High strength a514 steel plate with low weld crack sensitivity and method of production

Abstract

The application discloses a 200-210mm thick high-strength low-welding-crack-sensitivity A514 steel plate and a production method thereof. The A514 steel plate comprises the following chemical components (unit, wt%): C: 0.10-0.12, Si: 0.10-0.15, Mn: 0.90-1.0, P≤0.010, S≤0.003, Als: 0.020-0.040, Nb: 0.040-0.050, V: 0.18-0.20, Cr: 0.7-0.75, Ni: 2.01-2.3, Cu: 0.22-0.26, Mo: 0.45-0.50, B: 0.002-0.0030, Re: 0.0015-0.0025, and other Fe and residual elements, and the carbon equivalent Ceq≤0.7%. The 1015-1125mm thick steel ingot is adopted to ensure a large compression ratio, the cogging rolling and the secondary heating rolling are adopted to fully break the original cast structure and refine the grains, so that the internal quality of the steel plate reaches the level of a forged piece, and the rolling is used to replace the forging; the twice circulating quenching is adopted to reduce the mechanical property difference of different thickness layers of the steel plate, the differential temperature tempering process is adopted after the quenching to eliminate the residual stress in the quenched steel, the impact toughness and the strength uniformity of the whole plate are improved, and the performance of the A514 heavy steel plate under the low-temperature condition meets the use requirement.

Description

Technical Field

[0001] This invention belongs to the field of extra-thick plate production, specifically relating to a high-strength, low-weld-crack-sensitivity A514 steel plate and its production method. Background Technology

[0002] Low temperatures significantly reduce the fracture toughness of steel, thereby lowering the critical size for fatigue crack propagation. Materials used in applications such as crane booms, excavator booms, sticks, and loader booms, in addition to meeting strength requirements, must also meet impact toughness and brittle fracture surface ratio requirements at low temperatures. To ensure the design and operational safety of special engineering machinery in frigid regions, it is necessary to further clarify the performance parameters and fracture mechanisms of engineering machinery under low-temperature conditions. While the mechanical properties of engineering machinery have received widespread attention and research at room temperature, corresponding studies on low-temperature toughness and weldability at temperatures below -50°C are still lacking. Therefore, conducting evaluation research on engineering machinery, especially welded engineering machinery, under low-temperature environments has significant academic value and engineering practical implications, and is conducive to promoting the application and popularization of special engineering machinery.

[0003] Chinese patent CN201610368478.9 discloses a method for producing ASTM A514 GRE high-strength steel plate. The chemical composition by mass percentage is: C: 0.14–0.20%, Si: 0.35–0.55%, Mn: 1.55–1.70%, P≤0.015%, S≤0.005%, Cr: 1.40–2.0%, Mo: 0.40–0.60%, Ti: 0.040–0.050%, with the balance being Fe and unavoidable impurities. The A514GRE produced using this composition and the die casting + type II controlled rolling process + double quenching + tempering meets the performance requirements. However, the thickness range of this produced steel plate is only 150 mm–177.8 mm, the impact temperature is only -20°C, the calculated actual carbon equivalent is approximately 0.74, and the weldability is relatively poor, failing to meet the requirements for use under lower temperature conditions.

[0004] In view of this, the present invention is proposed. Summary of the Invention

[0005] To address the aforementioned problems, the purpose of this invention is to provide a high-strength A514 steel plate with low weld crack sensitivity. This steel plate possesses high strength, high toughness, fatigue resistance, resistance to lamellar tearing, and good weldability, and can meet the requirements for long-term service under low-temperature environments.

[0006] Another object of the present invention is to provide a method for producing high-strength A514 steel plates with low weld crack sensitivity.

[0007] To achieve the above objectives, the technical solution adopted by the present invention is: a high-strength, low-weld-crack-susceptibility A514 steel plate, wherein the thickness of the steel plate is 200-210mm, and it contains the following chemical composition by mass percentage (unit, wt%): C: 0.10-0.12, Si: 0.10-0.15, Mn: 0.90-1.0, P≤0.010, S≤0.003, Als: 0.020-0.040, Nb: 0.040-0.050, V: 0.18-0.20, Cr: 0.7-0.75, Ni: 2.01-2.3, Cu: 0.22-0.26, Mo: 0.45-0.50, B: 0.002-0.0030, RE: 0.0015-0.0025, the others being Fe and residual elements, and the carbon equivalent Ceq≤0.7%;

[0008] The steel plate is in a quenched and tempered state;

[0009] The steel plate has a uniform and fine tempered bainitic structure, with a tensile strength ≥690MPa, elongation after fracture ≥14%, longitudinal and transverse impact energy ≥60J at -50℃, and surface hardness 250~280HB.

[0010] The production method of the aforementioned high-strength, low-weld-crack-susceptibility A514 steel plate involves casting 1015-1125mm steel ingots using water-cooled ingot molds, rolling the ingots into steel plates, and requires ingot heating, billet rolling, secondary heating, and secondary rolling, specifically as follows:

[0011] a. Ingot heating: Total heating time 15 min / cm. The furnace temperature during charging should be ≤750℃. The ingot should be held for 2-4 hours to reduce the temperature difference between the furnace and the ingot, thereby reducing the stress difference caused by the temperature difference between the inside and outside of the ingot. Then, the temperature is increased to the high-temperature section. The target holding temperature in the high-temperature section is 1280-1300℃, and the total holding time is 14-15 hours. The ingot should be turned over four hours before tapping to ensure uniform and thorough heating. It should be noted that a reasonable heating temperature can promote the full dissolution of Nb and Mo alloying elements, but excessively high heating temperature will promote the growth of austenite grains, leading to coarsening of ferrite grains. Coarse ferrite structure will reduce its overall performance.

[0012] b. Initial rolling: The initial rolling temperature is 1050-1100℃, and the reduction per pass is controlled at 60-70mm. This ensures that the deformation penetrates to 1 / 2 of the thickness, achieves effective welding of the loose structure inside the steel ingot, fully breaks down the as-cast structure, and promotes uniform and refined grains in the steel plate. The steel is released when it reaches 600mm, and the rolling end temperature is 950-980℃.

[0013] c. Secondary heating: After the intermediate billet is cut into four pieces, it is heated a second time. The target temperature of the high-temperature section is 1220-1240℃ and the total holding time is 6-7 hours.

[0014] d. Secondary rolling: Initial rolling temperature ≥1020℃, pass reduction ≥30±5mm, rolling to finished thickness, final rolling temperature ≤950℃. High-pressure water cooling is used during rolling, and the hard shell rolling method is adopted to ensure that deformation penetrates to 1 / 2 of the thickness, promoting further grain refinement. Rolling can only begin after the steel plate surface has fully turned red. The total rolling time is controlled within 4-5 minutes. After rolling, ACC is turned on and the steel plate is immersed in water twice to quickly reduce the surface and core temperature to the non-recrystallized area. This is used to reduce the temperature difference between the center area and the surface of the steel plate, inhibit austenite grain growth and recrystallization, and promote grain refinement and uniformity. Immersion temperature is 900-950℃, and reddening temperature is 800-830℃.

[0015] After secondary rolling, the steel plate undergoes a dual-phase heat treatment process, as detailed below:

[0016] e. High-temperature quenching: Heat the steel plate to 920-940℃, hold for a time equal to the actual thickness of the steel plate x 2.4 min / mm. After the hot steel plate is taken out of the furnace, it is suspended into the quenching tank for water cooling within 3 minutes. The temperature of the steel plate when it is immersed in the water is 850-870℃, the water temperature is 30-40℃, and it is cooled to room temperature within 120 minutes.

[0017] f. Critical quenching: Heat the steel plate to 880-890℃, hold for a time equal to the actual thickness of the steel plate x 2 min / mm. After the hot steel plate is taken out of the furnace, it is suspended into the quenching tank for water cooling within 3 minutes. The temperature of the steel plate when it is immersed in the water is 730-760℃, the temperature of the circulating water is 15-18℃, and it is cooled to room temperature within 120 minutes. The temperature rise of the water during the quenching process is controlled within 5℃.

[0018] g. Tempering: After quenching, the steel plate is placed in an external mechanized furnace for tempering at 650±20℃, and then air-cooled to 400℃ and pressed flat while still warm. When loading the tempering furnace, the ends of the steel plates are interchanged, and the cap part of the steel plate with relatively severe positive segregation is placed at the back of the furnace. Taking advantage of the fast heating rate and good heat preservation effect at the back of the furnace, stress is fully released, toughness is improved, and the uniformity of the overall plate strength is improved.

[0019] The metallographic structure of the steel plate obtained according to the above scheme is a uniform and fine tempered bainite structure, A+B+C+D<1.5, banded structure grade less than 1.0. After testing, its tensile strength is ≥690MPa, elongation after fracture is ≥14%, longitudinal and transverse impact energy at -50℃ is ≥60J, and surface hardness is 250~280HB.

[0020] It should be noted that, to ensure the high strength, high low-temperature impact toughness, weldability, and other special requirements of A514, this design utilizes a reasonable ratio of Cu, Cr, Ni, and Mo elements to guarantee the toughness of high-strength steel. Specifically, increasing the nickel content significantly enhances the strength of extra-thick steel plates while maintaining a high level of toughness. Chromium carbides, being the smallest of all carbides, can be uniformly distributed within the steel body; increasing the Cr content makes a significant contribution to improving strength, hardness, and wear resistance. Microalloying elements such as Nb and V form various M (C, N) nano-precipitates during heat treatment, improving toughness, strength, and weldability. The low-carbon design is beneficial for improving low-temperature impact toughness and weldability.

[0021] The heat treatment process employs a double-cycle quenching and tempering process to reduce the differences in mechanical properties between different thickness layers of the steel plate. The first high-temperature quenching and critical quenching promote the dissolution of more hardenable alloying elements in the steel plate, resulting in a redistribution of carbides, a more uniform chemical composition, and greater stability of austenite. This leads to a lower nucleation rate of diffusion-type phase transformations during cooling, a longer transformation incubation period, a rightward shift of the C-curve, and a lower critical cooling threshold. After quenching, a differential temperature tempering process is used to eliminate residual stress in the quenched steel, improving impact toughness and the overall uniformity of plate strength.

[0022] The beneficial effects of this invention are as follows: by rationally controlling the amount of hardenable elements added, and by using rare earth elements to modify inclusions and purify molten steel; by using steel ingots of reasonable thickness to increase the rolling compression ratio, and by using open rolling + secondary heating rolling to fully break the original cast structure and refine the grains, the internal quality of the steel plate reaches the level of forgings, thus realizing rolling instead of forging; by using two-cycle quenching to reduce the difference in mechanical properties between different thickness layers of the steel plate, and by using differential temperature tempering after quenching to eliminate residual stress in the quenched steel, thereby improving impact toughness and the uniformity of overall plate strength, ensuring that the various properties of the A514 extra-thick steel plate meet the requirements. Attached Figure Description

[0023] The technical features of the present invention will be further described below with reference to the accompanying drawings and embodiments.

[0024] Figure 1 This is a schematic diagram of the metallographic structure of a thickness section according to an embodiment of the present invention (metallographic microscope, ×100).

[0025] Figure 2 This is a schematic diagram of the metallographic structure of a thickness section according to an embodiment of the present invention (metallographic microscope, ×200).

[0026] Figure 3 This is a schematic diagram of the metallographic structure of a thickness section according to an embodiment of the present invention (metallographic microscope, ×500). Detailed Implementation

[0027] The present invention will be further described below with reference to the embodiments.

[0028] A 200mm thick high-strength, low-weld-crack-susceptibility A514 steel plate for large engineering machinery was prepared, comprising the following chemical composition (wt%): C: 0.10, Si: 0.12, Mn: 0.93, P: 0.006, S: 0.001, Als: 0.035, Nb: 0.043, V: 0.187, Cr: 0.73, Ni: 2.26, Cu: 0.24, Mo: 0.457, B: 0.0026, RE: 0.0017, with the remainder being Fe and residual elements, and a carbon equivalent Ceq = 0.697%.

[0029] The steel plate is in a quenched and tempered state; the steel plate has a uniform and fine tempered bainitic structure, with a tensile strength ≥690MPa, elongation after fracture ≥14%, longitudinal and transverse impact energy ≥60J at -50℃, and surface hardness 250~280HB.

[0030] The steel plate production method includes the following process flow: KR molten iron pretreatment, converter smelting, VD decarburization, LF refining, VD refining, casting process, ingot cleaning and coating, heating and billet rolling, secondary heating and rolling, and heat treatment. Details are as follows:

[0031] 1. KR molten iron pretreatment: The slag before the molten iron arrives at the station must be completely removed to ensure that the slag layer thickness on the liquid surface is ≤25mm. After KR treatment, the sulfur content of the molten iron is controlled below 0.005%, and the desulfurization temperature drop is ≤25℃.

[0032] 2. Converter smelting: The molten iron charged into the furnace has S≤0.005% and P≤0.050%, and the molten iron temperature≥1280℃. The scrap steel added to the converter must be dry, high-quality offcuts and nickel-containing special scrap steel. The converter charging amount is controlled according to the casting residue of 6-8 tons. The tapping temperature is ≥1600℃, 0.04%≤C≤0.07%, and P≤0.007%. After tapping, slag is blocked by a slag-blocking cone. If the slag blocking fails, the furnace must be lifted in advance to ensure that the slag thickness at the bottom of the converter is controlled below 20mm to avoid slag back to P.

[0033] 3. VD decarburization: Ensure that the VD temperature is ≥1540℃. After the molten steel is placed in the VD furnace, add carbon powder appropriately according to the C content of the tapped steel. Then cover the furnace and perform vacuum operation. During the vacuum stage of the VD furnace, the argon gas volume should be increased to fully achieve the purpose of vacuum decarburization. The pressure holding time should be ≥10min, and the temperature at the station should be ≥1535℃.

[0034] 4. LF Refining: Slag preparation is carried out according to the large slag volume process standard. The amount of lime added is 1000-1200 kg, and the alkalinity is controlled at 4.0-6.0. After heating for 3 minutes, first add 50 kg of calcium carbide, then add 20-40 kg of aluminum granules. After that, add 2-4 shovels of aluminum granules to the ladle every 2 minutes until the slag turns white. For the second heating, add 10-30 kg of calcium carbide each time according to the submerged arc effect, and at the same time add 1-3 shovels of aluminum granules to the ladle every 2 minutes to ensure that the white slag is maintained throughout the second heating process. For the third heating, the deoxidizer is added according to the slag color, just enough to maintain the white slag. The white slag should be maintained for 30-35 minutes during the refining process.

[0035] 5. VD refining: The VD holding time should be controlled at ≥18min. It is required that the molten steel tumbling effect is good during the holding process under a vacuum of 67Pa, and the H content should be ≤1.2PPm. After breaking the vacuum, add rice husk soft blowing for 5-8min in time, and then prepare to lift the ladle to leave the station. The temperature at the station should be controlled at about 1578-1583℃.

[0036] 6. Casting Process: The temperature of the heat-insulating cap is greater than 150℃. A 50-ton fixed-width and adjustable-thickness water-cooled steel ingot mold is used. The thickness of the steel ingot body is 1025-1055mm, the width is 2200mm, and the height is 2800mm. During the mold placement process, it is necessary to ensure that all auxiliary materials are dry and that the gating system is dry and clean. The water-cooled mold must ensure a taper (30mm). At the same time, an extrusion mechanism is set on the side to extrude the wide edge of the ingot in the later stage of solidification, further forming a dense structure. Proper heat preservation measures are taken at the opening of the steel ingot cap. During the solidification process, a feeding channel with a larger upper part and a smaller lower part can be formed inside the steel ingot, avoiding the generation of shrinkage cavities, reducing the porosity and segregation inside the steel ingot, and promoting a dense internal structure and uniform composition. The pouring temperature should be controlled between 1560-1565℃. If the temperature of the molten steel is suitable upon arrival at the station (within the upper limit of the standard minus the upper limit plus 3℃), it should be calmed and the temperature measured to confirm that it meets the pouring requirements before pouring begins. If the temperature of the molten steel upon arrival at the station does not meet the aforementioned standard, soft argon blowing should be performed immediately, and pouring should begin 5 minutes after the argon blowing is completed and calming is resumed. After pouring, the heat preservation effect of the cap must be ensured.

[0037] 7. Steel ingot cleaning and coating: The steel ingots shall be cleaned within 24 hours after demolding, with a cleaning temperature of ≥200℃. After cleaning, the ingots shall be stacked in a slow cooling pit and slow cooled to 50-100℃ before spraying with anti-oxidation coating.

[0038] 8. Heating and billet rolling: The total heating time for the steel ingot is 15 min / cm. The furnace temperature during steel loading is ≤750℃. The steel is kept in the furnace for 2-4 hours to reduce the temperature difference between the furnace and the steel ingot, thereby reducing the stress difference caused by the temperature difference between the inside and outside of the steel ingot. The target holding temperature in the high-temperature section is 1280-1300℃, and the total holding time is 14-15 hours. The initial rolling temperature is 1050-1100℃, and the single-pass reduction is controlled at 60-70mm to ensure that the deformation penetrates to 1 / 2 of the thickness, so as to achieve effective welding of the loose structure inside the steel ingot, fully break the as-cast structure, and promote the uniform and refined grain of the steel plate. The steel is released when it is rolled to a thickness of 600mm, and the rolling end temperature is 950-980℃.

[0039] 9. Secondary heating and rolling: After the intermediate billet is cut into four sections, it is placed in the soaking pit. The target temperature of the high-temperature section is 1220-1240℃, and the total holding time is 6-7 hours. The initial rolling temperature is ≥1020℃, and the reduction per pass is ≥30±5mm. High-pressure water is used to cool the billet during rolling. Rolling can only begin after the steel plate surface has fully turned red. The total rolling time is controlled within 4-5 minutes, and the final rolling temperature is ≤950℃. After rolling, the ACC is turned on and the steel plate is cooled twice in water to quickly reduce the surface and core temperature to the non-recrystallization zone. The water temperature is 900-950℃, and the reddening temperature is 800-830℃.

[0040] 10. Heat Treatment: A two-cycle quenching + tempering process is adopted, namely, a combination of high-temperature quenching, critical quenching, and tempering. The high-temperature quenching heating temperature is 920-940℃, and the holding time is equal to the steel plate thickness x 2.4 min / mm. The hot steel plate is then immersed in a water-cooling quenching tank within 3 minutes of being removed from the furnace. The steel plate temperature upon immersion is 850-870℃, and the water temperature is 30-40℃. Cooling to room temperature is completed within 120 minutes. The critical quenching heating temperature is 880-890℃, and the holding time is equal to the steel plate thickness x 2 min / mm. Within 3 minutes of being removed from the furnace, the steel plate is hoisted into a quenching tank for water cooling. The initial temperature of the steel plate upon immersion is 730-760℃, and the circulating water temperature is 15-18℃. During quenching, the water temperature rise is controlled within 5℃, and the plate is cooled to room temperature within 120 minutes. The tempering temperature is 650±20℃, followed by air cooling to 400℃ and then pressing flat. When loading the tempering furnace, the ends of the steel plates are interchanged, with the cap portion of the steel plate, which has relatively severe positive segregation, placed at the rear of the furnace. This utilizes the advantages of the rapid heating rate and good heat preservation effect at the rear of the furnace to achieve full stress release, improve toughness, and enhance the uniformity of the overall plate strength.

[0041] Testing and analysis: Mechanical properties were tested according to GB / T 2975, low-temperature impact toughness was tested according to GB / T 229, and tensile properties were tested according to GB / T 228. A comprehensive inspection of the mechanical properties and metallographic structure of the steel plate in the supplied condition was carried out.

[0042]

[0043] Based on the data above, the steel plate has good low-temperature impact toughness, and the mechanical properties of the steel plate are relatively similar at the beginning and end. It also has good resistance to lamellar tearing. All properties and indicators meet the requirements of A514 steel for engineering machinery.

[0044] The metallographic structure of the 200mm thick A514 steel plate section obtained in this example is shown in the attached figure. Figure 1-3 As shown.

[0045] Metallographic test results: (1) Inclusions A+B+C+D ≤ 1.5 grade; (2) Microstructure: tempered bainitic microstructure;

[0046] External inspection and flaw detection: The external inspection pass rate of the developed steel plate is 100%, and the final flaw detection of the steel plate meets the Class I flaw detection requirements of GB / T2970 "Inspection Method for Thick Steel Plates".

[0047] The above description is only a preferred embodiment of the present invention. The above specific embodiments are not intended to limit the present invention. Any modifications, alterations or equivalent substitutions made by those skilled in the art based on the above description shall fall within the protection scope of the present invention.

Claims

1. A high strength, low weld crack sensitive A514 steel plate characterized by: The thickness of the steel plate is 200-210 mm, and the steel plate contains the following chemical components (unit, wt%): C: 0.10-0.12, Si: 0.10-0.15, Mn: 0.90-1.0, P≤0.010, S≤0.003, Als: 0.020-0.040, Nb: 0.040-0.050, V: 0.18-0.20, Cr: 0.7-0.75, Ni: 2.01-2.3, Cu: 0.22-0.26, Mo: 0.45-0.50, B: 0.002-0.0030, RE: 0.0015-0.0025, and the rest is Fe and residual elements, and the carbon equivalent Ceq≤0.7%; The steel plate is a uniform fine tempered bainite structure, and has a tensile strength≥690 MPa, an elongation after fracture≥14%, and a longitudinal and transverse impact energy at-50 ℃≥60 J, and a surface hardness of 250-280 HB; The steel plate is in a quenched and tempered state.

2. The method of producing a high-strength low-hydraulic-fracture-sensitivity A514 steel plate according to claim 1, characterized by: The steel plate is cast into a 1015-1125 mm steel ingot by using a water-cooled ingot mold, and is rolled from the steel ingot, and needs to be subjected to steel ingot heating, blooming rolling, secondary heating and secondary rolling, and the specific process is as follows: a. Steel ingot heating: total heating time 15 min / cm, furnace temperature≤750 ℃ when the steel is loaded, and after stewing the steel for 2-4 hours, the temperature is increased to the high-temperature section, the target temperature of the high-temperature section is 1280-1300 ℃, the total holding time is 14-15 hours, and the steel ingot is uniformly and thoroughly burned by turning the steel for four hours before the steel is discharged; b. Blooming rolling: the blooming temperature is 1050-1100 ℃, and the pass reduction amount is controlled to be 60-70 mm, and the rolling is stopped when the thickness is 600 mm, and the end rolling temperature is 950-980 ℃; c. Secondary heating: the intermediate blank is cut into four parts, and then is subjected to secondary heating, the target temperature of the high-temperature section is 1220-1240 ℃, and the total holding time is 6-7 hours; d. Secondary rolling: the blooming temperature is≥1020 ℃, the pass reduction amount is≥30±5 mm, the rolling is performed to the finished thickness, the final rolling temperature is≤950 ℃, high-pressure water is used for cooling during the rolling, the rolling is started after the surface of the steel plate is fully red, the total rolling time is controlled to be 4-5 minutes, and the surface and core of the steel plate are rapidly cooled to the unrecrystallization zone after the rolling is completed, the water inlet temperature is 900-950 ℃, and the red temperature is 800-830 ℃.

3. The method of producing a high strength low weld crack sensitive A514 steel plate according to claim 2, characterized by: After the secondary rolling of the steel plate through step d), the heat treatment of the steel plate adopts a dual-phase zone heat treatment process, and the specific process is as follows: e. High-temperature quenching: the steel plate is heated to 920-940 ℃, the holding time is equal to the actual thickness of the steel plate x 2.4 min / mm, the hot steel plate is lifted into a quenching pool for water cooling within 3 minutes after being discharged, the temperature of the steel plate when entering the water is 850-870 ℃, the water temperature is 30-40 ℃, and the steel plate is cooled to room temperature within 120 minutes. f. Critical quenching: heat the steel plate to 880-890 °C, holding time = steel plate thickness x 2 min / mm, after the hot steel plate is discharged, it is lifted into the quenching pool water within 3 minutes, the temperature of the steel plate is 730-760 °C when entering the water, the circulating water temperature is 15-18 °C, and it is cooled to room temperature within 120 minutes, and the water temperature rising amplitude is controlled within 5 °C during quenching; g. Tempering: after quenching, the steel plate is loaded into an external mechanized furnace at 650±20 °C for tempering, and air cooling is performed to 400 °C with temperature pressing, the head and tail of the steel plate are interchanged when tempering is loaded, and the cap portion with relatively serious normal segregation of the steel plate is loaded at the rear of the furnace.

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