Low-cost q690d steel plate and on-line quenching-tempering production method thereof

Abstract

A low-cost Q690D steel plate and its online quenching-tempering production method belong to the field of steel heat treatment technology. Its chemical composition and mass content are: C 0.14-0.17%, Si 0.25-0.35%, Mn 1.35-1.55%, P≤0.01%, S≤0.005%, Cr 0.30-0.50%, Nb 0.020-0.035%, Ti 0.015-0.025%, V 0.02-0.04%, B 0.0015-0.0025%, with the remainder being iron and unavoidable impurities. The production method includes controlled rolling, online quenching, and tempering processes. This invention does not add precious metal elements such as Ni, Mo, and Cu, resulting in low alloy cost, short process time, and a steel plate with a yield strength ≥690MPa and an impact energy ≥120J at -20℃, exhibiting good strength and toughness matching.

Description

Technical Field

[0001] This invention belongs to the field of steel heat treatment technology, and relates to a low-cost Q690D steel plate and its online quenching-tempering production method. Background Technology

[0002] With the development of lightweight construction machinery, Q690 grade steel plates are widely used in hydraulic supports, cranes, mining machinery, excavators, and other construction machinery. Traditional Q690D steel plates are usually produced using a process of "controlled rolling + offline quenching and tempering heat treatment." To ensure a high carbon equivalent to improve strength and toughness, a large amount of alloying elements such as Ni and Mo need to be added to the steel, resulting in a long production process and high alloying costs.

[0003] In recent years, with the development of ultrafast cooling technology and the demand for low-cost production processes, some steel mills have tried to use the "online quenching + tempering" process to replace the traditional offline quenching and tempering process in the production of Q690D steel plates. However, the existing technology still has the following shortcomings: Patent applications with application numbers CN201410467565.0, CN201811375040.9, and CN202511014587.6 require the addition of precious metal elements such as Mo and / or Ni to compensate for the limitations of online quenching cooling capacity, resulting in relatively high costs.

[0004] Patent application CN202210843516.7 introduces an online induction heating device for quenching rolled Q690 steel plates. This equipment is specific to the production line and lacks general applicability. Furthermore, the process is relatively complex, requiring precise temperature control, which can have a certain impact on the rolling rhythm in actual production.

[0005] While patent application CN202311556522.5 discloses a method for improving the low-temperature impact toughness of online-quenched Q690D steel, involving 20mm and 25mm thick Q690D steel plates, it does not mention the relevant components and requires the steel plate to be rolled at a temperature 50°C above the non-recrystallization temperature, placing high demands on the rolling process. Thin-gauge steel plates experience rapid temperature drops during actual rolling, making it difficult to meet the rolling process requirements mentioned in the patent.

[0006] In summary, existing online quenching production technologies still suffer from problems such as high component content or complex production processes. Therefore, there is an urgent need to develop a low-cost and simple production process for Q690D steel plates and its production method. Summary of the Invention

[0007] The present invention aims to provide a low-cost, high-performance Q690D steel plate and its online quenching-tempering production method. The steel plate does not contain precious metal elements such as Ni, Mo, and Cu, has a yield strength ≥690MPa, and also has good toughness.

[0008] To achieve the above-mentioned objectives, the technical solution adopted by the present invention is as follows: A low-cost Q690D steel plate has the following chemical composition and mass percentage: C 0.14-0.17%, Si 0.25-0.35%, Mn 1.35-1.55%, P≤0.01%, S≤0.005%, Cr 0.30-0.50%, Nb 0.020-0.035%, Ti 0.015-0.025%, V 0.02-0.04%, B 0.0015-0.0025%, with the remainder being iron and unavoidable impurities.

[0009] The thickness of the steel plate is 20-40 mm.

[0010] The steel plate has a yield strength of 790–850 MPa, a tensile strength of 840–890 MPa, an elongation of 16–18%, and an impact energy of ≥120 J at -20℃.

[0011] The online quenching-tempering production method for low-cost Q690D steel plates includes controlled rolling, online quenching, and tempering processes; wherein: Controlled rolling process: Two-stage controlled rolling is adopted. After the first stage of rolling, the thickness of the steel plate to be heated is 2.5 to 3.0 times the thickness of the finished steel plate. Online quenching process: After rolling, the steel plate is put into an ultra-fast cooling equipment at 800-830℃ for online quenching until the steel plate temperature is ≤200℃; Tempering process: heating temperature is 550~600℃.

[0012] The controlled rolling process has a two-stage initial rolling temperature of 930–950°C and a two-stage final rolling temperature of 850–870°C.

[0013] In the online quenching process, the ultra-fast cooling water pressure is ≥0.4MPa, and the acceleration of the steel plate running in the ultra-fast cooling equipment is 0.006m~0.008m / s². 2 .

[0014] The heat preservation coefficient for the tempering process is 3 to 3.5 min / mm.

[0015] The design concept of this invention is as follows: To ensure the toughness of steel plates after tempering using the "online quenching-tempering" process, the C content in Q690D steel plates is generally controlled below 0.1% under this process, while a certain amount of Mo is added to improve the hardenability and toughness of the steel plate after tempering.

[0016] This invention improves the hardenability of steel plates by increasing the carbon content and adding appropriate amounts of Cr and B elements, ensuring strength after online quenching. Simultaneously, the addition of Nb and Ti elements refines the grain structure, ensuring strength and toughness after tempering. For thicker steel plates, a certain amount of V is added to further guarantee post-temper strength. Compared to the microalloying elements Nb and Ti, V has a lower precipitation temperature and can precipitate in large quantities during tempering, thus providing precipitation strengthening.

[0017] Based on the above compositional system design, a reasonable controlled rolling process is adopted: the first-stage rolling refines the grains through recrystallization, and the second-stage rolling breaks up the austenite grains, increasing the nucleation rate of the quenched structure and refining the martensite laths. Simultaneously, the post-rolling water immersion temperature is increased to ensure the formation of a fully martensitic structure after online quenching.

[0018] This invention, through alloy composition design, eliminates the need for adding costly Mo element. Combined with a reasonable online quenching and tempering process, tempered sorbite is obtained, achieving low-cost production of Q690D steel plates. The process is simple and has high economic value. The resulting steel plates have a yield strength of 790–850 MPa, a tensile strength of 840–890 MPa, an elongation of 16–18%, and an impact energy of ≥120 J at -20℃. Attached Figure Description

[0019] Figure 1 This is a typical metallographic structure diagram (500×) of the steel plate obtained in Example 1 of the present invention. Detailed Implementation Example 1

[0020] In this embodiment, the thickness of the Q690D steel plate is 20mm, and its chemical composition and mass percentage are shown in Table 1.

[0021] Based on the composition design, a continuously cast billet is obtained through smelting, then heated and subjected to controlled rolling, online quenching, and tempering. Among these processes: The controlled rolling process adopts a two-stage controlled rolling process. After the first stage rolling, the thickness to be heated is 3.0 times the thickness of the finished steel plate. The second stage rolling temperature is 950℃, and the second stage final rolling temperature is 865℃.

[0022] After rolling, the steel plate enters an ultra-fast cooling equipment for online quenching. The water temperature is 800℃, the ultra-fast cooling water pressure is set to 0.4MPa, and the acceleration of the steel plate in the ultra-fast cooling equipment is 0.007m / s². 2 The steel plate was quenched online to a temperature of 70°C.

[0023] The tempering process is heated to 600℃ with a holding coefficient of 3.5 min / mm. Example 2

[0024] In this embodiment, the thickness of the Q690D steel plate is 25mm, and its chemical composition and mass percentage are shown in Table 1.

[0025] Based on the composition design, a continuously cast billet is obtained through smelting, then heated and subjected to controlled rolling, online quenching, and tempering. Among these processes: The controlled rolling process adopts a two-stage controlled rolling process. After the first stage rolling, the thickness to be heated is 3.0 times the thickness of the finished steel plate. The second stage rolling temperature is 940℃, and the second stage final rolling temperature is 870℃.

[0026] After rolling, the steel plate enters an ultra-fast cooling equipment for online quenching. The water temperature is 815℃, the ultra-fast cooling water pressure is set to 0.4MPa, and the acceleration of the steel plate in the ultra-fast cooling equipment is 0.008m / s². 2 The steel plate is quenched online to a temperature of 100℃.

[0027] The tempering process is heated to 600℃ with a holding coefficient of 3.5 min / mm. Example 3

[0028] In this embodiment, the thickness of the Q690D steel plate is 30mm, and its chemical composition and mass percentage are shown in Table 1.

[0029] Based on the composition design, a continuously cast billet is obtained through smelting, then heated and subjected to controlled rolling, online quenching, and tempering. Among these processes: The controlled rolling process adopts a two-stage controlled rolling process. After the first stage rolling, the thickness to be heated is 3.0 times the thickness of the finished steel plate. The second stage rolling temperature is 935℃, and the second stage final rolling temperature is 860℃.

[0030] After rolling, the steel plate enters an ultra-fast cooling equipment for online quenching. The water temperature is 825℃, the ultra-fast cooling water pressure is set to 0.4MPa, and the acceleration of the steel plate in the ultra-fast cooling equipment is 0.008m / s². 2 The steel plate was quenched online to a temperature of 125℃.

[0031] The tempering process is heated at 580℃ with a holding coefficient of 3.4 min / mm. Example 4

[0032] In this embodiment, the thickness of the Q690D steel plate is 35mm, and its chemical composition and mass percentage are shown in Table 1.

[0033] Based on the composition design, a continuously cast billet is obtained through smelting, then heated and subjected to controlled rolling, online quenching, and tempering. Among these processes: The controlled rolling process adopts a two-stage controlled rolling process. After the first stage rolling, the thickness to be heated is 3.0 times the thickness of the finished steel plate. The second stage rolling temperature is 930℃, and the second stage final rolling temperature is 850℃.

[0034] After rolling, the steel plate enters an ultra-fast cooling equipment for online quenching. The water temperature is 830℃, the ultra-fast cooling water pressure is set to 0.5MPa, and the acceleration of the steel plate in the ultra-fast cooling equipment is 0.006m / s². 2 The steel plate is quenched online to a temperature of 150℃.

[0035] The tempering process is heated at 580℃ with a holding coefficient of 3.0 min / mm. Example 5

[0036] In this embodiment, the thickness of the Q690D steel plate is 40mm, and its chemical composition and mass percentage are shown in Table 1.

[0037] Based on the composition design, a continuously cast billet is obtained through smelting, then heated and subjected to controlled rolling, online quenching, and tempering. Among these processes: The controlled rolling process adopts a two-stage controlled rolling process. After the first stage rolling, the thickness to be heated is 2.5 times the thickness of the finished steel plate. The second stage rolling temperature is 930℃, and the second stage final rolling temperature is 850℃.

[0038] After rolling, the steel plate enters an ultra-fast cooling equipment for online quenching. The water temperature is 830℃, the ultra-fast cooling water pressure is set to 0.5MPa, and the acceleration of the steel plate in the ultra-fast cooling equipment is 0.006m / s². 2 The steel plate was quenched online to a temperature of 190℃.

[0039] The tempering process is heated at 550℃ with a holding coefficient of 3.1 min / mm.

[0040] The chemical composition and mass percentage of Q690D steel plates in each embodiment are shown in Table 1, and the specifications and properties of the steel plates are shown in Table 2.

[0041] Table 1. Chemical composition and mass percentage (%) of steel plates in each embodiment

[0042] In Table 2, the balance is Fe and unavoidable impurities.

[0043] Table 2 Specifications and mechanical properties of steel plates in each embodiment

[0044] Depend on Figure 1 It can be seen that the typical metallographic structure of the steel plate obtained by the present invention is tempered sorbite, with fine and uniform grain size, no obvious coarse structure, and excellent overall microstructure uniformity.

Claims

1. A low-cost Q690D steel plate, characterized in that, The chemical composition and mass percentage of the steel plate are as follows: C 0.14-0.17%, Si 0.25-0.35%, Mn 1.35-1.55%, P≤0.01%, S≤0.005%, Cr 0.30-0.50%, Nb 0.020-0.035%, Ti 0.015-0.025%, V 0.02-0.04%, B 0.0015-0.0025%, with the remainder being iron and unavoidable impurities.

2. The low-cost Q690D steel plate according to claim 1, characterized in that, The thickness of the steel plate is 20-40 mm.

3. The low-cost Q690D steel plate according to claim 2, characterized in that, The steel plate has a yield strength of 790–850 MPa, a tensile strength of 840–890 MPa, an elongation of 16–18%, and an impact energy of ≥120 J at -20℃.

4. The online quenching-tempering production method for low-cost Q690D steel plates according to any one of claims 1-3, characterized in that, This includes controlled rolling, online quenching, and tempering processes; among which: Controlled rolling process: Two-stage controlled rolling is adopted. After the first stage of rolling, the thickness of the steel plate to be heated is 2.5 to 3.0 times the thickness of the finished steel plate. Online quenching process: After rolling, the steel plate is put into an ultra-fast cooling equipment at 800-830℃ for online quenching until the steel plate temperature is ≤200℃; Tempering process: heating temperature is 550~600℃.

5. The online quenching-tempering production method for low-cost Q690D steel plates according to claim 4, characterized in that, The controlled rolling process has a two-stage initial rolling temperature of 930–950°C and a two-stage final rolling temperature of 850–870°C.

6. The online quenching-tempering production method for low-cost Q690D steel plates according to claim 4, characterized in that, In the online quenching process, the ultra-fast cooling water pressure is ≥0.4MPa, and the acceleration of the steel plate running in the ultra-fast cooling equipment is 0.006m~0.008m / s². 2 .

7. The online quenching-tempering production method for low-cost Q690D steel plates according to claim 4, characterized in that, The heat preservation coefficient for the tempering process is 3 to 3.5 min / mm.

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