DE10107027A1 - Process for the production of metallic strips with sections of different material properties - 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 the Quality of Directly Cast Steel Strips, Produced According to the Two-Roll Strip Casting Process", Shaker Verlag, Aachen 1998 , it is known that on the one hand, for the production of a cast strip with a homogeneous distribution of properties, the force exerted on the strip in the casting gap Forces and, on the other hand, the temperatures at which the strip leaves the casting gap have a direct influence on its material properties. In order to set a cast strip that has a uniform structure and is easy to process, it is therefore necessary to bring the strip to a high temperature. Take care of the belt as it emerges from the casting gap and to avoid excessive forces in the casting gap to reduce the formation of segregations and to avoid a non-uniform temperature distribution on the belt surface. If necessary, the cast strip must also be actively cooled.

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 supported by so-called "tailored Blanks "has been fulfilled 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.

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

• - Melt a low alloy or micro alloy steel,
• - Pouring the steel in between moving walls Casting gap formed into a casting gap cast tape, the one in the casting gap on the tape applied force measured across the width of the belt is more than 100 kN / m,
• Cooling the cast strip.

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 have. 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% by weight Cr, ≦ 0.10% by weight Al, ≦ 0.80% by weight 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 are used, 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 contain 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 of P, up to 0.2% by weight of Ni, up 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 tape. 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 Composition 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.

Further advantageous embodiments of the invention are in the dependent claims and are given in Connection with the following based on a drawing described embodiment explained in more detail. It show schematically:

Figure 1 shows a two-roll casting machine in a detail in a side view.

Figure 2 shows a detail of a cast in the casting machine Zweirollen- strip in plan view.

Fig. 3, the cast strip of Figure 2 in a cross section along the indicated line in Fig 2 XX..;

Fig. 4-6 different components in cross section.

The two-roll casting machine 1 has two casting rolls 3 , 4 arranged axially parallel to one another and delimiting a casting gap 2 along its longitudinal sides, which are cooled from the inside with cooling water by means of a controllable cooling device, not shown. The rollers 3 , 4 are preferably made of a copper alloy and can be provided with a nickel coating.

The center distance of the casting rolls 3 , 4 is fixed so that the depth of the casting gap 2 corresponds to the predetermined thickness D of the cast strip B to be produced. The speeds of the casting rollers 3 , 4 which feed into the casting gap 2 in the opposite direction in the conveying direction F of the belt B can be adjusted by means of a control (not shown ) .

The cast strip B is conveyed downwards from the casting gap 2 in the vertical conveying direction F. In the conveying direction F behind the casting gap 2 , a cooling device 5 , 6 extends below the casting rolls 3 , 4 across the width of the belt B. Each of the nozzles of the cooling devices 5 , 6 can also be controlled via a control device (not shown), so that nozzles arranged in certain rows or columns each deliver cooling fluid K, such as water, as a group, while no cooling fluid K is released from the other at this time exit.

To cast a strip B, melt S is poured into the casting gap 2 , so that a melt pool 7 is formed above the casting gap 2 . The melt S solidifies on the rotating casting rolls 3 , 4 to form trays N1, N2, which are conveyed into the casting gap 2 by the casting rolls 3 , 4 . The thickness of the shells N1, N2 when entering the casting gap 2 is dependent on their dwell time on the casting rolls 3 , 4 , that is to say the time which elapses until they arrive in the casting gap 2 .

The shells N1, N2 meet in the casting gap 2 and are squeezed together between the casting rollers 3 , 4 . The casting forces G1, G2 acting across the width of the strip B and acting essentially normal to the strip surface due to the fixed setting of the center distance of the casting rolls 3 , 4 are at least 200 kN / m. As a result of these high casting forces G1, G2, a core region C is formed in the cast strip B, which has different properties than the outer regions A1, A2 of the cast strip B adjoining it.

The cast strip B then passes through the cooling devices 5 , 6 . In the example explained here, two strip regions S2, S4, which extend in the longitudinal direction L of the strip B and are distributed over the width of the strip B, are subjected to particularly intensive cooling, which is brought about by direct application of cooling fluid K, while in the strip strips S2, S4 or strip areas S1, S3, S5 arranged between them does not directly cool. In this way, zones of different material properties which essentially coincide with the strip areas S1-S5 are formed in the outer areas A1, A2.

Starting from a molten steel which contains 0.01-0.2 wt% C, 0.3-1.0 wt% Mn, up to 0.5 wt% Cr, up to 0.1 wt .-% Al and up to 0.8 wt .-% Si, produced cast tape has 0.04 wt .-% carbon when leaving the casting gap 2 in the strip core and 0.08 wt on the surface of the outer areas A1, A2 .-% carbon. The austenite in the core region C is converted into ferrite and pearlite by strong cooling in the cooling devices 5 , 6 , which is far above 30 K / s. In contrast, bainite is formed in the directly and consequently particularly intensively cooled strip areas S2, S4 of the outer areas A1, A2, while no formation of bainite or martensite occurs in the non-directly cooled strip areas S1, S3, S5. Despite the differences in their chemical composition (including a higher carbon content), the material properties of these strip areas S1, S3, S5 therefore essentially correspond to those of the core area C. After exiting the cooling devices 5 , 6 , the tape in the area of the tape core C and the not directly cooled strip areas S1, S3, S5 have soft ferritic phases, while hard phases are present in the directly cooled strip areas 52 , 54 in the outer areas A1, A2 of the strip B.

If a steel melt is assumed that 0.1-0.8% by weight C, 1.0-5.0 wt% Mn (in the concrete 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, the zones Zh, in which the relevant components 8 , 9 , 10 are subjected to high loads in practical use, have particularly high strengths, while the other zones Zg, in which during manufacture the components 8 , 9 , 10 a particularly large deformation takes place, are soft and stretchy. In this way, the components 8 , 9 , 10 can be produced in a simple manner in a material-saving and cost-effective manner and can nevertheless 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 Direction of delivery F
G1, G2 casting forces
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 stripe areas
Zh zones of high load
Zg zones of lower load

Claims (17)

1. A method for producing metallic strips (B) with sections (S1-S5) of different material properties, comprising the following steps:

• Melting a low-alloy or micro-alloyed steel,
• - Pouring the steel in the casting gap ( 2 ) formed between moving walls of a casting machine ( 1 ) to form a cast strip (B), the pressure exerted on the strip (B) in the casting gap ( 2 ) being measured across the width of the strip (B) Force (G1, G2) is more than 100 kN / m,
• - Cool the cast tape (B).
• - 2 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.

3. The method according to claim 2, characterized characterized that the steel 0.01-0.20% by weight of carbon and 0.30-1.00% by weight Contains manganese. 4. The method according to claim 2, characterized characterized that the steel 0.10-0.8% by weight carbon and 1.00-5.00% by weight Contains manganese. 5. The method according to any one of the preceding claims, characterized in that the casting machine ( 1 ) is a two-roll casting machine. 6. The method according to any one of the preceding claims, characterized in that the force (G1, G2) exerted in the casting gap ( 2 ) on the belt (B) is at least 200 kN / m. 7. The method according to any one of the preceding claims, characterized in that to adjust the force exerted on the belt (B) in the casting gap ( 2 ) force (G1, G2), the speed at which the casting gap ( 2 ) delimiting walls is varied move. 8. The method according to any one of the preceding claims, characterized in that adjacent sections (S1-S5) of the cast Band (B) with different Cooling rates are cooled. 9. The method according to claim 8, characterized characterized that the differently cooled sections (S1-S5) striped in the longitudinal direction (L) of the cast Extend band (B). 10. The method according to claim 8, characterized characterized that the differently cooled sections in strips in the transverse direction of the cast strip (B) extend. 11. The method according to any one of the preceding claims, characterized in that the cast strip (B) at a speed of at least 30 K / s amount cooling rate is cooled.   12. The method according to any one of the preceding claims, characterized in that the cooling of the cast strip (B) in at least two successive stages with different cooling rates he follows. 13. The method according to claim 12, characterized characterized that the Cooling rate in from the pouring heat outgoing and reaching up to 800 ° C Temperature range is at least 30 K / s. 14. The method according to any one of claims 12 or 13, characterized in that the cooling rate in the from 800 ° C to 300 ° C temperature range at least 10 K / s however, is at most 500 K / s. 15. Use of a tape produced according to the method claimed in one of claims 1 to 14 for the production of automotive body components ( 8-10 ). 16. Use of a strip produced according to the method claimed in one of claims 1 to 14 for the production of components ( 8-10 ) intended for scaffolding. 17. Use of one according to one of claims 1 to 14 claimed method produced tape as Material for flexible rolling. 18. Use of one according to one of claims 1 to 14 claimed processes for producing tape Manufacture of wear-resistant components.