Research On The Key Technology Of Structure Ultrafining Of High Grain Uniformity Microalloyed Steel Plate

Currently, the only way to improve the strength, ductility and impact toughness of steel is grain refinement. And the study on low carbon microalloyed steel is mainly focused on using deformation induced phase transformation to obtain ultrafine grain structure. However, a lot of researches show that the micro-alloyed steels obtained by the means of deformation induced phase transformation, especially the fine grain rate of vanadium micro-alloyed steels is not ideal, and there is a certain degree of mixed grains phenomenon, both are combined to cause the steel mechanical properties far from the expected goal, the specific performance is to obtain high strength in steel while other properties are low, especially for the low elongation. Which has become a key problem that restricts the effective application of low-carbon microalloyed ultrafine grain steel in mechanical engineering structure.Therefore, this paper mainly studies low carbon microalloyed steel, making full use of single pass and multi-pass thermal simulation experiment, and with the assistance of microscopic structure, transmission electron microscopy(TEM), scanning electron microscope and microstructure quantitative image analysis methods, this paper is focused on the important accompanying phase transformation(reverse transformation and meta-dynamic transformation) of deformation induced phase transformation, so as to acquire the effects of accompanying phase transformation on the microstructure and mechanical properties and the fine grain rate of steel, adjusting the microstructure and mechanical properties of test steel to achieve the expectation goals.The main research contents and innovative work of this paper are as follows:(1) In view of the existing status of microstructure grain size in ultrafine grain steel has great differences but can not quantitative description precisely and difficult in carrying out effective process control subsequently, this paper condensed out the new concept of grain size uniformity from a large number of steel grain image, and establish the calculating formula correspondingly, expecting to measure and calculate the grain size uniformity simply, rapidly and accurately. This method can realize the efficient measurement and accurate classification of grain size and uniformity of steel automatically, the measuring accuracy is as high as ±0.001μm.(2) Study on the reverse transformation phenomenon by the way of single pass and multi-pass thermal simulation test. The essence of reverse transformation is that ultrafine grained ferrite transforms to ultrafine grained austenite in the isothermal process of multi-pass rolling interval, and then transforms to the finer ferrite during the following rolling process. Results show that the increase of deformation and holding time is beneficial to improve the fine grain rate(up to 8.91%) of single pass test steel. The increase of the multi-pass test interval time is also beneficial to the improvement of the fine grain rate, and the grain size uniformity can be as high as 1.32.(3) The influence of meta-dynamic transformation and internal precipitate phase on fine grain rate of the steel was investigated through the single pass thermosimulation test. The essence of meta-dynamic transformation is the ferrite of main phase nucleation continue to grow up to ultrafine grained ferrites during the isothermal process after the main phase. Results show that, the fine grain rate of metadynamic transformation increases with the increase of the holding time after deformation, which is mainly due to the formation of a large number of internal ferrite by the internal precipitate phase.The results show that the highest fine grained rate of meta-dynamic transformation in single-pass deformation can reach 9.41%, and the grain size uniformity can be as high as 1.38.Finally, based on the above research results, by means of a laboratory reversible mill, has prepared the microalloy ultrafine grain steel plate successfully which the grain size and its uniformity were up to 1.21μm,1.26, the fine grain rate of ultrafine grain ferrite is up to 96.26%, tensile strength reaches up to 753 MPa.