The present invention relates to a process for producing an extra or ultra low carbon steel slab, strip or sheet, and to a slab, strip or sheet produced thereby.

Canmaking via the DWI (Drawing and Wall Ironing) or DRD (Draw and Redrawing) process takes place at high speed and involves severe plastic strain. Also for deep-drawing or forming of inner and outer panels for automotive applications the demands on formability, particularly high-speed formability, increase. In addition, weldability is also an issue. The steel therefore needs to be of the highest quality and a very low level of non-metallic inclusions is essential to the efficient operation of these processes. However, care must be taken to avoid an excessively large ferrite grain which can give rise to an orange peel effect and a poor surface for lacquering. DWI cans are, for instance, used for beer and soft- drinks, pet foods and human foodware, but also for battery cans. DRD cans are, for instance, used for pet foods and human foodware. Low levels of non-metallic inclusions are also very important for electrical steels and steels for automotive applications, not only for improving formability, but also for improving weldability.

Steels currently in production rely on the use of small precipitates to prevent the grains from becoming too large. However, the disadvantage is that the formability may be adversely affected by the presence of the precipitates. Also, the presence of precipitates adversely affects the magnetic properties for transformer steels because the precipitates hamper the motion of magnetic domain walls.

To prevent the formation of the non-metallic inclusions as a result of clogging, it has been proposed that a calcium treatment would prevent clogging. However, it was found that in calcium treated aluminium-killed titanium alloyed ultra low carbon steels calcium aluminate inclusions are frequently encountered. These inclusions are so large that they can be seen on the surface with the naked eye. Apparently this is a problem linked to the ultra low carbon content in combination with the titanium content in the steel, because in low carbon calcium treated steels these large inclusions do not occur.

It is an object of the invention to provide a process for producing an extra or ultra-low-carbon steel strip with a reduced amount of alumina inclusions or without alumina inclusions. It is also an object of the invention to provide an extra or ultra-low-carbon steel which has a shorter annealing time and/or a reduced annealing temperature in a recrystallisation annealing process, particularly in a batch annealing process.

Note that in the following the word "reductor' is used for a metal or metallic compound that reacts with oxygen in the steel but the redactor is not used to deoxidise or kill the steel in the sense of creating an oxygen potential that is so low (e.g < 5 ppm oxygen activity) that during solidification no extra oxide are formed other than the reductor; in the case of using a reductor the oxygen activity is higher than in fully killed steel (> 10 ppm) and other alloy metals that may be present in the liquid steel such as Si, Cr, Ti and Mn may form oxides as well during solidification of the slab.

One or more of these objects are reached with a process according to claim 1. Preferred embodiments of the process are in the dependent claims.

With the process according to the invention a steel slab or strip can be produced substantially without or completely without alumina containing inclusions.

The clear difference with the conventional process for producing an ultra- low-carbon steel strip or sheet is that the ladle treatment of the melt during the vacuum-degassing step, e.g. in an RH-process or another suitable vacuum- degassing process, does not target a removal of the oxygen by killing it by adding excess aluminium to form alumina particles, but a process wherein the oxygen content of the melt is monitored and controlled, and a dedicated amount of a first reductor (such as aluminium) is optionally added if the oxygen content is too high, i.e. not between 18 and 60 ppm. If the actual oxygen content is already within these boundaries, then the addition of the first reductor is not required, hence the optional character of the addition of the first reductor. The first reductor is added in a targeted amount based on the measurement of the actual oxygen content of the melt so as to avoid the addition of excess first reductor to the melt which would be present in the final steel, in the case of aluminium as a first reductor as acid soluble aluminium (i.e. in the form of metallic aluminium, not as alumina). This is the justification for the lower value of 18 ppm of oxygen activity for the first level. Steel melts with an oxygen activity or dissolved oxygen content below 10 ppm are considered killed steels. Full killing with the first reductor is not desired. It is therefore not a killed steel in the sense of EN 10130. The oxides formed during the ladle treatment float to the slag and the level of first reductor in the melt is quickly reduced as a result of the flotation. The addition of the precise amount of first reductor ensures that substantially all oxides formed during the ladle treatment are removed from the melt prior to solidification during continuous casting, so that the resulting steel contains substantially no oxides of the first reductor. The degassing of the molten steel may be made by any conventional methods such as the RH method or the RH-OB method. The oxygen content or activity of the liquid steel may be measured using expendable oxygen sensors or by permanent oxygen sensors. According to the process of the invention the oxygen activity or dissolved oxygen content of the melt must be reduced to the first level of between 18 and 60 ppm when measured 2 to 5 minutes after the addition of the first reductor. This range is chosen to obtain a steel with the lowest amount of dissolved first reductor to achieve that the liquid steel is not fully killed, but still contains small amounts of oxygen.

The phrase "at most 0.02% in total of the first, second and third reductor in their metallic form" intends to mean that the reductor is not present bound to oxygen or as a non-metallic form, but that the atoms are still in the steel as atoms. In case of using aluminium as a reductor the usual term for aluminium in the steel in its metallic form is "acid-soluble aluminium", so the term "in their metallic form" is to mean the equivalent term to "acid-soluble aluminium" for other reductors such as zirconium etc.

In an embodiment of the invention preferably at most 0.01%, in total of the first, second and third reductor in their metallic form is present in the slab, strip or sheet. The excess of reductors is to be minimized to avoid over alloying (costs) and to avoid any deleterious effects of the excess such as the formation of precipitates or attack of the refractory material of the vessels and tundish.

In an embodiment of the invention aluminium is used as a first reductor. Aluminium is cheap and its effects are well understood.

The oxygen activity can now be controlled between 15 and 40 ppm by the use of a second reductor which produces more stable oxides than alumina, such as nongaseous elements such as zirconium (Zr) or cerium (Ce) or tantalum (Ta), or a reductor which is gaseous at the steelmaking temperature such as calcium (Ca), selenium (Se), barium (Ba), strontium (Sr). For the purpose of this invention, 1600°C is defined as a typical steelmaking temperature.

Reductors which are gaseous at the steelmaking temperature, such as Ca, Se, Sr and Ba, can be injected into the melt, preferably with a cored wire. Addition directly in the circulating RH(-OB) vessel, e.g. as pellets or gas, is also possible because the flow of the melt in the vessel is such that these gaseous metals (i.e. gaseous at the steelmaking temperature) will have a recovery which, although lower than when injecting cored wire in the melt, is such that the total alloy costs are lower because the alloy can be added in the vessel without a cored wire. For calcium recovery levels of 4 to 12% have been measured.

In the process according to the invention Zr is used as second and (if necessary) third reductor.

The zirconium must be added in a suitable form, such as FeZr (80%) to bring down the oxygen activity or dissolved oxygen content of the melt to the second level of between 15 and 40 ppm.

Alloying elements are subsequently added (in addition to those elements which were added to the melt earlier) to create the final composition of the melt.

After stirring or post-circulation for a selected time, e.g between 2 and 10 minutes, the oxygen activity or dissolved oxygen content of the melt may optionally be measured and the results of the measurements may be used to bring the final level of the oxygen activity or dissolved oxygen to at most 35 ppm by adding a third reductor, such as zirconium, in a suitable form and amount to the melt. Again in the case of Zr this may be in the form of FeZr (80%). The total Zr content in the steel should preferably not exceed 0.018 wt%. The alloying elements referred to may be provided in the form of FeTi and/or FeNb for Ti and/or Nb alloyed ULC steels and/or FeP for IF steels requiring more strength. For ordinary ULC steels the alloying elements may e.g. be Mn (e.g. added as FeMn alloy), P (e.g. added as FeP alloy), Cr (e.g. added as FeCr alloy) and/or B (e.g. added as FeB alloy).

After the desired final level of the oxygen activity or dissolved oxygen content of the melt is reached, the steel melt is sent to the caster to be cast into a thick or thin slab or into a strip. The final solidified steel slab, strip or sheet of ultra-low-carbon steel comprises at most 0.002% of acid soluble aluminium and at most 0.030% silicon and a total oxygen content of at most 150 ppm. The process according to the invention will not lead to clogging because of alumina, because the alumina residu in the liquid steel is minimised or absent, and the wettability of alumina in steel is adequate to avoid build-ups of alumina in the casting system (tundish nozzle, SES/SEN).

The carbon content of steel melt, and thus of the solidified steel, is limited to at most 0.0100 wt% because when a higher carbon content is used, the carbon forms carbon monoxide in the manufacturing stage during casting and solidification, and that CO in turn remains as blow-hole defects in the solidified steel. Moreover, the boiling effect may cause operational problems during casting. It should be noted that the silicon in the solidified steel may be present as silicon oxide and/or as metallic silicon. Steels with a carbon content of below 0.010 are called Extra Low Carbon (ELC) steels. An ultra low carbon (ULC) steel has a carbon content of at most 0.006%, preferably at most 0.003%.

As a result of this process the grain boundaries are very clean and the recrystallisation temperature of the steel is much lower than conventional ultra- low carbon steels. This phenomenon is attributed to the extremely low levels of silicon and acid soluble aluminium in the final steel strip or sheet and the presence of finely dispersed oxide particles, such as titaniumoxides, tiny titanium-manganese-oxy-sulphides or titanium oxynitrides some with residual alumina and some residual Zr-oxides and their respective oxy-sulphides (sulphides form during solidification) and some traces of alumina (resulting from the use of aluminium as first reductor and/or from carry-over of alumina from earlier batches in the ladle or tundish, which do not hinder recrystallisation to the extent that small precipitated aluminium nitrides do.

Even in Ti stabilised ULC or ELC steels a low amount of aluminium nitrides formed in the hot strip mill can hinder recrystallisation of the strip after cold rolling. As a result of the low recrystallisation temperature of the steel the annealing temperatures can be reduced as well, leading to a more economical process as well as a reduced tendency for grain growth in the product. The reduced annealing temperatures also prevent sticking in batch annealing processes and reduce the risk of rupture in continuous annealing. A further advantage of the very clean grain boundaries is the strongly reduced susceptibility to corrosion on the grain boundaries. This is especially relevant for the application of the steel in the production of battery cases. The coating systems used in the production of batteries may be leaner (e.g. thinner coating layers or fewer coating layers) when using a substrate with a better corrosion resistance. The very clean steels are also beneficial for transformer or other electrical applications. For transformer steels punchability is important, hence the maximum phosphorous content of 0.2%. A suitable maximum value for phosphorous is 0.15%. For other applications of cold-rolled steels the phosphorous content should preferably be at most 0.025wt%, preferably at most 0.020%.

During casting very little and preferably no Al is left in the steel, and as a consequence the Si pick-up, which normally occurs according to the following reaction (Al _stee i + Si0 ₂ - Al ₂ 0 ₃ + Si _stee i) does not occur due to the low Al-content. The maximum value for silicon is 0.030%, but a suitable maximum value for silicon was found to be 0.003%.

It is preferable that the strip or sheet of ultra-low-carbon steel produced according to the invention comprises at most 0.001% of acid soluble aluminium and/or at most 0.002% silicon. Even more preferable the silicon content is at most 0.001%. Ideally, there is no acid soluble aluminium and no silicon in the solidified steel.

In an embodiment the first reductor to be added in a suitable form to the melt to bind oxygen to reduce the oxygen activity or dissolved oxygen content of the melt to a first level is aluminium.

In an embodiment of the invention the second reductor is zirconium.

It is possible to conduct the process according to the invention also such that the first and second and third reductor is zirconium. In this case the entire process is zirconium based. This means that the likelihood of clogging due to alumina is eliminated. Although technically feasible and attractive from a clogging point of view, the process is less attractive from a cost point of view as aluminium is, at present, less expensive as a reductor than zirconium. It is then a matter of weighing the benefits of the full elimination of alumina clogging against the costs of aluminium versus zirconium as a first reductor. It is noted that the deoxidation with zirconium can be performed during the vacuum-degassing process step or during the stirring. In any case, the zirconia is allowed to float during the post- circulation after the vacuum-degassing process step or during the stirring step. Usually about 75% or more of the zirconia is removed from the melt that way. Alloying elements are subsequently added as before to create the final composition of the melt. After stirring for a selected time, e.g. between 2 and 10 minutes, the oxygen activity or dissolved oxygen content of the melt may optionally be measured and the results of the measurements may be used to bring the final level of the oxygen activity or dissolved oxygen to at most 35 ppm by adding a third reductor in a suitable form and amount to the melt followed by casting as before. If deemed necessary there can also be additions of the third reductor in the tundish, i.e. immediately prior to casting to fine tune the oxygen activity. Preferably the second and third reductors are the same, and preferably they are zirconium. The final solidified steel slab, strip or sheet of ultra-low- carbon steel comprises no acid soluble aluminium (unless present as an inevitable impurity as a result of carry-over from earlier melts in the steelmaking equipment) and at most 0.030% silicon and a total oxygen content of at most 150 ppm.

Preferably the final level of the oxygen activity or dissolved oxygen content of the melt must be reduced to at most 35 ppm. The steel only contains a small amount of the reductor which produces more stable oxides than alumina. Excess reductor, such as Zr, may result in a reaction of the reductor with the refractory of the ladle wall and casting system so that break-outs may occur (e.g. in ladle, tundish, slide gates, stopper area).

Preferably the oxygen activity or dissolved oxygen content of the melt in the tundish of the casting machine has been reduced to at most 45 ppm.

The advantages of the steels produced by the process according to the invention are manifold. The strong reduction or even absence of clogging due to a reduced amount or complete elimination of alumina formed during the process is evident, leading to fewer or no alumina based inclusions and thereby to fewer of no alumina related steel cleanness or surface issues of the final product.

The reduction of the oxygen activity or dissolved oxygen content caused by the additions of reductor such as zirconium decreases the risk of pin hole defects because the likelihood of gas bubbles forming in the steel during its solidification is strongly reduced as a result of the lower oxygen activity. The same is true for the inevitable forming of titanium oxides during solidification of the slab of which their growth is largely suppressed because of the reduced oxygen activity.

An unexpected benefit is that the batch annealing temperature of the cold- rolled material (after first having been hot rolled, cooled, coiled and pickled in a conventional manner usual for ULC or stabilised ULC steels) is much lower than that of conventionally fully aluminium killed ULC or stabilised ULC steels because of the absence of the AIN precipitates that are normally present in the steel. The annealing temperature of the cold-rolled material produced according to the invention is between about 50 to 100°C lower than when recrystallisation annealing conventional fully aluminium killed ULC or stabilised ULC steels. The annealing time during batch annealing can be reduced by several hours.

The absence of metallic aluminium in the steel slab or strip prevents the formation of aluminium-nitride precipitates at later stages of the process and therefore provides clean grain boundaries. Moreover, the absence of AIN also prevents many problems associated with the dissolution and precipitation characteristics of AIN in the hot strip process such as inhomogeneities of the microstructure and properties over length and width of the strip as a result of the difference in thermal path of different positions of the hot rolled strip in coiled form. There is no need to dissolve the AIN in the reheating furnace of a hot strip mill so a lower furnace temperature can be used, nor is there a need to use a high coiling temperature to allow the AIN to precipitate in the coil. In the Ti- stabilised IF steels or partially stabilised Ti-IF steels, the TiN-particles are formed in the slab and reheating furnace. These are rather large. The amount of small oxides in the steel creates a stable and predictable nitride precipitation. The low coiling temperature in turn leads to an improved pickling ability. The chemistry of the slab or strip results in the formation of finely dispersed oxides, comprising mainly of small titanium oxides, titanium oxy-sulphides, titanium oxy-nitides, tiny zirconium oxides or titanium-manganese oxy-sulphides (the sulphides form during solidification in the slab) of which several have an zirconium oxide nucleus. Of these inclusions, relatively large size inclusions act as nuclei for the recrystallisation during annealing of cold-rolled steel, while relatively small size inclusions may act to become appropriate barriers with respect to grain coarsening caused after the recrystallisation to thereby control the grain size of the steel.

In an embodiment the carbon content of the steel is at most 0.01 wt%. Chromium is added to the steel to at most 0.05 wt% to suppress CO formation during solidification of the slab.

In an embodiment a process is provided for producing a slab or strip wherein the slab, strip or sheet comprises one or more of:

o at most 0.006% carbon, and/or

o at least 0.05 and/or at most 0.35% manganese, and/or

o at most 0.006% nitrogen, and/or

o at most 0.025% phosphorus, and/or

o at most 0.020% sulphur, and/or

o at most 0.05% titanium, and/or

o at most 0.05% niobium, and/or

o at most 0.018% zirconium, and/or

o at most 0.10% vanadium

In an embodiment the steel slab or strip comprises one or more of:

at most 5 ppm B, or if the steel comprises B as an alloying elements the

B-level is between 10 and 30 ppm B and/or

at most 0.004% carbon, preferably at most 0.003%, preferably at most 0.0028%, preferably at most.0025% or even preferably at most 0.002% carbon and/or

at most 0.005% nitrogen, preferably at most 0.004 and/or more preferably between 0.0012 and 0.0030% nitrogen. A suitable upper boundary for nitrogen is 0.0030%.

Preferably the boron free steel comprises at most 1 ppm B. Preferably the Boron containing steel comprises between 10 and 25 ppm B. The maximisation of the carbon content of at most 0.004% carbon, or any of its preferable lower maximum values mentioned above is intended to minimise the risk of CO- formation, carbide formation and carbon ageing issues. In an embodiment the manganese content is between 0.10 and 0.35%. Suitable maximum values for P and S in the solidified steel are 0.020 and 0.010 respectively.

Preferably, the sulphur content is at most 0.010%, more preferably at most 0.005%.

In an embodiment a process is provided wherein the steel slab or strip according to the invention is subjected to

hot-rolling the slab or strip at a temperature above Ar3 to obtain a hot- rolled strip;

- coiling the hot-rolled strip;

optionally followed by:

cold-rolling the hot-rolled strip with a cold rolling reduction of between 40 and 95% to obtain an intermediate cold-rolled strip;

annealing the intermediate cold-rolled strip;

- optionally subjecting the intermediate cold-rolled strip to a second cold rolling down to a final sheet thickness;

optionally cutting the strip into sheets or blanks;

The slab may be heated and hot-rolled prior to hot rolling in a known way. Alternatively, the warm slab may be heated or the hot slab may be hot-rolled directly. In order to save energy and, hence, to achieve a greater economy, the preheating of the steel prior to the hot-rolling is made at a relatively low temperature of 1150°C or lower, although the invention does not exclude the use of higher preheating temperatures.

The optional second cold rolling may be a conventional temper rolling step, preferably at a reduction of between 0.5 to 5%. However, the second cold rolling may also involve a substantially higher cold rolling reduction of preferably between 5 and 50% to produce a steel with a higher yield strength.

In an embodiment the intermediate cold-rolled steel strip or sheet is subjected to a recrystallisation treatment by continuously annealing or by batch- annealing. The annealing process conditions are chosen such that a recrystallised microstructure is obtained throughout the strip. One of the characteristic features of the invention is that the coiling temperature is limited neither to high temperature nor to low temperature. Namely, according to the invention, the steel may be coiled up at temperatures between 500 and 700°C. When the coiling temperature is higher than the above mentioned temperature range, the pickling is impeded due to a too large scale thickness. In an embodiment the coiling temperature is between 530 and 700°C, preferably between 550 and 650°C. A suitable minimum coiling temperature is 570°C, and a suitable maximum is 640°C. The lower coiling temperature can be chosen because there is no AIN- precipitation to be controlled by it. As a result the oxide layer on the strip is thinner and easier to remove by pickling .

In an embodiment the hot-rolled sheet has a thickness of between 2.0 and 3.5 mm, the hot-rolled strip is cold rolled with a reduction ratio of between 85 and 96%, preferably between 85 and 95%, and wherein the second cold rolling reduction is between 0.5 and 10%. Preferably the reduction ratio is between 87 and 93% . For double cold rolled steels the second cold rolling reduction is preferably between 5 and 50%

According to a second aspect the invention is embodied in an extra-low- carbon or ultra-low-carbon steel slab, strip or sheet according to claim 11 and produced by the process according to the invention .

In an embodiment the ultra-low-carbon steel strip or sheet according to the invention comprises at most 0.001% titanium and at most 0.001% niobium weight, and at most 0.001% vanadium by weight. It is important that the amount of elements causing deoxidation are minimised . Hence the silicon content of the melt is preferably minimised to 0.030 or even 0.020%.

The steel may be coated with a metallic and/or polymer coating system.

According to a third aspect the ultra-low carbon steel sheet according to the invention is used in packaging applications such as cans for packaging foodstuff or beverages or in packaging applications such as batteries or as electrical steels for applications such as electromagnets or as steels for automotive applications.

In an embodiment the ultra-low carbon steel sheet according to the invention is used as enamelling steel . The presence of the finely dispersed particles and the clean matrix results in an ability to store hydrogen during the enamelling process and avoids surface defects like fish-scale on the enamelled product, such as a bath tub.

The invention will now be illustrated by means of non-limitative examples. Continuously cast slabs were produced of the steel grades listed in table 1.

Table 1 - Composition in 1/1000 wt.% except Zr-tot, C, N, Ca, O-tot and B in ppm (indicated with *) ppm = 1/10,000 wt%

A 330 ton steel melt was prepared . Prior to the addition of the first uctor (aluminium in this case) the oxygen content of the melt was 460 ppm. After addition of the first reductor, the measured first level 2 to 5 minutes after addition of the reduction dropped to 32 ppm. The second reductor (zirconium) was added in the form of 55 kg FeZr (80% Zr) and resulted in a second level of 18 ppm. The alloying with the alloying elements resulted in an increase of the oxygen content because (e.g.) FeTi contains some oxygen, and there may have been some oxygen pick-up from the slag. Again measuring the oxygen content and adding the third reductor (Zr again) resulted in a final level of the total oxygen content of the melt at the end of the ladle treatment of 20 ppm. These steels were successfully cast into thick (225 mm) slabs. No clogging occurred during casting of the steels according to the invention. The total oxygen content of the solidified slab was 45 ppm. The composition of the solidified steel of this particular trial is given in Table 1 (Inv). The reference steel (Ref) is an aluminium-killed Nb-Ti stabilised ultra low carbon steel.

The slabs were reheated and hot-rolled in the hot strip mill of Tata Steel in IJmuiden. The finish hot rolling temperature was above Ar3 in all cases (950 +/- 15 °C for both materials) and the thickness of the hot rolled strip was 3.6 mm in all cases. The coiling temperature was 735 +/- 5 °C. After cooling to ambient temperatures the coils were uncoiled, pickled and cold rolled. The cold rolling reductions were 80% resulting in a final thickness of 0.7mm.

These cold rolled strips were subsequently annealed. Continuous annealing showed that there appears to be a difference in recrystallisation in the rapid annealing cycle of the continuous annealing line because the degree of recrystallisation at 770°C appears to be higher than in case of the reference steel. This is reflected in slightly lower strength values for 770°C for the inventive steel than for the reference steel, although it is risky to draw statistically significant conclusions from these measurements. The chemical analysis in terms of strength increasing elements is comparable, so it is believed that the slightly lower strength at 770°C is the result of a more advanced recrystallisation when compared to the reference steel. The better r-value for the inventive steel is believed to be caused by the reduced amount of large inclusions thereby preventing the particle stimulated nucleation mechanism which is believed to be responsible for a reduction of the (l l l)-texture component in the final recrystallised structure. A reduction of this favourable component causes a reduction of the r-value for the reference steel in comparison to the inventive steel. Table 2 - Mechanical properties determined according to EN-ISO-6892-1 ; 2009 from specimens with a gauge length of 80

The average r-value ((r ₀ + r ₉₀ + 2r ₄₅ )/4) is 1.81 vs 1.90 for T_top of 770°C and 1.96 vs 2.04 for T_top of 820°C. ΔΓ ((r ₀ + r ₉₀ - 2r ₄₅ )/4) is the same for both materials at 0.23 for 770°C and 0.28 for 820°C. These are important parameters for deep-drawing, and these values are very good values.

This perceived acceleration of the recrystallisation in continuous annealing is stronger when the duration of the annealing takes longer, such as in batch annealing. Although the strength of the samples in case of the inventive steels and the reference are similar after batch annealing, because of the similarity of the alloys in terms of strength increasing elements (such as Mn and Si), the increased speed of the recrystallisation due to the absence of AIN in the coil is notable. Batch annealing comprises stacking several coils on a furnace base, covered by a cylindrical cover and furnace which externally heats the cover which heats the contents of the cylindrical cover. Normal batch annealing cycles typically take about 30 hours to heat to the temperature where the cold spot of the coils reaches the desired minimum temperature and another 10 to 30 hours to cool down again. The slight decrease in annealing temperature as observed in the continuous annealing experiments is enlarged by the slow reheating in the batch annealing process and therefore the heating phase of the batch annealing process can be reduced. This also impacts the cooling phase. The result is that the coils are processed faster, and that a more efficient use can be made of the batch annealing furnace at a lower energy costs. Subjecting the cold-rolled strips to BA showed that the reduction in annealing time of the heating phase is in the order of 10% or more which in a typical BA-cycle of amounts to a reduction of 3 hours or more in the heating phase.

In any case, the steel according to the invention is very clean, as can be seen in the micrographs, and there are a lot of very small precipitates visible within the grains (figure 1). Figure 2 shows the continuously annealed cold rolled specimens of the inventive steel and the reference steel for top temperatures of 770 and 820°C. The microstructure at 770°C shows a homogeneous microstructure for the inventive steel, and a less homogeneous microstructure for the reference steel. For the higher annealing temperature there is no difference. This difference is also visible in the mechanical properties (see table 2) where the steel made with the inventive process shows stable mechanical properties whereas those for the reference steel are still affected by the less than 100% complete recrystallisation at 770°C.