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[Dharmendu007]

For purlin and runner specification:
     In Table 1 of IS 1079: 2009, chemical composition of HR0 to HR5 designation steel are given. And in Table 3, HR5 is subdivided into YST255 to YST500, and the mechanical properties are given. Now the chemical composition of YST255 to YST500 is essentially same, i.e., that of HR5.      Then how they are having such a wide variation of mechanical properties, for example minimum tensile strength of YST255 is 320MPa and maximum tensile strength of YST500 is 680MPa, but all are having same chemical composition!