A production method for improving the -100°C impact energy of extra-thick shipbuilding steel by using the large-angle grain boundary control technology

Abstract

The present invention provides a production method for improving the -100°C impact energy of extra-thick shipbuilding steel by using the large-angle grain boundary control technology. The steps include: heating and holding the continuous casting billet for the first time in the austenite recrystallization zone, and then directly water-cooling it to room temperature; heating and holding the continuous casting billet for the second time in the austenite non-recrystallization zone, and then performing rolling; using a laminar flow cooling device to reduce the surface temperature of the rolled continuous casting billet to the recalescence temperature of 320 ± 40°C. The production method for improving the -100°C impact energy of extra-thick shipbuilding steel by using the large-angle grain boundary control technology provided by the present invention has strong operability, high production efficiency, and low implementation difficulty, can achieve a high proportion of large-angle grain boundaries, refine the effective grains, significantly improve the low-temperature toughness of the extra-thick shipbuilding steel, and ensure that the -100°C impact energy ≥ 182 J.

Description

Technical Field

[0001] The present invention relates to the technical field of improving the properties of extra-thick shipbuilding steel, and particularly relates to a production method for improving the -100°C impact energy of extra-thick shipbuilding steel by using a large-angle grain boundary control technology. Background Art

[0002] In recent years, the development trend of ultra-large container ships has been significant. To ensure the safe navigation of ships, the requirements for the thickness specifications and low-temperature toughness of shipbuilding steel have become increasingly stringent. The TMCP rolling process with refined grains as the core has the characteristics of high production efficiency, low cost, and remarkable effects. The essence of the TMCP technology is to regulate the austenite grain size and distortion state in the non-recrystallization zone, obtain a refined phase transformation microstructure (ferrite, bainite) and a good match of the relative content, and ensure that the steel plate has a good strength and toughness match.

[0003] In the TMCP process, there are two ways to refine austenite grains: 1) Achieve full recrystallization of austenite grains through deformation; 2) Use a lower austenitizing temperature to limit the growth of austenite grains and obtain refined austenite grains. However, for conventional extra-thick shipbuilding steel during the TMCP rolling process, due to the existing continuous casting billet thickness, when producing extra-thick steel plates, a smaller rolling reduction ratio increases the difficulty of regulating the austenite grain size and distortion state before phase transformation, making it difficult to obtain a refined multiphase structure and a high proportion of large-angle grain boundaries (HAGB), and even unable to ensure the low-temperature toughness of the steel plate core.

[0004] Therefore, for extra-thick shipbuilding steel, it is necessary to develop a processing technology with strong operability, high production efficiency, and low implementation difficulty to achieve a high proportion of large-angle grain boundaries, refine effective grains, and significantly improve the -100°C low-temperature toughness of extra-thick shipbuilding steel. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide a production method for improving the -100°C impact energy of extra-thick shipbuilding steel by using a large-angle grain boundary control technology, which has strong operability, high production efficiency, and low implementation difficulty, so as to achieve a high proportion of large-angle grain boundaries, refine effective grains, significantly improve the low-temperature toughness of extra-thick shipbuilding steel, and ensure that the -100°C impact energy ≥ 182 J.

[0006] To solve the above technical problem, the present invention provides a production method for improving the -100°C impact energy of extra-thick shipbuilding steel by using a large-angle grain boundary control technology, including the following steps:

[0007] Heat and hold the continuous casting billet for the first time in the austenite recrystallization zone, and then directly water-cool to room temperature;

[0008] Heat and hold the continuous casting billet for the second time in the austenite non-recrystallization zone, and then perform rolling;

[0009] The surface temperature of the continuous casting billet after rolling is reduced to the recrystallization temperature of 320±40°C by using a laminar flow cooling device.

[0010] Furthermore, the heating temperature for the first heating and holding of the continuous casting billet is 1200±50°C, and the holding time is 2.5 - 4 h. After the first heating and holding of the continuous casting billet, it is directly water-cooled to room temperature by an ultra-fast cooling device.

[0011] Furthermore, the heating temperature for the second heating and holding of the continuous casting billet is 1000±150°C, and the holding time is 1.5 - 3 h.

[0012] Furthermore, the starting rolling temperature of the continuous casting billet during rolling is 780 - T nr -20°C, T nr which is the austenite recrystallization temperature, T nr and is calculated using the following formula T nr =887 + 446ω c +890ω Ti +363ω Al -357ω Si +6445ω Nb - 644ω Nb 1 / 2 +732ω V -230ω V 1 / 2 to obtain.

[0013] Furthermore, the total reduction ratio during the rolling of the continuous casting billet is controlled below 3.5, and the total reduction amount is controlled at 50 - 70%.

[0014] Furthermore, the matrix structure at the 1 / 2 position of the continuous casting billet after rolling and cooling is a mixed structure composed of ferrite + bainite + small block-shaped M / A islands, and the proportion of large-angle grain boundaries at the 1 / 2 position is 66.4%.

[0015] Furthermore, the effective grain distribution at the 1 / 2 position of the continuous casting billet after rolling and cooling is uniform, and the effective grain size is 3.8μm.

[0016] Furthermore, the impact energy of the continuous casting billet after rolling and cooling is ≥200J at -80°C and ≥182J at -100°C.

[0017] Under normal circumstances, the bainite grain boundaries are small-angle grain boundaries, while the ferrite grain boundaries are mostly large-angle grain boundaries. A high proportion of large-angle grain boundaries can improve the low-temperature toughness of the steel plate. The region surrounded by large-angle grain boundaries is the effective grain, and the equivalent circle diameter of which is called the effective grain size. The smaller the effective grain size of the microstructure, the better the low-temperature toughness of the steel plate. Moreover, there is a close relationship among the large-angle grain boundaries, the effective grain size and the austenite state before water cooling in the steel plate. By controlling the austenite non-recrystallization region stage of TMCP, applying a large amount of deformation to form a sufficient number of nucleation points for diffusion phase transformation products such as dislocations and deformation bands inside the flattened austenite grains, and further controlling the post-rolling cooling schedule, the proportion of large-angle grain boundaries and the effective grain size can be controlled, so as to improve the low-temperature impact toughness of the steel plate at -100°C by using a high proportion of large-angle grain boundaries and refined effective grains.

[0018] Therefore, a production method for improving the -100°C impact energy of extra-thick shipbuilding steel by using the large-angle grain boundary control technology provided by the present invention comprehensively considers the main ways to improve the low-temperature toughness of extra-thick shipbuilding steel and the production characteristics, uses the continuous casting billet with the existing alloy composition system, adopts the two-stage heating and one-stage controlled rolling process, and realizes a high proportion of large-angle grain boundaries and refines the effective grains under the conditions of low reduction ratio and high finish rolling temperature, so as to improve the low-temperature toughness of extra-thick shipbuilding steel, and realize that the -80°C impact energy of the continuous casting billet is ≥200 J and the -100°C impact energy is ≥182 J.

[0019] In addition, a production method for improving the -100°C impact energy of extra-thick shipbuilding steel by using the large-angle grain boundary control technology provided by the present invention has strong operability, high production efficiency and low implementation difficulty, and is suitable for mass production. Moreover, without modifying the current production line, it solves the urgent production problem of the lack of extra-thick continuous casting billets in domestic steel mills. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 It is a flow chart of the production method for improving the -100°C impact energy of extra-thick shipbuilding steel by using the large-angle grain boundary control technology provided by the embodiment of the present invention;

[0021] Figure 2 It is a typical microstructural diagram of the core of the continuous casting billet steel plate prepared by the production method for improving the -100°C impact energy of extra-thick shipbuilding steel by using the large-angle grain boundary control technology provided by Embodiment 1 of the present invention;

[0022] Figure 3 It is a grain boundary diagram of the core of the continuous casting billet steel plate prepared by the production method for improving the -100°C impact energy of extra-thick shipbuilding steel by using the large-angle grain boundary control technology provided by Embodiment 1 of the present invention;

[0023] Figure 4The distribution map of the effective grain size at the center of the continuous casting billet steel plate prepared by the production method for improving the -100 °C impact energy of extra-thick shipbuilding steel using the large-angle grain boundary control technology provided in Embodiment 1 of the present invention;

[0024] Figure 5 The grain boundary map at the center of the continuous casting billet steel plate prepared in Comparative Example 1 of the present invention;

[0025] Figure 6 The distribution map of the effective grain size at the center of the continuous casting billet steel plate prepared in Comparative Example 1 of the present invention;

[0026] Figure 7 The grain boundary map at the center of the continuous casting billet steel plate prepared in Comparative Example 2 of the present invention;

[0027] Figure 8 The distribution map of the effective grain size at the center of the continuous casting billet steel plate prepared in Comparative Example 2 of the present invention. Detailed implementation manners

[0028] See Figure 1 , a production method for improving the -100 °C impact energy of extra-thick shipbuilding steel using the large-angle grain boundary control technology provided in the embodiments of the present invention, includes the following steps:

[0029] Step 1) Heat and hold the continuous casting billet in the austenite recrystallization zone for the first time, and then directly water-cool it to room temperature through an ultra-fast cooling device. Among them, the first heating temperature is controlled at 1200 ± 50 °C, and the holding time is 2.5 - 4 h.

[0030] Step 2) Heat and hold the continuous casting billet in the austenite non-recrystallization zone for the second time, and then directly roll the continuous casting billet.

[0031] Among them, the second heating temperature is controlled at 1000 ± 150 °C, and the holding time is 1.5 - 3 h.

[0032] Among them, the starting rolling temperature during rolling of the continuous casting billet is controlled at 780 - T nr -20 °C, the total compression ratio during the rolling process is controlled below 3.5, and the total reduction in the austenite non-recrystallization zone is controlled at 50 - 70%.

[0033] Among them, T nr is the austenite recrystallization temperature, and the austenite recrystallization temperature T nr Adopt the formula T nr = 887 + 446ω c + 890ω Ti + 363ω Al - 357ω Si +6445ωNb - 644ω Nb 1 / 2 +732ω V -230ω V 1 / 2 Calculated

[0034] Step 3) Use the laminar flow cooling device to reduce the surface temperature of the rolled continuous casting billet to the recrystallization temperature of 320 ± 40 °C.

[0035] Among them, the matrix structure at the 1 / 2 position of the continuous casting billet after final rolling and cooling is a mixed structure composed of ferrite + bainite + small block-shaped M / A islands, and the proportion of large-angle grain boundaries at the 1 / 2 position is 66.4%.

[0036] Among them, the effective grain distribution at the 1 / 2 position of the continuous casting billet after final rolling and cooling is uniform and the effective grain size is 3.8 μm.

[0037] After the above processing, the impact energy of the continuous casting billet obtained after final rolling and cooling is ≥200 J at -80 °C and ≥182 J at -100 °C.

[0038] The following specifically describes a production method for improving the impact energy of extra-thick shipbuilding steel at -100 °C using the large-angle grain boundary control technology provided by the present invention through examples and comparative examples.

[0039] Among them, both the examples and the comparative examples use industrial EH47 continuous casting billets as raw materials.

[0040] Example 1

[0041] 1) Heat the industrial EH47 continuous casting billet at a heating temperature of 1200 °C in the austenite recrystallization region for the first time and hold for ≥2.5 h, and then directly cool the continuous casting billet to room temperature through an ultra-fast cooling device.

[0042] 2) Heat the continuous casting billet at a heating temperature of 1050 °C in the austenite non-recrystallization region for the second time and hold for ≥1.5 h, and then directly roll the continuous casting billet. Among them, the starting rolling temperature during the rolling of the continuous casting billet is controlled at 810 °C, the total compression ratio during the rolling process is controlled below 3.5, and the total reduction is controlled at 69%.

[0043] 3) After rolling, use the laminar flow cooling device to reduce the surface temperature of the continuous casting billet to the recrystallization temperature of 350 °C. Among them, the water inlet temperature of the rolled continuous casting billet in the laminar flow cooling device is controlled at 790 °C. The thickness of the continuous casting billet finally obtained in this example is 40 mm.

[0044] The relevant processing processes of the continuous casting billet in the example of the present invention are shown in Table 1. The low-temperature impact properties of the continuous casting billet finally obtained in this example are shown in Table 2.

[0045] The proportion of large-angle grain boundaries and the effective grain size at the core of the continuous casting slab steel plate finally obtained in the embodiments of the present invention are shown in Table 3.

[0046] The typical microstructure at the core of the continuous casting slab steel plate finally obtained in the embodiments of the present invention is as Figure 2 shown. From Figure 2 it can be seen that the metallographic structure of the continuous casting slab steel plate finally obtained in the embodiments of the present invention is mainly a mixed structure of ferrite + bainite + small block-shaped M / A islands.

[0047] The grain boundary map at the core of the continuous casting slab steel plate finally obtained in the embodiments of the present invention is as Figure 3 shown. From Figure 3 it can be seen that the entire field of view of the grain boundaries at the core of the continuous casting slab steel plate finally obtained in the embodiments of the present invention is covered by large-angle grain boundaries, and the proportion of large-angle grain boundaries is 66.4%.

[0048] The distribution of the effective grain size at the core of the continuous casting slab steel plate finally obtained in the embodiments of the present invention is as Figure 4 shown. From Figure 4 it can be seen that in the distribution of the effective grain size at the core of the continuous casting slab steel plate finally obtained in the embodiments of the present invention, the proportion of those below 5 μm is as high as 76%.

[0049] Comparative Example 1

[0050] The difference between this comparative example and Example 1 is that a two-stage heating and secondary controlled rolling process is adopted, and one-stage rolling is carried out in the austenite recrystallization zone after the first heating and heat preservation. The total reduction ratio of this comparative example is the same as that of Example 1. The thickness of the continuous casting slab finally obtained in this comparative example is 40 mm. The relevant processing processes of the continuous casting slab in this comparative example are shown in Table 1. The low-temperature impact properties of the continuous casting slab finally obtained in this comparative example are shown in Table 2.

[0051] The proportion of large-angle grain boundaries and the effective grain size at the core of the continuous casting slab steel plate finally obtained in this comparative example are shown in Table 3.

[0052] The grain boundary map at the core of the continuous casting slab steel plate finally obtained in this comparative example is as Figure 5 shown.

[0053] The distribution of the effective grain size at the core of the continuous casting slab steel plate finally obtained in this comparative example is as Figure 6 shown.

[0054] Comparative Example 2

[0055] The difference between this comparative example and Example 1 lies in that the traditional two-stage rolling process is adopted. After the first-stage rolling in the austenite recrystallization zone, no cooling is carried out, and the second-stage rolling is directly carried out in the austenite non-recrystallization zone. The total reduction ratio of this comparative example is the same as that of Example 1. The thickness of the continuously cast slab finally obtained in this comparative example is 40 mm. The relevant processing techniques of the continuously cast slab in this comparative example are shown in Table 1. The low-temperature impact properties of the continuously cast slab finally obtained in this comparative example are shown in Table 2.

[0056] The proportion of large-angle grain boundaries and the effective grain size at the core of the continuously cast slab steel plate finally obtained in this comparative example are shown in Table 3.

[0057] The grain boundary map at the core of the continuously cast slab steel plate finally obtained in this comparative example is as Figure 7 shown.

[0058] The distribution of the effective grain size at the core of the continuously cast slab steel plate finally obtained in this comparative example is as Figure 8 shown.

[0059] Table 1

[0060]

[0061] Note: “—” in the table represents that this step of the experiment was not carried out.

[0062] Table 2

[0063]

[0064] Table 3

[0065]

[0066] The comparison results show that the low-temperature toughness under the condition of -100 °C in Example 1 is the most excellent. At the same time, the proportion of large-angle grain boundaries at the core of the steel plate is the highest, the effective grain size is the smallest and evenly distributed. And, the impact energy at -60 °C ≥ 200 J, and the impact energy at -80 °C ≥ 182 J. In Comparative Example 1 and Comparative Example 2, the proportion of large-angle grain boundaries is generally lower than that in Example 1, the effective grain size is larger and unevenly distributed, and the impact energy is also lower than that in Example 1.

[0067] In Example 1 of the present invention, when measuring the surface temperature of the steel plate, in order to reduce the experimental error, the same temperature measuring device, the same temperature measuring position and the same temperature measuring method are adopted. To ensure the same cooling schedule, during the water cooling process, the cooling system maintains a consistent inlet water temperature, boiling time and water flow rate.

[0068] In the examples of the present invention, the impact properties are all carried out according to the national standard GB-T229-2007. The notch of the impact specimen is a V-notch. At the same time, to ensure the reliability of the experimental results, the average value of three test data is taken as the final result.

[0069] From the comparison between the embodiments of the present invention and the two comparative examples, it can be seen that a production method for improving the -100°C impact energy of extra-thick shipbuilding steel by using the large-angle grain boundary control technology provided by the embodiments of the present invention uses billets of the existing alloy composition system, and realizes a high proportion of large-angle grain boundaries and refines the effective grains under the conditions of low reduction ratio and high finish rolling temperature, improving the low-temperature toughness of the extra-thick shipbuilding steel, and achieving that the impact energy of the steel plate at -80°C is ≥200 J and the impact energy at -100°C is ≥182 J.

[0070] Finally, it should be noted that the above specific embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to the examples, those of ordinary skill in the art should understand that the technical solutions of the present invention can be modified or equivalently replaced without departing from the spirit and scope of the technical solutions of the present invention, and they should all be covered within the scope of the claims of the present invention.

Claims

1. A production method for improving the -100°C impact energy of extra-thick shipbuilding steel by using large-angle grain boundary control technology, characterized in that, It includes the following steps: The continuous casting billet is heated and held for the first time in the austenite recrystallization zone, and then directly water-cooled to room temperature by an ultra-fast cooling device; The continuous casting billet is heated and held for the second time in the non-recrystallized austenite zone, and then rolled; The surface temperature of the rolled continuous casting billet is reduced to the return red temperature of 320±40°C by a laminar flow cooling device; The starting rolling temperature during the rolling of the continuous casting billet is 780 - T nr - 20 °C, where T nr is the austenite recrystallization temperature. T nr is calculated using the following formula: T nr = 887 + 446ω c + 890ω Ti + 363ω Al - 357ω Si + 6445ω Nb - 644ω Nb 1 / 2 + 732ω V - 230ω V 1 / 2 and is obtained by calculation; The total reduction ratio during the rolling of the continuous casting billet is controlled below 3.5, and the total reduction amount is controlled at 50-70%; 2. The production method for improving the -100°C impact energy of extra-thick shipbuilding steel by using the large-angle grain boundary control technology according to claim 1, characterized in that: The heating temperature for the first heating and holding of the continuous casting billet is 1200±50°C, and the holding time is 2.5-4 h; 3. The production method for improving the -100°C impact energy of extra-thick shipbuilding steel by using the large-angle grain boundary control technology according to claim 1, characterized in that: The heating temperature for the second heating and holding of the continuous casting billet is 1000±150°C, and the holding time is 1.5-3 h; 4. The production method for improving the -100 °C impact energy of extra-thick shipbuilding steel by using the large-angle grain boundary control technology according to claim 1, characterized in that: The matrix structure at the 1 / 2 position of the rolled and cooled continuous casting billet is a mixed structure composed of ferrite + bainite + small blocky M / A islands, and the proportion of large-angle grain boundaries at the 1 / 2 position is 66.4%; 5. The production method for improving the -100°C impact energy of extra-thick shipbuilding steel by using the large-angle grain boundary control technology according to claim 1, characterized in that: The effective grain distribution at the 1 / 2 position of the rolled and cooled continuous casting billet is uniform, and the effective grain size is 3.8 μm; 6. The production method for improving the -100°C impact energy of extra-thick shipbuilding steel by using the large-angle grain boundary control technology according to claim 1, characterized in that: The impact energy of the rolled and cooled continuous casting billet at -80°C is ≥200 J, and the impact energy at -100°C is ≥182 J;

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