Title of Invention	THERMO MECHANICALLY TREATABLE NB-FREE MN-RICH LOW ALLOY STEEL WITH HIGH STRENGTH / DUCTILITY AND ITS PROCESS OF MANUFACTURE
Abstract	A thermo-mechanically treatable steel having selective low alloy composition free of micro alloying and high Manganese content to achieve selective microstructure adapted to provide higher improved strength-ductility combination and corrosion resistance. The desired properties of strength, ductility, weldability and impact toughness of said low alloy steel are achieved through a process starting with a preferred steel composition followed by suitable rolling parameters adapted to provide the mixed microstructure consisting of fine ferrite, pearlite and acicular ferrite resulting in the maximum strength-ductility combination. Such microstructure is achieved after austenitization at 1000°C and rolling at lower temperature such as at 800°C, in non-recrystallization region on laboratory scale. The invention further provide remarkable ferrite grain refinement at 800°C rolling as the grain size reduces from 9.5 μm at 25 % deformation to 6 μm at 50 % deformation. The steel is amenable to thermomechanical treatment even without microalloying due to synergistic effect of Mn rich multi-alloying.

Title of Invention

THERMO MECHANICALLY TREATABLE NB-FREE MN-RICH LOW ALLOY STEEL WITH HIGH STRENGTH / DUCTILITY AND ITS PROCESS OF MANUFACTURE

Abstract

A thermo-mechanically treatable steel having selective low alloy composition free of micro alloying and high Manganese content to achieve selective microstructure adapted to provide higher improved strength-ductility combination and corrosion resistance. The desired properties of strength, ductility, weldability and impact toughness of said low alloy steel are achieved through a process starting with a preferred steel composition followed by suitable rolling parameters adapted to provide the mixed microstructure consisting of fine ferrite, pearlite and acicular ferrite resulting in the maximum strength-ductility combination. Such microstructure is achieved after austenitization at 1000°C and rolling at lower temperature such as at 800°C, in non-recrystallization region on laboratory scale. The invention further provide remarkable ferrite grain refinement at 800°C rolling as the grain size reduces from 9.5 μm at 25 % deformation to 6 μm at 50 % deformation. The steel is amenable to thermomechanical treatment even without microalloying due to synergistic effect of Mn rich multi-alloying.

Full Text	FIELD OF THE INVENTION The present invention relates to a variety of thermo-mechanically treatable low alloy steel having a microstructure adapted to provide combination of higher strength, ductility and corrosion resistance. The desired properties of strength, ductility, weldability and impact toughness of said low alloy steel is based on a selective steel composition followed by suitable rolling parameters adapted to provide the mixed microstructure providing for desired maximum strength-ductility combination. The invention would favor achieving simply and cost-effectively steel amenable to thermo mechanical treatment even without micro alloying due to synergistic effect of Mn rich multi-alloying. The microstructure is further directed to exhibit the pearlite lamellae and non-polygonal ferrite with high dislocation density, which is characterized as the acicular ferrite. The low alloy thermo- mechanically treatable Mn rich steel with the above microstructure is directed to achieve far superior strength with required ductility of the steel and thus providing wide industrial application in railways carriages/coaches and the like where processing of parts involves moderate forming and welding which are the basic processes in the application of steel work fabrication. BACKGROUND ART It is well known in the application of rolled sections for steel structural fabrication that are exposed to open weather, the resistance to atmospheric corrosion is an essential property to be obtained in said steel along with strength, toughness, weldability and ductility. Conventionally, some of such properties are being maintained in the existing multi alloyed high strength low alloy steel (SAILCOR) with ferrite-pearlite microstructure which is in use for similar application. This particular grade of steel is consumed mainly by the Indian 2 Railways and other subsidiary organizations for manufacturing the superstructure and underframes/carriages of wagons and coaches or similar other equipments, generally exposed to open weather. General composition and strength properties of this existing SAILCOR grade is as given below: Chemical composition (wt% of elements): ---- C Mn S P Si Cr Ni Cu Al 0.08- 0.25- 0.04- 0.075- 0.28- 0.35- 0.28- 0.30- 0.10 0.45 (max) 0.115 0.72 0.49 0.49 0.45 YS: 345 MPa (Min) UTS: 480 MPa (Min) % elongation: 21(Min) In addition to atmospheric corrosion resistance, high strength, impact toughness and ductility are properties also required essentially for this steel because this grade of steel involves moderate to high degree of forming during production of components/parts in application. Although the existing grades of steel could provide the desired corrosion resistance and moderate strength properties, the same could not meet the requirement of other criteria of strength, toughness, ductility and weldability as necessary in application of steel structural. There had been therefore a persistent need to develop a grade of steel, for catering to the steel work fabrication and construction industries, that is capable of providing on one hand the desired improved strength, impact toughness, ductility and weldability in said steel which would be substantially formable and weldable without developing any defects during processing and at the same time exhibit favorable atmospheric corrosion resistance. 3 OBJECTS OF THE INVENTION It is thus the basic object of the present invention to develop a grade of steel having selective chemistry and with controlled rolling parameters providing the desired properties of corrosion resistance as well as improved strength, ductility, toughness and weldability. A further object of the present invention is directed to a process for developing the steel having the abovesaid properties by selective modification of existing multi alloyed low steel with suitable rolling parameters to achieve mixed microstructure comprising fine grained ferrite, pearlite and acicular ferrite in said steel and thus realizing much superior strength with required ductility of steel. A still further object of the present invention is directed to a process for developing an improved quality low alloy steel through change in chemistry and development of a mixed microstructure consisting of ferrite, pearlite and acicular ferrite after thermomechanical treatment. A further object of the present invention is directed to developing a grade of high manganese steel with improved impact toughness and weldability having said preferred mixed microstructure. A still further object of the present invention is directed to additional amount of Mn alloying in SAILCOR steel with suitable rolling parameters for producing the mixed microstructure comprising ferrite, pearlite and acicular ferrite in the present steel such that newly developed thermomechanically treatable steel with the above microstructure has far superior strength with required ductility of the steel and the same is extendable to actual shop production scale. 4 SUMMARY OF THE INVENTION Thus according to the basic aspect of the present invention there is provided a thermo mechanically treatable Nb free low alloy steel with high strength and ductility comprising: a Mn rich multi alloying and a mixed microstructure of fine ferrite, pearlite and acicular ferrite adapted for synergistically superior strength-ductility combination. A still further aspect of the present invention is directed to a thermo mechanically treatable Nb free low alloy steel comprising a chemical composition of steel having: C: 0.08 to 0.12 % by wt. preferably about 0.10 % by wt; Si:0.18 to 0.72 % by wt. preferably about 0.19 % by wt.; S:0.04 % (max)by wt. preferably about 0.025 % by wt.; P: 0.075 to 0.12% by wt. preferably about 0.12 % by wt.; Mn : 0.8 to 1.0% by wt. preferably about 0.88 % by wt.; Ni :0.28 to 0.49 % by wt. preferably about 0.34 % by wt.; Cr :0.35 to o.49% by wt. preferably about 0.48% by wt.; Cu :0.30 to 0.47% by wt. preferably about 0.47% by wt.; and Al.:0.005 to 0.025 % by wt. preferably about 0.006 % by wt. According to another aspect of the present invention of a thermo mechanically treatable Nb free Mn rich low alloy steel comprising: YS at about 800°C rolling temperature of 415 to 440 MPa preferably about 425 MPa; UTS at about 800°C rolling temperature of 525 to 550 MPa preferably about 535Mpa; elongation at about 800°C roiling temperature of 25 to 30 preferably about 27 %;and hardness at about 800°C rolling temperature of 150 to 170 VHN preferably about 160 VHN. 5 A still further aspect of the present invention directed to a thermo mechanically treatable Nb free low alloy steel wherein the mixed microstructure consisting of fine ferrite, pearlite and acicular ferrite is achieved after austenitization at 1000°C and rolling at lower temperature of about 800°C in nonrecrystallization region. A still further aspect of the present invention directed to a process for the manufacture of a thermomechanically treatable Nb free low alloy steel with high strength and ductility comprising: providing a Mn rich multi alloying and providing a mixed microstructure of fine ferrite, pearlite and acicular ferrite adapted for synergistically superior strength-ductility combination. A still further aspect of the present invention directed to a process for the manufacture of a thermomechanically treatable Nb free Mn rich low alloy steel, wherein the steel composition comprises selective amounts of C: 0.08 to 0.12 % by wt. preferably about 0.10 % by wt; Si: 0.18 to 0.72 % by wt. preferably about 0.19 % by wt.; S: 0.04 % (max) by wt. preferably about 0.025 % by wt.; P: 0.075 to 0.12 % by wt. preferably about 0.12 % by wt.; Mn.: 0.8 to 1.0% by wt. preferably about 0.88 % by wt.; Ni: 0.28 to 0.49% by wt. preferably about 0.34 % by wt.; Cr.: 0.35 to o.49% by wt. preferably about 0.48% by wt.; Cu; 0.30 to 0.47% by wt. preferably about 0.47% by wt.; and Al.: 0.005 to 0.025 % by wt. preferably about 0.006 % by wt. 6 A still further aspect of the present invention directed to said process comprising developing the said selective microstructure of fine ferrite, pearlite and acicular ferrite after austenitization at 1000°C and rolling at 800° in nonrecrystallization region. A still further aspect of the present invention directed to a process for the manufacture of a thermomechanically treatable Nb free Mn rich low alloy steel, wherein the steel is obtained in air induction furnace, ingots were soaked at about 1250°C for about 2 Hrs and hot rolled to plates with a final desired thickness over an approximate temperature range of 1200° - 900°C. According to yet another aspect of the present invention directed to said process for manufacturing said thermo mechanically treatable steel, wherein the steel is amenable to thermo mechanical treatment without micro alloying by way of synergistic effect of Mn rich multi alloying. The present invention and its objects and advantages are described in further details with reference to the following non limiting exemplary illustrations. BRIEF DESCRIPTION OF THE FIGURES Figure 1: is the illustration of the composite micrographs of the thermomechanically treated steel showing ferrite grain refinement at higher deformation at 800°C (a) when 25% deformed, and (b) when 50% deformed. Figure 2: is the illustration of TEM micrographs showing (a) Pearlite, and (b) acicular ferrite. 7 DETAILED DESCRIPTION WITH REFERENCE TO THE ACCOMPANYING FIGURES The present invention is directed to developing an improved grade of steel as compared to multi alloyed high strength low alloy steel (SAILCOR) with ferrite-pearlite microstructure, presently being in regular use. For manufacturing Railways wagons and coaches and other industrial structural items exposed to open weather, a steel having high atmospheric corrosion resistance coupled with higher strength and ductility as well as toughness and weldability is required. Such properties are essentially required to be present in the steel because this steel involves moderate to high degree of forming/joining during its application. Impact toughness and weldability are equally important during applications of this steel. A mixed microstructure comprising ferrite, pearlite and acicular ferrite is suitable for ensuring the above properties in steel. The present invention is directed to achieve said improved properties by providing an additional amount of Mn alloying in SAILCOR steel maintaining suitable subsequent rolling parameters to make it capable of producing the mixed microstructure comprising ferrite, pearlite and acicular ferrite in the present steel. The newly developed thermomechanically treatable steel with the above microstructure has far superior strength with required ductility of the steel. The process of obtaining the Mn rich low alloy steel of the present invention having high strength and ductility is further described through the following Example. EXAMPLE Laboratory experimentations were carried out to study the influence of higher Mn en the microstructure and mechanical properties of the commercially available multi alloyed SAILCOR steel. In order to ascertain desired properties in resulting steel different 8 microstructures were developed after austenitization at 1000 °C and rolling at 900-740 °C. It has been found experimentally, that the preferred mixed microstructure of fine ferrite, pearlite and acicular ferrite resulted in the maximum strength-ductility combination. Such microstructure was achieved after austenitization at 1000°C and rolling at lower temperature (800°C) in non-recrystallization region. The experimentation was carried out through following steps: 1. Firstly, the steel is made in a 100 kg. air induction furnace. 2. The steel metal was cast into ingots that are soaked at 1250 °C for 2 hours; 3. said ingots were hot rolled to plates with a final thickness of 12 mm over an approximate temperature range of 1200-900 °C; The preferred chemistry of the steel product thus obtained is as follows: Chemical composition of the steel (Weight percent) is provided in Table 1 hereunder: TABLE 1 c Si S P Mn Ni Cr Cu Al 0.10 0.19 0.025 0.116 0.88 0.34 0.48 0.47 0.006 4. Next, the strips of 150x25x12 mm were cut from the rolled plates for controlled rolling. 5. A 10 mm deep hole of 1.5 mm diameter was drilled in the centre of the length and thickness of the sample steel strips and a stainless steel sheathed (mineral insulated) chromel-alumel thermocouple was embedded into the hole so that the temperature of the steel could be recorded during controlled rolling of the steel. 6. The samples were next soaked at 1000 °C in a silicon carbide muffle furnace for 40 minutes and were quickly removed from the furnace (along with the embedded 9 thermocouple) and were cooled in air to get to the temperature of either 900, 800 and 740 °C, before rolling in single pass to the required reduction in thickness by either 25% or 50%. 7. The samples were then cooled in air after rolling. One sample was also air cooled, without any deformation, to room temperature as a controlled sample. 8. The tensile properties of all rolled samples were then evaluated with an Instron machine (model 1273) at a constant cross head speed of 2 mm/min. The YS, UTS and elongation achieved at 800 °C rolling temperature are 425 MPa, 535 MPa and 27 % respectively. The measured hardness of steel is 160 VHN at this rolling temperature. These improved properties are achieved at lower rolling temperature, preferably at 800 °C, which are far superior as compared to existing SAILCOR. 9. The resulting alternative microstructures showing the mixed fine grains of ferrite, pearlite and the acicular ferrite are studied and micrographed. The optical and TEM micrographs of the steel samples are shown in accompanying Figure 1 and 2 respectively. The accompanying Figure 1 clearly shows remarkable ferrite grain refinement at 800 °C rolling as the grain size reduces from 9.5 urn at 25 % deformation [1(a)], to 6 urn at 50 % deformation [1(b)]. Such experimental observation exhibits and confirms that the steel is amenable to thermomechanical treatment due to synergistic effect of higher Mn addition such as at least 0.88wt% in the multialloyed steel, even without microalloying such as element Nb, resulting in synergistically superior strength-ductility combination of the steel. Reference is now invited to accompanying Figure 2 that illustrates the presence and co- existence of pearlite lamellae (Fig. 2 a) and nonpolygonal ferrite with high dislocation density (Fig. 2 b) in the micrograph which is the characteristic of acicular ferrite. 10 It is thus possible by way of this invention to develop a process for obtaining steel of mixed microstructure comprising fine ferrite, pearlite and acicular ferrite in low alloy steel with higher Mn content (wt%) and maintaining suitable rolling parameters, even in absence of microalloying, such that the resulting steel exhibits improved properties of strength-ductility combination, Impact toughness and weldability apart from resistance to atmospheric corrosion as this steel is thermomechanically treatable. It is thus suitable for application in components/parts for fabrication of steel structures subjected to substantial forming or welding during processing or assembly. 11 WE CLAIM: 1. A thermo mechanically treatable Nb free low alloy steel with high strength and ductility comprising: a Mn rich multi alloying and a mixed microstructure of fine ferrite, pearlite and acicular ferrite adapted for synergistically superior strength-ductility combination. 2. A thermo mechanically treatable Nb free low alloy steel as claimed in claim 1 comprising a chemical composition of steel having: C: 0.08 to 0.12 % by wt. preferably about 0.10 % by wt.; Si:0.18 to 0.72 % by wt. preferably about 0.19 % by wt.; S:0.04% (max) by wt. preferably about 0.025 % by wt.; P:0.075 to 0.12% by wt. preferably about 0.12 % by wt.; Mn: 0.8 to 1.0% by wt. preferably about 0.88 % by wt.; Ni: 0.28 to 0.49 % by wt. preferably about 0.34 % by wt.; Cr:0.35 to 0.49% by wt. preferably about 0.48% by wt.; Cu:0.30 to 0.47% by wt. preferably about 0.47% by wt.; and Al:0.005 to 0.025% by wt. preferably about 0.006 % by wt. 3. A thermo mechanically treatable Nb free low alloy steel as claimed in anyone of claims 1 or 2 comprising: YS at about 800°C rolling temperature of 415 to 440 MPa, preferably about 425 MPa; UTS at about 800°C rolling temperature of 525 to 550 MPa, preferably about 535 Mpa; elongation at about 800°C rolling temperature of 25 to 30% preferably about 27 %; and 12 hardness at about 800°C rolling temperature of 150 to 170 preferably about 160 VHN. 4. A thermo mechanically treatable Nb free low alloy steel as claimed in anyone of claims 1 to 3 wherein the mixed microstructure consisting of fine ferrite, pearlite and acicular ferrite is achieved after austenitization at 1000°C and rolling at lower temperature of about 800°C in nonrecrystallization region. 5. A process for the manufacture of a thermomechanically treatable Nb free low alloy steel with high strength and ductility as claimed in anyone of claims 1 to 4 comprising: providing a Mn rich multi alloying and providing a mixed microstructure of fine ferrite, pearlite and acicular ferrite adapted for synergistically superior strength-ductility combination. 6. A process as claimed in claim 5 wherein the steel composition comprises selective amounts of C: 0.08 to 0.12 % by wt. preferably about 0.10 % by wt; Si: 0.18 to 0.72 % by wt. preferably about 0.19 % by wt.; S:0.04%(max) by wt. preferably about 0.025 % by wt.; P: 0.075 to 0.12% by wt. preferably about 0.12 % by wt.; Mn: 0.8 to 1.0% by wt. preferably about 0.88 % by wt.; Ni: 0.28 to 0.49 % by wt. preferably about 0.34 % by wt.; Cr: 0.35 to 0.49% by wt. preferably about 0.48% by wt.; Cu: 0.30 to 0.47% by wt. preferably about 0.47% by wt.; and Al: 0.005 to 0.025% by wt. preferably about 0.006 % by wt. 13 7. A process as claimed in anyone of claims 5 or 6 comprising developing the said selective microstructure of fine ferrite, pearlite and acicular ferrite after austenitization at 1000°C and rolling at 800° in nonrecrystallization region. 8. A process as claimed in anyone of claims 5 to 7 wherein the steel is made in air induction furnace, ingots were soaked at about 1250°C for about 2 Hrs and hot rolled to plates with a final desired thickness over an approximate temperature range of 1200°-900°C . 9. A process as claimed in anyone of claims 5 to 8 wherein the steel is amenable to thermo mechanical treatment without micro alloying by way of synergistic effect of Mn rich multi alloying. 10. A thermo mechanically treatable Nb free low alloy steel with high strength and ductility and its process of manufacture substantially as herein described and illustrated with reference to the accompanying figures. Dated this 12th day of January,2008 14 A thermo-mechanically treatable steel having selective low alloy composition free of micro alloying and high Manganese content to achieve selective microstructure adapted to provide higher improved strength-ductility combination and corrosion resistance. The desired properties of strength, ductility, weldability and impact toughness of said low alloy steel are achieved through a process starting with a preferred steel composition followed by suitable rolling parameters adapted to provide the mixed microstructure consisting of fine ferrite, pearlite and acicular ferrite resulting in the maximum strength-ductility combination. Such microstructure is achieved after austenitization at 1000°C and rolling at lower temperature such as at 800°C, in non-recrystallization region on laboratory scale. The invention further provide remarkable ferrite grain refinement at 800°C rolling as the grain size reduces from 9.5 μm at 25 % deformation to 6 μm at 50 % deformation. The steel is amenable to thermomechanical treatment even without microalloying due to synergistic effect of Mn rich multi-alloying.

Full Text

FIELD OF THE INVENTION
The present invention relates to a variety of thermo-mechanically treatable low alloy steel
having a microstructure adapted to provide combination of higher strength, ductility and
corrosion resistance. The desired properties of strength, ductility, weldability and impact
toughness of said low alloy steel is based on a selective steel composition followed by
suitable rolling parameters adapted to provide the mixed microstructure providing for
desired maximum strength-ductility combination. The invention would favor achieving
simply and cost-effectively steel amenable to thermo mechanical treatment even without
micro alloying due to synergistic effect of Mn rich multi-alloying. The microstructure is
further directed to exhibit the pearlite lamellae and non-polygonal ferrite with high
dislocation density, which is characterized as the acicular ferrite. The low alloy thermo-
mechanically treatable Mn rich steel with the above microstructure is directed to achieve
far superior strength with required ductility of the steel and thus providing wide industrial
application in railways carriages/coaches and the like where processing of parts involves
moderate forming and welding which are the basic processes in the application of steel
work fabrication.
BACKGROUND ART
It is well known in the application of rolled sections for steel structural fabrication that are
exposed to open weather, the resistance to atmospheric corrosion is an essential property
to be obtained in said steel along with strength, toughness, weldability and ductility.
Conventionally, some of such properties are being maintained in the existing multi alloyed
high strength low alloy steel (SAILCOR) with ferrite-pearlite microstructure which is in use
for similar application. This particular grade of steel is consumed mainly by the Indian
2

Railways and other subsidiary organizations for manufacturing the superstructure and
underframes/carriages of wagons and coaches or similar other equipments, generally
exposed to open weather. General composition and strength properties of this existing
SAILCOR grade is as given below:
Chemical composition (wt% of elements): ----
C Mn S P Si Cr Ni Cu Al
0.08- 0.25- 0.04- 0.075- 0.28- 0.35- 0.28- 0.30-
0.10 0.45 (max) 0.115 0.72 0.49 0.49 0.45
YS: 345 MPa (Min)
UTS: 480 MPa (Min)
% elongation: 21(Min)
In addition to atmospheric corrosion resistance, high strength, impact toughness
and ductility are properties also required essentially for this steel because this grade of
steel involves moderate to high degree of forming during production of components/parts
in application. Although the existing grades of steel could provide the desired corrosion
resistance and moderate strength properties, the same could not meet the requirement of
other criteria of strength, toughness, ductility and weldability as necessary in application of
steel structural.
There had been therefore a persistent need to develop a grade of steel, for catering
to the steel work fabrication and construction industries, that is capable of providing on
one hand the desired improved strength, impact toughness, ductility and weldability in
said steel which would be substantially formable and weldable without developing any
defects during processing and at the same time exhibit favorable atmospheric corrosion
resistance.
3

OBJECTS OF THE INVENTION
It is thus the basic object of the present invention to develop a grade of steel having
selective chemistry and with controlled rolling parameters providing the desired properties
of corrosion resistance as well as improved strength, ductility, toughness and weldability.
A further object of the present invention is directed to a process for developing the steel
having the abovesaid properties by selective modification of existing multi alloyed low
steel with suitable rolling parameters to achieve mixed microstructure comprising fine
grained ferrite, pearlite and acicular ferrite in said steel and thus realizing much superior
strength with required ductility of steel.
A still further object of the present invention is directed to a process for developing an
improved quality low alloy steel through change in chemistry and development of a mixed
microstructure consisting of ferrite, pearlite and acicular ferrite after thermomechanical
treatment.
A further object of the present invention is directed to developing a grade of high
manganese steel with improved impact toughness and weldability having said preferred
mixed microstructure.
A still further object of the present invention is directed to additional amount of Mn
alloying in SAILCOR steel with suitable rolling parameters for producing the mixed
microstructure comprising ferrite, pearlite and acicular ferrite in the present steel such that
newly developed thermomechanically treatable steel with the above microstructure has far
superior strength with required ductility of the steel and the same is extendable to actual
shop production scale.
4

SUMMARY OF THE INVENTION
Thus according to the basic aspect of the present invention there is provided a thermo
mechanically treatable Nb free low alloy steel with high strength and ductility comprising:
a Mn rich multi alloying and
a mixed microstructure of fine ferrite, pearlite and acicular ferrite adapted for
synergistically superior strength-ductility combination.
A still further aspect of the present invention is directed to a thermo mechanically
treatable Nb free low alloy steel comprising a chemical composition of steel having:
C: 0.08 to 0.12 % by wt. preferably about 0.10 % by wt;
Si:0.18 to 0.72 % by wt. preferably about 0.19 % by wt.;
S:0.04 % (max)by wt. preferably about 0.025 % by wt.;
P: 0.075 to 0.12% by wt. preferably about 0.12 % by wt.;
Mn : 0.8 to 1.0% by wt. preferably about 0.88 % by wt.;
Ni :0.28 to 0.49 % by wt. preferably about 0.34 % by wt.;
Cr :0.35 to o.49% by wt. preferably about 0.48% by wt.;
Cu :0.30 to 0.47% by wt. preferably about 0.47% by wt.; and
Al.:0.005 to 0.025 % by wt. preferably about 0.006 % by wt.
According to another aspect of the present invention of a thermo mechanically treatable
Nb free Mn rich low alloy steel comprising:
YS at about 800°C rolling temperature of 415 to 440 MPa preferably about 425 MPa;
UTS at about 800°C rolling temperature of 525 to 550 MPa preferably about 535Mpa;
elongation at about 800°C roiling temperature of 25 to 30 preferably about 27 %;and
hardness at about 800°C rolling temperature of 150 to 170 VHN preferably about 160
VHN.
5

A still further aspect of the present invention directed to a thermo mechanically treatable
Nb free low alloy steel wherein the mixed microstructure consisting of fine ferrite, pearlite
and acicular ferrite is achieved after austenitization at 1000°C and rolling at lower
temperature of about 800°C in nonrecrystallization region.
A still further aspect of the present invention directed to a process for the manufacture of
a thermomechanically treatable Nb free low alloy steel with high strength and ductility
comprising:
providing a Mn rich multi alloying and
providing a mixed microstructure of fine ferrite, pearlite and acicular ferrite adapted for
synergistically superior strength-ductility combination.
A still further aspect of the present invention directed to a process for the manufacture of
a thermomechanically treatable Nb free Mn rich low alloy steel, wherein the steel
composition comprises selective amounts of
C: 0.08 to 0.12 % by wt. preferably about 0.10 % by wt;
Si: 0.18 to 0.72 % by wt. preferably about 0.19 % by wt.;
S: 0.04 % (max) by wt. preferably about 0.025 % by wt.;
P: 0.075 to 0.12 % by wt. preferably about 0.12 % by wt.;
Mn.: 0.8 to 1.0% by wt. preferably about 0.88 % by wt.;
Ni: 0.28 to 0.49% by wt. preferably about 0.34 % by wt.;
Cr.: 0.35 to o.49% by wt. preferably about 0.48% by wt.;
Cu; 0.30 to 0.47% by wt. preferably about 0.47% by wt.; and
Al.: 0.005 to 0.025 % by wt. preferably about 0.006 % by wt.
6

A still further aspect of the present invention directed to said process comprising
developing the said selective microstructure of fine ferrite, pearlite and acicular ferrite
after austenitization at 1000°C and rolling at 800° in nonrecrystallization region.
A still further aspect of the present invention directed to a process for the manufacture of
a thermomechanically treatable Nb free Mn rich low alloy steel, wherein the steel is
obtained in air induction furnace, ingots were soaked at about 1250°C for about 2 Hrs and
hot rolled to plates with a final desired thickness over an approximate temperature range
of 1200° - 900°C.
According to yet another aspect of the present invention directed to said process for
manufacturing said thermo mechanically treatable steel, wherein the steel is amenable to
thermo mechanical treatment without micro alloying by way of synergistic effect of Mn rich
multi alloying.
The present invention and its objects and advantages are described in further details with
reference to the following non limiting exemplary illustrations.
BRIEF DESCRIPTION OF THE FIGURES
Figure 1: is the illustration of the composite micrographs of the thermomechanically
treated steel showing ferrite grain refinement at higher deformation at 800°C (a) when
25% deformed, and (b) when 50% deformed.
Figure 2: is the illustration of TEM micrographs showing (a) Pearlite, and (b) acicular
ferrite.
7

DETAILED DESCRIPTION WITH REFERENCE TO THE ACCOMPANYING FIGURES
The present invention is directed to developing an improved grade of steel as compared to
multi alloyed high strength low alloy steel (SAILCOR) with ferrite-pearlite microstructure,
presently being in regular use. For manufacturing Railways wagons and coaches and other
industrial structural items exposed to open weather, a steel having high atmospheric
corrosion resistance coupled with higher strength and ductility as well as toughness and
weldability is required. Such properties are essentially required to be present in the steel
because this steel involves moderate to high degree of forming/joining during its
application. Impact toughness and weldability are equally important during applications of
this steel. A mixed microstructure comprising ferrite, pearlite and acicular ferrite is suitable
for ensuring the above properties in steel.
The present invention is directed to achieve said improved properties by providing an
additional amount of Mn alloying in SAILCOR steel maintaining suitable subsequent rolling
parameters to make it capable of producing the mixed microstructure comprising ferrite,
pearlite and acicular ferrite in the present steel. The newly developed thermomechanically
treatable steel with the above microstructure has far superior strength with required
ductility of the steel. The process of obtaining the Mn rich low alloy steel of the present
invention having high strength and ductility is further described through the following
Example.
EXAMPLE
Laboratory experimentations were carried out to study the influence of higher Mn en the
microstructure and mechanical properties of the commercially available multi alloyed
SAILCOR steel. In order to ascertain desired properties in resulting steel different
8

microstructures were developed after austenitization at 1000 °C and rolling at 900-740 °C.
It has been found experimentally, that the preferred mixed microstructure of fine ferrite,
pearlite and acicular ferrite resulted in the maximum strength-ductility combination. Such
microstructure was achieved after austenitization at 1000°C and rolling at lower
temperature (800°C) in non-recrystallization region. The experimentation was carried out
through following steps:
1. Firstly, the steel is made in a 100 kg. air induction furnace.
2. The steel metal was cast into ingots that are soaked at 1250 °C for 2 hours;
3. said ingots were hot rolled to plates with a final thickness of 12 mm over an
approximate temperature range of 1200-900 °C;
The preferred chemistry of the steel product thus obtained is as follows:
Chemical composition of the steel (Weight percent) is provided in Table 1 hereunder:
TABLE 1

c Si S P Mn Ni Cr Cu Al
0.10 0.19 0.025 0.116 0.88 0.34 0.48 0.47 0.006
4. Next, the strips of 150x25x12 mm were cut from the rolled plates for controlled
rolling.
5. A 10 mm deep hole of 1.5 mm diameter was drilled in the centre of the length and
thickness of the sample steel strips and a stainless steel sheathed (mineral
insulated) chromel-alumel thermocouple was embedded into the hole so that the
temperature of the steel could be recorded during controlled rolling of the steel.
6. The samples were next soaked at 1000 °C in a silicon carbide muffle furnace for 40
minutes and were quickly removed from the furnace (along with the embedded
9

thermocouple) and were cooled in air to get to the temperature of either 900, 800
and 740 °C, before rolling in single pass to the required reduction in thickness by
either 25% or 50%.
7. The samples were then cooled in air after rolling. One sample was also air cooled,
without any deformation, to room temperature as a controlled sample.
8. The tensile properties of all rolled samples were then evaluated with an Instron
machine (model 1273) at a constant cross head speed of 2 mm/min. The YS, UTS
and elongation achieved at 800 °C rolling temperature are 425 MPa, 535 MPa and
27 % respectively. The measured hardness of steel is 160 VHN at this rolling
temperature. These improved properties are achieved at lower rolling temperature,
preferably at 800 °C, which are far superior as compared to existing SAILCOR.
9. The resulting alternative microstructures showing the mixed fine grains of ferrite,
pearlite and the acicular ferrite are studied and micrographed. The optical and TEM
micrographs of the steel samples are shown in accompanying Figure 1 and 2
respectively. The accompanying Figure 1 clearly shows remarkable ferrite grain
refinement at 800 °C rolling as the grain size reduces from 9.5 urn at 25 %
deformation [1(a)], to 6 urn at 50 % deformation [1(b)]. Such experimental
observation exhibits and confirms that the steel is amenable to thermomechanical
treatment due to synergistic effect of higher Mn addition such as at least 0.88wt%
in the multialloyed steel, even without microalloying such as element Nb, resulting
in synergistically superior strength-ductility combination of the steel. Reference is
now invited to accompanying Figure 2 that illustrates the presence and co-
existence of pearlite lamellae (Fig. 2 a) and nonpolygonal ferrite with high
dislocation density (Fig. 2 b) in the micrograph which is the characteristic of
acicular ferrite.
10

It is thus possible by way of this invention to develop a process for obtaining steel of
mixed microstructure comprising fine ferrite, pearlite and acicular ferrite in low alloy steel
with higher Mn content (wt%) and maintaining suitable rolling parameters, even in
absence of microalloying, such that the resulting steel exhibits improved properties of
strength-ductility combination, Impact toughness and weldability apart from resistance to
atmospheric corrosion as this steel is thermomechanically treatable. It is thus suitable for
application in components/parts for fabrication of steel structures subjected to substantial
forming or welding during processing or assembly.
11

WE CLAIM:
1. A thermo mechanically treatable Nb free low alloy steel with high strength and ductility
comprising:
a Mn rich multi alloying and
a mixed microstructure of fine ferrite, pearlite and acicular ferrite adapted for
synergistically superior strength-ductility combination.
2. A thermo mechanically treatable Nb free low alloy steel as claimed in claim 1
comprising a chemical composition of steel having:
C: 0.08 to 0.12 % by wt. preferably about 0.10 % by wt.;
Si:0.18 to 0.72 % by wt. preferably about 0.19 % by wt.;
S:0.04% (max) by wt. preferably about 0.025 % by wt.;
P:0.075 to 0.12% by wt. preferably about 0.12 % by wt.;
Mn: 0.8 to 1.0% by wt. preferably about 0.88 % by wt.;
Ni: 0.28 to 0.49 % by wt. preferably about 0.34 % by wt.;
Cr:0.35 to 0.49% by wt. preferably about 0.48% by wt.;
Cu:0.30 to 0.47% by wt. preferably about 0.47% by wt.; and
Al:0.005 to 0.025% by wt. preferably about 0.006 % by wt.
3. A thermo mechanically treatable Nb free low alloy steel as claimed in anyone of claims
1 or 2 comprising:
YS at about 800°C rolling temperature of 415 to 440 MPa, preferably about 425 MPa;
UTS at about 800°C rolling temperature of 525 to 550 MPa, preferably about 535 Mpa;
elongation at about 800°C rolling temperature of 25 to 30% preferably about 27 %;
and
12

hardness at about 800°C rolling temperature of 150 to 170 preferably about 160 VHN.
4. A thermo mechanically treatable Nb free low alloy steel as claimed in anyone of claims
1 to 3 wherein the mixed microstructure consisting of fine ferrite, pearlite and acicular
ferrite is achieved after austenitization at 1000°C and rolling at lower temperature of
about 800°C in nonrecrystallization region.
5. A process for the manufacture of a thermomechanically treatable Nb free low alloy
steel with high strength and ductility as claimed in anyone of claims 1 to 4 comprising:
providing a Mn rich multi alloying and
providing a mixed microstructure of fine ferrite, pearlite and acicular ferrite adapted for
synergistically superior strength-ductility combination.
6. A process as claimed in claim 5 wherein the steel composition comprises selective
amounts of
C: 0.08 to 0.12 % by wt. preferably about 0.10 % by wt;
Si: 0.18 to 0.72 % by wt. preferably about 0.19 % by wt.;
S:0.04%(max) by wt. preferably about 0.025 % by wt.;
P: 0.075 to 0.12% by wt. preferably about 0.12 % by wt.;
Mn: 0.8 to 1.0% by wt. preferably about 0.88 % by wt.;
Ni: 0.28 to 0.49 % by wt. preferably about 0.34 % by wt.;
Cr: 0.35 to 0.49% by wt. preferably about 0.48% by wt.;
Cu: 0.30 to 0.47% by wt. preferably about 0.47% by wt.; and
Al: 0.005 to 0.025% by wt. preferably about 0.006 % by wt.
13

7. A process as claimed in anyone of claims 5 or 6 comprising developing the said
selective microstructure of fine ferrite, pearlite and acicular ferrite after austenitization
at 1000°C and rolling at 800° in nonrecrystallization region.
8. A process as claimed in anyone of claims 5 to 7 wherein the steel is made in air
induction furnace, ingots were soaked at about 1250°C for about 2 Hrs and hot rolled
to plates with a final desired thickness over an approximate temperature range of
1200°-900°C .
9. A process as claimed in anyone of claims 5 to 8 wherein the steel is amenable to
thermo mechanical treatment without micro alloying by way of synergistic effect of Mn
rich multi alloying.
10. A thermo mechanically treatable Nb free low alloy steel with high strength and
ductility and its process of manufacture substantially as herein described and
illustrated with reference to the accompanying figures.

Dated this 12th day of January,2008
14

A thermo-mechanically treatable steel having selective low alloy composition free of micro
alloying and high Manganese content to achieve selective microstructure adapted to
provide higher improved strength-ductility combination and corrosion resistance. The
desired properties of strength, ductility, weldability and impact toughness of said low alloy
steel are achieved through a process starting with a preferred steel composition followed
by suitable rolling parameters adapted to provide the mixed microstructure consisting of
fine ferrite, pearlite and acicular ferrite resulting in the maximum strength-ductility
combination. Such microstructure is achieved after austenitization at 1000°C and rolling at
lower temperature such as at 800°C, in non-recrystallization region on laboratory scale.
The invention further provide remarkable ferrite grain refinement at 800°C rolling as the
grain size reduces from 9.5 μm at 25 % deformation to 6 μm at 50 % deformation. The
steel is amenable to thermomechanical treatment even without microalloying due to
synergistic effect of Mn rich multi-alloying.

Documents:

00098-kol-2008-abstract.pdf

00098-kol-2008-claims.pdf

00098-kol-2008-correspondence others.pdf

00098-kol-2008-description complete.pdf

00098-kol-2008-drawings.pdf

00098-kol-2008-form 1.pdf

00098-kol-2008-form 2.pdf

00098-kol-2008-form 3.pdf

98-KOL-2008-CORRESPONDENCE OTHERS 1.1.pdf

98-KOL-2008-FORM 18.pdf

98-KOL-2008-PA.pdf

abstract-00098-kol-2008.jpg

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Patent Number

263486

Indian Patent Application Number

98/KOL/2008

PG Journal Number

44/2014

Publication Date

31-Oct-2014

Grant Date

30-Oct-2014

Date of Filing

14-Jan-2008

Name of Patentee

STEEL AUTHORITY OF INDIA LIMITED

Applicant Address

RESEARCH & DEVELOPMENT CENTRE FOR IRON & STEEL, DORANDA, RANCHI

Inventors:

#	Inventor's Name	Inventor's Address
1	PRASAD SHAMBHU NATH	RESEARCH & DEVELOPMENT CENTRE FOR IRON & STEEL, DORANDA, RANCHI-834002

PCT International Classification Number

B21D37/20,C21D8/00

PCT International Application Number

N/A

PCT International Filing date

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1			NA