U.S. patent application number 13/812438 was filed with the patent office on 2013-06-13 for method of producing cold-rolled steel sheet as well as cold-rolled steel sheet and members for automobile.
This patent application is currently assigned to JFE STEEL CORPORATION. The applicant listed for this patent is Satoru Ando, Hiroyuki Masuoka, Shunsuke Yamamoto. Invention is credited to Satoru Ando, Hiroyuki Masuoka, Shunsuke Yamamoto.
Application Number | 20130149529 13/812438 |
Document ID | / |
Family ID | 45772723 |
Filed Date | 2013-06-13 |
United States Patent
Application |
20130149529 |
Kind Code |
A1 |
Masuoka; Hiroyuki ; et
al. |
June 13, 2013 |
METHOD OF PRODUCING COLD-ROLLED STEEL SHEET AS WELL AS COLD-ROLLED
STEEL SHEET AND MEMBERS FOR AUTOMOBILE
Abstract
In a method of producing a cold-rolled steel sheet being
excellent in not only the phosphate treatability but also the
corrosion resistance after coating under severe corrosion
environment such as hot salt water immersion test or composite
cycle corrosion test, a continuously annealed steel sheet after
cold rolling is pickled with a mixture of nitric acid and
hydrochloric acid having a nitric acid concentration of more than
100 g/L but not more than 200 g/L and a ratio R (HCl/HNO.sub.3) of
hydrochloric acid concentration to nitric acid concentration of
0.01-0.25 to remove Si-containing oxide formed on the steel sheet
surface by continuous annealing, and a ratio of covering the
surface of the steel sheet with an iron-based oxide formed by the
pickling is not more than 85% and preferably a maximum thickness of
the iron-based oxide existing on the surface of the steel sheet is
not more than 200 nm.
Inventors: |
Masuoka; Hiroyuki; (Tokyo,
JP) ; Ando; Satoru; (Tokyo, JP) ; Yamamoto;
Shunsuke; (Tokyo, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Masuoka; Hiroyuki
Ando; Satoru
Yamamoto; Shunsuke |
Tokyo
Tokyo
Tokyo |
|
JP
JP
JP |
|
|
Assignee: |
JFE STEEL CORPORATION
Tokyo
JP
|
Family ID: |
45772723 |
Appl. No.: |
13/812438 |
Filed: |
August 25, 2011 |
PCT Filed: |
August 25, 2011 |
PCT NO: |
PCT/JP2011/069192 |
371 Date: |
January 25, 2013 |
Current U.S.
Class: |
428/336 ;
134/2 |
Current CPC
Class: |
C21D 8/0236 20130101;
C22C 38/005 20130101; C22C 38/34 20130101; B21B 3/02 20130101; C22C
38/12 20130101; C22C 38/22 20130101; C22C 38/06 20130101; C22C
38/002 20130101; C22C 38/16 20130101; C22C 38/38 20130101; C22C
38/04 20130101; C22C 38/32 20130101; C23G 1/00 20130101; C22C 38/00
20130101; Y10T 428/265 20150115; C22C 38/14 20130101; C23G 1/08
20130101; C23G 1/085 20130101; C21D 9/46 20130101; C22C 38/001
20130101; C21D 8/0273 20130101; C21D 8/0205 20130101; C22C 38/02
20130101; C21D 8/0226 20130101 |
Class at
Publication: |
428/336 ;
134/2 |
International
Class: |
C23G 1/08 20060101
C23G001/08 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 31, 2010 |
JP |
2010-193179 |
Nov 30, 2010 |
JP |
2010-266123 |
Aug 16, 2011 |
JP |
2011-177865 |
Claims
1. A method of producing a cold-rolled steel sheet having Si with a
range of 0.5-3.0 mass %, characterized in that a continuously
annealed steel sheet after cold rolling is pickled with a mixture
of nitric acid and hydrochloric acid having a nitric acid
concentration of more than 100 g/L but not more than 200 g/L and a
ratio R (HCl/HNO.sub.3) of hydrochloric acid concentration to
nitric acid concentration of 0.01-0.25.
2. A method of producing a cold-rolled steel sheet according to
claim 1, wherein the mixture of nitric acid and hydrochloric acid
has a nitric acid concentration of more than 110 g/L but not more
than 140 g/L and a ratio R (HCl/HNO.sub.3) of hydrochloric acid
concentration to nitric acid concentration of 0.03-0.25.
3. A method of producing a cold-rolled steel sheet according to
claim 1, wherein the pickling is carried out at a temperature of a
pickling solution of 20-70.degree. C. for 3-30 seconds.
4. A method of producing a cold-rolled steel sheet according to
claim 1, wherein the steel sheet has a chemical composition
comprising, in addition to Si, C: 0.01-0.30 mass %, Mn: 1.0-7.5
mass %, P: not more than 0.05 mass %, S: not more than 0.01 mass %
and Al: not more than 0.06 mass % and the remainder being Fe and
inevitable impurities.
5. A method of producing a cold-rolled steel sheet according to
claim 1, wherein the steel sheet comprises, in addition to the
above chemical composition, one or more selected from the group
consisting of Nb: not more than 0.3 mass %, Ti: not more than 0.3
mass %, V: not more than 0.3 mass %, Mo: not more than 0.3 mass %,
Cr: not more than 0.5 mass %, B: not more than 0.006 mass % and N:
not more than 0.008 mass %.
6. A method of producing a cold-rolled steel sheet according to
claim 1, wherein the steel sheet comprises, in addition to the
above chemical composition, one or more selected from the group
consisting of Ni: not more than 2.0 mass %, Cu: not more than 2.0
mass %, Ca: not more than 0.1 mass % and REM: not more than 0.1
mass %.
7. A cold-rolled steel sheet produced by a method as claimed in
claim 1, characterized in that a Si-containing oxide layer is
removed from the surface of the steel sheet by pickling after
continuous annealing and a ratio of covering the surface of the
steel sheet with an iron-based oxide formed by the pickling is not
more than 85%.
8. A cold-rolled steel sheet according to claim 7, wherein a
maximum thickness of the iron-based oxide existing on the surface
of the steel sheet is not more than 200 nm.
9. A member for automobile characterized by using a cold-rolled
steel sheet as claimed in claim 7.
10. A method of producing a cold-rolled steel sheet according to
claim 2, wherein the pickling is carried out at a temperature of a
pickling solution of 20-70.degree. C. for 3-30 seconds.
11. A method of producing a cold-rolled steel sheet according to
claim 2, wherein the steel sheet has a chemical composition
comprising, in addition to Si, C: 0.01-0.30 mass %, Mn: 1.0-7.5
mass %, P: not more than 0.05 mass %, S: not more than 0.01 mass %
and Al: not more than 0.06 mass % and the remainder being Fe and
inevitable impurities.
12. A method of producing a cold-rolled steel sheet according to
claim 3, wherein the steel sheet has a chemical composition
comprising, in addition to Si, C: 0.01-0.30 mass %, Mn: 1.0-7.5
mass %, P: not more than 0.05 mass %, S: not more than 0.01 mass %
and Al: not more than 0.06 mass % and the remainder being Fe and
inevitable impurities.
13. A method of producing a cold-rolled steel sheet according to
claim 2, wherein the steel sheet comprises, in addition to the
above chemical composition, one or more selected from the group
consisting of Nb: not more than 0.3 mass %, Ti: not more than 0.3
mass %, V: not more than 0.3 mass %, Mo: not more than 0.3 mass %,
Cr: not more than 0.5 mass %, B: not more than 0.006 mass % and N:
not more than 0.008 mass %.
14. A method of producing a cold-rolled steel sheet according to
claim 3, wherein the steel sheet comprises, in addition to the
above chemical composition, one or more selected from the group
consisting of Nb: not more than 0.3 mass %, Ti: not more than 0.3
mass %, V: not more than 0.3 mass %, Mo: not more than 0.3 mass %,
Cr: not more than 0.5 mass %, B: not more than 0.006 mass % and N:
not more than 0.008 mass %.
15. A method of producing a cold-rolled steel sheet according to
claim 4, wherein the steel sheet comprises, in addition to the
above chemical composition, one or more selected from the group
consisting of Nb: not more than 0.3 mass %, Ti: not more than 0.3
mass %, V: not more than 0.3 mass %, Mo: not more than 0.3 mass %,
Cr: not more than 0.5 mass %, B: not more than 0.006 mass % and N:
not more than 0.008 mass %.
16. A method of producing a cold-rolled steel sheet according to
claim 2, wherein the steel sheet comprises, in addition to the
above chemical composition, one or more selected from the group
consisting of Ni: not more than 2.0 mass %, Cu: not more than 2.0
mass %, Ca: not more than 0.1 mass % and REM: not more than 0.1
mass %.
17. A method of producing a cold-rolled steel sheet according to
claim 3, wherein the steel sheet comprises, in addition to the
above chemical composition, one or more selected from the group
consisting of Ni: not more than 2.0 mass %, Cu: not more than 2.0
mass %, Ca: not more than 0.1 mass % and REM: not more than 0.1
mass %.
18. A method of producing a cold-rolled steel sheet according to
claim 4, wherein the steel sheet comprises, in addition to the
above chemical composition, one or more selected from the group
consisting of Ni: not more than 2.0 mass %, Cu: not more than 2.0
mass %, Ca: not more than 0.1 mass % and REM: not more than 0.1
mass %.
19. A method of producing a cold-rolled steel sheet according to
claim 5, wherein the steel sheet comprises, in addition to the
above chemical composition, one or more selected from the group
consisting of Ni: not more than 2.0 mass %, Cu: not more than 2.0
mass %, Ca: not more than 0.1 mass % and REM: not more than 0.1
mass %.
20. A cold-rolled steel sheet produced by a method as claimed in
claim 2, characterized in that a Si-containing oxide layer is
removed from the surface of the steel sheet by pickling after
continuous annealing and a ratio of covering the surface of the
steel sheet with an iron-based oxide formed by the pickling is not
more than 85%.
Description
TECHNICAL FIELD
[0001] This invention relates to a method of producing a
cold-rolled steel sheet as well as a cold-rolled steel sheet and a
member for automobile, and more particularly to a method of
producing a cold-rolled steel sheet being excellent in not only the
phosphate treatability but also the corrosion resistance after
coating as evaluated by a hot salt water immersion test or a
composite cycle corrosion test, a cold-rolled steel sheet produced
by this method as well as a member for automobile using the
cold-rolled steel sheet. Moreover, the cold-rolled steel sheet
according to the invention can be preferably used in a
high-strength cold-rolled steel sheet containing Si and having a
tensile strength TS of not less than 590 MPa.
BACKGROUND ART
[0002] Recently, it is strongly demanded to improve fuel
consumption of an automobile from a viewpoint of the protection of
global environment. Also, it is strongly demanded to improve the
safety of the automobile from a viewpoint of ensuring the safe of
crew members at the time of impact. In order to meet these demands,
it is required to simultaneously attain weight reduction and
high-strengthening of a vehicle body in the automobile, while the
thinning associated with the high strengthening is positively
proceeding in cold-rolled steel sheets as a starting material in
the member for automobile. However, many members for automobile are
manufactured by forming the steel sheet, so that these steel sheets
are required to have an excellent formability in addition to the
high strength.
[0003] There are various methods for enhancing the strength of the
cold-rolled steel sheet. As a method increasing the strength
without largely damaging the formability is mentioned a
solid-solution strengthening method through addition of Si.
However, when a greater amount of Si, particularly not less than
0.5 mass % of Si is added to a cold-rolled steel sheet, it is known
that Si-containing oxides such as SiO.sub.2, Si--Mn based composite
oxide and the like are formed on the surface of the steel sheet
during slab heating or during annealing after hot rolling or after
cold rolling. Since the Si-containing oxide considerably
deteriorates the phosphate treatability, the high-strength
cold-rolled steel sheets containing a great amount of Si have
problems that the phosphate treatability is poor but the coating
peeling is easily caused to deteriorate the corrosion resistance
after the coating as compared with the commonly used steel sheets
when the steels sheet after electrodeposition coating is subjected
to severer corrosion environment as in a hot salt water immersion
test or a composite cycle corrosion test repeating cycle of
wetting-drying.
[0004] As a countermeasure for these problems, for example, Patent
Document 1 proposes a high-strength cold-rolled steel sheet
obtained by heating a slab at a temperature of higher than
1200.degree. C. in hot rolling, descaling under high pressure,
polishing the surface of the hot-rolled steel sheet with a nylon
brush containing abrasion grains prior to pickling and then
immersing in a bath of 9% hydrochloric acid twice to perform
pickling to lower the Si concentration on the surface of the steel
sheet. Also, Patent Document 2 proposes a high-strength cold-rolled
steel sheet wherein the corrosion resistance is improved by
rendering line width of Si-containing linear oxide observed in 1-10
.mu.m from the surface of the steel sheet into not more than 300
nm.
[0005] However, in the high-strength cold-rolled steel sheet
disclosed in Patent Document 1, even if the Si concentration on the
surface of the steel sheet is reduced before the cold rolling, the
Si-containing oxide is formed on the surface of the steel sheet by
annealing after cold rolling, so that the improvement of the
corrosion resistance after coating is not desired. Also, in the
high-strength cold-rolled steel sheet disclosed in Patent Document
2, there is no problem in the corrosion resistance under corrosion
environment as in a salt spray test defined according to JIS Z2371,
but sufficient corrosion resistance after coating is not obtained
under severer corrosion environment as in a hot salt water
immersion test or a composite cycle corrosion test. That is, the
high-strength cold-rolled steel sheet having an excellent corrosion
resistance after coating can not be obtained only by reducing the
Si concentration on the surface of the steel sheet after hot
rolling or by reducing the Si-containing linear oxide.
[0006] As a technique for solving the above problems, Patent
Document 3 discloses a technique wherein the Si-containing oxide
enriched on the surface of the steel sheet by annealing step or the
like is removed by pickling and further an S-based compound is
applied to the surface to enhance the reactivity with a phosphate
treating solution to thereby improve the phosphate treatability.
Also, Patent Document 4 discloses a technique wherein a P-based
compound is applied instead of the S-based compound of the above
technique.
PRIOR ART ARTICLES
Patent Document
[0007] [Patent Document 1] JP-A-2004-204350
[0008] [Patent Document 2] JP-A-2004-244698
[0009] [Patent Document 3] JP-A-2007-217743
[0010] [Patent Document 4] JP-A-2007-246951
SUMMARY OF THE INVENTION
Problems to be Solved by the Invention
[0011] In recent years, for the purpose of reducing industrial
wastes (suppression of sludge formation) and cutting down running
cost, it is proceeded to lower the temperature of the phosphate
treating solution, and hence the reactivity of the phosphate
treating solution to the steel sheet is largely lowered as compared
with the conventional phosphate treating conditions. The lowering
of the temperature of the treating solution does not come into
problem when the surface adjusting technique prior to the phosphate
treatment is improved in the common steel sheet having a less
addition amount of alloy usually used. However, in the
high-strength cold-rolled steel sheet added with a great amount of
Si, the reactivity with the phosphate treating solution is
considerably deteriorated by the influence of the Si-containing
oxide formed on the surface of the steel sheet at an annealing
step, so that it is required to enhance the reactivity from the
steel sheet side in some way. On the other hand, the techniques
disclosed in Patent Documents 3 and 4 are effective to the
conventional common steel sheets, but can not expect the sufficient
improving effect capable of lowering the temperature of the
phosphate treating solution for the high-strength cold-rolled steel
sheets containing a great amount of Si.
[0012] The invention is made in view of considering the above
problems inherent to the cold-rolled steel sheet containing a great
amount of Si and is to provide a method of producing a cold-rolled
steel sheet being excellent in not only the phosphate treatability
even when using a phosphate treating solution at a lower
temperature but also in the corrosion resistance after coating as
evaluated by a hot salt water immersion test or a composite cycle
corrosion test, a cold-rolled steel sheet produced by this method
as well as a member for automobile using the cold-rolled steel
sheet.
Means for Solving Problems
[0013] The inventors have made detailed analysis on surface
properties of steel sheets after annealing in order to solve the
above problems and various studies on a method of enhancing the
reactivity between the surface of the steel sheet and the phosphate
treating solution. As a result, it has been found that it is very
important to subject the continuously annealed steel sheet surface
to strong pickling after the cold rolling to thereby remove
Si-containing oxide layer formed on the surface of the steel sheet
during the annealing but also reduce a ratio of covering the
surface of the steel sheet with an iron-based oxide formed on the
steel sheet surface by the strong pickling, and consequently the
invention has been accomplished.
[0014] That is, the invention proposes a method of producing a
cold-rolled steel sheet, characterized in that a continuously
annealed steel sheet after cold rolling is pickled with a mixture
of nitric acid and hydrochloric acid having a nitric acid
concentration of more than 100 g/L but not more than 200 g/L and a
ratio R (HCl/HNO.sub.3) of hydrochloric acid concentration to
nitric acid concentration of 0.01-0.25.
[0015] The mixture of nitric acid and hydrochloric acid in the
production method of the invention is characterized to have a
nitric acid concentration of more than 110 g/L but not more than
140 g/L and a ratio R (HCl/HNO.sub.3) of hydrochloric acid
concentration to nitric acid concentration of 0.03-0.25.
[0016] Also, the production method of the invention is
characterized in that the pickling is carried out at a temperature
of a pickling solution of 20-70.degree. C. for 3-30 seconds.
[0017] The steel sheet in the production method of the invention is
characterized by a chemical composition comprising, in addition to
Si, C: 0.01-0.30 mass %, Mn: 1.0-7.5 mass %, P: not more than 0.05
mass %, S: not more than 0.01 mass % and Al: not more than 0.06
mass % and the remainder being Fe and inevitable impurities.
[0018] Also, the cold-rolled steel sheet in the production method
of the invention is characterized by comprising, in addition to the
above chemical composition, one or more selected from the group
consisting of Nb: not more than 0.3 mass %, Ti: not more than 0.3
mass %, V: not more than 0.3 mass %, Mo: not more than 0.3 mass %,
Cr: not more than 0.5 mass %, B: not more than 0.006 mass % and N:
not more than 0.008 mass %.
[0019] Furthermore, the cold-rolled steel sheet in the production
method of the invention is characterized by comprising, in addition
to the above chemical composition, Ni: not more than 2.0 mass %,
Cu: not more than 2.0 mass %, Ca: not more than 0.1 mass % and REM:
not more than 0.1 mass %.
[0020] Moreover, the invention is a cold-rolled steel sheet
produced by any aforementioned method, which is a cold-rolled steel
sheet characterized in that a Si-containing oxide layer is removed
from the surface of the steel sheet by pickling after continuous
annealing and a ratio of covering the surface of the steel sheet
with an iron-based oxide formed by the pickling is not more than
85%.
[0021] The cold-rolled steel sheet according to the invention is
characterized in that a maximum thickness of the iron-based oxide
existing on the surface of the steel sheet is not more than 200
nm.
[0022] Further, the invention is a member for automobile
characterized by using any aforementioned cold-rolled steel
sheet.
Effect of the Invention
[0023] According to the invention, there can be provided a
cold-rolled steel sheet which is excellent in the phosphate
treatability even when Si is contained as large as 0.5-3.0 mass%
and when using a phosphate treating solution at a lower temperature
but also is excellent in the corrosion resistance after coating
under severer corrosion environment as in a hot salt water
immersion test or a composite cycle corrosion test. According to
the invention, therefore, it is possible to largely improve the
phosphate treatability and corrosion resistance after coating in
the high-strength cold-rolled steel sheets containing a greater
amount of Si and having a tensile strength TS of not less than 590
MPa, so that it can be preferably used in strong members and the
like in a vehicle body of an automobile.
BRIEF DESCRIPTION OF THE DRAWINGS
[0024] FIG. 1 shows reflection electron microphotographs of steel
sheet surfaces of standard cold-rolled steel sheet sample Nos. a
and b for determining a surface covering ratio with an iron-based
oxide.
[0025] FIG. 2 shows a histogram of pixel number to gray value in
the reflection electron microphotographs of the standard
cold-rolled steel sheet sample Nos. a and b.
[0026] FIG. 3 is a photograph of a section of a coating on a
surface of a steel sheet after pickling observed by means of a
transmission electron microscope.
[0027] FIG. 4 is a graph showing energy dispersion type X-ray (EDX)
analytical results of an iron-based oxide observed in FIG. 3.
[0028] FIG. 5 is a graph of depth distribution of O, Si, Mn and Fe
on a surface of a test specimen in Comparative Example (No. 1) and
Invention Example (No. 18) of Example 1 as measured by GDS.
EMBODIMENTS FOR CARRYING OUT THE INVENTION
[0029] First, the basic technical idea of the invention will be
described.
[0030] In an annealing step using a continuous annealing furnace
for recrystallizing a cold-rolled steel sheet after cold rolling to
impart desired structure, strength and workability, a non-oxidizing
or reducing gas is usually used as an atmosphere gas, and also a
dew point is strictly controlled. In the commonly general
cold-rolled steel sheet having a less amount of an alloy added,
therefore, the oxidation of the steel sheet surface is controlled.
However, in the steel sheet containing not less than 0.5 mass % of
Si or Mn, even if component or dew point of the atmosphere gas in
the annealing is strictly controlled, it can not be avoided that
Si, Mn and the like being easily oxidizable as compared with Fe are
oxidized to form a Si-containing oxide such as Si oxide
(SiO.sub.2), Si--Mn based composite oxide or the like on the
surface of the steel sheet. The construction of these oxides varies
depending on components of the steel sheet, annealing atmosphere
and the like, but both the oxides are typically and frequently
existent in a mixture. Also, since the Si-containing oxide is
formed not only the surface of the steel sheet but also in the
interior of the steel matrix, it is known that the oxide obstructs
the etching property on the surface of the steel sheet in the
phosphate treatment (treatment with zinc phosphate) made as an
underlaying treatment for electrodeposition coating and badly
affects the formation of sound phosphate treated coating.
[0031] In recent years, the lowering of the temperature of the
phosphate treating solution is proceeding for the purpose of
reducing the sludge amount generated in the phosphate treatment and
the running cost, and hence the phosphate treatment is carried out
under a condition that the reactivity of the phosphate treating
solution to the steel sheet is considerably low as compared with
the conventional technique. The change of the phosphate treating
condition is not particularly questioned by the improvement of the
surface adjusting technique or the like in the conventionally used
common steel sheets having a less addition amount of an alloy. In
the steel sheet having a greater addition amount of alloying
component, particularly a high-strength cold-rolled steel sheet
attempted to increase the strength by adding a greater amount of
Si, however, the influence of changing the phosphate treating
condition as mentioned above is very large. In the cold-rolled
steel sheet having a greater amount of Si, therefore, it is
required that the surface of the steel sheet itself is activated in
correspondence with the deterioration of the phosphate treating
condition to enhance the reactivity with the phosphate treating
solution.
[0032] The inventors have made various investigations on a method
of improving the phosphate treatability for corresponding to the
deterioration of the phosphate treating condition as mentioned
above. As a result, it has been found out that it is effective to
conduct strong pickling of the surface of the cold-rolled steel
sheet after continuous annealing with nitric acid as a pickling
solution to remove a Si-containing oxide layer formed on the
surface of the steel sheet by continuous annealing and the like
after cold rolling. The term "Si-containing oxide" used herein
means SiO.sub.2 or Si--Mn base composite oxide formed on the
surface of the steel sheet or along crystal grain boundary inside
the steel sheet in the slab heating or after hot rolling or in
annealing after cold rolling. The thickness of the layer containing
these Si-containing oxides varied depending upon components of the
steel sheet or the annealing condition (temperature, time,
atmosphere), but is usually about 1 .mu.m from the surface of the
steel sheet. Also, the term "removal of the Si-containing oxide
layer" according to the invention means that the pickling is
carried out to remove the Si-containing oxide layer to a level that
peaks of Si, O do not appear when the surface of the steel sheet is
analyzed in depth direction by means of GDS (glow discharge optical
emission spectroscopy).
[0033] Moreover, the reason why nitric acid is used as the pickling
solution is due to the fact that the Si--Mn based composite oxide
among the Si-containing oxides is easily dissolved in the acid, but
SiO.sub.2 is hardly soluble, and in order to remove the latter,
nitric acid as a strong-oxidizing acid must be used to remove the
Si-containing oxide on the surface of the steel sheet together with
the steel matrix.
[0034] According to the inventors' studies, however, it can be seen
that the phosphate treatability is largely improved by conducting
strong pickling with nitric acid after the continuous annealing to
remove the Si-containing oxide layer existing on the surface of the
steel sheet but the phosphate treatability may be deteriorated at
moments. As the cause is further investigated, it is newly found
that although the Si-containing oxide layer is removed by the
strong pickling with nitric acid, Fe dissolved from the surface of
the steel sheet by the pickling separately produces an iron-based
oxide, which is settled and precipitated on the surface of the
steel sheet so as to cover the steel sheet surface to thereby
deteriorate the phosphate treatability.
[0035] Furthermore, it has been found that in order to suppress the
oxidation of the steel sheet surface by the above pickling with
nitric acid to mitigate the bad influence upon the phosphate
treatability, it is important to suppress the formation of the
iron-based oxide on the steel sheet surface to reduce the ratio of
covering the steel sheet surface with the iron-based oxide to not
more than 85% and that it is important as means for attaining the
above to control the concentration of nitric acid to an adequate
range to suppress the oxidation with nitric acid and to conduct the
pickling with a mixture of nitric acid and hydrochloric acid at a
given mixing ratio of hydrochloric acid having an effect of
breaking an oxide film.
[0036] The inventors have also found that the phosphate
treatability is more improved and the corrosion resistance is
further improved when the covering ratio of the iron-based oxide
generated on the surface of the steel sheet by pickling is not more
than 85% and further the maximum thickness of the iron-based oxide
is not more than 200 nm and that it is effective as means for
attaining the above to control the concentration of hydrochloric
acid having the effect of breaking an oxide film, which is used in
a part of the pickling solution, to an adequate range for
pickling.
[0037] Moreover, the iron-based oxide in the invention means an
oxide composed mainly of iron wherein an atomic concentration ratio
of iron is not less than 30% as an element other than oxygen
constituting the oxide. The iron-based oxide is existent on the
surface of the steel sheet at an uneven thickness, which is
different from a natural oxide film existing uniformly and in layer
at a thickness of few nm. The iron-based oxide generated on the
surface of the cold-rolled steel sheet is confirmed to be amorphous
from the observation by means of a transmission electron microscope
(TEM) and analysis results of diffraction pattern (analytical
diagram) through an electron diffractometry.
[0038] The invention is accomplished by conducting further
examinations on the above new knowledge.
[0039] The reason why the chemical composition of the cold-rolled
steel sheet according to the invention is limited to the above
range will be described below.
[0040] Si: 0.5-3.0 mass %
[0041] Si is an element effective for attaining the increase of the
strength of the steel because the effect of enhancing the strength
of steel (solid-solution strengthening ability) is large without
largely damaging the workability, but is also an element adversely
exerting on the phosphate treatability and the corrosion resistance
after coating. When Si is added as means for attaining a high
strength, the addition of not less than 0.5 mass % is necessary. If
the Si content is less than 0.5 mass %, the influence due to the
deterioration of the phosphate treating conditions is less. On the
other hand, when the Si content exceeds 3.0 mass %, the hot rolling
property and cold rolling property are largely deteriorated, which
is adversely influenced on the productivity and leads to the
deterioration of ductility of the steel sheet itself. Therefore, Si
is added within a range of 0.5-3.0 mass %. Preferably, it is a
range of 0.8-2.5 mass %.
[0042] The cold-rolled steel sheet of the invention is an essential
feature to include Si in the above range. The other components are
acceptable as far as they are included within composition ranges in
the common cold-rolled steel sheet, and are not particularly
limited. However, the cold-rolled steel sheet of the invention is
preferable to have the following component composition when it is
applied to a high-strength cold-rolled steel sheet having a tensile
strength of not less than 590 MPa for use in vehicle bodies for
automobiles and so on.
[0043] C: 0.01-0.30 mass %
[0044] C is an element effective for enhancing the strength of
steel and further is an element effective for producing residual
austenite having an effect of TRIP (Transformation Induced
Plasticity), bainite and martensite. When C content is not less
than 0.01 mass %, the above effect is obtained, while when C
content is not more than 0.30 mass %, the deterioration of the
weldability is not caused. Therefore, C is added preferably within
a range of 0.01-0.3 mass %, more preferably within a range of
0.10-0.20 mass %.
[0045] Mn: 1.0-7.5 mass %
[0046] Mn is an element having an action for solid-solution
strengthening steel to increase the strength and enhance the
hardenability and promoting the formation of residual austenite,
bainite and martensite. Such effects are developed by the addition
of not less than 1.0 mass %. On the other hand, when Mn content is
not more than 7.5 mass %, the above effect is obtained without the
increase of the cost. Therefore, Mn is added preferably within a
range of 1.0-7.5 mass %, more preferably within a range of 2.0-5.0
mass %.
[0047] P: not more than 0.05 mass %
[0048] P is an element damaging no drawability though the
solid-solution strengthening ability is large and is also an
element effective for attaining a high strength, so that it is
preferable to be included in an amount of not less than 0.005 mass
%. However, P is an element damaging the spot weldability, but
there is no problem when it is not more than 0.05 mass %.
Therefore, P is preferably not more than 0.05 mass %, more
preferably not more than 0.02 mass %.
[0049] S: not more than 0.01 mass %
[0050] S is an impurity element inevitably incorporated, and is a
harmful element which is precipitated in steel as MnS to
deteriorate the stretch-flanging property. In order to prevent the
deterioration of the stretch-flanging property, S is preferably not
more than 0.01 mass %, more preferably not more than 0.005 mass %,
further preferably not more than 0.003 mass %.
[0051] Al: not more than 0.06 mass %
[0052] Al is an element added as a deoxidizer at steel-making step,
and is also an element effective for separating non-metallic
inclusion, which deteriorates the stretch-flanging property, as a
slug, so that it is preferable to be included in an amount of not
less than 0.01 mass %. When Al content is not more than 0.06 mass
%, the above effect is obtained without the increase of cost for
material. Therefore, Al is preferable to be not more than 0.06 mass
%, More preferably, it is a range of 0.02-0.06 mass %.
[0053] In addition to the above components, the cold-rolled steel
sheet of the invention may contain one or more selected from Nb:
not more than 0.3 mass %, Ti: not more than 0.3 mass %, V: not more
than 0.3 mass %, Mo: not more than 0.3 mass %, Cr: not more than
0.5 mass %, B: not more than 0.006 mass % and N: not more than
0.008 mass %.
[0054] Nb, Ti and V are elements forming carbide and nitride to
suppress ferrite growth at a heating stage in the annealing and
finely divide the structure to improve the formability,
particularly stretch-flanging property, and also Mo, Cr and B are
elements improving the hardenability of steel and promoting the
formation of bainite and martensite, so that they can be added
within the above ranges. Also, N is an element forming nitrides
with Nb, Ti and V or solid-soluting in steel to contribute to the
increase of the strength of steel, so that when it is not more than
0.008 mass %, a greater amount of the nitride is not formed, and
hence the breakage due to the formation of voids in the press
forming can be suppressed to obtain the above effect.
[0055] In addition to the above components, the cold-rolled steel
sheet of the invention may contain one or more selected from Ni:
not more than 2.0 mass %, Cu: not more than 2.0 mass %, Ca: not
more than 0.1 mass % and REM: not more than 0.1 mass %.
[0056] Ni and Cu promote the formation of the low-temperature
transformation phase to develop the effect of increasing the
strength of steel, so that they can be added within the above
ranges. Also, Ca and REM are elements controlling the form of the
sulfide base inclusion to improve the stretch-flanging property of
the steel sheet, so that they can be added within the above
ranges.
[0057] In the cold-rolled steel sheet of the invention, the
remainder other than the above components is Fe and inevitable
impurities. However, other components may be optionally added
within a scope of not damaging the action and effect of the
invention.
[0058] The surface properties of the cold-rolled steel sheet of the
invention will be described below.
[0059] As mentioned above, the cold-rolled steel sheet of the
invention is necessary to have a steel sheet surface obtained after
the removal of Si-containing oxide layer such as SiO.sub.2 or
Si--Mn based composite oxide layer formed on the surface layer of
the steel sheet during annealing. For this end, it is necessary to
conduct strong pickling with a pickling solution of mixed nitric
acid and hydrochloric acid to dissolve and remove the Si-containing
oxide formed on the surface of the steel sheet and in the grain
boundary portion in the vicinity of the surface together with the
steel matrix.
[0060] Furthermore, in the cold-rolled steel sheet of the
invention, it is necessary to reduce the ratio of covering the
surface of the steel sheet with iron-based oxide generated on the
steel sheet surface by the strong pickling with nitric acid to not
more than 85% as an area ratio in addition to the removal of the
Si-containing oxide layer. When the surface covering ratio of the
iron-based oxide exceeds 85%, the dissolving reaction of iron in
the phosphate treatment is inhibited to suppress the crystal growth
of phosphate such as zinc phosphate or the like. It is preferably
not more than 80%.
[0061] In the invention, the surface covering ratio of the
iron-based oxide is determined as follows:
[0062] The surface of the steel sheet after the pickling is
observed at about 5 fields with a ultra-low acceleration voltage
scanning type electron microscope (ULV-SEM) capable of detecting
information of an extremely surface layer under conditions of
acceleration voltage: 2 kV, operating distance: 3.0 mm and
magnification: about 1000 times and spectroscopy is conducted with
an energy dispersion type X-ray spectrometer (EDX) to obtain a
reflection electron image. The reflection electron image is
binarized with an image analysis software, e.g. Image J to measure
an area ratio of a black portion. The measured results on the
fields can be averaged to obtain a surface covering ratio of the
iron-based oxide. Moreover, as the ultra-low acceleration voltage
scanning type electron microscope
[0063] (ULV-SEM) may be mentioned, for example, ULTRA 55 made by
SEISS, and as the energy dispersion type X-ray spectrometer (EDX)
may be mentioned, for example, NSS 312E made by Thermo Fisher.
[0064] Here, threshold value in the binarization will be
described.
[0065] A steel slab of Steel symbol G shown in Table 3 of the
following example is subjected to hot rolling, cold rolling and
continuous annealing under conditions of No. 7 in Table 4 of the
following example to obtain a cold-rolled steel sheet of 1.8 mm in
thickness, and then the cod-rolled steel sheet after the continuous
annealing is subjected to pickling and repickling under conditions
shown in Table 1, washed with water, dried and subjected to 0.7%
temper rolling to obtain two cold-rolled steel sheets of Nos. a and
b having different iron-based oxide amounts on their steel sheet
surfaces. Then, the cold-rolled steel sheet of No. a is a standard
sample having a large amount of iron-based oxide and the
cold-rolled steel sheet of No. b is a standard sample having a
small amount of iron-based oxide, and each of these steel sheets is
observed with the scanning type electron microscope under the
aforementioned conditions to obtain a reflection electron image.
FIG. 1 shows photographs of reflection electron images of steel
sheets Nos. a and b, and FIG. 2 shows a histogram of pixel number
to a gray value in the photographs of the reflection electron
images of the steel sheets Nos. a and b. In the invention, a gray
value (Y point) corresponding to an intersecting point (X point) of
the histograms of Nos. a and b shown in FIG. 2 is defined as a
threshold value. Incidentally, when the surface covering ratio of
the iron-based oxide in the steel sheets Nos. a and b is determined
with the above threshold value, it is 85.3% in the steel sheet No.
a and 25.8% in the steel sheet No. b.
TABLE-US-00001 TABLE 1 Pickling conditions Treating Repickling
conditions Surface covering ratio Steel Acid concentration
Temperature time Acid concentration Temperature Treating time of
iron-based oxide sheet (g/l) (.degree. C.) (Seconds) (g/l)
(.degree. C.) (Seconds) (%) a Nitric acid: 250 + 40 10 -- -- --
85.3 Hydrochloric acid: 25 b Nitric acid: 150 + 40 10 Hydrochloric
acid: 10 40 30 25.8 Hydrochloric acid: 15
[0066] In order to more improve the phosphate treatability and
corrosion resistance in the cold-rolled steel sheet of the
invention, it is preferable that the maximum thickness of the
iron-based oxide is not more than 200 nm in addition that the
covering ratio of the iron-based oxide produced on the steel sheet
surface by pickling is not more than 85%. When the maximum
thickness of the iron-based oxide is not more than 200 nm, the
dissolving reaction of iron through the phosphate treatment is not
inhibited locally and also the precipitation of crystal of
phosphate such as zinc phosphate or the like is not inhibited
locally. More preferably, it is not more than 180 nm.
[0067] The maximum thickness of the iron-based oxide is measured as
follows.
[0068] First, 10 extraction replicas are prepared from the surface
of the steel sheet after the pickling by a focused ion beam (FIB)
work for observing a section of about 8 .mu.m relative to the
widthwise direction of the steel sheet. Then, the section of 8
.mu.m in the each replica is continuously shot by means of a
transmission electron microscope (TEM) provided with an energy
dispersion type X-ray spectrometer
[0069] (EDX) capable of checking local information of the section
at an acceleration voltage of 200 kV and a magnification of 100000
times. As an example, FIG. 3 is a photograph showing a section of a
covering layer existing on the surface of the steel sheet and
generated by pickling as observed by TEM, and FIG. 4 shows
analytical results of the covering layer by EDX. As seen from FIG.
4, the covering layer is an iron-based oxide composed mainly of
iron. Therefore, the interval between a line A showing a surface of
the steel sheet and a line B showing a thickest portion of an oxide
layer shown by the photograph of the section in FIG. 3 is measured
with respect to the 10 replicas, and a maximum thickness among them
is a maximum thickness of the iron-based oxide. Moreover, the size
and numbers of the replicas, measuring conditions by TEM and the
like as mentioned above are merely exemplified, and may be properly
modified as a matter of course.
[0070] The production method of the cold-rolled steel sheet
according to the invention will be described below.
[0071] The production method of the cold-rolled steel sheet of the
invention is necessary to be a method wherein a steel material
(slab) having Si: 0.5-3.0 mass % is heated, hot rolled, cold
rolled, continuously annealed and then pickled with a pickling
solution of mixed nitric acid and hydrochloric acid, whereby a
Si-containing oxide layer is removed from a surface layer portion
of the steel sheet and a surface covering ratio of an iron-based
oxide generated on the surface of the steel sheet by pickling can
be made to not more than 85%. Further, it is preferable to be a
method wherein a maximum thickness of the iron-based oxide can be
made to not more than 200 nm. Therefore, the procedure ranging from
the steel-making step to the continuous annealing step after the
cold rolling can be carried out according to the usual manner, but
the pickling after the continuous annealing is preferable to be
conducted under the following conditions.
[0072] Pickling conditions after continuous annealing
[0073] On the surface layer of the steel sheet after the continuous
annealing is produced a greater amount of the Si-containing oxide
such as SiO.sub.2, Si--Mn based composite oxide or the like, so
that the phosphate treatability and the corrosion resistance after
coating are considerably deteriorated. In the production method of
the invention, therefore, it is necessary that the cold-rolled
steel sheet after the annealing is strongly pickled with a pickling
solution of mixed nitric acid and hydrochloric acid, whereby the
Si-containing oxide layer on the surface of the steel sheet is
removed with the steel matrix but also the formation of the
iron-based oxide settled and precipitated on the surface of the
steel sheet by pickling is suppressed.
[0074] As previously mentioned, Si--Mn based composite oxide among
the Si-containing oxides is easily dissolved in an acid, but
SiO.sub.2 is insoluble in an acid. Therefore, in order to remove
the Si-containing oxide including SiO.sub.2 by pickling, the steel
matrix is required to be removed with nitric acid as a strong acid.
In order to conduct the strong pickling for removing the oxide
layer including the steel matrix, it is necessary that the
concentration of nitric acid is more than 100 g/L. However, since
nitric acid is a strongly oxidizing acid, Fe eluted is oxidized to
form an iron-based oxide, which is precipitated on the surface of
the steel sheet and adversely affects the phosphate treatability
and the corrosion resistance after coating. In order to suppress
this adverse effect, therefore, it is necessary to control the
concentration of nitric acid to not more than 200 g/L. Thus, the
concentration of nitric acid is more than 100 g/L but not more than
200 g/L. Preferably, it is a range of 110-150 g/L.
[0075] However, when the concentration of nitric acid is merely
limited to the above range, it is difficult to stably control the
surface covering ratio of the iron-based oxide generated on the
surface of the steel sheet by pickling with nitric acid to not more
than 85%. In the invention, therefore, in order to more surely
suppress the formation of the iron-based oxide on the surface of
the steel sheet by strong pickling with nitric acid, the pickling
is carried out with such a mixed acid that the concentration of
nitric acid is limited to the above range but also a ratio R
(HCl/HNO.sub.3) of a concentration of a chloride ion having an
effect of breaking an oxide film, i.e. hydrochloric acid to the
concentration of nitric acid is a range of 0.01-0.25. When the
ratio R is less than 0.01, the effect of suppressing the formation
of the iron-based oxide is small, while when it exceeds 0.25, the
amount of the steel sheet dissolved is reduced and hence the
Si-containing oxide layer cannot be removed.
[0076] In order to more improve the phosphate treatability and the
corrosion resistance, it is desirable to render the maximum
thickness of the iron-based oxide generated on the surface of the
steel sheet by pickling into not more than 200 nm. For this end,
the mixed acid pickling solution of nitric acid and hydrochloric
acid is preferable to have a concentration of nitric acid within a
range of more than 110 g/L but not more than 140 g/L and a ratio R
(HCl/HNO.sub.3) of hydrochloric acid concentration to nitric acid
concentration within a range of 0.03-0.25. When the concentrations
are satisfied within the above ranges, it is possible to stably
make the thickness of the iron-based oxide to not more than 200 nm,
and hence the phosphate treatability and the corrosion resistance
after coating are not deteriorated.
[0077] Moreover, it is preferable that the pickling with the mixed
pickling solution of nitric acid and hydrochloric acid is carried
out at a temperature of the pickling solution of 20-70.degree. C.
for a pickling time of 3-30 seconds. When the temperature of the
pickling solution is not lower than 20.degree. C. and the pickling
time is not less than 3 seconds, the Si-containing oxide layer
formed in the surface layer of the steel sheet in the annealing can
be removed sufficiently, and the phosphate treatability and the
corrosion resistance after coating are never deteriorated. While,
when the temperature of the pickling solution is not higher than
70.degree. C. and the time is not more than 30 seconds, there is no
phenomenon due to excessive pickling that the surface of the steel
sheet becomes coarse and the phosphate coating becomes non-uniform
and the surface covering ratio of the iron-based oxide becomes
high, and the phosphate treatability and the corrosion resistance
after coating are never deteriorated.
[0078] The cold-rolled steel sheet, wherein the covering ratio of
the steel sheet surface with the iron-based oxide is made to not
more than 85% by pickling after the continuous annealing as
mentioned above, or alternately the cold-rolled steel sheet,
wherein the maximum thickness of the iron-based oxide is made to
not more than 200 nm, is subsequently subjected to usual treating
steps such as temper rolling and the like to provide products.
Example 1
[0079] A steel comprising C: 0.125 mass %, Si: 1.5 mass %, Mn: 2.6
mass %, P: 0.019 mass %, S: 0.008 mass %, Al: 0.040 mass % and the
remainder being Fe and inevitable impurities is prepared according
to common refining process such as melting in a converter,
degassing treatment and the like and continuously cast into a steel
material (slab). The slab is reheated to a temperature of
1150-1170.degree. C., hot rolled at a terminating temperature of
finish rolling of 850-880.degree. C. and coiled at a temperature of
500-550.degree. C. to obtain a hot-rolled steel sheet having a
thickness of 3.about.4 mm. Then, the hot-rolled steel sheet is
pickled to remove scales and thereafter cold rolled to obtain a
cold-rolled steel sheet having a thickness of 1.8 mm. Next, the
cold-rolled steel sheet is subjected to such a continuous annealing
that it is heated to a soaking temperature of 750-780.degree. C.
and held at this temperature for 40-50 seconds and then cooled at a
rate of 20-30.degree. C./second from the soaking temperature to a
cooling stop temperature of 350-400.degree. C. and held at the
cooling stop temperature range for 100-120 seconds, and then the
steel sheet surface is pickled under conditions shown in Table 2,
washed with water, dried and subjected to a temper rolling at a
stretching ratio of 0.7% to obtain cold-rolled steel sheets Nos.
1-25 shown in Table 2.
[0080] A test specimen is sampled from each of the above
cold-rolled steel sheets and observed at 5 fields of the steel
sheet surface with a scanning type electron microscope of ultra-low
acceleration voltage (ULV-SEM; made by SEISS; ULTRA 55) at an
acceleration voltage of 2 kV, an operating distance of 3.0 mm and a
magnification of 1000 times. And analyzed with an energy dispersion
X-ray spectrometer (EDX; made by Thermo Fisher; NSS 312E) to obtain
a reflection electron image. The reflection electron image is
binarized with an image analyzing software (Image J) with respect
to gray value (Y-point) corresponding to intersect point (X-point)
and threshold value defined in histograms of the aforementioned
standard samples Nos. a and b to measure an area ratio of a black
portion. The values measured at 5 fields are averaged as a surface
covering ratio of iron-based oxide.
[0081] Also, a test specimen is sampled from each of the above
cold-rolled steel sheets and subjected to a phosphate treatment and
a coating treatment under the following conditions and then
subjected to three corrosion tests of hot salt water immersion
test, salt water spray test and composite cycle corrosion test to
evaluate a corrosion resistance after coating. Further, a
distribution of O, Si, Mn and Fe in depth direction on the surface
of the test specimen sampled from each cold-rolled steel sheet is
measured with GDS.
[0082] (1) Phosphate Treating Conditions
[0083] The test specimen sampled from each cold-rolled steel sheet
is subjected to a phosphate treatment with a degreasing agent:
FC-E2011, a surface regulator: PL-X and a phosphate treating agent:
PALBON PB-L3065, which are made by Nihon
[0084] Parkerizing Co., Ltd., so as to provide a phosphate coating
adhered amount of 1.7-3.0 g/m.sup.2 under two conditions of the
following standard condition and comparative condition of lowering
the phosphate treating temperature to a low temperature.
<Standard Condition>
[0085] Degreasing step: treating temperature 40.degree. C.,
treating time 120seconds Spray degreasing, surface regulating step:
pH 9.5, Treating temperature room temperature, treating time 20
seconds Phosphate treating step: temperature of phosphate treating
solution 35.degree. C., treating time 120 seconds
<Low Temperature Condition>
[0086] Condition of lowering the temperature of the phosphate
treating solution in the above standard condition to 33.degree.
C.
[0087] (2) Corrosion Test
[0088] The surface of the test specimen subjected to the phosphate
treatment is electrodeposited with an electrodeposition paint :
V-50 made by Nippon Paint Co., Ltd. so as to have a coating
thickness of 25 .mu.m and then subjected to the following three
corrosion tests.
<Hot Salt Water Immersion Test>
[0089] The test specimen (n=1) subjected to the phosphate treatment
and electrodeposition is provided on its surface with a crosscut
flaw of 45 mm in length by means of a cutter, and thereafter
immersed in a solution of 5 mass % NaCl (60.degree. C.) for 240
hours, washed with water, and dried. After an adhesive tape is
attached to a cut flaw portion, a test of peeling off the tape is
carried out to measure a maximum peeled full width combining either
side of the cut flaw portion. When the maximum peeled full width is
not more than 5.0 mm, the corrosion resistance can be evaluated to
be good in the hot slat water immersion test.
<Salt Water Spray Test (SST)>
[0090] The test specimen (n=1) subjected to the phosphate treatment
and electrodeposition is provided on its surface with a crosscut
flaw of 45 mm in length by means of a cutter, and thereafter
subjected to a salt water spray test with an aqueous solution of 5
mass % NaCl for 1000 hours according to a neutral salt water spray
test defined in JIS Z2371:2000,and then a tape peeling test on a
crosscut flaw portion is conducted to measure a maximum peeled full
width combining either side of the cut flaw portion. When the
maximum peeled full width is not more than 4.0 mm, the corrosion
resistance can be evaluated to be good in the salt water spray
test.
<Composite Cycle Corrosion Test (CCT)>
[0091] The test specimen (n=1) subjected to the phosphate treatment
and electrodeposition is provided on its surface with a crosscut
flaw of 45 mm in length by means of a cutter, and thereafter
subjected to a corrosion test that one cycle of salt water spraying
(aqueous solution of 5 mass % NaCl: 35.degree. C., relative
humidity: 98%) for 2 hours.fwdarw.drying (60.degree. C., relative
humidity: 30%) for 2 hours.fwdarw.wetting (50.degree. C., relative
humidity: 95%) for 2 hours is repeated 90 cycles, washed with water
and dried, and then a tape peeling test on a cut flaw portion is
conducted to measure a maximum peeled full width combining either
side of the cut flaw portion. When the maximum peeled full width is
not more than 6.0 mm, the corrosion resistance can be evaluated to
be good in the composite cycle corrosion test.
[0092] The test results are also shown in Table 2. As seen from
these results, the steel sheets of Invention Examples subjected to
the pickling under the conditions adequate for the invention after
the continuous annealing are small in the maximum peeled full width
on all of the hot salt water immersion test, salt water spray test
and composite cycle corrosion test and show the good corrosion
resistance after coating. On the contrary, all of the steel sheets
of Comparative Examples not satisfying the pickling conditions of
the invention for removing the Si-containing oxide layer on the
surface of the steel sheet or having the surface covering ratio of
the iron-based oxide of more than 85% are confirmed to be poor in
the corrosion resistance after coating. Moreover, as the
distribution in depth direction of O, Si, Mn and Fe on the surface
of each steel sheet in Table 2 is measured with GDS, it has been
confirmed that in the steel sheets pickled under the conditions
adequate for the invention, peaks of Si and O do not appear and the
Si-containing oxide layer is removed sufficiently. As a reference,
FIG. 5 shows the profile in depth direction of O, Si, Mn and Fe as
surface-analyzed with GDS with respect to the test specimens of
Comparative Example No. 1 and Invention Example No. 18 in Table
2.
TABLE-US-00002 TABLE 2 Full width peeled after corrosion test (mm)
Temperature of Pickling conditions phosphate treating solution
Concentration Surface properties 35.degree. C. 33.degree. C.
Concentration of hydrochloric Surface covering Hot salt Salt
Composite of nitric acid acid Temperature Treating ratio of water
water cycle C.sub.HNO3 C.sub.HCl Ratio R of acid solution time
iron-based oxide immersion spray corrosion No. (g/l) (g/l)
C.sub.HCl/C.sub.HNO3 (.degree. C.) (seconds) (%) test test test
Remarks 1 20 2.0 0.10 40 10 92.3 7.2 6.5 7.3 7.8 Comparative
example 2 100 5.0 0.05 40 10 91.5 6.9 6.1 7.3 7.6 Comparative
example 3 100 1.0 0.01 40 10 86.1 5.3 4.4 6.2 6.3 Comparative
example 4 100 25.0 0.25 40 10 88.3 5.9 5.3 6.5 6.9 Comparative
example 5 110 0.5 0.005 40 10 85.5 5.3 4.3 6.2 6.3 Comparative
example 6 110 1.1 0.01 40 10 70.8 4.6 3.7 5.7 5.8 Invention example
7 110 11.1 0.10 20 10 57.8 4.4 3.5 5.5 5.7 Invention example 8 110
11.1 0.10 40 3 65.0 4.6 3.5 5.6 5.7 Invention example 9 110 11.1
0.10 40 10 59.4 4.6 3.5 5.6 5.6 Invention example 10 110 11.1 0.10
40 30 56.3 4.5 3.3 5.4 5.6 Invention example 11 110 11.1 0.10 70 10
71.7 4.6 3.6 5.8 5.9 Invention example 12 110 22.2 0.20 40 10 52.9
4.3 3.4 5.4 5.5 Invention example 13 110 27.5 0.25 40 10 45.6 4.3
3.2 5.4 5.5 Invention example 14 150 1.5 0.01 40 3 72.7 4.8 3.6 5.8
5.8 Invention example 15 150 1.5 0.01 40 10 71.7 4.7 3.7 5.7 5.7
Invention example 16 150 1.5 0.01 40 30 67.5 4.7 3.7 5.7 5.7
Invention example 17 150 15.0 0.10 20 10 59.4 4.5 3.6 5.6 5.6
Invention example 18 150 15.0 0.10 40 10 72.6 4.5 3.5 5.6 5.6
Invention example 19 150 15.0 0.10 70 10 78.9 4.8 3.7 5.9 5.8
Invention example 20 150 30.0 0.20 40 10 56.3 4.4 3.3 5.5 5.6
Invention example 21 150 37.5 0.25 40 10 52.9 4.4 3.3 5.6 5.4
Invention example 22 200 2.0 0.01 40 10 78.4 4.7 3.7 5.8 5.9
Invention example 23 200 50.0 0.25 40 10 51.2 4.3 3.3 5.5 5.5
Invention example 24 300 30.0 0.10 40 10 85.5 5.2 4.2 6.1 6.3
Comparative example 25 300 75.0 0.25 40 10 85.1 5.1 4.3 6.2 6.4
Comparative example
Example 2
[0093] Each of steels A-X having a chemical composition shown in
Table 3 is prepared according to common refining process such as
melting in a converter, degassing treatment and the like and
continuously cast into a steel slab. The steel slab is hot rolled
under hot rolling conditions shown in Table 4 to obtain a
hot-rolled steel sheet having a thickness of 3-4 mm, which is
pickled to remove scales on the surface of the steel sheet and
thereafter cold rolled to obtain a cold-rolled steel sheet having a
thickness of 1.8 mm. Next, the cold-rolled steel sheet is
continuously annealed under the conditions shown in Table 4,
pickled under conditions shown in Table 5, washed with water, dried
and subjected to a temper rolling at a stretching ratio of 0.7% to
obtain cold-rolled steel sheets Nos. 1-30.
TABLE-US-00003 TABLE 3 Steel Chemical composition (mass %) symbol C
Si Mn P S Al Si/Mn Nb, Ti, V, Mo, Cr, B, N Ni, Cu, Ca, REM Remarks
A 0.11 1.25 1.55 0.018 0.001 0.032 0.81 -- -- Invention steel B
0.15 1.30 1.80 0.019 0.002 0.033 0.72 -- -- Invention steel C 0.15
1.20 1.95 0.017 0.001 0.033 0.62 -- -- Invention steel D 0.09 1.45
1.40 0.017 0.002 0.028 1.04 -- -- Invention steel E 0.18 1.11 1.36
0.018 0.001 0.032 0.82 -- -- Invention steel F 0.16 1.41 1.23 0.017
0.001 0.041 1.15 -- -- Invention steel G 0.14 1.65 1.33 0.018 0.002
0.035 1.24 -- -- Invention steel H 0.12 1.45 2.10 0.017 0.001 0.042
0.69 -- -- Invention steel I 0.17 0.90 1.40 0.017 0.002 0.044 0.64
-- -- Invention steel J 0.13 1.20 1.89 0.018 0.001 0.041 0.63 -- --
Invention steel K 0.15 1.20 1.85 0.017 0.001 0.034 0.65 -- --
Invention steel L 0.03 1.25 3.25 0.018 0.001 0.005 0.38 -- --
Invention steel M 0.22 3.30 1.15 0.018 0.001 0.027 2.87 -- --
Comparative steel N 0.06 1.28 2.12 0.025 0.003 0.040 0.60 Nb: 0.1,
Ti: 0.2 Cu: 0.15 Invention steel O 0.18 1.21 1.97 0.015 0.002 0.035
0.61 V: 0.1, Mo: 0.2 Ni: 0.13 Invention steel P 0.18 1.56 2.58
0.010 0.002 0.030 0.60 Cr: 0.2, B: 0.005 Ca: 0.003 Invention steel
Q 0.13 1.32 1.32 0.030 0.001 0.040 1.00 N: 0.007 REM: 0.002
Invention steel R 0.07 1.26 2.10 0.025 0.002 0.040 0.60 Nb: 0.1 --
Invention steel S 0.06 1.28 2.12 0.025 0.003 0.040 0.60 Nb: 0.1,
Ti: 0.2 -- Invention steel T 0.17 1.23 1.99 0.015 0.002 0.050 0.62
-- Ni: 0.13 Invention steel U 0.18 1.22 1.97 0.015 0.003 0.040 0.62
-- Ni: 0.13, Ca: 0.003 Invention steel V 0.18 1.21 1.98 0.015 0.002
0.035 0.61 V: 0.1 Ni: 0.13 Invention steel W 0.18 1.56 2.58 0.010
0.002 0.030 0.60 Mo: 0.1, Cr: 0.2, B: 0.005 Ca: 0.003 Invention
steel X 0.13 1.32 1.32 0.030 0.001 0.040 1.00 Nb: 0.1, N: 0.007 Cu:
0.2, REM: 0.002 Invention steel
TABLE-US-00004 TABLE 4 Continuous annealing conditions Hot rolling
conditions Cooling Heating Finish Coiling Cold Heating stop tem-
tem- Cooling tem- rolling tem- Holding Cooling tem- Holding Cooling
Strength Steel perature perature rate perature reduction perature
time rate perature time rate TS No. symbol (.degree. C.) (.degree.
C.) (.degree. C./s) (.degree. C.) (%) (.degree. C.) (Seconds)
(.degree. C./s) (.degree. C.) (Seconds) (.degree. C./s) (MPa)
Remarks 1 A 1150 850 25 620 60 780 45 20 350 100 40 625 Invention
example 2 B 1150 820 31 400 60 780 40 20 400 100 50 821 Invention
example 3 C 1140 850 26 600 60 760 50 20 350 100 45 814 Invention
example 4 D 1150 840 33 530 60 730 40 20 350 110 40 623 Invention
example 5 E 1150 850 30 580 55 750 35 20 400 110 50 836 Invention
example 6 F 1150 850 25 620 60 750 50 20 350 120 50 634 Invention
example 7 G 1150 850 33 550 60 750 30 20 400 100 50 632 Com-
parative example 8 G 1150 850 33 550 60 750 30 20 400 100 50 635
Com- parative example 9 G 1150 850 33 550 60 750 30 20 400 100 50
631 Invention example 10 G 1150 850 33 550 60 750 30 20 400 100 50
633 Com- parative example 11 H 1130 820 28 570 60 780 50 15 370 150
50 840 Invention example 12 I 1150 840 34 530 55 780 50 15 350 120
55 812 Invention example 13 J 1140 850 28 600 60 770 60 20 300 100
45 836 Invention example 14 K 1150 850 25 620 60 780 45 20 350 100
40 650 Invention example 15 L 1100 850 33 550 60 750 50 20 450 150
50 960 Com- parative example 16 L 1100 850 33 550 60 750 50 20 450
150 50 959 Com- parative example 17 L 1100 850 33 550 60 750 50 20
450 150 50 963 Invention example 18 L 1100 850 33 550 60 750 50 20
450 150 50 962 Com- parative example 19 M 1120 830 31 550 55 720 50
15 410 190 50 1124 Com- parative example 20 N 1120 830 33 550 60
750 30 20 400 100 50 613 Invention example 21 O 1150 850 32 560 60
750 35 20 350 100 50 776 Invention example 22 P 1130 840 33 550 55
780 30 20 400 110 50 1152 Invention example 23 Q 1140 850 33 580 60
750 40 20 400 120 45 586 Invention example 24 R 1120 830 33 550 60
750 30 20 400 100 50 611 Invention example 25 S 1120 830 32 550 60
750 35 20 410 100 50 621 Invention example 26 T 1150 850 32 560 60
750 35 20 350 100 50 773 Invention example 27 U 1140 850 33 550 60
750 35 20 400 100 50 785 Invention example 28 V 1150 850 32 550 60
750 40 20 380 100 50 770 Invention example 29 W 1130 840 33 550 55
780 30 20 400 110 50 1156 Invention example 30 X 1140 850 33 580 60
750 40 20 400 120 45 585 Invention example
TABLE-US-00005 TABLE 5 Full width peeled Surface after corrosion
test(mm) properties Temperature of phosphate Pickling conditions
Surface treating solution Concentration covering 35.degree. C.
33.degree. C. Concentration of hydrochloric Temperature ratio of
Hot salt Salt Composite of nitric acid acid of Treating iron-based
water water cycle Steel C.sub.HNO3 C.sub.HCl Ratio R acid solution
time oxide immersion spray corrosion No. symbol (g/l) (g/l)
C.sub.HCl/C.sub.HNO3 (.degree. C.) (Seconds) (%) test test test
Remarks 1 A 150 15 0.10 40 10 54.6 4.3 3.8 5.7 5.8 Invention
example 2 B 150 15 0.10 40 10 57.8 4.2 3.7 5.6 5.8 Invention
example 3 C 150 15 0.10 40 10 52.9 4.2 3.5 5.3 5.5 Invention
example 4 D 150 15 0.10 40 10 54.6 4.3 3.6 5.5 5.6 Invention
example 5 E 150 15 0.10 40 10 49.3 4.2 3.5 5.6 5.7 Invention
example 6 F 150 15 0.10 40 10 56.3 4.4 3.7 5.5 5.7 Invention
example 7 G 50 5 0.10 40 10 92.5 7.3 6.6 7.5 7.8 Comparative
example 8 G 150 1 0.007 40 10 85.6 5.5 4.3 6.1 6.6 Comparative
example 9 G 150 15 0.10 40 10 57.8 4.4 3.6 5.6 5.7 Invention
example 10 G 250 25 0.10 40 10 85.3 5.1 4.2 6.2 6.3 Comparative
example 11 H 150 15 0.10 40 10 56.3 4.1 3.9 5.5 5.6 Comparative
example 12 I 150 15 0.10 40 10 66.3 4.4 3.8 5.3 5.5 Invention
example 13 J 150 15 0.10 40 10 62.3 4.3 3.5 5.1 5.5 Invention
example 14 K 150 15 0.10 40 10 57.4 4.1 3.2 5.6 5.1 Invention
example 15 L 50 5 0.10 40 10 92.3 7.2 6.5 7.3 7.8 Comparative
example 16 L 150 1 0.007 40 10 85.5 5.3 4.3 6.2 6.3 Comparative
example 17 L 150 15 0.10 40 10 59.4 4.5 3.6 5.8 5.6 Invention
example 18 L 250 25 0.10 40 10 85.5 5.2 4.2 6.1 6.3 Comparative
example 19 M 150 15 0.10 40 10 87.0 5.3 4.2 6.2 6.5 Comparative
example 20 N 150 15 0.10 40 10 53.9 4.3 3.5 5.4 5.5 Invention
example 21 O 150 15 0.10 40 10 55.5 4.1 3.6 5.7 5.4 Invention
example 22 P 150 15 0.10 40 10 54.2 4.4 3.5 5.6 5.6 Invention
example 23 Q 150 15 0.10 40 10 54.8 4.3 3.6 5.5 5.5 Invention
example 24 R 150 15 0.10 40 10 53.6 4.2 3.3 5.6 5.4 Invention
example 25 S 150 15 0.10 40 10 55.5 4.1 3.5 5.6 5.6 Invention
example 26 T 150 15 0.10 40 10 54.5 4.3 3.4 5.5 5.7 Invention
example 27 U 150 15 0.10 40 10 54.7 4.2 3.5 5.4 5.5 Invention
example 28 V 150 15 0.10 40 10 54.5 4.2 3.5 5.5 5.7 Invention
example 29 W 150 15 0.10 40 10 54.7 4.2 3.6 5.6 5.5 Invention
example 30 X 150 15 0.10 40 10 54.3 4.3 3.5 5.6 5.5 Invention
example
[0094] A test specimen is sampled from each of the cold-rolled
steel sheets and subjected to the following tensile test and test
for the corrosion resistance after coating after the surface
covering ratio of iron-based oxide on the steel sheet surface after
the pickling is measured in the same manner as in Example 1. Also,
the distribution in depth direction of O, Si, Mn and Fe on the
surface of the test specimen sampled from each of the cold-rolled
steel sheets is measured with GDS.
[0095] (1) Mechanical Properties
[0096] A tensile test specimen of JIS No. 5 (n=1) sampled in a
direction (C-direction) parallel to the rolling direction according
to JIS Z2201:1998 is subjected to a tensile test according to JIS
Z2241:1998 to measure tensile strength TS.
[0097] (2) Corrosion Resistance After Coating
[0098] A test specimen is prepared by subjecting the test specimen
sampled from each of the cold-rolled steel sheet to phosphate
treatment and electrodeposition under the same conditions as in
Example 1 and then subjected to three corrosion tests of hot salt
water immersion test, salt water spray test (SST) and composite
cycle corrosion test (CCT) likewise Example 1 to evaluate the
corrosion resistance after coating.
[0099] The results of the above tests are shown in Tables 4 and 5.
As seen from these results, the high-strength cold-rolled steel
sheets of Invention Examples containing Si of not less than 0.5
mass % and pickled under the conditions adequate for the invention
are excellent in the corrosion resistance after coating but also
have a tensile strength TS of not less than 590 MPa. Moreover, as
the distribution in depth direction of O, Si, Mn and Fe is measured
with GDS, it has been confirmed that in all of the steel sheets
pickled under the conditions adequate for the invention, peaks of
Si and O do not appear and the Si-containing oxide layer is removed
sufficiently.
Example 3
[0100] A steel comprising C: 0.125 mass %, Si: 1.5 mass %, Mn: 2.6
mass %, P: 0.019 mass %, S: 0.008 mass %, Al: 0.040 mass % and the
remainder being Fe and inevitable impurities is melted and
continuously cast into a steel material (slab). The slab is
reheated to a temperature of 1150-1170.degree. C., hot rolled at a
terminating temperature of finish rolling of 850-880.degree. C. and
coiled at a temperature of 500-550.degree. C. to obtain a
hot-rolled steel sheet having a thickness of 3-4 mm. The hot-rolled
steel sheet is pickled to remove scales and thereafter cold rolled
to obtain a cold-rolled steel sheet having a thickness of 1.8 mm.
Next, the cold-rolled steel sheet is subjected to such a continuous
annealing that it is heated to a soaking temperature of
750-780.degree. C. and held at this temperature for 40-50 seconds
and then cooled at a rate of 20-30.degree. C./second from the
soaking temperature to a cooling stop temperature of
350-400.degree. C. and held at the cooling stop temperature range
for 100-120 seconds, and then the steel sheet is pickled under
conditions shown in Table 6, washed with water, dried and subjected
to a temper rolling at a stretching ratio of 0.7% to obtain
cold-rolled steel sheets Nos. 1-12 shown in Table 6.
[0101] A test specimen is sampled from each of the above
cold-rolled steel sheets to measure a surface covering ratio and
maximum thickness of iron-based oxide generated on the surface of
the steel sheet by pickling through the aforementioned methods.
[0102] Also, the test specimen is sampled from each of the above
cold-rolled steel sheets and subjected to phosphate treatment and
coating treatment under the following conditions and then subjected
to three corrosion tests of hot salt water immersion test, salt
water spray test and composite cycle corrosion test to evaluate the
corrosion resistance after coating.
[0103] Also, the test specimen is sampled from each of the above
cold-rolled steel sheets and subjected to phosphate treatment and
coating treatment under the following conditions and then subjected
to three corrosion tests of hot salt water immersion test, salt
water spray test and composite cycle corrosion test to evaluate the
corrosion resistance after coating. Further, the distribution in
depth direction of O, Si, Mn and Fe on the surface of the test
specimen sampled from each of the cold-rolled steel sheets is
measured with GDS.
[0104] (1) Phosphate Treating Conditions
[0105] The test specimen sampled from each cold-rolled steel sheet
is subjected to a phosphate treatment with a degreasing agent:
FC-E2011, a surface regulator: PL-X and a phosphate treating agent:
PALBOND PB-L3065, which are made by Nihon Parkerizing Co., Ltd., so
as to provide a phosphate coating adhered amount of 1.7-3.0 g/m2
under two conditions of the following standard condition and
comparative condition of lowering the phosphate treating
temperature to a low temperature.
<Standard Condition>
[0106] Degreasing step: treating temperature 40.degree. C.,
treating time 120 seconds Spray degreasing, surface regulating
step: pH 9.5, Treating temperature room temperature, treating time
20 seconds Phosphate treating step: temperature of phosphate
treating solution 35.degree. C., treating time 120 seconds <Low
temperature condition> Condition of lowering the temperature of
the phosphate treating solution in the above standard condition to
33.degree. C.
[0107] (2) Corrosion Test
[0108] The surface of the test specimen subjected to the phosphate
treatment is electrodeposited with an electrodeposition paint :
V-50 made by Nippon Paint Co., Ltd. so as to have a coating
thickness of 25 .mu.m and then subjected to the following three
corrosion tests with more strict condition than the one with
Example 1.
<Hot Salt Water Immersion Test>
[0109] The test specimen (n=1) subjected to the phosphate treatment
and electrodeposition is provided on its surface with a crosscut
flaw of 45 mm in length by means of a cutter, and thereafter
immersed in a solution of 5 mass % NaCl (60.degree. C.) for 360
hours, washed with water, and dried. After an adhesive tape is
attached to a cut flaw portion, a test of peeling off the tape is
carried out to measure a maximum peeled full width combining either
side of the cut flaw portion. When the maximum peeled full width is
not more than 5.0 mm, the corrosion resistance can be evaluated to
be good in the hot slat water immersion test.
<Salt Water Spray Test (SST)>
[0110] The test specimen (n=1) subjected to the phosphate treatment
and electrodeposition is provided on its surface with a crosscut
flaw of 45 mm in length by means of a cutter, and thereafter
subjected to a salt water spray test with an aqueous solution of 5
mass % NaCl for 1200 hours according to a neutral salt water spray
test defined in JIS Z2371:2000,and then a tape peeling test on a
crosscut flaw portion is conducted to measure a maximum peeled full
width combining either side of the cut flaw portion. When the
maximum peeled full width is not more than 4.0 mm, the corrosion
resistance can be evaluated to be good in the salt water spray
test.
<Composite Cycle Corrosion Test (CCT)>
[0111] The test specimen (n=1) subjected to the phosphate treatment
and electrodeposition is provided on its surface with a crosscut
flaw of 45 mm in length by means of a cutter, and thereafter
subjected to a corrosion test that one cycle of salt water spraying
(aqueous solution of 5 mass % NaCl: 35.degree. C., relative
humidity: 98%) for 2 hours.fwdarw.drying (60.degree. C., relative
humidity: 30%) for 2 hours.fwdarw.wetting (50.degree. C., relative
humidity: 95%) for 2 hours is repeated 120 cycles, washed with
water and dried, and then a tape peeling test on a cut flaw portion
is conducted to measure a maximum peeled full width combining
either side of the cut flaw portion. When the maximum peeled full
width is not more than 6.0 mm, the corrosion resistance can be
evaluated to be good in the composite cycle corrosion test.
[0112] The test results are also shown in Table 6. As seen from
these results, the steel sheets of Invention Examples, wherein the
surface of the steel sheet after annealing is subjected to the
pickling under the conditions that the surface covering ratio of
the iron-based oxide on the surface of the steel sheet after
pickling is not more than 85% and the maximum thickness of the
iron-based oxide is not more than 200 nm, are small in the maximum
peeled full width on all of the hot salt water immersion test, salt
water spray test and composite cycle corrosion test and show the
very good corrosion resistance after coating. Moreover, as the
distribution in depth direction of O, Si, Mn and Fe is measured
with GDS, it has been confirmed that in the steel sheets pickled
under the conditions adequate for the invention, peaks of Si and O
do not appear and the Si-containing oxide layer is removed
sufficiently.
TABLE-US-00006 TABLE 6 Full width peeled after corrosion test (mm)
Surface properties Temperature of phosphate Pickling conditions
Surface Maximum treating solution Concentration covering thickness
35.degree. C. 33.degree. C. Concentration of hydrochloric
Temperature ratio of of Hot salt Salt Composite of nitric acid acid
of Treating iron-based iron-based water water cycle C.sub.HNO3
C.sub.HCl Ratio R acid solution time Oxide oxide immersion spray
corrosion No. (g/l) (g/l) C.sub.HCl/C.sub.HNO3 (.degree. C.)
(Seconds) (%) (nm) test test test Remarks 1 110 1.1 0.01 40 10 70.8
212 6.6 5.9 8.2 8.4 Invention example 2 110 3.3 0.03 40 10 67.3 196
5.0 3.9 5.9 6.0 Invention example 3 110 11.1 0.10 40 10 59.4 179
4.8 3.7 5.8 5.7 Invention example 4 110 22.2 0.20 40 10 52.9 167
4.6 3.5 5.7 5.7 Invention example 5 110 27.5 0.25 40 10 45.6 161
4.5 3.5 5.5 5.6 Invention example 6 140 1.4 0.01 40 10 71.4 216 6.7
5.8 8.2 8.2 Invention example 7 140 4.2 0.03 40 10 68.1 198 4.9 4.0
6.0 5.8 Invention example 8 140 14.0 0.10 40 10 64.8 184 4.9 3.9
5.8 6.0 Invention example 9 140 28.0 0.20 40 10 55.1 172 4.6 3.7
5.7 5.8 Invention example 10 140 35.0 0.25 40 10 51.7 167 4.6 3.6
5.7 5.6 Invention example 11 150 15.0 0.10 40 10 72.6 214 6.3 5.5
7.9 8.3 Invention example 12 150 37.5 0.25 40 10 52.9 203 6.1 5.3
7.6 8.1 Invention example
INDUSTRIAL APPLICABILITY
[0113] The cold-rolled steel sheets produced according to the
invention not only are excellent in the corrosion resistance after
coating but also have a high strength and a good workability, so
that they can be preferably used as not only a starting material
used in members of the automotive vehicle body but also a starting
material for applications requiring the same properties such as
household electrical goods, building members and so on.
* * * * *