FIELD OF THE INVENTION
The present invention relates to a cold-rolled micro-alloyed high-strength steel
grade for automobiles by batch-annealing technology. The present invention
further relates to a process of producing cold-rolled micro-alloyed high strength
steel grade for automotive applications.
BACKGROUND OF THE INVENTION
High-strength steels are used for automobile manufacturers for applications such
as the structural components to meet the objective of reducing weight. Modern
cars are required to be light in body weight so as to achieve better fuel-
economy, faster speed and lower green-house gas emission. This requires
designing new steels with increased tensile strength without compromising
ductility and surface quality for automotive applications. It is known that addition
of alloying elements such as manganese, phosphorous, silicon increases the
tensile strength of the steel. But these elements have adverse effects on the
surface characteristics and ductility. Addition of silicon is in particular not
encouraged for these steels because the adherent silicon-scale makes pickling of
the hot-band difficult resulting into bad surface. Similarly, the amount of
manganese is restricted in order to minimize the chance of surface oxidation
after batch-annealing. Phosphorous increases strength dramatically but at the
same time rapidly reduces ductility. It is known from prior publications and
practices that micro-alloying elements increase strength of the steel by refining
the grain size and also forming uniformly distributed fine precipitates which was
originally developed to increase the strength of hot-rolled steels. However, in
case of cold-rolled steels annealed in batch process, the annealing temperature
of the material when micro-alloyed, gets restricted when compared with the
open coil annealing technology. Micro-alloying addition therefore reveals
diversing trends of the mechanical properties in cold-rolled steels because these
micro-alloy elements exert profound influence on the kinetics of recrystallisation
and grain-growth of the material.
OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose a process for producing
high-strength steels using micro-alloying elements for automotive applications by
batch-annealing after cold-rolling.
Another object of the present invention is to propose a new high-strength grade
steel with tensile strength > 440 MPa, yield strength < 375 MPa and total
elongation > 27% after cold-rolling and batch annealing using micro-alloying
elements.
SUMMARY OF THE INVENTION
Accordingly, there is provided a cold-rolled micro-alloyed high strength-steel
grade for automotive application, wherein composition of the steel in wt% of the
elements is C<0.09, Mn<2.5, S<0.02, P<0.06, Al<. 0.009, Ti<0.05,
Nb <. 0.05, wherein the microstructure of the steel after cold-rolling and batch-
annealing comprises predominantly ferrite grains with uniformly fine size of the
order of ASTM number 9-10, the grains possessing equi-axed or polygonal
shape, and wherein the hot-rolling and cold-rolling process parameters adapted
being 850-950°C, 630-750°C and 540-740°C respectively for finish rolling, coiling,
and batch annealing, and 40 to 70% and 0.2 to 2% respectively being cold
rolling deformation, and skin pass elongation.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figures 1 (a) - (c) illustrate the mechanical properties of the high strength steel
according to the invention, subsequent to cold-rolling and batch annealing, figure
1(a) showing UTS VS MPa, Figure 1(b) showing YS VS MPa, and Figure 1(c)
showing % elongation.
Figure 2 illustrates the microstructure of the high strength steel of the invention
after cold-rolling and batch-annealing revealed with optical microscope.
DETAILED DESCRIPTION OF THE INVENTION
The new steel was made in LD (Linz and Donawitz) vessel and continuously
casted into slabs. The material was further processed downstream into hot-rolled
coils. These were then pickled and cold-rolled into final section size through
batch annealing and skin pass rolling route.
Mechanical properties of all the cold-rolled and batch-annealed coils are
presented in Figure 1(a) - (c). The results indicate that the new steel with the
optimum chemistry and process parameters recorded consistent properties
conforming to the specification of automotive customers. The high tensile
strength of the steel would help reduce the gage of the components or the
panels and hence the weight of the car.
In order to limit the forming load in the press, the yield strength of the material
must conform to a maximum limit. The coils were all found to record yield
strength below the maximum limit of the specification, enabling the press-
forming to be performed with a nominal load.
The material was found to possess sufficient ductility required to form the
components. The combination of mechanical properties of the new steel was
thus found to match the requirements of material intended by automotive
applications.
Microstructure of the new material after cold-rolling and batch-annealing is
shown in Figure 2. The steel is found to be predominantly comprised of ferrite
grains with uniformly fine size of the order ASTM number 9 - 10. The grains
were all found to possess equi-axed or polygonal shape. The fine scale of the
microstructure has been primarily due to the micro-alloying elements besides the
process parameters. Apparently , there has been no remnant of the deformed
material in the microstructure, confirming that the re-crystallization of the cold-
rolled steel has been complete. The process parameters during the entire
process chain of the material achieved an optimum microstructure, resulting into
an excellent combination of mechanical properties.
Table 1; Chemistry of the new steel specified in wt% of the elements

Process route:
(i) Primary steel making by LD
(ii) Secondary steel making in Ladle furnace for alloying
(iii) Continuous slab casting
(iv) Hot tolling and cooling with an optimum processing parameters
(v) Cold rolling and batch annealing followed by skin pass rolling
The optimum processing parameters are shown in Table 2.
Table 2: Hot rolling and cold rolling parameters specified for processing
of the new high-strength steel

Advantages of the invention
(1) Optimum chemistry and process parameters of the steel resulted in
consistent and favorable mechanical properties suitable for high-end
automotive steel products.
(2) The material achieved high tensile strength without exceeding the
maximum limit of yield strength and the minimum limit of elongation.
(3) Minimum usage of micro-alloying and other elements in the steel
resulted into excellent balance of strength and ductility required by
automotive manufacturers.
(4) Uniformly fine scale of the microstructure has been achieved with
the appropriate process parameters and chemistry of the steel, without
sacrificing the mill productivity.
WE CLAIM:
1. A cold-rolled micro-alloyed high strength steel grade for automotive
application wherein composition of the steel in wt% of the elements is
C<0.09, Mn<2.5, S< 0.02, P< 0.06, Al< 0.009, Ti< 0.05, Nb< 0.05,
wherein the microstructure of the steel after cold-rolling and batch-
annealing comprises predominantly ferrite grains with uniformity fine size
of the order of ASTM number 9-10, the grains possessing equi-axed or
polygonal shape, and wherein the hot-rolling and cold-rolling process
parameters adapted being 850-950°C, 630-750°C and 540-740°C
respectively for finish rolling, coiling, and batch annealing, and 40 to 70%,
and 0.2 o 2% respectively being cold rolling deformation, and skin pass
elongation.
2. The steel grade as claimed in claim 1, wherein the tensile strength, yield
strength and total elongation after cold-rolling and batch annealing is
>440 MPa, <375 MPa, and >27% respectively.

ABSTRACT

This invention relates to a cold-rolled micro-alloyed high strength steel grade for
automotive application , wherein composition of the steel in wt% of the elements
is C<0.09, Mn < 2.5, S<0.02, P < 0.06, Al < 0.009, Ti < 0.05, Nb < 0.05,
wherein the microstructure of the steel after cold-rolling and batch-annealing
comprises predominantly ferrite grains with uniformly fine size of the order of
ASTM number 9-10, the grains possessing equi-axed or polygonal shape, and
wherein the hot-rolling and cold-rolling process parameters adapted being 850
950°C, 630-750°C and 540-740°C respectively for finish rolling, coiling, and batch
annealing, and 40 to 70% and 0.2 to 2% respectively being cold rolling
deformation, and skin pass elongation.