Steel alloy for producing a profiled beam and method for producing same

Abstract

The invention relates to a steel alloy for the production of a profile beam, comprising: C: 0.01% - 0.16% Si: 0.1% - 0.5% Mn: 1% - 1.7% Cr: 0% - 0.3% Mo: 0% - 0.1% Ni: 0% - 0.4% Cu: 0% - 0.55% N: 0% - 0.015% Nb: 0.02% - 0.07% Ti: 0.005% - 0.05% V: 0.02% - 0.06% Al: 0% - 0.014% P: 0% - 0.02% P: 0% - 0.02% S: 0% - 0.02% Remainder: Fe and unavoidable impurities.

Description

[0001] The invention relates to a steel alloy for manufacturing a highly stressed profile beam, in particular a guide beam. Such a guide beam is, for example, a lifting mast of a forklift truck. The invention further relates to a method for manufacturing a profile beam.

[0002] Profile beams are beams for carrying loads that typically have a complex cross-section, such as an H-profile, U-profile, C-profile, etc. The profiling gives such a beam an improved section modulus, so that it can carry essentially the same loads at a lower weight compared to a solid beam.

[0003] Guide rails are used in a variety of technical applications, particularly in industrial trucks and cranes, to ensure precise guidance, high stability, and reliable load transfer. In industrial trucks, especially in the masts of forklifts and other lifting vehicles, guide rails ensure precise movement of the telescopic sections and guarantee an even distribution of forces during the lifting process. This prevents the masts from tilting or jamming and enables safe and efficient handling of loads. In cranes, guide rails ensure the stable guidance of loads along the crane runway and, thanks to their robust construction, minimize vibrations and torsional forces. Further applications for guide rails are also known.

[0004] Guide beams, especially lifting masts, are therefore highly stressed components in operation: Not only must they withstand high bending forces while maintaining the lowest possible weight, but also rolling over them. A lifting mast is required to withstand approximately 500,000 loaded rolling cycles. Considering the Hertzian contact pressure, these rolling cycles result in a highly localized compressive load on the profile. This necessitates a high surface hardness of the profile to ensure the required dimensional accuracy even under heavy loads.

[0005] In addition to the required high hardness, sufficient toughness is also required so that suddenly introduced loads, for example in an accident or in case of incorrect operation, do not lead to the profile beam breaking.

[0006] Finally, good machinability, especially for cutting, is essential for the production of guide carriers. This includes both rolling into a specific profile shape and subsequent machining.

[0007] Various alloys are known in the prior art. CN 116445810 A discloses a steel plate for wind turbines with a specific composition. In particular, this document emphasizes that the aluminum content must be present in the alloy on a relatively large scale so that the titanium is not bound and can thus be used for other purposes.

[0008] CN 112553519 A discloses a steel plate for a building component. Here too, the deoxidizing effect of aluminum is mentioned, and it is therefore named as a mandatory component.

[0009] DE 11 2021 006 024 T5 concerns a steel plate for thick-walled pressure vessels. A relatively high aluminum content is also required here.

[0010] EP 3 872 206 B1 concerns a process for the production of flat steel. Here, too, the aluminium content, which is defined as mandatory, is to be considered relatively high.

[0011] The object of the invention is to provide an alloy that fulfills the aforementioned requirements. Furthermore, the invention provides a use of the steel alloy and a method for manufacturing a steel profile beam from the steel alloy.

[0012] The alloy-related problem is solved by a steel alloy having the features of claim 1. A preferred use is specified in claim 7. The method is specified in claim 8.

[0013] Advantageous embodiments result from the dependent claims and the description. Where a percentage is specified in these documents with regard to alloying elements, the corresponding content refers to wt.%. The alloying elements are referred to by their SI names.

[0014] The core of the invention lies in the realization that the specified alloy can meet the increased demands placed on a guide beam, particularly a lifting mast, despite the alloy being relatively inexpensive in terms of material usage and easy to process. The inventors consider it a particular advantage that the aluminum content in the alloy is significantly reduced and merely tolerated. Preferably, within the limits of what is technically feasible, the alloy is free of aluminum using scrap commonly used in steel production, meaning an aluminum content of < 0.010%. It has been observed that aluminum also binds the nitrogen contained in the alloy during cooling at an early stage (i.e., even at high temperatures during solidification).This is undesirable in the present case, so that the nitrogen is available for precipitation processes occurring at lower temperatures in conjunction with Nb and V.

[0015] The optional and reduced use of aluminum minimizes the formation of coarse aluminum oxides, which increases the purity of the steel and improves its workability. This is particularly advantageous for applications requiring high toughness and ductility in addition to high hardness.

[0016] In contrast, titanium is added in smaller quantities, which is beneficial. This serves, on the one hand, to limit the deoxidation of the steel and thus to stabilize the melt. Additionally, the properties of titanium that positively influence grain size predominate.

[0017] Si is also added in the specified quantity to stabilize the steel. Preferably, the Si content is limited to 0.2% - 0.5%.

[0018] Preferably, the alloy contains 0.02% to 0.04% vanadium and 0.025% to 0.05% nitrogen. With these amounts, in combination with the specified nitrogen, preferably 0.008% to 0.015%, particularly effective precipitation hardening is achieved, which can also be accomplished in conjunction with hot rolling of a profile. For particularly effective utilization of this effect, an nitrogen content of 0.035% to 0.05% is advantageous.

[0019] In a preferred embodiment, the Ti content is 0.006% to 0.015%. It has been found that the advantages of the invention are achieved even with a small amount of Ti.

[0020] The proposed alloy composition combines three hardening mechanisms in a steel alloy in a balanced ratio, so that the final product exhibits the necessary machinability – especially for machining processes – while also meeting the requirements for mechanical strength and, furthermore, can be formed into complex cross-sections with reasonable effort. The three hardening mechanisms of the present steel alloy are solid solution hardening, precipitation hardening, and grain fineness formation.

[0021] Solid solution hardening is achieved through the specified range of carbon, silicon, and manganese, which increase strength through solid dissolution in the iron. Chromium and molybdenum also enhance hardenability by delaying the transformation of austenite to soft phases and promoting the formation of martensite.

[0022] Copper and nickel improve strength and toughness through solid dissolution and positively influence corrosion resistance.

[0023] The micro-alloyed elements Nb and V form the basis for effective precipitation hardening. During precipitation hardening, finely dispersed carbides and nitrides are formed, which further increase the hardness and strength of the final product. Sufficient carbon (preferably 0.11%–0.16%) and nitrogen are essential for successful precipitation hardening. This effect is positively influenced by the appropriately specified, controlled addition of nitrogen and the correspondingly reduced use of aluminum.

[0024] Nb, Ti, V, N and Al together form nitrides and carbon nitrides which are important for fine grain formation, especially in the hot rolling process.

[0025] The alloy product may therefore preferably contain niobium and vanadium nitrides or carbonitrides. These increase the yield strength and tensile strength and improve the toughness-to-hardness ratio.

[0026] Although manganese improves hardenability by increasing the case depth and counteracts embrittlement, an interaction with the reduced aluminum content can be observed. It also increases tensile strength and toughness. A content of 1.2–1.65% is preferred, more preferably 1.55–1.65%.

[0027] Copper increases strength. Especially in combination with the specified amount of phosphorus, a content of 0–0.45% is considered preferable.

[0028] The different elements each have several tasks that are activated at different times during the cooling process.

[0029] The inventors further recognized that achieving the effects according to the invention does not necessarily require a specific, relatively tightly toleranced alloy composition, but rather that the ratio of the elements to one another has a decisive influence on the mechanical properties. A formula was developed that establishes a ratio between certain alloy components and whose limits include particularly advantageous alloy compositions. This formula is: 0,42 ≤ C + Si 8 + Mn 6 + Cr + Mo + V 5 + Ni + Cu 15 ≤ 0,55

[0030] It has been shown that the alloy space defined by the formula relationship unites those alloys that exhibit particularly outstanding properties and are especially suitable for the application of a guide profile. The formula relationship specifies the composition with which the individual, multidimensional tasks of the individual alloying elements can be most advantageously harmonized.

[0031] It is preferred to define the limits specified above as 0.44 to 0.54.

[0032] The use of the specified steel alloy is particularly preferred for lifting masts of industrial trucks. The exceptionally high demands on hardness and fatigue strength can be met with the steel alloy according to the invention.

[0033] To produce a steel alloy product, preferably a profile beam made from the alloy according to the invention, the specified alloying elements are cast in a first step into semi-finished products, for example as ingots or billets. Preferably, this is then normalized and rolled into the shape of a profile, usually above the Acs temperature (approximately 20° to 70° above). The profile is then cooled in moving air.

[0034] In addition to the precipitation hardening achieved through the aforementioned process, a particularly fine microstructure is produced. The grain size is approximately 10 to 20 µm (according to DIN ISO 643), typically in a row-like structure. The ferrite content is then typically >1.3 according to steel-iron test sheet 1520.

[0035] In particularly stressed areas of the profile, it can be further hardened or tempered if necessary.

[0036] The invention is explained in more detail using various exemplary embodiments.

[0037] The following alloy compositions were investigated: 01 02 03 See above. C 0,12 0,14 0,14 0,12 Si 0,32 0,44 0,45 0,30 Mn 1,56 1,52 1,60 1,50 Cr 0,26 0,29 0,29 0,11 Mon 0,10 0,06 0,10 0,09 Ni 0,36 0,24 0,19 0,04 Cu 0,16 0,11 0,14 0,25 N 0,012 0,010 0,010 0,007 Note 0,040 0,038 0,041 0,030 Ti 0,010 0,008 0,007 0,003 V 0,026 0,021 0,027 0,044 Al 0,005 0,006 0,013 0,032 P 0,017 0,015 0,012 0,012 S 0,004 0,008 0,014 0,004 Fe rest rest rest rest

[0038] Alloys 01 to 03 are according to the invention. "Cf." denotes a comparison alloy not according to the invention.

[0039] The comparison alloy differs significantly from the alloys according to the invention by a considerably higher Al content.

[0040] The semi-finished product provided after casting was normalized by rolling into H-profiles and U-profiles.

[0041] Subsequently, various mechanical properties of these alloys were tested. These can be summarized as follows: 01 02 03 See above. ReH [MPa] 504 499 516 438 Rm [MPa] 638 644 657 550 A [%] 23 23 22 25 KV @ -20°C [J] 72 66 54 118 Hardness [HV] 195 197 201 163

[0042] It was found that the products based on the alloy compositions according to the invention, in contrast to a composition according to a reference alloy, exhibit significantly higher hardness, which is accompanied by exceptional resistance to rolling. The higher tensile strength results in less surface damage and also in significantly higher safety margins when used as profile beams.

[0043] The improved mechanical properties result from the altered alloy composition and the resulting hardening processes during the cooling process, as described above. These effects can be largely attributed to the reduction in the aluminum content of the alloy.

[0044] The invention has been described with reference to exemplary embodiments. Without departing from the scope of protection described by the applicable claims, numerous further embodiments of the inventive concept would be apparent to a person skilled in the art, without these needing to be explained in more detail within the scope of these explanations.

Claims

1. Steel alloy for the manufacture of a profile beam, comprising: C: 0.01% - 0.16% Si: 0.1% - 0.5% Mn: 1% - 1.7% Cr: 0% - 0.3% Mo: 0% - 0.1% Ni: 0% - 0.4% Cu: 0% - 0.55% N: 0% - 0.015% Nb: 0.02% - 0.07% Ti: 0.005% - 0.05% V: 0.02% - 0.06% Al: 0% - 0.014% P: 0% - 0.02% P: 0% - 0.02% S: 0% - 0.02% Remainder: Fe and unavoidable impurities.

2. Steel alloy according to claim 1, characterized by the fact that the Al content is 0 - 0.010%.

3. Steel alloy according to one of claims 1 or 2, characterized by the fact that The Nb content is 0.025% - 0.05%, preferably 0.035% - 0.05%, and the V content is 0.02% - 0.04%.

4. Steel alloy according to one of claims 1 to 3, characterized by the fact that The N content is 0.008 - 0.015%.

5. Steel alloy according to one of claims 1 to 4, characterized by the fact that The Ti content is 0.006 - 0.015%.

6. Steel alloy according to any one of claims 1 to 5, characterized by the fact thatthe Mn content is 1.2 - 1.65%, more preferably 1.55 - 1.65%.

7. Steel alloy according to any one of claims 1 to 6, characterized by the fact that The copper content is 0 - 0.45%.

8. Steel alloy according to any one of claims 1 to 5, characterized by the fact that The alloy composition must meet the following condition: 0,42 ≤ C + Si 8 + Mn 6 + Cr + Mo + V 5 + Ni + Cu 15 ≤ 0,55 9. Steel alloy according to claim 8, characterized by the fact that where the lower limit is 0.44 and / or the upper limit is 0.

54.

10. Use of the steel alloy according to one of claims 1 to 9 for a guide profile, in particular a lifting mast for a forklift truck.

11. Method for producing a steel alloy product from a steel alloy according to any one of claims 1 to 9, characterized by the fact that The steel alloy is cast into semi-finished products in a first step and then, in a second step, via A C3 -The material is rolled at a warm temperature and then cooled in moving air.

12. Method according to claim 11, characterized by the fact that After rolling, a machining process is carried out.

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