FIELD OF THE INVENTION
The invention relates to an improved grade of high-strength interstitial-free (HS-
IF) steel for producing skin-panel of automobiles. The invention further relates to
a process for producing an improved grade of HS-IF steel.
BACKGROUND OF THE INVENTION
Interstitial-free (IF) steels are used by automobile manufacturers for applications
such as the outer panels for car bodies, primarily due to the high formability of
the material. The 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, designated high-strength interstitial-free or
HS-IF steels, with increased tensile strength without compromising formability
and surface standards for skin-panel applications in automobiles.
Formability of steels, as measured by the Lankford parameter or r-bar value,
depends on the crystallographic texture of the material. Steels with a strong
intensity of gamma-fiber texture records high r-bar value. The favorable texture
and superior r-bar value of IF steels evolve mainly due to the pure form of
lattice, free of carbon and nitrogen atoms. This is conventionally achieved by
alloying the steel with titanium and/or niobium that helps scavenge the
interstitials forming carbide, nitride and/or carbonitrides. Addition of titanium and
niobium in excess of the stoichiometry to fix the interstitials is known to be
beneficial for the r-bar value. Cold-rolled coils of IF-grade steels are routinely
being produced at Tata Steel India through batch-annealing route with a
minimum tensile strength of 270 MPa and r-bar valve of 1.8.

Addition of alloying elements such as manganese, phosphorous, silicon helps
increase the tensile strength of IF steels to 340 MPa. But these elements have
adverse effects on other characteristics such as formability and surface of the
product. Addition of phosphorous is known to deteriorate r-bar value by affecting
the crystallographic texture of the steel. This is due to the generation of complex
phosphide precipitates of iron and titanium, during hot-rolling or batch-annealing
after cold-rolling. Likewise, addition of silicon and manganese deteriorates
surface quality. Adherent silicon-scale makes pickling of the hot-hand difficult
while manganese results into oxidation-mark after batch-annealing.
Development of HS-IF steel for high-end applications in automotive market
would therefore need a judicious selection of alloying elements to achieve the
most favorable combination of mechanical properties and surface quality.
OBJECTS OF THE INVENTION
It is therefore, an object of the invention is to propose an improved grade of
high-strength interstitial-free (HS-IF) steel for application in automobile
manufacturing.
Another object of the invention is to propose a process to produce improved
grade of HS-IF Steel.
SUMMARY OF THE INVENTION
The innovative steel grade is made through LD-RH degassing route and
continuously cast into slabs. The slab material is further processed downstream

into hot-rolled coils. The hot-rolled coils are then pickled and cold-rolled into final
section size through batch annealing and skin pass rolling route.
The final product is inspected manually at the recoiling and inspection lines. The
oxidation mark in the coils is found to be almost negligible. Samples are collected
form the middle portion of the coils. The samples are then tested in laboratory to
determine the mechanical properties and carry-out metallographic investigation.
MECHANICAL TESTS AND METALLOGRAPHY :
The tensile properties are measured using test specimens with 50 mm gage
length, fitted with an extensometer. All tests are performed at room
temperature. The r-bar values are also tested using tensile test method and an
extensometer. The r-bar values recorded constitute the average of the r values
in the parallel, normal and diagonal to the rolling directions, namely, r-bar = (r0
+ 2r45 + r90)/4.
Metallographic samples prepared are etched with 5% nital. A simple light optical
microscope is used to record the size of the grains comprising the material. The
crystallographic texture of the material is also studied using an x-ray
diffractometer fitted with a goniometer.
MECHANICAL PROPERTIES
Mechanical properties of all the cold-rolled and batch-annealed coils are
evaluated and presented in Table 3. The results indicate that the new HS-IF steel
with the optimum chemistry exhibit consistent properties in all coils. The high
tensile strength of the steel when used to manufacture a car body reduces the

gage of the body panels and hence the weight of the car. It is also established
that this advantage can be achieved without compromising the formability of the
steel. The coils exhibit a minimum r-bar value of 1.6. The combination of
mechanical properties is found to match the high-end requirements of material
intended for outer panel applications in modern cars with reduced body weight.
The new HS-IF steel is found to be comprised of equaixed grains of ferrite with a
very fine size of ASTM number 9.
The material is found to posses strong intensity of gamma-fiber texture which is
good for the r-bar value.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 shows the microstructure of the HS-IF steel in cold-rolled and annealed
condition according to the invention.
Figure 2 shows the crystallographic texture of the HS-IF steel of the invention.
DETAIL DESCRIPTIONS OF THE INVENTION
Material specification in supply condition :
(i) Steels coils of 0.5-1.5 mm thickness and 1000 - 1200 mm width,
(ii) Chemistry of steel specified is given in Table 1
(iii) Mechanical properties of the product specified are as follows :
YS ≥ 180 MPa, UTS ≥ 340 MPa,%EL≥ 38, r-bar ≥ 1.6


Process Route:
(i) Steel making by LD
(ii) Secondary steel making : RH degassing
(iii) Continuous slab casting
(iv) Hot rolling and cooling with an optimum processing parameters
(v) Cold rolling and batch-annealing followed by skin pass rolling

Field of Application:
The HS-IF steel grade produced according to the invention, can be sued in
manufacture of outer body panels for modern automobiles.

Advantages of the invention
(1) Optimum chemistry of the steel resulted in consistent and favorable
mechanical properties suitable for high-end automotive steel products.
(2) Judicious selection of alloying elements helped achieve high tensile
strength of the steel without compromising formability.
(3) Minimum usage of titanium and niobium content in the steel resulted into
consistent and adequate formability required by automotive
manufacturers.
(4) Relying both on phosphorus and manganese for the purpose of
strengthening helped avoid a large amount of either of the elements and
hence their adverse effects.
(5) Microstructure and texture of the newly developed steel helped achieve a
favorable combination of mechanical properties.
(6) Minimum manganese of the steel results into reduction in the oxidation
mark severity of the product.
(7) High tensile strength of the material when used for automobile
manufacture would allow usage of thinner gage outer panels and help
reduce the weight of the car body.
(8) The superior r-bar value of the material allows easy forming of auto
components with complicated shapes.

(9) The new grade of steel product having reduced oxidation mark severity
enhances the painting performance.
(10) The new grade of steel possesses a favorable combination of mechanical
properties and surface quality suitable for high-end products in
automotive market.
(11) Due to minimum usage of various ferroalloys, the cost of production of
the new grade steel is reduced.

WE CLAIM :
1. An improved grade of high strength interstitial-free (HS-IF) Steel for
producing skin-panel of automobiles, having a composition consisting of
(wt%):
C ≤ 0.005
Mn ≤ 0.5
S ≤ 0.012
P ≤ 0.05
Si ≤ 0.05
AL ≤ 0.06
N ≤ 0.004
Ti ≤ 0.04
Nb ≤ 0.04
B ≤ 0.003
Wherein the temperature ranges for finish rolling, coiling and, batch
annealing are 830 - 950°C, 630 - 750°C and 640 - 740°C respectively,
wherein the parameters of cold rolling deformation and skin pass
elongation are maintained during hot and cold rolling respectively
between 50-90%, and 0.2 to 2.0%, and wherein the mechanical
properties for the new grade steel are achieved to YS ≥ 180 MPa, UTS ≥
340 MPa, Elongation ≥ 38%, and r-bar ≥ 1.6.
2. The HS-IF Steel as claimed in claim 1, wherein the steel in coil form is
produced with thickness of 0.5 -1.5 mm, and width between 1000 -1200
mm.

3. A process for producing a HS-IF Steel grade as claimed in claim 1,
comprising the steps of:
making the steel by LD route;
adapting RH-degassing process for Secondary Steel making;
producing Steel Slabs by continuous casting;
hot rolling and cold rolling of cast-slab with optimum parameters; and
cold rolling and batch annealing of the rolled product followed by skin-
passing.

The invention relates to an improved grade of high strength interstitial-
free (HS-IF) Steel for producing skin-panel of automobiles, having a
composition consisting of (wt%):
C ≤ 0.005
Mn ≤ 0.5
S ≤ 0.012
P ≤ 0.05
Si ≤ 0.05
AL ≤ 0.06
N ≤ 0.004
Ti ≤ 0.04
Nb ≤ 0.04
B ≤ 0.003
Wherein the temperature ranges for finish rolling, coiling and, batch
annealing are 830 - 950°C, 630 - 750°C and 640 - 740°C respectively,
wherein the parameters of cold rolling deformation and skin pass
elongation are maintained during hot and cold rolling respectively
between 50-90%, and 0.2 to 2.0%, and wherein the mechanical
properties for the new grade steel are achieved to YS ≥ 180 MPa, UTS ≥
340 MPa, Elongation ≥ 38%, and r-bar ≥ 1.6.