METHOD OF AND FOR PRODUCING HEAVY PLATES

Abstract

In the context of a method for producing heavy plate (4) from a steel alloy, comprising the continuous casting of a steel melt and primary forming of an obtained casting strand to produce a slab, and then forming or hot rolling the slab from the casting heat in multiple forming steps to produce a desired heavy plate dimension, followed immediately by a heat treatment of the heavy plate (4), effecting a targeted cooling of the obtained heavy plate (4), wherein the heavy plate (4) is cut to a desired individual plate length before or after its heat treatment as seen in the production direction (3), a solution is provided for producing heavy plate that permits the flexible production of heavy plate of variable qualities. This is achieved by carrying out the heat treatment in the temperature range of 150° C.-1100° C. as a combination of a targeted cooling of the obtained heavy plate (4) from the rolling heat to a desired first temperature, followed immediately by a targeted heating of the heavy plate (4) to a desired second temperature and an immediately subsequent cooling of the heavy plate (4) to a desired third temperature.

Claims

1-17. (canceled)

18. A method of producing heavy plates from a steel alloy, comprising the steps of continuous casting of a steel melt and primary forming of an obtained casting strand to produce a slab, and then forming or hot rolling the slab from the casting heat in multiple forming steps to produce a desired heavy plate dimension, followed immediately by a heat treatment of the heavy plate (4) that effects a targeted cooling of the obtained heavy plate (4), wherein the heavy plate (4) is cut to a desired individual plate length before or after its heat treatment as viewed in the direction of production (3), and the heat treatment is carried out immediately from the rolling heat in the temperature range of 150° C.-1100° C. as a combination of a targeted cooling of the obtained heavy plate (4) from the rolling heat to a desired first temperature, followed immediately by a targeted heating of the heavy plate (4) to a desired second temperature, and an immediately subsequent cooling of the heavy plate (4) to a desired third temperature, characterized in that, after forming or hot rolling, the heavy plate (4) is cooled in the immediately subsequent heat treatment step 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), and is then 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 is cooled to the ambient temperature as the desired third temperature, or in that, after forming or hot rolling, the heavy plate (4) is cooled in the immediately subsequent heat treatment step from the rolling heat to the first desired temperature, which is below the martensite formation temperature, after which it is heated by heating 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 is cooled to the ambient temperature as the desired third temperature, or in that, after forming or hot rolling, the heavy plate (4) is cooled in the immediately subsequent heat treatment step from the rolling heat to the first desired temperature, which is below the bainite formation temperature, after which it is held by heating at a temperature that is within the bainite transformation range as the desired second temperature, and is finally cooled to the ambient temperature as the desired third temperature.

19. The method according to claim 18, characterized in that the final cooling to the ambient temperature as the desired third temperature is carried out as air cooling.

20. The method according to claim 18, characterized in that the cooling of the heavy plate (4) from the rolling heat to the first desired temperature in the heat treatment step immediately following the forming or hot rolling of the heavy plate (4) is carried out by means of forced water or air cooling.

21. The method according to claim 18, characterized in that a portion of the heat treatment, which comprises in particular additional cooling and/or heating steps, is carried out after the heavy plate (4) has passed through a straightening unit (6) and before it is stacked.

22. The method according to claim 18, characterized in that after being heated to the desired second temperature, the heavy plate (4) is straightened and then stacked.

23. The method according to claim 18, characterized in that in the multiple forming steps, a heavy plate (4) having a thickness greater than 8 mm, in particular a thickness of 40 mm-400 mm, and a width greater than 1200 mm is produced.

24. An apparatus (1, 1′) for producing heavy plates from a steel alloy, comprising a casting machine (8) for the immediately consecutive processing and/or machining of a steel melt in which the steel melt can be continuously cast, a primary forming unit (9) in which a casting strand cast from the steel melt can be formed into a slab, a forming unit or rolling mill (11) in which the slab can be rolled from the casting heat in multiple forming steps to form a heavy plate (4) having the desired heavy plate dimensions, and a heat treatment unit (5) and a heavy plate cutting device (2) disposed upstream or downstream of the heat treatment unit (5) in the direction of production (3), wherein the heat treatment unit (5) allows a heavy plate (4) to be fed to a heat treatment immediately from the rolling heat and comprises at least one cooling device (12) and at least one heating device (13), and is designed to be used for carrying out a heat treatment of the heavy plate (4) within a temperature range of 150° C.-1100° C. in the form of a combination of a targeted cooling of the obtained heavy plate (4) from the rolling heat to the desired first temperature, followed immediately by a targeted heating of the heavy plate (4) to the desired second temperature and an immediately subsequent cooling of the heavy plate (4) to the desired third temperature, characterized in that the heavy plate (4) that is fed in can be treated variably by means of the heat treatment unit (5), wherein heavy plate (4) that is fed in may be cooled by the at least one cooling device (12) to a temperature below the Ac1 temperature for the steel alloy in question or to a temperature below the temperature that characterizes the start of transformation to a martensite structure for the steel alloy in question, or to a temperature between the temperature that characterizes the start of bainite transformation and the temperature that characterizes the start of transformation to a martensite structure for the steel alloy in question as the desired first temperature, and by means of the at least one heating device (13), the heavy plate (4) that has been cooled to the temperature below the Ac1 temperature for the steel alloy in question may be heated to a temperature that is above the AC3 temperature for the steel alloy in question, and the heavy plate (4) that has been cooled to the temperature below the temperature that characterizes the start of transformation to a martensite structure may be heated to a temperature that is below the Ac1 temperature for the steel alloy in question as the desired second temperature, and the heavy plate (4) that has been cooled to a temperature that is between the temperature that characterizes the start of bainite transformation and the temperature that characterizes the start of transformation to a martensite structure may be held within this temperature range as the second temperature.

25. The apparatus (1, 1′) according to claim 24, characterized in that the cooling of the heavy plate (4) to the desired third temperature is carried out at least partially outside of the heat treatment unit (5).

26. The apparatus (1, 1′) according to claim 24, characterized in that the heat treatment unit (5) is designed such that, after forming or hot rolling, in an immediately subsequent heat treatment step, the heavy plate (4) can be cooled by means of the cooling device (12) 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 (4) can then be heated by means of the heating device (13) 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 (4) can be cooled to the ambient temperature as the desired third temperature.

27. The apparatus (1, 1′) according to claim 24, characterized in that the heat treatment unit (5) is designed such that, after forming or hot rolling, in an immediately subsequent heat treatment step, the heavy plate (4) can be cooled by means of the cooling device (12) from the rolling heat to the first desired temperature, which is below the bainite formation temperature, in that the heavy plate (4) can then be heated by means of the heating device (13) and held at a temperature that is within the bainite transformation range as the desired second temperature, and in that finally, the heavy plate (4) can be cooled to the ambient temperature as the desired third temperature.

28. The apparatus (1, 1′) according to claim 24, characterized in that the heat treatment unit (5) is designed such that, after forming or hot rolling, in the immediately subsequent heat treatment step, the heavy plate (4) can be cooled by means of the cooling device (12) from the rolling heat to the first desired temperature, which is below the martensite formation temperature, in that the heavy plate (4) can then be heated by means of the heating device (13) 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 in that finally, the heavy plate (4) can be cooled to the ambient temperature as the desired third temperature.

29. The apparatus (1, 1′) according to claim 24, characterized in that it is designed and configured to produce a heavy plate (4) having a thickness greater than 8 mm, in particular a thickness of 40 mm-400 mm, and a width greater than 1200 mm in the plurality of forming steps of the rolling mill (11).

30. The apparatus (1, 1′) according to claim 24, characterized in that it comprises a straightening unit (6) disposed downstream of the heating device (13) in the direction of production (3) of the heavy plate (4).

31. The apparatus (1, 1′) according to claim 30, characterized in that it comprises one or more additional heat treatment unit(s) disposed downstream of the straightening unit (6) in the direction of production (3) of the heavy plate (4), each of said units preferably comprising at least one additional cooling device and at least one additional heating device.

Description

[0027] The invention will be described in greater detail below, in reference to the set of drawings. The drawings show

[0028] FIG. 1 in the upper part of the drawing, a schematic diagram of an apparatus according to the invention for the production of continuous heavy plate, and in the lower part of the drawing, a schematic diagram illustrating the temperature curve in the respectively produced flat product over the length of the apparatus and the individual system components of the apparatus, and

[0029] FIG. 2 in the upper part of the drawing, a schematic diagram of an apparatus according to the invention for the production of individual heavy plate, and in the lower part of the drawing, a schematic diagram illustrating the temperature curve in the respectively produced flat product over the length of the apparatus and the individual system components of the apparatus.

[0030] The apparatuses depicted in the form of a production system in FIGS. 1 and 2 and designated as a whole as either 1 or 1′ differ only in that, in the embodiment shown in FIG. 1, a cutting device 2 is provided on the output side, downstream of a heat treatment unit 5 in the direction of production of the heavy plate 4 as indicated by the arrow 3, and in the embodiment example shown in FIG. 2, such a cutting device is disposed on the input side, upstream of a heat treatment unit 5. The apparatus 1 according to FIG. 1 thus enables the production of heavy plate 4 with the continuous operation of the production system, since the flat material produced as a continuous heavy plate is cut by the cutting device 2 into individual plate lengths only after it undergoes a heat treatment carried out in heat treatment unit 5. Thus in this case, a continuous flat material passes through the individual system components or system units of the apparatus 1, configured in the form of a production system, until it reaches the point downstream of heat treatment unit 5. Beyond that point, the heavy plates 4, which have been cut into individual plate lengths, pass through a straightening unit 6, and are then stacked in a conventional manner by means of a discharge unit 7. In the embodiment shown in FIG. 2, the continuous flat material that is produced is cut into individual plate lengths before entering the heat treatment unit 5, so that heavy plates 4 that have already been separated into individual plate lengths pass through the heat treatment unit 5, followed by the straightening unit 6 and the discharge unit 7. The production of heavy plate in the individual plate operation in apparatus 1′ according to FIG. 2 is advantageous in particular for the production and processing of quenched and tempered plate with ultra-high shear strengths. Heavy plates of lower strength, which have lower strength values after passing through the heat treatment unit 5, can be produced both in individual plate operation using the system shown in FIG. 2 and in continuous plate operation using the system of FIG. 1.

[0031] 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 FIG. 1, the pre-formed and formed heavy plate 4 then passes through the heat treatment unit 5, after which it is cut to individual lengths by means of the cutting device 2, which is a cross-cutting device. In the apparatus 1′ according to FIG. 2, the pre-formed and formed heavy plate 4 product is divided into individual plate lengths by means of cutting device 2, likewise embodied as a cross-cutting device, before entering heat treatment unit 5, and is conveyed through heat treatment unit 5 in the form of individual plates. In both of apparatuses 1 and 1′, heat treatment unit 5 comprises a cooling device 12 and a heating device 13. The heavy plates 4 then pass through straightening unit 6 and are stacked in discharge unit 7. In a manner not shown here, additional heat treatment units 5 may also be provided between straightening unit 6 and discharge unit 7 if this should be necessary for qualities that require special heat treatment. These units may also each comprise a cooling device and a heating device, so that these heat treatment units can likewise be used for carrying out a combination of intensified air cooling or water cooling followed by heating to a tempering temperature.

[0032] 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.

[0033] The embodiment of apparatus 1 according to FIG. 1 is suitable for producing heavy plate with low thicknesses and low shear strengths, with which high shear rates can be achieved. The embodiment of apparatus 1′ according to FIG. 2 can also be used to produce heavy plate with ultra-high strength values, since the heavy plate is cut in cutting device 2 immediately after leaving the rolling heat of the rolling mill 11, and thus in a high temperature range, in the direction of production 3. At these temperatures, the materials that are usually processed to produce heavy plate still have low shear strengths.

[0034] With the apparatuses 1 and 1′ shown in FIGS. 1 and 2 and described above, heavy plates can be produced directly from the casting heat of a continuous casting machine. Cooling to room temperature for more than five minutes does not occur. The cast slabs have a thickness of at least 40 mm, with the maximum possible slab thickness being limited by the design and the technical parameters of the forming unit and rolling mill 11 downstream, and by the required thickness of the heavy plate 4 at the outlet of said units. The slab width is not specifically limited, and instead corresponds to slab widths that can customarily be achieved according to the prior art with systems of this type. The continuous casting machine may be a curved system, a vertical system with bending, a vertical system or a horizontal continuous casting machine. A reduction in the thickness of the casting strand by deformation with a liquid core (liquid core reduction) or by minimal forming in the region of the final solidification of the casting strand (dynamic soft reduction) is not necessary for carrying out the method according to the invention, but may be carried out if desired.

[0035] 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.

[0036] 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.

[0037] In the lower portions of FIGS. 1 and 2, the temperature curves (T) for a flat product produced in the respective apparatus 1 or 1′ over the length of the system (l), and thus over the respective production time (s), are plotted. With heat treatment unit 5, heavy plates 4, which up to that point have undergone the same temperature and cooling process, may be treated differently, allowing different grades and strength values to be produced.

[0038] 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 A.sub.C3 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 A.sub.C3 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.

[0039] 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.

[0040] The additional temperature profile curve 19 indicated by dashed lines in the lower portions of FIGS. 1 and 2 indicates the cooling and heating profile of heavy plate 4 when an ultra-high strength heavy plate 4 is to be produced. For this purpose, the rolled heavy plate 4 is cooled relatively intensely in cooling device 12 of heat treatment unit 5 to temperatures that are below temperature line 18, which characterizes the beginning of martensite transformation. The heavy plate 4 is then tempered in heating device 13 to a temperature that is below the A.sub.C1 temperature of the respective material and thus below the temperature that marks the end of the transformation range of γ-iron to α-iron. Subsequently and finally, heavy plate 4 is again cooled in air to the ambient temperature after leaving the heat treatment unit 5.

[0041] 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.

[0042] 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.

LIST OF REFERENCE SIGNS

[0043] 1, 1′ apparatus [0044] 2 heavy plate cutting device [0045] 3 direction of production [0046] 4 heavy plate [0047] 5 heat treatment unit [0048] 6 straightening unit [0049] 7 casting machine [0050] 8 primary forming unit [0051] 9 cooling device [0052] 10 heating device