Morphological Changes And Mechanical Properties Of Intragranular Ferrite In Extra-thick Plates

With the development of industries such as ships and offshore platforms,the application proportion of extra-thick steel plates has gradually increased.However,there are still some problems in the production process of extra-thick steel plates,such as the large difference between the microstructure and properties of the core and the surface layer and 1/4,and the excessive thickness of the continuous casting slab leads to an increase in defects(segregation,inclusions),etc.These problems cannot be completely solved by the rolling process alone.Among them,inclusions will increase the possibility of unqualified flaw detection or mechanical properties of extra-thick steel plates,but they can also promote ferrite nucleation to improve the structure and properties of the steel plate,and the promotion of ferrite nucleation is also affected by the cooling rate of the steel plate.For this reason,this paper takes low-alloy highstrength steel as the research object,and uses methods such as theoretical research and experimental detection to study the defects in continuous casting billets.The effects of cooling rate,deformation and tempering temperature of Q420 extra-thick plate on the morphology and properties of intragranular ferrite were also analyzed.The main research contents and conclusions of the paper are as follows:(1)Taking S355 J and Q420 continuous casting billets as the research objects,the macroscopic morphology of the continuous casting billets was observed,and the types and sizes of the inclusions in the billets were analyzed.The results show that the equiaxed crystallinity rate of the S355 J continuous casting billet is 43.4%,and that of the Q420 continuous casting billet is 35.1%;the inclusions in the two continuous casting billets are mainly sulfides,nitrides and oxides,and the proportion of inclusion size ≤10μm in S355 J is 96.41%,and the proportion of inclusion size ≤10μm in Q420 is 95.62%;(2)Taking Q420 continuous casting billet as the research object,the effects of cooling rate and deformation on the morphology of intragranular ferrite were simulated by using the Gleeble thermal simulation machine.the result shows,When not deformed,when the cooling rate is 1°C/s～5°C/s,the ferrite shape induced by inclusions is needle-like;when the true strain is 0.5,the cooling rate is 2°C/s～15°C/s,the shape of ferrite induced by inclusions is needle-like;(3)In laboratory rolling,the effect of cooling rate on the morphology of intragranular ferrite in steel was investigated.the result shows,When the cooling rate increased from 0.08℃/s to 16℃/s,the ferrite induced by inclusions transformed from equiaxed ferrite to acicular ferrite;the average grain size of ferrite during air cooling was the smallest,is 10.5μm;(4)In order to further improve the mechanical properties of the experimental steel,the quenched steel is tempered.the result shows,The average size of the precipitates in the quenched steel is 45.78 nm.After tempering at 600 °C and holding for 50 min,the average size of the precipitates in the steel is 151.72 nm.The yield strength and tensile strength of the steel are 517 MPa and 732 MPa,and the elongation and impact energy are 16.6%.and 115 J,the comprehensive mechanical properties are the best at this time.