Study On The Mechanism Of Surface Cracks Of Cr-Nb Microalloyed Steel

Niobium microalloyed steel has good comprehensive properties and is widely used in construction,oil,shipping and other fields.However,niobium microalloyed steel is prone to corner transverse cracks during conventional continuous casting and shaped billet continuous casting,which seriously affects Hot ductility of cast slab and comprehensive mechanical properties of steel.In-depth study of crack formation mechanism,adjustment of molten steel composition,and optimization of industrial parameters are necessary to provide theoretical support for improving the comprehensive performance of cast slabs in steel production.This paper focuses on the crack generation mechanism of the chromium-niobium microalloyed steel casting billet.With the help of Thermo-Calc and other thermodynamic software,relevant theoretical calculations and corresponding laboratory experiments are carried out to obtain the effect of chromium content on the transformation process and precipitation of austenite to ferrite.The influence of the distribution law of niobium is analyzed by electron backscattering technology,electron probe,high temperature confocal and transmission electron microscopy technology to analyze the influence of chromium content on the third brittle interval,macrostructure characteristics,and microstructure characteristics of niobium microalloyed steel.The effect of cooling rate and strain rate on the high-temperature phase transformation characteristics of chromium-niobium microalloyed steel was also analyzed,and the optimal addition range of chromium and the field process conditions of chromium-niobium microalloyed steel element/austenite were finally determined.The main conclusions of the high temperature phase transformation crack formation mechanism are as follows.(1)The study results of the thermodynamic calculations and the calculation results of the high temperature tensile test of niobium microalloyed steel with different chromium content are as follows.The hot ductility of 0.12%Cr steel is obviously better than that of 0Cr steel and 0.45%steel.With the increase of Cr content,the stability of undercooled austenite increases,the incubation period of grain boundary ferrite precipitation increases but the precipitation temperature of ferrite reduces.With the increase of Cr content,the CCT diagrams shifts to the right,which improves the hardenability of niobium microalloyed steel.The addition of 0.12wt%Cr in Nb microalloyed steel has the smallest low temperature brittleness range of 740?-780?.At the same time,the reduction of area at 750? increased from 33%to 38%.This is because with the increase of Cr content,the proportion of grain boundary ferrite decreases.The thickness of grain boundary ferrite of 1#,2#and 3#steel is 49.1 ?m,12.9?m and 8.7?m,respectively.(2)Transmission electron microscopy results confirmed that the addition of Cr also affects the distribution of precipitates in niobium microalloyed steel.As the Cr content increases from 0%to 0.45%,the number of NbC or Nb(C,N)precipitates decreases,but the average size increases.In 0 Cr steel,the diffusion of C element is not affected,and it is easier to diffuse from newly formed ferrite to the forefront austenite/ferrite phase boundary.The concentration of C element at the phase boundary increases.Furthermore,the driving force for niobium carbonitride precipitation at the phase boundary is increased,and a large amount of niobium carbonitride precipitates are formed near the phase boundary.However,when the temperature drops to 750?,the diffusion rate of C,N,and Nb elements is lower than that at high temperatures,and the growth of niobium carbonitride precipitates is inhibited.The number of precipitates increases sharply,and the average size of precipitates is smaller.The 0.12%and 0.45%steel contains chromium element,which inhibits the diffusion process of C element and weakens the transformation process of austenite to ferrite.The 0.12%Cr and 0.45%Cr steels provide less driving force for the precipitation of niobium carbonitride,which makes the number of precipitates smaller than that of 1#steel.(3)Analysis of the effect of cooling rate on high-temperature phase transformation and precipitation distribution of 0.12%Cr Niobium microalloyed steel from the perspectives of thermodynamics and kinetics.When the cooling rate is 1?/s,the initial precipitation temperature of ferrite is higher,and preferentially precipitates in the form of flakes at the position of the austenite trigeminal grain boundary.The temperature span of the phase transformation process is large,and it is easy to form MnS and Nb(C,N).)And aggregated in the grain boundary position in a chain shape;when the cooling rate is 3?/s,the initial precipitation temperature of ferrite is lower at 665?,and the temperature span of the phase transformation process is small.Mainly Nb(C,N)precipitates,and are not easy to accumulate at the grain boundary position,the size of the precipitate is 35.3 nm;at the cooling rate of 1?/s,while applying a strain rate of 0.001 s-1 to the sample,the grain boundary ferrite The thickness and volume fraction are the largest,47.4 ?m and 78.2%,respectively.The average size of precipitates is 30 nm,and the number of precipitates is relatively large.In addition,it is found that under the current experimental conditions,the cooling rate has a greater influence on the transformation process of austenite to ferrite at high temperature than the strain rate.(4)The mechanism of crack formation of 0.12%Cr niobium microalloyed steel at different tensile temperatures and different strain rates was studied through high-temperature in-situ tensile experiments.The results show.The size of Nb(C,N)precipitates is in the range of 10?20 nm,with an average size of 25.3 nm;while the size of complex precipitates of MnS and Nb(C,N)is relatively large,with an average size of 108.9 nm.The transformation process of austenite to ferrite provides 3 favorable conditions for the formation of cracks.(5)In the transformation process of austenite into ferrite,the growth rate of ferrite first increases and then decreases.As the strain rate increases,the growth rate of grain boundary ferrite also increases.When the strain rate is 0.0003 s-1,the deformed ferrite has enough time to recover or recrystallize to form ferrite,which makes the grain boundary ferrite softer and leads to intergranular fracture.When the strain rate is 0.003 s-1,due to the work hardening of ferrite,it should be uniformly distributed on the austenite and ferrite,thus avoiding premature failure and improving the hot ductility of the steel.As the strain rate increases,the dimple size gradually decreases,the proportion of ferrite increased from 11%to 27%,and the ductility of steel increases.