Göran Engberg

Physical Modeling on Recrystallization of Austenite in Steels in Thermo-Mechanical Processing

A physical model for austenite recrystallization of steel concerning TMCP is developed. Dislocation density plays a key role as recrystallization driving force. The dislocation density change is a result of competition between dislocation generation and dynamic recovery. Recrystallization is described as a nucleation-growth process. An abnormal subgrain growth mechanism is introduced for nucleation. A few subgrains fulfilling abnormal growth conditions will stand out and become nuclei of recrystallization. The recrystallized grain grows to the deformed materials driven by the stored energy. Oswald ripening occurs for grains surrounded by recrystallized grains. The models were verified by laboratory simulation results for selected austenite stainless steels. It showed good agreement between predicted and experimental results.

A Model for Particle Dissolution and Precipitation in HSLA Steels

Precipitation of carbonitrides has been studied in as-cast slabs of one Nb and one Nb and Ti containing HSLA steel. The precipitates have been quantified using LOM and TEM. The measured size and number distributions was then compared to model calculations of precipitate nucleation and growth using estimates of the cooling rates in the austenitic range (1490oC to 800oC) during casting. Both average size and number distributions could be modelled with good agreement using identical model parameters (except for individual diffusion coefficients for the participating species). The model is based on classic nucleation rate theory and a quasistationary approximation for growth of spherical particles. Local equilibrium is assumed at the phase boundary.

Microstructure Evolution in an Austenitic Stainless Steel during Wire Rolling

A wire rod block at Fagersta Stainless AB, Sweden, consists of eight pairs of rolls with consecutive round-oval-round grooves. Test bars of an austenitic stainless steel of type AISI 304L that had been preheated to 930±70°C were manually fed into the wire block. By entering a guide after one of the roll pair, the bar was led out from the block into a water-filled tube for rapid quenching. The guide was moved successively from the first to the last pair of rolls and test bars were collected after each roll pair. In order to characterize the original structure one bar was preheated and directly water quenched without rolling. The aim of this study was to characterize the microstructure evolution during the wire rod rolling using electron backscatter diffraction. The size evolution for all grains, the recrystallized grains and for the subgrains in the deformed grains has been estimated and the fraction of recrystallized grains has been determined. During the first 3 passes almost no recrystallization is observed and strain accumulates. Partial recrystallization then occurs and for the last 3 passes the recrystallization is almost complete and the texture is nearly random.

Prediction of the microstructural evolution during hot strip rolling of Nb microalloyed steels

A physically based model is used to describe the microstructural evolution of Nb microalloyed steels during hot rolling. The model is based on a physical description of dislocation density evolution, where the generation and recovery of dislocations determines the flow stress and also the driving force for recrystallization. In the model, abnormally growing subgrains are assumed to be the nuclei of recrystallized grains and recrystallization starts when the subgrains reach a critical size and configuration. The model is used to predict the flow stress during rolling in SSAB Tunnplåt’s hot strip mill. The predicted flow stress in each stand was compared to the stresses calculated by a friction-hill roll-force model. Good fit is obtained between the predicted values by the microstructure model and the measured mill data, with an agreement generally within the interval ±15%.

Modeling microstructure development during hot working of an austenitic stainless steel

A number of physically based models are combined in order to predict microstructure development during hot deformation. The models treat average values for the generation and recovery of vacancies and dislocations, recrystallization and grain growth and the dissolution and precipitation of second phase particles. The models are applied to a number of laboratory experiments made on 304 austenitic stainless steel and the model parameters are adjusted from those used for low alloyed steel mainly in order to obtain the right kinetics for the influence of solute drag on climb of dislocations and on grain growth. The thermodynamic data are obtained using Thermo-Calc© to create solubility products for the possible secondary phases. One case of wire rolling has been analyzed mainly concerning the evolution of recrystallization and grain size. The time, temperature and strain history has been derived using process information. The models are shown to give a fair description of the microstructure development during hot working of the studied austenitic stainless steel.