Annealing of Cold Rolled Steel

Annealing of Cold Rolled Steel

Annealing is a generic term denoting a treatment which consists of heating to and holding at a suitable temperature followed by cooling at an appropriate rate, primarily for the softening of metallic materials. In the plain carbon steels, annealing normally produces a ferrite-pearlite microstructure.

The normal method of manufacturing cold rolled sheet is to produce a hot rolled coil, pickle it to remove scale (oxide), and cold roll it to the desired final gauge. Cold rolling can reduce the thickness of the hot rolled sheet in excess of 90 %, which increases the hardness and strength of the steel but severely decreases its ductility. If any large amount of subsequent cold working (such as forming, drawing etc.) is to be done, the ductility of the steel is to be restored.

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 which have buildup within the microstructure during the process of cold rolling.

During the cold rolling of steel, extensive deformation given to the steel at room temperature considerably reduces the ductility and formability of cold rolled sheets. This necessitates annealing, where the cold rolled sheets get stress relieved through the mechanisms of recovery, recrystallization, and grain growth. Annealing is one of the most important processes in the cold rolling mill since it determines the quality of cold rolled steel sheet. In fact, it is an important process for controlling the mechanical properties of the cold rolled sheet.

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 different sizes. Annealing of a heavily cold worked steel sheet can be divided into three physically distinct but normally overlapping stages, namely recovery, recrystallization, and grain growth.

As annealing continues, the process of recrystallization occurs, and new, more equiaxed ferrite grains are formed from the elongated grains. During recrystallization, strength decreases rapidly, with a corresponding increase in ductility. Further time at temperature causes some of the newly formed grains to grow at the expense of other grains. This is termed grain growth and results in modest decreases in strength and small (but frequently considerable) increases in ductility. Majority of plain carbon steels are given an annealing treatment which promotes full recrystallization, but care is to be taken to avoid excessive grain growth, which can lead to surface defects (such as orange peel) in formed parts.

The rates at which the annealing process proceeds are functions of both the chemical composition and the prior history of the steel being annealed. For example, small amounts of elements such as aluminum, titanium, niobium, vanadium, and molybdenum can decrease the rate at which the steel recrystallizes, making the annealing response sluggish and hence necessitating either higher temperatures or longer annealing times to produce the same properties. Although the presence of these alloying elements is normally the result of deliberate additions intended to modify the properties of the sheet (as in the case of aluminum, titanium, niobium, and vanadium), some elements can be present as residual elements (molybdenum, for example) in quantities large enough to modify the response to annealing.

On the other hand, larger amounts of cold working (greater cold reductions) accelerate the annealing response. Hence, it is not possible to specify a single annealing cycle which produces a particular set of mechanical properties in all steels and the chemical composition and the amount of cold working is also  to be taken into account.

Annealing of the cold rolled steel normally is designed to produce a recrystallized ferrite microstructure from the highly elongated, stressed grains resulting from cold rolling. Fig 1 shows the effect of annealing on the microstructure of low-carbon cold rolled steel. The cold rolled microstructure is shown in Fig 1(a) in contrast to the partially and fully recrystallized microstructure in Fig 1(b) and 1(c).

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

During heating of the steel, and in the first segment of the holding portion of the annealing cycle, the first metallurgical process to occur is recovery. During this process, internal strains are relieved (although little change in the microstructure is evident), ductility is moderately increased, and strength is slightly decreased.

Recovery dominates at relatively low temperatures and includes the migration of vacancies and of dislocations introduced by cold deformation, leading to the annihilation or rearrangement of a certain portion of them. In more general terms, however, recovery relates to any modification of properties, during annealing, which occurs before the appearance of new strain free recrystallized grains. In other words, recovery does not involve migration of high angle grain boundaries (HAGBs). It is normal to find that during recovery both the mechanical and the physical properties of materials show some change from their values in the cold worked state. Normally the restoration of a mechanical property, such as hardness, yield strength, or ductility to its fully annealed value, is only about one-fifth completed during recovery.

During stress relief which takes place at 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 completely 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 normally during the period of soaking.

During the annealing operation, deformed microstructures of the cold rolled sheets are recovered and recrystallization takes place. Annealing of cold rolled steel sheets are carried out either in a batch annealing furnace where the annealing of cold rolled sheet is done in coil form or in a continuous annealing furnace, where the annealing of the cold rolled sheet is carried in sheet form.

Structure and resulting material properties are considerably influenced by cold rolling because in the process of cold rolling no recrystallization can occur. During the cold rolling process, extension of grains in the direction of rolling occurs and the arrangement of crystallographic lattice gets a directional character. Banding character of other structural phases, such as of inclusions, pearlitic blocks, etc. also gets developed. Three types of texture (i.e. deformation, structural, and crystallographic texture) arise, which yields in a directional character of mechanical properties.  Annealing of the steel sheet after its cold rolling is done for the removal of anisotropy of properties. The microstructure of the cold rolled sheet after its annealing depends on such factors as (i) the initial material structure before cold rolling, (ii) the total cold reduction, and (iii) annealing conditions (temperature and time), and (iv) cooling speed of the steel. With the introduction of high strength low alloy, high strength and advanced high strength materials for the deep drawing of the cold rolled sheet, there are further complex issues arise during the annealing of the cold rolled sheets.

Normally, when the cold deformation of the steel before annealing is higher, the initial temperature of recrystallization is then lower.    Also, at low temperatures the time necessary for finishing of recrystallization is much higher and the required spheroidizing of carbides cannot be completed. Strength or hardness properties of the steel normally decrease with increasing temperature of annealing, whereas plastic properties increase. Substantial lowering of strength values occurs at temperatures which are close to 600 deg C. Further, the higher is the previous cold deformation, the more important is this fall in the strength values.

The final mechanical properties and the microstructure of the steel are largely dependent upon the annealing process since it influences considerably 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 normally 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 normally preferred. Mixture of the two gases is obtained through cracking of ammonia (5 % hydrogen and 95 % nitrogen).

For the production of cold rolled steel normally two types of annealing processes which are used are (i) soft annealing, and (ii) recrystallization annealing. In the soft annealing of steel, the precipitation of cementite and pearlite takes place due to which the strength of the steel is reduced This facilitates the forming operation. Normal temperature for this annealing ranges from 680 deg C to 780 deg C. In the case of recrystallization annealing, the annealing reconstitutes the crystals 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.

It is well known that metals are strengthened by cold working. During cold rolling which is a necessary stage in sheet production, the steel becomes intensely hardened but loses almost all its ductility. During conventional processing the cold rolled steel is fully recrystallized by annealing with the purpose of restoring ductility, but at the expense of strength. Then, if higher strength is needed it is normally achieved by either alloying or by special heat treatments at higher temperatures. The above procedures entail wastage of energy and increased costs. On the other hand, by controlled low temperature annealing, it is possible to retain as much as possible of the strength from cold rolling while at the same time restoring adequate ductility. This yields a steel with properties which are a compromise between the high strength – low ductility of fully cold rolled sheet and the low strength – high ductility of fully recrystallized sheet. Such a process has been variously termed as ‘back annealing’, ‘partial annealing’, ‘recovery annealing’, ‘stress relief annealing’, ‘temper annealing’ or ‘controlled incomplete annealing’.

Basically, the ‘back annealing process consists of two stages namely deformation and annealing. An alternative approach called ‘temper rolling’ or ‘rolling to temper’, seeks to obtain the desired strength and ductility by plastically deforming the already fully annealed material, while in ‘back-annealing’, a heavily deformed material is either subjected to recovery or partial recrystallization. The objectives in both the cases are the same. It has been claimed that at all strength levels the ductility of the ‘back-annealed’ material is always superior to that of ‘temper rolled’ material.

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

Batch annealing process is the older of the two processes. In this process, the annealing of the cold rolled sheet is carried out in coil form. Normally, 3 to 4 cylindrical steel coils (typically having weight of 10 tons to 20 tons each) are stacked on the base unit of the process for annealing under a protective gas (hydrogen) atmosphere. The process is preferred where large ferrite grains are needed as in the case of electrical steels. The design of a batch annealing unit depends on the steel to be annealed.

Basic equipment which are needed for the batch annealing of the cold rolled steel sheet are (i) a base unit provided with a recirculation fan, (ii) circular convector plates for coil separation, (iii) a protective gas tight cylindrical cover, (iv) a heating hood or heating furnace (also known as bell furnace because of its shape) with burners arranged tangentially, and (v) a cooling hood. The convector plates between the coils are used to improve the heat flow. Fig 2 shows basic equipment for the batch annealing process.

Fig 2 Basic equipment for batch annealing process

For carrying out the annealing process, at first, 3 to 4 cold rolled steel coils are placed on the base unit one above the other, separated by convector plates as shown in Fig 2. These cylindrical shaped coils are called furnace charge or charge material.  After loading the base with the coils, a protective cover is put in place and the protective gas is circulated with in this enclosure. Protective gas atmosphere is applied to avoid oxidation on steel strip surface at high temperature and transfer heat from furnace pass through steel coil.

A heating furnace is then placed over the protective cover. Burners of the heating 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 to the coil inner surface by the circulating protective gas. The inner and outer surfaces of the coils then get heated by the convection from circulating protective gas and by radiation between the cover and the coil. The inner portions of the coil are heated by conduction.

There are three stages of the annealing cycle. The first stage is the heating stage in which the temperature is raised upto the target temperature. The second stage is the soaking stage in which the temperature is held for getting the homogeneous temperature between outside and inside of steel coils. The third and the last stage is cooling rage when the temperature is decreased slowly in the furnace.

After heating phase, the heating hood is replaced with a cooling hood and circulating of protective gas is continued. 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.

The large thermal mass and low conductivity arising from the air gaps between the sheets result in a large thermal lag between cylindrical surfaces of the coil and coil core. The coil surface with the highest temperature during the heating cycle is called hot spot and coil core with the lowest temperature is called cold spot. During the heating stage, for full recrystallization, cold and hot spots of the coil are required to be raised to a desired annealing temperature. The needed time to reach cold spot temperature to a desired temperature is the heating time. Longer heating time results in better uniformity of microstructural and mechanical properties, but reduces the productivity of the heating furnace. Fig 3 shows schematic representation and the sectional view of the batch annealing process.

Fig 3 Batch annealing process for cold rolled coils

The annealing cycle of the cold rolled coil varies with the steel composition, cold reduction, and steel grade desired. However, typical batch annealing temperatures range from 620 deg C to 690 deg C (just below Ac1 temperature) for the coldest point in the charge. Cycle times vary with the grade desired and the size of the charge, but total time (from the beginning of heating to removal of the steel from the furnace) can be as long as one week.

During the batch annealing, recrystallization of the deformed structure begins to take place at temperatures of around 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 is to be limited to around 730 deg C since higher temperature causes coarse carbide formation detrimental to forming and can cause sticking of the adjacent layers of the coil. Fig 4(a) shows a typical heating and cooling cycle for batch-annealing coils of low carbon cold rolled steel sheet.

Fig 4 heating and cooling cycles and hot and cold points in a coil during annealing

In a study done on batch annealing for getting the homogeneous temperature between outside and inside of steel coils, the annealing process has been controlled by using thermocouples sensor attached on the steel strip. In this study, it has been found that the cold rolled steel coil has hot point at the outside of steel coil and cold point at  the 2/5 of side wall from inside coil. These positions are shown in Fig 4(b).

During the cooling cycle, coil core is warmer than other coil spots. The air gaps between the sheets in the coils result in very low radial thermal conductivity. As a result, spatial variation in temperature is prevalent during the annealing process. The outer surfaces of the coil, referred to as hotspots, are heated faster and achieve the annealing temperature in a shorter time as compared to the inner core of the coil, normally referred to as cold spot. Since recrystallization and grain growth are thermally activated processes, such thermal lag leads to spatial variation in microstructure, with an associated variation in the mechanical properties within a coil. This variation in the mechanical properties occurs at each position (outside, middle, and inside of steel coil) of the length of the steel coil. Due to this reason, some portion of steel coil has the mechanical properties problems. In addition, due to the axial temperature variation along the furnace, there are coil-to-coil variations in their microstructure and mechanical properties.

In the batch annealing, temperature of steel strip depends upon thickness, width, weight, and stack position of the coil. Width range and the weight range of the coil have significant effect on the mechanical properties variation in batch annealing. Decreasing the width range leads to the decreasing the spread in yield strength within the stack. Number of coils in batch annealing furnace slightly affects the difference in temperature between hot point and cold point. In the annealing stage, heating rate and soaking time obviously have higher intrinsic effects on formability properties. The coil position in the batch annealing furnace has a slight effect on ‘n’ value but more effect on ‘r’ value of the steel.

Even though an increase in soaking time normally results in reduction in the variations of the microstructural and mechanical properties, it also reduces the furnace productivity. Hence, the selection of the soaking time in the batch annealing process needs an optimization between productivity and quality. In addition, appropriate selection of the heating rate, has a metallurgical implication on precipitation and recrystallization kinetics, as well as annealing temperature form the core of the design of the batch annealing thermal cycle.

The batch annealing operation has a considerable influence on all the important performance parameters of the cold rolling mill. These include energy consumption, plant productivity and emissions, as well as quality parameters such as strength, ductility, drawability, and formability of the steel sheet. In view of its relevance on all these key parameters, it is necessary that the batch annealing operation parameters are optimized for the achievement of the maximum productivity and minimum energy consumption, while maintaining the required product quality.

Despite being a critical operation, the batch annealing cycles at the industrial scale are normally designed through plant trials and empirical methods, which in addition to being time consuming and expensive, can at best provide sub-optimal result. Instead, the process cycle can be effectively optimized through an annealing model, which can emulate the batch annealing operation, thus reducing the number of plant trials needed for the plant optimization.

In one of the study on the batch annealing process, it has been concluded that the annealing temperature and the soaking time have considerable effect on the mechanical properties variation in batch annealing. The findings of the study has been summarized as (i) increasing the annealing temperature can remarkably decrease the yield strength, tensile strength, and hardness, whereas the % elongation can be increased, (ii) increasing the soaking time can have slight effect on mechanical properties (iii)  the annealing temperature of 650 deg C with soaking time of 2 hours is to be applied to provide the mechanical properties close to nominal value which provide yield strength of 220 MPa, tensile strength of 305 MPa, % elongation of 43 %, and hardness of 46 HRB, and (iv) grain size of the work piece trended to grow from the annealing temperature of 610 deg C.

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 the poor cold forming characteristics and poor ageing of the annealed steel. The carbon solute in the ferrite precipitated as cementite in batch annealing while it remained in super-saturation 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 which improved the properties of steel after continuous annealing.

Continuous annealing cycles are of shorter duration and are conducted at higher temperatures than batch annealing cycles. In some applications, the annealing temperature can exceed Ac1. Typical cycles are 40 seconds at 700 deg C for cold rolled commercial quality steel and 60 seconds at 800 deg C for the drawing quality special killed sheet. Majority of continuous annealing of cold rolled sheet includes an over-aging treatment designed to precipitate carbon and nitrogen from solution in the ferrite and to reduce the likelihood of strain aging. Over-aging for 3 minutes to 5 minutes at 300 deg C to 450 deg C accomplishes the desired precipitation of carbon and nitrogen. Batch annealing and continuous annealing differ slightly in the properties they produce in annealed steel. Typical average properties of the batch annealed cold rolled commercial quality plain carbon steel sheets are yield strength 210 MPa and elongation 43 % while those of continuous-annealed are yield strength 228 MPa and elongation 41.7 %.

Cold rolled steel coil which is 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 the 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. Fig 5 shows a schematic diagram of a continuous annealing line.

Fig 5 Schematic diagram of a continuous annealing line

In addition, the application of over-aging treatment (maintaining the sheet temperature at 400 deg C to 450 deg C after heating, soaking and rapidly cooling) to the annealing process makes it possible to have super-saturated solute carbon precipitate rapidly in the form of cementite, and to obtain suppressed-aging properties. Continuous annealing line can also integrate electrolytic cleaning line thus enabling the manufacture of steel sheets having very good material homogeneity, surface quality, and shape at lower costs and in a shorter time than what can be realized by employing conventional batch annealing process.

For the primary cooling of the sheet, continuous annealing line can also adopt the gas-jet cooling method whereby the furnace atmosphere gas is cooled and blown to the sheet surfaces. The cooling rate which is realized using this method is roughly 10 deg C per second, but for this cooling rate, solute carbon precipitates at the grain boundaries and the distance between the precipitates is large. As a result, over-aging treatment takes a long time. On the other hand, another continuous annealing process which has been developed uses water quenching for the primary cooling. With this method, the cooling rate is around 1,000 deg C per second, at which rate carbides precipitate in fine particles inside the crystal grains and at their boundaries, making the distance between the precipitates smaller, and reducing the over-aging time, although the figures of elongation, ‘n’ value, etc. tends to be lower.

To solve the above problem, in the early 1980s, a new cooling method has been developed which is called the ‘Accelerated Cooling’ process. With this method, the sheet is cooled by blasts of air-water mixture sprayed from cooling nozzles, each having a water header and a gas header. The cooling rate is around 100 deg C per second, and the rate and the final temperature of cooling are both controllable by changing parameters such as the quantity of water and gas and the number of headers. This method has made it possible to shorten the time for over-aging treatment and to achieve the needed properties of the steel. The high cooling rate and the temperature control capacity of the ‘Accelerated Cooling’ process has made it possible to efficiently produce high-tensile sheets of the solid-solution hardening and the precipitation hardening steels, as well as those of the steels developed by transformation strengthening, which have strengths of 1,180 MPa. Fig 6 shows schematic of accelerated cooling unit.

Fig 6 Accelerated cooling unit and continuous annealing cycle

Typical continuous cooling cycle is shown in Fig 6.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 needed 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.

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.

Nowadays majority 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, over-ageing and final cooling). Further continuous annealing line is lesser flexible since a change in soaking / over-ageing temperature takes a long transition time leading to an important yield loss.

High strength cold rolled sheet is growing in importance due to its high load-bearing capacity. Strength of sheet can be increased through modifications of chemical composition and / or selection of different annealing cycles, but these methods result in decreased ductility. Plain carbon steel, produced by conventional techniques, can be batch annealed or continuous annealed under conditions which result only in recovery or partial recrystallization. Typical batch annealing cycles of this type use soaking temperatures of 425 deg C to 480 deg C and various soaking times.

High strength low alloy (HSLA) steels containing alloying elements such as niobium, vanadium, and titanium also can be produced as cold rolled steel grades. The additional alloying produces stronger hot rolled steel, which is strengthened even more by cold rolling. Cold rolled HSLA steels can be recovery annealed to produce higher strength grades or recrystallization annealed to produce lower strength grades. Successful production of cold rolled HSLA steel needs selection of the appropriate combination of steel composition and hot rolled strength, amount of cold reduction, and type of annealing cycle.

Another series of high strength sheet steels are the dual phase (DP) steels. These steels are normally annealed for a short period (normally less than 5 minutes) in the inter-critical range, followed by rapid cooling. The resulting microstructure is 10 % to 20% martensite by volume in a matrix of ferrite. The continuous annealing process is ideal for producing DP sheet grades. DP steels are unique in that they deform by a continuous yielding behaviour since the martensite is a continuous source of dislocations during plastic deformation.

Majority of other low carbon steels which display a yield point upon deformation need to be skin passed or temper rolled to provide a source of dislocations for continuous yielding behaviour. Steels displaying a yield point are undesirable for many forming operations because of the formation of Luders bands which blemish the surface.

Hot dip galvanized products are produced on lines which process either pre-annealed (batch annealed) or full hard coils. Lines for processing full hard coils incorporate an in-line annealing capability so that annealing and hot dip galvanizing can be accomplished in a single pass through the line. This in-line annealing, like continuous annealing of uncoated steel, shows normally slightly higher strength and slightly lower ductility than batch annealing. Maximum temperatures are below the Ac1 temperature for commercial quality steel, but temperatures in excess of 845 deg C are needed for the drawing quality special killed grades. Galvanizing of pre-annealed steel results in properties which are similar to the properties of the steel not galvanized.

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