U.S. patent application number 16/678694 was filed with the patent office on 2020-03-05 for method of producing hot-dip aluminum-based alloy-coated steel sheet.
This patent application is currently assigned to Nisshin Steel Co., Ltd.. The applicant listed for this patent is Shinya Furukawa, Yasunori Hattori, Koutarou Ishii. Invention is credited to Shinya Furukawa, Yasunori Hattori, Koutarou Ishii.
Application Number | 20200071808 16/678694 |
Document ID | / |
Family ID | 57937434 |
Filed Date | 2020-03-05 |
United States Patent
Application |
20200071808 |
Kind Code |
A1 |
Furukawa; Shinya ; et
al. |
March 5, 2020 |
METHOD OF PRODUCING HOT-DIP ALUMINUM-BASED ALLOY-COATED STEEL
SHEET
Abstract
Provided is (i) a hot-dip Al-based alloy-coated steel sheet
which includes a coated layer having a surface on which fine
spangles are stably and sufficiently formed and which has a
beautiful surface appearance due to the fine spangles thus formed
on the surface of the coated layer, and (ii) a method of producing
such a hot-dip Al-based alloy-coated steel sheet. The hot-dip
Al-based alloy-coated steel sheet includes: a substrate steel
sheet; and a hot-dip aluminum-based alloy coated layer which is
formed on a surface of the substrate steel sheet and which contains
boron at an average concentration of not less than 0.005 mass % and
contains potassium at an average concentration of not less than
0.0004 mass %.
Inventors: |
Furukawa; Shinya; (Tokyo,
JP) ; Ishii; Koutarou; (Tokyo, JP) ; Hattori;
Yasunori; (Tokyo, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Furukawa; Shinya
Ishii; Koutarou
Hattori; Yasunori |
Tokyo
Tokyo
Tokyo |
|
JP
JP
JP |
|
|
Assignee: |
Nisshin Steel Co., Ltd.
Tokyo
JP
|
Family ID: |
57937434 |
Appl. No.: |
16/678694 |
Filed: |
November 8, 2019 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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16083743 |
Sep 10, 2018 |
|
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|
PCT/JP2016/074058 |
Aug 18, 2016 |
|
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16678694 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C22C 21/00 20130101;
C23C 2/40 20130101; C23C 2/12 20130101 |
International
Class: |
C23C 2/12 20060101
C23C002/12; C23C 2/40 20060101 C23C002/40 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 11, 2016 |
JP |
2016-048879 |
Claims
1-3. (canceled)
4. A method of producing a hot-dip aluminum-based alloy-coated
steel sheet, comprising: a coating step of dipping a substrate
steel sheet in a hot-dip aluminum-based alloy-coating bath which
contains aluminum as a main component, the hot-dip aluminum-based
alloy-coating bath containing boron at a concentration of not less
than 0.005 mass % and not more than 3.0 mass % and containing
potassium at a concentration of not less than 0.0004 mass % and not
more than 0.02 mass %.
5. The method as set forth in claim 4, wherein the hot-dip
aluminum-based alloy-coating bath contains boron at a concentration
of not less than 0.02 mass % and not more than 3.0 mass % and
contains potassium at a concentration of not less than 0.0008 mass
% and not more than 0.02 mass %.
6. The method as set forth in claim 4, further comprising: a
composition adjusting step of adjusting a composition of the
hot-dip aluminum-based alloy-coating bath, the composition
adjusting step including adding an aluminum master alloy containing
boron and potassium.
7. The method as set forth in claim 5, further comprising: a
composition adjusting step of adjusting a composition of the
hot-dip aluminum-based alloy-coating bath, the composition
adjusting step including adding an aluminum master alloy containing
boron and potassium.
Description
TECHNICAL FIELD
[0001] The present invention relates to a hot-dip Al-based
alloy-coated steel sheet and a method of producing the hot-dip
Al-based alloy-coated steel sheet. More specifically, the present
invention relates to (i) a hot-dip Al-based alloy-coated steel
sheet which has spangles having a minute size and has a beautiful
surface appearance due to such spangles, and (ii) a method of
producing such a hot-dip Al-based alloy-coated steel sheet.
BACKGROUND ART
[0002] A hot-dip aluminum-based alloy-coated steel sheet
(hereinafter referred to as a "hot-dip Al-based alloy-coated steel
sheet") includes a steel sheet whose surface is coated with an
alloy, which contains aluminum (Al) as a main component, by a
hot-dip method so that the steel sheet can have higher corrosion
resistance and/or higher heat resistance. Such a hot-dip Al-based
alloy-coated steel sheet has been widely used mainly for members
that are required to have heat resistance, such as exhaust gas
members of automobiles and members of combustion devices.
[0003] Note that the hot-dip Al-based alloy-coated steel sheet has
a coated layer having a surface on which a spangle pattern appears,
the spangle pattern being formed due to dendrites, which are
structures obtained by solidification of Al. The spangle pattern is
a characteristic geometric pattern or a flower pattern, and each
region (i.e., spangle) of the spangle pattern is constituted by
dendrites.
[0004] A spangle grows during solidification of Al after coating.
Growth of the spangle progresses as below. First, the nucleus of
the spangle (i.e., spangle nucleus) occurs. Then, a primary
dendrite arm grows from the spangle nucleus. Subsequently, a
secondary dendrite arm develops from the primary dendrite arm.
Growth of such dendrite arms stops due to a collision between
adjacent spangles. It follows that presence of more spangle nuclei
in the coated layer causes an increase in number of spangles. This
causes each spangle to have a minute size.
[0005] The presence of such a spangle does not adversely affect a
quality (e.g., corrosion resistance) of the hot-dip Al-based
alloy-coated steel sheet. Note, however, that in the market, a
hot-dip Al-based alloy-coated steel sheet is preferred which has
spangles having a minute size and thus has a surface skin having an
inconspicuous spangle pattern.
[0006] Under the circumstances, proposed is, for example, a method
of producing a hot-dip aluminum-zinc alloy-coated steel sheet which
includes a coated layer made of an aluminum-zinc alloy. According
to this method, for the purpose of formation of fine spangles,
titanium (Ti), zirconium (Zr), niobium (Nb), boron (B), a boride
such as aluminum boride (AlB.sub.2 or AlB.sub.12), titanium carbide
(TiC), titanium boride (TiB.sub.2), or titanium aluminide
(TiAl.sub.3) is added to a coating bath so that more substances
each acting as a spangle nucleus are obtained. Such a method is
disclosed in, for example, Patent Literatures 1 to 3.
CITATION LIST
Patent Literatures
[0007] [Patent Literature 1]
[0008] Japanese Patent Application Publication Tokukai No.
2004-115908 (Publication date: Apr. 15, 2004)
[0009] [Patent Literature 2]
[0010] Japanese Patent Application Publication Tokukai No.
2006-22409 (Publication date: Jan. 26, 2006)
[0011] [Patent Literature 3]
[0012] Japanese Patent No. 3751879 (Publication date: Dec. 16,
2005)
[0013] [Patent Literature 4]
[0014] Japanese Patent No. 5591414 (Publication date: Sep. 17,
2014)
SUMMARY OF INVENTION
Technical Problem
[0015] Note, however, that use of the above method to produce a
hot-dip Al-based alloy-coated steel sheet has the following
problems.
[0016] Specifically, since aluminum (having a specific gravity of
2.7) is one of the lightweight metals, molten aluminum is lower in
specific gravity than an aluminum-zinc alloy (having a specific
gravity of 7.1). Thus, any of substances, such as Ti, titanium
carbide (TiC), titanium boride (TiB.sub.2), and titanium aluminide
(TiAl.sub.3), which are higher in specific gravity than a hot-dip
Al-based alloy-coating bath, easily precipitates into a bath
bottom, so that it is difficult for such a substance to be
uniformly dispersed in the hot-dip Al-based alloy-coating bath.
This causes a problem of difficulty in stable formation of fine
spangles on surfaces of hot-dip Al-based alloy-coated steel sheets
which are continuously produced as in an industrial continuous
operation.
[0017] Meanwhile, B and aluminum boride (AlB.sub.2 or AlB.sub.12)
are less different in specific gravity from an aluminum bath and
thus are less likely to precipitate into a bath bottom. Note,
however, that, as compared with, for example, TiB.sub.2, B and
aluminum boride (AlB.sub.2 or AlB.sub.12) are unfortunately bring
about a less satisfactory effect of finer spangles.
[0018] For example, Patent Literature 4 discloses, as a
B-containing hot-dip Al-based alloy-coated steel sheet, a hot-dip
Al-based alloy-coated steel sheet which contains B at a
concentration of 0.002 mass % to 0.080 mass %. Note, however, that
according to the technique disclosed in Patent Literature 4, B
which is unevenly distributed over a surface of a coated layer of a
hot-dip Al-based alloy-coated steel sheet allows the coated layer
to be more slidable against a mold, and consequently allows the
coated layer to be more resistant to galling. It follows that
Patent Literature 4 fails to disclose that fine spangles are formed
so that a hot-dip Al-based alloy coated layer has a beautiful
surface appearance.
[0019] The present invention has been made in view of the problems,
and an object of the present invention is to provide (i) a hot-dip
Al-based alloy-coated steel sheet which includes a coated layer
having a surface on which fine spangles are stably and sufficiently
formed and which has a beautiful surface appearance due to the fine
spangles thus formed on the surface of the coated layer, and (ii) a
method of producing such a hot-dip Al-based alloy-coated steel
sheet.
Solution to Problem
[0020] The inventors of the present invention carried out a
diligent study and finally accomplished the present invention by
finding that, as compared with a hot-dip Al-based alloy-coated
steel sheet obtained with use of a coating bath to which B or
aluminum boride (AlB.sub.2 or AlB.sub.12) is added alone or
titanium boride (TiB.sub.2) and titanium aluminide (TiAl.sub.3) are
added, a hot-dip Al-based alloy-coated steel sheet obtained with
use of a hot-dip Al-based alloy-coating bath containing both boron
(B) and potassium (K) in proper amounts exhibits a more remarkable
effect of finer spangles.
[0021] That is, a hot-dip aluminum-based alloy-coated steel sheet
in accordance with an embodiment of the present invention includes:
a substrate steel sheet; and a hot-dip aluminum-based alloy coated
layer which is formed on a surface of the substrate steel sheet and
which contains boron at an average concentration of not less than
0.005 mass % and contains potassium at an average concentration of
not less than 0.0004 mass %.
Advantageous Effects of Invention
[0022] The present invention brings about an effect of providing
(i) a hot-dip Al-based alloy-coated steel sheet which includes a
coated layer having a surface on which fine spangles are stably and
sufficiently formed and which has a beautiful surface appearance
due to the fine spangles thus formed on the surface of the coated
layer, and (ii) a method of producing such a hot-dip Al-based
alloy-coated steel sheet.
BRIEF DESCRIPTION OF DRAWINGS
[0023] FIG. 1 is an optical photomicrograph of a state in which the
outermost surface of a hot-dip Al-based alloy-coated steel sheet in
accordance with an embodiment of the present invention has been
polished so that a dendrite structure is made observable.
DESCRIPTION OF EMBODIMENTS
[0024] The following description will discuss an embodiment of the
present invention. Note that, unless otherwise specified, the
present invention is not limited to the following description,
which is provided so that subject matters of the present invention
are better understood. Note also that a numerical expression such
as "A to B" as used herein means "not less than A and not more than
B".
[0025] First, the following description will schematically discuss
knowledge of the present invention before discussing a hot-dip
Al-based alloy-coated steel sheet in accordance with an embodiment
of the present invention and a method of producing such a hot-dip
Al-based alloy-coated steel sheet.
[0026] (Schematic Description of Knowledge of Present
Invention)
[0027] As described earlier, a spangle pattern formed due to
dendrites commonly appears on a surface of a hot-dip Al-based alloy
coated layer. In order that such a spangle pattern is made
inconspicuous, various approaches have been taken. The spangle
pattern can be made inconspicuous by, for example, a method of
carrying out a surface treatment as a post treatment, e.g.,
carrying out skin-pass rolling many times after coating. However,
such a method needs to be carried out with use of a major apparatus
or by a special process. This results in an increase in production
cost.
[0028] In view of the above problem, a method has been proposed in
which the spangle pattern is made inconspicuous by causing each
spangle to have a minute size. In order to cause spangles to have a
minute size, it is only necessary to cause spangle nuclei which are
formed at an early stage of growth of the spangles to be highly
dense. That is, the spangles can have a minute size by
heterogeneous nucleation of spangle nuclei.
[0029] For example, known is a technique in which a substrate steel
sheet is dipped in and taken out of a coating bath, and then fine
mist or fine metal oxide powder is sprayed over a surface of an
unsolidified coated layer. Note, however, that such a technique may
(i) prevent, due to flapping of a steel sheet in a continuous
hot-dip aluminum-coating line, spangles from being stably made
finer and/or (ii) necessitate an apparatus for carrying out a
spraying process and an apparatus for monitoring the spraying
process.
[0030] In view of the above problems, as described earlier, a
technique has been proposed in which a substance acting as a
spangle nucleus is added to a coating bath. According to this
technique, fine spangles are obtained by dipping a substrate steel
sheet in a coating bath whose components have been adjusted. Thus,
this technique is low in cost and highly convenient. Note, however,
that use of such a technique to produce a hot-dip aluminum-coated
steel sheet causes such problems as described earlier.
[0031] Under the circumstances, the inventors of the present
invention carried out detailed research on how various components
that can be added to a coating bath influence fine spangles of a
hot-dip Al-based alloy-coated steel sheet. As a result, the
inventors found that a coating bath containing both B and K brings
about a remarkable effect of finer spangles. That is, as compared
with a hot-dip Al-based alloy-coated steel sheet obtained with use
of a coating bath containing B or K alone, a hot-dip Al-based
alloy-coated steel sheet obtained with use of a coating bath
containing both B and K allows spangle nuclei formed on a surface
of a coated layer to be denser. In particular, the research
revealed that, as compared with a hot-dip Al-based alloy-coated
steel sheet obtained with use of a coating bath to which B or
aluminum boride (AlB.sub.2 or AlB.sub.12) is added alone or
titanium boride (TiB.sub.2) and titanium aluminide (TiAl.sub.3) are
added, a hot-dip Al-based alloy-coated steel sheet obtained with
use of a hot-dip Al-based alloy-coating bath containing B at a
concentration of not less than 0.005 mass % and K at a
concentration of not less than 0.0004 mass % exhibits a more
remarkable effect of finer spangles.
[0032] A specific mechanism by which a coating bath containing both
B and K enhances an effect of finer spangles is still unclear.
However, it is clear that, as compared with a coating bath to which
B or aluminum boride is added alone, a coating bath containing both
B and K even in very small amounts brings about a more remarkable
effect of finer spangles. It has been known that B is enriched
(unevenly distributed) over a surface of a coated layer. However, a
coating bath containing B alone is insufficient to bring about a
satisfactory effect of finer spangles. In view of this, examples of
the mechanism by which a coating bath containing both B and K
enhances the effect of finer spangles include a mechanism in which
B and K form clusters and the clusters are unevenly distributed
over a surface of a coated layer so as to each serve as a spangle
nucleus.
[0033] Meanwhile, in a case where a coating bath contains both B
and K but an amount in which the coating bath contains K is not
excessive, (i) an effect, brought about by a hot-dip Al-based alloy
coated layer, of improving corrosion resistance (red rust
resistance) of a steel sheet and (ii) intrinsic workability of an
Al-coated layer are maintained as in the case of a coating bath not
containing both B and K.
[0034] The above knowledge of the present invention is novel in the
field of hot-dip Al-based alloy-coated steel sheets, and the
knowledge is great in terms of the following points. According to
an embodiment of the present invention, by adjusting a composition
of a hot-dip Al-coating bath, it is possible to easily and stably
produce a hot-dip Al-based alloy-coated steel sheet which has
spangles whose size has been made sufficiently minute and which has
a beautiful surface skin due to such spangles. Furthermore, B and
K, which are neither rare metals nor heavy metals, are abundant in
the natural world and are harmless to human bodies. Moreover, B and
K are less likely to precipitate into the bottom of a hot-dip
Al-based alloy-coating bath. This makes it possible to stably
produce hot-dip Al-based alloy-coated steel sheets by an industrial
continuous operation. Thus, in another aspect, an embodiment of the
present invention makes it possible to provide (i) a hot-dip
Al-based alloy-coated steel sheet which can be produced at low
cost, which is highly suitable for industrial and practical use,
and which has spangles having a minute size and has a beautiful
surface appearance due to such spangles, and (ii) a method of
producing such a hot-dip Al-based alloy-coated steel sheet.
[0035] The foregoing description has schematically discussed the
knowledge of the present invention. Next, a hot-dip Al-based
alloy-coated steel sheet in accordance with an embodiment of the
present invention will be discussed below.
[0036] (Hot-Dip Al-Based Alloy-Coated Steel Sheet)
[0037] The hot-dip Al-based alloy-coated steel sheet in accordance
with an embodiment of the present invention will be discussed below
with reference to FIG. 1. FIG. 1 is an optical photomicrograph of a
state in which the outermost surface of the hot-dip Al-based
alloy-coated steel sheet in accordance with an embodiment of the
present invention has been polished so that a dendrite structure is
made observable.
[0038] Schematically, the hot-dip Al-based alloy-coated steel sheet
is produced by dipping a substrate steel sheet in a hot-dip
Al-based alloy-coating bath, which contains aluminum as a main
component, so as to form a hot-dip Al-based alloy coated layer on a
surface of the substrate steel sheet. During the production, Al and
iron (Fe) interdiffuse, so that an Al--Fe alloy coated layer is
also formed between (on a boundary between) (i) a steel base
material of the substrate steel sheet and (ii) the hot-dip Al-based
alloy coated layer. On a surface of the hot-dip Al-based alloy
coated layer, dendrites having grown from spangle crystal nuclei
are present (see FIG. 1). The density of the spangle crystal nuclei
present on the surface of the hot-dip Al-based alloy coated layer
will be discussed later.
[0039] [Substrate Steel Sheet]
[0040] The substrate steel sheet can be selected from commonly-used
substrate steel sheets in accordance with a purpose for which the
substrate steel sheet is used. In a case where the substrate steel
sheet is used while corrosion resistance is considered important, a
stainless steel sheet is applicable. The substrate steel sheet can
have a thickness of, for example, 0.4 mm to 2.0 mm. The substrate
steel sheet as used herein encompasses a substrate steel strip.
[0041] [Al--Fe Alloy Layer]
[0042] The Al--Fe alloy layer is made mainly of an Al--Fe-based
intermetallic compound. Note here that the hot-dip Al-based
alloy-coating bath preferably contains silicon (Si). An
Al--Fe-based alloy layer formed with use of an Si-containing
Al-based alloy-coating bath contains a large amount of Si. Both an
Si-free Al--Fe-based alloy layer and a so-called Al--Fe--Si-based
alloy layer containing Si are herein collectively referred to as an
Al--Fe-based alloy layer.
[0043] In a case where the Al--Fe-based alloy layer, which is made
of a brittle intermetallic compound, has a greater thickness, the
coated layer is made less adhesive. This leads to inhibition of
press workability. From the viewpoint of press workability, the
Al--Fe-based alloy layer preferably has a thickness that is as
small as possible. However, a technique of a too large reduction in
thickness of the Al--Fe-based alloy layer increases the process
load, and such a technique is uneconomical. Generally, the
Al--Fe-based alloy layer only needs to have an average thickness of
not less than 0.5 .mu.m.
[0044] [Composition of Hot-Dip Al-Based Alloy Coated Layer]
[0045] The hot-dip Al-based alloy coated layer has a chemical
composition that is substantially identical to the composition of
the coating bath. The composition of the coated layer can thus be
controlled by adjusting the composition of the coating bath.
[0046] Note that the hot-dip Al-based alloy coated layer, which
refers to a coated layer formed on the surface of the substrate
steel sheet, encompasses the Al--Fe-based alloy layer. An aluminum
oxide layer formed on the topmost surface of the hot-dip Al-based
alloy-coated steel sheet causes no particular problem because the
aluminum oxide layer is very thin. The aluminum oxide layer is
therefore assumed to be encompassed in the hot-dip Al-based alloy
coated layer. In a case where, for example, a film layer such as an
organic film is further formed on the surface of the hot-dip
Al-based alloy-coated steel sheet by a post treatment, such a film
layer is, as a matter of course, not encompassed in the hot-dip
Al-based alloy coated layer.
[0047] As such, the "average concentration" of a substance
contained in the hot-dip Al-based alloy coated layer as used herein
refers to an average of concentrations of the substance which
concentrations are measured, in a direction in which the depth of
the hot-dip Al-based alloy coated layer extends, from the surface
of the substrate steel sheet of the hot-dip Al-based alloy-coated
steel sheet to the outer surface of the hot-dip Al-based alloy
coated layer of the hot-dip Al-based alloy-coated steel sheet.
Specifically, as described later, the average concentration of a
substance is measured by carrying out concentration analysis with
respect to a measurement solution in which all the hot-dip Al-based
alloy coated layer has been melted. That is, the average
concentration of B, which is an element enriched on the surface of
the hot-dip Al-based alloy coated layer, refers to the
concentration of B contained in the hot-dip Al-based alloy coated
layer, the concentration being obtained by averaging concentrations
of B assuming that no B is enriched on the surface of the hot-dip
Al-based alloy coated layer. Furthermore, the concentration of B
contained in the hot-dip Al-based alloy-coating bath is reflected
in the average concentration of B contained in the hot-dip Al-based
alloy coated layer formed through coating.
[0048] The hot-dip Al-based alloy coated layer at least contains B
and K while containing Al as a main component. Note, however, that
the hot-dip Al-based alloy coated layer can contain other
element(s).
[0049] Si is an additive element that is necessary for inhibition
of growth of the Al--Fe alloy layer during hot-dip coating. The
Al-based alloy-coating bath to which Si is added has a lower
melting point. This is effective in reducing a temperature at which
coating is carried out. In a case where the coating bath contains
Si at a concentration of less than 1.0 mass %, the Al--Fe-based
alloy layer is formed thick during hot-dip coating due to
interdiffusion of Al and Fe. This causes peeling off in the coated
layer during processing such as press forming. Meanwhile, in a case
where the coating bath contains Si at a concentration of more than
12.0 mass %, the coated layer is cured. This makes it impossible to
prevent cracking in a bent part of the coated layer and
consequently causes the bent part to have lower corrosion
resistance. Therefore, the coating bath preferably contains Si at a
concentration of 1.0 mass % to 12.0 mass %. In particular, the
coating bath which contains Si at a concentration of less than 3.0
mass % (i) allows an Si phase to be formed in a smaller amount
during solidification of the coated layer and (ii) allows softening
of a primary crystal Al phase. Such a coating bath is thus more
effective in applications in which bending workability is
considered important.
[0050] In the hot-dip Al-based alloy-coating bath, Fe is mixed
which comes from the substrate steel sheet and/or a constituent
member(s) of a hot-dip coating tank. Generally, the hot-dip
Al-based alloy coated layer contains Fe at a concentration of not
less than 0.05 mass %. Note that Fe is permitted to be contained in
the hot-dip Al-based alloy coated layer at a concentration of up to
3.0 mass %, but more preferably not more than 2.5 mass %.
[0051] Besides the above elements, an element(s) (such as strontium
(Sr), sodium (Na), calcium (Ca), antimony (Sb), phosphorus (P),
magnesium (Mg), chromium (Cr), manganese (Mn), Ti, Zr, and/or
vanadium (V)) may be intentionally added to the hot-dip Al-based
alloy-coating bath as necessary, or the above element(s) coming
from, for example, a raw material may be mixed in the hot-dip
Al-based alloy-coating bath. The hot-dip Al-coated steel sheet in
accordance with an embodiment of the present invention can also
contain such an element that has been conventionally commonly
permitted. Specifically, for example, hot-dip Al-coated steel sheet
can contain Sr at a concentration falling within the range of 0
mass % to 0.2 mass %, Na at a concentration falling within the
range of 0 mass % to 0.1 mass %, Ca at a concentration falling
within the range of 0 mass % to 0.1 mass %, Sb at a concentration
falling within the range of 0 mass % to 0.6 mass %, P at a
concentration falling within the range of 0 mass % to 0.2 mass %,
Mg at a concentration falling within the range of 0 mass % to 5.0
mass %, Cr at a concentration falling within the range of 0 mass %
to 1.0 mass %, Mn at a concentration falling within the range of 0
mass % to 2.0 mass %, Ti at a concentration falling within the
range of 0 mass % to 0.5 mass %, Zr at a concentration falling
within the range of 0 mass % to 0.5 mass %, and/or Vat a
concentration falling within the range of 0 mass % to 0.5 mass
%.
[0052] The remainder, different from the foregoing elements, of the
hot-dip Al-based alloy-coating bath can be constituted by Al and
unavoidable impurities.
[0053] As described earlier, a hot-dip aluminum-based alloy-coated
steel sheet in accordance with an embodiment of the present
invention includes: a substrate steel sheet; and a hot-dip
aluminum-based alloy coated layer which is formed on a surface of
the substrate steel sheet and which contains boron at an average
concentration of not less than 0.005 mass % and contains potassium
at an average concentration of not less than 0.0004 mass %.
[0054] In a case where the hot-dip aluminum-based alloy coated
layer which contains B at a concentration falling within the above
range and contains K at a concentration falling within the above
range, not less than 100 spangle crystal nuclei can be present per
square centimeter surface area of the hot-dip Al-based alloy coated
layer. This makes it possible to produce a hot-dip Al-based
alloy-coated steel sheet which includes a coated layer having a
surface on which fine spangles are sufficiently formed and which
has a beautiful surface appearance due to the fine spangles thus
formed on the surface of the coated layer. Such a hot-dip Al-based
alloy-coated steel sheet can be obtained by (i) adjusting the
respective concentrations of B and K which are contained in the
coating bath and (ii) dipping the substrate steel sheet in the
coating bath. This makes it possible to achieve the hot-dip
Al-based alloy-coated steel sheet in which fine spangles are stably
formed.
[0055] By referring to FIG. 1 again, the following description will
discuss the density of spangle crystal nuclei. As illustrated in
FIG. 1, the spangles are non-uniform and irregular in size.
However, spangle crystal nuclei are still distinguishable when
viewed through, for example, an optical microscope.
[0056] Therefore, the number of spangle crystal nuclei per visual
field area can be understood by counting the number of spangle
crystal nuclei present in that visual field area. From the number
of spangle crystal nuclei per visual field area, it is possible to
roughly calculate the number of spangle crystal nuclei present per
square centimeter surface area of the hot-dip Al-based alloy coated
layer. Note that such a counting method as described above is
merely an example, and the number of spangle crystal nuclei can be
counted by any other method.
[0057] The hot-dip Al-based alloy coated layer which contains B at
an average concentration of less than 0.005 mass % makes it
impossible to achieve a satisfactory effect of finer spangles.
Meanwhile, the hot-dip Al-based alloy coated layer which contains B
at an average concentration of more than 0.50 mass % causes the
effect of finer spangles to reach a maximum, and no superiority is
displayed by the hot-dip Al-based alloy coated layer in which the
average concentration of B is further increased.
[0058] The hot-dip Al-based alloy coated layer which contains B at
an average concentration of more than 3.0% may cause a decrease in
corrosion resistance. Therefore, from the viewpoint of corrosion
resistance of the hot-dip Al-based alloy-coated steel sheet, the
hot-dip Al-based alloy coated layer preferably contains B at an
average concentration of 0.005 mass % to 3.0 mass %.
[0059] The hot-dip Al-based alloy coated layer which contains K at
an average concentration of less than 0.0004 mass % makes it
impossible to achieve a satisfactory effect of finer spangles.
Meanwhile, the hot-dip Al-based alloy coated layer which contains K
at an average concentration of more than 0.05 mass % causes the
effect of finer spangles to reach the maximum. The hot-dip Al-based
alloy coated layer which contains K at an average concentration of
not less than 0.03 mass % causes a decrease in corrosion
resistance. Therefore, from the viewpoint of corrosion resistance
of the hot-dip Al-based alloy-coated steel sheet, the hot-dip
Al-based alloy coated layer preferably contains K at an average
concentration of 0.0004 mass % to 0.02 mass %.
[0060] From the viewpoint of corrosion resistance of the hot-dip
Al-based alloy-coated steel sheet, the hot-dip Al-based alloy
coated layer is preferably configured to contain B at an average
concentration of 0.005 mass % to 3.0 mass % and contain K at an
average concentration of 0.0004 mass % to 0.02 mass %. The
configuration makes it possible to produce a hot-dip Al-based
alloy-coated steel sheet which has a beautiful surface appearance
and excellent corrosion resistance.
[0061] As described earlier, the effect of finer spangles reaches
the maximum in a case where the respective average concentrations
of B and K which are contained in the hot-dip Al-based alloy coated
layer are increased to some extent. Therefore, according to an
embodiment of the present invention, it is unnecessary to set
respective upper limits of those concentrations.
[0062] The hot-dip Al-based alloy coated layer is preferably
configured to contain B at an average concentration of not less
than 0.02 mass % and contain K at an average concentration of not
less than 0.0008 mass %. The configuration allows not less than 200
spangle crystal nuclei to be present per square centimeter surface
area of the hot-dip Al-based alloy coated layer. This makes it
possible to produce a hot-dip Al-based alloy-coated steel sheet
which has a more beautiful surface appearance.
[0063] The hot-dip Al-based alloy coated layer of the hot-dip
Al-based alloy-coated steel sheet does not necessarily need to be
provided on both sides of the substrate steel sheet, and only needs
to be provided on at least one side of the substrate steel
sheet.
[0064] (Method of Producing Hot-Dip Al-Based Alloy-Coated Steel
Sheet)
[0065] A hot-dip Al-based alloy-coated steel sheet in accordance
with an embodiment of the present invention can be produced by a
hot-dip method with use of a coating bath containing B and K at
respective adjusted concentrations. For example, the hot-dip
Al-based alloy-coated steel sheet can be produced in an
experimental line and by a common continuous Al-coating production
process (production apparatus). Alternatively, the hot-dip Al-based
alloy-coated steel sheet in accordance with an embodiment of the
present invention can be produced by applying the present invention
to any method, known to a skilled person, of producing a hot-dip
Al-coated steel sheet.
[0066] A method of producing a hot-dip aluminum-based alloy-coated
steel sheet in accordance with an embodiment of the present
invention includes a coating step of dipping a substrate steel
sheet in a hot-dip aluminum-based alloy-coating bath which contains
aluminum as a main component, the hot-dip aluminum-based
alloy-coating bath containing boron at a concentration of not less
than 0.005 mass % and containing potassium at a concentration of
not less than 0.0004 mass %.
[0067] The average concentration of each component contained in the
hot-dip Al-based alloy coated layer formed through the coating step
is substantially identical to the composition of the hot-dip
Al-based alloy-coating bath (i.e., the concentration of each
component contained in the hot-dip Al-based alloy-coating bath).
The configuration makes it possible to produce a hot-dip Al-based
alloy-coated steel sheet including a hot-dip Al-based alloy coated
layer which contains B at an average concentration of not less than
0.005 mass % and contains K at an average concentration of not less
than 0.0004 mass %.
[0068] From this, it is preferable that, as with the hot-dip
Al-based alloy-coated steel sheet, the hot-dip Al-based
alloy-coating bath contain B at a concentration of not less than
0.02 mass % and contain K at a concentration of not less than
0.0008 mass %. Note that the hot-dip Al-based alloy-coating bath
preferably contains B at a concentration of 0.005 mass % to 3.0
mass %. Note also that the hot-dip Al-based alloy-coating bath
preferably contains K at a concentration of 0.0004 mass % to 0.02
mass %.
[0069] At least prior to the coating step, a composition adjusting
step of adjusting a composition of the hot-dip Al-based
alloy-coating bath is carried out by adjusting respective
concentrations of elements contained in the hot-dip Al-based
alloy-coating bath. In the composition adjusting step, the
composition of the hot-dip Al-based alloy-coating bath can be
adjusted as below.
[0070] The concentration of B contained in the hot-dip Al-based
alloy-coating bath is preferably configured to be adjusted by
adding an aluminum master alloy containing B. The configuration
allows suitable dispersion of B in the hot-dip Al-based
alloy-coating bath. The concentration of B contained in the hot-dip
Al-based alloy-coating bath can alternatively be adjusted by adding
B alone or a boride such as aluminum boride (AlB.sub.2 or
AlB.sub.12), and a method of adjusting the concentration is not
limited to any particular method. The hot-dip Al-based
alloy-coating bath which contains such a raw material needs to be
subjected to a process for uniformly dispersing B in the hot-dip
Al-based alloy-coating bath.
[0071] Similarly, the concentration of K contained in the hot-dip
Al-based alloy-coating bath is preferably configured to be adjusted
by adding an aluminum master alloy containing K. The configuration
allows suitable dispersion of K in the hot-dip Al-based
alloy-coating bath. The concentration of K contained in the hot-dip
Al-based alloy-coating bath can alternatively be adjusted by adding
K alone or a compound such as KF, KBF.sub.4, or
K.sub.2AlF.sub.6AlB.sub.2, and a method of adjusting the
concentration is not limited to any particular method. The hot-dip
Al-based alloy-coating bath which contains such a raw material
needs to be subjected to a process for uniformly dispersing K in
the hot-dip Al-based alloy-coating bath.
[0072] The respective concentrations of B and K which are contained
in the hot-dip Al-based alloy-coating bath are preferably
configured to be adjusted by adding an aluminum master alloy
containing B and K. With the configuration, the addition of such an
aluminum master alloy allows B and K to be easily and suitably
dispersed in the hot-dip Al-based alloy-coating bath. In this case,
the respective concentrations of B and K which are contained in the
aluminum master alloy have a ratio that is substantially equal to a
ratio between the respective concentrations of B and K which are
contained in the hot-dip Al-based alloy-coating bath.
Alternatively, the respective concentrations of B and K which are
contained in the hot-dip Al-based alloy-coating bath can be
configured to be adjusted as desired by adding a plurality of
aluminum master alloys which differ from each other in amount of B
contained and in amount of K contained. The configuration can be
summarized as below. The method of producing the hot-dip
aluminum-based alloy-coated steel sheet preferably further includes
a composition adjusting step of adjusting a composition of the
hot-dip aluminum-based alloy-coating bath, the composition
adjusting step including adding an aluminum master alloy containing
boron and potassium.
[0073] In a case where the hot-dip Al-based alloy-coating bath
contains Si, the concentration of Si is preferably adjusted by
adding an aluminum master alloy containing Si. Furthermore, it is
only necessary that other element(s) that can be contained in the
hot-dip Al-based alloy-coating bath be added by a well-known method
so that a concentration(s) of the element(s) is/are adjusted.
[0074] Note here that an industrial continuous Al-coating producing
apparatus is configured such that substrate steel sheets are
continuously dipped in a hot-dip Al-based alloy-coating bath so
that hot-dip Al-based alloy-coated steel sheets are continuously
produced. During the production, each component contained in the
hot-dip Al-based alloy-coating bath is gradually reduced by an
amount in which the substrate steel sheets are coated with the each
component. This makes it necessary to compensate for the reduction
in hot-dip Al-based alloy-coating bath by any method.
[0075] As described earlier, the respective concentrations of B and
K which are contained in the hot-dip Al-based alloy-coating bath
can be adjusted by adding an aluminum master alloy containing B and
K. This makes it possible to easily compensate for the reduction in
hot-dip Al-based alloy-coating bath by using an aluminum master
alloy containing B and K in desired amounts, or using a plurality
of aluminum master alloys which differ from each other in amount B
contained and in amount of K contained. In a case where the hot-dip
Al-based alloy-coating bath contains Si, it is only necessary to
simultaneously add an aluminum master alloy containing Si. By thus
carrying out the composition adjusting step concurrently with the
coating step, it is possible to continuously and stably produce
hot-dip Al-based alloy-coated steel sheets each having a beautiful
surface appearance.
[0076] As described earlier, a hot-dip aluminum-based alloy-coated
steel sheet in accordance with an embodiment of the present
invention includes: a substrate steel sheet; and a hot-dip
aluminum-based alloy coated layer which is formed on a surface of
the substrate steel sheet and which contains boron at an average
concentration of not less than 0.005 mass % and contains potassium
at an average concentration of not less than 0.0004 mass %.
[0077] The hot-dip aluminum-based alloy-coated steel sheet in
accordance with an embodiment of the present invention is
configured such that not less than 100 spangle crystal nuclei are
present on a surface of the hot-dip aluminum-based alloy coated
layer per square centimeter surface area of the hot-dip
aluminum-based alloy coated layer.
[0078] The hot-dip aluminum-based alloy-coated steel sheet in
accordance with an embodiment of the present invention is
preferably configured such that the hot-dip aluminum-based alloy
coated layer contains boron at an average concentration of not less
than 0.02 mass % and contains potassium at an average concentration
of not less than 0.0008 mass %.
[0079] A method of producing a hot-dip aluminum-based alloy-coated
steel sheet in accordance with an embodiment of the present
invention includes a coating step of dipping a substrate steel
sheet in a hot-dip aluminum-based alloy-coating bath which contains
aluminum as a main component, the hot-dip aluminum-based
alloy-coating bath containing boron at a concentration of not less
than 0.005 mass % and containing potassium at a concentration of
not less than 0.0004 mass %.
[0080] The method of producing the hot-dip aluminum-based
alloy-coated steel sheet in accordance with an embodiment of the
present invention is preferably configured such that the hot-dip
aluminum-based alloy-coating bath contains boron at a concentration
of not less than 0.02 mass % and contains potassium at a
concentration of not less than 0.0008 mass %.
[0081] The method of producing the hot-dip aluminum-based
alloy-coated steel sheet in accordance with an embodiment of the
present invention preferably further includes a composition
adjusting step of adjusting a composition of the hot-dip
aluminum-based alloy-coating bath, the composition adjusting step
including adding an aluminum master alloy containing boron and
potassium.
[0082] The present invention is not limited to the embodiments, but
can be altered by a skilled person in the art within the scope of
the claims. The present invention also encompasses, in its
technical scope, any embodiment derived by combining technical
means disclosed in differing embodiments.
EXAMPLES
[0083] A hot-dip Al-based alloy-coated steel sheet (test sample)
was produced as below in an experimental line with use of coating
experimental equipment by using, as a substrate steel sheet, a
cold-rolled annealed steel sheet having a thickness of 0.8 mm and
having the chemical composition shown in Table 1. Specifically, the
hot-dip Al-based alloy-coated steel sheet was produced by (i)
dipping the substrate steel sheet in a hot-dip Al-based
alloy-coating bath prepared as described later, (ii) taking out the
substrate steel sheet thus dipped, and (iii) solidifying a coated
layer at a given cooling rate.
[0084] As the hot-dip Al-based alloy-coating bath, hot-dip Al-based
alloy-coating baths having various compositions were prepared as
below.
[0085] The concentration of Si contained in the coating bath was
adjusted to 0 mass % to 14.0 mass % with use of an Al-20 mass % Si
master alloy (an Al master alloy containing Si at a concentration
of 20 mass %). Then, the concentration of B contained in the
coating bath was adjusted to 0 mass % to 3.0 mass % by adding, to
the coating bath, a given amount of an Al-4 mass % B master alloy
(an Al master alloy containing B at a concentration of 4 mass %).
Furthermore, the concentration of K contained in the coating bath
was adjusted to 0.0001 mass % to 0.05 mass % by adding a given
amount of KF to the coating bath. Assuming that Fe coming from the
substrate steel sheet and/or a constituent member(s) of a pot
during continuous production was unavoidably mixed in the coating
bath, the concentration of Fe contained in the coating bath was
adjusted to 2.0 mass % by melting, in the coating bath, the
cold-rolled annealed steel sheet serving as the substrate steel
sheet. The remainder of the coating bath was constituted by Al and
unavoidable impurities.
[0086] The substrate steel sheet was dipped in the coating bath,
set at a temperature of 650.degree. C. to 680.degree. C., for two
seconds, was taken out of the coating bath, and then was cooled at
a cooling rate of 13.degree. C./sec. Respective amounts
(concentrations) of Si, B, and K which were contained in the coated
layer of each Example are shown in Table 2. Coating had, per
surface thereof, a thickness of approximately 20 .mu.m.
TABLE-US-00001 TABLE 1 Chemical composition (mass %) C Si Mn P S Al
O N 0.033 <0.01 0.23 <0.01 0.013 0.01 0.0027 0.0025
A resultant coated steel sheet was subjected to the following
examinations.
[0087] (Analysis of Components of Coated Layer by ICP)
[0088] First, the coated layer was melted by the following
procedure so that each component of the coated layer was
quantified.
[0089] Test samples produced with use of the foregoing hot-dip
Al-based alloy-coating baths having various compositions were each
cut into a piece having a given size, so that a test sample piece
was prepared. The test sample piece was put into an NaOH solution
(10 ml) at a concentration of 25%, was left to stand still, and
then was heated so that the coated layer was completely melted in
the solution. After it was confirmed that the coated layer had been
completely melted, the test sample piece, from which the coated
layer had been removed by being melted, was taken out of the
solution. Subsequently, the solution was further heated so that the
liquid was evaporated to dryness. A product obtained as a result of
evaporation to dryness was dissolved in a mixed acid (a mixed
solution of 40 ml of nitric acid and 10 ml of hydrochloric acid)
while being heated, and ultrapure water was added to a resultant
solution so that the volume of the solution was adjusted to a
constant volume of 250 ml. The solution which had been obtained
from the test sample piece and whose volume had been thus made
constant was used as a solution for use in measurement of the
composition of each test sample.
[0090] Thereafter, the solution for use in measurement of the
composition of each test sample was subjected to the following two
types of quantitative analyses so that the composition of the
coated layer was found.
[0091] The quantitative analysis of Si, B, and Fe was carried out
by an inductively coupled plasma atomic emission spectrometry
method (ICP-AES method). The quantitative analysis of K was carried
out by an inductively coupled plasma mass spectrometry method
(ICP-MS method).
[0092] (Number of Spangle Crystal Nuclei on Surface of Coated
Layer)
[0093] A dendrite structure was made observable by buffing the
surface of each test sample so as to make smoother the outermost
surface layer extending from the surface of the coated layer to the
depth of 5 .mu.m. Then, the number of spangle crystal nuclei
present per square centimeter surface area of the coated layer was
calculated with use of an optical microscope. The coated layer was
evaluated based on the following criteria, and the coated layer
evaluated as "Good" or "Excellent" was regarded as acceptable.
Excellent: Not less than 200 spangle crystal nuclei were present
per square centimeter surface area of the coated layer. Good: Not
less than 100 and less than 200 spangle crystal nuclei were present
per square centimeter surface area of the coated layer. Poor: Not
less than 50 and less than 100 spangle crystal nuclei were present
per square centimeter surface area of the coated layer. Very Poor:
Less than 50 spangle crystal nuclei were present per square
centimeter surface area of the coated layer.
[0094] (Corrosion Resistance of Coated Layer)
[0095] An untreated hot-dip Al-based alloy coated layer of each
test sample was subjected to a neutral salt spray test (NSS test),
specified by JIS Z2371:2000, so that a ratio of an area of white
rust formation to the entire coated layer was measured. Corrosion
resistance of the coated layer was evaluated based on the following
criteria, and the coated layer evaluated as "Good" was regarded as
acceptable.
Good: The ratio of the area of white rust formation to the entire
coated layer was not less than 0% and less than 5%. Fair: The ratio
of the area of white rust formation to the entire coated layer was
not less than 5% and less than 20%. Poor: The ratio of the area of
white rust formation to the entire coated layer was not less than
20%.
[0096] Results of the above examinations are shown in Table 2.
TABLE-US-00002 TABLE 2 Amount (Concentration) of Component
Contained in Coating Layer Density of Grade of (mass %) Spangles
Surface Corrosion Class No. Si B K (per cm.sup.2) Appearance
Resistance Examples of 1 0 0.01 0.0004 120 Good Good present 2 0.5
0.02 0.0005 150 Good Good invention 3 1 0.02 0.0005 150 Good Good 4
2 0.025 0.0008 400 Excellent Good 5 3 0.02 0.0008 200 Excellent
Good 6 5 0.02 0.0008 200 Good Good 7 8.8 0.018 0.0005 180 Good Good
8 8.9 0.01 0.0004 120 Good Good 9 9 0.02 0.0004 150 Good Good 10
9.1 0.02 0.0008 200 Excellent Good 11 9.2 0.02 0.0015 200 Excellent
Good 12 9.2 0.05 0.002 500 Excellent Good 13 9.5 0.5 0.02 500
Excellent Good 14 10 1 0.02 200 Excellent Good 15 12.3 2 0.02 200
Excellent Good 16 13.1 0.03 0.02 400 Excellent Good 17 5 3 0.02 500
Excellent Good 18 9 0.022 0.03 300 Excellent Poor 19 9.2 0.05 0.05
500 Excellent Poor Comparative 20 2 0.002 0.0004 20 Very Poor Good
Examples 21 8.8 0.02 0.0001 60 Poor Good 22 9 0.02 0.0003 80 Poor
Good 23 9.2 0 0.0001 5 Very Poor Good 24 12 0 0.0004 5 Very Poor
Good 25 9.2 0.002 0.0001 8 Very Poor Good 26 9.5 0.015 0.0001 70
Poor Good 27 9 0.022 0.0001 60 Poor Good 28 8.9 0.03 0.0001 70 Poor
Good 29 9.1 0.05 0.0001 80 Poor Good
[0097] As shown in Nos. 1 to 19 of Table 2, according to Examples
in each of which the coated layer contained B and K at respective
average concentrations falling within the ranges defined by an
embodiment of the present invention, not less than 100 spangle
crystal nuclei were present per square centimeter surface area of
the coated layer. This brought about a good effect of finer
spangles. Examples of the present invention reveal that an
embodiment of the present invention makes it possible to obtain a
hot-dip Al-based alloy-coated steel sheet which includes a coated
layer having a surface on which fine spangles are stably and
sufficiently formed and which has a beautiful surface appearance
due to the fine spangles thus formed on the surface of the coated
layer.
[0098] Examples of Nos. 4, 5, and 10 to 19 reveal (i) that not less
than 200 spangle crystal nuclei were present per square centimeter
surface area of the coated layer which contained B at an average
concentration of not less than 0.02 mass % and contained K at an
average concentration of 0.0008 mass % and (ii) that such a coated
layer makes it possible to obtain a hot-dip Al-based alloy-coated
steel sheet which has a more beautiful surface appearance.
[0099] Examples of Nos. 1 to 17 reveal that the coated layer which
contained K at an average concentration of 0.0004 mass % to 0.02
mass % had good corrosion resistance and makes it possible to
obtain a hot-dip Al-based alloy-coated steel sheet which has a
beautiful surface appearance and has excellent corrosion
resistance.
[0100] In contrast, according to Comparative Examples Nos. 20 to 29
in each of which the coated layer contained B and K at respective
average concentrations outside (less than lower limits of) the
ranges defined by an embodiment of the present invention, less than
100 spangle crystal nuclei were present per square centimeter
surface area of the coated layer. This (i) revealed that the effect
of finer spangles was not good enough and (ii) resulted in
obtainment of the hot-dip Al-based alloy-coated steel sheets each
having a poor surface appearance.
[0101] Note that, as shown in Nos. 1 to 29 of Table 2, the average
concentration of Si contained in the coated layer did not
particularly affect the effect of the present invention.
* * * * *