Laying your new mattress on the few slats that came with the metal bed frame will find you slipping into crevices during the night, and may invalidate your mattress' warranty. Putting a mattress on a metal bed frame without a box spring takes ingenuity, but it can be done.
The Purpose of a Box Spring
While the mattress supports you when you sleep, the box spring supports the mattress. Think of the box spring as a shock absorber similar to those used in cars. When you bounce down on the bed, the box spring absorbs the shock of the sudden weight, relieving the mattress' springs of heavy wear and tear. It also raises the mattress to a height comfortable for most people when getting in and out of bed. With deep mattresses, some box-spring manufacturers make a low-profile product to accommodate the mattress height; it's thinner but as effective as thicker box springs.
Analyze the Metal Bed Frame
Some metal bed frames, including antique iron beds, have several metal slats running horizontally across the frame and a vertical slat going from the headboard to the foot of the bed. That may not be enough to sufficiently hold your mattress without it losing its shape. You'll need to add slats or a plank.
Antique Iron Bed Challenge
Most antique iron beds have side rails into which bed slats must be fitted to hold a box spring. Wooden 2-by-4s must be cut, and depending on the shape of the iron rails, can be laid on top of the rail or in the case of a flat-top rail, must be fitted with screws to attach to the rail.
Bed Frame Plank
Measure the inside of your metal bed frame, from inside edge to inside edge, both horizontally and vertically. Most metal bed frames have a lip along the edges of the frame to keep the mattress in place. A lumberyard or big box hardware store can cut a piece of plywood to be placed on top of the slats, within the given frame. The plywood gives your mattress support.
With the inside-edge measurements in hand, have the hardware store cut 2-by-4 slats for the metal frame. Depending on the size of the bed, order enough slats to sufficiently cover the space. Place them within the lip-edged frame, 3 to 5 inches apart.
Today's Box Springs
Unlike box springs of recent years, economics have forced manufacturers to redesign the box spring, including renaming it a foundation. Not as bouncy as the original because they do not contain springs, they do help redistribute the weight of the mattress with bodies on it. They also cost less to manufacture. Most European beds use a foundation.