Role of delamination fracture for enhanced impact toughness in 0.05 %P doped high strength steel with ultrafine elongated grain structure

Ultrafine elongated grain (UFEG) structures with strong 〈110〉// rolling direction (RD) fiber deformation texture were produced by warm caliber-rolling at 773 K, namely tempforming in the 1200 MPa-class medium-carbon low-alloy steel with phosphorous (P) contents of 0.001 and 0.053 mass%. Charpy impact tests were performed at temperature range of -196 to 150 °C on the UFEG structure. Regardless of P content, high upper shelf energy about 145 J and a very low ductile to brittle transition temperature (DBTT) of around -175 °C were obtained. P segregation embrittlement completely disappeared in the 0.053 %P steel and both steels showed ductile fracture on the planes normal to RD at temperature range of -150 to 150 °C. The main reason for the high upper shelf energy and very low DBTT in the 0.053 %P steel would be delamination fracture along RD when both 0.001 and 0.053 %P steels showed quite similar microstructures including texture. Since the occurrence of delamination requires relatively weak interfaces or planes, P segregated to the ferrite grain boundaries and interfaces of cementite particles-ferrite matrix and made them feasible paths for crack branching and consequently delamination occurred. We showed in this work the advantage of delamination (crack arrester-type) on the high absorbed energy obtained by 0.053 %P steel in comparison with 0.001 %P steel.