A 60-80 mm thick high-toughness 980 mpa steel plate and a production method thereof

Abstract

This invention discloses a 60-80mm thick high-strength and high-toughness 980MPa steel plate, comprising the following chemical composition by mass percentage (wt%): C: 0.06-0.08, Si: 0.20-0.25, Mn: 0.60-0.70, P≤0.010, S≤0.002, Ni: 4.6-4.8, Cr: 0.5-0.6, Mo: 0.40-0.50, V: 0.05-0.06, with the remainder being Fe. After tempering and quenching treatment, its microstructure consists of tempered sorbite with a small amount of retained austenite. Its yield strength is 840–920 MPa, tensile strength is 940–1000 MPa, elongation at break is ≥16%, reduction of area is ≥50%, impact energy at -84℃ is ≥250 J, shear area ratio is ≥90%, impact energy at -100℃ is ≥200 J, shear area ratio is ≥80%. After fracture testing, the fracture surface is fibrous, without white spots, bright lines, or penetrating cracks. It exhibits excellent overall performance, fully meeting the quality requirements for high-strength and high-toughness 980 MPa steel plates.

Description

Technical Field

[0001] This invention relates to the field of medium and heavy plate production, specifically to a 60-80mm thick 980MPa steel plate with high strength and toughness and its production method. Background Technology

[0002] As is well known, low-carbon high-strength steel is one of the most important structural materials in engineering, widely used in oil and gas pipelines, offshore platforms, shipbuilding, large hydropower stations, and other projects. With the continuous advancement of metallurgical technology, higher requirements are being placed on the toughness, plasticity, and weldability of ultra-high-strength steel. Specifically, the steel plate must possess resistance to brittle fracture and ductile instability fracture at ultra-low temperatures, while simultaneously achieving a fracture elongation and uniform elongation at the level of steel plates with a tensile strength of 900 MPa. This is especially true in deep-sea working environments, where the requirements for the steel plate's resistance to high pressure and low-temperature impact are even higher.

[0003] Patent document CN 113403551 B discloses a method for manufacturing a high yield strength ratio and hydrogen embrittlement resistant cold-rolled DH980 steel plate. After cold rolling and annealing, the strength can reach more than 1000MPa. However, the steel plate is too thin and belongs to the category of cold-rolled steel, which is not suitable for the production of medium and heavy plates. Moreover, the steel plate obtained by this method does not have low-temperature impact toughness.

[0004] Patent document CN 112143958 B discloses a method for producing 1000MPa grade steel plates with ultra-thickness, ultra-high toughness, and excellent weldability. After quenching and tempering, the tensile strength of the steel plate reaches over 950MPa, and the transverse Charpy impact energy at -60℃ reaches over 100J. However, it still cannot meet the requirements for use in environments below -80℃. As the thickness of the steel plate gradually increases, the hardenability becomes increasingly poor, and the performance differences along the entire thickness direction of the steel plate also become increasingly greater. The heat treatment technology disclosed in the patent document cannot meet the requirement of uniform performance of the steel plate along the entire thickness direction, and the performance of the core half position is difficult to guarantee.

[0005] In view of this, the present invention is proposed. Summary of the Invention

[0006] One objective of this invention is to provide a 60-80mm thick high-strength and high-toughness 980MPa steel plate; another objective of this invention is to provide a method for producing a 60-80mm thick high-strength and high-toughness 980MPa steel plate.

[0007] This invention is achieved in the following manner:

[0008] A 60-80mm thick high-strength and high-toughness 980MPa steel plate contains the following chemical composition by mass percentage (wt%): C: 0.06-0.08, Si: 0.20-0.25, Mn: 0.60-0.70, P≤0.010, S≤0.002, Ni: 4.6-4.8, Cr: 0.5-0.6, Mo: 0.40-0.50, V: 0.05-0.06, with the remainder being Fe and residual elements.

[0009] Regarding composition design, it should be noted that Ni can lower the critical cooling rate of steel, improve its hardenability, expand the austenite region, and is an effective element for austenitization, increasing the strength of steel without reducing its plasticity and improving its low-temperature toughness. When Ni > 4.5%, it can prevent pearlite formation, stabilize austenite, and allow martensitic transformation to occur at low temperatures or in the as-cast state. However, excessive Ni content can lead to excessive hardenability and deteriorate toughness; therefore, the Ni content should be controlled between 4.6% and 4.8%.

[0010] Carbon (C) can improve the strength and hardenability of steel plates, but excessive C content can affect the low-temperature impact toughness of the steel plate. In particular, when the carbon content exceeds 0.10%, the number of small-angle grain boundaries in the steel increases, which is detrimental to improving impact toughness. Furthermore, the higher the C content, the lower the shear fiber cross-sectional area. Given that high-Ni steel has strong hardenability, the C content should be controlled between 0.06% and 0.08%.

[0011] Mo is an element that promotes ferrite formation and can increase the hardenability of steel. At the same time, the combination of Mo and Ni is more conducive to preventing the formation of pearlite. However, if the Mo content is too high, it will further lead to excessive hardenability and thus deteriorate toughness. Therefore, the Mo content should be controlled at 0.40% to 0.50%.

[0012] Preferably, when the C content is 0.06%–0.07%, the Mn content is 0.65%–0.70%, the Cr content is 0.55%–0.60%, and the Mo content is 0.45%–0.50%; when the C content is 0.071%–0.08%, the Mn content is 0.60%–0.64%, the Cr content is 0.50%–0.54%, and the Mo content is 0.40%–0.44%. It should be noted that within a limited range of elements, the C content plays a crucial role in the hardenability of the steel plate. When the C content is low, the hardenability needs to be compensated for by increasing the Mn, Cr, and Mo content. Conversely, when the C content is high, to avoid over-quenching on the steel plate surface, which is detrimental to low-temperature impact toughness, the Mn, Cr, and Mo content needs to be reduced to balance hardenability.

[0013] Furthermore, the steel plate is in a quenched and tempered state, and its microstructure is tempered sorbite with a small amount of retained austenite; its yield strength is 840-920 MPa, tensile strength is 940-1000 MPa, elongation at break is ≥16%, reduction of area is ≥50%, impact energy at -84℃ is ≥250 J, shear area ratio is ≥90%, impact energy at -100℃ is ≥200 J, shear area ratio is ≥80%, and the fracture surface after the fracture test is fibrous, without white spots, bright lines or penetrating cracks.

[0014] A method for producing 60-80mm thick high-strength and high-toughness 980MPa steel plates includes critical quenching + sub-temperature quenching and two tempering processes, as detailed below:

[0015] a. Critical quenching: The total heating time is 80-100 min, the holding temperature is 830-850℃, the holding time is 1.2 min / mm, and after taking it out of the furnace, it is quenched to room temperature. The quenching water temperature is 18-25℃, and the quenching roller speed is 2.3 m / min.

[0016] b. Sub-temperature quenching: The total heating time is 80-100 min, the holding temperature is 790-810℃, the holding time is 1.0 min / mm, and after taking it out of the furnace, it is quenched to room temperature. The quenching water temperature is 18-25℃, and the quenching roller speed is 2.3 m / min.

[0017] c. First tempering: The holding temperature is set at 600-620℃, the total tempering coefficient is 3.5min / mm, and after exiting the furnace, it is water-cooled to below 200℃;

[0018] d. Secondary tempering: The holding temperature is set to 620-640℃, the heating time is 60-80 minutes, the holding time is 30-40 minutes, and the furnace is air-cooled after being taken out.

[0019] Preferably, when the C content is ≤0.07%, the temperature for the first tempering holding stage is set to 600–610℃, and the temperature for the second tempering holding stage is set to 620–630℃; when the C content is >0.07%, the temperature for the first tempering holding stage is set to 611–620℃, and the temperature for the second tempering holding stage is set to 631–640℃. It should be noted that when the C content is low, the hardenability of the steel plate is slightly poor, requiring a lower tempering temperature to ensure the strength of the steel plate; when the C content is high, the hardenability of the steel plate is strong, requiring an increase in the tempering temperature to ensure the low-temperature impact toughness of the steel plate, achieving a balance between strength and toughness.

[0020] It is important to note the following: 1. After critical quenching and sub-temperature quenching, the number of austenite nucleation sites in the steel increases significantly, resulting in a noticeable grain refinement effect. After tempering, tempered sorbite with the best strength-toughness matching effect is obtained, with a small amount of retained austenite. The retained austenite will produce the TRIP effect during mechanical property testing, inducing stress-induced martensitic transformation, thereby improving the strength and toughness of the steel plate; 2. The purpose of the first tempering of the steel plate is to fully transform the microstructure into tempered sorbite. Water cooling to below 200℃ after exiting the furnace can avoid the tempering brittleness range of the steel plate. The purpose of the second tempering is to increase the tempering temperature and shorten the tempering time to fully temper the surface layer without reducing the strength of the center of the steel plate, since the surface layer of the steel plate has excessive hardenability.

[0021] Furthermore, the method for producing the steel plate also includes steel smelting, casting, ingot heating and rolling, and intermediate billet heating and rolling, as detailed below:

[0022] a. Steelmaking: The converter uses high-alloy steel scrap as waste steel, with alloy content of Cr ≥ 1.0%, Mo ≥ 0.5%, and Ni ≥ 1.5%. Top and bottom combined oxygen blowing is used for smelting, and the C content of the steel tapped from the converter is ≤ 0.05%. The process route of VD decarbonization + LF refining + VD vacuum degassing is adopted. After VD decarbonization, C is 0.02-0.03%. During the LF refining process, white slag is generated to adsorb inclusions, and Ca is added to modify the inclusions. After VD vacuum degassing, H in the molten steel is ≤ 1.0 ppm, O is ≤ 25 ppm, and N is ≤ 23 ppm.

[0023] b. Casting: Use water-cooled ingot molds with a thickness of 800-840mm for casting, ensuring a compression ratio ≥10, a casting superheat of 30-40℃, a casting time of 16-18min for the main body, a casting time of 6-8min for the riser, and demolding 10-12h after casting.

[0024] c. Steel ingot heating and rolling: The steel ingot is heated at ≤500℃ for 24-36 hours, then heated to 1160~1180℃ at a rate of ≤60℃ / h and held for 5 hours, then heated to 1180~1200℃ and held for 5 hours, and finally held at 1160~1180℃ for 2 hours before being rolled into a 300mm thick intermediate billet;

[0025] d. Heating and rolling of intermediate billet: The intermediate billet is heated to 1180-1200℃ and held for 5-7 hours, then held at 1160~1180℃ for 0.5-1.0 hours. After holding, the billet is rolled to a thickness of 140-160mm in the first stage. After rolling, it enters ACC controlled cooling. When the billet is heated to 820-860℃, the second stage rolling to the finished plate begins. The final rolling temperature is 800-820℃. After rolling, it enters ACC cooling again, and the final cooling temperature is ≤300℃.

[0026] The beneficial effects of this invention are as follows: Through the rational design of the steel plate composition, the control of the steel plate production process, and the precise control of the heat treatment process, the obtained steel plate has a microstructure of tempered sorbite + a small amount of retained austenite; its yield strength is 840-920MPa, tensile strength is 940-1000MPa, elongation at break is ≥16%, reduction of area is ≥50%, impact energy at -84℃ is ≥250J, shear area ratio is ≥90%, impact energy at -100℃ is ≥200J, shear area ratio is ≥80%, and the fracture surface after the fracture test is fibrous, without white spots, bright lines, or penetrating cracks. The steel plate has a uniform microstructure and stable performance throughout the thickness direction, and the performance degradation with thickness is small. Detailed Implementation

[0027] To better understand the present invention, the following embodiments further illustrate the content of the invention, but the scope of protection of the present invention is not limited to the following embodiments. Numerous specific details are set forth in the following description to provide a more thorough understanding of the invention. However, it will be apparent to those skilled in the art that the present invention can be practiced without one or more of these details.

[0028] Example 1:

[0029] The chemical composition (in wt%) is as follows: C: 0.062, Si: 0.23, Mn: 0.68, P: 0.004, S: 0.001, Ni: 4.72, Cr: 0.57, Mo: 0.48, V: 0.053, with the remainder being Fe and residual elements.

[0030] a. Steelmaking: The converter uses high-alloy steel scrap as waste steel, with alloy content reaching Cr: 1.23%, Mo: 0.65%, and Ni: 1.92%. Top and bottom combined oxygen blowing is used, resulting in a C content of 0.03% in the converter tapped steel. A process route of VD decarburization + LF refining + VD vacuum degassing is adopted, resulting in a C content of 0.02% after VD decarburization. During LF refining, aluminum granules and calcium carbide are added as deoxidizers. After slag formation, the slag is required to turn white. Heating time is controlled in three stages: the first heating time is 15 minutes, during which low-carbon ferromanganese is added, and the steel temperature ends at 1575℃; the second heating time is 25 minutes, during which ferrovanadium, nickel plates, and ferromolybdenum are added, and the Cr, V, Ni, and Mo element content is adjusted according to the weight of the steel, ending at 1625℃; the third heating time is 10 minutes, during which the alloy composition of the steel is fine-tuned. Argon is blown from the bottom during heating to stir the molten steel, adsorb inclusions, and Ca wire is added to modify the inclusions. After refining, the C content of the molten steel is controlled at 0.06%. After VD vacuum degassing, the H content in the molten steel is 0.82 ppm, O is 22 ppm, and N is 20 ppm.

[0031] b. Casting: Water-cooled ingot mold with a thickness of 820mm is used for casting, with a compression ratio of 10.2-13.6, a casting superheat of 35℃, a casting time of 17min for the main body, a casting time of 8min for the riser, and demolding 12h after casting.

[0032] c. Steel ingot heating and rolling: The steel ingot is heated at 460℃ for 30 hours with a heating rate of 48.3℃ / h, held at 1160~1180℃ for 5 hours, heated again and held at 1180~1200℃ for 5 hours, and then held at 1160~1180℃ for 2 hours before being tapped and rolled, with an intermediate billet thickness of 300mm.

[0033] d. Heating and rolling of intermediate billet: The intermediate billet is heated to 1180-1200℃ and held for 6 hours, followed by heating at 1160-1180℃ for 0.5 hours. After heating, the intermediate billet is rolled to a thickness of 160mm in the first stage. After rolling, it enters ACC controlled cooling and is heated to 845℃ to start the second stage rolling to the finished plate. The final rolling temperature is 813℃. After rolling, it enters ACC cooling again, with a final cooling temperature of 260-280℃.

[0034] e. Heat treatment: A two-stage quenching + two-stage tempering process is adopted. ① Critical quenching: The total heating time is 90 min, the holding temperature is 840℃, the holding time is 1.2 min / mm, and after taking it out of the furnace, it is quenched to room temperature with a quenching water temperature of 23℃ and a quenching roller speed of 2.3 m / min; ② Sub-critical quenching: The total heating time is 85 min, the holding temperature is 800℃, the holding time is 1.0 min / mm, and after taking it out of the furnace, it is quenched to room temperature with a quenching water temperature of 22℃ and a quenching roller speed of 2.3 m / min; ③ First tempering: The holding temperature is set at 605℃, the total tempering coefficient is 3.5 min / mm, and after taking it out of the furnace, it is water-cooled to below 200℃; ④ Second tempering: The holding temperature is set at 625℃, the heating time is 70 min, the holding time is 35 min, and after taking it out of the furnace, it is air-cooled.

[0035] Its mechanical properties are shown in Table 1:

[0036] Table 1 Mechanical Properties

[0037]

[0038] Example 2:

[0039] The chemical composition (in wt%) is as follows: C: 0.075, Si: 0.25, Mn: 0.62, P: 0.005, S: 0.001, Ni: 4.75, Cr: 0.52, Mo: 0.43, V: 0.055, with the remainder being Fe and residual elements.

[0040] a. Steelmaking: The converter uses high-alloy steel scrap as waste steel, with alloy content reaching Cr: 1.35%, Mo: 0.60%, and Ni: 1.88%. Top and bottom combined oxygen blowing is used, resulting in a C content of 0.03% in the converter tapped steel. A process route of VD decarburization + LF refining + VD vacuum degassing is adopted, resulting in a C content of 0.03% after VD decarburization. During the LF refining process, aluminum granules and calcium carbide are added as deoxidizers for the molten steel. After slag formation, the slag is required to turn white. The heating time is controlled in three stages: the first heating time is 15 minutes, during which low-carbon ferromanganese is added, and the molten steel temperature reaches 1572℃ at the end of the first heating; the second heating time is 28 minutes, during which ferrovanadium, nickel plates, and ferromolybdenum are added, and the Cr, V, Ni, and Mo element content is adjusted according to the weight of the molten steel, reaching a molten steel temperature of 1628℃ at the end of the second heating; the third heating time is 12 minutes, during which the alloy composition of the molten steel is fine-tuned. Argon is blown from the bottom during heating to stir the molten steel, adsorb inclusions, and Ca wire is added to modify the inclusions. After refining, the C content of the molten steel is controlled at 0.072%. After VD vacuum degassing, the H content in the molten steel is 0.9 ppm, O is 25 ppm, and N is 22 ppm.

[0041] b. Casting: Water-cooled ingot mold with a thickness of 840mm is used for casting, with a compression ratio of 10.5-14.0, a casting superheat of 33℃, a casting time of 18min for the main body, a casting time of 7min for the riser, and demolding 12h after casting.

[0042] c. Steel ingot heating and rolling: The steel ingot is simmered at 450℃ for 36 hours, with a heating rate of 55℃ / h, held at 1160~1180℃ for 5 hours, then heated again and held at 1180~1200℃ for 5 hours, and then held at 1160~1180℃ for 2 hours before being tapped and rolled, with an intermediate billet thickness of 300mm.

[0043] d. Heating and rolling of intermediate billet: The intermediate billet is heated at 1180-1200℃ for 6.5 hours, followed by heating at 1160-1180℃ for 0.5 hours. After heating, the intermediate billet is rolled to a thickness of 160 mm in the first stage. After rolling, it enters ACC controlled cooling. When it is heated to 835℃, the second stage rolling to the finished plate begins. The final rolling temperature is 810℃. After rolling, it enters ACC cooling again, with a final cooling temperature of 282-298℃.

[0044] e. Heat treatment: A two-stage quenching + two-stage tempering process is adopted. ① Critical quenching: The total heating time is 95 min, the holding temperature is 840℃, the holding time is 1.2 min / mm, and after taking it out of the furnace, it is quenched to room temperature with a quenching water temperature of 22℃ and a quenching roller speed of 2.3 m / min. ② Sub-critical quenching: The total heating time is 90 min, the holding temperature is 800℃, the holding time is 1.0 min / mm, and after taking it out of the furnace, it is quenched to room temperature with a quenching water temperature of 23℃ and a quenching roller speed of 2.3 m / min. ③ First tempering: The holding temperature is set at 615℃, the total tempering coefficient is 3.5 min / mm, and after taking it out of the furnace, it is water-cooled to below 200℃. ④ Second tempering: The holding temperature is set at 635℃, the heating time is 65 min, the holding time is 35 min, and after taking it out of the furnace, it is air-cooled.

[0045] Its mechanical properties are shown in Table 2:

[0046] Table 22 Mechanical Properties

[0047]

[0048] In summary, the production method of this invention is highly effective in producing 60-80mm thick high-strength and high-toughness 980MPa steel plates. The various steps work together synergistically to achieve excellent overall performance, resulting in 60-80mm thick high-strength and high-toughness steel plates.

[0049] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Any other modifications or equivalent substitutions made by those skilled in the art to the technical solutions of the present invention, as long as they do not depart from the spirit and scope of the technical solutions of the present invention, should be covered within the scope of the claims of the present invention.

Claims

1. A method for producing a 60-80mm thick high-strength, high-toughness 980MPa steel plate, characterized in that, The steel plate contains the following chemical composition by mass percentage: C: 0.06–0.08, Si: 0.20–0.25, Mn: 0.60–0.70, P≤0.010, S≤0.002, Ni: 4.6–4.8, Cr: 0.5–0.6, Mo: 0.40–0.50, V: 0.05–0.06, with the remainder being Fe and residual elements; The steel plate is in a quenched and tempered state, and its microstructure is tempered sorbite with a small amount of retained austenite; its yield strength is 840-920 MPa, tensile strength is 940-1000 MPa, elongation at break is ≥16%, reduction of area is ≥50%, impact energy at -84℃ is ≥250 J, shear area ratio is ≥90%, impact energy at -100℃ is ≥200 J, shear area ratio is ≥80%, and the fracture surface after the fracture test is fibrous, without white spots, bright lines or penetrating cracks; The production method for the aforementioned 60-80mm thick high-strength and high-toughness 980MPa steel plate includes critical quenching + sub-temperature quenching and two tempering processes, as detailed below: a. Critical quenching: The total heating time is 80-100 min, the holding temperature is 830-850℃, the holding time is 1.2 min / mm, and after taking it out of the furnace, it is quenched to room temperature. The quenching water temperature is 18-25℃, and the quenching roller speed is 2.3 m / min. b. Sub-temperature quenching: The total heating time is 80-100 min, the holding temperature is 790-810℃, the holding time is 1.0 min / mm, and after taking it out of the furnace, it is quenched to room temperature. The quenching water temperature is 18-25℃, and the quenching roller speed is 2.3 m / min. c. First tempering: The holding temperature is set at 600-620℃, the total tempering time is 3.5 min / mm, and after taking it out of the furnace, it is water cooled to below 200℃; d. Secondary tempering: The holding temperature is set to 620-640℃, the heating time is 60-80 minutes, the holding time is 30-40 minutes, and the furnace is air-cooled after being taken out.

2. The method for producing a 60-80mm thick high-strength and high-toughness 980MPa steel plate according to claim 1, characterized in that, When the C content is 0.06%–0.07%, the Mn content is 0.65%–0.70%, the Cr content is 0.55%–0.60%, and the Mo content is 0.45%–0.50%; when the C content is 0.071%–0.08%, the Mn content is 0.60%–0.64%, the Cr content is 0.50%–0.54%, and the Mo content is 0.40%–0.44%.

3. The method for producing a 60-80mm thick high-strength and high-toughness 980MPa steel plate according to claim 1, characterized in that, When the C content is ≤0.07%, the temperature of the first tempering and holding stage is set to 600~610℃, and the temperature of the second tempering and holding stage is set to 620~630℃; when the C content is >0.07%, the temperature of the first tempering and holding stage is set to 611~620℃, and the temperature of the second tempering and holding stage is set to 631~640℃.

4. The method for producing a 60-80mm thick high-strength and high-toughness 980MPa steel plate according to claim 1, characterized in that, The steel plate production method also includes steel smelting, casting, steel ingot heating and rolling, and intermediate billet heating and rolling, as detailed below: a. Steelmaking: The converter uses high-alloy steel scrap as waste steel, with alloy content of Cr≥1.0%, Mo≥0.5%, and Ni≥1.5%; top and bottom combined oxygen blowing smelting, the C content of the converter tapped steel ≤0.05%; the process route of VD decarbonization + LF refining + VD vacuum degassing is adopted. After VD decarbonization, C: 0.02-0.03%; during the LF refining process, white slag is generated to adsorb inclusions, and Ca is added to modify the inclusions; after VD vacuum degassing, H ≤1.0 ppm, O ≤25 ppm, N ≤23 ppm in the molten steel; b. Casting: Use water-cooled ingot molds with a thickness of 800-840mm for casting, ensuring a compression ratio ≥10, a casting superheat of 30-40℃, a casting time of 16-18min for the main body, a casting time of 6-8min for the riser, and demolding 10-12h after casting. c. Steel ingot heating and rolling: The steel ingot is heated at ≤500℃ for 24-36 hours, then heated to 1160~1180℃ at a rate of ≤60℃ / h and held for 5 hours, then heated to 1180~1200℃ and held for 5 hours, and finally held at 1160~1180℃ for 2 hours before being rolled into a 300mm thick intermediate billet; d. Heating and rolling of intermediate billet: The intermediate billet is heated to 1180-1200℃ and held for 5-7 hours, then held at 1160~1180℃ for 0.5-1.0 hours. After holding, the billet is rolled to a thickness of 140-160mm in the first stage. After rolling, it enters ACC controlled cooling. When the billet is heated to 820-860℃, the second stage rolling to the finished plate begins. The final rolling temperature is 800-820℃. After rolling, it enters ACC cooling again, and the final cooling temperature is ≤300℃.

Images