CA2338743C - Method and installation for producing dual-phase steel - Google Patents
Method and installation for producing dual-phase steel Download PDFInfo
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- CA2338743C CA2338743C CA2338743A CA2338743A CA2338743C CA 2338743 C CA2338743 C CA 2338743C CA 2338743 A CA2338743 A CA 2338743A CA 2338743 A CA2338743 A CA 2338743A CA 2338743 C CA2338743 C CA 2338743C
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- cooling
- ferrite
- stage
- dual
- austenite
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- 238000000034 method Methods 0.000 title claims description 21
- 229910000885 Dual-phase steel Inorganic materials 0.000 title abstract description 5
- 238000009434 installation Methods 0.000 title description 2
- 238000001816 cooling Methods 0.000 claims abstract description 187
- 229910000859 α-Fe Inorganic materials 0.000 claims abstract description 56
- 229910000831 Steel Inorganic materials 0.000 claims abstract description 21
- 239000010959 steel Substances 0.000 claims abstract description 21
- 229910000734 martensite Inorganic materials 0.000 claims abstract description 20
- 238000004519 manufacturing process Methods 0.000 claims abstract description 14
- 238000005096 rolling process Methods 0.000 claims abstract description 6
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 claims description 23
- 229910001566 austenite Inorganic materials 0.000 claims description 22
- 230000015572 biosynthetic process Effects 0.000 claims description 12
- 238000006243 chemical reaction Methods 0.000 claims description 2
- 238000010583 slow cooling Methods 0.000 claims description 2
- 230000009466 transformation Effects 0.000 description 25
- 239000002826 coolant Substances 0.000 description 5
- 229910001562 pearlite Inorganic materials 0.000 description 3
- OKTJSMMVPCPJKN-UHFFFAOYSA-N Carbon Chemical compound [C] OKTJSMMVPCPJKN-UHFFFAOYSA-N 0.000 description 2
- RYGMFSIKBFXOCR-UHFFFAOYSA-N Copper Chemical compound [Cu] RYGMFSIKBFXOCR-UHFFFAOYSA-N 0.000 description 2
- 229910001563 bainite Inorganic materials 0.000 description 2
- 229910052799 carbon Inorganic materials 0.000 description 2
- 229910052802 copper Inorganic materials 0.000 description 2
- 239000010949 copper Substances 0.000 description 2
- 239000003923 scrap metal Substances 0.000 description 2
- 230000035508 accumulation Effects 0.000 description 1
- 238000009825 accumulation Methods 0.000 description 1
- 230000009286 beneficial effect Effects 0.000 description 1
- 238000009749 continuous casting Methods 0.000 description 1
- 230000003111 delayed effect Effects 0.000 description 1
- 238000011982 device technology Methods 0.000 description 1
- 238000005098 hot rolling Methods 0.000 description 1
- 239000000203 mixture Substances 0.000 description 1
- 239000002994 raw material Substances 0.000 description 1
- 238000000926 separation method Methods 0.000 description 1
- 238000001228 spectrum Methods 0.000 description 1
Classifications
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
- C21D1/02—Hardening articles or materials formed by forging or rolling, with no further heating beyond that required for the formation
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
- C21D1/18—Hardening; Quenching with or without subsequent tempering
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
- C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
- C21D8/0226—Hot rolling
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/001—Austenite
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/002—Bainite
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/005—Ferrite
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/008—Martensite
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- Chemical & Material Sciences (AREA)
- Engineering & Computer Science (AREA)
- Organic Chemistry (AREA)
- Thermal Sciences (AREA)
- Crystallography & Structural Chemistry (AREA)
- Mechanical Engineering (AREA)
- Materials Engineering (AREA)
- Metallurgy (AREA)
- Physics & Mathematics (AREA)
- Heat Treatment Of Strip Materials And Filament Materials (AREA)
- Metal Rolling (AREA)
- Heat Treatment Of Sheet Steel (AREA)
- Heat Treatment Of Steel (AREA)
- Heat Treatments In General, Especially Conveying And Cooling (AREA)
- Heat Treatment Of Articles (AREA)
- Control Of Heat Treatment Processes (AREA)
Abstract
The invention relates to the production of dual-phase steel (2) which is obtained by targeted cooling after the final deformation step in a rolling stand (1), whereby a dual-phase structure of between 70 and 90 % ferrite and between 30 and 10 % martensite is adjusted. So as to be independent of steel geometry and strip speed the invention provides for cooling to take the form of fluidized-bed cooling carried out in a cooling line (6) comprising water-cooling stages (7) arranged one behind the other.
Description
Method and Installation for Producing Dual-Phase Steel The invention relates to a method and a device for producing dual-phase steel with a two-phase microstructure of 70 to 90 % ferrite and 30 to 10 % martensite from the hot-rolled state by a controlled temperature guiding and defined cooling strategy during the coolirg of the steels, inter alia by means of water cooling after their finish rolling, wherein in a first cooling stage the cooling curve enters the ferrite region and, after reaching the required ferrite contents, further cooling to temperatures below the martensite starting temperature is carried out in a second cooling stage.
The targeted structural transformation by a corresponding cooling of the steels is known. For example, in DE 44 16 752 Al a method for generating hot wide strip is described in which, before the first transformation, between the continuous casting device and a compensation furnace, the surface temperature of the slab is reduced to a sufficient depth (at least 2 mm) so that a structural transformation from austenite to ferrite/pearlite is achieved. In this context, the cooling time is selected such that at least 70 %
austenite is transformed into ferrite/pearlite. A renewed transformation into austenite with new orientation of the austenite grain boundaries is carried out subsequently in the compensation furnace. In this way, it is to be achieved that even scrap metal of second quality, in particular, scrap metal with copper contents, can be used as a raw material without undesirable accumulations of copper on the grain boundaries of the primary austenite.
When manufacturing dual-phase steels, one takes also advantage of an occurring structural transformation by means of a targeted cooling, but now temporally after the transformation has occurred.
The adjustment of a dual-phase microstructure depends in this connection significantly on the cooling speeds made possible by the device techriology arid on the SLeel composition. Important for the manufacturc of dual-phasc steels is a sufficient ferrite formation in the first cooling stage.
With respect to device technology, a sufficient ferrite forrriaLiuli is achieved, for example, by cooling with water to a-~emperature of approximately 620 - 650 C with subsequent air cooling. The duration of air cooling (approximately 8 seconds) is selected such that at least 70 % of the austenite is transtormed into ferrite before Llie second cooling stage begins. A transformation into the pearlitc stagc should be avoided during the first cooling stage as well as during air cooling.
In the second cooling stage there must still be so much cooling capacity present that hasp temperatures below the martensi.te starting temperature are achieved. Only then the formation of a dual-phase microstructure with ferrite and martensite components is ensured. This known manufacture presents no problem For small strip speeds because sufficierit cuoling capacities for the martensite transformation are available at the end of the first cooling stage.
For very high strip speeds, however, the beginning of the second cooling stage can be displaced within the current cooling stretch to such an extent that the s~~hgequent martensite formation occurs only incompletely or not at all because thcn the cooling capacity for adjustment of the required low-temperature (< 2.20 C) is tlc longer sufficient. A mixed microstructure of ferrite, bainite and proportions of martensite will result that cannot fulfill the desired mechanical properties of a pure dual-phase microstructure.
From EP-A-0 747 495 a method for manufacture of hot-rolled steel sheet is known whose structure comprises at least '/5 % territe and at least 10 % martensite. For its manufacture, the steel is cooled in a targeted fashion after hot-roll-ing, in particular, in a first cooling stage with a cooling rate of 2 to 15 (7/s within a time period of 8 to 40 seconds to a temperature between Ar, point and 730 C and thereafter in a second cooling stage with a cooling rate of 20 to 150 per secoiid to a temperature of. 300 C. As an alternativc, a quick cooling with a cooling rate of 20 to 150 C/s is used before the f i rst cooling stage that leads to a temperature below the Ar; point.
Frum the printed publication Patent Abstracts of Japan vol. 006, No. 191(C-127), 30 September 1982, and JP 57 104650 A (Kobe Steel Ltd.), 29 June 1982, a method for manufacturing a hot-rolled steel sheet comprised of ferrite and a proportion of 1 to 30 % martensite is known which is also generated by a two-stage cooling. According to this method, cooling i's carried out slowly to a temperature between the Ar: point and 550 C at a cooling rate of 5 to 30 C/second and, subsequently, cooling is carried out with a fast cooling rate of > 30 C/s to a temperature in Lhe range of 350 to 500 C in a second cooling stage.
2a Based on this known prior art, it is an object of the invention to provide a method and a device for producing dual-phase steel wherein a fast and quantitatively sufficient structural transformation of the austenite into ferrite is possible even at high strip speeds.
The above object is solved according to the invention in that during the first cooling stage the cooling curve of the steels is adjusted with such a low cooling speed of 20 K/s to 30 K/s that the cooling curve enters the ferrite region with a temperature still so high that the ferrite formation can take place quickly and that already at least 70 % of the austenite has been transformed into ferrite before the beginning of the second cooling stage.
With the inventively slower cooling with a cooling speed lower than in known methods, the cooling curve enters the ferrite region temporally later but at a higher temperature than in the known methods, i.e., the transformation of the austenite into ferrite begins slightly delayed but at a higher temperature than in the known methods and it occurs also faster as a result of the higher temperature. It is especially beneficial when the ferrite region is reached as quickly as possible while at the same time the transformation temperature is high.
The targeted structural transformation by a corresponding cooling of the steels is known. For example, in DE 44 16 752 Al a method for generating hot wide strip is described in which, before the first transformation, between the continuous casting device and a compensation furnace, the surface temperature of the slab is reduced to a sufficient depth (at least 2 mm) so that a structural transformation from austenite to ferrite/pearlite is achieved. In this context, the cooling time is selected such that at least 70 %
austenite is transformed into ferrite/pearlite. A renewed transformation into austenite with new orientation of the austenite grain boundaries is carried out subsequently in the compensation furnace. In this way, it is to be achieved that even scrap metal of second quality, in particular, scrap metal with copper contents, can be used as a raw material without undesirable accumulations of copper on the grain boundaries of the primary austenite.
When manufacturing dual-phase steels, one takes also advantage of an occurring structural transformation by means of a targeted cooling, but now temporally after the transformation has occurred.
The adjustment of a dual-phase microstructure depends in this connection significantly on the cooling speeds made possible by the device techriology arid on the SLeel composition. Important for the manufacturc of dual-phasc steels is a sufficient ferrite formation in the first cooling stage.
With respect to device technology, a sufficient ferrite forrriaLiuli is achieved, for example, by cooling with water to a-~emperature of approximately 620 - 650 C with subsequent air cooling. The duration of air cooling (approximately 8 seconds) is selected such that at least 70 % of the austenite is transtormed into ferrite before Llie second cooling stage begins. A transformation into the pearlitc stagc should be avoided during the first cooling stage as well as during air cooling.
In the second cooling stage there must still be so much cooling capacity present that hasp temperatures below the martensi.te starting temperature are achieved. Only then the formation of a dual-phase microstructure with ferrite and martensite components is ensured. This known manufacture presents no problem For small strip speeds because sufficierit cuoling capacities for the martensite transformation are available at the end of the first cooling stage.
For very high strip speeds, however, the beginning of the second cooling stage can be displaced within the current cooling stretch to such an extent that the s~~hgequent martensite formation occurs only incompletely or not at all because thcn the cooling capacity for adjustment of the required low-temperature (< 2.20 C) is tlc longer sufficient. A mixed microstructure of ferrite, bainite and proportions of martensite will result that cannot fulfill the desired mechanical properties of a pure dual-phase microstructure.
From EP-A-0 747 495 a method for manufacture of hot-rolled steel sheet is known whose structure comprises at least '/5 % territe and at least 10 % martensite. For its manufacture, the steel is cooled in a targeted fashion after hot-roll-ing, in particular, in a first cooling stage with a cooling rate of 2 to 15 (7/s within a time period of 8 to 40 seconds to a temperature between Ar, point and 730 C and thereafter in a second cooling stage with a cooling rate of 20 to 150 per secoiid to a temperature of. 300 C. As an alternativc, a quick cooling with a cooling rate of 20 to 150 C/s is used before the f i rst cooling stage that leads to a temperature below the Ar; point.
Frum the printed publication Patent Abstracts of Japan vol. 006, No. 191(C-127), 30 September 1982, and JP 57 104650 A (Kobe Steel Ltd.), 29 June 1982, a method for manufacturing a hot-rolled steel sheet comprised of ferrite and a proportion of 1 to 30 % martensite is known which is also generated by a two-stage cooling. According to this method, cooling i's carried out slowly to a temperature between the Ar: point and 550 C at a cooling rate of 5 to 30 C/second and, subsequently, cooling is carried out with a fast cooling rate of > 30 C/s to a temperature in Lhe range of 350 to 500 C in a second cooling stage.
2a Based on this known prior art, it is an object of the invention to provide a method and a device for producing dual-phase steel wherein a fast and quantitatively sufficient structural transformation of the austenite into ferrite is possible even at high strip speeds.
The above object is solved according to the invention in that during the first cooling stage the cooling curve of the steels is adjusted with such a low cooling speed of 20 K/s to 30 K/s that the cooling curve enters the ferrite region with a temperature still so high that the ferrite formation can take place quickly and that already at least 70 % of the austenite has been transformed into ferrite before the beginning of the second cooling stage.
With the inventively slower cooling with a cooling speed lower than in known methods, the cooling curve enters the ferrite region temporally later but at a higher temperature than in the known methods, i.e., the transformation of the austenite into ferrite begins slightly delayed but at a higher temperature than in the known methods and it occurs also faster as a result of the higher temperature. It is especially beneficial when the ferrite region is reached as quickly as possible while at the same time the transformation temperature is high.
In comparison to the known methods, a degree of transformation of at least 70 ~ is reached so early that there is sufficient cooling capacity in a given cooling stretch for the subsequent martensite formation. This means that at the end of the first cooling stage a sufficiently large quantity of austenite has been transformed into ferrite so that the conventionally performed air cooling can be eliminated and the second cooling stage can follow immediately after the first cooling stage.
In order to perform the cooling with the desired low cooling speed, the principle of a dispersed cooling is applied according to the invention. This is a water cooling process in which water is applied to the goods to be cooled by water cooling stages arranged successively at a spacing. By adjusting the number of the water cooling stages, their spacing from one another, as well as the effective length of the water cooling stages, the cooling speed as well as the applied water quantity can be optimally adjusted to the goods to be cooled (the mass of the goods to be cooled and/or the surface of the goods to be cooled) . The cooling can also be realized by a cooling medium quantity that can be adjusted continuously.
As a result of the adjustment to the goods to be cooled, the dispersed cooling can be temporally expanded until the desired degree of transformation has been reached without there being the risk that, as in the known methods of fast cooling, the cooling curve leaves the ferrite region already beforehand as a result of cooling that is too intensive.
In order to perform the cooling with the desired low cooling speed, the principle of a dispersed cooling is applied according to the invention. This is a water cooling process in which water is applied to the goods to be cooled by water cooling stages arranged successively at a spacing. By adjusting the number of the water cooling stages, their spacing from one another, as well as the effective length of the water cooling stages, the cooling speed as well as the applied water quantity can be optimally adjusted to the goods to be cooled (the mass of the goods to be cooled and/or the surface of the goods to be cooled) . The cooling can also be realized by a cooling medium quantity that can be adjusted continuously.
As a result of the adjustment to the goods to be cooled, the dispersed cooling can be temporally expanded until the desired degree of transformation has been reached without there being the risk that, as in the known methods of fast cooling, the cooling curve leaves the ferrite region already beforehand as a result of cooling that is too intensive.
In comparison to cooling according to the prior art, by means of a dispersed cooling or cooling with a continuously adjustable cooling medium quantity, less water is applied until the transformation temperature is reached. This differential water quantity can now be applied during the transformation in order to force the carbon separation from the ferrite into the residual austenite and to thereby accelerate the ferrite formation. The residual austenite regions are enriched with carbon to such an extent that they transform into martensite already at cooling speeds of 20 - 30 K/s.
Since a defined holding period for the cooling in air is no longer needed in order to ensure a sufficient ferrite formation, the production of dual-phase steels can be realized on a portion of the cooling stretch. In this connection, the employed portion of the cooling stretch is very much shorter than in known methods with air cooling.
When the required microstructure components for dual-phase steels can be adjusted without air cooling, this results in significant advantages for the operator. Fewer device components are required for the production of dual-phase steels. At the same time, the production spectrum can be broadened in comparison to the prior art with changed process and strip parameters (for example, higher strip speed).
A device for performing the method of the invention is characterized by a cooling stretch arranged behind the last finishing roll stand and comprised of several water cooling stages positioned successively at a spacing or cooling systems with a continuously adjustable cooling medium quantity. The number of water cooling stages, their effective length and their spacing from one another are changeable according to the invention so that this cooling stretch can be adapted in a simple way to changing geometries of the goods to be cooled as well as to different strip speeds.
In one aspect, the present invention resides in a method for producing dual-phase steels from the hot-rolled state with a two-phase microstructure of 70 to 90 % ferrite and 30 to 10 %
martensite by a controlled temperature guiding and defined cooling strategy during the cooling of the steels, inter alia by means of water cooling after their finish rolling, wherein in a first cooling stage at a cooling rate of < 30 K/s the cooling curve enters the ferrite region and, after reaching the required ferrite contents, further cooling is carried out in a second cooling stage at a cooling rate of > 30 K/s to temperatures below the martensite starting temperature, characterized in that a) the first cooling stage (14) is carried out in a cooling stretch of water cooling stages (7), arranged successively at a spacing, or in a cooling system with continuously changeable cooling medium quantity with a cooling rate of 30 K/s adjusted such b) that the cooling curve (10) enters the ferrite region a temperature still so high that the ferrite formation can take place quickly; and c) before begin of the second cooling stage (16), which follows without intermediate air cooling and holding time directly after the first cooling stage (14), already at least 70 % of the austenite is transformed to ferrite.
In another aspect, the present invention resides in a method for the manufacture of dual-phase steels from the hot-rolled state with a two-phase microstructure of 70 to 90 % ferrite and 30 % to 10 %
martensite by controlled temperature application and defined cooling strategy during the cooling of the steels, inter alia by means of water cooling after finish rolling whereby, in a first cooling stage with slow cooling rate, the cooling curve runs into the ferrite sector, and in a second cooling stage at a higher cooling rate is further cooled to temperatures below the martensite start temperature, wherein the first cooling stage (14) is carried out in an adjusted manner in a cooling stretch consisting of water cooling stages (7) arranged at intervals behind one another, with a cooling rate of 20-30 K/s, such that a) the cooling curve (10) runs into the ferrite sector still at such a high temperature that the ferrite formation can take place rapidly; and b) before the start of the second cooling stage (16), which follows immediately from the first cooling stage (14), without intermediate air cooling and dwell time, already at least 70 % of the austenite is converted into ferrite, and, during the conversion of the austenite into ferrite, the cooling of the first cooling stage is continued until the desired ferrite content of at least 70% is attained.
Further advantages, details, and features of the invention result from the following description of an embodiment schematically illustrated in the drawings.
It is shown in:
Fig. 1 a schematic illustration of the fast cooling and the dispersed cooling as well as their arrangement in a mill train;
Fig. 2 a time-temperature-transformation curve;
Fig. 3 the degree of austenite transformation for fast transformation;
Fig. 4 the degree of austenite transformation for dispersed cooling.
In Fig. 1 the end of a mill train is schematically illustrated. It is comprised of the last finish roll stand (1), the rolling stock or goods to be cooled (2), and a hasp (3) with deflection rolls or drivers (4) . Above this part of a mill train two different cooling stretches are shown. With the cooling stretch (5) according to the prior art an early, fast cooling of the goods to be cooled (2) is 6a realized by a continuous water supply. In the cooling stretch (6) according to the invention water cooling stages (7) are arranged successively at a spacing so that the cooling is "dispersed".
The different transformation results caused by the differenr cooling methods (5, 6) are represented in an exemplary fashion in the following schematic illustrations.
In Fig. 2, a time-temperature-transformation curve of the course of the cooling curve (9) for cooling according to known methods a:.d the cooling curve (10) for a dispersed cooling are illustrated, wherein on the abscissa the time (Z) in seconds and on the ordinate the temperature (T) in C are indicated.
The cooling curve (9) shows the cooling course for the strategy conventionally employed nowadays (early, fast cooling to a certain holding temperature with subsequent air cooling, followed by further cooling to lower temperatures below the martensite starting temperature). The first cooling stage (11) of the cooling curve reaches relatively early the transformation region for the ferrite formation (F = ferrite region) at the point (8) and also remains in this region (F) for a relatively long time as a result of the holding time (12) with air cooling before a further cooling to a temperature below the martensite starting temperature (M =
martensite, B = bainite, P = pearlite) takes place by means of the second cooling stage (13) starting at the point (17).
In contrast, with the dispersed cooling the cooling curve (10) with its first cooling stage (14) reaches the ferrite region (F) at the point (15) later in comparison to the cooling curve (9) Since after reaching the ferrite region (F) the dispersed cooling is initially maintained, no time-consuming waiting period with air cooling is required, and the cooling curve (10) leaves the ferrite region (F) earlier.
The dispersed cooling is maintained within the ferrite region (F) until the desired degree of transformation has been reached. The further cooling by means of the second cooling stage (16) is carried out directly thereafter.
The austenite transformation rates which can be achieved with the described different cooling strategies, i.e., the known fast cooling and the dispersed cooling, can be seen in the two next illustrations of Figs. 3 and 4. The cooling time (Z) in seconds and the degree of transformation (U) of the austenite transformation into ferrite are illustrated on the abscissa and on the ordinate, respectively.
In the fast cooling (Fig. 3), during the first cooling stage (11) of the cooling curve (9) first a strong ferrite formation up to approximately 53 % takes place which then increases during the following air cooling (12) to approximately 62 %. However, this is not sufficient for the production of dual-phase steels.
In contrast thereto, with the dispersed cooling (Fig. 4) according to cooling curve (10) a considerably higher ferrite contents has already been formed in the first cooling stage (14) in the same time period and approximately 82 % austenite has already been transformed into ferrite before the second cooling stage (16) occurs (the dual-phase steels produced nowadays have generally a contents of > 80 % ferrite).
The invention is not limited to the exemplary cooling curves described in the illustrations; other cooling curves as, for example, in cooling systems with a continuously changing cooling medium quantity are possible which, in keeping with the invention, result in higher transformation temperatures. Also, the invention is not limited to water cooling; other cooiing systems can also be employed which lead to an earlier reaching of the ferrite region at high temperatures.
Since a defined holding period for the cooling in air is no longer needed in order to ensure a sufficient ferrite formation, the production of dual-phase steels can be realized on a portion of the cooling stretch. In this connection, the employed portion of the cooling stretch is very much shorter than in known methods with air cooling.
When the required microstructure components for dual-phase steels can be adjusted without air cooling, this results in significant advantages for the operator. Fewer device components are required for the production of dual-phase steels. At the same time, the production spectrum can be broadened in comparison to the prior art with changed process and strip parameters (for example, higher strip speed).
A device for performing the method of the invention is characterized by a cooling stretch arranged behind the last finishing roll stand and comprised of several water cooling stages positioned successively at a spacing or cooling systems with a continuously adjustable cooling medium quantity. The number of water cooling stages, their effective length and their spacing from one another are changeable according to the invention so that this cooling stretch can be adapted in a simple way to changing geometries of the goods to be cooled as well as to different strip speeds.
In one aspect, the present invention resides in a method for producing dual-phase steels from the hot-rolled state with a two-phase microstructure of 70 to 90 % ferrite and 30 to 10 %
martensite by a controlled temperature guiding and defined cooling strategy during the cooling of the steels, inter alia by means of water cooling after their finish rolling, wherein in a first cooling stage at a cooling rate of < 30 K/s the cooling curve enters the ferrite region and, after reaching the required ferrite contents, further cooling is carried out in a second cooling stage at a cooling rate of > 30 K/s to temperatures below the martensite starting temperature, characterized in that a) the first cooling stage (14) is carried out in a cooling stretch of water cooling stages (7), arranged successively at a spacing, or in a cooling system with continuously changeable cooling medium quantity with a cooling rate of 30 K/s adjusted such b) that the cooling curve (10) enters the ferrite region a temperature still so high that the ferrite formation can take place quickly; and c) before begin of the second cooling stage (16), which follows without intermediate air cooling and holding time directly after the first cooling stage (14), already at least 70 % of the austenite is transformed to ferrite.
In another aspect, the present invention resides in a method for the manufacture of dual-phase steels from the hot-rolled state with a two-phase microstructure of 70 to 90 % ferrite and 30 % to 10 %
martensite by controlled temperature application and defined cooling strategy during the cooling of the steels, inter alia by means of water cooling after finish rolling whereby, in a first cooling stage with slow cooling rate, the cooling curve runs into the ferrite sector, and in a second cooling stage at a higher cooling rate is further cooled to temperatures below the martensite start temperature, wherein the first cooling stage (14) is carried out in an adjusted manner in a cooling stretch consisting of water cooling stages (7) arranged at intervals behind one another, with a cooling rate of 20-30 K/s, such that a) the cooling curve (10) runs into the ferrite sector still at such a high temperature that the ferrite formation can take place rapidly; and b) before the start of the second cooling stage (16), which follows immediately from the first cooling stage (14), without intermediate air cooling and dwell time, already at least 70 % of the austenite is converted into ferrite, and, during the conversion of the austenite into ferrite, the cooling of the first cooling stage is continued until the desired ferrite content of at least 70% is attained.
Further advantages, details, and features of the invention result from the following description of an embodiment schematically illustrated in the drawings.
It is shown in:
Fig. 1 a schematic illustration of the fast cooling and the dispersed cooling as well as their arrangement in a mill train;
Fig. 2 a time-temperature-transformation curve;
Fig. 3 the degree of austenite transformation for fast transformation;
Fig. 4 the degree of austenite transformation for dispersed cooling.
In Fig. 1 the end of a mill train is schematically illustrated. It is comprised of the last finish roll stand (1), the rolling stock or goods to be cooled (2), and a hasp (3) with deflection rolls or drivers (4) . Above this part of a mill train two different cooling stretches are shown. With the cooling stretch (5) according to the prior art an early, fast cooling of the goods to be cooled (2) is 6a realized by a continuous water supply. In the cooling stretch (6) according to the invention water cooling stages (7) are arranged successively at a spacing so that the cooling is "dispersed".
The different transformation results caused by the differenr cooling methods (5, 6) are represented in an exemplary fashion in the following schematic illustrations.
In Fig. 2, a time-temperature-transformation curve of the course of the cooling curve (9) for cooling according to known methods a:.d the cooling curve (10) for a dispersed cooling are illustrated, wherein on the abscissa the time (Z) in seconds and on the ordinate the temperature (T) in C are indicated.
The cooling curve (9) shows the cooling course for the strategy conventionally employed nowadays (early, fast cooling to a certain holding temperature with subsequent air cooling, followed by further cooling to lower temperatures below the martensite starting temperature). The first cooling stage (11) of the cooling curve reaches relatively early the transformation region for the ferrite formation (F = ferrite region) at the point (8) and also remains in this region (F) for a relatively long time as a result of the holding time (12) with air cooling before a further cooling to a temperature below the martensite starting temperature (M =
martensite, B = bainite, P = pearlite) takes place by means of the second cooling stage (13) starting at the point (17).
In contrast, with the dispersed cooling the cooling curve (10) with its first cooling stage (14) reaches the ferrite region (F) at the point (15) later in comparison to the cooling curve (9) Since after reaching the ferrite region (F) the dispersed cooling is initially maintained, no time-consuming waiting period with air cooling is required, and the cooling curve (10) leaves the ferrite region (F) earlier.
The dispersed cooling is maintained within the ferrite region (F) until the desired degree of transformation has been reached. The further cooling by means of the second cooling stage (16) is carried out directly thereafter.
The austenite transformation rates which can be achieved with the described different cooling strategies, i.e., the known fast cooling and the dispersed cooling, can be seen in the two next illustrations of Figs. 3 and 4. The cooling time (Z) in seconds and the degree of transformation (U) of the austenite transformation into ferrite are illustrated on the abscissa and on the ordinate, respectively.
In the fast cooling (Fig. 3), during the first cooling stage (11) of the cooling curve (9) first a strong ferrite formation up to approximately 53 % takes place which then increases during the following air cooling (12) to approximately 62 %. However, this is not sufficient for the production of dual-phase steels.
In contrast thereto, with the dispersed cooling (Fig. 4) according to cooling curve (10) a considerably higher ferrite contents has already been formed in the first cooling stage (14) in the same time period and approximately 82 % austenite has already been transformed into ferrite before the second cooling stage (16) occurs (the dual-phase steels produced nowadays have generally a contents of > 80 % ferrite).
The invention is not limited to the exemplary cooling curves described in the illustrations; other cooling curves as, for example, in cooling systems with a continuously changing cooling medium quantity are possible which, in keeping with the invention, result in higher transformation temperatures. Also, the invention is not limited to water cooling; other cooiing systems can also be employed which lead to an earlier reaching of the ferrite region at high temperatures.
Claims (3)
1. A method for the manufacture of dual-phase steels from the hot-rolled state with a two-phase microstructure of 70 to 90 % ferrite and 30 % to 10 % martensite by controlled temperature application and defined cooling strategy during the cooling of the steels, inter alia by means of water cooling after finish rolling whereby, in a first cooling stage with slow cooling rate, the cooling curve runs into the ferrite sector, and in a second cooling stage at a higher cooling rate is further cooled to temperatures below the martensite start temperature, wherein the first cooling stage (14) is carried out in an adjusted manner in a cooling stretch consisting of water cooling stages (7) arranged at intervals behind one another, with a cooling rate of 20-30 K/s, such that a) the cooling curve (10) runs into the ferrite sector still at such a high temperature that the ferrite formation can take place rapidly; and b) before the start of the second cooling stage (16), which follows immediately from the first cooling stage (14), without intermediate air cooling and dwell time, already at least 70 % of the austenite is converted into ferrite, and, during the conversion of the austenite into ferrite, the cooling of the first cooling stage is continued until the desired ferrite content of at least 70% is attained.
2. A system for the manufacture of dual-phase steels from the hot-rolled state for the performance of the method according to claim 1, wherein the cooling stretch (6) is arranged behind the last stand of finishing rollers (1), with the several water cooling stages (7) arranged at intervals behind one another.
3. The system according to claim 2, wherein the number of the water cooling stages (7), their effective length, and the distance between them are changeable or infinitely adjustable with a volume regulation device.
Applications Claiming Priority (3)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| DE19833321A DE19833321A1 (en) | 1998-07-24 | 1998-07-24 | Method and installation to produce dual phase steels out of hot-rolled strip, with cooling rate at first cooling stage set sufficiently low to obtain temperature which is sufficiently high for rapid transformation of austenite into ferrite |
| DE19833321.8 | 1998-07-24 | ||
| PCT/EP1999/005113 WO2000005422A1 (en) | 1998-07-24 | 1999-07-17 | Method and installation for producing dual-phase steel |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| CA2338743A1 CA2338743A1 (en) | 2000-02-03 |
| CA2338743C true CA2338743C (en) | 2010-03-30 |
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ID=7875154
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA2338743A Expired - Lifetime CA2338743C (en) | 1998-07-24 | 1999-07-17 | Method and installation for producing dual-phase steel |
Country Status (11)
| Country | Link |
|---|---|
| EP (1) | EP1108072B1 (en) |
| JP (1) | JP2002521562A (en) |
| KR (1) | KR100578823B1 (en) |
| CN (1) | CN1173048C (en) |
| AT (1) | ATE224959T1 (en) |
| BR (1) | BR9912310A (en) |
| CA (1) | CA2338743C (en) |
| DE (2) | DE19833321A1 (en) |
| MY (1) | MY124339A (en) |
| RU (1) | RU2225453C2 (en) |
| WO (1) | WO2000005422A1 (en) |
Families Citing this family (10)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| BE1013359A3 (en) * | 2000-03-22 | 2001-12-04 | Centre Rech Metallurgique | Method for manufacturing a multi-band steel hot rolled. |
| NL1016042C2 (en) * | 2000-08-29 | 2001-07-24 | Corus Technology B V | Hot rolled dual phase steel band for, e.g., automotive parts contains vanadium in place of chromium |
| AUPR048000A0 (en) * | 2000-09-29 | 2000-10-26 | Bhp Steel (Jla) Pty Limited | A method of producing steel |
| KR100516519B1 (en) * | 2001-12-26 | 2005-09-26 | 주식회사 포스코 | A method for manufacturing the dual phase carbon steel wire by using controlled rolling and rapid cooling |
| KR100521596B1 (en) * | 2002-11-20 | 2005-10-12 | 현대자동차주식회사 | Processing method of automotive parts by direct resistance heating |
| DE10327383C5 (en) * | 2003-06-18 | 2013-10-17 | Aceria Compacta De Bizkaia S.A. | Plant for the production of hot strip with dual phase structure |
| DE102006001198A1 (en) * | 2006-01-10 | 2007-07-12 | Sms Demag Ag | Method and device for setting specific property combinations in multiphase steels |
| CN104001742A (en) * | 2014-05-21 | 2014-08-27 | 中冶南方工程技术有限公司 | Method for achieving controlled cooling on rolled pieces between and after bar finishing mill units |
| CN104384207A (en) * | 2014-10-22 | 2015-03-04 | 南京钢铁股份有限公司 | Cooling control method for hot rolling bearing steel bar and process arrangement of hot rolling bearing steel bar |
| KR102440768B1 (en) * | 2020-12-18 | 2022-09-08 | 주식회사 포스코 | Apparatus and method for manufacturing thick steel plate |
Family Cites Families (13)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JPS5818970B2 (en) * | 1978-08-31 | 1983-04-15 | 川崎製鉄株式会社 | Method for manufacturing high-strength thin steel sheets with excellent cold workability |
| US4388122A (en) * | 1980-08-11 | 1983-06-14 | Kabushiki Kaisha Kobe Seiko Sho | Method of making high strength hot rolled steel sheet having excellent flash butt weldability, fatigue characteristic and formability |
| JPS57104650A (en) * | 1980-12-19 | 1982-06-29 | Kobe Steel Ltd | High-strength hot-rolled steel plate and its manufacture |
| JPS57137426A (en) * | 1981-02-20 | 1982-08-25 | Kawasaki Steel Corp | Production of low yield ratio, high tensile hot rolled steel plate by mixed structure |
| JPS57137452A (en) * | 1981-02-20 | 1982-08-25 | Kawasaki Steel Corp | Hot rolled high tensile steel plate having composite structure and its manufacture |
| JPS61170518A (en) * | 1985-01-25 | 1986-08-01 | Kobe Steel Ltd | Production of high-strength hot rolled steel sheet having excellent formability |
| JPS6293006A (en) * | 1985-10-18 | 1987-04-28 | Kobe Steel Ltd | Production of high strength hot rolled steel sheet |
| IT1244295B (en) * | 1990-07-09 | 1994-07-08 | Giovanni Arvedi | PROCESS AND PLANT FOR THE OBTAINING OF WRAPPED STEEL BELTS, WITH CHARACTERISTICS OF COLD ROLLED PRODUCTS OBTAINED DIRECTLY IN HOT ROLLING LINE |
| JP3039842B2 (en) * | 1994-12-26 | 2000-05-08 | 川崎製鉄株式会社 | Hot-rolled and cold-rolled steel sheets for automobiles having excellent impact resistance and methods for producing them |
| RU2075865C1 (en) * | 1995-02-13 | 1997-03-20 | АО "Магнитогорский металлургический комбинат" | METHOD FOR PRODUCING RENT FROM CARBON STEEL |
| FR2735148B1 (en) * | 1995-06-08 | 1997-07-11 | Lorraine Laminage | HIGH-STRENGTH, HIGH-STRENGTH HOT-ROLLED STEEL SHEET CONTAINING NIOBIUM, AND METHODS OF MAKING SAME. |
| KR100301994B1 (en) * | 1996-12-24 | 2001-11-22 | 이구택 | Uniform Cooling Method of Hot Rolled Wire |
| KR100747495B1 (en) * | 2006-11-10 | 2007-08-08 | 삼성전자주식회사 | Variable capacity rotary compressor |
-
1998
- 1998-07-24 DE DE19833321A patent/DE19833321A1/en not_active Withdrawn
-
1999
- 1999-07-07 MY MYPI99002855A patent/MY124339A/en unknown
- 1999-07-17 JP JP2000561368A patent/JP2002521562A/en active Pending
- 1999-07-17 EP EP99938282A patent/EP1108072B1/en not_active Expired - Lifetime
- 1999-07-17 AT AT99938282T patent/ATE224959T1/en not_active IP Right Cessation
- 1999-07-17 KR KR1020017000810A patent/KR100578823B1/en not_active Expired - Lifetime
- 1999-07-17 WO PCT/EP1999/005113 patent/WO2000005422A1/en not_active Ceased
- 1999-07-17 BR BR9912310-0A patent/BR9912310A/en not_active IP Right Cessation
- 1999-07-17 DE DE59902877T patent/DE59902877D1/en not_active Expired - Lifetime
- 1999-07-17 RU RU2001105194/02A patent/RU2225453C2/en active
- 1999-07-17 CA CA2338743A patent/CA2338743C/en not_active Expired - Lifetime
- 1999-07-17 CN CNB998091405A patent/CN1173048C/en not_active Expired - Lifetime
Also Published As
| Publication number | Publication date |
|---|---|
| MY124339A (en) | 2006-06-30 |
| WO2000005422A1 (en) | 2000-02-03 |
| ATE224959T1 (en) | 2002-10-15 |
| DE19833321A1 (en) | 2000-01-27 |
| DE59902877D1 (en) | 2002-10-31 |
| KR100578823B1 (en) | 2006-05-11 |
| EP1108072B1 (en) | 2002-09-25 |
| CN1311826A (en) | 2001-09-05 |
| BR9912310A (en) | 2004-08-31 |
| CA2338743A1 (en) | 2000-02-03 |
| CN1173048C (en) | 2004-10-27 |
| RU2225453C2 (en) | 2004-03-10 |
| EP1108072A1 (en) | 2001-06-20 |
| JP2002521562A (en) | 2002-07-16 |
| KR20010071978A (en) | 2001-07-31 |
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