【正文】 C to allow for the increase from 10?2 to 102 s?1. Thus, warm rolling takes place at temperatures above the DSA peak [9]. The influence of DSA on the flow stress at two different strain rates is shown schematically in Fig. 5, from which it can be seen that the rate sensitivity is negative in the vicinity of the lowtemperature DSA peak and more highly positive than expected immediately above the hightemperature peak. Fullsize image (4K) Fig. 5. Schematic representation of temperature vs. flow stress data for two strain rates. The peak in the LC steel curves is a consequence of DSA. In order for this diagram to apply to rolling conditions, it must be understood that the lower curve applies to the strain rate of the bulk material, which is deforming homogeneously. By contrast, the upper (high strain rate) curve pertains to the material in a potential shear band. In the temperature range where m0, potential shear bands will form readily as the shearing material has a lower flow stress than the homogeneously deforming material. This is because the mobile dislocations can easily break away from their trailing atmospheres by speeding up. Conversely, in the temperature range where m is unusually high, shear bands can form only with difficulty, if at all. This is because much higher flow stresses are required to move the relevant dislocations under DSA (. solute interaction) conditions. 18 In the temperature ranges where m has a “normal” (pure metal) value, shear bands will only form if some other softening mechanism (equivalent to m0) is operative. This is generally considered to be texture softening assisted by highly localized deformation heating [10]. The texture softening involves a change in strain path from planestrain rolling (pure shear) to simple shear. This mechanism is particularly applicable to grains of {1 1 1} 1 1 2 orientation, as they have the highest resistance to rolling (highest dislocation densities) and a reduced resistance to simple shear. . Rate sensitivity The dependence of the rate sensitivity on temperature discernible in a general way in Fig. 5 is shown in more detail in Fig. 6. This diagram summarizes the results obtained in the investigation described above. Here two contrasting behaviours are pared that of: (i) a “pure” material, in which the rate sensitivity simply has a “l(fā)owtemperature” and a “hightemperature” range, and (ii) the present type of DSA material, in which departures are observed from the basic trends. The first class of behaviour is displayed by steels that do not contain solute C or N, . as in fully stabilized IF steels, where the C and N are removed from solution by the addition of Ti and/or Nb. In the second class of material, the concentration of C in solution at warm rolling temperatures is higher because of the absence of stabilizing alloying additions and depends on the cooling rate from the hot rolling range or on the holding time after reheating. Fullsize image (6K) Fig. 6. Influence of homologous temperature on strain rate sensitivity (m) for a number of different materials [5 and 12]. The value of m is considerably higher in LC steels, pared to IF materials, at warm rolling temperatures (. homologous temperatures between and ). If no alloying additions have been made to the steel, then the equilibrium amount of C that is expected to be in solution in the WR temperature range (600–800176。C and in the IF steel under all conditions (which is absent in the warmrolled and annealed LC steels) is attributable to nucleation in the vicinity of shear bands of moderate intensity。C rolling), an additional Goss ponent ({0 1 1} 1 0 0 ) is present, and (ii) under warm rolling conditions (. 700176。C (intensity levels 2, 3, 4, 5, etc.) [6]. 3. Discussion It is clear from the results described above that the temperature dependence of the deformed state in the two LC grades differs sharply from that of the IF material. In particular, the influence of rolling temperature is far greater in the LC grades. The possible reasons for this are examined below in terms of shear band nature and density, the characteristics of the texture, and of the effect of the above parameters on rav and therefore on the formability. . Nature and density of the shear bands It was evident from Fig. 2 that, at temperatures below 400176。C texture determined in the LC steel are significant and these will be taken up later in Section 3. By contrast, the maximum intensities were similar in the IF steel over the whole rolling temperature range. . Annealing textures Some examples of the textures measured after annealing at 700176。C is typical of all rolling temperatures. In this micrograph, the shear taking place along individual bands is made evident by the grain boundary displacements. The fraction of grains containing shear bands was determined 14 by a point counting technique [5]. The resulting data are presented in Fig. 2, from which it can be seen that the intensity of the banding, like its nature, was unaffected by the rolling temperature in the IF material. In the LC grades, however, the intensity of the banding was highly temperaturesensitive, droppingoff sharply when rolling was carried out above 400176。C is depicted in Fig. 1a. (Similar results were obtained in the low Mn variant.) Only a few banded grains can be seen, and the bands themselves are quite thin and short, indicating that flow along them was retarded. These “stunted” bands were unique to the LC samples rolled at temperatures above 550176。C, in IF steels, does not involve increases in rolling load above the design limit for the mill in question. While both plain C and IF steels can be readily warm rolled, only the latter materials permit the attainment of high rvalues, as indicated in Table 2. The high rvalues in turn require the presence of des