Monday, November 28, 2016

Characteristics of Work rolls for TMT Bar Rolling mills

Keywords: Adamite Rolls, Alloy cast steel rolls, SG Iron Rolls, DPIC Spun cast rolls, SG Acicular rolls

This blog is a simple overview of the kind of rolls that are currently being used in Rolling mills in this case a bar mill; mill dynamics remain similar but with the change in the end product of the mill i.e wire rod, section, plates, strips, TMT bar etc. the rolls could vary. 

Broadly, a TMT bar rolling mill like any other mill can be broken down into 3 main parts – The roughing mill, the intermediate mill and the finishing mill. 

The Roughing Stands: As the name implies the roughing stands mainly use billets as an input in a 3-High stand. Depending upon the output size of the bar i.e. 20 inch to 16 inch, the number of stands in the roughing mill vary from a single stand to 3 different stands in case of 16 inch.
Characteristics of rolls used for roughing stands – Increased strength and ability to absorb jerks as there is high pressure on the rolls due to the large reduction in size of the bar. So rolls used are Forged steel, Adamite steel in the first two stands and in some mills even spheroidal graphite (SG).

The Intermediate stands: These are 2- High stands which form the mill for solely reduction purposes i.e. have rolls with qualities both of strength and good decent hardness as the inter stands are the building blocks for the bar before they enter the finishing stands. Ideally, the later stands of intermediate should have rolls with high hardness but due to costing reasons the rolls used in these stands are SG Iron or Single poured cast iron (ICCI) rolls or even DPIC rolls now.

The Finishing Stands: The bar sizes of 8 mm, 10mm or even 12 mm are the final sizes. As the name suggests, finishing rolls should be such that provide a shine on the product with great impression and perfect sizes. Thus the rolls for the finishing stands should be very high on hardness with great biting qualities. The work rolls ideally used in finishing stands of TMT bar mills are either SG Acicular (ASG) or Spun cast Double poured solid rolls (DPIC). 

Check the video of how hardness as a factor of depth has greatly improved with new technologies for casting of rolls hardness testing of roll for finshing stands

In my next blog I would be stressing on the composition of the rolls and how they vary with different mills.

Monday, November 14, 2016

Spun Cast DPIC Solid rolls for finishing mills

Keywords: Spun cast DPIC rolls, SG acicular rolls, Enhanced carbide rolls, SG Iron
Generalizing, the pass life for rolls for finishing stands relative to each other on a scale increasing from left to right

Pass life of the roll has become the defining factor for rolls in Rolling mills. A Roll is intended to have better performance at a sustainable price. A roll manufacturer would agree that quality comes at a price but just adding alloys into the rolls is not the right way to go. There needs to be a reasonable understanding of what kind of an element has what kind of an effect on the wear resistance of the roll and for what stands. 

In this blog I would be alluding to the work rolls for finishing mills.

Though rolls have come a long way from the simple static cast Pearlitic structures a technology of the 1950’s to centrifugally cast double poured solid rolls (1980’s). There was also a time for shaft fitted double poured barrels but with the increase in production and subsequently the speed of the mills, the shaft fitted barrels often lead to slippage causing mill accidents. 

To counter this problem spun cast DPIC solid Rolls are used for finishing stands of rolling mills. They have the right mix with an adequate hardness of around 80 +/- 2 sH with a bainitic structure to provide a polish on the rolled product along with the necessary alloying to have a longer pass life as it portrays properties of increased wear resistance.

DPIC Rolls can be classified into 3 main categories: Normal spun cast DPIC rolls with nickel content in range of 2.5 – 4 % and then Enhanced Carbide DPIC with further added alloys in the range of 0.5 - 1% and SG Acicular rolls with the outer working layer made up of highly alloyed Spheroidal Graphite SG iron