EP3853389A1

EP3853389A1 - Steel for surface hardening with high edge hardness and with a fine ductile core structure

Info

Publication number: EP3853389A1
Application number: EP19773026.0A
Authority: EP
Inventors: Serosh Engineer
Original assignee: Ezm Edelstahlzieherei Mark GmbH
Current assignee: Ezm Edelstahlzieherei Mark GmbH
Priority date: 2018-09-18
Filing date: 2019-09-17
Publication date: 2021-07-28
Also published as: US20220074034A1; CN112714799A; WO2020058269A1

Abstract

The invention relates to a steel, which during surface hardening without subsequent expansion annealing not only has the potential to develop a hardened surface layer having a high surface, in particular amounting to more than 820 HV1, but also has a viscous, fine-grained core structure and simultaneously has good weldability. For this purpose, a steel according to the invention consists of (in % by weight) C: 0.10 - 0.19%, Si: ≤ 0.15%, Mn: ≤ 1.0%, P: ≤ 0.015%, S: ≤ 0.015%, Cr: 0.2 - 1.0%, Ni: 0.7 - 2.0%, Mo: 0.5 - 1.0%, N: ≤ 0.015%, AI: 0.010 - 0.060%, Cu: ≤ 0.20%, B: ≤ 0.005%, and also in each case optionally one or more elements from the group "W, Ti, Nb, V, Ta" in contents according to the following proportions W: 0.15 - 0.65 %, Ti: 0.01 - 0.04 %, Nb: 0.015 - 0.05 %, Ta: 0.01 - 0.04 %, V: 0.04 - 0.12 %, and the remainder consists of iron and unavoidable contaminants.

Description

Steel for surface hardening with high surface hardness and with one

fine ductile core structure

The invention relates to a steel which is suitable for surface hardening and has a high hardness on the surface and a high

Depth of hardening enables.

In particular, the invention relates to a steel that has the potential to

Development of a hardened surface layer with a surface hardness of more than 820 HV1 in accordance with DIN EN ISO 2639 and at the same time has a fine core structure bearing the hardened surface layer, which is characterized by high toughness and thus enables the steel to withstand high sudden stresses.

Procedures for case hardening are explained in leaflet 452, "Case hardening", edition 2008, ISSN 0175-2006, published by the Steel Information Center. In so-called "direct hardening", the steel is heated to a carburizing temperature, which is usually between 900 and 1050 ° C, and then cooled down quickly. In the case of gas carburizing under vacuum, the material is cooled down by blowing in helium at high pressure. Relaxation annealing often follows, which serves to reduce internal stresses and increase ductility.

Another process for hardening the surface layer, in which the hardness of the surface layer is also increased by carburizing, is the so-called "single hardening". In this process, the material is heated again to a temperature after cooling from the operating heat Edge carbon content is adjusted to be quickly cooled afterwards.

In practice, "direct hardening" is increasingly used because it is the more economical process. The aim is to save on the relaxation annealing that is usually done after hardening.

However, direct hardening without subsequent stress relief annealing places higher demands on the material than single hardening. In order to obtain the highest possible hardness, the chemical composition of the material for the direct hardening carried out without subsequent stress relief annealing must be coordinated so that the residual austenite content remaining in the hardened zone is as low as possible. At the same time, the core structure of the steel must remain very ductile or tough to withstand sudden stresses. Furthermore, the chemical

Composition of the material also have sufficient hardenability.

In today's practice, case-hardened components are manufactured, for example, from the steel known under the name "14CrNiMo5", which according to the directional analysis consists of (in% by weight) 0.12% C, 1.40% Cr, 0.30% Mo and 1 , 6% Ni, balance iron and unavoidable impurities. However, this steel does not achieve the required high surface hardness with the necessary certainty after direct hardening.

The German standard DIN 17115 (current edition 2012 - 07) deals with the technical delivery conditions for steels for welded

Round steel chains and individual chain parts. Among other things, this standard also lists stainless steels for tempering treatment. The steel 15CrNi6 (material no.1.5919: (content in% by weight) C: 0.12 - 0.18, Si: <0.25, Mn: 0.40 - 0.70, P: <0.020, S: <0.015, Cr: 1.35-1.65,

Ni: 1.35-1.65, N: <0.012, Cu: <0.020; Rest Fe) is a material that is used for the production of case-hardened round steel chains can be used. But even this steel does not achieve the minimum surface hardness of> 820 HV1 after direct hardening. The materials listed in the delivery conditions, such as 20MnCrMo3-2 (1.6522: (content in% by weight) C: 0.17 - 0.23, Si: <0.40, Mn: 0.60 - 0.95, P <0.025, S: <0.015, Cr: 0.35-0.65, Mo: 0.15-0.25, No: 0.40-0.70, AI: <0.050, Cu: <0.30 , Rest Fe), 23MnNiCrMo5-3 (1.6540:

(Content in% by weight) C: 0.20-0.26, Si: <0.25, Mn: 1.1-1.40, P: <0.020, S: <0.015, Cr: 0.4 - 0.6, Mo: 0.20 - 0.30, Ni: 0.70 - 0.90, N: <0.012, AI: 0.025 - 0.50, Cu: <0.20, balance Fe) or 23MnNiCrMo5 -4

(1.6758: (content in% by weight) C: 0.20-0.26, Si: 0.25, Mn: 1, 10-1.40, P: <0.020, S: <0.015, Cr: 0 , 40 - 0.60, Ni: 0.90 - 1, 10, N: <0.012, AI: 0.025 - 0.050, Cu: 0.20, rest Fe) are more suitable for chains that are used in tempered condition.

EP 1 905 857 B1 is another high-strength steel with (in% by weight) C: 0.15 - 0.3%, Si: 0.1 - 0.5%, Mn: 0.6 - 1, 8%, Cr: 1, 0 - 1, 8%, Mo: 0.10 - 0.50%, Ni: up to 0.50%, Nb: 0.030 - 0.150%, Ti: 0.020 - 0.060%, AI: 0.010 - 0.060%, N: 0.008 - 0.030%, P: <0.030%, S: <0.030%, balance iron and unavoidable impurities known. This steel also does not achieve the required high surface hardness of more than 820 HV1 during direct hardening from the case heat. In addition, the tests show that the hardness of the core structure at 425 HV is too high and, as a result, the toughness of the core structure is too low to withstand high sudden break stresses.

Against this background, the task arose to develop a steel that can be used for surface hardening without a subsequent one

Relaxation annealing not only has the potential to develop a hardened surface layer with a high surface hardness, in particular more than 820 HV1, but also has a tough, fine-grained core structure and at the same time is easy to weld. Its properties are intended to make a steel according to the invention particularly suitable for the production of chains and individual chain parts for areas of application of the type described in DIN 17115.

The invention has achieved this object by means of a steel having the composition specified in claim 1.

A steel according to the invention which meets the above-mentioned requirements accordingly consists of (in% by weight)

C: 0.10 - 0.19%

Si: <0.15%

Mn: <1.0%

P: <0.015%

S: <0.015%

Cr: 0.2-1.0%

Ni: 0.7 - 2.0%

Mo: 0.5-1.0%

N: <0.015%

AI: 0.010 - 0.060%

Cu: <0.20%

B: <0.005%

as well as optionally one or more elements from the group "W, Ti, Nb, V, Ta" in contents in accordance with the following stipulations

W: 0.15 - 0.65%

Ti: 0.01 - 0.04%

Nb: 0.015 - 0.05%

Ta: 0.01 - 0.04%

V: 0.04 - 0.12%

Rest of iron and unavoidable impurities.

The steel according to the invention is composed such that it

required high hardness of more than 820 HV1 on the surface by a Surface hardening, which is carried out, for example, as case hardening, nitriding or nitrocarburizing, is achieved.

At the same time, even in a large hardening depth, there is still a high hardness, as is regularly required when processing the steel according to the invention into heavy chains and individual parts of chains

for example as conveyor or transport chains in mining, in

Mechanical engineering or the like should be used.

Thus, when hardening the surface layer of a sample rod formed from a steel according to the invention, a hardening depth of 0.30-0.45 mm is achieved

Rod dimensions up to 45 mm in diameter achieved with a hardness of 550 HV.

After the surface layer hardening, the steel according to the invention typically has a structure in the core that essentially consists of

fine-grained, ductile martensite and bainite and is characterized by a grain size of 6 and finer, determined according to the ASTM E112 standard.

A steel according to the invention has good weldability. For example, it can be used for the manufacture of chain links

Flash butt welding or butt welding.

To develop the required hardness on the surface of the hardened surface layer, at least 0.10% by weight of carbon ("C") is required. Tests have shown that low C contents of up to 0.13% by weight are favorable for many uses of the steel according to the invention. In other applications, the results are optimal

Presence of C in the steel according to the invention if the C content is at least 0.12% by weight. If the C content is above 0.19% by weight, there is a risk that the core hardness of the steel will increase too much and the ductility or toughness of the core structure is impaired. Furthermore, the limitation to at most 0.19% by weight of C, in particular less than 0.19% by weight of C, contributes to the good weldability of the steel. An optimized embodiment of the invention provides that the C content is limited to at most 0.16% by weight, in particular at most 0.13% by weight, in order to minimize any negative influence of C on the properties of the steel.

The silicon ("Si") content of a steel according to the invention should be as low as possible, since silicon leads to a solidification of the structure. Such an increase in hardening proves to be particularly harmful if no stress relief annealing is carried out after the surface hardening.

Manganese ("Mn") can be added to the steel according to the invention to increase its hardenability. Since Mn lowers the transition temperatures into the ferrite / pearlite and martensite stage, the Mn content is

according to the invention limited to a maximum of 1.0% by weight. In order to safely use the advantageous effects of Mn in a steel according to the invention, its Mn content can be increased to at least 0.7% by weight of Mn.

The content of phosphorus ("P") is kept as low as possible in a steel according to the invention, since P causes a strong hardening of the structure after cooling from the case hardening heat in the case of the surface layer hardening. To prevent this, the P content of the steel according to the invention is limited to at most 0.015% by weight, in particular less than 0.015% by weight.

The sulfur (“S”) content of a steel according to the invention should also be set as low as possible, since S reduces the ductility or toughness of the structure. To prevent this, the S content of the Steel according to the invention is limited to at most 0.015% by weight, in particular less than 0.015% by weight.

The chromium ("Cr") content in the steel according to the invention is limited to a maximum of 1.0% by weight in order to ensure a low residual austenite content in the hardened outer layer after hardening the outer layer. This effect can be achieved particularly reliably if the Cr content of the steel is limited to at most 0.6% by weight, in particular 0.5% by weight. The loss of hardenability of the steel to be expected as a result of the low Cr content is compensated for in the steel according to the invention by adjusting the contents of the other alloying elements. At the same time, at least 0.2% by weight, in particular at least 0.3% by weight or at least 0.4% by weight, of Cr is provided in the steel in order to make use of the contribution to hardness that Cr can make.

Nickel ("Ni") in the contents of 0.7-2.0% by weight provided according to the invention contributes to increasing the hardenability and increasing the ductility or toughness. In connection with their lower carbon contents, a martensitic structure with a slight distortion of the hardening structure is sometimes achieved in the steels according to the invention. However, the Ni contents, like the Mn contents, must be within certain limits, since Ni and Mn shift the transition points to lower temperatures. The martensite start temperature (Ms temperature) can be adjusted according to that of Dr. Helmut Brandis in Thyssen Edelstahl -Technischeberichte 1, volume 1975, issue 1, page 8 -10, specified formula. In the case of steel according to the invention, it is approximately 440 ° C. and is therefore still so high that the converted martensite / bainite can self-start during cooling from the carburizing heat. This effect increases ductility or toughness as well as the fatigue strength of the converted hardening structure and reduces warpage on the component. At the same time, it is ensured with the Ni contents according to the invention that the hardness of the core structure remains low, so that the Steel, even in the surface hardened state, high abrupt

Can withstand stress. This property makes the

Steel according to the invention is particularly suitable for the components which, after cooling from the heat of hardening, are not subjected to stress relief annealing. By increasing the Ni content to at least 0.9% by weight, in particular at least 1.5% by weight, the advantageous effects of Ni can be used particularly safely. Limiting the Ni content to at most 1.8% by weight, on the other hand, particularly reliably prevents negative influences of the presence of Ni.

Molybdenum ("Mo") hardly changes the transition temperatures, but increases the conversion to the bainite stage after cooling from the heat of the heat treatment carried out for surface hardening. A fine-grained hardening structure made of bainite increases ductility and toughness in the

Core area of the component formed from steel according to the invention due to the lower distortion of the structure. Molybdenum also improves the wear behavior of the hardened surface layer. The positive effects of Mo on the steel according to the invention can be exploited by Mo contents of at least 0.5% by weight. In contrast, negative influences of the presence of Mo in the steel according to the invention are excluded by the fact that the Mo content is at most 1.0% by weight,

in particular at most 0.9% by weight or at most 0.65% by weight.

Nitrogen ("N") should be as low as possible in the steel according to the invention in order to be able to optimally use the hardness-increasing effect of the boron content provided according to the invention in the steel according to the invention. The N content of a steel according to the invention is therefore limited to at most 0.015% by weight, in particular at most 0.010% by weight. To in the case of his

Presence in the steel according to the invention to ensure the full hardenability-increasing effect of boron, the N content can be achieved by alloying

Micro alloy elements such as Al and Ti are bound. Aluminum ("AI") is used in steel production for deoxidation. For this purpose, Al contents of 0.010-0.060% by weight, in particular at least 0.015% by weight or at most 0.040% by weight, are required and can simultaneously be used to set excess nitrogen and to increase the fine grain size.

Copper ("Cu") is an undesirable accompanying element that occurs in the

Melting of the steel via the scrap used in the process

Melt arrives. In order to avoid negative influences of Cu on the properties of the steel according to the invention, the Cu content is limited to a maximum of 0.20% by weight.

The boron (“B”) content optionally provided according to the invention also serves to increase the hardenability. In order for the B content to be effective, the N content must be as low as possible and the nitrogen present in the steel must be bound by aluminum or other elements, such as, for example, the optionally added titanium, niobium or vanadium. The positive influences of B can be used particularly safely if the B content is at least 0.001% by weight, in particular at least 0.002% by weight. The B content is limited to a maximum of 0.005% by weight in order to prevent the formation of boron-containing precipitates

Avoid grain boundaries.

To increase the fine grain of the structure, which is essential for ductility or

Toughness of a component formed from a steel according to the invention is of great importance, elements such as niobium ("Nb"), tantalum ("Ta"), vanadium ("V"), titanium ("Ti") or tungsten ("W. ") - alone or in combination. If a particularly fine-grained structure is to be secured, at least one of the elements W, Ti, Nb, Ta or V is accordingly present in the steel according to the invention in accordance with the invention, a combination of those in Elements W, Nb and V added according to the invention have proven to be particularly practical.

The optionally added Nb contents are 0.015-0.05% by weight, in particular 0.015-0.03% by weight.

The optionally added Ti contents are 0.01-0.04% by weight, in particular 0.015-0.035% by weight.

The optionally added V contents are 0.04-0.12% by weight, in particular 0.05-0.12% by weight or 0.08-0.12% by weight.

If Nb and V are added at the same time, their contents are optimally 0.015-0.03% by weight of Nb and 0.08-0.12% by weight of V in order to use the combined effect of their presence particularly effectively.

The optionally added contents of Ta are 0.01-0.04 weight percent.

The optionally added W contents can be 0.15-0.65% by weight, in particular 0.15-0.35% by weight. The addition of W not only has a refining effect, but also a higher wear resistance and a greater hardening depth after carburizing.

The content of all other elements not mentioned here that can occur in a steel according to the invention are to be attributed to the impurities and their content is limited so that they have no influence on the properties of the steel according to the invention. For a steel according to the invention, the contents of the

Contamination limited according to DIN 17 115 (edition 2012 - 07).

Carburizing processes such as those mentioned above are particularly suitable for the surface hardening of a steel according to the invention

Leaflet 452 are explained. The steel according to the invention can be used in particular for the production of case-hardened gear parts and other case-hardened parts

Components, e.g. of high-strength, weldable round steel chains. No subsequent stress relief annealing is required to ensure the required ductility of the material in the respective component. Steel according to the invention is particularly suitable for the production of case-hardened components which require a high surface hardness and high ductility in the core area of the components. Examples of this are the already mentioned round steel chains and their individual parts, if these are particularly suitable for use as conveyor chains in cement production, in mining or in the processing of coal, especially in the case of

Coal gasification are provided.

Even components that are used for other surface treatments

Surface hardening such as e.g. nitriding or

Nitrocarburizing can be produced from the steel according to the invention, with no relaxation annealing after the

Surface hardening must be carried out.

The invention is particularly suitable for the production of heavy

Case-hardened round steel chains for use in mining, in the cement industry or in coal gasification.

Highly wear-resistant and resilient drive chains for vehicles, in particular motor vehicles, for motorcycles and bicycles can also be produced from steel according to the invention.

The surface hardness of the steel can be brought to values of more than 820 HV1 by cooling the component formed from the steel according to the invention and hardened by case hardening in oil or helium after hardening. In the core structure consisting of bainite and martensite, such a surface layer hardened from existing steel sample rod with a

Diameters of up to 45 mm typically have a hardness of 200-350 HV in this case. The hardening depth for bars up to 45 mm in diameter is 0.30 - 0.45 mm.

Accordingly, a steel according to the invention has a hardness of over 820 HV1, for example, in the carburized surface layer without subsequent relaxation. With a hardening depth of 0.12% of the diameter of the particular rod-shaped component, there is still a hardness of at least 550 HV.

With chain links formed from steel according to the invention, which are surface hardened by case hardening and have been cooled in oil or helium after surface hardening in a vacuum, a breaking stress of at least 440 MPa can be achieved in a chain test carried out in accordance with DIN 22252.

A steel according to the invention, after blind hardening, i.e. case hardening, in which the steel is heated to the hardening temperature without carburizing, one determined according to the standard DIN EN ISO 148-1

Impact energy of at least 50 joules, in particular at least 90 joules, whereby impact energy values of 110-130 joules are regularly achieved.

The invention is explained below using an exemplary embodiment.

In order to check the properties of a steel according to the invention, a hot-rolled bar steel with a circular diameter of 30 mm, that is to say a diameter which is representative of chain links or the like, was produced in a conventional manner and has the composition given in Table 1 which meets the requirements of the invention had (data in% by weight, rest iron and unavoidable

Impurities):

Table 1.

The following properties arise on the steel bars:

After direct case hardening at 950 ° C followed by cooling in oil, the rod has a fine-grained structure of at least ASTM 6 in the case-hardened surface layer with a surface hardness of 840 HV1.

A hardness of 30HRc is achieved in the core of the sample, whereas a hardness of 560 HV is present at a hardening depth of 0.38 mm.

The composition of the steel examined here ensures such high ductility of the material that it is not necessary to subsequently relax it to adjust the mechanical properties. This results in impact energy values with an ISO-V notch of 110 to 130 joules for the steel bars examined.

The steel according to the invention achieves a tensile strength of 985 MPa determined in accordance with DIN EN ISO 17022-3 in the “blind-hardened state” (ie hardening of 950 ° C. with subsequent cooling in oil, no carburization). These determined properties make the steel according to the invention particularly suitable for the production of round steel chains, the one

Reach tensile strength of more than 440 MPa in the chain test according to DIN 22252.

Claims

PATENT CLAIMS

1. Steel with (in% by weight)

C: 0.10 - 0.19%,

Si: <0.15%,

Mn: <1.0%,

P: <0.015%,

S: <0.015%,

Cr: 0.2-1.0%,

Ni: 0.7 - 2.0%,

Mo: 0.5 - 1.0%,

N: <0.015%,

AI: 0.010 - 0.060%,

Cu: <0.20%,

B: <0.005%

as well as optionally one or more elements from the group "W, Ti, Nb, V, Ta" in contents in accordance with the following provisions W: 0.15 - 0.65%,

Ti: 0.01-0.04%,

Nb: 0.015 - 0.05%,

Ta: 0.01-0.04%,

V: 0.04 - 0.12%,

Rest of iron and unavoidable impurities.

2. Steel according to claim 1, characterized in that its C content is at most 0.13% by weight.

3. Steel according to one of the preceding claims, characterized

characterized in that its Cr content is at least 0.3% by weight.

4. Steel according to one of the preceding claims, characterized

characterized in that its Cr content is at most 0.5% by weight.

5. Steel according to one of the preceding claims, characterized

characterized in that its Ni content is at least 1.5% by weight.

6. Steel according to one of the preceding claims, characterized

characterized in that its Mo content is at most 0.65% by weight.

7. Steel according to one of the preceding claims, characterized

characterized that its W content is at most 0.35 wt .-%.

8. Steel according to one of the preceding claims, characterized

characterized in that its Nb content is at most 0.03% by weight.

9. Steel according to one of the preceding claims, characterized

characterized in that its V content is at least 0.08% by weight.

10. Steel according to one of the preceding claims, characterized

characterized that its V content is at most 0.12% by weight.

11. Steel according to one of the preceding claims, characterized

characterized in that it has a surface hardness of at least 820 HV1 in a carburized surface layer without subsequent relaxation, and a hardness depth of 0.12% of the diameter of the respective component has a hardness of at least 550 HV in the case-hardened state.

12. Steel according to one of the preceding claims, characterized

characterized that it has a tensile strength of at least 950 MPa in the blind hardened state.

13. Steel according to one of the preceding claims, characterized

characterized that in the blind-hardened state notched impact work with an ISO-V notch of over 90 joules is achieved.

14. Steel according to one of the preceding claims, characterized

characterized that he was in the chain test of

Case-hardened round steel chains according to DIN 22252 have a breaking stress of at least 440 MPa.

15. Use of one according to one of the preceding claims

trained steel for the manufacture of case hardened

Round steel chains and their individual parts.