Carbon steel plate with thickness of 200 mm and excellent high-temperature die welding performance for medium and low temperature pressure vessel and manufacturing method of carbon steel plate

Abstract

The invention relates to a 200mm-thick carbon steel plate for a medium and low temperature pressure vessel with excellent high-temperature die welding performance. The steel plate comprises the following chemical components: 0.12 to 0.18 percent of C, 0.20 to 0.40 percent of Si, 1.20 to 1.50 percent of Mn, less than or equal to 0.015 percent of P, less than or equal to 0.001 percent of S, 0.10 to 0.40 percent of Ni, 0.05 to 0.10 percent of Mo, 0.10 to 0.30 percent of Cr, 0.001 to 0.03 percent of Nb, 0.001 to 0.03 percent of V, 0.01 to 0.03 percent of Ti, 0.02 to 0.05 percent of Alt and the balance of Fe and inevitable impurity elements. Meanwhile, the chemical components meet the requirements that the carbon equivalent Ceq is smaller than or equal to 0.45%, and Ceq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15. The steel plate is formed by rolling a composite blank, normalizing and tempering are sequentially carried out after rolling is completed, and the steel plate enters a water tank to be cooled after normalizing treatment. The high-temperature die welding mechanical property of the carbon steel plate for the pressure vessel is guaranteed by strictly controlling the purity of molten steel and the quality of a continuous casting billet and adopting the processes of high-temperature diffusion heating, high-permeability rolling and normalizing, water accelerated cooling and tempering heat treatment.

Description

Technical Field

[0001] The invention belongs to the technical field of special steel smelting, and specifically relates to a 200mm thick carbon steel plate for medium and low temperature pressure vessels with excellent high temperature die welding performance and a manufacturing method thereof Background Art

[0002] Post-weld heat treatment (PWHT) is a very common process technology for improving or restoring the mechanical properties of materials. It is often used in the manufacturing process of special equipment. It is an important process technology for the manufacturing process of pressure-bearing special equipment, but it is also one of the weakest links. After the pressure vessel equipment is processed or in a certain link of the processing, it is usually necessary to perform post-weld heat treatment on it. The main purposes of doing so are: to improve the use properties of the welded joint part, soften the hard zone, reduce the hardness of the weld and its heat-affected zone, and improve the plasticity and toughness of the welded joint, that is, to improve the impact toughness and fracture toughness of the welded joint, so that during the use process, it can be ensured that the equipment will not be damaged due to excessive brittleness; reduce or eliminate the residual stress caused by welding, avoid equipment cracks, and effectively increase the service life of the equipment.

[0003] Carbon steel plate SA-516Gr.70 for medium and low temperature pressure vessels is a material grade in the iron-based materials of Volume II Material A of ASME (American Society of Mechanical Engineers) Boiler and Pressure Vessel Code. It is widely used in petroleum, chemical, boiler pressure vessel, hydropower and nuclear power industries. It is mainly used to manufacture reactors, crude oil storage tanks, spherical tanks, gas tanks, liquefied gas tanks, nuclear power plant reactor containment, steam-water separation reheater, core water tank, injection tank, and hydropower station high-pressure water pipes, turbine volutes and other equipment and components. The simulated post-weld heat treatment requirements of SA516Gr70 steel plates are user-specific requirements specified in the standard. Users can select the temperature and time of post-weld simulated heat treatment according to different post-weld heat treatment and processing methods, and require that the strength of the steel plate after simulated post-weld heat treatment can still meet the standard requirements. The strength of ordinary SA516Gr70 steel plates decreases after post-weld heat treatment, especially when thick-gauge steel plates are subjected to high-temperature and long-term die welding under the condition of limiting carbon equivalent, the strength decreases greatly and cannot meet the standard requirements.

[0004] As modern pressure vessel equipment gradually develops towards large-scale, the demand for thick carbon steel plates for pressure vessels is increasing, and the design requirements are also constantly improving. With the increase of steel plate thickness, under the condition of limiting carbon equivalent ≤ 0.45%, after high temperature and long-term die welding, the strength and toughness of the steel plate are difficult to meet the standard requirements, and some also require to ensure the performance of 1 / 2 of the plate thickness.

[0005] Based on the above reasons, this application proposes a carbon steel plate for medium and low temperature pressure vessels with excellent high temperature mold welding performance. The steel plate thickness is 200mm and still has good mechanical properties after 650℃ high temperature simulated post-weld heat treatment. Summary of the invention

[0006] The technical problem to be solved by the present invention is to provide a 200mm thick carbon steel plate for medium and low temperature pressure vessels with excellent high-temperature mold welding performance and a manufacturing method thereof in response to the above-mentioned prior art. The steel plate has good internal quality, and after long-term simulated post-weld heat treatment at high temperature, the strength and low-temperature impact toughness of the steel plate are not significantly weakened, thereby meeting the manufacturing process requirements of thick-walled pressure vessels.

[0007] The technical solution adopted by the present invention to solve the above problems is: a 200mm thick carbon steel plate for medium and low temperature pressure vessels with excellent high temperature die welding performance, the chemical composition of the steel plate is C: 0.12-0.18%, Si: 0.20-0.40%, Mn: 1.20-1.50%, P: ≤0.015%, S: ≤0.001%, Ni: 0.10-0.40%, Mo: 0.05-0.10%, Cr: 0.10-0.30%, Nb: 0.001-0.03%, V: 0.001-0.03%, Ti: 0.01-0.03%, Alt: 0.02-0.05%, the balance is Fe and unavoidable impurity elements. At the same time, the chemical composition must meet the carbon equivalent Ceq≤0.45%, Ceq=C+Mn / 6+(Cr+Mo+V) / 5+(Ni+Cu) / 15.

[0008] The main chemical elements in steel and their functions are as follows: C can significantly improve the strength and hardness of the steel plate, but as the carbon content increases, the toughness and plasticity of the steel plate decrease. In the present invention, the C content is set to 0.12-0.18%.

[0009] Mn can improve the strength and toughness of steel by refining the grains, improve the hardenability of steel, and improve the hot working properties of steel plates. Mn also tends to increase the coarsening of steel grains and the sensitivity to temper brittleness. In the present invention, the Mn content is set to 1.20-1.50%.

[0010] Si is mainly used as a reducing agent and deoxidizing agent during steelmaking. In the present invention, the Si content is set to 0.20-0.40%.

[0011] Cr is an element that improves hardenability, so that the steel has better comprehensive mechanical properties after quenching and tempering. The Cr content control range of this application is 0.10-0.30%.

[0012] Ni is a solid solution strengthener in steel and a good hardenability additive. At the same time, Ni can effectively improve the low-temperature toughness of steel. The Ni content control range of this application is 0.10-0.40%.

[0013] Mo improves hardenability, prevents temper brittleness, and improves carbide stability, thereby increasing the strength of steel. The Mo content control range of this application is 0.05-0.10%.

[0014] Alt: Aluminum is mainly used for deoxidation and grain refinement. The Al content in this patent is controlled at 0.02-0.05%.

[0015] Nb: Niobium can significantly increase the austenite recrystallization temperature of steel, expand the range of the recrystallization zone, and facilitate high-temperature rolling. Niobium can also inhibit the growth of austenite grains, and has the effects of fine grain strengthening and precipitation strengthening. The Nb content of this patent is controlled at 0.01-0.02%.

[0016] V, Ti: form carbonitrides with C and N elements, which have the effect of delaying austenite recrystallization and refining ferrite grains, and can simultaneously improve the strength and toughness of the steel plate. In this patent, the V content is controlled at 0.010-0.02%, and the Ti content is controlled at 0.010-0.02%.

[0017] P and S are harmful elements. P increases the low-temperature brittleness of steel and is a harmful element that causes serious segregation in steel. The higher the S content, the more it affects the lamellar tear resistance of the extra-thick steel plate. In the present invention, the S content is set to ≤0.001%, and the P content is set to ≤0.015%.

[0018] The steel plate product of the present invention is produced by composite billets, and the main process steps are smelting and casting, vacuum welding and combined billet making, composite billet heating and rolling, and heat treatment. The specific process flow is as follows: (1) Smelting and casting The continuous casting slabs of the same size produced by the same furnace are used as the base of the composite billet. The production process is: KR pretreatment → converter smelting → LF refining → RH refining → continuous casting. The pure steel smelting process is used to control the non-metallic inclusions of Class A, Class B, Class C and Class D in the molten steel to be ≤1.5 level; after RH is broken, the soft blowing time of argon is more than 20 minutes, and the overheating degree of molten steel in the continuous casting process is 20-30℃. Through the argon protection casting process and dynamic light pressure technology, the center segregation of the continuous casting billet is guaranteed to be less than Class C 1.0 level, the looseness is less than 0.5 level, and the continuous casting billet is slowly cooled after it comes off the line.

[0019] (2) Vacuum welding combined blank making After the surface of the continuous casting billet is milled, it is electron beam welded and assembled under vacuum conditions, and the composite billet is made using a conventional process that has been disclosed.

[0020] (3) Heating and rolling of composite billets The billet is heated in sections in a soaking furnace to ensure uniform and thorough burning, so that the billet segregation can be fully diffused; at the same time, high-temperature heating and long-term heat preservation make the metals in close contact with each other at the composite interface form metal bonds first, and through diffusion migration, most of the holes between the metals disappear; The "low speed, high reduction" rolling process is adopted to increase the stress of the composite interface, form recrystallization at the slab interface, re-nucleate and grow above the recrystallization temperature to form new grains, completely eliminate the bonding interface, and achieve a strong metallurgical bond. At the same time, the rolling force is allowed to penetrate into the core of the steel plate as much as possible, fully crush the cast structure, and press the loose defects. In order to avoid mixed crystals, the single-pass reduction rate is controlled at more than 8%; Furthermore, the composite billet adopts a segmented heating process: steel is loaded below 550°C, the low temperature section is heated to 820-860°C, and kept warm for 2-6 hours; the medium temperature section is heated to 980-1020°C, and kept warm for 1-4 hours; the high temperature section is heated to 1220-1250°C, and kept warm for 8-12 hours.

[0021] Rolling is divided into two stages: rough rolling and finish rolling. Rolling deformation is mainly concentrated in the rough rolling stage. In this stage, high permeability rolling process is adopted to control the single-pass reduction of at least 2 passes to be ≥45mm, and the starting temperature of rough rolling is 1200℃-1100℃. Finishing rolling starts at above 1000℃, mainly to accurately control the thickness tolerance and obtain a good plate shape. After the steel plate comes off the line, it is piled and slowly cooled, and the steel plate obtains uniform ferrite and pearlite structure after rolling.

[0022] (4) Heat treatment The steel plate is subjected to normalizing and tempering treatment in sequence, and water is used to accelerate cooling in the water tank after normalizing. Further, the normalizing heating temperature is 860-930℃, the holding time coefficient is 1.8-2.0min / mm, and the furnace is water-cooled. By controlling the cooling water temperature and flow rate, the cooling rate is 10-20℃ / s, and the final cooling temperature is 600-680℃; tempering: tempering temperature is 610-660℃, and the holding time coefficient is 3.0-4.0min / mm. After the heat treatment, the steel plate is cooled on the cooling bed to below 200℃ to ensure that the structure of the steel plate is consistent in the thickness direction, which is uniform and fine ferrite + tempered bainite + a small amount of pearlite.

[0023] Compared with the prior art, the advantages of the present invention are: The present invention relates to a carbon steel plate for medium and low temperature pressure vessels with large thickness and excellent high temperature die welding performance, with a thickness of 200 mm. The steel plate has well-matched mechanical properties, and the strength and low temperature impact toughness of the steel plate are not significantly weakened after high temperature and long-term simulated post-weld heat treatment. The mechanical properties are tested after 650±10℃×24h simulated post-weld heat treatment, and the yield strength is ≥320Mpa, the tensile strength is ≥500Mpa, the core -30℃ transverse Charpy impact energy single value is ≥100J, the Z-direction tensile section shrinkage is ≥35%, and the steel plate surface Brinell hardness is ≤180HB. DETAILED DESCRIPTION

[0024] The technical solution of the present invention is described in more detail in conjunction with the preferred embodiments of the present invention. However, these embodiments are only descriptions of the preferred implementation methods of the present invention and cannot impose any limitation on the scope of the present invention.

[0025] The carbon steel plate for medium and low temperature pressure vessels with excellent high temperature die welding performance in this embodiment has a thickness of 200 mm, and the chemical composition is calculated by weight percentage as follows: C: 0.14%, Si: 0.30%, Mn: 1.30%, P: 0.012%, S: 0.0008%, Ni: 0.30%, Mo: 0.07%, Cr: 0.18%, Nb: 0.013%, V: 0.015%, Ti: 0.015%, Alt: 0.035%, and the balance is Fe and unavoidable impurity elements. At the same time, the chemical composition must meet the carbon equivalent Ceq≤0.45%, Ceq=C+Mn / 6+(Cr+Mo+V) / 5+(Ni+Cu) / 15.

[0026] The manufacturing process of the steel plate is as follows: The steel plate product of the present invention is produced by composite billets, and the main process steps are smelting and casting, vacuum welding and combined billet making, composite billet heating and rolling, and heat treatment. The specific process flow is as follows: (1) Smelting and casting The continuous casting slab with the same cross-sectional size of 450mm produced by the same furnace number is used as the base billet of the composite billet. The production process is as follows: KR pretreatment → converter smelting → LF refining → RH refining → continuous casting. The pure steel smelting process is adopted to control the non-metallic inclusions of Class A, Class B, Class C and Class D in the molten steel to be ≤1.5 level; after RH is broken, the soft blowing time of argon is 30min, and the casting superheat of the molten steel in the continuous casting process is 25℃. Through the argon protection casting process and dynamic light pressure technology, the center segregation of the continuous casting billet is Class C 1.0, the looseness is 0.5, and the continuous casting billet is slowly cooled for 50h after it comes off the line.

[0027] (2) Vacuum welding combined blank making After the surface of the continuous casting billet is milled, it is electron beam welded and assembled under vacuum conditions. The composite billet is made using a conventional process that has been made public. The thickness of the composite billet is 865 mm.

[0028] (3) Heating and rolling of composite billets The billet is heated in sections in a soaking furnace to ensure uniform and thorough burning, so that the billet segregation can be fully diffused; at the same time, high-temperature heating and long-term heat preservation make the metals in close contact with each other at the composite interface form metal bonds first, and through diffusion migration, most of the holes between the metals disappear; The "low speed, high reduction" rolling process is adopted to increase the stress of the composite interface, form recrystallization at the slab interface, re-nucleate and grow above the recrystallization temperature to form new grains, completely eliminate the bonding interface, and achieve a strong metallurgical bond. At the same time, the rolling force is allowed to penetrate into the core of the steel plate as much as possible, fully crush the cast structure, and press the loose defects. In order to avoid mixed crystals, the single-pass reduction rate is controlled at more than 8%; Furthermore, the composite billet adopts a segmented heating process: steel is loaded below 530°C, the low temperature section is heated to 840°C and kept warm for 4 hours; the medium temperature section is heated to 1010°C and kept warm for 2 hours; the high temperature section is heated to 1230°C and kept warm for 11 hours.

[0029] Rolling is divided into two stages: rough rolling and finish rolling. Rolling deformation is mainly concentrated in the rough rolling stage. High permeability rolling process is adopted in this stage. The single pass reduction of 2 passes is ≥45mm, which are 46mm and 47mm respectively. The main task of finish rolling is to accurately control the thickness tolerance and obtain a good plate shape. The steel plate is piled and slowly cooled after it comes off the line.

[0030] (4) Heat treatment The steel plate is subjected to normalizing and tempering treatment in sequence, and after normalizing, water is used in the water tank to accelerate cooling. In particular, the normalizing heating temperature is 900°C, the holding time coefficient is 2.0min / mm, and the steel plate is water-cooled after leaving the furnace. By controlling the cooling water temperature and flow rate, the cooling rate and final cooling temperature of the steel plate are guaranteed; tempering: tempering temperature is 620°C, and the holding time coefficient is 3.5min / mm. After the heat treatment, the steel plate is cooled on the cooling bed to below 200°C and then removed from the production line.

[0031] The 200mm thick carbon steel plate for medium and low temperature pressure vessels produced by the above manufacturing process has good mechanical properties. After long-term simulated post-weld heat treatment at high temperature, the strength and low-temperature impact toughness of the steel plate are not significantly weakened. Its main mechanical properties are shown in Table 1. The flaw detection of the steel plate meets the TI level requirements of the NB / T47013.3 standard.

[0032] Table 1 Mechanical properties of the steel plates produced in the examples

[0033] Note: Simulated post-weld heat treatment: 650±10℃×24h.

[0034] Although the preferred embodiments of the present invention are described in detail above, it should be clearly understood that various modifications and variations are possible for those skilled in the art. Any modification, equivalent substitution, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A 200mm thick carbon steel plate for medium and low temperature pressure vessels with excellent high temperature die welding performance, characterized in that: C: 0.12~0.18%, Si: 0.20~0.40%, Mn: 1.20~1.50%, P: ≤0.015%, S: ≤0.001%, Ni: 0.10~0.40%, Mo: 0.05~0.10%, Cr: 0.10~0.30%, Nb: 0.001~0.03%, V: 0.001~0.03%, Ti: 0.01~0.03%, Alt: 0.02~0.05%, the balance is Fe and unavoidable impurity elements.

2. The 200 mm thick carbon steel plate for medium and low temperature pressure vessels with excellent high temperature die welding performance according to claim 1, characterized in that: The chemical composition of the steel plate satisfies the carbon equivalent Ceq≤0.45%, Ceq=C+Mn / 6+(Cr+Mo+V) / 5+(Ni+Cu) / 15.

3. The 200 mm thick carbon steel plate for medium and low temperature pressure vessels with excellent high temperature die welding performance according to claim 1, characterized in that: The yield strength of the steel plate is ≥320Mpa, the tensile strength is ≥500Mpa, the single value of the transverse Charpy impact energy at -30°C in the core is ≥100J, the Z-direction tensile section shrinkage is ≥35%, and the Brinell hardness of the steel plate surface is ≤180HB.

4. A method for manufacturing a 200 mm thick carbon steel plate for medium and low temperature pressure vessels with excellent high temperature die welding performance as claimed in claim 1, characterized in that: The method comprises the following steps: (1) Smelting and casting The continuous casting slabs with the same size produced by the same furnace are used as the base of the composite billet. The production process is as follows: KR pretreatment → converter smelting → LF refining → RH refining → continuous casting; the continuous casting billet is slowly cooled after it comes off the line; (2) Vacuum welding combined blank making After the surface of the continuous casting billet is milled, it is assembled by electron beam welding under vacuum conditions; (3) Heating and rolling of composite billets The billet is heated in sections in a soaking furnace to ensure uniform and thorough burning, so that the billet segregation can be fully diffused; at the same time, high-temperature heating and long-term heat preservation make the metals in close contact with each other at the composite interface form metal bonds first, and through diffusion migration, most of the holes between the metals disappear; Adopting the "low speed, high reduction" rolling process, the single-pass reduction rate is controlled at more than 8%; (4) Heat treatment The steel plates are subjected to normalizing and tempering treatments in sequence. After normalizing, water is used in a water tank to accelerate cooling. After the heat treatment, the steel plates are cooled on a cooling bed to below 200°C before being removed from the production line.

5. The method for manufacturing a 200 mm thick carbon steel plate for medium and low temperature pressure vessels with excellent high temperature die welding performance according to claim 4, characterized in that: Step (3) adopts a pure steel smelting process to control the non-metallic inclusions of Class A, Class B, Class C and Class D in the molten steel to be ≤1.5 level; after RH is broken, the soft blowing time of argon is more than 20 minutes, the molten steel casting superheat in the continuous casting process is 20-30°C, and the argon protection casting process and dynamic soft pressure reduction technology are used to ensure that the center segregation of the continuous casting billet is less than Class C level 1.0 and the looseness is less than 0.5 level.

6. The method for manufacturing a steel forging for a wind power bearing ring with a thickness of 250 mm and a low-fire rate according to claim 4, characterized in that: The composite billet in step (3) adopts a segmented heating process: steel is charged below 550°C, the low temperature section is heated to 820-860°C, and kept warm for 2-6 hours; the medium temperature section is heated to 980-1020°C, and kept warm for 1-4 hours; the high temperature section is heated to 1220-1250°C, and kept warm for 8-12 hours.

7. The method for manufacturing a steel forging for a wind power bearing ring with a thickness of 250 mm and a low-fire rate according to claim 4, characterized in that: In step (3), the rolling is divided into two stages: rough rolling and finish rolling. The rolling deformation is mainly concentrated in the rough rolling stage. In this stage, a high permeability rolling process is adopted to control the single-pass reduction of at least two passes to be ≥45 mm. The rough rolling start temperature is 1200°C-1100°C. The finish rolling starts at a temperature above 1000°C, mainly to accurately control the thickness tolerance and obtain a good plate shape. After the steel plate is off the line, it is piled and slowly cooled. After rolling, the steel plate obtains a uniform ferrite and pearlite structure.

8. The method for manufacturing a steel forging for a wind power bearing ring with a thickness of 250 mm and a low-fire rate according to claim 4, characterized in that: In step (4), the normalizing heating temperature is 860-930°C, the holding time coefficient is 1.8-2.0 min / mm, and the steel plate is water-cooled after exiting the furnace. The cooling rate and final cooling temperature of the steel plate are ensured by controlling the cooling water temperature and flow rate. Tempering: the tempering temperature is 610-630°C, and the holding time coefficient is 3.0-4.0 min / mm.

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