Patent Application
20250059617
The invention relates to a thermomechanical treatment method for the production of a heavy plate from 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 heavy plate 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 a high proportion of internal stresses, depending on their manufacturing route. This has a negative impact on processability—for example during flame cutting.
The object of the invention, therefore, is to create a thermomechanical treatment method for the production of a heavy plate with which a low-stress heavy plate with high toughness values can be produced in a reproducible way.
Because a cooling from a second final rolling temperature, more particularly ≥Ar3, of the second rolling procedure to a first temperature between 250 and 500° C., more particularly 300 to 450° C., is carried out at a first cooling rate KR1 in a first stage after the second rolling procedure and a cooling to room temperature is carried out at a second cooling rate KR2 in a subsequent second stage, wherein a straightening procedure, more particularly a hot straightening procedure, is carried out after the first stage, more particularly during the second stage, and wherein the first cooling rate KR1 is >the second cooling rate KR2, it is possible to ensure a sufficiently high tensile strength Rm and also to achieve an optimized comparatively high yield strength Rp0.2.
This optimized, comparatively high yield strength ratio Rp0.2/Rm, which can for example be between 0.70 and 0.90, offers the advantage that the formability of the material increases as the yield strength ratio increases, but it should not be too high, so that a certain “safety cushion” with regard to material overstressing or cracks can still be provided in the event of overloading.
More particularly, the accelerated cooling can further positively influence the structure of the steel material and increase the proportion of bainitic structures. The heavy plate according to the invention with a steel alloy containing the following, each in wt %
and 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 a comparatively high yield strength ratio Rp0.2/Rm according to the invention while having comparatively high toughness values.
Optionally, this steel alloy can also contain individual elements from the following group or combinations thereof, each in wt %:
In a preferred embodiment, the steel alloy contains the following (each in wt %)
This can further improve the mechanical properties.
Optionally, the steel alloy can contain individual elements from the following group or combinations thereof (each in wt %):
If the ratio of the first cooling rate KR1 to the second cooling rate KR2 is at least 2:1, then this can make it possible to produce the desired structure, consisting of ferrite, bainite, and possibly martensite, in a more reproducible way and thus achieve a high yield strength ratio Rp0.2/Rm together with high toughness. This is particularly the case if the ratio of the first cooling rate KR1 to the second cooling rate KR2 is at least 3:1. The structure formation can be shifted further toward martensite, which can be significantly influenced by the KR1—thus the higher the cooling rate ratio is selected, the higher the tensile strength value can be. This can, however, negatively influence the yield strength. The cooling rate ratio can be selected depending on customer requirements for the mechanical values of the desired product.
If the second cooling rate (KR2) is ≤5° C./s, more particularly ≤3° C./s, then there is comparatively little effect on the structure formation and the mechanical properties.
For example, after the end of the rapid cooling, i.e. the cooling at the first cooling rate (KR1), i.e. at a temperature in the vicinity of T1 or slightly below, a straightening, more particularly a hot straightening procedure, is carried out with a degree of plasticization of 40 to 80%. This can result in an optimum ratio of flatness (a high degree of plasticization is advantageous), yield strength, and toughness. An excessively high degree of plasticization could lead to undesirably high values for the yield strength ratio. Surprisingly, this “hot” straightening, i.e. a straightening at significantly higher than room temperature (for example in the range from 250 to 500° C.), can produce good mechanical properties.
It is assumed that although less work hardening is produced than with “cold” straightening, the effect of precipitation on the other hand is much more pronounced due to the “hot” straightening and this can surprisingly also lead to the optimized properties of good forming properties.
It should be noted in general that plasticizing, by contrast with rolling, does not essentially involve a reduction in thickness, i.e. the sheet thickness remains essentially the same after the straightening procedure. A percentage in the thickness direction of the heavy plate is plastically deformed without significantly reducing the thickness of the heavy plate. A degree of plasticization is therefore a measure for a plastic deformation of the material that is to be straightened and indicates the percentage of the strip cross-section that is plastically deformed during straightening (e.g. cf. DE602004010293T2 or EP3964592A1).
Preferably, final forming is carried out to a thickness of the heavy plate in the range of 8 to 150 mm (millimeters), more particularly in the range of 25 to 120 mm.
Preferably, the straightening procedure is carried out at a straightening temperature within a temperature range from the first temperature (T1) to the first temperature (T1) minus 100° C.
Another object of the invention is to produce a heavy plate that has a comparatively high yield strength ratio Rp0.2/Rm 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 <0.9. Preferably, the yield strength ratio Rp0.2/Rm is <0.90.
In a preferred embodiment, the heavy plate has a yield strength ratio Rp0.2/Rm of >0.70.
Preferably, the heavy plate has a yield strength ratio Rp0.2/Rm in the range from 0.75 to 0.85. In this case, it can be particularly advantageous if the heavy plate has a yield strength ratio Rp0.2/Rm in the range from 0.75 to 0.80.
Preferably, the heavy plate has a thickness in the range from 8 to 150 mm, more particularly in the range from 25 to 120 mm.
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) 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:
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) in 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 RT (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 final rolling procedure W2, the starting material of the heavy plate A is cooled from the second final rolling temperature to a temperature T, namely 100° C., in an accelerated manner, namely quenched, by means of water quenching at 20° C./s. Then the starting material is straightened at this temperature T. The quenching is followed by cooling at 0.1° C./s in still air at ambient temperature to room temperature RT as the subsequent second stage 7b of the multi-stage cooling 3.
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 final rolling procedure W2 in a first stage 8a, the starting material is cooled in an accelerated way, or more precisely quenched, at a first cooling rate KR1, namely 20° C./s, by means of water quenching from the second final rolling temperature EW2 to a first temperature T1, namely 420° C. (degrees Celsius).
Then the starting material is hot straightened, namely with a degree of plasticization of 50%. During the straightening procedure, the starting material has a temperature of about 420° C. (degrees Celsius). It is not possible, though, to rule out a certain amount of cooling up to the start of the straightening procedure. At the latest, the hot straightening takes place at a starting material temperature of 320 degrees Celsius.
In a subsequent second stage 8b, the starting material is cooled to room temperature (RT) at a second cooling rate KR2, namely 0.1° C./s, in still air at ambient temperature. The hot straightening can, however, also take place in the second stage.
As is also clear from
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 cited mechanical characteristics for the heavy plates. 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.
Heavy plate A (method with temperature profile 1 not according to the invention) has the following values:
Heavy plate B (method with temperature profile 2 according to the invention) has the following values:
Thus in comparison to heavy plate A, heavy plate B has higher toughness values and a desired yield strength ratio Rp0.2/Rm in the range from 0.70 to 0.90.
Other exemplary embodiments are shown in
Alloy 1 has somewhat higher values of carbon and chromium and can therefore achieve higher mechanical properties. Depending on customer requirements, however, a lower tensile strength can also be desired, as exhibited by alloy 2. Heavy plates were produced with these alloys 1 and 2.
In the examples according to the invention (respectively shown on the right side in
In addition, the first cooling rate KR1 was varied; in the examples with a thickness of 25 mm, which is shown in
It can be clearly seen in
This can be implemented with both alloys, where in this case, alloy 2 has comparatively lower mechanical properties, more particularly tensile strength.
The inventors have recognized that by setting the cooling stop temperature (T1) appropriately, the mechanical properties can be influenced and, more particularly, the yield strength ratio (STV), namely Rp0.2/Rm, can be optimized. In this case, if the first temperature T1 is too low, for example below 250° C., then the tensile strength will be too high and/or the yield strength too low. On the other hand, if the first temperature T1 is too high, then the tensile strength in turn decreases or the yield strength may even be too high, making the yield strength ratio comparatively unfavorable. An optimum range for T1 must therefore be selected, which according to the invention is in the range from 250° C. to 500° C., more particularly in the range from 300° C. to 450° C.
If necessary, the mechanical properties can be further influenced by suitable adjustment of the straightening temperature—and more particularly, the yield strength ratio (STV), namely Rp0.2/Rm, can be further optimized.
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 |
|---|---|---|---|
| 21218235.6 | Dec 2021 | EP | regional |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2022/088053 | 12/29/2022 | WO |
