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Sgarabotto, Francesco (2014) Investigation of tribological properties of coated high strength steels in hot stamping. [Tesi di dottorato]

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Abstract (inglese)

The demand for weight reduction in vehicles has increased significantly over the last ten years, since the new regulations for CO2 emissions control were introduced. Furthermore, the enhanced safety requirements have promoted the development of new materials with high strength-to-weight ratio such as the high strength steel (HSS) formed at high temperatures. By using HSS hot stamped several improvements have been made – if compared with cold forming process – such as the forming forces reduction, the achievement of more complex features, the springback phenomena reduction and the component mechanical properties increase. Therefore, it has been possible to significantly reduce automobiles weight, maintaining the structural strength and safety requirements.
The process that used to form the boron steel grade 22MnB5 HSS is the hot stamping, which consists in heating a metal sheet up to austenitization temperature and then a simultaneous forming and quenching phase in closed dies to obtain martensite microstructure on the final components. Thus, ultimate tensile strength passes from 600MPa to 1500MPa.
Nevertheless hot stamping involves also some critical aspects, such as the severe tribological conditions dealing with the elevated temperature of die-blank sliding surfaces. Problems such as oxidation, material transfer, surfaces damage, wear and high friction influence the workpieces surface quality, dies durability and overall process.
A review of the literature has shown that friction behaviour of uncoated and Al-Si coated HSS has been studied by using mainly strip drawing simulative testing apparatus. The influence of different process parameters on friction coefficient – such as temperature and contact pressure – have been investigated, but results are still incomplete or not comparable.
Instead, the studies of wear mechanisms characterizing hot forming dies have revealed to be almost unexplored in several aspects. The reported studies have been focusing on conventional wear testing configurations, which are suitable to achieve fundamental knowledge on wear evolution and mechanisms, but fail in replicating the thermal and mechanical conditions to which the forming dies are subject during the industrial process. In fact, they usually do not apply any thermal cycle to the material acting as the tool, carrying out the tests at constant temperature. Otherwise, the wear phenomena has been studied through costly and time- consuming industrial trials, without any control on process parameters.
Therefore, the main target of this PhD thesis is devoted to the development of innovative approaches, based on the design of both experimental apparatus and procedures, to accurately describe and investigate tribological phenomena of sliding surfaces during hot stamping process of coated high strength steel.
To this aim, a novel simulative testing apparatus was designed to evaluate the influence of different process parameters on the friction coefficient during the hot forming process. Furthermore, a new approach of wear test based on pin-on-disc was implemented in order to:
• evaluate the main mechanisms responsible of tools wear during industrial press hardening process;
• apply controlled thermo-mechanical stress on pin materials during the test.
In order to prove the proposed procedures, diffusion and friction tests were performed in hot stamping conditions by using Al-Si and Zn hot-dip galvanized 22MnB5 high strength steel sheet. Results showed the reliability of experimental apparatus and the influence of process parameters on friction coefficients and diffusion phenomena on the coating. Furthermore Zn coating revealed a lower friction coefficient than the Al-Si one.
The new approach to wear test in hot stamping condition was proved in terms of replication of thermo-mechanical stress applicable on pin surfaces. Afterwards, three different types of die materials were tested. By using specific surface investigation tools, it was possible to analyze the main wear mechanisms, allowing a better understanding of the fundamentals of friction and wear phenomena characterizing hot stamping process of coated high strength steels. Finally the best performing material in terms of wear resistance was identified.

Abstract (italiano)

Negli ultimi anni l’esigenza di ridurre il peso dei veicoli è notevolmente aumentata in seguito all’entrata in vigore di nuove regole per il controllo delle emissioni di CO2; inoltre, le continue richieste di miglioramento della sicurezza dei passeggeri hanno promosso lo sviluppo di nuovi materiali con elevato rapporto peso-resistenza, come gli acciai alto resistenziali formati ad alte temperature. Grazie all’utilizzo di questi materiali stampati a caldo sono stati possibili notevoli miglioramenti - anche rispetto ai processi di formatura a freddo - come la riduzione delle forze di formatura, la possibilità di ottenere geometrie più complesse, la riduzione dei fenomeni di ritorno elastico e il miglioramento delle proprietà meccaniche finali del componente. Questo ha portato ad una significativa riduzione del peso delle autovetture, pur mantenendo inalterate la resistenza della struttura e i requisiti di sicurezza imposti.
Il processo usato per formare gli acciai alto resistenziali a base di boro 22MnB5 è lo stampaggio a caldo, il quale prevede il riscaldamento di un foglio di lamiera al di sopra della temperatura di austenitizzazione e le successive operazioni di formatura e tempra all’interno di stampi chiusi che avvengono simultaneamente. L’obiettivo è quello di ottenere una microstruttura martensitica nel componente finale, che consente di aumentare il carico di rottura finale da 600MPa fino a 1500MPa.
Tuttavia, il processo di stampaggio a caldo presenta una serie di aspetti molto critici, quali per esempio le condizioni tribologiche connesse alle elevate temperature tra lamiera e superficie dello stampo. Problematiche come l’ossidazione, il trasferimento di materiale, il danneggiamento delle superfici, l’usura e l’alto attrito influenzano la qualità superficiale del componente, la durabilità degli stampi e l’interno processo.
Uno sguardo alla letteratura rivela che lo studio dei fenomeni di attrito di acciai alto resistenziali rivestiti e non rivestiti sono valutati utilizzando principalmente l’apparato sperimentale “strip drawing”. L’influenza dei differenti parametri di processi sul coefficienti di attrito – quali temperatura e pressione normale – è stata individuata, ma i risultati non sono completi e poco comparabili tra loro.
Lo studio dei fenomeni di usura che caratterizzano gli stampi usati nello stampaggio a caldo si è invece rivelato essere un campo per numerosi aspetti ancora inesplorato. I principali studi presenti in letteratura hanno focalizzato l’attenzione su test di usura convenzionali, molto utili per ottenere conoscenze sui fondamenti dell’evoluzione dell’usura, ma incapaci di riprodurre le stesse condizioni meccaniche e termiche a cui sono realmente sottoposti gli stampi durante il processo industriale. Infatti, nel corso di questi test non vengono solitamente applicati cicli termici al materiale, mantenendo la temperatura costante. In alternativa, lo studio dei fenomeni di usura viene eseguito con costose e lunghe prove industriali, che non consentono però di controllare separatamente alcun parametro di processo.
L’obbiettivo di questa tesi di dottorato è dunque rivolto principalmente allo sviluppo di approcci innovativi, basati sulla progettazione sia di apparati sperimentali che di procedure, per descrivere e studiare accuratamente i fenomeni tribologici sulle superfici di scorrimento durante i processi di stampaggio a caldo di acciai alto resistenziali rivestiti.
Per raggiungere questo scopo, è stata progettata una nuova macchina di prova simulativa che consente di valutare l’influenza dei differenti parametri di processo sul coefficiente di attrito durante lo stampaggio a caldo. Inoltre, è stato implementato un nuovo approccio basato sul pin-on-disc test, al fine di:
• valutare i principali meccanismi responsabili dell’usura degli stampi durante lo stampaggio a caldo;
• applicare stress meccanici e termici controllati sul materiale del pin durante la prova.
Al fine di validare le procedure proposte sono stati eseguiti test di diffusione e di attrito in condizioni di stampaggio a caldo utilizzando lamiere alto resistenziali rivestite con Al-Si e Zn (applicati con processi di deposizione galvanica a caldo). I risultati ottenuti hanno dimostrato l’efficacia del nuovo apparato sperimentale e l’influenza dei parametri di processo sui coefficienti di attrito e sui fenomeni diffusivi nel rivestimento. Inoltre è emerso che il rivestimento a base di Zn permette l’ottenimento di coefficienti di attrito inferiori rispetto a quello a base di Al-Si.
Il nuovo approccio per test di usura in condizioni di stampaggio a caldo è stato validato in termini di replicazione degli stress termo-meccanici applicati sulla superficie del pin e, successivamente, sono stati testati anche tre differenti materiali per stampi. Utilizzando specifici mezzi di investigazione delle superficie, è stato possibile analizzare i principali meccanismi di usura, permettendo una migliore comprensione di principi che stanno alla base dei fenomeni di attrito e usura nello stampaggio a caldo di acciai alto resistenziali rivestiti. Infine è stato individuato il materiale più performante in termini di resistenza all’usura.

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Tipo di EPrint:Tesi di dottorato
Relatore:Ghiotti, Andrea
Dottorato (corsi e scuole):Ciclo 26 > Scuole 26 > INGEGNERIA INDUSTRIALE > INGEGNERIA CHIMICA, DEI MATERIALI E DELLA PRODUZIONE
Data di deposito della tesi:26 Gennaio 2014
Anno di Pubblicazione:26 Gennaio 2014
Parole chiave (italiano / inglese):Tribologia/tribology, stampaggio a caldo/hot stamping, alta temperatura/high temperature, attrito/friction, usura/wear, acciai alto resistenziali/high strength steel
Settori scientifico-disciplinari MIUR:Area 09 - Ingegneria industriale e dell'informazione > ING-IND/17 Impianti industriali meccanici
Area 09 - Ingegneria industriale e dell'informazione > ING-IND/16 Tecnologie e sistemi di lavorazione
Struttura di riferimento:Dipartimenti > Dipartimento di Ingegneria Industriale
Codice ID:6381
Depositato il:04 Nov 2014 12:18
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