Patent Application
20170275729
The invention is directed to a method for producing heavy plate from a steel alloy, comprising continuously casting a steel melt and subjecting the resulting casting strand to primary forming to produce a slab, then forming or hot rolling the slab from the casting heat in multiple forming steps to produce a desired heavy plate dimension, and immediately thereafter, subjecting the heavy plate to a heat treatment that effects a targeted cooling of the obtained heavy plate, wherein the heavy plate is cut to a desired individual plate length in the direction of production, before or after it is subjected to the heat treatment.
The invention is further directed to an apparatus for carrying out such a method, which comprises a system for the immediately consecutive processing and/or machining of a steel melt, which system comprises a casting machine, in which the steel melt can be continuously cast, a primary forming unit, in which a casting strand that has been cast from the steel melt can be formed into a slab, a forming unit or rolling mill, in which the slab from the casting heat can be rolled in multiple forming steps to form a heavy plate with a desired heavy plate dimension, a heat treatment unit, and a heavy plate cutting device disposed upstream or downstream of the heat treatment unit.
In the manufacturing of hot-rolled strip by the single-strip method, the semi-continuous strip method or the continuous method, it is routine for a slab that has been produced by the continuous casting method to be fed directly from the casting heat to a reheating or equalizing furnace, in which the temperature desired for the subsequent finish rolling is established. In the finishing mill, the slab is rolled in several passes to the desired final thickness.
A method for producing steel sheet is known from EP 0415987 B1. In this method, a slab is produced by means of a continuous casting process, and is then passed through a first forming stage to reduce the slab thickness and then heated in a furnace, after which it enters a finishing mill in which the slab is rolled into the steel sheet or steel strip. In the rolling mill that comprises multiple forming steps, the steel strip is subjected to a treatment consisting of a combination of targeted forming steps and targeted cooling and heating steps. The desired mechanical properties of the steel strip are adjusted by this combination of forming at selectively adjusted temperatures, degrees of forming that are carried out at a specific temperature, and forming rates. Once the strip has passed through the finishing rolling mill, the desired sheet lengths are cut from the finish rolled steel sheet and stacked. After the steel sheet has passed through the finishing train, the resulting steel sheet is not subjected to any further targeted heat treatment.
From WO 2006/106376 A1 a method for producing strips and sheets having a thickness of 10-100 mm is known; said strips and sheets are produced from a continuously cast strand followed by hot rolling. The hot rolling may be combined with cooling devices arranged between the individual roll stands and a cooling of the rolled steel strip that can be effected with said devices. After passing through the mill train, the steel sheet is conveyed through a heat treatment unit in the form of a cooling unit, after which plates of sheet steel are cut off in the desired length.
It is a problem with this known prior art method that grain-refining processes occur very rapidly due to the rapid recrystallization of steels in the temperature range above 1100° C., and that in practice, a temperature field that leads to grain growth due to the localized temperature after each grain-refining forming step is always established in the slab. The localized differences in grain growth lead to a formation of a varied structure, at least across the width of the sheet that is rolled from the slab, with said structure being expressed in the finished sheet as an inhomogeneous mechanical property, which is determined by the strength value and/or the toughness value in each case. It is therefore not possible to reliably produce steel sheets and strips, in particular heavy plates, that meet high toughness requirements with consistent quality using the methods and apparatus known from the prior art. It also is not possible to produce steel plates of delivery quality +QT (quenched and tempered) using the methods and apparatus heretofore disclosed in the prior art for the production of heavy plate, which comprise merely a combination of forming/heating/cooling followed by a cooling process. Steels that are required by their delivery standards to undergo a targeted heat treatment, normalization or tempering likewise cannot be produced using the known systems.
The object of the invention is to provide a solution for producing heavy plate that does not entail the aforementioned disadvantages and that enables the flexible production of different qualities of heavy plate.
In the method defined at the outset, the object is achieved in that the heat treatment is carried out in the temperature range of 150° C.-1100° C. as a combination of targeted cooling of the obtained heavy plate from the rolling heat to a desired first temperature, followed immediately by a targeted heating of the heavy plate to a desired second temperature and an immediately subsequent cooling of the heavy plate to a desired third temperature.
In an apparatus of the type defined at the outset, the object is achieved in that the heat treatment unit comprises at least one cooling device and at least one heating device, which are designed to be used for carrying out a heat treatment of the heavy plate in the temperature range of 150° C.-1100° C. in the form of a combination of a targeted cooling of the obtained heavy plate from the rolling heat to a desired first temperature, followed immediately by a targeted heating of the heavy plate to a desired second temperature and an immediately subsequent cooling of the heavy plate to a desired third temperature.
Because the invention provides that a heat treatment that comprises a targeted cooling, a targeted reheating and a repeated targeted cooling of the heavy plate in multiple steps is carried out immediately after the rolling process is completed, and thus after the slab has been rolled to the heavy plate dimensions, a temperature field that is consistent even across the width of the heavy plate, with a resulting uniform structure in the finished heavy plate, is formed in the finished heavy plate, and thus the heavy plate has a homogeneous mechanical property, i.e. a homogeneous strength and/or toughness distribution. The method and the apparatus according to the invention may also be used to produce heavy plate in the normalized state, i.e. steels of high toughness, and heavy plate in the quenched and tempered state, i.e. steels of high strength, as well as special steels in the tempered state, i.e. steels that have been subjected to a tempering treatment.
Heavy plate in the scope of this application is understood as a flat product which is defined as heavy plate according to Standard EN 10029. With the described method, an extremely wide range of steel grades can be produced, such as those designated by symbols in the standard EN 10025-2:2004. Heavy plate “as rolled” (symbol +AR) may be produced, as can normalized heavy plate (symbol +N) and thermomechanically rolled heavy plate (symbol+M) and quenched and tempered heavy plate (symbol +QT). And all of this using the same apparatus or system.
An advantageous procedure for producing normalized heavy plate according to the embodiment of the invention may comprise the measure that, after forming or hot rolling, in the immediately subsequent heat treatment step, the heavy plate is cooled from the rolling heat to the first desired temperature, which is below the full transformation temperature of γ-iron (γ mixed crystal; austenite) to α-iron (α mixed crystal; ferrite), after which the heavy plate is heated to the desired second temperature, the normal annealing temperature, which is above the full transformation temperature of γ-iron to α-iron, the AC3 temperature, in the temperature range for homogeneous austenite, and finally, the heavy plate is cooled to the ambient temperature as the desired third temperature.
In contrast, if a high strength heavy plate is to be produced, the embodiment of the invention provides the measures that, after forming or hot rolling, in the immediately subsequent heat treatment step, the heavy plate is cooled from the rolling heat to the first desired temperature, which is below the bainite formation temperature, after which the heavy plate is heated to and held at a temperature that is within the bainite transformation range as the desired second temperature, and finally, the heavy plate is cooled to the ambient temperature as the desired third temperature.
It is also possible to produce ultra-high strength plates by the method according to the invention. In this case, the embodiment of the invention is characterized by the measures that, after forming or hot rolling, in the immediately subsequent heat treatment step, the heavy plate is cooled from the rolling heat to the first desired temperature, which is below the martensite formation temperature, after which the heavy plate is heated to a tempering temperature that is below the transformation temperature of γ-iron to α-iron, the Ac1 temperature, as the desired second temperature, and finally, the heavy plate is cooled to the ambient temperature as the desired third temperature.
The provided heat treatment advantageously and practically comprises a cooling to room temperature. The invention therefore also specifies that the final cooling to the ambient temperature as the desired third temperature is carried out by air cooling.
Forced cooling is practical for cooling the heavy plate from the rolling heat. The embodiment of the invention therefore further provides that the cooling of the heavy plate from the rolling heat to the first desired temperature during the heat treatment step performed immediately after the forming or hot rolling step is carried out by means of forced water or air cooling.
It is also possible, however, to perform some or additional cooling and/or heating steps on the manufactured or produced heavy plate only or even if the heavy plate has already passed through a straightening unit or a straightening device. In this regard, a refinement of the invention likewise provides that a portion of the heat treatment that comprises in particular additional cooling and/or heating steps is carried out after the heavy plate has passed through a straightening unit and before it is stacked.
It is further advantageous and practical for the heavy plate to be straightened and then stacked after it has been heated to the desired second temperature, which is likewise specified by the invention.
The method can be carried out particularly practically for producing heavy plate, and therefore the invention is further characterized in that in the multiple forming steps, a heavy plate having a thickness of more than 8 mm, in particular a thickness of 40 mm-400 mm, and a width of more than 1200 mm is produced.
For carrying out the method of the invention, the apparatus of the invention is characterized in a practical and advantageous embodiment in that the cooling of the heavy plate to the desired third temperature is carried out at least partially outside of the heat treatment unit.
For advantageously carrying out the method of the invention for producing normalized plate, the embodiment of the apparatus is characterized in that the heat treatment unit is designed such that, after forming or hot rolling, in an immediately subsequent heat treatment step, the heavy plate can be cooled by means of the cooling device from the rolling heat to the first desired temperature, which is below the full transformation temperature of γ-iron (γ mixed crystal; austenite) to α-iron (α mixed crystal; ferrite), in that the heavy plate can then be heated by means of the heating device to the desired second temperature, the normal annealing temperature, which is above the full transformation temperature of γ-iron to α-iron, the AC3 temperature, in the temperature range for homogeneous austenite, and in that finally, the heavy plate can be cooled to the ambient temperature as the desired third temperature.
To enable the method of the invention to be used for producing high-strength steel plate that has its functional characteristics when its structure is in the bainitic state, the apparatus according to the invention is further characterized in that the heat treatment unit is designed such that, after forming or hot rolling, in an immediately subsequent heat treatment step, the heavy plate can be cooled by means of the cooling device from the rolling heat to the first desired temperature, which is below the bainite formation temperature, in that the heavy plate can then be heated by means of the heating device and held at a temperature that lies in the bainite transformation range as the desired second temperature, and in that finally, the heavy plate can be cooled to the ambient temperature as the desired third temperature.
A particularly practical embodiment of the apparatus for carrying out the method according to the invention for the production of ultra-high strength steel plate is characterized in that, after forming or hot rolling, in an immediately subsequent heat treatment step, the heavy plate can be cooled by means of the cooling device from the rolling heat to the first desired temperature, which is below the martensite formation temperature, in that the heavy plate can then be heated by means of the heating device to a tempering temperature which is below the transformation temperature of γ-iron to α-iron, the Ac1 temperature, as the desired second temperature, and in that finally, the heavy plate can be cooled to the ambient temperature as the desired third temperature.
The apparatus is advantageously designed such that it is configured and embodied for the production of a heavy plate having a thickness of more than 8 mm, in particular a thickness of 40 mm-400 mm, and a width of more than 1200 mm, in the plurality of forming steps of the rolling mill, which is likewise specified by the invention.
It is further advantageous for the apparatus according to the invention to comprise a straightening unit, disposed downstream of the heating device in the direction of production of the heavy plate, which is likewise specified by the invention.
Finally, the invention is also characterized in that it comprises one or more additional heat treatment unit(s) disposed downstream of the straightening unit in the direction of production of the heavy plate, each of which preferably comprises at least one additional cooling device and at least one additional heating device. This embodiment of the invention makes it possible to carry out heat treatment procedures or heat treatment measures on the rolled heavy plate, even after the produced heavy plate has been straightened.
The invention will be described in greater detail below, in reference to the set of drawings. The drawings show
The apparatuses depicted in the form of a production system in
In apparatuses 1 and 1′ according to the invention, a steel melt is cast continuously in a casting machine 8 embodied as a continuous casting machine, in immediately consecutive processing and/or machining. The resulting casting strand is formed in the customary manner in a primary forming unit 9, which is part of the casting machine 8, into a slab, which is then passed through a descaling unit 10, and is then rolled from the casting heat in multiple forming steps, in a rolling mill 11 that forms a forming unit, to form a heavy plate 4 that has the respectively desired heavy plate dimensions. In the system according to
In a manner not shown here, the forming unit or rolling mill 11 is equipped in the customary manner with means for adjusting profile and surface evenness, and with means for controlling the surface temperature of the rolled material.
The embodiment of apparatus 1 according to
With the apparatuses 1 and 1′ shown in
The continuously cast strand exits the outlet area of the primary forming unit 9 at an average temperature of 1150° C. to 1300° C. and with a fully solidified core. Steel grades or materials that tend strongly toward scale formation are then descaled in descaling unit 10. For highly alloyed materials, this process step may optionally be omitted, however in that case it is recommended that the surface of these materials be cleaned following a first forming step in the subsequent forming unit or rolling mill 11. Materials or steel grades with a higher tendency toward scale formation are likewise cleaned in the conventional manner prior to the first forming step in the forming unit or rolling mill 11, usually using pressurized water or steam, although alternative methods such as cleaning with dry ice or water mixed with abrasive ingredients are also possible. In descaling with pressurized water, it is advantageous for rotating systems to be provided.
The forming process carried out in the casting machine 8 and in the primary forming unit 9 is then followed by a forming or a forming process in the forming unit or rolling mill 11, which comprises at least two forming steps with an accumulated logarithmic degree of formation of greater than 0.7. The cast structure of the slab is thereby transformed into a formed structure, so that the basic toughness of the steel material is achieved and any possible core porosities are closed. Depending on the final thickness that is required, the rolling mill 11 may comprise more than two stands that perform at least two forming steps, so that additional forming steps may be added depending on the required final thickness of the heavy plate. The heavy plate 4 that has been formed and rolled to the desired heavy plate dimensions after passing through rolling mill 11 exits the rolling mill 11 at a temperature in the range of 950° C. to 1100° C. At that point, the temperature of heavy plate 4 is therefore above the optimal temperature for normalizing rolling. To nevertheless allow the desired final properties of the heavy plate 4 or of each heavy plate 4 to be adjusted, a heat treatment by means of heat treatment unit 5, to which heavy plate 4 is fed immediately after forming and rolling, immediately follows the rolling heat. The rolled material, i.e. the heavy plate 4, remains in the production system or the apparatus 1 or 1′ for this purpose.
In the lower portions of
In the temperature and process sequence indicated by the solid line 14, normalized heavy plate 4 is produced. For this purpose, the rolled heavy plate 4 is fed to cooling device 12 in heat treatment unit 5, where it is cooled to a temperature (T) at which the transformation of γ-iron, i.e. the formed γ mixed crystal or austenite, to α-iron, i.e. the formed a mixed crystal or ferrite, is completed. Heavy plate 4 is thus cooled to a temperature that is below temperature line 15, which represents the minimum final temperature of the transformation range of γ-iron to α-iron. The cooling rates are such that no fractions of harder structures such as martensite or bainite are formed. The heavy plate 4 thus cooled is then subjected to normal annealing, for which purpose the temperature of the heavy plate 4 is raised above the AC3 temperature for the respective material and held there for a short time. This is accomplished by means of heating device 13, through which heavy plate 4 passes after passing through cooling device 12 in the direction of production 3. In heating device 13, heavy plate 4 can be heated as desired to a temperature above the AC3 temperature by means of a conventional roller hearth furnace with open gas firing or radiant tube heating, by means of induction heating or by the application of “direct flame inpingement”, or by a combination of all of these firing options or heating device options. Heating device 13 is designed to be operated within a temperature range of 150° C. to 1100° C. This is followed by a cooling of the produced heavy plate 4 in air to the ambient temperature, which is usually carried out once the heavy plate 4 has exited heat treatment unit 5. The method characterized by the shape of curve 14 and described above, in particular the heat treatment method, is suitable for all types of metal plate that are to be in the +N “normalized” condition.
The profile of the dashed cooling curve and heat treatment line 16 indicates the temperature profile during the production of high strength steel plates, the use properties of which are characterized by a bainitic microstructure. To produce such qualities, the rolled heavy plate 4 is cooled in heat treatment unit 5 by means of cooling device 12 to temperatures that lie between temperature line 17, which characterizes the start of bainite transformation, and temperature line 18, which characterizes the start of the transformation of the structure to a martensite structure. Heating device 13, which is entered by heavy plate 4 after it passes through the cooling device 12 in the direction of production 3, also holds heavy plate 4 in this temperature range that lies between temperature lines 17 and 18 within heat treatment unit 5, so that the transformation of the structure to a bainite structure continues. With this method, in particular this heat treatment method, high strength and ultra-high strength heavy plate 4 with good forming and wear properties and heavy plate 4 with defined residual austenite content can be produced. After leaving heat treatment unit 5, the heavy plate 4 produced by the method or by the curve of temperature line 16 also cools in air to the ambient temperature.
The additional temperature profile curve 19 indicated by dashed lines in the lower portions of
In addition to the above-described heat treatment method, additional combinations of cooling and heating, i.e. cooling stop temperatures and tempering and annealing temperatures, can be carried out in the apparatuses 1 and 1′ according to the invention. For example, a solution annealing of highly alloyed steels, or a precipitation hardening of previously quenched heavy plate 4 may be carried out in apparatuses 1 and 1′. In this case, it is also possible for additional optionally multiple-stage treatments of heavy plate 4 to be carried out in corresponding heat treatment units between the straightening unit 6 in each case and the discharge unit 7 to which the heavy plates 4 of a stack are to be fed. These heat treatment units advantageously also comprise a cooling device with a heating device disposed downstream thereof in production direction 3.
The method according to the invention and the apparatus according to the invention allow an energy savings to be realized in the production of heavy plate, which when produced by the methods known from the prior art otherwise requires a subsequent heat treatment that is not integrated directly into the process of producing heavy plate in individual lengths. Apparatuses 1 and 1′ according to the invention can be used to produce heavy plate 4 in the normalized condition, for example the grade S355J2+N, and to produce quenched and tempered heavy plate, for example of grade S355+QT, of a steel grade or a steel condition in a casting sequence. In other words, the system can be used to produce heavy plate 4 in a single casting sequence from steel melts, the chemical composition or basic composition of which is the same, but the heat treatment of which is different. In the methods that can be carried out using apparatuses 1 and 1′ according to the invention, the cooling of the produced slabs to room temperature, the reheating thereof prior to hot rolling and also the cooling and heating thereof prior to a heat treatment, according to the prior art, are dispensed with. A further advantage of apparatuses 1 and 1′ according to the invention is that they can be used to produce heavy plate 4 made of high-strength steels with a bainitic structure by annealing in the bainite stage.
| Number | Date | Country | Kind |
|---|---|---|---|
| 102014221068.2 | Oct 2014 | DE | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2015/073943 | 10/15/2015 | WO | 00 |
