TREA

COMPONENT MADE OF B-ZR ALLOYED STEEL

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Information

  • Patent Application
  • 20250019811
  • Publication Number
    20250019811
  • Date Filed
    December 01, 2022
    2 years ago
  • Date Published
    January 16, 2025
    2 days ago
Information
Abstract
Component with a steel part, wherein the steel, among other things, 0.30-0.50 wt. % C,0.05-1.3 wt. % Mn,0.001-0.015 wt. % P,0.001-0.015 wt. % S,0.01-0.8 wt. % Si,0.3-1.5 wt. % Cr,0.005-0.40 wt. % V,0.0008-0.0050 wt. % B,0.02-0.35 wt. % Al,0.0001-0.0200 wt. % N,0.01-0.08 wt. % Ti, and0.0030-0.0800 wt. % Zr;comprises the rest iron and unavoidable impurities,wherein the B content in the steel at a depth of 5-60 μm is ≥80% of the B content in the steel at a depth of 500 μm.
Description
TECHNICAL FIELD

The invention relates to a component with a steel part, in which the steel is alloyed with boron (hereinafter also referred to as “B”), among other things. In particular, the invention relates to a fastening means such as a screw or a nut.


BACKGROUND

Boron is often used as a cost-effective alloying element to improve the hardenability of steel component, particularly high-strength and ultra-high-strength components. Steels alloyed with boron are described, for example, in WO 2021/009705 A1 and WO 2008/142275 A2.


However, components made of boron-alloyed steels, such as screws or nuts, often show a drop in hardness in the edge area after heat treatment, in particular isothermal heat treatment in a salt bath to create a bainitic structure, in particular to a depth of up to 300 μm below the surface, which limits their applicability for high-strength and ultra-high-strength products, such as high-strength and ultra-high-strength screws.


Usually, steels comprising boron are additionally alloyed with titanium and aluminum in order to keep boron in the dissolved state and not to precipitate it in the form of nitrides, carbides, carbonitrides, silicides or oxides. However, this is not sufficient to reduce the hardness inhomogeneity in the edge area described above.


SUMMARY

Component with a steel part, where the steel, among other things, includes 0.30-0.50 wt. % C, 0.05-1.3 wt. % Mn, 0.001-0.015 wt. % P, 0.001-0.015 wt. % S, 0.01-0.8 wt. % Si, 0.3-1.5 wt. % Cr, 0.005-0.40 wt. % V, 0.0008-0.0050 wt. % B, 0.02-0.35 wt. % Al, 0.0001-0.0200 wt. % N, 0.01-0.08 wt. % Ti, and 0.0030-0.0800 wt. % Zr, with the rest iron and unavoidable impurities. The B content in the steel at a depth of 5-60 μm is ≥80% of the B content in the steel at a depth of 500 μm.





BRIEF DESCRIPTION OF THE DRAWINGS


FIG. 1 shows the hardness profile in the edge area of a screw according to the invention.



FIG. 2 shows the hardness curve in the edge area of a screw of the prior art.





DETAILED DESCRIPTION

The present invention is therefore based on the task of reducing the loss of hardness in the edge area of components made of boron-alloyed steels.


This problem is solved by a component with a steel part according to claim 1 and a method of manufacture according to claim 14. Further features, embodiments and advantages are shown in the dependent claims, the description and the figures.


One aspect of the invention relates to a component comprising a steel part, wherein the steel

    • 0.30-0.50 wt. % C,
    • 0.05-1.3 wt. % Mn,
    • 0.001-0.015 wt. % P,
    • 0.001-0.015 wt. % S,
    • 0.01-0.8 wt. % Si,
    • 0.3-1.5 wt. % Cr,
    • 0.005-0.40 wt. % V,
    • 0.0008-0.0050 wt. % B,
    • 0.02-0.35 wt. % Al,
    • 0.0001-0.0200 wt. % N,
    • 0.01-0.08 wt. % Ti, and
    • 0.0030-0.0800 wt. % Zr;
    • optionally
    • 0.01-0.20 wt. % Mo,
    • 0.01-0.50 wt. % Ni,
    • 0.01-0.50 wt. % Cu, and/or
    • 0.001-0.010 wt. % Ca,
    • and the rest iron and unavoidable impurities,
    • wherein the part of the component has a steel surface and the B content (boron content) in the steel at a depth of 5-60 μm is ≥80% of the B content in the steel at a depth of 500 μm, the depth being measured perpendicular to the steel surface.


Another aspect of the invention relates to a method of manufacturing a component with a steel part, comprising the steps of:

    • providing the steel with the aforementioned composition,
    • subsequent forming of a component with a part from the steel, and
    • optional heat treatment.


Surprisingly, the composition according to the invention, in particular the zirconium added to the B-comprising steel in combination with the other alloying elements in the component according to the invention with a steel part, counteracts the decrease in hardness in the edge area, in particular if the steel part is heat-treated. Another surprising advantage of the component according to the invention with a steel part is the improved resistance to hydrogen embrittlement. Surprisingly, significantly higher strengths can thus be achieved.


Improving the hardness in the edge area and also reducing hydrogen embrittlement is particularly advantageous in fasteners, which usually exhibit high and often dynamic axial stresses, as the fasteners, which can be bolts or nuts, for example, are essential for many assemblies. Failure of a fastener can have drastic consequences for man or machine, for example in the case of a bridge bolt, a chassis bolt, an engine head bolt or similar. The invention can therefore also relate to a vehicle, an engine, a cylinder head, a chassis arrangement or a battery arrangement with a component according to the invention, in particular a fastening means.


In a preferred embodiment of the invention, the component comprises a steel part, wherein the steel comprises the following components:

    • 0.30-0.50 wt. % C,
    • 0.05-1.3 wt. % Mn,
    • 0.001-0.015 wt. % P,
    • 0.001-0.015 wt. % S,
    • 0.01-0.80 wt. % Si,
    • 0.3-1.5 wt. % Cr,
    • 0.005-0.40 wt. % V,
    • 0.0008-0.0050 wt. % B,
    • 0.02-0.35 wt. % Al,
    • 0.0001-0.0200 wt. % N,
    • 0.01-0.08 wt. % Ti, and
    • 0.0030-0.08 wt. % Zr;
    • optionally
    • 0.01-0.20 wt. % Mo,
    • 0.01-0.50 wt. % Ni,
    • 0.01-0.50 wt. % Cu, and/or
    • 0.0010-0.0100 wt. % Ca;
    • and the balance iron and unavoidable impurities, wherein the part of the component has a steel surface and the B content in the steel at a depth of 5-60 μm is ≥80% of the B content in the steel at a depth of 500 μm, the depth being measured perpendicular to the steel surface. Preferably, each impurity is comprised in ≤0.01 wt. %.


Another preferred option is that the steel can be

    • 0.01-0.10 wt. % Bi,
    • 0.01-0.30 wt. % Co,
    • 0.01-0.06 wt. % Nb,
    • 0.01-0.10 wt. % Se,
    • 0.01-0.10 wt. % Te,
    • 0.01-0.3 wt. % W,
    • 0.01-0.04 wt. % As,
    • 0.01-0.02 wt. % Sn,
    • 0.01-0.20 wt. % Ta,
    • 0.01-0.20 wt. % Ce,
    • 0.01-0.50 wt. % Sn,
    • 0.01-0.40 wt. % Sb,
    • 0.01-0.20 wt. % Hf, and/or
    • one or more lanthanides, each in an amount of 0.01-0.02 wt. %.


Further preferably, the invention relates to a component comprising a steel part, wherein the steel

    • 0.30-0.48 wt. % C,
    • 0.2-1.3 wt. % Mn,
    • 0.001-0.015 wt. % P,
    • 0.001-0.015 wt. % S,
    • 0.01-0.70 wt. % Si,
    • 0.3-1.4 wt. % Cr,
    • 0.005-0.38 wt. % V,
    • 0.0010-0.0050 wt. % B,
    • 0.02-0.30 wt. % Al,
    • 0.0010-0.0180 wt. % N,
    • 0.012-0.07 wt. % Ti, and
    • 0.0040-0.0600 wt. % Zr;
    • optionally
    • 0.01-0.18 wt. % Mo,
    • 0.01-0.45 wt. % Ni,
    • 0.01-0.40 wt. % Cu, and/or
    • 0.0010-0.0090 wt. % Ca;


and the balance iron and unavoidable impurities, wherein the part of the component has a steel surface and the B content in the steel at a depth of 5-60 μm is ≥80% of the B content in the steel at a depth of 500 μm, the depth being measured perpendicular to the steel surface. Preferably the steel consists of the above components. Preferably, each impurity is present in ≤0.01 wt. %.


In a further preferred embodiment, the invention relates to a component comprising a steel part, wherein the steel

    • 0.30-0.46 wt. % C,
    • 0.3-1.3 wt. % Mn,
    • 0.001-0.015 wt. % P,
    • 0.001-0.015 wt. % S,
    • 0.01-0.60 wt. % Si,
    • 0.3-1.3 wt. % Cr,
    • 0.005-0.35 wt. % V,
    • 0.0012-0.0050 wt. % B,
    • 0.02-0.25 wt. % Al,
    • 0.0020-0.0150 wt. % N,
    • 0.014-0.060 wt. % Ti, and
    • 0.0050-0.0500 wt. % Zr;
    • optionally
    • 0.01-0.16 wt. % Mo,
    • 0.01-0.40 wt. % Ni,
    • 0.01-0.30 wt. % Cu, and/or
    • 0.0010-0.0080 wt. % Ca;
    • and the rest iron and unavoidable impurities,
    • wherein the part of the component has a steel surface and the B content in the steel at a depth of 5-60 μm is ≥80% of the B content in the steel at a depth of 500 μm, the depth being measured perpendicular to the steel surface. Preferably the steel consists of the above components. Preferably, each impurity is present in ≤0.01 wt. %.


In the most preferred embodiment of the invention, a component is provided with a steel part, wherein the steel is

    • 0.34-0.42 wt. % C,
    • 0.45-0.90 wt. % Mn,
    • 0.001-0.015 wt. % P,
    • 0.001-0.015 wt. % S,
    • 0.02-0.50 wt. % Si,
    • 0.60-1.00 wt. % Cr,
    • 0.08-0.25 wt. % V,
    • 0.0012-0.0050 wt. % B,
    • 0.02-0.25 wt. % Al,
    • 0.0025-0.0090 wt. % N,
    • 0.015-0.060 wt. % Ti, and
    • 0.0050-0.0500 wt. % Zr;
    • optionally
    • 0.01-0.16 wt. % Mo,
    • 0.01-0.40 wt. % Ni,
    • 0.01-0.30 wt. % Cu, and/or
    • 0.0010-0.0060 wt. % Ca;
    • and the rest iron and unavoidable impurities,
    • wherein the part of the component has a steel surface and the B content in the steel at a depth of 5-60 μm is ≥80% of the B content in the steel at a depth of 500 μm, the depth being measured perpendicular to the steel surface. Preferably the steel consists of the above components. Preferably, each impurity is present in ≤0.01 wt. %.


Further preferably, the steel according to the above compositions can also optionally be

    • 0.01-0.10 wt. % Bi,
    • 0.01-0.30 wt. % Co,
    • 0.01-0.06 wt. % Nb,
    • 0.01-0.10 wt. % Se,
    • 0.01-0.10 wt. % Te,
    • 0.01-0.3 wt. % W,
    • 0.01-0.04 wt. % As,
    • 0.01-0.02 wt. % Sn,
    • 0.01-0.20 wt. % Ta,
    • 0.01-0.20 wt. % Ce,
    • 0.01-0.50 wt. % Sn,
    • 0.01-0.40 wt. % Sb,
    • 0.01-0.20 wt. % Hf, and/or
    • one or more lanthanides, each in an amount of 0.01-0.02 wt. %.


A component with a steel part is therefore preferred, for example, wherein the steel

    • 0.30-0.50 wt. % C,
    • 0.05-1.3 wt. % Mn,
    • 0.001-0.015 wt. % P,
    • 0.001-0.015 wt. % S,
    • 0.01-0.8 wt. % Si,
    • 0.3-1.5 wt. % Cr,
    • 0.005-0.40 wt. % V,
    • 0.0008-0.0050 wt. % B,
    • 0.02-0.35 wt. % Al,
    • 0.0001-0.0200 wt. % N,
    • 0.01-0.08 wt. % Ti, and
    • 0.0030-0.0800 wt. % Zr;
    • optionally
    • 0.01-0.20 wt. % Mo,
    • 0.01-0.50 wt. % Ni,
    • 0.01-0.50 wt. % Cu,
    • 0.0010-0.0100 wt. % Ca,
    • 0.01-0.10 wt. % Bi,
    • 0.01-0.30 wt. % Co,
    • 0.01-0.06 wt. % Nb,
    • 0.01-0.40 wt. % Pb,
    • 0.01-0.10 wt. % Se,
    • 0.01-0.10 wt. % Te,
    • 0.01-0.3 wt. % W,
    • 0.01-0.04 wt. % As,
    • 0.01-0.02 wt. % Sn,
    • 0.01-0.20 wt. % Ta,
    • 0.01-0.20 wt. % Ce,
    • 0.01-0.50 wt. % Sn,
    • 0.01-0.40 wt. % Sb,
    • 0.01-0.20 wt. % Hf, and/or
    • comprises one or more lanthanides, each in an amount of 0.01-0.02 wt. %, the remainder being iron and unavoidable impurities, preferably consisting of these components, each impurity being ≤0.01 wt. %.


With the above-mentioned preferred and particularly preferred compositions for the steel, the drop in hardness in the edge area of the components can be reduced particularly effectively. In addition, the hydrogen embrittlement of the steel is greatly reduced.


The components Mo, Ni, Cu and Ca are optional, i.e. they may not be present independently of one another or, if they are present, they may be present in the steel independently of one another in the specified amounts of, for example, 0.01-0.20 wt. % Mo, 0.01-0.50 wt. % Ni, 0.01-0.50 wt. % Cu and/or 0.0010-0.0100 wt. % Ca. In a preferred embodiment, the components Mo, Ni Cu and Ca are comprised in the steel independently of one another. It is thus preferred that the steel comprises 0.01-0.20 wt. % Mo, 0.01-0.50 wt. % Ni, 0.01-0.50 wt. % Cu and/or 0.0010-0.0100 wt. % Ca, further preferably 0.01-0.16 wt. % Mo, 0.01-0.40 wt. % Ni, 0.01-0.30 wt. % Cu and/or 0.0010-0.0080 wt. % Ca.


The components Bi, Co, Nb, Pb, Se, Te, W, As, Sn, Ta, Ce, Sn, Sb, Hf and/or lanthanoids may also optionally be present in the steel, i.e. they may or may not be present independently of one another. If they are present, they may be present independently of each other, preferably in the amounts indicated.


Zirconium is a microalloying element in the steel of the component according to the invention, i.e. it has an effect even in very small quantities, in particular below 0.05 wt. %. Boron, titanium and vanadium are also microalloying elements. In the composition according to the invention, zirconium interacts with the other alloying elements, for example vanadium.


According to the invention, the B content in the steel at a depth of 5-60 μm is ≥ 80%, preferably ≥90%, of the B content in the steel at a depth of 500 μm (micrometers), the depth being measured perpendicular to the surface of the steel. This means that the B content at any point at a depth of 5-60 μm is ≥80% of the B content in the steel at a depth of 500 micrometers. In other words, over the depth range of 5-60 μm, the minimum B content in the steel at a depth of 5-60 μm is ≥80% of the boron content in the steel at a depth of 500 μm, preferably ≥90%, particularly preferably ≥95%.


The B content is the concentration of boron in percent by weight, based on the total weight of the steel. As the value of ≥80%, for example, is a relative value of two B contents in each case, the B content does not have to be in percent by weight, but can also be specified in percent by volume or atom, for example.


According to the invention, the B content is determined using GDOES (Glow Discharge Optical Emission Spectroscopy) (apparatus: GDA 750 HR from Spectruma Analytik GmbH). The surface of the sample material (steel) is removed with the aid of an Ar plasma, the sample atoms are brought into the gas phase (cathode sputtering or sputtering) and quantitatively determined there by spectroscopy. The B content is measured spectroscopically at every depth, for example over a depth range of 0-500 μm. The measurement result is a so-called B depth profile. In this way, the B content is determined at every depth, for example over a depth range of 0-500 μm. The ratio is then determined using the quotient of the B content at a certain depth (for example 10 μm) and at the depth of 500 μm, and the percentage value is thus determined, which according to the invention is ≥80%.


Furthermore, according to the invention, the boron content in the steel at a depth of 140-220 μm is additionally preferably ≥80% of the boron content in the steel at a depth of 500 μm. Both at a depth of 5-60 μm and at a depth of 140-220 μm, the boron content is independently preferably ≥90%, further preferably ≥95%, further preferably ≥98%, still further preferably ≥100%, most preferably 100-1000% of the boron content of the steel at a depth of 500 μm, the depth being measured perpendicular to the surface of the steel. For example, if the boron content (B concentration) in the steel is 0.0030-0.0033 wt. % at a depth of 5-60 μm and 0.0033 wt. % at a depth of 500 μm, this would be 90.9%-100%.


The other chemical elements of the steel are measured as usual using classical optical emission spectrometry on the surface of a cross-section of the steel part (so-called piece analysis). The stated weight percentages of the chemical elements of the steel are related to the total weight of the steel.


Investigations of the alloy composition of known boron steels in the context of the invention showed that there is a boron reduction in the edge region compared to the boron content at greater depths. Without being bound by the invention, it is assumed that the low or reduced drop in the boron concentration in the edge region according to the invention leads to a surprisingly low drop in hardness in the edge region of the steel part and to a surprising reduction in hydrogen embrittlement in the edge region. This is attributed to a combination of the amounts of zirconium according to the invention and the other alloying elements, for example vanadium. In addition to the composition of the steel, the heat treatment or tempering at the end of the manufacturing process, in particular salt bath tempering, which leads to these advantageous properties in the edge area of the steel part in the component according to the invention, is also advantageous for the low or non-existent drop in the boron concentration. The edge area is understood in particular to be the area at a depth of 0-300 micrometers, measured from the surface of the steel.


According to the invention, the zirconium in combination with the other alloying elements in particular counteracts the decrease in hardness in the edge area and leads to a reduction in hydrogen embrittlement in the edge area.


For the purposes of the invention, an impurity is understood to be an element that is present in an amount≤0.01 wt. %. Preferably, therefore, the steel comprises unavoidable impurities, each in an amount≤0.01 wt. %.


Furthermore, it was found in the context of the invention that the drop in hardness in the edge area of the components can be reduced particularly effectively if the ratio of (Zr+Ti+Al) to N is in a range of 2.7 to 150, more preferably 2.8 to 130, particularly preferably 3 to 100. The respective weight percentages of Zr, Ti, Al and N are used in the above formula.


The component according to the invention with a steel part is preferably a fastening means, particularly preferably selected from the group consisting of screws, nuts, rivets, bolts and chains.


A steel part within the meaning of the invention can be understood in particular to mean that at least a part of the component, i.e. a volume area, is made of steel. It is preferred that the steel part makes up ≥80 wt. %, further preferred ≥90 wt. %, particularly preferred ≥95 wt. % of the component. This means that the component consists of ≥80 wt. %, more preferably ≥90 wt. %, particularly preferably ≥95 wt. % of steel. This allows a particularly good mechanical strength of the component, especially the fastener, to be achieved. In order to increase the mechanical strength, it is particularly preferable if the steel part is in one piece. In particular, “one-piece” can be understood to mean that at least the one-piece part has been created in a forming process and/or is cohesive.


Preferably, the component according to the invention, in particular a screw, is a high-strength or ultra-high-strength component, preferably with strengths≥800 MPa (so-called high-strength components), particularly preferably above 1200 MPa, further preferably ≥1400 MPa (so-called ultra-high-strength components), particularly preferably 1200-1900 MPa, in particular 1400-1900 MPa. Strength classes according to ISO 898-1 in its version valid in January 2021. Preferred high-strength and ultra-high-strength components are high-strength or ultra-high-strength bolts, nuts, chain drives, formed components and/or structural components. Further or alternatively preferred is the component according to the invention, in particular the high-strength or ultra-high-strength component, preferably a welded component, an additive-manufactured component or a case-hardened component.


In a further preferred embodiment, in the component according to the invention with a steel part, the component or the steel is heat-treated, a so-called quenching and tempering, for example by salt bath quenching and tempering, in order to set a preferred microstructure. In a preferred embodiment, the microstructure of the steel is ≥70% by volume, more preferably ≥80% by volume, particularly preferably ≥90% by volume bainitic and/or martensitic, especially after tempering such as heat treatment. The proportion of the microstructure in percent by volume can be determined, for example, in microscopic images of micrographs, as the surfaces reflect the volumes on average over several micrographs. For this purpose, the areas are determined in several micrographs and the arithmetic mean is formed. As the densities of the microstructures of the steel are relatively similar, it is also preferred that the microstructure of the steel is ≥70 wt. %, further preferred ≥80 wt. %, particularly preferred ≥90 wt. % bainitic and/or martensitic. The proportion of austenite (retained austenite) is also preferably ≤20% by volume or weight, in particular ≤10% by volume or weight. These microstructures give the component according to the invention a particularly high strength and toughness. They can be subjected to high and often dynamic axial stress. Before tempering, the microstructure of the component according to the invention is preferably ≥90% by volume ferritic and/or pearlitic. Preferably, the microstructure of the component according to the invention is ≥90 wt. % ferritic and/or pearlitic before tempering.


Further preferably, the component according to the invention is a formed component. In particular, a formed component is to be understood as a component which has been formed by means of a forming step, in particular a cold forming process. The reduction of hydrogen embrittlement is particularly advantageous in the case of a formed component without heat treatment, because formed components already have a certain degree of brittleness due to the accumulated forest dislocations (e.g. two or more dislocations that run onto each other transversely or perpendicularly on different slip planes).


An above-mentioned structural component within the meaning of the invention is present in particular if the component is a load-bearing component. In particular, this structural component has two load-introducing sections, which advantageously have load-introducing structures, such as mounting recesses or openings, and a transmission region arranged between the load-introducing sections, which can and/or does transmit a load, in particular a bending load and/or tensile load, from one load-introducing section to the other load-introducing section.


The improvement in resistance to hydrogen embrittlement is attributed to the fact, without being linked to the invention, that additional binding sites for diffusible hydrogen are created in the component in the microstructure, in particular a heat-treated microstructure of the steel, in particular by precipitate-forming elements such as Al, Cu, Mo, V, Zr, Ti, B with C, N, O, Si and/or due to the microstructure adjusted by heat treatment.


As described above, in a preferred embodiment, the component according to the invention with a steel part is a fastening means. The fastening means according to the invention can in particular be non-positive fastening means, such as screws, bolts or nuts. Force-locking fastening means are characterized in particular by the fact that they have a threaded section for bracing or fastening, in particular with an external thread or an internal thread. For example, the threaded section can therefore be an external thread or an internal thread. Advantageously, this threaded section is incorporated in a component of the fastening means, which is made of steel. Conveniently, the fastener can have a shank area. This shank area can be formed adjacent to the threaded section and/or a drive area, in particular a head, of the fastening means. Preferably, the shank area can be threadless and/or designed as a cylindrical section. The diameter of the shank can be larger, smaller or equal to the thread diameter in the threaded section. The screws are advantageously high-strength or ultra-high-strength screws.


In a particularly preferred embodiment of the invention, the component is a high-strength or ultra-high-strength screw: A high-strength screw is understood to be a screw with a tensile strength of at least 800 MPa. High-strength screws are, for example, screws of strength classes 8.8, 10.9 and 12.9. In particular, the strength classes of the invention correspond to ISO 898-1 in its version valid in January 2021. An ultra-high-strength screw is understood to be a screw with a tensile strength of at least 1200 MPa in particular and/or advantageously at least 1400 MPa. Ultra-high-strength screws are, for example, screws of strength classes 12.8, 12.9, 14.8, 14.9, 15.8, 15.9, 16.8, 16.9, 17.8 and 12.8 U, 12.9 U, 14.8 U, 14.9 U, 15.8 U, 15.9 U, 16.8 U, 17.8 U. A high-strength screw is a screw that is at least high-strength, but it can also be ultra-high-strength. Preferably, it is a high-strength or ultra-high-strength screw with a strength of over 1000 MPa. The screw can have a head with tool-engaging surfaces, wherein these tool-engaging surfaces form an internal or external hexagon in particular.


The invention also relates to a method for manufacturing the component according to the invention. For production, the individual alloying elements are first alloyed to a steel in a known manner. The method according to the invention for manufacturing a component with a steel part comprises the following steps:

    • providing a steel with the composition described above,
    • subsequent forming of a component with a steel part and
    • optional heat treatment.


In a preferred embodiment, the method according to the invention for manufacturing a component with a steel part comprises the steps of:

    • a) providing a steel with the composition described above
    • b) rolling, in particular thermomechanical rolling of the steel
    • c) producing a wire or bar of steel,
    • d) optional GKZ annealing,
    • e) wire drawing,
    • f) forming and
    • g) optional heat treatment.


The above steps are carried out in the order indicated. In each step, the product obtained from the immediately preceding step is processed further.


The preferred method according to the invention has the advantage of a resource-saving and cost-efficient process route, since, for example, a wire rod can be processed directly without the need for GKZ annealing in between. In this way, a ferrite-pearlite microstructure can be achieved in the wire rod state by means of TM rolling (thermomechanical rolling). Preferably, thermomechanical rolling is carried out in step b). Particularly preferred is thermomechanical rolling in which the material is rolled at a final forming temperature in a range of Ar3-50° C. and +100° C., where Ar3 in Fe—C diagram is referred to as the austenite-proeutectoid transformation temperature. A microstructure consisting predominantly of ferrite and pearlite is particularly preferred, especially with an average secondary grain size of 8 or finer according to ASTM E112.


GKZ annealing (annealing to spheroidal cementite) is understood to mean heating with the aim of forming spheroidal cementite. In step d), the optional GKZ annealing, it is preferred that the steel is annealed for 6-10 hours, preferably 7-9 hours, for example 8 hours, at a holding temperature of 700-750° C., for example 735° C. The steel is then preferably cooled to below 100° C., particularly preferably below 50° C., especially to room temperature. Annealing (heating) advantageously produces a microstructure of ferrite and spheroidal cementite.


The forming and/or the optional heat treatment can be followed by further steps, in particular a quenching and tempering step, wherein the known quenching and tempering of steels can be considered. Alternatively, or additionally preferably, a tempering step can also take place during and/or at the same time as the heat treatment step. In other words, quenching and tempering and heating can take place together in one step. The optional heat treatment and/or tempering in step g) is preferably a salt bath tempering, particularly preferably at a temperature of 200-450° C. for 10 minutes to 3 hours.


The microstructure of the steel part after rolling in step b), in particular thermomechanical rolling, and before heat treatment in step f) is predominantly ferritic-pearlitic, bainitic and/or a mixed microstructure. Preferably, the microstructure of the steel is ≥80% by volume, particularly preferably ≥90% by volume ferritic-pearlitic, bainitic and/or a mixed microstructure. After heat treatment, the microstructure of the component in a preferred embodiment is predominantly martensitic and/or bainitic, as described above. In a preferred embodiment, the microstructure of the steel part in the component according to the invention is ≥70% by volume, more preferably ≥80% by volume, particularly preferably ≥90% by volume bainitic or martensitic, as described above. It is further preferred that the microstructure of the steel in the edge region, in particular the region from the surface of the steel part to a depth of 15 μm, preferably up to 12 μm, particularly preferably up to 10 μm, measured perpendicularly from the surface of the steel part, is predominantly ferritic and/or pearlitic, preferably ≥80% by volume, particularly preferably ≥90% by volume ferritic and/or pearlitic. The steel below the above-mentioned depths, i.e. below a depth of 15 μm, preferably below a depth of 12 μm, particularly preferably below a depth of 10 μm, preferably has the microstructure described above, i.e. preferably ≥70% by volume, more preferably ≥80% by volume, particularly preferably ≥90% by volume bainitic or martensitic.


In a preferred embodiment of the invention, the steel part has a Vickers hardness of ≥350 HV 0.3, further preferably ≥400 HV 0.3, particularly preferably ≥430 HV 0.3, especially ≥450 HV 0.3, in the edge region, in particular at a depth of 30-100 μm, preferably 50-150 μm, measured from the surface perpendicular to the surface of the steel part.


In a further preferred embodiment of the invention, the steel part has a Vickers hardness at a depth of 30-100 μm, preferably 40-120 μm, particularly preferably 50-150 μm, measured from the surface perpendicular to the surface of the steel part, which is less than 150 HV 0.3 below the Vickers hardness HV 0.3 of the steel part at a depth of 300-400 μm, in particular at a depth of 400 μm, particularly preferably at a depth of ¼ of the diameter of the steel part. This describes the reduced drop in hardness in the edge area of the steel compared to the core area, which is preferred according to the invention. Further preferably, the steel part has a Vickers hardness at a depth of 30-100 μm, preferably 40-120 μm, particularly preferably 50-150 μm, measured from the surface perpendicular to the surface of the steel part, which is less than 100 HV 0.5, further preferably less than 60 HV 0.5, in particular less than 30 HV 0.5, is below the Vickers hardness HV 0.5 of the steel part at a depth of 300-400 μm, also measured from the surface into the depth of the steel part, perpendicular to the surface of the steel part, in particular at a depth of 400 μm, especially preferably at a depth of ¼ of the diameter of the steel part.


The invention also relates to a component with a steel part, obtainable by the method according to the invention. Advantageously, the component and/or the steel part may also have the aforementioned features with regard to the method.


It is understood that the above-mentioned features and the features to be explained below can be used not only in the combinations indicated, but also in other combinations or on their own, without going beyond the scope of the present invention. The aforementioned advantages of features or of combinations of several features are merely exemplary and can take effect alternatively or cumulatively. The combination of features of different embodiments of the invention or of features of different patent claims is possible in deviation from the selected references of the patent claims.


The invention is explained further below with reference to the figures.



FIG. 1 shows the hardness of the steel as a function of the edge distance, measured perpendicular to the surface, of a screw according to the invention, and it can be seen that there is only a slight drop in hardness.



FIG. 2 shows the hardness profile of the steel in the edge area of a bainitically heat-treated B-alloyed screw, wherein a conventional B-alloyed steel was used. It can be seen that there is a significant drop in hardness in the edge area in conjunction with a greater depth of hardness drop, measured perpendicular to the surface of the screw.


The above example according to FIG. 1 and the comparative example according to FIG. 2 thus show the advantageous effect of the steel composition according to the invention in the component with a steel part.

Claims
  • 1. A component with a steel part, the component comprising: steel having 0.30-0.50 wt. % C,0.05-1.3 wt. % Mn,0.001-0.015 wt. % P,0.001-0.015 wt. % S,0.01-0.8 wt. % Si,0.3-1.5 wt. % Cr,0.005-0.40 wt. % V,0.0008-0.0050 wt. % B,0.02-0.35 wt. % Al,0.0001-0.0200 wt. % N,0.01-0.08 wt. % Ti, and0.0030-0.0800 wt. % Zr;wherein the part of the component has a steel surface and the B content in the steel at a depth of 5-60 μm is ≥80% of the B content in the steel at a depth of 500 μm, the depth being measured perpendicular to the steel surface.
  • 2. The component of claim 1, wherein the B content in the steel at a depth of 5-60 μm is ≥90% of the B content in the steel at a depth of 500 μm.
  • 3. The component of claim 1, wherein the B content in the steel at a depth of 140-220 μm is ≥80% of the B content in the steel at a depth of 500 μm, the depth being measured perpendicular to the steel surface.
  • 4. The component of claim 1, wherein the B content in the steel at a depth of 140-220 μm is ≥90% of the B content of the steel at a depth of 500 μm.
  • 5. The component of claim 1, wherein the steel consists of unavoidable impurity that amounts to ≤0.01 wt. %.
  • 6. The component of claim 1, wherein the steel comprises: 0.30-0.46 wt. % C,0.01-0.60 wt. % Si,0.3-1.3 wt. % Cr,0.005-0.35 wt. % V,0.0012-0.0050 wt. % B,0.02-0.25 wt. % Al,0.0020-0.0150 wt. % N,0.014-0.060 wt. % Ti, and0.0050-0.0500 wt. % Zr.
  • 7. The component of claim 1, wherein the steel comprises: 0.01-0.20 wt. % Mo,0.01-0.50 wt. % Ni,0.01-0.50 wt. % Cu,0.0010-0.0100 wt. % Ca,0.01-0.10 wt. % Bi,0.01-0.30 wt. % Co,0.01-0.06 wt. % Nb,0.01-0.40 wt. % Pb,0.01-0.10 wt. % Se,0.01-0.10 wt. % Te,0.01-0.3 wt. % W,0.01-0.04 wt. % As,0.01-0.02 wt. % Sn,0.01-0.20 wt. % Ta,0.01-0.20 wt. % Ce,0.01-0.50 wt. % Sn,0.01-0.40 wt. % Sb,0.01-0.20 wt. % Hf, and/orone or more lanthanoid(s), each in an amount of 0.01-0.02 wt. %.
  • 8. The component of claim 1, wherein (Zr+Ti+Al)/N is in a range from 2.7 to 150.
  • 9. The component of claim 1, wherein the steel part has a Vickers hardness at a depth of 30-100 μm, preferably 40-120 μm measured from the surface perpendicular to the surface of the steel part, which is less than 150 HV 0.3 below the Vickers hardness HV 0.3 of the steel part at a depth of 400 μm.
  • 10. The component of claim 1, wherein the component is a fastening means selected from the group consisting of screws, nuts, rivets, bolts and chains.
  • 11. The component of claim 1, wherein the steel makes up at least 90 wt. % of the component and/or the component has a tensile strength according to ISO 898-1:2021 of ≥800 MPa.
  • 12. The component of claim 1, wherein the microstructure of the steel is bainitic and/or martensitic after tempering ≥90% by volume.
  • 13. The component of claim 1, wherein the microstructure of the steel without tempering is ≥90% by volume ferritic-pearlitic.
  • 14. A method of manufacturing a component with a steel part, the method comprising the steps of: Providing a steel with a composition having 0.30-0.50 wt. % C, 0.05-1.3 wt. % Mn, 0.001-0.015 wt. % P, 0.001-0.015 wt. % S, 0.01-0.8 wt. % Si, 0.3-1.5 wt. % Cr, 0.005-0.40 wt. % V, 0.0008-0.0050 wt. % B, 0.02-0.35 wt. % Al, 0.0001-0.0200 wt. % N, 0.01-0.08 wt. % Ti, and 0.0030-0.0800 wt. % Zr; andForming at least a part of a component with the steel.
  • 15. The method of claim 14, further comprising the steps of: a) thermomechanical rolling of the steel,b) production of a wire or bar of steel, andc) wire drawing.
  • 16. The method of claim 14, further comprising the step of heat treatment and/or tempering the steel.
  • 17. The method of claim 14, further comprising the step of heating to 700-750° C. for 6-10 hours.
Priority Claims (1)
Number Date Country Kind
21211997.8 Dec 2021 EP regional
CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a U.S. National Stage Application under 35 U.S.C. § 371 of International Application No. PCT/EP2022/084020, filed Dec. 1, 2022, which claims the benefit of and priority to European Patent Application No. EP 21 211 997.8, filed Dec. 2, 2021, the contents of which are hereby incorporated herein by reference in their entireties.

PCT Information
Filing Document Filing Date Country Kind
PCT/EP2022/084020 12/1/2022 WO