Research On Hot Rolling Technique For Manufacturing Fine-Grained Dual Phase Steel Containing Nb And Ti

This paper is written to describe the experiment of low Si-Mn dual phase steel containing Nb,Ti. Two kinds of this steel are hot rolled and cooled. By the control rolling and control cooling designed to be two sections or three sections, fine microstructures that martensite exist in ferrite is gotten, of which the ratio ofσs/σb is low,σb is 550MPa,and the average size of grains is 5.5μm.The influence of rolling parameters and compound (Nb,Ti)(C,N) on the properties of steel is also analysized, and provide the theory basis for manufacture.The main works and corresponding results as follows:1.In order to roll dual phase steel at a small cost, a kind of low Si-Mn steel containing Nb,Ti is designed to be hot rolled, of which its ingredients of Nb,Ti is 0.02%,0.013%,by this way,σb of the steel can get to 550MPa with fine microstructures and good properties.2.The continuous cooling transformation experiment and single-pass hot simulation experiment of low Nb steel without deformation were carried out at the Gleeble 1500 thermo-mechanical simulator, and continuous cooling transformation (CCT) curves were gotten. The influence of deformation temperature,deformation speed on dynamic recrystallization discussed, the matrix of yield resistance is constructed.3.If CCT curves under the condition of a fixed cooling rate is used to show the beginning temperature and finishing temperature of transformation under the condition of a changing cooling rate, it is necessary to correct the results.In this paper, a new method is described that can show such temperatures and transformation time exactly and clearly, and it is easy, practical, credible.4. Through two-section cooling or three-section cooling experiment in laboratory, this paper investigate relationship between finish temperature,start water-cooling temperature and coiling temperature and the properties of both kinds of steel. The microstructures of the tested steels were observed by TEM.5.The small grains of second phase such as TiN,NbC,NbN and TiC precipitate at different temperatures calculated by iteration, TiN appears at the highest, NbC the second, NbN the third,TiC is difficult to equilibriumly precipitate.6.The dissolve of (Nb,Ti)(C,N) is investigated, Ti can get into (NbxTi1-x)(CyN1-y) when the temperatures is very high, (Ti,Nb)(C,N) can be the core of more complex compound.By the analysis above, the conclusion is that low Si-Mn dual phase steel containing Nb,Ti can be processed in two ways:in two sections cooling, finish temperature is 830-850℃,start water-cooling 700-730℃,coiling temperature below 250℃,in three sections cooling, finish temperature is 850℃,start water-cooling 660～680℃,coiling temperature 200-250℃, coiling temperature is the most important among them.