| Direct Solution Treatment (DST) is an innovative technology, which brings great benefits to the stainless steel wire producers. Linde is the market leader and has during the last years made such installations at Fagersta Stainless, Sweden, POSCO Specialty Steel, Korea, and Dongbei Special Steel, China.
Less process steps
In the DST process the annealing and quenching take place fully in-line with the hot rolling operation. The wire products will leave the hot rolling line as annealed and can be further processed without the conventional heat treatment in batch furnaces. Thus also the amount of products in work, needed labour force and internal transportations are reduced.
As the REBOX® DST roller hearth furnace is placed directly after the laying head in the rolling line, the energy utilization is substantially improved; when entering typically the wire rod has a temperature of 900°C and only some 200°C has to be added to reach the annealing temperature. The energy savings are further improved by use of energy-efficient flameless oxyfuel combustion, which also provides uniform heating and very low emissions of CO2, and NOX.
Higher yield and guaranteed better quality
The formation of scales mainly depends on the time the material is kept at high temperatures. In REBOX DST the annealing takes only 2-3 minutes compared to a much longer time in batch furnaces: the scaling losses could be reduced by 10 tons or more per 1,000 tons of steel produced.
Great benefits are gained in the quality of the wire rod. A tensile strength deviation of less than +/-10 N/mm2 within a coil of wire rod is achieved; this is a great improvement to off-line annealing. Also, the surface is smoother and since the amount of process steps is less the risk of damaging the surface is lower. Typical guarantee parameters at a REBOX DST installation include grain size, tensile strength deviation, throughput capacity, fuel consumption, and NOX emission. Low calorific fuels can be used; at Dongbei the fuel contains about 6 MJ/Nm3.
DST furnace at Fagersta Stainless. Note that it can be shifted in a sideway position so that the roller table on the side will replace the furnace in the line.
Easy retrofit in existing hot rolling lines
The REBOX DST roller hearth furnace is followed by a water spray cooling unit where the wire rod is cooled. Water is sprayed uniformly over the loops of wire rod and also between the layers. This method gives very uniform and rapid cooling, creating good properties of the wire products.
When installing DST the air cooling (Stelmor Conveyor) is replaced by the DST, all other equipment will be kept as is. Steel grades not suitable for DST treatment are processed outside the DST on a parallel roller table or air cooling unit (Stelmor Conveyor). Either the DST furnace is moved out of the line when not used or the wire rod flow is by-passed next to the furnace.
In the installations REBOX® DST has demonstrated important advantages over conventional wire rod annealing. As have been proven, it suits well both for greenfield plants and retrofits. |
