Materials Science Forum Vols. 638-642

Abstract: The values of nitrogen solubility in the liquid Fe-N-Cr alloy and in liquid chromium were determined experimentally. The alloy of a chromium content of 8; 13; 18; 25; 35; and 69, respectively, and chromium of purity of 99.7 wt % were subjected to experiment. The original data on nitrogen solubility in liquid iron and the data on nitrogen solubility in liquid chromium (the authors' own studies) were converted in relation to the new standard state that is represented by pure nitrogen. The energy parameter, h Fe-Cr, was determined based on literature data. Using the coefficient values for the Fe-Cr alloy, calculated on the basis of our own investigation results, the values of the energy parameter h were determined. The determined energy parameter h in the form of a temperature function relationship will enable the determination of the numerical values of the for the Fe-Cr alloy of any arbitrary composition.

Abstract: Cast structure of ferric, austenitic and dual-phase high-nitrogen alloys and steels saturated with nitrogen at pressures of up to 3.2 MPa and solidified at cooling rates of 2 to 4•104 K/s has been investigated. Increasing the nitrogen concentration in the alloy and decreasing the cooling rate results in smaller nitrogen content in α and γ solutions and in larger content of crystallization nitrides.

Abstract: For the constant transmission of gas through a pipeline, steel weight decreases linearly with an increase in the strength of the linepipe irrespective of pipe size and internal pressure. Thus, high-strength large-diameter linepipe up to X120 has been developed and is now being applied to reduce pipe costs, transportation costs and construction costs. To meet the excellent weldability and low production costs required for the linepipe application of bainite produced through using Thermo-Mechanical Control Processing (TMCP) from low carbon chemistry is essential. Dual phase steel made by means of the introduction of ferrite in the bainite matrix mitigates the inferior properties of bainite. Herein, the production parameters affecting the microstructure and the properties are overviewed.

Abstract: The effect of four different niobium(From 0-0.1%) addition on the mechanical properties of allotriomorphic ferrite (FGBA)/ granular bainite (BG) air cooling bainitic steels has been investigated in this paper. The results show that (1) The 0.06%Nb steel acquired superior strength and toughness combination by applying 1250°C×60min solution treated, finish rolling at 850°C, and air cooling. The corresponding mechanical property of the thick plate (30mm) is: σb>1050MPa, σ0.2>700MPa, δ5>17%, Akv>90J. (2) The addition of niobium refine the grain size of FGBA, and promoted the transformation of bainite structure. With the increase of niobium content, the refinement of ferrite grain and bainitic cluster is improved. (3) More refined M-A island is acquired by the small addition of niobium. According to M-A Analysis tools and transversal methods, with the rise of niobium content, the volume fraction of M-A island increase from 21% to 35%, and the average size of M-A island decrease from 1.1μm to 0.7um. (4) It is suggested that 0.02-0.06% niobium can improve the mechanical properties of the steel obviously. However, excess addition of Nb (0.1%) deteriorates the impact toughness obviously. (5) Under the synthetic roles of the microstructure refinement and precipitation strengthen, 60-160MPa yield strength improvement has been acquired in the low carbon air cooling bainitic steel by the small addition of niobium. (6) This steel is with low production cost since the alloying element Mn is cheap.

Abstract: Grain refinement in lath martensite and bainite structures, which is important for strengthening and toughening, are discussed in various aspects. Strain accommodation plays important roles to determine final crystal sizes of bainitic ferrite (BF) and martensite. There is strong variant selection of BF by natures of the austenite grain boundary where it nucleates. For small undercooling, such variant selection leads to coarse bainite block and packet sizes. More BF variants are formed by increasing undercooling, which leads to nucleation of BF variants of less potency, and by increasing strength of materials, which results in more self-accommodation of transformation strain due to suppression of plastic accommodation. In lath martensite, there seems to be similar variant selection at austenite grain boundaries. However, packet/block sizes in lath martensite decreases with an increase in carbon content because of more extensive self-accommodation due to lower transformation temperatures than bainite.

Abstract: Electrochemical measurement, metallographic observation and x-ray diffraction analysis were employed to investigate the further corrosion behaviors of low carbon bainitic steel in the environment containing Cl-, after its original rust layers had been damaged on different ways. It was found the damnification of rust layers on the low carbon bainitic steel (LCBS) and steels utilized as contrasts, i.e. low carbon ferrite steel (LCS) and a commercial weathering steel 09CUPCrNi (09Cu), could be rapidly self-repaired in the further corrosion process. When damnification degree and further corrosion time were same, the resistance of rust layers and the repair degree of damnification of the low carbon bainitic steel were higher than those of contrasts. The repair ratio of inside damnification is always higher than that of crossed damnification, due to faster formation of rust layer at damaged site, in which NaCl aqueous solution is reserved after dropping. Alloy elements such as Cu and Cr obviously enhance protection of rust layer newly formed at damaged sites. These results indicate that the alloy element content in weathering steel is not enough to improve obviously corrosion resistance of steel matrix, while it is sufficient to enhance protection of rust layer.

Abstract: Pseudoelastic behaviors were observed in compressive and tensile loading-unloading tests at room temperature. The large anelastic strains were observed in compressive stress-strain hysteresis loops. The fatigue limit of axial load-controlled fatigue test at 107 cycles was 90MPa at room temperature. The deformation twins were observed in the specimen subjected to the higher stress amplitude of fatigue limit and free deformation twins were observed in the specimen subjected to the lower stress amplitude of fatigue limit. Stress-strain hysteresis loops were linear in tensile and compressive phases at the lower stress amplitude of fatigue limit and the complicated pseudoelastic deformations were observed in tensile and compressive phases at the higher stress amplitude of fatigue limit of axial load-controlled fatigue tests. Compressive mean strain generated by cyclic pseudoelastic deformations at the higher stress amplitude of fatigue limit. Fatigue cracks initiated at the secondary particle/matrix interface or broken secondary particle near the surface. Subsequently, small cracks tended to grow through transgranular.

Abstract: The investigations deal with processes leading to local effects of strengthening in multiphase steels, being characterized by good formability, continuous yielding, high strength and a strong bake hardening and ageing effect. Dual phase and complex phase steels are under investigation to examine the effect of thermo-mechanical processing parameters on local ageing ability and its use for designed properties. For this purpose local heat treatment by laser are studied, as well as stability of local ageing on the adjusted strength. A remarkable increase of the hardness in the heat treated zone was observed. Stability of the local strengthening effect could be confirmed. Partial heat treatment of multiphase steels by laser can open a new field of application for the local use of the strengthening effect to influence only relevant areas, thus giving potential for energy saving.

Abstract: The phase transformation and the final microstructure were studied in a pipeline steel grade API-X80 by carrying out a number of physical simulations of the industrial hot rolling schedules. The deformation and the cooling parameters were simulated by means of hot torsion and dilatometry experiments. Torsion deformations in the same range as in the hot rolling schedule were applied in a multi-deformation cycle at various temperatures in the austenite region. Subsequently the following parameters were varied with respect to a reference status: the reheating temperature from 900 to 1200°C, the deformation step from 0.6 to 0.15 von Misses strain, the strain rate from 1 to 10 s-1, the inter-pass time from 0.4 to 2 s, the deformation temperature from 1,100 to 850°C, the cooling rate from 0.1 to 100°C/s and the cooling stop temperature from 650 to 25°C. The transformation product microstructures were observed with optical microscopy, scanning electron microscopy and electron backscatter diffraction. The experimental data were used to study the microstructure evolution of none-deformed austenite and highly deformed austenite (Von Misses strain of 3.2), and the corresponding CCT diagrams were constructed. The detailed microstructure characteristics obtained from the present work as well as the data from the CCT diagrams for undeformed and deformed austenite could be used to optimize the mechanical properties, strength and toughness of pipeline steel grades by thermo-mechanical control process.

Abstract: Ultra high-strength TRIP-aided steel consisting of bainitic ferrite matrix and interlath retained austenite films (TBF steel) possesses high toughness and fatigue strength, as well as high resistance against hydrogen embrittlement. In this study, to improve further these mechanical properties, the effects of hot forging and subsequent isothermal transformation holding process (FIT process) on microstructure, retained austenite characteristics, tensile properties and toughness of the TBF steel with chemical composition of 0.4%C, 1.5%Si, 1.5%Mn, 0.5%Cr, 0.2%Mo, 0.05%Nb and 0.5%Al (mass%) were investigated. The FIT process brought on an excellent combination of tensile strength of 1350-1550 MPa and Charpy impact absorbed value of 100-110 J/cm2 in the developed TBF steel, exceeding so much that of SCM440 steel. The excellent combination was mainly caused by (i) refined mixed structure of bainitic ferrite and retained austenite and (ii) the increased mechanical stability of retained austenite due to the FIT process.