Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
Title of the Invention
Process for preparing alloyed-zinc-plated titanium-killed steel
sheet having excellent deep-drawability
Field of the Invention
S This invention relates to a process for preparing alloyed-
zinc(Zn)-plated titanium(Ti)-killed steel sheet having excellent deep-
drawability (good workability and powdering resistance) by vacuum
vapor deposition.
Background of the Invention
An alloyed plated steel sheet such as an alloyed-Zn-plated steel
sheet is better than ordinary (Zn-)plated steel sheet in that
continuous spot-welding can be easily carried out, the adhesion of
electrophoretic coating film is good, the corrosion resistance is
better, etc. and, therefore, sheet of this type is being widely used
15- in the automobile industry and other fields.
Conventionally, alloyed-Zn-plated steel sheet is manufactured by
heat-treating Zn-plated steel sheet prepared by hot-dip plating or
electrolytic plating. However, hot-dip plating is not suitable for
thin coating of less than 30g/ml and one side plating, and hot-dip-
plated steel sheet is inferior in uniformity in thickness of the
plating layer in the londitudinal direction and the transverse
direction. Electrolytic plating is not suitable for plating of ~'
thicker than 50g/m~ because the plating cost rises steeply with the
~ increase in coating weight. In contrast, the vacuum vapor deposition
-~ 25 process is advantageous in that the coating weight can be relatively
easily controlled and uniform thickness is easily achieved.
- Therefore, processes for preparing alloyed Zn-plated steel sheet
utilizing vacuum vapor deposition have been proposed and being
` practiced (JP-A-61-195965 for instance). According to this process,
cold-rolled steel coil is plated with Zn on one or both sides by
vacuum vapor deposition and, thereafter, is heat-treated for alloying
at 250-350~ for 1-15 hours in a non-oxidizing or weakly reducing
atmosphere in a batch annealing furnace.
In the meanwhile, for manufacturing steel sheets for deep-
drawing, aluminumtAl)-killed steel and Ti-killed steel are used.
; With Al-killed steel sheet, alloyed-Zn-plated steel sheet can be
manufactured by vacuum vapor deposition and heat treatment in
accordance with the above-described known process. ~ith Ti-killed
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steel sheet, however, there is a problem that the formed alloy layer
is inferior in powdering resistance and easily scales off.
For ordinary deep drawing purposes, Al-killed steel sheet
suffices. Recently, however, more and more complicated shaping is
being required. For such purposes, Ti-killed steel, which has better
workability and formability, must be used. Therefore, alloyed-Zn-
plated~steel sheet made of Ti-killed steel sheet also must be
provided with good powdering resistance.
~e have conducted studies to determine the cause of the inferior
powdering resistance of alloyed-Zn-plated steel sheet made of Ti-
killed steel sheet focusing our attention on the structure of alloyed
plating layer. As a result of our studies, we have found that in the
case of Ti-killed steel, very brittle intermetallic compounds of
iron(Fe) and Zn are formed when the Zn-plated steel sheet is heated
lS to temperatures outside of a specific temperature range and that this
specific temperature range differs depending upon the Ti content.
That is, the temperature is in excess of 320~ when the Ti content is
0.05~ and in excess of 260~ when the Ti content is 0.3%. Further,
once a brittle intermetallic compound layer is formed, deterioration
of powdering resistance is inevitable irrespective of the alloying
conditions uner which the plated steel sheet is treated. Thus we
have found that the aforesaid problem can be solved by maintaining
- the substrate Ti-killed steel sheet at a low temperature set relative
to the Ti content during the vacuum vapor deposition and heating the
plated steel sheet in a non-oxidizing atmosphere, preferably in a
batch heat treatment annealing furnace, at a temperature within the
temperature range in which the Fe content of the formed alloyed layer
is wisthin a prescribed range.
~` Summary of the Inven~ion
This invention provides a process for preparing alloyed-zinc-
plated steel she~t having excellent deep drawability, which comprises
adjusting the temperature of substrate steel sheet of titanium-killed '~`~
;~ steel essentially consisting of C~0.01~, Si<0.15%, Mn: 0.15-0.85%,
Ti: 0.05-0.30$, P~0.02%, S50.02~, Al50.05~ and balance of Fe, to a
temperature within a range Tt~) defined as 180~T~-240X~292, wherein
~(~) is the Ti content of the substrate steel sheet, subjecting said
steel sheet to Zn-plating by vacuum vapor deposition, and thereafter
maintaining the plated steel sheet for alloying the plating zinc
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layer and the substrate steel at 220-320~ for 1-50 hours in
accordance with Ti content.
Throughout the specification, percent(%) means that by weight.
Ti-killed steel for manufacturing steel sheet suitable for deep
drawing is a low carbon(C) steel, the C content of which is usually
not more than 0.01%. The silicon(Si) content should preferably be
less than 0.15~ bevcause with a Si content in excess of 0.15~ the
adhesion of plating layer is unsatisfactory. Manganese(Mn), an
element which enhances the strength of steels, should preferably be
contained in the range of 0.15-0.85%. When the Mn content is less
than 0.15~, satisfactory strength is not obtained. On the other
hand, the strength of steels is not further improved by a Mn content
in excess of 0.85%. The Ti content in Ti-killed steel for manufac-
turing sheet for deep drawing is usually 0.05-0.30~. Ti is an
element which fixes C in steel and improves ~orkability of the steel
and is thought to be required in an amount of at least about four
times the content of C. This element also fixes nitrogen(~) which is
an impurity in steel. Therefore, Ti is contained in an amount in
excess of 0.05% or more in consideration of the N content. On the
other hand, no advantage commensurate with the rise in the manufactur-
ing cost is obtained if the Ti content is in excess of 0.3~.
As impurities, phosphorus(P), sulfur(S) and Al may be contained
respectively in an amount of 50.02%, S0.02~ and S0.05%. These
amounts are the same as the ordinary impurity contents of plain
carbon steels.
The above-described Ti-killed steel sheet is maintained at a
temperature T ~ defined as 180~TS-240xW+292 in accordance with the
Ti content W(%) and then subjected to plating with Zn by vacuum vapor
deposition.
In vacuum vapor deposition plating, Zn vapor which condenses on
the surface of the steel strip to form a plating layer raises the
temperature of the steel strip by 40~ at most. If the temperature
of the substrate steel (T) of Ti-killed steel exceeds the temperature
defined as 180~T~-240XW+292 in accordance with the Ti content N(~)
before being subjected to vacuum vapor deposition, during vacuum
vapor deposition, the temperature of the steel sheet reaches a tem-
perature at which brittle intermetallic compounds are formed because
of temperature rise caused by heat of condensation of the Zn vapor.
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Thus the powdering resistance of the formed alloyed layer
deteriorates.
The upper limit of the substrate temperature differs depending
upon Ti content as described above. Specifically, when the Ti
content is 0.3%, it is not higher than 220~ as calculated by the
above formula; and when the Ti content is 0.05%, it is not higher
than 280~ That is, the substrate temperature must be adiusted so as
not to exceed the upper limit as defined by the above formula prior
to vacuum vapor deposition.
If the substrate temperature is too low, the adhesion of the
formed plating Zn layer is poor. The lower limit is 180~ which is
the same as in the case of plain carbon steel containing no Ti.
The steel strip plated with Zn by vacuum vapor deposition is
heat-treated for alloying. The formed alloyed layer should
preferably contain 8~12% of Fe. If the Fe content is less than 6%,
unalloyed n-Zn remains on the surface of the plating layer and
affects the coatability, weldability, etc. of the plated steel sheet.
If the Fe content e~ceeds 14%, the powdering resistance of the
alloyed layer deteriorates.
As the heating means, a batch annealing furnace can be used. In
the alloying treatment with a batch annealing furnace, generally
steel sheets are heated in a non-oxidizing atmosphere in order to
prevent oxidation of the steel strip (or sheet). Heating can be
carried out with a strip in the form of a coil, a tight coil or an
open coil.
The heating temperature and time can be varied in accordance
with intended coating weight and average Fe content, but the heating
temperature must be lower by 30~ than the alloying temperature for
Zn-plated Ti-free steel sheet. The upper limit of the heating
temperature varies depending upon the Ti content. It is 280~ when
the Ti content is 0.3%, and is 320~ when the Ti content is 0.05~. If
the-heating temperature exceeds the above-described upper limit,
powdering resistance deteriorates, since brittle alloyed layer is
formed. On the other hand, if the heating temperature is below the
above-described temperature range, the Zn plating layer is not well
alloyed. Heating for shorter than one hour does not fully raise the
substrate temperature and satisfactory heating cannot be effected.
On the other hand, heating for longer than 50 hours lowers
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productivity.
It is possible to continuously carry out heating by means of an
annealing iurnace provided in a continuous plating line, but the
heating conditions will dii~er irom those in the case oi a batch
annealing furnace. Employment of a batch annealing iurnace is
preferred since temperature control is easy.
The temperature raising rate and cooling rate are not
specifically limited and these conditions are determined with
consideration to the performance of ordinary industrial batch
annealing furnaces.
Brief E~planation of the Attached Drawings
Fig. 1 is a diagram showing the ranges oi substrate sheet
temperature and Ti contents in substrate steel in which brittle Fe-Zn
intermetallic compounds are not formed at the interface oi the steel
substrate and the plating layer when the substrate is plated with Zn
by vacuum vapor deposition.
Fig. 2 is a diagram showing ranges oi treating temperature and
time in which the average Fe in the alloyed layer can be controlled
to 8-12.0S.
Specific Description of the Invention
~ow the invention will be speciiically described by way oi
- experimental results, working e~amples and comparative e~amples.
~e plated ordinary Ti-~illed steel strips containing 0.05-0.3S
Ti with Zn by vacuum vapor deposition using a continuous vacuum vapor
deposition apparatus as disclosed in Nisshin Giho No. 51,1954, 1~84
with various substrate temperatures.
The other operation conditions were as follows:
Steel strip: 0.8mm thic~ and 1200mm wide Ti-killed steel strips
Operation speed: 80m/min
Pressure in the deposition chamber: 0.01Torr
Alloying temperature: 270~
Thus we checked the relation between Ti content of substrate steels
and substrate temperature ior obtaining an alloyed layer containing
no intermetallic compounds. The results are summarized in Fig. 1.
Fig. 1 shows ranges of substrate temperature and Ti content in
substrate steel in which fonmation of Fe-Zn intenmetallic comQounds does not
occur at the interface of the substrate and the plating layer. In the
drawing, the solid line a represents upper limit temperatures below
~33~
which brittle Fe-Zn intermetsllic compounds are not formed. The line
is e~pressed by the relation T=-240xW+292~, wherein T is the
substrate temperature in ~ and ~ is Ti content in ~. The solid line
b represents lower limit temperatures above which the adhesion of the
formed plating layer is satisiactory. This temperature is constantly
180~ regardless of Ti content.
The Zn-plated steel strips prepared as described above (not yet
alloyed) were maintained at various temperatures for vaiious times
and Fe contents of the formed alloy layers were checked. It was
found that there is a relation between the Ti content and the upper
limit temperature. The results are summarized in Fig. 2.
Fig. 2 is a diagram which shows ranges of heat-treating
temperatures and times in which the average Fe content in the alloyed layer
can be controlled to 8-12.0~. In the drawing, the area surrounded by
broken lines is the range in which the Fe content of the formed alloy
layer falls between 8S and 12~. As seen in this figure, the strip
can be heated to 320~ when the Ti content is 0.05~ and 280~ when the
Ti content is 0.3~. Those skilled in the art will be able to
determine the upper limit temperature for Ti contents between 0.05~
and 0.3S from this figure. The Fe content of the alloyed layer of Ti-
killed steel sheet falls between 8~ and 12~ when the Zn-plated steel
sheet is heated at 220-320~ according to the Ti content.
In Fig. 2, the area surrounded by solid lines is the range known
for Al-killed steel. That is, the sloped line on the bottom left
side is ~nown by those skilled in the art.
E~amples and Comparative Examples
The above-described Ti-killed steel strips containing 0.05~ and
0.3$ were plated with Zn on both sides by vacuum vapor deposition as
described above under various conditions indicated in Table 1.
, The thus Zn-plated steel coils were heat treated for alloying in
a batch annealing furnace in an atmosphere consisting of 3~ H, and
97~ N~ and having a dew point (D.P.) of -25~ under the temperature
and time conditions indicated in Table 1.
The suriace condition and powdering resistance oi the thus
produced alloyed Zn-plated steel sheets were checked. The powdering
resistance was judged by bending specimens with a radius oi curvature
of 3 times the thic~ness oi the sheet up to 180 (6t bending) and
observing whether scaling-oii of the plating layers too~ place inside
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The results are summarized in Table 1. The terms used in Table
1 means as follows:
With respect to surface condition,
Good: Ho~ogeneous alloy were fonmed.
Zn remaining: ~-Zn remained.
With respect to powdering resistance,
Good: No powdering occurred.
Poor: Powdering occurred.
Poor adhesion: Plating layer easily peeled off.
The asterisked values are outside the conditions defined in the
claim.
As is apparent from the above-described results, the products of
the present invention were provided with good surface conditions and
i5 powdering resistance, while the products of the comparative examples
were inferior because of the existence of unalloyed Zn or formation
of intermetallic compounds.
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