Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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S~EL DRAWING SHEE~ NI~H ME~AL COA~ING
BACKGROUND TO THE I NV~NTI ON
Field of the Invention
This invention relates to drawing sheet with a metal
coating. It also relates to a process for the
manuf~cture of such shest.
The sheets to which the invention relates include sheets
coated with zinc or aluminum, whether by dipping or by
electrodeposition, and sheets coated with an allot of
zinc with another metal, particularly aluminum, nickel,
or iron. Such sheets are intended particularly for the
manufacture of visible parts in motor vehicle bodywork.
They generally have a thickness between 0.6mm and lmm.
In order to fix ideas and by way of a single example
intended purely to illustrate the invention, the
description which follows will make more particular
reference to steel sheet with a dip-galvani~ed zinc
coating, referred to for the sake of simpIicity as
galvanised sheet.
What is desired is a sheet of this type which has good
resistance to binding or seizing during drawing and
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excellent appearance after painting, the latter property
applying both to the sheet in the undeformed state and
to drawn components.
Tests which we have carried out have demonstrated that
the behavior of a coated sheet during drawing is
substantially different from the behavior of an uncoated
steel sheet. Although the reason for this has not been
clearly e~tablished at present, it would appear that the
difficulties encountered are due at least in part to the
fact that the coating metals are appreciably less hard
than cold rolled steel.
It is in fact found that these sheets, and in particular
galvanised sheets, are very much more susceptible to
binding than steel sheets.
Also the drawing of a galvanised sheet gives rise to
powdering.
Description of Prior art
Binding and powdering are well known. A~ a brief
remlnder, both conslst o~ the detachment of met~l
partlcle~ from the surface of ths sheet by the drawing
tool as a re~ult of the frictlon force~ cau~ed by
drawlng. These are thersfore two phenomana of the ~ame
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type, which differ es 9 entially in the siz 8 of the metal
particles torn off. They occur to a greater extend with
galvanised sheets than with steel sheets.
In the case of galvanised sheets they cause a build-up
of particles at the base of the drawing tools, i.e. in
places where binding of steel sheets generally does not
occur; this results in additional defects in the drawn
components. Finally, defects in galvanised sheets
cannot be repaired by grinding as in the case with
steel, as this would result in loss of the corrosion
protection provided by the coating.
It has already been proposed, particularly in BE-A-870
609, that a relief consisting of regularly distributed
plateaus and valleys should be imprinted onto the
surface of the sheet to provide a specific roughness by
means of a suitably treated finishing roll. These
valleys form a reservoir in which the lubricating oil
flows as it carries the particles torn off in the
process of drawing. The application of this method to
sheets coated with metals such as zinc or aluminum
nevertheless runs into the difficulties which have been
mentioned above, because of the very much lesser
hardness of these metals. Torn off particles cannot be
completely removed by the oil flowing in ths valleys of
the rough surface; as a consequence they stick to the
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tools and damage he surface of the sheets.
It is also known, particularly through LU-A-86 784, that
small isolated wells, i.e. wells which do not
communicate with each other, can ~e imprinted onto the
surface of sheets. This arrangement makes it possible
to work with higher drawing pres~ures than in the case
of communicating valleys, because the small wells act
a~ "oil pockets~ prsventing exae~lvely rapid expul~ion
of the lubricant under high pres~ures. It has been
found, however, that this arrangement cannot be ussd
directly with sheets coated with metals such as 2inc or
aluminum; not only is roughness not regularly imprinted
on the coated sheet, but the sheet is still particularly
susceptible to binding.
DISCLOSURE OF THE INVENTION
The ob~ect of thl~ lnventlon 1~ to provide a coated
sheet whloh doea not hava the aforementloned
disadvantage3 and whlch through approprlats roughness
ensures satl~f~ctory rasi3tance to blnding and excellent
appearance after palntlng.
According to thls lnventlon a steel drawing ~heet
provided wlth a metal coatlng on at lea~t one ~lde ha~ a
roughness on that 31de oon~l~tlng of regularly
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distributed isolated wells, the said wells have a depth
of between 5 ~m and 25 ~m ancL a base diameter of
between 80 ~m and 200 ~m, the said wells are
distributed with a density of between 50 and 150 wells
per linear inch in at least t:wo perpendicular directions
on the said side of the sheet:, the said side has an
arithmetic mean roughness Ra at the 0.8 mm cut-off
between 0.5 ~m and 2 ~m and the arithmetic mean
roughness ~a of the said side at the 8 mm cut-off does
not differ by more than 0.3 ~m, from the value of the
arithmetic mean roughness at the 0.8 mm cut-off.
According to preferred embodiment, the depth of the
wells lies preferably between 7 ~m and 20 ~m, and
their base diameter lies preferably between 100 ~m and
150 ~m.
In this respect it is appropriate to state that the base
diameter considered here is the diameter of the
cross-section of a well in the plane of the sheet
surface. Similarly the base diameter of a boss on a
roll in a rolling mill is the diameter of the
cross-section of that boss at the surface of the roll,
this cross-section being likened to a plane
cross-section because its dimensions are very small in
comparison with the diameter of the roll.
It is also to be noted, in the context of this
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invention, that the cross-sec:tion of a well and boss
respectively is not necessarily circular or capable of
b~ing directly likened to a circle; for a cross-section
of any shape, the diameter wi.ll be the diameter of tha
best possible circle circumscribed about the
cross-section.
Finally it should not be forgotten that the cut-off
exprssses the cut-off wavelength used for the roughness
measurement; this expression means that, when measuring
surface roughness, surface undulations having a
wavelength greater than the value indicated, for example
0.8 mm and 8 mm in this case, are not taken into account.
Preferably, the density of the wells advantageously lies
between 90 and 120 wells per linear inch, in at least
one of the aforementioned directions.
Furthermore the arithmetic mean roughness Ra at the 0.8
mm cut-off prsferably lies between 0.8 ~m and 1.4 ~m.
The metal coating on the same side preferably has a
thickness of between 7 ~m and 25 ~m.
Likewise in a preferential manner the said metal coating
may be a zinc coating deposited by dip galvanising.
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It has also been found that constancy of the thickness
of the metal coating is an important factor in obtaining
the desired roughness. In this respect it is
advantageous in accordance with the invention that ~ha
sum of the thicknesses of the metal coatings on the two
sides of the sheet, measured at the sama point, should
not differ by more than 30% and preferably ~y not more
than 15%, from the sum of the nominal values of these
two thicknes 8 e8 .
It has also been found that the best performance in
finishing rolling, as regards compliance with reduction
ratios and coating thicknesses, was obtained with steels
which have no or virtually no drawing plateau. In this
respect ultra-low carbon steels micro-alloyed with
titanlum have proved particularly useful. Typically
these steels have a carbon content of between 0.005% and
0.030% and a titanium content of between 0.050% and
0.150% by weight.
Another aspect of the invention relates to a process for
the manufacture of a drawing sheet having the
characteri~tics described above. In accordance with the
invention the metal coating i~ deposited on at least one
side of the said sheet and the coated sheet is rolled
using rolls in which at least one roll corresponding to
the said coated side has on its æurface a pluralit~ of
isolated bosæes regularly di-stributed with a density of
between 50 and 150 bosses per linear inch in at least
two perpendicular directions, and preferably b~tween 90
and 120 bosses per linear inch in at least one of the
aforesaid directions, the said bosses having the shape
of spherical segments of a helight between 15 ~m and 30
~m and a base diameter lying between 100 ~m and 150
~m, the surface of the said roll also having an
arithmetic mean roughness Ra of less than 0.4 ~m and
preferably less than 0.2 ~m at the 0.8 mm cut-off
between the said bosses.
In accordance with a particular embodiment of this
process the said sheet is rolled with a reduction ratio
of less than 1%, preferably between 0.4% and 0.8%.
The rolls used for finishing rolling are advantageously
treated by means of a high energy beam, such as a laser
beam or an electron beam, in accordance with processes
which have been developed previously.
Prefer~bly, the said metal coating is daposited by
dipping the sheet in a bath of zinc and the thicXness of
the said coating is adjusted to a value lying between
7 ~m and 25 ~m, the sum of the thicknesses of the
metal coatings on the two sides of the sheet measured at
the same point differing by no more than 30% and
preferably by not more than 15% from the sum of the
nominal values of these two thicknesse~.
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Other features and advantages of this invention will be
revealed by the example embodiments described below and
illustrated by the accompanying drawings.
BRIEF DFSCRIPTION OF THE DRANINGS
Figures 1 a-c show three photomicrographic views on
different scales, of the surface of a galvanised
drawing sheet according to the invention in its initial
condition;
Figures 2 a-c show three photomicrographlc views on the
same scales as in Figures 1 a-c, of the same sheet after
it has been subjected to a friction test by drawing
between planar jaws;
Figures 3 a-c show three photomicrographic views on the
same scales as in Figures 1 a-c and 2 a-c, of the same
sheet after it has been subjected to a friction test by
passage through a device called a "bead simulator"; and
Figure 4 is a graph which illustrates the distinctness
of image (DOI) of a sheet according to the invention
after painting by conventional means.
The photomicrographs are scanning electron microscope
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photographs.
DETAILED D~SCRIP~ION OF PREFERRED EMBODIMEN~
The sheet used in this examp:Le was a deep drawing steel
of the St 14 type. It was galvanised on both sides by
dipping in a lead-free bath, with 140 g/m2 of 7inc on
each side. The thickness of the zinc layer was from
8 ~m to 10 ~m on each side. Fini3hing rolling wa~
by means of rolls which had been dressed so as to have a
roughness of less than 0.2 ~m at the 0.8 mm cut-off.
The rolls had then been textured by means of a laser
beam so that their surface had bosses in the form of
spherical segments of a height between 16 ~m and
20 ~m and a base diameter between 100 ~m and 120
~m. The bosses were distributed in accordance with a
predetermined pattern with a density RL = 90 bosses
per linear inch in the circumferential direction of the
roll and a density ~d = 120 bosses per linear inch in
the axial direction of the roll. Finishing rolling was
performed with a reduction ratio of 0.8%. After this
operation the sheet had a total thickness of 0.76 mm.
Figure 1 shows the surface of the sheet at the time of
acceptance, i.e. after galvanising and finishing
rolling. Figure la shows the regularity of the surface
distribution of the pattern, each unit of which consists
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of a well and a plateau. Figure lb shows a portion of
the surface of the sheet on a larger scale. The wells
correspond to the darker part of each pattern unit. The
lighter part of these pattern units represents the
plateau, whose height does not exceed 5 ~m. Finally
Figure lc illustrates the shape of a well, which in this
case can be likened to that of a circle, and the base
diameter D of the well.
In this condition the sheet has an effectively planar
surface in which wells of a depth of some 12 ~m and a
base diameter of some 100 ~m are formed. The
distribution density of these wells is 120 and 90 per
linear inch in the transverse and longitudinal
directions of the sheet respectively. The sheet has an
arithmetic mean roughness Ra, measured at tha 0.8mm
cut-off, of the order of 1.2 ~m. A useful feature i8
that this roughness is not greatly affectsd by the level
of the cut-off. Thus is does not excsed 1.5 ~m at the
8 mm cut-off.
The drawability of the sheet was evaluated in relation
to the most critical problem encountered with galvanised
products, namely friction behavior and tendency to bind.
Friction behavior was evaluated on the basis of a
traction test between two planar jaws gripping the
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sheet; the sheet was slightly lubricated with protective
oil. The sheet showed very satisfactory behavior: the
depth and diameter of the wells wera not affected; the
height of the plateaus became less than 3 ~m. Figures
2 a-c show the surface of the sheet after this test.
A test was also performed in a device called a bead
simulator. This test consists of drawing the sheet
across a set of three succe~sive rollers in such a way
that it is bent in two opposite ways, under traction and
wi.h friction. This test caused no change in the shape
or dimensions of the wells was illu~trated in Figures 3
a-c. The arithmetic mean roughness Ra at the 0.8 mm
cut-of fell to 0.9 ~m.
As far as the tendency to binding is concerned, the
sheet was subjected to a U forming test, i.e. the
drawing currently performed by the user. 15 blanks were
drawn in succession without any sign of binding, whereas
with a conventional galvanised sheet only 5 to 6 blanks
can be drawn.
The sheet was also subjected to a painting test,
performed under conditions typical of the process for
the manufacture of motor ~ehicle coachwork sheets:
deposition of a layer of paint having a thickness of
some 30 ~m by cataphoresis followed by stoving at
180-C
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for 30 minutes; deposition of a surface coat having a
thickness of some 20 ~m followed by stoving at 150-C
for 30 minutes; and finally deposition of a coat of
enamel some 45 ~m thick followed by stoving at 125-C
for 30 minutes.
The distinctness of image, DOI, was determined by means
of a standard procedure well-known to those skilled in
the art under the name of the Ford test. This index
varies appreciably in relation the undulation of the
surface before paint is applied. This undulation can in
particular be expressed by the quantity
W = (Raa~ 8mm _ RaC-o 0.8 mm)
which represents the difference between the arithmetic
mean roughness Ra at the 8 mm and 0.8 mm cut offs
respectively, these roughnesses being measured on the
unpainted galvanised sheets.
Figure ~ shows the change in the Ford distinctness of
image index (DOI) for the painted sheet in relation to
the undulation W of the surface of the sheet before
painting. The two straight lines (a) and (b) bound a
zone in which the values (square points) corresponding
to a series of conventional sheets are scattered. It
can be seen that among the best of these the
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distinctness of image index l.ies between 8 and 9 for an
extremely small undulation (W = 0.2).
Values obtained with sheets in accordance with the
invention have besn plotted on this diagram as round
points (I and II).
In its acceptance condition, i.e. galvanised and rolled,
the sheet has an undulation W = 0.25, and ater painting
by the process described above its Ford distinctness of
image index is 10 (point I).
A painting test was also performed with a sheet
according to the invention deformed by 10% by expansion
along two axes in order to simulate drawing. The
surface undulation of the sheet increase slightly, to
W = 0.4; the Ford distinctness of image index
nevertheless remained greater than 9 (point II).
The galvanised sheet according to the invantion thus has
remarkable friction behavior as well as a very small
tendency to bind; also it ha a very high Ford
distinctness of image index, both in the undeformed and
the deformed state. These properties makes it
particularly useful for the manufacture of visible parts
of vehicles, in particular motor vehicle bodywork sheets.
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