FIELD OF THE INVENTION
The present invention relates to hot rolled steel and ,in particular, to developing fine
grained (<3 micron) micro-alloyed hot rolled steel composition in order to achieve fine
ferrite grains in hot rolled steel strip with improved toughness and strength properties and
a process for its production. More particularly, the present invention is directed to
developing microalloyed hot rolled strip steel and a process for its production involving
selective control on niobium and silicon and subjecting slabs cast from such grades to
controlled/optimized hot rolling process parameters to achieve desired average ferrite
grain size of less than 3 micron in said strip. Advantageously, the invention is targeted at
obtaining said fine grain size uniformly across through-thickness of the hot rolled strip
following the present process. Importantly thus, the present invention is directed to
developing selective alloy composition for hot rolled steel strip with fine ferrite grains
capable of providing combination of improved strength and toughness properties in
resulting steel product adapted to suit a number of industrial/structural application.
BACKGROUND ART
It is well known in the existing art of production of micro-alloyed steel that improvement
of strength and toughness of steel is the most important aspect for applications in steel
structural. It is also known in the prevailing art that both the properties are not compatible
with each other and as such grain refinement has always been the preferred practice for
strengthening steel as it enhances both strength and toughness simultaneously.
Conventionally, addition of micro-alloying elements like niobium, vanadium and titanium
coupled with thermo-mechanical controlled processing lead to grain refinement in hot
rolled strip and plates. Also in the conventional processing of micro-alloyed steel, hot
rolled high strength formable grades are produced with higher Nb (> 0.04 %) and
restricted Si (< 0.05 %). Traditionally, silicon is being effectively used for the development
of Advance Class of High Strength Steel (AHSS) like Dual Phase and TRIP (Transformation
Induced Plasticity) steels because of its scavenging and solid solution strengthening effect.
The existing processes however suffer from limitations in terms of grain refinement to the
tune of 5-6 micron in hot rolled micro-alloyed steel employing roll finish in austenitic
region. There has thus been the requirement of further improvements in properties of
micro-alloyed steel by way of further refining of grain structure for a number of
industrial/structural applications.
The present invention is directed to developing a process for achieving innovative alloy
composition with selective higher silicon contents for Nb micro-alloyed steel and respective
batch heats made through BOF-LF-CC route, capable of resulting ferrite grain size less
than 3 microns and commensurate improvements in combination of toughness and
strength of hot rolled steel strip products.
OBJECTS OF THE INVENTION
It is thus the basic object of the present invention to develop micro-alloyed steel
composition which would ensure selective fine ferrite grain < 3 micron through controlled
finish hot rolling and coiling parameters in order to achieve enhanced toughness and
strength properties in resulting steel grade.
A further object of the present invention is directed to developing selective alloy steel
composition and a process for its production directed to fine ferrite grains by selective
controlled composition with controlled processing/hot rolling parameters which could
confirmatively and distinctly contribute towards obtaining fine ferrite grains.
Yet another object of the present invention is directed to said process for developing
selective steel composition with fine ferrite grains by selective control on Nb and Si wt
percent with improved mechanical properties.
A further object of the present invention is directed to a process for developing selective
alloy steel composition with fine ferrite grains by selective control on Nb and Si wt percent
with controlled hot rolling/coiling processing parameters and advantageously attain
Carbonitride precipitation directed to influences microstructural evolution during hot rolling
of microalloyed steels.
A still further object of the present invention is directed to a process for developing
selective alloy steel composition with fine ferrite grains by selective control on Nb and Si
with selective control on processing parameters wherein hot rolling/finishing and coiling
parameters comprising temperatures, minimum draft, strain , inter-pass time and strain
rate in the finishing zone directed to facilitate the accelerated precipitation of Nb(CN) and
thus achieving desired finer ferrite grains of less than 3 micron size.
A still further object of the present invention is directed to a process for developing
selective alloy steel composition with fine ferrite grains by selective control on Nb and Si
wt percent with selective control on processing parameters wherein ferrite grain size of
newly developed hot rolled steel with higher silicon content is found to be less than 3
micron advantageously uniform across through-thickness in hot rolled strip steel of 2-
4mm thickness, favoring homogeneous structure and strength properties in the alloy steel
product.
A still further object of the present invention is directed to a process for developing
selective alloy steel composition with fine ferrite grains by selective control on Nb and Si
wt percent with selective control on processing parameters wherein to experimentally
establish a favourable value of correlation between Si addition and ferrite grain refinement
uniformly across through thickness.
SUMMARY OF THE INVENTION
Thus according to the basic aspect of the present invention there is provided a fine grain
< 3 micron hot rolled steel comprising a micro-alloyed steel composition having:
C= 0.06- 0.08 wt.%;
Mn= 0.9-1.1 wt.%;
S= 0.02 wt.% (max.);
P= 0.025 wt.% (max.);
Si= 0.25 - 0.35 wt.%;
Al= 0.02 wt.% (min.); and
Nb.= 0.035- 0.045 wt.%.
A further aspect of the present invention is directed to said fine grain < 3 micron uniformly
across through-thickness in hot rolled strip steel of thickness 2 to 4 mm.
According to another aspect of the invention there is provided a fine grain < 3 micron hot
rolled steel wherein the toughness range at room temperature from 100 J to 130 J and
yield strength properties range from 500 MPa to 550 MPa.
According to yet another aspect of the present invention is directed to a method of
manufacture of said fine grain < 3 micron hot rolled steel comprising:
(a) providing a selective chemical composition for the micro-alloyed steels comprising
C= 0.06- 0.08 wt.%;
Mn.= 0.9-1.1 wt.%;
S= 0.02 wt.% (max.);
P= 0.025 wt.% (max.);
Si= 0.25 - 0.35 wt.%;
Al= 0.02 wt.% (min.); and
Nb.= 0.035- 0.045 wt.%.
(b) obtaining concast slabs and processing into hot rolled coils with finish rolling and
coiling temperatures of 860° ± 10°C and of 630° ± 10°C respectively and minimum draft
of 85% in the finishing zone.
A still further aspect of the present invention is directed to a method of providing a fine
grain < 3 micron hot rolled steel comprising the step of controlling the Carbonitride
precipitation to favour microstructural evolution during hot rolling of the microalloyed
steel.
A still further aspect of the present invention is directed said method of providing a fine
grain < 3 micron hot rolled steel comprising providing for larger availability of carbon for
enhanced precipitation of Nb(CN) during hot rolling into fine ferrite grains.
Yet further aspect of the present invention is directed to said method of providing a fine
grain < 3 micron hot rolled steel comprising the step of synergistically enhancing the
precipitation of Nb(CN) during hot rolling by selective effect of silicon in the presence of
Niobium.
According to another important aspect of the present invention is directed to said method
of providing a fine grain < 3 micron hot rolled steel comprising maintaining selective
parameters preferably 0.3-0.4 as strain, 1.5-2.5 sec as inter pass time and strain rate of
30-55/sec in the finishing zone to facilitate the accelerated precipitation of Nb(CN).
The present invention and its objects and advantages are described in greater details with
reference to the following accompanying non- limiting illustrative figures and example.
BRIEF DESCRIPTION OF THE ACCOMPANYING FIGURES
Figure 1: is the graphical illustration of the effect of Silicon on ferrite grain size in Niobium
micro-alloyed steel.
Figure 2(a): is the image of the microstructure of the conventional Nb micro-alloyed steel
showing coarser (5-6 microns) non uniform grain distribution.
Figure 2(b): is the image of the microstructure of newly developed fine grained steel
produced following the process of the invention, showing uniform finer (< 3 micron) ferrite
grain distribution across through-thickness of 3.8 mm steel strip.
DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO THE
ACCOMPANYING FIGURES
The present invention is directed to a process for developing fine grained (< 3micron) hot
rolled micro-alloyed steel having selective composition comprising selective range of wt %
of Nb along with Si so that the fine ferrite grains of less than 3 micron size is uniformly
achieved across through-thickness of the hot rolled steel strip of 2-4 mm thickness. In the
present work, innovative alloys chemistry has been designed with niobium contents in the
range of ~0.035-0.045 wt % and silicon contents in the range of ~ 0.25 - 0.35 wt % and
processed by adhering to optimized hot rolling parameters for hot rolled strip of less than
4 mm to achieve average ferrite grain size of less than 3 micron. The alloy chemistry
according to the invention with higher silicon contents was designed and heats were made
through BOF-LF-CC route.
An exemplary embodiment of the steel composition and illustrative process steps to
achieve the grain refinement and improved strength and tough ness properties in the alloy
steel is given in following Example:
EXAMPLE: 1
The Chemical compositions (wt %) of microalloyed steel and steps involved in the process
adapted to provide desired finer ferrite grain size and improved mechanical properties in
the hot rolled steel strip product, is as given below:
1. Providing the developed steel grade having the composition comprising: C: 0.06-0.08;
Mn: 0.9-1.1; S: 0.02; P: 0.025; Si: 0.25-0.35; Al:0.02 min; 1Mb: 0.035-0.045 through trial
heats;
2. Niobium contents are selectively varied from 0.035-0.045 wt% and silicon from 0.25-
0.35 wt% respectively;
3. Heats are made through experimental trials in LD converter and processed through
ladle refining to achieve desired chemical composition as given above, before being
continuously cast in the slab of 210 mm thickness;
4. The Concast slabs are processed into 3.8 mm hot rolled coils in hot strip mill by
adhering to finish rolling and coiling temperatures of 860 ± 10 °C and of 630 ± 10 °C
respectively and minimum draft of 85 % in the finishing zone.
5. The parameters such as strain (0.3-0.4), inter pass time (1.5-2.5 sec) and strain rate
(30-55/sec) maintained in the finishing zone that facilitated the accelerated precipitation of
Nb(CN).
6. Role of Carbonitride precipitation influences microstructural evolution during hot rolling
of microalloyed steels. The synergic effect of silicon in presence of Nb has contributed to
larger availability of carbon for enhanced precipitation of Nb(CN) during hot rolling
resulting into fine ferrite grains.
7. Hot rolled steel strips produced through the above mentioned steps helped achieving
fine ferrite grain (< 3 micron) obtained in hot rolled strip of thickness 2 to 4 mm.
8. Microstructure of the processed and hot rolled strip is studied and compared with
comparable existing variety of micro-alloyed steel that revealed the uniform distribution of
finer ferrite grains of size less than 3 microns in newly developed steel strip across through
thickness.
The effect of Si addition in presence of selective Nb microalloyed steel composition is
studied for a number of sample of heats vis-a-vis the ferrite grain refining in order to
improve the toughness and strength properties in hot rolled steel strip produced through
controlled finish rolling and coiling temperatures and other variable parameters e.g. the
strain rate, inter-pass time and the like.
Reference is now invited to the accompanying Figure 1 that illustrates graphically the
effect of Silicon on ferrite grain size in Niobium micro-alloyed steel.
It is clearly apparent from the accompanying Figure 1 that effect on the ferrite grain size
of steels with fixed niobium (0.04 %) with varying silicon of newly developed alloy steel
strip product have been measured for comparing with that of conventional Nb-
microalloyed grade. A definite trend emerged while co-relating silicon vis-a-vis grain size,
showing an excellent correlation coefficient e.g. R2 = 0.98. It is clearly evident that a
correlation analysis of the data plot established the relation of the two variables
mathematically as:
y=-12.434 x + 6.0427
where V represent resulting grain size in microns depending on the independent variable
Si wt % as V, with some defined wt % of Nb e.g. 0.04 through the number of heats under
experimental observation.
Reference is now invited to the accompanying Figures 2(a) and 2(b) that shows the
images of the microstructures of resulting steel strip products wherein Figure 2(a) is the
image of the microstructure of the conventional Nb micro-alloyed steel showing coarser
(5-6 microns) non uniform grain distribution, while the accompanying Figure 2(b) shows
the image of the microstructure of newly developed fine grained steel produced following
the process of the invention, showing uniform finer < 3 micron ferrite grain distribution
across through-thickness of 2-4 mm, preferably 3.8 mm steel strip. Such fine ferrite grains
is achieved in hot rolled steel strip by controlling the weight percent of alloying elements
and processing parameters. The present invention overcomes the limitation of such grains
of less than 3 micron that suffer conventionally from large variation in grain size along the
thickness of the strip, and achieve desired uniformity of ferrite grain distribution across
through-thickness ensuring homogeneous property improvements.
It is thus possible by way of the present invention to developing a process for production
of fine grained hot rolled steel strips having ferrite grains of size less than 3 micron in hot
rolled steel strips in the thickness range of 2-4 mm and preferably 3.8mm thick wherein
the process parameters for hot rolling/finishing and coiling parameters are selectively
controlled, using a steel composition comprising Niobium contents in the range from
0.035-0.045 wt% and Silicon varied from 0.25-0.35 wt % respectively and adhering to
finish rolling and coiling temperatures of 860°C ± 10 °C and of 630°C ± 10 °C respectively
and minimum draft of 85 % in the finishing zone. The hot rolling and coiling parameters
such as strain (0.3-0.4), inter pass time (1.5-2.5 sec) and strain rate (30-55/sec) in the
finishing zone selectively controlled that have facilitated the accelerated precipitation of
Nb(CN), whereby desired fine ferritic grain structure is distributed uniformly across
through-thickness of the hot rolled steel strip favor achieving the desired combination of
improved strength and toughness properties capable of meeting the requirements of
various industrial/steel structural applications.
WE CLAIM:
1. A fine grain < 3 micron hot rolled steel comprising a micro-alloyed steel composition
having:
C= 0.06- 0.08 wt.%;
Mn= 0.9-1.1 wt.%;
S= 0.02 wt.% (max.);
P= 0.025 wt.% (max.);
Si= 0.25 - 0.35 wt.%;
Al= 0.02 wt.% (min.); and
Nb.= 0.035- 0.045 wt.%.
2. A fine grain < 3 micron hot rolled steel as claimed in claim 1 comprising said fine grain
< 3 micron uniformly across through-thickness in hot rolled strip steel of thickness 2 to 4
mm.
3. A fine grain < 3 micron hot rolled steel as claimed in anyone of claims 1 or 2 wherein
the toughness range at room temperature from 100 to 130 J and yield strength properties
range from 500 to 550 MPa.
4.A method of manufacture of a fine grain < 3 micron hot rolled steel as claimed in anyone
of claims 1 to 3 comprising:
(a) providing a selective chemical composition for the micro-alloyed steels comprising
C= 0.06- 0.08 wt.%;
Mn.= 0.9-1.1 wt.%;
S= 0.02 wt.% (max.);
P= 0.025 wt.% (max.);
Si= 0.25 - 0.35 wt.%;
Al= 0.02 wt.% (min.); and
Nb.= 0.035- 0.045 wt.%.
(b) obtaining concast slabs and processing into hot rolled coils with finish rolling and
coiling temperatures of 860° ± 10°C and of 630° ± 10°C respectively and minimum draft
of 85% in the finishing zone.
5. A method as claimed in claim 4 comprising the step of controlling the Carbonitride
precipitation to favour microstructural evolution during hot rolling of the microalloyed
steel.
6. A method as claimed in anyone of claims 4 or 5 comprising providing for larger
availability of carbon for enhanced precipitation of Nb(CN) during hot rolling into fine
ferrite grains.
7.A method as claimed in anyone of claims 4 to 6 synergistically enhancing the
precipitation of Nb(CN) during hot rolling by selective effect of silicon in the presence of
Niobium.
8. A method as claimed in anyone of claims 4 to 7 comprising maintaining selective
parameters preferably 0.3-0.4 as strain, 1.5-2.5 sec as inter pass time and strain rate of
30-55/sec in the finishing zone to facilitate the accelerated precipitation of Nb(CN).
9. A fine grain < 3 micron hot rolled steel comprising a micro-alloyed steel and its method
of manufacture substantially as hereindescribed and illustrated with reference to the
accompanying figures/examples.

Fine grained (<3 micron) micro-alloyed hot rolled steel composition directed to provide fine ferrite grains in hot rolled steel strip with improved toughness and strength properties and a process for its production. Importantly, the steel composition for such purpose involve a synergistic and selective control on niobium and silicon in the range of 0.035-0.045 wt % and ~0.25 - 0.35 wt % respectively. The slabs cast with heats comprising desired composition is hot rolled with controlled finishing and coiling temperatures and selective minimum draft in finishing zone to thickness 2-4 mm, having average ferrite grain size of less than 3 micron in said strip, uniformly across through-thickness of the hot rolled strip. Importantly, selective silicon addition in presence of Nb micro-alloying in steel contributed to larger availability of carbon for enhanced precipitation of Nb(CN) during hot rolling with controlled parameters resulting into finer ferrite grains. The improved strength and toughness properties in resulting hot rolled steel strip product adapted to suit a number of industrial/structural application.