The third great iron and steel emporium was Union Cerrajera, the most important lockmaker in the Upper Deba region. It had its origins in a small workshop founded in 1869 by the Vergarajauregui and Resusta families, following acquisition by the Count of Monterrón of the Olazar forge in the district of Zaldibar. In a few short years, the success of this initiative was consolidated with the take-over of the factory in Aretxabaleta Hijos de Echeverría y Cía (1890) and more importantly, with the installation of a blast furnace in Bergara (1901).

In the same year, however, La Cerrajera Guipuzcoana, was founded, also in Arrasate-Mondragón, and it soon began to rival Vergarajauregui, Resusta and Cía in the field of hardware. The owners saw of the two competitors realised that they it was in their mutual interest to join forces, thus extending production and reducing overheads, and the companies merged in 1906 to form Unión Cerrajera.

Between them, the original companies had land and facilities in Arrasate-Mondragón, Bergara and Aretxabaleta. The new business was excessively fragmented and rationalisation was required, focusing on just two sites: Zaldibar and Altos Hornos de Bergara. Three Siemens Martin furnaces were added to the primitive charcoal crucibles in 1906, 1912 and 1914. These were used to transform the pig iron into steel, which was then worked in the workshops in Arrasate-Mondragón and Aretxabaleta.

The Deba valley now had a self-contained industry performing the entire production process, with a wide product range covering everything from locks to stoves, clothes irons and galvanised braces, sleeper screws and bolts for railways, plough components, etc. The high costs of producing pig iron and the more competitive prices of the iron manufactured in Bizkaia dissuaded the management of Cerrajera from extending their work to include this process, which was considered to have more drawbacks than advantages. Consequently, from 1928 on, the company bought in its pig iron from Altos Hornos de Vizcaya.

With the abandonment of pig iron manufacture, the blast furnaces were removed. They were followed in 1962 by the Siemens furnaces, which were replaced by new electric ones. The former production centre, on the side of the Madrid-Irun road, with its offices, chapel, schools and houses for the workers, was given over entirely to rolling operations.