With  increasing  the  strength  and  toughness  of pipeline  steel

the  corresponding  evolution  of structure  is pearlite-ferrite(X65    grade)

acicular    ferite(X80    grade)

particle    bainite-ferrite(X100    grade)and    lower    bainite(X120 grade).Grain refining and dispersed secondary phase are also important measures to increase the strength and toughness of pipeline steel. In general the structure of pipeline steel is improved by optimizing composition of steel and increasing cool- ing speed of control rolling process.