EP3017074B1 - Wear-resistant uncoated steel part - Google Patents

Description

The invention relates to a wear-resistant, uncoated steel part consisting of a hardenable steel grade, which has been produced by hot forming and / or curing of a semi-finished product. In addition, the invention relates to a method for producing a wear-resistant, at least partially uncoated processing, conveying and / or emetic agent of agricultural machinery, conveyors, mining equipment or construction machinery from a semi-finished product, in which the semi-finished product is heated to a temperature of more than the Ac1 transformation temperature is and then hot worked and / or cured.

Wear-resistant, at least partially uncoated steel parts, which must have high strengths and are simultaneously exposed to abrasive forces, for example, for the provision of agricultural machinery, especially plows but also for blades of an excavator or screw conveyors for abrasive materials, such as the screw conveyor of a concrete mixer needed. In order to realize the necessary, high strengths in the applications mentioned, they are preferably subjected to hot working, in which the semi-finished products from which the steel parts are produced, are first heated to a temperature above the Ac1 transformation temperature point, so that by hot forming and subsequent Hardening, ie rapid cooling, a transformation hardening of the microstructure occurs and arises in the material martensitic structure. The martensitic structure has a much higher hardness but also a significantly higher mechanical strength, for example tensile strength. Corresponding steel parts are for example from the German patent DE 10 2010 050 499 B3 known. The German patent describes a manufacturing method for excavator buckets, concrete mixer screw conveyors, screw conveyor blades or other transport blades of conveyors, in which the components are hot-formed and press-hardened.

However, it has been found that the components produced in this way, in particular in contact with abrasive materials, have problems in terms of wear resistance despite the hardening process during production.

The German patent application DE 10 2010 017 354 A1 deals with the problem of hot forming of galvanized flat steel products into high- and high-strength steel components. When the melting temperature of the metal of the protective coating is exceeded, there is a risk of the so-called "liquid metal embrittlement", which is caused due to the penetration of the molten metal of the coating in the resulting during the forming of the flat steel product notches or cracks. The liquid metal entering the steel substrate deposits there at grain boundaries and there reduces the maximum absorbable tensile or compressive stress. As a solution, the patent publication offers a nitration of the edge layer areas, so that finely structured edge layer areas are produced.

Further, in Japanese Laid-Open Publication JP 2011 032536 A a wear-resistant, uncoated steel part described from a hardenable steel grade, which is nitrided before hardening at the surface. From the Chinese disclosure CN 101 805 823 A it can be seen how a conical steel part is heat-treated, in particular hardened by carburizing and quenching.

By contrast, the present invention deals with the problem that hot-formed and / or hardened steel parts in the uncoated areas do not have the desired wear resistance and are thus not optimally suited for use as conveying means, for example when in contact with abrasive materials. The object of the present invention is therefore to propose at least partially uncoated steel parts, the suitability of which is improved for use with abrasive materials. In addition, a cost-effective production of appropriate steel parts to be proposed.

The stated object is achieved for a steel part in that the steel part at least partially has a surface area which up to a depth of maximum 100 microns, preferably up to a depth of up to 40 microns, has been cured by surface hardening before hot working and / or curing.

It has been found that heating the semifinished products to produce the steel parts at a temperature greater than the Ac1 transformation temperature or above the Ac3 temperature prior to hot working and / or curing results in decarburization of near-surface areas such that the carbon content of these areas is significantly lower than the carbon content of the base material. As a result, the near-surface area of up to 100 microns depth, in particular the area up to 40 microns depth during hot working and / or curing can not be cured to the required extent. However, it has been found that at least partial surface hardening of the uncoated regions of the semifinished product prior to hot working and / or hardening to the steel part results in both the surface region and the base material despite the decarburization of the near surface regions due to the high temperatures during hot working or hardening , have very high hardness. As a result, a steel part is available which at least partially has a surface area which is preferably cured to a depth of 100 microns or in the range to 40 microns depth and thus significantly more wear resistant than the previously known, at least partially uncoated steel parts.

According to a first embodiment, the hardened surface area of the steel part is hardened by nitriding. This method offers the possibility of hardening near-surface areas of the steel part before hot forming or hardening. Nitriding also has the advantage that the hardness is not reduced during hot forming. Carburization increases the carbon content in the surface areas, but decreases again due to hot working.

Preferably, according to a further embodiment, after hot working and / or hardening, the hardened surface area of the steel part has at least the hardness of the base material of the steel part lying below the surface area. Preferably, the wear resistance of the steel part can also be improved in that the hardness of the surface portion of the steel part is greater than the hardness of the base material. It has been found that in particular the hardness of the surface areas are responsible for the wear resistance of the steel part in contact with highly abrasive materials, so that even with a slightly softer base material, a very wear-resistant steel part can be produced.

It follows according to the invention that the steel part is designed for use as processing, conveying and / or breaking means in agricultural machines, conveyors, mining machines or construction machines, wherein at least the abrasive forces exposed areas of the steel part are surface hardened.

In addition, manganese-boron steels, dual-phase steels or TRIP steels in which a particularly pronounced development of the martensite formation or the transformation of residual austenitic parts into martensite makes it possible to increase the hardness are also particularly advantageous.

According to a further embodiment of the steel part, the surface area of the steel part hardened before the hot working and / or hardening has a hardness of 400 to 700 HV at least in some areas. As a rule, these values are only achieved by ultrahigh-strength steel grades after hot forming or hardening in the base material. Surface hardening before hot working or hardening in particular offers the possibility of providing the starting material for the production of the steel components on a coil.

According to a further teaching of the present invention, the above-described object is achieved by a method for producing a wear-resistant, at least partially uncoated steel part for machining, conveying and / or breaking means of Agricultural machinery, conveyors, mining equipment or construction machinery from a semi-finished product, wherein the semi-finished product is at least partially heated to a temperature of more than the Ac1 transformation temperature and then hot-formed and / or cured, solved in that the semifinished product before hot forming and / or Hardening is at least partially subjected to a surface hardening, in which a surface area is cured to a maximum depth of 100 microns. Preferably, a surface area of up to 40 microns depth is hardened, in which usually the decarburization processes take place during hot working. The depth of the surface area to be cured is controlled by the exposure time of the curing treatment. In particular, it has been found that, despite the heating to a temperature above the Ac1-Umwendelungstemperaturpunktes the surface hardened areas of the steel part remain stable in terms of their surface hardness, so that after the hot forming and / or curing high surface hardness can be achieved. As a result, the steel parts in contact with abrasive materials for machining, conveying and / or breaking means of agricultural machines, conveyors, mining machines or construction machines have a reduced wear.

The hardening of the surface areas before hot forming or before hardening makes it possible to carry out the surface hardening on coilable materials, ie on the steel strip, so that a particularly economical production of wear-resistant, at least partially uncoated steel parts made of semi-finished products is made possible. According to the invention, the hardening of the surface area is carried out by nitriding. This method makes it possible to provide a higher hardness in the surface area, which after hot working and / or after hardening, enables a higher wear resistance of the surface of the hot-worked steel part, wherein an annealing treatment in an annealing atmosphere of up to 25 vol. -% H ₂ , 0.1 - 10 vol .-% NH ₃ , H ₂ O and balance N ₂ and unavoidable impurities at a holding temperature of 600 ° C to 900 ° C is performed. Preferably, the dew point of the annealing atmosphere is between -50 ° C and -5 ° C, so that the effect of humidity on the curing process is reduced. Be preferred over it addition, a maximum of 10 vol .-% H ₂ , a maximum of 5 vol .-% NH ₃ allowed and set the dew point to a dew point temperature of -40 ° C to -15 ° C at a temperature of 680 to 840 ° C. The latter process parameters resulted in improved and more uniform surface hardening.

The depth of the surface hardening can be adjusted by the duration of exposure to the holding temperature. During the surface hardening, the duration at which the semifinished product has the holding temperature is preferably set to 5 s to 600 s, preferably to 30 s to 120 s.

Preferably, the surface hardening is carried out in a continuous furnace, so that, for example, a strip-shaped semifinished product, ie a coilable semifinished product is surface-hardened and can be supplied to the further hot-forming and / or press-hardening steps. However, it is also conceivable surface hardening in a chamber furnace.

As already stated, semifinished products such as manganese-boron steels, dual-phase steels and TRIP steels on the one hand show a particularly high increase in strength during hot forming or hardening and on the other hand the possibility of nitriding to near-surface regions in the range from 400 to 700 HV bring to. As a result, steel parts can be produced in a cost effective manner, which are very resistant to wear and have particularly high strengths.

Fig. 1

schematisch ein Ausführungsbeispiel des Verfahrens zur Herstellung eines verschleißfesten, zumindest teilweise unbeschichteten Stahlteils,

Fig. 2

den Schichtaufbau des entsprechend dem Ausführungsbeispiel auf Fig. 1 behandelten Halbzeugs bzw. Stahlteils in schematischer Darstellung,

Fig. 3, 4

Ausführungsbeispiele eines Stahlteils für Landmaschinen und Fördermaschinen und

Fig. 5

in einem Diagramm den Härteverlauf in Abhängigkeit vom Abstand zur Oberfläche von zwei Ausführungsbeispielen und einem Vergleichsbeispiel.

In addition, the invention will now be explained in more detail with reference to embodiments in conjunction with the drawings. The drawing shows in

Fig. 1

1 shows an exemplary embodiment of the method for producing a wear-resistant, at least partially uncoated steel part,

Fig. 2

the layer structure of according to the embodiment Fig. 1 treated semi-finished or steel part in a schematic representation,

Fig. 3, 4

Embodiments of a steel part for agricultural machinery and conveyors and

Fig. 5

in a diagram, the hardness curve as a function of the distance to the surface of two embodiments and a comparative example.

In Fig. 1 is first very schematically an embodiment for producing a wear-resistant, at least partially uncoated steel part shown in a schematic representation. The semifinished product 1, which consists of a steel, for example of a manganese-boron steel, dual-phase steel or TRIP steel, is first supplied to a surface hardening 2. If a strip-shaped semifinished product is unwound from a coil 1a and fed to the surface hardening 2, it is advantageous, for example, to carry out the surface hardening, for example in the case of nitriding, in a continuous strip furnace, at the end of which, for example, the strip-shaped semifinished product 1 is now provided with a surface hardening a coil (not shown) can be wound up. The thus surface-hardened band-shaped semi-finished product is cut to length and fed to a hot forming and / or hardening 3, so that a transformed, at least partially uncoated steel part 4 can be produced by method step 3, which is used for processing, conveying and / or breaking means of agricultural machines, conveyors , Mining or construction equipment is suitable. On the one hand, the correspondingly produced steel part 4 is characterized by high strength values due to the hot forming and / or hardening step. On the other hand, the surface area of the steel part also has an increased hardness due to the nitriding of the surface taking place before hot working and / or before hardening. As already stated above, with the method according to the invention, the decarburization of the surface areas, which takes place to a depth of 100 μm, can be counteracted, in which the surface area surface hardened up to 100 μm depth or in a range up to 40 μm depth. Preferably, the surface hardening is carried out by nitriding. Surface hardening in process step 2 is preferably carried out by an annealing treatment in an annealing atmosphere with up to 25% by volume H ₂ , 0.1-10% by volume NH ₃ , H ₂ O and the radical N ₂ and unavoidable impurities at a holding temperature of 600 ° C to 900 ° C performed. A reduction of the hydrogen concentration to a maximum of 10% by volume or a limitation of the NH ₃ concentration to a maximum of 5% by volume lead to a further improvement in the nitration result.

Over the duration of the surface hardening, for example at a holding temperature of 5s to 600s, the depth of the surface hardening can be adjusted. Preferably, the surface is nitrided with a holding temperature of 30s to 120s, wherein the temperature is 680 ° C to 840 ° C if possible. Carrying out the surface hardening before hot forming or hardening has the advantage that an annealing process with a band-shaped semifinished product, for example, can be carried out much more efficiently in a continuous strip furnace or a circuit board in a continuous furnace than with deformed steel parts having differently shaped and different geometries. The quality of the surface hardening can also be more easily ensured by the use of ribbon-shaped or semi-finished products designed as a board.

In Fig. 2 is now shown schematically a cross section of the semifinished product at three different times of the process. First of all, the semifinished product 1 exhibits a more or less homogeneous, for example ferritic, structure 1 a corresponding to the production process, which is due to the combination of the production process and the steel composition. Surface hardening hardens the surface region 1b by diffusion of nitrogen during nitriding, whereby the microstructure changes there. The thickness of the surface region 1b depends on the duration of the annealing. Usually, the surface area is up to a maximum of 100 μm, in which the hardness of the semifinished product is changed becomes. A preferred range, which is a compromise of sufficient surface hardening and duration of annealing for surface hardening, has a thickness of 20 to 40 microns. The duration of the surface hardening, for example during nitriding, is then preferably from 30 seconds to 120 seconds. The microstructure of the material 1a remaining below the surface region 1b remains substantially unchanged during the annealing treatment.

In the hot-forming step, the microstructure of the base material 1a is first converted into austenite and later partly into martensite by hardening. As a result, large hardnesses and high mechanical strengths in the base material 1c are achieved. The surface area 1b remains unchanged apart from the decarburization of these layers. By nitriding, the surface area can remain hardened. The formed steel part 4 thus has a hardened portion 1b and a portion 1c hardened by hot working and hardening.

3 and 4 show typical applications of the wear-resistant, at least partially uncoated steel part in the form of a screw conveyor 5 in Fig. 3 and a ploughshare 6 for agricultural plows in Fig. 4 , Both components are typical representatives of processing, conveying and / or breaking agents, which are used in agricultural machinery, conveyors, mining equipment or construction machinery, such as concrete mixers, and are exposed to highly abrasive materials. The use of hot-formed and / or press-hardened steel parts was previously not very advantageous due to their increased susceptibility to wear. The surface hardening of the area decarburized during hot working and / or hardening gives the hot working steels an increased field of application. Table 1 Measurement HV 0.01 depth μm Sample A (1% NH ₃₎ Sample B (4% NH ₃ ) 5 460 546 10 404 490 15 436 447 20 333 415 25 409 394 30 479 453 35 453 479 40 436 485 45 492 466

Table 1 now shows measurements of the hardness of samples A and B, which consist of a grade 22MnB5 steel. Samples A and B were subjected to surface nitriding in an annealing atmosphere containing 1% by volume of NH ₃ and 4% by volume of NH ₃ at 760 ° C. and 90 s, respectively. The surface nitriding was carried out at intercritical temperatures (T> Ac1) because the austenite can dissolve more nitrogen than the ferrite. Subsequently, the samples were thermoformed and cured. Cuts were made from the hot-formed or hardened steel parts and measured at a distance of 5 μm from the surface to the hardness HV 0.01 (DIN EN ISO 6507-1). The microhardness measurement showed in the samples depending on the content of NH ₃ in the annealing atmosphere at the same Glühparametern, ie holding time and holding temperature, a higher hardness at higher NH ₃ content of the annealing atmosphere.

The hardness of the sample A initially decreases from the value 460 HV measured at the surface to a value of 333 HV at a depth of 20 μm. Thereafter, the hardness increases again to a value of about 492 HV, indicating that the decarburization of the base material stops here. Due to the surface hardening in particular the uppermost range of 5 to 15 microns was significantly hardened. It can be seen from sample B that, with increased NH ₃ content, the surface hardening is more pronounced both in the amplitude and in the depth of the hardening. This is due to the fact that due to the higher NH ₃ concentration of the annealing atmosphere a stronger diffusion of Nitrogen was introduced into the surface of the steel part. The values of Sample B begin at 546 in 5 μm depth and sink to a depth of 25 μm down to a value of 394. Subsequently, the values rise again to about 466 in 45 μm depth. It can be clearly seen that the surface is made harder than the base material at 45 μm depth. A similar picture shows the in Fig. 5 shown measurements on two other embodiments in comparison with a comparative example. The comparative example shown by a dotted line shows in the range of 5 to 35 microns a reduced hardness, which is below 400 HV 1 (DIN EN ISO 6507-1). The reduction in hardness compared to the base material, which is between 450 HV 1 and 500 HV 1, is explained by the decarburization during hot forming. The two comparative examples with two different Nitrierungsvarianten turn 1% NH ₃ -Glühatmosphäre or even 4% NH ₃ -Glühatmosphäre differ especially in this near-surface region, since hardnesses of above 500 could be measured here. Thus, not only the particularly high tensile strength values and the hot-formed and / or hardened steel parts can be provided for wear-resistant, at least partially uncoated steel parts, but also a high wear resistance due to high surface hardness in the range of, for example 500 to 700 HV.

Claims (8)

1. Wear-resistant, uncoated steel part (4) consisting of a hardenable steel grade which has been produced from a semifinished part (1) by hot forming and/or hardening, characterized in that at least part of the steel part (4) has a surface region (1b) which has been hardened to a depth of not more than 100 µm by surface hardening by nitriding before hot forming and/or hardening and the steel part (4) is configured for use as processing, conveying and/or crushing means (5, 6) in agricultural machines, conveying machines, mining machines or building machines, with at least the regions of the steel part which are subjected to abrasive forces having been surface-hardened
2. Steel part according to Claim 1, characterized in that after hot forming and/or press hardening, the hardened surface region (1b) of the steel part has at least the hardness of the base material of the steel part located under the surface region.
3. Steel part according to Claim 1 or 2, characterized in that the steel part (4) consists of a manganese-boron steel, a dual-phase steel or a TRIP steel.
4. Steel part according to any of Claims 1 to 3, characterized in that the surface region (1b) of the steel part which has been hardened before hot forming and/or hardening has, at least in regions, a hardness of from 400 to 700 HV.
5. Process for producing a wear-resistant, uncoated steel part for processing, conveying and/or crushing means of agricultural machines, conveying machines, mining machines or building machines from a semifinished part consisting of a hardenable steel grade, in which the semifinished part is heated, at least in regions, to a temperature above the Ac1 transformation temperature and is subsequently hot formed and/or hardened in particular for producing a steel part according to any of Claims 1 to 4, characterized in that at least part of the semifinished part is subjected to surface hardening in which a surface region is hardened to a depth of not more than 100 µm before hot forming and/or hardening, where the surface hardening is effected by a heat treatment in a heat treatment atmosphere comprising up to 25% by volume of H₂, 0.1-10% by volume of NH₃, H₂O and balance N₂ and also unavoidable impurities at a hold temperature of from 600°C to 900°C.
6. Process according to Claim 5, characterized in that, during surface hardening, the time for which the semifinished part has the hold temperature is from 5 s to 600 s, preferably from 30 s to 120 s.
7. Process according to Claim 5, characterized in that the surface hardening is carried out in a continuous hardening furnace.
8. Process according to any of Claims 5 to 7, characterized in that a semifinished part consisting of a manganese-boron steel or a TRIP steel is surface-hardened.

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