[Technical Field of the Invention]
[0001]
The present invention relates to a hot-dip plated steel sheet and a method of
manufacturing the hot-dip plated steel sheet.
Priority is claimed on Japanese Patent Application No. 2020-075495, filed
April 21, 2020, the content of which is incorporated herein by reference.
[Related Art]
[0002]
A Zn-Al-Mg-based hot-dip plated steel sheet has high corrosion resistance.
Further, a Zn-Al-Mg-Si-based hot-dip plated steel sheet, which is obtained in a case
where a small amount of Si is further contained in the Zn-Al-Mg-based hot-dip plated
steel sheet, is excellent on both corrosion resistance and workability. For this reason,
the Zn-Al-Mg-based hot-dip plated steel sheet and the Zn-Al-Mg-Si-based hot-dip
plated steel sheet are used in various technical fields, such as the field of building
materials, the field of home appliances, and the field of vehicles.
[0003]
Patent Document 1 discloses a highly corrosion-resistant hot-dip galvanized
steel sheet that is excellent in external appearance evenness and includes a hot-dip
galvanized layer provided on the surface thereof and containing Al: 4 to 22 mass%, Mg:
1 to 6 mass%, and a remainder consisting of Zn and unavoidable impurities. The
degree of non-recrystallization of a surface layer of a plating base sheet is 30% or more,
and an average grain size of a ternary eutectic phase of Al/MgZn2/Zn among component
- 1 -
phases of a plating layer is in a range of 10 to 100 ~m.
[0004]
Patent Document 2 discloses a highly corrosion-resistant hot-dip galvanized
steel sheet that is excellent in external appearance evenness and includes a plating layer
formed on the surface thereof and containing Al: 4 to 22 mass%, Mg: 1 to 6 mass%, Si:
0.001 to 1 mass%, and a remainder consisting of Zn and unavoidable impurities.
Mg2Si phases and Ca phases, which contain Ca or Ca compounds as main components,
are present at an interface between the plating layer and a base steel sheet, and at least
part of the Mg2Si phases are precipitated using the Ca phases as nuclei.
[0005]
Patent Document 3 discloses an aluminum-plated steel sheet that is excellent in
corrosion resistance and external appearance and includes a plating layer formed on at
least one surface of a steel sheet and containing, by mass%, Si: 2% or more and 11% or
less, Mg: 3% or more and 9% or less, Ca: 0.1% or more and 5% or less, Ti: 0.005% or
more and 0.05% or less, and a remainder consisting of Aland unavoidable impurities.
Mg2Si particles, of which a diameter on a major axis is 10 ~m or less and an aspect ratio
as a ratio of a diameter on the major axis to a diameter on a minor axis is 1 or more and
3 or less, are present in the plating layer.
[Prior Art Document]
[Patent Document]
[0006]
[Patent Document 1] Japanese Unexamined Patent Application, First
Publication No. 2013-14794
[Patent Document 2] Republished Japanese Translation W02013/002358 of
the PCT International Publication for Patent Applications
- 2 -
[Patent Document 3] Republished Japanese Translation W02013/008341 of
the PCT International Publication for Patent Applications
[Disclosure of the Invention]
[Problems to be Solved by the Invention]
[0007]
The present inventors found that a coating film is likely to be peeled off from a
coated steel sheet obtained in a case where coating is performed on the Zn-Al-Mg-Sibased
hot-dip plated steel sheet. As the result of the detailed investigation of the
coated steel sheet from which a coating film is peeled off, it was clarified that the
peeling of a coating film tends to occur at an interface between plating and a base steel
sheet. This result shows that there is a concern about the plating adhesion of a worked
portion of the Zn-Al-Mg-Si-based hot-dip plated steel sheet. The plating adhesion of a
worked portion means the adhesion of plating at a portion to which machining, such as
bending and drawing, has been applied.
[0008]
There are few examples where focus has been on the plating adhesion of a
worked portion in the related art of a Zn-Al-Mg-based hot-dip plated steel sheet or a ZnAl-
Mg-Si-based hot-dip plated steel sheet. Among the above-mentioned prior art
documents, Patent Document 3 discloses that spherical Mg2Si particles reduce pressure
concentration in forming with high stress and suppress the occurrence and propagation
of potential cracks. However, the peeling of plating has not been examined in Patent
Document 3 at all.
[0009]
The present invention has been made in consideration of the above-mentioned
circumstances and an object of the present invention is to provide a highly corrosion-
- 3 -
resistant hot -dip plated steel sheet that has excellent plating adhesion of a worked
portion and a method of manufacturing the highly corrosion-resistant hot-dip plated
steel sheet.
[Means for Solving the Problem]
[0010]
The gist of the present invention is as follows.
(1) A hot-dip plated steel sheet according to an aspect of the present invention
includes a base steel sheet and a hot-dip plating layer; a chemical composition of the
hot-dip plating layer contains Al: 4.0 to 22 mass%, Mg: 1 to 10 mass%, Si: 0.0001 to 2
mass%, and a remainder consisting of Zn and impurities; a coating weight of the hot-dip
plating layer is in a range of 40 to 600 g/m2 on both surface in total; a total value of an
interface contact length of interface Mg2Si phases present at an interface between the
base steel sheet and the hot-dip plating layer is 20% or less of the visual field, in which
the total value is measured in a visual field having a length of 10 mm in a vertical cross
section; and a number density of the interface Mg2Si phases having an equivalent circle
diameter of 30 ~m or more present at the interface between the base steel sheet and the
hot-dip plating layer is 10 pieces/mm2 or less, in which the number density is measured
in a plan view.
(2) In the hot-dip plated steel sheet according to (1), a maximum value of the
interface contact length of the interface Mg2Si phase may be 50 ~m or less, in which the
maximum value is measured in the visual field having a length of 10 mm in the vertical
cross section.
(3) In the hot-dip plated steel sheet according to (1) or (2), a ratio b/a of a
length b of the interface Mg2Si phase in an interface-horizontal direction to a length a of
the interface Mg2Si phase in a plating -depth direction may be 0.1 or more and 10 or less
- 4 -
in the visual field having a length of 10 mm in the vertical cross section.
(4) In the hot-dip plated steel sheet according to any one of (1) to (3), the
chemical composition of the hot-dip plating layer may contain 0.001 to 2 mass% of one
or more selected from Fe, Sb, Pb, Sn, Ca, Co, Mn, P, B, Bi, Cr, Group 3 elements, REM,
and Hf in total, instead of part of the Zn.
(5) In the hot-dip plated steel sheet according to any one of (1) to (4), the
chemical composition of the hot-dip plating layer may contain 0.001 to 2 mass% of one
or more selected from Ni, Ti, Zr, and Sr in total, instead of part of the Zn.
(6) A method of manufacturing a hot-dip plated steel sheet according to another
aspect of the present invention is a method of manufacturing the hot-dip plated steel
sheet according to any one of (1) to (5). The method includes: alkaline degreasing a
base steel sheet using alkaline degreasing liquid containing 0.5 to 5.0 mass% of a
surfactant; water rinsing the base steel sheet after the alkaline degreasing; annealing the
base steel sheet after the water rinsing; and immersing the base steel sheet in a hot-dip
plating bath containing Al: 4.0 to 22 mass%, Mg: 1 to 10 mass%, Si: 0.0001 to 2
mass%, and a remainder consisting of Zn and impurities after the annealing, to form a
hot-dip plating layer. A pH of rinsing water is always set to 8.7 or more and 12 or less
in the water rinsing.
(7) In the method of manufacturing the hot-dip plated steel sheet according to
(6), the hot-dip plating bath may contain 0.001 to 2 mass% of one or more selected from
Fe, Sb, Pb, Sn, Ca, Co, Mn, P, B, Bi, Cr, Group 3 elements, REM, and Hf in total,
instead of part of the Zn.
(8) In the method of manufacturing the hot-dip plated steel sheet according to
(6) or (7), the hot-dip plating bath may contain 0.001 to 2 mass% of one or more
selected from Ni, Ti, Zr, and Sr in total, instead of part of the Zn.
- 5 -
[Effects of the Invention]
[0011]
According to the present invention, it is possible to provide a highly corrosionresistant
hot -dip plated steel sheet that has excellent plating adhesion of a worked
portion and a method of manufacturing the highly corrosion-resistant hot-dip plated
steel sheet.
[Brief Description of the Drawings]
[0012]
FIG. 1 is a schematic cross-sectional view of a highly corrosion-resistant hotdip
plated steel sheet, which has excellent plating adhesion of a worked portion,
according to an aspect of the present invention.
FIG. 2 is a schematic cross-sectional view of the highly corrosion-resistant hotdip
plated steel sheet, which has excellent plating adhesion of a worked portion,
according to the aspect of the present invention.
[Embodiments of the Invention]
[0013]
The present inventors have performed the detailed structural analysis of plating
peeling that occurs in a worked portion in a case where a plated steel sheet is applied to
a test simulating forming with stress, such as a 180° -bending test. As a result, it has
been clarified that the crystallization form of Mg2Si phases at an interface between a
base steel sheet and plating has a great influence on the plating adhesion of a worked
portion.
[0014]
In a case where the cross-sectional observation of Mg2Si phases (hereinafter,
referred to as "interface Mg2Si phases") present at an interface between a base steel
- 6 -
sheet and a hot-dip plating layer in a normal plated steel sheet is performed, the
interface Mg2Si phases are present along the interface. The present inventors have
found that the plating adhesion of a worked portion is improved in a case where the
lengths of the interface Mg2Si phases along the interface (interface contact lengths) are
reduced. Further, it can be confirmed that the interface Mg2Si phases adhere to the
base steel sheet in a case where the surface of the base steel sheet is observed using
SEM after the removal of the plating of the plated steel sheet. The present inventors
have found that the plating adhesion of a worked portion is further improved in a case
where the number density of interface Mg2Si phases, which have an equivalent circle
diameter of 30 ~m or more, (coarse interface Mg2Si phases) of the interface Mg2Si
phases is reduced.
[0015]
In addition, the present inventors have found that there is a close relationship
between the interface contact length of the interface Mg2Si phases measured in a cross
section and rinsing conditions before the plating of the base steel sheet, and between the
number density of the coarse interface Mg2Si phases measured in a plan view and
rinsing conditions before the plating of the base steel sheet. Then, the present
inventors have clarified rinsing conditions that allow the state of the interface Mg2Si
phases to be in a desirable range.
[0016]
As shown in, for example, FIG. 1, a highly corrosion-resistant hot-dip plated
steel sheet 1, which is excellent in plating adhesion, according to an aspect of the
present invention (a hot-dip plated steel sheet 1 according to the present embodiment),
obtained from the above-mentioned knowledge includes a base steel sheet 11 and a hotdip
plating layer 12, in which the chemical composition of the hot-dip plating layer 12
- 7 -
contains Al: 4.0 to 22 mass%, Mg: 1 to 10 mass%, Si: 0.0001 to 2 mass%, and a
remainder consisting of Zn and impurities; the coating weight of the hot-dip plating
layer 12 is in a range of 40 to 600 g/m2 on both surface in total; the total interface
contact length of interface Mg2Si phases 13 is 20% or less of the visual field, in which
the total value is measured in a visual field having a length of 10 mm in a vertical cross
section; and the number density of the interface Mg2Si phases 13 having an equivalent
circle diameter of 30 ~m or more present at an interface between the base steel sheet 11
and the hot-dip plating layer 12 is 10 pieces/mm2 or less, in which the total value is
measured in a plan view.
[0017]
The hot-dip plated steel sheet 1 according to the present embodiment includes
the base steel sheet 11. The type of the base steel sheet 11 is not particular! y limited.
Various components, thicknesses, metallographic structures, mechanical properties, and
the like can be applied to the base steel sheet 11 depending on the use of the hot -dip
plated steel sheet 1.
[0018]
The hot-dip plated steel sheet 1 according to the present embodiment includes a
hot-dip plating layer 12 provided on the surface of the base steel sheet 11. The hot-dip
plating layer 12 may be provided on one surface of the base steel sheet 11 and may be
provided on both surfaces of the base steel sheet 11. First, the chemical composition
of the hot-dip plating layer 12 will be described below. Hereinafter, the unit"%" of the
content of each element means mass% unless otherwise specified.
[0019]
(Al: 4.0 to 22 mass%)
In the hot-dip plating layer 12, the Al content is set to 4.0 mass% or more and
- 8 -
22 mass% or less. It is considered that an effect of improving corrosion resistance is
not sufficiently obtained in a case where the Al content is less than 4.0 mass%. It is
considered that an effect of improving corrosion resistance is saturated in a case where
the Al content is 22 mass% or more. In order to further improve corrosion resistance,
the Al content may be 5 mass% or more or 10 mass% or more. Further, in order to
lower the melting point of a plating bath or improve plating adhesion, the Al content
may be set to 20 mass% or less or 15 mass% or less.
[0020]
(Mg: 1 to 10 mass%)
In the hot-dip plating layer 12, the Mg content is set to 1 mass% or more and
10 mass% or less. It is considered that an effect of improving corrosion resistance is
not sufficiently obtained in a case where the Mg content is less than 1 mass%. Further,
it is considered that the plating layer is embrittled and plating adhesion deteriorates in a
case where the Mg content exceeds 10 mass%. In order to further improve corrosion
resistance, the Mg content may be set to 2 mass% or more or 3 mass% or more. In
order to further improve the adhesion of the plating layer, the Mg content may be 9
mass% or less, 7 mass% or less, 5 mass% or less, 4.5 mass% or less, or 4 mass% or less.
[0021]
(Si: 0.0001 to 2 mass%)
In the hot-dip plating layer 12, the Si content is set to 0.0001 mass% or more
and 2 mass% or less. Si has an effect of improving the corrosion resistance of the hotdip
plating layer 12, but it is considered that it is industrially difficult to control the Si
content to less than 0.0001 mass%. Further, it is considered that an effect of
improving corrosion resistance is saturated in a case where the Si content is 2 mass% or
more. The Si content may be set to 0.001 mass% or more or 0.01 mass% or more.
- 9 -
The Si content may be set to 1.5 mass% or less or 1.0 mass% or less.
[0022]
(Remainder: Zn and impurities)
The remainder of the chemical composition of the hot-dip plating layer 12 is
Zn and impurities. The impurities are components which is mixed in the hot-dip
plating layer 12 due to various factors of, for example, raw materials for hot-dip
galvanizing or a manufacturing step, and which are allowed within a range where the
hot-dip plated steel sheet 1 according to the present embodiment is not adversely
affected.
[0023]
The chemical composition of the hot-dip plating layer 12 may contain 0.001 to
2 mass% of one or more selected from Fe, Sb, Pb, Sn, Ca, Co, Mn, P, B, Bi, Cr, Group 3
elements (for example, Sc and the like), REM, and Hf in total, instead of part of Zn of
the remainder. In a case where the content of these elements is 2 mass% or less,
various properties, such as corrosion resistance, of the hot-dip plating layer 12 do not
deteriorate. Further, there is also a possibility that the corrosion resistance of the hotdip
plating layer 12 will be improved by these elements. The total content of one or
more selected from Fe, Sb, Pb, Sn, Ca, Co, Mn, P, B, Bi, Cr, Group 3 elements, REM,
and Hf may be 0.002 mass% or more, 0.01 mass% or more, or 0.1 mass% or more.
The total content of one or more selected from Fe, Sb, Pb, Sn, Ca, Co, Mn, P, B, Bi, Cr,
Group 3 elements, REM, and Hf may be 1.5 mass% or less, 1.0 mass% or less, or 0.5
mass% or less.
[0024]
Further, the chemical composition of the hot-dip plating layer 12 may contain
0.001 to 2 mass% of one or more selected from Ni, Ti, Zr, and Sr in total, instead of part
- 10 -
of Zn of the remainder. In a case where the content of these elements is 0.001 mass%
or more in total, the intermetallic compound of these elements and Al is crystallized and
surface smoothness is improved. However, in a case where the total content of these
elements exceeds 2 mass%, there is a concern that the external appearance of plating is
rough and poor external appearance occurs. The total content of one or more selected
from Ni, Ti, Zr, and Sr may be 0.002 mass% or more, 0.01 mass% or more, or 0.1
mass% or more. The total content of one or more selected from Ni, Ti, Zr, and Sr may
be 1.5 mass% or less, 1.0 mass% or less, or 0.5 mass% or less.
[0025]
Next, the coating weight of the hot-dip plating layer 12 will be described.
The coating weight of the hot-dip plating layer 12 is set in a range of 40 to 600 g/m2 on
both surface in total. In a case where the coating weight of the hot-dip plating layer 12
is set to 40 g/m2 or more, high corrosion resistance can be given to the hot-dip plated
steel sheet 1. On the other hand, in a case where the coating weight of the hot-dip
plating layer 12 is set to 600 g/m2 or less, plating adhesion (for example, the plating
adhesion of a worked portion) can be ensured. The coating weight of the hot-dip
plating layer 12 may be set to 50 g/m2 or more, 100 g/m2 or more, or 200 g/m2 or more
on both surface in total. The coating weight of the hot-dip plating layer 12 may be set
to 550 g/m2 or less, 500 g/m2 or less, or 300 g/m2 or less on both surface in total.
[0026]
In the hot-dip plated steel sheet 1 according to the present embodiment, Mg2Si
phases may be present at the interface between the base steel sheet 11 and the hot-dip
plating layer 12. Hereinafter, the Mg2Si phases present at the interface between the
base steel sheet 11 and the hot-dip plating layer 12 are referred to as the interface Mg2Si
phases 13. An alloy layer having a thickness of about several hundred nm may be
- 11 -
formed at the interface between the base steel sheet 11 and the hot-dip plating layer 12.
In this case, all of Mg2Si phases in contact with an interface between the base steel sheet
11 and the alloy layer, Mg2Si phases in contact with an interface between the alloy layer
and the hot -dip plating layer 12, and Mg2Si phases in contact with both these interfaces
are regarded as the interface Mg2Si phases 13.
[0027]
In the hot -dip plated steel sheet 1 according to the present embodiment, the
total value of an interface contact length of the interface Mg2Si phases 13 is 20% or less
of the visual field, in which the total value is measured in a visual field having a length
of 10 mm in a vertical cross section.
[0028]
Here, the interface contact lengths of the interface Mg2Si phases 13 are the
lengths of portions, which are included in the interface Mg2Si phases, of the interface
between the base steel sheet 11 and the hot-dip plating layer 12. A description will be
given with reference to FIG. 1 by way of example. The interface between the base
steel sheet 11 and the hot-dip plating layer 12 is in contact with each interface Mg2Si
phase 13 at two points. A distance between the two points is the interface contact
length of the interface Mg2Si phase 13.
[0029]
The total interface contact length of the interface Mg2Si phases 13, which is
measured in a visual field having a length of 10 mm in a vertical cross section, is a total
value of the interface contact lengths of the interface Mg2Si phases 13 that are included
in an any visual field in a cross section perpendicular to the surface of the hot-dip plated
steel sheet 1. Here, the number of visual fields is set to 5 and the average value of the
total values of the interface contact lengths in these visual fields is calculated. Further,
- 12 -
the shape of a visual field is a 10 mm-by-1 0 mm square, and the interface between the
base steel sheet 11 and the hot-dip plating layer 12 is substantially parallel to the
horizontal side of the visual field. A description will be given with reference to FIG. 1
by way of example. Three interface Mg2Si phases 13 are included in the visual field
shown in FIG. 1 and the interface contact lengths thereof are L1 to L3. The total
interface contact length of the interface Mg2Si phases 13, which is measured in a visual
field having a length of 10 mm in a vertical cross section, is L1 + L2 + L3.
[0030]
In the hot -dip plated steel sheet 1 according to the present embodiment, the
state of the interface Mg2Si phases 13 is defined on the basis of a ratio of the total
interface contact length to the width (1 0 mm) of the visual field. In a case where the
total interface contact length (an average value in five visual fields) of the interface
Mg2Si phases 13 is set to 20% or less of the visual field, the area of a region in which
plating peeling is likely to occur is reduced. As a result, high plating adhesion of a
worked portion is obtained. The total interface contact length of the interface Mg2Si
phases 13 may be set to 18% or less, 15% or less, or 10% or less of the visual field.
The lower limit of the total interface contact length of the interface Mg2Si
phases 13 is not particular! y limited in terms of ensuring the plating adhesion of a
worked portion. Accordingly, the total interface contact length of the interface Mg2Si
phases 13 may be 0% of the visual field. However, the total interface contact length of
the interface Mg2Si phases 13 may be set to 0.5% or more of the visual field. The total
interface contact length of the interface Mg2Si phases 13 may be set to 1.0% or more,
2.0% or more, or 5.0% or more of the visual field.
[0031]
Further, in the hot-dip plated steel sheet 1 according to the present
- 13 -
embodiment, the number density of the interface Mg2Si phases having an equivalent
circle diameter of 30 ~m or more present at the interface between the base steel sheet
and the hot-dip plating layer (hereinafter, referred to as "coarse interface Mg2Si phases")
is 10 pieces/mm2 or less, in which the number density is measured in a plan view.
"Plan view" is a visual field in a case where an object is viewed in a direction
perpendicular to the interface or the base steel sheet. It should be noted here that the
number density of the coarse interface Mg2Si phases is measured on the surface of the
base steel sheet in a plan view in contrast to the fact that the above-mentioned interface
contact lengths of the interface Mg2Si phases 13 are measured in the cross section of the
hot-dip plated steel sheet 1. Specifically, the number density of the coarse interface
Mg2Si phases is measured according to the following procedure.
1. The hot-dip plated steel sheet is immersed in 0.5% hydrochloric acid
containing an inhibitor. Accordingly, the hot-dip plating layer can be dissolved and
removed from the hot-dip plated steel sheet. On the other hand, the interface Mg2Si
phases present at the interface between the base steel sheet and the hot-dip plating layer
remain on the surface of the base steel sheet.
2. The number of interface Mg2Si phases (coarse interface Mg2Si phases),
which are included in an any 1 mm-by-1 mm square region on the surface of the base
steel sheet and have an equivalent circle diameter of 30 ~m or more, is counted through
SEM observation. Here, the number of any 1 mm-by-1 mm square regions to be
observed is set to 5, and the average value of the numbers of coarse interface Mg2Si
phases of these regions is calculated. This average value is defined as the number
density of the interface Mg2Si phases (coarse interface Mg2Si phases). Here, the
equivalent circle diameter of the interface Mg2Si phases is an equivalent circle diameter
in a case where the base steel sheet is viewed in a plan view.
- 14 -
There is a concern that the coarse interface Mg2Si phases are not captured in
the above-mentioned cross-sectional observation. There is a possibility that coarse
interface Mg2Si phases not appearing in the cross section are present even though an
interface contact length measured through the cross-sectional observation is in the
above-mentioned range. There is a concern that the plating adhesion of a worked
portion of the hot-dip plated steel sheet deteriorates in a case where such coarse
interface Mg2Si phases are present. As the result of evaluation of various hot-dip
plated steel sheets performed by the present inventors, it has been confirmed that the
number density of coarse interface Mg2Si phases to be measured in a plan view may be
increased even in a hot-dip plated steel sheet having a small interface contact length of
interface Mg2Si phases in the cross section.
For this above-mentioned reason, in the hot-dip plated steel sheet according to
the present embodiment, the total interface contact length of the interface Mg2Si phases
is in the above-mentioned range and the number density of the coarse interface Mg2Si
phases measured in a plan view is 10 pieces/mm2 or less. In a case where the number
density of the coarse interface Mg2Si phases measured in a plan view exceeds 10
pieces/mm2, sufficient plating adhesion of a worked portion cannot be ensured. The
number density of the coarse interface Mg2Si phases measured in a plan view may be 9
pieces/mm2 or less, 8 pieces/mm2 or less, or 7 pieces/mm2 or less. Since a smaller
number density of the coarse interface Mg2Si phases measured in a plan view is more
preferable, the lower limit thereof is not particularly limited. The number density of
the coarse interface Mg2Si phases measured in a plan view may be, for example, 0
piece/mm2 or more, 1 piece/mm2 or more, or 2 pieces/mm2 or more.
[0032]
In addition, in the hot-dip plated steel sheet 1 according to the present
- 15 -
embodiment, it is preferable that the maximum value of the interface contact length of
the interface Mg2Si phase 13 is 50 ~m or less, in which the maximum value is measured
in a visual field having a length of 10 mm in a vertical cross section. The maximum
value of the interface contact length of the interface Mg2Si phase 13, which is measured
in a visual field having a length of 10 mm in a vertical cross section, is the maximum
value among the interface contact lengths of these interface Mg2Si phases 13 that are
included in an any visual field in a cross-section perpendicular to the surface of the hotdip
plated steel sheet 1. The number of visual fields is set to 5 and the average value
of the maximum values of the interface contact lengths of the interface Mg2Si phases 13
in these visual fields is calculated. A description will be given with reference to FIG. 1
by way of example. The maximum value among the interface contact lengths L 1 to L3
of three interface Mg2Si phases 13 included in the visual field shown in FIG. 1 is L3.
Accordingly, the maximum value of the interface contact length of the interface Mg2Si
phase 13, which is measured in the visual field shown in FIG. 1, is L3.
[0033]
It is supposed that plating peeling is more likely to occur in the case of an
interface Mg2Si phase 13 having a longer interface contact length. Accordingly, it is
considered that plating peeling can be more effectively suppressed in a case where not
only a ratio of the total value of the interface contact lengths of the interface Mg2Si
phases 13, which are included in the visual field, to the visual field but also the interface
contact length of each interface Mg2Si phase 13 is reduced. For this above-mentioned
reason, it is preferable that the maximum value of the interface contact length of the
interface Mg2Si phase 13 is 50 ~m or less. The maximum value of the interface
contact length of the interface Mg2Si phase 13 may be 45 ~m or less, 40 ~m or less, or
30 ~m or less.
- 16 -
[0034]
The lower limit of the maximum value of the interface contact length of the
interface Mg2Si phase 13 is not particularly limited in terms of ensuring the plating
adhesion of a worked portion. Accordingly, the maximum value of the interface
contact length of the interface Mg2Si phase 13 may be 0 ~m. In a case where the
interface Mg2Si phase 13 is not included in a measurement visual field at all, the
maximum value of the interface contact length of interface Mg2Si phase 13 is 0 ~m.
However, in consideration of the capacity of manufacturing equipment, the maximum
value of the interface contact length of the interface Mg2Si phase 13 may be 1 ~m or
more, 2 ~m or more, or 5 ~m or more.
[0035]
In addition, in the hot-dip plated steel sheet 1 according to the present
embodiment, a ratio b/a of a length b of the interface Mg2Si phase 13 in an interfacehorizontal
direction to a length a of the interface Mg2Si phase 13 in a plating-depth
direction may be 0.1 or more and 10 or less in a visual field having a length of 10 mm in
a vertical cross section. The length a of the interface Mg2Si phase 13 in the platingdepth
direction is the size of the interface Mg2Si phase 13 measured in the plating -depth
direction as shown in FIG. 2. The length b of the interface Mg2Si phase 13 in the
interface-horizontal direction is the size of the interface Mg2Si phase 13 measured in a
horizontal direction along the interface (that is, a direction perpendicular to the platingdepth
direction) as shown in FIG. 2. Hereinafter, the ratio of the length b of the
interface Mg2Si phase 13 in the interface-horizontal direction to the length a of the
interface Mg2Si phase 13 in the plating-depth direction may be referred to as the aspect
ratio of the interface Mg2Si phase 13. The aspect ratio of the interface Mg2Si phase 13
is evaluated in five visual fields. In a case where two or more interface Mg2Si phases
- 17 -
13 are included in five visual fields, "the ratio b/a of the length b of the interface Mg2Si
phase 13 in the interface-horizontal direction to the length a of the interface Mg2Si
phase 13 in the plating -depth direction is 0.1 or more and 10 or less in a visual field
having a length of 10 mm in a vertical cross section" means that the aspect ratios of all
interface Mg2Si phases 13 included in the five visual fields are in a range of 0.1 or more
and 10 or less.
[0036]
An interface Mg2Si phase 13 having a high aspect ratio has a shape extending
along the interface between the base steel sheet 11 and the hot-dip plating layer 12. In
a case where the aspect ratio of the interface Mg2Si phase 13 is set to 10 or less, plating
peeling can be more effectively suppressed. The aspect ratio of the interface Mg2Si
phase 13 may be set to 9 or less, 8 or less, or 5 or less.
[0037]
Further, there is a concern that an interface Mg2Si phase 13 having a low aspect
ratio serves as a crack propagation path of the plating layer during the working of the
hot-dip plated steel sheet 1 and causes plating cracks and the deterioration of corrosion
resistance. In a case where the aspect ratio of the interface Mg2Si phase 13 is set to 0.1
or more, the corrosion resistance and the like of the hot-dip plating layer 12 can be
further improved. The aspect ratio of the interface Mg2Si phase 13 may be set to 0.2
or more, 0.5 or more, or 1.0 or more.
[0038]
The hot -dip plated steel sheet 1 according to the present embodiment may
include a chemical conversion film layer, a coating film layer, and the like on the
surface of the hot-dip plating layer 12 in order to improve, for example, design,
corrosion resistance, and the like. Here, the types of the chemical conversion film
- 18 -
layer and the coating film layer are not particularly limited and chemical conversion
film layers and coating film layers having been publicly known can be applied. Even
in this case, the number density of the coarse interface Mg2Si phases can be easily
measured in a case where the chemical conversion film layer, the coating film layer, and
the like are appropriately removed using a publicly known method before the
dissolution of the hot-dip plating layer.
[0039]
Next, a method of manufacturing a highly corrosion-resistant hot-dip plated
steel sheet, which is excellent in plating adhesion, according to another aspect of the
present invention will be described. The method of manufacturing a highly corrosionresistant
hot-dip plated steel sheet, which is excellent in plating adhesion, according to
the present embodiment (hereinafter, this may be abbreviated to a method of
manufacturing a hot-dip plated steel sheet according to the present embodiment)
includes: alkaline de greasing a base steel sheet ll ; water rinsing the base steel sheet 11
after the alkaline degreasing; annealing the base steel sheet ll after the water rinsing;
and immersing the base steel sheet 11 in a hot-dip plating bath containing Al: 4.0 to 22
mass%, Mg: 1 to 10 mass%, Si: 0.0001 to 2 mass%, and a remainder consisting of Zn
and impurities after the annealing, to form a hot-dip plating layer 12, in which the pH of
rinsing water is always set to 8.7 or more and 12 or less.
[0040]
In the method of manufacturing a hot-dip plated steel sheet according to the
present embodiment, at first, the base steel sheet 11 is alkaline degreased. The alkaline
degreasing is performed using alkaline degreasing liquid containing 0.5 to 5.0 mass% of
a surfactant. In a case where the concentration of the surfactant is set to 0.5 mass% or
more, materials that may serve as the nuclei of crystallization for the interface Mg2Si
- 19 -
phases 13 can be well removed from the surface of the base steel sheet 11. However,
in a case where the concentration of the surfactant exceeds 5.0 mass%, there is a
concern that carbon, by which the surfactant adhering to the surface of the base steel
sheet 11 is configured, remains on the surface of the base steel sheet 11 even after the
annealing of the base steel sheet 11 to be described later. There is a concern that this
carbon serves as the nuclei of crystallization for the interface Mg2Si phases 13 and
increases the interface contact length of the interface Mg2Si phase. For this abovementioned
reason, the concentration of the surfactant in the alkaline de greasing liquid,
which is used for alkaline de greasing, is set in a range of 0.5 to 5.0 mass%.
[0041]
Then, the base steel sheet 11 after the alkaline degreasing is rinsed with water.
Water rinsing is performed to remove the alkaline degreasing liquid from the surface of
the base steel sheet 11. The pH of liquid for this water rinsing is set to 8. 7 or more and
12 or less. Accordingly, it is possible to prevent components, which are caused by the
liquid for the water rinsing and may serve as the nuclei of crystallization for the
interface Mg2Si phases 13, from adhering to the base steel sheet 11.
It should be noted here that the pH of the rinsing water is always set to 8.7 or
more and 12 or less during the water rinsing. In normal cascade type water rinsing,
neutral water having a pH of about 7 is used to perform finish rinsing. However,
neutral water is not used during the water rinsing in the method of manufacturing a hotdip
plated steel sheet according to the present embodiment. In a case where, for
example, a water rinsing tank is used to rinse the base steel sheet 11 after alkaline
de greasing with water, the pH of the rinsing water in contact with the base steel sheet 11
is set to 8.7 or more and 12 or less in the entire region from the entrance side to the exit
side of the water rinsing tank. In a case where the pH of the rinsing water is
- 20 -
inappropriate, that is, the pH of the rinsing water is less than 8.7 in part or all of the
water rinsing tank, it is not possible to sufficiently prevent components, which may
serve as the nuclei of crystallization for the interface Mg2Si phases 13, from adhering to
the base steel sheet 11. On the other hand, in a case where the pH of the rinsing water
exceeds 12, there is a concern that dissolution on the surface of the base steel sheet 11 is
not uniform and unevenness occurs in the adhesion of hot-dip plating. For this reason,
it is not preferable that the pH of the rinsing water exceeds 12.
In a case where the pH of the rinsing water in contact with the base steel sheet
11 is set to 8.7 or more and 12 or less in the entire region from the entrance side to the
exit side of the water rinsing tank as described above, it is possible to prevent
components, which may serve as the nuclei of crystallization for the interface Mg2Si
phases 13, from adhering to the base steel sheet 11. For this reason, the formation of
nuclei of crystallization for the interface Mg2Si phases 13 is suppressed in the
subsequent step. Accordingly, the formation of the interface Mg2Si phases 13 and the
coarsening of the interface Mg2Si phases 13 caused by pile-up or the like can be
suppressed.
[0042]
Then, the base steel sheet 11 after water rinsing is annealed. In addition, the
base steel sheet 11 after annealing is immersed in the hot-dip plating bath to form the
hot-dip plating layer 12 on the surface of the base steel sheet 11. Annealing conditions
are not particular! y limited, and various conditions can be employed according to the
components, use, thickness, metallographic structure, mechanical properties, and the
like of the base steel sheet 11. The components of the hot-dip plating bath may be the
same as those of the hot-dip plating layer 12 of the hot-dip plated steel sheet according
to the present embodiment, that is, the hot-dip plating bath may contain Al: 4.0 to 22
- 21 -
mass%, Mg: 1 to 10 mass%, Si: 0.0001 to 2 mass%, and a remainder consisting of Zn
and impurities. The preferred upper and lower limits of the content of each element of
the hot-dip plating bath are based on the preferred upper and lower limits of the content
of each element of the hot-dip plating layer 12. The hot-dip plating bath may contain
0.001 to 2 mass% of one or more selected from Fe, Sb, Pb, Sn, Ca, Co, Mn, P, B, Bi, Cr,
Group 3 elements, REM, and Hf in total, instead of part of Zn. The chemical
composition of the hot-dip plating bath may contain 0.001 to 2 mass% of one or more
selected from Ni, Ti, Zr, and Sr in total, instead of part of Zn.
[0043]
In a case where a chemical conversion layer is to be formed on the surface of
the hot-dip plating layer 12, chemical conversion is performed on the hot-dip plated
steel sheet on which the hot-dip plating layer is formed. The type of the chemical
conversion is not particularly limited, and publicly known chemical conversion can be
applied. Further, in a case where a coating film layer is to be formed on the surface of
the hot-dip plating layer 12, the surface of the chemical conversion layer, and the like,
coating treatment is performed on the hot-dip plated steel sheet on which the hot-dip
plating layer or the chemical conversion layer is formed. The type of the coating is not
particularly limited, and publicly known coating can be applied.
[Examples]
[0044]
An effect of an aspect of the present invention will be more specifically
described using Examples. However, conditions in Examples are merely one condition
example employed to confirm the feasibility and effects of the present invention. The
present invention is not limited to this condition example. The present invention may
employ various conditions to achieve the object of the present invention without
- 22 -
departing from the scope of the present invention.
[0045]
Various hot-dip plated steel sheets were manufactured through alkaline
de greasing a base steel sheet, water rinsing the base steel sheet after the alkaline
de greasing with water, annealing the base steel sheet after the water rinsing, and
immersing the base steel sheet in a hot-dip plating bath after the annealing to form a
hot-dip plating layer.
A cold-rolled steel sheet to which cold rolling oil adhered and which had a
thickness of 0.8 mm was used as the base steel sheet.
When the base steel sheet was alkaline de greased, the concentration of a
surfactant included in alkaline degreasing liquid was as shown in Table 1.
In a case where the base steel sheet was to be rinsed with water, the pH of the
rinsing water was set to the following three types. Rinsing water used in the
manufacture of each of Examples was shown in Table 1.
A: The pH of the rinsing water was always 8.7 or more and 12 or less during
water rinsing.
B: The pH of the rinsing water was always less than 8.7 during water rinsing.
C: The pH of the rinsing water was mainly 8.7 or more and 12 or less during
water rinsing and the pH of part of the rinsing water was less than 8. 7.
The base steel sheet after water rinsing was annealed, and then, the base steel
sheet was immersed in the hot-dip plating bath to form the hot-dip plating layer on the
surface of the base steel sheet. The components of the hot -dip plating bath were as
shown in Table 1. Since being substantially the same as that of the composition of the
plating bath, the composition of the obtained plating obtained in this way was omitted in
Table 1 or 2.
- 23 -
The coating weight of the hot-dip plating layer was adjusted by gas wiping
after plating.
In Table 1, an underline was given to a value out of the range of the present
invention. Further, the content of an element not intentionally added to the plating
bath (and the plating) was left blank in Table 1.
[0046]
- 24 -
[Table 1]
bi) ,.......,
bi) .s 0l
il) 1=1 I=:·,.., VJ ..§
....... VJ 1=1 .......... c aJO.. ro......-~ ,._,.0 '"d ..... > ~ o,.... s 0 ,._,. ;...,Cij u
~
lj)C'l s 0 ~OJ)
0 s::S ......
...... >< ,._,. ~ ro
ro z ::8 ~
1 10.5 3 27 1.5 B A Inventive Example
2 6.5 5 34 1.1 B A Inventive Example
3 4.5 1 18 2.4 A B Inventive Example
4 8.0 2 21 0.8 A A Inventive Example
5 5.0 0 16 3.1 A A Inventive Example
6 11.0 4 33 1.7 A A Inventive Example
7 19.5 6 42 9.7 A B Inventive Example
8 17.0 6 47 8.5 A B Inventive Example
9 0.6 0 7 2.7 A A Inventive Example
10 8.0 2 11 2.3 A A Inventive Example
11 1.5 1 6 1.2 A A Inventive Example
12 9.5 5 37 0.6 A A Inventive Example
13 4.5 4 27 1.4 A A Inventive Example
14 12.0 4 24 0.2 A B Inventive Example
15 14.5 3 15 1.7 A A Inventive Example
16 24.0 23 61 10.4 A D Comparative Example
17 27.0 8 36 4.1 A c Comparative Example
18 25.5 19 54 11.3 A D Comparative Example
19 12.5 9 52 8.4 c c Comparative Example
20 20.5 24 55 11.0 A D Comparative Example
21 17.4 13 54 9.1 A c Comparative Example
[0051]
In Comparative example 16, the concentration of a surfactant in alkaline
- 28 -
degreasing liquid was insufficient. In Comparative example 16, a ratio of the total
interface contact length of interface Mg2Si phases, which were present at the interface
between a base steel sheet and a hot-dip plating layer, to a visual field was excessive
and the number density of the interface Mg2Si phases having an equivalent circle
diameter of 30 ~m or more present at the interface between the base steel sheet and the
hot-dip plating layer, (the number density of coarse interface Mg2Si phases), which was
measured in a plan view, was excessive. As a result, workability deteriorated.
In Comparative example 17, the concentration of a surfactant in alkaline
de greasing liquid was excessive. In Comparative example 17, a ratio of the total
interface contact length of interface Mg2Si phases, which were present at the interface
between a base steel sheet and a hot-dip plating layer, to a visual field was excessive.
As a result, workability deteriorated.
In Comparative example 18, the concentration of a surfactant in alkaline
de greasing liquid was excessive and the pH of rinsing water used for the water rinsing
of a base steel sheet subjected to alkaline degreasing was insufficient. In Comparative
example 18, a ratio of the total interface contact length of interface Mg2Si phases, which
were present at the interface between a base steel sheet and a hot-dip plating layer, to a
visual field was excessive and the number density of coarse interface Mg2Si phases was
excessive. As a result, workability deteriorated.
In Comparative example 19, the amounts of Al and Mn included in plating
were insufficient. In Comparative example 19, both corrosion resistance and
workability deteriorated.
In Comparative example 20, the pH of rinsing water used for the water rinsing
of a base steel sheet subjected to alkaline degreasing was insufficient. In Comparative
example 20, a ratio of the total interface contact length of interface Mg2Si phases, which
- 29 -
were present at the interface between a base steel sheet and a hot-dip plating layer, to a
visual field was excessive and the number density of coarse interface Mg2Si phases was
excessive. As a result, workability deteriorated.
In Comparative example 21, the pH of rinsing water used for the water rinsing
of a base steel sheet subjected to alkaline degreasing was insufficient in part of a rinsing
step. In Comparative example 21, the number density of coarse interface Mg2Si
phases was excessive. As a result, workability deteriorated.
[0052]
On the other hand, since Examples Nos. 1 to 15 of the present invention were
subjected to degreasing and water rinsing under appropriate conditions, the total value
of an interface contact length of interface Mg2Si phases present at the interface between
a base steel sheet and a hot-dip plating layer was 20% or less of the visual field, in
which the roral value was measured in a visual field having a length of 10 mm in a
vertical cross section, and the number density of the interface Mg2Si phases having an
equivalent circle diameter of 30 ~m or more present at the interface between the base
steel sheet and the hot-dip plating layer was 10 pieces/mm2 or less, in which the number
density was measured in a plan view. Further, Examples Nos. 1 to 15 of the present
invention were excellent in both corrosion resistance and the plating adhesion of a
worked portion.
[Industrial Applicability]
[0053]
According to the present invention, it is possible to provide a highly corrosionresistant
hot -dip plated steel sheet that has excellent plating adhesion of a worked
portion and a method of manufacturing the highly corrosion-resistant hot-dip plated
steel sheet. According! y, the present invention has high industrial applicability.
- 30 -
[Brief Description of the Reference Symbols]
[0054]
1: hot-dip plated steel sheet
11: base steel sheet
12: hot-dip plating layer
13: interface Mg2Si phase
a: maximum length of interface Mg2Si phase in plating -depth direction
b: maximum length of interface Mg2Si phase in interface-horizontal direction
L 1 to L3: interface contact length of interface Mg2Si phase

CLAIMS
1. A hot-dip plated steel sheet comprising:
a base steel sheet; and
a hot-dip plating layer,
wherein a chemical composition of the hot-dip plating layer contains Al: 4.0 to
22 mass%, Mg: 1 to 10 mass%, Si: 0.0001 to 2 mass%, and a remainder consisting of
Zn and impurities,
a coating weight of the hot-dip plating layer is in a range of 40 to 600 g/m2 on
both surface in total,
a total value of an interface contact length of interface Mg2Si phases present at
an interface between the base steel sheet and the hot-dip plating layer is 20% or less of
the visual field, in which the total value is measured in a visual field having a length of
10 mm in a vertical cross section,and
a number density of the interface Mg2Si phases having an equivalent circle
diameter of 30 ~m or more present at the interface between the base steel sheet and the
hot-dip plating layer is 10 pieces/mm2 or less, in which the number density is measured
in a plan view.
2. The hot-dip plated steel sheet according to claim 1,
wherein a maximum value of the interface contact length of the interface
Mg2Si phase is 50 ~m or less, in which the maximum value is measured in the visual
field having a length of 10 mm in the vertical cross section.
3. The hot-dip plated steel sheet according to claim 1 or 2,
wherein a ratio b/a of a length b of the interface Mg2Si phase in an interfacehorizontal
direction to a length a of the interface Mg2Si phase in a plating-depth
direction is 0.1 or more and 10 or less in the visual field having a length of 10 mm in the
- 32 -
vertical cross section.
4. The hot-dip plated steel sheet according to any one of claims 1 to 3,
wherein the chemical composition of the hot-dip plating layer contains 0.001 to
2 mass% of one or more selected from Fe, Sb, Pb, Sn, Ca, Co, Mn, P, B, Bi, Cr, Group 3
elements, REM, and Hf in total, instead of part of the Zn.
5. The hot-dip plated steel sheet according to any one of claims 1 to 4,
wherein the chemical composition of the hot-dip plating layer contains 0.001 to
2 mass% of one or more selected from Ni, Ti, Zr, and Sr in total, instead of part of the
Zn.
6. A method of manufacturing the hot-dip plated steel sheet according to any
one of claims 1 to 5, the method comprising:
alkaline degreasing a base steel sheet using alkaline degreasing liquid
containing 0.5 to 5.0 mass% of a surfactant;
water rinsing the base steel sheet after the alkaline degreasing;
annealing the base steel sheet after the water rinsing; and
immersing the base steel sheet in a hot-dip plating bath containing Al: 4.0 to 22
mass%, Mg: 1 to 10 mass%, Si: 0.0001 to 2 mass%, and a remainder consisting of Zn
and impurities after the annealing, to form a hot-dip plating layer,
wherein a pH of rinsing water is always set to 8.7 or more and 12 or less in the
water rinsing.
7. The method of manufacturing the hot-dip plated steel sheet according to
claim 6,
wherein the hot-dip plating bath contains 0.001 to 2 mass% of one or more
selected from Fe, Sb, Pb, Sn, Ca, Co, Mn, P, B, Bi, Cr, Group 3 elements, REM, and Hf
in total, instead of part of the Zn.
- 33 -
8. The method of manufacturing the hot-dip plated steel sheet according to
claim 6 or 7,
wherein the hot-dip plating bath contains 0.001 to 2 mass% of one or more of
Ni, Ti, Zr, and Sr in total, instead of part of the Zn.