Laser cutting of low-carbon or structural steel plates, sheets or tubes involves using a multi-focal optical device and an assisting gas

摘要

Laser cutting of a low-carbon or structural steel article of thickness 1.5-5 mm is carried out by using a transparent or reflecting multifocal optical device, preferably a lens, for focussing at least one laser beam, and at least one assisting gas comprising nitrogen with or a mixture of nitrogen and oxygen containing at least 90 % nitrogen. Preferred Features: The optical device can be selected from lenses and/or mirrors, and is preferably a bifocal lens. Preferably, when the assisting gas is nitrogen, it contains less than 150 ppm (more preferably, less than 100 ppm) in volume of oxygen as impurity. When the assisting gas is a mixture of nitrogen and oxygen, the amount of oxygen is above 0 volume % and less than 8 volume %, (more preferably, 150 ppm to 5 volume %). The mixture of nitrogen and oxygen can be obtained from atmospheric air using a membrane system. The optical device is fitted so as obtain at least a first focussing point located close to, and preferably coinciding with, the upper surface of the article to be cut, or in the thickness of the article in a zone close to the upper surface of the article, and a second focussing point located close to the lower surface of the article, either within or outside its thickness. An Independent claim is given for a laser cutting installation used for implementing the above process.