The physical properties of a cold-rolled steel plate can be improved through various heat treatments.
In recent years, high-toughness steel plates have been used as steel plates for vehicles. Bainitic steel is a representative example of the high-toughness steel plates. The bainitic steel has high strength and ductility, and is thus suitable for non-molded vehicle body parts to increase crash characteristics.
In order to maximize the advantage of the bainitic steel, it is necessary to increase a volume fraction of bainite phase in a fine structure.
Three conditions are known to be satisfied in which i) a carbon content is low, ii) an isothermal transformation temperature is low, and iii) an isothermal transformation temperature holding time is long, in order to increase the volume fraction of bainite phase. Most steel plate manufacturers may apply the conditions i) and ii), whereas it is, in fact, difficult for them to apply the condition iii) due to the limitation of continuous annealing apparatus data. For this reason, the commercial steel plate cannot obtain a maximum expectable effect due to having a low phase fraction of bainite.
The annealing method of the cold-rolled steel plate is largely classified into a continuous annealing line (CAL) type and a batch annealing furnace (BAF) type.
The continuous annealing line type is advantageous in that a steel plate can be uniformly heat treated to thereby have a small physical property variation, rapid heating and cooling can be performed, and it can be maintained at a high temperature of 900° C. or more, but it is disadvantageous in that an isothermal holding time is short.
The batch annealing furnace type is advantageous in that heat treatment can be performed in an isothermal holding state for a long time, but it is disadvantageous in that a variation in temperature between inner and outer diameters of each laminated coil occurs and hence a physical property variation between the coils is wide after heat treatment.
Accordingly, a continuous annealing apparatus, which is capable of strengthening advantages and compensating for heat treatment methods of two continuous annealing line (CAL) type and batch annealing furnace (BAF) type by a combination of these two types, is necessary such that annealing is maintained at high temperature for a long time as occasion demands to manufacture a steel plate with high toughness.
The foregoing is intended merely to aid in the understanding of the background of the present disclosure, and is not intended to mean that the present disclosure falls within the purview of the related art that is already known to those skilled in the art.