Microstructure and Property of Coarse Grain HAZ X80 Pipeline Steel

The coarse grain HAZ microstructure and property of X80 pipeline steel with different carbon content was investigated. The weld thermal simulation test was carried out on Gleeble 1500 thermal mechanical test machine. The Charpy tests were completed at --20 ℃ for evaluating the toughness of coarse grain heat affected zone （CGHAZ）. The microstructure was examined by optical microscope （OM） and transmission electron microscopy （TEM）, and the austenite constituent was quantified by X-ray diffraction. The results showed that the ultra-low carbon can improve the toughness of CGHAZ by suppressing the formation of carbide, decreasing the martensite austenite （M-A） constituent and increasing the residual austenite in the M A.

LEDGE STRUCTURE ON BROAD FACES OF FERRITE PLATES IN LOWER BAINITE

The ferrite/austenite interfacial structure in lower bainite has been studied in a hypereutectoid steel by means of electron microscopy.It is found that the mobile growth ledges and superledges and their three-dimensional morphologies exist on the broad faces of ferrite plates.Structural ledges were also observed on the broad faces of lower bainite.Based on TEM observations,the viewpoint that the growth ledges can also evolve from structural ledges is proposed.These observations provide evidences to the ledge growth mechanism of lower bainite.

The controlling factors on metallurgical properties of advanced high strength steel sheets

Recent progress in production process for advanced high strength steel sheets（AHSS） is described focussing on Bainitic ferrite steel,transformation induced plasticity（TRIP） steel and precipitation hardened steel by NANO sized precipitates are reviewed. In higher tensile strength（TS） grade,elongation（El）,hole expansion ratio（λ） and their balance are eagerly requested to improve by customers.Bainitic ferrite steel which has shear deformed lath with various distribution of iron carbides and retained austenite should be distinguished to produce different balance of El/λ. Chemical composition and heat cycle are reviewed to produce various kinds of metallurgical phase and mechanical property.5%Mn -2%Si TRIP steel is introduced in which a plenty of retained austenite at about 30%and larger absorbed energy（AE） more than 3 000 kgf/mm;TS·%El（gauge length:50 mm） can be obtained. That steel also showed excellent upper shelf energy in Charpy test of 140 J/cm;,as well as excellent property of 20%El and 1 400 MPa TS after forging. A new type of precipitation hardened steel having NANO sized（Ti,Mo） C in ferrite matrix is introduced.It satisfies the both of excellent El andλat the same time. Bending property and holes expansion property obey the same kind of deforming criteria which depends on the forming capability of work hardened parts.Homogeneity of structure is their controlling factor and a new parameterαwhich is obtained by gauge length dependency of total elongation is proposed.

Factors Affecting the Mechanical Properties of Ultra-high-strength Bainitic Steel Containing W and 0.33 mass% C

A novel ultra-high-strength bainitic steel was designed.The analysis of its mechanical properties by quasistatic testing showed that upper bainitic steel exhibited an ultimate tensile strength of 2 260 MPa（engineering stress）and an ultimate compressive strength of more than 2 700MPa（true stress）.The ultra-high strength of upper bainitic steel was mainly attributed to untempered martensite and upper bainite with a feather-like microstructure.Moreover,lower bainitic steel demonstrated an ultimate tensile strength of 1 922 MPa（engineering stress）and an ultimate compressive strength of 2 500MPa（true stress）.The ultra-high strength of lower bainitic steel was primarily due to untempered martensite and lower bainite with an acicular microstructure.The untempered martensite in the two kinds of bainitic steels was produced in different ways.The dynamic test results showed that the ultimate compressive strengths of the two bainitic steels were maintained at 1 600MPa（true stress）under high strain rates（1 100and2 200s-1）at 600℃,because of the added tungsten,confirming the satisfactory hot hardness property of the steel.Furthermore,lower bainitic steel showed better comprehensive mechanical properties than upper bainitic steel.

An Investigation on Rotating Bending Fatigue Behavior of Nanostructured Low-Temperature Bainitic Steel

The aim of this work is to investigate the fatigue behavior of a new class of nanostructured, low-temperature bainitic steels, where rotating bending fatigue tests were conducted on samples isothermally transformed at 300, 250, and 200 ℃, after which, fracture surfaces were examined using scanning electron microscopy. Results showed that, fatigue limits of about 820, 945, and 1,005 MPa were achieved for the samples transformed at each transformation temperature, respectively. Moreover, according to the SEM micrographs, secondary crack initiation was observed in the high carbon retained austenite blocks for samples transformed at 300 ℃ and at the interface of blocky austenite-bainitic ferrite for samples transformed at 250 and 200 ℃.

Continuous Cooling Bainite Transformation Characteristics of a Low Carbon Microalloyed Steel under the Simulated Welding Thermal Cycle Process

Continuous cooling transformation of a low carbon microalloyed steel was investigated after it was subjected to the simulation welding thermal cycle process and the interrupted cooling test. Microstructure observation was performed by optical microscopy and transmission electron microscopy. On the basis of the dilatometric data and microstructure observation, the continuous cooling transformation （CCT） diagram was determined, which showed that the main microstructure changes from a mixture of lath martensite and bainitic ferrite to full granular bainite with the increase in the cooling time t8/5 from 10 to 600 s, accompanied with a decrease in the microhardness. The interrupted cooling test confirmed that the bainitic ferrite can form attached to grain boundaries at the beginning of transformation even if the final microstructure contains a mixture of granular bainite and bainitic ferrite.

Micromechanical Behavior and Failure Mechanism of F / B Multi-phase High Performance Steel

The deformation and micro-voids formation mechanisms in ferrite / bainite（ F / B） multi-phase steel with the volume fraction of bainite less than 50% were studied by numerical simulation and experimental observation. The results show that the micro-strain concentrates at the soft / hard phase（ F / B） interface in the multi-phase steel,which should be correlated with the mechanism of incoordinate deformation. During the necking of the steel,the micro-voids initially form around the F / B interface,which also form in ferrite and bainite with the severe strain. The micro-voids in bainite are more dense and finer than those in ferrite. The failure mechanism of bainite is the coalescence of micro-voids,and the failure mechanism of ferrite is the growth and tearing of micro-voids. Due to the different failure mechanisms of ferrite and bainite,a suitable part of soft phase would be beneficial to the capability of anti-failure of F / B multi-phase steel during the ductile fracture.