In Situ Quantitative Assessment of the Role of Silicon During the Quenching and Partitioning of a 0.2C Steel
- PDF / 2,143,849 Bytes
- 9 Pages / 593.972 x 792 pts Page_size
- 41 Downloads / 271 Views
TION
THE quenching and partitioning (Q&P) process was first proposed by Speer et al. in 2001 as a heat treatment designed to stabilize austenite in steels at room temperature.[1] It consists of two main steps. The first step after intercritical annealing or full austenitization is an interrupted quench between the martensite-start temperature (Ms) and the martensite-finish temperature (Mf), in order to form a controlled fraction of martensite. This is followed by a partitioning step during which the untransformed austenite is stabilized through carbon partitioning from martensite.[1,2] Q&P steels typically exhibit a good combination of strength and ductility, the latter being due to the transformation of retained
PIERRE HUYGHE, SYLVAIN DE´PINOY, and STE´PHANE GODET are with the 4MAT, Materials Engineering, Characterization, Processing and Recycling, Universite´ Libre de Bruxelles, 50 Avenue FD Roosevelt, CP194/03, 1050 Brussels, Belgium. Contact e-mail: [email protected] MATTEO CARUSO and JEAN-LOUIS COLLET are with the CRM Group, Avenue du Bois Saint Jean 21 P59, 4000 Lie`ge, Belgium. Manuscript submitted November 21, 2018. Article published online May 21, 2019 3486—VOLUME 50A, AUGUST 2019
austenite upon straining (TRIP effect).[3] The intensity of the TRIP effect depends on the amount and on the stability of the retained austenite, which in turn depends on its carbon content, size, surrounding phases, and stress state.[4–8] To improve austenite stabilization, carbon partitioning from martensite to austenite should be promoted, and consequently, competing mechanisms such as carbide precipitation and austenite decomposition should be suppressed. This is achieved through optimization of the Q&P thermal cycle or optimization of the steel composition.[3,9–11] Silicon is an essential alloying element in Q&P as it delays and/or suppresses the precipitation of carbides in martensite.[12–16] However, there are strong industrial motivations to reduce the silicon content in Q&P steels for automotive applications, since silicon has a detrimental effect on several steps of the industrial process (slab embrittlement, descaling issues, difficult pickling, and poor coatability).[17] It is thus of prime interest to investigate how the silicon influences the microstructural evolution during a Q&P treatment in order to optimize its content in future commercial grades. The relationship between silicon content, carbide precipitation, and the amount of retained austenite in Q&P steels is not straightforward. The recent literature,
METALLURGICAL AND MATERIALS TRANSACTIONS A
Chemical Compositions (in Weight Percent) of the Investigated Steels. Ac1 and Ac3 Temperatures Were Calculated Using Equations from Reference [34] and Bs and Ms from Reference [33]
Table I. Steel
C
Si
Mn
Cr
P
S
N
Fe
Ac1
Ac3
Bs
Ms
0.4Si 0.8Si 1.5Si
0.2 0.2 0.2
0.4 0.8 1.5
2.4 2.3 2.3
0.2 0.2 0.2
0.003 0.003 0.003
0.002 0.003 0.002
0.005 0.007 0.005
balance balance balance
985 K (712 C) 998 K (725 C) 1018 K (745 C)
1075 K (802 C) 1124 K (851 C) 1
Data Loading...
