EP3924528A1

EP3924528A1 - Method for producing a sheet steel component

Info

Publication number: EP3924528A1
Application number: EP20705035.2A
Authority: EP
Inventors: David Pieronek; Roger Bernd Roßmann
Original assignee: ThyssenKrupp Steel Europe AG
Current assignee: ThyssenKrupp Steel Europe AG
Priority date: 2019-02-13
Filing date: 2020-02-11
Publication date: 2021-12-22
Also published as: WO2020165116A1; DE102019201883A1; CN113439126A

Abstract

The invention relates to a method for producing a sheet steel component. The invention further relates to the use of the sheet steel component.

Description

Method for producing a sheet steel component

The invention relates to a method for producing a sheet steel component. The invention also relates to a use of the sheet steel component.

The production of sheet steel components by means of hot forming has already established itself industrially, in particular for the production of body parts such as for example for the production of safety-relevant B-pillars, etc. Sheet steel components can be produced in direct as well as in indirect hot forming processes. Flat blanks (direct) or pre-formed or near-net-shape (cold) formed semi-finished products (indirect) made from a steel workpiece, in particular from a hardenable steel workpiece, are heated to a temperature at which, depending on the composition of the steel workpiece used, a structural transformation occurs within of the steel workpiece occurs. With Acl, the microstructure transformation into austenite begins and when Ac3 or above is reached, an essentially completely austenitic structure is present. The heating is also called “austenitizing” in specialist circles, especially when a complete transformation into austenite is to take place. After the heating, the warm (austenitized) steel workpiece is placed in a forming tool and hot formed. In the course of or after the hot forming, the steel workpiece is cooled in this way, preferably within the forming tool, which is preferably actively cooled, so that the structure is converted into a hard structure made of martensite and / or bainite, preferably essentially made of martensite. In specialist circles, the cooling or quenching of the steel workpiece within the forming tool or by the action of a (hardening) tool, which has the final contour of the sheet metal component to be produced, is also called "press hardening". Alternatively, cooling / quenching can take place outside a forming tool / hardening tool, in particular in a (cold) medium, for example in an oil bath, and is referred to as “hardening”. Heating and cooling curves for setting the required microstructure are dependent on the chemical composition of the hardenable steel workpiece used and can be seen or derived from so-called ZTA or ZTU diagrams. The setting of an essentially martensitic structure with high strengths is possible by means of hot forming.

It is known from the prior art that an aluminum-based coating, preferably an aluminum-silicon coating, is suitable for direct hot forming in order to ensure passive corrosion protection and to prevent scaling of the steelworks. pieces to avoid. This eliminates the need to use protective gas during austenitization. In indirect hot forming, steel workpieces with a zinc-based coating and uncoated steel workpieces are used.

The production of body parts by means of direct hot forming and the use of aluminum (AI) -silicon (Si) -coated steel workpieces is known from the prior art, especially when using steel workpieces with a thickness of up to 3 mm, cf. EP 2 086 755 Bl, EP 2 242 863 Bl. An AISi coating is not suitable for the production of sheet steel components which are used in cyclically loaded areas of a vehicle, in particular as part of a chassis component, since the brittle silicon components in the coating on the surface of the sheet steel component are negative in the case of cyclical loading affect, cf. Figure 4 (sample 11).

Sheet steel components can alternatively be produced from uncoated steel workpieces by means of hot forging, whereby the risk of scaling of the surface of the steel workpiece during austenitization is higher than with coated steel workpieces, so that uncoated steel workpieces are austenitized in a protective gas atmosphere, for example in a furnace flooded with protective gas , the protective gas atmosphere inside the furnace is an inert gas. The protective gas atmosphere is conventionally based on nitrogen with a nitrogen content of almost 100%, which can prevent oxidation (scaling) on the surface of the steel workpiece. However, the use of a nitrogen-based protective gas leads to an effusion of carbon during austenitization on the surface of a steel workpiece. In professional circles, this phenomenon is known as edge decarburization, whereby the decrease in the carbon content (decarburization) correlates with a decrease in hardness on the surface of the steel workpiece, especially in the case of directly or indirectly hot-formed and / or press-hardened and / or hardened, uncoated sheet steel parts. For the production of sheet steel components that are used in cyclically loaded areas of a vehicle, in particular as part of a chassis component, excessive edge decarburization has a negative effect on the fatigue strength, cf. Figure 4 (sample 15).

The object is therefore to provide a method which allows a sheet steel component to be produced in such a way that the resulting sheet steel component has an improved property compared to the prior art. The object is achieved with a method for producing a sheet steel component with the characteristics of claim 1 and with a use with the features of claim 13.

To produce a sheet steel component, the method according to the invention comprises the following steps: - providing a hardenable, uncoated steel workpiece, the steel workpiece having a thickness of 3.0 to 15.0 mm, - at least partial austenitizing of the steel workpiece at a temperature of at least Ac3, - Feeding the at least partially austenitized steel workpiece to a unit for performing one of steps i) to v), steps i) to v) comprising: i) hot forming and press hardening or ii) hot forming and subsequent press hardening or iii) hot forming and subsequent Hardening or iv) press hardening or v) hardening of the at least partially austenitized steel workpiece for the production of a sheet steel component.

The temperature control unit comprises in particular a furnace through which the provided steel workpiece can be carried out and / or in which the provided steel workpiece can be received, which is flooded with an in particular regulated protective gas and is designed, for example, in the form of a roller hearth furnace, chamber furnace, etc. , or alternatively operated as an oven under vacuum. In particular, when operating a furnace with in particular special regulated protective gas, atmospheres can be set within the furnace, which u. a. substantially avoid oxidation of the uncoated steel workpiece. In particular, nitrogen-based protective gas atmospheres are used, preferably with up to 100% nitrogen and unavoidable impurities.

“Hardenable” is to be understood as meaning that the steel workpiece as a result of targeted heat treatment, as is carried out in direct and indirect hot forming for the production of a sheet steel component, the sheet steel component at least partially (partially / locally) has a higher hardness than the steel workpiece provided.

The steel workpiece can be a flat sheet steel workpiece or preferably a preformed sheet steel workpiece with a thickness of up to 15.0 mm, in particular of up to 14.0 mm, preferably of up to 12.0 mm. The steel workpiece has a thickness of at least 3.0 mm, in particular of at least 3.50 mm, preferably of at least 4.0 mm. The sheet steel workpiece is in particular made from a (hot) rolled flat steel product. tailored to speak. Depending on the thickness, the flat steel product is a heavy plate or a hot-rolled, uncoated strip (hot strip).

The steel workpiece is austenitized at least partially or only in one or more areas of the steel workpiece at a temperature of at least Ac3. Depending on the requirements, the steel workpiece can also be completely austenitized at a temperature of at least Ac3.

According to the invention, austenitizing is carried out at a temperature between 840 and 940 ° C in a temperature control unit, the steel work piece staying in the temperature control unit as follows, depending on the thickness of the steel workpiece:

al) with a thickness of 3.0 to <5.0 mm for a duration between 180 and 480 s; bl) with a thickness of 5.0 to <7.0 mm for a duration between 210 and 640 s; cl) with a thickness of 7.0 to <9.0 mm for a duration between 240 and 690 s; dl) with a thickness of 9.0 to <11.0 mm for a duration between 300 and 800 s; el) with a thickness of 11.0 to 15.0 mm for a duration between 360 and 1200 s.

The dwell times and temperatures in the temperature control unit are coordinated with one another as a function of the thickness of the steel workpiece used in such a way that a structural conversion, at least partially, preferably completely in austenite, can be ensured over the thickness of the steel workpiece in order to meet the requirements of the sheet steel component to be produced changed properties, in particular a desired hardness, to be able to set. The dwell times are limited as a function of the thickness to the extent that edge decarburization is limited to a minimum, which would otherwise have a negative effect on the use of such manufactured sheet steel components, in particular for cyclically loaded load cases. Furthermore, in addition to the aforementioned negative effect, too long dwell times would also lead to short cycle times, which in turn can have a negative effect on profitability. The minimum level of edge decarburization can, for example, be with a degree of decarburization of up to -20%, in particular up to -15%, preferably up to -10% of the carbon content of the steel workpiece provided (carbon content in the initial state = actual carbon content) on the surface of the steel workpiece or on the surface of the sheet steel component. In order to determine the extent to which edge decarburization has taken place, the carbon content on the surface can be measured before and after the heat treatment, for example using GDOES “Glow Discharge Optical Emission Spectroscopy” or other known methods for determining the carbon content the surface or in the area of a workpiece close to the surface. Since a decrease in carbon (edge decarburization) correlates with a decrease in hardness, the minimum level of hardness of up to -10%, in particular up to -7%, preferably up to -5%, preferably up to -3 % of the hardness on the surface of the sheet steel part, i.e. on the surface after heat treatment or after hardening or press hardening, compared to the hardness, in relation to the remaining area based on the thickness of the sheet steel component, the hardness, in particular via the component cross section , according to Vickers HV0.1 can be determined in accordance with DIN EN ISO 6507.

The surface or near-surface area is an area starting from the surface of the steel workpiece or sheet steel component to a depth of a maximum of 100 μm, in particular a maximum of 70 μm, preferably a maximum of 50 μm, preferably a maximum of 30 μm, particularly preferably a maximum 20 pm, understand.

The at least partially austenitized steel workpiece becomes a unit for performing one of steps i) to v), steps i) to v) being defined as follows.

Direct hot forming takes into account the use of a flat steel workpiece, which, after at least partial austenitization, is either i) hot formed and press hardened in a tool by means of hot forming and press hardening to form a sheet steel part with an at least partially hard structure, or ii) hot in a hot forming tool using hot forming formed and then press hardened in a hardening tool with at least partially hard structure, or iii) hot formed in a hot forming tool by means of hot forming and then hardened in a medium, in air or in a liquid medium, with at least partially hard structure.

Indirect hot forming takes into account a steel workpiece that has already been cold pre-formed or has been formed close to the final dimensions, which after at least partial austenitization is either iv) press-hardened in a hardening tool with at least partially hard structure or v) in a medium, in air or in a liquid medium is hardened at least partially hard structure. Optionally, the hardening process can be preceded by additional hot forming.

A certain degree of deformation in the hardening tool, cf. ii) and iv), is permitted and is minimal in comparison to the degree of deformation during hot forming or in relation to the deformation degree for creating the preform or near-net-shape geometry and can essentially correspond to a calibration step for producing the final geometry.

Further advantageous refinements and developments can be found in the description below. One or more features from the claims, the description and also the drawing can be combined with one or more other features from them to form further embodiments of the invention. One or more features from the independent claims can also be linked by one or more other features.

In order to austenitize the steel workpiece at least partially, in particular completely, to a temperature of at least Ac3, the steel workpiece is heated at an average heating rate between 0.5 and 7.5 K / s according to one embodiment of the method according to the invention. To determine the average heating rate, the temperature intervals from 100 ° C to 750 ° C, 825 ° C and 900 ° C are considered. Here, a linear gradient in K / s is determined, which is present on average. Since there are different temperatures during heating and the chemical composition of the steel workpieces in particular also have certain influences, a slope range is shown, which is specified as follows depending on the thickness of the steel workpiece:

a2) with a thickness of 3.0 to <5.0 mm:

100 ° C -> 750 ° C an average heating rate of 2 K / s to 7.5 K / s,

100 ° C -> 825 ° C an average heating rate of 1.75 K / s to 5.25 K / s,

100 ° C -> 900 ° C an average heating rate of 1.25 K / s to 3.5 K / s;

b2) with a thickness of 5.0 to <7.0 mm:

100 ° C -> 750 ° C an average heating rate of 2 K / s to 5.5 K / s,

100 ° C -> 825 ° C an average heating rate of 1.75 K / s to 4.5 K / s,

100 ° C -> 900 ° C an average heating rate of 1 K / s to 3 K / s;

c2) for a thickness of 7.0 to <9.0 mm:

100 ° C -> 750 ° C an average heating rate of 1.75 K / s to 4.25 K / s

100 ° C -> 825 ° C an average heating rate of 1.5 K / s to 3.75 K / s

100 ° C -> 900 ° C an average heating rate of 1 K / s to 2.5 K / s

d2) for a thickness of 9.0 to <11.0 mm:

100 ° C -> 750 ° C an average heating rate of 1.5 K / s to 3.75 K / s,

100 ° C -> 825 ° C an average heating rate of 1.25 K / s to 3 K / s,

100 ° C -> 900 ° C an average heating rate of 1 K / s to 2 K / s; e2) with a thickness of 11.0 to 15.0 mm:

100 ° C -> 750 ° C an average heating rate of 1 K / s to 2.5 K / s,

100 ° C -> 825 ° C an average heating rate of 0.75 K / s to 1.75 K / s,

100 ° C -> 900 ° C an average heating rate of 0.5 K / s to 1 K / s.

According to one embodiment of the method according to the invention, the at least partially austenitized steel workpiece is removed from the temperature control unit and fed to the unit for performing one of steps i) to v) within a period of between 3 and 20 s. This can ensure that the at least thin-walled (3 to <5mm), at least partially austenitized steel workpieces do not cool too much and still at a sufficient temperature, preferably above the Ms temperature, of the unit to carry out one of steps i) to v) can be supplied so that the process can be carried out reliably and the required properties can be achieved in the component to be produced.

According to one embodiment of the method according to the invention, the at least partially austenitized steel workpiece does not come into contact with the ambient atmosphere while it is removed from the temperature control unit and fed to the unit for performing one of steps i) to v), in particular the at least partially austenitized steel workpiece below Protective gas atmosphere is removed and supplied. If there is no contact between the at least partially austenitized, in particular completely austenitized, steel workpiece and the surroundings, scaling can occur during the transfer between the two units, in particular before it is inserted into the tool for hot forming and / or press hardening, cf. i), ii) and iv), in order not to negatively influence the service life of the tool, for example.

According to one embodiment of the method according to the invention, the at least partially austenitized steel workpiece is cooled in the unit for performing one of steps i) to v) to a temperature below Ms. This ensures that, depending on the chemical composition of the steel workpiece used, the corresponding Martensite start (Ms) temperature is fallen below in order to force the formation of a hard structure of full austenite in martensite. The conversion to martensite is completed when the martensite finish (Mf) temperature is reached or not reached. According to one embodiment of the method according to the invention, the at least partially austenitized steel workpiece is cooled at an average cooling rate between 5 and 60 K / s. To determine the average cooling rate, the temperature intervals from 750 ° C to 300 ° C, 200 ° C and 175 ° C are considered. Here, a linear decrease in K / s is determined, which is present on average. Since there are different temperatures during cooling and the chemical composition of the steel workpieces also have certain influences, an acceptance range is shown, which is specified as follows depending on the thickness of the steel workpiece:

a3) with a thickness of 3.0 to <5.0 mm:

750 ° C -> 300 ° C an average cooling rate of 20 K / s to 60 K / s,

750 ° C -> 200 ° C an average cooling rate of 17.5 K / s to 55 K / s,

750 ° C -> 175 ° C an average cooling rate of 15 K / s to 45 K / s;

b3) with a thickness of 5.0 to <7.0 mm:

750 ° C -> 300 ° C an average cooling rate of 20 K / s to 60 K / s,

750 ° C -> 200 ° C an average cooling rate of 17.5 K / s to 50 K / s,

750 ° C -> 175 ° C an average cooling rate of 15 K / s to 40 K / s;

c3) for a thickness of 7.0 to <9.0 mm:

750 ° C -> 300 ° C an average cooling rate of 20 K / s to 45 K / s,

750 ° C -> 200 ° C an average cooling rate of 15 K / s to 30 K / s,

750 ° C -> 175 ° C an average cooling rate of 10 K / s to 22.5 K / s;

d3) for a thickness of 9.0 to <11.0 mm:

750 ° C -> 300 ° C an average cooling rate of 15 K / s to 30 K / s,

750 ° C -> 200 ° C an average cooling rate of 12.5 K / s to 25 K / s,

750 ° C -> 175 ° C an average cooling rate of 7.5 K / s to 17.5 K / s;

e3) with a thickness of 11.0 to 15.0 mm:

750 ° C -> 300 ° C an average cooling rate of 7.5 K / s to 20 K / s,

750 ° C -> 200 ° C an average cooling rate of 5 K / s to 15 K / s,

750 ° C -> 175 ° C an average cooling rate of 5 K / s to 12.5 K / s.

In order to set the desired property in the sheet steel component, a hard structure is set in the sheet steel component, which over the thickness of the sheet steel component (component cross-section) comprises at least 70% martensite and / or bainite, in particular at least 80% martensite and / or bainite , preferably at least 90% by area comprising martensite and / or bainite, it being possible for remaining structural constituents to be present in the form of ferrite, pearlite, cementite, austenite and / or retained austenite. Loading Preferably, a hard structure is set over the thickness of the sheet steel component (component cross-section) with at least 70 area% martensite, in particular at least 80 area% martensite, preferably at least 90 area% martensite, with the remaining structural components in the form of ferrite, pearlite, bainite , Cementite, austenite, retained austenite may be present. The specified structural components are determined by evaluating light or electron microscopic examinations and are therefore to be understood as area proportions in area%. An exception to this is the structural component austenite or retained austenite, which is specified as a volume percentage in% by volume.

According to one embodiment of the method according to the invention, the at least partially austenitized steel workpiece is cooled in the unit for performing one of steps i) to v) to a temperature between 100 and 300 ° C, the steel workpiece as depending on the thickness of the steel workpiece follows dwell in the unit: a4) with a thickness of 3.0 to <5.0 mm for a period of up to 50 s;

b4) with a thickness of 5.0 to <7.0 mm for a duration of up to 60 s;

c4) with a thickness of 7.0 to <9.0 mm for a duration of up to 80 s;

d4) with a thickness of 9.0 to <11.0 mm for a duration of up to 120 s;

e4) with a thickness of 11.0 to 15.0 mm for a duration of up to 150 s.

Depending on the thickness of the steel workpiece, it can be ensured that the dwell times are matched to the temperatures in such a way that, in particular, complete conversion from austenite to martensite is possible. Longer dwell times would lead to a shorter cycle time, which in turn would have a negative effect on profitability. The residence time is at least 5 s, in particular at least 10 s, preferably at least 15 s, preferably at least 20 s.

According to one embodiment of the method according to the invention, the at least partially austenitized steel workpiece, in particular completely austenitized steel workpiece, is partially hardened or partially press-hardened. Partial hardening or partial press hardening has the advantage that different properties, for example hard and soft zones, can be set partially and in particular simultaneously in the sheet steel component depending on the requirement and / or intended use. Alternatively, complete hardening or press hardening is possible.

According to one embodiment of the method according to the invention, the sheet steel component is fed to at least one blasting step. The at least one beam step can Surface or edge hardness can be increased at least partially, in particular completely on the sheet steel part by mechanical action, since the entry of compressive stresses into the surface, locally or globally, on the sheet steel component, can cause strain hardening on the surface or in the area near the surface. In the event that the surface of the sheet steel component nevertheless scales, the surface can be descaled at the same time by the blasting step and / or process-related impurities can be removed. All granular abrasives are suitable as blasting material, for example steel balls, steel scrap, blast furnace slag, sand, corundum, glass, etc., which can solidify or compact the surface. The increase in hardness on the surface or in the area close to the surface can advantageously counteract the formation of cracks. Preferably, alternatively or additionally, the surface of the sheet steel component is conditioned by the at least one blasting step in such a way that the surface has a roughness of less than Ra = 2.5 miti, in particular less than Ra = 2.0 miti, preferably less than Ra = 1.75 miti, preferably smaller Ra = 1.5, miti. Such a roughness can have a positive effect on the surface appearance, especially after painting, as well as the service life under cyclical stress.

According to one embodiment of the method according to the invention, the sheet steel component is fed to at least one painting step. In particular, the sheet steel component is coated in the at least one painting step by means of a paint layer in the KTL process.

According to one embodiment of the method according to the invention, the sheet steel component is tempered, in particular at a temperature between 150 and 300 ° C. and a dwell time between 5 and 200 minutes. As a result of a tempering step, the sheet steel component slightly loses its hardness set by the preceding hardening or press hardening, but it can lead to an increase in the yield point of the hardened sheet steel component and / or to a reduction in crack sensitivity. In particular, the tempering can also take place in the course of a lacquer layer preferably applied beforehand, for example in the KTL process, by baking the lacquer layer.

According to one embodiment of the method according to the invention, a steel workpiece with the following chemical composition in% by weight is provided:

C = 0.08 to 0.60; in particular 0.1 to 0.5; preferably 0.1 to 0.3;

Si = 0.05 to 0.80; in particular 0.1 to 0.5; preferably 0.1 to 0.35;

Mn = 0.1 to 2.2; in particular 0.3 to 1.8; preferably 0.8 to 1.6; P to 0.1; in particular up to 0.05; preferably up to 0.03;

s to 0.1; in particular up to 0.05; preferably to 0.01;

N to 0.1; in particular up to 0.01; preferably up to 0.001;

and optionally one or more of the following elements:

AI up to 0.5; in particular 0.005 to 0.2; preferably 0.01 to 0.15;

Cr to 1.0; in particular 0.01 to 0.5; preferably 0.08 to 0.35;

Cu to 0.5; in particular up to 0.3; preferably up to 0.15;

Mo to 0.3; in particular up to 0.15; preferably to 0.07

Ni to 0.3; in particular up to 0.2; preferably up to 0.14;

Nb to 0.2; in particular up to 0.1; preferably up to 0.035;

Ti up to 0.2; in particular up to 0.1; preferably to 0.05; preferably 0.001 to

0.05;

V to 0.2; in particular up to 0.1; preferably up to 0.012;

B to 0.01; in particular 0.0005 to 0.008; preferably 0.001 to 0.005;

Sn to 0.1; in particular up to 0.07; preferably up to 0.05;

Remainder Fe and unavoidable impurities. Steel workpieces with a chemical composition within the specified limits can be hardened and are particularly suitable for hot forming and / or hardening or press hardening, cf. i) to v). Furthermore, they can be mass-produced inexpensively and with their potential, particularly in automobile construction, they can cover a broad spectrum.

The sheet steel components produced according to the invention have improved properties compared to the sheet steel components known from the prior art, in particular due to reduced (more) edge decarburization they have improved fatigue strength, as studies have shown. Such a sheet steel component produced according to the invention is outstandingly suitable as a chassis component or as a part thereof. Chassis parts are exposed to enormous cyclical loads during operation. Sheet steel components produced according to the invention can absorb these cyclical stresses or vibrations better than conventional sheet steel components, without premature component failure.

According to a preferred embodiment, sheet steel components produced according to the invention are used as a wheel or as part of the wheel, in particular as a wheel disc, as a wishbone or as part of the wishbone, as a torsion arm or as part of the torsion arm. In the following, specific embodiments of the invention are explained in more detail with reference to the drawing. The drawing and accompanying description of the resulting features are not to be read in a restrictive manner to the respective configurations, but serve to illustrate exemplary configurations. Furthermore, the respective features can be used with each other as well as with features of the above description for possible further developments and improvements of the invention, especially in the case of additional configurations which are not shown.

The drawing shows in

Figure 1) a schematic flow chart of an embodiment of the invention

Procedure, in

Figure 2) depending on the thickness of the steel workpieces used, different time-temperature ranges for austenitizing the steel workpieces, in Figure 3) depending on the thickness of the steel workpieces used, different time-temperature ranges for cooling the austenitized steel workpieces and in

Fig. 4) an illustration of a fatigue strength test according to Wöhler on different sheet steel components.

In Figure 1, a schematic flow chart of an embodiment of the method according to the invention is shown. In a first step (I) a hardenable, uncoated steel workpiece is provided. The steel workpiece is preferably a preformed or near-net-shape sheet steel workpiece with a thickness between 3.0 and 15.0 mm.

In a second step (II), the steel workpiece is at least partially, preferably completely, austenitized at a temperature of at least Ac3 in a temperature control unit. The temperature control unit can be designed, for example, as a continuous furnace with an in particular regulated protective gas atmosphere. The protective gas atmosphere is based on nitrogen, for example, preferably it consists of nitrogen and unavoidable impurities. Depending on the thickness of the steel workpieces used, the steel workpieces are austenitized in the following time-temperature ranges: - with a thickness of 3.0 to <5 , 0 mm, at 840 ° C for a period between 300 and 480 s, at 900 ° C for a period between 220 and 420 s, at 940 ° C for a period between 180 and 360 s; - with a thickness of 5.0 to <7.0 mm, at 840 ° C for a duration between 300 and 640 s, at 900 ° C for a duration between 240 and 480 s, at 940 ° C for a duration between 210 and 390 s; - at a Thickness from 7.0 to <9.0 mm, at 840 ° C for a period between 390 and 690 s, at 900 ° C for a period between 300 and 540 s, at 940 ° C for a period between 240 and 450 s ; - at a thickness of 9.0 to <11.0 mm, at 840 ° C for a duration between 420 and 800 s, at 900 ° C for a duration between 360 and 640 s, at 940 ° C for a duration between 300 and 560 s; - with a thickness of 11.0 to 15.0 mm, at 840 ° C for a period between 560 and 1200 s, at 900 ° C for a period between 420 and 900 s, at 940 ° C for a period between 360 and 720 s, cf. Fig. 2.

If the steel workpieces provided are austenitized in the areas defined in FIG. 2) by the time-temperature diagram as a function of the thickness of the steel workpiece, it can be ensured that edge decarburization is tolerated to a certain extent or that the properties on Sheet steel component, in particular with regard to vibration resistance on the surface, are not significantly negatively affected.

In the third step (III) the at least partially austenitized steel workpiece becomes a unit for performing one of steps i) to v), steps i) to v) comprising: i) hot forming and press hardening or ii) hot forming and subsequent press hardening or iii) hot forming and subsequent hardening or iv) press hardening or v) hardening of the at least partially austenitized steel workpiece for the production of a sheet steel component. If the steel workpiece provided is preferably a preformed or near-net-shape sheet steel workpiece, step iv) is preferably implemented, the at least partially austenitized steel workpiece being placed in a flattening tool and calibrating in the tool being permitted. The at least partially austenitized steel workpiece, for example, does not come into contact with the ambient atmosphere between the furnace outlet and the tool, the at least partially austenitized steel workpiece being fed to the (hardening) tool under a protective gas atmosphere.

The press-hardened sheet steel component is cooled to a temperature below Ms, preferably below Mf, in order to set a hard structure. Depending on the thickness of the steel workpieces used, the steel workpiece remains in the following time-temperature ranges: - at a thickness of 3.0 to <5.0 mm, at 300 ° C for a period between 5 and 30 s, at 200 ° C for a period between 10 and 40 s, at 100 ° C for a period between 15 and 50 s; - with a thickness of 5.0 to <7.0 mm, at 300 ° C for a period between 5 and 40 s, at 200 ° C for a period between 10 and 50 s, at 100 ° C for a period between 15 and 60 s; - at a thickness of 7.0 to <9.0 mm, at 300 ° C for a duration between see 10 and 50 s, at 200 ° C for a period between 15 and 65 s, at 100 ° C for a period between 20 and 80 s; - with a thickness of 9.0 to <11.0 mm, at 300 ° C for a period between 15 and 60 s, at 200 ° C for a period between 20 and 100 s, at 100 ° C for a period between 25 and 120 s; - with a thickness of 11.0 to 15.0 mm, at 300 ° C for a period between 20 and 70 s, at 200 ° C for a period between 25 and 120 s, at 100 ° C for a period between 30 and 150 s, cf. Fig. 3.

Depending on the requirements and / or use, different properties can be formed in the sheet steel component, so that only partial press hardening can take place. Alternatively, the sheet steel component can also be completely press-hardened and thus have a hard structure, in particular with an essentially martensitic structure, over the entire cross section.

To further increase the clarity on the surface or in the near-surface area, in particular when the sheet steel component is preferably used as a chassis component or part thereof, the sheet steel component is fed to at least one blasting step in a fourth step (IV), with a blasting agent applied locally or globally to the surface of the sheet steel component acts and this is cold-hardened or compressed. Sheet steel components blasted in this way are more resistant to cyclical loads and have higher resistance in terms of crack formation and crack propagation. The surface of the sheet steel component is preferably conditioned by the at least one blasting step in such a way that a roughness of less than Ra = 2.5 with i is established on the surface.

As a fifth step (V), the sheet steel component can be subjected to a tempering step, in particular at a temperature between 150 and 300 ° C. and a dwell time between 5 and 200 minutes. Alternatively, however, steps (V) and (IV) can also be exchanged so that the at least one tempering step can take place before the at least one blasting step.

An investigation with different parameters has been carried out. Uncoated steel workpieces in different thicknesses and with different compositions were provided. These were cut from corresponding flat products, for example from hot-rolled flat steel products. The first steel workpieces were made from a steel of the quality 24MnCr5-5. The thicknesses were 4 mm (sample 1), 6 mm (sample 2), 8 mm (sample 3), 10 mm (sample 4) and 12 mm (sample 5). More steel work pieces made of a steel of the quality 8MnCr3 with a thickness of 5 mm (sample 6), from one Steel of the quality 22MnB5 with a thickness of 3 mm (sample 7) and 5 mm (sample 8), made of a steel of the quality 27MnCrB5 with a thickness of 5 mm (sample 9) and of a steel of the quality 37MnB4 with a thickness of 6 mm (sample 10) were provided. In addition, an AlSi-coated steel of the quality 22MnB5 with a thickness of 3 mm (sample 11).

All steel workpieces were completely austenitized in a furnace with a nitrogen-based protective gas atmosphere with up to 5% by volume hydrogen and the remainder nitrogen and unavoidable impurities at a dew point temperature of <+ 5 ° C. All of the austenitized steel workpieces were then hot-formed and press-hardened in a hot-forming and press-hardening tool, step (i). Removal from the furnace and transfer to the mold took place in a normal atmosphere. The mean cooling rate was chosen in such a way that a martensitic structure has formed in all sheet steel components. The sheet steel components produced had a geometry as disclosed in the laid-open specification EP 3 115 767 A1, and is shown in particular in FIGS.

It was found that if the time-temperature ranges specified in FIG. 2) are adhered to depending on the thickness, a high level of fatigue strength compared to processing according to the prior art can be achieved in a cyclic load test according to Wöhler could, despite a tolerable edge decarburization, which was determined on the surface of the sheet steel components by means of GDEOS or by hardness testing according to Vickers HV0.1.

The dwell times or temperatures in the oven were varied, and it was found that with a dwell time and / or at temperatures which were outside the defined time-temperature range in FIG. 2), i.e. to the left of the defined range, over the thickness considered, no complete structural transformation into austenite could be ensured, which was subsequently determined in investigations, in particular on the basis of grinding over the thickness / cross-section and thus no hard structure or no maximum hardness could be generated over the thickness. If the dwell time and / or temperatures were in particular to the right of the defined area, edge decarburization with a degree of decarburization of in particular more than -20% of the provided carbon content of the steel workpiece used on the sheet steel component, in particular regardless of the composition, was observed. From the quality 22MnB5 with a thickness of 3 mm (sample 7) a peripheral decarburization with a degree of decarburization of -10% (sample 12), -15% (sample 13), -20% (sample 14) and -25% ( Sample 15) the carbon content of the provided Steel workpiece examined. In Fig. 4 is a representation of a Schwingstandsuntersu tion according to Wühler on the aforementioned samples. Samples 1 to 10 were austenitized depending on their thickness from the above-mentioned time-temperature ranges. For the sake of clarity, the results of samples 1 to 10 are shown in the dashed area. The aforementioned disadvantages can be seen in samples 11 and 15.

In a further investigation, some of the sheet steel components were (further) increased on the surface through mechanical action as a result of at least one blasting step, measured in HVO, 1 according to DIN EN ISO 6507, in particular one by at least 15%, preferably one by at least 20% higher cleared compared to the rest of the area (center or thickness of the sheet steel component). Steel balls were used for blasting.

Claims

Expectations

1. A method for producing a sheet steel component, comprising the following steps:

- Provision of a hardenable, uncoated steel workpiece, the steel workpiece having a thickness of 3.0 to 15.0 mm,

- At least partial austenitizing of the steel workpiece at a temperature of at least Ac3,

- Feeding the at least partially austenitized steel workpiece to a unit for carrying out one of steps i) to v), steps i) to v) comprising: i) hot forming and press hardening or ii) hot forming and subsequent press hardening or iii) hot forming and subsequent hardening or iv) press hardening or v) hardening of the at least partially austenitized steel workpiece to produce a sheet steel component,

characterized in that

Austenitizing is carried out at a temperature between 840 and 940 ° C in a temperature control unit, with the steel work piece staying in the temperature control unit as follows, depending on the thickness of the steel workpiece:

2. The method according to claim 1, wherein the steel workpiece is heated with an average heating rate between 0.5 and 7.5 K / s.

3. The method according to any one of the preceding claims, wherein the at least partially austenitized steel workpiece is removed from the temperature control unit and fed within a period of between 3 and 20 s to the unit for performing one of steps i) to v).

4. The method according to any one of the preceding claims, wherein the at least partially austenitized steel workpiece does not come into contact with the ambient atmosphere while it is removed from the temperature control unit and the unit for implementation one of steps i) to v) is supplied, in particular the at least partially austenitized steel workpiece is removed and supplied in a protective gas atmosphere.

5. The method according to any one of the preceding claims, wherein the at least partially austenitized steel workpiece is cooled in the unit for performing one of steps i) to v) to a temperature below Ms.

6. The method according to any one of the preceding claims, wherein the at least partially austenitized steel workpiece is cooled at an average cooling rate between 5 and 60 K / s.

7. The method according to any one of the preceding claims, wherein a cooling of the at least partially austenitized steel workpiece in the unit for carrying out one of steps i) to v) to a temperature between 100 and 300 ° C is carried out, depending on the thickness the steel workpiece remains in the unit as follows:

a4) with a thickness of 3.0 to <5.0 mm for a duration of up to 50 s;

b4) with a thickness of 5.0 to <7.0 mm for a duration of up to 60 s;

c4) with a thickness of 7.0 to <9.0 mm for a duration of up to 80 s;

d4) with a thickness of 9.0 to <11.0 mm for a duration of up to 120 s;

e4) with a thickness of 11.0 to 15.0 mm for a duration of up to 150 s.

8. The method according to any one of the preceding claims, wherein the at least partially austenized steel workpiece is partially hardened or partially press-hardened in the unit for performing one of steps i) to v).

9. The method according to any one of the preceding claims, wherein the sheet steel component is fed to at least one blasting step.

10. The method according to claim 9, wherein the upper surface of the sheet steel component is conditioned by the at least one blasting step in such a way that a roughness less than Ra = 2.5 miti is established on the surface.

11. The method according to any one of the preceding claims, wherein the sheet steel component is tempered, in particular at a temperature between 150 and 300 ° C and a dwell time between 5 and 200 min.

12. The method according to any one of the preceding claims, wherein a steel workpiece is provided with the following chemical composition in wt .-%:

C 0.08 to 0.60

Si 0.05 to 0.80

Mn 0.1 to 2.2,

P to 0.1,

S to 0.1,

N to 0.1,

and optionally one or more of the following elements:

AI up to 0.5,

Cr up to 1.0,

Cu up to 0.5,

Mon to 0.3,

Ni up to 0.3,

Nb up to 0.2,

Ti up to 0.2,

V to 0.2,

B to 0.01

Sn up to 0.1,

Remainder Fe and unavoidable impurities.

13. Use of a sheet steel component, produced according to one of the preceding claims, as a chassis component or as part thereof.

14. Use according to claim 13 as a wheel or as part of the wheel, in particular as a wheel disc, as a wishbone or as part of the wishbone, as a twist beam or as part of the twist beam.