New bainite steel

Abstract

The present disclosure relates to a new bainite steel for use in manufacturing drill parts, such as drill rods, or other parts for which such steels are useful. The disclosure further relates to drill parts comprising the bainite steel. The bainite steel has the following composition, in weight percent (wt%): C 0.33-0.40, Si 0.60-1.45, Mn 0.25-≦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.

Description

[Technical Field]

[0001] The present disclosure relates to a new bainite steel used to manufacture drill 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. [Background technology]

[0002] During rock drilling, shock waves and rotations are transmitted to the cemented carbide-equipped drill bit via one or more rods or tubes, which means that the drill rod is subjected to severe mechanical loads. Therefore, one challenge with drill rods is that they are subject to extensive wear, deformation, fatigue, and chipping. This results in a relatively short service life, which in turn necessitates the replacement of the drill rod at repeated intervals during the drilling process, directly impacting the total cost of the drilling operation. Another challenge is the unexpected rod breakage during drilling, as it can take a considerable amount of time to retrieve the broken rod from the drill hole. Therefore, the hardness, tensile strength, and impact toughness of the drill rod are particularly important.

[0003] Consequently, it is an aspect of the present disclosure to solve or at least mitigate the above-mentioned problems. In particular, it is an aspect of the present disclosure to provide an improved bainite steel composition that allows for the manufacture of drill rods with a microstructure that provides the bainite steel with balanced and optimized mechanical properties, thereby resulting in drill rods with an extended and predictable service life. A further aspect of the present disclosure is to obtain cost-effective drill components. Yet another aspect of the present disclosure relates to the use of the improved bainite steel in rock drilling components. Summary of the Invention

[0004] Accordingly, the present disclosure relates to a bainitic steel comprising the following composition in weight percent (wt%): C 0.33~0.40, Si 0.60~1.45, Mn 0.25~≦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, The balance is Fe and unavoidable impurities.

[0005] The steel of the present invention has a bainite microstructure, which means that the microstructure is essentially composed of dislocation-rich ferrite and cementite formed during the transformation of bainite, and retained austenite. Furthermore, the steel of the present invention may contain small amounts of proeutectoid ferrite and / or martensite, but it is important to keep these amounts low in order to avoid too low strength and hardness or too high brittleness, respectively.

[0006] 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 drill 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. Therefore, the bainite steel defined above and below has a unique combination of properties that are desirable for drill applications, thereby overcoming or at least alleviating the above-mentioned challenges.

[0007] The present disclosure also relates to the use of a bainitic steel as defined above or below for manufacturing a drill part, such as a drill rod, e.g. a tophammer drill rod, or other drill part comprising said bainitic steel. DETAILED DESCRIPTION OF THE INVENTION

[0008] The present disclosure relates to a bainitic steel for drill applications comprising the following elements in weight percent (wt%): C 0.33~0.40, Si 0.60~1.45, Mn 0.25~≦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, The balance is Fe and unavoidable impurities.

[0009] The inventors have therefore surprisingly found that a bainitic steel comprising alloying elements in the ranges defined above or below has a combination of mechanical properties suitable for drilling applications, whereby the inventive bainitic steel comprises a combination of good hardness, good yield strength, good ultimate tensile strength, good impact toughness and good tempering resistance in order to be able to withstand wear, plastic deformation, load fluctuations, embrittlement and softening at high service temperatures.

[0010] The alloying elements of the steels 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 specific elements should not be considered exhaustive.

[0011] Carbon (C): 0.33~0.40% by weight Carbon is included in the bainite steel of the present invention not only to enhance strength and hardness, but also to determine the desirable microstructure of the steel that forms during continuous air cooling following the final hot rolling operation. For example, carbon slows down the formation of proeutectoid ferrite during cooling, which could otherwise affect bainite formation at low temperatures. Furthermore, carbon provides improved mechanical properties in the bainite structure due to extended interstitial solid solution strengthening, precipitation hardening, and suppression of the Bs temperature, which is the transformation temperature at which bainite begins to form during cooling. The suppression of bainite formation affects both bainite nucleation and bainite growth rate, resulting in a refinement of the bainite microstructure.

[0012] Therefore, if the carbon content is too low, the mechanical properties of the bainite microstructure will be reduced. However, if the C content is too high, the quench hardenability will be too high, resulting in excessive suppression of the Bs temperature, which will increase the martensite content during air cooling. This will lead to incomplete bainite transformation, thereby forming a microstructure with impaired mechanical properties, such as reduced ductility and impact toughness. Therefore, the C content in the bainite steel of the present invention is set to 0.33-0.40 wt.%. According to one embodiment, the C content is set to 0.35-0.39 wt.% to achieve the best mechanical properties.

[0013] 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 present invention. Furthermore, since silicon is a solution strengthening element, it has an important effect, apparently improving the strength of the bainite microstructure. Si has been shown to be particularly important for improving the mechanical properties of the steel of the present invention, such as the ductility and impact toughness of the steel, by delaying cementite formation during cooling, thereby increasing the amount of retained austenite in the bainite microstructure. To have any of the desired effects, the Si content must be at least 0.60 wt.%.

[0014] However, since Si stabilizes the pro-eutectoid ferrite during cooling and the steel of the present invention must have a predominantly bainite microstructure, too much Si will result in the formation of excess pro-eutectoid ferrite during air cooling, which may lead to reduced hardness and strength since pro-eutectoid ferrite has inferior mechanical properties compared to a bainite microstructure.

[0015] Therefore, it is very important to carefully select the range of Si in the steel of the present invention, so the amount of silicon is selected in the range of 0.60-1.45 wt%. According to an embodiment, in order to have the best hardness and strength, the silicon content is 1.00-1.45 wt%.

[0016] Manganese (Mn): 0.25 to ≦0.80 wt% Mn is included in the steel of the present invention primarily to reduce hot cracking by forming MnS with sulfur, thereby avoiding the harmful formation of FeS. Therefore, Mn must be included in an amount of at least 0.25 wt.% to ensure MnS-type sulfides. Furthermore, Mn contributes to solid solution strengthening of the bainite microstructure, thereby having a favorable effect on the mechanical properties of the steel of the present invention. Mn also lowers the Bs temperature, thereby promoting the formation of a finer bainite microstructure, thereby improving both strength and ductility.

[0017] However, Mn reduces the austenitizing temperature, thereby reducing the austenite grain size during hot rolling. As a result, Mn also promotes pearlite formation upon cooling, despite being a hardenable element, at the expense of subsequent bainite formation. Mn also enhances work hardening and has a negative effect on the overall susceptibility to embrittlement, especially temper embrittlement. Furthermore, Mn can enhance softening when the steel is exposed to high service temperatures, which impairs hardness and strength.

[0018] Even small amounts of Mn have a strong hardening effect, and too much manganese leads to too high a quench hardenability and the formation of a high content of martensite during air cooling, which results in a decrease in ductility and impact toughness.

[0019] Due to these drawbacks, it has been found important to limit the amount of Mn in the steel of the present invention in order to allow for increased addition of other alloying elements and thereby avoid an excessive increase in hardenability. A careful selection of the Mn range is very important, so the Mn content is ≦0.80 wt.%. In one embodiment, Mn is ≦0.70 wt.%.

[0020] Chromium (Cr): 1.00 to 1.50% by weight Cr contributes to the solid solution strengthening of the bainite microstructure, thereby improving the mechanical properties of the steel of the present invention. It also improves the quench hardenability and reduces the Bs temperature. This reduces the Bs temperature, thereby improving the mechanical properties, particularly the strength and ductility.

[0021] In the steels of the present invention, Cr has been found to be a critical alloying element compared to the alloying elements Mn, Ni, and Si. Although a hardenable element, Cr has been found to have a much weaker hardenability effect at lower temperatures compared to higher temperatures, thus delaying the formation of pearlite, but avoiding the same limitations on bainite formation compared to Mn and Ni. Cr has also been found to add additional strength to the bainite microstructure compared to Ni, and not promote proeutectoid ferrite formation in the same way as Si.

[0022] However, too much chromium can lead to excessive hardenability, resulting in an increased content of martensite formed during air cooling, leading to a microstructure with impaired mechanical properties, such as reduced ductility and impact toughness. Too much chromium further increases the risk of grain boundary carbide precipitation during cooling, which can adversely affect ductility. On the other hand, too little chromium reduces the mechanical properties of the bainite microstructure. The chromium content should be 1.00-1.50 wt. %. Furthermore, the chromium content may be 1.10-1.50 wt. % for optimal mechanical properties.

[0023] Nickel (Ni): 0.10 to 0.60 wt% Nickel increases the quench hardenability of steel and improves the strength of the bainitic microstructure, but it also induces a solid-solution strengthening effect with a particularly strong toughening effect. The toughening effect increases impact strength, especially at low service temperatures. To ensure sufficient impact strength of the steel, the Ni content should be at least 0.10 wt.%. However, on the other hand, too much Ni can lead to an excessive amount of retained austenite, thereby reducing hardness and strength. At high service temperatures, a high Ni content can also impair tempering resistance, thereby reducing the hardness and strength of the steel over time. Furthermore, too much Ni can excessively increase quench hardenability, resulting in an increased martensite content during air cooling and a microstructure with impaired mechanical properties, such as reduced ductility and impact toughness. Therefore, in the steel of the present invention, the Ni content must be limited to 0.60 wt.%. Furthermore, Ni is an expensive alloying element, so it should be added in as small a quantity as possible and in a balanced manner. According to one embodiment, the Ni content may be 0.10 to 0.50 wt %.

[0024] 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 efficient at delaying the formation of pearlite during cooling and also suppresses temper embrittlement that can occur during slow cooling. Mo is particularly advantageous for reducing softening during use, i.e., improving temper resistance when the steel is exposed to high temperatures, and therefore helps to maintain hardness and strength. However, Mo is also an expensive element, and therefore is preferably kept as low as possible, but still added in amounts that affect properties. To ensure that Mo has these favorable effects, the amount should be at least 0.40 wt.%, with an upper limit of 0.80 wt.% for molybdenum.

[0025] Nitrogen (N): ≦0.020% by weight N may be added to the steel of the present invention because it has both interstitial solid solution strengthening and precipitation hardening effects, thereby improving the strength of the steel, particularly its 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 high a N content can also degrade hot workability during forging and rolling. Therefore, the upper limit is set to ≦0.020 wt%. When added, the N content is set to 0.005-0.020 wt%.

[0026] Phosphorous (P): ≦0.03% by weight P is an optional element and is considered an impurity since it is usually considered a deleterious element due to its embrittlement effect. It is therefore desirable to include ≦0.03 wt.% P.

[0027] Sulfur (S): ≦0.03% by weight S is an optional element that may be included to improve machinability. However, it is often considered an impurity because it can form grain boundary segregation and inclusions, thereby limiting hot workability and mechanical properties and causing increased anisotropy. Therefore, the S content should be ≤0.03 wt%. If added, the S content is set at 0.01-0.03 wt%.

[0028] Aluminum (Al): ≦0.05% by weight Al may be used as a deoxidizer, but it may also be added for grain refinement because it readily combines with nitrogen to form stable AlN precipitates, promoting toughness, especially at low temperatures. However, too much Al may degrade mechanical properties by reducing ductility. When added, the Al content is set to 0.01-0.05 wt%.

[0029] Optionally, small amounts of other alloying elements may be added to the bainitic steel of the present invention as defined above or below to improve its hot working properties, such as for example 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 is limited to a maximum of 0.05 wt. % while B is limited to a maximum of 0.005 wt. %.

[0030] The bainitic steel of the present invention may contain trace amounts of trace elements, such as tungsten (W), cobalt (Co), copper (Cu), titanium (Ti) and tantalum (Ta), vanadium (V) and / or niobium (Nb). Such trace elements should be considered as impurities, i.e., not intentionally added, which means that these elements are only allowed to be present in the steel in amounts such that the final properties of the steel are not affected. Impurities are therefore elements and / or compounds that are not intentionally added but cannot be completely avoided, for example because they are normally present as impurities in raw materials.

[0031] When the terms "up to" or "≦" are used, it will be understood by those skilled in the art that the lower limit of the range is 0% by weight, unless another number is specifically stated.

[0032] Other elements of the steel defined above or below are iron (Fe) and the impurities normally present, as mentioned above.

[0033] Thus, the present inventors have surprisingly discovered that the specific elemental composition of the present disclosure results in a bainite steel that provides wear resistance and embrittlement resistance. Furthermore, the bainite steel composition of the present invention provides reduced fatigue cracking and plastic deformation. Therefore, the composition of alloying elements is carefully tailored so that an object constructed from the bainite steel of the present invention contains the desired bainite content, i.e., a balanced content of ductile phases, and as little brittle or mechanically weak phases as possible. As such, the bainite steel of the present invention is suitable for dill applications.

[0034] According to one embodiment, the bainitic steel of the present invention consists of or comprises all of the elements mentioned herein and in the different ranges mentioned herein.

[0035] According to an embodiment, the bainitic steel comprises or consists of the following elements in % by weight: TIFF0007729838000001.tif70170

[0036] The balance is Fe, inevitable impurities, and optional elements, as described above. As described above, S, Al, and N may also be intentionally added.

[0037] According to an embodiment, the steel of the present invention has a chromium equivalent (Cr) of at least 2.70. eq ), it is believed that a desirable bainite microstructure is obtained and that the steel of the present invention has good strength (R p0.2) and a combination of good ductility, good impact toughness, and good hardness (hardness 3). eq ) is calculated according to the Schaeffler formula, and the units of the figures are weight percent. Cr eq =Cr+(1.5*Si)+(1*Mo)+(0.5*Nb)

[0038] According to one embodiment, the alloy of the invention as defined above or below 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 Si content is 1.00 to 1.45 wt.%.

[0039] According to an embodiment, the bainite steel as defined above or below has a yield strength (R ) of 1000 MPa or more for as-received drill rod samples. p0.2 The term "as received" means that the drill rod has been hot rolled and straightened.

[0040] According to an embodiment, the bainitic steel as defined above or below has a tensile strength (Rm) of 1400 MPa or more for as-received drill rod samples.

[0041] According to an embodiment, the impact toughness (IT) of the bainitic steel as defined above or below is >13 J at room temperature when using as-received drill rod samples.

[0042] According to one embodiment, the hardness after hardening, i.e. austenitizing and water quenching, when performed on as-received drill rod samples of bainitic steel as defined above or below, is in the range of 56-62 HRC (hardness 2).

[0043] The bainitic steels defined above or below and the drill rods produced therefrom can be produced using conventional steel manufacturing and steel machining and conventional drill rod manufacturing and machining.

[0044] An object or component comprising a bainitic steel as defined above or below is austenitized, hot rolled and air cooled to room temperature, thereby obtaining the desired bainitic microstructure during continuous cooling.

[0045] The mechanical properties of the surface of parts made of bainitic 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.

[0046] The steel according to the present disclosure, as referred to herein, is intended for the manufacture of drill parts, such as drill rods, for example tophammer drill rods.

[0047] The present disclosure is further illustrated by the following non-limiting examples. [Example]

[0048] Example 1 All of the alloys in Table 1, except for Alloy 7, were produced by melting scrap iron and alloy in a high-frequency furnace, followed by casting into ingots using a 9-inch steel mold. The compositions of the resulting alloys were as shown in Table 1. The balance consisted of iron and unavoidable impurities.

[0049] The weight of the ingot was approximately 270 kg. The ingot was heat-treated at 600-700°C for 4-8 hours, then air-cooled to room temperature, and the ingot surface was ground. The ingot was then heated to 1100-1250°C and hammer-forged into a rod with a circular dimension of approximately 130 mm. The rod was then air-cooled, heat-treated at 600-700°C for 4-8 hours, and air-cooled to room temperature.

[0050] In the next step, the bar was straightened, cut, turned, drilled, and a core inserted. The resulting round bar was then hot-rolled in a rolling mill at 1100-1250 °C to produce hexagonal hollow bars measuring 20-25 mm. After hot rolling, the bar was continuously air-cooled to room temperature. The core was removed, and the bar was cut to length and then straightened.

[0051] Example 2 Alloy 7 was produced by melting in a 75 MT electric arc furnace and then continuously casting to a 365 x 265 mm bloom. The alloy composition is shown in Table 1. The bloom was then heated to 1100-1250 °C and hot rolled to a diameter of approximately 125 mm.

[0052] The rods were heat treated at 700-850°C for 3-6 hours, then furnace cooled to 600°C and then air cooled to room temperature, after which they were straightened, cut, turned, drilled, and cores inserted.

[0053] The resulting round bars were then hot-rolled at 1100-1250°C in a rolling mill to produce hexagonal hollow bars with dimensions of 20-25 mm. After hot-rolling, the bars were continuously air-cooled to room temperature. The cores were removed, and the bars were cut to length and then straightened.

[0054] Example 3 - Mechanical Testing The results of the mechanical tests are shown in Table 2.

[0055] Hardness Test Three types of hardness measurements were performed at room temperature as HRC tests according to ASTM E 18-19. - hardness of as-rolled drill rod samples, That is, "as rolled" means after hot rolling and air cooling (hardness 1). - hardness of hardened drill rod samples, That is, it was hardened at 1000°C for 20 minutes and then water quenched (hardness 2). Hardening was performed on drill rod samples in the as-received condition, where the term "as-received" means that the drill rod was hot rolled and straightened before the sample was removed from the drill rod. - hardness of tempered drill rod samples, That is, it was tempered at 650°C for 30 minutes and then air cooled (hardness 3). Tempering was performed on both the as-received drill rod samples (hardness 3a) and the hardened drill rod samples (hardness 3b) and reported separately.

[0056] Hardness was measured on the longitudinal section of the 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 locations for all alloys except for Alloys 8 and 9, which were tested at one drill rod location. For hardened and tempered drill rod samples, hardness was measured at the cross section of the drill rod sample. All values ​​presented are based on the average of three or more indentations at each drill rod location. Tempering tests were not performed for Alloys 8, 9, and 10.

[0057] As can be seen from the examples, alloys 1-3 and 6-7 of the present invention have excellent hardness 3b. This means that these alloys have a superior 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. These hardness results mean that the tempering resistance is very good in both the as-received and hardened condition, without being bound by any theory, and in the hardened and as-rolled condition. It should be emphasized that when evaluating the alloys of the examples, the combined results of all the various mechanical tests performed were considered.

[0058] As can be seen from Table 2, the as-rolled hardness for all heats within the scope of the present invention will be 41-47 HRC (Hardness 1), which is the desired hardness to have optimal properties for the applications mentioned herein.

[0059] Tensile test Tensile test results 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, Figure 8 [E8M] using specimen 4. Values ​​presented are based on an average of two or more specimens. All alloys were tested in two drill rod locations except for alloy 10, which was tested in one drill rod location.

[0060] 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 in accordance with ISO 148-1:2016(E). 10 x 5 x 55 mm specimens with a V-notch were used in accordance with ISO 148-1:2016(E). The values ​​presented 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 test results, all of the alloys of the present invention had good results. Even if one of the reference alloys has a value close to one of the alloys of the present invention, all mechanical properties of each alloy must be considered when determining whether the alloy is good or poor.

[0061] Therefore, as can be seen from the results in Table 2, the bainite steel of the present invention has an optimized hardness to resist wear and reduce brittleness in drill applications.

[0062] Furthermore, as can be seen from Table 2, the bainite steel of the present invention has a balanced and optimized combination of mechanical properties such as hardness, tensile strength, and impact toughness to resist wear, deformation, fatigue, and softening caused by increased surface temperature due to frictional heat during drilling. TIFF0007729838000002.tif250170TIFF0007729838000003.tif250170

Claims

1. A bainitic steel for drill applications, comprising: C 0.33 to 0.40% by mass, Si 0.60-1.45% by mass, Mn 0.25-0.80% by mass, P≦0.03% by mass, S ≦0.03% by mass, Cr 1.00-1.50% by mass, Ni 0.10-0.60% by mass, Mo 0.40-0.80% by mass, N≦0.020% by mass, Al ≦0.05% by mass, The balance is Fe and unavoidable impurities Bainite steel consisting of

2. 2. The bainite steel according to claim 1, wherein the Mn content is 0.25 to 0.70 mass%.

3. The bainite steel according to claim 1 or 2, wherein the Ni content is 0.10 to 0.50 mass%.

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

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

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

7. 2. The bainite steel according to claim 1, wherein the Ni content is 0.10 to 0.40 mass%, the Mn content is 0.25 to 0.55 mass%, and the Mo content is 0.55 to 0.80 mass%.

8. The bainite steel according to claim 7, wherein the Si content is 1.00 to 1.45 mass%.

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

10. A drill part comprising a bainitic steel according to any one of claims 1 to 8.

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