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Funding Information: This research was conducted within the framework of the Genome of Steel project funded by the Academy of Finland (Project #311934). Publisher Copyright: © 2021, The Author(s). Copyright: Copyright 2021 Elsevier B.V., All rights reserved. New continuous cooling transformation (CCT) equations have been optimized to calculate the start temperatures and critical cooling rates of phase formations during austenite decomposition in low-alloyed steels. Experimental CCT data from the literature were used for applying the recently developed method of calculating the grain boundary soluble compositions of the steels for optimization. These compositions, which are influenced by solute microsegregation and precipitation depending on the heating/cooling/holding process, are expected to control the start of the austenite decomposition, if initiated at the grain boundaries. The current optimization was carried out rigorously for an extended set of steels than used previously, besides including three new solute elements, Al, Cu and B, in the CCT-equations. The validity of the equations was, therefore, boosted not only due to the inclusion of new elements, but also due to the addition of more low-alloyed steels in the optimization. The final optimization was made with a mini-tab tool, which discarded statistically insignificant parameters from the equations and made them prudently safer to use. Using a thermodynamic-kinetic software, IDS, the new equations were further validated using new experimental CCT data measured in this study. The agreement is good both for the phase transformation start temperatures as well as the final phase fractions. In addition, IDS simulations were carried out to construct the CCT diagrams and the final phase fraction diagrams for 17 steels and two cast irons, in order to outline the influence of solute elements on the calculations and their relationship with literature recommendations.

Roughing has been simulated with the Finite element software AbaqusTM to replicate an industrial-scale process. The model has been made to be as close as possible to its real counterpart. For this purpose, an automated controlling logic has been created to simulate the multiple passes as well as inter-pass times for roughing. Simulating multiple passes with FEM is computationally very demanding, so new methods to reduce computing times are worth considering. During a roll pass an explicit solver is necessary due to high deformation amounts and rates. An explicit solver is tied to a very small time increment, so it takes a long time. On the other hand, inter-pass periods do not include any deformation or roller contact, so an implicit solver is quite capable of computing this portion of the simulation. An implicit solver can speed up the time increment considerably when compared to the explicit solver, so using it potentially saves a significant amount of computing time. Unfortunately, Abaqus does not include any methods to change the solver during a single simulation. Instead it is possible to communicate between the two solver types by manually importing data from a completed simulation to a new simulation model. A new method to change solvers automatically using a self-made Python code is proposed in this paper.

We have determined different phase fractions from microscopy images using semi-automated image analysis fitting technique, and in addition we have classified each phase according to its hardness. The distribution of grayscale pixels of different phases is first characterised separately for each phase, which are sampled from the microscope image. After this the distributions of the separate phases are fitted to give the corresponding distribution of the whole image. The microhardness measurement provides reliability on the classification of the different phases to ferrite, bainite or martensite. In addition to describing the applied techniques in detail, we present the results obtained from the analysis for one steel subjected to isothermal holding experiments at different temperatures.

To reach high demands of a stainless steel surface quality the location of a slab edge is optimized utilizing multiphysical finite element (FE) analysis. The slab edge forms in roughing process when the longitudinal edge of the stainless steel slab moves parallelly towards the center of a transfer strip surface due to several rough and edge rolling passes. Strip spreading and location of the slab edge are managed by edge rolling process which is accomplished concurrently with roughing. Deformation resistance has a significant role characterizing the strip spreading and material flow in the roll bite, thus experimental material compression testing was carried out and the results fitted to the Hensel-Spittel equation. Multiple edger roll profiles were designed, and the most feasible details of the roll profile were iteratively utilized for the new profiles. In this way the location of the slab edge was optimized closer to the edge of the transfer strip by developing a new edger roll profile and resetting edge rolling passes according to results of FE-simulations. To mimic an industrial-scale roughing process an automated pass schedule control was developed in the FE-model. Therefore, multipass simulations require only a pass schedule data to run simulation.

Accurate control of the temperature of the workpiece is crucial when carrying out thermal processing of steel. Effective heat transfer coefficient for water cooling of hot rolled steel was calibrated using spray nozzles with different amounts of water flux. Austenitic stainless steel was chosen for the studies in order to avoid the release of latent heat, which would affect the result in the case of carbon steels. Three different measurement sites acquired with thermocouples were used, namely in the middle and at the quarter distance from both top and bottom surfaces. The heat transfer coefficient was fitted to the experimental data using a computational model, which calculated the time-dependent temperature distribution within the steel slab. The model was validated by altering the experimental exposure time to the water spray and compared the model prediction to the observed result. The calibrated model provides the capability to design and carry out cooling routes in thermomechanical processing of steel in practice.

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..

Forming limit curves (FLCs) are widely used for predicting failure in sheet metal forming operations. FLCs are determined in the laboratory according to ISO 12004‑2 using the Nakajima test. The testing procedure involves stretching a series of dog-bone shaped blanks with a hemispherical punch until failure occurs. This measurement procedure is extremely time-consuming. Consequently, a variety of statistical models have been proposed for predicting FLCs based on tensile test data. Unfortunately, practically all existing models are targeted for carbon and alloy steels, and thereby these models cannot be directly applied to stainless steels. In the present study, an approach for prediction of coil specific forming limit curves is developed. Predictive equations are derived based on statistical correlations between measured FLCs and steel properties. The application of the method to prediction of FLCs of stabilized ferritic stainless steels is described.

Hollow sections and cold-formed steels have a key role in modern structures and machinery. In addition, to benefit from full potentials of cold-formed steels, it is usually required to weld them to other steel parts of structures. However, data provided by relevant standards, such as Eurocode 3, do not cover newly developed high strength and ultra-high strength grades of this material. Thus, further study is critical to complete available data in literature and standards. Regarding this matter, having a good weldability for cold-formed high and ultra-high strength steels is vital for development of contemporary steel structures. Thus, newly developed steels S700MC and S1100 were selected to be investigated in this study. To do so, bended base metals with different degrees of cold-forming were welded to their straight (virgin) counterparts. Next, welded joints were investigated via microstructural analysis, hardness measurement, tensile test, and Charpy impact test to assess the weldabilities of the cold-formed base metals. Results show that the final joints had acceptable characteristics, and the cold-formed base metals showed good weldability. However, bending and pre-straining criteria recommended by the manufacturer must be satisfied to have an acceptable joint after welding. Beyond that criteria, fracture elongation and notch toughness of the welded joints decreased, and some welded samples failed from their cold-formed base metals. Post-print / Final draft

Variety of ultra-high strength steels (UHSS) with different microstructural characteristics is becoming available with continuous development of the manufacturing process in the steel industries. In order to effectively design structures made of such steel grades, a detailed knowledge of the mechanical properties is vital. Fire safety design is one of the areas in which such knowledge is essential. Welding process is indispensable in construction of steels structures with inevitable welding-induced degradation of mechanical properties of UHSSs. Thus, conducting experimental research on elevated-temperature constitutive mechanical behavior of welded joints made of UHSSs is of paramount importance. This study addresses elevated-temperature mechanical properties of as-received and as-welded S960 (manufactured via direct quenching technique) and S1100 (quenched and tempered) steel grades. A fully automated gas metal arc welding (GMAW) process with low heat input value was utilized to join the steel plates. Next, steady-state uniaxial tensile tests in the temperature range between room temperature (RT) and 900 °C were carried out. Accordingly, reduction factor-temperature relations for each tested steel in both as-received and as-welded forms are discussed and compared with several design standards, as well as with previous studies in the literature. Finally, predictive equations are proposed to estimate the elevated-temperature mechanical properties reduction factors of the tested UHSSs in as-received and as-welded forms. Publishers version

Air bending is a widely used method for forming ultra-high strength steels (UHSS). However, the limited formability of UHSS poses some challenges for the bending process in the form of strain localisation, surface defects, punch detachment (multi-breakage) and pseudo-polygonal “nut-like” shape of the bend. In this study, the bendability of three UHSS grades (700, 900 and 1100 MPa) is investigated with 3-point bending tests, utilising Digital Image Correlation (DIC) for measuring the strain distributions on the outer curvature. The differences in the extent of multi-breakage and the bend shapes are also studied, and these observations are correlated with the findings from the bending force and strain measurements. The differences between the investigated UHSS grades are significant. The 900 MPa grade produces more localised strain distributions and pronounced multi-breakage compared to the other grades, along with a more polygonal “nut-like” geometry. The reasons and effects of the multi-breakage phenomenon, as well as the causes for the observed differences in the behaviour of the materials are discussed in this paper. The presented results and the measurement data provide more information about the behaviour of the investigated materials in bending, and can be used for improving bending simulation, numerical models, and workshop instructions.

In this study, we present the effect of enhanced cooling on the mechanical properties of a high-strength low-alloy steel (having a yield strength of 700 MPa) following a single-pass weld process. The properties evaluated in this study include uniform elongation, impact toughness, yield, tensile and fatigue strengths alongside the cooling time of the weld. With the steel used in this study, the enhanced cooling resulted in a weld joint characterized with excellent cross-weld uniform elongation, yield and fatigue strength. The intensified cooling reduced the time it takes for the weld to reach 100 °C by around 190 s. Not only the fusion line of the weld was less pronounced, but also the grain size of the CGHAZ was greatly refined as a result of the enhanced cooling. The results indicate that combining external cooling to the welding processes can be beneficial for the studied high-strength steel.

The effect of forced cooling using heat sinks on the mechanical properties and interpass waiting time of two-pass welds has been studied for a martensitic steel with a yield strength of 960 MPa when the interpass temperature was 100 °C. Cross-weld tensile and − 40 °C Charpy-V impact toughness properties were examined. The use of heat sinks is shown to result in a beneficial increase of the cross-weld yield strength but at the expense of the yield-to-tensile strength ratio. Due to its particularly detrimental effect on the heat-affected zone (HAZ) toughness of multipass welds, special attention was given in the Charpy-V toughness of the intercritically reheated coarse-grained HAZ (ICCGHAZ) by also testing simulated ICCGHAZs. It is shown that forced cooling has a beneficial effect in respect of the toughness of this simulated subzone and on the Charpy-V toughness of the HAZ of the actual welds. The interpass cooling time during the two-pass welding was reduced by 37%. The results indicate that, in the case of high-strength steels, it may be possible to simultaneously improve both welding productivity and mechanical properties by using forced cooling down to 100 °C to reduce waiting time between weld passes.

The effects of forced cooling, meaning forced cooling rate and forced cooling finish temperature, on the tensile and impact toughness properties of simulated weld coarse-grained heat-affected zones have been studied for a commercial grade martensitic steel with a yield strength of 960 MPa. The simulations were done by using a Gleeble 3800 to give forced cooling finish temperatures of 500, 400, 300, 200, and 100 °C and forced cooling rates of 50 and 15 °C/s. For the steel studied, strength significantly increased with no significant negative effects on impact toughness when the steel was cooled rapidly to 200 or 100 °C at 15 °C/s. The results indicate that it may be possible to improve welding productivity and mechanical properties of the steel by using forced cooling down to 100 °C to reduce waiting time between weld passes.

The usage of the high-strength steels in air bending is limited due to the insufficient cold formability of these materials. One way to extend the air bending process range is to expose the bent plates to high temperatures. In this contribution, radiant heating by means of infrared lamps is utilized for locally heating the plates. A radiant heating setup with two infrared lamps has been developed and mounted on a die with an opening of 60 mm. Plates of a cold-rolled ultra-high-strength structural steel Strenx 1300 with a thickness of 6 mm have been used for the testing purposes. The forming process has been performed by a punch with a radius of 10 mm. The forming fails when performed at ambient temperature because of plate fracture due to limited bendability of material. An increase of the forming temperature makes the forming possible, and at the same time decreases the bending force, springback, and bend allowance. The deterioration of the material strength was estimated by means of hardness measurements. The work hardening due to the deformation is shown to be not significant enough to compensate for the loss of strength due to the exposure to the elevated temperatures. The lowest temperature at which plate fracture does not occur is 400 °C and this case delivers the highest residual strength. In conclusion, with the present material and tooling dimensions, a forming temperature of 400 °C is recommended for successful forming with limited loss of strength characteristics.

Hot deformation and cooling together with the chemical composition define the final mechanical properties of the steel product. In order to control and optimize the desired properties, quantitative models are needed. In this article we present our phase transformation model, which has been fitted to experimental continuous cooling data and coupled to our own heat conduction computer simulation software. The fitting of the model to experimental data enables accurate modeling. The coupled heat transfer and conduction models allow the simulation of non-uniform cooling rates and temperature distribution. The development of our own software enables full control and easy linking of the models in order to use them in modeling and optimizing cooling procedures. The presented phase transformation model also provides information on the onset and kinetics, which can be used in more detailed microstructure models. The transformation model can also be introduced in other simulation softwares as a subroutine.

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.

New CCT equations have been developed and optimized to simulate the start temperatures of the austenite decomposition process in low-alloyed steels using experimental CCT data published in the literature. Exceptionally, this optimization does not apply the nominal compositions of the steels, but the corresponding soluble compositions of the grain boundaries calculated using IDS software, depending on the reported austenitization treatments of the steels. These compositions, rather than the nominal ones, are expected to control the start of the austenite decomposition, which usually initiates at the grain boundaries. The new optimization treatment takes into account the solute microsegregation and the possible precipitate formation. Using IDS software, the new equations were validated with new experimental CCT data. Agreement was good not only for the austenite decomposition start temperatures, but also for the final phase fractions, indicating fairly reasonable predictions of phase transformation kinetics by the IDS. In addition, IDS simulations were compared with the experimental CCT data of five high-carbon steels, applying both the new equations based on grain boundary soluble compositions as well as the equations based on the nominal compositions. With the same experimental CCT data used in optimization, better agreement was obtained with the new equations, indicating the importance of determining the soluble compositions at the grain boundaries where the austenite decomposition process is likely to begin.

Bendability is an important property of ultrahigh-strength steels since the typical applications of such materials include structures manufactured by air-bending. Conventional methods to evaluate bendability, such as the bending test according to the standard VDA-238 or the conventional tensile test do not provide sufficient information to evaluate bendability of ultrahigh-strength steels due to the average nature of the material response in these tests. In this study, the mechanical properties were determined using thin tensile specimens cut from the surface of the sheet and the evaluation of bendability was carried out using frictionless bending tests. The results of the experiments and FE-modelling presented in this paper reveal that the mechanical properties of the sheet surface have a significant impact on bendability. Novel ultrahigh-strength steel with better work-hardening capacity at the surface caused by a layer of relatively soft ferrite and lower bainite has good bendability, especially when the bend line is aligned transverse to the rolling direction. Microstructural investigations reveal that in a conventional steel with a relatively hard surface microstructure, the deformation localizes into shear bands that eventually lead to fracture, but similar shear banding was not present in the novel steel surface. This can be attributed to the better work-hardening capacity which delays the onset of shear localization and fracture.

The current trend in temper rolling is to make a small reduction to steel strip in order to achieve higher strength with good formability and toughness. In addition, very high strength steels can be cold rolled twice with very small reductions. This causes problems in setup values for cold rolling. Rolling models are usually overestimating roll flattening in the case of small reductions.In temper rolling thickness reduction is small (0.5 – 3%) and the elastic deformation of the work roll should be taken into account [3]. However, standard circular arc roll gap models (e.g. Bland Ford Ellis combined with Hitchcock model) fail to predict the roll flattening and thus the rolling force [4]. In this work, finite element method has been used to define a simplified model for work roll flattening and contact length. Model describes the effect of reduction, strength of steel strip and roll radius.

This paper presents an investigation into the mechanical properties of ultra-high strength steels (UHSSs) (with nominal yield stress of 960 and 1100 MPa) after welding. Seven weld thermal cycles were simulated using a Gleeble 3800 thermal-mechanical machine. These cycles represented the temperature-time history of the joint at various distances from the weld fusion line (FL) in a typical gas metal arc welding (GMAW) process. The mechanical properties such as Vickers surface hardness, uniaxial tensile behavior and Charpy impact toughness were examined. Microstructural evaluation using field emission scanning electron microscopy (FESEM) was also conducted. According to the results, the S960 which was a direct-quenched type of steel, showed a considerable reduction in its hardness and tensile strength up to 29 and 32%, respectively. On the other hand, the S1100 which was manufactured via quenched and tempered process, showed only a minor degree of softening at far distances from the weld FL (up to 4%) followed by 2% tensile strength reduction, and hardening close to the FL (up to 13%). Microstructure analysis has been carried out to provide insight to the change of mechanical properties in UHSS after welding.

The objective of this study is developing a thermo-metallurgical-mechanical finite element (FE) model incorporating the effect of solid-state phase transformation (SSPT) to accurately simulate deformations for single bead-on-plate welding of an ultra-high strength carbon steel. Comprehensive phase transformation modeling including both diffusive and diffusionless (displacive) transformation kinetics, was performed and the effect of SSPT on welding-induced deformations was investigated. Modelling the heat source and thermal boundary conditions were accomplished in the ABAQUS user subroutines, the former based on the Goldak’s double ellipsoidal heat source model. An ABAQUS user subroutine was developed in which kinetics of diffusive and diffusionless transformations based on Machnienko model and Koistinen-Marburger formula, respectively, were implemented. Modification of strains due to volumetric change as a result of SSPT was accomplished using an ABAQUS user-defined subroutine. A comparison between the temperature histories from thermal simulations (with isotropic as well as anisotropic conductivities) and measurement with thermocouples shows that much better verification with experiments can be obtained when anisotropic conductivity is applied. From the results of the mechanical simulations (with and without considering the effect of SSPT) and comparison with measured deformations, it is observable that more accurate prediction of welding-induced angular and bending distortions is possible when the effect of SSPT is incorporated for the material under investigation. Post-print / Final draft

A finite element (FE) simulation model illustrating the stress relaxation test was established with the Abaqus TM software. The microstructural evolution of steel during relaxation includes the complex phenomena of recrystallization. While the compression introduces the planned deformation and stress into the test piece, subsequent softening relieves the stress and at the same time creates microstructural reconstitution and refinement. In this study, a model was developed to simulate the kinetics of static recrystallization taking place during holding, using a technique based on FE-simulation. The simulation results have been compared to the experimental stress relaxation data obtained on a Gleeble TM 3800 thermo-mechanical simulator. The model can be used to estimate the recrystallization kinetics throughout the test piece. In the future, these results can be used for estimating the required rolling forces for multi-pass roughing with reasonable accuracy, for instance. The modelling methodology can be extended to other steels too, with or without microalloying additions.

Air-bending is a widely used forming process for ultra-high strength steel because it is fast, cost-effective and flexible way to form material. The bendability of a material can be expressed by minimum bending radius R/tmin, which is the relation of the smallest inner radius to the sheet thickness the material can be bent without damage. Damage usually occurs on the outer surface of the bend in the form of intense strain localization that further progresses to cracking. The minimum bending radius contains no other information that affects the bendability such as the lower tool width or the desired bending angle. Hence, developing more detailed test procedure is critical to better describe the behavior of ultra-high strength steel sheet in bending. In this paper, a method for more detailed assessment of bendability for ultra-high strength structural steel is presented. Using optical strain measuring techniques and digital image correlation coupled with bending tests in a universal tensile test machine one can measure the strain evolution at the outer surface of the bend and determine the critical strains that limit the bendability of these materials

In order to study the coupled effect of rolling deformation, heat transfer and water cooling on microstructure formation of a steel strip, a multiscale and multiphysical FE-model is developed. The deformation state and temperature distribution of the steel strip after the last rolling pass are required to calculate the final microstructure. The sub-model predicting final microstructure on the steel strip after accelerated water cooling is implemented numerically. Final phase fractions are calculated with the Johnson-Mehl-Avrami-Kolmogorov equation for different parts of the object, taking into account non-uniform temperature distribution.

In modeling of hot rolling pass schedules the heat transfer phenomena have to be known. Radiation to ambient, between rolls and a steel slab as well as heat transfer in contacts must be considered to achieve accurate temperature distribution and thereby accurate material behavior in simulations. Additional heat is generated by friction between the slab and the work roll and by plastic deformation. These phenomena must be taken into account when the effective heat transfer coefficient is determined from experimental data. In this paper we determine the effective heat transfer coefficient at the contact interface and emissivity factor of slab surface for 1100MPa strength carbon steel for hot rolling simulations. Experimental pilot rolling test were carried out and slab temperatures gathered right below the interface and at the mid thickness of the slab. Emissivity factor tests were carried out in the same manner but without rolling. Experimental data is utilized to derive contact heat transfer coefficient at the interface and emissivity factor of slab surface. Pilot rolling test is reproduced in FE-analysis to further refine the heat transfer coefficient and emissivity factor. Material mechanical properties at rolling temperatures were determined by Gleeble™ thermo-mechanical simulator and IDS thermodynamic-kinetic-empirical software.

The effects of forced cooling, i.e., forced cooling rate and forced cooling finish temperature, on the tensile and impact toughness properties of simulated weld coarse-grained heat-affected zones has been explored in the case of a low-carbon thermomechanically processed steel with a yield strength of 700 MPa. The forced cooling finish temperatures that were studied were 400, 300, 200, and 100 °C and the forced cooling rates were 50 and 15 °C/s. Coarse-grained heat-affected zones were simulated using a Gleeble 3800 thermomechanical simulator. For the steel concerned, strength and impact toughness improved significantly when the steel was cooled rapidly to 200 or 100 °C. The results indicate that it may be possible to substantially improve welding productivity by using forced cooling to reduce interpass times.

A novel two-dimensional cellular automata model for simulating the formation of lath martensite and bainite in steels during cooling is presented. The model is parameterized using fitted Johnson-Mehl-Avrami-Kolmogorov (JMAK) and Koistinen-Marburger equations and by comparing predictions to scanning electron microscopy images of the actual microstructures after cooling. The results of this simulation can be used to estimate the fractions, shapes and sizes of bainite and martensite for different cooling rates, which should offer new possibilities for the qualitative estimation of the mechanical properties of high-strength steels with bainitic – martensitic microstructures formed from recrystallized or thermomechanically deformed austenite.

We present computer simulations of austenite decomposition to ferrite and bainite during cooling. The phase transformation model is based on Johnson-Mehl-Avrami-Kolmogorov type equations. The model is parameterized by numerical fitting to continuous cooling data obtained with Gleeble thermo-mechanical simulator and it can be used for calculation of the transformation behavior occurring during cooling along any cooling path. The phase transformation model has been coupled with heat conduction simulations. The model includes separate parameters to account for the incubation stage and for the kinetics after the transformation has started. The incubation time is calculated with inversion of the CCT transformation start time. For heat conduction simulations we employed our own parallelized 2-dimensional finite difference code. In addition, the transformation model was also implemented as a subroutine in commercial finite-element software Abaqus which allows for the use of the model in various engineering applications.

Additive Manufacturing technology offers almost unlimited capacity when manufacturing parts with complex geometries which could be impossible to get with conventional manufacturing processes. This paper is based on the study of a particular real part which has been redesigned and manufactured using an AM process. The challenge consists of redesigning the geometry of an originally aluminium made part, in order to get a new stainless steel made model with same mechanical properties but with less weight. The new design is the result of a structural optimization process based on Finite Element simulations which is carried out bearing in mind the facilities that an AM process offers. The results of the structural optimization showed that the mechanical properties can be achieved but a lighter model made of stainless steel instead of aluminium was not possible to produce.

A model for static recrystallization by Zurob et al. [1] has been fitted to experimental stress relaxation [2] data obtained on a low-alloyed steel using a Gleeble thermomechanical simulator. The model has been implemented as an algorithm that calculates the stress relaxation as a function of time, including physical descriptions of the recovery and recrystallization processes. The activation energy and volume were used as fitting parameters for recovery, and the activation energy of diffusion and nucleation site density were used as the fitting parameters for recrystallization. The four fitting parameters were determined from the experimental data by applying the Nelder-Mead algorithm within Matlab software. It can be concluded from the preliminary results that Zurob’s model can be successfully fitted to the stress relaxation data in order to illustrate the static restoration characteristics and kinetics in carbon steels using these fitting parameters.

In this paper, authors present a highly flexible tooling system based on reconfigurable multi-point thermoforming (MPTF) methodology, which has been developed within an EU-granted FP7 project. The MPTF technology employs an actuated-punch matrix to dynamically configure a controllable tool working surface through digitally adjusting relative displacement of each punch in the matrix. Novel MPTF methods have been proposed through re-changing configurations of actuated-punch tooling system according to rapid thermoforming principles and relevant cladding applications. The tooling system includes an industrial-scale prototype of an MPTF tooling integrated with functional CAD/CAE/CAT software interfaces. The numerical simulation with an explicit FEM predicts the unexpected deformation defects of dimples and wrinkles regarding to discrete contact boundaries between punches and the sheet blank. Innovative techniques of variable blank-holder and deformable cushion have been implemented to suppress wrinkling and eliminate dimpling effectively. The tooling system has been successfully applied to manufacture complex double-curved panels, which are described as application examples. Compared with conventional fixed moulds, the flexible tooling offers robust, rapid and re-changeable means to make mould-less manufacturing large freeform panels.

In high- and ultrahigh-strength steel welding, interpass cooling time is an important factor affecting productivity and welding costs. Usually, welding heat input is restricted to meet the relatively short recommended cooling times between 800 and 500 °C (t8/5), which are prescribed by the need to meet weld strength and toughness properties. This, in turn, leads to the need for multipass welding with the interpass waiting times needed for the weld to cool to a sufficiently low interpass temperature. Welding productivity is affected by both the number of passes and the interpass waiting time. With a view to minimizing the total number of passes needed for a given preparation, it is beneficial for the interpass temperature to be as low as possible as this permits higher heat input for a given t8/5. On the other hand, low interpass temperature requires longer interpass waiting times. Therefore, this research concerns the potential of introducing copper heat sinks adjacent to the weld to reduce the time it takes for the weld to cool down to the interpass temperature. It is demonstrated that, in the case of a butt weld in a 6 mm thick base plate MAG welded with a weld energy of 1 kJ/mm and an interpass temperature of 100 °C, copper heat sinks almoust halve the interpass waiting time. This can have a marked effect on the overall productivity when welding highand ultrahigh-strength steels and increase their attractiveness for steel construction.

In temper rolling thickness reduction is small (0,5 – 2%) and the elastic deformation of the work roll should be taken into account. During finishing rolling, classical cold rolling theories fail to predict the roll flattening and thus the rolling force. Numerous different mathematical models have been developed for temper rolling process. However, often the non-circular theories are computationally expensive and the range of usability is questionable. In this study, elasto- plastic finite element analysis, laboratory rolling tests and inverse computing from skin pass mill process data has been carried out. The aim is to find the roll shape, contact length and the rolling force to create a simplified model for controlling of temper rolling. In addition, the ability of different skin pass rolling theories has been compared with measured process values.

The reliability of a press-fit connector depends on the strength of the contact in the connector. Therefore, an ability to predict the contact strength would enable optimization of the connectors and their production conditions. However, predicting the contact strength is a difficult, non-trivial task. In the present work, FE simulations were carried out using Abaqus software to study a standard press-fit connector. Numerical analysis of the mechanical behavior of the connector requires constitutive and tribological data on the CuSn 6 pin and the copper socket, the nickel and gold plating and the printed circuit board.

It is important to predict the mechanical properties of thin steel strip when it is produced by rolling. For this purpose we have used artificial neural networks (ANN) and the data acquisition system of Rautaruukki Hameenlinna Works. This paper shows that it is possible to predict accurately the mechanical properties thin steel strip, by using ANN modelling and the measurement data of the process. The prediction model can be used in different ways. For example the effects of process parameters can be evaluated and the controllable parameters can be changed in such a way that the process yields the desired mechanical properties.

In this study physical models and a neural network theory have been integrated to a program package in order to predict rolling force in cold rolling. The parameters required by the model such as the friction parameter and the deformation resistance of the materials have been determined from measured rolling parameters and materials alloying elements by applying the Bland-Ford-Ellis (BFE) rolling force model and an artificial neural network model (ANN). Measured data of over 6000 coils have been used in the training of the ANN. The calculated results were in good agreement with measurements.

The thickness profile of hot strip is of importance to profile, flatness and shape of the final cold rolled product. In this work, strip thickness and flatness profiles are decomposed into independent components by solving Chebyshev polynomials coefficients using matrix calculation. Four terms are used to characterize most common shapes of thickness and flatness profile. The calculated Chebyshev coefficients from different line measurements are combined together and analysed using neural network tools. The most common types of shapes are classified.

IDDRG - Conference Proceedings - Tools and Technologies for the Processing of ultra high strength steels

Forming and fracture limits of AA 3104 aluminum alloy and AISI 304 type stainless steel were studied. Hydraulic bulge testing was employed in the stretch forming experiments. Marciniak-type in-plane tests and tensile testing were carried out to study the limit strains in the deep drawing region. Both screen-printed and laser-marked grids were used to measure the surface strains. Although the forming limits of the AA 3104 were generally much lower than the AISI 304 stainless steel, they both failed in stretch forming by ductile shearing in the through thickness direction without any visible local necking. The preferred failure direction for AA 3104 alloy was transverse to the rolling direction, as could be observed in the bulge tests with a circular die. In the Marciniak-type in-plane tests the AA 3104 alloy showed multiple necking similar to what has previously been reported for low-carbon steel. In the tensile testing, central cracking and subsequent local necking as well as local necking without previous central crack formation were observed both for the AA 3104 aluminum alloy and the AISI 304 stainless steel.

There is urgent need to develop new advanced fast and cost‐effective mass‐production methods for small sheet metal components. Traditionally progressive dies have been designed by using various CAD techniques. Recent results in mass production of small sheet metal parts using progressive dies and a transfer press showed that the tool design time may be cut in up to a half by using 3D finite element simulation of forming. In numerical simulation of sheet metal forming better constitutive models are required to obtain more accurate results, reduce the time for tool design and cut the production costs further. Accurate models are needed to describe the initial yielding, subsequent work hardening and to predict the formability. In this work two commercially available finite element simulation codes, PAM‐STAMP and LS‐DYNA, were compared in forming of small austenitic stainless steel sheet part for electronic industry. Several constitutive models were used in both codes and the results were compared. Comparisons were made between the same models in each of the codes and also between different models in the same code. Material models ranged from very simple to advanced ones, which took into account anisotropy and both isotropic and kinematic hardening behavior. In order to make a valid comparison we employed similar finite element meshes. The effects of the material models parameters were studied and the results were compared with experiments. The effects of the computational time were also studied.

Cold forming of magnesium and titanium is difficult due to their hexagonal crystal structure and limited number of available slip systems. However, high velocity deformation can be quite effective in increasing the forming limits. In this study, electromagnetic forming (EMF) of thin AZ31B-O magnesium and CP grade 1 titanium sheets were compared with normal deep drawing. Same dies were used in both forming processes. Finite element (FE) simulations were carried out to improve the EMF process parameters. Constitutive data was determined using Split Hopkinson Pressure Bar tests (SHPB). To study formability, sample sheets were electromagnetically launched to the female die, using a flat spiral electromagnetic coil and aluminum driver sheets. Deep drawing tests were made by a laboratory press-machine. Results show that high velocity forming processes increase the formability of Magnesium and Titanium sheets although process parameters have to be carefully tuned to obtain good results.

This chapter discusses the application of advanced modelling tools in the plastic forming and processing of metals. The tools include numerical modelling techniques, such as the finite element method (FEM), and more recently, artificial intelligence techniques, such as various types of artificial neural networks as well as data mining. Examples of real production processes, such as the production of sheet by rolling, deep drawing of stainless steel kitchen sinks, continuous extrusion of electrical conductors as well cutting of metal and tube hydroforming, will be presented. It is shown that defects can be avoided and tool life increased through optimization of plastic forming processes.

Summary Finite element modeling of the hot rolling of steel plates was studied. The finite element code ABAQUS was complemented by a recrystallization model called MICROPLA. It appeared that the rolling force as well as the final grain size and the yield strength of the plate could be predicted with a fairly good accuracy. To study the accelerated cooling after rolling. the phase transformations were taken into account approximately by considering the different cooling rates in the center and at the edges of the plate. The predicted distortion of the plate was reduced about tenfold after the phase transformations were taken into account.

Applications of neural networks in the rolling of steel are reviewed. The first papers on the topic were published in 1991 and since then the number of publications has steadily increased. In most applications today, so-called back propagation networks are used. After briefly reviewing the various neural network types, the results of two case studies at Rautaruukki cold strip mill are presented. In the first case an efficiency model for tandem cold rolling was developed. By using the model it is possible to study whether a new product with a given width, strength or thickness can be produced, and the optimised mill settings can then be determined. A 1.8% improvement in efficiency was obtained with the model. The second case concerns the prediction of the mechanical properties of steel strips and temper rolling force by using neural network modelling and measured process data. The location of the coils in annealing stacks and their vanadium content were found to explain the deviation in mechanical properties. The temper rolling force could be predicted with good accuracy, which can be exploited in determining mill pre-settings.