Development of grain boundary allotriomorphic ferrite/granular bainite duplex steel

A new hot-rolled low alloy high strength steel with grain boundaryallotriomorphic ferrite/granular bainite duplex microstructure has been developed through novelmicrostructure and alloying designs without any noble metal elements such as nickel and molybdenum.Its as-rolled microstructure and mechanical properties, fatigue crack propagation behavior comparedwith single granular bainitic steel as well as continuous cooling transformation, were investigatedin detail. The measured result of CCT (continuous cooling transformation) curve shows that suchduplex microstructure can be easily obtained within a wide air-cooling rate range. More importantly,this duplex microstructure has much better combination of toughness and strength than the singlegranular bainite microstructure. It is found that the grain boundary allotriomorphic ferrite in thisduplex microstructure can blunt the microcrack tip, cause fatigue crack propagation route branchingand curving, and thus it increases the resistance to fatigue crack propagation, improves steeltoughness. The mechanical properties of the above commercial duplex steel plates have achieved orexceeded 870 MPa ultimate tensile strength, 570 MPa yield strength, 18 percent elongation and 34 JCharpy V-notch impact energy at -40 deg C, showing good development potential.

DEFORMATION AND MICROFRACTURE OF GRANULAR BAINITE

Relationship among deformation history,fracture process and stress distribution of granular bainite has been investigated.The main process of fracture of granular bainite is the forma- tion.growth and coalescence of the microvoids.Even though the microcracks have formed at the earlier stage of deformation,they are not fateful for fracture because in the successive deformation stage the microcracks change their orientation toward the tensile axis.The strain hardening rate of granular bainite has a minimal value during the deformation process. Before and after the strain of the minimal value,the material shows different stress distribu- tion and microfracture mechanism.

FATIGUE BEHAVIOUR OF GRANULAR BAINITE STRUCTURE

The strain fatigue,impact fatigue and rotation beam fatigue behaviour of granular bainitic structure has been studied.The results show that the strain fatigue properties and the impact fatigue properties of granular bainite are superior to that of tempered martensite under the condition that the ultimate tensile strength is equal.The impact.fatigue life increases with in- creasing amount of granular bainite,because the M-A islands might retard the propagation of fatigue crack.The rotation beam fatigue properties of granular hainite are similar to that of tempered martensite.The relationship between.fatigue limit S_f,yield strength σ_y and frac- ture strength S_k may be expressed as S_f=4.651+0.1411(σ_y+S_k)

RELATIONSHIP BETWEEN M-A CONSTITUENT AND CLEAVAGE FRACTURE BEHAVIOUR OF GRANULAR BAINITIC WELD METAL

The cleavage fracture mechanism of granular bainitie weld metal in 15MnVN steel has been investigated.SEM observation of a special specimen for revealing simultaneously fracture sur- face and microstructure shows that the cleavage of the weld is initiated by M-A constituent. The 95th percentile equivalent diameter of M-A constituent controls the cleavage fracture stress of the weld metal.

Effect of Cerium on Structure and Properties of Granular Bainitic Steel

The alloying effect of cerium in low carbon granular bainitic steel has been studied. Cerium increases the incubation time of granular bainitic transformation, exerts an influence on the morphology of nucleation and growth of bainitic transformation, and refines the structure of normalized granular bainite.

Effect of La_2O_3 on granular bainite microstructure and wear resistance of hardfacing layer metal

The purpose of this work was to investigate the effect of La2O3 on the granular bainite microstructure and wear resistance of hardfacing layer metal. The hardfacing layer metals with different contents of La2O3 were prepared. The microstructures of the hardfacing layer metals were observed by field emission scanning electron microscopy （FESEM） and transmission electron micros-copy （TEM）. The hardness and wear resistance of the hardfacing layer metals were measured respectively. The results indicated that with the increasing content of La2O3, the amount of granular bainite increased, while that of martensite decreased and that of retained austenite did not change obviously. When the content of La2O3 was 2.55 wt.%, the volume fraction of the granular bainite in the hardfacing layer metal was 73.2%. Meanwhile, the wear resistance of the hardfacing layer metal was the largest, which was 12100 min/g. The mismatch between the face （100） of LaAlO3 and the face （100） ofδ-Fe was 7.1%. Therefore, LaAlO3 could act as moder-ate effective heterogeneous nuclei ofδ-Fe and the granular bainite could be refined.

Microstructure and mechanical properties of F_(GBA)/B_G air cooling bainitic steels containing niobium

The effect of different Nb additions on the mechanical properties and microstrucmre evolution of grain boundary allotriomorphic ferrite （FGBA） / granular bainite （BG） air cooling bainitic steels was investigated. The results indicate that the tensile strength and yield strength increase by 157 and 97 MPa, respectively with the addition of 0.02wt% Nb. The steel acquires superior strength and toughness with the addition of 0.06wt% Nb. The results of scanning electron microscopy and transmission electron microscopy reveal that the addition of Nb not only refines the size of granular bainite but also increases the volume fraction of granular bainite in FGBA/BG steels. The refinement effect of granular bainite is improved with the increase of Nb content.

Effects of continuous cooling rate on morphology of granular bainite in pipeline steels

The morphology and characteristics of granular bainite (GB) in pipeline steels at different continuous cooling rates were investigated by scanning electron microscopy, transmission electron microscopy and electron back-scattered diffraction (EBSD). The results show that the morphology of ferrite matrix in GB turned from the lath sheaf structure into the nearly equiaxed large grain with the cooling rate decreasing from high (60℃/s) to low (5-10℃/s). At the medium cooling rate (20-40℃/s), GB consisted of the irregular ferrite matrix, the granular martensite/austenite (M/A) constituents and abundant substructures inside. The formation of the irregular ferrite and substructure was attributed to the high-temperature recovery which occurred at relatively high-temperature stage before phase transformation. The granular morphology of M/A constitu-ents was formed from the carbon-rich triple junctions which were produced by the multidirectional substructure interfaces converged with each other. Particularly, some martensite in M/A constituents was misoriented from the adjacent ferrite by very small misorientation angle, which could be characterized by the mean band contrast function of EBSD qualitatively or semiquantitatively.

Non-Steady Equilibrium during Pro-Eutectoid Ferrite Formation in Fe-0.2C Alloy

As rolled microstructure and mechanical properties of a new kind of grain boundary allotriomorphic ferrite/granular bainite duplex steel plate and its crack propagation behavior were investigated in comparison with simple granular bainitic steel plate. These new duplex plate steels possess better combination of strength and toughness than granular bainite plate steels under the conditions of conventional rolling and air cooling. The observation of fatigue crack propagation behaviors showed that the existence of proper grain boundary allotriomorphic ferrite increases the compatible deformation ability of duplex microstructure and leads to the formation of crack branching and curving route, and it has an evidently blunting effect on microcrack tip and results in higher impact toughness. In addition, the new duplex steel plate also has good weldability as hot rolled high strength low alloy structural steel.

Mechanism of Stress Relief Cracking in a Granular Bainitic Steel

Coarse-grained heat-affected zone（CGHAZ） of a low alloyed,granular bainitic steel T24 was simulated in a Gleeble apparatus.The stress relief of the CGHAZ was analyzed by annealing the samples.The morphology and behavior of the microstructure near the grain boundaries during stress relief were investigated by means of focused ion beam,in situ tensile testing,transmission electron microscopy,scanning electron microscopy and electron back-scatter diffraction.It was observed that there were large martensite/austenite islands distributed along the grain boundaries of CGHAZ.During stress relief at elevated temperature,the retained austenite at the grain boundaries decomposed into M3C carbides and a ferrite forming softening zone.Together with the stress relief,piled up of dislocations occurred within the ferrite in the area adjacent to the ferrite/M3C interface,which resulted in high level of stress accumulation and caused crack initiation along the grain boundaries.These results indicate that the stress relief induced the grain boundary crack is controlled by other mechanisms rather than the creep-like grain boundary sliding.

Effects of rare earth on microstructure and impact toughness of low alloy Cr-Mo-V steels for hydrogenation reactor vessels

The effects of rare earth(RE) on the microstructure and impact toughness of low alloy Cr-Mo-V bainitic steels have been investigated where the steels have RE content of 0 to 0.048 wt.%. The results indicate that the normalized microstructures of the steels are typical granular bainite(GB) composed primarily of bainitic ferrite and martensite and/or austenite(M-A) constituents. The M-A constituents are transformed into ferrite and carbides and/or agglomerated carbides after tempering at 700℃ for 4 h. The addition of RE decreases the onset temperature of bainitic transformation and results in the formation of finer bainitic ferrite, and reduces the amount of carbon-rich M-A constituents. For the normalized and tempered samples, the ductile-to-brittle transition temperature(DBTT) decreases with increasing RE content to a critical value of 0.012 wt.%. Lower DBTT and higher upper shelf energy are attributed to the decreased effective grain size and lower amount of coarse agglomerated carbides from the decomposition of massive M-A constituents. However, the addition of RE in excess of 0.012 wt.% leads to a substantial increase in the volume fraction of large-sized inclusions, which are extremely detrimental to the impact toughness.

CCT Curves of Low-Carbon Mn-Si Steels and Development of Water-Cooled Bainitic Steels

CCT curves of Mn-Si steels with different manganese contents or carbon contents were determined. The results show that the transformation range of bainite can be separated from that of ferrite when the manganese content approaches a certain content, and the incubation period of ferrite increases more significantly than that of bainite transformation with the increase of carbon content in Mn-Si steels. Furthermore, water-cooled bainitic steels without adding expensive alloying element were developed. Granular bainite was obtained when a bar with diameter of 300 mm was cooled by water, and a mixed microstructure of granular bainite and martensite was obtained in watercooled plate with thickness of 40 mm. The developed water-cooled bainitic Steels containing no expensive alloying element showed a good combination of strength and toughness. The tensile strength, yield strength, and toughness （AKU at --20℃ ） of bar with diameter of 300 mm after water cooling were higher than 850 MPa, 620 MPa, and 65 J, respectively, and those of plate with thickness of 40mm after water cooling were higher than 1000 MPa, 800 MPa, and 50J, respectively.

Microstructures and Mechanical Properties of X100 Pipeline Steel Strip

Dynamic CCT （continuous cooling transformation） curves and effects of microstructures and M/A islands of X100 hot rolled strip on mechanical properties were studied by means of a thermal simulator, a scanning electron mi- croscope （SEM）, a transmission electron microscope （TEM）, a digital micro-hardness tester and Image-Pro Plus analysis software. The results show that high contents of C and Mo can make transformation lines of acicular ferrite and bain- ite shift rightward. High Mo tested strip has higher strength and micro-hardness and lower toughness than the other one because lath bainite appears instead of quasi-polygonal ferrite. Tested strip with granular bainite, lath bainite and M/A islands has better mechanical properties and in the microstructures lath bainite content is around 36.50% and M/A islands are fine and disperse in the matrix.

Microstructure and Mechanical Properties of Precipitation Strengthened Fire Resistant Steel Containing High Nb and Low Mo

Through the thermo-mechanical control process （TMCP）, a high Nb low Mo fire resistant steel with the yield strength （YS） of 521 MPa at room temperature （RT） and 360 MPa at elevated temperature （ET） of 600 ℃ was developed based on MX （M=Nb, V, Mo; X=C,N） precipitation strengthening. A series of tensile and con- stant load tests were conducted to study the mechanical properties at ET. The dynamic continuous cooling transfor- mation （CCT） as well as precipitation behavior of microalloy carbonitride was investigated by means of thermal sim- ulator and electron microscopy approaches. Results showed that the failure temperature of tested steel was deter- mined as 653 ℃, and the granular bainite was obtained when the cooling rate was higher than 10 ℃/s. In the rolled state, a certain amount of M/A islands was observed. During heating from RT to ET, M/A islands disappeared, and cementites and high dense compound precipitates （Nb, Mo, V）C with size of less than 10 nm precipitated in ferrite at ET （600 ℃）, which resulted in precipitation strengthening at ET.

Relationship between microstructure and corrosion behavior of high-grade pipeline steel in a low-temperature environment

The relationship between micro structure and corrosion behavior of high-grade pipeline steel under low-temperature conditions was comparatively investigated by the potentiodynamic polarization and electrochemical impedance spec-troscopy test combining with optical micrographs,scanning electron microscopy,transmission electron microscopy and electron back-scattered diffraction.The results showed that,compared with room and high temperature,low temperature could influence the corrosion behavior of high-grade pipeline steel,which means that the corrosion potential and current density decreased and the corrosion resistance increased significantly.Moreover,double layer structures of the interface became thicker and more compact.Under low-temperature environment,acicular ferrite had the martensite/austenite constituents with less amount and smaller size,a higher density of low angle grain boundaries and smaller effective grain sizes compared with granular bainite,which demonstrated higher corrosion resistance.

Mechanical properties of a microalloyed bainitic steel after hot forging and tempering

Mechanical properties of a newly developed microalloyed bainitic steel were investigated after the hot forging, air cooling and tempering process. The microstructure of the as forged bainitic steel mainly consists of granular bainite and -20 vol. % martensite. The fraction of retained austenite remains unchanged until tempering at 200 ℃, above which it decreases significantly. The increase of tempering temperature leads to decreases of both ultimate tensile strength and total elongation but decreases of both yield strength and reduction of area. The maximum and mini- mum values of impact toughness were observed after tempering at around 200 and 400 ℃, respectively. These effects are mainly attributed to the decomposition of martensite/austenite con stituents and the tempering effects in martensite. The tempering of the forged bainitic steel at around 200 ℃ results in an excellent combination of strength and toughness, which is comparable to that of the conventional quenched and-tempered 40Cr steel. Therefore, low-tempering treatment coupled with post-forging residual stress relieving is a feasible method to further improve the mechanical prooerties of the bainitic foging steel.

Research on Alloying Design and Process Control for X100 Hot-Rolled Strips

The influence of alloying design,controlled rolling technology and cooling system of X100 hot-rolled strips on microstructure and micro-hardness has been discussed in detail in this paper.The results show that optimal chemical composition range of X100 hot-rolled strips is obtained on the basis of X80 by increasing contents of C,Mn,Ni and Mo of X80.The granular bainite microstructure could be refined and the volume fraction and size of M-A islands could be controlled in a reasonable range by reasonably regulating cooling rate and coiling temperature of X100 pipeline steel,which ensure high strength and toughness of X100 pipeline steel.