DE102017202294A1 - Process for producing a hot-formed and press-hardened sheet steel component - Google Patents

Abstract

The invention relates to a method for producing a hot - worked and press - hardened steel sheet component (7) made of hardenable steel having a tensile strength (Rm) greater than 1500 MPa, preferably> 1650 to 2250 MPa, in which method the steel sheet (6) in a heat treatment step over the material-specific Austenitisierungtemperatur Ac3 is heat treated, in a subsequent insertion the steel sheet (6) is inserted with an insertion temperature (θ) in a forming tool (3), the steel sheet component (7) hot-formed in a press hardening step and at the same time cooled to a withdrawal temperature (θ) is removed, and in a removal step, the steel sheet member (7) with the removal temperature (θ) from the open forming tool (3) is removed. According to the invention, the steel sheet component (7) is cooled in the press-hardening step with closed forming tool (3) to a removal temperature (θ) which is in a temperature range close to the material-specific martensite finish temperature Mf, at which austenite to martensite transformation is for the most part is completed, in particular in a temperature range of the martensite finish temperature Mf of +/- 10%.

Description

The invention relates to a method for producing a hot-formed and press-hardened sheet steel component according to the preamble of claim 1. The sheet steel component is made of a hardenable steel having a tensile strength Rm greater than 1500 MPa, in particular> 1650 MPa to 2250 MPa.

Hot-forming steels, such as 22MnB5, exhibit an almost complete martensitic structure with a nominal tensile strength of 1500 MPa after the hot-forming process. The ductility of this structure is often characterized by the elongation at break. This is approximately between 4 to 6% at 22MnB5. In addition, the bending angle is given in a so-called platelet bending test according to VDA 238-100 as a further characteristic value for characterizing the ductility. For example, for 22MnB5 bending angles between 50 ° and 65 ° are to be expected. Currently, strengths of these hot-forming steels are further increased by, for example, increasing the carbon content up to 2000 MPa. At higher strengths, however, the hot-formed steel sheets are increasingly sensitive, that is, more brittle, to stresses. In the case of steel sheet materials (sheet thickness of, for example, 1.5 mm) with strengths Rm greater than 1500 MPa, a subsequent time-consuming tempering treatment is often required to reduce the brittleness. However, such additional tempering treatment increases the process time during hot working and results in additional tooling for performing the tempering treatment.

Exemplary is from the DE 10 2013 010 946 B3 a generic method for producing a hot-formed and press-hardened sheet steel component known. This is heat treated in a heat treatment step above the material specific austenitizing temperature. This is followed by an insertion step in which the steel sheet is inserted into a hot-forming tool at an insertion temperature which corresponds approximately to the austenitizing temperature. In the following press hardening step, the steel sheet is hot formed and cooled down to a withdrawal temperature. In the DE 10 2013 010 946 B3 the cooling process is interrupted above the so-called martensite finish temperature at which a transformation from austenite to martensite is largely completed, whereby a small proportion of retained austenite is retained in the microstructure. Subsequently, the steel sheet component is removed in a removal step from the open forming tool, and at a take-off temperature above the martensite finish temperature, and transferred to a heating device, wherein cooling of the sheet steel component is less than 200 ° C is avoided. This allows the retained austenite to stabilize particularly well. In the heating device is a tempering treatment in which the retained austenite is stabilized in the sheet steel component and even after further cooling of the sheet steel component to room temperature in which the steel sheet component structure is maintained. The retained austenite reduces the stress between the martensite needles and, at high strengths, causes the structure to have a significantly increased elongation at break or ductility compared to martensite.

The object of the invention is to provide a method for producing a hot-formed and press-hardened sheet steel component in which the ductility of sheet steel components is increased at higher strengths Rm greater than 1500 MPa, compared to the above prior art at a reduced process time and in reduced use of tools.

The object is solved by the features of claim 1. Preferred embodiments of the invention are disclosed in the subclaims.

According to the characterizing part of claim 1, the steel sheet member is cooled in the press-hardening step with the forming tool closed to a take-off temperature which is in a temperature range close to the material-specific martensite finish temperature Mf, specifically in a temperature range of the martensite finish temperature Mf + / - 10%. Such a temperature range is uncritical in view of disadvantageous sheet steel component distortion after the press hardening step.

The yield strength or a 0.2% yield strength or elastic limit of the sheet steel component may be> 1100 MPa, preferably 1250 MPa to 1950 MPa.

Particularly preferred is a process control in which the removal temperature of the sheet steel component is a temperature offset below the material-specific martensite finish temperature. In the removal step, therefore, the steel sheet component is removed with the removal temperature below the martensite finish temperature. In this way, it is ensured that the martensite formation is completed during the steel sheet component removal, whereby the component no longer warps in the further course of the process due to residual stresses that could otherwise occur in a further transformation of austenite to martensite.

A purely martensitic structure in the sheet steel component leads to a high steel sheet component strength, but at the same time to an increase in brittleness, so that depending on the carbon content only a slight deformation with elongations at break, usually less than 7% is possible. On the other hand, the elongation at break of a sheet steel component can be increased if a proportion of retained austenite remains in the martensitic microstructure of the sheet steel component, as can also be seen from FIG DE 10 2013 010 946 B3 evident. This is done by an incomplete transformation of austenite into martensite, so that retained austenite is retained in the finished sheet steel component.

Such an incomplete transformation of austenite to martensite is accomplished according to the invention by a specific time course of the cooling in the press hardening step. Preferably, the time course of the cooling can be followed by a cooling curve in which the steel sheet component temperature is reduced from a closing time, at which the forming tool is closed, to a transition time with a large first cooling rate, in particular greater than 27 ° C / s, and is further cooled in the further time after the transition time with a greatly reduced second transition cooling rate until a flat cooling line G with a slope, ie with a third cooling rate. By way of example, the third cooling rate may be 15 ° C / s to 3 ° C / s. It is preferred in terms of structural composition, if the sheet steel component temperature at the transition time (or at the beginning of the cooling line G) by a temperature offset above the martensite finish temperature, but below the material-specific martensite start temperature, in the a transformation from austenite to martensite begins. In such a cooling curve, on the one hand ensures that the austenite is almost completely converted into martensite. On the other hand, due to the greatly reduced cooling rate after the transition time, a slow cooling is achieved by means of which the formation of retained austenite is promoted.

With regard to a process-technically simple design of the cooling curve described above, it is preferred if the forming tool has a heating device with which the, the steel sheet component facing tool surface is heated or tempered. In this case, the forming tool can be heated in the press hardening step, in particular to a tool temperature in the range of 100 ° C to 250 ° C, which can be easily set in the Abkühlkurve Presshärteschritt. In the further course of the process after removal from the forming tool, the cooling process can be continued without interruption and continuously, more preferably with a fourth cooling rate corresponding to or less than the third cooling rate, that is, for example, at 1 ° C./s to 5 ° C / s. This fourth cooling phase thus represents an annealing of the component in order to further reduce the brittleness.

To continue the cooling process, the steel sheet component can be transferred after the press hardening step in a storage station in which the cooling process is continued by air cooling with ambient air.

The above method is particularly applicable to a steel sheet metal component having a sheet thickness of exemplarily 1.5 mm, which has a carbon content C of more than 0.25% by weight, in particular> 0.27% by weight to 0.40% by weight. %. In addition, the steel sheet may contain alloying constituents which contribute to austenite stabilization and thus lead to a component structure in which, in addition to the martensite, retained austenite is also contained.

Hereinafter, an embodiment of the invention will be described with reference to the figures.

• 1 eine Anlagenskizze, anhand der die in der 2 angedeutete Prozessabfolge zur Herstellung eines warmumgeformten und pressgehärteten Stahlblechbauteils veranschaulicht ist;
• 2 in einem Blockschaltdiagramm die Prozessabfolge zur Herstellung des Stahlblechbauteils; und
• 3 ein Diagramm, dass den zeitlichen Verlauf der Stahlblechbauteil-Temperatur beim Einlegen in das Umformwerkzeug und beim anschließenden Presshärten zeigt.

Show it:

• 1 an investment sketch, on the basis of which in the 2 indicated process sequence for producing a hot-formed and press-hardened sheet steel component is illustrated;
• 2 in a block diagram the process sequence for the production of the sheet steel component; and
• 3 a diagram showing the time course of the sheet steel component temperature when placed in the forming tool and the subsequent press hardening.

In the 1 is roughly outlined schematically a plant, based on the first basic process sequence for the production of a hot-formed and press-hardened sheet steel component is explained. The plant exemplifies a continuous furnace 1 , a forming tool 3 for hot forming and press hardening of sheet steel components as well as a storage station 5 on, in which the sheet steel components are stored. First, a steel sheet 6 made of a hardenable steel in the continuous furnace 1 and heated there over the material-specific Austenitisierungstemperatur Ac3 of the steel used, which may be 930 ° C by way of example. The heated steel sheet 6 is in the hot state in the forming tool 3 transferred and hot formed there in a press hardening step to a sheet steel component 7 and at the same time deterred. The high steel sheet component temperature during hot forming ensures outstanding forming behavior. In the 1 has the forming tool 3 also only hinted heating unit 9 whose mode of action is described later.

Hereinafter, the process flow during the inserting step, the press hardening step and the taking step will be described with reference to the temperature-time diagram of FIG 3 explained in more detail, in which the solid lines, the time courses of the sheet steel component temperature and the Umformwerkzeug temperature are shown. In addition, by means of a dotted line, an opening state and a closing state of the forming tool 3 indicated. As a result, the forming tool becomes 3 in a tool closing phase Δt _too , closed, which can be for example 2 to 5 s and the forming tool 3 during the press hardening step in a tool holding phase Δt _H kept closed. The tool holding phase Δt _H is in the 3 exemplary about 8s. Alternatively, the tool holding phase Δt _H at for example 15s lie. The tool holding phase Δt _H may generally be of any length of time, and more particularly in a range of 2 to 30 seconds. Subsequently, the forming tool 3 in a mold opening phase Δt _on opened, which can be 2 to 5 s.

With reference to the 3 is after the heat treatment in the oven 1 the steel sheet 6 at a time of insertion t ₁ and an insertion temperature θ ₁ (in this example is approximately at 930 ° C) in the open forming tool 3 inserted. Subsequently, the tool closing phase takes place Δt _too that at a closing time t ₂ is completed. In the further course of the process, the press hardening step takes place in the tool holding phase Δt _h , ie the hot forming with simultaneous cooling. After the press hardening step is in a mold opening phase Δt _on the forming tool 3 opened and at a withdrawal time t ₄ the sheet steel component 7 taken and to the storage station 5 transferred. The above process sequence is common practice in hot forming. The essence of the invention lies in the following described interpretation of the time course of the sheet steel component cooling, which is superimposed on the forming process in time:

So is in the 3 the press hardening step is designed so that the time course of the cooling follows a cooling curve at which the sheet steel component temperature from the closing time t ₂ until a transitional period t ₃ is reduced with an extremely large first cooling rate a ₁ , which is greater than 27 ° C / s. Later in the time after the transition t ₃ comes with a greatly reduced second cooling rate a ₂ Continue to cool until a flat cooling line G with a third cooling rate a ₃ established. The third cooling rate a ₃ is exemplary in particular less than 15 ° C / s to 3 ° C / s. Like from the 3 As can be seen, the sheet steel component temperature is at the beginning of the cooling line G still above the Mf temperature. At the time t ₅ cuts the cooling line G the Mf temperature, ie the sheet steel component temperature falls below the Mf temperature. By means of the flat cooling line G So is the sheet steel component temperature at the transition time t ₃ still a temperature offset Δθ ₁ above the martensite finish temperature Mf, slowly to a withdrawal temperature θ ₄ reduced by a temperature offset Δθ ₂ below the martensite finish temperature Mf that is in the 3 is about 300 ° C.

The in the 3 Cooling curve shown is process technology simply by a corresponding temperature of the forming tool 3 achieved. For this purpose, the heating unit 9 during the press-hardening step, preferably heated to a tool temperature θ _W in the range of 100 ° C to 250 ° C. In this way, on the one hand - despite heated forming tool 3 - Guarantees the above extremely large first cooling rate a ₁ at the beginning of the cooling curve, with simultaneous increase of the tool temperature θ _W to, for example, up to 600 ° C (at the surface). On the other hand, by the temperature of the forming tool 3 Ensures that the cooling rate after the transition time t ₃ is delayed very much. It should be emphasized that in the 3 the steel sheet component temperature θ ₃ at the transitional time t ₃ is above the martensite finish temperature Mf and that the transition time t ₃ within the tool holding phase Δt _H lies.

In the further course of the process after the press hardening step, the sheet steel component becomes 7 in the storage station 5 transferred. In the storage station 5 the cooling process is continued uninterrupted and continuously, with a fourth cooling rate a ₄ , which may be at 1 ° C / s to 5 ° C / s. This is preferably achieved by air cooling with ambient air.

LIST OF REFERENCE NUMBERS

1

Continuous furnace

3

forming tool

5

deposit station

6

Steel sheet

7

Sheet steel component

9

heating unit

t ₁

Open time point

t ₂

Closing time

t ₃

Transition date

t ₄

Taking time

t ₅

Time at which the steel sheet component temperature falls below the Mf temperature

a _1, a ₂ , a ₃

a ₄ Cooling rates

θ ₁

insertion temperature

θ ₂

closing temperature

θ ₃

Transition temperature

θ ₄

removal temperature

Δt _too

Mold closing phase

Δt _H

Tool holding phase

Δt _on

Mold opening phase

Δθ ₁ , Δθ ₂

Temperaturversätze

G

Abkühlgerade

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 102013010946 B3 [0003, 0009]

Claims (9)

A method of making a hot worked and press hardened hardenable steel sheet steel component (7) having a tensile strength (Rm) greater than 1500 MPa, preferably> 1650 to 2250 MPa, in which process the steel sheet (6) is heat treated above the material specific austenitizing temperature Ac3 in a heat treatment step is inserted in a subsequent insertion step, the steel sheet (6) with an insertion temperature (θ ₁ ) in a forming tool (3), in a press-hardening step, the steel sheet component (7) is hot-formed and at the same time to a withdrawal temperature (θ ₄ ) is cooled, and in a removal step the steel sheet component (7) with the removal temperature (θ ₄ ) is removed from the opened forming tool (3), characterized in that the steel sheet component (7) in the press hardening step with closed forming tool (3) to a removal temperature (θ _4th ) is cooled, which in a temperature range na of the material-specific martensite finish temperature Mf at which austenite-martensite transformation is largely completed, particularly in a temperature range of the martensite finish temperature Mf of +/- 10%. Method according to Claim 1 , characterized in that the removal temperature (θ ₄ ) by a temperature offset (Δθ ₂ ) below the martensite finish temperature Mf, and that in the removal step, the steel sheet component (7) with the removal temperature (θ ₄ ) below the martensite finish Temperature Mf is taken. Method according to Claim 1 or 2 characterized in that the press hardening step starts with a closing time (t ₂ ) to which the forming tool (3) is closed, and that the press hardening step is designed such that the time course of the cooling follows a cooling curve at which the sheet steel component temperature from the closing time (t ₂ ) to a transition time (t ₃ ) with a large first cooling rate (a ₁ ) is reduced, in particular greater than 27 ° C / s, and in the further time after the transition time (t ₃ ) with a is strongly cooled second transition cooling rate (a ₂ ) is further cooled until a flat cooling line (G) with a third cooling rate (a ₃ ), which is in particular at 15 ° C / s to 3 ° C / s. Method according to Claim 3 , characterized in that the sheet steel component temperature at the transition time (t ₃ ), in particular at the beginning of the cooling (G), by a temperature offset (Δθ ₁ ) above the martensite finish temperature Mf, but below the material-specific martensite start temperature Ms is where a transformation from austenite to martensite begins. Method according to one of the preceding claims, characterized in that the forming tool (3) has a heating unit (9), with which, the steel sheet member (7) facing tool surface is heated or tempered, and in particular that the forming tool (3) heated during the press hardening step is, to a tool temperature (θ _W ) below the martensite finish temperature Mf, in particular in a range of 100 ° C to 250 ° C, whereby the cooling curve is adjustable in the press hardening step. Method according to one of the preceding claims, characterized in that in the further course of the process after the removal step (t ₄ ) the cooling process is continued without interruption and continuously, in particular with a fourth cooling rate (a ₄ ) compared to the third cooling rate (a ₃ ) is further reduced, in particular to 1 ° C / s to 5 ° C / s. Method according to one of the preceding claims, characterized in that in the further course of the process after the removal step (t ₄ ), the steel sheet component (7) is transferred to a storage station (5), in which the cooling process is continued by air cooling with ambient air. Method according to one of the preceding claims, characterized in that the method is applicable to a steel sheet metal part (7) having a carbon content C of greater than 0.25% by weight, preferably between C> 0.27-0.40% by weight, and that alloying components are preferably included which contribute to austenite stabilization and thereby give a component structure in which residual austenite besides martensite is contained, for example Si> 0.5% by weight. Method according to one of the preceding claims, characterized in that the yield strength or a 0.2% yield strength or elastic limit (Rp) is greater than 1100 MPa, preferably 1250 MPa to 1950 MPa.

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