Who makes very large ship keels?

There's no more keel like the Titanic had..

It's all modular construction.

See time lapse keel laying.

Modern ships no longer have keels, they have as part of the fabrication a flat steel plate known as the keel plate. It is positioned in the same position as a keel of old, ie on the bottom of the ship, dead centre and running from fore to aft.

Ships have either side a bilge keel, this is a small plate tack welded on to the hull at the turn of the bilge. It is tack welded so that it can tare off without causing damage to the hull. It also runs from forred to aft but only along the flat side and not on the shaped sides. It helps to prevent some of the rolling motion of the ship.

Now days, the ships' keel is laid and built as part of the building frame on which the entire vessel is built. The building frame and the 'laying of the keel' are now one process - which is the very 1st step in the actual building of the ship - and officially marks the start of the construction process.

FYI - China is now the worlds larges ship builder.

The steel parts are contracted to a steel company. The keel is shipped to where the ship is getting constructed in pieces then assembled on site.

For steel ships keels are fabricated and welded together.