Production method of super-thick low-carbon equivalent high-strength low-temperature steel Q500F

Abstract

The application discloses a production method of a super-thick low-carbon-equivalent high-strength low-temperature steel Q500F, the thickness of the steel is 220-270 mm, and the steel contains the following chemical components in percentage by mass (unit, wt%): C: 0.08-0.10, Si: 0.05-0.10, Mn: 1.30-1.40, P <=0.010, S <=0.003, Nb: 0.015-0.030, V: 0.030-0.040, Ni: 0.35-0.40, Mo: 0.18-0.22, Als: 0.015-0.030, and other Fe and residual elements, and the carbon equivalent Ceq of the steel is 0.36-0.40; compared with the prior art, the application has outstanding advantages in alloy cost, can save more than 1000 yuan per ton of steel due to the fact that a large amount of precious alloy is not needed, the thickness of the finished product reaches 220-270 mm and can meet the requirement of large thickness, the simplified heat treatment process can further reduce the production cost and shorten the production flow, and the finished product obtained meets the requirement that the transverse-60 DEG C impact performance is greater than 160 J, and has good market competition advantages.

Description

Technical Field

[0001] This invention belongs to the field of metallurgical technology, specifically relating to a production method of extra-thick, low-carbon equivalent high-strength low-temperature steel Q500F. Background Technology

[0002] The current production of thick Q500F high-strength low-temperature steel typically involves adding large amounts of expensive hardenability-enhancing alloying elements such as Cr, Ni, and Mo to ensure hardenability. This inevitably increases production costs, hindering market competitiveness, and also leads to a significant increase in carbon equivalent, severely impacting the weldability of the steel plate and necessitating preheating before fire cutting or welding. Furthermore, to ensure the low-temperature performance of even thicker steel plates, the common practice is to employ a double quenching and tempering process in the heat treatment stage. This complex heat treatment process lengthens the production cycle, further increasing production costs.

[0003] For example, the patent with publication number CN111826585, "A thick and high-toughness S500QL1 steel plate and its production method", discloses a steel plate with a thickness of 200-300mm and its production method. The performance meets the requirements of Q500F, but the carbon equivalent is too high, at 0.50-0.55%, and it is necessary to produce steel ingots by electroslag remelting. The heat treatment process requires a secondary quenching and tempering process, which is complicated and requires the addition of more alloys. Not only is the alloy cost high, but the process cost is also high, and it will also cause relatively difficult welding problems. Summary of the Invention

[0004] To address the aforementioned technical deficiencies, the present invention aims to provide a production method for extra-thick, low-carbon equivalent high-strength low-temperature steel Q500F. This method produces thick Q500F low-carbon equivalent high-strength low-temperature steel with a carbon equivalent of only 0.36-0.40% using lower alloy costs, significantly improving the weldability of the steel plate. Preheating is not required before welding, and the heat treatment process only involves a single quenching and tempering process, further reducing the production cost of the steel.

[0005] To achieve the above objectives, the technical solution adopted by the present invention is as follows: A method for producing extra-thick, low-carbon equivalent high-strength low-temperature steel Q500F, characterized in that the thickness of the steel is 220-270mm, and it contains the following chemical composition by mass percentage (unit, wt%): C: 0.08-0.10, Si: 0.05-0.10, Mn: 1.30-1.40, P≤0.010, S≤0.003, Nb: 0.015-0.030, V: 0.030-0.040, Ni: 0.35-0.40, Mo: 0.18-0.22, Als: 0.015-0.030, the others being Fe and residual elements, and its carbon equivalent Ceq: 0.36-0.40.

[0006] The overall composition design adopts a low-C + Mn + appropriate amounts of Ni and Mo alloy. Specifically: C is a key element for improving the strength of steel plates, but as the C content increases, the toughness of the steel plate decreases. Therefore, C is controlled at 0.08-0.10. Too high a C content will not meet the requirements for transverse low-temperature impact performance at -60℃, while too low a C content will compromise strength. Mn can improve the strength of steel and its hardenability, and stabilize and expand the austenite region. However, too much Mn can easily cause Mn segregation. In low-C steel, the low Si content can raise the recrystallization temperature of Tnr, which is beneficial for rolling temperature control. Temperature control prevents the formation of mixed crystals during the rolling process. Ni can improve low-temperature toughness. For every 1% increase in nickel content, the critical brittle transition temperature can be reduced by about 20°C. However, excessive Ni will increase the carbon equivalent. To maintain performance, appropriate addition of alloying elements such as Nb, V, Ni, and Mo can fully utilize the dual effects of solid solution strengthening and grain refinement strengthening, solving the problems of hardenability and hardenability of steel plates. Combined with appropriate heat treatment processes, the microstructure of the steel plate achieves a reasonable combination of granular bainite, polygonal ferrite, and pearlite with suitable morphology, ensuring that the strength and toughness of the steel plate meet the standard requirements.

[0007] The production of the aforementioned steel includes ingot heating, rolling, and heat treatment, as detailed below:

[0008] a. Steel ingot heating: 950-1050mm thick water-cooled mold steel ingots are heated in a furnace at 300℃-400℃ for 2-3 hours. After heating, the ingots are heated to 1000℃ at a rate of ≤100℃ / h, and then heated to 1250℃ at a rate of more than 100℃ / h, and held at that temperature for 13-15 hours. The ingots are turned over once 2 hours before tapping to ensure uniform temperature on both the top and bottom surfaces.

[0009] b. Rolling: After descaling the steel ingot, rolling begins. The initial rolling temperature is ≥1050℃. The reduction in the first few passes is 50-55mm. The reduction in the last two passes is reduced to achieve the final product thickness. The final rolling temperature is 950-1000℃.

[0010] c. After rolling, the steel plate is rapidly cooled multiple times using ACC, with the reddening temperature below 200℃; by utilizing the residual heat of the steel plate and using ACC cooling, the grain pre-refinement effect of the steel plate before heat treatment can be increased.

[0011] d. Heat treatment: Quenching and tempering with air cooling is adopted. The quenching temperature is 920-930℃, the holding time is T=2.2min / mm, and the temperature is water-cooled to room temperature. The tempering temperature is 600-610℃, the holding time is 4min / mm, and the temperature is air-cooled to below 200℃ after tempering.

[0012] The steel plate obtained by the above treatment has a microstructure mainly consisting of bainite (60-70%) + ferrite (15-25%) + a small amount of pearlite.

[0013] The steel plate obtained using the above method meets the strength requirements of Q500F grade, and its transverse impact performance at -60℃ is greater than 160J, which is significantly better than the standard ≥47J, and its low-temperature resistance is significantly enhanced. With a carbon equivalent of only 0.36-0.40%, this method allows for welding at room temperature. Simulated post-weld heat treatment performance tests at 580±10℃ and T=4min / mm showed good weldability in both the weld and the affected zone, and mechanical properties close to those of the steel plate. Experience indicates that when CE≤0.4%, the steel has good weldability; when CE>0.4%, the weldability deteriorates, requiring appropriate preheating before welding; and when CE>0.6%, the weldability is poor, classifying it as a difficult-to-weld material, requiring higher preheating temperatures and stricter process methods. Compared with existing technologies, this solution has a significant advantage in alloy cost. Because it does not require the addition of a large amount of precious alloys, it can save more than 1,000 yuan per ton of steel. Secondly, the finished product thickness of 220-270mm can meet the requirements of large thicknesses. The simplified heat treatment process can further reduce production costs and shorten the production process, giving it a good competitive advantage in the market. Attached Figure Description

[0014] The technical features of the present invention will be further described below with reference to the accompanying drawings and embodiments.

[0015] Figure 1 This is a schematic diagram (×100) of the microstructure of a 1 / 4 thickness section in Example 1 of the present invention.

[0016] Figure 2 This is a schematic diagram (×200) of the microstructure of a 1 / 4 thickness section in Example 1 of the present invention.

[0017] Figure 3 This is a schematic diagram (×500) of the simulated post-weld microstructure of a 1 / 4 thickness section in Example 1 of the present invention.

[0018] Figure 4 This is a schematic diagram (×500) of the simulated post-weld microstructure of a 1 / 4 thickness section in Example 1 of the present invention. Detailed Implementation

[0019] The present invention will be further described below with reference to the embodiments.

[0020] A method for producing an extra-thick, low-carbon equivalent high-strength low-temperature steel Q500F, comprising the following chemical composition by mass percentage (wt%): C: 0.08–0.10, Si: 0.05–0.10, Mn: 1.30–1.40, P≤0.010, S≤0.003, Nb: 0.015–0.030, V: 0.030–0.040, Ni: 0.35–0.40, Mo: 0.18–0.22, Als: 0.015–0.030, with the remainder being Fe and residual elements, Ceq: 0.36–0.40;

[0021] Its production methods include KR desulfurization of molten iron, converter smelting, LF refining, VD vacuum degassing, ingot casting, ingot heating, rolling, and heat treatment, specifically:

[0022] a. KR desulfurization: After the molten iron arrives at the station, slag removal is carried out to ensure the smallest possible slag layer thickness. After KR deep desulfurization, the S content of the molten iron entering the furnace is ensured to be ≤0.005%, and the slag layer thickness in the molten iron ladle after desulfurization is ≤25mm.

[0023] b. Converter smelting: All scrap steel added to the furnace is selected from high-quality scraps; the scrap steel ratio is reasonably matched according to the composition and temperature of the molten iron to ensure that the carbon content of the tapped steel is controlled at 0.05-0.08%, and the P and S content of the tapped steel is controlled as low as possible below 0.010%. Slag is blocked in time during the tapping process to avoid a large amount of slag falling.

[0024] c.LF Refining: Argon is blown throughout the refining process. A large slag volume refining process is adopted. Deoxidizer is added at the same time. Heating is done in two stages, with each stage lasting about 20 minutes. The slag must turn white or yellowish-white at the end of the process. The white slag must be maintained for ≥20 minutes. Deoxidizer is added during the second heating process. Argon gas is turned off before leaving the station. A silicon-calcium line is added, and the steel temperature is well controlled.

[0025] d. VD vacuum degassing: VD vacuum degree ≤65Pa, pressure holding time must be ≥15min, soft blowing for 2-5min after vacuum breaking, molten steel must not be exposed during soft blowing, and the covering agent must be fully covered on the surface of molten steel after vacuuming.

[0026] e. Ingot casting: A 950-1050mm thick water-cooled mold with adjustable width is used. The casting process is carried out at low temperature and fast pouring. The initial pouring temperature is controlled at 1560±5℃. The pouring time of the main body is controlled within 20 minutes, and the pouring time of the cap is controlled within 3 minutes. Slag is added in time during the pouring process to avoid the molten steel being exposed. After the molten steel is poured, a sufficient amount of rice husks is added in time to ensure the heat preservation effect of the cap and prevent the cap from turning red.

[0027] f. Steel ingot heating: The steel ingot is heated at a low temperature of 300℃-400℃ for 2-3 hours in the furnace. After the simmering process is completed, it is heated to 1000℃ at a heating rate of ≤100℃ / h, and then heated to 1250℃ at a heating rate of more than 100℃ / h. It is held at this temperature for 13-15 hours. The steel ingot is turned over once 2 hours before tapping to ensure that the temperature of the top and bottom of the steel ingot is uniform.

[0028] g. Rolling: After descaling the steel ingot, rolling begins. The initial rolling temperature is ≥1050℃. The reduction in the first few passes is 50-55mm. The reduction in the last two passes is reduced to achieve the final product thickness. The final rolling temperature is 950-1000℃.

[0029] h. After rolling, the steel plate is rapidly cooled multiple times using ACC, with the red-hot temperature below 200℃;

[0030] i. Heat treatment: The heat treatment adopts the quenching + tempering and air cooling process. The quenching holding temperature is 920-930℃, the holding time is 2.2min / mm, and the temperature is water-cooled to room temperature; the tempering holding temperature is 600-610℃, the holding time is 4min / mm, and the temperature is air-cooled to below 200℃ after tempering.

[0031] Example 1

[0032] The chemical composition obtained by desulfurization of molten iron using KR, converter smelting, LF refining, and VD vacuum degassing is as follows (unit, wt%): C: 0.08, Si: 0.10, Mn: 1.39, P: 0.007, S≤0.001, Nb: 0.028, V: 0.035, Ni: 0.37, Mo: 0.21, Als: 0.020, with the remainder being Fe and residual elements, Ceq: 0.39;

[0033] a. Ingot casting: A water-cooled mold with fixed thickness and adjustable width is used. The mold cavity thickness is 1050mm. The casting process is low temperature and fast pouring. The initial pouring temperature is 1563℃. The main body pouring time is 20min. The cap pouring time is 3min. Slag is added multiple times to cover the molten steel during the pouring process. After the molten steel is poured, 8 bags of rice husks are added. No red color is seen at the cap.

[0034] b. Steel ingot heating: The steel ingot is placed in the furnace and heated at 330℃ for 3 hours. The heating rate is 70-100℃ / h when the temperature is below 1000℃ and 100-150℃ / h when the temperature is above 1000℃. The maximum heating temperature is 1250℃. The temperature is maintained for 14 hours. The steel is turned over once 2 hours before tapping.

[0035] c. Rolling: Hot rolling process is adopted, with an initial rolling temperature of 1100℃, a reduction of 50-55mm in the first pass after steel transfer, a reduction of 15mm and 10mm in the two passes before the final rolling, and the finished product is rolled to 270mm with a final rolling temperature of 990℃.

[0036] d. After rolling, the steel plate immediately enters ACC cooling, which is carried out 5 times in total. After each cooling, the steel plate is reddened for 3 minutes, and the final reddening temperature is 190℃.

[0037] e. Heat Treatment: The heat treatment adopts a quenching + tempering and air cooling process. For quenching, the holding temperature is 930℃, the holding coefficient is T=2.2min / mm, and it is water-cooled to room temperature. For tempering, the holding temperature is 610℃, the holding time is 4min / mm, and after tempering, it is air-cooled to 200℃.

[0038] The 270mm thick Q500F low-carbon equivalent high-strength low-temperature steel produced using the above method, after testing, exhibits a yield strength of 528-533MPa, tensile strength of 624-628MPa, elongation of 25-26%, and an impact value greater than 165J at -60℃. Simulated post-weld heat treatment at 580±10℃, with a temperature T=4min / mm, demonstrates satisfactory performance. Specific performance data are shown in Table 1 below.

[0039]

[0040] Example 2

[0041] The chemical composition obtained by desulfurization of molten iron in KR, smelting in a converter, refining in LF, and vacuum degassing in VD is as follows (unit, wt%): C: 0.09, Si: 0.08, Mn: 1.37, P: 0.008, S≤0.001, Nb: 0.016, V: 0.036, Ni: 0.38, Mo: 0.20, Als: 0.020, the others are Fe and residual elements, Ceq: 0.39;

[0042] a. Ingot casting: A fixed-width, adjustable-thickness water-cooled mold was used, with a mold cavity thickness of 950mm, a pouring temperature of 1563℃, a body pouring time of 16min, a cap pouring time of 2min, and multiple slag replenishments to cover the molten steel during the pouring process. After the molten steel was poured, 5 bags of rice husks were added, and the cap did not turn red.

[0043] b. Steel ingot heating: The steel ingot is placed in the furnace at 400℃ and simmered for 2 hours. When the temperature is below 1000℃, the heating rate is 70-100℃ / h, and when the temperature is above 1000℃, the heating rate is 100-150℃ / h. The maximum heating temperature is 1250℃. The temperature is maintained for 13 hours. The steel is turned over once 2 hours before tapping.

[0044] c. Rolling: Hot rolling process is adopted, with an initial rolling temperature of 1106℃, a reduction of 50-55mm after steel transfer, a reduction of 10mm and 6mm in the two passes before final rolling, and a finished product thickness of 220mm. The final rolling temperature is 980℃.

[0045] d. After rolling, the steel plate immediately enters ACC cooling, which is carried out three times in total. After each cooling, the steel plate is heated back to red for 3 minutes, and the final temperature is heated back to red for 196℃.

[0046] e. Heat Treatment: The heat treatment adopts a quenching + tempering and air cooling process. For quenching, the holding temperature is 920℃, the holding coefficient is T=2.2min / mm, and it is water-cooled to room temperature. For tempering, the holding temperature is 600℃, the holding time is 4min / mm, and after tempering, it is air-cooled to 190℃.

[0047] The 220mm thick Q500F low-carbon equivalent high-strength low-temperature steel produced using the above method, after testing, exhibits a yield strength of 569-572MPa, tensile strength of 681-683MPa, elongation of 25%, and an impact value greater than 220J at -60℃. Simulated post-weld heat treatment at 580±10℃, with a temperature T=4min / mm, demonstrates satisfactory performance. Specific performance data are shown in Table 2 below.

[0048]

[0049] Example 3

[0050] The chemical composition obtained by desulfurization of molten iron in KR, smelting in a converter, refining in LF, and vacuum degassing in VD is as follows (unit, wt%): C: 0.08, Si: 0.07, Mn: 1.33, P: 0.007, S≤0.001, Nb: 0.029, V: 0.033, Ni: 0.36, Mo: 0.18, Als: 0.020, the others are Fe and residual elements, Ceq: 0.37;

[0051] a. Ingot casting: A fixed-width, adjustable-thickness water-cooled mold was used, with a mold cavity thickness of 950mm, a pouring temperature of 1563℃, a body pouring time of 19min, a cap pouring time of 2min, and multiple slag replenishments to cover the molten steel during the pouring process. After the molten steel was poured, 6 bags of rice husks were added, and the cap did not turn red.

[0052] b. Steel ingot heating: The steel ingot is placed in the furnace and heated at 380℃ for 2 hours. After heating, it is heated to 1000℃ at a rate of 70-100℃ / h, and then heated to 1250℃ at a rate of 100-150℃ / h, and held at that temperature for 14 hours. The steel is turned over once 2 hours before tapping.

[0053] c. Rolling: Hot rolling process is adopted, with an initial rolling temperature of 1098℃, a reduction of 50-55mm in the first pass after steel transfer, a reduction of 13mm and 8mm in the two passes before the final rolling, and a finished product thickness of 255mm. The final rolling temperature is 998℃.

[0054] d. After rolling, the steel plate immediately enters ACC cooling, which is carried out 5 times in total. After each cooling, the steel plate is reddened for 3 minutes, and the final reddening temperature is 170℃.

[0055] e. Heat Treatment: The heat treatment adopts a quenching + tempering and air cooling process. For quenching, the holding temperature is 920℃, the holding coefficient is T=2.2min / mm, and it is water-cooled to room temperature. For tempering, the holding temperature is 610℃, the holding time is 4min / mm, and after tempering, it is air-cooled to 180℃.

[0056] The 255mm thick Q500F low-carbon equivalent high-strength low-temperature steel produced using the above method, after testing, has a yield strength of 543-544MPa, tensile strength of 643-650MPa, elongation of 26%, and an impact value of ≥200J at -60℃. It also passed simulated post-weld heat treatment at 580±10℃ with a temperature of T=4min / mm. Specific performance data are shown in Table 3 below.

[0057]

[0058] Metallographic analysis of the steel plates obtained in Examples 1-3 showed that their microstructure was mainly bainite (60-70%) + ferrite (15-25%) + a small amount of pearlite, with a grain size of 8-10, which meets the parameter requirements for steel plates in this invention.

Claims

1. A production method of a super-thick low-carbon equivalent high-strength low-temperature steel Q500F, characterized in that, The steel has a thickness of 220-270 mm and contains the following chemical composition by mass percentage: C: 0.08-0.10, Si: 0.05-0.10, Mn: 1.30-1.40, P≤0.010, S≤0.003, Nb: 0.015-0.030, V: 0.030-0.040, Ni: 0.35-0.40, Mo: 0.18-0.22, Als: 0.015-0.030, with the remainder being Fe and residual elements, and its carbon equivalent Ceq: 0.36-0.40; The steel production method includes steel ingot heating, rolling, and heat treatment, as detailed below: a. Steel ingot heating: 950-1050mm thick water-cooled mold steel ingots are heated in a furnace at 300℃-400℃ for 2-3 hours. After heating, the ingots are heated to 1000℃ at a rate of ≤100℃ / h, and then heated to 1250℃ at a rate of more than 100℃ / h, and held at that temperature for 13-15 hours. The ingots are turned over once 2 hours before tapping to ensure uniform temperature on both the top and bottom surfaces. b. Rolling: After descaling, rolling begins on the steel ingot. The initial rolling temperature is ≥1050℃. The reduction in the first few passes is 50-55mm. The reduction in the last two passes is reduced to achieve the final product thickness. The final rolling temperature is 950-1000℃. c. After rolling, the steel plate is rapidly cooled multiple times using ACC, with the red-hot temperature below 200℃; d. Heat treatment: Quenching and tempering with air cooling is adopted. The quenching temperature is 920-930℃, the holding time is T=2.2min / mm, and the temperature is water-cooled to room temperature. The tempering temperature is 600-610℃, the holding time is 4min / mm, and the temperature is air-cooled to below 200℃ after tempering.

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