Study On The Microstructure And Properties Of Low Carbon Low Silicon TRIP Steel Containing Vanadium

The relationship among heat treatment, microstructure and mechanical property in low carbon low silicon TRIP steel containing vanadium and aluminium was exploited successfully in the dissertation. The product of strength and ductility of studied TRIP steel was up to 22000MPaï¼…, which was equal to traditional TRIP steel containing high silicon. The static and dynamic mechanical properties were investigated and retained austenite transformation kinetics during static tensile and dynamic tensile test were studied. Moreover, thermal decomposition of retained austenite and galvanizability of TRIP steels were also investigated.The main points of the dissertation include:In order to study effects of the addition of vanadium on microstructure and mechanical property of TRIP steel, the relationship between microstructure and annealing temperature in TRIP steel with vanadium were studied compared with that of TRIP steel without vanadium. It was found that the addition of vanadium increased the volume fraction of retained austenite in TRIP steel. For TRIP steel without vanadium, the volume fraction of austenite after intercritical annealed at 780℃for 5 minutes and subsequently austempered at 400℃for 5 minutes reached its highest value 10ï¼…. For TRIP steel with vanadium, the volume fraction of austenite after intercritical annealed at 740℃for 5 minutes and subsequently austempered at 400℃for 5 minutes or after intercritical annealed at 800℃for 5 minutes and subsequently austempered at 400℃for 5 minutes was the highest, which was up to 15ï¼….The relationship between mechanical property and annealing temperature in TRIP steel containing vanadium and that without vanadium was studied. It was found that the addition of vanadium increased the strength of TRIP steel, but deteriorated the elongation. The addition of vanadium by 0.1wtï¼…increased the strength by 60Mpa, while reduced the elongation rate by 0.1. It was also found that the stain-stress curves of TRIP steel all satisfied the rule ofσ=kεⁿ. The product of strength and elongation in TRIP steel containing vanadium or without vanadium after TRIP treatment were all higher than the steel after DP treatment at the same annealing temperature, so retained austenite exhibited much effect on improving the product of strength and elongation in TRIP steel.Low carbon low silicon TRIP steel containing vanadium and aluminium was exploited successfully with the composition(in wtï¼…): C: 0.11ï¼…, Mn: 0.8ï¼…, Si: 0.55ï¼…, Al: 1.0ï¼…and V: 0.1ï¼…. The variation of microstructure, mechanical property regarding annealing temperature in test TRIP steel containing vanadium and aluminium was studied. The highest volume fraction of retained austenite in test TRIP steel containing vanadium and aluminum was up to18ï¼…, which showed that the addition of vanadium and aluminum simultaneously increased the volume fraction of retained austenite. The product of strength and elongation rate of test TRIP steel containing vanadium and aluminum after intercritical annealed at 760℃for 5 minutes and subsequently austempered at 400℃for 5 minutes was the biggest, which was up to 22000MPaï¼…and equal to the traditional TRIP steel with high silicon. The test TRIP steel containing vanadium and aluminum exhibited higher n value of 0.24. It followed from the fact stated above that the test TRIP steel containing vanadium and aluminum was of good work-hardening, an important behavior for steel applied in automobile.The variation rule of dynamic mechanical property in test TRIP steel containing vanadium and aluminum was studied by tensile test on dynamic impact tensile machine and the strain rate ranged from 400/s to 1500/s. It showed that the test TRIP steel containing vanadium and aluminum was sensitive to strain rate. With increasing strain rate, the strength and elongation increased and the highest product of strength and elongation approximated to 30000MPaï¼…, which was much higher than that obtained in static mechanical test.Transformation kinetics of retained austenite in test TRIP steel was studied during static and dynamic tensile test. The function y=1-e^-Ax represented the retained austenite transformation kinetics, while A the velocity of transformation. The smaller the A value, the slower the velocity. Much retained asutenite transformation occured at the larger strain, which improved the product of strength and ductility of TRIP steel. The retained austenite kinetics(A value) was a more effective factor for the product of strength and ductility of TRIP steel in comparison with its volume fraction. A value was different after different heat treatment conditions, so mechanical property could be improved by reducing A value.Thermal stability of retained austenite in test TRIP steel was studied by DSC method. Comparison between DSC curves of samples with retained austenite and without retained austenite determined decomposition curve of retained austenite. The thermal decomposition temperature of retained austenite in the tested TRIP steel was in the range of 400～600℃, being higher than that in quenched carbon steel. Kissinger function was used to determine the decomposition activation energies of retained austenite, ranging from 120 to 137KJ/mol. Kinetic parameters of retained austenite heat decomposition were obtained by fitting JMA kinetic equation. Good correlation was gained between heat decomposition curve of retained austenite, which was calculated according to JMA kinetic equation, and the curve measured by DSC test. The present work thus provided a simple and credible method for research on thermal stability of retained austenite in TRIP steel.Different TRIP steels were hot-dip galvanized by means of a laboratory hot-dip simulator and the test TRIP steel with silicon of 0.55wtï¼…was hot-dip galvanized successfully under appropriate dew point of the annealing atmosphere. Effect of different compositions of galvanization liquid on the visual appearance and structure of the galvanized panels were investigated, which showed that the composition of galvanization liquid had much effect on the visual appearance and structure of the galvanized panels. During immersion in the 0.5ï¼…Al-containing Zn bath, the appearance of the galvanized panels was 50μm equiaxed grain and the structure contains pure Zn, Zn+Al+Fe and Zn+Fe. During immersion in the 5ï¼…Al-containing Zn bath, the appearance of the galvanized panels was 1～3mm fir-tree crystal and the structure contained Zn+Al and Zn+Al+Fe, without Zn+Fe, which showed that Al of galvanization liquid inhibit the formation of Zn+Fe.