This disclosure relates to cold-rolled and annealed interstitial-free high-
strength steel.
Background
Interstitial-free (IF) steels form a material of choice majorly for
automotive components requiring a large value of formability namely the normal
anisotropy. This originates mainly from the crystallographic texture of the steel
due to the extremely low content of the interstitial elements such as carbon and
nitrogen. But this makes the material soft. In order to elevate the strength of the
steels, alloying elements like manganese, phosphorous or silicon are generally
added to these steels.
A US patent application no. US 2015/0047752 Al discloses a high
strength interstitial free low density steel. However the above mentioned
development does not provide strength upto a level of 500 MPa minimum.
An Indian patent application no. 306/KOL/2015 on similar concept has
been filed by the same inventors and the applicant. However the application is
more suitable to bake hardening in the automotive.
Objects
In view of the foregoing limitations inherent in the prior-art, it is an object
of the disclosure is to propose a method for making high-strength interstitial-free
steel after cold-rolling and annealing. Another object is to propose a process for
producing high-strength interstitial-free steel having YS = 330-360 MPa, UTS =
540-560 MPa, %EI = 31-34 and r-bar = 1.6-1.7.
SUMMARY OF THE DISCLOSURE
In one aspect, the disclosure provides a method for making an interstitial-
free steel comprising steps of casting a steel with required composition, soaking
the steel at 1200-1220C; hot rolling the steel at FRT 870-920t; coiling the steel
at 640-7001; cold rolling the steel with 70-90% reduction and continuous
annealing the steel at 820 - 8801 for 1-5 mins.

In another aspect, the disclosure provides an interstitial-free steel having
following composition of Carbon (C) = 0.001-0.003, Manganese (Mn) = 1.5-2.5,
Sulphur (S) = 0.003-0.012, Phosphorus (P) = 0.07-0.12, Silicon (Si) = 0.05-0.4,
Aluminum (Al) = 0.03-0.1, Nitrogen (N) = 0.005-0.009, Titanium (Ti) = 0.01-
0.05, Niobium (Nb) = 0.01- 0.05, Boron (B) = 10-40 ppm, rest Iron (Fe) (all in
wt. %).
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING
FIG. 1 shows flow diagram illustrating various steps for manufacturing an
interstitial free steel in accordance with an embodiment of the disclosure.
FIGS. 2(a) - 2(d) shows SEM image, EBSD plot, Grain size distribution and
Micro-texture analysis of the interstitial free steel in accordance with an
embodiment of the disclosure.
Detailed DPcrrip*;?»
Various embodiments of the disclosure provide method for making an
interstitial-free steel, the method comprising steps of: casting a steel using
composition of Carbon (C) = 0.001-0.003, Manganese (Mn) = 1.5-2.5, Sulphur
(S) = 0.003-0.012, Phosphorus (P) = 0.07-0.12, Silicon (Si) =0.05-0.4, Aluminum
Al) = 0.03-0.1, Nitrogen (N) = 0.005-0.009, Titanium (Ti) = 0.01-0.05, Niobium
(Nb) = 0.01- 0.05, Boron (B) = 10-40 ppm, rest Iron (Fe) (al. in wt. o/o), soaklng
the steel at 1200-1220C, hot rolling the steel at FRT 870-920C, coi.ing the steel
at 640-700JC, co.d rolling the steel with 70-90*0 reduction and continuous
annealing the steel at 820 - 8801 for 1-5 mins.
Another embodiment of the disclosure provide an interstitia,-free steel
oToTr iQ = a001-°-003' ^^ ^ " 1-5"2-5' S*" CS) =
or2Nh° (P) = °-07-0-12' S»iCOn <» - «^ Aluminum (A,)
: o oi o 0;TT=a005"0-009' Tltanlum ™ ■ aoi-°-°5'^ i
0.01 0.05, Boron (B) = 10-40 ppm, rest Iron (Fe) (a,l in wt. %).
Stoel (IFS)l ^ method comprises steps of:

Step (104): casting a steel using following composition:
Carbon (C) = 0.001- 0.003, Manganese (Mn) = 1.5-2.5, Sulphur (S) = 0.003-
0.012, Phosphorus (P) = 0.07-0.12, Silicon (Si) =0.05-0.4, Aluminum (Al) = 0.03-
0.1, Nitrogen (N) = 0.005-0.009, Titanium (Ti) = 0.01-0.05, Niobium (Nb) =
0.01- 0.05, Boron (B) = 10-40 ppm, rest Iron (Fe) (all in wt. %)
step (108): soaking the steel at 12OO-1220C;
step (112): hot rolling the steel at FRT 870-920 deg. C;
step (116): coiling the steel at 640-700 deg. C;
step (120): cold rolling the steel with 70-90% reduction; and
step (124): continuous annealing the steel again at 820-880 deg. C for l-5mins.
The obtained is the interstitial-free steel (IFS).
The steel, at step (104), is casted in a vacuum induction furnace. In some
embodiment the steel can be manufactured in some other furnaces as per
requirement.
The composition of the steel as mentioned in step (104) provides no bake
hardening properties. For present grade steel, the composition is selected in such
a way that all the carbon is tied up with carbide forming elements such as Nb/Ti.
Therefore, no additional bake hardening properties could be obtained during
paint baking.
The strength of the steel mainly rely on the sufficient alloying addition of
manganese and phosphorous. In addition, the anisotropy value i.e. r-bar arises
mainly from the ultra-low level of carbon present in the steel, preferably all in the
combined form. The latter would be adjusted with microalloying additions such
as titanium and niobium in order to lock all nitrogen atoms as nitride or
carbonitirde. Higher amount of Phosphorous sometimes makes the steel brittle
by segregating in the grain boundaries. Hence small amount of Boron is added in
the steel to mitigate the deleterious effects of Phosphorous by restricting the
segregation of Phosphorous. Al and S are present in trace amount. Sulphur
should be as low as possible, if restricted below lOppm is good for the properties
of the steel.

The preferred composition of the steel can be shown in Table 1.
Table 1 (all in wt.%)

In an embodiment the soaking of the steel is done for 1 hr. for 25mm
stock.
At the hot rolling step (112), the final thickness of the steel strip is in the
range of 3-5 mm.
The coiling of the steel, at step (116), is performed in a muffle furnace.
Between the coiling step (116) and cold rolling step (120), pickling of the
steel is done. The hot-rolled steel is pickled in HCI solution to remove the surface
scale. The concentration of the HCI solution is kept at 10% by vol.
The cold rolling, at step (120), the reduction of the steel is in the range of
70-90%.
FIGS. 2(a) - 2(d) shows SEM image, EBSD plot, Grain size distribution
and Micro-texture analysis of the interstitial free steel respectively.
The mechanical properties of the steel obtained by means of the process
(100) are shown in Table 2.


For present grade steel r-bar value which dictates the drawability of the
material higher amount of precipitates may deteriorate the r-bar value and
therefore in the present disclosure the carbon content is restricted to 10-30ppm.
An Interstitial-free steel is also obtained via the process (100) which is
unique in its composition. The interstitial-free steel comprises following
composition of Carbon (C) = 0.001-0.003, Manganese (Mn) = 1.5-2.5, Sulphur
(S) = 0.003-0.012, Phosphorus (P) = 0.07-0.12, Silicon (Si) = 0.05-0.4,
Aluminum (Al) = 0.03-0.1, Nitrogen (N) = 0.005-0.009, Titanium (Ti) = 0.01-
0.05, Niobium (Nb) = 0.01- 0.05, Boron (B) = 10-40 ppm, rest Iron (Fe) (all in
wt. %).
Advantages:
The newly developed interstitial-free steel can be used in manufacture of
critical parts of automobiles such as the exterior panels.
High tensile strength of the material would allow usage of thinner gage
outer panels and help reduce the weight of the car body
Adequate r-bar of the material would ensure critical formability of the
parts under high speed stamping operations.
Example:
New steel with the optimum chemical composition (Table 1) was
produced and processed in laboratory. The steel was made in a vacuum
induction furnace and cast into a 40 kg ingot. The material was then soaked at
1200 C for more than 10 hours in a muffle furnace and rolled 20 mm plates.
These plates were further hot rolled into 3 mm thick plates in an experimental
hot rolling mill. The finish rolling temperature was maintained at 900 C and then
transferred into a salt bath furnace for coiling simulation at 680 C. After about
holding for an hour the material was cooled to ambient temperature in air.
The hot-rolled material was subsequently pickled in HCL solution to
remove the surface scale. Further, the plates were cold-rolled to thin sheets of
thickness 0.8 mm. The cold rolled thin sheets were then annealed at a
temperature about 820-8801 for adequate time. Samples were then tested for
mechanical properties and assessed for microstructural details.

The following mechanical properties have been obtained for samples following
above mentioned process as shown in Table 3.


WE CLAIM
1. A method for making an interstitial-free steel, the method comprising
steps of:
casting a steel using following composition
Carbon (C) = 0.001-0.003, Manganese (Mn) = 1.5-2.5, Sulphur (S) =
0.003-0.012, Phosphorus (P) = 0.07-0.12, Silicon (Si) = 0.05-0.4,
Aluminum (Al) = 0.03-0.1, Nitrogen (N) = 0.005-0.009, Titanium (Ti) =
0.01-0.05, Niobium (Nb) = 0.01- 0.05, Boron (B) = 10-40 ppm, rest Iron
(Fe) (all in wt. %);
soaking the steel at 1200-1220C;
hot rolling the steel at FRT 870-920C;
coiling the steel at 640-700 °C;
cold rolling the steel with 70-90% reduction; and
continuous annealing the steel at 820 - 8801 for 1-5 mins.
2. The method for making the interstitial-free steel as claimed in claim 1,
wherein composition of the steel is C = 0.002, Mn = 2.02, S =0.006, P =
0.115, Si = 0.052, Al = 0.08, N = 0.007, Ti = 0.012, Nb = 0.024, B = 12
ppm rest Iron (Fe) (all in wt. %).
3. The method for making the interstitial-free steel as claimed in claim 1,
wherein the steel is casted in vacuum induction furnace.
4. The method for making the interstitial-free steel as claimed in claim 1,
wherein the soaking of the steel is done for 1 hr. for 25mm stock.
5. The method for making the interstitial-free steel as claimed in claim 1,
wherein the steel is coiled in a muffle furnace.
6. The method for making the interstitial-free steel as claimed in claim 1,
wherein pickling of the steel is performed between the steps of coiling
and cold rolling.
7. The method for making the interstitial-free steel as claimed in claim 6,
wherein pickling is done in cone. HCI (10% by vol.).

8. The method for making the interstitial-free steel as claimed in claim 1,
wherein reduction in size of the steel in hot rolling is upto 3-5 mm.
9. The Interstitial-free steel obtained from process as claimed in anyone of
claims 1-8 has r-bar of 1.6-1.7.
10. The Interstitial-free steel obtained from process as claimed in anyone of
claims 1-8 has ultimate tensile strength (UTS) of 540-560 MPa.
11. The Interstitial-free steel obtained from process as claimed in anyone of
claims 1-8 has yield strength (YS) of 330-360 MPa.
12. The Interstitial-free steel obtained from process as claimed in anyone of
claims 1-8 has El% of 31-34.
13. An interstitial-free steel comprising:
Carbon (C) = 0.001-0.003, Manganese (Mn) = 1.5-2.5, Sulphur (S) =
0.003-0.012, Phosphorus (P) = 0.07-0.12, Silicon (Si) = 0.05-0.4,
Aluminum (Al) = 0.03-0.1, Nitrogen (N) = 0.005-0.009, Titanium (Ti) =
0.01-0.05, Niobium (Nb) = 0.01- 0.05, Boron (B) = 10-40 ppm, rest Iron
(Fe) (all in wt. %).