Metal Sheet Component, Manufactured by Hot Forming a Flat Steel Product and Method for Its Manufacture
A metal sheet component including (in % by weight) C: up to 0.5%, Si: 0.05-1%, Mn: 4-12%, Cr: 0.1-4%, Al: up to 3.5%, N: up to 0.05%, P: up to 0.5%, S: up to 0.01%, Cu, Ni: in total up to 2%, Ti, Nb, V: in total up to 0.5%, rare earth metals: up to 0.1%, and as a remainder, Fe and unavoidable impurities, wherein the content % C of C and the content % Cr of Cr meet the following condition: (10×% C)+% Cr<5.5%. In order to manufacture a metal sheet component, a flat steel product is heated to a heating temperature, which is at least 200° C. and at most 800° C., and then formed into the component by hot forming the flat steel product heated to the heating temperature, with the structure of the hot-formed metal sheet component having 5-50% by volume austenite and, as the remainder, martensite, tempered martensite and/or ferrite, wherein the ferrite proportion can also be 0, and the average grain diameter of the structure is less than 5 μm.
The invention relates to a metal sheet component, manufactured by hot forming a flat steel product.
The invention further relates to a method for manufacturing a component according to the invention.
If information about the alloy contents of individual elements in the steel according to the invention is given in this text, this always relates to the weight (information in % by weight), unless otherwise indicated.
Information on the constituents of the structure of the steel, a flat steel product or a component formed therefrom refer here, by contrast, always to the volume (information in % by volume). If mentioned, the proportions of austenite have been measured via XRD with Fe-filtered Co-Kα radiation. The XRD measuring method is described in the following source: DIN EN 13925 X-ray diffraction from polycrystalline and amorphous materials Part 1 and 2 from 2003, 7, Part 3 from 2005. The further constituents, if mentioned, have in each case been identified by means of light microscope following nital etching.
The flat steel products according to the invention are rolled products, such as steel strips, steel sheets or cut-outs and panels obtained therefrom, whose thickness is essentially lower than their width and length.
The mechanical properties mentioned in the present text, tensile strength Rm, yield strength Rp0.2 and elongation at break A80 have been determined according to DIN EN ISO 6892-1:2017-02.
Examples of higher strength, Mn-containing steels are known from EP 2 383 353 A2 which have an elongation at break A80 of at least 4% and a tensile strength of 900-1500 MPa as a coated or uncoated hot or cold strip. These steels contain, in addition to iron and unavoidable impurities, (in % by weight) C: up to 0.5, Mn: from 4 to 12%, Si: up to 1.0%, Al: up to 3%, Cr: from 0.1 to 4%, Cu: up to 2.0%, Ni: up to 2.0%, N: up to 0.05%, P: up to 0.05%, S: up to 0.01% and optionally one or a plurality of elements from the group “V, Nb, Ti”, with the total of the contents of these elements being at most equal to 0.5%. Furthermore, a method for manufacturing a coated or uncoated hot or cold strip is presented in EP 2 383 353 A2. According to this method, in order to produce a starting product, a steel melt composed in the manner indicated above is cast into a strand or strip which is then subjected to a heat treatment in order to heat it to a hot rolling temperature of 1150-1000° C.
Thereafter, the respective starting product is hot rolled into a hot strip. The finished hot strip is then wound into a coil. Optionally, annealing the hot strip, cold rolling the annealed hot strip, annealing the cold strip and coating the surface of the hot or cold strip can follow this work step in each case.
A method for manufacturing a component by hot press forming a steel sheet after heating in a two-phase region is known from EP 2 778 247 A1, i.e. after heating to a temperature which is between the Ac1 and the Ac3 temperature of the respective steel alloy. According to this method, a slab which consists of iron, unavoidable impurities and (in % by weight) C: 0.01-0.5%, Si: up to 3.0%, Mn: 3-15%, P: 0.0001-0.1%, S: 0.0001-0.03%, Al: up to 3% and N: up to 0.03%, is heated to 1000-1400° C., hot rolled and then finish hot rolled in a temperature range from the Ar3-temperature of the steel to 1000° C. The hot-rolled strip obtained is wound, annealed and then cold rolled. After this, the hot strip is heated to a temperature which is between the Ac1 and the Ac3 temperature of the respective steel alloy and hot press formed. The structure of the component obtained in this manner consists of 5-50% by volume residual austenite and as the remainder of martensite, tempered martensite, bainite or ferrite.
A further possibility of manufacturing ultra high strength components is hot press hardening conventional hot forming steels. Plates consisting of these steels are heated for the hot press forming to high temperatures such that their structure is fully austenitic. After quenching, the obtained components then have a martensitic structure which also has a relatively low residual deformability. What is problematic here is that owing to the high austenitisation temperatures, cathodic protection of the sheets by a metallic corrosion protection coating is not possible.
Against the background of the above explained prior art, the object was to provide a metal sheet component which enables an energy saving through lower forming temperatures in comparison to conventionally manufactured metal sheet components, permits an increased residual elongation with high strengths and ensures the highest possible potential for cathodic corrosion protection.
A method for manufacturing such a metal sheet component should also be indicated.
A metal sheet component achieving this object has according to the invention at least the features indicated in claim 1.
A method solving the above-mentioned problem according to the invention is specified in claim 9.
Advantageous embodiments of the invention are defined in the dependent claims and, like the general concept of the invention, are explained in detail in the following. A metal sheet component according to the invention is accordingly manufactured by hot forming a flat steel product which consists of (in % by weight) C: up to 0.5%, Si: 0.05-1%, Mn: 4-12%, Cr: 0.1-4%, Al: up to 3.5%, N: up to 0.05%, P: up to 0.05%, S: up to 0.01%, in total up to 2% Cu or Ni, in total up to 0.5% of Ti, Nb or V, rare earth metals: up to 0.1%, and Fe as a remainder and unavoidable impurities.
In this case, the content % C of C and the content % Cr of Cr of the steel of the flat steel product meet the following condition: (10×% C)+% Cr<5.5% by weight.
At the same time, the flat steel product according to the invention has, after the hot forming into the metal sheet component, a bending angle determined according to VDA 238-100: 2010-12 of more than 60°.
The structure of the hot-formed metal sheet component according to the invention consists of 5-50% by volume austenite and as the remainder of martensite, tempered martensite or ferrite, wherein the ferrite proportion can also be “0”. In this case, the average grain diameters of the grains of the structure are below 5 μm, preferably below 2 μm.
The flat steel product formed according to the invention into the metal sheet component consists of a steel which is to be assigned to the class of so-called middle-manganese steels which usually have Mn contents of 4-12% by weight, in particular 4-9% by weight. The austenitisation temperature is reduced and the conversion of ferrite, perlite and bainite is delayed by manganese “Mn”. The holding temperature in the furnace can therefore also be reduced before hot forming. The advantages obtained are further strengthened by holding and hot forming in the two-phase region. In the subsequent cooling, a higher austenite proportion is retained. This leads to a very high residual elongation at break and a high possible bending angle up to first cracks and therefore to a higher energy absorption in the event of a crash. The Mn contents of a flat steel product processed according to the invention are set here with 4-12% by weight such that the required minimum strengths of a steel according to the invention are reliably achieved and at the same time a high residual austenite proportion is retained, which ensures optimal elongation properties.
Carbon “C” determines, in the case of the steel of a flat steel product formed according to the invention into the component, on the one hand, the strength of martensite and, on the other hand, the quantity and the stability of the residual austenite. In the case of excessively high carbon contents, the weldability and toughness of the steel is negatively influenced e.g. by formation of Cr carbides. Therefore, the carbon content of Mn steels of the type selected according to the invention is at most 0.5% by weight, with lower C contents of less than 0.5% by weight, in particular of up to 0.3% by weight proving particularly favourable. However, in the case of excessively low carbon content, the quantity and stability of the remaining residual austenite is affected. Therefore, the C content of a steel according to the invention is at least 0.02% by weight.
Aluminium “Al” and silicon “Si” are strong ferrite formers. Both elements counteract the influence of the austenite formers C and Mn. The essential object of the elements Si and Al, in the steel of a flat steel product hot formed according to the invention into the metal sheet component consists of suppressing the carbide precipitation and therefore promoting the stability of the residual austenite. At the same time, Si and Al lead to a solid solution hardening and reduce the specific weight of the steel. In the case of excessively low Si- and Al content, the carbide precipitation may, however, possibly not be effectively suppressed. In the case of excessively high contents of Si and Al, by contrast, processing is made more difficult in the case of producing via a continuous casting and via a strip casting process. Therefore, the invention provides for the Si content to be limited to max. 1° A) by weight, wherein the positive effects of the presence of Si can already be effectively utilised if the Si content of the steel of the flat steel product, from which the component according to the invention is hot formed, is at least 0.05% by weight.
In particular higher Al contents of the steel of the flat steel product used according to the invention for the hot forming of the component according to the invention reduce the thickness of the steel significantly, but lead to increased ferrite proportions in the structure and to a decrease in strength associated therewith. In the case of excessively high Al contents, the welding suitability also decreases, since stable welding slag forms in the case of the welding operation and the electric welding resistance is increased. At the same time, the Ac3 temperature is increased by high Al contents to the extent that a low hot forming temperature, as it is desired by the invention, is no longer achievable. The risk of the development of tension crack corrosion is specifically reduced by the presence of chromium “Cr” in contents of 0.1-4% by weight in a steel according to the invention. Cr and Al hinder hydrogen-induced crack formation. In addition, Cr contributes to the strength increase. Furthermore. Cr also reduces the Ms temperature (martensite starting temperature) and therefore supports the residual austenite stabilisation. These positive effects are to be observed from a content of 0.1% by weight Cr, but in particular from Cr contents of at least 2.2% by weight. The ignition resistance is also improved in the uncoated state from Cr contents of 2.2% by weight. In the case of flat steel products, which are provided with a metallic corrosion protection coating, a positive effect on the layer can be utilised, such as for example the effect as a diffusion barrier for the diffusion of iron into the protection layer. The Cr content of the steel of a flat steel product hot formed into the component according to the invention is limited to max. 4% by weight, because in the case of higher Cr contents carbides could result which would negatively influence the ductility of the steel.
Similarly, in regards to avoiding the development of higher Cr carbide quantities, the invention provides that the content “% C” of carbon “C” and the content “% Cr” of chromium “Cr” of the steel of the flat steel product formed according to the invention into the component must meet the condition (10×% C)+% Cr<5.5% by weight.
By adding copper “Cu” or nickel “Ni” to the steel of the flat steel product hot formed according to the invention, the resistance to different corrosion mechanisms can be improved. The positive effect of Cu and Ni can be particularly reliably used since these elements are added in contents in which they are technically effective. This is to be expected when in the steel of the component according to the invention the total of the contents of Cu and Ni is at least >0.04% by weight. By contrast, negative impacts, such as higher costs and hot crack brittleness in the case of high Cu contents or the individual or combined presence of Cu or Ni in steels according to the invention are reliably avoided since the total of the contents of Cu and Ni is limited to max. 2% by weight.
The micro alloy elements Ti, Nb and V can be present in the steel of the flat steel product, from which the component according to the invention is formed, in contents of in total up to 0.5% by weight. These micro alloy elements contribute to the grain refinement and strength increase. Contents in total above 0.5% by weight of Ti, Nb and V, however, do not lead to an increase in this effect, whereas the positive effects of Ti, Nb and V in the steel of the component according to the invention can be reliably used when their content is in total at least 0.05% by weight.
The austenitic structure can also be stabilised by adding nitrogen “N” in contents of up to 0.05% by weight. In the case of excessively high N content, the processability deteriorates in the case of continuous casting and an embrittling quantity of nitrides results.
The contents of phosphorous “P” of the steel of a component according to the invention are limited to max. 0.05% by weight in order to reliably rule out negative influences of this element.
For the same reason, the content of sulphur “S” of a steel according to the invention is limited to max. 0.01% by weight.
Rare earth metals (REMs) can, in the steel of the component according to the invention, contribute to grain refinement by forming oxides and improve the isotropy of the mechanical technological properties via the texture. The two rare earth metals cerium and lanthanum are chemically virtually identical and therefore always occur in nature together. They are very hard and therefore costly to separate due to their chemical similarity. They have the same effect here. The rare earth metals can be freely substituted for the use in the steel. However, in the case of contents over 0.1% by weight, the risk of clogging, i.e. blockage of the cast moulds by locally hardening melts results, amongst other things, in the case of large-scale casting of steel. The advantages of the presence of the REMs can still be reliably used since the content of the steel of a component according to the invention is at least 0.0005% by weight.
The bending angle determined according to VDA 238-100: 2010-12 is a measurement for the folding behaviour of the material in the event of a crash and therefore an indicator of the ductility which a hot-formed component has. Components according to the invention are characterised by a high bending angle of at least 60°, in particular at least 80° or more than 80°, such as for example at least 85° after hot forming. The uniform, very fine structure plays a supporting role here. A high austenite content has advantageous effects, as is present, when the hot forming takes place at temperatures which are in the two-phase mixed region of the steel (or deeper), of which the flat steel product consists, from which the component is formed.
Components according to the invention are characterised in that they have a structure which consists of at least 5% by volume austenite, wherein the austenite proportion of the structure can be up to 50% by volume. The residual structure of the component consists of strength-increasing proportions of martensite and tempered martensite. Ferrite may also be contained. The quantity of other technically unavoidably present constituents is so low that they are ineffective in regard to the properties of the component according to the invention. The method according to the invention for manufacturing a metal sheet component provided according to any one of the preceding claims comprises the following work steps:
- a) Providing a flat steel product made of a steel, which (in %—by weight) consists of
- C: up to 0.5%,
- Si: 0.05-1%,
- Mn: 4-12%,
- Cr: 0.1-4%,
- Al: up to 3.5%,
- N: up to 0.05%,
- P: up to 0.05%,
- S: up to 0.01%,
- in total up to 2% Cu or Ni,
- in total up to 0.5% of Ti, Nb or V,
- REM: up to 0.1%,
- and Fe as a remainder and unavoidable impurities,
- wherein the content % C of C and the content % Cr of Crmeet the following condition:
(10×% C)+% Cr<5.5%,
- b) Heating up the flat steel product to a heating temperature which is at least 200° C. and at most 800° C.;
- c) Hot forming the flat steel product heated to the heating temperature into the component.
The cooling speed at which the obtained hot-formed component is cooled, is not subject to any limitations.
The essential possibilities of producing flat steel products, which are suitable for the purposes according to the invention and are provided in work step a) of the method according to the invention, are described in EP 2 383 353 A2, whose content is incorporated in the present application by reference. The different methods available in practice for producing flat steel products are represented in the diagram reproduced there and the associated sections [0031] to [0040] of EP 2 383 353 A2 which are suitable for producing components according to the invention.
In addition, there is the possibility of supplying the rolled strip directly, i.e. without a prior annealing step, to the process of hot forming. Typical protection layers, which are present on components according to the invention and with which the flat steel products, from which components according to the invention are formed, can be coated, are zinc-based protection coats applied by melt dip coating, such as e.g. Zn coats (“Z”), zinc-iron coats (“ZF”), zinc-magnesium-aluminium coats (“ZM”), zinc-aluminium coats (“ZA”). Moreover, aluminium-based protection coats can be used, such as aluminium-zinc coats (“AZ”), aluminium-silicon coats (“AS”). Similarly, electrolytically applied Zn-based protection coats can be provided, such as e.g. pure zinc (“ZE”) coats or zinc-nickel coats (“ZN”). However, metallic corrosion protection coats known per se are also possible which are applied by precipitating methods, such as PVD, CVD or vapour spraying.
Proceeding from here, the invention shows a way in which a component can be produced by resource-sparing hot forming which has optimal mechanical properties after its hot forming and owing to these properties and its other usage properties also holds up to high requirements in the case of crash loading of the component.
The high manganese content of flat steel products processed according to the invention enables lower hot forming temperatures than with conventional hot forming steels. Thus, the invention allows energy and costs to be saved.
The heating temperatures for hot forming should not be more than 60° C. above the Ac3 temperature of the respective steel of the flat steel product in order to obtain the desired positive properties.
The heating temperatures can be particularly low when the forming is supposed to take place in the two-phase region or at temperatures below this. In this case, the residual austenite proportion in the obtained component is above 20% by volume and the elongation at break A80 above 15%. The hot forming according to the invention takes place here at heating temperatures which are typically above the Ac1 temperature and below the Ac3 temperature of the respective steel of the flat steel product, wherein in the case of a deformation in the two-phase region, heating temperatures prove particularly favourable which are higher by at least 10° C. than the Ac1 temperature and are lower by at least 50° C. than the Ac3 temperature of the respective steel of the flat steel product.
If deforming is carried out at temperatures which are below the temperature range in which a two-phase structure is present in the flat steel product, then the heating temperature can, to this end, be below the Ac1 temperature of the respective steel of which the flat steel product hot-formed according to the invention consists in each case. While in the case of annealing at heating temperatures above the Ac1 temperature, the austenite proportion is not of relevance before the hot forming, the desired proportion must be set in the case of forming below Ac1 in a preceding annealing step. The heating temperature in the case of this additional annealing should be at least as high such that the forming forces are positively differentiated from those of cold forming. Accordingly, the heating temperature should in this case be set such that the forming forces of the hot forming are max. 85% of the forming forces at room temperature. This is ensured in the case of heating temperatures of above 200° C., in particular of above 400° C.
A structure is obtained by the approach according to the invention which is characterised by optimised austenite proportions and as a result of this has very good mechanical properties, in particular a high residual elongation and a high energy absorption in the event of crash loading. The comparatively low heating temperatures in this region at which the hot forming of the component according to the invention takes place, have also proven particularly advantageous when the flat steel product processed according to the invention is supposed to have a cathodic corrosion protection.
The annealing times typically required for heating-up in the work step b) are usually up to 60 mins, and in practice annealing times of up to 20 mins, in particular up to 10 mins have proven particularly economic. The heating-up can be carried out in conventional chamber furnaces or roller kilns in which the flat steel products to be hot deformed are brought to the heating temperature during the cycle or in batches. Since, in the case of compositions according to the invention of the flat steel product deformed into the component the properties are formed virtually independently of heating and cooling speed, it may, however, also prove favourable when the heating is carried out by conductive or inductive heating or also for example by means of solid contact or in a fluidised bed. Shorter annealing times can be achieved by the alternative methods to the conventional furnace heating in comparison to the pure radiation heating in a conventional furnace. At the same time, the alternative methods allow more precisely controlled heating cycles since the course of heating can follow precise specifications in these cases. The further advantage of the use of alternative heating methods is that there can be a quick response to production changes, which are typical of small manufacturing quantities with different sheet thicknesses. Adjustments of the heating parameters to the respectively changed requirements can accordingly be carried out quickly.
The hot forming (work step c)) of the flat steel product heated to the respective heating temperature into the component according to the invention can be carried out in conventional hot forming tools available for this purpose in the prior art. In this case, the hot forming takes place as directly as possible after the heating-up (work step b)) such that the temperature at which the flat steel product enters the hot forming corresponds to the heating temperature up to a technically negligible difference. However, strong cooling is also permissible, as long as the forming forces and springback are advantageous compared to cold forming.
The cooling of the component after hot forming can take place in a manner similarly known per se in the hot forming tool. Alternatively, the component can, however, also be removed after hot forming in a suitable short time interval from the hot forming tool and cooled outside of the tool. Since the cooling speed is not limited, it can even be lower than 10K/s.
As already mentioned, the invention has a particularly positive impact in the case of producing components made of flat steel products which are coated with a metallic protection layer in order to protect them against corrosion or other attacks. It has been shown here that by means of the comparatively low required heating temperatures, at which the hot forming of the component according to the invention can be carried out, alloying of the protection coating by diffusing alloy constituents from the steel substrate takes place at best in a reduced manner, such that the protection coating maintains its cathodic protection effect even after the hot forming of the component. The protection layers present on the flat steel product processed in each case according to the invention and hot formed into the component according to the invention typically have a near-surface boundary layer adjoining the steel substrate of the flat steel product before the hot forming which consists of metallic and/or oxidic iron and, if applicable, metallic and/or oxidic manganese and further alloy constituents of the base material. After the hot forming into the component, owing to the low heating temperatures used according to the invention, at which the hot forming according to the invention takes place, a proportion of brittle phases, which is reduced compared to the conventional approach providing higher forming temperatures, is present in the boundary layer region since, owing to the heating temperature, reduced according to the invention, of the hot forming, only a minimised alloying of the protection coating results with elements coming from the steel substrate. The potential of the cathodic corrosion protection through Zn-rich phases is therefore retained.
The parameters of the approach according to the invention allow the cathodic protection effect of a Zn-containing layer present on the flat steel product to be retained and critical cracks in the case of hot forming of more than 10 μm to be avoided.
In the case of the comparatively low heating or forming temperatures provided in the case of the method according to the invention, the damaging consequences are avoided which would occur in the case of melting the Zn layer. Due to the diffusion of Fe from the substrate into the layer, its melting point is raised to a sufficient extent.
However, in order to ensure cathodic corrosion protection, a restriction of the Fe proportion in the coating is required, so that, after the hot forming, sufficient Zn-rich phases are still retained. The Fe—Zn phases present in the coat were determined for the examples by X-ray diffraction and are compiled in Table 3.
The comparative steel V conventionally used in hot forming is annealed to set the mechanical target properties typically at 870-950° C. In this case, the forming of a ┌/┌1 phase results, which is comparatively stable in temperature which restricts the proportion of resulting liquid Zn and therefore curbs the risk of any occurring liquid metal embittlement. The high Fe proportion contained in the ┌/┌1 phase, however, significantly restricts the active corrosion protection of the layer.
In the case of the samples according to the invention of middle-manganese+Z, owing to the notably lower furnace temperature for setting the mechanical target properties, the notably Zn-richer δ-phase is also retained which leads to an improved corrosion protection potential. Owing to the alloying-related layer construction, the layer system is sufficiently stable in temperature such that in the case of hot forming temperatures according to the invention there is no critical crack formation above 10 μm deep by liquid Zn, in the case of which a crack growth would be expected when the component is stressed.
A manganese-containing layer is also formed on the free surface of the protection coat in a manner known per se (see EP 2 290 133 B1) in a metallic and/or oxidic form on the free surface of the component by way of which the effectiveness of the protection coating is further increased.
Components produced according to the invention, as a result of their deformation at temperatures which are below the greatest limit corresponding to the Ac3-temperature of the respective steel+60° C., have an optimised combination of high strength values which represent tensile strengths Rm of typically at least 1000 MPa, and optimised elongation properties which are expressed in elongations at break A80 of normally more than 10%. The product Rm x A80 is, in the case of components according to the invention, accordingly similarly normally in the range of 13000-35000 MPa %. In contrast, the tensile strengths Rm in the case of components, which were manufactured from conventional steels for hot forming, are at temperatures at which a fully austenitic structure is present, namely typically in the case of at least 1200 MPa, since they are fully martensitic after quenching. However, these components achieve only notably lower elongation at break values A80 such that in the case of these components the product Rm×A80 is generally only 6000-11000 MPa %.
The invention is explained in greater detail below using exemplary embodiments.
Three melts S1-S3 corresponding to the measurements of the invention and a comparative melt V have been melted, whose compositions are in each case indicated in % by weight in Table 1. In addition, the Ac1 and Ac3 temperatures in ° C. determined for the steels S1-S3 and V according to SEP 1680:1990-12 are mentioned in Table 1.
The comparative melt V is, owing to its excessively low Mn content and the presence of B, outside of the specifications of the invention.
Metal sheet blanks have been manufactured from the steels S1-S3 and V.
In example 1, 4, 11 and 8, metal sheet samples were examined which have been cut from hot strips, which have been hot rolled from a primary product produced in a conventional manner to a thickness “d” (state “WW”) and then annealed under a cover (state “HG”) or in a continuous furnace (state “DO”). In the case of the examples 2 and 5, the metal sheet samples were cut from strips, which have been produced from hot strips, which have also been cold rolled to a thickness of “d” (state “KW”). Prior to the metal sheet cutting, some of the cold-rolled strips were to some extent annealed, like the examples 2, 6, 12, batch-annealed (state “HG”) or like the examples 7, 9, 10, 13-16 annealed in a continuous furnace (state “DO”). Some of the metal sheet blanks have also been coated with a pure zinc layer electrolytically (“ZE”) or hot-dip-coated (“Z”), with a zinc-iron layer (“ZF”) or with an aluminium-silicone layer (“AS”).
The metal sheet blanks are in each case heated in a conventional furnace to a heating temperature Tew, then hot formed in a conventional hot forming tool into a cap profile and then cooled in the air.
The tensile strength Rm, the yield strength Rp0.2, the elongation at break A80, the product Rm×A80 and the bending angle determined on the component obtained in each case are indicated in Table 2. Moreover, as far as these features have been determined, structural characteristic values of the component obtained in each case are indicated there.
Moreover, as far as these features have been determined, the austenite proportions of the component obtained in each case and the estimated grain size and the crack depths at the critical point of the cap profile are indicated, as they were measured in the cross-sectional polish under the light microscope.
It has been shown that in the case of the examples according to the invention the elongations at break A80 are above 10% and the products Rm×A80 are more than 14000 MPa %. At the same time, the examples have bending angles of above 60°. In the case of the examples 1-3, a largely austenitic structure has been set in the case of heating which largely converts into martensite in the case of cooling, which leads to the high strengths.
In the case of the examples 4-13, the austenite proportion was optimised by heating in the two-phase region such that particularly high products Rm×A80 and high bending angles were obtained.
A particularly fine structure can be achieved by alloying micro alloy elements and rare earth metals.
In the examples 14-16, the austenite content was set in the two-phase region by the annealing processes preceding the metal sheet cutting.
In the case of hot forming below Ac1, substantially only the martensite is tempered. The latter method, in addition to good mechanical properties, in particular has advantages in relation to the coating. Since the temperatures are below the melting temperature of the coat, cracks can be largely avoided in the substrate due to penetrating zinc in the case of hot forming.
However, the coat is also provided in the case of heating temperatures in the two-phase region (examples 8-10) such that cracks remain in an acceptable range of at most 10 μm.
Claims
1. A metal sheet component, manufactured by hot forming a flat steel product, consisting (in % by weight)
- C: up to 0.5%,
- Si: 0.05-1%,
- Mn: 4-12%,
- Cr: 0.1-4%,
- Al: up to 3.5%,
- N: up to 0.05%,
- P: up to 0.05%,
- S: up to 0.01%,
- Cu, Ni: in total up to 2%,
- Ti, Nb, V: in total up to 0.5%,
- Rare earth metals: up to 0.1%,
- and a remainder, Fe and unavoidable impurities,
- wherein the content % C of C and the content % Cr of Cr meet the following condition: (10×% C)+% Cr<5.5%,
- wherein the flat steel product after forming into the metal sheet component has a bending angle of more than 60°,
- wherein a structure of the hot-formed metal sheet component comprises 5-50% by volume austenite and as a remainder, martensite, tempered martensite, and/or ferrite,
- wherein the ferrite proportion can be 0, and
- wherein an average grain diameter of the structure is less than 5 μm.
2. The metal sheet component according to claim 1, wherein the C content is at least 0.02% by weight.
3. The metal sheet component according to claim 1, wherein the C content is up to 0.3% by weight.
4. The metal sheet component according to claim 1, wherein the Cr content is at least 2.2% by weight.
5. The metal sheet component according to claim 1, wherein the average grain diameter is below 2 μm.
6. The metal sheet component according to claim 1, wherein the bending angle is more than 80°.
7. The metal sheet component according to claim 1, wherein after the hot forming tensile strength Rm of the flat steel product is at least 1000 MPa, elongation at break A80 of the flat steel product is more than 10%, and a product Rm*A80 formed of the tensile strength Rm and the elongation at break A80 of the flat steel product is more than 13000 MPa %.
8. The metal sheet component according to claim 1, wherein the metal sheet component is provided with a metallic protective coating.
9. A method for manufacturing a metal sheet component, comprising the following steps:
- a) providing a flat steel product made of a steel, comprising (in %—by weight):
- C: up to 0.5%,
- Si: 0.05-1%,
- Mn: 4-12%,
- Cr: 0.1-4%,
- Al: up to 3.5%,
- N: up to 0.05%,
- P: up to 0.05%,
- S: up to 0.01%,
- in total up to 2% Cu or Ni,
- in total up to 0.5% of Ti, Nb or V,
- REM: up to 0.1%,
- and as a remainder, Fe and unavoidable impurities,
- wherein the content % C of C and the content % Cr of Cr meet the following condition: (10×% C)+% Cr<5.5%
- b) heating the flat steel product to a heating temperature which is at least 200° C. and at most equal to the Ac3 temperature +60° C. of the flat steel product; and
- c) hot forming the flat steel product heated to the heating temperature into the component.
10. The method according to claim 9, wherein the heating temperature is at most 800° C.
11. The method according to claim 9, wherein the heating temperature is above the Ac1 temperature and below the Ac3 temperature of the flat steel product.
12. The method according to claim 9, wherein the heating temperature is below the Ac1 temperature of the flat steel product, and wherein austenite setting takes place in an annealing step before the hot forming step.
13. The method according to claim 9, wherein the flat steel product provided in step a) has a metallic corrosion protection layer.
14. The method according to claim 9, wherein the heating in step b) is carried out by a conductively or inductively acting heating method.
Type: Application
Filed: Jul 25, 2017
Publication Date: Mar 25, 2021
Inventors: Brigitte Hammer (Voerde), Harald Hofmann (Dortmund), Thomas Heller (Duisburg), Sebastian Stille (Dortmund), Georg Parma (Dortmund), Janko Banik (Altena), Jonas Schwabe (Duisberg)
Application Number: 16/634,029
