WO2023018101A1 - High-hardness bulletproof steel having excellent low-temperature toughness, and manufacturing method therefor - Google Patents

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

High-hardness bulletproof steel with excellent low-temperature toughness and its manufacturing method

The present invention relates to high-hardness bulletproof steel with excellent low-temperature toughness and a manufacturing method thereof. More specifically, it relates to high-hardness bulletproof steel with excellent low-temperature toughness that can be preferably used for armored vehicles, explosion-proof structures, and the like, and a manufacturing method thereof.

In order to stably prevent the risk of human casualties by blocking bullets on the battlefield, high hardness of bulletproof steel materials that require protection such as armored vehicles and explosion-proof structures are required. This is because high hardness is a factor that increases resistance to prevent bullets from penetrating the material. However, since materials with high hardness can be broken relatively easily, it is necessary to develop materials that can simultaneously secure high hardness and fracture resistance to external impact in order to secure more stable protection.

Patent Document 1 relates to steel materials used for crushing of industrial waste and wear parts of grinders, and seeks to secure surface hardness by actively utilizing Nb together with a large amount of Cr and Mo, but it is difficult to secure low-temperature toughness due to excessive content. There are limits.

Patent Document 2 is a technique for securing high hardness by causing the retained austenite structure to cause plastic-induced machining hardening after forming the steel or when the steel is applied to a product and in use, or in the case of bulletproof steel, deformation / impact by bullets The speed is very high, and it is difficult to obtain the effect by the above phenomenon.

[Prior art literature]

(Patent Document 1) Japanese Unexamined Patent Publication No. 10-102185

(Patent Document 2) Japanese Unexamined Patent Publication No. 07-173571

One aspect of the present invention is to provide a high-hardness bulletproof steel with excellent low-temperature toughness and a manufacturing method thereof.

One embodiment of the present invention, in weight%, carbon (C): 0.28 ~ 0.32%, silicon (Si): 0.5% or less (excluding 0%), manganese (Mn): 0.2 ~ 1.1%, nickel (Ni) : 0.7~1.2%, Chromium (Cr): 0.2~1.1%, Phosphorus (P): 0.03% or less (excluding 0%), Sulfur (S): 0.015% or less (excluding 0%), Nitrogen (N) : 0.006% or less (excluding 0%), 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%, the balance includes Fe and other unavoidable impurities, and the unavoidable impurities include Nb: 0.0015% or less and B: 0.0008% or less, satisfy the following relational expression 1, in area%, tempered martensite contains a microstructure of 90% or more (including 100%), the average effective grain size of the tempered martensite is 20 μm or less, and the average size in the grains of the tempered martensite is 30 nm or less V (C, Provided is a high-hardness bulletproof steel having excellent low-temperature toughness including N)-based precipitates.

[Relational Expression 1] 225[C]-11.3[Mn]+37.5[Cr]-2.5[Ni]+228.6[V]+338.8[Mo] ≥ 220

(However, in the relational expression 1, the content of each element means % by weight.)

Another embodiment of the present invention, in weight percent, carbon (C): 0.28 to 0.32%, silicon (Si): 0.5% or less (excluding 0%), manganese (Mn): 0.2 to 1.1%, nickel (Ni) : 0.7~1.2%, Chromium (Cr): 0.2~1.1%, Phosphorus (P): 0.03% or less (excluding 0%), Sulfur (S): 0.015% or less (excluding 0%), Nitrogen (N) : 0.006% or less (excluding 0%), 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%, the balance includes Fe and other unavoidable impurities, and the unavoidable impurities include Nb: 0.0015% or less and B: 0.0008% or less, heating a slab satisfying the following relational expression 1 at 1050 to 1250 ° C. step; Obtaining a bar by rough rolling the heated slab at 950 to 1150 ° C; After finishing hot rolling the bar at 850 ~ 950 ℃ to obtain a hot-rolled steel sheet, cooling to room temperature; A first heat treatment step 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; And a second heat treatment step of tempering the first heat-treated hot-rolled steel sheet for 1.5t + 32 minutes (t: sheet thickness (mm)) to 1.5t + 60 minutes to satisfy the condition of the following relational expression 2; low-temperature toughness including Provides a method for manufacturing this excellent high hardness bulletproof steel.

[Relational Expression 1] 225[C]-11.3[Mn]+37.5[Cr]-2.5[Ni]+228.6[V]+338.8[Mo] ≥ 220

[Relational Expression 2] 200 ≤ tempering heat treatment temperature ≤ T _max

(However, in the above relational expression 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 the above relational expression 1 and T _max The content of each element means % by weight.)

According to one aspect of the present invention, it is possible to provide a high-hardness bulletproof steel with excellent low-temperature toughness and a manufacturing method thereof.

Hereinafter, high-hardness bulletproof steel excellent in low-temperature toughness according to an embodiment of the present invention will be described. First, the alloy composition of the present invention will be described. The content of the alloy composition described below refers to % by weight.

Carbon (C): 0.28 to 0.32%

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

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

Silicon (Si) is an effective element for improving hardness due to solid solution strengthening along with a deoxidation effect, and is particularly advantageous for securing low-temperature toughness by suppressing the formation of coarse cementite in tempered martensite. However, if the content is excessive, surface quality and weldability may be rapidly deteriorated. Therefore, in the present invention, the content of Si is limited to 0.5% or less. The Si content is more 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) is an element advantageous for securing hardness by improving hardenability of steel, suppressing the formation of ferrite and promoting the formation of martensite. If the content is less than 0.2%, it is difficult to obtain the above-mentioned effects, and if it exceeds 1.1%, the weldability of the steel is deteriorated and center segregation is promoted, so there is a risk of lowering the toughness of the center of the steel. Therefore, the Mn content is preferably in the range of 0.2 to 1.1%. The lower limit of the Mn content is more preferably 0.22%, even more preferably 0.25%, and most preferably 0.3%. The upper limit of the Mn content is more preferably 1.08%, even more preferably 1.05%, and most preferably 1.0%.

Nickel (Ni): 0.7 to 1.2%

Nickel (Ni) is an advantageous element for simultaneously improving the strength and toughness of steel. If the content is less than 0.7%, it is difficult to obtain the above-mentioned effect, and if it exceeds 1.2%, since Ni is an expensive element, economic efficiency may be reduced and weldability may be deteriorated. Therefore, the Ni content is preferably in the range of 0.7 to 1.2%. The lower limit of the Ni content is more preferably 0.72%, even more preferably 0.75%, and most preferably 0.8%. The upper limit of the Ni content is more preferably 1.18%, even more preferably 1.15%, and most preferably 1.1%.

Chromium (Cr): 0.2~1.1%

Chromium (Cr) is an element that improves strength by increasing hardenability of steel and effectively contributes to securing the hardness of the surface and center of steel. In addition, since it is a relatively inexpensive element, it is also an element that can secure hardness and toughness economically. If the content is less than 0.2%, it is difficult to obtain the above-mentioned effect, and if it exceeds 1.1%, weldability may be deteriorated. Therefore, the Cr content is preferably in the range of 0.2 to 1.1%. The lower limit of the Cr content is more preferably 0.22%, even more preferably 0.25%, and most preferably 0.3%. The upper limit of the Cr content is more preferably 1.08%, more preferably 1.05%, and most preferably 1.0%.

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

Phosphorus (P) is an impurity that is unavoidably contained, and since it inhibits the weldability of steel and is a major cause of increasing brittleness by segregating at grain boundaries, it is desirable to control its content as low as possible. Theoretically, it is advantageous to limit the content of P to 0%, but it is inevitably contained in the manufacturing process. Therefore, it is important to manage the upper limit, and in the present invention, the P content is limited to 0.03% or less. The P content is more 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) is an impurity that is unavoidably contained like the phosphorus (P), and it is preferable to suppress the content as much as possible because it combines with Mn and the like to form non-metallic inclusions, thereby greatly reducing the toughness of steel. Theoretically, it is advantageous to limit the content of S to 0%, but it is inevitably contained in the manufacturing process. Therefore, it is important to manage the upper limit, and in the present invention, the content of S is limited to 0.015% or less. The S content is more 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) contributes somewhat to securing the hardness of the steel, but it is difficult to control, and like phosphorus (P), it is segregated at the grain boundary and serves to increase the brittleness of the steel. Theoretically, it is advantageous to limit the content of N to 0%, but it is inevitably contained in the manufacturing process. Therefore, it is important to manage the upper limit, and in the present invention, the N content is limited to 0.006% or less. The N content is more 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) is an element added for deoxidation of molten steel. However, if the content exceeds 0.05%, it may be advantageous to increase strength by grain refinement, but there is a problem of causing nozzle clogging during steelmaking or continuous casting. Therefore, in the present invention, the Al content is limited to 0.05% or less. The Al content is more 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) increases the hardenability of steel, and is particularly advantageous for improving low-temperature toughness. If the content is less than 0.2%, it is difficult to obtain the above-mentioned effects, and if it exceeds 1.0%, manufacturing costs may increase as well as poor weldability. Therefore, the Mo content is preferably in the range of 0.2 to 1.0%. The lower limit of the Mo content is more preferably 0.22%, even more preferably 0.25%, and most preferably 0.3%. The upper limit of the Mo content is more preferably 0.98%, more preferably 0.95%, and most preferably 0.9%.

Vanadium (V): 0.02 to 0.5%

Vanadium (V) is an element advantageous for securing hardness and toughness by suppressing the growth of austenite crystal grains by forming VC carbide during reheating after hot rolling and improving hardenability of steel. If the content is less than 0.02%, it is difficult to obtain the above-mentioned effect, and if it exceeds 0.5%, since V is an expensive element, economic efficiency may be reduced and toughness may also be deteriorated. Therefore, the content of V is preferably in the range of 0.02 to 0.5%. The lower limit of the V content is more preferably 0.022%, even more preferably 0.025%, and most preferably 0.03%. The upper limit of the V content is more preferably 0.48%, even more preferably 0.45%, and most preferably 0.4%.

Calcium (Ca): 0.0005 to 0.004%

Calcium (Ca) has a good binding force with sulfur (S), so it generates CaS around (circumference) MnS to suppress elongation of MnS and is an advantageous element for improving toughness in a direction perpendicular to the rolling direction. In addition, CaS produced by the addition of calcium (Ca) can increase resistance to corrosion in a humid external environment. If the content is less than 0.0005%, it is difficult to obtain the above-mentioned effects, and if it exceeds 0.004%, defects such as nozzle clogging may be caused during steelmaking operations. Therefore, the content of Ca is preferably in the range of 0.0005 to 0.004%. The lower limit of the Ca content is more preferably 0.0006%, more preferably 0.0007%, and most preferably 0.0008%. The upper limit of the Ca content is more preferably 0.0038%, more preferably 0.0035%, and most preferably 0.003%.

In addition, the remaining components of the present invention are iron (Fe). However, since unintended impurities from raw materials or the surrounding environment may inevitably be mixed in a normal manufacturing process, this cannot be excluded. Since the above impurities can be known to anyone skilled in the art, all of them are not specifically mentioned in the present invention.

However, in the present invention, the unavoidable impurities may include Nb: 0.0015% or less and B: 0.0008% or less. The Nb may form coarse precipitates after the QT heat treatment to reduce low-temperature toughness, and if the B is non-uniformly segregated, it causes a difference in the phase transformation time in the quenching heat treatment process. may cause poor low-temperature toughness, and may cause shape defects. Therefore, in the present invention, low-temperature toughness can be improved by controlling the upper limit of the amount that can be included as an impurity without intentionally adding Nb and B. The Nb content is more preferably 0.0012% or less, and even more preferably 0.001% or less. The B content is more preferably 0.0006% or less, and even more preferably 0.0005% or less.

On the other hand, the bulletproof steel of the present invention preferably satisfies the above-described alloy composition and at the same time satisfies the following relational expression 1. The following Relational Equation 1 is an equation derived in order to sufficiently obtain an effect of improving hardness, and when Relational Equation 1 is not satisfied, it is difficult to secure high hardness. The value of the following relational expression 1 is more preferably 225 or more, more preferably 230 or more, and most preferably 235 or more. On the other hand, the higher the value of the relational expression 1, the more advantageous it is to secure high hardness. In the present invention, the upper limit of the value of the relational expression 1 is not particularly limited. However, in terms of manufacturing cost, the upper limit of the value of Equation 1 below may be 600. The upper limit of the value of the following relational expression 1 is more preferably 580, even more preferably 550, and most preferably 500.

[Relational Expression 1] 225[C]-11.3[Mn]+37.5[Cr]-2.5[Ni]+228.6[V]+338.8[Mo] ≥ 220

(However, in the relational expression 1, the content of each element means % by weight.)

The bulletproof steel of the present invention preferably includes a microstructure in which tempered martensite is 90% or more (including 100%) by area%. The tempered martensite is a structure that is advantageous for securing excellent levels of hardness and low-temperature toughness at the same time. When the 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 is advantageous that the fraction of the tempered martensite is theoretically 100%, but a secondary phase may inevitably be formed in the manufacturing process. The secondary phase may be one or more of retained austenite, bainite, pearlite, and 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.

The average effective grain size of the tempered martensite is preferably 20 μm or less. In the present invention, the low-temperature toughness can be improved to a more excellent level by minimizing the average effective grain size of the tempered martensite, and when the size exceeds 20 μm, it may be difficult to sufficiently obtain the above-described effect. The average effective grain size of the tempered martensite is more preferably 18 μm or less, even more preferably 15 μm or less, and most preferably 12 μm or less. On the other hand, the average effective grain size means the average size of grains having a grain boundary with a high angle of 15° or more.

The KAM of the tempered martensite is preferably 0.3 to 3.0. The KAM is an index for estimating the dislocation density. It is interpreted that the higher the KAM, the higher the dislocation density. In the present invention, 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. The lower limit of the KAM is more preferably 0.4, even more preferably 0.45, and most preferably 0.5. The upper limit of the KAM is more preferably 2.8, even more preferably 2.5, and most preferably 2.0.

V(C,N)-based precipitates are included in the grains of the tempered martensite, and the average size of the V(C,N)-based precipitates is preferably 30 nm or less. In this way, by miniaturizing the size of V(C,N)-based precipitates, high hardness can be secured through precipitation strengthening. On the other hand, when the average size of the V(C,N)-based precipitates exceeds 30 nm, it may be difficult to sufficiently obtain the above-described effects. The average size of the V(C,N)-based precipitates 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 precipitates may include V-based carbides, V-based nitrides, and V-based carbonitrides.

In the V(C,N)-based precipitates, it is preferable that the ratio of the number of precipitates having a size greater than 100 nm to the total number of precipitates is 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 the precipitation hardening effect may be reduced, but also the 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.

The bulletproof steel according to one embodiment of the present invention 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 invention will be described.

First, a slab satisfying the above alloy composition and relational expression 1 is heated at 1050 to 1250 ° C. If the slab heating temperature is less than 1050 ° C., the deformation resistance of the steel increases and the subsequent rolling process cannot be effectively performed. On the other hand, if it exceeds 1250 ° C., austenite crystal grains become coarse and low-temperature toughness may deteriorate. The lower limit of the slab heating temperature is more preferably 1060 ° C, more preferably 1070 ° C, and most preferably 1080 ° C. The upper limit of the slab heating temperature is more preferably 1240 ° C, more preferably 1230 ° C, and most preferably 1220 ° C.

Thereafter, the heated slab is crudely rolled at 950 to 1150° C. to obtain a bar. If the rough rolling temperature is less than 950 ° C., the rolling load increases and the deformation is not sufficiently transmitted to the center in the thickness direction of the slab as the rolling load increases and the reduction is relatively weak, and as a result, there is a risk that defects such as voids may not be removed. On the other hand, when the temperature exceeds 1150 ° C., the recrystallized grain size becomes excessively coarse, and there is a possibility that the toughness may deteriorate. The lower limit of the rough rolling temperature is more preferably 960°C, even more preferably 970°C, and most preferably 980°C. The upper limit of the rough rolling temperature is more preferably 1140°C, even more preferably 1130°C, and most preferably 1120°C.

Thereafter, the bar is finished hot-rolled at 850 to 950° C. to obtain a hot-rolled steel sheet, and then cooled to room temperature. If the finish hot rolling temperature is less than 850 ° C, two-phase rolling is performed and ferrite may be generated in the microstructure. On the other hand, if it exceeds 950 ° C, the grain size of the final microstructure becomes coarse, resulting in poor low-temperature toughness. . The lower limit of the finish hot rolling temperature is more preferably 860°C, even more preferably 870°C, and most preferably 880°C. The 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 do Heating during the primary heat treatment is for reverse transformation of the hot-rolled steel sheet in which the microstructure is composed of ferrite and pearlite into an austenite single phase. When the heating temperature during the first heat treatment is less than 880° C., austenitization is not sufficiently achieved and coarse soft ferrite is mixed, and thus the hardness of the final product may be lowered. On the other hand, when the temperature exceeds 930 ° C., the austenite grains become coarse and hardenability increases, but the low-temperature toughness deteriorates and there is a disadvantage in terms of thermal efficiency during mass production. The 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. The 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 the heating time during the first heat treatment is less than 1.3 t + 30 minutes, austenitization does not sufficiently occur, and thus phase transformation by rapid cooling, that is, martensitic structure cannot be sufficiently obtained. In addition, since the coarse V (C, N)-based precipitates exceeding 100 nm precipitated in the hot rolling process cannot be re-dissolved, the formation of fine precipitates in the subsequent tempering process is insufficient, so that the precipitation strengthening effect cannot be sufficiently obtained, and low-temperature toughness also degrades The heating time in the first heat treatment is more preferably 1.3t + 35 minutes or more, more preferably 1.3t + 40 minutes or more, and most preferably 1.3t + 45 minutes or more. On the other hand, since the longer the heating time during the primary heat treatment is, the more favorable it is for austenitization and the re-dissolution of coarse V(C,N)-based precipitates, so the upper limit of the heating time is not particularly limited in the present invention. However, 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 1.3t + 60 minutes. The cooling is intended to transform the austenitized microstructure into martensite. The cooling is preferably rapid cooling through water cooling. When the cooling rate is less than 10 °C/s or the 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. On the other hand, since the faster the cooling rate is, the more advantageous the martensite transformation is, in the present invention, the upper limit of the cooling rate is not particularly limited. However, due to the limitations of the facility, it is 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 invention, the lower limit of the cooling end temperature is not particularly limited, and may be, for example, room temperature.

Thereafter, a second heat treatment is performed on the hot-rolled steel sheet subjected to the first heat treatment by tempering heat treatment for 1.5t + 32 minutes (t: sheet thickness (mm)) to 1.5t + 60 minutes to satisfy the condition of the following relational expression 2. The tempering heat treatment releases the 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 fine V (C, N)-based precipitates. will be. If the tempering heat treatment temperature is less than 200 ° C., it is possible to prevent the decrease in hardness, but there is a disadvantage in securing low-temperature toughness because the internal stress is not sufficiently released after quenching. On the other hand, if it exceeds T _max , the dislocation density inside martensite The hardness of the final product may be reduced due to excessive reduction. The lower limit of the tempering heat treatment temperature is more preferably 202°C, even more preferably 205°C, and most preferably 210°C. The 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 1.5t + 32 minutes, the center of the thickness compared to the surface may not be sufficiently heated as the heat treatment is performed for a short time. In addition, since the formation of fine precipitates is insufficient, the precipitation strengthening effect cannot be sufficiently obtained, and the low-temperature toughness is also reduced. On the other hand, when the tempering heat treatment time exceeds 1.5 t + 60 minutes, it may not be easy to secure a desired high hardness because the dislocation inside martensite is reduced. The lower limit of the tempering heat treatment time is more preferably 1.5t + 35 minutes, more preferably 1.5t + 38 minutes, and most preferably 1.5t + 40 minutes. The upper limit of the tempering heat treatment time is more preferably 1.5t + 57 minutes, more preferably 1.5t + 55 minutes, and most preferably 1.5t + 50 minutes.

[Relational Expression 2] 200 ≤ tempering heat treatment temperature ≤ T _max

(However, in relational expression 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 the T _max Content means % by weight.)

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

(Example)

After preparing the slabs having the alloy compositions shown in Tables 1 and 2 below, according to the manufacturing conditions shown in Tables 3 and 4 below, the slab heating - rough rolling - finish hot rolling - primary heat treatment - secondary heat treatment are performed to produce bulletproof steel did At this time, air cooling was applied to room temperature after water cooling to the cooling end temperature during the first heat treatment, and air cooling was applied to room temperature after the second heat treatment. The microstructure and mechanical properties of the bulletproof steel thus prepared were measured, and the results are shown in Table 5 below.

The microstructure was prepared by cutting the manufactured steel sheet into an arbitrary size to make a specimen, mirror-processing it, corroding it using a nital etchant, and using a scanning electron microscope to examine the 1/4t (t: thickness) portion of the steel sheet. Observed. In addition, the 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 the polished surface of the cross section in the rolling direction of the steel sheet, in a field of view of 100 μm × 100 μm EBSD (Electron Backscatter Diffraction analysis (measurement step: 0.05 μm) was performed at a position of 1/4 of the plate thickness, and the average value of the orientation difference (°) between each pixel in the crystal grain and the adjacent pixel was calculated.

The V(C,N)-based precipitates formed in the tempered martensite grains are 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㎛ × 2 μm) at 5 random areas, and then the average value was calculated.

Hardness was measured 3 times using a Brinell hardness tester (load 3000 kgf, 10 mm tungsten indentation) after milling the surface of the steel plate by 2 mm, and then expressed as an average value.

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

Steel grade No.	Alloy composition (% by weight)
	C	Si	Mn	P	S	Ni	Cr
invention steel 1	0.3	0.3	0.5	0.007	0.002	1.0	0.5
invention steel 2	0.3	0.3	0.5	0.007	0.002	1.0	0.5
invention steel 3	0.3	0.3	0.5	0.007	0.002	1.0	0.5
Invention Steel 4	0.3	0.3	0.5	0.007	0.002	1.0	0.7
invention steel 5	0.3	0.3	0.5	0.007	0.002	1.0	0.5
invention 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 lecture 3	0.3	0.3	0.5	0.007	0.002	1.0	0.5
comparative lecture 4	0.3	0.3	0.5	0.007	0.002	1.0	0.5

Steel grade No.	Alloy composition (% by weight)
	Mo	V	Al	N	Nb	B	Equation 1	Equation 2
invention steel 1	0.5	0.40	0.03	0.004	0.0008	0.0001	338.9	369
invention steel 2	0.5	0.05	0.03	0.004	0.0009	0.0003	258.9	329
invention steel 3	0.5	0.10	0.03	0.004	0.0006	0.0002	270.4	335
Invention Steel 4	0.8	0.20	0.03	0.004	0.0007	0	402.4	401
invention steel 5	0.5	0.10	0.03	0.004	0.0006	0.0003	270.4	335
invention 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 lecture 3	0.5	0.40	0.03	0.004	0.015	0.0001	338.9	369
comparative lecture 4	0.5	0.40	0.03	0.004	0.0008	0.0012	338.9	369
[Equation 1] = 225 [C] -11.3 [Mn] + 37.5 [Cr] -2.5 [Ni] + 228.6 [V] + 338.8 [Mo] [Equation 2] = 112.5 [C] -5.7 [Mn] + 18.8 [Cr] -1.3 [Ni] + 114.3 [V] + 169.4 [Mo] + 200

division	Steel grade No,	slab Heating temperature (℃)	rolling		steel thickness (mm)
			Rough rolling temperature (℃)	Finish rolling temperature (℃)
Invention example 1	invention steel 1	1159	1059	890	12
Invention example 2	invention steel 2	1125	1018	924	18
Invention Example 3	invention steel 3	1176	1063	899	15
Invention example 4	Invention Steel 4	1164	1022	909	25
Comparative Example 1	invention steel 5	1165	1040	887	12
Comparative Example 2	invention 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 lecture 3	1156	1041	906	17
Comparative Example 6	comparative lecture 4	1157	1037	900	18
Comparative Example 7	invention steel 1	1156	1039	903	20
Comparative Example 8	invention steel 1	1158	1041	906	16
Comparative Example 9	invention steel 1	1155	1038	900	19

division	Steel grade No.	1st heat treatment				2nd heat treatment
		heating temperature (℃)	heating time (minute)	cooling rate (℃/s)	Cooling down Temperature (℃)	Tempering heat treatment Temperature (℃)	Tempering heat treatment time (minutes)
Invention example 1	invention steel 1	911	51	65	25	250	53
Invention example 2	invention steel 2	910	58	39	29	200	62
Invention example 3	invention steel 3	900	55	48	26	200	58
Invention example 4	Invention Steel 4	907	68	44	29	350	73
Comparative Example 1	invention steel 5	909	51	46	23	130	53
Comparative Example 2	invention 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 lecture 3	907	57	27	51	240	61
Comparative Example 6	comparative lecture 4	910	58	28	50	250	62
Comparative Example 7	invention steel 1	909	10	28	48	230	65
Comparative Example 8	invention steel 1	907	56	27	49	250	10
Comparative Example 9	invention steel 1	910	60	28	50	250	100

division	microstructure						surface Hardness (H-B)	Shock tenacity (J, @-40℃)
	tempered Marten site (area%)	tempered martensite average effective Grain size (μm)	KAM	precipitate average size (nm)	Ratio (%) of the number of precipitates whose size exceeds 100 nm to the total number of precipitates	2nd prize (area%)
Invention example 1	98	4.6	0.99	8	4	2	503	28
Invention Example 2	97	3.5	1.42	13	3	3	492	36
Invention example 3	99	4.6	1.06	11	2	One	498	37
Invention 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	One	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, and ferrite

As can be seen from Tables 1 to 5, in the case of Inventive Examples 1 to 4 satisfying the alloy composition and manufacturing conditions of the present invention, the surface hardness and impact toughness are excellent by obtaining the microstructure to be obtained by the present invention. Able to know.

On the other hand, in the case of Comparative Example 1, the alloy composition of the present invention is satisfied, but the tempering heat treatment temperature during the secondary heat treatment is low, so the average size of precipitates proposed by the present invention, the ratio of the number of precipitates whose size exceeds 100 nm to the total number of precipitates , it can be seen that the low-temperature toughness is low because it does not satisfy the KAM.

In the case of Comparative Example 2, the alloy composition of the present invention is satisfied, but as the tempering heat treatment temperature during the secondary heat treatment is high, the average precipitate size, KAM, proposed by the present invention is not satisfied, so it can be seen that the hardness is low.

In the case of Comparative Example 3, the manufacturing conditions of the present invention are satisfied, but the V content and the conditions of relational expression 1 are not satisfied, so it can be seen that the hardness is low.

In the case of Comparative Example 4, since the condition of relational expression 1 of the present invention is not satisfied, and the temperature of the tempering heat treatment during the secondary heat treatment is also low, the average size of the precipitates proposed by the present invention and the total number of precipitates are larger than 100 nm. It can be seen that the hardness and low-temperature toughness are low because the ratio of numbers and KAM are not satisfied.

In the case of Comparative Example 5, as the Nb content is high, the ratio of the average size of precipitates and the number of precipitates having a size exceeding 100 nm to the total number of precipitates proposed by the present invention is not satisfied, indicating that the low-temperature toughness is low.

In the case of Comparative Example 6, it can be seen that the low-temperature toughness is low as the content of B is high.

In the case of Comparative Example 7, the alloy composition of the present invention is satisfactory, but the average effective crystal grain size of tempered martensite increases as the heating time is not sufficient during the first heat treatment, and the average size of precipitates and total precipitates proposed by the present invention It can be seen that the low-temperature toughness is low because the ratio of the number of precipitates having a size of more than 100 nm to the number is not satisfied.

In the case of Comparative Example 8, the alloy composition of the present invention is satisfactory, but the tempering heat treatment time during the secondary heat treatment is not sufficient, so the average size of precipitates proposed by the present invention, the ratio of the number of precipitates whose size exceeds 100 nm to the total number of precipitates , it can be seen that the low-temperature toughness is low because it does not satisfy the KAM.

In the case of Comparative Example 9, the alloy composition of the present invention is satisfied, but as the tempering heat treatment time during the secondary heat treatment is excessive, the average precipitate size and KAM are not satisfied, so it can be seen that the hardness is low.

Claims (7)

1. 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): 0.2~1.1%, 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%, balance Contains Fe and other unavoidable impurities,

   The unavoidable impurities include Nb: 0.0015% or less and B: 0.0008% or less,

   Satisfies the following relational expression 1,

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

   The average effective grain size of the tempered martensite is 20 μm or less,

   High-hardness bulletproof steel with excellent low-temperature toughness including V (C, N)-based precipitates having an average size of 30 nm or less in grains of the tempered martensite.

   [Relational Expression 1] 225[C]-11.3[Mn]+37.5[Cr]-2.5[Ni]+228.6[V]+338.8[Mo] ≥ 220

   (However, in the relational expression 1, the content of each element means % by weight.)
2. The method of claim 1,

   The microstructure is high-hardness bulletproof steel with excellent low-temperature toughness, including at least one of retained austenite, bainite, pearlite, and ferrite so that the sum thereof is 10 area% or less.
3. The method of claim 1,

   The tempered martensite is a high-hardness bulletproof steel with excellent low-temperature toughness with a KAM of 0.3 to 3.0.
4. The method of claim 1,

   The V (C, N)-based precipitate is a high-hardness bulletproof steel having excellent low-temperature toughness in which the ratio of the number of precipitates having a size exceeding 100 nm to the total number of precipitates is 10% or less.
5. The method of claim 1,

   The bulletproof steel is a high-hardness bulletproof steel with excellent low-temperature toughness having a surface hardness of 480 to 530HB and an impact absorption energy at -40 ° C of 16J or more.
6. The method of claim 1,

   The bulletproof steel is a high-hardness bulletproof steel having excellent low-temperature toughness having a thickness of 5 to 40 mm.
7. 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): 0.2~1.1%, 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%, balance including Fe and other unavoidable impurities, wherein the unavoidable impurities include Nb: 0.0015% or less and B: 0.0008% or less, and heating a slab satisfying the following relational expression 1 at 1050 to 1250 ° C;

   Obtaining a bar by rough rolling the heated slab at 950 to 1150 ° C;

   After finishing hot rolling the bar at 850 ~ 950 ℃ to obtain a hot-rolled steel sheet, cooling to room temperature;

   A first heat treatment step 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; and

   A second heat treatment step of tempering the first heat-treated hot-rolled steel sheet for 1.5t + 32 minutes (t: sheet thickness (mm)) to 1.5t + 60 minutes to satisfy the condition of Equation 2 below; Method for manufacturing excellent high-hardness bulletproof steel.

   [Relational Expression 1] 225[C]-11.3[Mn]+37.5[Cr]-2.5[Ni]+228.6[V]+338.8[Mo] ≥ 220

   [Relational Expression 2] 200 ≤ tempering heat treatment temperature ≤ T _max

   (However, in the above relational expression 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 the above relational expression 1 and T _max The content of each element means % by weight.)