On-site nitrogen generator (PSA nitrogen generator) in annealing of cold rolled steel

Annealing of cold rolled steel

Annealing is a generic term denoting a treatment that involves heating to and holding at a suitable temperature and then cooling at a suitable rate， primarily for softening metallic materials. In plain carbon steels， annealing usually produces a ferrite - pearlite microstructure.

The normal method of manufacturing cold rolled sheet is to produce hot rolled coils that are pickled to remove scales (oxides) and then cold rolled to the desired final specification. Cold rolling can reduce the thickness of a hot rolled sheet by more than 90%， which increases the hardness and strength of the steel but severely reduces its ductility. The ductility of the steel is restored if any significant subsequent cold working (e.g. forming， drawing， etc.) is to be performed.

Cold rolling of steel is carried out at temperatures below the recrystallization temperature. The reduction in thickness during cold rolling is due to the plastic deformation that occurs through dislocation movement. Due to the accumulation of these dislocations， the steel hardens. These dislocations reduce the ductility of the cold rolled steel and make it unusable for forming operations. To restore ductility， cold rolled steel needs to be annealed to remove the stresses that build up in the microstructure during the cold rolling process.

During cold rolling of steel， the extensive deformation of the steel at room temperature greatly reduces the ductility and plasticity of the cold rolled sheet. This requires annealing， where the cold rolled sheet gets stress relief through the mechanisms of recovery， recrystallization and grain growth. Annealing is one of the most important processes in a cold rolling mill because it determines the quality of the cold rolled steel sheet. In fact， it is an important process for controlling the mechanical properties of cold rolled sheet.

Annealing involves heating the steel above the recrystallization temperature， dipping it at that temperature， and then cooling it again. During annealing， the steel is heated to favor the movement of iron， leading to the disappearance of dislocations and the formation and growth of new grains of different sizes. Annealing of heavily cold worked steel plates can be divided into three physically distinct but usually overlapping stages， namely recovery， recrystallization and grain growth.

As annealing continues， the process of recrystallization takes place and new， more equiaxed ferrite grains form from the elongated grains. During the recrystallization process， the strength decreases rapidly and the ductility increases accordingly. Further time at temperature causes some of the newly formed grains to grow at the expense of other grains. This is called grain growth and results in a moderate decrease in strength and a small (but often considerable) increase in ductility. Most plain carbon steels are annealed to promote complete recrystallization， but care is taken to avoid excessive grain growth， which can lead to surface defects (such as orange peel) in formed parts.

The rate at which the annealing process proceeds is a function of the chemical composition and prior history of the steel being annealed. For example， small amounts of elements such as aluminum， titanium， niobium， vanadium and molybdenum can reduce the rate of recrystallization of the steel and retard the annealing response， thus requiring higher temperatures or longer annealing times to produce the same properties. Although the presence of these alloying elements is usually the result of intentional additions designed to alter the properties of the plate (as in the case of aluminum， titanium， niobium and vanadium)， some elements can be present as residual elements (such as molybdenum) in quantities sufficient to alter the annealing response.

On the other hand， extensive cold working (greater cold reduction) can accelerate the annealing reaction. Therefore， it is not possible to specify a single annealing cycle that produces a specific set of mechanical properties in all steels， with the chemical composition and amount of cold working also taken into account.

Annealing of cold-rolled steel is usually done to produce a recrystallized ferrite microstructure from the highly elongated stress grains produced by cold rolling. Figure 1 shows the effect of annealing on the microstructure of low-carbon cold-rolled steel. Figure 1(a) shows the cold-rolled microstructure in contrast to the partially and fully recrystallized microstructure in Figures 1(b) and 1(c).

Fig. 1 Effect of annealing on the microstructure of cold rolled steel

The first metallurgical process that occurs during the heating of the steel， and in the first part of the holding portion of the annealing cycle， is recovery. During this process， internal strain is relieved (although microstructural changes are not significant)， ductility is moderately improved， and strength is slightly reduced.

Recovery dominates at relatively low temperatures and involves the migration of vacancies and dislocations introduced by cold deformation， leading to the annihilation or rearrangement of some part of them. However， from a broader perspective， recovery involves any change to the properties during annealing， which occurs before the appearance of new strain-free recrystallized grains. In other words， recovery does not involve the migration of high-angle grain boundaries (HAGBs). During the recovery process， it is normal to find some changes in both mechanical and physical properties of the material compared to the values in the cold worked state. Typically， the recovery of mechanical properties， such as hardness， yield strength， or ductility to fully annealed values， is only about one-fifth complete during the recovery process.

During the stress relief process from about 480 degrees C to 500 degrees C， the atoms move only a small distance， being pushed and pulled by surrounding atoms into a configuration where the internal stresses are reduced but the boundaries between the crystals remain unchanged. The recrystallization stage occurs at about 550 degrees Celsius， where new crystals begin to form at the boundaries of the original rolled grains. These crystals grow roughly into spheres， rearranging the atoms of the cold rolled grains until their boundaries meet the boundaries of the other newly formed grains. Once the cold worked grains are completely consumed， the steel is fully recrystallized. In the third stage of grain growth， the steel becomes softened as the grains consume other newly formed crystals and increase in size. This stage usually occurs during immersion.

During the annealing operation， the deformed microstructure of the cold rolled plate is restored and recrystallization occurs. Annealing of cold-rolled steel sheets can be performed in batch annealing furnaces， where the cold-rolled sheet is annealed in coils， or in continuous annealing furnaces， where the cold-rolled sheet is annealed in plates.

Since recrystallization is not possible during the cold rolling process， the structure and the resulting material properties are greatly influenced by cold rolling. During cold rolling， grain extension occurs in the rolling direction and the arrangement of the lattice has a directional character. Banding features of other structural phases， such as inclusions and pearlite blocks， are also developed. Three types of textures (i.e. deformation， structural and crystallographic textures) appear， which lead to directional characteristics of the mechanical properties. The annealing of cold rolled steel sheets after cold rolling is done to eliminate the anisotropic properties. The microstructure of cold rolled sheet after annealing depends on (i) the initial material structure prior to cold rolling， (ii) the total cold reduction， (iii) the annealing conditions (temperature and time)， and (iv) the cooling rate of the steel. With the introduction of high strength low alloy， high strength and advanced high strength materials into the deep drawing of cold rolled sheet， further complications arise in the annealing process of cold rolled sheet.

Typically， when the cold deformation of the steel is high before annealing， the initial temperature of recrystallization is subsequently reduced.   In addition， at low temperatures， the time required to complete recrystallization is much longer and the required carbide spheroidization cannot be completed. The strength or hardness properties of the steel usually decrease as the annealing temperature increases， while the plastic properties increase. The significant reduction in strength values occurs at temperatures close to 600 degrees Celsius. In addition， the higher the previous cold deformation， the more important the decrease in strength values.

The final mechanical properties and microstructure of the steel depend to a large extent on the annealing process， as it has a strong influence on the crystalline structure of the steel. Further precipitates break down into solute atoms， which are subsequently dissolved into the steel matrix during heating and holding， and then reprecipitate in different sizes and distributions depending on the cooling rate. These changes in the size and distribution of grains and precipitates also affect the hardness of the steel.

Annealing is usually done under a protective gas atmosphere to prevent surface oxidation in order to meet the high requirements for the surface of cold rolled steel. The protective gas atmosphere consists of nitrogen， hydrogen or a mixture of these two gases in various proportions. Hydrogen has a higher electrical conductivity and is therefore usually preferred. A mixture of these two gases is obtained by cracking ammonia (5% hydrogen and 95% nitrogen).

For the production of cold rolled steel， two types of annealing processes are usually used: (i) soft annealing， and (ii) recrystallization annealing. In soft annealing of steel， precipitation of scheelite and pearlite occurs， which reduces the strength of the steel， and this facilitates the forming operation. The normal temperature range for this annealing is 680 degrees C to 780 degrees C. In the case of recrystallization annealing， the annealing reorganizes the crystals to their pre-rolling state. In this type of annealing， the steel is heated to between 550 degrees Celsius and 700 degrees Celsius， slightly above the recrystallization temperature. Recrystallization depends on the material of the steel and the degree of deformation during the rolling process.

It is well known that metals are strengthened by cold working. During the cold rolling process， which is a necessary stage in the production of plates， the steel becomes very hard， but loses almost all of its ductility. In conventional processing， cold-rolled steel is completely recrystallized by annealing， with the aim of restoring ductility， but at the expense of strength. Then， if higher strength is required， it is usually achieved by alloying or special heat treatment at higher temperatures. The above procedures result in wasted energy and increased costs. On the other hand， by controlled low-temperature annealing， it is possible to retain as much of the strength of cold-rolled as possible while restoring sufficient ductility. This produces a steel with a compromise between the high strength-low ductility of a fully cold-rolled plate and the low strength-high ductility of a fully recrystallized plate. This process is known as "annealing"， "partial annealing"， "restorative annealing"， "stress relief annealing"， "tempering" or "controlled incomplete annealing".

Basically， the "annealing" process consists of two stages， namely deformation and annealing. Another method， called "temper rolling" or "rolling and tempering"， aims to obtain the desired strength and ductility by plastic deformation of the already fully annealed material， while in "annealing" the severely deformed material is subjected to recovery or partial recrystallization. The goal is the same in both cases. It is claimed that the ductility of "annealed" material is always better than that of "tempered" material at all strength levels.

Two processes are used to anneal cold rolled steel. They are (i) the batch annealing process and (ii) the continuous annealing process.

Batch annealing process

The batch annealing process is the earlier of these two processes. In this process， the cold rolled sheet is annealed in the form of coils. Typically， three to four cylindrical coils (usually weighing 10 to 20 tons each) are stacked on the basic cell of the process and annealed under a protective gas (hydrogen) atmosphere. The process is preferred in cases where large ferrite grains are required， such as in the case of electrical steels. The design of the batch annealing unit depends on the steel to be annealed.

The basic equipment required for batch annealing of cold rolled steel sheets are (i) basic equipment equipped with circulating fans， (ii) circular convection plates for coil separation， (iii) protective gas-tight cylindrical hood， (iv) heating hood or heating furnace (also known as bell furnace because of its shape) with burners arranged in a tangential direction， and (v) cooling hood. Convection plates between the coils are used to improve the heat flow. Figure 2 shows the basic equipment for the batch annealing process.

Figure 2 Basic equipment for batch annealing process

In order to perform the annealing process， three to four cold rolled steel coils are first placed on a base unit， separated by convection plates as shown in Figure 2. These cylindrical steel coils are called furnace charge or furnace material. After the coils are loaded on the base unit， a protective hood is put in place and a protective gas is circulated through this hood. The protective gas is applied to avoid oxidation of the steel strip surface at high temperatures and to transfer the heat from the furnace through the coils.

A heating furnace is then placed over the protective hood. The burner of the heated hood is ignited and the heat from the burner heats up the inner hood. The heat from the hood is radiated to the steel coils， heating them up. The heat is also transferred to the inner surface of the coil by the circulating protective gas. The inner and outer surfaces of the coil are then heated by convection of the circulating protective gas and by radiation between the cover and the coil. The inner part of the coil is then heated by conduction.

The annealing cycle has three stages. The first stage is the heating stage， where the temperature is raised to the target temperature. The second stage is the soaking stage， where the temperature is maintained to obtain a uniform temperature inside and outside the coil. The third and final stage is the cooling stage， where the temperature inside the furnace slowly decreases.

At the end of the heating stage， the heating hood is replaced by a cooling hood and the protective gas continues to circulate. The coil is then allowed to cool to room temperature. Longer heating， soaking and cooling times are required to ensure that the entire coil reaches the required temperature.

The large thermal mass and low conductivity created by the air gap between the plates results in a large thermal lag between the cylindrical surface of the coil and the coil core. The coil surface with the highest temperature during the heating cycle is referred to as the hot spot and the coil core with the lowest temperature is referred to as the cold spot. During the heating phase， the cold and hot spots of the coil need to be raised to the desired annealing temperature in order to fully recrystallize. The time required to raise the cold spot temperature to the desired temperature is the heating time. A longer heating time results in a more uniform microstructure and mechanical properties， but reduces the productivity of the heating furnace. Figure 3 shows a schematic and cross-sectional view of the batch annealing process.

Figure 3 Batch annealing process of cold rolled coil

The annealing cycle for cold rolled coils varies with the steel composition， cold reduction and the desired steel grade. However， typical batch annealing temperatures range from 620°C to 690°C (slightly below the Ac1 temperature)， which is the coldest point of the charge. The cycle time varies with the desired steel grade and charge size， but the total time (from the start of heating to the removal of the steel from the furnace) can be up to one week.

During batch annealing， recrystallization of the deformed structure begins by nucleation and nucleation growth at a temperature of about 550 degrees Celsius. This process utilizes the energy stored within the grains and reduces the grain density. Before the steel coil reaches this temperature， aluminum nitride precipitates on the deformed subgrain boundaries. This precipitate retards the recrystallization process by inhibiting the nucleation of new grains and causing the final grains to become larger. The presence of aluminum nitride also helps to produce the structure required for molding. The coiling temperature of the hot strip mill is an important parameter when considering the formation of aluminum nitride precipitates. It has to be low (usually around 560 degrees Celsius) so that the aluminum is present in solid solution form prior to the annealing process. For larger grain sizes， a higher soaking temperature is usually used， but limited to about 730 degrees Celsius， since higher temperatures can lead to coarse carbide formation， which is detrimental to forming and can lead to adhesion of adjacent layers of the coil. Figure 4(a) shows a typical heating and cooling cycle for batch annealing of low-carbon cold-rolled steel sheets.

Fig. 4 Heating and cooling cycles and hot and cold spots in the steel coil during annealing

In a study on batch annealing， the annealing process was controlled by using thermocouple sensors attached to the steel strip in order to obtain a uniform temperature inside and outside the coil. In this study， we found that the hot spots of the cold rolled coils were on the outer side of the coils and the cold spots were at 2/5 of the inner wall of the coils. These locations are shown in Figure 4(b).

During the cooling cycle， the core of the coil is warmer than the other coil points. The air gap between the sheets in the coil results in a very low radial thermal conductivity. Therefore， spatial variations in temperature are prevalent during the annealing process. The outer surface of the coil， called the hot spot， heats up faster and reaches the annealing temperature in a shorter period of time than the inner core of the coil， often called the cold spot. Since recrystallization and grain growth are thermally activated processes， this thermal hysteresis leads to spatial changes in the microstructure and the associated changes in the mechanical properties within the coil. This change in mechanical properties occurs at every location along the length of the coil (outside， middle and inside the coil). Due to this， some parts of the steel coil have mechanical property problems. In addition， there are changes in the microstructure and mechanical properties between the steel coils due to temperature variations along the axial direction of the furnace.

In batch annealing， the temperature of the steel strip depends on the thickness， width， weight and stacking position of the coils. The width range and weight range of the coils have a significant effect on the variation of mechanical properties in batch annealing. Reduction of the width range leads to a reduction of the yield strength distribution within the stack. The number of coils in the batch annealer has a slight effect on the temperature difference between hot and cold spots. During the annealing stage， the heating rate and soaking time clearly have a high intrinsic effect on formability. The position of the coils in the batch annealing furnace has a slight influence on the 'n' value， but has a greater influence on the 'r' value of the steel.

Although an increase in soaking time usually leads to a reduction in microstructural and mechanical property changes， it also reduces furnace productivity. Therefore， the choice of soak time needs to be optimized between productivity and quality in the batch annealing process. In addition， proper selection of heating rate， which is metallurgically significant for precipitation and recrystallization kinetics， and annealing temperature form the core of batch annealing thermal cycle design.

The batch annealing operation has a considerable impact on all important performance parameters of the cold rolling mill. These parameters include energy consumption， plant productivity and emissions， as well as quality parameters such as the strength， ductility， tensile properties and formability of the steel sheet. Given its relevance to all these key parameters， it is necessary to optimize the parameters of the batch annealing operation to achieve maximum productivity and minimum energy consumption while maintaining the required product quality.

Despite being a critical operation， industrial-scale batch annealing cycles are typically designed through plant trials and empirical methods that， in addition to being time-consuming and expensive， provide sub-optimal results at best. In contrast， process cycles can be effectively optimized by an annealing model that simulates the batch annealing operation， thereby reducing the number of plant trials required for plant optimization.

Fig. 4 Heating and cooling cycles and hot and cold spots in the steel coil during annealing

In a study on batch annealing， the annealing process was controlled by using thermocouple sensors attached to the steel strip in order to obtain a uniform temperature inside and outside the coil. In this study， we found that the hot spots of the cold rolled coils were on the outer side of the coils and the cold spots were at 2/5 of the inner wall of the coils. These locations are shown in Figure 4(b).

During the cooling cycle， the core of the coil is warmer than the other coil points. The air gap between the sheets in the coil results in a very low radial thermal conductivity. Therefore， spatial variations in temperature are prevalent during the annealing process. The outer surface of the coil， called the hot spot， heats up faster and reaches the annealing temperature in a shorter period of time than the inner core of the coil， often called the cold spot. Since recrystallization and grain growth are thermally activated processes， this thermal hysteresis leads to spatial changes in the microstructure and the associated changes in the mechanical properties within the coil. This change in mechanical properties occurs at every location along the length of the coil (outside， middle and inside the coil). Due to this， some parts of the steel coil have mechanical property problems. In addition， there are changes in the microstructure and mechanical properties between the steel coils due to temperature variations along the axial direction of the furnace.

In batch annealing， the temperature of the steel strip depends on the thickness， width， weight and stacking position of the coils. The width range and weight range of the coils have a significant effect on the variation of mechanical properties in batch annealing. Reduction of the width range leads to a reduction of the yield strength distribution within the stack. The number of coils in the batch annealer has a slight effect on the temperature difference between hot and cold spots. During the annealing stage， the heating rate and soaking time clearly have a high intrinsic effect on formability. The position of the coils in the batch annealing furnace has a slight influence on the 'n' value， but has a greater influence on the 'r' value of the steel.

Although an increase in soaking time usually leads to a reduction in microstructural and mechanical property changes， it also reduces furnace productivity. Therefore， the choice of soak time needs to be optimized between productivity and quality in the batch annealing process. In addition， proper selection of heating rate， which is metallurgically significant for precipitation and recrystallization kinetics， and annealing temperature form the core of batch annealing thermal cycle design.

The batch annealing operation has a considerable impact on all important performance parameters of the cold rolling mill. These parameters include energy consumption， plant productivity and emissions， as well as quality parameters such as the strength， ductility， tensile properties and formability of the steel sheet. Given its relevance to all these key parameters， it is necessary to optimize the parameters of the batch annealing operation to achieve maximum productivity and minimum energy consumption while maintaining the required product quality.

Despite being a critical operation， industrial-scale batch annealing cycles are typically designed through plant trials and empirical methods that， in addition to being time-consuming and expensive， provide sub-optimal results at best. In contrast， process cycles can be effectively optimized by an annealing model that simulates the batch annealing operation， thereby reducing the number of plant trials required for plant optimization.

Fig. 5 Schematic diagram of a continuous annealing line

In addition， the application of an over-aging treatment during the annealing process (maintaining the steel plate temperature at 400 degrees C to 450 degrees C after heating， soaking and rapid cooling) allows the rapid precipitation of supersaturated solute carbon in the form of hydromet and the acquisition of ageing-inhibiting properties. The continuous annealing line can also be integrated with an electrolytic cleaning line， thus making it possible to manufacture steel plates with very good material homogeneity， surface quality and shape at a shorter cost and time than can be achieved using the conventional batch annealing process.

For the primary cooling of the plates， continuous annealing lines can also be used with the gas jet cooling method， where the furnace atmosphere gas is cooled and blown onto the plate surface. The cooling rate achieved with this method is about 10 degrees Celsius per second， but for this cooling rate， solute carbon precipitates at the grain boundaries and the precipitates are separated by large distances. Therefore， the over-aging treatment takes a long time. On the other hand， another continuous annealing process that has been developed uses water quenching for primary cooling. With this method， the cooling rate is about 1000 degrees Celsius per second， at which carbides precipitate in fine particles within the grain and at the grain boundaries， making the distance between precipitates smaller and reducing the over-aging time， although figures such as elongation and 'n' values tend to be lower.

To solve these problems， a new cooling method， called the "accelerated cooling" process， was developed in the early 1980s. With this method， the sheet is cooled by jets of air-water mixture from cooling nozzles， each with a water head and a gas head. The cooling rate is about 100 degrees Celsius per second， and both the rate of cooling and the final temperature can be controlled by varying parameters such as the amount of water and gas and the number of nozzles. This method makes it possible to reduce the time of the over-aging treatment and to achieve the desired steel properties. The high cooling rate and temperature control capabilities of the "accelerated cooling" process make it possible to efficiently produce high-tensile plates of solution-hardened and precipitation-hardened steels， as well as those developed by transformation strengthening， with strengths up to 1，180 MPa. Figure 6 shows a schematic diagram of the accelerated cooling unit.

Figure 6 Accelerated cooling unit and continuous annealing cycle

A typical continuous cooling cycle is shown in Figure 6. The temperature to which the steel is heated and the rate at which it is heated depends on the chemical composition of the steel， its previous processing and the desired properties. Once the strip is heated， it is subjected to a full immersion in the soak zone. After soaking， the steel is cooled to precipitate more carbides in the microstructure of the steel. The steel is then reheated to overage temperatures to accelerate aging. This allows the carbides to coarsen at a greater rate. After this， the steel is cooled to room temperature.

Today's continuous annealing lines integrate an electrolytic degreasing line at the entry end and temper rolling (skin pass) at the exit end of the annealing line.

Most steel grades are now annealed on continuous annealing lines. The advantages of the continuous annealing process compared to the batch annealing process are (i) better uniformity of properties along the coil， (ii) better shape and surface properties and cleaner surfaces， (iii) shorter processing times leading to higher productivity， and (iv) the possibility to produce high strength grades at low cost. The disadvantages of continuous annealing lines are the huge investment costs required and the significant length due to the presence of different sections (heating， cooling， secondary cooling， overage and final cooling). In addition， continuous annealing lines are less flexible because changing the immersion/over-aging temperature requires long transition times resulting in important yield losses.

High strength cold rolled sheets are becoming increasingly important due to their high load bearing capacity. The strength of the plate can be improved by modifying the chemical composition and/or selecting different annealing cycles， but these methods lead to a decrease in ductility. Plain carbon steel produced by conventional techniques can be batch annealed or continuously annealed under conditions that result in only recovery or partial recrystallization. Typical batch annealing cycles of this type use soaking temperatures of 425 degrees Celsius to 480 degrees Celsius and various soaking times.

High Strength Low Alloy (HSLA) steels containing alloying elements such as niobium， vanadium and titanium can also be produced as cold rolled grades. The additional alloying produces a stronger hot-rolled steel that is even stronger by cold rolling. Cold-rolled HSLA steels can be produced in higher strength grades by restorative annealing or in lower strength grades by recrystallization annealing. The successful production of cold rolled HSLA steels requires the selection of the appropriate steel composition and combination of hot rolled strength， cold reduction and annealing cycle type.

Another family of high-strength plate steels is the dual-phase (DP) steels. These steels are typically annealed in the critical range for short periods of time (usually less than 5 minutes) followed by rapid cooling. The resulting microstructure is 10% to 20% martensite by volume in a ferrite matrix. The continuous annealing process is ideal for producing DP plate grades. DP steels are unique in that they deform through continuous yielding behavior， as martensite is a continuous source of dislocations during plastic deformation.

Most other mild steels show yield points when deformed and require debarking or temper rolling to provide a source of dislocation for continuous yielding behavior. Steels showing yield points are undesirable in many forming operations because of the formation of ludus strips， which give a defective surface.

Hot dip galvanized products are produced on lines that process pre-annealed (batch annealed) or fully hardened steel coils. Lines that process fully hardened steel coils have in-line annealing capability so that annealing and hot dip galvanizing can be done in one pass on the line. This in-line annealing， like the continuous annealing of uncoated steel， is typically slightly stronger and slightly less ductile than batch annealing. Maximum temperatures are below Ac1 temperature for commercial quality steels， but temperatures in excess of 845 degrees Celsius are required for special kill grades of drawing quality. Galvanizing pre-annealed steel results in properties similar to those of ungalvanized steel.