Formability Investigations of High-Strength Dual-Phase Steels

doi:10.1007/s40195-015-0346-1

Abstract

Abstract:

Car manufacturing is always regarded as the key industry behind sheet metal forming, and thus, the requirements of and developments in car manufacturing play a decisive role in the development of sheet metal forming. The automotive industry is faced with contradictory demands and requirements: better performance with lower consumption and less harmful emissions, more safety and comfort; these are extremely difficult to supply simultaneously with conventional materials and conventional manufacturing processes. The fulfillment of these often contradictory requirements is one of the main driving forces in the automotive industry and thus in the material and process developments in sheet metal forming, as well. In recent years, significant developments can be observed in the application of high-strength steels. In this respect, the application of various dual-phase steels is one of the best examples. However, the application of these high-strength steels often leads to formability and manufacturing problems. One formability problem is the springback occurring after sheet metal forming. In the current research, we have dealt mainly with advanced high-strength steels, primarily with dual-phase steels. When applying them, the springback phenomenon is one of the most critical issues. To reduce the tremendous amount of experimental work needed, we also applied numerical simulation using isotropic-kinematic hardening rules. The isotropic-kinematic hardening behavior of a given material in the applied AutoForm numerical package may be characterized with three independent material parameters γ, χ and K (a detailed explanation of their meaning will be given in the main part of this paper). However, we found that the material data included in simulation packages for these new high-strength steels are not fully adequate. For the determination of more reliable material parameters and to achieve better simulation results, a new testing device was developed. Numerical simulations were performed using the material parameters determined by the new device to show the sensitivity of springback behavior to these material parameters.

Key words: Springback, Large-strain cyclic deformation, High-strength dual-phase steels

Miklós Tisza, Zsolt Lukács. Formability Investigations of High-Strength Dual-Phase Steels[J]. Acta Metallurgica Sinica (English Letters), 2015, 28(12): 1471-1481.

Figures/Tables 18

Fig.1 Steel development trends during the last 40 years

Fig.2 Elongation versus tensile strength for conventional and advanced high-strength steels [2]

Fig.3 Schematic representation of the AutoForm model concept [11]

Fig.4 Supporting frame with two rows of plates (1—specimen; 2 and 3—frames with the supporting comb-shaped plates): a schematic diagram; b photo

Fig.5 Experimental device for cyclic tension-compression test (1—two-column guided tool; 2, 3—clamping frame; 4—prestressing unit; 5—extensometer; 6—plate spring; 7—supporting pad)

Table 1 Chemical composition of the experimental materials (in wt%)

Material	C	Si	Mn	P	Cr	Ni	Al	Co	Fe
DP600	0.118	0.217	0.785	0.022	0.023	0.040	0.051	0.015	Bal.
DP800	0.012	0.198	1.43	0.020	0.027	0.032	0.037	0.014	Bal.
DP1000	0.142	0.472	1.47	0.015	0.017	0.032	0.052	0.014	Bal.

Table 2 Mechanical properties of tested materials

Material	t (mm)	R_eH (MPa)	R_m (MPa)	Martensite (%)	E₀ (GPa)
DP600	1.0	351	670	18	206
DP800	1.0	555	847	32	206
DP1000	1.0	780	1004	50	206

Fig.6 Different deformation strategies during cyclic tension-compression tests: a samples subjected to larger and larger cyclic strain; b samples subjected to the same cyclic compression-tension strain

Fig.7 Hysteresis curves for DP600 dual-phase steel determined with different prestraining (εmean=0.0–0.10; εc=±0.02)

Fig.8 Explanation of the method for the determination of apparent elasticity modulus

Fig.9 Variation of apparent elasticity modulus for the material grade DP600

Table 3 γ and χ values for three different DP materials in the I-IV sections

	DP600				DP800				DP1000
	I	II	III	IV	I	II	III	IV	I	II	III	IV
γγ	0.22	0.177	0.154	0.123	0.22	0.161	0.131	0.113	0.20	0.146	0.117	0.094
χχ	49	45	30	37	82	65	54	46	80	68	60	57

Fig.10 Explanation of the meaning of K material parameter

Fig.11 Microstructure of the investigated DP steels

Fig.12 Comparison of the springback for three different DP steels (γ=0.13, χ=40, K=0.014)

Fig.13 Effect of the γ parameter on the springback for DP600 dual-phase steel

Fig.14 Effect of the χ parameter on the springback for DP600 dual-phase steel

Fig.15 Effect of the K parameter on the springback for DP600 dual-phase steel

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