U.S. patent application number 13/141798 was filed with the patent office on 2011-10-20 for steel sheet annealing device, device for producing plated steel sheet comprising the same, and production method for plated steel sheet using the same.
This patent application is currently assigned to POSCO. Invention is credited to Kwang-Geun Chin, Man-Young Park, Rho-Bum Park.
Application Number | 20110252849 13/141798 |
Document ID | / |
Family ID | 42288243 |
Filed Date | 2011-10-20 |
United States Patent
Application |
20110252849 |
Kind Code |
A1 |
Park; Rho-Bum ; et
al. |
October 20, 2011 |
STEEL SHEET ANNEALING DEVICE, DEVICE FOR PRODUCING PLATED STEEL
SHEET COMPRISING THE SAME, AND PRODUCTION METHOD FOR PLATED STEEL
SHEET USING THE SAME
Abstract
Provided is an annealing device which comprises at least one
section and in which the at least one section is filled with a gas
constituting a non-reducing atmosphere or a weakly reducing
atmosphere, as well as a device for producing plated steel sheet
comprising the annealing device, and a method for producing plated
steel sheet by means of such a device to substantially improve the
quality of plating onto hot-dipped steel sheet, including the
plating properties, alloying properties, anti-pickup properties,
plating adhesion properties, anti-flaking properties,
anti-cratering properties and anti-ash properties, by using
prior-art annealing equipment and heat-treatment cycle without any
additional oxidation-reduction heat treatment process or large
quantities of high-cost alloying elements.
Inventors: |
Park; Rho-Bum; (Gwangyang,
KR) ; Chin; Kwang-Geun; (Gwangyang, KR) ;
Park; Man-Young; (Gwangyang, KR) |
Assignee: |
POSCO
Pohang
KR
|
Family ID: |
42288243 |
Appl. No.: |
13/141798 |
Filed: |
December 8, 2009 |
PCT Filed: |
December 8, 2009 |
PCT NO: |
PCT/KR2009/007326 |
371 Date: |
June 23, 2011 |
Current U.S.
Class: |
72/38 |
Current CPC
Class: |
C23C 2/28 20130101; Y02P
10/253 20151101; C21D 1/26 20130101; C21D 1/42 20130101; C23C 26/00
20130101; C21D 1/84 20130101; C23C 2/06 20130101; C23C 2/12
20130101; C23C 2/26 20130101; C22C 38/02 20130101; C21D 6/005
20130101; C21D 8/02 20130101; C21D 9/46 20130101; C22C 38/60
20130101; C21D 1/34 20130101; C22C 38/04 20130101; C21D 6/008
20130101; C21D 1/613 20130101; C21D 9/54 20130101; C23C 2/02
20130101; Y02P 10/25 20151101; C21D 9/005 20130101 |
Class at
Publication: |
72/38 |
International
Class: |
B21B 9/00 20060101
B21B009/00 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 26, 2008 |
KR |
10-2008-0134884 |
Claims
1. An annealing device for a steel sheet including at least one
section, the device characterized in that the at least one section
is filled with a gas constituting anon-reducing atmosphere or a
weakly reducing atmosphere.
2. The device of claim 1, wherein the steel sheet comprises at
least one of Si, Mn, Al and B.
3. The device of claim 1, wherein the steel sheet is one of a hot
rolled steel sheet, a full hard steel sheet and a cold rolled steel
sheet.
4. The device of claim 1, wherein the gas contains hydrogen
(H.sub.2) of 3 vol % or less and nitrogen (N.sub.2) of 97 vol % or
more.
5. The device of claim 1, wherein the annealing device includes at
least one or two among a pre heating section, a heating section, a
soaking section, a slow cooling section, a rapid cooling section,
and a final cooling section.
6. The device of claim 1, wherein the annealing device further
includes at least one of an overaging section and a re-heating
section.
7. The device of claim 5, wherein the pre heating section or the
heating section is an induction heater, an infrared heater, a
radiant tube burner, an ultrasonic wave burner, or a device having
a combination thereof.
8. The device of claim 6, wherein the re-heating section is the
induction heater, the infrared heater, the radiant tube burner, the
ultrasonic wave burner, or a device having a combination
thereof.
9. A plated steel sheet manufacturing apparatus comprising the
annealing device for a steel sheet according to claim 1.
10. A plated steel sheet manufacturing method including: performing
a series of processes for manufacturing a plated steel sheet by
performing an annealing process on a steel sheet in an annealing
device, the annealing device including at least one section, and
the annealing process being performed in the at least one section
containing a gas constituting a non-reducing atmosphere or a weakly
reducing atmosphere.
11. The method of claim 10, wherein the annealing process includes
at least one or two among pre heating, heating, soaking, slow
cooling, rapid cooling, and final cooling operations.
12. The method of claim 11, wherein the annealing process further
includes at least one of overaging and re-heating operations.
13. The method of claim 11, wherein the pre heating operation or
the heating operation is an induction heating scheme, an infrared
heating scheme, a radiant tube burning scheme, an ultrasonic wave
burning scheme, or a scheme having a combination thereof.
14. The method of claim 12, wherein the re-heating operation is the
induction heating scheme, the infrared heating scheme, the radiant
tube burning scheme, the ultrasonic wave burning scheme, or a
scheme having a combination thereof.
15. The method of claim 11, wherein at least one or two cooling
operations among the slow cooling operation, the rapid cooling
operation, and the final cooling operation are a cooling scheme
using at least one of nitrogen, liquid nitrogen, and nitrogen
containing hydrogen.
16. The method of claim 10, wherein the gas contains hydrogen
(H.sub.2) of 3 vol % or less and nitrogen (N.sub.2) of 97 vol % or
more.
17. The method of claim 10, wherein the plated steel sheet is one
of a hot-dip galvanized steel sheet, a galvannealed steel sheet, a
galvanium plated steel sheet, an aluminum plated steel sheet and a
zinc-magnesium-based plated steel sheet.
Description
TECHNICAL FIELD
[0001] The present invention relates to a steel sheet annealing
device for manufacturing a plated steel sheet such as a high
strength hot-dip galvanized steel sheet and a galvannealed steel
sheet, a plated steel sheet manufacturing apparatus including the
same, and a plated steel sheet manufacturing method using the same.
More particularly, the present invention relates to a steel sheet
annealing device for manufacturing a hot-dip plated steel sheet
such as a hot-dip galvanized steel sheet and a galvannealed steel
sheet, a plated steel sheet manufacturing apparatus including the
same, and a plated steel sheet manufacturing method using the
same.
BACKGROUND ART
[0002] A recent environmental problem includes the issue concerning
the fuel consumption efficiency of a vehicle, and, as environmental
protection is becoming important, fuel consumption regulations for
vehicles have been strengthened. As a scheme for improving fuel
consumption efficiency, various methods for reducing the weight of
a vehicle have been considered from various angles. A current trend
is for steel manufacturers to attempt to manufacture a high
strength steel to secure safety while reducing the weight of steel
sheet used as a material of an automobile.
[0003] Recently, according to the current trend, the demand for a
high strength hot dip galvanized steel sheet for an automobile is
greatly increasing. In general, however, while a method of
manufacturing high strength steel using a solid solution
strengthening element such as P, Mn, or the like may go some way
toward strengthening the steel and reducing the weight thereof, the
method has a limitation in processing various forms of vehicle
components.
[0004] Therefore, at the time of producing vehicle components,
steel, which is able to be used for vehicle components having
complicated forms through excellent processibility and to provide
relatively high strength characteristics after the completion of
the process, is required. As this sort of steel there is Advanced
High Strength Steel (AHSS) such as dual phase steel (DP steel),
transformation induced plasticity steel (TRIP steel), or the like.
The AHSS may contain large quantities of elements such as Si, Mn,
Al, and the like. Si is an element capable of maintaining ductility
in steel without significantly reducing the strength thereof. Due
to this reason, Si may be frequently used.
[0005] However, when Si as an alloying element of steel is added in
an amount of approximately 0.1 wt % or more, a hot dip galvanized
steel sheet manufactured through a general method may result in the
occurrence of defects such as a bare spot or defects on an external
part thereof occurring relatively easily. These defects are
generated in an annealing process atmosphere as one of a plurality
of processes for manufacturing the hot dip galvanized steel sheet.
In the case of the annealing process, a heat treatment at a high
temperature of approximately 800.degree. is performed while
maintaining a reduction atmosphere containing hydrogen of 5 vol %
or more and nitrogen with regard to the remainder thereof (please
refer to JP1999-323443 and US 5137586). During the high temperature
heat treatment process, Si may diffuse on the surface of the steel
sheet. That is, the density of Si on the steel surface may be 10 to
100 times higher than an average density of Si in the entire steel
sheet, and the surface of the steel sheet, enriched with Si, may
react to moisture or impurities in an atmosphere of a furnace to
form an SiO.sub.2 oxide film.
[0006] The SiO.sub.2 oxide film formed on the surface of the steel
sheet in the process for manufacturing the hot dip galvanized steel
sheet may seriously degrade a specific characteristic, the
wettability of the steel sheet, such that it is therefore difficult
to secure excellent wettability of the steel sheet, by which a
bundle form of a bare spot effect occurs, or even when the plating
process is properly undertaken on the surface thereof, the adhesion
extent with regard to the plating state may become seriously
degraded. That is, at the time of conversion processing the steel
into components, the SiO.sub.2 oxide film may be a factor of a
plating peeling phenomenon in which a plating layer is
detached.
[0007] For reference, surface enrichment due to Si, Mn, or the like
may occur by the following reaction formula.
Fe.sub.2O.sub.3+3H.sub.2->2Fe+3H.sub.2O
Si+2H.sub.2O->SiO.sub.2+2H.sub.2
Mn+H.sub.2O->Mn O+H.sub.2 [Reaction Formula]
(Here, Fe.sub.2O.sub.3: FeO, Fe.sub.3O.sub.4, Fe(OH).sub.x, O and
other oxides)
[0008] In order to prevent defects from occurring due to the
decrease in wettability of a high strength steel sheet due to Si,
Mn, or the like, various techniques have been proposed, for
example, there was provided the technique of increasing an amount
of Al in the hot dip galvanizing bath to increase a production
amount of a Zn--Fe--Al--Si lased and Fe--Al--Si based alloying
layer on an interface between Fe and an alloying layer. Since the
alloying layer resolves an oxidized layer of an annoying element, a
hot dip plating wettability decrease occurring due to the oxidized
film of the alloying element on the interface can be suppressed.
However, absolutely increasing the amount of Al within the plating
bath may be undesirable, as the increase of Al may be a factor in
intergranular corrosion, together with Pb inevitably added as an
impurity to the plating bath at the time of manufacturing a
mini-spangle steel sheet. The intergranular corrosion may cause the
plating peeling, and moreover, since the increase of Al within the
plating bath is not good for welding at the time of processing the
steel sheet, the above-mentioned technique according to the related
art actually has difficulties when practically applied.
[0009] In addition, according to another technique of the related
art, in order to improve the wettability of Si-containing steel,
there has been proposed a technique in which surplus air is
introduced to a direct fired furnace to form an oxidized film, and
then, a reduction process is performed in the heating furnace RTS
of a 10 vol % H.sub.2-90 vol % H.sub.2 reduction atmosphere, to
greatly increase wettability. As an example, when the thickness of
an iron oxide may be increased by increasing the rate of air from
the general rate of 0.9 to 1.05 in the direct fired furnace, and
when a reduction heat treatment is performed therein, a pure iron
layer is formed on the surface of a steel sheet; stabilized
wettability can be secured. However, this technique according to
the related art also has technical defects, that is, when the
thickness of the oxidized film cannot be precisely controlled,
plating peeling may occur due to the thickened film layer. To the
contrary, since the oxidized film is thin and thus completely
returned by the reduction process, Si is enriched intact on the
surface of the steel sheet such that a zinc plating layer cannot be
strongly adhered to the surface of the steel sheet or bare plating
may occur thereon. Therefore, the thickness of the iron oxide
should be precisely controlled in the direct fired furnace.
[0010] One of the above-mentioned techniques according to the
related art regarding the oxidation-reduction heat treatment is
disclosed in JP2001-226742. In this case, the oxidized film is
formed with a thickness ranging from 0.02 .mu.m to 1 .mu.m during
an oxidation heat process, and is then completely resolved during
the reduction process, to secure the wettability thereof. In
addition, in the cases of JP1994-172953 and JP1994-172954, an
oxidized film retains a thickness ranging from 0.02 .mu.m to 0.2
.mu.m after the reduction heat treatment, but it is shown to be
completely resolved by Al within the plating bath.
[0011] However, in the case of the above patent technologies with
regard to the oxidation-reduction method, precisely controlling a
composition of iron oxide formed at the time of oxidation heat
treatment, and a thickness thereof, as well as a composition of
iron oxidized film remaining after the reduction heat treatment,
the porosity thereof, and the like is not easy. Therefore, a large
difference in wettability is inevitable, according to working
conditions or other external factors.
DISCLOSURE
Technical Problem
[0012] An aspect of the present invention provides a steel sheet
annealing device for providing an advanced hot-dip plated steel
sheet and a galvannealed steel sheet, a plated steel sheet
manufacturing apparatus including the same, and a plated steel
sheet manufacturing method using the same.
Technical Solution
[0013] According to an aspect of the present invention, there is
provided an annealing device including at least one section, the at
least one section being filled with a gas constituting a
non-reducing atmosphere or a weakly reducing atmosphere, and a
plated steel sheet manufacturing apparatus including the same. In
this case, the gas may be a weekly reducing gas containing nitrogen
(N.sub.2) of 100 vol %, or hydrogen (H.sub.2) of 3 vol % or less
and nitrogen (N.sub.2) of 97 vol % or more. In addition, the
annealing device may include at least one section among a
preheating section, a heating section, a soaking section, a slow
cooling section, a rapid cooling section, an overaging section, a
re-heating section, and a final cooling section. Further, the gas
may be filled in at least one section among the preheating section,
the heating section, the soaking section, the slow cooling section,
the rapid cooling section, the overaging section, the re-heating
section, and the final cooling section.
[0014] According to another aspect of the present invention, there
is provided a plated steel sheet manufacturing method including:
performing a series of processes for manufacturing a hot-dip plated
steel sheet or a galvannealed steel sheet by performing an
annealing process in at least one section of an annealing device,
the at least one section being filled with a gas constituting a
non-reducing atmosphere or a weakly reducing atmosphere, and the
annealing device including the at least one section. The at least
one section may be classified according to a heat treatment or a
cooling scheme, and further, a method of manufacturing a hot-dip
plating material steel sheet produced in a continuous annealing
line (CAL) having at least one section that contains a gas
constituting a non-reducing atmosphere or a weekly reducing
atmosphere, in the annealing device. In this case, the annealing
process may be a series of processes including at least one of a
pre heating operation, a heating operation, a soaking operation, a
slow cooling operation, a rapid cooling operation, an overaging
operation, a re-heating operation, and a final cooling operation.
Further, the gas may be a nitrogen gas (N.sub.2) of 100 vol %, or a
gas containing hydrogen gas (H.sub.2) of 3 vol % or less and
nitrogen gas (N.sub.2) of 97 vol % or more.
Advantageous Effects
[0015] According to an embodiment of the present invention, the
plating qualities of a hot dip plated steel sheet, such as
wettability on a hot-dip plated steel sheet, including alloying
reactivity, anti-pickup properties, plating adhesion properties,
anti-flaking properties, anti-cratering properties and anti-ash
properties, or the like, may be significantly improved. In
addition, an excellent quality of hot dip plated steel sheet may be
secured economically and easily such that the use thereof is varied
and is effective in terms of a reduction in cost.
DESCRIPTION OF DRAWINGS
[0016] 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:
[0017] FIG. 1 illustrates a schematic cross-section of a continuous
annealing line for continuously performing a continuous annealing
process on steel sheets in a general annealing process;
[0018] FIG. 2 is a graph showing the comparison of heat treatment
cycles in an annealing process according to the related art and an
embodiment of the present invention;
[0019] FIG. 3 is a graph showing a comparison of heat treatment
cycles in the annealing process according to the related art and an
embodiment of the present invention in detail;
[0020] FIG. 4 is a graph showing evaluation results of plating
quality effects according to a hydrogen gas concentration in the
inside of an annealing device;
[0021] FIG. 5 shows graphs illustrating the diffusion extent of Si,
Mn, Al, B, and the like, through an annealing process under a
nitrogen atmosphere (a) and a nitrogen-hydrogen atmosphere (b)
according to an embodiment of the present invention and under a
reducing atmosphere (c) of a comparative example; and
[0022] FIG. 6 shows photos for a comparison between surfaces (a and
b) of a steel sheet processed according to an embodiment of the
present invention and surfaces (c and d) of a steel sheet processed
according to the related art.
MODE FOR INVENTION
[0023] Embodiments of the present invention will now be described
in detail with reference to the accompanying drawings such that
they could be easily practiced by those having skill in the art to
which the present invention pertains. However, in describing the
embodiments of the present invention, detailed descriptions of
well-known functions or constructions will be omitted so as not to
obscure the description of the present invention with unnecessary
detail.
[0024] In addition, like reference numerals denote like elements
throughout the drawings.
[0025] Unless explicitly described to the contrary, the word
"comprise" and variations such as "comprises" or "comprising," will
be understood to imply the inclusion of stated elements but not the
exclusion of other elements.
[0026] According to an embodiment of the present invention, a
hot-dip plated steel sheet or a galvannealed steel sheet
manufacturing apparatus and a process thereof, and a continuous
annealing device and a process thereof for supplying a hot-dip
plating material, or a hot-dip plating apparatus or a process
therefor, are provided to anneal a steel sheet by technically
controlling at least one section among a plurality of sections of
an annealing device included in the above apparatus to maintain an
interior atmosphere of the annealing device as a non-reducing
atmosphere or a weekly reducing atmosphere. Hereinafter,
embodiments of the present invention will be described in detail
with reference to the accompanying drawings.
[0027] An annealing process in a hot-dip plating line and a
continuous annealing line is widely well-known as a process to
secure wettability by performing a heat treatment on a steel sheet
to secure material properties and resolve the surface of the steel
sheet
[0028] The heat treatment performed on the steel sheet in a general
annealing process is principally performed under a reduction
atmosphere with hydrogen (H.sub.2) of 5 vol % or more and nitrogen
(N.sub.2) with regard to the remainder thereof. That is, when the
steel sheet continuously passes through a pre heating section 101
(PHS) to a final cooling section 106 (FCS) shown in FIG. 1,
respective sections may be passed through a portion of or overall
among a pre heating operation, a heating operation, a soaking
operation, a slow cooling operation, a rapid cooling operation and
a final cooling operation, while being maintained as a reduction
atmosphere. Through this series of operations, the steel sheet may
be heated or cooled to implement a continuous annealing process
according to each operation performed.
[0029] However, the inventors of the present invention have studied
and found the fact that moisture was generated during a reduction
of oxidized Fe under a reduction atmosphere in which the heat
treatment of the annealing process was performed and the generated
moisture reacted to Si, Mn, Al, B, and the like, added as an
alloying element of steel. The alloying elements reacting to
moisture formed a oxide film such as SiO.sub.2, MnO, MnO.sub.2,
Mn.sub.2SiO.sub.4, B.sub.2O.sub.3, Al.sub.2O.sub.3, or the like, on
a surface of the steel sheet. It was therefore found that the oxide
film could significantly degrade plating properties or wettability
with hot-dip zinc during the hot-dip galvanizing process.
[0030] As a result, a general annealing process is performed to
resolve an oxidized surface of a steel sheet and a pollutant
material on the surface thereof such that wettability on the
surface of steel sheet is improved as the purposes of the process,
but it results in that under this hydrogen atmosphere, the
annealing process diffuses strength oxidation elements such as Si,
Mn, Al, B, and the like, existing in the interior of the steel
sheet, toward far end surface of the steel sheet, with reference to
FIGS. 5A, 5B and 5C. Further, as a density level of the alloying
elements moved to the far end surface thereof is increased, it
results in the oxidation on the surface of the steel sheet being
accelerated. In this case, due to the oxide film formed on the far
end surface, a reverse effect such as a decrease in wettability may
be caused and there may be difficulties in attaining some level of
satisfactory quality in the plated steel sheet as shown in FIGS. 6C
and 6D.
[0031] Therefore, in a case in which a material undergoes a heat
treatment in a series of processes and a device therefor, for
performing the annealing process using the continuous hot-dip
plating device and the continuous annealing device, the inventors
of the present invention studied a case in which the heat treatment
was performed by composing the entire or partial part of the
interior of the annealing device to have a non-reducing atmosphere
as shown in FIGS. 5A and 5B, and through this method, the study
resulted in a situation in which a device and a method able to
manufacture a plate steel sheet excellent in wettability were
completed. An applicable steel sheet is not particularly limited,
but may be one of a GI steel sheet (Zn-0.2% Al, hot-dip galvanized
steel sheet), a GA steel sheet (Zn-0.2% Al, galvannealed steel
sheet), a GL steel sheet (Zn-55% Al, a galvanium plated steel
sheet), an AL steel sheet (Al-5% Si, an aluminum plated steel
sheet) and an MG steel sheet (Zn-3% Mg-2% Al, a
zinc-magnesium-based plated steel sheet).
[0032] That is, an experiment with regard to a method of
manufacturing a steel sheet having excellent wettability was
conducted by forming respective sections in the interior of the
annealing device under various atmospheres. It proved by the
experiment that when at the time of annealing, the atmosphere was
composed as a non-reducing atmosphere containing a nitrogen gas
(N.sub.2) of 100 vol % or a weekly reducing atmosphere containing a
hydrogen gas (H.sub.2) of 3 vol % or less and a nitrogen gas
(N.sub.2) of 97 vol % or more, a plated steel sheet having
relatively excellent wettability could be manufactured.
[0033] When a hydrogen concentration in the interior of the
annealing device was maintained as 3 vol % or less, or as Ovol %, a
covering percentage of an oxide on the surface of the steel sheet
was significantly reduced. When hydrogen concentration exceeds 3
vol % and nitrogen concentration is decreased to below 97 vol %, a
covering percentage of an oxide on the surface of the steel sheet
which has undergone the annealing process may be increased such
that wettability may be decreased at the time of performing the
hot-dip plating process.
[0034] Moreover, while the steel sheets are continuously passed
through respective sections that are divided according to a heat
treatment or a cooling scheme of the annealing device so as to
undergo the heat treatment; a non-reducing atmosphere or a weekly
reducing atmosphere may be applied to at least one section among
the plurality of sections. Though applying the non-reducing
atmosphere or the weekly reducing atmosphere to multiple sections
may be excellent in terms of efficiency, it is also applicable that
an extremely oxidized material steel sheet is annealed, or a
partial application in consideration of work terms, for example, an
oxidation atmosphere formation prevention in the interior of an
annealing line, anti-ash properties, or the like, may be provided
in at least one continuous or interrupted section provided to
correspond to a plating process, the scale thereof, or the
like.
[0035] It was proved by experimentation, that in the case of the
annealing line shown in FIG. 1, the application in at least one
section among the sections from the pre heating section 101 to the
slow cooling section (SCS) 104 resulted in providing a more
excellent result than that in the application in the rapid cooling
section (RCS) 105 to the final cooling section 106.
[0036] Furthermore, in addition to the general continuous annealing
line shown in FIG. 1, in the process which further includes the
overaging section (OAS) (309), the re-heating section (RHS) 310, or
other kind of a section having a heating function, a maintenance
function and a cooling function between the rapid cooling section
105 and the final cooling section 106 shown in FIG. 3; the
application may be also provided.
[0037] In addition, heating, maintenance and cooling devices and
methods may be different from one another according to respective
sections of the continuous annealing line, and if necessary, an
additional section may be more provided, but there is no big
difference. That is, the non reducing or weekly reducing atmosphere
may be applied to an additional section not shown in FIG. 1, and
may be also applied to an annealing line from which a portion of
six sections shown in FIG. 1 is excluded.
[0038] As a heating device such as the pre heating section 101, the
heating section (HS) 102, the re-heating section 310, and the like;
an induction heater, an infrared heater, a radiant tube burner, an
ultrasonic wave burner, or any other kinds of heating devices may
be used or a device having a combination thereof may be used.
Further, as a cooling material used for the slow cooling section
104, the rapid cooling section 105, the final cooling section 106,
or the line; any other materials, for example, nitrogen, liquid
nitrogen, and nitrogen containing hydrogen, or the like, may be
used, and in addition, a mixed cooling scheme including at least
one thereof may be used.
[0039] Hereinafter, embodiments of the present invention will be
described in detail.
First Embodiment
[0040] A full hard steel sheet (F/H) with a thickness of 1.0 mm and
a tensile strength of 590 MPa grade TRIP (Containing Si of 1.5%, Mn
of 1.6%, C of 0.08%, Sb of 0.02%, or the like, as a main alloying
element) was subjected to a heat treatment process in an annealing
device that including the section of FIG. 1. Some specimens were
manufactured as a galvannealed steel sheet through a galvannealed
process.
[0041] At this time, the annealing condition changed hydrogen,
nitrogen and dewpoint atmosphere gases, and a heat treatment cycle
was first provided under the condition of a pre
heating-heating-soaking-slow cooling-rapid cooling-final cooling
cycle (hereinafter, referred to as `cycle A`) shown in FIG. 2, but
with regard to Inventive example 5, Inventive example 5 was
performed under the conditions of a pre
heating-heating-soaking-slow cooling-rapid
cooling-overaging-reheating-final cooling cycle (hereinafter,
referred to as `cycle B`) shown in FIG. 3.
[0042] A portion in the specimens completed in the annealing
process was evaluated with regard to anti-pickup properties, and
with regard to the remainder, a galvannealing process was performed
to evaluate wettability and alloying reactivity according to
respective annealing conditions, and the results were represented
in the following Table 1 and Table 4.
TABLE-US-00001 TABLE 1 Annealing Conditions Plating Quality Dew-
**Alloying Classification Hydrogen Nitrogen point cycle
*Wettability Reactivity ***Anti-pickup Inventive example 1 0% 100%
-30.degree. C. A 1 1 1 Inventive example 2 1% 99% -30.degree. C. A
2 1 1 Inventive example 3 2% 98% -30.degree. C. A 2 2 1 Inventive
example 4 3% 97% -30.degree. C. A 3 2 2 Comparative 4% 96%
-30.degree. C. A 4 3 3 Example 1 Comparative 5% 95% -30.degree. C.
A 5 3 3 Example 2 Comparative 10% 90% -30.degree. C. A 5 4 4
Example 3 Comparative 20% 80% -30.degree. C. A 5 5 4 Example 4
Comparative 40% 60% -30.degree. C. A 5 5 4 Example 5 Comparative
60% 40% -30.degree. C. A 4 5 5 Example 6 Comparative 80% 20%
-30.degree. C. A 3 5 5 Example 7 Comparative 100% 0% -30.degree. C.
A 3 5 5 Example 8 Inventive example 5 0% 100% -30.degree. C. B 1 1
1 Inventive example 6 0% 100% -10.degree. C. A 1 1 1 Inventive
example 7 0% 100% -50.degree. C. A 1 1 2
(% of gas is vol %, equally applied below) In the case of the
present embodiment, a method and a criterion of evaluating a
plating quality (wettability, alloying reactivity and anti-pickup
properties) are as follows. *Wettability: A hot-dip plating
material has the properties of being easily plated on a steel
sheet. In the present embodiment, an outer appearance of a plated
steel sheet was observed by the naked eye and the evaluation
criterion is as follows. Grade 1: No occurrence of bare spot, level
for use of automobile outer pannel Grade 2: Extremely small bare
spot observed, level for use of automobile inner pannel Grade 3:
Small bare spot observed, General level except an automobile Grade
4: Moderate bare spot observed, Unavailable as product Grade 5:
Large bare spot observed, Unavailable as product ** Alloying
reactivity: When a hot dip galvanized steel sheet was rapidly
re-heated, a base material, Fe, was diffused to a zinc plating
layer in such extent that an Fe--Zn alloying plated layer was
uniformly formed. In the present embodiment, an outer appearance of
a plated steel sheet was observed by the naked eye and the
evaluation criterion is as follows. Grade 1: Considerably even
alloying surface layer, level for use of automobile outer pannel
Grade 2: Even alloying surface layer, level for use of automobile
inner pannel Grade 3: Comparatively even alloying surface layer,
General level except an automobile Grade 4: Comparatively uneven
alloying surface layer, Unavailable as product Grade 5: Uneven
alloying surface layer, Unavailable as product *** Anti-pickup: The
anti-pickup properties are to prevent an annealed oxide from being
picked up on a hearth roll surface of an annealing device. While a
steel sheet is heat treated at a relatively high temperature in the
annealing device, Si, Mn, Al, and the like as steel components are
diffused to the surface of the steel sheet to form the annealed
oxide, and when the annealed oxide is picked up by the hearth roll
surface, a defect such as a dent may be caused. Therefore, the
anti-pickup properties are excellent when a covering percentage of
the annealed oxide diffusion-formed on the steel sheet surface is
smaller. In the case of the covering percentage of the annealed
oxide, a surface of a specimen completed in the annealing process
was photographed 30,000 times by using a field emission-SEM, and
the covering percentage was then represented using an image
analyzer. In addition, a thickness of an annealed oxide film was
also measured using a glow discharge spectrometer (GDS) for
reference of the present evaluation. The evaluation criterion was
as follows. Grade 1: Annealed oxide film covering percentage of 5%
or less Grade 2: Annealed oxide film covering percentage of 20% or
less Grade 3: Annealed oxide film covering percentage of 35% or
less Grade 4: Annealed oxide film covering percentage of 45% or
less Grade 5: Annealed oxide film covering percentage of 45% or
more
[0043] As shown in the above Table 1, Inventive examples 1 to 7
according to the conditions of the present invention were all
recorded as Grades 1 to 3 in the plating quality and represented
the quality sufficient to be used as a product.
[0044] These results are also shown in FIGS. 7A and 7B. In
particular, a relatively excellent plating quality was represented
in a state in which hydrogen was not contained at all in the equal
manner to Inventive example 1.
[0045] In addition, when hydrogen is not contained, it may be more
satisfied, but a production is producible even up to a level of 3
vol % of hydrogen. Further, if a gas atmosphere was satisfied, in a
case in which a heat treatment cycle was varied into a cycle form
of FIG. 3 and experimented as Inventive example 5, or in a case in
which a dewpoint was diversely varied to -10.degree. C. and
-50.degree. C. and experimented as Inventive examples 6 and 7; in
these cases, an excellent plating quality was also represented.
Therefore, it could be appreciated that the satisfactory effect
could be obtained by controlling a gas atmosphere even due to other
external factors.
[0046] Meanwhile, in the case of comparative examples 1 to 8, an
unsatisfactory plating quality of Grade 4 or 5 was represented in
at least one quality property among wettability, alloying
reactivity and anti-pickup properties. In particular, in a case in
which a hydrogen concentration is considerably high, such as a
level of 20 vol % or more, a low wettability was represented as up
to Grade 3 as the hydrogen concentration increased, but the
alloying reactivity and the anti-pickup properties were
significantly decreased. Therefore, a distinct quality improvement
effect could not be expected.
Second Embodiment
[0047] A full hard steel sheet (F/H) material having a thickness of
1.0 mm and a TRIP (Containing Si of 1.5%, Mn of 1.6%, C of 0.08%,
Sb of 0.02%, or the like, as a main alloying element) was subjected
to an annealing process in a heat treatment cycle shown in FIG. 2.
In particular, in the present embodiment, an optional section was
selected from six annealing sections shown in FIG. 1, and only a
corresponding section was filled with a non reducing atmosphere of
nitrogen gas of 100 vol % and a dewpoint of -30.degree. C. and then
a heat treatment was performed thereupon. A galvannealing process
was performed on the heat treated steel sheet to evaluate
wettability, alloying reactivity, and anti-pickup properties, and
the results were shown in Inventive examples 8 to 12 of the
following Table 2.
[0048] In addition, in order to determine whether the method
according to the embodiment of the present invention is applicable
according to materials and a plating condition, the annealing
section of FIG. 1 was filled with a non reducing atmosphere gas of
nitrogen of 100 vol % and a dewpoint of -30.degree. C., and in this
state, a heat treatment was performed while varying a steel type, a
type of a material and a kind of plating bath, and then, a
galvannealing process was performed to evaluate wettability,
alloying reactivity, and anti-pickup properties, and the results
were shown in Inventive examples 13 to 21 of the following Table
2.
[0049] In addition, in Comparative examples 9 to 15, an entire
section of a form of an annealing device the same as that of FIG. 1
was filled with an atmosphere containing hydrogen of 5 vol %, and
annealing and plating processes were performed. Respective results
were shown in the following Table 2.
TABLE-US-00002 TABLE 2 Annealing Device Non-reducing atmosphere
Application Material Plating Plating Quality**** Section (FIG.
Steel Bath Alloying Classification 1 Basis) Type* Kind** Kind***
Wettability Reactivity Anti-pickup Inventive 101~104 TRIP F/H GA 2
1 1 example 8 Inventive 102~105 TRIP F/H GA 2 1 1 example 9
Inventive 101~103, 106 TRIP F/H GA 2 1 1 example 10 Inventive 102,
103 TRIP F/H GA 2 2 1 example 11 Inventive 102 TRIP F/H GA 3 2 2
example 12 Inventive 101~106 TRIP CR GA 1 1 1 example13 Inventive
101~106 TRIP HR GA 2 2 1 example 14 Inventive 101~106 TWIP F/H GA 3
2 3 example 15 Inventive 101~106 DP F/H GA 1 1 1 example 16
Inventive 101~106 DQ F/H GA 1 1 1 example 17 Inventive 101~106 TRIP
F/H GI 1 -- 1 example 18 Inventive 101~106 DQ F/H GL 1 -- 1 example
19 Inventive 101~106 DQ F/H AL 2 -- 1 example 20 Inventive 101~106
DQ F/H MG 3 -- 1 example 21 Comparative Non-application TRIP CR GA
4 3 2 Example 9 of non Comparative reducing TRIP HR GA 5 5 3
Example 10 atmosphere Comparative (nitrogen-5 vol TWIP F/H GA 5 5 5
Example 11 % hyodrogen) Comparative DP F/H GA 4 4 5 Example 12
Comparative TRIP F/H GI 5 -- 3 Example 13 Comparative DQ F/H GL 3
-- 1 Example 14 Comparative DQ F/H AL 4 -- 1 Example 15 Comparative
DQ F/H MG 5 -- 1 Example 16 *A symbol for a steel type denotes a
tensile strength 980 MPa-grade high manganese steel (Twin Induced
Plasticity, TWIP), 780 MPa-grade heteroplasm steel (Dual Phase,
DP), and 300 MPa-grade steel sheet for processing (Drawing Quality:
DQ). **Kind of a material denotes full hard steel sheets (F/H),
cold rolled steel sheets (CR) and hot rolled steel sheets (HR).
***Kind of a plating bath indicates that respective annealing and
plating processes were performed on GI (Zn--0.2% Al), GL (Zn--55%
Al), AL (Al--5% Si) and Zn--3% Mg--2% Al). ****In the present
embodiment, a method and a criterion of evaluating a plating
quality were the same as that of the first embodiment.
[0050] In analyzing the result shown in the above Table 2,
Inventive examples 8 to 12 in which the non reducing atmosphere
according to the embodiment of the present invention was applied
and the heat treatment was performed in at least one section among
the plurality of sections, were shown to have excellent
wettability, alloying activity and anti-pickup properties in the
range of respective grades 1 to 3.
[0051] Further, Inventive example 8 in which a relatively large
amount of sections having a non reducing atmosphere applied thereto
were provided, was shown as providing a more excellent plating
quality than Inventive example 12, in which a relatively small
quantity of sections were provided. In addition, in comparison with
a case of being applied to a cooling processing section, a case in
which at least one section among sections 101 to 104 performing a
heat processing was selected and applied, was shown to have a
minute level but more excellent plating qualities.
[0052] Furthermore, when the kind of material was varied from
existing F/H to a cold rolled steel sheet (CR) and a hot rolled
steel sheet (HR), the plating quality was more improved (Inventive
examples 13 and 14), meanwhile, in a reducing atmosphere according
to the related art, a degraded plating quality of grade 4 or lower
was shown from at least one quality property among plating
qualities (Comparative examples 9 and 10).
[0053] Even in a case in which respective kinds were varied to
TWIP, DP and DQ, an excellent plating quality was shown as being
within respective grades 3 by being passed through processes
according to the embodiment of the present invention (Inventive
examples 15 to 17), meanwhile, in a reducing atmosphere according
to the related art, a relatively degraded plating quality as
compared to a quality property according to the embodiment of the
present invention was represented from at least one quality
property among plating qualities (Comparative examples 11 and
12).
[0054] Further, a case in which a GI material was manufactured
using a TRIP steel material was also shown to have an excellent
plating quality as in all grades 1 in the embodiment of the present
invention (Inventive example 18), meanwhile, in a reducing
atmosphere according to the related art, a plating quality was
shown as grade 5 with regard to wettability, and was significantly
degraded as compared to that of the embodiment of the present
invention (Comparative example 13). Further, in a case in which a
plating bath was varied to respective GL, AL and MG by using a DQ
steel material (Inventive examples 19 to 21 and Comparative
examples 14 to 16), the plating quality was excellent as respective
grades 1 to 3 according to the embodiment of the present invention,
meanwhile, in a reducing atmosphere according to the related art, a
relatively degraded plating quality as compared to a quality
property according to the embodiment of the present invention was
shown from at least one quality property among plating
qualities.
[0055] While the present invention has been shown and described in
connection with the 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.
* * * * *