DE102008032024B4 - Density-reduced UHC steels - Google Patents

Abstract

UHC lightweight steel, characterized by a composition in% by weight C: 0.8 to 1.4 Al: 5.5 to 7.0 Cr: 0 to 0.2 Si: 0.01 to 0.6 stabilizing alloying elements below 1 % By weight and the remainder iron as well as customary steel-accompanying impurities.

Description

The invention relates to UHC lightweight steels (Ultra High Carbon - lightweight steels) with improved scale resistance, according to the subject-matter of claim 1 and their thermomechanical processing method according to the preamble of claim 7.

UHC (Ultra High Carbon) steels are characterized, above all, by the carbon content which is rather high for a conventional steel, which in the case of unalloyed steels ranges from about 0.8 to 2.1% by weight. Such steels sometimes have superplastic properties after suitable pretreatment.

From the DE 869494 For example, an iron alloy containing 1.2 to 2% carbon, 3.5 to 4.5% aluminum, and 4.5 to 6.5% silicon is known. This is a cast iron, which contains no perlite when not annealed.

In the US 4,533,390 It is proposed to achieve a UHC steel with up to 7% Si by alloy additions of Cr, Mo, W, Ti and their combinations, an increase of the A1 temperature, a stabilization of the structure against graphitization, grain growth and an improvement of the superplastic properties. The high Si contents render the steels comparatively brittle under conditions of use.

From the DE 10 2005 027 258 A1 A density-reduced high-carbon or UHC steel is known which, in addition to iron and common steel-accompanying impurities, has the following alloy constituents: 0.8 to 2.5% C, 3.5 to 15% Al, 0.5 to 4% Cr, 0, 01 to 4% Si, up to 4% Ni, Mn, Mo, Nb, Ta, V and / or W, and 0.1 to 0.85 Sn, and 0 to 3% of Ti, Be and / or Ga.

From the DE 10 2006 041 902 A1 is a UHC lightweight steel with improved scale resistance having the composition in wt.% C: 1 to 1.6, Al: 5 to 10, Cr: 0.5 to 3, Si: 0.1 to 2.8, balance iron and usual steel-accompanying impurities known.

For the thermomechanical treatment for setting a suitable structure for superplastic forming different approaches are known. For example, in US patents US 4,448,613 . US 4,533,390 . US 5,445,685 . US 4,769,214 and US 3,951,697 given different thermomechanical treatments for different UHC steels.

The superplastic deformation, in which the thermo-mechanically pretreated UHC steels are formed into the finished workpieces, runs in a narrow process window from temperature and deformation 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. If the UHC steel is exposed to the high forming temperatures or too high temperatures for too long, the graphitization of the carbides is to be feared in particular. This can be counteracted by the addition of carbide formers, such as Cr.

The very low forming rates of superplastic forming are not acceptable for mass production of components. From the DE 10 2006 041 902 A1 Also, the processing of UHC lightweight steel is known by a semi-warm forming process, which has the usual in steel processing for massive forming processes or forging high forming speeds.

The listed UHC steels have the disadvantage of containing the expensive alloying element Cr, often in amounts of 1 to 2% or more.

It is therefore an object of the invention to provide cost-effective alloy compositions for UHC steels, which can be processed by hot forging into components, as well as to provide methods for adjusting suitable for semi-warm forming structure.

The object is achieved according to the invention by a UHC lightweight structural steel with improved resistance to scale having a composition (in% by weight) of 0.7 to 1.6 C, 5 to 12 Al, 0 to 0.4 Cr, 0.01 to 2, 8 Si, stabilizing alloying elements below 1% by weight and the remainder iron and conventional steel-accompanying impurities with the features of claim 1,
and a method for the production of hot-formed components made of UHC lightweight steels, in which the first set by thermomechanical treatment a microstructure with fine spheroid carbides with average cross-sectional areas below 10 microns ² and then hot forming the component at a temperature of 800 to 1100 ° C. Air is carried out with the features of claim 8.

For the invention, it is of crucial importance that the content of the expensive alloying element Cr is markedly reduced compared with the hitherto known UHC steels. Due to the high contents of Al and Si, however, undesirable graphite is formed during hot forming of the UHC steels at elevated temperature, in particular at the temperatures of thermal deformation, such as forging or superplastic forming. The associated microstructural transformations make high deformation rates and fast forming speeds impossible. In contrast to the known UHC steels, however, an alloying of a carbide-stabilizing and graphite-precipitating element such as Cr is dispensed with in the present case. It was surprisingly found that the Cr content can be reduced very much without detriment to the semi-hot forming or for the material properties of the steel components formed in this case. The UHC steel according to the invention is therefore very suitable for semi-hot forming after prior thermal processing for setting a microstructure with fine shaved carbide precipitation. Especially for semi-hot forming in the temperature range from 850 to 1150 ° C.

The Cr content is preferably below 0.5% by weight. Particularly favorable Cr contents are in the range from 0.01 to 0.4% by weight or 0.05 to 0.3% by weight. In a further preferred variant, it is entirely possible to dispense with the addition of Cr, so that Cr is present only as an impurity or in traces in the steel. However, it may be advantageous if small quantities of other carbide-stabilizing alloying elements are added.

Another particularly favorable composition is given by
C: 0.8 to 1.4
Al: 5.5 to 7.0
Cr: 0 to 0.2
Si: 0.01 to 0.6
Remaining iron and common steel-accompanying impurities, each in wt.%.

It has been shown that the alloying elements Al and Si influence each other favorably. Preferably, an Al / Si ratio between 10 and 20 is selected.

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%, more preferably 0.4 to 0.8.

As further alloy components, so-called stabilizing alloying elements, one or more elements from the group Nb, Ti, Mg and / or N in an amount of 0.02 to 0.8 wt.% Are suitable.

The content of Ni, Mo and / or V is preferably below 0.15% by weight.

A preferred embodiment of the UHC steel is given by a structure with fine spheroid carbides and / or correspondingly fine perlite. The microstructure can be adjusted by thermomechanical treatment and preferably resembles the UHC steels suitable for superplastic forming. The microstructure preferably has fine spheroidal, ie spherical or roundish carbides with average cross-sectional areas of less than 10 μm ² . Preferably, the average area of the carbide particles is in the range of 0.5 to 5 μm. The content of fine spheroid carbides is preferably 15 to 35% by volume, more preferably 20 to 28% by volume.

Another aspect of the invention is a process for hot working these UHC steels, which have a structure with fine spherical carbides. Here, a microstructure with fine spherical carbides with average cross-sectional areas below 10 microns ² is first set by thermomechanical treatment. Partly the structure also contains correspondingly fine pearlite structures. Then the actual transformation takes place to the finished component or further to be processed semi-finished product.

It is essential that the hot forming of the component takes place quickly and is carried out at moderate temperatures in the range of 800 to 1100 ° C (warm forging). The deformation rate or strain rate (ε '= relative change in length / initial length per unit time) is particularly preferably above 0.1 / s. Particularly preferred transformation rates are above 0.5 / s.

In the case of semi-hot forming, a process pressure below 150 to 180 MPa is preferably selected. 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. In the case of semi-hot forming, expansions of the blank in the range from 50 to 300% are preferably carried out

In the case of semi-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 Al and Si content, the listed UHC steels are not dependent on a special protective gas atmosphere during hot forming. Hot forming can therefore take place in the presence of air.

For the processing of the UHC steel to form the structure can in principle be applied to the different prior art thermomechanical treatments.

• a) Erwärmung des UHC-Stahls auf eine Homogenisierungstemperatur die in einem Bereich von bis zu 200°C oberhalb der Austenitisierungstemperatur (A_cm) liegt
• b) Heißumformung mit einem Umformgrad > 1,2 unter stetiger Abkühlung auf eine Temperatur, die im Bereich bis zu 200°C unterhalb A₁ (A₁-Temperatur) liegt,
• c) Warmumformung bei einer Temperatur in einem oberen Temperaturband von A1 bis A₁ + 200°C und
• d) Abkühlung auf eine Temperatur unterhalb A₁.

However, a thermomechanical treatment which comprises the following steps is particularly suitable:

• a) Heating of the UHC steel to a homogenization temperature which is in a range of up to 200 ° C above the Austenitisierungstemperatur (A _cm )
• b) hot forming with a degree of deformation> 1.2 while continuously cooling to a temperature which is in the range up to 200 ° C. below A ₁ (A ₁ temperature),
• c) hot forming at a temperature in an upper temperature range from A1 to A ₁ + 200 ° C and
• d) cooling to a temperature below A ₁ .

For the thermomechanical treatment, it is of particular importance that after hot forming in step b) a hot forming (step c) takes place in a temperature band above A ₁ . The temperature band is for the UHC steel according to the invention at 200 ° C from the A ₁ temperature. Too high temperatures and too long residence times above A ₁ lead to damage to the microstructure. Even slight deviations here can have a major influence on the optimum hot forming conditions (temperature and forming forces) of the hot forming to be carried out later. The residence time is therefore preferably limited to below 2 minutes. Particularly preferably, the process duration in the upper temperature band is less than 1 min. Not only the temperature range in process steps a) and b) but also the degree of mechanical deformation is of particular importance for the setting of the optimum microstructure.

The hot forming (b) should have a degree of deformation (φ)> 1.2. During the forming, the steel must cool down to temperatures well below A ₁ . The cooling rate must correspond to at least one cooling by ambient air. Quenching may also be appropriate. Forming degrees significantly above φ> 1.2 can be set, but for cost reasons they must be limited to the minimum required. Preferably, the hot forming is carried out with a degree of deformation of 1.2 to 2. A range of φ = 1.3 to 1.7 is particularly useful.

The degree of deformation (φ) always indicates the ratio of the characteristic dimensions of the steel part immediately before and immediately after the corresponding forming step. For example, in the case of round steel parts, which undergo a reduction in diameter during forming, the characteristic dimension of the degree of deformation is the cross-sectional area (A). In the case of square or rectangular steel parts, which are rolled out into sheets, the characteristic dimension is typically the height or thickness (h). The degree of deformation (φ) is the logarithmic ratio of initial dimension to final dimension, correspondingly φ = ln (A ₀ / A) or φ = ln (h ₀ / h).

Of particular importance is the size of the degree of deformation in step c). Since the steel is already at a comparatively very low temperature level relative to the forging temperature, the forming forces are very high. The process step is accordingly cost-intensive and must therefore be limited to the lowest possible degree of deformation. Preferably, a degree of deformation φ> 0.5 is selected. Conveniently, degrees of deformation in the range of φ = 0.5 to 1. Particularly preferably, the hot working in the upper temperature band is performed with a degree of deformation of 0.8 to 1.5.

As Gesamtumformgrad from the process steps b) and c) φ = 1.7 to 2.5 are preferred.

The process step c) is preferably carried out quasi-isothermally. This means that during the forming of the steel part on the average a constant temperature prevails, which changes only by a few ° C. If the forming is done in a rolling mill, this can be done if necessary, between several successive stitches an intermediate heating. The isothermal process control is required for a uniform formation of the microstructure. The upper temperature band is preferably limited to A ₁ + 50 ° C to A ₁ + 150 ° C. In this thermomechanical treatment, the lamellar pearlite structure is dissolved, without the C atoms can be homogeneously distributed in the austenite grains. This thermomechanical treatment produces a homogeneous, fully spheroidized microstructure that exhibits a high degree of ductility both slightly below and slightly above the A _c1 temperature.

After process step c), it is to be cooled relatively slowly below A1 in step d). The cooling rate is preferably below 20 ° C / min until reaching a temperature below A ₁ . This can be cooled quickly, for example by air quenching.

In an alternative to the described hot forming in an upper temperature band, it is also possible to thermomechanically treat the UHC steel in a lower temperature band.

The homogenization treatment in step a) is the same and does not change in comparison to the above-described process. Also in step b), the hot forming the same Umformtemperaturverläufe and degrees of deformation are to be selected. At the end of process step b), the temperature should be in the range of 50 to 200 ° C. below A1 (A1 temperature) in order to reach the range of the initial temperature for the following hot-forming step in a lower temperature band. The hot working c) takes place in this variant in a lower temperature range of A ₁ -100 ° C to A ₁ . Due to the lower temperatures, higher forming forces occur here than in the first variant of hot forming. On the other hand, at the lower temperature, the undesired diffusion of the carbon atoms into the austenite becomes less apparent. After the thermo-mechanical treatment in the lower temperature band, a rapid cooling to room temperature is to be carried out with cooling rates above 20 K / min (step d).

Particularly advantageous in hot forming in the lower temperature band is a higher minimum degree of deformation than in the upper temperature band, which is φ> 1.5. Preference is given to values of φ = 1.7 to 3 and more preferably 1.7 to 2.2. As Gesamtumformgrad from the process steps b) and c) here φ = 2.7 to 3 are preferred.

For d), cooling rates above 20 ° C / min to room temperature are to be provided. Once the thermomechanical treatment is completed, the semi-finished steel can also be quenched. Thermo-mechanical stresses due to high cooling rates in the semifinished product are of no importance if the subsequent forming process involves hot forming at superplastic temperatures.

In both process variants, with hot forming in the upper or lower temperature band, a microstructure is set, which can be characterized in particular by the crystallite sizes of the phases ferrite and carbide. Preferably, the average particle size, or the average particle diameter of ferrite <5 microns, of proeutectoid carbides <3 microns and eutectoid carbides <1 micron. The volume fraction of the carbide phases is preferably in the range of 15 to 35% of fine spheroidal (eutectoid) carbides and 20 to 30% of coarser spheroidal (proeutectoid) carbides.

Preferably, the process in steps b) and c) is driven so that the proportion of proeutectoid carbides deviates only slightly from that of the eutectoid. Preferably, the ratio of proeutectoid / eutectoid carbides ranges from 0.8 to 1.2.

The UHC steels are particularly suitable for suspension components, gear parts, gears or engine components for motor vehicles. On the one hand, there is a high potential for saving weight and, on the other hand, there are also process simplifications and savings in the application of semi-hot forming.

Claims (16)

UHC lightweight steel, characterized by a composition in% by weight C: 0.8 to 1.4 Al: 5.5 to 7.0 Cr: 0 to 0.2 Si: 0.01 to 0.6 Stabilizing alloying elements below 1 wt.% And the remainder of iron and common steel-accompanying impurities. UHC lightweight steel according to claim 1, characterized in that the Cr content is 0.01 to 0.1 wt.%. UHC lightweight steel according to claim 1 or 2, characterized in that the UHC lightweight steel stabilizing alloying elements selected from the group Nb, Ti, Mg and / or N in an amount of 0.02 to 0.8 wt.% Contains. UHC lightweight steel according to one of the preceding claims, characterized in that the Ni, Mo and / or V content is below 0.15 wt.%. UHC lightweight steel according to one of the preceding claims, characterized that the structure has fine spheroid carbides with average cross-sectional areas below 10 microns ² and partially a fine pearlite structure. UHC lightweight steel according to one of the preceding claims, characterized in that the volume fraction of carbides is 15 to 35%. Process for the production of hot-formed components from UHC lightweight steels having a composition according to one of claims 1 to 6, characterized in that first a structure with fine spheroid carbides having average cross-sectional areas below 10 μm ^{2 is} set by thermomechanical treatment, and then a semi-warm Conversion to the component at a temperature of 800 to 1100 ° C and at strain rates (ε ') above 0.1 / s takes place. The method of claim 7, wherein the thermomechanical treatment comprises the steps of: a) heating the UHC steel to a homogenization temperature in a range of up to 200 ° C above the austenitizing temperature (A _cm ) b) hot working with a degree of deformation> 1 , 2 with continuous cooling to a temperature in the range up to 200 ° C below A ₁ (A ₁ temperature), c) hot working at a temperature in an upper temperature range from A1 to A ₁ + 200 ° C and d) Cooling to a temperature below A ₁ . A method according to claim 8, characterized in that the cooling rate in step d) is below 20 ° C / min and that the hot forming is quasi-isothermal, has a degree of deformation> 0.5 and the residence time of the steel at the hot forming temperature is below 2 min , The method of claim 9, wherein the thermomechanical treatment comprises the steps of: a) heating the steel to a homogenization temperature which is in a range of 5 to 200 ° C above the Austenitisierungstemperatur b) hot forming with a degree of deformation> 1.2 while continuously cooling to a temperature which is in the range of 50 to 200 ° C. below A1 (A1 temperature), c) hot working at a temperature in a lower temperature range from A1-100 ° C to A1, d) rapid cooling to room temperature. A method according to claim 10, characterized in that in step d), the cooling rates above 20 ° C / min, the hot forming is quasi-isothermal and has a degree of deformation> 1.5. Method according to one of claims 8 to 11, characterized in that the deformation rate (ε ') of the hot working in step c) is set to values above 0.1 / s. Method according to one of claims 7 to 12, characterized in that the hot forming in step b) is carried out with a degree of deformation of 1.5 to 4. Method according to one of claims 7 to 12, characterized in that the hot deformation of the blank in step c) at least partially up to a degree of deformation> 2 is performed. Method according to one of claims 7 to 14, characterized in that the half-warm forming chassis components, transmission parts, gears or engine components for motor vehicles are formed. Method according to one of claims 7 to 14, for the production of steel components for internal combustion engines and transmission components of motor vehicles.