A production method for controlling banded structure of CrNiMo carburized drill rod

Abstract

This invention discloses a method for producing CrNiMo carburized drill rods with controlled banded microstructure. The method uses continuously cast billets as raw material and employs the following production process: continuously cast square billet – rolling – drilling and core loading – strip core rolling – core removal – drill bit making – carburizing, quenching, and tempering. Using continuously cast billets as raw material, the steel is rolled with a pressure ratio of 8 to improve its density. The rolled material is then drilled and cored, with the core coated with a lime solution. By controlling the heating temperature and time, strip core rolling is performed to produce hollow steel for the drill rod. After rolling, rapid water cooling reduces the banded microstructure to below grade 1.5. Following the subsequent drill bit making process, surface carburizing is performed to control the carburized layer to 0.9–1.1 mm with a carbon concentration of 0.5–0.7%. After homogenization at 860℃ for 30 minutes, the rod is quenched in a cooling tower. Rapid cooling at a rate greater than 20℃ / min is used between 500 and 860℃, with the cooling rate controlled at 5.5℃ / min in the low-temperature range. The drill rod produced using this method has a matrix structure banding level controlled within 1.0, good microstructure properties, high homogeneity, and significantly improved drilling life.

Description

Technical Field

[0001] This invention relates to a method for producing rock drilling rods, and more particularly to a method for producing CrNiMo carburized drill rods with controlled banded structure, belonging to the field of steel production technology. Background Technology

[0002] The drill rod is an important component of rock drilling tools. During operation, it transmits impact energy to the drill bit for rock drilling. The core hole is circulated with high-pressure water or compressed air for cooling the drill bit and removing slag. The drill rod is a slender rod that mainly bears bending fatigue stress, high-frequency tensile and compressive fatigue impact, and torsional stress. The core hole is also subject to stress corrosion. The working environment is extremely harsh, and the uniformity of the drill rod's structure has a significant impact on its fatigue performance and corrosion resistance.

[0003] Currently, CrNiMo alloy structural steel is used for carburizing treatment both domestically and internationally, with 22CrNi3Mo being the primary choice. Major material manufacturers include Sandvik and OVAKO, while domestic suppliers typically include steel mills such as Shanghai Baosteel or Daye. However, due to differences in manufacturing processes, the matrix microstructure of the drill rod has a high banding grade, which not only significantly impacts the drill rod's lifespan but also results in poor stability and short drilling life for domestically produced drill rods, lagging far behind imported products. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to provide a production method for controlling the banded structure of CrNiMo carburized drill rods, thereby reducing the banded structure level of the drill rod matrix and improving the stability and drilling life of the drill rod.

[0005] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0006] A method for producing CrNiMo carburizing drill rods with controlled banded structure, using continuously cast billet as raw material, and the production process route is as follows: continuously cast billet – drilling – core loading – hot rolling of strip with core – core pulling – drill making – carburizing, quenching and tempering.

[0007] The method uses continuously cast billets as raw material, and produces drill rods through processes such as drilling, core loading, hot rolling with core, core pulling, drill bit making, and carburizing heat treatment. Using continuously cast billets as raw material offers advantages such as high compositional uniformity, ensuring drill rod density to improve bending fatigue performance, and low production cost. Hot rolling with core after drilling is used to produce hollow steel raw material for drill bit making, while simultaneously controlling the banding level of the drill rod base material. Post-rolling water cooling improves the uniformity of material grain size and reduces the banding level. During carburizing, quenching, and tempering, staged quenching is used to increase the quenching rate in the 500–860℃ range, achieving the removal of banding levels in the finished drill rod.

[0008] This invention is mainly for low-carbon carburizing drill rod steel, with 22CrNi3Mo as the representative steel grade. Other similar low-carbon carburizing drill rods can also be applied.

[0009] This invention discloses a method for controlling the banded microstructure of a CrNiMo carburized drill rod, comprising the following specific steps:

[0010] (1) Continuous casting billets are used as raw materials, and the rolling ratio of the continuously cast billets is required to be greater than 8. The dimensions of the rolled material are φ160~180 and the length is 1.5~3 meters.

[0011] (2) The loose core material is removed by mechanical drilling of the continuously cast billet. The drilling size is φ43~67. After drilling, a mandrel is inserted into the inner hole. The gap between the mandrel and the mandrel is 1~1.5mm. The surface of the mandrel is coated with lime water with a thickness of 0.2~0.4mm.

[0012] (3) The raw material with the mandrel installed is put into the heating furnace for heating. The temperature of the preheating section is below 550℃, the temperature of the first heating section is controlled at 940~1100℃, the temperature of the second heating section is controlled at 1120~1200℃, the temperature of the soaking section is controlled at 1140~1200℃, the heating time is controlled at 2.8~3.5h, and the furnace exit temperature is controlled at 1060~1140℃.

[0013] (4) The rolling process adopts semi-continuous rolling. The initial rolling temperature is controlled at 1050-1100℃, and the continuous rolling temperature is controlled at 940℃~980℃. The final rolling temperature is 890-950℃, and the rolling specifications are hollow steel of H35, R38, R46 and R52.

[0014] (5) After rolling, the bars are rapidly cooled by water quenching. The surface temperature of the bars exiting the quencher is controlled at 680-740℃, and the temperature entering the bed is controlled below 740℃. The material entering the bed is collected quickly, and the collection temperature is controlled above 300℃. The rolled material is collected quickly and then slowly cooled in the pit, with a heat preservation time of 24 hours. After heat preservation, the hollow steel is cut to length and the core material is extracted.

[0015] (6) After the hollow steel is cut to length, the parts that need to be upset are induction heated to 1050-1150℃ and then forged. Immediately after forging, the two ends are softened and annealed at 300mm from the end face. The annealing temperature is 680℃±10℃ and the annealing time is 2.5 hours. The connecting threads are then machined.

[0016] (7) After cold working, the drill rod is carburized at a temperature of 910℃±10℃ using a strong carburizing + diffusion process. The carbon potential is 1.0~1.1 during the strong carburizing period and 0.7 during the diffusion period. After diffusion, the drill rod is cooled to 860℃ for homogenization and held at that temperature for 30 minutes. The homogenized material is then removed from the furnace and placed in an air-cooling tower for quenching. The cooling rate is controlled at 20℃ / min or higher between 500℃ and 860℃, and at 5.5℃ / min between 300℃ and 500℃. Below 300℃, rapid cooling is used with a cooling rate greater than 5.4℃ / min to room temperature. The quenched material is then tempered in a furnace at 160~200℃.

[0017] Furthermore, the production method uses continuously cast billets as raw materials, with a rolling ratio greater than 8, and then proceeds to drill cores, hot-roll the core strip, and core-pulling before brazing.

[0018] Furthermore, the production method requires hot rolling of the strip with a core, a gap of 1 to 1.5 mm between the mandrel and the base material, and the surface of the mandrel must be coated with a layer of paint with a thickness of 0.2 to 0.4 mm to prevent welding between the mandrel and the base material during the later rolling process.

[0019] Furthermore, the production method employs staged quenching during carburizing and quenching, with the cooling rate controlled at 20℃ / min or higher between 500℃ and 860℃, and at 5.5℃ / min between 300℃ and 500℃. Below 300℃, rapid cooling is employed, with a cooling intensity greater than 5.4℃ / min to room temperature.

[0020] Optionally, the coating applied to the surface of the mandrel in the production method is a lime water solution with a concentration of 50-70%. The thickness of the lime water coating is 0.2-0.4 mm.

[0021] Beneficial effects: Compared with existing technologies, this invention develops a new drill rod manufacturing process. Drill rods produced using this method have a uniform matrix structure, with the banding level controlled within 0.5 grade, significantly improving the uniformity of the structure. This greatly enhances the longitudinal and transverse properties of the material, as well as the bending fatigue performance of the threaded portion and the stress corrosion resistance of the inner bore.

[0022] The drill rod produced using this method has a stripe grade controlled at 0.5, a grain size controlled at 8.5 or higher, and a carburized layer thickness controlled at less than 2.0. The drill rod's rock-drilling life reaches advanced levels both domestically and internationally. By rapidly cooling the steel after rolling with water, the stripe grade is reduced to below 1.5. Subsequent drill-making processes, including surface carburizing, control the carburized layer at 0.9–1.1 mm and the carbon concentration at 0.5–0.7%, significantly improving the rock-drilling life. Detailed Implementation

[0023] The present invention will be described in detail below with reference to specific embodiments and examples, thereby making the advantages and various effects of the present invention more clearly apparent. Those skilled in the art should understand that these specific embodiments and examples are for illustrative purposes only and are not intended to limit the present invention.

[0024] The technical solution of this application embodiment is to solve the above-mentioned technical problems, and the general idea is as follows:

[0025] According to a typical embodiment of the present invention, a method for producing a controlled banded microstructure of a CrNiMo carburizing drill rod is provided, comprising the following steps:

[0026] (1) Continuous casting billet is used as raw material, and the size of the continuous casting billet is controlled at φ160~180, with a pressure ratio greater than 8.

[0027] (2) After drilling, core loading and hot rolling are performed, followed by water cooling.

[0028] (3) Hot-rolled finished products are cut to length and cored.

[0029] (4) Wire rod manufacturing.

[0030] (5) Carburizing, quenching and tempering.

[0031] The production method uses continuously cast billets as raw materials, with a pressure ratio greater than 8 to ensure the density of the raw materials and control the strip grade of the base material. Then, the process involves drilling, core loading, hot rolling with the core, and core pulling to produce the drill bit.

[0032] In the aforementioned production method, a coating of 0.2–0.4 mm is required on the surface of the mandrel, and the gap between the mandrel and the base material is controlled at 1–1.5 mm. This ensures the roundness of the mandrel hole while preventing the base material from welding to the mandrel during the rolling process.

[0033] The method describes a rolling heating process where the temperature of the second stage is controlled at 1120–1200℃, the temperature of the soaking stage is controlled at 1140–1200℃, and the heating time is controlled at 2.8–3.5 h.

[0034] The reason for controlling the rolling heating temperature and time is to ensure high-temperature diffusion of the billet, reduce component segregation caused by casting, and ensure uniform temperature between the core and surface of the billet. Temperatures exceeding 1200℃ for extended periods may produce overheated structures, exacerbate decarburization, and affect the surface quality of the rolled product. Heating times within 2.8 hours result in uneven temperatures inside and outside the billet, while heating times exceeding 3.5 hours can also lead to severe decarburization and oxidation.

[0035] The production method employs rapid water cooling of the bars after rolling, with the surface temperature of the bar exiting the water cooler controlled at 680–740°C and the temperature entering the bed controlled below 740°C.

[0036] By employing post-rolling water quenching, the material is rapidly passed through the two-phase region, preventing grain growth and the precipitation of the second phase, thereby achieving the goal of controlling the banded shape.

[0037] The production method employs staged quenching during the quenching process after carburizing, with the cooling rate controlled at 20-25℃ / min between 500℃ and 860℃.

[0038] The cooling rate is controlled within the range of 500–860℃ to rapidly pass through the two-phase region, achieving uniform microstructure control. Cooling rates exceeding 25℃ / min increase the thermal stress on the drill rod, leading to uneven temperature distribution across different parts of the rod within the furnace and increasing its bending degree.

[0039] As an optional rolling process, a semi-continuous rolling process is adopted, with the initial rolling temperature controlled at 1050-1100℃, the continuous rolling temperature controlled at 940℃~980℃, and the final rolling temperature at 890-950℃, producing hollow steel with specifications of H35, R38, R46 and R52.

[0040] Controlling the initial rolling temperature at 1050-1100℃ is to ensure the optimal temperature for continuous rolling and final rolling, and to control the grain size of the material. The final rolling temperature needs to be controlled within the range of 850℃-950℃ because when rolling at temperatures below 850℃ or above 950℃, low-carbon high-nickel steel is prone to developing coarse grains and mixed grain structures.

[0041] Alternatively, a lime solution can be used to coat the surface of the mandrel, with a thickness of 0.2–0.4 mm.

[0042] The surface of the mandrel is coated with lime water to a thickness of 0.2–0.4 mm to prevent the mandrel and base material from welding and sticking together during subsequent high-temperature rolling. A thickness greater than 0.4 mm increases the difficulty of core loading.

[0043] The production method of the steel for the shank of the rock drill in this application will be described in detail below with reference to specific embodiments.

[0044] Example 1:

[0045] This embodiment provides a method for producing a controlled banded microstructure on a CrNiMo carburized drill rod, comprising the following steps:

[0046] (1) φ180 continuously cast billet is used as raw material, and the rolling pressure ratio of the billet is greater than 8.

[0047] (2) Drill a hole in a 2.75-meter φ180 rolled steel bar. The drilling size is as follows:

[0048] (3) Using φ47 0 -0.5The bar stock is coated with lime water and loaded into the base material before being placed into the heating furnace for heating. The preheating section temperature is 530℃, the first heating section temperature is controlled at 965℃, the second heating section temperature is controlled at 1160℃, the soaking section temperature is controlled at 1180℃, the heating time is controlled at 2.8h, and the furnace exit temperature is 1080℃.

[0049] (4) Initial rolling temperature 1050℃, continuous rolling temperature 960℃, final rolling temperature 910℃

[0050] (5) After rolling, the water is passed through the water tank. The temperature of the water exiting the water tank is 690℃, and the temperature of the water entering the bed is 725℃. After cutting to length, the water is placed in the heat preservation pit for 24 hours.

[0051] (6) After the rolled material is cut to length, the core is removed. The cored material is then brazed and carburized. After carburizing, it is directly heated to 860℃ for half an hour and then quenched and tempered in a cooling tower. The first cooling rate is 21℃ / min. When the temperature drops to 520℃, the cooling rate is reduced to 5℃ / min. When the temperature drops to 300℃, rapid cooling is performed, and the cooling rate is increased to 5.5℃ / min.

[0052] Example 2:

[0053] This embodiment provides a method for producing a controlled banded microstructure on a CrNiMo carburizing drill rod, comprising the following steps:

[0054] (1) φ165 continuously cast billet is used as raw material, and the rolling pressure ratio of the billet is greater than 8.

[0055] (2) Drill a hole in a 2.75-meter φ165 rolled steel bar. The drilling size is as follows:

[0056] (3) Adopt The bar stock is coated with lime water and loaded into the base material before being placed into the heating furnace for heating. The preheating section temperature is 550℃, the first heating section temperature is controlled at 980℃, the second heating section temperature is controlled at 1140℃, the soaking section temperature is controlled at 1160℃, the heating time is controlled at 2.9h, and the furnace exit temperature is 1080℃.

[0057] (4) Initial rolling temperature 1080℃, continuous rolling temperature 970℃, final rolling temperature 915℃

[0058] (5) After rolling, the water temperature is 700℃ when exiting the water tank and 730℃ when entering the bed. After cutting to length, place in the heat preservation pit for 24 hours.

[0059] (6) After the rolled material is cut to length, the core is extracted, the cored material is brazed and carburized. After carburizing, it is directly heated to 860℃ for half an hour and then sent to a cooling tower for quenching and tempering. The first cooling rate is 22℃ / min. When the temperature drops to 520℃, the cooling rate is reduced to 4.5℃ / min. When the temperature drops to 300℃, rapid cooling is performed and the cooling rate is increased to 5.8℃ / min.

[0060] Comparative Example 1:

[0061] The production of ordinary drill rods includes the following steps:

[0062] (1) φ180 continuously cast billet is used as raw material, and the rolling pressure ratio of the billet is greater than 8.

[0063] (2) Drill a hole in a 2.75-meter φ165 rolled steel bar. The drilling size is as follows:

[0064] (3) Adopt The bar stock is loaded into the base material and then fed into the heating furnace for heating. The preheating section temperature is 550℃, the first heating section temperature is controlled at 980℃, the second heating section temperature is controlled at 1060℃, the soaking section temperature is controlled at 1070℃, the heating time is controlled at 2.3h, and the furnace exit temperature is 1000℃.

[0065] (4) Initial rolling temperature 980℃, continuous rolling temperature 930℃, final rolling temperature 890℃

[0066] (5) After cutting to length, place in an insulated pit for 24 hours.

[0067] (6) After the rolled material is cut to length, the core is extracted, the core material is used for brazing and carburizing. After carburizing, it is directly heated to 860℃ for half an hour and then sent to a cooling tower for quenching and tempering. The first section uses a constant wind speed and the cooling rate of the brazing rod is 4.5~18℃ / min. After the temperature drops to 300℃, it is rapidly cooled to increase the cooling rate to 5.8℃ / min.

[0068] The CrNiMo alloy steels provided in Examples 1-2 and Comparative Example 1 were prepared as the same steel for the drill bit, and physical testing was conducted for comparison. The specific results are shown in the table below.

[0069]

[0070]

[0071] As can be seen from the table above, the steel materials for the drill bit provided in Examples 1-2 of this application are superior to those in Comparative Example 1 in terms of uniformity of microstructure, such as grain size, infiltration layer structure, and banding level. The drilling life of the drill bit products manufactured from these materials is also significantly improved.

[0072] Throughout this specification, unless otherwise specified, the terminology used herein should be understood as having the meaning commonly used in the art. Therefore, unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. In case of any conflict, this specification shall prevail. The technical terms used herein are for the purpose of describing particular embodiments only and are not intended to limit the scope of protection of this invention. Unless otherwise specified, all raw materials, reagents, instruments, and equipment used in this invention are commercially available or can be prepared by existing methods.

Claims

1. A method for producing a CrNiMo carburized drill rod with controlled banded microstructure, characterized in that, Includes the following steps: (1) Continuous casting billet rolling is used as raw material, and the rolling ratio of continuous casting billet is required to be greater than 8; (2) Mechanical drilling is used to remove the loose material in the core of the continuously cast billet. After drilling, a mandrel is inserted into the inner hole, and the gap between the mandrel and the core hole is 1 to 1.5 mm. Before inserting the mandrel, a coating with a thickness of 0.2 to 0.4 mm is applied to the surface of the mandrel. (3) The raw material with the mandrel loaded is put into the heating furnace for heating. The temperature of the preheating section is below 550℃, the temperature of the first heating section is controlled at 940~1100℃, the temperature of the second heating section is controlled at 1120~1200℃, the temperature of the soaking section is controlled at 1140~1200℃, the heating time is controlled at 2.8~3.5h, and the furnace outlet temperature is controlled at 1050~1150℃. (4) The rolling process adopts semi-continuous rolling, with the initial rolling temperature controlled at 1050-1100℃, the continuous rolling temperature controlled at 940℃~980℃, and the final rolling temperature at 850-950℃. The rolling specifications are H35, R38, R46 and R52 hollow steel. (5) After rolling, the bar is rapidly cooled by water quenching. The surface temperature of the water quencher is controlled at 680-740℃, and the temperature of the bar entering the bed is controlled below 740℃. The material entering the bed is quickly collected, and the collection temperature is controlled above 300℃. The rolled material is rapidly collected and slowly cooled in the pit. The heat preservation time is 24 hours. After heat preservation, the hollow steel is cut to length and the core material is extracted. (6) After the hollow steel is cut to length, the parts that need to be upset are induction heated to 1050-1150℃ and then forged. Immediately after forging, the two ends are softened and annealed at 300mm from the end face. The annealing temperature is 680℃±10℃ and the annealing time is 2.5 hours. After annealing, the connecting threads are machined. (7) After cold working, the drill rod is carburized at a temperature of 910℃±10℃. The strong carburizing + diffusion process is used for carburizing. The carbon potential is 1.0~1.1 during the strong carburizing period and 0.7 during the diffusion period. After diffusion, the drill rod is cooled to 860℃ for homogenization. After reaching the temperature, it is held for 30 minutes. After homogenization, the material is taken out of the furnace and put into the air-cooling tower for quenching. The cooling rate is controlled at 20~25℃ / min during the period of 500℃~860℃ and at 5.5℃ / min during the period of 300~500℃. Rapid cooling is used below 300℃. The cooling rate is greater than 5.4℃ / min to room temperature. After quenching, the material is put into the furnace for tempering at 160~200℃.

2. The method for producing a controlled banded microstructure on a CrNiMo carburizing drill rod according to claim 1, characterized in that, In step (1), the rolled material has a size of φ160~180 and a length of 1.5~3 meters.

3. The method for producing a controlled banded microstructure on a CrNiMo carburizing drill rod according to claim 1, characterized in that, In step (2), the drilling size is φ43~67; the coating is a lime water solution with a concentration of 50~70%.

Images