EP1169486A1

EP1169486A1 - Method of producing a hot-rolled strip

Info

Publication number: EP1169486A1
Application number: EP00906372A
Authority: EP
Inventors: Rudolf Kawalla; Hans Pircher; Thomas Heller; Bernhard Engl; Pino Tes
Original assignee: ThyssenKrupp Stahl AG
Current assignee: ThyssenKrupp Steel Europe AG
Priority date: 1999-03-13
Filing date: 2000-02-24
Publication date: 2002-01-09
Anticipated expiration: 2020-02-24
Also published as: WO2000055381A1; US6855218B1; DE50001976D1; EP1169486B1; JP2002539330A; ATE239097T1; DE19911287C1; ES2195867T3

Abstract

The invention relates to a method for producing hot strip which features good forming ability and increased strength. This is achieved in that a hot strip (W) which is produced in particular from continuous casting in the shape of reheated slabs or slabs obtained directly from the casting heat, from thin slabs or cast strip, based on a steel comprising (in mass %) C: 0.001-1.05%; Si: <=1.5%; Mn: 0.05-3.5%; Al: <=2.5%, if necessary further elements such as Cu, Ni, Mo, N, Ti, Nb, V, Zn, B, P, Cr, Ca and/or S, with the remainder being iron as well as the usual accompanying elements, is continuously finish rolled and subsequently continuously cooled, with cooling taking place in at least two subsequent cooling phases (tCK, tLK) of accelerated cooling, to a final temperature; with the first cooling phase (tCK) of accelerated cooling starting at the latest three seconds after the last pass of finish rolling; and with the hot strip (W) during the first cooling phase (tCK) of accelerated cooling being cooled at a cooling rate of at least 150° C./s.

Description

Process for producing a hot strip

The invention relates to a method for producing a steel hot strip, in which the hot strip is subjected to cooling in several stages after finish rolling.

The cooling of a hot strip after the finish rolling, which usually takes place in several passes, is of considerable importance with regard to the material properties of the strip. The use of suitable cooling can be used, among other things, to influence the structure of the structure as such and the proportions of the individual structure types in this structure. For example, it is possible to influence the strength, toughness and hardness of a hot strip by cooling.

In the article "Hot rolled coils for special applications", A. De Vito et al., BTF - Special Issue 1986, pages 137-141, various studies are described which demonstrate the influence of cooling in hot strip production. These studies have shown that, for example in the production of a dual-phase hot strip steel (DP hot strip steel), it is expedient to carry out the cooling which takes place after the finish rolling in three stages. In the first and the last of these three stages, the belt runs through two conventionally designed, laminar cooling sections which are spaced apart from one another and in which cooling liquid is applied to the belt in the form of a plurality of veils arranged one behind the other in the direction of the belt is sprayed. The cooling rate achieved in the first stage of cooling is around 70 ° C / s. The cooling of the strip in the third stage is slower than in the first stage.

In the intermediate stage passed between the laminar cooling sections, the cooling takes place in air in the known method, the cooling rate achieved in this stage again being much lower than in the last stage of the cooling.

It has been shown that the known method described above can be used to produce DP hot-rolled steels without the presence of molybdenum in their composition, in which pronounced martensite and ferrite fractions are present. The hot strips in question have increased strength and toughness.

At the same time, however, a loss of ductility has to be accepted. In addition, it has been found that the improvements achieved with the known method are not sufficient to meet the requirements, particularly with regard to the hardness, of hot strips produced in this way.

The object of the invention is to provide a method with which hot strips can be produced which have a high formability and an increased strength.

This object is achieved according to the invention by a method for producing a hot strip, which in particular is made from continuous casting in the form of reheated or slabs used directly from the casting heat, from thin slabs or from cast strip based on a steel that is (in mass%) 0.001 - 1.05% C, ≤ 1.5% Si, 0.05 - 3.5 % Mn, <2.5% Al, optionally further elements such as Cu, Ni, Mo, N, Ti, Nb, V, Zn, B, P, Cr, Ca and / or S, and the balance iron and usual accompanying elements includes the following steps:

- continuous finishing of the hot strip,

- continuous cooling of the hot strip in at least two successive cooling phases of accelerated cooling to a final temperature,

- The first cooling phase of accelerated cooling begins no later than three seconds after the last pass of the finish rolling and

- The hot strip is cooled during the first cooling phase of accelerated cooling with a cooling speed of at least 150 ° C / s.

According to the invention, the hot strip is also cooled in at least two successive stages. The hot strip in the first cooling phase is cooled considerably faster than in the prior art. This compact cooling during the first cooling phase has the result that the γ / α conversion of the strip hot-rolled in the γ region is effectively and specifically suppressed to lower temperatures. In the subsequent second cooling phase with accelerated cooling, the strip is then brought to the desired final temperature. In this cooling phase the hardness-increasing second phases of the hot strip structure, such as martensite, bainite and residual austenite, are adjusted. (The final temperature reached at the end of the second cooling phase of accelerated cooling can of course be the reel temperature required depending on the desired processing results.)

Depending on the desired material properties, the steel used for the production of the hot strip can optionally contain additional elements. In the event of their presence, the proportion (in mass--) of Cu, Ni, Mo should not be greater than 0.8%, that of N, Ti, Nb, V, Zn, B should not be greater than 0.5% P is not more than 0.09%, Cr is not more than 1.5% and S is not more than 0.02%.

Experiments have shown that, among other things, those of the type mentioned above are particularly suitable for carrying out the process according to the invention which have a mass of 0.005 to 0.4. Contain silicon.

The method according to the invention is suitable, on the one hand, for producing hot strips which are produced on the basis of steels with low carbon contents. An advantageous variant of the method according to the invention is characterized in that the steel (in mass%) is not more than 0.07% C, not more than 0.2% Si, not more than 0.6% Mn and not more than Contains 0.08% AI, the hot strip is rolled in the austenite area during finish rolling, the hot strip is cooled in the first cooling phase of accelerated cooling from a temperature above 850 ° C to a temperature of 680 to 750 ° C, the hot strip in the second Cooling phase of accelerated cooling to a temperature of is cooled to less than 600 ° C and is finally coiled.

The method according to the invention is also suitable for producing DP hot strip steels. A corresponding embodiment of the method according to the invention is characterized in that the steel (in mass%) 0.04-0.09% C, not more than 0.2% Si, 0.5-2.0% Mn, 0, 02 - 0.09% P and not more than 0.9% Cr, and that the hot strip after the finish rolling in the first cooling phase of accelerated cooling is cooled from a temperature above 800 ° C to a temperature of 650 to 730 ° C that the hot strip is cooled to less than 500 ° C. in the second cooling phase of accelerated cooling and that the hot strip is then coiled.

With steels according to the invention, improvements in the material properties can also be achieved with steels with higher carbon proportions. According to a further embodiment of the invention, a hot strip is based on a steel with (in mass%) 0.25-1.05% C, not more than 0.25% Si and not more than 0.6% Mn , after the finish rolling in the first cooling phase accelerated cooling from a temperature above 800 ° C to a temperature of 530 to 620 ° C, in the second cooling phase accelerated cooling cooled to less than 500 ° C and then coiled. A hot strip produced in this way also has improved hardness and better shaping properties compared to conventionally produced strips.

In the case of an aluminum-containing TRIP hot strip which contains (in mass%) 0.12-0.3% C, 1.2-3.5% Mn and 1.1-2.2% Al, and in the manner according to the invention after this Finishing rolls in the first cooling phase are cooled from a temperature which is between the Ar ₃ temperature and a temperature of Ar ₃ + 150 ° C. to a temperature which is up to 50 ° C. below the Ar ₃ temperature In the second cooling phase, if it is cooled to 350 to 550 ° C and then coiled, improvements in strength and high formability can also be found.

A further advantageous variant of the method according to the invention is characterized in that the steel (in mass%) 0.04-0.09% C, 0.5-1.5% Si, 0.5-2.0% Mn, 0.4 - 2.5% AI, not more than 0.09% P and not more than 0.9% Cr contains that the hot strip after the finish rolling in the first cooling phase accelerated cooling from a temperature above 800 ° C a temperature of 650 to 730 ° C is cooled, that the hot strip is cooled to less than 500 ° C in the second cooling phase of accelerated cooling and that the hot strip is then coiled. Such a hot strip has DP and TRIP properties.

Structural steel with an increased ferrite content and consequently particularly good formability can be produced by the steel (in mass%) 0.07-0.22% C, 0.1-0.45% Si and 0.2 - 1.5% Mn contains that the hot strip is cooled after finishing rolling in the first cooling phase accelerated cooling from a temperature above 800 ° C to a temperature of 650 to 730 ° C, that the hot strip accelerated cooling in the second cooling phase is cooled less than 500 ° C and that the hot strip is then coiled. With the same steel composition, hot strip with improved hardness can be achieved in that the hot strip after finish rolling in the first Cooling phase of accelerated cooling is cooled from a temperature above 800 ° C to a temperature of 580 to 650 ° C, that the hot strip is cooled in the second cooling phase of accelerated cooling to less than 500 ° C and that the hot strip is then coiled. The hot strip cooled in this way has a higher proportion of bainite and martensite with a reduced proportion of ferrite.

According to an expedient embodiment of the invention, the hot strip runs through an intermediate cooling phase between the first cooling phase and the second cooling phase of accelerated cooling, during which the hot strip is exposed to air cooling. This intermediate cooling phase should last for at least one second. In the course of becoming more compact, i.e. greatly accelerated cooling subsequent intermediate phase, in which the cooling takes place in air, the austenite conversion into ferrite sets in more quickly and reaches a greater extent than in the prior art, with a strong grain-refining effect being observed at the same time.

Surprisingly, it has been found that a hot strip can be produced by the procedure according to the invention which, compared to a hot strip of the same composition cooled in air with intermediate cooling by the conventional method, has increased hardness and a finer-grained structure than in the conventional method. At the same time, the strip produced by the method according to the invention has high strength and, unlike the strips produced by the known method, has good formability. In order to safely suppress the γ / α conversion down to lower temperatures, the compact cooling phase should take place at the highest possible cooling rates and as close as possible to the last pass of the finish rolling. According to a preferred embodiment of the invention, the first cooling phase therefore begins at the latest two seconds after the last pass of the finish rolling, and the

Cooling rate in the first cooling phase is at least 250 ° C / s.

A further advantageous embodiment of the process, with which a hot strip of particularly good formability can be produced, is characterized in that at least one of the rolling passes is carried out during the finish rolling in the austenite region below a temperature of Ar ₃ + 80 ° C and that the total pass decrease during of finish rolling is more than 30%.

Depending on the nature and composition of the steel used to produce the hot strip, it is expedient if the steel, which in particular is introduced as thin slab primary material into the respective rolling mill, is treated in the liquid phase with Ca or Ca carrier alloys.

Depending on the desired work result, it can finally be advantageous if the hot strip is cooled in the second cooling phase at a cooling rate of at least 30 ° C./s.

The invention is explained in more detail below on the basis of a drawing illustrating an exemplary embodiment. In a schematic representation: 1 shows the end section of a line for the production of hot strips comprising a cooling section in a side view;

2 shows a diagram in which the temperature curve during cooling is shown within the cooling zone;

Fig. 3 is a diagram in which the converted

Portions of a steel used to produce a hot strip are shown above the temperature in the conventional and in the inventive method.

Line 1 for producing a hot strip W comprises a series of several finished roll stands, of which only the last stand 2 is shown here. In the finishing mill, hot strip W is rolled to its desired final thickness.

A compact cow device 3 is arranged at a short distance behind the last finished rolling stand 2. This compact cow device 3 comprises nozzles, not shown, via which cooling liquid, preferably water, is brought under increased pressure to the top and bottom of the hot strip W. The volume flow of the cooling liquid can be adjusted so that 3 cooling speeds of 150 ° C / s to 1000 ° C / s can be achieved within the compact cooling device.

A second cow device 4 is arranged in the forward direction F of the hot strip W at a distance from the compact cow device 3. The second cow device 4 works in the manner of a conventional laminar cooling, in which the cooling liquid is flowed through by several in the direction F consecutively arranged nozzles, also not shown here, are brought onto the hot strip W in a fan-like manner. The number of nozzles charged in each case and / or the volume flow of the cooling liquid applied in the area of the laminar cooling device 4 can be regulated in such a way that 4 cooling speeds of 30 to 150 ° C./s are achieved in the area of the laminar cooling device.

In the conveying direction F of the strip behind the laminar cooling device 4, a reel device 5 is arranged, in which the hot strip W is wound into a coil.

A hot strip W produced, for example, from a multi-phase steel is rolled in the finishing mill only in the austenite area with a total pass reduction of more than 30%. If necessary, the hot strip W is subjected to a thermomechanical treatment during rolling.

After the hot strip W has left the last stand 2 of the finished rolling mill, it reaches the compact cow device 3 within a transfer phase t _z , which is shorter than two seconds. Upon entering the compact cow device 3, the hot strip W becomes a first cooling phase t _C κ continuously exposed to a compact cooling, during which the hot strip W is cooled from an inlet _temperature ET _C κ to an outlet temperature AT _CK . The cooling rates achieved are between 250 and 1000 ° C / s. As a result of the accelerated cooling of the compact cow device 3 within a short time t _z after the exit from the finished rolling mill Hot strip W suppresses the γ / α conversion of the hot strip steel.

The hot strip W then runs through a free path in which it is cooled in air for an intermediate _{cooling phase} t _AUSE . The duration of the intermediate _{cooling phase} t _PAUSE is at least one second. During this time, there is a partial conversion of the hot-rolled steel.

Finally, the hot strip W reaches the laminar cooling device 4. In this, it is cooled from an inlet temperature ET _LK to an outlet temperature AT _LK within a second cooling phase t _LK . The set cooling rate is between 30 and 150 ° C / s. Depending on the respective chemical composition of the steel and the selected cooling rate, second phases (bamite, martensite or residual austenite) are formed, through which the properties of the hot strip W are influenced. The excretion state of the hot strip W is also controlled in this way.

Finally, the hot strip W cooled in this way is coiled in the reel device 5.

In Table 1, the microstructure proportions and the hardness of hot strips produced from steel "Stahl" - "Stahl2", which were produced by the above-described method according to the invention, are compared with the structural proportions and hardness of hot strips of the same composition, which are produced in a conventional manner have been cooled in air in two laminar cooling devices with cooling in between.

Table 1

The compositions of the steels "Stahl" and "Stahl2" used to produce the hot strips are given in Table 2.

Table 2

In Fig. 3 is for the steel in a solid line the course CLK of that structural change, which occurs when a hot strip first in the manner according to the invention for the time t _C κ a compact cooling with a cooling rate of 250 ° C / s, then _undergoes an intermediate _{cooling phase} t _PAUSE and finally a Lammar cooling for the time t _K , contrasted with the LLK course of the structural transformation drawn in dashed line, which occurs in a conventional combination of two laminar cooling systems with intermediate cooling in air.

It can be clearly seen that the upstream compact cooling means that the proportion of hard phases, i.e. those that convert at low temperatures increases. Thus, with the sequence of compact / air / laminar cooling according to the invention, the converted proportion UA of austenite at a temperature of 450 ° C is only about 60%. The conversion of the remaining portions of the austenite then begins to a greater extent at temperatures below 400 ° C and is only completed at a temperature of 320 ° C. In contrast, in the case of conventional laminar / air / laminar cooling at 400 ° C., the converted proportion UA has already reached approximately 90%. The transformation of the remaining austenite is already complete at 350 ° C.

Table 1 confirms the statement in FIG. 3. In each of the hot strips examined, using the method according to the invention, compared to conventionally cooled strips, a shift in the structural components in favor of the harder martensite phases has been achieved. With unchanged composition, this led to a significant increase in the hardness of the respective hot strip.

At the same time, the samples produced according to the invention have a microstructure with a finer grain structure than those produced by the conventional method. This has the consequence that the hot strips produced according to the invention have good formability despite the increased proportions of the hard phases. This fact was also confirmed for a TRIP steel ((in mass%) C: 0.2%, AI: 1.8 %, Mn: 1.6%). Such a steel had an average ferrite grain diameter of 6-7 μm according to the conventional production method. In the procedure according to the invention, this diameter is reduced to less than 3 μm.

REFERENCES

F direction of conveyance,

W hot strip,

1 line for producing a hot strip,

2 finishing mill stands,

3 compact cow device,

4 laminar cooling device,

5 reel device, t _z transfer phase between the exit from the

Finishing mill stand 2 and the beginning of the

Compact cooling, t κ first cooling phase, which the hot strip W needs to cover the length of the compact cooling device 3, ET _C κ entry _{temperature of} the hot strip W when entering the compact cooling device 3, AT _C κ exit temperature of the hot strip W when leaving the compact -Cooling device 3, tp _A us _E intermediate cooling phase, during which the hot strip W on

Air is cooled, t _LK second cooling phase, in which the hot strip W in the

Laminar cooling device 4 is cooled, ET _LK inlet temperature of the hot strip W when entering the laminar cooling device 4, AT _LK outlet temperature of the hot strip W when leaving the laminar cooling device 4, CLK course of the structural transformation that occurs when a hot strip is first undergoes a compact cooling and then a laminar cooling, LLK course LLK of the structural transformation, which occurs in a

Combination of two laminar cooling sets, UA respective converted proportion of austenite.

Claims

P A T E N T A N S P R Ü C H E

Process for producing a hot strip (W), which is produced in particular from continuous casting in the form of reheated slabs or slabs inserted directly from the casting heat, from thin slabs or from cast strip based on a steel which (in mass%)

C: 0.001 - 1.05%,

Si: <1.5%,

Mn: 0.05 - 3.5%,

AI: <2.5%,

optionally further elements, such as Cu, Ni, Mo, N, Ti, Nb, V, Zn, B, P, Cr, Ca and / or S, and

the remainder contains iron and usual accompanying elements,

comprising the following steps:

- Continuous finish rolling of the hot strip (W),

- Continuous cooling of the hot strip (W) in at least two successive cooling phases

(cκrt _LK ) accelerated cooling to a final temperature, - The first cooling phase (t _C κ) of accelerated cooling begins no later than three seconds after the last pass of the finish rolling and

- The hot strip (W) during the first cooling phase (tc_κ) accelerated cooling with a

Cooling rate of at least 150 ° C / s is cooled.

2. Method according to 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 the steel optionally (in mass%)

Cu, Ni, Mo with a share <0.8%,

N, Ti, Nb, V, Zn, B with a share <0.5%,

P with a share <0.09%,

Cr with a share <1.5% and / or

S with a share <0.02%

contains.

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 steel contains 0.005 to 0.4 mass% silicon.

4. 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,

- d a ß the steel (in mass%)

C: <0.07%, Si: <0.2%, Mn: <0.6%, AI: <0.08%

contains

- that the hot strip (W) is rolled in the austenite area during finish rolling,

the hot strip (W) is cooled in the first cooling phase (t _Cκ ) of accelerated cooling from a temperature above 850 ° C. to a temperature of 680 to 750 ° C.,

- Since ß the hot strip (W) in the second cooling phase (t _LK ) accelerated cooling is cooled to a temperature of less than 600 ° C and

- that the hot strip (W) is then coiled.

5. The method of 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, d a ß the steel (in mass.)

C: 0.04 - 0.09 o,

° T

Si: <0.2 0

⁰ /

Mn: 0.5 - 2.0 o. ° t

P: 0.02 - 0.09 o

° /

Cr: <0, 9 o,

0

contains the hot strip (W) after the finish rolling in the first cooling phase (t _C κ) of accelerated cooling is cooled from a temperature above 800 ° C to a temperature of 650 to 730 ° C,

- Since ß the hot strip (W) in the second cooling phase accelerated cooling (t _LK ) is cooled to less than 500 ° C and

- that the hot strip (W) is then coiled.

6. The method according to any one of claims 1 or 2, d a d u r c h g e k e n n z e i c h n e t, d a ß the steel (in mass%)

C: 0.25 - 1.05%, Si: <0.25%, Mn: <0.6%

contains

the hot strip (W) after the finish rolling in the first cooling phase (t _C κ) accelerated cooling is cooled from a temperature above 800 ° C to a temperature of 530 to 620 ° C,

- Since ß the hot strip (W) is cooled in the second cooling phase (t _K ) accelerated cooling to less than 500 ° C and

- Because the hot strip (W) is then coiled.

7. 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, d a ß the steel (in mass%)

C: 0.12 - 0.3%, Mn: 1.2 - 3.5%, AI: 1.1 - 2.2%

contains

- Since the hot strip (W) after the finish rolling in the first cooling phase (t _C κ) accelerated cooling from a temperature which is between the Ar ₃ temperature and a temperature of Ar ₃ + 150 ° C, cooled to a temperature which is up to 50 ° C below the Ar ₃ temperature,

- Since ß the hot strip (W) in the second cooling phase (t _LK ) accelerated cooling is cooled to 350 to 550 ° C and

- a ß the hot strip (W) is then coiled.

8. 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,

- d a ß the steel (in mass%)

C: 0.04 ^■ - 0.09%,

Si: 0.5 - 1.5%,

Mn: 0.5 - - 2.0%,

AI: 0.4 - - 2.5%, P: <0.09%,

Cr: <0.9%,

contains

the hot strip (W) after the finish rolling in the first cooling phase (t _C κ) of accelerated cooling is cooled from a temperature above 800 ° C to a temperature of 650 to 730 ° C,

- Since ß the hot strip (W) in the second cooling phase (t _L κ) accelerated cooling is cooled to less than 500 ° C and

- that the hot strip (W) is then coiled.

9. 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,

- d a ß the steel (in mass%)

C: 0.07-0.22%, Si: 0.1-0.45%, Mn: 0.2-1.5%,

contains

since the hot strip (W) after the _{finish rolling} in the first cooling phase (t _cκ ) accelerated cooling is cooled from a temperature above 800 ° C to a temperature of 650 to 730 ° C, - Since ß the hot strip is cooled in the second cooling phase (t _LK ) accelerated cooling to less than 500 ° C and

- that the hot strip is then coiled.

10. 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,

- d a ß the steel (in mass%)

C: 0.07-0.22%, Si: 0.1-1.45%, Mn: 0.2-1.5%,

contain,

that the hot strip (W) after the finish rolling in the first cooling phase (t _C κ) accelerated cooling is cooled from a temperature above 800 ° C to a temperature of 580 to 650 ° C,

- Since ß the hot strip (W) is cooled in the second cooling phase (t _LK ) accelerated cooling to less than 500 ° C and

- that the hot strip (W) is then coiled.

11. The method according to any one of the preceding claims, characterized in that ß the hot strip (W) between the first cooling phase (t _C κ) accelerated cooling and the second cooling phase (t _LK ) accelerated cooling undergoes an intermediate cooling _phase (t _PA u _sE ) during which the hot strip (W) is exposed to air cooling.

12. The method according to claim 11, characterized in that ß the intermediate cooling phase (tp _A usε) lasts at least one second.

13. The method according to any one of the preceding claims, characterized in that ß the first cooling phase (t _C κ) accelerated cooling begins at the latest two seconds after the last pass of the finish rolling and that ß the cooling rate during the first cooling phase

(t _C κ) accelerated cooling is at least 250 ° C / s.

14. The method according to any one of the preceding claims, characterized in that ß at least one of the pass passes during finish rolling in the austenite region below a temperature of Ar ₃ + 80 ° C and a total pass reduction of more than 30% is achieved.

15. The method according to any one of the preceding claims, characterized in that the steel is treated in the liquid phase with Ca or Ca carrier alloys.

6. 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 the hot strip (W) in the second cooling phase (tκ) accelerated cooling with a

Cooling rate of at least 30 ° C / s is cooled.