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Can any body tell me the difference b/w plain carbon steel & killed carbon steel? What is the speciality of KCS & why it is called so? What are the specific applications of KCS? It would be a great help if some one can also tell me the criteria of Pre & PWHT requirements of KCS w.r.t relevant codes/ standards?

Killed carbon steel is basically deoxidation process. In KCS, oxygen removes by the addition of silicon or Aluminum (de-oxidizing agent) to make the steel clean, without voids and refines the microstructure.
In practice, when a carbon steel contains a residual content of 0.10 Silicon in the chemical composition, it is considered as a Killed carbon steel. Examples are ASTM specifications A 516 Gr. 60, 70 (Plates), A 333 (Pipe); A 350 Gr. LF2 (forgings) etc.

The benefit of killing is to reduce the number of gas pockets present in the steel, which can adversely affect the mechanical properties of the steel, including ductility and toughness, as well as reduce the number of oxide-type inclusions in the steel.

In case of KCS plates to fabricate Pressure vessels, heat treatment requirements are listed in ASME Sec VIII, Div. 1, Part UCS-56 & UCS-79. In these sections formulas & thickness limitations are given to analyze the requirement of Heat treatment.

Hope this helps!!!

Specific applications of killed carbon steel include handling of low temperature services like liquid ammonia mainly in petrochemical industry.
For guidelines of welding of killed carbon steel piping, best reference is API 582. Pre and post weld heat treatment depends on minimum design temperature of service against pipe thickness; all KCS piping to handle temp. below -35 deg C requires PWHT.