EP1399598A1

EP1399598A1 - Method for producing high-strength cold-formed steel products from a hot rolled strip, said products exhibiting good malleability

Info

Publication number: EP1399598A1
Application number: EP02738172A
Authority: EP
Inventors: Thomas Heller; Bernhard Engl; Günter STICH
Original assignee: ThyssenKrupp Stahl AG
Current assignee: ThyssenKrupp Steel Europe AG
Priority date: 2001-06-26
Filing date: 2002-06-21
Publication date: 2004-03-24
Anticipated expiration: 2022-06-21
Also published as: EP1399598B2; ATE279540T1; ES2230500T3; WO2003002772A1; DE10130774C1; DE50201321D1; EP1399598B1

Abstract

The invention relates to a method for producing a high-strength cold-formed product from a hot-rolled strip, said product exhibiting good malleability. According to said method, a steel comprising (in wt.%): C: 0.01 0.25 %, Si: 0.01 1.50 %, Mn: 0.50 2.00 %, P: <= 0.08 %, S: <= 0.01 %, Al: 0.001 1.50 %, Cr: <= 0.60 %, Mo: <= 0.60 %, N: < 0.02 %, at least one micro-alloy element from the group Ti: <= 0.20 %, Nb: <= 0.06 %, V: <= 0.15 %, the remainder being iron and the usual impurities is cast to form an input stock such as slabs, thin slabs or a cast strip. Starting from an initial hot-roll temperature, at which the micro-alloy elements remain essentially dissolved, the input stock is hot-rolled to form a hot-rolled strip. Said strip is firstly wound at a winding temperature of less than 600 DEG C and then cold formed into a product at a cold formation degree which is not less than 2 % and not greater than the deformation limit. The cold-formed product is annealed at a temperature and for a period which are less those required for complete recrystallization. The inventive method enables a cold formed product to be produced, said product exhibiting high strength, malleability and plasticity in the cold formed state.

Description

Process for the production of high-strength, cold-formed steel products from a hot strip with good

ductility

The invention relates to a method for producing a high-strength, cold-formed steel product from a hot strip with good formability.

A method for producing high-strength hot strip is known from DE 197 10 125 AI. According to the known method, a (in% by weight) 0.1-0.2% C, 0.3-0.6% Si, 1.5-2.0% Mn, ≤ 0.08% P, 0.3 to 0.8% Cr, <0.4% Mo, <0.2% Ti,

<0.08% Nb and the rest iron and complex phase steel containing unavoidable impurities were melted, poured into slabs and then rolled out into hot strip, the hot rolling end temperature being above 800 ° C. The hot strip obtained is then cooled at a cooling rate of at least 30 ° C./s and then coiled at a coiling temperature of at most 600 ° C. With hot strip obtained in this way, the strength is at least 900 MPa due to the relatively low coiling temperature. In addition, the hot strip produced by the known method has good cold formability.

This formability can be according to the

DE 197 10 125 AI known methods can be further increased in that the hot strip after coiling subjected to annealing in the temperature range from 500 ° C to 850 ° C. This annealing achieves a material condition that offers advantages for components that, overall, must still have high strength, especially the yield strength, and good formability. By choosing higher annealing temperatures, particularly high strengths can be achieved with exceptionally low yield strength ratios or the same amount of hardening with good elongation values.

According to DE 197 10 125 AI, the hot strip treated in this way is suitable for cold rolling, in which a degree of deformation of at least 30% is achieved. By subsequently annealing the cold-formed strip in such a way that it recrystallises at temperatures of 700 ° C. to 900 ° C., a cold-formed sheet product of high strength, further improved yield strength ratio and good hardening behavior can be produced.

However, it has been shown that the elongation behavior of the products produced by cold working in accordance with the known processes is not sufficient for many applications. For example, it is difficult with components made from such steel, which are to be used to protect the occupants of a vehicle in the event of an accident, to meet the requirements imposed on such parts with regard to strength on the one hand and energy absorption capacity on the other. The components produced by the known method do not have the necessary deformability even after the cold forming. If they undergo recrystallizing annealing after cold working, increases Although the formability increases, the strength is then no longer sufficient.

The object of the invention was to provide a method for producing a cold-formed product which, with increased strength in the cold-formed state, has high ductility and, accordingly, good formability.

Starting from the prior art explained above, this object is achieved by a method for producing a high-strength, cold-formed product from a hot strip with good ductility, in which a steel with (in% by weight) 0.01-0.25% C, 0.01-1.50% Si, 0.50-2.00% Mn, <0.08% P, <0.01% S, 0.001

- 1.50% Al, <0.60% Cr, <0.60% Mo, <0.02% Mo, and at least one microalloying element from the group Ti, Nb, V, the content of Ti <0.20 %, the content of Nb <0.06% and the content of V <0.15%, balance iron and usual impurities, is cast into a raw material, such as slabs, thin slabs or cast strip, in which the raw material starts from a hot rolling starting temperature at which the

Microalloying elements remain essentially dissolved, hot-rolled to hot strip, in which the hot strip is coiled at a coiling temperature of less than 600 ° C., in which the hot strip is then cold-formed into a product with a degree of cold deformation of at least 2% and at most corresponding to the limit deformation, and in which the product produced by cold working at temperatures and times below that for the complete The temperatures and times required for recrystallization are annealed.

The invention is based on the surprising finding that the strength-increasing properties of the microalloying elements contained in steels according to the invention are activated when the hot strip is produced according to the invention by annealing the product obtained from the hot strip after coiling by cold deformation, and cold-formed products with particularly high strengths and one Allow good forming properties to be produced even in the cold-formed state, without requiring a technically complex remuneration.

As a result of the heat treatment carried out according to the invention after cold working, the loss of deformability which is regularly associated with cold working is compensated for without loss of strength. This also applies, for example, to hardening in the area of the cut edges when cutting or punching and the associated increasing sensitivity to crack formation. Instead, it is shown that in the course of the heat treatment carried out after the cold working there is a further increase in strength, so that high strength is specifically combined with high forming capacity in a cold-formed product produced according to the invention. This combination of advantageous properties makes it possible, for example, to cold-deform the cold-formed and then heat-treated product in order to bring it into its final shape. In addition, the good formability, for example, is relevant for crash-relevant components such as Bumpers, side impact beams, reinforcing elements, are available in full in order to be able to convert kinetic energy into forming energy on a large scale in the event of a collision with other objects.

Likewise, the method according to the invention is particularly suitable for producing components whose dimensional and dimensional stability are just as demanding as their strength in use. A typical example of such an application are frames for screens which have to hold the shadow mask in a tube of television sets or computer monitors due to the heat load during their assembly and their operation under uniform tension in a defined level. The required high strength is then given to the cold-formed component in the course of the heat treatment carried out according to the invention after the cold-forming, so that the result is an exactly shaped, high-strength component with a high yield strength. Additional compensation, which can lead to warpage of the component and impairment of the surface, is then no longer necessary.

A prerequisite for the success achieved by the invention is that during the hot processing of the steel used according to the invention, the operating parameters, including the coiling temperature, are selected such that the hardness increase per se due to the presence of a sufficient amount of microalloying elements is not achieved in the course of coiling, but rather that the microalloying elements are present in the undissolved, dissolved state after reeling. For this reason, the reel temperature is according to the invention always chosen so that it lies at a clear distance below the temperature at which the maximum precipitation of the microalloying elements would be reached.

The steel alloy used according to the invention comprises steel compositions from which, depending on the particular composition and the production conditions, dual-phase steels, bainitic steels or complex-phase steels can be produced. Basically, the aim is to have steels with a fine-grained, multi-phase structure, in which bainite, ferrite, martensite and residual austenite are contained, depending on the respective type of steel.

After a starting material has been produced from a steel with a composition used according to the invention, this starting material is hot-rolled into hot strip. The hot rolling starting temperature is chosen so that the micro-alloying elements remain largely dissolved in the steel so that they are available for the effects brought about in the last step of the method according to the invention.

For conventionally cast slabs, the initial rolling temperature should be at least 1050 ° C for this purpose. However, thin slab or thin strip casting processes are particularly suitable for the production of the primary material, with which it is possible with particular reliability due to the fact that there is no need to reheat the primary material or a solution of the microalloying elements and on the other hand the cooling rates of the strand can be high. the To keep micro-alloy elements largely dissolved until the start of the hot rolling process.

The hot rolling end temperature should preferably be in the austenite region, that is to say above the Ar ₃ temperature, in order to maintain the fine-grained, multi-phase structure sought after according to the invention.

The exact composition of the structure of the hot strip obtained is determined by the chemical properties, the heating, rolling and cooling conditions and the reel temperatures. It has been shown that the effect of increasing the strength and formability achieved with dual-phase steels achieved according to the invention is particularly reliable when the reel temperature is at most 300 ° C. In steels with a predominantly bainitic structure, this effect is reliably achieved at reel temperatures that are 350 ° C to 450 ° C. Complex phase steels should preferably be coiled at temperatures from 450 ° C to 550 ° C.

Even in the hot-formed state, steels of the type used according to the invention have a high strength of at least 550 N / mm ² . In the course of the subsequent cold forming, the steels harden very strongly. At the same time, the elongation decreases significantly, so that a product that is high-strength but difficult to deform is obtained. Because of their reduced formability, cold-formed components of this type have only a small forming reserve and a correspondingly reduced energy absorption capacity. As such, for example, they are only suitable to a limited extent for the production of components which are used as bumpers in crash endangered areas of an automobile.

By annealing the product obtained at temperatures and for times during which the complete recrystallization is reliably avoided, the yield strength of the steel used is further increased, on the one hand, by the fact that the product obtained is cold annealed. On the other hand, the annealing increases the elongation values almost to the level at which the ductility of the hot strip was before the cold working. In this way, the products that were originally difficult to form immediately after cold forming can be made into components that are not only high-strength but also have a high formability due to their good elongation properties. As such, they are suitable for those applications for which they were not suitable before the annealing treatment.

Typical annealing temperatures during the annealing following the cold deformation are in the range from 450 ° C. to 700 ° C., preferably in the range from 550 ° C. to 650 ° C. The annealing time and annealing temperature are interchangeable to a limited extent. This means that at high temperatures only short holding times are required and vice versa. Accordingly, the heat treatment can be carried out as a separately carried out treatment step in a hood or in a continuous annealing. Annealing treatment in a conventional manner under conditions that would lead to complete recrystallization would nullify the effect achieved by the invention. It is particularly advantageous to incorporate the annealing step into a heat treatment to which the product cold-formed from the hot strip is subjected in any case in the course of its further processing. This step can be, for example, a zinc coating or another method for coating or passivating the surface of the cold-formed product, which is carried out at the range of the temperatures provided for the annealing treatment.

The degrees of deformation achieved in the course of cold deformation amount to at least 2% up to the change in limit shape, the effects achieved by the procedure according to the invention having an effect particularly when the degree of cold deformation is 5% to 20%, in particular 5% to 10%. Cold forming can be carried out, for example, as roll profiling, drawing, pressing or hydroforming. Cold rolling with relatively low degrees of deformation is also suitable for this.

At least one of the elements Ti, Nb and / or V is used as the microalloying element in steel according to the invention. Of course, these elements can be added combined in amounts sufficient to trigger the excretion process effected at the end of the method according to the invention. However, it is particularly preferred to use Ti, which is distinguished by a particularly favorable excretion behavior. If Ti is used as the only microalloying element according to a preferred embodiment of the invention, its content should be at least 3.4 times the content of N in order to have a sufficient solution potential of the Ensure microalloying element. The Ti content of the steel for this purpose is preferably in the range 0.07 to 0.15% by weight, while the nitrogen content is preferably less than 0.007% by weight. With Ti contents of this size, the formation of fine deposits with a hardening effect is optimally supported.

Depending on the type of steel processed, the Al content is preferably in the range from 0.015 to 0.08% by weight. or 0.5 to 1.5% by weight.

Steel sheets produced according to the invention are preferably suitable for cold forming into components, the strength of which, along with good elongation behavior and, accordingly, good deformation behavior and high geometric dimensional stability, are subject to high demands. Components produced from sheet metal according to the invention are notable for their low weight and high energy absorption capacity.

An example of an advantageous use of steel sheets produced according to the invention is the production of frames for flat screens. Likewise, components for bodywork of motor vehicles or add-on parts can be produced from steel sheets produced according to the invention, which on the one hand have to be particularly strong and on the other hand, in the event of a collision with another vehicle, must convert the kinetic energies released in the event of an impact into deformation energy. These include longitudinal members, so-called "crash boxes", chassis parts, hollow structure parts and reinforcement parts, such as the A, B or C pillar of the automobile body. Hot strip produced according to the invention is basically suitable for galvanizing. However, hot-dip galvanizing, which takes place at relatively low temperatures, in particular piecewise, is preferable, galvanizing at temperatures above 750 ° C. is less suitable for this, since the critical temperatures with regard to the effect achieved according to the invention can be exceeded in the workpiece being processed ,

The invention is explained in more detail below on the basis of exemplary embodiments.

A steel composed according to Table 1 was cast on a continuous casting plant to a pre-slab, which was then hot-rolled as a starting material at a temperature of 1125 ° C. in a hot strip mill to a hot strip. The hot rolling end temperature was 925 ° C. The reel temperature was 475 ° C. After coiling, the hot strip is pickled and delivered to a cold rolling mill in the pickled state.

Table 1

There, various strip samples of the hot strip were cold rolled at cold rolling degrees of 5%, 10%, 20%, 30%, 40%. Groups G1 to G5 were then put together from the cold strips obtained in this way, each group G1 to G5 not having one Cold-formed (cold deformation degree = 0%), one with 5%, one with 10%, one with 20%, one with 30% and one with 40% cold deformation degree cold-formed cold strip has been assigned. For comparison purposes, the plates of group G1 have not been subjected to any annealing following the cold deformation, while the plates of group G2 at 500 ° C, the plates of group G3 at 550 ° C, the plates of group G4 at 600 ° C and the plates Group G5 were annealed at 650 ° C in a bell-type furnace for one hour each and then cooled in air.

The material properties determined in the tensile test are entered in Table 2 for each of the sheets in groups Gl - G5.

") The examples marked" E "in column A ₈₀ resulted in a final break.

Table 2

It can be seen that in the sheets processed in the manner according to the invention, as a result of the heat treatment carried out after the cold deformation, the strength values obtained in each case are markedly improved compared to the only cold-formed and not heat-treated sheets Gl.l-G1.5. At the same time, a uniform expansion Agi is achieved by the heat treatment carried out after the cold deformation, which corresponds approximately to the uniform expansion Agi of the non-cold-formed and non-heat-treated sheet Gll.O. Accordingly, the cold-formed and subsequently heat-treated steels produced according to the invention have particularly good formability and an equally good energy absorption capacity.

Table 3 shows a second example of a steel composition used according to the invention.

Table 3

A correspondingly alloyed steel was melted and cast into slabs. The slabs are then reheated to a temperature of more than 1150 ° C and with a hot rolling temperature of at least 850 ° C has been hot rolled. After coiling at a temperature of less than 600 ° C, for a first untreated strip sample T2.0 of the strip, for a strip sample annealed at 580 ° C for 30 minutes and then cooled in air, there is T2.1 for one with a degree of cold deformation of 10 % only cold-rolled strip sample T2.2 and for a strip sample T2.3 cold-rolled in the manner according to the invention with a degree of cold deformation of 10%, then annealed for 30 minutes at 580 ° C. and then cooled in air, the mechanical properties R _p o, ₂ , R _m , Re / R _m , A ₅ , A _so or A _{80 have been} determined. The determined values of the mechanical properties are given in Table 4.

Table 4

These examples also demonstrate that the combination of cold forming and subsequent heat according to the invention produces high-strength steels which, because of their good ductility, have a particularly high formability. The strength of the tape sample T2.3 produced according to the invention is far superior to the strength of the untreated tape sample T2.0, while its elongation values correspond to those of the undeformed tape sample T2.0. Likewise, the only cold-deformed tape sample T2.2 not only exhibits inferior extensibility compared to the tape sample T2.3 produced according to the invention, but also also lower strength. With the only heat-treated tape sample T2.1, the extensibility is at a level comparable to the tape sample T2.3 produced according to the invention. At the same time, however, the strength of the tape sample T2.1 is far inferior to the strength of the tape sample produced according to the invention.

The steel composed according to the alloy specified in Table 5 was also melted and cast into slabs.

Table 5

Subsequently, as in the example given in Tables 3/4, these slabs were reheated to a temperature of more than 1150 ° C., with a hot rolling temperature of at least 850 ° C. and at a temperature of less than 600 ° C. Then six tape samples T3.0 to T3.5 were then taken.

The further processing steps carried out with the relevant tape samples T3.0 to T3.2 as well as the associated values of the mechanical properties for a sample taken from the tape center are given in Table 6.

¹ ) Annealing time 30 minutes each, cooling in air

² ) The examples marked with "E" in column A ₈₀ resulted in a final break.

Table 6

In this case too, the strip sample T3.2 clearly shows that high-strength cold-formed sheet products can be obtained with the procedure according to the invention, which, as a comparison of the ductility values shows, are superior in their formability to the undeformed strip sample T3.0. Likewise, according to the invention

Property improvements are achieved when cold rolled strip is used for much longer times at comparatively low temperatures, e.g. 450 ° C to 550 ° C, can be annealed in a hood.

An example of a particularly advantageous use of a steel produced according to the invention consists in the production of frames in which the shadow masks are held by television tubes. In order to blacken the shadow mask stretched in such a frame, the frame with the shadow mask is exposed to a temperature which is typically approximately 570 ° C. for a sufficient time. By producing the frame from a hot strip produced in the manner according to the invention, taking advantage of the deformability of the hot strip which is outstanding in this state, by cold working and then in the course of the blackening of the shadow mask held by it for a sufficient time the temperature required for blackening, the strength of the frame increases to a level at which the frame can safely and dimensionally bear the heat load exerted on it during the manufacture and operation of the television tube and can ensure that the shadow mask is free from distortion.

Another advantageous application of the method according to the invention consists in the production of galvanized side impact beams or other crash-relevant components for vehicle bodies. First, the respective component is cold formed from hot strip produced according to the invention, for example by a deep-drawing operation. The cold-formed component is then subjected to piece galvanizing at temperatures in the range of 470 ° C. In the course of this galvanizing process, the component is also kept at a temperature level at which the increase in ductility and strength achieved according to the invention occurs. The side impact beam obtained in this way has a particularly high energy absorption capacity due to its good elasticity and high strength, by means of which it converts a large part of the kinetic energy released in the event of a collision with another vehicle into deformation energy and that required for the survival of the occupants Can secure space.

Claims

P AT E N T AN S P RÜ C H E

Process for producing a high-strength, cold-formed product from a hot strip with good forming capacity, in which a steel with (in% by weight):

C: 0.01-0.25%

Si: 0.01 - 1.50%

Mn: 0.50 - 2.00%

P: <0.08%

S: <0.01%

AI: 0.001 - 1.50%

Cr: <0.60%

Mo: <0.60%

N: <0.02% and at least one microalloying element from the group

Ti: <0.20%,

Nb: <0.06%,

V: <0.15%, remainder iron and usual impurities, is cast into a starting material, such as slabs, thin slabs or cast strip, in which the starting material is hot-rolled into hot strip starting from a hot rolling starting temperature at which the microalloying elements remain essentially dissolved becomes, in which the hot strip is coiled at a coiling temperature of less than 600 ° C, in which the hot strip is subsequently at a change of at least 2% and at most corresponding to the limit shape change

Degree of cold deformation into a product is cold worked, and at which the product produced by cold working is annealed at temperatures and times which are below the temperatures and times required for complete recrystallization.

2. The method of 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 steel as the only microalloying element contains Ti and that the ratio of the content of Ti based on the content of N is at least 3.4.

3. The method according to any one of the preceding claims, characterized in that the hot rolling end temperature is at least equal to the Ar ₃ temperature.

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, that the reel temperature is less than 300 ° C.

5. The method according to any one of claims 1 to 3, dadurchgekennzeic hn et that the reel temperature is 350 ° C - 450 ° C.

6. The method according to any one of claims 1 to 3, d a d u r c h g e k e n n z e i c h n e t, that the reel temperature is 450 ° C - 550 ° C.

7. 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 degree of cold deformation is 5% - 20%, in particular 5% to 10%.

8. 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 annealing temperature is 550 ° C - 600 ° C.

9. 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 annealing is carried out in one pass.

10. 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 annealing is carried out in the hood furnace.

11. 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 primary material is produced as a cast strip on a casting and rolling plant.

12. The method according to any one of the preceding claims, characterized in that the product produced by cold working is a frame for flat screens.

13. The method according to any one of claims 1 to 11, d a d u r c h g e k e n n z e i c h n e t that the product produced by cold forming is a component for the production of motor vehicle bodies or an add-on part.

14. 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 product is produced by cold internal high pressure forming.

15. 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 cold-formed product is subjected to galvanizing.

16. The method according to claim 15, d a d u r c h g e k e n n z e i c h n e t, that the galvanizing is carried out as hot-dip galvanizing.

17. The method of claim 15 or 16, d a d u r c h g e k e n n z e i c h n e t, that the galvanizing takes place at temperatures around 460 ° C.