EP0885974B1 - Process for rolling wide hot strip in a compact strip production line - Google Patents

Description

The invention relates to a method for rolling hot wide strip from continuously cast thin slabs of ferritic-pearlitic micro-alloyed Structural steels with a micro alloy with vanadium and / or with niobium and / or with Titan in a CSP plant, whereby the cast slab strand, divided into rolling lengths, fed to a multi-stand CSP rolling mill via a compensating furnace and rolled out there continuously to form hot wide strip in a cooling section is cooled and coiled.

EP 05 95 282 A1 describes a process for the production of hot-rolled steel strip from continuously cast material, preferably Thin slabs with one step at a time in a heat CSP method known, first of all leaving a compensating furnace Thin slabs after the first roughing pass in a roughing mill to temperatures be heated above 1150 ° C.

The roughing mill is a recrystallization zone with, for example, a winding device and downstream of an unwinding device in which possible material-specific longer dwell time recrystallizes the rolling stock, see above that unconsolidated relaxed material is available for the finishing train.

Then the pre-rolled strip is cooled so that the temperature for the first Stitch in the finishing train according to the necessary temperature profile can be optimally adjusted in the finishing train for all steel grades. It follows then finish rolling in the finishing train.

Rolling hot wide strip in a CSP system (CSP = Compact Strip Production), with continuously cast primary material after subdivision into rolling lengths Is fed directly to the rolling mill via a compensating furnace is known from EP-A-0368048 known, a multi-stand rolling mill being used as the rolling mill is in which to a temperature of 1100 ° C to 1130 ° C in the compensating furnace brought rolling lengths in successive steps with in between horizontal descaling can be finished rolled.

To improve the strength and toughness properties as well as the associated significant increase in the yield strength and the Notched impact strength of a rolled product made of steel achieve, is proposed in EP-A-0413163, the rolling stock treat thermomechanically.

In thermomechanical forming, in contrast to normalizing forming, in which the final forming in the area the normalizing temperature with complete recrystallization of austenite takes place, temperature ranges for one Targeted forming rate observed, at which the austenite is not or not substantially recrystallized, d. H. In any case lies before the actual thermomechanical treatment of the Rolled goods an austenite structure that either no or only small proportions of germs or structural components of the contains lower temperature resistant phase.

The setting of this initial structure can be immediate from the casting heat or in a preheating furnace from Room temperature or an intermediate temperature from.

According to the method proposed in EP-A-0413163, the forming of the rolling stock begins in the temperature range of the stable austenite and is continued until just above the A _r3 temperature.

To be the cheapest for thermomechanical rolling Temperature range to come, the piercing temperature of the Rolled goods depending on the desired degree of forming fixed.

The essential feature of thermomechanical treatment is Use of plastic deformation not only for manufacturing a defined product geometry, but especially for Setting a desired real structure and thus for Guarantee of defined material properties, but not recrystallized austenite to polymorphic gamma (gamma) - aLpha (α) conversion comes (in the normalizing Austenite is already recrystallized).

• Schmelze (L) → Ferrit (δ) → Austenit A₁ (gamma) →
• → Ferrit (α) → Austenit A₂ (gamma)

während für die CSP-Technologie gilt:

• Schmelze (L) → Ferrit (δ) → Austenit A₁ (gamma)

mit einer höheren übersättigung des Mischkristalls Austenit und einem erhöhten Ausscheidungspotential für Karbonitride aus dem Austenit.Conventional slabs are subjected to the polymorphic conversions when used cold before being formed in a conventional rolling mill:

• Melt (L) → ferrite (δ) → austenite A ₁ (gamma) →
• → ferrite (α) → austenite A ₂ (gamma)

while for CSP technology:

• Melt (L) → ferrite (δ) → austenite A ₁ (gamma)

with a higher supersaturation of the austenite mixed crystal and an increased precipitation potential for carbonitrides from the austenite.

It is an object of the invention to provide a specific Process strategy for thermomechanical rolling in CSP systems to develop the peculiarities of the Structure development and the derived ones Material properties through direct rolling without Intercooling and subsequent reheating during rolling optimal use of CSP slabs in CSP plants.

a) Änderung der Gussstruktur durch Einstellung definierter Temperatur- und Formänderungsbedingungen bei der ersten Umformung, wobei die Temperatur oberhalb der Rekristallisations-Stop-Temperatur (T_R) liegt, so dass ohne Unterbrechung des kontinuierlichen Wabuns während und/oder nach der ersten Umformung eine vollständige (dynamische und/oder meta-dynamische und/oder statische) Rekristallisation des Gussgefüges vor Beginn des zweiten Umformschrittes erfolgt;

b) Umformung in den letzen Walzgerüsten bei Temperaturen unterhalb der T_R-Temperatur, wobei die Umformung einen Betrag von 30 % nicht unterschreiten soll und die Endwalztemperatur nahe bei der A_r3-Temperatur (Temperatur der Austenit/Ferrit-Umwandlung) liegt;

c) gesteuerte Abkühlung der Warmbreitbänder in der Kühlstrecke, vorzugsweise einer Laminar-Kühlstrecke, wobei bei einer Temperatur, die zwischen der A_r3-Temperatur und der B_S-Temperatur (Bainit-Start-Temperatur) liegt, die polymorphe Umwandlung des Austenits erfolgt.

In terms of process technology, the object is achieved by the measures of claim 1. In order to achieve optimal mechanical properties on the hot wide strip, a controlled microstructural development is carried out by thermomechanical rolling when the thin slabs pass through the CSP system with the following process steps:

a) Changing the cast structure by setting defined temperature and shape change conditions during the first forming, the temperature being above the recrystallization stop temperature (T _R ), so that a complete without interrupting the continuous waving during and / or after the first forming (dynamic and / or meta-dynamic and / or static) recrystallization of the cast structure takes place before the start of the second forming step;

b) Forming in the last rolling stands at temperatures below the T _R temperature, the forming should not be less than 30% and the finish rolling temperature is close to the A _r3 temperature (temperature of the austenite / ferrite transformation);

c) Controlled cooling of the hot wide strips in the cooling section, preferably a laminar cooling section, the polymorphic transformation of the austenite taking place at a temperature which is between the A _r3 temperature and the B _S temperature (bainite start temperature).

• in die Fertigstraße eines konventionellen Walzwerkes läuft eine in der Vorstraße vorgewalzte (plastisch deformierte) Bramme mit rekristallisiertem Gefüge ein,
• in die CSP-Fertigstraße wird die Dünnbramme mit Gussgefüge eingeführt,
• die Oberflächenbeschaffenheit einer CSP-Dünnbramme unterscheidet sich deutlich von einer vorgewalzten Bramme (beispielsweise durch ihre Topologie).

The measures according to the invention align the thermomechanical forming in an optimal manner to the specific process parameters of the CSP process with its specific thermal history. When setting the temperature and the shape change conditions, the fundamental differences to conventional rolling must be observed:

• In the finishing train of a conventional rolling mill, a pre-rolled (plastically deformed) slab with a recrystallized structure enters the roughing train.
• the thin slab with cast structure is introduced into the CSP finishing train,
• The surface quality of a CSP thin slab differs significantly from that of a pre-rolled slab (for example due to its topology).

• unterschiedliche Beweglichkeit der Großwinkelgrenzen,
• unterschiedliches Mischkristall- und Ausscheidungsverhalten,
• unterschiedlicher Diffusionsmechanismus und -kinetik durch den unterschiedlichen Charakter der Grenzflächen und chemischen Inhomogenitäten, die gleichfalls bei der Einstellung der Verfahrensparameter zu beachten sind.

These differences also result in differences in the solid-state reactions triggered by hot forming, for example

• different mobility of the large-angle borders,
• different mixed crystal and precipitation behavior,
• Different diffusion mechanisms and kinetics due to the different character of the interfaces and chemical inhomogeneities, which must also be taken into account when setting the process parameters.

According to the invention, the first shaping is carried out at a temperature above the recrystallization stop temperature (T _R ), so that a complete recrystallization of the cast structure takes place during and / or after this first shaping. The recrystallization can take place dynamically and / or metadynamically and / or statically.

It is important according to the invention that this Recrystallization is fully completed before the next forming is carried out. If the distance of the Stands and the rolling speed for the required Time is not sufficient, then can be advantageous Embodiment of the invention, the next roll stand opened so that to the next but one roll stand, in which then the second forming is done, enough time to Available. The opening of the roll stand closes his Use as a driver is not enough.

The further forming in the last rolling stands of the CSP rolling mill then takes place at temperatures below the recrystallization stop temperature (TR) in order to solidify the austenite before its polymorphic transformation. The austenite-hardening forming should not be less than 30%. The finish rolling temperature is close to the A _r3 temperature.

The polymorphic transformation of the austenite then takes place during the final cooling in, for example, a laminar cooling section at a temperature which lies between the A _r3 temperature (temperature of the austenite-ferrite transformation) and the B _S temperature (bainite starting temperature).

Can further improve the mechanical properties by further controlled cooling of the wound coil, with the elimination processes being specifically targeted be influenced, achieved.

According to the invention, the second forming, which may only be carried out in the third roll stand, can preferably be used to initiate a second recrystallization cycle, which leads to further structural refinement and structural homogenization before the forming is carried out again. For this purpose, the subsequent mill stand can also be opened, which can then also be used as a driver if required. The temperature in this second transformation is also above the T _R temperature.

A plant for carrying out the method of the invention consists of a CSP plant in which cast thin slabs in the Direct use (without intermediate cooling and subsequent Reheating) in a multi-stand CSP mill be reshaped, and in which a controlled Microstructure development in the CSP rolling mill, in the cooling section and in the reel to achieve optimal mechanical Properties on hot broadband is possible, in particular between the first and the second forming as well as if necessary also between the second and third forming one for a complete recrystallization required variable Period is adjustable.

The method is described below using a drawing figure the invention explained in more detail with a case study.

In the drawing figure, a CSP system is shown in the an approximately 6 mm thick hot strip made of high-strength structural steel is produced by thermomechanical rolling.

The thin slabs (13) emerging from the casting plant (1) with a cutting device (2) in rolling lengths divided and introduced into a compensating furnace (3) in which they are brought to a temperature of about 1130 ° C.

The first forming is carried out with a stitch reduction of 50% in the first roll stand (4) at a forming temperature of 1080 ° C carried out. To complete the desired recrystallization to complete by the second transformation is that second roll stand (5) opened and serves only as Driver.

The second forming is then carried out in the third roll stand (6) a stitch reduction of 40% at a forming temperature of 1030 ° C carried out. Since here the transformation to another Recrystallization is used, the following is fourth Roll stand (7) also opened and only serves as a driver.

• dritte Umformung im fünften Walzgerüst (8) mit einer Stichabnahme von 30 % bei einer Umformtemperatur von 900 °C,
• vierte Umformung im sechsten Walzgerüst (9) mit einer Stichabnahme von 25 % bei einer Umformtemperatur von 840 °C,
• fünfte Umformung im siebten Walzgerüst (10) mit einer Stichabnahme von 15 % bei einer Umformtemperatur von 800 °C.

The other forming stages are

• third forming in the fifth roll stand (8) with a pass reduction of 30% at a forming temperature of 900 ° C,
• fourth forming in the sixth roll stand (9) with a pass reduction of 25% at a forming temperature of 840 ° C,
• fifth forming in the seventh roll stand (10) with a pass reduction of 15% at a forming temperature of 800 ° C.

Then the hot wide strip is in a laminar cooling section (11) cooled to 600 ° C (reel temperature) and in one Underfloor reel system (12) wound into a bundle.

The corresponding temperature ranges are shown in the drawing figure for the individual process steps. The time period (I) between the first and the second forming serves for a first recrystallization phase, the temperature T being greater than the T _R temperature.

The time period (II) between the second forming and the third forming serves the second recrystallization phase with a temperature T that is also greater than the T _R temperature.

The period of time (III) from the third transformation until the last transformation is used for solidification of the austenite at a temperature T between the T _R temperature, and the A _r3 temperature.

The period (IV) after the last forming in which cooling is used serves for the polymorphic transformation of the austenite. The temperature T is between the A _r3 temperature and the B _S temperature.

The parameters listed in the case example above only represent possible parameters for a certain steel grade, where other parameters, such as roll diameter, Rolling speed, distances of the roll stands from each other must be taken into account in order to optimize the microstructure to achieve by thermomechanical forming.

Claims (4)

1. Method of rolling hot wide strip from continuously cast thin slabs (13) of ferritic/pearlitic micro-alloyed constructional steels with a micro-alloying with vanadium and/or niobium and/or titanium in a compact strip production plant, wherein the cast slab strip (13), divided into roll lengths, is fed with cast structure by way of a compensating oven (3) of a multi-stand compact strip production rolling train (4 to 10), in which several roll stands are arranged directly one after the other without a preliminary stand and without an interposed thermal treating device, and continuously rolled out there into hot wide strip, cooled in a cooling path (11) and wound up into coils (12), wherein for attainment of optimum mechanical properties at the hot wide strip a controlled structural development is carried out by thermo-mechanical rolling during passage of the thin slabs through the compact strip production plant by the method steps:

   a) changing the cast structure by setting defined temperature and shape change conditions in the first reshaping (4), wherein the temperature lies above the recrystallisation strip temperature (T_R) so that a complete (dynamic and/or metadynamic and/or static) recrystallisation of the cast structure takes place before the beginning of the second reshaping step (6), without interruption of the continuous rolling, during and/or after the first reshaping (4);

   b) reshaping in the last roll stands (8 to 10) at temperatures below the T_R temperature, wherein the reshaping does not go below an amount of 30% and the final rolling temperature lies near the A_r3 temperature (temperature of the austenitic/ferritic conversion);

   c) controlled cooling of the hot wide strips in the cooling path (11), preferably a laminar cooling path, wherein the polymorphic conversion of the austenite takes place at a temperature which lies between the A_r3 temperature and the B_S temperature (bainite start temperature).
2. Method according to claim 1, characterised in that if needed the second roll stand (5) is opened for providing the time required for the recrystallisation of the first conversion and if needed is used only as a driver.
3. Method according to claim 1 or 2, characterised in that after recrystallisation of the cast structure in consequence of the first shaping (4) a second recrystallisation is triggered by the second reshaping (6).
4. Method according to claim 3, characterised in that the following roll stand (7) is opened for providing the time required for the recrystallisation by the second reshaping step (6) and if needed is used only as a driver.

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