FIELD OF THE INVENTION:
This invention relates to a method for strengthening hot-rolled strips of low
carbon.
BACKGROUND OF THE INVENTION:
Low-carbon steels possess an essentially ferritic microstructure and are
generally not very strong. This is by the virtue of the carbon content of these
steels that seldom exceeds 0.05 wt%. Strength of these materials can however
be boosted remarkably in the presence of strong carbide forming elements such
as titanium, niobium or vanadium. These elements generate various types of
precipitates such as carbide or carbonitride and thus help refining the grain size
of ferrite. In such a scenario, both the fine grain size of ferrite and the precipitates
contribute to the strength. Steels in hot-rolled condition with yield strength in
excess of 500 Mpa are being routinely produced in mass scale by this technique,
which is often referred as controlled rolling. However, in order to achieve a fine
scale of the microstructure, the temperature during hot rolling of these steels
must be maintained low which increases the load required for rolling.
Additionally, the material after rolling must be cooled at a faster rate. These
sometimes result in constraints on the capability of commercial mills. The
purpose of the present work was to investigate for an alternate strategy to make
low-carbon steel products that would be strong in hot rolled condition but neither
resorting to rapid cooling nor increasing the rolling load.
Hot rolling is carried out at temperatures where the steel exists as austenite.
During cooling after rolling, the austenite transforms into ferrite. This phase
transition takes place by destroying all the atomic bonds in
the austenite and rearranging the atoms in the

form of the ferrite lattice, aided by diffusion. Alternatively, the transformation of
austenite can be accomplished by a coordinated movement of the atoms without
any need of diffusion. The process is accompanied by a large shear strain, which
is accommodated in the form of defects such as dislocations in the lattice of the
product phase that takes the shape of laths or plates. Bainite in steels forms in
this way and derives its strength mainly from the thickness of the plates and the
density of defects. Both these are controlled solely by the reaction temperature,
through its effect on the strength of the austenite and the driving force for
transformation. Low-carbon steels can therefore be made strong with bainite,
rather ferrite, simply by allowing the reaction to occur at a low temperature,
without depending on the prior austenite grain size. For this, the alloy has to be
designed such that bainite forms during the course of cooling from the austenitic
state preferably at a slow rate.
OBJECTS OF THE INVENTION:
An object of this invention is to propose a method for strengthening hot-rolled
strips of low carbon steel;
Another object of this invention is to impart strength to low-carbon steels with
bainite;
Further, object of this invention is to propose a very simple method for
strengthening hot-rolled strip;
Still further object of this invention is to propose a method of strengthening low
carbon steel without relying on a rapid cooling at run out table (ROT) or low finish
roll temperature (FRT);

Still another object of this invention is to design an alloy which allows
transforming the austenite into bainitic during the course of cooling.
SUMMARY OF THE INVENTION:
According to this invention there is provided a method for strengthening hot-
rolled strips of low-carbon steel comprising: heating steels containing
molybdenum and boron to an elevated temperatures and then cooling the same
in air.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS:
Figure 1: A typical dilatation curve obtained using Gleeble-1500;
Figures 2a & b: FEGSEM images showing bainite plates constituting the
microstructure of the samples;
Figure 3: Engineering stress-strain curve of the bainitic steel sample.
DETAILED DESCRIPTION OF THE INVENTION:
Steels containing some molybdenum and boron allow transforming the austenite
into bainite during the course of cooling from elevated temperatures. While
molybdenum generally increases the hardenability of austenite, boron even in
only minute quantities segregates to the austenite grain boundaries and is
extremely effective in retarding the formation of ferrite at high temperatures.
Bainite can form only after cooling below a critical temperature called bainite start
temperature or Bs. The bainite plates would be fine at a low reaction temperature
and therefore effective in imparting high strength to the steel. Addition of

manganese can reduce the empirical B2 to low temperature. Boron containing
steels are susceptible to iron borocarbide (Fe23CB6) formation at the elevated
temperatures. In order to suppress the loss of boron in the form of borocarbide,
some niobium was added in the alloy so that carbon would remain locked with
niobium and would not facilitate the reaction of iron with boron.
About 25 kg of the material with composition Fe-(0.01-0.1)C-(1-3)Mn-(0.05-
0.75)Mo-(0.001-0.005)B-(0.01-0.15)Nb was made in laboratory using a simple air
induction furnace. The steel was made with a little titanium and was fully killed
with aluminium to protect boron from reacting with nitrogen and oxygen
respectively. The material was cast in a preheated metal mould. The cast ingot
was then forged at a temperature of about 1200°C and cooled in air. The forged
material was used to machine samples for subsequent tests.
Hollow cylindrical samples with 8mm outer diameter, 4 mm inner diameter and
70mm length were prepared for dilatometry tests using Gleeble-1500 machine.
Samples were heated at 1°Cs-1 to a temperature of 960°C, hold there for a
minute and then cooled to room temperature at 5°Cs-1 in one case while 20°Cs-1
in another. The dilatation data were recorded with the change in temperature for
each of the samples. These were then cut across the cross-section of the heated
zone and observed using a field emission gun scanning electron microscope
(FEGSEM). Hardness measurements of the samples were also made in Vickers
scale with 3 kg load. Rectangular blocks of dimensions 12mmx12mmx120mm
were further machined from the forged material and heat-treated using a muffle
furnace. The blocks were heated to 960°C, soaked for 15 minutes and then
cooled in air and oil. These were then used to prepare tensile test specimens of 5

mm diameter and 25 mm gage length. The tensile test was carried out at room
temperature using a strain rate of 3.33x10-6s-1.
A typical dilatation curve is presented in Figure 1. During cooling, dilatation was
initially negative due to thermal contraction but changed its sign on the onset of
the transformation of austenite. This is because of the strain due to
transformation of austenite exceeding the thermal contraction of the material.
The transformation start temperature was taken as the point where the first
deviation from the linearity was detected in the dilatation curve. Similarly,
transformation finish temperature was considered to be the point where the
dilatation curve became linear during subsequent cooling. The transformation
temperatures, as obtained using the dilatation data for the samples cooled at two
different rates, are shown in Table 1. The temperature at which transformation
began was found to be low. The transformation occurred in a range of
temperature that remained almost same at the two different cooling rates.
Table 1: Transformation temperatures and hardness values for the samples
cooled at two different rates, hardness value is the average of five readings with
the error as the standard deviation.

The samples were inspected using FEGSEM. The images presented in Figure
2a-b reveal clearly the laths or plates of bainitic ferrite to constitute the
microstructure of the samples. The fine scale of the microstructure must have
been responsible for the enormous hardening achieved in the alloy (Table 1).

Hardness values were found not to be affected significantly by the rate of cooling.
The mechanical properties of the heat-treated samples are shown in Table 2.
Tensile strength up to 780 Mpa can be easily achieved in these samples with a
total elongation of about 16%. However, it appears from the tensile curve (Figure
3) that the alloy is not capable of work hardening itself efficiently during the
course of plastic deformation. This resulted in extensive necking with a reduction
in area of about 70%.
Table 2: Mechanical property data

In summary, it has been possible to design an alloy that is capable of hardening
itself enormously without resorting to rapid cooling or low austenitising
temperatures. The strength of the steel was derived mainly from the fine plates of
bainitic ferrite formed from the austenite during cooling.

WE CLAIM:
1. A method for strengthening hot-rolled strips of low-carbon steel
comprising:
heating steels containing molybdenum and boron to an elevated
temperatures and then cooling the same in air.
2. The method as claimed in claim 1, wherein the elevated temperature is
about 1200°C.
3. The method as claimed in claim 1, wherein austenite changes to bainite in
the steel during the course of cooling.
4. The method as claimed in claim 3, wherein Bainite is formed only after
cooling below a critical temperature called bainite start temperature or Bs.
5. The method as claimed in claim 3, wherein bainite plates are fine at a low
reaction temperature and are effective in imparting high strength to the
steel.
6. The method as claimed in claim 1, wherein the tensile test was carried out
at room temperature using a strain rate of 3.33x10-6s-1.

A method for strengthening hot-rolled strips of low-carbon steel comprising:
heating steels containing molybdenum and boron to an elevated temperatures
and then cooling the same in air.