PG2 steel forged thick-walled pipe material with large length-diameter ratio and heat treatment process

Abstract

The application relates to PG2 steel forged large-length-ratio thick-wall pipe material capable of bearing high temperature, high strength and high-low temperature impact and a heat treatment process. The material is PG2 steel, and the chemical components include the following according to mass percentage: C: 0.28-0.33%; Si: 0.15-0.35%; Mn: 0.75-0.95%; Cr: 1.80-2.10%; Ni: 3.00-3.50%; Mo: 1.20-1.40%; V: 0.20-0.35%; W: 0.20-0.50%; P: <=0.010%; S: <=0.005%; Cu: <=0.10%; Al: <=0.030%, and the balance is iron. Through sectional heating grain refinement and structure regulation technology, and in combination with the use of an inner hole cooling special tool installed at the bottom of a water tank, the PG2 steel forged large-length-ratio thick-wall pipe is uniformly and rapidly cooled, so that the mechanical properties and grain size of the PG2 steel forged large-length-ratio thick-wall pipe meet the design target, the material has high hardenability, the steel has normal temperature and 700 DEG C strength and low temperature impact toughness after quenching and tempering, and the requirements of high strength and high toughness are met.

Description

Technical Field

[0001] This invention belongs to the field of production technology of thick-walled tube materials with large length-to-diameter ratio, and specifically relates to a PG2 steel forged thick-walled tube material with large length-to-diameter ratio that can withstand high temperature, high strength and high and low temperature impact and heat treatment process. Background Technology

[0002] The mechanical properties of existing P-steel thick-walled tubes with large length-to-diameter ratios are no longer sufficient for harsh service environments (requiring high strength at room temperature and high strength at high temperatures, as well as high toughness at low temperatures to improve safety and service life). Therefore, designing new materials and their heat treatment processes has become an urgent priority. Currently, there are many types of new materials available, but conventional heat treatment processes cannot meet the requirements for both high-temperature strength and low-temperature toughness. Summary of the Invention

[0003] The purpose of this invention is to overcome the shortcomings of the existing technology and provide a material and heat treatment process for forging PG2 steel for large length-to-diameter ratio thick-walled tubes with high strength and toughness, thereby improving the service life of the tubes and meeting the requirements of high-temperature use.

[0004] The objective of this invention is achieved as follows:

[0005] A type of PG2 steel forged thick-walled pipe material with a large length-to-diameter ratio is disclosed. The material is PG2 steel, which has high hardenability. After quenching and tempering, this steel exhibits high strength at room temperature and 700℃, as well as low-temperature impact toughness, meeting the requirements of high strength and high toughness. Its chemical composition, by mass percentage, includes the following: C: 0.28~0.33%; Si: 0.15~0.35%; Mn: 0.75~0.95%; Cr: 1.80~2.10%; Ni: 3.00~3.50%; Mo: 1.20~1.40%; V: 0.20~0.35%; W: 0.20~0.50%; P: ≤0.010%; S: ≤0.005%; Cu: ≤0.10%; Al: ≤0.030%, with the balance being iron.

[0006] A heat treatment process for PG2 forged thick-walled tubes with large length-to-diameter ratios, the manufacturing process of which includes the following steps:

[0007] Step 1) Place the workpiece formed by the precision forging machine on the material rack and air cool it to 500-550℃. Then place it in a waiting furnace at 600℃-650℃ to avoid prolonged high temperature in the core of the workpiece, which would cause grain coarsening. Hold it at 600℃-650℃ for 1-2 hours per 100mm to eliminate forging stress, structural stress, and temperature stress. Air cool it to 280℃-320℃ and hold it for 1.5-2 hours per 100mm to make the internal and external temperatures of the workpiece more uniform and remove hydrogen. Increase the temperature at a rate of ≤70℃ / h to 860℃-910℃ and hold it at a uniform temperature. Heat 1-2 h / 100 mm, air cool to 350-400℃, then air cool to 280-320℃ before furnace entry. Hold at 280-320℃ for 1.5-2 h / 100 mm to achieve uniform and refined microstructure, reduce internal stress, and remove hydrogen. Increase the temperature at ≤70℃ / h to 650-680℃ and hold for 8-15 h / 100 mm to achieve uniform and refined microstructure, reduce internal stress, and remove hydrogen. Furnace cool to ≤150℃ before air cooling. This process lays the microstructure foundation for subsequent performance heat treatment to obtain high strength and high toughness.

[0008] Step 2), Quenching: The workpiece after Step 1) is machined and non-destructive tested; the machined and inspected large length-to-diameter ratio thick-walled tube blank is vertically suspended in the effective heating zone of a pit-type resistance furnace using special tooling, with an initial furnace temperature ≤450℃. Heating is done in stages: the temperature is increased at ≤50℃ / h to 600~650℃ for 2 hours to remove machining stress; then the temperature is rapidly increased to 950~1000℃ and held for 2~3 hours to dissolve the carbides; the temperature is then reduced to 860~910℃ and held for 1~2 hours to homogenize the composition before being taken out of the furnace for quenching; the cooling medium is water.

[0009] Step 3) Performance tempering: Place the workpiece from step 2) into a tempering furnace for performance tempering. The furnace temperature is ≤450℃. Increase the temperature at ≤50℃ / h to 550~610℃ and hold for 4 hours to remove structural stress and temperature stress. Remove from the furnace for hot straightening. Put it back into the furnace and hold at 550~610℃ to remove straightening stress and structural stress generated by the decomposition of residual austenite. Air cool.

[0010] In step 2), to obtain ultra-high strength and high toughness, the inner hole is cooled using a special tooling cooling method after quenching. The specific quenching cooling method includes the following steps:

[0011] Step a) Air cooling: ≤60S, including workpiece hoisting time; hoist the thick-walled tube into the well-type water tank with the stirring system turned on, align the inner hole of the thick-walled tube of the workpiece with the inner hole cooling nozzle installed at the bottom of the water tank, and cool the inner wall of the workpiece.

[0012] Step b) Water cooling: Water cooling is performed at a rate of 2.5 to 6 min / 100 mm, with an initial water temperature of 20 ± 5℃.

[0013] The beneficial effects of this invention are as follows:

[0014] 1. This invention provides an ultra-high strength PG2 forged thick-walled tube material with a large length-to-diameter ratio and a heat treatment process. While ensuring the forging does not crack, it achieves a reasonable balance of strength and toughness, meeting the design and usage requirements of the thick-walled tube material. The thick-walled tube material of this invention uses PG2 steel. Through heat treatment design, it meets the design and usage requirements of the thick-walled tube material.

[0015] 2. The material involved in this invention has high hardenability. After quenching and tempering, the steel exhibits high strength at room temperature and 700℃, as well as low-temperature impact toughness, meeting the requirements of high strength and high toughness. V primarily refines the grain size and combines with carbon to form dispersed VC, resulting in secondary hardening. Cr is a weak carbide-forming element; chromium carbides are generally fine and uniformly distributed in the steel, giving it high strength, hardness, and wear resistance. Mo mainly increases hardenability, improves tempering stability, reduces austenite grain growth tendency, and forms carbides with C, resulting in secondary hardening. The addition of Mn and Si aims to improve material strength, and Mn and Si respectively enhance hardenability and oxidation resistance. Nickel is a non-carbide-forming element; it exists in steel by dissolving into the matrix lattice, forming a substitutional solid solution, which improves the hardenability and toughness of the steel. Tungsten is an important element in PG2; its addition to the steel improves its thermal stability and hardness. W can combine with C to form tungsten carbide, increasing the hardness and wear resistance of steel and giving it better high-temperature resistance.

[0016] 3. The production process route of this invention is designed as follows: normalizing after forging + dehydrogenation annealing → roughing → quenching and tempering heat treatment; the implementation of the technical solution mainly lies in controlling the range of chemical composition, the pre-heat treatment after forging, and the final heat treatment process. The chemical composition is designed as PG2 steel, which has a high alloy content, far exceeding the requirements of hardenability for thick-walled pipe materials. After quenching and tempering, this steel has excellent room temperature and low temperature impact performance, and can achieve the requirements of high strength and high toughness.

[0017] 4. Design of pre-forging heat treatment: This steel grade belongs to No. IV steel, which is highly sensitive to white spots and prone to mixed crystals. After forging, normalizing + dehydrogenation annealing + two over-holding processes are adopted to obtain a balanced structure of ferrite + carbides, which facilitates hydrogen overflow and isolates the inheritance of the structure. Through normalizing + dehydrogenation annealing, the grains can be refined, the internal structure can be improved, forging stress can be eliminated, white spots can be avoided, and the inheritance of the structure can be isolated.

[0018] 5. Final heat treatment design: Segmented heating to a higher austenitizing temperature with a shorter holding time is employed to dissolve carbides without causing grain coarsening; cooling to the normal quenching temperature and holding there ensures composition homogenization; and a special internal cooling fixture is used for uniform and strong cooling. Through tempering treatment, fine grains and a uniform tempered sorbite structure are obtained.

[0019] 6. The PG2 steel thick-walled pipe produced by the process of this invention has a room temperature yield strength ≥1173Mpa, a V-shaped impact energy at -40℃ ≥20J, a high temperature tensile strength at 700℃ ≥350Mpa, and an actual grain size ≥6.

[0020] 7. This invention designs a heat treatment process for the new material, fully exploring its potential and meeting the design and usage requirements of thick-walled tubes. The mechanical properties of conventional P-steel and those required by this patent are shown in Table 1.

[0021] Table 1

[0022] Rp0.1 (MPa) Z(%) -40℃ KV2(J) 700℃ Rm (MPa) Grain size (grade) Conventional P steel ≥1104 ≥40 ≥20 200~300 ≥5 levels PG2 conventional process ≥1173 ≥40 ≥20 300~350 ≥5 levels PG2 This invention process ≥1173 ≥40 ≥20 ≥350 ≥6 levels Detailed Implementation

[0023] Example 1: A forged PG2 steel material for thick-walled pipes with a large length-to-diameter ratio, characterized in that: the material is selected as PG2 steel, which has high hardenability. After quenching and tempering, this steel has high strength at room temperature and 700℃, as well as low-temperature impact toughness, meeting the requirements of high strength and high toughness; its chemical composition, by mass percentage, includes the following: C: 0.28~0.33%; Si: 0.15~0.35%; Mn: 0.75~0.95%; Cr: 1.80~2.10%; Ni: 3.00~3.50%; Mo: 1.20~1.40%; V: 0.20~0.35%; W: 0.20~0.50%; P: ≤0.010%; S: ≤0.005%; Cu: ≤0.10%; Al: ≤0.030%, with the balance being iron. The length-to-diameter ratio is 30~45, the minimum wall thickness is 65mm, and the length is greater than 10m.

[0024] A heat treatment process for the aforementioned PG2 steel forged large length-to-diameter ratio thick-walled tube material includes the following manufacturing steps:

[0025] Step 1) After the workpiece is formed by the hydraulic press and precision forging machine, the preparatory heat treatment adopts normalizing + dehydrogenation annealing: Place the workpiece formed by the precision forging machine on the material rack and air cool to 500-550℃, then place it in a waiting furnace at 600℃-650℃ to avoid prolonged high temperature in the core of the workpiece, which would cause grain coarsening. Hold at 600℃-650℃ for 4 hours to eliminate forging stress, structural stress and temperature stress. Air cool to 280℃-320℃ and hold for 7 hours to make the internal and external temperatures of the workpiece tend to be uniform and remove hydrogen, with a temperature ≤70℃ / h. The workpiece is heated to 860℃~910℃, held at a uniform temperature for ≥6 hours (visually assessed), and then air-cooled to 350℃~400℃ before being placed in the furnace at 280℃~320℃. It is then held at 280℃~320℃ for 10 hours to ensure uniform internal and external temperatures and remove hydrogen. The temperature is then increased at ≤70℃ / h to 650℃~680℃ and held for 40 hours to achieve a uniform and refined microstructure, reduce internal stress, and remove hydrogen. Finally, it is furnace-cooled to ≤150℃ and then air-cooled. This process lays the microstructural foundation for subsequent performance heat treatment to achieve high strength and high toughness.

[0026] Step 2) Quenching: The forgings completed in Step 1) are rough-machined and non-destructive tested. The qualified large length-to-diameter ratio thick-walled tube blanks are vertically suspended in the effective heating zone of the pit-type resistance furnace using special tooling. The furnace temperature is ≤450℃ (elastic deformation zone). Heating is done in stages: the temperature is increased at ≤50℃ / h to 600~650℃ for 2 hours to remove processing stress; then the temperature is rapidly increased to 950~1000℃ and held for 2~3 hours to dissolve the carbides; the temperature is then reduced to 860~910℃ and held for 1~2 hours to homogenize the composition before quenching. The cooling medium is water.

[0027] Step 3) Performance tempering: The thick-walled tube from step 2) is placed into a tempering furnace for performance tempering. The furnace temperature is ≤450℃, and the temperature is increased at ≤50℃ / h to 550~610℃ for 4 hours. The temperature is held for 4 hours to remove structural stress and temperature stress. After removing from the furnace, the tube is hot-calibrated and then placed back into the furnace at 550~610℃ for 4 hours to remove straightening stress and structural stress generated by the decomposition of residual austenite. Finally, the tube is air-cooled.

[0028] In step 2), to obtain ultra-high strength and high toughness, the inner hole is cooled using a special cooling fixture after immersion in water. The specific quenching and cooling method includes the following steps: a) Air cooling: ≤60S (workpiece hoisting time); the thick-walled tube is hoisted into a well-type water tank with the stirring system turned on, and the inner hole of the thick-walled tube is aligned with the nozzle of the special inner hole cooling fixture installed at the bottom of the water tank to uniformly cool the inner wall. b) Water cooling: 2.5~6min / 100mm, with an initial water temperature of 20±5℃;

[0029] After quenching and strong cooling treatment, the martensitic transformation and decomposition of retained austenite are completed. Subsequently, high-temperature tempering is performed to obtain a fine-grained and uniform tempered sorbite structure. The transverse mechanical properties and grain size at both ends of the thick-walled tube meet the design and usage requirements. The mechanical properties of Example 1 are shown in Table 2.

[0030] Table 2

[0031] Rp0.1 (MPa) Z(%) -40℃ KV2 (J) 700℃ Rm (MPa) Grain size (grade) Example 1 Tail end 1230 / 1210 54 / 55 22 / 22 425 / 430 7.5 Example 1: The cap 1220 / 1220 57 / 56 25 / 25 390 / 408 7 .

Claims

1. A PG2 steel forged thick-walled pipe material with a large length-to-diameter ratio, characterized in that: The material used is PG2 steel, which has high hardenability. After quenching and tempering, this steel exhibits high strength at room temperature and 700℃, as well as low-temperature impact toughness, meeting the requirements for high strength and high toughness. Its chemical composition, by mass percentage, includes the following: C: 0.28~0.33%; Si: 0.15~0.35%; Mn: 0.75~0.95%; Cr: 1.80~2.10%; Ni: 3.00~3.50%; Mo: 1.20~1.40%; V: 0.20~0.35%. W: 0.20~0.50%; P: ≤0.010%; S: ≤0.005%; Cu: ≤0.10%; Al: ≤0.030%, balance being iron; the PG2 steel forged large length-to-diameter ratio thick-walled pipe material is manufactured according to the following steps: Step 1) Place the workpiece formed by the precision forging machine on the material rack and air cool it to 500-550℃. Then place it in a waiting furnace at 600℃-650℃ to avoid prolonged high temperature in the core of the workpiece, which would cause grain coarsening. Hold it at 600℃-650℃ for 1-2 hours per 100mm to eliminate forging stress, structural stress, and temperature stress. Air cool it to 280℃-320℃ and hold it for 1.5-2 hours per 100mm to make the internal and external temperatures of the workpiece more uniform and remove hydrogen. Increase the temperature at a rate of ≤70℃ / h to 860℃-910℃ and hold it at a uniform temperature. Heat 1-2 h / 100 mm, air cool to 350-400℃, then air cool to 280-320℃ before furnace entry. Hold at 280-320℃ for 1.5-2 h / 100 mm to achieve uniform and refined microstructure, reduce internal stress, and remove hydrogen. Increase the temperature at ≤70℃ / h to 650-680℃ and hold for 8-15 h / 100 mm to achieve uniform and refined microstructure, reduce internal stress, and remove hydrogen. Furnace cool to ≤150℃ before air cooling. This process lays the microstructure foundation for subsequent performance heat treatment to obtain high strength and high toughness. Step 2), Quenching: The workpiece after Step 1) is machined and non-destructive tested; the machined and inspected large length-to-diameter ratio thick-walled tube blank is vertically suspended in the effective heating zone of a pit-type resistance furnace using special tooling, with an initial furnace temperature ≤450℃. Heating is done in stages: the temperature is increased at ≤50℃ / h to 600~650℃ for 2 hours to remove machining stress; then the temperature is rapidly increased to 950~1000℃ and held for 2~3 hours to dissolve the carbides; the temperature is then reduced to 860~910℃ and held for 1~2 hours to homogenize the composition before being taken out of the furnace for quenching; the cooling medium is water. Step 3) Performance tempering: Place the workpiece from step 2) into a tempering furnace for performance tempering. The furnace temperature is ≤450℃. Increase the temperature at ≤50℃ / h to 550~610℃ and hold for 4 hours to remove structural stress and temperature stress. Remove from the furnace for hot straightening. Put it back into the furnace and hold at 550~610℃ to remove straightening stress and structural stress generated by the decomposition of residual austenite. Air cool.

2. The PG2 steel forged thick-walled tube material with a large length-to-diameter ratio as described in claim 1, characterized in that: In step 2), to obtain ultra-high strength and high toughness, the inner hole is cooled using a special tooling cooling method after quenching. The specific quenching cooling method includes the following steps: Step a) Air cooling: ≤60S, including workpiece hoisting time; hoist the thick-walled tube into the well-type water tank with the stirring system turned on, align the inner hole of the thick-walled tube of the workpiece with the inner hole cooling nozzle installed at the bottom of the water tank, and cool the inner wall of the workpiece. Step b) Water cooling: Water cooling is performed at a rate of 2.5 to 6 min / 100 mm, with an initial water temperature of 20 ± 5℃.