TITLE:
A Nb-microalloyed steel and a method for producing the same.
FIELD OF INVENTION:
This invention relates to a Nb-microalloyed steel and a method for
producing the same.
BACKGROUND OF THE INVENTION:
Problems of high Mill loads are encountered while rolling thinner
sections & wider width in a number of Nb-microalloyed steel grades.
Additionally due to high mill loads, rolls wear is early and also uneven
leading to problems of salt and pepper type scale in these grades. Till
date the problem is tackled by following measures:
1) Intermediate roll changing in stands
2) Restricting the numbers of slabs of these grades to five to seven
only.
3) Restricting the size of the schedule to 55 nos. of slabs max instead
of 70 or more.
However these measures lead to reduced Hot Strip Mill productivity. It is
thus necessary to design chemistry/process parameters to reduce the
rolling load of these grades.

Keeping these in mind, a chemistry and process parameters have been
suggested which can lead to the lowering of mill load of these grades.
OBJECTS OF THE INVENTION:
An object of this invention is to propose a method for improving surface
finish of Nb-microalloyed steel;
Another object of this invention is to propose a method for producing Nb-
microalloyed steel having improved surface quality of strip and will also
reduce wear and tear of rolls;
Further object of this invention is to propose Nb-microalloyed steel
having reduced mill load;
Yet another object of this invention is to propose a Nb-microalloyed steel
having improved surface finish.
BRIEF DESCRIPTION OF THE INVENTION:
According to this invention there is provided composition of Nb-
microalloyed steel with improved surface quality comprising:
C: 0.020-0.10
Mn: 1.5 or less
Si: 0.50 or less
S: 0.020 or less
P: 0.025 or less
Al: 0.015-0.050

V: 0.2 or less
Cr: 0.1 or less
Nb: 0.01-0.03
Ti: 0.005-0.025
In Accordance with this invention there is also provided a method for
producing Nb-microalloyed steel comprising:
adding titanium to low carbon Nb microalloyed steel during pre-heating
of the slabs;
subjecting the pre-heated slabs to the step of rolling in roughing mill and
then in finishing mill;
deforming the strip obtained from the mill to achieve the required
thickness;
cooling the strip and cooling in between 500-650°C.
DETAILED DESCRIPTION OF THE INVENTION:
Ti has been added to low carbon Nb microalloyed steels and the same
has been given a critical amount of hot deformation during stands.
Addition of Ti controls the austenite grain size during reheating of the
slabs and promotes recrystallisation during deformation of the same. The
cooling on the Run-Out-Table (ROT) has been 'Near grooved'. The strip
has been coiled in-between a critical temperature range to obtain the
final property.

The steel studied contain (in wt%):
C:0.020-0.10 P:0.025 or less
Mn:1.5 or less Al:0.015-0.050
Si:0.50 or less V:0.2 or less
S:0.020 or less Cr:0.1 or less
Nb:0.01-0.03 Ti:0.005-0.025
The steel was made in LD furnace in the plant and slabs were reheated
to 1200°C for 1-3 hrs. Slabs were then rolled in roughing mill followed by
rolling in finishing mill. The entry temperature of finishing mill was kept
between 1000-1100°C. In finishing mill stand 1 a reduction of 50-70%
were given. The speed of finishing mill at stand 1 was kept around 1-2
m/sec. The strip is then deformed as per the required amount to achieve
the required thickness at the end. The strip was 'near grooved' cooled in
the ROT table and coiled in-between 550-650° temperature. The load
experienced by the mill was 4-5 MN less than what has been predicted.
The strips yield YS: 420-450 MPa; UTS:465-480 MPa and elongation
between 28-33%.
Addition of Ti restricts the growth of prior austenite grain size to ~
100µm from being ~ 300 µm during reheating of the slabs. Smaller
austenite grain size leads to the lower amount of critical strain for
dynamic recrystallisation. More the recrystallisation, softer the material
becomes and this leads to the lowering of mill load and subsequent
better surface finish or both strip and roll.

WE CLAIM:
1. Composition of Nb-microalloyed steel with improved surface quality
comprising:
C: 0.020-0.10
Mn: 1.5 or less
Si: 0.50 or less
S: 0.020 or less
P: 0.025 or less
A1: 0.015-0.050
V: 0.2 or less
Cr: 0.1 or less
Nb: 0.01-0.03
Ti: 0.005-0.025
2. A method for producing Nb-microalloyed steel comprising:
adding titanium to low carbon Nb microalloyed steel during pre-heating
of the slabs;
subjecting the pre-heated slabs to the step of rolling in roughly mill and
then in finishing mill;
deforming the strip obtained from the mill to achieve the required
thickness;
cooling the strip and cooling in between 500-650°C.

3. The method as claimed in claim 2, wherein the temperature for
reheating is 1200°C for a duration of 1 to 3 hrs.
4. The method as claimed in claim 2, wherein the temperature for
rolling is between 1000-1100°C.
5. The method as claimed in claim 2, wherein the strip is cooled in
the ROT table.
6. The method as claimed in claim 2 wherein the titanium is added in
a range of 0.005-0.025 wt%.

Composition of Nb-microalloyed steel with improved surface quality
comprising:
C: 0.020-0.10
Mn: 1.5 or less
Si: 0.50 or less
S: 0.020 or less
P: 0.025 or less
Al: 0.015-0.050
V: 0.2 or less
Cr: 0.1 or less
Nb: 0.01-0.03
Ti: 0.005-0.025