Patent Application
20250051890
The invention relates to heavy plate and thermomechanical treatment method for a starting material, more particularly a slab ingot, for the production of a heavy plate consisting of a steel alloy.
In order to increase the toughness, in particular the low-temperature toughness, of a heavy plate made of a steel alloy, WO2011/079341A2 has disclosed a thermomechanical treatment method in which the starting material is hot-rolled in several stages and cooled to below the Ar3 temperature in an accelerated manner between two hot-rolling passes and inductively heated to above the Ac3 temperature. After the last hot rolling pass, a two-stage cooling to room temperature takes place, first with an accelerated cooling rate by water quenching to a cooling stop temperature below Ar3 and then a cooling to room temperature. Despite their high low-temperature toughness, these heavy plates have the disadvantage of low uniform elongation Ag, which limits their use in earthquake zones, for example.
The object of the invention, therefore, is to create a thermomechanical treatment method for the production of a heavy plate with which an improved uniform elongation Ag in the heavy plate can be reproducibly achieved despite high toughness values.
In that a cooling from a second final rolling temperature, more particularly ≥Ar3, of the second rolling procedure to a first temperature between Ar3 and Ar1 is carried out at a first cooling rate KR1 in a first stage of a multi-stage cooling process after the second rolling procedure and in that a cooling from the first temperature to a second temperature <Ar1 is carried out at a second cooling rate KR2 in a subsequent second stage, where the first cooling rate KR1 is < the second cooling rate KR2, a delayed structural transformation in contrast to the prior art can be initiated, which leads to a considerable improvement in the uniform elongation Ag of the heavy plate.
It is assumed that due to the comparatively low first cooling rate KR1, ferrite—which has a lower solubility for carbon (C) than is the case with austenite—can precipitate out from the austenitic structure. Carbon can therefore shift from the ferrite into the remaining austenite and accumulate there. If a comparatively high second cooling rate KR2 is used for further cooling of the starting material, a multiphase structure consisting of ferrite and bainite or ferrite and martensite (depending on the alloy, thickness of the heavy plate, cooling rate KR2, and first temperature for this cooling start) can then form. A first cooling rate to a temperature between Ar3 and Ar1 that is reduced compared to the second cooling rate specifically enables the structural transformation that is decisive for the uniform elongation. In the subsequent second stage, the starting material is cooled at an accelerated rate, for example is quenched, which can limit the loss of toughness in the heavy plate. Depending on requirements (i.e. optionally), the multi-stage cooling can also include a straightening procedure or other process steps. The heavy plate according to the invention with a steel alloy containing the following (each in wt %)
residual iron (Fe) and inevitable production-related impurities, for example each amounting to a maximum of 0.05 wt % and collectively amounting to a maximum of 0.15 wt %, can therefore, in contrast to the prior art, also reproducibly have comparatively high toughness values and also a particularly high uniform elongation. Optionally, this steel alloy can also contain individual elements from the following group or combinations thereof:
In a preferred embodiment, the steel alloy contains the following (each in wt %)
Optionally, the steel alloy can contain individual elements from the following group or combinations thereof (each in wt %):
For example, in that the second cooling rate (KR2) in degrees Celsius is
where C1min=0.5, C2min=69.7, C3min=0.02148, Nmin=0.3 and Thickness=thickness of the heavy plate in millimeters, with this lower limit in the process, the desired microstructure consisting of ferrite, bainite, and possibly martensite can be produced more reproducibly in the steel alloy according to the invention in order to achieve high uniform elongation Ag with high toughness.
For example, for a heavy plate with a thickness of 25 mm, this lower limit for the cooling rate KR2 of the starting material can be 16° C./s. For a heavy plate with a thickness of 80 mm, this lower limit for the cooling rate KR2 of the starting material can be 3.9° C./s, for example.
It can prove to be sufficient if the second cooling rate (KR2) in degrees Celsius is
where C1max=1.5, C2max=12294.5, C3max=0.02264, Nmax=1.43, and Thickness=thickness of the heavy plate in millimeters. With this upper limit, the desired microstructure can be produced particularly reproducibly with the method in order to achieve high uniform elongation Ag with high toughness.
An example of an upper limit (using the above formula) for the cooling rates KR2 can be 71.5° C./s for a heavy plate with a thickness of 25 mm—and 5.3° C./s for a heavy plate with a thickness of 80 mm.
In combination, this means that in this example, the range for the second cooling rate KR2 for the thinner heavy plate (starting material after final forming) can be in the range from 16° C./s to 78.6° C./s and for the thicker heavy plate (starting material after final forming) can be in the range from 3.9° C./s to 5.9° C./s according to the invention. The thicknesses in mm (millimeters), the temperatures in ° C., and the cooling rates in ° C./s apply to all formula specifications.
If the ratio of the second cooling rate KR2 to the first cooling rate KR1 is at least 2:1, then this can further increase the uniform elongation Ag with a sufficiently high toughness. More particularly, if the ratio of the second cooling rate KR2 to the first cooling rate KR1 is at least 3:1, then this can lead to an optimum balance of toughness and uniform elongation Ag in the claimed steel alloy or alloys.
Preferably, the first cooling rate (KR1) is 5° C./s, particularly preferably 3° C./s, which makes the method easier to implement and can also further improve the reproducibility for a high uniform elongation Ag with a high toughness.
Preferably, the first temperature T1 in degrees Celsius is
A first temperature T1 in this range can, among other things, reproducibly lead to a maximum of uniform elongation Ag in the steel alloy according to the invention.
Preferably, the second temperature T2 is in the range from 450° C. to 100° C., preferably in the range from 400° C. to 150° C., particularly preferably in the range from 400° C. to 250° C., in order to set the microstructure for a high toughness and thus achieve an optimum combination of high strength, uniform elongation, and toughness.
If the cooling from the second temperature to room temperature is carried out at a third cooling rate KR3 in a third stage where the third cooling rate KR3 is < the second cooling rate KR2, then this can further improve the mechanical characteristics of the steel alloy.
For example, the third cooling rate KR3 is 5° C./s, preferably 3° C./s, because from a process standpoint, this makes it easier to cool in air without acceleration.
It is preferable if the heating to the second initial rolling temperature for a second rolling procedure is carried out at a heating rate of at least 12° C./min.
For example, the starting material can be heated inductively for this purpose, which can be advantageous for the overall duration of the production route since this allows the cycle time to be reduced and also allows the heating rate to be set very high. Alternatively, heating can also be carried out using radiant heat—this is a particularly simple and robust method of heating.
Preferably, final forming is carried out to a thickness of the heavy plate in the range of 8 to 150 mm (millimeters), more particularly to a thickness of the heavy plate in the range of 25 to 120 mm.
Another object of the invention is to create a heavy plate that has an improved uniform elongation Ag despite high toughness values.
The heavy plate produced by means of the thermomechanical treatment method according to the invention can, for example, have a yield strength ratio (Rp0.2/Rm) of <0.7. Preferably, this heavy plate can have a yield strength ratio (Rp0.2/Rm) of <0.70. Particularly preferably, this heavy plate can have a yield strength ratio (Rp0.2/Rm) of <0.65.
For example, the heavy plate has a thickness in the range of 8 to 150 mm. More particularly, the heavy plate can have a thickness in the range of 25 to 120 mm.
Preferably, the heavy plate has a yield strength Rp0.2>550 N/mm2 (Newton per square millimeter), more particularly a yield strength Rp0.2 (0.2% offset yield strength)>590 N/mm2, in order to be able to ensure a comparatively high strength.
This makes this heavy plate particularly suitable for use in a longitudinally welded pipe for a natural gas pipeline or in a construction material, especially in a seismically active region.
It should be noted in general that because of the comparatively high thickness of the starting material, a wide variety of cooling rates and/or heating rates develop across the thickness of the starting material. For example, a cooling rate on the outside of the starting material can be significantly higher than the cooling rate at its core. The respective cooling rate (KR1, KR2, KR3) or heating rate from the initial temperature to the final temperature is thus an average value, namely a cooling rate or heating rate that is averaged over the thickness of the starting material from the initial temperature to the final temperature.
The subject of the invention is shown in greater detail in the figures based on an embodiment. In the drawings:
and residual iron (Fe) and inevitable production-related impurities, each amounting to a maximum of 0.05 wt % and collectively amounting to a maximum of 0.15 wt %. The thickness of the heavy plates A and B is 25 mm (millimeters) in each case.
The starting material, namely the slab ingot, of the respective heavy plate A, B undergoes a heating 4 to above Ac3 temperature, namely 1100° C. (degrees Celsius), for example by means of a device for slab ingot heating.
The starting material is then partially formed by a first rolling procedure W1.
This is followed by an accelerated cooling 5, namely quenching, preferably water quenching, with which the starting material is cooled from the first final rolling temperature, which is above Ac3, to below the Ar3 temperature; specifically—as is clear from
This is immediately followed by a rapid, preferably inductive, heating 6 to above the Ac3 temperature, at which temperature, as the initial rolling temperature, the starting material is finally formed to a thickness of the heavy plate (final thickness of the starting material) by a second rolling procedure W2.
The starting material leaves the second rolling procedure W2 with a second final rolling temperature EW2≥Ar3, namely 830° C. Instead of inductive heating, other heating sources are also conceivable, for example sources with radiant heat. This rapid heating, whether inductive or with radiant heat, etc., takes place at a rate of at least 12° C./min.
This second rolling procedure W2, which can also be referred to as final rolling, is followed by two different multi-stage cooling processes 3 to room temperature (which is usually between 0 and 60 degrees Celsius in these processes, for example 20 degrees Celsius).
In a first stage 7a of the cooling 3 after the second rolling procedure W2, the starting material of the heavy plate A is cooled from the second final rolling temperature to a temperature below Ar1 in an accelerated manner, namely quenched, by means of water quenching at 30° C./s. This is followed by cooling at 0.1° C./s in still air at ambient temperature as the second immediately subsequent second stage 7b of cooling 3 to room temperature RT.
The multi-stage cooling 3 according to the invention is shown in the case of the starting material of the heavy plate B. In this case, after the second rolling procedure W2 in a first stage 8a, the starting material is cooled at a first cooling rate KR1, namely 0.6° C./s, from the second final rolling temperature EW2 to a first temperature T1 between Ar3 and Ar1, namely to 720° C. (degrees Celsius).
In an immediately subsequent second stage 8b, the starting material is quenched at a second cooling rate KR2, namely 30° C./s, from the first temperature T1 to a second temperature T2<Ar1, namely 150° C. It can, however, also be quenched to room temperature RT, which has is not described in more detail here.
As is clear from
A third stage 8c with a third cooling rate KR3 from the second temperature T2 to room temperature RT in still air at ambient temperature is shown in the multi-stage cooling process in
It should be noted in general that an accelerated cooling is understood to be a cooling that is faster than cooling at room temperature in still air, which is often also referred to as quenching.
It is also conceivable to use a block or a billet as a starting material.
In addition, the first and/or second rolling procedure can consist of one or more partial rolling procedures with possibly several partial rolling steps (passes), which is possible, for example, by means of a reversing rolling procedure.
This process difference in the multi-stage cooling 3 leads to the mechanical characteristics listed in Table 2 for the heavy plates A, B. Stress σ and elongation ε were determined by means of a tensile test (tensile testing according to the standard DIN EN 10002-1) and the toughness was determined by means of a notched bar impact bending test according to the standard DIN EN ISO 148-1.
As is clear from Table 1 and
At low strains (cf. 0.2% offset yield strength Rp0.2), heavy plate B does indeed undergo plastic deformation more quickly than heavy plate A (cf. 0.2% offset yield strength Rp0.2), but failure occurs much later with heavy plate B (cf. Ag).
This property is particularly advantageous in applications in earthquake-prone regions or seismically active regions where the dissipation capacity of the material is crucial.
The heavy plate B produced according to the invention is therefore particularly suit-able, for example, for use in longitudinally welded pipes for natural gas pipelines or in steel construction in seismically active regions. Due to their high uniform elongations, components made from this heavy plate B have a high energy dissipation capacity. It is also conceivable for it to be used as a building material in steel construction in the production of welded I-beams with advantageous behavior in the event of hole bearing failure.
It should be noted in general that the following definitions exist according to DIN EN 10052:
Ac3: temperature at which the transformation of ferrite into austenite ends during a heating process.
Ar1: temperature at which the transformation of austenite into ferrite or into ferrite and cementite ends during a cooling process.
Ar3: temperature at which the formation of ferrite begins during a cooling process.
It should be noted in general that the term heavy plate is known, for example, from DIN EN 10079.
It should be noted in general that the German expression “insbesondere” can be translated as “more particularly” in English. A feature that is preceded by “more particularly” is to be considered an optional feature, which can be omitted and does not thereby constitute a limitation, for example, of the claims. The same is true for the German expression “vorzugsweise”, which is translated as “preferably” in English.
| Number | Date | Country | Kind |
|---|---|---|---|
| 21218236.4 | Dec 2021 | EP | regional |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2022/088050 | 12/29/2022 | WO |
