Sheet Metal Part Formed from a Steel Having a High Tensile Strength and Method for Manufacturing Said Sheet Metal Part

A sheet metal part having a tensile strength Rm≥1000 MPa and a bending angle >70° formed from a flat steel product including, in % by weight: C: 0.10-0.30%, Si: 0.5-2.0%, Mn: 0.5-2.4%, Al: 0.01-0.2%, Cr: 0.005-1.5%, P: 0.01-0.1%, and optionally one or more of Ti, Nb, V, B, Ni, Cu, Mo, and W, with Ti: 0.005-0.1%, Nb: 0.005-0.1%, V: 0.001-0.2%, B: 0.0005-0.015%, Ni: 0.05-0.4%, Cu: 0.01-0.8%, Mo: 0.01-1.0&, and W: 0.001-1.0%, and remainder iron and unavoidable impurities, wherein the structure of the sheet metal part is 40-100% by area plate-shaped bainite, 70-95% of which is made of ferrite, 2-30% of high carbon phases that are plate-shaped with the remainder made up of other components and the remainder of the structure consists of <40% by area of the total structure of non-plate-shaped bainite, of which is made of ferrite, 2-30% of high carbon phases and <5% of other components. Also, a method for manufacturing the sheet metal part.

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Description
CROSS-REFERENCE TO RELATED APPLICATION

This application is the United States national phase of International Application No. PCT/EP2018/063356 filed May 22, 2018, the disclosure of which is hereby incorporated by reference in its entirety.

BACKGROUND OF THE INVENTION Field of the Invention

The invention relates to a sheet metal part formed from a steel having a high tensile strength Rm of at least 1000 MPa and a bending angle of more than 70°.

The invention further relates to a method for manufacturing a sheet metal part of this type.

Description of Related Art

When reference is made in the following to a flat steel product or a “sheet metal product”, then this means rolled products such as steel strips or sheets from which blank cuts or panels are separated for the manufacture of, for example, bodywork parts. “Sheet metal parts” or “sheet metal components” of the type according to the invention are manufactured from sheet steel or sheet metal products of this type, with the terms “sheet metal part” and “sheet metal component” being used interchangeably.

All information on contents of the steel compositions indicated in the present application are related to weight, unless explicitly otherwise mentioned. All % information not determined in detail and in relation to a steel alloy should therefore be understood as information in “% by weight”. With the exception of the information on the residual austenite content of the structure of a steel product according to the invention based on volume (indicated in % by volume), the information on the contents of various structural components is based, in each case, on the area of a ground part of a sample of the respective product (information in “% by area”) unless expressly indicated otherwise.

References to the contents of the components of an atmosphere in this text relate to the volume (information in “% by volume”).

Mechanical properties such as tensile strength, yield strength and elongation that are reported here were determined in a tensile test according to DIN EN ISO 6892-1:2009 unless expressly indicated otherwise.

The structure was determined on cross sections that had been subjected to etching with 3% nital (alcoholic nitric acid). The structural determination was carried out in the electron microscope grid at 5000× magnification to determine the percentage of the plate-shaped and other non-plate-shaped bainite and at 20,000× to 50,000× magnification for the determination of plate length and width and the plate distance. The percentage of residual austenite was determined using x-ray diffractometry.

A sheet metal part and a method for manufacturing a steel sheet metal part of this type with a tensile strength of at least 980 MPa are known from EP 2 719 786 B1. The sheet metal part consists of a steel which, in addition to iron and unavoidable impurities, is composed of (in % by mass) 0.15-0.4% C, 0.5-3% Si, 0.5-2% Mn, up to 0.05% P, up to 0.05% S, 0.01-0.1% Al, 0.01-1% Cr, 0.0002-0.01% B, 0.001-0.01% N and Ti such that the Ti content is at least four times the N content and a maximum of 0.1%. According to the known method, a cut metal sheet created from a steel composed in this way is heated to a temperature that is not less than the Ac3 temperature of the respective steel and is not more than 1000° C. and then hot formed in a pressing tool to form the hot press formed sheet metal part. During the shaping, the sheet metal part is cooled at an average cool rate of at least 20° C./s or higher in the molding tool. As a target temperature range for this cooling, a range is mentioned that starts 100° C. below the bainite starting temperature “BS”, in other words 100° C. below the temperature from which bainite forms in the structure of the steel and ends at the martensite starting temperature, in other words the temperature from which martensite forms in the structure of the steel. The sheet metal part is held in this temperature range for at least 10 s to set the properties of the molded part. The holding in this range can be an isothermal holding, a cooling or a reheating, provided it takes place in the temperature range mentioned. The steel product obtained in this way should be a structure that (in % by area) is made of 70-97% bainitic ferrite, up to 27% of martensite and 3-20% of residual austenite, wherein the remaining components of the structure make up a maximum of 5%.

SUMMARY OF THE INVENTION

Against the background of the prior art, the object was to provide a sheet metal part which can be manufactured using hot forming such as press hardening that has optimized strength in combination with an optimal energy absorption capacity in the event of sudden deformation load, as is the case when an automobile crashes.

The object was also to provide a method by means of which a sheet metal part of this type can be manufactured in a practice-oriented manner.

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 sheet metal part according to the invention correspondingly has a tensile strength Rm of at least 1000 MPa and a bending angle of more than 70° and is formed from a flat steel product consisting of (in % by weight):

    • C: 0.10-0.30%,
    • Si: 0.5-2.0%,
    • Mn: 0.5-2.4%,
    • Al: 0.01-0.2%,
    • Cr: 0.005-1.5%,
    • P: 0.01-0.1%,
    • as well as, in each case optionally, additionally of one or more elements from the group “Ti, Nb, V, B, Ni, Cu, Mo, W”, provided that
      • Ni: 0.005-0.1%,
      • Nb: 0.005-0.1%,
      • V: 0.001-0.2%,
      • B: 0.0005-0.01%,
      • Ni: 0.05-0.4%,
      • Cu: 0.01-0.8%,
      • Mo: 0.01-1.0%,
      • W: 0.001-1.0%,
        and the remainder of iron and unavoidable impurities,
        less than 0.05% of which belong to S and less than 0.01% belong to N,
    • wherein the structure of the sheet metal part is 40-100% by area made of plate-shaped bainite, which is formed from
    • 70-95% ferrite,
    • 2-30% high carbon phases which are designed to be at least 70% plate-shaped with a plate length PL of at least 200 nm with a ratio of the plate length PL to the plate width PB of the plate-shaped high carbon phase PL/PB of at least 1.7 and are arranged at a distance of 50 nm to 2 μm, and
    • the remainder of less than 5% other
      components,
    • wherein the remainder of the structure of the sheet metal part which is not taken by the plate-shaped bainite is made of up to 40% by area of non-plate-shaped bainite,
      • which is formed 70-95% of ferrite,
      • 2-30% of high carbon phases and
      • less than 5% of other components,
    • wherein the sum of the shares of the plate-shaped and non-plate-shaped bainite in the structure of the sheet metal part makes up at least 60% by area,
    • wherein the remaining austenite content of the structure of the sheet metal part is 2-20% by volume,
      and
    • wherein the remainder of the structure of the sheet metal part not taken up by the bainite components consists of one or more components of the following group: “martensitic or austenitic components, proeutectoid ferrite, iron carbide, iron nitride, transition metal carbide, transition metal nitride, non-metal carbide, non-metal nitride, metal or non-metal inclusions, sulfide and other unavoidable impurities”.

The remainder which does not consist of ferrite and high carbon phases and which takes in less than 5% of the plate-shaped bainite comprises for example nitride from microalloying elements or other inclusions.

“Bainite” means the conversion product which forms from the austenite in the structure during cooling of the steel from which the sheet metal part according to the invention is made. The bainite is not a single phase. Rather, bainite always consists of at least bainitic ferrite and one or more high carbon phases.

“plate-shaped bainite” here means a mixture of ferrite and high carbon phases that are designed to be at least 70% plate-shaped and up to 5% remaining components.

In this case, the term “high carbon phases” means austenite, cementite and other carbides.

Ferrite can easily be represented in the ground part pattern of a sample of the respective sheet metal part by means of etching with 3% nital solution.

The high carbon phases can also be identified in the ground part pattern after etching with 3% nital solution and can be determined using a grid electron microscope. While ferrite is significantly remove by the etching agent, the high carbon phases broadly remain in their original form as they are hardly etched at all. When quantifying the high carbon phases by shape, size and position relative to one another, only the phases that remain after etching are taken into account in other words those that were ground before the etching. Any carbides at a greater depth that have only been released by the ferrite being etched off are not taken into account. Otherwise the result would depend on the depth of the ferrite that has been etched away.

The remaining percentage of austenite in the overall structure is generally determined using microdiffractometry. Cementite is the most stable and most significant iron carbide with the stoichiometric composition Fe3C.

As part of the high carbon phases, cementite is not determined separately but rather is determined along with all of the high carbon phases.

The structure of a sheet metal part according to the invention consists of:

  • i) plate-shaped bainite (percentage in the total structure 40-100% by area), wherein 70-95% of the respective proportion of the plate-shaped bainite is taken in by ferrite, 2-30% of the respective proportion of the plate-shaped bainite is taken in by high carbon phases, 70% of which are designed to be plate-shaped with a plate length of at least 200 nm, a ratio of plate length to plate width of at least 1.7 and are arranged at a distance of 50 nm to 2 μm, and as a remainder less than 5% of the proportion of the plate-shaped bainite is taken in by other components, which could be nitrides of microalloying elements or other inclusions;
  • ii) other bainite, which means bainite that is non-plate-shaped, such as globular bainite, wherein this can take in other non-plate-shaped bainite up to 40% by area of the total structure and wherein here too 70-95% of the non-plate-shaped bainite is taken in by ferrite, 2-30% of the non-plate-shaped bainite is taken up by high carbon phases and as a remainder less than 5% of the respective proportion of the plate-shaped bainite is taken up by other components such as nitrides of the microalloying elements or other inclusions;
    and
  • iii) as a remainder from martensitic or austenitic components including tempered martensite, non-tempered martensite or austenite, and as a further remainder could include proeutectoid ferrite, iron carbides, iron nitrides, transition metal carbides, transition metal nitrides, non-metal carbides, non-metal nitrides such as boron carbonitride, metallic inclusions, non-metallic inclusions, sulfides and unavoidable impurities, wherein it is evident that the proportion of the affected remainder in the total structure can also be “0”, in other words practically not detectable or so minimal that it has no technical effect.

The bainite proportions (proportion of the plate-shaped bainite and proportion of the other, non-plate-shaped bainite) in the structure of a sheet metal part according to the invention defined above under i) and ii) are set such that they total at least 60% by area of the structure of the sheet metal part. In addition to the bainite proportions specified according to the invention, martensite proportions of up to 30% by area of the structure of the sheet metal part according to the invention can be tolerated, wherein the proportion of martensite ideally is as low as possible, in particular less than 20% by area or less than 5% by area.

It is therefore key to the invention that the bainite present in the structure of a sheet metal part according to the invention is, to a substantial degree, ideally more than 50%, plate-shaped. This means that the components of the bainite in question are plates of bainitic ferrite and high carbon phases such as residual austenite and cementite.

Reference is made to the attached FIGS. 1A and 1B to explain the fundamental construction of the structure of a sheet metal part according to the invention. These indicate possible configurations of the high carbon phases in black in each case. The area shown in white between the black, high carbon phases is the ferrite. Any number of additional segregations can take place in the white area, the maximum length of which segregations in the cut part is 200 nm.

As shown using FIGS. 1A and 1B, at least 70% of the high carbon phases of the plate-shaped bainite according to the invention is plate-shaped. These 70% plate-shaped, high carbon phases have a length PL of at least 200 nm and a ratio of the length PL to the width PB that is at least 1.7 times greater than the width PB of the respective plate (PL/PB>1.7). The size of the plates of the high carbon phases of the plate-shaped bainite is set such that the ferrite plates that lie between them are sufficiently far away from one another to avoid simple bypassing by displacement. Stretched plates (PL/PB>1.7; FIGS. 1A and 1B) are therefore necessary to obtain the ductility. A block-like shape (PL/PB<1.7) would lead to an increased risk of cracking at shear stresses.

The latter would be particularly disadvantageous under a bending load. The distance PA between two adjacent plates aligned in parallel to one another of the high carbon phase must be at least 50 nm, preferably at least 100 nm, and a maximum of 2 μm. The distance PA is the effective grain size of the bainitic ferrite. The smaller the grain size, the higher the resistance to deformation and therefore the strength of the affected structural component. In order to ensure sufficient strength, the distance may not be more than 2 μm, preferably not more than 1.2 μm. If the distance PA were under 50 nm, the strength would increase so significantly that this area would barely deform any longer as the critical tearing tension in the entire structure would have been reached. This would result in brittle material failure, which should be avoided. Two plates K are deemed to be “aligned in parallel to one another” if the alignment of the longest sides of the plates deviates by less than 25°.

The structural quality according to the invention has several advantages which lead to an exceptional combination of strength and flexibility:

    • i. The high strength of at least 1000 MPa is achieved by the fineness of the structure and not by brittle components such as martensite. According to the Hall-Petch relationship, the strength increases as the grain size decreases. In the sheet metal part according to the invention, the maximum orthogonal distance between two closest high carbon plates reflects the effective grain size. The two necessary components of the structure are austenite and bainitic ferrite, both of which have a high degree of deformability. If cementite is also formed, this is even finer still than the austenite. This has barely any detrimental effect on the bending properties, even though cementite itself represents a very hard and brittle phase. When considered over a wider range (>100 μm), the structure is in turn very homogeneous, which is critical to good flexibility. FIG. 2 shows a light microscope image of a ground part of a sample of a steel processed and composed according to the invention magnified 1000×. The very good macroscopic homogeneity can be clearly seen.
    • ii. The function of the austenite in the structure of a sheet metal part according to the invention is predominantly to prevent the free movement of displacements by the ferrite and therefore to achieve a higher resistance to deformation (=strength). At the same time, cracks that occur during deformation are caught by the layered structure and therefore not grow to a critical crack length and lead to a premature failure on bending.
    • iii. In addition to the high carbon content set by partitioning the carbon at 350-450° C., the high level of fineness of the austenite also ensures that it is mechanically stabilized against the formation of martensite that is induced by deformation. The formation of coarse, brittle martensite would worsen the flexibility significantly. The danger of larger percentages of martensite forming in the structure of a sheet metal part according to the invention is limited twice by the particular fineness of the remaining austenite: on the one hand the small grain size leads to a further reduction of the Ms temperature, so less austenite is converted into martensite during deformation. If martensite does form, this is on the other hand so fine that the negative impact on the mechanical properties is limited.

When it comes to the plate-shaped ferrite in the structure of a sheet metal part according to the invention, this is so regularly interrupted by plates in the high carbon phase that at any given point a high carbon plate is a maximum of 1 μm, preferably a maximum of 0.6 μm, away. This measure restricts the range of movement of displacements in the ferrite sufficiently for the exceptionally high strength of sheet metal parts according to the invention is achieved as a result of the very fine effective grain size.

As a result of its relatively low Si content of up to 2% by weight, preferably up to 1.4% by weight, particularly preferably up to 1% by weight, the invention allows for a hot-dip refined coat on the sheet metal part, in particular with an aluminum-based protective coating. In the event that a coating of this kind is applied, the deformation process can be carried out in an operationally safe manner in atmospheric air without this resulting in scaling and the associated problems.

The composition of the flat steel product from which the sheet metal part according to the invention is shaped is selected such that a strength of at least 1000 MPa, in particular at least 1100 MPa, can be achieved at optimal deformability of the flat steel product, wherein strengths of 1200 MPa and above are regularly achieved.

At the same time, sheet metal parts according to the invention have a bending angle determined according to VDA 238-100 of more than 70° . A high bending angle of this type means that a high energy absorption occurs on a sheet metal part according to the invention used as a body component in a passenger or transport vehicle as a result of bending if the sheet metal part is exposed to a sudden, sharp deformation load such as that which occurs if an obstacle is hit or the like, in other words in a typical accident situation.

The combination of properties set out above can in particular be achieved by a component according to the invention being hot stamped, in other words, as explained in greater detail below, heat is removed from it so quickly that the structure specified according to the invention is set, as a result of which the conditions for the properties achieved according to the invention are created.

Their special properties make sheet metal parts according to the invention particularly suitable for use as a part of a body or a chassis of a vehicle, in particular a land-bound vehicle.

Specifically, the steel of a sheet metal part according to the invention, as the basis for this combination of properties, contains the following compulsory components (C, Si, Mn, AL, Cr, P, Fe) and optionally added, in other words optional components that do not have to be present (Ti, Nb, V, B, Ni, Cu, Mo, W). The contents of the individual components of the steel from which the sheet metal part according to the invention is made are determined as follows:

0.10-0.30% by weight carbon (“C”) is found in the steel from which the sheet metal parts according to the invention are made. C contents set in this way contribute to the durability of the steel by delaying ferrite and bainite formation and stabilizing the remaining austenite in the structure. A carbon content of at least 0.10% by weight is needed to achieve a sufficient durability and the associated high strength. From a C content of above 0.30% by weight, however, the bainite formation is delayed too greatly and sufficient conversion is not ensured during the hold time provided for according to the invention or the air cooling. A low conversion temperature is needed to achieve a particularly high level of strength of the bainite. This is in turn limited in a downwards direction by the conversion of martensite, which in turn is shifted to lower temperatures by C. C in the contents provided for according to the invention decreases the Ac3 conversion temperature and the martensite starting temperature MS. C contents of at least 0.13% by weight, in particular at least 0.15% by weight can be provided in order to make use of the positive effects of the presence of C in a particularly safe manner. At these contents and taking into account the further stipulations of the invention, strengths of at least 1000 MPa, in particular at least 1100 MPa can be reached safely. If negative impacts of the presence of high C contents on the properties of a sheet metal part according to the invention are avoided, this can be achieved by limiting the C content to a maximum of 0.25% by weight, in particular a maximum of 0.20% by weight. Compliance with the lower upper limits for the C content contribute in particular to the improvement in weldability as at lower C contents greater differences in hardness between the welding spot and the surrounding material of the sheet metal part are avoided.

Silicone (“Si”) is used in the steel of a flat steel product according to the invention at contents of 0.5-2.0% by weight to suppress the precipitation of cementite. Si is practically insoluble in cementite, so nucleation is reduced significantly in the presence of sufficient Si contents. An Si content of less than 0.5% by weight would not be sufficient to suppress the precipitation of cementite from bainitic ferrite at the holding temperatures specified according to the invention. The remaining austenite can also be stabilized by the Si content specified according to the invention of at least 0.5% by weight. This effect can be further intensified by increasing the Si content to at least 0.6% by weight, in particular at least 0.7% by weight. Contents of at least 0.7% by weight Si open up a greater process window in the heat deformation by slowing the remaining austenite decay considerably. At an Si content exceeding 2.0% by weight, however, the surface quality and coatability of a sheet metal part obtained according to the invention would decrease too significantly. If a sheet metal part according to the invention or the flat steel product from which the sheet metal part is shaped is costed with a hot dipped coating, it can be expedient to limit the Si content to a maximum of 1.4% by weight, in particular a maximum of 1.0% by weight to avoid coating problems. This applies in particular if the hot dip coating is to be carried out with a molten mass with an Al base which contains Si. At the same time, lower Si contents enable a flat steel product from which the sheet metal part according to the invention is to be formed to austentise at lower temperatures. Surprisingly, it has been shown here that in a low alloy steel composed according to the invention, it is possible to stabilize significant quantities of austenite at Si contents of less than 1% by weight.

Manganese (“Mn”) is contained in the sheet metal part according to the invention in contents of 0.5-2.5% by weight. Mn is a hardening element in that it delays the formation of ferrite and bainite significantly. It also stabilizes the remaining austenite (austenite generator) and inhibitors a breakdown of the remaining austenite into cementite and ferrite downstream of the bainite conversion. At a manganese content of less than 0.5% by weight, the austenite would not be sufficiently stabilized so there would be a downstream breakdown of the austenite at the Si contents used. By increasing the Mn contents to at least 0.9% by weight, in particular at least 1.1% by weight, the austenite stability can be further significantly increased because in this way, in combination with the other alloy elements provided according to the invention, it is possible to prevent larger structural proportions forming at a holding temperature above the maximum provided for according to the invention. If the manganese content is increased to more than 2.4% by weight, however, the bainite conversion slows so significantly that during the method according to the invention the holding temperature specified according to the invention needs to be maintained for too long to achieve the conversion desired according to the invention of the structure of a sheet metal part according to the invention into a structure that is preferably more than 60% by area bainitic according to the invention. If optimized weldability is to be achieved at the same time, this can be done by limiting the Mn content to a maximum of 2.0% by weight, in particular a maximum of 1.8% by weight. Mn contents of a maximum of 1.6% by weight, in particular less than 1.6% by weight have proven to be particularly favorable, as then the bainitic conversion occurs so quickly that as a result of the associated recalescence after removal from the mold tool there is no need to additionally provide the heat for the workpiece that may be necessary to keep the sheet metal part at the bainitic conversion temperature for a sufficiently long period of time after the molding. In many applications, when the Mn content is set in this way the additional heating can be avoided entirely by selecting a suitable C content of a maximum of 0.2% by weight and a suitable sheet thickness of at least 1.2 mm.

Aluminum (“Al”) is used as a deoxidizing agent in contents of 0.01-0.2% by weight when generating the steel from which a sheet metal part according to the invention is made. At least 0.01% by weight Al is needed to ensure secure binding of the oxygen contained within the molten steel. Al can also be used to harden the N contents in the sheet metal product according to the invention that are undesirable but unavoidable as a result of manufacturing. At the same time, Al inhibits the creation of cementite in the sheet metal part structure. However, Al contents that are too high would shift the Ac3 temperature significantly upwards. From a content of more than 0.2% by weight, Al would impede the austenisation too greatly. In order to avoid the negative impacts of Al in the steel of the sheet metal part according to the invention safely, the Al content can be limited to a maximum of 0.1% by weight.

Chromium (“Cr”) contributes to the hardness of the steel in a sheet metal part according to the invention by slowing diffusive conversions during cooling to the holding temperature specified according to the invention, thereby ensuring a stable hot forming process. This favorable effect starts at a content of 0.005% by weight, with a content of at least 0.15% by weight having proven effective for safe process control in practice. A Cr content that is too high, however, impairs the coatability of the steel. The Cr content of the steel of a sheet metal part according to the invention is therefore limited to a maximum of 1.5% by weight, in particular 0.75% by weight.

Phosphorous (“P”) is needed in the steel of a sheet metal part according to the invention in contents of 0.01-0.1% by weight to as a balance for the reduced Si content to suppress the nucleation of cementite. P segregates to grain boundaries, gaps in the lattice structure and other places that typically function as nucleation points for cementite. In this way, P suppresses the carbon present in the locations in question, thereby decreasing the carbon concentration locally at possible cementite nucleation points resulting in the thermodynamic driving force for precipitation being reduced there, causing cementite precipitation to be suppressed. This effect starts from a P content of at least 0.01% by weight and increases as the P content increases. A phosphorous content that is too high, however, would impair the weldability, coatability and notch impact energy of the steel from which a sheet metal part according to the invention is formed. The maximum P content of this is therefore limited according to the invention to 0.1% by weight.

Titanium (“Ti”) is optionally found in the steel of a sheet metal part according to the invention in contents of 0.005-0.1% by weight to harden nitrogen and in this way to enable the boron which is also optionally present in effective quantities to exert its effect of significantly inhibiting the formation of ferrite. At the same time, Ti as a microalloying element contributes to grain refinement. In order to make use of these positive impacts, a Ti content of at least 0.005% by weight can be provided, with the Ti content optimally being set such that it corresponds to at least 3.42 times the N content of the steel. However, Ti also tends to form coarse Tin and can decrease the cold rolling and recrystallisation capacity significantly. The Ti content is therefore limited to a maximum of 0.1% by weight.

Like Ti, niobium (Nb) can also optionally be added to the steel of a sheet metal part according to the invention in contents of 0.005-0.1% by weight for grain refinement and reduction of the cementite precipitation. However, Nb also worsens the recrystallisation ability at contents of more than 0.1% by weight.

Vanadium (V) can also be optionally added in contents of 0.001-0.2% by weight to further increase the strength of the steel in a sheet metal part according to the invention. V contributes to stabilizing the remaining austenite. In cold strip production, however, V forms vanadium carbide, which must trigger during the austenisation of the material before the hot forming. This ensures that the V content is limited to a maximum of 0.2% by weight. The vanadium in solutions is precipitated during bainite formation in a size of a few nanometers and therefore contributes to the strength by means of precipitation hardening. V contents of at least 0.001% by weight, in particular more than 0.01% by weight are needed for a sufficient driving force.

Boron (“B”) can optionally also be present in the steel of the component according to the invention in contents of 0.0005-0.01% by weight to increase the durability of the steel. B lies on the grain boundaries, thereby decreasing their energy. This suppresses the nucleation of ferrite. B contents of at least 0.0005% by weight are needed for a significant effect. At contents of more than 0.01% by weight, however, increased numbers of boron carbides, boron nitrides or boron nitrocarbides form, which in turn preferably form nucleation points for the nucleation of ferrite and further decrease the hardening effect.

Nickel (“Ni”), which is also optionally present in the steel of a component according to the invention, is an austenite former that improves the stability of austenite and therefore the process stability over longer holding times during bainite formation. In the event that copper is present in the steel of the component according to the invention, the simultaneous presence of Ni can cancel out the negative impact of copper on the hot rolling. Even small quantities of Ni of at least 0.05% by weight can help here. Ni contents of more than 0.4% by weight, however, can result in a slowing of the bainite formation.

Optionally, copper (“Cu”) can also be added to the steel of a component according to the invention to increase the durability. At least 0.01% by weight Cu are sufficient for this. Cu further improves resistance against atmospheric corrosion in uncoated sheets. Cu contents of above 0.8% by weight, however, worsen the hot rolling significantly due to the low-melting Cu phases on the surface.

Molybdenum (“Mo”) can optionally be present in the steel of a sheet metal part according to the invention in contents of 0.01-1.0% by weight to improve the process stability. Mo slows ferrite formation significantly and only has a minimal effect on bainite formation in the temperature range specified according to the invention. From contents of at least 0.01% by weight, dynamic molybdenum-carbon clusters up to ultrafine molybdenum carbides form on the grain boundaries, effectively preventing the movement of the grain boundary and therefore diffusive phase conversions. The gain boundary energy and therefore the nucleation rate of ferrite is also decreased. At contents of more than 1.0% by weight, there is no significant increase in the effects of Mo that are used here.

Tungsten (“W”) can optionally be present in the steel of a sheet metal part according to the invention. It works in a similar manner to Mo, but is effective at lower levels. There is a positive effect on the durability from a W content of 0.001% by weight. From a content of 1.0% by weight, no significant increase in the efficacy of W on the properties that are the focus here can be identified. Nitrogen (“N”) and sulphur (“S”) are fundamentally undesirable as they have a negative impact on the properties of the steel of a sheet metal part according to the invention. N and S are, however, unavoidable in the steel as a result of the manufacturing process. They are therefore attributed to the unavoidable impurities of the steel, which should per se be kept as low as possible (N content <0.01% by weight; S content <0.05% by weight) to ensure that they do not have a negative effect on the properties of the steel.

According to the explanations above, in a steel from which the flat steel product from which the sheet metal part is formed is made, the C, Si, Mn, Al and Cr contents are each set so that they fall in the following content ranges (in % by weight):

  • C: 0.13-0.25%, in particular 0.15-0.20%,
  • Si: 0.6-1.4%, in particular 0.7-1.0%,
  • Mn: 0.9-1.8%, in particular 1.1-1.6%,
  • Al: 0.01-0.1%,
  • Cr: 0.15-0.75%,

In the method according to the invention for manufacturing a sheet metal part obtained according to the invention in the manner set out above, at least the following work steps are undergone:

  • a) Provision of a cut metal sheet consisting of a steel of the following composition (in % by weight): C: 0.10-0.30%, Si: 0.5-2.0%, Mn: 0.5-2.4%, Al: 0.01-0.2%, Cr: 0.005-1.5%, P: 0.01-0.1%, and in each case optionally additionally from one or more elements in the group “Ti, Nb, V, B, Ni, Cu, Mo, W”, with the proportions Ti: 0.005 -0.1%, Nb: 0.005-0.1%, V: 0.001-0.2%, B: 0.0005-0.01%, Ni: 0.05-0.4%, Cu: 0.01-0.8%, Mo: 0.01-1.0%, W: 0.001-1.0% and as a remainder iron and unavoidable impurities, wherein the impurities are made up of less than 0.05% S and less than 0.01% N;
  • b) Heating of the cut sheet such that at least 30% of the volume of the cut sheet, when inserted into a forming tool intended for hot press shaping (work step c)), has a temperature T_Aust above the Ac1 temperature, wherein the Ac1 temperature is determined according to the formula


Ac1=[739−22*% C−7*% Mn+2*% Si+14*% Cr+13*% Mo+13*% Ni]° C.

where % C=C content, % Si=Si content, % Mn=Mn content, % Cr=Cr content, % Mo=Mo content and % Ni=Ni content of the respective steel of the cut sheet;

  • c) Insertion of the heated cut sheet into the forming tool heated to a tool temperature T_WZ of 200-430° C., wherein the transfer time t_Trans for the removal and insertion of the cut sheet is a maximum of 20 s;
  • d) Hot pressing of the cut sheet into the sheet metal part, wherein over the course of the hot pressing the cut sheet is cooled for a time t_WZ of 1-50 s at a cooling speed r_WZ of more than 10 K/s to the cooling stop temperature T_coolstop and optionally held there;
  • e) Removal of the sheet metal part cooled to the cooling stop temperature T_coolstop from the tool;
  • f1) Optional: Holding of the sheet metal part at a holding temperature T_Halt of 300-450° C. for a holding time t_Halt of up to 100 s;
  • f2) Optional: heating of the sheet metal part to a homogenization temperature of 380-500° C. within 1-10 s;
  • f3) Optional: further deformation of the sheet metal part, wherein the deformation can in particular be designed as a calibration step to improve the dimensional tolerance of the sheet metal part;
  • g) Optional: trimming of the sheet metal part;
  • h) Cooling of the sheet metal part to a cooling temperature T_AB of less than 200° C. within a cooling time t_AB of 0.5-200 s.

In the method according to the invention, a cut sheet made of steel composed in a suitable manner in line with the explanations above is provided (work step a)) and is then heating in a known manner such that at least 30%, in particular at least 60%, of its volume has an austenitic structure when it is subsequently inserted into the respective forming tool. In other words, the conversion of ferritic into austenitic structures must not yet be complete on insertion into the forming tool. Rather, up to 70% of the volume of the cut part can, on insertion into the forming tool, be made of other structural components such as tempered bainite, tempered martensite and/or non-recrystallized and/or partially recrystallized ferrite. To this end, certain areas of the cut sheet can be kept at a lower temperature level than others in a targeted manner during the heating. In order to do this, the supply of heat can be guided to just certain sections of the cut sheet or the parts that are to be heated less can be shielded from the supply of heat. In the part of the cut sheet material, the temperature of which remains below the minimum temperature specified for the temperature T_Aust, no or significantly less bainite is generated over the course of deformation in the tool, so the structure is significantly softer there than in the respective other parts in which there is a bainitic structure. In this way a softer area can be created in the respective shaped sheet metal part, in which soft area for example there is optimal ductility for the respective purpose of use, while the other areas of the sheet metal part have maximum strength.

Maximum strength properties of the sheet metal part obtained can be achieved by the cut sheet being heated to an austenisation temperature T_Aust in work step b) for which the following applies


Ac3<T_Aust≤1250° C.,

wherein the minimum temperature Ac3 to be exceeded by the temperature T_Aust in this variant is determined according to the formula indicated by HOUGARDY, H. P. in Material Sciences: Steel, volume 1: Fundamental Principles, Verlag Stahleisen GmbH, Düsseldorf, 1984, p. 229.


Ac3=(902-225*% C+19*% Si−11*% Mn−5*% Cr+13*% Mo−20*% Ni+55*% V)° C.

where % C=respective C content, % Si=respective Si content, % Mn=respective Mn content, % Cr=respective Cr content, % Mo=respective Mo content, % Ni=respective Ni content and % V=respective V content of the steel from which the cut sheet is made.

An optimal, even distribution of properties can be achieved by the cut sheet being fully heated through in work step b).

In order to do this, the duration of the austenisation treatment carried out in work step b) can be set such that on the one hand a restriction to 1000 s means a grain coarsening is avoided but on the other hand the speed of the austenitic conversion is taken into account and in particular increases significantly if the cut steel sheet is heated above the Ac3 temperature. In the event of heating above the Ac3 temperature, the austenisation temperature T_Aust is optionally at least 30° C., in particular at least 50° C., above the respective Ac3 temperature of the steel from which the cut sheet to be deformed is made. At a high austenisation temperature of this type, the austenitic conversion happens so quickly that once the temperature in question has been reached it is no longer necessary to hold at this temperature to achieve the full conversion of the structure into austenite. Instead, the cut sheet can be fed on for further processing as soon as the austenisation temperature has been reached.

The cut sheet heated in this way is removed from the respective heating device, which could for example be a conventional heating furnace or a known induction heating device or a conventional device for keeping steel components warm and quickly transported into the forming tool such that the temperature on arrival in the tool is, at 600-900° C., still significantly above the temperature that is critical for the invention of at least 450° C.

In work step c), the transfer of the austenitic cut sheet from the heating device used to the forming tool is preferably completed within less than 20 s. Rapid transport of this type is necessary to avoid cooling before deformation being too great.

When the cut sheet is inserted, the tool is preferably held at a temperature of 200-430° C., preferably 300-400° C., particularly preferably 320-380° C., that is below the cooling stop temperature T_coolstop. The tool temperature T_WZ which can be chosen specifically in each case on insertion of the cut sheet can be determined as followed depending on the cooling stop temperature T_coolstop at which the sheet metal part is removed from the tool and the sheet thickness D of the cut sheet to be deformed into the sheet metal part:

T_coolstop - 100 ∘ C . * ( D 1.4 mm ) 1 2 < T_WZ < T_coolstop - 20 ∘ C . * ( D 1.4 mm ) 1 2

In the subsequent cooling carried out during the deformation of the cut sheet into the sheet metal part, a cooling speed of at least 10 K/s, in particular at least 20 K/s or at least 30 K/s is needed to prevent a conversion of the austenite before the holding temperature is reached. Conversion into both ferrite and bainite at a temperature that is more than 100° C. above the target holding temperature T_Halt of 300-450° C., in particular 320-430° C., in particular 320-400° C., is undesirable as the conversion products would have a significantly lower strength. This would also lead to low overall strength and to decreased flexibility.

In the tool, the cut sheet is therefore not only deformed into the sheet metal part but simultaneously cooled to a cooling stop temperature in the range from 450-300° C., preferably 430-320° C., particularly preferably 400-320° C. To this end, the tool is set to a tool temperature T_WZ of 200-430° C. At the cooling stop temperature, some martensite may already have formed in the structure of the sheet metal part that can function as a nucleation point. The majority of the structure at this point, however, is still made of unstable austenite, which subsequently converts to fine bainite extremely quickly. The addition of Si, Al and P according to the invention slows the formation of carbides so either no or only fine carbides are precipitated. The conversion, enabled by the alloying specifically determined by the invention, is carried out so quickly that a long holding time in the temperature range of 450-300° C., in particular 430-320° C., in particular 400-320° C., is not necessary. Specifically, the invention provides for a time t_WZ of 1-50 s for the cooling and where applicable holding at the temperature T_coolstop in the closed tool reached after cooling in the forming tool. The manufacturing process according to the invention can as such be easily integrated into a short, timed work cycle.

After the cooling, the sheet metal part obtained is optionally held at a holding temperature of 300-450° C., in particular 320-430° C., in particular 320-400° C., over a period of time t_Halt sufficient for conversion into the desired structure. This holding can be carried out both in the forming tool before removal or in a separate device after removal from the forming tool.

A particular advantage of the combination of the material according to the invention with the method according to the invention is the brief nature of the holding time t_Halt needed in practice to form the bainitic structure. Tests have shown that more than 50% of the austenite is converted even after just a few seconds. This leads to a high dimensional tolerance of the sheet metal part produced according to the invention with short processing times and excellent mechanical properties. A holding time t_Halt of longer than 100 s would be both inefficient and disadvantageous for the constitution of the structure. If the holding times t_Halt are too long, this would lead to an increased conversion of the remaining austenite to cementite, which would primarily worsen the tensile properties by decreasing the elongation at break.

If work step f1) is carried out in particular, it can be expedient to carry out an additional deformation optionally as a further work step, which for example contributes to an improvement in the dimensional tolerance of the sheet metal part.

If there is no separate holding, in other words work step f1) is not completed (t_Halt=0 seconds), a first part of the conversion takes place in the tool while a second part of the conversion takes place during the cooling in work step h), which is preferably carried out as air cooling in this case.

The invention therefore provides a method by means of which sheet metal parts can be produced, the structures of which are characterized by a plate-like structure. The plate-like structure of this type results in a combination of high tensile strength (>1000 MPa, in particular >1100 MPa) and a very high bending angle (uncoated >80°). Sheet metal components can therefore be manufactured particularly quickly using the method according to the invention. When the approach according to the invention is used, sheet metal parts with a high level of strength and optimal energy absorption capacities can be produced within total times of less than one minute.

In order to initiate the conversion processes used according to the invention in an operationally safe manner, the composition of the steel can be set such that the activation energy required for bainite formation Qb is sufficiently low. In order to do this, the C content % C, the Mn content % Mn, the Mo content % Mo, the Cr content % Cr, the Ni content % Ni and the Cu content % Cu can be set depending on the B content of the steel, in each case in % by weight, such that the activation energy Qb of the bainite formation is less than 45 kJ, in particular less than 40 kJ, particularly preferably less than 35 kJ to achieve sufficiently rapid bainite conversion, within the limits set out above and specified according to the invention. Qb can be calculated for B contents of up to 0.0005% by weight using the formula


Qb[kJ]=(90*% C+10*(% Mn+% Mo)+2*(% Cr+% Ni)+1*% Cu)[kJ/% by weight],

and for B contents of more than 0.0005% by weight according to the formula


Qb[kJ]=(90*% C+10*(% Mn+% Mo)+2*(% Cr+% Ni)+1*% Cu+2)[kJ/% by weight], .

wherein the C contents % C, the Mn content % Mn, the Mo content % Mo, the Cr content % Cr, the Ni content % Ni and the Cu content % Cu, in each case in % by weight, are used in these formulas.

Qb values of less than 40 kJ have proven to be advantageous for sheet thicknesses of more than 2 mm, Qb values of less than 35 kJ have proven to be advantageous for sheet thicknesses of 1-2 mm and Qb values of less than 34 kJ have proven to be advantageous for sheet thicknesses of less than 1 mm.

A low activation energy Qb is in particular helpful if the holding time t_Halt needs to be kept low, in particular when the holding time t_Halt should be 0 seconds. The heat output, in other words the specific conversion heat per time unit, is significant and counteracts any cooling, particularly if the Qb is less than 38 kJ. This release of heat is sufficient to keep the component at temperature even when the Qb values are low by means of the conversion heat. It is also possible for the component to increase in temperature slightly and only cool after conversion has been completed without heat being applied from the outside.

The lower limit of the range specified according to the invention for the holding temperature T_Halt is 300° C., as temperatures of below this amount would fall significantly below the martensite starting temperature MS even at maximum exploitation of the alloying options within the scope of the invention. Since, however, as much bainite as possible needs to be formed, the formation of martensite should be avoided as far as possible. Proportions of martensite of more than 30% by area would lead to a significant worsening of the properties of a sheet metal part according to the invention. The process control should therefore be selected to make the martensite proportion of the structure of a sheet metal part according to the invention as minimal as possible. The range of the holding temperature T_Halt is restricted to a maximum of 450° C. because the strength of the bainite would drop too quickly at higher temperatures.

According to CAPDEVILA, C. et al. Determination of Ms Temperature in Steels: A Bayesian Neural Network Model. ISIJ International, 42:8, August 2002, 894-902, the martensite starting temperature of a steel within the specifications can be calculated according to the invention according to the formula


Ms[° C.]=(490.85-302.6% C−30.6% Mn−16.6% Ni−8.9% Cr+2.4% Mo−11.3% Cu+8.58% Co+7.4% W−14.5% Si)[° C./% by weight],

wherein C % represents the C content, % Mn represents the Mn content, % Mo represents the Mo content, % Cr represents the Cr content, % Ni represents the Ni content, % Cu represents the Cu content, % Co represents the Co content (not found in steel with a composition according to the invention), % W represents the W content and % Si represents the Si content of the respective steel in % by weight.

If different local contact pressure occur in the tool during the deformation into the sheet metal part, there may be an inhomogeneous distribution of temperature in the sheet metal part when it is removed from the tool. In order to ensure even and complete bainitic conversion even in a case of this type, optional additional heating can homogenize the temperature distribution such that the temperature over the entire sheet metal part is in a temperature range from Ms−20 to Ms+100° C., in particular Ms to Ms+80° C.

At low holding temperatures T_Halt in particular, it is possible for the conversion to happen quickly, which is desirable, but for the remaining austenite not to remain stable down to room temperature due to an inhomogeneous distribution of the carbon within it. In order to remedy this and to ensure homogeneous C distribution in the remaining austenite, the sheet metal part obtained can be heated to a temperature of 380-500° C. after the holding time necessary for the formation of the bainitic structure that may go beyond “0” (work step f2)). In the cooling that occurs after the homogenization temperature in question is reached, the carbon has sufficient time and thermal energy to redistribute. The upper limit of 500° C. for the homogenization temperature results from the fact that a significant softening would occur at even higher temperatures. The diffusion rate would be too low at homogenization temperatures of less than 380° C. A temperature range of 420-470° C. has proven to be particularly advantageous for the homogenization temperature.

Even after the optional work step f2) it can be expedient to carry out an additional deformation which for example can be completed as a calibration step to improve the dimensional tolerance of the sheet metal part.

Laser trimming of the press hardened components in press hardening is very expensive. In conventional press hardening, it is necessary for the component to only be removed from the tool below the martensite finishing temperature, which is generally below 200° C. In contrast to this, in the method according to the invention the component can be removed from the component at above 300° C., preferably at least 350° C. The decreased hardness and high ductility at the higher temperature means that at this point in the method according to the invention the workpiece can also be cut warm (work step g)).

The latter is very cost effective compared to laser trimming and leads to greatly simplified logistics. Cycle times of 1-20 s, in particular 1-10 s can be achieved using the approach according to the invention. If the temperature homogenization (work step d)-f2)) requires more than 20 s, the work steps in question can be sub-divided into several steps that are carried out one after the other in different aggregates.

In the cooling in the subsequent step h) of the method according to the invention, the sheet metal part obtained is cooled to a cooling temperature T_AB of less than 200° C. within a time t_AB of 0.5-200 s. The time t_AB needed for the cooling and accordingly the speed at which the cooling is carried out are adjusted depending on the progression of the conversion of the structure in the previous work steps. If some of the conversion still needs to be carried out during the air cooling, the cooling is carried out under a medium such as stationary or moving air in which comparatively low cooling rates are achieved. It is necessary to note that the subsequent cooling is a decisive factor in the homogeneous distribution of carbon in the remaining austenite. While the bainite conversion in the steel according to the invention is carried out so quickly that good dimensional tolerance is achieved, the dwell time t_WZ is, on average, not sufficient to achieve homogeneous carbon distribution. Where there are local carbon contents of <0.8% in the remaining austenite, there is also a risk of martensitic conversion before room temperature is reached. This martensite which is then present in the structure is very hard due to its carbon content resulting in very poor bending behavior and must therefore be preventing. The invention therefore preferably provides for a comparatively slow cooling within a cooling duration of at least 5 seconds. The slower cooling times t_AB of at least 5 s also contribute to the dimensional tolerance of the components by avoiding warping. The upper limit of 200 s for the range over which the cooling time t_AB extends ensures an acceptable cycle time in the component manufacturing.

In order to prevent the formation of scales during austenisation and protect the sheet metal part according to the invention against corrosion, a metallic corrosion protection coat can be applied to the flat steel product from which the sheet metal part is made. Coats based on aluminum are particularly suitable for this, with coats of this type typically having Si contents to optimize their protective effect (known as “AS coats”). Al-based protective coats can be applied to a flat steel product processed according to the invention in a particularly economical manner using hot dip coating. The following composition (in % by weight) has proven to be particularly suitable for protecting a flat steel product processed according to the invention and correspondingly a sheet metal part according to the invention: 3-15% Si, 1-15% Fe, optionally up to 40% Zn, in particular up to 10% Zn, optionally up to 1% Mg, preferably 0.11-0.5% Mg, and Al and unavoidable impurities as the remainder. Typical layer thicknesses of a coast of this type in the cut sheet deformed into the sheet metal part according to the invention before hot shaping are 2 μm-40 μm, in particular 10 μm-35 μm.

The plate-shaped structure results in a combination of higher tensile strength (>1100 MPa) and a very high bending angle (uncoated significantly >80°, see example applications). While plate-shaped, bainitic structures have already been adequately described in the prior art, the fact that this results in very good bending properties is absolutely novel. The fact that a low-cementite, plate-like bainite structure with tensile strengths >1000 MPa and excellent bending properties can be created in very short holding times or only with air cooling and within times of <1 minute is also absolutely novel.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is explained in greater detail below using exemplary embodiments. In the figures:

FIG. 1A is a schematic view of the structure of a sample taken from a ground part according to the invention;

FIG. 1B is a schematic view of the structure of a sample taken from a second ground part according to the invention;

FIG. 2 shows a light microscope image of a ground part of a sample of a steel processed and composed according to the invention magnified 1000×;

FIG. 3 shows a grid electron microscopic image of a ground part of a sample produced according to the invention;

FIG. 4 shows a light microscopic figure of a ground part of a sample produced according to the invention;

FIG. 5 is a diagram showing the progression of the bainite conversion from the austenite over time in an alloy composed according to the invention at 400° C. in the dilatometer.

 DESCRIPTION OF THE PREFERRED EMBODIMENTS

Melts A-O have been created to test the invention, each of which was composed according to the specifications of the invention, with the compositions listed in Table 1.

Cold-rolled steel strips have been produced from the melts composed in this way in the conventional manner. Some of the steel strips were also hot dip coated with what is known as an AS coat, also in the conventional manner. The AS coating consisted of 3-15% by weight Si, 3% by weight Fe and the remainder Al and unavoidable impurities at a coat thickness of 22 μm per side of the cut sheet.

Cut sheets were divided off the steel strips and were used for further tests. In these tests, sheet metal part samples 1-24 in the form of 200×300 mm2 large plates were hot press molded from the respective cut sheets. In order to do this, the cut sheets were heated in a heating device, for example a conventional heating furnace, from room temperature to an austenisation temperature T_Aust at which they were heated and held for an austenisation time t_Aust. The cut sheets are then removed from the heating device and inserted into a forming tool heated to a tool temperature T_WZ. The transfer time consisting of the removal from the heating device, the transport to the tool and the insertion into the tool was 7 s in each case.

The cut sheets were reshaped into the respective sheet metal part in the forming tool.

With the exception of samples 5, 22 and 23, the sheet metal parts obtained were then removed from the forming tool and held at a holding temperature T_Halt in a temperature control tool and held at the temperature T_Halt over a holding time t_Halt to ensure homogenization of the temperature distribution and even conversion of bainite.

Sample 5 was not subjected to temperature homogenization (work step f1) of the method according to the invention), but instead after a transfer carried out within a transfer time t_trans was subjected to rapid heating in a rapid heating tool, by means of which it was heated to a homogenization temperature T_HOM at a heating speed HR.

The samples were then cooled to room temperature. Cooling was either carried out in stationary air at a cooling rate of 7 K/s or in compressed air at 30 K/s.

Samples 22 and 23 were only removed from the tool after the cooling stop temperature had been reached and cooled in stationary air.

Some of the samples were also subjected to cathodic dip painting (KTL) both to test their ability to be painted and to test whether the KTL treatment changes the mechanical properties. A comparison of samples 3 and 4 shows that KTL itself has barely any impact on the mechanical properties of samples cooled in stationary air.

The parameters provided for or set in the processing of samples 1-24 (“coating”, austenisation temperature “T_Aust”, austenisation time “t_Aust”, tool temperature “T_WZ”, duration of the cooling process “t_WZ” in the forming tool, cool stopping temperature T_coolstop, holding temperature “T_Halt”, holding time “t_Halt”, transfer time “t_Trans”, heating speed “HR”, homogenization temperature “T_HOM”, “air cooling” and cathodic dip painting “KTL”) are shown in Table 2.

Table 3 also shows the sheet thickness D of the cut sheets from which the individual samples 1-24 are produced and, for the samples 1-24 obtained, the yield strength Rp02 determined according to DIN EN ISO 6892-1:2009, tensile strength Rm and elongation at break A50, the direction of the tensile specimen relative to the rolling direction or the bending axis relative to the rolling direction (“Q”=transverse), the bending angle BW_Fmax determined according to VDA standard 238-100, the formula indicated in the standard from which the punch movement is calculated (the angle BW_Fmax is the bending angle at which the force in the bending test is at a maximum) and the corrected bending angle BW are indicated. The corrected bending angle BW_korr is calculated according to the formula:


BW_korr=BW*/Thickness

and considers the fact that the bending angle is highly depending on the thickness. The impact of the thickness is eliminated in the corrected bending angle.

Finally, Table 4 indicates certain structural parts in the samples 1-24 obtained, wherein the column “total bainite” shows the percentage of bainite, the column “plate-like bainite” shows the percentage of the plate-shaped bainite in the sense of the invention, the “martensite” column shows the percentage of martensitic components and the RA column shows the percentage of total remaining austenite in the total structure.

FIGS. 1A, 1B and 2 have already been explained above.

In the example in FIG. 3, a grid electron microscope image of a cut sheet of a structural ground part of a sample obtained from the alloy of molten mass A, embodiment 1, the areas removed by etching are bainitic ferrite (bF). The remaining areas are one of the high carbon phases of remaining austenite (RA) or cementite (Z). What these have in common is that they are made of at least 85% by weight Fe and at least 0.6% by weight C. The high carbon content means they can barely be etched and protrude almost up to the polishing level. Both high carbon phases prevent displacement movements in bainitic ferrite, thereby resulting in an increase in strength. At even higher resolution, the remaining austenite and the cementite can be distinguished by the fact that cementite is more resistant to the etching agent than the remaining austenite as a result of its higher carbon content, so the cementite portions have a smooth surface while the surface of austenite appears to be rough.

FIG. 5 demonstrates, using the example of the alloy “O” that the conversion of austenite into bainite is particular rapid in steel alloyed according to the alloy concept according to the invention. This not only has process technology-related advantages, it also makes it possible to use very low Si contents.

The latter enables an AS coat that adheres well to the respective steel substrate as demonstrated in FIG. 4 by means of a light microscopic figure of a ground part that comes from the cut sheet metal from which sheet metal part sample number 7 was created.

FIG. 5 shows the dilatometer curve of bainitic conversion at 400° C. using the example of the alloy “O”. According to this, the bainitic conversion is at 25% after just 10 s and after a further 10 s has progressed to 66%.

TABLE 1 Alloy C Si Mn Al P Cr Ti B Nb Ni Cu Mo A 0.22 0.8 1.2 0.03 0.014 0.23 0.03 0.0027 — — — 0.2  B 0.22 0.8 1.2 0.03 0.03 0.23 0.03 0.0027 — — — 0.2  C 0.19 0.8 0.8 0.03 0.014 0.5 0.03 0.0027 — — — — D 0.22 0.8 0.8 0.1 0.014 0.5 — 0.0027 — — — — E 0.22 0.8 1.2 0.03 0.014 0.5 — — — — — — F 0.22 0.8 1.2 0.03 0.03 0.23 0.03 — — — — — G 0.22 0.8 1.2 0.03 0.03 0.23 0.03 0.0023 — — — — H 0.19 0.8 1.2 0.03 0.014 0.23 — — 0.025 — — — I 0.19 0.8 1.2 0.03 0.014 0.23 — — 0.025 0.1 0.2 — J 0.19 0.8 1.2 0.03 0.014 0.23 — — 0.025 0.1 0.2 0.15 K 0.19 0.8 1.2 0.03 0.03 0.23 — — 0.025 0.1 0.2 0.15 L 0.22 0.8 1 0.03 0.014 0.5 — — 0.025 — — 0.15 M 0.22 0.8 1 0.03 0.03 0.5 — — 0.025 — — 0.15 O 0.18 0.8 1.5 0.03 0.014 0.23 — 0.0023 — — — — FIGURES in % by weight, the remainder iron and unavoidable impurities: “—” not present indicates data missing or illegible when filed

TABLE 2 Sample T_Aust t_Aust T_WZ t_WZ T_coolstop T_Halt t Halt t_Transfer no. Melt Coating [° C.] [s] [° C.] [s] [° C.] [° C.] [° C.] [s] 1 A — 925 300 350 9 400 400 100 2 A AS 925 300 300 10 350 400 30 3 B — 925 300 350 9 400 400 100 4 B — 925 300 350 9 400 400 100 5 B AS 925 300 375 8 425 — — 7 6 C — 925 300 350 9 400 400 30 7 C AS 925 300 300 10 350 400 30 8 D — 925 300 400 8 450 400 30 9 E — 925 300 350 9 400 400 100 10 E AS 925 300 400 8 450 400 30 11 E — 925 300 350 9 400 400 30 12 F — 925 300 350 9 400 400 100 13 G AS 925 300 350 9 400 400 25 14 G — 925 300 350 9 400 400 100 15 H — 925 300 350 9 400 400 30 16 I — 925 300 350 9 400 400 30 17 J — 925 300 350 9 400 400 30 18 K AS 925 300 350 9 400 400 100 19 K — 925 300 350 9 400 400 30 20 L — 925 300 400 8 450 400 30 21 M — 925 300 400 8 450 400 30 22 O AS 925 300 350 10 400 — — 23 O AS 925 300 350 20 400 — — 24 O AS 925 300 350 9 400 400 40 indicates data missing or illegible when filed

TABLE 3 Sample Thickness Rp02 Rm A50 BW_Fmax BW_korr number [mm] [MPa] [MPa] [%] Direction [°] [°] 1 1.56 911 1240 7.8 Q 104.3 130.3 2 1.49 1081 1287 8.1 Q 76.8 93.7 3 1.54 941 1276 9.4 Q 100.9 125.2 4 1.54 947 1261 8.6 Q 98.5 122.2 5 1.54 1143 1474 6.7 Q 71.9 89.2 6 1.5 925 1162 8.4 Q 121.9 149.3 7 1.51 1061 1249 7.4 Q 88.5 108.7 8 1.51 821 1151 10.8 Q 112.5 138.2 9 1.56 954 1251 8.8 Q 98.0 122.4 10 1.54 1069 1468 7.2 Q 72.1 89.5 11 1.53 1089 1425 6.8 Q 84.1 104 12 1.52 893 1210 8.5 Q 107.0 131.9 13 1.53 1102 1350 7.9 Q 79.3 98.1 14 1.53 932 1252 8.3 Q 101.3 125.3 15 1.52 870 1150 10.2 Q 115.2 142.0 16 1.54 878 1153 10.2 Q 112.0 139.0 17 1.52 949 1210 9.5 Q 105.0 129.5 18 1.51 970 1241 9.8 Q 84.9 104.3 19 1.51 1006 1305 9.3 Q 94.0 115.5 20 1.5 932 1208 9.1 Q 105.0 128.6 21 1.5 945 1252 9.4 Q 99.1 121.4 22 1.58 917 1227 8.8 Q 78.2 98.3 23 1.58 923 1196 8.5 Q 84.9 106.7 24 1.58 937 1197 8.4 Q 89.0 111.9 indicates data missing or illegible when filed

TABLE 4 Total Plate-like bainite bainite Sample [% [%] Martensite RA number by area] by area [%] [%]  1  95 60  5 4  2  80 50 20 3.5  3  95 55  5 4  4  95 55  5 5  5  70 50 30 3  6  95 70  5 3.5  7  95 70  5 1.5  8  95 75  5 5.5  9  95 50  5 4.5 10  75 65 25 3.5 11  80 50 20 3.5 12  80 50 15 4 13  70 40 30 4.5 14  90 55 10 3.5 15 100 60  0 3 16  95 60  5 4 17  95 70  5 4.5 18  95 65  5 5 19  70 55 30 3.5 20  90 60 10 3.5 21  90 55 10 4.5

Claims

1. A sheet metal part having a tensile strength Rm of at least 1000 MPa and a bending angle of more than 70° that is made from a flat steel product comprising (in % by weight):

C: 0.10-0.30%,
Si: 0.5-2.0%,
Mn: 0.5-2.4%,
Al: 0.01-0.2%,
Cr: 0.005 -1.5%,
P: 0.01 -0.1%,
as well as, in each case optionally, additionally of one or more elements from the group consisting of Nb, V, B, Ni, Cu, Mo, and W provided that
Ti: 0.005-0.1%,
Nb: 0.005-0.1%,
V: 0.001-0.2%,
B: 0.0005-0.01%,
Ni: 0.05-0.4%,
Cu: 0.01-0.8%,
Mo: 0.01-1.0%,
W: 0.001-1.0%,
and the remainder of iron and unavoidable impurities, wherein the unavoidable impurities comprise less than 0.05% S and less than 0.01% N, wherein the structure of the sheet metal part is 40-100% by area plate-shaped bainite, which is formed from
70-95% ferrite,
2-30% high carbon phases which are designed to be at least 70% plate-shaped with a plate length PL of at least 200 nm with a ratio of the plate length PL to the plate width PB of the plate-shaped high carbon phase PL/PB of at least 1.7 and are arranged at a distance of 50 nm to 2 μm, and
the remainder of less than 5% other components,
wherein the remaining structure of the sheet metal part which is not taken up by the plate-shaped bainite consists of up to 40% by area of the total structure of non-plate-shaped marked bainite,
which is formed 70-95% of ferrite,
2-30% of high carbon phases and
less than 5% of other components,
wherein the sum of the shares of the plate-shaped and non-plate-shaped, bainite in the structure of the sheet metal part makes up at least 60% by area,
wherein the remaining austenite content of the structure of the sheet metal part is 2-20% by volume,
and
wherein the remainder of the structure of the sheet metal part not taken up by the bainite components consists of one or more components selected from the group consisting of martensitic or austenitic components, proeutectoid ferrite, iron carbide, iron nitride, transition metal carbide, transition metal nitride, non-metal carbide, non-metal nitride, metal or non-metal inclusions, sulfide and other unavoidable impurities.

2. The sheet metal part according to claim 1, wherein the C content % C, the Mn content % Mn, the Mo content % Mo, the Cr content % Cr, the Ni content % Ni and the Cu content % Cu are set depending on the B content of the steel in each case in % by weight such that the activation energy Qb of the bainite formation is <45 kJ, wherein the following applies for Qb in B contents of up to 0.0005% by weight: and the following applies for B contents of more than 0.0005% by weight:

Qb[kJ]=(90*% C+10*(% Mn+% Mo)+2*(% Cr+% Ni)+1*% Cu)[kJ/% by weight],
Qb [kJ] =(90*% C+10*(% Mn+% Mo)+2*(% Cr+% Ni)+1*% Cu+2)[kJ/% by weight].

3. The sheet metal part according to claim 1, wherein the flat steel product used to form the sheet metal contains (in % by weight) 0.13-0.25% C, 0.6-1.4% Si, 0.9-1.8% Mn, 0.01-0.1% Al and 0.15-0.75% Cr.

4. The sheet metal part according to claim 1, wherein the flat steel product used to form the sheet metal part contains (in % by weight) 0.15-0.20% C, 0.7-1.0% Si and 1.1-1.6% Mn.

5. The sheet metal part according to claim 1, wherein the sheet metal part is provided with a metallic corrosion protection coating.

6. The sheet metal part according to claim 5, wherein the metallic corrosion protection coating consists of (in % by weight) 3-15% Si, 1-3.5 Fe, optionally up to 40% Zn, up to 0.5% of one or more alkaline or alkaline earth metals, optionally up to 1% Mg, with the remainder Al and unavoidable impurities.

7. The sheet metal part according to claim 1, wherein the sheet metal part is hot stamped.

8. A method for producing a sheet metal part, comprising the following steps: where % C=C content, % Si=Si content, % Mn=Mn content, % Cr=Cr content, % Mo=Mo content and % Ni=Ni content of the respective steel of the cut sheet;

a) Provision of a cut metal sheet consisting of a steel of the following composition (in % by weight):
C: 0.10-0.30%,
Si: 0.5-2.0%,
Mn: 0.5-2.4%,
Al: 0.01-0.2%,
Cr: 0.005-1.5%,
P: 0.01-0.1%,
as well as, in each case optionally, additionally of one or more elements from the group. Nb, V, B, Ni, Cu, Mo, and W, provided that
Ni: 0.005-0.1%,
Nb: 0.005-0.1%,
V: 0.001-0.2%,
B: 0.0005-0.01%,
Ni: 0.05-0.4%,
Cu: 0.01-0.8%,
Mo: 0.01-1.0%,
W: 0.001-1.0%,
and the remainder of iron or unavoidable impurities, wherein the unavoidable impurities comprise less than 0.05% S and less than 0.01% N;
b) heating the cut sheet such that at least 30% of the volume of the cut sheet, when inserted into a forming tool intended for hot press shaping, has a temperature T_Aust above the Ac1 temperature, wherein the Ac1 temperature is determined according to the formula Ac1=[739−22*% C−7*% Mn+2*% Si+14*% Cr+13*% Mo+13*% Ni]° C.
c) inserting the heated cut sheet into the forming tool tempered to a tool temperature T_WZ of 200-430° C., wherein the transfer time t_Trans needed to remove and insert the cut sheet is a maximum of 20 s;
d) hot pressing the cut sheet into the sheet metal part, wherein over the course of the hot pressing the cut sheet is cooled for a time t_WZ of 1-50 s at a cooling speed r_WZ of more than 10 K/s to a cooling stop temperature T_coolstop and optionally held there;
e) removing the sheet metal part cooled to the cooling stop temperature T_coolstop from the tool;
f1) optionally, holding the sheet metal part at a holding temperature T_Halt of 300-450° C. for a holding time t_Halt of up to 100 s;
f2) optionally, heating the sheet metal part to a homogenization temperature of 380-500° C. within 1-10 s;
f3) optionally, further reshaping of the sheet metal part;
g) optionally, trimming the sheet metal part;
h) optionally, the sheet metal part to a cooling temperature T_AB of less than 200° C. within a cooling time t_AB of 0.5-200 s.

9. The method according to claim 8, wherein the following applies for the temperature T_Aust reached in work step b): where % C=C content, % Si=Si content, % Mn=Mn content, % Cr=Cr content, % Mo=Mo content % Ni=Ni content and % V=V content of the respective steel of the cut sheet.

Ac3<T_Aust≤1250° C.,
wherein
Ac3=[902-225*% C+19*% Si−11*% Mn−5*% Cr+13*% Mo−20*% Ni+55*% V]° C.

10. The method according to claim 8, wherein the cut sheet is heated to the temperature T_Aust in work step b).

11. The method according to either claim 9, wherein for the heating in work step b) a heating time t_Aust is

1000s/(T_Aust/° C.−Ac3/° C.+10){circumflex over ( )}2≤t_Aust≤1000 s

12. The method according to claim 8, wherein the cooling speed r_WZ in work step d) is more than 20 K/s.

13. The method according to claim 8, wherein the time t_WZ in work step d) is a maximum of 20 s.

14. The method according to claim 8, wherein the temperature T_WZ of the tool on insertion of the cut sheet depending on the cooling stop temperature T_coolstop and the thickness D of the cut sheet to be formed into the sheet metal part is determined as follows: T_coolstop - 100 ∘   C. * ( D 1.4   mm ) 1 2 < T_WZ < T_coolstop - 20 ∘   C. * ( D 1.4   mm ) 1 2

15. The method according to claim 8, wherein the temperature T_coolstop of the sheet metal part on removal from the tool is 300-450° C.

16. The method according to claim 8, wherein the cooling takes place in air in work step h).

17. A body of a vehicle comprising a sheet metal part according to claim 1.

18. A chassis of a vehicle comprising a sheet metal part according to claim 1.

19. A sheet metal part according to claim 1 that is made from a flat steel product consisting of (in % by weight):

C: 0.10-0.30%,
Si: 0.5-2.0%,
Mn: 0.5-2.4%,
Al: 0.01-0.2%,
Cr: 0.005-1.5%,
P: 0.01-0.1%,
as well as, in each case optionally, additionally of one or more elements selected from the group consisting of Nb, V, B, Ni, Cu, Mo, and W provided that
Ti: 0.005-0.1%,
Nb: 0.005-0.1%,
V: 0.001-0.2%,
B: 0.0005-0.01%,
Ni: 0.05-0.4%,
Cu: 0.01-0.8%,
Mo: 0.01-1.0%,
W: 0.001-1.0%,
and the remainder of iron and unavoidable impurities comprising less than 0.05% S and less than 0.01% N.
Patent History
Publication number: 20210189517
Type: Application
Filed: May 22, 2018
Publication Date: Jun 24, 2021
Inventors: Thomas Gerber (Dortmund), Ilse Heckelmann (Kempen), Bernd Linke (Duisburg)
Application Number: 17/057,299
Classifications
International Classification: C21D 9/46 (20060101); C21D 8/02 (20060101); C21D 1/673 (20060101); C21D 1/19 (20060101); C21D 6/00 (20060101); C22C 38/54 (20060101); C22C 38/50 (20060101); C22C 38/48 (20060101); C22C 38/44 (20060101); C22C 38/42 (20060101); C22C 38/06 (20060101); C22C 38/04 (20060101); C22C 38/02 (20060101); C22C 38/46 (20060101);