US20160145707A1

US20160145707A1 - Process and Installation for Producing a Press-Hardened Sheet Steel Component

Info

Publication number: US20160145707A1
Application number: US14/900,588
Authority: US
Inventors: Peter Feuser; Bohuslav Masek; Thomas Schweiker
Original assignee: Daimler AG
Current assignee: Mercedes Benz Group AG
Priority date: 2013-06-28
Filing date: 2014-05-17
Publication date: 2016-05-26
Also published as: EP3013988A1; DE102013010946B3; CN105358718A; CN105358718B; WO2014206514A1

Abstract

A method and a system for producing a press-hardened sheet steel component is disclosed. The method includes: a) heating of a component blank formed from a hot-formable steel material at least to the austenitising temperature of the steel material by a heating device; b) hot forming of the component blank by a forming tool; c) cooling of the component blank in the forming tool to a temperature above the material-specific martensite finish temperature; d) bringing of the component blank from the forming tool to a warming device; and e) annealing of the component Hank, stabilizing the austenite in the component blank by the warming device. The component blank is brought directly from the forming tool to the waning device, preventing a cooling of the component Hank to less than the material-specific martensite finish temperature.

Description

BACKGROUND AND SUMMARY OF THE INVENTION

The invention relates to a method to produce a press-hardened sheet steel component as well as a system to produce such a press-hardened sheet steel component.
From the mass production of motor vehicles, in particular passenger vehicles, the use of hot-formed components made from the material 22MnB5 is sufficiently known. Such hot-formed components made from hot-formable steel, in particular from 22MnB5, are installed at the present time internationally and across manufacturers in quantities of more than 100 million pieces per year. Press-hardened sheet steel components are used in bodies of motor vehicles which, in the event of an accident, are to have a high stability and no or only very slight deformations.
However, the relatively low elongation at break of the press-hardened components made from 22MnB5, which, for example, lies in a range from 5 to 7%, is critical here. Therefore, kinetic energy or accident energy can only be dissipated in very low quantities by plastic deformation of the press-hardened components. An overloading of the components can therefore, for example, lead to tearing of the respective component or to a failure of this.
Therefore, for vehicle applications with requirements for a particularly high deformation capability, completely hardened components made from 22MnB5 cannot be used. At the current time, alternatives to this are hot-formed components made from microalloyed steel or from tailored welded blanks with regions made from microalloyed steel which is able to be press hardened. The low strength of the microalloyed steel after the hot-forming is, however, disadvantageous in this approach. Therefore, the strength after the hot forming amounts, for example, to only approximately 600 megapascals. Therefore, greater sheet thicknesses are required in comparison to stronger materials with similar ductility.
Sheet or sheet steel components which have a high elongation or elongation at break of 10% or more—measured in conformity with ISO 6892-1—as well as a high strength, for example in a range from 1,200 to 2,000 megapascals inclusive, are desirable for vehicle applications. Due to such a high elongation or elongation at break and due to the high strength, such components would have very good accident properties and are suggested for the implementation of shell constructions in light-weight construction, in particular in the field of passenger motor vehicles and commercial vehicles. Such mechanical properties would enable a clearly greater absorption of impact energy in the event of an accident, whereby a particularly high passenger protection would result. At the same time, however, the achievement of only a low carbon content is desirable in comparison to components of forging, in order to ensure weldability.
A practical development of such sheet steel components was previously not possible (or only with a very high expenditure) due to the unavailability of corresponding semi-finished products or component blanks and system techniques for the processing of the components.
US 2012/0273096 A1 discloses a device and a method to produce a press-hardened sheet steel component, wherein a component blank formed from a hot-formable steel material, from which component blank the sheet steel component is produced, is heated at least to the austenitising temperature of the steel material by means of a heating device. Then the component blank is hot formed by means of a forming tool. Then the component blank is cooled in one component region to at least 200° Celsius in the forming tool, wherein a different component region is kept at a temperature above 200° Celsius by tool measures. In a further step, the component blank is brought from the forming tool to a warming device. Finally, the component blank is annealed by stabilizing the austenite by means of the warming device.
The processing route proposed in US 2012/0273096 A1 in combination with the proposed tool, however, does not represent a solution to the problem described at the beginning concerning the production of a homogenous component with particularly high elongation as well as, at the same time, particularly high strength. In particular, such component regions which are quenched according to US 2012/0273096 A1 to a temperature below 200° C. have a very high strength on the finished part in combination with a very low elongation at break of less than 10%. It is additionally disadvantageous in US 2012/0273096 A1 that tool regions must be warmed to 550° C. in order to cause an increase of the ductility in partial regions by bainitic or perlitic ferritic phase transformations. Such a tool temperature has the consequence that specific and comparably expensive tool materials must be used. Besides the costs for the heat energy, a further disadvantage exists in the extended cycle time for the production of such a component. As the bainitic and/or perlitic ferritic phase transformations clearly proceed more slowly in comparison to martensitic transformation, the dwell time of the component in the tool is extended. The cycle time is reduced by exactly this proportion, which causes additional costs.
It is therefore the object of the present invention to further develop a method and a system of the type referred to at the beginning in such a way that press-hardened sheet steel components having particularly high ductility and at the same time particularly high strength can be produced in a simple, time- and cost-effective manner.
In order to create a method, by means of which press-hardened components can be produced having a particularly high ductility and, at the same time, having a particularly high strength in a time- and cost-effective manner, it is provided in the method according to the invention that the component blank is brought directly from the forming tool to the warming device, preventing a cooling of the component blank to less than the martensite finish temperature Mf, preferably to less than 200° Celsius. Due to this direct bringing or due to the direct transfer from the forming tool to or into the warming device for annealing, an excessive cooling of the component blank can be prevented. Due to very high cooling rates during forming, i.e. during press-hardening of the component with an extremely quick transfer into an isothermal holding phase at defined temperatures, the transfer from the hot forming tool into the warming device for annealing and the prevention of the cooling of the component blank to less than 200° Celsius plays an important role in order to be able to produce press-hardened components having a high ductility and a high strength in the scope of mass production in a cost-effective manner. This is achievable by means of the method according to the invention such that press-hardened components having a high ductility, for example having an elongation at break of 10% or more, as well as having a high strength, for example having a strength in a range from 1,200 megapascals to 2,000 megapascals inclusive can be produced in a time- and cost-effective manner. In particular, an elongation at break in a range from 10% to 20% inclusive can be achieved. As a consequence of the high elongation at break or the high ductility, the press-hardened sheet steel components which are able to be produced by means of the method according to the invention have a very high energy absorption capability as a consequence of plastic deformation, such that they, for example in the event of an accident of a motor vehicle, can convert a particular high amount of impact energy into deformation energy. At the same time, the press-hardened sheet steel components have an improved crash robustness due to the improved ductility, from which a particularly advantageous accident behavior results to achieve very good passenger protection. In comparison to components produced in another way, for example by roll forming of martensite phase steels, an improved passenger protection can therefore be achieved with the same or even lower individual part weight. Compared to hot-formed components made from 22MnB5 and in particular from microalloyed steel, the wall thickness can be further reduced such that press-hardened sheet steel components can be implemented having a very low wall thickness and therefore having a very low weight.
In comparison to conventional sheet steel components, the method according to the invention enables a further increase of the strength by the use of martensitic steel. The usual martensitic structure is the hardest structure variant for steels. At the same time, a purely martensitic structure is very brittle and, depending on the carbon content, only enables a low deformation, such that elongation values or elongation at break values usually lie below 7%.
The invention is therefore based on the idea and the knowledge that for an increase of the elongation or elongation at break, it is required to reduce the tension between the martensitic needles and thereby to achieve better conditions for plastic behavior of the sheet steel components. A possibility for this consists in forming thin austenitic foils between the martensitic needles. This is, for example, technically possible due to an incomplete conversion from the austenitic phase to the martensite. In the case of an interruption in the cooling above the so-called martensite finish temperature Mf, the austenite converts into martensite, but a small proportion of the austenite remains. The martensite finish temperature Mf is therein the temperature at which the greatest part of the martensite conversion is concluded. If, directly after this, the structure of the sheet steel component or of the component blank is kept at a slightly increased temperature, the carbon migrates from the supersaturated martensite by diffusion into the austenite. In order to achieve this, the formed component blank is transferred directly from the hot forming tool in a warming direction, wherein a cooling of the component blank to less than 200° Celsius is prevented. The austenite can hereby be particularly well stabilized as, due to the direct bringing, residual heat from the hot forming process can be used during annealing. Due to the direct transfer of the component blank from the hot forming tool into the warming device (used to anneal the component), the austenite in the component blank is stabilized and also remains in the component structure after a further cooling of the component blank or of the completely produced sheet steel component to room temperature. This so-called residual austenite reduces the tension between the martensite needles and causes the structure to have substantially better elongation or ductility compared to the martensite at the same time as high strengths. The use of a steel alloy having the following alloy elements has been shown to be particularly advantageous as a starting material for the production of the component blank:

- carbon (C) in a range from 0.2 to 0.5 percent by weight (% by weight) inclusive,
- silicon (Si) in a range from 0.5 to 2.9% by weight inclusive,
- manganese in a range from 0.7 to 4.1% by weight inclusive,
- up to 0.1% by weight phosphorous (P),
- up to 0.1% by weight sulphur (S),
- aluminum (Al) in a range from 0.001 to 0.5% by weight inclusive,
- chromium (Cr) in a range from 0.1 to 1.5% by weight inclusive,
- titanium (Ti) in a range from 0.01 to 0.2% by weight inclusive,
- boron (B) in a range from 0.01 to 0.03% by weight inclusive,
- and up to 0.025% by weight nitrogen (N).

A system is included in the invention, wherein it is provided according to the invention that the warming device is indirectly connected to the forming tool such that the component blank is able to be brought directly from the forming tool to the, and in particular into, the warming device, preventing a cooling of the component blank to less than 200° Celsius. Advantageous embodiments of the method according to the invention can be viewed as advantageous embodiments of the system according to the invention and vice versa. Press-hardened sheet steel components having a particularly high strength and at the same time having a particularly high ductility can be produced in a simple, time- and cost-effective manner, in particular in the scope of mass production, by means of the system according to the invention. In particular it is possible to prevent a particularly high number of pieces with only a low level of rejection, as an excessive cooling of the component blank is prevented after the cooling in the forming tool and before the annealing.
Compared to conventional press-hardening, for example of components made from 22MnB5, the warming device is provided as a further heating device to stabilize the residual austenite. The warming device is preferably a roller hearth furnace or walking beam furnace.
The forming tool is, in the method, preferably able to anneal to temperatures in a range from 25° to 500° Celsius inclusive in order to thereby control and targetedly adjust the quenching temperature and therefore the residual austenite content of the component blank functioning as a semi-finished product. During transfer from the forming tool to or into the warming device, the cooling of the component blank by measures such as radiant heaters and/or deflector plates can be prevented or kept low such that a cooling of the component blank to less than 200° Celsius is prevented.
In the warming device, the component blank(s) can be supported on goods carriers, by means of which, for example, the component blank or the component blanks is or are conveyed through the heating device. The goods carrier can thereby preferably counteract a thermal distortion of the component.
The following steel alloys have proved to be particularly advantageous as materials for the component blank and therefore for the sheet steel component or sheet component:
(0.25-0.35)% by weight C+(0.5-0.7)% by weight Mn+(1.5-2.5)% by weight Si+(0.5-1.5)% by weight Cr+(0.001-0.008)% by weight B+max. 0.01% by weight N+(0.015-0.08) % by weight Al+(0.001-0.009)% by weight Ti+(0.010-0.025)% by weight P+max. 0.010% by weight S,
or
(0.25-0.35)% by weight C+(1.2-1.8)% by weight Mn+(1.0-2.0)% by weight Si+(0.3-1.0) % by weight Cr+(0.001-0.008)% by weight B+max. 0.01% by weight N+(0.015-0.08)% by weight Al+(0.001-0.009)% by weight Ti+(0.010-0.025)% by weight P+max. 0.010% by weight S,
or
(0.25-0.35)% by weight C+(1.2-1.8)% by weight Mn+(1.0-2.0) % by weight Si+(0.10-0.30)% by weight Cr+(0.001-0.008)% by weight B+max. 0.01% by weight N+(0.015-0.08)% by weight Al+(0.001-0.009)% by weight Ti+(0.010-0.025)% by weight P+max. 0.010% by weight S.
Further advantages, features and details of the invention result from the following description of preferred exemplary embodiments as well as by means of the drawings:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic depiction of a method and a system to produce press-hardened sheet steel components, having a forming tool to form a component blank and having a warming device to anneal the component blank, which is brought from the forming tool into the warming device, wherein the warming device connects directly to the forming tool such that the component blank is brought directly from the forming tool to the warming device, preventing a cooling of the component blank to less than the martensite finish temperature, preferably to less than 200° Celsius;

FIG. 2 is a schematic time-temperature course of the component blank in the scope of the implementation of the method according to a first embodiment;

FIG. 3 is a schematic depiction of the system according to a second embodiment;

FIG. 4 is a schematic depiction of the system according to a third embodiment; and

FIG. 5 is a schematic time-temperature course of the component blank in the scope of the implementation of the method according to its second embodiment.

DETAILED DESCRIPTION OF THE DRAWINGS

In the Figures, identical or functionally identical elements are provided with the same reference numeral.
FIG. 1 shows, in a schematic depiction, the sequence of a method to produce a press-hardened sheet steel component in the form of a sheet component made from a component blank which is formed from a hot-formable steel material. The component blank is also referred to as a semi-finished product. For reasons of clarity, the method is described by means of the production of a sheet steel component or sheet component made from a component blank. The method is, however, readily particularly well suited for the mass production of such press-hardened sheet steel components.
To implement the method, a system referred to in FIG. 1 as a whole with 10 is provided. By means of the system 10, the component blank undergoes the method, wherein the component blank is heated and cooled in the course of the method. This heating and cooling is particularly well recognizable from FIG. 2. FIG. 2 shows a diagram 12 in which a time-temperature course of the component blank is recorded. The time t is laid out on the abscissa 16 of the diagram 12, wherein the temperature is laid out on the ordinate 18 of the diagram 16. It is therefore recognizable by means of the time-temperature course at which temperature the component blank is heated or cooled respectively and how long the component blank is held, if necessary, at a respective temperature.
As is recognizable from FIG. 1, the system 10 includes a heating device 20, for example in the form of a furnace, in particular a rolling hearth furnace, wherein the component blank is brought into the heating device 20 and if necessary is conveyed through this. As is recognizable when viewed together with FIG. 2, in a first step S1 of the method, the component blank is heated at least to, preferably above, the austenitising temperature of the steel material from which the component blank is formed by means of the heating device 20. In other words, the heating device 20 serves to austenitise the component blank in the first step S1.
The component blank can, for example, be present in the form of a plate.
As is recognizable from FIG. 2, the component blank is heated by means of the heating device 20 to a temperature above 900° Celsius, wherein in FIG. 2, the temperature of 900° Celsius is marked by means of a dashed line 22.
The system 10 furthermore comprises a forming tool 24 which, for example, is integrated into a hydraulic press. The heated component blank is brought, in particular conveyed, from the heating device 20 or from this to the forming tool 24 or into this, and is hot formed by means of the forming tool 24 in a second step S2 of the method. In the course of this hot forming and in the subsequent maintenance phase, the hot-formed component blank is cooled in the forming tool 24, however not cooled below 200° Celsius (step S3). It can therefore be provided that the component blank is cooled to a temperature between 200° Celsius and 500° Celsius. In other words, the component blank is cooled in such a way that the component temperature is not less than 200° Celsius and not more than 500° Celsius after the forming.
The system 10 additionally comprises a further heating device in the form of a warming device 26 which can be formed as a furnace. The heating device 20 and/or the warming device 26 can be a rolling hearth furnace, a lifting beam furnace, a chain conveyor furnace or a rotary hearth furnace. However, the use of other heating devices is also conceivable. For example, other possibilities such as contact plate heating, heating by radiant heaters, inductive heating, conductive heating, infrared heating are likewise possible in order to heat up or to heat the component blank. In particular in the case of the use of furnaces, the warming device 26 can be heated by exhaust heat of the heating device 20.
After the cooling of the component blank in the forming tool 24 (step S3), the component blank is brought from the forming tool 24 to or into the warming device 26 (step 24). The warming device 26 therein connects directly to the forming tool 24 such that the component blank is brought directly from the forming tool 24 to or into the warming device 26, preventing a cooling of the component blank to less than 200° Celsius. This transfer can thereby preferably be carried out by multi-axle industry robots or feeder systems.
The warming device 26 can in particular be designed as a continuous furnace such that the component blank is conveyed through the warming device 26. The component blank is annealed by means of the warming device 26 in a fifth step S5 of the method, stabilizing the austenite in the structure of the component blank. In order to prevent intake of atomic hydrogen, the dew point in the heating device and in the warming device is preferable controlled and adjusted to values smaller than 5° C. Preferably to values smaller than −5° C.
As is by the warming device from FIG. 2, the component blank is heated again slightly in the scope of the annealing from the temperature to which the component blank was cooled in the third step S3. Presently, the component blank is cooled to 250° Celsius, wherein it is heated in the fifth step S5 to more than 200° Celsius and less than 500° Celsius and is held in this temperature range for a time period between 2 and 15 minutes.
After the annealing, the component blank is brought from the warming device 26 to or into a cutting device 28 of the system 10, wherein the component blank, in particular in the form of a plate, is cut by means of the cutting device 28 and is cooled in this to room temperature (sixth step S6). Finally, the component blank is brought from the cutting device 28 in the scope of a chain link to a final trimming device 30 and is finally trimmed and cleaned by means of this in a seventh step S7 of the method.
The time-temperature course 14 illustrates the method according to a first exemplary embodiment, wherein other temperatures can also be adjustable.
Therefore, direct or indirect press-hardening of the component blank is able to be depicted by means of the method, which is preferably formed from a boron-manganese steel. The component blank can therein be uncoated or coated. Preferably the component blank is aluminized or galvanized. The sheet thickness of the component blank can lie in a range from 0.5 millimetres to 3 millimetres inclusive. Press-hardened sheet steel components having a high elongation at break and therefore ductility as well as at the same time having a very high strength are able to be produced in a cost-effective as well as time-effective manner.
FIG. 3 shows the system 10 according to a second embodiment. A so-called passage direction of the component blank is illustrated in FIG. 3 by directional arrows, in which the component blank passes through the system 10.
The system 10 according to the second exemplary embodiment includes a receiving roller bed 32 arranged in the passage direction before the heating device 20 for austenitising, by means of which receiving roller bed 32 the component blank is conveyed into the heating device 20. An emitting roller bed 34 connects to the heating device 20, by means of which emitting roller bed 34 the component blank is conveyed from the heating device 20. The receiving roller bed 32 and the emitting roller bed 34 can therein be components of the heating device 20.
Furthermore, the system 10 according to the second exemplary embodiment includes a receiving roller bed 36, by means of which the component blank is conveyed into the warming device 26 after the cooling, i.e. after the forming tool 24. A further emitting roller bed 36 connects to the warming device 26, by means of which the component blank is conveyed from the warming device 26 after the annealing. The receiving roller bed 36 and the emitting roller bed 38 can also be components of the warming device 26. Therefore, the component blank can be brought from the forming tool 24 into the warming device 26 for austenite stabilization directly after the cooling and without being cooled to less than 200° Celsius.
FIG. 4 shows the system 10 according to a third embodiment in which further presses 40, 42, 44 are connected to the emitting roller bed 38 to process the component blank. The component blank is, for example, perforated by means of the press 40. The component blank is trimmed by means of the press 42, wherein the component bank is trimmed a further time by means of the press 44.
FIG. 5 shows the time-temperature course 14 for the method according to a second exemplary embodiment. In the first step S1, the component blank is heated to the austenitising temperature referred to with A with a heating rate T_target1for the austenitising and is kept at the austenitising temperature A during an austenitising time B. A cooling rate is referred to with T_target2with which the component is cooled during and/or after its forming, i.e. during the second step S2 and/or during the third step S3, wherein a critical cooling rate for the development of martensite is marked with T_crit-MS.
After the forming and after the cooling, the component blank is kept in the forming tool 24 at a maintenance temperature C during a maintenance time D1. Subsequently, the component blank is transferred from the forming tool 24 into the warming device 26, wherein this transfer lasts for a transfer time D2. During this transfer time D2 it is prevented that the component blank cools to less than 200° Celsius.
The component blank is heated to an annealing temperature E in the warming device 26 with a heating rate T_target3and is kept at the annealing temperature E during an annealing time F. After the annealing, the component blank is cooled with a cooling rate T_target4, for example to room temperature.

Claims

1-6. (canceled)

7. A method for producing a press-hardened sheet steel component, comprising the steps of:

a) heating of a component blank formed from a hot-formable steel material at least to an austenitising temperature of the steel material by a heating device;

b) hot forming of the component blank by a forming tool;

c) cooling of the component blank in the forming tool to a temperature above a material-specific martensite finish temperature;

d) bringing the component blank from the forming tool to a warming device; and

e) annealing of the component blank by the warming device which stabilizes an austenite in the component blank;

wherein the component blank is brought directly from the forming tool to the warming device preventing a cooling of the component blank to less than the material-specific martensite finish temperature.

8. The method according to claim 7, wherein the component blank is quenched in step e) in the forming tool to a temperature in a range from 200° C. to 500° C. inclusive.

9. The method according to claim 7, wherein the component blank is heated in step a) to a temperature between 800° C. and 1000° C.

10. The method according to claim 7, wherein the steel material is an alloy having:

0.2 to 0.5% by weight carbon;

0.5 to 2.9% by weight silicon;

0.7 to 4.1% by weight manganese;

up to 0.1% by weight phosphorous;

up to 0.1% by weight sulphur;

0.0001 to 0.5% by weight aluminum;

0.1 to 1.5% by weight chromium;

0.001 to 0.2% by weight titanium;

0.001 to 0.03% by weight boron; and

up to 0.025% by weight nitrogen.

11. The method according to claim 7, wherein the warming device heats the component blank in step e) relative to a temperature to which the component blank is cooled in step c).

12. A system for producing a press-hardened sheet steel component, comprising:

a heating device, wherein the heating device heats a component blank formed from a hot-formable steel material at least to an austenitising temperature of the steel material;

a forming tool, wherein the component blank is hot formed by the forming tool after the heating device heats the component blank and wherein the forming tool cools the component blank to a temperature not below 200° C. in the forming tool after the component blank is hot formed; and

a warming device, wherein the component blank is annealed by the warming device to stabilize an austenite in the component blank after the forming tool cools the component blank;

wherein the warming device is connected directly to the forming tool such that the component blank is transferable directly from the forming tool to the warming device to prevent a cooling of the component blank to less than 200° C.