Page 27 - MetalForming May 2011

Relationship Between Strength and Elongation for DP and TRIP AHSS Grades
70
60
50
40
30
20
10
0
0 200 500 800 1100 1400 1700
DP = Dual phase steel
TRIP = Transformation induced plasticity
Constant strength increased stretchability
TRIP
HSLA
D
P
Fig. 1
Yield Strength (MPa)
1) Steel grades with increased formability for the same ini- tial yield or tensile strength. This improvement is shown for total elongation as a function of yield stress (Fig. 1). Similar curves can be drawn for tensile strength and/or work-hard- ening exponent (n-value). Compared to HSLA steels of the same strength, the increased total
elongation of a dual-phase (DP)
AHSS allows smaller ratios of
punch radius to thickness. DP
steels also can greatly restrict the
localization of deformation
(strain) at high stress locations.
These strain gradients are
extremely variable and are a
major cause of product dimen-
sional variations.
In addition, peak strains can dramatically increase and reach failure conditions at less than home depth. DP steels strength- en at the onset of the gradient and minimize or even prevent their development. These improvements result from the introduction of islands of marten- site in the microstructure.
Fig. 2 illustrates the results of a forming study on a steel front bumper with various HSLA steels as well as a DP steel. Two sharp strain gradients were measured across the front of the bumper. Note the dramatic change in the gra- dients when testing DP steel—the effect of the DP steel is exhausted around 7 to 9 percent strain. Since the effect of adding islands of martensite are so important, metallurgists found a way to replenish fresh islands of martensite and produce even better steels—transformation induced plas- ticity (TRIP). These grades provide significantly greater abil- ity to flatten gradients than do DP steels. In addition, the maximum allowable strain at local necking (forming limit curve) is increased.
2) Increased yield or tensile strength for the same formability. Martensitic steel (MS, Fig. 3) is a single-phase steel of hard martensite. Depending on the temper, MS
total elongation may range from 15 to 5 percent. Most parts made with MS are rollformed.
The other AHSS grade in this category are complex- phase (CP) steels, which gain their strength through extremely fine grain size and a microstructure contain- ing bainite, martensitic, retained austenite and pearlite. CP is characterized by high energy absorption and high residual-deformation capacity.
3) Unique processing to obtain final properties. Prod- uct designs can generate parts previously thought impos- sible to make. In the real world, a purchase order might require forming a very complex part that requires the extremely high strengths of MS. The purchase order may
not require deforming incoming MS—only that the end product exhibits those properties.
Many parts are made today using hot forming of boron- based steel with yield strength of 50 to 70 ksi, and 18-percent total elongation. The steel is heated to 850 to 900 C, placed
in a press and formed. Forming yield strength is almost a con- stant 6 to 13 ksi and more than 50- percent total elongation. Very severe deformation can be obtained to create complex fea- tures in the part. After quick form- ing, the cold die quenches the part to the final strength of MS. While much slower than normal stamping, the one-hit process can make some parts that prove impossible to make using more traditional forming processes.
A Look Ahead
The trend away from HSLA steels to AHSS (Fig. 4) is note- worthy. Use of AHSS in North American light vehicles is expect- ed to grow from 9.5 percent in
Forming a Front Bumper with HSLA Steels and a DP Steel
70 60 50 40 30 20 10
0
Fig. 2
45(310) ksi (MPa) Yield strength
55 (380) Maximum Allowable
65(450) Stretch 80 (550)
DP
Centers of Original Circles
The Advantages of CP and MS Grades
70
60
50
40
30
20
10
S
CP = Complex phase steel MS = Martensitic steel
H
LA
CP
Constant formability increased strength
MS
Fig. 3
Yield Strength (MPa)
0
0 200 500 800 1100 1400 1700
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Total Elongation (%)
Percent Stretch
Total Elongation (%)