FIELD OF INVENTION
The invention relates to a galvannealed dual phase steel. Particularly the invention relates to a process for manufacturing it.
BACKGROUND
Dual phase steels are increasingly becoming popular in automotive market because of the advantage of reducing the car body weight and thereby improving on the fuel economy, carbon emission without compromising on the safety requirements of the passenger cars. These materials are produced in hot-rolled conditions mainly for the wheels and suspension parts. However, dual phase steels produced in cold rolled conditions are mainly suited for structural applications such as A-pillar or B-pillars as well as front and rear bumper reinforcements. The most attractive of these grades are those containing an additional zinc-iron alloy coating layer and are known as galvannealed dual phase steels. The coating layer helps to inhibit corrosion of the steel substrate thereby increasing the service life of the components.
The US patent US8337643B2 discloses a hot rolled steel sheet having a dual phase microstructure with the martensite content of less than 35 vol% and ferrite more than 50 vol%; a composition containing by percent weight: 0.01≦C≦0.2; 0.3≦Mn≦3; 0.2≦Si≦2; 0.2≦Cr+Ni≦2; 0.01≦Al≦0.10; Mo less than about 0.2%, 0.0005≦Ca≦0.01, with the

balance iron and incidental ingredients. Hot rolled sheet for cold rolling, the silicon range may be from about 0.05% to about 2%, and the amount of molybdenum may be up to 0.5%. Also, the hot rolled steel sheet has a tensile strength of at least 500 MPa, a hole expansion ratio more than about 50%, and a yield strength/tensile strength ratio less than 70%. It may be noted that in context of automotive steels, dual phase steels gain prominence with strength level in excess of 600 MPa rather than only 500 MPa.
Another patent, US 20120255654 A1 relates to a dual phase steel sheet and a method for manufacturing the same. The steel sheet possesses a chemical composition of C: 0.05˜0.10 % wt %, Si: 0.03˜0.50 wt %, Mn: 1.50˜2.00 wt %, P: greater than 0 wt %˜0.03 wt %, S: greater than 0 wt %˜0.003 wt %, Al: 0.03˜0.50 wt %, Cr:0.1˜0.2 wt %, Mo: 0.1˜0.20 wt %, Nb: 0.02˜0.04 wt %, B: greater than 0 wt %˜0.005 wt %, N: greater than 0 wt %˜0.01 wt %, and the balance of Fe and other unavoidable impurities. The steel was made such that it possessed excellent formability, bake hardenability, dent resistance, high R value and plating characteristics to the steel sheet for exterior and interior panels of automobiles, the steel sheet is processed to have a dual phase structure through cold rolling, annealing, and hot-dip galvanizing. However, the chemistry of the material appears to be very rich containing the expensive miroalloying elements. Further, it contains silicon which is notoriously known to deteriorate the surface finish of the product.

OBJECTIVES OF THE INVNETION
In view of the foregoing limitations inherent in the prior-art, it is an object of the invention to develop a new process for manufacturing a galvannealed dual phase steel.
Another object of the invention is to develop the galvannealed dual phase steel such that the ultimate tensile strength (UTS)>600MPa.
Still another object of the invention is to develop the galvannealed dual phase steel having minimal silicon (Si) content and use comparatively less expensive alloys.
SUMMARY OF THE INVENTION
In one aspect, the invention provides a process for making a galvannealed
dual phase steel the process comprising steps of casting a steel of
thickness >200-250 mm with composition of Carbon (C) = 0.06-0.12,
Manganese (Mn) = 1.2-2.4, Sulphur (S) = 0.003-0.012, Phosphorus (P) =
0.007-0.012, Silicon (Si) = 0.02-0.08, Aluminum (Al) = 0.03-0.1, Nitrogen
(N) = 0.002-0.008, Chromium Cr 0.3-0.7, rest ron Fe and incidental
ingredients all in wt. % , soa ing the steel at 1200-1220C for 1.5-2.5hrs,
hot rolling the steel at F 870-920C with reduced thickness achieved of
3-6mm with speed of 80-90 meters per minute mpm , coiling the steel at 640-700 C, cold rolling the steel with 50-65% reduction, continuous annealing the steel at 750 - 850 C at speed of 60-80 mpm, galvanizing the steel at 460-480 oC in zinc bath, and galvannealing the steel at 500-520 oC for 20-30 s.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS FIG. 1 shows flow diagram of process illustrating various steps for manufacturing a galvannealed dual phase steel in accordance with an embodiment of the invention.
FIG. 2 show the microstructure of the galvannealed dual-phase steel processed as an experiment.
DETAILED DESCRIPTION OF THE INVENTION
Various embodiment of the invention provide a process for making a
galvannealed dual phase steel, the process comprising steps of casting a
steel of thickness >200-250 mm with following composition Carbon (C) =
0.06-0.12, Manganese (Mn) = 1.2-2.4, Sulphur (S) = 0.003-0.012,
Phosphorus (P) = 0.007-0.012, Silicon (Si) = 0.02-0.08, Aluminum (Al) =
0.03-0.1, Nitrogen (N) = 0.002-0.008, Chromium (Cr) = 0.3-0.7, rest Iron
(Fe) and incidental ingredients all in wt. % ; soa ing the steel at 1200-
1220C for 1.5-2.5hrs; hot rolling the steel at F 870-920C with thic ness
achieved of 3-6mm with speed of 80-90 meters per minute mpm ; coiling
the steel at 640-700 C; cold rolling the steel with 50-65% reduction;
continuous annealing the steel at 750 - 850 C at speed of 60-80 mpm;
galvanizing the steel at 460-480 oC in zinc bath; and galvannealing the
steel at 500-520 oC for 20-30 s.
Shown in FIG. 1 is a flow a diagram of a process (100) illustrating various steps for manufacturing a galvannealed dual-phase steel (DP). The process comprises various steps from 104 to 132.

At step (104) a steel is casted of thickness >200-250 mm with following composition:
Carbon (C) = 0.06-0.12, Manganese (Mn) = 1.2-2.4, Sulphur (S) = 0.003-0.012, Phosphorus (P) = 0.007-0.012, Silicon (Si) = 0.02-0.08, Aluminum (Al) = 0.03-0.1, Nitrogen (N) = 0.002-0.008, Chromium (Cr) = 0.3-0.7, rest Iron (Fe) and incidental ingredients (all in wt. %).
The steel is refined in BOF converter and casted in continuous casting strands. In some embodiment the steel can be manufactured in other furnaces as per requirement.
At step (108) the steel is soaked at 1200-1220C for 1.5-2.5hrs.
At step (112) the steel is hot rolled at F 870-920C with reduced
thickness achieved as 3-6mm. The speed is maintained at 80-90 meters per minute (mpm). The hot rolling of the steel is done by 6 stands. The number of the stands can be varied as per the requirement keeping the reduction of steel upto 3-6mm.
At step (116) the steel is coiled at 640-700 C. The coiling of the steel, at step (116), is performed in the real hot strip mill condition.
Pickling of the steel is performed after coiling step. The pickling is done with HCl acid to remove scales from the surface. The concentration of the HCl used is 10% by vol.

At step (120) the steel is cold rolled with 50-65% reduction. The cold rolling of the steel is done by 5 stands. The number of the stands can be varied as per the requirement keeping the reduction upto 50-65%.
At step (124) the steel is continuously annealed at 750 - 850 C at speed of 60-80 mpm.
At step (128) the steel is further galvanized at 460-480 oC in zinc bath. The concentration of the zinc bath is maintained at 99.00% by vol.
At step (132) the steel is galvannealed at 500-520 oC. The duration for galvannealing is 20-30 sec.
The preferred composition of the steel can be shown in Table 1.

The composition of the steel as mentioned in step (104) provides the extra hardenability requirement in a situation where the rate of cooling from continuous annealing temperature to that of galvanizing as mentioned in step (124) – (132) is not very fast such as less than 20 oC/s. In accordance with an embodiment of the invention, the composition is

selected in such a way that the required dual phase microstructure containing ferrite and 10-20 vol.% martensite (by vol.) is formed even with a very slow cooling rate of 7 – 13 oC/s.
The strength of the steel mainly relies on the volume fraction of the hard martensite phase dispersed in the soft and ductile ferrite. In order to ensure that the austenite formed during continuous annealing actually transforms to martensite, it is necessary to add sufficient quantities of manganese and chromium in the steel. These elements increase the hardenability and avoid the formation of the high temperature phases efficiently by shifting the continuous cooling transformation (CCT) curve to the right.
However, it is important to note that other elements such as silicon or phosphorous also play a similar role but are restricted in accordance with an embodiment the invention. This is mainly to ensure a good surface wettability of the substrate in order to achieve uniform and superior coating layer quality as well as surface free of bare spots.
Concentration of silicon is controlled by maintaining the optimum slag basicity during steel making process, where lime is added to the liquid bath for reacting with silica thereby facilitating extra removal of Si. It is done to ascertain the superior product quality such as strength, ductility as well surface appearance after galvannealing.
The obtained mechanical properties of the galvannealed dual phase steel obtained by means of the process (100) are shown in Table 2.


The galvannealed dual phase steel obtained from the process has 10-20% martensite and rest is ferrite (by vol.).
In accordance with an embodiment of the invention the yield ratio (YR) which dictates the forming characteristics of the material may deteriorate with the amount of martensite in the microstructure and therefore the martensite content is restricted to only 10-15 vol. %.
Example:
The above mentioned process for making galvannealed dual phase steel and its properties can be validated by the following examples. The following examples should not be construed to limit the scope of invention.
New steel with the optimum chemical composition mentioned in (Table 1) was produced and processed. The steel was made in BOF converter without any vacuum treatment and cast into slabs of 250 mm thickness. The material was then soaked at 1200 oC for about 2 hours in reheating furnace and hot-rolled into 3.2 mm thick strips. The speed maintained was

84 mpm. he finish rolling temperature was maintained at 900 C and coiling was done at 660 oC.
The hot-rolled material was subsequently pickled in HCL solution 10% by vol. to remove the surface scale. Further, the strips were cold-rolled into thin sheets of thickness 1.2 mm. The cold rolled coils were then annealed at a temperature of about 760-800 C with line speed of 70 mpm. subsequently the strip was passed through molten zinc bath maintained at the temperature of 470 deg. C followed by galvannealing at 510 deg. C for 25 sec.
Samples were then tested for mechanical properties and assessed for microstructural details.
FIGS. 2 show the microstructure of the steel processed in the full scale industrial continuous galvanizing line.
The following mechanical properties have been obtained for samples following above mentioned process as shown in Table 3.


Advantages:
The UTS of the galvannealed dual phase steel is > 600MPa using low Si content. Further the alloys used are comparatively less expensive in light of prior art.
The newly developed galvannealed dual phase steel can be used in manufacture of critical parts of automobiles such as the A-pillar, B-pillar, front and rear bumper reinforcement parts.

High tensile strength of the material would allow usage of thinner gage outer panels and help reduce the weight of the car body
Low yield ratio of the material would ensure critical formability of the parts under high speed forming operations.

We claim:
1. A process for making a galvannealed dual phase steel, the process comprising steps of:
casting a steel of thickness >200-250 mm with following composition
Carbon (C) = 0.06-0.12, Manganese (Mn) = 1.2-2.4, Sulphur (S) = 0.003-0.012, Phosphorus (P) = 0.007-0.012, Silicon (Si) = 0.02-0.08, Aluminum (Al) = 0.03-0.1, Nitrogen (N) = 0.002-0.008, Chromium (Cr) = 0.3-0.7, rest Iron (Fe) and incidental ingredients (all in wt. %);
soa ing the steel at 1200-1220oC for 1.5-2.5hrs;
hot rolling the steel at FRT 870-920oC with speed of 80-90 meters per minute (mpm) achieving reduced thickness of 3-6 mm;
coiling the steel at 640-700oC;
cold rolling the steel with 50-65% reduction;
continuous annealing the steel at 750 - 850 C at speed of 60-80 mpm;
galvanizing the steel at 460-480 oC in zinc bath; and
galvannealing the steel at 500-520 oC for 20-30 s.

2. The process for making the galvannealed dual phase steel as claimed in claim 1, wherein composition of the steel is Carbon (C) = 0.09, Manganese (Mn) = 1.7, Sulphur (S) = 0.006, Phosphorus (P) = 0.010, Silicon (Si) = 0.05, Aluminum (Al) = 0.04, Nitrogen (N) = 0.003, Chromium (Cr) = 0.6, rest Iron (Fe) and incidental ingredients (all in wt. %).
3. The process for making the galvannealed dual phase steel as claimed in claim 1, wherein the steel is continuously cast after refining in BOF converter.
4. The process for making the galvannealed dual phase steel as claimed in claim 1, wherein pickling of the steel is performed after step of coiling and before cold rolling.

5. The process for making the galvannealed dual phase steel as claimed in claim 4, wherein pickling is done in conc. HCl (10% by vol.).
6. The process for making the galvannealed dual phase steel as claimed in claim 1, wherein the hot rolling of the steel is done by 6 stands.
7. The process for making the galvannealed dual phase steel as claimed in claim 1, wherein the cold rolling of the steel is done by 5 stands.
8. The galvannealed dual phase steel obtained from the process as claimed in anyone of claims 1-7 has ultimate tensile strength (UTS) > 600 MPa.

9. The galvannealed dual phase steel obtained from the process as claimed in anyone of claims 1-7 has yield strength (YS) of 360-440 MPa.
lO.The galvannealed dual phase steel obtained from the process as claimed in anyone of claims 1-7 has El% of 20-26.
ll.The galvannealed dual phase steel obtained from the process as claimed in anyone of claims 1-7 has yield ratio (YR) 0.60-0.65.
12.The galvannealed dual phase steel obtained from the process as claimed in anyone of claims 1-7 has 10-20% martensite and rest is ferrite (by vol.)