[Technical Field of the Invention]
[0001]
The present invention relates to a galvannealed steel sheet having excellent
coating adhesion, and a method of manufacturing the same.
[Related Art]
[0002]
In recent years, particularly in the field of vehicle technology, there has been
an increasing demand for high-strength steel sheets from the viewpoint of a reduction
in the weight of a vehicle body for the purpose of energy saving by fuel efficiency
enhancement. In response to this demand, for example, in Patent Document 1, a steel
sheet having a structure in which three phases including ferrite, bainite, and austenite
are mixed, as its steel sheet structure is disclosed. In addition, it is disclosed that this
steel sheet is a steel sheet which uses transformation-induced plasticity that exhibits
high ductility due to the transformation of retained austenite into martensite during
forming work.
[0003]
This type of steel sheet contains, for example, 0.05 mass% to 0.4 mass% of C,
0.2 mass% to 3.0 mass% of Si, and 0.1 mass% to 2.5 mass% of Mn and has a
composite structure formed by annealing in a dual-phase region and thereafter
controlling the temperature pattern in a cooling process. Therefore, the steel sheet is
characterized in that necessary properties can be ensured without using expensive alloy
elements.
- 1 -
[0004]
In a case where zinc plating is performed on the steel sheet by a continuous
hot dip zinc plating facility in order to impart a rust preventive function thereto,
coating wettability is significantly degraded when the Si content of the steel sheet is
higher than 0.3 mass%. Therefore, in the Sendzimir method in which a typical
molten zinc bath containing Al is used, non-coating defects are generated, and there is
a problem in that the quality of the external appearance is degraded.
[0005]
It is said that this is because an external oxide film including oxides
containing Si or Mn which have poor wettability to molten Zn is generated on the
surface of the steel sheet during reduction annealing.
[0006]
As means for solving this problem, in Patent Document 2, a method of
heating a steel sheet in advance in an atmosphere with an air ratio of 0.9 to 1.2 to form
Fe oxides, controlling the thickness of the oxides to 500 A or smaller in a reduction
zone containing H2, and thereafter performing coating in a bath to which Mn and Al
are added is proposed. However, in an actual production line, various steel sheets
containing various addition elements pass therethrough and thus it is difficult to
accurately control the thickness of the oxides.
[0007]
As another means for limiting non-coating defects, in Patent Document 3, a
method of applying a specific coating to a lower layer to improve co.ating properties is
disclosed. However, in this method, there is a need to newly provide a coating
facility in the front stage of an annealing furnace in a hot dip zinc plating line or to
perform a coating process in advance in an electro coating line. In either case, a
- 2 -
significant increase in manufacturing costs is expected.
[0008]
On the other hand, in Patent Document 4, a method of manufacturing a
galvannealed steel sheet by adjusting the oxygen potential in an annealing atmosphere
during annealing so as not to oxidize the Fe in a steel sheet is disclosed. In this
method, easily oxidizable elements such as Si and Mn in steel are allowed to be
internally oxidized by controlling the oxygen potential in the atmosphere such that the
formation of an external oxide film is limited and the enhancement of coating
properties is achieved.
[0009]
According to applying this method, the steel sheet is re-heated after coating
and a Zn coating layer and the steel sheet are allowed to react with each other.
Therefore, a Zn-Fe alloying reaction can uniformly proceed when an alloy coating
layer made of a Zn-Fe alloy is formed. However, although sufficient adhesion is
ensured during typical work, an effect of improving coating adhesion during heavy
duty working cannot be obtained.
[0010]
A high-strength steel sheet used as a vehicle reinforcing member is generally
worked by working mainly including bending. In a case where a high-strength steel
sheet having a relatively high C content is used as a starting sheet, since the starting
sheet itself is hard, cracks may be easily initiated in the surface layer of the steel sheet
during bending. Such cracks are the cause of cracking of the steel sheet in the
through-thickness direction during the use of the steel sheet.
[0011]
In order to solve this problem with bendability, in Patent Document 5, the
- 3 -
applicant suggests a technique of controlling the oxygen potential in an annealing
atmosphere to enhance coating properties, reducing the C content in the surface of a
steel sheet to enhance the ductility of the outermost surface layer and limit the
initiation of cracks, and allowing the oxides to limit the propagation of cracks even
when cracks are initiated and ensuring the bendability of the steel sheet by generating
oxides of Si and Mil in the vicinity of the surface layer of the steel sheet.
[0012]
However, in the technique of Patent Document 5, even when the steel sheet is
annealed under conditions such that the internal oxidation occurs, not all oxides that
are generated at the interface between the coating and the steel sheet are removed.
Therefore, depending on the properties and state of the interface between the coating
layer and the steel sheet caused by the generation behavior of the oxides, there may be
a case where the adhesion between the steel sheet and the coating layer is deteriorated
and there is a problem in that the coating is peeled off during working.
[0013]
In a case where a coated steel sheet is manufactured by using such methods,
as described in Patent Document 4, after a galvannealing, the particles of oxides
containing Si or Mn are dispersed in the coating layer (Zn-Fe alloy coating layer)
containing Zn-Fe alloy phases generated by reactions between Zn infiltrating from the
coating layer during the galvannealing and Fe in the steel sheet.
[0014]
In the Zn-Fe alloy coating layer, a plurality of Zn-Fe alloy phases such as £, Si,
T, and Fi phases are present in ascending order in terms of Fe content. In general, the
Zn-Fe alloy phase is hard and brittle as the Fe content is increased. In addition, when
the oxide particles are dispersed in the Zn-Fe alloy phase, the plastic deformability of
- 4 -
the Zn-Fe alloy phase is reduced. Therefore, when stress is applied to the coating
layer, the coating layer is more likely to be cracked or peeled off.
[0015]
Regarding problems such as coating peeling or deterioration in powdering
resistance that occur when a galvannealed steel sheet is manufactured by using a highstrength
steel sheet as a starting sheet, for example, in Patent Document 6, there is
disclosed a technique of, focusing on the shape of a structure including Si-Mn oxides
and Zn-Fe iutermetallic compounds generated at the interface between the coating
layer and the steel sheet, the interface between the coating layer and the steel sheet to
enhance adhesion between the coating layer and the steel sheet by controlling the size
of convex-concave portions in the structure.
[0016]
However, in the technique of Patent Document 6, during annealing before
coating, a process of heating the steel sheet in an oxidizing atmosphere and holding the
steel sheet in a reduction atmosphere for a predetermined time is employed.
Therefore, the annealing atmosphere has to be strictly controlled in order to cause the
state of the interface between the coating layer and the steel sheet to be in a
predetermined state after the galvannealing.
[0017]
In Patent Document 7, a technique of controlling the infiltration depth of Zn-
Fe intermetallic compounds in a depth direction toward the steel sheet from the
interface between the coating layer and the steel sheet to 10 urn or smaller to enhance
powdering resistance and coating adhesion is disclosed. However, in recent years,
higher workability has been required of a high-strength galvannealed steel sheet for
automotive applications and the like. Therefore, it is difficult to ensure coating
- 5 -
adhesion that can withstand heavy duty processing only by controlling the maximum
infiltration depth of the Zn-Fe intermctallic compounds. For example, when a heavy
duty forming process is performed using a die, powdering in which coatings on the
surface are peeled off may occur, and in the related art, it is difficult to eliminate the
occurrence of powdering.
[Prior Art Document]
[Patent Document]
[0018]
[Patent Document 1]
Publication No. H05-59429
[Patent Document 2]
Publication No. H04-276057
[Patent Document 3]
Publication No. 2003-105514
[Patent Document 4]
[Patent Document 5]
WO2011/025042
[Patent Document 6]
Publication No. 2011-127216
[Patent Document 7]
Publication No. 2011-153367
[Disclosure of the Invention]
[Problems to be Solved by the Invention]
[0019]
The present invention has been made taking the above-described problems
Japanese Unexamined Patent Application, First
Japanese Unexamined Patent Application, First
Japanese Unexamined Patent Application, First
Japanese Patent No. 4718782
Pamphlet of PCT International Publication No.
Japanese Unexamined Patent Application, First
Japanese Unexamined Patent Application, First
- 6 -
associated with a high-strength galvannealed steel sheet into consideration. That is,
an object of the present invention is to provide a galvannealed steel sheet having
excellent coating adhesion and a method of manufacturing the same.
[Means for Solving the Problem]
[0020]
The inventors intensively examined a method of enhancing the coating
adhesion of a galvannealed steel sheet (hereinafter, also referred to as "coated steel
sheet"). As a result, it was newly found that in the vicinity of the interface between
the coating layer and the steel sheet in a coated steel sheet after a coating treatment, (i)
the state of a structure and oxides formed on the steel sheet side, and (ii) the
morphology of an existent Zn-Fe alloy phase generated by infiltration of Zn into the
steel sheet from the coating layer side have a significant effect on the enhancement of
coating adhesion.
[0021]
Furthermore, based on the findings, the inventors found that the above
problems can be solved by controlling the structure in the vicinity of the interface
between the coating layer and the steel sheet.
[0022]
The present invention is based on the findings, and the summary is as follows.
[0023]
(1) A galvannealed steel sheet according to an aspect of the present invention
includes: a steel sheet; a coating layer on a surface of the steel sheet; and a mixed layer
formed between the steel sheet and the coating layer, in which the steel sheet contains,
in terms of mass%, C: 0.050% or more and 0.50% or less, and Mn: 0.01% or more and
3.00% or less, further contains one type or two or more types of Si: 0.01% or more and
- 7 -
3.00% or less, Al: 0.010% or more and 2.00% or less, and Cr: 0.01% or more and
2.00% or less, limits amounts of P, S, O, N, Ti, Nb, Mo, Cu, Ni, and B to P: 0.100% or
less, S: 0.0200% or less, O: 0.0100% or less, N: 0.0100% or less, Ti: 0.150% or less,
Nb: 0.150% or less, Mo: 1.00% or less, Cu: 2.00% or less, Ni: 2.00% or less, B:
0.0100% or less, satisfies the following Expression 1 when the Mn content, the Si
content, the Al content, and the Cr content are respectively expressed by [Mn], [Si],
[Al], and [Cr] in terms of mass%, and contains a remainder including Fe and
unavoidable impurities, the coating layer is a galvannealed layer containing, in terms
ofmass%',Fe: 7.0% or more and 15.0% or less, Al: 0.01% or more and 1.00% or less,
and a remainder including Zn and unavoidable impurities, and the mixed layer includes
a base iron portion having fine grains having a size of greater than 0 um and equal to
or smaller than 2 mn, a Zn-Fe alloy phase, and oxides containing one or more types of
Mn, Si, Al, and Cr, and in the mixed layer, the oxides and the Zn-Fe alloy phase are
present in grain boundaries that form the fine grains and the Zn-Fe alloy phase is
tangled with the base iron portion.
[Mn]+ [Si] + [Al] + [Cr] > 0.4 ... (Expression 1)
[0024]
(2) In the galvannealed steel sheet described in (1), surface layer region on the
coating layer which is a region of 1 um or smaller from the surface of the coating layer
may be a Zn-Fe alloy phase which contains a ^ phase that does not contain the oxides.
, [0025]
(3) In the galvannealed steel sheet described in (1) or (2), an average thickness
of the mixed layer in a direction along a through-thickness direction of the steel sheet
may be 10 um or smaller.
[0026]
- 8 -
(4) In the galvannealed steel sheet described in any one of (1) to (3), the Zn-
Fe alloy phase in the mixed layer may have a shape that protrudes in a V-shape toward
a thickness center of the steel sheet from the coating layer when viewed in a crosssection
in the through-thickness direction of the steel sheet.
[0027]
(5) In the galvannealed steel sheet described in any one of (1) to (4), when 10
or more visual fields of the mixed layer are observed along an interface between the
mixed layer and the coating layer by using a scanning electron microscope at a
magnification of 5,000-fold, the fine grains having the grain boundaries in which the
Zn-Fe alloy phase is present in the mixed layer may be observed in 20% or greater of
the entirety of the observed visual fields.
[0028]
(6) In the galvannealed steel sheet described in any one of (1) to (5), the Zn-
Fe alloy phase in the mixed layer may be generated by a reaction between Zn
infiltrating from the coating layer during a galvannealing and Fe in the steel sheet.
[0029]
(7) A method of manufacturing a galvannealed steel sheet according to
another aspect of the present invention includes: a first temperature rising process of
heating the steel sheet having the composition described in (1), in an atmosphere which
contains 0.1 vol.% or more and 50 vol.% or less of hydrogen and a remainder
including nitrogen and unavoidable impurities and has a dew point of higher than -
30°C and equal to or lower than 20°C at a first temperature rising rate of 0.2 °C/sec or
higher and 6 °C/sec or lower, which is an average temperature rising rale between
650°C and 740°C; a second temperature rising process of heating the steel sheet from
740°C to an annealing temperature of 750°C or higher and 900°C or lower in the
- 9 -
atmosphere same as that of the first temperature rising process, after the first
temperature rising process; an annealing process of allowing the steel sheet to be
retained in the atmosphere same as that of the second temperature rising process at the
annealing temperature for 30 seconds or longer and 300 seconds or shorter, after the
second temperature rising process; a cooling process of cooling the steel sheet after the
annealing process; and a galvannealing process comprising of; a plating process of
performing hot dip zinc plating on the steel sheet after the cooling process; and a
heating process of performing a heating on the steel sheet at a temperature of 420°C to
550°C after the plating process.
[0030]
(8) In the method of manufacturing a galvannealcd steel sheet described in (7),
the temperature in the heating process ma}' be 420°C or higher and 500°C or lower.
[0031]
(9) In the method of manufacturing a galvannealed steel sheet described in (7)
or (8), a heavy duty grinding process of performing a heavy duty grinding under a
• 0 0
condition of a grinding amount of 0.01 g/m to 3.00 g/m before the first temperature
rising process may further be included.
[0032]
(10) In the method of manufacturing a galvannealed steel sheet described in
any one of (7) to (9), an average cooling rate between 740°C and 650°C in the cooling
process may be 0.5 °C/sec or higher.
[0033]
(11) In the method of manufacturing a galvannealed steel sheet described in
any one of (7) to (10), the annealing process may be performed in all radiant tube
furnace of a continuous hot dip coating facility.
- 10 -
[0034]
(12) In the method of manufacturing a galvannealed steel sheet described in
any one of (7) to (11), the steel sheet may be immersed in a molten zinc bath which
contains 0.01% or more and 1.00% or less of Al and has a bath temperature of 430°C
or higher and 500°C or lower in the plating process.
[0035]
(13) In the method of manufacturing a galvannealed steel sheet described in
any one of (7) to (12), in the heating process, an average temperature rising rate
between 420°C and 460°C may be 20 °C/sec or higher and 100 °C/sec or lower, and an
average temperature rising rate from 460°C to 550°C may be 2 °C/sec or higher and
40 °C/sec or lower.
[Effects of the Invention]
[0036]
According to the aspects of the present invention, a galvannealed steel sheet
in which coating adhesion is enhanced compared to that in the related art can be
provided.
[Brief Description of the Drawings]
[0037]
FIG. 1A is a view schematically showing a mechanism of significantly
enhancing coating adhesion and is a view showing an aspect in which zinc plating is
performed on a steel sheet having a fine structure in which oxides arc present in grain
boundaries.
FIG. IB is a view schematically showing the mechanism of significantly
enhancing coating adhesion and is a view showing the form of a V-shaped (wedgeshaped)
Zn-Fe alloy phase generated in the vicinity of the oxides that are present in the
- 11 -
/
grain boundaries by reactions between Zn infiltrating from a coating layer and Fe in
the steel sheet (subsequent to FIG. 1 A).
FIG. 1C is a view schematically showing the mechanism of significantly
enhancing coating adhesion and is a view showing an aspect of a Zn-Fe coating layer
formed by the galvannealing (subsequent to FIG. IB).
FIG. 2A is a view showing a correlation between "the fine structure in which
the oxides are present in the grain boundaries" formed in the vicinity of the surface of
the steel sheet and the coating layer, and is a view schematically showing an aspect of
"the fine structure in which the oxides are present in the grain boundaries" formed in
the vicinity of the surface of the steel sheet.
FIG. 2B is a view showing a correlation between "the fine structure in which
the oxides are present in the grain boundaries" formed in the vicinity of the surface of
the steel sheet and the coating layer, and is a view schematically showing an aspect of
"the fine structure in which the oxides are present in the grain boundaries" after
coating.
FIG. 3 is a view showing the fine structure after annealing.
FIG. 4 is a view showing the fine structure after a galvannealing.
FIG. 5 is a view showing a £ phase generated when the galvannealing is
performed at a low temperature.
[Embodiments of the Invention]
[0038]
Hereinafter, a galvannealed steel sheet according to an embodiment of the
present invention will be described in detail.
[0039]
The galvannealed steel sheet according to the embodiment of the present
- 12 -
invention (hereinafter, also referred to as a coated steel sheet according to this
embodiment) includes: a steel sheet; a coating layer on the surface of the steel sheet;
and a mixed layer formed between the steel sheet and the coating layer, in which the
steel sheet contains, in terms of mass%, C: 0.050% or more and 0.50% or less, and
Mn: 0.01% or more and 3.00% or less, further contains one type or two or more types
of Si: 0.01% or more and 3.00% or less, Al: 0.010% or more and 2.00% or less, and
Cr: 0.01% or more and 2.00% or less, limits the amounts of P, S, O, N, Ti, Nb, Mo, Cu,
Ni, and B to P: 0.100% or less, S: 0.0200% or less, O: 0.0100% or less, N: 0.0100% or
less, Ti: 0.150% or less, Nb: 0.150% or less, Mo: 1.00% or less, Cu: 2.00% or less, Ni:
2.00% or less, and B: 0.0100% or less, satisfies the following Expression 1 when the
Mn content, the Si content, the Al content, and the Cr content are respectively
expressed by [Mn], [Si], [Al], and [Cr] in terms of mass%, and contains a remainder
including Fe and unavoidable impurities, the coating layer is a galvannealed layer
containing, in terms of mass%, Fe: 7.0% or more and 15.0% or less, Al: 0.01% or more
and 1.00% or less, and a remainder including Zn and unavoidable impurities, and the
mixed layer includes a base iron portion having fine grains having a size of greater
than 0 um and equal to or smaller than 2 urn, a Zn-Fe alloy phase, and oxides
containing one or more types of Mn, Si, Ai, and Cr, and in the mixed layer, the oxides
and the Zn-Fe alloy phase are present in grain boundaries that form the fine grains and
the Zn-Fe alloy phase is tangled with the base iron portion.
[Mn]+ [Si] + [Al] + [Cr] > 0.4 ... (Expression 1)
[0040]
The thickness (mm) of the steel sheet subjected to zinc plating is not
particularly limited. Typically, the thickness of the steel sheet subjected to zinc
plating is 0.4 mm to 3.2 mm. However, in consideration of the load or productivity of
- 13 -
a rolling mill, the thickness is preferably 1.0 mm to 3.2 mm.
[0041]
First, a reason that the chemical composition of the steel sheet which is a
material to be coated (may also be referred to as a steel sheet according to this
embodiment) in the coated steel sheet according to this embodiment is limited will be
described. Here, % associated with the composition represents mass%.
[0042]
C: 0.050% or more and 0.5% or less
C is an effective element for ensuring the strength of steel. However, when
the C content is less than 0.050%, the strength enhancing effect may not be expected.
On the other hand, when the C content is more than 0.5%, weldability is deteriorated
and the utilization of the steel sheet of the present invention is degraded. Therefore,
the C content is 0.050% or more and 0.5% or less. The C content is preferably
0.100% or more and 0.4% or less.
[0043]
Mn: 0.01% or more and 3.00% or less
Mn is an effective element for ensuring the strength of steel. In addition, Mn
is an element that forms oxides which suppress coarsening of grains in the vicinity of
the surface of the steel sheet during annealing. However, when the Mn content is less
than 0.01%, an effect of added Mn may not be expected. On the other hand, when the
Mn content is more than 3.00%, weldability is deteriorated and the utilization of the
steel sheet of the present invention is degraded. Therefore, the Mn content is 0.01%
or more and 3.00% or less. The Mn content is preferably 0.07% or more and 3.00%
or less,
[0044]
- 14 -
Furthermore, the steel sheet needs to contain one type or two or more types
selected from Si, Al, and Cr in the following ranges.
[0045]
Si: 0.01% or more and 3.00% or less
Si is an element that ensures the strength of steel. In addition, Si is an
element that forms oxides which limit coarsening of grains in the vicinity of the
surface of the steel sheet during annealing. In order to obtain this effect, 0.01% or
more of Si needs to be contained in the steel. Therefore, the lower limit of the Si
content in a case where Si is added is 0.01%o. On the other hand, when the Si content
is more than 3.00%, coarse oxides are generated, and the coating layer is easily peeled
off. Therefore, the upper limit of the Si content is 3.00%. The upper limit of Si
content is preferably 2.00%.
[0046]
Al: 0.010% or more and 2.00% or less
Al is an element that deoxidizes steel. In addition, Al is an element that
forms oxides which limit coarsening of grains in the vicinity of the surface of the steel
sheet during annealing. In order to obtain this effect, 0.010% or more of Ai needs to
be contained in the steel. Therefore, the lower limit of the AI content in a case where
Al is added is 0.010%). On the other hand, when the Al content is more than 2.00%o,
coarse inclusions and oxides are generated, workability is degraded, and the coating
layer is easily peeled off. Therefore, the upper limit of the Al content is 2.00%>.
From the viewpoint of ensuring high workability, a preferable upper limit thereof is
1.50%.
[0047]
Cr: 0.01% or more and 2.00% or less
- 15 -
Cr is an effective element for ensuring the strength of steel without damaging
the workability, particularly, the elongation of the steel sheet. In addition, Cr is an
element that forms oxides which limit coarsening of grains in the vicinity of the
surface of the steel sheet during annealing. In order to obtain this effect, 0.01% or
more of Cr needs to be contained in the steel. Therefore, the lower limit of the Cr
content in a case where Cr is added is 0.01%. On the other hand, when the Cr content
is more than 2.00%, the grain boundaries are embrittled due to boundary segregation,
and the alloying rate is reduced. Therefore, the upper limit of the Cr content is 2.00%.
A preferable upper limit thereof is 1.50%.
[0048]
Mn + Si + Al + Cr: 0.400% or more
As described above, all of Mn, Si, Al, and Cr are elements that form oxides
which limit coarsening of grains in the vicinity of the surface of the steel sheet during
annealing. However, when Mn + Si + Al + Cr is less than 0.400%, the amount of
generated oxides is insufficient, and grains in the vicinity of the surface of the steel
sheet are coarsened. Accordingly, a desired fine structure is not obtained. Therefore,
Mn + Si + Al + Cr is more than 0.400%. Mn + Si + Al + Cr is preferably 0.900% or
more. The upper limit thereof is not particularly limited and may be the sum of the
upper limits of the elements. However, in order to limit excessive generation of
oxides, the upper limit thereof is preferably 6.000% or less.
[0049]
Here, oxides which limit coarsening of grains as described above are oxides
of Mn, Si, Al, or Cr, or composite oxides containing two or more types of Mn, Si, Al,
and Cr.
[0050]
- 16 -
Examples of the oxides include Si oxides, Mn oxides, Si-Mn oxides, Al
oxides, Al-Si composite oxides, Al-Mn composite oxides, Al-Si-Mn composite oxides,
Cr oxides, Cr-Si composite oxides, Cr-Mn composite oxides, Cr-Si-Mn composite
oxides, Cr-Al composite oxides, Cr-AI-Si composite oxides, Cr-Al-Mn composite
oxides, and Cr-Al-Mn-Si composite oxides. In addition, the oxides may also contain
Fe.
[0051]
The size of the oxides is preferably not greater than 1 um in terms of average
equivalent circle diameter so as not to deteriorate the elongation, and is preferably 10
nm or greater in order to exhibit an effect of limiting the movement of the grain
boundaries of the steel sheet. The size of the oxides may be obtained by observing a
cross-section polished sample at a SEM (scanning electron microscope) magnification
of 50,000-fold and obtaining equivalent circle diameters through image analysis. The
number of oxides is not particularly limited, and it is preferable that one or more
oxides are present in a length of 100 um of the cross-section at a depth d (um) in the
through-thickness direction during the cross-sectional observation.
[0052]
The steel sheet according to this embodiment is based on the composition
containing the above-mentioned elements and the remainder including iron and
unavoidable impurities. However, the steel sheet may further contain P, S, O, N, Ti,
Nb, Mo, Cu, Ni, and B in the following content ranges as necessary. The lower limits
of the elements are 0%. However, in order to obtain desired effects, the following
lower limits may be employed.
[0053]
P: 0.100% or less
- 17 -
P is an element that increases the strength of steel and is also an element that
segregates to a thickness center portion of the steel sheet and causes embrittlement of
welds. Therefore, the P content is limited to 0.100%o or less. The P content is
preferably 0.080% or less. The lower limit thereof is not particularly limited.
However, in order to ensure an effect of enhancing strength, the steel preferably
contains 0.001% or more of P.
[0054]
S: 0.0200% or less
S has an adverse effect on weldability and manufacturability during casting
and hot rolling. Therefore, the upper limit of the S content is 0.0200%. In addition,
S is bonded to Mn and forms coarse MnS and thus reduces ductility and stretch
flangeability. Therefore, the upper limit thereof is preferably 0.0050% or less and
more preferably 0.0025% or less. The effects of the present invention are exhibited
even when the lower limit of the S content is not particularly defined. However,
setting the S content to be less than 0.0001% causes a significant increase in
manufacturing costs, and thus the lower limit thereof is preferably is 0.0001% or more.
[0055]
O: 0.0100% or less
O forms oxides and deteriorates ductility and stretch flangeability, and thus
the O content needs to be limited. When the O content is more than 0.0100%, stretch
flangeability is significantly deteriorated, and thus the upper limit of the O content is
0.0100%. The O content is preferably 0.0080% or less and more preferably 0.0060%
or less. The effects of the present invention are exhibited even when the lower limit
of the O content is not particularly defined. However, setting the O content to be less
than 0.0001% causes a significant increase in manufacturing costs, and thus the lower
- 18 -
limit thereof is preferably 0.0001% or more.
[0056]
N: 0.0100% or less
N forms coarse nitrides and deteriorates ductility and stretch flangeability, and
thus the N content needs to be limited. When the N content is more than 0.0100%,
this tendency becomes significant, and thus the range of the N content is set to be
0.0100% or less. In addition, N causes the generation of blowholes during welding
and thus N content is preferably as small as possible. The effects of the present
invention are exhibited even wheii the lower limit of the N content is not particularly
defined. However, setting the N content to be less than 0.0001% causes a significant
increase in manufacturing costs, and thus the lower limit thereof is preferably 0.0001%
or more.
[0057]
Ti: 0.150% or less
Ti is an element which contributes to an increase in the strength of the steel
sheet (base metal steel sheet) which is a material to be coated, due to precipitate
strengthening, fine grain strengthening through the limitation of the growth of fcrrite
grains, and dislocation strengthening through the limitation of recrystallization.
However, when the Ti content is more than 0.150%, a large amount of carbonitrides
are precipitated, and thus formabilily is deteriorated. Therefore, the Ti content is
preferably 0.150% or less. From the viewpoint of formability, the Ti content is more
preferably 0.120% or less, and even more preferably 0.100% or less. The effects of
the present invention are exhibited even when the lower limit of the Ti content is not
particularly defined. However, in order to sufficiently obtain the effect of increasing
strength by Ti, the Ti content is preferably 0.005% or more. For an increase in the
- 19 -
strength of the base metal steel sheet, the Ti content is more preferably 0.010% or more,
and even more preferably 0.015% or more.
[0058]
Nb: 0.150% or less
Nb is an element which contributes to an increase in the strength of the base
metal steel sheet due to precipitate strengthening, fine grain strengthening through the
limitation of the growth of ferrite grains, and dislocation strengthening through the
limitation of recrystallization. However, when the Nb content is more than 0.150%, a
large amount of carbonitrides are precipitated, and thus formability is deteriorated.
Therefore, the Nb content is preferably 0.150% or less. From the viewpoint of
formability, the Nb content is more preferably 0.120% or less, and even more
preferably 0.100% or less. The effects of the present invention are exhibited even
when the lower limit of the Nb content is not particularly defined. However, in order
to sufficiently obtain the effect of increasing strength by Nb, the Nb content is
preferably 0.005% or more. For an increase in the strength of the base metal steel
sheet, the Nb content is more preferably 0.010% or more, and even more preferably
0.015% or more.
[0059]
Mo: 1.00% or less
Mo is an element which limits phase transformation at a high temperature and
is effective in increasing strength. Therefore, Mo may be added instead of a portion
of C and/or Mn. When the Mo content is more than 1.00%, hot workability is
damaged and thus productivity is reduced. Therefore, the Mo content is preferably
1.00% or less. The effects of the present invention are exhibited even when the lower
limit of the Mo content is not particularly defined. However, in order to sufficiently
- 20 -
obtain the effect of increasing strength by Mo, the Mo content is preferably 0.01% or
more.
[0060]
Cu: 2.00% or less
Cu is an element which is present in steel as fine particles and increases
strength. Therefore, Cu may be added instead of a portion of C and/or Mn. When
the C.u content is more than 2.00%, weldabihty is damaged, and thus the Cu content is
preferably 2.00% or less. The effects of the present invention are exhibited even
when the lower limit of the O content is not particularly defined. However, in order
to sufficiently obtain the effect of increasing strength by Cu, the Cu content is
preferably 0.01% or more.
[0061]
Ni: 2.00% or less
Ni is an element which limits phase transformation at a high temperature and
is effective in increasing strength. Therefore, Ni may be added instead of a portion of
C and/or Mn. When the Ni content is more than 2.00%, weldabihty is damaged, and
thus the Ni content is preferably 2.00% or less. The effects of the present invention
are exhibited even when the lower limit of the Ni content is not particularly defined.
However, in order to sufficiently obtain the e 11 eel of increasing strength by Ni, the Ni
content is preferably 0.01% or more.
[0062]
B: 0.0100% or less
B is an element which strengthens grain boundaries and improves secondary
workability. However, B is also an element that deteriorates coating properties.
Therefore, the upper limit thereof is 0.0100%, and preferably 0.0075%. The lower
- 21 -
limit thereof is not particularly limited, and is preferably 0.0001% or more in order to
ensure the improvement effect.
[0063]
The effects of the present invention are exhibited even when the steel sheet
according to this embodiment fiirther contains, as unavoidable impurity elements other
than the above-mentioned elements, one type or two or more types of W, Co, Sn, V, Ca,
and REM.
[0064]
Next, a reason that the composition of the coating layer formed on the surface
of the steel sheet in the coated steel sheet according to this embodiment is limited will
be described. Here, % associated with the composition represents mass%.
[0065]
Fe: 7.0% or more and 15.0% or less
When the Fe content in the coating layer is less than 7.0%, portions which are
not alloyed are generated and thus the appearance of the surface is poor, and flaking
resistance during pressing is deteriorated. On the other hand, when the Fc content in
the coating layer is more than 15.0%, over-alloyed portions are generated, and
powdering resistance during pressing is deteriorated. Therefore, the Fe content (Fe
concentration) in the coating layer is 7.0% or more and 15.0% or less. Here, the Fe
content in the coating layer indicates the ratio (mass%) of contained Fe, in a case
where the sum of the coating amounts of the Zn-Fe alloy phase that is present in the
galvannealed layer and the mixed layer, is the denominator.
[0066]
Al: 0.01% to 1.00%
When the Al content (Al concentration) in the coaling layer is less than 0.01%,
- 22 -
an alloying reaction of Zn and Fe excessively proceeds in the coating layer during the
manufacture of the steel sheet. In addition, when the AI content (Al concentration) in
the coating layer is more than 1,00%, an effect of limiting the Zn-Fe alloying reaction
by AI is significantly exhibited, and thus the line speed has to be reduced in order to
allow the Zn-Fe reaction to proceed, resulting in the deterioration of productivity.
Therefore, the Al content in the coating layer is 0.01% or more and 1.00% or less.
[0067]
In the coaled steel sheet according to this embodiment, the mixed layer, which
contains the base iron portion, the Fe-Zn phase, and the oxides containing one or more
types of Mn, Si, Al, and Cr, is formed between the above-described steel sheet and the
coating layer by the galvannealing.
[0068]
Next, the structural characteristics of the coated steel sheet according to this
embodiment will be described.
[0069]
When the galvannealed steel sheet is manufactured, in a case where annealing
is performed on the steel sheet which is a material to be coated in an all radiant tube
furnace (RTF) type line, by adjusting (he oxygen potential in the annealing furnace,
easily oxidizablc elements Mn, Si, Al, and Cr in the steel sheet can be oxidized and
form oxides while oxide films that are present on the surface of the steel sheet are
reduced.
[0070]
The structure of the steel sheet before annealing is typically an as rolled
structure and in many cases, the grains thereof are formed of fine grains of the
submicron order. When the fine structure is heated in the annealing furnace and
- 23 -
reaches a certain temperature, grain growth occurs and the grains gradually coarsen.
[0071]
However, when the oxygen potential or the temperature rising pattern in the
annealing furnace is adjusted, Mn, Si, Al, and Cr (easily oxidizable elements) in the
steel sheet can be preferentially oxidized (preferential oxidation) in the grain
boundaries of the steel sheet before the grains in the vicinity of the surface of the steel
sheet coarsen.
[0072]
Oxides generated through preferential oxidation limit the movement of the
grain boundaries. Therefore, by adjusting the oxygen potential or the temperature
rising pattern in the annealing furnace as described above, a fine structure in which
oxides are present in the grain boundaries can be formed while maintaining the fine
rolled structure in the vicinity of the surface of the steel sheet being fine.
[0073]
In the coated steel sheet according to this embodiment, hot dip zinc plating is
performed on the steel sheet after annealing. Accordingly, the coating layer is formed
on the surface of the steel sheet. Moreover, in the coated steel sheet according to this
embodiment, the heating is performed on the steel sheet having the coating layer. By
the heating, the mixed layer is formed between the steel sheet and the alloyed coating
layer (galvannealed layer). The mixed layer is formed as Zn infiltrates into the grain
boundaries of the fine structure of the steel sheet from the coating layer. Therefore,
the mixed layer includes the base iron portion (the steel sheet portion), the Zn-Fe alloy
phase, and the oxides formed in the grain boundaries of the steel sheet during
annealing. In addition, the Zn-Fe alloy phase in the mixed layer is generated as Zn
infiltrates from the coating layer into the grain boundaries of the fine structure in the
- 24 -
steel sheet obtained due to the action of limiting the grain growth of the oxides formed
during annealing and Zn infiltrating from the coating layer reacts with Fe in the steel
sheet. In addition, since the Zn-Fe alloy phase in the mixed layer is formed along the
grain boundaries of the steel sheet, the Zn-Fe alloy phase and the base iron portion are
in a tangled shape. Therefore, the adhesion between the steel sheet and the coating
layer is significantly enhanced. Particularly, in the coated steel sheet according to this
embodiment, the Zn-Fe alloy phase in the mixed layer preferably has a shape that
protrudes in a V-shape (so-called wedge shape) toward the thickness center of the steel
sheet from the coating layer when viewed in a cross-section in the through-thickness
direction. This adhesion enhancing mechanism will be described with reference to
the drawings.
[0074]
FIGS. lAto IC schematically illustrate a mechanism of significantly
enhancing coating adhesion. FIG. 1A shows an aspect in which zinc plating is
performed on the steel sheet having the fine structure in which the oxides are present in
the grain boundaries (containing the oxides). FIG. IB shows an aspect of the wedgeshaped
Zn-Fe alloy phase generated in the vicinity of the oxides that arc present in the
grain boundaries by reactions between Zn infiltrating from the coating layer and Fe in
the steel sheet. FIG IC shows an aspect of a Zn-Fe coating layer (alloy coating layer)
formed by the galvannealing.
[0075]
As shown in FIG. 1 A, hot dip zinc plating is performed on the steel sheet
having a fine structure 1 in which oxides 4 are present in the grain boundaries, thereby
forming a coating layer 2. The oxides 4 are present in most of the grain boundaries,
and Zn easily infiltrates into the grain boundaries in which the oxides 4 are present
- 25 -
from the coating layer 2. By a heating after the coating, Zn infiltrating from the
coating layer 2 is bonded to Fc in the steel sheet in some of the grain boundaries in
which the oxides 4 are present. In addition, as shown in FIG. IB, the Zn-Fe alloy
phase (intermetallic compounds) 5 which is present between the steel sheet and the
coating layer and has a shape that protrudes in a V-shape (wedge shape) toward the
steel sheet is formed in the around of the oxides 4.
[0076]
Furthermore, as shown in FIG. 1C, as the heating proceeds, the coating layer 2
is alloyed from a side close to the interface with the steel sheet, thereby forming an
alloy coating layer (galvannealed layer) 3. In addition, the alloy coating layer 3
incorporates the fine structure 1 in the vicinity of the surface of the steel sheet and
grows toward the steel sheet. This region becomes a mixed layer 13 described above.
The inventors found that the mixed layer 13 is present between the alloy coating layer
and the steel sheet, in the mixed layer since the Zn-Fe alloy phase 5 (intermetallic
compounds) is tangled with a base iron portion 11, the alloy coating layer 3 and the
steel sheet are securely bonded to each other, and thus the adhesion between the alloy
coating layer 3 and the steel sheet is dramatically increased. This point is the finding
for the base of the present invention.
[0077]
As described above, by performing the heating, the Zn-Fe alloy phase is
generated in not only the mixed layer 13 but also the alloy coating layer 3. It is
preferable that the Zn-Fe alloy phase in the mixed layer is formed as described above.
Furthermore, the inventors also found that when the Zn-Fe alloy phase in a coating
surface layer region which is a region of 1 um or smaller from the surface of the alloy
coating layer 3 (on the opposite side to the steel sheet) in the structure 3 is a Zn-Fe
- 26 -
alloy phase which contains a C, phase that does not contain the oxides, strength in
adhesion to other members can be further increased.
[0078]
As described above, a portion of the fine structure in the vicinity of the
surface of the steel sheet is incorporated into the alloy coating layer from the surface
side of the steel sheet and becomes the mixed layer by the heating. The inventors
found that controlling the process of internal oxidation by adjusting the annealing
atmosphere and the heating rate is important to form the mixed layer. The adjustment
of the annealing atmosphere and the heating rate will be described later.
[0079]
In the steel sheet, when the fine structure in which the oxides are present in
the grain boundaries is formed to a certain thickness, alloying of the interface between
the steel sheet and the coating layer proceeds rapidly and a desired mixed layer is
obtained after the heating ends.
[0080]
FIGS. 2A and 2B illustrate a correlation between "the'fine structure in which
the oxides are present in the grain boundaries" formed in the vicinity of the surface of
the steel sheet and the coating layer. FIG. 2A schematically shows an aspect of "the
fine structure in which the oxides arc present in the grain boundaries" formed in the
vicinity of the surface of the steel sheet, and FIG. 2B schematically shows an aspect of
"the fine structure in which the oxides are present in the grain boundaries" in the
mixed layer.
[0081]
When the coating layer is formed and the heating is performed on the surface
of the steel sheet shown in FIG. 2 A, the alloy coating layer incorporates "the fine
- 27 -
structure in which the oxides are present in the grain boundaries" and grows toward the
steel sheet as shown in FIG. 2B. As a result, in the coated steel sheet according to this
embodiment, the mixed layer including "the fine structure in which the oxides are
present in the grain boundaries" is formed. In addition, the Zn-Fe phase is formed in
the grain boundaries.
[0082]
The "wedge-shaped Zn-Fe alloy phase" that is present in the grain boundaries
of "the fine structure in which the oxides are present in the grain boundaries" in the
mixed layer has a function of structurally connecting the alloy coating layer and the
steel sheet to each other, and thus the coating adhesion of the steel sheet of the present
invention is dramatically enhanced.
[0083]
In order to ensure significant enhancement of the coating adhesion, in the
coated steel sheet according to this embodiment, the above-described mixed layer is
formed between the steel sheet and the coating layer. In addition, the abovedescribed
mixed layer is formed to include the base iron portion having fine grains
(fine structure) having a size of greater than 0 urn and equal to or smaller than 2 um,
the Zn-Fe alloy phase, and oxides containing one or more types of Mn, Si, Al, and Cr.
Furthermore, in the above-described mixed layer, the oxides and the Zn-Fc alloy phase
are present in the grain boundaries that form the fine grains, and the Zn-Fc alloy phase
is formed in a shape of being tangled with the base iron portion.
[0084]
As described above, the structure of the steel sheet before annealing is
typically an as rolled structure and in many cases, the grains thereof are formed of fine
grains of the submicron order. On the basis of this, in order to form a sufficient
- 28 -
amount of "wedge-shaped Zn-Fe alloy phase" in the grain boundaries in the mixed
layer, the fine structure of the base iron portion was specified to a fine structure having
fine grains with a grain size of 2 um or smaller. The grain size of the fine structure is
preferably 1 um or smaller. In addition, although the lower limit thereof does not
need to be particularly specified, due to the necessity for the presence of the fine
structure, the lower limit thereof is greater than 0 um.
[0085]
The mixed layer 13 is more brittle than the steel sheet 1 and the alloy coating
layer 3. Therefore, when the thickness of the mixed layer is greater than 10 um,
cracking easily occurs during bending. Therefore, the thickness of the mixed layer is
preferably 10 um or smaller.
[0086]
In order to ensure sufficient bendability, the thickness of the mixed layer is
more preferably 5 urn or smaller.
[0087]
In order to obtain sufficient coating adhesion, when 10 or more visual fields
of the mixed layer are observed along the interface between the mixed layer and the
coating layer by using a scanning electron microscope at a magnification of 5,000-fold,
one or more fine grains having the grain boundaries in which the Zn-Fe alloy phase is
present are observed in 20% or greater of the entirety of the observed visual fields.
In a case where the ratio of the visual fields in which the fine grains having
the grain boundaries in which the Zn-Fe alloy phase is present are observed is lower
than 20%, sufficient coating adhesion can be ensured in a press working range in
which a typical vehicle internal plate is postulated. However, in a case where more
heavy duly work such as bending, unbending, or sliding is applied, for example, a
- 29 -
vehicle external plate is postulated, there is concern that coating adhesion may be
insufficient, and thus there is a possibility that applications and uses of the present
invention may be limited, which is not preferable.
[0088]
In a case of further enhancing adhesion strength, it is preferable that by
reducing the temperature in the heating process, the surface layer region on the coating
layer which is a region of 1 um or smaller from the surface of the coating layer has the
Zn-Fe alloy phase which contains a £ phase 21 that does not contain the oxides, as
shown in FIG. 5.
In the Zn-Fe alloy phase, the C, phase is relatively soft and does not contain the
oxides, and thus has a certain degree of deformability. Therefore, when stress is
applied to the surface layer of the coating layer, the surface layer region on the coating
layer can be deformed to a certain degree. Accordingly, when the surface layer
region on the coating layer is adhered to another member with an adhesive, the
adhesion to the member is dense.
The reason that the £ phase does not contain oxides is not clear. However, it
is thought that the C, phase is not generated during the heating, and a Zn-Fe alloy phase
containing the C, phase is precipitated by a reaction between Fe eluted from the surface
of the steel sheet into a molten zinc bath during immersion into the molten zinc bath
and Zn in the bath.
[0089]
Next, a method of manufacturing the coated steel sheet according to this
embodiment will be described.
[0090]
The manufacturing of the coated steel sheet according to this embodiment
- 30 -
includes: a first temperature rising process of heating the steel sheet having the abovedescribed
composition in an atmosphere which contains 0.1 vol.% or more and 50
vol.% or less of hydrogen and the remainder including nitrogen and unavoidable
impurities and has a dew point of higher than -30°C and equal to or lower than 20°C at
a first temperature rising rate of 0.2 °C/sec or higher and 6 °C/sec or lower, which is an
average temperature rising rate between 650°C and 740°C; a second temperature rising
process of heating the steel sheet from 740°C to an annealing temperature of 750°C or
higher and 900°C or lower in the atmosphere same as that of the first temperature
rising process, after the first temperature rising process; an annealing process of
allowing the steel sheet to be retained in the atmosphere same as that of the second
temperature rising process at the annealing temperature for 30 seconds or longer and
300 seconds or shorter after the second temperature rising process; a cooling process of
cooling the steel sheet after the annealing process; and a galvannealing process
comprising of; a plating process of performing hot dip zinc plating on the steel sheet
after the cooling process; and a heating process of performing a heating on the steel
sheet at a temperature of 420°C to 550°C after the plating process.
[0091]
It is preferable that (he annealing is performed in an all radiant tube furnace of
a continuous hot dip coaling facility. The reduction annealing atmosphere before
coating is an atmosphere in which the ratio of hydrogen to the atmosphere gas is 0.1
vol.% to 50 vol.% and the remainder contains nitrogen and unavoidable impurities.
When the hydrogen content is less than 0.1 vol.%, oxide films that are present on the
surface of the steel sheet cannot be sufficiently reduced, and coating wettability cannot
be ensured. Therefore, the hydrogen content of the reduction annealing atmosphere is
0.1 vol.% or more.
- 31 -
[0092]
When the hydrogen content in the reduction annealing atmosphere is more
than 50 vol.%, the dew point (corresponding to the water vapor pressure PH2O) thereof
is excessively increased, and thus there is a need to introduce a facility that prevents
dew condensation. The introduction of a new facility leads to an increase in
production cost, and thus the hydrogen content of the reduction annealing atmosphere
is 50 vol.% or less. The hydrogen content is preferably 0.1 vol.% or more and 40
vol.% or less.
[0093]
The dew point of the annealing reduction atmosphere is higher than -30°C and
equal to or lower than 20°C. When the dew point thereof is -30°C or lower, it
becomes difficult to ensure a necessary oxygen potential for internally oxidizing easily
oxidizable elements such as Si and Mn are in steel. The dew point thereof is
preferably -25°C or higher. On the other hand, when the dew point thereof is higher
than 20°C, dew concentration significantly occurs in a pipe through which the
reduction gas flows, and thus stable atmosphere control is difficult. Therefore, the
dew point thereof is 20°C or lower. . The dew point thereof is preferably 15°C or
lower.
[0094]
Furthermore, it is preferable that the log(PH20/PH2) of the reduction
annealing atmosphere is adjusted to be 0 or lower. When the log(PH20/PH2) thereof
is increased, alloying is accelerated. However, when the log(PH20/PH2) thereof is
higher than 0, oxides that arc generated on the surface of the steel sheet before
annealing cannot be sufficiently reduced. As a result, coating wettability cannot be
ensured. Therefore, the upper limit of the log(PH20/PH2) thereof is preferably 0.
- 32 -
The upper limit thereof is more preferably -0.1 or lower.
[0095]
The composition and the dew point of the reduction annealing atmosphere and
the heating rate and the annealing temperature of the steel sheet are important to allow
the oxides and the Zn-Fe alloy phase to be present in the grain boundaries that form the
fine grain in the mixed layer and to form the mixed layer in which the Zn-Fe alloy
phase in the mixed layer is tangled with the base iron portion.
[0096]
The steel sheet is heated in the reduction annealing atmosphere at a first
temperature rising rate of 0.2 °C/sec or higher and 6 °C/sec or lower, which is an
average temperature rising rate between 650°C and 740°C (first temperature rising
process). After the first temperature rising process, the steel sheet is heated from
740°C to an annealing temperature of 750°C or higher and 900°C or less in the
atmosphere (second temperature rising process). When the first temperature rising
rate (heating rate) is higher than 6 °C/sec, the temperature rising rate is too high and
grains in the steel sheet are coarsened before internal oxidation sufficiently proceeds.
Accordingly, a structural morphology needed for the present invention is not obtained.
Therefore, the first temperature rising rale is 6 °C/scc or lower. The first temperature
rising rate is preferably 4 °C/scc or lower. The lower limit thereof is preferably
0.2 °C/sec or higher from the viewpoint of productivity.
The temperature rising rate in the second temperature rising process does not
need to be particularly limited. However, from the viewpoint of productivity, it is
preferable that the temperature rising rate is equal to or higher than 0.2 °C/sec and
equal to or lower than the upper limit of the facility ability. By controlling the
heating rate to 740°C as described above, oxides are generated in a region which is to
- 33 -
become the mixed layer when coating is performed in a subsequent process before
transformation due to internal oxidation in ferrite having a high diffusion rate.
Therefore, it is thought that the above-described mixed layer can be generated.
[0097]
After the second temperature rising process, annealing in which the steel sheet
is retained at an annealing temperature of 750°C or higher and 900°C or less for 30
seconds or longer and 300 seconds or shorter is performed (annealing process). Here,
retention does not represent only isothermal holding and may also allow a temperature
change in the above temperature range. When the annealing temperature is lower
than 750°C, the oxides film generated on the surface of the steel sheet before the
annealing cannot be sufficiently reduced, and there may be cases where coating
wettability cannot be ensured. When the annealing temperature is higher than 900°C,
press formability is deteriorated, and a necessary heat amount for heating is increased,
resulting in an increase in manufacturing costs. In addition, at an annealing
temperature of 900°C or higher, coarsening of the grains is likely to significantly
proceed, and there is concern that the fine structure that is formed on the surface of the
steel sheet once may be dissipated. Therefore, the annealing temperature is 750°C or
higher and 900°C or lower. A preferable annealing temperature is 760°C or higher
and 880°C or lower.
[0098]
After the annealing process, cooling is performed (cooling process). The
cooling rate is not particularly limited. However, from the viewpoint of material
properties, an average cooing rate between 740°C and 650°C is 0.5 °C/sec or higher.
When the upper limit of the cooling rate is 20 °C/sec, the grain boundaries in a region
which becomes the mixed layer when coating is subsequently performed are likely to
- 34 -
undergo component segregation and subsequently, the mixed layer is easily generated.
Therefore, it is preferable that an average cooing rate between 740°C and 650°C is
0.5 °C/scc or higher and 20 °C/sec or lower. The average cooing rate is more
preferably 15 °C/sec or lower and even more preferably 6 °C/sec or lower.
[0099]
Regarding the coated steel sheet according to this embodiment, hot dip zinc
plating is performed on the steel sheet subjected to the cooling after the annealing in
order to form the coating layer (plating process). It is preferable that hot dip zinc
plating is performed by using a molten zinc bath containing 0.01% to 1.00% of Al at a
bath temperature of 430°C to 500°C.
[0100]
When the Al content is less than 0.01%, the Zn-Fe alloy layer in the molten
zinc bath rapidly grows, there may be cases where a desired coating layer cannot be
formed, for example, the Fe concentration in the coating layer is excessively increased,
only by controlling an immersion time depending on the steel type. In addition, the
amount of bottom dross generated in the molten zinc bath is increased, and surface
defects caused by the dross are generated. Therefore, there is concern that failure in
the external appearance may occur in the steel sheet.
[0101]
On the other hand, when the Al content is more than 1.00%, an effect of
limiting the Zn-Fe alloying reaction by Al is significantly exhibited, and thus the line
speed has to be reduced in order to allow the Zn-Fe reaction to proceed, resulting in the
deterioration of productivity.
[0102]
When the bath temperature of the molten zinc bath is lower than 430°C, since
- 35 -
the melting point of zinc is about 420°C, there is concern that bath temperature control
is unstable and a portion of the bath may solidify. When the bath temperature thereof
is higher than 500°C, the life span of facilities such as a sink roll or a zinc pot is
reduced. Therefore, the bath temperature of the molten zinc bath is preferably 430°C
to 500°C. The bath temperature thereof is more preferably 440°C to 480°C.
[0103]
A coating amount is not particularly limited, and is preferably 1 um or greater
in terms of one surface coating amount from the viewpoint of corrosion resistance. In
addition, the one surface coating amount is preferably 20 urn or smaller from the
viewpoint of workability, weldability, and economic efficiency.
[0104]
The heating is performed at 420°C to 550°C (heating process). When the
temperature in the heating process is lower than 420°C, the progress of alloying is
delayed, and there is a possibility that a Zn layer may remain on the coating surface
layer. The temperature in the heating process is preferably 450°C or higher. On the
other hand, when the temperature in the heating process is higher than 550°C, alloying
excessively proceeds, and a V phase which is brittle is thickened at the interface
between the coating and the steel sheet, and thus coating adhesion during work is
degraded.
[0105]
It is preferable that, during the heating, the average temperature rising rate
from 420°C to 460°C is 20 °C/sec or higher and 100 °C/sec or lower, and the average
temperature rising rate from 460°C to the 550°C is 2 °C/sec or higher and 40 °C/sec or
lower.
By performing heating at such a temperature rising rate, the C, phase is easily
- 36 -
formed on the surface layer of the coating layer.
Here, in a case where the temperature in the heating process is 460°C or lower,
the average temperature rising rate from 420°C to the temperature in the heating
process may be 20 °C/sec or higher and 100 °C/sec or lower.
[0106]
In a case where the £ phase is formed on the surface layer of the coating layer
in order to enhance strength in adhesion to other members, the temperature in the
heating process is preferably 420°C or higher and 500°C or lower. When the
temperature in the heating process is higher than 500°C, the £ phase becomes unstable
and is divided into a Si phase and a Zn phase.
[0107]
Furthermore, it is preferable to provide a heavy duty grinding process of
performing heavy duty grinding before the first temperature rising process. By
performing heavy duty grinding, the grain size of the base iron fine grains in the mixed
layer can be further refined.
As for the heavy duty grinding conditions, the grinding amount is preferably
in a range of 0.01 g/m to 3.00 g/m . When the grinding amount is smaller than 0.01
g/m , an effect of refining the base iron grains by the heavy duty grinding is not
exhibited. When the grinding amount is greater than 3.00 g/m , there is a possibility
that the external appearance may be adversely affected. Even when the heavy duty
grinding is performed, the roughness of the base iron formed during the heavy duty
grinding is smoothened through subsequent processes from annealing to hot dip zinc
plating. That is, when the mixed layer is formed as described in this specification, Fe
of the steel sheet diffuses into the zinc coating and moves toward the interface between
iron and the coating as shown in FIG. 1. Therefore, even when heavy duty grinding is
- 37 -
performed, the convex-concave portions (roughness) of the surface of the steel sheet
are not maintained while being in the state after the heavy duty grinding.
In addition, the surface of the steel sheet undergoes strong shearing and is
plastically deformed by heavy duty grinding, and thus a large amount of dislocations
are introduced and the diffusion speed of atoms is increased. As a result, it is thought
that internal oxidation further proceeds in ferrite.
[0108]
In addition, performing coating of an upper layer on the coated steel sheet of
the present invention for the purpose of improving coating properties and weldability,
or performing various chemical conversion processes such as a phosphate treatment, a
weldability enhancing treatment, and a lubricity enhancing treatment does not depart
from the present invention.
[Examples]
[0109]
Next, Examples of the present invention will be described. The conditions
of Examples are only a conditional example employed to check the applicability and
effects of the present invention, and the present invention is not limited to the
conditional example. The present invention can employ various conditions without
departing from the spirit of the present invention as long as the object of the present
invention is accomplished.
[0110]
(Example)
Cold-rolled steel sheets having a thickness of 0.4 mm to 3.2 mm and
compositions shown in Table 1 were used as starting sheets, and galvannealed steel
sheets were manufactured by using a vertical type hot dip coating simulator.
- 38 -
Reduction annealing conditions before coating are shown in Table 2. The maximum
arrival temperature was 800°C, and the holding temperature at the maximum arrival
temperature was 100 seconds.
[0111]
The steel sheet was cooled to 450°C in nitrogen gas subsequently to annealing
and was immersed in a molten zinc bath containing 0.13% of AI for 3 seconds. The
temperature of the molten zinc bath was 450°C which was the same as the temperature
at which the steel sheet enters the bath.
[0112]
After coating, the zinc coating amount was adjusted to 5 uin to 15 um by a
gas wiper, and a heating process was performed. The temperature in the heating
process was a temperature shown in Table 2, and the Fe amount in the coating layer
was set as shown in Table 2. After the heating, the steel sheet was cooled to room
temperature in the nitrogen gas. The composition of the coating layer was measured
by melting the coating layer with an acid and performing chemical analysis using ICP.
[0113]
In addition, observation of the structure of the interface between the coating
layer and the steel sheet was performed by processing the steel sheet that was cut into
10 mm x 10 mm using a cross-section polisher and thereafter observing 20 or more
visual fields of each sample at a magnification of 5,000-fold to 50,000-fold using an
FE-SEM. The obtained image data was subjected to image analysis, and for the
structure of the interface between the coating and the steel sheet on the steel sheet side,
grain sizes in a direction parallel to the initial interface of the steel sheet were
measured. A structure having a grain size of 2 urn or smaller was specified as a fine
structure.
- 39 -
[0114]
FIG. 3 shows a fine structure in which oxides are present in grain boundaries
after annealing, and FIG. 4 shows a fine structure in a mixed layer after the
galvannealing. It can be seen from FIG. 3 that the fine structure in which oxides are
present in grain boundaries is formed in the vicinity of the surface of the steel sheet.
In addition, it can be seen from FIG 4 that a mixed layer having the fine structure in
which oxides are present in grain boundaries is formed between the steel sheet and the
alloy coating layer.
[0115]
When a fine structure having grains with a grain size of 2 urn or smaller could
not be seen, the average grain size of the fine structure was not measured. In the
tables, regarding the average grain size of the fine structure, "-" represents that the fine
structure was not observed. In addition, from the image data, presence or absence of
the infiltration of the Zn-Fe alloy layer into the grain boundaries of the fine structure as
shown in FIG 1C was checked.
[0116]
For the steel sheets, powdering resistance, tensile strength, and adhesion
strength were examined. The results are shown in Table 2 together with the reduction
annealing conditions, the observation results of the interface structures, and the like.
In all of examples (Test Nos. 1 to 19, 21, 22, 27 to 32, 35 to 42, and 48) which
satisfied the conditions of the present invention, powdering resistance was excellent.
In a case where a £ phase was formed on the coating surface layer, higher
adhesion strength could be obtained.
- 40 -
[0119]
An evaluation method of powdering resistance was as follows.
[0120]
Powdering Resistance
The galvannealed steel sheet manufactured in the above-described method
was cut into a size of 40-mm width x 250-mm length, and was processed into a molded
height of 65 mm using a die of half-round beads having a size of r=5 mm with a punch
shoulder radius of 5 mm and a die shoulder radius of 5 mm. During the processing,
coating layers that were peeled off were measured and evaluated according to the
following criteria.
In Test No. 45, non-coating defects were generated.
[0121]
Evaluation Criteria
Amount of peeled coating:
smaller than 3 g/m2: VG (VERY GOOD)
3 g/m or greater and smaller than 6 g/m : G (GOOD)
6 g/m2 or greater and smaller than 10 g/m2: NG (NO GOOD)
[0122]
In addition, a tensile test was performed in a method according to JIS Z 2241
to obtain tensile strength.
[0123]
In addition, an evaluation method of adhesion strength was performed as
follows using a tensile shear test.
The galvannealed steel sheet manufactured in the above-described method
was cut into a size of 25-mm width* 100-mm length, two sheets having the size were
- 45 -
prepared, and an adhesive agent was applied to overlapping portions of the sheets to be
bonded to each other in a state where the sheets were shifted from each other by 12.5
mm in the through-length direction.
A commercially available epoxy-based adhesive was used as the adhesive
agent, and was applied to the adhesion surface of 25 mm x 12.5 mm so that a thickness
is about 100 um. The prepared test pieces were cooled and left for 5 hours, and were
pulled at a rate of 50 m/min in an atmosphere of 0°C for the tensile shear test. The
maximum load until breakage was measured, and the adhesion strength was measured
by using a tensile shear strength obtained by dividing the maximum load by a shear
area (adhesion area).
Evaluation Criteria
Tensile shear strength:
180 Kgf/mm2 or higher: VG
140 Kgf7mm2 or higher and lower than 180 Kgf/mm2: G
lower than 140 Kgf/mm2: NG
[Industrial Applicability]
[0124]
As described above, according to the present invention, a galvannealed steel
sheet having dramatically enhanced coating adhesion can be provided. Therefore, the
present invention is high applicable to the galvanized steel sheet manufacturing
industry.
[Brief Description of the Reference Symbols]
[0125]
1: FINE STRUCTURE (FINE GRAINS)
2: COATING LAYER
- 46 -
i
3: ALLOY COATING LAYER
4: OXIDE
5: Zn-Fe ALLOY PHASE
6: OXIDE FILM
11: BASE IRON PORTION
13: MIXED LAYER
21: C PH

[Document Type] CLAIMS
1. A galvannealed steel sheet comprising:
a steel sheet;
a coating layer on a surface of the steel sheet; and
a mixed layer formed between the steel sheet and the coating layer,
wherein the steel sheet contains, in terms of mass%,
C: 0.050% or more and 0.50% or less, and
Mn: 0.01% or more and 3.00% or less,
. further contains one type or two or more types of
Si: 0.01% or more and 3.00% or less,
Al: 0.010% or more and 2.00% or less, and
Cr: 0.01% or more and 2.00% or less,
limits amounts of P, S, O, N, Ti, Nb, Mo, Cu, Ni, and B to
P: 0.100% or less,
S: 0.0200% or less,
0:0.0100% or less,
N: 0.0100% or less,
Ti: 0.150% or less,
Nb: 0.150% or less,
Mo: 1.00% or less,
Cu: 2.00% or less,
Ni: 2.00% or less, and
B: 0.0100% or less,
satisfies the following Expression 1 when the Mn content, the Si
content, the Al content, and the Cr content are respectively expressed by [Mn], [Si],
- 48 -
[Al], and [Cr] in terms of mass%, and
contains a remainder including Fe and unavoidable impurities,
the coating layer is a galvanncaled layer containing, in terms of mass%,
Fe: 7.0% or more and 15.0% or less,
Al: 0.01% or more and 1.00% or less, and
a remainder including Zn and unavoidable impurities,
the mixed layer includes
a base iron portion having fine grains having a size of greater than 0
um and equal to or smaller than 2 urn,
a Zn-Fe alloy phase, and
oxides containing one or more types of Mn, Si, Al, and Cr, and
in the mixed layer, the oxides and the Zn-Fe alloy phase are present in grain
boundaries that form the fine grains, and the Zn-Fe alloy phase is tangled with the base
iron portion.
[Mn]+ [Si] + [Al] + [Cr] > 0.4 ...(Expression 1)
2. The galvannealed steel sheet according to claim 1,
wherein surface layer region on the coating layer which is a region of 1 um or
smaller from the surface of the coating layer is a Zn-Fe alloy phase which contains a t,
phase that does not contain the oxides.
3. The galvannealed steel sheet according to claim 1 or 2,
wherein an average thickness of the mixed layer in a direction along a
through-thickness direction of the steel sheet is 10 um or smaller.
4. The galvannealed steel sheet according to claim 1 or 2,
wherein the Zn-Fe alloy phase in the mixed layer has a shape that protrudes in
a V-shape toward a thickness center of the steel sheet from the coating layer when
- 49 -
viewed in a cross-section in the through-thickness direction of the steel sheet.
5. The galvannealed steel sheet according to claim 1 or 2,
wherein, when 10 or more visual fields of the mixed layer are observed along
an interface between the mixed layer and the coating layer by using a scanning electron
microscope at a magnification of 5,000-fold, the fine grains having the grain
boundaries in which the Zn-Fe alloy phase is present in the mixed layer are observed in
20% or greater of the entirety of the observed visual fields.
6. The galvannealed steel sheet according to claim 1 or 2,
wherein the Zn-Fe alloy phase in the mixed layer is generated by a reaction
between Zn infiltrating from the coating layer during a galvannealing and Fe in the
steel sheet.
7. A method of manufacturing a galvannealed steel sheet comprising;
a first temperature rising process of heating the steel sheet having the
composition according to claim 1, in an atmosphere which contains 0.1 vol.% or more
and 50 vol.% or less of hydrogen and a remainder including nitrogen and unavoidable
impurities and has a dew point of higher than -30°C and equal to or lower than 20°C at
a first temperature rising rate of 0.2 °C/sec or higher and 6 °C/sec or lower, which is an
average temperature rising rate between 650°C and 740°C;
a second temperature rising process of heating the steel sheet from 740°C to
an annealing temperature of 750°C or higher and 900°C or lower in the atmosphere
same as that of the first temperature rising process, after the first temperature rising
process;
an annealing process of allowing the steel sheet to be retained in the
atmosphere same as that of the second temperature rising process at the annealing
temperature for 30 seconds or longer and 300 seconds or shorter, after the second
- 50 -
temperature rising process;
a cooling process of cooling the steel sheet after the annealing process; and
a galvannealing process comprising of;
a plating process of performing hot dip zinc plating on the steel sheet after the
cooling process and
a heating process of performing a heating on the steel sheet at a temperature
of 420°C to 550°C after the plating process.
8. The method of manufacturing a galvannealed steel sheet according to claim 7,
wherein the temperature in the heating process is 420°C or higher and 500°C
or lower.
9. The method of manufacturing a galvannealed steel sheet according to claim 7
or 8, further comprising:
a heavy duty grinding process of performing a heavy duty grinding under a
condition of a grinding amount of 0.01 g/m to 3.00 g/m before the first temperature
rising process.
10. The method of manufacturing a galvannealed steel sheet according to claim 7,
wherein an average cooling rate between 740°C and 650°C in the cooling
process is 0.5 °C/sec or higher.
11. The method of manufacturing a galvannealed steel sheet according to claim 7,
wherein the annealing process is performed in all radiant tube furnace of a
continuous hot dip coating facility.
12. The method of manufacturing a galvannealed steel sheet according to claim 7
or8,
wherein the steel sheet is immersed in a molten zinc bath which contains
0.01% or more and 1.00% or less of Al and has a bath temperature of 430°C or higher
- 51 -
and 500°C or lower in the plating process.
13. The method of manufacturing a galvannealed steel sheet according to claim 7
or 8,
wherein, in the heating process, an average temperature rising rate between
420°C and 460°C is 20 °C/sec or higher and 100 °C/sec or lower, and
an average temperature rising rate from 460°C to 550°C is 2 °C/sec or higher
and 40 °C/sec or lower.