WO1997026377A1

WO1997026377A1 - Process for the hot rolling of steel bands

Info

Publication number: WO1997026377A1
Application number: PCT/EP1997/000107
Authority: WO
Inventors: Manfred Espenhahn; Rudolf Kawalla; Hans Pircher; Waldemar Wolpert
Original assignee: Thyssen Stahl Ag
Priority date: 1996-01-14
Filing date: 1997-01-11
Publication date: 1997-07-24
Also published as: AU1440897A; CA2242728A1; DE19600990C2; JP2000503345A; MX9805594A; KR19990077215A; DE19600990A1; EP0873428A1

Abstract

The invention relates to a process for the hot rolling of bands from unalloyed or low-alloy steel, in particular extra low carbon, ultra low carbon and interstitial free steel, in a multiple-stand finishing group, starting at a temperature in the steel austenite range and ending at a temperature in the steel ferrite range. Structural transformation of the steel is completed by cooling of the band between stands of the finishing group. The invention is characterised in that the band is rolled in the austenitic phase to an intermediate thickness ranging from 2 to 12 mm, is cooled for ferrite transformation in a single cooling stage, and subsequently rolled to size in the ferritic phase.

Description

Process for hot rolling steel strips

The invention relates to a method for hot rolling strips of unalloyed or low-alloy steel, in particular ELC, ULC and IF steel, in one

Multi-frame finished series, starting at a temperature in the austenite area of the steel and ending at a temperature in the ferrite area of the steel

Temperature, the structural transformation of the steel by cooling the strip between the scaffolding

Finished relay is carried out.

There are basically three different processes for hot rolling strips of unalloyed or low-alloy steel, primarily ELC- (Extra Low Carbon with 0.02 - 0.04% C), ULC- (Ultra Low Carbon with <0.005% C) and IF - (Interstitial Free through Nb and / or Ti addition) steels. One way is the finish rolling of steel strips in the austenite area. Here are the beginning and roll

final temperature in the austenite area of the respective steel, i.e. in the temperature range above their A3 point, which depends on the steel composition, usually above 800 ° C. When hot rolling strips

is only in the austenite area

Temperature control with thin strip thicknesses below 2 mm is extremely difficult. It also exists with

Increasing thickness reduction of the strip increases the risk of uncontrolled unwanted transformation from austenite to ferrite. The second way is the hot rolling of steel strip in the finishing line exclusively in the ferrite area, which has been given the term "ferrite rolling". The

Ferrite rolling takes place, beginning and ending in the ferrite area of the steel, i.e. at a temperature below the A3 point of the steel. It will

mainly used for ELC, ULC and IF steels. The manufacturing advantages of ferrite rolling mainly lie in the significantly reduced rolling force despite the low temperature, which is due to the lower deformation resistance of ferrite compared to austenite. This effect is used in practice to expand the range of dimensions of the strips with respect to greater width and smaller thickness and at the same time to increase plant productivity. In addition, product-specific advantages both in the

Processing of hot strip as well as hot strip expected for further processing either by cold rolling or by surface finishing. The positive

Possibilities of the previously known hot strip rolling in the ferrite area are limited. The advantageously lower deformation resistance of ferrite compared to austenite is limited to a narrow temperature range of usually only around 100 to 150 ° C. The benefits are greatest immediately after full

Phase transformation from austenite to ferrite. They decrease with falling temperature in the ferrite area. The thinner the rolling stock, the greater the disadvantageous

Temperature losses and the more difficult to prevent them. The one from the diminished

The advantage that resists deformation is also limited by the strengthening of the ferrite as the degree of deformation increases. The strain hardening is not or only slightly degradable during continuous rolling. Another limitation arises for ferrite rolling on conventional ones

Finishing scales due to the fact that the rolling stock has unfavorable texture components when the rolling temperature drops and the total thickness is reduced, which either cannot be corrected or can only be corrected by complex interventions in the frictional relationship between the roll and the rolling stock.

The third route is hot rolling, which partly takes place in the austenite area and partly in the ferrite area. The hot strip runs into the finished section at a temperature in the austenite area of the steel.

The austenitic structure is then converted into the during the hot rolling in the finishing mill

ferritic structure, and the strip is then heated to a temperature in the ferrite region of the steel

Final dimension rolled. Such a procedure has already been described in EP 0 504 999 A2. The strip produced in the continuous casting process with a thickness of e.g. 60 mm is hot rolled in a mill stand to a 20 mm pre-strip thickness. Either immediately afterwards or after reheating, the preliminary strip is then in a multi-stand at a temperature lying in the austenite area of the steel

Finished sheet rolled to a thickness close to the final dimension of 1.5 mm. It is then cooled in a cooling section to carry out the structural transformation from austenite to ferrite and in another stand on the

Final rolled thickness of 0.7 mm. A disadvantage of this known method of austenite-ferrite hot rolling is that the structural transformation from austenite to ferrite can take place in an uncontrolled manner during the rolling in the finishing season, that is to say even before the cooling section in which it is actually supposed to take place. On the one hand, this leads to undesirable, because irregular rolling force drops. On the other hand, a temporary or permanent effect occurs over the pre-strip width and during finish rolling

- Thick heterogeneous transformation state negatively affects both the strip shape and the mechanical technological properties of the hot strip. For this reason, hot rolling on continuous or semi-continuous hot strip mills is usually only carried out in the ferrite area, with the austenite-ferrite conversion before entering the multi-stand mill

Finished relay.

The invention is based on the object

to improve processes of the generic type in such a way that the restrictions or disadvantages described above do not occur during rolling first in the austenite area and then in the ferrite area, i.e. a controlled conversion of austenite to ferrite in the course of finish rolling is to be ensured while maintaining the advantages of rolling in the austenite area and

then in the ferrite area.

To solve this problem, it is proposed according to the invention in the generic method that the strip is rolled austenitic to an intermediate thickness in the range from 2 to 12 mm in the finished season and then for

Ferrite transformation is cooled in a single cooling step, whereupon it is rolled ferritically to the final dimension in the finishing line.

The invention solves the task, because when

Austenite rolling up to an intermediate thickness in the range of 2 to 12 mm does not pose the risk of uncontrolled conversion of austenite into ferrite, which the disadvantages described in the rolling causes. The ferrite rolling according to the invention, which follows the structural change in a cooling section connected between the stands of the finished stack, in at least two stages takes advantage of the better deformability of the

Steel strip in the ferrite area without it

unwanted textures or solidifications must come. The invention thus sees an advantageous division of the thickness reduction in the austenite region on the one hand and

then on the other hand in the ferrite area.

According to a preferred embodiment of the

The process according to the invention is intended to austenitically roll the strip to an intermediate thickness in the range from 3 to 8 mm and finish-roll ferritically to a final thickness of less than 1.5 mm.

According to a further preferred embodiment of the method according to the invention, the austenitically pre-rolled strip is to be ferritically rolled to the final dimension in at least 2 passes.

According to a further preferred embodiment of the method according to the invention, the strip is to be cooled at a speed of more than 30 ° C./s for ferrite conversion. This ensures intensive cooling of the steel strip, resulting in a complete phase transformation from austenite to ferrite that takes place within a short time.

According to the invention, it is further provided in the claimed method that the strip, after passing through the cooling section for ferrite conversion, passes through a compensation section of a few meters in which a Temperature compensation between core and

Surface temperature and the full

Structural transformation takes place before it enters the first stand for ferrite rolling. In this

Compensating distance, the steel strip has enough time to complete the phase change over the cross section of the steel strip, which is particularly the case

larger strip thicknesses is appropriate.

It is essential in the invention that the first section of finish rolling is carried out in the austenite area and that the brief cooling required for the phase change of the preliminary strip in the continuous rolling process takes place at an intermediate thickness in the range from 12 mm to 2 mm, preferably 8 to 3 mm. With this intermediate thickness, the necessary strip cooling is short-range and requires little coolant

to reach. In addition, a short distance is sufficient

Temperature compensation over the strip thickness before that

Hot rolling is recorded in the ferrite area.

The control problems in the rolling process are eliminated by the targeted intermediate cooling of the strip in the continuous flow of the finished batch

excluded that arise from uncontrolled and thus uncontrolled rolling force drops when the

Austenite / ferrite conversion can take place without steering during rolling in the finishing mill.

After leaving the cooling section, the strip edges can be heated in order to compensate for any temperature losses there. The method according to the invention can be used in conventional hot strip mills with continuous slab casting as the starting material and also in new types of continuous casting and rolling plants with a preliminary strip produced directly from the casting heat and strip processed further inline by rolling. Is the inventive method in

If conventional hot strip mills are used, the heating temperature of the slabs can be selected according to the criteria of optimal product quality.

Productivity losses due to commuting between

Pre-rolling and finish-rolling phases are avoided. The need for ferrite rolling according to the state of the art to use slabs which were reduced by 100 to 200 ° C. compared to the conventional one

The blast furnace temperature is not required.

In the claimed process, the preliminary strip has a temperature of usually more than 900 ° C. when it enters the finishing line. It has an austenitic structure over the entire thickness and width

Pre-rolling a uniformly recrystallized

Has grain structure. This advantageous soft, because softened structure allows high degrees of deformation already during the first phase of finish rolling.

The state of the material solidifies with the degree of deformation. The phase transformation austenite / ferrite now used deliberately causes softening. On the one hand, this is due to the fact that a recrystallized grain structure is formed. A significant contribution to the softening results from the lower compared to austenite

Resistance to deformation of the ferrite. The turn

advantageously soft, because softened ferritic Structural condition allows high degrees of deformation even during the second phase of finish rolling. The one for the

Phase transformation from austenite to ferrite required

The temperature jump is usually not more than 150 ° C for ELC steels, and usually not more than 80 ° C for ULC and IF steels. The ferrite rolling is preferably carried out at a temperature between the Ar1 temperature and 150 ° C., preferably 100 ° C. below.

In order to avoid the formation of unfavorable textures, lubrication when rolling the strip is advantageous.

The finished rolled strip is cooled to a temperature of at least 10 ° C / s starting at the latest 2 s after the end of the rolling with liquid and / or gaseous cooling media, such as water and / or water-air mixtures, with a cooling rate in the core the Arl temperature falls below 150 ° C. A hot strip produced in this way, the structure of which was frozen by the abrupt cooling from the final roll temperature, that is to say is not recrystallized, is particularly suitable for the production of cold-rolled strip, preferably for cold forming by pressing and

Deep drawing. Alternatively, the strip can also be kept after the hot rolling at a temperature which falls below the Ari temperature by less than 100 ° C. for the purpose of recrystallization, e.g. B. in the reel.

In Table 1, the data, d = strip thickness,

T = rolling stock temperature after each pass and

Vw = belt speed, for the known

Austenite rolling on the one hand and the known ferrite rolling on the other hand. This is compared with the data for hot rolling in Table 2 way according to the invention. The data in Tables 1 and 2 apply to an ELC steel with an initial strip thickness d = 20 mm and passed on to 1.5, 1.2 and 1 mm

End tape thickness in a five-stand prefabricated suite.

In a departure from the known austenite and ferrite rolling, the austenite / ferrite microstructure transformation takes place in the method according to the invention when a strip thickness of 5 mm is reached after the second pass through targeted cooling. The temperature drops from 870 ° C after the second stitch to around 800 ° C before the third stitch. The final thickness is reached after the fifth and last stitch in the finished season.

That provided in the method according to the invention

final ferrite rolls in one pass or in

Several passes after a previous austenite roll, followed by a controlled phase change, offers the possibility of optimizing the strip shape at the

Adequate consideration of stitch schedule design.

The procedure according to the invention can also be practiced in existing systems by installing a cooling section. The existing systems thus supplemented can be used at full capacity, not only for the method according to the invention, but also for all conventional hot rolling programs.

Overall, the application of

method a whole series

remarkable advantages, namely - No control problems in the rolling process, because there are no uncontrolled drops in rolling force;

- Applicable to both reheating as well

when rolling directly from the first heat;

- No restrictions other than quality

regarding heating temperature;

- No loss of productivity due to commuting times;

- No additional cooling water capacity;

- Uniform structure over width / thickness

both in austenite and ferrite;

- recrystallized in austenite and ferrite,

soft initial state, therefore high stitch decreases in single stitch by double tapping in

recrystallized state possible;

- Avoiding temperature losses in the

Finished scale, therefore lower energy consumption; - Cheap forming without harmful texture because

lower degree of deformation in ferrite;

- Favorable final thicknesses outside the usual

Spectrum;

- Small scale thickness on the finished strip because of

Descaling when cooling the strip rolled to intermediate thickness, therefore savings in the pickling line and better surface quality;

- Realizable with little investment in new and existing systems;

- New and conventional hot rolling programs without

Limitation of time utilization possible.

The invention is explained in more detail with the aid of the drawing,

Fig. 1 shows the temperature dependence of

UmformwideStandes k _f for ELC, ULC and IF steel grades with constant

Degree of deformation (φ = 0.6). Below the diagram are the

Compositions of the steels listed. The

The temperature interval in which the phase change of the ELC steel takes place is indicated in FIG. 1.

The illustration shows the advantageously low deformation resistance of the compared to austenite

Ferrits. This advantage is greatest, i.e. the

Resistance to deformation is lowest immediately after the complete phase transformation from austenite to ferrite. The favorable temperature range is approx. 100 to 150 ° C.

Fig. 2 shows a section of the

Finished relay with between two

Scaffolding switched on cooling section.

Fig. 2 shows the last stand F2 of the first part of the finishing stage, in which the austenite rolling is carried out, behind the cooling section, in which cooling to carry out the austenite / ferrite phase transformation in the steel strip and then the stand F3 as the first stand of the last Section of the finishing line, which should advantageously contain two, better three roll stands, in which the ferrite rolling is carried out. Below the diagram is the temperature curve along the band section shown on the

Surface (solid line) and in the core

(dashed line) shown as an example. The austenitic rolled strip of a ULC steel enters the cooling section at a temperature of around 920 ° C. Due to the intensive cooling within the cooling section, the surface temperature drops steeply to around 805 ° C, while the core temperature naturally drops somewhat less and is 860 ° C at the end of the cooling section. Between the end of the cooling section and the subsequent stand F3, the belt can compensate for the core and surface temperature in air. After about 2.3 m, the temperature equalization is complete, as the diagram in FIG. 2 illustrates. A device (not shown) for heating the strip edge can be arranged in front of the stand F3. By switching them on, any major temperature losses that occur in the strip edge area can be compensated.

Claims

claims

1. Process for hot rolling from strips

unalloyed or low-alloy steel, in particular ELC, ULC and IF steel, in a multi-stand

Finished scale, starting at a temperature in the austenite area of the steel and ending at an in

Temperature of the ferrite of the steel, the structural transformation of the steel being carried out by cooling the strip between the frameworks of the prefabricated structure, characterized in that the strip is rolled austenitically to an intermediate thickness in the range from 2 to 12 mm and then cooled in a single cooling step for ferrite conversion is what it is ferritically rolled to the final dimension.

2. The method according to claim 1

d a d u r c h g e k e n n z e i c h n e t that the strip is rolled austenitically to an intermediate thickness in the range of 3 to 8 mm and finish-rolled ferritically to a final thickness of less than 1.5 mm.

3. The method according to claim 1 or 2,

d a d u r c h g e k e n n z e i c h n e t that the strip is ferritically rolled to the final dimension in at least 2 passes.

4. The method according to any one of claims 1 to 3, characterized in that the band is cooled at a speed of more than 30 ° C / s for ferrite conversion.

5. The method according to any one of claims 1 to 4, d a d u r c h g e k e n n z e i c h n e t that the belt after passing through the cooling section

Ferrite transformation an equalization distance of some

Meters passes in which a temperature equalization

between core and surface temperature and the complete structural transformation takes place before it enters the first stand for ferrite rolling.

6. The method according to claim 1 to 5,

d a d u r c h g e k e n n z e i c h n e t that the edges of the steel strip after passing through the

Cooling section and be heated before entering the first stand of the ferrite mill.

7. The method according to any one of claims 1-6,

d a d u r c h g e k e n n z e i c h n e t that the strip is rolled with the addition of lubricants.

8. The method according to any one of claims 1-7,

d a d u r c h g e k e n n z e i c h n e t that the strip is finish rolled in the ferrite mill at a temperature between the Ar1 temperature and a 150 ° C, preferably 100 ° C below.

9. The method according to any one of claims 1 to 8, 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 strip, starting at the latest 2 s after

Completion of the rolling with liquid and / or gaseous cooling media, such as water and / or water-air mixtures, is cooled at a core cooling rate of over 10 ° C / s to a temperature which the Ar1 temperature by more than 150 ° C falls below.

10. The method according to any one of claims 1 to 8, that the strip is kept for recrystallization after hot rolling at a temperature which falls below the Ar1 temperature by less than 100 ° C.

11. Application of the method according to any one of claims 1-10 to the production of good cold-formable

cold rolled steel strips.

12. The method according to any one of claims 1-10, d a d u r c h g e k e n n e e c h n e t that it is used in the production of the steel strip, starting from the continuous casting and in-line further processing by rolling.