U.S. patent application number 12/484553 was filed with the patent office on 2010-01-14 for alloyed hot-dip galvanized steel sheet and production method thereof.
This patent application is currently assigned to Kabushiki Kaisha Kobe Seiko Sho (Kobe Steel, Ltd.). Invention is credited to Kazutaka Kunii, Yoshihiro Miyake, Shigenobu Namba, Mikako Takeda, Fumio Yuse.
Application Number | 20100006184 12/484553 |
Document ID | / |
Family ID | 41060031 |
Filed Date | 2010-01-14 |
United States Patent
Application |
20100006184 |
Kind Code |
A1 |
Takeda; Mikako ; et
al. |
January 14, 2010 |
ALLOYED HOT-DIP GALVANIZED STEEL SHEET AND PRODUCTION METHOD
THEREOF
Abstract
Disclosed is an alloyed hot-dip galvanized steel sheet
containing 2.0 to 3.5 percent by mass of Mn. The steel sheet
includes a base steel sheet and a galvanized zinc-coat layer
thereon, in which MnO particles are present in an average number of
10 or less per micrometer on a straight line lying in an interface
between the galvanized zinc-coat layer and the steel sheet, an
Fe--Al--O alloy layer is present at the interface between the MnO
particles and the steel sheet, and the length of the Fe--Al--O
alloy layer is less than 10% of the overall length of the
interface. The alloyed hot-dip galvanized steel sheet, even though
having a high Mn content, is resistant to uneven alloying and
excels in surface appearance, because the amounts of the MnO
particles and the Fe--Al--O alloy layer that cause uneven alloying
are controlled.
Inventors: |
Takeda; Mikako; (Kobe-shi,
JP) ; Yuse; Fumio; (Kobe-shi, JP) ; Namba;
Shigenobu; (Kobe-shi, JP) ; Kunii; Kazutaka;
(Kobe-shi, JP) ; Miyake; Yoshihiro; (Kakogawa-shi,
JP) |
Correspondence
Address: |
OBLON, SPIVAK, MCCLELLAND MAIER & NEUSTADT, L.L.P.
1940 DUKE STREET
ALEXANDRIA
VA
22314
US
|
Assignee: |
Kabushiki Kaisha Kobe Seiko Sho
(Kobe Steel, Ltd.)
Kobe-shi
JP
|
Family ID: |
41060031 |
Appl. No.: |
12/484553 |
Filed: |
June 15, 2009 |
Current U.S.
Class: |
148/242 ;
148/332; 148/333; 148/334 |
Current CPC
Class: |
C21D 9/46 20130101; C22C
38/38 20130101; C21D 8/0478 20130101; C21D 9/48 20130101; C21D
8/0278 20130101; C22C 38/06 20130101; C21D 6/005 20130101; C23C
2/02 20130101; C23C 2/06 20130101; C21D 1/74 20130101 |
Class at
Publication: |
148/242 ;
148/333; 148/332; 148/334 |
International
Class: |
C23C 22/72 20060101
C23C022/72; C22C 38/18 20060101 C22C038/18; C22C 38/20 20060101
C22C038/20; C22C 38/22 20060101 C22C038/22 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 14, 2008 |
JP |
2008-182910 |
Claims
1. An alloyed hot-dip galvanized steel sheet, including a base
steel sheet and a galvanized zinc-coat layer on the steel sheet,
the galvanized zinc-coat layer and the steel sheet being alloyed, a
steel constituting the base steel sheet comprising, on the mass
basis, 0.02% to 0.2% of carbon (C), 2.0% to 3.5% of manganese (Mn),
0.03% to 0.5% of chromium (Cr), 0.01% to 0.15% of aluminum (Al),
0.04% or less (including 0%) of silicon (Si), 0.03% or less
(including 0%) of phosphorus (P), and 0.03% or less (including 0%)
of sulfur (S), with the remainder including iron (Fe) and
inevitable impurities, wherein MnO particles are present on an
arbitrary straight line lying in an interface between the
galvanized zinc-coat layer and the steel sheet in an average number
of 10 or less per micrometer of the straight line, wherein an
Fe--Al--O alloy layer is present at an interface between the MnO
particles and the steel sheet, and wherein the length of the
Fe--Al--O alloy layer is less than 10% of the overall length of the
interface, each of the length and the overall length being measured
on the arbitrary straight line.
2. The alloyed hot-dip galvanized steel sheet according to claim 1,
wherein the steel further comprises, on the mass basis, a total of
0.003%, to 1.0% of at least one member selected from the group
consisting of 0.003% to 0.5% of copper (Cu), 0.003% to 1.0% of
nickel (Ni), and 0.003% to 1.0% of titanium (Ti).
3. The alloyed hot-dip galvanized steel sheet according to claim 1,
wherein the steel further comprises, on the mass basis, at least
one member selected from the group consisting of 0.003% to 1.0% of
vanadium (V), 0.003% to 1.0% of niobium (Nb), 0.0002% to 0.1% of
boron (B), and 0.003% to 1.0% of molybdenum (Mo).
4. The alloyed hot-dip galvanized steel sheet according to claim 2,
wherein the steel further comprises, on the mass basis, at least
one member selected from the group consisting of 0.003% to 1.0% of
vanadium (V), 0.003% to 1.0% of niobium (Nb), 0.0002% to 0.1% of
boron (B), and 0.003% to 1.0% of molybdenum (Mo).
5. The alloyed hot-dip galvanized steel sheet according to claim 1,
wherein the steel further comprises, on the mass basis, at least
one member selected from the group consisting of 0.0005% to 0.005%
of calcium (Ca) and 0.0005% to 0.001% of magnesium (Mg).
6. The alloyed hot-dip galvanized steel sheet according to claim 2,
wherein the steel further comprises, on the mass basis, at least
one member selected from the group consisting of 0.0005% to 0.005%
of calcium (Ca) and 0.0005% to 0.001% of magnesium (Mg).
7. The alloyed hot-dip galvanized steel sheet according to claim 3,
wherein the steel further comprises, on the mass basis, at least
one member selected from the group consisting of 0.0005% to 0.005%
of calcium (Ca) and 0.0005% to 0.001% of magnesium (Mg).
8. The alloyed hot-dip galvanized steel sheet according to claim 4,
wherein the steel further comprises, on the mass basis, at least
one member selected from the group consisting of 0.0005% to 0.005%
of calcium (Ca) and 0.0005% to 0.001% of magnesium (Mg).
9. A method for producing the alloyed hot-dip galvanized steel
sheet of claim 1, the method comprising the steps of: carrying out
annealing of the steel sheet under such conditions that an oxygen
partial pressure PO.sub.2 (in units of atmospheric pressure (atm))
satisfies the following condition: -log(PO.sub.2).gtoreq.20;
galvanizing the annealed steel sheet to form the galvanized
zinc-coat layer on the surface of the steel sheet; and alloying the
steel sheet bearing the galvanized zinc-coat layer.
10. A method for producing the alloyed hot-dip galvanized steel
sheet of claim 2, the method comprising the steps of: carrying out
annealing of the steel sheet under such conditions that an oxygen
partial pressure PO.sub.2 (in units of atmospheric pressure (atm))
satisfies the following condition: -log(PO.sub.2).gtoreq.20;
galvanizing the annealed steel sheet to form the galvanized
zinc-coat layer on the surface of the steel sheet; and alloying the
steel sheet bearing the galvanized zinc-coat layer.
11. A method for producing the alloyed hot-dip galvanized steel
sheet of claim 3, the method comprising the steps of: carrying out
annealing of the steel sheet under such conditions that an oxygen
partial pressure PO.sub.2 (in units of atmospheric pressure (atm))
satisfies the following condition: -log(PO.sub.2).gtoreq.20;
galvanizing the annealed steel sheet to form the galvanized
zinc-coat layer on the surface of the steel sheet; and alloying the
steel sheet bearing the galvanized zinc-coat layer.
12. A method for producing the alloyed hot-dip galvanized steel
sheet of claim 3, the method comprising the steps of: carrying out
annealing of the steel sheet under such conditions that an oxygen
partial pressure PO.sub.2 (in units of atmospheric pressure (atm))
satisfies the following condition: -log(PO.sub.2).gtoreq.20;
galvanizing the annealed steel sheet to form the galvanized
zinc-coat layer on the surface of the steel sheet; and alloying the
steel sheet bearing the galvanized zinc-coat layer.
13. A method for producing the alloyed hot-dip galvanized steel
sheet of claim 4, the method comprising the steps of: carrying out
annealing of the steel sheet under such conditions that an oxygen
partial pressure PO.sub.2 (in units of atmospheric pressure (atm))
satisfies the following condition: -log(PO.sub.2).gtoreq.20;
galvanizing the annealed steel sheet to form the galvanized
zinc-coat layer on the surface of the steel sheet; and alloying the
steel sheet bearing the galvanized zinc-coat layer.
14. A method for producing the alloyed hot-dip galvanized steel
sheet of claim 5, the method comprising the steps of: carrying out
annealing of the steel sheet under such conditions that an oxygen
partial pressure PO.sub.2 (in units of atmospheric pressure (atm))
satisfies the following condition: -log(PO.sub.2).gtoreq.20;
galvanizing the annealed steel sheet to form the galvanized
zinc-coat layer on the surface of the steel sheet; and alloying the
steel sheet bearing the galvanized zinc-coat layer.
15. A method for producing the alloyed hot-dip galvanized steel
sheet of claim 6, the method comprising the steps of: carrying out
annealing of the steel sheet under such conditions that an oxygen
partial pressure PO.sub.2 (in units of atmospheric pressure (atm))
satisfies the following condition: -log(PO.sub.2).gtoreq.20;
galvanizing the annealed steel sheet to form the galvanized
zinc-coat layer on the surface of the steel sheet; and alloying the
steel sheet bearing the galvanized zinc-coat layer.
16. A method for producing the alloyed hot-dip galvanized steel
sheet of claim 7, the method comprising the steps of: carrying out
annealing of the steel sheet under such conditions that an oxygen
partial pressure PO.sub.2 (in units of atmospheric pressure (atm))
satisfies the following condition: -log(PO.sub.2).gtoreq.20;
galvanizing the annealed steel sheet to form the galvanized
zinc-coat layer on the surface of the steel sheet; and alloying the
steel sheet bearing the galvanized zinc-coat layer.
17. A method for producing the alloyed hot-dip galvanized steel
sheet of claim 8, the method comprising the steps of: carrying out
annealing of the steel sheet under such conditions that an oxygen
partial pressure PO.sub.2 (in units of atmospheric pressure (atm))
satisfies the following condition: -log(PO.sub.2).gtoreq.20;
galvanizing the annealed steel sheet to form the galvanized
zinc-coat layer on the surface of the steel sheet; and alloying the
steel sheet bearing the galvanized zinc-coat layer.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to an alloyed hot-dip
galvanized steel sheet which excels in surface appearance and is
used typically in automobiles, home appliances, and building
materials; and to a method for producing the alloyed hot-dip
galvanized steel sheet.
BACKGROUND OF THE INVENTION
[0002] Galvanized steel sheets are widely used typically in
automobiles, home appliances, and building materials. Among them,
alloyed hot-dip galvanized steel sheets excel in corrosion
resistance and spot weldability and are thereby widely used as
automobile steel sheets. The alloyed hot-dip galvanized steel
sheets are more and more demanded to have higher strength and
smaller thickness, with recent requirements for automobiles to have
higher collision safety and lighter body weight so as to have
higher fuel efficiency.
[0003] To meet these requirements and to ensure satisfactory
balance between strength and ductility, many of currently used
alloyed hot-dip galvanized steel sheets are those further
containing easily oxidizable elements such as silicon (Si) and
manganese (Mn). These easily oxidizable elements, however, are
known to be selectively oxidized upon annealing carried out before
galvanization of a steel sheet to thereby significantly impair
wettability in galvanization and alloying performance, and it is
very difficult to control these elements. Accordingly, stable
production of alloyed hot-dip galvanized steel sheets is difficult
according to known techniques.
[0004] Under these circumstances, a variety of proposals have been
made on alloyed hot-dip galvanized steel sheets.
[0005] Japanese Unexamined Patent Application Publication (JP-A)
No. 200711/2005 discloses an alloyed hot-dip galvanized steel sheet
and a production method thereof, in which a reaction product
between an added element in a steel sheet and a component in an
annealing atmosphere is formed during an annealing process. JP-A
No. 279410/2001 discloses a galvanized steel sheet and a production
method thereof, in which a sulfur-containing ammonium salt is
attached to the surface of a high tensile strength steel sheet
containing manganese, and the steel sheet is then subjected to a
heat treatment and subsequently to a galvanization process.
[0006] JP-A No. No. 88193/1994 discloses a method for producing an
alloyed hot-dip galvanized steel sheet, in which the surface layer
of a steel sheet is dry-etched before immersing the steel sheet in
a plating bath. JP-A No. 328036/2003 discloses a method for
improving the plating quality of an alloyed hot-dip galvanized
steel sheet, in which a steel sheet after annealing is cooled in a
controlled manner in order to reduce grain boundary
segregation.
[0007] JP-A No. 263271/2004 discloses a method for producing a high
tensile strength galvanized steel sheet, in which 70% or more of a
surface enriched layer containing silicon (Si), manganese (Mn), and
aluminum (Al) after annealing is removed by acid pickling, and the
treated steel sheet is galvanized.
[0008] These methods for producing alloyed hot-dip galvanized steel
sheets, however, are all complicated in their steps and are
difficult to produce alloyed hot-dip galvanized steel sheets in a
simple and easy manner. In addition, they are not intended to
produce an alloyed hot-dip galvanized steel sheet that is derived
from a steel sheet having a high manganese content and excels in
surface appearance.
SUMMARY OF THE INVENTION
[0009] Under these circumstances, an object of the present
invention is to provide an alloyed hot-dip galvanized steel sheet
which is resistant to uneven alloying and excels in surface
appearance even when the base steel has a high manganese content,
by controlling the amounts of MnO and an Fe--Al--O alloy layer that
cause uneven alloying and whereby accelerating the alloying of the
galvanized steel sheet. Another object of the present invention is
to provide a method for producing the alloyed hot-dip galvanized
steel sheet with superior surface appearance.
[0010] According to an embodiment of the present invention, there
is provided an alloyed hot-dip galvanized steel sheet which
includes a base steel sheet and a galvanized zinc-coat layer on the
steel sheet, the galvanized zinc-coat layer and the steel sheet are
alloyed, and a steel constituting the base steel sheet contains, on
the mass basis, 0.02% to 0.2% of carbon (C), 2.0% to 3.5% of
manganese (Mn), 0.03% to 0.5% of chromium (Cr), 0.01% to 0.15% of
aluminum (Al), 0.04% or less (including 0%) of silicon (Si), 0.03%
or less (including 0%) of phosphorus (P), and 0.03% or less
(including 0%) of sulfur (S), with the remainder including iron
(Fe) and inevitable impurities, in which MnO particles are present
on an arbitrary straight line lying in an interface between the
galvanized zinc-coat layer and the steel sheet, which MnO particles
are present in an average number of 10 or less per micrometer of
the straight line, an Fe--Al--O alloy layer is present at an
interface between the MnO particles and the steel sheet, and the
length of the Fe--Al--O alloy layer is less than 10% of the overall
length of the interface, each of the length and the overall length
being measured on the arbitrary straight line.
[0011] The steel sheet may further contain, on the mass basis, a
total of 0.003% to 1.0% of at least one member selected from the
group consisting of 0.003% to 0.5% of copper (Cu), 0.003% to 1.0%
of nickel (Ni), and 0.003% to 1.0% of titanium (Ti).
[0012] The steel sheet may further contain, on the mass basis, at
least one member selected from the group consisting of 0.003% to
1.0% of vanadium (V), 0.0039% to 1.0% of niobium (Nb), 0.0002% to
0.1% of boron (B), and 0.003% to 1.0% of molybdenum (Mo).
[0013] The steel sheet may further contain, on the mass basis, at
least one member selected from the group consisting of 0.0005% to
0.005% of calcium (Ca) and 0.0005% to 0.001% of magnesium (Mg).
[0014] According to another embodiment of the present invention,
there is provided a method for producing any of the above-mentioned
alloyed hot-dip galvanized steel sheets, the method includes the
steps of carrying out annealing of the steel sheet under such
conditions that an oxygen partial pressure PO.sub.2 (in units of
atmospheric pressure (atm)) satisfies the following condition:
-log(PO.sub.2).gtoreq.20; galvanizing the annealed steel sheet to
form the galvanized zinc-coat layer on the surface of the steel
sheet; and alloying the steel sheet bearing the galvanized
zinc-coat layer.
[0015] The alloyed hot-dip galvanized steel sheets according to the
present invention are alloyed hot-dip galvanized steel sheets that
are resistant to uneven alloying and excel in surface appearance,
even through they have a high manganese content of 2.0 to 3.5
percent by mass, because the amounts of MnO and an Fe--Al--O alloy
layer that cause uneven alloying are controlled.
[0016] Additionally, the method according to the present invention
can produce alloyed hot-dip galvanized steel sheets which are
resistant to uneven alloying and excel in surface appearance even
when their base steels have a high manganese content of 2.0 to 3.5
percent by mass, by controlling the amounts of MnO and an Fe--Al--O
alloy layer that cause uneven alloying and thereby accelerating the
alloying of the galvanized steel sheet.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] FIGS. 1A, 1B, and 1C are vertical sectional views of a steel
sheet illustrating how uneven alloying occurs in the production of
an alloyed hot-dip galvanized steel sheet, in which FIG. 1A
illustrates the generation of large amounts of MnO particles on the
surface of the steel sheet, FIG. 1B illustrates the generation of
an Fe--Al--O alloy layer at the interface between the steel sheet
and the galvanized zinc-coat layer, and FIG. 1C illustrates that
the Fe--Al--O alloy layer acts as a barrier against the diffusion
of iron and causes uneven alloying; and
[0018] FIG. 2 is a vertical sectional view showing an alloyed
hot-dip galvanized steel sheet according to an embodiment of the
present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0019] In an annealing process conducted in common processes for
producing an alloyed hot-dip galvanized steel sheet, iron as a main
component of the base steel sheet is not oxidized, but easily
oxidizable elements such as silicon and manganese, if added, are
selectively oxidized and diffuse to the surface layer of the steel
sheet. The surface of the steel sheet thereby bears oxides
containing each of these easily oxidizable elements alone
(single-component oxide) or in combination (multiple-component
oxide).
[0020] Of the easily oxidizable elements, silicon, if enriched on
the surface, forms a thin oxide layer in the outermost surface of
the steel sheet and/or causes internal oxidation and thereby
significantly impairs the plating quality and suitability for
alloying. Therefore, of the easily oxidizable elements, manganese
is positively added, but silicon is not positively added to the
steel in the present invention, although contamination of silicon
as an inevitable impurity is accepted.
[0021] On the other hand, manganese is also enriched in the surface
layer of the steel sheet but less acts as a barrier than silicon,
because manganese grows not as an oxide layer and an internal
oxidation layer as with silicon but grows as a granular oxide (MnO)
(oxide particles) and less impedes the outward diffusion of iron
during alloying. Contrarily, a small amount of manganese rather
increases the alloying rate. However, manganese should be added in
a large amount because of its low reinforcing efficiency, but such
a large amount of manganese may often cause MnO particles in the
surface layer of the steel sheet. This complicates the alloying
behavior and makes it difficult to control the alloying.
[0022] In consideration of these conditions, the present inventors
made investigations while focusing attention on the relationship
between the formation mechanism of MnO and alloying and, as a
result, succeeded in revealing a detailed formation mechanism of
uneven alloying.
[0023] The detailed mechanism will be explained with reference to
FIGS. 1A, 1B, and 1C. When a steel sheet 1 containing a large
amount of manganese is annealed under a high oxygen partial
pressure, a large amount of granular oxide MnO (MnO particles) is
initially formed in the outermost surface of the steel sheet 1
(FIG. 1A). When the steel sheet 1 in this state is dipped in a
galvanization bath (zinc plating bath) for hot dip galvanization,
aluminum contained in the galvanization bath immediately reacts
with oxygen of the MnO particles in the surface layer of the steel
sheet 1 and with iron diffused from inside of the steel sheet 1 to
form an Fe--Al--O alloy layer at the interface between the steel
sheet 1 and a galvanized zinc-coat layer 2 (FIG. 1B). The present
inventors found that the Fe--Al--O alloy layer acts as a barrier
against the diffusion of iron from the steel sheet 1 during
alloying and inhibits the alloying of the steel sheet 1 (FIG. 1C).
The Fe--Al--O alloy layer causes uneven alloying and impairs the
surface appearance of the alloyed hot-dip galvanized steel
sheet.
[0024] After detailed investigations on how the alloying behavior
varies depending on the state of MnO, the present inventors
conceived that uneven alloying can be controlled to thereby give a
good appearance of the galvanized zinc-coat surface, by allowing
MnO to be dispersed in the steel sheet through internal oxidation
and/or by suppressing the oxidation of Mn in the surface layer.
[0025] The present inventors focused attention on the state of MnO
in the surface layer of the steel sheet after annealing; prepared a
series of steel sheets having different manganese contents by
carrying out annealing under different oxygen partial pressures;
observed cross sections of steel sheets suffering from uneven
alloying and those of steel sheets without uneven alloying with a
scanning electron microscope (SEM) and a transmission electron
microscope (TEM); observed cross-sectional structures of these
steel sheets after galvanization; and analyzed the iron contents in
the galvanized zinc-coat layers. As a result, they succeeded in
revealing how the state of uneven alloying varies depending on the
state of MnO in the surface layer of the steel sheet after
annealing.
[0026] Initially, reasons for specifying contents of components of
a base steel (material steel) for use in the present invention will
be described. Hereinafter all percentages regarding contents of
elements are by mass, unless otherwise specified.
[0027] Carbon (C) content: 0.02to 0.2%
[0028] The carbon (C) element significantly affects the strength of
the steel and affects the amounts and shapes of low-temperature
transformation products to thereby affect the extensibility
(elongation properties) and stretch flange formability of the
steel. A steel, if having a carbon content of less than 0.02%, may
not give a satisfactory high-strength steel sheet for automobiles.
A steel, if having a carbon content of more than 0.2%, may show
insufficient weldability. The carbon content is therefore 0.02% and
preferably 0.04% in its lower limit and is 0.2% and preferably
0.15% in its upper limit.
[0029] Manganese (Mn) content: 2.0% to 3.5%
[0030] Manganese (Mn) element acts as a reinforcing element and
should be contained in a content of at least 2.0% or more for high
strength and for properties as a high-strength steel sheet with
very superior workability. In contrast, the manganese content
should be 3.5% or less, because manganese, if present in an
excessively large content, may impair extensibility (elongation
properties) or increase carbon equivalent to thereby adversely
affect the weldability. Accordingly, the manganese content should
be from 2.0% to 3.5%.
[0031] Chromium (Cr) content: 0.03% to 0.5%
[0032] Chromium (Cr) element is effective for increasing the
hardenability to reinforce the texture. Chromium helps carbon to be
enriched and thereby stabilized in austenite and accelerates the
formation of martensite. In addition, chromium affects the plating
quality by allowing an oxide to form on the surface of the steel
sheet. The lower limit of the chromium content should be 0.03%,
because chromium, if contained in a content of less than 0.03%, may
not effectively improve the hardenability. In contrast, the upper
limit of the chromium content should be 0.5%, because if chromium
is contained in a content of more than 0.5%, the effect of
improving the hardenability may be saturated and the cost may be
disadvantageously increase. The upper limit of the chromium content
is preferably 0.3%, because chromium, if contained in a content of
more than 0.3%, may impair the plating quality.
[0033] Aluminum (Al) content: 0.01% to 0.15%
[0034] Aluminum (Al) element is effective as a deoxidizer during
steel making and should be added in a content of 0.01% or more.
However, aluminum, if contained in a content of more than 0.15%,
may not only impair the surface appearance but also cause increased
production cost. Accordingly, the aluminum content should be 0.01%
to 0.15%.
[0035] Silicon (Si) content: 0.04% or less (including 0%)
[0036] Silicon (Si) element acts to reduce dissolved carbon content
in alpha phase to thereby improve the workability such as
extensibility (elongation properties); but it forms an oxide film
on the surface of steel sheet, significantly impairs the
wettability of the galvanized zinc-coat layer, and is not
positively added. However, this element may be contained as an
inevitable impurity in the steel, and the Si content should be
controlled to 0.04% or less so as to avoid adverse effects thereof,
and is preferably controlled to 0.03% or less.
[0037] Phosphorus (P) content: 0.03% or less (including 0%)
[0038] Phosphorus (P) element is effective for obtaining a
high-strength steel sheet. However, phosphorus, if contained in a
content of more than 0.03%, may often cause uneven plating, may
impede alloying, and is not positively added. However, this element
may be contained as an inevitable impurity in the steel, and the
phosphorus content should be controlled to 0.03% or less.
[0039] Sulfur (S) content: 0.03% or less (including 0%)
[0040] Sulfur (S) element causes hot crack during hot rolling and
significantly impairs spot crack resistance. Sulfur is fixed as a
precipitate in the steel, but, if contained in a large content, may
impair the extensibility (elongation properties) and stretch flange
formability, and is not positively added. However, this element may
be contained as an inevitable impurity in the steel, and the sulfur
content should be controlled to 0.03% or less.
[0041] The material steel for use herein contains the above
elements with the remainder being iron (Fe) and inevitable
impurities. Where necessary, the steel may further contain elements
mentioned below.
[0042] Copper (Cu): 0.003% to 0.5%, Nickel (Ni): 0.003% to 1.0%
Copper (Cu) and nickel (Ni) elements are effective to increase the
strength of the material steel itself and to improve the plating
quality. Copper and nickel, if enriched in the surface layer of the
steel sheet, serve to change the shapes (states) of oxides of
silicon (Si) and manganese (Mn) to thereby prevent deterioration in
plating quality, because copper and nickel are less oxidizable than
iron mainly constituting the material steel. For sufficiently
exhibiting these advantages, they are preferably contained in
contents of 0.003% or more. However, these elements, if contained
in excessively large contents, may impair the workability and cause
the cost to increase, and the upper limits of the copper and nickel
contents are preferably 0.5% and 1.0%, respectively.
[0043] Titanium (Ti): 0.003% to 1.0%
[0044] Titanium (Ti) element forms a carbide and is thereby
effective for strengthening the steel. This element also fixes
carbon and nitrogen to thereby increase the gamma value of the
steel sheet. For satisfactorily exhibiting these advantages, the
titanium content is preferably 0.003% or more. However, this
element, if present in an excessively large content, may impair the
workability and cause the production cost to increase, and the
upper limit of its content is preferably 1.0%.
[0045] Copper, nickel, and titanium, if contained in combination,
improve the surface cleanliness of the steel sheet and form
multiple oxides with iron upon melting of iron so as to improve the
plating quality. The total content of these elements, if two or
more of them are contained in combination, is preferably 0.003% to
1.0% in consideration of the upper and lower limits of their
contents in the case of single use.
[0046] Vanadium (V): 0.003%, to 1.0%, Niobium (Nb): 0.003% to
1.0%
[0047] Vanadium (V) and niobium (Nb) elements form carbides and are
thereby effective for increasing the strength of the steel. For
satisfactorily exhibiting these advantages, their contents are
preferably 0.003% or more, respectively. However, these elements,
if contained in excessively large contents, may impair the
workability and cause the production cost to increase, and the
upper limits of their contents are preferably 1.0%,
respectively.
[0048] Boron (B): 0.0002% to 0.1%
[0049] Boron (B) element serves to improve the weldability and to
increase the hardenability. For effectively exhibiting these
advantages, the boron content is preferably 0.0002% or more.
However, if this element is present in an excessively large
content, the effects thereof may be saturated and, in addition, the
element may impair the ductility and the workability. Accordingly,
the upper limit of its content is preferably 0.1%.
[0050] Molybdenum (Mo): 0.003% to 1.0%
[0051] Molybdenum (Mo) element is effective for solid-solution
strengthening without impairing the plating quality. For
satisfactorily exhibiting these advantages, the molybdenum content
is preferably 0.003% or more. However, this element, if present in
an excessively large content, may cause the production cost to
increase, and the upper limit of its content is preferably
1.0%.
[0052] Calcium (Ca): 0.0005% to 0.005%, Magnesium (Mg): 0.0005% to
Calcium (Ca) element acts to control the shapes of inclusions to
thereby increase the ductility and to increase the workability. For
effectively exhibiting these advantages, the calcium content is
preferably 0.0005% or more. However, this element, if present in an
excessively large content, may increase the amounts of inclusions
in the steel to thereby reduce the ductility and to impair the
workability. The upper limit of its content is therefore preferably
0.005%. Magnesium (Mg) acts in the steel in the same manner as with
calcium. The magnesium content is therefore preferably from 0.0005%
to 0.001% for the same reasons as with calcium.
[0053] Based on the investigations on how the generation of uneven
alloying varies depending on the state of MnO in the surface layer
of the steel sheet after annealing, as used herein the term
"alloyed hot-dip galvanized steel sheet having good surface
appearance with less uneven alloying" refers to an alloyed hot dip
galvanized steel sheet, in which MnO particles are present on an
arbitrary straight line lying in an interface between the
galvanized zinc-coat layer 2 and the steel sheet 1 in an average
number of 10 or less per micrometer of the straight line, an
Fe--Al--O alloy layer is present at an interface between the MnO
particles and the steel sheet 1, and the length of the Fe--Al--O
alloy layer is less than 10% of the overall length of the
interface, in which each of the length and the overall length are
measured on the arbitrary straight line, as illustrated typically
in FIG. 2.
[0054] In this connection, it requires much efforts and time and is
practically difficult to observe the entire interface between the
galvanized zinc-coat layer and the steel sheet typically with a
scanning electron microscope (SEM). Accordingly, it is enough to
observe only part of the interface. However, if the area to be
observed is excessively narrow, the obtained data may significantly
differ from actual data, and the interface should be observed in a
length of at least 500 .mu.m. As used herein the "overall length of
an interface" refers to the length of the interface in an area to
be observed.
[0055] The length of the Fe--Al--O alloy layer in FIG. 2 is not
illustrated as being less than 10% of the overall length of the
interface on an arbitrary straight line. FIG. 2 is, however, an
exemplary diagram in order to illustrate the present invention,
and, in actual, the length of the Fe--Al--O alloy layer on the
arbitrary straight line is less than 10% of the overall length of
the interface. Typically, the area shown in FIG. 2 is the area to
be observed; the overall longitudinal width of FIG. 2 is the
overall length of the interface; the total length of three parts of
the Fe--Al--O alloy layer is the length of the Fe--Al--O alloy
layer; and the percentage of the length of the Fe--Al--O alloy
layer is calculated according to the formula: (Length of the
Fe--Al--O alloy layer)/(Overall length of the
interface).times.100.
[0056] When MnO particles are generated in the surface layer of the
steel sheet and the steel sheet is dipped in a galvanization bath,
aluminum contained in the galvanization bath immediately reacts
with oxygen of the MnO on the surface of the steel sheet and with
iron diffused from inside of the steel sheet to form an Fe--Al--O
alloy layer at the interface between the steel sheet and a
galvanized zinc-coat layer. The Fe--Al--O alloy layer acts as a
barrier to inhibit the diffusion of iron during alloying, thus
causing uneven alloying.
[0057] The average number of MnO particles on an arbitrary straight
line lying in the interface between the galvanized zinc-coat layer
2 and the steel sheet 1 is preferably 10 or less per micrometer of
the straight line. MnO particles, if present in an average number
of more than 10 per micrometer, may accelerate uneven alloying of
the alloyed hot-dip galvanized steel sheet, and MnO particles, if
present in an average number of more than 20 per micrometer, may
further impair the wettability in galvanization, cause bare spots
when the steel sheet is dipped in a galvanization bath, and this
impedes the production of a galvanized steel sheet. The average
number of MnO particles is more preferably 5 or less per
micrometer.
[0058] The length of the Fe--Al--O alloy layer generated at an
interface between the MnO particle and the steel sheet 1 is
preferably less than 107, of the overall length of the interface.
An Fe--Al--O alloy layer, if present in a length of 10% or more of
the overall length of the interface, may accelerate uneven alloying
of the alloyed hot-dip galvanized steel sheet and significantly
impair the surface appearance thereof. The resulting alloyed hot
dip galvanized steel sheet is not suitable for use typically as an
automotive outside sheathing. The length of the Fe--Al--O alloy
layer is more preferably less than 5% of the overall length of the
interface.
[0059] Such an alloyed hot dip galvanized steel sheet according to
the present invention may be prepared in the following manner. In
the alloyed hot dip galvanized steel sheet, MnO particles are
present on an arbitrary straight line lying in the interface
between the galvanized zinc-coat layer 2 and the steel sheet 1 in
an average number of 10 or less per micrometer, and the length of
an Fe--Al--O alloy layer present at an interface between the MnO
particle and the steel sheet 1 on the arbitrary straight line is
less than 10% of the overall length of the interface. Specifically,
an annealing process is preferably carried out under such
conditions that an oxygen partial pressure PO.sub.2 (in units of
atmospheric pressure (atm)) satisfies the following condition:
-log(PO.sub.2).gtoreq.20. This annealing process is carried out
prior to a galvanizing process to form a galvanized zinc-coat layer
2 on the surface of the annealed steel sheet 1. For further
reducing the average number of the MnO particles to 5 or less per
micrometer, the annealing process is more preferably carried out
under such conditions that the oxygen partial pressure PO.sub.2 (in
units of atmospheric pressure (atm)) satisfies the following
condition: -log(PO.sub.2).gtoreq.23.
[0060] As has been described above, by suppressing the generation
of MnO particles, aluminum in the galvanization bath less reacts
with oxygen in the MnO particles, this suppresses the generation of
an Fe--Al--0 alloy layer at an interface between the galvanized
zinc-coat layer 2 and the steel sheet 1, and the resulting
Fe--Al--O alloy layer, even if generated, less works as a barrier
against the diffusion of iron. Thus, the diffusion of iron smoothly
proceeds during alloying to thereby give an alloyed hot-dip
galvanized steel sheet with superior surface appearance and with
less uneven alloying.
[0061] Next, a method according to an embodiment of the present
invention for producing an alloyed hot-dip galvanized steel sheet
will be illustrated below with production conditions.
[0062] Initially, a slab of steel containing the components is
hot-rolled, wound as a coil, subjected to acid pickling of the
surface according to necessity, cold-rolled, and thereby yields a
base steel sheet (steel sheet).
[0063] Next, annealing of the base steel sheet is conducted in a
continuous hot-dip galvanizing line. For example, the annealing in
the annealing process may be conducted at a temperature of from
750.degree. C. to 900.degree. C. for a duration of 200 seconds or
less. The annealing process is conducted under such conditions that
an oxygen partial pressure PO.sub.2 (in units of atmospheric
pressure (atm)) in the atmosphere satisfies the following
condition: -log(PO.sub.2).gtoreq.20.
[0064] After the completion of the annealing process, galvanization
(hot-dip zinc plating) is conducted as a galvanizing process. A
plating bath for use herein may be a galvanization bath containing
0.05 to 0.20 percent by mass of aluminum. The temperature of the
steel sheet upon dipping in the galvanization bath herein is
440.degree. C. or higher and lower than 480.degree. C., as being
substantially equal to the temperature of the hot-dip galvanization
bath. The dipping duration of the steel sheet in the galvanization
bath is, for example, 5 seconds or less.
[0065] The steel sheet after dipping is retrieved from the
galvanization bath, and the amount of the zinc coat deposited on
the surface of the steel sheet is adjusted. Typically, the amount
of the zinc coat is adjusted to a suitable amount of 60.+-.5
g/m.sup.2 typically with a gas wiper.
[0066] After the completion of the galvanizing process, alloying of
the galvanized steel sheet is conducted as an alloying process to
give an alloyed hot-dip galvanized steel sheet. Typically, the
alloying process is conducted at a temperature of from 450.degree.
C. to 600.degree. C. for a duration of 60 seconds or less. Through
these processes (steps), an alloyed hot-dip galvanized steel sheet
with superior surface appearance and with less uneven alloying can
be produced.
EXAMPLES
[0067] The present invention will be illustrated in further detail
with reference to several examples below. It should be noted,
however, the following examples are never intended to limit the
scope of the present invention, and appropriate modifications and
variations without departing from the spirit and scope of the
present invention set forth above and below are possible and fall
within the technical scope of the present invention.
[0068] In the following tests, test pieces of alloyed hot-dip
galvanized steel sheets were prepared by working cold-rolled steel
sheets having component compositions given in Table 1 to a size of
100 mm wide and 250 mm long, and sequentially subjecting the same
to annealing, galvanizing, and alloying with a hot-dip
galvanization simulator.
TABLE-US-00001 TABLE 1 Steel type C Si Mn P S Al Cr Cu Ni Ti V Nb
Mo B Ca Mg A 0.070 0.02 2.1 0.011 0.002 0.043 0.18 B 0.110 0.02
2.70 0.011 0.002 0.045 0.18 C 0.093 0.02 3.50 0.012 0.002 0.043
0.17 D 0.095 0.02 2.56 0.011 0.002 0.043 0.18 0.2 E 0.080 0.02 2.64
0.013 0.002 0.045 0.18 0.3 F 0.120 0.02 2.12 0.011 0.002 0.043 0.23
0.4 0.02 G 0.093 0.02 2.30 0.011 0.002 0.043 0.18 0.5 0.2 0.07 H
0.094 0.02 3.21 0.013 0.002 0.043 0.18 0.1 I 0.120 0.02 2.31 0.011
0.002 0.043 0.18 0.1 J 0.093 0.02 2.98 0.011 0.002 0.043 0.20 0.07
K 0.095 0.02 2.32 0.011 0.002 0.045 0.18 0.0009 L 0.130 0.02 2.30
0.011 0.002 0.043 0.17 0.1 0.15 M 0.094 0.02 2.45 0.011 0.002 0.043
0.18 0.3 0.05 0.31 0.0004 N 0.093 0.02 2.31 0.011 0.002 0.043 0.18
0.0012
[0069] Initially, the surfaces of the cold-rolled steel sheets
having component compositions given in Table 1 were cleaned by acid
pickling, and annealing was conducted in an atmosphere of N-3%
H.sub.2. The annealing conditions are shown in Table 2. The
annealing was conducted at temperatures of from 750.degree. C. to
900.degree. C. for a duration of 120 seconds. The parameter
-log(PO.sub.2) was controlled by setting the annealing temperature
within a range of from 750.degree. C. to 900.degree. C. and varying
the dew point within a range of from -75.degree. C. to 0.degree.
C., as shown in Table 2.
TABLE-US-00002 TABLE 2 Annealing conditions Annealing temperature
Dew H.sub.2 concentration log -log Type (.degree. C.) point
(.degree. C.) (% by volume) (H.sub.2O/H.sub.2) (PO.sub.2) a 750 -75
3 -4.399 28.5 b 750 -50 3 -2.887 25.5 c 750 -25 3 -1.68 23.1 d 750
0 3 -0.697 21.1 e 800 -75 3 -4.399 27.3 f 800 -50 3 -2.887 24.3 g
800 -25 3 -1.68 21.9 h 800 0 3 -0.697 19.9 i 850 -50 3 -2.887 24.3
j 850 -25 3 -1.68 20.8 k 850 0 3 -0.697 18.8 l 900 -75 3 -4.399
25.2 m 900 -50 3 -2.887 22.2 n 900 -25 3 -1.68 19.8 o 900 0 3
-0.697 17.8
[0070] The annealed steel sheets were dipped in a hot-dip
galvanization bath containing 0.13 percent by mass of aluminum to
thereby form a galvanized zinc-coat layer on the surface of the
steel sheets. The dipping was conducted at a temperature of the
steel sheet of 460.degree. C. for a duration of 2 seconds. After
the formation of the galvanized zinc-coat layer, the amount of zinc
coat was adjusted to 60 g/m.sup.2 with a gas wiper, and thereby
yielded galvanized steel sheets. The temperature of the
galvanization bath was set to the same temperature as that of the
steel sheet.
[0071] The alloying was conducted with an infrared heating furnace
in the galvanizing simulator immediately after the galvanization.
The alloying was conducted at a temperature of 550.degree. C. for a
duration of 10 seconds. In the tests, the test pieces of the
alloyed hot-dip galvanized steel sheets were examined on properties
and alloying performance of the steel sheets after alloying. The
results are shown in Table 3.
TABLE-US-00003 TABLE 3 Properties and alloying performance of steel
sheet after annealing Number of MnO particles at interface between
galvanized zinc-coat Surface Length of layer and appearance
Fe--Al--O steel Fe (uneven Sample Steel Annealing alloy layer (per
(percent alloying, number type condition (%) micrometer) by mass)
bare spots) Remarks 1 A b 1 0.2 11.5 Excellent Example 2 A e 0.8
0.1 9.9 Excellent Example 3 A h 15 19 4.1 Poor Com. Ex. 4 A i 1.2
0.9 10.6 Excellent Example 5 A j 7.2 8.8 9.1 Good Example 6 A k 19
13.3 7.2 Fair Com. Ex. 7 A n 25 12 3.3 Poor Com. Ex. 8 A o 23 15
4.5 Poor Com. Ex. 9 B c 1.9 2.5 10.9 Excellent Example 10 B d 8.6
5.7 9.3 Good Example 11 B f 2.2 3.2 10.5 Excellent Example 12 B g
7.3 6.9 9.0 Good Example 13 B h 14 15 4.5 Poor Com. Ex. 14 B i 2.4
1.6 10.7 Excellent Example 15 B j 8.2 7.9 8.8 Good Example 16 B k
22 18 2.6 Poor Com. Ex. 17 B m 6.4 7.7 9.4 Good Example 18 B n 21
13 2.9 Poor Com. Ex. 19 B o 18 16 2.1 Poor Com. Ex. 20 C a 4.2 3.8
10.4 Excellent Example 21 C b 4.6 4.2 10.7 Excellent Example 22 C d
7.9 8.7 8.9 Good Example 23 C e 4.7 4.1 9.9 Excellent Example 24 C
g 7.2 6.9 9.2 Good Example 25 C i 4.5 3.3 10.6 Excellent Example 26
C k 26 16.8 1.8 Poor Com. Ex. 27 C l 4.8 3.7 9.2 Excellent Example
28 C n 53 29 -- Bare spot Com. Ex. 29 C o 65 32 -- Bare spot Com.
Ex. 30 D a 4.3 4.2 8.9 Excellent Example 31 E a 3.6 4.1 9.2
Excellent Example 32 F a 3.1 3.8 9.3 Excellent Example 33 G a 3.8
3.1 9.6 Excellent Example 34 H a 2.2 2.9 8.8 Excellent Example 35 I
a 2.6 4.3 8.7 Excellent Example 36 J a 2.9 3.2 9.1 Excellent
Example 37 K a 3.7 4.4 9.5 Excellent Example 38 L a 1.5 1.1 9
Excellent Example 39 M a 0.8 1 10.6 Excellent Example 40 N a 1.2
1.6 9.7 Excellent Example
[0072] Initially, cross-sectional specimens were prepared by
cutting out 10-mm square pieces from the center of the test pieces
after alloying. Iron (Fe) contents (percent by mass) of the
galvanized zinc-coat layers were analyzed by scanning electron
microscopy-energy dispersive X-ray spectroscopy (SEM-EDX). The iron
contents indicate to which extent the alloying proceeds, and
whether uneven alloying occurs or not may be supposed based on the
magnitude of the iron content. Specifically, a small iron content
indicates insufficient alloying; and a large iron content indicates
delamination of the zinc coat due to excessive alloying.
[0073] To which extent uneven alloying occurs was evaluated
according to the following criteria:
[0074] Excellent: no uneven alloying occurs,
[0075] Good: uneven alloying occurs in an area ratio of less than
10%,
[0076] Fair: uneven alloying occurs in an area ratio of 10% or more
and less than 30%, and
[0077] Poor: uneven alloying occurs in an area ratio of 30% or
more.
[0078] When no galvanized zinc-coat layer was formed, the sample
was indicated as "Bare spot". Samples evaluated as "Excellent" or
"Good" are accepted herein.
[0079] Next, enrichment of manganese (Mn) and aluminum (Al) were
determined and the lengths of manganese- and aluminum-enriched
portions were measured through electron prove microanalysis (EPMA),
so as to identify or detect MnO particles at the interface between
the galvanized zinc-coat layer and the steel sheet and to detect an
Fe--Al--O alloy layer. After identifying the enrichment of
manganese and aluminum, the number of the MnO particles was
measured, and an area where the Fe--Al--O alloy layer is present
was identified. For these measurements, a 5-.mu.m square cross
section sample was sampled at random from an area of 500 .mu.m
length including the interface between the galvanized zinc-coat
layer and the steel sheet of each alloyed hot-dip galvanized steel
sheet, according to a focused ion beam (FIB) microsampling process.
The sample was machined to a thickness of about 0.1 .mu.m through
focused ion beam micromachining and used as a test specimen to be
observed in the tests.
[0080] The test specimen was observed with a TEM (JEM-2010F)
equipped with a high angle annular dark field detector (HAADF)
(EM-24015BU) at an accelerating voltage of 200 kV, to determine the
number of the MnO particles, and the percentage of the length of
the Fe--Al--O alloy layer to the overall length of the
interface.
[0081] Samples Nos. 1, 2, 4, 5, 9-12, 14, 15, 17, 20-25, 27, and
30-40 each had an average number of MnO particles present on an
arbitrary straight line lying in the interface between the
galvanized zinc-coat layer and the steel sheet of 10 or less per
micrometer and each had a length of the Fe--Al--O alloy layer
present at the interface between the series of the MnO particles
and the steel sheet of less than 10% of the overall length of the
interface. These samples are therefore examples according to the
present invention. These examples, in which uneven alloying occurs
in a quantity of less than 10%, are resistant to uneven alloying
and excel in surface appearance.
[0082] In contrast, Samples Nos. 3, 6-8, 13, 16, 18, 19, and 26
each had an average number of MnO particles present on an arbitrary
straight line lying in the interface between the galvanized
zinc-coat layer and the steel sheet of more than 10 and equal to or
less than 20 per micrometer and each had a length of an Fe--Al--O
alloy layer present at the interface between the series of the MnO
particles and the steel sheet of 10% or more of the overall length
of the interface (Comparative Examples). These comparative examples
are poor in surface appearance. This is probably because, when the
sample was dipped in the galvanization bath, aluminum contained in
the galvanization bath immediately reacted with oxygen in the MnO
particles on the surface of the sample and with iron diffused from
inside of the sample to form an Fe--Al--O alloy layer in a wide
region at the interface, and the Fe--Al--O alloy layer inhibited
the diffusion of iron during alloying to thereby cause uneven
alloying.
[0083] Samples Nos. 28 and 29 each had an average number of MnO
particles present on an arbitrary straight line lying in the
interface between the galvanized zinc-coat layer and the steel
sheet of more than 20 per micrometer (Comparative Examples). These
comparative examples had further deteriorated wettability in
galvanization and suffered from bare spots when they were dipped in
the galvanization bath. Thus, galvanized steel sheets were not
produced from these samples.
* * * * *