FIELD OF THE INVENTION
The present invention relates to an aluminum-killed ultra-low carbon based bake-
hardening grade steel for producing outer panels of automobiles.
BACKGROUND OF THE INVENTION
Bake-hardening (BH) grade steel relies on the controlled amount of solute carbon
and nitrogen atoms to enhance yield strength (YS) after completion of paint-
bake stage of automobile components. This is conventionally achieved by
adjusting the chemical composition of the steel in order to retain some carbon
atoms in the solution for the BH property, without deteriorating the original
properties of the steel, in particular the formability. BH steel is therefore required
to have an original set of mechanical properties such as YS, UTS, %Elongation
and r-bar in as-supplied condition. Additionally, the steel is required to have
sufficient bake-hardening index (BHI) for measuring the ability of the material to
enhance the YS after paint-baking of the formed panels.
The BHI property makes these steels more dent-resistant compared to the
conventional interstitial-free (IF) grade steels used for automotive outer panels.
The latter material is selected for such applications mainly because of the
superior drawability but suffers from the limitation of being prone to form dents.
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 value of 1.8.
The excellent r-bar value of IF steels originate from a favorable crystallographic
texture of the steels. This is possible because the interstitial atoms namely
carbon and nitrogen exist as precipitates such as carbide, nitride and/or
carbonitrides due to sufficient titanium and/or niobium content of these steels.
Adjusting the titanium and niobium content would leave a small portion of
carbon atoms in solution resulting into the BH effect. Conventionally, BH grade
steels are processed in continuous annealing lines after cold-rolling, similar to IF
grade steels in order to attain a favorable combination of properties.
OBJECT OF THE INVENTION
The object of the present inventions is to propose an aluminium-killed ultra-low
carbon based bake-hardening grade steel for automotive outer-panel application
with YS = 195 MPa, UTS = 320 MPa, %EI = 36, r-bar = 1.6, and BHI=30 MPa.
SUMMARY OF THE INVENTION
According to the invention, a BH grade steel is produced based on aluminum-
killed ultra-low carbon based material, without any addition of titanium and
niobium. The material is produced through cold-rolling and batch-annealing route
with an improved combination of strength and formability as well as a sufficiently
high BH index, suitable for automotive outer panel applications.
An aluminium-killed ultra-low carbon-based bake-hardening steel grade for
producing outer panels of automobiles, comprising a steel grade with chemistry
of C, Mn, S, P, Si, Al, N, Ti, and Nb (wt%) respectively at = 0.0035, = 0.7, =
0.012, = 0.07, = 0.1, = 0.08, = 0.006, = 0.001 and = 0.001, the process route
being steel making by LD converter, secondary steel making by RH degassing
continuous slab, hot rolling and cooling with optimum process parameters, cold
rolling and batch-annealing, and skin pass rolling, wherein the starting steel
material being supplied in the form of steel coils, and wherein the mechanical
properties achieved for the new grade of steel being YS = 195 MPa, UTS = 320
MPa, %EL = 36, r-bar = 1.6, BHI = 30 MPa.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 - shows a schematic representation of BHI-measurement of the new
grade steel.
Figure 2 - photographic views of microstructure of the steel grade according to
the invention in cold-rolled and batch-annealed condition after skin pass rolling.
Figure 3 - shows a texture data showing a favorable orientation (222) of the
grains present in the new grade steel material.
DETAILED DESCRIPTION OF THE INVENTION
Material specification in supply condition:
(i) Steel coils of 0.5-1.5 mm thickness and 1000 - 1300 mm width
(ii) Chemistry of steel specified is given in Table 1
(iii) Mechanical properties of the product specified are as follows:
YS=195 MPa, UTS=320 MPa, %E1=36, r-bar= 1.6, BHI= 30 MPa
Table 1: Chemistry of steel specified for the new BH-195 steel (wt%)
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
Table 2 : Hot rolling and cold rolling parameters specified for
processing of the new BH -195 steel
The BH-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 coils are then picked, cold-rolled into final section size and processed
through batch annealing and skin pass rolling route.
The final product when inspected exhibited negligible oxidation mark in the coils.
Samples were collected form the middle portion of the coils. These were then
used for tests in laboratory to determine the mechanical properties and carry-out
metallographic investigation.
Mechanical tests and metallography:
The tensile properties of the invented steel grade have been 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 BHI of the steel is tested with 2% pre-
strain followed by ageing at 170°C for 20 minutes, as illustrated schematically in
Figure-1. The r-bar values recorded are the average of the r values in the
parallel, normal and diagonal to the rolling directions, namely, r-bar = (rO + 2r45
+ r90)/4
Metallographic samples are prepared and etched with 5% natal. A light optical
microscope is used to record the size of the grains comprising the material. The
crystallographic texture of the material is studied using an x-ray diffractometer
fitted with a goniometer.
Results:
Mechanical properties
Mechanical properties of the material as evaluated in cold-rolled and batch-
annealed condition are presented in Table 3. The results indicate that the new
grade BH-195 steel with an optimum chemistry recorded consistent properties in
all coils. The high tensile strength of the steel enables reduction in the gage of
the body panels ad hence the weight of the car. The coils exhibit r-bar value
above 1.6. The BHI of the material recorded to be consistently above 30 MPa.
Table 3 : Mechanical properties of the cold-rolled and annealed coils
tested after skin pass deformation
Metallography
Microstructure of the new grade BH-195 steel in cold-rolled and annealed
condition is shown in Figure-2. The steel is found to be comprised of pancaked
grains of ferrite with a fine size of ASTM number 8. The crystallographic texture
of the steel is presented in Figure 3. The material is found to possess a strong
intensity of the gamma-fiber texture which is good for the r-bar value.
The invention is based on the following process parameters and activities.
(1) Selection of an optimum chemistry of the steel results in favorable
mechanical properties suitable for high-end automotive steel products
(2) Judicious selection of alloying elements allow achievement of high YS
and UTS of the steel without compromising formability
(3) Elimination by way of addition, the titanium and niobium in the steel
results into free carbon and nitrogen that provides sufficient value of BHI
(4) A combination of an optimum chemical composition and processing
parameters enables to achieve superior r-bar value or the formability
(5) Microstructure and texture of the new grade of BH-steel was found to be
favorable for mechanical properties
(6) Minimum manganese content of the steel results into reduced severity of
oxidation mark in the product.
Advantages
1) High tensile strength of the material would allow usage of thinner gage
outer panels and help reduce the weight of the car body
2) The superior r-bar value of the material would allow easy forming of
components designed with complicated shapes
3) Adequate BHI of the material shall reduce the susceptibility of the panels
to form dents during service life of the car
WE CLAIM
1. An aluminium-killed ultra-low carbon-based bake-hardening steel grade
for producing outer panels of automobiles, comprising:
- providing a steel grade with chemistry of C, Mn, S, P, Si, Al, N, Ti,
and Nb (wt%) respectively at = 0.0035, = 0.7, = 0.012, = 0.07, = 0.1,
= 0.08, = 0.006, = 0.001 and = 0.001, the process route being steel
making by LD converter, secondary steel making by RH degassing
continuous slab, hot rolling and cooling with optimum process
parameters, cold rolling and batch-annealing, and skin pass rolling,
wherein the starting steel material being supplied in the form of steel
coils, and wherein the mechanical properties achieved for the new
grade of steel being YS = 195 MPa, UTS = 320 MPa, %EL = 36, r-bar
= 1.6, BHI = 30 MPa.
2. The steel grade as claimed in claim 1, wherein the optimum process
parameters are : temperatures of finish rolling, coiling, batch annealing
respectively being 830 - 950 °C, 530 - 650 °C, and 640 - 740 °C, and
the cold rolling deformation and skin pass elongation being 50 - 90%,
and 0.4 - 2% respectively.
3. An aluminium-killed ultra-low carbon-based bake-hardening steel grade
for producing outer panels of automobiles as substantially described and
illustrated herein with reference to the accompanying drawings.

The invention relates to an aluminium-killed ultra-low carbon-based bake-
hardening steel grade for producing outer panels of automobiles, comprising
providing a steel grade with chemistry of C, Mn, S, P, Si, Al, N, Ti, and Nb (wt%)
respectively at ≤ 0.0035, ≤ 0.7, ≤ 0.012, ≤ 0.07, ≤ 0.1, ≤ 0.08, ≤ 0.006, ≤ 0.001
and ≤ 0.001, the process route being steel making by LD converter, secondary
steel making by RH degassing continuous slab, hot rolling and cooling with
optimum process parameters, cold rolling and batch-annealing, and skin pass
rolling, wherein the starting steel material being supplied in the form of steel
coils, and wherein the mechanical properties achieved for the new grade of steel
being YS ≥ 195 MPa, UTS ≥ 320 MPa, %EL ≥ 36, r-bar ≥ 1.6, BHI ≥ 30 MPa.