Low-carbon Bainitic Steel Organizations And The Second Phase Of Precipitates

This article conducted a thermodynamic analysis to second phase precipitation in continuous metal cast process; by microscope, Sem and Tem, experimented and analysed the casting blank and the rolled steel, discussed and validated the results.Thermodynamics calculation indicates that:in liquid steel, all the second phase cannot precipitate; in solidifying process, the precipitation temperature of TiN is 1779K. MnS precipitates a little at 1720K; the total solutiong temperature is 1776 K; in austenite phase, the precipitation sequence is MnS, TiC0.003N0.997, Cu2S, NbC0.76N0.24, NbC, NbN, TiC, A1N.By researching the isothermal transformation structure of casting blank at 1200℃,1000℃,800℃and 600℃, this article discussed the system of hot transporting and hot charging of casting blank before rolling: increase the hot charging temperature in low temperature zone as much as possible and the hot charging temperature should be under Ar1 and approaching Ar1 as much as possible.By researching the continuous cooling transformation structure of rolled steel, the bainite is generated by cooling rate of 15℃/s-30℃/s. In industry, cooling rate of 15℃/s～30℃/s should be adopted in rolling process.By researching the second phase precipitation of casting blank, lots of (Ti,Nb)(C,N) precipitation generated row by row is found and they precipitate in transgranular and grain boundary; less non-matal inclusions are found in the steel with the size of 2μm.This article discussed the precipitation behavior of CuS as well:CuS has the lowset misfit with acicular ferrite at each directions, "the lowest misfit theory"makes an important role in that CuS inhances the incleating of acicular ferrite.By researching the second phase precipitation of steel in different condition, the conclusion is:in precipitation density and volume fraction, the precipitation of casting blank is smallest, and the precipitation density of rolled+tempered steell is one number order more than that of casting blank with volume fraction 2-3 times more than that of casting blank。Because the number of precipitation increases,the second phase strengthening is enhancing, which is propitious to enhance the capability of the low-carbon bainite steel.