WO2021224423A1

WO2021224423A1 - A new bainitic steel

Info

Publication number: WO2021224423A1
Application number: PCT/EP2021/062067
Authority: WO
Inventors: Jan HARALDSSON; Lars NYLÖF; Sofia WESTHOLM
Original assignee: Sandvik Materials Technology Rock Drill Steel Ab
Priority date: 2020-05-06
Filing date: 2021-05-06
Publication date: 2021-11-11
Also published as: CN115552051A; EP4146838A1; BR112022022553A2; JP2023525516A

Abstract

The present disclosure relates to a new bainitic steel to be used for manufacturing a drill component, such as a drill rod, or in any other components wherein such steels are useful. The present disclosure further relates to a drill component comprising the bainitic steel. The bainitic steel comprises the following composition in weight %: C 0.33 to 0.40; Si 0.60 to 1.45; Mn 0.25 to ≤ 0.80; P ≤ 0.03; S ≤ 0.03; Cr 1.00 to 1.50; Ni 0.10 to 0.60; Mo 0.40 to 0.80; N ≤ 0.020; Al ≤ 0.05; balance Fe and unavoidable impurities.

Description

A new bainitic steel

TECHNICAL FIELD

The present disclosure relates to a new bainitic steel to be used for manufacturing a drill component, such as a drill rod, or in any other components wherein such steels are useful. The present disclosure further relates to a drill component comprising the bainitic steel.

BACKGROUND

During rock drilling, shock waves and rotation are transferred via one or more rods or tubes to a cemented carbide equipped drill bit which means that the drill rod will be subjected to severe mechanical loads. Thus, one problem relating to drill rods is their exposure to extensive wear, deformation, fatigue and chipping, resulting in a relatively short service life, which requires a replacement of the drill rods in recurring intervals during drilling, and that will have a direct impact on the total cost of the drilling operation. Another problem is unexpected rod breakages during drilling, as it may take considerable time to retrieve a broken rod from the drill hole. Therefore, hardness, tensile strength and impact toughness of the drill rod are especially important.

Consequently, it is an aspect of the present disclosure to solve or at least reduce the above- mentioned problems. In particular, it is an aspect of the present disclosure to provide an improved bainitic steel composition which will allow the manufacturing of a drill rod having a microstructure which will provide the bainitic steel with well-balanced and optimized mechanical properties, thereby resulting in a drill rod with an extended and predictable service life. A further aspect of the present disclosure is to achieve a cost-efficient drill component. Additionally, yet another aspect of the present disclosure relates to the use of the improved bainitic steel in a rock drilling component.

SUMMARY

The present disclosure therefore relates to a bainitic steel comprising the following composition in weight % (wt %): c 0.33 to 0.40;

Si 0.60 to 1.45;

Mn 0.25 to < 0.80;

P < 0.03;

S < 0.03;

Cr 1.00 to 1.50; Ni 0.10 to 0.60;

Mo 0.40 to 0.80;

N < 0.020;

A1 < 0.05; balance Fe and unavoidable impurities.

The present steel will have a bainitic microstructure meaning that the microstructure will essentially consist of dislocation-rich ferrite and cementite and retained austenite, which are formed during the bainitic transformation. Further, the present steel may also contain small amounts of proeutectoid ferrite and/or martensite but it is important to keep these amounts low in order to avoid a too low strength and hardness or a too high brittleness respectively.

Thus, due to its inventive composition and specific microstructure, the present bainitic steel will withstand wear and reduce brittleness in drill applications. Furthermore, both fatigue cracking and plastic deformation will also be reduced, especially during load peaks. Additionally, the present bainitic steel will also have a good resistance against softening due to overheating because of its tempering resistance, i.e. how well the steel will maintain its hardness at elevated service temperatures. Hence, the bainitic steel as defined hereinabove or hereinafter will have a unique combination of desired properties for drill applications and thereby overcome or at least reduce the problems mentioned above.

The present disclosure also relates to the use of the bainitic steel as defined hereinabove or hereinafter for manufacturing a drill component, for example a drill rod, such as a top hammer drill rod, or any other drill component comprising said bainitic steel.

DETAILED DESCRIPTION

The present disclosure relates to a bainitic steel for drill applications comprising the following elements in weight % (wt %):

C 0.33 to 0.40;

Si 0.60 to 1.45;

Mn 0.25 to < 0.80;

P < 0.03;

S < 0.03;

Cr 1.00 to 1.50;

Ni 0.10 to 0.60; Mo 0.40 to 0.80;

N < 0.020;

A1 < 0.05; balance Fe and unavoidable impurities.

Thus, the inventors have surprisingly found that a bainitic steel having the alloying element in the ranges as defined hereinabove or hereinafter will have a combination of suitable mechanical properties for drilling applications, which will provide the present bainitic steel with a combination of good hardness, good yield strength, good ultimate tensile strength, good impact toughness and good tempering resistance so that it will be able to withstand wear, plastic deformation, load variations, embrittlement as well as softening at elevated service temperatures.

The alloying elements of the steel according to the present disclosure will now be described. The terms “weight %” and “wt %” are used interchangeably. Also, the list of properties or contributions mentioned for a specific element should not be considered exhaustive.

Carbon (C): 0.33 to 0.40 wt %

Carbon is included in the present bainitic steel for increasing strength and hardness but also for governing the desired microstructure of the steel, which will be formed during continuous air cooling subsequent to the final hot rolling operation. For instance, C will slow down the formation of proeutectoid ferrite on cooling, which otherwise may have an impact on bainite formation at lower temperatures. Further, C will provide improved mechanical properties in the bainitic structure due to extended interstitial solid solution strengthening and due to precipitation hardening and also due to suppression of the Bs-temperature. The Bs- temperature is the transformation temperature from which the bainite starts to form on cooling. A suppression of the bainite formation causes a finer bainitic microstructure, as both bainite nucleation and bainite growth rate will be affected.

A too low content of carbon will therefore result in inferior mechanical properties of the bainitic microstructure. However, a too high content of C will increase the hardenability too much and result in a high martensite content during air cooling as the Bs-temperature will be suppressed too much. This will lead to an incomplete bainitic transformation whereby a microstructure with impaired mechanical properties, such as decreased ductility and reduced impact toughness will be formed. Thus, the content of C in the present bainite steel is between 0.33 to 0.40 wt %. According to one embodiment and in order to have the best mechanical properties, the content of C is 0.35 to 0.39 wt %.

Silicon (Si): 0.60 to 1.45 wt %

Silicon is used as a deoxidizing element in the manufacturing process and some amounts of silicon is therefore always present in the present steel. Further, silicon has an important effect as it is a solution strengthening element and will clearly improve the strength of the bainitic microstructure. It has been shown that Si will be especially important for improving the mechanic properties of the present steel, such the ductility and impact toughness of the present steel by retarding cementite formation during cooling thereby increasing the amount of retained austenite in the bainitic microstructure. In order to have any of the desired effects, the content of Si must be at least 0.60 wt %.

However, Si will stabilize proeutectoid ferrite on cooling and as the present steel should have a predominantly bainitic microstructure, too high amounts of Si will lead to the formation of too much proeutectoid ferrite during air cooling. This may also lead to a decrease in hardness and strength as the proeutectoid ferrite has inferior mechanical properties compared to the bainitic microstructure.

It is therefore very important to carefully select the range of Si for the present steel, the amount of silicon is therefore selected to 0.60 to 1.45 wt %. According to embodiment and in order to have the best hardness and strength, the content of silicon is 1.00 to 1.45 wt %.

Manganese (Mn): 0.25 to < 0.80 wt %

Mn is primarily included in the present steel in order to reduce hot cracking by forming MnS with sulphur, which will avoid the harmful formation of FeS. Mn should therefore be included in an amount of at least 0.25 wt % in order to ensure MnS-types of sulphides. Furthermore, Mn has a positive effect on the mechanical properties of the present steel as it will contribute to solid solution strengthening of the bainitic microstructure. Mn also lowers the Bs- temperature and thus favoring the formation of a finer bainitic microstructure, which improves both strength and ductility.

However, Mn will lower the austenitizing temperature and thereby the austenite grain size during hot rolling. As a consequence of this, Mn will also promote pearlite formation on cooling, although being a hardenability element, at the expense of the subsequent bainite formation. Mn will also increase work hardening and have a negative impact on the general susceptibility to embrittlement, especially temper embrittlement. Additionally, Mn may increase softening when the steel is exposed to elevated service temperatures which impairs hardness and strength.

Mn has a strong hardening effect, already at low amounts and too high amount of manganese will result in a too high hardenability, which leads to the formation of a high martensite content during air cooling and consequently to a decrease in ductility and impact toughness.

Due to these disadvantages, in the present steel, it has been found to be crucial to restrict the amount of Mn in order to allow for higher additions of other alloying elements and thereby to avoid a too high hardenability. It is very important to carefully select the range of Mn and the content of Mn is therefore < 0.80 wt %. In one embodiment, Mn is < 0.70 wt %.

Chromium (Cr): 1.00 to 1.50 wt %

Cr will contribute to the solid solution strengthening of the bainitic microstructure and thus will improve the mechanical properties of the present steel. It will also increase hardenability and suppress the Bs-temperature. The suppressed Bs-temperature will improve the mechanical properties, especially the strength and ductility properties.

In the present steel, it has been found that Cr is an important alloying element compared to the alloying elements Mn, Ni and Si. Although being a hardenability element, it has been found that Cr has a much weaker hardenability effect at lower temperatures compared to higher temperatures and will therefore retard the formation of pearlite but will avoid the same restriction of the bainite formation if compared with Mn and Ni. It has also been found that Cr will add more strength to the bainitic microstructure if compared with Ni and will not promote proeutectoid ferrite formation similar to Si.

However, in excessive amounts, chromium may increase the hardenability too much, which will result in a high martensite content formed during air cooling and a microstructure with impaired mechanical properties, such as decreased ductility and reduced impact toughness. A too high Cr content may further increase the risk for precipitation of grain boundary carbides on cooling, with a negative impact on ductility. A too low Cr content will, on the other hand, result in inferior mechanical properties of the bainitic microstructure. The Cr content is from 1.00 to 1.50 wt %. Further, in order to have the best mechanical properties, the Cr content may be between 1.10 to 1.50 wt %. Nickel (Ni): 0.10 to 0.60 wt %

Nickel increases the hardenability of the steel and causes a solid solution strengthening effect which improves the strength of the bainitic microstructure but has above all a strong toughening effect. The toughening effect will increase the impact strength, especially at low service temperatures. In order to ensure a sufficient impact strength of the steel, the Ni content should be at least 0.10 wt %. However, a too high content of Ni could on the other hand lead to a too high amount of retained austenite and thereby a reduced hardness and strength. At elevated service temperatures, a high Ni content may also impair tempering resistance, whereby the hardness and the strength of the steel will be reduced with time. A too high Ni content may as well increase the hardenability too much, resulting in a high martensite content during air cooling and a microstructure with impaired mechanical properties, such as decreased ductility and reduced impact toughness. The Ni content should therefore be limited to 0.60 wt % in the present steel. Ni is further an expensive alloying element and should for that reason be added in as low and well-balanced amounts as possible. According to one embodiment, the content of Ni may be 0.10 to 0.50 wt %.

Molybdenum (Mo): 0.40 to 0.80 wt %

Molybdenum will improve the strength of the bainitic microstructure by solid solution strengthening and precipitation hardening. Mo is very efficient in retarding the formation of perlite during cooling and also suppresses tempering embrittlement, which may occur during slow cooling. Mo is especially advantageous in reducing softening during service, i.e. improving tempering resistance, when the steel is exposed to elevated temperatures, and thus will help to maintain hardness and strength. However, Mo is also an expensive element and is therefore preferably kept as low as possible but still added in an amount wherein it will have an impact on the properties. To ensure that Mo has these positive effects, the amount is at least 0.40 wt % and the upper limit for molybdenum is 0.80 wt %.

Nitrogen (N): < 0.020 wt %

N may be added to the present steel as it both has an interstitial solid solution strengthening effect, as well as a precipitation hardening effect, which improves the strength of the steel, especially the yield strength. N may contribute to grain refinement as nitrides and thereby further improves the mechanical properties of the steel. However, N is generally regarded as an undesirable impurity in steels, as it gives rise to embrittlement and strain ageing effects, which are specifically deleterious to ductility, formability, and impact toughness at room temperature. A too high content of N may also reduce the hot working properties during forging and rolling. The upper limit is therefore set to < 0.020 wt %. If added, the N content is set to be 0.005 to 0.020 weight %.

Phosphorous (P): < 0.03 wt %

P is an optional element and is considered to be an impurity as it is normally regarded as a harmful element, due to its embrittling effect. Therefore, it is desirable to have < 0.03 wt % P.

Sulphur (S): < 0.03 wt %

S is an optional element but may be included in order to improve the machinability. However, it is often regarded as an impurity, as S may form grain boundary segregations and inclusions and will therefore restrict the hot working properties as well as the mechanical properties, causing an increased anisotropic behaviour. Hence, the content of S should be < 0.03 wt %. If added, the S content is set to be 0.01 to 0.03 wt %.

Aluminium (Al): < 0.05 wt %

A1 may be used as a deoxidizing agent, but may also be added for grain refinement, as it easily combines with nitrogen forming stable AlN-precipitates, which promotes toughness, especially at low temperatures. However, a too high content of Al may reduce the mechanical properties, by a decreased ductility. If added, the content of Al is set to be 0.01 to 0.05 wt %.

Optionally small amounts of other alloying elements may be added to the present bainitic steel as defined hereinabove or hereinafter in order to improve e.g. the machinability or the hot working properties, such as the hot ductility. Example, but not limiting, of such elements are Ca, Mg, B, Pb and/or Ce. The amounts of one or more of these elements are of max. 0.05 wt %, except for B which is of max. 0.005 wt %.

The present bainitic steel may contain traces of trace elements, for example Tungsten (W), Cobalt (Co), Cupper (Cu), Titanium (Ti) and Tantalum (Ta), Vanadium (V) and/or Niobium (Nb). Such trace elements are to be considered as impurities, i.e. not intentionally added, meaning that they are allowed to be present in the steel but only in such amount that the final properties of the steel will not be affected. Thus, impurities are elements and/or compounds which have not been added on purpose but cannot be fully avoided as they normally occur as impurities in e.g. the raw material. When the terms “max” or “< ” are used, the skilled person knows that the lower limit of the range is 0 wt % unless another number is specifically stated.

The remainder of elements of the steel as defined hereinabove or hereinafter is Iron (Fe) and normally occurring impurities as discussed above. Thus, the present inventors have surprisingly found that by the specific element composition of the present disclosure, a bainitic steel will be obtained which will provide wear and embrittlement resistance. Furthermore, the present bainite steel composition will provide for a reduction of fatigue cracking and plastic deformation. Hence, the composition of the alloying elements has been carefully adapted so that an object composed of the present bainite steel will have the desired bainite content, i.e. a balanced content of a ductile phase, and as low as possible content of brittle or mechanical weak phases. Thus, the present bainitic steel will be suitable for dill applications.

According to one embodiment, the present bainitic steel consists or comprises of all the elements mentioned herein and in the different ranges as mentioned herein. According to embodiments, the bainitic steel comprises or consists of the following elements in weight %:

The balance is Fe and unavoidable impurities and optional elements as described above. Also, S, Al, and N may be purposively be added as described above. According to embodiments, it has also been found that if the present steel also fulfills the requirements of having a Chromium -equivalent (Cr_eq) of at least 2.70, it will ensure that the desired bainitic microstructure will be obtained and that the present steel will have a combination of good strength (Rpo_.i), good ductility, good impact toughness and good hardness (Hardness 3). The Chromium equivalent (Cr_eq) is calculated according to Schaeffler's formula, wherein the numbers are in weight %:

Cr_eq=Cr+(1.5*Si) + (l*Mo) + (0.5*Nb).

According to one embodiment, the present alloy as defined hereinabove or hereinafter has a Ni content of 0.10 to 0.40 wt %, a Mn content of 0.25 to 0.55 wt % and a Mo content of 0.55 to 0.80 wt %. According to another embodiment, the content of Si is from 1.00 to 1.45 wt %.

According to embodiments, the bainitic steel as defined hereinabove or hereinafter has a yield strength (Rpo_.2) of >1000 MPa, regarding as-received drill rod samples. By the term “as- received” is meant that the drill rod has been hot rolled and straightened.

According to embodiments, the bainitic steel as defined hereinabove or hereinafter has a tensile strength (Rm) of > 1400 MPa, regarding as-received drill rod samples.

According to embodiments, the impact toughness (IT) of the bainitic steel as defined hereinabove or hereinafter is > 13 J at room temperature, when using as-received drill rod samples.

According to one embodiment, the hardness after hardening, i.e. austenitizing and water quenching, when performed on as-received drill rod samples of the bainitic steel as defined hereinabove or hereinafter is within 56 to 62 HRC (Hardness 2).

The bainitic steel as defined hereinabove or hereinafter and a drill rod manufactured thereof may be manufactured by using conventional steel production and steel machining processes and conventional drill rod production and machining processes.

An object or a component comprising the bainitic steel as defined hereinabove or hereinafter is austenitized, hot rolled and subjected to air cooling to room temperature, whereby the desired bainitic microstructure is obtained during the continuous cooling.

The mechanical properties of the surface of a component composed of the bainitic steel as defined hereinabove or hereinafter may be further improved by induction hardening or by applying a surface treatment method, such as but not limited to shoot peening. The steel according to the present disclosure is intended, as mentioned herein, for manufacturing of a drill component, such as for example a drill rod, such as for example a top hammer drill rod.

The present disclosure is further illustrated by the following non-limiting examples.

EXAMPLES

Example 1

All alloys of Table 1, except alloy 7, were produced by melting in a high frequency furnace by melting scrap and alloys and thereafter ingot cast using 9” steel moulds. The obtained alloys had the compositions as shown in Table 1. The balance is iron and unavoidable impurities.

The weights of the ingots were approximately 270 kg. The ingots were heat treated at 600 to 700 °C for 4 to 8 hours and then air cooled to room temperature followed by grinding of the ingot surface. Thereafter, the ingots were heated to 1100 to 1250 °C and forged in a hammer to bars having a round dimension of approx. 130 mm. The bars were then air cooled and heat treated at 600 to 700 °C for 4 to 8 hours and air cooled to room temperature.

In the next step, the bars were straightened, cut, turned, drilled and a core was inserted. The obtained round bars were then hot rolled at 1100 to 1250 °C in a rolling mill to hexagonal hollow bars with a dimension of 20 to 25 mm. After hot-rolling, the bars were continuously air cooled to room temperature. The core was removed, and the bars were cut in lengths and thereafter straightened.

Example 2

Alloy 7 was produced by melting in a 75 MT electric arc furnace and thereafter continuous cast to 365x265 mm blooms. The alloy composition is shown in Table 1. Thereafter, the blooms were heated to 1100 to 1250 °C and hot rolled to a diameter of approx. 125 mm.

The bars were heat treated at 700 to 850 °C for 3 to 6 hours, followed by furnace cooling to 600 °C and then air cooled to room temperature, thereafter straightened, cut, turned, drilled and a core was inserted.

The obtained round bars were then hot rolled at 1100 to 1250 °C in a rolling mill to hexagonal hollow bars with a dimension of 20 to 25 mm. After hot-rolling, the bars were continuously air cooled to room temperature. The core was removed, and the bars were cut in lengths and thereafter straightened. Example 3 Mechanical Testing

The results of the mechanical testing are shown in Table 2.

Hardness testing

Three types of hardness measurements were performed as HRC tests at room temperature, according to ASTM E 18-19.

- Hardness of as-rolled drill rod samples, i.e. “as-rolled” is meant after hot-rolling and air-cooling (Hardness 1).

- Hardness of hardened drill rod samples, i.e. 1000 °C during 20 minutes and then water quenching (Hardness 2)

Hardening was performed on as-received drill rod samples. The term “as-received” means that the drill rod has been hot rolled and straightened before a sample was taken from the drill rod.

- Hardness of tempered drill rod samples, i.e. 650 °C during 30 minutes and then air cooling (Hardness 3).

Tempering was performed both on as-received drill rod samples (Hardness 3a), as well as hardened drill rod samples (Hardness 3b), and reported separately.

Hardness was measured in the longitudinal section of the as-rolled drill rod samples. Prior to the measurements the surface was grinded to a depth of 0.5 mm. The as-rolled hardness was tested in two drill rod positions of all alloys, except for Alloy 8 and 9, which were tested in one drill rod position. In the hardened drill rod samples as well as the tempered drill rod samples, hardness was measured in the cross section of the drill rod samples. All presented values are based on the average of three or more indentations in each drill rod position. No tempering tests were performed on Alloy 8, 9 and 10.

As can be seen from the Examples, Hardness 3b is excellent for inventive Alloys 1 to 3 and 6 to 7. This means that these alloys have a superior ability to withstand softening when exposed to elevated temperatures, compared to the other alloys. Furthermore, as can be seen from the Examples, also Hardness 3a is very good for these alloys. These hardness results mean that their tempering resistance will be very good both in their as-received and in their hardened conditions and without being bound to any theory as well as in their hardened and as-rolled condition condition. It should be emphasis that when evaluation the alloys of the Examples, a combination of all results of the different performed mechanical tests have been taken into consideration.

As can be seen from Table 2, the as-rolled hardness of all heats within the present invention is between 41 to 47 HRC (Hardness 1) which is the desired hardness for having optimal properties for the applications mentioned herein.

Tensile testing

The results for tensile testing included both measurements on yield strength and ultimate tensile strength. The tests were performed on as-received drill rod samples at room temperature according to ASTM E8/E8M-16a, Figure 8 [E8M], with Specimen 4. The presented values are based on the average of two or more specimens. All alloys were tested in two drill rod positions, except for Alloy 10, which was tested in one drill rod position.

Impact toughness testing (IT)

The results for impact toughness were based on the total impact energy measured during Charpy-V testing. The tests were performed on as-received drill rod samples at room temperature according to ISO 148-1 :2016(E). Specimens 10x5x55 mm with a V notch were used, also according to ISO 148-1 :2016(E). The presented values are based on the average of two or more specimens. All alloys were tested in two drill rod positions, except for Alloy 10, which was tested in one drill rod position. As can be seen from the testing results, all inventive alloys had good results. Even though one of the reference alloys has a value close to one of the inventive alloys, all the mechanical properties of each alloy have to be considered when reviewing if an alloy is good or bad.

Hence, as can be seen from the results of Table 2, the present bainitic steel will have an optimized hardness to withstand both wear and reduce brittleness in the drill application.

Furthermore, as can be seen from Table 2, the present bainitic steel will have a combination of well-balanced and optimized mechanical properties, such as hardness, tensile strength and impact toughness, in order to withstand wear, deformation and fatigue, as well as softening caused by elevated surface temperatures, due to frictional heat during drilling. Table 1 The compositions of the alloys of the Examples. Alloy 1 to 3 and 6 to 7 are inventive alloys and within the scope of the claims and marked with a Alloys 4 to 5 and 8 to 10 are included as reference alloys. The balance is Fe and unavoidable impurities.

Table 2 Results from the mechanical testing

Claims

1. A bainitic steel for drill applications comprising the following composition:

C 0.33 to 0.40;

Si 0.60 to 1.45;

Mn 0.25 to < 0.80;

P < 0.03;

S < 0.03;

Cr 1.00 to 1.50;

Ni 0.10 to 0.60;

Mo 0.40 to 0.80;

N < 0.020;

A1 < 0.05; balance Fe and unavoidable impurities.

2. The bainitic steel according to claim 1, wherein the content of Mn is 0.25 to < 0.70 wt %.

3. The bainitic steel according to claim 1 or 2, wherein the content of Ni is from 0.10 to 0.50 wt %.

4. The bainitic steel according to any one of claims 1 to 3, wherein the content of C is 0.35 to 0.39 wt %.

5. The bainitic steel according to any one of claims 1 to 4, wherein the content of Cr is 1.10 to 1.50 wt %.

6. The bainitic steel according to any one of claims 1 to 5, wherein the content of Si is 1.00 to 1.45 wt %.

7 The bainitic steel according to claim 1, wherein content of Ni is from 0.10 to 0.40 wt %, the content of Mn is from 0.25 to 0.55 wt % and the content of Mo is from 0.55 to 0.80 wt %.

8. The bainitic steel according to claim 7, wherein the content of Si is from 1.00 to 1.45 wt%.

9. Use of the bainitic steel according to any one of claims 1 to 8 for manufacturing a drill component.

10. A drill component comprising the bainitic steel according to any one of claims 1 to 8.