Materials Science Forum Vols. 539-543

Abstract: The precipitation behavior of cementite in low carbon steels at various heating rates from 0.3 to 100 K/s has been studied using a high-frequency induction heating apparatus. The materials used in this study were steel plates
for welded structures: 610 and 780 MPa class steel plates with a mixed microstructure of bainite and martensite.
Cementite was observed using a carbon extraction replica method and the hardness and toughness were also examined.
When heated at the conventional slow rate of 0.3 K/s, relatively large cementite particles with an average diameter of 72 nm precipitated at the lath boundaries, whereas when heated at a rapid rate over 3.0 K/s, cementite precipitated both within the laths and at the lath boundaries, and the cementite was refined down to an average diameter of 54 nm. With such refinement of the cementite, the toughness was improved. On the other hand, the hardness was irrespective of the heating rate and was dependent on the tempering parameter. TEM observations of the cementite precipitation behavior during the rapid heating process revealed that cementite begins to precipitate at the lath boundaries at about 773 K and within the laths at about 873 K. It is concluded that rapid heating especially from 773 to 873 K contributes to the cementite refinement and consequently the improvement in toughness. The effect of alloying elements such as chromium, molybdenum or silicon on the cementite growth during the rapid heating and tempering treatment is also discussed.

Abstract: Higher strength and higher thickness are ongoing demands on plates for pipes and structural applications. At the same time other properties like toughness and weldability must be kept or even be improved. As a consequence these demands must be achieved with limited addition of alloying. Specific aspects must be incorporated in the design of thick plates. To exploit the mechanisms of property achievement effectively and to compensate certain disadvantages of thick plate production appropriate mill equipment is necessary. Utilization of TMCP provide the basis to meet these goals.

Abstract: New conceptual TMCP process for manufacturing high strength steel plates, which is applied an on-line heat treatment immediately after accelerated cooling (ACC), was developed. Transformation and precipitation behavior in the new TMCP process was investigated and compared with those in conventional ACC process and quenching and tempering process (Q+T). In the ACC process and Q+T process, microstructures were consisted of bainitic ferrite and second phase, such as cementite or martensite-austenite constituent (MA). And fine carbides, which were formed randomly, were observed in Q+T steel. On the other hand, in the new TMCP process polygonal ferrite was observed in addition to bainitic ferrite and cementite, and two kinds of precipitation forms, random precipitation and row precipitation, were observed. It was found that ferrite transformation is promoted during heating after accelerated cooling, which brings row precipitation of fine carbides. Furthermore, Control of the formation of MA this new TMCP process. In the conventional ACC process, MA constituents are formed from carbon enriched untransformed austenite during air cooling after ACC, and formation of MA is hard to prevent for higher strength steels. On the other hand, carbon enrichment to untransformed austenite can be prevented by carbide formation during on-line heat treatment after ACC. It was demonstrated that homogeneous microstructure with very low amount of MA constituents was achieved by the new TMCP process. And, absence of brittle phase brought excellent resistance to hydrogen induced cracking in NACE sour environment. In this paper, details of the metallurgical and mechanical feature of this new TMCP steel were discussed, and application to sour resistant linepipe steel was introduced.

Abstract: The required complex of properties of modern pipe steels, including improved weldability and cold resistance, can be achieved due to formation of fine-grained ferrite-bainite microstructure of steel. Effect of various designs of additional alloying of steel on phase transformations with use of deformation CCT diagrams is investigated. Key parameters of TMCPtechnology, necessary for obtaining the best complex of properties of plate and strip products are determined. Results of industrial trials in production of steel with ferrite-bainite microstructure of Russian steel manufacturers are presented.

Abstract: The X70 grade and X80 grade pipeline steel strip with acicular ferrite microstructure have been researched and developed. The properties of the steel with acicular ferrite were studied by using tensile test, Charpy impact test, CTOD test and DWTT test, and compared to that of the steel with ferrite and pearlite. The microstructure of acicular ferrite was analyzed by using optical metallographic microscopy, scanning electron microscopy and transmission electron microscopy. It shows that the acicular ferrite pipeline steel has high strength, high toughness, low ductile-brittle transition temperature and high fracture-arrest toughness, the excellent properties are benefit from the uniform microstructure, clean steel, and the low carbon acicular ferrite in which consist of interaction of very fine precipitated particles and high-density dislocation.

Abstract: Azovstal Iron & Steel Works (Ukraine) together with I.P. Bardin Central Research Institute (Russia) carry out research intended for development and realization of new HSLA-steels for large diameter gas pipelines of strength categories X60 through X80. A concept that has been recently adopted for creation of new pipeline steels calls for lower carbon content, bigger role of Nb microalloying, economical alloying with elements improving austenite resistance to perlite transformation, thermomechanical controlled rolling with strain finishing in two-phase γ+α-region. This paper describes the research results obtained for commercial lots of rolled steel plates of various alloying systems intended for X65 and X80 longitudinally electric welded pipes.

Abstract: Orientations of both the α and γ phases in a multi-phase commercial steel were measured by means of electron backscatter diffraction (EBSD) techniques. Using the average orientation of each austenite grain as the reference frame, the orientation relationships between the two lattices were compared with the common orientation relationships (i.e. the Kurdjumov-Sachs and Nishiyama-Wassermann) in Rodrigues-Frank space. The occurrence of variant selection in individual austenite grains was examined using a recent dislocation-based model. This model considers the role of the slip systems that were active during prior deformation, as well as those of in-plane reactions, cross-slip and the partial dislocations that are linked to specific variants. It also unites the competing K-S and N-W relationships through the dissociation of perfect dislocations. Reasonably good agreement was observed between the predictions and the observations. Possible explanations for some of the discrepancies are also presented.

Abstract: The new phase equilibrium of Fe-C diagram under magnetic field has been theoretically calculated. Results show that the magnetic field mainly shifts the γ⁄α+γ equilibrium line and the eutectoid point to the high carbon and high temperature sides. Based on this result, an experimental setup has been launched to investigate the effect of magnetic field on austenite decomposition in medium carbon and high carbon steels. The thermodynamic and kinetic effects of the high magnetic field on proeutectoid transformation at different cooling rates have been studied. It was found that for medium carbon steels, the magnetic field increases the amount of proeutectoid ferrite and accelerates the diffusional decomposition of austenite at medium and relatively fast cooling rates (10°C/min and 46°C/min). But there is no special grain growth along the field direction. The results led to a proposal of a new rapid annealing under a high magnetic field. However, when cooling is slow (2°C/min), the magnetic field shows a strong tendency to promote the proeutectoid ferrite grains to grow along the field direction through the magnetic dipolar interaction, which leads to the formation of an elongated grain structure. Moreover, the magnetic field also exhibits influence on the austenite decomposition in hypereutectoid steel by changing the amount of secondary cementite and lamellar spacing of pearlite.