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Thermo-simulation test, metallographic analysis, measurement of hardness, TEM(including diffraction contrast technique and nano-beam anylasis)along with calculating software of Metlab, etc. were used to study the influence of RPC parameters(relaxation time, aging time)on the precipitation of Nb in microalloyed steel. Following work was undertaken in this study:Strain-induced precipitation in Nb-bearing multi-microalloyed steel was monitered and analyzed. The interaction between dislocation evolution and precipitates in deformed austenite was directly observed(an Fe-Ni-Nb alloy was employed to simulate austenite in steel). The(a low carbon microalloyed steel only by Nb was adopted)effect of relaxation of deformed austenite on aging behavior occurring in bainite was investigated. Finally, the thermal stability and variation of composition of carbonitride precipitates during dissolving process in Nb-Ti microalloyed steel were alsodetected. These results indicate:(1)During the stress relaxation, the process of precipitation happened in deformed austenite of multi-microalloyed steel can be divided into three stages, i. e. nucleation, growth and coarsening. In the final stage of relaxation, Mo atoms would enter the lattice of(Nb,Ti)(C,N)precipitates and its concentration in the precipitates increases with the relaxation time.(2)In the earlier stage of relaxation, the dislocations in deformed austenite demonstrate twisted and distribute randomly. The precipitates have the tendency to occur along those dispersed dislocations. The dilocation cells would form gradually with the relaxing time prolonged and would become more stable by the subsequent precipitates formed on them in the later relaxation period. The paralleled dislocations in dislocation cells exhibit the same Burgers vector. The dislocations get rid of pinning by the bypassing mechanism.(3)When the samples are reheated to hold isothermally at 525℃,which are relaxed for different time in deformed austenite followed by accelerated cooling, their microstructures evolve very slowly with the aging time but two hardness peaks can be found one each hardness-time curves. The position of hardness peaks nearly raimains unchanged while their height varies obviously with the relaxation time. In no relaxtion samples, only one hardness peak could be observed. During the process of aging at 525℃, The growth and coarsening of as-existed strain-induced precipitates formed in deformed austenite contribute to formation of the first hardness peaks. While the second hardness peaks result from the further precipitates which nucleate in a phase after γ→α transformation(after aging for about 10 hours).(4)It was found that the precipitates formed during solidification are monophase(Ti,Nb)(C.N)rather than TiN enclosed with NbC. With addition of Ti to the steel, the thermal stability of the carbonitrides would enhance remarkably.
Thermo-simulation test, metallographic analysis, measurement of hardness, TEM (including diffraction contrast technique and nano-beam anylasis) along with calculating software of Metlab, etc. were used to study the influence of RPC parameters (relaxation time, aging time) on the precipitation of Nb in microalloyed steel. Following work was undertaken in this study: Strain-induced precipitation in Nb-bearing multi-microalloyed steel was monitered and analyzed. The interaction between dislocation evolution and precipitates inaled austenite was directly observed (an Fe- Ni-Nb alloy was employed to simulate austenite in steel). The (a low carbon microloyloyed steel only by Nb was adopted) effect of relaxation of deformed austenite on aging behavior occurring in bainite was investigated. Finally, the thermal stability and variation of composition of carbonitride precipitates during dissolving process in Nb-Ti microalloyed steel were alsodetected. These results indicate: (1) During the stress relax In the final stage of relaxation, the process of precipitation in a deformed austenite of multi-microalloyed steel can be divided into three stages, ie nucleation, growth and coarsening. In the final stage of relaxation, Mo atoms would enter the lattice of (Nb, Ti) (C (2) In the earlier stage of relaxation, the dislocations in deformed austenite demonstrated twisted and distributed randomly. The precipitates have the tendency to occur along those dispersed dislocations. The dilition cells would form gradually with the relaxing time prolonged and would become more stable by the subsequent precipitates formed would they become more stable by the bypassing mechanism (3) When the samples are reheated to hold isothermally at 525 ℃, which are relaxed for different time in deformed austenite follow ed by accelerated cooling, their microstructures evolve very slowly with the aging time but two hardness peaks can be found one each hardness-time curves. The position of hardness peaks nearly raimains unchanged while their height varies obviously with the relaxation time. During the aging of 525 ° C, the growth and coarsening of as-existed strain-induced precipitates were formed in the first austenite contribute to formation of the first hardness peaks. While the second hardness peaks result from the further precipitates which nucleate in a phase after γ → α transformation (after aging for about 10 hours). (4) It was found that the precipitates formed during solidification are monophase (Ti, Nb) (CN) rather than TiN enclosed with NbC With addition of Ti to the steel, the thermal stability of the carbonitrides would enhance remarkably.