WO2007013028A2

WO2007013028A2 - Process for continuous control of transitory phases on continuous annealing lines of steels

Info

Publication number: WO2007013028A2
Application number: PCT/IB2006/052544
Authority: WO
Inventors: Alessandro Dulcetti; Baldo Gurreri; Luca Lattanzi; Stefano Luperi
Original assignee: Centro Sviluppo Materiali S.P.A.
Priority date: 2005-07-25
Filing date: 2006-07-25
Publication date: 2007-02-01
Also published as: WO2007013028A8; ITRM20050396A1

Abstract

A process for continuous control of transitory phases on continuous annealing lines of stainless steels ensures elevated qualitative standard of the treated product, both in terms of mechanical properties and of surface aspect, in the operation transients generating on any industrial plant until restoring of normal process conditions, employing the input data related to the part of material processed and on the basis of the logics and parameters subject-matter of the present invention, continuously and automatically adjusting the productivity of the process depending on the extent of the transient-induced disturbance, so as to give to the material a thermal cycle close to the reference one until restoring normal process conditions.

Description

PROCESS FOR CONTINUOUS CONTROL OF TRANSITORY PHASES ON CONTINUOUS ANNEALING LINES OF STEELS

DESCRIPTION

The present invention relates to a process for continuous control of transitory phases on continuous annealing lines of stainless steels, which applies in particular in continuous annealing treatments of plane rolled metal materials, e.g. steel, both hot- and cold-rolled, on furnaces implementing an annealing line.

Steel annealing is performed to give adequate mechanical features to the products. In fact, the annealing treatment is aimed at recrystallization and grain growth. The metallurgical state, and therefore the mechanical features deriving from the thermal treatment of stainless steels, is as important as the knowledge of the composition and annealing is therefore required to make viable the subsequent processing of the material.

In general, the parameters ensuring the attainment of the desired mechanical properties are the maximum temperature and the time of treatment above transformation temperatures. Therefore, observance of the thermal cycles, implementable with the different existing technologies, is a fundamental prerogative for assuring the adequate qualitative level of the materials. hi oxidizing atmosphere furnaces, on the surface of the product it is generated a layer of scale that is subsequently removed by a pickling process, hi reducing atmosphere furnaces it is necessary to monitor the dew-point, both on the heating section and the cooling one, to prevent the forming of surface oxides. In all cases, there have been developed methods and techniques to control the annealing process, to improve the pickling ability of the products or reduce their costs, improve the surface appearance, etc. hi any plant in which continuous annealing treatments are carried out transients are generated, owing to production needs as well as occasional malfunctioning, e.g. by productivity variation such as thickness changes and/or width changes, changes of materials to be treated, changes of furnace temperature sets, re- starts after stops, whose duration substantially depends on the thermal inertia of the furnace.

In. these cases, even when only part of the material is concerned by the transient, there is the risk of a quality downgrading of the treated material. Therefore, it is necessary to control such process situations in order to limit their effects on the quality of the products and accordingly limit the economic losses associated thereto, maximizing material yields both in qualitative and in quantitative terms.

A management of transients during the annealing is described in the Japanese patent JP-60135531. In this document, strip temperature control in a radiant tube furnace occurs by adjustment of the fuel flow rate, resultant of a set value under steady state and of a transient-related term calculated by means of a relation with the strip temperature. However, such a methodology provides less than satisfactory results. The technical problem underlying the present invention consists in providing a process for control of transitory phases on annealing lines capable of improving the results attained according to the known art.

Such a problem is solved by a process as defined in the appended claim 1. Said process ensures elevated qualitative standards of the treated product, both in terms of mechanical properties and of surface aspect, in the operation transients generating on any industrial plant until restoring of normal process conditions, hi general, during the transients, more or less protracted in connection with the thermal inertia of the industrial plants, there are carried out annealing treatments unsuitable from the standpoint of the mechanical and qualitative surface properties, even merely of a part of the material, and this is often reason of quality degrading of all of the processed material.

According to the process subject-matter of the present invention, through the input data related to the part of material processed and on the basis of the logics and parameters subject-matter of the present invention, the productivity of the process is continuously and automatically adjusted depending on the extent of the transient- induced disturbance, so as to give to the material a thermal cycle close to the reference one until restoring normal process conditions.

Hereinafter, it will be given, by way of example and without limitative purposes, a detailed description of the process according to the present invention, with reference to the examples reported herein and to the annexed figures, wherein:

* Fig. 1 is a block diagram illustrating a general chart of the process according to the invention;

* Fig. 2 is a block diagram illustrating an example of the operation logic of the process according to the invention;

* Fig. 3 is a diagram illustrating the pattern of some parameters in a process according to the invention applied to an annealing line re-start after a stop; and

* Fig. 4 is a diagram illustrating the pattern of some parameters in a process according to the invention applied to a thickness change. The present invention applies on all process conditions and on all conditions of variation from standard operating procedures originating transients in continuous annealing processes of plane rolled metallic materials, like stainless steels, both hot- rolled and cold-rolled, on furnaces utilizing existing heating processes and technologies, e.g. burner furnaces fed with any fuel, resistor furnaces, radiant tube-, muffler-, oxidizing atmosphere furnaces, reducing or inert atmosphere furnaces, bright annealing furnaces, etc. The transient in the annealing treatment should be construed in a general sense; in fact, several are the cases in which a transient is originated, e.g.:

* due to production treatment needs, like the mass flow variation, e.g. in thickness changes, width being equal, in width changes, thicknesses being equal, in concomitant changes of thickness and width, in treatment speed changes with consequent variation of energy output (fuel flow rate, electric power, etc.), in changes of atmosphere composition inside furnace, e.g. in the variation of O₂ content in the combustion atmosphere in order to adjust or modify the oxidizing power, in air/fuel ratio changes, in thermal zones set changes;

* following occasional malfunctioning, blowing-out of one/some burners of one or more thermal zones, malfunctioning of one or more resistors, line slowing-down or stops; or

* on re-starts after a scheduled short- or longtime stop, for plant maintenance, etc.

The transient, whose duration depends also on the thermal inertia of the industrial furnaces and on their ability to comply over short times with new production needs and/or allow the resetting of normal process conditions, may lead to a downgrading of the qualitative level of the products, and accordingly to economic losses.

Therefore, the process according to the invention applies on a furnace implementing an annealing line. By an automatic controller, a control action is exerted on the basis of input data related to the part of material processed in the annealing section. Said data relate to the process speed and the material temperature, metered e.g. by one or more pyrometers inside of said annealing section, detected at a specific distance from the furnace inlet, preferably of from 25 to 75% of the annealing section length. The automatic controller receives the material temperature-related data, processes them to provide an output value and, in connection with the deviation of said value with respect to a reference value of the material temperature, on the basis of the logics of the process subject-matter of the invention, exerts the control by continuously adjusting the line productivity, increasing or reducing it, so as to nullify said deviation.

The processing of the material temperature metering, carried out with plural instruments or by detecting instruments along the width of the strips, is carried out by averaging the metered values and optionally leaving out the value differing from the average for a set percentage, e.g. the 15%.

If only one instrument for metering the material temperature is available, the meter value of said instrument will be compared to the reference value. To the ends of the management of the line transients, it is necessary to suitably define the position of the instruments for metering the material temperature and the reference values for the control action.

Hence, the position of the detecting instruments, depending on the type of material and the desired thermal cycle) is singled out by relation (1) below, which takes into account the furnace length and the maximum speed of the line. With this relation, it is singled out the furnace section where the material reaches a temperature relevant to the ends of the qualitative features in connection with the process to be carried out and the furnace type, and therefore where it is convenient to locate one or more instruments. (l) X = L - A v_Max where

X is the distance, expressed in m, from the inlet of the furnace in which the metering instruments are to be located,

L, expressed in m, is the furnace length, V_Max, expressed in m/min, is the maximum speed at which there can be treated the material in said furnace,

A, expressed in min, is a process parameter depending on the material to be treated. The values of said parameter are comprised in a range of from 0.03 to 1.00. E.g., for hot-rolled products said value is preferably comprised in a range of from 0.03 to 0.45 and, for cold-rolled products, of from 0.133 to 1.00.

Moreover, the reference value (T_REF) for the control action, set-point to be input in the HMI (Human Machine Interface), depends on the material to be treated and on the thermal cycle, and is calculated by the following relation (2):

where

T_Max, in ⁰C, is the maximum temperature of the material in the furnace in the thermal cycle implementing the desired qualitative features in terms of mechanical properties as well as of surface aspect.

By way of example, TMax values may be deemed as comprised in the following ranges:

- for austenitic steels, T_Max = 1000 - 1200 ⁰C

- for ferritic steels, T_Maχ = 800 - 1050 ⁰C B, expressed in °C, is a material-dependent process parameter, comprised in a range of from 0 to 150⁰C and preferably of from 10 to 150°C.

The mode by which line productivity is modified is proportional to the deviation [T(X) - TREF] between the material temperature metered in the position located at a distance X from the furnace inlet, T(X), and the set temperature, (TREF), according to the following relation (3): (3) P₁ = (C [T(X) - TREF] + I) Po where

Po, expressed in t/h, is the productivity calculated at the time of detection of the deviation.

P₁, expressed in t/h is the new productivity to which the line is driven by effect of the deviation. Said quantity should anyhow be always comprised in a range of from 15% to 100% of the maximum productivity of the plant used and for the material processed. C is a process parameter expressed in ⁰C^"1, function of the response time of the furnace for adjusting to productivity variations. This experimental constant is comprised in the range of 0.0005 - 0.01.

By the process described, during process transients the material follows a thermal cycle close to that envisaged under steady state conditions, so as to minimize off-standard treatment of the material and reduce negative effects on quality.

When the material temperature T(X), metered with the detecting instrument, is higher than TREF, T(X) > TREF and the line productivity is lower than the maximum productivity for the material to be treated, according to the process subject-matter of the invention, the control system increases the line productivity by acting on the process speed.

Vice versa, when T(X) < TREF and the line productivity is greater than the minimum productivity for the material to be treated, the speed of the material is reduced.

When the deviation between the metered temperature and the reference one persists after the adjusting of the line productivity, upon having attained the maximum productivity (or the minimum productivity), an alarm system will activate. In the carrying out of the method, for austenitic steels in particular said TM_8X preferably has values comprised in a range of from 1000 to 1200 ⁰C, whereas for ferritic steels T_Max preferably has values comprised in a range of from 800 to 1050 ⁰C.

The process proves particularly advantageous when performed on plane rolled sections, preferably strips. In case of hot-rolled sections, the process is advantageous for thicknesses comprised in a range of from 1.5 to 6.0 mm, whereas for cold-rolled sections the preferred thickness is comprised in a range of from 0.3 to 4.5 mm. Examples Example 1 In this example it is described the application of the invention on an industrial plant processing plane cold-rolled stainless steels. It is an annealing and pickling line for austenitic steel strips. The annealing section of this line is 30 m long; the maximum process speed is 30 m/min.

The position of the instrument for metering the temperature of the strip (pyrometer) was singled out by assigning a value of parameter A equal to 0.167. Hence, according to relation (1), the optimal position of the pyrometer is 25 m from the inlet of the material into the furnace.

The thermal cycle carried out on this line for AISI 304 steels of thickness equal to 1 mm provides a T_Max = 1110 °C, with 15 s treatment times at temperatures higher than T=1050°C. For reference parameter B, a value equal to 30 °C has been assumed, therefore the set value for the control system was set at TR_EF =1080 ⁰C.

The maximum productivity of the line with strips of 1270-mm width is of 18 t/h, the value of the process parameter C is equal to 0.0065.

In the example at issue the material under treatment was concerned by a stop for a brief maintenance intervention. After line re-start, transient control (TC) was activated, which in this transitory phase allowed to obtain a strip with the desired mechanical properties. In Fig. 3 it is shown the action of the transient control on the line productivity, by acting on the treatment speed according to relation (4) and in which it is highlighted the pattern, during the transient, of the size of the grain and the acceptability range of the latter. (4) V_P = P/(S*L*d) Where:

Vp — treatment speed; P - productivity; S — thickness

L - width; D - density Example 2

In this example it is described the application of the invention in a transient due to a thickness change from 1.0 mm to 0.7 mm of cold-rolled plane stainless steels on an industrial annealing and pickling plant. The annealing section of this line is 55 m long; the maximum process speed is 75 m/min. For the location of the pyrometer metering the strip temperature a value A = 0.18 was applied, therefore it was located 41.5 m from the inlet of the material in the furnace. The treatment of the 1-mm AISI 304 strips provides a 50 m/min annealing cycle, with T_Max ⁼ H lO ⁰C, whereas for the 0.7-mm thick strips it provides the same maximum temperature, with the same set of furnace temperatures, at 70 m/min. The reference parameter, B, was assumed equal to 55°C for the 1-mm strip and 3O⁰C for the 0.7-mm strip, therefore the set value for the control system was set at T_REF =1055 ⁰C and T_REF =1080 ⁰C, respectively. The value of the parameter C is equal to 0.005.

Li the absence of corrective actions, the initial length of the 0.7-mm strip processed at the speed of the strip of greater thickness causes a mass flow variation in the furnace, resulting in temporary transients in the methane supply and in the temperature of the thermal zones. This gives to the strip of thinner thickness a thermal cycle with higher treatment times and maximum temperature until the overcoming of the transitory phase. In Fig. 4, the adjusting of the transient, according to the process subject- matter of the invention, which acts on the line productivity by control of the treatment speed according to relation (4), enabled to keep a nearly constant grain size during the entire transitory phase, allowing the two strips to follow a thermal cycle close to the reference one. The present invention has hereto been described according to a preferred embodiment thereof, given by way of example and without limitative purposes.

It is understood that other embodiments might be envisaged, all to be construed as falling within the protective scope thereof, as defined by the appended claims.

Claims

1. Process for continuous control of transitory phases on continuous annealing lines of stainless steels, characterized in that it comprises the following steps: defining the positioning of at least one temperature detecting instrument along a control-subjected continuous annealing furnace, by the following relation

X = L - A V_Max where

X is the distance in meters from the inlet of the furnace line, where said at least one temperature detecting instrument is to be located, L is the furnace length, in meters,

V_MaX5 expressed in rn/min, is the maximum speed at which there can be treated the material in said furnace,

A, expressed in min, is a process parameter whose values are comprised in a range of from 0.03 to 1.0; calculating a value of the reference temperature (TREF) by the following relation:

B where

T_Max, expressed in °C, is the maximum temperature of the material in the furnace in the thermal cycle implementing the desired qualitative features, whose values are comprised in a range of 800 -1200⁰C,

B, expressed in °C, is a process parameter comprised in a range of 0 - 150°C and preferably of 10 - 100°C, modifying line productivity by the following relation P₁ = (C [T(X) - TREF] + I) P₀ where

T(X), expressed in ⁰C, is the material temperature metered in the position located at a distance X from the furnace inlet,

P₀, expressed in t/h, is the productivity calculated at the time of detection of the temperature, P₁, expressed in t/h, is the new productivity to which the line is driven by effect of the deviation, kept comprised in a range of from 15% to 100% of the maximum productivity of the plant used and for the material processed, C is a process parameter expressed in 1/°C, function of the response time of the furnace for adjusting to productivity variations, comprised in the range of 0.0005 - 0.01.

2. The process according to claim 1, wherein the process parameter A for hot- rolled strips has values comprised in a range of from 0.03 to 0.45.

3. The process according to claim 1, wherein the process parameter A for cold- rolled strips has values comprised in a range of 0.133 to 1.

4. The process according to claim 1, wherein said TM_3X has, for austenitic steels, values comprised in a range of 1000 to 1200 °C.

5. The process according to claim 1, wherein said T_M3X, has, for ferritic steels, values comprised in a range of 800 to 1050 ⁰C.

6. The process according to any one of the preceding claims, wherein said stainless steel is in the form of a plane rolled section.

7. The process according to claim 6, wherein said plane rolled section is a strip.

8. The process according to any one of the preceding claims, wherein said stainless steel is in the form of a hot-rolled section.

9. The process according to claim 8 wherein said hot-rolled section has thicknesses comprised in a range of 1.5 to 6.0 mm.

10. The process according to any one of the preceding claims, wherein said steel is in the form of a cold-rolled section.

11. The process according to claim 10, wherein said cold-rolled section has thicknesses comprised in a range of 0.3 to 4.5 mm.