Friday, October 4, 2013

Ladle & Tundish Repair Section Steel Making Dept. Pakistan Steel.

Aqil Khan

Ladle & Tundish Repair & Relining Section
(Steel Making Department Pakistan Steel).

The Steel Making Department is the main consumer of high quality refractories which directly affects the pace of production, quality and cost per ton of steel.

Ladle Repair Section.
The ladle repair and relining section is a vital integral unit of Steel Making Department Pakistan Steel which is located in the aisle of Continuous Casting bay at the back of 1300 tonne Mixer section.

After casting a heat, the empty steel ladle from Continuous Caster is sent back to Ladle Repair Section where they are inspected for refractory wear / mechanical fitness and then prepared for subsequent  LD-Converter’s steel tapping and Continuous casting operation cycle.

As per project ,the working refractory lining of Steel Casting ladles (SMD) was comprised of Fire clay bricks ( lining life was 9-11 heats), later ( with the efforts of Pakistan Steel’s engineers) switched over to local Tar-dolomite bricks (25-30heats) in the year 1988-89, which brought improvements and smoothness in the steel production cycle.

As per project, the steel teeming ladles of steel making department were provided with sleeve’s stopper and nozzle mechanism which were inferior against resistance to aggressive slags and were unable to cope up the teeming demand of high quality aggressive steel grades.

Later in the year 1984-85, the steel teeming mechanism was replaced with imported hydraulically operated Ladle slidegate mechanism with its superior quality shaped refractories (Al2O3 content 70% or higher) shown below.

A similar slidegate system was also proposed to replace the stoppers and nozzles system for steel dispensing from tundish to moulds but due to high maintenance cost it was not found economically viable for present level of production.

Tundish Repair Section.
Tundish repair & relining section is located adjacent to ladle repair section. The tundish is a boat shaped refractory lined intermediate vessel (specially designed for continuous casting machines), where liquid steel goes down from steel ladle through slide gate and falls on its middle portion (impact pad) and exits out through bottom metering nozzles of tundish to feed the liquid steel to water cooled copper moulds for acquiring desired shapes ( Slabs, Blooms or Billets ).

 After completion of refractory life campaign, the tundishes are de-skulled and prepared  for relining work.

Due to work load, steel making department has its own trained masonry workforce which remains continuously engaged in ladles and tundish relining work.

Tundish Refractories.
The tundish distributes liquid steel to the moulds through metering nozzles located along its bottom thereby provides stable stream to moulds and keep steel flow constant in continuous casting process.

As per project, the working lining of tundishes were provided with fireclay brick work. Every time the tundish had to be freshly relined with new bricks and then dried and preheated for 5-6 hours before start of casting process which was extensively labour, material and time consuming process.

Later in the year 1985-86, it was decided to switch-over from Brick lining to Silica boards lining which significantly reduced relining and preheating cycle time for each tundish.

For tonnage steel grades, where steel quality is not so critical, silica board lining is very attractive as low cost lining system. With less labour requirement, the lining boards are quick and easy to install.

 Monoblock Stoppers and Tundish Metering Nozzles.

Mostly the campaign life of a tundish is determined by the endurance of refractories of stoppers & metering nozzles. Tundish metering nozzles in combination with mono-block stopper controls the flow of stream of liquid steel before it exits the tundish.

As per project, the sleeve stoppers and low  Zirconia metering nozzles were provided for steel casting tundishes, caused frequent tundish failures specially during casting of low carbon Aluminium Killed steel grades.

During continuous casting the deposition of deoxidation and reoxidation products (mainly aluminium oxide inclusions) inside metering nozzle causes clogging. The severe nozzle clogging needs to be removed by oxygen lancing. Extensive O2 lancing leads to failure of metering nozzle due to fast erosion and results in replacement of tundish during casting operation. Consequently these events bloat additional costs for tundish refurbishment and sometime termination of continuous casting process which reduce the net casting throughput and productivity.

In order to overcome the above problems, the data of failures of local production practice was investigated and analyzed and lastly the high Alumina - graphite refractory monoblock stoppers of present dimensions and high zirconia (ZrO% ≥  67  ) refractory metering nozzles were  brought in practice in 1986.

The above combinations of refractories have proved to be good and helped to stabilize present production pace but Steel production is facing an increasing demand for high cleanliness with high quality and high performance.

So  future demand of higher quality steel grades with increase production rate will be needing to switch over to further superior quality refractories which may be mostly fulfilled by exploiting local resources and upgrading the manufacturing capabilities of the Refractories production plant at Pakistan Steel.

Wednesday, October 2, 2013

1300 Tonne Mixer Section (Steel Making Department).

1300 Tonne Mixer Section
(Steel Making Department Pakistan Steel).

The Hot metal (molten pig iron) is supplied to steel making department from different tappings of Blast Furnaces, stored in 1300 Tonne Mixer. 

1300 Tonne Mixer section is located in the charging aisle of LD-Converters of steel making department.

As the tapping schedule of Blast furnace does not match with the charging of steel making converters, Mixer acts as a reserve stock of molten pig iron at steel making department. Besides it equalizes the varying temperature and chemical composition of different tappings of Blast furnaces and helps the LD-Converter operators to set the operating parameters for steady pace of production cycle. 

There are two railway tracks for transportation and receiving the hot metal from Blast Furnaces of Iron Making Department  to Steel Making Department.

Mixer is made in cylindrical shape with the length to diameter ratio of around 1:3.
Mixer rests on special pads fastened on a strong reinforced-concrete foundation and when being tilted, it turns on its horizontal axes with the aid of rollers. The rollers are mounted in cast steel frames attached to the shell of the Mixer. A huge rocker arm and pinion system allows mixer tilt for pouring out hot metal.

As per project the safety lining of 1300 tonne Mixer is of heat insulating fireclay bricks which diminishes the heat loss to the surroundings. The roof is lined with heavy duty fireclay  bricks and inner working lining (900 mm thick) is made up of  two layers of Magnesite bricks.

The permanent pre- heating burners are provided at both the end faces and nozzle of the Mixer to keep the temperature of hot metal intact. As per project design, for Phase- ii expansion to 2.2 million tone, a premise adjacent to existing Mixer was left for installation of second 1300 Tonne Mixer.

Capital Repair of 1300 Tons Mixer.
The campaign lining life of Mixer is about 0.9 million tons hot metal holding cycle and after that Mixer is stopped for Capital repair for about two months.

 The main causes for stopping Mixer for relining are collapse of roof fire clay brick work, wearing of slag line and tap hole zone which need improvement. In order to enhance the lining life, safety and improve the service condition of hot metal Mixer, the conventional brickwork has been replaced with monolithic lining in developed countries.

Capital repair of Mixer is a labour and material intensive project. The refractory demolishing and relining work is carried out by R. R. Dept. Pakistan Steel. During capital repair of Mixer, besides the relining work, the electrical, mechanical, heat & power equipments and automation instruments pertaining to mixer operation are repaired and overhauled.

During capital repair of Mixer, the BF hot metal ladles are directly poured into hot metal charging ladle of Converter which increases the use of overhead charging cranes thereby interrupts the pace of production and affects the quality and yield of steel production.

 Hot metal mixer plays important role in achieving pace of production and quality in the Steel making shops which are operating without Torpedo vessels and computer aided dynamic models.

 Sulfur and silica are harmful components in Mixer slag. The unwanted Mixer Slag is skimmed off from Hot Metal Ladles before dispatching the hot metal to converter to prevent its passage into the LD- Converter.

As per demand of Converter operator, hot metal is poured out from 1300 tonne Mixer into a refractory lined ladle positioned under the Mixer spout on a weigh scale platform. The Mixer Section has arrangements for weighing hot metal, temperature measuring and taking samples for hot metal analysis.

Technical specifications of Hot metal for steel making. 

As per Pakistan Steel’s specifications, the average Physical  temperature of hot metal poured out of Mixer should not be less than 1320 ÂșC and the Chemical composition is kept controlled within the limits as under:

-          Carbon not less than 4%
-          Si   : 0.4 – 0.8%
-          Mn : 0.5 – 1.0%
-          P    : 0.2% max
-          S    : 0.04% max

On the basis of Physical temperature and hot metal analysis, the ratio of the amounts of liquid pig iron and scrap and the quantity of additives ( lime, dolomite,CaF2  etc) and amount of required O2 are calculated and adjusted by the Converter operator to achieve the targeted end point temperature and chemical composition for each batch (heat).