may be a relatively inexpensive element, Cr may be also an element capable of securing hardness and toughness economically. When an amount thereof is less than 0.2%, it may be difficult to obtain the above-mentioned effects, and when an amount thereof exceeds 1.1%, weldability may be deteriorated. Therefore, an amount of Cr may be 0.2 to 1.1%. A lower limit of the amount of Cr is preferably 0.22%, even more preferably 0.25%, and most preferably 0.3%. An upper limit of the amount of Cr is preferably 1.08%, more preferably 1.05%, and most preferably 1.0%.

Phosphorus (P): 0.03% or less (excluding 0%)

Phosphorus (P) may be an impurity that may be unavoidably contained, and since P may be an element that inhibits weldability of steel and may be a major cause of increasing brittleness by segregating at grain boundaries, it may be desirable to control an amount thereof as low as possible. Theoretically, it may be advantageous to limit an amount of P to 0%, but it may be inevitably contained in the manufacturing process. Therefore, it may be important to manage an upper limit thereof, and in the present disclosure, an amount of P may be limited to 0.03% or less. The amount of P is preferably 0.025% or less, even more preferably 0.02% or less, and most preferably 0.015% or less.

Sulfur (S): 0.015% or less (excluding 0%)

Sulfur (S) may be an impurity that may be unavoidably contained like phosphorus (P), and is preferable to suppress an amount thereof as much as possible because it combines with Mn or the like to form a non-metallic inclusion, thereby greatly reducing toughness of steel. Theoretically, it may be advantageous to limit an amount of S to 0%, but it may be inevitably contained in the manufacturing process. Therefore, it may be important to manage an upper limit, and in the present disclosure, an amount of S may be limited to 0.015% or less. The amount of S is preferably 0.01% or less, even more preferably 0.008% or less, and most preferably 0.006% or less.

Nitrogen (N): 0.006% or less (excluding 0%)

Nitrogen (N) may contribute somewhat to securing hardness of steel, but it may be difficult to control, and, like phosphorus (P), segregates at grain boundaries and serves to increase brittleness of the steel. Theoretically, it may be advantageous to limit an amount of N to 0%, but it may be inevitably contained in the manufacturing process. Therefore, it may be important to manage an upper limit thereof, and in the present disclosure, an amount of N may be limited to 0.006% or less. The amount of N is preferably 0.0058% or less, even more preferably 0.0055% or less, and most preferably 0.005% or less.

Aluminum (Al): 0.05% or less (excluding 0%)

Aluminum (Al) may be an element added for deoxidation of molten steel. When an amount thereof exceeds 0.05%, it may be advantageous to increase strength by grain refinement, but there may be a problem of causing nozzle clogging during steelmaking or continuous casting. Therefore, in the present disclosure, an amount of Al may be limited to 0.05% or less. The amount of Al is preferably 0.048% or less, even more preferably 0.045% or less, and most preferably 0.04% or less.

Molybdenum (Mo): 0.2 to 1.0%

Molybdenum (Mo) may be an element increasing quenchability of steel, and particularly advantageous for improving low-temperature toughness. When an amount thereof is less than 0.2%, it may be difficult to obtain the above-mentioned effects, and when an amount thereof exceeds 1.0%, manufacturing costs may increase as well as poor weldability. Therefore, an amount of Mo may be 0.2 to 1.0%. A lower limit of the amount of Mo is preferably 0.22%, even more preferably 0.25%, and most preferably 0.3%. An upper limit of the amount of Mo is preferably 0.98%, more preferably 0.95%, and most preferably 0.9%.

Vanadium (V): 0.02 to 0.5%

Vanadium (V) may be an element advantageous for securing hardness and toughness by forming VC carbide during reheating after hot-rolling to suppress growth of an austenite crystal grain and improving quenchability of steel. When an amount thereof is less than 0.02%, it may be difficult to obtain the above-mentioned effects, and when an amount thereof exceeds 0.5%, since V may be an expensive element, economic efficiency may be reduced and toughness may also be deteriorated. Therefore, an amount of V may be 0.02 to 0.5%. A lower limit of the amount of V is preferably 0.022%, even more preferably 0.025%, and most preferably 0.03%. An upper limit of the amount of V is preferably 0.48%, even more preferably 0.45%, and most preferably 0.4%.

Calcium (Ca): 0.0005 to 0.004%

Calcium (Ca) may be an element having a good binding force with sulfur (S) to generate CaS around (circumference of) MnS, to suppress elongation of MnS, and advantageous for improving toughness in a direction, perpendicular to a rolling direction. In addition, CaS produced by addition of calcium (Ca) may increase resistance to corrosion in a humid external environment. When an amount thereof is less than 0.0005%, it may be difficult to obtain the above-mentioned effects, and when an amount thereof exceeds 0.004%, defects such as nozzle clogging or the like may be caused during a steelmaking operation. Therefore, an amount of Ca may be 0.0005 to 0.004%. A lower limit of the amount of Ca is preferably 0.0006%, more preferably 0.0007%, and most preferably 0.0008%. An upper limit of the amount of Ca is preferably 0.0038%, more preferably 0.0035%, and most preferably 0.003%.
In addition, a remaining component of the present disclosure may be iron (Fe). Since unintended impurities from raw materials or surrounding environment may inevitably be mixed in a normal manufacturing process, this cannot be excluded. Since the above impurities may be known to anyone skilled in the art, all of them may not be specifically mentioned in the present disclosure.
In the present disclosure, the unavoidable impurities may include Nb: 0.0015% or less and B: 0.0008% or less. The Nb may form coarse precipitates after QT heat treatment to reduce low-temperature toughness, and when the B is non-uniformly segregated, as a difference in phase transformation time in a quenching heat treatment process causes poor low-temperature toughness, unevenness in effective grain size of tempered martensite may be generated to degrade low-temperature toughness and cause a shape defect. Therefore, in the present disclosure, low-temperature toughness may be improved by controlling an upper limit of amounts that may be included as an impurity without intentionally adding Nb and B. The Nb is preferably 0.0012% or less, and even more preferably 0.001% or less. The B is preferably 0.0006% or less, and even more preferably 0.0005% or less.
Bulletproof steel of the present disclosure may satisfy the above-described alloy composition, and may satisfy the following relationship 1 at the same time. The following relationship 1 may be an equation derived in order to sufficiently obtain an effect of improving hardness, and when relationship 1 is not satisfied, it may be difficult to secure high hardness. A value of the following relationship 1 is preferably 225 or more, more preferably 230 or more, and most preferably 235 or more. The higher the value of the relationship 1, the more advantageous it may be to secure high hardness. In the present disclosure, an upper limit of the amount of value of the relationship 1 may not be particularly limited. However, in terms of manufacturing cost, the upper limit of the amount of value of Equation 1 below may be 600. An upper limit of the amount of value of the following relationship 1 is more preferably 580, even more preferably 550, and most preferably 500. 225[C] - 11.3[Mn] + 37.5[Cr] - 2.5[Ni] + 228.6[V] + 338.8[Mo] ≥ 220
in the relationship 1, amounts of elements are expressed in wt%.
The bulletproof steel of the present disclosure may include a microstructure with tempered martensite at least 90% (including 100%), by area. The tempered martensite may be a structure that may be advantageous for securing excellent levels of hardness and low-temperature toughness at the same time. When a fraction of the tempered martensite is less than 90%, there may be some advantages in improving low-temperature toughness by the secondary phase, but it may be disadvantageous in securing hardness. It may be advantageous that the fraction of the tempered martensite is theoretically 100%, but a secondary phase may inevitably be formed in a manufacturing process. The secondary phase may be one or more of retained austenite, bainite, pearlite, or ferrite, and the fraction thereof is preferably 10 area% or less in total. When the fraction of the secondary phase exceeds 10%, there may be some advantages in improving low-temperature toughness, but it may be disadvantageous in securing hardness. The fraction of the tempered martensite is more preferably 92% or more, even more preferably 94% or more, and most preferably 95% or more.
An average effective grain size of the tempered martensite may be 20 um or less. In the present disclosure, the low-temperature toughness may be improved to a more excellent level by minimizing the average effective grain size of the tempered martensite, and when the size exceeds 20 um, it may be difficult to sufficiently obtain the above-described effect. The average effective grain size of the tempered martensite is more preferably 18 um or less, even more preferably 15 um or less, and most preferably 12 um or less. The average effective grain size means an average size of grains having a grain boundary with a high angle of 15° or more.
KAM of the tempered martensite may be 0.3 to 3.0. The KAM may be an index for estimating a dislocation density. It may be interpreted that the higher the KAM, the higher the dislocation density. In the present disclosure, when the KAM is less than 0.3, it may be difficult to secure sufficient hardness due to low dislocation density, and when it exceeds 3.0, it may be difficult to secure low-temperature toughness. A lower limit of KAM is more preferably 0.4, even more preferably 0.45, and most preferably 0.5. An upper limit of KAM is more preferably 2.8, even more preferably 2.5, and most preferably 2.0.
A V(C,N)-based precipitate may be included in a grain of the tempered martensite, and an average size of the V(C,N)-based precipitate may be preferably 30 nm or less. In this manner, a size of V(C,N)-based precipitate may be miniaturized to secure high hardness through precipitation strengthening. When the average size of the V(C,N)-based precipitate exceeds 30 nm, it may be difficult to sufficiently obtain the above-described effects. The average size of the V(C,N)-based precipitate is more preferably 28 nm or less, even more preferably 25 nm or less, and most preferably 20 nm or less. The V(C,N)-based precipitate may include a V-based carbide, a V-based nitride, a V-based carbonitride, or the like.
In the V(C,N)-based precipitate, it is preferable that a ratio of the number of precipitates having a size greater than 100 nm relative to the total number of precipitates may be 10% or less. When the number of V(C,N)-based precipitates exceeding 100 nm in size exceeds 10% relative to the total number of precipitates, not only a precipitation hardening effect may be reduced, but also low-temperature toughness may be deteriorated. The number of precipitates having a size exceeding 100 nm is more preferably 8% or less, more preferably 6% or less, and most preferably 5% or less, relative to the total number of precipitates.
Bulletproof steel according to an embodiment of the present disclosure provided as described above may have a surface hardness of 480 to 530 HB, an impact absorption energy of 16 J or more at -40°C, and a thickness of 5 to 40 mm.
Hereinafter, a method for manufacturing high-hardness bulletproof steel having excellent low-temperature toughness according to an embodiment of the present disclosure will be described.
First, a slab satisfying the above alloy composition and relationship 1 may be heated at 1050 to 1250°C. When a heating temperature of the slab is less than 1050°C, deformation resistance of steel increases and a subsequent rolling process may not be effectively performed. When a heating temperature of the slab exceeds 1250°C, austenite crystal grains become coarse and low-temperature toughness may deteriorate. A lower limit of the heating temperature of the slab is more preferably 1060°C, more preferably 1070°C, and most preferably 1080°C. An upper limit of the heating temperature of the slab is more preferably 1240°C, more preferably 1230°C, and most preferably 1220°C.
Thereafter, the heated slab may be rough-rolled at 950 to 1150°C to obtain a bar. When a rough-rolling temperature is less than 950°C, a rolling load may increase and deformation may not be sufficiently transmitted to a center of the slab in a thickness direction, as the rolling load increases and reduction is relatively weak. As a result, there may be a risk that defects such as voids may not be removed. When the temperature exceeds 1150°C, a recrystallized grain size may become excessively coarse, and there may be a possibility that toughness may deteriorate. A lower limit of a rough-rolling temperature is more preferably 960°C, even more preferably 970°C, and most preferably 980°C. An upper limit of a rough-rolling temperature is more preferably 1140°C, even more preferably 1130°C, and most preferably 1120°C.
Thereafter, the bar may be finish hot-rolled at 850 to 950°C to obtain a hot-rolled steel sheet, and then cooled to room temperature. When a finish hot-rolling temperature is less than 850°C, two-phase rolling may be performed and ferrite may be generated in a microstructure. When a finish hot-rolling temperature exceeds 950°C, a grain size of a final microstructure becomes coarse, resulting in poor low-temperature toughness. A lower limit of the finish hot-rolling temperature is more preferably 860°C, even more preferably 870°C, and most preferably 880°C. An upper limit of the finish hot-rolling temperature is more preferably 945°C, even more preferably 940°C, and most preferably 935°C.
Thereafter, a first heat treatment of heating the cooled hot-rolled steel sheet to 880 to 930°C for 1.3 t + 30 minutes or more (t: sheet thickness (mm)) and then cooling to 150°C or less at a cooling rate of 10°C/s or more may be performed. Heating during the first heat treatment may be for reverse transformation of the hot-rolled steel sheet in which the microstructure includes ferrite and pearlite into an austenite single phase. When a heating temperature during the first heat treatment is less than 880°C, austenitization may not be sufficiently achieved and coarse soft ferrite may be mixed, and thus hardness of a final product may be lowered. When the temperature exceeds 930°C, an austenite grain may become coarse and quenchability may increase, but low-temperature toughness may deteriorate and there may be a disadvantage in terms of thermal efficiency during mass production. A lower limit of the heating temperature during the first heat treatment is more preferably 882°C, more preferably 885°C, and most preferably 890°C. An upper limit of the heating temperature during the first heat treatment is more preferably 928°C, more preferably 925°C, and most preferably 920°C. When a heating time during the first heat treatment is less than 1.3 t + 30 minutes, austenitization may not sufficiently occur, and thus phase transformation by rapid cooling, e.g., martensitic structure may not be sufficiently obtained. In addition, since a coarse V(C,N)-based precipitate exceeding 100 nm precipitated in the hot-rolling process may not be re-dissolved, formation of a fine precipitate in a subsequent tempering process may be insufficient, such that a precipitation strengthening effect may not be sufficiently obtained, and low-temperature toughness may also degrade. The heating time in the first heat treatment is more preferably 1.3 t + 35 minutes or more, more preferably 1.3 t + 40 minutes or more, and most preferably 1.3 t + 45 minutes or more. Since the longer the heating time during the first heat treatment is, the more favorable for austenitization and re-dissolution of coarse V(C,N)-based precipitate. Therefore, an upper limit of the heating time is not particularly limited in the present disclosure. In terms of preventing deterioration of low-temperature toughness and productivity, the upper limit of the heating time during the first heat treatment may be 1.3 t + 60 minutes. The cooling may be intended to transform an austenitized microstructure into martensite. The cooling may be rapid cooling through water cooling. When the cooling rate is less than 10°C/s or a cooling end temperature exceeds 150°C, ferrite or bainite may be excessively formed during cooling. The cooling rate is more preferably 12°C/s or more, more preferably 15°C/s or more, and most preferably 20°C/s or more. Since the faster the cooling rate is, the more advantageous the martensite transformation is, in the present disclosure, an upper limit of the cooling rate is not particularly limited. Due to limitations of a facility, it may be difficult for the cooling rate to exceed 150°C/s. The cooling end temperature is more preferably 100°C or less, even more preferably 80°C or less, and most preferably 50°C or less. In the present disclosure, a lower limit of the cooling end temperature is not particularly limited, and may be, for example, room temperature.
Thereafter, a second heat treatment may be performed on the hot-rolled steel sheet subjected to the first heat treatment by tempering heat treatment for 1.5 t + 32 minutes (t: sheet thickness (mm)) to 1.5 t + 60 minutes to satisfy the following relationship 2. The tempering heat treatment may release an internal stress of the hot-rolled steel sheet in which the microstructure is transformed into martensite by the first heat treatment to secure excellent low-temperature toughness, and to secure high hardness by precipitating a fine V(C,N)-based precipitate. When the tempering heat treatment temperature is less than 200°C, it is possible to prevent the decrease in hardness, but there may be a disadvantage in securing low-temperature toughness because the internal stress may not be sufficiently released after quenching. This may decrease hardness of the final product. A lower limit of the tempering heat treatment temperature is more preferably 202°C, even more preferably 205°C, and most preferably 210°C. An upper limit of the tempering heat treatment temperature is more preferably T_max-5°C, more preferably T_max-10°C, and most preferably T_max-15°C. When the tempering heat treatment time is less than 1.5 t + 32 minutes, a central portion of a thickness, compared to a surface, may not be sufficiently heated, as heat treatment is performed for a short time. In addition, since formation of a fine precipitate may be insufficient, an precipitation strengthening effect may not be sufficiently obtained, and low-temperature toughness may be also reduced. When the tempering heat treatment time exceeds 1.5 t + 60 minutes, it may not be easy to secure a desired high hardness because dislocation in martensite may be reduced. A lower limit of the tempering heat treatment time is more preferably 1.5 t + 35 minutes, more preferably 1.5 t + 38 minutes, and most preferably 1.5 t + 40 minutes. An upper limit of the tempering heat treatment time is more preferably 1.5 t + 57 minutes, more preferably 1.5 t + 55 minutes, and most preferably 1.5 t + 50 minutes. 200 ≤ tempering heat treatment temperature ≤ T_max
in relationship 2, T_max is 112.5[C]-5.7[Mn]+18.8[Cr]-1.3[Ni]+114.3[V]+169.4[Mo]+200, and in relationship 1 and T_max, amounts of elements are expressed in wt%.

Mode for Invention

Hereinafter, the present disclosure will be described in more detail through examples. However, the following examples may be only examples for explaining the present disclosure in detail, and do not limit the scope of the present disclosure.

(Example)

After preparing slabs having alloy compositions illustrated in Tables 1 and 2 below, according to manufacturing conditions illustrated in Tables 3 and 4 below, slab heating - rough-rolling - finish hot-rolling - 1st heat treatment - 2nd heat treatment were performed to produce bulletproof steel. In this case, after water cooling to a cooling end temperature during the first heat treatment, air cooling was applied to room temperature, and air cooling was applied to room temperature after the second heat treatment. A microstructure and mechanical properties of the bulletproof steel thus prepared were measured, and results thereof were illustrated in Table 5 below.
The microstructure was prepared by cutting a manufactured steel sheet into an arbitrary size to make a specimen, mirror-processing the same, corroding the same using a nital etchant, and a 1/4t (t: thickness) portion of the steel sheet was observed using a scanning electron microscope. In addition, an average effective grain size of tempered martensite was measured based on a misorientation angle of 15° or more using EBSD. In addition, KAM of tempered martensite was examined using a scanning electron microscope (SEM) JSM-7001F manufactured by JEOL Co., Ltd., on a polished surface of a cross section in a rolling direction of the steel sheet, in a field of view of 100 µm × 100 um electron backscatter diffraction (EBSD) analysis (measurement step: 0.05 um) was performed at a position of 1/4 of a plate thickness, and an average value of the orientation difference (°) between each pixel in a crystal grain and an adjacent pixel was calculated.
V(C,N)-based precipitates formed in grains of the tempered martensite were prepared by cutting the manufactured steel sheet into a random size and then preparing a specimen, and then at a magnification of 50,000 times (2 um × 2 um) at 5 random areas, and then an average value thereof was calculated.
Hardness was measured 3 times using a Brinell hardness tester (load 3000 kgf, 10 mm tungsten indentation) after milling a surface of the steel sheet by 2 mm, and then expressed as an average value.

The low-temperature toughness was measured 3 times at -40°C after processing a 1/4t (t: thickness) portion of the steel sheet into a specimen using a Charpy impact tester, and then expressed as an average value of impact energy.

[Table 1]

Steel Type No.	Alloy Composition (wt%)
Steel Type No.	C	Si	Mn	P	S	Ni	Cr
Inventive Steel 1	0.3	0.3	0.5	0.007	0.002	1.0	0.5
Inventive Steel 2	0.3	0.3	0.5	0.007	0.002	1.0	0.5
Inventive Steel 3	0.3	0.3	0.5	0.007	0.002	1.0	0.5
Inventive Steel 4	0.3	0.3	0.5	0.007	0.002	1.0	0.7
Inventive Steel 5	0.3	0.3	0.5	0.007	0.002	1.0	0.5
Inventive Steel 6	0.3	0.3	0.5	0.007	0.002	1.0	0.7
Comparative Steel 1	0.3	0.3	0.5	0.007	0.002	1.0	0.5
Comparative Steel 2	0.3	0.3	0.8	0.007	0.002	1.0	0.5
Comparative Steel 3	0.3	0.3	0.5	0.007	0.002	1.0	0.5
Comparative Steel 4	0.3	0.3	0.5	0.007	0.002	1.0	0.5

[Table 2]

Steel Type No.	Alloy Composition (wt%)
Steel Type No.	Mo	V	Al	N	Nb	B	Relationship 1	Relationship 2
Inventive Steel 1	0.5	0.40	0.03	0.004	0.0008	0.0001	338.9	369
Inventive Steel 2	0.5	0.05	0.03	0.004	0.0009	0.0003	258.9	329
Inventive Steel 3	0.5	0.10	0.03	0.004	0.0006	0.0002	270.4	335
Inventive Steel 4	0.8	0.20	0.03	0.004	0.0007	0	402.4	401
Inventive Steel 5	0.5	0.10	0.03	0.004	0.0006	0.0003	270.4	335
Inventive Steel 6	0.8	0.20	0.03	0.004	0.0009	0.0001	402.4	401
Comparative Steel 1	0.4	0.01	0.03	0.004	0.0008	0	215.9	308
Comparative Steel 2	0.4	0.03	0.03	0.004	0.0007	0.0001	217.1	308
Comparative Steel 3	0.5	0.40	0.03	0.004	0.015	0.0001	338.9	369
Comparative Steel 4	0.5	0.40	0.03	0.004	0.0008	0.0012	338.9	369
[Relationship 1] = 225[C]-11.3[Mn]+37.5[Cr]-2.5[Ni]+228.6[V]+338.8[Mo]
[Relationship 2] = 112.5[C]-5.7[Mn]+18.8[Cr]-1.3[Ni]+114.3[V]+169.4[Mo]+200

[Table 3]

	Steel Type No,	Slab Heating Temp. (°C)	Rolling		Steel Material Thickness (mm)
	Steel Type No,	Slab Heating Temp. (°C)	Rough Rolling Temp. (°C)	Finish Rolling Temp. (°C)	Steel Material Thickness (mm)
Inventive Example 1	Inventive Steel 1	1159	1059	890	12
Inventive Example 2	Inventive Steel 2	1125	1018	924	18
Inventive Example 3	Inventive Steel 3	1176	1063	899	15
Inventive Example 4	Inventive Steel 4	1164	1022	909	25
Comparative Example 1	Inventive Steel 5	1165	1040	887	12
Comparative Example 2	Inventive Steel 6	1165	1025	923	25
Comparative Example 3	Comparative Steel 1	1149	1029	928	25
Comparative Example 4	Comparative Steel 2	1147	1055	861	10
Comparative Example 5	Comparative Steel 3	1156	1041	906	17
Comparative Example 6	Comparative Steel 4	1157	1037	900	18
Comparative	Inventive	1156	1039	903	20
Example 7	Steel 1
Comparative Example 8	Inventive Steel 1	1158	1041	906	16
Comparative Example 9	Inventive Steel 1	1155	1038	900	19

[Table 4]

	Steel Type No.	1^st Heat Treatment				2^nd Heat Treatment
	Steel Type No.	Heat Temp. (°C)	Heat Time (min)	Cooling rate (°C/s)	Cooling End Temp. (°C)	Tempering Heat Treatment Temp. (°C)	Tempering Heat Treatment Time (min)
Inventive Example 1	Inventive Steel 1	911	51	65	25	250	53
Inventive Example 2	Inventive Steel 2	910	58	39	29	200	62
Inventive Example 3	Inventive Steel 3	900	55	48	26	200	58
Inventive Example 4	Inventive Steel 4	907	68	44	29	350	73
Comparative Example 1	Inventive Steel 5	909	51	46	23	130	53
Comparative Example 2	Inventive Steel 6	910	68	45	23	500	73
Comparative Example 3	Comparative Steel 1	913	68	46	37	250	73
Comparative Example 4	Comparative Steel 2	909	48	65	30	100	50
Comparative Example 5	Comparative Steel 3	907	57	27	51	240	61
Comparative Example 6	Comparative Steel 4	910	58	28	50	250	62
Comparative Example 7	Inventive Steel 1	909	10	28	48	230	65
Comparative Example 8	Inventive Steel 1	907	56	27	49	250	10
Comparative	Inventive	910	60	28	50	250	100
Example 9	Steel 1

[Table 5]

	Microstructure						Surf ace Hard ness (HB)	Impact Toughne ss (J, @-40°C)
	Tempered Martensi te (area$)	Tempered Martensite Average Effective Grain Size (µm)	KAM	Precip itate Averag e Size (nm)	Ratio of Number of Precipitates having Size exceeding 100 nm relative to Total Number of Precipitates (%)	Secon dary Phase (area %)	Surf ace Hard ness (HB)	Impact Toughne ss (J, @-40°C)
Inventive Example 1	98	4.6	0.99	8	4	2	503	28
Inventive Example 2	97	3.5	1.42	13	3	3	492	36
Inventive Example 3	99	4.6	1.06	11	2	1	498	37
Inventive Example 4	97	3.2	1.35	7	4	3	507	29
Comparative Example 1	98	7.7	3.35	153	88	2	525	6
Comparative Example 2	97	5.7	0.23	47	8	3	470	59
Comparative Example 3	97	6.3	0.91	33	14	3	446	49
Comparative Example 4	99	5.0	3.43	146	86	1	457	3
Comparative Example 5	98	3.9	1.01	210	54	2	500	11
Comparative Example 6	98	6.2	1.33	18	7	2	505	10
Comparative Example 7	97	22.0	0.91	161	42	3	495	7
Comparative Example 8	97	4.0	3.16	125	64	3	515	5
Comparative Example 9	98	8.3	0.21	46	9	2	466	61
Secondary Phase: at least one of retained austenite, bainite, pearlite, or ferrite

As can be seen from Tables 1 to 5, in Inventive Examples 1 to 4 satisfying the alloy composition and the manufacturing conditions of the present disclosure, surface hardness and impact toughness may be confirmed to be excellent by acquiring the microstructure to be obtained by the present disclosure.
In Comparative Example 1, as the alloy composition of the present disclosure was satisfactory, but a tempering heat treatment temperature during the secondary heat treatment was low, it can be seen that low-temperature toughness was low, since an average size of precipitates, a ratio of the number of precipitates having a size exceeding 100 nm relative to the total number of precipitates, and KAM, proposed by the present disclosure, were not satisfied.
In Comparative Example 2, as the alloy composition of the present disclosure was satisfactory, but a tempering heat treatment temperature during the secondary heat treatment was high, it can be seen that hardness was low, since an average size of precipitates and KAM, proposed by the present disclosure, were not satisfied.
In Comparative Example 3, as the manufacturing conditions of the present disclosure were satisfied, but the V content and the conditions of relationship 1 were not satisfied, it can be seen that hardness was low.
In Comparative Example 4, as the condition of relationship 1 of the present disclosure was not satisfied, and a temperature of a tempering heat treatment during a secondary heat treatment were also low, it can be seen that hardness and low-temperature toughness were low, since an average size of precipitates, a ratio of the number of precipitates having a size exceeding 100 nm relative to the total number of precipitates, and KAM, proposed by the present disclosure, were not satisfied.
In Comparative Example 5, as the Nb content was high, it can be seen that low-temperature toughness was low, since a ratio of the number of precipitates having a size exceeding 100 nm relative to the total number of precipitates proposed by the present disclosure was not satisfied.
In Comparative Example 6, it can be seen that low-temperature toughness was low, as an amount of B was high.
In Comparative Example 7, as the alloy composition of the present disclosure was satisfactory, but an average effective crystal grain size of tempered martensite increases as a heating time was not sufficient during a first heat treatment, it can be seen that low-temperature toughness was low, since an average size of precipitates, and a ratio of the number of precipitates having a size exceeding 100 nm relative to the total number of precipitates, proposed by the present disclosure, were not satisfied.
In Comparative Example 8, as the alloy composition of the present disclosure may be satisfactory, but a tempering heat treatment time during a secondary heat treatment may not be sufficient, it can be seen that low-temperature toughness was low, since an average size of precipitates, a ratio of the number of precipitates having a size exceeding 100 nm relative to the total number of precipitates, and KAM, proposed by the present disclosure, were not satisfied.
In Comparative Example 9, as the alloy composition of the present disclosure was satisfied, but a tempering heat treatment time during a secondary heat treatment was excessive, it can be seen that hardness was low, since the average precipitate size and KAM were not satisfied.

Claims

High-hardness bulletproof steel having excellent low-temperature toughness, comprising:
by weight, carbon (C): 0.28 to 0.32%, silicon (Si): 0.5% or less (excluding 0%), manganese (Mn): 0.2 to 1.1%, nickel (Ni): 0.7 to 1.2%, chromium (Cr): 0.2 to 1.1%, phosphorus (P): 0.03% or less (excluding 0%), sulfur (S): 0.015% or less (excluding 0%), nitrogen (N): 0.006% or less (0% excluding), aluminum (Al): 0.05% or less (excluding 0%), molybdenum (Mo): 0.2 to 1.0%, vanadium (V): 0.02 to 0.5%, calcium (Ca): 0.0005 to 0.004%, and a remainder of Fe and unavoidable impurities, the unavoidable impurities including Nb: 0.0015% or less and B: 0.0008% or less,

satisfying the following relationship 1,

including a microstructure with tempered martensite at least 90% (including 100%), by area,

wherein an average effective grain size of the tempered martensite is 20 um or less,

including a V(C,N)-based precipitate having an average size of 30 nm or less in a grain of the tempered martensite: $225 [C] - 11.3 [Mn] + 37.5 [Cr] - 2.5 [Ni] + 228.6 [V] + 338.8 [Mo] \geq 220$
in relationship 1, amounts of elements are expressed in wt%.
The bulletproof steel of claim 1, wherein at least one of retained austenite, bainite, pearlite, or ferrite is included in the microstructure to have 10 area% or less in total.
The bulletproof steel of claim 1, wherein KAM of the tempered martensite is 0.3 to 3.0.
The bulletproof steel of claim 1, wherein, in the V(C,N)-based precipitate, a ratio of the number of precipitates having a size exceeding 100 nm relative to the total number of precipitates is 10% or less.
The bulletproof steel of claim 1, having a surface hardness of 480 to 530HB and an impact absorption energy at -40°C of 16J or more.
The bulletproof steel of claim 1, having a thickness of 5 to 40 mm.
A method for manufacturing a high-hardness bulletproof steel having excellent low-temperature toughness, comprising:
heating a slab including, by weight, carbon (C): 0.28 to 0.32%, silicon (Si): 0.5% or less (excluding 0%), manganese (Mn): 0.2 to 1.1%, nickel (Ni): 0.7 to 1.2%, chromium (Cr): 0.2 to 1.1%, phosphorus (P): 0.03% or less (excluding 0%), sulfur (S): 0.015% or less (excluding 0%), nitrogen (N): 0.006% or less (0% excluding), aluminum (Al): 0.05% or less (excluding 0%), molybdenum (Mo): 0.2 to 1.0%, vanadium (V): 0.02 to 0.5%, calcium (Ca): 0.0005 to 0.004%, and a remainder of Fe and unavoidable impurities, and satisfying the following relationship 1, at 1050 to 1250°C;

rough-rolling the heated slab at 950 to 1150°C to obtain a bar;

finish hot-rolling the bar at 850 to 950°C to obtain a hot-rolled steel sheet, and cooling the same to room temperature;

performing a first heat treatment operation of heating the cooled hot-rolled steel sheet to 880 to 930°C for 1.3 t + 30 minutes or more (t: sheet thickness (mm)) and then cooling the same to 150°C or less at a cooling rate of 10°C/s or more; and

performing a second heat treatment operation of tempering the first heat-treated hot-rolled steel sheet for 1.5 t + 32 minutes (t: sheet thickness (mm)) to 1.5 t + 60 minutes to satisfy the following relationship 2: $225 [C] - 11.3 [Mn] + 37.5 [Cr] - 2.5 [Ni] + 228.6 [V] + 338.8 [Mo] \geq 220$
$200 \leq tempering heat treatment temperature \leq T_{\max}$

in relationship 2, T_max is 112.5[C]-5.7[Mn]+18.8[Cr]-1.3[Ni]+114.3[V]+169.4[Mo]+200, and

in relationship 1 and T_max, amounts of elements are expressed in wt%.