JP2023525516A - new bainite steel - Google Patents

Abstract

The present disclosure relates to new bainite steels used to make drilling parts such as drill rods or other parts where such steels are useful. The present disclosure further relates to drill components comprising bainite steel. Bainite steel comprises the following composition in weight percent (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, Al ≤ 0.05, balance Fe and unavoidable impurities. [Selection figure] None

Description

The present disclosure relates to new bainite steels used to make drilling parts such as drill rods or other parts where such steels are useful. The present disclosure further relates to drill parts comprising the bainite steel.

During rock drilling, shock waves and rotation are transmitted through one or more rods or tubes to the cemented carbide-equipped drill bit, which means that the drill rod is subjected to severe mechanical loads. Therefore, one problem with drill rods is that they are subject to extensive wear, deformation, fatigue and chipping. The result is a relatively short service life, which leads to the need to replace the drill rod at repeated intervals during drilling, directly affecting the total cost of the drilling operation. Another challenge is unexpected rod breakage during drilling. This is because it can take a considerable amount of time to retrieve the broken rod from the drilled hole. Therefore, drill rod hardness, tensile strength and impact toughness are of particular importance.

As a result, it is an aspect of the present disclosure to solve, or at least mitigate, the problems set forth above. In particular, one aspect of the present disclosure enables the production of drill rods with microstructures that provide balanced and optimized mechanical properties in bainite steels, thereby extending and predicting service life. To provide an improved bainite steel composition that yields a viable drill rod. A further aspect of the present disclosure is to obtain a cost effective drill part. Yet another aspect of the present disclosure relates to the use of modified bainite steel in rock drilling components.

Accordingly, the present disclosure relates to a bainite steel comprising the following composition in weight percent (wt%).
C 0.33-0.40,
Si 0.60-1.45,
Mn 0.25 to ≦0.80,
P ≤ 0.03,
S ≤ 0.03,
Cr 1.00-1.50,
Ni 0.10-0.60,
Mo 0.40-0.80,
N≤0.020,
Al≤0.05,
Balance Fe and unavoidable impurities.

The steel of the present invention has a bainite microstructure, which means that the microstructure consists essentially of dislocation-rich ferrite and cementite formed during the transformation of bainite, and retained austenite. means. Furthermore, the steel according to the invention may contain small amounts of proeutectoid ferrite and/or martensite, but these amounts should be is important to keep .

Therefore, due to the composition and specific microstructure of the present invention, the bainite steel of the present invention resists wear and reduces brittleness in drilling applications. Furthermore, both fatigue cracking and plastic deformation are suppressed, especially during load peaks. The bainite steel of the present invention also has good resistance to softening due to overheating due to its tempering resistance which indicates how well the hardness of the steel is maintained at high service temperatures. Accordingly, the bainite steels defined above or below possess a unique combination of properties that are desirable for drilling applications, thereby overcoming or at least mitigating the above challenges.

The present disclosure also relates to the use of a bainite steel as defined above or below for manufacturing drilling parts, such as drill rods, for example tophammer drill rods, or other drilling parts comprising said bainite steel.

The present disclosure relates to a bainite steel for drilling applications containing the following elements in weight percent (wt%):
C 0.33-0.40,
Si 0.60-1.45,
Mn 0.25 to ≦0.80,
P ≤ 0.03,
S ≤ 0.03,
Cr 1.00-1.50,
Ni 0.10-0.60,
Mo 0.40-0.80,
N≤0.020,
Al≤0.05,
Balance Fe and unavoidable impurities.

Accordingly, the inventors have found that bainite steels containing alloying elements within the ranges defined above or below possess a combination of mechanical properties suitable for drilling applications, thereby rendering the bainite steels of the present invention However, in order to withstand wear, plastic deformation, load variation, embrittlement and softening at high service temperatures, good hardness, good yield strength, good ultimate tensile strength, good impact toughness, It has surprisingly been found to provide a combination with tempering resistance.

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

Carbon (C): 0.33 to 0.40% by weight
Carbon is included in the bainite steel of the present invention not only to increase strength and hardness, but also to determine the desired microstructure of the steel that is formed during continuous air cooling following the final hot rolling operation. For example, C slows down the formation of proeutectoid ferrite on cooling and can otherwise affect bainite formation at low temperatures. Furthermore, C provides improved mechanical properties in the bainite structure due to extended interstitial solid solution strengthening and due to precipitation hardening and due to suppression of the Bs temperature. The Bs temperature is the transformation temperature at which bainite begins to form upon cooling. Suppression of bainite formation affects both bainite nucleation and bainite growth rate, thus causing refinement of the bainite microstructure.

Therefore, if the carbon content is too low, the mechanical properties of the bainite microstructure deteriorate. However, if the C content is too high, the quench hardenability becomes too high, and as a result, the Bs temperature is excessively suppressed, which increases the martensite content during air cooling. This leads to incomplete bainite transformation, thereby forming a microstructure with compromised mechanical properties such as reduced ductility and reduced impact toughness. Therefore, the C content in the bainite steel of the present invention is set to 0.33 to 0.40% by weight. According to one embodiment, the C content is between 0.35 and 0.39% by weight in order to have the best mechanical properties.

Silicon (Si): 0.60 to 1.45% by weight
Since silicon is used as a deoxidizing element in the manufacturing process, some amount of silicon is always present in the steel of the invention. Moreover, since silicon is a solution-strengthening element, it has an important effect and clearly improves the strength of the bainite microstructure. Si improves the mechanical properties of the steel of the invention, such as the ductility and impact toughness of the steel of the invention, by retarding cementite formation during cooling, thereby increasing the amount of retained austenite in the bainite microstructure. have been shown to be particularly important for To have any of the desired effects, the Si content should be at least 0.60% by weight.

However, Si stabilizes proeutectoid ferrite during cooling, and the steel of the present invention must have a predominantly bainite microstructure. Ferrite is formed. This can lead to lower hardness and strength, since the pro-eutectoid ferrite has inferior mechanical properties compared to the bainite microstructure.

It is therefore very important to carefully select the range of Si in the steels of the invention, so the amount of silicon is selected between 0.60 and 1.45% by weight. According to embodiments, the silicon content is between 1.00 and 1.45% by weight in order to have the best hardness and strength.

Manganese (Mn): 0.25 to ≦0.80% by weight
Mn is primarily included in the steel of the present invention to reduce hot cracking by forming MnS with sulfur, thereby avoiding the detrimental formation of FeS. Therefore, Mn should be contained in an amount of at least 0.25% by weight to ensure MnS type sulfides. Moreover, Mn contributes to solid-solution strengthening of the bainite microstructure and thus has a positive effect on the mechanical properties of the steel of the invention. Mn also lowers the Bs temperature, thus promoting the formation of a finer bainite microstructure, thereby improving both strength and ductility.

However, Mn lowers the austenitizing temperature and thus reduces the austenite grain size during hot rolling. As a result of this, Mn also promotes pearlite formation on cooling, despite being a hardening element, at the expense of subsequent bainite formation. Mn also enhances work hardening and adversely affects overall susceptibility to embrittlement, particularly temper embrittlement. Additionally, Mn can enhance softening when the steel is exposed to high service temperatures that impair its hardness and strength.

Even a small amount of Mn has a strong hardening effect, and if the amount of manganese is too high, the hardenability will be too high, leading to the formation of a high content of martensite during air cooling, As a result, ductility and impact toughness are reduced.

Due to these drawbacks, in the steels of the present invention, it is necessary to limit the amount of Mn in order to allow the addition of increased amounts of other alloying elements, thereby avoiding an excessive increase in hardenability. turned out to be important. Careful selection of the range of Mn is very important, so the content of Mn should be ≤0.80% by weight. In one embodiment, Mn is ≤0.70 wt%.

Chromium (Cr): 1.00 to 1.50% by weight
Cr contributes to solid-solution strengthening of the bainite microstructure and thus improves the mechanical properties of the steel of the present invention. In addition, it improves hardenability and suppresses the Bs temperature. Suppressing the Bs temperature improves mechanical properties, especially strength and ductility properties.

In the steel according to the invention Cr has been found to be an important alloying element compared to the alloying elements Mn, Ni and Si. Although Cr is a hardenable element, the hardenability effect of Cr is much weaker at lower temperatures compared to higher temperatures, thus retarding pearlite formation, but bainite formation when compared to Mn and Ni. was found to circumvent a similar limitation of It has also been found that Cr adds additional strength to the bainite microstructure compared to Ni and does not promote proeutectoid ferrite formation similar to Si.

However, too much chromium can lead to excessive hardenability, resulting in increased martensite content formed during air cooling and mechanical stresses such as reduced ductility and reduced impact toughness. A microstructure with impaired physical properties results. Too high a Cr content further increases the risk of precipitation of grain boundary carbides on cooling, which can adversely affect ductility. On the other hand, if the Cr content is too low, the mechanical properties of the bainite microstructure deteriorate. The Cr content should be 1.00 to 1.50% by weight. Furthermore, the Cr content may be 1.10-1.50 wt% in order to have the best mechanical properties.

Nickel (Ni): 0.10 to 0.60% by weight
Nickel increases the hardenability of the steel and improves the strength of the bainite microstructure, but causes a solid-solution strengthening effect that has a particularly strong toughening effect. The toughening effect increases impact strength, especially at low service temperatures. The Ni content should be at least 0.10% by weight to ensure sufficient impact strength of the steel. On the other hand, however, if the Ni content is too high, the amount of retained austenite will be too high, thereby reducing hardness and strength. At high service temperatures, high Ni content can also compromise tempering resistance, thereby reducing the hardness and strength of the steel over time. Moreover, too much Ni content can lead to excessive hardenability, resulting in increased martensite content during air cooling and reduced mechanical properties such as reduced ductility and impact toughness. resulting in a microstructure in which the Therefore, in the steel according to the invention, the Ni content must be limited to 0.60% by weight. Furthermore, Ni is an expensive alloying element, so it should be added in as small a quantity as possible and in a well-balanced manner. According to one embodiment, the content of Ni may be 0.10-0.50% by weight.

Molybdenum (Mo): 0.40 to 0.80% by weight
Molybdenum improves the strength of the bainite microstructure through solid solution strengthening and precipitation hardening. Mo is very effective in retarding the formation of pearlite during cooling and also suppresses possible temper embrittlement during slow cooling. Mo is particularly advantageous in reducing softening in service, ie improving tempering resistance when the steel is exposed to high temperatures, thus helping to maintain hardness and strength. However, Mo is also an expensive element, so it is preferred to keep it as small as possible, but still add an amount that affects the properties. To ensure that Mo has these favorable effects, the amount should be at least 0.40 wt% and the upper limit for molybdenum should be 0.80 wt%.

Nitrogen (N): ≤ 0.020% by weight
Since N has both an interstitial solid-solution strengthening effect and a precipitation hardening effect, it may be added to the steel of the present invention, thereby improving the strength of the steel, especially the yield strength. N can contribute to grain refinement as a nitride, thereby further improving the mechanical properties of the steel. However, N is generally considered an undesirable impurity in steel because it causes embrittlement and strain aging effects that are detrimental to ductility, formability and impact toughness, especially at room temperature. Too much N content can also degrade the hot working properties during forging and rolling. Therefore, the upper limit is set to ≦0.020% by weight. If added, the N content is set at 0.005-0.020 wt%.

Phosphorous (P): < 0.03 wt%
P is an arbitrary element and is considered an impurity because it is usually considered a detrimental element due to its embrittlement effect. Therefore, it is desirable to include ≤0.03 wt% P.

Sulfur (S): ≤ 0.03% by weight
S is an optional element, but may be included to improve machinability. However, S is often considered an impurity because it can form grain boundary segregation and inclusions, which limit hot working and mechanical properties and cause an increase in anisotropic properties. Therefore, the content of S should be ≤0.03% by weight. If added, the S content is set at 0.01-0.03 wt%.

Aluminum (Al): ≤ 0.05% by weight
Al may be used as a deoxidizer, but may also be added for grain refinement because it readily combines with nitrogen to form stable AlN precipitates and promotes toughness, especially at low temperatures. . However, if the Al content is too high, mechanical properties may deteriorate due to reduced ductility. When added, the content of Al is set to 0.01-0.05 wt%.

Minor amounts of other alloying elements may optionally be added to the bainitic steel of the present invention as defined above or below, for example to improve hot working properties such as machinability or hot ductility. Such elements include, for example, but are not limited to Ca, Mg, B, Pb and/or Ce. The amount of one or more of these elements should be up to 0.05% by weight, while B should be up to 0.005% by weight.

The bainite steel of the present invention contains trace elements such as tungsten (W), cobalt (Co), copper (Cu) (Cu), titanium (Ti) and tantalum (Ta), vanadium (V) and/or niobium (Nb). ) may be contained in a trace amount. Such trace elements should be regarded as impurities, i.e. not intentionally added, which means that these elements are present in the steel only in such amounts that the final properties of the steel are not affected. means that it is permissible to Impurities are therefore elements and/or compounds which are not intentionally added but which cannot be completely avoided, for example because they are usually present as impurities in raw materials.

It is known to those skilled in the art that when the term "maximum" or "≤" is used, the lower limit of the range is 0% by weight, unless another number is specifically stated.

Other elements of steel defined above or below are, as mentioned above, iron (Fe) and commonly present impurities.

Accordingly, the inventors have surprisingly found that the specific elemental composition of the present disclosure results in a bainite steel that provides resistance to wear and embrittlement. Additionally, the bainite steel composition of the present invention provides reduced fatigue cracking and plastic deformation. Therefore, the composition of the alloying elements should be such that the body composed of the bainite steel of the invention has the desired bainite content, i.e. a balanced content of ductile phases and as little brittle or mechanically weak phases as possible. are carefully adapted to include the amount and Thus, the bainite steel of the present invention is suitable for dill applications.

According to one embodiment, the bainitic steel of the present invention consists of or contains all the elements mentioned herein and in the different ranges mentioned herein.

According to embodiments, the bainite steel comprises or consists of the following elements in weight percent:

The balance is, as described above, Fe, unavoidable impurities, and arbitrary elements. Also, as described above, S, Al, and N may be intentionally added.

According to an embodiment, if the steel of the invention also meets the requirement to have a chromium equivalent (Cr _eq ) of at least 2.70, a desirable bainite microstructure is obtained and the steel of the invention exhibits good strength (R _p0.2 ), it was also found to ensure a combination of good ductility, good impact toughness and good hardness (hardness 3). The chromium equivalent (Cr _eq ) is calculated according to the Schaeffler formula and the numerical unit is % by weight.
Cr _eq =Cr+(1.5*Si)+(1*Mo)+(0.5*Nb)

According to one embodiment, the alloy of the invention as defined above or below has a Ni content of 0.10-0.40% by weight, a Mn content of 0.25-0.55% by weight, a Mo The amount is 0.55-0.80% by weight. According to another embodiment, the Si content is between 1.00 and 1.45% by weight.

According to an embodiment, the bainite steel defined above or below has a yield strength (R _p0.2 ) of 1000 MPa or more for as-received drill rod samples. The term "as received" means that the drill rod has been hot rolled and straightened.

According to embodiments, the bainite steel defined above or below has a tensile strength (Rm) of 1400 MPa or more for as-received drill rod samples.

According to an embodiment, the impact toughness (IT) of the bainite steel defined above or below is >13 J at room temperature when using as-received drill rod samples.

According to one embodiment, the hardness after hardening, i.e. austenitization and water quenching, is between 56 and 62 HRC when performed on as-received drill rod samples of bainite steel as defined above or below. (Hardness 2).

Bainite steels and drill rods made therefrom as defined above or below can be manufactured using conventional steel manufacturing and steel machining and conventional drill rod manufacturing and machining.

An object or component comprising a bainite steel as defined above or below is austenitized, hot rolled and air cooled to room temperature to obtain the desired bainite microstructure during continuous cooling.

The surface mechanical properties of parts composed of bainite steel as defined above or below may be further improved by induction hardening or by applying surface treatment methods such as, but not limited to, shoot peening. .

The steel according to the present disclosure, as referred to herein, is intended for the production of drilling components such as drill rods, for example tophammer drill rods.

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

Example 1
All alloys in Table 1, except Alloy 7, were produced by high frequency furnace melting by melting scrap iron and alloys, followed by ingot casting using a 9 inch steel mold. The composition of the obtained alloy was as shown in Table 1. The balance is iron and unavoidable impurities.

The weight of the ingot was approximately 270 kg. After the ingot was heat-treated at 600-700° C. for 4-8 hours, it was air-cooled to room temperature and the ingot surface was ground. The ingots were then heated to 1100-1250° C. and hammer forged into bars with a circular dimension of about 130 mm. The bars were then air cooled, heat treated at 600-700° C. for 4-8 hours, and air cooled to room temperature.

The next step was straightening the rod, cutting it, turning it, drilling a hole, and inserting the core. The resulting round bar was then hot rolled at 1100-1250° C. in a rolling mill to obtain a hexagonal hollow bar with dimensions of 20-25 mm. After hot rolling, the bars were continuously air cooled to room temperature. The core was removed and the bars were cut in length and then straightened.

Example 2
Alloy 7 was produced by continuous casting to a 365 x 265 mm bloom after melting in a 75 MT electric arc furnace. Table 1 shows the composition of the alloy. The bloom was then heated to 1100-1250° C. and hot rolled to a diameter of about 125 mm.

After heat treating the bars at 700-850° C. for 3-6 hours, they were furnace cooled to 600° C. and then air cooled to room temperature before straightening, cutting, rolling, drilling and core insertion.

The resulting round bar was then hot rolled at 1100-1250° C. in a rolling mill to obtain a hexagonal hollow bar with dimensions of 20-25 mm. After hot rolling, the bars were continuously air cooled to room temperature. The core was removed and the bars were cut in length and then straightened.

Example 3 - Mechanical Testing The mechanical testing results are shown in Table 2.

Hardness Test Three hardness measurements were performed at room temperature as HRC tests according to ASTM E 18-19.
- the hardness of the as-rolled drill rod sample,
That is, "as-rolled" means after hot rolling and air cooling (hardness 1).
- the hardness of the hardened drill rod sample,
That is, it was cured at 1000° C. for 20 minutes and then water quenched (hardness 2).
Curing was performed on as-received drill rod samples. The term "as received" means that the drill rod has been hot rolled and straightened before the sample is removed from the drill rod.
- the hardness of the tempered drill rod sample,
That is, it was tempered at 650° C. for 30 minutes and then air-cooled (hardness 3).
Tempering was performed on both as-received drill rod samples (hardness 3a) and hardened drill rod samples (hardness 3b) and reported separately.

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

As can be seen from the examples, the alloys 1-3 and 6-7 of the invention have an excellent hardness of 3b. This means that these alloys have an excellent ability to resist softening when exposed to high temperatures compared to other alloys. Furthermore, as can be seen from the examples, hardness 3a is also very good for these alloys. Without being bound by any theory, these hardness results indicate that the tempering resistance is in both the as-received and hardened conditions, and in the hardened and as-rolled conditions. means very good in It should be emphasized that when evaluating the alloys of the Examples, the combined results of all of the various mechanical tests that were performed were considered.

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

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

Impact toughness test (IT)
Impact toughness results were based on the total impact force measured during the Charpy V test. Testing was performed at room temperature on as-received drill rod samples according to ISO 148-1:2016(E). 10×5×55 mm specimens with V notches were used according to ISO 148-1:2016(E). Values presented are based on the average of two or more specimens. All alloys were tested with two drill rod positions, except Alloy 10, which was tested with one drill rod position. As can be seen from the test results, all alloys of the invention had good results. Even if one of the reference alloys has values close to one of the alloys of the invention, all mechanical properties of each alloy should be considered when considering whether an alloy is good or bad.

Therefore, as can be seen from the results in Table 2, the bainite steels of the present invention have hardness optimized to resist wear and reduce brittleness in drilling applications.

Furthermore, as can be seen from Table 2, the bainite steel of the present invention has a hardness, tensile It has a balanced and optimized combination of mechanical properties such as strength and impact toughness.

Claims (10)

A bainite steel for drilling,
C 0.33-0.40,
Si 0.60-1.45,
Mn 0.25 to ≦0.80,
P ≤ 0.03,
S ≤ 0.03,
Cr 1.00-1.50,
Ni 0.10-0.60,
Mo 0.40-0.80,
N≤0.020,
Al≤0.05,
A bainite steel containing a composition of balance Fe and unavoidable impurities. Bainite steel according to claim 1, wherein the content of Mn is from 0.25 to ≤0.70 wt%. Bainite steel according to claim 1 or 2, wherein the Ni content is 0.10-0.50% by weight. Bainite steel according to any one of claims 1 to 3, wherein the content of C is 0.35-0.39% by weight. Bainite steel according to any one of the preceding claims, wherein the Cr content is between 1.10 and 1.50% by weight. Bainite steel according to any one of the preceding claims, wherein the Si content is between 1.00 and 1.45% by weight. The Ni content is 0.10-0.40 wt%, the Mn content is 0.25-0.55 wt%, and the Mo content is 0.55-0.80 wt%. A bainite steel according to claim 1. Bainite steel according to claim 7, wherein the Si content is 1.00-1.45% by weight. Use of the bainite steel according to any one of claims 1 to 8 for producing drill parts. A drill part comprising the bainite steel according to any one of claims 1-8.