A steel plant for the production of long products, in particular bars, rods, profiles, wires and the like, or for flat products, in particular strips, sheets and the like comprising: (a) a plurality of machinery (10, 12, 36, 38, 42; 86, 88, 90, 92; 134, 136, 138, 166, 168; 224, 226, 228, 186, 188, 190) equipped with direct cooling systems and divided into at least a first (10, 12; 86, 88; 134, 136, 138; 224, 226, 228) and a second group (36, 38, 42; 90, 92; 166, 168; 186, 188, 190) of machinery, each group comprising one or more pieces of machinery, wherein the groups of machinery are distinguished by the different higher or lower concentration of scale released into the cooling water accumulated during the use of each group of machinery; (b) a circuit (C, C", C4) of treatment of direct cooling waters exiting said direct cooling systems, comprising a plurality of cooling water treatment devices (44, 76, 50, 52; 94, 102, 210, 212; 194, 200, 211, 213) characterized in that (i) said direct cooling systems of said two groups of machinery are connected in series, wherein said direct cooling systems of said second group of machinery (36, 38, 42; 90, 92; 166, 168; 186, 188, 190) producing cooling waters with a higher scale concentration are situated downstream of said direct cooling systems of said first group of machinery (10, 12; 86, 88; 134, 136, 138; 224, 226, 228) producing cooling waters with a lower concentration of scale, so that said direct cooling systems of said first group of machinery feed said direct cooling systems of said second group of machinery; and in that (ii) said direct cooling water treatment circuit (C, C", C4) is arranged downstream of said second group of machinery (a) a plurality of machinery (10, 12, 36, 38, 42; 86, 88, 90, 92; 134, 136, 138, 166, 168; 224, 226, 228, 186, 188, 190) equipped with direct cooling systems and divided into at least a first (10, 12; 86, 88; 134, 136, 138; 224, 226, 228) and a second group (36, 38, 42; 90, 92; 166, 168; 186, 188, 190) of machinery, each group comprising one or more pieces of machinery, wherein the groups of machinery are distinguished by the different higher or lower concentration of scale released into the cooling water accumulated during the use of each group of machinery; (b) a circuit (C, C", C4) of treatment of direct cooling waters exiting said direct cooling systems, comprising a plurality of cooling water treatment devices (44, 76, 50, 52; 94, 102, 210, 212; 194, 200, 211, 213) (i) said direct cooling systems of said two groups of machinery are connected in series, wherein said direct cooling systems of said second group of machinery (36, 38, 42; 90, 92; 166, 168; 186, 188, 190) producing cooling waters with a higher scale concentration are situated downstream of said direct cooling systems of said first group of machinery (10, 12; 86, 88; 134, 136, 138; 224, 226, 228) producing cooling waters with a lower concentration of scale, so that said direct cooling systems of said first group of machinery feed said direct cooling systems of said second group of machinery; and in that (ii) said direct cooling water treatment circuit (C, C", C4) is arranged downstream of said second group of machinery