Harold C. Simmons began his tender offer for NL.

The Dallas financier, who already owns 20.4% of NL Industries’ 60 million common shares outstanding, is offering $15.12 1/2 a share for the remaining stock. Simmons had announced plans for the hostile tender offer after NL’s directors rejected his initial proposal to buy the chemicals and oil-services concern for $15 a share. The offer is valued at about $723 million.