Process for producing steel components with highest stability and plasticity

Abstract

Processes for manufacture of metal components with high hardness and plasticity by deforming with high degree of deformation of metals, in particular steels, of which the deformation leads to a hardening by TWIP (Twinning Induced Plasticity) or SIP (Shearband Induced Plasticity) Effect, wherein the metal after the final step of annealing or crystallization annealing is deformed in at least one step into a semi finished product or the finished metal component, wherein the total elongation is in the range of 10 to 70%, as well as semi finished products, in particular continuous sheets, of steel with TWIP (Twinning Induced Plasticity) or SIP (Shearband Induced Plasticity) Effect, wherein the semi finished product exhibits a tensile strength of greater than 800 MPa and an elongation of greater than 35%.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention concerns a process for production of metal components, in particular steel components, with highest strength and plasticity, by a multiple deformation of metals of which the deformation leads to an increase in hardening (work hardening), in particular of steels with TWIP-(Twinning Induced Plasticity) or SIP-(Shearband Induced Plasticity) Effect, as well as semi finished products such as continuous sheets of steel with TWIP-(Twinning Induced Plasticity) or SIP-(Shearband Induced Plasticity) Effect.

2. Related Art of the Invention

High hardness steels are developed for the motor vehicle industry, for the construction industry, as well as air and space travel applications with various characteristics, and are already employed in manufacturing processes. Herein there is, in particular for employment in the motor vehicle industry, increasingly the desire in the foreground, to undertake a weight reduction of the vehicle by the use of new materials. The goal therein is the manufacture of specific lighter steel alloys, of which the otherwise hitherto desirable characteristics remain retained or as the case may be are further improved, or of high hardness steels, in which a weight reduction is achievable by reduction of the component cross section. Of substantial importance herein is the achievement of component hardness while maintaining of good deformability or, as the case may be, plasticity of the steel semi finished products as well as the finished components.

The conventional deforming processes in the motor vehicle industry include the deformation of semi-finished products, in particular continuous sheets (coils), for example by stamping, stretching or deep drawing. These deformation processes require a semi-finished product with comparatively high plasticity. In the case of insufficient plasticity there is, among other things, the danger of a tearing of the steel component and a high tool friction wear.

The increase of the component hardness can be achieved by the employment of high hardness multi-phase, complex phase or martensitic steels, as well as air hardened steels, such as for example BAS 100 or press hardened steels, such as for example Uisbor 1500 or BTR 165. If components made of these materials are incorporated into vehicles, then these still exhibit in general only plasticity reserves below approximately 10%. This is likewise critical for components relevant to operational hardness, since crack propagation can occur, as well as for crash relevant vehicle components, since the low flexibility values can lead to brittle material failures and low deformation energy absorption.

From DE 197 27 759 A1 the use of a cold deformable, in particular good deep draw capable, austensitic/ferritic light steel (duplex-steel) is known, which exhibits TRIP and/or TWIP characteristics. A preferred supposition includes 1 to 6% Si, 1 to 8% Al, wherein (Al+Si)<12%, 10 to 30% Mn and as remainder essentially iron, inclusive of conventional steel component elements. One area of application the material is used for stiffening or reinforcing body sheet metal or panels. On the basis of their TWIP characteristics, it is possible to achieve with these steels tensile strengths of up to 1100 MPa and maximal elongation of 90%. This type of duplex light steel exhibits, besides reduced weight, also high hardness and very good deep or stretch draw characteristics.

From DE 102 311 25 A1 high hardness α/γ-duplex or α/γ/ε-triplex light steels are known, which have a specific weight of less than 7 g/cm³. They exhibit TWIP characteristics. The preferred composition (content in weight %) is 18 to 35% Mn, 8 to 12% Al, Si, wherein Al+Si>12%, at least 0.5% C, at most 0.05% B, with the rest being essentially iron, inclusive of conventional steel component elements. Further alloy elements include 0.03 to 2% Ti, as well as less than 0.3% N, less than 0.5% Nb and less than 0.5% V.

The highest hardness of this type of TWIP-steel is only achieved by a deformation process, in which by the stretching of the steel material a mechanical twinning or duplex formation is induced in the austenitic phase. This twinning formation in particular leads to a strong increase in hardness. In particular in the case of steels with triplex microstructure the stress induced hardening is introduced by the formation of homogenous shearbands, the so called SIP-Effect (Shearband Induced Plasticity).

DE 100 60 948 A1 discloses a process for production of a hot rolled strip of a steel with high Mn-content. This steel is cast very thin and thereupon is continuously further process to a hot rolled strip, in that in a single hot rolling pass it is rolled to the final thickness of the hot rolled strip. The thereby obtainable continuous sheet has TWIP and TRIP-characteristics. This type of TWIP steel however still does not exhibit after manufacture the extremely high hardness. Rather, these steels are characterized by extreme high plasticity. Thereby on the one hand a very good deformability (deep draw ability) may be produced, however the required hardness cannot be achieved.

SUMMARY OF THE INVENTION

It is thus the task of the invention to provide a process for providing steel components, which makes it possible to utilize conventional steel deformation techniques and which leads to a high component hardness.

The task is inventively solved by a process for production of metal components, in particular steel components, with high hardness and plasticity by deforming metals of which the deforming leads to a hardness, in particular of steels with TWIP (Twinning Induced Plasticity) or SIP (Shearband Induced Plasticity) Effect, with the characterizing features of claim 1, as well as by a semi finished product, in particular a continuous sheet, of steel with TWIP (Twinning Induced Plasticity) or SIP (Shearband Induced Plasticity) Effect, with the characterizing features of claim 10, as well as motor vehicle components obtainable therefrom.

In a first aspect of the invention a process is envisioned, which inclueds a one-time or repeated deformation of metals which are stress or deformation induced hardness type metals, in particular steels. Therein in particular the use of TWIP (Twinning Induced Plasticity) or SIP (Shearband Induced Plasticity) Effect is of significance. In accordance with the invention it is therewith provided, that the stress or, as the case may be, deformation induced hardened metals, in particular steels, are deformed by a deformation with elongation in the range of 10 to 60% in the semi finished product or metal component. The semi finished component is deformed or, as the case may be, finish formed with less stretching in a subsequent deformation process into metal components, in particular steel components.

The inventive process has the advantage, that by the deformation process with high elongation very high hardness values can be established. Nevertheless there remains, despite the high hardness, a plasticity reserve which makes possible in a conventional subsequent final deformation the manufacture of components by means of conventional deformation techniques. With the inventive process there is achieved the highest possible hardness values with high deep-drawing ability. In the final component there remains, beyond this, the highest hardness and yet substantial plasticity reserves, which substantially exceed the plasticity of known highly hardened steels.

The inventive solution can also been seen as pre-stretching of metals, in particular steels. While typical TWIP or SIP steels exhibit, without pre-stretching, a hardness in the range of approximately 350 to maximally 500 MPa, with the inventive stretching of the steels a hardness level of above 800 MPa, in extreme cases up to 1500 MPa, can be achieved. Therein it is nevertheless of substantial significance that the draw stretching of the pre-hardened steels does not drop to an undesirably low level. Rather, a high residual plasticity is retained. This is of significance in particular in comparison to known high hardened steels on the basis of martensitic or heat deformed steels with draw elongation substantially below 10%.

Thereby it is in particular also possible, to provide the inventive pre-hardened steels for use in the, in principle already known, steel structures and methods of construction, without adapting the deformation technology to reduced deformation paths or material plasticities. Likewise they can be employed with advantage as crash relevant components of motor vehicles. The crash relevant components include in principle the total body shell work.

The inventive pre-hardening of the steel is concerned with the steel in its manufactured condition, that is, essentially steel following casting, hardening and in certain cases rolling out. In the following this will be referred to as the condition subsequent to the last stage of annealing or crystallization annealing.

It is thus immaterial for the inventive step whether the deformation with an elongation of up to 60% occurs in one single process or is divided into multiple sequential processes with cumulative elongation. In general it is useful to separate the deformation into at least two steps, in which first a pre-elongated semi-finished product is produced and in a subsequent final shaping process the finished component is produced with substantially less stretching.

For introduction of the pre-stretching or pre-elongation, in principle all deformation techniques are suitable which leave the main deformation mechanism to the mechanical twinning formation or shearband formation and do not reverse or undo the twinning formation or shearband formation. The preferred processes for introduction of the pre-elongation include cold milling, stretch forming or deep drawing. In steels, deformation temperatures of approximately 850° C. are still referred to as cold processes, since up to the region of these temperature small noticeable crystallographic or microstructure changes occur. Warm deformation processes are in general not suited or, as the case may be, not necessary, since they allow the re-crystallization of the adjusted microstructure.

Also, warm deformation processes are suited, however, are in general not necessary. This represents a substantial process simplification in the deformation process.

The degree or magnitude of the pre-stretching can be variously selected for the various metals, in particular TWIP or SIP steels, in certain cases depending upon the type of application. Of substantial importance herein is that a sufficient plasticity reserve remains in the finished component. Preferably the pre-stretching is carried out by cold deforming maximally to the extent that in the resulting steel or steel semi-finished product a draw elongation of greater than 20% remains.

The pre-stretching is thus preferably so adjusted, that at least in one spatial direction a stretching in the range of 10 to 60%, particularly preferably 15 to 35%, results.

In a further variant of the inventive process the magnitude of the first cold deformation, as the case may be pre-stretching, is determined by the hardness of the thereby achievable semi-finished product. Preferably the pre-stretching is carried out up to a value, which results in a hardness increase of the steel semi-finished product of at least 20% of the starting value. It is particularly preferred to increase the hardness by means of pre-stretching by at least 300 MPa.

The semi-finished products provided with the pre-stretching provide as a rule the not-yet-finished components. Rather, in accordance with the invention it is envisioned that these semi-finished products are subjected to a final shaping. Therein in principle the known deformation techniques can be employed. Also, during the final shaping a component hardening by TWIP or SIP Effect can occur. Both stretching as well as hardening are therein as a rule however essentially less than the fist step of the pre-hardening. Preferably the total of pre-stretching and stretching during final shaping is never greater than 60%, particularly preferably in the range of 15 to 45%. It is therein of advantage to keep the stretching of the final shaping to less than 10%.

With regard to the selection of suitable metals, light-metal-poor or light-metal-free steels on the basis of Fe/Mn/C with TWIP or SIP Effect are preferred. By appropriate cold deforming, for example by cold milling, the starting yield stress these steels of approximately 350 to 550 MPa can be increased to a component hardness greater than 1000 MPa, while the remaining plasticity is reduced only insignificantly. Therewith the inventive treated steel components exhibit a multiple of the plasticities achievable with known high hardened steels with comparable hardness.

Further preferred representatives of the TWIP or SIP steels comprise, besides Fe and conventional minor or secondary ingredients of steel, the following alloy components in wt. %:

1 to 6 Si,

1 to 8 Al, and

10 to 30 Mn,

or

2 to 3.5 Si,

2 to 3.5 Al, and

12 to 30 Mn,

or

0.1 to 6 Si,

8 to 12 Al, wherein Al+Si>12,

18 to 35 Mn,

0.5 to 2 C, and

at least one of the elements Mg, Ga, Be being up to 3,

or

3 to 6 Si,

8 to 12 Al, wherein Al+Si>12,

18 to 35 Mn,

0.5 to 2 C,

at most 0.05 B,

at most 3 Ti and at least one of the elements Mg, Ga, Be with a content of respectively 0.3 to 3,

or

0.1 to 0.25 Si,

0 to 0.01 Al,

18 to 25 Mn,

0.4 to 0.9 C,

0 to 0.01 N,

or

0.05 to 1 Si,

0 to 0.008 Al,

15 to 30 Mn,

0.4 to 0.7 C,

0.001 to 0.01 N.

Further suitable steels exhibit a duplex microstructure, with austensitic and ferritic crystal components, preferably in an amount of respectively 40 to 60%. The particularly suited steels further include triplex steels with a microstructure of austensitic, ferritic and perovskite crystallites.

Likewise, there are preferred also special austensitic nickel poor stainless steels with, to a certain extent, TWIP characteristics.

A further aspect of the invention concerns semi-finished products of steel with TWIP (Twinning Induced Plasticity) or SIP (Shearband Induced Plasticity) Effect, which in accordance with the invention are adjusted to a tensile strength above 800 MPa and elongations of longer than 25%. These types of semi-finished products are particularly suited for manufacture of body components, in particular for crash relevant areas. In contrast to the semi-finished products of the conventional high hardened steels, the inventive semi-finished products exhibit both for the subsequent final shaping to the component, in particular body components, as well as for the use as components, a sufficient plasticity or as the case may be plasticity reserve. It is particularly preferred when the tensile elongation of the semi-finished product is adjusted to values in the range of 25 to 55%.

The inventive semi finished products are particularly preferably formed by TWIP or SIP steels with a pre-stretching corresponding to a cold deformation of 10 to 40% in at least one spatial direction.

The particularly suited deformation techniques of the semi-finished products for manufacture of components for motor vehicle construction include rolling and deep drawing.

By the variant of the roller profiling there can supplementally be achieved a local stiffening of the material in that varying material stretching is realized in the component. This can lead for example to the manufacture of shaped parts which have in certain areas higher and in other areas a lower plasticity reserve however higher hardness. Thereby it is possible to produce in advantageous manner also possible integral components with local varying adapted crash behavior.

In a further embodiment of the invention the sheets or coils (continuous bands) are first pre-stretched in a body press and thereupon deformed in the same press to a final component.

Claims

1. A process for manufacture of metal components or semi-finished products with high hardness and plasticity by the cold deforming of steel, wherein the degree of deformation lies at a total elongation in the range of 10 to 70%, comprising:

selecting a metal of which the deformation leads to a hardening by TWIP (Twinning Induced Plasticity) or SIP (Shearband Induced Plasticity) Effect, and

cold deforming following the last stage of annealing or crystallization annealing to the extent that a hardness increase of at least 30% of the start value is imparted and the residual tensile elongation of the metal is reduced by less than 20%.

2. The process according to claim 1, wherein the cold deforming is carried out in a first step with an elongation of 10 to 60% and with a final deformation in a subsequent step with an elongation of less than 10%.

3. The process according to claim 1, wherein the deforming is carried out as cold deforming with an elongation in at least one spatial direction of 10 to 35%.

4. The process according to claim 1, wherein the deforming is carried out to the extent until a hardness increase of at least 300 MPa is imparted.

5. The process according to claim 1, wherein the sum of the elongation of the deformation steps does not exceed 50%.

6. The process according to claim 1, wherein the metal is selected from steels with the following composition (amounts in wt. %): 1 to 6 Si, 1 to 8 Al, and 10 to 30 Mn, or 2 to 3.5 Si, 2 to 3.5 Al, and 12 to 30 Mn, or 0.1 to 6 Si, 8 to 12 Al, wherein Al+Si>12, 18 to 35 Mn, 0.5 to 2 C, and at least one of the elements Mg, Ga, Be being up to 3, or 3 to 6 Si, 8 to 12 Al, wherein Al+Si>12, 18 to 35 Mn, 0.5 to 2 C, at most 0.05 B, at most 3 Ti and at least one of the elements Mg, Ga, Be with a content of respectively 0.3 to 3, or 0.1 to 0.25 Si, 0 to 0.01 Al, 18 to 25 Mn, 0.4 to 0.9 C, 0 to 0.01 N, or 0.05 to 1 Si, 0 to 0.008 Al, 15 to 30 Mn, 0.4 to 0.7 C, 0.001 to 0.01 N, besides iron and conventional minor components of steel.

7. The process according to claim 1 or 2, wherein the metal is steel with a duplex microstructure with austensitic and ferritic crystallites, or with a triplex microstructure with austensitic, ferritic and perovskite crystallites.

8. A semi-finished product, or motor vehicle component produced by a process comprising:

selecting a metal of which deformation leads to a hardening by TWIP (Twinning Induced Plasticity) or SIP (Shearband Induced Plasticity) Effect, and

cold deforming following the last stage of annealing or crystallization annealing to the extent that a hardness increase of at least 30% of the start value is imparted and the residual tensile elongation of the metal is reduced by less than 20%,

wherein the semi-finished product exhibits a tensile strength of greater than 800 MPa and an elongation of greater than 35%.

9. The semi-finished product or motor vehicle component according to claim 8, wherein it exhibits a tensile strength of greater than 1000 MPa and a elongation in the range of 35 to 55%.

10. The semi-finished product or motor vehicle component according to claim 8, wherein the steel is pre-stretched by deforming in at least one spatial direction by 10 to 40%.