Technical Field
[0001] The present invention relates to a zinc and zinc-alloy hot-dip-coated steel sheet
having a decreased number of bare spots and excellent coating adhesion, and a method
for manufacturing the same.
Background Art
[0002] Zinc and zinc-alloy hot-dip-coated steel sheets are mainly used for automobile bodies
because of low cost and excellent corrosion resistance, and in addition to the corrosion
resistance due to coating, coating adhesion during press working is required for applying
the steel sheets to automobile bodies. When coating adhesion deteriorates, coated
layers peel as a powder or blocks, which phenomenon sometimes causes galling in press
forming or deteriorates corrosion resistance of the portions from which the coated
layer peels; and also, peeled fragments disadvantageously inflict the steel sheet.
[0003] As a conventional technique for improving coating adhesion, Japanese Patent Laid-Open
No. 61-276961 discloses a technique in which alloying Fe with Zn at a high temperature
ranging from 700 to 850°C is performed after zinc hot-dip-coating. However, alloying
at a high temperature lead to not only higher costs but also increased expenses for
equipment such as rolls.
[0004] Additionally, in Japanese Patent Laid-Open No. 3-232926, steel contains at least
one of Zr, La, Ce, Y, and Ca, and the cooling rate from recrystallization annealing
to coating is set to not less than 50°C/sec. The cost is raised due to the addition
of Zr or the like to steel and productivity deteriorates because the sheet-feeding
rate has to be lowered due to the cooling capacity.
[0005] Furthermore, in Japanese Patent Laid-Open No. 2-163356, the O, Al, and N contents
in steel are set to not more than 0.0045 wt%, (25 x N wt%) to 0.15 wt%, and not more
than 0.0030%, respectively. Moreover, restrictions on the Ti, Si, and P contents,
and Si (wt%) + P (wt%) ≥ Ti (wt%) must be satisfied according to Japanese Patent Laid-Open
No. 6-81101. Anyway, the desired steel-sheet properties such as strength and drawing
cannot be always achieved by such content restrictions, and there is a possibility
that coating adhesion will deteriorate because of deviations from a predetermined
composition range.
[0006] In Japanese Patent Laid-Open No. 4-333552, coating adhesion is improved by carrying
out Ni pre-plating before galvanizing. However, in general, a continuous galvanizing
line (hereinafter referred to as "CGL" ) does not have such equipment, and a large
investment is required for improving equipment or the like.
[0007] Meanwhile, automobile bodies are required to be lighter because of recent regulations
for exhaust gas. Thinning the steel sheets is a method for lightening the automobile
bodies. According to this method, it is necessary for ensuring safety to increase
steel-sheet strength corresponding to the decreased thickness. Thus, high tensile-strength
steel sheets have been developed for strengthening the steel sheets by increasing
the steel contents of elements such as Si, Mn, and P. Since steel sheets for automobiles
are subjected to press forming, excellent material characteristics with a high r-value
(high Lankford value) are required, and in particular, the addition of these elements
is essential for high-tensile strength steel sheets.
[0008] In the case of zinc hot-dip-coating such steel sheets, recrystallization annealing
at a high temperature ranging from approximately 700 to 900°C is necessary to attain
excellent material characteristics. In the CGL, recrystallization annealing is generally
carried out under a nitrogen atmosphere in the presence of hydrogen (hereinafter referred
to as reduction annealing), and although this atmosphere is a reducing atmosphere
for Fe, it is an oxidizing atmosphere for some elements such as Si, Mn, and P. Thus,
elements such as Si, Mn, and P (referred to as readily oxidizable elements) which
are more oxidizable than Fe externally diffuse during reduction annealing and bond
to oxygen on the surface of steel sheets to form oxides (called as "surface segregated
layer"). Since these oxides significantly impede wettability between molten zinc and
the steel sheets, so-called bare spots, i. e., defects occurring when zinc does not
adhere to the steel sheets, are seen.
[0009] For overcoming such problems, Japanese Patent Examined Publication No. 61-9386 proposes
a method of pre-plating the surface of steel sheets with Ni before the zinc hot-dip-coating
process. However, according to this method, when steel contains at least Si and one
more element among 0.2 to 2.0 wt% of Si, 0.5 to 2.0 wt% of Mn, and 0.1 to 20 wt% of
Cr, Ni plating of not less than 10 g/m
2 is necessary, resulting in an increased cost. In addition, although such a large
quantity of Ni plating improves the wettability between the zinc hot-dip-coating and
the steel sheet, disadvantageously, defects caused by Si and Ni on the coated surface
frequently appear during the alloying process.
[0010] Furthermore, for example, Japanese Patent Laid-Open No. 57-70268 proposes a method
of pre-plating the surface of steel sheets with Fe before the zinc hot-dip-coating
process. According to this method, bare spots in Si-containing steel are preventable
by pre-plating, however, not less than 5 g/m
2 of Fe plating is required, which fact is extremely uneconomical.
[0011] Additionally, other methods are disclosed in Japanese Patent Laid-Open Nos. 55-122865
and 4-254531. In these methods, steel sheets are oxidized beforehand to form a Fe
oxide film on their surface and then subjected to reduction annealing. However, according
to these methods, alloy elements such as Si are segregated on the surface to form
an oxide film because of excess reducing during reduction annealing, causing a problem
of inferior coating. For preventing such excess reducing, a large amount of Fe oxide
is necessary. However, if the amount of Fe oxide film is exceedingly large, the Fe
oxide film peels due to rolling or the like, thus on the contrary, a surface segregated
layer is produced and results in impeded coating or adverse effects on operation because
the fragments of the peeled Fe oxide-film are scattered inside a furnace.
[0012] In addition, concerning known proposals for the steel composition and hot-rolling
conditions for zinc hot-dip-coating the high-tensile steel sheets, Japanese Patent
Laid-Open No. 6-158172 discloses a method in which a steel containing Si≤0.2 and Mn≤1.5
by wt% is wound at a temperature not less than 650°C followed by acid washing, cold-rolling,
annealing, and zinc hot-dip-coating; and Japanese Patent Laid-Open No. 6-179943 discloses
a method in which a steel containing 0.10 to 1.5 wt% of Si and 1.00 to 3.5 wt% of
Mn is wound at a temperature ranging from 500°C to 680°C, both inclusive, followed
by acid washing, cold-rolling, annealing, and zinc hot-dip-coating.
[0013] Although these methods give specified processing conditions, such as the steel composition
and hot-rolling conditions, for a series of manufacturing steps, they cannot suppress
the surface segregated layer formed during reduction annealing or improve bare spots
or coating adhesion.
Disclosure of the Invention
[0014] As a result of detailed experiments, the inventors of the present invention have
found that bare spots and coating adhesion are remarkably improved by providing oxides
of readily oxidizable elements just under a coated layer of a zinc and zinc-alloy
hot-dip-coated steel sheet.
[0015] In other words, the present invention provides a zinc and zinc-alloy hot-dip-coated
steel sheet having oxides of readily oxidizable elements just under a coated layer.
[0016] Moreover, in the zinc and zinc-alloy hot-dip-coated steel sheet, the oxygen concentration
is preferably not less than 1 ppm, more preferably, 2 to 200 ppm, and further more
preferably, 3 to 100 ppm, in a region of from the surface layer of a steel-sheet substrate
just under the coated layer to 3 µm deep in the sheet-thickness direction.
[0017] In addition, such hot-dip-coated steel sheets are preferably further subjected to
heat-alloying after zinc hot-dip-coating, and excellent alloyed zinc and zinc-alloy
hot-dip-coated steel sheets are thereby obtained. Also in the alloyed zinc and zinc-alloy
hot-dip-coated steel sheets, the oxygen concentration is preferably not less than
1 ppm, more preferably, 2 to 200 ppm, and further more preferably, 3 to 100 ppm, in
a region of from the surface layer of a steel-sheet substrate just under the coated
layer to 3 µm deep in the sheet-thickness direction.
[0018] Furthermore, each of the zinc and zinc-alloy hot-dip-coated steel sheets and alloyed
zinc and zinc-alloy hot-dip-coated steel sheets is preferably contains at least one
element selected from the group consisting of Si, Mn, and P as a steel component in
the following ranges:
0.001 ≤ Si ≤ 3.0 Wt%
0.05 ≤ Mn ≤ 2.0 Wt%
0.005 ≤ P ≤ 0.2 Wt%
[0019] Additionally, the present invention provides a method for producing the above-mentioned
zinc and zinc-alloy hot-dip-coated steel sheets or the alloyed zinc-alloy hot-dip-coated
steel sheets both of which show a decreased number of base spots and excellent coating
adhesion. In other words, the present invention provides a method having:
a step A for forming oxides just under the scale, which oxides are formed from elements
more oxidizable than iron, by setting a temperature of a steel strip to not less than
600°C and setting the mean slow-cooling rate up to 540°C to not more than (CT - 540)0.9 ÷ 40 (°C/min) during coiling the steel strip hot-rolled; and
a step B for zinc and zinc-alloy hot-dip-coating the steel strip. According to this
method, the step B follows the step A, and other steps may be also employed between
the steps A and B. In general, steps of pickling, degreasing, cold rolling, annealing,
and the like may appropriately be used as such intermediate steps.
[0020] In addition, according to a method of the present invention, the oxides formed in
the step A preferably remain after a pre-treatment step carried out after the step
A until treatment conducted in an annealing furnace immediately before the step B.
[0021] Furthermore, according to these methods, a slab subjected to hot rolling preferably
contains at least one element selected from the group consisting of Si, Mn, and P
as a steel component in the following ranges:
0.001 ≤ Si ≤ 3.0 Wt%
0.05 ≤ Mn ≤ 2.0 Wt%
0.005 ≤ P ≤ 0.2 Wt%
[0022] Moreover, according to any of the above-mentioned methods, an alloyed zinc and zinc-alloy
hot-dip-coated steel sheet can be produced by employing heat-alloying treatment after
the step B.
[0023] Next, oxides of readily oxidizable elements positioned just under a coated layer
will be explained.
[0024] These oxides of readily oxidizable elements are formed during hot-rolling, in particular,
the oxides are grown when the temperature (hereinafter referred to as "CT" ) during
coiling is high and the cooling rate after coiling is low.
[0025] The oxides formed during hot-rolling are observed just under the scale, as is shown
in figure 6. Meanwhile, in a conventional hot-rolled sheet, no oxide is observed just
under the scale, as is shown in figure 7. The oxides observed during hot-rolling are
analyzed by using an electron probe microanalyzer (hereinafter referred to as "EPMA"
) and the results are shown in figure 1. Since Mn, P, Al, and O show peaks, it is
understood that oxides of these elements are formed. Steel sheets shown in figures
6 and 1 contain 0.1 wt% of Mn, 0.006 wt% of P, and 0.03 wt% of Al, and they do not
contain a particularly large amount of Mn, P, or Al.
[0026] The oxides positioned just under a coated layer of a zinc hot-dip-coated steel sheet
or an alloyed zinc hot-dip-coated steel sheet of the present invention are produced
such that oxides formed just under the scale during the hot-rolling process remain
even after post-treatment steps such as pickling and coating.
[0027] The mechanism of producing oxides just under the scale is as follows: oxygen in a
scale layer essentially consisting of iron oxide which has been formed during hot-rolling
internally diffuses into steel during or after the coiling process, and then, forms
an oxide of a readily oxidizable element in the steel. Therefore, oxides are produced
even when only a trace amount of readily oxidizable elements is contained in the steel.
[0028] Although oxides of elements more oxidizable than iron exist just under the zinc and
zinc-alloy hot-dip-coating according to the present invention, an oxide of an element
less oxidizable than iron oxide or iron may also be contained. In addition, such an
oxide is preferably formed in grain boundaries of a hot-rolled steel sheet.
[0029] As a result of studies and investigations conducted on various types of steel sheets,
the inventors of the present invention have found oxides of Si-O, Mn-O, Al-O, P-O,
and Fe-Si-O in the steel sheets.
[0030] Figure 2 shows the result of elemental analysis of a conventional steel sheet and
figure 3 shows that of an unannealed cold-rolled steel sheet wherein oxides were observed,
which analysis was carried out in a region of from the surface of each steel sheet
to approximately 10 µm in the depth direction by glow-discharge spectroscopy (hereinafter
referred to as "GDS"). The peaks of Mn, Al, P, and O observed at the depth of approximately
0.3 to 4 µm from the surface layer correspond to the oxides.
[0031] Figure 4 shows the result of elemental analysis of a conventional steel sheet and
figure 5 shows that of an annealed cold-rolled steel sheet wherein oxides were observed,
which analysis was carried out by GDS in a region of from the surface of each steel
sheet to approximately 10 µm in the depth direction. A large amount of surface segregated
substances generated by reduction annealing is observed in the conventional steel
sheet of figure 4, meanwhile the generation of surface segregation products is suppressed
and hardly observed in the steel sheet with oxides produced during hot-rolling.
[0032] Next, oxides of the present invention which exist in a surface layer of a steel sheet
(surface layer of a steel-sheet substrate) just under a coated layer can optical-microscopically
be observed by etching the steel sheet with a 1% nital solution for several to several
dozen of seconds.
[0033] Figure 8 (photograph) and figure 9 (photograph) show a conventional alloyed zinc
hot-dip-coated steel sheet not containing oxide and an alloyed zinc hot-dip-coated
steel sheet containing oxides incorporated in the present invention, respectively.
Figures 8 and 9 are cross-sectional optical micrographs of alloyed zinc hot-dip-coated
steel sheets taken at a magnification of x1,000. Black ribbon-like materials observed
just under the coated layer are oxides (shown by arrows).
[0034] In addition, the formation of oxides can also be confirmed by analyzing oxygen contained
in steel. Concerning technique, oxygen in steel is analyzed in the total sheet-thickness
direction using a hot-rolled steel sheet whose scale layer has been removed by pickling
after coiling, a steel sheet obtained by dissolving only a coated layer of a zinc
and zinc-alloy hot-dip-coated steel sheet, an unannealed cold-rolled steel sheet,
or an annealed steel sheet, and the resulting values are compared with those of steel
sheets obtained by grinding the surface layer in which oxides are formed. The steel
sheets in which oxides are formed have larger oxygen values analyzed in the total
sheet-thickness direction as compared with those of the ground sheets.
[0035] Next, the mechanism of improving bare spots and coating adhesion by forming oxides
just under a coated layer will be investigated.
[0036] First, concerning improvement in bare spots, it was found that surface segregation
of readily oxidizable elements is suppressed during reduction annealing in the CGL
when oxides are produced just under the scale by internal oxygen diffusion during
or after coiling, as is above-mentioned.
[0037] This phenomenon is assumed to be due to following: the amount of readily oxidizable
elements in the surface layer decreases because the readily oxidizable elements already
precipitate as oxides during or after coiling; the formed oxides impede transfer (external
diffusion) of the readily oxidizable elements from bulk steel to the steel sheet surface;
and oxidation-reduction occurs inside the steel sheet, in other words, a Fe-containing
oxide produced during or after coiling changes to an oxide of readily oxidizable element
during reduction annealing.
[0038] Therefore, the surface segregated substances of the readily oxidizable elements,
which substances impede wettability between molten zinc and the steel sheet, extremely
decrease, thereby remarkably improving bare spots.
[0039] Next, coating adhesion will be explained.
[0040] It has been known that coating peels mainly due to compressive stress during press
forming.
[0041] Since a steel sheet having oxides just under a coated layer, i. e. a steel sheet
of the present invention, has spaces between oxide crystals, zinc more readily penetrates
into the steel sheet as compared with conventional steel sheets not containing oxides.
As a result, the interface between the coated layer and the steel sheet is significantly
roughened so that the coated layer can tightly adhere to the steel sheet. As a result,
a zinc hot-dip-coated steel sheet and an alloyed zinc hot-dip-coated steel sheet both
incorporated in the present invention acquire excellent coating adhesion during press
forming.
[0042] Figures 10 and 11 show the observation results obtained from a steel sheet using
a SEM, a coated layer of which steel sheet has been forcibly dissolved to the iron
potential according to a galvanostatic process (4% methyl salicylate, 1% salicylic
acid, and 10% potassium iodide/methanol solution; 5 mA/cm
2) so as to expose the steel sheet. It is understood that the interface between the
coated layer and the steel sheet is apparently more roughened as compared with the
conventional steel sheet not containing oxides.
[0043] In addition, the technique disclosed by the present invention exhibits more excellent
effects when a steel sheet contains at least one component selected from the group
consisting of Si, Mn, and P as a steel component in the following ranges:
0.001 ≤ Si ≤ 3.0 Wt%
0.05 ≤ Mn ≤ 2.0 Wt%
0.005 ≤ P ≤ 0.2 Wt%
Problems such as bare spots and decreased coating adhesion hardly occur in steel
sheets not containing the above elements, thus the lower limits for these elements
are preferably 0.001 wt% for Si, 0.05 wt% for Mn, and 0.005 wt% for P. Meanwhile,
the upper limit for each element is determined considering the preferable ranges for
both the maximum effect for strengthening and cost.
[0044] Furthermore, the technique disclosed by the present invention exhibits sufficient
effects on both bare spots and coating adhesion when even a small amount of oxides
is observed by an optical microscope in a cross-section of a zinc and zinc-alloy hot-dip-coated
steel sheet etched by 1% nital.
[0045] In addition, according to an oxygen analysis of steel, sufficient effects are shown,
particularly on bare spots and coating adhesion when the value of the following formula
is not less than 1 ppm:

[0046] Next, a technique for manufacturing the above-described coated steel sheets will
be disclosed. It is required that the temperature for coiling after hot-rolling be
high and cooling after the coiling be slow, and a detailed explanation will be given
below.
[0047] The temperature for coiling after hot-rolling must be 600°C or more to produce oxides
and the cooling rate up to 540°C after coiling must be not more than the following:

Oxides are not formed at not more than 540°C even when slow-cooling is further carried
out.
[0048] In addition, although pickling and/or grinding is generally carried out to remove
the scale before coating, and sometimes, equipment for electrolytic degreasing or
pickling is also provided for the CGL inlet side, the oxides produced in the surface
layer of a steel sheet during or after coiling in the hot-rolling process must remain
after the above treatment.
[0049] Zinc and zinc-alloy hot-dip-coating of the present invention is a general term for
molten zinc containing zinc and may include not only zinc hot-dip-coating but also
galfan and galvalume, in both of which Si is contained in zinc. Moreover, Pb, Mg,
Mn, etc. may be further contained. Therefore, conditions for a zinc bath are not particularly
restricted.
[0050] Other conditions for the coated layer are not particularly limited, however, considering
corrosion resistance and the like, the preferred amount of zinc and zinc-alloy coating
is approximately 25 to 90 g/m
2 and the preferred iron content in a coated layer in an alloyed zinc hot-dip-coated
steel sheet is 8 to 13 wt%.
[0051] Furthermore, both hot-rolled steel sheets and cold-rolled steel sheets can be used
as a material for coating.
Brief Description of Drawings
[0052]
[Figure 1] An EPMA analysis chart of oxides observed just under the scale during hot-rolling.
[Figure 2] A graph showing the result of elemental analysis of a conventional unannealed
cold-rolled steel sheet, which analysis was carried out by GDS from the surface to
approximately 10 µm in the depth direction.
[Figure 3] A graph showing the result of elemental analysis of an unannealed cold-rolled
steel sheet of the present invention, which analysis was carried out by GDS from the
surface to approximately 10 µm in the depth direction.
[Figure 4] A graph showing the result of elemental analysis of a conventional annealed
cold-rolled steel sheet, which analysis was carried out by GDS from the surface to
approximately 10 µm in the depth direction.
[Figure 5] A graph showing the result of elemental analysis of an annealed cold-rolled
steel sheet of the present invention, which analysis was carried out by GDS from the
surface to approximately 10 µm in the depth direction.
[Figure 6] A cross-sectional optical micrograph, taken at a magnification of x1,000,
showing oxides positioned just under the scale of a hot-rolled sheet of an example.
[Figure 7] A cross-sectional optical micrograph, taken at a magnification of x1,000,
showing oxides positioned just under the scale of a conventional hot-rolled sheet.
[Figure 8] A cross-sectional optical micrograph, taken at a magnification of x1,000,
showing an example alloyed zinc hot-dip-coated steel sheet containing oxides.
[Figure 9] A cross-sectional optical micrograph, taken at a magnification of x1,000,
showing a conventional alloyed zinc hot-dip-coated steel sheet not containing oxides.
[Figure 10] A SEM photograph, taken at a magnification of x1,500, showing an example
steel sheet whose coated layer has been dissolved.
[Figure 11] A SEM photograph, taken at a magnification of x1,500, showing a conventional
steel sheet whose coated layer has been dissolved.
[Reference Numerals]
[0053] 1, untreated steel-sheet portion; 2, scale; 3, oxides; 4, a coated layer; and 5,
oxides.
Best Mode for Carrying Out the Invention
[0054] An example of the present invention will be shown below:
[0055] Each sample shown in Table 1 was melted by a converter and formed into a slab by
continuous casting. Each of the resulting slabs was hot-rolled to 1.2 to 3.5 mm thick
at a slab-heating temperature of 1150 to 1200°C, and with a finishing temperature
of 900 to 920°C, and a coiling temperature and a cooling rate which are shown in Table
2. After that, the resulting sheets were pickled for 5 to 15 seconds at 80°C in an
aqueous 5% HCl solution to remove scale layers, and then, divided into two groups
one of which was directly subjected to the CGL and the other was cold-rolled into
0.7 mm thick. Furthermore, in the CGL inlet side, the following methods were also
used in combination as a pre-treatment for removing the surface layer of a steel sheet,
if required.
[0056] Electrolytic degreasing: electrolysis at 60°C in an aqueous 3% NaOH solution for
approximately 10 seconds.
[0057] Pickling: pickling at 60°C in an aqueous 5% HCl solution for approximately 3 seconds.
[0058] Brushing roll: a brushing roll with abrasive grains.
[0059] In the CGL, both the hot-rolled sheet and the cold-rolled sheet were zinc hot-dip-coated
at 470°C after annealing at 800 to 850°C. In addition, alloyed zinc hot-dip-coated
steel sheets were obtained by successively subjecting the annealed sheets to an alloying
process conducted at 480 to 530°C for 15 to 30 seconds.
o Evaluation method for oxide
Observation method for oxides in hot-rolled sheets
[0060] A cross section of each hot-rolled sheet with the scale was ground and, without being
etched, subjected to optical-microscopic observation so as to measure the depth of
oxide invasion. The preferred magnification of the optical microscopy was 1,000.
Quantitative evaluation of oxides in hot-rolled sheets
[0061] The following value was obtained:

Quantitative evaluation of oxides in coated sheets
[0062] Each of the coated sheets was immersed in the solutions shown below until the end
of the dissolving reaction of coating and then the concentration of the oxide-derived
oxygen in a region of from the surface of the steel sheet to 3 µm in the sheet-thickness
direction was calculated according to the following formula:

o Evaluation method for bare-spots
[0063] Each of the coated sheets was evaluated by macroscopic observation.
- Bare spots
- not observed: rank 1
a few were observed: rank 2
a small number were observed: rank 3
observed: rank 4
o Test for coating-adhesion evaluation
[0064] Each of the coated sheets was subjected to a Dupont impact test using a 1/2-inch
punch and occurrence of peeling was confirmed by macroscopic observation.
Peeling was not observed: o
Peeling was observed: x
[0065] Each alloyed zinc alloy hot-dip-coated steel sheet was bent to 90°, bent back, and
then the compressed side of the steel sheet was peeled by a tape so as to measure
the peeled amount of zinc by fluorescent X ray.
[0066] Count number of:
less than 500: rank 1 (good)
not less than 500 to less than 1,000: rank 2
not less than 1,000 to less than 2,000: rank 3
not less than 2,000 to less than 3,000: rank 4
not less than 3,000 : rank 5
[0067] Table 3 shows the results of the zinc hot-dip-coated steel sheet and Table 4 shows
those of the alloyed zinc hot-dip-coated steel sheets.
Table 1
Sample-Steel Composition |
Symbol |
C wt% |
Si wt% |
Mn wt% |
P wt% |
A |
0.105 |
0.010 |
0.08 |
0.008 |
B |
0.070 |
0.10 |
0.10 |
0.01 |
C |
0.070 |
0.50 |
2.0 |
0.07 |
D |
0.010 |
1.50 |
0.10 |
0.05 |
E |
0.003 |
0.003 |
0.05 |
0.005 |
F |
0.003 |
0.01 |
0.20 |
0.01 |
G |
0.003 |
0.30 |
0.50 |
0.04 |
H |
0.003 |
0.05 |
1.95 |
0.20 |
Table 2
Coiling conditions, depth of oxide invasion into hot-rolled sheet, and oxide amount
in hot-rolled sheet |
Sample steel |
CT °C |
Mean cooling rate to 540°C °C/min |
Depth of oxide invasion into hot-rolled sheet µm |
Oxide amount in hot-rolled sheet ppm |
Sample steel No. |
A |
540 |
1.0 |
0 |
0 |
1 |
A |
600 |
1.0 |
1 |
1 |
2 |
A |
600 |
1.5 |
0 |
<1 |
3 |
A |
700 |
2.0 |
7 |
5 |
4 |
B |
650 |
1.5 |
8 |
8 |
5 |
C |
650 |
1.5 |
6 |
7 |
6 |
D |
580 |
1.0 |
0 |
0 |
7 |
D |
620 |
1.2 |
<1 |
<1 |
8 |
E |
650 |
1.2 |
5 |
5 |
9 |
E |
650 |
1.6 |
<1 |
1 |
10 |
E |
650 |
1.8 |
0 |
<1 |
11 |
F |
650 |
1.0 |
10 |
11 |
12 |
G |
650 |
1.0 |
12 |
15 |
13 |
H |
600 |
1.8 |
0 |
0 |
14 |
H |
650 |
1.0 |
12 |
18 |
15 |

Industrial availability
[0068] The technique disclosed by the present invention relates to a zinc hot-dip-coated
steel sheet and an alloyed zinc hot-dip-coated steel sheet showing a decreased number
of bare spots and excellent coating adhesion, and are appropriately used mainly for
steel sheets of automobile bodies.