Materials Science Forum Vols. 539-543

Abstract: Series of trials were conducted on a laboratory rolling mill to evaluate the influence of intercritical rolling on the microstructure and texture of steel with 0.082%C, 1.54% Mn, 0.35% Si, 0.055%Nb and 0.078%V. Two parallel rolling schedules A and B were designed on the base of the experimentally deduced CCT diagram of the steel. In rolling schedule A the material was subjected to accelerated cooling and coiling simulation after final rolling in the intercritical region, whereas in rolling schedule B the last rolling pass in the intercritical region was replaced by a water quench at the same temperature of the intercritical rolling pass in schedule A. Microstructure and texture were characterized by means of light optical microscopy, scanning electron microscopy, EBSD and XRD. It was found that the average grain diameter and the texture depend significantly on the final rolling temperature in the intercritical region. The decrease of the intercritical rolling temperature leads to an increase of the {111}〈uvw〉 /{001}〈uvw〉 ratio, but at the same time the increase of the average ferrite grain size was also observed. A phenomenological model based on the K–S orientation relationships was used to predict the texture formation in the intercritical region.

Abstract: New type of IF cold-rolled high strength steels (HSSs) with the strength level of 390 and 440MPa have been developed under the chemistry of the extra-low carbon steel containing around 60ppm C with an intentional addition of niobium by hybridizing the precipitation hardening with niobium carbides and the supplemental solid-solution hardening. In this steel, Precipitation Free Zone (PFZ) nearby recrystallized grain boundaries forms during continuous annealing. This structure leads to unique mechanical properties such as lower yielding and superior anti-secondary-work embrittlement under fine grain structure strictly required for the exposed panels in Body-in-White. Principles of the unique mechanical properties of the steel are introduced related with the formation of PFZ during annealing, and the results of further approach to improve them as the state-of-the-art product, which is widely used for the exposed panels in Body in White, are introduced in the paper.

Abstract: The multiphase steels have complex microstructures containing polygonal ferrite, martensite, bainite, carbide and a small amount of retained austenite. This microstructure provides these steels with a high mechanical strength and good ductility. Different thermal cycles were simulated in the laboratory in order to create the microstructures with improved mechanical properties. The samples were heated to various annealing temperatures (740, 760 or 780°C), held for 300 s, and then quickly cooled to 600 or 500°C, where they were soaked for another 300 s and then submitted to the accelerated cooling process, with the rates in the range of 12-30°C/s. The microstructure was examined at the end of each processing route. The mechanical behavior evaluation was made by microhardness testing. The microstructural characterization involved optical microscopy (OM), X-ray diffraction (XRD), scanning electron microscopy (SEM) with electron backscattering diffraction (EBSD) and transmission electron microscopy (TEM). The use of multiple regression analysis allowed the establishment of quantitative relationship between the microstructural parameters, cooling rates and mechanical properties of the steel.

Abstract: Innovative technological processes for a new integrated deformation-thermal production of flat rolled stock imparting enhanced physical-mechanical properties while minimized alloy additions has been developed on the basis of recently discovered metallophysical laws of influence, through hot plastic rolling deformation, upon microstructure-phase conversions and states of steel in metallurgical products.

Abstract: There has been a big demand for increased vehicle safety and weight reduction of auto-bodies. An extensive use of high strength steels is one of the ways to answer the requirement. Since the crashworthiness is improved by applications of higher strength steels to crashworthiness conscious structural components, various types of advanced high strength steels have been developed. The crash energy during frontal collisions is absorbed by the buckling and bending deformations of thin wall tube structures of the crushable zone of auto-bodies. In the case of side collision, on the other hand, a limited length of crushable zone requires the components to minimize the deformation during the collision. The lower the strength during press forming, the better the press formability is expected. However, the higher the strength at a collision event, the better the crashworthiness can be obtained. It can, therefore, be concluded that steels with higher strain rate sensitivities are desired. Combinations of soft ferrite phase and other hard phases were found to improve the strain rate sensitivity of flow stresses. Bake hardening is also one of the ways to improve the strain rate sensitivity of flow stresses.

Abstract: The microstructural evolution has been studied for hot rolling of a dual-phase steel with a lean C-Mn-Si chemistry. This study includes the investigation of austenite grain growth during reheating, constitutive behaviour and static recrystallization kinetics of austenite, and austenite decomposition during simulated run-out table cooling conditions. To develop and validate the microstructure models for these phenomena, experimental studies have been carried out in the laboratory using a Gleeble 3500 thermomechanical simulator. The hyperbolic sine relationship between flow stress and Zener-Hollomon parameter is employed to describe the constitutive behaviour. The Johnson-Mehl-Avrami-Kolmogorov (JMAK) theory is used to predict the static recrystallization kinetics. Ferrite transformation start is described with an approach that considers early growth of corner nucleated ferrite. The fraction of ferrite transformed from austenite during continuous cooling is described using the JMAK approach in combination with the additivity rule. The ferrite grain size is quantified as a function of the transformation start temperature. The overall microstructure model has been validated based on a number of laboratory simulations of the entire hot strip rolling and controlled cooling process with an emphasis on industrially relevant run-out table cooling strategies.

Abstract: Thermomechanical processing (TMP) involves both thermal and mechanical treatments that define both product shape and microstructure/properties. Since the industrial revolution, machines of augmented power, size and precision have given rise to TMP that challenged explanation of the crystal mechanisms. In wrought iron, lamellar ferrite exhibited high transverse crack resistance due to fine slag stringers that as flux facilitated welding of puddled bars in forging of shafts or rolling of plates for bell-welding into pressure tight pipes; the substructure developed in the iron as working continued below 900°C strengthened it. Patenting of high C steel wire led to an optimum cold-drawn structure for outstanding strength and toughness. Hot forming technology, combined with the refining potential for austenite decomposition gave rise to controlled rolling for enhanced ferrite nucleation, ausforming to refine martensite and intercritical rolling to deform the ferrite or to spheroidize the carbides. Cold rolling and annealing have been scheduled to impart suitable strength, grain size, substructure and texture.

Abstract: Nowadays, it is not evident to produce in a robust way cold rolled and annealed/galvanised high strength steels based on lean chemistries and with consistent mechanical properties over the coil length. The reasons behind this are first the low cooling rates available on the lines which require the use of a large amount of alloying elements for avoiding the unwanted phases such as the pearlite and secondly the difficult control of soaking temperature and time in a narrow range, resulting in a variable austenite content at the end of the soaking and then in a dispersion of the obtained mechanical properties. By considering high speed cooling technologies on a compact annealing/galvanizing line, this production becomes possible. After rapid heating by means of induction heating and short holding at a high soaking temperature, the strip is cooled down to an intermediate temperature, where it is held for a short period for obtaining a stable ferrite-austenite structure. Then it is rapidly cooled by means of cold water (Twice) in the case of a continuous annealing line or by the zinc quench process in the case of the galvanizing line. These rapid cooling equipments allow reaching high tensile strength levels with a significant reduction of the addition of expensive elements (Mn, Cr and Mo).

Abstract: Dual Phase (DP) steel sheets, mainly 590 MPa TS grade steel, have been applied to structural parts of automobile because of their good formability, large bake hardenability and high crash worthiness. Although the concept of DP steel was established as early as the mid-1970th, the literature contains little discussion of Zn-coated DP steel, which has been the main application in recent years. In manufacturing Zn-coated DP steel, chemical composition which secures adequate Zn coatability and appropriate heat cycles, including galvannealing, must be considered. In this paper, the effect of Nb on the tensile properties, stretch flange formability and bake hardenability of 590 MPa TS grade DP steel is discussed. The base chemical composition was 0.05%C-2%Mn-0.5%Cr steel. The effect of Nb on the above-mentioned properties was studied using 0.05%Nb added steel and found that the addition of 0.05%Nb results in improved elongation, stretch flangeability and bake hardenability with higher tensile strength under both simulated GA and GI heat cycles. These improved properties by the addition of Nb are brought about by the grain refinement of ferrite matrix and finer dispersion of martensite.

Abstract: Applying the severe deformation, the mechanical properties related to secondary hardening were investigated. The Mo-Cr-Co-Ni steels containing (5-13)wt% Co and (8-14)wt% Ni were severely rolled at 850 °C, followed by direct quenching(DQ). DQ specimens were then isothermally aged at 475°C. Ni additions promoted aging kinetics, accompanied by a little enhancement in hardness. In contrast, Co additions enhanced the peak hardness, but did not induce a remarkable aging acceleration. In the 13Co addition group, an actual secondary hardening, that is, the peak hardness is higher than the as-quenched hardness. Variation in peak hardness(Rc) in alloys is summarized as follows; 14Ni-13Co(57.2) ≒ 11Ni-13Co(57.1) > 8Ni-13Co(56.5) > 11Ni- 9Co(56.2) > 8Ni-9Co(54.4) > 11Ni-5Co(53.4).