PROCESS FOR MANUFACTURING A HIGH STRENGTH FORMABLE AND
FATIGUE RESISTANT STEEL
Field of the invention
The present invention relates to a process for manufacturing a high strength,
formable and fatigue resistant steel. The steel manufactured by the process of the
present invention is particularly useful in the automobile sector, especially in the
manufacture of automobile chassis, wheel disc and rims.
Background of the invention
Several high strength steels are known in the art which have applications in specific
fields and are manufactured by various processes in order to optimize selected
physical properties of steels so produced, such as the yield strength, fracture
toughness and corrosion resistance.
JP06081080 teaches steel sheet excellent in fatigue properties and deep drawing
ability using IF steel having a specified composition and reducing its ferritic grain
diameter into specified value or below. The steel comprises by weight, <0.0050%C,
<0.3% Si, 0.10 to 0.5% Mn, <0.03%P, <0.015%S, <0.10%sol. Al and <0.0040%N
and Ti and/or Nb so as to satisfy 1 <(12Ti*)/(48C)+(12Nb)/(93C) <2, and the balance
Fe, and in which the ferritic grain diameter is regulated to <10mm; in the inequality,
Ti*=Ti-48/14N-48/32. Since this steel sheet is constituted of IF steel, high (r) value
can be secured. Furthermore, by regulating the ferritic grain diameter after annealing
to <10mm, its yield strength can be increased and its fatigue strength is more
increased the higher the yield strength is regulated, so that fatigue strength equal to
that of a low carbon aluminum killed steel sheet can be obtained.
JP07166236 teaches a process to produce a high strength hot rolled steel sheet
excellent in fatigue properties and stretch-flanging properties by subjecting a steel
having specified amounts of C, Si, Mn, S, P and Al to hot rolling, triple stage cooling
and coiling in succession under specified conditions. The steel comprising, by
weight, 0/05 to 0.20% C, 0.20% to 2.0%Si, 0.60 to 2.5%Mn, <0.005%S, <0.1%P and
<0.1%AI, and the balance Fe with inevitable impurities is subjected to hot rolling to
regulate the finishing temperature to Ar3+50°C to the Ar3-20°C. Next as for triple
stage cooling is executed at ~15°C/sec cooling rate, secondary cooling is executed
in such a manner that it is gradually cooled in the temperature range of 600 to 750°C
at <15°C cooling rate for 3 to 15 sec and third cooling is executed at ~15°C/sec. and
after that, it is coiled from the Ms point or above to <500°C.
JP2000290745 teaches a high strength steel sheet for working, excellent in fatigue
characteristic and safety against collision and capable of sufficiently meeting the
demands from the viewpoint of stabilization of material quality and reduction of costs,
and its manufacturing method. The steel sheet has, before forming, a microstructure
which contains ferrite of >180 Vickers hardness as a principal phase and also
contains retained austenito (y) in an amount of ~3% by volume fraction and in which
the volume fraction of the balance (secondary phase) of the structure is regulated to
<6% or <3%. Moreover, in the microstructure after 5% forming, remained austenite
(?) is contained in an amount of ~3% by volume fraction, and further, -0.130 work
hardening exponent is provided.
For the purpose of auto chassis production, it is required that the steel possess
special characteristics such as a low ratio of Yield Strength to Ultimate Tensile
Strength (YS/UTS ratio), good formability and high fatigue resistance. None of the
aforesaid patents teach steel having optimum combination of properties desired for
auto chassis production. Furthermore, the prior art specify a wide range of chemistry
along with very wide range of phases achieved for the properties. Steel having
specific characteristics for the purpose of auto chassis production is not known.
Furthermore, the steels available in the market as well as the ones taught in the
above prior art comprise costly alloying elements such as Niobium, Molybdenum,
Nickel, Copper etc. to achieve the desired properties and are therefore, not
economic. Moreover, the sulphur and phosphorous contents of these steels are not
low enough for good elongation property and other mechanical properties. Steels in
general having sulphur >0.015% and phosphorus >0.020% do not meet the
requirement of high formability and fatigue resistance. Anisotropy of the steel is also
worst affected by high sulphur and phosphorous.
Therefore, there is a need of a process for manufacturing a high strength, formable
and fatigue resistant steel which meets the requirements for production of
automobile chassis, wheel disc and rims, and such other items.
Objects of the invention
The object of the present invention is to provide a process for manufacturing a steel
which has high strength with good formability and high resistance to fatigue.
Another object of the present invention is to provide a process for manufacturing of
a steel which has a uniform polygonal ferrite-pearlite structure.
A further object of the present invention is to provide a process for manufacturing of
a steel which exhibits higher percentage elongation.
A further object of the present invention is to provide a process for manufacturing of
a steel which is based on a lean chemistry and is thus economical.
The applicants have now developed a thermo-mechanical process for
manufacturing high strength, formable and fatigue resistant steel wherein the
desired properties in steel plates are achieved by selection of steel chemistry, hot
rolling parameters and roll crown.
Summary of the invention
According to the present invention there is provided a process for manufacturing of
a high strength, formable and fatigue resistant steel comprising the steps of.
i) heating steel slabs having a steel composition comprising 0.02 to 0.18 wt
%carbon, 0.01 to 0.14 wt % manganese, 0 to 0.07 wt % silicon, 0 to 0.02 wt
% sulfur, 0 to 0.02 wt %, phosphorous, 0 to 0.05 wt%, aluminum, the rest
being iron, at a temperature between 1100 to 1350° C.
ii) rolling of said slabs in five or six stands in a hot rolling mill wherein the
finishing temperature is maintained between 850 to 950 ° C ; and
iii) coiling said rolled steel strip at a temperature between 600 to 750 ° C.
Detailed description of the invention
The various physical properties of steel of the present invention are optimized by
selecting a specific steel chemistry as well as selecting the heating temperature of
steel slabs, finishing temperature, coiling temperature of rolled strip in the hot strip
mill. The steel chemistry has been selected such that sulfur and phosphorous levels
are below 0.02 wt %. The process controls pearlite/ferrite ratio in steel in the
range 0.05 - 0.10 and the restriction on sulfur and phosphorous levels makes the
steel free from large size inclusions. The steel is weldable and free from harmful
oxides. The modified steel chemistry and the microstructure enhance the
formability and fatigue resistance together. Although, the processing cost of the
steel is marginally higher due to lower sulfur and phosphorous levels than in the
conventional steels, the critical forming of components oversheds the cost of
production.
The process firstly comprises making steel of following preferred composition:
0.05 to 0.15 wt% carbon, 0.03 to 1.2 wt% manganese, 0 to 0.05 wt% silicon, 0 to
0.01 wt% sulfur, 0 to 0.02 wt% phosphorus, 0 to 0.03 wt% aluminum, the balance
being iron.
The desired composition may be achieved by partial addition of Ferro-alloys
during steel refining.
The slabs are heated at a temperature preferably between 1200 to 1250°C for a
predetermined time, after which the heated slabs are rolled in at least 5 stands
under strict control of finishing and coiling temperatures. The finishing
temperature of the hot rolling is preferably maintained at temperature between
880 to 950°C. Roll crown and profile at different stands and temperature gradient
across the work rolls are controlled by rolling speed and quantity of water
sprayed on work rolls. Preferably 6 roll stands are used wherein the roll crowns
have values of 0.5, 0.45, 0.35, 0.35 and 0.2 mm (+ 5% range). Such parameters
reduce the variation in the width and thickness of the coil. Reduction in thickness
at each roll pass is fed to computer, which maintains the final thickness of the
plate. During the rolling the thickness of the steel is reduced substantially. After
the steps of rolling, the steel strips so formed are coiled wherein the temperature
are maintained such that the steel has 5 to 10% pearlite in ferritic structure.
Preferably, the coiling temperature is maintained between 650 and 700°C. The
steel so formed has low value of the ratio of yield stress to ultimate tensile
strength in order to arrest the fatigue crack growth.
The process of the invention will now be described with reference to a non-
limiting example.
Example
Steel slabs having chemical composition comprising 0.15 wt% carbon, 1.1 wt%
manganese, 0.03 wt% silicon, 0.02 wt% sulfur, 0.01 wt% phosphorous, 0.02 wt%
aluminum (the rest being iron), are heated in a reheating furnace at temperature
of 1225 C for 3.5 - 4 hrs. The heated slabs are then rolled in a six-stand rolling
mill wherein the finished temperature is maintained at 900 C. The roll crown
values for the stands are selected as 0.5, 0.45, 0.4, 0.35, 0.35 and 0.2 mm. Initial
thickness of slab is 180 mm which is reduced to 32 mm before finish rolling. The
thickness of steel strips after rolling was 5 to 10 mm. The rolled steel strips were
then coiled at temperature of 675 C. The strip is coiled on a coiler which is made
for coiling the long strips.
The steel composition made by the above process was found to have yield
strength of 310 MPa/mm2 and ultimate tensile strength of 470 MPa/mm2. The
percentage elongation was found to be 35%. The steel was found to possess 5
to 10% pearlite structure in the ferrite matrix. This steel is found to have sufficient
fatigue resistance for use in making vehicles, particularly automobile chassis.
We claim:
1. A process for manufacturing a high strength, formable and fatigue resistant
steel comprising the steps of:
(i) heating steel slabs having a steel composition comprising 0.02 to 0.18
wt% carbon, 0.03 to 1.2 wt % manganese, 0 to 0.07 wt% silicon, 0 to
0.02 wt% sulfur, 0 to 0.02 wt% phosphorous, 0 to 0.05 wt% aluminum,
the rest being iron, at a temperature between 1100 to 1350 C for a
predetermined time based on composition;
(ii) rolling of said slabs in five or six stands in a hot rolling mill wherein the
finishing temperature is maintained between 850 to 950 C by
maintaining the temperature of feed plate from 1020 to 1050°C; and
(iii) coiling of rolled steel strip at a temperature between 600 to 750 °C.
2. A process according to claim 1, wherein said steel composition comprises
0.05 to 0.15 wt% carbon, 0.03 to 1.2 wt % manganese, 0 to 0.05 wt% silicon,
0 to 0.01 wt % sulfur, 0 to 0.02 wt% phosphorus, 0 to 0.03 wt% aluminum, the
balance being iron.
3. A process according to any of claims 1 or 2, wherein said steel composition
comprises 0.15 wt% carbon, 1.1 wt% manganese, 0.03 wt% silicon, 0.01 wt%
sulfur, 0.01 wt% phosphorous, 0.02 wt% aluminum, the balance being iron.
4. A process according to claim, wherein said heating of steel slabs is carried
out at temperature between 1200 to 1250 C for 3.5 - 4hrs.
5. A process according to claim 4, wherein said temperature is maintained at
1225°C.
6. A process according to claim 1, wherein said rolling is carried out in at least
five stands.
7. A process according to claim 1, wherein said rolling is carried out in a six-stand
rolling mill.
8. A process according to claim 7, wherein roll crowns of said six roll stands have
values of 0.5, 0.45, 0.40, 0.35; 0.35 and 0.20 mm (+ 5% range).
9. A process according to claim 1, wherein said finishing temperature is
maintained between 880 to 950 C.
10. A process according to claim 8, wherein said temperature is maintained at
900°C.
11. A process according to claim 1, wherein said coiling is carried out at
temperature between 650 to 700 °C.
12. A process according to claim 11, wherein said temperature is maintained at
675°C.

A process for manufacturing high strength, formable and fatigue resistant steel
particularly useful in the production of automobile chassis, wheel disc and rims, and
such other items. The process comprises the steps of heating steel slabs having a
steel composition comprising 0.02 to 0.18 wt% carbon, 0.01 to 0.14 wt%
manganese, 0 to 0.07 wt% silicon, 0 to 0.02 wt% sulfur, 0 to 0.02 wt%
phosphorous, 0 to 0.05 wt% aluminum, the rest being iron, at a temperature
between 1100 to 1350°C, rolling of the slabs in 5 or 6 stands in a hot rolling mill
wherein the finishing temperature is maintained between 850 to 950°C and finally
coiling of rolled steel strip at a temperature between 600 to 750°C. The process
controls pearlite/ferrite ratio in the range of 0.05 to 1.0 and is free from large size
inclusions. The steel has desirably low ratio of yield strength to ultimate strength for
arresting fatigue crack growth. The steel exhibits high percentage elongation and is
weldable and free from harmful oxides.