Page 51 - MetalForming May 2016

The Science of Forming
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0
0 200
500 800
Tensile Strength (MPa)
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HS
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1100 1400 1700
Fig. 3—This graph shows the total elongation and tensile strength of martensitic steel as it is created from Mn/B steel [1], formed into stampings at low strength and high total elongation [2] and quenched to strong martensite [3]. WorldAutoSteel—AHSS Application Guidelines Ver. 5.0
four and five. Annealing the stamped part after the third die should return its properties to those of the initial coil or blank. However, any additional annealing can be disastrous. Anneal- ing depends on two sources of energy—the external applied heat and (often overlooked) the stored energy due to defor- mation of the sheetmetal.
The amount of energy available for annealing varies throughout the stamping. In Fig. 2, for example, the graph shows the number of grains that have a specific grain size within the stamping. The first section shows a large number
of grains that have undergone no changes during annealing. This resulted from low strain energy and insufficient heat energy.
A major problem is the steep climb of one grain to a larg- er grain size. The heat and strain energies total just enough for one grain to form and then grow as large as possible. No other grains had yet formed to surround it and restrict its growth. The large core of the grain is weak and subject to easy tearing.
As total energy increases, more grains form simultaneous- ly. This forces the creation of smaller grains with larger grain boundaries and minimal core volume. These smaller, stronger grains result in increased strength of the stamped parts.
Hot Forming
Hot-formed stampings depend on heat to reduce the strength of the steel during forming. Parts then are quench- hardened to eliminate elastic springback. The process (Fig. 3) starts with a manganese-boron (22Mn5B) steel with the properties shown at location [1] on the graph. This steel is heated to 850 C [2], which decreases its yield strength to 100 MPa and allows complex deformation. After forming, the stamped part is immediately quenched in the die to form martensite. Yield strength reaches 1000-1250 MPa, and ten-
sile strength is 1400-1700 MPa.
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Total Elongation (%)
2