EP1565588B1 - Use of a steel alloy as a material for producing thick-walled tubular parts for motor vehicles - Google Patents

Description

Torques transmitting components in motor vehicles, such as drive shafts, transmission shafts, camshafts and other highly stressed components, such as suspension stabilizers and damper piston rods are usually made of solid material. Drive shafts, which transmit engine power to the wheels of a motor vehicle, generate torques of up to 2000 Nm. At the same time drive shafts are easily excitable to vibrations. Although drive shafts made of solid material are relatively inexpensive to manufacture, they have a very low first bending natural frequency and a high weight considered to be particularly disadvantageous. To reduce weight hollow drilled waves are known. The problem is the selection of materials, since the most cost-effective use of alternative materials of sufficient strength is desired. Because an increase in the diameter of the drive shafts due to the tight space in a motor vehicle is out of the question, excrete low-strength materials anyway.

In the meantime, even with camshafts, it is now state of the art to replace solid cast shafts with so-called built-in camshafts and thus achieve weight savings of 25% to 40%.

Even chassis stabilizers in tube construction are known, but the material utilization must be raised to a previously unattained in the automotive industry, compared to tube stabilizers made of solid material to a 40% higher voltage level. Weight advantages in the double-digit percentage range can be achieved.

Tubes for hollow beads are usually made from water-hardened tempered steels, eg from 34 MnB5. Such pipes according to DIN 2393 or DIN 2394 are usually deformed in normal or annealed condition by rotary swaging. This is followed by a heat treatment (water quenching). Finally, the finishing takes place. The plant concept for pipes produced in this way from water-hardenable tempered steels with the process steps hardening, water quenching, tempering, is relatively complex and therefore prone to failure. The water quenching following the hardening is additionally associated with the disadvantage that very high residual stresses are induced, which can lead to a hardening distortion of the workpiece. Subsequent straightening and, if appropriate, mechanical post-processing of geometrically critical regions is thereby unavoidable. In the case of rotating components, a distortion or a deviation of the component geometry also leads to an oscillation-inducing imbalance. Due to the nature of the heat treatment, it also leads to an undesirable scaling of the treated component areas. Another disadvantage is that the heat treatment on high-carbon steels such as 34 MnB5 always leads to a high hardness and a decrease in strength, whereby a subsequent tempering is imperative. The stresses within the pipe components also lead to stress peaks, which can be the cause of cracking, which ultimately lead to failure of the pipe components in practice. Of course, cracking can not be undone by subsequent heat treatment. It is therefore a relatively complex quality assurance and strict sorting of missing parts required.

EP 0 753 597 A2 relates to the use of a steel alloy, expressed in terms of weight percent, of carbon (C) 0.12% -0.25%, silicon (Si) 0.10% -0.40%, manganese (Mn) 1.30% - 2.00%, phosphorus (P) max. 0.025%, sulfur (S) max. 0.025%, chromium (Cr) 1.60% - 2.50%, molybdenum (Mo) 0.40% - 1.00%, aluminum (Al) 0.010% - 0.050%, remainder iron including impurities caused by melting. This material is characterized by a good deformability and its toughness behavior. An additional fee is not required. Since the material is weldable, it can be used in addition to the use of seamless pipes for the production of longitudinally welded pipes, which are better suited for the intended use as a starting material for stabilizers.

WO 03/069005 A2 published in the priority interval relates to the use of a steel alloy consisting of 0.09% -0.13% carbon (C), 0.15% -0.50% silicon (Si), 1.10% 1.80% manganese (Mn), max. 0.02% phosphorus (P), max. 0.02% sulfur (S), 1.00% - 2.00% chromium (Cr), 0.20% - 0.60% molybdenum (Mo), 0.02% - 0.06% aluminum (Al) , 0.10% - 0.25% vanadium (V) and iron and the usual impurities as a remainder, as a material for air under inert gas-hardened tubes for the production of compressed gas containers or as a material for the production of molded parts in lightweight steel construction.

Proceeding from this, the present invention seeks to show a steel alloy for use as a material for the production of thick-walled pipe components for motor vehicles, which allows compared to previously used materials higher strength, better ductility and a simplified heat treatment, to simplify in this way thick-walled pipe components and cheaper to produce.

The object is achieved by the use of a steel alloy having the features of patent claim 1.

The use of the proposed steel alloy is particularly suitable due to the much simpler heat treatment to achieve the desired component strength, and meets the total in the

Task specified requirements. The steel alloy described is based on an alloying concept that allows for air hardening or air treatment. A previously required Wasserabschreckung deleted without replacement. An air cooling of the thick-walled pipe components after hardening, in contrast to a water quenching to less high residual stress within the pipe components, so that it may even be possible to dispense with the previously required subsequent tempering of the workpieces. With suitable temperature control, subsequent straightening and machining operations can be largely restricted. The degree of scaling is significantly lower in air-hardened or air-tempered pipe components than in a water quench. Scaling can even be completely avoided under a protective gas atmosphere.

The investment technical effort for the production of thick-walled pipe components can be reduced as a result of the use of the proposed steel alloy to a considerable degree. Since possible sources of error are eliminated by reducing the necessary process steps, the entire manufacturing process is less prone to failure. As a result, a higher quality standard can be ensured thereby.

The lower internal stresses within the pipe components lead to less toughness compared to water-resistant steels. The improvement of the toughness as well as the reduction of notch sensitivity allows larger manufacturing tolerances and thus a simpler process management, resulting in better structural design options. The greater plasticizability of the steel alloys used makes far less likely in the heat treatment cracks on manufacturing critical points, which is reflected in a lower reject rate of the pipe components, or reduces the cost of manufacturing the pipes, especially in terms of the test. In summary, thick-walled pipe components for motor vehicles using the proposed steel alloys are much cheaper to manufacture than comparable ones due to the resulting manufacturing advantages Pipe-building site from previously used steel alloys, in particular from high-carbon water-treatable steel alloys.

The pipe components produced using the claimed steel alloy also have better vibration resistance than comparable pipe components made of other steel alloys. Further, due to the lower carbon content, better ductility is possible, e.g. by swaging in soft annealed condition.

It is known that the lowest nitrogen contents sustainably damage the mechanical properties of a steel, increase yield strength and strength, greatly reduce the deformability and the notched impact strength and at the same time have an aging effect on the steel. In the context of the invention it has been found that the targeted addition of nitrogen leads to the formation of vanadium (-carbo) nitrides, which have pronounced positive properties on the steel alloy used and for the inventive use of the steel alloy. The vanadium (-carbo) nitrides formed by the targeted addition of nitrogen contribute to precipitation strengthening and refinement. It has been found that when mass fractions of nitrogen in a range of 0.005% and 0.05% on the one hand sufficient carbonitrides are formed and on the other hand, the nitrogen is sufficiently bound by vanadium.

Thick-walled pipe components in the sense of the invention are those in which the ratio of the outer diameter (AD) of the pipe component to the wall thickness (WD) of the pipe component is less than 14 (AD / WD <14) and preferably less than 8 (AD / WD <8 ). The ratio AD / WD can also be less than 6. The ratio AD / WD can vary over the length of the pipe component, but the thick wall is retained in the above sense.

Under thick-walled pipe components are torque-transmitting pipe components in the form of drive shafts, gear shafts, camshafts to understand.

The steel alloy used can have tensile strengths of Rm> 950 N / mm ² and yield strengths of Rp0.2> 700 N / mm ² at an elongation at break A 5> 14% be set. Air hardening is preferably carried out at 950 ° C ± 15 ° C in a continuous furnace under inert gas.

In the air-tempered state, the steel alloy achieves a tensile strength Rm> 850 N / mm ² and a yield strength of Rp0.2> 700 N / mm ² with an elongation at break of A5> 15%.

Claims (3)

1. Use of a steel alloy which consists of, in mass terms,
   0.09 - 0.12% of carbon (C),
   0.15 - 0.30% of silicon (Si),
   1.45 - 1.60% of manganese (Mn),
   max. 0.015% of phosphorus (P),
   max. 0.011% of sulphur (S),
   1.25 - 1.50% of chromium (Cr),
   0.40 - 0.60% of molybdenum (Mo),
   0.020 - 0.060% of aluminium (Al),
   0.12 - 0.20% of vanadium (V),
   0.005 - 0.05% of nitrogen (N)
   remainder iron and melting-related impurities, as a material for air-hardened or air-treated tubes for producing thick-walled, torque-transmitting tubular components in the form of drive shafts, transmission shafts or camshafts for motor vehicles.
2. Use of a steel alloy according to Patent Claim 1, characterized in that the ratio of the external diameter (ED) of the tubular component to the wall thickness (WT) of the tubular component is less than 14 (ED/WT < 14).
3. Use of a steel alloy according to Patent Claim 1 or 2, characterized in that the ratio of the external diameter (ED) of the tubular component to the wall thickness (WT) of the tubular component is less than 8 (ED/WT < 8).