Banded Microstructure Formation and Its Effect on the Performance of 06Ni9 Steel

Focusing on the banded microstructure formed during the production of 06Ni9 steels for cryo-LNG,this paper examines its formation,distribution of alloying elements,structure,hardness,and low-temperature property.The results show that the banded microstructure formed after hot-rolling and cooling of the steel binct in which the element segregation occurred during solidification.The phase change during heat treatment also can cause the formation of the banded microstructure of 06Ni9 steel.The white bands are mainly composed of ferrite and reversed austenite,and the black bands are mainly composed of reversed austenite and a certain amount of ferrite.Element segregation and formation of more carbide caused some black regions to appear.Grain refinement of 06Ni9steel is beneficial to the formation of reversed austenite,the redistribution of alloying elements,improving the stability of austenite and the low-temperature toughness of steel.This steel easily undergoes nickel segregation;thus,undergoing a secondary quenching and tempering process is recommended.The refinement of martensite quenching above A c3,the martensite that is rich in nickel and carbon,residual austenite and a few little of ferrite after secondary quenching lower than A c3 are beneficial to the formation of high stability austenite.Thus,this can meet the strength and toughness requirement of the low temperature 06Ni9 steel.

Influence of band microstructure on carbide precipitation behavior and toughness of 1 GPa-grade ultra-heavy gauge low-alloy steel

This study investigated the influence of band microstructure induced by centerline segregation on carbide precipitation behavior and toughness in an 80 mm-thick 1 GPa low-carbon low-alloy steel plate.The quarter-thickness(1/4t)and half-thickness(1/2t)regions of the plate exhibited similar ductility and toughness after quenching.After tempering,the 1/4t region exhibited~50%and~25%enhancements in both the total elongation and low-temperature toughness at-40°C,respectively,without a decrease in yield strength,whereas the toughness of the 1/2t region decreased by~46%.After quenching,both the 1/4t and 1/2t regions exhibited lower bainite and lath martensite concentrations,but only the 1/2t region exhibited microstructure bands.Moreover,the tempered 1/4t region featured uniformly dispersed short rod-like M_(23)C_(6)carbides,and spherical MC precipitates with diameters of~20–100 nm and<20 nm,respectively.The uniformly dispersed nanosized M_(23)C_(6)carbides and MC precipitates contributed to the balance of high strength and high toughness.The band microstructure of the tempered 1/2t region featured a high density of large needle-like M3C carbides.The length and width of the large M3C carbides were~200–500 nm and~20–50 nm,respectively.Fractography analysis revealed that the high density of large carbides led to delamination cleavage fracture,which significantly deteriorated toughness.

MODELING OF 'BANDING' MICROSTRUCTURE FORMATION IN CENTRIFUGALLY SOLIDIFIED Ti-6Al-4V ALLOY

Numerical investigations of the ＇banding＇ microstructure formation during solidification of Ti-6Al-4 V alloy in the centrifugal casting are conducted using a multi-scale model, which combines the finite difference method （FDM） at the macroscale with a cellular automaton （CA） model at the microscale. The macro model is used to simulate the fluid flow and heat transfer throughout the casting. The micro model is used to predict the nucleation and growth of microstructures. With the proposed model, numerical simulations are performed to study the influences of the nucleation density, mould rotation speed, and casting size upon the ＇banding＇ microstructure formation. It is noted that changing the nucleation density has a minor effect on the microstructure formation. The rotation speed promotes the formation of ＇banding＇ microstructure, which is more noticeable for larger size castings. The ＇major mechanism responsible for this ＇banding＇ phenomenon is the spatial variation in cooling rates created by centrifugal force.

THE MICROSTRUCTURAL BANDING IN THE CENTER OF HOT ROLLLNG STRIP

The microstructural banding in steels is often found in hot rolling strips, which plays a very important role in mechanical properties. Much work has been done to investigate how the microstructural banding is formed during hot rolling. In the present study, the microstructure of hot rolling strips was examined in term of optical microscopy and transmission electron microscopy. Electron probe microanalysis was also used to decide the distribution of microchemical bands, by this means, the phases in these strips were found to be ferrite and pearlite. The average distance between the carbon lamellas in pearlite is about 0.06-0.1μm. It is also shown that microstructural banding in hot rolled carbon steel was closely related to the segregation of manganese and silicon into those bands. Based on the transformation kinetic, the simulated results pointed out that the thermodynamic stability of austenite would increase with the increasing of Mn, which led to a decrease of ferrite growth rate. The effect of Mn on the decomposition of austenite is attributed to segregation of Mn atoms along the ferrite/austenite phase boundary which causes a strong solute drag effect. The addition of Mn to steel decreases the activity of austenite, thereby it is beneficial to the formation of non-equilibrium phase, such as degenerate pearlite. The formation of banded structure on the hot rolled process was discussed.

Effects of TMCP Parameters on Microstructure and Mechanical Properties of Hot Rolled Economical Dual Phase Steel in CSP

Effects of chemical compositions, finish rolling temperature, isothermal temperature on runout table and coiling temperature on microstructure and mechanical properties of economical dual phase steel produced on CSP line were investigated. Experimental results showed that martensite volume fraction could be enhanced and banding mi- crostructure could be reduced by controlling Mn, Si contents and applying proper finish rolling temperature. Opti- mized processing-parameters were obtained for DP580 production on CSP line of Wuhan Iron and Steel （group） Co （WISCO） in China. Optimal mierostructure and mechanical properties could be achieved when the strip was finished rolling at the range of 790 to 830 ~C, isothermally holding at 680 to 740 ~C and coiling below 250 ~C.