A PROCESS FOR PRODUCING FINE GRAINED (2.5-3.5 MICRON) CARBON-
MANGANESE STEEL PLATES WITH OR WITHOUT NIOBIUM, OR VANADIUM
BY MULTI-PASS AIR-COOLED ROLLING ROUTE
FIELD OF INVENTION
This invention relates to a process for producing fine grained (2.5-3.5 µm)
carbon-manganese (C-Mn) steel plates with or without niobium (Nb) or vanadium
(V) by multi-pass air-cooled rolling route.
DESCRIPTION OF RELATED ART
Over the years a number of methods have been proposed for the
development of steel microstructures with a ferrite grain size of the order of 1 µm.
Essentially fine-grained microstructure can be achieved by two methods, namely
thermo-mechanically controlled processing (TMCP) and deformation induced
ferrite transformation (DIFT).
A lot of research works have been carried out on TMCP for refinement of
ferrite grain size during 60-70's. In TMCP, a fine-grained microstructure is
achieved by controlling the deformation temperature in the regions of austenite
recrystallization and non-recrystallization followed by accelerated cooling.
However, the TMCP process has its limitations. It can be applied to the
microalloyed steels only for microstructural refinement and minimum ferrite grain
size attainable by this process is 4-5 µm. For plain carbon (C-Mn) steels since
there is no microalloy (Nb,Ti,V) additions grain coarsening in the austenite
recrystallization and non-recrystallization zones are not avoidable. For plain
carbon steel, the technique of deformation induced ferrite transformation is found
to be more suitable. This approach has been given a number of names: Dynamic
Strain Induced Transformation (DSIT), Dynamic Transformation (DT). The

common feature in all cases is that a substantial amount of transformation occurs
dynamically, or 'during' deformation.
The document CN 1789466 discloses the high-intensity fine grain
polyphase steel and preparing method, utilizing the induced deformation ferrite
phase transformation phenomenon, adopting the low cost C-Mn steel, and
controlling the DIFT technology and continuous cooling technology to get the fine
grain polyphase steel organization. The method comprises the 0.03-0.15wt% C,
1.00-2.00wt% Mn, 0.10-1.50wt% Si, less 0.020wt% S, less 0.030wt% P, 0.020-
0.080wt% Nb, less 0.020wt% N, less 0.020wt% O and iron. The method
comprises the following steps: a. smelting, casting; b. heating to 1150-
1250Deg.C; c. rough rolling at the temperature between 920-1100Deg.C; d. fine
rolling at the temperature between 790-920Deg.C; e. cooling at the speed of 0.5-
30i[mu]/s to less 600Deg.C, and batching.
The document US 6027587 (A) discloses Steel with ultrafine grains is
produced by altering the transformation from one which normally proceeds with
grain boundary nucleation followed by intragranular nucleation at deformation
bands and other defects, to one which induces a substantially instantaneous
transformation homogeneously over the austenite grain. This is favoured by a
reduction or minimisation of grain boundary nucleation, (for example by
enlargement of the austenite grain size), prior to or during the transformation. In
an embodiment, partially cooled austenite phase steel is deformed in a single
pass at a temperature in the range of 700-950 DEG C. to obtain ferrite grain size
of 5 mu m or less.
The document US 4466842 (A) discloses hot-rolled ferritic steel
composed of 70% or more of equiaxed ferrite grains having an ultra-fine grain
size of 4 mu m or less. This steel is produced by a hot working at approximately
the Ar3 point and by one or more passes of the hot working having a total
reduction ratio of at least 75%. When a plurality of passes is carried out, the time

between passes is less than one second. Due to hot working, dynamic
transformation of austenite and/or dynamic recrystallization of ferrite takes place.
The total reduction ratio may be at least 35% for a high purity steel.
The document RU 2321670 discloses fine-grain iron-base martensite
alloys with number of grain according to ASTM no less than five, method for
producing such alloys, where the alloy contains, mass%: carbon, approximately
from 0.05 till 0.5; chrome, at least 5; nickel, at least 0.5; cobalt, no more than 15;
copper, no more than 8; manganese, no more than 8; silicon, no more than 4;
molybdenum and tungsten, no more than 6; titanium, no more than 1.5;
vanadium, no more than 3; niobium, no more than 1.7; aluminum, no more than
0.2; zirconium less than 2; tantalum, less than 4; hafnium, less than 4; nitrogen,
less than 0.1; each component of group containing calcium, cerium, magnesium,
scandium, yttrium, lanthanum, beryllium and boron, less than 0.1; each
component of group containing sulfur, phosphorus, tin, antimony, oxygen and
lead, less than 0.1; iron, at least approximately 40. Method comprises steps of
producing alloy, subjecting it to hot working at temperature more than 800° C till
real deformation exceeding 7.5%; cooling alloy till 20° C. The alloy thus produces
has improved mechanical properties and corrosion resistance of alloy.
However, ultra-fine grains achieved through these processes in plain
carbon steels upto the level of 3-5 urn in flat as well as long products to a limited
extent of thickness. Variation in the microstructure as well as grain sizes is
observed in these steels in through-thickness direction. The steel structure at the
centre is found to be different and the grain size is coarser compared to the
surface. This sort of structural inhomogeneity leads to a wide scatter in
mechanical properties and the benefit of ultra-fine grains produced at the surface
layers get restricted.
The invention therefore proposes a novel method to overcome the
aforesaid disadvantages of conventional process of producing fine grained C-Mn

steel plates and to ensure uniform structure and through-thickness grain size to
the desired level in the experimental hot-rolled steel plate upto 4 mm thickness.
SUMMARY OF THE INVENTION
In view of the foregoing disadvantages inherent to the known types of the
processes for achieving fine grained structures in steel plates the present
invention relates to an improved and easier technique for producing fine grained
structure in carbon-manganese steel plates with through thickness uniformity.
The proposed improved and novel process for producing fine grained carbon-
manganese steel plates with or without addition of vanadium and, or niobium
comprising the steps of:
Heat making and hot rolling
Dilatometry studies
Hot compression studies
Microstructural examination and grain size measurement
Property evaluation
Scanning and transmission electron microscopy.
Therefore the principal objective of the present invention is to provide an
improved method for producing fine grain structure in carbon-manganese steel
plates having minimum 2.5-3.5µm size grains with through thickness uniformity.
The second objective of the present invention is to provide an improved
method for producing fine grained carbon-manganese steel plates with or without
addition of vanadium or niobium.
The third objective of the present invention is to provide an improved
method for producing fine grained carbon-manganese steel plates through multi-
pass hot rolling and subsequent air cooling.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Fig 1 illustrates the chemical composition of the experimental steels
(weight %).
Fig 2 illustrates the transformation temperatures calculated from the
dilatometry plots.
Fig 3 illustrates the optical micrographs showing through thickness fine
grained structures in steel 1 at (a) edge (b) quarter thickness and (c) centre;
500X.
Fig 4 illustrates the optical micrographs showing through thickness fine
grained structures in steel 2 at (a) edge (b) quarter thickness and (c) centre;
500X.
Fig 5 illustrates the optical micrographs showing through thickness fine
grained structures in steel 3 at (a) edge (b) quarter thickness and (c) centre;
500X.
Fig 6 illustrates the grain sizes of the hot-rolled steels.
Fig 7 illustrates the bar chart showing grain size distribution near surface
of steel 1.
Fig 8 illustrates the bar chart showing grain size distribution near surface
of steel 2.
Fig 9 illustrates the bar chart showing grain size distribution near surface
of steel 3.

Fig 10 illustrates the SEM micrographs of steel 1 showing fine grains at (a)
surface quarter thickness and (c) centre; 1000X.
Fig 11 illustrates the SEM micrographs of steel 2 showing fine grains at (a)
surface quarter thickness (c) centre; 1000X.
Fig 12 illustrates the SEM micrographs of sample#3 showing fine grains at
(a) surface quarter thickness (c) centre ; 1000X.
Fig 13 illustrates TEM micrographs of steel 1 showing (a) fine ferrite grain;
40K, (b) deformed pearlite; 12K, (c) undeformed parallel pearlite lamella; 15K
Fig 14 illustrates TEM micrographs of steel 2 showing (a) fine ferrite grain;
6K, (b) deformed pearlite; 15K, (c) undeformed parallel pearlite lamella; 15K, (d)
Nb (C, N) precipitates; 30K, (e) corresponding SAD pattern.
Fig 15 illustrates TEM micrographs of steel 3 showing (a) fine ferrite
grains; 10K, (b) deformed pearlite; 10K, (c) VC precipitates; 25K, (d)
corresponding SAD pattern.
Fig 16 illustrates the mechanical properties of the as-rolled plates.
DETAILED DESCRIPTION OF THE INVENTION
In consideration of that state of the art the inventors set themselves the
aim of improving the known methods, avoiding the disadvantages noted, and
dealing with the task, which is considerably different in comparison with the
conventional process of producing fine grained steel plates.
The first operation is the making of laboratory heats of different
chemistries such as with plain C-Mn, C-Mn-Nb and C-Mn-V compositions and

cast into ingots. The defective portions of the ingots, i.e., 25% from the top and
10% from the bottom are then discarded. The remaining portion of the ingots are
soaked in furnace at 1200°C for 3 hours and rolled in 2 Hi/ 4 Hi experimental
rolling mill into 20 mm thick plates. Samples for dilatometry and hot compression
studies are prepared from as-rolled plates. After determining the optimum
processing window for obtaining fine ferrite grains, the plates are further rolled
into 4 mm plates in 3 passes with finish rolling temperature of 820°C and
subsequently cooled in air.
The second operation is the dilatometry studies of the cylindrical steel
samples of 6mmΦ x 85mm dimension using a dynamic thermomechanical
simulator. Specimens are electrically heated and subsequently cooled at different
cooling rates of 1°C/s, 10°C/s, 20°C/s and 50°C/s to obtain the transformation
start and finish temperatures and consequently, to asses the nature of phase
transformation occurring at various cooling rates.
The third operation is the hot compression studies of the cylindrical steel
samples of 10mmΦ x 15mm dimension using a dynamic thermomechanical
simulator at a strain of 0.5, strain rate of 5s"1 and finish rolling temperature of
820°C followed by air cooling at the rates of 5/10°C/s.
The fourth operation is the microstructural examination under metallurgical
microscope and grain size measurement of the rolled as well as of the samples
pertaining to the dilatometry and hot compression studies using image analyzer.
The L-T section of all the hot rolled samples and transverse sections of the
dilatometric and hot compression studied samples are ground and mechanically
polished following conventional metallographic procedures. The polished
samples are etched in 2% nital solution for microstructural examination. Grain
sizes are measured at surface, quarter thickness and centre of the hot
compressed samples at various spots.

The fifth operation is the property evaluation of the hot rolled steel
samples through tensile testing and Charpy impact testing. Tensile tests are
carried out in a universal testing machine using flat specimens of 25 mm gauge
length in accordance with ASTM A 370 standard. Charpy impact tests are carried
out at 25°C, 0°C and -20°C using sub-sized specimens as per ASTM E23
specification. For sub-zero temperature testing (0°C and -20°C), methanol and
liquid nitrogen mixture bath is used and each sample is soaked for 25 minutes in
the bath before testing.
The sixth operation is the scanning electron microscopy and transmission
electron microscopy of the as-rolled steel samples.
Dilatometric studies shows that at a slower cooling rate of 1°C/s, the
transformation is mostly ferrito-pearlitic and Ar3 and Ar1 temperatures varies
between 758-784°C and 640-680°C respectively. At a faster cooling rate of 10°
C/s, the transformation is largely bainitic and Bs and Bf temperatures varies
between 531-648°C and 412-566°C. At a cooling rate of 20°C/s and 50°C/s, the
transformation was however, martensitic and Ms and Mf temperatures varied
between 348-521 °C and 291-408°C. Dilatometry indicates lowering of
transformation temperatures with faster cooling rate. Fine polygonal ferrito-
pearlitic microstructure is achieved at a cooling rate of 1°C/s. Beyond that
microstructure is largely acicular ferrite, upper bainite or martensite.
The scanning electron micrographs of the as-rolled 4 mm plates of steel 1
to 3 at surface, quarter thickness and centre are shown in Figure 20 to 25
respectively at 1000X magnification. The structures at surface, quarter thickness
and centre shows fine equiaxed ferrite grains. Here also finer and uniform
surface to centre grain sizes can be observed in higher Mn steel (>1.19 wt%)
confirming the observations made by optical microscopy.

In order to assess the distribution of grain sizes near the surface, bar
charts were prepared for all the 3 steels using the image analysis data as shown
in Figure 26 to 31. It can be observed that the grain sizes in the range of 2.63 to
3.46 urn are 64, 69 and 80% in steel 1, 2 and 3 respectively which confirms the
presence of larger volume fraction of finer grains (<3.46 µrn) in higher Mn steels.
Transmission Electron Microscopy (TEM) of the as-rolled plate of steel 4
reveals a small equiaxed ferrite grain of approximate diameter 3.25 µm as shown
in Fig. 13 (a) at 40.000X magnification. Fragmented pearlite lamellae as well as
aligned pearlite lamellae could be observed as shown in Figure 13(b) and (c)
respectively at 12,000X and 15,000X magnifications. Interlamellar spacing was
measured and found to 0.25µm.
TEM micrographs of steel 5 are shown in Figure 14. A small equiaxed
ferrite grain of diameter approximately 2.21 µm can be observed in Figure 14 (a)
at 6,000X magnification. Fragmented pearlite as well as aligned pearlite lamalle
can be visible in the structure as shown in Figure 14 (b) and (c) at 15.000X
magnification. Pearlite Lamellar spacing in this steel was measured to be 0.22
pm. A typical globular Nb (C,N) precipitate and its corresponding SAD pattern are
shown in Figure 14(d) and (e) respectively.
The TEM micrographs taken in steel 3 are shown in Figure 15. Figure
15(a) shows a typical elongated ferrite grain of average grain diameter 1.02 pm.
Fragmented pearlite lamalle can be observed in Figure 15(b) at 10,000X
magnification. The average lamellar spacing is measured as 0.18 pm. An oval-
shaped large precipitate of vanadium carbide can be observed at 25,000X
magnification and corresponding SAD pattern are shown in in Figure 15(c) and
15(d) respectively.
Tensile and Charpy impact toughness properties of the as-rolled steels 1
to 3 are provided in Figure 5. The average yield strength (YS) varies between

422 to 594 MPa. Yield strength in Nb and V microalloyed steels (steels 2 and 3)
are 472 and 594 MPa respectively, which clearly shows the effectiveness of Nb
and V in enhancing ferrite grain size refinement as well as inducing precipitation
hardening. The ultimate tensile strength (UTS) of these steels varies between
587 to 750 MPa. The pet. elongation (%EL) varies between 22-23% based on the
chemical composition of the steels. As expected, matrix hardening led to
reduction in elongation%. However, even at very high tensile strength at the level
of 750 MPa, in V microalloyed steel (steel 3), the %EL is maintained at the level
of 23% which is quite acceptable from application point of view. The reduction in
area (RA) percentage varies between 55-61%.
The Charpy V-notch (CVN) energies of steels 1 to 3 varies between 56-
68J at RT 48-56J at 0°C, 44-68J at -20°C. Among the three steels, steel 2 (Nb-
microalloyed) shows maximum CVN energies at all test temperatures followed by
steel 3 (V-microalloyed) in comparison to steel 1 (C-Mn). Nb is found to increase
impact toughness more compared to V- microalloyed steels. Overall impact
toughness of all the 3 steels are found to be satisfactory and do not deteriorate
much at sub zero temperature of -20°C. Thus, grain refinement not only
enhanced the strength of the steels but also assists to achieve better impact
toughness properties.

WE CLAIM
1. A process for producing fine grained 2.5-3.5 µm carbon-manganese steel
plates with or without niobium or vanadium by multi-pass air-cooled rolling
route with through thickness uniformity comprising the steps of:
(i) heat making and hot rolling;
(ii) dilatometry studies;
(iii) hot compression studies;
(iv) microstructural examination and grain size measurement and
(v) Property evaluation.
2. A process for producing fine grained 2.5-3.5 µm carbon-manganese steel
plates as claimed in claim 1, where in rolling of the steel is continued after
soaking at a temperature of 1200°C for 3 hours and rolled in rolling mill
into 20 mm thick plates.
3. A process for producing fine grained 2.5-3.5 µm carbon-manganese steel
plates as claimed in claim 2, where in the steel plates are further rolled
into 4 mm plates in 3 passes with finish rolling temperature of 820°C and
subsequently cooled in air.
4. A process for producing fine grained 2.5-3.5 µm carbon-manganese steel
plates as claimed in claim 1, where in dilatometry studies of the cylindrical
steel using a dynamic thermomechanical simulator.
5. A process for producing fine grained 2.5-3.5 µm carbon-manganese steel
plates as claimed in claim 5, where in the cylindrical steel are electrically
heated and subsequently cooled at different cooling rates of 1°C/s, 10°C/s,
20°C/s and 50°C/s to obtain transformation start and finish temperature.

6. A process for producing fine grained 2.5-3.5 µm carbon-manganese steel
plates as claimed in claim 6, where in at a slower cooling rate of 1°C/s, the
transformation is mostly ferrito-pearlitic and Ar3 and Ar1 temperatures
varies between 758-784°C and 640-680°C respectively.
7. A process for producing fine grained 2.5-3.5 µm carbon-manganese steel
plates as claimed in claim 6, where in at a faster cooling rate of 10° C/s,
the transformation is largely bainitic and Bs and Bf temperatures varies
between 531-648°C and 412-566°C.
8. A process for producing fine grained 2.5-3.5 |j,m carbon-manganese steel
plates as claimed in claim 6, where in at cooling rate of 20°C/s and
50°C/s, the transformation was martensitic and Ms and Mf temperatures
varied between 348-521 °C and 291-408°C.
9. A process for producing fine grained 2.5-3.5 µm carbon-manganese steel
plates as claimed in claim 1, where in the hot compression studies of the
cylindrical steel is made using a dynamic thermomechanical simulator at a
strain of 0.5.
10. A process for producing fine grained 2.5-3.5 µm carbon-manganese steel
plates as claimed in claim 5, where in strain rate is 5s"1 and finish rolling
temperature of 820°C followed by air cooling at the rates of 5/10°C/s.
11. A process for producing fine grained 2.5-3.5 µm carbon-manganese steel
plates as claimed in claim 1, where in the microstructural examination of
the steel plate is made under metallurgical microscope and grain size
measurement studied using image analyzer.

12. A process for producing fine grained 2.5-3.5 µm carbon-manganese steel
plates as claimed in claim 1, where in property evaluation of the hot rolled
steel samples through tensile testing and Charpy impact testing.
13. A process for producing fine grained 2.5-3.5 µm carbon-manganese steel
plates as claimed in claim 1, where in the hot rolled steel plates are
scanned by electron microscopy and transmission electron microscopy.
14. A process for producing fine grained 2.5-3.5 µm carbon-manganese steel
plates as claimed in claim 1, wherein the steel comprise of 0.27 to 0.29
wt% of carbon, 0.37 to 0.40 wt% of silicon, 1.19 to 1.24 wt% of
Manganese, 0.010 to 0.013 wt% of sulphur, 0.006 to 0.008 wt% of
phosphorus, 0.032 wt% of niobium, 0.21 wt% of vanadium and 0.023 to
0.048 wt% of aluminium.

The present invention relates to a process for producing fine grained 2.5-3.5 µm
carbon-manganese steel plates with or without niobium or vanadium by multi pass
air-cooled rolling route with through thickness uniformity comprising the
steps of heat making and hot rolling; dilatometry studies; hot compression
studies; microstructural examination and grain size measurement and Property
evaluation.