EP0579702A1

EP0579702A1 - Dynamic casting speed control method for a skinning over cycle flollowing sticking in a continuous steel casting process.

Info

Publication number: EP0579702A1
Application number: EP92908866A
Authority: EP
Inventors: Andre Klein; Manfred Michael Wolf
Original assignee: Techmetal Promotion SA
Current assignee: Techmetal Promotion SA
Priority date: 1991-04-10
Filing date: 1992-03-30
Publication date: 1994-01-26
Anticipated expiration: 2012-03-30
Also published as: FI934393A; PT100355A; FR2675062B1; MX9201617A; AU651883B2; FI934393A0; KR100230888B1; CN1046875C; US5449034A; FR2675062A1; FI97782B; ATE115019T1; DE69200848D1; CN1065613A; WO1992018273A1; AU1646492A; CA2108127A1; EP0579702B1; TW206171B; DE69200848T2

Abstract

PCT No. PCT/FR92/00286 Sec. 371 Date Oct. 8, 1993 Sec. 102(e) Date Oct. 8, 1993 PCT Filed Dec. 29, 1993 PCT Pub. No. WO92/18273 PCT Pub. Date Oct. 29, 1992.On detection of an occurrence of skin sticking in the mould, the withdrawal speed is subjected to a cyclic variation which comprises a ramp from the cruising speed to a reduced, decelerated speed, a healing plateau, and an acceleration ramp from the reduced speed to the cruising speed, measures are taken to determine the ferritic potential (PF) of the steel which is being cast, to determine the gradients (d, a) of one of the two ramps as a function of this ferritic potential, and to determine the length (tr) of the healing plateau as a function of the difference between the liquidus and solidus temperatures of said steel.

Description

METHOD FOR DYNAMICALLY CONTROLLING THE EXTRACTION SPEED DURING A HEALING CYCLE AFTER GLUING IN DN

CONTINUOUS STEEL CASTING PROCESS

The invention relates to a method of dynamic control of the speed of extraction in a process of continuous casting of a steel of the type according to which, when detecting a bonding of skin in ingot molds, the speed of extraction of a cycle composed of a deceleration ramp from cruising speed to a reduced speed, a healing plateau and an acceleration ramp from reduced speed to cruising speed.

Skin bonding in ingot molds of a continuous casting machine is very dangerous since it can lead to breakthroughs. It is known, in particular by the system developed by SOLLAC under the name of SAPSOL, to signal these collages by an alarm system based on the monitoring of the temperatures of the walls of the mold by thermocouples inserted at two heights in the thickness walls of the vertical ingot mold, below the meniscus. Other alarm systems have been proposed. Initially, after a detected alarm, the casting operation was stopped for a time deemed sufficient for healing. Later, it was proposed to adjust the extraction speed on a healing cycle as defined above, which avoids the complete stopping of the machine. However, this cycle and more particularly its slowdown period is not without consequence on the surface quality of the product as well as on the productivity of the machine.

There are also known, in horizontal continuous casting installations, methods for detecting breakthrough by measurement of stress (EP-A-111 000) and methods teaching not to interrupt the extraction of the product unless a fall in pressure is detected. temperature in the mold (DE-A-33 07 176).

The object of the invention is to replace the management of this cycle by a dynamic control adapted to the behavior of the steel and minimizing the slowdown period to the minimum time to heal the collage.

The invention achieves its object in that the ferritic potential of the cast steel is determined and in that at least the slope of the acceleration and deceleration ramps is determined as a function of this ferritic potential.

The invention is indeed based on the discovery based both on scientific considerations and on real experiences, according to which the ferritic potential, which will be defined later, can be considered as the determining factor in the speed regulation d during the healing cycle.

It also appeared advantageous to determine the length of the healing plate as a function of the difference between the liquidus temperature and the solidus temperature of the cast steel.

Advantageously, the reduced speed in the event of bonding is of the order of 0.2 to 1 m / minute to heal the bonding.

Other characteristics and advantages of the invention will appear on reading the following detailed description. Reference is made to the appended drawings in which:

FIG. 1 is a graph of the speeds during the healing cycle,

FIG. 2 superimposes three graphs expressing from top to bottom: the healing time in minutes as a function of the interval of solidification temperatures in degrees, the slope of the deceleration ramp in m / min ² as a function of the ferritic potential, and the slope of the acceleration ramp in m / min ² as a function of the ferritic potential,

- Figure 3 is a graph similar to that of Figure 1 showing the healing cycle of three grades of steel X, B, D according to the invention and grade X ', analogous to X, in the traditional way. The graph in FIG. 1 represents, schematically, the extraction speed V (in m / min) as a function of time t (in min), around and during the healing cycle. Before and after this cycle, the extraction speed has a cruising value V _c . In the event of an detected alarm, it is lowered at a reduced speed V _r , during a decrease time t _d and according to an average decrease value D = (V _r -V _c ) / t _d , ie the slope of the deceleration ramp. After a healing time or waiting time t _r , the speed rises to its value V _c in a time t _a and according to an acceleration A = (V _c -V _r ) / t _a .

According to the invention, it has been discovered that:

- t _a and D are strongly influenced by the tendency for slab swelling between rollers, itself a function of the creep behavior of the skin at high temperature; a ferritic grade with low creep resistance requires a long duration t _d (and a low level for D), while the opposite applies to an austenitic grade;

- t _r is mainly related to the solidification interval, ie the difference in temperatures between liquidus and solidus, T _L -T _s (in K); therefore a highly alloyed shade with a high value for T _L -T _s requires a corresponding increase in t _r and vice versa,

- _ta and A require adaptation to the sticking tendency is strong for shades is fully ferritic or fully austenitic and is against lower for a mixed structure ie austenitic-ferritic skin temperature regime.

All these aspects are largely a function of microsegregation phenomena within the matrix, and ultimately depend on the ferritic or austenitic nature of the grade of cast steel, to the extent that studies have shown that the presence of ferrite during the solidification phase has a very favorable influence to minimize microsegregation. Given the evolution of the ratio of solid ferrite and austenite fractions as a function of the carbon content in the case of non-alloyed or low-alloyed steels, it appears possible to define a "ferritic potential" PF expressing the fraction of ferrite formed during solidification. So :

PF = 2.5 (0.5 -% Cp)

where% Cp represents a carbon equivalent to the peritectic reaction, that is to say a C content corrected to take account of the influence of the other alloying elements. In practice, we retain the formula:

% Cp =% c + 0.02% Mn + 0.04% Ni-0.1% Si

-0.04% Cr-0.1% Mo.

The value 1 of the ferritic potential, or higher values, signifies a completely ferritic solidification. On the contrary, the negative values of the ferritic potential indicate a totally austenitic solidification.

The formula for calculating the ferritic potential for stainless steels to be used is:

PF = 5.26 (0.74 - [% Ni '/% Cr']) Where% Ni '=% Ni + 0.31% Mn + 22% C + 14.2% N +

% Cu

% Cr '=% Cr + 1.5% Si + 1.4% Mo + 3% Ti +

2% Nb

On the basis of a classification of steels produced from the ferritic potential thus defined, it appeared possible, essentially from experience data, to determine the ideal accelerations A and decelerations D of the healing cycle after alarm. This is shown by the two lower curves in Figure 2.

So the curve at the bottom of the figure shows that The acceleration A, expressed in m / min ² as a function of the ferritic potential, is an increasing function from a value slightly less than 0.1 · m / min ² for very positive potentials up to a maximum of approximately 0, 7 m / min ² for a potential close to 1, then decreasing from this maximum to a value slightly less than 0.2 m / min ² for negative potentials.

A polynomial approximation of the values of A as a function of PF gives:

with:

for PF> 1 to ₀ = 5.9

a ₁ = -10.635

a ₂ = 7.82

a ₃ = -2.8459

a ₄ ≈ 0.5091

a ₅ ≈ -0.0357

for PF <1 a ₀ ≈ 0.3116

a ₁ = 0.2075

a ₂ = 0.15

a ₃ = 0.0471

a ₄ = 0.0051

The preferred durations t _a for acceleration are between 60 and 600 s.

In fact, the durations t _a (which theoretically result from the calculation (V _c -V _r ) / A) are advantageously arranged to also take into account other alloying elements which promote bonding by affecting the viscosity of the slag in ingot mold. The multiplication factors to remember (corresponding to similar division factors for A) are: Element,% content

0.05 0.1 0.5 or greater

S 1 2 3

Al 1 2 3

Ti 1.5 3 6

Zr and / or REM 2 4 10

As for deceleration, here again, a polynomial approximation is possible:

with:

a ₀ = - 57.15

a ₁ = 21

a ₂ = 7.68

a ₃ = - 1.83

a ₄ = 0.822

a ₅ = 0.0531

a ₆ = 0.0289

The durations ta. preferred for deceleration are in the range of 0.5 to 30 s.

As for the waiting time during the healing plateau, it is, as has been said, linked to the solidification interval T _L - T _s , where T _L and T _s are the liquidus and solidus temperatures . Consider the actual solidus temperatures, for the given steel grade, i.e. the temperatures changed from the solidus temperatures theoretical equilibrium, so as to take into account the effects of elements with low solubility such as phosphorus or sulfur which cause a certain fall in the solidus.

In practice, the liquidus temperature T _{L is} worth: for PF> 0: T _L = 1538 - 90 (% C) - [% X] for PF <0: T _L = 1528 - 60 (% C) - [% X ] and the solidus temperature, T _s is _equal to:

for PF> 1: T _s = 1538 - 450 (% C) - [% X] for PF <1: T _s = 1528 - 180 (% C) - [% X] where the coefficient X of the elements and alloys represents respectively : 10Si, 5Mn, 2Cr, 3Ni, 3Mo, 3Cu, 8Nb, 14Ti, 3A1, 2V, 60B, 1W, ICo, 34P, 40S, 14As, 10Sn, 36Se.

The upper graph of FIG. 2 shows that the waiting time t _r is an increasing function of the solidification interval, from values of approximately 15 s to values of approximately 6 min, the preferred durations being of the order of 30 to 300 s.

A polynomial approximation of t _r is as follows:

with

a ₀ = 0.351

a ₁ = - 0.0194

a ₂ = 0.000572

a ₃ = - 0.1715.10 ^-5

All of these curves are advantageously programmed in a computer or microprocessor which automatically manages the dynamic control of the cicratrisation cycle in conjunction with the sticking alarm system. It goes without saying that the indicated values of D and A are average values, and that they can be modified around these average values by approximately 20%, in particular to effect nonlinear speed changes. Table I below, as well as FIG. 2, shows by way of example the values determined for six alloys A, B, C, D, E and F in a typical case of continuous casting of slabs of 250 mm × 1800 mm.

TABLE I

Steel grade A B C D E F

Analysis in%

C 0.05 0.02 0.005 1.0 0.12 0.35

If 0.5 3.0 0.20

Mn 1.5 0.30 0.50

Cr 18.0 1.5

Ni 10.5

Ti 0.05

Al 0.03

Characteristic values:

PF 0.53 1.95 1.24 -1.06 0.94 0.34

T _L (ºC) 1460 1506 1537 1465 1526 1502

T _S (ºC) 1408 1499 1535 1344 1504 1458

T _L -T _S (K) 52 7 2 121 22 44

Dynamic control criteria *):

D <m / min ² ) -44 -12 -20 -68 -30 -52 t _d (s) 1 _. 4 5.0 3.0 o. 9 2.0 1.2 t _r (min) 0.7 0.3 0.3 3, 6 0.3 0.5

A (m / min ² ) 0.45 0.16 0.38 0.22 0.72 0.38 t _a (min) 2.2 6.2 2.6 *** 4, 5 1.4 2, 6 t _a + t _r (min **) 2.9 6.5 4.2 8, 1 1.7 3.1

*) V _c = 1.5 m / min; V _r = 0.5 m / min

**) total time at reduced speed (healing period)

***) correction of t _a for Ti: 2.6 × 1.5 = 3.9 min The advantage of the invention will be better illustrated with the following examples comprising, on the one hand, the conventional control and the dynamic control in accordance with the invention for the same steel grade X (0.06% C; 0, 30% Mn, 0.015% P; 0.010% S; 0.040% Al; PF = 1.085; T _L - T _S = 1531 - 1508 = 23 K) and on the other hand, dynamic control for three different shades B, D and X.

The same FIG. 3 shows the typical healing cycle of a bonding for the mild steel grade X according to the invention and according to a conventional method X ', as well as for a grade with a high silica content for sheets. electrical (steel B) and for a hard steel grade, type 100 C 6 (steel D). The different cycle parameters are shown in Table 2 below.

As can be seen, according to a traditional process, the cycle X 'requires a total t _a + t _r of 7 min, to which is added 0.9 s of deceleration. This results in a loss of productivity as well as a deterioration of the surface quality.

In addition, a similar cycle is applied in the traditional way for all steel grades because we do not know how to distinguish their behavior with regard to healing of bonding.

On the contrary, according to the invention, it can be seen that it is possible to reduce the healing cycle t _a + t _r to only about 1 minute, which corresponds to a productivity gain of almost 90% and to a part whose quality is only affected on a very short surface.

A similar gain is observed for steel B.

On the other hand, shade D requires a much longer cycle; the conventional method has insufficient security to effectively heal the bonding. These examples clearly show how much the invention makes it possible to gain both in safety and in productivity.

As mentioned, the reduced speed V _r is advantageously between 0.2 and 1 m / min for most practical cases. Nevertheless, its determination preferably obeys the following criteria: the reduced speed of the healing cycle is substantially equal to the greater of the two values obtained by taking 70% of the cruising speed and a speed obtained relative to the useful length of the ingot mold at the length t _r of the healing plate. In other words, a speed V _r substantially equal to 70% of V _c is chosen if this is compatible with the possibility of scarring over the useful length L of the mold which extends between the second mold height and the outlet of the ingot mold. For example, an ingot mold with a total height of 0.90 meters, the second thermocouple height of which is 0.3 meters, has a useful length of 0.6 m.

For grade X, the time t _r given in FIG. 2 is 0.23 min and the speed 70% of V = gives a theoretical speed V _r of 1 min. Furthermore, we can calculate a maximum useful time of L / V _r = 0.6 / 1 = 0.6 min, greater than 0.23 which shows that the theoretical value of V _{r is} suitable.

On the other hand, for grade D, the value of t _r = 3.65 min is greater than the maximum useful time obtained with a speed V _r of 1 m / min. The authorized speed V _r is only V _r = L / t _r . = 0.6 / 3.6 = 0.15 m / min as used in figure 3. TABLE II

Grade X B D X '

V _c , m / min 1, 4 1.4 1.4 1.4

D, m / min ^a -26 -12 -68 -1.3 t _d , min (s) 0.015 (0.9) 0.033 (2.0) 0.02 (1.2) (0.9) V _r , m / min 1.0 1.0 0.15 0.1 t _r min 0.23 0.3 3.6 2.0

A, m / min ² 0.58 0.16 0.22 0.26 t _a , min 0.7 2.5 6.1 5.0 t _r + t _a , min 0.93 2.8 9.7 7.0

Claims

1. Method for dynamic control of the speed of extraction in a process of continuous casting of a steel, of the type according to which, upon detection of a bonding of skin in an ingot mold, the speed of extraction is imposed by a cycle consisting of a ramp from cruising speed to a reduced deceleration speed, a healing plateau and an acceleration ramp from reduced speed to cruising speed, characterized in that 'the ferritic potential of the cast steel is determined and at least the slope (D, A) of one of the two ramps is determined as a function of this ferritic potential.

2. Method according to claim 1, characterized in that the length (tr) of the healing plate is determined as a function of the difference between the liquidus temperature and the solidus temperature of the cast steel.

3. Method according to one of claims 1 or 2 characterized in that the ferritic potential of the low alloyed steels is chosen at a value in accordance with that which the formula would give:

PF = 2.5 (0.5 -% C _p )

where C _p is the equivalent of carbon at the peritectic reaction, calculated according to the formula:

% C _p =% C + 0.02% Mn + 0.04% Ni-0.1% Si

-0.04% Cr-0.1% Mθ.

and the ferritic potential of the stainless steels is chosen at a value corresponding to that which the formula would give:

% Cu

% Cr '=% Cr + 1.5% Si + 1.4% Mo + 3% Ti +

2% Nb

4. Method according to any one of claims 1 to 3, characterized in that one determines the length (t _r ) of the healing plateau and the slopes D and A of the deceleration and acceleration ramps substantially from the curves in Figure 2.

5. Method according to any one of claims 1 to 4, characterized in that the duration (t "0 deceleration is of the order of 0.5 to 30 s, the waiting time (tr) at reduced speed of the order of 30 to 300 s, and the duration (t _a ) of acceleration of the order of 60 to 600 s.

6. Method according to any one of claims 1 to 5, characterized in that the control of the extraction speed is carried out by a computer integrating the calculation of the speed as a function of the cast steel.

7. Method according to any one of claims 1 to 6, characterized in that the reduced speed of the healing cycle is substantially equal to the greater of the two values obtained by taking 70% of the cruising speed and a speed obtained by ratio of the useful length of the mold to the length t _r of the healing plate.