Influence Of Thermomechanical Treatment Processing On Microstructures And Properties Of Lath Martensitic Steel

The continuous cooling transformation（CCT）curve of low carbon micro-alloyed steel 20CrNi2Mo was drawn by dilatometer method in this paper.Then the microstructures and properties of the tested steel were studied after thermal-mechanical treatment which deformation temperature ranged from 900°C to1050°C and the strain rates varied from 0.001 s^-11 to1 s^-1.Finally the grain refinement mechanism of 20CrNi2Mo was explored after cyclic quenching followed by rolling with large plastic The main research contents and conclusions are as follows:（1）It can be seen that the areas of ferrite and pearlite were very small,while the area of bainite was larger in the CCT curve.The content of the pearlite decreased more rapidly than that of the ferrite when the cooling rate was 0.05⁰.7℃/s and the micro-hardness of the samples increase when the cooling rate was increased.But it hardly changed when the cooling rate was over 30℃/s and there only existed lath martensite at this cooling rate.With the increasing of cooling rate,the size of the lath martensite was gradually refined.（2）The constitutive equation of the maximum deformation resistance,strain compensation constitutive equation and hot working processing maps of 20CrNi2Mo steel were established at the deformation temperatures ranging from 900 to 1050°C and strain rates from 0.001 to1 s^-1.The correlation（R）and the average relative error（AARE）are 0.9921 and 3.0192%respectively between the magnitudes of the flow stress calculated by the proposed model and the experimental results.There were two peaks in the energy dissipation processing maps of the tested steel with the true strain at the range of 0.1 s^-1 to 0.5 s^-1.With the increasing of the true strain,peak I moves towards the higher temperature,and peak II moves towards the higher strain rate.However,the area of the instability zone decreased slightly before gradually increased with the increase of the strain,and the instability zone reached minimum value at the true strain 0.4.Therefore,the optimum hot working processes of the tested steel are as follows which the deformation temperature were 940⁹60℃with the strain rate0.001s^-1 or 1025¹050℃with 0.01⁰.06 s^-1.（3）The dynamic recrystallization of the tested steel was easy to happen during the process of compression.The critical dynamic recrystallization model was established after the inflection points were solved by fitting a third order polynomial to theθ-σcurves.With the increase of deformation beforeε_c,the marensite packet size and block width are increased and decreased,respectively while the prior austenite grain is almost invariant.Meanwhile,the number of the low angle boundaries increased with the opposite changes of the high angle boundaries.However,with the increase of deformation afterε_c,the sizes of prior austenite grain,packet and block were refined gradually,while the number of the low angle boundaries firstly decreases and then increases.The pole figure analysis showed that the preferential distribution of the block orientation of the 20CrNi2Mo steel appears during the deformation.（4）The grain structure and sub-microstructures Of the tested steel had been significantly refinement and their strength has been greatly improved after 5 cyclic quenching and subsequent large cold rolling deformation.The analysis on the anti-pole diagram and pole figure showed that cold rolling deformation caused preferential orientation of lath martensite.