EP4265785A1

EP4265785A1 - High-hardness bullet-proof steel with excellent low-temperature impact toughness and method for manufacturing same

Info

Publication number: EP4265785A1
Application number: EP21906840.0A
Authority: EP
Inventors: Seng-Ho YU; Nam-Young Cho
Original assignee: Posco Co Ltd
Current assignee: Posco Holdings Inc
Priority date: 2020-12-18
Filing date: 2021-11-04
Publication date: 2023-10-25
Also published as: AU2021400778A1; AU2021400778A9; WO2022131535A1; KR102498147B1; KR20220088239A

Abstract

The present invention can provide a bullet-proof steel capable of providing excellent bullet-proof properties by having high hardness characteristics and excellent low-temperature impact toughness, and a method for manufacturing same.

Description

Technical Field

The present disclosure relates to a material appropriate for armored vehicles, explosion-proof structures, and the like, and more particularly to bullet-proof steel having excellent low-temperature impact toughness and having high hardness, and a method for manufacturing the same.

Background Art

Bullet-proof steel is a material of which a surface is made very hard for its main function of blocking bullets, and is used where protection is required, such as for an exterior of armored vehicles used on the battlefield. Since bulletproof performance is directly related to human life, research to improve the performance of bulletproof materials has been actively conducted in the past, and recently, a non-ferrous material such as titanium and aluminum has been developed.
The non-ferrous material has an advantage of weight reduction compared to a steel material, but is relatively expensive and has poor workability. As compared to the non-ferrous material, since the steel material is relatively inexpensive and may change physical properties such as harness relatively easily, the steel material is widely used as a material for self-propelled artillery, wheeled armored vehicles, and the like.
Hardness is one of the most important physical properties for securing the performance of bullet-proof 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.

(Prior art Document)

(Patent Document) Korean Patent Publication No. 10-2018-0043788 (published on April 30, 2018 )

Summary of Invention

Technical Problem

An aspect of the present disclosure is to provide bullet-proof steel having high hardness characteristics and excellent low-temperature impact toughness and a method for manufacturing 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.

Solution to Problem

According to an aspect of the present disclosure, a high- hardness bullet-proof steel having excellent low-temperature impact toughness includes, by weight: 0.18 to 0.25% 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. $10 * [C] * [Si] \geq 2.5$
In Relational Expression 1, [C] and [Si] refer to contents (weight %) of carbon (C) and silicon (Si) included in the steel sheet, and 0 is substituted when the corresponding element is not intentionally added.
The bullet-proof steel may further include, by weight: at least one of 0.005 to 0.025% of titanium (Ti) and 0.2% or less of vanadium (V).
A fraction of the tempered martensite may be 90% or more by area, and a fraction of the retained austenite may be 0.5% by area to 5% by area.
The bullet-proof steel may have a surface hardness of 360 to 440 HB, and an impact absorption energy of 27 J or more at -40°C.
The bullet-proof steel may have a thickness of 5 to 20 mm.
According to an aspect of the present disclosure, a manufacturing method for a high-hardness bullet-proof steel having excellent low-temperature impact toughness includes operations of: preparing a steel slab including, by weight: 0.18 to 0.25% of carbon (C), 1.0 to 2.0% of silicon (Si), 0.5 to 1.6% of manganese (Mn), 0.5 to 1.2% of nickel (Ni), 0.4 to 1.5% of chromium (Cr), 0.05% or less of phosphorus (P), 0.02% or less of sulfur (S), 0.006% or less of nitrogen (N), 0.07% or less of aluminum (Al) (excluding 0%), 0.1 to 0.5% of molybdenum (Mo), 0.01 to 0.05% of niobium (Nb), 0.0002 to 0.005% of boron (B), 0.0005 to 0.004% of calcium (Ca), with a balance of iron (Fe) and inevitable impurities, satisfying the following [Relational Expression 1]; heating the steel slab in a temperature range of 1050 to 1250 °C; rough rolling the heated steel slab in a temperature range of 950 to 1150 °C; subjecting the heated steel slab to finish hot rolling in a temperature range of 850 to 950 °C after the rough rolling to manufacture a hot-rolled steel sheet; performing a primary heat-treatment of heating the hot-rolled steel sheet to a temperature range of 880 to 930 °C and then cooling the same to a cooling end temperature of 150 °C or lower at a cooling rate of 10 °C/s or more; and performing a secondary heat-treatment of heating the primarily heat-treated hot-rolled steel sheet to a temperature range of 350 °C or lower and maintaining the same. $10 * [C] * [Si] \geq 2.5$
In Relational Expression 1, [C] and [Si] refer to contents (weight %) of carbon (C) and silicon (Si) included in the steel slab, and 0 is substituted when the corresponding element is not intentionally added.
The steel slab may further include, by weight: at least one of 0.005 to 0.025% of titanium (Ti) and 0.2% or less of vanadium (V).
A residence time during the first heat treatment may be 1.3t * 10 minutes (t: plate thickness (mm)) or more.
A holding time during the secondary heat treatment may be 1.9t+10 minutes (t: plate thickness (mm)) or more.
The solution to the above problems does not enumerate all the features of the present disclosure, and various beneficial merits of the present disclosure are not limited to the descriptions above, and may be more easily understood in a process of describing specific exemplary embodiments in the present disclosure.

Advantageous Effects of Invention

As set forth above, according to the present disclosure, bullet-proof steel having excellent low-temperature toughness while having ultra-high hardness may be provided.
The present disclosure may provide bullet-proof steel having a target level of physical properties without performing a further heat treatment from optimization of alloy compositions and manufacturing conditions, and thus, is economically favorable.
The effect of the present disclosure is not limited to the above, and may be interpreted as including an effect that can be inferred from the description described below by those skilled in the art.

Best Mode for Invention

The present disclosure relates to a high-hardness bullet-proof steel having excellent low-temperature impact toughness and a method for manufacturing the same . Hereinafter, 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 tracked armored vehicles, explosion-proof structures, and the like.
In particular, the bulletproof performance of the steel material was intended to be improved by an economically favorable method, and thus, the present disclosure was provided.
Hereinafter, bullet-proof steel according to an aspect of the present disclosure will be described in more detail.
Hereinafter, a steel composition of the present disclosure will be described in more detail. Hereinafter, % represents a content of each element based on weight, unless otherwise particularly specified.

Carbon (C): 0.18 to 0.25%

Carbon (C) is an element which is effective for improving strength and hardness in steel having a low-temperature transformation phase such as a martensite or bainite phase, and is effective for improving hardenability. In order to sufficiently obtain the effect described above, 0.18% or more of carbon (C) may be included. Preferably, a lower limit of a content of carbon (C) may be 0.19%, and more preferably, the lower limit of the content of carbon (C) may be 0.20%. However, when carbon (C) is excessively added, there may be a concern that weldability and toughness of steel are deteriorated, so in the present disclosure, an upper limit of the content of carbon (C) may be limited to 0.25%. Preferably, the upper limit of the content of carbon (C) may be 0.24%.

Silicon (Si): 1.0 to 2.0%

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. In particular, since residual austenite which is uniformly distributed in steel having a low-temperature transformation phase such as martensite and bainite may effectively contribute to improvement of impact toughness without strength reduction. Therefore, in order to sufficiently obtain the effect described above, in the present disclosure, 1.0% or more of silicon (Si) may be included. Preferably, 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%. However, when silicon (Si) is excessively added, weldability may be rapidly deteriorated, so in the present disclosure, an upper limit of the content of silicon (Si) may be limited to 2.0%. Preferably, 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): 0.5 to 1.6%

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. In order to obtain a target level of hardness in the present disclosure, 0.5% or more of manganese (Mn) may be included. Preferably, 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%. However, when manganese (Mn) is excessively added, there may be a concern that weldability is deteriorated and center segregation is encouraged to deteriorate the physical properties in the center part of steel, so in the present disclosure, an upper limit of the content of manganese (Mn) may be limited to 1.6%. Preferably, the upper limit of the content of manganese (Mn) may be 1.5%, and more preferably, the upper limit of the content of manganese (Mn) may be 1.45%.

Nickel (Ni): 0.5 to 1.2%

Nickel (Ni) is an element favorable to improve both strength and toughness of steel. In order to obtain the effect described above, in the present disclosure, 0.5% or more of nickel (Ni) may be included. Preferably, a lower limit of a content of nickel (Ni) may be 0.6%, and more preferably, the lower limit of the content of nickel (Ni) may be 0.7%. However, nickel (Ni) is an expensive element, when nickel (Ni) is excessively added, manufacturing costs may be greatly increased, so in the present disclosure, an upper limit of the content of nickel (Ni) may be limited to 1.2%. Preferably, the upper limit of the content of nickel (Ni) may be 1.17%, and more preferably, the upper limit of the content of nickel (Ni) may be 1.15%.

Chromium (Cr): 0.4 to 1.5%

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. In addition, since chromium (Cr) is a relatively inexpensive element, chromium (Cr) is also an element for economically securing hardness and toughness. In order to obtain the effect described above, in the present disclosure, 0.4% or more of chromium (Cr) may be included. Preferably, a lower limit of a content of chromium (Cr) may be 0.45%. However, when chromium (Cr) is excessively added, weldability may be deteriorated, so in the present disclosure, an upper limit of the content of chromium (Cr) may be limited to 1.5%. Preferably, 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. In the present disclosure, even in the case of including phosphorus (P) up to 0.05%, the physical properties of the steel are not significantly influenced, and thus, 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): 0.02% or less

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. In the present disclosure, even in the case of including sulfur (S) up to 0.02%, the physical properties of the steel are not significantly influenced, and thus, 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): 0.006% or less

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. In the present disclosure, there is no difficulty in securing strength even when N is not contained, so in the present disclosure, 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): 0.07% or less (excluding 0%)

Aluminum (Al) is an element effective for lowering an oxygen content in molten steel as a deoxidizing agent of steel. However, when aluminum (Al) is excessively added, cleanliness of steel may be impaired, so in the present disclosure, an upper limit of a content of aluminum (Al) may be limited to 0.07%.
On the other hand, when the content of aluminum (Al) is excessively lowered, a load may occur in a steelmaking process and manufacturing costs may be increased, and thus, in the present disclosure, 0% may be excluded from a lower limit of the content of aluminum (Al), and the lower limit thereof may be 0.01%.

Molybdenum (Mo): 0.1 to 0.5%

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 0.11%, and more preferably, the lower limit of the content of molybdenum (Mo) may be 0.12%. However, when molybdenum (Mo) is excessively added, not only manufacturing costs may be increased, but also weldability may be deteriorated, so in the present disclosure, an upper limit of the content of molybdenum (Mo) may be limited to 0.5%. Preferably, 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): 0.01 to 0.05%

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). In order to sufficiently obtain the effect described above, in the present disclosure, 0.01% or more of niobium (Ni) may be included. However, when niobium (Ni) is excessively added, coarse precipitates may be formed to become a starting point of brittle fracture, so in the present disclosure, an upper limit of a content of niobium (Nb) may be limited to 0.05%. Preferably, the upper limit of the content of niobium (Nb) content may be 0.04%, and more preferably, the upper limit of the content of niobium (Nb) may be 0.03%.

Boron (B): 0.0002 to 0.005%

Boron (B) is an element effectively contributing to strength improvement by increasing quenching properties of steel even with a small addition amount thereof. In order to sufficiently obtain the effect, in the present disclosure, 0.0002% or more of boron (B) may be contained. Preferably, 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%. However, when boron (B) is excessively added, toughness and weldability of steel may be rather deteriorated, so in the present disclosure, an upper limit of the content of boron (B) may be limited to 0.005%. Preferably, 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): 0.0005 to 0.004%

Calcium (Ca) an element having a good binding force with sulfur (S) and producing CaS on the periphery (around) MnS, thereby suppressing elongation of MnS to improve toughness in a direction perpendicular to a rolling direction. In addition, CaS produced by adding Ca has an effect of increasing corrosion resistance under a humid external environment. In order to sufficiently obtain the effect described above, 0.0005% or more of Ca may be included. Preferably, a lower limit of a content of calcium (Ca) may be 0.001%. However, when calcium (Ca) is excessively added, defects such as nozzle clogging, or the like, may be caused in a steelmaking operation, so in the present disclosure, an upper limit of the content of calcium (Ca) may be limited to 0.004%. Preferably, the upper limit of the content of calcium (Ca) may be 0.003%.
In addition to the above-described alloy composition, bullet-proof steel of the present disclosure may further include the following elements for the purpose of favorably securing target physical properties.
Specifically, the bullet-proof steel of the present disclosure may further include at least one of titanium (Ti) and vanadium (V).

Titanium (Ti): 0.005 to 0.025%

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%.

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. However, since 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 bullet-proof 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. However, since in the common manufacturing process, unintended impurities may be inevitably incorporated from raw materials or the surrounding environment, the component may not be excluded. Since these impurities are known to any person skilled in the common manufacturing process, the entire contents thereof are not particularly mentioned in the present specification. In addition, further addition of effective ingredients other than the above-mentioned ingredients is not entirely excluded.
The bullet-proof steel according to an aspect of the present disclosure may satisfy the following [Relational Expression 1]. $10 * [C] * [Si] \geq 2.5$
In Relational Expression 1, [C] and [Si] refer to contents (weight %) of carbon (C) and silicon (Si) included in the steel sheet, and 0 is substituted if the corresponding element is not intentionally added.
The inventors of the present disclosure have conducted in-depth research on a method capable of securing high-hardness characteristics and excellent low-temperature impact toughness of a steel sheet at the same time, and have derived that it is effective to control not only a content range of each respective alloy composition, but also a relative content range of the specific alloy composition included in the steel sheet. In the present disclosure, not only the content range of each respective alloy composition included in the steel sheet is controlled to be within a certain range, but also the relative content range of carbon (C) and silicon (Si) is controlled to be within a certain range, as illustrated in [Relational Expression 1], so that high hard characteristics and excellent low-temperature impact toughness may be effectively compatible. 10*[C]*[Si] defined by the Relational Expression 1 may be 2.9 or more.
The bullet-proof steel of the present disclosure having the alloy composition described above may have a tempered martensite base structure including retained austenite as a microstructure, and may further include other inevitable structures. In this case, preferably, 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. For this effect, 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. On the other hand, when the retained austenite is excessively formed, the low-temperature impact toughness greatly increases, but it is difficult to secure target hardness characteristics. Therefore, in the present disclosure, 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.
Meanwhile, the bullet-proof steel of the present disclosure may have the above-described microstructural configuration over the entire thickness.
The bullet-proof steel of the present disclosure having the above-described alloy composition and the proposed microstructure may have a thickness of 6 to 40 mm and a surface hardness of the bullet-proof steel may be 360 to 440HB, exhibiting ultra-high hardness, and may have an impact absorption energy of 27 J or more at -40 °C, exhibiting excellent low-temperature toughness.
Here, 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).
Hereinafter, a method for manufacturing bullet-proof steel according to an aspect of the present disclosure will be described in more detail.
A steel slab having a predetermined component is prepared. Since the steel slab of the present disclosure has an alloy composition corresponding to the alloy composition of the hot-rolled steel sheet described above (including [Relational Expression 1]), a description of the alloy composition of the steel slab is substituted for the description of the alloy composition of the above-described hot-rolled steel sheet.
In brief, the bullet-proof steel may be manufactured by preparing a steel slab satisfying the alloy composition described above, and then subjecting the steel slab to the processes of [heating - rolling - primary heat treatment (quenching) - secondary heat treatment (tempering)]. Hereinafter, each process condition will be described in detail.

[Steel slab heating process]

First, 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.
When the temperature is lower than 1050°C during the heating, deformation resistance of steel is increased, so that a subsequent rolling process may not be effectively performed. On the other hand, when the temperature is higher than 1250°C, austenite crystal grains are coarsened, so that non-uniform structure may be formed.
Therefore, the steel slab may be heated in a temperature range of 1050 to 1250°C.

[Rolling process]

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.
First, 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.
When the temperature is lower than 950°C during the rough rolling, a rolling load is increased to reduce the steel slab relatively weakly, and thus, deformation is not sufficiently transferred to a center of the slab in the thickness direction, and as a result, defects such as voids may not be removed. On the other hand, when the temperature thereof is higher than 1150°C, recrystallization granularity is coarsened, which may be harmful to toughness.
When the temperature is lower than 850°C during the finish hot rolling, two-phase region rolling is performed, so that there is a concern that ferrite may be produced in the microstructure. On the other hand, when the temperature thereof is higher than 950°C, the granularity of the final structure is coarsened to deteriorate low-temperature toughness.

[Primary heat treatment (quenching) process]

The hot-rolled steel sheet manufactured through the rolling process described above is air cooled to room temperature, and then heated to a residence time of 1.3t + 10 minutes (t: plate thickness (mm)) or more in a temperature range of 880 to 930°C.
The heating is performed for reverse transformation of a hot-rolled steel sheet composed of ferrite and pearlite into an austenite single phase. When a heating temperature is lower than 880°C, austenitization is not sufficiently achieved, and coarse soft ferrite is mixed, and thus hardness of a final product may be lowered. On the other hand, when the temperature thereof is higher than 930°C, there is an effect of increasing quenching properties due to coarse austenite crystal grains, but there is a disadvantage in terms of thermal efficiency during mass production. Therefore, heating during a primary heat treatment is preferably performed in a range of 880 to 930°C. A lower limit of the heating temperature is more preferably 885 °C, even more preferably 890°C, and most preferably 895°C. In addition, an upper limit of the heating temperature is more preferably 925 °C, even more preferably 920 °C, and most preferably 915 °C.
Meanwhile, when a residence time during the primary heat treatment is less than 1.3t + 10 minutes (t: plate thickness (mm)), austenitization does not sufficiently occur, so that phase transformation by subsequent rapid cooling, that is, a martensitic structure cannot be sufficiently obtained. Therefore, the residence time during the heating is preferably 1.3t + 10 minutes (t: plate thickness (mm)) or more. The residence time during the heating is more preferably 1.3t + 12 minutes (t: plate thickness (mm)) or more, even more preferably 1.3t + 13 minutes (t: plate thickness (mm)) or more, and most preferably 1.5t + 15 minutes (t: plate thickness (mm)) or more.
In the present disclosure, an upper limit of the residence time during the primary heat treatment is not particularly limited. However, when the residence time during the heating exceeds 1.3 t + 60 minutes (t: plate thickness (mm)), austenite crystal grains become coarse, so that quenching properties may increase, but there may be a disadvantage in that productivity is relatively lowered. Therefore, the residence time during the heating is preferably 1.3t + 60 minutes (t: plate thickness (mm)) or less. The residence time during the heating is more preferably 1.3t + 50 minutes (t: plate thickness (mm)) or less, even more preferably 1.3t + 40 minutes (t: plate thickness (mm)) or less, and most preferably 1.3t + 30 minutes (t: plate thickness (mm)) or less.
Thereafter, the heated hot-rolled steel sheet may be cooled to 120 °C or lower at a cooling rate of 10°C/s or more with respect to a plate thickness center portion(e.g., 1/2t point, t: plate thickness (mm)). In this case, the cooling is preferably rapid cooling through water cooling. When the cooling rate is less than 10°C/s or a cooling end temperature is higher than 120 °C, there is a concern that a ferrite phase may be formed or a bainite phase may be excessively formed during cooling. Therefore, the cooling is preferably performed to 120 °C or lower at a cooling rate of 10°C/s or more. The faster the cooling rate, the more favorable it is to form the microstructure to be obtained in the present disclosure. Meanwhile, in the present disclosure, an upper limit of the cooling rate is not particularly limited, and it may be appropriately set in consideration of facility limitations by any person skilled in the art. The cooling end temperature is more preferably 100°C or lower, even more preferably 80°C or lower, and most preferably 50°C or lower.

[Secondary heat treatment (tempering) process]

The hot-rolled steel sheet manufactured through the above-described primary heat treatment may be heated to a temperature range of 350 °C or lower and maintained for 1.9t+10 minutes (t: plate thickness (mm)) or more.
The reheating is performed to release internal stress of a hot-rolled steel sheet fully composed of martensite after quenching. As dislocation density inside a material thereof decreases through such a tempering heat treatment, hardness may decrease somewhat, but toughness may be secured. In the case of bullet-proof steel, since it is necessary not only to have high hardness but also to have excellent low-temperature impact toughness, such a subsequent tempering heat treatment is essential if no special element is added. When a tempering temperature is higher than 350 °C, the dislocation density inside the martensite is excessively reduced, and thus hardness of a final product may be lowered. On the other hand, when the tempering temperature is too low, the decrease in hardness can be prevented, but the dislocation density becomes too high, which is disadvantageous in terms of securing impact toughness. In the present disclosure, a lower limit of the tempering temperature is not separately specified, but is preferably 100 °C or higher in order to obtain the effect described above. It is preferable to perform tempering, more preferably at a temperature of 125 °C or higher, and even more preferably 150 °C or higher.
In the present disclosure, a lower limit of a holding time during the tempering is not particularly limited. However, when the holding time during the tempering is less than 1.9t + 10 minutes (t: plate thickness (mm)), a thickness center portion compared to the surface thereof may not be sufficiently heated, as the heat treatment is performed at a relatively low temperature for a short time. Therefore, the holding time during the tempering is preferably 1.9t + 10 minutes (t: plate thickness (mm)) or more. The holding time during the tempering is more preferably 1.9t + 12 minutes (t: plate thickness (mm)) or more, more preferably 1.9t + 15 minutes (t: plate thickness (mm)) or less, and most preferably 1.9t + 20 minutes (t: plate thickness (mm)) or more.
After tempering, it can be cooled to room temperature by air cooling. Through such a tempering process, the hot-rolled steel sheet according to an aspect of the present disclosure may have softened tempered martensite as a base structure.
The hot-rolled steel sheet obtained through the above-described series of manufacturing processes is a steel material having a thickness of 6 to 40 mm, and can provide excellent bullet-proof resistance by securing high hardness and high toughness.

Mode for Invention

Hereinafter, bullet-proof steel of the present disclosure and a method for manufacturing the same will be described in more detail through specific examples. It should be noted that the following examples are only for understanding of the present disclosure, and are not intended to specify the scope of the present disclosure. The scope of the present disclosure may be determined by the matters described in the claims and the matters reasonably inferred therefrom.

(Example)

After preparing a steel slab having alloy compositions of Table 1 below, [heating - rolling - heat treatment (quenching) - heat treatment (tempering)] were performed according to process conditions illustrated in Table 2 below to manufacture each hot-rolled steel sheet. In this case, after being water cooled to a cooling end temperature, air cooling was applied to room temperature. The tempering performed thereafter also applied air cooling to room temperature after performing a heat treatment at a specific temperature. Alloy compositions not listed in Table 1 refer to unavoidable impurities and iron (Fe). In addition, a part marked with"-" in Table 1 means that the corresponding component was not intentionally added, which is preferable to be interpreted as 0% by weight within an error range. Specimen 11 in Table 2 refers to a specimen that was not subjected to the second heat treatment (tempering) after the first heat treatment (quenching).

[Table 1]

St ee 1 ty pe	Alloy composition(weight %)															Relation al Expressi on 1
St ee 1 ty pe	C	Si	Mn	P*	S*	Ni	Cr	Mo	Nb	V	Al	Ca*	Ti	B*	N*	Relation al Expressi on 1
A	0.3 6	1.3 7	1.4 5	76	19	1.1 3	0.6 8	0.2 8	0.0 3	-	0.0 4	16	0.01 2	21	45	4.93
B	0.1 6	0.2 9	1.0 8	72	20	0.4 2	0.4 6	0.1 3	0.0 1	0.0 2	0.0 3	18	0.01 7	23	48	0.46
C	0.2 4	1.3 9	1.1 2	73	18	0.9 6	0.5 5	0.3 9	0.0 2	-	0.0 3	22	-	21	44	3.34
D	0.2 0	1.6 5	1.2	76	19	1.1 4	0.8 1	0.4 4	0.0 3	0.0 4	0.0 3	21	-	19	46	3.30
E	0.2 3	1.2 6	0.9 6	77	17	1.0 6	0.7 3	0.4 2	0.0 3	-	0.0 3	19	0.01 5	18	46	2.90
F	0.1 8	1.1 2	0.6 3	80	18	0.7 1	0.6 2	0.2 7	0.0 2	-	0.0 3	20	0.01 4	20	43	2.02
P, S, Ca, B, and N* are represented in units of ppm

[Table 2]

Spe cim en No.	Stee 1 type	Thic knes s (mm)	Slab heatin 9 (°C)	Rolling		Primary heat treatment (quenching)				Secondary heat treatment (tempering)
Spe cim en No.	Stee 1 type	Thic knes s (mm)	Slab heatin 9 (°C)	Rough rollin 9 (°C)	Finish hot rollin 9 (°C)	Heati ng tempe ratur e (°C)	Resid ence time (min. )	Cooli ng end tempe ratur e (°C)	Coolin g rate (°C/s)	Heating tempera ture (°C)	Holdin g time (min.)
1	A	10	1147	1055	861	909	36	30	65.2	208	39
2	A	25	1149	1029	928	913	54	37	45.9	227	60
3	A	50	1155	1044	910	904	85	23	12.5	212	95
4	B	15	1162	1025	899	907	43	24	44.1	301	39
5	B	30	1148	1035	923	905	60	32	31.9	254	75
6	B	60	1153	980	915	904	89	43	7.6	203	114
7	C	8	1185	1059	886	913	32	19	62.2	215	36
8	C	25	1129	1033	925	916	54	25	43.3	391	60
9	C	50	1131	965	911	907	86	21	9.4	224	116
10	D	12	1145	1046	924	905	40	18	50.5	209	45
11	D	25	1138	1030	922	914	55	19	40.7	-	-
12	D	40	1149	991	916	900	72	20	25	216	93
13	D	60	1142	984	900	912	93	285	7.7	200	127
14	E	20	1163	1030	934	917	47	30	43.8	209	59
15	E	40	1154	1004	927	903	75	25	20.6	231	97
16	E	40	1152	1018	923	912	74	29	21.9	208	15
17	E	60	1120	977	909	906	100	16	7.1	220	140
18	F	40	1145	982	914	908	85	41	9.0	256	118

Thereafter, a microstructure and mechanical properties of each hot-rolled steel sheet were measured, and the results thereof were shown in Table 3.
The 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.

In addition, 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, respectively. In this case, in the surface hardness, an average value of three measurements after a milling process of the surface of the hot-rolled sheet at 2 mm was used, and in the Charpy impact test, a specimen was collected at 1/4t point in the thickness direction, and then an average value of three measurements at -40°C was used.

[Table 3]

Specim en	Steel type	Microstructure (area %)			Surface hardness	Impact toughness (J, @-40°C)
No.		TM	B	R-γ	(HB)
1	A	98	-	2	526	24
2	A	97	-	3	493	28
3	A	98	-	2	511	26
4	B	99	-	1	329	29
5	B	97	-	3	337	34
6	B	98	-	2	355	31
7	C	96	-	4	424	45
8	C	100	-	0	308	63
9	C	97	-	3	417	37
10	D	96	-	4	379	58
11	D	100	-	0	470	20
12	D	97	-	3	409	33
13	E	53	B: 47	0	302	77
14	E	97	-	3	435	32
15	E	97	-	3	414	34
16	E	100	-	0	461	24
17	E	96	-	4	416	40
18	F	98	-	2	347	30
TM: Tempered martensite, B: Bainite, R-γ: Retained austenite

As shown in Tables 1 to 3, it can be seen that specimens satisfying the alloy composition and process conditions of the present disclosure have a surface harness of 360 to 440 HB and an impact absorption energy of 27 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 360 to 440 HB or an impact absorption energy of 27 J or more at -40°C at the same time.
While example embodiments have been shown and described above, it will be apparent to those skilled in the art that modifications and variations could be made without departing from the scope of the present disclosure as defined by the appended claims.

Claims

A high-hardness bullet-proof steel having excellent low-temperature impact toughness comprising, by weight: 0.18 to 0.25% 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, $10 * [C] * [Si] \geq 2.5$
in Relational Expression 1, [C] and [Si] refer to contents (weight %) of carbon (C) and silicon (Si) included in the steel, and 0 is substituted when the corresponding element is not intentionally added.
The high-hardness bullet-proof steel having excellent low-temperature impact toughness of claim 1, wherein the bullet-proof steel further comprises, by weight:
at least one of 0.005 to 0.025% of titanium (Ti) and 0.2% or less of vanadium (V).
The high-hardness bullet-proof steel having excellent low-temperature impact toughness of claim 1, wherein a fraction of the tempered martensite is 90% or more by area, and a fraction of the retained austenite is 0.5% by area to 5% by area.
The high-hardness bullet-proof steel having excellent low-temperature impact toughness of claim 1, wherein the bullet-proof steel has a surface hardness of 360 to 440 HB and an impact absorption energy of 27 J or more at -40°C.
The high-hardness bullet-proof steel having excellent low-temperature impact toughness of claim 1, wherein the bullet-proof steel has a thickness of 6 to 40 mm.
A manufacturing method for a high-hardness bullet-proof steel having excellent low-temperature impact toughness, comprising operations of: preparing a steel slab including, by weight: 0.18 to 0.25% of carbon (C), 1.0 to 2.0% of silicon (Si), 0.5 to 1.6% of manganese (Mn), 0.5 to 1.2% of nickel (Ni), 0.4 to 1.5% of chromium (Cr), 0.05% or less of phosphorus (P), 0.02% or less of sulfur (S), 0.006% or less of nitrogen (N), 0.07% or less of aluminum (Al) (excluding 0%), 0.1 to 0.5% of molybdenum (Mo), 0.01 to 0.05% of niobium (Nb), 0.0002 to 0.005% of boron (B), 0.0005 to 0.004% of calcium (Ca), with a balance of iron (Fe) and inevitable impurities, satisfying the following [Relational Expression 1];
heating the steel slab in a temperature range of 1050 to 1250 °C;

rough rolling the heated steel slab in a temperature range of 950 to 1150 °C;

subjecting the heated steel slab to finish hot rolling in a temperature range of 850 to 950 °C after the rough rolling to manufacture a hot-rolled steel sheet;

performing a primary heat treatment of heating the hot-rolled steel sheet to a temperature range of 880 to 930 °C and then cooling the same to a cooling end temperature of 150 °C or lower at a cooling rate of 10 °C/s or more; and

performing a secondary heat treatment of heating the primarily heat-treated hot-rolled steel sheet to a temperature range of 350 °C or lower and maintaining the same, $10 * [C] * [Si] \geq 2.5$

in Relational Expression 1, [C] and [Si] refer to contents (weight %) of carbon (C) and silicon (Si) included in the steel slab, and 0 is substituted when the corresponding element is not intentionally added.
The manufacturing method for a high-hardness bullet-proof steel having excellent low-temperature impact toughness of claim 6, wherein the steel slab further comprises, by weight: at least one of 0.005 to 0.025% of titanium (Ti) and 0.2 % or less of vanadium (V).
The manufacturing method for a high-hardness bullet-proof steel having excellent low-temperature impact toughness of claim 6, wherein a residence time during the primary heat treatment is 1.3t + 10 minutes (t: plate thickness (mm)) or more.
The manufacturing method for a high-hardness bullet-proof steel having excellent low-temperature impact toughness of claim 6, wherein a holding time during the secondary heat treatment is 1.9t + 10 minutes (t: plate thickness (mm)) or more.