Production and Processing of Armour Steel

Production and Processing of Armour Steel

In the present day environment, there are accelerated efforts to deliver lightweight armour technologies which can defeat armour-piercing (AP) projectiles at reduced areal weights. While many of these efforts involve the application of lower density metals such as aluminum and titanium, the selection of steel alloys continues to be competitive for many ballistic and structural applications, because of its ability to fabricate armour components in both commercial and military operational areas with available equipment and personnel. This is a major advantage of steel solutions.

Steel is the best all round performing armour material in spite of its high density because of its properties like toughness, ready availability, low cost, castability, and weldability etc. Armour steels are not ordinary steels but they have high strength combined with hardness and fracture toughness. They are used to protect objects from projectile damage or pressure during combat. These steels are generally used in the form of hot rolled plate normally in the manufacture of armoured vehicles.

The main properties of the armour steel such as toughness, hardness, good fatigue strength, ease of fabrication and joining and having relative low cost make the steel a popular material for the armoured vehicles. The important requirement of this steel is that it is to maintain structural integrity even at the sub-zero temperatures when impacted by overmatching artillery rounds. Hence, this steel is required to have low temperature impact strength. Other important considerations for the armour steel plates are that they are to be amenable to modern fabrication and construction techniques and be readily weldable and capable of being produced in a variety of shapes.

It is well known that the chemical composition, austenitization and tempering temperature, and grain size of the steel affects its mechanical properties of steel and hence it’s ballistic performance. It has also been established that the mechanical properties and the ballistic performance of martensitic steels can be optimized by controlling the chemical composition and the heat treatment parameters.

Armour steel is basically a high strength low alloy structural steel which has been treated to have property of very high resistance to penetration. This property to the steel is usually imparted by the heat treatment usually by the thermo mechanical treatment. It is well known that the resistance to penetration of steel can be improved by increasing its texture intensity which can be obtained by thermo-mechanical treatment. The mass effectiveness of the armour increases with the hardness of the material. However, very hard armour tends to be brittle and to shatter when hit.

The main alloying elements of the armour steel are nickel (Ni), chromium (Cr), and molybdenum (Mo). The phosphorus (P) and sulphur (S) contents of this steel are to be very low (preferably less than 0.015 % of each element). Also there is to very low value of the dissolved gases like nitrogen (N2), oxygen (O2), and hydrogen (H2) in this steel. Further, the steel is to be very clean steel with very low level of inclusions. It is also to be free from segregation.

Steel armour can be classified into four main groups. These groups are (i) rolled homogeneous armour (RHA), (ii) high hardness armour (HHA), (iii) variable hardness steel armour, and (iv) perforated armour. Out of these four types, RHA steels are usually being considered as a benchmark material. RHA steel has been considered as the conventional armour for light armoured vehicles. It is a high quality alloy steel which is rolled out before being heat treated to give it an optimum combination of strength and toughness.

Rolled homogeneous armour (RHA) steel has remained the standard armour world over on most of the tanks. Its low cost, reliability, availability of production infrastructure, concurrent utility as a structural material and its ease of fabrication have enabled this steel to hold on to its prime position. This amour steel continues to be used in the tempered martensitic microstructure after heat treatment which involves hardening to increase its resistance to penetration by projectiles and then tempering to make it tougher and therefore enhance the energy absorbing capability against impacting projectiles.

The HHA specification allows modern continuous processing technologies to be used efficiently and offers a new class of auto-tempered high-hard steels. Variable hardness armour steel is also known as the dual hardness armour (DHA) steel which is produced by roll bonding a high hardness front plate to a lower hardness back plate.  The roll-bonded DHA steels are complex to produce and have known production limitations.  Studies have been done to produce DHA steels by electro-slag remelting processes, but producing DHA steels continues to be difficult.

There are several efforts are being made to develop monolithic ultra-high hardness armour (UHHA) steels with a hardness of 600 BHN (Brinell hardness number) or greater and significant advancements in steel metallurgy have been made in this direction. The improved ballistic resistance of steel as a function of increasing hardness is well established in the ballistic community. UHHA steels are expected to increase AP bullet defeat, reduce armour weight, and eliminate the manufacturing difficulties inherent in DHA.

Quenching and tempering, defined as a combination of heating and cooling of a metal or alloy, changes the microstructure of the steel and improves the strength, hardness and toughness of the materials being treated. The cooling rate during quenching of the steel which is in the austenitic range is to be such that it cools the steel below the Ms (start of martensite formation) temperature. After the entire microstructure of the steel gets converted into martensite, tempering of the martensite is done. During the tempering process, the temperature of the steel is raised to a temperature where the martensitic structure of the steel gets tempered. In the quenching and tempering process during the hot rolling of the plate, the finish rolling temperatures and quenching and tempering rates are to be controlled to obtain the optimum quality grades of steel with low alloy content. The resulting products of low alloy quenched and tempered steel offer designers of the armoured vehicles the strength to weight advantages and wear resistant properties which are normally not available in conventional steels.

Production process for armour steels

The technology used in the manufacture of armour plate need to be of a very high nature, since the demands of high strength and high hardness steels dictate the need for one of the most stringent process routes which is to be utilized for the production of the steel plate. The primary steelmaking of the armour steel can be carried out either in the basic oxygen furnace (BOF) or in the electric arc furnace (EAF).

While in the basic oxygen furnace mostly hot metal (liquid iron from a blast furnace) and scrap are used as the raw materials for making steel, the electric arc furnace can use scrap, direct reduced iron, and hot metal based on their availability. The quality of raw materials used for primary steelmaking is required to be controlled. In case of use of hot metal during steelmaking, it is desirable to carry out de-sulphurization, de-phosphorization, and de-siliconization of hot metal as per the process requirement for ensuring low levels of sulphur, phosphorus, and silicon in the hot metal. The removal of these elements from hot metal assists the quality of slag formation during the basic oxygen steelmaking process. Scrap used for steel making is to be clean and is to be of high density. Also, the amount of tramp elements in the scrap is to be very low.

The flowsheets of the processes for the production and processing of armour steels are given in Fig 1 and Fig 2. Fig 1 gives the processes used upto the stage of production of slabs while Fig 2 gives the processes of rolling, thermal processing, and inspection of the plates.

Fig 1 Flowsheet of production and processing of armour steel plate (upto slab production)

  Fig 2 Flowsheet of production and processing of armour steel plate (rolling of slab into plates)

Making and casting of the armour steel

After the liquid steel has been made in the basic oxygen furnace or the electric arc furnace, it is treated in the secondary steelmaking units. The objectives of secondary steelmaking include (i) homogenization of chemical composition and temperature of liquid steel in the ladle, (ii) deoxidization or killing which means removal of oxygen, (iii) superheat adjustment which means heating or cooling of the liquid steel to a temperature suitable for its continuous casting, (iv) additions of ferro-alloys and carbon with the purpose of making adjustments in the chemistry of liquid steel, (v) vacuum degassing of the steel for the removal of hydrogen and nitrogen gases, (vi) removal of undesirable nonmetallic compounds by floating them into slag, and (vii) changing of the composition of the remaining impurities to improve the microstructure of the steel. Secondary steelmaking is necessary for the achievement of the desired mechanical properties in the steel after rolling.

During the production of the armour quality steels, one or more of the secondary steelmaking processes which are normally used include vacuum degassing, ladle furnace and ladle degassing, vacuum arc degassing, and electro-slag re-melting. During the secondary steelmaking processes also the rinsing or stirring of the liquid steel along with injections of different materials are carried out in the ladle for homogenization and the refining of the liquid steel. Secondary steel making is also needed for smooth casting of the liquid steel in the continuous casting machines as well as for the production of the sound quality of the slabs.

Continuous casting of steel is a process whereby liquid steel is solidified into a semi-finished steel product namely slab (in the case of armour steel) for its subsequent rolling in the rolling mill. The operation of the continuous casting converts the liquid steel of a given composition into a strand of slab of a given size through a group of operations like mould operation, spray cooling zone, and straightening zone operation etc. The thickness of the cast slab is to be such that a minimum level of reduction takes place during the rolling of plate.

The main equipments of a continuous casting machine constitute (i) ladle turret along with turret weighing system and ladle cover manipulator, (ii) tundish and tundish car along with tundish weighing system, tundish preheater and dryer, (iii) mould and mould oscillation along with mould level control and electromagnetic stirrer, (iv) secondary cooling consisting of strand cooling, strand containment and guiding, (v) withdrawal and straightening unit, (vi) dummy bar, dummy bar parking and dummy bar disconnect roll unit, (vii) pinch roll and torch cut off unit, (viii) product identification system, and (ix) roller table and product discharge system consisting of cooling bed , roller table and discharge grid.

For the casting of the low alloy armour steels, the stress is on the production of clean steels. Also, there are higher requirements for the microstructure and the composition homogenization of the cast product. The chemical composition, solidification conditions and the nature of the liquid steel flow in the mould affects the surface quality and the inner structure of the cast product. The application of electromagnetic stirring (EMS) technique promotes the formation of an equiaxed crystallic zone in the strand. It causes the refinement of the solidification structure, the reduction in the content of inclusions and improvement in the quality of the surface, sub surface and the inner structure of the cast slab.

The slabs for the armour steels can also be produced by following the route of electro-slag remelting, casting into wide ingots and then forging the ingots into slabs. Since during the ingot casting, segregation of carbon takes place in the direction of solidification as the solidification of liquid steel proceeds, there is necessity for the process of electro- slag remelting. However, this route increases the cost of production of the slabs. This route of production is mainly suited for low capacities. Further, since the slabs are produced by the process of forging, they have sound inner structure.

Modern slab continuous casting machines equipped with all the types of controls right from ladle turret to the cast product discharge normally produce very sound slabs with practically no surface and subsurface defects. However, because of the importance of the armour steels, slabs after cooling down are subjected to visual, magnaflux and ultrasonic inspection for identifying of the possible surface, sub-surface, and internal defects. The internal defects in continuously cast slabs can have a strong effect both on the performance of the steel during thermo-mechanical processing and/or the mechanical properties of the final product. Hence, it is important to identify, quantify and characterize the defects.  The characterization of the defects includes the density, distribution, type and location of the anomalies.

The inspection helps in the segregation of the cast slabs into three categories namely (i) prime slabs which can be sent for rolling, (ii) slabs with minor surface defects which can be removed either by scarfing or by grinding to make them fit for rolling, and (iii) slabs with unacceptable sub-surface and internal defects stands rejected for rolling and are scrapped for remelting.

Rolling of slabs in plate mill

The slabs after inspection are heated in a reheating furnace to temperatures of around 1150 deg C to 1200 deg C which is suitable for plastic deformation of steel and hence for rolling of the steel in the rolling mill. The walking beam type of reheating furnace is preferred since it is energy efficient and ensures uniform heating of the slabs. The reheating furnace is required to have all the facilities for the waste heat recovery. It is also to be equipped with the combustion controls needed for the control of the slab temperature as well as other controls required for its efficient operation.

The heated slab is then rolled in a plate rolling mill. The plate mill is normally a four high reversing rolling mill with either a single stand configuration or with a two stands configurations. The rolling stand is normally having attached edger rolls for controlling the plate width. Plates are generally rolled to the prescribed thickness in the reversing rolling stand (i.e., repeatedly passing the plate back and forth through the roll stand) while progressively reducing the gap between the top and bottom rolls in a stepwise manner, and generally requires a number of rolling passes. The action of passing a plate through the roll gap is called a pass, and the amount of reduction of the plate thickness in each pass is called rolling reduction. The thickness reduction during the rolling is distributed into several rolling passes. The process by which the number of passes and the rolling reduction in each pass from the slab thickness to the product thickness are decided is the rolling pass schedule. The finish rolling temperature affects the number of passes needed due to the material properties, where the cooler material gets harder.

In the case of normal thickness products (i.e., flat plates), the same thickness is obtained over the entire length by controlling the mill so that the gap between the top and bottom rolls does not change during a rolling pass.

The rolling start and finish temperatures determine the process stability, where cooler material needs more rolling force than the hotter one. Hence, thin plate which has higher cooling rate than thick plate can make the rolling process unstable, especially for the low thickness plate where the temperature drop is high.

Mill stands and plate cooling systems as well as all downstream mill sections have to be designed such that high-strength plates can be produced and processed to obtain top-quality final products

Screw down and automatic gap control is the main parts of the rolling mill to adjust the roll gap in accordance with the set thickness. Each of the plate sizes has its own pass schedule calculation including the appropriate roll gap, roll force, and mill modulus.

For the rolling of the thin plate, the plate mill is required to be equipped with facilities for automatic shape control, flatness control, and gauge control. Rolling of the thin plates generally need two stands with the finishing stand giving the final pass. Also needed is n online gauge measurement instrument for thickness measurement.

The rolled plate is subjected to leveling in hot leveler before it enters the thermo-processing section and a cold leveler after the thermo-processing section. Good flatness of a steel plate is desired as during the process of cooling, flatness influences the distance for the water to collide with the steel plate and influences the flow of water on the steel plate. The function of the hot leveler installed before cooling equipment is to flatten the steel plate before cooling. On the other hand, the cold leveler installed after the thermo-processing section is intended to flatten the plate to rectify the shape deteriorated by cooling for easy transfer to subsequent process.

During the rolling of the armour steel, the finish rolling temperature is set at a lower value than that in the case of conventional hot rolled plates. This means sometimes waiting time for temperature adjustment is necessary during the rolling process, and the waiting time tends to become longer with thicker products.

Thermo processing section is very important in the production and the processing of the armour steel since the final properties of the steel is obtained during the process ing of the plate in this section. To meet the requirements during the production of armour steels, three approaches are being used.

In the first approach, the thermo processing is carried out off-line of the plate mill, In this approach, the plate is heated to a desired temperature in the austenizing range. Heating control is important to for avoiding the grain growth. After the plate achieves a homogenized austenitic structure, it is quenched with water at a predetermined cooling rate to get a martensitic steel structure. The quenched steel is then tempered at low temperature for achieving the desired properties.

In the second approach, the rolled steel immediately after rolling is subjected to the accelerated control cooling. In this case the martensitic structure is avoided, and a very fine grained bainitic structure is the aim. In this approach very high hardness in the steel cannot be achieved but the steel gets high strength coupled with good toughness.

In the third approach, the quenching and tempering operations are carried out on line immediately after rolling. In this approach the heating of the plate gets eliminated, but in this approach, either the rolling rate of the plate in the mill and the quenching and tempering times are to match, or otherwise one operation has to wait for the other operation to get completed. Also, adequate controls with generous use of pyrometers are to be provided for the control of the thermo processing parameters. Further, thermo processing section is to be closely linked with the rolling operations through a control system for effective control of the properties of the armour plates.

Pinch rolls during the quenching and tempering operations have the function of holding the steel plate in between, promoting uniform cooling/heating by suppressing the plate deformation during cooling/heating, improving plate form, and securing cooling zones. In case of quenching of the steel plates, finish cooling temperature and cooling time vary greatly depending on the size and the aimed material. Hence, it becomes necessary to adjust the length of the cooling zone of the cooling equipment. The pinch rolls determines the cooling water flow to downstream of the cooling zone, preventing non-uniform cooling due to sojourning water on the steel plate outside the cooling zone, thereby separating the cooling zone from non-cooling zone.

The water quenching unit needs to be properly designed since it is influenced by the water boiling curve. The cooling capacity of water in the case of cooling a steel plate at high temperature shows a characteristic behaviour as expressed by what is known as the boiling curve (shown in Fig 3). In the high temperature region, a steam vapour film exists between the steel plate and the water, causing a state termed as film boiling and, despite high temperature in the region, the cooling capacity becomes slightly lower. As the steel plate temperature goes down, contacting of water with the steel plate starts and, as the steel plate temperature further goes down, the area of contact of water with the steel plate expands and the state of cooling enters the transition boiling region where the cooling capacity increases. As the temperature of the plate further goes down, the state of cooling goes into the nucleate boiling region where bubbles generated play a major role. In cooling of the steel plates, cooling in the transition boiling region is crucial. In this region, as the cooling capacity increases along with the decrease in plate temperature, uneven temperature distribution within a steel plate developed in the earlier cooling is enlarged and, finish cooling temperature also varies for each steel plate.

Fig 3 Boiling curve of water

The water quenching unit needs to be properly designed since it is influenced by the water boiling curve. The functions needed for the water quenching unit are wide ranged to have fast cooling capability. Hence, the unit is to be equipped with many spray nozzles because it needs a high water flow rate for uniform fast cooling of the wide plate. Also, under the operating conditions and depending on the aimed quality of the steel plate, the finish cooling temperature (Mf temperature) is to be set in somewhere in the transition boiling region.

After the plate has been subjected to the needed thermo processing for achieving the desired properties of strength, hardness, and toughness, the steel plate is straightened again in a straightener and then the finishing activities such as shearing and cutting, sample cutting, testing and inspection and if required shot blasting and coating and dying are carried out as shown in the flowsheet. The plate is subjected to final inspection before its dispatch.

During testing all the tests needed as per the standard are required to be conducted for ensuring that the plates are conforming to the values specified in the standard with respect to the dimensions, dimensional tolerances, micro-structure, strength, hardness, and toughness. The rolling mill laboratory is to be equipped with all the needed testing and the inspection facilities so that the required testing and inspection of the rolled plates can be carried out.

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