If Carbon + Manganese with Sulfur levels are increased it may lead to these type of cracks. Photograph is at low magnification i am unable to see step wise cracks. If it exists then crack is due to Hydrogen levels in steel. Micro etch shows heavy segregation try seeing Micro-structure near the center portion with crack at center. if heavy pearlitic banding is seen then cracks will form. If Nb is not given time for proper dissolution in laddle furnace then these crack may occur, you may even observe tensile splitting cases also.

Dear Raghu,

thanks for the reply. Have you gone through the microstructure available in the word document file. Can you suggest a line of investigation that I should follow

 Hello Sir,

Today i have seen the word file.. 

Comments on Rolling

"The plate is 50mm thick which was rolled from 20mm thick slabs." I think 200mm slab

Rolling pattern should have atleast 10%-12%  minimum in first pass in roughing mill and pattern should follow Reverse V pattern. Transfer bar thickness needs to be kept 3 times the final thickness. Rolling would be secondary every plant has it capacity to design rolling schedule.

Comments on Metallography/Casting/ Steel Melt Shop

Unetched Microstructure shows MnS (Grayish Once) and they does seem to be elongated which means reduction was less  and did not reach the center. From segregation point of view. Heavy segregation has occurred in the cast sequence (Some casting parameters needs to checked)  If you see in the unetched microstructure you can see some whitish cuboidal or rectangular or round features those are probably Nb which were not dissolved in ladle furnace. (I may be wrong as optical image does not confirm it, I use to locate it with my experience). To confirm or locate Nb at decently high magnification (500X to 1000X) scan the center area use back scattered mode in SEM instead of Secondary electron mode. Fe-Nb takes time to dissolve in Steel, Sequencing of addition needs to be checked. If not Nb MnS and heavy alumina can create cracks with Carbon 0.16 and Mn 1.53. Chained structure is also seen Alumina inclusion levels are also on higher side. De oxidation practice needs to be taken care. 

I do not find it as hydrogen crack as it will give you steps and in NDT peaks it will be shown as ups and down with disturbance in between. Those are not pits it is basically a de-cohesion between MnS or Nb or Alumina with Matrix.

Crack has occurred due heavy segregation at center. Steel needs to be clean, Rolling can help to reduce the effect of segregation but cannot remove it. 

Seems like Boiler plate with strength 350+ MPa. Strength also has an effect on creating cracks. Cleanliness needs to improved for high strength. 

Hydrogen cracking is normally delayed and as noted above, will give a stepped crack. so I would not expect it to be Hydrogen cracking

I'd agree with Raghu, that the cracking was due to heavy segregation at the centre. The unetched micrograph shows significant inclusions, so there is probably other non-metallic inclusions, such as Oxides, including the MnS. Pointing to a steelmaking issue.

The structure has a high degree of martensite/pearlite, 'hard' phases, which increases the propensity for centreline cracking.

Thanks Raghu and Kristin

Your answers have been really helpful

Regards

Raju