Is there such a word as "semiproduct"?

There's no such word as semiproduct in normal English. Regardless of whether it is the finished product or not, a steel sheet is still a product; it has been produced from raw materials.

I would think 'component' would be more appropriate, but there are lots of industrial in-words, so as Tragicomic says, there's no reason you can't use semiproduct so long as your colleagues understand it.

The word you are asking for is semifinished or semiprocessed, which are adjectives; the nouns you would use are semifinished product, or semiprocessed product.

To tackle this from yet another angle for good measure, there is the word prefabrication, which refers to the process of producing "semiproducts" which are later assembled to the final product on site:

The most widely-used form of prefabrication in building and civil engineering is the use of prefabricated concrete and prefabricated steel sections in structures where a particular part or form is repeated many times. [...]

Prefabricated bridge elements and systems offer bridge designers and contractors significant advantages in terms of construction time, safety, environmental impact, constructibility, and cost. [... S]mall, commonly-used structures such as concrete pylons are in most cases prefabricated.

Radio towers for mobile phone and other services often consist of multiple prefabricated sections. Modern lattice towers and guyed masts are also commonly assembled of prefabricated elements.

Prefabrication has become widely used in the assembly of aircraft and spacecraft, with components such as wings and fuselage sections often being manufactured in different countries or states from the final assembly site.

One more possibility, depending on the nature of the material, would be "assembly" or even "sub-assembly", especially when such a thing can be serviced or replaced in isolation. A car, for instance, might have a fusebox assembly, which can be removed as a unit, or a fuel sensor sub-assembly, which includes not only the sensor itself but also the connecting wire, mounting bracket, tube, and screws.