US20160145705A1

US20160145705A1 - Wear-resistant, partially uncoated steel parts and methods of producing same

Info

Publication number: US20160145705A1
Application number: US14/903,022
Authority: US
Inventors: Sascha Sikora; Janko Banik; Thiemo Wuttke
Original assignee: ThyssenKrupp Steel Europe AG
Current assignee: ThyssenKrupp Steel Europe AG
Priority date: 2013-07-05
Filing date: 2014-06-24
Publication date: 2016-05-26
Also published as: CN105358720A; EP3017074B1; JP2016528381A; WO2015000740A1; CA2916155A1; EP3017074A1; CA2916155C; KR20160029102A; DE102013107100A1; ES2704437T3

Abstract

A wear-resistant steel part may be formed by hot forming and/or hardening a hardenable steel grade semifinished part. The steel part may be used, for example, as a processing, conveying, and/or crushing mechanism in agricultural machines, conveying machines, mining machines, or building machines. The semifinished part may be heated to a temperature above an Ac1 transformation temperature and then subsequently hot formed and/or hardened. The steel part may be particularly suitable for use with abrasive materials. To that end, the steel part may have at least one region that has been hardened to a depth of not more than 100 microns by surface hardening before the semifinished part is hot formed or hardened.”

Description

The invention relates to a wear-resistant, at least partly uncoated steel part consisting of a hardenable steel grade which has been produced from a semifinished part by hot forming and/or hardening. In addition, the invention relates to a process for producing a wear-resistant, at least partly uncoated processing, conveying and/or crushing means of agricultural machines, conveying machines, mining machines or building machines from a semifinished part, in which the semifinished part is heated to a temperature above the Ac1 transformation temperature and is subsequently hot formed and/or hardened.
Wear-resistant, at least partly uncoated steel parts which have to have high strengths and at the same time are subjected to abrasive forces are required, for example, for the production of agricultural machines, in particular plows, and also for buckets of a dredge or conveying screws for abrasive materials, for example the conveying screw of a concrete mixer. In order to achieve the necessary high strengths in the abovementioned applications, the parts are preferably subjected to hot forming in which the semifinished parts from which the steel parts are produced are firstly heated to a temperature above the Ac1 transformation temperature point, so that transformation hardening of the microstructure is effected by hot forming and subsequent hardening, i.e. rapid cooling, and a material having a martensitic microstructure is formed. The martensitic microstructure has a significantly greater hardness but also a significantly greater mechanical strength, for example tensile strength. Corresponding steel parts are known, for example, from the German patent DE 10 2010 050 499 B3. The German patent describes a process for producing dredger buckets, concrete mixer conveying screws, conveying screw blades or other transport blades of conveying plants, in which the components are hot formed and press hardened.
However, it has been found that the components produced in this way have problems in respect of the wear resistance despite the hardening process during production, especially on contact with abrasive materials.
The German first publication DE 10 2010 017 354 A1 is concerned with the problem of hot forming of zinc-plated flat steel products to produce high-strength or very high-strength steel components. When the melting point of the metal of the protective coating is exceeded, there is a risk of “liquid metal embrittlement” which is caused by penetration of the molten metal of the coating into the notches or cracks arising in forming of the flat steel product. The liquid metal which has penetrated into the steel substrate deposits at grain boundaries and there reduces the maximum tensile or compressive stress which can be withstood. As a solution, the patent publication offers nitriding of the outer layer regions, so as to produce finely structured outer layer regions.
The present invention is, in contrast, concerned with the problem that hot-formed and/or hardened steel parts do not have the desired wear resistance in the uncoated regions and are therefore not optimally suited for use as conveying means, for example on contact with abrasive materials. It is therefore an object of the present invention to propose at least partly uncoated steel parts having improved suitability for use with abrasive materials. In addition an inexpensive production process for corresponding steel parts should be proposed.
The object indicated is achieved, for a steel part, the steel part at least partially having a surface region which has been hardened to a depth of not more than 100 μm, preferably to a depth of up to 40 μm, by surface hardening before hot forming and/or hardening.
It has been found that the heating of the semifinished parts for production of the steel parts to a temperature above the Ac1 transformation temperature or above the Ac3 temperature before hot forming and/or hardening leads to decarburization of regions close to the surface, so that the carbon content of these regions is significantly lower than the carbon content of the base material. As a result, the region close to the surface up to a depth of 100 μm, in particular the region up to a depth of 40 μm, cannot be hardened to the required degree during hot forming and/or hardening. However, it has been found that at least partial surface hardening of the uncoated regions of the semifinished parts before hot forming and/or hardening to give the steel part leads to both the surface region and the base material having very high hardness despite the decarburization of the regions close to the surface as a result of the high temperatures during hot forming or hardening. This provides a steel part of which at least partially has a surface region which has been hardened to a depth of preferably 100 μm or in the region down to a depth of 40 μm and is therefore significantly more wear resistant than the at least partly uncoated steel parts known hitherto.
In a first embodiment, the hardened surface region of the steel part is hardened by carburization or nitriding. Both processes offer the opportunity of hardening regions close to the surface of the steel part in a targeted manner before hot forming or hardening. In addition, nitriding has the advantage that the hardness is not reduced during hot forming. In the case of carburization, the carbon content in the surface regions is increased but decreases again due to hot forming.
In a further embodiment, after hot forming and/or hardening the hardened surface region of the steel part preferably has at least the hardness of the base material of the steel part located under the surface region.
The wear resistance of the steel part can preferably also be improved by the hardness of the surface region of the steel part being greater than the hardness of the base material. It has been found that, in particular, the hardness of the surface regions is responsible for the wear resistance of the steel part on contact with highly abrasive materials, so that a very wear-resistant steel part can be produced even when using a somewhat softer base material.
Consequently, the steel part is, according to a further embodiment of the steel part, configured for use as processing, conveying and/or crushing means 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 being surface-hardened.
In addition, manganese-boron steels, dual-phase steels or TRIP steels, in which particularly pronounced martensite formation or transformation of residual austenitic components into martensite makes an increase in the hardnesses possible, are also particularly advantageous.
In a further embodiment of the steel part, the surface region 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. These values are generally achieved only by very high-strength steel grades after hot forming or hardening in the base material. The surface hardening before hot forming or hardening offers, in particular, the opportunity of providing the starting material for production of the steel components on a coil.
According to further teaching of the present invention, the abovementioned object is achieved by a process for producing a wear-resistant, at least partly uncoated steel part for processing, conveying and/or crushing means of agricultural machines, conveying machines, mining machines or building machines from a semifinished part, 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 that the semifinished part at least partially 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. Preference is given to hardening a surface region having a depth of up to 40 μm, in which decarburization processes usually take place during hot forming. The depth of the surface region which is to be hardened is controlled by the duration of the hardening treatment. It has been found, in particular, that despite heating to a temperature above the Ac1 transformation temperature point, the surface-hardened regions of the steel part remain stable in respect of the surface hardness, so that high surface hardness can be achieved after hot forming and/or hardening. This leads to the steel parts of processing, conveying and/or crushing means of agricultural machines, conveying machines, mining machines or building machines which are in contact with abrasive materials displaying reduced wear.
The hardening of the surface regions before hot forming or before hardening makes it possible to carry out the surface hardening on coilable materials, i.e. on steel strip, so that particularly economical production of wear-resistant, at least partly uncoated steel parts from semifinished parts is made possible. In a preferred embodiment of the process, hardening of the surface region is effected by nitriding or by carburization. Both processes make it possible to provide a higher hardness in the surface region, which after hot forming and/or after hardening make a higher wear resistance of the surface of the hot-formed or hardened steel part possible.
The surface hardening is, in a further embodiment, particularly preferably carried out 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 a balance N₂and also unavoidable impurities at a holding temperature of from 600° C. to 900° C. The dew point of the heat treatment atmosphere is preferably in the range from −50° C. to −5° C., so that the effect of atmospheric moisture on the hardening process is reduced. In addition, preference is given to a maximum of 10% by volume of H₂and a maximum of 5% by volume of NH₃being permitted and the dew point being set to a dew point temperature of from −40° C. to −15° C. at a temperature of from 680 to 840° C. The latter process parameters gave improved and more uniform surface hardening.
The depth of the surface hardening can be set via the time for which the holding temperature is maintained. The time for which the semifinished part has the holding temperature during surface hardening is preferably set to from 5 s to 600 s, preferably from 30 s to 120 s.
The surface hardening is preferably carried out in a continuous hardening furnace, so that, for example, a strip-like semifinished part, i.e. a coilable semifinished part, is also surface-hardened and can be fed to the further hot forming and/or press hardening steps. However, surface hardening in a chamber furnace is also conceivable.
As indicated above, semifinished parts such as manganese-boron steels, dual-phase steels and TRIP steels firstly display a particularly high strength increase during hot forming or during hardening and secondly provide the opportunity of bringing the regions close to the surface to identical hardness in the range from 400 to 700 HV by nitriding. As a result, steel parts which are very wear-resistant and have particularly high strengths can be produced inexpensively.

In the following, the invention will be illustrated with the aid of examples in conjunction with the drawing. In the drawing,

FIG. 1 schematically shows an example of the process for producing a wear-resistant, at least partly uncoated steel part,

FIG. 2 shows the layer structure of the semifinished part or steel part treated as per the example in FIG. 1 in a schematic illustration,

FIGS. 3, 4 shows examples of a steel part for agricultural machines and conveying machines and

FIG. 5 shows a graph of the hardness profile as a function of the distance from the surface for two examples and a comparative example.

FIG. 1 firstly shows, very schematically, an example of the production of a wear-resistant, at least partly uncoated steel part in a schematic illustration. The semifinished part 1, which consists of a steel, for example a manganese-boron steel, dual-phase steel or TRIP steel, is firstly fed to surface hardening 2. If a strip-like semifinished part is reeled off a coil 1 a and fed to surface hardening 2, it is, for example, advantageous to carry out surface hardening, for example in the case of nitriding, in a continuous hardening furnace at the end of which, for example, the strip-like semifinished part 1, now provided with a hardened surface, can be wound up on a coil (not shown). The surface-hardened strip-like semifinished part is cut to length and fed to hot forming and/or hardening 3, so that process step 3 can produce a formed, at least partly uncoated steel part 4 which is suitable for processing, conveying and/or crushing means of agricultural machines, conveying machines, mining machines or building machines. Firstly, the steel part 4 produced in this way characterizes high strength values owing to the hot forming and/or hardening step. Secondly, the surface region of the steel part also has an increased hardening due to the nitriding of the surface which has taken place before hot forming and/or before hardening. As indicated above, the process of the invention enables the decarburization of the surface regions, which takes place to a depth of 100 μm, to be countered by the surface region being surface-hardened to a depth of 100 μm or in a region down to a depth of 40 μm. The surface hardening is preferably carried out by nitriding. However, carburization of the surface region is also conceivable.
The surface hardening in process step 2 is preferably carried out by means of 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 N₂and also unavoidable impurities at a holding temperature of from 600° C. to 900° C. Reduction of the hydrogen concentration to a maximum of 10% by volume or limiting of the NH₃concentration to a maximum of 5% by volume also leads to a further improvement of the nitriding result.
The depth of the surface hardening can be set via the duration of the surface hardening, for example at a holding temperature of from 5 s to 600 s. The surface is preferably nitrided at a holding temperature of from 30 s to 120 s, with the temperature being from 680° C. to 840° C. Carrying out the surface hardening before hot forming or hardening has the advantage that a heat treatment process can be carried out significantly more efficiently using a, for example, strip-like semifinished part in a continuous hardening furnace or a plate in a continuous hardening furnace than when using formed steel parts which have different shapes and different geometries. The quality of the surface hardening can likewise be ensured more easily by the use of strip-like semifinished parts or semifinished parts configured as a blank.
FIG. 2 then schematically shows a cross section of the semifinished part at three different points in time during the process. At first, the semifinished part 1 has a more or less homogeneous, for example ferritic microstructure 1 a corresponding to the production process, which is determined by the combination of production process and steel composition. As a result of the surface hardening, the surface region lb is hardened by inward diffusion of nitrogen in the case of nitriding or carbon in the case of carburization, with the microstructure changing there. The thickness of the surface region 1 b depends on the duration of the heat treatment. The surface region is usually up to a maximum of 100 μm in which the hardness of the semifinished part is altered. A preferred region, which is a compromise between sufficient surface hardening and duration of the heat treatment for surface hardening, has a thickness of from 20 to 40 μm. The duration of surface hardening, for example in nitriding, is then preferably from 30 s to 120 s. The microstructure of the material 1 a remaining underneath the surface region 1 b remains essentially unchanged during the heat treatment.
In the hot forming step, the microstructure of the base material 1 a is then firstly converted into austenite and, by means of hardening, later partially into martensite. In this way, high hardness and good mechanical strengths are achieved in the base material 1 c. The surface region 1 b remains unchanged except for carburization of these layers. As a result of nitriding, the surface region can continue to remain hardened. In the case of targeted carburization of the surface region 1 b instead of nitriding, decarburization can be countered, so that an increase in the hardness is also achievable here. The formed steel part 4 thus has a hardened region 1 b and also a region 1 c which has been hardened by the hot forming and hardening.
FIGS. 3 and 4 show typical fields of application for the wear-resistant, at least partially uncoated steel part in the form of a conveying screw 5 in FIG. 3 and a plowshare 6 for agricultural plows in FIG. 4. Both components are typical representatives of processing, conveying and/or crushing means which are used in agricultural machines, conveying machines, mining machines or building machines, for example concrete mixers, and are exposed to highly abrasive materials. The use of hot formed and/or press hardened steel parts has hitherto not been very advantageous because of the increased susceptibility to wear. Due to the surface hardening of the region which is decarburized during hot forming and/or hardening, the hot forming steels gain an enlarged range of uses.

TABLE 1

Measurement of HV 0.01	Sample A	Sample B
depth μm	(1% NH₃)	(4% NH₃)

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 shows measurements of the hardness of samples A and B which consist of a steel of grade 22MnB5. The samples A and B were subjected to surface nitriding in a heat treatment atmosphere comprising 1% by volume of NH₃or 4% by volume of NH₃at 760° C. and 90 s in each case. The surface nitriding was carried out at inter-critical temperatures (T>Ac1) since austenite can dissolve more nitrogen than ferrite. The samples were subsequently hot formed and hardened. Polished sections were made from the hot formed or hardened steel parts and the hardness HV 0.01 (DIN EN ISO 6507-1) was measured at a distance of 5 μm from the surface. The microhardness measurement on the samples as a function of the content of NH₃in the heat treatment atmosphere had a greater hardness at a higher NH₃content of the heat treatment atmosphere at the same heat treatment parameters, i.e. hold time and hold temperature.
The hardness of sample A firstly decreases from the value of 460 HV measured at the surface to a value of 333 HV at a depth of 20 μm. The hardness then increases again to a value of about 492 HV, which indicates that the decarburization of the base material ceases here. The uppermost region, in particular, from 5 to 15 μm was significantly hardened by the surface hardening. It can be seen from sample B that the surface hardening is more pronounced, both in terms of the amplitude and the depth of hardening, at an increased NH₃content. This can be attributed to greater diffusion of nitrogen into the surface of the steel part taking place due to the higher NH₃concentration in the heat treatment atmosphere. The values for sample B start at 546 at a depth of 5 μm and decrease to a value of 394 at a depth of 25 μm. The values subsequently increase again to about 466 at a depth of 45 μm. It can clearly be seen that the surface is harder than the base material at a depth of 45 μm.
A similar picture is shown by the measurements on two further examples shown in FIG. 5 compared to a comparative example. The comparative example illustrated by a dotted line displays a reduced hardness below 400 HV 1 (DIN EN ISO 6507-1) in the region of 5 to 35 μm. The reduction in the hardness compared to the base material, which is in the range from 450 HV 1 to 500 HV 1, is explained by decarburization during hot forming. The two comparative examples with two different nitriding variants, once again 1% strength NH₃heat treatment atmosphere or 4% strength NH₃heat treatment atmosphere, differ especially in this region close to the surface, since hardness of above 500 could be measured here. In this way, it is possible, in the case of wear-resistant, at least partly uncoated steel parts, to provide not only the particularly high tensile strength values of the hot formed and/or hardened steel parts but also a high wear resistance due to greater surface hardness in the range from, for example, 500 to 700 HV.

Claims

1.-12. (canceled)

13. A method for producing a wear-resistant, uncoated steel part for a processing, conveying, and/or crushing mechanism of an agricultural machine, a conveying machine, a mining machine, or a building machine from a semifinished part comprising a hardenable steel grade, the method comprising:

heating at least regions of the semifinished part to a temperature above an Ac1 transformation temperature;

at least partially subjecting the semifinished part to surface hardening by hardening a surface region to a depth of not more than 100 microns by heat treating the semifinished part 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 impurities at a hold temperature of 600-900 degrees Celsius; and

at least one of hot forming or hardening the semifinished part.

14. The method of claim 13 wherein hardening the surface region comprises nitriding or carburizing.

15. The method of claim 13 wherein hardening the surface region comprises maintaining the semifinished part at the holding temperature for 30 to 120 seconds.

16. The method of claim 13 wherein the surface hardening is performed in a continuous hardening furnace.

17. The method of claim 13 wherein the semifinished part to be surface hardened comprises a manganese-boron steel or a TRIP steel.

18. A wear-resistant, uncoated steel part formed of a hardenable steel grade and produced by hot forming and/or hardening a semifinished part, wherein the wear-resistant, uncoated steel part comprises a surface region that has been hardened to a depth of not more than 100 microns by surface hardening by nitriding prior to hot forming and/or hardening the semifinished part, wherein the wear-resistant, uncoated steel part is configured for use as a processing, conveying, and/or crushing mechanism in agricultural machines, conveying machines, mining machines, or building machines, wherein at least regions of the wear-resistant, uncoated steel part that are to be subjected to abrasive forces have been surface hardened.

19. The wear-resistant, uncoated steel part of claim 18 wherein after hot forming and/or hardening the steel part has at least the hardness of a base material of the steel part located under the surface region.

20. The wear-resistant, uncoated steel part of claim 18 wherein the steel part comprises at least one of a manganese-boron steel, a dual-phase steel, or a TRIP steel.

21. The wear-resistant, uncoated steel part of claim 18 wherein the surface region that has been hardened has before hot forming and/or hardening at least in some regions a hardness of 400-700 HV.