WO2008135022A1

WO2008135022A1 - Microalloy (afp) steel with a low ti content and component made of said steel for use in internal combustion engines

Info

Publication number: WO2008135022A1
Application number: PCT/DE2008/000737
Authority: WO
Inventors: Andreas Seeger-Van Nie
Original assignee: Mahle International Gmbh
Priority date: 2007-05-03
Filing date: 2008-04-30
Publication date: 2008-11-13
Also published as: DE102007021101A1

Abstract

The invention relates to an alloy pearlitic ferritic steel with the following composition, listed in % by weight: carbon: 0.35 - 0.50; silicon: 0.15 - 0.80; manganese: 1.20 - 2.00; phosphorus: 0.00 - 0.0025; nitrogen: 0.010 - 0.035; chromium: 0.00 - 0.50; molybdenum: 0.00 - 0.050; nickel: 0.00 - 0.15; copper: 0.00 - 0.40; vanadium: 0.13 - 0.40; titanium: 0.001 - 0.004; aluminium: 0.00 - 0.04; niobium: 0.00 - 0.05; at least two of the elements vanadium, aluminium and niobium being included. Said steel is used in a component for internal combustion engines, in particular for pistons or piston parts.

Description

MICRO-LUBRICATED AFP STEEL WITH LOW TI CONTINUITY AND COMPONENT MANUFACTURED FROM THIS STEEL FOR INTERNAL COMBUSTION ENGINES

The present invention relates to an alloyed steel and its use.

The material steel is used extensively in the automotive industry, where it is the most important material. For highly loaded vehicle components, especially in the chassis and internal combustion engine, tempered steels are usually used because they meet the mechanical requirements for such components. The tempered steels are tempered after the manufacturing process (eg forging process) in a separate heat treatment step to the desired strength properties out. However, the production of such vehicle components is complicated because of the necessary compensation step and therefore costly. Therefore, for certain applications, the tempered steels are replaced by precipitation-hardening ferritic-pearlitic steels (in the following: AFP steels) according to DIN EN 10267 (formerly: Stahl-Eisen-Werkstoffblatt 101). These steels, for example of the 38MnVS6 type, can be cooled in a controlled manner directly from forging heat and thus adjusted to the required strength. In addition to the elimination of the costly compensation process, the better cutting performance leads to significant cost savings.

Forged pistons and piston parts are usually tempered steels according to DIN EN 10083 used, for example. Type 42CrMo4. Here, too, it is known to replace the tempering steel by an AFP steel in individual parts of the piston. DE 41 09 628 C2 discloses to use AFP steel of the type 38MnSiVS5 in a piston with forged steel areas in the areas adjacent to the combustion chamber, at the hubs and in the area of the piston skirt. EP 1 161 624 B1 describes a built-up piston whose lower piston part is made of AFP steel.

The mechanical properties of AFP steels are determined, among other things, by the grain size of the structural constituents and by the effectiveness of the precipitation - -

hardening of fine nitrides and carbonitrides of intentionally added micro-alloying elements determined. The micro-alloying elements include the elements vanadium, niobium, aluminum and titanium, although not necessarily all elements must be alloyed simultaneously. In particular, the setting of a fine-grained austenite during the heating, rolling and forging process is of particular importance.

However, the mechanical properties that can be achieved in today known AFP steels used for piston parts do not correspond to the values of tempered steels in all parameters. While the tensile strength nearly reaches the tensile strength of known tempered steels, the 0.2% yield strength and the cyclic flexural strength are still below the level of tempered steels. In addition, the main stresses in a piston caused by the stress during operation can also be oriented transversely to the fiber flow of the forging blank. Therefore, the strength values transversely or tangentially to the fiber flow must meet the requirements of a suitable material. Therefore, pistons and piston parts with very high mechanical stresses are still made of tempered steel, preferably of 42CrMo4 steel grade.

The object of the present invention is to provide an alloyed steel based on an AFP steel whose mechanical properties are comparable to the mechanical properties of tempered steels and suitable for use in internal combustion engines.

The solution consists in an alloyed steel with the features of claim 1.

The alloy steel according to the invention is characterized by the following composition, indicated in each case in% by weight: carbon: 0.35-0.50; Silicon: 0.15-0.80; Manganese: 1.20 - 2.00; Phosphorus: 0.00 - 0.0025; Nitrogen: 0.010-0.035; Chromium: 0.00-0.50; Molybdenum: 0.00-0.050; Nickel: 0.00 - 0.15; Copper: 0.00-0.40; Vanadium: 0.13-0.40; Titanium: 0.001-0.004; Aluminum: 0.00-0.04; Niobium: 0.00 - - -

0.05; wherein at least two of the elements vanadium, aluminum and niobium are included.

It has been found that a low titanium content of 0.001 to 0.004% by weight is a decisive factor of the solution according to the invention. Although a higher titanium content improves the fine granularity of the austenite by precipitation of fine titanium nitrides, because the titanium nitrides hinder the grain growth of the structural constituents. However, some of the nitrogen and vanadium contained in the alloy are permanently set as titanium nitride and titanium-vanadium nitride precipitate and remain ineffective for precipitation hardening, i.e., precipitation hardening. there are too few nitrides and carbonitrides of the other micro-alloying elements. However, especially the nitrides and carbonitrides of vanadium and niobium contribute to an increase in the strength values, in particular the 0.2% yield strength, to a particularly great extent. The composition of the steel alloyed according to the invention thus causes sufficient nitrides and carbonitrides to form the other micro-alloying elements present. At the same time, a sufficiently fine-grained microstructure is created. The result is an alloyed steel based on an AFP steel whose mechanical properties are comparable to the mechanical properties of a tempered steel.

Furthermore, during casting of the primary material and during solidification in the strand, fewer titanium nitrides precipitate directly on the grain boundaries of the austenite, so that no fine fracture networks at the near-surface grain boundaries arise during rolling directly from the casting heat. This eliminates the previously required further α / γ microstructure transformation during further processing to the rolled billet.

Advantageous developments emerge from the subclaims.

A preferred development is that the boundary condition Al [wt .-%] x N [wt .-%]> 2 x 10 ^"4 (I) is satisfied.As aluminum nitrides are redissolved at a higher temperature, the proportion of remaining undissolved aluminum nitrides are adjusted on the basis of the temperature-dependent solubility product taking into account the forging temperature. - -

ensures an optimum amount of aluminum nitrides in the manufacturing process. Since aluminum nitrides also hinder the grain growth of the structural components, it is ensured that an optimized fine-grained microstructure is formed. If niobium partially or completely takes the place of aluminum in the alloy according to the invention, this boundary condition for the niobium content applies correspondingly on the basis of the solubility product of the niobium nitrides.

It is also advantageous if the boundary condition Al [wt .-%]: N [wt .-%] <2 (II) is satisfied. Thus, the proportion of free, available for precipitation hardening, nitrogen is optimized in the alloy, so that an optimum content of vanadium nitrides or vanadium carbonitrides results. If niobium takes the place of aluminum in the alloy according to the invention, this boundary condition for the niobium content applies correspondingly taking into account the specific atomic weight.

With the fulfillment of the boundary condition V [wt .-%]: N [wt .-%]> 5 (III) produces an optimized amount of vanadium nitrides and vanadium carbonitrides, so that an alloyed steel results with particularly good mechanical properties. If niobium partly or wholly takes the place of vanadium in the alloy according to the invention, this boundary condition applies to the niobium content corresponding to Nb [wt%]: N [wt%]> 1.

Finally, the boundary condition Mn [wt .-%] + 2.5 x Si [wt .-%]> 2.3 (IV) may be satisfied. Since silicon and manganese contribute to solid solution hardening, the adherence to this boundary condition optimizes solid solution hardening.

The best results are achieved if all boundary conditions (I), (II), (III) and (IV) are cumulatively met.

The alloyed steel according to the invention preferably contains at most 0.004% by weight of titanium, more preferably at most 0.003% by weight of titanium. It has been found that a steel according to the invention with this particularly low titanium content has particularly good mechanical properties. The alloy steel of the present invention may contain sulfur in a proportion of 0.005 wt% to 0.10 wt% for the purpose of improving the workability. However, during the rolling or forging process, elongated lines of sulfide may be produced which impair the mechanical properties of the steel. The addition of calcium in a proportion of not more than 60 ppm total and / or tellurium in a proportion of not more than 0.2%, the sulfide form is specifically influenced and this effect avoided. This applies in particular if, in addition to calcium, a maximum of 60 ppm of oxygen is added.

An embodiment of the present invention will be explained in more detail below.

A piston head for a two-piece piston of an internal combustion engine was made of the alloy steel according to the invention. In this embodiment, it is a piston head for a so-called "Ferrotherm®" piston, in which the piston head, together with the piston rings, performs the sealing function with respect to the combustion chamber and forwards the gas forces to the crank mechanism and withstand thermal stresses.

The starting point for the preparation was a starting material having the composition according to Table 1. The composition was determined by means of a test analysis.

Table 1

From the starting material, a piston blank was forged.

For comparison, an identical piston blank for a "Ferrotherm®" piston made of the material 42CrMo4 was produced according to DIN EN 10083 and heat treated according to specifications of this standard.

For both piston blanks the characteristic values according to Table 2 were determined for the same sample position.

Table 2

1) Sampling above the hub bore, transverse to the fiber direction - -

2) bending fatigue strength; Circulation bending test, sampling at the piston bottom, transverse to the fiber direction

The alloyed steel according to the invention thus also exhibits, transverse to the fiber direction, strength properties which are comparable to those of a conventional tempered steel

The alloyed steel according to the invention is used for producing a component, in particular a piston part, such as a piston head or piston upper part and / or piston lower part (two-part steel pistons) and / or a one-piece piston.

Claims

- Patent claims

1. Alloy steel, having the following composition, indicated in each case in% by weight:

Carbon: 0.35-0.50

Silicon: 0.15 - 0.80

Manganese: 1, 20 - 2.00

Phosphorus: 0.00 - 0.0025

Nitrogen: 0.010-0.035

Chromium: 0.00 - 0.50

Molybdenum: 0.00-0.050

Nickel: 0.00 - 0.15

Copper: 0.00 - 0.40

Vanadium: 0.13-0.40

Titanium: 0.001-0.004

Aluminum: 0.00-0.04

Niobium: 0.00 - 0.05; wherein at least two of the elements vanadium, aluminum and niobium are included.

2. Alloy steel according to claim 1, characterized in that the following boundary condition (I) is fulfilled:

Al [weight%] × N [weight%]> 2 × 10 ^-4 (I)

3. Alloy steel according to claim 1 or 2, characterized in that the following boundary condition (II) is fulfilled:

Al [wt%]: N [wt%] <2 (II)

4. Alloy steel according to one of the preceding claims, characterized in that the following boundary condition (III) is satisfied:

V [% by weight]: N [% by weight]> 5 (III) - -

5. Alloy steel according to one of the preceding claims, characterized in that the following boundary condition (IV) is fulfilled:

Mn [wt%] + 2.5 x Si [wt%]> 2.3 (IV)

6. An alloy steel according to any one of the preceding claims, characterized in that it contains at most 0.004 wt .-%, preferably at most 0.003 wt .-% titanium.

7. An alloy steel according to any one of the preceding claims, characterized in that it contains 0.005 wt .-% to 0.10 wt .-% sulfur.

8. An alloy steel according to claim 7, characterized in that it contains maximum calcium in a proportion of not more than 60 ppm in total and / or tellurium in a proportion of not more than 0.2%.

9. Alloyed steel according to one of the preceding claims, characterized in that it contains a maximum of 60 ppm oxygen.

10. A component made of an alloy steel according to one of claims 1 to 9, characterized in that it is used in internal combustion engines.

11. Component according to claim 10, namely a piston or a piston part.