U.S. patent application number 13/812774 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 | 20130149526 13/812774 |
Document ID | / |
Family ID | 45772724 |
Filed Date | 2013-06-13 |
United States Patent
Application |
20130149526 |
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 preferably including 0.5-3.0 mass % of Si is pickled
to remove a Si-containing oxide layer on a surface layer of the
steel sheet and further repickled so that a surface covering ratio
of an iron-based oxide on the surface of the steel sheet is not
more than 40% and preferably a maximum thickness of the iron-based
oxide is not more than 150 nm, as well as a cold-rolled steel sheet
produced by this method and a member for automobile using the
cold-rolled steel sheet.
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: |
45772724 |
Appl. No.: |
13/812774 |
Filed: |
August 25, 2011 |
PCT Filed: |
August 25, 2011 |
PCT NO: |
PCT/JP2011/069193 |
371 Date: |
January 28, 2013 |
Current U.S.
Class: |
428/332 ;
428/472.2; 72/39 |
Current CPC
Class: |
C22C 38/04 20130101;
C22C 38/34 20130101; C21D 8/0236 20130101; C22C 38/005 20130101;
C22C 38/14 20130101; C22C 38/16 20130101; C23G 1/08 20130101; C23G
1/086 20130101; C23G 1/00 20130101; C22C 38/38 20130101; C22C
38/001 20130101; C22C 38/06 20130101; C22C 38/00 20130101; C21D
8/0278 20130101; C22C 38/32 20130101; C22C 38/02 20130101; C23G
1/081 20130101; B21B 45/0269 20130101; C22C 38/12 20130101; C23G
1/088 20130101; Y10T 428/26 20150115; C22C 38/002 20130101; B21B
3/02 20130101; C23G 1/085 20130101; C22C 38/08 20130101; C22C 38/22
20130101 |
Class at
Publication: |
428/332 ; 72/39;
428/472.2 |
International
Class: |
C23G 1/08 20060101
C23G001/08; C22C 38/00 20060101 C22C038/00; B21B 45/02 20060101
B21B045/02 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 31, 2010 |
JP |
2010-193182 |
Nov 30, 2010 |
JP |
2010-266125 |
Aug 16, 2011 |
JP |
2011-177861 |
Claims
1. A method of producing a cold-rolled steel sheet, comprising
steps of cold rolling a steel sheet, continuously annealing,
pickling and further repickling it.
2. A method of producing a cold-rolled steel sheet according to
claim 1, wherein the repickling uses a non-oxidizable acid
different from an acid used in the pickling before repickling.
3. A method of producing a cold-rolled steel sheet according to
claim 2, wherein the non-oxidizable acid is any of hydrochloric
acid, sulfuric acid, phosphoric acid, pyrophosphoric acid, formic
acid, acetic acid, citric acid, hydrofluoric acid, oxalic acid and
a mixed acid of two or more thereof.
4. A method of producing a cold-rolled steel sheet according to
claim 2, wherein the non-oxidizable acid is any of hydrochloric
acid with a concentration of 0.1-50 g/L, sulfuric acid with a
concentration of 0.1-150 g/L and a mixed acid of 0.1-20 g/L of
hydrochloric acid and 0.1-60 g/L of sulfuric acid.
5. A method of producing a cold-rolled steel sheet according to
claim 1, wherein the repickling is carried out at a temperature of
a repickling solution of 20-70.degree. C. for 1-30 seconds.
6. A method of producing a cold-rolled steel sheet according to
claim 1, wherein the pickling is carried out with any of nitric
acid, hydrochloric acid, hydrofluoric acid, sulfuric acid and a
mixed acid of two or more thereof.
7. A method of producing a cold-rolled steel sheet according to
claim 1, wherein the pickling is carried out with any of a mixed
acid of nitric acid and hydrochloric acid wherein a concentration
of nitric acid is more than 50 g/L but not more than 200 g/L and a
ratio (HCl/HNO.sub.3) of hydrochloric acid concentration to nitric
acid concentration is 0.01-1.0, or a mixed acid of nitric acid and
hydrofluoric acid wherein a concentration of nitric acid is more
than 50 g/L but not more than 200 g/L and a ratio (HF/HNO.sub.3) of
hydrofluoric acid concentration to nitric acid concentration is
0.01-1.0.
8. A method of producing a cold-rolled steel sheet according to
claim 1, wherein the steel sheet comprises 0.5-3.0 mass % of
Si.
9. A method of producing a cold-rolled steel sheet according to
claim 8, 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 %,
Al: not more than 0.06 mass % and the remainder being Fe and
inevitable impurities.
10. A method of producing a cold-rolled steel sheet according to
claim 8, wherein the steel sheet contains, in addition to the
chemical composition, 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 %.
11. A method of producing a cold-rolled steel sheet according to
claim 8, wherein the steel sheet contains, in addition to the
chemical composition, 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 %.
12. 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 surface covering ratio of an iron-based
oxide existing on the surface of the steel sheet after repickling
is not more than 40%.
13. A cold-rolled steel sheet according to claim 12, wherein a
maximum thickness of the iron-based oxide existing on the surface
of the steel sheet after repickling is not more than 150 nm.
14. A member for automobile characterized by using a cold-rolled
steel sheet as claimed in claim 12 13.
15. A method of producing a cold-rolled steel sheet according to
claim 2, wherein the repickling is carried out at a temperature of
a repickling solution of 20-70.degree. C. for 1-30 seconds.
16. A method of producing a cold-rolled steel sheet according to
claim 3, wherein the repickling is carried out at a temperature of
a repickling solution of 20-70.degree. C. for 1-30 seconds.
17. A method of producing a cold-rolled steel sheet according to
claim 4, wherein the repickling is carried out at a temperature of
a repickling solution of 20-70.degree. C. for 1-30 seconds.
18. A method of producing a cold-rolled steel sheet according to
claim 2, wherein the pickling is carried out with any of nitric
acid, hydrochloric acid, hydrofluoric acid, sulfuric acid and a
mixed acid of two or more thereof.
19. A method of producing a cold-rolled steel sheet according to
claim 3, wherein the pickling is carried out with any of nitric
acid, hydrochloric acid, hydrofluoric acid, sulfuric acid and a
mixed acid of two or more thereof.
20. A method of producing a cold-rolled steel sheet according to
claim 4, wherein the pickling is carried out with any of nitric
acid, hydrochloric acid, hydrofluoric acid, sulfuric acid and a
mixed acid of two or more thereof.
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.
[0002] 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
[0003] 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.
[0004] 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.
[0005] 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
pm from the surface of the steel sheet into not more than 300
nm.
[0006] 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.
[0007] 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]
[0008] [Patent Document 1] JP-A-2004-204350
[0009] [Patent Document 2] JP-A-2004-244698
[0010] [Patent Document 3] JP-A-2007-217743
[0011] [Patent Document 4] JP-A-2007-246951
SUMMARY OF THE INVENTION
[Problems to be Solved by the Invention]
[0012] 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.
[0013] 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]
[0014] 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.
[0015] That is, the invention proposes a method of producing a
cold-rolled steel sheet, comprising steps of cold rolling a steel
sheet, continuously annealing, pickling and further repickling
it.
[0016] The repickling in the production method of the invention is
characterized in that a non-oxidizable acid is used instead of an
acid used in the pickling prior to the repickling.
[0017] The non-oxidizable acid in the production method of the
invention is characterized to be any of hydrochloric acid, sulfuric
acid, phosphoric acid, pyrophosphoric acid, formic acid, acetic
acid, citric acid, hydrofluoric acid, oxalic acid and a mixed acid
of two or more thereof.
[0018] The non-oxidizable acid in the production method of the
invention is characterized to be any of hydrochloric acid with a
concentration of 0.1-50 g/L, sulfuric acid with a concentration of
0.1-150 g/L and a mixed acid of 0.1-20 g/L of hydrochloric acid and
0.1-60 g/L of sulfuric acid.
[0019] Also, the production method of the invention is
characterized in that the repickling is carried out at a
temperature of a repickling solution of 20-70.degree. C. for 1-30
seconds.
[0020] Furthermore, the production method of the invention is
characterized in that the pickling is carried out with any of
nitric acid, hydrochloric acid, hydrofluoric acid, sulfuric acid
and a mixed acid of two or more thereof.
[0021] Moreover, the production method of the invention is
characterized in that the pickling is carried out with any of a
mixed acid of nitric acid and hydrochloric acid wherein a
concentration of nitric acid is more than 50 g/L but not more than
200 g/L and a ratio (HCl/HNO.sub.3) of hydrochloric acid
concentration to nitric acid concentration is 0.01-1.0, or a mixed
acid of nitric acid and hydrofluoric acid wherein a concentration
of nitric acid is more than 50 g/L but not more than 200 g/L and a
ratio (HF/HNO.sub.3) of hydrofluoric acid concentration to nitric
acid concentration is 0.01-1.0.
[0022] The steel sheet in the production method of the invention is
characterized by comprising 0.5-3.0 mass % of Si.
[0023] Also, the steel sheet in the production method of the
invention is characterized by having 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 %,
Al: not more than 0.06 mass % and the remainder being Fe and
inevitable impurities.
[0024] Furthermore, the steel sheet in the production method of the
invention is characterized by containing, in addition to the above
chemical composition, 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 %.
[0025] Moreover, the steel sheet in the production method of the
invention is characterized by containing, in addition to the
aforementioned chemical composition, 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 %.
[0026] The invention is a cold-rolled steel sheet produced by any
one of the aforementioned methods, characterized in that a
Si-containing oxide layer is removed from a surface layer of the
steel sheet by pickling after continuous annealing and a surface
covering ratio of an iron-based oxide existing on the surface of
the steel sheet after repickling is not more than 40%.
[0027] Also, the cold-rolled steel sheet of the invention is
characterized in that a maximum thickness of the iron-based oxide
existing on the steel sheet surface after repickling is not more
than 150 nm.
[0028] Further, the invention is a member for automobiles,
characterized by using a cold-rolled steel sheet as described in
any one of the above.
[Effect Of The Invention]
[0029] 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
[0030] 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.
[0031] 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.
[0032] FIG. 3 is a photograph of a section of a coating on a
surface of a steel sheet after repickling observed by means of a
transmission electron microscope.
[0033] FIG. 4 is a graph showing energy dispersion type X-ray (EDX)
analytical results of an iron-based oxide observed in FIG. 3.
[0034] 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. 9) of Example 1 as measured by GDS.
EMBODIMENTS FOR CARRYING OUT THE INVENTION
[0035] First, the basic technical idea of the invention will be
described.
[0036] 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.
[0037] 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.
[0038] 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 or the like 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).
[0039] The reason why a strong acid such as nitric acid or the like
is used as the pickling solution is due to the fact that among the
Si-containing oxides, Si--Mn based composite oxide is easily
dissolved in an acid, but SiO.sub.2 is hardly soluble, and in order
to remove the latter, the Si-containing oxide on the surface of the
steel sheet should be removed together with the steel matrix.
[0040] According to the inventors' studies, however, it can be seen
that the phosphate treatability is largely improved by removing the
Si-containing oxide layer existing on the steel sheet surface
through strong pickling with nitric acid or the like after the
continuous annealing 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 or the like, 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.
[0041] And, it has been found that in order to suppress the
oxidation of the steel sheet surface by the above strong pickling
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 40% and
that it is effective as means for attaining the above to further
conduct repickling under adequate conditions after the pickling to
dissolve and remove the iron-based oxide precipitated on the
surface of the steel sheet.
[0042] Further, the inventors have found that the phosphate
treatability is more improved and the corrosion resistance is
further improved when the maximum thickness of the iron-based oxide
is not more than 150 nm in addition to the fact that the covering
ratio of the iron-based oxide generated on the surface of the steel
sheet by pickling is not more than 40% and that it is effective as
means for attaining the above to conduct the repickling by properly
increasing the concentration of the acid used in the
repickling.
[0043] 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.
[0044] The invention is accomplished by conducting further
examinations on the above new knowledge.
[0045] 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.
[0046] Si: 0.5-3.0 mass %
[0047] 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 %.
[0048] 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.
[0049] C: 0.01-0.30 mass %
[0050] 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 %.
[0051] Mn: 1.0-7.5 mass %
[0052] 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 %.
[0053] P: not more than 0.05 mass %
[0054] 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 %.
[0055] S: not more than 0.01 mass %
[0056] 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 %.
[0057] Al: not more than 0.06 mass %
[0058] 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 %.
[0059] 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:
[0060] not more than 0.5 mass %, B: not more than 0.006 mass % and
N: not more than 0.008 mass %.
[0061] 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.
[0062] 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 %.
[0063] 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.
[0064] 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.
[0065] The surface properties of the cold-rolled steel sheet of the
invention will be described below.
[0066] 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 formed on the surface layer of the
steel sheet during annealing. For this end, it is necessary to
conduct strong pickling with nitric acid or the like 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.
[0067] 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 or the
like 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 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. However, in case of using a
phosphate treating solution of a lower temperature, the covering
ratio of not more than 85% is insufficient in cold-rolled steel
sheets used in applications requiring an extremely severe corrosion
resistance after coating such as leg members for vehicle bodies
particularly subjected to severe corrosion, so that it should be
further reduced to not more than 40%, preferably not more than
35%.
[0068] In the invention, the surface covering ratio of the
iron-based oxide is determined as follows:
[0069] 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 (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.
[0070] Here, threshold value in the binarization will be
described.
[0071] 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. 8 in Table 4 of the
following example to obtain a cold-rolled steel sheet of 1.8 mm in
thickness, and then the cold-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 Surface Pickling conditions Repickling
conditions covering Acid Treating Acid Treating ratio of Steel
concentration Temperature time concentration Temperature time
iron-based sheet (g/l) (.degree. C.) (Seconds) (g/l) (.degree. C.)
(Seconds) oxide (%) a Nitric acid: 40 10 -- -- -- 85.3 250 +
Hydrochloric acid: 25 b Nitric acid: 40 10 Hydrochloric 40 30 25.8
150 + acid: 10 Hydrochloric acid: 15
[0072] In order to more improve the phosphate treatability and
hence the 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 150 nm in addition that the
covering ratio of the iron-based oxide produced on the steel sheet
surface by repickling is not more than 40%. When the maximum
thickness of the iron-based oxide is not more than 150 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 130 nm.
[0073] The maximum thickness of the iron-based oxide is measured as
follows. 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 pm 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 (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.
[0074] The production method of the cold-rolled steel sheet
according to the invention will be described below.
[0075] 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 strong-pickled with nitric
acid or the like to remove Si-containing oxide layer on a surface
layer portion of the steel sheet and further repickled to render a
surface covering ratio of an iron-based oxide not more than 40%
generated on the steel sheet surface by the above strong pickling.
Further, it is preferable to be a method wherein a maximum
thickness of the iron-based oxide can be made to not more than 150
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.
[0076] Pickling Conditions After Continuous Annealing
[0077] 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 nitric
acid or the like, whereby the Si-containing oxide layer on the
surface of the steel sheet is removed with the steel matrix.
[0078] 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, it is necessary to
remove the oxide layer together with the steel matrix of the steel
sheet by the strong pickling. As the acid usable in the strong
pickling can be preferably used nitric acid as a strong oxidizable
acid, but hydrofluoric acid, hydrochloric acid, sulfuric acid or
the like may be used as long as the Si-containing oxide layer can
be removed, so that the kind of the acid is particularly no matter.
Also, it is effective to add a pickling promoting agent to the
acid, or to co-use an electrolytic treatment to promote the
dissolution of the steel matrix.
[0079] Moreover, in order to remove the Si-containing oxide layer
from the surface layer of the steel sheet after the continuous
annealing and mitigate the load of the following repickling, it is
preferable to suppress the amount of the iron-based oxide generated
on the steel sheet surface by the strong pickling after the
continuous annealing and before the repickling. For this end, it is
preferable to conduct the pickling with a pickling solution having
a nitric acid concentration of more than 50 g/L but not more than
200 g/L wherein hydrochloric acid having an effect of breaking the
oxide is mixed so that a ratio R (HCl/HNO.sub.3) of hydrochloric
acid concentration to nitric acid concentration is a range of
0.01-1.0 or hydrofluoric acid is mixed so that a ratio
(HF/HNO.sub.3) of hydrofluoric acid concentration to nitric acid
concentration is a range of 0.01-1.0. In case of using the above
pickling solution, it is preferable that a temperature of the
pickling solution is 20-70.degree. C. and a pickling time is 3-30
seconds.
[0080] Repickling Conditions After the Pickling
[0081] However, when only the strong pickling is carried out with
the pickling solution obtained by mixing nitric acid and
hydrofluoric acid or nitric acid and hydrofluoric acid as mentioned
above, it is difficult to stably control the surface covering ratio
of the iron-based oxide generated on the surface of the steel sheet
to not more than 40%. In the invention, therefore, in order to more
surely reduce the iron-based oxide generated on the surface of the
steel sheet by the strong pickling, the iron-based oxide is
dissolved and removed by further repickling the steel sheet pickled
after the continuous annealing with a non-oxidizable acid.
[0082] The non-oxidizable acid usable in the repickling includes
hydrochloric acid, sulfuric acid, phosphoric acid, pyrophosphoric
acid, formic acid, acetic acid, citric acid, hydrofluoric acid,
oxalic acid and a mixed acid of two or more thereof. Any of these
may be used, but hydrochloric acid or sulfuring acid commonly used
in the iron-making industry may be preferably used. Among them,
hydrochloric acid is preferable because it is a volatile acid and
hardly remains a residue on the steel sheet surface after washing
with water different from sulfuric acid retaining sulfuric acid
root and is large in the effect of breaking the oxide by chloride
ion. Also, a mixed acid of hydrochloric acid and sulfuric acid may
be used.
[0083] When hydrochloric acid is used as the pickling solution in
the repickling, it is preferable that a concentration of
hydrochloric acid is 0.1-50 g/L, while in case of using sulfuric
acid, it is preferable that a concentration of sulfuric acid is
0.1-150 g/L. Also, when the mixed acid of hydrochloric acid and
sulfuric acid is used in the repickling, it is preferable to use a
mixed acid having a hydrochloric acid concentration of 0.1-20 g/L
and a sulfuric acid concentration of 0.1-60 g/L. Also, the
repickling of the invention is preferable to be conducted at a
temperature of a repickling solution of 20-70.degree. C. for a
treating time of 1-30 seconds even in case of using any of the
repickling solutions. When the concentration of the repickling
solution is more than the above lower limit and the liquid
temperature is not lower than 20.degree. C. and the treating time
is not less than 1 second, it is sufficient to remove the
iron-based oxide existing on the steel sheet surface, while when
the concentration of the repickling solution is not more than the
above upper limit and the temperature is not higher than 70.degree.
C. and the treating time is not more than 30 seconds, the
dissolution of the steel sheet surface becomes not excessive and a
new surface oxide film is not formed.
[0084] In order to obtain steel sheets being more excellent in the
phosphate treatability and corrosion resistance, it is preferable
that the maximum thickness of the iron-based oxide existing on the
steel sheet surface after the pickling is surely thinned to not
more than 150 nm.
[0085] For this end, it is preferable to properly increase the
concentration of the pickling solution used in the repickling. For
example, it is preferable that when hydrochloric acid is used in
the repickling, the concentration of hydrochloric acid is 3-50 g/L,
while when sulfuric acid is used in the repickling, the
concentration of sulfuric acid is 8-150 g/L. On the other hand,
when a mixture of hydrochloric acid and sulfuric acid is used as a
pickling solution in the repickling, it is preferable to use a
mixed acid having a hydrochloric acid concentration of 3-20 g/L and
a sulfuric acid concentration of 8-60 g/L. In any case, when the
acid concentration is within the above range, the iron-based oxide
can be surely thinned to not more than 150 nm, whereby the
phosphate treatability and the corrosion resistance after coating
are improved. Also, when the acid concentration is within the above
range, the dissolution of the steel sheet surface becomes not
excessive, and hence new surface oxide film is never formed.
[0086] 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 40% by pickling and repickling 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 150 nm, is subsequently subjected to usual
treating steps such as temper rolling and the like to provide
products.
EXAMPLE 1
[0087] 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). Then, 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 3-5 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 is
pickled and further repickled 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.
1-85 shown in Table 2.
[0088] 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 micrcope 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.
[0089] 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.
[0090] (1) Phosphate Treating Conditions
[0091] 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/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>
[0092] Degreasing step: treating temperature 40.degree. C.,
treating time 120 seconds [0093] Spray degreasing, surface
regulating step: pH 9.5, Treating temperature room temperature,
treating time 20 seconds [0094] Phosphate treating step:
temperature of phosphate treating solution 35.degree. C., treating
time 120 seconds
<Low Temperature Condition>
[0095] Condition of lowering the temperature of the phosphate
treating solution in the above standard condition to 33.degree.
C.
[0096] (2) Corrosion Test
[0097] 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>
[0098] 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)>
[0099] 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)>
[0100] 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 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.
[0101] The test results are also shown in Table 2. As seen from
these results, the steel sheets of Invention Examples subjected to
the pickling and repickling 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. Particularly, all of the
cold-rolled steel sheets having the surface covering ratio of the
iron-based oxide of not more than 40% are excellent in the
corrosion resistance after coating under severe corrosion
environment. 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. 9 in Table 2.
TABLE-US-00002 TABLE 2-1 Surface properties Surface Pickling
condition Repickling condition covering Acid Treating Acid Treating
ratio of concentration Temperature time concentration Temperature
time iron-based No (g/l) (.degree. C.) (seconds) (g/l) (.degree.
C.) (seconds) oxide (%) 1 Nitric acid: 40 10 -- -- -- 72.6 2 150 +
Hydrochloric 40 1 39.5 3 Hydrochloric acid: 0.1 10 35.3 4 acid: 15
30 30.4 5 Hydrochloric 20 1 39.1 6 acid: 10 10 36.1 7 30 32.3 8
Hydrochlori 40 1 35.2 9 acid: 10 10 30.3 10 30 25.9 11 Hydrochloric
70 1 30.9 12 acid: 10 10 25.1 13 30 22.3 14 Hydrochloric 40 1 30.1
15 acid: 50 10 26.2 16 30 21.2 17 Hydrochloric 40 1 49.8 18 acid:
100 10 54.5 19 30 59.8 20 Nitric acid: 40 10 Hydrochloric 40 1 39.7
21 50 + acid: 0.1 10 36.1 22 Hydrofluoric 30 32.1 23 acid: 50
Hydrochloric 20 1 39.5 24 acid: 10 10 37.2 25 30 33.6 26
Hydrochloric 40 1 36.2 27 acid: 10 10 32.4 28 30 28.3 29
Hydrochloric 70 1 32.1 30 acid: 10 10 26.8 31 30 24.1 32
Hydrochloric 40 1 31.2 33 acid: 50 10 25.6 34 30 22.3 35
Hydrochloric 40 1 45.9 36 acid: 100 10 55.3 37 30 62.1 Full width
peeled after corrosion test (mm) Temperature of phosphate treating
solution 35.degree. C. 33.degree. C. Hot salt water Salt water
Composite cycle No Immersion test Spray test corrosion test Remarks
1 6.3 5.5 7.9 8.3 Comparative example 2 4.9 4.0 5.8 5.8 Invention
example 3 4.6 3.8 5.6 5.6 Invention example 4 4.4 3.6 5.0 5.0
Invention example 5 4.9 4.0 5.8 5.8 Invention example 6 4.8 3.7 5.4
5.5 Invention example 7 4.6 3.6 5.2 5.0 Invention example 8 4.6 3.7
5.3 5.6 Invention example 9 4.5 3.6 4.8 5.0 Invention example 10
4.0 3.1 4.4 4.5 Invention example 11 4.4 3.5 4.8 4.9 Invention
example 12 4.0 3.2 4.1 4.5 Invention example 13 3.7 3.0 4.0 4.1
Invention example 14 4.3 3.5 4.8 4.8 Invention example 15 4.1 3.2
4.3 4.5 Invention example 16 3.5 3.0 3.6 3.6 Invention example 17
5.5 4.4 6.7 6.8 Comparative example 18 5.7 4.7 7.1 7.4 Comparative
example 19 5.9 5.1 7.3 7.6 Comparative example 20 4.9 3.9 5.9 5.8
Invention example 21 4.8 3.8 5.4 5.4 Invention example 22 4.2 3.5
5.0 5.0 Invention example 23 5.0 4.0 5.6 5.6 Invention example 24
4.7 3.9 5.6 5.4 Invention example 25 4.6 3.6 5.3 5.1 Invention
example 26 4.8 3.8 5.5 5.8 Invention example 27 4.6 3.7 5.3 5.5
Invention example 28 4.2 3.5 4.8 4.6 Invention example 29 4.6 3.6
5.0 5.3 Invention example 30 4.2 3.3 4.5 4.6 Invention example 31
3.9 3.2 4.2 4.3 Invention example 32 4.3 3.5 4.6 5.0 Invention
example 33 4.0 3.3 4.1 4.6 Invention example 34 3.7 3.1 3.6 4.2
Invention example 35 5.3 4.2 6.2 6.4 Comparative example 36 5.8 4.7
7.1 7.3 Comparative example 37 6.0 5.1 7.3 7.7 Comparative
example
TABLE-US-00003 TABLE 2-2 Surface properties Surface Pickling
condition Repickling condition covering Acid Treating Acid Treating
ratio of concentration Temperature time concentration Temperature
time iron-based No (g/l) (.degree. C.) (seconds) (g/l) (.degree.
C.) (seconds) oxide (%) 38 Nitric acid: 40 10 Sulfuric 40 1 39.5 39
150 + acid: 0.1 10 35.3 40 Hydrochloric 30 30.4 41 acid: 15
Sulfuric 20 1 39.1 42 acid: 75 10 36.1 43 30 32.3 44 Sulfuric 40 1
35.2 45 acid: 75 10 30.3 46 30 25.9 47 Sulfuric 70 1 30.9 48 acid:
75 10 25.1 49 30 22.3 50 Sulfuric 40 1 30.1 51 acid: 150 10 26.2 52
30 21.2 53 Sulfuric 40 1 49.9 54 acid: 200 10 55.0 55 30 62.1 56
Nitric acid: 40 10 Sulfuric 40 1 39.7 57 50 + acid: 0.1 10 36.1 58
Hydrofluoric 30 32.1 59 acid: 50 Sulfuric 20 1 39.5 60 acid: 75 10
37.2 61 30 33.6 62 Sulfuric 40 1 36.2 63 acid: 75 10 32.4 64 30
28.3 65 Sulfuric 70 1 32.1 66 acid: 75 10 26.8 67 30 24.1 68
Sulfuric 40 1 31.2 69 acid: 150 10 25.6 70 30 22.3 71 Sulfuric 40 1
50.1 72 acid: 200 10 55.3 73 30 61.5 Full width peeled after
corrosion test (mm) Temperature of phosphate treating solution
35.degree. C. 33.degree. C. Hot salt water Salt water Composite
cycle No Immersion test Spray test corrosion test Remarks 38 4.8
4.0 5.7 5.9 Invention example 39 4.7 3.9 5.6 5.7 Invention example
40 4.6 3.7 5.1 5.2 Invention example 41 4.8 4.1 5.9 5.9 Invention
example 42 4.7 3.8 5.6 5.6 Invention example 43 4.5 3.7 5.2 5.4
Invention example 44 4.8 3.9 5.6 5.6 Invention example 45 4.6 3.6
5.2 5.1 Invention example 46 4.3 3.3 4.8 4.8 Invention example 47
4.6 3.8 5.2 5.2 Invention example 48 4.2 3.5 4.7 4.7 Invention
example 49 3.9 3.2 4.5 4.6 Invention example 50 4.6 3.5 5.2 5.2
Invention example 51 4.3 3.3 4.6 4.8 Invention example 52 4.0 3.2
4.2 4.5 Invention example 53 5.4 4.4 6.6 6.8 Comparative example 54
5.7 4.7 7.1 7.4 Comparative example 55 6.0 5.2 7.4 7.6 Comparative
example 56 5.0 3.8 5.9 6.0 Invention example 57 4.7 3.7 5.7 5.8
Invention example 58 4.7 3.6 5.5 5.6 Invention example 59 4.9 4.2
6.0 6.0 Invention example 60 4.8 4.0 5.8 5.8 Invention example 61
4.5 3.7 5.6 5.6 Invention example 62 4.8 3.9 5.6 5.7 Invention
example 63 4.6 3.6 5.3 5.4 Invention example 64 4.5 3.5 4.9 5.2
Invention example 65 4.5 3.7 5.3 5.3 Invention example 66 4.3 3.4
4.6 4.9 Invention example 67 4.2 3.3 4.4 4.7 Invention example 68
4.5 3.6 5.2 5.3 Invention example 69 4.2 3.5 4.8 4.7 Invention
example 70 4.2 3.3 4.4 4.6 Invention example 71 5.4 4.5 6.6 6.8
Comparative example 72 5.8 4.8 7.2 7.5 Comparative example 73 5.9
5.2 7.4 7.7 Comparative example
TABLE-US-00004 TABLE 2-3 Surface properties Surface Pickling
condition Repickling condition covering Acid Treating Acid Treating
ratio of concentration Temperature time concentration Temperature
time iron-based No (g/l) (.degree. C.) (seconds) (g/l) (.degree.
C.) (seconds) oxide (%) 74 Nitric acid: 40 10 Hydrochloric 40 1
35.5 75 150 + acid: 5 + 10 30.6 76 Hydrochloric Sulfuric 30 26.3
acid: 15 acid: 5 77 Nitric acid: 40 10 Hydrochloric 40 1 33.2 78
150 + acid: 10 + 10 30.1 79 Hydrochloric Sulfuric 30 25.6 acid: 15
acid: 50 80 Nitric acid: 40 10 Hydrochloric 40 1 35.7 81 50 + acid:
5 + 10 30.9 82 Hydrofluoric Sulfuric 30 27.0 acid: 50 acid: 5 83
Nitric acid: 40 10 Hydrochloric 40 1 34.6 84 50 + acid: 5 + 10 30.1
85 Hydrofluoric Sulfuric 30 26.5 acid: 50 acid: 5 Full width peeled
after corrosion test (mm) Temperature of phosphate treating
solution 35.degree. C. 33.degree. C. Hot salt water Salt water
Composite cycle No Immersion test Spray test corrosion test Remarks
74 4.5 3.7 5.4 5.8 Invention example 75 4.4 3.5 4.9 5.0 Invention
example 76 3.9 3.2 4.6 4.6 Invention example 77 4.4 3.6 5.3 5.8
Invention example 78 4.2 3.5 4.9 5.0 Invention example 79 3.8 3.3
4.5 4.5 Invention example 80 4.6 3.8 5.4 5.7 Invention example 81
4.5 3.6 4.9 5.3 Invention example 82 4.1 3.2 4.7 4.8 Invention
example 83 4.5 3.8 5.2 5.6 Invention example 84 4.4 3.5 5.0 5.2
Invention example 85 4.1 3.1 4.6 4.7 Invention example
EXAMPLE 2
[0102] 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 and repickled 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. 1-39.
TABLE-US-00005 TABLE 3 Chemical composition (mass %) Steel Nb, Ti,
V, Mo, Ni, Cu, Symbol C Si Mn P S Al Si/Mn Cr, B, N Ca, REM A 0.11
1.25 1.55 0.018 0.001 0.032 0.81 -- -- B 0.15 1.30 1.80 0.019 0.002
0.033 0.72 -- -- C 0.15 1.20 1.95 0.017 0.001 0.033 0.62 -- -- D
0.09 1.45 1.40 0.017 0.002 0.028 1.04 -- -- E 0.18 1.11 1.36 0.018
0.001 0.032 0.82 -- -- F 0.16 1.41 1.23 0.017 0.001 0.041 1.15 --
-- G 0.14 1.65 1.33 0.018 0.002 0.035 1.24 -- -- H 0.12 1.45 2.10
0.017 0.001 0.042 0.69 -- -- I 0.17 0.90 1.40 0.017 0.002 0.044
0.64 -- -- J 0.13 1.20 1.89 0.018 0.001 0.041 0.63 -- -- K 0.15
1.20 1.85 0.017 0.001 0.034 0.65 -- -- L 0.03 1.25 3.25 0.018 0.001
0.005 0.38 -- -- M 0.22 3.30 1.15 0.018 0.001 0.027 2.87 -- -- N
0.06 1.28 2.12 0.025 0.003 0.040 0.60 Nb: 0.1, Cu: 0.15 Ti: 0.2 O
0.18 1.21 1.97 0.015 0.002 0.035 0.61 V: 0.1, Ni: 0.13 Mo: 0.2 P
0.18 1.56 2.58 0.010 0.002 0.030 0.60 Cr: 0.2, Ca: 0.003 B: 0.005 Q
0.13 1.32 1.32 0.030 0.001 0.040 1.00 N: 0.007 REM: 0.002 R 0.07
1.26 2.10 0.025 0.002 0.040 0.60 Nb: 0.1 -- S 0.06 1.28 2.12 0.025
0.003 0.040 0.60 Nb: 0.1, -- Ti: 0.2 T 0.17 1.23 1.99 0.015 0.002
0.050 0.62 -- Ni: 0.13 U 0.18 1.22 1.97 0.015 0.003 0.040 0.62 --
Ni: 0.13, Ca: 0.003 V 0.18 1.21 1.98 0.015 0.002 0.035 0.61 V: 0.1
Ni: 0.13 W 0.18 1.56 2.58 0.010 0.002 0.030 0.60 Mo: 0.1, Ca: 0.003
Cr: 0.2, B: 0.005 X 0.13 1.32 1.32 0.030 0.001 0.040 1.00 Nb: 0.1,
Cu: 0.2, N: 0.007 REM: 0.002 Y 0.01 0.02 0.25 0.020 0.012 0.040
0.08 -- -- Z 0.11 0.45 1.50 0.020 0.003 0.030 0.30 -- --
TABLE-US-00006 TABLE 4-1 Hot rolling conditions Cold Continuous
annealing conditions Heating Finish Cooling Coiling rolling Heating
Holding Cooling Steel Temperature Temperature rate temperature
reduction temperature time rate No symbol (.degree. C.) (.degree.
C.) (.degree. C./s) (.degree. C.) (%) (.degree. C.) (Second)
(.degree. C./s) 1 A 1150 850 25 620 60 780 45 20 2 B 1150 820 31
400 60 780 40 20 3 B 1150 820 31 400 60 780 40 20 4 C 1140 850 26
600 60 760 50 20 5 D 1150 840 33 530 60 730 40 20 6 E 1150 850 30
580 55 750 35 20 7 F 1150 850 25 620 60 750 50 20 8 G 1150 850 33
550 60 750 30 20 9 G 1150 850 33 550 60 750 30 20 10 G 1150 850 33
550 60 750 30 20 11 G 1150 850 33 550 60 750 30 20 12 G 1150 850 33
550 60 750 30 20 13 H 1130 820 28 570 60 780 50 15 14 I 1150 840 34
530 55 780 50 15 15 J 1140 850 28 600 60 770 60 20 16 K 1150 850 25
620 60 780 45 20 17 L 1100 850 33 550 60 750 50 20 18 L 1100 850 33
550 60 750 50 20 19 L 1100 850 33 550 60 750 50 20 20 L 1100 850 33
550 60 750 50 20 21 L 1100 850 33 550 60 750 50 20 22 M 1120 830 31
550 55 720 50 15 Continuous annealing conditions Cooling stop
Holding Cooling Strength temperature time rate TS No (.degree. C.)
(second) (.degree. C./s) (MPa) Remarks 1 350 100 40 625 Invention
example 2 400 100 50 821 Invention example 3 400 100 50 819
Invention example 4 350 100 45 814 Invention example 5 350 110 40
623 Invention example 6 400 110 50 836 Invention example 7 350 120
50 634 Invention example 8 400 100 50 632 Comparative example 9 400
100 50 635 Invention example 10 400 100 50 631 Invention example 11
400 100 50 633 Invention example 12 400 100 50 634 Comparative
example 13 370 150 50 840 Invention example 14 350 120 55 812
Invention example 15 300 100 45 836 Invention example 16 350 100 40
650 Invention example 17 450 150 50 960 Comparative example 18 450
150 50 959 Invention example 19 450 150 50 963 Invention example 20
450 150 50 962 Invention example 21 450 150 50 961 Comparative
example 22 410 190 50 1124 Comparative example
TABLE-US-00007 TABLE 4-2 Hot rolling conditions Cold Continuous
annealing conditions Heating Finish Cooling Coiling rolling Heating
Holding Cooling Steel Temperature Temperature rate temperature
reduction temperature time rate No symbol (.degree. C.) (.degree.
C.) (.degree. C./s) (.degree. C.) (%) (.degree. C.) (Second)
(.degree. C./s) 23 N 1120 830 33 550 60 750 30 20 24 O 1150 850 32
560 60 750 35 20 25 P 1130 840 33 550 55 780 30 20 26 Q 1140 850 33
580 60 750 40 20 27 R 1120 830 33 550 60 750 30 20 28 S 1120 830 32
550 60 750 35 20 29 T 1150 850 32 560 60 750 35 20 30 U 1140 850 33
550 60 750 35 20 31 V 1150 850 32 550 60 750 40 20 32 W 1130 840 33
550 55 780 30 20 33 X 1140 850 33 580 60 750 40 20 34 Y 910 630 22
550 80 750 40 20 35 Y 905 650 24 560 83 750 40 20 36 Y 910 640 24
550 85 750 35 20 37 Z 990 690 25 540 75 750 35 20 38 Z 970 710 28
540 70 750 35 20 Continuous annealing conditions Cooling stop
Holding Cooling Strength temperature time rate TS No (.degree. C.)
(second) (.degree. C./s) (MPa) Remarks 23 400 100 50 613 Invention
example 24 350 100 50 776 Invention example 25 400 110 50 1152
Invention example 26 400 120 45 586 Invention example 27 400 100 50
611 Invention example 28 410 100 50 621 Invention example 29 350
100 50 773 Invention example 30 400 100 50 785 Invention example 31
380 100 50 770 Invention example 32 400 110 50 1156 Invention
example 33 400 120 45 585 Invention example 34 370 100 50 285
Invention example 35 400 100 50 279 Invention example 36 400 100 50
290 Invention example 37 400 100 50 785 Invention example 38 400
100 50 790 Invention example
TABLE-US-00008 TABLE 5-1 Surface properties Surface Pickling
conditions Repickling conditions covering Acid Treating Acid
Treating ratio of Steel concentration Temperature time
concentration Temperature time iron-based No Symbol (g/l) (.degree.
C.) (seconds) (g/l) (.degree. C.) (seconds) Oxide (%) 1 A Nitric
Acid: 40 10 Hydrochloric 40 10 30.1 150 + Acid: 1 2 B Hydrochloric
40 10 Hydrochloric 40 10 30.5 Acid: 15 Acid: 10 3 B NitricAcid: 40
10 Hydrochloric 40 10 30.2 50 + Acid: 10 Hydrofluoric acid: 50 4 C
Nitric acid: 40 10 Hydrochloric 40 10 29.9 150 + Acid: 10 5 D
Hydrochloric 40 10 Hydrochloric 40 10 30.7 acid: 15 Acid: 10 6 E 40
10 Hydrochloric 40 10 30.2 Acid: 10 7 F 40 10 Hydrochloric 40 10
30.3 Acid: 10 8 G 40 10 Hydrochloric 10 1 74.3 Acid: 10 9 G 40 10
Hydrochloric 40 1 35.5 Acid: 10 10 G 40 10 Hydrochloric 40 30 25.8
Acid: 10 11 G 40 10 Sulfuric 40 30 26.4 Acid: 75 12 G 40 10
Hydrochloric 40 10 54.5 Acid: 100 13 H 40 10 Hydrochloric 40 10
30.5 Acid: 10 14 I 40 10 Hydrochloric 40 10 30.8 Acid: 10 15 J 40
10 Hydrochloric 40 10 29.8 Acid: 10 16 K 40 10 Hydrochloric 40 10
30.1 Acid: 10 17 L 40 10 Sulfuric 10 1 75.2 Acid: 75 18 L 40 10
Sulfuric 40 1 35.3 Acid: 75 19 L 40 10 Sulfuric 40 30 25.5 Acid: 75
20 L 40 10 Hydrochloric 40 30 25.4 Acid: 10 21 L 40 10 Sulfuric 40
10 55.0 Acid: 200 22 M 40 10 Hydrochloric 40 10 41.2 Acid: 10 Full
width peeled after corrosion test(mm) Temperature of phosphate
treating solution: 35.degree. C. 33.degree. C. Hot salt water Salt
water Composite cycle No immersion test spray test corrosion test
Remarks 1 4.4 3.7 4.8 4.9 Invention example 2 4.3 3.7 4.6 5.0
Invention example 3 4.5 3.8 5.1 5.3 Invention example 4 4.4 3.6 4.7
5.2 Invention example 5 4.4 3.9 4.9 5.2 Invention example 6 4.3 3.8
4.8 5.3 Invention example 7 4.6 3.5 4.8 5.1 Invention example 8 6.5
5.3 7.7 8.0 Comparative example 9 4.5 3.9 5.2 5.5 Invention example
10 4.0 3.0 4.5 4.6 Invention example 11 4.3 3.3 4.8 4.9 Invention
example 12 5.7 4.7 7.1 7.4 Comparative example 13 4.2 3.9 4.8 5.2
Invention example 14 4.2 3.8 5.0 5.2 Invention example 15 4.3 3.9
4.9 5.1 Invention example 16 4.1 4.0 4.7 5.2 Invention example 17
6.4 5.5 7.8 8.2 Comparative example 18 4.4 3.9 5.3 5.4 Invention
example 19 4.4 3.3 4.9 5.2 Invention example 20 4.5 3.2 5.0 5.1
Invention example 21 5.7 4.7 7.1 7.4 Comparative example 22 5.2 4.1
6.3 6.5 Comparative example
TABLE-US-00009 TABLE 5-2 Surface properties Surface Pickling
conditions Repickling conditions covering Acid Treating Acid
Treating ratio of Steel concentration Temperature time
concentration Temperature time iron-based No Symbol (g/l) (.degree.
C.) (seconds) (g/l) (.degree. C.) (seconds) Oxide (%) 23 N
Nitricacid: 40 10 Hydrochloric 40 10 30.8 150 + acid: 10 24 O
Hydrochloric 40 10 Hydrochloric 40 10 31.3 acid: 15 acid: 10 25 P
40 10 Hydrochloric 40 10 30.9 acid: 10 26 Q Nitric acid: 40 10
Hydrochloric 40 10 31.0 50 + acid: 10 27 R Hydrochloric 40 10
Hydrochloric 40 10 30.7 acid: 5 acid: 10 28 S Nitric acid: 40 10
Hydrochloric 40 10 31.1 150 + acid: 10 29 T Hydrochloric 40 10
Hydrochloric 40 10 31.4 acid: 15 acid: 10 30 U Nitric acid: 40 10
Hydrochloric 40 10 31.4 50 + acid: 10 31 V Hydrochloric 40 10
Hydrochloric 40 10 30.9 acid: 5 acid: 10 32 W Nitric acid: 40 10
Hydrochloric 40 10 30.5 150 + acid: 10 33 X Hydrochloric 40 10
Hydrochloric 40 10 31.4 acid: 15 acid: 10 34 Y 40 10 Hydrochloric
40 10 29.8 acid: 10 35 Y 40 10 Hydrochloric 40 10 29.3 acid: 10 36
Y 40 10 Hydrochloric 40 10 28.3 acid: 10 37 Z 40 10 Hydrochloric 40
10 37.4 acid: 10 38 Z 40 10 Hydrochloric 40 10 35.3 acid: 10 39 Z
40 10 Hydrochloric 40 10 29.9 acid: 10 Full width peeled after
corrosion test(mm) Temperature of phosphate treating solution:
35.degree. C. 33.degree. C. Hot salt water Salt water Composite
cycle No immersion test spray test corrosion test Remarks 23 4.4
3.9 5.1 5.2 Invention example 24 4.3 3.9 5.1 5.3 Invention example
25 4.4 3.7 5.0 5.2 Invention example 26 4.4 3.8 4.9 5.1 Invention
example 27 4.4 3.8 5.2 5.3 Invention example 28 4.4 4.0 5.1 5.2
Invention example 29 4.3 3.8 5.2 5.3 Invention example 30 4.3 3.8
5.3 5.3 Invention example 31 4.4 3.9 5.2 5.3 Invention example 32
4.2 3.7 5.1 5.1 Invention example 33 4.3 3.9 4.9 5.2 Invention
example 34 4.2 3.6 4.7 5.2 Invention example 35 4.3 3.5 4.7 5.2
Invention example 36 4.1 3.5 4.5 5.1 Invention example 37 4.4 3.9
5.3 5.4 Invention example 38 4.3 3.9 5.3 5.3 Invention example 39
4.4 3.7 4.7 5.2 Invention example
[0103] 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 repickling 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.
[0104] (1) Mechanical Properties
[0105] 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.
[0106] (2) Corrosion Resistance After Coating
[0107] 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.
[0108] 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 and repickled under the conditions adequate for
the invention to render the surface covering ratio of the
iron-based oxide into not more than 40% 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
[0109] 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 and
repickled 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. 1-61 shown in Table
6.
[0110] 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.
[0111] 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.
[0112] (1) Phosphate Treating Conditions
[0113] 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>
[0114] Degreasing step: treating temperature 40.degree. C.,
treating time 120 seconds [0115] Spray degreasing, surface
regulating step: pH 9.5, Treating temperature room temperature,
treating time 20 seconds [0116] Phosphate treating step:
temperature of phosphate treating solution 35.degree. C., treating
time 120 seconds
<Low Temperature Condition>
[0117] Condition of lowering the temperature of the phosphate
treating solution in the above standard condition to 33.degree.
C.
[0118] (2) Corrosion Test
[0119] 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 under more strict conditions than the one with
Example 1.
<Hot Salt Water Immersion Test>
[0120] 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 480
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)>
[0121] 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 1400 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)>
[0122] 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 drying (60.degree. C., relative
humidity: 30%) for 2 hours wetting (50.degree. C., relative
humidity: 95%) for 2 hours is repeated 150 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.
[0123] 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 and repickling under the conditions that the surface
covering ratio of the iron-based oxide on the surface of the steel
sheet after repickling is not more than 40% and the maximum
thickness of the iron-based oxide is not more than 150 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-00010 TABLE 6-1 Surface properties Surface Pcckling
condition Repickling condition covering Maximum Acid Treating Acid
Treating ratio of thickness of concentration Temperature time
concentration Temperature time iron-based iron-based No (g/l)
(.degree. C.) (seconds) (g/l) (.degree. C.) (seconds) Oxide (%)
Oxide (nm) 1 Nitric acid: 40 10 -- -- -- 72.6 214 2 150 + 40 10
Hydrofluoric 40 1 39.5 158 3 Hydrochloric acid: 0.1 10 35.3 157 4
acid: 15 30 30.4 162 5 40 10 Hydrofluoric 40 1 38.2 149 6 acid: 3
10 33.1 148 7 30 27.8 144 8 40 10 Hydrofluoric 40 1 35.2 119 9
acid: 10 10 30.3 114 10 30 25.9 124 11 40 10 Hydrofluoric 40 1 30.1
91 12 acid: 50 10 26.2 88 13 30 21.2 83 14 Nitric acid: 40 10
Hydrofluoric 40 1 39.7 163 15 50 + acid: 0.1 10 36.1 167 16
Hydrofluoric 30 32.1 159 17 acid: 50 40 10 Hydrofluoric 40 1 37.8
148 18 acid: 3 10 34.3 147 19 30 28.5 149 20 40 10 Hydrofluoric 40
1 36.2 146 21 acid: 10 10 32.4 144 22 30 28.3 148 23 40 10
Hydrofluoric 40 1 31.2 115 24 acid: 50 10 25.6 120 25 30 22.3 119
Full width peeled after corrosion test (mm) Temperature of
phosphate treating solution: 35.degree. C. 33.degree. C. Hot salt
water Salt water Composite cycle No Immersion test Spray test
corrosion test Remarks 1 6.5 5.8 8.2 8.4 Comparative example 2 5.2
4.3 6.2 6.3 Invention example 3 5.2 4.1 6.1 6.3 Invention example 4
5.3 4.2 6.3 6.4 Invention example 5 4.9 4.0 5.9 5.9 Invention
example 6 5.0 3.9 5.8 5.9 Invention example 7 4.8 4.0 5.7 5.9
Invention example 8 4.7 3.8 5.5 5.6 Invention example 9 4.7 3.7 5.6
5.4 Invention example 10 4.8 3.8 5.6 5.6 Invention example 11 4.4
3.6 5.1 5.2 Invention example 12 4.3 3.4 4.9 5.2 Invention example
13 4.2 3.1 4.6 4.4 Invention example 14 5.3 4.4 6.3 6.2 Invention
example 15 5.5 4.3 6.3 6.4 Invention example 16 5.4 4.4 6.2 6.2
Invention example 17 5.0 3.9 5.9 6.0 Invention example 18 4.8 3.9
5.8 6.0 Invention example 19 4.9 4.0 5.8 5.9 Invention example 20
4.9 4.0 5.9 5.9 Invention example 21 4.9 3.9 5.7 6.0 Invention
example 22 4.8 3.9 5.9 6.0 Invention example 23 4.5 3.7 5.5 5.5
Invention example 24 4.6 3.8 5.6 5.4 Invention example 25 4.4 3.8
5.5 5.6 Invention example
TABLE-US-00011 TABLE 6-2 Surface properties Surface Pcckling
condition Repickling condition covering Maximum Acid Treating Acid
Treating ratio of thickness of concentration Temperature time
concentration Temperature time iron-based iron-based No (g/l)
(.degree. C.) (seconds) (g/l) (.degree. C.) (seconds) Oxide (%)
oxide (nm) 26 Nitric acid: 40 10 Sulfuric 40 1 39.5 165 27 150 +
acid: 0.1 10 35.3 168 28 Hydrochloric 30 30.4 170 29 acid: 15
Sulfuric 40 1 38.5 148 30 acid: 8 10 33.1 146 31 30 27.6 149 32
Sulfuric 40 1 35.2 121 33 acid: 75 10 30.3 118 34 30 25.9 117 35
Sulfuric 40 1 30.1 90 36 acid: 150 10 26.2 81 37 30 21.2 86 38
Nitric acid: 40 10 Sulfuric 40 1 39.7 170 39 50 + acid: 0.1 10 36.1
169 40 Hydrofluoric 30 32.1 174 41 acid: 50 Sulfuric 40 1 38.7 149
42 acid: 8 10 33.9 148 43 30 27.4 148 44 Sulfuric 40 1 36.2 145 45
acid: 75 10 32.4 148 46 30 28.3 147 47 Sulfuric 40 1 31.2 118 48
acid: 150 10 25.6 115 49 30 22.3 113 Full width peeled after
corrosion test (mm) Temperature of phosphate treating solution:
35.degree. C. 33.degree. C. Hot salt water Salt water Composite
cycle No Immersion test Spray test corrosion test Remarks 26 5.4
4.4 6.2 6.4 Invention example 27 5.5 4.4 6.4 6.5 Invention example
28 5.7 4.5 6.3 6.5 Invention example 29 4.9 4.0 5.8 5.9 Invention
example 30 4.8 3.9 5.8 5.7 Invention example 31 4.8 3.8 5.7 5.9
Invention example 32 4.6 3.9 5.6 5.7 Invention example 33 4.6 3.7
5.5 5.4 Invention example 34 4.4 3.6 5.6 5.8 Invention example 35
4.6 3.6 5.2 5.2 Invention example 36 4.3 3.4 5.0 4.9 Invention
example 37 4.4 3.3 4.7 5.0 Invention example 38 5.6 4.6 6.4 6.5
Invention example 39 5.5 4.6 6.6 6.8 Invention example 40 5.8 4.7
6.7 6.8 Invention example 41 4.8 4.0 5.9 6.0 Invention example 42
4.9 3.9 5.8 5.9 Invention example 43 5.0 4.0 5.8 5.9 Invention
example 44 4.9 4.0 5.8 5.9 Invention example 45 4.8 3.9 5.9 5.8
Invention example 46 4.8 3.9 5.7 6.0 Invention example 47 4.6 3.6
5.5 5.6 Invention example 48 4.4 3.7 5.4 5.7 Invention example 49
4.5 3.5 5.3 5.4 Invention example
TABLE-US-00012 TABLE 6-3 Surface properties Surface Pcckling
condition Repickling condition covering Maximum Acid Treating Acid
Treating ratio of thickness of concentration Temperature time
concentration Temperature time iron-based iron-based No (g/l)
(.degree. C.) (seconds) (g/l) (.degree. C.) (seconds) Oxide (%)
oxide (nm) 50 Nitric acid: 40 10 Hydrofluoric 40 1 35.5 152 51 150
+ acid: 5 + 10 30.6 155 52 Hydrochloric Sulfuric 30 26.3 154 acid:
15 acid: 5 53 Nitric acid: 40 10 Hydrofluoric 40 1 34.3 146 54 150
+ acid: 3 + 10 29.5 145 55 Hydrochloric Sulfuric 30 25.6 143 acid:
15 acid: 8 56 Nitric acid: 40 10 Hydrofluoric 40 1 36.5 155 57 50 +
acid: 5 + 10 31.6 158 58 Hydrofluoric Sulfuric 30 28.4 156 acid: 50
acid: 5 59 Nitric acid: 40 10 Hydrofluoric 40 1 35.7 148 60 50 +
acid: 3 + 10 30.9 147 61 Hydrofluoric Sulfuric 30 27.0 148 acid: 50
acid: 8 Full width peeled after corrosion test (mm) Temperature of
phosphate treating solution: 35.degree. C. 33.degree. C. Hot salt
water Salt water Composite cycle No Immersion test Spray test
corrosion test Remarks 50 5.1 4.1 6.1 6.2 Invention example 51 5.1
4.2 6.2 6.2 Invention example 52 5.2 4.2 6.3 6.4 Invention example
53 4.8 3.9 5.9 5.9 Invention example 54 4.8 3.8 5.9 5.7 Invention
example 55 4.6 3.7 5.7 5.8 Invention example 56 5.2 4.2 6.2 6.2
Invention example 57 5.3 4.4 6.3 6.2 Invention example 58 5.3 4.2
6.3 6.4 Invention example 59 4.9 3.8 5.9 5.9 Invention example 60
4.9 4.0 5.9 5.8 Invention example 61 5.0 3.9 5.8 6.0 Invention
example
INDUSTRIAL APPLICABILITY
[0124] 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.
* * * * *