U.S. patent application number 16/061844 was filed with the patent office on 2018-12-20 for hot press formed product having excellent corrosion resistance and method for preparing same.
The applicant listed for this patent is POSCO. Invention is credited to Hyeon-Seok HWANG, Jong-Sang KIM, Il-Ryoung SOHN.
Application Number | 20180363117 16/061844 |
Document ID | / |
Family ID | 59089563 |
Filed Date | 2018-12-20 |
United States Patent
Application |
20180363117 |
Kind Code |
A1 |
SOHN; Il-Ryoung ; et
al. |
December 20, 2018 |
HOT PRESS FORMED PRODUCT HAVING EXCELLENT CORROSION RESISTANCE AND
METHOD FOR PREPARING SAME
Abstract
Provided is a hot press formed product, which is prepared by
means of hot press forming of a Zn--Al--Mg-based plated steel
material comprising base iron and a Zn--Al--Mg-based plated layer,
and a method for preparing the same, the hot press formed product
comprising an oxide layer formed on the surface thereof, wherein
the content ratio of Al to Mg (Al/Mg) in the oxide layer is 0.8 or
more.
Inventors: |
SOHN; Il-Ryoung;
(Gwangyang-si, KR) ; HWANG; Hyeon-Seok;
(Gwangyang-si, KR) ; KIM; Jong-Sang;
(Gwangyang-si, KR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
POSCO |
Pohang-si |
|
KR |
|
|
Family ID: |
59089563 |
Appl. No.: |
16/061844 |
Filed: |
December 20, 2016 |
PCT Filed: |
December 20, 2016 |
PCT NO: |
PCT/KR2016/014937 |
371 Date: |
June 13, 2018 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C21D 8/0226 20130101;
C22C 38/04 20130101; B21D 37/16 20130101; C21D 8/0247 20130101;
C23C 2/06 20130101; C21D 9/46 20130101; B21D 53/88 20130101; C22C
38/00 20130101; C23C 2/28 20130101; C21D 8/0236 20130101; B21D
22/022 20130101; C21D 8/0278 20130101; B21D 22/02 20130101; C23C
2/02 20130101; C22C 38/02 20130101; B21D 35/005 20130101 |
International
Class: |
C23C 2/06 20060101
C23C002/06; C23C 2/02 20060101 C23C002/02; C23C 2/28 20060101
C23C002/28; C22C 38/04 20060101 C22C038/04; C22C 38/02 20060101
C22C038/02; B21D 22/02 20060101 B21D022/02 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 22, 2015 |
KR |
10-2015-0183502 |
Claims
1. A hot press formed product prepared by hot-press forming a
Zn--Al--Mg-based plated steel material including base iron and a
Zn--Al--Mg-based plating layer, wherein the hot press formed
product comprises an oxide layer formed on a surface, and a content
ratio of Al to Mg (Al/Mg) in the oxide layer is 0.8 or more.
2. The hot press formed product of claim 1, wherein the content
ratio of Al to Mg (Al/Mg) in the oxide layer is 0.9 or more.
3. The hot press formed product of claim 1, wherein a total coating
weight of Zn, Al and Mg in the oxide layer is 700 mg/m.sup.2 or
less (exclusive of 0 mg/m.sup.2).
4. The hot press formed product of claim 1, wherein the oxide layer
contains one or two or more selected from the group consisting of
Mn, Si and Fe, and a sum of contents of Mn, Si and Fe in the oxide
layer is 50% or less relative to a total contents of metals in the
oxide layer.
5. The hot press formed product of claim 1, wherein a ratio of a
total amount of Mg (Mg.sub.o) contained in the oxide layer relative
to a total amount of Mg (Mg.sub.c) contained in the plating layer
of the hot press formed product is 1 or less.
6. The hot press formed product of claim 1, wherein an alloying
degree of Fe in the plating layer of the hot press formed product
is 20-70%.
7. The hot press formed product of claim 1, wherein the
Zn--Al--Mg-based plating layer contains: 0.9-3.5% by weight of Mg,
and 1.0-15% by weight of Al, with a balance of Zn and other
unavoidable impurities.
8. The hot press formed product of claim 1, wherein the base iron
contains: 0.15-0.35% by weight of C, 0.5% by weight or less
(exclusive of 0% by weight) of Si, 0.5-8.0% by weight of Mn, and
0.0020-0.0050% by weight of B, with a balance of Fe and unavoidable
impurities.
9. The hot press formed product of claim 1, wherein a maximum
corrosion depth of a base member after a salt spray test for 1200
hours according to KS R 1127 is 0.5 mm or less.
10. The hot press formed product of claim 1, wherein tensile
strength is 1300 MPa or more.
11. A method for preparing a hot press formed product, comprising:
immersing base iron in a Zn--Al--Mg-based plating bath, and
performing plating to obtain a Zn--Al--Mg-based plated steel
material; heating the Zn--Al--Mg-based plated steel material to a
heating temperature of 600-950.degree. C. at a rate of 10.degree.
C./sec or more in a heating furnace; and forming the
Zn--Al--Mg-based plated steel material which has reached the
heating temperature with a mold simultaneously with quenching,
wherein a residence time is 120 seconds or less, the residence time
representing a time during which the Zn--Al--Mg-based plated steel
material which has reached the heating temperature resides in the
heating furnace.
12. The method of claim 11, wherein the heating temperature is
800.degree. C. or more and 950.degree. C. or less, an average
heating rate to the heating temperature is 20.degree. C./sec or
more, and the residence time is 60 seconds or less.
13. The method of claim 11, wherein the heating is carried out by
any one method of radiant heating, high-frequency induction heating
and ohmic heating.
14. The method of claim 11, wherein the heating is carried out
under an inert gas atmosphere.
15. The method of claim 11, wherein the content ratio of Al to Mg
(Al/Mg) in the Zn--Al--Mg-based plating bath is 0.8 or more.
16. The method of claim 11, wherein the base iron is a cold rolled
steel plate, and the cold rolled steel plate has a surface
roughness of 2.0 .mu.m or less before plating.
17. The method of claim 11, wherein the base iron contains:
0.15-0.35% by weight of C, 0.5% by weight or less (exclusive of 0%)
of Si, 0.5-8.0% by weight of Mn, and 0.0020-0.0050% by weight of B,
with a balance of Fe and unavoidable impurities.
18. The method of claim 17, further comprising the following before
obtaining the plated steel material: pre-plating one or more metals
selected from the group consisting of Fe, Ni, Cu, Sn and Sb to an
average thickness of 5-100 nm on a surface of the base iron; and
annealing the pre-plated base iron.
19. The method of claim 18, wherein the annealing is carried out
under 1-15% by volume of hydrogen gas and remaining nitrogen gas.
Description
TECHNICAL FIELD
[0001] The present disclosure relates to a hot press formed product
having excellent corrosion resistance and a method for preparing
the same.
BACKGROUND ART
[0002] Recently, high-strength steel is increasingly being utilized
for lightening the weight of cars, but such high-strength steel may
be easily abraded or fractured when processed at room temperature.
In addition, since spring back also occurs at the time of
processing, precise dimension processing is difficult, and thus, it
is difficult to mold a product having a complicated shape.
Accordingly, as a preferable method for processing high-strength
steel, hot press forming (HPF) is being applied.
[0003] Hot press forming (HPF) is a method of processing steel into
a complicated shape at high temperature, using the nature of the
steel of being softened and highly ductile at high temperature, and
more specifically, steel is subjected to processing, simultaneously
with quenching in the state of being heated equal to or higher than
the austenite region to transform the structure of steel to
martensite, thereby preparing a high-strength product having a
precise shape.
[0004] However, when heating a steel material to a high
temperature, there may be corrosion or decarburization on the
surface of the steel material, and in order to prevent this
phenomenon, a zinc-based plated steel material having a zinc-based
plating layer formed on the surface is currently attracting
attention, as a material for hot press forming.
[0005] However, in the case of a general zinc-based plated steel
material, zinc may be excessively oxidized during heating for hot
press forming, so that the effective thickness of the plating layer
may be decreased, or the content of zinc in the zinc-based plating
layer maybe excessively decreased, so that corrosion resistance
after forming is deteriorated.
[0006] Meanwhile, recently, for further improving the corrosion
resistance of the zinc-based plated steel material, there has been
suggested a technique to add magnesium to the plating layer. When
adding magnesium to the plating layer, a magnesium-based corrosion
product is densely formed below the corrosive environment to
decrease a corrosion rate, thereby obtaining an effect of improving
corrosion resistance. However, this magnesium is rapidly oxidized
at high temperature to greatly damage the plating layer, and thus,
the addition of magnesium to the zinc-based plated steel material
for hot press forming is currently limited.
DISCLOSURE
Technical Problem
[0007] An aspect of the present disclosure is to provide a hot
press formed product having excellent corrosion resistance and a
method for preparing the same.
Technical Solution
[0008] According to an aspect of the present disclosure, a hot
press formed product is prepared by hot-press forming a
Zn--Al--Mg-based plated steel material including base iron and a
Zn--Al--Mg-based plating layer, wherein the hot press formed
product includes an oxide layer formed on a surface thereof, and
the content ratio of Al to Mg (Al/Mg) in the oxide layer is 0.8 or
more.
[0009] According to another aspect of the present disclosure, a
method for preparing a hot press formed product includes immersing
base iron in a Zn--Al--Mg-based plating bath and performing plating
to obtain a Zn--Al--Mg-based plated steel material; adjusting a
plated coating weight of the Zn--Al--Mg-based plated steel material
and then performing cooling; heating the cooled Zn--Al--Mg-based
plated steel material to a heating temperature of 600-950.degree.
C. in a heating furnace; and forming the Zn--Al--Mg-based plated
steel material which has reached the heating temperature with a
mold simultaneously with quenching, wherein a residence time is 120
seconds or less, the residence time representing a time during
which the Zn--Al--Mg-based plated steel material which has reached
the heating temperature resides in the heating furnace.
Advantageous Effects
[0010] As set forth above, according to an exemplary embodiment in
the present disclosure, the hot press formed product prepared
according to the present disclosure has very good corrosion
resistance.
DESCRIPTION OF DRAWINGS
[0011] FIG. 1 is a scanning electron microscope (SEM) image
observing a section of the hot press formed product according to
Inventive Example 5, and FIG. 2 is a SEM image observing a section
of the hot press formed product according to Comparative Example
5.
BEST MODE FOR INVENTION
[0012] Hereinafter, a hot press formed product having excellent
corrosion resistance, an aspect of the present disclosure, will be
described in detail.
[0013] The hot press formed product of the present disclosure is
prepared by hot-press forming a Zn--Al--Mg-based plated steel
material including base iron and a Zn--Al--Mg-based plating layer.
Here, the base iron may be a steel plate or a steel wire rod.
[0014] The composition of the base iron is not particularly limited
in the present disclosure, however, as an example, it may contain:
0.15-0.35% by weight of C, 0.5% by weight or less (exclusive of 0%)
of Si, 0.5-8.0% by weight of Mn, and 0.0020-0.0050% by weight of B,
with a balance of Fe and unavoidable impurities.
[0015] C: 0.15-0.35% by Weight
[0016] Carbon, an element for stabilizing austenite, is added for
securing quenching properties, and securing strength of a formed
product after hot press forming. When the content of carbon is
unduly low, the product may lack quenching properties, resulting in
a difficulty in securing the target strength. Accordingly, in the
present disclosure, preferably 0.15% by weight or more, more
preferably 0.18% by weight or more of C is contained. However, when
the content of carbon is unduly high, toughness and weldability
degradation may be caused, and due to an excessive increase in
strength, there may be demerits in the manufacturing process, such
as threading hinderance in annealing and plating processes.
Accordingly, in the present disclosure, preferably 0.35% by weight
or less, more preferably 0.32% by weight or less of C is
contained.
[0017] Si: 0.5% by Weight or Less (Exclusive of 0% by Weight)
[0018] Silicon is a component added for deoxidation, however, when
the content is unduly high, a large amount of SiO.sub.2 is produced
on the surface of steel at the time of annealing, thereby causing
unplating. Accordingly, in the present disclosure, preferably 0.5%
by weight or less, more preferably 0.4% by weight or less of Si is
contained.
[0019] Mn: 0.5-8.0% by Weight
[0020] Manganese not only greatly contributes to a strength
increase as a solid solution strengthening element, but also plays
an important role in delaying transformation from austenite to
ferrite. When the content of manganese is unduly low, a
transformation temperature (Ae3) from austenite to ferrite is
raised, so that an excessively high heat treatment temperature is
required for hot press processing in the austenite single phase
region. Accordingly, in the present disclosure, preferably 0.5% by
weight or more, more preferably 1.0% by weight or more of Mn is
contained. However, when the content of manganese is unduly high,
weldability, hot rolling properties and the like may be
deteriorated. Accordingly, in the present disclosure, preferably
8.0% by weight or less, more preferably 7.8% by weight or less of
Mn is contained.
[0021] B: 0.0020-0.0050% by Weight
[0022] Boron serves to delay transformation from austenite to
ferrite. In order to obtain this effect in the present disclosure,
preferably 0.0020% by weight or more, more preferably 0.0022% by
weight or more of B is contained. However, when the content is
excessive, the effect is not only saturated, but also deteriorates
hot workability. Accordingly, in the present disclosure, preferably
0.0050% by weight or less, more preferably 0.0045% by weight or
less of B is contained.
[0023] In addition to the above composition, the remaining is Fe.
However, since in the common manufacturing process, unintended
impurities may be inevitably incorporated from raw materials or the
surrounding environment, they may not be excluded. Since these
impurities are known to any person with ordinary knowledge in the
art, the entire contents thereof are not particularly mentioned in
the present specification.
[0024] However, as a representative example of these impurities,
Al, P and S may be mentioned, and when the content of Al in the
base iron is increased, steelmaking cracks may be caused, and thus,
it is preferable to adjust the content of Al to 0.2% by weight or
less, and when the contents of P and S are increased, ductility may
be deteriorated, and thus, it is preferable to adjust the contents
of P and S to 0.03% by weight or less, and 0.001% by weight or
less, respectively.
[0025] The Zn--Al--Mg-based plating layer is formed on the surface
of base iron to serve to prevent the corrosion of the iron base
under the corrosive environment, and may contain: 0.9-3.5% by
weight of Mg, and 1.0-15% by weight of Al, with a balance of Zn and
other unavoidable impurities.
[0026] Mg is an essential element, added for improving the
corrosion resistance of a hot press formed product, and forms a
dense corrosive product on the surface of plating layer, thereby
effectively preventing the corrosion of the hot press formed
product. Meanwhile, Mg in the Zn--Al--Mg-based plating layer is
partially oxidized and lost in the course of hot pressing, and the
Zn--Al--Mg-based plating layer is alloyed with Fe to decrease the
content of Mg in the entire plating layer, and thus, in order to
secure the corrosion resistance equivalent to a common plated steel
material, a larger amount of Mg may be contained. In order to
secure the corrosion resistance effect required in the present
disclosure, 0.9% by weight or more, more preferably 0.95% by weight
or more of Mg should be contained. However, when the content is
excessive, oxidation of Mg on the surface of the plating bath
becomes significant so that plating workability is deteriorated,
and also excessive MgO is formed in the course of hot pressing to
promote the oxidation and volatilization of Zn, thereby
deteriorating the corrosion resistance of the hot press formed
product. In terms of preventing this, 3.5% by weight or less, more
preferably 3.3% by weight or less of Mg should be contained.
[0027] Al forms a stable Al.sub.2O.sub.3 layer on the surface in
the course of hot pressing to suppress the oxidation and
volatilization of Zn, thereby contributing the improvement of
corrosion resistance of the hot press formed product. In order to
obtain this effect in the present disclosure, 1.0% by weight or
more, more preferably 1.1% by weight or more of Al should be
contained. However, when the content is excessive, the thermal
resistance of the surface may become better, but the melting
temperature of the plating bath is unduly raised at the time of
hot-dip coating, causing a difficulty in operation. In terms of
preventing this, 15% by weight or less of Al should be
contained.
[0028] The hot press formed product of the present disclosure
includes an oxide layer formed on the surface, and it is
characterized in that the content ratio of Al to Mg (Al/Mg) in the
oxide layer is 0.8 or more. The content ratio is preferably in a
range of 0.85 or more, more preferably 0.9 or more.
[0029] As a result of research of the present inventors, the
Mg-based oxide coat is not physically stable, and thus, it is
easily broken to promote the oxidation and volatilization of Zn in
the plating layer. However, the Al-based oxide coat is physically
very stable, and thus, when an Al-based oxide coat is stably
produced on the surface, not only the oxidation and volatilization
of Zn in the plating layer is prevented, but also the amount of
oxide itself is significantly decreased, thereby greatly improving
the corrosion resistance of the hot press formed product. In order
to obtain this effect in the present disclosure, the content ratio
of Al to Mg (Al/Mg) in the oxide layer is needed to be controlled
to 0.8 or more.
[0030] In the present disclosure, any specific device or method for
measuring the contents of Mg and Al in the oxide layer, and the
like is not particularly limited; however, for example, it may be
measured using GDOES (glow discharge optical emission
spectrometry). Herein, it is preferable to analyze the element to
be analyzed after calibrating the analysis equipment using a
standard specimen.
[0031] According to an exemplary embodiment, the total coating
weight of Zn, Al and Mg may be 700 mg/m.sup.2 or less (exclusive of
0 mg/m.sup.2), more preferably 500 mg/m.sup.2 or less (exclusive of
0 mg/m.sup.2), still more preferably 100 mg/m.sup.2 or less
(exclusive of 0 mg/m.sup.2).
[0032] The surface oxide increases surface resistance at the time
of spot welding to cause welding spatter, thereby rendering welding
to be difficult or impossible, and when the total coating weight of
the oxide is 700 mg/m.sup.2 or less as described above, excellent
weldability may be secured. According to an exemplary embodiment,
when performing spot welding according to the relevant procedure
such as KS B ISO 15609, in the case that the total coating weight
of the oxide as the above is suppressed to 700 mg/m.sup.2 or less,
a weldable current range of 0.5 KA or more is obtained, however, in
the case that the total coating weight of the oxide is above the
range, the weldable current range of 0.5 KA or less is obtained, or
the weldable current range is not obtainable.
[0033] According to an exemplary embodiment, the oxide layer may
contain one or two or more selected from the group consisting of
Mn, Si and Fe, and the sum of these contents may be 50% or less,
more preferably 30% or less, still more preferably 10% or less
relative to the total content of metal in the oxide layer. There
are concerns that the above elements form physical or chemical
defects in the oxide layer to hinder an improvement effect of
thermal resistance at high temperature. Accordingly, it is
preferable to suppress the content as much as possible.
[0034] According to an exemplary embodiment, a ratio
(Mg.sub.o/Mg.sub.c) of the total amount of Mg (Mg.sub.o) contained
in the oxide layer of the hot press formed product to the total
amount of Mg (Mg.sub.c) contained in the plating layer of the hot
press formed product may be 1 or less, more preferably 0.5 or less,
still more preferably 0.3 or less.
[0035] Mg contained in the plating layer greatly contributes to the
improvement of the corrosion resistance of the hot press formed
product, and thus, for securing excellent corrosion resistance, it
is preferable that the oxidation of Mg is suppressed in the course
of hot pressing, so that Mg is maintained in the form of being
solid solubilized in the plating layer as much as possible. When
the total amount ratio (Mg.sub.o/Mg.sub.c) is controlled to 1 or
less, the corrosion resistance of the hot press formed product may
be further significantly increased.
[0036] According to an exemplary embodiment, an alloying degree of
Fe in the plating layer of the hot press formed product may be
20-70%, more preferably 25-65%, still more preferably 30-60%. When
the alloying degree of Fe satisfies the above range, the occurrence
of the oxide coat during a heating process may be effectively
suppressed, and the corrosion resistance property by a sacrifice
way becomes excellent. When the alloying degree of Fe is less than
20%, some regions of the plating layer in which Zn is concentrated
are present as a liquid phase, causing liquid embrittlement cracks
upon processing. Meanwhile, the alloying degree of Fe is more than
70%, the corrosion resistance may be decreased.
[0037] The hot press formed product as described above may be
prepared in various ways, and the preparation method thereof is not
particularly limited. However, as an exemplary embodiment, it may
be prepared by the following method.
[0038] Hereinafter, a method for preparing a hot press formed
product having excellent corrosion resistance, another aspect of
the present disclosure, will be described in detail.
[0039] First, base iron is immersed in a Zn--Al--Mg-based plating
bath, and plating is performed to obtain a Zn--Al--Mg-based plated
steel material. The specific method for obtaining a plated steel
material is not particularly limited in the present disclosure,
however, in order to further significantly increase the effect of
the present disclosure, the following method may be used:
[0040] (a) Type of Base Iron and Control of Surface Roughness
[0041] According to the research results of the present inventors,
the surface roughness of base iron before plating has an influence
on the activity of Al in the plating layer, and in particular,
lower surface roughness of base iron increase the activity of Al,
and thus, is advantageous for stably forming Al.sub.2O.sub.3 on the
surface of the hot press formed product. In order to obtain this
effect in the present disclosure, it is preferable to use a cold
rolled steel plate having a surface roughness (Ra) controlled to
2.0 .mu.m or less as the base iron. Meanwhile, since lower surface
roughness is advantageous for increasing the activity of Al, the
lower limit of the surface roughness is not particularly limited in
the present disclosure, however, when the surface roughness of the
base iron is unduly low, sliding of a steel material during rolling
may interfere with the operation, and thus, for preventing this,
the lower limit may be limited to 0.3 .mu.m.
[0042] (b) Control of Plating Bath Composition
[0043] According to the research results of the present inventors,
when Al and Mg are added to the plating bath in combination, the
content ratio of Al and Mg also has an influence on the activity of
Al, and in particular, a higher Al/Mg ratio increases the activity
of Al, and thus, is advantageous for stably forming Al.sub.2O.sub.3
on the surface of the hot press formed product. In order to obtain
this effect in the present disclosure, it is preferable to control
the Al/Mg ratio in the plating bath to 0.8 or more. Meanwhile,
since the higher Al/Mg ratio is advantageous for increasing the
activity of Al, the lower limit thereof is not particularly limited
in the present disclosure.
[0044] (c) Formation of Pre-Plating Layer and Control of Annealing
Conditions
[0045] According to the research results of the present inventors,
when base iron contains a large amount of pro-oxidizing elements
such as Mn, diffusion of the pro-oxidizing elements into the
plating layer significantly occurs, and the diffused pro-oxidizing
element into the plating layer as such lowers the activity of Al,
thereby interfering with stable formation of an Al.sub.2O.sub.3
coat.
[0046] In order to prevent this, according to an exemplary
embodiment, after pre-plating one or more metals selected from the
group consisting of Fe, Ni, Cu, Sn and Sb on the surface, plating
may be performed on base iron subjected to annealing. Meanwhile,
the method of pre-plating is not particularly limited in the
present disclosure, and for example, it may be formed by an
electroplating method.
[0047] Herein, it is preferable that the thickness of a pre-plating
layer is 5-100 nm. When the thickness is less than 5 nm, it is
difficult to effectively suppress the diffusion of the
pro-oxidizing element into the plating layer, however, when the
thickness is more than 100 nm, it may be effective in surface oxide
suppression, but securing economical efficiency is difficult.
[0048] Meanwhile, an annealing treatment is carried out for
recovery of recrystallization of a base iron structure, and may be
carried out at a temperature of 750-850.degree. C. at which the
recrystallization of the base iron structure is sufficiently
recovered.
[0049] According to an exemplary embodiment, the annealing
treatment may be carried out under an atmosphere of 1-15% by volume
of hydrogen gas and remaining nitrogen gas. When the hydrogen gas
is less than 1% by volume, it may be difficult to effectively
perform the suppression of the surface oxide, however, when the
hydrogen gas is more than 20% by volume, the cost is increased due
to the increased hydrogen content, and a danger of explosion is
also excessively increased.
[0050] Next, the Zn--Al--Mg-based plated steel material is heated
to a predetermined heating temperature in a heating furnace.
[0051] Herein, it is preferable that a residence time representing
a time during which the Zn--Al--Mg-based plated steel material
which has reached the heating temperature resides in the heating
furnace is controlled to 120 seconds or less.
[0052] According to the research results of the present inventors,
the higher the temperature of the material is, the more active the
production of MgO is, and in particular, since Mg is more easily
oxidized than other elements, as the material resides at high
temperature for a longer time, the oxides by other elements are
reduced to increase the ratio of Mg in the oxide layer. In this
case, due to the formation of the physically unstable oxide layer,
volatilization and oxidization of Zn is promoted, resulting in
deterioration of the corrosion resistance of the hot press formed
product. Thus, the residence time is controlled to 120 seconds or
less in the present disclosure.
[0053] Meanwhile, according to further research results of the
present inventors, a heating temperature and a heating rate have an
influence on the formation of the desired oxide layer.
[0054] As a result of research of the present inventors, at the
time of heating for hot press forming, an Al.sub.2O.sub.3 coat is
stably produced initially, and as the heating proceeds, and the
temperature of the material is raised, MgO is produced and already
produced Al.sub.2O.sub.3 is reduced. Thus, in order to prevent the
production of MgO and the reduction of Al.sub.2O.sub.3, the heating
rate is needed to be controlled to be high at 10.degree. C./sec or
more.
[0055] Meanwhile, when general hot press forming, the heating
temperature of the material is 600-950.degree. C., and when the
heating temperature is 800.degree. C. or more and 950.degree. C. or
less, it is preferable that the heating rate is controlled to be
higher at 20.degree. C./sec or more, and at the same time the
residence time is controlled to be shorter at 60 seconds or less.
The reason why the heating rate is controlled to be higher, and the
residence time is controlled to be shorter as such is that the
production of MgO is excessive in the high temperature region as
described above. Herein, the residence time is controlled to more
preferably 40 seconds or less, still more preferably 20 seconds or
less, most preferably 15 seconds or less.
[0056] The heating rate is significantly high as compared with the
case of using a common thermostatic furnace such as an electric
furnace, and according to an exemplary embodiment, the heating may
be carried out by any one method of radiant heating, high-frequency
induction heating and ohmic heating.
[0057] The heating is possible even in the atmosphere, but in order
to suppress surface oxidation by impurities and promote production
of Al.sub.2O.sub.3, heating may be performed under the inert gas
(e.g., nitrogen, argon, etc.) atmosphere.
[0058] Next, the Zn--Al--Mg-based plated steel material which has
reached the heating temperature is formed with a mold,
simultaneously being quenched, thereby obtaining a hot press formed
product.
MODE FOR INVENTION
[0059] Hereinafter, the present disclosure will be specifically
described through the following Examples. However, it should be
noted that the following Examples are only for embodying the
present disclosure by illustration, and not intended to limit the
right scope of the present disclosure. The reason is that the right
scope of the present disclosure is determined by the matters
described in the claims and reasonably inferred therefrom.
[0060] After preparing a steel material having the composition (%
by weight) of the following Table 1, the steel material was
processed into a cold rolled steel plate having a thickness of 1.5
mm. Thereafter, the steel material was subjected to annealing heat
treatment at a temperature up to 780.degree. C. for 40 seconds
under the nitrogen gas atmosphere containing 5% by volume of
hydrogen, and immersed in a zinc-based plating bath to obtain a
plated steel material. Herein, the temperature of the zinc plating
bath was adjusted to constant 450.degree. C.
[0061] Thereafter, each plated steel material was heated under the
conditions of Table 3, and then formed with a mold simultaneously
with being quenched to prepare a formed product.
[0062] Thereafter, for each formed product, the tensile strength
was measured, corrosion resistance and weldability were evaluated,
and the results are shown in the following Table
[0063] 3. For the corrosion resistance, a salt spray test according
to KS R 1127 was used, and after corroding the formed product for
1200 hours and removing the surface corrosion product therefrom,
the maximum corrosion depth of a base member was measured. In
addition, weldability was evaluated according to KS B ISO 15609, by
performing spot welding, and then measuring a weldable current
range.
TABLE-US-00001 TABLE 1 Base iron components (% by weight) Steel
type C Si Mn P S Al B Steel 1 0.18 0.25 1.3 0.01 0.001 0.02 0.0025
Steel 2 0.2 0.3 7.5 0.02 0.003 0.1 0.0040 Steel 3 0.31 0.3 2.2 0.01
0.003 0.05 0.0025
TABLE-US-00002 TABLE 2 Plating bath Plating bath components (% by
weight) type Mg Al Plating bath 1 0.97 1.1 Plating bath 2 1.41 1.43
Plating bath 3 1.45 15 Plating bath 4 3.12 2.54 Plating bath 5 0
0.2
TABLE-US-00003 TABLE 3 Pre-plated Plating Press Plating Surface
coating layer Heating Heating Residence starting Classifi- Steel
bath roughness Pre- weight thickness rate temperature time
temperature cation type type (Ra) plating (mg/m.sup.2) (.mu.m)
(.degree. C./s) (.degree. C.) (sec) (.degree. C.) Inventive Steel 1
Plating 0.3 Fe 150 6 15 880 10 750 Example 1 bath 1 Inventive Steel
1 Plating 0.9 -- -- 8 20 900 10 750 Example 2 bath 2 Inventive
Steel 1 Plating 0.9 -- -- 8 120 950 10 500 Example 3 bath 3
Inventive Steel 1 Plating 0.9 -- -- 8 15 870 10 750 Example 4 bath
4 Inventive Steel 2 Plating 2.0 -- -- 4 4 610 120 500 Example 5
bath 4 Inventive Steel 3 Plating 1.5 Fe--Ni 300 5 4 780 10 500
Example 6 bath 4 Inventive Steel 2 Plating 1.2 -- -- 8 4 700 10 500
Example 7 bath 3 Inventive Steel 2 Plating 1.2 -- -- 8 30 770 10
500 Example 8 bath 4 Inventive Steel 3 Plating 1.5 -- -- 8 30 770
10 500 Example 9 bath 3 Inventive Steel 3 Plating 1.5 -- -- 8 4 770
20 550 Example 10 bath 4 Inventive Steel 3 Plating 1.5 Ni 250 8 4
770 20 550 Example 11 bath 4 Comparative Steel 1 Plating 0.9 -- --
8 4 900 180 750 Example 1 bath 1 Comparative Steel 1 Plating 0.9 --
-- 8 4 900 300 750 Example 2 bath 2 Comparative Steel 1 Plating 0.9
-- -- 8 4 900 300 750 Example 3 bath 3 Comparative Steel 1 Plating
0.9 -- -- 8 4 930 300 750 Example 4 bath 5 Comparative Steel 2
Plating 1.2 -- -- 8 4 800 300 500 Example 5 bath 4 Comparative
Steel 3 Plating 1.5 -- -- 8 4 770 300 500 Example 6 bath 5
TABLE-US-00004 TABLE 4 Al/Mg Total coating Weldable Maximum content
weight of Tensile current corrosion Classifi- ratio in Zn, Mg and
Al strength range depth cation oxide layer Mg.sub.o/Mg.sub.c
(mg/m.sup.2) (Mpa) (kA) (mm)* Inventive 1.0 0.8 450 1480 1.0 0.5
Example 1 Inventive 0.9 0.7 540 1510 1.0 0.4 Example 2 Inventive
1.5 0.8 290 1530 1.1 0.4 Example 3 Inventive 1.2 0.5 250 1490 1.2
0.5 Example 4 Inventive 1.3 0.3 90 1310 1.4 0.5 Example 5 Inventive
1.0 0.9 600 1510 0.6 0.3 Example 6 Inventive 1.3 0.2 70 1490 1.5
0.4 Example 7 Inventive 1.1 0.4 60 1510 1.8 0.5 Example 8 Inventive
1.0 0.3 90 1480 1.0 0.3 Example 9 Inventive 0.7 0.6 250 1530 0.9
0.4 Example 10 Inventive 0.9 0.4 100 1530 1.2 0.3 Example 11
Comparative 0.3 220 1700 1550 0 0.7 Example 1 Comparative 0.4 345
2300 1520 0 -- Example 2 Comparative 0.4 1.5 900 1490 0.2 --
Example 3 Comparative 0.5 300 2500 1480 0 -- Example 4 Comparative
0.7 1.1 800 1520 0.2 0.8 Example 5 Comparative -- -- 1700 1510 0
0.7 Example 6
[0064] Referring to Table 4, it is confirmed that Inventive
Examples 1 to 11 satisfying all of the conditions proposed in the
present disclosure all represented the Al/Mg content ratio in the
oxide layer of 0.8 or more, and accordingly, the maximum corrosion
depth of a base member after a salt spray test for 1200 hours was
0.5 mm or less, and thus, corrosion resistance was excellent. In
addition, it is confirmed that the weldable current range was 0.5
kA or more, and thus, weldability was excellent.
[0065] [69] In Table 4, no description for Mg.sub.o/Mg.sub.c means
that there was no Mg in the plating bath like plating bath 5, or Mg
in the base iron was all consumed and did not remain. In addition,
no description for maximum corrosion depth means that penetration
corrosion occurred through a specimen thickness so that the
corrosion depth was not able to be measured.
[0066] [70] Meanwhile, FIG. 1 is a scanning electron microscope
(SEM) image observing a section of the hot press formed product
according to Inventive Example 5. FIG. 2 is a SEM image observing a
section of the hot press formed product according to Comparative
Example 5.
* * * * *