Characteristics of Work rolls for TMT Bar Rolling mills

 By SARJIT ROLLS

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.

Rolling mills: Roll Hardness and it characteristics

Hardness of rolls is considered to be of prime importance for roll users these days rather it has become the only defining factor to judge the quality of the rolls.

This has been proved to be incorrect.

There are a set of important parameters for rolls which together define the quality of rolls

1        The composition i.e. the carbon content and the alloy factor are of utmost importance. The carbide formation (MC) is dictated by the carbon content which varies from rolls to roll and its application in the rolling mill. Addition of alloying metals like Ni, Cr, Mo, etc. increase the alloy factor of rolls which further increases the wear resistance.

2       The Micro-structure i.e. the way the Fe atoms align in the roll. This parameter is the most difficult to control within a roll; standardization of the micro-structure is what differentiates the good roll makers from the normal ones. The micro-structure could be one which imparts strength to absorb shocks or have a fine structure which imparts polish, etc.

3        Hardness of rolls is a property of its cooling rate during casting along with the carbide content in the rolls. So, a Roll with high carbide formation and fast rate of cooling would have a higher hardness than slow cooling rate or lower carbide formation. On the same lines, Cast iron roll with higher carbon content would have more carbide formation and hardness than steel base rolls with lower carbon content. eg. DPIC cast iron or Acicular SG iron (high carbon) material compared to Adamite and SG Steel (low carbon)

In a rolling mill the above 3 parameter should vary as below for the different areas in a mill are required as below: e.g. in a roughing mill the carbon content required is low, rolls like Adamite rolls, SG Steel Rolls, while for a finishing mill it should be higher for better carbide formation and better hardness, rolls like DPIC Rolls, High Chrome rolls, Acicular SG Iron rolls

                                    ROUGHING MILL                    INTERMEDIATE MILL                   FINISHING MILL

COMPOSITION:                                LOW ------------------ CARBON CONTENT ------------------ HIGH

MICRO STRUCTURE:                      HIGH --------------------       STRENGTH      --------------------  LOW

MICRO STRUCTURE:                      LOW -------------------- FINE STRUCTURE -------------------- HIGH

HARDNESS:                                       LOW --------------------      HARDNESS       -------------------- HIGH

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