Materials Science Forum Vol. 762

Abstract: Within the techniques and equipments used to simulate industrial thermomechanical processing of High Strength Low Alloy (HSLA) pipeline steels, hot rolling laboratory mill equipped with cooling bed and coiling simulation furnace allows, not only accurate control of strains, temperatures, inter-pass times, and cooling rates but also enough amount of processed material for micro-structural characterisation and mechanical testing. Despite some differences with the industrial rolling, laboratory rolling offers a better simulation of the industrial rolling conditions than other thermo-mechanical simulators in terms of deformation mechanisms and processing constrains. This paper presents the results of simulation of different rolling schedules applied on pipeline grades in order to better understand the influence of the finishing rolling parameters namely: finish rolling temperature (FRT) and cooling routes on the microstructure and mechanical properties. It was observed that FRT and cooling rate have a strong impact on both grain refinement and precipitation behaviour, which leads to significant differences in strength and toughness. Furthermore variations of the above mentioned rolling parameters produce distinct fractions and distributions of austenite transformation products, variations in the final crystallographic texture and trigger diverse strengthening mechanisms (i.e. dislocation hardening). It was found that the accelerated cooling in a combination with a coiling simulation results in formation of microstructures with well developed low angle grain boundaries in comparison to the simulation made with air cooling. As a consequence the strength of the plates after accelerated cooling increases without changes in the Charpy impact toughness. It has been shown that the understanding of the effect of processing parameters on the microstructure of these steels is a key aspect for the optimization of their mechanical properties.

Abstract: The thermomechanical processing of certain AHSS still represents a challenge due to the lack of complete data on their hot deformation behaviour. Therefore, the aim of this study was to provide data on the hot-working behaviour of four model steels of the type 0.17C-3Mn-1.5Al and 0.17C-5Mn-1.5Al with or without Nb microaddition. The paper presents the results of hot strip rolling simulated by multi-step compression tests using a Gleeble simulator. Analysis of microstructural features of steels with focusing on Mn and Nb contents was carried out using X-ray, LM (light microscopy) and SEM (scanning electron microscopy). It has been shown that the applied deformation schedule allows to develop very fine-grained transformation products of supercooled austenite and controlled cooling with isothermal holding at 400°C enables to retain from 13 to 18% of retained austenite with the blocky or lath-type morphology. Mn alloying in the amount of 3 to 5 wt.% does not affect hot deformation resistance contrary to Nb microaddition, which raises flow stress levels. Influences of Mn and Nb on the retained austenite content and its carbon content are discussed.

Abstract: This study presents the use of physical and laboratory rolling simulations for the development of a novel direct quenching and partitioning (DQ&P) process for the development of tough ductile ultra-high strength structural steels with yield strengths ~1100 MPa and reasonable ductility and toughness. Suitable compositions were designed based on high silicon and/or aluminium content. The DQ&P parameters were established with the aid of physical simulation on a Gleeble simulator. Two types of dilatation tests were carried out: with or without prior straining in the no-recrystallization regime to establish the influence of controlled deformation on subsequent transformation structures and properties. Based on dilatation results, simulated rolling trials were conducted on a laboratory rolling mill and the rolled samples were direct quenched in water to the desired quench stop temperatures followed by partitioning in a furnace held at this temperature. Detailed microstructural examination confirmed that the desired martensite-austenite microstructures were achieved. Besides high strengths, the ductility (including uniform elongation) and impact toughness were quite improved in comparison to that of a direct quenched carbon steel in the same strength class.

Abstract: The C, Mn and Si impart high strength to steels through solid solution strengthening. On hardening and tempering, these elements further increase the strength substantially by exploiting hardenability potentials. With this point in view, a microalloyed C-Mn-Si steel was designed to have elements in the range of C:0.24-0.28, Mn:1.30-1.50, Si:1.30-1.50, Cr.0.10-0.20, Ni:0.20-0.40, Mo:0.15-0.25, P:0.015max, S:0.010max and H:2ppm max. Aiming at this range of chemistry, steel was made in an electric arc furnace, refined in VAD and continuous cast to 170mm x 1160 mm slabs. Prior to hot rolling of the slabs, hot deformation and dilatation studies were carried out in a Gleeble-3500C system for simulating hot rolling and heat treatment schedules. Employing the designed parameters, the slabs were hot rolled to 5 mm x 1135 mm x 4600 mm plates, which were subsequently hardened and tempered to obtain hardness levels >460 BHN, YS >1300 MPa, UTS >1500 MPa, elongation >8% and reduction in area >25% in the material. The plates contained >95% martensite in the microstructure and observed to have adequately smooth surfaces free from defects such as holes, roll marks, scabs and rolled-in scale, etc. Keywords: Hardening, Tempering, Quenching, Simulation, Strategic Application

Abstract: In this work, the investigation of transmission electron microscopy has elucidated the morphologies of the interphase precipitated carbides in an experimental Ti-Mo-bearing steel into three types: (1) planar interphase precipitation with regular sheet spacing (designated as PIP), (2) curved interphase precipitation with regular sheet spacing (designated as Regular CIP), and (3) curved interphase precipitation with irregular sheet spacing (designated as Irregular CIP). The planar sheets of carbides have also been analyzed and found to be oriented close to ferrite planes {211}, {210} and {111}; the results of transmission electron microscopy provide strong evidence to suggest that the development of interphase-precipitated carbides can be associated with the growth of incoherent ferrite/austenite interface by the ledge mechanism. The sheet spacing and inter-carbide spacing in the sheet have been measured and estimated in this work. The sheet spacing is found to be finer than the inter-carbide spacing in the sheet for all samples investigated. The result reflects that the distribution of interphase-precipitated carbides is anisotropic and cannot be considered random distribution. The relevance of the Orowan mechanism to the non-random distribution of interphase-precipitated carbides has been considered. The contribution of the dispersion of interphase-precipitated carbides to the yield strength of the steel studied has been estimated. It is revealed that an optimum component about 400 MPa contributed by interphase-precipitated carbides can be achieved, and the finding is consistent with the hardness data. Other examples of the different alloy steels are also addressed.

Abstract: nfluences of various heating processes on the amount of retained austenite after an intercritical annealing were investigated experimentally in a Mn-alloyed TRIP steel. The heating with a higher rate or an interruption at lower temperature can lead to the more rapid formation of austenite during intercritical annealing and hence more retained austenite in final microstructure. Such an enhanced austenitization kinetics is attributed to the rapid dissolution of fine carbides having previously precipitated out at low temperature during the heating, which is also confirmed by the numerical simulations.

Abstract: The effects of cooling rate and austenite structure on bainite formation was investigated by means of electron backscatter diffraction analysis and processing of obtained orientation data. Variant pairing tendency of bainitic ferrite was found to depend on the austenite grain size, austenite plastic deformation and cooling rate. In the bainite formed at low cooling rate the variant pairs having the same Bain axis correspondence are more frequent, while at high cooling rate the variant pairs having the same parallel correspondence of close-packed planes are formed side by side preferably. At the same time, these features are influenced significantly by structural state of parent austenite.

Abstract: An integral mathematical physically based model is developed for prediction of the microstructure and mechanical properties of steels processed in accordance with a given hot deformation and accelerated cooling regimes. The model predicts austenite microstructure evolution under hot deformation, as well as its transformation during subsequent cooling with account of formation of ferrite, pearlite, bainite and martensite. Structure-property relationships are developed using an extensive experimental database chemical composition - microstructure - mechanical properties obtained for 10 steel grades. Austenite transformation depending on grain size, cooling rate and preliminary plastic deformation was investigated with the help of Gleeble 3800 system to obtain a set of practically important morphologically different microstructures for each steel grade. A quantitative analysis of the microstructures was performed using optical and scanning electron microscopy (EBSD-method). Investigation of the mechanical properties of steels with wide spectrum of obtained microstructures was carried out on the double-samples processed using Gleeble 3800. The predicted microstructure parameters for investigated steels obtained using the developed model, as well as their mechanical properties, are in good agreement with the experimental data.

Abstract: The numerical modelling of heat treatment has become an essential tool in understanding distortion potentials for case hardening. When looking at other surface hardening processes such as induction or laser hardening, high heating and cooling rates automatically lead to higher strain rates during the heat treatment cycle. So far, there have been almost no investigations on the strain rate as well as temperature dependency of the mechanical properties of supercooled austenite. In this paper, the typical induction and laser hardening steel AISI 4140 has been used in order to determine the influence of strain rate and temperature on the mechanical behaviour. The experiments are based on tensile tests, using a specifically designed thermo-mechanical simulator. The experimental results show that a positive strain rate sensitivity for strain rates up to 1 s^-1 results. Especially in the temperature interval where austenite formation occurs during heating, the strain rate sensitive flow stress might lead to an alteration of the plastic strains in comparison to conventional heat treatments at low heating rates. The material model presented in this paper allows a good reproduction of the experimental data over a wide range of strain rates and temperatures.

Abstract: Double-hit isothermal deformation and multi-pass continuous cooling hot compression tests were carried out to study the recrystallization behavior of a 0.11 (in mass %) Ti-microalloyed complex phase (CP) steel. The influence of different deforming temperatures and holding times on microstructure evolution was investigated. The results showed that a pronounced austenite grain refinement after appropriate recrystallization process has been detected. The grain size decreases continuously from 176μm to 20μm after four-pass compression. It has been verified that non-crystallization temperature (T_nr) of the experimental steel is about 975°C under the deformation conditions. Based on the stress-strain curves, a kinetic method was established to predict the non-recrystallization temperature of the studied steel during nonisothermal continuous hot deformation.