U.S. patent application number 13/822153 was filed with the patent office on 2013-12-12 for high strength steel sheet and method for manufacturing the same.
This patent application is currently assigned to JFE Steel Corporation. The applicant listed for this patent is Yusuke Fushiwaki, Yoshitsugu Suzuki. Invention is credited to Yusuke Fushiwaki, Yoshitsugu Suzuki.
Application Number | 20130327452 13/822153 |
Document ID | / |
Family ID | 45892180 |
Filed Date | 2013-12-12 |
United States Patent
Application |
20130327452 |
Kind Code |
A1 |
Fushiwaki; Yusuke ; et
al. |
December 12, 2013 |
HIGH STRENGTH STEEL SHEET AND METHOD FOR MANUFACTURING THE SAME
Abstract
The invention provides a high strength steel sheet which
exhibits excellent chemical convertibility and corrosion resistance
after electrodeposition coating even in the case where the steel
sheet has a high Si content, and a method for manufacturing such
steel sheets. The method includes 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 .ltoreq.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.degree.
C. when the heating furnace inside temperature is in the range of
not less than A.degree. C. and not more than B.degree. C. during
the course of heating (A: 600.ltoreq.A.ltoreq.780, B:
800.ltoreq.B.ltoreq.900).
Inventors: |
Fushiwaki; Yusuke;
(Fukuyama-shi, JP) ; Suzuki; Yoshitsugu;
(Fukuyama-shi, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Fushiwaki; Yusuke
Suzuki; Yoshitsugu |
Fukuyama-shi
Fukuyama-shi |
|
JP
JP |
|
|
Assignee: |
JFE Steel Corporation
Chiyoda-ku, Tokyo
JP
|
Family ID: |
45892180 |
Appl. No.: |
13/822153 |
Filed: |
September 30, 2010 |
PCT Filed: |
September 30, 2010 |
PCT NO: |
PCT/JP2010/067612 |
371 Date: |
May 21, 2013 |
Current U.S.
Class: |
148/579 ;
148/320; 148/337 |
Current CPC
Class: |
C22C 38/12 20130101;
C21D 9/46 20130101; C23C 22/184 20130101; C22C 38/06 20130101; C22C
38/16 20130101; C22C 38/02 20130101; C23C 8/10 20130101; C21D 9/561
20130101; C22C 38/18 20130101; C21D 8/0478 20130101; C22C 38/002
20130101; C21D 8/0447 20130101; C23C 8/14 20130101; C21D 9/56
20130101; C22C 38/04 20130101; C22C 38/08 20130101; C22C 38/14
20130101 |
Class at
Publication: |
148/579 ;
148/320; 148/337 |
International
Class: |
C21D 9/46 20060101
C21D009/46; C22C 38/04 20060101 C22C038/04; C22C 38/02 20060101
C22C038/02; C22C 38/06 20060101 C22C038/06 |
Claims
1. A method for manufacturing high strength steel sheets,
comprising 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
.ltoreq.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.degree. C. when the heating
furnace inside temperature is in the range of not less than
A.degree. C. and not more than B.degree. C. during the course of
heating wherein A is 600.ltoreq.A.ltoreq.780 and B is
800.ltoreq.B.ltoreq.900.
2. The method for manufacturing high strength steel sheets
according to claim 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 %.
3. The method for manufacturing high strength steel sheets
according to claim 1, further comprising, after the continuous
annealing, electrolytically pickling the steel sheet in an aqueous
solution containing sulfuric acid.
4. A high strength steel sheet manufactured by the method described
in claim 1 in which a surface portion of the steel sheet extending
from the steel sheet surface within a depth of 100 .mu.m 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/m.sup.2 per single
side surface, and in which with respect to a region extending from
the steel sheet surface within a depth of 10 .mu.m, a crystalline
Si/Mn oxide is present in grains that are within 1 .mu.m from
crystal grain boundaries of the steel sheet.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This application is the U.S. National Phase application of
PCT International Application No. PCT/JP2010/067612, filed Sep. 30,
2010, the disclosure of which is incorporated herein by reference
in its entirety for all purposes.
FIELD OF THE INVENTION
[0002] 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 OF THE INVENTION
[0003] 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.
[0004] 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.
[0005] 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 N.sub.2 H.sub.2 gas atmosphere which does
not induce the oxidation of Fe (which reduces Fe oxides). As a
result, a silicon oxide (SiO.sub.2) is formed on the outermost
surface of a steel sheet. This SiO.sub.2 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.
[0006] 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/m.sup.2 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.
[0007] Further, patent documents 2 and 3 provide an 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.
[0008] Patent document 4 discloses a method in which the dew-point
temperature during annealing is controlled to be -25 to 0.degree.
C. so as to form an internal oxide layer which includes a
Si-containing oxide within a depth of 1 .mu.m 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
.mu.m 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 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.
[0009] Further, patent document 5 describes a method in which the
steel sheet temperature is brought to 350 to 650.degree. 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.
[0010] 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.degree. 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.degree. 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 N.sub.2 H.sub.2 gas atmosphere
which reduces the iron oxide, thereby forming an iron oxide layer
on the outermost surface while suppressing the oxidation of
SiO.sub.2 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 SiO.sub.2 comes to
be present on the surface of the steel sheet after the reduction,
thus possibly resulting in a region which may not be covered with a
chemical film.
PATENT DOCUMENT
[0011] [Patent document 1] Japanese Unexamined Patent Application
Publication No. 5-320952 [0012] [Patent document 2] Japanese
Unexamined Patent Application Publication No. 2004-323969 [0013]
[Patent document 3] Japanese Unexamined Patent Application
Publication No. 6-10096 [0014] [Patent document 4] Japanese
Unexamined Patent Application Publication No. 2003-113441 [0015]
[Patent document 5] Japanese Unexamined Patent Application
Publication No. 55-145122 [0016] [Patent document 6] Japanese
Unexamined Patent Application Publication No. 2006-45615
SUMMARY OF THE INVENTION
[0017] 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 manufacturing such steel
sheets.
[0018] 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 out in such a manner that the dew-point
temperature of the atmosphere is controlled to become not less than
-10.degree. C. when the heating furnace inside temperature is in a
limited range of not less than A.degree. C. and not more than
B.degree. C. during the course of heating (A:
600.ltoreq.A.ltoreq.780, B: 800.ltoreq.B.ltoreq.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.
[0019] 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 .mu.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/m.sup.2 per single side
surface, and in which a region extending from the steel sheet
surface to a depth of 10 .mu.l is such that a crystalline Si/Mn
oxide has been precipitated in base iron grains that are within 1
.mu.m from grain boundaries. Because of this configuration,
deterioration in corrosion resistance after electrodeposition
coating is realized and excellent chemical convertibility is
obtained.
[0020] The present invention is based on the aforementioned
findings. Features of embodiments of the invention are as described
below.
[0021] [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
.ltoreq.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.degree. C. when the heating
furnace inside temperature is in the range of not less than
A.degree. C. and not more than B.degree. C. during the course of
heating wherein A is 600.ltoreq.A.ltoreq.780 and B is
800.ltoreq.B.ltoreq.900.
[0022] [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 %.
[0023] [3] The method for manufacturing high strength steel sheets
described in [1] or [2], further including, after the continuous
annealing, electrolytically pickling the steel sheet in an aqueous
solution containing sulfuric acid.
[0024] [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 .mu.m 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/m.sup.2 per single side surface, and in which with respect
to a region extending from the steel sheet surface within a depth
of 10 .mu.m, a crystalline Si/Mn oxide is present in grains that
are within 1 .mu.m from crystal grain boundaries of the steel
sheet.
[0025] 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.
[0026] 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 sheet has a high Si content.
DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION
[0027] The present invention will be described in detail
hereinbelow in terms of exemplary embodiments. 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.
[0028] 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.
[0029] 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.degree. C. when the heating furnace inside temperature is in a
limited range of not less than A.degree. C. and not more than
B.degree. C. during the course of heating in an annealing furnace
(A: 600.ltoreq.A.ltoreq.780, B: 800.ltoreq.B.ltoreq.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 .mu.m (hereinafter, such oxides will be referred to as
internal oxides), thereby 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.
[0030] The lower limit temperature A is limited to be
600.ltoreq.A.ltoreq.780 for the following reasons. When the
temperature is in the range of less than 600.degree. 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.degree. 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.degree. C. at
least from when the temperature is in the range of not more than
780.degree. C. Thus, the acceptable range of A is
600.ltoreq.A.ltoreq.780. For the above reason, it is preferable
that A be a temperature as low as possible within this range.
[0031] The upper limit temperature B is limited to be
800.ltoreq.B.ltoreq.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
.mu.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.ltoreq.B.ltoreq.900. If the temperature is less than
800.degree. C., internal oxides are not formed sufficiently. If the
temperature exceeds 900.degree. C., internal oxides are formed in
excessively large amounts and serve as starting points of a
deterioration in corrosion resistance after electrodeposition
coating.
[0032] The dew-point temperature is controlled to become not less
than -10.degree. C. when the temperature is in the range of not
less than A.degree. C. and not more than B.degree. C. for the
following reasons. Increasing the dew-point temperature increases
the potential of O.sub.2 generated by the decomposition of
H.sub.2O, and therefore internal oxidation can be promoted. The
amount of formed internal oxides is small if the dew-point
temperature is in the range of below -10.degree. 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.degree. C., causing a risk that
annealing furnace walls or rollers may be degraded. Thus, the
dew-point temperature is preferably not more than 90.degree. C.
[0033] Next, the chemical composition of the high strength steel
sheets of interest according to embodiments of the present
invention will be described.
C: 0.01 to 0.18%
[0034] 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%.
Si: 0.4 to 2.0%
[0035] 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 the present invention,
silicon is preferably 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%.
Mn: 1.0 to 3.0%
[0036] 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%.
Al: 0.001 to 1.0%
[0037] Aluminum is added for the purpose of deoxidation of molten
steel. The deoxidation effect for molten steel is 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%.
P: 0.005 to not more than 0.060%
[0038] 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%.
S: S.ltoreq.0.01%
[0039] 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%.
[0040] 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.
[0041] The appropriate amounts of these optional elements are
limited for the following reasons.
B: 0.001 to 0.005%
[0042] 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%.
Nb: 0.005 to 0.05%
[0043] 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%.
Ti: 0.005 to 0.05%
[0044] 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%.
Cr: 0.001 to 1.0%
[0045] 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%.
Mo: 0.05 to 1.0%
[0046] 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%.
Cu: 0.05 to 1.0%
[0047] The effect in promoting the formation of a retained
.gamma.-phaseis 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%.
Ni: 0.05 to 1.0%
[0048] The effect in promoting the formation of a retained 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%.
[0049] The balance after the deduction of the aforementioned
elements is represented by Fe and inevitable impurities.
[0050] Next, there will be described an embodiment of 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 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.degree. C.
when the heating furnace inside temperature is in the range of not
less than A.degree. C. and not more than 3.degree. C. during the
course of heating (A: 600.ltoreq.A.ltoreq.780, B:
800.ltoreq.B.ltoreq.900). This is the most important aspect 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.
Hot Rolling
[0051] Hot rolling may be performed under usual conditions.
Pickling
[0052] It is preferable to perform a pickling treatment after hot
rolling. In the pickling step, black scales formed on the surface
are removed and the steel sheet is subjected to cold rolling.
Pickling conditions are not particularly limited.
Cold Rolling
[0053] 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.
[0054] The steel sheet that has been cold rolled or hot rolled is
annealed and then subjected to a chemical conversion treatment.
[0055] 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 prescribed time. Next, a cooling
step is performed.
[0056] 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.degree. C. when the heating
furnace inside temperature is in the range of not less than
A.degree. C. and not more than B.degree. C. (A:
600.ltoreq.A.ltoreq.780, B: 800.ltoreq.B.ltoreq.900). Except when
the temperature is in the range of not less than A.degree. C. and
not more than B.degree. C., the dew-point temperature of the
atmosphere in the annealing furnace is not particularly limited,
but is preferably in the range of -50.degree. C. to -10.degree.
C.
[0057] 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
volt, 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 volt. 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 inevitable impurity gases.
Other gas components may be present as long as they are not
detrimental in achieving the advantageous effects of the
invention.
[0058] After the steel sheet is cooled from the temperature range
of not less than 750.degree. 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.degree. C. The reasons are
because elongation tends to be deteriorated if the temperature is
less than 150.degree. C. as well as because hardness tends to be
decreased if the temperature is in excess of 400.degree. C.
[0059] 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.
[0060] 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 than 1 A/dm.sup.2 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.
[0061] 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.
[0062] The temperature of the electrolytic solution is preferably
not less than 40.degree. C. and not more than 70.degree. 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.degree. C. Further, maintaining the
temperature below 40.degree. C. is sometimes difficult.
Furthermore, a temperature exceeding 70.degree. C. is not
preferable in view of the durability of the lining of the
electrolytic cell.
[0063] The high strength steel sheets of the present invention are
obtained in the above manner.
[0064] As a result, the inventive steel sheet has a characteristic
structure of the surface described below.
[0065] A surface portion of the steel sheet extending from the
steel sheet surface within a depth of 100 .mu.m 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/m.sup.2 per single side surface. Further, with respect to a
region extending from the steel sheet surface to a depth of 10
.mu.m, a crystalline Si/Mn complex oxide is present in base iron
grains that are within 1 .mu.m from grain boundaries.
[0066] In a high strength steel sheet containing Si and a 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 .mu.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/m.sup.2 per single side surface.
The effects are saturated even when such oxides are present in
excess of 0.50 g/m.sup.2. Thus, the upper limit is specified to be
0.50 g/m.sup.2.
[0067] 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.degree. C. when the heating
furnace inside temperature is in the range of not less than
A.degree. C. and not more than B.degree. C. (A:
600.ltoreq.A.ltoreq.780, B: 800.ltoreq.B.ltoreq.900). In detail, a
crystalline Si/Mn complex oxide is caused to be present in base
iron grains that are within 1 .mu.m from grain boundaries in a
region extending from the steel sheet surface to a depth of 10
.mu.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.
[0068] 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 problem even when the
oxides have been grown so as to extend to a region that is more
than 100 .mu.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
.mu.m away from grain boundaries in a region extending from the
steel sheet surface to a depth in excess of 10 .mu.m.
Example 1
[0069] Hereinbelow, the present invention will be described in
detail based on EXAMPLES.
[0070] 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.
TABLE-US-00001 TABLE 1 (mass %) Steel code C S i Mn Al P S Cr Mo B
Nb Cu Ni Ti A 0.04 0.1 1.9 0.04 0.01 0.003 -- -- -- -- -- -- -- 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 -- -- -- -- --
-- -- F 0.08 2.0 2.1 0.03 0.01 0.003 -- -- -- -- -- -- -- G 0.11
1.3 2.8 0.04 0.01 0.003 -- -- -- -- -- -- -- H 0.12 1.3 2.0 0.95
0.01 0.003 -- -- -- -- -- -- -- I 0.12 1.3 2.0 0.04 0.06 0.004 --
-- -- -- -- -- -- J 0.12 1.3 2.1 0.03 0.01 0.008 -- -- -- -- -- --
-- K 0.12 1.3 1.9 0.02 0.01 0.003 0.7 -- -- -- -- -- -- L 0.12 1.3
2.0 0.04 0.01 0.003 -- 0.12 -- -- -- -- -- 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 0.20 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 -- -- --
-- -- -- -- U 0.12 1.3 2.0 1.10 0.01 0.004 -- -- -- -- -- -- -- V
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".
[0071] 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.degree. C. for 140 seconds. Subsequently,
electrolytic pickling was performed by alternating electrolysis in
a 5 mass % aqueous sulfuric acid solution at 40.degree. 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.degree. 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 %.
[0072] 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, 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 .mu.m (the internal oxidation amount). The
measurement methods and the evaluation criteria are described
below.
Chemical Convertibility
[0073] Chemical convertibility was evaluated by the following
method.
[0074] 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.
[0075] 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.degree. C. for 120 seconds, washed with water and
dried with hot air.
[0076] The sample sheet after the chemical conversion treatment was
observed with a scanning electron microscope (SEM) at 500.times.
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.
[0077] .largecircle.: not more than 10%
[0078] x: more than 10%
Corrosion Resistance after Electrodeposition Coating
[0079] A 70 mm.times.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.degree. C..times.20 min, film
thickness: 25 .mu.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.degree.), thereby preparing a sample.
[0080] Next, the sample was soaked in a 5 mass % aqueous NaCl
solution (55.degree. C.) for 240 hours, removed from the solution,
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 .largecircle. indicates an
acceptable level.
[0081] .largecircle.: The exfoliation width from each cut line was
less than 2.5 mm.
[0082] x: The exfoliation width from each cut line was 2.5 mm or
more.
Workability
[0083] To evaluate workability, a JIS No. 5 tensile test piece was
sampled from the sample sheet in a direction that was 90.degree.
relative to the rolling direction. The test piece was subjected to
a tensile test at a constant cross head speed of 10 mm/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.times.El.gtoreq.22000 and to be bad when TS.times.El<22000.
For steel sheets with TS of 650 MPa to 900 MPa, workability was
evaluated to be good when TS.times.El.gtoreq.20000 and to be bad
when TS.times.El<20000. For steel sheets with TS of not less
than 900 MPa, workability was evaluated to be good when
TS.times.El.gtoreq.18000 and to be bad when
TS.times.El<18000.
Internal Oxidation Amount in Region from Steel Sheet Surface to
Depth of 100 .mu.m
[0084] 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 .mu.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 m.sup.2) on one surface,
thereby determining the internal oxidation amount (g/m.sup.2).
[0085] The results and the manufacturing conditions are described
in Table 2.
TABLE-US-00002 TABLE 2 Internal oxide in region from immediately
under coating to depth of 10 .mu.m Annealing furnace Internal
oxidation Presence of Dew-point amount (g/m.sup.2) in intragranular
temp. (.degree. C.) region from precipitate Steel at between
Maximum immediately immediately under Steel Si Mn Steel Temp. A
Temp. B temp. A temp. under coating to coating at depth within No.
code (mass %) (mass %) sheet (.degree. C.) (.degree. C.) and temp.
B (.degree. C.) depth of 100 .mu.m Presence 1 um from grain 1 D 1.3
2.0 Cold 600 700 -5 850 0.004 X X rolled 2 D 1.3 2.0 Cold 600 790
-5 850 0.009 X X rolled 3 D 1.3 2.0 Cold 600 800 -5 800 0.021
.largecircle. .largecircle. rolled 4 D 1.3 2.0 Cold 600 800 -5 830
0.025 .largecircle. .largecircle. rolled 5 D 1.3 2.0 Cold 600 800
-5 860 0.028 .largecircle. .largecircle. rolled 6 D 1.3 2.0 Cold
600 800 -5 890 0.033 .largecircle. .largecircle. rolled 7 D 1.3 2.0
Cold 650 850 -5 850 0.022 .largecircle. .largecircle. rolled 8 D
1.3 2.0 Cold 700 850 -5 850 0.020 .largecircle. .largecircle.
rolled 9 D 1.3 2.0 Hot 700 850 -5 850 0.123 .largecircle.
.largecircle. rolled 10 D 1.3 2.0 Cold 750 850 -5 850 0.015
.largecircle. .largecircle. rolled 11 D 1.3 2.0 Cold 780 850 -5 850
0.012 .largecircle. .largecircle. rolled 12 D 1.3 2.0 Cold 790 850
-5 850 0.007 X X rolled 13 D 1.3 2.0 Cold 700 850 -35 850 0.006 X X
rolled 14 D 1.3 2.0 Cold 700 850 -15 850 0.008 X X rolled 15 D 1.3
2.0 Cold 700 850 -10 850 0.011 .largecircle. .largecircle. rolled
16 D 1.3 2.0 Cold 700 850 0 850 0.068 .largecircle. .largecircle.
rolled 17 D 1.3 2.0 Cold 700 850 20 850 0.221 .largecircle.
.largecircle. rolled 18 D 1.3 2.0 Cold 700 850 60 850 0.436
.largecircle. .largecircle. rolled 19 D 1.3 2.0 Cold 700 850 -5 850
0.021 .largecircle. .largecircle. rolled 20 D 1.3 2.0 Cold 700 850
-5 850 0.019 .largecircle. .largecircle. rolled 21 D 1.3 2.0 Cold
700 850 -5 850 0.020 .largecircle. .largecircle. rolled 22 A 0.1
1.9 Cold 700 850 -5 850 0.021 .largecircle. .largecircle. rolled 23
B 0.4 2.0 Cold 700 850 -5 850 0.009 .largecircle. .largecircle.
rolled 24 C 0.9 2.1 Cold 700 850 -5 850 0.011 .largecircle.
.largecircle. rolled 25 E 1.7 1.9 Cold 700 850 -5 850 0.030
.largecircle. .largecircle. rolled 26 F 2.0 2.1 Cold 700 850 -5 850
0.039 .largecircle. .largecircle. rolled 27 G 1.3 2.8 Cold 700 850
-5 850 0.021 .largecircle. .largecircle. rolled 28 H 1.3 2.0 Cold
700 850 -5 850 0.051 .largecircle. .largecircle. rolled 29 I 1.3
2.0 Cold 700 850 -5 850 0.022 .largecircle. .largecircle. rolled 30
J 1.3 2.1 Cold 700 850 -5 850 0.015 .largecircle. .largecircle.
rolled 31 K 1.3 1.9 Cold 700 850 -5 850 0.016 .largecircle.
.largecircle. rolled 32 L 1.3 2.0 Cold 700 850 -5 850 0.013
.largecircle. .largecircle. rolled 33 M 1.3 2.1 Cold 700 850 -5 850
0.014 .largecircle. .largecircle. rolled 34 N 1.3 2.0 Cold 700 850
-5 850 0.016 .largecircle. .largecircle. rolled 35 O 1.3 1.9 Cold
700 850 -5 850 0.015 .largecircle. .largecircle. rolled 36 P 1.3
1.9 Cold 700 850 -5 850 0.013 .largecircle. .largecircle. rolled 37
Q 1.3 2.0 Cold 700 850 -5 850 0.017 .largecircle. .largecircle.
rolled 38 R 1.3 2.1 Cold 700 850 -5 850 0.019 .largecircle.
.largecircle. rolled 39 S 2.1 1.9 Cold 700 850 -5 850 0.052
.largecircle. .largecircle. rolled 40 T 1.3 3.1 Cold 700 850 -5 850
0.016 .largecircle. .largecircle. rolled 41 U 1.3 2.0 Cold 700 850
-5 850 0.051 .largecircle. .largecircle. rolled 42 V 1.3 1.9 Cold
700 850 -5 850 0.033 .largecircle. .largecircle. rolled 43 W 1.3
2.1 Cold 700 850 -5 850 0.020 .largecircle. .largecircle. rolled
Corrosion resistance after Electrolytic Current density Chemical
electrodeposition TS No. pickling A/dm.sup.2 convertibility coating
MPa El % TS .times. El Workability Remarks 1 Not -- X X 1051 20.8
21861 Good COMP. EX. performed 2 Not -- X X 1029 21.1 21712 Good
COMP. EX. performed 3 Not -- .largecircle. .largecircle. 1031 20.4
21032 Good INV. EX. performed 4 Not -- .largecircle. .largecircle.
1025 20.3 20808 Good INV. EX. performed 5 Not -- .largecircle.
.largecircle. 1021 20.2 20624 Good INV. EX. performed 6 Not --
.largecircle. .largecircle. 1029 20.0 20580 Good INV. EX. performed
7 Not -- .largecircle. .largecircle. 1034 20.7 21404 Good INV. EX.
performed 8 Not -- .largecircle. .largecircle. 1039 20.6 21403 Good
INV. EX. performed 9 Not -- .largecircle. .largecircle. 1040 20.3
21112 Good INV. EX. performed 10 Not -- .largecircle. .largecircle.
1024 20.4 20890 Good INV. EX. performed 11 Not -- .largecircle.
.largecircle. 1031 20.8 21445 Good INV. EX. performed 12 Not -- X X
990 20.9 20691 Good COMP. EX. performed 13 Not -- X X 991 20.7
20514 Good COMP. EX. performed 14 Not -- X X 1159 18.0 20862 Good
COMP. EX. performed 15 Not -- .largecircle. .largecircle. 1044 19.9
20776 Good INV. EX. performed 16 Not -- .largecircle. .largecircle.
1033 20.4 21073 Good INV. EX. performed 17 Not -- .largecircle.
.largecircle. 1050 20.6 21630 Good INV. EX. performed 18 Not --
.largecircle. .largecircle. 1051 20.1 21125 Good INV. EX. performed
19 Performed 1 .largecircle. .largecircle. 1041 20.0 20820 Good
INV. EX. 20 Performed 5 .largecircle. .largecircle. 1042 20.7 21569
Good INV. EX. 21 Performed 10 .largecircle. .largecircle. 1044 20.9
21820 Good INV. EX. 22 Not -- .largecircle. .largecircle. 712 26.5
18868 Bad COMP. EX. performed 23 Not -- .largecircle. .largecircle.
1010 20.9 21109 Good INV. EX. performed 24 Not -- .largecircle.
.largecircle. 1021 21.4 21849 Good INV. EX. performed 25 Not --
.largecircle. .largecircle. 1036 22.8 23621 Good INV. EX. performed
26 Not -- .largecircle. .largecircle. 1029 20.5 21095 Good INV. EX.
performed 27 Not -- .largecircle. .largecircle. 1064 19.7 20961
Good INV. EX. performed 28 Not -- .largecircle. .largecircle. 1066
20.3 21640 Good INV. EX. performed 29 Not -- .largecircle.
.largecircle. 1145 20.1 23015 Good INV. EX. performed 30 Not --
.largecircle. .largecircle. 1044 19.9 20776 Good INV. EX. performed
31 Not -- .largecircle. .largecircle. 1063 19.4 20622 Good INV. EX.
performed 32 Not -- .largecircle. .largecircle. 1052 19.5 20514
Good INV. EX. performed 33 Not -- .largecircle. .largecircle. 1037
20.9 21673 Good INV. EX. performed 34 Not -- .largecircle.
.largecircle. 1077 20.4 21971 Good INV. EX. performed 35 Not --
.largecircle. .largecircle. 1078 21.0 22638 Good INV. EX. performed
36 Not -- .largecircle. .largecircle. 811 26.7 21654 Good INV. EX.
performed 37 Not -- .largecircle. .largecircle. 1055 19.7 20784
Good INV. EX. performed 38 Not -- .largecircle. .largecircle. 1066
12.8 13645 Bad COMP. EX. performed 39 Not -- X .largecircle. 1212
16.4 19877 Good COMP. EX. performed 40 Not -- .largecircle.
.largecircle. 1125 13.4 15075 Bad COMP. EX. performed 41 Not -- X X
1079 21.4 23091 Good COMP. EX. performed 42 Not -- X .largecircle.
1144 19.4 22194 Good COMP. EX. performed 43 Not -- .largecircle. X
1079 20.3 21904 Good COMP. EX. performed Underlines indicate that
manufacturing conditions are outside the inventive ranges.
[0086] From Table 2, the high strength steel sheets manufactured by
the inventive method were shown to be 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.
[0087] 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.
[0088] 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.
* * * * *