The Science of Forming


 

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How Heat Can Affect Steel Performance

By: Stuart Keeler

Sunday, May 1, 2016
 

The microstructures of the different types of steel comprise a variety of grain sizes, and grain size impacts an alloy’s mechanical properties. For example, the schematic in Fig. 1 portrays the grains found in a dual-phase (DP) advanced high-strength steel (AHSS). The grains making up the ferrite phase have two important, controlling features—the boundaries and cores of the grain. The grain boundaries are stronger than the cores, so to increase the strength of the steel, smaller grains are formed to reduce the size of the weaker cores. This increases the volume of the stronger grain boundaries. Grain size depends on the available heat.

Fig. 1—Schematic of a ferrite microstructure with a highlighted grain boundary. The 10-percent martensite grains exist in dual-phase (DP) steels. AHSS Application Guidelines, Ver. 5.0.
Steel types ranging from common mild steel to high-strength low-alloy (HSLA) grades are based on a ferrite microstructure. AHSS grades comprise one or more phases other than ferrite. DP steels (Fig. 1), for example, comprise a martensitic structure.

Excessive Annealing

Annealing of steels used to involve placing stacked coils onto a fire-resistant base. A metal tube with a closed upper end was placed over the stack to protect the steel from direct flames. Then, the annealing unit—another tube with attached burners—was placed over the original metal tube. With this process, the steel wraps in the middle of the coil underwent a different heating process than did the ends of the coil, the outer wraps and the inner wraps. This caused mechanical properties to vary along the coil and from edge to edge.

Fig. 2—High heat and strain energy are needed to recreate original smaller grain size in a partially formed stamping.
Steel companies now use continuous-annealing lines to achieve well-controlled and consistent grain size and other properties. The speed of the coil in the heat zone remains constant to achieve uniform properties. However, one day a real disaster occurred, when a steel supplier’s annealing rules were violated. As a result, when a metalforming company attempted to stamp the material, very large tears formed. From the bottom edge was a wide tear in the sheet, which ran up through the center of the stamping. From the main tear were a number of side tears that turned into other multiple tears. In between the long tears were groups of small tears. An examination of the surface showed rough pebbles over the flat steel. And, metallurgical examination of the stamping revealed that large grains had formed completely out of control.

The cause of the problem? The steel assigned for the stamping had been heated in the annealing furnace zone for more than an hour. When a continuous-annealing line stops for any length of time, the material within the annealing furnace must be cut out of the coil. That did not happen in this case.

Annealing Intermediate Stampings

Now consider a stamped part that must form in five dies. Attempts have been made to restore formability to a third die, allowing the stamped part to survive additional stretch in dies 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. Annealing depends on two sources of energy—the external applied heat and (often overlooked) the stored energy due to deformation of the sheetmetal.

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
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 larger 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 simultaneously. 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 tensile strength is 1400-1700 MPa. MF

 

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