Note: Descriptions are shown in the official language in which they were submitted.
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DESCRIPTION
[Title of Invention] HIGH STRENGTH STEEL SHEET AND METHOD
FOR MANUFACTURING THE SAME
[Technical Field]
[0001]
The present invention relates to a high strength steel
sheet having excellent chemical convertibility and corrosion
resistance after electrodeposition coating even in the case
where the steel sheet has a high Si content, as well as to a
method for manufacturing such steel sheets.
[Background Art]
[0002]
From the viewpoint of the improvements in automobile
fuel efficiency and crash safety of the automobiles, there
have recently been increasing demands for car body materials
to be increased in strength for thickness reduction in order
to reduce the weight and increase the strength of car bodies
themselves. For this purpose, the use of high strength
steel sheets in automobiles has been promoted.
In general, automotive steel sheets are painted before
use. As a pretreatment before painting, a chemical
conversion treatment called phosphatization is performed.
The chemical conversion treatment for steel sheets is one of
the important treatments for ensuring corrosion resistance
after painting.
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[0003]
The addition of silicon is effective for increasing the
strength and the ductility of steel sheets. During
continuous annealing, however, silicon is oxidized even if
the annealing is performed in a reductive N2 H2 gas
atmosphere which does not induce the oxidation of Fe (which
reduces Fe oxides). As a result, a silicon oxide (Si02) is
formed on the outermost surface of a steel sheet. This Si02
inhibits a reaction for forming a chemical conversion film
during a chemical conversion treatment, thereby resulting in
formation of a microscopical region where any chemical
conversion film is not generated. (Hereinafter, such a
region will be sometimes referred to as "non-covered
region"). That is, chemical convertibility is lowered.
[0004]
Among conventional techniques directed to the
improvement of chemical convertibility of high-Si containing
steel sheets, patent document 1 discloses a method in which
an iron coating layer is electroplated at 20 to 1500 mg/m2
onto a steel sheet. However, this method entails the
provision of a separate electroplating facility and
increases costs correspondingly to an increase in the number
of steps.
[0005]
Further, patent documents 2 and 3 provide an
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improvement in phosphatability by specifying the Mn/Si ratio
and by adding nickel, respectively. However, the effects
are dependent on the Si content in a steel sheet, and a
further improvement will be necessary for steel sheets
having a high Si content.
[0006]
Patent document 4 discloses a method in which the dew-
point temperature during annealing is controlled to be -25
to 0 C so as to form an internal oxide layer which includes
a Si-containing oxide within a depth of 1 gm from the
surface of a steel sheet base as well as to control the
proportion of the Si-containing oxide to be not more than
80% over a length of 10 gm of the surface of the steel sheet.
However, the method described in patent document 4 is
predicated on the idea that the dew-point temperature is
controlled with respect to the entire area inside a furnace.
Thus, difficulties are encountered in controlling the dew-
point temperature and ensuring stable operation. If
annealing is performed while the controlling of the dew-
point temperature is unstable, the distribution of internal
oxides formed in a steel sheet becomes nonuniform to cause a
risk that chemical convertibility may be variable in a
longitudinal direction or a width direction of the steel
sheet (non-covered regions may be formed in the entirety or
a portion of the steel sheet). Even though an improvement
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in chemical convertibility is attained, a problem still
remains in that corrosion resistance after electrodeposition
coating is poor because of the presence of the Si-containing
oxide immediately under the chemical conversion coating.
[0007]
Further, patent document 5 describes a method in which
the steel sheet temperature is brought to 350 to 650 C in an
oxidative atmosphere so as to form an oxide film on the
surface of the steel sheet, and thereafter the steel sheet
is heated to a recrystallization temperature in a reductive
atmosphere and subsequently cooled. With this method,
however, it is often the case that the thickness of the
oxide film formed on the surface of the steel sheet is
variable depending on the oxidation method and that the
oxidation does not take place sufficiently or the oxide film
becomes excessively thick with the result that the oxide
film leaves residue or is exfoliated during the subsequent
annealing in a reductive atmosphere, thus resulting in a
deterioration in surface quality. In EXAMPLES, this patent
document describes an embodiment in which oxidation is
carried out in air. However, oxidation in air causes
problems such as giving a thick oxide which is hardly
reduced in subsequent reduction or requiring a reductive
atmosphere with a high hydrogen concentration.
[0008]
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Furthermore, patent document 6 describes a method in
which a cold rolled steel sheet containing, in terms of
mass%, Si at not less than 0.1% and/or Mn at not less than
1.0% is heated at a steel sheet temperature of not less than
400 C in an iron-oxidizing atmosphere to form an oxide film
on the surface of the steel sheet, and thereafter the oxide
film on the surface of the steel sheet is reduced in an
iron-reducing atmosphere. In detail, iron on the surface of
the steel sheet is oxidized at not less than 400 C using a
direct flame burner with an air ratio of not less than 0.93
and not more than 1.10, and thereafter the steel sheet is
annealed in a N2 H2 gas atmosphere which reduces the iron
oxide, thereby forming an iron oxide layer on the outermost
surface while suppressing the oxidation of Si02 which lowers
chemical convertibility from occurring on the outermost
surface. Patent document 6 does not specifically describe
the heating temperature with the direct flame burner.
However, in the case where Si is present at a high content
(generally, 0.6% or more), the oxidation amount of silicon,
which is more easily oxidized than iron, becomes large so as
to suppress the oxidation of Fe or limit the oxidation of Fe
itself to a too low level. As a result, the formation of a
superficial reduced Fe layer by the reduction becomes
insufficient and Si02 comes to be present on the surface of
the steel sheet after the reduction, thus possibly resulting
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in a region which may not be covered with a chemical film.
[Citation List] .
,
[Patent Document]
[0009]
[Patent document 1] Japanese Unexamined Patent
Application Publication No. 5-320952
[Patent document 2] Japanese Unexamined Patent
Application Publication No. 2004-323969
[Patent document 3] Japanese Unexamined Patent
Application Publication No. 6-10096
[Patent document 4] Japanese Unexamined Patent
Application Publication No. 2003-113441
[Patent document 5] Japanese Unexamined Patent
Application Publication No. 55-145122
[Patent document 6] Japanese Unexamined Patent
Application Publication No. 2006-45615
[Summary of Invention]
[Technical Problem]
[0010]
The present invention has been made in view of the
circumstances described above. It is therefore an object of
the invention to provide a high strength steel sheet which
exhibits excellent chemical convertibility and corrosion
resistance after electrodeposition coating even in the case
of a high Si content, as well as to provide a method for
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manufacturing such steel sheets.
[Solution to Problem] .
,
[0011]
Conventional approaches were based on simply increasing
the water vapor partial pressure or the oxygen partial
pressure in the entire inside of an annealing furnace so as
to raise the dew-point temperature or the oxygen
concentration and thereby to produce excessive internal
oxidation of a steel sheet. Consequently, as mentioned
above, various problems have been encountered such as
difficulties in controlling the dew-point temperature or the
oxidation, the occurrence of uneven chemical convertibility
and a decrease in corrosion resistance after
electrodeposition coating. Thus, the present inventors
studied a novel approach based on an unconventional idea
capable of solving the above problems. As a result, the
present inventors have found that because a deterioration in
corrosion resistance after electrodeposition coating can
originate from a surface portion of a steel sheet, more
sophisticated controlling of the microstructure and
configuration of the surface portion of the steel sheet
allows for obtaining a high strength steel sheet having
excellent chemical convertibility and corrosion resistance
after electrodeposition coating. In detail, a chemical
conversion treatment is performed after annealing is carried
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out in such a manner that the dew-point temperature of the
atmosphere is controlled to become not less than -10 C when
the heating furnace inside temperature is in a limited range
of not less than A C and not more than 3 C during the course
of heating (A: 600 5_ A 780, B: 800 B 900). In this
manner, selective surface oxidation and surface segregation
can be suppressed, resulting in a high strength steel sheet
exhibiting excellent chemical convertibility and corrosion
resistance after electrodeposition coating. Here, the term
"excellent chemical convertibility" means that a steel sheet
having undergone a chemical conversion treatment has an
appearance without any non-covered regions or uneven results
of the chemical conversion treatment.
[0012]
A high strength steel sheet obtained in the above
manner comes to have a microstructure and configuration in
which a surface portion of the steel sheet extending from
the steel sheet surface within a depth of 100 m contains an
oxide of at least one or more selected from Fe, Si, Mn, Al
and P, as well as from B, Nb, Ti, Cr, Mo, Cu and Ni at 0.010
to 0.50 g/m2 per single side surface, and in which a region
extending from the steel sheet surface to a depth of 10 m
is such that a crystalline Si/Mn oxide has been precipitated
in base iron grains that are within 1 m from grain
boundaries. Because of this configuration, deterioration in
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corrosion resistance after electrodeposition coating is
realized and excellent chemical convertibility is obtained.
The present invention is based on the aforementioned
findings. Features of the invention are as described below.
[0013]
[1] A method for manufacturing high strength steel
sheets, including continuous annealing of a steel sheet
which includes, in terms of mass%, C at 0.01 to 0.18%, Si at
0.4 to 2.0%, Mn at 1.0 to 3.0%, Al at 0.001 to 1.0%, P at
0.005 to 0.060% and S at 5 0.01%, the balance being
represented by Fe and inevitable impurities, while
controlling the dew-point temperature of the atmosphere to
become not less than -10 C when the heating furnace inside
temperature is in the range of not less than A C and not
more than 3 C during the course of heating wherein A is 600
A 5 780 and B is 800 5 B 5 900.
[0014]
[2] The method for manufacturing high strength steel
sheets described in [1], wherein the chemical composition of
the steel sheet further includes one or more elements
selected from B at 0.001 to 0.005%, Nb at 0.005 to 0.05%, Ti
at 0.005 to 0.05%, Cr at 0.001 to 1.0%, Mo at 0.05 to 1.0%,
Cu at 0.05 to 1.0% and Ni at 0.05 to 1.0% in terms of mass%.
[0015]
[3] The method for manufacturing high strength steel
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sheets described in [1] or [2], further including, after the
continuous annealing, electrolytically pickling the steel
sheet in an aqueous solution containing sulfuric acid.
[0016]
[4] A high strength steel sheet manufactured by the
method described in any of [1] to [3] in which a surface
portion of the steel sheet extending from the steel sheet
surface within a depth of 100 pm contains an oxide of at
least one or more selected from Fe, Si, Mn, Al, P, B, Nb, Ti,
Cr, Mo, Cu and Ni at 0.010 to 0.50 g/m2 per single side
surface, and in which with respect to a region extending
from the steel sheet surface within a depth of 10 pm, a
crystalline Si/Mn oxide is present in grains that are within
1 pm from crystal grain boundaries of the steel sheet.
[0017]
In the present invention, the term "high strength"
means that the tensile strength TS is not less than 340 MPa.
The high strength steel sheets in the invention include both
cold rolled steel sheets and hot rolled steel sheets.
[Advantageous Effects of Invention]
[0018]
According to the present invention, a high strength
steel sheet is obtained which exhibits excellent chemical
convertibility and corrosion resistance after
electrodeposition coating even in the case where the steel
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sheet has a high Si content.
[Description of Embodiments] ,
[0019]
The present invention will be described in detail
hereinbelow. In the following description, the unit for the
contents of individual elements in the chemical composition
of steel is "mass%" and is indicated simply as "%" unless
otherwise mentioned.
[0020]
First, there will be described annealing atmosphere
conditions that are the most important requirement in the
invention and determine the structure of the surface of the
steel sheet.
A chemical conversion treatment is performed after a
steel sheet is continuously annealed in such a manner that
the dew-point temperature of the atmosphere is controlled to
become not less than -10 C when the heating furnace inside
temperature is in a limited range of not less than A C and
not more than 3 C during the course of heating in an
annealing furnace (A: 600 5. A 5 780, B: 800 5 B 5 900). In
this manner, oxides of easily oxidized elements (such as Si
and Mn) are allowed to be present in appropriate amounts
inside a surface portion of the steel sheet extending from
the surface within a depth of 10 m (hereinafter, such
oxides will be referred to as internal oxides), thereby
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making it possible to suppress selective surface oxidation
of such elements as Si and Mn on the steel sheet surface
that deteriorate the chemical convertibility of the steel
after annealing (hereinafter, this oxidation will be
referred to as "surface segregation") from occurring in the
surface portion of the steel sheet.
[0021]
The lower limit temperature A is limited to be 600 A
780 for the following reasons. When the temperature is in
the range of less than 600 C, the amount of surface
segregation is inherently small. Thus, a deterioration in
chemical convertibility is not caused in such a temperature
range even if the dew-point temperature is not controlled
and internal oxides are not formed. If the temperature is
raised to above 780 C without controlling of the dew-point
temperature, the amount of surface segregation is so
increased that the inward diffusion of oxygen is inhibited
and internal oxidation is unlikely to occur. It is
therefore necessary to control the dew-point temperature to
become not less than -10 C at least from when the
temperature is in the range of not more than 780 C. Thus,
the acceptable range of A is 600 A 5_ 780. For the above
reason, it is preferable that A be a temperature as low as
possible within this range.
[0022]
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The upper limit temperature B is limited to be 800 B
900 for the following reasons.. The formation of internal
oxides decreases the amount of easily oxidized elements
(such as Si and Mn) present as solutes inside a surface
portion of the steel sheet extending from the surface within
a depth of 10 m (hereinafter, such a portion will be
referred to as "deficient layer"), and thereby the easily
oxidized elements are suppressed from diffusing from the
inside of steel toward the surface. In order to form such
internal oxides as well as to form the deficient layer
enough to suppress the occurrence of surface segregation,
the temperature B needs to satisfy 800 B 900. If the
temperature is less than 800 C, internal oxides are not
formed sufficiently. If the temperature exceeds 900 C,
internal oxides are formed in excessively large amounts and
serve as starting points of a deterioration in corrosion
resistance after electrodeposition coating.
[0023]
The dew-point temperature is controlled to become not
less than -10 C when the temperature is in the range of not
less than A C and not more than B C for the following
reasons. Increasing the dew-point temperature increases the
potential of 02 generated by the decomposition of H20, and
therefore internal oxidation can be promoted. The amount of
formed internal oxides is small if the dew-point temperature
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is in the range of below -10 C. The upper limit of the dew-
point temperature is not ,particularly limited. However, the
amount of oxidation of iron increases if the dew-point
temperature is in excess of 90 C, causing a risk that
annealing furnace walls or rollers may be degraded. Thus,
the dew-point temperature is preferably not more than 90 C.
[0024]
Next, the chemical composition of the high strength
steel sheets of interest according to the present invention
will be described.
[0025]
C: 0.01 to 0.18%
Carbon increases workability by forming phases such as
martensite in the steel microstructure. In order to obtain
this effect, carbon needs to be contained at not less than
0.01%. On the other hand, containing carbon in excess of
0.18% causes a decrease in elongation as well as
deteriorations in quality and weldability. Thus, the C
content is limited to be not less than 0.01% and not more
than 0.18%.
[0026]
Si: 0.4 to 2.0%
Silicon increases the strength and the elongation of
steel and is therefore an effective element for achieving a
good quality. In order to obtain the objective strength in
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the present invention, silicon needs to be contained at not
less than 0.4%. Steel sheets having a Si content of less
than 0.4% cannot achieve a strength of interest in the
invention and are substantially free of problems in terms of
chemical convertibility. On the other hand, containing
silicon in excess of 2.0% results in the saturation of steel
strengthening effects as well as the saturation of
elongation enhancement, and achieving an improvement of
chemical convertibility becomes difficult. Thus, the Si
content is limited to be not less than 0.4% and not more
than 2.0%.
[0027]
Mn: 1.0 to 3.0%
Manganese is an effective element for increasing the
strength of steel. In order to ensure mechanical
characteristics and strength, the Mn content needs to be not
less than 1.0%. On the other hand, containing manganese in
excess of 3.0% causes difficulties in ensuring weldability
as well as in ensuring the balance between strength and
ductility. Thus, the Mn content is limited to be not less
than 1.0% and not more than 3.0%.
[0028]
Al: 0.001 to 1.0%
Aluminum is added for the purpose of deoxidation of
molten steel. The deoxidation effect for molten steel is
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obtained by adding aluminum at not less than 0.001%. On the
other hand, adding aluminum in excess of 1.0% increases
costs and further results in an increase in the amount of
surface segregation of aluminum, thereby making it difficult
to improve chemical convertibility. Thus, the Al content is
limited to be not less than 0.001% and not more than 1.0%.
[0029]
P: 0.005 to not more than 0.060%
Phosphorus is one of elements that are inevitably
present in steel. An increase in cost is expected if the P
content is reduced to below 0.005%. Thus, the P content is
specified to be not less than 0.005%. On the other hand,
any P content exceeding 0.060% leads to a decrease in
weldability and causes a marked deterioration in chemical
convertibility to such an extent that it becomes difficult
to improve chemical convertibility even by the present
invention. Thus, the P content is limited to be not less
than 0.005% and not more than 0.060%.
[0030]
S: 0.01%
Sulfur is one of inevitable elements. The lower limit
is not particularly limited. However, the presence of this
element in a large amount causes decreases in weldability
and corrosion resistance. Thus, the S content is limited to
be not more than 0.01%.
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[0031]
In order to control the balance between strength and
ductility, one or more elements selected from 0.001 to
0.005% of B, 0.005 to 0.05% of Nb, 0.005 to 0.05% of Ti,
0.001 to 1.0% of Cr, 0.05 to 1.0% of Mo, 0.05 to 1.0% of Cu
and 0.05 to 1.0% of Ni may be added as required.
The appropriate amounts of these optional elements are
limited for the following reasons.
[0032]
B: 0.001 to 0.005%
The effect in promoting hardening is hardly obtained if
the B content is less than 0.001%. On the other hand,
adding boron in excess of 0.005% results in a decrease in
chemical convertibility. Thus, when boron is contained, the
B content is limited to be not less than 0.001% and not more
than 0.005%.
[0033]
Nb: 0.005 to 0.05%
The effect in adjusting strength is hardly obtained if
the Nb content is less than 0.005%. On the other hand,
containing niobium in excess of 0.05% results in an increase
in cost. Thus, when niobium is contained, the Nb content is
limited to be not less than 0.005% and not more than 0.05%.
[0034]
Ti: 0.005 to 0.05%
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The effect in adjusting strength is hardly obtained if
the Ti content is less than 0..005%. On the other hand,
containing titanium in excess of 0.05% results in a decrease
in chemical convertibility. Thus, when titanium is
contained, the Ti content is limited to be not less than
0.005% and not more than 0.05%.
[0035]
Cr: 0.001 to 1.0%
The effect in promoting hardening is hardly obtained if
the Cr content is less than 0.001%. On the other hand,
containing chromium in excess of 1.0% results in the surface
segregation of chromium and a consequent decrease in
weldability. Thus, when chromium is contained, the Cr
content is limited to be not less than 0.001% and not more
than 1.0%.
[0036]
Mo: 0.05 to 1.0%
The effect in adjusting strength is hardly obtained if
the Mo content is less than 0.05%. On the other hand,
containing molybdenum in excess of 1.0% results in an
increase in cost. Thus, when molybdenum is contained, the
Mo content is limited to be not less than 0.05% and not more
than 1.0%.
[0037]
Cu: 0.05 to 1.0%
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The effect in promoting the formation of a retained y-
phase is hardly obtained .if the Cu content is less than
0.05%. On the other hand, containing copper in excess of
1.0% results in an increase in cost. Thus, when copper is
contained, the Cu content is limited to be not less than
0.05% and not more than 1.0%.
[0038]
Ni: 0.05 to 1.0%
The effect in promoting the formation of a retained 7-
phase is hardly obtained if the Ni content is less than
0.05%. On the other hand, containing nickel in excess of
1.0% results in an increase in cost. Thus, when nickel is
contained, the Ni content is limited to be not less than
0.05% and not more than 1.0%.
The balance after the deduction of the aforementioned
elements is represented by Fe and inevitable impurities.
[0039]
Next, there will be described a method for
manufacturing the high strength steel sheets according to
the invention as well as the reasons why the conditions in
the method are limited. For example, a steel having the
above-described chemical composition is hot rolled and is
thereafter cold rolled, and subsequently the steel sheet is
annealed in a continuous annealing facility and is subjected
to a chemical conversion treatment. Here, in the present
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invention, the annealing is carried out in such a manner
that the dew-point temperature. of the atmosphere is
controlled to become not less than -10 C when the heating
furnace inside temperature is in the range of not less than
A C and not more than B C during the course of heating (A:
600 A 780, B: 800 B 900).
This is the most important
requirement in the invention. By controlling the dew-point
temperature, namely, the oxygen partial pressure in the
atmosphere during the annealing step, the oxygen potential
is increased with the result that easily oxidized elements
such as Si and Mn are internally oxidized beforehand
immediately before a chemical conversion treatment and the
activities of Si and Mn in the surface portion of the steel
sheet are lowered. Consequently, the external oxidation of
these elements is suppressed, resulting in an improvement in
chemical convertibility. In the above processing of steel,
it is possible to anneal the hot rolled steel sheet without
subjecting it to cold rolling.
[0040]
Hot rolling
Hot rolling may be performed under usual conditions.
[0041]
Pickling
It is preferable to perform a pickling treatment after
hot rolling. In the pickling step, black scales formed on
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the surface are removed and the steel sheet is subjected to
cold rolling. Pickling conditions are not particularly
limited.
[0042]
Cold rolling
Cold rolling is preferably carried out with a draft of
not less than 40% and not more than 80%. If the draft is
less than 40%, the recrystallization temperature becomes
lower and the steel sheet tends to be deteriorated in
mechanical characteristics. On the other hand, because the
steel sheet of the invention is a high strength steel sheet,
cold rolling the steel sheet with a draft exceeding 80%
increases not only the rolling costs but also the amount of
surface segregation during annealing, possibly resulting in
a decrease in chemical convertibility.
[0043]
The steel sheet that has been cold rolled or hot rolled
is annealed and then subjected to a chemical conversion
treatment.
[0044]
In an annealing furnace, the steel sheet undergoes a
heating step in which the steel sheet is heated to a
predetermined temperature in an upstream heating zone and a
soaking step in which the steel sheet is held in a
downstream soaking zone at a predetermined temperature for a
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prescribed time. Next, a cooling step is performed.
As described above, the annealing is carried out in
such a manner that the dew-point temperature of the
atmosphere is controlled to become not less than -10 C when
the heating furnace inside temperature is in the range of
not less than A C and not more than 3 C (A: 600 A 5 780,
B: 800 B 5 900). Except when the temperature is in the
range of not less than A C and not more than B C, the dew-
point temperature of the atmosphere in the annealing furnace
is not particularly limited, but is preferably in the range
of -50 C to -10 C.
[0045]
The gas components in the annealing furnace include
nitrogen, hydrogen and inevitable impurities. Other gas
components may be present as long as they are not
detrimental in achieving the advantageous effects of the
invention. If the hydrogen concentration in the annealing
furnace atmosphere is less than 1 vol%, the activation
effect by reduction cannot be obtained and chemical
convertibility is deteriorated. Although the upper limit is
not particularly limited, costs are increased and the effect
is saturated if the hydrogen concentration exceeds 50 vol%.
Thus, the hydrogen concentration is preferably not less than
1 vol% and not more than 50 vol%. The gas components in the
annealing furnace except hydrogen gas are nitrogen gas and
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inevitable impurity gases. Other gas components may be
present as long as they are not detrimental in achieving the
advantageous effects of the invention.
After the steel sheet is cooled from the temperature
range of not less than 750 C, hardening and tempering may be
performed as required. Although the conditions for these
treatments are not particularly limited, it is desirable
that tempering be performed at a temperature of 150 to 400 C.
The reasons are because elongation tends to be deteriorated
if the temperature is less than 150 C as well as because
hardness tends to be decreased if the temperature is in
excess of 400 C.
[0046]
According to the present invention, good chemical
convertibility can be ensured even without performing
electrolytic pickling. However, it is preferable that
electrolytic pickling be performed in order to remove trace
amounts of oxides that have been inevitably generated by
surface segregation during annealing and thereby to ensure
better chemical convertibility.
The electrolytic pickling conditions are not
particularly limited. However, in order to efficiently
remove the inevitably formed surface segregation of silicon
and manganese oxides formed during the annealing,
alternating electrolysis at a current density of not less
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than 1 A/dm2 is desirable. The reasons why alternating
electrolysis is selected are because the pickling effects
are low if the steel sheet is fixed to a cathode as well as
because if the steel sheet is fixed to an anode, iron that
is dissolved during electrolysis is accumulated in the
pickling solution and the Fe concentration in the pickling
solution is increased with the result that the attachment of
iron to the surface of the steel sheet causes problems such
as dry contamination.
[0047]
The pickling solution used in the electrolytic pickling
is not particularly limited. However, nitric acid or
hydrofluoric acid is not preferable because they are highly
corrosive to a facility and require careful handling.
Hydrochloric acid is not preferable because chlorine gas can
be generated from the cathode. In view of corrosiveness and
environment, the use of sulfuric acid is preferable. The
sulfuric acid concentration is preferably not less than 5
mass% and not more than 20 mass%. If the sulfuric acid
concentration is less than 5 mass%, the conductivity is so
lowered that the bath voltage is raised during electrolysis
possibly to increase the power load. On the other hand, any
sulfuric acid concentration exceeding 20 mass% leads to a
cost problem because a large loss is caused due to drag-out.
[0048]
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The temperature of the electrolytic solution is
preferably not less than 40 C and not more than 70 C.
Because the bath temperature is raised by the generation of
heat by continuous electrolysis, the pickling effect may be
lowered if the temperature is less than 40 C. Further,
maintaining the temperature below 40 C is sometimes
difficult. Furthermore, a temperature exceeding 70 C is not
preferable in view of the durability of the lining of the
electrolytic cell.
[0049]
The high strength steel sheets of the present invention
are obtained in the above manner.
As a result, the inventive steel sheet has a
characteristic structure of the surface described below.
A surface portion of the steel sheet extending from the
steel sheet surface within a depth of 100 gm contains an
oxide of one or more selected from Fe, Si, Mn, Al and P, as
well as from B, Nb, Ti, Cr, Mo, Cu and Ni at a total amount
of 0.010 to 0.50 g/m2 per single side surface. Further, with
respect to a region extending from the steel sheet surface
to a depth of 10 gm, a crystalline Si/Mn complex oxide is
present in base iron grains that are within 1 gm from grain
boundaries.
[0050]
In a high strength steel sheet containing Si and a
CA 02811489 2013-03-15
- 26 -
large amount of Mn, more sophisticated controlling of the
microstructure and configuration of a surface portion of the
steel sheet which can be an origin of corrosion or cracks is
necessary in order to achieve satisfactory corrosion
resistance after electrodeposition coating. For the purpose
of ensuring chemical convertibility, the present invention
first provides that the dew-point temperature is controlled
as described hereinabove in order to increase the oxygen
potential in the annealing step. As a result of the oxygen
potential being increased, easily oxidized elements such as
Si and Mn are internally oxidized beforehand immediately
before a chemical conversion treatment and the activities of
Si and Mn in the surface portion of the steel sheet are
lowered. Consequently, the external oxidation of these
elements is suppressed, resulting in improvements in
chemical convertibility and corrosion resistance after
electrodeposition coating. These improvements are obtained
by configuring the steel sheet such that the surface portion
of the steel sheet extending from the steel sheet surface
within a depth of 100 m contains an oxide of at least one
or more selected from Fe, Si, Mn, Al and P, as well as from
B, Nb, Ti, Cr, Mo, Cu and Ni at not less than 0.010 g/m2 per
single side surface. The effects are saturated even when
such oxides are present in excess of 0.50 g/m2. Thus, the
upper limit is specified to be 0.50 g/m2.
CA 02811489 2013-03-15
- 27 -
[0051]
In the case where internal oxides are present only at
grain boundaries and not in grains, the intergranular
diffusion of easily oxidized elements in steel can be
suppressed but the intragranular diffusion thereof may not
be suppressed sufficiently. Thus, as described hereinabove,
the present invention provides that internal oxidation is
caused to take place not only at grain boundaries but also
in grains by controlling the dew-point temperature of the
atmosphere to become not less than -10 C when the heating
furnace inside temperature is in the range of not less than
A C and not more than B C (A: 600 A 780, B: 800 B
900). In detail, a crystalline Si/Mn complex oxide is
caused to be present in base iron grains that are within 1
m from grain boundaries in a region extending from the
steel sheet surface to a depth of 10 m. Because of the
oxide being present in base iron grains, the amount of
solute silicon and manganese in base iron grains in the
vicinity of the oxide is decreased. As a result, the
surface segregation of Si and Mn due to intragranular
diffusion can be suppressed.
[0052]
The structure of the surface of the high strength steel
sheet obtained by the manufacturing method according to the
present invention is as described above. There is no
CA 02811489 2013-03-15
- 28 -
problem even when the oxides have been grown so as to extend
to a region that is more than 100 m away from the steel
sheet surface. Further, no problems are caused even when
the crystalline Si/Mn complex oxide is caused to be present
in base iron grains that are more than 1 m away from grain
boundaries in a region extending from the steel sheet
surface to a depth in excess of 10 m.
[EXAMPLE 1]
[0053]
Hereinbelow, the present invention will be described in
detail based on EXAMPLES.
Hot rolled steel sheets with a steel composition
described in Table 1 were pickled to remove black scales and
were thereafter cold rolled to give cold rolled steel sheets
with a thickness of 1.0 mm. Cold rolling was omitted for
some of the steel sheets. That is, as-descaled hot rolled
steel sheets (thickness: 2.0 mm) were also provided.
- 29 -
.
,
,
[0054]
[Table 1]
Table 1
(mass%)
Steel code C Si Mn Al P S Cr Mo B
Nb Cu Ni Ti
,
A 0.04 0.1 1.9 0.04 0.01 0.003 - - -
- - - -
1
, B 0.03 0.4 2.0 0.04 0.01 0.003 - - -
- - - -
C 0.09 0.9 2.1 0.03 0.01 0.004 - - -
- - - -
.
_ _
D 0.13 , 1.3 2.0 0.03 0.01 0.003 -
- - - - - -
._
E 0.09 1.7 1.9 0.03 0.01 0.003 - -
- - - - -
_ _
_ 0
- -
- -
F 0.08 2.0 2.1 0.03 0.01 0.003 -
- -
- _
0
G 0.11 1.3 2.8 0.04 0.01 0.003 - - -
- - - - "
0 ,
.
-
H
H 0.12 1.3 2.0 0.95 0.01 0.003 - - F -
- - - -
_
H P
I 0.12 1.3 2.0 0.04 0.06 0.004 - - -
- - - - ko
_
J 0.12 1.3 2.1 0.03 0.01 0.008 - - -
- - ,
-
- I.)
0
H
K 0.12 1.3 1.9 0.02 0.01 0.003 0.7 -
- - - - - LO
1
-
- 0
L 0.12 1.3 2.0 0.04 0.01 0.003 -
0.12 - - - - - LO
I
H
M 0.12 1.3 2.1 0.03 0.01 0.003 - - 0.005
- - - -
_
N 0.12 1.3 2.0 0.05 , 0.01 0.003 -
- 0.001 0.04 - - -
-
O 0.12 1.3 1.9 0.03 0.01 0.004 -
0.11 - - 0.2 0.3 -
P 0.12 1.3 1.9 0.04 0.01 0.003 - -
0.003 - - - 0.03
_
_
Q 0.12 1.3 2.0 0.03 0.01 0.004 - -
- - - - 0.05
_
-
R 020 1.3 2.1 0.04 0.01 0.003 - - -
- - - -
_
S 0.12 2.1 1.9 0.04 0.01 0.003 - -
- - - - -
T 0.12 1.3 3.1 0.04 , 0.01 0.004 - - -
- - - 1
-
U 0.12 1.3
2.0 1.10 0.01 0.004 - - - - , - - -
/ 0.12 1.3 1.9 0.03_ 0.07 0.003 -
- - - - - -
W 0.12 1.3 2.1 0.04 0.01 0.015 - - -
- - - -
Underlines indicate "outside the inventive range".
CA 02811489 2013-03-15
- 30 -
Next, the cold rolled steel sheets and the hot rolled
steel sheets obtained above were introduced into a
continuous annealing facility. The steel sheet was passed
through the annealing facility while controlling the heating
furnace inside temperature and the dew-point temperature as
described in Table 2. The annealed steel sheet was
thereafter subjected to water hardening and then to
tempering at 300 C for 140 seconds. Subsequently,
electrolytic pickling was performed by alternating
electrolysis in a 5 mass% aqueous sulfuric acid solution at
40 C under current density conditions described in Table 2
while switching the polarity of the sample sheet between
anodic and cathodic alternately each after 3 seconds. Thus,
sample sheets were prepared. The dew-point temperature in
the annealing furnace was basically set at -35 C except when
the dew-point temperature was controlled as described above.
The gas components in the atmosphere included nitrogen gas,
hydrogen gas and inevitable impurity gases. The dew-point
temperature was controlled by dehumidifying the atmosphere
or by removing water in the atmosphere by absorption. The
hydrogen concentration in the atmosphere was basically set
at 10 vol%.
With respect to the obtained sample sheets, TS and El
were measured in accordance with a tensile testing method
for metallic materials described in JIS Z 2241. Further,
CA 02811489 2013-03-15
- 31 -
the sample sheets were tested to examine chemical
convertibility and corrosion resistance, as well as the
amount of oxides present in a surface portion of the steel
sheet extending immediately from the surface of the steel
sheet to a depth of 100 m (the internal oxidation amount).
The measurement methods and the evaluation criteria are
described below.
[0055]
Chemical convertibility
Chemical convertibility was evaluated by the following
method.
A chemical conversion treatment liquid (PALBOND L3080
(registered trademark)) manufactured by Nihon Parkerizing
Co., Ltd. was used. A chemical conversion treatment was
carried out in the following manner.
The sample sheet was degreased with degreasing liquid
FINE CLEANER (registered trademark) manufactured by Nihon
Parkerizing Co., Ltd., and was thereafter washed with water.
Subsequently, the surface of the sample sheet was
conditioned for 30 seconds with surface conditioning liquid
PREPAREN Z (registered trademark) manufactured by Nihon
Parkerizing Co., Ltd. The sample sheet was then soaked in
the chemical conversion treatment liquid (PALBOND L3080) at
43 C for 120 seconds, washed with water and dried with hot
air.
CA 02811489 2013-03-15
- 32 -
The sample sheet after the chemical conversion
treatment was observed with a scanning electron microscope
(SEM) at 500x magnification with respect to randomly
selected five fields of view. The area ratio of the regions
that had not been covered with the chemical conversion
coating was measured by image processing. Chemical
convertibility was evaluated based on the area ratio of the
non-covered regions according to the following criteria.
The symbol 0 indicates an acceptable level.
0: not more than 10%
x: more than 10%
[0056]
Corrosion resistance after electrodeposition coating
A 70 mm x 150 mm test piece was cut out from the sample
sheet that had been subjected to the above chemical
conversion treatment. The test piece was cationically
electrodeposition coated with PN-150G (registered trademark)
manufactured by NIPPON PAINT Co., Ltd. (baking conditions:
170 C x 20 min, film thickness: 25 m). Thereafter, the
edges and the non-test surface were sealed with an Al tape,
and the test surface was cut deep into the base steel with a
cutter knife to create a cross cut pattern (cross angle:
60 ), thereby preparing a sample.
Next, the sample was soaked in a 5 mass% aqueous NaC1
solution (55 C) for 240 hours, removed from the solution,
CA 02811489 2013-03-15
- 33 -
washed with water and dried. Thereafter, an adhesive tape
was applied to the cross cut pattern and was peeled
therefrom. The exfoliation width was measured and was
evaluated based on the following criteria. The symbol 0
indicates an acceptable level.
0: The exfoliation width from each cut line was less
than 2.5 mm.
x: The exfoliation width from each cut line was 2.5 mm
or more.
[0057]
Workability
To evaluate workability, a JIS No. 5 tensile test piece
was sampled from the sample sheet in a direction that was
90 relative to the rolling direction. The test piece was
subjected to a tensile test at a constant cross head speed
of 10 ram/min in accordance with JIS Z 2241, thereby
determining the tensile strength (TS/MPa) and the elongation
(El %). For steel sheets with TS of less than 650 MPa,
workability was evaluated to be good when TS x El 22000
and to be bad when TS x El < 22000. For steel sheets with
TS of 650 MPa to 900 MPa, workability was evaluated to be
good when TS x El 20000 and to be bad when TS x El < 20000.
For steel sheets with TS of not less than 900 MPa,
workability was evaluated to be good when TS x El ?. 18000
and to be bad when TS x El < 18000.
CA 02811489 2013-03-15
,
- 34 -
[0058]
Internal oxidation amount in region from steel sheet surface
to depth of 100 m
The internal oxidation amount was measured by an
"impulse furnace fusion-infrared absorption method". It
should be noted that the amount of oxygen present in the
starting material (namely, the high strength steel sheet
before annealing) should be subtracted. Thus, in the
invention, surface portions on both sides of the
continuously annealed high strength steel sheet were
polished by at least 100 m and thereafter the oxygen
concentration in the steel was measured. The measured value
was obtained as the oxygen amount OH of the starting
material. Further, the oxygen concentration was measured
across the entirety of the continuously annealed high
strength steel sheet in the sheet thickness direction. The
measured value was obtained as the oxygen amount OI after
internal oxidation. The difference between OI and OH (= OI
- OH) was calculated wherein OI was the oxygen amount in the
high strength steel sheet after internal oxidation and OH
was the oxygen amount in the starting material. The
difference was then converted to an amount per unit area
(namely, 1 m2) on one surface, thereby determining the
internal oxidation amount (g/m2).
The results and the manufacturing conditions are
CA 02811489 2013-03-15
- 35 -
described in Table 2.
- 36 -
[0059]
,
%
[Table 2]
..,
No. Steel Internal oxidation Internal
oxide in region from Electrolytic Current Chemical Corrosion TS El
TSxEl Workability Remarks
Annealing furnace amount (g/m2) in immediately
under coating to pickling density convertibility resistance
after MPa %
region from depth of 10pm
A/dm2 electrodeposition
Steel Si Mn Steel Temp. Temp. Dew-point Maximum
immediately Presence Presence of coating
code (mass%) (mass%) sheet A B temp. (t)
temp. (') under coating to intragranular
("C) ("C) at between depth of 100pm precipitate
temp. A immediately
under
and temp. coating at depth
within
B 1um from arain
Cold
Not _
1 D 1.3 2.0 600 700 -5 850 0.004 x
x - x x 1051 20.8 21861 Good COMP. EX.
rolled
performed
.
.
Cold
Not
2 D 1.3 2.0 600 790 -5 850 0.009 x
x - x x 1029 21.1 21712 Good COMP. EX.
rolled
performed
-
Cold
Not
3 D 1.3 2.0 600 800 -5 800 0.021 0
0 - 0 0 1031 20.4 21032 Good INV. EX.
rolled
performed .
_
.
Cold
Not
4 D 1.3 2.0 600 800 -5 830 0.025 0
0 - 0 0 1025 20.3 20808 Good INV. EX.
rolled
performed 0
Cold
Not
D 1.3 2.0 600 800 -5 860 0.028 0 0
- 0 0 1021 20.2 20624 Good INV. EX.
rolled
performed 0
_
.
Cold
Not - n)co
6 0 1.3 2.0 600 800 -5 890 0.033 0
0 - 0 0 1029 20.0 20580 Good INV. EX. F-,
rolled
performed H
-
-
Cold
Not
7 D 1.3 2.0 650 850 -5 850 0.022 0
0 - 0 0 1034 20.7 21404 Good INV. EX. CO
rolled
Performed q3.
t---,
_______________________________________________________________________________
____________________________________
Cold
Not
8 D 1.3 2.0 700 850 -5 850 0.020 0
0 - 0 0 1039 20.6 21403 Good INV. EX.
cl`D)
rolled
performed H
- .
.
Hot
Not u.)
9 D 1.3 2.0 700 850 -5 850 0.123 0
0 - 0 0 1040 20.3 21112 Good INV. EX. i
rolled
performed 0
Cold
Not u.)
D 1.3 2.0 750 850 -5 850 0.015 0 0
- 0 0 1024 20.4 20890 Good INV. EX. I
rolled
oerformed H
-
Ui
Cold
Not
11 D 1.3 2.0 780 850 -5 850 0.012 0 ,
0 - 0 0 1031 20.8 21445 Good INV. EX.
rolled
performed
Cold
Not
12 D 1.3 2.0 790 850 -5 850 0.007 x
x - x x 990 20.9 20691 Good COMP. EX.
rolled
Performed
Cold
Not
13 D 1.3 2.0 700 850 -35 850 0.006 x
x - x x 991 20.7 20514 Good COMP. EX.
rolled
, performed
Cold
Not
14 D 1.3 2.0 700 850 -15 850 0.008 x
x - x x 1159 18.0 20862 Good COMP. EX.
rolled
Performed
Cold
Not
D 1.3 2.0 700 850 -10 850 0.011 0 0
- 0 0 1044 19.9 20776 Good INV. EX.
rolled
Performed
Cold
Not
16 D 1.3 2.0 700 850 0 850 0.068 0
0 - 0 0 1033 20.4 21073 Good INV. EX.
rolled
Performed
-
Cold
Not
17 D 1.3 2.0 700 850 20 850 0.221 0
0 - 0 0 1050 20.6 21630 Good INV. EX.
rolled
Performed
Cold
Not
18 D 1.3 2.0 700 850 60 850 0.436 0
0 - 0 0 1051 20.1 21125 Good INV. EX.
rolled
performed
Cold
19 D 1.3 2.0 700 850 -5 850 0.021 0
0 Performed 1 0 0 1041 20.0 20820 Good
INV. EX.
rolled
Cold
D 1.3 2.0 700 850 -5 850 0.019 0 0
Performed 5 0 0 1042 20.7 21569 Good INV.
EX.
rolled
Cold
21 D 1.3 2.0 700 850 -5 850 0.020 0
0 Performed 10 0 0 1044 20.9 21820 Good
INV. EX.
rolled
_
_
Underlines indicate that manufacturing conditions are outside the inventive
ranges.
- 37 -
[0060]
,
.,
[Table 2 (Cont.)]
,
Table 2 (Cont.)
No. Steel Internal
oxidation Internal oxide in region from Electrolytic Current
Chemical Corrosion TS El TSxEl Workability Remarks
Annealing furnace amount (g/m2) in immediately under coating
to depth of pickling density convertibility resistance after MPa %
region from 101m1
A/dm2 electrodeposition
-
Steel Si Mn Steel Temp. Temp. Dew-point Maximum
immediately Presence Presence of intragranular coating
code (mass%) (mass%) sheet A B temp. (t) at temp. (t) under
coating to precipitate immediately
("C) (C) between temp. depth of 100um under coating at depth
_A_and tpmn Rwithin 11,m_from min
. - _
Cold
Not
22 A 0.1 1.9 700 850 -5 850 0.021
0 0 - 0 0 712 26.5 18868 Bad
COMP. EX.
_
_ rolled
performed ,
-
_
Cold
Not -
23 B 0.4 2.0 rolled 700 850 -5 850
0.009 0 0 Performed - 0 0 1010 20.9 21109
Good INV. EX.
_
_
- -
Cold
Not
24 C 0.9 2.1 rolled 700 850 -5 850
0.011 0 0 prform - 0 0 1021 21.4 21849
Good INV. EX.
_
eed
_
-
_
25 E 1.7 1.9 Cold 700 850 -5 850 0.030
0 0 Not - 0 0 1036 22.8 23621 Good
INV. EX.
rolled
Performed
_
Cold
Not n
26 F 2.0 2.1 rolled 700 850 -5 850
0.039 0 0 performed - 0 0 1029 20.5 21095
Good INV. EX.
Cold
Not - o
27 G 1.3 2.8 700 850 -5 850 0.021
0 0 - 0 0 '1064 19.7 20961 Good
INV. EX. "
rolled
performed co
. H
Cold
Not
28 H 1.3 2.0 700 850 -5 850 0.051 0
0 - 0 0 "- 1066 20.3 21640 Good INV.
EX. ,1-1,-;
rolled ,
performed_
. ,
, _ co
29 I 1.3 2.0 . Cold
rolled
700 850 -5 850 0.022 0 0 Not
performed
-
0 0 '1145 20.1 23015 Good INV. EX. ko
_
_ iv
,
Cold
Not o
30 J 1.3 2.1 _ rolled 700 850 -5 850
0.015 0 0 performed_ - 0 0 1044 19.9
20776 Good INV. EX. H
,
u.)
' Cold .
Not
31 K 1.3 1.9 700 850 -5 850 0.016
0 0 - 0 0 1063 19.4 20622 Good
INV. Ex. S
rolled
Performed
. _
1
Cold
Not H
32 L 1.3 2.0 700 850 -5 850 0.013
0 0 - 0 0 1052 19.5 20514 Good
INV. EX. in
_ rolled performed
.
- _
Cold
Not
33 M 1.3 2.1 rolled Performed 700 850 -5
850 0.014 0 0 - 0 0 1037 20.9 21673
Good INV. a.
Cold - . -
Not .
-
34 N 1.3 2.0 rolled performed 700 850 -5
850 0.016 0 0 - . 0 0 1077 20.4 21971
Good INV. EX.
.
_ .
Cold
Not
35 0 1.3 1.9 700 850 -5 850 0.015
0 0 - 0 0 1078 21.0 22638 Good
INV. a.
.rolledCol d performed _ .
_
_
Not
36 P 1.3 1.9 .rolled 700 850 -5 850
0.013 0 0 performed - 0 0 811 26.7 21654
Good INV. a.
Cold
Not
37 Q 1.3 2.0 rolled 700 850 . Performed -5
850 0.017 0 0 - 0 0 1055 19.7 20784
Good INV. EX.
Cold
Not
38 R 1.3 2.1 rolled performed 700 850 -5
850 0.019 0 0 - 0 0 1066 12.8 13645
Bad COMP. a.
Cold
Not
39 S 2.1 1.9 rolled performed 700 850 -5
850 0.052 0 0 - x 0 1212 16.4 19877
Good COMP. EX.
Cold
Not
40 T 1.3 3.1 _ rolled 700 850 -5 850
0.016 0 0 performed - 0 0 1125 13.4 15075
Bad COMP. EX.
Cold
Not
41 U 1.3 2.0 , rolled 700 850 -5 850
0.051 0 0 Pei-formed - x x 1079 21.4 23091
Good COMP. EX.
_
Cold
Not
42 V 1.3 1.9 rolled 700 850 -5 850
0.033 0 0 performed - x 0 1144 19.4 22194
Good COMP. a.
Cold
Not -
43 W 1.3 2.1 rolled 700 850 -5 850
0.020 0 0 performed - 0 x 1079 20.3 21904
Good COMP. LX.
Underlines indicate that manufacturing conditions are outside the inventive
ranges.
CA 02811489 2013-03-15
- 38 -
From Table 2, the high strength steel sheets
manufactured by the inventive method were shown to be
= 0 =
excellent in chemical convertibility, corrosion resistance
after electrodeposition coating and workability in spite of
the fact that these high strength steel sheets contained
large amounts of easily oxidized elements such as Si and Mn.
On the other hand, the steel sheets obtained in
COMPARATIVE EXAMPLES were poor in at least one of chemical
convertibility, corrosion resistance after electrodeposition
coating and workability.
[Industrial Applicability]
[0061]
The high strength steel sheets according to the present
invention are excellent in chemical convertibility,
corrosion resistance and workability, and can be used as
surface-treated steel sheets for reducing the weight and
increasing the strength of bodies of automobiles. Besides
automobiles, the inventive high strength steel sheets can be
used as surface-treated steel sheets having corrosion
resistance on the base steel sheet in a wide range of
applications including home appliances and building
materials.
