Field of the invention

The invention relates to a method for controlling the cooling of a steel strip by a hot strip mill run out table to produce a hot rolled steel strip, wherein the cooling on the run-out table until the coiling of the strip is controlled.

Background of the invention

The standard process in a hot rolling mill can be described as follows. Slight variations in this process are possible.

After the molten steel has been prepared in the steel plant with a certain composition, the steel is cast in a continuous casting machine. At the end of the casting process, the cast strand is cut into slabs.

The slabs are transported to the hot strip mill and first reheated to a temperature of approximately 1150 - 1270 °C in a reheating furnace. At these temperatures the microstructure of the steel slab is austenitic. The slab usually has a thickness of approximately 250 mm.

The reheated slab is then rolled by roughing mills, of which usually five stands with horizontal and sometimes also vertical rolls are present. The thickness of the strip is thereby reduced to approximately 40 mm. After the roughing mills the temperature of the slab is approximately 1050 - 1120 °C.

Afterthe rolling in the roughing mills the slab is transported to the finishing mills. Between the roughing and finishing mills the velocity of the slab is controlled. The velocity, thickness and temperature of the slab are measured before the slab enters the finishing mill stands.

Usually, seven finishing mill stands are present. The velocity at the entry of the finishing mill stands is determined on the basis of the required thickness of the strip after the last finishing mill stand. The thickness of the strip afterthe last finishing mill stand is also calculated on the basis of the measured thickness after the roughing mills.

Directly after the finishing mills the temperature of the strip and the velocity of the strip are measured. The thickness of the strip after the finishing mills is calculated as described above. During the thickness reduction in the finishing mill stands the velocity of the strip entering the finishing mills can be increased as soon as the head of the strip has entered the last finishing mill stand. Thus, the velocity of the strip after the last finishing mill stand usually is not constant during hot strip rolling.

After the strip has left the finishing mills the strip is cooled on a run out table. At the end of the run-out table, the strip is coiled. On entering the run-out table (or ROT) the strip has a temperature of approximately 900 °C or more, so the strip still has an austenitic microstructure.

At coiling the temperature of the strip is usually roughly between 550 and 700 °C. Most steels have a predominantly ferritic microstructure at this temperature. The coiling temperature is realized by the cooling on the ROT (referred to as run out table cooling (ROT-cooling)). The ROT-cooling consists of a number of cooling banks, for instance 60 cooling banks, of which the first 52 form the main cooling and the last 8 are called the trimmers, intended to fine-tune the cooling. The trimmers are used to ensure that the coiling temperature is within the required range. The cooling banks are present both above and below the pass line of the strip. The cooling banks can usually only be open, half-open or closed, but nowadays cooling banks can also be fully variably open. The cooling banks may provide cooling water at high pressure.

To cool the strip on the ROT in a standard hot rolling mill only a limited number of cooling patterns is available, for instance five cooling patterns, in which a number of cooling banks are available to provide cooling water. A cooling pattern is a selection of all the cooling banks of the ROT-cooling that is used to cool the strip. These cooling banks can be fully or partly open. For instance, the cooling banks are alternately available and fully open and not available.

As the result of the cooling pattern that is used, the strip on the ROT will have a temperature profile along its length on the ROT. This temperature profile is called the cooling path of the strip. The cooling path thus starts with the temperature of the strip directly after the hot strip finishing mills, and ends with the coiling temperature of the strip at (or just before) the coiling of the strip. In between these temperatures, the cooling path is seen as a smooth temperature curve, though in practice the impingement of the waterjets of cooling water from the cooling banks results in peaks in the cooling path, particularly when looking at the surface temperature.

A cooling pattern is chosen on the basis of the composition of the strip and the required coiling temperature, consisting of determined cooling banks. Once a specific cooling pattern is chosen, the cooling on the ROT is mainly controlled on the basis of the finishing temperature as measured, the velocity as measured and the thickness as controlled and determined by the finishing rolls. Since especially the velocity will change during hot rolling, the control of the ROT-cooling determines how many cooling banks are actually used from the selected cooling banks made available by the cooling pattern that has been chosen.

The control of the ROT-cooling is not only determined feed-forward by the three input parameters as described above, but also in a feed-back loop by the coiling temperature that is measured at coiling.

Furthermore, if an intermediate temperature is measured, for instance between the main cooling and the trimmers, this intermediate temperature of the strip is used as a feed-forward to calculate and control the coiling temperature, and as a feed-back loop. The intermediate temperature is for instance important when dual-phase steel is hot rolled. In some cases two intermediate temperatures of the strip are measured.

The ROT-cooling is controlled so as to reach a targeted coiling temperature and cooling rate, and - if present - a targeted intermediate temperature.

This standard cooling process works for standard products, for which only the coiling temperature is important. However, for the new high strength steel products that are developed in the last ten to twenty years, not only the coiling temperature of a hot rolled strip is important. Dual phase strip, for instance, poses requirements on other properties as well, for instance the microstructure, and the surface temperature is also important.

To be able to hot roll such products with the required properties, the standard cooling processes do not suffice.

Objects of the invention

It is an object of the invention to provide a method for controlling a hot strip mill run out table line to produce a hot rolled strip, wherein a cooling path of the strip through the run-out table is determined and controlled such that a required material property of the strip is obtained.

It is another object of the invention to provide such a method, wherein the cooling process is controlled using several input parameters.

It is a further object of the invention to provide such a method, wherein more than one required material property of the strip is obtained.

It is moreover an object of the invention to provide such a method, wherein not only a required material property of the strip at coiling is controlled, but also at one or more other locations in the run-out table.

It is still another object of the invention to provide such a method, wherein certain boundary conditions are observed.

Description of the invention

According to the invention one or more of these objects are reached by providing a method for controlling the cooling of a steel strip on a hot strip mill run out table comprising a plurality of cooling banks to produce a hot rolled steel strip, wherein the cooling on the run-out table until the coiling of the strip is controlled using the following input parameters: 1. surface temperature T1 of the hot rolled strip directly after hot strip finishing mill stands.

2. strip thickness d1 of the hot rolled strip directly after hot strip finishing mill stands

3. velocity v1 of the hot rolled strip directly after hot strip finishing mill stands

4. steel chemistry of the strip, wherein an austenite grain size G3 of the steel strip directly after the finishing mills is used as an additional input parameter, wherein a cooling pattern of selected cooling banks of a cooling installation in the runout table and a cooling flow of each selected cooling bank is determined on the basis of the input parameters 1 - 4 and the austenite grain size G3 of the steel strip directly after the finishing mills, and on the basis of a targeted aspect of microstructure M4 at the coiling of the strip, and wherein the number of selected cooling banks that is actually used to cool the strip is controlled on the basis of the input parameters 1 - 4 during cooling of the strip, and on the basis of the grain size G3 of the steel strip directly after the finishing mills, and on the basis of that aspect of microstructure M4 as measured at the coiling of the strip.

Preferably the grain size G3 is the average austenite grainsize of the steel as it is present directly after leaving the last finishing mill. Within the context of this invention it is noted that the term “directly after the hot strip finishing mill” intends to mean “as soon as possible after the strip leaves the finishing mill” and in any case before the first cooling bank of the ROT- cooling. For a normal hot rolling practice, where all stands of the hot strip mill are used, the term “directly after the hot strip finishing mill” means within 2 seconds after leaving the last finishing stand. The thinner d1 , the higher the velocity v1 , and therefore also the shorter the time between leaving the hot strip finishing mill. For a hot rolling practice, where not all stands of the hot strip mill are used, and where the last stand or last stands are not used, the time increases and may be more than 2 seconds, but still it means “as soon as possible after the strip leaves the finishing mill” and in any case before the first cooling bank of the ROT-cooling.

The invention thus provides a method that makes it possible to provide any cooling pattern on the ROT between the situation wherein no cooling by the cooling banks is used, and the situation wherein all the cooling banks are switched on at their maximum water flow capacity. In between these two limits, any cooling pattern of selected cooling banks and cooling flow of the selected individual cooling banks can be provided to determine a cooling path so as to reach the targeted aspect of the microstructure as accurate as possible, preferably real time.

The cooling pattern is determined on the basis of a number of input parameters. These are the usual input parameters 1 - 4 as indicated above, but as an additional input parameter the grain size G3 of the hot-rolled strip directly after the finishing mills is used. This grain size is often determined in a hot strip mill, but the method according to the invention uses the grain size G3 also as an input parameter for determining the cooling pattern.

Furthermore, the cooling pattern is determined on the basis of a targeted aspect of microstructure M4 at the coiling of the strip. This makes the method according to the invention suitable for the high strength steel strips that have been developed in the past years, which high strength steels need to have a specified microstructure, for instance having a certain percentage of pearlite and a certain percentage of ferrite.

On the basis of these input parameters a cooling pattern is determined that will enable reaching the targeted aspect of microstructure M4.

Furthermore, during the cooling of the strip on the ROT the cooling of the strip is controlled on the basis of a number of input parameters. This is normal practice in many hot rolling mills, but according to the method of the invention also the grain size G3 and a measured aspect of the microstructure M4 of the steel strip at the coiling of the strip are used as input parameters to control the cooling. Controlling the cooling means that the number of selected cooling banks in the cooling pattern is changed, thus switching on or off one or more cooling banks, and/or changing the cooling flow of water of the selected cooling banks in the cooling pattern.

In the above the term microstructure of the steel strip encompassed the following aspects:

• the grain size of the austenite and/or ferrite in the strip,

• the phases that are present in the strip, such as the amount of austenite, ferrite, pearlite, bainite and/or martensite in the strip

• the percentage of recrystallisation of the strip

• the amount of precipitates in the strip

• the type of precipitates in the strip.

According to the method of the invention, at least one aspect of the microstructure is a target to be reached at the coiling of the strip, and that measured aspect of the microstructure at the coiling of the strip is used to control the selected cooling banks. At present it is not possible to measure all these aspects during hot rolling on-line, but these aspects of the microstructure can be calculated off-line on the basis of the strip input parameters and the measured process parameters and measured aspects.

According to a preferred embodiment of the method according to the invention the cooling pattern of selected cooling banks is also determined on the basis of a targeted coiling temperature Tct of the strip, wherein the number of selected cooling banks is also controlled on the basis of a measured surface temperature Tc of the strip at coiling. In many (if not all) hot rolling mills that are in use nowadays a targeted coiling temperature Tct is used as input parameter, and the surface temperature Tc of the strip at coiling is measured, but this is not done in combination with the grain size directly after the finishing mills and/or one or more aspects of the microstructure at coiling of the strip.

Preferably the strip surface temperature T1 and velocity v1 are measured directly after the hot strip mill finishing stands. These measurements make the process according to the invention more reliable than the use of a predicted temperature T1 and velocity v1 based on a predictive model that uses the temperatures before or during the use of the finishing mills.

According to a preferred embodiment of the method of the invention, the strip chemistry is determined by analysis of a sample taken from the molten steel that is used to cast a strand from which a slab is cut as input to produce the steel strip in the hot strip mill. The input parameter number 4 of all the input parameters thus is preferably not determined at any point in the hot rolling mill, and also is preferably not a chemistry that is a targeted chemistry for the steel strip, but is measured during the steel making process that precedes the hot rolling of the steel.

In a preferred embodiment an aspect of the microstructure M4 at the coiling of the strip is the percentage of the austenite phase that is transformed into another phase, such as pearlite and/or ferrite, wherein preferably either the amount of transformed austenite is measured or the amount of transformed austenite is calculated based on at least three measured surface temperatures of the strip along the length of the run-out table.

It is possible to determine the percentage of transformed austenite in the strip at the coiling of the strip; measuring devices to do so are commercially available.

It is also possible to calculate the amount of transformed austenite in the strip based on the surface temperature of the strip that is measured at least at three spots along the length of the ROT. On the basis of the measured or calculated amount of transformed austenite, the cooling pattern can be determined and the selected cooling banks can be controlled.

According to a preferred embodiment as a further input parameter the percentage of recrystallized austenite in the strip directly after the hot strip mill finishing stands and/or the amount of precipitates in the strip directly after the hot strip mill finishing stands can be used, and preferably also the type of precipitates in the strip. These input parameters can be determined on the basis of the percentage of recrystallization and the amount of precipitates and the type of precipitates determined in the stages of hot rolling before the ROT and on the basis of the chemistry of the steel strip. These parameters are determined in many hot rolling mills and the person skilled in the art knows how to determine these.

According to a preferred embodiment the following measured parameters: o strip temperature TO before hot strip finishing mill stands o strip thickness dO before hot strip finishing mill stands o strip velocity vO before hot strip finishing mill stands and a calculated grain size G2 before hot strip finishing mill stands are used to determine the thickness d1 and the grain size G3 of the steel strip directly after the finishing mills. If it is not possible to measure the grain size G3 and the thickness d1 directly after the finishing mills, the measured parameters TO, dO and vO and the calculated grain size G2 are used to calculate d1 and G3. This calculation is normal practice in many hot rolling mills today.

Preferably the grain size G2 is calculated on the basis of measured grain sizes of slabs with a corresponding composition, and on the basis of the time/temperature path in the reheating furnace of the slab that is processed into the steel strip in the roughing mills, and on the basis of the the time/temperature path and thickness reduction of the slab in the roughing mills. This means that the grain size G2 is calculated with the available input to do so. This calculation is normal practice in many hot rolling mills today.

According to a preferred embodiment, in addition to the measured aspect of the microstructure M4 at the coiling of the strip and optionally the measured coiling temperature Tc at the coiling of the strip, one or more of the parameters strip temperature T, strip velocity v and one or more aspects of strip microstructure M are calculated and/or measured at one or more places along the length of the run out table, and used to control the number of selected cooling banks and/or the cooling flow of each selected cooling bank that is used to cool the strip. These additional input parameters for controlling the number of cooling banks and/or cooling flow of these cooling banks make it possible to further improve control of the cooling of the strip over the length of the ROT, resulting in a microstructure M4 at the coiling of the strip that is closer to the targeted aspect of M4, and optionally a temperature Tc of the strip at coiling that is closer to the targeted Tct as well.

Preferably the parameters strip temperature T and/or one or more aspects of strip microstructure M are calculated and/or measured at one or more places over the width of the strip, at one or more places along the length of the run-out table line. This means that for instance not only the temperature and/or microstructure are calculated and/or measured at the centre line of the strip, as is the usual practice, but also for instance near the edge of the strip.

According to a preferred embodiment one or more of the following targeted parameters are used as additional input parameter for determining the cooling pattern of the selected cooling banks of the cooling installation in the run-out table and/or the cooling flow of each selected cooling bank:

• Average temperature of the strip Ta over the thickness of the strip at one or more spots along the run-out table

• Surface temperature of the strip Ts at one or more spots along the run-out table

• Cooling rate Vc over the length of the run-out table.

These input parameters can influence the cooling pattern and/or the cooling flow that is chosen, for instance when the cooling rate should be comparatively low, the number of selected cooling banks will be higher and the cooling flow will be lower.

It is furthermore preferred when the cooling pattern of selected cooling banks in the runout table and/or the cooling flow of each selected cooling bank is controlled such that the cooling of the strip meets one or more of the following requirements:

• A predetermined temperature gradient in the strip

• A predetermined surface temperature of the strip at one or both sides of the strip

• A predetermined homogeneous cooling of the strip at high cooling rate

These requirements are relevant for the quality of the steel strip having undergone the cooling on the ROT. For instance, too high a temperature gradient in the strip can result in cracks in the strip or coarsening of the surface.

Preferably the input parameter grain size G3 is determined real time so as to control the selected cooling banks real time. Since the grain size G3 that is used as input is based on a model to predict the grain size G3 directly after the finishing mills, it is advantageous to predict the grain size G3 real time so as to determine the cooling pattern and control the selected cooling banks as accurately as possible.

It is preferable to be able to adjust the cooling flow of the cooling banks, it is advantageous when one or more cooling banks in the run-out table have two or more different cooling capacity settings, and/or wherein one or more cooling banks in the run-out table have continuously adjustable cooling capacity settings. With such cooling banks, the cooling of the steel strip on the ROT can be performed as accurately as possible. It is preferable that all cooling banks have two or more different cooling capacity settings or that all cooling banks have continuously adjustable cooling capacity settings.

According to a preferred embodiment two or more targeted or measured parameters are used as input parameters with a weighing factor when these parameters are used to determine the selected cooling banks of the cooling pattern and/or the cooling flow of the selected cooling banks, and/or to control the number of selected cooling banks that is used to cool the strip. Usually it is not possible to reach two targeted parameters, for instance an aspect of microstructure M4 and coiling temperature Tc both exactly at coiling of the strip. When determining the cooling pattern and/or cooling flow, and when controlling the selected cooling banks, it is therefore advantageous when it is determined beforehand which parameter to be reached should be more relevant in comparison to another parameter to be reached.

For instance, it can be preferred that at least the targeted input properties coiling temperature Tct and an aspect of microstructure M4 are used with a weighing factor, for instance such that the microstructure M4 is more important than the coiling temperature Tc.

Drawings

The invention is elucidated with a number of examples showing the result of the use of the method according to the invention.

Fig. 1 shows the cooling pattern of the ROT-cooling for Example 1.

Fig. 2 shows the selected cooling banks of the ROT-cooling used for Example 1.

Fig. 3 shows the calculated temperatures for Example 1 .

Fig. 4 shows the resulting transformation for Example 1 .

Fig. 5 shows the cooling pattern of the ROT-cooling for Example 2.

Fig. 6 shows the selected cooling banks of the ROT-cooling used for Example 2.

Fig. 7 shows the calculated temperatures for Example 2.

Fig. 8 shows the resulting transformation for Example 2.

Fig. 9 shows the cooling pattern of the ROT-cooling for Example 3.

Fig. 10 shows the selected cooling banks of the ROT-cooling used for Example 3.

Fig. 11 shows the calculated temperatures for Example 3.

Fig. 12 shows the resulting transformation for Example 3.

Furthermore, the method according to the invention will be elucidated with reference to a number of schematic drawings.

Fig. 13 schematically shows the cooling banks of the ROT-cooling and the range of cooling paths that is possible with these cooling banks.

Fig. 14 shows the production line to produce a coiled strip with the different phases in the production process and the settings and parameters used to calculate the input parameters for the method according to the invention.

Fig. 15 shows the process to determine the cooling pattern of cooling banks and the flow of these cooling banks, and the control of the cooling pattern and flow of the cooling banks. Fig. 13 schematically shows at the top a number of cooling banks that is available in a run out table cooling. The graph below the cooling banks shows that cooling in the ROT- cooling can be performed between an upper limit where none of the cooling banks is used (Cmin), and a lower limit where all cooling banks are used with maximal flow (Cmax). These limits result in a maximum coiling temperature (Tc max) and a minimum coiling temperature (Tc min) that can be reached for a certain strip.

In between the upper limit and lower limit any cooling path can be chosen that is physically possible (heating of the strip is only possible when transformation energy of the strip is used). Fig. 13 shows an example wherein a cooling pattern (PT) is chosen as indicated by the black boxes representing cooling banks that are used to cool. As a result of the use of this cooling pattern the Cooling Path (CP) as indicated in the graph results. Since the method according to the invention makes it possible to use a cooling pattern that can choose any of the cooling banks in the ROT-cooling, any cooling path that is physically possible between the upper and lower limit can be obtained.

This is important because for high strength steels not only the coiling temperature Tc usually has to be within certain limits, but especially the microstructure at coiling of the strip has to fulfil predetermined criteria. For instance, for a dual phase steel the microstructure at coiling should contain predetermined amounts of pearlite and ferrite. Furthermore, often the grain size is important.

Thus, not only the temperature Tc at coiling of the strip is usually important. If only the Tc would be important, the cooling path to reach that Tc would not be relevant and many cooling patterns could be used to reach that Tc. However, when the microstructure of the strip at coiling is relevant, also the cooling path is important because the cooling path determines the start of the phase transformation in the strip, the evolution of the phase transformation and the resulting phases. The method according to the invention makes it possible to use the cooling path that is required to achieve the microstructure of the strip at coiling.

To determine the required cooling path, a number of input parameters has to be provided. Fig. 14 shows how and where these input parameters can be measured or calculated based on earlier input during the production process.

In Fig. 14 the following abbreviations are used:

S: Mill I Installation settings

Me: Measurements (M, v, d, T)

P0..P4: Position O to 4

SL: Slab

RH: Reheating Furnace RHM: Reheating Furnace Model

MM: Micro-structure Model

RM: Roughing Mill

RMM: Roughing Mill Model

T(t): Temperature as function of time

£(t): Strain as function of time

M0..M4: Micro-structure or one of its aspects

FM: Finishing Mill

FMM: Finishing Mill Model

ROT: Run Out Table cooling

ROTM: Run Out Table Model

OR: Coiler.

Fig. 14 shows that the production process of a hot rolled strip starts with the providing of a slab SL. This slab is re-heated in a reheating furnace RH to provide it with the required temperature, after which it is transported to the roughing mill RM. After the first reduction of its thickness in the roughing mill the slab is transported to the finishing mill FM, where the thickness is reduced such that it has the required thickness of that specific hot rolled strip. Usually, a hot rolled strip has a thickness between 2 and 10 mm, but it can be thinner or thicker. After the finishing mill the strip is cooled on the run-out table ROT and thereafter coiled on a coiler CR. These installations for producing the hot rolled strip are well known to the person skilled in the art.

Fig. 14 also shows that in a hot strip mill nowadays a number of models is used. Each of the above-mentioned installations in the production process uses its own model to calculate the microstructure of the slab or strip resulting from the use of that installation. To calculate the microstructure, measurements in and after each installation are used, and the model also generates settings for the installations.

As shown in Fig. 14, for each installation there is a model using the measurements as input and generating the settings as output. There is also a separate model to calculate the microstructure. In this way each subsequent installation uses the resulting microstructure M generated by the model for the preceding installation. Only the first input microstructure MO is not calculated based in the slab that is hot rolled, but is based on a general model made on the basis of earlier produced slabs, having the same or almost the same composition. Hence the dotted line in Fig. 14.

Fig. 14 shows five positions P0 to P4. Each position has its own microstructure MO to M4 , before or after one of the installations in the hot strip mill. Nowadays most hot strip mills use the models as shown in Fig. 14 and whereas the models itself may vary, the resulting microstructure can be calculated by the person skilled in the art.

Fig. 15 shows how the input parameters M, d, v and T are used to generate a cooling pattern together with the targeted material property or properties, and how these parameters are also used to control the cooling banks of the cooling pattern.

In Fig. 15 the following abbreviations are used:

H: strip Head

M: micro-structure d: thickness v: strip speed

T: temperature

TF7: temperature measured after FM

FM: Finishing Mill

CT: coiling temperature measurement

A: Delta.

Fig. 15 shows that a reiterating loop is used to calculate a cooling pattern, and that this cooling pattern is used to calculate what the resulting material properties will be. By comparing the calculated material properties with the required material properties, the cooling pattern can be optimised.

A reiterating loop is also used to control the use of the selected cooling pattern, by measuring or calculating (based on other measurements along the ROT) targeted material properties just before the coiling of the strip and comparing these with the required material properties. In this way the number of active cooling banks in the selected cooling pattern and the flow thereof can be optimised.

Fig. 15 shows that for both reiterating loops the same input parameters temperature T3, strip thickness d3, velocity v3 and microstructure M3 are used, as well as the targeted microstructure M4. According to the invention the grain size is the aspect of the microstructure used as input parameter; as targeted microstructure M4 different aspects of the microstructure can be used, as indicated above.

Apart from these targets also other targets can be used such as the coiling temperature Tc or the surface quality on the ROT, as indicated above.

With the above elucidation of the invention the below description of examples will be readily understood.

The examples described below all relate to the cooling of a High Carbon steel grade. High Carbon steel grades are characterised by a carbon content above 0.5 wt.%. The High Carbon steel grade used in the below Examples has the following composition (in wt.%):

C 0.7

Mn 0.7

Si 0.2

Al 0.006

Cr 0.2.

Nb, Ti, S, B, Cu, Ni, Mo, V and N are present as an impurity, that means present at a maximum of 0.01 wt.% each. Other possible elements are unavoidable impurities, the remainder being iron. This is a commercially available grade.

Examples

Example 1 : In Figure 1 the lay-out of a ROT-cooling is shown as used for the present examples. The ROT-cooling is in general indicated with the number 10, and the strip 1 that is coiled into a coil 2 is shown, departing from the last stand 5 of the finishing mill as shown in Figure B. The cooling banks 1 to 54 of the main cooling are each indicated, and the trimmers 55 to 62 as well, according to the ROT-cooling of the present examples. The lay-out of the ROT-cooling as shown in Figure 1 is used for all examples.

The inputs for the use of the invention are a Finishing Mill Temperature T1 of 880° C, a strip thickness d3 after the Finishing Mill of 2.5 mm, a strip speed v3 of 13.0 m/s and a grain size G3 of 11 .5 pm. These input variables are shown in Figure C.

In this example 1 the only targeted material property is a targeted pearlite fraction of 100%. The targeted material property that 100% pearlite is reached is chosen so as to get mechanical properties and homogeneity of the strip that is as good as possible, since all austenite will then have transformed into pearlite before coiling.

On the basis of the strip temperature T2, the strip thickness d2, the strip velocity v2 and the grain size G2 as measured or calculated at Position P2 between the roughing mills and the finishing mills of the hot mill, see Figure B, and on the basis of the targeted material property, predicted values for the strip temperature T3, strip thickness d3, grain size G3 and velocity v3 are calculated for the strip when the strip enters the ROT at Position P3, see Figure B. Based on these calculated input variables and the targeted material property, the ROT- cooling control system determines a cooling pattern for the cooling banks in the ROT-cooling which is used as starting point for the cooling process. This cooling pattern is calculated as soon as the input parameters temperature T3, strip thickness d, grain size G3 and optionally strip velocity v are measured or calculated when the head of the strip passes the spot where the temperature T3 is measured.

In accordance with the predicted input parameters and the targeted material property for this example 1 , the cooling pattern as shown in Figure 1 is calculated. This cooling pattern is used for the cooling of this strip, but the cooling banks that are actually used are determined on the basis of the measurements/calculations. Figure 1 shows that the calculated cooling pattern makes no use of the trimmers 55 - 62. Figure 1 shows the measurement of the finishing temperature T1 , the measurement of the temperature before the trimming section (Tbt) and the measurement of the coiling temperature (Tct).

When the strip 1 enters the ROT-cooling 10, the temperature T3, the velocity v3 and the thickness d3 are measured, and the grain size G3 is calculated. Based on these actual values, the cooling path is calculated, which means that it is calculated which cooling banks have to be actually used from the cooling pattern that has been calculated. In this example all cooling banks can be used from cooling bank 1 to cooling bank 9; none of the trimmers will be used. Moreover, for the cooling banks 1 - 9 a flow rate of 100% is determined. This is shown in Figure 1 with the hatched blocks. Figure 2 shows the actual cooling path that is calculated for cooling the strip using the method according to the invention, based on the above input values T3, d3, G3 and v3 for this example 1. The cooling banks 1 - 8 are used at 100% flow rate, shown as the black blocks, and as indicated above none of the trimmers. Less or more cooling banks can be used depending on the velocity of the strip, in accordance with the cooling banks determined in the cooling pattern.

It should be understood that according to the invention the input parameter T2, d2, v2 and G2 are not necessary. The method according to the invention can be used with only the input parameters T3, d3, G3 and optionally v3, but then a powerful computer is needed to directly calculate the actual cooling path of the cooling banks when the head of the strip passes the spot where T1 is measured, since otherwise a first part of the strip will not be accurately cooled. Thus, it is preferred to use T2, d2, v2 and G2 to predict T3, d3, G3 and optionally v3 so as to determine a cooling pattern before the strip enters the ROT-cooling.

For the actual cooling path shown in Figure 2 the calculated temperatures of the strip are shown in Figure 3 as function of the location along the run-out table. Shown is the average temperature of the strip, halfway the upper and the lower surface of the strip. With the method according to the invention the temperature at the upper and the lower surface of the strip can also be calculated. The dotted lines at 645° C and 665° C show the temperature limits between which the Coiling Temperature Tc should preferably be kept. However, the Coiling Temperature Tc is not a targeted material property in this example 1. Figure 3 shows the calculated cooling temperatures. It is clear that a very fast cooling of the strip is obtained directly from the start of the cooling on the ROT. The strip is even cooled to a temperature of about 50° C below the preferred Coiling Temperature Tc, but the strip is heated again in its course over the ROT due to the transformation energy that is released as a result of the transformation from austenite into pearlite (and ferrite, if present).

This is shown in Figure 4. All or almost all austenite is transformed into perlite, about 95%, and the remainder is transformed into ferrite. Thus, the austenitic strip is completely or almost completely transformed before the strip is coiled, resulting in mechanical properties of the strip that are as homogeneous as possible.

It is interesting to see that, even though it was not a targeted material property, the realised Coiling Temperature Tc is within the desired limits of 655° ± 10° C, as shown in Figure 3.

Example 2: In example 2 there are two targeted material properties. One is the Targeted Coiling Temperature TCT, which is 655°. The other targeted material property is a perlite fraction target of 100% just before the coiling of the strip, as in example 1. All other input variables are the same as in example 1 , thus also the grain size of 11.5 pm. Both targeted material properties are given equal weight, so the ROT-cooling control system should try to reach both material properties at the same time.

Figure 5 shows the cooling pattern of the ROT-cooling. This cooling pattern is determined in the same way as in example 1 . It shows that of the banks 1 to 31 only the first out of three can be used, and that all the trimmers 55 - 62 can be used. Banks and trimmers will be used at 50% flow rate. This is shown as the hatched blocks in Figure 5.

Figure 6 shows the actual cooling path that is calculated for cooling the strip using the method according to the invention, based on the above input values for this example 2. Only the cooling banks 1 - 22 are used, and none of the trimmers.

This cooling path results in the calculated cooling temperatures as shown in Figure 7. In comparison to Example 1 , it can be seen that in this example 2 the strip is cooled slower, and also to a temperature below the Targeted Coiling Temperature, as in example 1. Due to the transformation heat, the Coiling Temperature Tc is well inside the temperature limits of 655° C ± 10° C.

Figure 8 shows that the transformation of the strip on the ROT is almost as high as in Example 1. More than 85% of the austenite is transformed into pearlite, and some 5% ferrite is formed just before the strip is coiled. The remainder is maintained as austenite just before the coiling of the strip.

It will be clear that in this case, wherein also a targeted perlite fraction of 100% is used as an input for the cooling method according to the present invention, about 90% transformation from austenite takes place. This means that only a partial, relatively minor transformation from austenite has to take place during or after coiling of the strip. This results in mechanical properties of the strip that are almost as good as the mechanical properties are when the cooling method according to Example 1 is followed.

Example 3: This last example shows how the method according to the invention works out when more than 2 targeted material properties are chosen. In this case, 4 targeted material properties are used:

• the Targeted Coiling Temperature T _C T of 655° C,

• a fast cooling rate target (dT/dt) to start with; this fast cooling rate should be as fast as possible,

• fast cooling until the average temperature of the strip is 800° C,

• a slow cooling rate (dT/dt) from the point that the average temperature of the strip is 800° C until Tc, of 100° C/s.

All four targeted material properties are given equal weight.

This results, using the input parameters T2, d2, v2, G2 and the above targeted material properties, in a cooling pattern of the ROT-cooling as shown in Figure 9, wherein all cooling banks 1 - 11 will be used, but wherein the cooling banks 1 and 2 will use a 100% water flow and the cooling banks 3 - 11 will use a 50% water flow. The trimmers will not be used.

Figure 10 shows the actual cooling path that is calculated for cooling the strip using the method according to the invention, based on the above input values for this example 3. The cooling banks 1 and 2 are used at 100% flow rate, and the cooling banks 3 - 10 are used at 50% flow rate. This is shown as the black blocks. As indicated above none of the trimmers is used.

Figure 11 shows the calculated cooling temperatures. It is clear that the use of 4 targeted material properties in the method according to the invention results in cooling that looks very much like the cooling in example 1 , where only the targeted transformation was used as target input. However, the calculated graph shows that the Coiling Temperature Tc is higher in this example 3. Moreover, the cooling is clearly not as fast after the temperature of the strip is below 800° C.

Though the transformation of the strip is not a targeted material property, the calculated transformation as shown in Figure 12 is almost as good as the transformation as in example 1.

The above examples 1 to 3 show that with the method according to the invention, several targeted material properties can be reached with the same steel grade.