EP1238727A2 - Process for manufacturing metallic strip having differing material properties through their section - Google Patents

Abstract

Production of metallic strips (B) having sections of different material properties comprises: (i) melting a low alloyed or micro-alloyed steel; (ii) casting the steel in the casting gap (2) formed between walls of a casting machine (1); and (iii) cooling the cast strip. The force (G1, G2) exerted on the strip in the casting gap and measured over the width of the strip is more than 100 kN/m. Preferred Features: The steel contains (in wt.%) 0.01-0.80 C, 0.30-5.00 Mn, ≤ 0.50 Cr, ≤ 0.10 Al, ≤ 0.80 Si, ≤ 0.05 Ti, ≤ 0.05 Nb, ≤ 0.05 Nb, ≤ 0.01 N, ≤ 0.02 S, ≤ 0.02 P, ≤ 0.2 Ni, ≤ 0.1 Mo, ≤ 0.1 W and a balance iron and unavoidable impurities. The casting machine is a two roller casting machine. The force exerted on the strip in the casting gap and measured over the width of the strip is at least 200 kN/m.

Description

For the production of cast tapes and from them obtained blanks are usually two-roll casting machines, so-called "double-roller" casting machines, used. These casting machines are parallel with two Casters arranged to each other equipped, between which a casting gap is formed. When pouring the In rotating rolls, the strip becomes molten steel from above placed in this gap. The melt solidifies on the Surfaces of the rolls to two shells, which are in the casting gap to be pressed together to form the band. Because the thickness of the Band is set, is the effective in the casting gap Force depends on the thickness of the tape pressed, solidified bowls on the rollers.

In addition to "double roller" casting machines are others Casting machines in testing or in development. What these machines have in common is that at least a wall of the casting gap is moved during casting and the tape by continuously pressing Shells of solidified molten metal is produced.

An essential goal in the production of cast In practice, tape is a high quality To produce tape material, its material properties are homogeneous over its length and cross section. On such an even distribution of its properties having band can be used both during the Cast the following rolling steps as well as during forming in the intended component shape well and with Process repeatable success.

From Ulrich Rudolphi "Contribution to the assessment of quality of directly cast steel strips, manufactured according to the Two-roll band casting process ", Shaker Verlag, Aachen 1998, it is known that for the production of a cast tape with a homogeneous Distribution of properties on the one hand in the casting gap the band exerted forces and on the other hand the Temperatures at which the strip leaves the casting gap, direct influence on its material properties to have. To set an even structure having cast, easily processable cast tape it is therefore necessary to maintain a high temperature of the Tape at the exit from the casting gap and to Reduction of the generation of segregations and Avoiding a non-uniform temperature distribution excessive forces in the casting gap to the belt surface avoid. There may also be an active one Perform post-cooling of the cast tape.

Especially in the field of automobile body construction the demand has been made to use materials for To have available at a reduced weight the manufacture and practical use of Motor vehicles to meet the requirements are. This requirement is met by so-called "tailored blanks" been fulfilled. Such sheets are concerned are boards that are made by combining having different properties Sheet metal blanks are made. That way you can Tailored blanks, for example, in each of the areas be trained particularly resilient, in which they as Body components during practical use or are subject to special requirements during production, while less stressful in everyone else Areas optimal in terms of their weight have reduced thickness.

The production of tailored blanks is technical consuming. Such boards are therefore currently in the Usually only used to manufacture products that have a high added value.

The task is to create a process which is the inexpensive manufacture of metal strip, steel strip in particular, enables one in terms of his later use optimized distribution of his Has material properties.

• Erschmelzen eines niedriglegierten oder mikrolegierten Stahls,
• Vergießen des Stahls im zwischen bewegten Wänden einer Gießmaschine ausgebildeten Gießspalt zu einem gegossenen Band, wobei die im Gießspalt auf das Band ausgeübte, über die Breite des Bandes gemessene Kraft mehr als 100 kN/m beträgt,
• Abkühlen des gegossenen Bandes.

This object is achieved by a method for producing strips with sections of different material properties, which comprises the following steps:

• Melting of a low-alloy or micro-alloyed steel,
• Casting the steel into a cast strip in the casting gap formed between moving walls of a casting machine, the force exerted on the strip in the casting gap and measured over the width of the strip being more than 100 kN / m,
• Cool the cast tape.

Tapes produced according to the invention and, if appropriate, therefrom obtained boards show already when leaving the Pouring a distribution of their composition on that is adapted to their respective purpose. This is on the one hand by a suitable dimensioning of the Casting gap effective forces and on the other hand by a targeted cooling of the belt achieved. The invention makes use of the knowledge that through a Control of the forces acting in the casting gap direct influence on the structure of the structure can be taken over which the cast tape has its cross section. That way it can cast tape so influenced already during casting be that it's his for construction materials significant properties such as strength, elongation, Abrasion resistance and the like already in the as-cast state has.

Due to the high level, especially in the case of forced cooling Solidification speed, which occurs during thin strip casting is typical of the process, the Diffusion compensation of the steel accompanying elements largely suppressed. This also applies to interstitially resolved ones Atoms with high diffusion rates. Both high band forming forces provided according to the invention there is a depletion of steel elements in the strip core on. This "segregation" or "depletion" is all the more larger, the stronger the respective element for segregation inclines. To achieve this effect are specific Belt forming forces of over 100 kN per meter of belt width required, preferably 200 kN / m and more be provided to ensure the desired result to reach.

In the casting gap they are placed on the casting gap band walls which solidify the bounding walls compressed. This creates a forming zone from which between the solidified dendritic proportions of the Shells existing, not yet solidified interdendritic melt due to the across the width of the band acting high forces back in the still liquid part of the over the casting gap Melt pools is squeezed. The remaining Dendrites are iron-rich and poorer Steel accompanying elements as the flowable residual melt between the dendrites.

Because of the high casting forces continuous backflow of the Melt-enriched steel is created in a central zone of dendrites and Dendritenfragmenten. This way, despite one homogeneously composed melt a tape with over cross-section of inhomogeneous composition, the band core contains fewer alloy elements than the rest of the band. After the original and reshaping of the Due to the small thickness, the tape is quick Cooling down from the pouring heat. As a result, the Band core different properties than the outer areas a tape cast according to the invention. In this way can be cost-effective, for example, simply by Targeted adjustment of the belt in the casting gap force acting cast a tape in which the required strength through its inner core zone is guaranteed, while its outer shells a low wear resistance with better formability possess.For the implementation of the invention Process can be low alloyed or use microalloyed steel alloys, which typically 0.01 - 0.80% by weight C, 0.30 - 5.00% by weight Mn, ≤ 0.50 wt% Cr, ≤ 0.10 wt% Al, ≤ 0.80 wt% Si and the balance iron and unavoidable impurities contain. If a steel with 0.01 - 0.20% by weight of C and 0.30-1.00% by weight of Mn, see above can be created a tape that in the core area higher elasticity and in its outer areas has a high strength. On the other hand, should be a tape generated, which in its inner core area higher strength and greater elasticity on the outside has, this can be done by using a Reach steel alloy, the 0.10 - 0.8 wt .-% C and 1.00 - 5.00 wt .-% Mn contains, this alloy preferably in addition to iron and impurities other than chromium preferably in concentrations of up to 0.5% by weight no further alloying elements are added. In addition, the steel according to the invention can be up to 0.05 % By weight Ti, up to 0.05% Nb, up to 0.01% N, up to 0.02 % By weight S, up to 0.02% by weight P, up to 0.2% by weight Ni, to 0.1% by weight of Mo and up to 0.1% by weight of W. The Origin of the respective areas of the cast tape can by the targeted cooling of the cast tape supported after its exit from the casting gap become.

Has to carry out the method according to the invention a two-roll casting machine has proven itself. Such Casting machine makes it easy to put on the tape exerts force in the casting gap vary that the speed is varied at which the walls bounding the pouring gap move, which in the case of the two-roll casting machine by the Circumferential surfaces of the rollers delimiting the casting gap be formed. The diameter of the rolls is preferably 200 mm to 2000 mm.

By changing the speed at which the walls that delimit the pouring gap become the on them solidifying melt more or less time for building a more or less thick shell given. This is how a two-roll casting machine performs Reduce the speed of the rollers to one Increasing the thickness of the shells. Because at the same time Thickness of the tape to be produced and thus the measure of narrowest point of the casting gap is set to rise the forces acting in the casting gap. The opposite Effect is increased by increasing the speed of the Casting rolls causes.

Alternatively or in addition to a variation of the Speed of the walls delimiting the casting gap the forces in the casting gap can also be Change in those taking place over the walls in question Cooling of the melt solidifying on them affected become. So a larger cooling causes a stronger one Increase in the thickness of the solidified shells and thus higher forces in the casting gap, while by a low Cooling on the walls of the casting gap the thickness of the solidified shells and consequently the casting forces can be reduced.

Is of particular importance in the implementation of the inventive method also following the Exit from the pouring gap cooling of the cast band. The achieved Cooling speeds should be at least until Reaching certain limit temperatures of at least 30 K / s be. This can cool the cast strip in at least two successive stages with different cooling rates, where the cooling rate is from the casting heat outgoing temperature range up to 800 ° C should be at least 30 K / s while in the of Temperature range from 800 ° C to 300 ° C preferably at least 10 K / s but at most 500 K / s is. The cooling can be done with splash water as well also done with gases. When cooling takes place due to the small amount made possible by strip casting Strip thicknesses from 0.5 mm to 4 mm in contrast to continuous casting cooling not only close to the surface, but almost at the same time across the entire cross-section of the belt. The in the casting gap due to the effect of the band forming forces generated non-uniform structural state is due to this Way frozen, so that an adjustment of the Characteristics of the individual areas different Compositions due to an otherwise occurring Temperature compensation is prevented.

One with regard to the diversity of the Products that can be produced according to the method of the invention is a particularly advantageous embodiment of the invention characterized in that adjacent portions of the cast tape with different Cooling rates are cooled. The different cooling in the different areas of the cast tape, in cooperation with the inhomogeneous distribution of the alloying elements that a different transformation in the areas is achieved. In this way you can cast Create tapes where not only the core area and the adjacent outdoor areas a have different characteristics, but it zones can also be used in the external areas create their properties, for example, as a result a specifically reduced cooling the properties of the Core area adjusted or as a result of a special strong cooling are clearly different from the Properties of the adjacent area.

The differently cooled sections can in the longitudinal or transverse direction of the cast tape extend. It is also conceivable to only use certain tightly delimited sections of the cast volume to cool so that the properties desired there locally narrowly trained, while the remaining band is cooled less, so that this less refrigerated areas other properties mint.

When using the procedure according to the invention receive cast strips and blanks made from them, comparable to "tailored blanks", locally have clearly specified certain properties. This can be achieved specifically, if a different cooling different Adjacent areas of the cast tape performed becomes. Sheets with such a distribution and Setting the properties are particularly suitable for Manufacture of automotive body components, at which certain zones are special in practical use Exposed to other zones special requirements with regard to their deformability be put. Likewise generated are according to the invention Tapes for the manufacture of scaffolding certain components, such as pipes, clamps and similar fasteners determined in which in a certain direction a particular strength, Dimensional stability or comparable is required. The distribution of properties can also be done Adjust the tapes produced according to the invention so that they to manufacture wear-stressed Use components such as channels for bulk goods leave the tapes a wear-resistant, hard Surface with a sufficiently stretchable at the same time Should have core area.

Fig. 1

eine Zweirollen-Gießmaschine in einem Ausschnitt in seitlicher Ansicht;

Fig. 2

einen Ausschnitt eines in der Zweirollen-Gießmaschine gegossenen Bandes in Aufsicht;

Fig. 3

das gegossene Band gemäß Fig. 2 in einem Querschnitt entlang der in Fig. 2 eingezeichneten Linie X-X;

Fig. 4-6

verschiedene Bauelemente im Querschnitt.

Further advantageous refinements of the invention are specified in the dependent claims and are explained in more detail in connection with the exemplary embodiment described below with reference to a drawing. They show schematically:

Fig. 1

a two-roll casting machine in a cutout in a side view;

Fig. 2

a section of a strip cast in the two-roll casting machine in supervision;

Fig. 3

2 in a cross section along the line XX drawn in FIG. 2;

Fig. 4-6

different components in cross section.

The two-roll casting machine 1 has two axially parallel arranged to each other, a casting gap 2 along its Longitudinal bounding casters 3.4 on a Adjustable cooling device, not shown, from the inside be cooled with cooling water. The roles 3.4 exist preferably made of a copper alloy and can with a Nickel coating.

The center distance of the casting rolls 3.4 is fixed set that the depth of the casting gap 2 of predetermined thickness D of the cast strip to be produced B corresponds. Are via a controller, not shown the speeds of the opposite direction in the conveying direction F of Band B in the casting gap 2 conveying casting rolls 3.4 adjustable.

The cast strip B is in the vertical conveying direction F conveyed downwards from the casting gap 2. In Delivery direction F extends behind the casting gap 2 below the casting rolls 3.4 across the width of the belt B a cooling device 5.6. Every single one of the nozzles of the Cooling devices 5,6 is also not one Control device shown controllable, so that in nozzles arranged in certain rows or columns apply cooling fluid K, such as water, together as a group, while none of the others at that time Cooling fluid K emerges.

To cast a strip B, melt S is placed in the Pouring gap 2 poured so that it is above the casting gap 2 forms a melt pool 7. The melt S solidifies doing so on the rotating casting rolls 3, 4 to shells N1, N2, from the casting rolls 3, 4 into the casting gap 2 be promoted. The thickness of the shells N1, N2 at Entry into the casting gap 2 depends on it Dwell time on the casting rolls 3, 4, i.e. the time that passes until they arrive in casting gap 2.

The shells N1, N2 meet in the casting gap 2 and are squeezed together between the casting rolls 3, 4. The due to the firm attitude of the Center distance of the casting rolls 3.4 over the width of the Band B acting, essentially normal to Casting forces G1, G2 acting on the belt surface at least 200 kN / m. As a result of these high casting forces G1, G2 forms a core area in the cast strip B. C, which has properties other than those of it adjacent outer areas A1, A2 of the cast strip B.

The cast strip B then passes through the Cooling devices 5.6. In the example explained here are two in the longitudinal direction L of the tape B extending, distributed over the width of the band B. Stripe areas S2, S4 of a particularly intense, through direct application of cooling fluid K brings about cooling subjected, while in the side of each Strip areas S2, S4 or arranged between them Strip areas S1, S3, S5 direct cooling omitted. This way, in the outer Areas A1, A2 with the stripe areas S1 - S5 in essentially matching zones of different Material properties trained.

Starting from a molten steel, which is 0.01 - 0.2 % By weight C, 0.3-1.0% by weight Mn, up to 0.5% by weight Cr, to contains 0.1% by weight of Al and up to 0.8% by weight of Si, Cast tape produced shows when leaving the Casting gap 2 in the strip core 0.04 wt .-% carbon and the surface of the outer areas A1, A2 0.08% by weight Carbon on. By a far above 30 K / s horizontal, strong cooling in the cooling devices 5,6 the austenite in the core area C in ferrite and pearlite converted. In the direct and consequently special intensely cooled strip areas S2, S4 of the outer Areas A1, A2, on the other hand, develop bainite during it in the strip areas S1, S3, S5 that are not directly cooled no bainite or martensite is formed. Despite the differences in their chemical composition (including higher carbon content) correspond to the Material properties of these strip areas S1, S3, S5 therefore essentially those of the core area C. According to The belt exits from the cooling devices 5, 6 in Area of the band core C and the not directly cooled Strip areas S1, S3, S5 soften ferritic phases, while in the directly cooled strip area S2, S4 in the outer areas A1, A2 of volume B hard phases available.

Assuming a molten steel, the 0.1 - 0.8 % By weight C, 1.0 - 5.0% by weight Mn (in the specific example 1.5 wt .-% Mn) and up to 0.5 wt .-% Cr, so remains in the due to the high cooling rate directly cooled strip areas S2, S4 of the areas A1, A2 near the surface predominantly get austenitic structure, which has a high Has elasticity. Here, manganese acts as Austenite former, the carbon remains in the Austenite in solution. In contrast, arises in the core area C and the not directly cooled strip areas S1, S3, S5 a pearlitic or martensitic phase that has a higher hardness with reduced toughness.

In the examples shown in FIGS. 4 to 6 point the zones Zh in which the relevant components 8,9,10 subjected to high loads in practical use are particularly high strengths, while the other zones Zg, in which the manufacture of the Components 8,9,10 a particularly large deformation takes place, are soft and stretchy. On this way, the components 8,9,10 in a simple manner are manufactured in a material-saving and cost-effective manner and can still be used as carriers for high loads.

REFERENCE NUMBERS

1

Two-roll casting machine

2

casting gap

3.4

casting rolls

5.6

cooling device

7

melt pool

8-10

components

A1, A2

outer areas

B

cast tape

C

core area

D

thickness

F

Delivery direction F

G1, G2

Gießkräfte

K

Cooling fluid K

L

Longitudinal direction of the tape B

N1, N2

from solidified melt S on the casting rolls 3.4 formed shells

S

melt

S1-S5

strip areas

Zh

High stress zones

zg

Less stressed zones

Claims (18)

Process for the production of metallic strips (B) with sections (S1 - S5) of different material properties, comprising the following steps: Melting of a low-alloy or micro-alloyed steel, Casting the steel in the casting gap (2) formed between moving walls of a casting machine (1) to form a cast strip (B), the force exerted on the strip (B) in the casting gap (2) measured over the width of the strip (B) (G1, G2) is more than 100 kN / m, Cool the cast tape (B). A method according to claim 1, characterized in that the steel (in wt .-%) C 0.01 - 0.80%, Mn 0.30 - 5.00%, Cr ≤ 0.50%, al ≤ 0.10%, Si ≤ 0.80%, Ti ≤ 0.05%, Nb ≤ 0.05%, N ≤ 0.01%, S ≤ 0.02% P ≤ 0.02% Ni ≤ 0.2% Mo ≤ 0.1% W ≤ 0.1% Rest contains iron and unavoidable impurities. Method according to claim 2, characterized in that the steel contains 0.01-0.20% by weight carbon and 0.30-1.00% by weight manganese. Process according to Claim 2, characterized in that the steel contains 0.10-0.8% by weight of carbon and 1.00-5.00% by weight of manganese. Method according to one of the preceding claims, characterized in that the casting machine (1) is a two-roller casting machine. Method according to one of the preceding claims, characterized in that the force (G1, G2) exerted on the strip (B) in the casting gap (2) is at least 200 kN / m. Method according to one of the preceding claims, characterized in that in order to adjust the force (G1, G2) exerted on the band (B) in the casting gap (2), the speed at which the walls delimiting the casting gap (2) move is varied. Method according to one of the preceding claims, characterized in that adjacent sections (S1 - S5) of the cast strip (B) are cooled at different cooling rates. Method according to Claim 8, characterized in that the differently cooled sections (S1 - S5) extend in the form of strips in the longitudinal direction (L) of the cast strip (B). Method according to Claim 8, characterized in that the differently cooled sections extend in the form of strips in the transverse direction of the cast strip (B). Method according to one of the preceding claims, characterized in that the cast strip (B) is cooled at a cooling rate of at least 30 K / s. Method according to one of the preceding claims, characterized in that the cast strip (B) is cooled in at least two successive stages with different cooling speeds. A method according to claim 12, characterized in that the cooling rate in the temperature range emanating from the casting heat and reaching up to 800 ° C is at least 30 K / s. Method according to one of claims 12 or 13, characterized in that the cooling rate in the temperature range from 800 ° C to 300 ° C is at least 10 K / s but at most 500 K / s. Use of one according to one of claims 1 to 14 claimed processes for producing tape Manufacture of automotive body components (8-10). Use of one according to one of claims 1 to 14 claimed processes for producing tape Manufacture of scaffolding Components (8-10). Use of one according to one of claims 1 to 14 claimed method produced tape as Material for flexible rolling. Use of one according to one of claims 1 to 14 claimed processes for producing tape Manufacture of wear-resistant components.

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