[Technical Field]
[0001] The present disclosure relates to a microcrack reduced hot press-formed article and
a method for manufacturing the same.
[Background Art]
[0002] In recent years, the use of high-strength steel has increased to reduce the weight
of automobiles, but such high-strength steel is easily worn or fractured at room temperature.
In addition, the occurrence of a spring back phenomenon during machining makes precise
dimension work difficult, which makes it difficult to form complex products. Thus,
hot press forming (HPF) has been used as a desirable method for machining high strength
steel.
[0003] Hot press forming (HPF) is a method of machining a steel sheet to have a complicated
shape at high temperatures by utilizing the properties of the steel sheet, soft-nitrides,
having high ductility at high temperature. More specifically, in a state in which
a steel sheet is heated to an austenite region or greater, the steel sheet is machined
and rapidly quenched simultaneously to transform the structure of the steel sheet
into martensite, thus producing a product having high strength and a precise shape.
[0004] However, when a steel material is heated to high temperatures, a phenomenon such
as corrosion or decarbonization may occur on a surface of the steel material. To prevent
this, plated steel materials having a zinc-based or aluminum-based plated layer formed
on a surface thereof are commonly used as materials for hot press forming. In particular,
a galvanized steel sheet having a zinc-based plated layer is a steel material having
corrosion resistance improved using the self-sacrificial corrosion resistance of zinc.
[0005] However, when such a plated steel material is subjected to hot press forming, cracks
are formed in the plated layer of a seam-processed part where surface friction is
severe due to direct contact between a mold and the plated layer, and microcracks
are formed even on a surface of the base steel sheet along the cracks formed in the
plated layer.
[0006] In order to solve the problem, Patent document 1 (
U.S. Patent Publication No. 6296805) proposes a technique of performing Al-based plating on a surface of a steel sheet.
As proposed in Patent document 1, as Al-based plating is performed, an oxidation reaction
on a surface of the steel sheet is suppressed, while the plated layer is maintained
in a heating furnace, and formation of a passivation film of Al is used to increase
corrosion resistance, but corrosion resistance of the Al plated steel sheet is drastically
lowered.
[0007] In order to solve the problem, research into Zn-plated hot-pressed steel sheets has
been conducted and reviewed, but there is a problem that microcracks are formed even
in a surface of the base steel sheet, due to a high temperature working environment
in which a temperature of the plated steel exceeds 900°C and stress caused by friction
between a Zn-Fe alloyed layer alloyed during hot press forming and a dice. Such microcracks
may act as a starting point for the propagation of cracks in the base steel sheet
or cause fatigue cracks, which may decrease durability of parts.
[Disclosure]
[Technical Problem]
[0008] An aspect of the present disclosure is to provide a microcrack reduced hot press-formed
article and a method for manufacturing the same.
[Technical Solution]
[0009] According to an aspect of the present disclosure, there is provided a hot press-formed
article manufactured by hot press forming a galvanized steel sheet including a base
steel sheet and a zinc-based plated layer formed on a surface of the base steel sheet,
wherein the zinc-based plated layer includes at least one element selected from the
group consisting of Sb, Sn, and Bi in a total amount of 0.05 to 2.0 wt%, a balance
of Zn, and inevitable impurities, and at least 70 wt% of the at least one element
selected from the group consisting of Sb, Sn, Bi is concentrated in a region 3 µm
or less away from a surface of an alloyed zinc-based plated layer, formed by alloying
the zinc-based plated layer, of the hot press-formed article.
[0010] According to another aspect of the present disclosure, a method for manufacturing
a hot press-formed article includes: preparing a zinc-based plated steel sheet; primarily
heating the zinc-based plated steel sheet to a temperature of 640 to 680°C at a rate
of 3.5 to 4.2°C/sec; secondarily heating the primarily-heated zinc-based plated steel
sheet to a temperature of 900 to 930°C at a rate of 1.1 to 1.6°C/sec; maintaining
the secondarily-heated zinc-based plated steel sheet at a constant temperature for
1 to 5 minutes; and molding the zinc-based plated steel sheet maintained at the constant
temperature with a die and simultaneously quenching the steel sheet, wherein the zinc-based
plated steel sheet includes a base steel sheet and a zinc-based plated layer formed
on a surface of the base steel sheet and including at least one element selected from
the group consisting of Sb, Sn, and Bi in a total amount of 0.05 to 2.0 wt%.
[Advantageous Effects]
[0011] As set forth above, in the hot press-formed article according to an exemplary embodiment
in the present disclosure, microcracks in a plated layer caused during hot press forming
is effectively restrained from propagating to the base steel sheet, obtaining excellent
durability.
[0012] Various and advantageous advantages and effects of the present inventive concept
are not limited to those described above and may be more easily understood in the
course of describing the specific example embodiment of the present inventive concept.
[Description of Drawings]
[0013] FIG. 1 shows observed microcracks of Comparative Example 1, FIG. 2 shows observed
microcracks of Inventive Example 1, FIG. 3 shows observed microcracks of Inventive
Example 3, FIG. 4 shows observed microcracks of Comparative Example 4, and FIG. 5
shows observed microcracks of Inventive Example 5.
[0014] FIG. 6 (a) is GDS data obtained by analyzing contents of Al, Mg, and Sn according
to depths of a plated layer in Inventive Example 1, FIG. 6 (b) is GDS data obtained
by analyzing contents of Al, Mg, and Sn according to depths of a plated layer in Inventive
Example 3, and FIG. 6 (c) is GDS data obtained by analyzing contents of Al, Mg, and
Sn according to depths of a plated layer in Inventive Example 5.
[Best Mode for invention]
[0015] The inventors of the present application have conducted research in depth to provide
a hot press formed article with suppressed microcracks and resultantly discovered
that propagation of microcracks in a plated layer to a base steel sheet could be effectively
blocked by using a galvanized steel sheet having a zinc-based plated layer containing
a proper amount of grain, a boundary segregation element as a material for hot press
forming and concentrating the boundary segregation element on a surface layer of a
plated layer by appropriately controlling heating conditions during the hot press
forming, thus completing the present disclosure.
[0016] Hereinafter, a hot press-formed article according to an aspect of the present disclosure
will be described in detail.
[0017] The hot press formed article as one aspect of the present disclosure is manufactured
by hot press forming a galvanized steel sheet including a base steel sheet and a zinc-based
plated layer formed on a surface of the base steel sheet to hot press molding.
[0018] In the present disclosure, the kind of the base steel sheet is not limited and may
be, for example, a hot-rolled steel sheet or a cold-rolled steel sheet used as a base
of a general galvanized steel sheet. However, in the case of a hot-rolled steel sheet,
a large amount of oxide scale is present on a surface thereof. Such an oxide scale
lowers plating adhesion to deteriorate quality of plating, and thus, a hot-rolled
steel sheet whose oxide scale has previously been removed by an acid solution may
be used as a base.
[0019] Meanwhile, the zinc-based plated layer is formed on one side or both sides of the
base steel sheet, and the zinc-based plated layer is alloyed at the time of heat treatment
for hot press forming to change into an alloyed zinc-based plated layer.
[0020] The zinc-based plated layer may include at least one element selected from the group
consisting of Sb, Sn, and Bi in a total amount of 0.05% to 2.0% by weight, a balanced
amount of Zn, and inevitable impurities.
[0021] Sb, Sn, and Bi are grain boundary segregation elements serving to inhibit formation
of an internal oxide due to penetration of oxygen into the grain boundary in a high-temperature
working environment. In order to exhibit such effects in the present disclosure, the
sum of the contents of the above elements is preferably 0.05 wt% or greater, and more
preferably 0.3 wt% or greater. However, if the content is excessive, formation of
an aluminum oxide film on the surface of the plated layer may be hindered to impair
a barrier function of aluminum and an effect is low relative to the increase in the
content, lowering economic efficiency. Therefore, the sum of the contents of the above
elements is preferably 2.0 wt% or less, more preferably 1.5 wt% or less.
[0022] According to an example, the zinc-based plated layer may further contain 0.1 to 5.0
wt% of Mg and 0.1 to 7.5 wt% of Al.
[0023] Mg is an element serving to improve corrosion resistance of a hot press-formed article.
In order to exhibit such an effect in the present disclosure, the Mg content is preferably
0.1 wt% or greater, and more preferably 1 wt% or greater. However, if the Mg content
is excessive, dross of a plating bath may be generated due to Mg oxidation in the
plating bath. Therefore, an upper limit of the magnesium content is preferably 5.0
wt%, more preferably 4.0 wt%, and even more preferably 3.0 wt%.
[0024] Al serves to suppress Mg oxide dross. If the Al content is too low, the effect of
preventing Mg oxidation in the plating bath may be insignificant. Therefore, a lower
limit of the aluminum content is preferably 0.1 wt%, and more preferably 1.5 wt%.
However, if the Al content is too excessive, a temperature of the plating bath must
be increased. If the temperature of the plating bath is high, the plating facility
may be eroded. Therefore, an upper limit of the aluminum content is preferably 7.5
wt%, and more preferably 7.2 wt%.
[0025] According to an example, a degree of alloying of Fe of the alloyed zinc-based plated
layer formed by alloying the zinc-based plated layer is preferably 30 to 85%, more
preferably 45 to 78%, and even more preferably 50 to 75%. When the degree of alloying
of Fe satisfies the above range, surface cracking during hot pressing may be effectively
prevented and corrosion resistance characteristics based on sacrificial corrosion
prevention is excellent. If the degree of alloying of Fe is less than 30%, a region
of the plated layer in which a part of Zn is concentrated may exist in a liquid phase,
causing a liquid embrittlement cracks during processing. Meanwhile, if the degree
of alloying of Fe degree exceeds 85%, corrosion resistance may be lowered.
[0026] The hot pressed-formed article of the present disclosure features that at least 70
wt% of at least one element selected from the group consisting of Sb, Sn, and Bi is
concentrated in a region 3 µm or less away from a surface of the alloyed zinc-based
plated layer.
[0027] When Sb, Sn and Bi are concentrated in a large amount on the surface of the alloyed
zinc-based plated layer as described above, Sb, Sn and Bi may settle on the surface
of the plated layer before oxygen penetrates from the surface of the plated layer
to cause grain boundary segregation to restrain formation of internal oxide to prevent
formation of boundary cracks in the plated layer, thus blocking propagation of microcracks
to the base member. Furthermore, microcracks are mainly formed in a location where
friction between the mold and the plated layer is severe. The oxide of Sb, Sn, and
Bi concentrated on the surface may reduce a coefficient of friction between the mold
and the plated layer to reduce formation of microcracks, thus improving durability
of the hot press-formed article.
[0028] Meanwhile, in the present disclosure, a specific method of measuring the content
of at least one element selected from the group consisting of Sb, Sn, and Bi concentrated
in a region 3 µm or less away from the surface of the alloyed zinc-based plated layer
is not particularly limited, but the following method may be used. That is, after
the hot press-formed article may be cut vertically, a distribution of at least one
element selected from the group consisting of Sb, Sn, and Bi in the cross-section
of the plated layer may be measured using a glow discharge emission spectrometry (GDS),
and an area thereof is integrated in a graph related to the content of at least one
element selected from the group consisting of Sb, Sn, and Bi relative to the depth
from the surface of the plated layer, whereby the content of at least one element
selected from the group consisting of Sb, Sn, and Bi concentrated in the region 3
µm or less away from the surface of the alloyed zinc-based plated layer may be measured.
[0029] The hot press-formed article of the present disclosure described above may be manufactured
by various methods, and the manufacturing method is not particularly limited. However,
the hot press-formed article may be manufactured by the following method as one embodiment.
[0030] Hereinafter, a method for manufacturing a hot press-formed article having excellent
durability, which is another aspect of the present disclosure, will be described in
detail.
[0031] First, a galvanized steel sheet having the above-described alloy composition is prepared.
In the present disclosure, a specific method for preparing a zinc-based plated steel
sheet is not particularly limited. The galvanized steel sheet may be manufactured
by a general method of manufacturing a hot dip galvanized steel sheet. For example,
a base steel sheet may be dipped in a zinc-based plating bath having the above-described
composition and subsequently cooled to prepare the galvanized steel sheet.
[0032] However, in order to further maximize the intended effect of the present disclosure,
it is preferable to perform bubbling by supplying an inert gas in advance in the zinc-based
plating bath before dipping the base steel sheet in the zinc-based plating bath. Here,
the inert gas may be one or more selected from the group consisting of nitrogen (N
2), argon (Ar), and helium (He).
[0033] Performing bubbling in the zinc-based plating bath prior to performing the plating
as described above may help uniformly distribute Sb, Sn, and Bi in the zinc-based
plating bath, help evenly distribute Sb, Sn, and Bi in the zinc-based plated layer
obtained by a plating operation (to be described hereinafter), and help concentrate
Sb, Sn, and Bi on the surface of the alloyed zinc-based plated layer of the hot press-formed
article which is resultantly obtained. This is because as the distribution of Sb,
Sn, and Bi in the plated layer prior to heating for hot press forming is uniform,
Sb, Si, and Bi may be easily concentrated on the surface.
[0034] Meanwhile, in order to obtain the above effect, supply of the inert gas is preferably
maintained for 1 hour or greater, and more preferably for 3 hours or greater. Meanwhile,
an increase in the supply time of the inert gas may be advantageous to evenly distribute
the components in the plating bath, and thus, an upper limit is not particularly limited.
[0035] Next, the zinc-based plated steel sheet is primarily-heated to be processed into
an article. This operation is performed in order to sufficiently impart the zinc content
of the plated layer in a follow-up heating process by increasing a melting point by
performing alloying with iron before zinc of the plated layer is oxidized in the atmosphere
[0036] During the primary heating, an average heating rate is preferably 3.5 to 4.2°C/sec.
If the average heating rate is lower than 3.5°C/sec, a rise time may be prolonged
to delay the effect of the increase in the melting point due to alloying to cause
excessive oxidation of zinc. Meanwhile, if the average heating rate exceeds 4.2°C/sec,
zinc on the surface may be first melted earlier than alloying of the material to increase
oxidation of the surface of the plated layer.
[0037] During the primary heating, a primary heating end temperature is preferably 640 to
680°C. If the temperature is lower than 650°C, a diffusion coefficient in the plated
layer may be too low due to the low temperature so the plated layer may not be uniformly
alloyed. Meanwhile, if the temperature exceeds 680°C, the plated layer may be liquefied
beyond the melting point of zinc delta and zinc may be vaporized to cause loss of
the plated layer.
[0038] Next, the primarily-heated zinc-based plated steel sheet is secondarily-heated. This
operation is performed so that added internal oxidation inhibiting materials are first
segregated to the grain boundary to prevent grain boundary oxidation due to oxygen
to suppress microcracks, while stably changing the plated layer, sufficiently changed
into delta phase, into Fe-alpha phase.
[0039] During the secondary heating, an average heating rate is preferably 1.1 to 1.6°C/sec.
If the average heating rate is less than 1.1°C/sec, an alloying time to the Fe-alpha
phase may be prolonged to cause a possibility of grain boundary oxidation based on
oxygen, rather than the grain boundary segregation element. Meanwhile, if the average
heating rate exceeds 1.6°C/sec, partial plated layer liquefaction may occur on the
surface of the plated layer at high temperatures to deteriorate quality due to a non-uniform
surface.
[0040] During the secondary heating, a secondary heating end temperature is preferably 900
to 930°C. If the temperature is lower than 900°C, sufficient austenite transformation
of the material may not be achieved, making it difficult to secure strength of a final
product. If the temperature exceeds 930°C, the plated layer may be entirely liquefied
to degrade the microcrack suppressing effect based on the added grain boundary oxidation
element.
[0041] Next, the secondarily-heated zinc-based plated steel sheet is kept at the secondary
heating end temperature for 1 to 5 minutes. If the holding time is less than 1 minute,
it may be difficult to secure a sufficient time for the austenite transformation of
the material due to the shortage of the total heating time. Meanwhile, if the holding
time exceeds 5 minutes, the plated layer may be excessively alloyed to lower the zinc
content in the plated layer to degrade corrosion resistance.
[0042] Thereafter, the secondarily-heated zinc-based plated steel sheet is molded by a die
and quenched at the same time. Here, the molding and quenching by the die may be sufficient
by the general hot press forming method, and therefore, it is not limited in the present
disclosure.
[Mode for invention]
[0043] Hereinafter, the present disclosure will be described more specifically by way of
examples. It should be noted, however, that the following embodiments are intended
to illustrate and specify the present disclosure and do not to limit the scope of
the present disclosure. The scope of the present disclosure is determined by the matters
described in the claims and the matters reasonably deduced therefrom.
[0044] A low carbon cold-rolled steel sheet having a thickness of 0.8 mm, a width of 100
mm, and a length of 200 mm, as a base steel sheet, was prepared as a test sample for
plating, dipped in acetone, and ultrasonically cleaned to remove foreign substances
such as rolling oil present on the surface thereof. Thereafter, the steel sheet was
subjected to a heat treatment in a reducing atmosphere at 750°C to secure the mechanical
properties of the steel sheet at the general hotdip plating site and subsequently
dipped in a zinc-based plating bath having the composition shown in Table 1 below
to manufacture a plated steel sheet. Thereafter, each of the manufactured plated steel
materials was gas-wiped to adjust the coating weight to 70g/m
2 per side and cooled at a rate of 12°C/sec.
[0045] Thereafter, each of the cooled plated steel materials was heated under the conditions
shown in Table 2 below and hot press-formed to obtain a hot press-formed article.
[0046] Thereafter, each of the hot press-formed articles was cut vertically, and a distribution
of grain boundary segregation elements in the plated layer was measured by a GDS analysis.
The results are shown in Table 2 below. A specific measurement method is as described
above.
[0047] Thereafter, a maximum depth of microcracks at a portion where tension and surface
friction were most severe during molding, and the results are shown in Table 2 below.
[Table 1]
Bath type |
Plating bath composition (wt%) (* balance is Zn) |
Sb |
Sn |
Bi |
Al |
Mg |
Bath 1 |
- |
- |
- |
0.21 |
- |
Bath 2 |
- |
0.7 |
- |
1.50 |
1.0 |
Bath 3 |
- |
0.4 |
- |
2.5 |
1.0 |
Bath 4 |
- |
- |
- |
2.5 |
1.0 |
Bath 5 |
- |
0.3 |
- |
7.0 |
3.0 |
Bath 6 |
0.3 |
- |
- |
1.5 |
1.0 |
Bath 7 |
- |
- |
0.5 |
2.5 |
1.0 |
[Table 2]
No |
Bath type |
Primary heating |
Secondary heating |
Main tain |
① |
Maximum depth of microc rack |
Remark |
Rate (°C /s) |
End temperature (°C) |
Rate (°C /s) |
End temperature (°C) |
Time (min .) |
1 |
Bath 1 |
3.8 |
640 |
1.2 |
900 |
5 |
- |
32.0 |
Comparative Example 1 |
2 |
Bath 1 |
4.2 |
670 |
1.6 |
910 |
5 |
- |
29.0 |
Comparative Example 2 |
3 |
Bath 2 |
3.5 |
650 |
1.2 |
900 |
5 |
78 |
2.5 |
Inventive Example 1 |
4 |
Bath 2 |
3.9 |
670 |
1.4 |
910 |
5 |
85 |
3.8 |
Inventive Example 2 |
5 |
Bath 3 |
3.7 |
650 |
1.3 |
910 |
5 |
99 |
3.0 |
Inventive Example 3 |
6 |
Bath 3 |
4.0 |
660 |
1.1 |
900 |
5 |
92 |
3.7 |
Inventive Example 4 |
7 |
Bath 4 |
4.0 |
660 |
1.3 |
900 |
5 |
- |
28.0 |
Comparative Example 3 |
8 |
Bath 4 |
3.9 |
660 |
1.5 |
915 |
5 |
- |
26.0 |
Comparative Example 4 |
9 |
Bath 5 |
3.7 |
650 |
1.2 |
910 |
5 |
77 |
7.0 |
Inventive Example 5 |
10 |
Bath 5 |
3.8 |
640 |
1.4 |
910 |
5 |
79 |
7.3 |
Inventive Example 6 |
11 |
Bath 6 |
4.0 |
650 |
1.5 |
900 |
5 |
83 |
5.2 |
Inventive Example 7 |
12 |
Bath 6 |
4.1 |
640 |
1.3 |
920 |
5 |
82 |
6.0 |
Inventive Example 8 |
13 |
Bath 7 |
3.9 |
650 |
1.3 |
910 |
5 |
89 |
8.0 |
Inventive Example 9 |
14 |
Bath 7 |
3.5 |
640 |
1.2 |
930 |
5 |
91 |
7.8 |
Inventive Example 10 |
Here, ① indicates the content (wt%) of at least one element selected from the group
consisting of Sb, Sn, and Bi concentrated a region 3µm or less away from the surface of the alloyed zinc-based plated layer |
[0048] Referring to Table 2, it can be seen that the maximum depth of microcracks in Inventive
Examples 1 to 10 satisfying all the conditions of the present disclosure was suppressed
to 10 µm or less.
[0049] FIG. 1 shows observed microcracks of Comparative Example 1, FIG. 2 shows observed
microcracks of Inventive Example 1, FIG. 3 shows observed microcracks of Inventive
Example 3, FIG. 4 shows observed microcracks of Comparative Example 4, and FIG. 5
shows observed microcracks of Inventive Example 5. Referring to FIGS. 1 to 5, it can
be seen that in the case of Inventive Examples, propagation of microcracks in the
plated layer to the base steel sheet was effectively blocked.
[0050] FIG. 6A is GDS data obtained by analyzing contents of Al, Mg, and Sn according to
depths of a plated layer in Inventive Example 1, FIG. 6B is GDS data obtained by analyzing
contents of Al, Mg, and Sn according to depths of a plated layer in Inventive Example
3, and FIG. 6C is GDS data obtained by analyzing contents of Al, Mg, and Sn according
to depths of a plated layer in Inventive Example 5.
1. A hot press-formed article manufactured by hot press forming a galvanized steel sheet
including a base steel sheet and a zinc-based plated layer formed on a surface of
the base steel sheet,
wherein the zinc-based plated layer includes at least one element selected from the
group consisting of Sb, Sn, and Bi in a total amount of 0.05 to 2.0 wt%, a balance
of Zn, and inevitable impurities, and
at least 70 wt% of the at least one element selected from the group consisting of
Sb, Sn, Bi is concentrated in a region 3 µm or less away from a surface of an alloyed
zinc-based plated layer, formed by alloying the zinc-based plated layer, of the hot
press-formed article.
2. The hot press-formed article of claim 1,
Wherein the zinc-based plated layer includes at least one element selected from the
group consisting of Sb, Sn, and Bi in a total amount of 0.3 to 1.5 wt%.
3. The hot press-formed article of claim 1,
Wherein the zinc-based plated layer further includes 0.1 to 5.0 wt% of Al and 0.1
to 5.0 wt% of Mg.
4. The hot press-formed article of claim 1,
wherein a degree of alloying of Fe of the alloyed zinc-based plated layer is 30 to
85%.
5. A method for manufacturing a hot press-formed article, the method comprising:
preparing a zinc-based plated steel sheet;
primarily heating the zinc-based plated steel sheet to a temperature of 640 to 680°C
at a rate of 3.5 to 4.2°C/sec;
secondarily heating the primarily-heated zinc-based plated steel sheet to a temperature
of 900 to 930°C at a rate of 1.1 to 1.6°C/sec;
maintaining the secondarily-heated zinc-based plated steel sheet at a constant temperature
for 1 to 5 minutes; and
molding the zinc-based plated steel sheet maintained at the constant temperature with
a die and simultaneously quenching the steel sheet,
wherein the zinc-based plated steel sheet includes a base steel sheet and a zinc-based
plated layer formed on a surface of the base steel sheet and including at least one
element selected from the group consisting of Sb, Sn, and Bi in a total amount of
0.05 to 2.0 wt%.
6. The method of claim 1,
Wherein the zinc-based plated layer includes at least one element selected from the
group consisting of Sb, Sn, and Bi in a total amount of 0.3 to 1.5 wt%.
7. The method of claim 1,
Wherein the zinc-based plated layer further includes 0.1 to 5.0 wt% of Al and 0.1
to 5.0 wt% of Mg.