WO2015000740A1

WO2015000740A1 - Wear-resistant, at least partially uncoated steel part

Info

Publication number: WO2015000740A1
Application number: PCT/EP2014/063259
Authority: WO
Inventors: Sascha Sikora; Janko Banik; Thiemo Wuttke
Original assignee: Thyssenkrupp Steel Europe Ag
Priority date: 2013-07-05
Filing date: 2014-06-24
Publication date: 2015-01-08
Also published as: JP2016528381A; DE102013107100A1; US20160145705A1; EP3017074A1; ES2704437T3; EP3017074B1; CN105358720A; CA2916155A1; CA2916155C; KR20160029102A

Abstract

The invention relates to a wear-resistant, at least partially uncoated steel part composed of a hardenable steel grade, which steel part has been produced from a semi-finished product by hot-forming and/or hardening. The invention further relates to a method for producing a wear-resistant, at least partially uncoated processing, conveying, and/or fracturing means of agricultural machines, conveying machines, mining machines, or construction machines from a semi-finished product, wherein the semi-finished product is heated to a temperature above the Ac1 conversion temperature and is subsequently hot-worked and/or hardened. The problem of proposing at least partially uncoated steel parts whose suitability for use with abrasive material is improved is solved for a steel part in that the steel part at least partially has a surface region that has been hardened to a depth of at most 100 μm, preferably to a depth of up to 40 μm, by means of surface hardening before the hot working and/or hardening.

Description

Wear-resistant, at least partially uncoated steel part

The invention relates to a wear-resistant, at least partially 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 Acl transformation temperature is and then hot worked and / or cured.

Wear resistant, at least partially uncoated steel parts, which high

Must have strength and at the same time 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, these are preferably subjected to hot forming, in which the semi-finished products from which the steel parts are produced, are first heated to a temperature above the Acl 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 known, for example, from German patent DE 10 2010 050 499 B3. The German patent specification describes a manufacturing method for excavator buckets, concrete mixer screw conveyors, Conveyor screw blades or other conveying vanes of conveyor systems, 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 AI deals with the problem of hot forming of galvanized flat steel products to high or 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 by the penetration of the molten metal of the coating into the notches or cracks produced during the forming of the flat steel product, The liquid metal entering the steel substrate deposits there at grain boundaries and reduces the maximum there

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 generated. On the other hand, 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 therefore 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 corresponding steel parts are proposed.

The stated object is achieved for a steel part in that the steel part at least partially has a surface area which is up to a depth of a maximum of 100 μπι, preferably up to a depth of up to 40 μπι, by a

Surface hardening has been cured before hot working and / or curing.

It has been found that heating the semi-finished products to produce the steel parts at a temperature greater than the Acl 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 region of up to 100 μπι depth, in particular the range to 40 μιη 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 before hot working and / or hardening to the steel part results in both the

Surface area as well as the base material despite the decarburization of

Near-surface areas due to the high temperatures during hot forming or curing, have very high hardness. The result is a steel part for

Available, which has at least partially a surface area, which is preferably hardened to a depth of 100 μπι or in the range to 40 μπι depth and thus significantly more resistant to wear than the previously known, at least partially uncoated steel parts.

According to a first aspect, the hardened surface portion of the steel part is hardened by carburizing or nitriding. Both methods offer the possibility of specifically hardening near-surface areas of the steel part prior to 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 the 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 was 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 that according to another embodiment of the steel part, the steel part for use as processing, conveying and / or emetic in

Agricultural machinery, conveyors, mining or construction machinery

is formed, 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 expression of the

Martening or the transformation of residual austenitic parts in martensite an increase in hardness allows.

According to a further embodiment of the steel part, the front of the

Hot working and / or hardening hardened surface area of the steel part at least partially a hardness of 400 to 700 HV. As a rule, these values are only achieved by ultrahigh-strength steel grades after hot forming or hardening in the base material. The surface hardening before the

Hot forming or curing offers in particular the possibility that

To provide 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 processing, conveying and / or breaking means of agricultural machines, conveyors, mining machines or construction machines from a semifinished product, in which the semifinished product at least partially heated to a temperature of more than the Acl transformation temperature and then hot worked and / or cured, achieved in that the semi-finished is at least partially subjected to a surface hardening prior to hot forming and / or curing, in which a surface area up to a Depth of a maximum of 100 μπι is cured. Preferably, a surface area of up to 40 μιη depth is hardened, in which usually the decarburization processes take place during hot forming. The depth of the surface area which is to be hardened is determined by the exposure time of the

Hardening treatment controlled. It has been shown, in particular, that despite the

Heating to a temperature above the Acl transformation temperature point, the surface hardened areas of the steel part with respect to their

Surface hardness remain stable, so that after the hot forming and / or curing high surface hardness can be achieved. This leads to the fact that the steel parts in contact with abrasive materials for processing, conveying and / or breaking means of agricultural machinery, conveyors, mining machinery 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 a preferred embodiment of the method, the hardening of the surface area is carried out by nitriding or by carburizing. Both methods make it possible to provide a higher hardness in the surface area, which after hot working and / or after hardening a higher Allow wear resistance of the surface of the hot-formed or hardened steel part.

Surface hardening is particularly preferred according to another

Embodiment by an annealing in an annealing atmosphere with up to 25 vol .-% H ₂ , 0.1 - 10 vol .-% NH ₃ , H2O and rest N ₂ and unavoidable impurities at a holding temperature of 600 ° C to 900 ° C. , 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. Preference is given to a maximum of 10 vol .-%, ₂ , a maximum of 5 vol .-% N H3 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 determined by the duration of the action of the

Hold temperature to be set. It is preferred during the

Surface hardening the duration at which the semi-finished product has the holding temperature, 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 coil-capable semifinished product is surface-hardened and the further hot-forming and / or

Press hardening steps can be supplied. But it is also conceivable one

Surface hardening in a chamber furnace.

As already carried out, semi-finished products such as manganese-boron steels, dual-phase steels and TRIP steels on the one hand show a particularly high level

Increase in strength during hot forming or hardening and on the other hand the possibility to bring the near-surface areas by nitriding to identical hardness in the range of 400 to 700 HV. As a result, you can Steel parts are produced in a cost effective manner, which are very resistant to wear and have particularly high strengths.

In addition, the invention will now be explained in more detail with reference to embodiments in conjunction with the drawings. The drawing shows schematically an embodiment of the method for producing a wear-resistant, at least partially uncoated steel part, Fig. 2 shows the layer structure of the according to the embodiment of FIG. 1 treated semi-finished or steel part in a schematic representation,

Fig. 3, 4 embodiments of a steel part for agricultural machinery and

Carriers and

5 is a graph showing the hardness curve as a function of the distance to

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 in one

schematic representation shown. 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 a nitration, in a strip continuous furnace at the end thereof

For example, the band-shaped semi-finished product 1 provided with a surface hardening now, can be wound on a (not shown) coil. The surface-hardened band-shaped semi-finished product is cut to length and a

Hot forming and / or hardening 3 supplied, so that by the method step 3 a formed, at least partially uncoated steel part 4 can be produced, which is suitable for processing, conveying and / or breaking means of agricultural machinery, conveyors, mining equipment or construction machinery. 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

Decarburization of the surface areas, which up to a depth of 100 μη

takes place, be counteracted, in which the surface area to 100 μιτι depth or surface hardened in a range up to 40 μπι depth. Preferably, the surface hardening is carried out by nitriding. However, it is also conceivable carburizing the surface area. The surface hardening in method step 2 is preferred by a

Annealing treatment in an annealing atmosphere with up to 25% by volume H2, 0.1-10% by volume

NH3, H ₂ 0 and rest N ₂ and unavoidable impurities in a

Holding temperature of 600 ° C to 900 ° C carried out. Also a reduction of

Hydrogen concentration to a maximum of 10% by volume or a limitation of 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. The implementation of

Surface hardening before hot forming or hardening has the advantage that an annealing process with, for example, a band-shaped semifinished product in one

Ribbon continuous furnace or a board in a continuous furnace can be performed much more efficient than with differently shaped and different geometries having formed steel parts. The quality of Surface hardening can also be more easily ensured by the use of ribbon-shaped or sheet-formed semi-finished products.

FIG. 2 shows schematically a cross-section of the semifinished product at three different times of the method. First, the semifinished product 1

according to the manufacturing process more or less homogeneous,

For example, ferritic microstructure la, which is characterized by the combination of

Manufacturing process and steel composition is conditional. By the

Surface hardening, the surface area lb is hardened by diffusion of nitrogen during nitriding or carbon during carburization, wherein the microstructure changes there. The thickness of the surface area lb depends on the duration of the annealing. The surface area is usually up to 100 μιη, in which the hardness of the semifinished product is changed. A preferred range, which is a compromise of sufficient surface hardening and duration of the annealing treatment for surface hardening, has a thickness of 20 to 40 μm. The duration of the

Surface hardening, for example in nitriding, is then preferably from 30 seconds to 120 seconds. The structure of the material la remaining below the surface area lb 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 partially into martensite by hardening.

This results in high hardness and high mechanical strength in the

Base material lc achieved. The surface area lb remains unchanged apart from the decarburization of these layers. By nitriding, while the

Surface area remains cured. In a targeted carburization of the surface area lb instead of nitriding the decarburization can be counteracted, so that here too an increase in hardness can be achieved. The formed steel part 4 thus has a hardened region 1b and a through the

Hot working and hardening hardened area lc on. 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 Emetics, which in

Agricultural machinery, conveyors, mining or construction equipment, such as concrete mixers are used 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

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> Acl) since the austenite can dissolve more nitrogen than the ferrite. Subsequently, the samples were thermoformed and cured. From the warm Sectioned or hardened steel parts sections were made and measured at a distance of 5 μπι 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 NH3 content of the annealing atmosphere.

The hardness of sample A first decreases from the value 460 HV measured at the surface to a value of 333 HV at 20 μm depth. Thereafter, the hardness increases again to a value of about 492 HV, indicating that the decarburization of the

Base material stops. As a result of the surface hardening, in particular the uppermost region of 5 to 15 μm was clearly 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 into the surface of the steel part took place. The values of sample B begin at 546 in 5 μιη depth and decrease to a depth of 25 μιη to a value of 394 from. Subsequently, the values rise again to about 466 in 45 μπι depth. It can be clearly seen that the surface is made harder than the base material at 45 μπι depth.

A similar picture is also shown by the measurements shown in FIG. 5 on two further exemplary embodiments in comparison with a comparative example. The

Comparative example represented by a dotted line shows in the range of 5 to 35 μπι a reduced hardness, which is below 400 HV 1 (DIN EN ISO 6507-1). The reduction in hardness compared to the base material, which lies between 450 HV 1 and 500 HV 1, is achieved by the final carbonization during the

Hot forming explained. The two comparative examples with two different Nitrierungsvarianten turn l% NH ₃ -Glühatmosphäre or 4% NH ₃ - annealing atmosphere differ mainly in this near-surface region, since hardnesses of above 500 could be measured here. This makes it possible for wear-resistant, at least partially uncoated steel parts not only the special high tensile strength values and the hot-worked and / or hardened steel parts, but also a high wear resistance due to high surface hardness in the range of, for example, 500 to 700 HV.

Claims

Patent claims

Wear-resistant, at least partially uncoated steel part (4) consisting of a hardenable steel grade which has been produced by hot forming and / or hardening from a semifinished product (1),

characterized in that

the steel part (4) at least partially has a surface area (lb) which has been hardened to a depth of a maximum of ΙΟΟμηι by surface hardening before hot working and / or hardening. Steel part according to claim 1,

characterized in that

the hardened surface area (lb) of the steel part is hardened by carburizing or nitriding.

Steel part according to claim 1 or 2,

characterized in that

after hot working and / or press hardening the hardened

Surface area (lb) of the steel part at least the hardness of the under the

Surface area lying base material of the steel part has.

Steel part according to one of claims 1 to 3,

characterized in that

the steel part (4) is designed for use as processing, conveying and / or breaking means (5, 6) in agricultural machines, conveyors, mining machines or construction machines, wherein at least the abrasive forces

exposed areas of the steel part are surface hardened. Steel part according to one of claims 1 to 4,

characterized in that

the steel part (4) consists of a manganese-boron steel, a dual-phase steel or a TRIP steel.

Steel part according to one of claims 1 to 5,

characterized in that

hardened before hot working and / or hardening

Surface area (lb) of the steel part at least partially a hardness of

400 to 700 HV.

Method for producing a wear-resistant, at least partially uncoated, processing, conveying and / or breaking means of

Agricultural machinery, conveyors, mining machines or construction machinery from a semi-finished product consisting of a hardenable steel grade, wherein the semi-finished product is at least partially heated to a temperature of more than the Acl transformation temperature and then hot-formed and / or cured, in particular for producing a steel part after a of claims 1 to 6,

characterized in that

the semi-finished product is at least partially subjected to a surface hardening before the hot forming and / or curing, in which a surface area is cured to a depth of at most 100 μπι.

Method according to claim 7,

characterized in that

the surface area is hardened by nitriding or carburizing.

9. The method according to claim 7 or 8,

characterized in that

the surface hardening by an annealing treatment in an annealing atmosphere with up to 25 vol .-% H ₂ , 0.1 - 10 vol .-% NH ₃ , H ₂ 0 and balance N ₂ and unavoidable impurities at a holding temperature of 600 ° C to 900 ° C takes place.

10. The method according to any one of claims 7 to 9,

characterized in that

During the surface hardening, the duration at which the semifinished product has the holding temperature is 5 s to 600 s, preferably 30 s to 120 s.

11. The method according to any one of claims 7 to 10,

characterized in that

the surface hardening is carried out in a continuous furnace.

12. The method according to any one of claims 7 to 11,

characterized in that

a semi-finished product consisting of a manganese-boron steel or a TRIP steel surface hardened.