KR20170077192A - Method for producing a component by subjecting a sheet bar of steel to a forming process - Google Patents

Abstract

The present invention relates to a method of manufacturing a component by applying a forming process to a steel sheet bar according to the preamble of claim 1, which enables molding to be applied to the edge of the cold-hardened, mechanically separated sheet, The edge of the sheet being cold-cured by cutting or punching operation at any desired point after cutting the sheet bar to an appropriate size and after any additional stamping or cutting operation, Is heated to a temperature of 600 DEG C or higher, and the temperature exposure time is 10 seconds or less.

Description

TECHNICAL FIELD [0001] The present invention relates to a method of manufacturing a component by applying a forming process to a steel sheet bar. BACKGROUND OF THE INVENTION 1. Field of the Invention [0002]

The present invention relates to a method of manufacturing a component by molding a steel plate in accordance with the preamble of claim 1, which enables high moldability of the strain hardened, mechanically separated plate edge.

Hereinafter, the term " part " refers to a part made of a metal plate molded into a molding tool at room temperature. Metal materials that can be used include all moldable metal materials, but especially steel. The metal plate may be uncoated, or may have a metal coating and / or an organic corrosion resistant coating.

These components are mainly used in the car body structure, but also in the household electric appliance industry, mechanical structure or construction industry.

Because of the highly competitive automotive market, manufacturers must constantly seek solutions to reduce their rapid consumption while maintaining the highest possible convenience and passenger protection. This, on the one hand, plays an important role in reducing the weight of all vehicle components, but ensures optimal behavior of the part during high static and dynamic stresses during operation and during impact.

Raw material suppliers strive to address the need for materials by providing high strength and ultra high strength steels that enable reduced wall thickness while providing improved component properties during manufacturing and operation.

Therefore, such steels must meet the relatively high requirements of strength, toughness, toughness, energy absorption, corrosion resistance, and the likelihood of processing during cold forming and welding, for example.

Among the embodiments described above, the manufacture of parts made of higher strength and higher strength steels with a yield strength exceeding 600 MPa is becoming increasingly important.

To make the part, the metal plate is first cut to a suitable size from hot strip or cold strip at room temperature. The cutting methods used for this purpose mainly include mechanical separation processes such as shearing or punching, and also include thermal separation processes such as laser cutting with less frequency. The thermal separation method is extremely expensive compared to the mechanical separation method and is therefore used only in exceptional cases.

After being cut to the appropriate size, the cut plate is placed in a forming tool, and finished parts such as, for example, a vehicle chassis are manufactured in a single step or multi-step molding process.

Before molding, various optional additional manufacturing steps such as, for example, punching and cutting operations are performed on the plate, and a flanging operation coupled during molding is performed in the punched section.

Particularly high stresses are applied to the cut edges during molding, for example when bent upwardly during flanging work on punched plates.

Pre-damage may be present at the severed edge. This may be due to the strain hardening of the material caused by mechanical separation, which on the one hand represents the total deformation until the material is separated. On the other hand, a notch effect that can be caused by the topography of the cut surface can occur.

Thus, especially for high strength and ultra high strength sheet metal materials, the likelihood of crack formation in the boundary regions of these cut edges during subsequent molding is increased.

The above-mentioned prior damage at the plate edge can lead to premature failure during subsequent molding operations or part operation. Testing of the forming properties of the cut edge of the plate in relation to the sensitivity to edge cracking is carried out with a hole expansion test according to ISO 16630.

During the hole expansion test, the circular hole is introduced into the metal by shearing, which is then expanded by a conical die. The measured variable is the change in the hole diameter to the starting diameter (the hole diameter at which the first crack occurs at the boundary of the hole).

A known approach for minimizing the edge cracking sensitivity described above during cold forming of sheared or punched plate edges is to change the alloy composition and to treat the material (e.g., targeted control of the bainitic microstructure) or (E. G., Through alteration of cut clearance, speed, multiple cut, etc.) during cold cutting of the plate.

Such measures are costly and inconvenient (e.g., multi-stage cutting operations, 3D cutting maintenance, etc.), or fail to provide optimal results.

It is also known from published document DE 10 2009 049 155 A1 that at least the area of the cut edge is heated to the specified temperature and the cut is performed at this temperature to improve the formability of the cut edge, Is known to be reduced or prevented. A disadvantage is the high technical and economic costs for heating the metal and the forced coupling immediately after heating and cutting of the plate, thereby reducing the flexibility of production.

It is known to cold-mold a plate cut from a shearing cut from DE 10 2011 212 904 and locally heat the strain hardened region with a laser aiming at partial softening. The disadvantages here are local softening, which is a disadvantage for high strength and ultra high strength materials which are frequently used in the case of load conditions and vibration stresses. In addition, the precise location at which heating occurs, or the specific temperature and time course of local heating, is unclear. It is also unclear how and to what extent partial softening can improve the forming ability of cold-formed plates.

It is an object of the present invention to disclose a method for manufacturing a cold-formed part from a metal plate sheared at room temperature into a potential further manufacturing step, for example, performed at room temperature, such as a hole punching or cutting operation, This method reduces or eliminates the effect of pre-cracking of the cut area, thus reducing or even eliminating engine crack susceptibility in subsequent cold forming of the metal plate. The method is simple, cost effective and on the one hand achieves equivalent and / or improved properties with respect to productivity, particularly with respect to the formability of the cut edges, on the one hand, in particular on static strength.

According to the teachings of the present invention, this object is achieved by a method of manufacturing a part by mechanically cutting or punching a plate from a steel at room temperature, the plate having edges of high moldability and reduced crack sensitivity, The plate is cut in advance to a suitable size from the strip or plate at room temperature, where punching or cutting operations to produce a recess or perforation on the sheet metal or plate, depending on the situation in question, The plate thus produced is subsequently molded at one or more stages in a single step at room temperature, and the method is carried out at a temperature of about < RTI ID = 0.0 > At any point in time, it may be subjected to a strain hardening by cutting or punching operation, The edge of the metal plate subjected to subsequent cold forming, and heated to a temperature of 600 ℃ during manufacture of the components, the time of the heat treatment is characterized in that less than 10 seconds.

The test does not require the cutting process itself to be performed at an increased temperature of the cut edge region to improve the hole expanding ability and does not require a shear- It has been shown that it is sufficient to heat the edge region cut by impact. According to the present invention, this can be done at any time before molding into the part, regardless of the cutting or punching process and subsequent manufacturing steps.

As a result, the thermal effect acts on the entire sheet thickness and in the plane direction of the plate in one region, which corresponds to a maximum sheet thickness. The duration of the thermal effect depends on the type of heat treatment method.

The heating itself may be carried out in any way, for example, by conduction heating through radiation heating and induction heating, or by laser treatment. Very suitable for heat treatment is, for example, conduction heating used in automobile manufacturing, for example in spot welding. Advantageously, for example, a spot welding machine with a relatively short impact time is suitable for holes punched into the plate, but an inductive method with a longer impact time to process longer edge sections, Processing is useful.

To protect the heated cut edge area from oxidation, an advantageous embodiment of the invention cleans these areas with an inert gas, such as argon. Here, the cleaning with an inert gas is carried out during the duration of the heat treatment, but if necessary, it can be carried out beforehand before the start of the heat treatment and / or within a limited time after the heat treatment.

Thus, the heat treatment is carried out very intensively in the sheared edge region subjected to shear impact, and thus is associated with a relatively small energy investment, in particular when compared to a method in which the entire metal plate is heated or stress-relieved annealing several orders of magnitude more time- .

Also, the management limit of the temperature reached in the cut edge region is very wide and includes a temperature range of up to about 1500 占 폚 to a solidus temperature of more than 600 占 폚.

Testing has also shown that only the removal of strain hardening is important for a significant improvement in hole expandability and that non-hardenable defects such as, for example, pores are less important.

This is irrelevant whether the heat treatment is carried out at or below the transformation temperature Ac1.

When a heat treatment is performed in excess of Ac1, a so-called quasi-normal transformation is generated in the transformable steel after the treatment during rapid cooling caused by the surrounding cool material. The microstructure formed thereon has an increased strength compared to the starting state.

Surprisingly, the microstructure transformation with the usually associated hardness and strength enhancement does not normally adversely affect the hole expandability, regardless of whether a low-hardness or low-toughness microstructure is formed, so the processing temperature of the cut edge is It is possible to reach the solidus temperature limit. An important factor is that in any case the strain hardening introduced by cutting is removed to a maximum extent.

In order to achieve the object according to the invention, it is not sufficient according to this test to heat for a duration of a few seconds to less than 600 ° C, since a significant reduction in the dislocation introduced by the mechanical separation process has to occur.

Compared to the known method for lowering the edge cracking sensitivity, the method according to the present invention has the advantage that only the shear-impacted edge region as a result of the heat treatment undergoes the microstructure change, so that the strength is not increased but increased. Thus, the resistance to edge cracks evidenced by the larger hole expandability can be improved by as much as two or even three times.

In the industrial application of the method according to the invention, the scrap of the molded part can be reduced due to the significantly increased formability of the plate edge area which is critically shear-impacted, and on the other hand the formation of the bearing part The flanging operation can be performed, and therefore, the currently required bonding operation can be performed.

Thus, the method according to the present invention allows for a more complicated part shape as a result of the improved forming ability of the cut edge area and thus greater structural freedom with the same material. Also, as predicted for example in a pronounced dual phase microstructure, such as a dual phase microstructure, the resulting microstructure can have a higher hardness relative to a more homogeneous starting state, so that the fatigue strength of the cold formed part is not reduced .

The heat treatment of the cut edge region to be cold-formed can be performed completely after the cutting or punching process and at any time before molding of the plate, or can be performed as an intermediate step of a multi-step molding operation in which the plate is printed with parts, The steps of cutting or punching, heat treating the cut edges, and shaping the plate into parts can be completely separated from each other. Thus, the present manufacturing is much more flexible than is possible according to the prior art, including the modification of the edge by heat treatment.

Due to the short treatment duration compared to known measures, the process can be integrated as an intermediate production stage of continuous production with a cycle time in the range of 0.1 to 10 seconds. Thus, in particular the intended application is the production of sheet metal parts in the automotive field of multiple subsequent stages.

In addition, the plate produced in this manner can be molded using a molding tool already provided in production, for example, since no additional heating device such as a heating furnace for heating the plate itself is required. This also contributes to cost-effective manufacturing and separates the manufacturing steps, making the manufacturing process highly flexible.

However, according to an advantageous embodiment of the present invention, heating of the cut edge, in accordance with the provided manufacturing sequence, can be carried out immediately after the mechanical cutting or punching process, Or may be performed immediately before molding. For example, the cutting and punching apparatus may be provided with a downstream heat treatment apparatus, or the latter heat treatment apparatus may be disposed immediately upstream of the forming apparatus for cold forming of the plate.

The plates themselves may be rolled, for example, flexibly to different thicknesses, or may be joined from cold strips or hot strips of the same or different thicknesses and / or grades. The invention can be used for hot or cold rolled steels made of soft steel with high yield strength, for example, with a yield strength of 140 MPa to 1200 MPa, which may have metal and / or organic coatings . The metal coating may be made of, for example, zinc or zinc alloy, magnesium, aluminum and / or silicon.

The suitability of the coated steel strip can be explained by the possibility of limiting the treatment of the edge region to a distance to the edge corresponding to a portion of the sheet thickness since there is a major portion of the harmful strain hardening during shearing in this region . Thus, in the case of sheet thicknesses in the area of a few millimeters thick, for example, the effective corrosion protection of the metal corrosion resistant layer is only affected to an insignificant or insignificant extent, so that the distance to the edge of several tens of micrometers may already be sufficient have.

As a higher strength steel, not only all single-phase steel but also polyphase steel can be used. This includes micro-alloy steels, higher strength steels as well as bainitic or martensitic steels and also duplex steels, composite steels and TRIP steels.

Further features, advantages and details of the present invention will become apparent from the following description of the drawings.

Figure 1 is a schematic view of the hole expansion test according to ISO 16630 on heat treated cut edges according to the present invention,
2 is a test fixture for conductive heat treatment of edges cut by shear-impact,
3 is the result of the hole expansion test according to ISO 16630 for a sample HDT780C that has not been coated after the conductive heat treatment of the edge cut by shear-impact,
4 is the result of the hole expansion test according to ISO 16630 for the melt coated sample HCT780CD and the uncoated sample HDT780C after the heat treatment of the edge cut by the shear-shock by the laser,
Figure 5 is a microstructure and hardness trend for a cut edge heat treated in accordance with the present invention.

Figure 1 schematically shows an hole expansion test according to ISO 16630 for a cut edge heat treated in accordance with the present invention.

According to the invention, this heat treatment is carried out only on the edge cut by shear-impact after cutting of the plate to an appropriate size and as an intermediate step before shaping of the area adjacent to the cut edge.

The test setup for the conductive heat treatment of shear-impact cut edges is shown in Fig.

In the present test, a conventional spot welding machine was used to weld a steel sheet used for manufacturing parts of a vehicle in the automobile industry in addition to a high output laser as a heating device. In this case, however, the sheet having the hole punched therein (step 1) is heat-treated in the region of the edge cut by the shear-impact according to Fig. 1 (step 2) . The actual hole expansion is then carried out by the die in step 3, which is determined in the tested probe.

As shown in Fig. 2, the opposing spot welding electrodes have a diameter larger than the hole punched so that the shear-impacted hole edge can be heat-treated. In addition, at the end contacting the boundary of the hole, the electrode has a semicircular shape so that the plate can be easily centered and, on the other hand, heat can be intensively introduced only in the shear-shocked region.

In order to impact the current only in the essentially shear-shocked region, the shape of the tip of the contact electrode must be adjusted for each geometry of the edge region.

For the test, HDT780C grade uncoated high strength, hot rolled bainite steel with a minimum yield strength of 680 MPa and a minimum tensile strength of 800 MPa was used. Hot dip galvanized cold rolled composite steels having a minimum yield strength of 500 MPa and a minimum tensile strength of 780 MPa of the HCT780CD grade were also used.

Depending on the method, the treatment duration, i. E. The duration of the current when the heating is conducted inductively and the duration of the power taken by the laser, or the exposure time to the other heat source is 20 ms to 10 s, Advantageously from 100 ms to 2000 ms. In any case, it is important to reach a temperature of 600 ° C or higher at the heat treatment site.

An important method parameter is the processing duration, and in the case of induction heating the current is in the range of 4 to 10 kA. In the case of heat treatment by laser, a laser output of 5 kW distributed over a circular area of about 12 mm so that the roughly ring shape with a 1 mm boundary width of the cut circular holes of the sample having a diameter of 10 mm is heat- .

The results of the hole expansion test according to ISO 16630 for the uncoated sample HDT780C after the conductive heat treatment of the edge cut by the shear-impact are shown in Figure 3 and the results of the hot dip galvanizing after the heat treatment of the edge cut by shear- The corresponding results obtained with the sample HCT780CD and the coated sample HDT780C are shown in Fig.

As shown in FIGS. 3 and 4, an increase in the hole expansion by two to three times or more as compared to the untreated reference sample can be achieved in most cases after the heat treatment. The difference in results is due to non-uniform heat treatment by the laser, especially due to unoptimized geometric conditions.

The upper left image of FIG. 5 shows a schematic plan view of a hole punched in a metal plate in which the hole edge region has been treated in accordance with the present invention. The microstructure formed in the heat affected zone is shown schematically in the upper right side image.

This is an illustrative example of the effect of heat treatment, and conclusions can be drawn regarding the dominant temperature. The illustrated results relate to a treatment duration of 500 ms of the HDT780C steel having a bainite-based microstructure and an inductive treatment using a current of 8 kA.

In an adjacent boundary region of about 0.5 mm, the microstructure is formed with 100% martensite. As a result, rapid cooling was performed after heating exceeding Ac3 was performed. As the distance to the edge increases, the ratio of bainite increases to a distance of up to about 2.5 mm beyond the distance that 100% bainite is present. Exceeding an edge distance of 2.5 mm results in a treatment temperature below Ac2 (about 700 ° C) because the microstructure no longer undergoes transformation.

The increase in hardness in the adjacent region of the hole edge (lower image in FIG. 5) is typical for micro-alloyed bainitic hot strips and is attributed to the subsequent precipitation of nanoparticles in the temperature range of about 500 ° C. to 700 ° C. do.

In summary, the advantages of the present invention can be summarized as follows.

- produces very good formable cut edges with reduced edge cracking sensitivity and high hole expandability, which enables the production of geometric shapes of complex parts and reduces the risk of scrap due to edge cracking during molding .

Produce a product optimized for light weight and cost by manufacturing geometric shapes of complex parts.

- Because the duration of the heat treatment is short and the temperature interval is very wide, there is a possibility to integrate this method into the multi-stage production of pressed parts.

- The method can be applied to corrosion resistant coated sheet metal due to localized and temporarily very limited heating.

- Heat-treated areas in the fabricable material are typically strengthened rather than softened relative to the starting material.

Claims (15)

CLAIMS What is claimed is: 1. A method of manufacturing a component by mechanically cutting or punching a plate having high formability and reduced crack sensitivity edges from steel at room temperature,
The plate is first cut from the strip or metal plate at room temperature and additional manufacturing steps are performed such as, for example, a punching or cutting operation to obtain a recess or perforation on the sheet metal or plate, depending on the circumstances And subsequently the thus produced plate is molded into a part at one or more steps at room temperature and the resulting plate is then molded into the part regardless of its molding into the part and after any cutting of the plate to an appropriate size and after optional further punching or cutting operation Wherein the edge region of the sheet metal subjected to subsequent cold forming during the manufacturing of the part is heated to a temperature of at least 600 DEG C and the temperature exposure time is less than or equal to 10 seconds,
A method of manufacturing a component. The method according to claim 1,
Wherein the temperature exposure time is 0.02 to 10 seconds,
A method of manufacturing a component. 3. The method of claim 2,
Wherein the temperature exposure time is between 0.1 and 2 seconds,
A method of manufacturing a component. 4. The method according to any one of claims 1 to 3,
Heating of the edge regions of the strain-hardened sheet metal is carried out up to a temperature of 600 ° C to a solidus temperature,
A method of manufacturing a component. 5. The method of claim 4,
Wherein heating of the edge region of the deformed and hardened sheet metal is carried out from the temperature of Ac1 to the solidus temperature,
A method of manufacturing a component. 6. The method according to any one of claims 1 to 5,
The heating is carried out up to a molding temperature by induction heating, conduction heating, radiation heating, or laser irradiation.
A method of manufacturing a component. The method according to claim 6,
The heating may be performed by a resistance welding device or a laser,
A method of manufacturing a component. 8. The method according to any one of claims 1 to 7,
Wherein the plate is formed in one step or in multiple steps,
A method of manufacturing a component. 9. The method according to any one of claims 1 to 8,
Said sheet metal plate having an organic coating and / or a metal coating,
A method of manufacturing a component. 10. The method of claim 9,
Wherein the metal coating comprises Zn and / or Mn and / or Al and / or Si,
A method of manufacturing a component. 11. The method according to any one of claims 1 to 10,
Wherein a heat treatment in a plane direction of the plate starting from an edge of the sheet metal is performed in a region corresponding to a maximum thickness of the sheet metal,
A method of manufacturing a component. 12. The method according to any one of claims 1 to 11,
Wherein the peripheral region of the heat treatment site is protected from oxidation,
A method of manufacturing a component. 13. The method according to any one of claims 1 to 12,
Wherein said heat-treating peripheral region is washed with an inert gas during at least thermal < RTI ID = 0.0 >
A method of manufacturing a component. 14. The method of claim 13,
Wherein a peripheral region of the region of the heat treatment is further washed with an inert gas before and /
A method of manufacturing a component. As a use of a plate made of steel for forming into parts at room temperature,
The plate is mechanically cut from the strip or metal plate to an appropriate size at room temperature and further punching or cutting operations to obtain a recess or perforation are optionally performed at room temperature and are pre- The edge of the punched sheet metal is subjected to a heat treatment at 600 占 폚 or more over a period of 0.02 to 10 seconds or 0.1 to 2 seconds,
The use of plates made of steel.

Images