Patent Application
20200017941
The invention relates to embodiments of a semifinished flat steel product comprising a first layer and at least one second layer fully and cohesively bonded to the first layer. The invention further relates to processes for producing a component from the semifinished flat steel product and to corresponding uses.
There is a search in the automobile industry for new solutions for reducing vehicle weight and accordingly for reducing fuel consumption. Lightweight construction is an essential tool here to be able to lower vehicle weight. One way of achieving this is the use of materials having increased strength. However, the bending capacity of materials generally decreases with rising strength. In order to ensure occupant protection, which is also required in the case of crash-relevant components, in spite of elevated strength for implementation of lightweight construction, it has to be ensured that the materials used can convert the energy introduced by a crash by deformation. This requires a high degree of shaping capacity, especially in the crash-relevant components of a vehicle structure or seat structure. One way of saving weight is to configure or to build, for example, the bodywork, frame, seat structure and/or chassis of a vehicle, especially in the case of electrical and/or hybrid vehicles, for example even the battery housing for accommodation of battery modules for the electrical drive, in even lighter form, by means of lightweight and innovative materials compared to the materials conventionally used. Conventional materials may thus be replaced by more lightweight materials having comparable properties in a component-specific manner. For example, there is ever greater use in the automotive industry of hybrid materials or material composites that are composed of two or more different materials, where each individual material has particular properties that are combined in the composite essentially to give antithetical properties, in order to achieve improved properties in the material composite compared to the individual monolithic materials. Material composites, especially made from different steel alloys, are known in the prior art, for example from German published specification DE 10 2008 022 709 A1.
Advantageous properties are possessed especially by steel alloys having a martensitic microstructure which, having high (tensile) strengths (Rm), are especially suitable for the production of cold-formed, crash-relevant components (component parts). Such steel alloys are sold by the applicant as martensite-phase steels under the “MS-W®” trade name, which, with the same properties, can be made thinner in terms of material thickness compared to conventional steel alloys, where the reduction in material thickness can have a positive effect on the weight of the component (component part) or total weight of the vehicle. Such steel alloys are therefore of excellent suitability for the automotive industry.
The potential, in terms of increasing strength in martensite-phase steels, has by no means been exhausted, and so, by means of appropriate alloy concepts or, alternatively or additionally, by means of optimization of the production route, there are possibilities of being able to achieve or to establish tensile strengths in martensite-phase steels of up to 2000 MPa or higher. However, steel alloys having an essentially martensitic microstructure (martensite-phase steels), owing to their chemical and physical properties, are only of limited coatability, especially with an anticorrosion coating. Since shaping capacity decreases with increasing strength, which is especially at the expense of bending angle, it is possible for microcracks/cracks to form in the surface or in the near-surface region of the steel material in the forming operation, depending on the geometry or complexity to be established, and these can lead in the worst case to premature component failure.
It is one object of the invention to provide embodiments of semifinished flat steel products having essentially improved properties which are easily coatable and especially have a lower tendency to crack, if any, in the forming operation and especially a higher bending angle, and also to specify processes for producing a component and a corresponding uses.
There follows a detailed explanation of embodiments of the invention with reference to a drawing that constitutes a working example. The FIGURE shows a schematic section through a semifinished flat steel product of the invention.
The inventor has found that it is possible, by the provision of at least a second layer of a soft steel alloy fully and cohesively bonded at least on one side to the first layer of a steel alloy having a martensitic microstructure and having a tensile strength of >1200 MPa and/or a hardness of >370 HV10. In some embodiments, the first layer of the steel alloy has a tensile strength of >1300 MPa and/or a hardness of >400 HV10, a tensile strength of >1400 MPa and/or a hardness of >435 HV10, a tensile strength of >1500 MPa and/or a hardness of >465 HV10, or even a tensile strength of >1600 MPa and/or a hardness of >490 HV10. Importantly, no direct contact with the first layer should be possible at least on one side when bonded to the soft steel alloy, such that the second layer of a soft steel alloy functions as a kind of functional layer. In the context of the invention, the soft steel alloy has a tensile strength of <600 MPa and/or a hardness of <190 HV10. In some embodiments, the soft steel alloy has a tensile strength of <550 MPa and/or a hardness of <175 HV10, a tensile strength of <450 MPa and/or a hardness of <140 HV10, or even a tensile strength of <380 MPa and/or a hardness of <120 HV10. The second layer, or the soft steel alloy, has properties that have a particularly positive effect with regard to coating and/or shaping capacity. The semifinished flat steel product of the invention can thus be integrated into existing standard processes, for example roll profiling, etc., without having to make changes in the process chain. The propensity to coating and/or shaping capacity is determined to a crucial degree by the properties at the surface of the semifinished flat steel product, which are determined in accordance with the invention by the second layer as functional layer.
The semifinished flat steel product may be executed, or provided to the further process steps, as a semifinished product in the form of strip, plate or sheet. According to embodiments of the invention, the semifinished flat steel product has at least two layers of different steel alloys.
The first layer of the semifinished flat steel product may include, as well as Fe and unavoidable preparation-related impurities, in % by weight, C: 0.15-0.6%, Si: 0.05-0.9%, Mn: 0.3-2.0%, Al: 0.01-2.0%, Cr+Mo: up to 1.5%, Nb+Ti: up to 0.2%, B: up to 0.02%, V: up to 0.25%, Cu: up to 0.2%, Ni: up to 0.3%, Sn: up to 0.05%, Ca: up to 0.01%, As: up to 0.02%, N: up to 0.01%, P: up to 0.06%, and/or S: up to 0.03%.
C is a strength-increasing alloy element and contributes to increasing strength with increasing content, and so a content of at least 0.15% by weight, and in some embodiments preferably of at least 0.2% by weight, is present in order to achieve or establish the desired strength. With higher strength, brittleness also increases, and so the content is limited to not more than 0.6% by weight. In some embodiments, it may be preferable for the C content to be limited to not more than 0.55% by weight, not more than 0.5% by weight, not more than 0.45% by weight, or even not more than 0.4% by weight, in order not to adversely affect the material properties and to ensure sufficient weldability.
Si is an alloy element that contributes to solid solution hardening and, according to its content, has a positive effect in an increase in strength, and so a content of at least 0.05% by weight is present. The alloy element is limited to not more than 0.9% by weight, and may even be limited to not more than 0.7% by weight, or not more than 0.5% by weight, in order to ensure sufficient rollability.
Mn is an alloy element that contributes to hardenability and has a positive effect on tensile strength, especially for binding of S to give MnS, and so a content of at least 0.3% by weight is present. The alloy element may be limited to not more than 2.0% by weight, and may be further limited to not more than 1.7% by weight, or even not more than 1.5% by weight, in order to ensure sufficient weldability.
Al as an alloy element contributes to deoxidation, where a content of at least 0.01% by weight, especially of 0.015% by weight, is present. In some embodiments, the alloy element is limited to not more than 2.0% by weight, not more than 1.0% by weight, not more than 0.5% by weight, or even not more than 0.1% by weight, in order to essentially reduce and/or to avoid precipitates in the material, especially in the form of nonmetallic oxidic inclusions, which can have an adverse effect on material properties. For example, the content of Al may be set between 0.02% and 0.06% by weight.
Cr as an alloy element, according to its content, can make a positive contribution to establishing strength, and especially a positive contribution to hardenability. The Cr content may be at least 0.05% by weight. The alloy element may be limited to not more than 1.5% by weight, not more than 1.2% by weight, or not more than 1.0% by weight, depending on the embodiment, in order to ensure adequate weldability.
B as an alloy element can contribute to hardenability, especially when N is bound, and is present with a content especially of at least 0.001% by weight. In embodiments, the alloy element is limited to not more than 0.02% by weight. In further embodiments, the alloy element may be limited to not more than 0.015% by weight, since higher contents have an adverse effect on material properties and would result in a reduction in hardness and/or strength in the material.
Ti and Nb as alloy elements may be included individually or in combination in the alloy for grain refining and/or N binding, especially when Ti is present with a content of at least 0.005% by weight. For complete binding of N, the content of Ti would have to be provided at an amount of at least 3.42*N. The alloy elements in combination may be limited to not more than 0.2% by weight, not more than 0.15% by weight, or even not more than 0.1% by weight, since higher contents have an adverse effect on material properties, especially an adverse effect on the toughness of the material.
Mo, V, Cu, Ni, Sn, Ca, As, N, P or S are alloy elements which, individually or in combination, if they are not specifically included in alloys to establish specific properties, can be counted among the impurities. The contents are limited to not more than 0.3% by weight of Mo, to not more than 0.25% by weight of V, to not more than 0.2% by weight of Cu, to not more than 0.3% by weight of Ni, to not more than 0.05% by weight of Sn, to not more than 0.01% by weight of Ca, to not more than 0.02% by weight of As, to not more than 0.01% by weight of N, to not more than 0.06% by weight of P, to not more than 0.03% by weight of S.
The second layer for formation of the functional layer at least on one side of the first layer preferably consists of a microalloyed steel alloy or dual-phase steel alloy that can be coated and/or formed easily and conventionally without difficulty. According to the invention, the second layer of the semifinished flat steel product, as well as Fe and unavoidable preparation-related impurities, includes, in % by weight, C: up to 0.2%, Si: 0.01-0.6%, Mn: 0.1-2.5%, Al: 0.01-2.0%, Cr+Mo: up to 1.4%, Nb+Ti: up to 0.25%, B: up to 0.02%, V: up to 0.05%, Cu: up to 0.2%, Ni: up to 0.2%, Sn: up to 0.05%, Ca: up to 0.01%, Co: up to 0.02%, N: up to 0.01%, P: up to 0.1%, S: up to 0.06%.
To increase formability and/or coatability, C as an alloy element may be limited to not more than 0.2% by weight. In some embodiments, C is limited to not more than 0.15% by weight, not more than 0.11% by weight, or even not more than 0.09% by weight, where C is present at in an amount of at least 0.001% by weight.
Si is an alloy element that contributes to solid solution hardening and has a positive effect in an increase in strength, and so a content of at least 0.01% by weight is present. The alloy element may be limited to not more than 0.6% by weight, not more than 0.5% by weight, or not more than 0.4% by weight, depending on the embodiment, in order to ensure sufficient rollability and/or surface quality.
Mn is an alloy element that contributes to hardenability and has a positive effect on tensile strength, especially for binding of S to give MnS, and so a content of at least 0.1% by weight may be present. The alloy element may be limited to not more than 2.5% by weight, not more than 2.0% by weight, or even not more than 1.5% by weight, in order to ensure sufficient weldability.
Al as an alloy element contributes to deoxidation, where a content of at least 0.01% by weight is present. In some embodiments, a content of at least 0.015% by weight is present. Especially in the case of dual-phase steel alloys that are especially hot-rolled in the two-phase region (austenite/ferrite), Al is included in the alloy in high contents in order to bring about widening of the two-phase region. In embodiments, the alloy element is limited to not more than 2.0% by weight. The alloy element may be further limited to not more than 1.8% by weight, or not more than 1.6% by weight, in order to essentially reduce and/or to avoid precipitates in the material, especially in the form of nonmetallic oxidic inclusions, which can have an adverse effect on material properties. The Al content, especially in the case of microalloyed steel alloys, may be limited to not more than 1.0% by weight, not more than 0.5% by weight, or not more than 0.2% by weight, in order essentially to avoid the aforementioned disadvantages.
Cr as an alloy element, according to its content, can also contribute to establishing strength, with a content especially of at least 0.1% by weight and limited to not more than 1.4% by weight. In some embodiments, the Cr content is limited to not more than 1.2% by weight, not more than 1.0% by weight, or even not more than 0.8% by weight, in order to be able to ensure essentially complete coatability of the surface.
B as an alloy element can contribute to hardenability, especially when N is bound, and is present in a content especially of at least 0.0002% by weight. The alloy element may be limited to not more than 0.02% by weight, not more than 0.015% by weight, not more than 0.01% by weight, or not more than 0.005% by weight, depending on the embodiment, since higher contents have an adverse effect on material properties and would result in a reduction in hardness and/or strength in the material.
Ti and Nb as alloy elements may be included in the alloy individually or in combination for grain refining and/or N binding, with contents especially of at least 0.001% by weight of Ti and/or of at least 0.001% by weight of Nb. For complete binding of N, the content of Ti would have to be provided at an amount of at least 3.42*N. The alloy elements in combination may be limited to not more than 0.25% by weight, not more than 0.2% by weight, or even not more than 0.15% by weight, since higher contents have an adverse effect on material properties, especially an adverse effect on the toughness of the material.
Mo, V, Cu, Ni, Sn, Ca, Co, N, P or S are alloy elements which, individually or in combination, if they are not specifically included in alloys to establish specific properties, can be counted among the impurities. The contents are limited to not more than 0.2% by weight of Mo, to not more than 0.05% by weight of V, to not more than 0.2% by weight of Cu, to not more than 0.2% by weight of Ni, to not more than 0.05% by weight of Sn, to not more than 0.01% by weight of Ca, to not more than 0.02% by weight of Co, to not more than 0.01% by weight of N, to not more than 0.1% by weight of P, to not more than 0.06% by weight of S.
In one configuration of the semifinished flat steel product, in the simplest execution, only a first layer with a second layer bonded to one side is provided. The free surface of the second layer may be coated with an anticorrosion coating based on zinc, and, in particular, alternatively or additionally, the free surface of the first layer may be coated with an anticorrosion layer based on zinc. The semifinished product may include two second layers arranged on either side of and fully and cohesively bonded to the first layer, and so it is possible to provide a sandwich material which, according to the application, may have a symmetric or asymmetric construction. Both free surfaces of the second layers may be coated with an anticorrosion coating, such as a coating based on zinc. In embodiments, the semifinished flat steel product, according to the execution, is provided on one or both sides with an electrolytic zinc coating. The performance of an electrolytic coating has the advantage that the properties of the first layer in particular are not adversely affected by thermal effects in particular, as occur, for example, in the performance of a hot dip coating operation.
In a further configuration of the semifinished flat steel product, the second layer of the soft steel alloy has a material thickness between 2% and 30%, between 5% and 20%, or between 7.5% and 12%, based on the total material thickness of the semifinished flat steel product. The soft steel alloy provided as functional layer should be of such a material thickness that, firstly, the positive properties of the first layer are essentially not adversely affected, where the material thickness of the second layer (per side) is limited to not more than 30%, not more than 20%, or even not more than 12% based on the total material thickness of the semifinished product. Secondly, the material thickness of the soft steel alloy provided as the functional layer should ensure that the first layer has a certain distance from the surface of the semifinished flat steel product, such that coating and/or forming is performable without drawbacks, where the material thickness of the second layer (per side) is at least 2%, and may even be at least 5%, or at least 7.5%, based on the total material thickness of the semifinished product. The semifinished flat steel product may have a total material thickness between 0.5 and 6.0 mm, between 0.8 and 4.0 mm, or between 1.2 and 3.0 mm, depending on the embodiment.
In a further configuration of the semifinished flat steel product, the semifinished flat steel product is produced by means of cladding, especially roll cladding, or by means of casting. The semifinished flat steel product of the invention may be preferably produced by means of hot roll cladding as disclosed, for example, in German patent specification DE 10 2005 006 606 B3, the contents of which are hereby incorporated into this application by reference. Alternatively, the semifinished flat steel product of the invention may be produced by means of casting, in which case one option for production thereof is disclosed in Japanese published specification JP-A 03 133 630. Metallic composite production is generally prior art.
Embodiments of the invention further relate to processes for producing a component for road vehicle construction, rail vehicle construction, shipbuilding or aerospace, wherein a semifinished flat steel product of the invention is cold-formed. Since the second layer of the semifinished flat steel product of the invention has particularly good deformability, for example consists of a microalloyed or dual-phase steel alloy, the deforming characteristics are optimal and the semifinished flat steel product of the invention can be formed in an essentially cracked-free manner and with a higher bending angle compared to a conventional martensite-phase steel of the same composition.
The cold forming of the semifinished flat steel product of the invention that can be provided in sheet or plate form can be effected, for example, in a discontinuous process by folding or U-O forming, preferably in conventional forming molds. Alternatively, the forming, for example of semifinished flat steel product in strip form, can be effected inexpensively by roll profiling on preferably conventional profiling plants. By the folding, U-O forming or roll profiling, it is possible to produce open or closed profiles with different cross-sectional geometry according to the requirement. The profiles produced may have a longitudinally constant or longitudinally variable cross section.
Further embodiments of the invention relate to various uses of a profile produced from the semifinished flat steel product of the invention. The profile can be used as crash profile in a vehicle, especially as profile in a battery housing of a vehicle, or the profile can be used as seat rail of a vehicle seat. The battery housing comprises at least one base, four walls and a lid that are assembled, and serves to accommodate battery modules. Particularly the walls are formed from profiles produced from the semifinished flat steel product according to the various embodiments of the invention. The battery housing is releasably bonded to the bodywork, for example in the floor region of a vehicle, and may deform only slightly, if at all, in the event of a crash. Semifinished flat steel products of the invention are of particularly good suitability for this application owing to their high tensile strength and/or hardness, especially when they are provided with an electrolytic zinc coating to increase corrosion protection due to use in the wet region of the vehicle. The vehicles are preferably hybrid vehicles or purely electrically driven vehicles, whether they be personal vehicles, utility vehicles or buses.
The profiles produced from the semifinished flat steel products of the embodiments of the invention may also be used as longitudinal or transverse beam in a motor vehicle, for example as profiles, especially as crash profile in the bumper, door sill, side impact beam, or in regions in which zero to low deformation/intrusion in the event of a crash is required, as, for example, in battery housings, seat structures, bodywork, chassis, roof frame etc.
The sole FIGURE shows a schematic sectional view through an inventive semifinished flat steel product (1). The inventive semifinished flat steel product (1) includes a first layer (1.1) of a steel alloy having a martensitic microstructure (martensite-phase steel) having a tensile strength of >1200 MPa and/or a hardness of >370 HV10. As described above, the first layer (1.1) may have a tensile strength of >1300 MPa and/or a hardness of >400 HV10, a tensile strength of >1400 MPa and/or a hardness of >435 HV10, a tensile strength of >1500 MPa and/or a hardness of >465 HV10, or even a tensile strength of >1600 MPa and/or a hardness of >490 HV10. The product (1) further includes two second layers (1.2, 1.2′) of a soft steel alloy having a tensile strength of <600 MPa and/or a hardness of <190 HV10. As described above, each second layer may have a tensile strength of <550 MPa and/or a hardness of <175 HV10, a tensile strength of <450 MPa and/or a hardness of <140 HV10, or even a tensile strength of <380 MPa and/or a hardness of <120 HV10. The second layers (1.2, 1.2′) are bonded fully and cohesively to the first layer (1.1) on either side respectively. According to the application and in the simplest execution, it is also possible for just one second layer (1.2) to be fully and cohesively bonded to the first layer (1.1); therefore, the second layer (1.2′) is represented by dotted lines.
The first layer (1.1), as well as Fe and unavoidable preparation-related impurities, includes, in % by weight, C: 0.15-0.6%, Si: 0.05-0.9%, Mn: 0.3-2.0%, Al: 0.01-2.0%, Cr+Mo: up to 1.5%, Nb+Ti: up to 0.2%, B: up to 0.02%, V: up to 0.25%, Cu: up to 0.2%, Ni: up to 0.3%, Sn: up to 0.05%, Ca: up to 0.01%, As: up to 0.02%, N: up to 0.01%, P: up to 0.06%, S: up to 0.03%. The second layers (1.2, 1.2), as well as Fe and unavoidable preparation-related impurities, include, in % by weight, C: up to 0.2%, Si: 0.01-0.6%, Mn: 0.1-2.5%, Al: 0.01-2.0%, Cr+Mo: up to 1.4%, Nb+Ti: up to 0.25%, B: up to 0.02%, V: up to 0.05%, Cu: up to 0.2%, Ni: up to 0.2%, Sn: up to 0.05%, Ca: up to 0.01%, Co: up to 0.02%, N: up to 0.01%, P: up to 0.1%, S: up to 0.06%, where they are preferably formed from a microalloyed steel alloy.
The material thickness of the second layer (1.2, 1.2), especially per side, is such that the positive properties of the first layer (1.1) are essentially not adversely affected, where the material thickness of the second layer (per side) is at least 2% and at most 30%. In some embodiments, the material thickness of the second layer (per side) is at least 7.5% and at most 12%, based on the total material thickness of the semifinished flat steel product (1). The semifinished flat steel product (1) may have, for example, a total material thickness between 0.5 and 6 mm. Since the second layers (1.2, 1.2), by comparison with the first layer (1.1) of the semifinished flat steel product, are suitable for coating, they have an anticorrosion coating based on zinc on their free surfaces, preferably an electrolytic zinc coating (1.3, 1.3′) in each case. The coating (1.3′) is represented by dotted lines since, for example in the simplest embodiment of the semifinished flat steel product (1) as already described further up, it is likewise absent in the absence of a second layer (1.2′).
The invention is not limited to the working example shown in the drawing and to the executions in the general description; instead, the semifinished flat steel product of the invention may also be formed from a tailored product, for example a tailored blank and/or tailored rolled blank.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2017 203 507.2 | Mar 2017 | DE | national |
This application is the U.S. National phase of, and claims priority to, International Application No. PCT/EP2018/054372, filed Feb. 22, 2018, which designated the U.S. and which claims priority to DE Application No. 10 2017 203 507.2, filed Mar. 3, 2017. These applications are each incorporated herein by reference in their entireties.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2018/054372 | 2/22/2018 | WO | 00 |
