4 0 mm thick by the third pass so as to assure the sheet temperature immediately after the completion of the third pass varies within the temperature range of from 700 u00b0 c to 900 u00b0 c then the hot rolled sheets thus obtained were cooled to 150 u00b0 c with an average cooling rate of 50 u00b0 c second and slowly cooled with a cooling rate of 20 u00b0 c hour this is a similation test for the coiling at 150 u00b0 c the results of the tensile tests of the sheets are shown in the drawing it is clearly understood from the drawing that the steel composition containing 0 05 c 1 mn and 0 5 si cannot provide a substantially low yield ratio without addition of boron and that the addition of boron to a similar steel composition can provide a remarkable lowering of yield ratio and a substantial increase of strength without adverse effects on the ductility the ft temperature which ensures the lowest yield ratio namely the best ft temperature for the above steel composition is 820 u00b0 c and the temperature range which can produce the desired low yield ratio ys ts u2266 0 6 is from best ft 20 u00b0 c to best ft 40 u00b0 c various steels shown in table 1 were subjected to continuous hot rolling experiments under similar conditions to those in example 1 and the results are shown in table 2 in table 2 the results obtained with the best finishing temperatures are shown but the relationship between the.