EP1633894B1 - Method and installation for the production of hot-rolled strip having a dual-phase structure - Google Patents

Description

The invention relates to a process for the production of hot strip with a dual phase structure of ferrite and martensite, wherein at least 70% of the austenite are converted into ferrite, from the hot rolled state by a controlled two-stage cooling after finish rolling to a strip temperature below the martensite start temperature in a Cooling section of spaced from each other arranged water cooling groups.

The targeted structural transformation by a controlled cooling of the steels is known, which is carried out for the production of dual-phase steels this controlled cooling time after the successful transformation of the hot strip. The setting of the achievable dual-phase microstructure depends essentially on the cooling technology's possible cooling rates and the chemical composition of the steel. In any case, sufficient ferrite formation of at least 70% in the first cooling stage is important. During this first cooling stage, a conversion of the austenite in the pearlite stage should be avoided.

The cooling capacity of the second cooling stage following the first cooling stage must be so great that reel temperatures below the martensite start temperature are achieved. Only then is the formation of a dual-phase microstructure with ferritic and martensitic constituents ensured.

The known production of dual-phase steels is unproblematic at low belt speeds or at sufficiently long cooling sections. At very high belt speeds, however, the beginning of the second cooling stage can be shifted so far in the existing cooling section, that the subsequent martensite is only imperfect or completely absent. This results in a mixed structure of ferrite, bainite and proportions Martensite, which does not achieve the desired mechanical properties of pure dual-phase microstructure.

In the EP 0 747 495 B1 describes a process for the production of high strength steel sheet having a structure of at least 75% ferrite, at least 10% martensite and optionally bainite and retained austenite. It is therefore not a microstructure of pure dual-phase steels. The alloy used is a niobium-microalloyed steel. For its production, the hot-rolled steel sheet is selectively cooled, with a slow cooling followed by a rapid cooling or alternatively the slow cooling is first preceded by a rapid cooling. For the first cooling stage, a cooling rate of 2 to 15 ° C / s is given within a period of 8 to 40 seconds cooling time to a final temperature between the Ar ₁ point and 730 ° C. The second cooling stage is performed at a cooling rate of 20 to 150 ° C / s up to a temperature of 300 ° C. Alternatively, the rapid cooling preceding the slow cooling is performed at a cooling rate of 20 to 150 ° C / s below the Ar ₃ point.

In the EP 1 108 072 B1 describes a process for the production of dual-phase steels in which, after the finish rolling with a two-stage cooling - first slowly, then quickly - a biphasic structure of 70 to 90% ferrite and 30 to 10% martensite is achieved. The first (slow) cooling is carried out in a cooling section in which the hot strip is cooled by spaced-apart water cooling zones with a cooling rate of 20 - 30 K / s defined. The cooling is adjusted so that the cooling curve enters the ferrite region with an even high temperature, so that ferrite formation can take place quickly. This first cooling is continued until at least 70% of the austenite has been converted to ferrite, followed by further (rapid) cooling immediately and without holding time.

Based on this described prior art with the indicated various possibilities of producing dual-phase structure, it is an object of the invention to provide a method and a system, with or in the production of hot strip with dual-phase structure in a conventional Gießwalzanlage with the local given there and thus time restrictions are feasible. The cooling section of such a system is characterized in that the total length usually does not exceed 50 m and no compact cooling is provided.

• 0,01 - 0,08% C, 0,9 % Si, 0,5 - 1,6 % Mn, 1,2 % Al, 0,3 - 1,2 % Cr, Rest Fe sowie übliche Begleitelemente, zur Erzielung eines Warmbandes mit einem zweiphasigen Gefüge aus 70 bis 95 % Ferrit und 30 bis 5 % Martensit mit hoher mechanischer Festigkeit und hohem Umformvermögen (Zugfestigkeit größer 600 MPa, Bruchdehnung mindestens 25 %) in der Kühlstrecke einer Gießwalzanlage die zweistufige kontrollierte Kühlung von einer Endwalz-Bandtemperatur T_finish mit A₃ - 100 K < T_finish < A₃ - 50 K auf eine Haspel-Bandtemperatur T_coiling < 300 °C (< Martensit-Starttemperatur) durchgeführt wird, wobei die Abkühlgeschwindigkeit V_1,2 in beiden Kühlstufen zwischen V = 30 - 150 K/s, vorzugsweise zwischen V = 50 - 90 K/s liegt, die erste Kühlstufe bis zum Eintritt der Kühlkurve in das Ferritgebiet durchgeführt wird und dann die durch Umwandlung des Austenits in Ferrit freigesetzte Umwandlungswärme zum isothermen Halten der erreichten Bandtemperatur mit einer Haltezeit 5 s bis zum Beginn der zweiten Kühlstufe genutzt wird.

This object is procedurally achieved with the characterizing features of claim 1, characterized in that starting from a steel having the chemical composition: consisting of:

• 0.01-0.08% C, 0.9% Si, 0.5-1.6% Mn, 1.2% Al, 0.3-1.2% Cr, remainder Fe and usual accompanying elements to achieve a hot strip with a two-phase structure of 70 to 95% ferrite and 30 to 5% martensite with high mechanical strength and high formability (tensile strength greater than 600 MPa, breaking elongation at least 25%) in the cooling section of a casting mill the two-stage controlled cooling of a final rolling strip temperature T _finish with A ₃ - 100 K <T _finish <A ₃ - 50 K to a reel belt temperature T _coiling <300 ° C (<martensite start temperature) is performed, wherein the cooling rate V _1.2 in both cooling stages between V = 30-150 K / s, preferably between V = 50-90 K / s, the first cooling stage is performed until the entry of the cooling curve in the ferrite region and then the heat of transformation released by conversion of the austenite into ferrite for isothermal holding of the band temperature achieved a holding time of 5 s to B Eginn the second cooling stage is used.

Due to the short length of conventional cooling sections in existing cast rolling mills, the production of hot strip with dual-phase structure is only possible with a special cooling strategy. In order for such a cooling strategy also to be practicable, it is absolutely necessary to comply with certain limit values of the chemical composition as listed in claim 1, to achieve the desired degree of conversion within the available short total cooling time.

The cooling strategy provides for a two-stage cooling with either different cooling rates, which is interrupted by an isothermal hold time of a maximum of 5 seconds. The beginning of the holding time, which corresponds to the end of the first cooling stage, is determined by the entry of the cooling curve into the ferrite region or the beginning of the austenite transformation into ferrite. In the short isothermal cooling break of a maximum of 5 seconds, while according to the invention the released heat of transformation is used to maintain the temperature at a constant value and an unavoidable air cooling is compensated for, the total desired conversion of the austenite to at least 70% ferrite. Subsequent to this holding time then immediately follows the second cooling stage with a cooling of the hot strip to a temperature below 300 ° C. Since this temperature is below the martensite start temperature, the second microstructure constituent is then obtained in the desired height with martensite in this cooling.

In addition to performing a short hold time, the cooling strategy is determined by a well-defined predetermined cooling rate for both cooling stages. This cooling rate is between V = 30 - 150 K / s, preferably between V = 50 - 90 K / s, depending on the hot strip geometry and the chemical composition of the steel grade used. With regard to these cooling rates, it should be noted that a cooling rate of less than 30 K / s is not possible because of the short time available in the conventional cooling section of a cast roll mill, while cooling rates of greater than 150 K / s can not be achieved in such cooling sections.

1. a) die Endwalztemperatur deutlich unterhalb der A₃-Temperatur liegt,
2. b) in der zweiten Kühlstufe bis zu einer Temperatur unterhalb von 300 °C gekühlt wird,
3. c) die Abkühlgeschwindigkeiten unterhalb von 150 K/s und oberhalb von 30 K/s liegen,
4. d) zwischen den beiden Kühlstufen eine mit maximal 5 Sekunden sehr kurze Haltezeit liegt, in der keine Kühlung erfolgt,
5. e) die Umwandlung zu Ferrit isotherm erfolgt.

Compared with the production of dual-phase hot strip according to the prior art, the method according to the invention is distinguished by a different one chemical composition of the parent steel characterized in that

1. a) the final rolling temperature is significantly below the A ₃ temperature,
2. b) is cooled in the second cooling stage to a temperature below 300 ° C,
3. c) the cooling rates are below 150 K / s and above 30 K / s,
4. d) between the two cooling stages is a very short hold time of up to 5 seconds, in which no cooling takes place,
5. e) the conversion to ferrite is isothermal.

A plant for carrying out the method of the invention is characterized by a conventional cooling section of a casting rolling plant, which is arranged behind the last finish rolling stand and has a plurality of controllable water cooling groups with water cooling bars arranged at a distance one behind the other. The cooling bars present in each cooling group are arranged in such a way that a certain amount of water is applied evenly to the upper side of the strip and the underside of the strip of the hot strip. The total amount of water is adjustable by switching individual chill bars on or off during rolling. The number and arrangement of the activated water cooling bar can be variably pre-set to optimally adapt the entire cooling section to the cooling conditions to be set

Further details, features and characteristics of the invention are explained in more detail below with reference to an exemplary embodiment shown in schematic drawing figures.

Fig. 1

eine Zeit-Temperatur-Abkühlkurve eines Warmbandes,

Fig. 2

ein Layout einer Kühlstrecke in einer Gießwalzanlage mit 6-gerüstiger Fertigstraße,

Fig. 3

ein Layout einer Kühlstrecke in einer Gießwalzanlage mit 7-gerüstiger Fertigstraße.

Show it:

Fig. 1

a time-temperature cooling curve of a hot strip,

Fig. 2

a layout of a cooling section in a casting rolling mill with 6-stand finishing train,

Fig. 3

a layout of a cooling section in a casting rolling mill with 7-stand finishing line.

• 0,06 % C, 0,1 % Si, 1,2 % Mn, 0,015 % P, 0,06 % S, 00,036 % Al, 0,15 % Cu, 0,054 % Ni, 0,71 % Cr, Rest Fe sowie übliche Begleitelemente wurde von einer eingestellten Endwalztemperatur T_finish von 800 °C in einer ersten Kühlstufe mit einer Abkühlgeschwindigkeit V₁ von 54 K/s auf eine Temperatur des Warmbandes von 670 °C abgekühlt, bei der die Kühlkurve in das Ferritgebiet eintrat. Während einer Haltezeit von etwa 4 Sekunden blieb die Warmbandtemperatur bei dieser Haltetemperatur T_const., bevor in einer zweiten Kühlstufe mit einer Abkühlgeschwindigkeit V₂ von 84 K/s auf eine Bandtemperatur unterhalb von 300 °C (ca. 250 °C Haspeltemperatur) fertig gekühlt wurde. An dem nach diesem Verfahren hergestellten Warmband mit einem Dualphasengefüge im angestrebten Bereich von mindestens 70 % Ferrit und weniger als 20 % Martensit wurde in Versuchen eine Zugfestigkeit von 620 MPa in Kombination mit einem Streckgrenzenverhältnis von 0,52 ermittelt.

In FIG. 1 is a cooling curve with the time-temperature curve of a hot strip shown by way of example, which was cooled by the inventive method on the outlet roller table in a cooling section 1. Hot strip with composition: consisting of:

• 0.06% C, 0.1% Si, 1.2% Mn, 0.015% P, 0.06% S, 00.036% Al, 0.15% Cu, 0.054% Ni, 0.71% Cr, remainder Fe and usual accompanying elements were cooled from a set final rolling temperature T _finish of 800 ° C in a first cooling stage at a cooling rate V ₁ of 54 K / s to a hot strip temperature of 670 ° C at which the cooling curve entered the ferrite region. During a holding time of about 4 seconds, the hot strip temperature remained at this holding temperature T _const. before cooling in a second cooling stage with a cooling rate V ₂ of 84 K / s to a strip temperature below 300 ° C (about 250 ° C coiler temperature) was finished. A tensile strength of 620 MPa in combination with a yield ratio of 0.52 was determined in tests on the hot strip having a dual-phase structure in the desired range of at least 70% ferrite and less than 20% martensite produced by this process.

In FIG. 2 the layout of an inventively designed cooling section 1 of a conventional casting rolling mill is shown by way of example. Between the last finishing stand 2 and the first water cooling group 3 ₁ is a temperature measuring point 6 for controlling the temperature of the incoming into the cooling section 1 hot strip 10th The cooling section 1 is made according to Fig. 2 from a total of eight cooling groups 3 _1-7 and 4, wherein the last is often designed as a trim zone 4. More general - depending on the respective casting rolling mill - belong between six and nine cooling groups to a conventional Kühlstrekke.

In the example shown the Fig. 2 This is the typical layout of a cooling line for a 6-stand cast rolling mill, which can be recognized by the gap between cooling groups 3 ₇ and 4. The subsequent expansion to a 7-stand finishing train often requires that, for example, the first cooling group (cooling zone) 3 ₁ to the rear in the structural gap between the cooling groups 3 ₇ and 4 must be added. In this case, a layout of a cooling section 1 'according to results FIG. 3 , which only by eliminating this structural gap between the cooling groups 3 ₇ and 4 from the layout of the cooling section 1 of Fig. 2 different. The reference numbers of the individual structural parts and assemblies of Fig. 3 therefore correspond to the corresponding reference numerals of Fig. 2 , An exception is the first cooling group 3 ₁ ', the upper chilled beam in contrast to the cooling beam of the cooling group 3 ₁ of Fig. 2 in the usual length of the cooling groups 3 ₂ to 3 _{7 is} formed.

As a rule, each cooling group has four cooling bars each on both the top and bottom sides. Each chilled beam, in turn, consists of two rows of water tubes for cooling the top of the belt 10 'and the bottom of the belt 10 " FIG. 2 illustrated cooling group 3 _{1 cut} for reasons of space on the top to a chilled beam.

In contrast to the front cooling groups 3 _1-7 , which have a switchable valve 7 per cooling beam, the trim zone 4 has two valves 7 for each beam. This means that in the trim zone each row of cooling tubes can be controlled individually and thus the amount of water can be controlled finer.

Depending on the rolled finished strip thickness, the outfeed speed of the strip changes from the finishing train and, accordingly, the operating mode of the cooling section must be adjusted in order to adjust the strip properties to be able to set the required time temperature control. For a strip thickness of, for example, 3 mm, the first necessary cooling stage is achieved with the cooling groups 3 ₁ and 3 ₂ , while the second cooling stage with the groups 3 ₅ , 3 ₆ , 3 ₇ and 4 is realized. With a 2.0 mm finished strip, only the cooling groups 3 ₆ , 3 ₇ and 4 are used for the second cooling stage due to the changed boundary conditions.

LIST OF REFERENCE NUMBERS

1

cooling section

2

last finishing stand

3 _1-7

Water cooler groups

4

Water cooling group (trim zone)

5

reel

6

Temperature measuring point

7

switchable valve

8th

transport direction

10

hot strip

10 '

Band top

10 "

Band bottom

V ₁

Cooling speed of the first cooling stage

V ₂

Cooling speed of the second cooling stage

T _finish

Belt temperature after the last finishing stand

T _const .

Belt temperature after the holding time

T _coiling

Belt temperature after the end of cooling (coil temperature)

Claims (3)

1. Method of producing hot strip (10) with a dual-phase structure of ferrite and martensite, wherein at least 70% of the austenite is converted into ferrite, from the hot-rolled state by controlled two-stage cooling down after rolling to finished state to a strip temperature below the martensite start temperature in a cooling path (1, 1') consisting of water cooling groups (3_1-7, 4), arranged one behind another at a spacing,
   characterised in that
   starting from a steel with the chemical composition consisting of:

   0.01 - 0.08% C, 0.9% Si,0.5 - 1.6% Mn, 1.2% Al, 0.3 - 1.2% Cr, the remainder Fe as well as usual incidental elements,

   in order to achieve a hot strip (10) with a two-phase structure of 70 to 95% ferrite and 30 to 5% martensite with a high mechanical strength and high deforming capability (tensile strength greater than 600 MPa, percent elongation failure at least 25%) in the cooling path of a casting and rolling plant:

   a) the two-stage controlled cooling is carried out from a final-rolling strip temperature T_finish wherein A₃ - 100 K < T_finish < A₃ - 50 K, to a coiler strip temperature T_coiling < 300° C (< martensite start temperature), wherein the cooling rate V_1,2 in the two cooling stages is between V = 30 - 150 K/s, preferably between V + 50 - 90 K/s, and

   b) the first cooling state is carried out up to entry of the cooling curve into the ferrite range and then the conversion heat liberated by conversion of the austenite into ferrite is used for isothermic maintenance of the attained strip temperature T_const. with a holding time 5 s up to the start of the second cooling stage.
2. Casting and rolling plant for producing hot strip (10) with dual-phase structure from the hot-rolled state with a cooling path (1, 1'), which is arranged after the last finishing stand (2), with several water cooling groups (3_1-7, 4) arranged in succession, for carrying out the method according to claim 1,
   characterised in that
   the cooling path (1, 1') has a length (< 50 m) usual for conventional casting and rolling plants and
   a corresponding number of regulable water cooling groups (3_1-17, 4) is arranged within the cooling path, each water cooling group (3_1-7, 4) including several cooling bars which are regulable by way of switchable valves (7) and by which the strip upper side (10') and the strip lower side (10") of the transiting hot strip (10) can be uniformly acted on by a water quantity, wherein the water quantities for the strip upper side (10') and the strip lower side (10") are also adjustable relative to one another.
3. Casting and rolling plant according to claim 2, characterised in that the last water cooling group (4) for cooling the strip upper side (10') and the strip lower side (10") respectively comprises eight switchable valves (7) for four cooling bars at the top and bottom.

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