Field of Invention
This invention relates to cold-rolled and annealed interstitial-free high-strength
steel.
Background of Invention
Interstitial-free (IF) steels form a material of choice 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.
Some of the developments that have happened are described below. JP2003293084A relates to provide a hot rolled steel sheet which exhibits a high increase of tensile strength in coating/baking treatment at 150[deg.]C, and to provide a production method thereof. SOLUTION: The hot rolled steel sheet having excellent baking hardenability has a composition containing, by mass, 0.035 to 0.16% C, <=0.5% Si, 0.05 to 1% Mn, <=0.08% P, <=0.02% S, 0.01 to 0.1% Al, 0.0025 to 0.009% N and 0.0005 to 0.005% B, and further containing solid solution carbon in a range of 40 to 70 ppm and solid solution nitrogen in a range of 15 to 25 ppm, and the balance Fe with inevitable impurities.
JP4176823A teaches to produce the steel plate with superior productivity by subjecting a steel, having a composition in which respective contents of C, Si, Mn, P, S, Al, N, Ti, and Nb are specified, to hot rolling, cooling, and coiling under respectively prescribed conditions and then exerting heating in a carburizing-atmosphere gas. CONSTITUTION:A steel having a composition consisting of, by weight, <=0.01% C, <=0.2% Si, <=0.05-l% Mn, <=0.1% P, <=0.02% S, 0.01-0.08% Sol Al, 40.005% N, further specific amounts of Ti and/or Nb, and the balance Fe is refined. This steel is heated at 1000-1250 deg. C, and hot rolling is

finished at a temp, between (Ar3-50) and 950 deg. C. Subsequently, the resulting plate is cooled within 2sec at >=10 deg. C/S cooling rate, and coiled at 400-800 deg. C. Then, this plate is pickled and heated in a carburizing-atmosphere gas, by which solid-solution C is provided by 10-30ppm.
CA 2297291 A1 also relates to high tensile strength hot-rolled steel sheet suitable for use in interior materials for automobiles and a method for producing the same, in which bake hardenability, fatigue resistance, crash resistance, and resistance to room temperature aging are improved, containing 0.01% to 0.12% by weight of carbon, 2.0% by weight or less of silicon, 0.01% to 3.0% by weight of manganese, 0.2% by weight or less of phosphorus, 0.001% to 0.1% by weight of aluminum, and 0.003% to 0.02% by weight of nitrogen and subjected to hot rolling and cooling at a cooling rate of 50.degree.C/s or more within 0.5 second after hot rolling; the hot-rolled steel sheet has a structure including a ferrite having an average grain diameter of 8 urn or less as a primary phase, the amount of solute Nitrogen ranges from 0.003% to 0.01%, and the ratio, Ngb/Ng, of an average concentration Ngb of nitrogen dissolved in the ferrite grain boundary to an average concentration Ng of nitrogen dissolved in ferrite grains ranges from 100 to 10,000.
W096/14444 teaches about bake hardenable vanadium containing steel has a composition consisting in weight percent of between 0.0005 and 0.1 % carbon, between zero and less than 0.04% nitrogen, between zero and less than 0.5 % of a nitride forming element, between zero and 0.5 % aluminum, between zero and up to 2.5 % manganese, between 0.005 and 0.6 % vanadium with the balance iron and inevitable impurities.
However the above mentioned developments do not provide the strength of upto a level of 500 MPa minimum and anisotropy index of 1.5 or more.

Objects of the Invention
In view of the foregoing limitations inherent in the prior-art, it is an object of the invention to propose a method of making high-strength interstitial-free steel after cold-rolling and annealing.
Another object of this invention 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 INVENTION
In one aspect, the invention provides method for making interstitial-free steel comprising steps of casting steel with required composition, soaking the steel at 1200-1220OC; hot rolling the steel at FRT 870-920oC; coiling the steel at 640-700° C; cold rolling the steel with 70-90% reduction and continuous annealing the steel at 820 - 880°C for 1-5 mins.
In another aspect, the invention provides interstitial-free steel having following composition of Carbon (C) = 0.005-0.007, 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 interstitial free steel in accordance with an embodiment of the invention.
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 invention.

Detailed Description of Invention
Various embodiments of the invention provide method for making interstitial-free steel, the method comprising steps of: casting steel using composition of Carbon (C) = 0.005-0.007, 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 1200-1220oC, hot rolling the steel at FRT 870-920oC, coiling the steel at 640-700° C, cold rolling the steel with 70-90% reduction and continuous annealing the steel at 820 - 880 °C for 1-5 mins.
Shown in FIG. 1 is a flow a diagram showing method (100) for manufacturing interstitial-free steel (IFS). The method comprises steps of:
Step (104): casting steel using following composition:
Carbon (C) = 0.005- 0.007, 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 1200-1220oC;
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 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.
The preferred composition of the steel can be shown in Table 1.

In an embodiment the soaking of the steel is done for 1 hr. for 25mm stock.
At 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.
After coiling step (116), 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.

While 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.

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) = 0.005-0.007, 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 25 kg ingot. The material was then soaked at 1200 C for more than 10 hours in a muffle furnace, forged and cut into in to 20 mm square bars. These bars were further hot rolled into 3 mm thick plates in an experimental hot rolling mill. The finish rolling temperature was maintained at above 900 C and then transferred into a muffle 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 820-880oC 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 interstitial-free steel, the method comprising steps
of:
casting steel using following composition
Carbon (C) = 0.005-0.007, 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 1200-1220oC;
hot rolling the steel at FRT 87O-920oC;
coiling the steel at 640-700°C;
cold rolling the steel with 70-90% reduction; and
continuous annealing the steel at 820 - 880oC for 1-5 mins.
2. The method for making interstitial-free steel as claimed in claim 1, wherein composition of the steel is C = 0.006, 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 (all in wt. %).
3. The method for making interstitial-free steel as claimed in claim 1, wherein the steel is casted in vacuum induction furnace.
4. The method for making 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 interstitial-free steel as claimed in claim 1, wherein the steel is coiled in muffle furnace.

6. The method for making 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 interstitial-free steel as claimed in claim 6, wherein the pickling is done in conc. HCI (10% by vol.).
8. The method for making interstitial-free steel as claimed in claim 1, wherein reduction in size of the steel in hot rolling is upto 3-5 mm.
9. Interstitial-free steel obtained from above mentioned process as claimed in claims 1-8 has r-bar of 1.6-1.7.
10. Interstitial-free steel obtained from above mentioned process as claimed in claims 1-8 has ultimate tensile strength (UTS) of 540-560 MPa.
11. Interstitial-free steel obtained from above mentioned process as claimed in claims 1-8 has yield strength (YS) of 330-360 MPa.
12. Interstitial-free steel obtained from above mentioned process as claimed in claims 1-8 has El% of 31-34.
13. An interstitial-free steel comprising:
Carbon (C) = 0.005-0.007, 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. %).