1. The Effect of Restoration Process on the Mechanical Behavior of Ultrafine Grain Size Nb-Ti Steel Processed by Warm Rolling and Sub and Intercritical Annealing
Hezio Rosa Silva
Gustavo Gonçalves Lourenço
Luciana Helena Reis Braga
Dagoberto Brandão Santos
Federal University of Minas Gerais - UFMG - Brazil
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2. Mechanical Behavior of High Strength Steels
Concepts Review
Ferrite grain size refining increases both mechanical strength and toughness of steels
Low carbon content enhances good welding characteristics
Mechanical Behavior of High Strength Steels
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3. Historical Review Development through the years
Dr. D. Ponge – Max Planck Inst.
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4. (σe )HSLA : 350 to 850 MPa = 3(σe)carbon steels without alloying
Introduction
%C %Mn
enhances welding
Nb, V, Ti
stable precipitates
microstructure control
Controlled rolling with accelerated cooling
refined ferrite
(σe )HSLA : 350 to 850 MPa = 3(σe)carbon steels without alloying
automotive industry
pipe lines for low temperatures operations
plates for the naval industry
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5. Objectives
Evaluate the restoration process in refinement of ferrite in low C-Mn steel microalloyed with Nb and Ti. Produce ferrite grain size of about 1m with different microstructural constitution:
Besides ferrite and MA constituent,
a combination of:
Ferrite + dispersed cementite particles
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6. Experimental Procedure
Chemical Composition (%)
C Mn Nb Ti
Following steps:
Specimens reheated at 900°C and then ice brine quenched;
Specimens reheated at 740°C, submitted to warm rolling at 700°C, with three equal pass reductions and then air cooled;
The annealing schedule was employed on all specimens at temperatures of 550ºC or 800ºC for different times, and after the soaking time the samples were air cooled.
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7. Experimental Procedure
900°C – 30 min
800°C
550°C
740°C – 30 min
Ac3
Ac1
Times: 5, 30, 60, 120 min
T (°C)
Time (min)
Three pass of 20% reduction each
Experimental Procedure
Final microstructure revealed by nital 2% and LePera etching.
Ferrite grain size and volume fractions were determined using image analyzer software and the ASTM standards.
Vickers microhardness measured using a 0.3 N (300 gf).
Tensile tests and sub-size Charpy impact tests at -20ºC
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8. Results and Discussion
Ice brine quenching of non-deformed samples
To obtain a metastable microstructure to increase the ferrite nucleation rate during the warm rolling and in the subsequent annealing.
Temperature (ºC)
Time (s)
Ar3
Ar1
Annealing
900ºC
740oC
800oC
Quenching
Warm rolling
550oC
50mm
Nital Etched, MO
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9. Results and Discussion
Ice brine quenching of non-deformed samples
The mean prior austenite grain sized was about 10.1 m.
Temperature (ºC)
Time (s)
Ar3
Ar1
Annealing
900ºC
740oC
800oC
Quenching
Warm rolling
550oC
50mm
Picral Etched, OM
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10. Results and Discussion
Warm Rolled Microstructure
Microstructure highly deformed due to the work hardening during warm rolling, and presents islands of martensite over a ferritic matrix.
Temperature (ºC)
Time (s)
Ar3
Ar1
Annealing
900ºC
740oC
800oC
Quenching
Warm rolling
550oC
20 m
Nital Etched, MO
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11. Results and Discussion
Warm Rolled Microstructure
Rolling temperature was not enough to start recrystallization;
The austenite formed in the ferrite grain boundaries.
Temperature (ºC)
Time (s)
Ar3
Ar1
Annealing
900ºC
740oC
800oC
Quenching
Warm rolling
550oC
2% nital etched, SEM micrographs
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12. Results and Discussion
Intercritical and Subcritical Annealing
550ºC, 300 s – OM and SEM respectively, nital 2% etched
Temperature (ºC)
Time (s)
Ar3
Ar1
Annealing
900ºC
740oC
800oC
Quenching
Warm rolling
550oC
7200 s
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13. Results and Discussion
Intercritical and Subcritical Annealing
800ºC, 300 s
Temperature (ºC)
Time (s)
Ar3
Ar1
Annealing
900ºC
740oC
800oC
Quenching
Warm rolling
550oC
800ºC, s
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14. Results and Discussion
Intercritical and Subcritical Annealing
Two types of ferrite grains were observed:
polygonal ferrite grains which were formed on cooling from annealing temperature;
ferrite sub-grains formed as a result of recovery and recrystallization of deformed grains.
Some ferrite grains with cell dislocation structure in which recovery took place are also present;
As soon as austenite become homogeneous, it has sufficient time to coalesce, providing larger MA islands.
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15. Results and Discussion
Intercritical and Subcritical Annealing
300 s
7200 s
550ºC
800ºC
OM, LePera etched
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16. Results and Discussion
Intercritical and Subcritical Annealing
Observed:
Some elongated grains with high dislocation density;
Islands and blocks of MA in the final microstructure;
Minor constituents present were granular bainite and pearlite;
Carbides prevail for subcritical annealing, while MA for intercritical annealing.
Fv(800ºC) for MA practically suffers no variation with the annealing time.
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17. Results and Discussion
Mechanical Behavior
As the annealing time increases, the hardness and tensile strength decreases…
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18. Results and Discussion
Mechanical Behavior
…whereas the average ferrite grain size increases continuously.
Result of the competition between three processes taking place:
I - recovery and recrystallization of ferrite;
II - grain growth of recrystallized ferrite grains;
III - austenite formation and their grain growth during intercritical annealing
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19. Results and Discussion
Mechanical Behavior
The beginning of the recrystallization can be observed by an inflection in the curve of hardness versus annealing temperature at approximately 550ºC.
Microhardness values for specimens annealed during 1800 s at different temperatures.
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20. Results and Discussion
Mechanical Behavior
The MA constituent formation is responsible for lower yield strength of the samples annealed at 800ºC as the same way that happen in DP steels. The higher ductility for samples annealed at 800ºC can be explained in the same way.
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21. Results and Discussion
Mechanical Behavior
The MA constituent formation is responsible for low energies of the samples annealed at 800ºC. Annealing at 500ºC led to an increase in strength, but low values for absorbed energies.
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22. Conclusion
The mean ferrite grain size obtained was between 1.3 and 3.8 µm, respectively, for the highest and lowest annealing times, respectively. Reaching the maximum level of refinement about 87% with an initial austenitic grain size of 10.1 µm.
The microhardness has changed from 175 to 220 VHN, that shows the influence of austenitic grain size prevail about MA volume fraction. There was an improvement of 20% on mechanical properties in comparison to initial sample (hot rolled – 187 VHN).
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23. Conclusion
The quenching from 900ºC led to the formation of a martensite homogeneous microstructure.
The results indicates it is possible to project an alloy with tensile strength near by 630 MPa for a 300 s annealing at 550ºC or a lower value for a 7200 s annealing (570 MPa). For annealing at 800ºC these values are lower.
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24. Conclusions
The low carbon steel has been subjected to warm rolling followed by intercritical and subcritical annealing. The results have shown the significant refinement of ferrite grain structure and corresponding improvement in mechanical properties.
The optimum combination of strength and ductility has been achieved in samples subjected to 900ºC austenitizing, quenching and then 0.66 reduction during warm rolling and 60 min annealing at 800ºC.
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25. Conclusion
The ferrite grain refinement led to an increase by 20% in mechanical properties compared to industrially hot rolled steel. This was accompanied by similar improvement in ductility and work hardening behaviour.
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26. Special Thanks To
Your attention Aknowledgements Conselho Nacional de Desenvolvimento Científico e Tecnológico
CNPq
Project FVA number: /2004-5
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