U.S. patent application number 10/998950 was filed with the patent office on 2005-06-30 for high-strength cold-rolled steel sheet excellent in coating film adhesion.
This patent application is currently assigned to Kabushiki Kaisha Kobe Seiko Sho (Kobe Steel, Ltd.). Invention is credited to Hashimoto, Ikurou, Kamura, Manabu, Kozuma, Shinji, Nomura, Masahiro, Omiya, Yoshinobu.
Application Number | 20050139293 10/998950 |
Document ID | / |
Family ID | 34545001 |
Filed Date | 2005-06-30 |
United States Patent
Application |
20050139293 |
Kind Code |
A1 |
Nomura, Masahiro ; et
al. |
June 30, 2005 |
High-strength cold-rolled steel sheet excellent in coating film
adhesion
Abstract
A cold-rolled steel sheet of DP (Dual Phase) type with a
specific composition meets the requirements: (I) In the surface of
the steel sheet, there exist Si--Mn complex oxides no larger than 5
.mu.m in diameter of the equivalent circle as many as 10 or more
per 100 .mu.m.sup.2 and the coverage of oxides composed mainly of
Si on the surface of steel sheet is no more than 10% of surface
area, and/or (II) The cross section near the surface of the steel
sheet does not show cracks with a width no larger than 3 .mu.m and
a depth no smaller than 5 .mu.m in arbitrary ten fields of
observation under an SEM with a magnification of 2000. A
high-strength cold-rolled steel sheet excellent in coating film
adhesion and having a tensile strength no lower than 550 MPa is
provided.
Inventors: |
Nomura, Masahiro; (Kobe-shi,
JP) ; Hashimoto, Ikurou; (Kobe-shi, JP) ;
Omiya, Yoshinobu; (Kakogawa-shi, JP) ; Kozuma,
Shinji; (Kakogawa-shi, JP) ; Kamura, Manabu;
(Kakogawa-shi, JP) |
Correspondence
Address: |
OBLON, SPIVAK, MCCLELLAND, MAIER & NEUSTADT, P.C.
1940 DUKE STREET
ALEXANDRIA
VA
22314
US
|
Assignee: |
Kabushiki Kaisha Kobe Seiko Sho
(Kobe Steel, Ltd.)
Kobe-shi
JP
|
Family ID: |
34545001 |
Appl. No.: |
10/998950 |
Filed: |
November 30, 2004 |
Current U.S.
Class: |
148/320 |
Current CPC
Class: |
C23C 30/00 20130101;
C21D 2211/008 20130101; C21D 8/0226 20130101; C22C 38/02 20130101;
C21D 2211/005 20130101; C23C 2/02 20130101; C22C 38/04 20130101;
C21D 8/0278 20130101; C21D 8/0236 20130101 |
Class at
Publication: |
148/320 |
International
Class: |
C22C 038/00 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 25, 2003 |
JP |
2003-429151 |
Claims
What is claimed is:
1. A high-strength cold-rolled steel sheet excellent in coating
film adhesion, which is a DP (Dual Phase) steel sheet of
ferrite-tempered martensite type containing no more than 1 mass %
of C (excluding 0 mass %), 0.05 to 2 mass % of Si, and 1 to 5 mass
% of Mn, having a tensile strength no lower than 550 MPa,
satisfying the equation (1) below, and being characterized by its
surface in which there exist Si--Mn complex oxides no larger than 5
.mu.m in diameter of the equivalent circle as many as 10 or more
per 100 .mu.m.sup.2 and the coverage of oxides composed mainly of
Si on the surface of steel sheet is no more than 10% of surface
area. [Si]/[Mn].ltoreq.0.4 (1) where [Si] denotes an Si content (in
mass %) and [Mn] denotes an Mn content (in mass %).
2. A high-strength cold-rolled steel sheet excellent in coating
film adhesion, which is a DP (Dual Phase) steel sheet of
ferrite-tempered martensite type containing no more than 1 mass %
of C (excluding 0 mass %), no more than 2 mass % of Si (excluding 0
mass %), and 1 to 5 mass % of Mn, having a tensile strength no
lower than 550 MPa, and being characterized by its surface whose
cross section does not show cracks with a width no larger than 3
.mu.m and a depth no smaller than 5 .mu.m in arbitrary ten fields
of observation under an SEM with a magnification of 2000.
3. A high-strength cold-rolled steel sheet excellent in coating
film adhesion, which is a DP (Dual Phase) steel sheet of
ferrite-tempered martensite type containing no more than 1 mass %
of C (excluding 0 mass %), 0.05 to 2 mass % of Si, and 1 to 5 mass
% of Mn, having a tensile strength no lower than 550 MPa,
satisfying the equation (1) below, and meeting the following
requirements (I) and (II). (I) In the surface of the steel sheet,
there exist Si--Mn complex oxides no larger than 5 .mu.m in
diameter of the equivalent circle as many as 10 or more per 100
.mu.m.sup.2 and the coverage of oxides composed mainly of Si on the
surface of steel sheet is no more than 10% of surface area. (II)
The cross section near the surface of the steel sheet does not show
cracks with a width no larger than 3 .mu.m and a depth no smaller
than 5 .mu.m in arbitrary ten fields of observation under an SEM
with a magnification of 2000. [Si]/[Mn].ltoreq.0.4 (1) where [Si]
denotes an Si content (in mass %) and [Mn] denotes an Mn content
(in mass %).
4. The high-strength steel sheets as defined in claim 1, which
satisfies the equations (2) and (3) below. [P]+3[S]+1.54[C]<0.25
(2) [C]+[Si]/30+[Mn]/20+2[P]+4[S]<0.34 (3) where [C], [Si],
[Mn], [P], and [S] denote the content (in mass %) of these
elements.
5. The high-strength steel sheets as defined in claim 2, which
satisfies the equations (2) and (3) below. [P]+3[S]+1.54[C]<0.25
(2) [C]+[Si]/30+[Mn]/20+2[P]+4[S]<0.34 (3) where [C], [Si],
[Mn], [P], and [S] denote the content (in mass %) of these
elements.
6. The high-strength steel sheets as defined in claim 3, which
satisfies the equations (2) and (3) below. [P]+3[S]+1.54[C]<0.25
(2) [C]+[Si]/30+[Mn]/20+2[P]+4[S]<0.34 (3) where [C], [Si],
[Mn], [P], and [S] denote the content (in mass %) of these
elements.
7. The high-strength steel sheets as defined in claim 1, wherein
the coverage of oxides composed mainly of Si on the surface of
steel sheet is no more than 5% of surface area.
8. The high-strength steel sheets as defined in claim 3, wherein
the coverage of oxides composed mainly of Si on the surface of
steel sheet is no more than 5% of surface area.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a high-strength cold-rolled
steel sheet excellent in coating film adhesion, and more
particularly, to a cold-rolled steel sheet which has a tensile
strength no lower than 550 MPa and is suitable for use as a steel
sheet for automobile parts on account of its excellent coating film
adhesion.
[0003] 2. Description of the Related Art
[0004] There is a growing demand for high-strength steel products
necessary for automobiles with less fuel consumption and lower
weight than before. This trend is prevailing also in the field of
cold-rolled steel sheet. On the other hand, cold-rolled steel
sheets are required to have sufficient ductility (such as
elongation) because they are formed into automotive parts by
pressing. Increase in strength can be effectively achieved by
incorporation with alloying elements; however, they adversely
affect ductility as their amount increases.
[0005] Of these alloying elements, Si is less influential in
reducing ductility and is effective in increasing strength while
retaining ductility. However, with an increased Si content, the
resulting steel sheet is poor in chemical treatability and hence in
coating film adhesion. Consequently, it was necessary to reduce Si
content in the case where chemical treatability is important.
Moreover, excess Si forms an Si-containing intergranular oxide on
the surface of steel sheet, thereby causing cracks to occur and
aggravating coating film adhesion.
[0006] One way to reconcile mechanical properties and chemical
treatability is by cladding a steel sheet of high Si content with a
layer of low Si content. Such a cladding layer contributes to
chemical treatability without adverse effect on the mechanical
properties of the steel sheet. (See Japanese Patent Laid-open No.
Hei-5-787452) And the steel sheet of high Si content ensures
sufficient mechanical properties. Unfortunately, cladding needs a
complex process which leads to an increased production cost.
[0007] There is a conventional technology of adding a special
alloying element, such as Ni and Cu, which prevents Si (detrimental
to chemical treatment) from concentrating in the surface of a steel
sheet. (See Japanese Patent No. 2951480 and Japanese Patent No.
3266328) This technology suffers a disadvantage of requiring
expensive Ni or Cu, which leads to an increase in production
cost.
[0008] The conventional technology mentioned above is concerned
with so-called IF (Interstitial Free) steel. IF steel is limited in
carbon content (no more than 0.005%) and has its texture controlled
by a specific recrystallization temperature, so that it is improved
in deep drawability. However, IF steel with a very low carbon
content will not achieve the high strength intended by the present
invention.
[0009] There is a technology of ensuring chemical treatability by
causing NbC to separate out and function as a site for nucleation
of zinc phosphate crystals. (See Japanese Patent No. 2003-3049147)
This technology is also designed to improve deep drawability by
keeping the carbon content low (no more than 0.02%) for texture
control. The steel of this technology has a slightly higher carbon
content than the above-mentioned IF steel, but is still
unsatisfactory in strength. Japanese Patent No. 3049147 discloses
two inventions which respectively achieve a strength of 539 MPa (55
kgf/mm.sup.2) and 588 MPa (60 kgf/mm.sup.2) which is in excess of
550 MPa. This strength has been realized by increasing the content
of P or Mo. Unfortunately, these elements are detrimental to
weldability.
[0010] There has been proposed a retained austenite-containing
steel sheet which has good chemical treatability owing to the
controlled ratio of SiO.sub.2/Mn.sub.2SiO.sub.4 in the surface
layer. (See Japanese Patent Laid-open No. 2003-201538) To obtain
this steel sheet, it is necessary to control oxides in the surface
layer, to perform pickling or brushing on the surface after
continuous annealing, thereby removing Si oxides and controlling
the Si/Fe ratio, and to keep the dew point above -30.degree. C. at
the temperature below the Ac.sub.1 transformation point, thereby
limiting the amount of Si oxides to be formed.
[0011] Unfortunately, pickling and brushing increase the number of
manufacturing steps, which leads to a higher production cost. In
addition, the control of dew point, which is accomplished in a
continuous annealing furnace, is not very effective so long as
Examples show in the document. According to data in the document,
the ratio of SiO.sub.2/Mn.sub.2SiO.sub.4 in the surface layer is
about 1.0. This value suggests that SiO.sub.2, which prevents the
formation of film crystals due to chemical treatment, occurs as
much as Mn.sub.2SiO.sub.4. Judging from these results, the
disclosed technology will not sufficiently improve chemical
treatability.
[0012] Moreover, the retained austenite-containing steel sheet
mentioned above contains such alloying elements as C, Si, Mn, and
Al in large amounts so as to secure retained austenite. Therefore,
it is poor in weldability.
[0013] There has been proposed another technology of improving
chemical treatability, which is intended to keep below 1 the Si/Mn
ratio in oxides determined by surface analysis with XPS (X-ray
photoelectron spectroscopy). (See Japanese Patent Laid-open No.
Hei-4-276060)
[0014] An example of steel having an Si/Mn ratio lower than 1 is
mild steel nearly free of Si, which is known to have good chemical
treatability. However, a certain amount of Si is necessary for
steel to have both high strength and good ductility, and hence
there is a limit of reducing the Si content to keep the Si/Mn ratio
below 1. Further, it turned out that a steel sheet does not always
exhibit good chemical treatability even though it has an Si/Mn
ratio lower than 1, for a certain Si content and an adequately
controlled Mn content.
SUMMARY OF THE INVENTION
[0015] The present invention was completed in view of the
foregoing. It is an object of the present invention to provide a
cold-rolled steel sheet characterized by a tensile strength no
lower than 500 MPa and excellent coating film adhesion and
weldability.
[0016] The present invention is directed to a high-strength
cold-rolled steel sheet excellent in coating film adhesion, which
is a DP (Dual Phase) steel sheet of ferrite-tempered martensite
type containing no more than 1 mass % of C (excluding 0 mass %),
0.05 to 2 mass % of Si, and 1 to 5 mass % of Mn, having a tensile
strength no lower than 550 MPa, satisfying the equation (1) below,
and being characterized by its surface in which there exist Si--Mn
complex oxides no larger than 5 .mu.m in diameter of the equivalent
circle as many as 10 or more per 100 .mu.m.sup.2 and the coverage
of oxides composed mainly of Si on the surface of steel sheet is no
more than 10% of surface area (requirement (I)). The equivalent
circle means the circle of the same area as the Si--Mn complex
oxide. (This steel sheet will be referred to as "Steel sheet 1 of
the present invention" hereinafter.)
[Si]/[Mn].ltoreq.0.4 (1)
[0017] where [Si] denotes an Si content (in mass %) and [Mn]
denotes an Mn content (in mass %).
[0018] The term "oxides composed mainly of Si" mentioned above
means those oxides in which Si (as one of the constituents
excluding oxygen) accounts for no less than 70% in atomic ratio.
Such oxides are considered to be amorphous according to the result
of analysis.
[0019] The ratio of the surface area of steel sheet which is
covered by the oxides composed mainly of Si was obtained by
observation under a TEM (Transmission Electron Microscope),
quantitative analysis and mapping of Si, O, Mn, and Fe by EDX
(Energy Dispersive X-ray), and image analysis of these data.
Observation under a TEM was accomplished by using an extraction
replica, which is explained in Examples given later. Observation
under a TEM for an extraction replica may be replaced by surface
mapping for Si, O, Mn, and Fe by AES (Auger Electron Spectroscopy)
at a magnification of 2000 to 5000, and the resulting data may be
used for image analysis.
[0020] The present invention is directed also to a high-strength
cold-rolled steel sheet excellent in coating film adhesion, which
is a DP (Dual Phase) steel sheet of ferrite-tempered martensite
type containing no more than 1 mass % of C (excluding 0 mass %), no
more than 2 mass % of Si (excluding 0 mass %), and 1 to 5 mass % of
Mn, having a tensile strength no lower than 550 MPa, and being
characterized by its surface whose cross section does not show
cracks with a width no larger than 3 .mu.m and a depth no smaller
than 5 .mu.m in arbitrary ten fields of observation under an SEM
(Scanning Electron Microscope) with a magnification of 2000
(requirement (II)). (This steel sheet will be referred to as "Steel
sheet 2 of the present invention" hereinafter.)
[0021] The width and depth of cracks are shown in FIG. 1 (which is
a schematic sectional view of the steel sheet). They are found by
observing the vicinity of the surface of the steel sheet under an
SEM with a magnification of 2000 (Model S-4500 of Hitachi
Ltd.).
[0022] The present invention is directed also to a high-strength
cold-rolled steel sheet excellent in coating film adhesion, which
is a DP (Dual Phase) steel sheet of ferrite-tempered martensite
type containing no more than 1 mass % of C (excluding 0 mass %),
0.05 to 2 mass % of Si, and 1 to 5 mass % of Mn, having a tensile
strength no lower than 550 MPa, satisfying the equation (1) above,
and meeting the above-mentioned requirements (I) and (II). (This
steel sheet will be referred to as "Steel sheet 3 of the present
invention" hereinafter.)
[0023] The steel sheets of the present invention should preferably
have a composition specified by the equations (2) and (3) below as
an additional requirement, so that they exhibit good
weldability.
[P]+3[S]+1.54[C]<0.25 (2)
[C]+[Si]/30+[Mn]/20+2[P]+4[S]<0.34 (3)
[0024] where [C], [Si], [Mn], [P], and [S] denote the content (in
mass %) of these elements.
[0025] The steel sheet according to the present invention has a
high strength in excess of 550 MPa, exhibits good chemical
treatability, and/or good coating film adhesion owing to controlled
fine cracks, and provides good weldability. It is suitable for
automotive parts. It can be produced without cladding or expensive
elements.
BRIEF DESCRIPTION OF THE DRAWINGS
[0026] FIG. 1 is a schematic diagram showing cracks in the cross
section of the steel sheet.
[0027] FIG. 2 is a diagram illustrating (part of) one manufacturing
process in Examples.
[0028] FIG. 3 is a diagram illustrating (part of) another
manufacturing process in Examples.
[0029] FIG. 4 is an electron micrograph (TEM) of sample in
experiment No. 1 in Examples. (Extraction replica,
.times.15000)
[0030] FIG. 5 is an electron micrograph (TEM) of sample in
experiment No. 29 in Examples. (Extraction replica,
.times.15000)
[0031] FIG. 6 is an electron micrograph (TEM) of sample in
experiment No. 34 in Examples. (Extraction replica,
.times.15000)
[0032] FIG. 7 is an electron micrograph (SEM) showing the cross
section near the surface of the steel sheet in experiment No. 1 in
Examples.
[0033] FIG. 8 is an electron micrograph (SEM) showing the cross
section near the surface of the steel sheet in experiment No. 29 in
Examples.
[0034] FIG. 9 is an electron micrograph (SEM) showing the cross
section near the surface of the steel sheet in experiment No. 34 in
Examples.
[0035] FIG. 10 is an electron micrograph (SEM) showing the surface
of the steel sheet (after chemical treatment) in experiment No. 1
in Examples.
[0036] FIG. 11 is an electron micrograph (SEM) showing the surface
of the steel sheet (after chemical treatment) in experiment No. 29
in Examples.
[0037] FIG. 12 is an electron micrograph (SEM) showing the surface
of the steel sheet (after chemical treatment) in experiment No. 34
in Examples.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0038] The results of investigation carried out to obtain a steel
sheet excellent in coating film adhesion revealed that the object
is achieved if the following requirements (I) and/or (II) are met.
This finding led to the present invention. The steel sheet that
meets these requirements and has a high strength (in excess of 550
MPa) and good ductility can be produced in specific compositions
under specific manufacturing conditions as mentioned later.
[0039] (I) In the surface of steel sheet:
[0040] (i) there should exist Si--Mn complex oxides no larger than
5 .mu.m in diameter of the equivalent circle as many as 10 or more
per 100 .mu.m.sup.2, and
[0041] (ii) the coverage of oxides composed mainly of Si on the
surface of steel sheet should be no more than 10% of the surface
area. ("Mainly" means that Si accounts for no less than 70% (in
atomic ratio) in the constituents of oxides other than oxygen.)
[0042] (II) The cross section of the surface of steel sheet should
not show cracks with a width no larger than 3 .mu.m and a depth no
smaller than 5 .mu.m in arbitrary ten fields of observation under
an SEM with a magnification of 2000.
[0043] The above-mentioned requirements (I) and (II) were
established for the following reasons.
[0044] Requirement that in the surface of steel sheet, there should
exist Si--Mn complex oxides no larger than 5 .mu.m in diameter of
the equivalent circle as many as 10 or more per 100
.mu.m.sup.2.
[0045] The present inventors have carried out a series of
researches to obtain a high-strength steel sheet excellent in
coating film adhesion and proposed a technique for improving the
chemical treatability of a steel sheet containing Si in a
comparatively large amount. (See Japanese Patent Application No.
2003-106152.) This technique is intended to improve chemical
treatability by finely dispersing amorphous Si oxides detrimental
to chemical treatability while controlling the annealing
atmosphere. However, major oxides that occur when the Si content is
relatively low (Si content: 0.05 to 2% as defined in the present
invention) are Si--Mn complex oxides rather than amorphous Si
oxides. It is considered that these complex oxides are also
detrimental to coating film adhesion. With this in mind, the
present inventors searched for the possibility of positively using
Si--Mn complex oxides for improvement of chemical treatability.
[0046] As the result, it turned out that chemical treatability
improves if Si--Mn complex oxides are finely dispersed into iron
oxides formed in the surface layer of steel sheet, so as to form
the "inhomogeneous field of oxide interface" which functions as the
nucleating site for zinc phosphate crystals (as mentioned later).
It is not yet elucidated why the Si--Mn complex oxides specified in
the present invention function as the nucleating site for zinc
phosphate crystals. A probable reason is as follows.
[0047] It is known that zinc phosphate crystals tend to form during
chemical treatment in the "electrochemical inhomogeneous field"
originating from the grain boundary or the periphery of the Ti
colloid which has been attached to the surface of steel sheet at
the time of surface preparation. It is considered that the Si--Mn
complex oxides specified in the present invention also create the
electrochemical inhomogeneous field around them, thereby helping
zinc phosphate crystals to stick easily at the time of chemical
treatment, which leads to improved chemical treatability.
[0048] It is considered that zinc phosphate crystals after chemical
treatment should preferably be no larger than several micrometers
from the standpoint of coating film adhesion. Consequently, it is
also considered that the electrochemical inhomogeneous field
mentioned above should preferably be of the same size. For this
reason, the present invention specifies that there should exist
Si--Mn complex oxides no larger than 5 .mu.m in diameter of the
equivalent circle as many as 10 or more per 100 .mu.m.sup.2 (or 1
per 10 .mu.m.sup.2 or more on average), with the average distance
between particles of complex oxides being several micrometers. This
condition is necessary for easy formation of the electrochemical
inhomogeneous field of the above-specified size.
[0049] The number of particles of Si--Mn complex oxides should
preferably be 50 or more per 100 .mu.m.sup.2, more preferably 100
per or more 100 .mu.m.sup.2, and most desirably 150 or more per 100
.mu.m.sup.2, because the electrochemical inhomogeneous field does
not necessarily occur in every particle of the Si--Mn complex
oxides which are present. An example of the Si--Mn complex oxides
is Mn.sub.2SiO.sub.4. It is considered that about 50 nm is the
maximum observable size of Si--Mn complex oxides.
[0050] Requirement that the coverage of oxides composed mainly of
Si on the surface of steel sheet should be no more than 10% of
surface area.
[0051] The Si--Mn complex oxides functioning as nucleating sites
for zinc phosphate crystals will not contribute to good chemical
treatability if there exist other substances detrimental to
chemical treatment. Hence, the resulting steel sheet will be poor
in coating film adhesion.
[0052] If oxides composed mainly of Si are present on the surface
of steel sheet, zinc phosphate crystals do not form on them, which
leads to considerably poor chemical treatability. Consequently, the
present invention requires that the coverage of oxides composed
mainly of Si on the surface of steel sheet should be no more than
10% of surface area.
[0053] The present inventors had previously proposed a technique of
improving chemical treatability by finely dispersing oxides
composed mainly of Si, as mentioned above. However, it turned that
that the presence of oxides should be minimized in the present
invention which is intended to utilize the action of Si--Mn complex
oxides as mentioned above. Therefore, the coverage of oxides
composed mainly of Si on the surface of steel sheet should
preferably be no more than 5% of surface area, most desirably 0% of
surface area.
[0054] Requirement that the cross section of the surface layer of
the steel sheet does not show cracks with a width no larger than 3
.mu.m and a depth no smaller than 5 .mu.m in arbitrary ten fields
of observation under an SEM with a magnification of 2000.
[0055] Sharp cracks present on the surface of the steel sheet
prevent zinc phosphate crystals from sticking to them at the time
of chemical treatment. As the result, corrosion readily proceeds
there, aggravating coating film adhesion. For this plausible
reason, it is important to minimize the occurrence of sharp cracks
in order to improve coating film adhesion.
[0056] The present inventors had previously proposed a technique of
improving coating film adhesion by restricting to 10 .mu.m or less
the depth of linear oxides (narrower than 30 nm) composed of Si and
oxygen. This technique is based on the assumption that continuous
annealing will not be followed by pickling. However, in common
practice, continuous annealing is followed by pickling, and
pickling removes linear oxides, thereby causing cracks to
occur.
[0057] How the depth of cracks relates with linear oxides is not
yet quantitatively elucidated. It is considered that linear oxides
dissolve in acid or mechanically drop off, thereby giving rise to
cracks. It is also considered that such cracks are deeper than the
size of linear oxides because they dissolve further in acid even
after linear oxides have been removed.
[0058] With the foregoing in mind, the present inventors conceived
that it would be possible to improve coating film adhesion more by
controlling cracks than by regulating the depth of linear oxides
(as in the technology they had previously proposed) and they
investigated the shape of cracks to be controlled. As the result,
it was found that zinc phosphate crystals hardly stick to cracks
having a width approximately equal to or smaller than their
particle diameter. This holds true particularly for cracks deeper
than 5 .mu.m. Thus, according to the present invention, cracks to
be controlled are limited to those which are narrower than 3 .mu.m
and deeper than 5 .mu.m.
[0059] Based on the foregoing is established the requirement that
the cross section of the surface layer of the steel sheet should
not show the above-specified cracks in arbitrary ten fields of
observation under an SEM with a magnification of 2000.
[0060] The steel sheet according to the present invention is
required to have the following chemical composition so that it has
controlled cracks for efficient deposition of the above-mentioned
oxides and it exhibits the characteristic properties of
high-strength steel sheet.
[Si]/[Mn].ltoreq.0.4 (1)
[0061] where [Si] denotes an Si content (in mass %) and [Mn]
denotes an Mn content (in mass %).
[0062] Since oxides composed mainly of Si adversely affect chemical
treatability, it is more desirable to suppress them as much as
possible rather than finely dispersing them. The object of
suppressing such oxides can be achieved if the [Si]/[Mn] ratio in
the chemical composition is no larger than 0.4, preferably no
larger than 0.3.
[0063] C: no larger than 1 mass % (excluding 0 mass %)
[0064] Carbon is essential for strength. The minimum carbon content
is 0.05 mass %. An excess carbon content aggravates weldability.
Therefore, the carbon content should be no larger than 1 mass %,
preferably no larger than 0.23 mass %, and more preferably no
larger than 0.15 mass %.
[0065] Si: 0.05 to 2 mass % (for steel sheets 1 and 3)
[0066] Si: no larger than 2 mass % (excluding 0 mass %) (for steel
sheet 2)
[0067] Since Si increases strength without decreasing ductility, it
may be contained in the steel sheet. A certain amount of Si is
necessary for the Si--Mn complex oxides with a diameter of the
equivalent circle no larger than 5 .mu.m to form as much as
specified by the requirement (I) mentioned above. A minimum amount
of Si for this purpose is 0.05 mass %. An adequate amount should be
no less than 0.15 mass %, preferably no less than 0.3 mass %, and
more preferably no less than 0.5 mass %. Si in an excess amount
brings about solid solution hardening more than necessary, which
leads to an increased rolling load. Therefore, the content of Si
should be no larger than 2 mass %, preferably no larger than 1.5
mass %.
[0068] Mn: 1 to 5 mass %
[0069] Mn is also essential for strength; however, excess Mn is
detrimental to ductility. An adequate content of Mn should be no
less than 1 mass %, preferably no less than 2 mass %, and no more
than 5 mass %, preferably no more than 3.5 mass %.
[0070] The steel sheet according to the present invention should
contain the above-mentioned elements, with the remainder being
substantially iron. It may contain inevitable impurities, such as
Al no more than 1 mass %, N no more than 0.01 mass %, and O no more
than 0.01 mass %, originating from raw materials or incorporated
depending on production conditions. It may be positively
incorporated with additional elements, such as Cr, Mo, Ni, Ti, Nb,
V, P, and B, in an amount not harmful to the effect of the present
invention.
[0071] The amount of these additional elements to strengthen the
steel sheet is specified as follows.
[0072] Cr: 0.1 to 1 mass %
[0073] Mo: 0.1 to 1 mass %
[0074] Ni: 0.1 to 1 mass %
[0075] Ti: 0.005 to 0.1 mass %
[0076] Nb: 0.005 to 0.1 mass %
[0077] V: 0.0005 to 0.01 mass %
[0078] P: 0.005 to 0.1 mass %
[0079] B: 0.0003 to 0.01 mass %
[0080] These additional elements will aggravate ductility and
weldability when added in an excess amount.
[0081] [P]+3[S]+1.54[C]<0.25 (2)
[C]+[Si]/30+[Mn]/20+2[P]+4[S]<0.34 (3)
[0082] where [C], [Si], [Mn], [P], and [S] denote the content (in
mass %) of these elements.
[0083] The left side of each of the equations (2) and (3) above is
known as the parameter to evaluate spot weldability. {Tanaka et
al., Nippon Koukan Gihou, No. 105 (1984); Heuschkel, J.: Weld J26
(10), P560 S(1947)} The more the parameter increases, the more the
weldability decreases. It was found in the present invention that
spot weldability decreases when the left hand in (2) and (3)
exceeds 0.25 and 0.34, respectively.
[0084] The present invention covers a steel sheet having a strength
no lower than 550 MPa (preferably no lower than 750 MPa, more
preferably no lower than 900 MPa). The steel sheet should contain
C, Si, and Mn (and optionally P) in an adequate amount as specified
below according to strength and weldability desired.
[0085] For tensile strength from 550 to 650 MPa.
[P]+3[S]+1.54[C]<0.14
[C]+[Si]/30+[Mn]/20+2[P]+4[S]<0.21
[0086] For tensile strength from 650 (exclusive) to 750 MPa.
[P]+3[S]+1.54[C]<0.18
[C]+[Si]/30+[Mn]/20+2[P]+4[S]<0.27
[0087] For tensile strength from 750 (exclusive) to 1050 MPa.
[P]+3[S]+1.54[C]<0.22
[C]+[Si]/30+[Mn]/20+2[P]+4[S]<0.30
[0088] For tensile strength in excess of 1050 MPa.
[P]+3[S]+1.54[C]<0.25
[C]+[Si]/30+[Mn]/20+2[P]+4[S]<0.34
[0089] The present invention covers a DP (Dual Phase) steel sheet
of ferrite-tempered martensite type. The steel may be composed
solely of ferrite and tempered martensite, or it may additionally
contain pearlite, bainite, and retained austenite in an amount not
harmful to the effect of the present invention. They inevitably
remain in the manufacturing process, bur they should be as little
as possible.
[0090] For the steel sheet to have good chemical treatability, the
shape of the oxides that separate out on the surface thereof should
be controlled according to the requirement (I) mentioned above.
This object is achieved not only by the controlled steel
composition as mentioned above but also by pickling (that follows
hot rolling) with hydrochloric acid (1 to 18 mass %) at 70 to
90.degree. C. for about 40 seconds or more (preferably 60 seconds
or more) and continuous annealing in an atmosphere with a dew point
no higher than -40.degree. C., preferably no higher than
-45.degree. C. Incidentally, in the case of pickling with several
acid baths for intermittent dipping, the total dipping time should
be 40 seconds at the minimum. Thus, in order to control the
formation of the oxides so as to satisfy the requirement (I), the
hot rolled steel sheet need undergo pickling.
[0091] For the steel sheet to be free of cracks as provided in the
requirement (II) mentioned above, the manufacturing process is
specified as follows:
[0092] The winding temperature for hot rolling should be no higher
than 500.degree. C., preferably no higher than 480.degree. C.; The
hot rolled steel sheet should be dipped in hydrochloric acid (1 to
18 mass %) at 70 to 90.degree. C. for 40 seconds or more,
preferably 60 seconds or more; Continuous annealing should be
performed in an atmosphere with a dew point no higher than
-40.degree. C., preferably no higher than -45.degree. C.; The
hardening start temperature at the time of annealing (which may be
referred to as "slow cooling end temperature") should be no higher
than 550.degree. C., preferably from 400 to 450.degree. C. Thus, in
order to restrict the formation of the cracks and to satisfy the
requirement (II), the production conditions should be such that
generation of grain boundary oxides which can become start points
for the cracks is restrained.
[0093] The present invention does not specify other manufacturing
conditions than mentioned above. Thus the steel sheet may be
produced in the usual way by melting, casting, and hot rolling.
Although the manufacturing process in Examples that follow involves
pickling that follows continuous annealing, pickling is not
mandatory in the present invention.
EXAMPLES
[0094] The invention will be described in more detail with
reference to the following examples, which are not intended to
restrict the scope thereof, and various changes and modifications
may be made in the invention without departing from the spirit and
scope thereof.
[0095] In each example, a steel with the chemical composition shown
in Table 1 was prepared by melting, and the resulting steel was
cast into a slab, which underwent hot rolling, followed by
pickling. Winding and pickling were performed under the conditions
shown in Tables 2 and 3. Pickling involved an aqueous solution of
hydrochloric acid (1 to 18 mass %) at 70 to 90.degree. C. Pickling
was followed by cold rolling, which gave a 1.4 mm thick steel
sheet.
[0096] The steel sheet underwent continuous annealing by either of
the processes shown in FIGS. 2 and 3. The process shown in FIG. 2
involves cooling with water quenching (WQ) that follows soaking and
slow cooling. The process shown in FIG. 3 involves cooling with
mist, gas blowing (GJ), or water-cooled roll quenching (RQ).
[0097] The heating temperature, slow cooling end temperature, and
tempering temperature shown in Tables 2 and 3 correspond to those
shown in FIGS. 2 and 3. The dew point is that of the atmosphere in
the continuous annealing furnace. After cooling, the steel sheet
underwent tempering. In the process shown FIG. 2, pickling was
carried out before and/or after tempering.
[0098] The thus obtained steel sheet was examined for mechanical
properties and coating film adhesion. All of the steel sheet
samples were found to be composed mainly of ferrite and tempered
martensite.
[0099] Mechanical properties were determined by measuring the
tensile strength (TS), total elongation (El), and yield ratio (YP)
of specimens (conforming to JIS No. 5) taken from the steel sheet.
A discoid specimen measuring 100 mm in diameter and 1.4 mm thick
was tested for stretch-flanging performance. The test method
consists of punching a hole (10 mm in diameter) in the specimen and
expanding the hole by a 60.degree. conical punch, with the burr
upward. The bore expanding ratio (.lambda.) was measured when the
conical punch passed through with cracking. (according to JFST 1001
provided by The Japan Iron and Steel Federation).
[0100] For evaluation of coating film adhesion, samples were
examined for chemical treatability and the presence of cracks in
the following manner. After chemical treatment, the surface of the
treated steel sheet was observed under an SEM with a magnification
of 1000 to confirm the presence of zinc phosphate crystals in ten
fields of observation. The result was rated according to the
following criterion.
[0101] .largecircle.: Zinc phosphate crystals are present uniformly
in all of ten fields of observation.
[0102] x: There is at least one field of observation in which zinc
phosphate crystals are not present.
[0103] Method for Chemical Treatment
[0104] Chemical treating solution: "Parbond L3020" from Nihon
Parkerizing Co., Ltd.
[0105] Process of chemical treatment: degreasing.fwdarw.water
washing.fwdarw.surface conditioning.fwdarw.chemical treatment
[0106] Method of Counting the Number of Si--Mn Oxide Particles
[0107] First, an extraction replica is prepared from the surface of
the steel sheet. Then, it is observed under a TEM (Model H-800 of
Hitachi, Ltd.) with a magnification of 15000. An average number of
particles (per 100 .mu.m.sup.2) is counted in arbitrary 20 fields
of observation.
[0108] The ratio of the surface area of steel sheet which is
covered by the oxides composed mainly of Si was obtained by
observation of a sample under a TEM and ensuing image analysis. The
sample was prepared by the extraction replica method consisting of
four steps (a) to (d) as explained in the following.
[0109] (a) Vacuum deposition of carbon on the surface of steel
sheet.
[0110] (b) Cross-cutting (2 to 3 mm square each) of the sample
surface.
[0111] (c) Corrosion with an etching solution composed of 10%
acetylacetone and 90% methanol. This corrosion brings carbon into
relief.
[0112] (d) Storing the sample in alcohol for observation.
[0113] The treated sample was photographed in ten fields of
observation (each measuring 13 by 11 cm) through a TEM with a
magnification of 15000. The resulting electron micrograph was
examined to measure the area covered by oxides composed mainly of
Si (or oxides in which Si as the constituents excluding oxygen
accounts for no less than 70% in atomic ratio). In this way there
was obtained the coverage of oxides composed mainly of Si.
[0114] The presence of cracks (width: no larger than 3 .mu.m,
depth: no smaller than 5 .mu.m) was examined by observing the cross
section of the surface layer of the steel sheet in ten fields of
observation (each measuring 13 by 11 cm) under an SEM (Model S-4500
of Hitachi Ltd.) with a magnification of 2000.
1 TABLE 1 Composition (mass %) Type C + Si/3O + of B N O P + 3S +
Mn/20 + steel C Si Mn P S Al Cr Mo Ti Nb V (ppm) (ppm) (ppm) Si/Mn
1.54C 2P + 4S 1 0.08 0.69 2.45 0.001 0.001 0.029 -- 0.2 -- -- -- --
18 21 0.282 0.13 0.23 2 0.08 1.01 3.11 0.008 0.003 0.033 -- -- --
-- -- -- 9 15 0.325 0.14 0.30 3 0.08 0.21 2.91 0.008 0.003 0.014 --
-- -- -- -- -- 18 31 0.072 0.14 0.26 4 0.08 0.59 2.99 0.007 0.003
0.015 -- -- -- -- -- -- 22 36 0.197 0.14 0.27 5 0.08 1.01 3.04
0.007 0.002 0.015 -- -- -- -- -- -- 23 32 0.332 0.13 0.29 6 0.10
0.47 1.61 0.011 0.002 0.024 -- 0.19 -- -- -- -- 30 28 0.292 0.17
0.23 7 0.10 0.66 2.10 0.013 0.003 0.026 -- 0.18 -- -- -- -- 28 23
0.314 0.18 0.27 8 0.13 0.19 2.68 0.010 0.011 0.019 -- -- 0.050 --
-- -- 34 24 0.071 0.22 0.30 9 0.08 0.73 2.39 0.006 0.002 0.047 --
0.2 -- 0.001 0.003 5 15 20 0.305 0.13 0.24 10 0.08 0.96 2.95 0.008
0.003 0.038 -- -- -- 0.003 0.005 6 20 23 0.325 0.14 0.29 11 0.05
1.02 2.98 0.003 0.005 0.066 -- -- -- -- -- -- 12 16 0.324 0.10 0.28
12 0.05 0.98 2.92 0.003 0.006 0.062 0.20 -- -- -- -- -- 15 15 0.336
0.10 0.27 13 0.05 0.98 2.87 0.002 0.007 0.060 0.40 -- -- -- -- -- 8
11 0.341 0.11 0.27 14 0.05 1.00 2.87 0.003 0.007 0.066 0.59 -- --
-- -- -- 13 12 0.341 0.11 0.28 15 0.05 1.00 3.05 0.011 0.007 0.064
0.80 -- -- -- -- -- 11 10 0.328 0.11 0.29 16 0.05 0.99 3.09 0.010
0.005 0.043 -- 0.19 -- -- -- -- 24 22 0.320 0.11 0.28 17 0.05 0.99
3.08 0.010 0.005 0.048 -- 0.47 -- -- -- -- 22 22 0.321 0.11 0.28 18
0.05 0.99 2.91 0.010 0.007 0.049 0.20 0.19 -- -- -- -- 22 23 0.340
0.11 0.28 19 0.05 0.98 2.85 0.010 0.006 0.044 0.21 0.47 -- -- -- --
21 21 0.343 0.11 0.27 20 0.11 1.01 2.93 0.011 0.007 0.092 -- -- --
-- -- -- 33 8 0.345 0.11 0.27 21 0.07 0.19 2.00 0.004 0.006 0.039
-- -- -- 0.020 -- -- 37 30 0.095 0.13 0.22 22 0.05 0.48 1.98 0.004
0.005 0.033 -- -- -- 0.022 -- -- 41 31 0.242 0.13 0.22 23 0.08 0.49
1.91 0.004 0.005 0.035 -- -- 0.010 0.020 -- -- 39 27 0.257 0.14
0.25 24 0.07 0.48 1.94 0.004 0.006 0.036 -- -- -- 0.035 -- -- 42 27
0.247 0.14 0.23 25 0.19 1.66 2.03 0.002 0.001 0.029 -- -- -- -- --
-- 22 29 0.818 0.30 0.35 26 0.10 0.82 1.86 0.010 0.002 0.067 -- --
0.066 -- -- -- 27 13 0.441 0.17 0.25 27 0.14 0.24 1.87 0.011 0.007
0.055 -- -- -- -- -- -- 30 17 0.128 0.25 0.29 28 0.11 1.05 2.31
0.004 0.002 0.037 -- -- 0.042 -- -- -- 28 26 0.455 0.18 0.28 29
0.10 0.01 2.57 0.005 0.003 0.038 0.10 0.09 0.008 0.008 -- -- 25 19
0.004 0.17 0.25 30 0.10 1.96 2.49 0.004 0.003 0.040 0.09 0.10 0.009
0.010 -- -- 23 20 0.787 0.17 0.31 31 0.10 0.65 1.55 0.004 0.003
0.039 0.10 0.09 0.007 0.008 -- -- 20 18 0.419 0.17 0.22 32 0.10
0.63 3.53 0.006 0.003 0.040 0.11 0.09 0.009 0.009 -- -- 26 18 0.178
0.17 0.32 33 0.16 0.63 2.59 0.011 0.005 0.057 0.021 -- -- -- 0.007
-- 17 20 0.243 0.27 0.35
[0115]
2 TABLE 2 Manufacturing conditions Surface oxides Slow Si--Mn Ex-
Wind- Pick- Cooling Tem- *1 peri- Steel ing ling Heating End pering
Dew Mechanical properties oxides Si *2 Film adhesion ment Kind
Temp. Time Temp. Temp. Cooling Temp. Point TS EL YP .lambda. (numb-
oxides Chemical No. No. (.degree. C.) (sec) (.degree. C.) (.degree.
C.) Method (.degree. C.) (.degree. C.) (MPa) (%) (MPa) (%) er) (%)
treatability Cracks 1 1 450 50 850 500 WQ 180 -50 985 17 639 42 20
0 .largecircle. none 2 2 450 50 850 440 WQ 200 -50 1007 18 697 52
41 0 .largecircle. none 3 3 500 50 830 500 WQ 200 -40 849 18 560 33
16 0 .largecircle. none 4 4 500 50 830 500 WQ 200 -40 984 16 679 36
26 0 .largecircle. none 5 5 500 50 830 500 WQ 200 -40 1017 16 651
28 43 3 .largecircle. none 6 6 500 50 830 450 WQ 180 -40 633 30 396
55 12 2 .largecircle. none 7 7 500 50 830 450 WQ 180 -40 802 23 529
42 24 3 .largecircle. none 8 8 500 50 830 450 WQ 250 -40 1203 12
866 31 13 0 .largecircle. none 9 9 500 50 830 450 WQ 180 -40 1004
17 664 43 18 3 .largecircle. none 10 10 500 50 830 500 WQ 200 -40
998 17 637 34 39 2 .largecircle. none 11 11 450 60 870 400 WQ 200
-50 835 22 502 35 35 2 .largecircle. none 12 12 450 60 870 420 WQ
200 -50 871 20 545 33 34 2 .largecircle. none 13 13 450 60 870 440
WQ 200 -50 902 19 599 30 35 4 .largecircle. none 14 14 450 60 870
460 WQ 200 -50 997 16 702 35 37 5 .largecircle. none 15 15 450 60
870 400 WQ 200 -50 1008 13 734 38 36 2 .largecircle. none 16 16 450
60 870 380 WQ 250 -50 981 16 667 45 35 3 .largecircle. none 17 17
450 60 870 380 WQ 250 -50 1012 12 784 40 35 3 .largecircle. none 18
18 450 60 870 380 WQ 180 -50 987 15 658 35 31 2 .largecircle. none
19 19 450 60 870 380 WQ 180 -50 1003 13 772 41 29 5 .largecircle.
none 20 20 450 60 870 450 WQ 400 -50 1052 14 910 35 38 4
.largecircle. none 21 21 400 40 850 520 WQ 400 -45 617 27 523 49 13
0 .largecircle. none 22 22 400 40 850 520 WQ 400 -45 608 28 499 58
18 0 .largecircle. none 23 23 400 40 850 520 WQ 400 -45 708 25 554
43 19 0 .largecircle. none *1 Number of particles of Si-Mn complex
oxides (smaller than 1.5 .mu.m) per 100 .mu.m.sup.2. *2 Ratio of
the surface area of steel sheet which is covered with oxides
composed mainly of Si.
[0116]
3 TABLE 3 Manufacturing conditions Surface oxides Slow Si--Mn Ex-
Wind- Pick- Cooling Tem- *1 peri- Steel ing ling Heating End pering
Dew Mechanical properties oxides Si *2 Film adhesion ment Kind
Temp. Time Temp. Temp. Cooling Temp. Point TS EL YP .lambda. (numb-
oxides Chemical No. No. (.degree. C.) (sec) (.degree. C.) (.degree.
C.) Method (.degree. C.) (.degree. C.) (MPa) (%) (MPa) (%) er) (%)
treatability Cracks 24 24 400 40 850 520 WQ 400 -45 611 26 527 45
20 0 .largecircle. none 25 1 450 50 850 500 RQ 180 -45 948 18 685
44 18 0 .largecircle. none 26 1 450 50 850 500 mist 180 -40 953 17
673 40 25 0 .largecircle. none 27 1 450 50 850 500 GJ 180 -40 902
19 667 48 22 0 .largecircle. none 28 33 480 50 850 450 WQ 200 -40
1285 13 882 25 21 0 .largecircle. none 29 25 550 50 880 630 WQ 230
-40 1006 19 649 26 8 55 X yes 30 26 600 50 880 630 WQ 400 -40 1215
11 972 28 9 25 X yes 31 28 600 40 850 650 mist 250 -40 990 17 599
18 8 33 X yes 32 27 650 50 850 650 GJ 500 -40 608 26 394 47 13 0
.largecircle. yes 33 1 450 50 850 500 WQ 180 0 972 16 659 38 5 5 X
yes 34 29 480 50 850 500 WQ 200 -45 1056 9 744 27 0 0 X none 35 30
480 50 850 500 WQ 200 -45 1227 11 937 25 7 40 X none 36 31 480 50
850 500 WQ 200 -45 735 20 558 42 9 20 X none 37 32 480 50 850 500
WQ 200 -45 1327 8 1002 19 30 0 .largecircle. none 38 5 700 50 830
500 WQ 200 -40 992 16 631 30 45 8 .largecircle. yes 39 5 500 5 830
500 WQ 200 -40 1021 15 660 27 9 12 X yes 40 5 500 50 830 700 WQ 200
-40 1217 11 1010 48 48 5 .largecircle. yes 41 5 500 50 830 500 WQ
200 -10 1007 16 654 33 8 15 X yes *1 Number of particles of Si-Mn
complex oxides (smaller than 1.5 .mu.m) per 100 .mu.m.sup.2. *2
Ratio of the surface area of steel sheet which is covered with
oxides composed mainly of Si.
[0117] The results shown in Tables 1 to 3 are discussed in the
following. (No. means Experiment No.)
[0118] Samples in Nos. 32, 38, and 40 meet the requirement for the
steel sheet 1 of the present invention and hence they are excellent
in chemical treatability and coating film adhesion. The results
suggest that it is necessary to control the winding temperature and
slow cooling end temperature for the steel sheet to have good
coating film adhesion, with cracks property controlled.
[0119] Samples in Nos. 34 to 36 meet the requirement for the steel
sheet 2 of the present invention and hence they are free of cracks
and excellent in coating film adhesion. The results suggest that it
is necessary to control the composition and the shape of the oxides
that separate out on the surface of the steel sheet for the steel
sheet to have good chemical treatability and coating film
adhesion.
[0120] By contrast, samples in Nos. 29, 30, 31, 33, 39, and 41 do
not meet the requirement for the steel sheets (1 to 3) of the
present invention and hence they are poor in coating film adhesion.
In other words, samples in Nos. 29 to 31 do not meet the
requirement for [Si]/[Mn] ratio and hence they do not give oxides
having the shape specified in the present invention. Moreover, they
have many cracks (because they are not produced under the desired
conditions) and they are poor in coating film adhesion.
[0121] Samples in Nos. 33, 39, and 41 are not produced under the
desirable conditions and hence they do not have oxides with the
shape specified in the present invention. They have cracks and are
poor in coating film adhesion.
[0122] Sample in No. 37 meets the requirements and hence is
excellent in coating film adhesion; but it cannot be formed
satisfactorily on account of its poor ductility.
[0123] Samples in Nos. 1 to 27 meet the requirement for the steel
sheet 3 of the present invention (or the requirements for the steel
sheets (1 and 2) of the present invention) and they also satisfy
the equations (2) and (3) and hence they are excellent in chemical
treatability, coating film adhesion (free of cracks), and
weldability.
[0124] Sample in No. 28 meets the requirements for the steel sheet
3 of the present invention; however, the results suggest that the
composition should satisfy the equations (2) and (3) for the steel
sheet to exhibit good weldability.
[0125] Samples in Nos. 1, 29, and 34 gave the extraction replicas
whose electron micrographs (by observation under a TEM) are shown
in FIGS. 4 to 6. FIG. 4 indicates that sample in No. 1 has fine
Si--Mn complex oxides but does not have oxides composed mainly of
Si. FIG. 5 indicates that sample in No. 29 is covered with oxides
composed mainly of Si. FIG. 6 indicates that sample in No. 34 does
not have fine Si--Mn complex oxides although it has particulate
matter (which is rust).
[0126] Samples in Nos. 1, 29, and 34 have the cross section near
the surface of the steel sheet whose electron micrographs (by
observation under an SEM) are shown in FIGS. 7 to 9. FIG. 7
indicates that sample in No. 1 is free from cracks. FIG. 8
indicates that sample in No. 29 has cracks, 5 .mu.m deep. FIG. 9
indicates that sample in No. 34 is free of cracks and hence
excellent in coating film adhesion.
[0127] Samples in Nos. 1, 29, and 34 have the surface texture whose
electron micrographs (by observation under an SEM) are shown in
FIGS. 10 to 12. FIG. 10 indicates that sample in No. 1 has fine
zinc phosphate crystals free of interstice. FIG. 11 indicates that
sample in No. 29 has small zinc phosphate crystals with large
interstices. FIG. 12 indicates that sample in No. 34 has large zinc
phosphate crystals with large interstices.
* * * * *