TITLE:
A hot-rolled bake-hardening steel for automotives.
FIELD OF THE INVENTION:
This invention relates to a hot-rolled Bake hardening steel for
automotives.
BACKGROUND OF THE INVENTION:
Bake-hardening (BH) grade steels rely on the controlled amount of
solute carbon and nitrogen atoms to enhance yield strength (YS) after
paint-bake stage of automobile components. This is conventionally
achieved by adjusting the chemical composition of the steels in order
to retain some carbon atoms in solution for the BH property.
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. ; COPYRIGHT: (C) 2004, JPO.
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, <=0.05-1% Mn, <=0.1% P, <=0.02%
S, 0.01-0.08% SolAI, 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 .mu.m 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 % aluminium, between zero and up to
2.5 % manganese, between 0.005 and 0.6 % vanadium with the
balance iron and inevitable impurities.
In the present invention, new bake-hardening grade steel is
developed based on simple aluminum-killed ultra-low carbon
chemistry containing manganese and phosphorous with ultimate
tensile strength of no less than 500 MPa. The material has been
produced through vacuum induction steel making furnace and a small
experimental hot-rolling unit. The material was tested and found to
record an attractive combination of strength, ductility and BH index.

OBJECTS OF THE INVENTION:
An object of this invention is to propose a hot-rolled bake-hardening
steel for automotives.
Another object of this invention is to propose a process for producing
hot-rolled bake-hardening steel having YS > 400MPa, UTS >
500MPa, %EI > 28 and BHI > 30MPa.
Further object of this invention is to propose a process for producing
hot-rolled bake-hardening steel plate of 3mm thickness.
BRIEF DESCRIPTION OF THE PRESENT INVENTION:
According to this invention there is provided a composition of hot-
rolled bake hardening steel for automotive comprises:
C < 0.01 Weight percentage
Mn < 2.5 Weight percentage
S < 0.012 Weight percentage
P < 0.12 Weight percentage
Si < 0.1 Weight percentage
Al < 0.1 Weight percentage
N < 0.009 Weight percentage

Ti < 0.1 Weight percentage
Nb < 0.1 Weight percentage
B 10-40 ppm
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS:
Figure 1 shows as Schematic illustration of Bake-hardening tests.
Figure 2 shows as Microstructure of the steel after hot rolling shows
only ferrite as the dominant phase, with some martensitic second
phase.
DETAILED DESCRIPTION OF THE INVENTION:
The present invention is directed to hot-rolled bake-hardening steel
(HRBS 500) for automotive structural and suspension part application
with YS > 400 MPa, UTS > 500 MPa, %EI > 28 and BHI > 30 MPa.
The strength of the steel will mainly rely on the sufficient alloying
addition of manganese and phosphorous. In addition, the bake-
hardening behavior of the material would arise from the free carbon
present in the steel. The latter would be adjusted with microalloying
additions such as titanium and niobium in order to lock all nitrogen
atoms as nitride or carbonitirde but keeping some solute carbon free
in solution.

Process steps for steel making:
i) Steel making by vacuum induction furnace
ii) Ingot casting
iii) Hot rolling and coiling
The 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 homogenized 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 rollng temperature was maintained at above
900 C and then transfered into a muffle furnace for coiling simulation
at 680 C. After about holdng for an hour the material was cooled to
ambient temperature in air.
Material Specification of the steel:
(i) Steel plate 3 mm thickness, as rolled in laboratory scale hot
rolling mill,
(ii) Chemistry of steel specified is given in Table 1
(iii) Mechanical properties of the product specified are as follows
(Table 2):
YS > 400 MPa, UTG > 500 MPa, %EI > 30, BHI > 30 MPa

Table 1: Chemistry of steel specified for the new HBHS 500 steel

The tensile properties were measured using test specimens with 50
mm gage length (prepared according to ASTM E8 specification),
fitted with an extensometer. All tests were performed at room
temperature. The BHI of the steel was tested with 6% pre-strain
followed by ageing at 170 C for 20 minutes, as illustrated
schematically in Fig 1.
The preferable range of Manganese (Mn) is 1.8 to 2.2 weight
percentage.
The preferable range of Phosphorous (P) is 0.08 to 0.12 weight
percentage.
The chemical composition of the alloy that revealed the best results is
given in Table 2. Mechanical properties of the material in hot-rolled
condition are presented in Table 3. The results indicate that the new

steel with the optimum chemistry recorded excellent combination of
strength and ductility. The high tensile strength of the steel would
help reduce the gage of the body panels and hence the weight of the
car. The BHI of the material was recorded consistently above 30
MPa. The material is also revealed to possess a single phase ferritic
microstructure, with a second martensitic phase of small fraction
(Figure 2).
Table 2: Chemical composition of the alloy that revealed the best
properties


Table 3: Mechanical properties of the hot-rolled and bake-
hardened material, notice the YS and UTS of the steel becomes
equal after bake-hardening

The newly developed HBHS500 grade steel can be used in
manufacture of structural and suspension parts of automobiles.
High tensile strength of the material would allow usage of thinner
gage outer panels and help reduce the weight of the car body
Adequate BHI of the material would ensure durability of the parts
under high service load conditions

WE CLAIM:
1. A composition of hot-rolled bake hardening steer for automotives
comprises:
C < 0.01 Weight %
Mn < 2.5 Weight %
S < 0.012 Weight %
P < 0.12 Weight %
Si < 0.1 Weight %
Al < 0.1 Weight %
N < 0.009 Weight %
Ti < 0.1 Weight %
Nb < 0.1 Weight %
B 10-40 ppm
2. The composition as claimed in claim 1 preferably comprises of :
C - 0.006 Weight %
Mn - 2.02 Weight %
S - 0.006 Weight %
P - 0.115 Weight %
Si - 0.052 Weight %
Al - 0.08 Weight %
N - 0.007 Weight %
Ti - 0.012 Weight %
Nb - 0.024 Weight %
B - 12 ppm

3. The composition as claimed in claim 1, preferably comprises
Manganese (Mn) in the range of 1.8 to 2.2 weight percentage.
4. The composition as claimed in claim 1, preferably comprises
Phosphorous (P) in the range of 0.08- 0.12 weight percentage.
5. The composition as claimed in claim 1 wherein the steel has
high YS almost equal to UTS of the steel, beyond 500 MPa.
6. The composition as claimed in claim 1 wherein the mechanical
properties possessed by the steel are as follows:
YS > 400 MPa, UTS > 500 MPa, % E1 > 30, BHI > 30 MPa
7. A process for making hot-rolled bake-hardening steel comprising
subjecting the steel obtained from vacuum induction furnace to
the step of Ingot casting, homogenizing the material at 1200°C
for more than 10 hrs in a muffle furnace,
forging and cutting into square bars, subjecting the square bars
to the step of hot rolling at above 900°C into 3mm thick plates,

transferring the plates into a muffle furnace for coiling at 680°C
for almost an hour, and cooling the material to the ambient
temperature in air.

ABSTRACT

A composition of hot-rolled bake hardening steel with YS ≥ 400 MPa,
UTS ≥ 500 MPa, % E1 ≥ 30, BHI ≥ 30 MPa for automotives
comprises:
C ≤ 0.01 Wt. %
Mn ≤ 2.5 Wt. %
S ≤ 0.012 Wt. %
P ≤ 0.12 Wt. %
Si ≤ 0.1 Wt. %
Al ≤ 0.1 Wt. %
N ≤ 0.009 Wt. %
Ti ≤ 0.1 Wt. %
Nb ≤ 0.1 Wt. %
B 10-40 ppm