SURFACE TREATED STEEL MATERIAL

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to German patent application no. 10 2024 205 368.6 filed on Jun. 11, 2024, and to German patent application no. 10 2024 207 866.2 filed on Aug. 19, 2024, the contents of which are fully incorporated herein by reference.

TECHNICAL FIELD

The present invention generally relates to a steel material for engineering components, a bearing or a bearing component composed of such a steel material, as well as to a method for manufacturing an engineering component, such as a bearing or a bearing component, using the steel material.

BACKGROUND OF THE INVENTION

The majority of engineering steels for engineering components, in particular those used for bearing components, including through-hardened and case-hardened variants, are non-stainless and therefore not very resistant to corrosion or fatigue. One way to improve corrosion-resistance and fatigue-resistance properties is to use stainless steels. However, stainless steel is costly to produce.

Recently, an increasing amount of steel is being produced from scrap steel. Producing steel from scrap metal instead of mining fresh ore is more environmentally friendly as it significantly reduces energy consumption, e.g., by about 60-75%, thereby reducing carbon dioxide emissions and the negative impacts of mining. Additionally, recycling scrap metal repurposes waste materials, preventing them from ending up in landfills and thereby conserving natural resources and reducing environmental pollution.

However, the use of steel scrap may introduce tramp elements such as copper into the recycled steel material. But, the copper content in engineering steels is often required to be minimal. Thus, only iron ore or high-quality steel scrap having a low copper content typically has been used in the past for the production of steel materials having a low copper content suitable for many engineering applications.

Furthermore, the surface hardness of the finished component is in many applications crucial for reliable operation of engineering components. One such example is bearing components, such as an inner ring, an outer ring or rolling elements of a bearing. Such components have high demands on corrosion-resistant and fatigue-resistant properties, as well as surface hardness. At the same time, there is a general desire to keep material costs and manufacturing costs low.

Consequently, there is a need for a steel material for engineering components which is cost-efficient, has good corrosion-resistance and fatigue-resistance properties, and is environmentally friendly.

SUMMARY

In view of the above, it is one non-limiting object of the present disclosure to disclose techniques for achieving a steel material for an engineering component that alleviates at least one of the above-mentioned drawbacks of the prior art. For example, it would be preferable to prepare a steel material that has improved fatigue-resistance and/or corrosion-resistance properties while also allowing for a relatively high copper content; e.g., the steel material may originate or be produced from recycled (scrap) steel. Such a steel material may be advantageously utilized to make bearing components.

According to a first aspect of the present teachings, a steel material for an engineering component has an alloy composition that comprises at least 0.3 wt % of copper (Cu) and preferably has a higher surface carbon content than its core carbon content and/or a higher surface nitrogen content than its core nitrogen content.

Such a steel material is capable of tolerating the inclusion of significant amounts of copper, thereby increasing the fatigue and corrosion properties of the steel material and at the same time providing a steel material that has a sufficient surface hardness.

In an embodiment of the present teachings, the surface carbon content and/or the nitrogen content may be at least 100% higher, or at least one 200% higher, or at least 300% higher, or at least 400% higher, or at least 600% higher, or at least 700% higher than the core carbon content and/or the core nitrogen content, respectively. When using such a steel material, the hardness of the surface of the engineering component can be increased.

In another embodiment of the present teachings, the steel material may have a surface carbon content of 0.58 wt % or more, preferably 0.7 wt % or more, and/or a surface nitrogen content of 0.1 wt % or more, preferably 0.2 wt % or more.

A surface carbon content of at least 0.58% is useful for obtaining a surface hardness of approximately 600 HV1. By further increasing the carbon content to 0.7%, a surface hardness of over 700 HV1 may be achieved.

In another embodiment of the present teachings, the alloy composition (steel material) may comprise or optionally consist of (in wt %):

	Cu	0.3-1.75
	C	<0.3
	Si	<0.4
	Mn	0.4-1.5
	Cr	0.4-1.8
	Ni	0.2-3.75

	Mo	<0.65
	Al	0.01-0.05
	Nb	<0.03
	V	<0.08
	Ti	<0.03
	B	<0.004
	N	<0.015
	P	<0.025
	S	<0.015

the balance iron (and unavoidable impurities).

In another embodiment of the present teachings, the higher surface carbon content and/or the higher surface nitrogen content may be created by a thermochemical surface enrichment treatment, such as carbonitriding, nitriding, case carburizing and/or nitrocarburizing.

In a second aspect of the present teachings, the steel material according to the first aspect may be used to form a bearing or bearing component.

By using such a steel material in a bearing, bearing components having increased corrosion and fatigue properties as well as high surface hardness may be obtained. Furthermore, such a bearing or bearing component may be environmentally friendly because it can be produced from recycled steel scrap.

In a third aspect according to the present teachings, a method of manufacturing an engineering component may comprise:

• • forming a portion of a steel material into a rough geometry of the intended engineering component, wherein the steel material has an alloy composition comprising at least 0.3 wt % of copper (Cu), and
  • subjecting the formed portion of the steel material to a thermochemical surface enrichment treatment, such that the formed portion of the steel material has a higher surface carbon content compared to its core carbon content and/or a higher surface nitrogen content compared to its core nitrogen content.

The advantages of the third aspect largely correspond to the advantages of the first aspect.

In another embodiment of the present teachings, the thermochemical surface enrichment treatment may comprise one or more of carbonitriding, case carburizing and/or nitrocarburizing.

In another embodiment of the present teachings, subjecting the formed portion of steel material to a thermochemical surface enrichment treatment may comprise:

• • carburizing the steel material during a predetermined period of time at a temperature of at least 850° C. at a carbon potential higher than 0.8%;
  • ramping down the carbon potential;
  • ramping down the temperature to <800° C.; and
  • direct quenching the steel material.

BRIEF DESCRIPTION OF THE DRAWINGS

Further details concerning additional embodiments of the present teachings will be described below with reference to the drawings, in which:

FIG. 1 shows the carbon content in a surface region of the steel material according to one embodiment of the present teachings;

FIG. 2 shows the hardness HV1 in a surface region of the disclosed steel material according to an embodiment;

FIG. 3 shows a flow chart of a method for manufacturing an engineering component according to another embodiment of the present teachings; and

FIG. 4 shows an exemplary bearing according to the present teachings.

The drawings show diagrammatic exemplifying embodiments of the present disclosure and are thus not necessarily drawn to scale. It shall be understood that the embodiments shown and described are exemplifying and that the disclosure is not limited to these embodiments. It shall also be noted that some details in the drawings may be exaggerated in order to better describe and illustrate the disclosure. Like reference characters refer to like elements throughout the description, unless expressed otherwise. Some of the reference characters in some of the drawings may have been omitted for the sake of clarity.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

When copper is added as an alloying element in a steel material, it provides advantages such as increased strength due to precipitation of copper. Copper addition in a mild steel also improves its resistance to atmospheric corrosion. Copper in solid solution of an iron matrix has been reported to increase the resistance of the steel to fatigue crack growth and to increase its ability to suppress hydrogen induced cracking. However, the strength level even with other alloying elements still cannot satisfy the requirements for roller bearing applications. Therefore, it has been determined that steels containing significant amounts of copper require some type of surface treatment to be performed, in order to produce suitable engineering components such as roller bearing components.

Thus, disclosed herein is a steel material for an engineering component having improved corrosion and fatigue properties as compared to non-stainless steels. Also, steel materials according to the present teachings enable an increased use of recycled steel scrap for the production thereof. More specifically, steel materials for an engineering component according to the present teachings have an alloy composition comprising at least 0.3 wt % of copper. Further, the steel material has a higher surface carbon content as compared to its core carbon content and/or a higher surface nitrogen content compared to its core nitrogen content. Herein, the term “surface content of carbon or nitrogen” refers to a layer close to the surface of the material that may have a thickness of approximately 1-2 mm from the outer surface of the material. In addition, the term “core content of carbon or nitrogen” refers to a core region farther away from the surface, for example more than 2 mm from the surface of the steel material (or engineering component).

The higher surface carbon/nitrogen content may be achieved by utilizing a thermochemical surface treatment. Non-limiting examples of surface treatments suitable for use with the present teachings are carbonitriding, nitriding, case carburizing and nitrocarburizing. During the thermochemical surface treatment, the steel material is exposed to a carbon and/or nitrogen atmosphere at high temperatures, e.g., above 850° C. Thereby, carbon and/or nitrogen diffuses into the surface of the steel material. An increased surface content of carbon and/or nitrogen increases the surface hardness of the material.

In a preferred embodiment, the alloy composition of the steel material may preferably comprise one or more (or all) of the following elements in the concentration ranges shown in Table 1 below.

	TABLE 1
	Element	wt %
	Cu	0.3-1.75
	C	<0.3
	Si	<0.4
	Mn	0.4-1.5
	Cr	0.4-1.8
	Ni	0.2-3.75

	Mo	<0.65
	Al	0.01-0.05
	Nb	<0.03
	V	<0.08
	Ti	<0.03
	B	<0.004
	N	<0.015
	P	<0.025
	S	<0.015
	Fe	balance (and unavoidable impurities)

In another preferred embodiment, the alloy composition of the steel material consists of the elements in the concentration ranges shown in Table 1 above.

The nickel (Ni) content may be chosen to be in a ratio to the copper content (i.e. Cu:Ni) within the range of 1.5:1 to 3:1, in order to mitigate the problem of red hot shortness.

FIGS. 1 and 2 respectively show examples of the carbon content and the corresponding hardness in a surface region, i.e. a layer, of a steel material after it has been case carburized. The steel material used in this example had a chemical composition (in wt %) consisting of 0.093 C, 0.2 Si, 1.11 Mn, 0.036 Al, 0.001 Nb, 0.012 V, 0.028 Ti, 0.4 Cu, 1.12 Cr, 0.2 Ni, 0.007 Mo, and 0.002 B, the balance being iron and unavoidable impurities. In the example, as can be seen in FIG. 1, the carbon content at the surface of the material exceeded 0.7 wt %, i.e. it is more than 700% higher than the core carbon content (0.093 wt %).

As can be seen in FIG. 1, the corresponding surface hardness HV1 exceeds 700 HV1, which is typically suitable for engineering components, such as roller bearing components, such as e.g. an outer ring, an inner ring or rolling elements of a roller bearing. Due to the relatively high copper content (0.4 wt % in this example), the material has improved corrosion and fatigue properties and is thus particularly suitable for engineering components, such as bearing components.

However, lower or higher surface carbon contents may be suitable for specific applications. For example, the surface carbon content may be 0.58 wt % or more, which may result in an approximate hardness of around 650 HV1. In the case of a higher nitrogen content at the surface (not shown in FIGS. 1-2), the surface nitrogen content may be, e.g., greater than 0.1 wt % or greater than 0.2 wt %.

Due to the copper content being greater than 0.3 wt % and the thermochemical surface treatment, the steel material may be compliant with the following limits on micro inclusion ratings of steel heats, when assessed according to ISO 4967 method A as shown in Table 2 below.

	TABLE 2
	Inclusion type	Thin	Heavy

	A	2.5	1.5
	B	2.0	1.0
	C	0.5	0.5
	D	1.5	1.0

	DS	2.0

FIG. 3 shows a flow chart of an exemplary method for manufacturing an engineering component according to the present teachings. The exemplary method comprises the following steps S1 and S2.

Step S1: forming a portion of a steel material into a rough geometry of the intended engineering component, wherein the steel material has an alloy composition comprising at least 0.3 wt % of copper (Cu).

By choosing a steel material having a copper content of at least 0.3 wt %, the corrosion and fatigue properties of the material are increased.

Step S2: subjecting the formed portion of steel material to a thermochemical surface enrichment treatment, such that the formed portion of the steel material has a higher surface carbon content compared to its core carbon content and/or a higher surface nitrogen content compared to its core nitrogen content.

This thermochemical surface enrichment treatment increases the surface hardness of the material.

As was mentioned above, the thermochemical surface treatment may be any suitable thermochemical surface treatment having the aim of increasing the surface hardness of the material, such as carbonitriding, nitriding, case carburizing and nitrocarburizing.

For example, if the formed portion of the steel material is subjected to case carburizing, step S2 may comprise the following sub-steps:

• • S2a: carburizing the steel material during a predetermined period of time at a temperature of at least 850° C. at a carbon potential higher than 0.8%;
  • S2b: ramping down the carbon potential;
  • S2c: ramping down the temperature to <800° C.; and
  • S2d: direct quenching the steel material.

For example, a more specific example of a case carburizing process according to the present teachings is as follows:

• • carburizing for 4 hours at 930° C. and a carbon potential of 1.1%,
  • ramping down the carbon potential from 1.1% to 0.5% over 30 minutes,
  • dwelling 1.5 hours,
  • ramping down the temperature to 880° C.,
  • direct quenching in 100° C. oil and stirring for 15 min, then air cooling for 5 minutes,
  • post-quenching in cold water for 15 minutes, and
  • tempering at 160° C. for 1.5 hours.

After the thermochemical surface treatment, the steel material may optionally be hard machined (S3) and then subjected to a second thermochemical surface treatment (S4), e.g. nitrocarburizing. The corrosion resistance of the surface may be further increased by such processes.

With reference to FIG. 4, a bearing 1 according to the present teachings may include an inner ring 2, an outer ring 3, and a plurality of rolling elements 4, such as balls, needles, cylinders, etc. A steel material according to the present teachings may be used to form one or more of the inner ring 2, the outer ring 3 and/or the rolling elements 4. Then, although the entire surface of the inner ring 2 and/or the entire surface of the outer ring 3 may be subjected to the thermochemical surface treatment, in some embodiments of the present teachings, it is sufficient if only the outer surface (layer) 5 of the inner ring 2 and/or the inner surface (layer) 6 of the outer ring 3 is subjected to the thermochemical surface treatment.

Representative, non-limiting examples of the present invention were described above in detail with reference to the attached drawings. This detailed description is merely intended to teach a person of skill in the art further details for practicing preferred aspects of the present teachings and is not intended to limit the scope of the invention. Furthermore, each of the additional features and teachings disclosed above may be utilized separately or in conjunction with other features and teachings to provide improved steel materials and improved bearing components made of such improved steel materials, as well as methods of making and using the same.

Moreover, combinations of features and steps disclosed in the above detailed description may not be necessary to practice the invention in the broadest sense, and are instead taught merely to particularly describe representative examples of the invention. Furthermore, various features of the above-described representative examples, as well as the various independent and dependent claims below, may be combined in ways that are not specifically and explicitly enumerated in order to provide additional useful embodiments of the present teachings.

All features disclosed in the description and/or the claims are intended to be disclosed separately and independently from each other for the purpose of original written disclosure, as well as for the purpose of restricting the claimed subject matter, independent of the compositions of the features in the embodiments and/or the claims. In addition, all value ranges or indications of groups of entities are intended to disclose every possible intermediate value or intermediate entity for the purpose of original written disclosure, as well as for the purpose of restricting the claimed subject matter.