ROLLING CONDITIONS FOR HIGH R-VALUE OF HOT ROLLED STEEL SHEETS ANALYZED BY COMPUTER SIMULATIONS MODEL FOR MICRO-STRUCTURE

Hot rolled steel sheets with high r-value were developed. The steel was rolled with more than 50% reduction at each final three stands near the A r temperature. The condition was investigated, by a computer simulation model for micro-structures and a transformation texture model. Dynamically recrystallized T with random texture transforms to a with random texture or that with {1101<001> orientation. AQerage r-alue reaches 1.0 or more with random orientation. On the other hand, more than 1.2 of average r-value was maintained when the {110}<001> a was hot rolled giving recrystallized 11121--{111}<011> a texture.


INTRODUCTIONS
Since hot rolled steel sheets with the same strength and elongation as cold roiled steel sheets don't show as good drawabilty as cold rolled one, cold rolled steel sheets are commonly used as drawable thin steel sheets. Recently, however, the demand for lower cost has roused a keen interest in employing hot rolled steel sheets in place of cold rolled steel sheets. Some trials, that increases an average r-value of hot roiled steel sheets, have been carried out. -In this paper, the relation between the r-value and the volume fractions of micro-structure which are calculated by a computer simulation model"-, and simulations by a transformation texture model-v are reported.

EXPERIMENTAL PROCEDURE
Chemical compositions of steels used in the experiments are shown in Table 1. Steels were hot rolled in laboratory and in mill scale. Typical schedules of the rolling test are shown in Table 2. After rolling, specimens with and wthout an one hour annealing at 750C for recrystallization were prepared. The effect of high reduction on texture and r-value were investigated.
The volume fractions of micro-structure after rolling were calculated, using the computer simulation model for micro-structures. "-Transformation textures of test specimens were calculated by the model -v> and investigated through electron channeling patterns. gradually and reaches the value 6 below 750C. The relative intensity of (100} of as-rolled sheets is between 1 and 2 when finishing temperature is above 850C, and it has a maximum value between 750 and 850C. No dependence of finishing temperature on the (110} intensity was observed. In the case of bigh reduction as-rolled steel sheets the {100) intensity above 750C is the same as the intensity of conventional draft steels. But the{liD intensity is higher than that of conventional draft steels.
Above 850C, the {100) and {111} Table 2. When each reduction of final three stands is more than 50% and bar thickness is over 40rnm, high average of r-values between 1.0 to 1.2 are obtained. Fig.3 depicts the r-value of test pieces which were rolled with high reduction at temperature in a region above 760C after a rough rolling at a low temperature in T region namely at 950C, with the aim of refining T grains. The rolling schedule was Test (g) in Table 2. Most of average r-values exceed 1.2. When T grains are refined, which transform to fine ferrite and make the recrystallization of ferrtte easy to take place. The r-value of cold rolled steel sheets is affected by solute carbon. - Fig.4 describes the influence of carbon content on the r-value of hot rolled steel sheets rolled with more than 50% reduction at each final three stands. (Test (D in  The authors have used this model to study transformation texture of high reduction steel sheets. In the high reduction tests, the rolling starting temperatures were in 7" region, and three finishing rolling temperature ranges, in T, in a + T and in a range were selected. The initial texture was assumed to be random, as shown in Fig. 5. Fig.5 shows the orientations of high reduction steels which were investigated by electron channeling patterns (ECP). The specimen was deformed more than 75 at 950C. Calculated micro-structures were all dynamically recrystallized T. The orientation of dynamically recrystal-Random F=0. S (100)P. F. F=0. 9 (II0)P. F. The T with random orientation by high reduction rolling in T region transform to a with random orientation or {110}<001>. The {110}<001> changes into {112}{111}<011> because it is rolled in a range and recrystallizes. That is the reason why the r-value of the high reduction steel sheets increases.

An application of a computer simulation model for micro-structure
The authors have been developed a computer simulation model for micro-structures. 4-> The model simulates the hot rolling processes; heating, rolling and coiling processes. After that mechanical properties (YP, TS,EI) are predicted. When calculation has been stopled at the end of the rolling, volume fractions of dynamically recrystallized T, statically recrystallized 7", non-recrystallized T, and in the case of tz + T rolling finishing range, recrystallized a and deformed a are calculated by this model. It has been well established that the r-value of a steel sheet is influenced by the texture. In the case of hot rolled steel sheets, the final texture will be determined by ferrite transformation and rolling. This transformation texture is influenced by the micro-structure before the transformation. Relationship between the calculated micro-structures and rvalue was investigated. Fig. 7 shows the calculation flow of the model. after high reduction rolling above 850C. 0.9 of micro-structure were dynamically recrystallized T at the end of the rolling. The average r-value becomes 1.04 after transformation. Fig.9 shows the calculated results for steels which were rolled isothermally with high reduction at lower roiling starting temperatures. After the Fl-stand rolling the micro-structures were dynamically recrystallized and non-recrystallization " The volume fraction of recrystallized a at the end of the rolling xceeded 0.5. The average r-value of this specimen was 1.2. Fig. 10 shows the relationship between average r-values and calculated volume fractions of micro-structures. As the volume fraction of dynamically recrystallizedincreases, the average r-value increases gradually and reaches about 1.0. When the volume fraction of recrystallized ferrite increases, the average r-value increases moreover.
But when the volume fraction of deformed ferrite goes up, the average r-value decreases remarkably. It means that volume fractions of dynamically recrystallized T with random orientation and recrystallized a with {112-(111<011> after finishing rolling are useful for the increase of the r-value. According to the results in Fig.lO where D T and D a indicate the volume fractions of dynamically recrystallized T and recrystallized a. Table 3 shows the calculated examples. Since high reductions help the increase in volume fractions of dynamically recrystallized T and reerystallized a, the average r-values become higher. The average rvalue goes down with the increase of volume fractions of nonrecrystallized T and deformed a.
The model can then be used for finding an optimum rolling condition to increase the r-value.

CONCLUSION
Three ranges of finishing temperature were characterized in relation to the r-value. Hot rolled steel sheets with high r-value could be produced by high reduction hot roiling at above 750C. Maor findings are summarized as follow; 1) When each of the inal three reductions is more than 50%, the average r-value rises about 1.O--1.2. Then r-value is not affected by carbon content. Refinement of T grains is effective for the acceleration o ferrite recrystallization giving high average r-values. 2) The raction of dynamically recrystallized T and recrystallized a at the end of finishing rolling, which can be calculated through the model o micro-structure, and the r-value correlated. As the volume fraction of dynamically recrystallized T goes up, the average r-value increases to 1.0. As the volume fraction of recrystallized a increases, the average r-value increases and exceeds 1.2. However, non-recrystallized T and deformed a at the end of finishing rolling decrease the average r-value.
3) The average r-value is predicted by equation (1) through the volume fractions of dynamically recrystallized T and recrystallized a.