JP2016528381A - Wear-resistant, at least partially uncoated steel parts - Google Patents

Abstract

The present invention relates to a wear-resistant, at least partially uncoated steel part of a hardenable steel grade, which is produced from a semi-finished part by hot forming and / or quenching. The invention further relates to a method for producing a wear-resistant and at least partly uncoated steel part for processing, conveying and / or crushing means in agricultural, transport, excavating or construction machinery. The finished part is heated to a temperature above the Ac1 transformation temperature and then hot formed and / or quenched. The proposed problem of improving the compatibility of at least partially uncoated steel parts with wearable materials is at least partially due to surface hardening prior to hot working and / or quenching. This is solved by having an uncoated steel part having a surface region hardened to a depth of up to 100 μm or less, preferably to a depth of 40 μm or less. [Selection] Figure 1

Description

  The present invention relates to a steel part composed of a hardened grade of wear-resistant, at least partly uncoated, which is produced from a semifinished part by hot forming and / or quenching. Furthermore, the invention produces a wear-resistant and at least partly uncoated steel part from semi-finished parts for the processing, conveying and / or crushing means of agricultural, transport, excavating or construction machinery. With respect to the method, the semi-finished part is heated at a temperature above the Ac1 transformation temperature and then hot formed and / or quenched.

  Wear-resistant and at least partially uncoated steel parts that are required to have high strength and are exposed to abrasive forces, such as agricultural machinery, especially plow manufacturing, and dredge buckets Or for the production of conveyor screws for materials with wear effects, for example concrete mixer conveyor screws. In order to achieve the high strength required in the above applications, the steel part is preferably subjected to hot forming, in which a semi-finished part (from which the semi-finished part is produced) First, heating is performed at a temperature higher than the Ac1 point transformation temperature, and as a result, transformation hardening of the microstructure is achieved by hot forming and subsequent quenching, ie, rapid cooling, and a material having a martensite microstructure is formed. In addition to having a very high hardness, the martensite microstructure also has a very high mechanical strength, for example a tensile strength. Corresponding steel parts are known, for example, from German patent DE 10200100499B3. This German patent describes a method of manufacturing dredger buckets, concrete mixer conveyor screws, conveyor screw blades, and other transfer blades of transfer equipment, and these components are hot formed and press quenched. ing.

  However, it has been found that components produced in this way have problems with regard to wear resistance, especially when in contact with substances that have an abrading action, despite being subjected to a curing method during production.

  German Offenlegungsschrift DE 10 201 0017 354 A1 addresses the problem of hot forming galvanized flat steel parts for producing high-strength or ultra-high-strength steel parts. When the metal melting point of the protective coating is exceeded, there is a risk of “liquid metal embrittlement”, which is caused by the molten metal of the coating entering the notches and cracks that occur during the forming of flat steel parts. . Liquid metal that penetrates into the steel substrate accumulates at grain boundaries, which reduces the maximum tensile or compressive stress that can be tolerated. As a solution, this patent publication proposes nitriding the outer layer region to provide an outer layer region having a microstructure.

  In contrast, the present invention does not have the desired wear resistance in areas where hot-formed and / or hardened steel parts are not coated, so that, for example, it is in contact with a material having an abrading action. It takes up the problem of not being optimal for use as a transport means. Accordingly, it is an object of the present invention to propose an at least partially uncoated steel part with improved suitability for use with an abrasive material. In addition, a cheap manufacturing method for the corresponding steel parts should be proposed.

  The indicated objectives are achieved by a steel part having a surface region which is at least partially hardened to a depth of 100 μm by surface hardening prior to hot forming and / or quenching and preferably to a depth of 40 μm. .

  Heating the semi-finished part at a temperature higher than the Ac1 transformation temperature or Ac3 transformation temperature prior to hot forming and / or quenching to produce a steel part results in decarburization in the region near the surface, resulting in It has been found that the carbon content of these regions is significantly lower than the carbon content of the substrate. As a result, regions near the surface up to a depth of 100 μm, in particular regions up to a depth of 40 μm, cannot be cured to the extent necessary during hot forming and / or quenching. However, the high temperature results during hot forming or quenching by at least partially surface hardening the uncoated areas of the semi-finished part prior to hot forming and / or quenching to produce steel parts. It has been found that both the surface region and the substrate provide very high hardness despite the fact that the region near the surface is decarburized. This preferably has a hardened surface area at least partially in the region of 100 μm depth or up to a depth of 40 μm, and thus over at least partially uncoated steel parts known in the art. A steel part is provided that is significantly more wear resistant.

  In the first embodiment, the hardened surface area of the steel part is hardened by carbonization or nitriding. Both processes offer the opportunity to harden areas near the surface of the steel part in a targeted manner prior to hot forming or quenching. In addition, nitriding has the advantage that the hardness does not decrease during hot forming. In the case of carbonization, the carbon content of the surface region increases but decreases again by hot forming.

  In a further embodiment, the surface area of the steel part hardened after hot forming and / or quenching preferably has at least the hardness of the steel part substrate located below the surface area.

  The wear resistance of the steel part can also be improved preferably by the fact that the hardness of the surface region of the steel part is higher than the hardness of the base material. Specifically, the hardness of the surface area contributes to the wear resistance of steel parts when in contact with highly abrasive materials, and as a result is high even with somewhat soft substrates. It has been found that it is possible to produce wear-resistant steel parts.

  Thus, according to a further embodiment of the steel part, the steel part is configured to be used as a processing, conveying and / or crushing means in an agricultural machine, conveying machine, excavating machine or construction machine, and at least a force with wear action. The area of the steel part that is exposed to is surface hardened.

  In addition, semi-finished parts such as manganese boron steel, duplex steel or TRIP steel, in particular because they increase the hardness as much as possible due to obvious martensite formation or transformation of residual austenite components to martensite, in particular. It is advantageous.

  In a further embodiment of the steel part, the surface area of the steel part hardened before hot forming and / or quenching has a hardness of 400 HV to 700 HV in at least some areas. These values are usually achieved only with very high strength steel grades after hot forming or quenching of the substrate. Surface hardening prior to hot forming or quenching offers the opportunity to provide starting materials for the production of steel components, especially in coils.

  In accordance with further teachings of the present invention, the above object is achieved from a semi-finished part, at least partially wear-resistant for processing, conveying and / or crushing means in agricultural machinery, conveying machinery, excavating machinery or construction machinery. A method for producing an uncoated steel part, wherein a semi-finished part is heated to a temperature above the Ac1 transformation temperature in at least some areas and then hot formed and / or quenched. The semi-finished part is achieved by being at least partially exposed to surface hardening and the surface area is hardened to a depth of 100 μm or less prior to hot forming and / or quenching. Preferably, it is chosen to cure surface areas up to 40 μm deep where the decarburization process normally occurs during hot forming. The depth of the surface area to be cured is controlled by the duration of the curing process. In particular, despite being heated to a temperature above the Ac1 transformation temperature point, the surface-hardened region of the steel part remains stable as far as surface hardening is concerned, and as a result after hot forming and / or quenching. It has been found that high surface hardness is achieved. This leads to less wear of the steel parts for processing, conveying and / or crushing means in agricultural machines, transport machines, excavating machines or construction machines that come into contact with the abrasive material.

  Curing the surface area prior to hot forming or quenching allows surface hardening to be performed on a material that can be coiled, i.e. a steel strip, so that at least a part that is wear resistant. Uncoated steel parts can be produced very economically from semifinished parts. In a preferred embodiment of the method, the hardening of the surface region is performed by nitriding or carbonizing. Both processes can give higher hardness to the surface area, thereby providing higher wear resistance of the surface of the hot formed or hardened steel part after hot forming and / or after quenching Is possible.

In a further embodiment, the surface hardening is particularly suitably, the holding temperature of 600 ° C. to 900 ° C., 25% by volume of _H 2, 0.1-10% by volume of _NH 3, _{H 2} O and balance _{N 2,} and The heat treatment is performed in a heat treatment atmosphere containing inevitable impurities. The dew point of the heat treatment atmosphere is preferably in the range of −50 ° C. to −5 ° C. As a result, the influence of atmospheric moisture on the curing process is reduced. Furthermore, preferably NH ₃ can be up to 5% by volume and H ₂ volume can be up to 10% by volume, and the dew point can be from -40 ° C. to -15 ° C. at a temperature of 680 ° C. to 840 ° C. It is selected to be set. The latter process parameter gave improved and more uniform surface hardening.

  The depth of surface hardening can be set by the time during which the holding temperature is maintained. The time during which the semi-finished part is kept at the holding temperature during the surface hardening is set to 5 seconds to 600 seconds, preferably 30 seconds to 120 seconds.

  The surface hardening is preferably carried out in a continuous hardening furnace, so that, for example, a strip-like semi-finished part, i.e. a semi-finished part that can be coiled, is surface hardened for further hot forming and / Or it can be made to transfer to the step of press hardening. However, surface hardening in a chamber furnace is also conceivable.

  As mentioned above, semi-finished parts such as manganese boron steel, duplex steel and TRIP steel first show a particularly high strength increase during hot forming or quenching, and then by nitriding to areas near the surface. Gives the opportunity to produce the same hardness in the range of 400 HV to 700 HV. As a result, it is possible to manufacture a steel part having excellent wear resistance and particularly high strength at a low cost.

Hereinafter, the present invention will be described with reference to the accompanying drawings.
FIG. 1 schematically illustrates one embodiment of a method for producing a wear-resistant, at least partially uncoated steel part. FIG. 2 shows the layer structure of a semi-finished or steel part processed as in the embodiment of FIG. FIG. 3 shows an example of a steel part for an agricultural machine. FIG. 4 shows an example of a steel part for a transport machine. FIG. 5 shows a graph of hardness characteristics according to the distance from the surface for two examples and one comparative example.

  First, FIG. 1 very schematically shows an example of the production of a steel part that is wear-resistant and at least partially uncoated. The semi-finished part 1 is made of steel such as manganese boron steel, duplex steel or TRIP steel, and is first conveyed to the surface hardening 2. When the strip-shaped semi-finished part is fed out of the coil 1a and sent to the surface hardening 2, for example in the case of nitriding, it is advantageous to carry out the surface hardening in a continuous hardening furnace, the end of the continuous hardening furnace. For example, the strip-shaped semi-finished part 1 to which the hardened surface is given at this point can be wound up on a coil (not shown). Semi-finished parts such as surface hardened strips are cut to a predetermined length and transported to hot forming and / or quenching 3 so that processing step 3 can be carried out in agricultural machinery, transport machinery, drilling machinery Alternatively, it is possible to produce a shaped steel part 4 which is at least partially uncoated and is suitable for processing, conveying and / or crushing means in construction machines. Firstly, the steel part 4 thus produced exhibits high strength value characteristics due to the hot forming and / or quenching steps. Secondly, surface nitridation performed before hot forming and / or prior to quenching also increases the hardness of the surface area of the steel part. As described above, by the method of the present invention, the decarburization of the surface region performed to a depth of 100 μm is offset by surface hardening of the surface region in the region up to a depth of 100 μm or a depth of 40 μm. Make it possible. Surface hardening is preferably carried out by nitriding. However, carbonization of the surface area is also conceivable.

The surface hardening of process step 2 is preferably carried out at a holding temperature of 600 ° C. to 900 ° C. with 25% by volume H ₂ , 0.1-10% by volume NH ₃ , H ₂ O and balance N ₂ , and inevitable. The heat treatment is performed in a heat treatment atmosphere containing impurities. By reducing the hydrogen concentration to a maximum of 10% by volume or limiting the NH ₃ concentration to a maximum of 5% by volume, the nitriding results are further improved.

  The depth of surface hardening can be set by the duration of surface hardening at a holding temperature of, for example, 5 seconds to 600 seconds. Preferably the surface is nitrided at a holding temperature of 680 ° C. to 840 ° C. for 30 seconds to 120 seconds. Performing surface hardening prior to hot forming or quenching uses shaped steel parts with different shapes and different geometries, for example by using strip-like semi-finished parts or plates in a continuous hardening furnace It has the advantage that the heat treatment process can be carried out much more efficiently than when doing so. Similarly, the quality of surface hardening can be more easily ensured by using strip-like semi-finished parts or semi-finished parts configured as blanks.

  FIG. 2 schematically shows a cross-section of a semi-finished part at three different times during processing. First, the semi-finished part 1 has a roughly uniform, for example ferrite microstructure 1a, corresponding to the manufacturing process, which is determined by a combination of the manufacturing process and the steel composition. As a result of surface hardening, the surface region 1b is hardened by inward diffusion of nitrogen in the case of nitridation and carbon in the case of carbonization, where the microstructure changes. The thickness of the surface region 1b depends on the duration of the heat treatment. The surface area is usually up to 100 μm, at which the hardness of the semi-finished part can be changed. A desirable region that provides a compromise between sufficient surface effect and a period of heat treatment for surface hardening has a thickness of 20 μm to 40 μm. For example, in nitriding, the duration of surface hardening is preferably 30 to 120 seconds. The microstructure of the material 1a remaining below the surface region 1b remains substantially unchanged during the heat treatment.

  In the hot forming step, the microstructure of the substrate 1a is first converted to austenite and then partially converted to martensite by quenching. Thus, high hardness and good mechanical strength are achieved with the substrate 1c. The surface region 1b remains unchanged except for the carbonization of these layers. As a result of nitriding, the surface region can remain cured. If the surface region 1b is carbonized instead of nitriding, decarbonization is offset, and as a result, an increase in hardness can also be achieved here. Accordingly, the formed steel part 4 has a hardened region 1b and a region 1c hardened by hot forming and quenching.

  3 and 4 show a general field of application of wear-resistant and at least partly uncoated steel parts, in the form of a conveyor screw 5 in FIG. 3 and an agricultural plow blade in FIG. Each of the six forms is shown. Both components are typical of processing, conveying and / or crushing means used in agricultural machines, transport machines, excavating machines or construction machines, such as concrete mixers, and are exposed to abrasive substances The use of hot-formed and / or press-hardened steel parts has not been very advantageous so far because it is very sensitive to wear. By surface hardening the areas that are decarburized during hot forming and / or quenching, hot formed steels can extend the range of use.

Table 1 shows the measured values of the hardness of Sample A and Sample B composed of steel type 22MnB5. Samples A and B were each exposed to surface nitridation in a heat treatment atmosphere consisting of 1% by volume of NH ₃ or 4% by volume of NH ₃ at 760 ° C. for 90 seconds. Since austenite can dissolve more nitrogen than ferrite, surface nitridation was performed at the temperature between transformations (T> Ac1). The sample was then hot formed and quenched. The parts to be polished consisted of hot-formed or hardened steel parts and the hardness of HV 0.01 (DIN EN ISO 6507-1) was measured at a distance of 5 μm from the surface. The measured value of the microhardness of the sample according to the NH ₃ content in the heat treatment atmosphere had a higher hardness with a higher NH ₃ content in the heat treatment atmosphere with the same heat treatment parameters, ie the same holding time and holding temperature. .

First, the hardness of Sample A decreased from a 460 HV value measured on the surface to a 333 HV value at a depth of 20 μm. Thereafter, the hardness increased again to a value of about 492 HV. This indicates that the decarbonization of the substrate ends here. The top region, specifically the 5 to 15 μm region, was markedly cured by surface curing. It can be seen from Sample B, which is clearer than surface hardening with respect to both hardness spread and depth at high NH ₃ content. This is believed to be due to the greater diffusion of nitrogen into the surface of the steel part due to the higher NH ₃ concentration in the heat treatment atmosphere. The value for Sample B starts at 546 at a depth of 5 μm and decreases to a value of 394 at a depth of 25 μm. This value then increases again to about 466 at a depth of 45 μm. It can be clearly seen that the surface is harder than the substrate at a depth of 45 μm.

Similar measurements are shown by the measurements in the two further examples shown in FIG. 5 compared to the comparative example. The comparative example indicated by the dotted line shows a reduced hardness lower than 400HV1 (DIN EN ISO6507-1) in the region of 5 to 35 μm. The decrease in hardness when compared to substrates in the range of 450 HV-1 to 500 HV1 is explained by decarburization during hot forming. Two compared examples with two different nitridation conditions, again a 1% concentration NH ₃ heat treatment atmosphere or a 4% concentration NH ₃ heat treatment atmosphere, measured hardness above 500 in the region near the surface. Because it is particularly different. Thus, in the case of wear-resistant at least partially uncoated steel parts, not only the tensile strength values of hot-formed and / or quenched steel parts are particularly high, but for example between 500 HV and 700 HV. Higher surface hardness in the range can also provide high wear resistance.

Claims (12)

In a steel part (4) which is wear-resistant and at least partly uncoated and is composed of a hardenable steel grade produced from a semi-finished part (1) by hot forming and / or quenching ,
The steel part (4) has at least partly a surface region (1b) hardened to a depth of 100 μm or less by surface hardening before hot forming and / or quenching. The steel part according to claim 1,
A steel part characterized in that the hardened surface region (1b) of the steel part is hardened by carbonization or nitriding. In the steel part according to claim 1 or 2,
After hot forming and / or press hardening, the hardened surface area (1b) of the steel part has at least the hardness of the base of the steel part located below the surface area parts. The steel part according to any one of claims 1 to 3,
The steel part (4) is configured to be used as a processing, conveying and / or crushing means (5, 6) in an agricultural machine, a transporting machine, an excavating machine or a construction machine, and is exposed to at least a force having a wear action. A steel part characterized in that the region of the steel part to be processed is surface hardened. In the steel part according to any one of claims 1 to 4,
The steel part (4) is composed of manganese boron steel, duplex steel or TRIP steel. The steel part according to any one of claims 1 to 5,
Steel part, characterized in that the surface area (1b) of the steel part hardened before hot forming and / or quenching has a hardness of 400 HV to 700 HV in at least some areas. From semi-finished parts composed of hardenable steel grades, wear-resistant and at least partly uncoated steel parts for the processing, conveying and / or crushing means of agricultural, transport, excavating or construction machinery A method of manufacturing, in particular for manufacturing a steel part according to any one of claims 1 to 6, wherein the semi-finished part is heated to a temperature higher than the Ac1 transformation temperature in at least some regions. After being hot formed and / or quenched,
The semi-finished part is at least partially exposed to a surface hardening, in which the surface region is hardened to a depth of 100 μm or less before hot forming and / or quenching. Method. The method of claim 7, wherein
The method wherein the surface region is cured by nitridation or carbonization. The method according to claim 7 or 8, wherein
The surface hardening is a heat treatment including a maximum of 25 volume% H ₂ , 0.1 to 10 volume% NH ₃ , H ₂ O and balance N ₂ , and inevitable impurities at a holding temperature of 600 ° C. to 900 ° C. A method characterized by being performed by heat treatment in an atmosphere. The method according to any one of claims 7 to 9,
The method wherein the semi-finished part has a temperature holding time of 5 to 600 seconds, preferably 30 to 120 seconds, during surface hardening. A method according to any one of claims 7 to 10,
A method characterized in that the surface curing is carried out in a continuous curing furnace. 12. The method according to any one of claims 7 to 11, wherein
A method characterized in that a semi-finished part made of manganese boron steel or TRIP steel is surface hardened.

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