Page 43 - MetalForming February 2022 Special Automotive Issue

Most conventional steels used in automotive 100 applications have a uniform distribution of a single
0
Metal Matters
Punched-Hole Expansion Ratio
Machined-Hole Expansion Ratio
Automotive AHSS Grades: Microstructure Matters
160 140 120
metallurgical phase called ferrite. While high-strength
low-alloy (HSLA) steels rely on precipitation-strength-
ened ferrite as their primary microstructural com-
ponent, AHSS grades rely on the full spectrum of
different phases in varying amounts and distributions associated with the specific type and strength level 20 of that particular grade. These phases include soft
80 60 40
ferrite as well as martensite, bainite and austenite. Dual-phase (DP) steels, the workhorse of the automotive AHSS grades, have a microstructure of hard martensite surrounded by a matrix of soft ferrite. Another AHSS grade, transformation- induced-plasticity or TRIP steels, also have soft fer- rite and hard martensite as microstructural com- ponents, along with retained austenite and bainite. This microstructural balance provides these grades with both high strength and high elongation, but also causes poor edge-stretching performance. The high hardness difference between ferrite and martensite leads to the formation of microscopic voids at the interface between the phases. Shearing to produce cut edges further damages the microstructure, with the shearing process work hardening the steel sheet at the edges. When these edges experience tensile stress—during stretch flanging or hole extrusion for example—these voids combine to form cracks. Therefore, these grades may not be the best choice for engineered parts
with expanded cut edges.
Fortunately, other options exist. For example,
Dual-Phase 638 MPa YS 1017 MPa TS 19% Elongation
Dual-Phase 783 MPa YS 1028 MPa TS 15% Elongation
Martensite 995 MPa YS 1068 MPa TS 5% Elongation
Fig. 2
130
120
110
100
90
80
70
60
50
40
30
20
10
0
10% clearance
15% clearance
20% clearance
Fig. 3
1.3mm 1200CP
some steel companies offer DP grades with modified compositions and mill processing to increase yield
strength while maintaining tensile strength. Metal-
lurgically accomplished by increasing the strength of the ferrite, the microstructural components now have a less-extreme dif- ference in hardness. This hardness uniformity holds the key to achieving higher stretched-edge flangeability, and explains why single-phase martensite provides reasonable edge ductility despite having high yield and tensile strength (Fig. 2).
Complex-phase (CP) steels represent another option for providing good edge ductility at high strength. The microstructure of CP grades consists of martensite and retained austenite in a ferrite-bainite matrix, but unlike with DP and TRIP steels, the ferrite in CP steels is fine- grained and precipitation strengthened, resulting in a high- hardness ferrite. Bainite is a high-strength phase but with a lower hardness than martensite. Again, with relatively uni- form hardness distribution of the metallurgical phases, CP steels possess good sheared-edge ductility.
Impact of Edge Condition
www.metalformingmagazine.com
MetalForming/February 2022 41
To quantify a material’s edge-stretching ability, we call on the hole-expansion test. After creating a hole of known diameter, a punch with specific dimensions expands that hole until an edge crack ends the test. The hole-expansion ratio represents the percent expansion of the initial hole diameter.
The international standard ISO 16630, “Metallic materi- als—Sheet and strip—Hole expanding test,” specifies several test parameters, including a 10-mm initial hole diameter created by shearing with a 12-percent clearance, and expanded with a conical punch with a 60-deg. apex angle. Locking in these values creates a way to compare performance between grades and suppliers. Larger initial-hole diameter appears to be asso- ciated with reduced hole expansion, and 12-percent clearance may not be the best choice to optimize edge stretchability.
For nearly 50 years, practitioners have known that a
Hole-Expansion Ratio (%)
Hole-Expansion Ratio (%)