Study On Crystallographic Characteristics Of High Strength Low Alloy Steel And Its Composition-process-Performance Relationship

The construction of offshore platforms,bridges,buildings,machinery and other projects is becoming more and more demanding for high strength low alloy steel,and the requirements for strength and toughness are getting higher and higher.Phase transformation is the key factor determining the performance of high strength low alloy steel.In order to meet the demand,this paper systematically studies the phase transformation behavior of high strength low alloy steel.The effects of composition and process on microstructure and properties were studied by means of crystallographic characterization method-quantification and visualization of microstructure.The mechanism of ??? phase transformation of steel and its effect on mechanical properties were studied from the perspective of crystallography and thermodynamics.Based on these results,an alloy design idea that satisfies the toughening and weldability of high strength low alloy steel was proposed.Firstly,the influence of process factors on the microstructure was clarified.It was found that the twin-related variant pair-V1/V2 is the main source of high angle grain boundaries(HAGBs),which contributes to the inhibition of crack propagation and the improvement of low temperature impact toughness.The reason why the performance of the core of the industrial high strength ultra-heavy plate steel is lower than that of the surface was that granular bainite structure is formed,mainly forming V1/V4 and V1/V8,etc.variant pairs with low misorientation angle.The surface formed lath bainite with a higher density of V1/V2 variant pairs and HAGBs.The method of refining prior austenite grains by lowering the austenitizing temperature also achieves an increase in strength and toughness.Studies have also shown that after the prior austenite grains are refined from 15 to 6.4 ?m,the number of packet units in the prior austenite grains is reduced,but more side-by-side block units are formed.The improvement of toughness is not only related to the grain refinement of the prior austenite grain,but also from the increase of the density of HAGBs.Through the study of continuous cooling transformation products of boron-bearing steel and boron-free steel,the relationship between hardness and density of high angle grain boundary is clarified.When martensite and bainite transformation occurs,the higher the density of HAGBs,the lower the hardness;and when only the bainite transformation occurs,the higher the density of HAGBs,the higher the hardness.Compared to B-free steel,the B-bearing steel reduces the phase transformation starting temperature at a low cooling rate,increases the phase transfomation driving force and produce higher density of HAGBs in the phase transformation product,thereby improving the hardenability.From the perspective of thermodynamics,the variant selection mechanism of martensite,lath bainite and granular bainite transformation was clarified.The reason for the martensite transformation to form a high density of V1/V4 variant pair is that the phase tansformation driving force is too large to form enough V1/V2 variant pair to adjust the phase transformation strain.It is found that a martensite and bainite mixed micro structure is formed between the Ms and Mf temperature;the bainite transformation near the Ms temperature forms a relatively high V1/V2 variant pair;the mixed microstructure of lath bainite and granular bainite forms above Ms temperature and fully granular bainite forms at a further higher temperature.It is concluded that the two favorable conditions for forming V1/V2 are a sufficiently high phase transformation driving force and carbon diffusion.By adopting the thermodynamic basis of the variant selection,the idea and method of alloy design for high strength ultra-heavy plate steel are proposed.By reducing the carbon content and optimizing the alloying elements,not only the strength of the martensite structure but also the excellent toughness can be achieved.Through adding alloy elements,the martensite transformation driving force was reduced to promote the generation of more V1/V2 variant pairs.This method helps to improve the weldability of the steel and the performance of the heat affected zone of the weld.