JP2016506451A - Bainite steel for rock drilling components - Google Patents

Abstract

In mass% (wt%), C: 0.16-0.23, Si: 0.8-1.0, Mo: 0.67-0.9, Cr: 1.10-1.30, V: 0.18 to 0.4, Ni: 1.60 to 2.0, Mn: 0.65 to 0.9, P: ≦ 0.020, S: ≦ 0.02, Cu: ≦ 0.20, N : Bainitic steel containing 0.005 to 0.012, balance iron, and inevitable impurities. [Selection] Figure 1

Description

  The invention relates to a bainite steel according to the preamble of claim 1. The invention further relates to a drill rod component according to the preamble of claim 7. The invention further relates to a method for manufacturing a drill rod component according to the preamble of claim 10. The invention also relates to the use of the new bainitic steel according to the preamble of claim 15.

  Drill rods for mining and construction operations typically comprise a central rod portion, a threaded male end and a threaded female end. During operation, a drill head or drill bit is screwed onto the male end of the rod and the drill head is driven into the rock or ground by a drill tool. One type of drill is called a so-called “top hammer drill” and is arranged so that the drill tool provides high speed rotational motion and vibration to the drill rod. As the drill hole length progresses, the drill rod can be stretched by further threading the drill rod onto the end of the preceding drill rod.

  Drill rods can be manufactured by forging and threading into male and female connectors that mate the ends of a steel rod. However, the most common practice today is to manufacture male and female connectors separately and then attach the connectors to each end of the steel rod by friction welding.

  One problem associated with drill rods is that they have a relatively short service life, as the rate at which the drill rods wear and need to be replaced directly affects the overall cost of the drilling operation. An additional problem is the strength of the rod. If the rod breaks, it can take a considerable amount of time to retrieve the rod from the drill hole.

  Some work has been done in the past to improve drill rods. For example, WO 97/27022 addresses the problem of soft material zones that occur at the interface between the connector and the central rod after friction welding. When the connector and central rod are friction welded together, heat is released into the interface between the connector and the central rod. The overheated zone is referred to as a “heat affected zone” (HAZ). The steel in the HAZ is tempered and a soft material zone is created at the interface between the rod and the connector. The soft area is the weakest part of the drill rod and is typically the position where the drill rod breaks. In order to solve this problem, WO 97/27022 discloses a steel having a balanced chemical composition so that the most tempered portion of the HAZ has a hardness equal to the core hardness of the drill rod. is suggesting.

  The steel described in WO 97/27022 has resulted in an improved service life of the drill rod, particularly in terms of interface failure between the connector and the central rod. However, the overall service life of the drill rod is still not sufficient.

  Field observations show that today's drill rod failure rarely occurs at the interface between the connector and the central rod. Instead, the longevity of the drill rod appears to be limited to breakage in the threaded portion of the connector.

International Publication No. 97/27022 Pamphlet

  The object of the present invention is therefore to solve at least one of the above-mentioned problems. In particular, it is an object of the present invention to achieve an improved steel composition that allows the production of drill rods with a long service life. An additional object of the present invention is to achieve a cost-effective drilling component that can be used over a long period of time. An additional object of the present invention is to achieve a method for manufacturing wear resistant drill components. Again, an additional object of the present invention relates to the use of an improved steel composition in rock drilling components.

According to the present invention, at least one of these purposes is (in mass% (wt%)):
C: 0.16-0.23,
Si: 0.8 to 1.0,
Mo: 0.67 to 0.9,
Cr: 1.10 to 1.30,
V: 0.18-0.4,
Ni: 1.60 to 2.0,
Mn: 0.65-0.9,
P: ≦ 0.020,
S: ≦ 0.02
Cu: ≦ 0.20,
N: 0.005 to 0.012% by mass
And is achieved by a bainite steel containing the balance iron and inevitable impurities.

  The new steel is primarily intended for the production of surface hardened components that undergo repeated wear at elevated temperatures, i.e., 300-500 ° C, such as surface hardened and threaded connectors in drill rods, for example. These components have a martensite surface area and a bainite-martensite core.

  Results from field tests conducted during top hammer drilling showed that surface hardened drill rods made from new steels surprisingly last longer than drill rods made from conventional steels.

  During top hammer drilling or soil excavation on the ground, the drill rod receives strong vibrations from the drill tool. The vibration causes a shock wave that travels down through the interconnected drill rod to the drill bit at the bottom of the hole. As the shock wave travels through the interconnected rods, approximately 5% of its energy is lost in the form of heat, which is primarily due to the male and female connectors of the interconnected drill rods. Release into the thread. As a result, during top hammer drilling, the working temperature in the connector is typically as high as 300 ° C, but can reach 500 ° C. During top hammer drilling on the ground, air is typically used to cool the drill rod and remove drill cuts. However, the air is not an effective cooling fluid and is sufficient to prevent the released heat from transforming the martensite surface in the drill rod connector threads into the softer phases cementite and ferrite. Never cool the rod. In conventional drill rods, the martensitic transformation can soften the surface of the thread and eventually cause the connector to wear. Because it is tacky, wear resistance is directly related to hardness.

  The reason for the surprisingly long service life of drill rods made from new steel is not fully understood. However, without being bound by theory, due to the balanced amount of alloying elements silicon, molybdenum, chromium and vanadium in the steel, the martensitic surface of the drill rod connector is at high working temperatures during top hammer drilling. Hardness is maintained.

  Silicon stabilizes the ε-carbide and thus delays the transformation of the hard martensite surface area of the connector into softer cementite and ferrite until a temperature of about 300 ° C. is reached. However, as the temperature in the connector increases during drilling, the martensitic phase in the surface of the surface hardened connector will eventually begin to transform into cementite and ferrite. The amount of martensite in the surface area of the connector is thereby reduced, and eventually the hardness of the surface area is also reduced. Carbon is released into the steel while martensite is transformed into cementite and ferrite.

  In the new steel, the alloying elements molybdenum, chromium and vanadium form hard and stable carbides containing excess carbon originating from the transformed martensite phase. Hard carbide precipitates in the remaining martensitic phase of the connector, thereby compensating for the hardness lost by the transformation of martensite to cementite.

  The core part of a connector consists of martensite and bainite. Bainite is a fine mixture of cementite and ferrite phases. Bainite is stable at high temperatures and is therefore still strong enough to support the hardened surface area of the connector at high processing temperatures.

  According to an alternative form, in the new steel, the Si content is between 0.85 and 0.95% by weight.

  According to an alternative form, in the new steel, the Mo content is 0.70 to 0.80 mass%.

  According to an alternative form, in the new steel, the Cr content is 1.20 to 1.25% by mass.

  According to an alternative form, in the new steel, the content of V is 0.20 to 0.30% by weight, preferably 0.2 to 0.25% by weight.

  According to an alternative form, in the new steel, the N content is 0.005 to 0.008% by mass, more preferably 0.008 to 0.012% by mass.

  The invention further relates to a rock drilling component comprising a novel steel.

  The component can be a threaded male connector for a drill rod or a threaded female connector.

  For example, the component is a drill rod comprising a threaded male connector and a threaded female connector.

The present invention further provides a method for producing a rock drilling component comprising:
a. Forming a rock drilling component from a new steel as described above;
b. Heating the component to an austenitizing temperature;
c. Maintaining the component at an austenitizing temperature in a carbon-containing atmosphere for a predetermined time;
d. Cooling the component.

  Preferably, the component is heated to a temperature of 900-1000 ° C.

Preferably, the component is heated in an atmosphere of CO and H _2.

  Preferably, the component is heated for 3-6 hours.

  Preferably, the component is cooled in air.

The invention further relates to the use of the new bainite steel in surface hardened connectors for drill rods during air cooled top hammer drilling on the ground.
Detailed Description of the Invention

  The new steel contains the following elements in mass% (wt%):

  Carbon (C). Carbon is included in the new steel for strength and dominates the final steel structure that should be bainite-based. Carbon is also added to the new steel to ensure the formation of carbides. Carbide provides a precipitation hardening effect for the bainite structure of steel. Carbide further prevents the grains in the steel from growing due to agglomeration, thereby ensuring fine grains in the steel and consequently high strength. The carbon content should therefore be at least 0.16% by weight in the steel. When there is too much carbon content, the impact strength of steel will be reduced. Therefore, carbon should be limited to 0.23 wt%. Preferably, the carbon is 0.18-0.20% by weight.

  Silicon (Si) is used as an oxygen scavenger in the manufacture of steel, so some amount of silicon is always present in the steel. Silicon has a positive effect on the new steel because it increases the hardenability, ie the rate at which the austenite phase transforms into martensite during quenching. In new steels, silicon is an important alloying element because it delays the transformation of martensite to cementite and ferrite.

  Martensite is an unstable phase, and when heated, it transforms into cementite and ferrite via various carbides, which causes the steel to decrease in hardness. Silicon stabilizes ε-carbide, one of the carbides that progresses to the cementite phase during the transformation of martensite, thereby delaying the transformation of martensite. Furthermore, during the decomposition of the martensite phase, the carbon must diffuse through the steel to the carbide in order for the carbide to grow. The presence of silicon in the steel increases the carbon activity in the steel, thereby also delaying the growth of already formed carbides and the nucleation of new carbides. In addition, this mechanism substantially delays the formation of martensite. Thus, silicon has a positive effect in maintaining the strength of the surface area of the new steel surface-hardened component at high temperatures.

  However, silicon stabilizes the ferrite so that too much silicon will lead to an increase in A1 temperature. This has a negative effect because the steel must be heated to a higher temperature during hardening, thereby growing the grains in the austenite phase and thus reducing the strength. Eventually, the amount of silicon is limited to 0.80-1.0% by weight in the new steel. Preferably, the amount of silicon is 0.85 to 0.95% by weight.

  Molybdenum, chromium and vanadium are important elements in the new steel because they form hard carbide that compensates for the decrease in hardness when the martensite phase transforms to cementite and ferrite. Different carbide formers, molybdenum, chromium and vanadium, form stable carbides at various temperatures. Therefore, mainly carbides with a high molybdenum content are deposited at low temperatures and therefore in the mild transformation of martensite. As the temperature increases, the martensitic transformation increases. However, at higher temperatures, carbides with a high chromium content will precipitate first, and then at higher temperatures, carbides with a high vanadium content will precipitate. This provides the effect that the martensite hardness at the surface of the connector remains substantially constant over a wide range of processing temperatures.

  Molybdenum (Mo) produces stable carbides with a high molybdenum content at temperatures from 300 ° C. to about 500 ° C., and compensates for a decrease in hardness when the martensite phase is transformed into cementite and ferrite. In order to ensure that a sufficient amount of carbide is precipitated, the amount of molybdenum should be at least 0.67% by weight. However, molybdenum stabilizes austenite and thus has a very strong effect on hardenability. Therefore, too much molybdenum content leads to the formation of martensite in the core of the connector, thereby making it easier to break the connector. High molybdenum content can cause further secondary maximum hardness. Therefore, the upper limit for molybdenum in the new steel is 0.9 mass%. Preferably, molybdenum is 0.67 to 0.83 mass% in the steel.

  Chromium (Cr) forms a stable chromium-rich carbide with carbon. Some high-carbide carbides precipitate even at low temperatures, ie even 300 ° C. However, most of the carbides with high chromium content are deposited at temperatures between 400-500 ° C. The new steel should contain at least 1.10% by weight chromium in order to ensure that a sufficiently high chromium content of carbide is formed. A very high chromium content can lead to the formation of a so-called secondary maximum hardness in the steel at high temperatures, typically above 600 ° C. This phenomenon is generally caused by the formation of carbides with a high chromium content, vanadium content and molybdenum content. However, as the temperature of the steel increases further, the hardness rapidly decreases due to the growth of precipitated carbide, which results in carbon steel from other precipitates in the steel. Therefore, chromium should be limited to 1.30% by weight. Preferably, the chromium content is 1.20 to 1.25 in the new steel to ensure that a sufficient amount of carbide is formed and the formation of secondary maximum hardness is avoided. .

  Vanadium (V) produces very fine carbides with a high vanadium content at temperatures of 550-600 ° C., and therefore compensates for the decrease in hardness when the martensite phase transforms to cementite and ferrite at high temperatures. The new steel should contain at least 0.18 wt% vanadium to ensure that a sufficient amount of vanadium carbide precipitates in the steel at high processing temperatures.

  Vanadium also forms vanadium carbonitride at high temperatures, ie, 900 ° C. or higher. Vanadium carbonitride is important because it prevents austenite grain growth during carburizing of the steel. Since the carbonitrides are very stable and do not decompose in the annealing step prior to hot working, too much vanadium content can cause problems during hot working of steel. Therefore, vanadium must be limited to 0.40 mass% in the new steel. Preferably, the vanadium is 0.18 to 0.30 wt%, more preferably 0.20 to 0.30 wt%, and even more preferably 0.20 to 0.25 wt%. It is.

  Manganese (Mn) is included in the new steel to form MnS with sulfur, which can be present as an impurity in the steel. Manganese lowers the Ms temperature, that is, the temperature at which martensite begins to form after austenitization, and thus has a positive effect on the hardenability of the steel. The low Ms temperature also results in a fine bainite structure in the core of the connector made from the new steel. This is a plus to ensure high strength in the connector core. Manganese should be included in an amount of at least 0.65% by weight to ensure a MnS type sulfide. Since manganese lowers the Ms temperature, a high content of manganese can lead to the formation of retained austenite in the steel. Therefore, manganese should be limited to 0.85% by weight. This content of manganese again guarantees a fine bainite structure in the new steel, so the manganese content is preferably 0.70 to 0.80% by weight in the steel.

  Phosphorus (P) is present as an impurity in the new raw materials for steel. Phosphorus separates into a liquid phase during solidification of the steel, resulting in a streak with a high phosphorus content in the solidified steel. Thus, a high phosphorus content adversely affects the ductility and impact toughness of the steel. Therefore, phosphorus should be limited to a maximum of 0.020 wt%, i.e. 0-0.020 wt% in the new steel.

  Sulfur (S) is also present as an impurity in the new steel raw materials. Sulfur forms sulfide inclusions in the steel that adversely affect the ductility and impact toughness of the steel. Therefore, sulfur should be limited to 0.02 wt%, i.e. 0 to 0.020 wt%, more preferably up to 0.015 wt% in the new steel.

  Nickel (Ni) is an important element in the new steel that increases the impact strength of the steel and thus targets drill rods. Nickel further reduces the Ms temperature of the steel, thereby increasing hardenability. In order to ensure sufficient impact strength in the steel, the nickel content should be at least 1.60% by weight. If the nickel content is too high, the Ms temperature will be too low, which can lead to the formation of retained austenite in the steel. Residual austenite can cause tensile stress in the martensite phase, which can reduce the strength of the steel. Therefore, the nickel content should be limited to 2.0% by weight in the new steel. Furthermore, nickel is an expensive alloying element and for that reason it should be present in as little content as possible. Preferably, the nickel content is between 1.70 and 1.90% by weight in the new steel, since this amount of nickel results in a cost effective steel with sufficient impact strength.

  Copper (Cu) is typically included in metal scrap used as a raw material. Copper can be tolerated with a content of up to 0.20% by weight, ie 0-0.20% by weight.

  Nitrogen (N). In order to ensure that a stable vanadium carbonitride is formed during carburization, preferably the new steel contains nitrogen. Preferably, the nitrogen content is 0.005% by mass, and more preferably 0.008% by mass. If the steel contains too much nitrogen, the vanadium carbonitride becomes too stable to decompose during heating to the hot working temperature of the steel. Therefore, the maximum nitrogen content is 0.012% by mass.

In the hot-rolled state, the new steel has a bainite structure throughout, ie cementite (Fe ₃ C) and ferrite (α-iron). “Hot rolling” means that a new steel is produced by casting and then heated to a temperature of about 1200 ° C., subjected to hot rolling and subsequently cooled in air.

  In the hardened state, the new steel has a martensite surface area and a bainite / martensite core.

1 is a schematic view of a manufactured rock drilling component including new steel. FIG. 2 is a graph showing the results from an experiment conducted on new steel. 2 is a table showing results from experiments conducted on new steel. FIG. 6 is a graph showing the surface hardness of samples of tests performed on new steel and comparative steel. It is a graph which shows the core part hardness of the sample of the test implemented with the novel steel and the comparison steel. FIG. 6 is a diagram produced in a ThermoCalc ™ simulation experiment conducted on a new steel and a comparative steel. FIG. 6 is a diagram produced in a ThermoCalc ™ simulation experiment conducted on a new steel and a comparative steel. FIG. 6 is a diagram produced in a ThermoCalc ™ simulation experiment conducted on a new steel and a comparative steel. FIG. 6 is a diagram produced in a ThermoCalc ™ simulation experiment conducted on a new steel and a comparative steel. FIG. 6 is a diagram produced in a ThermoCalc ™ simulation experiment conducted on a new steel and a comparative steel.

  FIG. 1 schematically shows a longitudinal cross section of a drill component according to a first embodiment of the invention. The drill component shown in FIG. 1 is an MF drill rod 1 with a central rod portion 10. The first end of the central rod 10 is provided with a male connector 20 and the second end of the central rod is provided with a female connector 30. The male connector 20 includes an outer thread 21 and the female connector includes an inner thread 31. The male and female connectors and the threads 21 and 31 are dimensioned so that the male connector 20 of the first MF rod can be received in the female connector 30 of the second MF rod. Is done. The MF rod further comprises a hole extending through the central channel 60, i.e. the entire MF rod. The channel includes one opening 61 in the center of the male connector and one opening 61 in the center of the female connector. In operation, a cooling fluid such as air is directed through channel 60.

  In FIG. 1, a male connector 20 and a female connector 30 are attached to the central rod portion 10 by friction welding indicated by a broken line 11. However, the MF rod of FIG. 1 can also be manufactured in one piece, ie the male connector 20 and female connector 30 can be formed by forging and threading the rod end.

  Connector 20 and connector 30 are manufactured from bainite steel according to the present invention. The central rod 10 can be made from another type of steel, such as conventional low alloy carbon steel. However, the central rod can also be manufactured from bainite steel according to the invention.

  Connector 20 and connector 30 are surface hardened and have a bainite core 40 and a martensite surface area 50. The martensite surface area is 1 to 3 mm thick and extends from the surface of the connector toward its core.

  Although the novel drill component has been described with respect to an MF rod, it is clear that the drill component may be any other type of component that is subject to repeated wear under high processing temperatures, such as a drifter rod. is there.

  Preferably, the novel drill component is manufactured by a method comprising the following steps.

  In the first step, the drill component is formed of bainite steel according to the present invention. This is typically accomplished by forging a new steel precursor into the male connector 20 and female connector 30 and threading. The precursor is typically part of a solid rod made from new steel.

  In the second step, the connector undergoes surface hardening. This means that the connector is heated in the furnace to an austenitizing temperature, above 900 ° C. for new steels. The furnace can be of any type, for example a pit furnace. To ensure complete austenitization of the connector and avoid adverse effects such as grain expansion, the connector should be heated to a temperature between 900 ° C. and 950 ° C., preferably 925 ° C.

The step of austenizing the connector is carried out in an atmosphere rich in carbon to ensure so-called carburization, where the carbon content increases in the surface area of the connector. Typically, the furnace atmosphere is a mixture of H ₂ gas and CO gas, such as cracked methane.

  The connector is kept in the furnace for 3-6 hours. Time governs the depth of the surface, ie the thickness of the martensite surface area. In order to ensure a sufficient surface depth, a time of 5 hours is preferred.

  When the heating temperature expires, the currently austenized connector is removed from the furnace and cooled with ambient air. Forced air cooling is employed by blowing air over the connector.

  During cooling, the carburized surface of the austenitized connector transforms into martensite and the connector core transforms into a mixture of bainite and martensite.

  The connector can then undergo a tempering step to optimize the hardness of the martensite surface. Therefore, tempering is performed at 200-300 ° C. for 1 hour.

  Finally, the connector is attached to the central rod portion by friction welding.

Four examples that do not limit the new steel will be described below.
Example 1

Example 1 illustrates the results of a field test conducted using a surface hardened drill rod made from a new bainitic steel.

  In the first step, a new steel melt was generated. Melting was generated by melting the metal scrap in an electric arc furnace, refining the molten steel in a CLU converter, and then casting into an ingot in a mold 24 ".

The resulting new steel had the following composition:

  Rods were made from new steel. Some rods were forged into threaded female connectors and some rods were forged into threaded male connectors.

The male connector and the female connector received surface hardening. In the first step, the connector was carburized in a pit furnace containing an atmosphere of CO and H ₂ at a temperature of 925 ° C. for 5 hours.

  After 5 hours, the connector was removed from the furnace and allowed to cool in air. Surface hardening resulted in a martensite layer extending from the surface of the connector toward the core having a bainite / martensite structure.

  The connector was then attached to the end of a steel rod, also made from a new steel material. A male connector was attached to one end of the rod and a female connector was attached to the other end. The connector was attached by friction welding.

  A field test was then carried out with drill rods from the new steel at two different locations, location A and location B. Drilling was carried out with a drill bit having a diameter of 115 mm and a Sandvik DP1500 type drill tool. The excavation speed was about 1 meter / min.

  For comparison, a conventional drill rod was also used. These rods were made from steel grade Sanbar 64.

Nine rods of each type (new and conventional) were used at location A and 4 rods of each type were used at location B. The drill rods were used until they were damaged, and the total number of meters drilled with each rod was recorded as the “drilling meter” (dm). Table 2 shows the test results as the average number of drilling meters drilled per rod at location A and location B.

As can be seen in Table 1, the new steel drill rod had a much longer working life than the rod of conventional material.
Example 2

  In the second example, the hardness reduction of the test specimens from the new steel was determined under laboratory conditions at various reheat temperatures.

  In the first step, a new steel melt was generated. Melting was generated by melting the metal scrap in an electric arc furnace, refining the molten steel in a CLU converter, and then casting into an ingot in a mold 24 ".

The resulting new steel had the following composition:

  The ingot was rolled into a bar, and the bar was cut into a 5 cm long cylinder and used as a sample.

  The sample was then subjected to a simulated curing process. This treatment includes heating to the austenitizing temperature, maintaining the austenitizing temperature for a given temperature, and then cooling in oil heated to room temperature. The cured sample was then reheated to simulate the temperature during the excavation operation. After reheating, the sample was cooled in air. After cooling of the reheated samples, measurements were taken at the surface, mid-radius, and center of each sample. Hardness was measured with Vickers (HV1).

  As a reference, one sample of each set was left cured but not reheated.

  Twelve samples were used for each austenitizing temperature. The austenitizing temperature was 860 ° C. with a holding time of 1 hour, 880 ° C. with a holding time of 1 hour, and 925 ° C. with a holding time of 20 minutes. After quenching in oil, the sample was reheated at the following temperature. No reheating, 200 ° C, 300 ° C, 400 ° C, 500 ° C, 550 ° C, 580 ° C, 600 ° C, 650 ° C, 675 ° C, and 700 ° C.

  The measurement results are shown graphically in FIG. FIG. 2 shows the results for each austenitizing temperature as an average value for the hardness measured at each reheating temperature. Specific measurements are shown in Table 4, see FIG.

  It should be noted that the experiment is performed on non-carburized samples. However, it is clear from the graph of FIG. 2 that the hardness of the three different sample sets is constant from 650 ° C. to the non-reheated sample. The constant hardness is attributed to the stabilizing effect of silicon on the martensite phase at low temperatures and is due to the hard and stable precipitation of chromium, molybdenum and vanadium at higher temperatures, thereby martensite Appears to compensate for transformation to cementite and ferrite. At 700 ° C., a secondary hardness maximum is formed, after which the hardness is rapidly reduced by agglomerating into a coarse precipitate with less Cr carbide, Mo carbide, and V carbide. The growth of Cr carbide, Mo carbide, and V carbide further causes the remaining martensite to decompose into cementite and ferrite, thereby further reducing the hardness.

It is clear that the carburized sample of the new steel material is harder than the non-carburized sample at all reheat temperatures. However, the hardness of the carburized sample appears to exhibit an essentially constant hardness up to about 650 ° C.
Example 3

  In the third example, a comparison was made of the surface hardness and core hardness of the hardened and tempered samples of the alloys according to the invention and the comparative alloys. The test simulates the tempering effect that occurs in a surface-hardened drill rod due to the heat released in the joint during drilling. For comparison, an alloy similar to the alloy disclosed in the literature of WO 97/27022 was selected. WO 97/27022 discloses an alloy that is optimized for friction welding and briefly discussed under the section “Background of the invention”, which is an application of the invention.

The chemical compositions of the new and comparative alloys are shown in Table 5 below. Comp 0.09 displays the comparative alloy and Inv 0.22 displays the new alloy.

  A 1 kg melt comparative alloy was produced by conventional methods including melting, refining, and casting of metal scrap in an induction furnace. The casting was preheated in a furnace at 700 ° C. for about 30 minutes and then hot rolled at 1200 ° C. into a square bar with dimensions of 13 mm. The rod was then slowly cooled in air and cut into 13 × 13 mm samples.

  75 tons of new alloys were produced by conventional methods used in manufacturing, including melting in an EA furnace, AoD processing, out-of-furnace refining, continuous casting and hot rolling. The resulting casting of the new material was hot rolled into a bar with a diameter of 40 mm.

  The new material bar was cut into samples measuring 40 × 130 mm.

The sample was then carburized and hardened by forced air cooling. The sample carburization was carried out in a propane / nitrogen / methanol atmosphere according to the following program. In step 1, the sample was first heated to a processing temperature of 925 ° C. for 150 minutes and then held at that temperature for 435 minutes.

The cured samples were then tempered at different temperatures. Prior to tempering, it was painted with No-Carb ™ to prevent decarburization. Table 7 below shows the tempering temperature for each sample. Each alloy of one sample was kept untempered. Each remaining sample was tempered for 30 minutes.

  After tempering, the hardness of the core and surface of each sample was measured. The surface hardness was measured by Rockwell hardness (HRC), and the core hardness was measured by Vickers hardness (HV30). The surface hardness of the various samples is shown in FIG. The core hardness of various samples is shown in FIG.

  From FIG. 4 it can be concluded that the non-tempered samples of the new alloy and the comparative alloy have similar surface hardness. This is due to the fact that each non-tempered sample surface structure consists essentially of martensite. The hardness of the sample that is not tempered decreases as the tempering temperature increases. However, it can be clearly seen from the graph of FIG. 4 that the surface hardness of the new alloy is higher than that of the comparative alloy for all tempering temperatures up to 600 ° C. That is, the new alloy has a higher tempering resistance than the comparative alloy.

  Surprisingly, the surface hardness of the new alloy remains much more stable than the surface hardness of the comparative alloy as the tempering temperature increases. As can be seen from FIG. 4, the surface hardness of the new alloy is essentially constant at Rockwell Hardness (HRC) 57 up to 200 ° C., but decreases to Rockwell Hardness 55 at 200 ° C. and then essentially up to 300 ° C. Progresses constantly. On the other hand, the surface hardness of the comparative alloy decreases continuously over the entire temperature interval.

  At higher temperatures, the martensite decomposition rate increases and vanadium carbide aggregates into coarse grains, thereby reducing the surface hardness. At 700 ° C., vanadium carbide becomes unstable and the surface hardness of both the new and comparative samples decreases rapidly.

From FIG. 5, it can be concluded that the core hardness in the new sample is slightly lower than the core hardness in the comparative sample. The main reason for the relatively low core hardness of the new alloy is that the high vanadium content combined with the selected nitrogen content produces a stable vanadium carbonitride during the sample carburizing step. is there. Less vanadium carbonitride prevents grain growth during the carburizing step and increases the impact toughness of the core. Small grains also reduce the hardenability of the alloy, thereby ensuring that after hardening, the core consists of bainite that is substantially less hard than martensite but is tougher.
Conclusion

  The results from the third example show better tempering resistance in the new alloy than in the comparative alloy. The surface hardness of the new alloy is more stable compared to the comparative material.

During rock drilling, the ability to have a stable surface hardness is important for wear resistance. Since adhesive wear resistance is directly related to hardness, materials that maintain surface hardness even when the temperature rises during excavation will better withstand wear. The relationship between surface hardness and core hardness is still an important factor for the threads used in drill rods. The desired relationship is a hard surface for better wear resistance along with a tough core for better impact resistance. A large difference between surface hardness and core hardness results in greater residual compressive stress, which increases fatigue life. Considering this, the new alloy with high vanadium content is advantageous over the comparative material with low vanadium content and has a higher surface hardness with a tougher core, which is the opposite of the comparative material. provide.
Example 4

  In the four examples, simulations were performed in the program of ThermoCalc ™ 3.0 and database TCFE7. The purpose of the simulation experiment was to confirm the measurement results of the core hardness of the new sample and the comparative sample in the third example. An additional objective was to confirm that good results for the core hardness of the new sample existed over the desired range of nitrogen and vanadium of the new alloy.

  Simulated experiments show the stability of vanadium carbonitrides at various temperatures in the new and comparative alloys. As explained further below, the presence of vanadium carbonitride at the carburizing or hot working temperature will have a significant effect on the microstructure in the final component of the core.

  FIG. 6 shows a first ThermoCalc ™ simulation of the stability of vanadium carbonitride formed in a novel alloy containing 0.2 wt% vanadium content and 0.005 wt% nitrogen content. The chart produced by experiment is shown. The overall composition of the alloy in the simulation is 0.019C, 0.9Si, 0.75Mo, 1.2Cr, 0.20V, 1.8Ni, 0.78Mn, 0.005N.

  FIG. 6 shows in moles the amount of various precipitated phases present in the alloy system at different temperatures. The y-axis indicates the amount of precipitated phase, and the x-axis indicates temperature. Line 1 shows the amount (in moles) of vanadium carbonitride present in the alloy system at various temperatures. The other lines in the chart show the other phases present in the new alloy system. These phases are not discussed in further detail.

  Focusing on line 1 in FIG. 6, it can be seen that the precipitation of vanadium carbonitrides increases with increasing temperature at temperatures in the range of 700-800 ° C. Above 800 ° C., the vanadium carbonitride stops precipitating and the precipitated vanadium carbonitride begins to decompose due to the equilibrium in the alloy system. Eventually, there may be less vanadium carbonitride at higher temperatures. Thus, the amount of carbonitride in the alloy system decreases with increasing temperature. In the alloy system of FIG. 6, it can be seen that there is a relatively large amount of vanadium carbonitride at temperatures ranging from 900 to 1000 ° C. The chart further shows that vanadium carbonitride completely decomposes at about 1100 ° C.

  The above vanadium carbonitride distribution should ensure good core properties in components made from the new alloy for the following reasons.

  First, in the manufacture of rock drilling components, the components are carburized and cured at 930 ° C. At this temperature, the grains in the steel try to agglomerate into a few large grains.

  In general, the grain size of steel affects the hardenability of steel in the sense that the hardenability of steel increases with increasing grain size. Thus, after hardening, steel with a small grain size will have a pronounced bainite structure, while steel with a large grain size will have a martensite structure.

  The presence of a relatively large amount of vanadium carbonitride at 930 ° C. in FIG. 6 effectively prevents grain growth in the new steel by preventing the alloy grains from agglomerating. It should be. This should result in small grains in the new alloy and a pronounced bainite structure in the core of the hardened component produced therefrom. This is important for core strength and impact toughness, and structural stability at high temperatures.

  Second, it can be concluded from FIG. 6 that all vanadium carbonitrides decompose at about 1100 ° C. Of course, this is important for the hot workability of steel. However, it is even more important that the vanadium carbonitride remaining after hot working does not have an adverse effect on the grain size during alloy hardening. In the curing step, the remaining vanadium carbonitride will aggregate into a small amount of very large grains. Since these grains have little effect on preventing grain growth during carburizing / hardening, the result is a core consisting mainly of a martensite structure with low toughness and thus weak impact strength Will bring the component.

  FIG. 7 shows a second ThermoCalc ™ simulation of the stability of vanadium carbonitride formed in a novel alloy containing 0.2 wt% vanadium content and 0.012 wt% nitrogen content. The chart produced by experiment is shown. This simulation confirms the results of the first simulation. Therefore, this simulation also shows that a sufficient amount of vanadium carbonitride is present in the alloy at a temperature interval of 900-1000 ° C. to ensure a bainite structure in the alloy core after hardening. Show. Furthermore, from the chart it can be concluded that the vanadium carbonitride completely decomposes at about 1130 ° C.

  It can be noted that the high nitrogen content in the second simulated experiment leads to more vanadium carbonitride precipitation at 930 ° C. compared to the first simulated experiment. Of course, this is a plus to guarantee the bainite structure of the core.

  FIG. 8 shows a third ThermoCalc ™ simulation of the stability of vanadium carbonitride formed in a novel alloy containing 0.3 wt% vanadium content and 0.005 wt% nitrogen content. The chart produced by experiment is shown. The simulated alloy contained the following composition: 0.019C, 0.9Si, 0.75Mo, 1.2Cr, 0.1V, 1.8Ni, 0.78Mn, 0.005N.

  This simulated experiment again shows that a sufficient amount of vanadium carbonitride was deposited at 900-1000 ° C. and that all vanadium carbonitride was decomposed at a temperature of 1120 ° C.

  Compared to the first and second simulation experiments, more vanadium carbonitride is precipitated in the third simulation experiment. This is because the alloy has a higher vanadium content.

  FIG. 9 shows a fourth ThermoCalc ™ simulation of the stability of vanadium carbonitride formed in a novel alloy containing 0.3 wt% vanadium content and 0.012 wt% nitrogen content. The chart produced by experiment is shown. The simulated alloy contained the following composition: 0.019C, 0.9Si, 0.75Mo, 1.2Cr, 0.1V, 1.8Ni, 0.78Mn, 0.005N.

  This simulated experiment again shows that a sufficient amount of vanadium carbonitride was present at 900-1000 ° C and that the vanadium carbonitride decomposed at temperatures below 1200 ° C.

  FIG. 10 shows the fifth ThermoCalc ™ of the stability of vanadium carbonitride formed in a comparative alloy containing a low vanadium content (0.1 wt%) and a nitrogen content of 0.005 wt% The chart produced by the simulation experiment is shown. The simulated alloy is similar to the alloy used in Example 3, with the following composition: 0.019C, 0.9Si, 0.75Mo, 1.2Cr, 0.1V, 1.8Ni, 0.78Mn 0.005N.

  From line 1 in FIG. 10, it can be concluded that there is a very low content of vanadium carbonitride in this alloy at a temperature interval of 900-1000 ° C. Since the vanadium carbonitride content in this alloy is too low, grain growth cannot be prevented during carburizing, and thereby in the core of the hardened component produced from this alloy. Will result in increased hardenability and martensite formation. Therefore, the simulation experiment confirms the measurements made for the core hardness of the comparative alloy of Example 3.

  In summary, from the results of five ThermoCalc ™ simulation experiments and physical experiment 3, it can be concluded that the optimal balance between surface hardness and core hardness is achieved in the novel alloy. . The optimum balance between surface hardness and core hardness makes the new alloy very suitable for use in rock drilling components.

Claims (15)

In mass% (wt%)
C: 0.16-0.23,
Si: 0.8 to 1.0,
Mo: 0.67 to 0.9,
Cr: 1.10 to 1.30,
V: 0.18-0.4,
Ni: 1.60 to 2.0,
Mn: 0.65-0.9,
P: ≦ 0.020,
S: ≦ 0.02
Cu: ≦ 0.20,
N: 0.005 to 0.012% by mass
And bainitic steel containing the balance iron and inevitable impurities.   The bainite steel according to claim 1, wherein the content of Si is 0.85 to 0.95 mass%.   The bainite steel according to claim 1 or 2, wherein the content of Mo is 0.70 to 0.80 mass%.   The bainite steel according to any one of claims 1 to 3, wherein the content of Cr is 1.20 to 1.25 mass%.   The bainite steel according to any one of claims 1 to 4, wherein the content of V is 0.20 to 0.30 mass%, preferably 0.2 to 0.25 mass%.   The bainite steel according to any one of claims 1 to 5, wherein the content of N is 0.008 to 0.012 mass%.   Rock drilling component (10, 20, 30) comprising the steel according to any one of claims 1 to 6.   The component according to claim 7, wherein the component is a threaded male connector or a threaded female connector (20, 30) for a drill rod (10).   9. Component according to claim 7 or 8, wherein the component is a drill rod (10) comprising a threaded male connector and a threaded female connector (20, 30). A method for producing a rock drilling component (10, 20, 30) comprising:
a. Forming the rock drilling component (10, 20, 30) according to any one of claims 7 to 9 with the steel according to any one of claims 1 to 6;
b. Heating the component (10, 20, 30) to an austenitizing temperature;
c. Maintaining the components (10, 20, 30) at austenitizing temperature in a carbon-containing atmosphere for a predetermined time;
d. Cooling the component.   The method of claim 10, wherein the component is heated to a temperature of 900-1000 ° C. The component is heated in an atmosphere of CO and H _2, The method of claim 10 or 11.   13. A method according to any one of claims 10 to 12, wherein the component is heated for 3 to 6 hours.   14. A method according to any one of claims 10 to 13, wherein the component is cooled in air.   Use of bainite steel according to any one of claims 1 to 6 in a surface hardened connector for a drill rod during air cooled top hammer drilling on the ground.

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