To study precipitation behavior of vanadium-rich carbides formed during the reheating processes and its influence on the formation of ultra-fine grained austenite microstructure and mechanical properties have been systematically investigated in a high strength low alloy martensite steel. reference Gengwei Yang et al. Materials and Design 50 (2013) 102–107

1. National Institute of Foundry and Forge Technology, Ranchi
Department of Metallurgy and Materials
Report Seminar
Presented by:- NITIN KUMAR

2. In recent years, Ultra High Strength and excellent toughness has been demanded for the development
of high-performance machines
• Low alloy martensite steel has potential of achieving high strength.
• However, the toughness of martensite steels is decreased as its strength increases.
What is the most effective way to acquire excellent balance between strength and toughness ?
Answer is, keeping Low carbon and doing grain refinement.
 Lath martensite generally appear in low carbon (0-.6 wt. % c) low alloy steels and consist of multi-scale
substructures such as packet, block and lath.
 The packet and block sizes are the important factors which can be adjusted for achieving both strength
and toughness of martensite steels.
 Since the sizes of packet and block in martensite strongly depend on the prior austenite grain size.
 The packet and block sizes of lath martensite were both refined as the austenite grain size
decreased.

3. Method of Prior austenite grain size Refinement.
1> controlled rolling.
a. recrystallization rolling (rolling above 950 °c in austenite recrystallization region)
Average grain size of austenite as small as 20μm was the refinement limitation.
b. non-recrystallization rolling (rolling below 850°c in un-crystallize region)
Average grain size of austenite is 5μm.
2> Heat treatment.
Rapid cyclic heat treatment could achieve ultra-fine grained austenite with the average grain
size smaller than 5μm. However, it was hardly applied in mass production for the limitations
of complex process and specimen dimension.
3> Combining controlled rolling and simple Heat treatment.
this is the recent method of achieving ultra fined grained austenite smaller than 5μm.
Here Vanadium instead of niobium is chosen as the microalloying element to restrain
austenite recrystallization and growth in controlled rolling and heat treatment.

4. 1. EXPERIMENTAL WORK
AIM:-
To study precipitation behavior of vanadium-rich carbides formed during the reheating processes and
its influence on the formation of ultra-fine grained austenite microstructure and mechanical properties
have been systematically investigated in a high strength low alloy martensite steel.
MATERIAL
Chemical composition in wt.%
C Mn Si V Ti N Fe
.26 1.1 .5 .28 .02 .002 balance

5. 2. PROCEDURE
the samples cut from the rolled plate were then reheated to different interested temperatures at the
rate of 5 °C/s. At this heating rate,Ac1and Ac3 of the studied steel are 725 ° C and 845 ° C respectively.
holding for 1 h, the samples were water quenched to ambient temperature. In order to identify the heating
process for vanadium precipitation, an additional sample was reheated to 880 ° C, followed by water
quenching without isothermal holding. The names and status of all the samples are listed in Table 1.

6. 1. The microstructure of samples was characterized by means of optical microscopy, The prior
austenite grain boundary was etched by a mixed solution of picric acid, hydrochloric acid and
surface active agent.
2. Precipitation particles were observed by transmission electron microscopy (TEM) ,TEM
observation was operated at 200 kV on carbon replicas, which was prepared through
electrolytically extracting in 4% nital at the voltage of 2 V after etching in the same etchant and
depositing a thin carbon film.
3. The particle size of precipitates was measured by a quantitative image analyzer.
4. phase structures of extracted particles were identified by X-ray diffraction (XRD).
5. The tensile tests were conducted on a universal materials tester in accordance with standard.
6. Charpy impact were conducted at -40 C using standard V-notched specimens in size of 10 mm10
mm 55 mm with 2 mm deep notch as per ISO.

7. 3. Results and discussion
3.1. Austenite grain refinement during different processes
Fig. 1 shows the prior austenite grain of the samples A, E, F and G.
Sample Austenite grain
size(μm)
A(controlled hot
rolling)
10.1
E (reheated to 880) 3.5
F (reheated to 900) 4.5
G(reheated to 950) 11.2

8. 3.2. The microstructure transformed from ultra-fine austenite
EBSD analysis was used to characterize the lath martensite, which was transformed from the ultra-fine
austenite.
1. Fig. 2 compares inverse-pole-figure (IPF) color maps of the three samples (E, F and G) reheated at 880 °C,
900 °C and 950 °C. The prior austenite grain boundaries indicated by the lath martensite boundaries
misorientated in the range from 15 °C to 50 °C were plotted as black lines in the IPF maps.
2. The packet and block sizes of lath martensite were both refined as the austenite grain size decreased.
Further more,
the mean number of packet in a single austenite grain decreased in the order of G, F and E.
Prior austenite grain boundary divided into  Packets  Blocks  laths.

9. shown in Fig. 2a, there was only one packet transformed from the small austenite grain, and the average
packet size approached to the size of refined prior austenite grain. As the austenite grain sizes of sample
E and F were much smaller than that of sample G, the proportion of the boundaries misoriented from 15
to 50, most of which were prior austenite grain boundaries, in samples E and F were much higher than
sample G. They were about 27%, 24% and 15% for samples E, F and G, respectively.

10. 3.3. Precipitation behaviors of (Ti, V)C during the reheating processes
Table 2 shows the quantitative analysis results of the precipitate compositions.. As vanadium carbonitride
has a large solubility product in austenite at relatively high temperature precipitates hardly formed in the
sample which was directly quenched after controlled rolling. Therefore, in the hot rolled steel, the main
vanadium remained in the transformed ferritic structure as solution condition. Since the solubility of (Ti, V)C
in ferrite is much lower than that in austenite, the supersaturated vanadium would precipitate in the further
reheating process. When the sample was tempered at 650 C (sample B), the content of vanadium
precipitated increased to 0.168%. As the reheating temperature increased to 780 C (sample C), the amount
of vanadium precipitated was up to 0.195%.

11. Thus, it can be concluded that the vanadium precipitates mainly formed during reheating process, then
was dissolved when it exposed at high temperature of austenitization.
It could be concluded that the amount of vanadium precipitates decreased and its size coarsened as the
soaking time was increased at 880 C
It is well known that the austenite grain refinement during the reheating process depends on the phase
transformation and the pinning effect by precipitates.
The pinning force can be expressed as
where
C is the interfacial energy of grain boundary, f and r are the
volume fraction and average radius of particles, respectively

12. When the sample was reheated in the austenite phase region, the mass fraction of (Ti, V)C decreased as
the reheating temperature increased. Some small precipitates would be redissolved but the bigger
particles coarsened at the higher reheating temperature.
Thus, the volume fraction of (Ti, V)C decreased but the radius of (Ti,V)C particles increased with the
reheating temperature increasing.
Therefore, the pinning force decreased with the reheating temperature increasing, as shown in Fig. 5.
The biggest pinning force was generated by (Ti, V)C particles in sample D that would result in the finest
austenite grain in sample D. The average size of austenite grain in sample D was 2μm, which was the
smallest among the samples D, E, F and G.

13. 3.4. Mechanical properties of the ultra-fine grained steels
Table 3 shows the mechanical properties of the sample E and G with the prior austenite grain size of 3.5
μm
and 11.2 μm, respectively.
the toughness of sample E was slightly improved in spite of the great increase of strength.
Thus, the low-temperature toughness could be improved mainly due to the refinement of the packet or
block size, which resulted from austenite grain refining

14. Grain refining strengthening could be described by the conventional Hall–Petch equation:
where d is the effective grain size,
Ky is the Hall–Patch coefficient
In lath martensite, a prior austenite grain can be divided into packets, and a packet is partitioned into
several blocks, each of which contains several laths.
Prior austenite grain boundary divided into  Packets  Blocks  laths.
the structural unit closely related to the toughness of lath martensite is martensitic packet.
The fracture stress is inversely to square root of packet size, and the ductile to brittle transition temperature
(DBTT) decreases with the grain size decreases

15. CONCLUSION
The nano-sized (Ti, V)C carbides mainly precipitated during the reheating process and inhibited austenite
grain growth effectively. Ultra-fine austenite grain with the average size of 3.5 μm was obtained during
austenitization at 880 °C for 1 h.
the mass fraction of (Ti, V)C decreased and the particle size coarsened with the increasing of reheating
temperature, the austenite grains coarsened significantly.
Excellent combined mechanical properties such as tensile strength 1670 MPa, yield strength 1460 MPa,
elongation 10% at room temperature and impact energy 57 J at -40 C were obtained in ultra fine grained
steel.

16. Thanks