WO2018091039A1

WO2018091039A1 - Method for producing wheel discs from a dual-phase steel with improved cold workability

Info

Publication number: WO2018091039A1
Application number: PCT/DE2017/100970
Authority: WO
Inventors: Ingwer Denks; Stefan MÜTZE; Christian PELZ
Original assignee: Salzgitter Flachstahl Gmbh
Priority date: 2016-11-15
Filing date: 2017-11-15
Publication date: 2018-05-24
Also published as: DE102016121905A1

Abstract

The invention relates to a method for producing a wheel disc from a dual-phase steel, having improved cold workability of cold-solidified, mechanically separated sheet metal edges, comprising the following method steps: - providing a hot-rolled strip or a hot-rolled strip sheet of the claimed alloy composition having a dual phase structure, consisting of a ferritic matrix, into which a predominantly martensitic second phase is embedded in an insular manner, - cutting a sheet at room temperature and optionally carrying out further punching or cutting operations, - heating exclusively the sheet metal edge regions of the blank, which have been cold-solidified by the cutting or punching operations, to a temperature of at least 700 °C with a dwell time of at most 10 seconds and subsequent cooling with air, and - cold forming the blank in one or more steps to form a wheel disc at room temperature.

Description

Method for producing dual-phase steel wheel discs with improved cold workability

description

The invention relates to a method for the production of Radschüsseln from dual-phase steel, with improved cold workability according to claim 1 made from

cold-formed blanks according to claim 1, wherein the blanks have an improved cold workability of work-hardened, mechanically separated edges.

The production of wheel shells of a vehicle wheel by cold forming is known for example from the patent DE 102013 114 245 B3. For cold forming, preference is given to using microalloyed dual-phase (DP) steels having a strength of 400 to 600 MPa. A concrete alloy composition is not specified.

To produce the Radschüssei first a metal sheet, mainly from

Hot strip cut to size at room temperature. The cutting process is usually mechanical separation processes, such. shearing or punching, but more rarely also thermal separation methods, such as e.g. laser cutting, for use. Thermal separation processes are significantly more cost-intensive compared to mechanical ones

Separation process, so that they are used only in exceptional cases.

After cutting, the cut board is placed in a Urnformwerkzeug and produced in single or multi-stage forming steps, the finished wheel.

Before the cold forming, various further manufacturing steps, e.g. Punching and cutting operations performed on the board and the application of various holes for receiving the wheel hub and the wheel bolts, and there are occasionally made during the forming combined Umsteif- or Aufweitoperationen at the perforated sections.

In the case of callus forming, the cut edges, especially if they open or

be raised, e.g. in collar operations in perforated boards, particularly stressed.

DP steels have a high strength during cold forming, which is technically described by the solidification exponent. A high one

Solidification exponent leads to particularly advantageous properties during molding planar areas of complex parts, where in particular a pronounced

Thermoformability is required. A high solidification exponent is achieved in the DP steels by a balanced ratio of the structural components Ferrit and Martenstt. In this case, ferrite is a soft component, martensite usually prevails as a hard component insular in a ferritic matrix. The resulting advantageous properties of the microstructure with regard to the cold forming in areal areas is simultaneous with a comparatively high limitation of the formability

sheared edges connected to DP steel. The reason is that at the

Forming of edges results in a massive local stress, which leads to an early high defect density at the boundary of martensite and ferrite grains, which in turn leads to a comparatively early failure of the sheared edge in the

subsequent transformation process has the consequence.

At the cutting edges, various previous damage may also be present. On the one hand due to a work hardening of the material, caused by the mechanical separation, which represents a total deformation to material separation. On the other hand, a notch effect can occur which results from the topography of the cut surface.

Therefore, especially in the case of very high and very high strength sheet materials, depending on the concrete alloy composition and the microstructure, an increased crack probability occurs in the edge regions of these cut edges in the subsequent shaping.

The aforementioned damage to the sheet edges can lead to premature failure in subsequent forming operations or while driving. The examination of the

Umformverhaltens cut sheet edges with regard to their

Edge crack sensation is performed with a hole expansion test according to ISO 16630.

When Lochaufweitversuch a circular hole is introduced into the sheet by shear cutting, which is then widened by a conical punch. The measured quantity is the change in the diameter of the hole in relation to the initial diameter at which the first crack occurs through the sheet at the edge of the hole.

In order to minimize the above-described edge crack sensitivity in the cold forming of sheared or punched bending edges, for example, approaches to changing the alloy composition and material processing (eg targeted setting of bainitic structures) or the process technology in cold cutting of Board (eg upper modifications of cutting gap, speed, multiple trimming, etc.) known.

These measures are either expensive and expensive (e.g., multi-level

Cutting operations, maintenance of 3-D cuts, etc.), or they do not yet provide optimal results.

Oesweiteren it is known from the published patent application DE 10 2009 049 155 A1, at least to heat the region of the cutting edge to a defined temperature and the

To perform cutting at this temperature in order to improve the formability of the cut edges and so to reduce or avoid the work hardening in the area of the cutting edge. The disadvantage here are the necessary for heating the sheet high technical and economic effort on the one hand and on the other hand for the

Forcing coupling of board heating and immediately subsequent cutting, making the production less flexible.

From DE 102011112904 A1, it is also known to cold-form a shear-cut sheet and, before further forming operations, to locally heat the work-hardened areas by means of a laser with the aim of partial softening. The disadvantage here is in particular the local softening, which represents a discontinuity in terms of the often used highly and extremely strong material, especially in stress situations and under oscillatory stress. Moreover, it is unclear exactly where the warming will take place and how the local warming with temperature and Zeitveriauf should be made concrete. Furthermore, it is unclear how and to what extent by the partial softening the forming capacity of the already cold-formed sheet can be improved.

The published patent application DE 10 2014 016614 A1 discloses a method for producing a cold-formed component from a sheet-metal blank which has been sheared at room temperature and optionally with various further production steps carried out at room temperature, such as e.g. Punching or cutting operations are known in which the cold edge hardened during the cutting or punching operations sheet edge areas, which undergo a subsequent cold working in the manufacture of the component, are heated to a temperature of at least 600 "C and the time of the temperature is less than 10 seconds This is intended to improve the cold workability of these cold worked

Sheet edges are significantly improved. Among other things, this method is used in dual-phase steels. However, there are no indications of a concrete Alloy composition or structure of the steels disclosed therein or the application of this method for the production of Radschüsseln.

Current state of the art is therefore an elaborate rework especially the seats of the wheel nuts or hub seats and the raised edges. A very high production rejection of different customers is common. In addition, special designs for new types of road bikes can not be implemented in series for named reasons, resulting in geometry adjustments or additional reworking steps.

The object of the present invention is to provide a process for the production of dual phase steel wheeled dishes with improved mat formability. made of cold-formed blanks, which have an improved formability of work-hardened, mechanically separated sheet edges.

According to the teachings of the invention, this object is achieved by a method with the following steps:

Providing a hot strip or a hot strip sheet having the following alloy composition in weight%: C: 0.04 to 0.2, Si: 0.05 to 0.5, Mn: 0.2 to 1.5. P: max. 0.085, S: max. 0.010, N: max. 0.009, AI: max. 0.060. Nb: max. 0.05, Ti: max. 0.05, and one or more of the elements of the sum of Cu + Cr + Ni + Mo: 0.1 to 1.3 with Cr: max. 0.9. Remainder iron and impurities caused by melting with a dual-phase structure consisting of a ferritic matrix, in which a predominantly martensitic second phase is insular,

- Cutting a board at room temperature and optionally performing further punching or cutting operations to achieve recesses, holes or

Breakthroughs on the board at room temperature

- Heating only the cold-strengthened by the cutting or punching operations sheet edge areas of the board to a temperature of at least 700 ° C with a holding time of at most 10 seconds and subsequent cooling in air

- Cold forming the board in one or more steps to a wheel at room temperature.

The content limits arise from the need to produce a dual-phase ferrite steel with embedded martensilins in the hot strip. In addition, the allow

Content limits advantageous the generation of a homogeneous martensitic microstructure in the heat tape side edge area, which ensures sufficient formability of the edge

In the following, the importance of compliance with the content limits is described in more detail: C-casings of> 0.04% are used to set sufficiently high levels of martensite in the ferritic matrix, with contents of less than 0.2% to ensure the weldability of the base material ,

The Si content should be less than 0.5% to avoid firmly adhering scale during hot rolling, as this can not be completely removed by pickling and thus adversely affects the performance characteristics of the component. On the other hand, contents of at least 0.05% are typical of steel.

An Mn content of at least 0.2% serves to increase the strength by

Mixed criticality formation and reduces the critical cooling rate for adjusting martensite. The Mn content should not exceed 1.5% as it tends to increase segregation. The result are midsite center lines. which adversely affect the performance characteristics.

The sum content of should be at least 0.1% to the critical

To reduce cooling rate for the adjustment of martensite and to influence the homogeneity of the structure favorable. Above 1.3%, a certain saturation of these effects sets in, so that they should not be exceeded for cost reasons.

The Cr content should be at most 0.8% since, similar to Mn Cr, the critical cooling rate for adjusting martensite decreases. Higher contents are not useful for cost reasons.

Wheel cups made in accordance with the present invention have the advantage that the present alloy composition of the dual phase composite material has a high tensile strength of up to 950 MPa and a low draw for drawdown applications

Yield ratio Re / Rm of 0.5-0.7.

In addition, the steel advantageously has a particularly high work hardening capacity, which has a positive effect on the mechanical properties of the finished molded wheel dish.

Surprisingly, it has been shown that the combination of the invention

Alloy composition of the DP steel with the described method for edge modification in an ideal manner, which allows shaping of flat areas with the transformation of sheared edges and so makes the construction of new components with a high functionality through complex shaping possible. In combination with the structure of the DP steel with the inventive

Heating the cutting and / or punching edges produces sheet edges that have a particularly high formability in Lochaufweitversuch without cracking on the sheet edges.

The proposed treatment of sheared edges of board areas experiencing significant cold deformation during forming into a wheel disc results in a marked reduction in crack formation in the manufacturing process.

Experiments have shown that it is not necessary to improve the Lochaufweitvermögens, the cutting process even at elevated temperature of

It is sufficient to heat only the work-hardened, shear-influenced cut edge areas in an unexpectedly short time interval in the range of less than 10 seconds, but usually between 0.1 and 2.0 seconds to a temperature of at least 700 ° C. According to the invention this can be done detached from the cutting or punching process and the subsequent manufacturing steps at any time before forming into a component.

The heat is applied over the entire sheet thickness and in the plane direction of the board in an area which corresponds at most to the thickness of the sieve

The effect of heat depends on the type of heat treatment process.

The heating itself can take place arbitrarily, for example, conductively, inductively via radiation heating or by means of laser processing. Excellent for the heat treatment is the conductive heating, as for example in the

Automotive production is often applied to the example of spot welds.

Advantageously, for example, is a spot welding machine with rather short

Contact times for the treatment of punched holes in the board, whereas inductive process, radiation heating or laser machining with longer exposure times is considered in the treatment of longer Kantenabschnttten.

In order to protect the heated cut edge regions from oxidation, an advantageous development of the invention provides for rinsing these areas with inert gases, for example argon. The inert gas purging takes place during the duration of the heat treatment but can also, if necessary, be made shortly before the beginning and / or in a limited period of time after the heat treatment has been carried out. Thus, the heat input is very concentrated in the shard-influenced

Cut edge areas and is therefore associated with a comparatively low energy consumption, in particular with regard to methods in which the entire Piatine is supplied to a heating or time-consuming by orders of magnitude

Stress relieving application finds

The process window for the temperature to be reached in the cutting edge area is also very large and covers a temperature range from above 700 ° C up to the

Solidus temperature of about 1500 * C.

The experiments have also shown that all but the elimination of strain hardening is crucial for a significant improvement in hole expanding capacity and the incurable discontinuities. such as. Pores, of subordinate importance.

This is regardless of whether the heat treatment below or above the

Conversion temperature Ac1 takes place.

If the heat treatment is carried out above Ad, it comes after treatment in the course of a rapid cooling due to the surrounding cold material

convertible steels for conversion into so-called metastable phases. The ensuing structure, which is largely martensitic in the present DP steel, will differ from the initial state in terms of increased strength.

A structural transformation with a hardness and

Surprisingly, however, the increase in strength does not have any negative influence on the hole expansion capacity, regardless of whether a harder and less tough microstructure is set compared to the starting structure, so that too

Treatment temperatures of the cut edges up to the solidus limit are possible.

In any case, the decisive factor is that the introduced by the cutting

Hardening is largely eliminated.

In order to achieve the objectives of the invention, it is sufficient according to the present

Investigations should not be carried out for a period of a few seconds, as there must be a marked reduction in the hardening brought about by the mechanical separation process. The heating according to the invention of the cut edges prior to the cold forming of the board has the advantage over the known measures for reducing the edge crack sensitivity that only the shard-influenced edge areas are microstructured by the heat treatment and the strength is not reduced as a rule, but increased. The insensitivity to edge cracks in the sense of a larger Lochaufweitvermögens can thus be improved by a factor of 2 or even more than 5.

In the industrial application of the cutting edge heating of inventive dual-phase steels for Radschüsseln can be significantly reduced due to the significantly increased formability of the critical shear-influenced sheet edge areas on the one hand, the Committee of reshaped Radschüssein, and on the other hand, previously necessary joining operations, for example by now feasible collar operations in the Ausbungung. from storage locations, can be saved.

The inventive method steps for the production of Radschüsseln in

Combination with the alloy composition and the dual-phase feed allows more complex ones due to the improved formability of the cut edge areas

Component geometries and thus greater design freedom when using the same materials. In addition, the fatigue strength of the cold-formed component is expected not to be reduced due to the self-adjusting, but possibly harder but homogeneous compared to the initial state but homogeneous structure, but with pronounced two-phase structures such. Dual phase structures increased.

The heat treatment of the cold-formed cut edge areas can be carried out completely at any time after the cutting or punching processes and before the forming of the board or as an intermediate step in multi-stage forming operations of the board for producing a wheel disc, so that the process steps cutting or punching the board, Heat treatment of the cutting edges and forming the board from each other are completely decoupled. Thus, the production is much more flexible than in the prior art in the integration of an edge modification by

Heat treatment is possible.

Due to the short duration of treatment compared with known measures, the method can be integrated as an intermediate production step in a series production, which specifies a cycle in the range of 0.1 to 10 seconds. In particular, the production of Sheet metal components in the automotive sector in several successive steps thus represents a predestined area of application.

In addition, the forming of the board prepared in this way can advantageously be carried out with the forming tools already in production, since no additional heating devices, such as e.g. Oven, necessary for heating the board itself. This allows a further cost-effective production and by the decoupling of the manufacturing steps a high flexibility in the production process.

According to an advantageous development of the invention, the heating of the cut edges, however, depending on the intended production process, if this appears advantageous, also immediately after the mechanical cutting or punching processes or immediately before the forming into a component, take place in a combined with the respective manufacturing process step. For example, the cutting and stamping devices may be provided with a downstream heat treatment device or this may be directly upstream of the forming device for cold forming of the board.

The invention is applicable to hot rolled steel strip having yield strengths of 280 MPa to 700 MPa and tensile strengths of 450 MPa to 950 MPa.

Claims

claims

1. A method for producing a wheel disc made of dual-phase steel with

improved cold workability of work hardened, mechanically separated sheet edges with the following process steps:

Providing a hot strip or a hot strip sheet comprising the following alloy composition in weight%: C: 0.04 to 0.2, Si: 0.05 to 0.5, Mn: 0.2 to 1.5, P: max , 0.085, S: max. 0.010, N: max. 0.009, AI: max. 0.060, Nb: max. 0.05, Ti: max. 0.05. and one or more of the sum of Cu + Cr + Ni + Mo: 0.1 to 1.3 with Cr: max. 0.9. The remainder of iron and impurities due to abrasion have a dual-phase structure, consisting of a ferritic matrix in which a predominantly martensitic second phase is embedded in the form of an oyster,

- Cutting a board at room temperature and optional weathering punching or cutting operations, to achieve recesses, holes or breakthroughs on the board at room temperature

- Cold forming the board in one or more steps to a wheel at room temperature

2. The method according to claim 1,

characterized,

the time of applying the temperature is 0.02 to 10 seconds.

3. The method according to claim 2,

characterized,

that time of the temperature application is 0.1 to 2 seconds

4. Process according to claims 1 to 3,

characterized,

that the heating of the work hardened sheet edge areas on a Temperature from 700 * 0 to solidus temperature.

5. The method according to claim 4,

characterized,

in that the heating of the work hardened edge areas to a temperature of Ac1 to solidus temperature.

6. Process according to claims 1 to 5,

characterized,

the heating to forming temperature takes place inductively, conductively, by means of radiation heating or medium laser radiation.

7. The method according to claim 6,

characterized,

the heating takes place by means of a resistance welding device or by means of a laser.

8. The method according to at least one of claims 1 to 7,

characterized,

that the board is reshaped in one or more steps.

9. The method according to at least one of claims 1 to 8,

characterized,

that the sheet metal plate has an organic and / or metallic coating.

10. The method according to claim 9,

characterized,

the metallic coating contains Zn and / or Mg and / or Al and / or Si.

1 1. A method according to any one of claims 1 to 10,

characterized,

that the heat treatment in the plane direction of the board, starting from the sheet edge, takes place in a region which corresponds at most to the sheet thickness.

12. The method according to any one of claims 1 to 11,

characterized, that the area around the site of the heat treatment is protected from oxidation.

13. The method according to any one of claims 1 to 12,

characterized,

in order to protect against oxidation, the area around the location of the heat treatment is rinsed at least during the heat action by means of an inert gas.

14. The method according to claim 13,

characterized,

the area around the location of the heat treatment is additionally rinsed before and / or after the heat action by means of an inert gas.

15. Use of a steel consisting of the following

Alloy composition in weight%: C: 0.04 to 0.12, Si: 0.05 to 0.5, Mn: 0.2 to 1.5, P: max. 0.085. S: max. 0.010, N: max. 0.009, AI: max. 0.050, Nb: max. 0.05, Ti: max. 0.05. and one or more of the sum of Cu + Cr + Ni + Mo: 0.1 to 1.3 with Cr: max. 0.9, remainder iron and melting impurities with a Oualphasengefüge, consisting of a ferritic matrix, in which a predominantly martensitic second phase is insular embedded, for producing a wheel by means of cold forming a circuit board, in which the board prior to forming from a strip or sheet mechanically cut at room temperature and occasionally further punching or cutting operations to achieve

Recesses or breakthroughs are carried out at room temperature, in which prior to forming a component on the cut or punched sheet edges, which have undergone work hardening, a

Heat treatment of at least 700 * C over a period of at most 10 seconds is performed.