Metal strip having a constant thickness and varying mechanical properties

Abstract

The invention relates to a metal strip which is produced from steel by hot rolling and by cold rolling of the metal strip. The invention also relates to a blank produced from the metal strip, the use of the metal strip, and to a method of producing it. The object of providing metal strip from which components of minimum weight which are adapted to specific loads can be produced with little cost or complication is achieved by metal strip of the generic kind which, after the cold rolling, is of constant thickness and has, section by section, regions whose mechanical properties vary. What after the cold rolling means, for the purposes of the present invention, is that, immediately on completion of the cold rolling, i.e. without any further treatment such as heat treatment, regions whose mechanical properties vary are present in the metal strip.

Claims

1. A metal strip comprising: steel and produced by hot rolling and by cold rolling of the metal strip, wherein after the cold rolling the metal strip is of constant thickness and has, section by section, regions whose mechanical properties vary; and, wherein the metal strip to be cold rolled is produced by one of flexible hot rolling, by flexible casting or by flexible cold rolling with subsequent annealing, the metal strip thus having regions of varying thicknesses before the cold rolling, and the regions of varying thickness thus having varying degrees of rolling-down after the cold rolling.

2. The metal strip according to claim 1, wherein the regions whose mechanical properties vary have different tensile strengths, yield points and/or elongations at rupture.

3. The metal strip according to claim 1, wherein there are arranged, between the regions of the metal strip whose mechanical properties vary, transitional regions in which the mechanical properties, and in particular the tensile strengths, yields points and/or elongations at rupture, at least partially change continuously.

4. The metal strip according to claim 1, wherein the thickness of the metal strip is 0.5 mm to 3 mm.

5. The metal strip according to claim 1, wherein the metal strip consists of one of manganese steels, stainless steels, retained-austenite steels or dual-phase steels.

6. The metal strip according to claim 5, wherein the metal strip has at least one of an organic or inorganic coating.

7. The metal strip of claim 5 wherein the manganese content is 22% by weight.

8. A blank produced from the metal strip according to claim 1, wherein the blank is of constant thickness, consists of a single steel material and has, section by section, regions whose mechanical properties vary.

9. The blank according to claim 8, wherein, between the regions whose mechanical properties vary, the blank has transitional regions in which the mechanical properties at least partially change continuously.

10. The blank according to claim 8, wherein the regions whose mechanical properties vary have different tensile strengths, yield points and/or elongations at rupture.

11. A method of use of a blank according to claim 8 in vehicle construction, motor vehicle construction and railway vehicle construction.

12. The method of use of a blank according to claim 11, wherein the blank is a formed structural component.

13. A method of producing steel metal strip according to claim 1, in which a metal strip having regions in which the thickness of the metal strip varies is produced from one of a steel slab by flexible hot rolling, by flexible casting or by flexible cold rolling with subsequent annealing, and the metal strip is cold rolled to a constant final thickness.

14. The method according to claim 13, wherein, in the regions of the metal strip of greater thickness, the degree of rolling-down in the final cold rolling is up to 50%.

15. The method according to claim 14, wherein the degree of rolling-down in the final cold rolling is up to 20%.

16. The method according to claim 13, wherein, in the regions of the metal strip of smaller thickness, the degree of rolling-down in the final cold rolling is 0% to 10%.

17. The method according to claim 13, wherein blanks are cut to size from the metal strip after the final cold rolling and the optional application of an organic and/or inorganic coating to the fully rolled metal strip.

18. The metal strip of claim 1 wherein the regions of varying thickness are in periodic arrangement.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention is explained in detail below by reference to the drawings and to the description of exemplary embodiments. In the drawings:

(2) FIG. 1 is a longitudinal section showing, in section, the shape of a hot-rolled strip after flexible hot rolling and showing, schematically, the corresponding use of working rolls in the hot rolling.

(3) FIG. 2 is a longitudinal section through a first exemplary embodiment of metal strip according to the invention, after the cold rolling.

(4) FIG. 3 is a graph of the associated tensile strength, yield point and elongation at rupture of the first exemplary embodiment of metal strip.

(5) FIG. 4 is a plan view of a blank produced from the first exemplary embodiment of metal strip.

(6) FIG. 5 is a longitudinal section through further hot-rolled strip which is used to produce a second exemplary embodiment of metal strip according to the invention.

(7) FIG. 6 is a longitudinal section through a second exemplary embodiment of metal strip according to the invention, which is produced from the hot-rolled strip shown in FIG. 5.

(8) FIG. 7 is a graph of the associated tensile strength, yield point and elongation at rupture of the second exemplary embodiment.

(9) FIG. 8 is a plan view of a blank according to the invention produced from the metal strip shown in FIG. 6.

DETAILED DESCRIPTION OF THE INVENTION

(10) Shown in FIG. 1 is hot-rolled strip 1 which has been hot rolled by means of the working rolls 7a, 7b by changing the roll gap during the rolling, thus producing regions 2, 3, 4, 5, 6 which have various thicknesses. In the transitional regions 3, 5 between the regions 2, 4, 6, there is a continuous change in the thickness of the metal strip 1. As can be seen from FIG. 1, the change in thickness can for example be made simply by changing the position of one working roll. It is however also conceivable for the positions of both the working rolls 7a, 7b to be displaceable.

(11) In the present exemplary embodiment, the material X-IP1000 was selected for the hot-rolled strip which, as well as iron, has a carbon content of 0.6% by weight, a manganese content of 22% by weight and a silicon content of 0.2% by weight as its main alloying constituents. This steel is thus one of the high manganese steels. However, as already mentioned, in principle all grades of steel are suitable for providing tensile strengths, yield points and/or elongations at rupture which vary as a result of strengthening during cold rolling.

(12) In the first exemplary embodiment, the transitional regions 3, 5 are selected to be of a length of 50 mm, wherein the regions 2, 6 where the thickness of the hot-rolled strip is reduced each have a length of 200 mm and the region 4 where the wall thickness is increased has a length of approximately 800 mm. The wall thicknesses envisaged are for example 1.8 mm in the regions where the wall thickness is reduced and 2 mm in the regions of greater wall thickness. As the second exemplary embodiment shows, other length ratios may of course be selected.

(13) FIG. 2 shows the hot-rolled strip shown in FIG. 1 after the cold rolling. The thickness of the metal strip 1 after the cold rolling is a constant 1.8 mm in the present exemplary embodiment and the degree to which the region of increased wall thickness 4 has been rolled down is thus approximately 10%. The degrees to which all the other regions have been rolled down is less than 10%, i.e. is 0%. In the transitional regions 3, 5 there is a continuous increase in wall thickness towards the region 4 of the metal strip and because of this after cold rolling there is also a continuous change in the degree of rolling-down, and hence in the mechanical properties such for example as the tensile strength, yield point and/or elongation at rupture, in these regions.

(14) This can be seen from the graph in FIG. 3. In the region 2, the tensile strength is initially more than 1000 MPa, which corresponds to the starting state at a degree of rolling-down of 0%, and in the transitional region 3 it rises to approximately 1200 MPa. In the region 4 where the degree of rolling-down was at a maximum, the tensile strength remains at more than 1200 MPa and only in the region 5 does it decline towards the original value, which is reached in the region 6. The yield point behaves in a similar way, with the change in the yield point at a degree of rolling-down of 10% being even more clearly apparent because in this region where the degree of rolling-down was higher there has been an increase in the yield point from 500 MPa to approximately more than 800 MPa. The changes produced by the strengthening very much depend on the material, which means that the values shown should be considered specific to the X-IP1000 material.

(15) Hence, what are produced by the flexible rolling are transitional regions in which the mechanical properties and, as FIG. 3 also shows, the elongation at rupture, change continuously. The relative changes are dependent in this case on the material which is used to produce the metal strip 1 according to the invention. Hence, in the metal strip 1 according to the invention there is then an alternation between sections where the tensile strength and yield point are increased while the elongation at rupture is reduced and regions where the elongation at rupture is higher and tensile strength is lower and so too is the yield point. Provision is preferably made for a periodic arrangement of these regions, thus enabling identical blanks to be produced easily from the metal strip.

(16) Such a blank is shown for example in plan in FIG. 4. The blank may for example be of a width of 400 mm and a length of 1300 mm. The regions 2 and 6 of reduced tensile strength and yield point and increased elongation at rupture are 200 mm long whereas the transitional regions 3, 5 which are arranged between regions 2 and 4, and 4 and 6, respectively of the blank are of a length of only 50 mm.

(17) A further exemplary embodiment is elucidated in FIGS. 5 to 8. What is shown first, in FIG. 5, is a flexibly cast hot-rolled strip 8 of a grade 1.4318 stainless steel which provides regions 9, 10, 11 of different thicknesses. In the region 9 for example the thickness of the hot-rolled strip 8 is 1.7 mm and in the region 11 it is 1.9 mm. After the cold rolling to a final thickness of 1.62 mm, the region 9 of the metal strip 8 has been rolled down with a degree of rolling-down of 5%, as FIG. 6 shows. In the transitional region 10, the degree of rolling-down then rises to 15%, as is reached for example in the region 11. Because of these different degrees of rolling-down, which on the one hand are 5% or more, the cold-rolled strip which is produced in this way achieves on the one hand properties which are considerably changed in comparison with the starting product before the cold rolling step (when Rp.sub.0.2=340 MPa, R.sub.m=730 MPa and A.sub.80=48%) and on the other hand properties which vary considerably between the individual regions in which the degrees of rolling-down differ. This can once again be seen from the graph, in FIG. 7, in which the tensile strength, yield point and elongation at rupture of the material in the different regions are shown.

(18) A plan view of the blank is then shown in FIG. 8. The blank 8a has a region rather more than 400 mm long in which the degree of rolling-down is relatively low and in which lower tensile strengths and yield points are obtained with an improved elongation at rupture. The transitional region 10 of the blank, which measures at least 100 mm, has a tensile strength and yield point which increase continuously and which finally move up to the values for the more severely rolled down region 11 of the blank. The region whose degree of rolling-down was as high as possible may for example be approximately 900 mm long.

(19) The blanks according to the invention which are shown in FIG. 4 and FIG. 8 may be processed into components for vehicle construction, in particular into structural components (not shown), wherein they are optimally adapted to the specific loads. For reasons of corrosion protection for example, the blanks may also be provided with an organic and/or inorganic coating (not shown), for example a galvanised coating, before or after the forming into a component.