Development of Newer Version of Work Roll for last

Finishing stand in HSM

at

Bharat Roll Industry, Kolkata

By

K P Verma S.Chatterjee Sanjay Kumar

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Reverse Roughing Hot Strip Mill

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Continuous Hot Strip Mill

F2 F8 F5F4 F7F6

Cast Roll Manufacturing Method

Centrifugally Cast Roll

Horizontal

Spin

Casting

Inclined

Spin

Casting

Vertical

Spin

Casting

COPE

CHILL

DRAG

BEND

SPRUE

Mould is static and metal is poured

either through sprue or from top.

This practice is more suitable for

casting roll up to barrel Hs 65 shC.

Gradual change in properties due to

gradually change of rate of

solidification.

Exceptions are Clear Chill roll

which has clear white iron at the

barrel surface & via narrow mottled

zone solidifies suddenly to grey

iron.

Generally it is a single composition

melt.

Mono Cast Static Roll

Double Poured Static Roll

Static cast double poured rolls are produced by

displacing and /or diluting the liquid metal inside the

mold with a second liquid metal with different

composition.

The first metal poured into the mold eventually form

the working layer or shell of the barrel.

Different techniques for displacement of parent metal

with controlled pouring rate & after certain time gap so

that required shell depth can be achieved and mixing

of shell metal with 2nd metal should be minimum.

Centrifugally Cast Roll

Horizontal Spin Casting processShell metal is poured into spinning

horizontal mold directly into the barrel

portion of the roll.

Temp. of the spinning shell metal is

monitored through radiation pyrometer.

After shell solidification mold is made

vertical & core metal is poured.

Horizontal Centrifugal Casting

Centrifugal Casting (Shell part)

Chill mould

Ladle

Funnel

Roller

Ladle

Chill mould

Top side mould

Bottom side mould

After the Chill mould is set up, and the core metal

is cast.

Need for Improvement

in

Work Roll Properties ?

Roughing Stand : Adamite

: Hi-Cr Steel

: Semi High Speed Steel

Initial Finishing Stand : Hi-Cr Iron

: Modified Hi-Cr Iron

: High Speed Steel

Final Finishing Stand : Normal ICDP

: Enhanced Carbide (ECIC)

: Spl. Enhanced Carbide

Development in Rolls For Flat Product

CHANGES IN MILL REQUIREMENT :

Demand for higher productivity. ICDP is the weakest link

in the whole stand.

Thiner gauge rolling.

Faster speed of rolling.

Prolong campaign life / Less frequent changing of rolls

Demand for less mill down time.

Improved Mill cooling / condition.

Energy saving

Less off-take during grinding

Better Strip Profile / Surface Finish of Strip

Improved compressive strength

Improved thermal fatigue

Expectations from Finishing Mill WR

Improved wear resistance

Improved tensile strength

Excellent resistance to slippage and sticking

Improved resistance to thermal fatigue

To have improved resistance to cracks

It is not possible to get all properties in one roll. Hence we need

to customize & optimized depending upon mill rolling condition.

Strip Thickness Wear Factor

Suppose for rolling

1.5 mm thick strip

m

So for 1.2 mm 40 % more of

m i.e.

m+ 0.40m

0.9 mm 160 % more

of m

i.e

m + 1.60 m

Modern DSP/CSP Mills are rolling strips < 1.0 mm thickness.

While rolling thin Gauges wear increases dramatically.

Mill Severity

Strategy for New Development in Rolls For Flat Rolling

Objective was to enhance the performance of last finishing stands

roll by 10/30 % in terms of Ts / mm to reduce mill down time due to

frequent change of roll without any additional risk involved.

Wear resistance can be improved by increasing the volumetric

fraction of harder carbide but this will reduce the fracture toughness

of shell material and in case of a mill incident depth of crack will be

more. Hence idea is to retain the same fracture toughness.

Similarly crack resistance can be improved by many ways but idea

is not to compromise on effective Ts / mm.

This has been achieved through

(A) Enhancement of carbide achieved through combination of spl.

Alloying elements ( V /Nb/W /B)

(B) Improved matrix hardness through introduction of spl. Carbides

forming elements like V,Nb,

(C) Use of spl. inoculant & modification in inoculation practices. For

improved graphite morphology.

(D) Modified heat treatment for minimizing residual stress.

Development of newer version of rolls

for last finishing stand in HSM

Spl. Enhanced Carbide Roll Characteristics

Different types to suit specific mill requirement

Able to meet specific barrel hardness

Improved wear resistance with same level of graphite

Improved crack resistance to suit mill requirement

Improved bending strength

Presence of additional MC type carbide in a Banitic/

Martensitic matrix

Scope for varying shape of Graphite from globular to lamellar

depending upon mill condition

Improved / optimized RA value

Improved campaign length

Less surface deterioration during rolling campaign leading to

minimal grinding after each campaign.

Controlled Hardness drop at scrap dia. 2-3 shC

Metallurgical difference among ICDP /ECIC /

Spl ECIC Rolls

• Wear resistance type

• Crack resistance type

Different type of Spl. Enhanced Carbide Roll

Characteristics of Special ECIC Roll

Un Etched X 50

ECICSpl. ECIC

Comparative Microstructure

ICDP

Comparative Microstructure

ECIC 100 X Spl. ECIC 100 XICDP 100 X

Spl. ECIC & Normal ICDP Hardness drop curve :

Hardness drop curve for Spl. ECIC roll along with Normal ICDP . Starting Hs

78-80 shc & Hs drop ~ 1 shc /10 mm. Along the barrel maximum Hs variation

is ~ 3-4 shc

Results obtained so far

The roll to be sent to rolling mill should be free from crack, marks,

chatter & wheel marks. In case of any crack visible on roll barrel

surface, this has to be removed completely.

Its better to remove 0.5 mm extra or optimized through ECT /UT

even if crack is removed completely so that crack is removed from

the in case of a known mill incident.

Min. 6 set of rolls per stand should be in circulation for avoiding

grinding of rolls in hot condition & hence reducing inbuilt stresses.

The defects found during / after rolling should be localized as far

position on the roll barrel and should be included in the roll history

card for follow up action.

These rolls should not be used in initial finishing stand because of

intrinsic mechanical & thermal properties being unsuitable for

initial finishing stand to avoid any failures.

Threshold of crack /bruise to be revised in case of known cobble /

mill incidents.

Roll Care in the Mill

Cobble rate : It should be as low as possible.

Tail Chew / Turned Tail :The frequency of this problem

should be tried to be as low as possible through more

disciplined approach.

Roll cooling has to be regulated after any interruption

in rolling process for over /under cooling of rolls.

Starting of mill should be slow for new rolls until the

rolls pick up a temp. of 60-65oC.

Over campaign life of BUR & WR results in deep

concave wear profile which leads to high contact

pressure at the ends, which should be avoided.

Back up roll grinding to be done so that work hardened

layer is removed from barrel surface.

Roll Care in the Mill

Flow of cooling water should not be interrupted

otherwise due to thermal stress roll may break.

Insufficient water causes increase in roll temp. Roll

temp. above 100 deg.C may cause unstable rolling

condition.

Local lack of cooling ( Blocked nozzle) causes

circumferential strip of different roll surface temp.

which may effect strip quality.

Local lack of cooling water and heating up of the roll

along one generating line may cause sticker.

Roll Care in the Mill

Thank You