Rushabh Project

8/8/2019 Rushabh Project

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PROJECT ON

CASTER SLAB DIMENSIONAL ACCURACY

TECHNIQUE

BY

RUSHABH PARVANI

B.E.-3,(METALLURGICAL AND MATERIAL SCI. ENGG.)

FACULTY OF TECH. AND ENGG.

M.S. UNIVERSITY OF BARODA

REF CODE-09VT137GE


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INTRODUCTION

Continuous casting transforms molten metal into solid on a continuous basis and

includes a variety of important commercial processes. These processes are the

most efficient way to solidify large volumes of metal into simple shapes for

subsequent processing. Most basic metals are mass-produced using a continuous

casting process, including over 500 million tons of steel, 20 million tons of

aluminum, and 1 million tons of copper, nickel, and other metals in the world each

year.

Continuous casting is distinguished from other solidification processes by its

steady state nature, relative to an outside observer in a laboratory frame of

reference. The molten metal solidifies against the mold walls while it issimultaneously withdrawn from the bottom of the mold at a rate which maintains

the solid / liquid interface at a constant position with time. The process works best

when all of its aspects operate in this steady-state manner.

Relative to other casting processes, continuous casting generally has a higher

capital cost, but lower operating cost. It is the most cost- and energy- efficient

method to mass-produce semi-finished metal products with consistent quality in a

variety of sizes and shapes. Cross-sections can be rectangular, for subsequentrolling into plate or sheet, square or circular for long products, and even dog-

bone shapes, for rolling into I or H beams.

Many different types of continuous casting processes exist.Vertical machines are

used to cast aluminum and a few other metals for special applications. Curved

machines are used for the majority of steel casting and require bending and / or

unbending of the solidifying strand. Horizontal casting features a shorter building

and is used occasionally for both nonferrous alloys and steel. Finally, thin strip

casting is being pioneered for steel and other metals in low-production markets in

order to minimize the amount of rolling required.

1. Steel Continuous CastingContinuous casting is a relatively new process in historical terms. Although the

continuous strip casting process was conceived by Bessemer in 1858, the


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continuous casting of steel did not gain widespread use until the 1960s. Earlier

attempts suffered from technical difficulties such as breakouts, where the

solidifying steel shell sticks to the mold, tears, and allows molten steel to pour out

over the bottom of the machine. This problem was overcome by Junghans in 1934

by vertically oscillating the mold, utilizing the concept of negative strip wherethe mold travels downward faster than the steel shell during some portion of the

oscillation cycle to dislodge any sticking.[1] Many other developments and

innovations have transformed the continuous casting process into the sophisticated

process currently used to produce over 90% of steel in the world today, including

plain carbon, alloy and stainless steel grades.[1]

FIGURE 1


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steel flows from a ladle, through a tundish into the mold. The tundish holds

enough metal to provide a continuous flow to the mold, even during an

exchange of ladles, which are supplied periodically from the steelmaking

process. The tundish can also serve as a refining vessel to float out

detrimental inclusions into the slag layer. If solid inclusion particles are

allowed to remain in the product, then surface defects such as slivers may

form during subsequent rolling operations, or they may cause local internalstress concentration, which lowers the fatigue life. To produce higher quality

product, the liquid steel must be protected from exposure to air by a slag

cover over the liquid surface in each vessel and by using ceramic nozzles

between vessels. If not, then oxygen in the air will react to form detrimental

oxide inclusions in the steel.

Once in the mold, the molten steel freezes against the water-cooled walls of

a bottomless copper mold to form a solid shell. The mold is oscillated

vertically in order to discourage sticking of the shell to the mold walls. Drive

rolls lower in the machine continuously withdraw the shell from the mold ata rate or casting speed that matches the flow of incoming metal, so the

process ideally runs in steady state. The liquid flow rate is controlled by

restricting the opening in the nozzle according to the signal fed back from a

level sensor in the mold.

The most critical part of the process is the initial solidification at the

meniscus, found at the junction where the top of the shell meets the mold,

and the liquid surface. This is where the surface of the final product is

created, and defects such as surface cracks can form, if problems such aslevel fluctuations occur. To avoid this, oil or mold slag is added to the steel

meniscus, which flows into the gap between the mold and shell. In addition

to lubricating the contact, a mold slag layer protects the steel from air,

provides thermal insulation, and absorbs inclusions.

Below mold exit, the thin solidified shell (6-20 mm thick) acts as a container

to support the remaining liquid, which makes up the interior of the strand.


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Water or air mist sprays cool the surface of the strand between the support

rolls. The spray flow rates are adjusted to control the strand surface

temperature with minimal reheating until the molten core is solid. After the

center is completely solid (at the metallurgical length of the caster, which

is 10 - 40m) the strand is cut with oxyacetylene torches into slabs or billetsof any desired length.

Different continuous casting processes exist to produce cross sections of

different shapes and sizes. Heavy, four-piece plate molds with rigid backing

plates are used to cast large, rectangular slabs, (50-250 mm thick and 0.5

2.2 m wide), which are rolled into plate or sheet. Similar molds are used for

casting relatively square blooms, which range up to 400 x 600 mm in

cross section. Single-piece tube molds are used to cast small, square billets

(100 - 200 mm thick) which are rolled into long products, such as bars,angles, rails, nails, and axles. The new strip casting process is being

developed using large rotating rolls


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FIGURE 2

as the mold walls to solidify 1-3mm thick steel sheet.

When casting large cross sections, such as slabs, a series of rolls must


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support the soft steel shell between mold exit and the metallurgical length, in

order to minimize bulging due to the internal liquid pressure. Extra rolls are

needed to force the strand to unbend through the transition from the

curved to the straight portion of the path shown in Fig. 1. If the roll support

and alignment are not sufficient, internal cracks and segregation may result.These defects will persist in the final product, even after many rolling and

other operations, so it is important to control the casting process.

The process is started by plugging the bottom of the mold with a dummy

bar. After enough metal has solidified like a conventional casting onto its

head, the dummy bar is then slowly withdrawn down through the continuous

casting machine and steady state conditions evolve. The process then

operates continuously for a period of one hour to several weeks, when the

molten steel supply is stopped and the process must be restarted. Themaximum casting speed of 1-8 m/min is governed by the allowable length of

the liquid core, and to avoid quality problems, which are generally worse at

higher speeds.

After the steel leaves the caster, it is reheated to a uniform temperature and

rolled into sheet, bars, rails, and other shapes. Modern steel plants position

the rolling operations close to the caster to save on reheating energy. The

application of computational models to understand and improve this process

is discussed in Model_Chap.


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Continuous casting process in ESSAR STEEL:-

ESSAR STEEL contains 3 continuous casting machines. All of them are

automated by the company SMS-DEMAG. They are namely CCM-1, CCM-2,

CCM-3. CCM-1 and CCM-2 are having bend rolled casting while CCM-3 has

vertical roll process.

In the continuous casting process in ESSAR STEEL the other difference between

the caster machines have oscillating moulds which enhances the speed of casting

and prevents solidification of metal in the moulds.

In CCM1 and CCM2 the mould oscillations are done by the means of mechanical

motors.While CCM3 has hydraulically operated machine mould oscillator. This

gives a constant force in every stroke of oscillation which enhances the

productivity of the machine.

PROFILE OF THE CASTER MACHINES IN ESSAR STEEL:-

1. CASTER-1WIDTH OF THE SLABS PRODUCES: 800-2000 MM

SEGMENTS:15


SEGMENTS:15


SEGMENTS:14


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The flow of the liquid metal from ladle to tundish is controlled by slide gate

mechanism, and the flow of metal from tundish to mould is controlled by stopper

mechanism. The liquid metal is then rolled under in between segments and cooled

simultaneously with help of water jets which are automatically controlled. The

speed at which the casting is done are of utmost controlling factor for the quality of

the slabs produces.

The slabs are normally produced with thickness of 220mm. these slabs are then

processed in HOT STRIP MILL in which the thickness is reduces from 220mm to

up to 0.6 mm. Hence a slightest inclusion or SLIVER in the slab will be increased

to a great extent in terms of surface area when the slab thickness is reduced. Hence

the main aim of concast machine is to produce a sound casting which is mainly

governed by factors like temperature and speed of casting.

PROJECT ON SLAB SHRINKAGE

Basic Concept

y It is observed in concast process that during water jet cooling the avg. speedof the casting process is the most important factor governing the Quality andDimensional accuracy of the casting slabs. If the speed is slow Avg.Width-

TheoreticalWidth then will come on negative side. If the speed is high then

Avg.Width-TheoreticalWidth will come on the positive side.AIM OF PROJECT:-

So now it is clear that a OPTIMUM THRESHOLD SPEED(OTS) must be

established in each GRADE CLASS of steel for different widths so that we get

very high dimensional accuracy as far as widths are concerned. This will result in

maximization of dimensional accuracy which will ultimately increase theproductivity and result in reduction of cost of the process as a whole. If the speed

prediction is available it will also help in REALISTIC prediction and planning of

future production rate and costing.

To establish a accurate OTS we must be well aware of the factors governing the

Avg. Speed of casting.


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Factors affecting the average speed:-

The avg speed of casting is calculated in unit of met./min. in ESSAR STEEL.

The main factors governing are :

y Set width of the slaby Grade class of the steely Type of casterIt may be observed that the type of caster is taken in consideration only to

maintain uniformity which will help in establishing individual relations with the

above factors.

Data available:-

The following data were available of January 2010 to June 2010:-

y Date of Castingy Castery Slab Noy Heat Noy Gradey GRADE CLASS

1. LCMA(LOW CARBON MICRO ALLOY)2. LC(LOW CARBON)3. HCMA(HIGH CARBON MICRO ALLOY)4. HC(HIGH CARBON)5. VHC(VERY HIGH CARBON)6.

PERI(PERITECTIC)7. PCMA(PERI CARBON MICRO ALLOY)

y SetWidthy Avg.Widthy TheoreticalWidthy Avg.Width-TheoreticalWidthy Avg. Speed


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Hence we have all the parameters that govern the OTS which will cause us to

establish individual relations with avg, speed.

Avg Speed and Set Width:

Initially all other factors were kept constant other than set width to observe a relation

between avg. speed and set width. It was observed that with increasing set width the

avg. speed decreases.

Grade class and Avg.Width-Theo. Width:

GRADE CLASS Avg.Width-Theo.Width (MM)

PCMA -3 -5 -3 - - -

LCMA 0 1 4 3 -2 -

HCMA -2 -3 -1 0 6 0

HC 2 1 3 4 11 3

LC 4 6 5 1 - 6

PERI 7 8 11 16 7 20

WIDTH(MM) 925-1125 1126-

1325

1326-

1526

1526-

1725

1726-

1925

2000

Hence from the above table we observe that HCMA grade has highest Avg.Width-

Theo.Width on negative side and PERI grade has highest Avg.Width-Theo.

Width on positive side.

It was observed that HC has Avg. Width-TheoreticalWidth nearest to zero.This is

due to the fact that as the carbon content increases the ductility decreases which

restricts the quantity to vary from zero. In LC the expansion was high. the highest

Avg.Width-Theo.Width was in PERI maybe due to the fact that when cooled it

forms a liquid and solid causing higher Avg.Width-Theo. Width.


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It was observed that Avg.Width-Theo. Width in microalloys like HCMA and

LCMA was highly on the negative side. Hence it gives the idea that micro alloy

addition causes great changes in the dimensional and elastic properties of steel.

Optimum Threshold Speed(OTS):

The OTS is the speed at which the particular grade class in a set width which

causes Avg.Width-Theo. Width tending to zero.

This can be achieved if we observe the scatter trend of avg. speed v/s Avg.Width-

Theo.Width any grade class at a particular class of width.

Then by plotting the moving average of the scatter graph we can get the OTS by

marking the point at which the moving average line intersects with the Zero Avg.

Width-Theo.Width line.

It must noted that in all cases the OTS must be such that it must have some room

for variation and the Avg.Width-Theo.Width must not vary greatly by the

slightest change in avg. speed.


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OTS(MM/MIN ) AT VARIOUS SET WIDTH OF SLAB(MM)

925-1025 1025-1125 1125-1225 1225-1325 1325-1525 1525-1725 1875 1900 1925 2

GRADECLASS

LCMA 1.43 1.1 1.275 1.2 1.19 1.56 0.88 - - -

LC 1.53 1.51 1.25 1.15 1.395 1 - - - -

HCMA 1.05 1.35 1.2 0.95 1.3 1.04 1.09 0

HC 0.83 1.2 1.15 1.18 1 1 0.91 0.89 1

PCMA 1.5 1.18 1.09 1.13 - 0.85 - - -

PERI 1.14 1.2 - 1.07 - - - - -

VHC 1 - - - - - - - - -

SIL - 1.3 - - - - - - -

he above chart gives the info of the OTS of different grade class at various widths.


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The above chart describes the variation of OTS in various grade classes.

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

0 2 4 6 8 10 12

LCMA

LC

HCMA

HC

PCMA

PERI

VHC

SIL


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CONCLUSION:-

The above results gives us various OTS values if we run the casting at this speed

we can get the Avg.Width-Theo. Width tending to zero.

It may also be noted that below the OTS the Avg. Width-Theo.Width will mostly

be on the negative side and above OTS the Avg.Width-Theo.Width will be on the

positive side. As noted if Avg.Width-Theo.Width is positive then we get the

freedom to set the width changes which can be frequently seen in grades which

needs scarfing. But if Avg.Width-Theo.Width is negative then the degree of

freedom is lost in grades that require scarfing such as HIGH CARBON grades.

The values which are not given are either very high value of Avg. Width-Theo.

Width is not casted at the particular range of set width.

Also with the help of correlation between charts of Avg.Width-Theo.Width and

avg. speed at various widths we can establish a relation between Avg.Width-Theo.

Width and avg. speed

The highest observed OTS was 1.56mm/min. in LCMA grade class in range 1525-

1725mm set width range.

With these speeds the almost Avg.Width-Theo.Width tending to zero can be

achieved which results in and reduction of costs. This will help enhance the

productivity of the continuous casting process.

Also it will result in sound casting slab at once with less number of crackes

frequently observed in that grades.


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THANKING NOTE

I would like to thank MR.S.BANERJEE(HOD SMP) for giving time, interest

and knowledge as my Vocational Training Guide and MR.R.Sanghwai(HOD

CASTER) as my project guide for giving me vast amount of knowledge andanswering all my queries.

I would also like to thank:-

MR.V.V.APPARAO(TECH CELL,CASTER)

MR.MURALI(TECH CELL,CASTER)

MR.AWINASH SAWRANKAR(ELC ACADEMY).

MR.KARTHIK(TECH CELL, CASTER)

MR.MAYUR(TECH CELL, CASTER)

AND ALL OTHER PEOPLEWHO GAVE ME KNOWLEDGE.

Without the help of whom the project would not have been possible.

Rushabh Project

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