Thermomechanical treatment process for steel plate and starting materials for the manufacture of steel plate

Abstract

A method for thermomechanical processing of a steel plate and a starting material, more specifically a slab ingot, for the production of the steel plate made of alloy steel is disclosed. In order to achieve advantageous mechanical properties in the steel plate, it is proposed to carry out a multi-stage cooling including at least one straightening step after the final forming. In the multi-stage cooling, in a first stage (8a) after the second rolling stage (W2), the steel plate is cooled at a first cooling rate (KR1) from a second final rolling temperature of the second rolling stage (W2), more specifically a temperature equal to or higher than Ar3, to a first temperature (T1) between Ar3 and Ar1, and in a subsequent second stage (8b), the steel plate is cooled at a second cooling rate (KR2) from the first temperature (T1) to a second temperature (T2) below Ar1. The first cooling rate (KR1) is slower than the second cooling rate (KR2).

Description

[Technical field]

[0001] The present invention relates to a steel plate and a thermomechanical processing method for a starting material for the production of steel plate, more specifically a slab ingot, made of an alloy steel. [Background technology]

[0002] Patent Document 1 discloses a thermomechanical processing method for increasing the toughness, particularly the low-temperature toughness, of thick steel plates made of alloy steel. In this method, the starting material is hot-rolled in several stages, quenched between two hot rolling passes below the Ar3 temperature, and then inductively heated above the Ac3 temperature. After the last hot rolling pass, two stages of cooling to room temperature are performed. First, the cooling rate is accelerated by water quenching to a cooling stop temperature below Ar3, and then cooled to room temperature. Although such thick steel plates have high low-temperature toughness, they have the disadvantage of low uniform elongation Ag, which limits their use in areas prone to earthquakes. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] International Publication No. 2011 / 079341 Summary of the Invention [Problem to be solved by the invention]

[0004] It is therefore an object of the present invention to provide a thermomechanical processing method in the manufacture of steel plate, which reproducibly achieves improved uniform elongation Ag in the steel plate whilst achieving high toughness values. [Means for solving the problem]

[0005] The above object of the present invention is achieved by the features of claim 1.

[0006] After the second rolling step, in the first step of the multi-stage cooling, cooling to the second final rolling temperature of the second rolling step, more specifically from a temperature equal to or higher than Ar3 to a first temperature between Ar3 and Ar1, is performed at a first cooling rate KR1. Then, in the subsequent second step, cooling from the first temperature to a second temperature less than Ar1 is performed at a second cooling rate KR2, where the first cooling rate KR1 is slower than the second cooling rate KR2. This allows a delayed structural transformation to be initiated, in contrast to the prior art, and the uniform elongation Ag of the steel plate is significantly improved.

[0007] It is believed that the relatively low first cooling rate KR1 allows ferrite, which has a lower solubility of carbon (C) than austenite, to precipitate from the austenite structure. Thus, carbon can migrate from ferrite to the remaining austenite and accumulate there. If the starting material is further cooled at a relatively high second cooling rate KR2, a multiphase structure consisting of ferrite and bainite or ferrite and martensite is formed (depending on the alloy, the thickness of the steel plate, the cooling rate KR2, and the first temperature at the start of cooling). In particular, cooling to a temperature between Ar3 and Ar1 at a first cooling rate slower than the second cooling rate can bring about a structural transformation that determines the uniform elongation. In the subsequent second stage, the starting material is cooled more rapidly, for example by quenching, so that the loss of toughness of the steel plate can be suppressed. If required (i.e., optional), the multi-stage cooling can also include straightening treatments and other processing steps. The steel plate according to the present invention uses an alloy steel containing the following (in weight percent): Carbon (C):0.01~0.20 Manganese (Mn): 0.5 to 2.50 Silicon (Si): 0.05 to 0.80 Aluminum (Al): 0.01 to 0.20 Phosphorus (P): <0.05 Sulfur (S): <0.01 The balance includes iron (Fe) and unavoidable impurities due to production, for example, up to 0.05% by weight each and up to 0.15% by weight in total. This makes it possible to reproducibly achieve relatively high toughness values ​​and particularly high uniform elongation, in contrast to the prior art. The alloy steel may also optionally contain individual elements or combinations thereof selected from the group consisting of: Chromium (Cr): 0~1.5 Molybdenum (Mo): 0~1.0 Copper (Cu): 0 to 1.0 Nickel (Ni): 0 to 5.0 Vanadium(V): 0~0.30 Titanium (Ti): 0~0.20 Niobium (Nb): 0~0.20 Boron (B): 0 to 0.005 Nitrogen (N): 0 to 0.015 Calcium (Ca): 0 to 0.01

[0008] In a preferred embodiment, the steel alloy contains the following (each in weight percent): Carbon (C):0.02~0.1 Manganese (Mn): 1.0~2.0 Silicon (Si): 0.1 to 0.80 Aluminum (Al): 0.010 to 0.15 Phosphorus (P): <0.050 Sulfur (S): <0.010

[0009] The steel alloy may optionally include, in weight percent, individual elements or combinations thereof selected from the group consisting of the following (each in weight percent): Copper (Cu): 0 to 0.75 Nickel (Ni): 0~3.0 Vanadium(V): 0~0.20 Boron (B): 0 to 0.003

[0010] For example, the second cooling rate (KR2) is expressed in degrees Celsius as follows, assuming that C1min = 0.5, C2min = 69.7, C3min = 0.02148, Nmin = 0.3, and thickness = thickness of the steel plate (in millimeters).

[0011]

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[0012] In the steel alloy according to the invention, the lower limit of this process allows the desired microstructure of ferrite, bainite and possibly martensite to be achieved by It can be produced more reproducibly and achieve high toughness and high uniform elongation Ag. For example, for a 25 mm thick steel plate, the lower limit of the cooling rate KR2 of the starting material can be 16°C / s. For a 80 mm thick steel plate, the lower limit of the cooling rate KR2 of the starting material can be 3.9°C / s.

[0013] It turns out that the second cooling rate (KR2) is sufficient if, with C1max = 1.5, C2max = 12294.5, C3max = 0.02264, Nmax = 1.43, and thickness = the thickness of the steel plate in millimeters, the following value in degrees Celsius is sufficient: This upper limit allows the desired microstructure to be produced particularly reproducibly and allows high toughness and high uniform elongation Ag to be achieved.

[0014]

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[0015] The upper limit for the cooling rate KR2 (using the formula above) is, for example, 71.5°C / s for a 25 mm thick steel plate and 5.3°C / s for an 80 mm thick steel plate.

[0016] According to the present invention, by way of example, combining the above, it can be seen that the range of the second cooling rate KR2 for the thinner steel plate (starting material after final forming) can be in the range of 16°C / s to 78.6°C / s, and for the thicker steel plate (starting material after final forming) can be in the range of 3.9°C / s to 5.9°C / s. The thickness expressed in mm (millimeters), the temperature expressed in degrees Celsius, and the cooling rate expressed in °C / s are common to all formulas.

[0017] If the ratio of the second cooling rate KR2 to the first cooling rate KR1 is at least 2:1, the uniform elongation Ag can still be achieved with sufficiently high toughness. More specifically, if the ratio of the second cooling rate KR2 to the first cooling rate KR1 is at least 3:1, an optimal balance between toughness and uniform elongation Ag can be achieved in the claimed alloy steel or alloy.

[0018] Preferably, the first cooling rate KR1 is 5° C. / sec or less, particularly preferably 3° C. / sec or less, which makes it easier to carry out the method and further improves the reproducibility of high toughness and high uniform elongation Ag.

[0019] Preferably, the first temperature T1 is expressed in degrees Celsius as follows:

[0020]

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[0021]

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[0022] A first temperature T1 in this range makes it possible, inter alia, to reproducibly achieve the maximum uniform elongation Ag in the steel alloy according to the invention.

[0023] In order to obtain a microstructure that provides high toughness and to achieve an optimal combination of high strength, uniform elongation, and toughness, the second temperature T2 is preferably in the range of 450°C to 100°C, more preferably in the range of 400°C to 150°C, and particularly preferably in the range of 400°C to 250°C.

[0024] Cooling from the second temperature to room temperature is performed in a third stage at a third cooling rate KR3, which is slower than the second cooling rate KR2, thereby further improving the mechanical properties of the alloy steel.

[0025] For example, the third cooling rate KR3 is equal to or less than 5° C. / s, preferably equal to or less than 3° C. / s, which from a process point of view facilitates cooling in air without acceleration.

[0026] Preferably, heating to the second initial rolling temperature in the second rolling step is carried out at a heating rate of at least 12°C / min.

[0027] For example, the starting materials can be heated inductively in this way, thus shortening the time required for the entire manufacturing process and allowing very high heating rates, which has an advantage in terms of the overall production time. Alternatively, heating can also be carried out using radiant heat, which is a particularly simple and robust heating method.

[0028] Preferably, the final forming is carried out for plate thicknesses in the range of 8 to 150 mm, more particularly for plate thicknesses in the range of 25 to 120 mm.

[0029] Another object of the present invention is to produce a steel plate having improved uniform elongation Ag despite high toughness values.

[0030] The present invention achieves this object with the features of claim 14.

[0031] In the steel plate manufactured by the thermomechanical processing method according to the present invention, for example, the yield strength ratio (Rp0.2 / R m ) can be less than 0.7. Preferably, in the case of a thick steel plate, the yield strength ratio (R p0.2 / R m ) can be less than 0.70. Particularly preferably, in the thick steel plate, the yield strength ratio (R p0.2 / R m ) may be less than 0.65.

[0032] For example, the thickness of the thick steel plate may be in the range of 8 to 150 mm, more specifically, the thickness of the thick steel plate may be in the range of 25 to 120 mm.

[0033] Preferably, the yield strength R of the thick steel plate p0.2 550N / mm 2 (Newtons per square millimeter), more specifically, the yield strength R p0.2 (0.2% offset yield strength) 590N / mm 2 This ensures a relatively high strength.

[0034] This makes the steel plate particularly suitable for use in longitudinally welded pipes for natural gas pipelines or as construction materials in seismically active regions.

[0035] In general, the starting material has a relatively large thickness, so that the cooling rate and / or heating rate may vary throughout the thickness of the starting material. For example, the cooling rate at the outside of the starting material may be much higher than the cooling rate at its center. Therefore, each cooling rate (KR1, KR2, KR3) or heating rate from the initial temperature to the final temperature was taken as an average value, i.e., the average value of the cooling rate or heating rate throughout the thickness of the starting material from the initial temperature to the final temperature. [Brief description of the drawings]

[0036] The object of the invention is illustrated in more detail on the basis of an embodiment in the figures, in which:

[0037] [Figure 1]Figure 1 shows the temperature profiles for the two thermomechanical processing methods. [Diagram 2] FIG. 2 shows the stress-strain diagrams of two steel plates, each produced by a thermomechanical processing method according to FIG. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0038] Figure 1 shows two temperature profiles 1 and 2 in a thermomechanical processing method for producing steel plates A and B from alloy steel. Both steel plates A and B (flat products according to the European harmonized material standard DIN EN 10079) use the same alloy steel and contain the following elements: Carbon (c): 0.04% by weight Manganese (Mn): 1.63% by weight Silicon (Si): 0.34% by weight Aluminum (Al): 0.04% by weight Phosphorus (P): 0.012% by weight Sulfur (S): 0.001% by weight Chromium (Cr): 0.17% by weight Molybdenum (Mo): 0.02% by weight Niobium (Nb): 0.035% by weight Titanium (Ti): 0.014% by weight Boron (B): 0.0003% by weight Nitrogen (N): 0.0045% by weight The balance includes iron (Fe) and unavoidable impurities associated with manufacturing, each of which contains a maximum of 0.05% by weight, and a maximum of 0.15% by weight in total. The thicknesses of the steel plates A and B were each 25 mm (millimeters).

[0039] 1 shows that at the end of the multi-stage cooling 3 to room temperature RT, the first temperature profile 1 and the second temperature profile 2 are different from each other. The steps of the method carried out were similar.

[0040] The starting materials, i.e. slab ingots, of the steel plates A and B, respectively, are subjected to heating 4, for example by means of a slab ingot heating device, to a temperature above Ac3 temperature, i.e. above 1100° C. (degrees Celsius).

[0041] The starting material is then partially shaped by a first rolling step W1.

[0042] This is followed by accelerated cooling 5, i.e. quenching, preferably water quenching, whereby the starting material is cooled from the first final rolling temperature (above Ac3) to below the Ar3 temperature. In particular, as is evident from FIG. 1, the starting material is cooled or quenched to below the Ar1 temperature.

[0043] This is followed by rapid, preferably inductive, heating 6 to a temperature above the Ac3 temperature, as the initial rolling temperature, at which the starting material is formed by a second rolling step W2 into a steel plate of final thickness (final thickness of the starting material).

[0044] After the second rolling step W2, the starting material is at a second final rolling temperature EW2 above Ar3, i.e. 830°C. Instead of induction heating, other heat sources are also conceivable, for example radiant heat sources. The rapid heating here (e.g. induction heating or radiant heating) is carried out at a rate of at least 12°C / min.

[0045] This second rolling step W2 is also called final rolling, after which two different multi-stage cooling steps 3 are carried out down to room temperature (usually between 0 and 60°C, for example 20°C, in such processes).

[0046] In the first stage 7a of cooling 3, after the second rolling step W2, the starting material of the steel plate A is quenched from the second final rolling temperature to a temperature below Ar1 in an accelerated manner, i.e. by water quenching at 30° C. / s. This is followed by Immediately following, in a second stage 7b of cooling 3, it is cooled down to room temperature RT at 0.1° C. / s in still air at ambient temperature.

[0047] The multi-stage cooling 3 according to the invention is shown with the case of a starting material for a steel plate B. In this case, after the second rolling procedure W2 in the first stage 8a, the starting material is cooled from the second final rolling temperature EW2 to a first temperature T1 between Ar3 and Ar1, i.e. 720°C, at a first cooling rate KR1, i.e. 0.6°C / s.

[0048] In the immediately following second stage 8b, the starting material is quenched from the first temperature T1 to a second temperature T2 less than Ar1, i.e. 150° C., at a second cooling rate KR2, i.e. 30° C. / s. It can also be quenched to room temperature RT, but this is not described here.

[0049] As is clear from FIG. 1, the first cooling rate KR1 is slower than the second cooling rate KR2, specifically, it is three times slower than the second cooling rate KR2.

[0050] A third stage 8c of cooling from the second temperature T2 to room temperature RT at a third cooling rate KR3 is shown in the multi-stage cooling process of Figure 1. Preferably, the third cooling rate KR3 is 0.1°C / s.

[0051] Generally, accelerated cooling is understood to be cooling faster than cooling at room temperature in still air, and is also called quenching.

[0052] It is also possible to use a block or billet as the starting material.

[0053] Furthermore, the first and / or second rolling procedure can consist of one or more partial rolling procedures and possibly several partial rolling steps (passes), which is made possible, for example, by a reversing rolling procedure.

[0054] Due to the difference in the process in multi-stage cooling 3, the mechanical properties of steel plates A and B were as shown in Table 2. The stress σ and elongation ε were determined by a tensile test (tensile test in accordance with the European Harmonized Material Standard DIN EN 10002-1), and the toughness was determined by a notched bar impact bending test in accordance with the European Harmonized Material Standard DIN EN ISO 148-1.

[0055] [Table 1]

[0056] As is clear from Table 1 and Figure 2, the uniform elongation Ag of steel plate B increases from 8.9% to 14.7%, i.e., an increase of 5.8%, compared to steel plate A. Therefore, steel plate B can exhibit significantly higher energy dissipation capacity and, therefore, energy absorption capacity.

[0057] Low strain (0.2% offset yield strength R p0.2 In fact, steel plate B is subject to plastic deformation at an earlier stage than steel plate A (0.2% offset yield strength R p0.2 However, the fracture occurred much later in plate B (see A g reference).

[0058] These properties are particularly advantageous where the absorbent capacity of the material is important, such as in earthquake-prone or seismically active regions.

[0059] The steel plate B manufactured according to the invention is therefore particularly suitable for use in longitudinally welded pipes for natural gas pipelines or in steel structures, for example in areas of high seismic activity. Due to its high uniform elongation, parts made from the steel plate B have a high energy dissipation capacity. It is also conceivable to use parts made from the steel plate B as building materials for steel structures in welded I-beams, due to their advantageous behavior in case of bore bearing failure.

[0060] In general, the European unified material standard DIN EN 10052 has the following definition: Ac3: The temperature at which the transformation of ferrite to austenite is completed during heat treatment Ar1: The temperature at which the transformation of austenite to ferrite or to ferrite and cementite during cooling is completed. Ar3: The temperature at which ferrite begins to form during the cooling process

[0061] In general, the term "thick steel plate" is known from, for example, the European Harmonized Material Standard DIN EN 10079.

[0062] In general, the German expression "insbesondere" can be translated into English as "more particularly". Features preceded by "more particularly" can be omitted and such features are optional and should not be considered as intended to limit the content of the claims. The same can be said for the German expression "vorzugsweise", which can be translated into English as "preferably".

Claims

1. A thermomechanical treatment method for a starting material, more specifically a slab ingot, for the manufacture of a thick steel plate made of an alloy steel, comprising: The alloy steel contains, by weight percent: Carbon (C): 0.01-0.20 Manganese (Mn): 0.5 to 2.50 Silicon (Si): 0.05 to 0.80 Aluminum (Al): 0.01 to 0.20 Phosphorus (P): <0.05 Sulfur (S): <0.01 and Chromium (Cr): 0 to 1.5 Molybdenum (Mo): 0 to 1.0 Copper (Cu): 0 to 1.0 Nickel (Ni): 0 to 5.0 Vanadium (V): 0 to 0.30 Titanium (Ti): 0 to 0.20 Niobium (Nb): 0 to 0.20 Boron (B): 0 to 0.005 Nitrogen (N): 0 to 0.015 Calcium (Ca): 0 to 0.01 and optionally containing individual elements or combinations thereof selected from the group consisting of: The heat treatment method is Heating the sheet to a temperature equal to or higher than Ac3 temperature and performing partial forming by a first rolling step (W1); After the first rolling step (W1), quenching from the first final rolling temperature to a temperature lower than Ar3, more specifically, to a temperature lower than Ar1; After quenching, the steel sheet is heated to a second initial rolling temperature higher than the Ac3 temperature for a second rolling step (W2), and is finally formed into a thickness of a thick steel plate by the second rolling step (W2); After final forming, multi-stage cooling is performed, including at least one straightening step; Including, When cooling in multiple stages, In a first stage (8a) after the second rolling step (W2), cooling is performed at a first cooling rate (KR1) from a second final rolling temperature of the second rolling step (W2), more specifically, a temperature equal to or higher than Ar3, to a first temperature (T1) between Ar3 and Ar1; In a subsequent second stage (8b), cooling is performed from the first temperature (T1) to a second temperature (T2) less than Ar1 at a second cooling rate (KR2), The first cooling rate (KR1) is slower than the second cooling rate (KR2); A thermomechanical processing method comprising the steps of:

2. The second cooling rate (KR2) is C1min=0.5 C2min=69.7 C3min=0.02148 Nmin=0.3 Thickness = thickness of said steel plate (in millimeters) In this case, in degrees Celsius, [0010] It is expressed as:

2. The thermomechanical processing method according to claim 1 .

3. The second cooling rate (KR2) is C1max=1.5 C2max=12294.5 C3max=0.02264 Nmax=1.43 Thickness = thickness of said steel plate (in millimeters) In this case, in degrees Celsius [0025] It is expressed as:

3. The thermomechanical processing method according to claim 1 or 2.

4. The ratio of the second cooling rate (KR2) to the first cooling rate (KR1) is at least 2:1, more specifically at least 3:1; 4. The thermomechanical processing method according to claim 1 ,

5. The first cooling rate (KR1) is 5 ° C / sec or less, particularly preferably 3 ° C / sec or less; 5. The thermomechanical processing method according to claim 1 ,

6. the first temperature (T1) in degrees Celsius; [0030] and [0045] It is expressed as:

6. The thermomechanical processing method according to claim 1 ,

7. The second temperature (T2) is in the range of 450°C to 100°C, more specifically, in the range of 400°C to 150°C, and preferably in the range of 400°C to 250°C; 7. A thermomechanical processing method according to claim 1, characterized in that

8. The cooling from the second temperature (T2) to room temperature is carried out in a third stage (8c) at a third cooling rate (KR3), the third cooling rate (KR3) being slower than the second cooling rate (KR2); A thermomechanical processing method according to any one of claims 1 to 7, characterized in that

9. The third cooling rate (KR3) is 5° C. / sec or less, preferably 3° C. / sec or less; 9. The thermomechanical processing method according to claim 8 .

10. The heating to the second initial rolling temperature in the second rolling step (W2) is carried out at a heating rate of at least 12 ° C / min; 10. The thermomechanical processing method according to claim 1 ,

11. The final forming is carried out for plate thicknesses in the range of 8 to 150 mm, more specifically for plate thicknesses in the range of 25 to 120 mm; A thermomechanical processing method according to any one of claims 1 to 10, characterized in that

12. The alloy steel comprises, in weight percent: Carbon (C): 0.02-0.1 Manganese (Mn): 1.0 to 2.0 Silicon (Si): 0.1 to 0.80 Aluminum (Al): 0.010 to 0.15 Phosphorus (P): <0.050 Sulfur (S): <0.010 Includes 12. A thermomechanical processing method according to any one of claims 1 to 11.

13. The alloy steel comprises, in weight percent: Copper (Cu): 0 to 0.75 Nickel (Ni): 0 to 3.0 Vanadium (V): 0 to 0.20 Boron (B): 0 to 0.003 Optionally, the composition may include any combination of elements selected from the group consisting of:

13. A thermomechanical processing method according to any one of claims 1 to 12, characterized in that

14. The yield strength ratio of the thick steel plate (R ｐ０．２ / R ｍ ) is less than 0.7, preferably less than 0.65; A thermomechanical processing method according to any one of claims 1 to 13, characterized in that

15. The thickness of the thick steel plate is in the range of 8 to 150 mm, more specifically, in the range of 25 to 120 mm; 15. The thermomechanical processing method according to claim 14,

16. The yield strength of the thick steel plate (R ｐ０．２ ) is 550 N / mm ２ Less than, more specifically, 590 N / mm ２ is less than 16. The thermomechanical processing method according to claim 14 or 15,

17. More specifically, the use of steel plates subjected to the thermomechanical treatment method according to any one of claims 14 to 16 in longitudinally welded pipes for natural gas pipelines or construction materials in areas of high seismic activity.

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