CA2242728A1 - Process for the hot rolling of steel bands - Google Patents
Process for the hot rolling of steel bands Download PDFInfo
- Publication number
- CA2242728A1 CA2242728A1 CA002242728A CA2242728A CA2242728A1 CA 2242728 A1 CA2242728 A1 CA 2242728A1 CA 002242728 A CA002242728 A CA 002242728A CA 2242728 A CA2242728 A CA 2242728A CA 2242728 A1 CA2242728 A1 CA 2242728A1
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- Prior art keywords
- strip
- temperature
- ferrite
- steel
- rolling
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- Abandoned
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Classifications
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
- B21B—ROLLING OF METAL
- B21B1/00—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations
- B21B1/22—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations for rolling plates, strips, bands or sheets of indefinite length
- B21B1/24—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations for rolling plates, strips, bands or sheets of indefinite length in a continuous or semi-continuous process
- B21B1/26—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations for rolling plates, strips, bands or sheets of indefinite length in a continuous or semi-continuous process by hot-rolling, e.g. Steckel hot mill
-
- 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
- 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
- C21D8/0294—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips involving a localised treatment
-
- 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
- C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
- C21D8/0421—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the working steps
- C21D8/0426—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
- C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
- C21D9/52—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for wires; for strips ; for rods of unlimited length
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
- B21B—ROLLING OF METAL
- B21B1/00—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations
- B21B1/46—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations for rolling metal immediately subsequent to continuous casting
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
- B21B—ROLLING OF METAL
- B21B2201/00—Special rolling modes
- B21B2201/04—Ferritic 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
- C21D2211/005—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
- C21D2221/00—Treating localised areas of an article
- C21D2221/02—Edge parts
-
- 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
- C21D8/0231—Warm rolling
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- Chemical & Material Sciences (AREA)
- Engineering & Computer Science (AREA)
- Mechanical Engineering (AREA)
- Physics & Mathematics (AREA)
- Thermal Sciences (AREA)
- Crystallography & Structural Chemistry (AREA)
- Materials Engineering (AREA)
- Metallurgy (AREA)
- Organic Chemistry (AREA)
- Metal Rolling (AREA)
- Heat Treatment Of Steel (AREA)
- Heat Treatment Of Sheet Steel (AREA)
Abstract
The invention relates to a process for the hot rolling of bands from unalloyed or low-alloy steel, in particular extra low carbon, ultra low carbon and interstitial free steel, in a multiple-stand finishing group, starting at a temperature in the steel austenite range and ending at a temperature in the steel ferrite range. Structural transformation of the steel is completed by cooling of the band between stands of the finishing group. The invention is characterised in that the band is rolled in the austenitic phase to an intermediate thickness ranging from 2 to 12 mm, is cooled for ferrite transformation in a single cooling stage, and subsequently rolled to size in the ferritic phase.
Description
CA 02242728 l998-07-lO
A PROCESS FOR HOT ROLLING STEEL STRIP
The invention relates to a process for hot rolling steel strip made of unalloyed or low-alloy steel, in particular extra low carbon, ultra low carbon and interstitial free steel, in a multiple-stand finishing group, starting at a temperature in the steel austenite range and ending at a temperature in the steel ferrite range; with the transformation of the steel's microstructure being completed by cooling the strip between stands of the finishing group.
There are basically three different processes for hot-rolling steel strip from unalloyed or low-alloy steel, principally ELC steels (Extra Low Carbon with 0.02 -0.04% C), ~LC steels (Ultra Low Carbon with < 0.005% C) and IF steels (Interstitial Free through the addition of Nb and/or Ti). One approach is the finish-roll-ng of steel strip in the austenite range. In this, both the starting and finishing temperature of rolling is in the austenite range of the respective steel, i.e. n the temperature range above its A3 point depending on the steel composition, as a rule above 800 ~C. During hot rolling of strip exclusively in the austenite range, temperature control with thin strip-end thickness below 2 mm is extraordinarily difficult. In addition, with increasing thickness reduction of the strip, there is the danger of uncontrolled transformation of austenite into ferrite.
The second approach is hot rolling steel strip in the finishing group exclusively in the ferrite range, called "ferrite rolling". Ferrite rolling takes place with both start and end of rolling being in the ferrite range of the steel, i.e. at a temperature below the A3 point of the steel. It is mainly used with ELC, ~LC and IF steels.
~rom the point of view of process technology, the main advantage of ferrite rolling is in the significantly reduced rolling force, in spite of lower temperature, due to the lower deformation resistance of the ferrite in comparison to austenite. This effect is utilised in practice to increase the dimensional range of the strip in regard to increased width and reduced thickness and at the same time to increase the productivity of the plant.
In addition, product-specific advantages are expected both during processing of hot strip and in the case of the hot strip for further processing either by cold rolling or by surface refinement. The positive possibilities of hitherto known hot-strip rolling in the ferrite range are limited. The advantageous low deformation resistance of ferrite, when compared to austenite, is limited to a narrow temperature range of mostly only about 100 to 150 ~C. The advantages are greatest immediately following complete phase transformation of austenite into ferrite. The advantages decrease as the temperature in the ferrite range drops.
The thinner the article to be rolled, the larger the disadvantageous temperature losses become, and the more difficult it becomes to prevent them. In addition, the advantage arising from reduced deformation resistance is limited by the solidification of the ferrite which increases in line with the degree of deformation. During continuous rolling, the increase in strength resulting from deformation cannot be reduced or can orly be reduced slightly. There is a further limitatior, for ferrite rolling on conventional finishing groups in that, in the case of decreased rolling temperature and increased overall-thickness reduction, the material to be rolled has unfavourable texture components which either cannot be corrected at all, or only by expensive intervention with the friction relationship between the roll and the material to be rolled.
The third approach is hot rolling taking place partly in the austenite range and partly in the ferrite range. In this, the hot strip enters the finishing group at a temperature in the austenite range of the steel.
Subsequently, during hot rolling in the finishing group, transformation of the austenitic microstructure into the ferritic microstructure takes place, and the strip is rolled to final dimensions at a tem?erature in the ferrite range of the steel. Su h a process was already described in E~ 0 504 999 A2. In this, the continuous-cast strip with a thickness of e.g. 60 mm is hot-rolled in a rolling stand to 20 mm thickness of the intermediate strip. Either immediately therea~ter or after re-heating, the intermediate strip is rolled, in a multi-stand finishing group at a temperature in the austenlte range of the steel, to a thickness of 1.5 mm which is close to its final dimensions. Subsequently, the strip is cooled in a cooling section to effect a change in the microstructure from austenite to ferrite, and in a further stand it is finish-rolled to a final thickness of 0.7 mm. This known process of austenite-ferrite-hot-rolling has the disadvantage in that the change in microstructure from austenite to ferrite can happen in an uncontrollable way during rolling in the finishing group, i.e. already prior to entry into the cooling section in which it is meant to happen. On the cne hand this leads to a significant drop in rolling fo~ce which is undesirable because it is uncontrollable. On the other hand a heterogeneous condition of transformation, either temporarily or permanently across the width and thickness of the intermediate strip during finish-rolling, has a negative effect both on the strip shape and the mechanical technological properties of the hot strip. For this reason, on continuous or semi-continuous hot-strip lines, most of the time hot rolling is only carried out in the ferrite range, with the austenite-ferrite transformation occurring prior to entry into the multi-stand finishing group.
It is the object of the inventlon to improve the generic process in such a way as to prevent the occurrence of the above-mentioned limitations or disadvantages during rolling at first in the austenite range and subsequently in the ferrite range. In other words, a controlled transformation from austenite to ferrite is to be ensured as part of the finish-rolling, maintaining the advantages of rolling in the austenite range and subsequently in the ferrite range.
In the generic process according o the invention, this object is solved in that the strip is rolled in the austenitic phase in the finishing group to an intermediate thickness ranging from 2 to 12 mm and subsequently, for ferrite transformation, cooled in a single cooling step whereupon it is further rolled to its final dimensions in the ferritic phase in the finishing group.
The invention meets the object, for during austenite rolling to an intermediate thickness ranging from 2 to 12 mm there is no danger of uncontrolled transformation of austenite into ferrite with the associated disadvantages, as mentioned, during rolling. The subsequent ferrite rolling according to the inventionr which takes place in at least two steps after the transformation in the microstructure in a cooling section between stands of the finishing group, takes advantage of the improved deformability of the steel strip in the ferrite range without inevitably resulting in undesirable textures or hardening. Thus the invention provides an advantageous division of thickness reduction in the austenite range on the one hand and subsequently in the ferrite range on the other hand.
According to a preferred embodiment of the process according to the invention, the strip lS rolled in the austenitic phase to an intermediate thickness ranging from 3 to 8 mm and finish-rolled in the ferritic phase to a final thickness of less than 1.5 mm.
According to a further preferred embodiment of the process according to the invention, the strip pre-rolled in the austenitic phase is rolled in the ferritic phase to its final dimensions in at least 2 passes.
According to a further preferred embodiment of the process according to the invention, the strip is cooled at a rate exceeding 30 ~C/s for ferrite transformation.
This ensures intensive cooling of the steel strip resulting in a complete phase transformatior from austenite to ferrite within a short ti~e.
According to the invention, the claimed process further provides for the strip, following passage through the cooling section for ferrite transformation, to pass an equalisation section of a few metres in which temperature equalisation between core temperature and surface temperature and complete microstructure transformation take place before it enters the first stand for ferrite rolling. In this equalisation section, there is sufficient time for completion of the phase transformation across the cross section of the steel strip. This is especially expedient for larger strip thicknesses.
A significant point of the invention is that the first section of finish rolling takes place in the austenite range and that the brief cooling necessary for phase transformation of the continuous-cast lntermediate strip takes place at an intermediate thickness ranging from 12 mm to 2 mm, preferably 8 to 3 mm. At this intermediate thickness, the necessary strip cooling can take place in a short distance and with little expense regarding cooling media. Besides, a short distance is sufficient for temperature equalisatlon across the strip thickness, prior to taking up hot rolling in the ferrite range.
Controlled in~ermediary cooling of the continuous cast strip in the finishing group prevents the control problems in the rolling process arising as a result of uncontrolled and thus uncontrollable significant drop in rolling force when the austenite/ferrite transformation can take place in an uncontrolled way during rolling in the finishing grou2 After leaving the cooling section, the strip edges can be heated if necessary, to compensate for temperature loss at the edges.
The process according to the inventior. can be used in conventional hot-strip mills with continuous slab casting as a starting material, as well as in modern continuous casting-rolling mills with intermediate strip directly produced from the casting heat and with the strip further processed in-line by rolling. If the process according to the invention is used in conventional hot-strip mills, the heating temperature of the slabs can be freely selected with optimal product quality in mind.
There are no productivity losses as a result of changeover times between preliminary rolling and finishing rolling phases. Nor is there any necessity, according to the state of the art for ferrite rolling, to use slabs which have to be heated to a pusher-type heating furnace temperature which is around 100 to 200 ~C
lower in comparison to the normal procedure.
In the process according to the invention, the temperature of the preliminary strip when entering the finishing group usually exceeds 900 ~C. Over the entire thickness and width it has an austenitlc microstructure which, due to pre-rolling has an evenly re-crystallised grain structure. This microstructure which is advantageous because it is softened, allows a high degree of deformation during the first phase of finish rolling already.
The condition of the materials hardens in line with the degree of deformation. The controlled phase transformation austenite/ferrite causes softening which in turn is due to the formation of a recrystallised grain structure. The deformation resistance cf the ferrite, which is reduced in comparison with austenite, results in a significant contribution to the softening. The ferritic microstructure which is advantageous as a result of the softening allows high degrees of deformation even during the second phase of finish rolling. As a rule, the jump in temperature required for the phase transformation from austenite to ferrite is no more than 1~0 ~C in the case of ELC steels, and in most cases not more than 80 CC in the case of ULC and IF steels. Preferably, ferrite rolling takes place at a temperature between the Arl temperature and a temperature lower than that by 150 ~C, preferably by 100 ~C.
To avoid unfavourable textures, lubrication when rolling the strip is advantageous.
The finish-ro led strip , starting at the latest after 2 s following completion of rolllng, is cooled wlth liquid and/or gaseous cooling media such as water and/or water-air mixtures, at a cooling rate in the core exceeding 10 ~C/s to a temperature which is more than 150 ~C lower than the Arl temperature. Hot strip manufactured in such a way, whose microstructure was frozen by sudden cooling from the finish-rolling temperature, i.e. which was not recrystallised, is particularly suitable for producing cold-rolled strip, preferably for cold-shaping by pressing and deep drawing. Alternatively, for recristallisation the strip can also be held after hot rolling at a temperature which is lowe~ thar. the Arl temperature by less than lOC ~C, e.g. in the coil.
Table 1 shows the data (d = strip thickness, T =
temperature of material to be rolled after each pass, and Vw = strip speed) for the known austenite rolling on the one hand and the known ferrite rolling on the other hand.
By contrast, Table 2 shows the data during hot rolling according to the invention. The data ir. tables 1 and 2 apply to an ELC steel with an ini,ial strip thickness d =
20 mm and rolling down to 1.5 or 1.2 or 1 ~ final strip thickness in a five-stand finishing group.
~nlike with known austenite and ferrite rolling, in the case of the process according to the invention, the austenite/ferrite microstructure transformation takes place when a strip thickness of 5 mm is attained, after the second pass, by targeted cooling. In this, the temperature drops from 870 ~C after the second pass to around 800 ~C before the third pass. In each instance, final thickness is attained after the fifth and last pass through the finishing group.
According to the process of the invention, finishing ferrite rollir.g in one pass or in several passes, after austenite rolling and followed by a controlled phase transformation offers the possibility, in spite of the high overall thickness reduction, of taking adequate account of optimising the strip shape when scheduling the passes.
By incorporating a cooling section, the process according to the invention can also be put into practice in existing plants. Existing plants supplemented in this way can be used at full capacity, not just for the process according to the invention but also fo- all conventional hot-rolling programs.
Overall, the use of the process accord-ng to the invention leads to a host of remarkable advantages, such as:
- No control problems during the roll_ng process because no uncontrolled significant drops in rolling force occur;
- Can be applied both during re-heating and during direct rolling from the first heat;
- No limits concerning heating temperature, apart from quality-related limits;
- No productivity losses as a result cf changeover timesi - No additional coolant capacity;
- Even microstructure across width/thickness both in the austenite and in the ferrite;
- Recrystallised soft initial state in the austenite and in the ferrite, thus high drops during individual passes, in the case of an individual pass two partial passes are possible in the recrystallised condition;
- Avoids temperature losses in the finishing group, thus reduced energy consumption;
- Favourable deformation without detrimental texture, due to the lower degree of deformation in the ferrite;
- Advantageous final thicknesses outside the normal spectrum;
- Reduced thickness of scale on the finished strip due to de-scaling during cooling of the strip rolled to intermediate thickness, resulting in savings in pickling lines and in improved surface quality;
- Can be realised with modest investment expenditure both in the case of new and existing plants;
- New and cor.ventional hot-rolling programs are possible without limiting the utilisation time.
Below, the invention is explained in more detail by means of a drawing as follows:
Fig. 1 shows the temperature relationship of the deformation resistance kf for ELC, ULC and IF
steel qualities at constant degree of deformation ~ = 0.6).
Below the diagram there is a table show ng the composition of the steels. Fig. 1 sets out the temperature interval in which phase transformation of the ELC steel takes place.
The diagram shows the advantageous low deformatio~.
resistance of ferrite when compared to austenite. This advantage is greatest (i.e. the deformation resistance is lowest) immediately following complete phase transfor-mation from austenite into ferrite. The favourable temperature range is approximately 100 to 150 ~C.
Fig. 2 shows a section of the finishins group with a cooling section incorporated in between two stands.
Fig. 2 shows the last stand F2 of the first part of the finishing group in which austenite rolling takes place.
Behind it is the cooling section in which cooling to carry out the austenite/ferrite phase transformation in the steel strip is to take place, followed by stand F3 being the first stand of the last section of the finishing group which advantageously should contain two, but preferably three rolling stands in which ferrite rolling takes place. Below the diagram, by way of example, the temperature gradient along the strip section shown is depicted, both at the surface (solid line) and in the core (dashed line). The strip pre-rolled in the austenitic phase of a ~LC steel enters the cooling section at a temperature of approx. 920 ~C. As a result of intense cooling within the cooling section, the surface temperature drops steeply to approx. 805 ~C, with the core temperature naturally dropping somewhat less to 860 ~C at the end of the cooling section.
Between the end of the cooling section and the subsequent stand F3, the strip has the opportunity of tem?erature equalisation between core temperature and surface temperature, through exposure to air. After approx.
2.3 m, the temperature equalisation is completed, as shown in the diagram of Fig. 2. A device (not shown) for heating the strip edge can be arranged in front of stand ~3. By activating the device, any major temperature losses that may have occurred in the region of the strip edges can be compensated for.
Table 1 Conventional rolling Austenite rolling Ferrite rolling Pass n-. Strip Temp. Strip Strip Temp. Strip thickness ~C speedthickness ~C speed (d) (Vw) (d) (Vw) mm mJs mm m/s 2 5 goo 2 5 785 2 3 2.8 885 3.6 2.8 770 3.6 4 1.8 870 5.6 1.6 760 6.3 1.5 860 6.7 .2 750 8.3 Table 2 Process according to the invention P~ss nr.Strip thickness Te~p.Strip speed ~Vw) (d) ~C m/s ~m 2 5 ~/0 2 Controlled ~ transformation 3 2.4 790 4.2 4 1.2 785 8.3
A PROCESS FOR HOT ROLLING STEEL STRIP
The invention relates to a process for hot rolling steel strip made of unalloyed or low-alloy steel, in particular extra low carbon, ultra low carbon and interstitial free steel, in a multiple-stand finishing group, starting at a temperature in the steel austenite range and ending at a temperature in the steel ferrite range; with the transformation of the steel's microstructure being completed by cooling the strip between stands of the finishing group.
There are basically three different processes for hot-rolling steel strip from unalloyed or low-alloy steel, principally ELC steels (Extra Low Carbon with 0.02 -0.04% C), ~LC steels (Ultra Low Carbon with < 0.005% C) and IF steels (Interstitial Free through the addition of Nb and/or Ti). One approach is the finish-roll-ng of steel strip in the austenite range. In this, both the starting and finishing temperature of rolling is in the austenite range of the respective steel, i.e. n the temperature range above its A3 point depending on the steel composition, as a rule above 800 ~C. During hot rolling of strip exclusively in the austenite range, temperature control with thin strip-end thickness below 2 mm is extraordinarily difficult. In addition, with increasing thickness reduction of the strip, there is the danger of uncontrolled transformation of austenite into ferrite.
The second approach is hot rolling steel strip in the finishing group exclusively in the ferrite range, called "ferrite rolling". Ferrite rolling takes place with both start and end of rolling being in the ferrite range of the steel, i.e. at a temperature below the A3 point of the steel. It is mainly used with ELC, ~LC and IF steels.
~rom the point of view of process technology, the main advantage of ferrite rolling is in the significantly reduced rolling force, in spite of lower temperature, due to the lower deformation resistance of the ferrite in comparison to austenite. This effect is utilised in practice to increase the dimensional range of the strip in regard to increased width and reduced thickness and at the same time to increase the productivity of the plant.
In addition, product-specific advantages are expected both during processing of hot strip and in the case of the hot strip for further processing either by cold rolling or by surface refinement. The positive possibilities of hitherto known hot-strip rolling in the ferrite range are limited. The advantageous low deformation resistance of ferrite, when compared to austenite, is limited to a narrow temperature range of mostly only about 100 to 150 ~C. The advantages are greatest immediately following complete phase transformation of austenite into ferrite. The advantages decrease as the temperature in the ferrite range drops.
The thinner the article to be rolled, the larger the disadvantageous temperature losses become, and the more difficult it becomes to prevent them. In addition, the advantage arising from reduced deformation resistance is limited by the solidification of the ferrite which increases in line with the degree of deformation. During continuous rolling, the increase in strength resulting from deformation cannot be reduced or can orly be reduced slightly. There is a further limitatior, for ferrite rolling on conventional finishing groups in that, in the case of decreased rolling temperature and increased overall-thickness reduction, the material to be rolled has unfavourable texture components which either cannot be corrected at all, or only by expensive intervention with the friction relationship between the roll and the material to be rolled.
The third approach is hot rolling taking place partly in the austenite range and partly in the ferrite range. In this, the hot strip enters the finishing group at a temperature in the austenite range of the steel.
Subsequently, during hot rolling in the finishing group, transformation of the austenitic microstructure into the ferritic microstructure takes place, and the strip is rolled to final dimensions at a tem?erature in the ferrite range of the steel. Su h a process was already described in E~ 0 504 999 A2. In this, the continuous-cast strip with a thickness of e.g. 60 mm is hot-rolled in a rolling stand to 20 mm thickness of the intermediate strip. Either immediately therea~ter or after re-heating, the intermediate strip is rolled, in a multi-stand finishing group at a temperature in the austenlte range of the steel, to a thickness of 1.5 mm which is close to its final dimensions. Subsequently, the strip is cooled in a cooling section to effect a change in the microstructure from austenite to ferrite, and in a further stand it is finish-rolled to a final thickness of 0.7 mm. This known process of austenite-ferrite-hot-rolling has the disadvantage in that the change in microstructure from austenite to ferrite can happen in an uncontrollable way during rolling in the finishing group, i.e. already prior to entry into the cooling section in which it is meant to happen. On the cne hand this leads to a significant drop in rolling fo~ce which is undesirable because it is uncontrollable. On the other hand a heterogeneous condition of transformation, either temporarily or permanently across the width and thickness of the intermediate strip during finish-rolling, has a negative effect both on the strip shape and the mechanical technological properties of the hot strip. For this reason, on continuous or semi-continuous hot-strip lines, most of the time hot rolling is only carried out in the ferrite range, with the austenite-ferrite transformation occurring prior to entry into the multi-stand finishing group.
It is the object of the inventlon to improve the generic process in such a way as to prevent the occurrence of the above-mentioned limitations or disadvantages during rolling at first in the austenite range and subsequently in the ferrite range. In other words, a controlled transformation from austenite to ferrite is to be ensured as part of the finish-rolling, maintaining the advantages of rolling in the austenite range and subsequently in the ferrite range.
In the generic process according o the invention, this object is solved in that the strip is rolled in the austenitic phase in the finishing group to an intermediate thickness ranging from 2 to 12 mm and subsequently, for ferrite transformation, cooled in a single cooling step whereupon it is further rolled to its final dimensions in the ferritic phase in the finishing group.
The invention meets the object, for during austenite rolling to an intermediate thickness ranging from 2 to 12 mm there is no danger of uncontrolled transformation of austenite into ferrite with the associated disadvantages, as mentioned, during rolling. The subsequent ferrite rolling according to the inventionr which takes place in at least two steps after the transformation in the microstructure in a cooling section between stands of the finishing group, takes advantage of the improved deformability of the steel strip in the ferrite range without inevitably resulting in undesirable textures or hardening. Thus the invention provides an advantageous division of thickness reduction in the austenite range on the one hand and subsequently in the ferrite range on the other hand.
According to a preferred embodiment of the process according to the invention, the strip lS rolled in the austenitic phase to an intermediate thickness ranging from 3 to 8 mm and finish-rolled in the ferritic phase to a final thickness of less than 1.5 mm.
According to a further preferred embodiment of the process according to the invention, the strip pre-rolled in the austenitic phase is rolled in the ferritic phase to its final dimensions in at least 2 passes.
According to a further preferred embodiment of the process according to the invention, the strip is cooled at a rate exceeding 30 ~C/s for ferrite transformation.
This ensures intensive cooling of the steel strip resulting in a complete phase transformatior from austenite to ferrite within a short ti~e.
According to the invention, the claimed process further provides for the strip, following passage through the cooling section for ferrite transformation, to pass an equalisation section of a few metres in which temperature equalisation between core temperature and surface temperature and complete microstructure transformation take place before it enters the first stand for ferrite rolling. In this equalisation section, there is sufficient time for completion of the phase transformation across the cross section of the steel strip. This is especially expedient for larger strip thicknesses.
A significant point of the invention is that the first section of finish rolling takes place in the austenite range and that the brief cooling necessary for phase transformation of the continuous-cast lntermediate strip takes place at an intermediate thickness ranging from 12 mm to 2 mm, preferably 8 to 3 mm. At this intermediate thickness, the necessary strip cooling can take place in a short distance and with little expense regarding cooling media. Besides, a short distance is sufficient for temperature equalisatlon across the strip thickness, prior to taking up hot rolling in the ferrite range.
Controlled in~ermediary cooling of the continuous cast strip in the finishing group prevents the control problems in the rolling process arising as a result of uncontrolled and thus uncontrollable significant drop in rolling force when the austenite/ferrite transformation can take place in an uncontrolled way during rolling in the finishing grou2 After leaving the cooling section, the strip edges can be heated if necessary, to compensate for temperature loss at the edges.
The process according to the inventior. can be used in conventional hot-strip mills with continuous slab casting as a starting material, as well as in modern continuous casting-rolling mills with intermediate strip directly produced from the casting heat and with the strip further processed in-line by rolling. If the process according to the invention is used in conventional hot-strip mills, the heating temperature of the slabs can be freely selected with optimal product quality in mind.
There are no productivity losses as a result of changeover times between preliminary rolling and finishing rolling phases. Nor is there any necessity, according to the state of the art for ferrite rolling, to use slabs which have to be heated to a pusher-type heating furnace temperature which is around 100 to 200 ~C
lower in comparison to the normal procedure.
In the process according to the invention, the temperature of the preliminary strip when entering the finishing group usually exceeds 900 ~C. Over the entire thickness and width it has an austenitlc microstructure which, due to pre-rolling has an evenly re-crystallised grain structure. This microstructure which is advantageous because it is softened, allows a high degree of deformation during the first phase of finish rolling already.
The condition of the materials hardens in line with the degree of deformation. The controlled phase transformation austenite/ferrite causes softening which in turn is due to the formation of a recrystallised grain structure. The deformation resistance cf the ferrite, which is reduced in comparison with austenite, results in a significant contribution to the softening. The ferritic microstructure which is advantageous as a result of the softening allows high degrees of deformation even during the second phase of finish rolling. As a rule, the jump in temperature required for the phase transformation from austenite to ferrite is no more than 1~0 ~C in the case of ELC steels, and in most cases not more than 80 CC in the case of ULC and IF steels. Preferably, ferrite rolling takes place at a temperature between the Arl temperature and a temperature lower than that by 150 ~C, preferably by 100 ~C.
To avoid unfavourable textures, lubrication when rolling the strip is advantageous.
The finish-ro led strip , starting at the latest after 2 s following completion of rolllng, is cooled wlth liquid and/or gaseous cooling media such as water and/or water-air mixtures, at a cooling rate in the core exceeding 10 ~C/s to a temperature which is more than 150 ~C lower than the Arl temperature. Hot strip manufactured in such a way, whose microstructure was frozen by sudden cooling from the finish-rolling temperature, i.e. which was not recrystallised, is particularly suitable for producing cold-rolled strip, preferably for cold-shaping by pressing and deep drawing. Alternatively, for recristallisation the strip can also be held after hot rolling at a temperature which is lowe~ thar. the Arl temperature by less than lOC ~C, e.g. in the coil.
Table 1 shows the data (d = strip thickness, T =
temperature of material to be rolled after each pass, and Vw = strip speed) for the known austenite rolling on the one hand and the known ferrite rolling on the other hand.
By contrast, Table 2 shows the data during hot rolling according to the invention. The data ir. tables 1 and 2 apply to an ELC steel with an ini,ial strip thickness d =
20 mm and rolling down to 1.5 or 1.2 or 1 ~ final strip thickness in a five-stand finishing group.
~nlike with known austenite and ferrite rolling, in the case of the process according to the invention, the austenite/ferrite microstructure transformation takes place when a strip thickness of 5 mm is attained, after the second pass, by targeted cooling. In this, the temperature drops from 870 ~C after the second pass to around 800 ~C before the third pass. In each instance, final thickness is attained after the fifth and last pass through the finishing group.
According to the process of the invention, finishing ferrite rollir.g in one pass or in several passes, after austenite rolling and followed by a controlled phase transformation offers the possibility, in spite of the high overall thickness reduction, of taking adequate account of optimising the strip shape when scheduling the passes.
By incorporating a cooling section, the process according to the invention can also be put into practice in existing plants. Existing plants supplemented in this way can be used at full capacity, not just for the process according to the invention but also fo- all conventional hot-rolling programs.
Overall, the use of the process accord-ng to the invention leads to a host of remarkable advantages, such as:
- No control problems during the roll_ng process because no uncontrolled significant drops in rolling force occur;
- Can be applied both during re-heating and during direct rolling from the first heat;
- No limits concerning heating temperature, apart from quality-related limits;
- No productivity losses as a result cf changeover timesi - No additional coolant capacity;
- Even microstructure across width/thickness both in the austenite and in the ferrite;
- Recrystallised soft initial state in the austenite and in the ferrite, thus high drops during individual passes, in the case of an individual pass two partial passes are possible in the recrystallised condition;
- Avoids temperature losses in the finishing group, thus reduced energy consumption;
- Favourable deformation without detrimental texture, due to the lower degree of deformation in the ferrite;
- Advantageous final thicknesses outside the normal spectrum;
- Reduced thickness of scale on the finished strip due to de-scaling during cooling of the strip rolled to intermediate thickness, resulting in savings in pickling lines and in improved surface quality;
- Can be realised with modest investment expenditure both in the case of new and existing plants;
- New and cor.ventional hot-rolling programs are possible without limiting the utilisation time.
Below, the invention is explained in more detail by means of a drawing as follows:
Fig. 1 shows the temperature relationship of the deformation resistance kf for ELC, ULC and IF
steel qualities at constant degree of deformation ~ = 0.6).
Below the diagram there is a table show ng the composition of the steels. Fig. 1 sets out the temperature interval in which phase transformation of the ELC steel takes place.
The diagram shows the advantageous low deformatio~.
resistance of ferrite when compared to austenite. This advantage is greatest (i.e. the deformation resistance is lowest) immediately following complete phase transfor-mation from austenite into ferrite. The favourable temperature range is approximately 100 to 150 ~C.
Fig. 2 shows a section of the finishins group with a cooling section incorporated in between two stands.
Fig. 2 shows the last stand F2 of the first part of the finishing group in which austenite rolling takes place.
Behind it is the cooling section in which cooling to carry out the austenite/ferrite phase transformation in the steel strip is to take place, followed by stand F3 being the first stand of the last section of the finishing group which advantageously should contain two, but preferably three rolling stands in which ferrite rolling takes place. Below the diagram, by way of example, the temperature gradient along the strip section shown is depicted, both at the surface (solid line) and in the core (dashed line). The strip pre-rolled in the austenitic phase of a ~LC steel enters the cooling section at a temperature of approx. 920 ~C. As a result of intense cooling within the cooling section, the surface temperature drops steeply to approx. 805 ~C, with the core temperature naturally dropping somewhat less to 860 ~C at the end of the cooling section.
Between the end of the cooling section and the subsequent stand F3, the strip has the opportunity of tem?erature equalisation between core temperature and surface temperature, through exposure to air. After approx.
2.3 m, the temperature equalisation is completed, as shown in the diagram of Fig. 2. A device (not shown) for heating the strip edge can be arranged in front of stand ~3. By activating the device, any major temperature losses that may have occurred in the region of the strip edges can be compensated for.
Table 1 Conventional rolling Austenite rolling Ferrite rolling Pass n-. Strip Temp. Strip Strip Temp. Strip thickness ~C speedthickness ~C speed (d) (Vw) (d) (Vw) mm mJs mm m/s 2 5 goo 2 5 785 2 3 2.8 885 3.6 2.8 770 3.6 4 1.8 870 5.6 1.6 760 6.3 1.5 860 6.7 .2 750 8.3 Table 2 Process according to the invention P~ss nr.Strip thickness Te~p.Strip speed ~Vw) (d) ~C m/s ~m 2 5 ~/0 2 Controlled ~ transformation 3 2.4 790 4.2 4 1.2 785 8.3
Claims (12)
1. A process for hot rolling steel strip made of unalloyed or low-alloy steel, in particular extra low carbon, ultra low carbon and interstitial free steel, in a multiple-stand finishing group, starting at a temperature in the steel austenite range and ending at a temperature in the steel ferrite range, with the transformation of the steel's microstructure being completed by cooling the strip between stands of the finishing group, characterised in that the strip is rolled in the austenitic phase to an intermediate thickness ranging from 2 to 12 mm and subsequently is cooled in a single cooling step for ferrite transformation, whereupon it is rolled in the ferritic phase to its final dimensions.
2. A process according to claim 1, characterised in that the strip is rolled in the austenitic phase to an intermediate thickness ranging from 3 to 8 mm and finish-rolled in the ferritic phase to a final thickness of less than 1.5 mm.
3. A process according to claim 1 or 2, characterised in that the strip is rolled in the ferritic phase to its final dimensions in at least 2 passes.
4. A process according to one of claims 1 to 3, characterised in that the strip is cooled at a rate exceeding 30 °C/s for ferrite transformation.
5. A process according to one of claims 1 to 4, characterised in that following passage through the cooling section for ferrite transformation the strip passes an equalisation section of a few metres in which temperature equalisation between core temperature and surface temperature and complete microstructural transformation take place before it enters the first stand for ferrite rolling.
6. A process according to claims 1 to 5, characterised in that the edges of the steel strip are heated after passing the cooling section and before entering the first stand of the ferrite rolling group.
7. A process according to one of claims 1 - 6, characterised in that the strip is rolled with the addition of lubricants.
8. A process according to one of claims 1 - 7, characterised in that the strip is finish-rolled in the ferrite rolling group at a temperature between the Ar1 temperature and a temperature 150 °C below Ar1, preferably 100 °C lower than Ar1.
9. A process according to one of claims 1 to 8, characterised in that the finish-rolled strip , starting at the latest after 2 s following completion of rolling, is cooled with liquid and/or gaseous cooling media such as water and/or water-air mixtures, at a cooling rate in the core exceeding 10 °C/s to a temperature which is more than 150 °C
lower than the Ar1 temperature.
lower than the Ar1 temperature.
10. A process according to one of claims 1 to 8, characterised in that the strip is held for recrystallisation after hot rolling at a temperature which is less than 100 °C below the Ar1 temperature.
11. The use of the process according to one of claims 1 - 10 in the production of cold-rolled steel strip with good cold-deformation properties.
12. A process according to one of claims 1 10, characterised in that it is applied in the production of the steel strip, starting from continuous casting and in-line further processing by rolling.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| DE19600990A DE19600990C2 (en) | 1996-01-14 | 1996-01-14 | Process for hot rolling steel strips |
| DE19600990.1 | 1996-01-14 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2242728A1 true CA2242728A1 (en) | 1997-07-24 |
Family
ID=7782645
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA002242728A Abandoned CA2242728A1 (en) | 1996-01-14 | 1997-01-11 | Process for the hot rolling of steel bands |
Country Status (8)
| Country | Link |
|---|---|
| EP (1) | EP0873428A1 (en) |
| JP (1) | JP2000503345A (en) |
| KR (1) | KR19990077215A (en) |
| AU (1) | AU1440897A (en) |
| CA (1) | CA2242728A1 (en) |
| DE (1) | DE19600990C2 (en) |
| MX (1) | MX9805594A (en) |
| WO (1) | WO1997026377A1 (en) |
Families Citing this family (15)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| NL1003293C2 (en) * | 1996-06-07 | 1997-12-10 | Hoogovens Staal Bv | Method and device for manufacturing a steel strip. |
| NL1007739C2 (en) | 1997-12-08 | 1999-06-09 | Hoogovens Staal Bv | Method and device for manufacturing a high strength steel strip. |
| DE19632448A1 (en) * | 1996-08-05 | 1998-02-12 | Mannesmann Ag | Process and plant for producing strip from low-carbon and ultra-low-carbon steels |
| KR100353570B1 (en) * | 1996-12-19 | 2002-09-19 | 코루스 스타알 베.뷔. | Process and device for producing a steel strip or sheet |
| DE19712616C2 (en) * | 1997-03-26 | 1999-07-15 | Thyssen Stahl Ag | Hot rolling of steel strip |
| NL1007731C2 (en) * | 1997-12-08 | 1999-06-09 | Hoogovens Staal Bv | Method and device for manufacturing a ferritically rolled steel strip. |
| DE19758108C1 (en) * | 1997-12-17 | 1999-01-14 | Mannesmann Ag | Method and installation for continuous production of hot rolled thin flat products |
| DE19807122C2 (en) | 1998-02-20 | 2000-03-23 | Thyssenkrupp Stahl Ag | Process for the production of non-grain oriented electrical sheet |
| WO1999050463A1 (en) * | 1998-03-27 | 1999-10-07 | Corus Staal Bv | Method for manufacturing a forming steel having good forming characteristics and low-carbon grade forming steel |
| IT1303873B1 (en) * | 1998-11-26 | 2001-03-01 | Demag Italimpianti Spa | HOT ROLLING MACHINE FOR THIN TAPES |
| DE19858073C2 (en) * | 1998-12-16 | 2003-04-24 | Max Planck Inst Eisenforschung | Process for the production of thin hot strips of steel with improved deep drawing ability |
| BE1012462A3 (en) * | 1999-02-05 | 2000-11-07 | Centre Rech Metallurgique | Process for producing a steel strip for stamping hot rolled. |
| ITRM20060262A1 (en) * | 2006-05-17 | 2007-11-18 | Ct Sviluppo Materiali Spa | PROCEDURE FOR THE PRODUCTION OF FINE CARBON STEEL RIBBONS AND RIBBONS AS SUCH POSSIBLE |
| AT509707B1 (en) | 2010-05-04 | 2011-11-15 | Siemens Vai Metals Tech Gmbh | METHOD FOR HOT ROLLING OF STEEL STRIPS AND HOT ROLLING STRIP |
| EP4364867A1 (en) * | 2022-11-04 | 2024-05-08 | Primetals Technologies Austria GmbH | Rolling of steel with metrological detection of phase transformation |
Family Cites Families (7)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| BE807881R (en) * | 1973-11-27 | 1974-05-27 | Centre Rech Metallurgique | PROCESS FOR THE MANUFACTURE OF SHEET FOR STAMPING |
| LU78401A1 (en) * | 1977-10-27 | 1979-06-01 | Centre Rech Metallurgique | PROCESS FOR MANUFACTURING A COLD FORMABLE STEEL |
| BE901292A (en) * | 1984-12-17 | 1985-06-17 | Centre Rech Metallurgique | PROCESS OF CONTROLLED LAMINATION OF A THICK PRODUCT. |
| NL8702050A (en) * | 1987-09-01 | 1989-04-03 | Hoogovens Groep Bv | METHOD AND APPARATUS FOR THE MANUFACTURE OF TIRE-DEFORMING STEEL WITH GOOD MECHANICAL AND SURFACE PROPERTIES. |
| US5200005A (en) * | 1991-02-08 | 1993-04-06 | Mcgill University | Interstitial free steels and method thereof |
| NL9100911A (en) * | 1991-03-22 | 1992-10-16 | Hoogovens Groep Bv | Mfg. hot-rolled steel strip with single pass - for the sole reduction means through two-high roll stand |
| EP0524162B1 (en) * | 1991-07-17 | 1998-11-11 | CENTRE DE RECHERCHES METALLURGIQUES CENTRUM VOOR RESEARCH IN DE METALLURGIE Association sans but lucratif | Method of manufacturing a thin sheet of low carbon steel |
-
1996
- 1996-01-14 DE DE19600990A patent/DE19600990C2/en not_active Expired - Fee Related
-
1997
- 1997-01-11 JP JP9525663A patent/JP2000503345A/en active Pending
- 1997-01-11 WO PCT/EP1997/000107 patent/WO1997026377A1/en not_active Application Discontinuation
- 1997-01-11 CA CA002242728A patent/CA2242728A1/en not_active Abandoned
- 1997-01-11 KR KR1019980705357A patent/KR19990077215A/en not_active Application Discontinuation
- 1997-01-11 EP EP97901001A patent/EP0873428A1/en not_active Ceased
- 1997-01-11 AU AU14408/97A patent/AU1440897A/en not_active Abandoned
-
1998
- 1998-07-10 MX MX9805594A patent/MX9805594A/en unknown
Also Published As
| Publication number | Publication date |
|---|---|
| DE19600990A1 (en) | 1997-07-17 |
| JP2000503345A (en) | 2000-03-21 |
| MX9805594A (en) | 1998-10-31 |
| AU1440897A (en) | 1997-08-11 |
| KR19990077215A (en) | 1999-10-25 |
| WO1997026377A1 (en) | 1997-07-24 |
| DE19600990C2 (en) | 1997-12-18 |
| EP0873428A1 (en) | 1998-10-28 |
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