A study on grain growth using a novel grain size calculation tool

Abstract The growth of prior austenite grains (PAG) of low alloyed martensitic steel is proven to be one of the key attributes contributing to the mechanical properties of ultrahigh-strength steels. The mean linear intercept -method (MLI) is traditionally used to acquire average PAG sizes from light optical microscopy images, which are from experimental test samples. The MLI -method is arduous and time-consuming as well as a highly generalizing method, where you lose information about the grain size distribution. Therefore, a more sophisticated and computerised method is in high demand among metallurgists. A program has been developed that encompasses an importing, digitalizing and calculating tool, which provides grain sizes and their distribution from multiple images. The tool mimics the workflow of manual MLI -method so the user sets the measure lines and marks all the linear intercepts. After this the tool calculates the MLI grain sizes and their 95 % confidence limits. Additionally, the tool provides the size of each intercepted grain and combines them to create a distribution. This information has been used to study the effects of holding temperature and time on grain sizes throughout the test samples in a case where abnormal grain growth at the centreline was expected. In the present study, PAG sizes were studied before and after deformation at ¼ and ½ thicknesses at various temperatures and holding times using the grain size calculation tool. The average MLI grain sizes show very little differences between temperatures and holding times, so information about grain size distribution is needed. Traditional presentation of the grain size distributions also shows too much variation to interpret the data properly. Instead, using the grain size distribution information and grouping grains to small, medium and large instances gives more profound data, especially in cases where grain size variation is significantly large. Distribution data from the test series also showed abnormal grain growth at the centreline of the test sample. The grain size calculation tool is used to quantify the effect of temperature and hold time on abnormal grain growth and its root cause is examined briefly

Microstructure evolution and static recrystallization kinetics in hot-deformed austenite of coarse-grained Mo-free and Mo containing low-carbon CrNiMnB ultrahigh-strength steels

Abstract The static recrystallization characteristics and microstructure evolution in hot-deformed austenite were evaluated for a newly developed low-carbon CrNiMnB ultrahigh-strength steel with and without molybdenum addition. The time for 50% static recrystallization (t50%) over a wide range of strains and hot-deformation temperatures were obtained using the stress-relaxation technique on Gleeble thermomechanical simulator. Moreover, effect of deformation parameters on the size distribution and average size of prior austenite grains are investigated. A novel semi-automatic stress relaxation test reading tool with a graphical user interface was created and used successfully for the current study. The obtained results of strain‘s power and the apparent activation energy are within the range stated in literature for C-Mn and microalloyed steels. Addition of molybdenum increase the power of strain and the apparent activation energy from − 1.9 to − 2.6 and 206 to 212 kJ/mol, respectively. The retardation effect of molybdenum addition was shown by a new regression equation devised for calculating t50%. The developed equations show a good agreement with the experimental data and can be used in the designing of roughing during thermomechanical processing. The deformation parameters i.e., temperature, strain and holding time have a significant effect on the size distribution and average size of prior austenite grains

Virtual rolling automation and setup calculations for six stands FEM finishing mill

Abstract Digitalization is becoming increasingly common in the steel industry. Formerly developed models of individual phenomenon or separate sub-processes are being further developed into wider complexes where multiple models are coupled together. Virtual rolling automation, which can be used to control a finite-element rolling model, is a new element in these complexes. The automation enables to model the variations caused by the process adjustment. It must be taken in the account that neither the model nor the industrial process are ideal, but there are limitations in the attainable accuracy in both cases. Inclusion of the new automation control in the FE-model introduces new requirements: the setup calculations for all six rolling stands and the automation logic adjustments must perform within the model. The focus of the current article is prediction of the roll force and the virtual rolling automation of six stand finishing mill

The effect of internal contact pressure on thermal contact conductance during coil cooling

Abstract Coil cooling process is an important step in production of certain steel grades. Phase transformations for dual phase steels and precipitations for precipitation hardened steels occur mainly during the coil cooling. Generally, a coil goes through a coil conveyance chain before arriving at the final cooling storage at a steel plant. This conveyance chain contains various thermal contacts with different types of conveyors. Ambient temperatures and weather conditions may also change considerably. Those variables are relatively easy to measure and define in a simulation model whereas internal stresses and contact pressure inside the coil are very challenging to measure in industrial scale process. Thermal conductance between adjacent strip revolutions is dependent of contact pressure. In addition, thermal conductance is influenced by the combined thermal conductivity of steel and oxide layer of contact interfaces as well as thickness profile. In this paper the internal contact pressure between strip revolutions due to strip coiling and gravity are solved and considered when defining thermal conductance. Heat transfer is computed using FE-model, and GAPCON subroutine in Abaqus is utilized to calculate thermal contact conductance, taking into consideration the contact pressure between the strip revolutions. Also, the whole coil conveyance chain commencing from downcoiler mandrel to coil field cooling is implemented.

Analysis of grain size distribution evolution of steel during recrystallization and grain growth

Abstract Controlling the hot rolling process requires a deep understanding of the underlying metallurgical phenomena. Quantitative methods are of paramount importance for achieving the capability of controlling microstructural evolution. Since the final mechanical properties of steel result from microstructural evolution in the whole process, analysis of the microstructure provides an important input for numerical simulations that can be used for tailoring the mechanical properties of steel. The evolution of grain size distribution of a low-carbon CrNiMnB ultrahigh-steel in austenitic state is studied in hot forming and annealing using experimental data obtained with the Gleeble 3800 thermo-mechanical simulator. A general method is described that can be utilized to systematically compare the grain size distributions obtained from the experimental studies. The experimental data has been obtained from laser scanning confocal microscopy images using the mean linear intercept method. A custom-made semi-automatic software has been utilized to process the data rapidly and reliably

Coupled heat transfer and phase transformations of dual-phase steel in coil cooling

Abstract Dual-phase steels are generally used in the car industry due to high tensile strength and good formability, which are obtained by a mixture of bainite and ferrite phases. This microstructure is achieved through slow rate coil cooling. However, the manufacturing of dual-phase steels introduces various challenges such as the instability of the cold rolling process. An important factor affecting this is the non-uniform coil cooling of a hot rolled strip. In coil cooling the cooling rates are not controlled and there are different thermal contacts during coil conveyance causing unequal cooling of the steel coil. Unequal cooling rates lead to non-uniform coil cooling, producing irregular phase transformations on different sides of the coil, which causes periodical variations of the phase fractions in the steel strip. Varying phase fractions cause thickness deviations in the strip during the cold rolling process. A three-dimensional transient heat transfer finite element model was developed and used for modeling the complete coil conveyance chain and coil field cooling of the coil on an industrial scale. A coupled phase transformation model is implemented as a subroutine into the finite element model for calculating the resulting phase fractions. It was found that the different thermal contacts during the coil conveyance produce uneven cooling rates causing length- and widthwise variations in the phase fractions. The heat transfer model is validated by comparing temperature profiles between the simulated and measured coil edges. The phase transformation model is fitted into experimental data and verification is carried out in industrial conditions by comparing the modeled phase fractions and test samples from a cooled and unwound steel coil

Numerical and experimental study on thermo-mechanical processing of medium-carbon steels at low temperatures for achieving ultrafine-structured bainite

Abstract A combination of experimental and numerical approaches was applied for constructing a dynamic model for thermomechanical processing, which was used for simulating laboratory rolling and cooling, and for designing a cooling path to enable phase transformation from austenite to ultrafine (~ 50–100 nm) bainitic laths. Physical thermomechanical simulation experiments were used for calibrating the numerical models. Hot rolling and water cooling experiments were conducted and they were numerically simulated. The calibrated numerical models were used for simulating the main processing stages affecting the final microstructure evolution during a laboratory scale processing, i.e. the low temperature (500 °C) ausforming and subsequent cooling schedules leading to the decomposition of austenite into bainite and martensite. The fitted model parameters and simulation results are presented for the laboratory rolling and two different cooling paths: (i) air cooling to 350 °C temperature with subsequent holding for 1–1.5 h, and (ii) water cooling close to martensite start temperature, and furnace holding for 1–1.5 h. Microstructural analysis was carried out using scanning electron microscopy combined with electron backscatter diffraction as well as X-ray diffraction and the structures were corroborated with mechanical properties evaluated in respect of hardness, tensile and impact toughness properties. The achieved mechanical properties and microstructures were further interpreted with the numerical simulation results. The results show that the calibrated numerical simulations provide an effective tool for designing suitable thermomechanical processing paths leading to desired microstructure

The effect of Pd and Ni coatings on hydrogen permeation experiments of as-quenched martensitic steel

Abstract Hydrogen permeation technique is a widely used testing method for the determination of hydrogen diffusion coefficient (D), which is an important parameter considering hydrogen embrittlement. A palladium (Pd) or nickel (Ni) coating is often utilised on the hydrogen detection side of the test specimens. Here, we investigate the effect of Pd and Ni coatings on hydrogen diffusion in a martensitic 500 HBW hardness low-alloy steel in the thickness range of 0.5–0.8 mm using a refined successive transient method and compare against an uncoated reference specimen. Both coatings yield similar average D values (6–6.6 × 10⁻⁷ cm²/s), but the best repeatability is achieved with Pd coating. With Ni coating, D values decrease with the increasing specimen thickness, which is partly caused by a slower hydrogen diffusion in Ni, and therefore a concentration gradient at the specimen-coating interface. The uncoated specimen has a poor transient fit, and significantly lower D (2.1 × 10⁻⁷ cm²/s) due to surface oxidation. With both coatings, the steepness of the last decay transient was highly affected by specimen thickness, and therefore the density of reversible hydrogen traps is only comparable for similar thicknesses

Effect of prior austenite grain morphology on hydrogen embrittlement behaviour under plastic straining in as-quenched 500 HBW steels

Abstract Prior austenite grain (PAG) structure is an important factor influencing hydrogen embrittlement (HE) susceptibility of ultrahigh-strength steels. In this study, the effect of PAG shape and size on HE behaviour is investigated using a novel tuning-fork testing method and hydrogen thermal desorption spectroscopy (TDS). Different PAG structures were acquired via re-austenitization (860°C = A860, 960°C = A960) and rapid quenching of an as-received 500 HBW direct-quenched (DQ) steel, which has an auto-tempered lath-martensitic microstructure and elongated PAG morphology. Fractography reveals different crack propagation mechanisms depending on the PAG shape. With the elongated PAG structure, hydrogen-induced crack propagation transverse to elongated PAGs was transgranular quasi-cleavage. Propagation was partially intergranular with the equiaxed PAG structures, regardless of the PAG size, leading to equally faster fracture. The TDS results show that there are no significant differences between the total hydrogen contents, but re-austenitized A860 and A960 steels contain a higher fraction of weakly trapped hydrogen. This indicates that the PAG boundaries are not the dominant hydrogen traps, and the different crack propagation mechanisms are rather linked to the geometrical shape of the grain structure