EP2111475B1 - Components made of steels with an ultrahigh carbon content and with a reduced density and high scaling resistance - Google Patents

Description

The invention relates to ultra-high carbon steels or Ultra High Carbon Steel (UHC) with reduced density and high scale resistance according to claim 1, and the manufacture of components by hot forging according to claim 8 and claim 9. UHC steels have been known for some time. They have been developed especially with regard to their superplastic properties. The superplastic deformation runs in a narrow process window of temperature and strain rate (strain rate (ε ')). In the superplastic forming strain values of some 100 to 1000% can be achieved. Typical here are a forming temperature above about 50% of the melting temperature (ideally in the range of α -> γ conversion) and a very low strain rate of about 10 ^-2 to 10 ^-5 s ^-1 . If the respective optimum temperature and / or forming speed are exceeded, destruction of the structure required for the good mechanical properties takes place. The ideal speeds of the superplastic forming are thus well below the limit of industrial acceptability for series products which is about 0.1 / s.
Unalloyed UHC steels, such as from the US 3,951,697 known to have only a small superplastic effect, since the structure is unstable to grain growth.

In the US 4,448,613 Production processes of the superplastic structure for UHC steels are described. It also describes the setting of superplastic structures for UHC steels with low alloying additions of Cr, Mn and Si.

In the US 4,533,390 It is proposed that a UHC steel having a very high Si content (3-7%), alloy additions of Cr, Mo, W, Ti and combinations thereof, an increase of the A1 temperature, a stabilization of the structure against grain growth and an improvement to achieve the superplastic properties. The high Si contents make the steels very brittle under use conditions.

In the US 4,769,214 UHC steels with a high Al content (preferably 0.5 up to 6.4%) are described. Good superplastic properties, in particular good formability under superplastic conditions (and oxidation resistance) are aimed at.Alloy additives from Cr and / or Mo are specified for stabilizing the structure.With an Al content greater than 6.4%, a strong reduction in hot and cold workability The preferred UHC steels have Al contents <6.4%.

Of particular importance for the economy of the molding process is the formability of the material. A good formability includes a high achievable without component damage degree of deformation, a low yield stress during forming and the lowest possible forming temperature. Only then are complex components made available in a few cost-effective forming steps.

In cold forging (cold extrusion), high dimensional accuracy, high surface quality and high component strength (by work hardening) are possible; but are the disadvantages of some very high required forming forces contrary.

In hot forging (about 1100 ° C - 1250 ° C), the materials show a high forming capacity (suitable for highly complex components), but only small dimensional accuracy and poorer surface quality is possible. Particularly disadvantageous is the high thermo-mechanical tool load or the correspondingly high tool wear. Shaping in the high temperature range, for example, forging temperature lead to high tooling costs, since either high wear is present or expensive high-temperature tools must be used. In addition, the reshaped blanks are processed for cost reasons in air and thereby oxidatively damaged. For example, this leads to scaling in the case of steels. Before the further processing of the components produced thereby must be reworked at least on the surface. A near net shape production of components is thereby very limited achievable at these temperatures.

Another important factor for mass production, especially in the automotive industry, is a high process speed in the forming process. The very low forming rates of superplastic forming are therefore not acceptable for mass production of components.

• C: 1 bis 1,6
• Al : 5 bis 10
• Cr: 0,5 bis 3
• Si: 0,1 bis 2,8

In the case of the known low-Al content UHC steels, about 750-950.degree. C. is present at the usual forming temperatures to fear significant scaling, which can lead to additional processing effort. These steels are not suitable for lightweight construction.
It is an object of the invention to provide a lightweight steel which can be processed at temperatures below the hot forging temperatures in air with the highest possible deformation rates, and show forming processes, the high forming speeds and the least possible deterioration of the mechanical properties of the steels, and the lowest possible thermomechanical Ensure loading of the forming tools.
The object is achieved by an ultra-high carbon or UHC lightweight steel with improved formability and scale resistance with the features of claim 1 and by a method for the production of hot-formed components made of UHC lightweight steels by hot forming at a temperature of 800 to 980 ° C in air the features of claim 8, or by a method for the production of hot-formed components from UHC lightweight steels by hot forming at a temperature of 880 to 1050 ° C in air with the features of claim 9.
In summary, the forming processes at a temperature in the range of 800 to 1050 ° C are referred to as hot forming below.
Thus, according to the invention, the following alloy composition according to claim 1 is provided for the UHC lightweight steel having improved scale resistance (compositions below in% by weight unless otherwise specified).

• C: 1 to 1.6
• Al: 5 to 10
• Cr: 0.5 to 3
• Si: 0.1 to 2.8

Remaining iron and usual steel-accompanying impurities.

Particular importance is attached to the alloying elements Al and Si, which are present next to one another in the UHC lightweight structural steel according to the invention.

Both Al and Si contribute to a significant reduction in the density of UHC steels. Thus, it is in particular for the automotive industry interesting lightweight steels.
Thus, the density for a UHC lightweight steel with 0.4% Si and 6.7% Al is 7 g / ccm compared to the conventional 25MoCr4 steel with a density of 7.8 g / ccm.

Surprisingly, it has been found that the Si can greatly affect the A1 transformation temperature in the given alloy composition. The high content of Al significantly increases the Si sensitivity of the alloy. In the Al-containing alloy, even a slight increase in Si alloy additions leads to a significant increase in the A1 transformation temperature. This means that the alloying of the Si achieves an increase in the optimum forming temperature. The optimum forming temperature is to be understood in particular as meaning the temperature which permits the highest possible forming speeds without damaging the microstructure.

For example, the A1 transformation temperature of about 820 ° C for a 6.5% Al, 1.5% Cr, 1.35% C, 0.04% Si UHC lightweight steel Increased by increasing the Si content to only 0.4% already at 865 ° C.

Increasing the A1 transformation temperature shifts the optimum forming temperature to higher values, with the temperature level of the hot working still remaining below the range of forging temperatures. This brings significant advantages for hot forming. Since the hot forming process can be carried out at higher temperatures, the yield stress of the UHC lightweight steel decreases. Overall, the forming capacity of the UHC lightweight steel improves at the optimum forming temperature. The temperatures of hot forging, which are unfavorable for components and tools, are not reached.

Compared with forged steels, there is a significant improvement in the mechanical properties at room temperature.

The Al content, in addition to reducing the density, also has the very significant effect of greatly reducing the scale formation at the hot working temperatures. Since only thin scale layers form, in which only a small surface finish is required, the UHC lightweight steels according to the invention are also particularly suitable for near-net-shape processes. In the case of the UHC lightweight steels according to the invention, it was possible to achieve a corrosion rate reduction of 92 to 99% compared to the conventional 25MoCr4 steels.

Surprisingly, the Si content also has a significant influence on the decrease in scaling.

With the addition of Si, the superplastic properties are retained, whereby in some cases a slight increase in the formability at high speed could be measured.
The mechanical properties at room temperature are not adversely affected by the usually strongly embrittling Si. The UHC lightweight steels according to the invention show only slightly reduced elongations at break compared to Si-free UHC steels.

In the steelmaking process, Si is usually added to the alloy melt without special precautions from the furnace lining during alloy melting. For Si-poor steel grades, this behavior is problematic and must be prevented by complex measures. By contrast, for the UHC lightweight steels according to the invention, this Si uptake no longer poses a problem because of their already high Si content. Cost-effective metallurgical production processes are therefore applicable.

It has been shown that the alloying elements Al and Si influence each other favorably. Therefore, the Al / Si ratio is of particular interest. Preferably, an Al / Si ratio between 10 and 20 is selected. More preferably, the Al / Si ratio is 14 to 16 at an Al content of 6 to 7%.

• C: 1,2 bis 1,4
• Al : 5,5 bis 7
• Cr: 1 bis 2
• Si: 0,3 bis 0,6

Since the elongation at break deteriorates due to a high Si content or the brittleness of the steel increases at room temperature, the Si alloy content for most applications is limited to values below approximately 2.8%.
The preferred Si content is a compromise between increasing the optimum forming temperature and degrading the mechanical properties and is preferably in the range of 0.3 to 1.2 wt.%, Particularly preferably 0.4 to 0.8.
Due to the high contents of Al and Si, there is a risk that unwanted graphite is formed in the alloy. This is countered by alloying a suitable amount of Cr, which must be matched to the individual components Si and Al.
A particularly preferred composition in% by weight is given by:

• C: 1.2 to 1.4
• Al: 5.5 to 7
• Cr: 1 to 2
• Si: 0.3 to 0.6

Remaining iron and common steel-accompanying impurities.

In the context of the alloy according to the invention, the steel-accompanying impurities may likewise be the typical steel alloy companion Ni, Mo, Nb and / or V. As a rule, fractions of these elements in an amount below 1% are not critical. According to claim 1, the Ni, Mo and / or V content is below 0.15 wt.%. Particular preference is given to setting at least Ni and / or V to less than 0.05%.

In the invention, the UHC lightweight steel contains further stabilizing alloying elements selected from the group Nb, Ti, Mg and / or N. The content of these alloying elements is limited according to claim 1 to values below 0.8, preferably below 0.5%. Particularly preferably, the sum of these elements in the range of 0.02 to 0.5 wt.%.

It should be regarded as a further advantage of the invention that can be dispensed with in the UHC lightweight steel according to the invention on the alloying of the very expensive alloying elements Ni, Mo and / or V.

After their metallurgical production, the UHC steels are generally not in a structural state that allows a high deformation rate of the hot forming. An ideal structure for this purpose typically corresponds to a structure with superplastic properties. However, for the preferred according to economic considerations for economic considerations hot working, instead of the superplastic forming, can be deviated from this optimum superplastic structure, however, within wide limits. It is important that there is a homogeneous, fine-grained, spheroidal carbide distribution stable against grain growth and graphite formation in a likewise fine-grained and grain-growth-stable ferrite matrix. The grain size of the microstructure is preferably below 10 μm. Particularly preferably, the average particle size is below 1.5 μm. The majority of the grains are preferably spheroidal, with small amounts of lamellar carbide being tolerable for the properties of the UHC steel.

Only by a special thermo-mechanical treatment is a structure formed which contains the required fine crystallites or grains. At least two phases must be formed which prevent grain growth.

The corresponding phases in the compositions according to the invention are composed essentially of the main phase α-ferrite and secondary phases of κ-carbides. Al and Si stabilize the structure against grain growth.

To adjust this structure, a relatively homogeneous material of pearlite is first prepared, which is a lamellar mixture of ferrite and cementite. In a second step, this perlite structure is converted into a microstructure in which the carbides are predominantly spheroidal and the ferrite ultrafine-grained.

The structure of the UHC lightweight steels preferably has fine spheroid carbides. The average cross-sectional area of the spheroid carbides is preferably below 8 μm ² , more preferably below 3 μm ² .

Preferably, the volume fraction of the fine spheroid carbides is 25 to 30%. The frequency of carbide particles or particles above 500 nm per surface element to be determined by light microscopy should be above 50,000 carbide particles / mm ² , preferably above 150,000 carbide particles / mm ² .

A spheroidal shape is much cheaper than a lamellar form of the carbide particles. The average elongation of the carbide particles is preferably below 1.8. Particularly preferably, very roundish particles are formed, with an average elongation between 1 and 1.5.

• A. Weitgehend vollständige Austenitisierung, je nach C, Si, Al und Cr-Gehalt insbesondere bei 1000-1150°C. Hierbei findet eine homogene Verteilung des Kohlenstoffs und der Begleitelemente in der groben γ-Phase statt.
• B. Abkühlung unter Warmumformung, gegebenenfalls mit zyklischer Temperaturführung im Temperaturbereich von 1100 bis 700°C; Hierbei werden sphäroide κ-Karbide erzeugt. Die Umformung findet nahe an der A1 Temperatur, oberhalb und unterhalb dieser statt. Gegebenenfalls kann zwischengeglüht werden, um die mittlere Abkühlgeschwindigkeit zu reduzieren oder die Temperatur um die A1-Temperatur pendeln zu lassen.
• C. Luftabkühlung; Hierbei findet keine Gefügeumwandlung mehr statt.

In the case of the UHC lightweight steels according to the invention, the following procedure can be used for the production of a superplastic structure:

• A. Almost complete austenitization, depending on C, Si, Al and Cr content, especially at 1000-1150 ° C. In this case, a homogeneous distribution of the carbon and the accompanying elements in the coarse γ-phase takes place.
• B. cooling with hot forming, optionally with cyclical temperature control in the temperature range of 1100 to 700 ° C; This produces spheroid κ-carbides. The transformation takes place close to the A1 temperature, above and below it. Optionally, it may be annealed to reduce the average cooling rate or to cause the temperature to oscillate around the A1 temperature.
• C. air cooling; Here, no structural transformation takes place.

In step B, typically, strain levels above 1.5 are used. Preferably, degrees of deformation at 1.7 to 4 are used.

The UHC lightweight steels according to the invention are preferably used for the production of suspension components, transmission parts, or gears for motor vehicles. A particularly demanding application are connecting rods, which have not been satisfactorily available as a lightweight component.

Further preferred applications are components for internal combustion engines and transmission components of motor vehicles.

Another aspect of the invention relates to methods of making hot formed UHC lightweight structural members.

• C: 1 bis 1,6
• Al : 5 bis 10
• Cr: 0,5 bis 3
• Si: 0,1 bis 0,8

For the manufacture of components, the invention provides a UHC lightweight steel
the composition (% by weight)

• C: 1 to 1.6
• Al: 5 to 10
• Cr: 0.5 to 3
• Si: 0.1 to 0.8

Remaining iron and common steel-accompanying impurities, at a temperature in the range of 800 to 980 ° C in air warm form.

• C: 1 bis 1,6
• Al : 5 bis 10
• Cr: 0,5 bis 3
• Si: 0,8 bis 2,8

In a further embodiment of the invention is provided for the production of components, on a UHC lightweight steel of the composition

• C: 1 to 1.6
• Al: 5 to 10
• Cr: 0.5 to 3
• Si: 0.8 to 2.8

The remainder of the iron and common steel-accompanying impurities are subjected to hot forming at a temperature in the range of 880 to 1050 ° C in air.

In hot forming, in principle, the various methods known in mechanical engineering can be used for the production of complex-shaped components made of metals. If necessary, make an appropriate adaptation of the cold process to hot forming. Suitable processes include, but are not limited to, hot extrusion, cross rolling, hot bore pressing, hot swaging, hot splining, hot swaging or hydroforming, and forging.

Due to their A1 and Si content, the listed UHC steels are not special in hot forming Inert gas atmosphere. Hot forming can therefore take place in the presence of air.

The temperatures of the hot working used according to the invention are significantly below the forging temperature of the respective alloy. These comparatively lower temperatures have a further significant advantage for the forming tools. Frequently, conventional steel tools can be used instead of the otherwise required high-temperature tools.

For the UHC lightweight steels, hot working is preferably carried out at a process pressure below 150 to 180 MPa and a deformation rate or strain rate (ε '= relative change in length / initial length per unit time) above 0.1 / s. The design of the process can be optimized to low process pressure or to high forming speeds, depending on the selected forming process or forming tool. Particularly preferred transformation rates are above 0.5 / s.

If UHC lightweight steels are used, which have a superplastic structure, very high degrees of deformation can be achieved even under the non-superplastic conditions of hot forming. In the case of hot forming, expansions of the blank in the range from 50 to 300% are preferably carried out.

In general, with the method according to the invention, even for complex components, no or at least significantly reduced machining further process steps are required, which also results in better material utilization. Likewise, if necessary to combine several separate rear-end stored forming processes into a single forming process according to the invention. With the UHC lightweight steels according to the invention, the scaling accumulated over several warm processes is also comparatively low. The process chain for producing the finished components is shortened in an advantageous manner.

After the hot forming of the UHC lightweight structural steel, it is particularly preferred that no chip-removing surface treatment is carried out to remove the scale layer.

The method according to the invention is preferably carried out as a near-net-shape method, so that the component is obtained in the most ready-to-use state after the forming and only has to be subsequently reworked on special functional surfaces. Cleaning and polishing the surfaces are considerably easier than with the known steels.

The UHC lightweight steels according to the invention likewise have good hardenability (up to> 60 HRC without case hardening).

Preferably, after the hot forming, a hardening process takes place. This is in particular conducted directly from the process heat of the forming process and under Luftabschreckung. Thereafter, it can be started in a known manner. For such tempered UHC lightweight steels, tensile strengths of 1500 MPa at an elongation of 8% were measured at room temperature.

Examples:

In the following example, the properties between a quasi Si-free UHC lightweight steel, a conventional Steel and a UHC lightweight steel according to the invention compared (see Table 1).

For the UHC steels, the Al and Cr content as well as the degree of manufacturing deformation were kept essentially the same and the Si content of 0.039% (UHC steel Si0.04) to 0.38% (UHC steel Si0.4 ) elevated.

In comparison to the 25MoCr4, there is a significant reduction in density with comparatively better high-temperature properties; Tensile strength and elongation at 910 ° C are significantly increased.

The comparison between UHC steel Si0.04 and UHC steel Si0.4 shows a significant favorable effect of Si addition on the A1 transformation temperature, which is increased from 805 to 865 ° C. The density is also lowered again, from 7.11 to 7.01 g / cc. The embrittlement, which is characterized by the reduction of the elongation at break from 12.4 to 6.5%, still remains within acceptable limits. <b> Table 1 </ b> UHC steel Si0.04 UHC steel Si0,4 Reference 25 MoCr 4 coarse grain annealing Si content 0.039 wt.% Si 0.380 wt.% Si - Al proportion 6.49% by weight Al 6.72% by weight Al - C-content 1.35% by weight of C 1.32% by weight of C Cr content 1.57% by weight Cr 1.54% by weight C Manufacturing Umformprad φ _H = 1.7 φ _H = 1.6 - BASIC FEATURES density 7.11 g / cm ³ 7.01 g / cm ³ 7.83 g / cm ³ Modulus 209.0 GPa - 209.9 GPa microhardness 301.0 HV30 311.0 HV30 252.0 HV30 Macro hardness 29.6 HRC 27.8 HRC 24.4 HRC STRUCTURE description Fine spheroid carbides Coarse spheroid carbides - statistics Volume fraction of carbide particles 04.20% 26.07% - Number of carbide particles 141,000 / mm ² 69,000 / mm ² - size Medium area carbide particles 1.4 μm ² 3.9 μm ² - Mean extent of carbide particles 1.9 μm 2.9 μm - shape Medium elongation carbide particles 1.80 1.76 - PHASE TRANSFORMATION Start (Ac1) about 820 ° C about 865 ° C - End (Acm) about 905 ° C about 1000 ° C - RT FEATURES Stretch limit 698 MPa 676 MPa - tensile strenght 934 MPa > 940 MPa - elongation 04.12% 06.05% - HT-PROPERTIES 25 MoCr 4 - cold shear capability 910 ° C - 1.0 / s tensile strenght 172 MPa 215 MPa 144 MPa elongation 74% 102% 60%

In Fig. 1 are the results of high-temperature corrosion resistance of the two UHC lightweight steels shown. The scale formation at 860 ° C and 910 ° C of the UHC steel with 0.04% Si against the UHC steel with 0.4% Si is shown (UHC steel Si0.04 versus UHC steel Si0.4).

Samples of 100x20x3mm were measured at 860 ° C and 910 ° C for up to 60 minutes in air. The increase in weight is an essential measure for the formation of scale.

The Si content has a significant influence on the decrease in scaling. At 910 ° C there is a 70% decrease in scaling at the transition from 0.04 to 0.4% Si. At the temperature of 860 ° C, which is particularly relevant for hot forming, the relative decrease in scaling is even 98%.

Comparative studies between UHC0, 4Si and 25MoCr4 are in Fig.2 shown. 25MoCr4 shows a 92 to 99% higher scaling at both temperatures compared to UHC0, 4Si.

Claims (15)

1. UHC light-weight steel with improved scale resistance, characterised by a composition in % w/w of

   C: 1 to 1.6

   Al: 5 to 10

   Cr: 0.5 to 3

   Si: 0.1 to 2.8

   if applicable stabilising alloying elements selected from the group Nb, Ti, Mg and/or N in a quantity of 0,02 to 0.8 % w/w,
   and if applicable the further alloying elements with an Ni, Mo and/or V content below 0.15 % w/w
   rest iron
   and the usual impurities which accompany steel.
2. UHC light-weight steel according to claim 1,
   characterised in that
   the Al/Si ratio is between 10 and 20.
3. UHC light-weight steel according to claim 1 or 2,
   characterised in that
   the UHC light-weight steel contains stabilising alloying elements selected from the group Nb, Ti, Mg and/or N in a quantity of 0.02 to below 0.8 % w/w.
4. UHC light-weight steel according to any of the preceding claims, characterised by a composition in % w/w of

   C: 1.2 to 1.4

   Al : 5.5 to 7.0

   Cr: 1 to 2.0

   Si: 0.3 to 0.6

   if applicable Ni, Mo and/or V with a content below 0.15 rest iron and the usual impurities which accompany steel.
5. UHC light-weight steel according to any of the preceding claims, characterised in that
   the Ni, Mo and/or V content is below 0.15 % w/w.
6. UHC light-weight steel according to any of the preceding claims, characterised in that
   the structure comprises fine spheroidal carbides with average cross-sectional areas below 8 µm².
7. UHC light-weight steel according to any of the preceding claims, characterised in that
   the structure comprises coarse spheroidal carbides at a volume fraction of 25% to 30%.
8. Method for producing hot-formed components from UHC light-weight steels according to any of the preceding claims with a UHC light-weight steel having an Si content below 0.8,
   characterised in that
   the hot forming process is carried out in air at a temperature of 800 to 980°C.
9. Method for producing hot-formed components from UHC light-weight steels according to any of the preceding claims with a UHC light-weight steel having an Si content above 0.8,
   characterised in that
   the hot forming process is carried out in air at a temperature of 880 to 1050°C.
10. Method according to claim 8 or 9,
    characterised in that
    the deformation rate (ε') of the hot forming process is set to values above 0.1/s.
11. Method according to any of claims 8 to 10,
    characterised in that
    before the hot-forming process, the UHC light-weight steel is subjected to a manufacturing hot forming process with a forming level of 1.5 to 4.
12. Method according to any of claims 8 to 11,
    characterised in that
    the hot forming process of the blank is at least partially carried out to a forming level > 2.
13. Method according to any of claims 8 to 12,
    characterised in that
    the formed component is hardened from the process hardness of the hot forming process with air quenching.
14. Method according to any of claims 8 to 13,
    characterised in that
    vehicle components, gearbox components, gears or a light-weight connecting rod for motor vehicles is/are formed.
15. Use of a UHC light-weight steel according to any of claims 1 to 7 for producing components for internal combustion engines and gearbox parts of motor vehicles.

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