Rapid cooling of high yield strength sheet steel

Abstract

Method for reducing unevenness in a strip subjected to cooling by spraying of liquid, or a mixture of gas and liquid, along a cooling zone of a continuous heat treatment one, the cooling intensity being adjusted in the direction of travel of the strip so as to achieve a relative position between the Leidenfrost temperature and at least one temperature at which the metallurgical structure changes such that said cooling intensity minimizes the internal stresses of the strip, and device for implementing the method.

Claims

1. A method for reducing unevenness defects of a strip subjected to cooling by spraying a fluid along a cooling zone of a continuous heat treatment line, said cooling zone having means for adjusting a cooling intensity along the cooling zone, the method comprising: determining, before the strip is subjected to the cooling along the cooling zone, a thermal profile to be applied to the strip along the cooling zone to cause, at a first portion of the strip, substantial concomitance between (i) a predetermined critical strip temperature, called Leidenfrost temperature, and (ii) a first temperature at which a change in metallurgical structure of the strip is initiated; cooling, at a first cooling intensity, at a first point along the cooling zone, and according to the thermal profile, the first portion of the strip to the predetermined critical strip temperature; cooling, at a second cooling intensity, at a second point downstream of the first point along the cooling zone, and according to the thermal profile, the first portion of the strip to the first temperature; and adjusting the second cooling intensity at the second point, including adjusting one or more of a flow rate of the fluid, a flow pressure of the fluid, and an amount of time the fluid is sprayed onto the strip at the second point; wherein the thermal profile is determined by: a minimum cooling rate to achieve desired metallurgical structure changes; a metallurgical transformation temperature for achieving desired metallurgical structure changes; thermal critical points for achieving the Leidenfrost temperature; and conditions for initiating adjusting the thermal profile.

2. The method according to claim 1, further comprising adjusting the cooling intensity along the cooling zone according to a minimum cooling rate to achieve a selected metallurgical structure change.

3. The method according to claim 1, further comprising adjusting the cooling intensity along the cooling zone such that a change in metallurgical structure begins at a selected temperature.

4. The method according to claim 1, further comprising adjusting the cooling intensity along the cooling zone such that the Leidenfrost temperature is equal to a predetermined value.

5. The method according to claim 1, further comprising adjusting the cooling intensity along the cooling zone such that either (a) the Leidenfrost temperature is equal to a metallurgical structure change onset temperature, or (b) the Leidenfrost temperature is at a temperature between a temperature at the onset of a first metallurgical structure change and a temperature at the onset of a last metallurgical structure change.

6. The method according to claim 1, wherein the change in metallurgical structure comprises a phase transformation from austenite to martensite, and wherein the cooling intensity is adjusted such that the Leidenfrost temperature is within a temperature range of plus or minus 50 C. from the martensitic structure change onset temperature.

7. The method according to claim 1, wherein the cooling intensity is adjusted along the cooling zone so that the Leidenfrost temperature is at a temperature midway between a temperature at the onset of a first metallurgical structure change and a temperature at the onset of a last metallurgical structure change.

8. The method according to claim 1, the fluid comprises a mixture of gas and liquid.

9. The method according claim 1, wherein the fluid is non-oxidizing to the strip.

10. The method according claim 1, wherein the first cooling intensity is different than the second cooling intensity.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) Apart from the arrangements set out above, the invention consists of a certain number of other arrangements that will be more explicitly discussed below with regard to embodiments described with reference to the appended drawings, but which are in no way limiting. In these drawings:

(2) FIG. 1 is a schematic view in longitudinal section of the strip in the cooling section according to an embodiment of the invention.

(3) FIG. 2 is a graph of the evolution of the heat flux as a function of the surface temperature, representative of the liquid spray cooling method.

(4) FIG. 3 is a graph of the evolution of the thermal expansion coefficient as a function of temperature, according to a first embodiment.

(5) FIG. 4 is a graph of the evolution of the strip temperature in the direction of travel, for a uniform pressure distribution in the spray nozzles.

(6) FIG. 5 is a graph of the evolution of the heat exchange coefficient corresponding to the thermal profiles of FIG. 3.

(7) FIG. 6 is a graph of the evolution of the longitudinal stress at the edge of the strip, for the thermal profile corresponding to the thermal profile of FIG. 3

(8) FIG. 7 is a graph of the evolution of the temperature of the strip in the direction of travel, for a uniform pressure distribution in the spray nozzles and an optimized pressure distribution in the spray nozzles according to a first embodiment

(9) FIG. 8 is a graph of the evolution of the heat exchange coefficient as a function of the temperature of the strip corresponding to the thermal profiles of FIG. 6.

(10) FIG. 9 is a graph of the evolution of the longitudinal stress at the edge of the strip, for the longitudinal thermal profile corresponding to the thermal profiles of FIG. 6.

(11) FIG. 10 is a graph of the evolution of the thermal expansion coefficient as a function of temperature, according to a second embodiment.

(12) FIG. 11 is a graph of the evolution of the temperature of the strip in the direction of travel, for a uniform pressure distribution in the spray nozzles and an optimized pressure distribution in the spray nozzles according to a second embodiment.

(13) FIG. 12 is a graph of the evolution of the heat exchange coefficient as a function of the temperature of the strip corresponding to the thermal profiles of FIG. 9.

(14) FIG. 13 is a graph of the evolution of the longitudinal stress at the edge of the strip, for the longitudinal thermal profile corresponding to the thermal profiles of FIG. 9.

DETAILED DESCRIPTION OF THE INVENTION

(15) According to a first embodiment, the rapid cooling zone of a continuous treatment line metal strips illustrated in FIG. 1 is arranged to cool the strip (1) by spraying it with a liquid, or with a mixture of a gas and a liquid, by means of nozzles arranged on either side of the strip with respect to its travel plane, and comprises, in the direction of travel (F) of the strip, three rows (2) of mono-fluid or bi-fluid nozzles, followed by a row of mono-fluid or bi-fluid nozzles with a range of spray flow rates greater than or equal to the previous one, the rows of nozzles being arranged transversely to the travel plane of the strip.

(16) For carbon steel composed of 0.1% Carbon, 1% Manganese and 1% Silicon, annealed at a temperature above 850 C. for complete austenitization, cooled from 650 to 100 C. in a rapid cooling section by spraying with an average cooling slope of about 500 C./s, a single slope break may be observed at 45015 C. on the evolution curve of the thermal expansion coefficient illustrated schematically by curve D2 in the attached FIG. 3, corresponding to the total martensitic transformation of the austenitic phase.

(17) The minimum cooling rate for complete martensitic transformation, i.e. without transformation of austenite into another phase such as bainite or pearlite, may be determined from the transformation curves established for the composition of the steel, i.e. 200 C./s for the example considered.

(18) The thermal profile illustrated by the curve in the appended FIG. 4 is obtained in a cooling section comprising 4 rows of nozzles for spraying liquid with a constant flow rate and pressure over the entire length of the cooling section, to cool a strip from 650 to 100 C. with an average cooling rate of 480 C./s for a strip traveling at 70 m/min. The critical points are located at 550 C. represented by point A, corresponding to the Leidenfrost temperature, and at 450 C. represented by point B, corresponding to the temperature at the onset of martensitic transformation.

(19) Points A and B are also represented on the evolution curve of the heat exchange coefficient between the strip and the sprayed liquid, illustrated by the curve of the appended FIG. 5. The curve shows an increase in the heat exchange coefficient between the two critical points. Thus, at point B of martensitic transformation, a low initial thermal heterogeneity will be amplified by the increase in the heat exchange coefficient, thus leading to a significant risk of desynchronization of martensitic transformations and an associated heterogeneity in the distribution of internal stresses.

(20) The evolution of the longitudinal stress at the edge of the strip represented by curve C1 in the appended FIG. 6, without taking into account the metallurgical transformation at point B, shows a compressive stress peak at the onset of cooling to 650 C., and at the thermal slope break at 150 C. on the cooling curve, corresponding to the decrease in the heat exchange coefficient at the end of the Leidenfrost transition zone.

(21) Taking into account the metallurgical transformation at 450 C. by a disturbance of the thermal expansion coefficient at this point, illustrated by curve D2 of FIG. 3, results in a compressive stress peak at the martensitic transformation point and an increase of the tensile stress upstream of the martensitic transformation point as shown by curve C2 in the appended FIG. 6. This result highlights the opposing effects of the contraction of the strip during cooling, which causes tensile stresses, and the relative increase in volume that accompanies the metallurgical transformation, which causes compressive stress.

(22) From the identification of the critical points, a second thermal profile is proposed with the same average cooling rate equal to 480 C./s, illustrated by curve T2 in the appended FIG. 7, the critical point C corresponding to the concomitance of the Leidenfrost temperature and the martensitic transformation temperature at 450 C.

(23) This optimized thermal profile is obtained by a different adjustment of the 2 successive zones of the cooling section: a first zone with a length equal to % of the total length of the cooling section, for which the pressure is limited to 1 bar to delay the appearance of the Leidenfrost point, a second zone with a length equal to of the total length of the cooling section, for which the maximum pressure of 8 bar is applied.

(24) Points A, B and C are also represented on the evolution curve of the heat exchange coefficient between the strip and the sprayed liquid, illustrated by curves H1 and H2 of the appended FIG. 8, highlighting, on curve H2, a reduction in the amplitude of variation of the heat exchange coefficient for a small variation in temperature of the strip in the martensitic transformation zone, in comparison with the slope observed at point B on curve H1.

(25) Similarly, the evolution of the longitudinal stress at the edge of the strip represented by curve C2 in the appended FIG. 9 highlights a more favorable distribution of stresses along the strip, with an increase in the tensile stress upstream of the non-critical martensitic transformation point and a reduction in the compressive stress at the origin of buckling phenomena and risk of irreversible deformation, downstream of the martensitic transformation point.

(26) Thus, the concomitance of the Leidenfrost temperature and the martensitic transformation temperature illustrated by point C in the appended FIG. 8, which induce internal stresses of opposite signs, that is to say, a contraction at the Leidenfrost point and an expansion at the metallurgical transformation point, allows a reduction of the amplitude of the internal stresses and the risk of associated unevenness defects.

(27) Moreover, the position of the martensitic transformation point at a more favorable point of the evolution curve of the heat exchange coefficient reduces the risk of amplification of local coupled phenomena of thermal and metallurgical heterogeneity.

(28) According to a second embodiment, for carbon steel composed of 0.25% Carbon, 1% Manganese and 1% Silicon, annealed at a temperature above 850 C. for complete austenitization, cooled from 650 to 100 C. in a rapid cooling section by spraying with an average cooling rate of about 100 C./s for a strip moving at 15 m/min, 2 slope breaks may be observed on the evolution curve of the thermal expansion coefficient, a first break at 550 C. corresponding to the bainitic transformation and a second at 400 C. corresponding to the martensitic transformation of the residual austenite, illustrated schematically by the curve of the appended FIG. 10, corresponding to the total martensitic transformation of the austenitic phase.

(29) The thermal profile obtained for uniform spraying along the length of the product, illustrated by curve T1 in the appended FIG. 11, highlights 3 critical points, the Leidenfrost transition point represented by point A at 600 C., the first metallurgical transformation point represented by point B at 550 C. and the second metallurgical transformation point represented by point C at 400 C.

(30) The thermal profile obtained for an optimized adjustment of the cooling distribution according to: a first zone with a length equal to of the total length of the cooling section, for which the pressure is limited to 0.5 bar, a second zone with a length equal to of the total length of the cooling section, for which the maximum pressure of 8 bar is applied.

(31) Curve T2 in the appended FIG. 11 illustrates, with highlighting of 3 critical points, the first metallurgical transformation point represented by the point D shifted to 560 C. to take account of the reduction in cooling rate, the second metallurgical transformation point represented by point F shifted to 410 C. and the Leidenfrost transition point represented by point E at 500 C. at an intermediate temperature between the 2 metallurgical transformation points.

(32) Points A, B, C, D, E and F are also represented on the evolution curves of the heat exchange coefficient between the strip and the sprayed liquid, illustrated by curves H1 and H2 of the appended FIG. 12, corresponding respectively to the thermal profiles T1 and 12 of the attached FIG. 11, highlighting the amplitude of variation of the heat exchange coefficient for a small variation in temperature of the strip at the metallurgical transformation points.

(33) The comparison of the longitudinal stresses at the edge of the strip calculated under these assumptions, illustrated by curves C1 and 02 on the graph of the appended FIG. 13 corresponding respectively to the thermal profiles T1 and T2 of FIG. 11, highlights an optimization of the distribution of the stresses along the strip, with a reduction in the compressive stress, at the 2 metallurgical transformation points and downstream of the last transformation point.

(34) The invention is not limited to the examples that have just been described, and numerous modifications may be made to these examples without departing from the scope of the invention. In addition, the various features, forms, variants, and embodiments of the invention may be grouped together in various combinations as long as they are not incompatible or mutually exclusive.