The invention relates to a method for processing a steel
product, in which the steel product is passed between a set of rotating rolls of
a rolling mill stand. This rolling mill stand may be part of a rolling mill device
consisting of one or more rolling mill stands.
Rolling is a very standard operation for imparting desired
dimensions and properties to metal in general and steel in particular. Apart from
obtaining the desired final geometry of the steel product, rolling also results
in an improvement to the structure as a result of the metallurgical processes taking
place during and after the rolling.
However, the conventional rolling, which for wide products
is usually considered to be a plane strain compression process, results in a considerable
change in thickness, which in some cases is undesirable or impossible. For example,
in heavy construction it is necessary to have steel plate with a thickness of 60
to 150 mm for, inter alia, the production of off-shore platforms or bridges. Since
cast steel slabs currently have a maximum thickness of less than 400 mm, the change
in thickness caused by the rolling to 150 mm would only amount to approximately
60%. Each pass through a conventional rolling mill stand usually results in a change
in thickness of 10 to 30%.
The casting of slabs sometimes results in the formation
of porosity in the slab, a characteristic which is inherent to the casting process.
This porosity is closed up by the pressure applied as a result of the slabs being
rolled a sufficient number of times. However, if it is necessary to form a plate
with a very high thickness, the rolling only closes up the pores in the outermost
layers of the slab, and not those in the core of the material. However, the pores
in the core of the material are highly disadvantageous for the mechanical properties
of the material, in particular for the toughness properties of the plate. Also,
grain refinement only occurs in the outermost layers of the plate. To close up the
pores by the application of pressure and to achieve grain refinement even in the
core of the plate, the degree of rolling through the thick slab therefore has to
be high, whereas the combination of starting thickness of the slab and final thickness
of the steel product do often not allow a large thickness reduction.
It is possible to introduce a large equivalent strain into
a product without imposing a large thickness reduction under laboratory conditions
using small samples with the Equal Channel Angular Extrusion (ECAE) method in which
extreme shear strains are applied without changing the specimen's dimension. In
ECAE a billet is extruded through a die with two channels of equal cross-section
that meet at an angle. Under ideal circumstances the billet is sheared on crossing
the plane of intersection of the channels by an amount determined by the angle between
the two channels. Since the cross section does not change during the process, it
can be repeated thereby accumulating strain. However, this laboratory technique
cannot be used for industrial production of steel products because of the very high
process forces required, and the impossibility to up-scale this process for flat
products of conventional dimensions.
a method of producing fine-grain sheet or plate, in particular fine-grain
plate with a thickness of more than 50 mm, of austenitic stainless steels by hot
rolling billets or slabs in a number of passes is proposed. In this method, the
mean velocity of the reduction in thickness is chosen in dependence upon the ferrite
content of the steel.
The method is particularly intended to prevent the formation
of coarse grains in austenitic stainless steel plates having a thickness of more
than 50 mm.
a method is proposed wherein a shearing deformation is imparted to a metal
cast piece by means of rolling between an upper and a lower roll having a different
It is an object of the invention to provide a method for
introducing a large equivalent strain into the steel product without imposing an
equivalent reduction in thickness of the product.
It is also an object of the invention to provide a method
for processing a steel product which allows the properties of the product produced
thereby to be improved.
Yet another object of the invention is to provide a method
for processing a steel product which results in grain refinement in the product
which is thereby produced.
Yet another object of the invention is to provide a method
for processing continuously cast steel by means of which the properties of the slab
or strip are improved.
It is another object of the invention to provide a method
for processing a continuously cast steel slab or strip with which it is possible
to close up pores in the cast material.
It is also an object of the invention to provide a steel
product with improved mechanical properties which is produced with the aid of this
In the context of this invention, steel should be considered
to comprise all ferrous alloys for example ultra-low carbon steels, low-carbon steels,
medium to high carbon steels, electrical steels, and stainless steels. A steel product
in the context of this invention comprises ingots, slabs, blooms, billets, bar,
rod, strip and profiled sections.
One or more of these objects are achieved by a method for
processing a continuously cast steel product, in which the steel is passed between
a set of rotating rolls of a rolling mill stand in order to roll the steel product,
wherein the rolls of the rolling mill stand have different peripheral velocities
such that one roll is a faster moving roll and the other roll is a slower moving
roll, wherein the peripheral velocity of the faster moving roll is at least 5% and
at most 100% higher that that of the slower moving roll, wherein the thickness of
the steel product is reduced by at most 15% for each pass, and wherein that the
rolling takes place at a maximum temperature of 1350°C.
As a result of the rolls being provided with a different
peripheral velocity, shearing occurs in the steel product and has been found to
occur throughout the entire thickness of the product. It has been found that this
requires a velocity difference of at least 5%. The shearing leads to pores in the
continuously cast material being closed up to a considerable extent. This does not
require a major change in thickness, but rather a change in thickness of at most
15% can suffice. Preferably this thickness reduction is at most 8% and more preferable
at most 5%. This is particularly advantageous in the processing of those steel products
where the dimensions of the steel product at the start of the process do not allow
a singificant reduction in the thickness direction, because the thickness is substantially
In addition, it is important that the rolling according
to the invention can result in a grain refinement which occurs throughout the entire
thickness of the rolled material, which is advantageous for the mechanical properties
of the slab or strip. Inter alia, the strength of the material increases. The beneficial
effects of smaller grain sizes are commonly known.
The rolling is preferably carried out at an elevated temperature.
However, the maximum temperature is limited to 1350°C because the formation
of low melting oxides on the surface of the steel product to be produced has to
be avoided. The elevated temperature makes the rolling run more smoothly.
It is also expected that the processing according to the
invention will result in a rolled sheet with less lateral spread.
The peripheral velocity of the faster moving roll is preferably
at most 50% higher and more preferably at most 20% higher than that of the slower
moving roll. If there is a high difference in velocity, there is a considerable
risk of slipping between the rolls and the steel product, which would result in
According to an advantageous embodiment, the rolling mill
is designed in such a manner that the rolls have different diameters. This makes
it possible to obtain the desired difference in peripheral velocity.
According to another advantageous embodiment, the rolls
have a different rotational speed. This too makes it possible to obtain the desired
difference in rotational speed.
It is also possible for these latter two measures to be
combined, i.e. rolls with different diameters and different rotational speeds in
order to obtain the desired difference in peripheral velocity of the rolls.
According to an advantageous embodiment of the method,
the steel product is introduced between the rolls at an angle of between 5 and 45°
with respect to the perpendicular to the plane through the center axes of the rolls.
Introducing the steel product between the rolls at an angle makes it easier for
the rolls to grip the steel product, with the result that the change in thickness
can be kept as low as possible. Experiments have also shown that after rolling the
steel product has an improved straightness if it is introduced at an angle between
the rolls. The steel product is preferably fed in at an angle of between 10 and
25°, and more preferably at angle of between 15 and 25°, since with such
an angle the steel product comes out of the rolling mill with a good level of straightness.
It should be noted that the latter effect is also dependent on the reduction in
the size of the steel product, the type of steel product and the alloy and the temperature.
For this purpose, after the rolling has been carried out
for the first time, the processing operating is preferably repeated one or more
times. For example, sufficiently good grain refinement is obtained by carrying out
the processing operating according to the invention three times. However, the number
of times that the processing operation has to be carried out depends on the thickness
of the steel product, the difference in peripheral velocity of the rolls and the
desired grain refinement. It is desirable for the steel product to be introduced
between the rolls at an angle of between 5 and 45°, preferably between 10 and
25° and more preferably between 15 and 25° during each processing operation.
If the processing operation according to the invention
is repeated a number of times, according to an advantageous embodiment the steel
product can be passed through the rolling mill stand in opposite directions for
each pass. The steel product then changes direction after each rolling operation
and is always passed through the same rolling mill stand. In this case, the rolls
have to rotate in opposite directions for each pass. In this case too, it is desirable
for the steel product in each case to be introduced at an angle between the rolls.
According to another advantageous embodiment, the steel
product is successively passed through two or more rolling mill stands. This method
is suitable primarily for strip material, which in this way can undergo the desired
processing operation very quickly.
According to a preferred embodiment of the invention the
rolling is carried out on a steel product of which at least a skin layer has a substantially
austenitic structure, and preferably on a steel product having a substantially austenitic
structure throughout. Typical minimum temperatures range from 900 °C for an
ultra low carbon steel to 800-870 °C for a low carbon steel (depending on the
chemical composition of course) to about 723 °C for a steel with 0.8 %C. In
all cases the maximum temperature is 1350 °C. In case of rolling an austenitic
stainless steel, the rolling always takes place on an austenitic structure.
According to a second preferred embodiment the rolling
is carried out on a steel product of which at least a skin layer has a substantially
austenitic-ferritic two-phase structure, and preferably on a steel product having
a substantially austenitic-ferritic two-phase structure throughout. Typical temperatures
range for a low carbon steel from 723 °C ending at 800-870 °C. The temperature
range decreases with increasing carbon contents to reduce to an eutectoid point
of about 723 °C for a steel with 0.8 %C.
According to a third preferred embodiment the rolling is
carried out on a steel product of which at least a skin layer has a substantially
ferritic structure, and preferably on a steel product having a substantially ferritic
structure throughout. For a low carbon steel with a carbon content higher than 0.02%
the maximum temperature is about 723 °C, whereas for steels with lower carbon
contents such as ultra low carbon steels the maximum temperature is about 850 °C.
It should be noted here that these temperature boundaries for the ferritic, ferritic-austenitic
and austenitic region depend on the composition of the steel and on the thermomechanical
history of the steel. The phase transformation is not instantaneous once a critical
temperature is exceeded and therefore a transforming steel may have a skin layer
of a different phase compared to the centre layer of the steel product.
According to a further advantageous embodiment of the invention
the rolling is performed at temperatures between 0 °C and 720°C. This
comprises not only the cold rolling of the ferritic steel product, but also the
advantageous rolling of steel with a martensitic structure or the austenitic stainless
It is possible for the method to be preceded or followed
by a rolling operation which is carried out using a rolling mill in which the rolls
have substantially identical péripheral velocities. In this way, by way of
example, an accurately desired thickness or smoothness can be imparted to the product.
According to another advantageous embodiment, a steel product
is produced according to a method comprising the steps of:
- continuous casting of a steel strand;
- optionally heating and/or temperature homogenising the steel strand between
a casting machine and a rolling device;
- optionally rolling the steel product in one or more rolling mill stands of the
rolling device with rolls having substantially identical peripheral velocities;
- optionally accelerated cooling after the last rolling step;
- optionally cutting the steel product into slabs or coils before or after rolling;
- optionally coiling the steel product
- cooling the steel product
The most commonly used method to produce steel slabs is
by continuous casting of a steel strand and cutting it into steel slabs with a thickness
of between 200 and 400 mm. After casting, these slabs are usually allowed to cool
down to ambient temperatures before being introduced in the furnaces of a hot strip
mill. In some cases the slabs can be introduced into the furnace while it is still
warm or hot from casting (respectively so-called "hot-charging" or "direct-charging").
The thickness of the continuously cast strand is preferably
below 150 mm, more preferably below 100 mm and even more preferably below 80 mm
for thin slab casting.
The cast strand may be cut after casting by means of a
cutting device. The thus obtained slabs may be stored for later processing and allowed
to cool down or they may be processed immediately. In the former case the slabs
may require reheating prior to rolling, in the latter case the slabs may require
to be homogenised in temperature. After finish rolling the rolled product may be
cooled using accelerated cooling and optionally coiled. After the final processing
step the steel product cools or is cooled to ambient temperatures. In case the cast
strand is not cut into slabs, but processed immediately by continuous, endless or
semi-endless rolling, the rolled product will be cut in a later stage of the rolling
process e.g. before the optional coiler. It will be obvious that the rolling according
to the invention may take place anywhere between the casting step and the final
cooling step, or even thereafter.
Prior to coiling the steel product may be subjected to
accelerated cooling. After the final processing step the steel product cools or
is cooled to ambient temperatures.
According to another embodiment of the invention the thickness
of the continuously cast strand is preferably below 20 mm, more preferably below
10 mm and even more preferably below 5 mm.
The cast strand having a cast microstructure may be cut
after casting by means of a cutting device. The thus obtained slabs may be stored
for later processing and allowed to cool down or they may be processed immediately.
In the former case the slabs may require reheating prior to rolling, or they may
be used as final product. In the latter case the slabs may require to be homogenised
in temperature. One drawback of the strip-cast steel products is that the end product
still largely has the cast microstructure, since the strip has scarcely been rolled.
Consequently, the mechanical properties of the end products are relatively poor,
and consequently the use of the end products is limited and do not meet the standards
of the products obtained through the conventional thick slab or even the more recent
thin slab route. During the rolling process according to the invention the microstructure
is transformed from a casting structure to a wrought microstructure without substantial
reduction in thickness thereby improving the final properties of the steel product
significantly. After finish rolling the rolled product may be cooled using accelerated
cooling and optionally coiled. After the final processing step the steel product
cools or is cooled to ambient temperatures. In case the cast strand is not cut into
slabs, but processed immediately by continuous, endless or semi-endless rolling,
the rolled product will be cut in a later stage of the rolling process e.g. before
the optional coiler. After finish rolling the rolled product may be cooled using
accelerated cooling. After the final processing step the steel product cools or
is cooled to ambient temperatures. Again, it will be obvious that the rolling according
to the invention may take place anywhere between the casting step and the final
cooling step, or even thereafter.
A further advantage is obtained if the steel product to
be processed according to the previous two embodiments is a stainless steel.
In the context of this invention, stainless steel comprises
both ferritic, austenitic-ferritic duplex steels and austenitic stainless steels.
These steels are commonly applied in application where the corrosion resistance
of unalloyed or low-alloy steel is inadequate. The combination of corrosion resistance,
high strength and good ductility usually associated with the duplex stainless steels
results in applications where the formability of ferritic and austenitic stainless
steels is inadequate. Typical examples of a ferritic stainless steels according
to EN 10088 (1995) are X2CrNi12- 1.4003 (410) X6Cr14 - 1.4016 (430), and of austenitic
stainless steels are X5CrNiMo17-12-2 1.4401 (316) X5CrNi18-10 - 1.4301 (304). These
steels are typically used as general-purpose stainless steels in plate, strip, semi-,
bar, rod and applied as construction steels for buildings, pipelines, kitchenware,
components in pumps and valves etc.
The thickness of the slab or strip is preferably reduced
by at most 15% for each pass, and preferably by at most 8% and more preferably by
at most 5% for each pass. Since the shearing and therefore the grain refinement
are brought about by the difference in peripheral velocity between the rolls, the
reduction in thickness of the material is not required to obtain grain refinement.
The reduction in thickness is required primarily in order to enable the rolls to
grip the material. This only requires a slight change in thickness, which is advantageous
in the case of thin continuously cast steel slab, strip cast material and strip
material. The smaller the reduction, the thicker the slab or strip remains after
each pass. The possible applications of continuously cast slabs and strip material
increase as a result. With the aid of the method according to the invention, better
mechanical properties can be imparted to the steel product, without the need for
a substantial reduction in thickness. Since the method according to the invention
can be used to impart better properties to an already relatively thin steel product,
it is to be expected that thicker continuously cast plate and strip material, now
with better mechanical properties, will also find industrial applications.
In the production of high strength steel strip microalloyed
with one or more of the elements Nb, V, Ti or B(these steelgrades are usually called
HSLA-steels (high strength, low alloy)), in a hot strip mill acording to the well-known
principles of thermomechanical rolling it is a problem to produce strip with a higher
thickness. The continuously cast slabs that are used to start the rolling process
with usually have a fixed thickness of between 200 and 350 mm, for example 225 mm.
The rolling mills also usually are divided in a roughing section where the slab
is rolled down in a number of passes, for example 5 passes, to a chosen thickness
of, for example, 36 mm. This so-called transfer bar thickness is usually a fixed
thickness within a given hot strip mill and the deviations from this fixed value
are minimal. Deviations from this value by increasing its value usually results
in rolling forces or torques in the finishing mill which exceed operational limits,
thereby causing risks to the rolling mill or resulting in unacceptable changes in
the shape and profile of the product. Decreasing the thickness of the transfer bar
usually results in rolling forces or torques in the roughing mill which exceed operational
limits. However, the fixed value of the transfer bar also causes a problem because
it results in different values of reduction for a thick strip of for example 18
mm and a thin strip of for example 4 mm. In the first case the total reduction in
the finishing mill is 50%, in the second case it is 89%. This has large repercussions
on the development of the microstructure of the steel during and after hot-rolling
because the thermomechanical conditions are quite different which results in different
recrystallisation of the deformed austenite and different precipitation kinetics
of micro-alloying elements. Consequently also the phase transformation during cooling
after rolling is affected. In an advantageous embodiment of the invention the degree
of deformation of the steel product can be increased without the need to increase
the transfer bar thickness, or the degree of deformation can be kept unchanged while
the final thickness of the steel product is increased.
With profiled sections the degree of deformation is essential
for the properties of the final product as well. For example, it is known that steel
billets which are rolled into profiled sections, such as H-sections, often have
a part which has undergone scarcely any rolling, with the result that little or
no grain refinement occurs in this part. Steel billets for sections usually have
a gauge between 200 and 400 mm, for example 230 mm or 310 mm. These are rolled in
the slab/bloom/billet stage after reheating to a temperature of maximal 1350°C.
Finish rolling occurs usually at a temperature where the steel is austenitic and
flange thicknesses range from 10 to 150 mm. Non-limitative examples for typical
steel grades used for these sections comprise CMn-steels and HSLA-steels. The process
according to the invention allows a finer grainsize of the billet because of the
larger degree of deformation in the billet, and also allows a reduction in the pore
size of the billet, resulting in better fracture toughness.
Recently it has become clear from the results of basic
research that properties such as strength, toughness and corrosion resistance can
be improved by reducing grain size. Steels have been developed with a very fine
grain size by controlling the structure of the grain. These steels not only provide
higher tensile strengths compared to conventional steel, but also improved toughness,
endurance and corrosion resistance. This technology has been implemented in the
hot strip mill by imposing a very large thickness reduction at low rolling temperatures,
as a result of which the rolling forces and torques increase to extremely high levels.
However, the proposed solution for obtaining ultra fine ferrite grains relies on
grain refinement by ordinary rolling (i.e. plane strain compression) at low hot
rolling temperatures and requires a very powerful rolling mill. Furthermore, a strong
thickness reduction is imposed to the material to attain the required levels of
deformation. In the process according to the invention, a significant grain reduction
can be achieved because of the accumulation of strain in the steel without substantially
reducing the thickness. The average grainsize of the steel product obtained is preferably
smaller than 5 µm, more preferably smaller than 2 µm and even more preferably
smaller than 1 µm.
According to another embodiment of the invention the properties
of complex phase steels are unexpectedly improved because of the accumulation of
strain in the steel without substantially reducing the thickness. When the steel
product is rolled in the austenitic state and subsequently acceleratedly cooled,
the large degree of accumulated deformation allows the steel to transform to a very
fine ferrite grain in combination with a very finely distributed fine-grained second
phase consisting of bainite or martensite. A small amount of carbides may also be
present. The ferrite content of this steel product is preferably at least 60%, more
preferably at least 70% and even more preferably at least 80%. The average grainsize
of the steel product obtained is preferably smaller than 5 µm, more preferably
smaller than 2 µm and even more preferably smaller than 1 µm.
In conventional production of steel plates, for example
of the carbon-manganese type or of the HSLA-type, the starting point is a continuously
cast slab with a typical thickness between 200 and 350 mm. These slabs are reheated
in a reheating furnace to a temperature between 1000 and 1350°C. After reheating
these slabs are rolled to a thickness of between 30 to 200 mm, preferably 40 to
150 mm and held at temperature, for instance by shielding it against cooling. During
this holding period at high temperature grain growth takes place as a result of
which the final mechanical properties of the finished plate may also deteriorate.
It is common knowledge that a larger grain size decreases the ductility properties
and the toughness of a steel product. It is also well known that the yield strength
decreases with an increase in grainsize. Consequently, grain growth during holding
should be avoided. Conventionally this is done by accelerated cooling. However,
the use of accelerated cooling has the disadvantage of enlarging the temperature
difference between the centre part of the slab and the surface part of the slab.
This temperature difference adversely affects the homogeneity of the final microstructure
of the slab.
In many cases the plate receives a heat treatment during
the production process. This may for example be a normalisation treatment wherein
the slab is reheated into the austenite region and allowed to cool down in still
air or a tempering anneal or stress relief anneal which both aim to reduce the level
of internal stresses. Another example of a heat treatment is the speroidisation
treatment in which elongated carbides are transformed into more or less spheroidal
particles. These carbides may be iron carbides (e.g. cementite) or other metal carbides
like chromium carbides. This type of annealing treatment is used often in steels
with carbon contents in excess of 0.8%. Unfortunately, the majority of these heat
treatments and particularly the spheroidisation treatments take a long time and
frequently lead to decarburisation of the surface part of the strip thereby adversely
affecting the properties.
The rolling according to the invention can also be carried
out at low temperatures between 0 and 720 °C. Special benefits from the rolling
can be expected when performed at low temperatures (i.e. cold rolling) because of
the resulting breaking up of undesired particles. As a result of the break up of
the particles the final properties of the steel product are improved. The shearing
as a result of the rolling process breaks up the particles in the steel products,
for example metal carbides like cementite or chromium carbides which may result
in an improved toughness. The break up of the particles also affects the heat treatment
response of the steel product. Different heating and cooling regimes can be employed
leading to improved throughput through the heat treatment stage, e.g. a spheroidisation
annealing treatment, or an improved product.
It is also possible for the method according to the invention
to be preceded or followed by a heat treatment of the steel product. Examples of
these heat treatments are the well known normalising treatment, stress relief annealing
treatment, temper annealing treatment or spheroidisation annealing treatment.
In the context of this invention, a steel product also
comprises a steel where one or both steel surfaces which are to be rolled are covered
with one or more layers prior to rolling according to the invention. This combination
of a steel product covered on one or both surfaces with one or more layers of metal
is commonly referred to as cladded plate or strip. In producing clad plate there
are three options by which the covering metal is bonded to the steel substrate:
explosive bonding, roll bonding and weld overlay. One of the important factors affecting
the quality of clad plate is the quality of the adhesion between the substrate and
the cladding layer. This is a particular problem for the clad plate which is produced
by roll bonding, because in conventional rolling the stress state at the interface
between the substrate and the cladding layer, or between cladding layers is compressive
only. According to an advantageous embodiment, a surface of the steel product which
is to be rolled is covered by one or more layers prior to rolling. The covering
layer can be a metal, preferably another steel, e.g. a steel with a different composition
or a stainless steel, Titanium, Nickel, Copper, Aluminium or alloys thereof. This
way it is possible, for example, to produce laminated material, such as what is
known as clad material for use in, for example, pipes and pipe lines, chemical plants,
power plants, vessels, pressure vessels.
The invention also relates to an improved metal plate or
strip which has been produced by continuous casting, preferably with the aid of
the method according to the first aspect of the invention, in which the pores in
the core of the plate or strip have a maximum dimension of less than 200 µm,
preferably less than 100 µm, more preferably less than 20 µm and even
more preferably less than 10 µm. As a result of the continuous casting, continuously
cast plate and strip material always has pores which can be significantly larger
than 200 µm. The standard rolling operations can only close up these pores
in the core to a slight extent or cannot do so at all. The rolling operation according
to the invention makes it possible to provide continuously cast plate and strip
material having pores which are much smaller.
The invention also relates to an improved metal plate or
strip which is produced by continuous casting, preferably with the aid of the method
according to the first aspect of the invention, in which the metal plate or strip,
after recrystallisation, has a substantially homogenous degree of recrystallisation
over its entire thickness. The fact that the grains have all been subjected to shearing
as a result of the rolling operation according to the invention, including those
in the core, means that the continuously cast plate and strip material will recrystallize
over the entire thickness.
It also relates to a steel product produced according to
the invention, in which the starting point is a steel ingot, and in which steel
product the pores in the core of the product preferably have a maximum dimension
of less than 200 µm, more preferably less than 100 µm, still more preferably
less than 20 µm and even more preferably less than 10 µm as well as to
a steel product produced by continuous casting and processed according to the invention,
in which the pores in the core of the plate or strip have a maximum dimension of
less than 200 µm, more preferably less than 100 µm, still more preferably
less than 20 µm and even more preferably less than 10 µm.
The invention also relates to a steel strip produced according
to the invention for use in for example parts of automobiles, transport equipment,
piling, buildings, construction and to a clad steel product for use in for example
pipes, chemical plants, power plants, vessels, pressure vessels and to a steel strip
wherein the steel is a HSLA-steel comprising at least one of the elements niobium,
titanium, vanadium or boron, or wherein the steel is an ultra low carbon steel,
preferably at least partly stabilised, preferably with at least one of the elements
titanium, niobium or boron.
The invention will be explained with reference to an exemplary
Experiments were carried out using slabs of a Titanium
stabilised ultra low carbon steel, carbon-manganese steels and Niobium microalloyed
The slabs were introduced at different angles varying between
5° and 45°. The temperature of the slabs when they were introduced into
the rolling device was approximately 1000 °C. The two rolls were driven at
a speed of 5 revolutions per minute.
After rolling, the slabs had a certain curvature, which
is highly dependent on the angle of introduction. The straightness of the slab after
rolling can to a large extent be determined by the angle of introduction, in which
context the optimum angle of introduction will be dependent on the degree of reduction
of the slab, the type of material and alloy, and the temperature. For the slabs
of steel which have been rolled in the experiment described above, an optimum introduction
angle is approximately 20°.
A shear angle of 20° was measured in the steel slabs
which were rolled in accordance with the experiment described above. Using this
measurement and the reduction in the size of the slab, it is possible to calculate
an equivalent strain in accordance with the following formula:
This formula is used to make it possible to present the
strain in one dimension and is known from the book "
Fundamentals of metal forming" by R.H. Wagoner and J.L. Chenot, John Wiley
& Sons, 1997
Therefore, in the slabs which have been rolled in accordance
with the experiment, the equivalent strain is
In the case of rolling with an ordinary rolling mill, shearing
does not take place across the thickness of the plate and the equivalent strain
is therefore only
(working on the basis of a uniform strain over the entire thickness of the steel
Therefore, the rolling using the method according to the
invention results in an equivalent strain which is three to four times higher than
with conventional rolling without any difference in peripheral velocity. A high
equivalent strain means less porosity in the slab, greater recrystalization and
therefore greater grain refinement, and more extensive breaking up of the second-phase
particles (constituent particles) in the slab. These effects are generally known
to the person skilled in this field of engineering if the equivalent strain increases.
Therefore, the rolling according to the invention means that the resulting properties
of the material are greatly improved as a result of the use of the method according
to the invention.