Specification
[Title of the Invention] STEEL SHEET FOR ELECTROPLATING,
ELECTROPLATED STEEL SHEET, AND METHODS FOR PRODUCING THE
SAME
[Technical Field of the Invention]
[0001]
The present invention relates to a steel sheet for electroplating, an
electroplated steel sheet, and methods for producing the same. More specifically, the
present invention relates to an electroplated steel sheet for press working which is
applied to the field of automobiles, the home electronics, and the like, a steel sheet for
electroplating which is a material of the electroplated steel sheet, and methods for
producing the same, and particularly relates to an electroplated steel sheet for press
working which is desirable for use in a fuel tank of a vehicle and is superior in plating
adhesion, a steel sheet for electroplating which is a material of the electroplated steel
sheet, and methods for producing the same.
Priority is claimed on Japanese Patent Application No. 2012-092341, filed on
April 13,2012, the content of which is incorporated herein by reference.
[Related Art]
[0002]
Recently, in the field of automobiles, improvement of fuel efficiency has been
progressing by reducing the vehicle weight. Regarding a fuel tank of a vehicle, the
shape of the file1 tank has become complicated in consideration of reduction in the
weight of a steel sheet for a tank, complication of a vehicle body design, and a tank
storage position. Therefore, superior formability is required for a steel sheet for a
tank.
[0003]
In order to satisfy such a requirement of formability, IF (Interstitial Free) steel
in which a carbonnitride-forming element such as Ti or Nb is added to ultra low carbon
steel has been developed. Further, for fuel tanks, a steel sheet has also been required
in which a corrosion product which causes the clogging of a filter is not produced by
gasoline, alcohol, or an organic acid produced by degradation of gasoline and thus
pitting corrosion does not occur.
[0004]
In order to satisfy this requirement, in the related ast, the plating of a Pb-Sn
alloy, an Al-Si alloy, a Sn-Zn alloy, a Zn-A1 alloy, or a Zn-Ni alloy on a steel sheet
surface has been proposed and applied. Therefore, a steel sheet which is a plating
substrate must have superior plating properties with the above alloys and superior
plating adhesion during press forming or in a usage environment.
[OOOS]
However, recently, along with complication of the shape of fuel tanks, in a
steel sheet subjected to harsh press forming, there has been a problem regarding plating
adhesion in that a plating layer is exfoliated. That is, even if the formability of a steel
sheet is improved, there has been a limit to the improvement of press formability due
to the restriction of plating adhesion.
[0006]
In order to solve such a problem, several methods for improving the plating
adliesion of electroplating have been proposed.
[0007]
For example, Patent Document 1 proposes a technique in which the
electroplating adhesion of Zn, a Zn alloy, Fe, or a Fe alloy is improved by subjecting a
steel sheet containing 11% or more of Cr to shot blasting in an atmosphere of inert gas
or reducing gas.
[OOOS]
Patent Document 2 proposes a technique in which the plating adhesion of Znalloy-
electroplating is improved by pickling a polished surface of a base metal to be
uniformly treated using a pickling solution to which thiouric acid is added.
[0009]
Patent Document 3 discloses a technique in which the plating adhesion of an
electroplated steel sheet is improved by annealing a steel sheet, which is rolled with a
bright roll having a small amount of surface convex-concave portions, polishing a
surface of the steel sheet as a plating pretreatment, and pickling the surface in a
hydrochloric acid solution containing sodium hydrogen fluoride.
[OOlO]
Patent Document 4 discloses a technique in which plating adhesion is
improved by polishing a surface of an annealed steel sheet as a plating pretreatment,
pickling the surface for 10 seconds or less, and forming an electroplating layer on the
surface.
[OOll]
Patent Document 5 discloses a technique in which the plating adhesion of a
Zn based-electroplated steel sheet is irnproved by polishing a non-plated surface of a
base metal with an abrasive material containing abrasive grains and forming a Znelectroplating
layer on the surface.
[Prior A1-t Document]
[Patent Document]
[0012]
[Patent Document 11 Japanese Unexamined Patent Application, First
Publication No. H04-362193
[Patent Document 21 Japanese Patent No. 3327222
patent Document 31 Japanese Unexamined Patent Application, First
PublicationNo. H05-320981
[Patent Document 41 Japanese Unexamined Patent Application, First
Publication No. H05-230689
[Patent Document 51 Japanese Unexamined Patent Application, First
Publication No. S63-140098
[Disclosure of the Invention]
[Problems to be Solved by the Invention]
[0013]
However, even with any of the above-described techniques, it is difficult to
secure plating adhesion when a steel sheet is press-formed into a complicated shape
such as a fuel tank.
[0014]
That is, the technique of Patent Document 1 is effective for, for example, a
steel sheet containing 11% or more of Cr on which a passive state film is formed.
This technique is a technique only for removing the passive state film which decreases
plating adhesion, not a technique for improving the plating adhesion of a steel sheet
containing a small Cr content.
[OOlS]
With the technique of Patent Document 2, the surface can be made to be
uniform, but plating adhesion cannot be secured when a steel sheet is pressed into a
complicated shape such as a fuel tank. With the technique of Patent Docunient 3, an
oxide can be removed with a small polishing amount, but plating adhesion cannot be
secured when a steel sheet is pressed into a complicated shape.
[00 161
With the technique of Patent Document 4, the surface can be made to be
smooth, but plating adhesion cannot be secured when a steel sheet is pressed into a
complicated shape such as a fuel tank. With the technique of Patent Document 5,
plating adhesion cannot be secured when a steel sheet is pressed into a complicated
shape, and it is necessary that the processing time and the size of facilities be increased
to completely remove an oxide film by polishing. Therefore, it is difficult to apply
this technique in practice.
[0017]
As described above, among the techniques of the related art, there is a
technique of improving plating adhesion, but there is no technique to be applied to a
fuel tank, in which plating adhesion is secured during complicated press forming.
[0018]
The present invention has been made to solve the above-described technical
problem, and an object thereof is to provide an electroplated steel sheet for press
working which can secure superior plating adhesion even during complicated press
forming and is desirable for use in the automobile field or the home electronics field,
particularly, for use in a fuel tank of a vehicle; a steel sheet for electroplating which is
a material of the electroplated steel sheet; and methods for producing the same.
[Means for Solving the Problem]
[0019]
In the present invention, in order to solve the above-described problem, the
plating adhesion of a steel sheet which is press-formed into a complicated shape such
as a fuel tank has been studied. As a result, it was found that superior plating
adhesion can be obtained by measuring a surface roughness, which is too fine to be
measured with a normal contact surface roughness metel; with a laser microscope and
adjusting the surface roughness to be within a desired range.
[0020]
The present invention has been made based on the above finding, and the
scope thereof is as follows.
[0021]
(1) According to a first aspect of the present invention, there is provided a
steel sheet for electroplating including, as a chemical composition, by mass%, C:
0.0005% to 0.0050%, Si: 0.20% to 1.0%, Mn: 0.40% to 2.5%, P: 0.05% or less, Ti:
0.010% to 0.050%, Nb: 0.010% to 0.040%, B: 0.0005% to 0.0030%, S: 0.02% or less,
Al: 0.01% to 0.30%, N: 0.0010% to 0.01%, and the balance including Fe and
impurities, in which in the chemical composition, when a Si content is represented by
[Si] and a Mn content is represented by [Mn], a value of "[Mn]+5[Si]" is 2.0 to 7.0,
and the steel sheet has a surface property in which an average of displace~nentso f a
measurement point that is obtained based on a moving average of continuous 31 points
in total including 15 front points and 15 back points in a cross-sectional profile of a
surface of the steel slieet obtained by measuring the average of displacements in an
evaluation length of 10 pm or more at an interval of 0.07 pm is 0.005 pm to 0.10 pm.
[0022]
(2) According to a second aspect of the present invention, there is provided
an electroplated steel slieet obtained by forming an electroplating layer on the surface
of the steel sheet for electroplating according to (1).
[0023]
(3) According to a third aspect of the present invention, there is provided a
method for producing a steel sheet for electroplating, the method including:
continuously casting molten steel having the chemical composition according to (1)
into a slab; heating the slab in a temperature range of 1000°C to 1300°C; hot-rolling
tlie heated slab under a finishing rolling temperature of As3 temperature to 1000°C and
a coiling temperature of 850°C or lower to obtain a hot-rolled steel sheet; removing a
surface scale of the hot-rolled steel sheet to obtain a pickled steel sheet, as a first
pickling process; cold-rolling the pickled steel sheet at a cold rolling reduction of 40%
or higher to obtain a cold-rolled steel sheet; annealing the cold-rolled steel sheet at a
recrystallization temperature or higher; and pickling a surface of the steel sheet using
10 g/l to 400 g/l of a sulfuric acid aqueous solution or using a pickling solution in
which one or more selected from the group consisting of nitrates, sulfates,
fluorosilicates, and fluoroborates are mixed with the sulfuric acid aqueous solution, as
a second pickling process.
[0024]
(4) According to a fourth aspect of the present invention, there is provided a
method for producing an electroplated steel sheet including forming an electroplating
layer, which contains 0% to 20% of Ni and the balance including Zn and impurities, on
the surface of the steel sheet for electroplating, which is obtained using the method
according to (3), in an amount of 3 g/m2 to 100 g/m2 per single surface.
[Effects of the Invention]
[0025]
According to the present invention, a steel slieet having superior plating
adliesion can be obtained, and it is possible to provide an electroplated steel sheet
which has superior press formability and plating adhesion in the field of automobiles
and home electronics, particularly, for use in a fuel tank of a vehicle; a steel sheet for
electroplating which is a material of the electroplated steel sheet; and methods for
producing the same. In addition, according to the present invention, the allowable
processing degree of a plated steel sheet is improved, and reduction in the weight of a
vehicle, particularly, a fuel tank and complication of a vehicle body design can be
realized.
[Brief Description of the Drawing]
[0026]
FIG. 1 is a drawing illustrating a relationship of the Si content and the Mn
content with plating adhesion.
FIG. 2 is a drawing illustrating a measurement example of surface fine
convex-concave portions (in the present invention, a difference between the height of
each measurement point measured at an intei-val of 0.07 pm and the average of heights
of 31 points on both sides in a measurement direction of the measurement point is
calculated, and an index is obtained using all the average values.)
FIG. 3 is an enlarged view of FIG. 2 illustrating "a measurement point (A)",
"the average (B) of heights of 3 1 points on both sides", and "a difference (C) between
the measurement point (A) and the average (B) of heights of 31 points on both sides".
[Embodiments of the Invention]
[0027]
Hereinafter, an embodiment of the present invention will be described in
detail. In the following description, "mass%" of a chemical composition will be
refelred to simply as "%.
[0028]
After diligent research, the present inventors have obtained an electroplated
steel sheet having superior plating adhesion even in press working into a complicated
shape such as a fuel tank which is extremely difficult to obtain in the related art. As a
result, it was found that superior plating adhesion can be obtained by measuring a
surface roughness, which is too fine to be measured with a normal contact surface
roughness meter, with a laser microscope and adjusting the surface roughness to be at
the level described later.
[0029]
It was also found that, to that end, superior plating adhesion can be realized by
controlling the Si content and the Mn content to be within a specific range and pickling
a non-plated surface of a steel sheet using 10 g/l to 400 g/l of a sulfuric acid aqueous
solution or using a pickling solution in which one or more selected fiom the group
consisting of nitrates, sulfates, fluorosilicates, and fluoroborates are mixed with the
sulfuric acid aqueous solution.
[0030]
It was found that from the above findings that press formability and plating
adhesion, which are applicable to, for example, the field of automobiles or home
electronics and particularly to a complicated shape such as a fuel tank of a vehicle, can
be realized, thereby completing the present invention.
[0031]
That is, an electroplated steel sheet according to the present invention
includes: a cold-rolled steel sheet; and a plating layer that is formed on a surface of the
cold-rolled steel sheet. The electroplated steel sheet contains, by mass%, C: 0.0005%
to 0.0050%, Si: 0.20% to 1.0%, Mn: 0.40% to 2.5%, P: 0.05% or less, Ti: 0.010% to
0.050%, Nb: 0.010% to 0.040%, B: 0.0005% to 0.0030%, S: 0.02% or less, Al: 0.01%
to 0.30%, N: 0.0010% to 0.01%, and the balance including Fe and impurities. In the
electroplated steel sheet, when a Si content is represented by [Si] and a Mn content is
represented by [Mn], a value of "[Mn]+S[Si]" is 2.0 to 7.0, and the steel sheet has a
surface property in which an average of displacements of a measurement point that is
obtained based on a moving average of continuous 31 points in total including 15 front
points and 15 back points in a cross-sectional profile of a surface of the steel sheet
obtained by measuring the average of displacements in an evaluation length of 10 pm
or more at an interval of 0.07 pm, is 0.005 pm to 0.10 pm.
[0032]
Hereinafter, the details of an experiment and analysis from which the above
facts were found by the present inventors will be described. The present inventors
produced a hot-rolled steel sheet by melting steel having a composition, which was
changed in ranges of C: 0.0005% to 0.0050%, Si: 0.06% to 1.4%, Mn: 0.1% to 3.5%,
P: 0.05% or less, Ti: 0.010% to 0.050%, Nb: 0.010% to 0.040%, B: 0.0005% to
0.0030%, S: 0.02% or less,Al: 0;01% to 0.30%, andN: 0.0010% to 0.01%, in a
vacuum melting furnace, heating the molten steel at 1230°C for 1 houi; and hot-rolling
the steel at a finishing rolling temperature of 850°C to 950°C.
[0033]
Next, this hot-rolled steel sheet was pickled and then was cold-rolled at a cold
rolling reduction of 50% to 90% to obtain a cold-rolled steel sheet. This cold-rolled
steel sheet was annealed in a cycle of being held at 800°C for 60 seconds in an
atmosphere containing 95% of nitrogen and 5% of hydrogen to obtain an annealed
steel sheet.
[0034]
Next, a surface of the annealed steel sheet was pickled using a sulfuric acid
aqueous solution and using a solution in which one to four kinds of nitrates, sulfates,
fluorosilicates, and fluoroborates was added to the sulfuric acid aqueous solution.
Next, a Zn electroplating layer or a Zn-Ni electroplating layer, which contained 0% to
20% of Ni and the balance including Z Ia~n d impurities, was formed on the surface of
the steel sheet to obtain an electroplated steel sheet.
[0035]
Next, a Dupont impact test was performed using the electroplated steel sheet
to evaluate the adhesion of the plating layer. In the Dupont impact test, assuming that
the steel sheet was pressed into a complicated shape such as a fuel tank, a weight
having a tip shape with a punch diameter of 4 mm and a weight of 500 g which was
more strict than a normal one (punch diameter: 5 rnm) was allowed to fall to the steel
sheet from a height of 1 m.
[0036]
The results are illustrated in FIG 1. In FIG. 1, the horizontal axis represents
the Si content, the vertical axis represents the Mn content, a case where there was no
plating exfoliation is represented by "o", and a case where there was plating exfoliation
is represented by "x" or "A". Whether or not there was plating exfoliation was
determined by attaching an adhesive tape on a sample surface and then peeling
therefio~na fter the test. A case where no plating layer was exfoliated was determined
as "no plating exfoliation (o)", and a case where a plating layer was exfoliated and
attached on the adhesive tape was determined as "plating exfoliation (x or A)".
Regarding a difference between x and A of the case of plating exfoliation, exfoliation
caused by a surface fine convex-concave portion (described below) of less than 0.005
pm was represented by "x", and exfoliation caused by a surface fine convex-concave
portion of more than 0.10 pm was represented by "A".
[0037]
It was found from the results of the Dupont test that superior plating adhesion
can be secured by controlling the Si content to be 0.20% to 1.0%, controlling the Mn
content to be 0.40% to 2.5%, and controlling a value of "[Mn] +5[Si]" to be 2.0 to 7.0
when the Si content is represented by [Si] and the Mn content is represented by [Mn].
[0038]
Furthel; in order to examine the mechanism of the above-described
phenomenon, the present inventors have thoroughly investigated the surface property
of a steel slieet immediately before plating. As a result, the present inventors have
found that there is a correlation between whether or not there is plating exfoliation and
surface fine convex-concave portions, thereby completing the present invention. The
surface fine convex-concave portions were measured as follows using shape
measurement laser microscopes VK-8700 and VK-8710 series (manufactured by
Keyence Corporation).
[0039]
That is, as illustrated in FIGS. 2 and 3, in a case where the surface fine
convex-concave portions of a steel sheet had a surface property in which an average of
displacements of a measurement point that is obtained based on a moving average of
continuous 31 points in total including 15 fiont points and 15 back points in a crosssectional
profile of a surface of the steel sheet obtained by measuring the average of
displacements in an evaluation length of 10 pni or more at an interval of 0.07 pm
(hereinafter, referred to as "fine convex-concave portion height") is 0.005 pm to 0.10
pm, plating exfoliation did not occur and superior plating adhesion was obtained.
However, when the fine convex-concave portion height was less than 0.005 pni (x in
FIG. 1) or was more than 0.10 pm (A in FIG. I), plating exfoliation occurred. In FIG.
3, "a measurement point (A)" corresponds to "the measurement point", "the average
(B) of heights of 3 1 points on both sides" corresponds to "the moving average of
continuous 31 points in total including 15 front points and 15 back points", and "a
difference (C) between the measurement point (A) and the average (B) of heights of 3 1
points on both sides" corresponds to "a displacement of a measurement point which is
obtained based on a moving average of continuous 31 points in total including 15 front
points and 15 back points".
[0040]
The reason is not clear but can be presumed to be as follows. When the fine
convex-concave portion height is less than 0.005 pm, an anchor effect of a plating
layer to a matrix is smaller than that of a case where the fine convex-concave portion
height is 0.005 pnl or more. On the other hand, when the fine convex-concave
portion height is more than 0.10 pm, the convex-concave portions are excessively
large, hydrogen is likely to be generated during elechoplating. In addition, hydrogen
is more likely to accumulate, and a plating layer is likely to be exfoliated when an
impact is applied thereto.
[0041]
Next, the reason for limiting the chemical composition of the steel sheet
according to the present invention will be described.
[0042]

C is an extremely important element. Specifically, C is an element which
binds to Nb or Ti to form a carbide and is effective for achieving high-strengthening.
Howevel; when the C content is more than 0.0050%, workability is decreased even
with the addition of Ti and Nb which are necessary for immobilizing C, and weld
toughness is decreased. Accordingly, the C content is controlled to be 0.0050% or
less.
[0043]
On the other hand, in the steel sheet according to the present invention, when
the C content is low, the strength can be complemented using other strengthening
methods. However, when the C content is less than 0.0005%, it is difficult to secure
the strength and the decarburization cost is increased during steelmaking.
[0044]
Accordingly, the C content is colitrolled to be 0.0050% or less. When
extremely high workability and weld toughness are required, the C content is
controlled to be preferably 0.0030% or less.
[0045]

Si is a solid-solution-strengthening element which is effective for highstrengthening.
The present inventors found that, when the Si content is 0.20% or
more and preferably 0.30% or more, plating adhesion after electroplating is improved.
[0046]
The reason is that fine convex-concave portions are formed on the surface
after pickling as described above. It is presumed that a Si oxide produced by Si on
the surface being oxidized is not completely coated on the surface in the layer form, is
non-uniformly distributed on the surface, and is non-uniformly dissolved during
pickling and the Si oxide forms surface convex-concave portions. As a result, plating
adhesion is improved. Accordingly, the Si content is colitrolled to be 0.20% or more
and preferably 0.30% or more.
[0047]
However, when the Si content is more than 1.0%, even if the other conditions
are in the ranges of the present invention, adhesion is decreased. The reason is
presumed to be as follows. The convex-concave portions are excessively large,
hydrogen is likely to be generated during electroplating. In addition, hydrogen is
more likely to accumulate, and a plating layer is likely to be exfoliated when an impact
is applied thereto. Accordingly, the Si content is controlled to be 1.0% or less.
[0048]

Mn is an element which increases the strength of steel sheet by solid solution
strengthening as in the case of Si and is one of the important elements to improve
plating adhesion.
[0049]
The present inventors found that Mn improves plating adhesion after
electroplating as in the case of Si. The reason is that surface convex-concave poi-tiol~s
are formed as describe above. It is presumed that a Mn oxide produced by Mn on the
surface being oxidized is not completely coated on the surface in the layer form, is
non-uniformly distributed on the surface, and is non-uniformly dissolved during
pickling and this Mn oxide forms surface convex-concave portions.
[0050]
Therefore, plating adhesion is improved. When the Mn content is 0.40% or
more, this effect is obtained. Therefore, the Mn content is controlled to be 0.40% or
more and preferably 0.50% or more.
[0051]
However, when the Mn content is more than 2.5%, even if the other
conditions are in the ranges of the present invention, adhesion is decreased. The
reason is presumed to be as follows. The convex-concave portions are excessively
large, hydrogen is likely to be generated during electroplating. In addition, hydrogen
is more likely to accumulate, and a plating layer is likely to be exfoliated when an
impact is applied thereto. Accordingly, the Mn content is controlled to be 2.5% or
less.
[0052]
Mn has a mechanism in which the strength is increased by structure
refinement and a mechanism in which the strength is increased by solid solution
strengthening. By controlling the Mn content to be 0.7% or more, a decrease in
secondary working brittleness resistance and weld toughness which may occur when
the strength is complemented with other elements can be easily avoided.
Accordingly, the Mn content is preferably 0.7% or more.
[0053]

P has a small decrease in workability even when being added and is an
element which is effective for increasing the strength by solid solution strengthening.
However, P is an element which segregates in grain boundaries to decrease secondary
working brittleness resistance, causes solidifying segregation to occur in a weld part,
and decreases weld toughness.
[0054]
When the P content is more than 0.05%, P segregation occurs. Accordingly,
the P content is controlled to be 0.05% or less. It is not necessary that the lower limit
of the P content be particularly limited. I-Iowever, when the lower limit of the P
content is less than 0.005%, the refining cost is increased. Therefore, the lower limit
of the P content is preferably 0.005% or more. When it is desired that a high-strength
steel sheet is produced, the P content is controlled to be preferably 0.02% or more from
the viewpoint of securing the strength.
[0055]
cTi: 0.010% to 0.050%>
Ti is an element which has high affinity to C and N, forms a carbonnitride
during solidification or hot rolling, reduces the amount of C and N solid-soluted in
steel, and increases workability.
[0056]
However, when Ti content is less than 0.010%, the above effects cannot be
obtained. Accordingly, the Ti content is controlled to be 0.010% or more. On the
other hand, when the Ti content is more than 0.050%, the weld toughness of a welded
joint is decreased. Accordingly,the Ti content is controlled to be 0.050% or less.
[0057]

As in the case of Ti, Nb is an element which has high affinity to C and N,
forms a carbonnitride during solidification or hot rolling, reduces the amount of C and
N solid-soluted in steel, and increases workability.
[0058]
However, when Nb content is less than 0.010%, the above effect cannot be
obtained. Accordingly, the Nb content is controlled to be 0.010% or more. On the
other hand, when the Nb content is more than 0.040%, the recrystallization
temperature is increased, high-temperature annealing is necessary, and the weld
toughness of a welded joint is decreased. Accordingly, the Nb content is controlled to
be 0.040% or less.
[0059]

B is an element which segregates in grain boundaries, increases the grain
boundary strength and improves secondary working brittleness resistance. Howevel;
when the B content is less than 0.0005%, the above effects cannot be obtained.
Accordingly, the B content is controlled to be 0.0005% or more.
[0060]
On the other hand, when the B content is more than 0.0030%, since B
segregates in y grain boundaries and suppresses ferritic transformation during welding,
a welded part and a heat-affected zone thereof have a low-temperature transformation
structure. Therefore, the welded part and the heat-affected zone are hardened, and the
weld toughness is decreased.
[0061]
In addition, when a large amount of B is added, ferritic transfornlation is
suppressed during hot rolling, a hot-rolled steel sheet having a low-temperature
transformation structure is produced, the strength of the hot-rolled steel sheet is
increased, and the load during cold rolling is increased. Further, when the B content
is more than 0.0030%, the recrystallization temperature is increased, and hightemperature
annealing is necessary. Therefore, the production cost is increased, and
the in-plane anisotropy of an r value which is an index of deep drawability is increased,
and press formability is decreased.
[0062]
Accordingly, the B content is controlled to be 0.0030% or less and preferably
0.0015% or less.
[0063]

S is an impurity which is unavoidably mixed during steel refining and is an
element which binds to Mn or Ti to form a precipitate and decreases workability.
Therefore, the S content is controlled to be 0.02% or less and preferably 0.01% or less.
It is not necessary that the lower limit of the S content be particularly limited.
However, when the lower limit of the S content is less than 0.0001%, the production
cost is increased. Therefore, the lower limit of the S content is preferably 0.0001% or
more.
[0064]

A1 is an element which is used as a deoxidation material during steel refining.
However, when Al content is less than O.Ol%, the above deoxidation effect cannot be
obtained. Accordingly, the A1 content is controlled to be 0.01% or more. On the
other hand, when the A1 content is more than 0.30%, the weld toughness and the
workability are decreased. Accordingly, the A1 content is controlled to be 0.30% or
less.
[0065]

N is an element whicli is unavoidably mixed during steel refining. Solidsoluted
N decreases workability, but a decrease in workability can be avoided if N
forms a nitride with Ti, Al, and Nb. Howevel; the weld toughness is decreased by this
precipitate. Therefore, the N content is controlled to be 0.01% or less. It is not
necessary that the lower limit of the N content be particularly limited. However,
when the N content is less than 0.0010%, the production cost is increased.
Accordingly, the N content is controlled to be 0.01% or less.
[0066]
The remainder of the steel sheet according to the present invention, that is,
conlponents other than the above-described elements include Fe and impurities.
[0067]
<[Mn]+S[Si]: 2.0 to 7.0>
Further, in the present invention, when the Si content is represented by [Si]
and the Mn content is represented by [Mn], a value of "[Mn]+5[Si]" is limited to be 2.0
to 7.0. The reason is as follows. Even if Si and Mn are in the ranges of the present
invention, when the value of "[Mn]+S[Si]" is less than 2.0, the fine convex-concave
portion height of the surface is less than 0.005 pm. On the other hand, when the
value of "[Mn]+S[Si]" is more than 7.0, the f i e convex-concave portion height of the
surface is more than 0.10 pm, plating adhesion is decreased.
[0068]
Next, a method for producing the steel sheet according to the present
invention will be described. When the steel sheet according to the present invention
is produced, first, raw materials which are adjusted to obtain the above-described
chemical composition are put into a converter or an electric furnace, followed by a
vacuum degassing treatment to obtain a slab.
[0069]
Next, this slab is hot-rolled under a heating temperature of 1000°C to 1300°C,
a finishing rolling temperature of & temperature to 1000°C, and a coiling temperature
of 850°C or less to obtain a hot-rolled coil. The heating temperature of hot rolling is
controlled to be 1000°C or higher to secure the rolling temperature.
[0070]
On the other hand, the heating temperature of hot rolling is controlled to be
1300°C or lower to suppress the production of coarse TiN which causes decrease in
toughness in hot rolling, to suppress the coarsening of austenite grains, and to suppress
the heating cost. -
[0071]
When the finishing rolling temperature of hot rolling is lower than As3
temperature, the workability of the steel sheet is decreased. Therefore, the finishing
rolling temperature of hot rolling is controlled to be As3 temperature or higher. In
addition, by controlling the finishing rolling temperature of hot rolling to be 1000°C or
lower, tlie structure of the steel sheet can be controlled to improve low-temperature
toughness. Further, when the coiling temperature after hot rolling is higher than
850°C, the strength of the steel sheet after cold rolling and annealing is decreased.
Therefore, the coiling temperature is controlled to be 850°C or lower. The lower
limit of the coiling temperature is not particularly limited but is preferably 400°C or
higher.
[0072]
Next, after descaling by pickling, the hot-rolled steel sheet prepared using the
above-described method is cold-rolled at a cold rolling reduction of 40% or higher to
obtain a cold-rolled steel sheet. At this time, when the cold rolling reduction is lower
than 40%, the strength of the annealed steel sheet is decreased, and deep drawability is
decreased. The cold rolling reduction is preferably 60% to 85%. As a result, a
plated steel sheet having superior strengtli and deep drawability can be obtained.
[0073]
Next, the cold-rolled steel sheet is annealed at a recrystallization temperature
or higher. At this time, when the annealing temperature is lower than the
recrystallization temperature, a superior texture is not developed, and deep drawability
is decreased. On the other hand, when the annealing temperature is increased, the
strength of the steel sheet is decreased. Therefore, it is preferable that the cold-rolled
steel sheet be annealed at 85OoC or lower.
[0074]
Next, in order to remove an oxide which is non-uniformly dispersed on the
surface and to obtain fine surface convex-concave portions, the non-plated surface of
the cold-rolled coil is pickled using 10 g/l to 400 g/l of a sulfuric acid aqueous solution
or using a pickling solution in which one or more selected from the group consisting of
nitrates, sulfates, fluorosilicates, and fluoroborates are mixed with the sulfuric acid
aqueous solution. Here, electrolytic pickling may be performed to increase the
efficiency of pickling.
[0075]
In the pickling solution, sulfuric acid which is used as a main agent is added
to obtain a lytic action on an oxide film and to improve electrical conductivity.
However, when sulfuric acid is coexistent with other solvents, the pickling efficiency is
significantly poor in a concentration of less than 10 g/l. On the other hand, in a
concentration of more than 400 g/l, pickling is excessive, and surface convex-concave
portions are coarsened. Accordingly, in the present invention, sulfuric acid is used in
a concentration of 10 g/l to 400 g/l.
[0076]
In order to improve the removal speed of an oxide film, one or more selected
from among fluorosilicates such as sodium fluorosilicate and potassium fluorosilicate
and fluoroborates such as sodium fluoroborate and ammonium fluoroborate may added
to the sulfuric acid aqueous solution.
[0077]
By the sulfuric acid aqueous solution containing fluorosilicates and
fluoroborates in each amount of 10 g/l, the removal speed of an oxide film can be
significantly improved. Accordingly, the content of any of fluorosilicates and
fluoroborates is controlled to be preferably 10 g/l or more. On the other hand, when
the content of any of fluorosilicates and fluoroborates is more than 100 g/l, the effect is
saturated. Accordingly, the content of any of fluorosilicates and fluoroborates is
controlled to be preferably 100 g/l or less.
[0078]
Nitrates have effects of improving the removal speed of an oxide film and
suppressing excessive pickling. As nitrates, sodium nitrate, potassium nitrate,
ammonium nitrate, and the like call be used. In order to reliably obtain the abovedescribed
effects, the content of nitrates is preferably 50 g/l or more. On the other
hand, when the content of nitrates is more than 200 dl, the effects are saturated.
Therefore, the content of nitrates is controlled to be preferably 200 g/l or less.
[0079]
Sulfates represented by sodium sulfate have an effect of suppressing excessive
pickling. In order to reliably obtain the above-described effect, the content of sulfates
is controlled to be preferably 50 g/l or more. On the other hand, when the content of
sulfates is more than 200 g/l, the effect is saturated. Therefore, the content of sulfates
is controlled to be preferably 200 g/l or less.
[OOSO]
Next, an electroplating layer is formed on the steel sheet to obtain an
electroplated steel sheet. It is preferable that the electroplating layer contain 0% to
20% of Ni atid the balance including Zn and impurities. When a corrosive
environment of a fuel tank is taken into consideration, the amount of Ni having an
effect of stabilizing a zinc corrosion product and itnproving corrosion resistance is
colitrolled to be preferably 2% or more and more preferably 5% or more. On the
other hand, from the viewpoint of cost, the Ni content is controlled to be preferably
15% or less.
[0081]
The plating deposition amount is controlled to be preferably 3 g/m2 to 100
g/m2 per single surface. By controlling the plating deposition amount to be 3 g/m2 or
more, higher corrosion resistance can be secured. By controlling the plating
depositio~a~m ount to be 100 g/m2 or less, a cost increase can be suppressed, pattern
defects caused by unevenness in thickness can be suppressed, and a decrease in
weldability can be suppressed. The plating deposition amount is controlled to be
preferably 3 g/n12 to 50 g/m2 per single surface.
[0082]
When a chemical conversion film, a chromate film, or various organic films
are formed on a plating layer, the same effects can be obtained. In addition, when a
coated steel sheet on which an organic film is formed is used, the same effects can be
obtained.
[Examples]
[0083]
Next, examples of the present invention will be described. Ilowevel;
conditions adopted in the examples are mere examples for confirming the operability
and the effects of the present invention. The present invention is not limited to these
conditions. The present invention may have various modifications without departing
from the scope oft he present invei~tioial s long as the object of the present invention
can be achieved.
[0084]
(Example 1)
- Steel having a chemical composition shown in Table lwas melted, was
heated and held at 1220°C, and was hot-rolled under a hot-rolling finishing rolling
temperature of AT3 temperature to 1000°C and a coiling temperature of 630°C to 670°C
to obtain a hot-rolled steel sheet having a thickness of 4 mm. Next, this hot-rolled
steel sheet was pickled and then cold-rolled to obtain a cold-rolled steel sheet having a
thickness of 1.2 mm.
[OOSS]
[Table 11
[0086] -
The balance of the chemical composition shown in Table 1 includes Fe and
impurities. The underline of Table 1 represents a numerical value being out of the
range of the present invention.
[0087]
Further, this cold-rolled steel sheet was annealed in a cycle of being held at
the recrystallization temperature or higher for 60 seconds to 120 seconds to obtain an
annealed steel sheet. A surface of this annealed steel sheet was pickled using a
pickling solution shown in Table 2 at 30°C to 50°C for 1 second to 30 seconds, and an
electroplating layer was formed thereon. The plating composition and the plating
deposition amount are as shown in Table 2.
[OOSS]
[Table 21
*KIND OF PICKLING SOLUTION AFTER ANNEALING
A : 50g/l OF SULFURIC ACID AQUEOUS SOLUTION
B:NITRATE
C : SULFATE
D : FLUOROSI LICATE
E: FLUOROBORATE
(KIND OF LIQUID SHOWN BELOW)
30
31
32
33
A+B+C+D
A
A+B+C+D+E
A+B+C
Zn-15XNi
Zn-1 OXNi
Zn-7XNi
Zn-2OXNi
20
30
40
10
[0089] -
Regarding the steel sheet immediately before plating, the fine convex-concave
portion height was measured in an evaluation length of 20 pm using the abovedescribed
shape measurement laser nlicroscopes VK-8700 and VK-8710 series
(manufactured by Keyence Corporation).
In addition using the plated steel sheets of Examples and Comparative
Examples prepared with the above-described method, a Dupont impact test was
performed to evaluate plating adhesion. In the Dupont impact test, assuming that the
steel sheet was pressed into a conlplicated shape such as a fuel tank, a weight having a
tip shape with a punch diameter of 4 mm and a weight of 500 g which was more strict
than a normal one was allowed to fall to the steel sheet from a height of 1 m. The test
results are shown in Table 3. A case where there was no plating exfoliation is
represented by "om, and a case where there was plating exfoliation is represented by
"x". Whether or not there was plating exfoliation was determined by attaching an
adhesive tape on a sample surface and then peeling therefrom after the test. A case
where no plating layer was peeled off was determined as "no plating exfoliation (o)",
and a case where a plating layer was peeled off and attached on the adhesive tape was
determined as "plating exfoliation (x)".
[0090]
[Table 31
DUPONT IMPACT TES
30
31
32
33
L 0 13
-0.18
-0.23
x
x
X
X
[0091] -
The underline of Table 3 represents a numerical value being out of the range
of the present invention.
[0092]
In the steel sheets of Examples No. 1 to 20 where the values are within the
range of the present invention, the Si content is 0.20% to 1.0% and the Mn content is
0.40% to 2.5%. In addition, when the Si content is represented by [Si] and the Mn
content is represented by [Mn], a value of "[Mn]+5[Si]" is 2.0 to 7.0. The fine
convex-concave portion height of the steel sheet surface immediately before plating
was 0.005 pm to 0.10 pm, and superior plating adhesion was shown in the Dupont
impact test.
[0093]
On the other hand, in the plated steel sheets of Comparative Examples No. 21
to 33 in which the Si content, the Mn content, and the value of "[Mn]+5[Si]" are out of
the range of the present invention, the fine convex-concave portion height of the steel
slieet surface immediately before plating was less than 0.005 pm or more than 0.10
pm, plating exfoliation was found in the Dupont impact test, and plating adhesion was
insufficient.
[Industrial Applicability]
[0094]
According to the present invention, a steel sheet having superior plating
adhesion can be obtained, and it is possible to provide an electroplated steel slieet
which has superior press formability and plating adhesion in the automobile field and
the home electronics field, particularly, for use in a fuel tank of a vehicle; a steel sheet
for electroplating which is a material of the electroplated steel slieet; and metliods for
producing the same. In addition, according to the present invention, the allowable
processing degree of a plated steel sheet is improved, and reduction in the weight of a
vehicle, particularly, a fuel tank and complication of a vehicle body design can be
realized. Accordingly, the present invention is highly applicable to the automobile
industry.
[Document Type] CLAIMS
[Claim 11
A steel sheet for electroplating comprising, as a chemical composition, by
mass%,
C: 0.0005% to 0.0050%,
Si: 0.20% to 1.0%,
Mn: 0.40% to 2.5%,
P: 0.05% or less,
Ti: 0.010% to 0.050%,
Nh: 0.010% to 0.040%,
B: 0.0005% to 0.0030%,
S: 0.02% or less,
Al: 0.01% to 0.30%,
N: 0.0010% to 0.01%, and
the balance including Fe and impurities,
wherein in the chemical composition, when a Si content is represented by [Si]
and a Mn content is represented by [Mn], a value of "[Mt1]+5[Si]" is 2.0 to 7.0, and
the steel sheet has a surface property in which an average of displacements of
a measurement point that is obtained based on a moving average of continuous 31
points in total including 15 fiont points and 15 back points in a cross-sectional profile
of a surface of the steel sheet obtained by measuring the average of displacements in
an evaluation length of 10 pm or more at an inte~lraol f 0.07 pm is 0.005 pln to 0.10
pnl.
[Claim 21
An electroplated steel sheet, wherein
an electroplating layer is formed on the surface of the steel sheet for
electroplating according to claim 1.
[Claim 31
A method for producing a steel sheet for electroplating, the method
comprising:
continuously casting molten steel having the chemical composition according
to Claim 1 into a slab;
heating the slab in a temperature range of 1000°C to 1300°C;
hot-rolling the heated slab under a finishing rolling temperature of As3
temperature to 1000°C and a coiling temperature of 850°C or lower to obtain a hotrolled
steel sheet;
removing a surface scale of the hot-rolled steel sheet to obtain a pickled steel
sheet, as a first pickling process;
cold-rolling the pickled steel sheet at a cold rolling reduction of 40% or higher
to obtain a cold-rolled steel sheet;
annealing the cold-rolled steel sheet at a recrystallization temperature or
higher; and
pickling a surface of the steel sheet using 10 g/l to 400 g/l of a sulfuric acid
aqueous solution or using a pickling solution in which one or more selected from the
group consisting of nitrates, sulfates, fluorosilicates, and fluoroborates are mixed with
the sulfuric acid aqueous solution, as a second pickling process.
[Claim 41
A method for producing an electroplated steel sheet, the method comprising
forming an electroplating layer, which contains 0% to 20% of Ni and the
balance including Zn and impurities, on the surface of the steel sheet for electroplating,
which is obtained using the method according to Claim 3, in an amount of 3 g/m2 to
100 g/m2 per single surface.