The effect of ultra-fast heat treatments on the microstructure evolusion of automotive steels

Abstract

This thesis aims to offer a detailed understanding of the transformation phenomena that occur when high heating rates are applied to automotive steel grades. It is focused on the evolution of the microstructure through the heating and cooling stages of the thermal treatment. By comprehending these phenomena, Ultra-Fast Heating treatments are better controlled with the purpose to improve existing grades and design new ones. Such grades can be incorporated into the current 3rd Generation Advanced High Strength Steels that consist of similar microstructures and exhibit desired mechanical properties. To that end, the effect of the heating rate is studied, using characterization techniques, on the cementite spheroidization and dissolution, the recrystallization of ferrite, and the nucleation and growth of austenite, which eventually lead to the formation of bainite and martensite, and the retainment of austenite upon quenching. To better analyze these mechanisms, three different approaches were made. On the first approach, the goal is to study the effect of the chemical composition, initial microstructure, heating rate, heating time, and soaking time on the microstructure when high heating rates are applied in laboratory-scale samples. The purpose of these experiments is to determine the optimal sample and treatment parameters that will lead to the desired microstructures. Therefore, three different automotive steel grades are used that undergo different UFHtreatments on a laboratory scale, and their microstructures are characterized. The results of these experiments set the stepping stone for the second approach, in which, an ultra-fast heating cycle was studied on industry scale samples with the scope to determine whether such treatments can be applied in the industry. The compositions and thermal treatments are chosen for this set according to the results of the laboratory-scale experiments. The final approach's scope is to check whether such heat treatments can replace currently used conventional steel grades. To that end, high heating rates were applied on commercial steel grades and the microstructure and properties were compared to the conventional ones. The first approach (Chapter 8.1) comprises a set of three different experiments. Its purpose is to study smallsized samples to comprehend the effect of the parameters mentioned above better. The heat treatments areconducted with the use of dilatometers. Three different automotive grades with medium carbon content were studied, the C45, 42CrMo4, and 42CrMn6. Also, the initial microstructure would consist either of ferrite and pearlite or ferrite and spheroidized cementite. The heating treatments consist of high heating rates (300 °C/s), up to the peak temperature (900-1080 °C), and either short (2 s) or long (300 s) soaking times followed by quenching. For comparison reasons, conventional heating treatments (10 °C/s heating rate) were also studied. The analysis of the microstructure took place with the use of electron microscopy techniques, such as Secondary Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), and Electron BackScatterDiffraction (EBSD). The characterization is focused on the microstructural constituents and their grain size. Simulation software is also used for analyzing the size and composition of the Parent Austenite Grains (PAGs). It is found that the dissolution of cementite is impeded to a large extent, depending on its initial shape and size. This undissolved cementite leads to the refinement of the parent austenite grains and heterogeneity in its chemical composition, creating gradients in the carbon content. These gradients can lead to the formation of bainite and retained austenite in the final microstructure, as has been indicated. Also, alloying elements, such as chromium and molybdenum, impede cementite dissolution through the solute drag effect. From these results, the optimal parameters were chosen for the second set of experiments on industrial scale samples. With the optimal parameters determined, high heating rates were applied (Chapter 8.2) on large-scale hollow tube samples, similar to those used in the industry, using a pilot setup designed by the supervisor. This approach aims to study whether such treatments are viable in a production line. The chosen composition is that of the 42CrMo4, with ferrite-pearlite initial microstructure. The heating rate used is 100 °C/s, up to the annealing temperature of 930 °C followed by subsequent quenching in water. The formation of lower bainite alongside the retainment of austenite films found in the dilatometry samples is now confirmed with the TEM. The presence of such constituents is attributed to the chemical heterogeneity that was found in the laboratory-scale samples. Regions that are not enriched in carbon favor bainite formation, while regions in small proximity to carbon sources favor the formation of martensite and retainment of austenite. The third and final approach (Chapter 8.3) aims to study whether the application of an ultra-fast heat treatment can replace conventional methods currently used in the automotive industry. Therefore, very high heating rates were applied in a commercial steel grade that is thoroughly used in vehicles' bodies. The microstructure features and the studied material properties are compared to the conventional, commercial grade. It is foundthat the partial spheroidization and dissolution of cementite found in the pearlitic colonies of the initial microstructure favors the refinement of the ferrite, parent austenite grains, and martensite of the final microstructure. The evolution of the microstructure is thoroughly studied. The recovery and recrystallization processes initiate at higher temperatures when high heating rates are applied, which overlap with the austenitization and are incomplete. The nucleation of austenite occurs inside the pearlitic colonies due to the high carbon concentration and ferritic areas without any carbon enrichment. Traces of retained austenite are found inside the partially dissolved pearlitic colonies. The ultra-fast heated samples show increased strength and elongation compared to the conventional ones. From this thesis, it is concluded that in ultra-fast heat treatments, due to cementite's impeded and incomplete dissolution, heterogeneity in the parent austenite's chemical composition and grain size is formed. This heterogeneity explains the bainite formation alongside martensite and the retainment of austenite. Such grades with mixed microstructures can be incorporated to the 3rd Generation AHSS that exhibit spectacular properties. In general, the ultra-fast heated steels exhibit better strength/ductility ratio than the conventional heated steels. The mechanical properties of these steels result from their refined mixed microstructures. In contrast, in the conventional heated steels, the microstructure consists only of martensite, and thus their ductility is expected relatively low. It is also proven that the strength of the steel can be increased without significant losses in the ductility, when high heating rates are applied. The experimental approach used in this thesis, confirms through characterization techniques the ability to produce mixed microstructures in a single step, by explaining the simultaneous formation of bainite and martensite, and the retainment of austenite in ultra-fast heating cycles, as has been predicted via simulation experiments.

Αυτή η διατριβή έχει ως στόχο τη λεπτομερή κατανόηση των φαινομένων μετασχηματισμού που λαμβάνουν χώρα όταν εφαρμόζονται υψηλοί ρυθμοί θέρμανσης σε τύπους χαλύβων της αυτοκινητοβιομηχανίας. Επικεντρώνεται στην εξέλιξη της μικροδομής μέσω των σταδίων θέρμανσης και ψύξης της θερμικής επεξεργασίας. Με την κατανόηση αυτών των φαινομένων, οι κατεργασίες Ultra-Fast Heating μπορούν αν ελεγχθούν καλύτερα με σκοπό τη βελτιστοποίηση των υπαρχόντων βαθμών και τη σχεδίαση νέων. Τέτοιοι βαθμοί μπορούν να ενσωματωθούν στους τρέχοντες Προηγμένους Χάλυβες Υψηλής Αντοχής 3ης γενιάς που συνίστανται από παρόμοιες μικροδομές και παρουσιάζουν ιδανικές μηχανικές ιδιότητες. Για το σκοπό αυτό,μελετάται η επίδραση του ρυθμού θέρμανσης, μέσω της χρήσης τεχνικών χαρακτηρισμού, στη σφαιροποίηση και διάλυση του σεμεντίτη, στην ανακρυστάλλωση του φερρίτη και στην πυρήνωση και ανάπτυξη του ωστενίτη, τα οποία τελικά οδηγούν στο σχηματισμό μπαινίτη και μαρτενσίτη, και σε υπολειπόμενο ωστενίτη κατά την βαφή. Από τα πειράματα, προκύπτει το συμπέρασμα ότι σε τάχιστες θερμικές επεξεργασίες, λόγω της καθυστερημένης και ατελούς διαλυτοποίησης του σεμεντίτη, εμφανίζεται ετερογένεια στη χημική σύσταση και στο μέγεθος κόκκου του πρότερου ωστενίτη. Αυτή η ετερογένεια έρχεται να εξηγήσει τον σχηματισμό του μπαινίτη παράλληλα με τον μαρτενσίτη και τη διατήρηση του ωστενίτη. Γενικά, οι χάλυβεςπου έχουν υποστεί τάχιστη θερμική κατεργασία εμφανίζουν καλύτερη αναλογία αντοχής / ολκιμότητας από τους συμβατικούς θερμαινόμενους χάλυβες. Αυτές οι μηχανικές ιδιότητες προκύπτουν από τις εκλεπτυσμένες μικτές μικροδομές τους. Αντιθέτως, στους χάλυβες που έχουν υποστεί συμβατικές θερμικές κατεργασίες, η μικροδομή αποτελείται μόνο από μαρτενσίτη, και έτσι η ολκιμότητα τους αναμένεται σχετικά χαμηλή. Αποδεικνύεται επίσης ότι η αντοχή του χάλυβα μπορεί να αυξηθεί χωρίς σημαντικές απώλειες στην ολκιμότητα, όταν εφαρμόζονται υψηλοί ρυθμοί θέρμανσης. Η πειραματική προσέγγιση που χρησιμοποιείται σε αυτή τη διατριβή, επιβεβαιώνει μέσω τεχνικών χαρακτηρισμού την ικανότητα παραγωγής μικτών μικροδομών σε ένα μόνο βήμα, εξηγώντας τον ταυτόχρονο σχηματισμό μπαινίτη και μαρτενσίτη, και τη διατήρηση του ωστενίτη σε εξαιρετικά γρήγορους κύκλους θέρμανσης, όπως έχει προβλεφθεί μέσωπειραμάτων προσομοίωσης.