EP3171997B1 - Method for producing a metal product - Google Patents

Description

The invention relates to a method for producing a metallic product in which in a continuous casting liquid metal as a slab from a mold vertically downward in a conveying direction, guided along a strand guide and is deflected in the horizontal, the slab behind the continuous casting in a Oven is heated.

When casting steel with higher contents of copper and tin surface defects occur, the so-called copper-red or hot break or the "hot-shortness". It is known that by a structural transformation of austenite to ferrite and back to austenite, the surface quality due to grain refining can be improved, then less and not so deep surface cracks occur on the slab, thin slab or hot strip.

There are still occasional cracks ("hot shortness") on the surface. The reason for this is that despite the microstructure transformation is still partially coarse, inhomogeneous structure. This was confirmed in experiments where intensive cooling was used in the upper strand guide. Sand-blasted hot strip samples from hot strip whose associated slabs had been intensively and normally cooled were visually evaluated for surface defect Cu hot brittleness over the hot strip width by a straightening series. This is in Fig. 1 illustrated. In concrete terms, a test is shown on a so-called CSP continuous casting plant with intensively and normally cooled slabs; the average values of the inspected hot strip samples are shown, where "0" stands for error-free and "3" for the worst surface.

From the illustration according to Fig. 1 It becomes clear that, on the one hand, the intensive cooling of the slab generally reduces the occurrence of Cu hot brittleness. On the other hand, one can see fluctuations in the infestation of "hot shortness" over the hot strip width. This is because the near-surface structure is not homogeneous. The coarser the near-surface microstructure, the greater the hot shortness infestation, because there are fewer grain boundaries for penetration of the Cu-containing phase. Repeated, two-fold intensive cooling causes a further refinement and homogenization of the surface-frame structure. Accordingly, the surface finish on hot shortness will continue to improve. The expected, improved surface result is also in Fig. 1 shown.

For processing steel is on the JP 2002 307 148 A , on the DE 694 31 178 T2 , on the WO 2010/003402 A1 , on the DE 10 2009 048 567 A1 , on the EP 1 937 429 B1 or. DE 10 2006 056 683 A1 and on the EP 0 686 702 A1 pointed. Similar solutions are from the JP 2007 245232 A , from the JP S55 14173 A , from the JP S63 112058 A and from the JP H07 197120 A known.

The invention has for its object to provide a generic method that allows a further reduction of surface cracks and thus improving the surface quality. In this case, a very fine and homogeneous structure in the material should be achieved.

1. a) in Förderrichtung hinter der Kokille in einer ersten Kühlzone: Intensivkühlung der Bramme dergestalt, dass in der oberflächennahen Randzone der Bramme eine Gefügeumwandlung von Austenit in Ferrit erfolgt;
2. b) in Förderrichtung hinter der ersten Kühlzone in einer ersten Erwärmungszone: Wiedererwärmen der Bramme dergestalt, dass in der oberflächennahen Randzone der Bramme eine Gefügeumwandlung von Ferrit in Austenit erfolgt, wobei die Wiedererwärmung der Bramme durch Wärmeausgleich in der Bramme erfolgt, indem ein Wärmestrom vom Inneren der Bramme zur Brammenoberfläche zugelassen wird;
3. c) in Förderrichtung hinter der ersten Erwärmungszone in einer zweiten Kühlzone: Intensivkühlung der Bramme dergestalt, dass in der oberflächennahen Randzone der Bramme eine Gefügeumwandlung von Austenit in Ferrit erfolgt;
4. d) in Förderrichtung hinter der zweiten Kühlzone in einer zweiten Erwärmungszone: Wiedererwärmen der Bramme dergestalt, dass in der oberflächennahen Randzone der Bramme eine Gefügeumwandlung von Ferrit in Austenit erfolgt,

wobei nach der Durchführung von Schritt d) noch mindestens eine weitere Intensivkühlung der Bramme dergestalt erfolgt, dass in der oberflächennahen Randzone der Bramme eine Gefügeumwandlung von Austenit in Ferrit auftritt, wobei nach der Durchführung der weiteren Intensivkühlung der Bramme noch mindestens eine weitere Erwärmung der Bramme dergestalt erfolgt, dass in der oberflächennahen Randzone der Bramme eine Gefügeumwandlung von Ferrit in Austenit auftritt,
wobei die Schritte a) bis c) erfolgen, während die Bramme noch in der Vertikalen orientiert ist und
wobei die letztmalige Erwärmung der Bramme im Ofen oder durch induktive Erwärmung erfolgt.The solution of this object by the invention is characterized in that the method comprises the steps of:

1. a) in the conveying direction behind the mold in a first cooling zone: intensive cooling of the slab in such a way that in the near-surface edge zone of the slab a structural transformation of austenite into ferrite takes place;
2. b) in the conveying direction behind the first cooling zone in a first heating zone: reheating the slab such that in the near-surface edge zone of the slab a microstructure transformation of ferrite into austenite takes place, wherein the reheating of the slab by heat balance in the slab, by allowing heat flow from the interior of the slab to the slab surface;
3. c) in the conveying direction behind the first heating zone in a second cooling zone: intensive cooling of the slab in such a way that in the near-surface edge zone of the slab a structural transformation of austenite into ferrite takes place;
4. d) in the conveying direction behind the second cooling zone in a second heating zone: reheating the slab in such a way that a microstructure transformation of ferrite into austenite takes place in the near-surface edge zone of the slab,

wherein, after carrying out step d), at least one further intensive cooling of the slab takes place in such a way that structural transformation of austenite into ferrite occurs in the near-surface edge zone of the slab, after at least one further heating of the slab after carrying out the further intensive cooling of the slab that a structural transformation of ferrite into austenite occurs in the near-surface edge zone of the slab,
wherein the steps a) to c) take place while the slab is still oriented in the vertical and
the last time the slab is heated in the furnace or by inductive heating.

The slab surface is preferably cooled in the above step a) and step c) to a temperature below the Ac1 temperature. Accordingly, it is preferably provided that the slab surface in the above step b) and step d) is heated to a temperature above the Ac3 temperature.

The last intensive cooling of the slab is carried out according to a possible embodiment of the invention, as soon as the slab is diverted into the horizontal.

Thus, the invention is based on a multiple near-surface structural transformation in the continuous casting, in order to improve the surface quality of the slab.

Repeating the microstructure transformation from austenite to ferrite, further to austenite and again to ferrite etc. in the near-surface edge zone of the slab. This leads to a refinement of the partial coarse, inhomogeneous structure. This results in a further reduction of the surface cracks and thus an improvement of the surface quality. This would correspond to a pendulum glow or a multiple normalization during the heat treatment. In order to achieve the desired homogeneous grain refining, the transformation must be completed at least twice.

A possible embodiment of the method may be such that a first pass through the austenite to ferrite transition to austenite in the surface area of the slab is accomplished by intensive cooling at the top of the continuous run of the continuous casting line, followed by reheating of the near-surface area of the slab by normal or weak cooling in the central region of the strand guide.

A second pass through the austenite to ferrite transition to austenite can be done before the bending driver by re-cooling and then rewarming.

Optionally, a third or second pass through the austenite to ferrite transition to austenite may occur before or after the straightening driver.

According to the invention, after leaving the mold within the strand guide of the continuous casting plant or after the shears or before entering the tunnel kiln or furnace, the slab is to undergo a multi-stage heat treatment with the aim of setting a fine and homogeneous microstructure in the near-surface edge zone.

After leaving the mold, the already solidified strand shell usually has an austenitic, inhomogeneous solidification structure, depending on the steel composition. By a time-defined, intensive cooling, the near-surface edge zone of the steel strand is cooled below the mold to a temperature below the Ac1 point, so that in the surface-boundary layers a first transformation of austenite into ferrite takes place. By the subsequent reheating of the ferritic, near-surface edge zone through the still existing core or heat of fusion from the slab interior to a temperature above Ac ₃ , a back transformation of the ferrite into austenite takes place. Both transformation processes are associated with a refinement of the structure.

However, inhomogeneities (partial coarseness) of the original austenite microstructure can be retained. This "inheriting" of the structural inhomogeneities can be eliminated by repeated, ie two or more stages through the transformation austenite - ferrite - austenite, so that in the end a fine, homogeneous austenitic structure is present.

In the context of the present invention, the two-stage transformation of austenite-ferrite-austenite-ferrite-austenite is in particular realized by intensive cooling below the mold in the upper part of the strand guide of the continuous casting plant (austenite close to surface converts to ferrite) and by rewarming of the near-surface boundary layer the core heat of the slab in the middle part of the strand guide (near-surface ferrite converts to austenite).

This is followed by intensive cooling in the lower part of the strand guide (near-surface austenite converts into ferrite) and rewarming after leaving the strand guide by the core heat (near-surface ferrite converts to austenite) or in a downstream heating furnace.

An alternative is that the second or even another stage of the transformation of austenite to ferrite by attaching additional chilled beams in a section outside the strand guide is realized. The required conversion of the near-surface ferrite into austenite would be done either by the core heat of the slab or in a downstream heating furnace.

Fig. 1

das Bewertungsergebnis der Cu-Warmbrüchigkeit eines Stahlbandes, über der Warmbandbreite für verschieden starke Intensitätsgrade der Kühlung,

Fig. 2

schematisch eine Stranggießanlage mit Illustration einer ersten Ausführungsform der Erfindung,

Fig. 3

schematisch eine Stranggießanlage mit Illustration einer nicht erfindungsgemäßen Ausführungsform der Erfindung und

Fig. 4

schematisch die Darstellung der Gefügeausbildung in einer oberflächennahen Randzone einer Bramme beim Durchlaufen eines Verfahrens gemäß der Erfindung.

In the drawings, embodiments of the invention are shown. Show it:

Fig. 1

the evaluation result of the Cu hot brittleness of a steel strip, above the hot strip width for different degrees of intensity of the cooling,

Fig. 2

schematically a continuous casting plant with illustration of a first embodiment of the invention,

Fig. 3

schematically a continuous casting plant with an illustration of a non-inventive embodiment of the invention and

Fig. 4

schematically the representation of the microstructure in a near-surface edge zone of a slab when passing through a method according to the invention.

The present invention relates to a process which is carried out in a continuous casting plant for steel. It can be made conventional slabs, thin slabs or medium-thick slabs.

In Fig. 2 a first embodiment of the invention can be seen. The continuous casting plant 1 has a mold 3, under which a strand guide 4 is arranged. Through the strand guide 4 and the downstream rollers, the cast slab 2 is deflected from the vertical V in the horizontal H. The slab 2 is thereby conveyed in a conveying direction F. After the slab 2 is deflected in the horizontal H, it is conveyed into a furnace 5.

It is essential that in the conveying direction F behind the mold 3 (that is, directly below the same) in a first cooling zone 6, an intensive cooling of the slab 2 takes place. For this purpose, water is sprayed onto the surface of the slab with a corresponding volume flow. The cooling takes place with an intensity such that a structural transformation of austenite into ferrite takes place in the near-surface edge zone of the slab 2.

Subsequently, the slab passes into a first heating zone 7, which is arranged in the conveying direction F behind the first cooling zone 6. Here, a reheating of the slab 2 takes place such that in the near-surface edge zone of the slab 2, a structural transformation takes place back from ferrite to austenite. In the heating zone 7 there is a normal or weak cooling, so that the said structural transformation can vonstattengehen.

In the conveying direction F of the first heating zone 7 is followed by a second cooling zone 8. Here is again an intensive cooling of the slab 2, in such a way that takes place in the near-surface edge zone of the slab 2, a structural transformation of austenite into ferrite.

Finally, in the conveying direction F behind the second cooling zone 8 follows a second heating zone 9. In this, reheating of the slab 2 takes place such that in the near-surface edge zone of the slab 2 is a structural transformation of ferrite into austenite.

Reference numeral 11 indicates that there are alternative positions for additional chill beams for intensive cooling to accomplish austenite to ferrite conversion.

Otherwise, it should be mentioned to the oven 5 that it can also be here that a conversion of ferrite to austenite can take place if a corresponding heating takes place.

In Fig. 2 is shown that the continuous casting 1 is designed as a vertical bending plant, wherein the bending of the slab from the vertical to the horizontal takes place with a solid slab core.

In Fig. 3 is provided as an alternative embodiment of the invention that a continuous casting 1 is used as a vertical bending plant, wherein the bending is done with liquid slab core.

The given reference numerals correspond to those according to Fig. 2 , The first heating zone 7 is here directly where the slab leaves the vertical V and is bent over. As the second heating zone 9, the oven 5 is provided.

In Fig. 4 is shown schematically how the microstructure changes when the respective transformations of austenite to ferrite and back done.

The slab surface 10 is indicated and the microstructure is sketched near the surface of the slab. Here, the respective grain diameters are indicated schematically and set in relation to each other. The last letters in the grain diameters D for three adjacent regions 1, 2 and 3 over the width of the slab indicates the respective status after the corresponding microstructural transformations.

It can be seen that, from phase transformation to phase transformation, the grain size not only becomes smaller, but also uniform.

For slabs, grain diameters are according to the ASTM grain size table of ASTM Nos. -3 to 0.

The transformation achieves the following grain sizes: D 1, 2, 3a: ASTM Nos. 0 to 2, D 1, 2, 3b: ASTM Nos. 2 to 4, D 1, 2, 3c: ASTM Nos. 4 to 6, D 1, 2, 3d: ASTM No .: 6 to 7

ASTM: American Society for Testing and Materials

LIST OF REFERENCE NUMBERS

1

continuous casting plant

2

slab

3

mold

4

strand guide

5

Oven / Induction heating

6

first cooling zone

7

first heating zone

8th

second cooling zone

9

second heating zone

10

slab surface

11

Chilled beams

V

vertical

H

horizontal

F

conveying direction

Claims (4)

1. Method of producing a metallic product in which in a continuous casting plant (1) liquid metal is extracted as a slab (2) from a mould (3) vertically (V) downwardly in a conveying direction (F), guided along a strip guide (4) and deflected into the horizontal (H), wherein the slab (2) is heated in an oven (5) behind the continuous casting plant (1),
   characterised in that
   the method comprises the steps:

   a) in a first cooling zone (6) behind the mould (3) in conveying direction (F): intensive cooling of the slab (2) to such an extent that a structural conversion from austenite into ferrite takes place in the edge zone of the slab (2) near the surface;

   b) in a first heating zone (7) behind the first cooling zone (6) in conveying direction (F): reheating of the slab (2) to such an extent that a structural conversion from ferrite into austenite takes place in the edge zone of the slab (2) near the surface, wherein the reheating of the slab (2) is carried out by heat equalisation in the slab (2) in that a heat flow is allowed from the interior of the slab (2) to the slab surface;

   c) in a second cooling zone (8) behind the first heating zone (7) in conveying direction (F): intensive cooling of the slab (2) to such an extent that a structural conversion from austenite into ferrite takes place in the edge zone of the slab (2) near the surface; and

   d) in a second heating zone (9) behind the second cooling zone (8) in conveying direction (F): reheating of the slab (2) to such an extent that a structural conversion from ferrite into austenite takes place in the edge zone of the slab (2) near the surface,

   wherein after step d) is carried out still at least one further intensive cooling of the slab (2) is carried out in such a way that a structural conversion from austenite into ferrite occurs in the edge zone of the slab (2) near the surface, wherein after the further intensive cooling of the slab (2) has been carried out still at least one further heating of the slab is carried out to such an extent that a structural conversion from ferrite into austenite occurs in the edge zone of the slab (2) near the surface,
   wherein the steps a) to c) are carried out while the slab (2) is still oriented in the vertical (V) and
   wherein the last heating of the slab (2) is carried out in the oven (5) or by inductive heating.
2. Method according to claim 1, characterised in that the slab surface in the case of steps a) and c) according to claim 1 is cooled to a temperature below the Ac1 temperature.
3. Method according to claim 1 or 2, characterised in that the slab surface in the case of steps b) and d) according to claim 1 is heated to a temperature above the Ac3 temperature.
4. Method according to any one of claims 1 to 3, characterised in that the last intensive cooling of the slab (2) is carried out as soon as the slab (2) is transferred to the horizontal (H).

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