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 FVA

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 FVA

Historical Review Development through the years Dr. D. Ponge – Max Planck Inst. FVA

(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 FVA

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 FVA

Experimental Procedure Chemical Composition (%) ------------------------------------------------------------------ C Mn Nb Ti 0.11 1.41 0.028 0.012 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. FVA

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 FVA

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 FVA

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 FVA

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 FVA

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 FVA

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 FVA

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, 7200 s FVA

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. FVA

Results and Discussion Intercritical and Subcritical Annealing 300 s 7200 s 550ºC 800ºC OM, LePera etched FVA

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. FVA

Results and Discussion Mechanical Behavior As the annealing time increases, the hardness and tensile strength decreases… FVA

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 FVA

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. FVA

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. FVA

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. FVA

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). FVA

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. FVA

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. FVA

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. FVA

Special Thanks To Your attention Aknowledgements Conselho Nacional de Desenvolvimento Científico e Tecnológico CNPq Project FVA number: 400609/2004-5 FVA