Indian Patents. 256039:TOOL FOR COLDFORMING OPERATIONS WITH IMPROVED PERFORMANCE

Title of Invention	TOOL FOR COLDFORMING OPERATIONS WITH IMPROVED PERFORMANCE
Abstract	The present invention relates to cemented carbide for steel tire cord drawing operations. The cemented carbide comprises WC with an ultra fine grain size and >5 but <10 weight-% Co, including grain growth inhibitors (V and/or Cr) and with a specific relation between HV30 and cobalt content.

Full Text	The present invention relates to a tool for coldforming and drawing operations particularly steel tire cord drawing operations. The performance of a drawing die in production of steel tire cord is improved by increasing the hardness of the cemented carbide. Coarse wire is usually dry drawn by grades with 10 wt-% or 6 wt-% Co and a hardness 1600 and 1750 Vickers respectively. Wet drawing from 1.5-2 mm down to final dimension, 0.15-0.3 mm, is usually made with drawing dies in grades having a hardness of from about 1900-2000HV and Co content In the 1980's a grade having only 3 wt-% Co and ultra fine grain size for tire cord drawing was introduced by Sandvik. It was later withdrawn due to the low strength and brittle behaviour leading to premature failures. In a European project, Wireman, (reported by A. M. Massai et al, "Scientific and technological progress in the field of steel wire drawing", Wire 6/1999), the conditions for drawing of tire cord were investigated. New cemented carbide grades were tested in the grain size range of 0.3-1 µm and a binder of 0.3-5 wt-% Co. A hardness increase was achieved by reducing the binder content and decreasing the grain size of WC. According to published results the grades did not completely satisfy the expectation on better performance, despite the high hardness achieved. The conclusion quotes: "The wear tests demonstrated that not only the hardness of the dies controls the die wear mechanism." According to US 6,464,748, beside hardness of cemented carbide, corrosion is a major factor controlling the wear resistance. Normally higher Co binder content leads to higher sensitivity to corrosion and said US-patent discloses improvements by low binder content and alh loying of the cobalt binder with nickel and chromium to make it cor- rosion resistant, i.e. a similar approach as in the above mentioned Wireman project. US 5,948,523 discloses a coldforming tool with an improved hard wearing surface zone. This has been achieved by a post-sintering heat treatment in a boron nitride containing environment of a hard metal of a suitable composition. The effect is most pronounced when the heat treatment is made of a hard metal which has previously been sin- tered to achieve a high carbon content through a suitable choice of chemical composition and processing conditions. During many years there has been an ongoing development of ce- mented carbide with finer and finer grain size. The extension of cemented carbide grain sizes into the ultra fine size range leads to a number of positive improvements regarding the wear processes. Attrition wear (or grain loss volume) may be reduced by an order of magnitude by little more than halving the sintered grain size (in the absence of other wear processes), since grain volume is related to the cube of diameter. Adhesive fracture is another dangerous kind of attrition wear, in which the separation of strongly welded tool-workmaterial inter- faces can induce tensile cleavage within the underlying carbide. Ul- tra fine hardmetals can resist the onset of such fractures better than coarser ones due to their greater rupture strength. Erosion/corrosion of the binder phase is said to be part of the wear mechanism in wire drawing. Even though the content of binder is increased in ultra fine cemented carbide the smaller WC grain size leads to thinner binder films, generally called binder free path. Thus resistance to selective erosion of the soft binder phase by wear particles is reduced. It is reasonable to believe that the thinner binder also leads to better oxidation/corrosion properties since the properties of the binder at the WC interface is different from the pure metal. From the above it seems that the main interest in developing finer sub-micron hardmetal, perhaps into the nanometer range, is to raise hardness, maximise attrition wear resistance and strength whilst as far as possible maintaining all other attributes at useful levels. It has now been found that use of ultra fine grained cemented carbide with a cobalt content >5 wt-% can lead to improved performance in steel tire cord production by the combination of the improvements in strength, hardness and toughness of ultra fine ce- mented carbide. It is an object of the present invention to provide a tool for coldforming and drawing operations particularly tire cord drawing operations with a further improved combination of high wear resistance, high strength and keeping a good toughness. Fig. 1 shows a drawing die in which A=cemented carbide nib and B=steel casing. Fig. 2 shows in 10000 times magnification the microstructure of a cemented carbide according to the present invention etched in Mura- kami. The structure contains WC and Co binder. It has now surprisingly been found that a tool for coldforming and drawing operations, particularly tire cord drawing operations with a better performance than prior art tools can be obtained if the tool is made of a cemented carbide with a Co content >5 wt-% but wt-% comprising WC with an ultra fine grain size. A combination of grain size and binder content that leads to better performance is represented by 6 wt-% Co with ultra fine WC having a hardness about 100-150HV higher than most used 3 wt-% Co binder grade having hardness of 1925HV. Another example of ultra fine cemented carbide successfully tested for tire cord drawing is characterized by having 9 wt-% of co- balt and ultra fine tungsten carbide grain size so that the hardness, HV30, is 1900. Thus the same hardness level as the conventional 3 wt- % Co grade is achieved by the ultra fine grain size. Improved wear resistance is achieved by decreasing the grain size and increasing the binder content so that the hardness as HV30 is maintained or even increased by having an ultra fine grain size of tungsten carbide. Thus the invention relates to the use as a cold forming tool of cemented carbide grades with increased Co binder content and very much decreased WC grain size, producing material with improved wear resistance for coldforming and drawing operations particularly tire cord drawing operations. It is a well known fact that hardness of cemented carbide is de- pendent on the binder content and tungsten carbide grain size. Gener- ally as grain size or binder content decreases the hardness increa- ses. In order to circumvent the well known difficulties in defining and measuring "grain size" in cemented carbide, and in this case to characterize "ultra fine cemented carbide", a hardness/binder content relation is used to characterise the cemented carbide according to the present invention. The invention thus relates to a cold forming tool of cemented carbide having a Co content >5 wt-% but the following relation between HV30 and Co-content in wt-%: HV30>2150-52wt-% Co preferably HV30>2200-52wt-% Co more preferably HV30>2250-52wt-% Co and most preferably the hardness HV30>1900. The cemented carbide is made by conventional powder metallurgi- cal techniques such as milling, pressing and sintering. The invention also applies to the use of the cemented carbide according to the invention particularly for the steel tire cord draw- ing operations but it can also be used for other coldforming and drawing operations such as deepdrawing of cans. Example 1 Steel wire drawing dies with inner diameters between 1.3 and 0 mm and A. WC-3 wt-% Co, submicron grain size, VC as grain growth inhibitor, prior art. B. Ultra fine cemented carbide consisting of WC-9 wt-% Co with and Cr carbide grain size inhibitor, invention. The Vickers hardness HV30 of the grades is 1925 and 1950 re- spectively. The tools were tested in the wire drawing of brass coate steel wires of high tensile strength for tire cord applications witl- the following results. Performance factor relates to the quantity of product (wire) as length of mass drawn through the different nibs relative to the prior art nib, A. Table 1 summarizes the results. Example 2 Steel wire-drawing dies with inner diameters between 1.3 and 0.175 mm and A. Same prior art grade as in Example 1. B. Ultra fine cemented carbide drawing die consisting of WC and 6 wt-% Co with grain size inhibitor V and Cr. The Vickers hardness HV30 of the grades are 1925 and 2050 re- spectively, tested in drawing of brass coated steel wire for tire cord: Example 3 Steel wire drawing dies with inner diameters between 1.7 and 0.3 mm and Same composition of cemented carbide as in Example 2 was tested in the drawing of brass coated steel wire for tire cord. It can be seen from the great differences in improvements, 15- 120%, that the conditions in the wire drawing operation, e.g. steel quality, lubrication, maintenance etc, factors outside the influence of the cemented carbide manufacturer, superimpose a great variation. Thus, the tests in the examples can not be compared more than within each test conditions. WE CLAIM 1. Die for steel wire drawing comprising ultra fine cemented carbide comprising WC, a binder phase of Co, and characterised by a Co content of >5 but HV30>2150-52wt-% Co. 2. The die according to claim 6, wherein a Vickers hardness, HV30>2200-52wt-% Co. 3. The die according to claim 6, wherein a Vickers hardness, HV30>2250-52wt-% Co. 4. The die according to claim 6, wherein a Vickers hardness HV30>1900. Abstract Tool for coldforming operations with improved performance The present invention relates to cemented carbide for steel tire cord drawing operations. The cemented carbide comprises WC with an ultra fine grain size and >5 but growth inhibitors (V and/or Cr) and with a specific relation between HV30 and cobalt content.

Full Text

The present invention relates to a tool for coldforming and
drawing operations particularly steel tire cord drawing operations.
The performance of a drawing die in production of steel tire
cord is improved by increasing the hardness of the cemented carbide.
Coarse wire is usually dry drawn by grades with 10 wt-% or 6 wt-% Co
and a hardness 1600 and 1750 Vickers respectively. Wet drawing from
1.5-2 mm down to final dimension, 0.15-0.3 mm, is usually made with
drawing dies in grades having a hardness of from about 1900-2000HV
and Co content In the 1980's a grade having only 3 wt-% Co and ultra fine grain
size for tire cord drawing was introduced by Sandvik. It was later
withdrawn due to the low strength and brittle behaviour leading to
premature failures.
In a European project, Wireman, (reported by A. M. Massai et al,
"Scientific and technological progress in the field of steel wire
drawing", Wire 6/1999), the conditions for drawing of tire cord were
investigated. New cemented carbide grades were tested in the grain
size range of 0.3-1 µm and a binder of 0.3-5 wt-% Co. A hardness
increase was achieved by reducing the binder content and decreasing

the grain size of WC. According to published results the grades did
not completely satisfy the expectation on better performance, despite
the high hardness achieved. The conclusion quotes: "The wear tests
demonstrated that not only the hardness of the dies controls the die
wear mechanism."
According to US 6,464,748, beside hardness of cemented carbide,
corrosion is a major factor controlling the wear resistance. Normally
higher Co binder content leads to higher sensitivity to corrosion and
said US-patent discloses improvements by low binder content and alh
loying of the cobalt binder with nickel and chromium to make it cor-
rosion resistant, i.e. a similar approach as in the above mentioned
Wireman project.

US 5,948,523 discloses a coldforming tool with an improved hard
wearing surface zone. This has been achieved by a post-sintering heat
treatment in a boron nitride containing environment of a hard metal
of a suitable composition. The effect is most pronounced when the
heat treatment is made of a hard metal which has previously been sin-
tered to achieve a high carbon content through a suitable choice of
chemical composition and processing conditions.
During many years there has been an ongoing development of ce-
mented carbide with finer and finer grain size.
The extension of cemented carbide grain sizes into the ultra
fine size range leads to a number of positive improvements regarding
the wear processes.
Attrition wear (or grain loss volume) may be reduced by an order
of magnitude by little more than halving the sintered grain size (in
the absence of other wear processes), since grain volume is related
to the cube of diameter.
Adhesive fracture is another dangerous kind of attrition wear,
in which the separation of strongly welded tool-workmaterial inter-
faces can induce tensile cleavage within the underlying carbide. Ul-
tra fine hardmetals can resist the onset of such fractures better
than coarser ones due to their greater rupture strength.
Erosion/corrosion of the binder phase is said to be part of the
wear mechanism in wire drawing. Even though the content of binder is
increased in ultra fine cemented carbide the smaller WC grain size
leads to thinner binder films, generally called binder free path.
Thus resistance to selective erosion of the soft binder phase by wear
particles is reduced. It is reasonable to believe that the thinner
binder also leads to better oxidation/corrosion properties since the
properties of the binder at the WC interface is different from the
pure metal.
From the above it seems that the main interest in developing
finer sub-micron hardmetal, perhaps into the nanometer range, is to
raise hardness, maximise attrition wear resistance and strength

whilst as far as possible maintaining all other attributes at useful
levels.
It has now been found that use of ultra fine grained cemented
carbide with a cobalt content >5 wt-% can lead to improved
performance in steel tire cord production by the combination of the
improvements in strength, hardness and toughness of ultra fine ce-
mented carbide.
It is an object of the present invention to provide a tool for
coldforming and drawing operations particularly tire cord drawing
operations with a further improved combination of high wear
resistance, high strength and keeping a good toughness.
Fig. 1 shows a drawing die in which A=cemented carbide nib and
B=steel casing.
Fig. 2 shows in 10000 times magnification the microstructure of
a cemented carbide according to the present invention etched in Mura-
kami. The structure contains WC and Co binder.
It has now surprisingly been found that a tool for coldforming
and drawing operations, particularly tire cord drawing operations
with a better performance than prior art tools can be obtained if the
tool is made of a cemented carbide with a Co content >5 wt-% but wt-% comprising WC with an ultra fine grain size. A combination of
grain size and binder content that leads to better performance is
represented by 6 wt-% Co with ultra fine WC having a hardness about
100-150HV higher than most used 3 wt-% Co binder grade having
hardness of 1925HV.
Another example of ultra fine cemented carbide successfully
tested for tire cord drawing is characterized by having 9 wt-% of co-
balt and ultra fine tungsten carbide grain size so that the hardness,
HV30, is 1900. Thus the same hardness level as the conventional 3 wt-
% Co grade is achieved by the ultra fine grain size.
Improved wear resistance is achieved by decreasing the grain
size and increasing the binder content so that the hardness as HV30

is maintained or even increased by having an ultra fine grain size of
tungsten carbide.
Thus the invention relates to the use as a cold forming tool of
cemented carbide grades with increased Co binder content and very
much decreased WC grain size, producing material with improved wear
resistance for coldforming and drawing operations particularly tire
cord drawing operations.
It is a well known fact that hardness of cemented carbide is de-
pendent on the binder content and tungsten carbide grain size. Gener-
ally as grain size or binder content decreases the hardness increa-
ses. In order to circumvent the well known difficulties in defining
and measuring "grain size" in cemented carbide, and in this case to
characterize "ultra fine cemented carbide", a hardness/binder content
relation is used to characterise the cemented carbide according to
the present invention.
The invention thus relates to a cold forming tool of cemented
carbide having a Co content >5 wt-% but the following relation between HV30 and Co-content in wt-%:
HV30>2150-52*wt-% Co
preferably
HV30>2200-52*wt-% Co
more preferably
HV30>2250-52*wt-% Co
and most preferably the hardness HV30>1900.
The cemented carbide is made by conventional powder metallurgi-
cal techniques such as milling, pressing and sintering.
The invention also applies to the use of the cemented carbide
according to the invention particularly for the steel tire cord draw-
ing operations but it can also be used for other coldforming and
drawing operations such as deepdrawing of cans.
Example 1

Steel wire drawing dies with inner diameters between 1.3 and 0
mm and
A. WC-3 wt-% Co, submicron grain size, VC as grain growth
inhibitor, prior art.
B. Ultra fine cemented carbide consisting of WC-9 wt-% Co with
and Cr carbide grain size inhibitor, invention.
The Vickers hardness HV30 of the grades is 1925 and 1950 re-
spectively. The tools were tested in the wire drawing of brass coate
steel wires of high tensile strength for tire cord applications witl-
the following results. Performance factor relates to the quantity of
product (wire) as length of mass drawn through the different nibs
relative to the prior art nib, A. Table 1 summarizes the results.

Example 2
Steel wire-drawing dies with inner diameters between 1.3 and
0.175 mm and
A. Same prior art grade as in Example 1.
B. Ultra fine cemented carbide drawing die consisting of WC and
6 wt-% Co with grain size inhibitor V and Cr.
The Vickers hardness HV30 of the grades are 1925 and 2050 re-
spectively, tested in drawing of brass coated steel wire for tire
cord:

Example 3
Steel wire drawing dies with inner diameters between 1.7 and 0.3
mm and
Same composition of cemented carbide as in Example 2 was tested
in the drawing of brass coated steel wire for tire cord.

It can be seen from the great differences in improvements, 15-
120%, that the conditions in the wire drawing operation, e.g. steel
quality, lubrication, maintenance etc, factors outside the influence
of the cemented carbide manufacturer, superimpose a great variation.
Thus, the tests in the examples can not be compared more than within
each test conditions.

WE CLAIM
1. Die for steel wire drawing comprising ultra fine cemented carbide comprising
WC, a binder phase of Co, and characterised by a Co content of >5 but HV30>2150-52*wt-% Co.
2. The die according to claim 6, wherein a Vickers hardness, HV30>2200-52*wt-%
Co.
3. The die according to claim 6, wherein a Vickers hardness, HV30>2250-52*wt-%
Co.
4. The die according to claim 6, wherein a Vickers hardness HV30>1900.

Abstract Tool for coldforming operations with
improved performance

The present invention relates to cemented carbide for steel tire
cord drawing operations. The cemented carbide comprises WC with an
ultra fine grain size and >5 but growth inhibitors (V and/or Cr) and with a specific relation between
HV30 and cobalt content.

TOOL FOR COLDFORMING OPERATIONS WITH IMPROVED PERFORMANCE

Documents:

Inventors:

PCT Conventions: