Technical Field
[0001] Embodiment s of the present disclosure relate to a hot stamping part, and more particularly,
to a hot stamping part of which a molded part after hot stamping exhibits excellent
mechanical properties, such as high strength and high toughness.
Background Art
[0002] For lightness and stability, high strength steel is applied to parts for vehicles.
Meanwhile, high strength steel may obtain high strength properties compared with weight,
but as strength increases, press formability deteriorates, causing a material to break
during processing or causing a springback phenomenon and thus making it difficult
to form products with complex and precise shapes.
[0003] Hot stamping is a method to improve these problems, and as interest in the method
increases, research on materials for hot stamping is also actively being conducted.
For example, as disclosed in
Korean Patent Publication No. 10-2017-0076009, the hot stamping method is a forming technique for manufacturing a high-strength
part by heating a steel plate for hot stamping to a high temperature and then rapidly
cooling the steel plate while forming the steel plate in a press mold.
[0004] Furthermore, as disclosed in
Korean Patent Publication No. 10-2019-0095858, so-called boron steel (22MnB5) containing carbon (C), and manganese (Mn), boron
(B), or the like, as an element for improving heat treatment performance, is used
as a typical example of a steel plate for hot stamping.
Disclosure of Invention
Technical Problem
[0005] However, in the hot stamping part according to the related art, due to a difference
in strength between regions generated by components and microstructure included in
a steel plate for hot stamping, there is a problem that mechanical properties, such
as tensile strength, bending properties, or the like, of a molded part after hot stamping
deteriorate.
[0006] To solve various problems including the above problem, embodiments of the present
disclosure are directed to provide a hot stamping part of which a molded part after
hot stamping exhibits excellent mechanical properties, such as high strength and high
toughness. However, such an objective is an example, and the scope of the present
disclosure is not limited thereby.
Solution to Problem
[0007] According to an aspect of the disclosure, provided is a hot stamping part including
a steel plate that includes carbon (C) in an amount of 0.26 to 0.40 wt%, silicon (Si)
in an amount of 0.02 to 2.0 wt%, manganese (Mn) in an amount of 0.3 to 1.60 wt%, phosphorus
(P) in an amount of 0.03 wt% or less, sulfur (S) in an amount of 0.008 wt% or less,
chromium (Cr) in an amount of 0.05 to 0.90 wt%, boron (B) in an amount of 0.0005 to
0.01 wt%, molybdenum (Mo) in an amount of 0.05 to 0.2 wt%, titanium (Ti) in an amount
of 0.001 to 0.095 wt%, niobium (Nb) in an amount of 0.001 to 0.095 wt%, vanadium (V)
in an amount of 0.001 to 0.095 wt%, the balance of iron (Fe), and other inevitable
impurities, the hot stamping part having tensile strength of 1,700 MPa or more and
yield strength of 1,150 MPa or more, wherein the hot stamping part includes microstructure
including austenite grains and carbon-based precipitates including at least one of
niobium (Nb), titanium (Ti), molybdenum (Mo), and vanadium (V), and an average size
of the austenite grains is 15 µm or less.
Advantageous Effects of Invention
[0008] According to an embodiment of the present disclosure configured as above, a hot stamping
part of which a molded part after hot stamping exhibits excellent mechanical properties,
such as high strength, high toughness, and the like, may be implemented.
[0009] In detail, by adjusting a difference in strength between regions of a steel plate
for hot stamping through control of the properties of components, microstructure,
and precipitates included in the steel plate for hot stamping, a steel plate for hot
stamping of which a molded part after hot stamping exhibits excellent mechanical properties,
such as high strength, high toughness, and the like, and a manufacturing method thereof,
may be implemented. According to an embodiment of the present disclosure, the scope
of the disclosure is not limited by the above effects.
Brief Description of Drawings
[0010]
FIG. 1 is an image showing a portion of microstructure of a steel plate before hot
stamping, according to an embodiment of the present disclosure.
FIG. 2 is an image showing the structure of a hot stamping part formed by hot stamping
the steel plate according to FIG. 1.
FIGS. 3 and 4 are graphs showing the measurements of austenite grain sizes and fractions
of a hot stamping part, according to embodiments of the present disclosure and comparative
examples.
Best Mode for Carrying out the Invention
[0011] According to the present embodiment, a fraction of the austenite grains having a
size of 10 µm or more may be 67% or less.
[0012] According to the present embodiment, a fraction of the austenite grains having a
size of 20 µm or more may be 10% or less.
[0013] According to the present embodiment, when contents of titanium (Ti), niobium (Nb),
vanadium (V), and molybdenum (Mo) included in the steel plate may be represented by
[Ti], [Nb], [V], and [Mo] in wt%, respectively, [Inequality 1] below may be satisfied,

[0014] According to the present embodiment, the amount of activated hydrogen of the hot
stamping part may be 0.6 wppm or less.
[0015] According to the present embodiment, an average particle size of the precipitates
may be 10 nm or less.
[0016] According to the present embodiment, the precipitates may include 50 wt% or less
of titanium (Ti) and 30 wt% or more of molybdenum (Mo).
[0017] According to the present embodiment, the average number of the precipitates per unit
area may be 10,000/100 µm
2 to 35,000/100 µm
2.
[0018] According to the present embodiment, an average gap between the precipitates may
be 0.1 nm to 100 nm.
[0019] According to the present embodiment, the hot stamping part may satisfy a bending
angle of 50° or more.
[0020] Other aspects, features, and advantages than those described above will become apparent
from the following drawings, claims, and detailed description of the disclosure
Mode for the Invention
[0021] Various modifications may be applied to the present embodiments, and particular embodiments
will be illustrated in the drawings and described in the detailed description section.
The effect and features of the present embodiments, and a method to achieve the same,
will be clearer referring to the detailed descriptions below with the drawings. However,
the present embodiments may be implemented in various forms, not by being limited
to the embodiments presented below.
[0022] In the following embodiment, it will be understood that although the terms first,
second, etc. may be used herein to describe various elements, these elements should
not be limited by these terms. These elements are only used to distinguish one element
from another.
[0023] In the following embodiment, as used herein, the singular forms "a," "an," and "the"
are intended to include the plural forms as well, unless the context clearly indicates
otherwise.
[0024] In the following embodiment, it will be further understood that the terms "comprises"
and/or "comprising," when used in this specification, specify the presence of stated
components or elements, but do not preclude the presence or addition of one or more
other components or elements thereof.
[0025] Sizes of elements in the drawings may be exaggerated for convenience of explanation.
For example, since sizes and thicknesses of elements in the drawings are arbitrarily
illustrated for convenience of explanation, the following embodiments are not limited
thereto.
[0026] When a certain embodiment may be implemented differently, a specific process order
may be performed differently from the described order. For example, two consecutively
described processes may be performed substantially at the same time or performed in
an order opposite to the described order.
[0027] In the specification, the expression such as "A and/or B" may include A, B, or A
and B. Also, the expression such as "at least one of A and B" may include A, B, or
A and B.
[0028] Hereinafter, embodiments will be described in detail with reference to the accompanying
drawings, and in the description with reference to the drawings, the same or corresponding
constituents are indicated by the same reference numerals and redundant descriptions
thereof are omitted.
[0029] FIG. 1 is an image showing a portion of microstructure of a steel plate before hot
stamping, according to an embodiment of the present disclosure.
[0030] In detail, FIG. 1 illustrates a steel plate for hot stamping which is controlled
and manufactured such that the content of a material forming a steel plate for hot
stamping, a configuration of microstructure of a steel plate for hot stamping, and
a process condition for manufacturing a steel plate for hot stamping satisfy preset
conditions.
[0031] The steel plate according to the present embodiment may be a steel plate manufactured
by performing a hot rolling process and/or a cold rolling process on a slab cast to
contain a predetermined content of a certain alloy element.
[0032] The steel plate may include carbon (C), silicon (Si), manganese (Mn), phosphorus (P),
sulfur (S), chromium (Cr), boron (B), calcium (Ca), molybdenum (Mo), titanium (Ti),
niobium (Nb), vanadium (V), the balance of iron (Fe), and other inevitable impurities.
In an embodiment, the steel plate for hot stamping may include carbon (C) in an amount
of 0.26 to 0.40 wt%, silicon (Si) in an amount of 0.02 to 2.0 wt%, manganese (Mn)
in an amount of 0.3 to 1.60 wt%, phosphorus (P) in an amount of 0.03 wt% or less,
sulfur (S) in an amount of 0.008 wt% or less, chromium (Cr) in an amount of 0.05 to
0.90 wt%, boron (B) in an amount of 0.0005 to 0.01 wt%, molybdenum (Mo) in an amount
of 0.05 to 0.2 wt%, titanium (Ti) in an amount of 0.001 to 0.095 wt%, niobium (Nb)
in an amount of 0.001 to 0.095 wt%, vanadium (V) in an amount of 0.001 to 0.095 wt%,
the balance of iron (Fe), and other inevitable impurities. Furthermore, selectively,
the steel plate for hot stamping may further include calcium (Ca) in an amount of
0.00001 to 0.0060 wt%.
[0033] Carbon (C) operates as an austenite stabilizing element in a steel plate. Carbon
is a major element in determining strength and hardness of a steel plate, and may
be added for the purpose of increasing hardenability and strength during heat treatment.
The carbon may be included in an amount of 0.26 to 0.40 wt% to the total weight of
a steel plate. When the carbon content is less than 0.26 wt%, it is difficult to obtain
a hard phase (e.g., martensite, etc.) so that it may be difficult to satisfy the mechanical
strength of a molded part after hot stamping. In contrast, the carbon content, which
exceeds 0.40 wt%, may cause deterioration of machinability of a steel plate or deterioration
of bending performance of a molded part after hot stamping.
[0034] Silicon (Si) operates as a ferrite stabilizing element in a steel plate. Silicon,
as a solid-solution strengthening element, increases strength of a steel plate, and
restricts formation of low-temperature carbide so as to increase carbon thickening
in austenite. Furthermore, silicon is a core element in hot-rolling, cold-rolling,
and hot-press structure homogenization and ferrite fine dispersion. Silicon operates
as a martensite strength inhomogeneous control element to improve crashworthiness.
The silicon may be included in an amount of 0.02 to 2.0 wt% to the total weight of
a steel plate. When the silicon content is less than 0.02 wt%, it may be difficult
to obtain the effect described above, and cementite formation and coarseness may occur
in martensite structure of a molded part after hot stamping. In contrast, when the
silicon content exceeds 2.0 wt%, load on hot rolling and cold rolling may increase,
and the plating properties of a steel plate may deteriorate.
[0035] Meanwhile, when silicon (Si) is added appropriately in an amount of 0.3 wt% or more,
by suppressing excessive formation of a pearlite region where pearlite is accumulated
in the hot stamping steel plate, the pearlite region may be formed within the hot
stamping steel plate with a minimum content.
[0036] Manganese (Mn) may operate as an austenite stabilizing element in a steel plate.
Manganese is added for the purpose of increasing hardenability and strength during
heat treatment. The manganese may be included in an amount of 0.3 to 1.60 wt% to the
total weight of a steel plate. When the manganese content is less than 0.3 wt%, a
hardenability effect is insufficient so that a hard phase fraction in a molded part
after hot stamping may be insufficient due to insufficient hardenability. In contrast,
when the manganese content exceeds 1.60 wt%, an area of concentrated manganese-segregated
pearlite occurs so that ductility and toughness may deteriorate, which cause deterioration
of the bending performance of a molded part after hot stamping and generation of inhomogeneous
microstructure.
[0037] Phosphorus (P) is an element that contributes to strength improvement. The phosphorus
may be included in an amount of greater than 0 to 0.03 wt% or less to the total weight
of a steel plate, to prevent deterioration of toughness of a steel plate. When the
phosphorus content exceeds 0.03 wt%, an iron phosphide compound is formed so that
toughness and weldability deteriorate, and cracks may be generated in a steel plate
during a manufacturing process.
[0038] Sulfur (S) is an element that contributes to improvement of machinability. The sulfur
may be included in an amount of greater than 0 to 0.008 wt% or less to the total weight
of a steel plate. When the sulfur content exceeds 0.008 wt%, hot machinability, weldability,
and shock-resistant properties may deteriorate, and due to generation of large inclusions,
surface defects, such as cracks and the like, may be generated.
[0039] Chromium (Cr) is added for the purpose of increasing hardenability and strength during
heat treatment. Chromium enables grain refinement and strength securement through
precipitation hardening. The chromium may be included in an amount of 0.05 wt% to
0.9 wt% to the total weight of a steel plate. When the chromium content is less than
0.05 wt%, precipitation hardening effect may be reduced. In contrast, when the chromium
content exceeds 0.9 wt%, Cr-based precipitates and matrix solid content increase so
that toughness is reduced, and production costs may increase due to an increased cost.
[0040] Boron (B), which secures a martensite structure by restricting ferrite, pearlite,
and bainite transformation, is added for the purpose of obtaining hardenability and
strength during heat treatment. Furthermore, boron is segregated at grain boundaries
and lowers grain boundary energy so as to increase hardenability, and has a grain
refinement effect by increasing the austenite grain growth temperature. The boron
may be included in an amount of 0.0005 wt% to 0.01 wt% to the total weight of a steel
plate. When the boron is included in the range described above, occurrence of hard
phase grain boundary brittleness may be prevented, and high toughness and bendability
may be secured. When the boron content is less than 0.0005 wt%, the hardenability
effect is insufficient. In contrast, when the boron content exceeds 0.01 wt%, solubility
is low so that the boron is easily precipitated at grain boundaries according to heat
treatment conditions, which causes deterioration of hardenability or high temperature
embrittlement, and as hard phase grain boundary brittleness occurs, toughness and
bendability may deteriorate.
[0041] Calcium (Ca) may be added for control of precipitates. Calcium has a high bonding
strength with sulfur so as to form CaS precipitates, which may suppress the generation
of MnS that impedes weldability. The calcium may be included in an amount of 0.00001
wt% to 0.006 wt% to the total weight of a steel plate. When the calcium content is
less than 0.00001 wt%, a MnS control effect deteriorates. When the calcium content
exceeds 0.006 wt%, continuous casting properties may deteriorate.
[0042] Titanium (Ti) may effectively contribute to grain refinement by forming precipitates
at high temperature. The titanium may be included in an amount of 0.001 wt% to 0.095
wt%, in particular 0.005 wt% to 0.06 wt%, to the total weight of a steel plate. When
the titanium is included in the content range, continuous casting defects and precipitate
coarseness may be prevented, the physical properties of structural steel may be easily
secured, and defects, such as crack generation and the like, on a surface of structural
steel may be prevented. When the titanium content falls below the lower limit, the
effect may not be appropriately achieved. In contrast, when the titanium content exceeds
the upper limit, precipitates become coarse so that an elongation rate and bendability
may be dropped.
[0043] Niobium (Nb) and vanadium (V) may increase strength and toughness according to a
decrease in the martensite packet size. The niobium and vanadium may each be included
in an amount of 0.005 wt% to 0.06 wt% to the total weight of a steel plate. When the
niobium is included in the above range, in hot rolling and cold rolling processes,
grain refinement effect of a steel plate is excellent, during steel making/continuous
casting, occurrence of cracks in a slab and brittle fracture of products may be prevented,
and generation of coarse precipitates in steel making may be minimized. When the niobium
content is less than 0.005 wt%, the effect may not be appropriately achieved. In contrast,
when the niobium content exceeds 0.06 wt%, strength and toughness do not improve further
with increasing niobium content, and the niobium exists as a state employed in ferrite
so that there is a risk that impact toughness may be rather reduced. Vanadium may
also have a tendency similar to the niobium described above.
[0044] Molybdenum (Mo), as a substitution element, improves strength of steel with a solid
strengthening effect. The molybdenum may be added for the purpose of reducing precipitate
coarseness and increasing hardenability. Furthermore, the molybdenum (Mo) may serve
to increase hardenability of steel. The molybdenum may be included in an amount of
0.05 wt% to 0.2 wt% to the total weight of a steel plate. When the molybdenum content
is less than 0.05 wt%, the effect may be appropriately achieved. In contrast, when
the molybdenum content exceeds 0.2 wt%, there is a risk of lowering of rolling productivity
and an elongation rate, and only the manufacturing costs are raised without an additional
effect.
[0045] The titanium (Ti), niobium (Nb), vanadium (V), and molybdenum (Mo) described above
may be used as elements to control the formation of precipitates in a molded part
after hot stamping. In an embodiment, a steel plate may appropriately include titanium
(Ti), niobium (Nb), and vanadium (V) each in an amount of 0.005 to 0.06 wt%. Accordingly,
when titanium (Ti), niobium (Nb), and vanadium (V) are each included in an amount
of 0.005 to 0.06 wt%, and molybdenum (Mo) is included in an amount of 0.05 to 0.2
wt%, a microstructure area in a steel plate before hot stamping may be easily controlled,
and furthermore, conditions that are easy to control precipitates in a hot stamping
part after hot stamping may be satisfied.
[0046] In an embodiment, when the contents of titanium (Ti), niobium (Nb), vanadium (V),
and molybdenum (Mo) included in a steel plate are represented by [Ti], [Nb], [V],
and [Mo] in wt%, respectively, the following Inequality 1 may be satisfied.

[0047] Accordingly, the shape of a microstructure formed in a steel plate may be controlled.
The microstructure may include, for example, an area where a pearlite structure is
locally accumulated (hereinafter, referred to as the pearlite region). The area where
a pearlite structure is accumulated affects a size of a grain and fraction coarseness
after hot stamping, which may deteriorate hydrogen embrittlement and bending angle
(e.g., V-bending angle) performance of a hot stamping part after hot stamping. Accordingly,
in a steel plate before hot stamping, when the contents of titanium (Ti), niobium
(Nb), vanadium (V), and molybdenum (Mo) satisfy the [Inequality 1], the grain refinement
of a hot stamping part after hot stamping may be easily controlled, and thus, hydrogen
embrittlement and bending angle performance may be secured.
[0048] The microstructure of a steel plate before hot stamping may include ferrite and pearlite.
In an embodiment, a steel plate may include ferrite: 50-99% and pearlite: 0.1-50%
at an area fraction. Furthermore, a steel plate may include other inevitable structures.
For example, a steel plate may include other inevitable structures of 0% or more and
less than 5%. Meanwhile, in an embodiment, an average grain size of ferrite included
in a steel plate before hot stamping may be controlled to satisfy a range of 2 µm
or more and 10 µm or less.
[0049] Carbon (C) and/or manganese (Mn) may be segregated in the pearlite, and thus, the
microstructure of a steel plate for hot stamping may include pearlite with relatively
high carbon content and/or manganese content. Furthermore, the pearlite with relatively
high carbon content and/or manganese content is locally accumulated in a steel plate
so as to form a pearlite region.
[0050] The steel plate before hot stamping according to an embodiment of the present disclosure
may be controlled such that the size, density, and area fraction of a pearlite region
that a steel plate for hot stamping has satisfy preset conditions, while including
carbon and manganese as much as the content optimized as described above. Accordingly,
the mechanical properties, such as tensile strength, yield strength, bending properties,
an elongation rate, and the like, of a molded part after hot stamping may be controlled.
[0051] The tensile strength of a hot stamping part formed by hot stamping a steel plate
may satisfy a range of 1,700 MPa or more, in particular 1,760 MPa or more and 1,950
MPa or less. Furthermore, the yield strength of a hot stamping part may satisfy a
range of 1,150 MPa or more, in particular 1,200 MPa or more and 1,350 MPa or less.
Furthermore, a hot stamping part may satisfy a bending angle of 50° or more, and may
have an elongation rate of 5% or more. Here, the "bending angle" may mean a V-bending
angle in a rolling direction (a rolling direction, RD).
[0052] The degree of influence on the mechanical properties of a hot stamping part after
hot stamping may vary depending on the content of carbon (C) and the content of manganese
(Mn) included in the pearlite accumulated in a pearlite region in a steel plate before
hot stamping. In detail, what affects the mechanical properties of a hot stamping
part is an area where pearlite including carbon of 0.27 wt% or more and manganese
of 1.0 wt% or more is locally concentrated. In contrast, the effect of an area where
pearlite having a carbon content of less than 0.27 wt% or a manganese content of less
than 1.0 wt% is locally concentrated area on the mechanical properties of a hot stamping
part is minimal. Accordingly, a steel plate for hot stamping according to an embodiment
of the present disclosure is controlled such that the size, density, and area fraction
of an area where pearlite including carbon of 0.27 wt% or more and manganese of 1.0
wt% or more is locally concentrated satisfy preset conditions.
[0053] The steel plate for hot stamping according to an embodiment of the present disclosure
may include a pearlite region in which pearlite including carbon (C) of 0.27 to 0.70
wt% and/or manganese (Mn) of 1.0 to 5.0 wt% locally accumulated. The size, shape,
and area fraction of the pearlite region may be controlled to satisfy preset conditions.
[0054] In an embodiment, the average length of the pearlite region may be controlled to
satisfy a range of 0.01 µm or more and 500 µm or less, in particular 0.1 µm or more
and 100 µm or less. Furthermore, the average thickness of the pearlite region may
be controlled to satisfy a range of 0.01 µm or more and 30 µm or less. Furthermore,
an average gap between pearlite regions may be controlled to be 0.01 µm or more and
10 µm or less.
[0055] In an embodiment, an area fraction of a pearlite region in a steel plate for hot
stamping may be controlled to satisfy a range of 0.1% or more and 15% or less.
[0056] The pearlite region may contain an area fraction of 50% or more pearlite and 5% or
less ferrite. Furthermore, selectively, a low temperature phase structure, such as
precipitate, martensite, and/or bainite, and the like may be included up to 5%.
[0057] FIG. 2 is an image showing the structure of a hot stamping part formed by hot stamping
the steel plate according to FIG. 1.
[0058] The hot stamping part may include martensite, bainite, ferrite, and/or austenite.
A ratio of microstructure and an average grain size of the microstructure of the hot
stamping part may be controlled to satisfy preset conditions. Accordingly, the mechanical
properties of the hot stamping part, such as tensile strength, yield strength, bending
properties, an elongation rate, and the like, may be controlled. For example, the
tensile strength of the hot stamping part may satisfy a range of 1,700 MPa or more,
in particular 1,760 MPa or more and 1,950 MPa or less. Furthermore, the yield strength
of the hot stamping part may satisfy a range of 1,150 MPa or more, in particular 1,200
MPa or more and 1,350 MPa or less. Furthermore, the hot stamping part may satisfy
a bending angle of 50° or more, and may have an elongation rate of 5% or more.
[0059] In an embodiment, the microstructure of the hot stamping part may include 70% or
more martensite, 30% or less bainite and ferrite, 5% or less residual carbide, and
retained austenite.
[0060] In an embodiment, the microstructure included in the hot stamping part may be refined.
In detail, the average grain size of the microstructure included in the hot stamping
part may be controlled to satisfy a range of 15 µm or less, in particular 2 µm or
more and 15 µm or less.
[0061] Referring to FIG. 2, the hot stamping part according to an embodiment of the present
disclosure may include microstructure including austenite grains. A steel plate may
include an area fraction of 70% or more martensite phase, the austenite grains may
be generally distributed in the martensite phase.
[0062] In an embodiment, an average size of austenite grains in the hot stamping part may
be about 15 µm or less, in particular 13 µm or less. When the average size of austenite
grains exceeds 15 µm, fracture may occur during hydrogen embrittlement evaluation.
By controlling an austenite grain size (AGS) to be below a certain level in the hot
stamping part, sensitivity to hydrogen embrittlement may be reduced. The austenite
grain size may be controlled through an element that forms precipitate within a steel
plate. As an example, for a steel plate including niobium (Nb), titanium (Ti), and
molybdenum (Mo), the refinement of austenite grains may be easily implemented within
a hot stamping part after hot stamping. Furthermore, the steel plate may further include
vanadium (V) other than niobium (Nb), titanium (Ti), and molybdenum (Mo).
[0063] FIGS. 3 and 4 are graphs showing the measurements of austenite grain sizes and fractions
of a hot stamping part, according to embodiments of the present disclosure and comparative
examples.
[0064] In detail, FIG. 3 is a graph of measuring a fraction of an austenite grain size of
about 10 µm or more of the hot stamping part according to the present embodiment,
and FIG. 4 is a graph of measuring a fraction of an austenite grain size of about
20 µm or more of the hot stamping part according to the present embodiment. Specimen
① and specimen ② are according to comparative examples, and specimen ③ and specimen
④ are according to embodiments of the present disclosure. Specimen ① and specimen
② according to comparative examples have fracture occurring during hydrogen embrittlement
evaluation, whereas specimen ③ and specimen ④ according to the present embodiment
do not have fracture occurring during hydrogen embrittlement evaluation, which satisfies
the performance to be implemented by the present disclosure.
[0065] In an embodiment, a fraction of an austenite grain size of about 10 µm or more of
the hot stamping part may be 67% or less, in particular 65% or less.
[0066] As illustrated in FIG. 3, specimen ① and specimen ② according to comparative examples
are formed such that a fraction of an austenite grain size of about 10 µm or more
exceeds 67%. Specimen ① and specimen ② according to comparative examples have fracture
occurring during hydrogen embrittlement evaluation, and thus, it can be seen that
specimen ① and specimen ② according to comparative examples fall short of the performance
to be implemented by the present disclosure. In contrast, specimen ③ and specimen
④ according to the present embodiment are formed such that a fraction of an austenite
grain size of about 10 µm or more is 67% or less. Specimen ③ and specimen ④ according
to the present embodiment do not have fracture occurring during hydrogen embrittlement
evaluation, and thus, it can be seen that the performance to be implemented by the
present disclosure is satisfied. In other words, when the average size of austenite
grains in the hot stamping part is about 15 µm or less, and furthermore, the specimen
is formed such that a fraction of an austenite grain size of about 10 µm or more is
67% or less, hydrogen embrittlement and bending angle performance may be satisfied.
[0067] In an embodiment, a fraction of an austenite grain size of about 20 µm or more of
the hot stamping part may be 10% or less, in particular 7% or less.
[0068] As illustrated in FIG. 4, specimen ① and specimen ② according to comparative examples
are formed such that a fraction of an austenite grain size of about 20 µm or more
exceeds 10%. Specimen ① and specimen ② according to comparative examples have fracture
occurring during hydrogen embrittlement evaluation, and thus, it can be seen that
specimen ① and specimen ② according to comparative examples fall short of the performance
to be implemented by the present disclosure. In contrast, specimen ③ and specimen
④ according to the present embodiment are formed such that a fraction of an austenite
grain size of about 20 µm or more is 10% or less. Specimen ③ and specimen ④ according
to the present embodiment do not have fracture occurring during hydrogen embrittlement
evaluation, and thus, it can be seen that the performance to be implemented by the
present disclosure is satisfied. In other words, when the average size of austenite
grains in the hot stamping part is about 15 µm or less, and furthermore, the specimen
is formed such that a fraction of an austenite grain size of about 20 µm or more is
10% or less, hydrogen embrittlement and bending angle performance may be satisfied.
Specimen ③ and specimen ④ according to the present embodiment may be formed such that
the average size of austenite grains in the hot stamping part is 13 µm or less.
[0069] Meanwhile, the hot stamping part according to an embodiment of the present disclosure
may include precipitates including at least one of at least one of niobium (Nb), titanium
(Ti), molybdenum (Mo), and vanadium (V). Niobium (Nb), titanium (Ti), molybdenum (Mo),
and vanadium (V) included in a steel plate are carbide generating elements that contribute
to the formation of precipitates. Titanium (Ti), niobium (Nb), and molybdenum (Mo)
form carbon (C)-based precipitates, and thus, strength, hydrogen embrittlement, and
bendability of a hot stamping part may be secured. The elements may function as a
hydrogen trap site effective for improving delayed fracture resistance. In other words,
the precipitates may be distributed within a steel plate to trap hydrogen. In other
words, the precipitates provide a trap site to hydrogen introduced into the interior
of a steel plate before hot stamping, and thus, hydrogen delayed fracture properties
of the hot stamping part may be improved.
[0070] As described above, niobium (Nb), titanium (Ti), and vanadium (V) may each be included
in an amount of 0.005 to 0.06 wt%, and molybdenum (Mo) may be included in an amount
of 0.05 to 0.2 wt%. In particular, niobium (Nb), titanium (Ti), molybdenum (Mo), and
vanadium (V) may satisfy the [Inequality 1] described above. In this case, as described
above, the average size of austenite grains in the hot stamping part is about 15 µm
or less, a fraction of an austenite grain size of about 10 µm or more of the hot stamping
part is 67% or less, and a fraction of an austenite grain size of about 20 µm or more
of the hot stamping part may be formed to be 10% or less. Accordingly, hydrogen embrittlement
and bending angle performance of the hot stamping part may be satisfied.
[0071] The precipitation behavior of the precipitates may be measured by a method of analyzing
a transmission electron microscopy (TEM) image. In detail, TEM images for certain
regions as many as a preset number may be obtained with respect to specimen, precipitates
may be extracted from the obtained images through an image analysis program and the
like, and the number of precipitates, an average distance between precipitates, diameters
of precipitates, and the like may be measured for the extracted precipitates.
[0072] Furthermore, during measurement of the diameters of precipitates, considering non-uniformity
in the form of precipitates, the shapes of precipitates are converted into circles
to calculate the diameters of the precipitates. In detail, the area of the extracted
precipitate may be measured using a unit pixel having a specific area, and the precipitate
may be converted into a circle having the same area as the measured area so as to
calculate the diameter of the precipitate.
[0073] In an embodiment, the average particle size (size, diameter) of precipitates may
be controlled to satisfy preset conditions. In detail, the average particle size of
precipitates formed in a hot stamping part may be 10 nm or less, in particular 1 nm
or more and 6 nm or less. Furthermore, the amount of activated hydrogen of a hot stamping
part including the precipitates may be 0.6 wppm or less.
[0074] As a comparative example, when niobium (Nb), titanium (Ti), and vanadium (V) are
each included exceeding 0.06 wt%, or molybdenum (Mo) is included to be less than 0.05
wt%, the average particle size of precipitates exceeds 10 nm so that the probability
of hydrogen embrittlement occurring in hot stamping part may increase.
[0075] In an embodiment, the component (that is, an average component) of precipitates may
include 50 wt% or less of titanium (Ti) and 30 wt% or more of molybdenum (Mo). In
order for the precipitates to satisfy the average particle size described above, the
component of precipitates satisfies 50 wt% or less of titanium (Ti) and 30 wt% or
more of molybdenum (Mo). As a comparative example, when the component of precipitates
includes titanium (Ti) exceeding 50 wt%, or molybdenum (Mo) to be less than 30 wt%,
the coarseness of precipitates occurs, and thus, designed strength may not be secured.
[0076] In an embodiment, a gap between adjacent precipitates, that is, an average distance,
may be controlled to satisfy a preset range. Here, the "average distance" may be measured
through a mean free path of precipitates.
In detail, the average distance between precipitates may be calculated using a particle
area fraction and the number of particles per unit length. However, a method of measuring
the precipitation behavior of the precipitates is not limited to the example described
above, and various methods may be employed therefor.
[0077] In detail, the average distance between precipitates may be 0.1 nm or more 100 nm
or less, in particular 0.1 nm or more and 50 nm or less, or 0.1 nm or more 10 nm or
less. When the average distance between microprecipitates is less than 0.1 nm, formability
to bendability may deteriorate. In contrast, when the average distance between microprecipitates
exceeds 100 nm, strength may deteriorate.
[0078] In an embodiment, the average number of precipitates per unit area may be controlled
to satisfy preset conditions. In detail, when the [Inequality 1] described above is
satisfied, refinement of precipitates may be carried out. In this case, the average
number of precipitates per unit area may be 10,000/100 µm
2 to 35,000/100 µm
2. As a comparative example, when the [Inequality 1] described above is unsatisfied,
coarseness of precipitates is carried out. In this case, the average number of precipitates
per unit area may be formed to be less than 10,000/100 µm
2.
[0079] In the following description, the present disclosure is described in detail through
embodiments and comparative examples. However, the embodiments and comparative examples
described below are to describe the present disclosure in more detail, and the scope
of the present disclosure is not limited by the embodiments and comparative examples
described below. The embodiments and comparative examples described below may be appropriately
corrected and modified by a person skilled in the art within the scope of the present
disclosure.
[Table 1]
C |
Si |
Mn |
P |
S |
Cr |
B |
Ca |
[Inequality 1] 0.25(Ti+Nb+V+ 0.25Mo) |
0.26-0.40 |
0.02-2.00 |
0.30-1.60 |
0.03 or less |
0.008 or less |
0.05-0.90 |
0.0005-0.01 |
0.00001 -0.0060 |
0.015-0.060 |
[Table 2]
Specimen |
Austenite Grain Size (AGS) (µm) |
After H/S Hot Stamping Part Hydrogen Embrittlement Evaluation |
Comparative Example 1 |
85 |
52 |
Fractured |
Comparative Example 2 |
83 |
53 |
Fractured |
Comparative Example 3 |
84 |
53 |
Fractured |
Comparative Example 4 |
83 |
52 |
Fractured |
Comparative Example 5 |
85 |
52 |
Fractured |
Comparative Example 6 |
84 |
52 |
Fractured |
Comparative Example 7 |
84 |
53 |
Fractured |
Comparative Example 8 |
85 |
57 |
Fractured |
Comparative Example 9 |
90 |
61 |
Fractured |
Comparative Example 10 |
89 |
62 |
Fractured |
Comparative Example 11 |
85 |
57 |
Fractured |
Comparative Example 12 |
84 |
58 |
Fractured |
Embodiment 1 |
65 |
9 |
Non-fractured |
Embodiment 2 |
64 |
9 |
Non-fractured |
Embodiment 3 |
66 |
10 |
Non-fractured |
Embodiment 4 |
63 |
6 |
Non-fractured |
Embodiment 5 |
65 |
8 |
Non-fractured |
Embodiment 6 |
64 |
8 |
Non-fractured |
Embodiment 7 |
64 |
7 |
Non-fractured |
Embodiment 8 |
65 |
8 |
Non-fractured |
Comparative Example 13 |
72 |
18 |
Fractured |
Comparative Example 14 |
70 |
15 |
Fractured |
Comparative Example 15 |
70 |
16 |
Fractured |
Comparative Example 16 |
71 |
17 |
Fractured |
[0080] Table 1 shows the composition of a steel plate for hot stamping, and Table 2 shows
measurement values for evaluation of an austenite grain size and hydrogen embrittlement
of specimens corresponding to a hot stamping part. Comparative Example 1 to Comparative
Example 12 are specimens that do not satisfy the value of [Inequality 1] among the
composition of Table 1, and Embodiment 1 to Embodiment 8 are specimens corresponding
to a hot stamping part formed by hot stamping a steel plate having such a composition
as Table 1.
[0081] An evaluation for hydrogen embrittlement of each specimen employs the ASTM G39-99
reference (4-point bending test) test method. In detail, a specimen is loaded in a
4-point bending tester, and a yield strength (YP) of stress 100% is applied thereto.
Next, the specimen is dipped in 0.1 N HCl of an aqueous solution for 100 hours, and
then, it is measured whether a crack, that is, fracture, has occurred in a surface
of the specimen.
[0082] Referring to Table 2, Comparative Example 1 to Comparative Example 12 are specimens
of a hot stamping part manufactured from a steel plate having a value of [Inequality
1], that is, 0.25(Ti+Nb+V+0.25Mo), of the composition of Table 1, that is 0.013 wt%,
which is out of a range of 0.015 to 0.060 wt%. Niobium (Nb), titanium (Ti), molybdenum
(Mo), and vanadium (V) are carbide generating elements that contribute to the formation
of precipitates, and Comparative Example 1 to Comparative Example 12, which do not
satisfy precipitate control conditions, show a result of coarseness of the austenite
grain size in a hot stamping part. Accordingly, it may be seen that, in Comparative
Example 1 to Comparative Example 12, a fraction of an austenite grain size of 10 µm
or more is formed to exceed 67% that is a reference value of the present disclosure.
Furthermore, it may be seen that, in Comparative Example 1 to Comparative Example
12, a fraction of an austenite grain size of 20 µm or more is formed to be about 50%
or more exceeding, by far, 10% that is a reference value of the present disclosure.
As a result, it may be seen that, in Comparative Example 1 to Comparative Example
12, fracture occurs in the hydrogen embrittlement evaluation, and thus, the design
conditions of the present disclosure are not satisfied.
[0083] Meanwhile, it may be seen that, in Embodiment 1 to Embodiment 8, which are embodiments
of the present disclosure, while satisfying the composition of Table 1, a fraction
of an austenite grain size of 10 µm or more is formed to be 67% or less, and a fraction
of an austenite grain size of 20 µm or more is formed to be 10% or less. As a result,
it may be seen that, in Embodiment 1 to Embodiment 8, no fracture occurs in the hydrogen
embrittlement evaluation so that the design conditions of the present disclosure are
satisfied.
[0084] In contrast, Comparative Example 13 to Comparative Example 16 are specimens that
satisfy the composition of Table 1, but do not satisfy the conditions of precipitate
and the like due to a difference in the process control conditions. In Comparative
Example 13 to Comparative Example 16, a fraction of an austenite grain size of 10
µm or more is formed to exceed 67%, and a fraction of an austenite grain size of 20
µm or more is formed to exceed 10%. As a result, it may be seen that, in Comparative
Example 13 to Comparative Example 16, fracture occurs in the hydrogen embrittlement
evaluation so that the design conditions of the present disclosure are unsatisfied.
[0085] In the above, although embodiments have been described, these are merely examples,
and those skilled in the art to which the present disclosure pertains could make various
modifications and changes from these descriptions. Therefore, various changes in form
and details may be made therein without departing from the spirit and scope of the
disclosure as defined by the following claims.