DE102016121905A1 - Method for producing dual-phase steel wheel discs with improved cold workability - Google Patents

Abstract

Method for producing a dual phase steel wheel disc having improved cold workability of work hardened, mechanically separated sheet edges comprising the steps of: providing a hot strip or hot strip sheet of the claimed alloy composition having a dual phase structure consisting of a ferritic matrix insular with a predominantly martensitic second phase, cutting one Board at room temperature and optional further punching or cutting operations, heating only the sheet edge areas of the board cold-strengthened by the cutting or punching operations to a temperature of at least 700 ° C with a holding time of at most 10 seconds and subsequent cooling in air cold forming of the board in one or more steps to a wheel dish at room temperature.

Description

The invention relates to a process for the production of wheel disks made of dual-phase steel, with improved cold workability according to claim 1, made of cold-formed blanks according to claim 1, wherein the blanks have improved cold workability of work hardened, mechanically separated edges.

The manufacture of Radschüsseln a vehicle by cold forming is for example from the patent DE 10 2013 114 245 B3 known. 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 wheel disc, a sheet metal blank is cut primarily from hot strip at room temperature to measure. 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 costly compared to mechanical separation processes, so that they are used only in exceptional cases.

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

Before cold working, various further manufacturing steps, such as e.g. Punching and cutting operations on the board and the application of various holes for receiving the wheel hub and the wheel bolts carried out, and there are occasionally carried out during the conversion combined Umfor- or Aufweitoperationen at the perforated sections.

In cold forming, the cut edges, especially when raised or raised, e.g. in collar operations in perforated boards, particularly stressed.

DP steels have high strength during cold forming, which is technically described by the solidification exponent. A high solidification exponent leads to particularly advantageous properties when forming areal areas of complex parts, in which in particular a pronounced deep drawability is required. A high solidification exponent is achieved in the DP steels by a balanced ratio of the structural components ferrite and martensite. In this case, ferrite is a soft component, martensite is usually present as a hard ingredient insular in a ferritic matrix. The resulting advantageous properties of the microstructure with regard to cold forming in flat areas is simultaneously associated with a comparatively high limitation of the formability of sheared edges in the DP steel. The reason is that the deformation of edges causes a massive local stress, which leads to an early high defect density in the boundary region of martensite islands and ferrite grains, which in turn results in a comparatively early failure of the sheared edge in the subsequent forming process.

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 testing of the forming behavior of cut sheet edges with regard to their edge crack sensitivity is carried out 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 working of sheared or punched sheet edges, e.g. Approaches for changing the alloy composition and material processing (e.g., targeted setting of bainitic structures) or the process technique in cold-cutting the board (e.g., via modifications of cutting gap, speed, multiple trimming, etc.) are known.

These measures are either expensive and expensive (eg multi-stage cutting operations, Maintenance of 3-D cuts, etc.), or they do not yet provide optimal results.

Furthermore, it is from the published patent application DE 10 2009 049 155 A1 It is known to heat at least the region of the cut edge to a defined temperature and to perform cutting at this temperature in order to improve the formability of the cut edges and thus to reduce or avoid strain hardening in the region of the cut edge. The disadvantage here is the necessary for heating the sheet high technical and economic effort on the one hand and on the other hand for the positive coupling of heating of the board and immediately subsequent cutting, which make the production inflexible.

From the DE 10 2011 121 904 A1 It is also known to cold form a sheared sheet and to heat the cold worked areas locally by means of a laser before further forming operations 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. In addition, it is unclear exactly where the warming will take place and how the local warming with temperature and time course should take place concretely. Furthermore, it is unclear how and to what extent by partial softening the forming capacity of the already cold-formed sheet can be improved.

From the publication DE 10 2014 016 614 A1 is a method for producing a cold-formed component from a sheet metal sheet sheared at room temperature with case-by-case various other manufacturing steps carried out at room temperature, such as punching or cutting operations, in which the sheet edge portions cold-worked during the cutting or punching operations, followed by cold forming during manufacture of the component, are heated to a temperature of at least 600 ° C and the time of the temperature application is less than 10 seconds. As a result, the cold workability of these cold-strengthened sheet edges should be significantly improved. This method is used, inter alia, in dual-phase steels. However, there are no indications of a specific 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 steel wheels can not be implemented as standard for named reasons, which results in geometry adjustments or additional work steps for reworking.

The object of the present invention is to provide a process for the production of wheel disks from dual-phase steel with improved cold workability, produced from cold-formed blanks, which have an improved formability of work-hardened, mechanically separated sheet metal edges.

• -Bereitstellen eines Warmbandes oder eines Warmbandblechs aufweisend folgende Legierungszusammensetzung in Gewichts.-%: C: 0,04 bis 0,2, Si: 0,05 bis 0,5, Mn: 0,2 bis 1,5, P: max. 0,085, S: max. 0,010, N: max. 0,009, Al: max. 0,060, Nb: max. 0,05, Ti: max. 0,05, sowie eines oder mehrere der Elemente aus der Summe von Cu+Cr+ Ni+Mo: 0,1 bis 1,3 mit Cr: max. 0,9, Rest Eisen und erschmelzungsbedingte Verunreinigungen mit einem Dualphasengefüge, bestehend aus einer ferritischen Matrix, in die eine überwiegend martensitische Zweitphase inselförmig eingelagert ist,
• - Zuschneiden einer Platine bei Raumtemperatur sowie optionaler Durchführung weiterer Stanz- oder Schneidoperationen, zur Erzielung von Aussparungen, Löchern oder Durchbrüchen an der Platine bei Raumtemperatur
• - Erwärmen ausschließlich der durch die Schneid- oder Stanzoperationen kalt verfestigten Blechkantenbereiche der Platine auf eine Temperatur von mindestens 700°Cmit einer Haltezeit von höchstens 10 Sekunden und anschließender Abkühlung an Luft
• - Kaltumformen der Platine in einem oder mehreren Schritten zu einer Radschüssel bei Raumtemperatur

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, Al: 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, remainder iron and impurities resulting from 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 ° Cmit 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

Wheel disks 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 yield ratio Re / Rm of 0.5-0.7, which is low for stretch-draw formations.

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 inventive alloy composition of the DP steel with the described method for edge modification in an ideal manner, allows the forming of flat areas with the transformation of sheared edges and so the construction of new components with a high functionality by complex Shaping possible.

In combination with the microstructure of the DP steel sheet edges are produced with the inventive heating of the cutting and / or punching edges, which have a particularly high formability in Lochaufweitversuch without cracking at 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 perform the cutting process even at elevated temperature of the cut edge areas to improve the Lochaufweitvermögen, but it is sufficient only the work hardened, shear-influenced cut edge areas in an unexpectedly short time interval in the range of less than 10 seconds, in the Usually, however, to heat up to a temperature of at least 700 ° C between 0.1 and 2.0 seconds. 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 a range which corresponds at most to the sheet thickness. The duration of the 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 it is often used in the automotive industry for example on the example of spot welds.

Advantageously, for example, is a spot welding machine with rather short exposure times for the treatment of punched holes in the board, whereas in indefinite to be treated edge portions, the inductive method, radiation heating or laser machining with longer exposure times in question.

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 shear-influenced cutting edge areas and is therefore associated with a relatively low energy consumption, in particular with regard to methods in which the entire board is supplied to a heating or by orders of magnitude temporally more expensive stress relief is applied.

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 approx. 1500 ° C.

The experiments have also shown that only the elimination of strain hardening is crucial for a marked improvement in hole-expanding capacity and that the incurable discontinuities, e.g. Pores, of subordinate importance.

This is independent of whether the heat treatment takes place below or above the transformation temperature Ac1.

If the heat treatment is carried out above Ac1, transformation into so-called metastable phases takes place after treatment in the course of a rapid cooling due to the surrounding cold material in the case of convertible steels. The resulting microstructure, which is largely martensitic in the present DP steel, will differ from the initial state in terms of increased strength.

However, surprisingly, a microstructure transformation with as a rule associated increase in hardness and strength has no negative influence on the Lochaufweitvermögen, regardless of whether a harder and less tough compared to the starting structure structure is adjusted so that treatment temperatures of the cut edges up to the solidus limit possible are. In any case, it is crucial that the strain hardening introduced by cutting is largely eliminated.

In order to achieve the objectives according to the invention, it is not sufficient, according to the present investigations, to carry out a heating below 700 ° C. for a period of a few seconds, since a significant reduction in the solidification introduced by the mechanical separation process must take place.

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 dual-phase steels for wheeled dishes according to the invention 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 Radschüsseln, and on the other hand, previously required joining operations, for example, by now feasible collar operations in the formation of e.g. of bearings, can be saved.

The process steps according to the invention for the production of wheel disks in combination with the alloy composition and the dual-phase structure allows more complex component geometries and thus greater design freedom when using the same materials due to the improved formability of the cut edge regions. 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 is possible in the prior art in integration of edge modification by heat treatment.

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 scope.

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. Ovens, to heat the board itself are necessary. 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.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• DE 102013114245 B3 [0002]
• DE 102009049155 A1 [0014]
• DE 102011121904 A1 [0015]
• DE 102014016614 A1 [0016]

Claims (15)

Method for producing a dual-phase steel wheel disc with improved cold workability of work hardened, mechanically separated sheet metal edges with the following method 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, remainder iron and impurities resulting from 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. Method according to Claim 1 , characterized in that the time of the temperature application is 0.02 to 10 seconds. Method according to Claim 2 , characterized in that the time of temperature application is 0.1 to 2 seconds Process according to claims 1 to 3, characterized in that the heating of the cold-worked sheet metal edge regions is carried out to a temperature of 700 ° C to solidus temperature. Method according to Claim 4 , characterized in that the heating of the cold-strengthened sheet edge areas is carried out to a temperature of Ac1 to solidus temperature. Process according to claims 1 to 5, characterized in that the heating to forming temperature is effected inductively, conductively, by means of radiation heating or by means of laser radiation. Method according to Claim 6 , characterized in that the heating takes place by means of a resistance welding device or by means of a laser. Method according to at least one of Claims 1 to 7 , characterized in that the board is formed in one or more steps. Method according to at least one of Claims 1 to 8th , characterized in that the sheet metal plate has an organic and / or metallic coating. Method according to Claim 9 , characterized in that the metallic coating contains Zn and / or Mg and / or Al and / or Si. Method according to at least one of Claims 1 to 10 , characterized in 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. Method according to one of Claims 1 biss 11, characterized in that the area around the location of the heat treatment is protected from oxidation. Method according to one of Claims 1 to 12 , characterized in that, 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. Method according to Claim 13 , characterized in that the region around the location of the heat treatment is additionally rinsed before and / or after the action of heat by means of an inert gas. 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, Al: 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, 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, for the production of a wheel by means of cold forming a board, in which the board before forming from a strip or sheet at Room temperature is mechanically cut and occasionally further punching or cutting operations to achieve recesses or openings are performed at room temperature, in which prior to forming a component on the cut or stamped sheet metal edges, which have undergone work hardening, a heat treatment of at least 700 ° C. above a period of at most 10 seconds is performed.