EP3016754B2 - Method for hot rolling steel strip - Google Patents

Description

The invention relates to a method for hot rolling steel strip, see e.g. EP 1 038 9178 A1 A hot rolling mill of the type in question here usually comprises a hot rolling mill with several rolling stands which are passed through one after the other in the conveying direction of the steel strip to be hot rolled and a cooling section for the intensive cooling of the hot-rolled steel strip emerging from the last rolling stand of the rolling mill.

Methods of the type according to the invention are used for rolling so-called "heavy plate" whose thickness is at least 15 mm. In the conventional production of such thick steel strips, the respective steel strip is thermomechanically rolled in a four-high stand. However, this rolling process takes much longer than hot rolling in a hot strip mill. The aim is therefore to also hot roll thick steel strips in a conventional hot rolling mill.

A particular challenge is the rolling of flat steel material intended for the manufacture of thick-walled pipelines, which have to meet the highest requirements in terms of toughness and resistance to cracking. These properties are usually assessed using the results of the so-called "Drop Weight Tear Test", or "DWTT" for short. The DWTT is described in the API 5L3 standard of the American Petroleum Institute, 3rd Edition, 02/1996, in ASTM E436, in DIN EN 10274 from 1999 and in the steel-iron test sheet SEP 1326. In this test, a test specimen of a defined weight is dropped from a similarly defined height onto a strip-shaped sheet metal sample, which has a defined groove-like notch on the side facing away from the impacting test specimen in the area of the expected fracture and is placed with its end sections on a support. As a rule, it is required that at a certain specified temperature, for example -35 °C, the ductile fracture fraction of the respective sample generated in this way is on average 85 %.

Attempts have been made to optimize the toughness of thick steel strips required for the manufacture of oil or gas pipelines by using certain hot rolling and cooling strategies. Various examples of these processes can be found in, for example, the EP 1 038 978 B1 The EP 1 038 978 B1 The newly described process itself enables the cost-effective production of high-strength hot strip with outstanding toughness. For this purpose, a preliminary material such as slabs, thin slabs or cast strip is produced from an unalloyed or low-alloyed steel with additions of micro-alloying elements, which then runs through a finishing line made up of several rolling stands. The preliminary material is introduced into the first rolling stand of the finishing line at a temperature that is at least 30 °C above the recrystallization stop temperature of the respective steel. The preliminary strip is then continuously hot rolled to form a hot strip in one or more passes. Hot rolling is carried out in a temperature range that includes the recrystallization range of the austenite. Between two rolling stands, the hot strip is then cooled by means of a cooling device to a temperature that is at least 20 °C below the recrystallization stop temperature, with the cooling rate being at least 10 °C/s. Rolling is then continued below the recrystallization stop temperature with a total deformation degree of at least 30% in the temperature range below the recrystallization stop temperature until the finished hot strip exits the hot rolling mill.

Like in the EP 1 038 978 B1 As also stated, steels for the manufacture of thick-walled pipes typically consist of an alloy containing, in addition to iron and unavoidable impurities (in % by weight), C: ≤ 0.18%, Si: ≤ 1.5%, Mn: ≤ 2.5%, P: 0.005 - 0.1%, S: ≤ 0.03%, N: ≤ 0.02%, Cr: ≤ 0.5%, Cu: ≤ 0.5%, Ni: ≤ 0.5%, Mo: ≤ 0.5%, Al ≤ 2%, up to a total of 0.3% of one or more of the elements B, Nb, Ti, V, Zr and Ca. These steels also include the steel grades known as "X70" and X80.

Practical experience shows that, despite the comparatively complex measures required for the temperature control required in each case in the state of the art, thick hot-rolled strips with increased strength can be produced using the processes known from practice, but that these hot-rolled strips do not meet the requirements for their toughness in the field of pipeline construction with the necessary reliability.

Against this background, the object of the invention was to create a hot rolling process based on a conventional hot rolling mill with which hot strips with a final thickness of more than 15 mm can be reliably produced, which also meet the highest requirements for their toughness.

The solution to the above-mentioned problem with regard to the method consists according to the invention in that the work steps specified in claim 1 are carried out during the production of thick hot strip.

Advantageous embodiments of the invention are specified in the dependent claims and are explained in detail below, as is the general inventive concept.

A plant for hot rolling steel strip therefore comprises, in accordance with the state of the art specified at the beginning, a hot rolling mill which has several rolling stands which are passed through one after the other in the conveying direction of the steel strip to be hot rolled. Typically, such a hot rolling mill consists of seven rolling stands which are lined up one after the other in the conveying direction and which are passed through one after the other by the steel strip to be hot rolled. As is usual with conventional hot rolling plants, the plant also has a cooling section for intensive cooling of the hot-rolled steel strip emerging from the last rolling stand of the rolling mill.

The cooling section now begins, in the direction of conveyance of the steel strip to be hot rolled, not behind the last rolling stand of the hot rolling stage, but before the end of the hot rolling stage. The start of the cooling section is set up in such a way that the cooling section begins immediately after the last rolling stand that was actively passed through before entering the cooling section. "Active" here means that hot rolling is still taking place in this rolling stand. "Inactive", on the other hand, are the rolling stands whose roll gap is opened so wide by a corresponding adjustment of the work rolls that the hot strip no longer experiences any deformation when passing through the relevant rolling stand. This means that when the hot strip leaves the last hot rolling stand in the direction of conveyance before the start of the cooling section, in which hot rolling is still taking place, it is immediately caught by the cooling fluid released by the cooling section and cooled at an accelerated rate.

In the hot rolling plant described above, the cooling section and the hot rolling stage overlap in such a way that the rolling stage is shortened by two rolling stands and the cooling section is extended into the rolling stage at least so far that, if several of the rolling stands passed through last in the conveying direction of the steel strip to be hot rolled are inactivated, cooling can take place directly behind the last rolling stand in which forming is still taking place.

The method according to the invention according to claim 1 for producing rolled steel strip accordingly provides that it is carried out on a previously described system and, during hot rolling, the roll gap is opened in inactive rolling stands to such an extent that no more deformation of the steel strip takes place in this rolling stand in the hot rolling stage, wherein the steel strip is cooled at an accelerated rate after leaving the last active rolling stand by applying a cooling fluid.

The invention is therefore based on the proposal to operate a conventional multi-stand rolling mill in such a way that the thickness of the steel strip is not reduced in each of the hot rolling stands it passes through. Instead, the steel strip is only deformed in the active rolling stands of the rolling mill. In the inactive rolling stands, the roll gap is opened so wide that its work rolls no longer touch the rolling stock, so that no more deformation can take place in it. At the same time, the start of the cooling section is shifted into the hot rolling mill so that, for example, in a hot rolling mill with seven hot rolling stands, the accelerated cooling can take place immediately after the fifth rolling stand and no more hot rolling takes place above the penultimate, i.e. the sixth, and last, i.e. the seventh, rolling stands.

This approach is based on the knowledge that when high-strength tube sheet grades with a thickness of more than 15 mm, whose toughness is subject to the highest requirements, are to be hot rolled in a hot rolling mill in which they pass through the rolling stands one after the other in a continuous sequence, only a limited number of hot deformations should be carried out in order to achieve, on the one hand, sufficient deformation per rolling pass by means of the activated rolling stands for good dimensional stability of the strip. On the other hand, the limited number of rolling passes with cooling starting directly after the last deformation makes it possible to shift the transition temperature of the toughness to lower temperatures. In this way, on the basis of conventional hot rolling mills modified in the manner according to the invention, steel sheets for pipes can be produced which not only have a high level of strength, such as the steel grades "X70" or "X80", but also have a low transition temperature of -10° C and less and high toughness requirements up to thicknesses of 25.4 mm.

When producing hot strip with a thickness of more than 18 mm according to the invention, bainitic steels can preferably be used in order to reliably meet the requirements to be met according to DWTT. By improving the transition temperatures as a result of the cooling that begins as quickly as possible after the last effective forming pass according to the invention, the application range of ferritic / pearlitic steels can be expanded to include greater thicknesses.

In contrast to conventional cooling after the last stand of a finishing line, the cooling according to the invention, which extends into the rolling line and starts early when rolling in thicknesses >15 mm, prevents unhindered access of oxygen and, as a result, severe post-scaling of the strip surfaces.

When operating a hot rolling mill according to the invention, the rolling speeds are low due to the early end of active forming and the low overall degrees of deformation achieved during hot rolling. They are typically in the range of less than 3 m/s.

By extending the cooling section into the finishing line, it is also possible to display cooling curves with holding times. The system configuration simply has to be designed in such a way that when rolling in a rolling line with seven rolling stands, of which only the first five are activated, the spraying starts directly behind the fifth stand, whereby the amount of cooling fluid applied in front of and behind the unused rolling stands can ideally be adjusted. In conjunction with another spraying system behind the seventh stand and/or a suitable cooling section behind the measuring house provided as standard in hot rolling mills of the type in question, different holding times can be achieved with the desired cooling curves.

For this purpose, in a hot rolling plant according to the invention, the cooling section can comprise several cooling units and a cooling unit can be arranged above the rolling stand passed through in the conveying direction behind the last one before entering the cooling section and each further rolling stand passed through thereafter.

The cooling that takes place after the last active rolling stand is not carried out using conventional laminar cooling, which is known from conventional hot rolling mills, but rather a particularly rapid cooling with a higher cooling rate of at least 80 K/s is used. Cooling rates of at least 130 K/s have proven particularly effective, although in practice the cooling rate is typically up to 160 K/s. The rapid cooling provided for in the invention limits grain growth in the hot-rolled steel strip and increases the cold toughness of the material, so that it reliably reaches maximum toughness values at low temperatures and accordingly has the highest mechanical properties.

In order to achieve the intensive cooling according to the invention, intensive cooling or compact cooling units can be used, for example. These should be designed in such a way that the cooling section is able to provide a cooling fluid output of at least 1000 m ³ /h, in particular up to 1500 m ³ /h. Cooling is preferably carried out from both the top and bottom of the strip to be cooled in order to ensure that the strip cross-section cools as quickly as possible and as evenly as possible. After each intensive cooling, the water remaining on the hot strip can be sprayed off by cross-spraying at high pressure before the hot strip passes through the next inactive rolling stand and is subsequently cooled further. This prevents water from standing on the hot strip after the respective cooling stage and ensures that a correspondingly controlled, gradual cooling of the hot strip is achieved.

Compact cooling units are particularly suitable for the accelerated cooling that is brought forward into the rolling mill according to the invention, and each of these units applies a cooling fluid jet concentrated on a specific section to the respective hot strip. Outside the rolling mill, however, the cooling units of the cooling section can be designed as conventional intensive cooling units, for example.

With regard to the specifically controlled manner in which the cooling is carried out according to the invention, it has proven to be optimal in this context if the length measured in the conveying direction of the steel strip to be hot-rolled, over which the cooling unit arranged behind one of the rolling stands within the rolling mill applies cooling fluid to the steel strip in the conveying direction, is at most 25% of the distance at which the rolling stands of the rolling mill arranged adjacent to one another are arranged one after the other in the conveying direction. In particular, if the length section over which the cooling fluid is applied is limited to 8 - 15% of the distance between the cooling units, the best working results are achieved in practice.

In this way, cooling between the rolling stands can be carried out in such a way that, due to the intensity of the cooling, no longer controlled deformation can take place in the austenite region of the steel being processed. In this respect, the cooling units provided according to the invention, which are designed in particular as compact cooling units, differ from those cooling devices that are used in conventional hot rolling mills to cool the strip to be hot rolled between two rolling stands. The cooling units used according to the invention from the last active rolling stand onwards cause such intensive strip cooling that no longer controlled deformation can take place in the austenite region.

Typically, when carrying out the hot rolling process according to the invention, the initial hot rolling temperature of the steel strip is above 800 °C and below 1050 °C. The exit temperature at which the steel strip enters the cooling section when leaving the last rolling stand over which it is hot-formed, on the other hand, is typically between 740 °C and 900 °C.

In order to achieve the desired toughness properties of the steel strip hot-rolled according to the invention, it may be expedient to interrupt the cooling of the steel strip at a cooling stop temperature when the steel strip has reached a cooling stop temperature of between 500 °C and 700 °C. It has also proven advantageous with regard to the development of the desired mechanical properties if the steel strip is cooled in air for 2 - 12 seconds without active cooling after reaching this cooling stop temperature.

After cooling as described above, the steel strip can be coiled at a coiling temperature of between 450 °C and 650 °C.

Thin slabs or pre-strips with a thickness of 50 - 100 mm are particularly suitable as preliminary products for the hot rolling according to the invention. In contrast, the final thickness of the steel strip hot-rolled according to the invention is typically more than 15 mm. Tests have shown that with the method according to the invention, heavy plates can be hot-rolled on hot rolling mills converted according to the invention in a continuous sequence of work steps. These plates are up to 25.4 mm thick and in the DWTT also meet the highest requirements for their toughness.

The method according to the invention is suitable for high-strength, micro-alloyed steels and steels according to DIN EN 10149. The method according to the invention is particularly suitable for processing steel strips made of the bainitic grades X60, X65, X70, X80 and other comparable steels that are usually used for heavy plate production. The steels particularly suitable for the process according to the invention can be summarized under the general alloying specification (in wt. %) C: ≤ 0.18%, Si: ≤ 1.5%, Mn: ≤ 2.5%, P: 0.005 - 0.1%, S: ≤ 0.03%, N: ≤ 0.02%, Cr: ≤ 0.5%, Cu: ≤ 0.5%, Ni: ≤ 0.5%, Mo: ≤ 0.5%, Al ≤ 2%, up to a total of 0.3% of one or more of the elements B, Nb, Ti, V, Zr and Ca, the remainder being iron and unavoidable impurities.

The invention provides a method which makes it possible in a variety of ways to produce hot-rolled steel strip of great thickness based on a conventional hot rolling mill, which not only has high strength values but also has optimal toughness. The steel strips produced in this way are particularly suitable for pipeline construction due to their property profile. A hot rolling mill designed as described above can also be used for other hot rolling tasks without any problem. To do this, the cooling units provided according to the invention in the overlap area between the cooling section and the hot rolling stage only have to be deactivated or operated in such a way that they meet the cooling requirements for conventional hot rolling.

Fig. 1

eine Anlage 1 zum Warmwalzen von Stahlband S mit einer Enddicke D von mehr als 15 mm mit Kühlung von oben und unten;

Fig. 2

zwei in der Anlage 1 vorgesehene Walzgerüste in seitlicher Ansicht;

Fig. 3

die beiden Walzgerüste gemäß Fig. 2 in einer Ansicht von oben;

Fig. 4

ein Diagramm, in dem für verschiedene Varianten einer in der Anlage 1 durchgeführten Abkühlung des Stahlbands der Temperaturverlauf über die Zeit dargestellt ist.

The invention is explained in more detail below using exemplary embodiments. Each of these shows schematically:

Fig.1

a plant 1 for hot rolling steel strip S with a final thickness D of more than 15 mm with cooling from above and below;

Fig.2

two rolling stands provided in Annex 1 in side view;

Fig.3

the two rolling stands according to Fig.2 in a view from above;

Fig.4

a diagram showing the temperature curve over time for different variants of cooling the steel strip carried out in Appendix 1.

The plant 1 comprises a hot rolling mill 2, which is formed in a conventional manner by seven rolling stands F1, F2, F3, F4, F5, F6, F7, which are arranged one after the other in the conveying direction F of the steel strip S to be hot rolled in the plant 1, a roller table 3, which follows the hot rolling mill 2 in the conveying direction F, a coiler device 4, which is positioned at the end of the roller table 3, seen in the conveying direction F, a measuring house M, which is arranged in the area of the roller table 3 adjacent to the end of the hot rolling mill 2, and a cooling section 5.

The cooling section 5 is formed by several cooling units K1, K2, K3, designed as compact cooling devices, arranged one behind the other in the conveying direction F, and cooling units K4, K5, K6, ..., Kn, designed as conventional cooling units, optionally as laminar cooling units, which are fed via a cooling fluid supply (not shown here) and whose cooling fluid output can be individually adjusted. The cooling fluid is applied by the respective cooling units K1 - Kn from below and from above to the respective bottom and top of the steel strip S. In order to ensure the required cooling fluid output, the cooling fluid flowing to the cooling units K1 - K3 can, for example, be pressurized if necessary by means of pumps (also not shown here).

The first cooling unit K1 of the cooling section 5 in the conveying direction F is arranged between the fifth rolling stand F5 and the sixth rolling stand F6 and the second cooling unit K2 of the cooling section 5 is arranged between the sixth rolling stand F6 and the seventh rolling stand F7 of the rolling mill 2, so that the cooling section 5 extends into the rolling mill 2 and accordingly the end section 6 of the rolling mill 2 and the beginning section 7 of the cooling section 5 overlap each other. The length section a, over which the cooling units K1, K2 and K3 arranged in the rolling mill apply cooling fluid to the steel strip S, is limited to approx. 10% of the distance A in which, as can be seen from the rolling stands F5 and F6 arranged one behind the other in the conveying direction F in the Figures 2 and 3 shown, the adjacent rolling stands F1 - F7 are arranged respectively.

Between the respective cooling units K1 and K2 arranged in the rolling mill 2 and the rolling stand F6, F7 arranged next in the conveying direction F and behind the cooling unit K3 provided after the rolling stand F7, a spraying device Q1, Q2, Q3 is provided which directs a high-pressure jet O aligned transversely to the conveying direction F and in the direction of the respective cooling unit K1, K2, K3 at least onto the upper side of the steel strip S in order to drive cooling fluid standing there from the relevant surface.

In principle, it is possible to operate the rolling stands F1 - F7 located further up in the hot rolling mill 2 inactively. However, practice shows that at least five of the rolling stands F1 - F7 must be active, whereby according to the invention in each case after the rolling stand in the conveying direction F last active rolling stand, but at the latest after the last rolling stand F7 of the hot rolling mill 2, the intensive compact cooling starts.

The cooling unit K1 arranged between the fifth rolling stand F5 and the sixth rolling stand F6 of the hot rolling mill 2 is set up in such a way that, when the cooling unit K1 is switched on, the coolant jets directed vertically downwards that it emits reach as far as the outlet from the rolling stand F5. In the same way, the cooling unit K2 arranged between the sixth rolling stand F6 and the seventh rolling stand F7 of the hot rolling mill 2 is set up in such a way that, when the cooling unit K2 is switched on, the coolant jets that it emits reach as far as the outlet from the rolling stand F6. Likewise, the cooling unit K3 arranged behind the seventh rolling stand F7 in the conveying direction F is set up in such a way that, when the cooling unit K3 is switched on, the coolant jets that it emits reach as far as the rolling stand F7.

In the embodiments described here, at least one of the cooling units K1 - K3 is in operation. Cooling in air can take place in the area of the cooling unit that is not active. The hot strip is cooled to the required coiling temperature HT using the conventional cooling units K4 - Kn, which are located behind the hot rolling mill 2 in the conveying direction F.

In practice, the thickness of the steel slabs processed in rolling mill 2 is typically in the range of 180 - 270 mm. Specifically, in the exemplary embodiments described here, 255 mm thick slabs were produced from the steels E1, E2, E3 specified in Table 1, which entered hot rolling mill 2 with a hot rolling start temperature WAT typically in the range of 800 - 1050 °C and were hot rolled there in a continuous sequence in the first five rolling stands F1, F2, F3, F4, F5 to form a steel strip S. The thickness D of the steel strips S hot-rolled from the steels E1, E2, E3 was 23 mm or 18 mm in each case. The hot rolling start temperatures WAT specifically set in the exemplary embodiments explained here are given in Table 3. In addition, the temperature TAF5 at the exit of the fifth rolling stand F5, the temperature WET at the exit of the finishing train and the coiler temperature HT are also specified there for the hot strip processed in each case and produced from the respective steel E1, E2 and E3.

The steel strips S emerging from the fifth rolling stand F5 have also passed through the last two rolling stands F6 and F7 of the hot rolling mill 2. However, in these rolling stands F6, F7, the work rolls were moved so far apart that the height of the roll gap delimited by them was greater than the thickness D of the steel strip S emerging from the fifth rolling stand F5. As a result, in the embodiments explained here, no further forming of the steel strip S took place via the last two rolling stands F6 and F7 of the rolling mill 2 as seen in the conveying direction F.

Since the rolling stands F6 and F7 were inactive and thus the rolling stand F5 was the last of the rolling stands F1 - F7 in the conveying direction F in which hot forming of the steel strip S took place, the cooling units K1 and K2 as well as all subsequent cooling units K3 - Kn of the cooling section 5 were activated. Accordingly, the steel strip S emerging from the last active rolling stand F5 in the conveying direction F was caught by the cooling fluid jet of the cooling unit K1 after it emerged from the working gap A5 and was intensively cooled on its way to the next rolling stand F6 until it reached the inlet E6 of the rolling stand F6. As soon as the steel strip S passed through the working gap A6 of the inactive rolling stand F6, it was immediately caught by the cooling fluid jet of the cooling unit K2 and cooled just as intensively until it reached the inlet E7 of the inactive rolling stand F7. Likewise, immediately after it has passed through the working gap A7 of the rolling stand F7, the steel strip S is caught by the cooling fluid jet of the cooling unit K3 and runs out onto the roller table 3, where it is further accelerated and cooled in a controlled manner by the additional cooling units K4 - Kn arranged there until a cooling stop temperature of 500 - 700 °C is reached.

When the cooling stop temperature is reached, the active cooling is stopped and the steel strip S runs out on the roller table 3 until it is reeled into a coil in the reeling device 4 at a reel temperature of 450 - 650 °C.

The cooling units K1 - Kn of the cooling section 5 have achieved a total output of up to 1500 m3/h, specifically 1400 ^m3 /h, of cooling fluid via the cooling section 5 at a cooling fluid pressure of more than 3 bar, specifically 3.2 bar, and a cooling fluid temperature of less than 40 °C, specifically ²⁵ °C.

In the embodiments described here, water was used as the cooling fluid. Of course, other cooling fluids can also be used to achieve the required cooling rate.

In Fig.4 For a 23 mm thick hot strip sample made of steel E1, the temperature curve over time t is shown as a solid line T1, which is achieved in the above-explained operating mode of plant 1 according to the invention.

For comparison, Fig.4 The dashed line T2 shows the temperature profile that is achieved during the production of a 23 mm thick hot strip sample made of steel E1 when the cooling already begins in the rolling mill 2 in accordance with the invention, but the cooling rate is less than 80 K/s.

In contrast, in a conventional hot rolling mill equipped with seven rolling stands, where the 23 mm thick hot strip made of E1 steel is rolled up to after the measuring house M in air and then cooled by means of a compact cooling system that only starts after the measuring house M, which is in Fig.4 The temperature curve shown by the dotted line T3 is achieved.

Finally, the Fig.4 The dotted line T4 also shows the temperature curve achieved in a conventional hot rolling mill equipped with seven rolling stands, in which the hot strip is cooled in air after leaving the last active rolling stand F5 up to the measuring house M and after the measuring house M by means of conventional laminar cooling.

In the diagram according to Fig.4 In addition, for each temperature curve T1 - T4, the respective temperature TAF5 that the hot strip has at the exit of the last active rolling stand F5 is symbolized by filled triangles, the respective temperature TAF6 that the hot strip has at the exit of the first inactive rolling stand F6 is symbolized by unfilled triangles, the respective temperature WET that the respective steel strip S had at the end of rolling mill 2 is symbolized by a square and the respective coiler temperature is symbolized by a circle.

It is shown that only with the procedure according to the invention a temperature profile of the cooling is established (line T1) in which the bainitic structure required for the desired toughness is reliably achieved.

Each of the steel strips S manufactured in this way from steels E1, E2 and E3 achieved the target values specified for the respective steel in terms of strength (steel E1: Rm at least 570 MPa, Rt0.5 at least 485 MPa; steel E2: Rm at least 570 MPa, Rt0.5 at least 485 MPa; steel E3: Rm at least 625 MPa, Rt0.5 at least 555 MPa).

The average transition temperatures Tue determined in the DWTT for the steel strips S produced from the steels E1, E2, E3 in the inventive manner described above, at which a matte fracture rate of on average more than 85% was present, as well as the specifically measured tensile strengths Rm and yield strengths Rp0.5 are given in Table 2. Thus, each of the steel strips S produced according to the invention also met the requirements placed on its toughness. Table 1 steel C Si Mn P S Al Cr Cu Mon N No Nb T V Sn 8th Approx CÄq* ¹⁾ PCM2 ⁾ E1 0.054 0.297 1,528 0.014 0.0013 0.036 0.221 0.026 0.103 0.0054 0.2053 0.0657 0.0191 0.0006 0.005 0.0004 0.0012 0.389 0.165 E2 0.075 0.388 1,633 0.014 0.0014 0.032 0.044 0.023 0.008 0.0063 0.0296 0.0564 0.0044 0.0826 0.0065 0.0004 0.0007 0.378 0.184 E3 0.046 0.290 1,690 0.012 0.001 0.035 0.280 0.040 0.110 0.0054 0.0500 0.0770 0.0130 0.0030 0.0030 0.0005 0.0011 0.412 0.167 Data in wt.%, balance iron and unavoidable impurities
¹⁾ CÄq = C + Mn/6 + (Ni + Cu)/15 + (Cr + Mo + V)/5 (according to International Institute of Welding (IIW))
²⁾ PCM = C + Si/30 + (Mn + Cu + Cr)/20 + Ni/60 + Mo/15 + V/10 + 5B ( according to ITO et al.: Weldability Formula of High Steels, Related to Heat-Affected Zone Cracking, Sumintomo Search, 1 (1969), H. 5, p. 59-70 ) steel Steel strip thickness [mm] Tue [°C] Rp0.5 [MPa] Rm [MPa] Matt breakage rate [%] E1 18 -20 530 630 >90 E1 23 0 530 630 >85 E2 18 -10 530 630 >85 E3 18 -20 650 650 >87 steel Steel strip thickness [mm] WAT [°C] TAF5 [°C] WET-T [°C] HT [°C] E1 18 900 820 730 550 E1 23 880 820 700 550 E2 18 900 820 730 550 E3 18 880 820 730 550

REFERENCE SIGNS

1

Plant for hot rolling of steel strip S

2

Hot rolling mill

3

Roller conveyor

4

Reel device

5

Cooling section

6

End section of rolling mill 2

7

Initial section of cooling section 5

A

Distance between two adjacent rolling stands F1 - F7

a

Length section over which the cooling units K1-K3 each apply cooling fluid to the steel strip S

A5

Working gap of the rolling stand F5

A6

Working gap of the rolling stand F6

A7

Working gap of the rolling stand F7

D

Thickness of steel strip S

E6

Entry of rolling stand F6

E7

Entry of rolling stand F7

F

Conveying direction of steel belt S

F1 - F7

Rolling stands of hot rolling mill 2

K1 - K3

Cooling units in the area of hot rolling mill 2

K4 - Kn

Cooling units in conveying direction F behind measuring house M

M

Measuring house

O

Fluid jet emitted by spray devices Q1,Q2

Q1,Q2,Q3

Spraying equipment

S

Steel band

T1-T4

Temperature curves during inventive driving

T

Temperature in °C

t

Time in s

Claims (9)

1. Process for the hot rolling of steel strip for the manufacture of thick-walled pipes, characterized in that it is based on an installation for hot rolling steel strip (S), having a hot rolling relay (2) consisting of seven rolling stands (F1 - F7) that are passed through successively in the conveying direction (F) of the steel strip (S) to be hot rolled, and having a cooling section (5) for intensively cooling the hot-rolled steel strip (S) exiting the final rolling stand (F7) of the rolling relay (2), wherein the start of the cooling section (5) is shifted to be the end of the hot-rolling relay (2) as seen in the conveying direction (F) of the steel strip (S) to be hot-rolled, wherein the cooling section (5) starts after the last rolling stand (F5) that is actively passed through before entry into the cooling section (5), in which hot rolling of the steel strip (S) to be hot rolled in each case takes place and that the cooling section (5) comprises a plurality cooling units (K1 - Kn) and that in the conveying direction (F) a respective cooling unit (K1, K2, K3) is arranged behind the last rolling stand (F5) that is passed through before entry into the cooling section (5) and of every further rolling stand (F6, F7) that is passed through and before entry first cooling unit K1 of the cooling section (5) in conveying direction F is arranged between the fifth rolling stand (F5) and the sixth rolling stand (F6) and the second cooling unit (K2) of the cooling section (5) is arranged between the sixth rolling stand (F6) and the seventh rolling stand (F7) of the rolling relay (2), and that during hot rolling the working gap (A6, A7) at the last two rolling stands (F6, F7) as seen in the conveying direction (F) is opened to such an extent that, starting from this rolling stand (A6, A7) in the hot rolling relay (2), no more deformation of the steel strip (S) occurs, five of the rolling stands (F1 - F5) being active, and in that after exiting the rolling stand (F5, F6) that is passed through before the respectively first opened rolling stand (F6, F7), the steel strip (S) is cooled in an accelerated manner at a cooling rate of at least 80 K/s by being subjected to a cooling fluid, the final thickness (D) of the steel strip (S) being at least 15 mm on exciting the hot rolling relay (2).
2. Method according to claim 1,
   characterized in that the final hot rolling speed is less than 3 m/s.
3. The method according to any one of claims 1 or 2,
   characterized in that the initial hot rolling temperature of the steel strip (S) is more than 800 °C and less than 1050 °C.
4. Method according to any one of claims 1 to 3,
   characterized in that the exit temperature at which the steel strip (S) enters the cooling section (5) on leaving the final rolling stand (F5), via which it is hot-formed, is between 740 °C and 900 °C.
5. Method according to one of claims 1 to 4,
   characterized in that the cooling of the steel strip (S) is stopped at a cooling-stop temperature of between 500 °C and 700 °C.
6. Process according to claim 5,
   characterized in that upon reaching the cooling-stop temperature, the steel strip (2) is kept at the particular temperature for 2-12 seconds.
7. Method according to any one of claims 1 to 6,
   characterized in that the steel strip (S) is coiled at a coiling temperature of between 450 °C and 650 °C.
8. A method according to any one of claims 1 to 7,
   characterized in that the thickness (D) of the steel strip (S) upon entering the hot rolling relay is 50 - 100 mm and upon leaving the hot rolling relay is > 15 - 25.5 mm.
9. Method according to any one of claims 1 to 8,
   characterized in that the steel strip (S) is produced from a steel which, in addition to iron and unavoidable impurities, consists of (in % by weight) C: < 0.18 %, Si: < 1.5 %, Mn: 2.5 %, P: 0.005 - 0.1 %, S: 0.03 %, N: < 0.02 %, Cr: 0.5 %, Cu: 0.5 %, Ni: 0.5 %, Mo: 0,5 %, Al < 2 %, up to a total of 0.5 % of one or more of the elements B, Nb, Ti, V, Zr, Ca.

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