| The role of nitrogen and niobium in low alloy steel as well as their effects on the mechanical properties were elaborated at home and aboard. Based on 20MnSiNb steel, two low alloy steels with different nitrogen content (130 ppm and 42 ppm) were designed. The influence of nitrogen content on the microstructure and mechanical properties of the niobium microalloyed steels was studied.To investigate the the effect of nitrogen on the precipitation behaviour of Nb (C, N), microstructure evolution, mechanical properties and alloyed cementite, two different trails, furnace cooling after solution treated at 1200? and 1150?, respectively. These experiments were also conducted under the condition of deformation and non-deformation, respectively.Micro-analyses including optical microscopy(OM), scanning electron microscopy(SEM), transmission electron microscopy(TEM), Carbon extraction replica, chemical analysis and X-ray diffraction were used to systematically study the morphology and distribution of Nb(C, N) precipitates and nitrogen content of alloyed cementite. Meanwhile, a comparision and analysis of mechanical properties were made to the test steels.Thermodynamic computation results showed that, with the increase in nitrogen content, the precipitation temperature of Nb(C, N) increased, but there is no significant increase in the amount of Nb(C, N) precipitates.The results demonstrated that the size, distribution and morphology of Nb(C, N) precipitates were influence by nitrogen content. After solution treated at 1150? where part of the Nb atoms are still out of solution, the Nb(C, N) precipitates in the high nitrogen test steel were independently distributed, some cuboidal and long needle-shape precipitates were detected. In the contrast, precipitates in the low nitrogen test steels are relatively very small and distributed in the form of clusters. After solution treated at 1200? where all the Nb atoms were in solid solution, in the high nitrogen test steels, the cuboidal and long needle-shape precipitates disappeared, the precipitates existed in the form of individual or complex. Whereas the precipitates in the low nitrogen test steels existed in the form of complex. The size distribution results demonstrated that, after solution treated at 1150?, precipitates size ranging 36-140 nm in the high nitrogen test steel exceeded 55%,in comparsion with low nitrogen test steel, the amount of super large precipitates was not increased. After solution treated at 1200?, the amount of small precipitates(< 36nm) were significantly increased which is more than 60%, while the amount of precipitates ranging 140-300nm was in a fluctuation of about 5%.The quantitative chemical analysis results showed that the nitrogen amount of Nb(C, N) increased to a small extent with the increse of nitrogen in the test steels. Although the amount of alloyed cementite was not affected by the nitrogen content, nitrogen was detected in the alloyed cementite, and the more nitrogen in the test steel, the more nitrogen in alloyed cementite.The mechanical properties results showed that the strength of high nitrogen test steels were a little higher than that of low nitrogen test steels, meanwhile, aging did not occurred in either steel.The continuous cooling transformation curves was significantly influenced by nitrogen content, the enhancement of nitrogen make a close of bainite regionThe influence of nitrogen on Nb (C, N) precipitation behavior indicates that nitrogen content affects the size, morphology and distribution of the Nb(C, N) precipitates, but did not affect the amount of Nb (C, N) precipitates. The mass fraction of fine particles (1-10nm) in the steel with high nitrogen content is less than that in that in the steel with low nitrogen content. Nb (C, N) particles in steel with high nitrogen content are in a homogeneous and independent arrangement, meanwhile, a large number of cubic and long needle-shaped particles were detected, However Nb(C, N) precipitates in the steel with low nitrogen content were very small and they were in a cluster-like distribution. After solution treated at 1200?,the mass fraction of fine precipitates increased in both the steels. Nitrogen in the steel existed in three forms including"free" nitrogen, alloyed cementite and Nb (C, N). Although nitrogen had no effect on the amount of alloyed cementite, it is the first time that nitrogen was detected in the alloyed cementite. What is more, the higher the nitrogen content in the steel, the greater the proportion of the nitrogen content of the alloyed cementite. Strength of the steel with high nitrogen content is slightly higher than that of steel with low nitrogen content, meanwhile, aging did not occur in either steel.The continuous cooling transformation (CCT) curves of the test steels was influenced by nitrogen content. The enhancement of nitrogen make a close to the bainite region, the formation of acicular ferrite was depressed at the slow cooling rates. At the same time, the ferrite-pearlite region was enlarged.The microscopic observation of the test steels solution treated at 1200? affirmed the above. When cooled at the rate of 200?/h or 100?/h, a great amount of acicular ferrite was observed in pearlite, and the large pearlite degenerated evidently, the cementite lamellae were discontinuous. While the pearlite in the high nitrogen test steel are relative small, where only few or no acicular ferrite was found. High density dislocations and fine Nb (C, N) precipitates (< 10 nm) were found inside the acicular ferrite which were not observed in the ferrite inside the pearlite beside the acicular ferrite. This demonstrated that the small Nb (C, N) precipitates involved the formation of the acicular ferrite.The above phenomena could be explained as follow:the small Nb (C, N) precipitates (< 10 nm) preferred to precipitating at the high density dislocation area. Instantaneous carbon-poor region will form thereby causing the stimulated nucleation of the acicular ferrite. Meanwhile, the interaction between the niobium and carbon inhibited the diffusion of carbon, resulting in the delay of pearlite transformation. In the steel with high nitrogen content, the interaction between between niobium and nitrogen is stronger than of the interaction between niobium and carbon, carbon-poor tendency in the high density dislocation region weakened as the increased nitrogen suppressed the formation of acicular ferrite. Compared with the steel with low nitrogen content, pearlite transformation is more easily.High and low temperature deformation on the microstructure and performance on the test steels showed that, under the same deformation condition, the microstructures of the steel with high nitrogen content is small and homogeneous compared with the steel with low nitrogen content. After high temperature deformed at 1100?, the microstructures were coarse, acicular ferrite and degenerated pearlite were detected in both the steels. Whereas the amount of acicular ferrite in the steel with high nitrogen content is far less than that of the steel with low nitrogen content. And the pearlite in the high nitrogen niobium steel is small and homogeneous. After low temperature deformed at 850?, ferrite-pearlite microstructures were obtained in both the steels. Microstructures in the steel with high nitrogen content were equiaxed while in the steel with low nitrogen content they had the characteristic of deformation bands. |
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