| Niobium possessing both grain refinement and precipitation strengthening function is a typical microalloying element. Strength and toughness of the low carbon steels addition of niobium can remarkably be improved. Therefore, research, development and application work on microalloyed steels containing niobium has been extensively carried out in the world. However, since the limitation of solid solubility of niobium in austenite, precipitation strengthening potential of niobium has not been commendably developed. Being a near neighbor element of niobium, molybdenum has some similar properties with niobium. By complex addition of molybdenum and niobium into steel, the precipitation strengthening effect of niobium can be increased. Meanwhile, acicular ferrite can be obtained by molybdenum. In a word, properties of low carbon steel can be further improved by complex microalloying of molybdenum and niobium and it has a broad application scope.In this thesis, three kinds of low carbon steels, which containing molybdenum and niobium weight percent as 0.021%Nb-0.24%Mo, 0.081%Nb-0.14%Mo and 0.17%Nb-0.12%Mo, respectively, were designed and adopted to investigate microalloying carbonitride precipitation and the effect of molybdenum and niobium on microstructure and properties. Thermal simulation experiments were used for investigation of isothermal non-deformed and deformed austenite transformation and microalloying carbonitride precipitation. Furthermore, the rolling experiments in laboratory were evaluated to study complex precipitation of molybdenum and niobium and the effect of complex precipitation of molybdenum and niobium on microstructure, mechanics properties and strengthen mechanism under the condition of controlled rolling and controlled cooling. Meanwhile, Nb-Mo steel was compared with a purified steel containing of Nb and Ti. The solubility formulae of molybdenum and carbide molybdenum with face center cubic structure in iron matrix have been obtained from the thermodynamic data of molybdenum and carbide molybdenum. The model of complex precipitation of molybdenum and niobium are also presented. Carbonitride niobium precipitation in austenite and the effect of molybdenum on it and carbonitride niobium and molybdenum precipitation in ferrite were analyzed and discussed from the theory calculation.The result of the effect of hot deformation on isothermal austenite to ferrite transformation kinetics in the low carbon Nb-Mo steels revealed was that the ferrite transformation is accelerated and ferrite grains are refined under hot deformation because incubation period of the transformation can be shortened by hot deformation, and acicular ferrite can be obtained in 0.021Nb-0.24Mo steel after isothermal holding at 650℃. The results of optical microstructure observation and quantitative metallography analysis showed that the kinetic of isothermal austenite to ferrite transformation in lower niobium steel with and without deformation is a stage mechanism with the linear relationship between holding time logarithm and ferrite volume fraction logarithm according to Avrami equation, while two stage mechanism in high niobium steel micrographs, that is, n value in the initial stage is lower, and in the second stage, n value is higher. The transformation was delayed due to carbionitride niobium precipitation pinning and the solute niobium drag.The result of the rolling experiment in laboratory showed the matrix microstructure in Nb-Mo steels are composed of polygonal ferrite and acicular ferrite. Meanwhile, the strength is increased with increase niobium content in the steels. Under the process condition of rolling deformation 50% at finishing temperature and coiling at 650℃,niobium precipitation ratio is 90% and yield strength grade of 600and 683 MPa for 0.081Nb-0.14Mo steel and 0.17Nb-0.12Mo steel , respectively, were grained. Strengthening mechanism analysis revealed grain refinement strengthening in lower niobium steel is primary strength mechanism, while refinement strengthening and precipitation hardening in higher niobium steel. Meanwhile, the analysis for strengthening mechanisms in three tested steels confirmed it is very little deferent in refine strengthening, but the contribution of precipitation hardening is obviously increased with increasing niobium content in the steels. This is due to the amount of fine carbionitride niobium precipitate are increased with increase niobium content in the experimental steels.Using physicochemical phase analysis, XRD, TEM and EDS, the distribution, morphology, composition, crystal structure and precipitates particle size were observed and identified in these steels samples isothermally held various time at different temperature after deformation at 875℃and controlled rolling and controlled cooling. The results revealed that isothermal temperature is higher, the size of precipitate particles is larger .The finer particles contained molybdenum show that molybdenum can precipitate with niobium at low temperature. The mass percent of the fine particles (<10nm in size) in 0.081Nb-0.14Mo steel and 0.17Nb-0.12Mo steel was 58.4% and 66.1%, which precipitation hardening increment is 180MPa and 261 MPa, respectively. The precipitate phase composition analysis has shown that the coarse particles do not contain molybdenum, while fine particles contain molybdenum, and meanwhile the atom ratio of Mo/Nb is about 0.41 and 0.22 for 0.081Nb-0.14Mo steel and 0.17Nb-0.12Mo steel, respectively. Therefore, the finer and denser microalloying carbonitride containing niobium and molybdenum caused a major precipitate-hardening effect in Nb-Mo steel. The theory analysis confirmed that the addition of molybdenum would increase the solubility of carbonitride mobium in austenite, which resulted in finer and denser carbonitride precipitated in ferrite. Furthermore, the observed presence of molybdenum in the precipitates can increase their strengthening effectiveness of niobium from refining size of precipitates and increasing coherency strains and increasing volume fraction of precipitates. It has been found that complex microalloying of molybdenum and niobium is much better to develop precipitation strengthen potential of niobium than that of titanium and niobium.The model of complex precipitation of molybdenum and niobium, where the form of carbonitride are taken for mutual solube NbC,NbN and MoC with face center cubic structure were also presented. According to the model, compositional assessment of complex precipitate of molybdenum and niobium are be close to the experimental value.Calculated and analyzed result of effect of molybdenum on carbonitride niobium and solubility of molybdenum and carbide molybdenum in austenite showed that molybdenum can not be precipitated in austenite on the whole, but molybdenum can decrease activity degree of element carbon, nitrogen and niobium and enhanced the solubility product of carbonitride niobium in austenite, and furthermore delay carbonitride niobium precipitation in austenite. The result of complex precipitation of Mo and Nb in ferrite revealed that carbonitride molybdenum and niobium particle of 5nm in size can be obtained because molybdenum delay niobium precipitation, critical nucleus size of carbonitride niobium and molybdenum is less than 1nm and particle size is 2-4nm after precipitation transformation finish, and coursing rate is lower.In this work, the origin of the high strength low alloy steel with 0.02-0.03%C,0.05-0.1%Nb,0.1%Mo,0.002-0.003%N was obtained, in which a lot of nano microalling carbonitride precipitate particles were distributed on the ferrite matrix. In the optimum rolling parameter, the finishing temperature was around 875℃, distortion amount in the finishing temperature was 50%, and coiling temperature was 650-700℃.Our result demonstrated that, microalling carbonitride in steels containing molybdenum and niobium can be stabilized in the range of nano scale and can exert a better precipitation strengthen effect of microalloying element niobium.
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