dello chassis. La sesta generazione di Volkswagwn Passat (Fig.4.2)del 2005 aveva 13 parti formate a caldo che andavano a costituire il 19% della massa dello chassis. Dal 2012 molti veicoli hanno sorpassato il 20% in massa dello chassis costituito da componenti stampati a caldo. La prima, nel 2012, fu la Volvo V40, poi la piatta- forma MQB di Volkswagen (24% - 28%) Al momento l’automobile in produzione con la maggior quantità di acciai per stampaggio al boro contenuta è la seconda generazione della Volvo XC90 che contiene ben il 38% in massa dello chassis. Questi dati ci fanno notare l’eccezionale successo commerciale che questo tipo di acciai sta avendo nell’industria automobilistica, preso atto di questo, si rende necessaria una trattazione sistematica circa la resistenza di questi acciai, particolarmente soggetti all’infragilimento da idrogeno.
granted in 1914 in Switzerland. The technique described in this patent has been used in agricultural products since 1930s[20,30,31].
Hot forming as we know it today started in 1970s in Sweden. In 1973, Norrbottens Ja¨rnverks (which later merged to form SSAB) applied for the first “modern hot forming” patent, which was granted in 1977 [32]. In 1975, a 6-year R&D project was started by Volvo Trucks, Norrbottens Ja¨rnverks, and Lulea˚ University of Technology[33].
The first automotive application was side impact door beams of the SAAB 9000 which went to production in 1984. In 1986 Jaguar XJ and in 1993 Ford Mondeo also used hot formed door beams [34,35]. Ford Mondeo’s North American versions Ford Contour and Mercury Mystique were the first American cars to have hot formed components[34,36]. In 1996, the first bumper beam was used by Renault [37]. In late 1990s the earliest hot formed B-pillar studies were done at SAAB [38]. The earliest hot formed A-pillar reinforcements were used in BMW E46 Cabrio (Production started in March 2000) and Citroe¨n C5 (Production started in March 2001) [39,40]. Peugeot 307 (Production started in 2001) had hot formed B-pillar reinforcements and roof rail[41].
One of the biggest breakthroughs was the first generation Volvo XC90, which was introduced in 2002. There were a total of 10 hot formed parts, which accounted for 7% mass of the body-in-white. The second breakthrough was the 6th generation Volkswagen Passat (Production started in 2005). This vehicle had a total of 13 hot formed parts (parts shown in Fig. 12.5), which accounted for 19% of the body mass[4244].
Since 2012, several vehicles have surpassed 20% barrier of hot formed body components. The first one was Volvo V40 (SOP 2012) with 20% hot stamped parts by mass. Later on several Volkswagen Group vehicles on MQB platform were introduced; these vehicles typically have 2428% of hot formed body by mass. These vehicles include, but are not limited to, Audi A3 (3rd generation), VW Golf 7, VW Passat B8, Seat Leon Mk3 and ˇSkoda Octavia Mk3 [45]. However, as of 2016, the highest usage of hot formed steels is in Volvo XC90 (2nd generation, production started in 2014), which accounts for 38% of the body mass [46].
Hot stamped boron steel
Other steels
81%
19%
Figure 12.5 Hot stamped boron steel components in Passat B6 (SOP 2005)[42].
391 Hot formed steels
Figura 4.2: Componenti realizzate con acciai da stampaggio a caldo su VW Passat 6th generazione
4.3
Acciai da stampaggio a caldo
Questa speciale famiglia di acciai AHSS fu usata inizialmente esclusivamente per le parti adibite alla costruzione della cella di sicurezza. Ovvero per quelle parti che dovevano garantire una elevata resistenza a rottura con modesti allungamenti, dato che non erano adibiti a scopi di assorbimento energetico da impatto. In seguito si sono resi disponibili materiali della stessa famiglia adatti anche ad assorbire grossi quantitativi di energia
4.3.1
Intrusion Resistant
Il primo acciaio di questa famiglia ad essere commercializzato è tuttora il più diffuso e usato. Si tratta del 22MnB5 con carichi di Yse Rm pari a 1100/1500 MPa a fronte di allungamenti percentuali inferiori al 7%
4 – Acciai automotive per formatura a caldo
I maggiori produttori di questo acciaio sono Arcelor Mittal con la linea USIBOR e Thyssen-Krupp con la linea MBW i rivestimenti commercializzati sono di Al-Si o Zn-Ni e verranno descritti nel capitolo 4.5
Dal primo 22MnB5 ne sono poi stati derivati altri variando le percentuali di elementi chimici all’interno e che sono in fase di pre-commercializzazione.
Questi acciai sono forniti in condizioni ferritico-perlitiche e devono essere auste- nitizzati in forni con atmosfera controllata prima di essere sottoposti al trattamento di tempra
In tabella 4.1 si riportano le percentuali di elementi chimici degli acciai ad ora presenti in questa famiglia evidenziandone le caratteristiche meccaniche in tabella 4.2 si riportano i valori di resistenza per il metallo come fornito e temprato
Steel Al B C Cr Mn Ni Si Ti
22MnB5 0.03 0.002 0.23 0.16 1.18 0.12 0.22 0.040
28MnB5 - 0.005 0.28 - 1.30 - 0.40 -
34MnB5 - 0.005 0.34 - 1.30 - 0.40 -
Tabella 4.1: Composizione chimica dei principali acciai Manganese-Boro
Steel YS (MPa) UTS (MPa) YS (MPa) UTS (MPa)As delivered Quenched
22MnB5 457 608 1010 1478
28MnB5 - - 1135 1740
37MnB4 580 810 1378 2040
Tabella 4.2: Composizione chimica dei principali acciai Manganese-Boro In figura 4.3 si confrontano tre acciai della famiglia. I problemi maggiori ai quali si va incontro con l’incremento delle proprietà meccaniche sono principalmente 3
1. La più bassa capacità di assorbire energia 2. I fenomeni di infragilimento da idrogeno 3. La minore saldabilità
4.3.2
Energy absorbing
Gli acciai discussi nel sottoparagrafo precedente dopo la tempra hanno un altissima resistenza ma una bassa duttilità che riduce la quantità di energia che possono disperdere durante un impatto. Per aumentare la quantità di energia assorbibile sono state abbassate le percentuali di carbonio nella composizione. Ci sono due aree di sviluppo in questo settore al momento.
4.3 – Acciai da stampaggio a caldo
of 600610 HV
[10]. Similarly, Overrath
[71]
found
B490 HV for MBW
s1500,
B530 and B580 HV for MBW
s1900.
The first commercially available “higher-strength hot forming steel” (i.e., UTS
. 1600 MPa) was Sumitomo Steel’s SQ1800. This steel was used in the front and
rear bumper beams of the Mazda CX-5 (production started in 2011). Studies have
shown that 1.4 mm SQ1800 has the same intrusion resistance as 1.6 mm SQ1500
(22MnB5). Thus, by using SQ1800, Mazda could save 4.8 kg per vehicle
[65,75].
In 2013, ThyssenKrupp started delivering MBW 1900 (34MnB5). Docol 1800
Bor from SSAB has 0.27 wt.% carbon. This steel has been available since 2013 and
is similar to 28MnB5 shown in
Table 12.1
and
Fig. 12.7 [74]. SSAB is also
developing a 37MnB4 grade with the commercial name Docol 2000 Bor
[76].
POSCO is working on HPF1800 and HPF2000, two new grades already available
for customer trials
[4]. HPF2000 was already demonstrated in a number of proto-
type car bodies, including POSCO’s own PBC-EV (POSCO Body Concept-Electric
0 5 10 15 20 25 0 500 1000 1500 2000 En g in e e ri n g st re s s ( M Pa ) Engineering strain (%) 22MnB5 (USIBOR 1500, MBW 1500, HPF 1470, BTR165, phs-ultraform) 34MnB5 / 37MnB4 (USIBOR 2000, MBW 1900, HPF 2000, Docol 2000 Bor) 28MnB5 / 30MnB5 (SQ1800, USIBOR 1800, Docol 1800 Bor)
Figure 12.7
Engineering strain-stress diagram for 22MnB5 and higher strength boron steels.
Source: Recreated after K. Hikita, et al., Properties of new TS1800 MPa grade hot staming
steel and components, in: Materials in Car Body Engineering 2012, May 1011, Bad
Nauheim, Germany, 2012; S. Graff, T. Gerber, F.-J. Lenze, S. Sikora, About the simulation
of microstructure evolution in the hot sheet stamping process and the correlation of resulting
mechanical properties and crash-performance, Proceedings of chs2 2011, 2011, pp.
323330; H. Lanzerath, et al., Simulation: enabler for the efficient design of lightweight
boron intensive body structures, in: Insight Edition Conference, September 2021,
Gothenburg, Sweden, 2011; J. Overrath, F.-J. Lenze, S. Sikora, Aktuelle Entwicklung der
Warmumformung im Automobilen Fahrzeugbau, (Current Developments of Hot Forming in
Automobiles in German), in 30. EFB-Kolloquium, March 23, Bad Boll, Germany, 2010;
H.W. Lee, K.-H. Chung, A new body concept for electric vehicle, in: Materials in Car Body
Engineering 2012, May 11, Bad Nauheim, Germany, 2012; H. Matsuoka, K. Fujihara, Mazda
CX-5, in: EuroCarBody 2011, October 1820, Bad Nauheim, Germany, 2011; and K.
Fahlstro¨m, Laser welding of boron steels for light-weight vehicle applications, Licentiate
Thesis, Ho¨gskolan Va¨st, 2015
[65,68,69,7174].
395 Hot formed steels
Figura 4.3: Possibili impieghi di acciai al boro in uno chassis automobilistico
1. Acciai con allungamenti a rottura maggiori del 15% e con Rm di 450 - 500 MPa come il DUCTIBOR 500
2. Acciai con allungamenti a rottura maggiori del 5% e con Rm di 1000 - 1300 MPa come il DUCTIBOR 1300
In tabella 4.3 sono riportate le composizioni chimiche di tre diversi acciai Ener- gy Adsorbing commercializzati con allungamenti a rottura >15% e in (Fig.4.4) è riportato il confronto tra i due tipi di DUCTIBOR e USIBOR 1500
Steel C Si Mn P S Al Nb
Ductibor 500 0.05 - 0.08 <0.5 <1.4 <0.03 <0.01 0.02 - 0.04 0.03 - 0.07 MBW500 <0.10 <0.35 <1.0 <0.03 <0.025 >0.015 <0.1 phs-ultraform <0.11 <0.5 <1.4 <0.03 <0.025 <0.015 <0.1 Tabella 4.3: Composizione chimica dei principali acciai Manganese-Boro per Energy Absorbing
4 – Acciai automotive per formatura a caldo
Vehicle) and Renault EOLAB concept car
[77,78]. ArcelorMittal is developing
USIBOR 1800 and USIBOR 2000
[59,73]. BaoSteel is also expected to roll an
1800 MPa hot forming steel
[79].
12.3.2
Energy absorbing hot forming steels
For the automotive applications, the main purpose of hot stamped components is to
improve crashworthiness and save weight. Therefore performance of these parts
should be tested in crash conditions. Hot formed steels discussed in 12.2.1 have
high strength but low ductility after quenching. This reduces the energy that can be
absorbed during crash.
To improve energy absorption, several lower carbon steels were developed.
Currently there are two separate areas of development: (1) 450500 MPa
strength level with
.15% total elongation, and (2) 10001300 MPa strength
level with
.5% elongation.
Fig. 12.8
shows approximate engineering stress-
strain curves of energy absorbing steels compared with 22MnB5, all after
quenching.
At present, several steelmakers offer 450500 MPa level steels, including:
1.
ArcelorMittal’s DUCTIBOR 450 and DUCTIBOR 500, offered with Al-Si coating
[60,82].
2.
ThyssenKrupp’s MBW500, uncoated or with Al-Si coating
[63]. MBW500 is currently
used in 6 components of the 2nd generation Volvo XC90
[46].
3.
Voestalpine’s phs-ultraform 490, offered with Zn coating
[83].
0 5 10 15 20 25 0 500 1000 1500 2000 E ngi neer ing s tr es s ( M P a) Engineering strain (%) 22MnB5 (USIBOR 1500, MBW 1500, HPF 1470, BTR165, phs-ultraform) DUCTIBOR 1300, B1200 DUCTIBOR 500, MBW 500, phs-ultraform 490
Figure 12.8
Engineering stress-strain curves of 22MnB5 in comparison with energy
absorbing grades
[3,59,80,81].
396 Automotive Steels
Figura 4.4: Possibili impieghi di acciai al boro in uno chassis automobilistico