Materials Science Forum Vols. 539-543

Abstract: The behavior of work hardening by cold rolling and tensile deformation was investigated in an ultralow carbon and carbon bearing martensitic steels, and then the effect of carbon on the work hardening behavior was discussed in terms of the change in dislocation density and the microstructure development during deformation. In the ultralow carbon 18%Ni steel (20ppmC), the hardness is almost constant irrespective of the reduction ratio. On the other hand, the carbon bearing 18%Ni steel (890ppmC) exhibits marked work hardening. The dislocation density of these specimens was confirmed to be never increased by cold rolling. It was also found that 10% cold rolling gives no significant influence on the morphology of martensite packet and block structure. TEM images of the 10% cold-rolled steels revealed that the martensite laths in the ultralow carbon steel are partially vanished, while those in the carbon bearing steel are stably remained. These results indicate that the solute carbon retards the movement of dislocations, which results in the high work hardening rate through the formation of fine dislocation substructure within laths.

Abstract: The work of Frommeyer on electrical conductivity measurements in pearlitic steels is reviewed to provide insight into microstructures developed during wire drawing. Electrical conductivity measurements were made as a function of drawing strain (up to ε = 6.0) for wires with strength exceeding 3500MPa. The results show that electrical conductivity increases during wiredrawing to a maximum value, then decreases with further deformation finally reaching a steady state value that is equal to the original conductivity. The initial increase is the result of pearlite plate orientation in the direction of wire-drawing, which makes the path of conduction through the ferrite plates more accessible. At a critical strain the cementite plates begin to fragment and the electrical conductivity decreases to a steady state value that is the same as that observed prior to wire drawing. With increasing strain, the cementite particles are refined and the strength increases due to the reduction in inter-particle spacing. It is concluded that the electrical conductivity of the wires is solely dependent on the amount of iron carbides provided they are randomly distributed as plates or as particles. An estimate was made that indicates the carbide particle size is approximately 3 - 5 nm in the steady state range of electrical conductivity.

Abstract: Martensitic microstructures in steels provide the strength and toughness required for the dynamic loads experienced by construction and mining machines. Such microstructures are produced with appropriate heat treatments. A physics based model has been developed to represent the microstructure evolution during the martensitic transformation. This modeling has been used to understand the role of as-quenched microstructure on subsequent processing. This paper describes modeling the martensitic transformation in steels under different cooling rates. The model described in this paper has been validated with a medium carbon, low alloy steel.

Abstract: Superplastic properties of fine-grained ultrahigh carbon steels (UHCS) have been greatly improved through the addition of 3 wt% Si (UHCS-3Si) and through improved processing conditions. This material showed an elongation to failure of 1300% under optimum superplastic conditions. It is also superplastic at very high strain rates, i.e. 10-2 s-1, in the temperature range between 800 and 825°C. An analysis of the effect of silicon additions on the UHCS and the influence of the introduction of temperatures regions in the phase diagram on the superplastic properties is made.

Abstract: This study aims to shorten the softening treatment period as possible in high strength structural steels. The steel used is SCM440 steel. As an initial microstructure, martensite, bainite, pearlite and complicated microstructure consisting of ultrafine polygonal, martensite and equiaxed cementite were extensively examined to understand their softening process on aging at 973K. These initial microstructures were prepared by heat or thermomechanical treatment. Their initial Vickers hardness (Hv(10kgf)) were 634, 281, 219 and 238, respectively. It is noteworthy that within five minutes on aging hardness of the complicated microstructure reached lower than Hv200, while it took more than several hours for other initial microstructures. A quantitative evaluation of microstructures appears to help in understanding the mechanism of the softening kinetics.

Abstract: This study deals with a relationship between strength and coiling temperature of high strength hot-rolled sheet steels consisting of ferrite and nanometer-sized carbides in order to evaluate the stability of the strength against the variation of the coiling temperature. Ti-Mo-bearing and Ti-bearing steels were prepared to form (Ti,Mo)C and TiC in ferrite matrix, respectively. Ti-Mo-bearing steel exhibited the high strength even under the high temperature coiling while the strength of Ti-bearing steel decreased significantly. Ti-bearing steel just after transforming at 923K had the same hardness as that at 898K. In addition, hardness of Ti-bearing steel coiled at 898K decreased significantly by holding at 923K for 8.64ks while Ti-Mo-bearing steel did not represent a large change in hardness. These results confirm that (Ti,Mo)C is not coarsened easily by Ostwald ripening at the high coiling temperature unlike TiC. Consequently the retardation of Ostwald ripening of (Ti,Mo)C is attributed to the small amount of titanium in solution in Ti-Mo-bearing steel.

Abstract: The microstructure following a new martensite heat treatment has been examined, principally by high-resolution microanalytical transmission electron microscopy and by atom probe tomography. The new process involves quenching to a temperature between the martensite-start (Ms) and martensite-finish (Mf) temperatures, followed by ageing either at or above, the initial quench temperature, whereupon carbon can partition from the supersaturated martensite phase to the untransformed austenite phase. Thus the treatment has been termed ‘Quenching and Partitioning’ (Q&P). The carbon must be protected from competing reactions, primarily carbide precipitation, during the first quench and partitioning steps, thus enabling the untransformed austenite to be enriched in carbon and largely stabilised against further decomposition to martensite upon final quenching to room temperature. This microstructural objective is almost directly opposed to conventional quenching and tempering of martensite, which seeks to eliminate retained austenite and where carbon supersaturation is relieved by carbide precipitation. This study focuses upon a steel composition representative of a TRIP-assisted sheet steel. The Q&P microstructure is characterised, paying particular attention to the prospect for controlling or suppressing carbide precipitation by alloying, through examination of the carbide precipitation that occurs.

Abstract: Thermomechanical processing allows the attainment of spheroidized microstructures that show improved mechanical properties. In this work, a thermomechanical processing route consisting of two steps was developed for two ultrahigh carbon steels (UHCS) containing 1.3 and 1.5%C. This route develops structures of fine spheroidized cementite particles in a fine-grained ferrite matrix. Spheroidized microstructures are formed by eutectoid carbide particles in the UHCS- 1.3C and by proeutectoid and eutectoid carbide particles in the UHCS-1.5C. In the latter steel, the proeutectoid carbide particle size is larger than the eutectoid carbide particle size. The carbide size distribution remains basically constant with austenitizing temperature for both steels. Plane-strain fracture toughness of spheroidized UHCS-1.3C is higher than for UHCS-1.5C, about 80 vs 40 MPa m1/2. These values do not vary significantly with austenitizing temperature which is attributed to the constancy of the mean proeutectoid and eutectoid carbide size.

Abstract: Microstructures formed by degenerate pearlite transformation in an Fe-0.38mass%C alloy were studied by means of scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Degenerate pearlite which contains fine cementite particles even at the growth front was observed with other structures such as proeutectoid ferrite, lamellar pearlite and bainite in a temperature range between 773K and 923K. As the isothermal transformation temperature is lowered, a fraction of the degenerate pearlite increases. The degenerate pearlite consists of ‘block’ (a region in which ferrite orientations are nearly the same) and ‘colony’ (a region containing cementite particles of nearly the same orientation), both of which are similar to those in lamellar pearlite. Block boundaries within an austenite grain are generally of high-angle type and their misorientations deviate largely from intervariant relationships for the K-S orientation relationship. In contrast, colony boundaries are of low-angle type. Cementite films are formed along those ferrite boundaries in the degenerate pearlite, presumably formed by encounter of the blocks or colonies.