KR20230118901A - How to control cooling of hot rolling mill runout table - Google Patents

Abstract

The present invention relates to a method for controlling a hot rolling mill run-out table to produce hot-rolled strip using a cooling process, wherein the cooling process at the run-out table is measured or model-based immediately after the hot strip finishing mill stands. Controlled using a number of input parameters, the cooling pattern of a cooling installation comprising a number of cooling banks in the runout table is determined.

Description

How to control cooling of hot rolling mill runout table

The present invention relates to a method for controlling the cooling of a steel strip by means of a hot rolling mill run out table to produce hot rolled steel strip, wherein the cooling in the run out table until coiling of the strip this is controlled

The standard process of hot rolling mill can be described as below. There may be some variations in this process.

After molten steel with a specific composition is prepared in a steel plant, 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 a hot rolling mill and first reheated in a reheat furnace to a temperature of approximately 1150 - 1270 °C. At this temperature, the microstructure of the steel slab is austenitic. The slabs typically have a thickness of approximately 250 mm.

The reheated slab is then rolled by a roughing mill. A roughing mill usually includes five stands, including horizontal rolls and sometimes vertical rolls as well. This reduces the thickness of the strip to approximately 40 mm. After passing through the roughing mill, the temperature of the slab is approximately 1,050 - 1,120 ° C.

After rolling in the roughing mill, the slabs are transported to a finishing mill. Between the roughing mill and the finishing mill, the speed of the slab is controlled. The speed, thickness and temperature of the slabs are measured before the slabs enter the finishing mill stand.

Typically, there are seven finish rolling stands. The entry speed into the finishing mill stands is determined based on the thickness of the strip required after the finishing mill stand. The thickness of the strip after the final finishing mill stand is also calculated based on the measured thickness after the roughing mill.

Immediately after the finishing mills, the temperature of the strip and the speed of the strip are measured. The strip thickness after finishing mills is calculated as described above. While reducing the thickness at the finishing mill stands, the speed of the strip entering the finishing mills can be increased as soon as the tip of the strip enters the finishing mill stand. Therefore, the speed of the strip after the finishing mill stand is usually not constant during hot strip rolling.

After the strip leaves the finishing mills, the strip is cooled on a run-out table. At the end of the runout table the strip is wound up. Upon entering the run-out table (ROT), the strip has a temperature above approximately 900° C., so the strip still has an austenitic microstructure.

When coiling, the temperature of the strip is typically between approximately 550°C and 700°C. Most steels have a predominantly ferritic microstructure at this temperature. The winding temperature is achieved by cooling at the ROT (referred to as runout table cooling (ROT-cooling)). ROT-cooling consists of multiple (eg 60) cooling banks. Of these cooling banks, the front 52 form the main cooling and the back eight are for fine-tuning the cooling and are called trimmers. A trimmer is used to ensure that the coiling temperature is within the required range. Cooling banks are disposed both above and below the pass line of the strip. Cooling banks can normally only be open, semi-open or closed, but nowadays they can also be fully open. The cooling banks may supply high pressure cooling water.

Only a limited number (eg 5) cooling patterns are available for cooling the strip at the ROT of a standard hot rolling mill, and in each pattern a number of cooling banks are available to supply cooling water. One cooling pattern is the selection of all cooling banks of ROT-cooling used to cool the strip. These cooling banks can be fully or partially open. For example, the cooling banks may be alternately available and completely open or unavailable.

As a result of the cooling pattern used, the strip in the ROT will have a temperature profile along its length in the ROT. This temperature profile is called the cooling path of the strip. This cooling path starts at the temperature of the strip immediately after the hot finishing mills and ends at the winding temperature of the strip at (or immediately before) winding the strip. The cooling path appears to be a smooth temperature curve, but in reality the impingement of water jets of cooling water from the cooling banks creates peaks in the cooling path, especially when looking at the surface temperature.

The cooling pattern is selected based on the composition of the strip and the required winding temperature and consists of the determined cooling banks. Once a particular cooling pattern is selected, cooling at the ROT is controlled primarily by the measured finishing temperature, measured speed, and finishing rolls and based on the determined thickness. In particular, since the speed will vary during hot rolling, the control of ROT-cooling determines how many of the selected cooling banks made available by the selected cooling pattern will actually be used.

Control of ROT-cooling is determined not only by feed-forward by the three input parameters described above, but also by a feed-back loop by the winding temperature measured during winding. It is decided.

In addition, if an intermediate temperature is measured between main cooling and the trimmers, for example, this intermediate temperature of the strip is used as a feed-forward and as a feed-back loop for calculating and controlling the winding temperature. The intermediate temperature is important, for example, when two-phase steel is hot rolled. In some cases, the two intermediate temperatures of the strip are measured.

The ROT-cooling is controlled to reach a target winding temperature and cooling rate and, if present, a target intermediate temperature.

This standard cooling process is valid for standard products where only the coiling temperature is important. However, in the new high-strength steel products developed in the last 10 to 20 years, not only the coiling temperature of the hot-rolled strip is important. For example, two-phase strips have requirements for other properties as well, such as microstructure, and surface temperature is also important.

To be able to hot-roll products of the required properties, standard cooling processes are not sufficient.

It is an object of the present invention to provide a method of controlling a hot rolling mill runout table line to produce hot rolled strip, wherein the cooling path of the strip through the runout table is determined and controlled so that the required material properties of the strip are obtained.

Another object of the present invention is to provide a method in which the cooling process is controlled using several input parameters.

Another object of the present invention is to provide a method by which two or more required material properties of a strip are obtained.

It is also an object of the present invention to provide a method in which the required material properties of the strip are controlled not only during winding, but also at one or more other positions of the run-out table.

Another object of the present invention is to provide a method by which certain boundary conditions are observed.

According to the present invention, one or more of the above objects are achieved by providing a method of controlling steel strip cooling in a hot rolling mill runout table comprising a plurality of cooling banks to produce a hot rolled steel strip, wherein said strip Cooling in the runout table until winding of is controlled using the following input parameters:

1. The surface temperature of the hot-rolled strip immediately after the stand of the hot-strip finishing mill (T1);

2. The thickness of the hot-rolled strip (d1) immediately after the stand of the hot-strip finishing mill;

3. The speed (v1) of the hot-rolled strip immediately after the stand of the hot-strip finishing mill;

4. Strong chemical properties of the strip.

Here, the austenite grain size (G3) of the steel strip immediately after the finishing mill is used as an additional input parameter,

Here, the cooling pattern of the selected cooling banks of the cooling equipment of the run-out table and the cooling flow rate of each selected cooling bank depend on the input parameters 1-4 and the austenite grain size (G3) of the steel strip immediately after the finishing mill and the microstructure during strip winding. determined based on the target aspect of (M4);

Here, the number of selected cooling banks actually used to cool the strip depends on the input parameters 1-4 during cooling of the strip, the grain size of the steel strip immediately after the finishing mill (G3), and the measured microstructure during strip winding (M4). It is controlled based on the aspect of.

Preferably, the grain size G3 is the average austenite grain size of the as-is steel immediately after leaving the final finishing mill. It should be noted that within the context of the present invention, the expression "immediately after the hot strip finishing mill" is intended to mean "as soon as the strip leaves the finishing mill" and, in any event, prior to the first cold bank of ROT-cooling. In normal hot rolling practice, where all stands of the hot strip mill are used, the expression "immediately after the hot strip finishing mill" means within 2 seconds after leaving the finishing stand. The thinner the thickness d1, the higher the speed v1, and therefore the shorter the time after exiting the hot strip finishing mill. In hot rolling practice where not all the stands of the hot strip mill are used and the final stand or the final stands are not used, the time may increase and exceed 2 seconds, but the expression is still "as soon as the strip leaves the finishing mill". , which means, anyway, before the first cooling bank of ROT-cooling.

Thus, the present invention provides a method that makes it possible to provide the ROT with any cooling pattern between a situation where cooling by the cooling banks is not being used and a situation where all cooling banks are operating at maximum water flow rate. Between these two limits, an arbitrary cooling pattern of selected cooling banks and a cooling flow rate of the selected individual cooling banks can be provided, thereby reaching the target aspect of the microstructure as accurately as possible, preferably in real time. Determine the cooling path.

The cooling pattern is determined based on a number of input parameters. These are the usual input parameters 1-4 as mentioned above, but as an additional input parameter, the grain size of the hot-rolled strip immediately after the finishing mills (G3) is used. This grain size is often determined in a hot strip mill, but the method according to the present invention uses the grain size G3 as an input parameter to determine the cooling pattern.

Further, the cooling pattern is determined based on the target aspect of the microstructure M4 when winding the strip. This makes the method according to the invention suitable for high-strength steel strips that have been developed in recent years, which must have a specific microstructure, for example with a specific proportion of pearlite and ferrite.

Based on these input parameters, a cooling pattern is determined that allows reaching the target aspect of the microstructure M4.

Additionally, during strip cooling at the ROT, strip cooling is controlled based on a number of input parameters. This is common practice in many hot rolling mills, but according to the method of the present invention, the measured aspects of the grain size (G3) and microstructure (M4) of the steel strip are also used as input parameters to control the cooling during strip winding. Controlling cooling means that the number of selected cooling banks in the cooling pattern is changed, thus starting or stopping operation of one or more cooling banks, and/or changing the cooling flow rate of water in selected cooling banks in the cooling pattern.

Above, the term microstructure of a steel strip includes the following aspects:

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

- phases present in the strip, such as the amount of austenite, ferrite, pearlite, bainite and/or martensite in the strip;

- the rate of recrystallization of the strip,

- the amount of precipitate in the strip,

- the type of precipitate in the strip.

According to the method of the present invention, one or more aspects of the microstructure are a target to be reached upon winding the strip, and such measured aspects of the microstructure upon winding the strip are used to control the selected cooling banks. While it is currently not possible to measure all aspects on-line during hot rolling, this aspect of the microstructure can be calculated off-line based on strip input parameters and measured process parameters and measured aspects.

According to a preferred embodiment of the method according to the invention, the cooling pattern of the selected cooling banks is also determined on the basis of the target winding temperature Tct of the strip, wherein the number of selected cooling banks also depends on the measured surface temperature of the strip during winding. (Tc). In many (if not all) hot rolling mills currently in use, a target coiling temperature (Tct) is used as an input parameter, and the surface temperature (Tc) of the strip is measured during coiling, but this is /or strip winding is not performed in conjunction with one or more aspects of the microstructure.

Preferably, the strip surface temperature (T1) and speed (v1) are measured immediately after the hot strip mill finishing stand. These measurements make the process according to the present invention more reliable than the use of the predicted temperature (T1) and speed (v1) based on a predictive model that uses the temperature before or during the use of the finishing mill.

According to one preferred embodiment of the method of the present invention, the chemistry of the strip is sampled from the molten steel used to cast the strand from which the slab is cut as input to the production of the steel strip in the hot strip mill. is determined by analyzing Therefore, among all input parameters, input parameter number 4 is preferably not determined at any point in the hot rolling mill, and is also preferably not a chemical property that is a target chemical property for the steel strip, but prior to hot rolling of the steel. It is measured during the steelmaking process.

In a preferred embodiment, the aspect of the microstructure (M4) upon winding of the strip is the proportion of the austenite phase converted to other phases such as pearlite and/or ferrite, wherein preferably the amount of converted austenite is measured or converted The amount of austenite released is calculated based on three or more surface temperatures of the strip measured along the length of the runout table.

When winding the strip, the proportion of converted austenite in the strip can be determined; Measuring devices for this are commercially available.

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

According to a preferred embodiment, as additional input parameters the proportion of recrystallized austenite in the strip immediately after the hot strip mill finishing stand and/or the amount of precipitate in the strip immediately after the hot strip mill finishing stand may be used, preferably the strip A type of precipitate can also be used. These input parameters can be determined based on the recrystallization ratio, amount and type of precipitates determined in the hot rolling steps prior to the ROT, and based on 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 them.

According to a preferred embodiment, the grain size (G2) calculated before the hot strip finishing mill stand and the following measured parameters are used to determine the thickness (d1) and grain size (G3) of the steel strip immediately after the finishing mills :

ㅇ strip temperature before hot strip finishing mill stands (TO);

ㅇ strip thickness (dO) before hot strip finishing mill stands; and

ㅇ Strip speed (vO) before hot strip finishing mill stands.

If it is not possible to measure the grain size (G3) and thickness (d1) immediately 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 common practice on many hot rolling mills today.

Preferably, the grain size (G2) is the measured grain size of the slabs with the corresponding composition, the time/temperature path in the reheating furnace of the slab processed into steel strip in the roughing mills, and the size of the slabs in the roughing mills. Calculated based on time/temperature path and thickness reduction rate. This means that the particle size G2 is calculated using the available inputs. This calculation is common practice in many hot rolling mills today.

According to a preferred embodiment, in addition to the measured microstructure (M4) aspect during strip winding and, optionally, the winding temperature (Tc) measured during strip winding, the strip temperature (T), strip speed (v) and strip microstructure ( M) wherein one or more parameters of the one or more aspects are calculated and/or measured at one or more locations along the length of the runout table, the number of selected cooling banks, and the cooling of each selected cooling bank used to cool the strip. It is used to control at least one of the flow rates. These additional input parameters for controlling the number of cooling banks and the cooling flow rate of the cooling banks allow for further improved control of the cooling of the strip over the entire length of the ROT, resulting in a microstructure (M4) closer to the target aspect of M4 ( at strip winding) and, optionally, a temperature (Tc) of the strip at winding closer to the target Tct.

Preferably, one or more aspects of the parameters strip temperature (T) and/or strip microstructure (M) are calculated at one or more places along the length of the runout table line and at one or more places across the width of the strip and/or or measured This means, for example, that the temperature and/or microstructure is not only calculated and/or measured at the center line of the strip, as is customary, but also calculated and measured, for example, near the edge of the strip.

According to a preferred embodiment, one or more of the following target parameters are used as additional input parameters to determine the cooling pattern of selected cooling banks of the cooling installation in the runout table and/or the cooling flow rate of each selected cooling bank: :

- the average temperature of the strip over the entire thickness of the strip at one or more points along the runout table (Ta);

- the surface temperature of the strip at one or more points along the runout table (Ts);

- Cooling rate (Vc) across the length of the runout table.

These input parameters may affect the cooling pattern and/or cooling flow rate selected; for example, when the cooling rate is to be kept relatively low, the higher the number of cooling banks selected and the lower the cooling flow rate will be.

It is also more desirable when the cooling pattern of selected cooling banks of the runout table and/or the cooling flow rate 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 of the strip,

- a predetermined surface temperature of the strip on one or both sides of the strip,

- predetermined uniform cooling of the strip at high cooling rates.

These requirements relate to the quality of the steel strip that has undergone ROT cooling. For example, if the temperature gradient of the strip is too high, cracks may be formed in the strip or the surface may be rough.

Preferably, the input parameter granularity G3 is determined in real time to control the selected cooling banks in real time. Since the grain size G3 used as input is based on a model that predicts the grain size G3 immediately after the finishing mill, it is necessary to predict the grain size G3 in real time in order to determine the cooling pattern and control the selected cooling banks as precisely as possible. it is advantageous

It is desirable to be able to adjust the cooling flow rate of the cooling banks, and it is advantageous when one or more cooling banks of the runout table have two or more different cooling capacity settings, and/or where one or more cooling banks of the runout table It has a continuously adjustable cooling capacity setpoint. With these cooling banks, cooling of the steel strip at the ROT can be performed as accurately as possible. Preferably, all cooling banks have at least two different cooling capacity settings, or all cooling banks have continuously adjustable cooling capacity settings.

According to a preferred embodiment, the two or more targets or measurement parameters are the number of selected cooling banks used to cool the strip and/or to determine the cooling flow rate of the selected cooling banks and/or the selected cooling banks of the cooling pattern. When used to control , these parameters can be used as input parameters along with weighting factors.

Usually, when winding a strip, it is not possible to achieve exactly both the two target parameters, eg the aspect of the microstructure M4 and the winding temperature Tc. Therefore, when determining the cooling pattern and/or cooling flow rate, and when controlling the selected cooling banks, it is advantageous to determine in advance which parameters to be achieved are more relevant when compared to other parameters to be achieved. do.

For example, the aspects of winding temperature (Tct) and microstructure (M4), which are at least the target input characteristics, are preferably used with weighting factors such that, for example, microstructure (M4) is more important than winding temperature (Tc). can do.

The invention is explained in more detail with a number of examples showing the results of the use of the method according to the invention.
Figure 1 shows the cooling pattern of ROT-cooling in Example 1.
Figure 2 shows selected cooling banks of ROT-cooling used in Example 1.
3 shows the temperature calculated in Example 1.
Figure 4 shows the resulting transformation in Example 1.
5 shows the cooling pattern of ROT-cooling in Example 2.
Figure 6 shows selected cooling banks of ROT-cooling used in Example 2.
7 shows the temperature calculated in Example 2.
8 shows the resulting transformation in Example 2.
9 shows the cooling pattern of ROT-cooling in Example 3.
Figure 10 shows selected cooling banks of ROT-cooling used in Example 3.
11 shows the temperature calculated in Example 3.
12 shows the resulting transformation in Example 3.
In addition, the method according to the invention will be explained in more detail with reference to a number of schematic diagrams.
Figure 13 schematically shows the cooling banks of ROT-cooling and the range of possible cooling paths with these cooling banks.
Figure 14 shows the settings and parameters used to calculate the input parameters for the method according to the invention and a production line producing wound strips with different phases in the production process.
15 shows the process of determining the cooling patterns of the cooling banks and the flow rates of the cooling banks, and the control of the cooling patterns and flow rates of the cooling banks.

At the top of FIG. 13 there is schematically shown a number of cooling banks usable in runout table cooling. The graph below the cooling banks shows that ROT-cooling can be performed between an upper limit at which none of the cooling banks are used (Cmin) and a lower limit at which all cooling banks are used at maximum flow (Cmax). These limits define the maximum winding temperature (Tc max) and minimum winding temperature (Tc min) that can be reached on a particular strip.

Between the above upper and lower limits, any cooling path that is physically possible (strip heating is only possible when the converted energy of the strip is used) can be selected. 13 shows an example in which a cooling pattern PT is selected as indicated by black boxes representing cooling banks used for cooling. As a result of using this cooling pattern, a cooling path CP as shown in the graph is obtained. Since the method according to the invention makes it possible to use a selectable cooling pattern for any of the cooling banks in ROT-cooling, all physically possible cooling paths between the upper and lower limits can be obtained.

This is important because in high-strength steels the winding temperature (Tc) usually has to be within certain limits as well as the microstructure must meet predetermined criteria, especially when winding the strip. For example, in the case of two-phase steel, when wound, the microstructure must contain predetermined amounts of pearlite and ferrite. Also, often the granularity is also important.

Therefore, only the temperature Tc at the time of winding the strip is usually not of importance. If only Tc is important, the cooling path to reach Tc will be meaningless and many cooling patterns can be used to reach this Tc. However, when winding the microstructure of the strip is concerned, the cooling path is also important. This is because the cooling path determines the onset of phase transformation in the strip, the evolution of phase transformation and the resulting phase. The method according to the invention makes it possible to use the cooling path required to obtain the microstructure of the strip during winding.

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

In Figure 14 the following abbreviations are used:

S: Rolling mill/equipment setpoint

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

P0...P4: position 0 to 4

SL: slab

RH: reheat furnace

RHMs: Reheat Furnace Model

MM: microstructure model

RM: roughing mill

RMM: Roughing mill model

T(t): Temperature as a function of time

ε(t): strain as a function of time

M0...M4: Microstructure or one of its variants

FM: finishing mill

FMM: Models of finishing mills

ROT: Runout table cooling

ROTM: runout table model

CR: winder

Figure 14 shows that the production process of the hot rolled strip begins with the provision of a slab (SL). This slab is reheated in a reheating furnace (RH) to provide the slab with the required temperature and then transferred to a roughing mill (RM). After the first reduction in thickness in the roughing mill, the slab is transferred to the finishing mill (FM) and reduced in thickness to have the required thickness of a specific hot-rolled strip. Typically, hot rolled strip has a thickness between 2 mm and 10 mm, but may be thinner or thicker. After the finishing mill, the strip is cooled on a run-out table (ROT) and then wound up on a winder (CR). Such installations for producing hot rolled strip are well known to the person skilled in the art.

14 also shows that many models are used in hot rolling mills these days. Each of the aforementioned facilities within the production process uses its own model to calculate the microstructure of the slab or strip obtained using that facility. To calculate the microstructure, measurements within and after each fixture are used, and the model also creates setpoints for the fixtures.

As can be seen in FIG. 14, there is a model that uses measured values as inputs for each facility and generates set values as outputs. There are also separate models for calculating the microstructure. In this way, each subsequent facility uses the resulting microstructure M generated by the model of the predecessor facility. Only the first input microstructure (M0) is not calculated on the basis of hot-rolled slabs, but is based on a generic model made on the basis of previously produced slabs having the same or nearly identical composition. Accordingly, a dotted line is indicated in FIG. 14 .

14 shows five positions P0 to P4. Each location has its own microstructure (M0 to M4), either before or after one of the installations in the hot strip mill. Currently, most hot strip mills use models as shown in Fig. 14, but the models themselves can be changed, and the resulting microstructure can be calculated by a person skilled in the art.

15 shows how the input parameters (M, d, v and T) are used to create a cooling pattern with a target material property or properties, and how these parameters are used to control the cooling banks of the cooling pattern. show how to use

In Figure 15 the following abbreviations are used:

H: strip tip

M: microstructure

d: thickness

V: strip speed

T: temperature

TF7: Temperature measured after FM

FM: finishing mill

CT: Winding temperature measurement

Δ: delta

15 shows that an iterative loop is used to calculate the cooling pattern, and 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 optimized.

An iterative loop is used to control the use of the selected cooling pattern by measuring or calculating target material properties immediately prior to strip winding (based on other measurements taken along the ROT) and comparing them to desired material properties. In this way, the number of active cooling banks of the selected cooling pattern and their flow rates can be optimized.

15 shows that the same input parameters, temperature (T3), strip thickness (d3), speed (v3), and microstructure (M3) and target microstructure (M4) are used in both iterative loops. According to the present invention, particle size is an aspect of the microstructure used as an input parameter; As the target microstructure M4, as mentioned above, various aspects of the microstructure can be used.

Besides these targets, as mentioned above, other targets such as winding temperature (Tc) or surface quality at ROT may also be used.

With the foregoing description of the invention, the description of the following examples will be readily understood.

The embodiments described below all relate to cooling high carbon steel grades. High carbon steel grades are characterized by a carbon content greater than 0.5% by weight.

The high carbon steel grade used in the examples below has the following composition (in weight percent):

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 impurities, meaning each present in a maximum content of 0.01% by weight. Other possible elements are unavoidable impurities, the remainder being iron. This is a commercially available grade.

Examples

Example 1: Figure 1 shows the arrangement of the ROT-cooling used in the present examples. The ROT-cooling is generally indicated by the number '10' and shows a strip 1 wound into a coil 2, starting from the final stand 5 of the finishing mill as shown in Figure B. According to the ROT-cooling of the present embodiments, cooling banks 1 to 54 and trimmers 55 to 62 of the main cooling are respectively indicated. An arrangement of ROT-cooling as shown in Figure 1 is used in all examples.

The inputs for use of the present invention are finishing mill temperature (T1) of 880° C., strip thickness (d3) after finishing mill (d3) of 2.5 mm, strip speed (v3) of 13.0 m/s, and grain size (G3) of 11.5 μm. These input variables are presented in Figure C.

In this Example 1, the only target material property is the target pearlite fraction of 100%. Then, since all austenite will be converted to pearlite prior to winding, the target material properties are selected such that 100% pearlite is achieved in order to obtain the mechanical properties and homogeneity of the strip as good as possible.

Based on the measured and calculated strip temperature (T2), strip thickness (d2), strip speed (v2) and grain size (G2) at the position (P2) between the roughing mills and the finishing mills of the hot rolling mill (see Fig. B) ) and based on the target material properties, the predicted values for strip temperature (T3), strip thickness (d3), particle size (G3), and velocity (v3), when the strip enters the ROT at position P3 ( see Figure B), calculated for the strip. Based on these calculated input parameters and target material properties, the ROT-cooling control system determines a pattern for the cooling banks of the ROT-cooling to be used as a starting point for the cooling process. As soon as the front end of the strip passes the point at which the temperature T3 is measured, the input parameters temperature T3, strip thickness d, particle size G3 and optionally strip speed v are measured or calculated. , the cooling pattern is calculated.

According to the input parameters and target material properties predicted in Example 1, a cooling pattern as shown in FIG. 1 is calculated. This cooling pattern is used for cooling the strip, but the cooling banks actually used are determined based on measured/calculated values. 1 shows that the calculated cooling pattern does not use the trimmers 55 to 62. Figure 1 shows the finishing temperature (T1) measurement, the temperature before trimming zone (Tbt) measurement, and the winding temperature (Tct) measurement.

When the strip (1) enters the ROT-cooling (10), the temperature (T3), speed (v3) and thickness (d3) are measured, and the grain size (G3) is calculated. A cooling path is calculated based on these actual values, which means that which cooling banks are actually to be used is calculated from the calculated cooling pattern. In this embodiment, all cooling banks from cooling bank 1 to cooling bank 9 can be used, and the trimmers will not be used. Also, a flow rate of 100% is determined for the cooling banks 1 to 9. In Fig. 1, it is represented by hatched blocks. Figure 2 shows the actual cooling path calculated for cooling the strip using the method according to the invention, based on the input values T3, d3, G3 and v3 for Example 1. Cooling banks 1 to 8 are used at 100% flow and are indicated by black blocks, and as mentioned above no trimmer is used. With the cooling banks determined in the cooling pattern, fewer or more cooling banks may be used depending on the strip speed.

It should be understood that according to the present invention, the input parameters T2, d2, v2 and G2 are not essential. The method according to the invention can be used with only the input parameters T3, d3, G3 and optionally v3, but in that case the actual cooling path of the cooling banks when the tip of the strip passes the point at which T1 is measured. You need a powerful computer to calculate it right away. Otherwise, the first part of the strip will not be cooled correctly. Therefore, it is desirable to determine the cooling pattern before the strip enters ROT-cooling by predicting T3, d3, G3 and optionally v3 using T2, d2, v2 and G2.

The calculated temperature of the strip for the actual cooling path shown in FIG. 2 is plotted in FIG. 3 as a function of the above in the runout table. The average temperature of the strip appears midway between the top and bottom surfaces of the strip. With the method according to the invention, the temperature at the upper and lower surfaces of the strip can also be calculated. The dotted lines at 645° C. and 665° C. preferably represent the temperature limits at which the coiling temperature (Tc) must be maintained. However, the coiling temperature (Tc) is not the target material property of Example 1.

3 shows the calculated cooling temperatures. It is clear that very fast cooling of the strip is obtained right from the start of cooling at the ROT. The strip is cooled to a temperature of about 50° C. below the desired coiling temperature (Tc). However, the strip heats up again throughout the ROT in its path because of the conversion energy released as a result of the conversion from austenite to pearlite (and, if present, ferrite).

This is shown in FIG. 4 . All or nearly all of the austenite is converted to about 95% pearlite and the remainder to ferrite. Thus, the austenitic strip is completely or almost completely converted before the strip is wound, resulting in mechanical properties of the strip that are as homogeneous as possible.

Although not a target material property, it is interesting that the realized coiling temperature (Tc) is within the desired limit of 655°C±10°C, as shown in FIG. 3 .

Example 2: In Example 2 there are two target material properties. One is the target winding temperature (Tct), which is 655°C. Another target material property, as in Example 1, is the pearlite fraction target of 100% immediately before strip winding. All other input parameters are the same as in Example 1, so the particle size is 11.5 μm. Since both target material properties are given equal weight, the ROT-cooling control system should try to reach both material properties simultaneously.

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

Figure 6 shows the actual cooling path calculated for cooling the strip using the method according to the invention, based on the input values for Example 2. Only cold banks from 1 to 22 are used, trimmers are not used.

This cooling path achieves the calculated cooling temperature as shown in FIG. 7 . Compared to Example 1, it can be seen that in Example 2 the strip cools more slowly, and also to a temperature below the target coiling temperature as in Example 1. Due to the heat of conversion, the winding temperature (Tc) is within the temperature limit of 655°C ± 10°C.

Figure 8 shows that the conversion of the strip at ROT is almost as high as in Example 1. More than 85% of the austenite is converted to pearlite, and some 5% of ferrite is formed immediately before the strip is wound. The remainder remains austenite immediately prior to strip winding.

It will be clear that in this case where a target pearlite fraction of 100% is used as input for the cooling method according to the invention as well, a conversion of about 90% from austenite takes place. This means that relatively few partial transformations from austenite must occur either during or after winding of the strip. This achieves mechanical properties of the strip that are as good as those when following the cooling method according to Example 1.

Example 3: This last example shows how the method according to the present invention works when two or more target material properties are selected. Here, four target material properties are used:

- target winding temperature (Tct) of 655 ° C;

- starting quench rate target (dT/dt): this quench rate should be as fast as possible;

- quenching until the strip average temperature is 800 ° C,

- An annealing rate (dT/dt) from the point where the average strip temperature is 800°C to Tc is 100°C/s.

Equal weighting is given to all four target material properties.

This, when using the input parameters (T2, d2, v2, G2) and the target material properties, yields a cooling pattern of ROT-cooling as shown in FIG. Here all cooling banks 1 to 11 are used, but cooling banks 1 and 2 use 100% flow rate and cooling banks 3 to 11 use 50% flow rate. Trimmers are not used.

Figure 10 shows the actual cooling path calculated for cooling the strip using the method according to the invention, based on the input values for Example 3. Cooling banks 1 and 2 are used at 100% flow rate and cooling banks 3-10 are used at 50% flow rate. This is indicated by a black block. As mentioned above, trimmers are not used.

11 shows the calculated cooling temperatures. It is clear that using the four target material properties in the method according to the present invention achieves a cooling that looks very similar to the cooling in Example 1 where only the target transformation is used as the target input. However, the calculated graph shows that the coiling temperature (Tc) is higher in Example 3. Also, the cooling rate slows down obviously after the temperature of the strip drops below 800°C.

Although the conversion of the strip is not a target material property, the calculated conversion as shown in Fig. 12 is almost as good as the conversion in Example 1.

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

Claims (15)

A method of controlling the cooling of steel strip in a hot strip mill runout table comprising a plurality of cooling banks to produce hot rolled steel strip, comprising:
Here, cooling in the runout table until winding of the strip is controlled using the following input parameters:
1. The surface temperature of the hot-rolled strip (T1) immediately after the stand of the hot-strip finishing mill
2. The thickness of the hot-rolled strip immediately after the stand of the hot-strip finishing rolling mill (d1)
3. Speed of hot-rolled strip immediately after hot-strip finishing mill stand (v1)
4. The strong chemistry of the strip;
The grain size (G3) of the steel strip immediately after the finishing mill is used as an additional input parameter,
The cooling pattern of the selected cooling banks of the cooling installation of the run-out table and the cooling flow rate of each of the selected cooling banks are based on the input parameters 1-4 and the grain size (G3) of the steel strip immediately after the finishing mill and the strip It is determined based on the target aspect of the microstructure (M4) at the time of winding,
wherein the cooling pattern of the selected cooling banks is determined based on the target winding temperature (Tct) of the strip, and the number of the selected cooling banks is controlled based on the measured surface temperature (Tc) of the strip during winding;
The number of the selected cooling banks actually used to cool the strip depends on the input parameters 1-4 during cooling of the strip, the grain size of the steel strip immediately after the finishing mills (G3), and the Controlled based on the aspect of the microstructure (M4) measured during winding of the strip. According to claim 1,
The cooling control method, wherein the grain size (G3) is the average austenite grain size of the steel as it is immediately after leaving the final finishing mill. According to claim 1 or 2,
wherein the strip surface temperature (T1) and speed (v1) are measured immediately after the hot strip finishing mill stands. According to any one of claims 1 to 3,
wherein the strip chemistry is determined by analysis of a sample taken from a molten steel used for casting a strand from which a slab is cut as an input to produce steel strip in the hot strip mill. According to any one of claims 1 to 4,
The aspect of the microstructure (M4) upon winding of the strip is the percentage of the austenite phase that is converted to other phases such as pearlite and/or ferrite, preferably the amount of converted austenite is measured, or the amount of converted austenite wherein the amount is calculated based on at least three measured surface temperatures of the strip along the length of the runout table. According to any one of claims 1 to 5,
As an additional input parameter, at least one of the percentage of recrystallization of austenite in the strip immediately after the hot mill finishing stands and the amount of precipitate in the strip immediately after the hot strip mill finishing stands is and, preferably, the type of precipitate in the strip is additionally used. According to any one of claims 1 to 6,
The following parameters were measured:
- the strip temperature (T0) before the hot strip finishing mill stands,
- the strip thickness before the hot strip finishing mill stands (dO), and
- the strip speed (vO) before the hot strip finishing mill stands,
The cooling control method of determining the thickness (d1) and the grain size (G3) of the steel strip immediately after the finishing mills using the calculated grain size (G2) before the hot strip finishing mill stands. According to claim 7,
The grain size G2 is the measured grain size of the slab having the corresponding composition, the time/temperature path in the reheating furnace of the slab converted to steel strip in the roughing mill, and the time/temperature path of the slab in the roughing mill. and the cooling control method, which is calculated based on the thickness reduction rate. According to any one of claims 1 to 8,
In addition to the measured aspect of the microstructure (M4) during winding of the strip and optionally the measured winding temperature (Tc) during winding of the strip, the parameters strip temperature (T), strip speed (v) and strip microstructure wherein at least one of the one or more aspects of (M) is calculated, measured, or calculated and measured at one or more locations along the length of the runout table, a number of selected cooling banks and each of the selected cooling banks being used to cool the strip; Cooling control method used to control at least one of the cooling flow rates of the. According to claim 9,
At least one of the one or more aspects of the parameters strip temperature (T) and strip microstructure (M) is calculated at one or more locations along the length of the runout table line and at one or more locations across the width of the strip. A cooling control method, either measured or computed and measured. According to any one of claims 1 to 10,
wherein one or more of the following target parameters is used as an additional input parameter to determine at least one of a cooling pattern of the selected cooling banks of the cooling installation in the runout table and a cooling flow rate of each of the selected cooling banks, Cooling control method:
- the average temperature of the strip at one or more points on the runout table (Ta)
- the surface temperature (Ts) of the strip at one or more points on the runout table;
- Cooling rate (Vc) over the length of the runout table. According to any one of claims 1 to 11,
At least one of the cooling pattern of the selected cooling banks of the runout table and the cooling flow rate of each of the selected cooling banks is controlled so that the cooling of the strip meets one or more of the following requirements:
- a predetermined temperature gradient of the strip;
- a predetermined surface temperature on one or both sides of the strip;
- Homogeneous cooling of the pre-determined strip at high cooling rates. According to any one of claims 1 to 12,
wherein the input parameter granularity (G3) is determined in real time to control the selected cooling banks in real time. According to any one of claims 1 to 13,
one or more cooling banks of the runout table have two or more different cooling capacities;
one or more cooling banks of the runout table have a continuously variable cooling capacity;
wherein one or more cooling banks of the runout table have two or more different cooling capacities, and one or more cooling banks of the runout table have continuously variable cooling capacities. According to any one of claims 1 to 14,
two or more goals for at least one of determining at least one of selected cooling banks of the cooling pattern and a cooling rate of the selected cooling banks, and controlling the number of selected cooling banks used to cool the strip; or When measurement parameters are used, said two or more target or measurement parameters are used as input parameters along with a weighting factor, and preferably, at least aspects of the target properties, winding temperature (Tct) and microstructure (M4) are: A cooling control method, for example used with a weighting factor such that the microstructure (M4) is more important than the coiling temperature (Tc).