EP3004401B1 - Method for producing a component by hot forming a pre-product made of steel - Google Patents

Description

The invention relates to a method for producing a component by hot forming a precursor of steel according to the preamble of claim 1. As precursors z. As sheets cut from the coil or as a blank board or seamless or welded tubes, which may be additionally cold drawn case by case, understood.

Such components are mainly used in the automotive and commercial vehicle industry, but also in mechanical engineering or construction offer opportunities.

The hotly contested market is forcing automakers to constantly seek solutions to reduce their fleet consumption while maintaining the highest possible comfort and occupant safety. On the one hand, the weight saving of all vehicle components plays a decisive role, on the other hand, but also a most favorable behavior of the individual components with high static and dynamic stress during operation as well as in the event of a crash.

The primary suppliers are trying to meet this need by reducing the wall thickness by providing high-strength and ultra-high-strength steels, while at the same time improving component behavior during production and operation.

These steels must therefore meet comparatively high requirements in terms of strength, ductility, toughness, energy absorption and corrosion resistance and their processability, for example in cold forming and welding.

Among the above-mentioned aspects, the production of components made of hot-processable steels is becoming increasingly important since they ideally meet the increased demands on the component properties with less material expenditure. The production of components by means of quenching precursors from press-hardenable steels by hot forming in a forming tool is known from US Pat DE 601 19 826 T2 known. Here is a previously heated above the Austenitisierungstemperatur to 800 - 1200 ° C and possibly provided with a metallic coating of zinc or based on zinc sheet metal blank formed in a case by case cooled tool by hot forming into a component, during the forming by rapid heat removal the Sheet metal or component undergoes quench hardening (press hardening) in the forming tool and thereby achieves the required microstructural and strength properties.

The metallic coating is applied as corrosion protection usually in the continuous hot dip process on a hot or cold strip or on the precursor produced therefrom, z. B. as hot dip galvanizing or Feueralumierung.

Subsequently, the board is cut to fit the hot forming forming tool. It is also possible to provide the respective workpiece to be formed, or the blank, with a hot-dip coating.

The application of a metallic coating on the preform to be formed prior to hot forming is advantageous in this method because the coating effectively prevents scaling of the base material and, due to an additional lubricating effect, excessive tool wear.

Known thermoformable steels for this application are z. B. the manganese-boron steel "22MnB5" and more recently also luftvergütbare steels according to the DE 10 2010 024 664 A1 ,

In order to achieve components with very high strengths of more than 980 MPa with still sufficiently high toughness, it is from the EP 2 546 375 A1 It is known to reshape a steel having a microstructure in the initial state by means of press-hardening and to adjust a structure of bainite, tempered martensite and retained austenite on the finished component by means of a step-shaped process control. In this case, the sheet to be formed is first heated to a temperature of 750 to 1000 ° C and held at this temperature for 5 to 1000 seconds, then formed at 350 to 900 ° C and cooled to 50 to 350 ° C. Finally, reheating to a temperature of 350 to 490 ° C, which is held in a period of 5 to 1000 seconds. The microstructure on the finished component consists of 10 to 85% martensite, 5 to 40% retained austenite and at least 5% bainite.

However, the production of a component by hot forming by means of press hardening has several disadvantages.

On the one hand, this process requires a great deal of energy by heating the precursor to austenitizing temperature and converting ferrite into austenite, which makes the process expensive and produces significant amounts of CO ₂ .

In addition, to avoid excessive scaling of the sheet surface as described above, an additional metallic protective layer or a lacquer-based protective layer is required or a considerable reworking of the surface rendered difficult by heating and deformation.

Since the forming takes place at temperatures above the Ac ₃ temperature usually well above 800 ° C, also extremely high demands are placed on the temperature stability of these layers.

A further disadvantage is that to obtain appropriate component strengths after press hardening only convertible steels can be used with a sufficient conversion inertia, which must have correspondingly expensive alloying additions for the structure to be achieved and hardness after forming.

In summary, it should be noted that the known method for producing steel components by hot working above the austenitizing temperature due to the required large furnaces associated with long heating times leads to high manufacturing and energy costs and thus high component costs.

To improve the formability of high strength steels is from DE 10 2004 028 236 B3 In addition, it is known to process workpieces instead of by cold forming by hot forming at temperatures of 400 to 700 ° C on to a component (warm forging). The disadvantage here is that the deformed component, in contrast to press-hardened components by the heating below the transformation temperature a softening experiences, the strength compared to the initial state is thus reduced.

From the DE 10 2011 108 162 A1 a method for producing a component by hot forging of a precursor of steel below the Ac ₁ transformation temperature is known, in which an increase in strength in the component is achieved by cold forming the precursor before heating to forming temperature. Optionally, an additional increase in strength in the component can be achieved by using higher-strength materials, such as bainitic, martensitic, microalloyed and dual or multiphase steels. The disadvantage here is the additional expense of the necessary cold forming before heating to forming temperature. Dual phase steels also have the disadvantage of hot edge deformation susceptibility to edge crack induced failure during forming.

References to specific alloy compositions or specifications for the structure of the precursor for the targeted adjustment of the mechanical properties of the component after hot forging when using high-strength steels are not disclosed.

The object of the invention is to provide a method for producing a component by hot working a precursor of steel at temperatures below the A _c1 conversion point, which is inexpensive and can be achieved with the comparable or improved properties of the formed component as in the known hot forming by press hardening. In particular, strengths of more than 800 MPa at yield strengths of more than 700 MPa and breaking elongations A ₈₀ of more than 8% on the finished component should be achieved with a ductile failure behavior of the component.

According to the teachings of the invention, this object is achieved by a method for producing a component by hot forming a precursor of steel, in which the precursor is heated to forming temperature and then formed, wherein the component after forming a bainitic microstructure with a minimum tensile strength of 800 MPa which is characterized in that the heating takes place at a temperature below the A _c1 transformation temperature, wherein even the precursor of a

• C: 0,02 bis 0,3
• Si: 0,01 bis 0,5
• Mn: 1,0 bis 3,0
• P: max. 0,02
• S: max. 0,01
• N: max. 0,01
• Al: bis 0,1
• Cu: bis 0,2
• Cr: bis 3,0
• Ni: bis 0,2
• Mo: bis 0,2
• Ti: bis 0,2
• V: bis 0,2
• Nb: bis 0,1
• B: bis 0,01

Steel having a microstructure of at least 70% bainite, and wherein the precursor has the following alloy composition in% by weight:

• C: 0.02 to 0.3
• Si: 0.01 to 0.5
• Mn: 1.0 to 3.0
• P: max. 0.02
• S: max. 0.01
• N: max. 0.01
• Al: to 0.1
• Cu: to 0.2
• Cr: up to 3.0
• Ni: up to 0.2
• Mo: to 0.2
• Ti: to 0.2
• V: to 0.2
• Nb: to 0.1
• B: to 0.01

Remaining iron and impurities caused by melting.

The inventive method has opposite to that of DE 601 19 826 T2 or the EP 2 546 375 A1 known method for producing a component by means of press hardening on the advantage that at significantly lower energy requirements for heating by the use of a bainitic steel already in the initial state, a component with mechanical characteristics is provided which are equal or even better than the mechanical properties in the initial state of the precursor. This saves energy costs.

Opposite the DE 10 2011 108 162 A1 can with the alloy composition according to the invention and a precursor, which already has a microstructure with at least 50% bainite in the initial state, waived the additional step of cold deformation of the precursor to increase the strength and the required mechanical properties of the component can be adjusted specifically after the warm forging.

The use of an already produced on the precursor bainitic steel with the specified alloy composition is of great advantage, since even the starting material has a high tensile strength and elongation, which are retained even after the (conversion) conversion or even higher.

The bainitic steel used for the process according to the invention obtains its microstructure according to the invention via an appropriate temperature control during the production process of the preliminary product. For hot strip, the microstructure z. B. on thermomechanical rolling, for cold strip z. B. done by the annealing process after cold rolling or hot dip galvanizing.

A "softening" after forming, as known with other high-strength steels, could not be observed with this bainitic steel. A "softening" is often associated with a structural transformation and is thus time and temperature critical. The use of the precursor of the invention of mainly bainitic steel, however, is largely insensitive, so that z. B. intended or unintended Zeitund temperature changes during heating and forming cause no deterioration of the mechanical properties. Due to this advantageous material behavior even demanding multi-stage process steps can be carried out reproducibly.

The particular advantage of using this alloy concept and the bainitic microstructure remains in a very fine and homogeneous microstructure with at least 70% bainite and only small amounts of ferrite, retained austenite and martensite. It is necessary for the achievement of the required mechanical properties that the structure has at least 70% bainite and the amounts of retained austenite + martensite are <10% and the remainder consists of ferrite.

• C: 0,02 bis 0,11%
• Si: 0,01 bis 0,5%
• Mn: 1,0 bis 2,0%
• P: max. 0,02%
• S: max. 0,01%
• N: max. 0,01%
• Al_min: 0,015 bis 0,1%
• B: max. 0,004%
• Nb+V+Ti: max. 0,2%

Particularly uniform and homogeneous material properties can be achieved if the bainitic steel of the precursor has the following alloy composition in% by weight:

• C: 0.02 to 0.11%
• Si: 0.01 to 0.5%
• Mn: 1.0 to 2.0%
• P: max. 0.02%
• S: max. 0.01%
• N: max. 0.01%
• Al _min : 0.015 to 0.1%
• B: max. 0.004%
• Nb + V + Ti: max. 0.2%

• C: 0,05 bis 0,11%
• Si: 0,1 bis 0,5%
• Mn: 1,0 bis 2,0%
• P: max. 0,02%
• S: max. 0,01%
• N: 0,003 bis 0,01%
• Al_min: 0,03 bis 0,1%
• B: max. 0,004%
• Mo: 0,04 bis 0,2
• Ti: 0,04 bis 0,2
• Nb+V+Ti: 0,1 bis 0,2%

In a further improved embodiment of the invention, the steel of the precursor has the following alloy composition in% by weight:

• C: 0.05 to 0.11%
• Si: 0.1 to 0.5%
• Mn: 1.0 to 2.0%
• P: max. 0.02%
• S: max. 0.01%
• N: 0.003 to 0.01%
• Al _min : 0.03 to 0.1%
• B: max. 0.004%
• Mo: 0.04 to 0.2
• Ti: 0.04 to 0.2
• Nb + V + Ti: 0.1 to 0.2%

The addition of nitrogen of at least 0.003 to 0.01 wt .-% in combination with carbon and a minimum content of titanium from 0.04 to 0.2 wt .-% by the formation of titanium carbonitrides advantageously for a fine-grained structure with high strength and toughness properties. In addition, the addition of molybdenum in contents of 0.04 to 0.2 wt .-%, the forming precipitates are advantageously kept very small.

Comparative tests were carried out on steels with the alloy compositions given in Table 1. The results for the mechanical properties before and after the warm forging are shown in Table 2.

Sheets were tested with a thickness of 1.8 to 2.25 mm, which were heated in the oven at a temperature of 600 ° C for 3 minutes and then cooled between two flat tool components in a forming press.

The materials tested are indicated in Tables 1 and 2 by the letters a, b, c, d, e and f. The alloy compositions of the materials correspond to the invention, but the microstructures were set differently in the initial state. Thus, in the initial state, steel a had a ferritic-bainitic basic structure "FB" before being heated to forming temperature, steel b a bainitic "B", steel c a mixed structure of martensite, bainite and ferrite "MBF", the martensite content being dominant. The steels d and e had a ferritic "F" and the steel f a martensitic matrix "M". For steels a and c, the bainite content in the microstructure was below 50% and in steel b over 50%. It can be seen from Table 2 that only the steel b with a predominantly bainitic starting structure of the precursor satisfies the requirements for mechanical properties with a minimum tensile strength of 800 MPa and a minimum elongation at break A80 of more than 8% after hot forging.

Typical applications for exploiting the high strength potential while at the same time saving weight on the component are mobile crane construction, longitudinal and transverse beams in trucks and trailers, safety and chassis parts in passenger cars and wagon construction.

The steel according to the invention or the component produced therefrom is characterized by a very high yield strength and tensile strength of more than 800 MPa with a sufficiently high elongation. Due to the chemical composition also good weldability is given.

The aforementioned steel can furthermore be provided in a known manner with a scale-inhibiting or corrosion-inhibiting layer based on lacquer or with a metallic coating. The metallic coating may contain zinc and / or magnesium and / or aluminum and / or silicon.

In contrast to conventional production routes, surface-finished hot or cold strip can already be used for forming following heating, since the adhesion and the ductility endure half-warm forming with low degrees of deformation. The metallic coating is resistant to short-term reheating of the substrate / coating combination (steel strip / coating) below the Ac ₁ temperature of the substrate to survive reheating prior to semi-hot forming and actual semi-hot forming.

Due to the relatively low amount of heat can be used on large-scale reheating units such. As tunnel kilns or chamber furnaces, in favor of fast and direct-acting systems (inductive, conductive and in particular radiation) are dispensed with.

In addition, the new method described with significantly less heat energy, or the energy efficiency is higher than during press hardening. This reduces process costs and reduces CO ₂ emissions.

Preferably, the reheating takes place before the half-warm forms by means of radiation, since the efficiency is significantly higher than when heated in an oven or conductive heating and the energy input into the material takes place faster and more effective depending on the surface condition.

The material is also very suitable for partial heating. By the use of z. B. emitters can be selectively heated individual areas of the preform to be formed in order to obtain formability optimized zones. This advantageously allows the use of conventional presses for cold forming, so that it is possible to dispense with a complex hot-forming installation, as is necessary in press-hardening.

For the transport between the heat source and the forming tool, it may also be useful, especially in the case of very thin sheets (eg <0.8 mm), to provide a profiling of the blanks to increase the local rigidity. This is not possible in conventional press hardening, since the strength to be achieved requires a rapid cooling, which excludes over mold in the tool due to the profiling.

In the method according to the invention, the precursor is heated to a temperature of below 720 ° C advantageously in a temperature range of 400 - 700 ° C and then formed into a component. The optimum forming temperature is dependent on the required strength of the component and is preferably between about 500 ° C and 700 ° C. Long holding times to obtain a bainitic structure as in the EP 2 546 375 A1 are not required, so that the process time for the production of the component is significantly shortened.

In an advantageous embodiment of the invention, when the precursor is heated to forming temperature, the temperature range of the semi-warming is locally exceeded in the austenitizing region in order to undertake local changes in properties (eg local hardening) which, in combination with the increase in the strength of the remaining material, later Demands of the component are adjusted. Tab. 1 material C Si Mn P S N al Cu Cr Ni V Ti Nb Not a word B FB a 0.07 0.08 1.4 0.01 0,002 0.005 0,041 0.03 0.04 0.04 0.05 - 0.04 - - B b 0.08 0.47 1.9 0.01 0.001 0,006 0.066 0.03 0.03 0.04 0.01 0.12 0.05 0.14 - MBF C 0.23 0.25 1.2 0.01 0,002 0.005 0,038 0.04 0.16 0.04 0.01 0.03 - - 0,003 F d 0.10 0.28 2.0 0.01 0.001 0,006 0,041 0.02 0.33 0.04 0.01 0.04 0.04 - 0,003 M f 0.15 0.12 1.7 0.01 0.001 0.005 0,045 0.02 0.33 0.04 0.01 0.02 - - - F e 0.09 0.25 1.8 0.01 0.001 0.005 0,041 0.03 0.33 0.04 0.01 - 0.01 - - material Tensile strength R _m [MPa] Yield strength R _p0.2 [MPa] Elongation at break A ₈₀ [%] before HWU to HWU before HWU to HWU before HWU to HWU FB a 591 632 552 589 19 16 B b 788 854 678 833 14 12 MBF C 982 979 915 922 7 7 F d 855 778 644 767 13 12 M f 1343 1246 1047 1173 6 1 F e 676 650 407 498 22 18

Claims (14)

1. A method for producing a component by hot forming a pre-product made of steel, in which the pre-product is heated to forming temperature and then formed, wherein the component after forming has a bainitic microstructure with a minimum tensile strength of 800 MPa,
   characterised in that
   the heating takes place to a temperature below the Ac1 transformation temperature, wherein the pre-product already consists of a steel having a structure of at least 70% bainite and the contents of residual austenite + martensite are <10% and the remainder consists of ferrite, and wherein the pre-product has the following alloy composition in wt. %:

   C: 0.02 to 0.3

   Si: 0.01 to 0.5

   Mn: 1.0 to 3.0

   P: max 0.02

   S: max 0.01

   N: max 0.01

   Al: up to 0.1

   Cu: up to 0.2

   Cr: up to 3.0

   Ni: up to 0.2

   Mo: up to 0.2

   Ti: up to 0.2

   V: up to 0.2

   Nb: up to 0.1

   B: up to 0.01

   remainder iron and impurities associated with smelting.
2. The method according to claim 1,
   characterised in that
   the steel comprises an alloy with the following composition in wt.%:

   C: 0.02 to 0.11%

   Si: 0.01 to 0.5%

   Mn: 1.0 to 2%

   P: max 0.02%

   S: max 0.01%

   Al: 0.015 to 0.1%

   B: max. 0.004%

   Nb+V+Ti: max 0.2%.
3. The method according to claim 1,
   characterised in that
   the steel comprises an alloy with the following composition in wt.%:

   C: 0.05 to 0.11%

   Si: 0.1 to 0.5%

   Mn: 1.0 to 2.0%

   P: max. 0.02%

   S: max 0.01%

   N: 0.003 to 0.01%

   Al min: 0.03 to 0.1%

   B: max 0.004%

   Mo: 0.04 to 0.2

   Ti: 0.04 to 0.2

   Nb+V+Ti: 0.1 to 0.2%.
4. The method according to one of claims 1 to 3,
   characterised in that
   the heating of the pre-product to hot forming temperature takes place only partially and the partial heating optionally takes place above the Ac1 transformation temperature.
5. The method according to at least one of claims 1 to 4,
   characterised in that
   the heating of the pre-product takes place to a temperature below 720°C.
6. The method according to claim 5,
   characterised in that
   the heating of the pre-product takes place to a temperature in a range from 400 to 700°C.
7. The method according to claim 6,
   characterised in that
   the heating of the pre-product takes place to a temperature in a range from 500 to 700°C.
8. The method according to one of claims 1 to 7,
   characterised in that
   prior to the heating step, the pre-product is provided with a metallic or lacquer-like coating.
9. The method according to claim 8,
   characterised in that
   the metallic coating contains Zn and/or Mg and/or Al and/or Si.
10. The method according to at least one of claims 1 to 9,
    characterised in that
    the heating to forming temperature is accomplished inductively, conductively or by means of radiation.
11. The method according to at least one of claims 1 to 10,
    characterised in that
    a sheet metal blank or a tube is used as the pre-product.
12. The method according to claim 11,
    characterised in that
    the sheet metal blank consists of hot-rolled strip or cold-rolled strip.
13. The method according to claim 11,
    characterised in that
    the tube is a seamlessly hot rolled tube or a welded tube made of hot-rolled strip or cold-rolled strip.
14. The method according to claim 13,
    characterised in that
    the tube is a seamlessly hot rolled tube or a welded tube made of hot-rolled strip or cold-rolled strip which has been subjected again to one or multiple drawing and/or annealing processes.