EP1228255A1

EP1228255A1 - Method for producing a hot strip

Info

Publication number: EP1228255A1
Application number: EP00966035A
Authority: EP
Inventors: Rudolf Kawalla; Bernhard Engl; Thomas Heller; Wolfgang Rasim; Eberhard Sowka
Original assignee: ThyssenKrupp Stahl AG
Current assignee: ThyssenKrupp Steel Europe AG
Priority date: 1999-10-20
Filing date: 2000-09-16
Publication date: 2002-08-07
Anticipated expiration: 2020-09-16
Also published as: AU7657400A; EP1228255B1; WO2001029273A1; ES2223585T3; DE19950502C1; US6835253B1; DE50007566D1

Abstract

The invention relates to a method for production of a hot rolled strip, in particular, for production of a hot rolled strip suitable for production of a cold rolled strip which can easily be deep drawn, whereby a steel melt, containing, in mass %, C: </= 0.07 %, Si: </= 0.5 %, Mn: </= 2.5 %, Al: </= 0.1 %, N: </= 0.01 %, P: </= 0.025 %, B: </= 0.05 %, optionally up to a total of 0,35 % of Nb, Ti and V, the remainder being iron and the usual impurities, is melted and continuously extruded from a casting mould (1). The cast bar (S) is led through a cooling line immediately after the exit from the casting mould (1), in which said cast bar (S) is intensively cooled with a cooling rate (aLM) of at least 3 K/s to a temperature of Ar1 +/- 25 K and, in addition, said cast bar (S) is cooled for at least 30 seconds in air after which the cast bar (S) itself or a thin slab (D) from said cast bar (S) is reheated in a soaking oven (5) before the cast bar (S) or the thin slab (D) is hot rolled to form a hot rolled strip. The inventive method permits a reduction in the necessary temperatures in the soaking oven during the processing of low-alloyed, low carbon content steels, such that the load on the oven is reduced, without leading to a reduction in the quality of the hot rolled strip or the cold strip prepared from said hot rolled strip.

Description

Process for producing hot strip

The invention relates to a method for producing a hot strip, in particular for producing a hot strip intended for producing a good deep-drawing cold strip, from a low-carbon, low-alloy steel, in which thin slabs are produced by continuous casting, in which the strand emerging from a casting mold during continuous casting passes through a cooling section and in which the cast strand itself or thin slabs separated from the strand are reheated in a compensating furnace before they are hot-rolled into hot strip.

In a known method of the type mentioned above, which has become known under the name "CSP method", thin slabs are separated from a steel strand produced in a continuous casting plant and, after temperature compensation in a tunnel kiln, continuously rolled into hot strip in a multi-stand rolling mill. In the known method, the thin slabs generally enter the compensating furnace at a temperature between 950 ° C. and 1100 ° C. and are reheated therein to temperatures which are between 1100 ° C. and 1200 ° C.

The known method makes use of the heat present in the strand after casting Production of hot strip with an energy expenditure which is reduced compared to other conventional processes of this type. However, the compensating furnace must be operated at very high temperatures. These high temperatures lead to premature wear of the furnace, which maintenance therefore consumes the energy savings achieved. However, the high temperatures of the compensating furnace are required in the prior art in order to bring or keep the alloy components of the steel strand in solution, which cause the formation of precipitates in the course of the further process steps in the production of the hot strip or the cold strip produced therefrom, which decisively influence the formation of a certain structural structure of the finished hot strip or the cold strip produced therefrom.

EP 0 686 702 A1 discloses a modification of the above-described method, in which the surface temperature of the slab between the casting mold and the compensating furnace is reduced to such an extent that a structural transformation of austenite into ferrite / pearlite occurs in the slab , It is also stated that the temperatures reached at a depth of 2 mm below the slab surface are preferably less than 600 ° C.

The aim of the measures described in EP 0 686 702 AI is that slabs containing copper in the course of the addition of larger amounts of secondary scrap to the melt in significant amount a state in the equalization furnace, through which an excessive accumulation of copper in the region of the grain boundaries of the primary austenite is prevented. Otherwise, these accumulations cause severe scale formation and can lead to a so-called "solder break" in the further course of hot strip production. By cooling the slabs to temperatures below the A _r3 temperature (the temperature below which the austenite converts to ferrite), a structural change is forced with a reorientation of the austenite grain boundaries in the course of reheating in the compensating furnace.

The slabs cooled in this way are heated in the compensating furnace to the high temperatures usually set in the compensating furnace. In order to have to use as little energy as possible for reheating, the depth of the cooling and the time provided for this are reduced to a minimum in the process known from EP 0 686 702 A1, so that the temperature inside the slab when it enters the compensating furnace is as high as possible.

Attempts to minimize the wear of the compensating furnace, the energy expenditure required for its operation by lowering the furnace temperature, have shown that such a reduction in temperature, particularly when processing low-alloy, low-carbon steels, has a negative effect on the formation of precipitates in the subsequent hot and cold strip production process. The object of the invention is to reduce the required temperatures in the compensating furnace in a process of the type mentioned in the course of processing low-alloy, low-carbon steels so that the load on the furnace is reduced without sacrificing the quality of the hot strip produced or a cold strip made from it comes.

This object is achieved according to the invention by a method for producing a hot strip, in particular for producing a hot strip intended for producing a cold strip which is capable of good deep drawing, in which a steel melt which (in mass%) C: <0.07%, Si: <0.5%, Mn: <2.5%, AI: <0.1%, N: <0.01%, P: <0.025, B: <0.05, possibly up to a total of 0.35% Nb, Ti and V, and the balance contains iron and usual impurities, in which the molten steel is continuously discharged from a casting mold in one strand, in which the cast strand is passed through a cooling section immediately after emerging from the casting mold, in which the Strand is cooled intensively at a cooling rate of at least 3 K / s to a temperature of A _r ι i 25 K, in which the strand is cooled in air for at least 30 seconds following its intensive cooling, and in the strand itself or by thin slabs divided into a strand in a meadow be reheated before the strand or thin slabs are hot rolled into hot strip.

By, according to the invention, the strand emerging from the casting mold of intensive cooling Cooling rates of at least 3 K s is exposed /, (is finished the temperature at which the transformation of austenite to ferrite) in which the strand below the A _r ι temperature is cooled, the the hot strip for the formation of the desired material properties are required Excretions are specifically induced in the area that is to be upstream of the compensating furnace. There is so much time available during the subsequent cooling in air after the intensive cooling that the excretion processes have essentially expired when entering the compensating furnace. At the same time, the temperature in the strand is homogenized during this time, so that there is a uniform temperature distribution when entering the furnace.

Since the formation of precipitates is essentially complete before entering the equalization furnace, the furnace temperature can be limited to temperatures which are lower than those required in the conventional procedure

Reheating temperatures. Appropriately, the equalization furnace temperature to be maintained according to the invention is in a range whose lower limit is determined by the A _r3 temperature and whose upper limit is 1150 ° C.

Reheating temperatures of a maximum of 1050 ° C. are sufficient if a cold strip is produced from the hot strip produced according to the invention, which is annealed in a continuous furnace after the cold rolling. In this case, preferably all during the Production of the hot and cold strip associated with the rewarming subsequent work steps no more precipitation processes take place, so that it is no longer necessary to dissolve alloy components involved in the formation of excretions.

If, on the other hand, a cold strip is rolled from the hot strip produced according to the invention, which is annealed after cold rolling in a hood furnace, the temperature when the strand or thin slabs are heated in the compensating furnace should be in the range from 1100 ° C. to 1150 ° C. At a heating temperature above 1100 ° C., Al nitride dissolves to an extent which is sufficient to produce a desired “pancake” structure in the course of the hood annealing.

It has been found that a strip produced in accordance with the invention has a fine-grained structure which has a favorable effect on the deep-drawing ability of a cold strip produced from the hot strip. As a result, the invention thus provides a method which makes it possible to reduce the temperature in the compensating furnace, so that its service life is increased and the economy of the method is improved compared to the conventional procedure. In addition, a method according to the invention provides a product which is outstandingly suitable for processing by deep drawing.

A plurality of pass passes are preferably carried out during hot rolling, the finished rolled hot strip having a thickness of 2 to 5 mm. It should a thickness decrease S> 15% can be achieved in the last pass of the rolling. The hot-rolled strip rolled in this way has a particularly fine-grained structure, which further improves its deep-drawing ability. In this context, the “change in shape β” is understood to mean the ratio of the decrease in thickness during the last pass to the thickness of the strip as it enters the last rolling stand of the hot rolling mill. Accordingly, a hot strip has a thickness ho, for example, before the last stitch. After the last stitch, the thickness of the tape is reduced to hi. By definition, this results in the shape change in the last pass S to (h ₀ - hi) / h ₀ > 15% with h ₀ = thickness of the hot strip when entering the last rolling stand and hi = thickness of the finished hot strip.

If hot rolling is to be carried out with the austenitic structure of the hot strip, the final rolling temperature at the end of hot rolling is preferably at least 20 ° C. above the A _r3 temperature. If, however, after hot rolling a substantially ferritic microstructure of the hot strip are present, so the final rolling temperature is preferably at the completion of the hot rolling less than the A _r χ temperature + 50 ° C.

A further improvement in the microstructure of a cold strip produced from the hot strip produced according to the invention with regard to the deep-drawing properties can be achieved in that it was achieved during the cold rolling of the hot strip Total shape change ε _{total is} at least 60%. The "overall change in shape ε _tot " is understood here to mean the ratio of the decrease in thickness during cold rolling to the thickness of the unwrought strip as it enters the cold rolling stand. According to this definition, a hot strip produced according to the invention has a thickness h _0, for example after hot rolling. After cold rolling, the thickness of the strip is reduced to hi.

By definition, this results in the total change in shape ε _tot zu (h ₀ - hi) / h ₀ with h ₀ = thickness of the hot strip when it enters the cold rolling mill and h _x = thickness of the finished rolled cold strip.

If, as mentioned above, the cold rolled strip produced from the hot strip is annealed in a continuous furnace after cold rolling, the finished rolled hot strip should be coiled at a coiling temperature of at least 650 ° C. By adhering to this minimum temperature, the formation of precipitates in the coiled hot strip is promoted, so that the recrystallization of the cold strip can proceed unhindered during the continuous annealing of the precipitates.

If, on the other hand, a cold strip is produced that is annealed after cold rolling in a bell-type furnace, then the finished rolled hot strip should first be coiled at a coiling temperature of at most 625 ° C. In this way, the rest of the alloy constituents still present in the dissolved state and involved in the formation of precipitates are kept in solution. During the hood annealing, during which the cold strip faces one another for a longer period of time is subjected to the continuous annealing at a lower temperature, the precipitates are formed in the cold strip, which are required for the formation of the desired pancake structure of the cold strip.

The invention is explained in more detail below on the basis of a drawing representing an exemplary embodiment and on the basis of diagrams. They show schematically:

Figure 1 shows the beginning of a production line for producing a hot strip from a cast steel strand in a side view.

Diag. 1 shows the course of the Arl and Ar3 temperatures as a function of the carbon content of a low-carbon steel;

Diag. 2 shows the course of the temperature of the strand in the region of the start of a production line shown in FIG. 1.

A melt of a low-carbon, low-alloy steel is cast via a casting mold 1 to form a steel strand S between 20 and 70 mm thick.

Immediately after emerging from the casting mold 1, the steel strand S is intensively cooled by cooling water, which is directed onto the steel strand S from cooling devices 2 arranged on both sides of the steel strand S, in the course of a “metallurgical length” LM. The cooling rate a _LM reached during the intensive cooling of the steel strand S within the metallurgical length _LM is at least 3 K / s, the cooling rate a _LM actually set being dependent on the respective thermal conductivity of the steel strand S and the required temperature T _LM at the end of the metallurgical length LM. The extent of the intensive cooling is in any case such that the steel strand S at the end of the metallurgical length LM has a temperature T _LM of A _r ι ± 25 ° C, for example 710 ° C. In Diag. 1, the position of the A _r ι Temperature is given in function of the carbon content of the composition of the steel strand S.

Following the metallurgical length LM with the cooling devices 2 positioned thereon, the steel strand S runs on a roller table 3 through a cooling section LT, in which the steel strand S is cooled in air. For the cooling section LT, the steel strand S requires at least 30 seconds, so that at the end of the cooling section LT the formation of precipitates in the steel strand S is essentially complete and there is a homogeneous temperature distribution. After passing through the cooling section LT, depending on the design of the production line, the steel strand S itself or thin slabs D divided by it by means of a dividing device 4 enter an equalizing furnace 5.

In the compensation furnace 5 designed as a tunnel furnace, the steel strand S or the thin slabs D are heated to a reheating temperature T _w which is above the A _r3 temperature but below 1100 ° C. The position of the A _r3 temperature is also in Diag. 1 in Depends on the carbon content of the steel composition.

The temperature T _w reached during the reheating in the compensating furnace 5 depends on the annealing treatment which is carried out during the further processing of the hot strip produced from the steel strand S or the thin slabs D into cold strip. If the cold-rolled cold-rolled strip is subjected to a hood anneal, the reheating temperature T _{w is} in the range of 1100 ° C. If, on the other hand, the cold strip passes through a continuous anneal, the reheating temperature T _{w is} approx. 1000 ° C.

In Diag. 2, in addition to the course of the temperature of the steel strand S shown in a solid line in the procedure according to the invention, the course of the steel strand S shown in broken lines, which occurs in the conventional manner of operation, is shown. It can be clearly seen that in the conventional processing of a strand from a low-carbon, low-alloy steel, the cooling rate a _M Sdτ in the region of the metallurgical length LM is significantly lower than in the process according to the invention, that the A _r3 temperature is not fallen below and that the upper limit the reheating _temperature T _{wstD is} significantly higher than the upper limit of the reheating of 1100 ° C in the invention. REFERENCE SIGN ISTE

1 casting mold

2 cooling devices

3 roller table

4 compartment facility

5 leveling furnace D thin slabs

LM metallurgical length

LT cooling section

L Path axis of the diag. 2

S steel strand

T Diag temperature axis. 2

T _M temperature at the end of the metallurgical length LM

T _w reheating temperature

Ts _t dT reheating temperature in the prior art

Claims

PATENT TO SPEECH

1. A method for producing a hot strip, in particular for producing a hot strip intended for producing a cold strip that is easy to deep-draw,

in which a molten steel, which (in mass%)

C: <0.07%

Si: <0.5%

Mn: <

AI: <0.1%

N: <0.01%

P: <0.025

B: <0.05, optionally until a total of 0.35% of Nb, Ti and V, and the remainder contains iron and usual impurities, is melted,

in which the molten steel is continuously discharged from a casting mold (1) in a strand (S),

in which the cast strand (S) is passed immediately after exiting the casting mold (1) through a cooling section (2), in which the strand (S) has a cooling rate (a _LM ) of at least 3 K / s 25 K is intensively cooled to a temperature of A _r ι i,

in which the strand (S) is cooled in air after its intensive cooling for at least 30 seconds,

in which the strand (S) itself or thin slabs (D) separated from the strand (S) are reheated in a compensating furnace (5) before the strand (S) or the thin slabs (D) are hot-rolled into hot strip.

2. The method according to claim 1, characterized in that the strand (S) or the thin slabs (D) in the compensating furnace (5) to a temperature above the A _r3 temperature, but not exceeding 1100 ° C (T _w ) become.

3. The method according to any one of the preceding claims, d a d u r c h g e k e n n z e i c h n e t, that the thickness of the thin slabs is 20-70 mm.

4. The method according to any one of the preceding claims, d a d u r c h g e k e n n z e i c h n e t, d a ß several hot passes are carried out during hot rolling and d a ß the finished rolled hot strip has a thickness of 2 to 5 mm.

5. The method according to claim 4, characterized in that ß in the last stitch hot rolling a reduction in thickness S> 15% is achieved.

6. The method according to any one of claims 4 or 5, characterized in that the final rolling temperature at the end of hot rolling is at least 20 ° C above the A _r3 temperature.

7. The method according to any one of claims 4 or 5, characterized in that ß the final rolling temperature at the end of hot rolling is less than the A _r ι temperature + 50 ° C.

8. The method according to any one of claims 4 to 7, characterized in that ß a cold strip is cold-rolled from the hot strip and that ß the total shape change ε _total achieved during cold rolling is at least 60%.

9. The method according to any one of claims 4 to 8, characterized in that ß the cold strip is annealed in a continuous furnace and ß the temperature when heating the strand (S) or the thin slabs (D) in the compensating furnace (5) 1050 ° C not exceeds.

10. The method according to claim 9, characterized in that the finished rolled hot strip is coiled at a coiling temperature of at least 650 ° C.

11. The method according to any one of claims 4 to 8, characterized in that ß the cold strip is annealed in a hood furnace and that ß the temperature during the heating of the strand (S) or the thin slabs (D) in the compensating furnace (5) in the range of 1100 ° C to 1150 ° C.

12. The method of claim 11, d a d u r c h g e k e n n z e i c h n e t, that the finished rolled hot strip is coiled at a coiling temperature of at most 625 ° C.