Annealing of Cold Rolled Steel
Annealing of Cold Rolled Steel
The cold rolling of steel is done at temperatures below the recrystallization temperature. During cold rolling process the reduction in thickness is due to plastic deformation which occurs by means of dislocation movement. Steel gets hardened because of the buildup of these dislocations. These dislocations reduce the ductility of cold rolled steel making it useless for forming operation. To recover the ductility, cold rolled steels need to undergo an annealing process for the relieving of the stresses that have buildup within the microstructure during the process of cold rolling.
Annealing consists of heating of the steel to above the recrystallization temperature, soaking at that temperature and then cooling it. Heating of the steel during annealing facilitates the movement of iron items, resulting in the disappearance of dislocations and formation and growth of new grains of various sizes. It has three different stages namely (i) stress relief, (ii) recrystallization, and (iii) grain growth. During stress relief which takes place around 480 deg C to 500 deg C, the atoms move only small distances, pushed and pulled by the surrounding atoms into a configuration in which the internal stresses are reduced but the boundary between the crystals remains unchanged. The stage of recrystallization takes place at around 550 deg C and during this stage new crystals begin to form at the boundary of the original rolled grain. These crystals grow roughly into spheres, realigning atoms from the cold rolled grains until their boundaries meet up with those of other newly formed grains. Once the cold worked grains are fully consumed, the steel is fully recrystallized. In the third stage of grain growth the steel gets softened as the grains consume other newly formed crystals and grow in size. This stage takes place usually during the period of soaking.
The final mechanical properties and the microstructure of the steel are largely dependent upon the annealing process since it significantly influences the crystallographic texture of the steel. Further precipitates decompose to solute atoms which subsequently dissolve into the steel matrix on heating and holding, then re-precipitate in various sizes and distributions, depending on the rate of cooling. These changes in the size and distribution of the grains and precipitates also affect the hardness of the steel.
The annealing is usually carried out under protective gas atmosphere for preventing surface oxidation in order to meet the high demand on the surface of the cold rolled steel. The protective gas atmosphere consists of nitrogen gas, hydrogen gas, or a mixture of these two gases in various proportions. Hydrogen gas has higher conductivity and hence it is preferred sometimes. Mixture of the two gases is obtained through cracking of ammonia (5 % H2 and 95 % N2).
For production of cold rolled steel usually two types of annealing processes are used. They are as follows.
- Soft annealing of steel – This type of annealing precipitation of cementite and pearlite to reduce the strength of the steel for facilitating the forming operation. Common temperature for this annealing ranges from 680 deg C to 780 deg C.
- Recrystallization annealing – This type of annealing reconstitutes the crystallites forms to their pre rolling state. The steel is heated in this type of annealing to a temperature between 550 deg C to 700 deg C, slightly above the recrystallization temperature. The recrystallization depends on the steel material and the degree of deformation during rolling.
There are two processes which are being used for the annealing of cold rolled steels. These are (i) batch annealing process and (ii) continuous annealing process.
Batch annealing process
This is the older of the two processes. The process is preferred where large ferrite grains are needed as in the case of electrical steels. The design of a batch annealing plant depends on the material to be annealed. Basic equipment needed for batch annealing of steel are as follows.
- Base unit provided with a circulation fan
- Protective gas tight cylindrical cover
- Heating hood or heating furnace with burners arranged tangentially. The furnace is also known as bell furnace due to its shape.
- Cooling hood
Cold rolled steel coils (charge material) are stacked over the base three or four high on top of each other, separated by convector plates. The interior cover is placed over the coils and its volume is filled with the protective gas to prevent the steel surface from getting oxidized under the high temperature. After this the heating hood is placed in position. Burners of the hood are lighted and the heat from the burners causes inner cover to heat up. Heat from the cover is radiated to the steel coils causing them to heat up. Heat is also transferred through convention to the coil inner surface by the circulating protective gas. The steel coils are then held or soaked at a temperature aimed for annealing. After this the furnace is removed and the cooled hood is placed and the steel coils are then left to cool to room temperature. For ensuring the required temperature being achieved through the complete steel coil, long heating, soaking and cooling times are needed. Typical schematics of batch annealing furnace and equipment along with annealing cycle are shown in Fig 1.
Fig 1 Typical schematics of batch annealing
During the batch annealing, recrystallization of the deformed structure begins to take place at temperatures about 550 deg C by means of nucleation and nuclei growth. This process uses the stored energy within the grains and reduces the grain density. Before steel coils reaches this temperature, aluminum nitride precipitates on the deformation sub grain boundaries. The precipitate leads to a retardation of the recrystallization process by inhibiting nucleation of the new grains leading to the final grain being large. The presence of the aluminum nitride also helps to produce the required structure needed for forming. When considering the formation of the aluminum nitride precipitates, the coiling temperature in the hot strip mill is an important parameter. It is to be low (typically around 560 deg C) so that aluminum is present in solid solution prior to the annealing process. For a larger grain size normally a higher soak temperature is aimed but it should be limited to around 730 deg C since higher temperature will cause coarse carbide formation detrimental to forming and may cause sticking of the adjacent layers of the coil.
Continuous annealing process
Continuous annealing of cold rolled steel was first introduced by the Armco steel corporation in the year 1936 as a process step in the production of hot dip galvanized steel. Since then several improvements have been made in the process which allows several types of steels to be processed by this method. Though the continuous annealing had several advantages over batch annealing still it was not used for all applications due to its poor cold forming characteristics and poor ageing. The carbon solute in the ferrite precipitated as cementite in batch annealing while it remained in supersaturation after cooling in continuous annealing due to higher cooling rate. This was the reason for poor ageing property of the continuous annealed sheet. This problem was overcome when Japanese introduced in 1970s an overage stage into the annealing process that improved the properties of steel after continuous annealing.
Nowadays most of the steel grades are annealed in a continuous annealing line. The advantages of continuous annealing process over batch annealing process are (i) better uniformity of the properties along the coil, (ii) better shape and surface properties and cleaner surface, (iii) short processing time leading to higher productivity, and (iv) possibility to produce lower cost high strength grades. The disadvantages of a continuous annealing line are that it needs a huge investment cost and has an important length due to the presence of the different sections (heating, cooling, secondary cooling, overageing and final cooling). Further continuous annealing line is lesser flexible since a change in soaking/overageing temperature takes a long transition time leading to an important yield loss.
A typical schematic diagram of a continuous annealing line is shown in Fig. 2.
Fig 2 Typical schematic diagram of a continuous annealing line
Steel strip coil which to be annealed is placed at the decoiler. Since the process is continuous, head end of the new coil is joined with the tail end of the previous coil. During the period when strip joining is taking place, the continuous movement of the strip in the heat treatment section is to be maintained. To ensure this, there is provision of two loopers or accumulators, one before and other after heat treatment and cooling section. These loopers consist of two parallel sets of rolls that can move apart from each other. As the entry looper moves apart it is able to hold more coil. When the joining operation is going on, the looper supplies the strip to the heat treatment section to maintain the continuity of movement. Similarly, when the shearing operation is going on at the exit end to separate two coils, the looper at the exit end receives the strip and accumulates it to maintain the continuity of the strip movement in the heat treatment section.
The temperature to which the steel is heated to and the rate of heating are dependent on the chemistry of steel, its prior processing and the required properties. Once the strip is heated it is to be soaked sufficiently in the soaking zone. The steel is cooled after soaking to precipitate a greater amount of carbides within the microstructure of steel. The steel is then reheated to the overage temperature for accelerated ageing. This allows the carbides to coarsen at a greater rate. After this the steel is cooled to room temperature. A typical annealing cycle in a continuous annealing line is shown in Fig 3.
Fig 3 Typical annealing cycle in a continuous annealing line
Nowadays continuous annealing lines are built integrating the electrolytic degreasing line at the entry end and the temper rolling (skin passing) at the exit end of the annealing line.