FIELD OF THE INVENTION
The present invention is directed to high Manganese and Niobium micro alloyed high
strength low alloy steel composition and a process for controlled rolling of ingots cast from
said steel grade adapted to provide higher strength, improved toughness and comparable
elongation properties. More particularly, the steel composition of selectively high
manganese and Nb microalloyed is controlled rolled after austenitising at 1000 or 1100°C,
subjected to deformation at specified temperature to achieve desired microstructure of said
controlled rolled steel comprising fine ferrite, pearlite and substantial amount of granular
bainite, favoring achieving the end properties. Importantly, for achieving desired
microstructure and strength by way of controlled rolling is effectively implemented with
higher austenitisation temperature (= 1100°C), lower rolling temperature (~800°C) and
higher rolling reduction (=50%) carried out to achieve maximum advantage of the Nb
microalloying addition in the steel composition, retaining the toughness of steel and
avoiding segregation even with high phosphorous contents beyond 0.05%. Advantageously,
the typical TEM micrographs of the steel grade obtained following the controlled rolling
according to the present invention, by soaking at 1100°C and being rolled 50% at 800°C
show the microstructure comprising ferrite, pearlite and granular bainite with martensite-
austenite constituents as dark patches and ferrite with high dislocation density and also
ferrite containing extremely fine precipitates of Nb (C, N) particles of about 100 Å size. The
low alloy weather resistant steel variety according to the invention favour achieving higher
YS: 450-470 MPa, UTS: 575-595 MPa and % El: 25-30 after controlled rolling with a
composition having increased Mn from 0.45% max. to about 1.0 % and addition of 0.02-
0.03 % Nb in the said steel composition, having enhanced strength and comparable
elongation properties, making it suitable for a wide range of industrial application such as in
railways or other out door steel structural fabrication work in economic and reliable manner.
BACKGROUND ART
High strength low alloy steel having ferrite-pearlitic microstructure is known and existing
under the brand SAILCOR meeting the requirements of the Indian Railways for use in
production of wagons and coaches. The multi-alloyed steel sections of this grade providing
resistance to atmospheric corrosion as well as moderately high strength and ductility as the
essential properties are directed to complying with the intended end applications and
service conditions. The existing SAILCOR grade steel is having moderate strength and is
generally alloyed with Cu, P, Cr, and Ni in selective weight proportions, for desired solid
solution strengthening and also for promoting weather resistance/atmospheric corrosion
resistance. As already known in the art of producing low alloy steel grades, among all the
alloying elements provided, phosphorous has the most effective role in solid solution
strengthening but P content beyond 0.05% invokes detrimental effect on physical properties
of the low alloy steel due to loss of toughness properties. Moreover, the use of phosphorous
as the alloying element is also restricted as it is prone to develop tendency of harmful
segregation in steel. The existing steel manufacturing process attempted to overcome the
limitations of the properties in prior art steel products by adopting controlled rolling with
selective deformation using specified rolling parameters/temperatures, directed to achieving
improved strength and toughness properties in resulting steel products by advantageous
ferrite grain refinement along with pearlite and granular bainite in resulting steel products.
The increase in hardenability as a consequence of alloying, affects phase transformation in
controlled rolled steel products and produces granular bainite also attaining the desired end
properties in rolled steel section.
There has thus been a consistent need in the art of steel making to developing a steel
composition and a method of controlled rolling of said steel grade which would on one hand
ensure attaining the desired atmospheric corrosion/weather resistance properties, on the
other hand would provide a steel composition with higher Mn content as well as Nb addition
as selective micro alloying to ensure achieving fine grained microstructure adapted to
improve strength and toughness with comparable elongation, avoiding segregation in the
end product obtained through controlled rolling route so as to favour wide industrial
application of such steel grades in railway wagons/coaches and the like outdoor weather
resistant steel structural applications. The present invention is directed to a process for
producing steel sections of given composition involving controlled rolling, wherein
investigation carried out on effect of soaking temperature and Nb addition in phosphorus
reduced Mn rich weather resistant steel has been evaluated to meet the end application.
OBJECTS OF THE INVENTION
The basic object of the present invention is thus directed to developing a Mn rich Nb micro
alloyed low alloy steel composition and a process for its production involving controlled
rolling adapted to provide desired microstructure resulting in improved strength and
toughness, meeting the requirements of ductility and weather resistance properties in the
end applications.
Another object of the present invention is directed to developing said high strength low alloy
Nb micro alloyed steel grade following the controlled rolling route to achieve desired fine
grained microstructure in the rolled steel products ensuring desired higher strength,
toughness and elongation properties while also meeting the atmospheric corrosion
resistance for outdoor application in a wide range of steel structures.
A further object of the present invention is directed to developing said low alloy Nb micro
alloyed steel grade following the controlled rolling route adapted to ensure a mixed micro
structure comprising fine ferrite, pearlite and granular bainite in the steel matrix consisting
of martensite/austenite constituent and fine ferrite with high dislocation density and ferrite
with extremely fine precipitates of Nb(C,N), ensuring the desired end properties after said
controlled rolling.
A still further object of the present invention is directed to developing said low alloy steel
grade following the controlled rolling route wherein effect of the soaking temperature and
Nb addition with reduced phosphorous content in Mn rich steel composition is selectively
used to achieve desired microstructure, solid solution strengthening, improved strength and
toughness with comparable elongation, atmospheric corrosion resistance to desired levels in
resulting controlled rolled steel sections.
A still further object of the present invention is directed to developing said Nb micro alloyed
low alloy steel grade with fine grained micro structure following the controlled rolling route
adapted to ensure the strength, ductility, toughness and corrosion resistance properties
wherein the influence of deformation at selective variable temperature range in combination
with selective soaking temperature have been studied for a given steel composition to arrive
at the desired end results/composition.
A still further object of the present invention is directed to developing said low alloy steel
grade following the controlled rolling route wherein the controlled rolling to be effective at
higher austenitization temperature (= 1100°C), lower rolling temperature (~800°C) and
higher rolling reduction (=50%) are observed favorable to take maximum advantage of Nb
micro alloy addition.
A still further object of the present invention is directed to developing said low alloy steel
grade following the controlled rolling route wherein substantial amount of granular bainite
comparising martensite-austenite constituent and acicular ferrite in addition to fine ferrite
and pearlite has been achieved in steel samples when rolled/deformed at 50% or above at
800°C, after soaking/austenitization at 1100°C.
A still further object of the present invention is directed to developing said low alloy steel
grade following the controlled rolling route wherein the micro structure adapted to ensure
desired end properties in the steel comprised ferrite grains obtained in non-polygonal form
with high dislocation density containing extremely fine precipitates of Nb(C,N) particles
when subjected to higher deformation (=50%) at lower rolling temperature (800 °C).
A still further object of the present invention is directed to developing said low alloy steel
grade following the controlled rolling route wherein the resulting properties in said
controlled rolled steel having selective composition achieving YS in the range of 450-470
MPa, UTS in the range of 575-595 MPa and %EI in the range of 25-30%.
SUMMARY OF THE INVENTION
The basic object of the present invention is directed to a high strength low alloy controlled
rolled steel comprising:
00.09 to 0.14 preferably about 0.11 wt %;
Si=0.20 to 0.30 preferably about 0.24 wt %;
S=0.015 to 0.045 preferably about 0.033 wt%;
P=0.045 to 0.075 preferably about 0.052 wt %;
Mn= 0.90 to 1.10.. preferably about 1.00 wt %;
Ni= 0.30 to 0.35 preferably about 0.33 wt %;
Cr= 0.45 to 0.50 preferably about 0.47 wt %;
Cu= 0.45 to 0.50 preferably about 0.47 wt %;
Al= 0.022 to 0.025 preferably about 0.023 wt %;
Nb= 0.022 to 0.027 preferably about 0.024 wt%; and
N= 0.007 to 0.008 preferably about 0.0077 wt %.
Another aspect of the present invention is directed to said high strength low alloy steel
comprising high YS: 450-470 MPa, UTS: 575-595 MPa and % elongation: 25-30.
A further aspect of the present invention is directed to a high strength low alloy steel
wherein the microstructure of the above controlled rolled steel comprises ferrite, pearlite
and granular bainite comprising of martensite/austenite constituents and ferrite with high
dislocation density.
A still further aspect of the present invention is directed to a high strength low alloy steel
comprising ferrite, pearlite and bainite in the steel rolled 25% and 50% at 800°C after
austenitisation at 1100°C, said ferrite grains, found in the range of 5-10 µm, were finer at
higher deformation and lower rolling temperature.
A still further aspect of the present invention is directed to a high strength low alloy steel
comprising a typical TEM micrograph of steel soaked at 1100°C and rolled 50% at 800°C
comprising pearlite, martensite-austenite (M/A) constituent (dark patches) and ferrite with
high dislocation density and ferrite containing extremely fine precipitates of Nb (C,N) of
about 100 Å size.
A still further aspect of the present invention is directed to a process for the manufacture of
a high strength low alloy controlled rolled steel comprising:
providing the selective steel composition and forming ingots therefrom, soaking the said
ingots at 1240 to 1260 °C for a period of 1 hour 50 minutes to 2 hour 10 minutes and finally
hot rolled to plates which are further controlled rolled such as to obtain said mixed
microstructure having fine ferrite, pearlite and granular bainite.
A still further aspect of the present invention is directed to a process for the manufacture of
multi alloyed high strength low alloy steel as claimed in claim 6 wherein for controlled
rolling to be effective higher austenitisation temperature (= 1100°C), lower rolling
temperature (~800°C) and higher rolling reduction (=50%) are involved.
A still further aspect of the present invention is directed to a process for the manufacture of
multi alloyed high strength low alloy steel wherein the steel is rolled after soaking at
1100°C.
The present invention and its objects and advantages are described in greater details with
reference to the following accompanying non limiting illustrative example and drawings.
BRIEF DESCRIPTION OF THE ACCOMPANYING FIGURES
Figure 1: is the illustration of composite micrographic images of the steel austenitised at
1100°C and rolled at 800°C and deformed (a) 25% and (b) 50%.
Figure 2: is the illustration of TEM micrographs of the steel showing (a) pearlite, (b)
Martensite/Austenite constituents dark patches) and ferrite with high dislocation density,
and (c) ferrite containing extremely fine Nb(C,N) particles of about 100A size.
DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO THE
ACCOMPANYING FIGURES
The present invention is directed to developing an improved quality low alloy steel
composition comprising reduced phosphorous, increased manganese content and selective
Niobium micro alloying so as to ensure a mixed microstructure consisting of fine ferrite,
pearlite and substantial amount of granular bainite with martensite-austenite constituents
when subjected to controlled rolling at lower rolling temperature of about 800°C and higher
rolling reduction of the order of over 50%, after soaking at or above 1100°C, adapted to
enhance the yield strength and ultimate tensile strength, as well as toughness with
associated elongation and resistance to atmospheric corrosion in the resulting steel products
for desired application in railway wagon/coaches or like steel structural fabrication for
outdoor applications. The micrograph also shows ferrite with high dislocation density and
ferrite containing extremely fine precipitates of Nb(C,N) particles.
The objects and advantages are described in greater details with reference to the following
accompanying non limiting illustrative Example:
Example I:
Experiments have been carried out in laboratory scale to study the influence of higher Mn
content and Nb microalloying in steel on the microstructure and mechanical properties of
controlled rolled low alloy steel. The following sequential steps has been implemented
during the experimental studies:
(a) The steel has been produced for experimentation in a 100 kg air induction furnace
and said steel having the composition comprising:
C: 0.09 to 0.14 and preferably about 0.11 wt%;
Si: 0.20 to 0.30 and preferably about 0.24 wt%;
S: 0.015 to 0.045 and preferably about 0.033 wt% ;
P: 0.045 to 0.075 and preferably about 0.052 wt%;
Mn: 0.90 to 1.10 and preferably about 1.00 wt%;
Ni: 0.30 to 0.35 and preferably about 0.33 wt%;
Cr: 0.45 to 0.50 and preferably about 0.47 wt%;
Cu: 0.45 to 0.50 and preferably about 0.47 wt%;
Al: 0.022 to 0.025 and preferably about 0.023 wt%;
Nb: 0.022 to 0.027 and preferably about 0.024 wt%; and
N:0.007 to 0.008 and preferably about 0.0077 at %;
And rest Fe.
(b) The ingots cast with steel having composition as above is soaked at 1250 °C for 2
hours and hot rolled to plates with a final thickness of 12 mm over an approximate
temperature range of 1200-900 °C.
(c) The sample steel strips of size 150x25x12mm are cut from the rolled plates for
conducting experimentation with controlled rolling.
(d) A hole of 1.5 mm diameter and 10 mm deep is drilled in the centre of the length and
thickness of the sample and a stainless steel sheathed (mineral insulated) Chromel-
Alumel thermocouple is embedded into the hole so that the temperatures of the
sample steel strip at different stages during controlled rolling are recorded.
(e) The sample steel strips as stated under preceding para (c), are soaked at 1000°C
and 1100°C in a silicon carbide muffle furnace for 40 minutes and are quickly
removed from the furnace along with the embedded thermocouple, and
(f) They are then cooled in air to get to the required temperature range of 700°C to
900°C before rolling in single pass to the required reduction in thickness by either
25% or 50%.
(g) The samples are cooled in air to room temperature after rolling.
Observations based on Testing of controlled rolled steel strip samples for Mechanical
properties are as follows:
1. The effect of deformations at 800, 900 and 1000°C after soaking at 1100°C
and that at 740, 800 and 900°C after 1000°C soaking on the YS, UTS and %
EL of the steel is studied;
2. The range of variation in YS values are found to be wide e.g. 335 to 460 MPa,
due to variation in rolling parameters, in which the lower values are for lower
deformation and higher rolling temperatures; The UTS varied only over a
narrow range of 530 to 585 MPa;
3. The % EL values are observed in the range 27 to 36%.
4. Soaking at 1100°C resulted in higher UTS and lower % EL as compared to
those for soaking at 1000°C.
5. The maximum hardness achieved is about 180 HV when rolled at 800-1000°C
after soaking at 1100°C.
Microstructure/Micrographic analysis of controlled rolled steel samples of composition as
given in step (a) above:
1. The optical microstructure found to consist of only ferrite-pearlite at 800 °C
and 900°C rolling temperatures, after austenitisation at 1000°C;
2. A sizable amount of granular bainite in addition to ferrite and pearlite has
been achieved in steel samples rolled after soaking at 1100°C;
3. Composite micrographs consisting of ferrite, pearlite and bainite in the steel
rolled with 25% and 50% reductions at 800°C after austenitisation at
1100°C, as shown in the accompanying Figures 1(a) and 1(b) respectively;
4. The ferrite grains obtained in the size range of 5-10 urn, are found to be finer
at higher deformation and lower rolling temperature;
5. The typical TEM micrographs as illustrated in accompanying Figure 2 for steel
soaked at 1100°C and rolled with 50% reduction/deformation at 800°C rolling
temperature shows most favourable microstructure;
6. The microstructure of the above controlled rolled steel shows ferrite, pearlite
and granular bainite consisting of martensite/austenite constituent and ferrite
with high dislocation density. The accompanying Figure 2(a), (b) and (c)
shows the respective TEM microhraphs of pearlite, M/A constituents(dark
patches) and ferrite with high dislocation density, ferrite containing extremely
fine Nb(C,N) particles of about 100 A size. EDX and SAD of these fine
precipitates confirm them to be Nb (C, N) particles.
The test results with the controlled rolled steel sample having composition comprising
reduced P, increased Mn content of about 1.0 wt % and micro alloy addition of Nb in the
range of 0.02-0.03 wt %, are higher YS: 450-470 MPa, UTS: 575-595 MPa and % El: 25-
30, with favorable toughness properties after controlled rolling. It has also been found
experimentally that for controlled rolling to be effective, higher austenitisation temperature
(= 1100°C), lower rolling temperature (~800°C) and higher rolling reduction (=50%)
favored achieving desired microstructure and end properties in controlled rolled strips in
order to derive maximum advantage of the Nb microalloying addition. The above stated
experimental results are also adapted to be scaled up to plant scale production of controlled
rolled steel products with similar advantages.
It is thus possible by way of the present invention to developing a composition for low alloy
steel with higher Mn, lower phosphorous content and selective Nb microalloying and
subjecting the same to controlled rolling so that the resulting steel provides desired fine
grained microstructure comprising pearlite, granular bainite consisting of
martensite/austenite constituents, fine ferrite with high dislocation density as well as ferrite
with extremely fine precipitates of Nb(C,N) after said controlled rolling with selective
temperature and deformation, after soaking at preferred austenitisation temperature. The
resulting steel having the composition and microstructure as stated is adapted to ensure
higher strength and toughness maintaining the % elongation as well as atmospheric
corrosion resistance making such steel more suitable for use in manufacturing of railway
wagons/coaches or similar other application in steel structural fabrication for outdoor
service conditions.
WE CLAIM:
1. A high strength low alloy controlled rolled steel comprising:
C= 0.09 to 0.14 preferably about 0.11 wt %;
Si= 0.20 to 0.30 preferably about 0.24 wt %;
S= 0.015 to 0.045 preferably about 0.033 wt%;
P= 0.045 to 0.075 preferably about 0.052 wt %;
Mn= 0.90 to 1.10. preferably about 1.00 wt %;
Ni= 0.30 to 0.35 preferably about 0.33 wt %;
Cr= 0.45 to 0.50 preferably about 0.47 wt %;
Cu= 0.45 to 0.50 preferably about 0.47 wt %;
Al= 0.022 to 0.025 preferably about 0.023 wt %;
Nb= 0.022 to 0.027 preferably about 0.024 wt%; and
N= 0.007 to 0.008 preferably about 0.0077 wt %.
2. A high strength low alloy steel as claimed in claim 1 comprising high YS: 450-470 MPa,
UTS: 575-595 MPa and % elongation: 25-30.
3. A high strength low alloy steel as claimed in anyone of claims 1 or 2 wherein the
microstructure of the above controlled rolled steel comprises ferrite, pearlite and granular
bainite comprising of martensite/austenite constituents and ferrite with high dislocation
density.
4. A high strength low alloy steel as claimed in anyone of claims 1 to 3 comprising ferrite,
pearlite and bainite in the steel rolled 25% and 50% at 800°C after austenitisation at
1100°C, said ferrite grains, found in the range of 5-10 urn, were finer at higher
deformation and lower rolling temperature.
5. A high strength low alloy steel as claimed in anyone of claims 1 to 4 comprising a typical
TEM micrographs of steel soaked at 1100°C and rolled 50% at 800°C comprising pearlite ,
martensite-austenite (M/A) constituent (dark patches) and ferrite with high dislocation
density and ferrite containing extremely fine Nb (C,N) precipitates of about 100 Å size.
6. A process for the manufacture of a high strength low alloy controlled rolled steel as
claimed in anyone of claims 1 to 5 comprising:
providing the selective steel composition and forming ingots therefrom, soaking the said
ingots at 1240 to 1260 °C for a period of 1 hour 50 minutes to 2 hour 10 minutes and finally
hot rolled to plates which are further controlled rolled such as to obtain said mixed
microstructure having fine ferrite, paerlite and granular bainite.
7. A process for the manufacture of a high strength low alloy steel as claimed in claim 6
wherein for controlled rolling to be effective higher austenitisation temperature (= 1100°C),
lower rolling temperature (~800°C) and higher rolling reduction (=50%) are involved.
8. A process for the manufacture of a high strength low alloy steel as claimed in anyone of
claims 6 or 7 wherein the steel is rolled after soaking at 1100°C.
9. A high strength low alloy controlled rolled steel and its process of manufacture
substantially as hereindescribed and illustrated with reference to the accompanying
examples and figures.

A high strength low alloy steel composition and a process of controlled rolling of ingots cast
from said steel grade adapted to provide the combination of weather resistance, higher
strength, improved toughness and comparable elongation properties. More particularly, said
manganese rich and Nb microalloyed controlled rolled steel obtained after austenitising
above 1100°C, selectively subjecting it to deformation at specified rolling temperature to
achieve desired microstructure comprising fine ferrite, pearlite and substantial amount of
granular bainite. Importantly, said controlled rolling to be effective at higher austenitisation
temperature (≥ 1100°C), lower rolling temperature (~800°C) and higher rolling reduction
(≥50%) favor achieving maximum advantage of the Nb micro alloy addition. The TEM
micrographs show microstructure comprising pearlite and granular bainite with martensite-
austenite constituents as dark patches and ferrite with high dislocation density and also
ferrite containing extremely fine precipitates of Nb (C, N). The low alloy weather resistant
steel favors achieving higher YS: 450-470 MPa, UTS: 575-595 MPa and % El: 25-30 after
controlled rolling, making it suitable for a wide range of industrial applications.