U.S. patent application number 13/824525 was filed with the patent office on 2013-07-11 for hot dip plated steel sheet having excellent plating adhesiveness and method of manufacturing the same.
This patent application is currently assigned to POSCO. The applicant listed for this patent is Sang-Kug Han, Jong-Sang Kim, Young-Ra Lee, Joong-Chul Park, Rho-Bum Park, Pan-Woo Seon, Hong-Jong Yoo. Invention is credited to Sang-Kug Han, Jong-Sang Kim, Young-Ra Lee, Joong-Chul Park, Rho-Bum Park, Pan-Woo Seon, Hong-Jong Yoo.
Application Number | 20130177780 13/824525 |
Document ID | / |
Family ID | 46383616 |
Filed Date | 2013-07-11 |
United States Patent
Application |
20130177780 |
Kind Code |
A1 |
Park; Rho-Bum ; et
al. |
July 11, 2013 |
Hot Dip Plated Steel Sheet Having Excellent Plating Adhesiveness
and Method of Manufacturing the Same
Abstract
Provided is a hot dip plated steel sheet used in automotive
materials and a method of manufacturing the same, and more
particularly, to a hot dip plated steel sheet having excellent
platability and plating adhesiveness in which a steel sheet
containing alloying elements forming oxides on a surface of the
steel sheet at high temperatures is used as an underlying steel
sheet, and a method of manufacturing the same. According to the
present invention, a hot dip plated steel sheet having excellent
platability and plating adhesiveness is provided, in which a steel
sheet containing alloying elements forming oxides on a surface of
the steel sheet at high temperatures is used as an underlying steel
sheet, and thus, limitations in added amounts of silicon (Si),
manganese (Mn), or aluminum (Al) may be mitigated. Therefore,
development of new steels may be accelerated.
Inventors: |
Park; Rho-Bum;
(Gwangyang-si, KR) ; Kim; Jong-Sang;
(Gwangyang-si, KR) ; Park; Joong-Chul;
(Gwangyang-si, KR) ; Lee; Young-Ra; (Gwangyang-si,
KR) ; Yoo; Hong-Jong; (Gwangyang-si, KR) ;
Seon; Pan-Woo; (Gwangyang-si, KR) ; Han;
Sang-Kug; (Gwangyang-si, KR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Park; Rho-Bum
Kim; Jong-Sang
Park; Joong-Chul
Lee; Young-Ra
Yoo; Hong-Jong
Seon; Pan-Woo
Han; Sang-Kug |
Gwangyang-si
Gwangyang-si
Gwangyang-si
Gwangyang-si
Gwangyang-si
Gwangyang-si
Gwangyang-si |
|
KR
KR
KR
KR
KR
KR
KR |
|
|
Assignee: |
POSCO
Pohang-si
KR
|
Family ID: |
46383616 |
Appl. No.: |
13/824525 |
Filed: |
December 5, 2011 |
PCT Filed: |
December 5, 2011 |
PCT NO: |
PCT/KR11/09361 |
371 Date: |
March 18, 2013 |
Current U.S.
Class: |
428/640 ;
427/226; 428/639 |
Current CPC
Class: |
C21D 1/74 20130101; C23C
2/28 20130101; C21D 1/52 20130101; C21D 1/68 20130101; Y10T
428/12667 20150115; Y10T 428/1266 20150115; C23C 2/02 20130101;
C23C 2/04 20130101; C21D 1/32 20130101 |
Class at
Publication: |
428/640 ;
428/639; 427/226 |
International
Class: |
C23C 2/02 20060101
C23C002/02 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 28, 2010 |
KR |
10-2010-0137323 |
Claims
1. A hot dip plated steel sheet having excellent plating
adhesiveness comprising: a steel sheet containing alloying elements
forming oxides on a surface of the steel sheet at high temperatures
as an underlying steel sheet; and a plating material plated on the
underlying steel sheet, wherein a discontinuous reduced iron (Fe)
layer and a Fe-plating material alloy phase are formed at an
interface of the underlying steel sheet and a plating layer.
2. The hot dip plated steel sheet having excellent plating
adhesiveness of claim 1, wherein the underlying steel sheet
comprises one or more alloying elements of silicon (Si), manganese
(Mn), and aluminum (Al).
3. The hot dip plated steel sheet having excellent plating
adhesiveness of claim 1, wherein the underlying steel sheet is any
one of a dual phase (DP) steel, a transformation induced plasticity
(TRIP) steel, a complex phase (CP) steel, a martensitic (MART)
steel; and a twinning induced plasticity (TWIP) steel.
4. The hot dip plated steel sheet having excellent plating
adhesiveness of claim 1, wherein the plating material comprises one
or more of zinc (Zn), aluminum (Al), and magnesium (Mg) as main
components.
5. The hot dip plated steel sheet having excellent plating
adhesiveness of claim 1, wherein a discontinuous Si--Mn oxide layer
is formed between the underlying steel sheet and the plating
layer.
6. The hot dip plated steel sheet having excellent plating
adhesiveness of claim 1, wherein the Fe-plating material alloy
phase is distributed within a thickness of 60% of a total thickness
of the plating layer from the interface of the plating layer and
the underlying steel sheet to a direction of a surface of the
plating layer.
7. The hot dip plated steel sheet having excellent plating
adhesiveness of claim 1, wherein a discontinuous Al--Fe inhibition
layer is distributed between the underlying steel sheet and the
plating layer.
8. The hot dip plated steel sheet having excellent plating
adhesiveness of claim 1, wherein a Si--Mn internal oxide is
distributed up to a depth of 7 .mu.m from the interface of the
plating layer and the underlying steel sheet to an inner direction
of the underlying steel sheet.
9. A method of manufacturing a hot dip plated steel sheet having
excellent plating adhesiveness, the method comprising: oxidizing a
steel sheet containing alloying elements forming oxides on a
surface of the steel sheet at high temperatures by using a direct
fired furnace at an air/fuel ratio or an air/gas ratio range of
about 1.01 to about 1.5 and a steel sheet temperature range at an
exit of the direct fired furnace of about 550.degree. C. to about
750.degree. C.; performing reduction annealing and hot dip plating;
and performing a slight annealing treatment in a temperature range
of 400.degree. C. to about 550.degree. C.
10. The method of claim 9, wherein an underlying steel sheet
comprises one or more of silicon (Si), manganese (Mn), and aluminum
(Al).
11. The method of claim 9, wherein the underlying steel sheet is
any one of a dual phase (DP) steel, a transformation induced
plasticity (TRIP) steel, a complex phase (CP) steel, a martensitic
(MART) steel, and a twinning induced plasticity (TWIP) steel.
12. The method of claim 9, wherein a temperature of the slight
annealing treatment is within a range of 440.degree. C. to about
500.degree. C.
13. The method of claim 9, wherein the plating material comprises
one or more of zinc (Zn), aluminum (Al), and magnesium (Mg) as a
main component.
Description
TECHNICAL FIELD
[0001] The present invention relates to a hot dip plated steel
sheet used in automotive materials and a method of manufacturing
the same, and more particularly, to a hot dip plated steel sheet
having excellent platability and plating adhesiveness, and a method
of manufacturing the same.
BACKGROUND ART
[0002] Vehicle fuel economy regulations have recently been
reinforced as environmental issues have become more important and
accordingly, various, weight reduction methods for automobiles have
been sought as measures to improve fuel economy.
[0003] For this purpose, steel manufacturers have tended to make
great efforts to manufacture high strength steels in order to
reduce the weight of steel sheets used in automotive materials,
while assuring passenger safety.
[0004] In accordance with this trend, demand for high strength hot
dip zinc (Zn) plated steel sheets for automotive bodies has greatly
increased in recent years. However, although a typical method of
manufacturing high strength steels using solid solution
strengthening elements such as phosphorous (P) and manganese (Mn)
is somewhat helpful for strengthening and lightening of plated
steel sheets for automotive bodies, there are limitations in that
the foregoing high strength steels are processed into automotive
components requiring various shapes. Therefore, steels able to be
formed as automotive components having complex shapes due to
excellent processability, as well as having characteristics
demonstrating high strength after forming, are required.
[0005] Such steels may include an advanced high strength steel
(AHSS) or the like, such as a dual phase (DP) steel and a
transformation induced plasticity (TRIP) steel, which have recently
been developed and partially commercialized.
Advanced high strength steel is characterized by containing large
amounts of elements, such as silicon (Si), manganese (Mn), and
aluminum (Al), and for example, since Si acts as an element to
maintain ductility without significantly reducing the strength
thereof, Si is frequently added to obtain the foregoing
property.
[0006] However, a hot dip Zn plated steel sheet manufactured
through a typical method of hot dip Zn plating by using a steel
sheet as an underlying steel sheet, in which Si is added as an
alloy element in an amount of about 0.1 wt % or more, may have
limitations in that bare spots or appearance defects may easily be
generated.
[0007] The foregoing limitations are due to an environment of an
annealing process, one of processes of manufacturing the hot dip Zn
plated steel sheet. In the annealing process, a heat treatment is
performed at a high temperature of about 800.degree. C. while a
reducing environment including 5% or more of hydrogen and nitrogen
as a remainder is maintained (see JP1999-323443 and U.S. Pat. No.
5,137,586), and Si diffuses to a surface of the steel sheet during
the foregoing high-temperature heat treatment process.
[0008] Therefore, a concentration of Si in the steel surface will
be about 10 to 100 times higher than an average concentration of Si
in the overall steel, and the Si concentrated on the surface of the
steel sheet may react with moisture or impurities in a furnace
atmosphere to form a SiO.sub.2 coating layer.
The SiO.sub.2 coating layer formed on the surface of the steel
sheet may significantly decrease wettability of the steel sheet,
and eventually, excellent platability may be difficult to secure.
That is, the SiO.sub.2 coating layer formed on the surface of the
steel sheet may generate a phenomenon of multiple bare spots or may
significantly deteriorate plating adhesiveness even in the case
that plating is possible, and therefore, may act as a cause of a
plating delamination phenomenon, in which a plating layer is
delaminated when the steel sheet is processed into a component.
[0009] Various techniques have been suggested for preventing the
foregoing phenomenon of bare spots or the deterioration of plating
adhesiveness caused by the foregoing oxide coating layer.
[0010] An example thereof may be a technique, in which amounts of
alloy layers of Zn--Fe--Al--Si and Fe--Al--Si systems at an
interface of underlying iron and plating layers are increased by
increasing an added amount of Al in a hot dip Zn plating bath.
[0011] Since such alloy layers may cause a reduction of an oxide
layer of an alloying element, the decrease of wettability due to
the oxide coating layer at the interface may be prevented. However,
a method of increasing Al in the plating bath may be inappropriate.
The reason for this is that the increase in Al may be a cause of
intergranular corrosion, together with lead (Pb) inevitably added
in the plating bath as an impurity during the manufacturing of a
mini spangle steel sheet.
Since intergranular corrosion may generate plating delamination and
the increase of Al in the plating bath may further deteriorate
weldability during processing of the steel sheet, there may be many
difficulties in actually applying the foregoing related art.
[0012] Another related art plating process may include a technique
in which an oxide coating layer is formed by introducing excessive
air into a direct fired furnace in order to improve the platability
of a Si-added steel, and a reduction treatment is then performed in
a heating furnace having a reducing environment, to significantly
improve platability.
[0013] An example of the foregoing technique was disclosed in
Japanese Patent Application Laid-Open Publication No. 2001-226742,
and according to the technique, stable platability may be secured
because a pure iron layer is formed on the surface of a steel
sheet, when a reduction heat treatment is performed after a
thickness of iron oxide is increased by increasing an air ratio in
a direct fired furnace from a typical value of 0.9 to 1.05.
[0014] However, in the related art, plating delamination may be
instead be generated by the thick coating layer when the thickness
of the oxide coating layer is not precisely controlled.
[0015] On the other hand, when the oxide coating layer is
completely reduced by the reduction treatment because the oxide
coating layer is thin, Si is concentrated as it is on the surface
of the steel sheet and thus, a Zn plating layer is not strongly
adhered to the surface of the steel sheet or bare spots may be
generated.
[0016] Therefore, the thickness of the iron oxide in the direct
fired furnace may be very precisely controlled.
[0017] Another related art may include a technique disclosed in
Japanese Patent Application Laid-Open Publication No. 2010-1156590,
the technique being a method of manufacturing a hot dip Zn plated
steel sheet by oxidizing, reduction annealing, and hot dip Zn
plating a cold-rolled steel sheet containing alloying elements such
as Si and Mn, wherein the steel sheet is heated to obtain a steel
sheet temperature of 550.degree. C. or more in an environment, in
which fuel gas including H.sub.2 and CH.sub.4 in a total amount of
50 vol % or more burns at an air-fuel ratio range of 1.01 to 1.5
during oxidation, and the steel sheet is also heated in an
environment, which includes 1 vol % to 20 vol % of hydrogen having
a dew point range of -50.degree. C. to -10.degree. C. and nitrogen
as a remainder, during reduction annealing.
[0018] However, plating adhesiveness may also be insufficiently
secured by means of the foregoing related art.
DISCLOSURE
Technical Problem
[0019] An aspect of the present invention provides a hot dip plated
steel sheet having excellent platability and plating adhesiveness,
in which a steel sheet containing alloying elements forming oxides
on a surface of the steel sheet at high temperatures is used as an
underlying steel sheet, and a method of manufacturing the hot dip
plated steel sheet.
Technical Solution
[0020] According to an aspect of the present invention, there is
provided a hot dip plated steel sheet having excellent plating
adhesiveness including: a steel sheet containing alloying elements
forming oxides on a surface of the steel sheet at high temperatures
as an underlying steel sheet; and a plating material plated on the
underlying steel sheet, wherein a discontinuous reduced iron (Fe)
layer and a Fe-plating material alloy phase are formed at an
interface of the underlying steel sheet and a plating layer.
[0021] Examples of the underlying steel sheet may be an annealed or
a full hard cold-rolled steel sheet including one or more of
silicon (Si), manganese (Mn), and aluminum (Al) in addition to
Fe.
[0022] An example of the underlying steel sheet may be a dual phase
(DP) steel, a transformation induced plasticity (TRIP) steel, a
complex phase (CP) steel, a martensitic (MART) steel, a twinning
induced plasticity (TWIP) steel, etc.
The plating material may be a material including one or more of
zinc (Zn), aluminum (Al), and magnesium (Mg) as main
components.
[0023] Also, examples of the hot dip plated steel may be a hot dip
Zn plated steel sheet, a hot dip Al plated steel sheet, a hot dip
Zn--Al plated steel sheet, and hot dip alloy plated steel sheets of
Al--Zn--Mg--Si, Al--Zn--Mg, Al--Mg, Zn--Mg, and the like.
[0024] According to another aspect of the present invention, there
is provided a method of manufacturing a hot dip plated steel sheet
having excellent plating adhesiveness including: oxidizing a steel
sheet containing alloying elements forming oxides on a surface of
the steel sheet at high temperatures by using a direct fired
furnace at an air/fuel ratio or an air/gas ratio range of about
1.01 to about 1.5 and a steel sheet temperature range at an exit of
the direct fired furnace of about 550.degree. C. to about
750.degree. C.; performing reduction annealing and hot dip plating;
and performing a slight annealing treatment in a temperature range
of 400.degree. C. to about 550.degree. C.
[0025] Examples of the underlying steel plate may be an annealed or
a full hard cold-rolled steel sheet including one or more of Si,
Mn, and Al in addition to Fe.
[0026] An example of the underlying steel sheet may be a DP steel,
a TRIP steel, a CP steel, a MART steel, a TWIP steel, or the
like.
[0027] Examples of the direct fired furnace may be a direct fired
furnace (DFF) or a direct flame impingement (DFI) furnace.
The plating material may be a material including one or more of Zn,
Al, and Mg as a main component.
[0028] Also, examples of the hot dip plated steel sheet may be a
hot dip Zn plated steel sheet, a hot dip Al plated steel sheet, a
hot dip Zn--Al plated steel sheet, and hot dip alloy plated steel
sheets of Al--Zn--Mg--Si, Al--Zn--Mg, Al--Mg, Zn--Mg, and the
like.
Advantageous Effects
[0029] According to the present invention, a hot dip plated steel
sheet having excellent platability and plating adhesiveness, in
which a steel sheet containing alloying elements forming oxides on
a surface of the steel sheet at high temperatures is used as an
underlying steel sheet, is provided, and thus, limitations in added
amounts of Si, Mn, or Al may be mitigated. Therefore, development
of new steels may be accelerated.
DESCRIPTION OF DRAWINGS
[0030] The above and other aspects, features and other advantages
of the present invention will be more clearly understood from the
following detailed description taken in conjunction with the
accompanying drawings, in which:
[0031] FIG. 1 is a schematic view illustrating a cross-section in a
thickness direction of a hot dip zinc plated steel sheet
manufactured according to a conventional method;
[0032] FIG. 2 is a schematic view illustrating a cross-section in a
thickness direction of a hot dip zinc plated steel sheet
manufactured according to an example of the present invention;
[0033] FIG. 3 are micrographs showing cross-sections in a thickness
direction of a hot dip zinc plated steel sheet after a slight
annealing treatment according to an example of the present
invention;
[0034] FIG. 4 is a micrograph showing a cross-section in a
thickness direction of a hot dip zinc plated steel sheet
manufactured according to a conventional method; and
[0035] FIG. 5 is a micrograph showing a cross-section in a
thickness direction of a hot dip zinc plated steel sheet
manufactured according to an example of the present invention.
BEST MODE
[0036] Hereinafter, the present invention is described in more
detail.
[0037] As described above, there are various types of hot dip
plated steel sheets which may be applicable to the present
invention, and hereinafter, a hot dip zinc (Zn) plated steel sheet,
known as the most typical hot dip plated steel sheet, is described
as an example.
[0038] When a steel sheet containing alloying elements, such as
silicon (Si) and manganese (Mn), easily forming oxides on a surface
of the steel sheet at high temperatures, is used as an underlying
steel sheet, a iron (Fe) oxide layer is formed on the surface of
the steel sheet in a direct fired furnace as in the foregoing
related art, a reduced Fe layer is formed by reducing the Fe oxide
layer in a reduction annealing process, and hot dip Zn plating is
then performed, an oxide layer is continuously or mostly formed
between the underlying steel sheet and a zinc plating layer as
shown in FIG. 1.
[0039] The oxide layer formed at this time is not necessarily
formed continuously, and any form may be applicable so long as the
oxide layer may cause bare spots or plating delamination even in
the case that the oxide layer is discontinuous.
[0040] Examples of the oxide layer formed between the underlying
steel sheet and the Zn plating layer may be a oxide layer formed of
mono oxide, in which Si, Mn, or aluminum (Al) are singly combined
with oxygen, or a complex oxide, in which two or more of Si, Mn, or
Al are combined with oxygen by being mixed with each other.
[0041] Hereinafter, the present invention and the related art are
described according to an example of an underlying steel sheet
containing Si and Mn as alloying elements among components easily
forming oxides on the surface of the steel sheet.
[0042] The oxide layer formed between the underlying steel sheet
and the Zn plating layer denotes an oxide layer formed singly of
mono oxides such as SiO.sub.2 and MnO or complex oxides such as
Mn.sub.2SiO.sub.4 and MnSiO.sub.3, or formed by mixing two or more
thereof.
[0043] Herein, an oxide layer, formed by continuous or
discontinuous concentration of the foregoing oxides on the surface,
is described by through denotation as an Si--Mn oxide layer for
convenience, as shown in FIGS. 1, 2, 4, and 5.
[0044] In the related art, although zinc platability is improved
because diffusion of a Si--Mn oxide layer to a surface layer
portion of a steel sheet is prevented due to a reduced Fe layer,
delamination at an interface between the Si--Mn oxide layer and the
reduced Fe layer may be facilitated as shown in FIG. 1 because the
Si--Mn oxide layer and the reduced Fe layer lack adhesiveness.
[0045] The objective of the present invention is to improve plating
adhesiveness as well as platability by using a hot dip Zn plated
steel sheet, in which a steel sheet containing alloying elements
forming oxides on a surface of the steel sheet at high temperatures
is used as an underlying steel sheet and a plating material
(hereinafter, Zn is described as an example) is plated on the
underlying steel sheet, and forming discontinuous reduced Fe layer
and Fe--Zn alloy phase, as shown in FIG. 2, at or near an interface
of the underlying steel sheet and the hot dip Zn plating layer.
[0046] The Fe--Zn alloy phase, as shown in FIG. 2, is distributed
from the interface of the plating layer/underlying steel sheet in a
direction of a surface of the plating layer, and may be distributed
so as not to be optically observed at the surface of the plated
steel sheet.
[0047] For example, the Fe--Zn alloy phase, as shown in FIG. 2, is
distributed from the interface of the plating layer/underlying
steel sheet Pinto the zinc plating layer by penetrating a
discontinuous oxide layer.
[0048] The Fe--Zn alloy phase may be distributed within a thickness
of 60% of a total thickness of the plating layer from the interface
of the plating layer/underlying steel sheet to a direction of the
surface of the plating layer.
[0049] When the thickness of the Fe--Zn alloy phase is greater than
60% of the total thickness of the plating layer, corrosion
resistance of the steel sheet may decrease because the alloy phase
may be partially formed on the surface of the plating layer.
[0050] The hot dip Zn plated steel sheet of the present invention
may include a discontinuous Si--Mn oxide layer and an Al--Fe
inhibition layer in addition to the discontinuous reduced Fe layer
and the Fe--Zn alloy phase between the underlying steel sheet and
the Zn plating layer.
[0051] The discontinuous Si--Mn oxide layer and Al--Fe inhibition
layer were layers observed in a relatively continuous form in the
related art as in FIG. 1, but the Si--Mn oxide layer and Al--Fe
inhibition layer are observed in a discontinuous form because the
foregoing layers are partially destructed or diffused into the zinc
plating layer while the Fe--Zn alloy phase is formed as in FIG.
2.
[0052] The hot dip Zn plated steel sheet of the present invention
may include Si--Mn internal oxide, for example, up to a depth of 7
pm from the interface of the plating layer/underlying steel sheet
in an inner direction of the underlying steel sheet.
[0053] Also, examples of the underlying steel sheet may be an
annealed or full hard cold-rolled steel sheet including one or more
of Si, Mn, and Al in addition to Fe.
[0054] An example of the underlying steel sheet may be a dual phase
(DP) steel, a transformation induced plasticity (TRIP) steel, a
complex phase (CP) steel, a martensitic (MART) steel, a twinning
induced plasticity (TWIP) steel, etc.
[0055] The plating material may be a material including one or more
of Zn, Al, and magnesium (Mg) as main components.
[0056] Also, examples of the hot dip plated steel may be a hot dip
Zn plated steel sheet, a hot dip Al plated steel sheet, a hot dip
Zn--Al plated steel sheet, and hot dip alloy plated steel sheets of
Al--Zn--Mg--Si, Al--Zn--Mg, Al--Mg, Zn--Mg, and the like.
[0057] As described above, excellent plating adhesiveness may be
secured in the hot dip Zn plated steel sheet of the present
invention, because wettability is improved due to the prevention of
a diffusion amount of the Si--Mn oxide to the surface portion of
the steel sheet by the reduced Fe layer, and also, after plating as
shown in FIG. 3, the Fe--Zn alloy phase acts as a bridge between
the steel sheet and the plating layer by destroying the continuous
oxide layer and being formed inside the zinc plating layer from the
interface of the plating layer/underlying steel sheet.
[0058] Hereinafter, a method of manufacturing a hot dip Zn plated
steel sheet having excellent plating adhesiveness is described
according to the present invention.
[0059] The method of manufacturing the hot dip Zn plated steel
sheet of the present invention includes a process of oxidizing an
underlying steel sheet by using a steel sheet containing alloying
elements forming oxides on a surface of the steel sheet at high
temperatures as the underlying steel sheet, a process of reduction
annealing, a process of hot dip Zn plating, and a process of
performing a slight annealing treatment.
[0060] Examples of the underlying steel plate may be an annealed or
full hard cold-rolled steel sheet including one or more of Si, Mn,
and Al in addition to Fe.
[0061] An example of the underlying steel sheet may be a dual phase
(DP) steel, a transformation induced plasticity (TRIP) steel, a
complex phase (CP) steel, a martensitic (MART) steel, a twinning
induced plasticity (TWIP) steel, etc.
[0062] The plating material may be a material including one or more
of Zn, Al, and Mg as main components.
[0063] Also, examples of the hot dip plated steel may be a hot dip
Zn plated steel sheet, a hot dip Al plated steel sheet, a hot dip
Zn--Al plated steel sheet, and hot dip alloy plated steel sheets of
Al--Zn--Mg--Si, Al--Zn--Mg, Al--Mg, Zn--Mg, and the like.
[0064] The oxidation process is performed under conditions of an
air-fuel ratio of 1.01 to 1.5 in a direct fired furnace and a
temperature of the steel sheet at an exit of the direct fired
furnace is within a range of 550.degree. C. to 750.degree. C.
[0065] Examples of the direct fired furnace may be a direct fired
furnace (DFF) or a direct flame impingement (DFI) furnace.
[0066] When the air-fuel ratio in the direct fired furnace is less
than 1.01, oxidation of Fe may be insufficient, and when the
air-fuel ratio is greater than 1.5, a backfire phenomenon of a
heating apparatus of the direct fired furnace may occur. Therefore,
the air-fuel ratio in the direct fired furnace may be limited to
within a range of 1.01 to 1.5.
[0067] When the temperature of the steel sheet at the exit of the
direct fired furnace is less than 550.degree. C., prevention of the
diffusion of oxide to a surface thereof may be difficult because
the generation of internal oxidation of the steel sheet is
insufficient, and when the temperature of the steel sheet at the
exit of the direct fired furnace is greater than 750.degree. C., an
amount of a concentrated product may be too large because a
diffusion rate of the oxide to the surface thereof is excessively
increased while an amount of production may decrease because a line
speed must be reduced in order to attain a target temperature.
Therefore, the temperature of the steel sheet at the exit of the
direct fired furnace may be limited to within a range of
550.degree. C. to 750.degree. C.
[0068] For example, when an inner portion of the direct fired
furnace is divided into four regions, an air-fuel ratio of a third
region from an entrance of an incoming steel sheet among the four
regions may be within a range of 1.1 to 1.4, and an air-fuel ratio
of a fourth region may be within a range of 1.1 to 1.3.
[0069] At this time, a DFF or DFI apparatus, the direct fired
furnace described in the present invention, is mainly positioned at
a first half part of a heating section. However, any position may
be used so long as a targeted heat treatment temperature may be
secured and a Fe oxide layer may be formed.
[0070] When the steel sheet is oxidized in the direct fired furnace
as described above, a Fe oxide layer is formed on the surface of
the steel sheet and the Fe oxide layer is reduced in a reduction
annealing process to be formed as a reduced Fe layer.
[0071] Diffusion of oxides of alloying elements contained in the
underlying steel sheet, for example, Si--Mn oxides, is prevented by
forming a reduced Fe layer and therefore, excellent platability may
be secured.
[0072] The reduction annealing process in the present invention is
not particularly limited and the reduction annealing process is
irrelevant to apparatus types and operating conditions so long as
mechanical properties of the steel sheet may be secured and the
reduced Fe layer may be formed by continuously performing heat
treatments.
[0073] For example, a general annealing heat treatment apparatus is
composed of a preheating section, a heating section, a soaking
section, a fast cooling section, a slow cooling section, an
overaging section, or a reheating section, and the arrangement and
numbers of the foregoing heat treatment sections may be changed if
necessary.
[0074] At this time, when oxidation and a reduction annealing heat
treatment are performed by using a steel sheet containing alloying
elements forming oxides on the surface of the steel sheet at high
temperatures as an underlying steel sheet, diffusion rates of
Si--Mn oxides to the surface portion of the steel sheet increase as
temperature increases.
[0075] For example, with respect to a hot dip Zn plating process, a
section having the highest heat treatment temperature is a soaking
section and a soaking heat treatment is performed while the
temperature thereof is typically maintained at a range of
780.degree. C. to 850.degree. C. for 50 seconds to 100 seconds.
[0076] The hot dip Zn plating process is not particularly limited
in the present invention, and the plating is typically performed by
dipping for about 3 seconds to 5 seconds in a plating bath having a
composition of 0.12% to 0.25% of Al and Zn as a remainder and a
temperature of 460.degree. C. to 470.degree. C.
[0077] At this time, there is a general operating condition with
respect to a hot dip plated steel sheet having a different plating
material. For example, the operating condition may be changed
according to melting points of alloying elements, such as a plating
bath having 5% to 12% of Si and Al as a remainder at a temperature
of 660.degree. C. to 680.degree. C. with respect to a hot dip Al
plated steel sheet and a plating bath having 50% to 60% of Al and
40% to 50% of Zn at a temperature of 590.degree. C. to 610.degree.
C. with respect to a hot dip Al--Zn plated steel sheet, and
capacity of an apparatus.
[0078] Also, in the present invention, the underlying steel sheet
is hot dip Zn plated as described above, and a slight annealing
treatment is then performed in order to form the reduced Fe layer
and Fe--Zn alloy phase at the interface between the underlying
steel plate and hot dip Zn plating layer. At this time, a
temperature of the slight annealing treatment may be limited within
a range of 400.degree. C. to 550.degree. C.
[0079] When the slight annealing treatment temperature is less than
400.degree. C., plating delamination may be generated because a
bridging effect_between the underlying steel sheet and the plating
layer is insufficient due to the less formation of the Fe--Zn alloy
phase at the interface of the underlying steel sheet/plating layer.
When the slight annealing treatment temperature is greater than
550.degree. C., a decrease in corrosion resistance and a
non-uniform appearance of the steel sheet may occur because the
thickness of the Fe--Zn alloy phase is greater than 60% of the
total thickness of the plating layer and the Fe--Zn alloy phase is
partially formed at the surface of the plating layer.
[0080] The slight annealing treatment temperature, for example, may
be within a range of 440.degree. C. to 500.degree. C.
[0081] Any apparatus may be used for the slight annealing treatment
so long as the apparatus may perform a slight annealing reaction by
being positioned at a certain position past an air knife and
heating the steel sheet to a temperature range of 400.degree. C. to
550.degree. C. within a few seconds after hot dip plating. However,
it may be effective to perform the slight annealing treatment by
using a GA heater installed in a typical plating line or an
apparatus having a function similar thereto.
[0082] Also, a discontinuous Si--Mn oxide layer and an Al--Fe
inhibition layer may exist at the interface between the plating
layer and the underlying steel sheet.
[0083] Hereinafter, the present invention is described in more
detail according to an example.
Example
[0084] In the present invention, 1.0 mm thick full hard cold-rolled
steel sheets having compositions of the following Table 1 were used
as underlying steel sheets and oxidized under the conditions of the
following Table 2 in a direct fired furnace, and reduction
annealing and hot dip plating were then performed under typical
operating conditions.
[0085] At this time, with respect to sample numbers 1 to 5 in the
following Table 1, the reduction annealing was performed in a mixed
gas environment of 10% H.sub.2-90% N.sub.2 having a dew point of
-45.degree. C., and in particular, a heat treatment condition of a
soaking section was maintained at 800.degree. C. for 1 minute.
[0086] Meanwhile, with respect to sample numbers 6 to 8 in the
following Table 1, the reduction annealing was performed in a mixed
gas environment of 25% H.sub.2-75% N.sub.2 having a dew point of
-45.degree. C., and a heat treatment condition of a soaking section
was also maintained at 800.degree. C. for 1 minute.
[0087] Also, the hot dip plating was performed by dipping in a
plating bath, which had plating material compositions of the
following Table 1 and was sufficiently melted, for 5 seconds.
[0088] Further, hot dip plated steel sheets manufactured under the
foregoing conditions were subjected to slight annealing treatments
with the conditions of Table 2 to manufacture respective hot dip Zn
plated steel sheets.
[0089] The presence of bare spots and plating adhesiveness were
investigated on the hot dip Zn plated steel sheets thus
manufactured and the results thereof are presented in the following
Table 2.
[0090] Herein, methods and criteria of evaluating plating quality
are as below, respectively.
[0091] Platability: as a property of easily coating a hot dip
plating material on an underlying steel sheet, an appearance of a
plated steel sheet was optically observed and evaluation criteria
are as below.
[0092] Grades 1 to 2: no bare spots, a level for an automotive
outer panel
[0093] Grades 3 to 5: observation of ultra fine bare spots, a level
for an automotive inner panel and other products
[0094] Non-grade: observation of small bare spots, not a product
level
[0095] Plating adhesiveness: as a property of evaluating whether a
plating layer adhered to an underlying steel sheet generates a
delamination phenomenon or not, whether or not a delaminated
plating layer peeled off with a tape was observed when the tape was
adhered to a bending portion and detached after O-T bending of a
plated steel sheet, and evaluation criteria are as below.
[0096] .circleincircle.: very good, no overall delamination [0097]
.largecircle.: good, generation of delamination only within spots 5
mm away from an edge portion (it is fine if a side trimming
treatment is performed during production of products)
[0098] X: generation of overall delamination
[0099] In consideration of other operability and stability, cases
of the generation of particular problems were also observed and
evaluation criteria are as below.
[0100] .largecircle.: fine
[0101] X: generation of problems (a back fire phenomenon in a
direct fired furnace)
[0102] Meanwhile, a cross-sectional micrograph of a hot dip Zn
plated steel sheet manufactured according to a conventional example
using a conventional method in a thickness direction was observed,
and the result thereof is presented in FIG. 4. A cross-sectional
micrograph of a hot dip Zn plated steel sheet manufactured
according to an example of the present invention in a thickness
direction was observed, and the result thereof is presented in FIG.
5.
TABLE-US-00001 TABLE 1 Underlying Steel Sheet Hot Dip Plated Steel
Sheet Chemical Plating Sample Composition (wt %) Material
Composition No. C Mn Si Al Steels Zn Al Mg Si Type 1 0.2 1.5 1.5
0.05 TRIP 99.8 0.2 -- -- Hot dip Zn plated steel sheet 2 0.1 2.0
0.1 0.05 DP 99.8 0.2 -- -- Hot dip Zn plated steel sheet 3 0.2 2.5
0.2 0.05 CP 99.8 0.2 -- -- Hot dip Zn plated steel sheet 4 0.2 1.6
0.05 0.02 MART 99.8 0.2 -- -- Hot dip Zn plated steel sheet 5 0.7
15.0 0.5 2.0 TWIP 99.8 0.2 -- -- Hot dip Zn plated steel sheet 6
0.2 1.5 1.5 0.05 TRIP -- 91 -- 9 Hot dip Al plated steel sheet 7
0.2 1.5 1.5 0.05 TRIP 43 55 -- 2 Hot dip Al--Zn plated steel sheet
8 0.2 1.5 1.5 0.05 TRIP 90 6 3 1 Hot dip alloy plated steel
sheet
TABLE-US-00002 TABLE 2 Direct fired furnace Slight Air-fuel
annealing Plating quality ratio Steel sheet Steel sheet Surface
Sample 3.sup.rd 4.sup.th temperature temperature appearance Plating
Others Example No. No. region region (exit, .degree. C.) (exit,
.degree. C.) (grade) adhesiveness (operability) Conventional 1 0.98
0.98 580 No No grade X .largecircle. Example1 1 treatment
Conventional 1 1.3 1.1 620 No 1-2 X .largecircle. Example1 2
treatment Inventive 1 1.01 1.3 620 460 1-2 .circleincircle.
.largecircle. Example1 1 Inventive 1 1.3 1.01 620 460 1-2
.circleincircle. .largecircle. Example1 2 Inventive 1 1.50 1.1 620
460 1-2 .circleincircle. .largecircle. Example1 3 Comparative 1
1.55 1.1 620 460 -- -- X(backfire) Example 1 Comparative 1 1.3 1.1
530 460 3-5 X .largecircle. Example 2 Inventive 1 1.3 1.1 550 460
1-2 .circleincircle. .largecircle. Example1 4 Inventive 1 1.3 1.1
750 460 1-2 .largecircle. .largecircle. Example1 5 Comparative 1
1.3 1.1 770 460 1-3 X .largecircle. Example 3 Comparative 1 1.3 1.1
620 380 1-3 X .largecircle. Example 4 Inventive 1 1.3 1.1 620 400
1-2 .largecircle. .largecircle. Example1 6 Inventive 1 1.3 1.1 620
550 3-5 .circleincircle. .largecircle. Example1 7 Comparative 1 1.3
1.1 620 570 No grade .circleincircle. .largecircle. Example 5
Inventive 2 1.3 1.1 620 460 1-2 .circleincircle. .largecircle.
Example1 8 Inventive 3 1.3 1.1 620 460 1-2 .circleincircle.
.largecircle. Example1 9 Inventive 4 1.3 1.1 620 460 1-2
.largecircle. .largecircle. Example1 10 Inventive 5 1.3 1.1 620 460
3-5 .largecircle. .largecircle. Example1 11 Inventive 6 1.3 1.1 620
460 3-5 .circleincircle. .largecircle. Example1 12 Inventive 7 1.3
1.1 620 460 3-5 .circleincircle. .largecircle. Example1 13
Inventive 8 1.3 1.1 620 460 3-5 .largecircle. .largecircle.
Example1 14
[0103] As shown in Table 2, with respect to the hot dip Zn plated
steel sheet manufactured according to Conventional Example 2 using
a conventional method, it may be understood that a surface
appearance of the plated steel sheet was grades 1 to 3 such that
platability was good, but plating adhesiveness was not.
[0104] Meanwhile, with respect to the hot dip Zn plated steel sheet
manufactured according to Inventive Examples corresponding to the
present invention, it may be understood that the plated steel sheet
had a good surface appearance as well as having good plating
adhesiveness.
[0105] Also, as shown in FIG. 4, a continuous and dense Si--Mn
oxide layer was observed in the hot dip Zn plated steel sheet
manufactured according to a conventional example, while it may be
understood that a Fe--Zn alloy phase and a discontinuous Si--Mn
oxide layer were observed in the hot dip Zn plated steel sheet
manufactured according to an inventive example as shown in FIG.
5.
[0106] As shown in FIG. 5, in the present inventive example, the
Si--Mn oxide layer was destroyed by the formation of the Fe--Zn
alloy phase and became discontinuous or weakened, and therefore,
plating adhesiveness of the hot dip Zn plated steel sheet may be
more improved.
[0107] While the present invention has been shown and described in
connection with the exemplary embodiments, it will be apparent to
those skilled in the art that modifications and variations can be
made without departing from the spirit and scope of the invention
as defined by the appended claims.
* * * * *